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Linux PPP HOWTO
Robert Hart, hartr@interweft.com.au
v3.0, 31 March 1997
This document shows how to connect your Linux PC to a PPP server, how
to use PPP to link two LANs together and provides one method of set­
ting up your Linux computer as a PPP server.The document also provides
help in debugging non-functional PPP connections.
______________________________________________________________________
Table of Contents
1. Introduction
1.1 Clients and Servers
1.2 Differences between Linux distributions
1.3 Distribution specific PPP configuration tools
2. IP Numbers
3. Aims of this Document
3.1 Setting up a PPP Client
3.2 Linking two LANs or a LAN to the Internet using PPP
3.3 Setting up a PPP server
3.4 Using PPP over a direct null modem connection
3.5 This document at present does NOT cover...
4. Software versions covered
5. Other Useful/Important Documents
5.1 Useful Linux Mailing Lists
6. Overview of what has to be done to get PPP working as a client
6.1 Obtaining/Installing the software
6.2 Compiling PPP support into the kernel
6.3 Obtaining information from your ISP
6.4 Configuring your modem and serial port
6.5 Setting up Name to Address Resolution (DNS)
6.6 PPP and root Privileges
6.7 Checking your distribution PPP Files and setting up the PPP Options
6.8 If your PPP server uses PAP (Password Authentication Protocol)
6.9 Connecting to the PPP server by hand
6.10 Automating your PPP Connection
6.11 Shutting down the link
6.12 If you have problems
6.13 After the link comes up
6.14 Problems with standard IP services on a Dynamic IP number PPP link
7. Configuring your Linux Kernel
7.1 Installing the Linux Kernel source
7.2 Knowing your hardware
7.3 Kernel compilation - the Linux 1.2.13 kernel
7.4 Kernel compilation - the Linux 1.3.x and 2.0.x kernels
7.5 Note on PPP-2.2 and
7.6 General kernel config considerations for PPP
8. Getting the Information you need about the PPP server
9. Configuring your modem and serial port
9.1 A note about serial ports and speed capabilities
9.2 Serial Port Names
9.3 Configuring your modem
9.4 Note on Serial Flow Control
9.5 Testing your modem for dial out
10. Setting up Name to Address Resolution (DNS)
10.1 The
10.2 The
11. Using PPP and root privileges
12. Setting up the PPP connection files
12.1 The supplied options.tpl file
12.2 What options should I use? (No PAP/CHAP)
13. If your PPP server uses PAP (Password Authentication Protocol)
13.1 Using MSCHAP
13.2 The PAP/CHAP secrets file
13.3 The PAP secrets file
13.4 The CHAP secrets file
13.5 Handling multiple PAP-authenticated connections
14. Setting up the PPP connection manually
15. Automating your connections - Creating the connection scripts
15.1 Connection scripts for User name/Password Authentication
15.2 The ppp-on script
15.3 Editing the supplied PPP startup scripts
15.3.1 The ppp-on script
15.3.2 The ppp-on-dialer script
15.4 What a Chat script means...
15.4.1 Starting PPP at the server end
15.5 A chat script for PAP/CHAP authenticated connections
15.6 The pppd
16. Testing your connection script
17. Shutting down the PPP link
18. Debugging
18.1 I have compiled PPP support into the kernel, but...
18.1.1 Are you booting the right kernel? Whilst you
18.1.2 Did you compile ppp kernel support as a module?
18.1.3 Are you using the correct version of PPP for your kernel?
18.1.4 Are you running pppd as root?
18.2 My modem connects but ppp never starts up
18.3 The syslog says "
18.3.1 You are not correctly logging into the server
18.3.2 You are not starting PPP on the server
18.3.3 The remote PPP process is slow to start
18.4 Default route not set
18.5 Other Problems
19. Getting Help when totally stuck
20. Common Problems once the link is working
20.1 I can't see beyond the PPP server I connect to
20.2 I can send email, but not receive it
20.3 Why can't people finger, WWW, gopher, talk etc to my machine?
21. Using Internet services with Dynamic IP numbers
21.1 Setting up email
21.2 Setting Up a local Name server
22. Linking two networks using PPP
22.1 Setting up the IP numbers
22.2 Setting up the routing
22.3 Network security
23. After the link comes up - the
23.1 Special routing
23.2 Handling email queues
23.3 A sample
23.4 Handling email
24. Using
25. Routing issues on a LAN
25.1 Note on Security
26. Setting up a PPP server
26.1 Kernel compilation
26.2 Overview of the server system
26.3 Getting the software together
26.4 Setting up standard (shell access) dialup.
26.5 Setting up the PPP options files
26.6 Setting pppd up to allow users to (successfully) run it
26.7 Setting up the global alias for pppd
27. Using PPP across a null modem (direct serial) connection
______________________________________________________________________
Copyright
This document is distributed under the terms of the GPL (GNU Public
License).
Distribution
This document will be posted to comp.os.linux.answers as new versions
of the document are produced. It is also available in HTML format at:-
· Linux Howto Index <http://sunsite.unc.edu/mdw/linux.html#howto>
· PPP-HOWTO <http://www.interweft.com.au/other/ppp-howto/ppp-
howto.html>
Other formats (SGML, ASCII, postscript, DVI) are available from Howtos
- other formats <ftp://sunsite.unc.edu/pub/Linux/docs/HOWTO/other-
formats>.
As sunsite.unc.edu carries a very heavy load, please use an
appropriate mirror site close to you.
Acknowledgements
A growing number of people have provided me with assistance in
preparing this document. Special thanks go to Al Longyear for the
guidance on PPP itself (if there are mistakes here, they are mine not
his), Greg Hankins (maintainer of the Linux Howto system)and Debi
Tackett (of MaximumAccess.com) for many helpful suggestions on style,
content order, logic and clarity of explanations.
Finally, to the many people who have contacted me by email offering
comments - my thanks. As with all HOWTO authors, the satisfaction of
helping is all the payment we receive and it is enough. By writing
this HOWTO I am repaying in a small way the debt I - and all other
Linux users - owe to the people who write and maintain our OS of
choice.
1. Introduction
PPP (the Point to Point Protocol) is a mechanism for creating and
running IP (the Internet Protocol) and other network protocols over a
serial link - be that a direct serial connection (using a null-modem
cable), over a telnet established link or a link made using modems and
telephone lines (and of course using digital lines such as ISDN).
Using PPP, you can connect your Linux PC to a PPP server and access
the resources of the network to which the server is connected (almost)
as if you were directly connected to that network.
You can also set up your Linux PC as a PPP server, so that other
computers can dial into your computer and access the resources on your
local PC and/or network.
As PPP is a peer-to-peer system, you can also use PPP on two Linux PCs
to link together two networks (or a local network to the Internet),
creating a Wide Area Network (WAN).
One major difference between PPP and an Ethernet connection is of
course speed - a standard Ethernet connection operates at 10 Mbs (Mega
- million bits per second) maximum theoretical throughput, whereas an
analogue modem operates at speeds up to 56 kbps (kilo - thousand bits
per second).
Also, depending on the type of PPP connection, there may be some
limitations in usage of some applications and services.
1.1. Clients and Servers
PPP is strictly a peer to peer protocol; there is (technically) no
difference between the machine that dials in and the machine that is
dialed into. However, for clarity's sake, it is useful to think in
terms of servers and clients.
When you dial into a site to establish a PPP connection, you are a
client. The machine to which you connect is the server.
When you are setting up a Linux box to receive and handle dial in PPP
connections, you are setting up a PPP server.
Any Linux PC can be both a PPP server and client - even simultaneously
if you have more than one serial port (and modem if necessary). As
stated above, there is no real difference between clients and servers
as far as PPP is concerned, once the connection is made.
This document refers to the machine that initiates the call (that
dials in) as the CLIENT, whilst the machine that answers the
telephone, checks the authentication of the dial in request (using
user names, passwords and possibly other mechanisms) is referred to as
the SERVER.
The use of PPP as a client to link one or more machines at a location
into the Internet is, probably, the one in which most people are
interested - that is using their Linux PC as a client.
The procedure described in this document will allow you to establish
and automate your Internet connection.
This document will also give you guidance in setting up your Linux PC
as a PPP server and in linking two LANs together (with full routing)
using PPP (this is frequently characterised as establishing a WAN -
wide area network - link).
1.2. Differences between Linux distributions
There are many different Linux distributions and they all have their
own idiosyncrasies and ways of doing things.
In particular, there are two different ways a Linux (and Unix)
computer actually starts up, configures its interfaces and so forth.
These are BSD system initialisation and System V system
initialisation. If you dip into some of the Unix news groups, you will
find occasional religious wars between proponents of these two
systems. If that sort of thing amuses you, have fun burning bandwidth
and join in!
Possibly the most widely used distributions are
· Slackware
which uses BSD style system initialisation
· Red Hat (and its former associate Caldera)
which use SysV system initialisation (although in a slightly
modified form)
· Debian
which uses SysV system initialisation
BSD style initialisation typically keeps its initialisation files in
/etc/... and these files are:-
______________________________________________________________________
/etc/rc
/etc/rc.local
/etc/rc.serial
(and possibly other files)
______________________________________________________________________
Of recent times, some BSD system initialisation schemes use a
/etc/rc.d... directory to hold the start up file rather than putting
everything into /etc.
System V initialisation keeps its initialisation files in directories
under /etc/... or /etc/rc.d/... and a number of subdirectories under
there:-
______________________________________________________________________
drwxr-xr-x 2 root root 1024 Jul 6 15:12 init.d
-rwxr-xr-x 1 root root 1776 Feb 9 05:01 rc
-rwxr-xr-x 1 root root 820 Jan 2 1996 rc.local
-rwxr-xr-x 1 root root 2567 Jul 5 20:30 rc.sysinit
drwxr-xr-x 2 root root 1024 Jul 6 15:12 rc0.d
drwxr-xr-x 2 root root 1024 Jul 6 15:12 rc1.d
drwxr-xr-x 2 root root 1024 Jul 6 15:12 rc2.d
drwxr-xr-x 2 root root 1024 Jul 18 18:07 rc3.d
drwxr-xr-x 2 root root 1024 May 27 1995 rc4.d
drwxr-xr-x 2 root root 1024 Jul 6 15:12 rc5.d
drwxr-xr-x 2 root root 1024 Jul 6 15:12 rc6.d
______________________________________________________________________
If you are trying to track down where your Ethernet interface and
associated network routes are actually configured, you will need to
track through these files to actually find where the commands are that
do this.
1.3. Distribution specific PPP configuration tools
On some installations (for example Red Hat and Caldera), there is a X
Windows configured PPP dial up system. This HOWTO does not cover these
distribution specific tools. If you are having problems with them,
contact the distributors directly!
For Red Hat 4.x users, there is now a Red Hat PPP-TIP
<http://www.interweft.com.au> in the Linux resources area and also
from Red Hat Software <http://www.redhat.com> in the support area.
2. IP Numbers
Every device that connects to the Internet must have its own, unique
IP number. These are assigned centrally by a designated authority for
each country.
If you are connecting a local area network (LAN) to the Internet, YOU
MUST use an IP number from your own assigned network range for all the
computers and devices you have on your LAN. You MUST NOT pick IP
numbers out of the air and use these whilst connecting to another LAN
(let alone the Internet). At worst this will simply not work at all
and could cause total havoc as your 'stolen' IP number starts
interfering with the communications of another computer that is
already using the IP number you have picked out of the air.
Please note that the IP numbers used throughout this document (with
some exceptions) are from the 'unconnected network numbers' series
that are reserved for use by networks that are not (ever) connected to
the Internet.
There are IP numbers that are specifically dedicated to LANs that do
not connect to the Internet. The IP number sequences are:-
· One A Class Network Address
10.0.0.0 (netmask 255.0.0.0)
· 16 B Class Network Addresses
172.16.0.0 - 172.31.0.0 (netmask 255.255.0.0)
· 256 C Class Network Addresses
192.168.0.0 - 192.168.255.0 (netmask 255.255.255.0)
If you have a LAN for which you have not been allocated IP numbers by
the responsible authority in your country, you should use one of the
network numbers from the above sequences for your machines.
These numbers should never be used on the Internet.
However, they can be used for the local Ethernet on a machine that is
connecting to the Internet. This is because IP numbers are actually
allocated to a network interface, not to a computer. So whilst your
Ethernet interface may use 10.0.0.1 (for example), when you hook onto
the Internet using PPP, your PPP interface will be given another (and
valid) IP number by the server. Your PC will have Internet
connectivity, but the other computers on your LAN will not.
However, using Linux and the IP Masquerade (also known as NAT -
Network address Translation) capabilities of the Linux and the ipfwadm
software, you can connect your LAN to the Internet (with some
restriction of services), even if you do not have valid IP numbers for
the machines on your Ethernet.
For more information on how to do this see the IP Masquerade mini-
HOWTO at Linux IP Masquerade mini HOWTO
<http://sunsite.unc.edu/mdw/HOWTO/mini/IP-Masquerade>
For most users, who are connecting a single machine to an Internet
service provider via PPP, obtaining an IP number (or more accurately,
a network number) will not be necessary.
If you wish to connect a small LAN to the Internet, many Internet
Service Providers (ISPs) can provide you with a dedicated subnet (a
specific sequence of IP numbers) from their existing IP address space.
Alternatively, use IP Masquerading.
For users, who are connecting a single PC to the Internet via an ISP,
most providers use dynamic IP number assignment. That is, as part of
the connection process, the PPP service you contact will tell your
machine what IP number to use for the PPP interface during the current
session. This number will not be the same every time you connect to
your ISP.
With dynamic IP numbers, you are not given the same IP number each
time you connect. This has implications for server type applications
on your Linux machine such as sendmail, ftpd, httpd and so forth.
These services are based on the premise that the computer offering the
service is accessible at the same IP number all the time (or at least
the same fully qualified domain name - FQDN - and that DNS resolution
of the name to IP address is available).
The limitations of service due to dynamic IP number assignment (and
ways to work around these, where possible) are discussed later in the
document.
3. Aims of this Document
3.1. Setting up a PPP Client
This document provides guidance to people who wish to use Linux and
PPP to dial into a PPP server and set up an IP connection using PPP.
It assumes that PPP has been compiled and installed on your Linux
machine (but does briefly cover reconfiguring/recompiling your kernel
to include PPP support).
Whilst DIP (the standard way of creating a SLIP connection) can be
used to set up a PPP connection, DIP scripts are generally quite
complex. For this reason, this document does NOT cover using DIP to
set up a PPP connection.
Instead, this document describes the standard Linux PPP software
(chat/pppd).
3.2. Linking two LANs or a LAN to the Internet using PPP
This document provides (basic) information on linking two LANs or a
LAN to the Internet using PPP.
3.3. Setting up a PPP server
This document provides guidance on how to configure your Linux PC as a
PPP server (allowing other people to dial into your Linux PC and
establish a PPP connection).
You should note that there are a myriad of ways of setting up Linux as
a PPP server. This document gives one method - that used by the author
to set up several small PPP servers (each of 16 modems).
This method is known to work well. However, it is not necessarily the
best method.
3.4. Using PPP over a direct null modem connection
This document provides a brief overview of using PPP to link two Linux
PCs via a null modem cable. It is possible to link other OS's to Linux
this way as well. To do so, you will need to consult the documentation
for the operating system you are interested in.
3.5. This document at present does NOT cover...
· Compiling the PPP daemon software
See the documentation that comes with the version of pppd you are
using.
· Connecting and configuring a modem to Linux (in detail)
See the Serial-HOWTO and for modem specific initialisation, see
Modem Setup Information <http://www.in.net/info/modems/index.html>
for information that may help you to configure your modem.
· Using DIP to make PPP connections
Use chat instead...
· Using socks or IP Masquerade
There are perfectly good documents already covering these two
packages.
· Using diald to set up an automated connection
See the diald documentation for information on this.
· Using EQL to gang together two modems into a single PPP link.
· Distribution specific PPP connection methods (such as the Red Hat
4.x network configuration tool.
See the distribution for documentation on the methods used.
· The growing number of tools available to automate PPP set up
See the appropriate documentation.
4. Software versions covered
This HOWTO assumes that you are using a Linux 1.2.x kernel with the
PPP 2.1.2 software or Linux 1.3.X/2.0.x and PPP 2.2.
At the time of writing, the latest official version of PPP available
for Linux is ppp-2.2f. The new version (ppp-2.3) is still in beta.
It is possible to use PPP 2.2.0 with kernel 1.2.13. This requires
kernel patches. It is recommended that version 1.2.13 kernel users
move up to ppp-2.2 as it includes several bug fixes and enhancements.
Also, you should particularly note that you cannot use the PPP 2.1.2
software with Linux kernel version 2.0.X.
Please note that this document does NOT cover problems arising from
the use of loadable modules for Linux kernel 2.0.x. Please see the
kerneld mini-HOWTO and the kernel/module 2.0.x documentation (in the
Linux 2.0.x source tree at /usr/src/linux/Documentation/...).
As this document is designed to assist new users, it is highly
recommended that you use a version of the Linux kernel and the
appropriate PPP version that are known to be stable together.
5. Other Useful/Important Documents
Users are advised to read :-
· the documentation that comes with the PPP package;
· the pppd and chat man pages;
(use man chat and man pppd to explore these)
· the Linux Network Administration Guide (NAG);
see The Network Administrators' Guide
<http://sunsite.unc.edu/mdw/LDP-books/nag-1.0/nag.html>
· the Net-2/3 HOWTO;
see Linux NET-2/3-HOWTO
<http://sunsite.unc.edu/mdw/HOWTO/NET-2-HOWTO.html>
· Linux kernel documentation installed in
/usr/src/linux/Documentation when you install the Linux source
code;
· The modem setup information page - see Modem Setup Information
<http://www.in.net/info/modems/index.html>
· The excellent Unix/Linux books published by O'Reilly and
Associates. See (O'Reilly and Associates On-Line Catalogue <
http://www.ora.com/>). If you are new to Unix/Linux, run (don't
walk) to your nearest computer book shop and invest in a number of
these immediately!
· The PPP-FAQ maintained by Al Longyear, available from Linux PPP-FAQ
<ftp://sunsite.unc.edu/pub/Linux/docs/faqs>.
This contains a great deal of useful information in question/answer
format that is very useful when working out why PPP is not working
(properly).
· The growing number of Linux books from various publishing houses
and authors;
You are actively encouraged to check the currency of these books.
Linux development and distributions tend to evolve fairly rapidly,
whilst the revision of books move (generally) much more slowly!
Buying an excellent book (and there are many) that is now out of
date will cause new users considerable confusion and frustration.
The best general starting point for Linux documentation is The Linux
Documentation Project Home Page <http://sunsite.unc.edu/mdw/>. The
HOWTO's tend to be revised reasonably regularly.
Whilst you can use this document to create your PPP link without
reading any of these documents, you will have a far better
understanding of what is going on if you do so! You will also be able
to address problems yourself (or at least ask more intelligent
questions on the comp.os.linux... newsgroups or Linux mailing lists).
These documents (as well as various others, including the relevant
RFCs) provide additional and more detailed explanation than is
possible in this HOWTO.
If you are connecting a LAN to the Internet using PPP, you will need
to know a reasonable amount about TCP/IP networking. In addition to
the documents above, you will find the O'Reilly books "TCP/IP Network
Administration" and "Building Internet Firewalls" of considerable
benefit!
5.1. Useful Linux Mailing Lists
There are many Linux mailing lists that operate as a means of
communication between users of many levels of ability. By all means
subscribe to those that interest you and contribute your expertise and
views.
A word to the wise: some lists are specifically aimed at "high
powered" users and/or specific topics. Whilst no-one will complain if
you 'lurk' (subscribe but don't post messages), you are likely to earn
heated comments (if not outright flames) if you post 'newbie'
questions to inappropriate lists.
This is not because guru level users hate new users, but because these
lists are there to handle the specific issues at particular levels of
difficulty.
By all means join the lists that offer open subscription, but keep
your comments relevant to the subject of the list!
A good starting point for Linux mailing lists is Linux Mailing List
Directory <http://summer.snu.ac.kr/~djshin/linux/mail-
list/index.shtml>
6. Overview of what has to be done to get PPP working as a client
This document contains a great deal of information - and with each
version it grows!
As a consequence, this section aims to provide a concise overview of
the actions you will need to take to get your Linux system connected
as a client to a PPP server.
6.1. Obtaining/Installing the software
If your Linux distribution does not include the PPP software, you will
need to obtain this from the Linux PPP daemon
<ftp://sunsite.unc.edu/pub/Linux/system/Network/serial/ppp/ppp-2.2.0f.tar.gz>.
This is the latest official version at the time of writing. However,
choose the latest version available from this site (ppp-2.3 is in beta
at the time of writing and should be released soon).
The PPP package contains instructions on how to compile and install
the software so this HOWTO does not!
6.2. Compiling PPP support into the kernel
Linux PPP operations come in two parts
· the PPP daemon mentioned above
· kernel support for PPP
Many distributions seem to provide PPP kernel support in their default
installation kernels, but others do not.
If at boot your kernel reports messages like
______________________________________________________________________
PPP Dynamic channel allocation code copyright 1995 Caldera, Inc.
PPP line discipline registered.
______________________________________________________________________
your kernel does have PPP support compiled in.
That said, you will probably want to compile your own kernel whatever
your distribution to provide the most efficient use of system
resources given your particular hardware configuration. It is worth
remembering that the kernel cannot be swapped out of memory and so
keeping the kernel as small as possible has advantages on a memory
limited machine.
This document provides minimal kernel re-compilation instructions at
section ``Configuring your Linux Kernel''.
For greater detail, see the Kernel-HOWTO at The Linux Kernel HOWTO
<http://sunsite.unc.edu/mdw/HOWTO/Kernel-HOWTO.html>
6.3. Obtaining information from your ISP
There are an almost infinite number of ways in which a PPP server can
be set up. In order to connect to your ISP (or corporate PPP server to
access your intranet), you will need to obtain information on how the
PPP server operates.
Because you are using Linux, you may have some difficulty with some
ISP help desks (and work site based PPP intranet servers) which know
only about MS Windows clients.
However, a rapidly growing number of ISPs use Linux to provide their
service - and Linux is also penetrating the corporate environment as
well, so you may be lucky if you do strike problems.
Section ``Getting the Information you need about the PPP server''
tells you what you need to know about the PPP server to which you are
going to connect - and how to find out the information you need to
know.
6.4. Configuring your modem and serial port
In order to connect to a PPP server and to obtain the best possible
data transfer rate, your modem needs to be configured correctly.
Similarly, the serial ports on your modem and computer need to be set
up correctly.
Section ``Configuring your modem and serial port'' provides
information on this.
6.5. Setting up Name to Address Resolution (DNS)
In addition to the files that run PPP and perform the automated log in
to the PPP server, there are a number of text configuration files that
have to be set up for your computer to be able to resolve names like
www.interweft.com.au to the IP address that is actually used to
contact that computer. These are:-
· /etc/resolv.conf
· /etc/host.conf
Section ``Setting up Name to Address Resolution'' for details on
setting this up.
In particular, you do NOT need to run a name server on your Linux PC
in order to connect to the Internet (although you may wish to). All
you need is to know the IP number of at least one name server that you
can use (preferably one at your ISPs site).
6.6. PPP and root Privileges
As establishing a PPP link between you Linux computer and another PPP
server requires manipulation of network devices (the PPP interface is
a network interface) and the kernel routing table, pppd requires root
privileges.
For details on this, see section ``Using PPP and root privileges''.
6.7. Checking your distribution PPP Files and setting up the PPP
Options
There are a number of configuration and dialer files that need to be
set up to get PPP operational. There are examples as part of the PPP
distribution and this section shows what files you should have:-
______________________________________________________________________
/etc/ppp/options
/etc/ppp/scripts/ppp-on
/etc/ppp/scripts/ppp-on-dialer
/etc/ppp/options.tpl
______________________________________________________________________
You may need to create some additional files depending on exactly what
you are aiming to achieve with PPP:-
______________________________________________________________________
/etc/ppp/options.ttyXX
/etc/ppp/ip-up
/etc/ppp/pap-secrets
/etc/ppp/chap-secrets
______________________________________________________________________
In addition, the PPP daemon can use a large number of command line
options and it is important to use the right ones; so this section
takes you through the standard PPP options and helps you choose the
options you should use.
For details on this, see ``Setting up the PPP connection files''.
6.8. If your PPP server uses PAP (Password Authentication Protocol)
Many ISPs and corporate PPP servers use PAP. If your server does not
require you to use PAP (if you can log in manually and receive the
standard user name/password text based prompts it does not use PAP),
you can safely ignore this section.
Instead of logging into such a server using a user name and password
when prompted to enter them by the server, a PPP server using PAP does
not require a text based login.
The user authentication information instead is exchanged as part of
the link control protocol (LCP) which is the first part of
establishing a PPP link.
Section ``If your PPP server uses PAP (Password Authentication
Protocol)'' provides information on the files you need to set up to
establish a PPP link using PAP.
6.9. Connecting to the PPP server by hand
Having set up the basic files, it is a good idea to test these by
connecting (using minicom or seyon) and starting pppd on your Linux PC
by hand.
See Section ``Setting up the PPP connection manually'' for full
details of setting this up.
6.10. Automating your PPP Connection
Once you are able to log in by hand, you can now move to setting up a
set of scripts that will automate the establishment of the connection.
Section ``Automating your connections - Creating the connection
scripts'' covers setting up the necessary scripts, with considerable
attention paid to chat and scripting the login process to the PPP
server.
This section discusses scripts for user name/password authentication
as well as scripts for PAP/CHAP authenticating servers.
6.11. Shutting down the link
Once your link is up and working, you need to be able to deactivate
the link.
This is covered in Section ``Shutting down the PPP link''.
6.12. If you have problems
Many people have problems getting PPP to work straight away. The
variation in PPP servers and how they require you to set up the
connection is enormous. Similarly, there are many options to PPP - and
some combinations of these just do not work together, ever.
In addition to the problems of logging in and starting the PPP
service, there are problems with the modems and the actual telephone
lines as well!
Section ``Fixing problems'' provides some basic information about
common errors, how to isolate these and fix them.
This is NOT intended to provide more than just the basics. Al Longyear
maintains the PPP-FAQ which contains much more information on this
topic!
6.13. After the link comes up
Once a PPP link is operational (specifically, once the IP layer is
operational), Linux PPP can automatically run (as the root user), a
script to perform any function you can write a script to accomplish.
Section ``After the link comes up'' provides information on the
/etc/ppp/ip-up script, the parameters it receives from PPP and how to
use it to do things like acquire your email from your ISP account,
send any queued email waiting transmission on your machine and such.
6.14. Problems with standard IP services on a Dynamic IP number PPP
link
As noted in the introduction, dynamic IP numbers affect the ability of
your Linux PC to act as a server on the Internet.
Section ``Problems with standard IP services on a Dynamic IP number
PPP link'' provides information on the (main) services affected and
what you can do (if anything) to overcome this.
7. Configuring your Linux Kernel
In order to use PPP, your Linux kernel must be compiled to include PPP
support. Obtain the Linux source code for your kernel if you do not
already have this - it belongs in /usr/src/linux on Linux's standard
file system.
Check out this directory - many Linux distributions install the source
tree (the files and subdirectories) as part of their installation
process.
At bootup, your Linux kernel prints out a great deal of information.
Amongst this is information about PPP support if the kernel includes
this. To view this information, look at your syslog file or use dmesg
| less to display the information to the screen. If your kernel
includes PPP support, you will see lines like
______________________________________________________________________
PPP Dynamic channel allocation code copyright 1995 Caldera, Inc.
PPP line discipline registered.
______________________________________________________________________
(this is for the Linux 2.0.x kernel series).
Linux kernel sources can be obtained by ftp from sunsite.unc.edu or
its mirror sites.
7.1. Installing the Linux Kernel source
The following are brief instructions for obtaining and installing the
Linux kernel sources. Full information can be obtained from The Linux
Kernel HOWTO <http://sunsite.unc.edu/mdw/HOWTO/Kernel-HOWTO.html>.
In order to install and compile the Linux kernel, you need to be
logged in as root.
1. Change directory to the /usr/src directory
cd /usr/src
2. Check in /usr/src/linux to see if you already have the sources
installed.
3. If you don't have the sources, get them from Linux kernel source
directory <ftp://sunsite.unc.edu/pub/Linux/kernel/v2.0> or your
nearest mirror.
If you are looking for earlier versions of the kernel (such as
1.2.X), these are kept in Old Linux kernel source directory
<ftp://sunsite.unc.edu/pub/Linux/kernel/old>.
4. Choose the appropriate kernel - usually the most recent one
available is what you are looking for. Retrieve this and put the
source tar file in /usr/src.
Note: a 'tar' file is an archive - possibly compressed (as are the
Linux kernel source tar files) containing many files in a number of
directories. It is the Linux equivalent of a DOS multi-directory
zip file.
5. If you already have the Linux sources installed but are upgrading
to a new kernel, you must remove the old sources. Use the command
rm -rf /usr/src/linux
6. Now uncompress and extract the sources using the command
tar xzf linux-2.0.XX.tar.gz
7. Now, cd /usr/src/linux and read the README file. This contains an
excellent explanation of how to go about configuring and compiling
a new kernel. Read this file (it's a good idea to print it out and
have a copy handy whilst you are compiling until you have done this
enough times to know your way around).
7.2. Knowing your hardware
You MUST know what cards/devices you have inside your PC if you are
going to recompile your kernel!!! For some devices (such as sound
cards) you will also need to know various settings (such as IRQ's, I/O
addresses and such).
7.3. Kernel compilation - the Linux 1.2.13 kernel
To start the configuration process, follow the instructions in the
README file to properly install the sources. You start the kernel
configuration process with
make config
In order to use PPP, you must configure the kernel to include PPP
support (PPP requires BOTH pppd AND kernel support for PPP).
______________________________________________________________________
PPP (point-to-point) support (CONFIG_PPP) [n] y
______________________________________________________________________
Answer the other make config questions according to the hardware in
your PC and the features of the Linux operating system you want. Then
continue to follow the README to compile and install your new kernel.
The 1.2.13 kernel creates only 4 PPP devices. For multi- port serial
cards, you will need to edit the kernel PPP sources to obtain more
ports. (See the README.linux file that comes as part of the PPP-2.1.2
distribution for full details of the simple edits you need to make).
Note: the 1.2.13 configuration dialogue does NOT allow you to go
backwards - so if you make a mistake in answering one of the questions
in the make config dialogue, exit by typing CTRL C and start again.
7.4. Kernel compilation - the Linux 1.3.x and 2.0.x kernels
For Linux 1.3.x and 2.0.x, you can use a similar process as for Linux
1.2.13. Again, follow the instructions in the README file to properly
install the sources. You start the kernel configuration process with
make config
However, you also have the choice of
make menuconfig
This provides a menu based configuration system with online help that
allows you to move backwards and forwards in the configuration
process.
There is also a highly recommended X windows based configuration
interface
make xconfig
You can compile PPP support directly into your kernel or as a loadable
module.
If you only use PPP some of the time that your Linux machine is
operating, then compiling PPP support as a loadable module is
recommended. Using 'kerneld', your kernel will automatically load the
module(s) required to provide PPP support when you start your PPP link
process. This saves valuable memory space: no part of the kernel can
be swapped out of memory, but loadable modules are automatically
removed if they are not in use.
To do this, you need to enable loadable module support:-
______________________________________________________________________
Enable loadable module support (CONFIG_MODULES) [Y/n/?] y
______________________________________________________________________
To add PPP kernel support, answer the following question:-
______________________________________________________________________
PPP (point-to-point) support (CONFIG_PPP) [M/n/y/?]
______________________________________________________________________
For a PPP loadable module, answer M, otherwise for PPP compiled in as
part of the kernel, answer Y.
Unlike kernel 1.2.13, kernel 2.0.x creates PPP devices on the fly as
needed and it is not necessary to hack the sources to increase
available PPP device numbers at all.
7.5. Note on PPP-2.2 and /proc/net/dev
If you are using PPP-2.2, you will find that a side effect of the 'on
the fly' creation of the PPP devices is that no devices show up if you
look in the /proc/net file system until a device is created by
starting up pppd:-
______________________________________________________________________
[hartr@archenland hartr]$ cat /proc/net/dev
Inter-| Receive | Transmit
face |packets errs drop fifo frame|packets errs drop fifo colls carrier
lo: 92792 0 0 0 0 92792 0 0 0 0 0
eth0: 621737 13 13 0 23 501621 0 0 0 1309 0
______________________________________________________________________
Once you have one (or more) ppp services started, you will see entries
such as this (from a ppp server):-
______________________________________________________________________
[root@kepler /root]# cat /proc/net/dev
Inter-| Receive | Transmit
face |packets errs drop fifo frame|packets errs drop fifo colls carrier
lo: 428021 0 0 0 0 428021 0 0 0 0 0
eth0:4788257 648 648 319 650 1423836 0 0 0 4623 5
ppp0: 2103 3 3 0 0 2017 0 0 0 0 0
ppp1: 10008 0 0 0 0 8782 0 0 0 0 0
ppp2: 305 0 0 0 0 297 0 0 0 0 0
ppp3: 6720 7 7 0 0 7498 0 0 0 0 0
ppp4: 118231 725 725 0 0 117791 0 0 0 0 0
ppp5: 38915 5 5 0 0 28309 0 0 0 0 0
______________________________________________________________________
7.6. General kernel config considerations for PPP
If you are setting up your Linux PC as a PPP server, you must compile
in IP forwarding support. This is also necessary if you want to use
Linux to link to LANs together or your LAN to the Internet.
If you are linking a LAN to the Internet (or linking together two
LANs), you should be concerned about security. Adding support for IP
fire walls to the kernel is probably a MUST!
You will also need this if you want to use IP masquerade to connect a
LAN that uses any of the above mentioned 'unconnected' IP network
numbers.
To enable IP Masquerade and IP fire walling, you MUST answer yes to
the first question in the make config process:-
______________________________________________________________________
Prompt for development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL)?
______________________________________________________________________
Whilst this may sound a bit off-putting to new users, many users are
actively using the IP Masquerade and IP fire walling features of the
Linux 2.0.XX kernel with no problems.
Once you have installed and rebooted your new kernel, you can start
configuring and testing your PPP link(s).
8. Getting the Information you need about the PPP server
Before you can establish a PPP connection with a server, you need to
obtain the following information (from the sysadmin/user support
people of the PPP server):-
· The telephone number(s) to dial for the service
If you are behind a PABX, you also need the PABX number that gives
you an outside dial tone - this is frequently digit zero (0) or
nine (9).
· Does the server use DYNAMIC or STATIC IP numbers?
If the server uses STATIC IP numbers, then you may need to know
what IP number to use for your end of the PPP connection. If your
ISP is providing you with a subnet of valid IP numbers, you will
need to know the IP numbers you can use and the network mask
(netmask).
Most Internet Service Providers use DYNAMIC IP numbers. As
mentioned above, this has some implications in terms of the
services you can use.
However, even if you are using STATIC IP numbers, most PPP servers
will never (for security reasons) allow the client to specify an IP
number as this is a security risk. You do still need to know this
information!
· What are the IP numbers of the ISPs Domain Name Servers?
There should be at least two although only one is needed.
There could be a problem here. The MS Windows 95 PPP setup allows
the DNS address to be passed to the client as part of its
connection process. So your ISP (or corporate help desk) may well
tell you you don't need the IP address of the DNS server(s).
For Linux, you DO need the address of at least one DNS. The linux
implementation of PPP does not allow the setting of the DNS IP
number dynamically at connection time - and quite possibly will
never do so.
Note: whilst Linux (as a PPP client) cannot accept the DNS address
from a server, it can, when acting as a server, pass this
information to clients using the dns-addr pppd option.
· Does the server require the use of PAP/CHAP?
If this is the case you need to know the "id" and "secret" you are
to use in connecting. (These are probably your user name and
password at your ISP).
· Does the server automatically start PPP or do you need to issue any
commands to start PPP on the server once you are logged in?
If you must issue a command to start PPP, what is it?
· Is the server a Microsoft Windows NT system and, if so, is it using
the MS PAP/CHAP system?
Many corporate LANs seem to use MS Windows NT this way for
increased security.
Carefully note down this information - you are going to use it!
9. Configuring your modem and serial port
You should make sure that your modem is correctly set up and that you
know which serial port it is connected to.
Remember:-
· DOS com1: = Linux /dev/cua0 (and /dev/ttyS0)
· DOS com2: = Linux /dev/cua1 (and /dev/ttyS1)
et cetera
It is also worth remembering that if you have 4 serial ports, the
standard PC set up is to have com1 and com3 share IRQ4 and com2 and
com4 share IRQ3.
If you have devices on standard serial ports that share an IRQ with
your modem you are going to have problems. You need to make sure that
your modem serial port is on its own, unique IRQ. Many modern serial
cards (and better quality motherboard serial ports) allow you to move
the IRQ of the serial ports around.
If you are running Linux kernel 2, you can check the in-use IRQs using
cat /proc/interrupts, which will produce output like
______________________________________________________________________
0: 6766283 timer
1: 91545 keyboard
2: 0 cascade
4: 156944 + serial
7: 101764 WD8013
10: 134365 + BusLogic BT-958
13: 1 math error
15: 3671702 + serial
______________________________________________________________________
This shows a serial port on IRQ4 (a mouse) and a serial port on IRQ15
(the permanent modem based PPP link to the Internet. (There is also a
serial port on com2, IRQ3 and com4 is on IRQ14, but as they are not in
use, they do not show up).
Be warned - you need to know what you are doing if you are going to
play with your IRQs! Not only do you have to open up you computer,
pull out cards and play with jumpers, but you need to know what is on
which IRQ. In my case, this is a totally SCSI based PC, and so I can
disable the on motherboard IDE interfaces that normally use IRQ14 and
15!
You should also remember that if your PC boots other operating
systems, moving IRQs around may well mean that OS cannot boot properly
- or at all!
If you do move your serial ports to non-standard IRQs, then you need
to tell Linux which IRQ each port is using. This is done using
setserial and is best done as part of the boot process in rc.local or
rc.serial which is called from rc.local or as part of the SysV
initialisation. For the machine illustrated above, the commands used
are
______________________________________________________________________
/bin/setserial -b /dev/ttyS2 IRQ 11
/bin/setserial -b /dev/ttyS3 IRQ 15
______________________________________________________________________
However, if you are using serial modules dynamically loaded when
required by the kerneld process, you cannot set and forget the IRQ etc
once at boot time. This is because if the serial module is unloaded,
Linux forgets the special settings.
So, if you are loading the serial module on demand, you will need to
reconfigure the IRQs etc each time the module is loaded.
9.1. A note about serial ports and speed capabilities
If you are using a high speed (external) modem (14,400 Baud or above),
your serial port needs to be capable of handling the throughput that
such a modem is capable of producing, particularly when the modems are
compressing the data.
This requires your serial port to use a modern UART (Universal
Asynchronous Receiver Transmitter) such as a 16550(A). If you are
using an old machine (or old serial card), it is quite possible that
your serial port has only an 8250 UART, which will cause you
considerable problems when used with a high speed modem.
Use the command
setserial -a /dev/ttySx
to get Linux to report to you the type of UART you have. If you do not
have a 16550A type UART, invest in a new serial card (available for
under $50). When you purchase a new card, make sure you can move the
IRQs around on it!
Note: the first versions of the 16550 UART chip had an error. This was
rapidly discovered and a revision of the chip was released - the
16550A UART. A relatively small number of the faulty chips did however
get into circulation. It is unlikely that you will encounter one of
these but you should look for a response that says 16550A,
particularly on serial cards of some vintage.
9.2. Serial Port Names
Historically, Linux used cuaX devices for dial out and ttySx devices
for dial in.
The kernel code that required this was changed in kernel version 2.0.x
and you should now use ttySx for both dial in and dial out. I
understand that the cuaX device names may well disappear in future
kernel versions.
9.3. Configuring your modem
You will need to configure your modem correctly for PPP - to do this
READ YOUR MODEM MANUAL! Most modems come with a factory default
setting that selects the options required for PPP. The minimum
configuration specifies:-
· Hardware flow control (RTS/CTS) (&K3 on many Hayes modems)
Other settings (in standard Hayes commands) you should investigate
are:-
· E1 Command/usr/src/linux-2.0.27/include/linux/serial.h Echo ON
(required for chat to operate)
· Q0 Report result codes (required for chat to operate)
· S0=0 Auto Answer OFF (unless you want your modem to answer the
phone)
· &C1 Carrier Detect ON only after connect
· &S0 Data Set Ready (DSR) always ON
· (depends) Data Terminal Ready
There is a site offering modem setups for a growing variety of modem
makes and models at Modem setup information
<http://www.in.net/info/modems/index.html> which may assist you in
this.
It is also worth while investigating how the modem's serial interface
between your computer and modem operates. Most modern modems allow you
to run the serial interface at a FIXED speed whilst allowing the
telephone line interface to change its speed to the highest speed it
and the remote modem can both handle.
This is known as split speed operation. If your modem supports this,
lock the modem's serial interface to its highest available speed
(usually 115,200 baud but maybe 38,400 baud for 14,400 baud modems).
Use your communications software (e.g. minicom or seyon) to find out
about your modem configuration and set it to what is required for PPP.
Many modems report their current settings in response to AT&V, but you
should consult your modem manual.
If you completely mess up the settings, you can return to sanity
(usually) by issuing an AT&F - return to factory settings. (For most
modem modems I have encountered, the factory settings include all you
need for PPP - but you should check).
Once you have worked out the modem setup string required write it
down. You now have a decision: you can store these settings in your
modem non-volatile memory so they can be recalled by issuing the
appropriate AT command. Alternatively you can pass the correct
settings to your modem as part of the PPP dialing process.
If you only use your modem from Linux to call into your ISP or
corporate server, the simplest set up will have you save your modem
configuration in non-volatile RAM.
If on the other hand, you modem is used by other applications and
operating systems, it is safest to pass this information to the modem
as each call is made so that the modem is guaranteed to be in the
correct state for the call. (This has the added advantage also of
recording the modem setup string in case the modem looses the
contents of its NV-RAM, which can indeed happen).
9.4. Note on Serial Flow Control
When data is traveling on serial communication lines, it can happen
that data arrives faster than a computer can handle it (the computer
may be busy doing something else - remember, Linux is a multi-user,
multi- tasking operating system). In order to ensure that data is not
lost (data does not over run in the input buffer and hence get lost),
some method of controlling the flow of data is necessary.
There are two ways of doing this on serial lines:-
· Using hardware signals (Clear To Send/Request to Send - CTS/RTS)
· Using software signals (control S and control Q, also known as
XON/XOFF).
Whilst the latter may be fine for a terminal (text) link, data on a
PPP link uses all 8 bits - and it is quite probable that somewhere in
the data there will be data bytes that translate as control S and
control Q. So, if a modem is set up to use software flow control,
things can rapidly go berserk!
For high speed links using PPP (which uses 8 bits of data) hardware
flow control is vital and it is for this reason that you must use
hardware flow control.
9.5. Testing your modem for dial out
Now that you have sorted out the serial port and modem settings it is
a good idea to make sure that these setting do indeed work by dialing
you ISP and seeing if you can connect.
Using you terminal communications package (such as minicom), set up
the modem initialisation required for PPP and dial into the PPP server
you want to connect to with a PPP session.
(Note: at this stage we are NOT trying to make a PPP connection - just
establishing that we have the right phone number and also to find out
exactly what the server sends to us in order to get logged in and
start PPP).
During this process, either capture (log to a file) the entire login
process or carefully (very carefully) write down exactly what prompts
the server gives to let you know it is time to enter your user name
and password (and any other commands needed to establish the PPP
connection).
If your server uses PAP, you should not see a login prompt, but should
instead see the (text representation) of the link control protocol
(which looks like garbage) starting on your screen.
A few words of warning:-
· some servers are quite intelligent: you can log in using text based
user name/passwords OR using PAP. So if your ISP or corporate site
uses PAP but you do not see the garbage start up immediately, this
may not mean you have done something wrong.
· some servers require you to enter some text initially and then
start a standard PAP sequence.
· Some PPP servers are passive - that is they simply sit there
sending nothing until the client that is dialing in sends them a
valid lcp packet. If the ppp server you are connecting to operates
in passive mode, you will never see the garbage!
· Some servers do not start PPP until you press ENTER - so it is
worth trying this if you correctly log in and do not see the
garbage!
It is worth dialing in at least twice - some servers change their
prompts (e.g. with the time!) every time you log in. The two critical
prompts your Linux box needs to be able to identify every time you
dial in are:-
· the prompt that requests you to enter your user name;
· the prompt that requests you to enter your password;
If you have to issue a command to start PPP on the server, you will
also need to find out the prompt the server gives you once you are
logged in to tell you that you can now enter the command to start ppp.
If your server automatically starts PPP, once you have logged in, you
will start to see garbage on your screen - this is the PPP server
sending your machine information to start up and configure the PPP
connection.
This should look something like this :-
______________________________________________________________________
~y}#.!}!}!} }8}!}$}%U}"}&} } } } }%}& ...}'}"}(}"} .~~y}
______________________________________________________________________
(and it just keeps on coming!)
On some systems PPP must be explicitly started on the server. This is
usually because the server has been set up to allow PPP logins and
shell logins using the same user name/password pair. If this is the
case, issue this command once you have logged in. Again, you will see
the garbage as the server end of the PPP connection starts up.
If you do not see this immediately after connecting (and logging in
and starting the PPP server if required), press Enter to see if this
starts the PPP server...
At this point, you can hang up your modem (usually, type +++ quickly
and then issue the ATHO command once your modem responds with OK).
If you can't get your modem to work, read your modem manual, the man
pages for your communications software and the Serial HOWTO! Once you
have this sorted out, carry on as above.
10. Setting up Name to Address Resolution (DNS)
Whilst we humans like to give names to things, computers really like
numbers. On a TCP/IP network (which is what the Internet is), we call
machines by a particular name - and every machine lives in a
particular "domain". For example, my Linux workstation is called
archenland and it resides in the interweft.com.au domain. Its human
readable address is thus archenland.interweft.com.au (which is known
as the FQDN - fully qualified domain name).
However, for this machine to be found by other computers on the
Internet, it is actually known by its IP number when computers are
communicating across the Internet.
Translating (resolving) machine (and domain) names into the numbers
actually used on the Internet is the business of machines that offer
the Domain Name Service.
What happens is this:-
· your machine needs to know the IP address of a particular computer.
The application requiring this information asks the 'resolver' on
your Linux PC to provide this information;
· the resolver queries the local host file (/etc/hosts and/or the
domain name servers it knows about (the exact behaviour of the
resolver is determined by /etc/host.conf);
· if the answer is found in the host file, this answer is returned;
· if a domain name server is specified, your PC queries this machine;
· if the DNS machine already knows the IP number for the required
name, it returns it. If it does not, it queries other name servers
across the Internet to find the information. The name server than
passes this information back to the requesting resolver - which
gives the information to the requesting application.
When you make a PPP connection, you need to tell your Linux machine
where it can get host name to IP number (address resolution)
information so that you can use the machine names but your computer
can translate these to the IP numbers it needs to do its work.
One way is to enter every host that you want to talk to into the
/etc/hosts file (which is in reality totally impossible if you are
connecting to the Internet); another is to use the machine IP numbers
as opposed to the names (an impossible memory task for all but the
smallest LANs).
The best way is to set up Linux so that it knows where to go to get
this name to number information - automatically. This service is
provided by the Domain Name Server (DNS) system. All that is necessary
is to enter the IP number(s) for the domain name servers into your
/etc/resolv.conf file.
10.1. The /etc/resolv.conf file
Your PPP server sysadmin/user support people should provide you with
two DNS IP numbers (only one is necessary - but two gives some
redundancy in the event of failure).
As previously mentioned, Linux cannot set its name server IP number in
the way that MS Windows 95 does. So you must insist (politely) that
your ISP provide you with this information!
Your /etc/resolv.conf should look something like :-
______________________________________________________________________
domain your.isp.domain.name
nameserver 10.25.0.1
nameserver 10.25.1.2
______________________________________________________________________
Edit this file (creating it if necessary) to represent the information
that your ISP has provided. It should have ownership and permissions
as follows :-
-rw-r--r-- 1 root root 73 Feb 19 01:46 /etc/resolv.conf
If you have already set up a /etc/resolv.conf because you are on a
LAN, simply add the IP numbers of the PPP DNS servers to your existing
file.
10.2. The /etc/host.conf file
You should also check that your /etc/host.conf file is correctly set
up. This should look like
______________________________________________________________________
order hosts,bind
multi on
______________________________________________________________________
This tells the resolver to use information in the host file before it
sends queries to the DNS for resolution.
11. Using PPP and root privileges
Because PPP needs to set up networking devices, change the kernel
routing table and so forth, it requires root privileges to do this.
If users other than root are to set up PPP connections, the pppd
program should be setuid root :-
-rwsr-xr-x 1 root root 95225 Jul 11 00:27 /usr/sbin/pppd
If /usr/sbin/pppd is not set up this way, then as root issue the
command:-
chmod u+s /usr/sbin/pppd
What this does is make pppd run with root privileges even if the
binary is run by an ordinary user. This allows a normal user to run
pppd with the necessary privileges to set up the network interfaces
and the kernel routing table.
Programs that run 'set uid root' are potential security holes and you
should be extremely cautious about making programs 'suid root'. A
number of programs (including pppd) have been carefully written to
minimise the danger of running suid root, so you should be safe with
this one (but no guarantees).
Depending on how you want your system to operate - specifically if you
want ANY user on your system to be able to initiate a PPP link, you
should make your ppp-on/off scripts world read/execute. (This is
probably fine if your PC is used ONLY by you).
However, if you do NOT want just anyone to be able to start up a PPP
connection (for example, your children have accounts on your Linux PC
and you do not want them hooking into the Internet without your
supervision), you will need to establish a PPP group (as root, edit
/etc/group) and :-
· Make pppd suid root, owned by user root and group PPP, with the
'other' permissions on this file empty. It should then look like
-rwsr-x--- 1 root PPP 95225 Jul 11 00:27 /usr/sbin/pppd
· Make the ppp-on/off scripts owned by user root and group PPP
· Make the ppp-on/off scripts read/executable by group PPP
-rwxr-x--- 1 root PPP 587 Mar 14 1995 /usr/sbin/ppp-on
-rwxr-x--- 1 root PPP 631 Mar 14 1995 /usr/sbin/ppp-off
· Make the other access rights for ppp-on/off nill.
· add the users who will be firing up PPP to the PPP group in
/etc/group
Even if you do this, ordinary users will STILL not be able to shut
down the link under software control! Running the ppp-off script
requires root privileges. However, any user can just turn off the
modem (or disconnect the telephone line from an internal modem).
An alternative (and better method) to this set up is to use the sudo
program. This offers superior security and will allow you to set
things up so that any (authorised) user can activate/deactivate the
link using the scripts. Using sudo will allow an authorised user to
activate/deactivate the PPP link cleanly and securely.
12. Setting up the PPP connection files
You now need to be logged in as root to create the directories and
edit the files needed to set up PPP, even if you want PPP to be
accessible to all users.
PPP uses a number of files to connect and set up a PPP connection.
These differ in name and location between PPP 2.1.2 and 2.2.
For PPP 2.1.2 the files are:-
______________________________________________________________________
/usr/sbin/pppd # the PPP binary
/usr/sbin/ppp-on # the dialer/connection script
/usr/sbin/ppp-off # the disconnection script
/etc/ppp/options # the options pppd uses for all connections
/etc/ppp/options.ttyXX # the options specific to a connection on this port
______________________________________________________________________
For PPP 2.2 the files are:-
______________________________________________________________________
/usr/sbin/pppd # the PPP binary
/etc/ppp/scripts/ppp-on # the dialer/connection script
/etc/ppp/scripts/ppp-on-dialer # part 1 of the dialer script
/etc/ppp/scripts/ppp-off # the actual chat script itself
/etc/ppp/options # the options pppd uses for all connections
/etc/ppp/options.ttyXX # the options specific to a connection on this port
______________________________________________________________________
Red Hat Linux users should note that the standard Red Hat 4.X
installation places these scripts in /usr/doc/ppp-2.2.0f-2/scripts.
In your /etc directory there should be a ppp directory:-
drwxrwxr-x 2 root root 1024 Oct 9 11:01 ppp
If it does not exist - create it with these ownerships and
permissions.
If the directory already existed, it should contain a template options
file called options.tpl. This file is included below in case it does
not.
Print it out as it contains an explanation of nearly all the PPP
options (these are useful to read in conjunction with the pppd man
pages). Whilst you can use this file as the basis of your
/etc/ppp/options file, it is probably better to create your own
options file that does not include all the comments in the template -
it will be much shorter and easier to read/maintain.
If you have multiple serial lines/modems (typically the case for PPP
servers), create a general /etc/ppp/options file containing the
options that are common for all the serial ports on which you are
supporting dial in/out and set up individual option files for each
serial line on which you will be establishing a PPP connection with
the individual settings required for each port.
These port specific option files are named options.ttyx1,
options.ttyx2 and so forth (where x is the appropriate letter for your
serial ports).
However, for a single PPP connection, you can happily use the
/etc/ppp/options file. Alternatively, you can put all the options as
arguments in the pppd command itself.
It is easier to maintain a setup that uses /etc/ppp/options.ttySx
files. If you use PPP to connect to a number of different sites, you
can create option files for each site in /etc/ppp/options.site and
then specify the option file as a parameter to the PPP command as you
connect (using the file option-file pppd option to pppd on the command
line).
12.1. The supplied options.tpl file
Some distributions of PPP seem to have lost the options.tpl file, so
here is the complete file. I suggest that you do NOT edit this file to
create your /etc/ppp/options file(s). Rather, copy this to a new file
and then edit that. If you mess up your edits, you can then go back to
the original and start again.
______________________________________________________________________
# /etc/ppp/options -*- sh -*- general options for pppd
# created 13-Jul-1995 jmk
# autodate: 01-Aug-1995
# autotime: 19:45
# Use the executable or shell command specified to set up the serial
# line. This script would typically use the "chat" program to dial the
# modem and start the remote ppp session.
#connect "echo You need to install a connect command."
# Run the executable or shell command specified after pppd has
# terminated the link. This script could, for example, issue commands
# to the modem to cause it to hang up if hardware modem control signals
# were not available.
#disconnect "chat -- \d+++\d\c OK ath0 OK"
# async character map -- 32-bit hex; each bit is a character
# that needs to be escaped for pppd to receive it. 0x00000001
# represents '\x01', and 0x80000000 represents '\x1f'.
#asyncmap 0
# Require the peer to authenticate itself before allowing network
# packets to be sent or received.
#auth
# Use hardware flow control (i.e. RTS/CTS) to control the flow of data
# on the serial port.
#crtscts
# Use software flow control (i.e. XON/XOFF) to control the flow of data
# on the serial port.
#xonxoff
# Add a default route to the system routing tables, using the peer as
# the gateway, when IPCP negotiation is successfully completed. This
# entry is removed when the PPP connection is broken.
#defaultroute
# Specifies that certain characters should be escaped on transmission
# (regardless of whether the peer requests them to be escaped with its
# async control character map). The characters to be escaped are
# specified as a list of hex numbers separated by commas. Note that
# almost any character can be specified for the escape option, unlike
# the asyncmap option which only allows control characters to be
# specified. The characters which may not be escaped are those with hex
# values 0x20 - 0x3f or 0x5e.
#escape 11,13,ff
# Don't use the modem control lines.
#local
# Specifies that pppd should use a UUCP-style lock on the serial device
# to ensure exclusive access to the device.
#lock
# Use the modem control lines. On Ultrix, this option implies hardware
# flow control, as for the crtscts option. (This option is not fully
# implemented.)
#modem
# Set the MRU [Maximum Receive Unit] value to <n> for negotiation. pppd
# will ask the peer to send packets of no more than <n> bytes. The
# minimum MRU value is 128. The default MRU value is 1500. A value of
# 296 is recommended for slow links (40 bytes for TCP/IP header + 256
# bytes of data).
#mru 542
# Set the interface netmask to <n>, a 32 bit netmask in "decimal dot"
# notation (e.g. 255.255.255.0).
#netmask 255.255.255.0
# Disables the default behaviour when no local IP address is specified,
# which is to determine (if possible) the local IP address from the
# hostname. With this option, the peer will have to supply the local IP
# address during IPCP negotiation (unless it specified explicitly on the
# command line or in an options file).
#noipdefault
# Enables the "passive" option in the LCP. With this option, pppd will
# attempt to initiate a connection; if no reply is received from the
# peer, pppd will then just wait passively for a valid LCP packet from
# the peer (instead of exiting, as it does without this option).
#passive
# With this option, pppd will not transmit LCP packets to initiate a
# connection until a valid LCP packet is received from the peer (as for
# the "passive" option with old versions of pppd).
#silent
# Don't request or allow negotiation of any options for LCP and IPCP
# (use default values).
#-all
# Disable Address/Control compression negotiation (use default, i.e.
# address/control field disabled).
#-ac
# Disable asyncmap negotiation (use the default asyncmap, i.e. escape
# all control characters).
#-am
# Don't fork to become a background process (otherwise pppd will do so
# if a serial device is specified).
#-detach
# Disable IP address negotiation (with this option, the remote IP
# address must be specified with an option on the command line or in an
# options file).
#-ip
# Disable magic number negotiation. With this option, pppd cannot
# detect a looped-back line.
#-mn
# Disable MRU [Maximum Receive Unit] negotiation (use default, i.e.
# 1500).
#-mru
# Disable protocol field compression negotiation (use default, i.e.
# protocol field compression disabled).
#-pc
# Require the peer to authenticate itself using PAP.
# This requires TWO WAY authentication - do NOT use this for a standard
# PAP authenticated link to an ISP as this will require the ISP machine
# to authenticate itself to your machine (and it will not be able to).
#+pap
# Don't agree to authenticate using PAP.
#-pap
# Require the peer to authenticate itself using CHAP [Cryptographic
# Handshake Authentication Protocol] authentication.
# This requires TWO WAY authentication - do NOT use this for a standard
# CHAP authenticated link to an ISP as this will require the ISP machine
# to authenticate itself to your machine (and it will not be able to).
#+chap
# Don't agree to authenticate using CHAP.
#-chap
# Disable negotiation of Van Jacobson style IP header compression (use
# default, i.e. no compression).
#-vj
# Increase debugging level (same as -d). If this option is given, pppd
# will log the contents of all control packets sent or received in a
# readable form. The packets are logged through syslog with facility
# daemon and level debug. This information can be directed to a file by
# setting up /etc/syslog.conf appropriately (see syslog.conf(5)). (If
# pppd is compiled with extra debugging enabled, it will log messages
# using facility local2 instead of daemon).
#debug
# Append the domain name <d> to the local host name for authentication
# purposes. For example, if gethostname() returns the name porsche,
# but the fully qualified domain name is porsche.Quotron.COM, you would
# use the domain option to set the domain name to Quotron.COM.
#domain <d>
# Enable debugging code in the kernel-level PPP driver. The argument n
# is a number which is the sum of the following values: 1 to enable
# general debug messages, 2 to request that the contents of received
# packets be printed, and 4 to request that the contents of transmitted
# packets be printed.
#kdebug n
# Set the MTU [Maximum Transmit Unit] value to <n>. Unless the peer
# requests a smaller value via MRU negotiation, pppd will request that
# the kernel networking code send data packets of no more than n bytes
# through the PPP network interface.
#mtu <n>
# Set the name of the local system for authentication purposes to <n>.
# This will probably have to be set to your ISP user name if you are
# using PAP/CHAP.
#name <n>
# Set the user name to use for authenticating this machine with the peer
# using PAP to <u>.
# Do NOT use this if you are using 'name' above!
#user <u>
# Enforce the use of the host name as the name of the local system for
# authentication purposes (overrides the name option).
#usehostname
# Set the assumed name of the remote system for authentication purposes
# to <n>.
#remotename <n>
# Add an entry to this system's ARP [Address Resolution Protocol]
# table with the IP address of the peer and the Ethernet address of this
# system.
#proxyarp
# Use the system password database for authenticating the peer using
# PAP.
#login
# If this option is given, pppd will send an LCP echo-request frame to
# the peer every n seconds. Under Linux, the echo-request is sent when
# no packets have been received from the peer for n seconds. Normally
# the peer should respond to the echo-request by sending an echo-reply.
# This option can be used with the lcp-echo-failure option to detect
# that the peer is no longer connected.
#lcp-echo-interval <n>
# If this option is given, pppd will presume the peer to be dead if n
# LCP echo-requests are sent without receiving a valid LCP echo-reply.
# If this happens, pppd will terminate the connection. Use of this
# option requires a non-zero value for the lcp-echo-interval parameter.
# This option can be used to enable pppd to terminate after the physical
# connection has been broken (e.g., the modem has hung up) in
# situations where no hardware modem control lines are available.
#lcp-echo-failure <n>
# Set the LCP restart interval (retransmission timeout) to <n> seconds
# (default 3).
#lcp-restart <n>
# Set the maximum number of LCP terminate-request transmissions to <n>
# (default 3).
#lcp-max-terminate <n>
# Set the maximum number of LCP configure-request transmissions to <n>
# (default 10).
# Some PPP servers are slow to start up. You may need to increase this
# if you keep getting 'serial line looped back' errors and your are SURE
# that you have logged in correctly and PPP should be starting on the server.
#lcp-max-configure <n>
# Set the maximum number of LCP configure-NAKs returned before starting
# to send configure-Rejects instead to <n> (default 10).
#lcp-max-failure <n>
# Set the IPCP restart interval (retransmission timeout) to <n>
# seconds (default 3).
#ipcp-restart <n>
# Set the maximum number of IPCP terminate-request transmissions to <n>
# (default 3).
#ipcp-max-terminate <n>
# Set the maximum number of IPCP configure-request transmissions to <n>
# (default 10).
#ipcp-max-configure <n>
# Set the maximum number of IPCP configure-NAKs returned before starting
# to send configure-Rejects instead to <n> (default 10).
#ipcp-max-failure <n>
# Set the PAP restart interval (retransmission timeout) to <n> seconds
# (default 3).
#pap-restart <n>
# Set the maximum number of PAP authenticate-request transmissions to
# <n> (default 10).
#pap-max-authreq <n>
# Set the CHAP restart interval (retransmission timeout for
# challenges) to <n> seconds (default 3).
#chap-restart <n>
# Set the maximum number of CHAP challenge transmissions to <n>
# (default 10).
#chap-max-challenge
# If this option is given, pppd will re-challenge the peer every <n>
# seconds.
#chap-interval <n>
# With this option, pppd will accept the peer's idea of our local IP
# address, even if the local IP address was specified in an option.
#ipcp-accept-local
# With this option, pppd will accept the peer's idea of its (remote) IP
# address, even if the remote IP address was specified in an option.
#ipcp-accept-remote
______________________________________________________________________
12.2. What options should I use? (No PAP/CHAP)
Well, as in all things that depends (sigh). The options specified here
should work with most servers.
However, if it does NOT work, READ THE TEMPLATE FILE
(/etc/ppp/options.tpl) and the pppd man pages and speak to the
sysadmin/user support people who run the server to which you are
connecting.
You should also note that the connect scripts presented here also use
some command line options to pppd to make things a bit easier to
change.
______________________________________________________________________
# /etc/ppp/options (NO PAP/CHAP)
#
# Prevent pppd from forking into the background
-detach
#
# use the modem control lines
modem
# use uucp style locks to ensure exclusive access to the serial device
lock
# use hardware flow control
crtscts
# create a default route for this connection in the routing table
defaultroute
# do NOT set up any "escaped" control sequences
asyncmap 0
# use a maximum transmission packet size of 552 bytes
mtu 552
# use a maximum receive packet size of 552 bytes
mru 552
#
#-------END OF SAMPLE /etc/ppp/options (no PAP/CHAP)
______________________________________________________________________
13. Protocol) If your PPP server uses PAP (Password Authentication
If the server to which you are connecting requires PAP or CHAP
authentication, you have a little bit more work.
To the above options file, add the following lines
______________________________________________________________________
#
# force pppd to use your ISP user name as your 'host name' during the
# authentication process
name <your ISP user name> # you need to edit this line
#
# If you are running a PPP *server* and need to force PAP or CHAP
# uncomment the appropriate one of the following lines. Do NOT use
# these is you are a client connecting to a PPP server (even if it uses PAP
# or CHAP) as this tells the SERVER to authenticate itself to your
# machine (which almost certainly can't do - and the link will fail).
#+chap
#+pap
#
# If you are using ENCRYPTED secrets in the /etc/ppp/pap-secrets
# file, then uncomment the following line.
# Note: this is NOT the same as using MS encrypted passwords as can be
# set up in MS RAS on Windows NT.
#+papcrypt
______________________________________________________________________
13.1. Using MSCHAP
Microsoft Windows NT RAS can be set up to use a variation on CHAP
(Challenge/Handshake Authentication Protocol). In your PPP sources tar
ball, you will find a file called README.MSCHAP80 that discusses this.
You can determine if the server is requesting authentication using
this protocol by enabling debugging for pppd. If the server is
requesting MS CHAP authentication, you will see lines like
______________________________________________________________________
rcvd [LCP ConfReq id=0x2 <asyncmap 0x0> <auth chap 80> <magic 0x46a3>]
______________________________________________________________________
The critical information here is auth chap 80.
In order to use MS CHAP, you will need to recompile pppd to support
this. Please see the instructions in the README.MSCHAP80 file in the
PPP source file for instructions on how to compile and use this
variation.
You should note that at present this code supports only Linux PPP
clients connecting to an MS Windows NT server. It does NOT support
setting up a Linux PPP server to use MSCHAP80 authentication from
clients.
13.2. The PAP/CHAP secrets file
If you are using pap or chap authentication, then you also need to
create the secrets file. These are:
______________________________________________________________________
/etc/ppp/pap-secrets
/etc/ppp/chap-secrets
______________________________________________________________________
They must be owned by user root, group root and have file permissions
740 for security.
The first point to note about PAP and CHAP is that they are designed
to authenticate computer systems not users.
"Huh? What's the difference?" I hear you ask.
Well now, once your computer has made its PPP connection to the
server, ANY user on your system can use that connection - not just
you. This is why you can set up a WAN (wide area network) link that
joins two LANs (local area networks) using PPP.
PAP can (and for CHAP DOES) require bidirectional authentication -
that is a valid name and secret is required on each computer for the
other computer involved. However, this is NOT the way most PPP servers
offering dial up PPP PAP-authenticated connections operate.
That being said, your ISP will probably have given you a user name and
password to allow you to connect to their system and thence the
Internet. Your ISP is not interested in your computer's name at all,
so you will probably need to use the user name at your ISP as the name
for your computer.
This is done using the name user name option to pppd. So, if you are
to use the user name given you by your ISP, add the line
______________________________________________________________________
name your_user name_at_your_ISP
______________________________________________________________________
to your /etc/ppp/options file.
Technically, you should really use user our_user name_at_your_ISP for
PAP, but pppd is sufficiently intelligent to interpret name as user if
it is required to use PAP. The advantage of using the name option is
that this is also valid for CHAP.
As PAP is for authenticating computers, technically you need also to
specify a remote computer name. However, as most people only have one
ISP, you can use a wild card (*) for the remote host name in the
secrets file.
It is also worth noting that many ISPs operate multiple modem banks
connected to different terminal servers - each with a different name,
but ACCESSED from a single (rotary) dial in number. It can therefore
be quite difficult in some circumstances to know ahead of time what
the name of the remote computer is, as this depends on which terminal
server you connect to!
13.3. The PAP secrets file
The /etc/ppp/pap-secrets file looks like
______________________________________________________________________
# Secrets for authentication using PAP
# client server secret acceptable_local_IP_addresses
______________________________________________________________________
The four fields are white space delimited and the last one can be
blank (which is what you want for a dynamic and probably static IP
allocation from your ISP).
Suppose your ISP gave you a user name of fred and a password of
flintstone you would set the name fred option in
/etc/ppp/options[.ttySx] and set up your /etc/ppp/pap-secrets file as
follows
______________________________________________________________________
# Secrets for authentication using PAP
# client server secret acceptable local IP addresses
fred * flintstone
______________________________________________________________________
This says for the local machine name fred (which we have told pppd to
use even though it is not our local machine name) and for ANY server,
use the password (secret) of flintstone.
Note that we do not need to specify a local IP address, unless we are
required to FORCE a particular local, static IP address. Even if you
try this, it is unlikely to work as most PPP servers (for security
reasons) do not allow the remote system to set the IP number they are
to be given.
13.4. The CHAP secrets file
This requires that you have mutual authentication methods - that is
you must allow for both your machine to authenticate the remote server
AND the remote server to authenticate your machine.
So, if your machine is fred and the remote is barney, your machine
would set name fred remotename barney and the remote machine would set
name barney remotename fred in their respective /etc/ppp/options.ttySx
files.
The /etc/chap-secrets file for fred would look like
______________________________________________________________________
# Secrets for authentication using CHAP
# client server secret acceptable local IP addresses
fred barney flintstone
barney fred wilma
______________________________________________________________________
and for barney
______________________________________________________________________
# Secrets for authentication using CHAP
# client server secret acceptable local IP addresses
barney fred flintstone
fred barney wilma
______________________________________________________________________
Note in particular that both machines must have entries for
bidirectional authentication. This allows the local machine to
authenticate itself to the remote AND the remote machine to
authenticate itself to the local machine.
13.5. Handling multiple PAP-authenticated connections
Some users have more than one server to which they connect that use
PAP. Provided that your user name is different on each machine to
which you want to connect, this is not a problem.
However, many users have the same user name on two (or more - even
all) systems to which they connect. This then presents a problem in
correctly selecting the appropriate line from /etc/ppp/pap-secrets.
As you might expect, PPP provides a mechanism for overcoming this. PPP
allows you to set an 'assumed name' for the remote (server) end of the
connection using the remotename option to pppd.
Let us suppose that you connect to two PPP servers using the username
fred. You set up your /etc/ppp/pap-secrets something like
______________________________________________________________________
fred pppserver1 barney
fred pppserver2 wilma
______________________________________________________________________
Now, to set connect to pppserver1 you would use name fred remotename
pppserver1 in your ppp-options and for pppserver2 name fred remotename
pppserver2.
As you can select the ppp options file to use with pppd using the file
filename option, you can set up a script to connect to each of your
PPP servers, correctly picking the options file to use and hence
selecting the right remotename option.
14. Setting up the PPP connection manually
Now that you have created your /etc/ppp/options and /etc/resolv.conf
files (and, if necessary, the /etc/ppp/pap|chap-secrets file), you can
test the settings by manually establishing a PPP connection. (Once we
have the manual connection working, we will automate the process).
To do this, your communications software must be capable of quitting
WITHOUT resetting the modem. Minicom can do this - ALT Q (or in older
version of minicom CTRL A Q)
Make sure you are logged in as root.
Fire up you communications software (such as minicom), dial into the
PPP server and log in as normal. If you need to issue a command to
start up PPP on the server, do so. You will now see the garbage you
saw before.
If you are using pap or chap, then merely connecting to the remote
system should start ppp on the remote and you will see the garbage
without logging in (although this may not happen for some servers -
try pressing Enter and see if the garbage starts up).
Now quit the communications software without resetting the modem (ALT
Q or CTL A Q in minicom) and at the Linux prompt (as root) type
______________________________________________________________________
pppd -d -detach /dev/ttySx 38400 &
______________________________________________________________________
The -d option turns on debugging - the ppp connection start up
conversation will be logged to your system log - which is useful if
you are having trouble.
Your modem lights should now flash as the PPP connection is
established. It will take a short while for the PPP connection to be
made.
At this point you can look at the PPP interface, by issuing the
command
______________________________________________________________________
ifconfig
______________________________________________________________________
In addition to any Ethernet and loop back devices you have, you should
see something like :-
______________________________________________________________________
ppp0 Link encap:Point-Point Protocol
inet addr:10.144.153.104 P-t-P:10.144.153.51 Mask:255.255.255.0
UP POINTOPOINT RUNNING MTU:552 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0
TX packets:0 errors:0 dropped:0 overruns:0
______________________________________________________________________
Where
· inet addr:10.144.153.10 is the IP number of your end of the link.
· P-t-P:10.144.153.5 is the SERVER's IP number.
(Naturally, ifconfig will not report these IP numbers, but the ones
used by your PPP server.)
Note: ifconfig also tells you that the link is UP and RUNNING!
If you get no ppp device listed or something like
______________________________________________________________________
ppp0 Link encap:Point-Point Protocol
inet addr:0.0.0.0 P-t-P:0.0.0.0 Mask:0.0.0.0
POINTOPOINT MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0
TX packets:0 errors:0 dropped:0 overruns:0
______________________________________________________________________
Your PPP connection has not been made...see the later section on
debugging!
You should also be able to see a route to the the remote host (and
beyond). To do this, issue the command
______________________________________________________________________
route -n
______________________________________________________________________
You should se something like:-
______________________________________________________________________
Kernel routing table
Destination Gateway Genmask Flags MSS Window Use Iface
10.144.153.3 * 255.255.255.255 UH 1500 0 1 ppp0
127.0.0.0 * 255.0.0.0 U 3584 0 11 lo
10.0.0.0 * 255.0.0.0 U 1500 0 35 eth0
default 10.144.153.3 * UG 1500 0 5 ppp0
______________________________________________________________________
Of particular importance here, notice we have TWO entries pointing to
our ppp interface.
The first is a HOST route (indicated by the H flag) and that allows us
to see the host to which we are connected to - but no further.
The second is the default route (established by giving pppd the option
defaultroute. This is the route that tells our Linux PC to send any
packets NOT destined for the local Ethernet(s) - to which we have
specific network routes - to the PPP server itself. The PPP server
then is responsible for routing our packets out onto the Internet and
routing the return packets back to us.
If you do not see a routing table with two entries, something is
wrong. In particular if your syslog shows a message telling you pppd
is not replacing an existing default route, then you have a default
route pointing at your Ethernet interface - which MUST be replaced by
a specific network route: YOU CAN ONLY HAVE ONE DEFAULT ROUTE!!!
You will need to explore your system initialisation files to find out
where this default route is being set up (it will use a route add
default... command). Change this command to something like route add
net....
Now test the link by 'pinging' the server at its IP number as reported
by the ifconfig output, i.e.
______________________________________________________________________
ping 10.144.153.51
______________________________________________________________________
You should receive output like
______________________________________________________________________
PING 10.144.153.51 (10.144.153.51): 56 data bytes
64 bytes from 10.144.153.51: icmp_seq=0 ttl=255 time=328.3 ms
64 bytes from 10.144.153.51: icmp_seq=1 ttl=255 time=190.5 ms
64 bytes from 10.144.153.51: icmp_seq=2 ttl=255 time=187.5 ms
64 bytes from 10.144.153.51: icmp_seq=3 ttl=255 time=170.7 ms
______________________________________________________________________
This listing will go on for ever - to stop it press CTRL C, at which
point you will receive some more information :-
______________________________________________________________________
--- 10.144.153.51 ping statistics ---
4 packets transmitted, 4 packets received, 0% packet loss
round-trip min/avg/max = 170.7/219.2/328.3 ms
______________________________________________________________________
So far so good.
Now try pinging a host by name (not the name of the PPP server itself)
but a host at another site that you KNOW is probably going to be up
and running...). For example
______________________________________________________________________
ping sunsite.unc.edu
______________________________________________________________________
This time there will be a bit of a pause as Linux obtains the IP
number for the fully qualified host name you have 'ping'ed from the
DNS you specified in /etc/resolv.conf - so don't worry (but you will
see your modem lights flash). Shortly you will receive output like
______________________________________________________________________
PING sunsite.unc.edu (152.2.254.81): 56 data bytes
64 bytes from 152.2.254.81: icmp_seq=0 ttl=254 time=190.1 ms
64 bytes from 152.2.254.81: icmp_seq=1 ttl=254 time=180.6 ms
64 bytes from 152.2.254.81: icmp_seq=2 ttl=254 time=169.8 ms
64 bytes from 152.2.254.81: icmp_seq=3 ttl=254 time=170.6 ms
64 bytes from 152.2.254.81: icmp_seq=4 ttl=254 time=170.6 ms
______________________________________________________________________
Again, stop the output by pressing CTRL C and get the statistics...
______________________________________________________________________
--- sunsite.unc.edu ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max = 169.8/176.3/190.1 ms
______________________________________________________________________
If you don't get any response, try pinging the IP address of the DNS
server at your ISP's site. If you get a result from this, then it
looks like you have a problem with /etc/resolv.conf.
If this doesn't work, you have a routing problem, or your ISP has a
problem routing packets back to you. Check your routing table as shown
above and if that is OK, contact your ISP. A good test of the ISP is
to use another operating system to connect. If you can get beyond your
ISP with that, then the problem is at your end.
If everything works, shut down the connection by typing
______________________________________________________________________
ppp-off
______________________________________________________________________
After a short pause, the modem should hang itself up.
If that does not work, either turn off your modem or fire up your
communications software and interrupt the modem with +++ and then hang
up with ATH0 when you receive the modem's OK prompt.
You may also need to clean up the lock file created by pppd
______________________________________________________________________
rm -f /var/lock/LCK..ttySx
______________________________________________________________________
15. Automating your connections - Creating the connection scripts
Whilst you can continue to log in by hand as shown above, it is much
neater to set up some scripts to do this automatically for you.
A set of scripts automates the log in and PPP start up so all you have
to do (as root or as a member of the PPP group) is issue a single
command to fire up your connection.
15.1. Connection scripts for User name/Password Authentication
If your ISP does NOT require the use of PAP/CHAP, these are the
scripts for you!
If the ppp package installed correctly, you should have two example
files. For PPP 2.1.2 they are in /usr/sbin and for PPP 2.2 they are
in /etc/ppp/scripts. They are called
for PPP-2.1.2
ppp-on
ppp-off
and for PPP-2.2
ppp-off
ppp-on
ppp-on-dialer
Now, if you are using PPP 2.1.2, I strongly urge you to delete the
sample files. There are potential problems with these - and don't tell
me they work fine - I used them for ages too (and recommended them in
the first version of this HOWTO)!
For the benefit of PPP 2.1.2 users, here are BETTER template versions,
taken from the PPP 2.2 distribution. I suggest you copy and use these
scripts instead of the old PPP-2.1.2 scripts.
15.2. The ppp-on script
This is the first of a PAIR of scripts that actually fire up the
connection.
______________________________________________________________________
#!/bin/sh
#
# Script to initiate a PPP connection. This is the first part of the
# pair of scripts. This is not a secure pair of scripts as the codes
# are visible with the 'ps' command. However, it is simple.
#
# These are the parameters. Change as needed.
TELEPHONE=555-1212 # The telephone number for the connection
ACCOUNT=george # The account name for logon (as in 'George Burns')
PASSWORD=gracie # The password for this account (and 'Gracie Allen')
LOCAL_IP=0.0.0.0 # Local IP address if known. Dynamic = 0.0.0.0
REMOTE_IP=0.0.0.0 # Remote IP address if desired. Normally 0.0.0.0
NETMASK=255.255.255.0 # The proper netmask if needed
#
# Export them so that they will be available to 'ppp-on-dialer'
export TELEPHONE ACCOUNT PASSWORD
#
# This is the location of the script which dials the phone and logs
# in. Please use the absolute file name as the $PATH variable is not
# used on the connect option. (To do so on a 'root' account would be
# a security hole so don't ask.)
#
DIALER_SCRIPT=/etc/ppp/ppp-on-dialer
#
# Initiate the connection
#
#
exec /usr/sbin/pppd debug /dev/ttySx 38400 \
$LOCAL_IP:$REMOTE_IP \
connect $DIALER_SCRIPT
______________________________________________________________________
Here is the ppp-on-dialer script:-
______________________________________________________________________
#!/bin/sh
#
# This is part 2 of the ppp-on script. It will perform the connection
# protocol for the desired connection.
#
/usr/sbin/chat -v \
TIMEOUT 3 \
ABORT '\nBUSY\r' \
ABORT '\nNO ANSWER\r' \
ABORT '\nRINGING\r\n\r\nRINGING\r' \
'' \rAT \
'OK-+++\c-OK' ATH0 \
TIMEOUT 30 \
OK ATDT$TELEPHONE \
CONNECT '' \
ogin:--ogin: $ACCOUNT \
assword: $PASSWORD
______________________________________________________________________
For PPP-2.2, the ppp-off script looks like:-
______________________________________________________________________
#!/bin/sh
######################################################################
#
# Determine the device to be terminated.
#
if [ "$1" = "" ]; then
DEVICE=ppp0
else
DEVICE=$1
fi
######################################################################
#
# If the ppp0 pid file is present then the program is running. Stop it.
if [ -r /var/run/$DEVICE.pid ]; then
kill -INT `cat /var/run/$DEVICE.pid`
#
# If the kill did not work then there is no process running for this
# pid. It may also mean that the lock file will be left. You may wish
# to delete the lock file at the same time.
if [ ! "$?" = "0" ]; then
rm -f /var/run/$DEVICE.pid
echo "ERROR: Removed stale pid file"
exit 1
fi
#
# Success. Let pppd clean up its own junk.
echo "PPP link to $DEVICE terminated."
exit 0
fi
#
# The ppp process is not running for ppp0
echo "ERROR: PPP link is not active on $DEVICE"
exit 1
______________________________________________________________________
15.3. Editing the supplied PPP startup scripts
As the new scripts come in two parts, we will edit them in turn.
15.3.1. The ppp-on script
You will need to edit the script to reflect YOUR user name at your
ISP, YOUR password at your ISP, the telephone number of your ISP.
Each of the lines like TELEPHONE= actually set up shell variables that
contain the information to the right of the '=' (excluding the
comments of course). So edit each of these lines so it is correct for
your ISP and connection.
Also, as you are setting the IP number (if you need to) in the
/etc/ppp/options file, DELETE the line that says
______________________________________________________________________
$LOCAL_IP:$REMOTE_IP \
______________________________________________________________________
Also, make sure that the shell variable DIALER_SCRIPT points at the
full path and name of the dialer script that you are actually going to
use. So, if you have moved this or renamed the script, make sure you
edit this line correctly in the ppp-on script!
15.3.2. The ppp-on-dialer script
This is the second of the scripts that actually brings up our ppp
link.
Note: a chat script is normally all on one line. the backslashes are
used to allow line continuations across several physical lines (for
human readability) and do not form part of the script itself.
However, it is very useful to look at it in detail so that we
understand what it is actually (supposed) to be doing!
15.4. What a Chat script means...
A chat script is a sequence of "expect string" "send string" pairs. In
particular, note that we ALWAYS expect something before we send
something.
If we are to send something WITHOUT receiving anything first, we must
use an empty expect string (indicated by "") and similarly for
expecting something without sending anything! Also, if a string
consists of several words, (e.g. NO CARRIER), you must quote the
string so that it is seen as a single entity by chat.
The chat line in our template is:-
______________________________________________________________________
exec /usr/sbin/chat -v
______________________________________________________________________
Invoke chat, the -v tells chat to copy ALL its I/O into the system log
(usually /var/log/messages). Once you are happy that the chat script
is working reliably, edit this line to remove the -v to save
unnecessary clutter in your syslog.
______________________________________________________________________
TIMEOUT 3
______________________________________________________________________
This sets the timeout for the receipt of expected input to three sec­
onds. You may need to increase this to say 5 or 10 seconds if you are
using a really slow modem!
______________________________________________________________________
ABORT '\nBUSY\r'
______________________________________________________________________
If the string BUSY is received, abort the operation.
______________________________________________________________________
ABORT '\nNO ANSWER\r'
______________________________________________________________________
If the string NO ANSWER is received, abort the operation
______________________________________________________________________
ABORT '\nRINGING\r\n\r\nRINGING\r'
______________________________________________________________________
If the (repeated) string RINGING is received, abort the operation.
This is because someone is ringing your phone line!
______________________________________________________________________
" \rAT
______________________________________________________________________
Expect nothing from the modem and send the string AT.
______________________________________________________________________
OK-+++\c-OK ATH0
______________________________________________________________________
This one is a bit more complicated as it uses some of chat's error
recovery capabilities.
What is says is...Expect OK, if it is NOT received (because the modem
is not in command mode) then send +++ (the standard Hayes-compatible
modem string that returns the modem to command mode) and expect OK.
Then send ATH0 (the modem hang up string). This allows your script to
cope with the situation of your modem being stuck on-line!
______________________________________________________________________
TIMEOUT 30
______________________________________________________________________
Set the timeout to 30 seconds for the remainder of the script. If you
experience trouble with the chat script aborting due to timeouts,
increase this to 45 seconds or more.
______________________________________________________________________
OK ATDT$TELEPHONE
______________________________________________________________________
Expect OK (the modem's response to the ATH0 command) and dial the
number we want to call.
______________________________________________________________________
CONNECT ''
______________________________________________________________________
Expect CONNECT (which our modem sends when the remote modem answers)
and send nothing in reply.
______________________________________________________________________
ogin:--ogin: $ACCOUNT
______________________________________________________________________
Again, we have some error recovery built in here. Expect the login
prompt (...ogin:) but if we don't receive it by the timeout, send a
return and then look for the login prompt again. When the prompt is
received, send the username (stored in the shell variable $ACCOUNT).
______________________________________________________________________
assword: $PASSWORD
______________________________________________________________________
Expect the password prompt and send our password (again, stored in a
shell variable).
This chat script has reasonable error recovery capability. Chat has
considerably more features than demonstrated here. For more
information consult the chat manual page (man 8 chat).
15.4.1. Starting PPP at the server end
Whilst the ppp-on-dialer script is fine for servers that automatically
start pppd at the server end once you have logged in, some servers
require that you explicitly start PPP on the server.
If you need to issue a command to start up PPP on the server, you DO
need to edit the ppp-on-dialer script.
At the END of the script (after the password line) add an additional
expect send pair - this one would look for your login prompt (beware
of characters that have a special meaning in the Bourne shell - such
as $ and [ or ] (open and close square brackets).
Once chat has found the shell prompt, chat must issue the ppp start up
command required for your ISPs PPP server.
In my case, my PPP server uses the standard Linux Bash prompt
______________________________________________________________________
[hartr@kepler hartr]$
______________________________________________________________________
and requires that I type
______________________________________________________________________
ppp
______________________________________________________________________
to start up PPP on the server.
It is a good idea to allow for a bit of error recovery here, so in my
case I use
______________________________________________________________________
hartr--hartr ppp
______________________________________________________________________
This says, if we don't receive the prompt within the timeout, send a
carriage return and looks for the prompt again.
Once the prompt is received, then send the string ppp.
Note: don't forget to add a \ to the end of the previous line so chat
still thinks the entire chat script is on one line!
Unfortunately, some servers produce a very variable set of prompts!
You may need to log in several times using minicom to understand what
is going on and pick the stable "expect" strings.
15.5. A chat script for PAP/CHAP authenticated connections
If your ISP is using PAP/CHAP, then your chat script is much simpler.
All your chat script needs to do is dial the telephone, wait for a
connect and then let pppd handle the logging in!
______________________________________________________________________
#!/bin/sh
#
# This is part 2 of the ppp-on script. It will perform the connection
# protocol for the desired connection.
#
exec /usr/sbin/chat -v \
TIMEOUT 3 \
ABORT '\nBUSY\r' \
ABORT '\nNO ANSWER\r' \
ABORT '\nRINGING\r\n\r\nRINGING\r' \
'' \rAT \
'OK-+++\c-OK' ATH0 \
TIMEOUT 30 \
OK ATDT$TELEPHONE \
CONNECT '' \
______________________________________________________________________
15.6. The pppd debug and file option_file options
As we have already seen, you can turn on debug information logging
with the -d option to pppd. The 'debug' option is equivalent to this.
As we are establishing a new connection with a new script, leave in
the debug option for now. (Warning: if your disk space is tight,
logging pppd exchanges can rapidly extend your syslog file and run you
into trouble - but to do this you must fail to connect and keep on
trying for quite a few minutes).
Once you are happy that all is working properly, then you can remove
this option.
If you have called your ppp options file anything other than
/etc/ppp/options or /etc/ppp/options.ttySx, specify the file name with
the file option to pppd - e.g.
______________________________________________________________________
exec /usr/sbin/pppd debug file options.myserver /dev/ttyS0 38400 \
______________________________________________________________________
16. Testing your connection script
Open a new root Xterm (if you are in X) or open a new virtual console
and log in as root.
In this new session, issue the command
tail -f /var/log/messages
(or whatever your system log file is).
In the first window (or virtual console) issue the command
ppp-on &
(or whatever name you have called your edited version of
/usr/sbin/ppp- on). If you do not put the script into the background
by specifying & at the end of the command, you will not get your
terminal prompt back until ppp exits (when the link terminates).
Now switch back to the window that is tracking your system log.
You will see something like the following (provided you specified -v
to chat and -d to pppd)....this is the chat script and responses being
logged to the system log file followed by the start up information for
pppd :-
______________________________________________________________________
Oct 21 16:09:58 hwin chat[19868]: abort on (NO CARRIER)
Oct 21 16:09:59 hwin chat[19868]: abort on (BUSY)
Oct 21 16:09:59 hwin chat[19868]: send (ATZ^M)
Oct 21 16:09:59 hwin chat[19868]: expect (OK)
Oct 21 16:10:00 hwin chat[19868]: ATZ^M^M
Oct 21 16:10:00 hwin chat[19868]: OK -- got it
Oct 21 16:10:00 hwin chat[19868]: send (ATDT722298^M)
Oct 21 16:10:00 hwin chat[19868]: expect (CONNECT)
Oct 21 16:10:00 hwin chat[19868]: ^M
Oct 21 16:10:22 hwin chat[19868]: ATDT722298^M^M
Oct 21 16:10:22 hwin chat[19868]: CONNECT -- got it
Oct 21 16:10:22 hwin chat[19868]: send (^M)
Oct 21 16:10:22 hwin chat[19868]: expect (ogin:)
Oct 21 16:10:23 hwin chat[19868]: kepler login: -- got it
Oct 21 16:10:23 hwin chat[19868]: send (hartr^M)
Oct 21 16:10:23 hwin chat[19868]: expect (ssword:)
Oct 21 16:10:23 hwin chat[19868]: hartr^M
Oct 21 16:10:23 hwin chat[19868]: Password: -- got it
Oct 21 16:10:23 hwin chat[19868]: send (??????^M)
Oct 21 16:10:23 hwin chat[19868]: expect (hartr)
Oct 21 16:10:24 hwin chat[19868]: [hartr -- got it
Oct 21 16:10:24 hwin chat[19868]: send (ppp^M)
Oct 21 16:10:27 hwin pppd[19872]: pppd 2.1.2 started by root, uid 0
Oct 21 16:10:27 hwin pppd[19873]: Using interface ppp0
Oct 21 16:10:27 hwin pppd[19873]: Connect: ppp0 <--> /dev/cua1
Oct 21 16:10:27 hwin pppd[19873]: fsm_sdata(LCP): Sent code 1, id 1.
Oct 21 16:10:27 hwin pppd[19873]: LCP: sending Configure-Request, id 1
Oct 21 16:10:27 hwin pppd[19873]: fsm_rconfreq(LCP): Rcvd id 1.
Oct 21 16:10:27 hwin pppd[19873]: lcp_reqci: rcvd MRU
Oct 21 16:10:27 hwin pppd[19873]: (1500)
Oct 21 16:10:27 hwin pppd[19873]: (ACK)
Oct 21 16:10:27 hwin pppd[19873]: lcp_reqci: rcvd ASYNCMAP
Oct 21 16:10:27 hwin pppd[19873]: (0)
Oct 21 16:10:27 hwin pppd[19873]: (ACK)
Oct 21 16:10:27 hwin pppd[19873]: lcp_reqci: rcvd MAGICNUMBER
Oct 21 16:10:27 hwin pppd[19873]: (a098b898)
Oct 21 16:10:27 hwin pppd[19873]: (ACK)
Oct 21 16:10:27 hwin pppd[19873]: lcp_reqci: rcvd PCOMPRESSION
Oct 21 16:10:27 hwin pppd[19873]: (ACK)
Oct 21 16:10:27 hwin pppd[19873]: lcp_reqci: rcvd ACCOMPRESSION
Oct 21 16:10:27 hwin pppd[19873]: (ACK)
Oct 21 16:10:27 hwin pppd[19873]: lcp_reqci: returning CONFACK.
Oct 21 16:10:27 hwin pppd[19873]: fsm_sdata(LCP): Sent code 2, id 1.
Oct 21 16:10:27 hwin pppd[19873]: fsm_rconfack(LCP): Rcvd id 1.
Oct 21 16:10:27 hwin pppd[19873]: fsm_sdata(IPCP): Sent code 1, id 1.
Oct 21 16:10:27 hwin pppd[19873]: IPCP: sending Configure-Request, id 1
Oct 21 16:10:27 hwin pppd[19873]: fsm_rconfreq(IPCP): Rcvd id 1.
Oct 21 16:10:27 hwin pppd[19873]: ipcp: received ADDR
Oct 21 16:10:27 hwin pppd[19873]: (10.144.153.51)
Oct 21 16:10:27 hwin pppd[19873]: (ACK)
Oct 21 16:10:27 hwin pppd[19873]: ipcp: received COMPRESSTYPE
Oct 21 16:10:27 hwin pppd[19873]: (45)
Oct 21 16:10:27 hwin pppd[19873]: (ACK)
Oct 21 16:10:27 hwin pppd[19873]: ipcp: returning Configure-ACK
Oct 21 16:10:28 hwin pppd[19873]: fsm_sdata(IPCP): Sent code 2, id 1.
Oct 21 16:10:30 hwin pppd[19873]: fsm_sdata(IPCP): Sent code 1, id 1.
Oct 21 16:10:30 hwin pppd[19873]: IPCP: sending Configure-Request, id 1
Oct 21 16:10:30 hwin pppd[19873]: fsm_rconfreq(IPCP): Rcvd id 255.
Oct 21 16:10:31 hwin pppd[19873]: ipcp: received ADDR
Oct 21 16:10:31 hwin pppd[19873]: (10.144.153.51)
Oct 21 16:10:31 hwin pppd[19873]: (ACK)
Oct 21 16:10:31 hwin pppd[19873]: ipcp: received COMPRESSTYPE
Oct 21 16:10:31 hwin pppd[19873]: (45)
Oct 21 16:10:31 hwin pppd[19873]: (ACK)
Oct 21 16:10:31 hwin pppd[19873]: ipcp: returning Configure-ACK
Oct 21 16:10:31 hwin pppd[19873]: fsm_sdata(IPCP): Sent code 2, id 255.
Oct 21 16:10:31 hwin pppd[19873]: fsm_rconfack(IPCP): Rcvd id 1.
Oct 21 16:10:31 hwin pppd[19873]: ipcp: up
Oct 21 16:10:31 hwin pppd[19873]: local IP address 10.144.153.104
Oct 21 16:10:31 hwin pppd[19873]: remote IP address 10.144.153.51
______________________________________________________________________
(Note - I am using STATIC IP numbers - hence my machine sent that to
the PPP server - you won't see this if you are using DYNAMIC IP
numbers.) Also, this server requires a specific command to start ppp
at its end.
This looks OK - so test it out as before with pings to IP numbers and
host names.
Fire up you web browser or whatever and go surfing - you are
connected!
17. Shutting down the PPP link
When you have finished with the PPP link, use the standard ppp-off
command to shut it down (remember - you need to be root or a member of
the PPP group!).
In your system log you will see something like:-
______________________________________________________________________
Oct 21 16:10:45 hwin pppd[19873]: Interrupt received: terminating link
Oct 21 16:10:45 hwin pppd[19873]: ipcp: down
Oct 21 16:10:45 hwin pppd[19873]: default route ioctl(SIOCDELRT): Bad address
Oct 21 16:10:45 hwin pppd[19873]: fsm_sdata(LCP): Sent code 5, id 2.
Oct 21 16:10:46 hwin pppd[19873]: fsm_rtermack(LCP).
Oct 21 16:10:46 hwin pppd[19873]: Connection terminated.
Oct 21 16:10:46 hwin pppd[19873]: Exit.
______________________________________________________________________
Don't worry about the SIOCDELRT - this is just pppd noting that it is
terminating and is nothing to worry about.
18. Debugging
There are any number of reasons that your connection does not work -
chat has failed to complete correctly, you have a dirty line, etc. So
check your syslog for indications.
18.1. I have compiled PPP support into the kernel, but...
A very common problem is that people compile PPP support into the
kernel and yet when they try to run pppd, the kernel complains that it
does not support ppp! There are a variety of reasons this can occur.
18.1.1. Whilst you have recompiled your kernel to support ppp, you
are not booting the new kernel. This can happen if you do not update
/etc/lilo.conf and rerun lilo. Are you booting the right kernel?
A good check on the kernel can be obtained by issuing the command
uname -a, which should produce a line like
______________________________________________________________________
Linux archenland 2.0.28 #2 Thu Feb 13 12:31:37 EST 1997 i586
______________________________________________________________________
This gives the kernel version and the date on which this kernel was
compiled - which should give you a pretty good idea of what is going
on.
18.1.2. Did you compile ppp kernel support as a module?
If you compiled your kernel ppp support as a module, but did not make
and install the modules, then you can get this error. Check the
kernel-HOWTO and the README file in /usr/src/linux!
Another module connected possibility is that you are expecting
required modules to be automatically loaded, but are not running the
kerneld daemon (which auto-loads and unloads modules on the fly).
Check the kerneld mini-HOWTO for information on setting up kerneld.
18.1.3. Are you using the correct version of PPP for your kernel?
You must use ppp-2.2 with kernel version 2.0.x. You can use ppp-2.2
with kernel version 1.2.x (if you patch the kernel) otherwise you must
use ppp-2.1.2.
18.1.4. Are you running pppd as root?
If you are not running pppd as the root user (and pppd is not suid to
root), you can receive this message.
18.2. My modem connects but ppp never starts up
There are innumerable variations on this (take a look in
comp.os.linux...).
A VERY common mistake is that you have mistyped something in your
scripts. The only thing to do here is to make sure you are logging the
chat conversation between you Linux PC and the server into your syslog
(/var/log/messages) and then go through this line by line to make.
You may need to dial into the ppp server manually to check things out
again.
You need to check the log against the actual prompts very carefully -
and bear in mind that we humans have a tendency to read what we THINK
we have typed - not what is actually there!
18.3. The syslog says " serial line is not 8 bit clean ..."
There are variations on this too - such as serial line looped back
etc., and the cause can be one (or a sequence) of a number of things.
To understand what is going on here, it is necessary to grasp a bit of
what is going on behind the scenes in pppd itself.
When pppd starts up, it sends LCP (link control protocol) packets to
the remote machine. If it receives a valid response it then goes on to
the next stage (using IPCP - IP control protocol packets) and only
when this negotiation completes is the actual IP layer started so that
you can use the PPP link.
If there is no ppp server operating at the remote end when your PC
sends lcp packets, these get reflected by the login process at the far
end. As these packets use 8 bits, reflecting them strips the 8th bit
(remember, ASCII is a 7 bit code). PPP sees this and complains
accordingly.
There are several reasons this reflection can occur.
18.3.1. You are not correctly logging into the server
When your chat script completes, pppd starts on your PC. However, if
you have not completed the log in process to the server (including
sending any command required to start PPP on the server), PPP will
not start.
So, the lcp packets are reflected and you receive this error.
You need to carefully check and correct (if necessary) your chat
script (see above).
18.3.2. You are not starting PPP on the server
Some PPP servers require you to enter a command and/or a RETURN after
completing the log in process before the remote end starts ppp.
Check your chat script (see above).
If you log in manually and find you need to send a RETURN after this
to start PPP, simply add a blank expect/send pair to the end of your
chat script (an empty send string actually sends a RETURN).
18.3.3. The remote PPP process is slow to start
This one is a bit tricksy!
By default, your Linux pppd is compiled to send a maximum of 10 lcp
configuration requests. If the server is a bit slow to start up, all
10 such requests can be sent before the remote PPP is ready to receive
them.
On your machine, pppd sees all 10 requests reflected back (with the
8th bit stripped) and exits.
There are two ways round this:-
Add lcp-max-configure 30 to your ppp options. This increases the
maximum number of lcp configure packets pppd sends before giving up.
For really slow server, you may need even more than this.
Alternatively, you can get a bit tricksy in return. You may have
noticed that when you logged in by hand to the PPP server and PPP
started there, the first character of the ppp garbage that appears was
always the tilde character (~).
Using this knowledge we can add a new expect/send pair to the end of
the chat script which expects a tilde and sends nothing. This would
look like:-
______________________________________________________________________
\~ ''
______________________________________________________________________
Note: as the tilde character has a special meaning in the shell, it
must be escaped (and hence the leading backslash).
18.4. Default route not set
If pppd refuses to set up a default route, it is because (quite
correctly) it refuses remove/replace an existing default route.
The usual reason that this error occurs is that some distributions set
up a default route via your Ethernet card as opposed to setting up a
specific network route.
See the Linux NAG and the Net2/3 HOWTOs for information on correctly
setting up your Ethernet card and associated routes.
An alternative to this is that your LAN uses a gateway/router already
and your routing table has been set up to point the default route at
this.
Fixing up this last situation can require a fair bit of IP networking
knowledge and is beyond the scope of this HOWTO. It is suggested you
obtain some expert advice (via the news groups of from someone locally
you can ask).
18.5. Other Problems
There are many reasons apart from these that ppp fails to connect
and/or operate properly.
Look in the PPP FAQ (which is really a series of questions and
answers). This is a very comprehensive document and the answers ARE
there! From my own (sad) experience, if the answer to your problems is
not there, the problem is NOT ppp's fault! In my case I was using an
ELF kernel that I had not upgraded to the appropriate kernel modules.
I only wasted about 2 days (and most of one night) cursing what had
been a perfect PPP server before the light dawned!
19. Getting Help when totally stuck
If you can't get your PPP link to work, go back through this document
and check everything - in conjunction with the output created by
"chat-v..." and "pppd -d" in you system log.
Also consult the PPP documentation and FAQ plus the other documents
mention herein!
If you are still stuck, try the comp.os.linux.misc and
comp.os.linux.networking newsgroups are reasonably regularly scanned
by people that can help you with PPP as is comp.protocols.ppp
You can try sending me personal email, but I do have a day job (and a
life) and I do not guarantee to respond quickly (if at all) as this
depends on my current work load and the state of my private life!
In particular - DO NOT POST REAMS OF DEBUGGING OUTPUT TO THE NEWS
GROUPS NOR SEND IT TO ME BY EMAIL - the former wastes huge amounts of
network bandwidth and the latter will be consigned to /dev/null
(unless I have specifically requested it).
20. Common Problems once the link is working
One problem you will find is that many service providers will only
support the connection software package that they distribute to new
accounts. This is (typically) for Microsoft Windows :-( - and many
service provider help desks seem to know nothing about Unix (or
Linux). So, be prepared for limited assistance from them!
You could of course do the individual a favour and educate then about
Linux (any ISP help desk person should be reasonably 'with it' in
Internet terms and that means they should have a home Linux box - of
course it does)!
20.1. I can't see beyond the PPP server I connect to
OK - your PPP connection is up and running and you can ping the PPP
server by IP number (the second or "remote" IP number shown by
ifconfig ppp0), but you can't reach anything beyond this.
First of all, try pinging the IP numbers you have specified in
/etc/resolv.conf as name servers. If this works, you can see beyond
your PPP server (unless this has the same IP number as the "remote" IP
number of your connection). So now try pinging the full Internet name
of your service provider - eg
ping my.provider.net.au
If this does NOT work, you have a problem with the name resolution.
This is probably because of a typo in your /etc/resolv.conf file.
Check this carefully against the information you acquired by ringing
your service provider. If all looks OK, ring your service provider and
check that you wrote down the IP numbers correctly.
If it STILL doesn't work (and your service provider confirms that his
name servers are up and running), you have a problem somewhere else -
and I suggest you check carefully through your Linux installation
(looking particularly for file permissions).
If you STILL can't ping your service provider's IP name servers by IP
number, either they are down (give them a voice call and check) or
there is a routing problem at your service provider's end. Again, ring
them and check this out.
One possibility is that the "remote end" is a Linux PPP server where
the IP forwarding option has not been specified in the kernel!
A good general test is to try connecting to your service provider
using the software that most supply for (gulp) Microsoft Windows. If
everything works from another operating system to exactly the same
account, then the problem is with your Linux system and NOT your
service provider.
20.2. I can send email, but not receive it
If you are using dynamic IP numbers, this is perfectly normal. See
"Setting up Services" below.
20.3. Why can't people finger, WWW, gopher, talk etc to my machine?
Again, if you are using dynamic IP numbers, this is perfectly normal.
See "Setting up Services" below.
21. Using Internet services with Dynamic IP numbers
If you are using dynamic IP numbers (and many service providers will
only give you a dynamic IP number unless you pay significantly more
for your connection), then you have to recognise the limitations this
imposes.
First of all, outbound service requests will work just fine. That is,
you can send email using sendmail (provided you have correctly set up
sendmail), ftp files from remote sites, finger users on other
machines, browse the web etc.
In particular, you can answer email that you have brought down to your
machine whilst you are off line. Mail will simply sit in your mail
queue until you dial back into your ISP.
However, your machine is NOT connected to the Internet 24 hours a day,
nor does it have the same IP number every time it is connected. So it
is impossible for you to receive email directed to your machine, and
very difficult to set up a web or ftp server that your friends can
access! As far as the Internet is concerned your machine does not
exist as a unique, permanently contactable machine as it does not have
a unique IP number (remember - other machines will be using the IP
number when they are allocated it on dial in).
If you set up a WWW (or any other server), it is totally unknown by
any user on the Internet UNLESS they know that your machine is
connected AND its actual (current) IP number. There are a number of
ways they can get this info, ranging from you ringing them, sending
them email to tell them or cunning use of ".plan" files on a shell
account at your service provider (assuming that your provider allows
shell and finger access).
Now, for most users, this is not a problem - all that most people want
is to send and receive email (using your account on your service
provider) and make outbound connections to WWW, ftp and other servers
on the Internet. If you MUST have inbound connections to your server,
you should really get a static IP number. Alternatively you can
explore the methods hinted at above...
21.1. Setting up email
Even for dynamic IP numbers, you can certainly configure sendmail on
your machine to send out any email that you compose locally.
Configuration of sendmail can be obscure and difficult - so this
document does not attempt to tell you how to do this. However, you
should probably configure sendmail so that your Internet service
provider is designated as your "smart relay" host (the sendmail.cf DS
option). (For more sendmail configuration info, see the sendmail
documents - and look at the m4 configurations that come with sendmail.
There is almost certain to be one there that will meet your needs).
There are also excellent books on Sendmail (notably the 'bible' from
O'Reilly and Associates), but these are almost certainly overkill for
most users!
Once you have sendmail configured, you will probably want to have
sendmail dispatch any messages that have been sitting in the outbound
mail queue as soon as the PPP connection comes up. To do this, add the
command
sendmail -q &
to your /etc/ppp/ip-up script (see below).
Inbound email is a problem for dynamic IP numbers. The way to handle
this is to:-
· configure your mail user agent so that all mail is sent out with a
"reply to" header giving your email address at your Internet
Service provider.
If you can, you should also set your FROM address to be your email
address at your ISP as well.
· use the popclient, fetchmail programs to retrieve your email from
your service provider. Alternatively, if your ISP is using IMAP,
use an IMAP enabled mail user agent (such as pine).
You can automate this process at dial up time by putting the necessary
commands in the /etc/ppp/ip-up script (see below).
21.2. Setting Up a local Name server
Whilst you can quite happily use the domain name servers located at
your ISP, you can also set up a local caching only (secondary) name
server that is brought up by the ip-up script. The advantage of
running a local (caching only) name server is that it will save you
time (and bandwidth) if you frequently contact the same sites during a
long on-line session.
DNS configuration for a caching only nameserver (that uses a
"forwarders' line in the named.boot file pointing at your ISPs DNS) is
relatively simple. The O'Reilly book (DNS and Bind) explains all you
want to know about this.
There is also a DNS-HOWTO available.
If you are running a small LAN that can access the Internet through
you Linux PC (using IP Masquerade for example), it is probably a good
idea to run a local name server (with a forwarders directive) whilst
the link is up as this will minimise the bandwidth and delays
associated with name resolution.
One point of Nettiquette: ask permission from your ISP before you
start using a secondary, caching only name server in your ISP's
domain. Properly configured, your DNS will not cause any problems to
your ISP at all, but if you get things wrong, it can cause problems.
22. Linking two networks using PPP
There is basically no difference between linking a single Linux PC to
a PPP server and linking two LANs using PPP on a machine on each LAN.
Remember, PPP is a peer to peer protocol.
However, you DEFINITELY need to understand about how routing is
established. Read the NET-2 howto and the Linux Network Administrator
Guide (NAG). You will also find " TCP/IP Network Administration"
(published by O'Reilly and Assoc - ISBN 0-937175-82-X) to be of
invaluable assistance.
If you are going to be sub networking an IP network number on either
side of the link, you will also find the Linux (draft) sub networking
mini-howto) to be of use. This is available at Linux Sub networking
mini-HOWTO <http://www.interweft.com.au/other/>.
In order to link two LANs, you must be using different IP network
numbers (or subnets of the same network number) and you will need to
use static IP numbers - or use IP masquerade. If you want to use IP
masquerade, see the IP masquerade mini-howto for instructions on
setting that up.
22.1. Setting up the IP numbers
Arrange with the network administrator of the other LAN the IP numbers
that will be used for each end of the PPP interface. If you are using
static IP numbers, this will also probably require you to dial into a
specific telephone number.
Now edit the appropriate /etc/ppp/options[.ttyXX] file - it's a good
idea to have a specific modem and port at your end for this
connection. This may well require you to change your /etc/ppp/options
file - and create appropriate options.ttyXX files for any other
connections!
Specify the IP numbers for your end of the PPP link in the appropriate
options file exactly as shown above for static IP numbers.
22.2. Setting up the routing
You must arrange that packets on your local LAN are routed across the
interface that the PPP link establishes. This is a two stage process.
First of all, you need to establish a route from the machine running
the PPP link to the network(s) at the far end of the link. If the link
is to the Internet, this can be handled by a default route established
by pppd itself at your end of the connection using the 'defaultroute'
option to pppd.
If however, the link is only linking two LANs, then a specific network
route must be added for each network that is accessible across the
link. This is done using a 'route' command for each network in the
/etc/ppp/ip-up script (see After the link comes up...) for
instructions on doing this.
The second thing you need to do is to tell the other computers on your
LAN that your Linux computer is actually the 'gateway' for the
network(s) at the far end of the ppp link.
Of course, the network administrator at the other end of the link has
to do all this too! However, as s/he will be routing packets to your
specific networks, a specific network route will be required, not a
default route (unless the LANs at the far and of the link are linking
into you to access the Internet across your connection).
22.3. Network security
If you are linking you LAN to the Internet using PPP - or even just to
a "foreign" LAN, you need to think about security issues. I strongly
urge you to think about setting up a firewall!
You should also speak to the LAN administrator at your site BEFORE you
start linking to foreign LANs or the Internet this way. Failure to do
so could earn you anything from no reaction to really serious trouble!
23. After the link comes up - the /etc/ppp/ip-up script
Once the PPP link is established, pppd looks for /etc/ppp/ip-up. If
this script exists and is executable, the PPP daemon executes the
script. This allows you to automate any special routing commands that
may be necessary and any other actions that you want to occur every
time the PPP link is activated.
This is just a shell script and can do anything that a shell script
can do (i.e. virtually anything you want).
For example, you can get sendmail to dispatch any waiting outbound
messages in the mail queue.
Similarly, you can insert the commands into ip-up to collect (using
pop) any email waiting for you at your ISP.
There are restrictions on /etc/ppp/ip-up:-
· It runs in a deliberately restricted environment to enhance
security. This means you must give a full path to binaries etc.
· Technically, /etc/ppp/ip-up is a program not a script. This means
it can be directly executed - and hence it requires the standard
file magic (#!/bin/bash) at the start of the first line and must be
readable and executable by root.
23.1. Special routing
If you are linking two LANs, you will need to set up specific routes
to the 'foreign' LANs. This is easily done using the /etc/ppp/ip-up
script. The only difficulty arises if your machine handles multiple
PPP links.
This is because the /etc/ppp/ip-up is executed for EVERY ppp
connection that comes up, so you need to carefully execute the correct
routing commands for the particular link that comes up - and not when
any other link comes up!
23.2. Handling email queues
When the link between two LANs comes up, you may well want to make
sure that email that is queued at either end is flushed - sent out to
its destination. This is done by adding the appropriate sendmail
invocation.
Using the bash 'case' statement on an appropriate parameter that pppd
passes into the script accomplishes this. For example, this is the
/etc/ppp/ip-up script I use to handle our WAN links and the link to my
home Ethernet (also handled on the same ppp server).
23.3. A sample /etc/ppp/ip-up script
The example below provides a variety of example uses.
______________________________________________________________________
#!/bin/bash
#
# Script which handles the routing issues as necessary for pppd
# Only the link to Newman requires this handling.
#
# When the ppp link comes up, this script is called with the following
# parameters
# $1 the interface name used by pppd (e.g. ppp3)
# $2 the tty device name
# $3 the tty device speed
# $4 the local IP address for the interface
# $5 the remote IP address
# $6 the parameter specified by the 'ipparam' option to pppd
#
case "$5" in
# Handle the routing to the Newman Campus server
202.12.126.1)
/sbin/route add -net 202.12.126.0 gw 202.12.126.1
# and flush the mail queue to get their email there asap!
/usr/sbin/sendmail -q &
;;
139.130.177.2)
# Our Internet link
# When the link comes up, start the time server and synchronise to the world
# provided it is not already running
if [ ! -f /var/lock/subsys/xntpd ]; then
/etc/rc.d/init.d/xntpd.init start &
fi
# Start the news server (if not already running)
if [ ! -f /var/lock/subsys/news ]; then
/etc/rc.d/init.d/news start &
fi
;;
203.18.8.104)
# Get the email down to my home machine as soon as the link comes up
# No routing is required as my home Ethernet is handled by IP
# masquerade and proxyarp routing.
/usr/sbin/sendmail -q &
;;
*)
esac
exit 0
______________________________________________________________________
As a result of bringing up the ppp link to our Newman campus and this
script, we end up with the following routing table entries (this
machine also is our general dial up PPP server AND handles our
Internet link). I have interspersed comments in the output to help
explain what each entry is) :-
______________________________________________________________________
[root@kepler /root]# route -n
Kernel routing table
Destination Gateway Genmask Flags MSS Window Use Iface
# the HOST route to our remote internet gateway
139.130.177.2 * 255.255.255.255 UH 1500 0 134 ppp4
# the HOST route to our Newman campus server
202.12.126.1 * 255.255.255.255 UH 1500 0 82 ppp5
# the HOST route to my home ethernet
203.18.8.104 * 255.255.255.255 UH 1500 0 74 ppp3
# two of our general dial up PPP lines
203.18.8.64 * 255.255.255.255 UH 552 0 0 ppp2
203.18.8.62 * 255.255.255.255 UH 552 0 1 ppp1
# the specific network route to the Newman campus LAN
202.12.126.0 202.12.126.1 255.255.255.0 UG 1500 0 0 ppp5
# the route to our local Ethernet (super-netting two adjacent C classes)
203.18.8.0 * 255.255.254.0 U 1500 0 1683 eth0
# the route to the loop back device
127.0.0.0 * 255.0.0.0 U 3584 0 483 lo
# the default route to the Internet
default 139.130.177.2 * UG 1500 0 3633 ppp4
______________________________________________________________________
23.4. Handling email
The previous section shows how to handle the outgoing mail - simply by
flushing the mail queue once the link is up.
If you are running a WAN link, you can arrange with the network
administrator of the remote LAN to do exactly the same thing. For
example, at the Newman Campus end of our WAN link, the /etc/ppp/ip-up
script looks like :-
______________________________________________________________________
#!/bin/bash
#
# Script which handles the routing issues as necessary for pppd
# Only the link to Hedland requires this handling.
#
# When the ppp link comes up, this script is called with the following
# parameters
# $1 the interface name used by pppd (e.g. ppp3)
# $2 the tty device name
# $3 the tty device speed
# $4 the local IP address for the interface
# $5 the remote IP address
# $6 the parameter specified by the 'ipparam' option to pppd
#
case "$5" in
203.18.8.4)
/usr/sbin/sendmail -q
;;
*)
esac
exit 0
______________________________________________________________________
If however you have only a dynamic IP PPP link to your ISP, you need
to get your email from the account on your ISPs machine. This is
usually done using the POP (Post Office Protocol). This process can be
handled using the 'popclient' program - and the ip-up script can
automate this process for you too!
Simply create a /etc/ppp/ip-up script that contains the appropriate
invocation of popclient. For my laptop that runs Red Hat Linux (which
I take on any travels), this is
______________________________________________________________________
popclient -3 -c -u hartr -p <password> kepler.hedland.edu.au |formail -s procmail
______________________________________________________________________
You could use slurp or whatever to do the same for news, and so forth.
Remember, the ip-up script is just a standard bash script and so can
be used to automate ANY function that needs to be accomplished every
time the appropriate PPP link comes up.
24. Using /etc/ppp/ip-down
You can create a script that will be executed once the link has been
terminated. This is stored in /etc/ppp/ip-down. It can be used to undo
anything special that you did in the corresponding /etc/ppp/ip-up
script.
25. Routing issues on a LAN
If you are connected to a LAN but still want to use PPP on your
personal Linux machine , you need to address some issues of the routes
packets need to take from your machine to reach your LAN (through your
Ethernet interface) and also to the remote PPP server and beyond.
This section does NOT attempt to teach you about routing - it deals
only with a simple, special case of (static) routing!
I strongly urge you to read the Linux Network Administrator Guide
(NAG) if you are NOT familiar with routing. Also the O'Reilly book
"TCP/IP Network Administration" covers this topic in a very
understandable form.
The basic rule of static routing is that the DEFAULT route should be
the one that points to the MOST number of network addresses. For other
networks, enter specific routes to the routing table.
The ONLY situation I am going to cover here is where your Linux box is
on a LAN that is not connected to the Internet - and you want to dial
out to the Internet for personal use whilst still connected to the
LAN.
First of all, make sure that your Ethernet route is set up to the
specific network addresses available across your LAN - NOT set to the
default route!
Check this by issuing a route command, you should see something like
the following:-
[root@hwin /root]# route -n
Kernel routing table
Destination Gateway Genmask Flags MSS Window Use Iface
loopback * 255.255.255.0 U 1936 0 50 lo
10.0.0.0 * 255.255.255.0 U 1436 0 565 eth0
If your Ethernet interface (eth0) is pointing at the default route,
(the first column will show "default" in the eth0 line) you need to
change your Ethernet initialisation scripts to make it point at the
specific network numbers rather than the default route (consult the
Net2 HOWTO and NAG).
This will allow pppd to set up your default route as shown below:-
[root@hwin /root]# route -n
Kernel routing table
Destination Gateway Genmask Flags MSS Window Use Iface
10.144.153.51 * 255.255.255.255 UH 488 0 0 ppp0
127.0.0.0 * 255.255.255.0 U 1936 0 50 lo
10.1.0.0 * 255.255.255.0 U 1436 0 569 eth0
default 10.144.153.51 * UG 488 0 3 ppp0
As you can see, we have a host route to the PPP server (
10.144.153.51) via ppp0 and also a default network route that uses the
PPP server as its gateway.
If your set up needs to be more complex than this - read the routing
documents already mentioned and consult an expert at your site!
If your LAN already has routers on it, you will already have gateways
established to the wider networks available at your site. You should
STILL point your default route at the PPP interface - and make the
other routes specific to the networks they serve.
25.1. Note on Security
When you set up a Linux box on an existing LAN to link into the
Internet, you are potentially opening your entire LAN to the Internet
- and the hackers that reside there. Before you do this, I strongly
urge you to consult your network administrator and site security
policy. If your PPP connection to the Internet is used to successfully
attack your site, you will at the very least earn the intense anger of
your fellow users, network and system administrators. You may also
find yourself in very much more serious trouble!
Before you connect a LAN to the Internet, you should consider the
security implications of even a DYNAMIC connection - hence the earlier
reference to the O'Reilly "Building Internet Firewalls"!
26. Setting up a PPP server
As already mentioned, there are many ways to do this. What I present
here is the way I do it (using a Cyclades multi-port serial card) and
a rotary dial in set of telephone lines.
If you don't like the method I present here, please feel free to go
your own way. I would however, be pleased to include additional
methods in future versions of the HOWTO. So, please send me your
comments and methods!
Please note, this section only concerns setting up Linux as a PPP
server. I do not (ever) intend to include information on setting up
special terminal servers and such.
Also, I have yet to experiment with shadow passwords (but will be
doing so sometime). Information currently presented does NOT therefore
include any bells and whistles that are required by the shadow suite.
26.1. Kernel compilation
All the earlier comments regarding kernel compilation and kernel
versions versus pppd versions apply. This section assumes that you
have read the earlier sections of this document!
For a PPP server, you MUST include IP forwarding in your kernel. You
may also wish to include other capabilities (such as IP fire walls,
accounting etc etc).
If you are using a multi-port serial card, then you must obviously
include the necessary drivers in your kernel too!
26.2. Overview of the server system
We offer dial up PPP (and SLIP) accounts and shell accounts using the
same user name/password pair. This has the advantages (for us) that a
user requires only one account and can use it for all types of
connectivity.
As we are an educational organisation, we do not charge our staff and
students for access, and so do not have to worry about accounting and
charging issues.
We operate a firewall between our site and the Internet, and this
restricts some user access as the dial up lines are inside our
(Internet) firewall (for fairly obvious reasons, details of our other
internal fire walls are not presented here and are irrelevant in any
case).
The process a user goes through to establish a PPP link to our site
(once they have a valid account of course) is :-
· Dial into our rotary dialer (this is a single phone number that
connects to a bank of modems - the first free modem is then used).
· Log in using a valid user name and password pair.
· At the shell prompt, issue the command ppp to start PPP on the
server.
· Start PPP on their PC (be it running Windows, DOS, Linux MAC OS or
whatever - that is their problem).
The server uses individual /etc/ppp/options.ttyXX files for each dial
in port that set the remote IP number for dynamic IP allocation. The
server users proxyarp routing for the remote clients (set via the
appropriate option to pppd). This obviates the need for routed or
gated.
When the user hangs up at their end, pppd detects this and tells the
modem to hang up, bringing down the PPP link at the same time.
26.3. Getting the software together
You will need the following software:-
· Linux, properly compiled to include the necessary options.
· The appropriate version of pppd for your kernel.
· A 'getty' program that intelligently handles modem communications.
We use getty_ps2.0.7h, but mgetty is highly thought of. I
understand that mgetty can detect a call that is using pap/chap
(pap is the standard for Windows95) and invoke pppd automatically,
but I have yet to explore this.
· An operational domain name server (DNS) that is accessible to your
dial up users.
You should really be running your own DNS if possible...
26.4. Setting up standard (shell access) dialup.
Before you can set up your PPP server, your Linux box must be capable
of handling standard dial up access.
This howto does NOT cover setting this up. Please see the
documentation of the getty of your choice and serial HOWTO for
information on this.
26.5. Setting up the PPP options files
You will need to set up the overall /etc/ppp/options with the common
options for all dial up ports. The options we use are:-
______________________________________________________________________
asyncmap 0
netmask 255.255.254.0
proxyarp
lock
crtscts
modem
______________________________________________________________________
Note - we do NOT use any (obvious) routing - and in particular there
is no defaultroute option. The reason for this is that all you (as a
PPP server) are required to do is to route packets from the ppp client
out across your LAN/Internet and route packets to the client from your
LAN and beyond.
All that is necessary for this is a host route to the client machine
and the use of the 'proxyarp' option to pppd.
The 'proxyarp' option sets up (surprise) a proxy ARP entry in the PPP
server's ARP table that basically says 'send all packets destined for
the PPP client to me'. This is the easiest way to set up routing to a
single PPP client - but you cannot use this if you are routing between
two LANs - you must add proper network routes which can't use proxy
ARP.
You will almost certainly wish to provide dynamic IP number allocation
to your dial up users. You can accomplish this by allocating an IP
number to each dial up port. Now, create a /etc/ppp/options.ttyXX for
each dial up port.
In this, simply put the local (server) IP number and the IP number
that is to be used for that port. For example
______________________________________________________________________
kepler:slip01
______________________________________________________________________
In particular, note that you can use valid host names in this file (I
find that I only remember the IP numbers of critical machines and
devices on my networks - names are more meaningful)!
26.6. Setting pppd up to allow users to (successfully) run it
As starting a ppp link implies configuring a kernel device (a network
interface) and manipulating the kernel routing tables, special
privileges are required - in fact full root privileges.
Fortunately, pppd has been designed to be 'safe' to run set uid to
root. So you will need to
______________________________________________________________________
chmod u+s /usr/sbin/pppd
______________________________________________________________________
When you list the file, it should then appear as
______________________________________________________________________
-rwsr-xr-x 1 root root 74224 Apr 28 07:17 /usr/sbin/pppd
______________________________________________________________________
If you do not do this, users will be unable to set up their ppp link.
26.7. Setting up the global alias for pppd
In order to simplify things for our dial up PPP users, we create a
global alias (in /etc/bashrc) so that one simple command will start
ppp on the server once they are logged in.
This looks like
______________________________________________________________________
alias ppp="exec /usr/sbin/pppd -detach"
______________________________________________________________________
What this does is
· exec : this means replace the running program (in this case the
shell) with the program that is run.
· pppd -detach : start up pppd and do NOT fork into the background.
This ensures that when pppd exits there is no process hanging
around.
When a user logs in like this, they will appear in the output of 'w'
as
______________________________________________________________________
6:24pm up 3 days, 7:00, 4 users, load average: 0.05, 0.03, 0.00
User tty login@ idle JCPU PCPU what
hartr ttyC0 3:05am 9:14 -
______________________________________________________________________
And that is it...I told you this was a simple, basic PPP server
system!
27. Using PPP across a null modem (direct serial) connection
This is very simple - there is no modem in the way so things are much
simpler.
First of all, choose one of the machines as a 'server', setting up a
getty on the serial port so you can test that you do have connectivity
using minicom to access the serial port on the 'client'.
Once you have this functioning, you can remove the getty UNLESS you
want to make sure that the connection is validated using user
name/password pairs as for a dial up connection. As you have 'physical
control' of both machines, I will presume that you do NOT want to do
this.
Now, on the server, remove the getty and make sure that you have the
serial ports on both machines configured correctly using 'setserial'.
All you need to do now is to start pppd on both systems. I will assume
that the connection uses /dev/ttyS34 on both machines. So, on both
machines execute the command:-
______________________________________________________________________
pppd -detach crtscts lock <local IP>:<remote IP> /dev/ttyS3 38400 &
______________________________________________________________________
This will bring up the link - but as yet you have no routing
specified. You can test the link by pinging to and fro to each
machine. If this works, bring down the link by killing one of the pppd
processes.
The routing you need will of course depend on exactly what you are
trying to do. Generally, one of the machines will be connected to an
Ethernet (and beyond) and so the routing required is exactly the same
as for a PPP server and client.
So on the Ethernet equipped machine, the pppd command would be
______________________________________________________________________
pppd -detach crtscts lock proxyarp <local IP>:<remote IP> /dev/ttyS3 38400 &
______________________________________________________________________
and on the other machine
______________________________________________________________________
pppd -detach crtscts lock defaultroute <local IP>:<remote IP> /dev/ttyS3 38400 &
______________________________________________________________________
If you are linking two networks (using a serial link!) or have more
complex routing requirements, you can use /etc/ppp/ip-up in exactly
the same way as mentioned earlier in this document.
Robert Hart
Port Hedland, Western Australia
Melbourne, Victoria, Australia August/October 1996 January/March 1997
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