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PPP for Linux Version 2.2.0
============= based on
ppp-2.2.0
Mar 1995
Michael Callahan callahan@maths.ox.ac.uk
Al Longyear longyear@netcom.com
Contents:
INTRODUCTION
CREDITS
CHANGES FROM THE PREVIOUS VERSION
FUTURE PLANS
INSTALLATION
PROBLEMS WHICH MAY OCCUR WHILE BUILDING THE KERNEL
A REFERENCE TO UNDEFINED _mod_use_count_
BLOCK ON FREELIST AT nnnnnn ISN'T FREE
GENERAL NETWORK CONFIGURATION
CONNECTING TO A PPP SERVER
IF IT WORKS
IF IT DOESN'T WORK
IF IT STILL DOESN'T WORK (OR, BUG REPORTS)
DYNAMIC ADDRESS ASSIGNMENT
SETTING UP A MACHINE FOR INCOMING PPP CONNECTIONS
SETTING UP A MACHINE FOR INCOMING PPP CONNECTIONS WITH DYNAMIC IP
ADDITIONAL INFORMATION
DIP SUPPORT
CONCLUSION
INTRODUCTION
This file is substantially derived from the previous version for
the pppd process 2.2.0. Michael Callahan wrote that version. This
particular version was written, modified, hacked, changed, whatever,
by Al Longyear. If you find errors in this document, they are probably
mine and not Michael's.
This is a PPP driver for Linux. It has been used by many people and
seems to be quite stable. It is capable of being used either as a
'client'--for connecting a Linux machine to a local Internet provider,
for example--or as a 'server'--allowing a Linux machine with a modem
and an Ethernet connection to the Internet to provide dial-in PPP
links. (In fact, the PPP protocol does not make the distinction
between client and server, but this is the way people often think
about it.)
The PPP protocol consists of two parts. One is a scheme for framing
and encoding packets, the other is a series of protocols called LCP,
IPCP, PAP and CHAP, for negotiating link options and for
authentication. This package similarly consists of two parts: a
kernel module which handles PPP's low-level framing protocol, and a
user-level program called pppd which implements PPP's negotiation
protocols.
The kernel module assembles/disassembles PPP frames, handles error
detection, and forwards packets between the serial port and either the
kernel network code or the user-level program pppd. IP packets go
directly to the kernel network code. So once pppd has negotiated the
link, it in practice lies completely dormant until you want to take
the link down, when it negotiates a graceful disconnect.
CREDITS
I (MJC) wrote the original kernel driver from scratch. Laurence
Culhane and Fred van Kempen's slip.c was priceless as a model (a
perusal of the files will reveal that I often mimicked what slip.c
did). Otherwise I just implemented what pppd needs, using RFC1331 as
a guide. For the most part, the Linux driver provides the same
interface as the free 386BSD and SunOS drivers. The exception is that
Linux has no support for asynchronous I/O, so I hacked an ioctl into
the PPP kernel module that provides a signal when packets appear and
made pppd use this instead.
Al Longyear ported version 2.2 of pppd (from the free package
ppp-2.2.0) to Linux. He also provided several enhancements to both
the kernel driver and the OS-independent part of pppd. His
contributions to Linux PPP have been immense, and so this release
is being distributed over both our names.
The pppd program comes from the free distribution of PPP for Suns and
386BSD machines, maintained by Paul Mackerras. This package lists
"thanks to" Brad Parker, Greg Christy, Drew D. Perkins, Rick Adams and
Chris Torek.
Jim Freeman added the code to support a ppp module and to dynamically
extend the number of ppp devices. All ppp devices listed in the Space.c
will be unlinked when the kernel is loaded. This feature makes the use
of '16 channel' support obsolete.
CHANGES FROM THE PREVIOUS VERSION
- The number of devices for the PPP device has been made dynamic. It was
previously configured with the default value of four devices.
- The problems dealing with other systems such as Windows NT and their
authenticiation has been corrected. It will now generate the proper
responses to allow the system to choose a valid authentication protocol.
- The kernel debug value has changed. Previously it was a level. It is now
a bit map with various bits meaning certain types of debug information.
0 - No debug information is generated
1 - Debug messages
2 - Log incoming packets
4 - Log outgoing packets
8 - Log tty output buffers
16 - Log tty input buffers
If you wish to use any combination, add the values together. For example,
'7' will log debug messages and incoming packages and outgoing packets.
The default setting is 0.
The simple IP trace which ppp.c performed when 'kdebug' was greater than
127 has been removed. You should use tcpdump for this type of trace
actions.
- Support is added for compression control protocol. At the present time
only the BSD-Compress compression protocol is supported.
- There are two queues for output frames. This avoids some problems which
occured with the previous version and some PPP packages which exchanged
echo frames with Linux.
- The echo frames are now proper. Previously, it would generate extra
characters and this caused some providers to not respond to the 'junk'.
- The max-echo-failure option will now properly disconnect the line.
- There are other changes which are listed in the general README file. Please
read that file as well for changes.
- There is no limit to the number of ppp devices which you may use. Jim Freeman
has added code to create them upon demand and to re-use the ones which have
been closed. There is no code, nor plans to write code, to remove (delete)
the un-used devices. So, if your system goes to a spurt and uses 256 ppp
devices, it will remain at that level until you next reload the kernel.
If you are using modules then you may use the additional setting of
max_dev=#
where # is the maximum number. The default is set by the define PPP_MAX_DEV
and this define may be altered if you are not using modules.
The BSD compressor may only be loaded as a module. Previous beta versions
permitted the compressor to be included into the kernel. This was removed
for several reasons, some technical, some less technical and more
political (legal).
FUTURE PLANS
The next version of pppd, 2.3, is designed to contain a demand dial
function directly.
INSTALLATION
This version of PPP has been tested on 1.1.x (x>=14) It will probably
not work on kernels much earlier than this due to a change in the
routing code. If you have an earlier kernel, please upgrade.
joining the PPP channel of linux-activists:
This isn't really part of installation, but if you DO use
Linux PPP you should do this. Send a message with the line
subscribe linux-ppp
contained in the body to majordomo@vger.rutgers.edu
You may use
subscribe linux-ppp myname@mail.address
if you wish the linux-ppp information sent to a different mail
address.
To leave the mail list, send 'unsubscribe linux-ppp' to the same
mail address.
You can send to the list by mailing to
linux-ppp@vger.rutgers.edu. This is a majordomo mailing list and
is unlike the earlier version on hut.fi. There is no magic header
required for this list. In addition, it is gated to the usenet
group linux.dev.ppp. You may choose to read the few messages posted
there.
Usenet News Groups
There are three applicable usenet news groups for the PPP code. Please
choose the group which applies the closest to the type of problem
which you are experiencing.
comp.os.linux.networking
- Trouble setting routes, running name services, using telnet, ftp,
news, etc.
- It will not compile.
comp.os.linux.setup
- Trouble installing the package from BINARIES only. This does *NOT*
include problems with compiling the package.
comp.protocols.ppp
- How do I use the package?
- How do I connect to .... services?
- Why does this not work?
- All other types of questions on how to use just the PPP code.
PLEASE don't assume that just because you are using PPP as your
network device driver that all problems with your networking are a
problem of PPP. PPP is *NOT* responsible for your modem disconnecting,
routing to other servers, running telnet, etc. Calling the problem
'ppp' on usenet may cause it to be ignored by the people who
actually work on the networking code.
Installation procedure:
The installation procedure has been totally revised for this
version. Due to feedback from other users, it was felt that a more
automated installation procedure be performed.
Use the following procedure for all kernel versions. There are six steps
numbered one through six. Please do them in order and not skip one.
1. Issue the command:
./configure
from the top level directory of pppd. This is the directory which
contains this README.linux file. The result of this will be to build a
set of symbolic links to the makefiles. They should link 'Makefile' to
'Makefile.linux' in each of the directories.
2. Issue the command:
make kernel
from the top level directory. This will install the various include
files and source files into the proper directory for the linux
kernel. If you don't have the kernel installed in the /usr/src/kernel
directory then it will not work. Instead it will print a message to
the effect that you need to specify the kernel location on the
kinstall command.
The actual message will say:
There appears to be no kernel source distribution in /usr/src/linux.
Give the top-level kernel source directory as the argument to
this script.
usage: kinstall.sh [linux-source-directory]
If, and only if, you receive this message, do the following:
a. Change to the 'linux' directory with the command:
cd linux
b. Issue the command:
./kinstall.sh /usr/src/linux
or use the proper location for the kernel rather than
/usr/src/linux. For example, if you have the kernel installed in
/usr1/kernel then the command would be:
./kinstall.sh /usr1/kernel
The script will validate that the kernel is properly installed into
that directory and check the level of the kernel. The installation
will not be accepted if your kernel is too early.
The installation procedure will copy only the files which are
needed. It will not replace any file which should not be
replaced. Please don't second-guess the installation script and
attempt to do the procedure on your own. There are some very subtle
dependencies and if you are not careful, the installation will not
work.
You are free to run the installation script as many times as you
wish. The additional executions will only change the files which have
not been changed.
3. Build the kernel.
You must rebuild the kernel with this package. The driver is totally
new and will not work with the older daemon and the newer daemon will
not work with the older kernel driver. If you don't know how to build
a kernel, then you should read the README file in the kernel source
directory.
If you wish module support then you need to have the 'modules-1.1.87'
package installed as the minimum version. Earlier versions of the module
support will not work properly.
As of this time, the current version for the modules package is
1.2.0. Even 1.1.87 is old. However, if you only have 1.1.87 then it
will do as it permits the symbol table references. Please consider
upgrading the module package however.
Instructions on building the kernel with modules are given in the
README.modules in the kernel source directory.
4. Install the kernel
If you are using the Yggdrasil distribution then you need to 'install'
the kernel at this point. Refer to their documentation on the procedures
to install the kernel.
Distributions other than the Yggdrasil will normally install the
kernel when you build it.
5. Build the programs.
The programs are built next. The command to build the programs is fairly
simple. Just issue the command:
make
from the top level directory where this README.linux file is located.
5a. Support for shadow passwords
To compile the pppd process with support for shadow passwords you must
first have compiled the shadow password suite and have it installed
into the /usr/include and /usr/lib directories. Rather than using the
simple make command mentioned above, issue the command:
make HAS_SHADOW=1
5b. Support for Microsoft DNS LCP tags
To compile the pppd process with support for the LCP tags used by
Microsoft's DNS tags in their Windows 95 and Windows NT operating
system platforms, use the command:
make USE_MS_DNS=1
If you wish both shadow support and MS DNS tags, then use both options in
either order.
6. Install the programs.
You may use the command
make install
to install the various programs. They will be installed into the
/usr/lib/ppp directory. You may not like this directory for the
executables. The directory name is called BINDIR and is set in the
file 'linux/Makefile.linux'.
7. Reboot to the new kernel.
After building the new kernel, you will need to actually use it. Reboot
the Linux system and you may then use the new pppd program.
8. Load optional modules.
If you are using loadable modules for the ppp then you must load them
after the kernel has been started. The following relative order must
be maintained.
Sequence Module Description
1 slhc.o VJ header compression
2 ppp.o PPP driver
3 bsd_comp.o BSD compression for PPP's compression protocol.
If you only have the bsd comprssor as a module then you may load it without
regard to any order.
You may elect not to load the BSD compression module if you desire. There
is a controversy regarding a Motorola software patent and while it is
believed that this code does not infringe upon the patent, it is however
an optional component.
In addition, if memory is a premium, do not run the BSD compression. It
may take large amounts of memory (up to 2.6 meg) for high compression
lengths to hold the compression dictionaries.
Without the BSD compression module, the PPP driver will not accept PPP's
compression control protocol for BSD compression.
PROBLEMS WHICH MAY OCCUR WHILE BUILDING THE KERNEL
A REFERENCE TO UNDEFINED _mod_use_count_
If you experience an error message that the variable "mod_use_count_" is
undefined then apply the patches in the linux/Other.Patches directory. There
is a version for the 1.2.13 kernel and a differnt one for the early 1.3
kernels.
The current 1.3 series kernels should not experience this problem as the
patch has already been incorporated.
BLOCK ON FREELIST AT nnnnnn ISN'T FREE
(where nnnnnn are a sequence of hexadecimal digits.)
While this is not really an error when the kernel is built, it is an error
which may occur when you actually run the system. The solution is a patch
in the linux/Other.Patches directory. (That is why I mention it here.)
The problem is in the VJ header compression module. It allocated a block
of memory and then used the wrong variable to determine the amount of memory
which should be reset. The patch is present in the 1.3.46 kernel and later
ones. However, versions prior to that may POSSIBLY have the problem depending
upon the number of slots which are allocated for the header compression logic.
GENERAL NETWORK CONFIGURATION
Since many people don't use the Linux networking code at all until
they get a PPP link, this section describes generally what's needed to
get things running. In principle none of this is special to PPP. For
more details, you should consult the relevant Linux HOWTOs. If you
already understand network setup, you can skip this section.
The first file that requires attention is the rc script that does
network configuration at boot time, called /etc/rc.net or
/etc/rc.d/rc.net.{1,2} or something similar, depending on your Linux
distribution. This file should 'ifconfig' the loopback interface lo,
and should add an interface route for it. These lines might look
something like this:
$CONFIG lo 127.0.0.1
$ROUTE add loopback
or
/sbin/ifconfig lo 127.0.0.1
/sbin/route add 127.0.0.1
However, it should *not* config an ethernet card or install any other
routes (unless you actually have an ethernet card, in which case I'll
assume you know what to do). Many distributions will provide scripts
that expect you to have an ethernet card.
You also need to decide whether you want to allow incoming
telnet/ftp/finger, etc. If so, you should have the rc startup script
run the 'inetd' daemon.
Next, you should set up /etc/hosts to have two lines. The first
should just give the loopback or localhost address and the second
should give your own host name and the IP address your PPP connection
will use. For example:
127.0.0.1 loopback localhost # useful aliases
192.1.1.17 billpc.president.whitehouse.gov bill # my hostname
192.1.1.23 chelseapc.president.whitehouse.gov chelseapc
where my IP address is 192.1.1.17 and my hostname is
billpc.president.whitehouse.gov. (Not really, you understand.) If
your PPP server does dynamic IP address assignment, give a guess as to
an address you might get (see also "Dynamic Address Assignment"
below).
Finally, you need to configure the domain name system by putting
appropriate lines in /etc/resolv.conf . It should look something like
this:
domain president.whitehouse.gov
search president.whitehouse.gov whitehouse.gov
nameserver 192.1.2.1
nameserver 192.1.2.10
Assuming there are nameservers at 192.1.2.1 and 192.1.2.10, then when
you get connected with PPP, you can reach hosts whose full names are
'hillarypc.president.whitehouse.gov' and 'chelseapc.whitehouse.gov' by
the names 'hillarypc' and 'chelseapc'. You can probably find out the
right domain name to use and the IP numbers of nameservers from
whoever's providing your PPP link.
CONNECTING TO A PPP SERVER
To use PPP, you invoke the pppd program with appropriate options.
Everything you need to know is contained in the pppd(8) manual page.
However, it's useful to see some examples:
Example 1: A simple dial-up connection.
Here's a command for connecting to a PPP server by modem.
pppd connect 'chat -v "" ATDT5551212 CONNECT "" ogin: ppp word: whitewater' \
/dev/cua1 38400 debug crtscts modem defaultroute 192.1.1.17
Going through pppd's options in order:
connect 'chat etc...' This gives a command to run to contact the
PPP server. Here the supplied 'chat' program is used to dial a
remote computer. The whole command is enclosed in single quotes
because pppd expects a one-word argument for the 'connect' option.
The options to 'chat' itself are:
-v verbose mode; log what we do to syslog
"" don't wait for any prompt, but instead...
ATDT5551212 dial the modem, then
CONNECT wait for answer
"" send a return (null text followed by usual return)
ogin: ppp word: whitewater log in.
/dev/cua1 specify the callout serial port cua1
38400 specify baud rate
debug log status in syslog
crtscts use hardware flow control between computer and modem
(at 38400 this is a must)
modem indicate that this is a modem device; pppd will hang up the
phone before and after making the call
defaultroute once the PPP link is established, make it the default
route; if you have a PPP link to the Internet this
is probably what you want
192.1.1.17 this is a degenerate case of a general option
of the form x.x.x.x:y.y.y.y . Here x.x.x.x is the local IP
address and y.y.y.y is the IP address of the remote end of the
PPP connection. If this option is not specified, or if just
one side is specified, then x.x.x.x defaults to the IP address
associated with the local machine's hostname (in /etc/hosts),
and y.y.y.y is determined by the remote machine. So if this
example had been taken from the fictional machine 'billpc',
this option would actually be redundant.
pppd will write error messages and debugging logs to the syslogd
daemon using the facility name "daemon". These messages may already be
logged to the console or to a file like /usr/adm/messages; consult
your /etc/syslog.conf file to see. If you want to make all pppd
messages go to the console, add the line
daemon.* /dev/console
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This is one or more tabs. Do not use spaces.
to syslog.conf; make sure to put one or more TAB characters between
the two fields.
Example 2: Connecting to PPP server over hard-wired link.
This is a slightly more complicated example. This is the script I run
to make my own PPP link, which is over a hard-wired Gandalf link to an
Ultrix machine running Morningstar PPP.
pppd connect /etc/ppp/ppp-connect defaultroute noipdefault debug \
kdebug 0 /dev/cua0 9600
Here /etc/ppp/ppp-connect is the following script:
#!/bin/sh
/etc/ppp/sendbreak
chat -v -t60 "" \; "service :" blackice ogin: callahan word: PASSWORD \
black% "stty -echo; ppp" "Starting PPP now" && sleep 5
This sends a break to wake up my terminal server, sends a semicolon
(which lets my terminal server do autobaud detection), then says we
want the service "blackice". It logs in, waits for a shell prompt
("black%"), then starts PPP. The -t60 argument sets the timeout to a
minute, since things here are sometimes very slow.
The "&& sleep 5" causes the script to pause for 5 seconds, unless chat
fails in which case it exits immediately. This is just to give the
PPP server time to start (it's very slow). Also, the "stty -echo"
turned out to be very important for me; without it, my pppd would
sometimes start to send negotiation packets before the remote PPP
server had time to turn off echoing. The negotiation packets would
then get sent back to my local machine, be rejected (PPP is able to
detect loopback) and pppd would fail before the remote PPP server even
got going. The "stty -echo" command prevents this confusion. This
kind of problem should only ever affect a *very* few people who
connect to a PPP server that runs as a command on a slow Unix machine,
but I wanted to mention it because it took me several frustrating
hours to figure out.
The pppd options are mostly familiar. Two that are new are
"noipdefault" and "kdebug 1". "noipdefault" tells pppd to ask the
remote end for the IP address to use; this is necessary if the PPP
server implements dynamic IP address assignment as mine does (i.e., I
don't know what address I'll get ahead of time). "kdebug 1" sets the
kernel debugging level to 1, enabling slightly chattier messages from
the ppp kernel code.
Anyway, assuming your connection is working, you should see chat dial
the modem, then perhaps some messages from pppd (depending on your
syslog.conf setup), then some kernel messages like this:
ppp: channel ppp0 mtu changed to 1500
ppp: channel ppp0 open
ppp: channel ppp0 going up for IP packets!
(These messages will only appear if you gave the option "kdebug 1" and
have kern.info messages directed to the screen.) Simultaneously, pppd
is also writing interesting things to /usr/adm/messages (or other log
file, depending on syslog.conf).
IF IT WORKS
If you think you've got a connection, there are a number of things you
can do to test it.
First, type
/sbin/ifconfig
(ifconfig may live elsewhere, depending on your distribution.)
This should show you all the network interfaces that are 'UP'. ppp0
should be one of them, and you should recognize the first IP address
as your own and the "P-t-P address" (or point-to-point address) the
address of your server. Here's what it looks like on my machine:
lo Link encap Local Loopback
inet addr 127.0.0.1 Bcast 127.255.255.255 Mask 255.0.0.0
UP LOOPBACK RUNNING MTU 2000 Metric 1
RX packets 0 errors 0 dropped 0 overrun 0
TX packets 0 errors 0 dropped 0 overrun 0
ppp0 Link encap Point-to-Point Protocol
inet addr 192.76.32.2 P-t-P 129.67.1.165 Mask 255.255.255.0
UP POINTOPOINT RUNNING MTU 1500 Metric 1
RX packets 33 errors 0 dropped 0 overrun 0
TX packets 42 errors 0 dropped 0 overrun 0
Now, type
ping z.z.z.z
where z.z.z.z is the address of your name server. This should work.
Here's what it looks like for me:
waddington:~$ ping 129.67.1.165
PING 129.67.1.165 (129.67.1.165): 56 data bytes
64 bytes from 129.67.1.165: icmp_seq=0 ttl=255 time=268 ms
64 bytes from 129.67.1.165: icmp_seq=1 ttl=255 time=247 ms
64 bytes from 129.67.1.165: icmp_seq=2 ttl=255 time=266 ms
^C
--- 129.67.1.165 ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 247/260/268 ms
waddington:~$
Try typing:
netstat -nr
This should show three routes, something like this:
Kernel routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
129.67.1.165 0.0.0.0 255.255.255.255 UH 0 0 6 ppp0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 129.67.1.165 0.0.0.0 UG 0 0 6298 ppp0
If your output looks similar but doesn't have the destination 0.0.0.0
line (which refers to the default route used for connections), you may
have run pppd without the 'defaultroute' option.
At this point you can try telnetting/ftping/fingering whereever you
want, bearing in mind that you'll have to use numeric IP addresses
unless you've set up your /etc/resolv.conf correctly.
IF IT DOESN'T WORK
If you don't seem to get a connection, the thing to do is to collect
'debug' output from pppd. To do this, make sure you run pppd with the
'debug' option, and put the following two lines in your
/etc/syslog.conf file:
daemon.* /dev/console
daemon.* /usr/adm/ppplog
This will cause pppd's messages to be written to the current virtual
console and to the file /usr/adm/ppplog. Note that the left-hand
field and the right-hand field must be separated by at least one TAB
character. After modifying /etc/syslog.conf, you must execute the
command 'kill -HUP <pid>' where <pid> is the process ID of the
currently running syslogd process to cause it to re-read the
configuration file.
Some messages to look for:
- "pppd[NNN]: Connected..." means that the "connect" script has
completed successfully.
- "pppd[NNN]: sent [LCP ConfReq"... means that pppd has attempted to
begin negotiation with the remote end.
- "pppd[NNN]: recv [LCP ConfReq"... means that pppd has received a
negotiation frame from the remote end.
- "pppd[NNN]: ipcp up" means that pppd has reached the point where
it believes the link is ready for IP traffic to travel across it.
If you never see a "recv" message then there may be serious problems
with your link. (For example, the link may not be passing all 8
bits.) If that's the case, it would be useful to collect a debug log
which contains all the bytes being passed between your computer and
the remote PPP server. To do this, alter your syslog.conf lines to
look like this
daemon.*,kern.* /dev/console
daemon.*,kern.* /usr/adm/ppplog
and HUP the syslog daemon as before. Then, run pppd with the option
"kdebug 25". Whatever characters arrive over the PPP terminal line
will appear in the debugging output.
Occasionally you may see a message like
ppp_toss: tossing frame, reason = 4
The PPP code is throwing away a packet ("frame") from the remote
server because of a serial overrun. This means your CPU isn't able to
read characters from the serial port as quickly as they arrive; the
best solution is to get a 16550A serial chip, which gives the CPU some
grace period. Reasons other than 4 indicate other kinds of serial
errors, which should not occur.
During the initial connection sequence, you may see one or more
messages which indicate "bad fcs". This refers to a checksum error in
a received PPP frame, and usually occurs at the start of a session
when the peer system is sending some "text" messages, such as "hello
this is the XYZ company". Messages of "bad fcs" once the link is
established and the routes have been added are not normal and indicate
transmission errors or noise on the telephone line.
IF IT STILL DOESN'T WORK (OR, BUG REPORTS)
If you're still having difficulty, send the linux-ppp list a bug
report. It is extremely important to include as much information as
possible; for example:
- the version number of the kernel you are using
- the version number of Linux PPP you are using
- the exact command you use to start the PPP session
- log output from a session run with the 'debug' option, captured
using daemon.*,kern.* in your syslog.conf file
- the type of PPP peer that you are connecting to (eg, Xyzzy Corp
terminal server, Morningstar PPP software, etc)
- the kind of connection you use (modem, hardwired, etc...)
DYNAMIC ADDRESS ASSIGNMENT
You can use Linux PPP with a PPP server which assigns a different IP
address every time you connect. This action is automatically performed
when you don't have a local IP address.
pppd connect 'chat -v "" ATDT5551212 CONNECT "" ogin: ppp word: whitewater' \
/dev/cua1 38400 noipdefault debug crtscts modem defaultroute
The noipdefault, added to the above example, suppresses the attempts
of pppd to deduce its own IP address by looking it up in the
/etc/hosts file. Since the process does not have an IP address, one
will be assigned to it from the configuration file on the remote
system.
Sometimes you may get an error message like "Cannot assign requested
address" when you use a Linux client (for example, "talk"). This
happens when the IP address given in /etc/hosts for our hostname
differs from the IP address used by the PPP interface. The solution
is to use ifconfig ppp0 to get the interface address and then edit
/etc/hosts appropriately.
SETTING UP A MACHINE FOR INCOMING PPP CONNECTIONS
Suppose you want to permit another machine to call yours up and start
a PPP session. This is possible using Linux PPP.
One way is to create an account named, say, 'ppp', with the login
shell being a short script that starts pppd. For example, the passwd
entry might look like this:
ppp:(encrypted password):102:50:PPP client login:/home/ppp:/usr/sbin/pppd
In addition, you would edit the file ~ppp/.ppprc to have the following
pieces of information:
-detach
modem
crtscts
lock
:192.1.2.23
Here we will insist that the remote machine use IP address 192.1.2.23,
while the local PPP interface will use the IP address associated with
this machine's hostname in /etc/hosts. The '-detach' option is required
for a server. It tells the pppd process not to terminate until the modem
is disconnected. Should it fork, the init process would restart the getty
process and the this would cause a severe conflict over the port.
The 'modem' option indicates that the connection is via a switched circuit
(using a modem) and that the pppd process should monitor the DCD signal
from the modem.
The 'crtscts' option tells the pppd process to use hardware RTS/CTS flow
control for the modem.
The 'lock' option tells pppd to lock the tty device. This will use the UUCP
style locking file in the lock directory.
This setup is sufficient if you just want to connect two machines so
that they can talk to one another. If you want to use Linux PPP to
connect a single machine to an entire network, or to connect two
networks together, then you need to arrange for packets to be routed
from the networks to the PPP link. Setting up a link between networks
is beyond the scope of this document; you should examine the routing
options in the manual page for pppd carefully and find out about
routed, etc.
Let's consider just the first case. Suppose you have a Linux machine
attached to an Ethernet, and you want to allow its PPP peer to be able
to communicate with hosts on that Ethernet. To do this, you should
have the remote machine use an IP address that would normally appear
to be on the local Ethernet segment and you should give the 'proxyarp'
option to pppd on the server. Suppose, for example, we have this
setup:
192.1.2.23 192.1.2.17
+-----------+ PPP link +----------+
| chelseapc | ------------------- | billpc |
+-----------+ +----------+
| Ethernet
----------------------------------- 192.1.2.x
Here the PPP and Ethernet interfaces of billpc will have IP address
192.1.2.17. (It's OK for one or more PPP interfaces on a machine to
share an IP address with an Ethernet interface.) There is an
appropriate entry in /etc/passwd on billpc to allow chelseapc to call
in. It will run pppd when the user signs on to the system and pppd will
take the options from the user option file.
In addition, you would edit the file ~ppp/.ppprc to have the following
piece of information:
-detach
modem
crtscts
lock
192.1.2.17:192.1.2.23
proxyarp
When the link comes up, pppd will enter a "proxy arp" entry for
chelseapc into the arp table on billpc. What this means effectively
is that billpc will pretend to the other machines on the 192.1.2.x
Ethernet that its Ethernet interface is ALSO the interface for
chelseapc (192.1.2.23) as well as billpc (192.1.2.17). In practice
this means that chelseapc can communicate just as if it was directly
connected to the Ethernet.
SETTING UP A MACHINE FOR INCOMING PPP CONNECTIONS WITH DYNAMIC IP
The use of dynamic IP assignments is not much different from that
using static IP addresses. Rather than putting the IP address into the
single file ~ppp/.ppprc, you would put the IP address for each of the
incoming terminals into the /etc/ppp/options.tty files. ('tty' is the
name of the tty device. For example /etc/ppp/options.ttyS0 is used for
the /dev/ttyS0 device.)
To each of the serial devices, you would attach a modem. To the
modems, attach the telephone lines. Place all of the telephone lines
into a hunt group so that the telephone system will select the
non-busy telephone and subsequently, the modem. By selecting the
modem, the user will select a tty device and the tty device will
select the IP address. Run a getty process against the tty device such
as /dev/ttyS0.
(The general consensus among the users is that you should *not* use
the agetty process to monitor a modem. Use either getty_ps' uugetty
process or mgetty from the mgetty+sendfax package.)
SECURITY CONCERNS ABOUT INCOMING PPP CONNECTIONS
The following security should be considered with the ppp connections.
1. Never put the pppd program file into the /etc/shells file. It is not
a legal shell for the general user. In addition, if the shell is missing
from the shells file, the ftpd process will not allow the user to access
the system via ftp. You would not want Joe Hacker using the ppp account
via ftp.
2. Ensure that the directory /etc/ppp is owned by 'root' and permits
only write access to the root user.
3. The files /etc/ppp/options must be owned by root and accessible only
from that user. Never permit any other user access to this file.
4. The files /etc/ppp/ip-up and /etc/ppp/ip-down will be executed by the
pppd process while it is root. Ensure that these files are writable only
from the root user.
5. If you use an incoming PPP connection, you should do the following as
the root user:
a) Invalidate the files for rhosts and forward
rm -f ~ppp/.rhosts ~ppp/.forward
touch ~ppp/.rhosts ~ppp/.forward
chmod 444 ~ppp/.rhosts ~ppp/.forward
b) Prevent users from sending mail to the user 'ppp'.
This is best performed by creating a system alias 'ppp' and have it
point to the name "THIS_USER_CANNOT_RECEIVE_MAIL". It has no special
meaning other than the obvious one.
For sendmail, the sequence is fairly easy. Edit the /etc/aliases file
and add the line:
ppp:THIS_USER_CANNOT_RECEIVE_MAIL
Then run the sendmail program with the option '-bi' to rebuild the
alias database.
c) Secure the ppp file properly.
chown root ~ppp/.ppprc
chmod 444 ~ppp/.ppprc
You may wish to extend the security by creating a group 'ppp' and putting
the ppp user into that group, along with the binaries for pppd and pppstats.
Then you may secure the binaries so that they are executable from the owner
(which should be root) and the group only. All other users would be denied
all access to the files and executables.
d) Prevent the motd file from being sent to the ppp user.
touch ~ppp/.hushlogin
chown root ~ppp/.hushlogin
chmod 444 ~ppp/.hushlogin
ADDITIONAL INFORMATION
Besides this document, additional information may be found in:
- The file README in the source package
- The PPP-HOWTO on sunsite.unc.edu
- The Net-2-HOWTO on sunsite.unc.edu
- The Network Administration Guide published by O'Rielly and Associates
Please consult these sources of information should you have questions
about the program. If you still can not find your answer then ask either
the usenet news groups or the mail list.
DIP SUPPORT
The dip program used by Linux is not directly supported by the PPP
package as such. Please don't ask the PPP porting group questions
about dip. It does work in two areas.
1. If you use it as a parameter to 'connect' then you can use the scripting
language and establish the connection. You would use the standard set of
PPP options.
2. dip-3.3.7m-uri and later versions support a 'mode ppp' function
which will invoke the pppd program. That is all that it does. It will
not pass any parameters to pppd other than its required '-detach' to
allow dip to detect the normal termination of pppd.
The following information comes from John Phillips in an article which he
posted to comp.os.linux.setup.
Assuming that you already know how dip supports SLIP, these points
are relative to a working SLIP set-up.
1. You need dip-3.3.7m-uri, and, of course, PPP compiled into the
kernel.
2. Make sure pppd is where dip thinks it is: /usr/lib/ppp/pppd, or
make a link from there to where pppd really is. (Or re-compile dip
to tell it where pppd is on your system - see pathnames.h).
3. The key differences between the dip script for PPP, compared to one
for SLIP are:
a. Use "mode PPP" instead of "mode SLIP"
b. Don't set certain options such as mtu and default - these are set
by pppd from the file /etc/ppp/options. Mine looks like this:
crtscts
modem
defaultroute
asyncmap 0x00000000
mru 576
mtu 576
The actual parameters and values may depend on your IP supplier
and his set-up.
c. Tell your IP supplier's start-up code to use ppp, not slip: I
use "send nolqm,idle=240\n" instead of "send slip,idle=240,mru=576\n"
at the "protocol: " prompt. ("nolqm" asks for ppp without the line
quality monitoring protocol, which is not - I think - supported in
Linux PPP.) This prompt may be different (or absent) with another
IP supplier.
d. You don't need "get $local <name>", since the ppp protocol
negotiates this at start-up. You still need "get $remote <name>".
(This may also vary with IP supplier - you may need to set some
more parameters in /etc/ppp/options to work with yours - see "man
pppd" for details of the options supported by pppd.)
4. The dip script will exit after dialling and starting up pppd. When
ppp negotiation is completed and IP comes up, pppd runs /etc/ppp/ip-up.
This file can contain things you want to run when the network comes up
(e.g. running the mail queue).
5. When IP goes down (e.g. after you close down the link with "dip -k"),
pppd runs /etc/ppp/ip-down, which can contain things you want to do on
close-down.
CONCLUSION
Good luck!
Al and Michael
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