510 lines
14 KiB
Plaintext
510 lines
14 KiB
Plaintext
%include "default.mgp"
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%default 1 bgrad
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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%nodefault
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%back "blue"
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%center
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%size 7
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Linux 2.4.x netfilter/iptables
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firewalling internals
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%center
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%size 4
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by
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Harald Welte <laforge@netfilter.org>
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Contents
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Introduction
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Netfilter hooks in protocol stacks
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Packet selection based on IP Tables
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The Connection Tracking Subsystem
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The NAT Subsystem based on netfilter + iptables
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Packet filtering using the 'filter' table
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Packet mangling using the 'mangle' table
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Advanced netfilter concepts
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Current development and Future
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Introduction
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Why did we need netfilter/iptables?
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Because ipchains...
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has no infrastructure for passing packets to userspace
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makes transparent proxying extremely difficult
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has interface address dependent Packet filter rules
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has Masquerading implemented as part of packet filtering
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code is too complex and intermixed with core ipv4 stack
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is neither modular nor extensible
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only barely supports one special case of NAT (masquerading)
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has only stateless packet filtering
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Introduction
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Who's behind netfilter/iptables
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Paul 'Rusty' Russell
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co-author of iptables in Linux 2.2
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was paid by Watchguard for about one Year of development
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James Morris
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userspace queuing (kernel, library and tools)
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Marc Boucher
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NAT and packet filtering controlled by one command
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Mangle table
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Harald Welte
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Conntrack+NAT helper infrastructure (newnat)
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Userspace packet logging (ULOG)
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Jozsef Kadlecsik
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TCP window tracking
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H.323 conntrack + NAT helper
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Non-core team contributors
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http://www.netfilter.org/scoreboard/
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Netfilter Hooks
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What is netfilter?
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System of callback functions within network stack
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Callback function to be called for every packet traversing certain point (hook) within network stack
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Protocol independent framework
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Hooks in layer 3 stacks (IPv4, IPv6, DECnet, ARP)
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Multiple kernel modules can register with each of the hooks
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Asynchronous packet handling in userspace (ip_queue)
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Traditional packet filtering, NAT, ... is implemented on top of this framework
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Can be used for other stuff interfacing with the core network stack, like DECnet routing daemon.
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Netfilter Hooks
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Netfilter architecture in IPv4
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%font "typewriter"
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%size 4
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--->[1]--->[ROUTE]--->[3]--->[4]--->
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v |
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[2] [5]
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v |
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%font "standard"
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1=NF_IP_PRE_ROUTING
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2=NF_IP_LOCAL_IN
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3=NF_IP_FORWARD
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4=NF_IP_POST_ROUTING
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5=NF_IP_LOCAL_OUT
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Netfilter Hooks
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Netfilter Hooks
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Any kernel module may register a callback function at any of the hooks
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The module has to return one of the following constants
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NF_ACCEPT continue traversal as normal
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NF_DROP drop the packet, do not continue
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NF_STOLEN I've taken over the packet do not continue
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NF_QUEUE enqueue packet to userspace
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NF_REPEAT call this hook again
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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IP tables
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Packet selection using IP tables
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The kernel provides generic IP tables support
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Each kernel module may create it's own IP table
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The three major parts of 2.4 firewalling subsystem are implemented using IP tables
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Packet filtering table 'filter'
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NAT table 'nat'
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Packet mangling table 'mangle'
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Can potentially be used for other stuff, i.e. IPsec SPDB
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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IP Tables
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Managing chains and tables
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An IP table consists out of multiple chains
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A chain consists out of a list of rules
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Every single rule in a chain consists out of
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match[es] (rule executed if all matches true)
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target (what to do if the rule is matched)
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%size 4
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matches and targets can either be builtin or implemented as kernel modules
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%size 5
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The userspace tool iptables is used to control IP tables
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handles all different kinds of IP tables
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supports a plugin/shlib interface for target/match specific options
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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IP Tables
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Basic iptables commands
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To build a complete iptables command, we must specify
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which table to work with
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which chain in this table to use
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an operation (insert, add, delete, modify)
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one or more matches (optional)
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a target
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The syntax is
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%font "typewriter"
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%size 3
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iptables -t table -Operation chain -j target match(es)
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%font "standard"
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%size 5
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Example:
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%font "typewriter"
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%size 3
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iptables -t filter -A INPUT -j ACCEPT -p tcp --dport smtp
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%font "standard"
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%size 5
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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IP Tables
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Matches
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Basic matches
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-p protocol (tcp/udp/icmp/...)
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-s source address (ip/mask)
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-d destination address (ip/mask)
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-i incoming interface
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-o outgoing interface
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Match extensions (examples)
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tcp/udp TCP/udp source/destination port
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icmp ICMP code/type
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ah/esp AH/ESP SPID match
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mac source MAC address
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mark nfmark
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length match on length of packet
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limit rate limiting (n packets per timeframe)
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owner owner uid of the socket sending the packet
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tos TOS field of IP header
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ttl TTL field of IP header
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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IP Tables
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Targets
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very dependent on the particular table.
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Table specific targets will be discussed later
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Generic Targets, always available
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ACCEPT accept packet within chain
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DROP silently drop packet
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QUEUE enqueue packet to userspace
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LOG log packet via syslog
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ULOG log packet via ulogd
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RETURN return to previous (calling) chain
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foobar jump to user defined chain
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Packet Filtering
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Overview
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Implemented as 'filter' table
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Registers with three netfilter hooks
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NF_IP_LOCAL_IN (packets destined for the local host)
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NF_IP_FORWARD (packets forwarded by local host)
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NF_IP_LOCAL_OUT (packets from the local host)
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Each of the three hooks has attached one chain (INPUT, FORWARD, OUTPUT)
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Every packet passes exactly one of the three chains. Note that this is very different compared to the old 2.2.x ipchains behaviour.
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Packet Filtering
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Targets available within 'filter' table
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Builtin Targets to be used in filter table
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ACCEPT accept the packet
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DROP silently drop the packet
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QUEUE enqueue packet to userspace
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RETURN return to previous (calling) chain
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foobar user defined chain
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Targets implemented as loadable modules
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REJECT drop the packet but inform sender
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MIRROR change source/destination IP and resend
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LOG log via syslog
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ULOG log via userspace
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Connection Tracking Subsystem
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Connection tracking...
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implemented seperately from NAT
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enables stateful filtering
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implementation
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hooks into NF_IP_PRE_ROUTING to track packets
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hooks into NF_IP_POST_ROUTING and NF_IP_LOCAL_IN to see if packet passed filtering rules
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protocol modules (currently TCP/UDP/ICMP)
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application helpers currently (FTP,IRC,H.323,talk,SNMP)
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divides packets in the following four categories
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NEW - would establish new connection
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ESTABLISHED - part of already established connection
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RELATED - is related to established connection
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INVALID - (multicast, errors...)
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does _NOT_ filter packets itself
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can be utilized by iptables using the 'state' match
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is used by NAT Subsystem
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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HA for netfillter/iptables
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Connection Tracking Subsystem
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Common structures
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struct ip_conntrack_tuple, representing unidirectional flow
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layer 3 src + dst
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layer 4 protocol
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layer 4 src + dst
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connetions represented as struct ip_conntrack
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original tuple
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reply tuple
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timeout
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l4 state private data
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app helper
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app helper private data
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expected connections
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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HA for netfillter/iptables
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Connection Tracking Subsystem
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Flow of events for new packet
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packet enters NF_IP_PRE_ROUTING
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tuple is derived from packet
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lookup conntrack hash table with hash(tuple) -> fails
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new ip_conntrack is allocated
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fill in original and reply == inverted(original) tuple
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initialize timer
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assign app helper if applicable
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see if we've been expected -> fails
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call layer 4 helper 'new' function
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...
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packet enters NF_IP_POST_ROUTING
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do hashtable lookup for packet -> fails
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place struct ip_conntrack in hashtable
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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HA for netfillter/iptables
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Connection Tracking Subsystem
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Flow of events for packet part of existing connection
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packet enters NF_IP_PRE_ROUTING
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tuple is derived from packet
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lookup conntrack hash table with hash(tuple)
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assosiate conntrack entry with skb->nfct
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call l4 protocol helper 'packet' function
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do l4 state tracking
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update timeouts as needed [i.e. TCP TIME_WAIT,...]
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...
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packet enters NF_IP_POST_ROUTING
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do hashtable lookup for packet -> succeds
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do nothing else
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Network Address Translation
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Overview
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Previous Linux Kernels only implemented one special case of NAT: Masquerading
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Linux 2.4.x can do any kind of NAT.
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NAT subsystem implemented on top of netfilter, iptables and conntrack
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NAT subsystem registers with all five netfilter hooks
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'nat' Table registers chains PREROUTING, POSTROUTING and OUTPUT
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Following targets available within 'nat' Table
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SNAT changes the packet's source whille passing NF_IP_POST_ROUTING
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DNAT changes the packet's destination while passing NF_IP_PRE_ROUTING
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MASQUERADE is a special case of SNAT
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REDIRECT is a special case of DNAT
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Network Address Translation
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Source NAT
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SNAT Example:
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%font "typewriter"
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%size 3
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iptables -t nat -A POSTROUTING -j SNAT --to-source 1.2.3.4 -s 10.0.0.0/8
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%font "standard"
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%size 4
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MASQUERADE Example:
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%font "typewriter"
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%size 3
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iptables -t nat -A POSTROUTING -j MASQUERADE -o ppp0
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%font "standard"
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%size 5
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Destination NAT
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DNAT example
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%font "typewriter"
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%size 3
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iptables -t nat -A PREROUTING -j DNAT --to-destination 1.2.3.4:8080 -p tcp --dport 80 -i eth1
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%font "standard"
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%size 4
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REDIRECT example
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%font "typewriter"
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%size 3
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iptables -t nat -A PREROUTING -j REDIRECT --to-port 3128 -i eth1 -p tcp --dport 80
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%font "standard"
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%size 5
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Packet Mangling
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Purpose of mangle table
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packet manipulation except address manipulation
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Integration with netfilter
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'mangle' table hooks in all five netfilter hooks
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priority: after conntrack
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Targets specific to the 'mangle' table:
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DSCP - manipulate DSCP field
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IPV4OPTSSTRIP - strip IPv4 options
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MARK - change the nfmark field of the skb
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TCPMSS - set TCP MSS option
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TOS - manipulate the TOS bits
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TTL - set / increase / decrease TTL field
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Simple example:
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%font "typewriter"
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%size 3
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iptables -t mangle -A PREROUTING -j MARK --set-mark 10 -p tcp --dport 80
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Advanced Netfilter concepts
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%size 4
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Userspace logging
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flexible replacement for old syslog-based logging
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packets to userspace via multicast netlink sockets
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easy-to-use library (libipulog)
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plugin-extensible userspace logging daemon (ulogd)
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Can even be used to directly log into MySQL
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Queuing
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reliable asynchronous packet handling
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packets to userspace via unicast netlink socket
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easy-to-use library (libipq)
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provides Perl bindings
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experimental queue multiplex daemon (ipqmpd)
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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netfilter/iptables in Linux 2.4
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Current Development and Future
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Netfilter (although it proved very stable) is still work in progress.
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Areas of current development
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infrastructure for conntrack manipulation from userspace
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failover of stateful firewalls (conntrack sync)
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making iptables layer3 independent (pkttables)
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new userspace library (libiptables) to hide plugins from apps
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more matches and targets for advanced functions (pool, hashslot)
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more conntrack and NAT modules (RPC, SNMP, SMB, ...)
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better IPv6 support (conntrack, more matches / targets)
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%page
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Future of Linux packet filtering
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Thanks
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Thanks to
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the BBS people, Z-Netz, FIDO, ...
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for heavily increasing my computer usage in 1992
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KNF
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for bringing me in touch with the internet as early as 1994
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for providing a playground for technical people
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for telling me about the existance of Linux!
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Alan Cox, Alexey Kuznetsov, David Miller, Andi Kleen
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for implementing (one of?) the world's best TCP/IP stacks
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Paul 'Rusty' Russell
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for starting the netfilter/iptables project
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for trusting me to maintain it today
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Astaro AG
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for sponsoring parts of my netfilter work
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%size 3
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The slides and the an according paper of this presentation are available at http://www.gnumonks.org/
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%size 3
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The netfilter homepage http://www.netfilter.org/
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