path: root/2002/netfilter-internals-lsm2002/netfilter-internals-lsm2002.mgp

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Linux 2.4.x netfilter/iptables 
firewalling internals


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by

Harald Welte <laforge@gnumonks.org>


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netfilter/iptables in Linux 2.4
Contents


	Introduction
	Netfilter hooks in protocol stacks
	Packet selection based on IP Tables
	The Connection Tracking Subsystem
	The NAT Subsystem based on netfilter + iptables 
	Packet filtering using the 'filter' table
	Packet mangling using the 'mangle' table
	Advanced netfilter concepts
	Current development and Future

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netfilter/iptables in Linux 2.4
Introduction

Why did we need netfilter/iptables?
Because ipchains...

		has no infrastructure for passing packets to userspace
		makes transparent proxying extremely difficult
		has interface address dependent Packet filter rules
		has Masquerading implemented as part of packet filtering
		code is too complex and intermixed with core ipv4 stack
		is neither modular nor extensible
		only barely supports one special case of NAT (masquerading)
		has only stateless packet filtering

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netfilter/iptables in Linux 2.4
Introduction 

Who's behind netfilter/iptables
		Paul 'Rusty' Russel
			co-author of iptables in Linux 2.2
			was paid by Watchguard for about one Year of development
		James Morris
			userspace queuing (kernel, library and tools)
			REJECT target
		Marc Boucher
			NAT and packet filtering controlled by one command
			Mangle table
		Harald Welte 
			Conntrack+NAT helper infrastructure (newnat)
			Userspace packet logging (ULOG)
			PPTP and IRC conntrack/NAT helpers
		Jozsef Kadlecsik
			TCP window tracking
			H.323 conntrack + NAT helper
			Continued newnat development
		Non-core team contributors
			http://www.netfilter.org/scoreboard/
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netfilter/iptables in Linux 2.4
Netfilter Hooks

What is netfilter?

		System of callback functions within network stack
		Callback function to be called for every packet traversing certain point (hook) within network stack
		Protocol independent framework
		Hooks in layer 3 stacks (IPv4, IPv6, DECnet, ARP)
		Multiple kernel modules can register with each of the hooks
		Asynchronous packet handling in userspace (ip_queue)

Traditional packet filtering, NAT, ... is implemented on top of this framework

Can be used for other stuff interfacing with the core network stack, like DECnet routing daemon.

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netfilter/iptables in Linux 2.4
Netfilter Hooks

Netfilter architecture in IPv4
%font "courier"

   --->[1]--->[ROUTE]--->[3]--->[4]--->
                 |            ^
                 |            |
                 |         [ROUTE]
                 v            |
                [2]          [5]
                 |            ^
                 |            |
                 v            |

%font "standard"
1=NF_IP_PRE_ROUTING
2=NF_IP_LOCAL_IN
3=NF_IP_FORWARD
4=NF_IP_POST_ROUTING
5=NF_IP_LOCAL_OUT
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netfilter/iptables in Linux 2.4
Netfilter Hooks

Netfilter Hooks

	Any kernel module may register a callback function at any of the hooks

	The module has to return one of the following constants

		NF_ACCEPT	 continue traversal as normal
		NF_DROP		 drop the packet, do not continue
		NF_STOLEN	 I've taken over the packet do not continue
		NF_QUEUE	 enqueue packet to userspace
		NF_REPEAT	 call this hook again

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netfilter/iptables in Linux 2.4
IP tables

Packet selection using IP tables

		The kernel provides generic IP tables support

		Each kernel module may create it's own IP table

		The three major parts of 2.4 firewalling subsystem are implemented using IP tables
			Packet filtering table 'filter'
			NAT table 'nat'
			Packet mangling table 'mangle'

		Can potentially be used for other stuff, i.e. IPsec SPDB

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netfilter/iptables in Linux 2.4
IP Tables

Managing chains and tables

	An IP table consists out of multiple chains
	A chain consists out of a list of rules
	Every single rule in a chain consists out of
		match[es] (rule executed if all matches true)
		target (what to do if the rule is matched)

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matches and targets can either be builtin or implemented as kernel modules

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	The userspace tool iptables is used to control IP tables
		handles all different kinds of IP tables 
		supports a plugin/shlib interface for target/match specific options

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netfilter/iptables in Linux 2.4
IP Tables

Basic iptables commands

	To build a complete iptables command, we must specify
		which table to work with
		which chain in this table to use
		an operation (insert, add, delete, modify)
		one or more matches (optional)
		a target

The syntax is
%font "typewriter"
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iptables -t table -Operation chain -j target match(es)
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Example:
%font "typewriter"
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iptables -t filter -A INPUT -j ACCEPT -p tcp --dport smtp
%font "standard"
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netfilter/iptables in Linux 2.4
IP Tables

Matches
		Basic matches
			-p			protocol (tcp/udp/icmp/...)
			-s			source address (ip/mask)
			-d			destination address (ip/mask)
			-i			incoming interface
			-o			outgoing interface

		Match extensions (examples)
			tcp/udp		TCP/udp source/destination port
			icmp			ICMP code/type
			ah/esp		AH/ESP SPID match
			mac		 	source MAC address
			mark 		nfmark
			length		match on length of packet
			limit			rate limiting (n packets per timeframe)
			owner	 	owner uid of the socket sending the packet
			tos			TOS field of IP header
			ttl			TTL field of IP header


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IP Tables

Targets 
	very dependent on the particular table.

	Table specific targets will be discussed later

	Generic Targets, always available
		ACCEPT		accept packet within chain
		DROP		silently drop packet
		QUEUE		enqueue packet to userspace
		LOG		log packet via syslog
		ULOG		log packet via ulogd
		RETURN		return to previous (calling) chain
		foobar		jump to user defined chain


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netfilter/iptables in Linux 2.4
Packet Filtering

Overview

	Implemented as 'filter' table
	Registers with three netfilter hooks

		NF_IP_LOCAL_IN (packets destined for the local host)
		NF_IP_FORWARD (packets forwarded by local host)
		NF_IP_LOCAL_OUT (packets from the local host)

Each of the three hooks has attached one chain (INPUT, FORWARD, OUTPUT)

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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Packet Filtering

Targets available within 'filter' table

	Builtin Targets to be used in filter table
		ACCEPT	accept the packet
		DROP	silently drop the packet 
		QUEUE	enqueue packet to userspace
		RETURN	return to previous (calling) chain
		foobar	user defined chain

	Targets implemented as loadable modules 
		REJECT	drop the packet but inform sender
		MIRROR	change source/destination IP and resend
		LOG  	log via syslog
		ULOG	log via userspace

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netfilter/iptables in Linux 2.4
Connection Tracking Subsystem

	Connection tracking...

		implemented seperately from NAT 
		enables stateful filtering 
		implementation
			hooks into NF_IP_PRE_ROUTING to track packets
			hooks into NF_IP_POST_ROUTING and NF_IP_LOCAL_IN to see if packet passed filtering rules
			protocol modules (currently TCP/UDP/ICMP)
			application helpers currently (FTP,IRC,H.323,talk,SNMP)
		divides packets in the following four categories
			NEW - would establish new connection
			ESTABLISHED - part of already established connection
			RELATED - is related to established connection
			INVALID - (multicast, errors...)
		does _NOT_ filter packets itself
		can be utilized by iptables using the 'state' match 
		is used by NAT Subsystem

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HA for netfillter/iptables
Connection Tracking Subsystem

Common structures
	struct ip_conntrack_tuple, representing unidirectional flow
		layer 3 src + dst 
		layer 4 protocol
		layer 4 src + dst


	connetions represented as struct ip_conntrack
		original tuple
		reply tuple
		timeout
		l4 state private data
		app helper
		app helper private data
		expected connections

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HA for netfillter/iptables
Connection Tracking Subsystem

Flow of events for new packet
	packet enters NF_IP_PRE_ROUTING
		tuple is derived from packet
		lookup conntrack hash table with hash(tuple) -> fails
		new ip_conntrack is allocated
			fill in original and reply == inverted(original) tuple
			initialize timer
			assign app helper if applicable
			see if we've been expected -> fails
			call layer 4 helper 'new' function

	...

	packet enters NF_IP_POST_ROUTING
		do hashtable lookup for packet -> fails
		place struct ip_conntrack in hashtable			


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Connection Tracking Subsystem

Flow of events for packet part of existing connection
	packet enters NF_IP_PRE_ROUTING
		tuple is derived from packet
		lookup conntrack hash table with hash(tuple)
		assosiate conntrack entry with skb->nfct
		call l4 protocol helper 'packet' function
			do l4 state tracking
			update timeouts as needed [i.e. TCP TIME_WAIT,...]

	...
	
	packet enters NF_IP_POST_ROUTING
		do hashtable lookup for packet -> succeds
		do nothing else



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Network Address Translation

Overview

		Previous Linux Kernels only implemented one special case of NAT: Masquerading
		Linux 2.4.x can do any kind of NAT.
		NAT subsystem implemented on top of netfilter, iptables and conntrack
		NAT subsystem registers with all five netfilter hooks
		'nat' Table registers chains PREROUTING, POSTROUTING and OUTPUT
		Following targets available within 'nat' Table
			SNAT changes the packet's source whille passing NF_IP_POST_ROUTING
			DNAT changes the packet's destination while passing NF_IP_PRE_ROUTING
			MASQUERADE is a special case of SNAT
			REDIRECT is a special case of DNAT


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Network Address Translation

	Source NAT
		SNAT Example:
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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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		MASQUERADE Example:
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iptables -t nat -A POSTROUTING -j MASQUERADE -o ppp0
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%size 5

	Destination NAT
		DNAT example
%font "typewriter"
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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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		REDIRECT example
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iptables -t nat -A PREROUTING -j REDIRECT --to-port 3128 -i eth1 -p tcp --dport 80
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Packet Mangling

	Purpose of mangle table
		packet manipulation except address manipulation

	Integration with netfilter
		'mangle' table hooks in all five netfilter hooks
		priority: after conntrack

	Targets specific to the 'mangle' table:
		DSCP - manipulate DSCP field
		IPV4OPTSSTRIP - strip IPv4 options
		MARK - change the nfmark field of the skb
		TCPMSS - set TCP MSS option
		TOS - manipulate the TOS bits 
		TTL - set / increase / decrease TTL field

Simple example:
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iptables -t mangle -A PREROUTING -j MARK --set-mark 10 -p tcp --dport 80


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Advanced Netfilter concepts

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	Userspace logging
		flexible replacement for old syslog-based logging
		packets to userspace via multicast netlink sockets
		easy-to-use library (libipulog)
		plugin-extensible userspace logging daemon (ulogd)
		Can even be used to directly log into MySQL

	Queuing
		reliable asynchronous packet handling 
		packets to userspace via unicast netlink socket
		easy-to-use library (libipq)
		provides Perl bindings
		experimental queue multiplex daemon (ipqmpd)


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Current Development and Future

Netfilter (although it proved very stable) is still work in progress. 

	Areas of current development
		infrastructure for conntrack manipulation from userspace
		failover of stateful firewalls
		making iptables layer3 independent (pkttables)
		new userspace library (libiptables) to hide plugins from apps
		more matches and targets for advanced functions (pool, hashslot)
		more conntrack and NAT modules (RPC, SNMP, SMB, ...)
		better IPv6 support (conntrack, more matches / targets)

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Future of Linux packet filtering
Thanks
	The slides and the an according paper of this presentation are available at http://www.gnumonks.org/

	The netfilter homepage http://www.netfilter.org/

	Thanks to
		the BBS people, Z-Netz, FIDO, ...
			for heavily increasing my computer usage in 1992
		KNF
			for bringing me in touch with the internet as early as 1994
			for providing a playground for technical people
			for telling me about the existance of Linux!
		Alan Cox, Alexey Kuznetsov, David Miller, Andi Kleen
			for implementing (one of?) the world's best TCP/IP stacks
		Paul 'Rusty' Russell
			for starting the netfilter/iptables project
			for trusting me to maintain it today
		Astaro AG
			for sponsoring parts of my netfilter work