path: root/2007/netfilter-iptables-tfh2007/netfilter-iptables.mgp

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netfilter/iptables
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Jan 29, 2007
TFH Berlin

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by

Harald Welte <laforge@netfilter.org>


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Contents

	Introduction
	Highly Scalable Linux Network Stack
	Netfilter Hooks
	Packet selection based on IP Tables
	The Connection Tracking Subsystem
	The NAT Subsystem
	Packet Mangling

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Introduction


Who is speaking to you?
		an independent Free Software developer
		who earns his living off Free Software since 1997
		who is one of the authors of the Linux kernel packet filter
		busy with enforcing the GNU GPL at gpl-violations.org
		working on Free Software for smartphones (openEZX, OpenMoko)`
		... and Free Software for RFID (librfid)
		... and Free Software for ePassports (libmrtd)
		... and Free Hardware for RFID (openpcd.org, openbeacon.org)

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Introduction

Linux and Networking
	Linux is a true child of the Internet
	Early adopters: ISP's, Universities
	Lots of work went into a highly scalable network stack
	Not only for client/server, but also for routers
	Features unheared of in other OS's 

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Introduction 

Did you know, that a stock 2.6.x linux kernel can provide

	a stateful packet filter ?
	fully symmetric NA(P)T ?
	policy routing ?
	QoS / traffic shaping ?
	IPv6 firewalling ?
	packet filtering, NA(P)T on a bridge ?
	layer 2 (mac) address translation ?
	packet forwarding rates of up to 2.1Mpps (in 2005)?

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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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Introduction 

Who's behind netfilter/iptables

	The core team
		Paul 'Rusty' Russel
			co-author of iptables in Linux 2.2
		James Morris
		Marc Boucher
		Harald Welte 
		Jozsef Kadlecsik
		Martin Josefsson
		Patrick McHardy

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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

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 Hooks

Netfilter architecture in IPv4
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in --->[1]--->[ROUTE]--->[3]--->[4]---> out
                 |            ^
                 |            |
                 |         [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 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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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 four major parts of the firewalling subsystem are implemented using IP tables
			Packet filtering table 'filter'
			NAT table 'nat'
			Packet mangling table 'mangle'
			The 'raw' table for conntrack exemptions

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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)
			implicit packet and byte counter

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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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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
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iptables -t table -Operation chain -j target match(es)
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Example:
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iptables -t filter -A INPUT -j ACCEPT -p tcp --dport smtp
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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

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IP Tables

	addrtype match
		matches source/destionation address type
		types are UNICAST/LOCAL/BROADCAST/ANYCAST/MULTICAST/...
	ah match
		matches IPSEC AH SPI (range)
	comment match
		always matches, allows user to place comment in rule
	connmark match
		connection marking, see later
	conntrack match
		more extended version of 'state'
		match on timeout, fine-grained state, original tuples
	dscp match
		matches DSCP codepoint (formerly-known as TOS bits)


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IP Tables

	ecn match
		matches ECN bits of tcp and ip header
	esp match
		matches IPSEC ESP SPI (range)
	hashlimit match
		dynamic limiting
	helper match
		allows matching of conntrack helper name
	iprange match
		match on arbitrary IP address ranges (not a mask)

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IP Tables

	length match
		match on packet length
	limit
		static rate limiting
	mac
		match on source mac address
	mark
		match on nfmark (fwmark)
	multiport
		match on multiple ports

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IP Tables

	owner
		match on socket owner (uid, gid, pid, sid, command name)
	physdev
		match underlying device in case of bridge
	pkttype
		match link-layer packet type (unicast,broadcast,multicast)
	realm
		match routing realm
	recent
		see special section below
	tcpmss 
		match on TCP maximum segment size

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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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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

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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/SCTP)
			application helpers currently (FTP,IRC,H.323,talk,SNMP)

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Connection Tracking Subsystem

	Connection tracking...

		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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Connection Tracking Subsystem

	State tracking for TCP is obvious
		TCP inherently stateful
		Two TCP state machines on each end have well-defined behaviour
		Passive tracking of state machines
		In more recent 2.6.x kernels, tracking of TCP window (seq/ack)
		Max idle timeout of fully-established session: 5 days


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Connection Tracking Subsystem

	State tracking for UDP: How is this possible?
		UDP itself not stateful at all
		However, higher-level protocols mostly match request-reply
		First packet (request) is assumed to be NEW
		First matching reply packet is assumed to confirm connection
		Further packets in either direction refresh timeout
		Timeouts: 30sec unreplied, 180sec confirmed

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Connection Tracking Subsystem

	State tracking on ICMP: What's that?
		ICMP Errors (e.g. host/net unreachable, ttl exceeded)
			They can always be categorized as RELATED to other connections
		ICMP request/reply (ECHO REQUEST, INFO REQUEST)
			can be treated like UDP request/reply case

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Connection Tracking Subsystem

	State tracking on SCTP: What's SCTP?
		Streaming Control Transfer Protocol
		Linux has SCTP in the network stack, so why should the packet filter not support it?
		Pretty much like TCP in most cases
		Doesn't support more advanced features such as failover of an endpoint

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Connection Tracking Subsystem

	State tracking on other protocols
		'generic' protocol: no layer-4 tuple information
		'gre' helper in patch-o-matic

	State tracking of higher-layer protocols
		implemented as 'connection tracking helpers'
		currently in-kernel: amanda, ftp, irc, tftp
		currently in patch-o-matic: pptp, h.323, sip, quake, ...
		have to be explicitly loaded (ip_conntrack_*.[k]o)
		work by issuing so-called "expectations"

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Connection Tracking Subsystem

	Exemptions to connection tracking
		Usually connection tracking is called first in PRE_ROUTING
		Sometimes, filtering is preferred before this conntrack lookup
			Therefore, the "raw" table was introduced
		In some rare cases, one might want to not track certain packets
			The NOTRACK can be used in the "raw" table

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Connection Tracking Subsystem

	Configuration / Tuning
		module parameter "hashsize"
			number of hash table buckets
		/proc/sys/net/ipv4/ip_conntrack_max
			maximum number of tracked connections
		/proc/sys/net/ipv4/ip_conntrack_buckets (read-only)
			number of hash table buckets
		/proc/net/ip_conntrack
			list of connections
		/proc/net/ip_conntrack_expect
			list of pending expectations

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Connection Tracking Subsystem

	Configuration / Tuning
		/proc/sys/net/ip_conntrack_log_invalid
			log invalid packets?
		/proc/sys/net/ip_conntrack_tcp_be_liberal
			basically disables window tracking, if "1"
		/proc/sys/net/ip_conntrack_tcp_loose
			how many packets required until sync in case of pickup
			if set to zero, disables pickup
		/proc/sys/net/ip_conntrack_tcp_max_retrans
			maximum number of retransmitted packets without seeing a n ACK
		/proc/sys/net/ip_conntrack_*timeout*
			timeout values of respective protocol states

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Network Address Translation

	Network Address Translation

		Previous Linux Kernels only implemented one special case of NAT: Masquerading
		Linux 2.4.x / 2.6.x can do any kind of NAT.
		NAT subsystem implemented on top of netfilter, iptables and conntrack
		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
			SAME
			NETMAP

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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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	Destination NAT
		DNAT example
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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

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

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Packet Mangling

	Targets specific to the 'mangle' table:
		DSCP
			manipulate DSCP field
		ECN
			manipulate ECN bits
		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
		CLASSIFY
			classify packet (for tc/iproute)
		CONNMARK
			set mark of connection

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The raw Table

	Purpose of 'raw' table
		to allow for filtering rules _before_ conntrack
	Targets specific to the 'raw' table:
		NOTRACK
			don't do connection tracking
	
	The table can also be useful for flood protection rules that happen before traversing the (computational) expensive connection tracking subsystem.

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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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Advanced Netfilter concepts

	Firewalling on a Bridge (ebtables + iptables)
		totally transparent to layer 2 and above
		no attack vector since firewall has no IP address
		even possible to do NAT on the bridge
		or even NAT of MAC addresses
	
	ipset - Faster matching
		iptables are a linear list of rules
		ipset represents a 'group' scheme
		Implements different data types for different applications	
			hash table (for random addresses)
			bitmask (for let's say a /24 network)

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Advanced Netfilter concepts

	ipv6 packet filtering
		ip6tables almost identical to iptables
		no connection tracking in mainline yet, but patches exist
		ip6_conntrack
			initial copy+paste 'port' by USAGI
			was not accepted because of code duplication
		nf_conntrack
			generalized connection tracking, supports ipv4 and ipv6
			mutually exclusive with ip_conntrack
			up to 2.6.20, no ipv4 nat on to of nf_conntrack

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Thanks

	Thanks to
		the BBS scene, Z-Netz, FIDO, ...
			for heavily increasing my computer usage in 1992
		KNF (http://www.franken.de/)
			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