path: root/2002/netfilter-curdevel-lk2002/netfilter-curdevel-lk2002.mgp

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The future of Linux packet filtering
targeted for kernel 2.6 and beyond


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by

Harald Welte <laforge@gnumonks.org>

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Future of Linux packet filtering
Contents


	Problems with current 2.4.x netfilter/iptables
		Solution to code replication
		Solution for dynamic rulesets
		Solution for API to GUI's and other management programs

	HA for stateful firewalling
		What's special about firewalling HA
		Poor man's failover
		Real state replication

	Other current work

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Future of Linux packet filtering
Problems with 2.4.x netfilter/iptables

	code replication between iptables/ip6tables/arptables
		iptables was never meant for other protocols, but people did copy+paste 'ports'
		replication of
			core kernel code
			layer 3 independent matches (mac, interface, ...)
			userspace library (libiptc)
			userspace tool (iptables)
			userspace plugins (libipt_xxx.so)

	doesn't suit the needs for dynamically changing rulesets
		dynamic rulesets becomming more common due (service selection, IDS)
		a whole table is created in userspace and sent as blob to kernel
		for every ruleset the table needs to be copied to userspace and back
		inside kernel consistency checks on whole table, loop detection

	too extensible for writing any forward-compatible GUI
		new extensions showing up all the time
		a frontend would need to know about the options and use of a new extension
		thus frontends are always incomplete and out-of-date
		no high-level API other than piping to iptables-restore

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Future of Linux packet filtering
Reducing code replication

	code replication is a real problem: unclean, bugfixes missed
	we need layer 3 independent layer for
		submitting rules to the kernel
		traversing packet-rulesets supporting match/target modules
		registering matches/targets
			layer 3 specific (like matching ipv4 address)
			layer 3 independent (like matching MAC address)

	solution
		pkt_tables inside kernel
			pkt_tables_ipv4 registers layer 3 handler with pkt_tables
			pkt_tables_ipv6 registers layer 3 handler with pkt_tables
			everybody registering a pkt_table (like iptable_filter) needs to specify the l3 protocol
		libraries in userspace (see later)

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Future of Linux packet filtering
Supporting dynamic rulesets

	atomic table-replacement turned out to be bad idea
	need new interface for sending individual rules to kernel
	policy routing has the same problem and good solution: rtnetlink
	solution: nfnetlink
		multicast-netlink based packet-orinented socket between kernel and userspace
		has extra benefit that other userspace processes get notified of rule changes [just like routing daemons]
		nfnetlink is a low-layer below all kernel/userspace communication
			pkttnetlink [aka iptnetlink]
			ctnetlink
			ulog
			ip_queue

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Future of Linux packet filtering
Communication with other programs

whole set of libraries
	libnfnetlink for low-layer communication
	libpkttnetlink for rule modifications
		will handle all plugins [which are currently part of iptables]
		query functions about avaliable matches/targets
		query functions about parameters
		query functions for help messages about specific match/parameter of a match
		generic structure from which rules can be built
		conversion functions to parse generic structure into in-kernel structure
		conversion functiosn to perse kernel structure into generic structure
		functions to convert generic structure in plain text
	libipq will stay API-compatible to current version
	libipulog will stay API-compatible to current version
	libiptc will go away [compatibility layer extremely difficult]


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HA for netfillter/iptables
Introduction

What is special about firewall failover?

	Nothing, in case of the stateless packet filter
		Common IP takeover solutions can be used
			VRRP
			Hartbeat

	Distribution of packet filtering ruleset no problem
		can be done manually
		or implemented with simple userspace process

	Problems arise with stateful packet filters
		Connection state only on active node
		NAT mappings only on active node


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HA for netfillter/iptables
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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HA for netfillter/iptables
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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HA for netfillter/iptables
Poor man's failover

Poor man's failover
	principle
		let every node do it's own tracking rather than replicating state
	two possible implementations
		connect every node to shared media (i.e. real ethernet)
			forwarding only turned on on active node
			slave nodes use promiscuous mode to sniff packets
		copy all traffic to slave nodes
			active master needs to copy all traffic to other nodes
			disadvantage: high load, sync traffic == payload traffic
			IMHO stupid way of solving the problem 
	advantages
		very easy implementation
			only addition of sniffing mode to conntrack needed
			existing means of address takeover can be used
		same load on active master and slave nodes
		no additional load on active master
	disadvantages
		can only be used with real shared media (no switches, ...)
		can not be used with NAT
	remaining problem
		no initial state sync after reboot of slave node!


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HA for netfillter/iptables
Real state replication

Parts needed
	state replication protocol
		multicast based
		sequence numbers for detection of packet loss
		NACK-based retransmission
		no security, since private ethernet segment to be used
	event interface on active node
		calling out to callback function at all state changes
	exported interface to manipulate conntrack hash table
	kernel thread for sending conntrack state protocol messages
		registers with event interface
		creates and accumulates state replication packets
		sends them via in-kernel sockets api
	kernel thread for receiving conntrack state replication messages
		receives state replication packets via in-kernel sockets
		uses conntrack hashtable manipulation interface


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HA for netfillter/iptables
Real state replication

	Flow of events in chronological order:
		on active node, inside the network RX softirq
			connection tracking code is analyzing a forwarded packet
			connection tracking gathers some new state information
			connection tracking updates local connection tracking database
			connection tracking sends event message to event API
		on active node, inside the conntrack-sync kernel thread
			conntrack sync daemon receives event through event API
			conntrack sync daemon aggregates multiple event messages into a state replication protocol message, removing possible redundancy
			conntrack sync daemon generates state replication protocol message
			conntrack sync daemon sends state replication protocol message
		on slave node(s), inside network RX softirq
			connection tracking code ignores packets coming from the interface attached to the private conntrac sync network
			state replication protocol messages is appended to socket receive queue of conntrack-sync kernel thread
		on slave node(s), inside conntrack-sync kernel thread
			conntrack sync daemon receives state replication message
			conntrack sync daemon creates/updates conntrack entry

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HA for netfillter/iptables
Neccessary changes to kernel

Neccessary changes to current conntrack core

	event generation (callback functions) for all state changes

	conntrack hashtable manipulation API
		is needed (and already implemented) for 'ctnetlink' API

	conntrack exemptions
		needed to _not_ track conntrack state replication packets
		is needed for other cases as well
		currently being developed by Jozsef Kadlecsik

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HA for netfillter/iptables
Other current work

	optimizing the conntrack code
		hash function optimization
			current hash function not good for even hash bucket count
			other hash functions in development
			hash function evaluation tool [cttest] avaliable
			introduce per-system randomness to prevent hash attack
		code optimization (locking/timers/...)

	getting our work submitted into the mainstream kernel
		turns out to be more difficult
			e.g. newnat api now waiting for three months

	discussions about multiple targets/actions per rule
		technical implementation easy
		however, not everybody convinced that it fits into the concept

	using tc for firewalling
		Jamal Hadi Selim uses iptables targets from within TC
		leads to discussion of generic classification engine API in kernel


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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