%include "default.mgp" %default 1 bgrad %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page %nodefault %back "blue" %center %size 7 The netfilter framework in Linux 2.4 %center %size 4 by Harald Welte %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 Contents Introduction PART I - Netfilter basics / concepts Part II - Packet filtering using iptables and netfilter Part III - NAT using iptables and netfilter Part IV - Packet mangling using iptables and netfilter Advanced netfilter concepts Current development and Future %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 Introduction What is netfilter More than a firewall subsystem Generalized Framework (protocol independend) Hooks in the Network stack Multiple kernel modules can register with the hooks Asynchronous packet handling in userspace Traditional packet filtering / NAT / ... implemented on top of this framework %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 Introduction Why did we need netfilter No infrastructure for passing packets to userspace Transparent proxying extremely difficult Packet filter rules depend on interface addresses Masquerading and packet filtering not implemented seperately Code too complex Neither modular nor extensible %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 Introduction Authors of netfilter Paul 'Rusty' Russel co-author of iptables in Linux 2.2 was paid by Watchguard for about one Year of development now works for Linuxcare James Morris userspace queuing (kernel, library and tools) REJECT target Marc Boucher NAT and packet filtering controlled by one comand Mangle table Non-core team contributors Philip Blundell (IPv6 integration) Jozsef Kadlecsik (full TCP window tracking) Harald Welte (IRC NAT helper, userspace logging) lots of people I probably forgot see http://netfilter.kernelnotes.org/scoreboard.html %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART I - Netfilter basics Netfilter architecture in IPv4 %font "typewriter" --->[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 6=NF_IP_LOCAL_OUT %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART I - Netfilter basics Netfilter base A Module registered to a hook 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 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART I - Netfilter basics Packet selection using IP tables Most modules registering for one of the netfilter hooks also use the generic IP tables infrastructure. The three major parts of 2.4 advanced packet handling are implemented using IP tables Packet filtering table 'filter' NAT table 'nat' Packet mangling table 'mangle' %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART I - Netfilter basics Connection tracking Implemented seperately from NAT to Provide a basis for statefule filtering using the 'state' match described later. The generic conntrack infrastructure is extensible via protocol helpers (currently TCP/UDP/ICMP) application helpers (currently FTP and IRC-DCC) Conntrack 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...) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART II - packet filtering Overview Packet filtering in IPv4 is implemented on top of three netfilter hooks NF_IP_LOCAL_IN (packets destined for the local host) NF_IP_FORWARD (packets forwarded by us) NF_IP_LOCAL_OUT (packets from the local host) %size 4 Each packet passes exactly one of the three hooks. Note that this is very different compared to the old 2.2 ipchains behaviour. On each of the three hooks is a chain (INPUT, FORWARD, OUTPUT) implemented on top of the IP table "filter" %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART II - packet filtering Managing chains and tables Each rule in a chain consists out of match (which packet match this rule) target (what to do if the rule is matched) %size 4 matches and targets can either be builtin or implemented as kernel modules %size 6 The userspace tool iptables is very flexible handles all different kinds of IP tables supports a plugin/shlib interface for target / match specific options %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART II - packet filtering Basic iptables commands To build a complete iptable command, we must specify which table to work with which chain in this table to use an operation (insert, add, delete, modify) a match a target The syntax is %font "typewriter" %size 3 iptables -t table -Operation chain -j target match(es) %font "standard" %size 5 Example: %font "typewriter" %size 3 iptables -t filter -A INPUT -j ACCEPT -p tcp --dport smtp %font "standard" %size 5 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART II - packet filtering Targets 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 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART II - packet filtering Matches Basic matches -p protocol (tcp/udp/icmp/...) -s source address (ip/mask) -d destination address (ip/mask) -i incoming interface -o outgoint interface Match extensions --dport destination port --sport source port --state (ESTABLISHED/RELATED/NEW/INVALID) --mac-source source MAC address --mark nfmark --tos --limit rate limiting (n packets per timeframe) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART III - NAT Overview Previous Linux Kernels only implemented one special case of NAT: Masquerading Netfilter enables Linux to do any kind of NAT. All matches from packet filtering are available for the nat tables, too We divide NAT into 'source NAT' and 'destination NAT' 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 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART III - NAT Source NAT SNAT Example: %font "typewriter" %size 3 iptables -t nat -A POSTROUTING -j SNAT --to-source 1.2.3.4 %font "standard" %size 4 Masquerading does almost the same as SNAT, but if the outgoing interfaces' address changes (in case we have a dialup with dynamic ip), the new address is used. MASQUERADE Example: %font "typewriter" %size 3 iptables -t nat -A POSTROUTING -j MASQUERADE -o ppp0 %font "standard" %size 5 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART III - NAT Destination NAT DNAT example: %font "typewriter" %size 3 iptables -t nat -A PREROUTING -j DNAT --to-destination 1.2.3.4:8080 -p tcp --dport 80 -i eth1 %font "standard" %size 4 REDIRECT is a special case of DNAT, which alters the destination to the address of the incoming interface. REDIRECT example: %font "typewriter" %size 3 iptables -t nat -A PREROUTING -j REDIRECT --to-port 3128 -i eth1 -p tcp --dport 80 %font "standard" %size 5 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 PART IV - Packet mangling Packet mangling enables us to change certain parts of a packet based on rules in IP tables. Of course, we again have all the matches available, as described in the packet filtering section. Currently, the supported packet mangling targets are: - TOS - manipulate the TOS bits - MARK - change the nfmark field of the skb Simple example: %font "typewriter" %size 3 iptables -t mangle -A PREROUTING -j MARK --set-mark 10 -p tcp --dport 80 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 Advanced Netfilter concepts %size 4 Connection tracking 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 already available Queuing reliable asynchronous packet handling packets to userspace via unicast netlink socket easy-to-use library (libipq) experimental queue multiplex daemon (ipqmpd) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %page netfilter in Linux 2.4 Current Development and Future Netfilter (although it proved very stable) is still work in progress. Areas of current development provide an unique libnfnetlink (like librtnetlink) infrastructure for conntrack/nat helpers in userspace full TCP sequence number tracking multicast support more flexible matches (MAXCONN, ...) more NAT modules (RPC, SNMP, SMB, ...) more IPv6 matches / targets