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diff --git a/2003/netfilter-programming-ols2003/netfilter-programming-ols2003_.mgp b/2003/netfilter-programming-ols2003/netfilter-programming-ols2003_.mgp new file mode 100644 index 0000000..8c32cbb --- /dev/null +++ b/2003/netfilter-programming-ols2003/netfilter-programming-ols2003_.mgp @@ -0,0 +1,615 @@ +%include "default.mgp" +%default 1 bgrad +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +#%deffont "typewriter" tfont "MONOTYPE.TTF" +%page +%nodefault +%back "blue" + +%center +%size 7 + + +Developing netfilter/iptables +extensions + + +%center +%size 4 +by + +Harald Welte <laforge@netfilter.org> + + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +Contents + + + Introduction + The netfilter/iptables architecture + 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 + Developing a netfilter module + Developing a new iptables match + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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. + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +Netfilter Hooks + +Netfilter architecture in IPv4 + +%font "typewriter" +%size 3 + --->[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 +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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 + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +Developing netfilter/iptables extensions +Developing a netfilter module + + Netfilter modules are very low-layer + Get called for every packet passing the hook in this l3prot + Examples of netfilter modules are: ip_tables, ip_conntrack, iptable_nat +%font "typewriter" +%size 3 + #include <linux/netfilter.h> +%size 3 + nf_register_hook(struct nf_hook_ops *reg) +%size 3 + nf_unregister_hook(struct nf_hook_ops *reg) +%size 3 + struct nf_hook_ops: +%size 3 + struct list_head list; /* list header */ +%size 3 + nf_hookfn *hook; /* the callback function */ +%size 3 + int pf; /* protocol family */ +%size 3 + int hooknum; /* hook to register with */ +%size 3 + int priority; /* priority (ordering) */ +%font "standard" + Example code see "nf_workshop.c" + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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' + + Could potentially be used for other stuff, i.e. IPsec SPDB + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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) + +%size 4 +matches and targets can either be builtin or implemented as kernel modules + +%size 5 + 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 + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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" +%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 +The netfilter/iptables architecture +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 + + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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 + + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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. + + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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 + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +Developing netfilter/iptables extensions +Developing an ip_tables match module + + ip_tables modules are at a high layer + Get called for every packet iterating a rule with this match + Examples of iptables modules are: ipt_ttl, ipt_tos, ipt_tcpmss +%font "typewriter" +%size 3 + #include <linux/netfilter_ipv4/ip_tables.h> +%size 3 + ipt_register_match(struct ipt_match *match) +%size 3 + ipt_unregister_match(struct ipt_match *match) +%size 3 + struct ipt_match: +%size 3 + struct list_head list; /* list header {NULL,NULL} */ +%size 3 + const char name[]; /* name of the match */ +%size 3 + int (*match); /* called when pkt is matched */ +%size 3 + int (*checkentry); /* called when entry inserted */ +%size 3 + void (*destroy); /* called when entry deleted */ +%size 3 + struct module *me; /* set to THIS_MODULE */ +%font "standard" + Example code see "ipt_workshop.c" + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +Developing netfilter/iptables extensions +Developing an iptables match module + + Something has to parse the commandline options for ipt_workshop.c + Solution: libpt_workshop.c as iptables plugin +%font "typewriter" +%size 3 + #include <iptables.h>: +%size 3 + register_match(struct iptables_match) +%size 3 + struct iptables_match: +%size 3 + struct iptables_match *next; /* next one */ +%size 3 + ipt_chainlabel name; /* name */ +%size 3 + const char *version; /* version */ +%size 3 + size_t size; /* size of match data */ +%size 3 + size_t userspacesize; /* size for userspace */ +%size 3 + void (*help); /* print help message */ +%size 3 + void (*init); /* init the matchinfo */ +%size 3 + int (*parse); /* parse getopt chars */ +%size 3 + void (*final_check); /* consistency check */ +%size 3 + void (*print); /* print (iptables -L) */ +%size 3 + void (*save); /* iptables-save */ +%size 3 + struct option extra_opts; /* getopt-style opts */ +%font "typewriter" + Example code see "libipt_workshop.c" + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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 + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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 + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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 + + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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 + + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +Writing extensions for the conntrack subsystem + + new l4 protocol modules are very rare + more common: application helpers for ftp,irc,h.323,quake,mms,... + API for conntrack helper modules: +%font "typewriter" +%size 3 + #include <linux/netfilter_ipv4/ip_conntrack_helper.h> +%size 3 + struct ip_conntrack_helper +%size 3 + struct list_head *list; +%size 3 + const char *name; +%size 3 + unsigned char flags; +%size 3 + struct module *me; +%size 3 + unsigned int max_expected; +%size 3 + unsigned int timeout; +%size 3 + struct ip_conntrack_tuple tuple; +%size 3 + struct ip_conntrack_mask mask; +%size 3 + int (*help)(const struct iphdr *iph, size_t, struct ip_conntrack, enum ip_conntrack_info); +%size 3 + int ip_conntrack_helper_register(struct ip_conntrack_helper); +%size 3 + void ip_conntrack_helper_unregister(struct ip_conntrack_helper); +%size 3 + int ip_conntrack_expect_related(struct ip_conntrack, struct ip_conntrack_expect); +%size 3 + int ip_conntrack_change_expect(struct ip_conntrack_expect, struct ip_conntrack_tuple); +%size 3 + void ip_conntrack_unexpect_related(struct ip_conntrack_expect); +%font "standard" + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +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 + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +Network Address Translation + +flow of events for NEW packet: + packet enters NF_IP_PRE_ROUTING after conntrack + resolve conntrack entry for packet + if (expectfn of helper) call it + else iterate over rules in PREROUTING chain of nat table + save respective NAT mappings in conntrack + apply the NAT mappings to the packet + call NAT helper function, if there is one for this proto + ... + packet enters NF_IP_POST_ROUTING + resolve conntrack entry for packet + iterate over rules in POSTROUTING chain of nat table + save respectiva NAT mappings in conntrack + apply the NAT mappings to the packet + call NAT helper function, if there is one for this proto + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +Network Address Translation + +flow of events for ESTABLISHED packets: + packet enters NF_IP_PRE_ROUTING after conntrack + reseolve conntrack entry for packet + apply the NAT mappings (read from conntrack entry) to the packet + call NAT helper function, if there is one for this proto + ... + packet enters NF_IP_POST_ROUTING + resolve conntrack entry for packet + apply the NAT mappings (read from conntrack entry) to the packet + call NAT helper function, if there is one for this proto + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +Developing a NAT helper module +Network Address Translation + +%font "typewriter" +%size 3 + #include <linux/netfilter_ipv4/ip_nat_helper.h> +%size 3 + struct ip_nat_helper +%size 3 + struct list_head list; +%size 3 + const char *name; +%size 3 + unsigned char *flags; +%size 3 + struct module *me; +%size 3 + struct ip_conntrack_tuple tuple; +%size 3 + struct ip_conntrack_tuple mask; +%size 3 + unsigned int (*help)(struct ip_conntrack *, struct ip_conntrack_expect *, struct ip_nat_info *, enum ip_conntrack_info, unsigned int hooknum, struct sk_buff **) +%size 3 + unsigned int (*expect)(struct sk_buff **, unsigned int hooknum, struct ip_conntrack, struct ip_nat_info *) +%size 3 + int ip_nat_helper_register(struct ip_nat_helper *); +%size 3 + void ip_nat_helper_unregister(struct ip_nat_helper *); +%size 3 + int ip_nat_mangle_tcp_packet(); +%size 3 + int ip_nat_mangle_udp_packet(); + + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +The netfilter/iptables architecture +Advanced Netfilter concepts + +%size 4 + 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) + + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%page +Developing netfilter/iptables extensions +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 (http://www.astaro.com/) + for sponsoring parts of my netfilter work + for sponsoring my travel cost to OLS + |