6 # ferm, a firewall setup program that makes firewall rules easy!
8 # Copyright (C) 2001-2012 Max Kellermann, Auke Kok
10 # Comments, questions, greetings and additions to this program
11 # may be sent to <ferm@foo-projects.org>
15 # This program is free software; you can redistribute it and/or modify
16 # it under the terms of the GNU General Public License as published by
17 # the Free Software Foundation; either version 2 of the License, or
18 # (at your option) any later version.
20 # This program is distributed in the hope that it will be useful,
21 # but WITHOUT ANY WARRANTY; without even the implied warranty of
22 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
23 # GNU General Public License for more details.
25 # You should have received a copy of the GNU General Public License
26 # along with this program; if not, write to the Free Software
27 # Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
32 B<ferm> - a firewall rule parser for linux
36 B<ferm> I<options> I<inputfile>
40 B<ferm> is a frontend for B<iptables>. It reads the rules from a
41 structured configuration file and calls iptables(8) to insert them
42 into the running kernel.
44 B<ferm>'s goal is to make firewall rules easy to write and easy to
45 read. It tries to reduce the tedious task of writing down rules, thus
46 enabling the firewall administrator to spend more time on developing
47 good rules than the proper implementation of the rule.
49 To achieve this, B<ferm> uses a simple but powerful configuration
50 language, which allows variables, functions, arrays, blocks. It also
51 allows you to include other files, allowing you to create libraries of
52 commonly used structures and functions.
54 B<ferm>, pronounced "firm", stands for "For Easy Rule Making".
59 This manual page does I<not> indend to teach you how firewalling works
60 and how to write good rules. There is already enough documentation on
66 Let's start with a simple example:
72 This will add a rule to the predefined input chain, matching and
73 accepting all tcp packets. Ok, let's make it more complicated:
75 chain (INPUT OUTPUT) {
76 proto (udp tcp) ACCEPT;
79 This will insert 4 rules, namely 2 in chain input, and 2 in chain
80 output, matching and accepting both udp and tcp packets. Normally you
83 iptables -A INPUT -p tcp -j ACCEPT
84 iptables -A OUTPUT -p tcp -j ACCEPT
85 iptables -A INPUT -p udp -j ACCEPT
86 iptables -A OUTPUT -p udp -j ACCEPT
88 Note how much less typing we need to do? :-)
90 Basically, this is all there is to it, although you can make it quite
91 more complex. Something to look at:
95 daddr 10.0.0.0/8 proto tcp dport ! ftp jump mychain sport :1023 TOS 4 settos 8 mark 2;
96 daddr 10.0.0.0/8 proto tcp dport ftp REJECT;
99 My point here is, that *you* need to make nice rules, keep
100 them readable to you and others, and not make it into a mess.
102 It would aid the reader if the resulting firewall rules were placed
103 here for reference. Also, you could include the nested version with
106 Try using comments to show what you are doing:
108 # this line enables transparent http-proxying for the internal network:
109 proto tcp if eth0 daddr ! 192.168.0.0/255.255.255.0
110 dport http REDIRECT to-ports 3128;
112 You will be thankful for it later!
116 interface (eth0 ppp0) {
117 # deny access to notorius hackers, return here if no match
118 # was found to resume normal firewalling
121 protocol tcp jump fw_tcp;
122 protocol udp jump fw_udp;
126 The more you nest, the better it looks. Make sure the order you
127 specify is correct, you would not want to do this:
131 proto tcp dport ftp ACCEPT;
134 because the second rule will never match. Best way is to specify
135 first everyting that is allowed, and then deny everything else.
136 Look at the examples for more good snapshots. Most people do
143 dport 1024:65535 ! syn ACCEPT;
147 =head1 STRUCTURE OF A FIREWALL FILE
149 The structure of a proper firewall file looks like simplified
150 C-code. Only a few syntactic characters are used in ferm-
151 configuration files. Besides these special caracters, ferm
152 uses 'keys' and 'values', think of them as options and
153 parameters, or as variables and values, whatever.
155 With these words, you define the characteristics of your firewall.
156 Every firewall consists of two things: First, look if network
157 traffic matches certain conditions, and second, what to do
160 You may specify conditions that are valid for the kernel
161 interface program you are using, probably iptables(8). For
162 instance, in iptables, when you are trying to match tcp
163 packets, you would say:
165 iptables --protocol tcp
167 In ferm, this will become:
171 Just typing this in ferm doesn't do anything, you need to tell
172 ferm (actually, you need to tell iptables(8) and the kernel) what
173 to do with any traffic that matches this condition:
175 iptables --protocol tcp -j ACCEPT
177 Or, translated to B<ferm>:
181 The B<;> character is at the end of every ferm rule. Ferm ignores line
182 breaks, meaning the above example is identical to the following:
187 Here's a list of the special characters:
193 This character finalizes a rule.
195 Separated by semicolons, you may write multiple rules in one line,
196 although this decreases readability:
198 protocol tcp ACCEPT; protocol udp DROP;
202 The nesting symbol defines a 'block' of rules.
204 The curly brackets contain any number of nested rules. All matches
205 before the block are carried forward to these.
207 The closing curly bracket finalizes the rule set. You should not write
208 a ';' after that, because that would be an empty rule.
212 chain INPUT proto icmp {
213 icmp-type echo-request ACCEPT;
217 This block shows two rules inside a block, which will both be merged
218 with anything in front of it, so you will get two rules:
220 iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT
221 iptables -A INPUT -p icmp -j DROP
223 There can be multiple nesting levels:
227 icmp-type echo-request ACCEPT;
230 daddr 172.16.0.0/12 REJECT;
233 Note that the 'REJECT' rule is not affected by 'proto icmp', although
234 there is no ';' after the closing curly brace. Translated to iptables:
236 iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT
237 iptables -A INPUT -p icmp -j DROP
238 iptables -A INPUT -d 172.16.0.0/12 -j REJECT
242 Variable expansion. Replaces '$FOO' by the value of the variable. See
243 the section I<VARIABLES> for details.
247 Function call. See the section I<FUNCTIONS> for details.
251 The array symbol. Using the parentheses, you can define
252 a 'list' of values that should be applied for the key to the
257 protocol ( tcp udp icmp )
259 this will result in three rules:
265 Only values can be 'listed', so you cannot do something like this:
267 proto tcp ( ACCEPT LOG );
271 chain (INPUT OUTPUT FORWARD) proto (icmp udp tcp) DROP;
273 (which will result in nine rules!)
275 Values are separated by spaces. The array symbol is both left- and
276 right-associative, in contrast with the nesting block, which is
277 left-associative only.
281 The comment symbol. Anything that follows this symbol up to
282 the end of line is ignored.
286 Execute the command in a shell, and insert the process output. See the
287 section I<backticks> for details.
291 Quote a string which may contain whitespaces, the dollar sign etc.
293 LOG log-prefix ' hey, this is my log prefix!';
297 Quote a string (see above), but variable references with a dollar sign
300 DNAT to "$myhost:$myport";
307 In the previous section, we already introduced some basic keywords
308 like "chain", "protocol" and "ACCEPT". Let's explore their nature.
310 There are three kinds of keywords:
316 B<location> keywords define where a rule will be created. Example:
321 B<match> keywords perform a test on all passing packets. The current
322 rule is without effect if one (or more) of the matches does not
323 pass. Example: "proto", "daddr".
325 Most matches are followed by a parameter: "proto tcp", "daddr
330 B<target> keywords state what to do with a packet. Example: "ACCEPT",
333 Some targets define more keywords to specify details: "REJECT
334 reject-with icmp-net-unreachable".
338 Every rule consists of a B<location> and a B<target>, plus any number
341 table filter # location
342 proto tcp dport (http https) # match
345 Strictly speaking, there is a fourth kind: B<ferm> keywords (which
346 control ferm's internal behaviour), but they will be explained later.
351 Many keywords take parameters. These can be specified as literals,
352 variable references or lists (arrays):
355 saddr $TRUSTED_HOSTS;
356 proto tcp dport (http https ssh);
357 LOG log-prefix "funky wardriver alert: ";
359 Some of them can be negated (lists cannot be negated):
362 proto udp dport !domain;
364 Keywords which take no parameters are negated by a prefixed '!':
368 Read iptables(8) to see where the B<!> can be used.
371 =head1 BASIC KEYWORDS
374 =head2 Location keywords
378 =item B<domain [ip|ip6]>
380 Set the domain. "ip" is default and means "IPv4" (iptables). "ip6" is
381 for IPv6 support, using "ip6tables".
383 =item B<table [filter|nat|mangle]>
385 Specifies which netfilter table this rule will be inserted to:
386 "filter" (default), "nat" or "mangle".
388 =item B<chain [chain-name]>
390 Specifies the netfilter chain (within the current table) this rule
391 will be inserted to. Common predefined chain names are "INPUT",
392 "OUTPUT", "FORWARD", "PREROUTING", "POSTROUTING", depending on the
393 table. See the netfilter documentation for details.
395 If you specify a non-existing chain here, ferm will add the rule to a
396 custom chain with that name.
398 =item B<policy [ACCEPT|DROP|..]>
400 Specifies the default policy for the current chain (built-in
401 only). Can be one of the built-in targets (ACCEPT, DROP, REJECT,
402 ...). A packet that matches no rules in a chain will be treated as
403 specified by the policy.
405 To avoid ambiguity, always specify the policies of all predefined
408 =item B<@subchain ["CHAIN-NAME"] { ... }>
410 Works like the normal block operators (i.e. without the I<@subchain>
411 keyword), except that B<ferm> moves rules within the curly braces into
412 a new custom chain. The name for this chain is chosen automatically by
415 In many cases, this is faster than just a block, because the kernel
416 may skip a huge block of rules when a precondition is false. Imagine
417 the following example:
419 table filter chain INPUT {
420 saddr (1.2.3.4 2.3.4.5 3.4.5.6 4.5.6.7 5.6.7.8) {
421 proto tcp dport (http https ssh) ACCEPT;
422 proto udp dport domain ACCEPT;
426 This generates 20 rules. When a packet arrives which does not pass the
427 B<saddr> match, it nonetheless checks all 20 rules. With B<@subchain>,
428 this check is done once, resulting in faster network filtering and
431 table filter chain INPUT {
432 saddr (1.2.3.4 2.3.4.5 3.4.5.6 4.5.6.7 5.6.7.8) @subchain {
433 proto tcp dport (http https ssh) ACCEPT;
434 proto udp dport domain ACCEPT;
438 Optionally, you may define the name of the sub chain:
440 saddr (1.2.3.4 2.3.4.5 3.4.5.6) @subchain "foobar" {
441 proto tcp dport (http https ssh) ACCEPT;
442 proto udp dport domain ACCEPT;
445 The name can either be a quoted string literal, or an expanded ferm
446 expression such as @cat("interface_", $iface) or @substr($var,0,20).
448 You can achieve the same by explicitly declaring a custom chain, but
449 you may feel that using B<@subchain> requires less typing.
451 =item B<@gotosubchain ["CHAIN-NAME"] { ... }>
453 Works like B<@subchain> except that instead of using B<jump> target it
454 uses B<goto> target. See discussion below for the difference between
460 =head2 Basic iptables match keywords
464 =item B<interface [interface-name]>
466 Define the interface name, your outside network card, like eth0,
467 or dialup like ppp1, or whatever device you want to match for
468 passing packets. It is equivalent to the C<-i> switch in
471 =item B<outerface [interface-name]>
473 Same as interface, only for matching the outgoing interface
474 for a packet, as in iptables(8).
476 =item B<protocol [protocol-name|protocol-number]>
478 Currently supported by the kernel are tcp, udp and icmp, or
479 their respective numbers.
481 =item B<saddr|daddr [address-spec]>
483 Matches on packets originating from the specified address (saddr) or
484 targeted at the address (daddr).
488 saddr 192.168/8 ACCEPT; # (identical to the next one:)
489 saddr 192.168.0.0/255.255.255.0 ACCEPT;
490 daddr my.domain.com ACCEPT;
494 Specify that only fragmented IP packets should be matched.
495 When packets are larger that the maximum packet size your
496 system can handle (called Maximum Transmission Unit or MTU)
497 they will be chopped into bits and sent one by one as single
498 packets. See ifconfig(8) if you want to find the MTU for
499 your system (the default is usually 1500 bytes).
501 Fragments are frequently used in DOS attacks, because there
502 is no way of finding out the origin of a fragment packet.
504 =item B<sport|dport [port-spec]>
506 Matches on packets on the specified TCP or UDP port. "sport" matches
507 the source port, and dport matches the destination port.
509 This match can be used only after you specified "protocol tcp" or
510 "protocol udp", because only these two protocols actually have ports.
512 And some examples of valid ports/ranges:
516 dport ssh:http ACCEPT;
517 dport 0:1023 ACCEPT; # equivalent to :1023
518 dport 1023:65535 ACCEPT;
522 Specify that the SYN flag in a tcp package should be matched,
523 which are used to build new tcp connections. You can identify
524 incoming connections with this, and decide wether you want
525 to allow it or not. Packets that do not have this flag are
526 probably from an already established connection, so it's
527 considered reasonably safe to let these through.
529 =item B<module [module-name]>
531 Load an iptables module. Most modules provide more match
532 keywords. We'll get to that later.
537 =head2 Basic target keywords
541 =item B<jump [custom-chain-name]>
543 Jumps to a custom chain. If no rule in the custom chain matched,
544 netfilter returns to the next rule in the previous chain.
546 =item B<goto [custom-chain-name]>
548 Go to a custom chain. Unlike the B<jump> option, B<RETURN> will not
549 continue processing in this chain but instead in the chain that called
554 Accepts matching packets.
558 Drop matching packets without further notice.
562 Rejects matching packets, i.e. send an ICMP packet to the sender,
563 which is port-unreachable by default. You may specify another ICMP
566 REJECT; # default to icmp-port-unreachable
567 REJECT reject-with icmp-net-unreachable;
569 Type "iptables -j REJECT -h" for details.
573 Finish the current chain and return to the calling chain (if "jump
574 [custom-chain-name]" was used).
583 =head1 ADDITIONAL KEYWORDS
585 Netfilter is modular. Modules may provide additional targets and match
586 keywords. The list of netfilter modules is constantly growing, and
587 ferm tries to keep up with supporting them all. This chapter describes
588 modules which are currently supported.
591 =head2 iptables match modules
597 Account traffic for all hosts in defined network/netmask. This is one
598 of the match modules which behave like a target, i.e. you will mostly
599 have to use the B<NOP> target.
601 mod account aname mynetwork aaddr 192.168.1.0/24 ashort NOP;
605 Check the address type; either source address or destination address.
607 mod addrtype src-type BROADCAST;
608 mod addrtype dst-type LOCAL;
610 Type "iptables -m addrtype -h" for details.
614 Checks the SPI header in an AH packet.
617 mod ah ahspi ! 0x200:0x2ff;
619 Additional arguments for IPv6:
621 mod ah ahlen 32 ACCEPT;
622 mod ah ahlen !32 ACCEPT;
627 Match using Linux Socket Filter.
629 mod bpf bytecode "4,48 0 0 9,21 0 1 6,6 0 0 1,6 0 0 0";
633 Adds a comment of up to 256 characters to a rule, without an effect.
634 Note that unlike ferm comments ('#'), this one will show up in
637 mod comment comment "This is my comment." ACCEPT;
641 Matches if a value in /proc/net/ipt_condition/NAME is 1 (path is
642 /proc/net/ip6t_condition/NAME for the ip6 domain).
644 mod condition condition (abc def) ACCEPT;
645 mod condition condition !foo ACCEPT;
649 Match by how many bytes or packets a connection (or one of the two
650 flows constituting the connection) have tranferred so far, or by
651 average bytes per packet.
653 mod connbytes connbytes 65536: connbytes-dir both connbytes-mode bytes ACCEPT;
654 mod connbytes connbytes !1024:2048 connbytes-dir reply connbytes-mode packets ACCEPT;
656 Valid values for I<connbytes-dir>: I<original>, I<reply>, I<both>; for
657 I<connbytes-mode>: I<packets>, I<bytes>, I<avgpkt>.
661 Module matches or adds connlabels to a connection.
663 mod connlabel label "name";
664 mod connlabel label "name" set;
668 Allows you to restrict the number of parallel TCP connections to a
669 server per client IP address (or address block).
671 mod connlimit connlimit-above 4 REJECT;
672 mod connlimit connlimit-above !4 ACCEPT;
673 mod connlimit connlimit-above 4 connlimit-mask 24 REJECT;
677 Check the mark field associated with the connection, set by the
680 mod connmark mark 64;
681 mod connmark mark 6/7;
685 Check connection tracking information.
687 mod conntrack ctstate (ESTABLISHED RELATED);
688 mod conntrack ctproto tcp;
689 mod conntrack ctorigsrc 192.168.0.2;
690 mod conntrack ctorigdst 1.2.3.0/24;
691 mod conntrack ctorigsrcport 67;
692 mod conntrack ctorigdstport 22;
693 mod conntrack ctreplsrc 2.3.4.5;
694 mod conntrack ctrepldst ! 3.4.5.6;
695 mod conntrack ctstatus ASSURED;
696 mod conntrack ctexpire 60;
697 mod conntrack ctexpire 180:240;
699 Type "iptables -m conntrack -h" for details.
703 Match cpu handling this packet.
709 Check DCCP (Datagram Congestion Control Protocol) specific attributes.
710 This module is automatically loaded when you use "protocol dccp".
712 proto dccp sport 1234 dport 2345 ACCEPT;
713 proto dccp dccp-types (SYNCACK ACK) ACCEPT;
714 proto dccp dccp-types !REQUEST DROP;
715 proto dccp dccp-option 2 ACCEPT;
719 Match the 6 bit DSCP field within the TOS field.
722 mod dscp dscp-class AF41;
726 Match the parameters in Destination Options header (IPv6).
729 mod dst dst-opts (type1 type2 ...);
733 Match the ECN bits of an IPv4 TCP header.
737 mod ecn ecn-ip-ect 2;
739 Type "iptables -m ecn -h" for details.
743 Checks the SPI header in an ESP packet.
745 mod esp espspi 0x101;
746 mod esp espspi ! 0x200:0x2ff;
750 "This module matches the EUI-64 part of a stateless autoconfigured
751 IPv6 address. It compares the EUI-64 derived from the source MAC
752 address in Ehternet frame with the lower 64 bits of the IPv6 source
753 address. But "Universal/Local" bit is not compared. This module
754 doesn't match other link layer frame, and is only valid in the
755 PREROUTING, INPUT and FORWARD chains."
761 "This module matches a rate limit based on a fuzzy logic controller [FLC]."
763 mod fuzzy lower-limit 10 upper-limit 20 ACCEPT;
767 Matches the Hop-by-Hop Options header (ip6).
769 mod hbh hbh-len 8 ACCEPT;
770 mod hbh hbh-len !8 ACCEPT;
771 mod hbh hbh-opts (1:4 2:8) ACCEPT;
775 Matches the Hop Limit field (ip6).
777 mod hl hl-eq (8 10) ACCEPT;
778 mod hl hl-eq !5 ACCEPT;
779 mod hl hl-gt 15 ACCEPT;
780 mod hl hl-lt 2 ACCEPT;
784 Checks which conntrack helper module tracks this connection. The port
785 may be specified with "-portnr".
787 mod helper helper irc ACCEPT;
788 mod helper helper ftp-21 ACCEPT;
792 Check ICMP specific attributes. This module is automatically loaded
793 when you use "protocol icmp".
795 proto icmp icmp-type echo-request ACCEPT;
797 This option can also be used in be I<ip6> domain, although this is
798 called B<icmpv6> in F<ip6tables>.
800 Use "iptables -p icmp C<-h>" to obtain a list of valid ICMP types.
804 Match a range of IPv4 addresses.
806 mod iprange src-range 192.168.2.0-192.168.3.255;
807 mod iprange dst-range ! 192.168.6.0-192.168.6.255;
811 Match on IPv4 header options like source routing, record route,
812 timestamp and router-alert.
814 mod ipv4options ssrr ACCEPT;
815 mod ipv4options lsrr ACCEPT;
816 mod ipv4options no-srr ACCEPT;
817 mod ipv4options !rr ACCEPT;
818 mod ipv4options !ts ACCEPT;
819 mod ipv4options !ra ACCEPT;
820 mod ipv4options !any-opt ACCEPT;
824 Matches the IPv6 extension header (ip6).
826 mod ipv6header header !(hop frag) ACCEPT;
827 mod ipv6header header (auth dst) ACCEPT;
831 Similar to 'mod limit', but adds the ability to add per-destination or
832 per-port limits managed in a hash table.
834 mod hashlimit hashlimit 10/minute hashlimit-burst 30/minute
835 hashlimit-mode dstip hashlimit-name foobar ACCEPT;
837 Possible values for hashlimit-mode: dstip dstport srcip srcport (or a
838 list with more than one of these).
840 There are more possible settings, type "iptables -m hashlimit -h" for
845 Match IPVS connection properties.
847 mod ipvs ipvs ACCEPT; # packet belongs to an IPVS connection
848 mod ipvs vproto tcp ACCEPT; # VIP protocol to match; by number or name, e.g. "tcp
849 mod ipvs vaddr 1.2.3.4/24 ACCEPT; # VIP address to match
850 mod ipvs vport http ACCEPT; # VIP port to match
851 mod ipvs vdir ORIGINAL ACCEPT; # flow direction of packet
852 mod ipvs vmethod GATE ACCEPT; # IPVS forwarding method used
853 mod ipvs vportctl 80; # VIP port of the controlling connection to match
857 Check the package length.
859 mod length length 128; # exactly 128 bytes
860 mod length length 512:768; # range
861 mod length length ! 256; # negated
865 Limits the packet rate.
867 mod limit limit 1/second;
868 mod limit limit 15/minute limit-burst 10;
870 Type "iptables -m limit -h" for details.
874 Match the source MAC address.
876 mod mac mac-source 01:23:45:67:89;
880 Matches packets based on their netfilter mark field. This may be a 32
881 bit integer between 0 and 4294967295.
887 Matches the mobility header (domain I<ip6>).
889 proto mh mh-type binding-update ACCEPT;
893 Match a set of source or destination ports (UDP and TCP only).
895 mod multiport source-ports (https ftp);
896 mod multiport destination-ports (mysql domain);
898 This rule has a big advantage over "dport" and "sport": it generates
899 only one rule for up to 15 ports instead of one rule for every port.
903 Match every 'n'th packet.
906 mod nth counter 5 every 2;
907 mod nth start 2 every 3;
908 mod nth start 5 packet 2 every 6;
910 Type "iptables -m nth -h" for details.
914 Match packets depending on the operating system of the sender.
917 mod osf ! genre FreeBSD ttl 1 log 1;
919 Type "iptables -m osf -h" for details.
923 Check information about the packet creator, namely user id, group id,
924 process id, session id and command name.
926 mod owner uid-owner 0;
927 mod owner gid-owner 1000;
928 mod owner pid-owner 5432;
929 mod owner sid-owner 6543;
930 mod owner cmd-owner "sendmail";
932 ("cmd-owner", "pid-owner" and "sid-owner" require special kernel
933 patches not included in the vanilla Linux kernel)
937 Matches the physical device on which a packet entered or is about to
938 leave the machine. This is useful for bridged interfaces.
940 mod physdev physdev-in ppp1;
941 mod physdev physdev-out eth2;
942 mod physdev physdev-is-in;
943 mod physdev physdev-is-out;
944 mod physdev physdev-is-bridged;
948 Check the link-layer packet type.
950 mod pkttype pkt-type unicast;
951 mod pkttype pkt-type broadcase;
952 mod pkttype pkt-type multicast;
956 Matches IPsec policy being applied to this packet.
958 mod policy dir out pol ipsec ACCEPT;
959 mod policy strict reqid 23 spi 0x10 proto ah ACCEPT;
960 mod policy mode tunnel tunnel-src 192.168.1.2 ACCEPT;
961 mod policy mode tunnel tunnel-dst 192.168.2.1 ACCEPT;
962 mod policy strict next reqid 24 spi 0x11 ACCEPT;
964 Note that the keyword I<proto> is also used as a shorthand version of
965 I<protocol> (built-in match module). You can fix this conflict by
966 always using the long keyword I<protocol>.
970 Detect TCP/UDP port scans.
972 mod psd psd-weight-threshold 21 psd-delay-threshold 300
973 psd-lo-ports-weight 3 psd-hi-ports-weight 1 DROP;
977 Implements network quotas by decrementing a byte counter with each packet.
979 mod quota quota 65536 ACCEPT;
983 Match a random percentage of all packets.
985 mod random average 70;
989 Match the routing realm. Useful in environments using BGP.
995 Temporarily mark source IP addresses.
998 mod recent rcheck seconds 60;
999 mod recent set rsource name "badguy";
1000 mod recent set rdest;
1001 mod recent rcheck rsource name "badguy" seconds 60;
1002 mod recent update seconds 120 hitcount 3 rttl;
1004 This netfilter module has a design flaw: although it is implemented as
1005 a match module, it has target-like behaviour when using the "set"
1008 L<http://snowman.net/projects/ipt_recent/>
1012 Checks a reply to the packet would be sent via the same interface it arrived on.
1013 Packets from the loopback interface are always permitted.
1015 mod rpfilter proto tcp loose RETURN;
1016 mod rpfilter validmark accept-local RETURN;
1017 mod rpfilter invert DROP;
1019 This netfilter module is the preferred way to perform reverse path filtering for
1020 IPv6, and a powerful alternative to checks controlled by sysctl
1021 I<net.ipv4.conf.*.rp_filter>.
1025 Match the IPv6 routing header (ip6 only).
1027 mod rt rt-type 2 rt-len 20 ACCEPT;
1028 mod rt rt-type !2 rt-len !20 ACCEPT;
1029 mod rt rt-segsleft 2:3 ACCEPT;
1030 mod rt rt-segsleft !4:5 ACCEPT;
1031 mod rt rt-0-res rt-0-addrs (::1 ::2) rt-0-not-strict ACCEPT;
1035 Check SCTP (Stream Control Transmission Protocol) specific attributes.
1036 This module is automatically loaded when you use "protocol sctp".
1038 proto sctp sport 1234 dport 2345 ACCEPT;
1039 proto sctp chunk-types only DATA:Be ACCEPT;
1040 proto sctp chunk-types any (INIT INIT_ACK) ACCEPT;
1041 proto sctp chunk-types !all (HEARTBEAT) ACCEPT;
1043 Use "iptables -p sctp C<-h>" to obtain a list of valid chunk types.
1047 Checks the source or destination IP/Port/MAC against a set.
1049 mod set set badguys src DROP;
1051 See L<http://ipset.netfilter.org/> for more information.
1055 Checks the connection tracking state.
1057 mod state state INVALID DROP;
1058 mod state state (ESTABLISHED RELATED) ACCEPT;
1060 Type "iptables -m state -h" for details.
1064 Successor of B<nth> and B<random>, currently undocumented in the
1065 iptables(8) man page.
1067 mod statistic mode random probability 0.8 ACCEPT;
1068 mod statistic mode nth every 5 packet 0 DROP;
1074 mod string string "foo bar" ACCEPT;
1075 mod string algo kmp from 64 to 128 hex-string "deadbeef" ACCEPT;
1079 Checks TCP specific attributes. This module is automatically loaded
1080 when you use "protocol tcp".
1082 proto tcp sport 1234;
1083 proto tcp dport 2345;
1084 proto tcp tcp-flags (SYN ACK) SYN;
1085 proto tcp tcp-flags ! (SYN ACK) SYN;
1086 proto tcp tcp-flags ALL (RST ACK);
1088 proto tcp tcp-option 2;
1091 Type "iptables -p tcp -h" for details.
1095 Check the TCP MSS field of a SYN or SYN/ACK packet.
1097 mod tcpmss mss 123 ACCEPT;
1098 mod tcpmss mss 234:567 ACCEPT;
1102 Check if the time a packet arrives is in given range.
1104 mod time timestart 12:00;
1105 mod time timestop 13:30;
1106 mod time days (Mon Wed Fri);
1107 mod time datestart 2005:01:01;
1108 mod time datestart 2005:01:01:23:59:59;
1109 mod time datestop 2005:04:01;
1110 mod time monthday (30 31);
1111 mod time weekdays (Wed Thu);
1112 mod time timestart 12:00 utc;
1113 mod time timestart 12:00 localtz;
1115 Type "iptables -m time -h" for details.
1119 Matches a packet on the specified TOS-value.
1121 mod tos tos Minimize-Cost ACCEPT;
1122 mod tos tos !Normal-Service ACCEPT;
1124 Type "iptables -m tos -h" for details.
1128 Matches the ttl (time to live) field in the IP header.
1130 mod ttl ttl-eq 12; # ttl equals
1131 mod ttl ttl-gt 10; # ttl greater than
1132 mod ttl ttl-lt 16; # ttl less than
1136 Compares raw data from the packet. You can specify more than one
1137 filter in a ferm list; these are not expanded into multiple rules.
1139 mod u32 u32 '6&0xFF=1' ACCEPT;
1140 mod u32 u32 ('27&0x8f=7' '31=0x527c4833') DROP;
1144 Matches packets which seem malformed or unusual. This match has no
1150 =head2 iptables target modules
1152 The following additional targets are available in ferm, provided that
1153 you enabled them in your kernel:
1159 Compute packet checksum.
1161 CHECKSUM checksum-fill;
1167 CLASSIFY set-class 3:50;
1171 Configure a simple cluster of nodes that share a certain IP and MAC
1172 address. Connections are statically distributed between the nodes.
1174 CLUSTERIP new hashmode sourceip clustermac 00:12:34:45:67:89
1175 total-nodes 4 local-node 2 hash-init 12345;
1179 Sets the netfilter mark value associated with a connection.
1181 CONNMARK set-xmark 42/0xff;
1182 CONNMARK set-mark 42;
1184 CONNMARK restore-mark;
1185 CONNMARK save-mark nfmask 0xff ctmask 0xff;
1186 CONNMARK save-mark mask 0x7fff;
1187 CONNMARK restore-mark mask 0x8000;
1188 CONNMARK and-mark 0x7;
1189 CONNMARK or-mark 0x4;
1190 CONNMARK xor-mark 0x7;
1191 CONNMARK and-mark 0x7;
1193 =item B<CONNSECMARK>
1195 This module copies security markings from packets to connections (if
1196 unlabeled), and from connections back to packets (also only if
1197 unlabeled). Typically used in conjunction with SECMARK, it is only
1198 valid in the mangle table.
1201 CONNSECMARK restore;
1203 =item B<DNAT to [ip-address|ip-range|ip-port-range]>
1205 Change the destination address of the packet.
1208 DNAT to 10.0.0.4:80;
1209 DNAT to 10.0.0.4:1024-2048;
1210 DNAT to 10.0.1.1-10.0.1.20;
1214 Provides stateless destination IPv6-to-IPv6 Network Prefix Translation.
1216 DNPT src-pfx 2001:42::/16 dst-pfx 2002:42::/16;
1220 This target allows to selectively work around known ECN blackholes.
1221 It can only be used in the mangle table.
1227 Modify the IPv6 Hop Limit field (ip6/mangle only).
1235 Like MARK, i.e. set the fwmark, but the mark is calculated from
1236 hashing packet selector at choice.
1238 HMARK hmark-tuple "src" hmark-mod "1" hmark-offset "1"
1239 hmark-src-prefix 192.168.1.0/24 hmark-dst-prefix 192.168.2.0/24
1240 hmark-sport-mask 0x1234 hmark-dport-mask 0x2345
1241 hmark-spi-mask 0xdeadbeef hmark-proto-mask 0x42 hmark-rnd 0xcoffee;
1245 This target can be used to identify when interfaces have been idle for
1246 a certain period of time.
1248 IDLETIMER timeout 60 label "foo";
1250 =item B<IPV4OPTSSTRIP>
1252 Strip all the IP options from a packet. This module does not take any
1259 This creates an LED-trigger that can then be attached to system
1260 indicator lights, to blink or illuminate them when certain packets
1261 pass through the system.
1263 LED led-trigger-id "foo" led-delay 100 led-always-blink;
1267 Log all packets that match this rule in the kernel log. Be carefull
1268 with log flooding. Note that this is a "non-terminating target",
1269 i.e. rule traversal continues at the next rule.
1271 LOG log-level warning log-prefix "Look at this: ";
1272 LOG log-tcp-sequence log-tcp-options;
1277 Sets the netfilter mark field for the packet (a 32 bit integer between
1288 Masquerades matching packets. Optionally followed by a port or
1289 port-range for iptables. Specify as "123", "123-456" or "123:456".
1290 The port range parameter specifies what local ports masqueraded
1291 connections should originate from.
1294 MASQUERADE to-ports 1234:2345;
1295 MASQUERADE to-ports 1234:2345 random;
1299 Experimental demonstration target which inverts the source and
1300 destination fields in the IP header.
1306 Map a whole network onto another network in the B<nat> table.
1308 NETMAP to 192.168.2.0/24;
1312 Disable connection tracking for all packets matching that rule.
1314 proto tcp dport (135:139 445) NOTRACK;
1318 RATEEST rateest-name "foo" rateest-interval 60s rateest-ewmalog 100;
1320 proto tcp dport (135:139 445) NOTRACK;
1324 Log packets over netlink; this is the successor of I<ULOG>.
1326 NFLOG nflog-group 5 nflog-prefix "Look at this: ";
1327 NFLOG nflog-range 256;
1328 NFLOG nflog-threshold 10;
1332 Userspace queueing, requires nfnetlink_queue kernel support.
1334 proto tcp dport ftp NFQUEUE queue-num 20;
1338 Userspace queueing, the predecessor to B<NFQUEUE>. All packets go to
1341 proto tcp dport ftp QUEUE;
1343 =item B<REDIRECT to-ports [ports]>
1345 Transparent proxying: alter the destination IP of the packet to the
1348 proto tcp dport http REDIRECT to-ports 3128;
1349 proto tcp dport http REDIRECT to-ports 3128 random;
1353 Similar to SNAT, but a client is mapped to the same source IP for all
1356 SAME to 1.2.3.4-1.2.3.7;
1357 SAME to 1.2.3.8-1.2.3.15 nodst;
1358 SAME to 1.2.3.16-1.2.3.31 random;
1362 This is used to set the security mark value associated with the packet
1363 for use by security subsystems such as SELinux. It is only valid in
1366 SECMARK selctx "system_u:object_r:httpd_packet_t:s0";
1368 =item B<SET [add-set|del-set] [setname] [flag(s)]>
1370 Add the IP to the specified set. See L<http://ipset.netfilter.org/>
1372 proto icmp icmp-type echo-request SET add-set badguys src;
1373 SET add-set "foo" timeout 60 exist;
1375 =item B<SNAT to [ip-address|ip-range|ip-port-range]>
1377 Change the source address of the packet.
1380 SNAT to 1.2.3.4:20000-30000;
1381 SNAT to 1.2.3.4 random;
1385 Provides stateless source IPv6-to-IPv6 Network Prefix Translation.
1387 SNPT src-pfx 2001:42::/16 dst-pfx 2002:42::/16;
1391 TCP 3-way handshake proxy: let the firewall handle the TCP 3-way handshake and
1392 only establish connection with the server socket once the client handshake has
1395 SYNPROXY wscale 7 mss 1460 timestamp sack-perm
1399 Alter the MSS value of TCP SYN packets.
1401 TCPMSS set-mss 1400;
1402 TCPMSS clamp-mss-to-pmtu;
1404 =item B<TCPOPTSTRIP>
1406 This target will strip TCP options off a TCP packet.
1408 TCPOPTSTRIP strip-options (option1 option2 ...);
1410 =item B<TOS set-tos [value]>
1412 Set the tcp package Type Of Service bit to this value. This will be
1413 used by whatever traffic scheduler is willing to, mostly your own
1414 linux-machine, but maybe more. The original tos-bits are blanked and
1415 overwritten by this value.
1417 TOS set-tos Maximize-Throughput;
1422 Type "iptables -j TOS -h" for details.
1426 Modify the TTL header field.
1429 TTL ttl-dec 1; # decrease by 1
1430 TTL ttl-inc 4; # increase by 4
1434 Log packets to a userspace program.
1436 ULOG ulog-nlgroup 5 ulog-prefix "Look at this: ";
1437 ULOG ulog-cprange 256;
1438 ULOG ulog-qthreshold 10;
1442 =head1 OTHER DOMAINS
1444 Since version 2.0, B<ferm> supports not only I<ip> and I<ip6>, but
1445 also I<arp> (ARP tables) and I<eb> (ethernet bridging tables). The
1446 concepts are similar to I<iptables>.
1448 =head2 arptables keywords
1452 =item B<source-ip>, B<destination-ip>
1454 Matches the source or destination IPv4 address. Same as B<saddr> and
1455 B<daddr> in the I<ip> domain.
1457 =item B<source-mac>, B<destination-mac>
1459 Matches the source or destination MAC address.
1461 =item B<interface>, B<outerface>
1463 Input and output interface.
1467 Hardware length of the packet.
1469 chain INPUT h-length 64 ACCEPT;
1473 Operation code, for details see the iptables(8).
1487 proto-type 0x800 ACCEPT;
1491 The keywords B<mangle-ip-s>, B<mangle-ip-d>, B<mangle-mac-s>,
1492 B<mangle-mac-d>, B<mangle-target> may be used for ARP mangling. See
1493 iptables(8) for details.
1497 =head2 ebtables keywords
1503 Matches the protocol which created the frame, e.g. I<IPv4> or B<PPP>.
1504 For a list, see F</etc/ethertypes>.
1506 =item B<interface>, B<outerface>
1508 Physical input and output interface.
1510 =item B<logical-in>, B<logical-out>
1512 The logical bridge interface.
1514 =item B<saddr>, B<daddr>
1516 Matches source or destination MAC address.
1518 =item B<Match modules>
1520 The following match modules are supported: 802.3, arp, ip, mark_m,
1521 pkttype, stp, vlan, log.
1523 =item B<Target extensions>
1525 The following target extensions are supported: arpreply, dnat, mark,
1528 Please note that there is a conflict between I<--mark> from the
1529 I<mark_m> match module and I<-j mark>. Since both would be
1530 implemented with the ferm keyword B<mark>, we decided to solve this by
1531 writing the target's name in uppercase, like in the other domains.
1532 The following example rewrites mark 1 to 2:
1538 =head1 ADVANCED FEATURES
1542 In complex firewall files, it is helpful to use variables, e.g. to
1543 give a network interface a meaningful name.
1545 To set variables, write:
1547 @def $DEV_INTERNET = eth0;
1548 @def $PORTS = (http ftp);
1549 @def $MORE_PORTS = ($PORTS 8080);
1551 In the real ferm code, variables are used like any other keyword
1554 chain INPUT interface $DEV_INTERNET proto tcp dport $MORE_PORTS ACCEPT;
1556 Note that variables can only be used in keyword parameters
1557 ("192.168.1.1", "http"); they cannot contain ferm keywords like
1558 "proto" or "interface".
1560 Variables are only valid in the current block:
1562 @def $DEV_INTERNET = eth1;
1565 @def $DEV_INTERNET = ppp0;
1566 interface $DEV_INTERNET dport http ACCEPT;
1568 interface $DEV_INTERNET DROP;
1571 will be expanded to:
1575 interface ppp0 dport http ACCEPT;
1577 interface eth1 DROP;
1580 The "def $DEV_INTERNET = ppp0" is only valid in the "proto tcp" block;
1581 the parent block still knows "set $DEV_INTERNET = eth1".
1583 Include files are special - variables declared in an included file are
1584 still available in the calling block. This is useful when you include
1585 a file which only declares variables.
1587 =head2 Automatic variables
1589 Some variables are set internally by ferm. Ferm scripts can use them
1590 just like any other variable.
1596 The name of the configuration file relative to the directory ferm was
1601 The base name of the configuration file.
1605 The directory of the configuration file.
1609 The current domain. One of I<ip>, I<ip6>, I<arp>, I<eb>.
1613 The current netfilter table.
1617 The current netfilter chain.
1621 The line of the current script. It can be used like this:
1624 LOG log-prefix "rule=$msg:$LINE ";
1629 &log("log message");
1635 Functions are similar to variables, except that they may have
1636 parameters, and they provide ferm commands, not values.
1638 @def &FOO() = proto (tcp udp) dport domain;
1641 @def &TCP_TUNNEL($port, $dest) = {
1642 table filter chain FORWARD interface ppp0 proto tcp dport $port daddr $dest outerface eth0 ACCEPT;
1643 table nat chain PREROUTING interface ppp0 proto tcp dport $port daddr 1.2.3.4 DNAT to $dest;
1646 &TCP_TUNNEL(http, 192.168.1.33);
1647 &TCP_TUNNEL(ftp, 192.168.1.30);
1648 &TCP_TUNNEL((ssh smtp), 192.168.1.2);
1650 A function call which contains a block (like '{...}') must be the last
1651 command in a ferm rule, i.e. it must be followed by ';'. The '&FOO()'
1652 example does not contain a block, thus you may write 'ACCEPT' after
1653 the call. To circumvent this, you can reorder the keywords:
1655 @def &IPSEC() = { proto (esp ah); proto udp dport 500; }
1656 chain INPUT ACCEPT &IPSEC();
1660 With backticks, you may use the output of an external command:
1662 @def $DNSSERVERS = `grep nameserver /etc/resolv.conf | awk '{print $2}'`;
1663 chain INPUT proto tcp saddr $DNSSERVERS ACCEPT;
1665 The command is executed with the shell (F</bin/sh>), just like
1666 backticks in perl. ferm does not do any variable expansion here.
1668 The output is then tokenized, and saved as a ferm list (array). Lines
1669 beginning with '#' are ignored; the other lines may contain any number
1670 of values, separated by whitespace.
1674 The B<@include> keyword allows you to include external files:
1676 @include 'vars.ferm';
1678 The file name is relative to the calling file, e.g. when including
1679 from F</etc/ferm/ferm.conf>, the above statement includes
1680 F</etc/ferm/vars.ferm>. Variables and functions declared in an
1681 included file are still available in the calling file.
1683 B<include> works within a block:
1686 @include 'input.ferm';
1689 If you specify a directory (with a trailing '/'), all files in this
1690 directory are included, sorted alphabetically:
1694 The function @glob can be used to expand wild cards:
1696 @include @glob('*.include');
1698 With a trailing pipe symbol, B<ferm> executes a shell command and
1701 @include "/root/generate_ferm_rules.sh $HOSTNAME|"
1703 B<ferm> aborts, if return code is not 0.
1707 The keyword B<@if> introduces a conditional expression:
1709 @if $condition DROP;
1711 A value is evaluated true just like in Perl: zero, empty list, empty
1712 string are false, everything else is true. Examples for true values:
1714 (a b); 1; 'foo'; (0 0)
1716 Examples for false values:
1720 There is also B<@else>:
1722 @if $condition DROP; @else REJECT;
1724 Note the semicolon before the B<@else>.
1726 It is possible to use curly braces after either B<@if> or B<@else>:
1735 Since the closing curly brace also finishes the command, there is no
1738 There is no B<@elsif>, use B<@else @if> instead.
1742 @def $have_ipv6 = `test -f /proc/net/ip6_tables_names && echo 1 || echo`;
1751 To run custom commands, you may install hooks:
1753 @hook pre "echo 0 >/proc/sys/net/ipv4/conf/eth0/forwarding";
1754 @hook post "echo 1 >/proc/sys/net/ipv4/conf/eth0/forwarding";
1755 @hook flush "echo 0 >/proc/sys/net/ipv4/conf/eth0/forwarding";
1757 The specified command is executed using the shell. "pre" means run
1758 the command before applying the firewall rules, and "post" means run
1759 the command afterwards. "flush" hooks are run after ferm has flushed
1760 the firewall rules (option --flush). You may install any number of
1763 =head1 BUILT-IN FUNCTIONS
1765 There are several built-in functions which you might find useful.
1767 =head2 @defined($name), @defined(&name)
1769 Tests if the variable or function is defined.
1772 @if @defined($a) good;
1773 @if @not(@defined($a)) bad;
1774 @if @defined(&funcname) good;
1778 Tests two values for equality. Example:
1780 @if @eq($DOMAIN, ip6) DROP;
1784 Similar to @eq, this tests for non-equality.
1788 Negates a boolean value.
1790 =head2 @resolve((hostname1 hostname2 ...), [type])
1792 Usually, host names are resolved by iptables. To let ferm resolve
1793 host names, use the function @resolve:
1795 saddr @resolve(my.host.foo) proto tcp dport ssh ACCEPT;
1796 saddr @resolve((another.host.foo third.host.foo)) proto tcp dport openvpn ACCEPT;
1797 daddr @resolve(ipv6.google.com, AAAA) proto tcp dport http ACCEPT;
1799 Note the double parentheses in the second line: the inner pair for
1800 creating a ferm list, and the outer pair as function parameter
1803 The second parameter is optional, and specifies the DNS record type.
1806 Be careful with resolved host names in firewall configuration. DNS
1807 requests may block the firewall configuration for a long time, leaving
1808 the machine vulnerable, or they may fail.
1810 =head2 @cat(a, b, ...)
1812 Concatenate all parameters into one string.
1814 =head2 @substr(expression, offset, length)
1816 Extracts a substring out of expression and returns it. First
1817 character is at offset 0. If OFFSET is negative, starts that far from
1818 the end of the string.
1820 =head2 @length(expression)
1822 Returns the length in characters of the value of EXPR.
1824 =head2 @basename(path)
1826 Return the base name of the file for a given path
1827 (File::Spec::splitpath).
1829 =head2 @dirname(path)
1831 Return the name of the last directory for a given path, assuming the
1832 last component is a file name (File::Spec::splitpath).
1836 Expand shell wildcards in the given paths (assumed to be relative to
1837 the current script). Returns a list of matching files. This function
1838 is useful as parameter of @include.
1840 =head2 @ipfilter(list)
1842 Filters out the IP addresses that obviously do not match the current
1843 domain. That is useful to create common variables and rules for IPv4
1846 @def $TRUSTED_HOSTS = (192.168.0.40 2001:abcd:ef::40);
1848 domain (ip ip6) chain INPUT {
1849 saddr @ipfilter($TRUSTED_HOSTS) proto tcp dport ssh ACCEPT;
1854 The F<./examples/> directory contains numerous ferm configuration
1855 which can be used to begin a new firewall. This sections contains more
1856 samples, recipes and tricks.
1858 =head2 Easy port forwarding
1860 Ferm function make routine tasks quick and easy:
1862 @def &FORWARD_TCP($proto, $port, $dest) = {
1863 table filter chain FORWARD interface $DEV_WORLD outerface $DEV_DMZ daddr $dest proto $proto dport $port ACCEPT;
1864 table nat chain PREROUTING interface $DEV_WORLD daddr $HOST_STATIC proto $proto dport $port DNAT to $dest;
1867 &FORWARD_TCP(tcp, http, 192.168.1.2);
1868 &FORWARD_TCP(tcp, smtp, 192.168.1.3);
1869 &FORWARD_TCP((tcp udp), domain, 192.168.1.4);
1871 =head2 Remote B<ferm>
1873 If the target machine is not able to run B<ferm> for some reason
1874 (maybe an embedded device without Perl), you can edit the B<ferm>
1875 configuration file on another computer and let B<ferm> generate a
1878 Example for OpenWRT:
1880 ferm --remote --shell mywrt/ferm.conf >mywrt/firewall.user
1881 chmod +x mywrt/firewall.user
1882 scp mywrt/firewall.user mywrt.local.net:/etc/
1883 ssh mywrt.local.net /etc/firewall.user
1891 Do not execute the iptables(8) commands, but skip instead. This way
1892 you can parse your data, use B<--lines> to view the output.
1896 Clears the firewall rules and sets the policy of all chains to ACCEPT.
1897 B<ferm> needs a configuration file for that to determine which domains
1898 and tables are affected.
1902 Show the firewall lines that were generated from the rules. They
1903 will be shown just before they are executed, so if you get error
1904 messages from iptables(8) etc., you can see which rule caused
1907 =item B<--interactive>
1909 Apply the firewall rules and ask the user for confirmation. Reverts
1910 to the previous ruleset if there is no valid user response within 30
1911 seconds (see B<--timeout>). This is useful for remote firewall
1912 administration: you can test the rules without fearing to lock
1915 =item B<--timeout S>
1917 If B<--interactive> is used, then roll back if there is no valid user
1918 response after this number of seconds. The default is 30.
1922 Show a brief list of available commandline options.
1926 Shows the version number of the program.
1930 Enable fast mode: ferm generates an iptables-save(8) file, and
1931 installs it with iptables-restore(8). This is much faster, because
1932 ferm calls iptables(8) once for every rule by default.
1934 Fast mode is enabled by default since B<ferm> 2.0, deprecating this
1939 Disable fast mode, i.e. run iptables(8) for every rule, and don't use
1940 iptables-restore(8).
1944 Generate a shell script which calls iptables-restore(8) and prints it.
1945 Implies --fast --lines.
1949 Generate rules for a remote machine. Implies B<--noexec> and
1950 B<--lines>. Can be combined with B<--shell>.
1952 =item B<--domain {ip|ip6}>
1954 Handle only the specified domain. B<ferm> output may be empty if the
1955 domain is not configured in the input file.
1957 =item B<--def '$name=value'>
1959 Override a variable defined in the configuration file.
1971 =head2 Operating system
1973 Linux 2.4 or newer, with netfilter support and all netfilter modules
1974 used by your firewall script
1978 iptables and perl 5.6
1984 If you find a bug, please tell us: ferm@foo-projects.org
1988 Copyright (C) 2001-2012 Max Kellermann <max@foo-projects.org>, Auke
1989 Kok <sofar@foo-projects.org>
1991 This program is free software; you can redistribute it and/or modify
1992 it under the terms of the GNU General Public License as published by
1993 the Free Software Foundation; either version 2 of the License, or (at
1994 your option) any later version.
1996 This program is distributed in the hope that it will be useful, but
1997 WITHOUT ANY WARRANTY; without even the implied warranty of
1998 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
1999 General Public License for more details.
2001 You should have received a copy of the GNU General Public License
2002 along with this program; if not, write to the Free Software
2003 Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
2008 Max Kellermann <max@foo-projects.org>, Auke Kok
2009 <sofar@foo-projects.org>