2 .\" $FreeBSD: src/sbin/ipfw/ipfw.8,v 1.63.2.33 2003/02/04 01:36:02 brueffer Exp $
3 .\" $DragonFly: src/sbin/ipfw/ipfw.8,v 1.3 2004/03/11 12:28:54 hmp Exp $
10 .Nd IP firewall and traffic shaper control program
25 .Brq Cm delete | zero | resetlog
30 .Brq Cm firewall | one_pass | debug | verbose | dyn_keepalive
33 .Brq Cm firewall | one_pass | debug | verbose | dyn_keepalive
36 .Cm set Oo Cm disable Ar number ... Oc Op Cm enable Ar number ...
40 .Ar number Cm to Ar number
42 .Cm set swap Ar number number
54 .Brq Cm delete | list | show
62 .Ar macro Ns Op = Ns Ar value
70 utility is the user interface for controlling the
77 .Bd -ragged -offset XXXX
79 this manual page documents the newer version of
83 CURRENT in July 2002, also known as
86 is a superset of the old firewall,
88 The differences between the two are listed in Section
89 .Sx IPFW2 ENHANCEMENTS ,
90 which you are encouraged to read to revise older rulesets and possibly
91 write them more efficiently.
93 .Sx USING IPFW2 IN FreeBSD-STABLE
94 for instructions on how to run
107 numbered from 1 to 65535.
108 Packets are passed to
110 from a number of different places in the protocol stack
111 (depending on the source and destination of the packet,
114 is invoked multiple times on the same packet).
115 The packet passed to the firewall is compared
116 against each of the rules in the firewall
118 When a match is found, the action corresponding to the
119 matching rule is performed.
121 Depending on the action and certain system settings, packets
122 can be reinjected into the firewall at some rule after the
123 matching one for further processing.
127 ruleset always includes a
129 rule (numbered 65535) which cannot be modified,
130 and matches all packets.
131 The action associated with the
137 depending on how the kernel is configured.
139 If the ruleset includes one or more rules with the
147 behaviour, i.e. upon a match it will create dynamic rules matching
148 the exact parameters (addresses and ports) of the matching packet.
150 These dynamic rules, which have a limited lifetime, are checked
151 at the first occurrence of a
156 rule, and are typically used to open the firewall on-demand to
157 legitimate traffic only.
159 .Sx STATEFUL FIREWALL
162 Sections below for more information on the stateful behaviour of
165 All rules (including dynamic ones) have a few associated counters:
166 a packet count, a byte count, a log count and a timestamp
167 indicating the time of the last match.
168 Counters can be displayed or reset with
172 Rules can be added with the
174 command; deleted individually or in groups with the
176 command, and globally with the
178 command; displayed, optionally with the content of the
184 Finally, counters can be reset with the
190 Also, each rule belongs to one of 32 different
194 commands to atomically manipulate sets, such as enable,
195 disable, swap sets, move all rules in a set to another
196 one, delete all rules in a set. These can be useful to
197 install temporary configurations, or to test them.
200 for more information on
203 The following options are available:
204 .Bl -tag -width indent
206 While listing, show counter values.
209 command just implies this option.
211 When entering or showing rules, print them in compact form,
212 i.e. without the optional "ip from any to any" string
213 when this does not carry any additional information.
215 While listing, show dynamic rules in addition to static ones.
217 While listing, if the
219 option was specified, also show expired dynamic rules.
221 Don't ask for confirmation for commands that can cause problems
224 If there is no tty associated with the process, this is implied.
226 Try to resolve addresses and service names in output.
234 be quiet about actions
237 This is useful for adjusting rules by executing multiple
241 .Ql sh\ /etc/rc.firewall ) ,
242 or by processing a file of many
244 rules across a remote login session.
247 is performed in normal (verbose) mode (with the default kernel
248 configuration), it prints a message.
249 Because all rules are flushed, the message might not be delivered
250 to the login session, causing the remote login session to be closed
251 and the remainder of the ruleset to not be processed.
252 Access to the console would then be required to recover.
254 While listing rules, show the
256 each rule belongs to.
257 If this flag is not specified, disabled rules will not be
260 While listing pipes, sort according to one of the four
261 counters (total or current packets or bytes).
263 While listing, show last match timestamp.
266 To ease configuration, rules can be put into a file which is
269 as shown in the last synopsis line.
273 The file will be read line by line and applied as arguments to the
277 Optionally, a preprocessor can be specified using
281 is to be piped through.
282 Useful preprocessors include
288 doesn't start with a slash
290 as its first character, the usual
292 name search is performed.
293 Care should be taken with this in environments where not all
294 file systems are mounted (yet) by the time
296 is being run (e.g. when they are mounted over NFS).
299 has been specified, optional
303 specifications can follow and will be passed on to the preprocessor.
304 This allows for flexible configuration files (like conditionalizing
305 them on the local hostname) and the use of macros to centralize
306 frequently required arguments like IP addresses.
313 commands are used to configure the traffic shaper, as shown in the
314 .Sx TRAFFIC SHAPER (DUMMYNET) CONFIGURATION
317 If the world and the kernel get out of sync the
319 ABI may break, preventing you from being able to add any rules. This can
320 adversely effect the booting process. You can use
324 to temporarily disable the firewall to regain access to the network,
325 allowing you to fix the problem.
327 A packet is checked against the active ruleset in multiple places
328 in the protocol stack, under control of several sysctl variables.
329 These places and variables are shown below, and it is important to
330 have this picture in mind in order to design a correct ruleset.
331 .Bd -literal -offset indent
334 +----------->-----------+
336 [ip_input] [ip_output] net.inet.ip.fw.enable=1
339 [ether_demux] [ether_output_frame] net.link.ether.ipfw=1
341 +-->--[bdg_forward]-->--+ net.link.ether.bridge_ipfw=1
346 As can be noted from the above picture, the number of
347 times the same packet goes through the firewall can
348 vary between 0 and 4 depending on packet source and
349 destination, and system configuration.
351 Note that as packets flow through the stack, headers can be
352 stripped or added to it, and so they may or may not be available
354 E.g., incoming packets will include the MAC header when
358 but the same packets will have the MAC header stripped off when
363 Also note that each packet is always checked against the complete ruleset,
364 irrespective of the place where the check occurs, or the source of the packet.
365 If a rule contains some match patterns or actions which are not valid
366 for the place of invocation (e.g. trying to match a MAC header within
368 ), the match pattern will not match, but a
370 operator in front of such patterns
374 match on those packets.
375 It is thus the responsibility of
376 the programmer, if necessary, to write a suitable ruleset to
377 differentiate among the possible places.
379 rules can be useful here, as an example:
380 .Bd -literal -offset indent
381 # packets from ether_demux or bdg_forward
382 ipfw add 10 skipto 1000 all from any to any layer2 in
383 # packets from ip_input
384 ipfw add 10 skipto 2000 all from any to any not layer2 in
385 # packets from ip_output
386 ipfw add 10 skipto 3000 all from any to any not layer2 out
387 # packets from ether_output_frame
388 ipfw add 10 skipto 4000 all from any to any layer2 out
391 (yes, at the moment there is no way to differentiate between
392 ether_demux and bdg_forward).
396 rules is the following:
397 .Bd -ragged -offset indent
399 .Op Cm set Ar set_number
400 .Op Cm prob Ar match_probability
403 .Op Cm log Op Cm logamount Ar number
407 where the body of the rule specifies which information is used
408 for filtering packets, among the following:
410 .Bl -tag -width "Source and dest. addresses and ports" -offset XXX -compact
411 .It Layer-2 header fields
415 .It Source and dest. addresses and ports
419 .It Transmit and receive interface
421 .It Misc. IP header fields
422 Version, type of service, datagram length, identification,
423 fragment flag (non-zero IP offset),
426 .It Misc. TCP header fields
427 TCP flags (SYN, FIN, ACK, RST, etc.),
428 sequence number, acknowledgment number,
434 When the packet can be associated with a local socket.
437 Note that some of the above information, e.g. source MAC or IP addresses and
438 TCP/UDP ports, could easily be spoofed, so filtering on those fields
439 alone might not guarantee the desired results.
440 .Bl -tag -width indent
442 Each rule is associated with a
444 in the range 1..65535, with the latter reserved for the
447 Rules are checked sequentially by rule number.
448 Multiple rules can have the same number, in which case they are
449 checked (and listed) according to the order in which they have
451 If a rule is entered without specifying a number, the kernel will
452 assign one in such a way that the rule becomes the last one
456 Automatic rule numbers are assigned by incrementing the last
457 non-default rule number by the value of the sysctl variable
458 .Ar net.inet.ip.fw.autoinc_step
459 which defaults to 100.
460 If this is not possible (e.g. because we would go beyond the
461 maximum allowed rule number), the number of the last
462 non-default value is used instead.
463 .It Cm set Ar set_number
464 Each rule is associated with a
466 in the range 0..31, with the latter reserved for the
469 Sets can be individually disabled and enabled, so this parameter
470 is of fundamental importance for atomic ruleset manipulation.
471 It can be also used to simplify deletion of groups of rules.
472 If a rule is entered without specifying a set number,
474 .It Cm prob Ar match_probability
475 A match is only declared with the specified probability
476 (floating point number between 0 and 1).
477 This can be useful for a number of applications such as
478 random packet drop or
481 to simulate the effect of multiple paths leading to out-of-order
483 .It Cm log Op Cm logamount Ar number
484 When a packet matches a rule with the
486 keyword, a message will be
492 The logging only occurs if the sysctl variable
493 .Em net.inet.ip.fw.verbose
495 (which is the default when the kernel is compiled with
496 .Dv IPFIREWALL_VERBOSE
497 ) and the number of packets logged so far for that
498 particular rule does not exceed the
503 is specified, the limit is taken from the sysctl variable
504 .Em net.inet.ip.fw.verbose_limit .
505 In both cases, a value of 0 removes the logging limit.
507 Once the limit is reached, logging can be re-enabled by
508 clearing the logging counter or the packet counter for that entry, see the
514 A rule can be associated with one of the following actions, which
515 will be executed when the packet matches the body of the rule.
516 .Bl -tag -width indent
517 .It Cm allow | accept | pass | permit
518 Allow packets that match rule.
519 The search terminates.
521 Checks the packet against the dynamic ruleset.
522 If a match is found, execute the action associated with
523 the rule which generated this dynamic rule, otherwise
524 move to the next rule.
527 rules do not have a body.
530 rule is found, the dynamic ruleset is checked at the first
536 Update counters for all packets that match rule.
537 The search continues with the next rule.
539 Discard packets that match this rule.
540 The search terminates.
541 .It Cm divert Ar port
542 Divert packets that match this rule to the
546 The search terminates.
547 .It Cm fwd | forward Ar ipaddr Ns Op , Ns Ar port
548 Change the next-hop on matching packets to
550 which can be an IP address in dotted quad format or a host name.
551 The search terminates if this rule matches.
555 is a local address, then matching packets will be forwarded to
557 (or the port number in the packet if one is not specified in the rule)
558 on the local machine.
562 is not a local address, then the port number
563 (if specified) is ignored, and the packet will be
564 forwarded to the remote address, using the route as found in
565 the local routing table for that IP.
569 rule will not match layer-2 packets (those received
570 on ether_input, ether_output, or bridged).
574 action does not change the contents of the packet at all.
575 In particular, the destination address remains unmodified, so
576 packets forwarded to another system will usually be rejected by that system
577 unless there is a matching rule on that system to capture them.
578 For packets forwarded locally,
579 the local address of the socket will be
580 set to the original destination address of the packet.
583 entry look rather weird but is intended for
584 use with transparent proxy servers.
585 .It Cm pipe Ar pipe_nr
589 (for bandwidth limitation, delay, etc.).
591 .Sx TRAFFIC SHAPER (DUMMYNET) CONFIGURATION
592 Section for further information.
593 The search terminates; however, on exit from the pipe and if
597 .Em net.inet.ip.fw.one_pass
598 is not set, the packet is passed again to the firewall code
599 starting from the next rule.
600 .It Cm queue Ar queue_nr
604 (for bandwidth limitation using WF2Q+).
610 Discard packets that match this rule, and if the
611 packet is a TCP packet, try to send a TCP reset (RST) notice.
612 The search terminates.
613 .It Cm skipto Ar number
614 Skip all subsequent rules numbered less than
616 The search continues with the first rule numbered
620 Send a copy of packets matching this rule to the
624 The search terminates and the original packet is accepted
628 .It Cm unreach Ar code
629 Discard packets that match this rule, and try to send an ICMP
630 unreachable notice with code
634 is a number from 0 to 255, or one of these aliases:
635 .Cm net , host , protocol , port ,
636 .Cm needfrag , srcfail , net-unknown , host-unknown ,
637 .Cm isolated , net-prohib , host-prohib , tosnet ,
638 .Cm toshost , filter-prohib , host-precedence
640 .Cm precedence-cutoff .
641 The search terminates.
644 The body of a rule contains zero or more patterns (such as
645 specific source and destination addresses or ports,
646 protocol options, incoming or outgoing interfaces, etc.)
647 that the packet must match in order to be recognised.
648 In general, the patterns are connected by (implicit)
650 operators -- i.e. all must match in order for the
652 Individual patterns can be prefixed by the
654 operator to reverse the result of the match, as in
656 .Dl "ipfw add 100 allow ip from not 1.2.3.4 to any"
658 Additionally, sets of alternative match patterns (
660 ) can be constructed by putting the patterns in
661 lists enclosed between parentheses ( ) or braces { }, and
666 .Dl "ipfw add 100 allow ip from { x or not y or z } to any"
668 Only one level of parentheses is allowed.
669 Beware that most shells have special meanings for parentheses
670 or braces, so it is advisable to put a backslash \\ in front of them
671 to prevent such interpretations.
673 The body of a rule must in general include a source and destination
677 can be used in various places to specify that the content of
678 a required field is irrelevant.
680 The rule body has the following format:
681 .Bd -ragged -offset indent
682 .Op Ar proto Cm from Ar src Cm to Ar dst
686 The first part (protocol from src to dst) is for backward
691 any match pattern (including MAC headers, IPv4 protocols,
692 addresses and ports) can be specified in the
696 Rule fields have the following meaning:
697 .Bl -tag -width indent
698 .It Ar proto : protocol | Cm { Ar protocol Cm or ... }
699 An IPv4 protocol (or an
701 with multiple protocols) specified by number or name
702 (for a complete list see
703 .Pa /etc/protocols ) .
708 keywords mean any protocol will match.
709 .It Ar src No and Ar dst : ip-address | Cm { Ar ip-address Cm or ... } Op Ar ports
714 containing one or more of them,
715 optionally followed by
719 An address (or set of addresses) specified in one of the following
720 ways, optionally preceded by a
723 .Bl -tag -width indent
725 matches any IP address.
727 matches any IP address configured on an interface in the system.
728 The address list is evaluated at the time the packet is
730 .It Ar numeric-ip | hostname
731 Matches a single IPv4 address, specified as dotted-quad or a hostname.
732 Hostnames are resolved at the time the rule is added to the firewall list.
733 .It Ar addr Ns / Ns Ar masklen
734 Matches all addresses with base
736 (specified as a dotted quad or a hostname)
740 As an example, 1.2.3.4/25 will match
741 all IP numbers from 1.2.3.0 to 1.2.3.127 .
742 .It Ar addr Ns / Ns Ar masklen Ns Cm { Ns Ar num,num,... Ns Cm }
743 Matches all addresses with base address
745 (specified as a dotted quad or a hostname)
746 and whose last byte is in the list between braces { } .
747 Note that there must be no spaces between braces, commas and
751 field is used to limit the size of the set of addresses,
752 and can have any value between 24 and 32.
754 As an example, an address specified as 1.2.3.4/24{128,35,55,89}
755 will match the following IP addresses:
757 1.2.3.128 1.2.3.35 1.2.3.55 1.2.3.89 .
759 This format is particularly useful to handle sparse address sets
760 within a single rule. Because the matching occurs using a
761 bitmask, it takes constant time and dramatically reduces
762 the complexity of rulesets.
763 .It Ar addr Ns : Ns Ar mask
764 Matches all addresses with base
766 (specified as a dotted quad or a hostname)
769 specified as a dotted quad.
770 As an example, 1.2.3.4/255.0.255.0 will match
772 We suggest to use this form only for non-contiguous
773 masks, and resort to the
774 .Ar addr Ns / Ns Ar masklen
775 format for contiguous masks, which is more compact and less
778 .It Ar ports : Oo Cm not Oc Bro Ar port | port Ns \&- Ns Ar port Ns Brc Op , Ns Ar ...
779 For protocols which support port numbers (such as TCP and UDP), optional
781 may be specified as one or more ports or port ranges, separated
782 by commas but no spaces, and an optional
787 notation specifies a range of ports (including boundaries).
791 may be used instead of numeric port values.
792 The length of the port list is limited to 30 ports or ranges,
793 though one can specify larger ranges by using an
801 can be used to escape the dash
803 character in a service name (from a shell, the backslash must be
804 typed twice to avoid the shell itself interpreting it as an escape
807 .Dl "ipfw add count tcp from any ftp\e\e-data-ftp to any"
809 Fragmented packets which have a non-zero offset (i.e. not the first
810 fragment) will never match a rule which has one or more port
814 option for details on matching fragmented packets.
816 .Ss RULE OPTIONS (MATCH PATTERNS)
817 Additional match patterns can be used within
818 rules. Zero or more of these so-called
820 can be present in a rule, optionally prefixed by the
822 operand, and possibly grouped into
825 The following match patterns can be used (listed in alphabetical order):
826 .Bl -tag -width indent
828 Matches only bridged packets.
829 .It Cm dst-ip Ar ip address
830 Matches IP packets whose destination IP is one of the address(es)
831 specified as argument.
832 .It Cm dst-port Ar source ports
833 Matches IP packets whose destination port is one of the port(s)
834 specified as argument.
836 Matches TCP packets that have the RST or ACK bits set.
838 Matches packets that are fragments and not the first
839 fragment of an IP datagram. Note that these packets will not have
840 the next protocol header (e.g. TCP, UDP) so options that look into
841 these headers cannot match.
843 Matches all TCP or UDP packets sent by or received for a
847 may be specified by name or number.
848 .It Cm icmptypes Ar types
849 Matches ICMP packets whose ICMP type is in the list
851 The list may be specified as any combination of ranges or
852 individual types separated by commas.
853 The supported ICMP types are:
857 destination unreachable
869 time-to-live exceeded
883 and address mask reply
886 Matches incoming or outgoing packets, respectively.
890 are mutually exclusive (in fact,
895 Matches IP packets whose
900 Matches IP packets whose total length, including header and data, is
903 .It Cm ipoptions Ar spec
904 Matches packets whose IP header contains the comma separated list of
907 The supported IP options are:
910 (strict source route),
912 (loose source route),
914 (record packet route) and
917 The absence of a particular option may be denoted
920 .It Cm ipprecedence Ar precedence
921 Matches IP packets whose precedence field is equal to
924 Matches IP packets whose
926 field contains the comma separated list of
927 service types specified in
929 The supported IP types of service are:
932 .Pq Dv IPTOS_LOWDELAY ,
934 .Pq Dv IPTOS_THROUGHPUT ,
936 .Pq Dv IPTOS_RELIABILITY ,
938 .Pq Dv IPTOS_MINCOST ,
941 The absence of a particular type may be denoted
945 Matches IP packets whose time to live is
947 .It Cm ipversion Ar ver
948 Matches IP packets whose IP version field is
951 Upon a match, the firewall will create a dynamic rule, whose
952 default behaviour is to match bidirectional traffic between
953 source and destination IP/port using the same protocol.
954 The rule has a limited lifetime (controlled by a set of
956 variables), and the lifetime is refreshed every time a matching
959 Matches only layer2 packets, i.e. those passed to
961 from ether_demux() and ether_output_frame().
962 .It Cm limit Bro Cm src-addr | src-port | dst-addr | dst-port Brc Ar N
963 The firewall will only allow
965 connections with the same
966 set of parameters as specified in the rule.
968 of source and destination addresses and ports can be
970 .It Cm { MAC | mac } Ar dst-mac src-mac
971 Match packets with a given
975 addresses, specified as the
977 keyword (matching any MAC address), or six groups of hex digits
979 and optionally followed by a mask indicating how many bits are
982 .Dl "MAC 10:20:30:40:50:60/33 any"
984 Note that the order of MAC addresses (destination first,
986 the same as on the wire, but the opposite of the one used for
988 .It Cm mac-type Ar mac-type
989 Matches packets whose Ethernet Type field
990 corresponds to one of those specified as argument.
992 is specified in the same way as
994 (i.e. one or more comma-separated single values or ranges).
995 You can use symbolic names for known values such as
996 .Em vlan , ipv4, ipv6 .
997 Values can be entered as decimal or hexadecimal (if prefixed by 0x),
998 and they are always printed as hexadecimal (unless the
1000 option is used, in which case symbolic resolution will be attempted).
1001 .It Cm proto Ar protocol
1002 Matches packets with the corresponding IPv4 protocol.
1003 .It Cm recv | xmit | via Brq Ar ifX | Ar if Ns Cm * | Ar ipno | Ar any
1004 Matches packets received, transmitted or going through,
1005 respectively, the interface specified by exact name
1006 .Ns No ( Ar ifX Ns No ),
1008 .Ns No ( Ar if Ns Ar * Ns No ),
1009 by IP address, or through some interface.
1013 keyword causes the interface to always be checked.
1020 then only the receive or transmit interface (respectively)
1022 By specifying both, it is possible to match packets based on
1023 both receive and transmit interface, e.g.:
1025 .Dl "ipfw add deny ip from any to any out recv ed0 xmit ed1"
1029 interface can be tested on either incoming or outgoing packets,
1032 interface can only be tested on outgoing packets.
1037 is invalid) whenever
1041 A packet may not have a receive or transmit interface: packets
1042 originating from the local host have no receive interface,
1043 while packets destined for the local host have no transmit
1046 Matches TCP packets that have the SYN bit set but no ACK bit.
1047 This is the short form of
1048 .Dq Li tcpflags\ syn,!ack .
1049 .It Cm src-ip Ar ip-address
1050 Matches IP packets whose source IP is one of the address(es)
1051 specified as argument.
1052 .It Cm src-port Ar ports
1053 Matches IP packets whose source port is one of the port(s)
1054 specified as argument.
1055 .It Cm tcpack Ar ack
1057 Match if the TCP header acknowledgment number field is set to
1059 .It Cm tcpflags Ar spec
1061 Match if the TCP header contains the comma separated list of
1064 The supported TCP flags are:
1073 The absence of a particular flag may be denoted
1076 A rule which contains a
1078 specification can never match a fragmented packet which has
1082 option for details on matching fragmented packets.
1083 .It Cm tcpseq Ar seq
1085 Match if the TCP header sequence number field is set to
1087 .It Cm tcpwin Ar win
1089 Match if the TCP header window field is set to
1091 .It Cm tcpoptions Ar spec
1093 Match if the TCP header contains the comma separated list of
1094 options specified in
1096 The supported TCP options are:
1099 (maximum segment size),
1101 (tcp window advertisement),
1105 (rfc1323 timestamp) and
1107 (rfc1644 t/tcp connection count).
1108 The absence of a particular option may be denoted
1112 Match all TCP or UDP packets sent by or received for a
1116 may be matched by name or identification number.
1119 Each rule belongs to one of 32 different
1122 Set 31 is reserved for the default rule.
1124 By default, rules are put in set 0, unless you use the
1126 attribute when entering a new rule.
1127 Sets can be individually and atomically enabled or disabled,
1128 so this mechanism permits an easy way to store multiple configurations
1129 of the firewall and quickly (and atomically) switch between them.
1130 The command to enable/disable sets is
1131 .Bd -ragged -offset indent
1133 .Cm set Oo Cm disable Ar number ... Oc Op Cm enable Ar number ...
1140 sections can be specified.
1141 Command execution is atomic on all the sets specified in the command.
1142 By default, all sets are enabled.
1144 When you disable a set, its rules behave as if they do not exist
1145 in the firewall configuration, with only one exception:
1146 .Bd -ragged -offset indent
1147 dynamic rules created from a rule before it had been disabled
1148 will still be active until they expire. In order to delete
1149 dynamic rules you have to explicitly delete the parent rule
1150 which generated them.
1153 The set number of rules can be changed with the command
1154 .Bd -ragged -offset indent
1157 .Brq Cm rule Ar rule-number | old-set
1161 Also, you can atomically swap two rulesets with the command
1162 .Bd -ragged -offset indent
1164 .Cm set swap Ar first-set second-set
1169 Section on some possible uses of sets of rules.
1170 .Sh STATEFUL FIREWALL
1171 Stateful operation is a way for the firewall to dynamically
1172 create rules for specific flows when packets that
1173 match a given pattern are detected. Support for stateful
1174 operation comes through the
1175 .Cm check-state , keep-state
1181 Dynamic rules are created when a packet matches a
1185 rule, causing the creation of a
1187 rule which will match all and only packets with
1191 .Em src-ip/src-port dst-ip/dst-port
1196 are used here only to denote the initial match addresses, but they
1197 are completely equivalent afterwards).
1198 Dynamic rules will be checked at the first
1199 .Cm check-state, keep-state
1202 occurrence, and the action performed upon a match will be the same
1203 as in the parent rule.
1205 Note that no additional attributes other than protocol and IP addresses
1206 and ports are checked on dynamic rules.
1208 The typical use of dynamic rules is to keep a closed firewall configuration,
1209 but let the first TCP SYN packet from the inside network install a
1210 dynamic rule for the flow so that packets belonging to that session
1211 will be allowed through the firewall:
1213 .Dl "ipfw add check-state"
1214 .Dl "ipfw add allow tcp from my-subnet to any setup keep-state"
1215 .Dl "ipfw add deny tcp from any to any"
1217 A similar approach can be used for UDP, where an UDP packet coming
1218 from the inside will install a dynamic rule to let the response through
1221 .Dl "ipfw add check-state"
1222 .Dl "ipfw add allow udp from my-subnet to any keep-state"
1223 .Dl "ipfw add deny udp from any to any"
1225 Dynamic rules expire after some time, which depends on the status
1226 of the flow and the setting of some
1230 .Sx SYSCTL VARIABLES
1232 For TCP sessions, dynamic rules can be instructed to periodically
1233 send keepalive packets to refresh the state of the rule when it is
1238 for more examples on how to use dynamic rules.
1239 .Sh TRAFFIC SHAPER (DUMMYNET) CONFIGURATION
1241 is also the user interface for the
1246 operates by first using the firewall to classify packets and divide them into
1248 using any match pattern that can be used in
1251 Depending on local policies, a flow can contain packets for a single
1252 TCP connection, or from/to a given host, or entire subnet, or a
1255 Packets belonging to the same flow are then passed to either of two
1256 different objects, which implement the traffic regulation:
1257 .Bl -hang -offset XXXX
1259 A pipe emulates a link with given bandwidth, propagation delay,
1260 queue size and packet loss rate.
1261 Packets are queued in front of the pipe as they come out from the classifier,
1262 and then transferred to the pipe according to the pipe's parameters.
1266 is an abstraction used to implement the WF2Q+
1267 (Worst-case Fair Weighted Fair Queueing) policy, which is
1268 an efficient variant of the WFQ policy.
1270 The queue associates a
1272 and a reference pipe to each flow, and then all backlogged (i.e.,
1273 with packets queued) flows linked to the same pipe share the pipe's
1274 bandwidth proportionally to their weights.
1275 Note that weights are not priorities; a flow with a lower weight
1276 is still guaranteed to get its fraction of the bandwidth even if a
1277 flow with a higher weight is permanently backlogged.
1282 can be used to set hard limits to the bandwidth that a flow can use, whereas
1284 can be used to determine how different flow share the available bandwidth.
1290 configuration commands are the following:
1291 .Bd -ragged -offset indent
1292 .Cm pipe Ar number Cm config Ar pipe-configuration
1294 .Cm queue Ar number Cm config Ar queue-configuration
1297 The following parameters can be configured for a pipe:
1299 .Bl -tag -width indent -compact
1300 .It Cm bw Ar bandwidth | device
1301 Bandwidth, measured in
1304 .Brq Cm bit/s | Byte/s .
1307 A value of 0 (default) means unlimited bandwidth.
1308 The unit must immediately follow the number, as in
1310 .Dl "ipfw pipe 1 config bw 300Kbit/s"
1312 If a device name is specified instead of a numeric value, as in
1314 .Dl "ipfw pipe 1 config bw tun0"
1316 then the transmit clock is supplied by the specified device.
1317 At the moment only the
1319 device supports this
1320 functionality, for use in conjunction with
1323 .It Cm delay Ar ms-delay
1324 Propagation delay, measured in milliseconds.
1325 The value is rounded to the next multiple of the clock tick
1326 (typically 10ms, but it is a good practice to run kernels
1328 .Dq "options HZ=1000"
1330 the granularity to 1ms or less).
1331 Default value is 0, meaning no delay.
1334 The following parameters can be configured for a queue:
1336 .Bl -tag -width indent -compact
1337 .It Cm pipe Ar pipe_nr
1338 Connects a queue to the specified pipe.
1339 Multiple queues (with the same or different weights) can be connected to
1340 the same pipe, which specifies the aggregate rate for the set of queues.
1342 .It Cm weight Ar weight
1343 Specifies the weight to be used for flows matching this queue.
1344 The weight must be in the range 1..100, and defaults to 1.
1347 Finally, the following parameters can be configured for both
1350 .Bl -tag -width XXXX -compact
1352 .It Cm buckets Ar hash-table-size
1353 Specifies the size of the hash table used for storing the
1355 Default value is 64 controlled by the
1358 .Em net.inet.ip.dummynet.hash_size ,
1359 allowed range is 16 to 65536.
1361 .It Cm mask Ar mask-specifier
1362 Packets sent to a given pipe or queue by an
1364 rule can be further classified into multiple flows, each of which is then
1368 A flow identifier is constructed by masking the IP addresses,
1369 ports and protocol types as specified with the
1371 options in the configuration of the pipe or queue.
1372 For each different flow identifier, a new pipe or queue is created
1373 with the same parameters as the original object, and matching packets
1378 are used, each flow will get the same bandwidth as defined by the pipe,
1381 are used, each flow will share the parent's pipe bandwidth evenly
1382 with other flows generated by the same queue (note that other queues
1383 with different weights might be connected to the same pipe).
1385 Available mask specifiers are a combination of one or more of the following:
1387 .Cm dst-ip Ar mask ,
1388 .Cm src-ip Ar mask ,
1389 .Cm dst-port Ar mask ,
1390 .Cm src-port Ar mask ,
1395 where the latter means all bits in all fields are significant.
1398 When a packet is dropped by a dummynet queue or pipe, the error
1399 is normally reported to the caller routine in the kernel, in the
1400 same way as it happens when a device queue fills up. Setting this
1401 option reports the packet as successfully delivered, which can be
1402 needed for some experimental setups where you want to simulate
1403 loss or congestion at a remote router.
1405 .It Cm plr Ar packet-loss-rate
1408 .Ar packet-loss-rate
1409 is a floating-point number between 0 and 1, with 0 meaning no
1410 loss, 1 meaning 100% loss.
1411 The loss rate is internally represented on 31 bits.
1413 .It Cm queue Brq Ar slots | size Ns Cm Kbytes
1418 Default value is 50 slots, which
1419 is the typical queue size for Ethernet devices.
1420 Note that for slow speed links you should keep the queue
1421 size short or your traffic might be affected by a significant
1423 E.g., 50 max-sized ethernet packets (1500 bytes) mean 600Kbit
1424 or 20s of queue on a 30Kbit/s pipe.
1425 Even worse effect can result if you get packets from an
1426 interface with a much larger MTU, e.g. the loopback interface
1427 with its 16KB packets.
1429 .It Cm red | gred Ar w_q Ns / Ns Ar min_th Ns / Ns Ar max_th Ns / Ns Ar max_p
1430 Make use of the RED (Random Early Detection) queue management algorithm.
1435 point numbers between 0 and 1 (0 not included), while
1439 are integer numbers specifying thresholds for queue management
1440 (thresholds are computed in bytes if the queue has been defined
1441 in bytes, in slots otherwise).
1444 also supports the gentle RED variant (gred).
1447 variables can be used to control the RED behaviour:
1448 .Bl -tag -width indent
1449 .It Em net.inet.ip.dummynet.red_lookup_depth
1450 specifies the accuracy in computing the average queue
1451 when the link is idle (defaults to 256, must be greater than zero)
1452 .It Em net.inet.ip.dummynet.red_avg_pkt_size
1453 specifies the expected average packet size (defaults to 512, must be
1455 .It Em net.inet.ip.dummynet.red_max_pkt_size
1456 specifies the expected maximum packet size, only used when queue
1457 thresholds are in bytes (defaults to 1500, must be greater than zero).
1461 Here are some important points to consider when designing your
1465 Remember that you filter both packets going
1469 Most connections need packets going in both directions.
1471 Remember to test very carefully.
1472 It is a good idea to be near the console when doing this.
1473 If you cannot be near the console,
1474 use an auto-recovery script such as the one in
1475 .Pa /usr/share/examples/ipfw/change_rules.sh .
1477 Don't forget the loopback interface.
1482 There are circumstances where fragmented datagrams are unconditionally
1484 TCP packets are dropped if they do not contain at least 20 bytes of
1485 TCP header, UDP packets are dropped if they do not contain a full 8
1486 byte UDP header, and ICMP packets are dropped if they do not contain
1487 4 bytes of ICMP header, enough to specify the ICMP type, code, and
1489 These packets are simply logged as
1491 since there may not be enough good data in the packet to produce a
1492 meaningful log entry.
1494 Another type of packet is unconditionally dropped, a TCP packet with a
1495 fragment offset of one.
1496 This is a valid packet, but it only has one use, to try
1497 to circumvent firewalls.
1498 When logging is enabled, these packets are
1499 reported as being dropped by rule -1.
1501 If you are logged in over a network, loading the
1505 is probably not as straightforward as you would think.
1506 I recommend the following command line:
1507 .Bd -literal -offset indent
1508 kldload /modules/ipfw.ko && \e
1509 ipfw add 32000 allow ip from any to any
1512 Along the same lines, doing an
1513 .Bd -literal -offset indent
1517 in similar surroundings is also a bad idea.
1521 filter list may not be modified if the system security level
1522 is set to 3 or higher
1525 for information on system security levels).
1527 .Sh PACKET DIVERSION
1530 socket bound to the specified port will receive all packets
1531 diverted to that port.
1532 If no socket is bound to the destination port, or if the kernel
1533 wasn't compiled with divert socket support, the packets are
1535 .Sh SYSCTL VARIABLES
1538 variables controls the behaviour of the firewall and
1539 associated modules (
1540 .Nm dummynet, bridge
1542 These are shown below together with their default value
1543 (but always check with the
1545 command what value is actually in use) and meaning:
1546 .Bl -tag -width indent
1547 .It Em net.inet.ip.dummynet.expire : No 1
1548 Lazily delete dynamic pipes/queue once they have no pending traffic.
1549 You can disable this by setting the variable to 0, in which case
1550 the pipes/queues will only be deleted when the threshold is reached.
1551 .It Em net.inet.ip.dummynet.hash_size : No 64
1552 Default size of the hash table used for dynamic pipes/queues.
1553 This value is used when no
1555 option is specified when configuring a pipe/queue.
1556 .It Em net.inet.ip.dummynet.max_chain_len : No 16
1557 Target value for the maximum number of pipes/queues in a hash bucket.
1559 .Cm max_chain_len*hash_size
1560 is used to determine the threshold over which empty pipes/queues
1561 will be expired even when
1562 .Cm net.inet.ip.dummynet.expire=0 .
1563 .It Em net.inet.ip.dummynet.red_lookup_depth : No 256
1564 .It Em net.inet.ip.dummynet.red_avg_pkt_size : No 512
1565 .It Em net.inet.ip.dummynet.red_max_pkt_size : No 1500
1566 Parameters used in the computations of the drop probability
1567 for the RED algorithm.
1568 .It Em net.inet.ip.fw.autoinc_step : No 100
1569 Delta between rule numbers when auto-generating them.
1570 The value must be in the range 1..1000.
1571 .It Em net.inet.ip.fw.curr_dyn_buckets : Em net.inet.ip.fw.dyn_buckets
1572 The current number of buckets in the hash table for dynamic rules
1574 .It Em net.inet.ip.fw.debug : No 1
1575 Controls debugging messages produced by
1577 .It Em net.inet.ip.fw.dyn_buckets : No 256
1578 The number of buckets in the hash table for dynamic rules.
1579 Must be a power of 2, up to 65536.
1580 It only takes effect when all dynamic rules have expired, so you
1581 are advised to use a
1583 command to make sure that the hash table is resized.
1584 .It Em net.inet.ip.fw.dyn_count : No 3
1585 Current number of dynamic rules
1587 .It Em net.inet.ip.fw.dyn_keepalive : No 1
1588 Enables generation of keepalive packets for
1590 rules on TCP sessions. A keepalive is generated to both
1591 sides of the connection every 5 seconds for the last 20
1592 seconds of the lifetime of the rule.
1593 .It Em net.inet.ip.fw.dyn_max : No 8192
1594 Maximum number of dynamic rules.
1595 When you hit this limit, no more dynamic rules can be
1596 installed until old ones expire.
1597 .It Em net.inet.ip.fw.dyn_ack_lifetime : No 300
1598 .It Em net.inet.ip.fw.dyn_syn_lifetime : No 20
1599 .It Em net.inet.ip.fw.dyn_fin_lifetime : No 1
1600 .It Em net.inet.ip.fw.dyn_rst_lifetime : No 1
1601 .It Em net.inet.ip.fw.dyn_udp_lifetime : No 5
1602 .It Em net.inet.ip.fw.dyn_short_lifetime : No 30
1603 These variables control the lifetime, in seconds, of dynamic
1605 Upon the initial SYN exchange the lifetime is kept short,
1606 then increased after both SYN have been seen, then decreased
1607 again during the final FIN exchange or when a RST is received.
1609 .Em dyn_fin_lifetime
1611 .Em dyn_rst_lifetime
1612 must be strictly lower than 5 seconds, the period of
1613 repetition of keepalives. The firewall enforces that.
1614 .It Em net.inet.ip.fw.enable : No 1
1615 Enables the firewall.
1616 Setting this variable to 0 lets you run your machine without
1617 firewall even if compiled in.
1618 .It Em net.inet.ip.fw.one_pass : No 1
1619 When set, the packet exiting from the
1621 pipe is not passed though the firewall again.
1622 Otherwise, after a pipe action, the packet is
1623 reinjected into the firewall at the next rule.
1625 Note: bridged and layer 2 packets coming out of a pipe
1626 are never reinjected in the firewall irrespective of the
1627 value of this variable.
1628 .It Em net.inet.ip.fw.verbose : No 1
1629 Enables verbose messages.
1630 .It Em net.inet.ip.fw.verbose_limit : No 0
1631 Limits the number of messages produced by a verbose firewall.
1632 .It Em net.link.ether.ipfw : No 0
1633 Controls whether layer-2 packets are passed to
1636 .It Em net.link.ether.bridge_ipfw : No 0
1637 Controls whether bridged packets are passed to
1641 .Sh USING IPFW2 IN FreeBSD-STABLE
1649 unless the kernel is compiled with
1654 .Nm /usr/lib/libalias
1657 and reinstalled (the same effect can be achieved by adding
1661 before a buildworld).
1663 .Sh IPFW2 ENHANCEMENTS
1664 This Section lists the features that have been introduced in
1666 which were not present in
1668 We list them in order of the potential impact that they can
1669 have in writing your rulesets.
1670 You might want to consider using these features in order to
1671 write your rulesets in a more efficient way.
1672 .Bl -tag -width indent
1673 .It Handling of non-IPv4 packets
1675 will silently accept all non-IPv4 packets (which
1678 .Em net.link.ether.bridge_ipfw=1 Ns
1681 will filter all packets (including non-IPv4 ones) according to the ruleset.
1682 To achieve the same behaviour as
1684 you can use the following as the very first rule in your ruleset:
1686 .Dl "ipfw add 1 allow layer2 not mac-type ip"
1690 option might seem redundant, but it is necessary -- packets
1691 passed to the firewall from layer3 will not have a MAC header,
1694 pattern will always fail on them, and the
1696 operator will make this rule into a pass-all.
1699 does not supports address sets (those in the form
1700 .Ar addr/masklen{num,num,...}
1703 .It Port specifications
1705 only allows one port range when specifying TCP and UDP ports, and
1706 is limited to 10 entries instead of the 15 allowed by
1710 you can only specify ports when the rule is requesting
1716 you can put port specifications in rules matching all packets,
1717 and the match will be attempted only on those packets carrying
1718 protocols which include port identifiers.
1722 allowed the first port entry to be specified as
1726 can be an arbitrary 16-bit mask.
1727 This syntax is of questionable usefulness and it is not
1728 supported anymore in
1732 does not support Or-blocks.
1735 does not generate keepalives for stateful sessions.
1736 As a consequence, it might cause idle sessions to drop because
1737 the lifetime of the dynamic rules expires.
1740 does not implement sets of rules.
1741 .It MAC header filtering and Layer-2 firewalling.
1743 does not implement filtering on MAC header fields, nor is it
1744 invoked on packets from
1747 .Cm ether_output_frame().
1749 .Em net.link.ether.ipfw
1750 has no effect there.
1752 The following options are not supported in
1755 .Cm dst-ip, dst-port, layer2, mac, mac-type, src-ip, src-port.
1757 Additionally, the following options are not supported in
1762 .Cm ipid, iplen, ipprecedence, iptos, ipttl,
1763 .Cm ipversion, tcpack, tcpseq, tcpwin .
1764 .It Dummynet options
1765 The following option for
1767 pipes/queues is not supported:
1771 There are far too many possible uses of
1773 so this Section will only give a small set of examples.
1775 .Ss BASIC PACKET FILTERING
1776 This command adds an entry which denies all tcp packets from
1777 .Em cracker.evil.org
1778 to the telnet port of
1780 from being forwarded by the host:
1782 .Dl "ipfw add deny tcp from cracker.evil.org to wolf.tambov.su telnet"
1784 This one disallows any connection from the entire cracker's
1787 .Dl "ipfw add deny ip from 123.45.67.0/24 to my.host.org"
1789 A first and efficient way to limit access (not using dynamic rules)
1790 is the use of the following rules:
1792 .Dl "ipfw add allow tcp from any to any established"
1793 .Dl "ipfw add allow tcp from net1 portlist1 to net2 portlist2 setup"
1794 .Dl "ipfw add allow tcp from net3 portlist3 to net3 portlist3 setup"
1796 .Dl "ipfw add deny tcp from any to any"
1798 The first rule will be a quick match for normal TCP packets,
1799 but it will not match the initial SYN packet, which will be
1802 rules only for selected source/destination pairs.
1803 All other SYN packets will be rejected by the final
1807 If you administer one or more subnets, you can take advantage of the
1809 syntax to specify address sets and or-blocks and write extremely
1810 compact rulesets which selectively enable services to blocks
1811 of clients, as below:
1813 .Dl "goodguys=\*q{ 10.1.2.0/24{20,35,66,18} or 10.2.3.0/28{6,3,11} }\*q"
1814 .Dl "badguys=\*q10.1.2.0/24{8,38,60}\*q"
1816 .Dl "ipfw add allow ip from ${goodguys} to any"
1817 .Dl "ipfw add deny ip from ${badguys} to any"
1818 .Dl "... normal policies ..."
1822 syntax would require a separate rule for each IP in the above
1825 In order to protect a site from flood attacks involving fake
1826 TCP packets, it is safer to use dynamic rules:
1828 .Dl "ipfw add check-state"
1829 .Dl "ipfw add deny tcp from any to any established"
1830 .Dl "ipfw add allow tcp from my-net to any setup keep-state"
1832 This will let the firewall install dynamic rules only for
1833 those connection which start with a regular SYN packet coming
1834 from the inside of our network.
1835 Dynamic rules are checked when encountering the first
1842 rule should usually be placed near the beginning of the
1843 ruleset to minimize the amount of work scanning the ruleset.
1844 Your mileage may vary.
1846 To limit the number of connections a user can open
1847 you can use the following type of rules:
1849 .Dl "ipfw add allow tcp from my-net/24 to any setup limit src-addr 10"
1850 .Dl "ipfw add allow tcp from any to me setup limit src-addr 4"
1852 The former (assuming it runs on a gateway) will allow each host
1853 on a /24 network to open at most 10 TCP connections.
1854 The latter can be placed on a server to make sure that a single
1855 client does not use more than 4 simultaneous connections.
1858 stateful rules can be subject to denial-of-service attacks
1859 by a SYN-flood which opens a huge number of dynamic rules.
1860 The effects of such attacks can be partially limited by
1863 variables which control the operation of the firewall.
1865 Here is a good usage of the
1867 command to see accounting records and timestamp information:
1871 or in short form without timestamps:
1875 which is equivalent to:
1879 Next rule diverts all incoming packets from 192.168.2.0/24
1880 to divert port 5000:
1882 .Dl ipfw divert 5000 ip from 192.168.2.0/24 to any in
1885 The following rules show some of the applications of
1889 for simulations and the like.
1891 This rule drops random incoming packets with a probability
1894 .Dl "ipfw add prob 0.05 deny ip from any to any in"
1896 A similar effect can be achieved making use of dummynet pipes:
1898 .Dl "ipfw add pipe 10 ip from any to any"
1899 .Dl "ipfw pipe 10 config plr 0.05"
1901 We can use pipes to artificially limit bandwidth, e.g. on a
1902 machine acting as a router, if we want to limit traffic from
1903 local clients on 192.168.2.0/24 we do:
1905 .Dl "ipfw add pipe 1 ip from 192.168.2.0/24 to any out"
1906 .Dl "ipfw pipe 1 config bw 300Kbit/s queue 50KBytes"
1908 note that we use the
1910 modifier so that the rule is not used twice.
1911 Remember in fact that
1913 rules are checked both on incoming and outgoing packets.
1915 Should we want to simulate a bidirectional link with bandwidth
1916 limitations, the correct way is the following:
1918 .Dl "ipfw add pipe 1 ip from any to any out"
1919 .Dl "ipfw add pipe 2 ip from any to any in"
1920 .Dl "ipfw pipe 1 config bw 64Kbit/s queue 10Kbytes"
1921 .Dl "ipfw pipe 2 config bw 64Kbit/s queue 10Kbytes"
1923 The above can be very useful, e.g. if you want to see how
1924 your fancy Web page will look for a residential user who
1925 is connected only through a slow link.
1926 You should not use only one pipe for both directions, unless
1927 you want to simulate a half-duplex medium (e.g. AppleTalk,
1929 It is not necessary that both pipes have the same configuration,
1930 so we can also simulate asymmetric links.
1932 Should we want to verify network performance with the RED queue
1933 management algorithm:
1935 .Dl "ipfw add pipe 1 ip from any to any"
1936 .Dl "ipfw pipe 1 config bw 500Kbit/s queue 100 red 0.002/30/80/0.1"
1938 Another typical application of the traffic shaper is to
1939 introduce some delay in the communication.
1940 This can significantly affect applications which do a lot of Remote
1941 Procedure Calls, and where the round-trip-time of the
1942 connection often becomes a limiting factor much more than
1945 .Dl "ipfw add pipe 1 ip from any to any out"
1946 .Dl "ipfw add pipe 2 ip from any to any in"
1947 .Dl "ipfw pipe 1 config delay 250ms bw 1Mbit/s"
1948 .Dl "ipfw pipe 2 config delay 250ms bw 1Mbit/s"
1950 Per-flow queueing can be useful for a variety of purposes.
1951 A very simple one is counting traffic:
1953 .Dl "ipfw add pipe 1 tcp from any to any"
1954 .Dl "ipfw add pipe 1 udp from any to any"
1955 .Dl "ipfw add pipe 1 ip from any to any"
1956 .Dl "ipfw pipe 1 config mask all"
1958 The above set of rules will create queues (and collect
1959 statistics) for all traffic.
1960 Because the pipes have no limitations, the only effect is
1961 collecting statistics.
1962 Note that we need 3 rules, not just the last one, because
1965 tries to match IP packets it will not consider ports, so we
1966 would not see connections on separate ports as different
1969 A more sophisticated example is limiting the outbound traffic
1970 on a net with per-host limits, rather than per-network limits:
1972 .Dl "ipfw add pipe 1 ip from 192.168.2.0/24 to any out"
1973 .Dl "ipfw add pipe 2 ip from any to 192.168.2.0/24 in"
1974 .Dl "ipfw pipe 1 config mask src-ip 0x000000ff bw 200Kbit/s queue 20Kbytes"
1975 .Dl "ipfw pipe 2 config mask dst-ip 0x000000ff bw 200Kbit/s queue 20Kbytes"
1977 To add a set of rules atomically, e.g. set 18:
1979 .Dl "ipfw disable set 18"
1980 .Dl "ipfw add NN set 18 ... # repeat as needed"
1981 .Dl "ipfw enable set 18"
1983 To delete a set of rules atomically the command is simply:
1985 .Dl "ipfw delete set 18"
1987 To test a ruleset and disable it and regain control if something goes wrong:
1989 .Dl "ipfw disable set 18"
1990 .Dl "ipfw add NN set 18 ... # repeat as needed"
1991 .Dl "ipfw enable set 18 ; echo done; sleep 30 && ipfw disable set 18"
1993 Here if everything goes well, you press control-C before the "sleep"
1994 terminates, and your ruleset will be left active. Otherwise, e.g. if
1995 you cannot access your box, the ruleset will be disabled after
1996 the sleep terminates thus restoring the previous situation.
2013 The syntax has grown over the years and sometimes it might be confusing.
2014 Unfortunately, backward compatibility prevents cleaning up mistakes
2015 made in the definition of the syntax.
2019 Misconfiguring the firewall can put your computer in an unusable state,
2020 possibly shutting down network services and requiring console access to
2021 regain control of it.
2023 Incoming packet fragments diverted by
2027 are reassembled before delivery to the socket.
2028 The action used on those packet is the one from the
2029 rule which matches the first fragment of the packet.
2031 Packets that match a
2033 rule should not be immediately accepted, but should continue
2034 going through the rule list.
2035 This may be fixed in a later version.
2037 Packets diverted to userland, and then reinserted by a userland process
2040 will lose various packet attributes, including their source interface.
2041 If a packet is reinserted in this manner, later rules may be incorrectly
2042 applied, making the order of
2044 rules in the rule sequence very important.
2046 .An Ugen J. S. Antsilevich ,
2047 .An Poul-Henning Kamp ,
2053 API based upon code written by
2059 traffic shaper supported by Akamba Corp.
2063 utility first appeared in
2068 Stateful extensions were introduced in
2071 was introduced in Summer 2002.