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IPFW(8)			  BSD System Manager's Manual		       IPFW(8)

     ipfw -- IP	firewall and traffic shaper control program

     ipfw [-cq]	add rule
     ipfw [-acdeftNS] {list | show} [number ...]
     ipfw [-f |	-q] flush
     ipfw [-q] {delete | zero |	resetlog} [set]	[number	...]

     ipfw set [disable number ...] [enable number ...]
     ipfw set move [rule] number to number
     ipfw set swap number number
     ipfw set show

     ipfw {pipe	| queue} number	config config-options
     ipfw [-s [field]] {pipe | queue} {delete |	list | show} [number ...]

     ipfw [-q] [-p preproc [-D macro[=value]] [-U macro]] pathname

     The ipfw utility is the user interface for	controlling the	ipfw(4)	fire-
     wall and the dummynet(4) traffic shaper in	FreeBSD.

     NOTE: this	manual page refers to the newer	version	of ipfw	introduced in
     July 2002,	also known as ipfw2.  The commands listed here are a superset
     of	the old	firewall, which	we will	call ipfw1 when	it is necessary	to
     distinguish between the two.

     ipfw2 is standard in FreeBSD CURRENT, whereas FreeBSD STABLE still	uses
     ipfw1 unless the kernel is	compiled with options IPFW2, and /sbin/ipfw
     and /usr/lib/libalias are recompiled with -DIPFW2 and reinstalled (the
     same effect can be	achieved by adding IPFW2=TRUE to /etc/make.conf	before
     a buildworld).

     See the IPFW2 ENHANCEMENTS	Section	for a list of features which are not
     present in	ipfw1.	This list can also be useful to	revise your rules and
     write them	more efficiently.

     An	ipfw configuration, or ruleset,	is made	of a list of rules numbered
     from 1 to 65535.  Packets are passed to ipfw in a number of different
     places in the protocol stack (depending on	the source and destination of
     the packet, it is possible	that ipfw is invoked multiple times on the
     same packet).  The	packet passed to the firewall is compared against each
     of	the rules in the firewall ruleset.  When a match is found, the action
     corresponding to the matching rule	is performed.

     Depending on the action and certain system	settings, packets can be rein-
     jected into the firewall at some rule after the matching one for further

     An	ipfw ruleset always includes a default rule (numbered 65535) which
     cannot be modified, and matches all packets.  The action associated with
     the default rule can be either deny or allow depending on how the kernel
     is	configured.

     If	the ruleset includes one or more rules with the	keep-state or limit
     option, then ipfw assumes a stateful behaviour, i.e. upon a match it will
     create dynamic rules matching the exact parameters	(addresses and ports)
     of	the matching packet.

     These dynamic rules, which	have a limited lifetime, are checked at	the
     first occurrence of a check-state,	keep-state or limit rule, and are typ-
     ically used to open the firewall on-demand	to legitimate traffic only.
     See the STATEFUL FIREWALL and EXAMPLES Sections below for more informa-
     tion on the stateful behaviour of ipfw.

     All rules (including dynamic ones)	have a few associated counters:	a
     packet count, a byte count, a log count and a timestamp indicating	the
     time of the last match.  Counters can be displayed	or reset with ipfw

     Rules can be added	with the add command; deleted individually or in
     groups with the delete command, and globally with the flush command; dis-
     played, optionally	with the content of the	counters, using	the show and
     list commands.  Finally, counters can be reset with the zero and resetlog

     Also, each	rule belongs to	one of 32 different sets , and there are ipfw
     commands to atomically manipulate sets, such as enable, disable, swap
     sets, move	all rules in a set to another one, delete all rules in a set.
     These can be useful to install temporary configurations, or to test them.
     See Section SETS OF RULES for more	information on sets.

     The following options are available:

     -a	     While listing, show counter values.  The show command just	im-
	     plies this	option.

     -c	     When entering or showing rules, print them	in compact form, i.e.
	     without the optional "ip from any to any" string when this	does
	     not carry any additional information.

     -d	     While listing, show dynamic rules in addition to static ones.

     -e	     While listing, if the -d option was specified, also show expired
	     dynamic rules.

     -f	     Don't ask for confirmation	for commands that can cause problems
	     if	misused, i.e. flush.  If there is no tty associated with the
	     process, this is implied.

     -N	     Try to resolve addresses and service names	in output.

     -q	     While adding, zeroing, resetlogging or flushing, be quiet about
	     actions (implies -f).  This is useful for adjusting rules by exe-
	     cuting multiple ipfw commands in a	script (e.g.,
	     `sh /etc/rc.firewall'), or	by processing a	file of	many ipfw
	     rules, across a remote login session.  If a flush is performed in
	     normal (verbose) mode (with the default kernel configuration), it
	     prints a message.	Because	all rules are flushed, the message
	     might not be delivered to the login session, causing the remote
	     login session to be closed	and the	remainder of the ruleset is
	     not processed.  Access to the console would then be required to

     -S	     While listing rules, show the set each rule belongs to.  If this
	     flag is not specified, disabled rules will	not be listed.

     -s	[field]
	     While listing pipes, sort according to one	of the four counters
	     (total and	current	packets	or bytes).

     -t	     While listing, show last match timestamp.

     To	ease configuration, rules can be put into a file which is processed
     using ipfw	as shown in the	first synopsis line.  An absolute pathname
     must be used.  The	file will be read line by line and applied as argu-
     ments to the ipfw utility.

     Optionally, a preprocessor	can be specified using -p preproc where
     pathname is to be piped through.  Useful preprocessors include cpp(1) and
     m4(1).  If	preproc	doesn't	start with a slash (`/') as its	first charac-
     ter, the usual PATH name search is	performed.  Care should	be taken with
     this in environments where	not all	file systems are mounted (yet) by the
     time ipfw is being	run (e.g. when they are	mounted	over NFS).  Once -p
     has been specified, optional -D and -U specifications can follow and will
     be	passed on to the preprocessor.	This allows for	flexible configuration
     files (like conditionalizing them on the local hostname) and the use of
     macros to centralize frequently required arguments	like IP	addresses.

     The ipfw pipe and queue commands are used to configure the	traffic
     shaper, as	shown in the TRAFFIC SHAPER CONFIGURATION Section below.

     ipfw can be invoked from multiple places in the protocol stack, under
     control of	several	system parameters, and it is important to understand
     when this occurs in order to design a proper ruleset. The places where
     ipfw is invoked are listed	below, together	with the sysctl	variables
     which control its invocation.

		 ^     to upper	layers	 V
		 |			 |
		 ^			 V
	    [ip_input]		    [ip_output]	  net.inet.ip.fw.enable=1
		 |			 |
		 ^			 V
	   [ether_demux]    [ether_output_frame]
		 |			 |
		 ^			 V
		 |	to devices	 |

     As	can be noted from the above picture, the number	of times the same
     packet goes through the firewall can vary between 0 and 4 depending on
     packet source and destination, and	system configuration.  In each of
     these places, the packet is passed	to ipfw	with all (and only) the	fields
     that belong to that level.	 That is, incoming packets will	include	the
     MAC header	when ipfw is invoked from ether_demux(), but the same packets
     will have the MAC header stripped off when	ipfw is	invoked	from
     The complete ruleset is always used, irrespective of the place where ipfw
     is	invoked, or the	source of the packet.  If a rule contains some match
     patterns or actions which are not valid for the place of invokation (e.g.
     trying to match a MAC header when ipfw is called from ip_input() )	the
     match pattern will	not match. However, a not operator in front of such
     patterns will cause the pattern to	always match on	those packets, which
     might cause undesired results.  It	is thus	responsibility of the program-
     mer, if necessary,	to write a suitable ruleset to differentiate among the
     possible places.  skipto rules can	be useful here,	as an example:

	   # packets from ether_demux or bdg_forward
	   ipfw	add 10 skipto 1000 all from any	to any layer2 in
	   # packets from ip_input
	   ipfw	add 10 skipto 2000 all from any	to any not layer2 in
	   # packets from ip_output
	   ipfw	add 10 skipto 3000 all from any	to any not layer2 out
	   # packets from ether_output_frame
	   ipfw	add 10 skipto 4000 all from any	to any layer2 out

     (yes, at the moment there is no way to differentiate between ether_demux
     and bdg_forward).

     The format	of ipfw	rules is the following:

	   [rule_number] [set set_number] [prob	match_probability]
	       action [log [logamount number]] body

     where the body of the rule	specifies which	information is used for	fil-
     tering packets, among the following:

	Layer-2	header fields		      When available
	IPv4 Protocol			      TCP, UDP,	ICMP, etc.
	Source and dest. addresses and ports
	Direction			      See Section PACKET FLOW
	Transmit and receive interface	      By name or address
	Misc. IP header	fields		      Version, type of service,	data-
					      gram length, identification,
					      fragment flag (non-zero IP off-
					      set), Time To Live
	IP options
	Misc. TCP header fields		      TCP flags	(SYN, FIN, ACK,	RST,
					      etc.), sequence number, acknowl-
					      edgment number, window
	TCP options
	ICMP types			      for ICMP packets
	User/group ID			      When the packet can be associate
					      to a local socket.

     Note that some of the above information, e.g. source MAC or IP addresses
     and TCP/UDP ports,	could easily be	spoofed, so filtering on those fields
     alone might not guarantee the desired results.

	     Each rule is associated with a rule_number	in the range 1..65535,
	     with the latter reserved for the default rule.  Rules are checked
	     sequentially by rule number.  Multiple rules can have the same
	     number, in	which case they	are checked (and listed) according to
	     the order in which	they have been added.  If a rule is entered
	     without specifying	a number, the kernel will assign one in	such a
	     way that the rule becomes the last	one before the default rule.
	     Automatic rule numbers are	assigned by incrementing the last non-
	     default rule number by the	value of the sysctl variable
	     net.inet.ip.fw.autoinc_step which defaults	to 100.	 If this is
	     not possible (e.g.	because	we would go beyond the maximum allowed
	     rule number), the same number of the last non-default value is
	     used instead.

     set set_number
	     Each rule is associated to	a set_number in	the range 0..31, with
	     the latter	reserved for the default rule.	Sets can be individu-
	     ally disabled and enabled,	so this	parameter is of	fundamental
	     importance	for atomic ruleset manipulation.  It can be also used
	     to	simplify deletion of groups of rules.  If a rule is entered
	     without specifying	a set number, set 0 will be used.

     prob match_probability
	     A match is	only declared with the specified probability (floating
	     point number between 0 and	1).  This can be useful	for a number
	     of	applications such as random packet drop	or (in conjunction
	     with dummynet(4)) to simulate the effect of multiple paths	lead-
	     ing to out-of-order packet	delivery.

     log [logamount number]
	     When a packet matches a rule with the log keyword,	a message will
	     be	logged to syslogd(8) with a LOG_SECURITY facility.  The	log-
	     ging only occurs if the sysctl variable net.inet.ip.fw.verbose is
	     set to 1 (which is	the default when the kernel is compiled	with
	     IPFIREWALL_VERBOSE	) and the number of packets logged so far for
	     that particular rule does not exceed ther logamount parameter.
	     If	no logamount is	specified, the limit is	taken from the sysctl
	     variable net.inet.ip.fw.verbose_limit.  In	both cases, a value of
	     0 removes the logging limit.

	     Once the limit is reached,	logging	can be re-enabled by clearing
	     the logging counter or the	packet counter for that	entry, see the
	     resetlog command.

     A rule can	be associated with one of the following	actions, which will be
     executed when the packet matches the body of the rule.

     allow | accept | pass | permit
	     Allow packets that	match rule.  The search	terminates.

	     Checks the	packet against the dynamic ruleset.  If	a match	is
	     found, execute the	action associated with the rule	which gener-
	     ated this dynamic rule, otherwise move to the next	rule.
	     Check-state rules do not have a body.  If no check-state rule is
	     found, the	dynamic	ruleset	is checked at the first	keep-state or
	     limit rule.

     count   Update counters for all packets that match	rule.  The search con-
	     tinues with the next rule.

     deny | drop
	     Discard packets that match	this rule.  The	search terminates.

     divert port
	     Divert packets that match this rule to the	divert(4) socket bound
	     to	port port.  The	search terminates.

     fwd | forward ipaddr[,port]
	     Change the	next-hop on matching packets to	ipaddr,	which can be
	     an	IP address in dotted quad or a host name.  The search termi-
	     nates if this rule	matches.

	     If	ipaddr is a local address, then	matching packets will be for-
	     warded to port (or	the port number	in the packet if one is	not
	     specified in the rule) on the local machine.
	     If	ipaddr is not a	local address, then the	port number (if	speci-
	     fied) is ignored, and the packet will be forwarded	to the remote
	     address, using the	route as found in the local routing table for
	     that IP.
	     A fwd rule	will not match layer-2 packets (those received on
	     ether_input, ether_output,	or bridged).
	     The fwd action does not change the	contents of the	packet at all.
	     In	particular, the	destination address remains unmodified,	so
	     packets forwarded to another system will usually be rejected by
	     that system unless	there is a matching rule on that system	to
	     capture them.  For	packets	forwarded locally, the local address
	     of	the socket will	be set to the original destination address of
	     the packet.  This makes the netstat(1) entry look rather weird
	     but is intended for use with transparent proxy servers.

     pipe pipe_nr
	     Pass packet to a dummynet(4) "pipe" (for bandwidth	limitation,
	     delay, etc.).  See	the TRAFFIC SHAPER CONFIGURATION Section for
	     further information.  The search terminates; however, on exit
	     from the pipe and if the sysctl(8)	variable
	     net.inet.ip.fw.one_pass is	not set, the packet is passed again to
	     the firewall code starting	from the next rule.

     queue queue_nr
	     Pass packet to a dummynet(4) "queue" (for bandwidth limitation
	     using WF2Q).

     reject  (Deprecated).  Synonym for	unreach	host.

     reset   Discard packets that match	this rule, and if the packet is	a TCP
	     packet, try to send a TCP reset (RST) notice.  The	search termi-

     skipto number
	     Skip all subsequent rules numbered	less than number.  The search
	     continues with the	first rule numbered number or higher.

     tee port
	     Send a copy of packets matching this rule to the divert(4)	socket
	     bound to port port.  The search terminates	and the	original
	     packet is accepted	(but see Section BUGS below).

     unreach code
	     Discard packets that match	this rule, and try to send an ICMP un-
	     reachable notice with code	code, where code is a number from 0 to
	     255, or one of these aliases: net,	host, protocol,	port,
	     needfrag, srcfail,	net-unknown, host-unknown, isolated,
	     net-prohib, host-prohib, tosnet, toshost, filter-prohib,
	     host-precedence or	precedence-cutoff.  The	search terminates.

     The body of a rule	contains zero or more patterns (such as	specific
     source and	destination addresses or ports,	protocol options, incoming or
     outgoing interfaces, etc.)	 that the packet must match in order to	be
     recognised.  In general, the patterns are connected by (implicit) and op-
     erators --	i.e. all must match in order for the rule to match.  Individ-
     ual patterns can be prefixed by the not operator to reverse the result of
     the match,	as in

	   ipfw	add 100	allow ip from not to any

     Additionally, sets	of alternative match patterns (	or-blocks ) can	be
     constructed by putting the	patterns in lists enclosed between parentheses
     ( ) or braces { },	and using the or operator as follows:

	   ipfw	add 100	allow ip from {	x or not y or z	} to any

     Only one level of parentheses is allowed.	Beware that most shells	have
     special meanings for parentheses or braces, so it is advisable to put a
     backslash \ in front of them to prevent such interpretations.

     The body of a rule	must in	general	include	a source and destination ad-
     dres specifier.  The keyword any can be used in various places to specify
     that the content of a required field is irrelevant.

     The rule body has the following format:

	   [proto from src to dst] [options]

     The first part (protocol from src to dst) is for backward compatibility
     with ipfw1.  In ipfw2 any match pattern (including	MAC headers, IPv4 pro-
     tocols, addresses and ports) can be specified in the options section.

     Rule fields have the following meaning:

     proto: protocol | { protocol or ... }
	     An	IPv4 protocol (or an or-block with multiple protocols) speci-
	     fied by number or name (for a complete list see /etc/protocols).
	     The ip or all keywords mean any protocol will match.

     src and dst: ip-address | { ip-address or ... } [ports]
	     A single ip-address , or an or-block containing one or more of
	     them, optionally followed by ports	specifiers.

	     An	address	(or set	of addresses) specified	in one of the follow-
	     ing ways, optionally preceded by a	not operator:

	     any     matches any IP address.

	     me	     matches any IP address configured on an interface in the
		     system.  The address list is evaluated at the time	the
		     packet is analysed.

	     numeric-ip	| hostname
		     Matches a single IPv4 address, specified as dotted-quad
		     or	a hostname.  Hostnames are resolved at the time	the
		     rule is added to the firewall list.

		     Matches all addresses with	base addr (specified as	a dot-
		     ted quad or a hostname) and mask width of masklen bits.
		     As	an example, will match all IP numbers from to .

		     Matches all addresses with	base address addr (specified
		     as	a dotted quad or a hostname) and whose last byte is in
		     the list between braces { } .  Note that there must be no
		     spaces between braces, commas and numbers.	 The masklen
		     field is used to limit the	size of	the set	of addresses,
		     and can have any value between 24 and 32.
		     As	an example, an address specified as{128,35,55,89} will match the following IP ad-
		     dresses: .
		     This format is particularly useful	to handle sparse ad-
		     dress sets	within a single	rule. Because the matching oc-
		     curs using	a bitmask, it takes constant time and dramati-
		     cally reduces the complexity of rulesets.

     ports: [not] {port	| port-port} [,...]
	     For protocols which support port numbers (such as TCP and UDP),
	     optional ports may	be specified as	one or more ports or port
	     ranges, separated by commas but no	spaces,	and an optional	not
	     operator.	The `-'	notation specifies a range of ports (including

	     Service names (from /etc/services)	may be used instead of numeric
	     port values.  The length of the port list is limited to 30	ports
	     or	ranges,	though one can specify larger ranges by	using an
	     or-block in the options section of	the rule.

	     A backslash (`\') can be used to escape the dash (`-') character
	     in	a service name (from a shell, the backslash must be typed
	     twice to avoid that the shell itself uses it as an	escape charac-

		   ipfw	add count tcp from any ftp\\-data-ftp to any

	     Fragmented	packets	which have a non-zero offset (i.e. not the
	     first fragment) will never	match a	rule which has one or more
	     port specifications.  See the frag	option for details on matching
	     fragmented	packets.

     Additional	match patterns can be used within rules. Zero or more of these
     so-called options can be present in a rule, optionally prefixed by	the
     not operand, and possibly grouped into or-blocks.

     The following match patterns can be used (listed in alphabetical order):

	     Matches only bridged packets.

     dst-ip ip address
	     Matches IP	packets	whose destination IP is	one of the address(es)
	     specified as argument.

     dst-port source ports
	     Matches IP	packets	whose destination port is one of the port(s)
	     specified as argument.

	     Matches TCP packets that have the RST or ACK bits set.

     frag    Matches packets that are fragments	and not	the first fragment of
	     an	IP datagram. Note that these packets will not have the next
	     protocol header (e.g. TCP,	UDP) so	options	that look into these
	     headers cannot match.

     gid group
	     Matches all TCP or	UDP packets sent by or received	for a group.
	     A group may be specified by name or number.

     icmptypes types
	     Matches ICMP packets whose	ICMP type is in	the list types.	 The
	     list may be specified as any combination of ranges	or individual
	     types separated by	commas.	 The supported ICMP types are:

	     echo reply	(0), destination unreachable (3), source quench	(4),
	     redirect (5), echo	request	(8), router advertisement (9), router
	     solicitation (10),	time-to-live exceeded (11), IP header bad
	     (12), timestamp request (13), timestamp reply (14), information
	     request (15), information reply (16), address mask	request	(17)
	     and address mask reply (18).

     in	| out
	     Matches incoming or outgoing packets, respectively.  in and out
	     are mutually exclusive (in	fact, out is implemented as not	in).

     ipid id
	     Matches IP	packets	whose ip_id field has value id.

     iplen len
	     Matches IP	packets	whose total length, including header and data,
	     is	len bytes.

     ipoptions spec
	     Matches packets whose IP header contains the comma	separated list
	     of	options	specified in spec.  The	supported IP options are:

	     ssrr (strict source route), lsrr (loose source route), rr (record
	     packet route) and ts (timestamp).	The absence of a particular
	     option may	be denoted with	a `!'.

     ipprecedence precedence
	     Matches IP	packets	whose precedence field is equal	to precedence.

     iptos spec
	     Matches IP	packets	whose tos field	contains the comma separated
	     list of service types specified in	spec.  The supported IP	types
	     of	service	are:

	     lowdelay (IPTOS_LOWDELAY),	throughput (IPTOS_THROUGHPUT),
	     reliability (IPTOS_RELIABILITY), mincost (IPTOS_MINCOST),
	     congestion	(IPTOS_CE).  The absence of a particular type may be
	     denoted with a `!'.

     ipttl ttl
	     Matches IP	packets	whose time to live is ttl.

     ipversion ver
	     Matches IP	packets	whose IP version field is ver.

	     Upon a match, the firewall	will create a dynamic rule, whose de-
	     fault behaviour is	to matching bidirectional traffic between
	     source and	destination IP/port using the same protocol.  The rule
	     has a limited lifetime (controlled	by a set of sysctl(8) vari-
	     ables), and the lifetime is refreshed every time a	matching
	     packet is found.

     layer2  Matches only layer2 packets, i.e. those passed to ipfw from
	     ether_demux() and ether_output_frame().

     limit {src-addr | src-port	| dst-addr | dst-port} N
	     The firewall will only allow N connections	with the same set of
	     parameters	as specified in	the rule.  One or more of source and
	     destination addresses and ports can be specified.

     { MAC | mac } dst-mac src-mac
	     Match packets with	a given	dst-mac	and src-mac addresses, speci-
	     fied as the any keyword (matching any MAC address), or six	groups
	     of	hex digits separated by	commas,	and optionally followed	by a
	     mask indicating how many bits are significant, as in

		   MAC 10:20:30:40:50:60/33 any

	     Note that the order of MAC	addresses (destination first, source
	     second) is	the same as on the wire, but the opposite of the one
	     used for IP addresses.

     mac-type mac-type
	     Matches packets whose Ethernet Type field corresponds to one of
	     those specified as	argument.  mac-type is specified in the	same
	     way as port numbers (i.e. one or more comma-separated single val-
	     ues or ranges).  You can use symbolic names for known values such
	     as	vlan, ipv4, ipv6.  Values can be enter as decimal or hexadeci-
	     mal (if prefixed by 0x), and they are always printed as hexadeci-
	     mal (unless the -N	option is used,	in which case symbolic resolu-
	     tion will be attempted).

     proto protocol
	     Matches packets with the corresponding IPv4 protocol.

     recv | xmit | via {ifX | if* | ipno | any}
	     Matches packets received, transmitted or be going through,	re-
	     spectively, the interface specified by exact name (ifX), by de-
	     vice name (if*), by IP address, or	through	some interface.

	     The via keyword causes the	interface to always be checked.	 If
	     recv or xmit is used instead of via, then only the	receive	or
	     transmit interface	(respectively) is checked.  By specifying
	     both, it is possible to match packets based on both receive and
	     transmit interface, e.g.:

		   ipfw	add deny ip from any to	any out	recv ed0 xmit ed1

	     The recv interface	can be tested on either	incoming or outgoing
	     packets, while the	xmit interface can only	be tested on outgoing
	     packets.  So out is required (and in is invalid) whenever xmit is

	     A packet may not have a receive or	transmit interface: packets
	     originating from the local	host have no receive interface,	while
	     packets destined for the local host have no transmit interface.

     setup   Matches TCP packets that have the SYN bit set but no ACK bit.
	     This is the short form of "tcpflags syn,!ack".

     src-ip ip-address
	     Matches IP	packets	whose source IP	is one of the address(es)
	     specified as argument.

     src-port ports
	     Matches IP	packets	whose source port is one of the	port(s)	speci-
	     fied as argument.

     tcpack ack
	     TCP packets only.	Match if the TCP header	acknowledgment number
	     field is set to ack.

     tcpflags spec
	     TCP packets only.	Match if the TCP header	contains the comma
	     separated list of flags specified in spec.	 The supported TCP
	     flags are:

	     fin, syn, rst, psh, ack and urg.  The absence of a	particular
	     flag may be denoted with a	`!'.  A	rule which contains a tcpflags
	     specification can never match a fragmented	packet which has a
	     non-zero offset.  See the frag option for details on matching
	     fragmented	packets.

     tcpseq seq
	     TCP packets only.	Match if the TCP header	sequence number	field
	     is	set to seq.

     tcpwin win
	     TCP packets only.	Match if the TCP header	window field is	set to

     tcpoptions	spec
	     TCP packets only.	Match if the TCP header	contains the comma
	     separated list of options specified in spec.  The supported TCP
	     options are:

	     mss (maximum segment size), window	(tcp window advertisement),
	     sack (selective ack), ts (rfc1323 timestamp) and cc (rfc1644
	     t/tcp connection count).  The absence of a	particular option may
	     be	denoted	with a `!'.

     uid user
	     Match all TCP or UDP packets sent by or received for a user.  A
	     user may be matched by name or identification number.

     Each rule belongs to one of 32 different sets , numbered 0	to 31.	Set 31
     is	reserved for the default rule.

     By	default, rules are put in set 0, unless	you use	the set	N attribute
     when entering a new rule.	Sets can be individually and atomically	en-
     abled or disabled,	so this	mechanism permits an easy way to store multi-
     ple configurations	of the firewall	and quickly (and atomically) switch
     between them.  The	command	to enable/disable sets is

     ipfw set disable number ... [enable number	...]

     where multiple enable or disable sections can be specified.  Command exe-
     cution is atomic on all the sets specified	in the command.	 By default,
     all sets are enabled.

     When you disable a	set, its rules behave as if they were not existing in
     the firewall configuration, with only one exception:

     o	 dynamic rules created from a rule before it had been disabled will
	 still be active until they expire. In order to	delete dynamic rules
	 you have to explicitly	delete the parent rule which generated them;
     The set number of rules can be changed with the command

     ipfw set move {rule rule-number | old-set}	to new-set

     Also, you can atomically swap two rulesets	with the command

     ipfw set swap first-set second-set

     See the EXAMPLES Section on some possible uses of sets of rules.

     Stateful operation	is a way for the firewall to dynamically create	rules
     for specific flows	when packets that match	a given	pattern	are detected.
     Support for stateful operation comes through the check-state, keep-state
     and limit options of rules.

     Dynamic rules are created when a packet matches a keep-state or limit
     rule, causing the creation	of a dynamic rule which	will match all and
     only packets with a given protocol	between	a src-ip/src-port
     dst-ip/dst-port pair of addresses ( src and dst are used here only	to de-
     note the initial match addresses, but they	are completely equivalent af-
     terwards).	 Dynamic rules will be checked at the first check-state,
     keep-state	or limit occurrence, and the action performed upon a match
     will be the same as in the	parent rule.

     Note that no additional attributes	other than protocol and	IP addresses
     and ports are checked on dynamic rules.

     The typical use of	dynamic	rules is to keep a closed firewall configura-
     tion, but let the first TCP SYN packet from the inside network install a
     dynamic rule for the flow so that packets belonging to that session will
     be	allowed	through	the firewall:

	   ipfw	add check-state
	   ipfw	add allow tcp from my-subnet to	any setup
	   ipfw	add deny tcp from any to any

     A similar approach	can be used for	UDP, where an UDP packet coming	from
     the inside	will install a dynamic rule to let the response	through	the

	   ipfw	add check-state
	   ipfw	add allow udp from my-subnet to	any
	   ipfw	add deny udp from any to any

     Dynamic rules expire after	some time, which depends on the	status of the
     flow and the setting of some sysctl variables.  See Section SYSCTL
     VARIABLES for more	details.  For TCP sessions, dynamic rules can be in-
     structed to periodically send keepalive packets to	refresh	the state of
     the rule when it is about to expire.

     See Section EXAMPLES for more examples on how to use dynamic rules.

     ipfw is also the user interface for the dummynet(4) traffic shaper.  The
     shaper operates by	dividing packets into flows according to a user-speci-
     fied mask on different fields of the IP header.  Packets belonging	to the
     same flow are then	passed to two different	objects, named pipe or queue.

     A pipe emulates a link with given bandwidth, propagation delay, queue
     size and packet loss rate.	 Packets transit through the pipe according to
     its parameters.

     A queue is	an abstraction used to implement the WF2Q+ (Worst-case Fair
     Weighted Fair Queueing) policy.  The queue	associates to each flow	a
     weight and	a reference pipe.  Then, all flows linked to the same pipe are
     scheduled at the rate fixed by the	pipe according to the WF2Q+ policy.

     The ipfw pipe configuration format	is the following:

	   pipe	number config pipe-configuration

     The ipfw queue configuration format is the	following:

	   queue number	config queue-configuration

     The following parameters can be configured	for a pipe:

     bw	bandwidth | device
	     Bandwidth,	measured in [K|M]{bit/s|Byte/s}.

	     A value of	0 (default) means unlimited bandwidth.	The unit must
	     follow immediately	the number, as in

		   ipfw	pipe 1 config bw 300Kbit/s

	     If	a device name is specified instead of a	numeric	value, then
	     the transmit clock	is supplied by the specified device.  At the
	     moment only the tun(4) device supports this functionality,	for
	     use in conjunction	with ppp(8).

     delay ms-delay
	     Propagation delay,	measured in milliseconds.  The value is
	     rounded to	the next multiple of the clock tick (typically 10ms,
	     but it is a good practice to run kernels with "options HZ=1000"
	     to	reduce the granularity to 1ms or less).	 Default value is 0,
	     meaning no	delay.

     The following parameters can be configured	for a queue:

     pipe pipe_nr
	     Connects a	queue to the specified pipe.  Multiple queues (usually
	     with different weights) can be connected to the same pipe,	which
	     specifies the aggregate rate for the set of queues.

     weight weight
	     Specifies the weight to be	used for flows matching	this queue.
	     The weight	must be	in the range 1..100, and defaults to 1.

     Finally, the following parameters can be configured for both pipes	and

     buckets hash-table-size
	     Specifies the size	of the hash table used for storing the various
	     queues.  Default value is 64 controlled by	the sysctl(8) variable
	     net.inet.ip.dummynet.hash_size, allowed range is 16 to 1024.

     mask mask-specifier
	     The dummynet(4) lets you to create	per-flow queues.  A flow iden-
	     tifier is constructed by masking the IP addresses,	ports and pro-
	     tocol types as specified in the pipe configuration.  Packets with
	     the same identifier after masking fall into the same queue.
	     Available mask specifiers are a combination of the	following:
	     dst-ip mask, src-ip mask, dst-port	mask, src-port mask, proto
	     mask or all, where	the latter means all bits in all fields	are
	     significant.  When	used within a pipe configuration, each flow is
	     assigned a	rate equal to the rate of the pipe.  When used within
	     a queue configuration, each flow is assigned a weight equal to
	     the weight	of the queue, and all flows insisting on the same pipe
	     share bandwidth proportionally to their weight.

	     When a packet is dropped by a dummynet queue or pipe, the error
	     is	normally reported to the caller	routine	in the kernel, in the
	     same way as it happens when a device queue	fills up. Setting this
	     option reports the	packet as successfully delivered, which	can be
	     needed for	some experimental setups where you want	to simulate
	     loss or congestion	at a remote router.

     plr packet-loss-rate
	     Packet loss rate.	Argument packet-loss-rate is a floating-point
	     number between 0 and 1, with 0 meaning no loss, 1 meaning 100%
	     loss.  The	loss rate is internally	represented on 31 bits.

     queue {slots | sizeKbytes}
	     Queue size, in slots or KBytes.  Default value is 50 slots, which
	     is	the typical queue size for Ethernet devices.  Note that	for
	     slow speed	links you should keep the queue	size short or your
	     traffic might be affected by a significant	queueing delay.	 E.g.,
	     50	max-sized ethernet packets (1500 bytes)	mean 600Kbit or	20s of
	     queue on a	30Kbit/s pipe.	Even worse effect can result if	you
	     get packets from an interface with	a much larger MTU, e.g.	the
	     loopback interface	with its 16KB packets.

     red | gred	w_q/min_th/max_th/max_p
	     Make use of the RED (Random Early Detection) queue	management al-
	     gorithm.  w_q and max_p are floating point	numbers	between	0 and
	     1 (0 not included), while min_th and max_th are integer numbers
	     specifying	thresholds for queue management	(thresholds are	com-
	     puted in bytes if the queue has been defined in bytes, in slots
	     otherwise).  The dummynet(4) also supports	the gentle RED variant
	     (gred).  Three sysctl(8) variables	can be used to control the RED

		     specifies the accuracy in computing the average queue
		     when the link is idle (defaults to	256, must be greater
		     than zero)

		     specifies the expected average packet size	(defaults to
		     512, must be greater than zero)

		     specifies the expected maximum packet size, only used
		     when queue	thresholds are in bytes	(defaults to 1500,
		     must be greater than zero).

     Here are some important points to consider	when designing your rules:

     o	 Remember that you filter both packets going in	and out.  Most connec-
	 tions need packets going in both directions.

     o	 Remember to test very carefully.  It is a good	idea to	be near	the
	 console when doing this.  If you cannot be near the console, use an
	 auto-recovery script such as the one in

     o	 Don't forget the loopback interface.

     o	 There are circumstances where fragmented datagrams are	uncondition-
	 ally dropped.	TCP packets are	dropped	if they	do not contain at
	 least 20 bytes	of TCP header, UDP packets are dropped if they do not
	 contain a full	8 byte UDP header, and ICMP packets are	dropped	if
	 they do not contain 4 bytes of	ICMP header, enough to specify the
	 ICMP type, code, and checksum.	 These packets are simply logged as
	 "pullup failed" since there may not be	enough good data in the	packet
	 to produce a meaningful log entry.

     o	 Another type of packet	is unconditionally dropped, a TCP packet with
	 a fragment offset of one.  This is a valid packet, but	it only	has
	 one use, to try to circumvent firewalls.  When	logging	is enabled,
	 these packets are reported as being dropped by	rule -1.

     o	 If you	are logged in over a network, loading the kld(4) version of
	 ipfw is probably not as straightforward as you	would think.  I	recom-
	 mend the following command line:

	       kldload /modules/ipfw.ko	&& \
	       ipfw add	32000 allow ip from any	to any

	 Along the same	lines, doing an

	       ipfw flush

	 in similar surroundings is also a bad idea.

     o	 The ipfw filter list may not be modified if the system	security level
	 is set	to 3 or	higher (see init(8) for	information on system security

     A divert(4) socket	bound to the specified port will receive all packets
     diverted to that port.  If	no socket is bound to the destination port, or
     if	the kernel wasn't compiled with	divert socket support, the packets are

     A set of sysctl(8)	variables controls the behaviour of the	firewall and
     associated	modules	( dummynet, bridge ).  These are shown below together
     with their	default	value (but always check	with the sysctl(8) command
     what value	is actually in use) and	meaning:

     net.inet.ip.dummynet.expire: 1
	     Lazily delete dynamic pipes/queue once they have no pending traf-
	     fic.  You can disable this	by setting the variable	to 0, in which
	     case the pipes/queues will	only be	deleted	when the threshold is

     net.inet.ip.dummynet.hash_size: 64
	     Default size of the hash table used for dynamic pipes/queues.
	     This value	is used	when no	buckets	option is specified when con-
	     figuring a	pipe/queue.

     net.inet.ip.dummynet.max_chain_len: 16
	     Target value for the maximum number of pipes/queues in a hash
	     bucket.  The product max_chain_len*hash_size is used to determine
	     the threshold over	which empty pipes/queues will be expired even
	     when net.inet.ip.dummynet.expire=0.

     net.inet.ip.dummynet.red_lookup_depth: 256

     net.inet.ip.dummynet.red_avg_pkt_size: 512

     net.inet.ip.dummynet.red_max_pkt_size: 1500
	     Parameters	used in	the computations of the	drop probability for
	     the RED algorithm.

     net.inet.ip.fw.autoinc_step: 100
	     Delta beween rule numbers when auto-generating them.  The value
	     must be in	the range 1..1000.

     net.inet.ip.fw.curr_dyn_buckets: net.inet.ip.fw.dyn_buckets
	     The current number	of buckets in the hash table for dynamic rules

     net.inet.ip.fw.debug: 1
	     Controls debugging	messages produced by ipfw.

     net.inet.ip.fw.dyn_buckets: 256
	     The number	of buckets in the hash table for dynamic rules.	 Must
	     be	a power	of 2, up to 65536.  It only takes effect when all dy-
	     namic rules have expired, so you are advised to use a flush com-
	     mand to make sure that the	hash table is resized.

     net.inet.ip.fw.dyn_count: 3
	     Current number of dynamic rules (read-only).

     net.inet.ip.fw.dyn_keepalive: 1
	     Enables generation	of keepalive packets for keep-state rules on
	     TCP sessions. A keepalive is generated to both sides of the con-
	     nection every 5 seconds for the last 20 seconds of	the lifetime
	     of	the rule.

     net.inet.ip.fw.dyn_max: 8192
	     Maximum number of dynamic rules.  When you	hit this limit,	no
	     more dynamic rules	can be installed until old ones	expire.

     net.inet.ip.fw.dyn_ack_lifetime: 300

     net.inet.ip.fw.dyn_syn_lifetime: 20

     net.inet.ip.fw.dyn_fin_lifetime: 1

     net.inet.ip.fw.dyn_rst_lifetime: 1

     net.inet.ip.fw.dyn_udp_lifetime: 5

     net.inet.ip.fw.dyn_short_lifetime:	30
	     These variables control the lifetime, in seconds, of dynamic
	     rules.  Upon the initial SYN exchange the lifetime	is kept	short,
	     then increased after both SYN have	been seen, then	decreased
	     again during the final FIN	exchange or when a RST is received.
	     Both dyn_fin_lifetime and dyn_rst_lifetime	must be	strictly lower
	     than 5 seconds, the period	of repetition of keepalives. The fire-
	     wall enforces that.

     net.inet.ip.fw.enable: 1
	     Enables the firewall.  Setting this variable to 0 lets you	run
	     your machine without firewall even	if compiled in.

     net.inet.ip.fw.one_pass: 1
	     When set, the packet exiting from the dummynet(4) pipe is not
	     passed though the firewall	again.	Otherwise, after a pipe	ac-
	     tion, the packet is reinjected into the firewall at the next

	     Note: bridged and layer 2 packets coming out of a pipe are	never
	     reinjected	in the firewall	irrespective of	the value of this

     net.inet.ip.fw.verbose: 1
	     Enables verbose messages.

     net.inet.ip.fw.verbose_limit: 0
	     Limits the	number of messages produced by a verbose firewall. 0
	     Controls whether layer-2 packets are passed to ipfw.  Default is
	     no. 0
	     Controls whether bridged packets are passed to ipfw.  Default is

     This Section lists	the features that have been introduced in ipfw2	and
     were not present in ipfw1.	 We list them in order of the potential	impact
     that they can have	in writing your	rulesets.  You might want to consider
     using these features in order to write your rulesets in a more efficient

     Handling of non-IPv4 packets
	     ipfw1 will	silently accept	all non-IPv4 packets (which ipfw1 will
	     only see when  ipfw2 will filter
	     all packets (including non-IPv4 ones) according to	the ruleset.
	     To	achieve	the same behaviour as ipfw1 you	can use	the following
	     as	the very first rule in your ruleset:

		   ipfw	add 1 allow layer2 not mac-type	ip

	     The layer2	options	might seem redundant, but it is	necessary --
	     packets passed to the firewall from layer3	will not have a	MAC
	     header, so	the mac-type ip	pattern	will always fail on them, and
	     the not operator will make	this rule into a pass-all.

     Address sets
	     ipfw1 does	not supports address sets (those in the	form
	     addr/masklen{num,num,...} ).

	     A minor difference	between	ipfw1 and ipfw2	is that	the former al-
	     lows addresses to be specified as ipno:mask where the mask	can be
	     an	arbitrary bitmask instead of a countiguous set of bits.	 ipfw2
	     no	longer supports	this syntax though it would be trivial to
	     reintroduce it as it is supported on the kernel side.

     Port specifications
	     ipfw1 only	allows one port	range when specifying TCP and UDP
	     ports, and	is limited to 10 entries instead of the	15 allowed by
	     ipfw2.  Also, in ipfw1 you	can only specify ports when the	rule
	     is	requesting tcp or udp packets. With ipfw2 you can put port
	     specifications in rules matching all packets, and the match will
	     be	attempted only on those	packets	carrying protocols which in-
	     clude port	identifiers.

	     Finally, ipfw1 allowed the	first port entry to be specified as
	     port:mask where mask can be an arbitrary 16-bit mask.  This syn-
	     tax is of questionable usefulness and it is not supported anymore
	     in	ipfw2.

	     ipfw1 does	not support Or-blocks.

	     ipfw1 does	not generate keepalives	for stateful sessions.	As a
	     consequence, it might cause idle sessions to drop because the
	     lifetime of the dynamic rules expires.

     Sets of rules
	     ipfw1 does	not implement sets of rules.

     MAC header	filtering and Layer-2 firewalling.
	     ipfw1 does	not implement filtering	on MAC header fields, nor it
	     is	invoked	on packets from	ether_demux() and
	     ether_output_frame(). The sysctl variable has
	     no	effect there.

	     The following options are not supported in	ipfw1

	     dst-ip, dst-port, layer2, mac, mac-type, src-ip, src-port.

	     Additionally, the following options are not supported in ipfw1
	     (RELENG_4)	rules:

	     ipid, iplen, ipprecedence,	iptos, ipttl, ipversion, .Cm tcpack,
	     tcpseq, tcpwin.

     Dummynet options
	     The following option for dummynet pipes/queues is not supported:

     There are far too many possible uses of ipfw so this Section will only
     give a small set of examples.

     This command adds an entry	which denies all tcp packets from to the telnet port of from	being for-
     warded by the host:

	   ipfw	add deny tcp from to telnet

     This one disallows	any connection from the	entire crackers	network	to my

	   ipfw	add deny ip from	to

     A first and efficient way to limit	access (not using dynamic rules) is
     the use of	the following rules:

	   ipfw	add allow tcp from any to any established
	   ipfw	add allow tcp from net1	portlist1 to net2 portlist2 setup
	   ipfw	add allow tcp from net3	portlist3 to net3 portlist3 setup
	   ipfw	add deny tcp from any to any

     The first rule will be a quick match for normal TCP packets, but it will
     not match the initial SYN packet, which will be matched by	the setup
     rules only	for selected source/destination	pairs.	All other SYN packets
     will be rejected by the final deny	rule.

     If	you administer one or more subnets, you	can take advantage of the
     ipfw2 syntax to specify address sets and or-blocks	and write extremely
     compact rulesets which selectively	enable services	to blocks of clients,
     as	below:

	   goodguys="{{20,35,66,18}	or{6,3,11} }"

	   ipfw	add allow ip from ${goodguys} to any
	   ipfw	add deny ip from ${badguys} to any
	   ... normal policies ...

     The ipfw1 syntax would require a separate rule for	each IP	in the above

     In	order to protect a site	from flood attacks involving fake TCP packets,
     it	is safer to use	dynamic	rules:

	   ipfw	add check-state
	   ipfw	add deny tcp from any to any established
	   ipfw	add allow tcp from my-net to any setup keep-state

     This will let the firewall	install	dynamic	rules only for those connec-
     tion which	start with a regular SYN packet	coming from the	inside of our
     network.  Dynamic rules are checked when encountering the first
     check-state or keep-state rule.  A	check-state rule should	be usually
     placed near the beginning of the ruleset to minimize the amount of	work
     scanning the ruleset.  Your mileage may vary.

     To	limit the number of connections	a user can open	you can	use the	fol-
     lowing type of rules:

	   ipfw	add allow tcp from my-net/24 to	any setup limit	src-addr 10
	   ipfw	add allow tcp from any to me setup limit src-addr 4

     The former	(assuming it runs on a gateway)	will allow each	host on	a /24
     network to	open at	most 10	TCP connections.  The latter can be placed on
     a server to make sure that	a single client	does not use more than 4 si-
     multaneous	connections.

     BEWARE: stateful rules can	be subject to denial-of-service	attacks	by a
     SYN-flood which opens a huge number of dynamic rules.  The	effects	of
     such attacks can be partially limited by acting on	a set of sysctl(8)
     variables which control the operation of the firewall.

     Here is a good usage of the list command to see accounting	records	and
     timestamp information:

	   ipfw	-at list

     or	in short form without timestamps:

	   ipfw	-a list

     which is equivalent to:

	   ipfw	show

     Next rule diverts all incoming packets from	to divert port

	   ipfw	divert 5000 ip from to any in

     The following rules show some of the applications of ipfw and dummynet(4)
     for simulations and the like.

     This rule drops random incoming packets with a probability	of 5%:

	   ipfw	add prob 0.05 deny ip from any to any in

     A similar effect can be achieved making use of dummynet pipes:

	   ipfw	add pipe 10 ip from any	to any
	   ipfw	pipe 10	config plr 0.05

     We	can use	pipes to artificially limit bandwidth, e.g. on a machine act-
     ing as a router, if we want to limit traffic from local clients on we do:

	   ipfw	add pipe 1 ip from to any out
	   ipfw	pipe 1 config bw 300Kbit/s queue 50KBytes

     note that we use the out modifier so that the rule	is not used twice.
     Remember in fact that ipfw	rules are checked both on incoming and outgo-
     ing packets.

     Should we like to simulate	a bidirectional	link with bandwidth limita-
     tions, the	correct	way is the following:

	   ipfw	add pipe 1 ip from any to any out
	   ipfw	add pipe 2 ip from any to any in
	   ipfw	pipe 1 config bw 64Kbit/s queue	10Kbytes
	   ipfw	pipe 2 config bw 64Kbit/s queue	10Kbytes

     The above can be very useful, e.g.	if you want to see how your fancy Web
     page will look for	a residential user which is connected only through a
     slow link.	 You should not	use only one pipe for both directions, unless
     you want to simulate a half-duplex	medium (e.g. AppleTalk,	Ethernet,
     IRDA).  It	is not necessary that both pipes have the same configuration,
     so	we can also simulate asymmetric	links.

     Should we like to verify network performance with the RED queue manage-
     ment algorithm:

	   ipfw	add pipe 1 ip from any to any
	   ipfw	pipe 1 config bw 500Kbit/s queue 100 red 0.002/30/80/0.1

     Another typical application of the	traffic	shaper is to introduce some
     delay in the communication.  This can affect a lot	applications which do
     a lot of Remote Procedure Calls, and where	the round-trip-time of the
     connection	often becomes a	limiting factor	much more than bandwidth:

	   ipfw	add pipe 1 ip from any to any out
	   ipfw	add pipe 2 ip from any to any in
	   ipfw	pipe 1 config delay 250ms bw 1Mbit/s
	   ipfw	pipe 2 config delay 250ms bw 1Mbit/s

     Per-flow queueing can be useful for a variety of purposes.	 A very	simple
     one is counting traffic:

	   ipfw	add pipe 1 tcp from any	to any
	   ipfw	add pipe 1 udp from any	to any
	   ipfw	add pipe 1 ip from any to any
	   ipfw	pipe 1 config mask all

     The above set of rules will create	queues (and collect statistics)	for
     all traffic.  Because the pipes have no limitations, the only effect is
     collecting	statistics.  Note that we need 3 rules,	not just the last one,
     because when ipfw tries to	match IP packets it will not consider ports,
     so	we would not see connections on	separate ports as different ones.

     A more sophisticated example is limiting the outbound traffic on a	net
     with per-host limits, rather than per-network limits:

	   ipfw	add pipe 1 ip from to any out
	   ipfw	add pipe 2 ip from any to in
	   ipfw	pipe 1 config mask src-ip 0x000000ff bw	200Kbit/s queue
	   ipfw	pipe 2 config mask dst-ip 0x000000ff bw	200Kbit/s queue

     To	add a set of rules atomically, e.g. set	18:

	   ipfw	disable	set 18
	   ipfw	add NN set 18 ...	  # repeat as needed
	   ipfw	enable set 18

     To	delete a set of	rules atomically the command is	simply:

	   ipfw	delete set 18

     To	test a ruleset and disable it and regain control if something goes

	   ipfw	disable	set 18
	   ipfw	add NN set 18 ...	  # repeat as needed
	   ipfw	enable set 18 ;	echo done; sleep 30 && ipfw disable set	18

     Here if everything	goes well, you press control-C before the "sleep" ter-
     minates, and your ruleset will be left active. Otherwise, e.g. if you
     cannot access your	box, the ruleset will be disabled after	the sleep ter-
     minates thus restoring the	previous situation.

     cpp(1), m4(1), bridge(4), divert(4), dummynet(4), ip(4), ipfirewall(4),
     protocols(5), services(5),	init(8), kldload(8), reboot(8),	sysctl(8),

     S.	Floyd and V. Jacobson, Random Early Detection gateways for Congestion
     Avoidance,	August 1993.

     B.	Braden,	D. Clark, J. Crowcroft,	B. Davie, S. Deering, D. Estrin, S.
     Floyd, V. Jacobson, G. Minshall, C. Partridge, L. Peterson, K.
     Ramakrishnan, S. Shenker, J. Wroclawski, and L. Zhang, Recommendations on
     Queue Management and Congestion Avoidance in the Internet,	April 1998,
     RFC 2309.

     The syntax	has grown over the years and sometimes it might	be confusing.
     Unfortunately, backward compatibility prevents cleaning up	mistakes done
     in	the definition of the syntax.

     !!! WARNING !!!

     Misconfiguring the	firewall can put your computer in an unusable state,
     possibly shutting down network services and requiring console access to
     regain control to it.

     Incoming packet fragments diverted	by divert or tee are reassembled be-
     fore delivery to the socket.  The action used on those packet is the one
     from the rule which matches the first fragment of the packet.

     Packets that match	a tee rule should not be immediately accepted, but
     should continue going through the rule list.  This	may be fixed in	a
     later version.

     Packets diverted to userland, and then reinserted by a userland process
     (such as natd(8)) will lose various packet	attributes, including their
     source interface.	If a packet is reinserted in this manner, later	rules
     may be incorrectly	applied, making	the order of divert rules in the rule
     sequence very important.

     Ugen J. S.	Antsilevich,
     Poul-Henning Kamp,
     Alex Nash,
     Archie Cobbs,
     Luigi Rizzo.

     API based upon code written by Daniel Boulet for BSDI.

     Work on dummynet(4) traffic shaper	supported by Akamba Corp.

     The ipfw utility first appeared in	FreeBSD	2.0.  dummynet(4) was intro-
     duced in FreeBSD 2.2.8.  Stateful extensions were introduced in
     FreeBSD 4.0.  ipfw2 was introduced	in Summer 2002.

BSD				August 13, 2002				   BSD


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