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

     ipfw -- IP	firewall and traffic shaper control program

     ipfw [-q] [-p preproc [-D macro[=value]] [-U macro]] pathname
     ipfw [-f |	-q] flush
     ipfw [-q] {zero | resetlog	| delete} [number ...]
     ipfw [-s [field]] [-adeftN] {list | show} [number ...]
     ipfw [-q] add [number] rule-body
     ipfw pipe number config pipe-config-options
     ipfw pipe {delete | list |	show} [number ...]
     ipfw queue	number config queue-config-options
     ipfw queue	{delete	| list | show} [number ...]

     ipfw is the user interface	for controlling	the ipfirewall(4) and the
     dummynet(4) traffic shaper	in FreeBSD.

     A firewall	configuration is made of a list	of numbered rules, which is
     scanned for each incoming or outgoing IP packet until a match is found
     and the relevant action is	performed.  Depending on the action and	cer-
     tain system settings, packets can be reinjected into the firewall at the
     rule after	the matching one for further processing.  All rules apply to
     all interfaces, so	it is responsibility of	the system administrator to
     write the ruleset in such a way as	to minimize the	number of checks.

     A configuration 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 or keep-state rule, and are typically
     used to open the firewall on-demand to legitimate traffic only.  See the
     RULE FORMAT and EXAMPLES sections below for more information 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 with	the
     delete command, and globally with the flush command; displayed, option-
     ally with the content of the counters, using the show and list commands.
     Finally, counters can be reset with the zero and resetlog commands.

     The following options are available:

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

     -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.  Note, if there is	no tty associated with
	     the process, this is implied.

     -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
	     cannot be delivered to the	login session.	This causes the	remote
	     login session to be closed	and the	remainder of the ruleset is
	     not processed.  Access to the console is required to recover.

     -t	     While listing, show last match timestamp.

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

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

     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	filesystems 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 commands are	used to	configure the traffic shaper, as shown
     in	the TRAFFIC SHAPER CONFIGURATION section below.

     The ipfw rule format is the following:

     [prob match_probability] action [log [logamount number]] proto from src
     to	dst [interface-spec] [options]

     Each packet can be	filtered based on the following	information that is
     associated	with it:

	   Transmit and	receive	interface     (by name or address)
	   Direction			      (incoming	or outgoing)
	   Source and destination IP address  (possibly	masked)
	   Protocol			      (TCP, UDP, ICMP, etc.)
	   Source and destination port	      (lists, ranges or	masks)
	   TCP flags
	   IP fragment flag
	   IP options
	   ICMP	types
	   User/group ID of the	socket associated with the packet

     Note that it may be dangerous to filter on	the source IP address or
     source TCP/UDP port because either	or both	could easily be	spoofed.

     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.


	     allow   Allow packets that	match rule.  The search	terminates.
		     Aliases are pass, permit and accept.

	     deny    Discard packets that match	this rule.  The	search termi-
		     nates.  drop is an	alias for deny.

	     reject  (Deprecated).  Discard packets that match this rule, and
		     try to send an ICMP host unreachable notice.  The search

	     unreach code
		     Discard packets that match	this rule, and try to send an
		     ICMP unreachable 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.

	     reset   TCP packets only.	Discard	packets	that match this	rule,
		     and try to	send a TCP reset (RST) notice.	The search

	     count   Update counters for all packets that match	rule.  The
		     search continues with the next rule.

		     Checks the	packet against the dynamic ruleset.  If	a
		     match is found then the search terminates,	otherwise we
		     move to the next rule.  If	no check-state rule is found,
		     the dynamic ruleset is checked at the first keep-state

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

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

	     fwd ipaddr[,port]
		     Change the	next-hop on matching packets to	ipaddr,	which
		     can be an IP address in dotted quad or a host name.  If
		     ipaddr is not a directly-reachable	address, the route as
		     found in the local	routing	table for that IP is used in-
		     stead.  If	ipaddr is a local address, then	on a packet
		     entering the system from a	remote host it will be di-
		     verted to port on the local machine, keeping the local
		     address of	the socket set to the original IP address the
		     packet was	destined for.  This is intended	for use	with
		     transparent proxy servers.	 If the	IP is not a local ad-
		     dress then	the port number	(if specified) is ignored and
		     the rule only applies to packets leaving the system.
		     This will also map	addresses to local ports when packets
		     are generated locally.  The search	terminates if this
		     rule matches.  If the port	number is not given then the
		     port number in the	packet is used,	so that	a packet for
		     an	external machine port Y	would be forwarded to local
		     port Y.  The kernel must have been	compiled with the

	     pipe pipe_nr
		     Pass packet to a dummynet(4) "pipe" (for bandwidth	limi-
		     tation, 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 limi-
		     tation using WF2Q).

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

     log [logamount number]
	     If	the kernel was compiled	with IPFIREWALL_VERBOSE, then when a
	     packet matches a rule with	the log	keyword	a message will be
	     logged to syslogd(8) with a LOG_SECURITY facility.	 Note: by de-
	     fault, they are appended to the /var/log/security file (see
	     syslog.conf(5)).  If the kernel was compiled with the
	     IPFIREWALL_VERBOSE_LIMIT option, then by default logging will
	     cease after the number of packets specified by the	option are re-
	     ceived for	that particular	chain entry, and
	     net.inet.ip.fw.verbose_limit will be set to that number.  How-
	     ever, if logamount	number is used,	that number will be the	log-
	     ging limit	rather than net.inet.ip.fw.verbose_limit, where	the
	     value "0" removes the logging limit.  Logging may then be re-en-
	     abled by clearing the logging counter or the packet counter for
	     that entry.

	     Console logging and the log limit are adjustable dynamically
	     through the sysctl(8) interface in	the MIB	base of

     proto   An	IP protocol specified by number	or name	(for a complete	list
	     see /etc/protocols).  The ip or all keywords mean any protocol
	     will match.

     src and dst:
	     any | me |	[not] <address/mask> [ports]

	     Specifying	any makes the rule match any IP	address.

	     Specifying	me makes the rule match	any IP address configured on
	     an	interface in the system.

	     The <address/mask>	may be specified as:

	     ipno	An IP number of	the form  Only	this exact IP
			number will match the rule.

	     ipno/bits	An IP number with a mask width of the form
			In this	case all IP numbers from to
			will match.

	     ipno:mask	An IP number with a mask of the	form	In this	case all IP numbers
			from to will	match.

	     The sense of the match can	be inverted by preceding an address
	     with the not modifier, causing all	other addresses	to be matched
	     instead.  This does not affect the	selection of port numbers.

	     With the TCP and UDP protocols, optional ports may	be specified


	     The `-' notation specifies	a range	of ports (including bound-

	     The `:' notation specifies	a port and a mask, a match is declared
	     if	the port number	in the packet matches the one in the rule,
	     limited to	the bits which are set in the mask.

	     Service names (from /etc/services)	may be used instead of numeric
	     port values.  A range may only be specified as the	first value,
	     and the length of the port	list is	limited	to IP_FW_MAX_PORTS
	     ports (as defined in /usr/src/sys/netinet/ip_fw.h).  A backslash
	     (`\') can be used to escape the dash (`-')	character in a service

		   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.

	     Some combinations of the following	specifiers are allowed:

	     in	       Only match incoming packets.

	     out       Only match outgoing packets.

	     via ifX   Packet must be going through interface ifX.

	     via if*   Packet must be going through interface ifX, where X is
		       any unit	number.

	     via any   Packet must be going through some interface.

	     via ipno  Packet must be going through the	interface having IP
		       address ipno.

	     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 100	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
	     used.  Specifying via together with xmit or recv is invalid.

	     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.


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

	     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

		     Matches only bridged packets.  This can be	useful for
		     multicast or broadcast traffic, which would otherwise
		     pass through the firewall twice: once during bridging,
		     and a second time when the	packet is delivered to the lo-
		     cal stack.

		     Apart from	a small	performance penalty, this would	be a
		     problem when using	pipes because the same packet would be
		     accounted for twice in terms of bandwidth,	queue occupa-
		     tion, and also counters.

	     frag    Match if the packet is a fragment and this	is not the
		     first fragment of the datagram.  frag may not be used in
		     conjunction with either tcpflags or TCP/UDP port specifi-

	     ipoptions spec
		     Match if the IP header contains the comma separated list
		     of	options	specified in spec.  The	supported IP options

		     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 `!'.

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

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

		     TCP packets only.	Match packets that have	the RST	or ACK
		     bits set.

	     setup   TCP packets only.	Match packets that have	the SYN	bit
		     set but no	ACK bit.

	     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	par-
		     ticular flag may be denoted with a	`!'.  A	rule which
		     contains a	tcpflags specification can never match a frag-
		     mented packet which has a non-zero	offset.	 See the frag
		     option for	details	on matching fragmented packets.

	     icmptypes types
		     ICMP packets only.	 Match if the ICMP type	is in the list
		     types.  The list may be specified as any combination of
		     ranges or individual types	separated by commas.  The sup-
		     ported ICMP types are:

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

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

	     gid group
		     Match all TCP or UDP packets sent by or received for a
		     group.  A group may be matched by name or identification

     The ipfw utility is also the user interface for the dummynet(4) traffic
     shaper.  The shaper operates by dividing packets into flows according to
     a user-specified mask on different	fields of the IP header.  Packets be-
     longing 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+ 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 ac-
     cording to	the WF2Q+ policy.

     The ipfw pipe configuration format	is the following:

     pipe number config	[bw bandwidth |	device]	[delay ms-delay] [queue	{slots
     | size}] [plr loss-probability] [mask mask-specifier] [buckets
     hash-table-size] [red | gred w_q/min_th/max_th/max_p]

     The ipfw queue configuration format is the	following:

     queue number config [pipe pipe_nr]	[weight	weight]	[queue {slots |	size}]
     [plr loss-probability] [mask mask-specifier] [buckets hash-table-size]
     [red | gred w_q/min_th/max_th/max_p]

     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 queue 50KBytes

	     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.

     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.

     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.

     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.

     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.

     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.

     red | gred	w_q/min_th/max_th/max_p
	     Make use of the RED queue management algorithm.  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 computed 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	behaviour:

		     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 is one kind of packet that the firewall will always discard,
	 that is a TCP packet's	fragment 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.
     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.fw.debug: 1
	     Controls debugging	messages produced by ipfw.

     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

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

     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.verbose_limit: 0
	     Limits the	number of messages produced by a verbose firewall.

     net.inet.ip.fw.dyn_buckets: 256

     net.inet.ip.fw.curr_dyn_buckets: 256
	     The configured and	current	size of	the hash table used to hold
	     dynamic rules.  This must be a power of 2.	 The table can only be
	     resized when empty, so in order to	resize it on the fly you will
	     probably have to flush and	reload the ruleset.

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

     net.inet.ip.fw.dyn_max: 1000
	     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

     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.

     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

     The number	of times a packet is processed by ipfw varies -- basically,
     ipfw is invoked every time	the kernel functions ip_input(), ip_output()
     and bdg_forward() are invoked.  This means	that packets are processed
     once for connections having only one endpoint on the local	host, twice
     for connections with both endpoints on the	local host, or for packet
     routed by the host	(acting	as a gateway), and once	for packets bridged by
     the host (acting as a bridge).

     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),

     The syntax	has grown over the years and it	is not very clean.


     This program can put your computer	in rather unusable state.  When	using
     it	for the	first time, work on the	console	of the computer, and do	NOT do
     anything you don't	understand.

     When manipulating/adding chain entries, service and protocol names	are
     not accepted.

     Incoming packet fragments diverted	by divert or tee are reassembled be-
     fore delivery to the socket.

     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.

     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.

BSD				 May 31, 2001				   BSD


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