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

NAME
     ipfw -- User interface for	firewall, traffic shaper, packet scheduler,
     in-kernel NAT.

SYNOPSIS
   FIREWALL CONFIGURATION
     ipfw [-cq]	add rule
     ipfw [-acdefnNStT]	[set N]	{list |	show} [rule | first-last ...]
     ipfw [-f |	-q] [set N] flush
     ipfw [-q] [set N] {delete | zero |	resetlog} [number ...]

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

	SYSCTL SHORTCUTS
     ipfw enable
	  {firewall | altq | one_pass |	debug |	verbose	| dyn_keepalive}
     ipfw disable
	  {firewall | altq | one_pass |	debug |	verbose	| dyn_keepalive}

	LOOKUP TABLES
     ipfw table	number add addr[/masklen] [value]
     ipfw table	number delete addr[/masklen]
     ipfw table	{number	| all} flush
     ipfw table	{number	| all} list

	DUMMYNET CONFIGURATION (TRAFFIC	SHAPER AND PACKET SCHEDULER)
     ipfw {pipe	| queue	| sched} number	config config-options
     ipfw [-s [field]] {pipe | queue | sched} {delete |	list | show}
	  [number ...]

	IN-KERNEL NAT
     ipfw [-q] nat number config config-options

     ipfw [-cfnNqS] [-p	preproc	[preproc-flags]] pathname

DESCRIPTION
     The ipfw utility is the user interface for	controlling the	ipfw(4)	fire-
     wall, the dummynet(4) traffic shaper/packet scheduler, and	the in-kernel
     NAT services.

     A firewall	configuration, or ruleset, is made of a	list of	rules numbered
     from 1 to 65535.  Packets are passed to the firewall from a number	of
     different places in the protocol stack (depending on the source and des-
     tination of the packet, it	is possible for	the firewall to	be invoked
     multiple times on the same	packet).  The packet passed to the firewall is
     compared against each of the rules	in the ruleset,	in rule-number order
     (multiple rules with the same number are permitted, in which case they
     are processed in order of insertion).  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
     processing.

     A ruleset always includes a default rule (numbered	65535) which cannot be
     modified or deleted, 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, the firewall will have a stateful behaviour, i.e.,	upon a match
     it	will create dynamic rules, i.e.	rules that match packets with the same
     5-tuple (protocol,	source and destination addresses and ports) as the
     packet which caused their creation.  Dynamic rules, which have a limited
     lifetime, are checked at the first	occurrence of a	check-state,
     keep-state	or limit rule, and are typically used to open the firewall on-
     demand to legitimate traffic only.	 See the STATEFUL FIREWALL 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
     commands.

     Each rule belongs to one of 32 different sets , and there are ipfw	com-
     mands 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.

     Rules can be added	with the add command; deleted individually or in
     groups with the delete command, and globally (except those	in set 31)
     with the flush command; displayed,	optionally with	the content of the
     counters, using the show and list commands.  Finally, counters can	be
     reset with	the zero and resetlog commands.

   COMMAND OPTIONS
     The following general options are available when invoking ipfw:

     -a	     Show counter values when listing rules.  The show command implies
	     this option.

     -b	     Only show the action and the comment, not the body	of a rule.
	     Implies -c.

     -c	     When entering or showing rules, print them	in compact form, i.e.,
	     omitting the "ip from any to any" string when this	does not carry
	     any additional information.

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

     -e	     When listing and -d is specified, also show expired dynamic
	     rules.

     -f	     Do	not 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.

     -i	     When listing a table (see the LOOKUP TABLES section below for
	     more information on lookup	tables), format	values as IP
	     addresses.	By default, values are shown as	integers.

     -n	     Only check	syntax of the command strings, without actually	pass-
	     ing them to the kernel.

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

     -q	     Be	quiet when executing the add, nat, zero, resetlog or flush
	     commands; (implies	-f).  This is useful when updating rulesets by
	     executing multiple	ipfw commands in a script (e.g.,
	     `sh /etc/rc.firewall'), or	by processing a	file with many ipfw
	     rules across a remote login session.  It also stops a table add
	     or	delete from failing if the entry already exists	or is not
	     present.

	     The reason	why this option	may be important is that for some of
	     these actions, ipfw may print a message; if the action results in
	     blocking the traffic to the remote	client,	the remote login ses-
	     sion will be closed and the rest of the ruleset will not be pro-
	     cessed.  Access to	the console would then be required to recover.

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

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

     -t	     When listing, show	last match timestamp converted with ctime().

     -T	     When listing, show	last match timestamp as	seconds	from the
	     epoch.  This form can be more convenient for postprocessing by
	     scripts.

   LIST	OF RULES AND PREPROCESSING
     To	ease configuration, rules can be put into a file which is processed
     using ipfw	as shown in the	last synopsis line.  An	absolute pathname must
     be	used.  The file	will be	read line by line and applied as arguments 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	does not 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, any additional	arguments are passed on	to the pre-
     processor for interpretation.  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.

   TRAFFIC SHAPER CONFIGURATION
     The ipfw pipe, queue and sched commands are used to configure the traffic
     shaper and	packet scheduler.  See the TRAFFIC SHAPER (DUMMYNET)
     CONFIGURATION Section below for details.

     If	the world and the kernel get out of sync the ipfw ABI may break, pre-
     venting you from being able to add	any rules.  This can adversely effect
     the booting process.  You can use ipfw disable firewall to	temporarily
     disable the firewall to regain access to the network, allowing you	to fix
     the problem.

PACKET FLOW
     A packet is checked against the active ruleset in multiple	places in the
     protocol stack, under control of several sysctl variables.	 These places
     and variables are shown below, and	it is important	to have	this picture
     in	mind in	order to design	a correct ruleset.

		  ^    to upper	layers	  V
		  |			  |
		  +----------->-----------+
		  ^			  V
	    [ip(6)_input]	    [ip(6)_output]     net.inet(6).ip(6).fw.enable=1
		  |			  |
		  ^			  V
	    [ether_demux]	 [ether_output_frame]  net.link.ether.ipfw=1
		  |			  |
		  +-->--[bdg_forward]-->--+	       net.link.bridge.ipfw=1
		  ^			  V
		  |	 to devices	  |

     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.

     Note that as packets flow through the stack, headers can be stripped or
     added to it, and so they may or may not be	available for inspection.
     E.g., 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 ip_input() or ip6_input().

     Also note that each packet	is always checked against the complete rule-
     set, irrespective of the place where the check occurs, or the source of
     the packet.  If a rule contains some match	patterns or actions which are
     not valid for the place of	invocation (e.g. trying	to match a MAC header
     within ip_input or	ip6_input ), the match pattern will not	match, but a
     not operator in front of such patterns will cause the pattern to always
     match on those packets.  It is thus the responsibility of the programmer,
     if	necessary, to write a suitable ruleset to differentiate	among the pos-
     sible 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).

SYNTAX
     In	general, each keyword or argument must be provided as a	separate com-
     mand line argument, with no leading or trailing spaces.  Keywords are
     case-sensitive, whereas arguments may or may not be case-sensitive
     depending on their	nature (e.g. uid's are,	hostnames are not).

     Some arguments (e.g. port or address lists) are comma-separated lists of
     values.  In this case, spaces after commas	',' are	allowed	to make	the
     line more readable.  You can also put the entire command (including
     flags) into a single argument.  E.g., the following forms are equivalent:

	   ipfw	-q add deny src-ip 10.0.0.0/24,127.0.0.1/8
	   ipfw	-q add deny src-ip 10.0.0.0/24,	127.0.0.1/8
	   ipfw	"-q add	deny src-ip 10.0.0.0/24, 127.0.0.1/8"

RULE FORMAT
     The format	of firewall rules is the following:

	   [rule_number] [set set_number] [prob	match_probability] action
	   [log	[logamount number]] [altq queue] [{tag | untag}	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 and IPv6 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
	IPv6 Extension headers		      Fragmentation, Hop-by-Hop
					      options, Routing Headers,	Source
					      routing rthdr0, Mobile IPv6
					      rthdr2, IPSec options.
	IPv6 Flow-ID
	Misc. TCP header fields		      TCP flags	(SYN, FIN, ACK,	RST,
					      etc.), sequence number, acknowl-
					      edgment number, window
	TCP options
	ICMP types			      for ICMP packets
	ICMP6 types			      for ICMP6	packets
	User/group ID			      When the packet can be associ-
					      ated with	a local	socket.
	Divert status			      Whether a	packet came from a
					      divert socket (e.g., natd(8)).
	Fib annotation state		      Whether a	packet has been	tagged
					      for using	a specific FIB (rout-
					      ing table) in future forwarding
					      decisions.

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

     rule_number
	     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 number of the last non-default value is used
	     instead.

     set set_number
	     Each rule is associated with a set_number in the range 0..31.
	     Sets can be individually 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.
	     Set 31 is special in that it cannot be disabled, and rules	in set
	     31	are not	deleted	by the ipfw flush command (but you can delete
	     them with the ipfw	delete set 31 command).	 Set 31	is also	used
	     for the default rule.

     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) to simulate	the effect of multiple paths leading
	     to	out-of-order packet delivery.

	     Note: this	condition is checked before any	other condition,
	     including ones such as keep-state or check-state which might have
	     side effects.

     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 the 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.

	     Note: logging is done after all other packet matching conditions
	     have been successfully verified, and before performing the	final
	     action (accept, deny, etc.) on the	packet.

     tag number
	     When a packet matches a rule with the tag keyword,	the numeric
	     tag for the given number in the range 1..65534 will be attached
	     to	the packet.  The tag acts as an	internal marker	(it is not
	     sent out over the wire) that can be used to identify these	pack-
	     ets later on.  This can be	used, for example, to provide trust
	     between interfaces	and to start doing policy-based	filtering.  A
	     packet can	have multiple tags at the same time.  Tags are
	     "sticky", meaning once a tag is applied to	a packet by a matching
	     rule it exists until explicit removal.  Tags are kept with	the
	     packet everywhere within the kernel, but are lost when packet
	     leaves the	kernel,	for example, on	transmitting packet out	to the
	     network or	sending	packet to a divert(4) socket.

	     To	check for previously applied tags, use the tagged rule option.
	     To	delete previously applied tag, use the untag keyword.

	     Note: since tags are kept with the	packet everywhere in ker-
	     nelspace, they can	be set and unset anywhere in the kernel	net-
	     work subsystem (using the mbuf_tags(9) facility), not only	by
	     means of the ipfw(4) tag and untag	keywords.  For example,	there
	     can be a specialized netgraph(4) node doing traffic analyzing and
	     tagging for later inspecting in firewall.

     untag number
	     When a packet matches a rule with the untag keyword, the tag with
	     the number	number is searched among the tags attached to this
	     packet and, if found, removed from	it.  Other tags	bound to
	     packet, if	present, are left untouched.

     altq queue
	     When a packet matches a rule with the altq	keyword, the ALTQ
	     identifier	for the	given queue (see altq(4)) will be attached.
	     Note that this ALTQ tag is	only meaningful	for packets going
	     "out" of IPFW, and	not being rejected or going to divert sockets.
	     Note that if there	is insufficient	memory at the time the packet
	     is	processed, it will not be tagged, so it	is wise	to make	your
	     ALTQ "default" queue policy account for this.  If multiple	altq
	     rules match a single packet, only the first one adds the ALTQ
	     classification tag.  In doing so, traffic may be shaped by	using
	     count altq	queue rules for	classification early in	the ruleset,
	     then later	applying the filtering decision.  For example,
	     check-state and keep-state	rules may come later and provide the
	     actual filtering decisions	in addition to the fallback ALTQ tag.

	     You must run pfctl(8) to set up the queues	before IPFW will be
	     able to look them up by name, and if the ALTQ disciplines are
	     rearranged, the rules in containing the queue identifiers in the
	     kernel will likely	have gone stale	and need to be reloaded.
	     Stale queue identifiers will probably result in misclassifica-
	     tion.

	     All system	ALTQ processing	can be turned on or off	via ipfw
	     enable altq and ipfw disable altq.	 The usage of
	     net.inet.ip.fw.one_pass is	irrelevant to ALTQ traffic shaping, as
	     the actual	rule action is followed	always after adding an ALTQ
	     tag.

   RULE	ACTIONS
     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.

     check-state
	     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 | tablearg[,port]
	     Change the	next-hop on matching packets to	ipaddr,	which can be
	     an	IP address or a	host name.  The	next hop can also be supplied
	     by	the last table looked up for the packet	by using the tablearg
	     keyword instead of	an explicit address.  The search terminates 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.

	     To	enable fwd a custom kernel needs to be compiled	with the
	     option options IPFIREWALL_FORWARD.

     nat nat_nr
	     Pass packet to a nat instance (for	network	address	translation,
	     address redirect, etc.): see the NETWORK ADDRESS TRANSLATION
	     (NAT) Section for further information.

     pipe pipe_nr
	     Pass packet to a dummynet ``pipe''	(for bandwidth limitation,
	     delay, etc.).  See	the TRAFFIC SHAPER (DUMMYNET) 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 ``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-
	     nates.

     reset6  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-
	     nates.

     skipto number | tablearg
	     Skip all subsequent rules numbered	less than number.  The search
	     continues with the	first rule numbered number or higher.  It is
	     possible to use the tablearg keyword with a skipto	for a computed
	     skipto, but care should be	used, as no destination	caching	is
	     possible in this case so the rules	are always walked to find it,
	     starting from the skipto.

     call number | tablearg
	     The current rule number is	saved in the internal stack and	rule-
	     set processing continues with the first rule numbered number or
	     higher.  If later a rule with the return action is	encountered,
	     the processing returns to the first rule with number of this call
	     rule plus one or higher (the same behaviour as with packets
	     returning from divert(4) socket after a divert action).  This
	     could be used to make somewhat like an assembly language
	     ``subroutine'' calls to rules with	common checks for different
	     interfaces, etc.

	     Rule with any number could	be called, not just forward jumps as
	     with skipto.  So, to prevent endless loops	in case	of mistakes,
	     both call and return actions don't	do any jumps and simply	go to
	     the next rule if memory can't be allocated	or stack over-
	     flowed/undeflowed.

	     Internally	stack for rule numbers is implemented using
	     mbuf_tags(9) facility and currently has size of 16	entries.  As
	     mbuf tags are lost	when packet leaves the kernel, divert should
	     not be used in subroutines	to avoid endless loops and other unde-
	     sired effects.

     return  Takes rule	number saved to	internal stack by the last call	action
	     and returns ruleset processing to the first rule with number
	     greater than number of corresponding call rule. See description
	     of	the call action	for more details.

	     Note that return rules usually end	a ``subroutine'' and thus are
	     unconditional, but	ipfw command-line utility currently requires
	     every action except check-state to	have body.  While it is	some-
	     times useful to return only on some packets, usually you want to
	     print just	``return'' for readability.  A workaround for this is
	     to	use new	syntax and -c switch:

		   # Add a rule	without	actual body
		   ipfw	add 2999 return	via any

		   # List rules	without	"from any to any" part
		   ipfw	-c list

	     This cosmetic annoyance may be fixed in future releases.

     tee port
	     Send a copy of packets matching this rule to the divert(4)	socket
	     bound to port port.  The search continues with the	next rule.

     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.

     unreach6 code
	     Discard packets that match	this rule, and try to send an ICMPv6
	     unreachable notice	with code code,	where code is a	number from 0,
	     1,	3 or 4,	or one of these	aliases: no-route, admin-prohib,
	     address or	port.  The search terminates.

     netgraph cookie
	     Divert packet into	netgraph with given cookie.  The search	termi-
	     nates.  If	packet is later	returned from netgraph it is either
	     accepted or continues with	the next rule, depending on
	     net.inet.ip.fw.one_pass sysctl variable.

     ngtee cookie
	     A copy of packet is diverted into netgraph, original packet con-
	     tinues with the next rule.	 See ng_ipfw(4)	for more information
	     on	netgraph and ngtee actions.

     setfib fibnum | tablearg
	     The packet	is tagged so as	to use the FIB (routing	table) fibnum
	     in	any subsequent forwarding decisions.  Initially	this is	lim-
	     ited to the values	0 through 15, see setfib(1).  Processing con-
	     tinues at the next	rule.  It is possible to use the tablearg key-
	     word with a setfib. If tablearg value is not within compiled FIB
	     range packet fib is set to	0.

     reass   Queue and reassemble ip fragments.	 If the	packet is not frag-
	     mented, counters are updated and processing continues with	the
	     next rule.	 If the	packet is the last logical fragment, the
	     packet is reassembled and,	if net.inet.ip.fw.one_pass is set to
	     0,	processing continues with the next rule, else packet is
	     allowed to	pass and search	terminates.  If	the packet is a	frag-
	     ment in the middle, it is consumed	and processing stops immedi-
	     ately.

	     Fragments handling	can be tuned via net.inet.ip.maxfragpackets
	     and net.inet.ip.maxfragsperpacket which limit, respectively, the
	     maximum number of processable fragments (default: 800) and	the
	     maximum number of fragments per packet (default: 16).

	     NOTA BENE:	since fragments	do not contain port numbers, they
	     should be avoided with the	reass rule.  Alternatively, direction-
	     based (like in / out ) and	source-based (like via ) match pat-
	     terns can be used to select fragments.

	     Usually a simple rule like:

		   # reassemble	incoming fragments
		   ipfw	add reass all from any to any in

	     is	all you	need at	the beginning of your ruleset.

   RULE	BODY
     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
     operators -- i.e.,	all must match in order	for the	rule to	match.	Indi-
     vidual patterns can be prefixed by	the not	operator to reverse the	result
     of	the match, as in

	   ipfw	add 100	allow ip from not 1.2.3.4 to any

     Additionally, sets	of alternative match patterns (or-blocks) can be con-
     structed 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
     address 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 (proto from	src to dst) is for backward compatibility with
     earlier versions of FreeBSD.  In modern FreeBSD any match pattern
     (including	MAC headers, IP	protocols, addresses and ports)	can be speci-
     fied in the options section.

     Rule fields have the following meaning:

     proto: protocol | { protocol or ... }

     protocol: [not] protocol-name | protocol-number
	     An	IP protocol specified by number	or name	(for a complete	list
	     see /etc/protocols), or one of the	following keywords:

	     ip4 | ipv4
		     Matches IPv4 packets.

	     ip6 | ipv6
		     Matches IPv6 packets.

	     ip	| all
		     Matches any packet.

	     The ipv6 in proto option will be treated as inner protocol.  And,
	     the ipv4 is not available in proto	option.

	     The { protocol or ... } format (an	or-block) is provided for con-
	     venience only but its use is deprecated.

     src and dst: {addr	| { addr or ...	}} [[not] ports]
	     An	address	(or a list, see	below) optionally followed by ports
	     specifiers.

	     The second	format (or-block with multiple addresses) is provided
	     for convenience only and its use is discouraged.

	     ip	| all

	     any     matches any IP address.

	     me	     matches any IP address configured on an interface in the
		     system.

	     me6     matches any IPv6 address configured on an interface in
		     the system.  The address list is evaluated	at the time
		     the packet	is analysed.

	     table(number[,value])
		     Matches any IPv4 address for which	an entry exists	in the
		     lookup table number.  If an optional 32-bit unsigned
		     value is also specified, an entry will match only if it
		     has this value.  See the LOOKUP TABLES section below for
		     more information on lookup	tables.

     addr-list:	ip-addr[,addr-list]

     ip-addr:
	     A host or subnet address specified	in one of the following	ways:

	     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.

	     addr/masklen
		     Matches all addresses with	base addr (specified as	an IP
		     address, a	network	number,	or a hostname) and mask	width
		     of	masklen	bits.  As an example, 1.2.3.4/25 or 1.2.3.0/25
		     will match	all IP numbers from 1.2.3.0 to 1.2.3.127 .

	     addr:mask
		     Matches all addresses with	base addr (specified as	an IP
		     address, a	network	number,	or a hostname) and the mask of
		     mask, specified as	a dotted quad.	As an example,
		     1.2.3.4:255.0.255.0 or 1.0.3.0:255.0.255.0	will match
		     1.*.3.*.  This form is advised only for non-contiguous
		     masks.  It	is better to resort to the addr/masklen	format
		     for contiguous masks, which is more compact and less
		     error-prone.

     addr-set: addr[/masklen]{list}

     list: {num	| num-num}[,list]
	     Matches all addresses with	base address addr (specified as	an IP
	     address, a	network	number,	or a hostname) and whose last byte is
	     in	the list between braces	{ } .  Note that there must be no spa-
	     ces between braces	and numbers (spaces after commas are allowed).
	     Elements of the list can be specified as single entries or
	     ranges.  The masklen field	is used	to limit the size of the set
	     of	addresses, and can have	any value between 24 and 32.  If not
	     specified,	it will	be assumed as 24.
	     This format is particularly useful	to handle sparse address sets
	     within a single rule.  Because the	matching occurs	using a	bit-
	     mask, it takes constant time and dramatically reduces the com-
	     plexity of	rulesets.
	     As	an example, an address specified as 1.2.3.4/24{128,35-55,89}
	     or	1.2.3.0/24{128,35-55,89} will match the	following IP
	     addresses:
	     1.2.3.128,	1.2.3.35 to 1.2.3.55, 1.2.3.89 .

     addr6-list: ip6-addr[,addr6-list]

     ip6-addr:
	     A host or subnet specified	one of the following ways:

	     numeric-ip	| hostname
		     Matches a single IPv6 address as allowed by inet_pton(3)
		     or	a hostname.  Hostnames are resolved at the time	the
		     rule is added to the firewall list.

	     addr/masklen
		     Matches all IPv6 addresses	with base addr (specified as
		     allowed by	inet_pton or a hostname) and mask width	of
		     masklen bits.

	     No	support	for sets of IPv6 addresses is provided because IPv6
	     addresses are typically random past the initial prefix.

     ports: {port | port-port}[,ports]
	     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
	     boundaries).

	     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 the	shell itself interpreting it as	an escape
	     character).

		   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.

   RULE	OPTIONS	(MATCH PATTERNS)
     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):

     //	this is	a comment.
	     Inserts the specified text	as a comment in	the rule.  Everything
	     following // is considered	as a comment and stored	in the rule.
	     You can have comment-only rules, which are	listed as having a
	     count action followed by the comment.

     bridged
	     Alias for layer2.

     diverted
	     Matches only packets generated by a divert	socket.

     diverted-loopback
	     Matches only packets coming from a	divert socket back into	the IP
	     stack input for delivery.

     diverted-output
	     Matches only packets going	from a divert socket back outward to
	     the IP stack output for delivery.

     dst-ip ip-address
	     Matches IPv4 packets whose	destination IP is one of the
	     address(es) specified as argument.

     {dst-ip6 |	dst-ipv6} ip6-address
	     Matches IPv6 packets whose	destination IP is one of the
	     address(es) specified as argument.

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

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

     ext6hdr header
	     Matches IPv6 packets containing the extended header given by
	     header.  Supported	headers	are:

	     Fragment, (frag), Hop-to-hop options (hopopt), any	type of	Rout-
	     ing Header	(route), Source	routing	Routing	Header Type 0
	     (rthdr0), Mobile IPv6 Routing Header Type 2 (rthdr2), Destination
	     options (dstopt), IPSec authentication headers (ah), and IPsec
	     encapsulated security payload headers (esp).

     fib fibnum
	     Matches a packet that has been tagged to use the given FIB	(rout-
	     ing table)	number.

     flow-id labels
	     Matches IPv6 packets containing any of the	flow labels given in
	     labels.  labels is	a comma	separated list of numeric flow labels.

     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.

     jail prisonID
	     Matches all TCP or	UDP packets sent by or received	for the	jail
	     whos prison ID is prisonID.

     icmptypes types
	     Matches ICMP packets whose	ICMP type is in	the list types.	 The
	     list may be specified as any combination of individual types
	     (numeric) separated by commas.  Ranges are	not allowed.  The sup-
	     ported 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).

     icmp6types	types
	     Matches ICMP6 packets whose ICMP6 type is in the list of types.
	     The list may be specified as any combination of individual	types
	     (numeric) separated by commas.  Ranges are	not allowed.

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

     ipid id-list
	     Matches IPv4 packets whose	ip_id field has	value included in
	     id-list, which is either a	single value or	a list of values or
	     ranges specified in the same way as ports.

     iplen len-list
	     Matches IP	packets	whose total length, including header and data,
	     is	in the set len-list, which is either a single value or a list
	     of	values or ranges specified in the same way as ports.

     ipoptions spec
	     Matches packets whose IPv4	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 IPv4 packets whose	precedence field is equal to
	     precedence.

     ipsec   Matches packets that have IPSEC history associated	with them
	     (i.e., the	packet comes encapsulated in IPSEC, the	kernel has
	     IPSEC support and IPSEC_FILTERTUNNEL option, and can correctly
	     decapsulate it).

	     Note that specifying ipsec	is different from specifying proto
	     ipsec as the latter will only look	at the specific	IP protocol
	     field, irrespective of IPSEC kernel support and the validity of
	     the IPSEC data.

	     Further note that this flag is silently ignored in	kernels	with-
	     out IPSEC support.	 It does not affect rule processing when given
	     and the rules are handled as if with no ipsec flag.

     iptos spec
	     Matches IPv4 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_ECN_CE).	 The absence of	a particular type may
	     be	denoted	with a `!'.

     ipttl ttl-list
	     Matches IPv4 packets whose	time to	live is	included in ttl-list,
	     which is either a single value or a list of values	or ranges
	     specified in the same way as ports.

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

     keep-state
	     Upon a match, the firewall	will create a dynamic rule, whose
	     default behaviour is to match 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.	Currently,
	     only IPv4 flows are supported.

     lookup {dst-ip | dst-port | src-ip	| src-port | uid | jail} N
	     Search an entry in	lookup table N that matches the	field speci-
	     fied as argument.	If not found, the match	fails.	Otherwise, the
	     match succeeds and	tablearg is set	to the value extracted from
	     the table.

	     This option can be	useful to quickly dispatch traffic based on
	     certain packet fields.  See the LOOKUP TABLES section below for
	     more information on lookup	tables.

     { 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	colons,	and optionally followed	by a
	     mask indicating the significant bits.  The	mask may be specified
	     using either of the following methods:

	     1.	     A slash (/) followed by the number	of significant bits.
		     For example, an address with 33 significant bits could be
		     specified as:

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

	     2.	     An	ampersand (&) followed by a bitmask specified as six
		     groups of hex digits separated by colons.	For example,
		     an	address	in which the last 16 bits are significant
		     could be specified	as:

			   MAC 10:20:30:40:50:60&00:00:00:00:ff:ff any

		     Note that the ampersand character has a special meaning
		     in	many shells and	should generally be escaped.

	     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
	     values or ranges).	 You can use symbolic names for	known values
	     such as vlan, ipv4, ipv6.	Values can be entered as decimal or
	     hexadecimal (if prefixed by 0x), and they are always printed as
	     hexadecimal (unless the -N	option is used,	in which case symbolic
	     resolution	will be	attempted).

     proto protocol
	     Matches packets with the corresponding IP protocol.

     recv | xmit | via {ifX | if* | ipno | any}
	     Matches packets received, transmitted or going through, respec-
	     tively, the interface specified by	exact name (ifX), by device
	     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
	     used.

	     A packet might 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 IPv4 packets whose	source IP is one of the	address(es)
	     specified as an argument.

     src-ip6 ip6-address
	     Matches IPv6 packets whose	source IP is one of the	address(es)
	     specified as an argument.

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

     tagged tag-list
	     Matches packets whose tags	are included in	tag-list, which	is
	     either a single value or a	list of	values or ranges specified in
	     the same way as ports.  Tags can be applied to the	packet using
	     tag rule action parameter (see it's description for details on
	     tags).

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

     tcpdatalen	tcpdatalen-list
	     Matches TCP packets whose length of TCP data is tcpdatalen-list,
	     which is either a single value or a list of values	or ranges
	     specified in the same way as ports.

     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
	     win.

     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.

     verrevpath
	     For incoming packets, a routing table lookup is done on the
	     packet's source address.  If the interface	on which the packet
	     entered the system	matches	the outgoing interface for the route,
	     the packet	matches.  If the interfaces do not match up, the
	     packet does not match.  All outgoing packets or packets with no
	     incoming interface	match.

	     The name and functionality	of the option is intentionally similar
	     to	the Cisco IOS command:

		   ip verify unicast reverse-path

	     This option can be	used to	make anti-spoofing rules to reject all
	     packets with source addresses not from this interface.  See also
	     the option	antispoof.

     versrcreach
	     For incoming packets, a routing table lookup is done on the
	     packet's source address.  If a route to the source	address
	     exists, but not the default route or a blackhole/reject route,
	     the packet	matches.  Otherwise, the packet	does not match.	 All
	     outgoing packets match.

	     The name and functionality	of the option is intentionally similar
	     to	the Cisco IOS command:

		   ip verify unicast source reachable-via any

	     This option can be	used to	make anti-spoofing rules to reject all
	     packets whose source address is unreachable.

     antispoof
	     For incoming packets, the packet's	source address is checked if
	     it	belongs	to a directly connected	network.  If the network is
	     directly connected, then the interface the	packet came on in is
	     compared to the interface the network is connected	to.  When
	     incoming interface	and directly connected interface are not the
	     same, the packet does not match.  Otherwise, the packet does
	     match.  All outgoing packets match.

	     This option can be	used to	make anti-spoofing rules to reject all
	     packets that pretend to be	from a directly	connected network but
	     do	not come in through that interface.  This option is similar to
	     but more restricted than verrevpath because it engages only on
	     packets with source addresses of directly connected networks
	     instead of	all source addresses.

LOOKUP TABLES
     Lookup tables are useful to handle	large sparse sets of addresses or
     other search keys (e.g. ports, jail IDs).	In the rest of this section we
     will use the term ``address'' to mean any unsigned	value of up to 32-bit.
     There may be up to	128 different lookup tables, numbered 0	to 127.

     Each entry	is represented by an addr[/masklen] and	will match all
     addresses with base addr (specified as an IP address, a hostname or an
     unsigned integer) and mask	width of masklen bits.	If masklen is not
     specified,	it defaults to 32.  When looking up an IP address in a table,
     the most specific entry will match.  Associated with each entry is	a
     32-bit unsigned value, which can optionally be checked by a rule matching
     code.  When adding	an entry, if value is not specified, it	defaults to 0.

     An	entry can be added to a	table (add), or	removed	from a table (delete).
     A table can be examined (list) or flushed (flush).

     Internally, each table is stored in a Radix tree, the same	way as the
     routing table (see	route(4)).

     Lookup tables currently support only ports, jail IDs and IPv4 addresses.

     The tablearg feature provides the ability to use a	value, looked up in
     the table,	as the argument	for a rule action, action parameter or rule
     option.  This can significantly reduce number of rules in some configura-
     tions.  If	two tables are used in a rule, the result of the second	(des-
     tination) is used.	 The tablearg argument can be used with	the following
     actions: nat, pipe, queue,	divert,	tee, netgraph, ngtee, fwd, skipto,
     setfib, action parameters:	tag, untag, rule options: limit, tagged.

     When used with fwd	it is possible to supply table entries with values
     that are in the form of IP	addresses or hostnames.	 See the EXAMPLES Sec-
     tion for example usage of tables and the tablearg keyword.

     When used with the	skipto action, the user	should be aware	that the code
     will walk the ruleset up to a rule	equal to, or past, the given number,
     and should	therefore try keep the ruleset compact between the skipto and
     the target	rules.

SETS OF	RULES
     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
     enabled or	disabled, so this mechanism permits an easy way	to store mul-
     tiple 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 do not	exist in the
     firewall configuration, with only one exception:

	   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 FIREWALL
     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
     denote the	initial	match addresses, but they are completely equivalent
     afterwards).  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 keep-state
	   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
     firewall:

	   ipfw	add check-state
	   ipfw	add allow udp from my-subnet to	any keep-state
	   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
     instructed	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.

TRAFFIC	SHAPER (DUMMYNET) CONFIGURATION
     ipfw is also the user interface for the dummynet traffic shaper, packet
     scheduler and network emulator, a subsystem that can artificially queue,
     delay or drop packets emulating the behaviour of certain network links or
     queueing systems.

     dummynet operates by first	using the firewall to select packets using any
     match pattern that	can be used in ipfw rules.  Matching packets are then
     passed to either of two different objects,	which implement	the traffic
     regulation:

	 pipe	 A pipe	emulates a link	with given bandwidth and propagation
		 delay,	driven by a FIFO scheduler and a single	queue with
		 programmable queue size and packet loss rate.	Packets	are
		 appended to the queue as they come out	from ipfw, and then
		 transferred in	FIFO order to the link at the desired rate.

	 queue	 A queue is an abstraction used	to implement packet scheduling
		 using one of several packet scheduling	algorithms.  Packets
		 sent to a queue are first grouped into	flows according	to a
		 mask on the 5-tuple.  Flows are then passed to	the scheduler
		 associated to the queue, and each flow	uses scheduling	param-
		 eters (weight and others) as configured in the	queue itself.
		 A scheduler in	turn is	connected to an	emulated link, and
		 arbitrates the	link's bandwidth among backlogged flows
		 according to weights and to the features of the scheduling
		 algorithm in use.

     In	practice, pipes	can be used to set hard	limits to the bandwidth	that a
     flow can use, whereas queues can be used to determine how different flows
     share the available bandwidth.

     A graphical representation	of the binding of queues, flows, schedulers
     and links is below.

			    (flow_mask|sched_mask)  sched_mask
		    +---------+	  weight Wx  +-------------+
		    |	      |->-[flow]-->--|		   |-+
	       -->--| QUEUE x |	  ...	     |		   | |
		    |	      |->-[flow]-->--| SCHEDuler N | |
		    +---------+		     |		   | |
			...		     |		   +--[LINK N]-->--
		    +---------+	  weight Wy  |		   | +--[LINK N]-->--
		    |	      |->-[flow]-->--|		   | |
	       -->--| QUEUE y |	  ...	     |		   | |
		    |	      |->-[flow]-->--|		   | |
		    +---------+		     +-------------+ |
					       +-------------+
     It	is important to	understand the role of the SCHED_MASK and FLOW_MASK,
     which are configured through the commands
	   ipfw	sched N	config mask SCHED_MASK ...
     and
	   ipfw	queue X	config mask FLOW_MASK ....

     The SCHED_MASK is used to assign flows to one or more scheduler
     instances,	one for	each value of the packet's 5-fuple after applying
     SCHED_MASK.  As an	example, using ``src-ip	0xffffff00'' creates one
     instance for each /24 destination subnet.

     The FLOW_MASK, together with the SCHED_MASK, is used to split packets
     into flows. As an example,	using ``src-ip 0x000000ff'' together with the
     previous SCHED_MASK makes a flow for each individual source address. In
     turn, flows for each /24 subnet will be sent to the same scheduler
     instance.

     The above diagram holds even for the pipe case, with the only restriction
     that a pipe only supports a SCHED_MASK, and forces	the use	of a FIFO
     scheduler (these are for backward compatibility reasons; in fact, inter-
     nally, a dummynet's pipe is implemented exactly as	above).

     There are two modes of dummynet operation:	``normal'' and ``fast''.  The
     ``normal''	mode tries to emulate a	real link: the dummynet	scheduler
     ensures that the packet will not leave the	pipe faster than it would on
     the real link with	a given	bandwidth.  The	``fast'' mode allows certain
     packets to	bypass the dummynet scheduler (if packet flow does not exceed
     pipe's bandwidth).	 This is the reason why	the ``fast'' mode requires
     less CPU cycles per packet	(on average) and packet	latency	can be signif-
     icantly lower in comparison to a real link	with the same bandwidth.  The
     default mode is ``normal''.  The ``fast'' mode can	be enabled by setting
     the net.inet.ip.dummynet.io_fast sysctl(8)	variable to a non-zero value.

   PIPE, QUEUE AND SCHEDULER CONFIGURATION
     The pipe, queue and scheduler configuration commands are the following:

	   pipe	number config pipe-configuration

	   queue number	config queue-configuration

	   sched number	config sched-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
	     immediately follow	the number, as in

		   ipfw	pipe 1 config bw 300Kbit/s

	     If	a device name is specified instead of a	numeric	value, as in

		   ipfw	pipe 1 config bw tun0

	     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).	 The default value is
	     0,	meaning	no delay.

     burst size
	     If	the data to be sent exceeds the	pipe's bandwidth limit (and
	     the pipe was previously idle), up to size bytes of	data are
	     allowed to	bypass the dummynet scheduler, and will	be sent	as
	     fast as the physical link allows.	Any additional data will be
	     transmitted at the	rate specified by the pipe bandwidth.  The
	     burst size	depends	on how long the	pipe has been idle; the	effec-
	     tive burst	size is	calculated as follows: MAX( size , bw *
	     pipe_idle_time).

     profile filename
	     A file specifying the additional overhead incurred	in the trans-
	     mission of	a packet on the	link.

	     Some link types introduce extra delays in the transmission	of a
	     packet, e.g. because of MAC level framing,	contention on the use
	     of	the channel, MAC level retransmissions and so on.  From	our
	     point of view, the	channel	is effectively unavailable for this
	     extra time, which is constant or variable depending on the	link
	     type. Additionally, packets may be	dropped	after this time	(e.g.
	     on	a wireless link	after too many retransmissions).  We can model
	     the additional delay with an empirical curve that represents its
	     distribution.

			 cumulative probability
			 1.0 ^
			     |
			 L   +-- loss-level	     x
			     |		       ******
			     |		      *
			     |		 *****
			     |		*
			     |	      **
			     |	     *
			     +-------*------------------->
					 delay
	     The empirical curve may have both vertical	and horizontal lines.
	     Vertical lines represent constant delay for a range of probabili-
	     ties.  Horizontal lines correspond	to a discontinuity in the
	     delay distribution: the pipe will use the largest delay for a
	     given probability.

	     The file format is	the following, with whitespace acting as a
	     separator and '#' indicating the beginning	a comment:

	     name identifier
		     optional name (listed by "ipfw pipe show")	to identify
		     the delay distribution;

	     bw	value
		     the bandwidth used	for the	pipe.  If not specified	here,
		     it	must be	present	explicitly as a	configuration parame-
		     ter for the pipe;

	     loss-level	L
		     the probability above which packets are lost.  (0.0 <= L
		     <=	1.0, default 1.0 i.e. no loss);

	     samples N
		     the number	of samples used	in the internal	representation
		     of	the curve (2..1024; default 100);

	     delay prob	| prob delay
		     One of these two lines is mandatory and defines the for-
		     mat of the	following lines	with data points.

	     XXX YYY
		     2 or more lines representing points in the	curve, with
		     either delay or probability first,	according to the cho-
		     sen format.  The unit for delay is	milliseconds.  Data
		     points do not need	to be sorted.  Also, tne number	of
		     actual lines can be different from	the value of the "sam-
		     ples" parameter: ipfw utility will	sort and interpolate
		     the curve as needed.

	     Example of	a profile file:

		   name	   bla_bla_bla
		   samples 100
		   loss-level	 0.86
		   prob	   delay
		   0	   200	   # minimum overhead is 200ms
		   0.5	   200
		   0.5	   300
		   0.8	   1000
		   0.9	   1300
		   1	   1300
		   #configuration file end

     The following parameters can be configured	for a queue:

     pipe pipe_nr
	     Connects a	queue to the specified pipe.  Multiple queues (with
	     the same or 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.

     The following parameters can be configured	for a scheduler:

     type {fifo	| wf2qp	| rr | qfq}
	     specifies the scheduling algorithm	to use.
	     cm	fifo
		     is	just a FIFO scheduler (which means that	all packets
		     are stored	in the same queue as they arrive to the	sched-
		     uler).  FIFO has O(1) per-packet time complexity, with
		     very low constants	(estimate 60-80ns on a 2Ghz desktop
		     machine) but gives	no service guarantees.
	     wf2qp   implements	the WF2Q+ algorithm, which is a	Weighted Fair
		     Queueing algorithm	which permits flows to share bandwidth
		     according to their	weights. Note that weights are not
		     priorities; even a	flow with a minuscule weight will
		     never starve.  WF2Q+ has O(log N) per-packet processing
		     cost, where N is the number of flows, and is the default
		     algorithm used by previous	versions dummynet's queues.
	     rr	     implements	the Deficit Round Robin	algorithm, which has
		     O(1) processing costs (roughly, 100-150ns per packet) and
		     permits bandwidth allocation according to weights,	but
		     with poor service guarantees.
	     qfq     implements	the QFQ	algorithm, which is a very fast	vari-
		     ant of WF2Q+, with	similar	service	guarantees and O(1)
		     processing	costs (roughly,	200-250ns per packet).

     In	addition to the	type, all parameters allowed for a pipe	can also be
     specified for a scheduler.

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

     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 65536.

     mask mask-specifier
	   Packets sent	to a given pipe	or queue by an ipfw rule can be	fur-
	   ther	classified into	multiple flows,	each of	which is then sent to
	   a different dynamic pipe or queue.  A flow identifier is con-
	   structed by masking the IP addresses, ports and protocol types as
	   specified with the mask options in the configuration	of the pipe or
	   queue.  For each different flow identifier, a new pipe or queue is
	   created with	the same parameters as the original object, and	match-
	   ing packets are sent	to it.

	   Thus, when dynamic pipes are	used, each flow	will get the same
	   bandwidth as	defined	by the pipe, whereas when dynamic queues are
	   used, each flow will	share the parent's pipe	bandwidth evenly with
	   other flows generated by the	same queue (note that other queues
	   with	different weights might	be connected to	the same pipe).
	   Available mask specifiers are a combination of one or more of the
	   following:

	   dst-ip mask,	dst-ip6	mask, src-ip mask, src-ip6 mask, dst-port
	   mask, src-port mask,	flow-id	mask, proto mask or all,

	   where the latter means all bits in all fields are significant.

     noerror
	   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	effects	can result if you get packets
	   from	an interface with a much larger	MTU, e.g. the loopback inter-
	   face	with its 16KB packets.	The sysctl(8) variables
	   net.inet.ip.dummynet.pipe_byte_limit	and
	   net.inet.ip.dummynet.pipe_slot_limit	control	the maximum lengths
	   that	can be specified.

     red | gred	w_q/min_th/max_th/max_p
	   Make	use of the RED (Random Early Detection)	queue management algo-
	   rithm.  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 specify-
	   ing thresholds for queue management (thresholds are computed	in
	   bytes if the	queue has been defined in bytes, in slots otherwise).
	   The dummynet	also supports the gentle RED variant (gred).  Three
	   sysctl(8) variables can be used to control the RED behaviour:

	   net.inet.ip.dummynet.red_lookup_depth
		   specifies the accuracy in computing the average queue when
		   the link is idle (defaults to 256, must be greater than
		   zero)

	   net.inet.ip.dummynet.red_avg_pkt_size
		   specifies the expected average packet size (defaults	to
		   512,	must be	greater	than zero)

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

     When used with IPv6 data, dummynet	currently has several limitations.
     Information necessary to route link-local packets to an interface is not
     available after processing	by dummynet so those packets are dropped in
     the output	path.  Care should be taken to insure that link-local packets
     are not passed to dummynet.

CHECKLIST
     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
	 /usr/share/examples/ipfw/change_rules.sh.

     +o	 Do not	forget the loopback interface.

FINE POINTS
     +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.  The fol-
	 lowing	command	line is	recommended:

	       kldload ipfw && \
	       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
	 levels).

PACKET DIVERSION
     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 divert module is not loaded, or if the kernel was not compiled
     with divert socket	support, the packets are dropped.

NETWORK	ADDRESS	TRANSLATION (NAT)
     ipfw support in-kernel NAT	using the kernel version of libalias(3).

     The nat configuration command is the following:

	   nat nat_number config nat-configuration

     The following parameters can be configured:

     ip	ip_address
	     Define an ip address to use for aliasing.

     if	nic  Use ip address of NIC for aliasing, dynamically changing it if
	     NIC's ip address changes.

     log     Enable logging on this nat	instance.

     deny_in
	     Deny any incoming connection from outside world.

     same_ports
	     Try to leave the alias port numbers unchanged from	the actual
	     local port	numbers.

     unreg_only
	     Traffic on	the local network not originating from an unregistered
	     address spaces will be ignored.

     reset   Reset table of the	packet aliasing	engine on address change.

     reverse
	     Reverse the way libalias handles aliasing.

     proxy_only
	     Obey transparent proxy rules only,	packet aliasing	is not per-
	     formed.

     skip_global
	     Skip instance in case of global state lookup (see below).

     Some specials value can be	supplied instead of nat_number:

     global  Looks up translation state	in all configured nat instances.  If
	     an	entry is found,	packet is aliased according to that entry.  If
	     no	entry was found	in any of the instances, packet	is passed
	     unchanged,	and no new entry will be created.  See section
	     MULTIPLE INSTANCES	in natd(8) for more information.

     tablearg
	     Uses argument supplied in lookup table. See LOOKUP	TABLES section
	     below for more information	on lookup tables.

     To	let the	packet continue	after being (de)aliased, set the sysctl	vari-
     able net.inet.ip.fw.one_pass to 0.	 For more information about aliasing
     modes, refer to libalias(3).  See Section EXAMPLES	for some examples
     about nat usage.

   REDIRECT AND	LSNAT SUPPORT IN IPFW
     Redirect and LSNAT	support	follow closely the syntax used in natd(8).
     See Section EXAMPLES for some examples on how to do redirect and lsnat.

   SCTP	NAT SUPPORT
     SCTP nat can be configured	in a similar manner to TCP through the ipfw
     command line tool.	 The main difference is	that sctp nat does not do port
     translation.  Since the local and global side ports will be the same,
     there is no need to specify both.	Ports are redirected as	follows:

	   nat nat_number config if nic	redirect_port sctp
	   ip_address [,addr_list] {[port | port-port] [,ports]}

     Most sctp nat configuration can be	done in	real-time through the
     sysctl(8) interface.  All may be changed dynamically, though the hash_ta-
     ble size will only	change for new nat instances.  See SYSCTL VARIABLES
     for more info.

SYSCTL VARIABLES
     A set of sysctl(8)	variables controls the behaviour of the	firewall and
     associated	modules	(dummynet, bridge, sctp	nat).  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.alias.sctp.accept_global_ootb_addip: 0
	     Defines how the nat responds to receipt of	global OOTB ASCONF-
	     AddIP:

	     0	     No	response (unless a partially matching association
		     exists - ports and	vtags match but	global address does
		     not)

	     1	     nat will accept and process all OOTB global AddIP mes-
		     sages.

	     Option 1 should never be selected as this forms a security	risk.
	     An	attacker can establish multiple	fake associations by sending
	     AddIP messages.

     net.inet.ip.alias.sctp.chunk_proc_limit: 5
	     Defines the maximum number	of chunks in an	SCTP packet that will
	     be	parsed for a packet that matches an existing association.
	     This value	is enforced to be greater or equal than
	     net.inet.ip.alias.sctp.initialising_chunk_proc_limit.  A high
	     value is a	DoS risk yet setting too low a value may result	in
	     important control chunks in the packet not	being located and
	     parsed.

     net.inet.ip.alias.sctp.error_on_ootb: 1
	     Defines when the nat responds to any Out-of-the-Blue (OOTB) pack-
	     ets with ErrorM packets.  An OOTB packet is a packet that arrives
	     with no existing association registered in	the nat	and is not an
	     INIT or ASCONF-AddIP packet:

	     0	     ErrorM is never sent in response to OOTB packets.

	     1	     ErrorM is only sent to OOTB packets received on the local
		     side.

	     2	     ErrorM is sent to the local side and on the global	side
		     ONLY if there is a	partial	match (ports and vtags match
		     but the source global IP does not).  This value is	only
		     useful if the nat is tracking global IP addresses.

	     3	     ErrorM is sent in response	to all OOTB packets on both
		     the local and global side (DoS risk).

	     At	the moment the default is 0, since the ErrorM packet is	not
	     yet supported by most SCTP	stacks.	 When it is supported, and if
	     not tracking global addresses, we recommend setting this value to
	     1 to allow	multi-homed local hosts	to function with the nat.  To
	     track global addresses, we	recommend setting this value to	2 to
	     allow global hosts	to be informed when they need to (re)send an
	     ASCONF-AddIP.  Value 3 should never be chosen (except for debug-
	     ging) as the nat will respond to all OOTB global packets (a DoS
	     risk).

     net.inet.ip.alias.sctp.hashtable_size: 2003
	     Size of hash tables used for nat lookups (100 < prime_number >
	     1000001).	This value sets	the hash table size for	any future
	     created nat instance and therefore	must be	set prior to creating
	     a nat instance.  The table	sizes may be changed to	suit specific
	     needs.  If	there will be few concurrent associations, and memory
	     is	scarce,	you may	make these smaller.  If	there will be many
	     thousands (or millions) of	concurrent associations, you should
	     make these	larger.	 A prime number	is best	for the	table size.
	     The sysctl	update function	will adjust your input value to	the
	     next highest prime	number.

     net.inet.ip.alias.sctp.holddown_time: 0
	     Hold association in table for this	many seconds after receiving a
	     SHUTDOWN-COMPLETE.	 This allows endpoints to correct shutdown
	     gracefully	if a shutdown_complete is lost and retransmissions are
	     required.

     net.inet.ip.alias.sctp.init_timer:	15
	     Timeout value while waiting for (INIT-ACK|AddIP-ACK).  This value
	     cannot be 0.

     net.inet.ip.alias.sctp.initialising_chunk_proc_limit: 2
	     Defines the maximum number	of chunks in an	SCTP packet that will
	     be	parsed when no existing	association exists that	matches	that
	     packet.  Ideally this packet will only be an INIT or ASCONF-AddIP
	     packet.  A	higher value may become	a DoS risk as malformed	pack-
	     ets can consume processing	resources.

     net.inet.ip.alias.sctp.param_proc_limit: 25
	     Defines the maximum number	of parameters within a chunk that will
	     be	parsed in a packet.  As	for other similar sysctl variables,
	     larger values pose	a DoS risk.

     net.inet.ip.alias.sctp.log_level: 0
	     Level of detail in	the system log messages	(0 - minimal, 1	-
	     event, 2 -	info, 3	- detail, 4 - debug, 5 - max debug). May be a
	     good option in high loss environments.

     net.inet.ip.alias.sctp.shutdown_time: 15
	     Timeout value while waiting for SHUTDOWN-COMPLETE.	 This value
	     cannot be 0.

     net.inet.ip.alias.sctp.track_global_addresses: 0
	     Enables/disables global IP	address	tracking within	the nat	and
	     places an upper limit on the number of addresses tracked for each
	     association:

	     0	     Global tracking is	disabled

	     >1	     Enables tracking, the maximum number of addresses tracked
		     for each association is limited to	this value

	     This variable is fully dynamic, the new value will	be adopted for
	     all newly arriving	associations, existing associations are
	     treated as	they were previously.  Global tracking will decrease
	     the number	of collisions within the nat at	a cost of increased
	     processing	load, memory usage, complexity,	and possible nat state
	     problems in complex networks with multiple	nats.  We recommend
	     not tracking global IP addresses, this will still result in a
	     fully functional nat.

     net.inet.ip.alias.sctp.up_timer: 300
	     Timeout value to keep an association up with no traffic.  This
	     value cannot be 0.

     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
	     reached.

     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.io_fast: 0
	     If	set to a non-zero value, the ``fast'' mode of dummynet opera-
	     tion (see above) is enabled.

     net.inet.ip.dummynet.io_pkt
	     Number of packets passed to dummynet.

     net.inet.ip.dummynet.io_pkt_drop
	     Number of packets dropped by dummynet.

     net.inet.ip.dummynet.io_pkt_fast
	     Number of packets bypassed	by the dummynet	scheduler.

     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.dummynet.pipe_byte_limit: 1048576

     net.inet.ip.dummynet.pipe_slot_limit: 100
	     The maximum queue size that can be	specified in bytes or packets.
	     These limits prevent accidental exhaustion	of resources such as
	     mbufs.  If	you raise these	limits,	you should make	sure the sys-
	     tem is configured so that sufficient resources are	available.

     net.inet.ip.fw.autoinc_step: 100
	     Delta between 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
	     (readonly).

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

     net.inet.ip.fw.default_rule: 65535
	     The default rule number (read-only).  By the design of ipfw, the
	     default rule is the last one, so its number can also serve	as the
	     highest number allowed for	a rule.

     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
	     dynamic rules have	expired, so you	are advised to use a flush
	     command 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
	     firewall 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.inet6.ip6.fw.enable: 1
	     provides the same functionality as	above for the IPv6 case.

     net.inet.ip.fw.one_pass: 1
	     When set, the packet exiting from the dummynet pipe or from
	     ng_ipfw(4)	node is	not passed though the firewall again.  Other-
	     wise, after an action, the	packet is reinjected into the firewall
	     at	the next rule.

     net.inet.ip.fw.tables_max:	128
	     Maximum number of tables (read-only).

     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.

     net.inet6.ip6.fw.deny_unknown_exthdrs: 1
	     If	enabled	packets	with unknown IPv6 Extension Headers will be
	     denied.

     net.link.ether.ipfw: 0
	     Controls whether layer-2 packets are passed to ipfw.  Default is
	     no.

     net.link.bridge.ipfw: 0
	     Controls whether bridged packets are passed to ipfw.  Default is
	     no.

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

   BASIC PACKET	FILTERING
     This command adds an entry	which denies all tcp packets from
     cracker.evil.org to the telnet port of wolf.tambov.su from	being for-
     warded by the host:

	   ipfw	add deny tcp from cracker.evil.org to wolf.tambov.su telnet

     This one disallows	any connection from the	entire cracker's network to my
     host:

	   ipfw	add deny ip from 123.45.67.0/24	to my.host.org

     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
     address sets and or-blocks	and write extremely compact rulesets which
     selectively enable	services to blocks of clients, as below:

	   goodguys="{ 10.1.2.0/24{20,35,66,18}	or 10.2.3.0/28{6,3,11} }"
	   badguys="10.1.2.0/24{8,38,60}"

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

     The verrevpath option could be used to do automated anti-spoofing by
     adding the	following to the top of	a ruleset:

	   ipfw	add deny ip from any to	any not	verrevpath in

     This rule drops all incoming packets that appear to be coming to the sys-
     tem on the	wrong interface.  For example, a packet	with a source address
     belonging to a host on a protected	internal network would be dropped if
     it	tried to enter the system from an external interface.

     The antispoof option could	be used	to do similar but more restricted
     anti-spoofing by adding the following to the top of a ruleset:

	   ipfw	add deny ip from any to	any not	antispoof in

     This rule drops all incoming packets that appear to be coming from
     another directly connected	system but on the wrong	interface.  For	exam-
     ple, a packet with	a source address of 192.168.0.0/24, configured on
     fxp0, but coming in on fxp1 would be dropped.

   DYNAMIC RULES
     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	usually	be
     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
     simultaneous 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 192.168.2.0/24	to divert port
     5000:

	   ipfw	divert 5000 ip from 192.168.2.0/24 to any in

   TRAFFIC SHAPING
     The following rules show some of the applications of ipfw and dummynet
     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
     192.168.2.0/24 we do:

	   ipfw	add pipe 1 ip from 192.168.2.0/24 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 want 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 who 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 want 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 significantly affect	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 band-
     width:

	   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 192.168.2.0/24 to any out
	   ipfw	add pipe 2 ip from any to 192.168.2.0/24 in
	   ipfw	pipe 1 config mask src-ip 0x000000ff bw	200Kbit/s queue
	   20Kbytes
	   ipfw	pipe 2 config mask dst-ip 0x000000ff bw	200Kbit/s queue
	   20Kbytes

   LOOKUP TABLES
     In	the following example, we need to create several traffic bandwidth
     classes and we need different hosts/networks to fall into different
     classes.  We create one pipe for each class and configure them accord-
     ingly.  Then we create a single table and fill it with IP subnets and
     addresses.	 For each subnet/host we set the argument equal	to the number
     of	the pipe that it should	use.  Then we classify traffic using a single
     rule:

	   ipfw	pipe 1 config bw 1000Kbyte/s
	   ipfw	pipe 4 config bw 4000Kbyte/s
	   ...
	   ipfw	table 1	add 192.168.2.0/24 1
	   ipfw	table 1	add 192.168.0.0/27 4
	   ipfw	table 1	add 192.168.0.2	1
	   ...
	   ipfw	add pipe tablearg ip from table(1) to any

     Using the fwd action, the table entries may include hostnames and IP
     addresses.

	   ipfw	table 1	add 192.168.2.0/24 10.23.2.1
	   ipfw	table 1	add 192.168.0.0/27 router1.dmz
	   ...
	   ipfw	add 100	fwd tablearg ip	from any to table(1)

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

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

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

     To	show rules of the specific set:

	   ipfw	set 18 show

     To	show rules of the disabled set:

	   ipfw	-S set 18 show

     To	clear a	specific rule counters of the specific set:

	   ipfw	set 18 zero NN

     To	delete a specific rule of the specific set:

	   ipfw	set 18 delete NN

   NAT,	REDIRECT AND LSNAT
     First redirect all	the traffic to nat instance 123:

	   ipfw	add nat	123 all	from any to any

     Then to configure nat instance 123	to alias all the outgoing traffic with
     ip	192.168.0.123, blocking	all incoming connections, trying to keep same
     ports on both sides, clearing aliasing table on address change and	keep-
     ing a log of traffic/link statistics:

	   ipfw	nat 123	config ip 192.168.0.123	log deny_in reset same_ports

     Or	to change address of instance 123, aliasing table will be cleared (see
     reset option):

	   ipfw	nat 123	config ip 10.0.0.1

     To	see configuration of nat instance 123:

	   ipfw	nat 123	show config

     To	show logs of all the instances in range	111-999:

	   ipfw	nat 111-999 show

     To	see configurations of all instances:

	   ipfw	nat show config

     Or	a redirect rule	with mixed modes could looks like:

	   ipfw	nat 123	config redirect_addr 10.0.0.1 10.0.0.66
			   redirect_port tcp 192.168.0.1:80 500
			   redirect_proto udp 192.168.1.43 192.168.1.1
			   redirect_addr 192.168.0.10,192.168.0.11
				   10.0.0.100 #	LSNAT
			   redirect_port tcp 192.168.0.1:80,192.168.0.10:22
				   500	      #	LSNAT

     or	it could be split in:

	   ipfw	nat 1 config redirect_addr 10.0.0.1 10.0.0.66
	   ipfw	nat 2 config redirect_port tcp 192.168.0.1:80 500
	   ipfw	nat 3 config redirect_proto udp	192.168.1.43 192.168.1.1
	   ipfw	nat 4 config redirect_addr
	   192.168.0.10,192.168.0.11,192.168.0.12
					10.0.0.100
	   ipfw	nat 5 config redirect_port tcp
			  192.168.0.1:80,192.168.0.10:22,192.168.0.20:25 500

SEE ALSO
     cpp(1), m4(1), altq(4), divert(4),	dummynet(4), if_bridge(4), ip(4),
     ipfirewall(4), ng_ipfw(4),	protocols(5), services(5), init(8),
     kldload(8), reboot(8), sysctl(8), syslogd(8)

HISTORY
     The ipfw utility first appeared in	FreeBSD	2.0.  dummynet was introduced
     in	FreeBSD	2.2.8.	Stateful extensions were introduced in FreeBSD 4.0.
     ipfw2 was introduced in Summer 2002.

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

     API based upon code written by Daniel Boulet for BSDI.

     Dummynet has been introduced by Luigi Rizzo in 1997-1998.

     Some early	work (1999-2000) on the	dummynet traffic shaper	supported by
     Akamba Corp.

     The ipfw core (ipfw2) has been completely redesigned and reimplemented by
     Luigi Rizzo in summer 2002. Further actions and options have been added
     by	various	developer over the years.

     In-kernel NAT support written by Paolo Pisati <piso@FreeBSD.org> as part
     of	a Summer of Code 2005 project.

     SCTP nat support has been developed by The	Centre for Advanced Internet
     Architectures (CAIA) <http://www.caia.swin.edu.au>.  The primary develop-
     ers and maintainers are David Hayes and Jason But.	 For further informa-
     tion visit: <http://www.caia.swin.edu.au/urp/SONATA>

     Delay profiles have been developed	by Alessandro Cerri and	Luigi Rizzo,
     supported by the European Commission within Projects Onelab and Onelab2.

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

     Incoming packet fragments diverted	by divert are reassembled before
     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 diverted to userland, and then reinserted by a userland process
     may lose various packet attributes.  The packet source interface name
     will be preserved if it is	shorter	than 8 bytes and the userland process
     saves and reuses the sockaddr_in (as does natd(8)); otherwise, it may be
     lost.  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.

     Dummynet drops all	packets	with IPv6 link-local addresses.

     Rules using uid or	gid may	not behave as expected.	 In particular,	incom-
     ing SYN packets may have no uid or	gid associated with them since they do
     not yet belong to a TCP connection, and the uid/gid associated with a
     packet may	not be as expected if the associated process calls setuid(2)
     or	similar	system calls.

     Rule syntax is subject to the command line	environment and	some patterns
     may need to be escaped with the backslash character or quoted appropri-
     ately.

     Due to the	architecture of	libalias(3), ipfw nat is not compatible	with
     the TCP segmentation offloading (TSO).  Thus, to reliably nat your	net-
     work traffic, please disable TSO on your NICs using ifconfig(8).

     ICMP error	messages are not implicitly matched by dynamic rules for the
     respective	conversations.	To avoid failures of network error detection
     and path MTU discovery, ICMP error	messages may need to be	allowed
     explicitly	through	static rules.

     Rules using call and return actions may lead to confusing behaviour if
     ruleset has mistakes, and/or interaction with other subsystems (netgraph,
     dummynet, etc.) is	used.  One possible case for this is packet leaving
     ipfw in subroutine	on the input pass, while later on output encountering
     unpaired return first.  As	the call stack is kept intact after input
     pass, packet will suddenly	return to the rule number used on input	pass,
     not on output one.	 Order of processing should be checked carefully to
     avoid such	mistakes.

FreeBSD	10.1			 June 29, 2011			  FreeBSD 10.1

NAME | SYNOPSIS | DESCRIPTION | PACKET FLOW | SYNTAX | RULE FORMAT | LOOKUP TABLES | SETS OF RULES | STATEFUL FIREWALL | TRAFFIC SHAPER (DUMMYNET) CONFIGURATION | CHECKLIST | FINE POINTS | PACKET DIVERSION | NETWORK ADDRESS TRANSLATION (NAT) | SYSCTL VARIABLES | EXAMPLES | SEE ALSO | HISTORY | AUTHORS | BUGS

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