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

     ipfw — IP firewall and traffic shaper control program

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

     ipfw is the user interface for controlling the ipfirewall(4)

     Each incoming or outgoing packet is passed through the ipfw rules.	 If
     the host is acting as a gateway, packets forwarded by the gateway are
     processed by ipfw twice.  When the host is acting as a bridge, packets
     forwarded by the bridge are processed by ipfw once.

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

     A configuration always includes a DEFAULT rule (numbered 65535) which
     cannot be modified by the programmer and always matches 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 option,
     then ipfw assumes a stateful behaviour, i.e. upon a match will create
     dynamic rules matching the exact parameters (addresses and ports) of the
     matching packet.

     These dynamic rules, which have a limited lifetime, are checked at the
     first occurrence of a check-state or keep-state rule, and are typically
     used to open the firewall on-demand to legitimate traffic only.  See the
     RULE FORMAT and EXAMPLES sections below for more information on the
     stateful behaviour of ipfw.

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

     Rules can be added with the add command; deleted individually with the
     delete command, and globally with the flush command; displayed, option‐
     ally with the content of the counters, using the show and list commands.
     Finally, counters can be reset with the zero and resetlog commands.

     The following options are available:

     -a	     While listing, show counter values.  See also the show command.

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

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

     -t	     While listing, show last match timestamp.

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

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

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

     The ipfw rule format is the following:

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

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

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

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

     prob match_probability
	     A match is only declared with the specified probability (floating
	     point number between 0 and 1).  This can be useful for a number
	     of applications such as random packet drop.


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

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

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

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

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

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

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

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

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

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

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

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

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

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

     src and dst:
	     any | me | [not] ⟨address/mask⟩ [ports]

	     Specifying any makes the rule match any IP number.

	     Specifying me makes the rule match any IP number configured on an
	     interface in the system.  This is a computationally semi-expen‐
	     sive check which should be used with care.

	     The ⟨address/mask⟩ may be specified as:

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

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

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

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

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


	     The ‘-’ notation specifies a range of ports (including bound‐

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

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

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

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

	     Some combinations of the following specifiers are allowed:

	     in	       Only match incoming packets.

	     out       Only match outgoing packets.

	     via ifX   Packet must be going through interface ifX.

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

	     via any   Packet must be going through some interface.

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

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

		   ipfw add 100 deny ip from any to any out recv ed0 xmit ed1

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

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


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

		     The actual behaviour can be modified by specifying a dif‐
		     ferent method, although at the moment only the default
		     one is specified.

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

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

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

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

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

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

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

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

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

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

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

		     echo reply (0), destination unreachable (3), source
		     quench (4), redirect (5), echo request (8), router adver‐
		     tisement (9), router solicitation (10), time-to-live
		     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).

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

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

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

     ·	 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.

     ·	 Don't forget the loopback interface.

     ·	 There is one kind of packet that the firewall will always discard,
	 that is a TCP packet's fragment with a fragment offset of one.	 This
	 is a valid packet, but it only has one use, to try to circumvent
	 firewalls.  When logging is enabled, these packets are reported as
	 being dropped by rule -1.

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

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

     A set of sysctl(8) variables controls the behaviour of the firewall.
     These are shown below together with their default value and meaning:

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

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

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

     net.inet.ip.fw.verbose_limit: 0
	     Limits the number of messages produced by a verbose firewall.

     net.inet.ip.fw.dyn_buckets: 256

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

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

     net.inet.ip.fw.dyn_max: 1000
	     Maximum number of dynamic rules.  When you hit this limit, no
	     more dynamic rules can be installed until old ones expire.

     net.inet.ip.fw.dyn_ack_lifetime: 300

     net.inet.ip.fw.dyn_syn_lifetime: 20

     net.inet.ip.fw.dyn_fin_lifetime: 20

     net.inet.ip.fw.dyn_rst_lifetime: 5

     net.inet.ip.fw.dyn_short_lifetime: 30
	     These variables control the lifetime, in seconds, of dynamic
	     rules.  Upon the initial SYN exchange the lifetime is kept short,
	     then increased after both SYN have been seen, then decreased
	     again during the final FIN exchange or when a RST

     This command adds an entry which denies all tcp packets from
     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 crackers network to my

	   ipfw add deny ip from to my.host.org

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

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

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

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

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

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

     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

     Next rule diverts all incoming packets from to divert port

	   ipfw divert 5000 ip from to any in

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

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


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

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

     Incoming packet fragments diverted by divert or tee are reassembled
     before delivery to the socket.

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

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

     API based upon code written by Daniel Boulet for BSDI.

     The ipfw utility first appeared in FreeBSD 2.0.  Stateful extensions were
     introduced in FreeBSD 4.0.

Darwin				 July 2, 2003				Darwin

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