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OpenDarwin 7.2.1 - man page for ipfw (opendarwin section 8)

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 parame-
     ters (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 informa-
     tion on the stateful behaviour of ipfw.

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

     Rules can be added with the add command; deleted individually with the delete command, and
     globally 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

     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 executing 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 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 doesn't start with a
     slash ('/') as its first character, 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 terminates.  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 terminates.

	     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 terminates.

	     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 rule.

	     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 terminates and the original packet is accepted (but
		     see section 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-reach-
		     able 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 speci-
		     fied) 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 IPFIREWALL_FORWARD option.

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

     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.  However, if logamount
	     number is used, that number will be the logging limit rather than
	     net.inet.ip.fw.verbose_limit, where the value ``0'' removes the logging limit.  Log-
	     ging 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 net.inet.ip.fw.

     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-expensive check which should be used with

	     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 modi-
	     fier, 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 as:


	     The '-' notation specifies a range of ports (including boundaries).

	     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

	     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 name:

		   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

	     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 behav-
		     iour 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 lifetime is refreshed
		     every time a matching packet is found.

		     The actual behaviour can be modified by specifying a different 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 specifications.

	     ipoptions spec
		     Match if the IP header contains the comma separated list of options speci-
		     fied 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 '!'.

	     tcpoptions spec
		     Match if the TCP header contains the comma separated list of options speci-
		     fied in spec.  The supported TCP options are:

		     mss (maximum segment size), window (tcp window advertisement), sack (selec-
		     tive 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 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.

	     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 supported ICMP types are:

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

	     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.

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

     o	 Remember that you filter both packets going in and out.  Most connections 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.

     o	 Don't forget the loopback interface.

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

     o	 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).

     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 dropped.

     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 ini-
	     tial 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 forwarded 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 host:

	   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 follow-
     ing 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 ini-
     tial SYN packet, which will be matched by the setup rules only for selected source/destina-
     tion 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 connection 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 rule-
     set.  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 informa-

	   ipfw -at list

     or in short form without timestamps:

	   ipfw -a list

     Next rule diverts all incoming packets from to divert port 5000:

	   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

     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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