*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->FreeBSD man pages -> ipfw (8)              



NAME    [Toc]    [Back]

     ipfw -- IP firewall and traffic shaper control program

SYNOPSIS    [Toc]    [Back]

     ipfw [-cq] add rule
     ipfw [-acdefnNStT] {list | show} [rule | first-last ...]
     ipfw [-f | -q] flush
     ipfw [-q] {delete | zero | resetlog} [set] [number ...]
     ipfw enable {firewall | one_pass | debug | verbose | dyn_keepalive}
     ipfw disable {firewall | one_pass | debug | verbose | dyn_keepalive}

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

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

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

DESCRIPTION    [Toc]    [Back]

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

	 NOTE: this manual page documents the newer version of ipfw introduced
	 in FreeBSD CURRENT in July 2002, also known as ipfw2.	ipfw2 is a
	 superset of the old firewall, ipfw1.  The differences between the two
	 are listed in Section IPFW2 ENHANCEMENTS, which you are encouraged to
	 read to revise older rulesets and possibly write them more efficiently.
  See Section USING IPFW2 IN FreeBSD-STABLE for instructions
	 on how to run ipfw2 on FreeBSD STABLE.

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

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

     An ipfw ruleset always includes a default rule (numbered 65535) which
     cannot be modified 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, then ipfw assumes a stateful behaviour, i.e. upon a match it will
     create dynamic rules matching the exact parameters (addresses and ports)
     of the matching packet.

     These dynamic rules, which have a limited lifetime, are checked at the
     first occurrence of a check-state, keep-state or limit rule, and are 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

     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.

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

     The following options are available:

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

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

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

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

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

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

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

     -q      While adding, zeroing, resetlogging or flushing, be quiet about
	     actions (implies -f).  This is useful for adjusting rules by 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
	     might not be delivered to the login session, causing the remote
	     login session to be closed and the remainder of the ruleset to
	     not be processed.	Access to the console would then be required
	     to recover.

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

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

     -t      While listing, show last match timestamp (converted with

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

     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 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 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 as passed on to the preprocessor
 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.

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

     If the world and the kernel get out of sync the ipfw ABI may break, preventing
 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    [Toc]    [Back]

     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_input]		    [ip_output]   net.inet.ip.fw.enable=1
		 |			 |
		 ^			 V
	   [ether_demux]    [ether_output_frame]  net.link.ether.ipfw=1
		 |			 |
		 +-->--[bdg_forward]-->--+	  net.link.ether.bridge_ipfw=1
		 ^			 V
		 |	to devices	 |

     As can be noted from the above picture, the number of times the same
     packet goes through the firewall can vary between 0 and 4 depending on
     packet source and destination, and system configuration.

     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().

     Also note that each packet is always checked against the complete ruleset,
 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() ), 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 possible places.
     skipto rules can be useful here, as an example:

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

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

SYNTAX    [Toc]    [Back]

     In general, each keyword or argument must be provided as a separate command
 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).

     In ipfw2 you can introduce spaces after commas ',' 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,
	   ipfw -q add deny src-ip,
	   ipfw "-q add deny src-ip,"

RULE FORMAT    [Toc]    [Back]

     The format of ipfw rules is the following:

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

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

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

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

	     Each rule is associated with a rule_number in the range 1..65535,
	     with the latter reserved for the default rule.  Rules are checked
	     sequentially by rule number.  Multiple rules can have the same
	     number, in which case they are checked (and listed) according to
	     the order in which they have been added.  If a rule is entered
	     without specifying a number, the kernel will assign one in such a
	     way that the rule becomes the last one before the default rule.
	     Automatic rule numbers are assigned by incrementing the last nondefault
 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

     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
	     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(4)) 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 logging
 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.

   RULE ACTIONS    [Toc]    [Back]
     A rule can be associated with one of the following actions, which will be
     executed when the packet matches the body of the rule.

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

	     Checks the packet against the dynamic ruleset.  If a match is
	     found, execute the action associated with the rule which generated
 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 continues
 with the next rule.

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

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

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

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

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

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

     reject  (Deprecated).  Synonym for unreach host.

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

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

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

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

   RULE BODY    [Toc]    [Back]
     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.  Individual
 patterns can be prefixed by the not operator to reverse the result
     of the match, as in

	   ipfw add 100 allow ip from not to any

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

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

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

     The body of a rule must in general include a source and destination
     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
     ipfw1.  In ipfw2 any match pattern (including MAC headers, IPv4 protocols,
 addresses and ports) can be specified in the options section.

     Rule fields have the following meaning:

     proto: protocol | { protocol or ... }

     protocol: [not] protocol-name | protocol-number
	     An IPv4 protocol specified by number or name (for a complete list
	     see /etc/protocols).  The ip or all keywords mean any protocol
	     will match.

	     The { protocol or ... } format (an or-block) is provided for convenience
 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

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

     addr: [not] {any | me | addr-list | addr-set}

     any     matches any IP address.

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

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

	     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.

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

		     Matches all addresses with base addr (specified as a dotted
 quad or a hostname) and the mask of mask, specified
		     as a dotted quad.	As an example,
		     will match 1.*.3.*.  We suggest to use this form only for
		     non-contiguous masks, and resort to the addr/masklen format
 for contiguous masks, which is more compact and less

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

     list: {num | num-num}[,list]
	     Matches all addresses with base address addr (specified as a dotted
 quad or a hostname) and whose last byte is in the list
	     between braces { } .  Note that there must be no spaces between
	     braces 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 bitmask,
 it takes constant time and dramatically reduces the complexity
 of rulesets.
	     As an example, an address specified as{128,35-55,89}
	     will match the following IP addresses:, to, .

     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

	     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

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

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

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

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

     // 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.

	     Matches only bridged packets.

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

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

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

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

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

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

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

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

     ipid id-list
	     Matches IP 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 IP header contains the comma separated list
	     of options specified in spec.  The supported IP options are:

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

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

     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_FILTERGIF 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 without
 IPSEC support.  It does not affect rule processing when given
	     and the rules are handled as if with no ipsec flag.

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

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

     ipttl ttl-list
	     Matches IP 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.

	     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) variables),
 and the lifetime is refreshed every time a matching
	     packet is found.

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

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

     { MAC | mac } dst-mac src-mac
	     Match packets with a given dst-mac and src-mac addresses, specified
 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 IPv4 protocol.

     recv | xmit | via {ifX | if* | ipno | any}
	     Matches packets received, transmitted or going through, respectively,
 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

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

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

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

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

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

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

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

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

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

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

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

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

	     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.

SETS OF RULES    [Toc]    [Back]

     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 multiple
 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 execution
 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    [Toc]    [Back]

     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 configuration,
 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

	   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.


     ipfw is also the user interface for the dummynet(4) traffic shaper.

     dummynet operates by first using the firewall to classify packets and
     divide them into flows, using any match pattern that can be used in ipfw
     rules.  Depending on local policies, a flow can contain packets for a
     single TCP connection, or from/to a given host, or entire subnet, or a
     protocol type, etc.

     Packets belonging to the same flow are then passed to either of two different
 objects, which implement the traffic regulation:

	 pipe	 A pipe emulates a link with given bandwidth, propagation
		 delay, queue size and packet loss rate.  Packets are queued
		 in front of the pipe as they come out from the classifier,
		 and then transferred to the pipe according to the pipe's

	 queue	 A queue is an abstraction used to implement the WF2Q+ (Worstcase
 Fair Weighted Fair Queueing) policy, which is an efficient
 variant of the WFQ policy.
		 The queue associates a weight and a reference pipe to each
		 flow, and then all backlogged (i.e., with packets queued)
		 flows linked to the same pipe share the pipe's bandwidth proportionally
 to their weights.	Note that weights are not priorities;
 a flow with a lower weight is still guaranteed to
		 get its fraction of the bandwidth even if a flow with a
		 higher weight is permanently backlogged.

     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 flow
     share the available bandwidth.

     The pipe and queue configuration commands are the following:

	   pipe number config pipe-configuration

	   queue number config queue-configuration

     The following parameters can be configured for a pipe:

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

	     A value of 0 (default) means unlimited bandwidth.	The unit must
	     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).  Default value is 0,
	     meaning no delay.

     The following parameters can be configured for a queue:

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

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

     buckets hash-table-size
	   Specifies the size of the hash table used for storing the various
	   queues.  Default value is 64 controlled by the sysctl(8) variable
	   net.inet.ip.dummynet.hash_size, allowed range is 16 to 65536.

     mask mask-specifier
	   Packets sent to a given pipe or queue by an ipfw rule can be further
 classified into multiple flows, each of which is then sent to
	   a different dynamic pipe or queue.  A flow identifier is constructed
 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 matching
 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

	   dst-ip mask, src-ip mask, dst-port mask, src-port mask, proto mask
	   or all,

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

	   When 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 maxsized
 ethernet packets (1500 bytes) mean 600Kbit or 20s of queue on
	   a 30Kbit/s pipe.  Even worse effect can result if you get packets
	   from an interface with a much larger MTU, e.g. the loopback interface
 with its 16KB packets.

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

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

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

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

CHECKLIST    [Toc]    [Back]

     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 such as the one in

     +o	 Don't forget the loopback interface.

FINE POINTS    [Toc]    [Back]

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

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

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

	       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

PACKET DIVERSION    [Toc]    [Back]

     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

SYSCTL VARIABLES    [Toc]    [Back]

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

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

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

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

     net.inet.ip.dummynet.red_lookup_depth: 256

     net.inet.ip.dummynet.red_avg_pkt_size: 512

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

     net.inet.ip.fw.autoinc_step: 100
	     Delta between rule numbers when auto-generating them.  The value
	     must be in the range 1..1000.  This variable is only present in
	     ipfw2, the delta is hardwired to 100 in ipfw1.

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

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

     net.inet.ip.fw.dyn_buckets: 256
	     The number of buckets in the hash table for dynamic rules.  Must
	     be a power of 2, up to 65536.  It only takes effect when all
	     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 connection
 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.inet.ip.fw.one_pass: 1
	     When set, the packet exiting from the dummynet(4) pipe is not
	     passed though the firewall again.	Otherwise, after a pipe
	     action, the packet is reinjected into the firewall at the next

     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.link.ether.ipfw: 0
	     Controls whether layer-2 packets are passed to ipfw.  Default is

     net.link.ether.bridge_ipfw: 0
	     Controls whether bridged packets are passed to ipfw.  Default is

USING IPFW2 IN FreeBSD-STABLE    [Toc]    [Back]

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

IPFW2 ENHANCEMENTS    [Toc]    [Back]

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

     Syntax and flags
	     ipfw1 does not support the -n flag (only test syntax), nor it
	     allows spaces after commas or supports all rule fields in a single

     Handling of non-IPv4 packets
	     ipfw1 will silently accept all non-IPv4 packets (which ipfw1 will
	     only see when net.link.ether.bridge_ipfw=1).  ipfw2 will filter
	     all packets (including non-IPv4 ones) according to the ruleset.
	     To achieve the same behaviour as ipfw1 you can use the following
	     as the very first rule in your ruleset:

		   ipfw add 1 allow layer2 not mac-type ip

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

	     ipfw1 does not supports address sets or lists of addresses.

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

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

	     ipfw1 does not support Or-blocks.

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

     Sets of rules
	     ipfw1 does not implement sets of rules.

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

	     In ipfw1, the following options only accept a single value as an

	     ipid, iplen, ipttl

	     The following options are not impleme

 Similar pages
Name OS Title
dummynet FreeBSD traffic shaper, bandwidth manager and delay emulator
iftcntl Tru64 View and modify network interface traffic control settings
pppctl FreeBSD PPP control program
camcontrol FreeBSD CAM control program
dtscp HP-UX DTS control program
pppctl OpenBSD PPP control program
cdscp HP-UX Starts the CDS control program
stacker IRIX robotics control program
rpccp HP-UX Starts the RPC control program
timedc FreeBSD timed control program
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service