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

     ipfw -- IP firewall and traffic shaper control program

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

     The ipfw utility is the user interface for controlling the ipfw(4) fire-
     wall 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 effi-
         ciently.  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 rein-
     jected 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 asso-
     ciated 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 typ-
     ically used to open the firewall on-demand to legitimate traffic only.
     See the STATEFUL FIREWALL 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

     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 pass-
             ing 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 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
             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 charac-
     ter, the usual PATH name search is performed.  Care should be taken with
     this in environments where not all file systems are mounted (yet) by the
     time ipfw is being run (e.g. when they are mounted over NFS).  Once -p
     has been specified, any additional arguments as passed on to the pre-
     processor for interpretation.  This allows for flexible configuration
     files (like conditionalizing them on the local hostname) and the use of
     macros to centralize frequently required arguments like IP addresses.

     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, pre-
     venting you from being able to add any rules.  This can adversely effect
     the booting process.  You can use ipfw disable firewall to temporarily
     disable the firewall to regain access to the network, allowing you to fix
     the problem.

     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]
                 |                       |
                 ^                       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 rule-
     set, irrespective of the place where the check occurs, or the source of
     the packet.  If a rule contains some match patterns or actions which are
     not valid for the place of invocation (e.g. trying to match a MAC header
     within ip_input() ), 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).

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

     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,"

     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 fil-
     tering 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, data-
                                              gram length, identification,
                                              fragment flag (non-zero IP off-
                                              set), Time To Live
        IP options
        Misc. TCP header fields               TCP flags (SYN, FIN, ACK, RST,
                                              etc.), sequence number, acknowl-
                                              edgment number, window
        TCP options
        ICMP types                            for ICMP packets
        User/group ID                         When the packet can be associ-
                                              ated 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 non-
             default rule number by the value of the sysctl variable
             net.inet.ip.fw.autoinc_step which defaults to 100.  If this is
             not possible (e.g. because we would go beyond the maximum allowed
             rule number), the number of the last non-default value is used

     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 lead-
             ing to out-of-order packet delivery.

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

     log [logamount number]
             When a packet matches a rule with the log keyword, a message will
             be logged to syslogd(8) with a LOG_SECURITY facility.  The log-
             ging only occurs if the sysctl variable net.inet.ip.fw.verbose is
             set to 1 (which is the default when the kernel is compiled with
             IPFIREWALL_VERBOSE ) and the number of packets logged so far for
             that particular rule does not exceed the logamount parameter.  If
             no logamount is specified, the limit is taken from the sysctl
             variable net.inet.ip.fw.verbose_limit.  In both cases, a value of
             0 removes the logging limit.

             Once the limit is reached, logging can be re-enabled by clearing
             the logging counter or the packet counter for that entry, see the
             resetlog command.

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

     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 gener-
             ated this dynamic rule, otherwise move to the next rule.
             Check-state rules do not have a body.  If no check-state rule is
             found, the dynamic ruleset is checked at the first keep-state or
             limit rule.

     count   Update counters for all packets that match rule.  The search con-
             tinues with the next rule.

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

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

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

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

     queue queue_nr
             Pass packet to a dummynet(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 termi-

     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.

     The body of a rule contains zero or more patterns (such as specific
     source and destination addresses or ports, protocol options, incoming or
     outgoing interfaces, etc.)  that the packet must match in order to be
     recognised.  In general, the patterns are connected by (implicit) and
     operators -- i.e. all must match in order for the rule to match.  Indi-
     vidual patterns can be prefixed by the not operator to reverse the result
     of the match, as in

           ipfw add 100 allow ip from not 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 proto-
     cols, 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 con-
             venience only but its use is deprecated.

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

             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 dot-
                     ted 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 dot-
                     ted 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 for-
                     mat 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 dot-
             ted 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 bit-
             mask, it takes constant time and dramatically reduces the com-
             plexity 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 sup-
             ported ICMP types are:

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

     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.

     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) vari-
             ables), and the lifetime is refreshed every time a matching
             packet is found.

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

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

     { MAC | mac } dst-mac src-mac
             Match packets with a given dst-mac and src-mac addresses, speci-
             fied as the any keyword (matching any MAC address), or six groups
             of hex digits separated by colons, and optionally followed by a
             mask indicating how many bits are significant, as in

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

             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 val-
             ues or ranges).  You can use symbolic names for known values such
             as vlan, ipv4, ipv6.  Values can be entered as decimal or hexa-
             decimal (if prefixed by 0x), and they are always printed as hexa-
             decimal (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, respec-
             tively, the interface specified by exact name (ifX), by device
             name (if*), by IP address, or through some interface.

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

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

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

             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) speci-
             fied 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.

     Each rule belongs to one of 32 different sets , numbered 0 to 31.  Set 31
     is reserved for the default rule.

     By default, rules are put in set 0, unless you use the set N attribute
     when entering a new rule.  Sets can be individually and atomically
     enabled or disabled, so this mechanism permits an easy way to store mul-
     tiple configurations of the firewall and quickly (and atomically) switch
     between them.  The command to enable/disable sets is

           ipfw set [disable number ...] [enable number ...]

     where multiple enable or disable sections can be specified.  Command exe-
     cution is atomic on all the sets specified in the command.  By default,
     all sets are enabled.

     When you disable a set, its rules behave as if they do not exist in the
     firewall configuration, with only one exception:

           dynamic rules created from a rule before it had been disabled will
           still be active until they expire. In order to delete dynamic rules
           you have to explicitly delete the parent rule which generated them.

     The set number of rules can be changed with the command

           ipfw set move {rule rule-number | old-set} to new-set

     Also, you can atomically swap two rulesets with the command

           ipfw set swap first-set second-set

     See the EXAMPLES Section on some possible uses of sets of rules.

     Stateful operation is a way for the firewall to dynamically create rules
     for specific flows when packets that match a given pattern are detected.
     Support for stateful operation comes through the check-state, keep-state
     and limit options of rules.

     Dynamic rules are created when a packet matches a keep-state or limit
     rule, causing the creation of a dynamic rule which will match all and
     only packets with a given protocol between a src-ip/src-port
     dst-ip/dst-port pair of addresses ( src and dst are used here only to
     denote the initial match addresses, but they are completely equivalent
     afterwards).  Dynamic rules will be checked at the first check-state,
     keep-state or limit occurrence, and the action performed upon a match
     will be the same as in the parent rule.

     Note that no additional attributes other than protocol and IP addresses
     and ports are checked on dynamic rules.

     The typical use of dynamic rules is to keep a closed firewall configura-
     tion, but let the first TCP SYN packet from the inside network install a
     dynamic rule for the flow so that packets belonging to that session will
     be allowed through the firewall:

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

     A similar approach can be used for UDP, where an UDP packet coming from
     the inside will install a dynamic rule to let the response through the

           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 dif-
     ferent 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+ (Worst-
                 case Fair Weighted Fair Queueing) policy, which is an effi-
                 cient 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 pro-
                 portionally to their weights.  Note that weights are not pri-
                 orities; 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 fur-
           ther classified into multiple flows, each of which is then sent to
           a different dynamic pipe or queue.  A flow identifier is con-
           structed by masking the IP addresses, ports and protocol types as
           specified with the mask options in the configuration of the pipe or
           queue.  For each different flow identifier, a new pipe or queue is
           created with the same parameters as the original object, and match-
           ing packets are sent to it.

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

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

     red | gred w_q/min_th/max_th/max_p
           Make use of the RED (Random Early Detection) queue management algo-
           rithm.  w_q and max_p are floating point numbers between 0 and 1 (0
           not included), while min_th and max_th are integer numbers specify-
           ing thresholds for queue management (thresholds are computed in
           bytes if the queue has been defined in bytes, in slots otherwise).
           The dummynet(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).

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

     +o   Remember that you filter both packets going in and out.  Most connec-
         tions need packets going in both directions.

     +o   Remember to test very carefully.  It is a good idea to be near the
         console when doing this.  If you cannot be near the console, use an
         auto-recovery script such as the one in

     +o   Don't forget the loopback interface.

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

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

     +o   If you are logged in over a network, loading the kld(4) version of
         ipfw is probably not as straightforward as you would think.  I recom-
         mend 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

     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 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 traf-
             fic.  You can disable this by setting the variable to 0, in which
             case the pipes/queues will only be deleted when the threshold is

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

     net.inet.ip.dummynet.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 con-
             nection every 5 seconds for the last 20 seconds of the lifetime
             of the rule.

     net.inet.ip.fw.dyn_max: 8192
             Maximum number of dynamic rules.  When you hit this limit, no
             more dynamic rules can be installed until old ones expire.

     net.inet.ip.fw.dyn_ack_lifetime: 300

     net.inet.ip.fw.dyn_syn_lifetime: 20

     net.inet.ip.fw.dyn_fin_lifetime: 1

     net.inet.ip.fw.dyn_rst_lifetime: 1

     net.inet.ip.fw.dyn_udp_lifetime: 5

     net.inet.ip.fw.dyn_short_lifetime: 30
             These variables control the lifetime, in seconds, of dynamic
             rules.  Upon the initial SYN exchange the lifetime is kept short,
             then increased after both SYN have been seen, then decreased
             again during the final FIN exchange or when a RST is received.
             Both dyn_fin_lifetime and dyn_rst_lifetime must be strictly lower
             than 5 seconds, the period of repetition of keepalives. The fire-
             wall 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. 0
             Controls whether layer-2 packets are passed to ipfw.  Default is
             no. 0
             Controls whether bridged packets are passed to ipfw.  Default is

     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).

     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 sin-
             gle argument.  ipfw1 does not allow the -f flag (force) in con-
             junction with the -p flag (preprocessor).  ipfw1 does not support
             the -c (compact) flag.

     Handling of non-IPv4 packets
             ipfw1 will silently accept all non-IPv4 packets (which ipfw1 will
             only see when  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 30 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 syn-
             tax 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 has
             no effect there.

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

             ipid, iplen, ipttl

             The following options are not implemented by ipfw1:

             dst-ip, dst-port, layer2, mac, mac-type, src-ip, src-port.

             Additionally, the RELENG_4 version of ipfw1 does not implement
             the following options:

             ipid, iplen, ipprecedence, iptos, ipttl, ipversion, tcpack,
             tcpseq, tcpwin.

     Dummynet options
             The following option for dummynet pipes/queues is not supported:

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

     This command adds an entry which denies all tcp packets from to the telnet port of from being for-
     warded by the host:

           ipfw add deny tcp from to telnet

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

           ipfw add deny ip from to

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

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

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

     If you administer one or more subnets, you can take advantage of the
     ipfw2 syntax to specify address sets and or-blocks and write extremely
     compact rulesets which selectively enable services to blocks of clients,
     as below:

           goodguys="{{20,35,66,18} or{6,3,11} }"

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

     The ipfw1 syntax would require a separate rule for each IP in the above

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

           ipfw add deny ip from any to any not verrevpath in

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

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

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

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

     To limit the number of connections a user can open you can use the fol-
     lowing type of rules:

           ipfw add allow tcp from my-net/24 to any setup limit src-addr 10
           ipfw add allow tcp from any to me setup limit src-addr 4

     The former (assuming it runs on a gateway) will allow each host on a /24
     network to open at most 10 TCP connections.  The latter can be placed on
     a server to make sure that a single client does not use more than 4
     simultaneous connections.

     BEWARE: stateful rules can be subject to denial-of-service attacks by a
     SYN-flood which opens a huge number of dynamic rules.  The effects of
     such attacks can be partially limited by acting on a set of sysctl(8)
     variables which control the operation of the firewall.

     Here is a good usage of the list command to see accounting records and
     timestamp information:

           ipfw -at list

     or in short form without timestamps:

           ipfw -a list

     which is equivalent to:

           ipfw show

     Next rule diverts all incoming packets from to divert port

           ipfw divert 5000 ip from to any in

     The following rules show some of the applications of ipfw and dummynet(4)
     for simulations and the like.

     This rule drops random incoming packets with a probability of 5%:

           ipfw add prob 0.05 deny ip from any to any in

     A similar effect can be achieved making use of dummynet pipes:

           ipfw add pipe 10 ip from any to any
           ipfw pipe 10 config plr 0.05

     We can use pipes to artificially limit bandwidth, e.g. on a machine act-
     ing as a router, if we want to limit traffic from local clients on we do:

           ipfw add pipe 1 ip from to any out
           ipfw pipe 1 config bw 300Kbit/s queue 50KBytes

     note that we use the out modifier so that the rule is not used twice.
     Remember in fact that ipfw rules are checked both on incoming and outgo-
     ing packets.

     Should we want to simulate a bidirectional link with bandwidth limita-
     tions, the correct way is the following:

           ipfw add pipe 1 ip from any to any out
           ipfw add pipe 2 ip from any to any in
           ipfw pipe 1 config bw 64Kbit/s queue 10Kbytes
           ipfw pipe 2 config bw 64Kbit/s queue 10Kbytes

     The above can be very useful, e.g. if you want to see how your fancy Web
     page will look for a residential user who is connected only through a
     slow link.  You should not use only one pipe for both directions, unless
     you want to simulate a half-duplex medium (e.g. AppleTalk, Ethernet,
     IRDA).  It is not necessary that both pipes have the same configuration,
     so we can also simulate asymmetric links.

     Should we want to verify network performance with the RED queue manage-
     ment algorithm:

           ipfw add pipe 1 ip from any to any
           ipfw pipe 1 config bw 500Kbit/s queue 100 red 0.002/30/80/0.1

     Another typical application of the traffic shaper is to introduce some
     delay in the communication.  This can significantly affect applications
     which do a lot of Remote Procedure Calls, and where the round-trip-time
     of the connection often becomes a limiting factor much more than band-

           ipfw add pipe 1 ip from any to any out
           ipfw add pipe 2 ip from any to any in
           ipfw pipe 1 config delay 250ms bw 1Mbit/s
           ipfw pipe 2 config delay 250ms bw 1Mbit/s

     Per-flow queueing can be useful for a variety of purposes.  A very simple
     one is counting traffic:

           ipfw add pipe 1 tcp from any to any
           ipfw add pipe 1 udp from any to any
           ipfw add pipe 1 ip from any to any
           ipfw pipe 1 config mask all

     The above set of rules will create queues (and collect statistics) for
     all traffic.  Because the pipes have no limitations, the only effect is
     collecting statistics.  Note that we need 3 rules, not just the last one,
     because when ipfw tries to match IP packets it will not consider ports,
     so we would not see connections on separate ports as different ones.

     A more sophisticated example is limiting the outbound traffic on a net
     with per-host limits, rather than per-network limits:

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

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

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

     To delete a set of rules atomically the command is simply:

           ipfw delete set 18

     To test a ruleset and disable it and regain control if something goes

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

     Here if everything goes well, you press control-C before the "sleep" ter-
     minates, and your ruleset will be left active. Otherwise, e.g. if you
     cannot access your box, the ruleset will be disabled after the sleep ter-
     minates thus restoring the previous situation.

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

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

     !!! WARNING !!!

     Misconfiguring the firewall can put your computer in an unusable state,
     possibly shutting down network services and requiring console access to
     regain control of it.

     Incoming packet fragments diverted by divert or tee are reassembled
     before delivery to the socket.  The action used on those packet is the
     one from the rule which matches the first fragment of the packet.

     Packets 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.

     Packets diverted to userland, and then reinserted by a userland process
     may lose various packet attributes.  The packet source interface name
     will be preserved if it is shorter than 8 bytes and the userland process
     saves and reuses the sockaddr_in (as does natd(8)); otherwise, it may be
     lost.  If a packet is reinserted in this manner, later rules may be
     incorrectly applied, making the order of divert rules in the rule
     sequence very important.

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

     API based upon code written by Daniel Boulet for BSDI.

     Work on dummynet(4) traffic shaper supported by Akamba Corp.

     The ipfw utility first appeared in FreeBSD 2.0.  dummynet(4) was intro-
     duced in FreeBSD 2.2.8.  Stateful extensions were introduced in
     FreeBSD 4.0.  ipfw2 was introduced in Summer 2002.

FreeBSD 4.10                    August 13, 2002                   FreeBSD 4.10


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