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

     ipfw - controlling utility for IP firewall

     ipfw [-q] file
     ipfw [-f | -q] flush
     ipfw [-q] zero [number ...]
     ipfw delete number ...
     ipfw [-aftN] list [number ...]
     ipfw [-ftN] show [number ...]
     ipfw [-q] add [number] action [log] proto from src to dst
          [via name | ipno] [options]

     If used as shown in the first synopsis line, the file will be read line
     by line and applied as arguments to the ipfw command.

     The ipfw code works by going through the rule-list for each packet, until
     a match is found.  All rules have two associated counters, a packet count
     and a byte count.  These counters are updated when a packet matches the

     The rules are ordered by a ``line-number'' from 1 to 65534 that is used
     to order and delete rules. Rules are tried in increasing order, and the
     first rule that matches a packet applies.  Multiple rules may share the
     same number and apply in the order in which they were added.

     If a rule is added without a number, it is numbered 100 higher than the
     previous rule. If the highest defined rule number is greater than 65434,
     new rules are appended to the last rule.

     The delete operation deletes the first rule with number number, if any.

     The list command prints out the current rule set.

     The show command is equivalent to `ipfw -a list'.

     The zero operation zeroes the counters associated with rule number

     The flush operation removes all rules.

     Any command beginning with a '#', or being all blank, is ignored.

     One rule is always present:

           65535 deny all from any to any

     This rule is the default policy, i.e., don't allow anything at all.  Your
     job in setting up rules is to modify this policy to match your needs.

     The following options are available:

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

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

     -q    While adding, zeroing 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, it prints a message.
           Because all rules are flushed, the message cannot be delivered to
           the login session, the login session is closed and the remainder of
           the ruleset is not processed.  Access to the console is required to

     -t    While listing, show last match timestamp.

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


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

         deny              Discard packets that match this rule.  The search
                           terminates.  Drop is an alias for deny.

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

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

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

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

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

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

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

     If a packet matches more than one divert and/or tee rule, all but the
     last are ignored.

     If the kernel was compiled with IPFIREWALL_VERBOSE, then when a packet
     matches a rule with the ``log'' keyword a message will be printed on the
     console.  If the kernel was compiled with the IPFIREWALL_VERBOSE_LIMIT
     option, then logging will cease after the number of packets specified by
     the option are received for that particular chain entry.  Logging may
     then be re-enabled by clearing the packet counter for that entry.

     Console logging and the log limit are adjustable dynamically through the
     sysctl(8) interface.


         ip                All packets match. The alias all has the same

         tcp               Only TCP packets match.

         udp               Only UDP packets match.

         icmp              Only ICMP packets match.

         _number|name_     Only packets for the specified protocol matches
                           (see /etc/protocols for a complete list).

     src and dst:

         _address/mask_ [ports]

     The _address/mask_ may be specified as:

         ipno              An ipnumber of the form  Only this exact
                           ip number match the rule.

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

         ipno:mask         An ipnumber with a mask width of the form
                   In this case all ip numbers
                           from to will match.

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

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


     Service names (from /etc/services) may be used instead of numeric port
     values.  A range may only be specified as the first value, and the length
     of the port list is limited to IP_FW_MAX_PORTS (as defined in
     /usr/src/sys/netinet/ip_fw.h) ports.

     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

     Rules can apply to packets when they are incoming, or outgoing, or both.
     The in keyword indicates the rule should only match incoming packets.
     The out keyword indicates the rule should only match outgoing packets.

     To match packets going through a certain interface, specify the interface
     using via:

         via ifX           Packet must be going through interface ifX.

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

         via any           Packet must be going through some interface.

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

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

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

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

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

     Additional options:

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

         in                Matches if this packet was on the way in.

         out               Matches if this packet was on the way out.

         ipoptions spec    Matches if the 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 ``!''.

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

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

         tcpflags spec     Matches if the TCP header contains the comma
                           separated list of flags specified in spec.  The
                           supported TCP flags are: fin, syn, rst, psh, ack,
                           and urg.  The absence of a particular flag may be
                           denoted with a ``!''.  A rule which contains a
                           tcpflags specification can never match a fragmented
                           packet which has a non-zero offset.  See the frag
                           option for details on matching fragmented packets.

         icmptypes types   Matches if the ICMP type is in the list types.  The
                           list may be specified as any combination of ranges
                           or individual types separated by commas.

     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.

         +o   Don't forget the loopback interface.

     There is one kind of packet that the firewall will always discard, that
     is an IP fragment with a fragment offset of one.  This is a valid packet,
     but it only has one use, to try to circumvent firewalls.

     If you are logged in over a network, loading the LKM version of ipfw is
     probably not as straightforward as you would think.  I recommend this
     command line:

           modload /lkm/ipfw_mod.o && \
           ipfw add 32000 allow all from any to any

     Along the same lines, doing an

           ipfw flush

     in similar surroundings is also a bad idea.

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

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

           ipfw add deny tcp from to 23

     This one disallows any connection from the entire hackers network to my

           ipfw addf deny all from to

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

           ipfw -at l

     or in short form without timestamps:

           ipfw -a l

     This rule diverts all incoming packets from to divert port

           ipfw divert 5000 all from to any in

     ip(4), ipfirewall(4), divert(4), protocols(5), services(5), reboot(8),
     syslogd(8), sysctl(8)


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

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

     Incoming packet fragments diverted by divert are reassembled before
     delivery to the socket, whereas fragments diverted via tee are not.

     Port aliases containing dashes cannot be first in a list.

     Ugen J. S. Antsilevich,
     Poul-Henning Kamp,
     Alex Nash,
     Archie Cobbs.
     Archie Cobbs.  API based upon code written by
     Daniel Boulet for BSDI.

     Ipfw first appeared in FreeBSD 2.0.

FreeBSD                          July 20, 1996                         FreeBSD


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