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

NAME
     ipfw - User interface for firewall, traffic shaper, packet scheduler, in-
     kernel NAT.

SYNOPSIS
   FIREWALL CONFIGURATION
     ipfw [-cq] add rule
     ipfw [-acdefnNStT] [set N] {list | show} [rule | first-last ...]
     ipfw [-f | -q] [set N] flush
     ipfw [-q] [set N] {delete | zero | resetlog} [number ...]

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

   SYSCTL SHORTCUTS
     ipfw enable
          {firewall | altq | one_pass | debug | verbose | dyn_keepalive}
     ipfw disable
          {firewall | altq | one_pass | debug | verbose | dyn_keepalive}

   LOOKUP TABLES
     ipfw table number add addr[/masklen] [value]
     ipfw table number delete addr[/masklen]
     ipfw table {number | all} flush
     ipfw table {number | all} list

   DUMMYNET CONFIGURATION (TRAFFIC SHAPER AND PACKET SCHEDULER)
     ipfw {pipe | queue | sched} number config config-options
     ipfw [-s [field]] {pipe | queue | sched} {delete | list | show}
          [number ...]

   IN-KERNEL NAT
     ipfw [-q] nat number config config-options

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

DESCRIPTION
     The ipfw utility is the user interface for controlling the ipfw(4)
     firewall, the dummynet(4) traffic shaper/packet scheduler, and the in-
     kernel NAT services.

     A firewall configuration, or ruleset, is made of a list of rules numbered
     from 1 to 65535.  Packets are passed to the firewall from a number of
     different places in the protocol stack (depending on the source and
     destination of the packet, it is possible for the firewall to be invoked
     multiple times on the same packet).  The packet passed to the firewall is
     compared against each of the rules in the ruleset, in rule-number order
     (multiple rules with the same number are permitted, in which case they
     are processed in order of insertion).  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 processing.

     A 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, the firewall will have a stateful behaviour, i.e., upon a match
     it will create dynamic rules, i.e., rules that match packets with the
     same 5-tuple (protocol, source and destination addresses and ports) as
     the packet which caused their creation.  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
     commands.

     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.

     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.

   COMMAND OPTIONS
     The following general options are available when invoking ipfw:

     -a      Show counter values when listing rules.  The show command implies
             this option.

     -b      Only show the action and the comment, not the body of a rule.
             Implies -c.

     -c      When entering or showing rules, print them in compact form, i.e.,
             omitting the "ip from any to any" string when this does not carry
             any additional information.

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

     -e      When listing and -d is specified, also show expired dynamic
             rules.

     -f      Do not 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.

     -i      When listing a table (see the LOOKUP TABLES section below for
             more information on lookup tables), format values as IP
             addresses.  By default, values are shown as integers.

     -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      Be quiet when executing the add, nat, zero, resetlog or flush
             commands; (implies -f).  This is useful when updating rulesets by
             executing multiple ipfw commands in a script (e.g.,
             `sh /etc/rc.firewall'), or by processing a file with many ipfw
             rules across a remote login session.  It also stops a table add
             or delete from failing if the entry already exists or is not
             present.

             The reason why this option may be important is that for some of
             these actions, ipfw may print a message; if the action results in
             blocking the traffic to the remote client, the remote login
             session will be closed and the rest of the ruleset will not be
             processed.  Access to the console would then be required to
             recover.

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

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

     -t      When listing, show last match timestamp converted with ctime().

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

   LIST OF RULES AND PREPROCESSING
     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 does not 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 are 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.

   TRAFFIC SHAPER CONFIGURATION
     The ipfw pipe, queue and sched commands are used to configure the traffic
     shaper and packet scheduler.  See the TRAFFIC SHAPER (DUMMYNET)
     CONFIGURATION Section below for details.

     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
     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(6)_input]           [ip(6)_output]     net.inet(6).ip(6).fw.enable=1
                  |                       |
                  ^                       V
            [ether_demux]        [ether_output_frame]  net.link.ether.ipfw=1
                  |                       |
                  +-->--[bdg_forward]-->--+            net.link.bridge.ipfw=1
                  ^                       V
                  |      to devices       |

     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() or ip6_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 or ip6_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
     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).

     Some arguments (e.g., port or address lists) are comma-separated lists of
     values.  In this case, spaces after commas ',' are allowed 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 10.0.0.0/24,127.0.0.1/8
           ipfw -q add deny src-ip 10.0.0.0/24, 127.0.0.1/8
           ipfw "-q add deny src-ip 10.0.0.0/24, 127.0.0.1/8"

RULE FORMAT
     The format of firewall rules is the following:

           [rule_number] [set set_number] [prob match_probability] action
           [log [logamount number]] [altq queue] [{tag | untag} 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 and IPv6 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
        IPv6 Extension headers                Fragmentation, Hop-by-Hop
                                              options, Routing Headers, Source
                                              routing rthdr0, Mobile IPv6
                                              rthdr2, IPSec options.
        IPv6 Flow-ID
        Misc. TCP header fields               TCP flags (SYN, FIN, ACK, RST,
                                              etc.), sequence number,
                                              acknowledgment number, window
        TCP options
        ICMP types                            for ICMP packets
        ICMP6 types                           for ICMP6 packets
        User/group ID                         When the packet can be
                                              associated with a local socket.
        Divert status                         Whether a packet came from a
                                              divert socket (e.g., natd(8)).
        Fib annotation state                  Whether a packet has been tagged
                                              for using a specific FIB
                                              (routing table) in future
                                              forwarding decisions.

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

     rule_number
             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
             instead.

     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
             used.
             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) 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]
             Packets matching a rule with the log keyword will be made
             available for logging in two ways: if the sysctl variable
             net.inet.ip.fw.verbose is set to 0 (default), one can use bpf(4)
             attached to the ipfw0 pseudo interface.  This pseudo interface
             can be created after a boot manually by using the following
             command:

                   # ifconfig ipfw0 create

             Or, automatically at boot time by adding the following line to
             the rc.conf(5) file:

                   firewall_logif="YES"

             There is no overhead if no bpf(4) is attached to the pseudo
             interface.

             If net.inet.ip.fw.verbose is set to 1, packets will be logged to
             syslogd(8) with a LOG_SECURITY facility up to a maximum of
             logamount packets.  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 means unlimited logging.

             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.

     tag number
             When a packet matches a rule with the tag keyword, the numeric
             tag for the given number in the range 1..65534 will be attached
             to the packet.  The tag acts as an internal marker (it is not
             sent out over the wire) that can be used to identify these
             packets later on.  This can be used, for example, to provide
             trust between interfaces and to start doing policy-based
             filtering.  A packet can have multiple tags at the same time.
             Tags are "sticky", meaning once a tag is applied to a packet by a
             matching rule it exists until explicit removal.  Tags are kept
             with the packet everywhere within the kernel, but are lost when
             packet leaves the kernel, for example, on transmitting packet out
             to the network or sending packet to a divert(4) socket.

             To check for previously applied tags, use the tagged rule option.
             To delete previously applied tag, use the untag keyword.

             Note: since tags are kept with the packet everywhere in
             kernelspace, they can be set and unset anywhere in the kernel
             network subsystem (using the mbuf_tags(9) facility), not only by
             means of the ipfw(4) tag and untag keywords.  For example, there
             can be a specialized netgraph(4) node doing traffic analyzing and
             tagging for later inspecting in firewall.

     untag number
             When a packet matches a rule with the untag keyword, the tag with
             the number number is searched among the tags attached to this
             packet and, if found, removed from it.  Other tags bound to
             packet, if present, are left untouched.

     altq queue
             When a packet matches a rule with the altq keyword, the ALTQ
             identifier for the given queue (see altq(4)) will be attached.
             Note that this ALTQ tag is only meaningful for packets going
             "out" of IPFW, and not being rejected or going to divert sockets.
             Note that if there is insufficient memory at the time the packet
             is processed, it will not be tagged, so it is wise to make your
             ALTQ "default" queue policy account for this.  If multiple altq
             rules match a single packet, only the first one adds the ALTQ
             classification tag.  In doing so, traffic may be shaped by using
             count altq queue rules for classification early in the ruleset,
             then later applying the filtering decision.  For example,
             check-state and keep-state rules may come later and provide the
             actual filtering decisions in addition to the fallback ALTQ tag.

             You must run pfctl(8) to set up the queues before IPFW will be
             able to look them up by name, and if the ALTQ disciplines are
             rearranged, the rules in containing the queue identifiers in the
             kernel will likely have gone stale and need to be reloaded.
             Stale queue identifiers will probably result in
             misclassification.

             All system ALTQ processing can be turned on or off via ipfw
             enable altq and ipfw disable altq.  The usage of
             net.inet.ip.fw.one_pass is irrelevant to ALTQ traffic shaping, as
             the actual rule action is followed always after adding an ALTQ
             tag.

   RULE ACTIONS
     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.

     check-state
             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 | tablearg[,port]
             Change the next-hop on matching packets to ipaddr, which can be
             an IP address or a host name.  For IPv4, the next hop can also be
             supplied by the last table looked up for the packet by using the
             tablearg keyword instead of an explicit address.  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.

     nat nat_nr | tablearg
             Pass packet to a nat instance (for network address translation,
             address redirect, etc.): see the NETWORK ADDRESS TRANSLATION
             (NAT) Section for further information.

     pipe pipe_nr
             Pass packet to a dummynet ``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 ``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.

     reset6  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 | tablearg
             Skip all subsequent rules numbered less than number.  The search
             continues with the first rule numbered number or higher.  It is
             possible to use the tablearg keyword with a skipto for a computed
             skipto, but care should be used, as no destination caching is
             possible in this case so the rules are always walked to find it,
             starting from the skipto.

     call number | tablearg
             The current rule number is saved in the internal stack and
             ruleset processing continues with the first rule numbered number
             or higher.  If later a rule with the return action is
             encountered, the processing returns to the first rule with number
             of this call rule plus one or higher (the same behaviour as with
             packets returning from divert(4) socket after a divert action).
             This could be used to make somewhat like an assembly language
             ``subroutine'' calls to rules with common checks for different
             interfaces, etc.

             Rule with any number could be called, not just forward jumps as
             with skipto.  So, to prevent endless loops in case of mistakes,
             both call and return actions don't do any jumps and simply go to
             the next rule if memory cannot be allocated or stack
             overflowed/underflowed.

             Internally stack for rule numbers is implemented using
             mbuf_tags(9) facility and currently has size of 16 entries.  As
             mbuf tags are lost when packet leaves the kernel, divert should
             not be used in subroutines to avoid endless loops and other
             undesired effects.

     return  Takes rule number saved to internal stack by the last call action
             and returns ruleset processing to the first rule with number
             greater than number of corresponding call rule.  See description
             of the call action for more details.

             Note that return rules usually end a ``subroutine'' and thus are
             unconditional, but ipfw command-line utility currently requires
             every action except check-state to have body.  While it is
             sometimes useful to return only on some packets, usually you want
             to print just ``return'' for readability.  A workaround for this
             is to use new syntax and -c switch:

                   # Add a rule without actual body
                   ipfw add 2999 return via any

                   # List rules without "from any to any" part
                   ipfw -c list

             This cosmetic annoyance may be fixed in future releases.

     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.

     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.

     unreach6 code
             Discard packets that match this rule, and try to send an ICMPv6
             unreachable notice with code code, where code is a number from 0,
             1, 3 or 4, or one of these aliases: no-route, admin-prohib,
             address or port.  The search terminates.

     netgraph cookie
             Divert packet into netgraph with given cookie.  The search
             terminates.  If packet is later returned from netgraph it is
             either accepted or continues with the next rule, depending on
             net.inet.ip.fw.one_pass sysctl variable.

     ngtee cookie
             A copy of packet is diverted into netgraph, original packet
             continues with the next rule.  See ng_ipfw(4) for more
             information on netgraph and ngtee actions.

     setfib fibnum | tablearg
             The packet is tagged so as to use the FIB (routing table) fibnum
             in any subsequent forwarding decisions.  In the current
             implementation, this is limited to the values 0 through 15, see
             setfib(2).  Processing continues at the next rule.  It is
             possible to use the tablearg keyword with setfib.  If the
             tablearg value is not within the compiled range of fibs, the
             packet's fib is set to 0.

     setdscp DSCP | number | tablearg
             Set specified DiffServ codepoint for an IPv4/IPv6 packet.
             Processing continues at the next rule.  Supported values are:

             CS0 (000000), CS1 (001000), CS2 (010000), CS3 (011000), CS4
             (100000), CS5 (101000), CS6 (110000), CS7 (111000), AF11
             (001010), AF12 (001100), AF13 (001110), AF21 (010010), AF22
             (010100), AF23 (010110), AF31 (011010), AF32 (011100), AF33
             (011110), AF41 (100010), AF42 (100100), AF43 (100110), EF
             (101110), BE (000000).  Additionally, DSCP value can be specified
             by number (0..64).  It is also possible to use the tablearg
             keyword with setdscp.  If the tablearg value is not within the
             0..64 range, lower 6 bits of supplied value are used.

     reass   Queue and reassemble IP fragments.  If the packet is not
             fragmented, counters are updated and processing continues with
             the next rule.  If the packet is the last logical fragment, the
             packet is reassembled and, if net.inet.ip.fw.one_pass is set to
             0, processing continues with the next rule.  Otherwise, the
             packet is allowed to pass and the search terminates.  If the
             packet is a fragment in the middle of a logical group of
             fragments, it is consumed and processing stops immediately.

             Fragment handling can be tuned via net.inet.ip.maxfragpackets and
             net.inet.ip.maxfragsperpacket which limit, respectively, the
             maximum number of processable fragments (default: 800) and the
             maximum number of fragments per packet (default: 16).

             NOTA BENE: since fragments do not contain port numbers, they
             should be avoided with the reass rule.  Alternatively, direction-
             based (like in / out ) and source-based (like via ) match
             patterns can be used to select fragments.

             Usually a simple rule like:

                   # reassemble incoming fragments
                   ipfw add reass all from any to any in

             is all you need at the beginning of your ruleset.

   RULE BODY
     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 1.2.3.4 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
     earlier versions of FreeBSD.  In modern FreeBSD any match pattern
     (including MAC headers, IP 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 IP protocol specified by number or name (for a complete list
             see /etc/protocols), or one of the following keywords:

             ip4 | ipv4
                     Matches IPv4 packets.

             ip6 | ipv6
                     Matches IPv6 packets.

             ip | all
                     Matches any packet.

             The ipv6 in proto option will be treated as inner protocol.  And,
             the ipv4 is not available in proto option.

             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
             specifiers.

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

     addr: [not] {any | me | me6 | table(number[,value]) | addr-list |
             addr-set}

             any     matches any IP address.

             me      matches any IP address configured on an interface in the
                     system.

             me6     matches any IPv6 address configured on an interface in
                     the system.  The address list is evaluated at the time
                     the packet is analysed.

             table(number[,value])
                     Matches any IPv4 address for which an entry exists in the
                     lookup table number.  If an optional 32-bit unsigned
                     value is also specified, an entry will match only if it
                     has this value.  See the LOOKUP TABLES section below for
                     more information on lookup tables.

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

     ip-addr:
             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.

             addr/masklen
                     Matches all addresses with base addr (specified as an IP
                     address, a network number, or a hostname) and mask width
                     of masklen bits.  As an example, 1.2.3.4/25 or 1.2.3.0/25
                     will match all IP numbers from 1.2.3.0 to 1.2.3.127 .

             addr:mask
                     Matches all addresses with base addr (specified as an IP
                     address, a network number, or a hostname) and the mask of
                     mask, specified as a dotted quad.  As an example,
                     1.2.3.4:255.0.255.0 or 1.0.3.0:255.0.255.0 will match
                     1.*.3.*.  This form is advised only for non-contiguous
                     masks.  It is better to resort to the addr/masklen format
                     for contiguous masks, which is more compact and less
                     error-prone.

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

     list: {num | num-num}[,list]
             Matches all addresses with base address addr (specified as an IP
             address, a network number, 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 1.2.3.4/24{128,35-55,89}
             or 1.2.3.0/24{128,35-55,89} will match the following IP
             addresses:
             1.2.3.128, 1.2.3.35 to 1.2.3.55, 1.2.3.89 .

     addr6-list: ip6-addr[,addr6-list]

     ip6-addr:
             A host or subnet specified one of the following ways:

             numeric-ip | hostname
                     Matches a single IPv6 address as allowed by inet_pton(3)
                     or a hostname.  Hostnames are resolved at the time the
                     rule is added to the firewall list.

             addr/masklen
                     Matches all IPv6 addresses with base addr (specified as
                     allowed by inet_pton or a hostname) and mask width of
                     masklen bits.

             No support for sets of IPv6 addresses is provided because IPv6
             addresses are typically random past the initial prefix.

     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
             boundaries).

             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
             character).

                   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.

   RULE OPTIONS (MATCH PATTERNS)
     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.

     bridged
             Alias for layer2.

     diverted
             Matches only packets generated by a divert socket.

     diverted-loopback
             Matches only packets coming from a divert socket back into the IP
             stack input for delivery.

     diverted-output
             Matches only packets going from a divert socket back outward to
             the IP stack output for delivery.

     dst-ip ip-address
             Matches IPv4 packets whose destination IP is one of the
             address(es) specified as argument.

     {dst-ip6 | dst-ipv6} ip6-address
             Matches IPv6 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.

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

     ext6hdr header
             Matches IPv6 packets containing the extended header given by
             header.  Supported headers are:

             Fragment, (frag), Hop-to-hop options (hopopt), any type of
             Routing Header (route), Source routing Routing Header Type 0
             (rthdr0), Mobile IPv6 Routing Header Type 2 (rthdr2), Destination
             options (dstopt), IPSec authentication headers (ah), and IPsec
             encapsulated security payload headers (esp).

     fib fibnum
             Matches a packet that has been tagged to use the given FIB
             (routing table) number.

     flow-id labels
             Matches IPv6 packets containing any of the flow labels given in
             labels.  labels is a comma separated list of numeric flow labels.

     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.

     jail prisonID
             Matches all TCP or UDP packets sent by or received for the jail
             whos prison ID is prisonID.

     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).

     icmp6types types
             Matches ICMP6 packets whose ICMP6 type is in the list of types.
             The list may be specified as any combination of individual types
             (numeric) separated by commas.  Ranges are not allowed.

     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 IPv4 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 IPv4 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 IPv4 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_FILTERTUNNEL 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 IPv4 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_ECN_CE).  The absence of a particular type may
             be denoted with a `!'.

     dscp spec[,spec]
             Matches IPv4/IPv6 packets whose DS field value is contained in
             spec mask.  Multiple values can be specified via the comma
             separated list.  Value can be one of keywords used in setdscp
             action or exact number.

     ipttl ttl-list
             Matches IPv4 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.

     keep-state
             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.  Currently,
             only IPv4 flows are supported.

     lookup {dst-ip | dst-port | src-ip | src-port | uid | jail} N
             Search an entry in lookup table N that matches the field
             specified as argument.  If not found, the match fails.
             Otherwise, the match succeeds and tablearg is set to the value
             extracted from the table.

             This option can be useful to quickly dispatch traffic based on
             certain packet fields.  See the LOOKUP TABLES section below for
             more information on lookup tables.

     { 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 IP protocol.

     recv | xmit | via {ifX | if* | table(number[,value | 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
             used.

             A packet might 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''.

     sockarg
             Matches packets that are associated to a local socket and for
             which the SO_USER_COOKIE socket option has been set to a non-zero
             value.  As a side effect, the value of the option is made
             available as tablearg value, which in turn can be used as skipto
             or pipe number.

     src-ip ip-address
             Matches IPv4 packets whose source IP is one of the address(es)
             specified as an argument.

     src-ip6 ip6-address
             Matches IPv6 packets whose source IP is one of the address(es)
             specified as an argument.

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

     tagged tag-list
             Matches packets whose tags are included in tag-list, which is
             either a single value or a list of values or ranges specified in
             the same way as ports.  Tags can be applied to the packet using
             tag rule action parameter (see it's description for details on
             tags).

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

     tcpdatalen tcpdatalen-list
             Matches TCP packets whose length of TCP data is tcpdatalen-list,
             which is either a single value or a list of values or ranges
             specified in the same way as ports.

     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 tcpwin-list
             Matches TCP packets whose  header window field is set to
             tcpwin-list, which is either a single value or a list of values
             or ranges specified in the same way as ports.

     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.

     verrevpath
             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 to reject all
             packets with source addresses not from this interface.  See also
             the option antispoof.

     versrcreach
             For incoming packets, a routing table lookup is done on the
             packet's source address.  If a route to the source address
             exists, but not the default route or a blackhole/reject route,
             the packet matches.  Otherwise, the packet does not match.  All
             outgoing packets match.

             The name and functionality of the option is intentionally similar
             to the Cisco IOS command:

                   ip verify unicast source reachable-via any

             This option can be used to make anti-spoofing rules to reject all
             packets whose source address is unreachable.

     antispoof
             For incoming packets, the packet's source address is checked if
             it belongs to a directly connected network.  If the network is
             directly connected, then the interface the packet came on in is
             compared to the interface the network is connected to.  When
             incoming interface and directly connected interface are not the
             same, the packet does not match.  Otherwise, the packet does
             match.  All outgoing packets match.

             This option can be used to make anti-spoofing rules to reject all
             packets that pretend to be from a directly connected network but
             do not come in through that interface.  This option is similar to
             but more restricted than verrevpath because it engages only on
             packets with source addresses of directly connected networks
             instead of all source addresses.

LOOKUP TABLES
     Lookup tables are useful to handle large sparse sets of addresses or
     other search keys (e.g., ports, jail IDs, interface names).  In the rest
     of this section we will use the term ``address''.  There may be up to
     65535 different lookup tables, numbered 0 to 65534.

     Each entry is represented by an addr[/masklen] and will match all
     addresses with base addr (specified as an IPv4/IPv6 address, a hostname
     or an unsigned integer) and mask width of masklen bits.  If masklen is
     not specified, it defaults to 32 for IPv4 and 128 for IPv6.  When looking
     up an IP address in a table, the most specific entry will match.
     Associated with each entry is a 32-bit unsigned value, which can
     optionally be checked by a rule matching code.  When adding an entry, if
     value is not specified, it defaults to 0.

     An entry can be added to a table (add), or removed from a table (delete).
     A table can be examined (list) or flushed (flush).

     Internally, each table is stored in a Radix tree, the same way as the
     routing table (see route(4)).

     Lookup tables currently support only ports, jail IDs, IPv4/IPv6
     addresses and interface names.  Wildcards is not supported for interface
     names.

     The tablearg feature provides the ability to use a value, looked up in
     the table, as the argument for a rule action, action parameter or rule
     option.  This can significantly reduce number of rules in some
     configurations.  If two tables are used in a rule, the result of the
     second (destination) is used.  The tablearg argument can be used with the
     following actions: nat, pipe, queue, divert, tee, netgraph, ngtee, fwd,
     skipto, setfib, action parameters: tag, untag, rule options: limit,
     tagged.

     When used with fwd it is possible to supply table entries with values
     that are in the form of IP addresses or hostnames.  See the EXAMPLES
     Section for example usage of tables and the tablearg keyword.

     When used with the skipto action, the user should be aware that the code
     will walk the ruleset up to a rule equal to, or past, the given number,
     and should therefore try keep the ruleset compact between the skipto and
     the target rules.

SETS OF 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
     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
     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
     firewall:

           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.

TRAFFIC SHAPER (DUMMYNET) CONFIGURATION
     ipfw is also the user interface for the dummynet traffic shaper, packet
     scheduler and network emulator, a subsystem that can artificially queue,
     delay or drop packets emulating the behaviour of certain network links or
     queueing systems.

     dummynet operates by first using the firewall to select packets using any
     match pattern that can be used in ipfw rules.  Matching packets are then
     passed to either of two different objects, which implement the traffic
     regulation:

         pipe    A pipe emulates a link with given bandwidth and propagation
                 delay, driven by a FIFO scheduler and a single queue with
                 programmable queue size and packet loss rate.  Packets are
                 appended to the queue as they come out from ipfw, and then
                 transferred in FIFO order to the link at the desired rate.

         queue   A queue is an abstraction used to implement packet scheduling
                 using one of several packet scheduling algorithms.  Packets
                 sent to a queue are first grouped into flows according to a
                 mask on the 5-tuple.  Flows are then passed to the scheduler
                 associated to the queue, and each flow uses scheduling
                 parameters (weight and others) as configured in the queue
                 itself.  A scheduler in turn is connected to an emulated
                 link, and arbitrates the link's bandwidth among backlogged
                 flows according to weights and to the features of the
                 scheduling algorithm in use.

     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 flows
     share the available bandwidth.

     A graphical representation of the binding of queues, flows, schedulers
     and links is below.

                            (flow_mask|sched_mask)  sched_mask
                    +---------+   weight Wx  +-------------+
                    |         |->-[flow]-->--|             |-+
               -->--| QUEUE x |   ...        |             | |
                    |         |->-[flow]-->--| SCHEDuler N | |
                    +---------+              |             | |
                        ...                  |             +--[LINK N]-->--
                    +---------+   weight Wy  |             | +--[LINK N]-->--
                    |         |->-[flow]-->--|             | |
               -->--| QUEUE y |   ...        |             | |
                    |         |->-[flow]-->--|             | |
                    +---------+              +-------------+ |
                                               +-------------+
     It is important to understand the role of the SCHED_MASK and FLOW_MASK,
     which are configured through the commands
           ipfw sched N config mask SCHED_MASK ...
     and
           ipfw queue X config mask FLOW_MASK ....

     The SCHED_MASK is used to assign flows to one or more scheduler
     instances, one for each value of the packet's 5-tuple after applying
     SCHED_MASK.  As an example, using ``src-ip 0xffffff00'' creates one
     instance for each /24 destination subnet.

     The FLOW_MASK, together with the SCHED_MASK, is used to split packets
     into flows.  As an example, using ``src-ip 0x000000ff'' together with the
     previous SCHED_MASK makes a flow for each individual source address.  In
     turn, flows for each /24 subnet will be sent to the same scheduler
     instance.

     The above diagram holds even for the pipe case, with the only restriction
     that a pipe only supports a SCHED_MASK, and forces the use of a FIFO
     scheduler (these are for backward compatibility reasons; in fact,
     internally, a dummynet's pipe is implemented exactly as above).

     There are two modes of dummynet operation: ``normal'' and ``fast''.  The
     ``normal'' mode tries to emulate a real link: the dummynet scheduler
     ensures that the packet will not leave the pipe faster than it would on
     the real link with a given bandwidth.  The ``fast'' mode allows certain
     packets to bypass the dummynet scheduler (if packet flow does not exceed
     pipe's bandwidth).  This is the reason why the ``fast'' mode requires
     less CPU cycles per packet (on average) and packet latency can be
     significantly lower in comparison to a real link with the same bandwidth.
     The default mode is ``normal''.  The ``fast'' mode can be enabled by
     setting the net.inet.ip.dummynet.io_fast sysctl(8) variable to a non-zero
     value.

   PIPE, QUEUE AND SCHEDULER CONFIGURATION
     The pipe, queue and scheduler configuration commands are the following:

           pipe number config pipe-configuration

           queue number config queue-configuration

           sched number config sched-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).  The default value is
             0, meaning no delay.

     burst size
             If the data to be sent exceeds the pipe's bandwidth limit (and
             the pipe was previously idle), up to size bytes of data are
             allowed to bypass the dummynet scheduler, and will be sent as
             fast as the physical link allows.  Any additional data will be
             transmitted at the rate specified by the pipe bandwidth.  The
             burst size depends on how long the pipe has been idle; the
             effective burst size is calculated as follows: MAX( size , bw *
             pipe_idle_time).

     profile filename
             A file specifying the additional overhead incurred in the
             transmission of a packet on the link.

             Some link types introduce extra delays in the transmission of a
             packet, e.g., because of MAC level framing, contention on the use
             of the channel, MAC level retransmissions and so on.  From our
             point of view, the channel is effectively unavailable for this
             extra time, which is constant or variable depending on the link
             type.  Additionally, packets may be dropped after this time
             (e.g., on a wireless link after too many retransmissions).  We
             can model the additional delay with an empirical curve that
             represents its distribution.

                         cumulative probability
                         1.0 ^
                             |
                         L   +-- loss-level          x
                             |                 ******
                             |                *
                             |           *****
                             |          *
                             |        **
                             |       *
                             +-------*------------------->
                                         delay
             The empirical curve may have both vertical and horizontal lines.
             Vertical lines represent constant delay for a range of
             probabilities.  Horizontal lines correspond to a discontinuity in
             the delay distribution: the pipe will use the largest delay for a
             given probability.

             The file format is the following, with whitespace acting as a
             separator and '#' indicating the beginning a comment:

             name identifier
                     optional name (listed by "ipfw pipe show") to identify
                     the delay distribution;

             bw value
                     the bandwidth used for the pipe.  If not specified here,
                     it must be present explicitly as a configuration
                     parameter for the pipe;

             loss-level L
                     the probability above which packets are lost.  (0.0 <= L
                     <= 1.0, default 1.0 i.e., no loss);

             samples N
                     the number of samples used in the internal representation
                     of the curve (2..1024; default 100);

             delay prob | prob delay
                     One of these two lines is mandatory and defines the
                     format of the following lines with data points.

             XXX YYY
                     2 or more lines representing points in the curve, with
                     either delay or probability first, according to the
                     chosen format.  The unit for delay is milliseconds.  Data
                     points do not need to be sorted.  Also, the number of
                     actual lines can be different from the value of the
                     "samples" parameter: ipfw utility will sort and
                     interpolate the curve as needed.

             Example of a profile file:

                   name    bla_bla_bla
                   samples 100
                   loss-level    0.86
                   prob    delay
                   0       200     # minimum overhead is 200ms
                   0.5     200
                   0.5     300
                   0.8     1000
                   0.9     1300
                   1       1300
                   #configuration file end

     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.

     The following case-insensitive parameters can be configured for a
     scheduler:

     type {fifo | wf2q+ | rr | qfq}
             specifies the scheduling algorithm to use.
             fifo    is just a FIFO scheduler (which means that all packets
                     are stored in the same queue as they arrive to the
                     scheduler).  FIFO has O(1) per-packet time complexity,
                     with very low constants (estimate 60-80ns on a 2GHz
                     desktop machine) but gives no service guarantees.
             wf2q+   implements the WF2Q+ algorithm, which is a Weighted Fair
                     Queueing algorithm which permits flows to share bandwidth
                     according to their weights.  Note that weights are not
                     priorities; even a flow with a minuscule weight will
                     never starve.  WF2Q+ has O(log N) per-packet processing
                     cost, where N is the number of flows, and is the default
                     algorithm used by previous versions dummynet's queues.
             rr      implements the Deficit Round Robin algorithm, which has
                     O(1) processing costs (roughly, 100-150ns per packet) and
                     permits bandwidth allocation according to weights, but
                     with poor service guarantees.
             qfq     implements the QFQ algorithm, which is a very fast
                     variant of WF2Q+, with similar service guarantees and
                     O(1) processing costs (roughly, 200-250ns per packet).

     In addition to the type, all parameters allowed for a pipe can also be
     specified for a scheduler.

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

     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
           following:

           dst-ip mask, dst-ip6 mask, src-ip mask, src-ip6 mask, dst-port
           mask, src-port mask, flow-id mask, proto mask or all,

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

     noerror
           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 effects can result if you get packets
           from an interface with a much larger MTU, e.g. the loopback
           interface with its 16KB packets.  The sysctl(8) variables
           net.inet.ip.dummynet.pipe_byte_limit and
           net.inet.ip.dummynet.pipe_slot_limit control the maximum lengths
           that can be specified.

     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 also supports the gentle RED variant
           (gred).  Three sysctl(8) variables can be used to control the RED
           behaviour:

           net.inet.ip.dummynet.red_lookup_depth
                   specifies the accuracy in computing the average queue when
                   the link is idle (defaults to 256, must be greater than
                   zero)

           net.inet.ip.dummynet.red_avg_pkt_size
                   specifies the expected average packet size (defaults to
                   512, must be greater than zero)

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

     When used with IPv6 data, dummynet currently has several limitations.
     Information necessary to route link-local packets to an interface is not
     available after processing by dummynet so those packets are dropped in
     the output path.  Care should be taken to ensure that link-local packets
     are not passed to dummynet.

CHECKLIST
     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
         /usr/share/examples/ipfw/change_rules.sh.

     +o   Do not forget the loopback interface.

FINE POINTS
     +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.  The
         following command line is recommended:

               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
         levels).

PACKET DIVERSION
     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 divert module is not loaded, or if the kernel was not compiled
     with divert socket support, the packets are dropped.

NETWORK ADDRESS TRANSLATION (NAT)
     ipfw support in-kernel NAT using the kernel version of libalias(3).

     The nat configuration command is the following:

           nat nat_number config nat-configuration

     The following parameters can be configured:

     ip ip_address
             Define an ip address to use for aliasing.

     if nic  Use ip address of NIC for aliasing, dynamically changing it if
             NIC's ip address changes.

     log     Enable logging on this nat instance.

     deny_in
             Deny any incoming connection from outside world.

     same_ports
             Try to leave the alias port numbers unchanged from the actual
             local port numbers.

     unreg_only
             Traffic on the local network not originating from an unregistered
             address spaces will be ignored.

     reset   Reset table of the packet aliasing engine on address change.

     reverse
             Reverse the way libalias handles aliasing.

     proxy_only
             Obey transparent proxy rules only, packet aliasing is not
             performed.

     skip_global
             Skip instance in case of global state lookup (see below).

     Some specials value can be supplied instead of nat_number:

     global  Looks up translation state in all configured nat instances.  If
             an entry is found, packet is aliased according to that entry.  If
             no entry was found in any of the instances, packet is passed
             unchanged, and no new entry will be created.  See section
             MULTIPLE INSTANCES in natd(8) for more information.

     tablearg
             Uses argument supplied in lookup table.  See LOOKUP TABLES
             section below for more information on lookup tables.

     To let the packet continue after being (de)aliased, set the sysctl
     variable net.inet.ip.fw.one_pass to 0.  For more information about
     aliasing modes, refer to libalias(3).  See Section EXAMPLES for some
     examples about nat usage.

   REDIRECT AND LSNAT SUPPORT IN IPFW
     Redirect and LSNAT support follow closely the syntax used in natd(8).
     See Section EXAMPLES for some examples on how to do redirect and lsnat.

   SCTP NAT SUPPORT
     SCTP nat can be configured in a similar manner to TCP through the ipfw
     command line tool.  The main difference is that sctp nat does not do port
     translation.  Since the local and global side ports will be the same,
     there is no need to specify both.  Ports are redirected as follows:

           nat nat_number config if nic redirect_port sctp
           ip_address [,addr_list] {[port | port-port] [,ports]}

     Most sctp nat configuration can be done in real-time through the
     sysctl(8) interface.  All may be changed dynamically, though the
     hash_table size will only change for new nat instances.  See SYSCTL
     VARIABLES for more info.

LOADER TUNABLES
     Tunables can be set in loader(8) prompt, loader.conf(5) or kenv(1) before
     ipfw module gets loaded.

     net.inet.ip.fw.default_to_accept: 0
             Defines ipfw last rule behavior.  This value overrides options
             IPFW_DEFAULT_TO_(ACCEPT|DENY) from kernel configuration file.

     net.inet.ip.fw.tables_max: 128
             Defines number of tables available in ipfw.  Number cannot exceed
             65534.

SYSCTL VARIABLES
     A set of sysctl(8) variables controls the behaviour of the firewall and
     associated modules (dummynet, bridge, sctp nat).  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.alias.sctp.accept_global_ootb_addip: 0
             Defines how the nat responds to receipt of global OOTB ASCONF-
             AddIP:

             0       No response (unless a partially matching association
                     exists - ports and vtags match but global address does
                     not)

             1       nat will accept and process all OOTB global AddIP
                     messages.

             Option 1 should never be selected as this forms a security risk.
             An attacker can establish multiple fake associations by sending
             AddIP messages.

     net.inet.ip.alias.sctp.chunk_proc_limit: 5
             Defines the maximum number of chunks in an SCTP packet that will
             be parsed for a packet that matches an existing association.
             This value is enforced to be greater or equal than
             net.inet.ip.alias.sctp.initialising_chunk_proc_limit.  A high
             value is a DoS risk yet setting too low a value may result in
             important control chunks in the packet not being located and
             parsed.

     net.inet.ip.alias.sctp.error_on_ootb: 1
             Defines when the nat responds to any Out-of-the-Blue (OOTB)
             packets with ErrorM packets.  An OOTB packet is a packet that
             arrives with no existing association registered in the nat and is
             not an INIT or ASCONF-AddIP packet:

             0       ErrorM is never sent in response to OOTB packets.

             1       ErrorM is only sent to OOTB packets received on the local
                     side.

             2       ErrorM is sent to the local side and on the global side
                     ONLY if there is a partial match (ports and vtags match
                     but the source global IP does not).  This value is only
                     useful if the nat is tracking global IP addresses.

             3       ErrorM is sent in response to all OOTB packets on both
                     the local and global side (DoS risk).

             At the moment the default is 0, since the ErrorM packet is not
             yet supported by most SCTP stacks.  When it is supported, and if
             not tracking global addresses, we recommend setting this value to
             1 to allow multi-homed local hosts to function with the nat.  To
             track global addresses, we recommend setting this value to 2 to
             allow global hosts to be informed when they need to (re)send an
             ASCONF-AddIP.  Value 3 should never be chosen (except for
             debugging) as the nat will respond to all OOTB global packets (a
             DoS risk).

     net.inet.ip.alias.sctp.hashtable_size: 2003
             Size of hash tables used for nat lookups (100 < prime_number >
             1000001).  This value sets the hash table size for any future
             created nat instance and therefore must be set prior to creating
             a nat instance.  The table sizes may be changed to suit specific
             needs.  If there will be few concurrent associations, and memory
             is scarce, you may make these smaller.  If there will be many
             thousands (or millions) of concurrent associations, you should
             make these larger.  A prime number is best for the table size.
             The sysctl update function will adjust your input value to the
             next highest prime number.

     net.inet.ip.alias.sctp.holddown_time: 0
             Hold association in table for this many seconds after receiving a
             SHUTDOWN-COMPLETE.  This allows endpoints to correct shutdown
             gracefully if a shutdown_complete is lost and retransmissions are
             required.

     net.inet.ip.alias.sctp.init_timer: 15
             Timeout value while waiting for (INIT-ACK|AddIP-ACK).  This value
             cannot be 0.

     net.inet.ip.alias.sctp.initialising_chunk_proc_limit: 2
             Defines the maximum number of chunks in an SCTP packet that will
             be parsed when no existing association exists that matches that
             packet.  Ideally this packet will only be an INIT or ASCONF-AddIP
             packet.  A higher value may become a DoS risk as malformed
             packets can consume processing resources.

     net.inet.ip.alias.sctp.param_proc_limit: 25
             Defines the maximum number of parameters within a chunk that will
             be parsed in a packet.  As for other similar sysctl variables,
             larger values pose a DoS risk.

     net.inet.ip.alias.sctp.log_level: 0
             Level of detail in the system log messages (0 - minimal, 1 -
             event, 2 - info, 3 - detail, 4 - debug, 5 - max debug).  May be a
             good option in high loss environments.

     net.inet.ip.alias.sctp.shutdown_time: 15
             Timeout value while waiting for SHUTDOWN-COMPLETE.  This value
             cannot be 0.

     net.inet.ip.alias.sctp.track_global_addresses: 0
             Enables/disables global IP address tracking within the nat and
             places an upper limit on the number of addresses tracked for each
             association:

             0       Global tracking is disabled

             >1      Enables tracking, the maximum number of addresses tracked
                     for each association is limited to this value

             This variable is fully dynamic, the new value will be adopted for
             all newly arriving associations, existing associations are
             treated as they were previously.  Global tracking will decrease
             the number of collisions within the nat at a cost of increased
             processing load, memory usage, complexity, and possible nat state
             problems in complex networks with multiple nats.  We recommend
             not tracking global IP addresses, this will still result in a
             fully functional nat.

     net.inet.ip.alias.sctp.up_timer: 300
             Timeout value to keep an association up with no traffic.  This
             value cannot be 0.

     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 reached.

     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.io_fast: 0
             If set to a non-zero value, the ``fast'' mode of dummynet
             operation (see above) is enabled.

     net.inet.ip.dummynet.io_pkt
             Number of packets passed to dummynet.

     net.inet.ip.dummynet.io_pkt_drop
             Number of packets dropped by dummynet.

     net.inet.ip.dummynet.io_pkt_fast
             Number of packets bypassed by the dummynet scheduler.

     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.dummynet.pipe_byte_limit: 1048576

     net.inet.ip.dummynet.pipe_slot_limit: 100
             The maximum queue size that can be specified in bytes or packets.
             These limits prevent accidental exhaustion of resources such as
             mbufs.  If you raise these limits, you should make sure the
             system is configured so that sufficient resources are available.

     net.inet.ip.fw.autoinc_step: 100
             Delta between rule numbers when auto-generating them.  The value
             must be in the range 1..1000.

     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
             (readonly).

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

     net.inet.ip.fw.default_rule: 65535
             The default rule number (read-only).  By the design of ipfw, the
             default rule is the last one, so its number can also serve as the
             highest number allowed for a rule.

     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.inet6.ip6.fw.enable: 1
             provides the same functionality as above for the IPv6 case.

     net.inet.ip.fw.one_pass: 1
             When set, the packet exiting from the dummynet pipe or from
             ng_ipfw(4) node is not passed though the firewall again.
             Otherwise, after an action, the packet is reinjected into the
             firewall at the next rule.

     net.inet.ip.fw.tables_max: 128
             Maximum number of tables.

     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.inet6.ip6.fw.deny_unknown_exthdrs: 1
             If enabled packets with unknown IPv6 Extension Headers will be
             denied.

     net.link.ether.ipfw: 0
             Controls whether layer-2 packets are passed to ipfw.  Default is
             no.

     net.link.bridge.ipfw: 0
             Controls whether bridged packets are passed to ipfw.  Default is
             no.

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

   BASIC PACKET FILTERING
     This command adds an entry which denies all tcp packets from
     cracker.evil.org to the telnet port of wolf.tambov.su from being
     forwarded by the host:

           ipfw add deny tcp from cracker.evil.org to wolf.tambov.su telnet

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

           ipfw add deny ip from 123.45.67.0/24 to my.host.org

     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
     address sets and or-blocks and write extremely compact rulesets which
     selectively enable services to blocks of clients, as below:

           goodguys="{ 10.1.2.0/24{20,35,66,18} or 10.2.3.0/28{6,3,11} }"
           badguys="10.1.2.0/24{8,38,60}"
           ipfw add allow ip from ${goodguys} to any
           ipfw add deny ip from ${badguys} to any
           ... normal policies ...

     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
     system 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.

     The antispoof option could be used to do similar but more restricted
     anti-spoofing by adding the following to the top of a ruleset:

           ipfw add deny ip from any to any not antispoof in

     This rule drops all incoming packets that appear to be coming from
     another directly connected system but on the wrong interface.  For
     example, a packet with a source address of 192.168.0.0/24, configured on
     fxp0, but coming in on fxp1 would be dropped.

     The setdscp option could be used to (re)mark user traffic, by adding the
     following to the appropriate place in ruleset:

           ipfw add setdscp be ip from any to any dscp af11,af21

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

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

     This will let the firewall install dynamic rules only for those
     connection which start with a regular SYN packet coming from the inside
     of our network.  Dynamic rules are checked when encountering the first
     occurrence of a check-state, keep-state or limit 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
     following 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 192.168.2.0/24 to divert port
     5000:

           ipfw divert 5000 ip from 192.168.2.0/24 to any in

   TRAFFIC SHAPING
     The following rules show some of the applications of ipfw and dummynet
     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
     acting as a router, if we want to limit traffic from local clients on
     192.168.2.0/24 we do:

           ipfw add pipe 1 ip from 192.168.2.0/24 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
     outgoing packets.

     Should we want to simulate a bidirectional link with bandwidth
     limitations, 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
     management 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
     bandwidth:

           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 192.168.2.0/24 to any out
           ipfw add pipe 2 ip from any to 192.168.2.0/24 in
           ipfw pipe 1 config mask src-ip 0x000000ff bw 200Kbit/s queue
           20Kbytes
           ipfw pipe 2 config mask dst-ip 0x000000ff bw 200Kbit/s queue
           20Kbytes

   LOOKUP TABLES
     In the following example, we need to create several traffic bandwidth
     classes and we need different hosts/networks to fall into different
     classes.  We create one pipe for each class and configure them
     accordingly.  Then we create a single table and fill it with IP subnets
     and addresses.  For each subnet/host we set the argument equal to the
     number of the pipe that it should use.  Then we classify traffic using a
     single rule:

           ipfw pipe 1 config bw 1000Kbyte/s
           ipfw pipe 4 config bw 4000Kbyte/s
           ...
           ipfw table 1 add 192.168.2.0/24 1
           ipfw table 1 add 192.168.0.0/27 4
           ipfw table 1 add 192.168.0.2 1
           ...
           ipfw add pipe tablearg ip from table(1) to any

     Using the fwd action, the table entries may include hostnames and IP
     addresses.

           ipfw table 1 add 192.168.2.0/24 10.23.2.1
           ipfw table 1 add 192.168.0.0/27 router1.dmz
           ...
           ipfw add 100 fwd tablearg ip from any to table(1)

     In the following example per-interface firewall is created:

           ipfw table 10 add vlan20 12000
           ipfw table 10 add vlan30 13000
           ipfw table 20 add vlan20 22000
           ipfw table 20 add vlan30 23000
           ..
           ipfw add 100 ipfw skipto tablearg ip from any to any recv
           'table(10)' in
           ipfw add 200 ipfw skipto tablearg ip from any to any xmit
           'table(10)' out

   SETS OF RULES
     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
     wrong:

           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"
     terminates, and your ruleset will be left active.  Otherwise, e.g. if you
     cannot access your box, the ruleset will be disabled after the sleep
     terminates thus restoring the previous situation.

     To show rules of the specific set:

           ipfw set 18 show

     To show rules of the disabled set:

           ipfw -S set 18 show

     To clear a specific rule counters of the specific set:

           ipfw set 18 zero NN

     To delete a specific rule of the specific set:

           ipfw set 18 delete NN

   NAT, REDIRECT AND LSNAT
     First redirect all the traffic to nat instance 123:

           ipfw add nat 123 all from any to any

     Then to configure nat instance 123 to alias all the outgoing traffic with
     ip 192.168.0.123, blocking all incoming connections, trying to keep same
     ports on both sides, clearing aliasing table on address change and
     keeping a log of traffic/link statistics:

           ipfw nat 123 config ip 192.168.0.123 log deny_in reset same_ports

     Or to change address of instance 123, aliasing table will be cleared (see
     reset option):

           ipfw nat 123 config ip 10.0.0.1

     To see configuration of nat instance 123:

           ipfw nat 123 show config

     To show logs of all the instances in range 111-999:

           ipfw nat 111-999 show

     To see configurations of all instances:

           ipfw nat show config

     Or a redirect rule with mixed modes could looks like:

           ipfw nat 123 config redirect_addr 10.0.0.1 10.0.0.66
                           redirect_port tcp 192.168.0.1:80 500
                           redirect_proto udp 192.168.1.43 192.168.1.1
                           redirect_addr 192.168.0.10,192.168.0.11
                                   10.0.0.100 # LSNAT
                           redirect_port tcp 192.168.0.1:80,192.168.0.10:22
                                   500        # LSNAT

     or it could be split in:

           ipfw nat 1 config redirect_addr 10.0.0.1 10.0.0.66
           ipfw nat 2 config redirect_port tcp 192.168.0.1:80 500
           ipfw nat 3 config redirect_proto udp 192.168.1.43 192.168.1.1
           ipfw nat 4 config redirect_addr
           192.168.0.10,192.168.0.11,192.168.0.12
                                        10.0.0.100
           ipfw nat 5 config redirect_port tcp
                          192.168.0.1:80,192.168.0.10:22,192.168.0.20:25 500

SEE ALSO
     cpp(1), m4(1), altq(4), divert(4), dummynet(4), if_bridge(4), ip(4),
     ipfirewall(4), ng_ipfw(4), protocols(5), services(5), init(8),
     kldload(8), reboot(8), sysctl(8), syslogd(8)

HISTORY
     The ipfw utility first appeared in FreeBSD 2.0.  dummynet was introduced
     in FreeBSD 2.2.8.  Stateful extensions were introduced in FreeBSD 4.0.
     ipfw2 was introduced in Summer 2002.

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

     API based upon code written by Daniel Boulet for BSDI.

     Dummynet has been introduced by Luigi Rizzo in 1997-1998.

     Some early work (1999-2000) on the dummynet traffic shaper supported by
     Akamba Corp.

     The ipfw core (ipfw2) has been completely redesigned and reimplemented by
     Luigi Rizzo in summer 2002.  Further actions and options have been added
     by various developer over the years.

     In-kernel NAT support written by Paolo Pisati <piso@FreeBSD.org> as part
     of a Summer of Code 2005 project.

     SCTP nat support has been developed by The Centre for Advanced Internet
     Architectures (CAIA) <http://www.caia.swin.edu.au>.  The primary
     developers and maintainers are David Hayes and Jason But.  For further
     information visit: <http://www.caia.swin.edu.au/urp/SONATA>

     Delay profiles have been developed by Alessandro Cerri and Luigi Rizzo,
     supported by the European Commission within Projects Onelab and Onelab2.

BUGS
     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 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 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.

     Dummynet drops all packets with IPv6 link-local addresses.

     Rules using uid or gid may not behave as expected.  In particular,
     incoming SYN packets may have no uid or gid associated with them since
     they do not yet belong to a TCP connection, and the uid/gid associated
     with a packet may not be as expected if the associated process calls
     setuid(2) or similar system calls.

     Rule syntax is subject to the command line environment and some patterns
     may need to be escaped with the backslash character or quoted
     appropriately.

     Due to the architecture of libalias(3), ipfw nat is not compatible with
     the TCP segmentation offloading (TSO).  Thus, to reliably nat your
     network traffic, please disable TSO on your NICs using ifconfig(8).

     ICMP error messages are not implicitly matched by dynamic rules for the
     respective conversations.  To avoid failures of network error detection
     and path MTU discovery, ICMP error messages may need to be allowed
     explicitly through static rules.

     Rules using call and return actions may lead to confusing behaviour if
     ruleset has mistakes, and/or interaction with other subsystems (netgraph,
     dummynet, etc.) is used.  One possible case for this is packet leaving
     ipfw in subroutine on the input pass, while later on output encountering
     unpaired return first.  As the call stack is kept intact after input
     pass, packet will suddenly return to the rule number used on input pass,
     not on output one.  Order of processing should be checked carefully to
     avoid such mistakes.

FreeBSD 11.0-PRERELEASE        October 25, 2012        FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | PACKET FLOW | SYNTAX | RULE FORMAT | LOOKUP TABLES | SETS OF RULES | STATEFUL FIREWALL | TRAFFIC SHAPER (DUMMYNET) CONFIGURATION | CHECKLIST | FINE POINTS | PACKET DIVERSION | NETWORK ADDRESS TRANSLATION (NAT) | LOADER TUNABLES | SYSCTL VARIABLES | EXAMPLES | SEE ALSO | HISTORY | AUTHORS | BUGS

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