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

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[править] NAME

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

[править] 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

[править] DESCRIPTION

    The ipfw utility is the user interface for controlling the ipfw(4) fire‐
    wall, 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 des‐
    tination 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 rein‐
    jected 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 com‐
    mands 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 pass‐
            ing 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 ses‐
            sion will be closed and the rest of the ruleset will not be pro‐
            cessed.  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 charac‐
    ter, the usual PATH name search is performed.  Care should be taken with
    this in environments where not all file systems are mounted (yet) by the
    time ipfw is being run (e.g. when they are mounted over NFS).  Once -p
    has been specified, any additional arguments are passed on to the pre‐
    processor for interpretation.  This allows for flexible configuration
    files (like conditionalizing them on the local hostname) and the use of
    macros to centralize frequently required arguments like IP addresses.
  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, pre‐
    venting you from being able to add any rules.  This can adversely effect
    the booting process.  You can use ipfw disable firewall to temporarily
    disable the firewall to regain access to the network, allowing you to fix
    the problem.

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

[править] SYNTAX

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

[править] 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 fil‐
    tering 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, data‐
                                             gram length, identification,
                                             fragment flag (non-zero IP off‐
                                             set), 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, acknowl‐
                                             edgment number, window
       TCP options
       ICMP types                            for ICMP packets
       ICMP6 types                           for ICMP6 packets
       User/group ID                         When the packet can be associ‐
                                             ated 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 (rout‐
                                             ing 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 avail‐
            able 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. 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 pack‐
            ets 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 ker‐
            nelspace, they can be set and unset anywhere in the kernel net‐
            work 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 misclassifica‐
            tion.
            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 gener‐
            ated this dynamic rule, otherwise move to the next rule.
            Check-state rules do not have a body.  If no check-state rule is
            found, the dynamic ruleset is checked at the first keep-state or
            limit rule.
    count   Update counters for all packets that match rule.  The search con‐
            tinues with the next rule.
    deny | drop
            Discard packets that match this rule.  The search terminates.
    divert port
            Divert packets that match this rule to the divert(4) socket bound
            to port port.  The search terminates.
    fwd | forward ipaddr | 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 ter‐
            minates if this rule matches.
            If ipaddr is a local address, then matching packets will be for‐
            warded to port (or the port number in the packet if one is not
            specified in the rule) on the local machine.
            If ipaddr is not a local address, then the port number (if speci‐
            fied) is ignored, and the packet will be forwarded to the remote
            address, using the route as found in the local routing table for
            that IP.
            A fwd rule will not match layer-2 packets (those received on
            ether_input, ether_output, or bridged).
            The fwd action does not change the contents of the packet at all.
            In particular, the destination address remains unmodified, so
            packets forwarded to another system will usually be rejected by
            that system unless there is a matching rule on that system to
            capture them.  For packets forwarded locally, the local address
            of the socket will be set to the original destination address of
            the packet.  This makes the netstat(1) entry look rather weird
            but is intended for use with transparent proxy servers.
            To enable fwd a custom kernel needs to be compiled with the
            option options IPFIREWALL_FORWARD.
    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 termi‐
            nates.
    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 termi‐
            nates.
    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 rule‐
            set 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 can't be allocated or stack over‐
            flowed/undeflowed.
            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 unde‐
            sired 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 some‐
            times 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 termi‐
            nates.  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 con‐
            tinues 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.  Initially this is lim‐
            ited to the values 0 through 15, see setfib(1).  Processing con‐
            tinues at the next rule.  It is possible to use the tablearg key‐
            word with a setfib. If tablearg value is not within compiled FIB
            range packet fib is set to 0.
    reass   Queue and reassemble ip fragments.  If the packet is not frag‐
            mented, 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, else packet is
            allowed to pass and search terminates.  If the packet is a frag‐
            ment in the middle, it is consumed and processing stops immedi‐
            ately.
            Fragments 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 pat‐
            terns 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.  Indi‐
    vidual patterns can be prefixed by the not operator to reverse the result
    of the match, as in
          ipfw add 100 allow ip from not 1.2.3.4 to any
    Additionally, sets of alternative match patterns (or-blocks) can be con‐
    structed 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 speci‐
    fied 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 con‐
            venience only but its use is deprecated.
    src and dst: {addr | { addr or ... }} [[not] ports]
            An address (or a list, see below) optionally followed by ports
            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 spa‐
            ces between braces and numbers (spaces after commas are allowed).
            Elements of the list can be specified as single entries or
            ranges.  The masklen field is used to limit the size of the set
            of addresses, and can have any value between 24 and 32.  If not
            specified, it will be assumed as 24.
            This format is particularly useful to handle sparse address sets
            within a single rule.  Because the matching occurs using a bit‐
            mask, it takes constant time and dramatically reduces the com‐
            plexity of rulesets.
            As an example, an address specified as 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 Rout‐
            ing 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 (rout‐
            ing 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 sup‐
            ported ICMP types are:
            echo reply (0), destination unreachable (3), source quench (4),
            redirect (5), echo request (8), router advertisement (9), router
            solicitation (10), time-to-live exceeded (11), IP header bad
            (12), timestamp request (13), timestamp reply (14), information
            request (15), information reply (16), address mask request (17)
            and address mask reply (18).
    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 with‐
            out 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 ‘!’.
    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) vari‐
            ables), and the lifetime is refreshed every time a matching
            packet is found.
    layer2  Matches only layer2 packets, i.e., those passed to ipfw from
            ether_demux() and ether_output_frame().
    limit {src-addr | src-port | dst-addr | dst-port} N
            The firewall will only allow N connections with the same set of
            parameters as specified in the rule.  One or more of source and
            destination addresses and ports can be specified.  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 speci‐
            fied 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, speci‐
            fied as the any keyword (matching any MAC address), or six groups
            of hex digits separated by colons, and optionally followed by a
            mask indicating 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* | ipno | any}
            Matches packets received, transmitted or going through, respec‐
            tively, the interface specified by exact name (ifX), by device
            name (if*), by IP address, or through some interface.
            The via keyword causes the interface to always be checked.  If
            recv or xmit is used instead of via, then only the receive or
            transmit interface (respectively) is checked.  By specifying
            both, it is possible to match packets based on both receive and
            transmit interface, e.g.:
                  ipfw add deny ip from any to any out recv ed0 xmit ed1
            The recv interface can be tested on either incoming or outgoing
            packets, while the xmit interface can only be tested on outgoing
            packets.  So out is required (and in is invalid) whenever xmit is
            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 avail‐
            able 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) speci‐
            fied 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 win
            TCP packets only.  Match if the TCP header window field is set to
            win.
    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.

[править] LOOKUP TABLES

    Lookup tables are useful to handle large sparse sets of addresses or
    other search keys (e.g. ports, jail IDs).  In the rest of this section we
    will use the term ``address to mean any unsigned value of up to 32-bit.
    There may be up to 128 different lookup tables, numbered 0 to 127.
    Each entry is represented by an addr[/masklen] and will match all
    addresses with base addr (specified as an IP address, a hostname or an
    unsigned integer) and mask width of masklen bits.  If masklen is not
    specified, it defaults to 32.  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 and IPv4 addresses.
    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 configura‐
    tions.  If two tables are used in a rule, the result of the second (des‐
    tination) 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 Sec‐
    tion 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.

[править] 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 mul‐
    tiple configurations of the firewall and quickly (and atomically) switch
    between them.  The command to enable/disable sets is
          ipfw set [disable number ...] [enable number ...]
    where multiple enable or disable sections can be specified.  Command exe‐
    cution is atomic on all the sets specified in the command.  By default,
    all sets are enabled.
    When you disable a set, its rules behave as if they do not exist in the
    firewall configuration, with only one exception:
          dynamic rules created from a rule before it had been disabled will
          still be active until they expire.  In order to delete dynamic
          rules you have to explicitly delete the parent rule which generated
          them.
    The set number of rules can be changed with the command
          ipfw set move {rule rule-number | old-set} to new-set
    Also, you can atomically swap two rulesets with the command
          ipfw set swap first-set second-set
    See the EXAMPLES Section on some possible uses of sets of rules.

[править] 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 configura‐
    tion, but let the first TCP SYN packet from the inside network install a
    dynamic rule for the flow so that packets belonging to that session will
    be allowed through the firewall:
          ipfw add check-state
          ipfw add allow tcp from my-subnet to any setup keep-state
          ipfw add deny tcp from any to any
    A similar approach can be used for UDP, where an UDP packet coming from
    the inside will install a dynamic rule to let the response through the
    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.

[править] 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 param‐
                eters (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, inter‐
    nally, 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 signifi‐
    cantly 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 effec‐
            tive burst size is calculated as follows: MAX( size , bw *
            pipe_idle_time).
    profile filename
            A file specifying the additional overhead incurred in the trans‐
            mission 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 probabili‐
            ties.  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 parame‐
                    ter 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 for‐
                    mat 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 cho‐
                    sen 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 "sam‐
                    ples" 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 parameters can be configured for a scheduler:
    type {fifo | wf2qp | rr | qfq}
            specifies the scheduling algorithm to use.
            cm fifo
                    is just a FIFO scheduler (which means that all packets
                    are stored in the same queue as they arrive to the sched‐
                    uler).  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.
            wf2qp   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 vari‐
                    ant 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 fur‐
          ther classified into multiple flows, each of which is then sent to
          a different dynamic pipe or queue.  A flow identifier is con‐
          structed by masking the IP addresses, ports and protocol types as
          specified with the mask options in the configuration of the pipe or
          queue.  For each different flow identifier, a new pipe or queue is
          created with the same parameters as the original object, and match‐
          ing packets are sent to it.
          Thus, when dynamic pipes are used, each flow will get the same
          bandwidth as defined by the pipe, whereas when dynamic queues are
          used, each flow will share the parent's pipe bandwidth evenly with
          other flows generated by the same queue (note that other queues
          with different weights might be connected to the same pipe).
          Available mask specifiers are a combination of one or more of the
          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 inter‐
          face 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 algo‐
          rithm.  w_q and max_p are floating point numbers between 0 and 1 (0
          not included), while min_th and max_th are integer numbers specify‐
          ing thresholds for queue management (thresholds are computed in
          bytes if the queue has been defined in bytes, in slots otherwise).
          The dummynet 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.

[править] CHECKLIST

    Here are some important points to consider when designing your rules:
    ·   Remember that you filter both packets going in and out.  Most connec‐
        tions need packets going in both directions.
    ·   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.
    ·   Do not forget the loopback interface.

[править] FINE POINTS

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

[править] 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.

[править] 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 per‐
            formed.
    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 vari‐
    able 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_ta‐
    ble size will only change for new nat instances.  See SYSCTL VARIABLES
    for more info.

[править] 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 mes‐
                    sages.
            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) pack‐
            ets 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 debug‐
            ging) 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 pack‐
            ets 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 traf‐
            fic.  You can disable this by setting the variable to 0, in which
            case the pipes/queues will only be deleted when the threshold is
            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 con‐
            figuring 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 sys‐
            tem 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 con‐
            nection every 5 seconds for the last 20 seconds of the lifetime
            of the rule.
    net.inet.ip.fw.dyn_max: 8192
            Maximum number of dynamic rules.  When you hit this limit, no
            more dynamic rules can be installed until old ones expire.
    net.inet.ip.fw.dyn_ack_lifetime: 300
    net.inet.ip.fw.dyn_syn_lifetime: 20
    net.inet.ip.fw.dyn_fin_lifetime: 1
    net.inet.ip.fw.dyn_rst_lifetime: 1
    net.inet.ip.fw.dyn_udp_lifetime: 5
    net.inet.ip.fw.dyn_short_lifetime: 30
            These variables control the lifetime, in seconds, of dynamic
            rules.  Upon the initial SYN exchange the lifetime is kept short,
            then increased after both SYN have been seen, then decreased
            again during the final FIN exchange or when a RST is received.
            Both dyn_fin_lifetime and dyn_rst_lifetime must be strictly lower
            than 5 seconds, the period of repetition of keepalives.  The
            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.  Other‐
            wise, 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 (read-only).
    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.

[править] 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 for‐
    warded 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 sys‐
    tem 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 exam‐
    ple, a packet with a source address of 192.168.0.0/24, configured on
    fxp0, but coming in on fxp1 would be dropped.
  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 connec‐
    tion which start with a regular SYN packet coming from the inside of our
    network.  Dynamic rules are checked when encountering the first
    check-state or keep-state rule.  A check-state rule should usually be
    placed near the beginning of the ruleset to minimize the amount of work
    scanning the ruleset.  Your mileage may vary.
    To limit the number of connections a user can open you can use the fol‐
    lowing type of rules:
          ipfw add allow tcp from my-net/24 to any setup limit src-addr 10
          ipfw add allow tcp from any to me setup limit src-addr 4
    The former (assuming it runs on a gateway) will allow each host on a /24
    network to open at most 10 TCP connections.  The latter can be placed on
    a server to make sure that a single client does not use more than 4
    simultaneous connections.
    BEWARE: stateful rules can be subject to denial-of-service attacks by a
    SYN-flood which opens a huge number of dynamic rules.  The effects of
    such attacks can be partially limited by acting on a set of sysctl(8)
    variables which control the operation of the firewall.
    Here is a good usage of the list command to see accounting records and
    timestamp information:
          ipfw -at list
    or in short form without timestamps:
          ipfw -a list
    which is equivalent to:
          ipfw show
    Next rule diverts all incoming packets from 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 act‐
    ing 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 outgo‐
    ing packets.
    Should we want to simulate a bidirectional link with bandwidth limita‐
    tions, the correct way is the following:
          ipfw add pipe 1 ip from any to any out
          ipfw add pipe 2 ip from any to any in
          ipfw pipe 1 config bw 64Kbit/s queue 10Kbytes
          ipfw pipe 2 config bw 64Kbit/s queue 10Kbytes
    The above can be very useful, e.g. if you want to see how your fancy Web
    page will look for a residential user who is connected only through a
    slow link.  You should not use only one pipe for both directions, unless
    you want to simulate a half-duplex medium (e.g. AppleTalk, Ethernet,
    IRDA).  It is not necessary that both pipes have the same configuration,
    so we can also simulate asymmetric links.
    Should we want to verify network performance with the RED queue manage‐
    ment algorithm:
          ipfw add pipe 1 ip from any to any
          ipfw pipe 1 config bw 500Kbit/s queue 100 red 0.002/30/80/0.1
    Another typical application of the traffic shaper is to introduce some
    delay in the communication.  This can significantly affect applications
    which do a lot of Remote Procedure Calls, and where the round-trip-time
    of the connection often becomes a limiting factor much more than band‐
    width:
          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 accord‐
    ingly.  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)
  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" ter‐
    minates, and your ruleset will be left active.  Otherwise, e.g. if you
    cannot access your box, the ruleset will be disabled after the sleep ter‐
    minates thus restoring the previous situation.
    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 keep‐
    ing 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

[править] 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)

[править] 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.

[править] 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 develop‐
    ers and maintainers are David Hayes and Jason But.  For further informa‐
    tion 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.

[править] 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, incom‐
    ing 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 appropri‐
    ately.
    Due to the architecture of libalias(3), ipfw nat is not compatible with
    the TCP segmentation offloading (TSO).  Thus, to reliably nat your net‐
    work 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 9.0 November 10, 2011 FreeBSD 9.0

Источник — «http://xgu.xgu.ru/wiki/man/orig/ipfw»