MirOS Manual: pf.conf(5)

PF.CONF(5)                   BSD Reference Manual                   PF.CONF(5)

NAME

     pf.conf - packet filter configuration file

DESCRIPTION

     The pf(4) packet filter modifies, drops or passes packets according to
     rules or definitions specified in pf.conf.

STATEMENT ORDER

     There are seven types of statements in pf.conf:

     Macros
           User-defined variables may be defined and used later, simplifying
           the configuration file. Macros must be defined before they are
           referenced in pf.conf.

     Tables
           Tables provide a mechanism for increasing the performance and flex-
           ibility of rules with large numbers of source or destination ad-
           dresses.

     Options
           Options tune the behaviour of the packet filtering engine.

     Traffic Normalization (e.g. scrub)
           Traffic normalization protects internal machines against incon-
           sistencies in Internet protocols and implementations.

     Queueing
           Queueing provides rule-based bandwidth control.

     Translation (various forms of NAT)
           Translation rules specify how addresses are to be mapped or
           redirected to other addresses.

     Packet Filtering
           Stateful and stateless packet filtering provides rule-based block-
           ing or passing of packets.

     With the exception of macros and tables, the types of statements should
     be grouped and appear in pf.conf in the order shown above, as this
     matches the operation of the underlying packet filtering engine. By de-
     fault pfctl(8) enforces this order (see set require-order below).

MACROS

     Much like cpp(1) or m4(1), macros can be defined that will later be ex-
     panded in context. Macro names must start with a letter, and may contain
     letters, digits and underscores. Macro names may not be reserved words
     (for example pass, in, out). Macros are not expanded inside quotes.

     For example,

           ext_if = "kue0"
           all_ifs = "{" $ext_if lo0 "}"
           pass out on $ext_if from any to any keep state
           pass in  on $ext_if proto tcp from any to any port 25 keep state

TABLES

     Tables are named structures which can hold a collection of addresses and
     networks. Lookups against tables in pf(4) are relatively fast, making a
     single rule with tables much more efficient, in terms of processor usage
     and memory consumption, than a large number of rules which differ only in
     IP address (either created explicitly or automatically by rule expan-
     sion).

     Tables can be used as the source or destination of filter rules, scrub
     rules or translation rules such as nat or rdr (see below for details on
     the various rule types). Tables can also be used for the redirect address
     of nat and rdr rules and in the routing options of filter rules, but only
     for round-robin pools.

     Tables can be defined with any of the following pfctl(8) mechanisms. As
     with macros, reserved words may not be used as table names.

     manually  Persistent tables can be manually created with the add or
               replace option of pfctl(8), before or after the ruleset has
               been loaded.

     pf.conf   Table definitions can be placed directly in this file, and
               loaded at the same time as other rules are loaded, atomically.
               Table definitions inside pf.conf use the table statement, and
               are especially useful to define non-persistent tables. The con-
               tents of a pre-existing table defined without a list of ad-
               dresses to initialize it is not altered when pf.conf is loaded.
               A table initialized with the empty list, { }, will be cleared
               on load.

     Tables may be defined with the following two attributes:

     persist  The persist flag forces the kernel to keep the table even when
              no rules refer to it. If the flag is not set, the kernel will
              automatically remove the table when the last rule referring to
              it is flushed.

     const    The const flag prevents the user from altering the contents of
              the table once it has been created. Without that flag, pfctl(8)
              can be used to add or remove addresses from the table at any
              time, even when running with securelevel(7) = 2.

     For example,

           table <private> const { 10/8, 172.16/12, 192.168/16 }
           table <badhosts> persist
           block on fxp0 from { <private>, <badhosts> } to any

     creates a table called private, to hold RFC 1918 private network blocks,
     and a table called badhosts, which is initially empty. A filter rule is
     set up to block all traffic coming from addresses listed in either table.
     The private table cannot have its contents changed and the badhosts table
     will exist even when no active filter rules reference it. Addresses may
     later be added to the badhosts table, so that traffic from these hosts
     can be blocked by using

           # pfctl -t badhosts -Tadd 204.92.77.111

     A table can also be initialized with an address list specified in one or
     more external files, using the following syntax:

           table <spam> persist file "/etc/spammers" file "/etc/openrelays"
           block on fxp0 from <spam> to any

     The files /etc/spammers and /etc/openrelays list IP addresses, one per
     line. Any lines beginning with a # are treated as comments and ignored.
     In addition to being specified by IP address, hosts may also be specified
     by their hostname. When the resolver is called to add a hostname to a
     table, all resulting IPv4 and IPv6 addresses are placed into the table.
     IP addresses can also be entered in a table by specifying a valid inter-
     face name, a valid interface group or the self keyword, in which case all
     addresses assigned to the interface(s) will be added to the table.

OPTIONS

     pf(4) may be tuned for various situations using the set command.

     set timeout

           interval   Interval between purging expired states and fragments.
           frag       Seconds before an unassembled fragment is expired.
           src.track  Length of time to retain a source tracking entry after
                      the last state expires.

           When a packet matches a stateful connection, the seconds to live
           for the connection will be updated to that of the proto.modifier
           which corresponds to the connection state. Each packet which
           matches this state will reset the TTL. Tuning these values may im-
           prove the performance of the firewall at the risk of dropping valid
           idle connections.

           tcp.first
                 The state after the first packet.
           tcp.opening
                 The state before the destination host ever sends a packet.
           tcp.established
                 The fully established state.
           tcp.closing
                 The state after the first FIN has been sent.
           tcp.finwait
                 The state after both FINs have been exchanged and the connec-
                 tion is closed. Some hosts (notably web servers on Solaris)
                 send TCP packets even after closing the connection. Increas-
                 ing tcp.finwait (and possibly tcp.closing) can prevent block-
                 ing of such packets.
           tcp.closed
                 The state after one endpoint sends an RST.

           ICMP and UDP are handled in a fashion similar to TCP, but with a
           much more limited set of states:

           udp.first
                 The state after the first packet.
           udp.single
                 The state if the source host sends more than one packet but
                 the destination host has never sent one back.
           udp.multiple
                 The state if both hosts have sent packets.
           icmp.first
                 The state after the first packet.
           icmp.error
                 The state after an ICMP error came back in response to an
                 ICMP packet.

           Other protocols are handled similarly to UDP:

           other.first
           other.single
           other.multiple

           Timeout values can be reduced adaptively as the number of state
           table entries grows.

           adaptive.start
                 When the number of state entries exceeds this value, adaptive
                 scaling begins. All timeout values are scaled linearly with
                 factor (adaptive.end - number of states) / (adaptive.end -
                 adaptive.start).
           adaptive.end
                 When reaching this number of state entries, all timeout
                 values become zero, effectively purging all state entries im-
                 mediately. This value is used to define the scale factor, it
                 should not actually be reached (set a lower state limit, see
                 below).

           These values can be defined both globally and for each rule. When
           used on a per-rule basis, the values relate to the number of states
           created by the rule, otherwise to the total number of states.

           For example:

                 set timeout tcp.first 120
                 set timeout tcp.established 86400
                 set timeout { adaptive.start 6000, adaptive.end 12000 }
                 set limit states 10000

           With 9000 state table entries, the timeout values are scaled to 50%
           (tcp.first 60, tcp.established 43200).

     set loginterface
           Enable collection of packet and byte count statistics for the given
           interface. These statistics can be viewed using

                 # pfctl -s info

           In this example pf(4) collects statistics on the interface named
           dc0:

                 set loginterface dc0

           One can disable the loginterface using:

                 set loginterface none

     set limit
           Sets hard limits on the memory pools used by the packet filter. See
           pool(9) for an explanation of memory pools.

           For example,

                 set limit states 20000

           sets the maximum number of entries in the memory pool used by state
           table entries (generated by keep state rules) to 20000. Using

                 set limit frags 20000

           sets the maximum number of entries in the memory pool used for
           fragment reassembly (generated by scrub rules) to 20000. Finally,

                 set limit src-nodes 2000

           sets the maximum number of entries in the memory pool used for
           tracking source IP addresses (generated by the sticky-address and
           source-track options) to 2000.

           These can be combined:

                 set limit { states 20000, frags 20000, src-nodes 2000 }

     set optimization
           Optimize the engine for one of the following network environments:

           normal
                 A normal network environment. Suitable for almost all net-
                 works.
           high-latency
                 A high-latency environment (such as a satellite connection).
           satellite
                 Alias for high-latency.
           aggressive
                 Aggressively expire connections. This can greatly reduce the
                 memory usage of the firewall at the cost of dropping idle
                 connections early.
           conservative
                 Extremely conservative settings. Avoid dropping legitimate
                 connections at the expense of greater memory utilization
                 (possibly much greater on a busy network) and slightly in-
                 creased processor utilization.

           For example:

                 set optimization aggressive

     set block-policy
           The block-policy option sets the default behaviour for the packet
           block action:

           drop      Packet is silently dropped.
           return    A TCP RST is returned for blocked TCP packets, an ICMP
                     UNREACHABLE is returned for blocked UDP packets, and all
                     other packets are silently dropped.

           For example:

                 set block-policy return

     set state-policy
           The state-policy option sets the default behaviour for states:

           if-bound     States are bound to interface.
           floating     States can match packets on any interfaces (the de-
                        fault).

           For example:

                 set state-policy if-bound

     set require-order
           By default pfctl(8) enforces an ordering of the statement types in
           the ruleset to: options, normalization, queueing, translation,
           filtering. Setting this option to no disables this enforcement.
           There may be non-trivial and non-obvious implications to an out of
           order ruleset. Consider carefully before disabling the order en-
           forcement.

     set fingerprints
           Load fingerprints of known operating systems from the given
           filename. By default fingerprints of known operating systems are
           automatically loaded from pf.os(5) in /etc but can be overridden
           via this option. Setting this option may leave a small period of
           time where the fingerprints referenced by the currently active ru-
           leset are inconsistent until the new ruleset finishes loading.

           For example:

                 set fingerprints "/etc/pf.os.devel"

     set skip on <ifspec>
           List interfaces for which packets should not be filtered. Packets
           passing in or out on such interfaces are passed as if pf was dis-
           abled, i.e. pf does not process them in any way. This can be useful
           on loopback and other virtual interfaces, when packet filtering is
           not desired and can have unexpected effects. For example:

                 set skip on lo0

     set debug
           Set the debug level to one of the following:

           none          Don't generate debug messages.
           urgent        Generate debug messages only for serious errors.
           misc          Generate debug messages for various errors.
           loud          Generate debug messages for common conditions.

TRAFFIC NORMALIZATION

     Traffic normalization is used to sanitize packet content in such a way
     that there are no ambiguities in packet interpretation on the receiving
     side. The normalizer does IP fragment reassembly to prevent attacks that
     confuse intrusion detection systems by sending overlapping IP fragments.
     Packet normalization is invoked with the scrub directive.

     scrub has the following options:

     no-df
           Clears the dont-fragment bit from a matching IP packet. Some
           operating systems are known to generate fragmented packets with the
           dont-fragment bit set. This is particularly true with NFS. Scrub
           will drop such fragmented dont-fragment packets unless no-df is
           specified.

           Unfortunately some operating systems also generate their dont-
           fragment packets with a zero IP identification field. Clearing the
           dont-fragment bit on packets with a zero IP ID may cause deleteri-
           ous results if an upstream router later fragments the packet. Using
           the random-id modifier (see below) is recommended in combination
           with the no-df modifier to ensure unique IP identifiers.

     min-ttl <number>
           Enforces a minimum TTL for matching IP packets.

     max-mss <number>
           Enforces a maximum MSS for matching TCP packets.

     random-id
           Replaces the IP identification field with random values to compen-
           sate for predictable values generated by many hosts. This option
           only applies to packets that are not fragmented after the optional
           fragment reassembly.

     fragment reassemble
           Using scrub rules, fragments can be reassembled by normalization.
           In this case, fragments are buffered until they form a complete
           packet, and only the completed packet is passed on to the filter.
           The advantage is that filter rules have to deal only with complete
           packets, and can ignore fragments. The drawback of caching frag-
           ments is the additional memory cost. But the full reassembly method
           is the only method that currently works with NAT. This is the de-
           fault behavior of a scrub rule if no fragmentation modifier is sup-
           plied.

     fragment crop
           The default fragment reassembly method is expensive, hence the op-
           tion to crop is provided. In this case, pf(4) will track the frag-
           ments and cache a small range descriptor. Duplicate fragments are
           dropped and overlaps are cropped. Thus data will only occur once on
           the wire with ambiguities resolving to the first occurrence. Unlike
           the fragment reassemble modifier, fragments are not buffered, they
           are passed as soon as they are received. The fragment crop reassem-
           bly mechanism does not yet work with NAT.

     fragment drop-ovl
           This option is similar to the fragment crop modifier except that
           all overlapping or duplicate fragments will be dropped, and all
           further corresponding fragments will be dropped as well.

     reassemble tcp
           Statefully normalizes TCP connections. scrub reassemble tcp rules
           may not have the direction (in/out) specified. reassemble tcp per-
           forms the following normalizations:

           ttl      Neither side of the connection is allowed to reduce their
                    IP TTL. An attacker may send a packet such that it reaches
                    the firewall, affects the firewall state, and expires be-
                    fore reaching the destination host. reassemble tcp will
                    raise the TTL of all packets back up to the highest value
                    seen on the connection.
           timestamp modulation
                    Modern TCP stacks will send a timestamp on every TCP pack-
                    et and echo the other endpoint's timestamp back to them.
                    Many operating systems will merely start the timestamp at
                    zero when first booted, and increment it several times a
                    second. The uptime of the host can be deduced by reading
                    the timestamp and multiplying by a constant. Also observ-
                    ing several different timestamps can be used to count
                    hosts behind a NAT device. And spoofing TCP packets into a
                    connection requires knowing or guessing valid timestamps.
                    Timestamps merely need to be monotonically increasing and
                    not derived off a guessable base time. reassemble tcp will
                    cause scrub to modulate the TCP timestamps with a random
                    number.
           extended PAWS checks
                    There is a problem with TCP on long fat pipes, in that a
                    packet might get delayed for longer than it takes the con-
                    nection to wrap its 32-bit sequence space. In such an oc-
                    currence, the old packet would be indistinguishable from a
                    new packet and would be accepted as such. The solution to
                    this is called PAWS: Protection Against Wrapped Sequence
                    numbers. It protects against it by making sure the times-
                    tamp on each packet does not go backwards. reassemble tcp
                    also makes sure the timestamp on the packet does not go
                    forward more than the RFC allows. By doing this, pf(4) ar-
                    tificially extends the security of TCP sequence numbers by
                    10 to 18 bits when the host uses appropriately randomized
                    timestamps, since a blind attacker would have to guess the
                    timestamp as well.

     For example,

           scrub in on $ext_if all fragment reassemble

     The no option prefixed to a scrub rule causes matching packets to remain
     unscrubbed, much in the same way as drop quick works in the packet filter
     (see below). This mechanism should be used when it is necessary to ex-
     clude specific packets from broader scrub rules.

QUEUEING

     Packets can be assigned to queues for the purpose of bandwidth control.
     At least two declarations are required to configure queues, and later any
     packet filtering rule can reference the defined queues by name. During
     the filtering component of pf.conf, the last referenced queue name is
     where any packets from pass rules will be queued, while for block rules
     it specifies where any resulting ICMP or TCP RST packets should be
     queued. The scheduler defines the algorithm used to decide which packets
     get delayed, dropped, or sent out immediately. There are three schedulers
     currently supported.

     cbq   Class Based Queueing. Queues attached to an interface build a tree,
           thus each queue can have further child queues. Each queue can have
           a priority and a bandwidth assigned. Priority mainly controls the
           time packets take to get sent out, while bandwidth has primarily
           effects on throughput. cbq achieves both partitioning and sharing
           of link bandwidth by hierarchically structured classes. Each class
           has its own queue and is assigned its share of bandwidth. A child
           class can borrow bandwidth from its parent class as long as excess
           bandwidth is available (see the option borrow, below).

     priq  Priority Queueing. Queues are flat attached to the interface, thus,
           queues cannot have further child queues. Each queue has a unique
           priority assigned, ranging from 0 to 15. Packets in the queue with
           the highest priority are processed first.

     hfsc  Hierarchical Fair Service Curve. Queues attached to an interface
           build a tree, thus each queue can have further child queues. Each
           queue can have a priority and a bandwidth assigned. Priority mainly
           controls the time packets take to get sent out, while bandwidth has
           primarily effects on throughput. hfsc supports both link-sharing
           and guaranteed real-time services. It employs a service curve based
           QoS model, and its unique feature is an ability to decouple delay
           and bandwidth allocation.

     The interfaces on which queueing should be activated are declared using
     the altq on declaration. altq on has the following keywords:

     <interface>
           Queueing is enabled on the named interface.

     <scheduler>
           Specifies which queueing scheduler to use. Currently supported
           values are cbq for Class Based Queueing, priq for Priority Queueing
           and hfsc for the Hierarchical Fair Service Curve scheduler.

     bandwidth <bw>
           The maximum bitrate for all queues on an interface may be specified
           using the bandwidth keyword. The value can be specified as an abso-
           lute value or as a percentage of the interface bandwidth. When us-
           ing an absolute value, the suffixes b, Kib, Mib, and Gib are used
           to represent bits, kibibits, mebibits, and gibibits per second,
           respectively. The value must not exceed the interface bandwidth. If
           bandwidth is not specified, the interface bandwidth is used.

     qlimit <limit>
           The maximum number of packets held in the queue. The default is 50.

     tbrsize <size>
           Adjusts the size, in bytes, of the token bucket regulator. If not
           specified, heuristics based on the interface bandwidth are used to
           determine the size.

     queue <list>
           Defines a list of subqueues to create on an interface.

     In the following example, the interface dc0 should queue up to 5 Mbit/s
     in four second-level queues using Class Based Queueing. Those four queues
     will be shown in a later example.

           altq on dc0 cbq bandwidth 5000000b queue { std, http, mail, ssh }

     Once interfaces are activated for queueing using the altq directive, a
     sequence of queue directives may be defined. The name associated with a
     queue must match a queue defined in the altq directive (e.g. mail), or,
     except for the priq scheduler, in a parent queue declaration. The follow-
     ing keywords can be used:

     on <interface>
           Specifies the interface the queue operates on. If not given, it
           operates on all matching interfaces.

     bandwidth <bw>
           Specifies the maximum bitrate to be processed by the queue. This
           value must not exceed the value of the parent queue and can be
           specified as an absolute value or a percentage of the parent
           queue's bandwidth. If not specified, defaults to 100% of the parent
           queue's bandwidth. The priq scheduler does not support bandwidth
           specification.

     priority <level>
           Between queues a priority level can be set. For cbq and hfsc, the
           range is 0 to 7 and for priq, the range is 0 to 15. The default for
           all is 1. Priq queues with a higher priority are always served
           first. Cbq and Hfsc queues with a higher priority are preferred in
           the case of overload.

     qlimit <limit>
           The maximum number of packets held in the queue. The default is 50.

     The scheduler can get additional parameters with <scheduler>(
     <parameters> ). Parameters are as follows:

     default     Packets not matched by another queue are assigned to this
                 one. Exactly one default queue is required.

     red         Enable RED (Random Early Detection) on this queue. RED drops
                 packets with a probability proportional to the average queue
                 length.

     rio         Enables RIO on this queue. RIO is RED with IN/OUT, thus run-
                 ning RED two times more than RIO would achieve the same ef-
                 fect. RIO is currently not supported in the GENERIC kernel.

     ecn         Enables ECN (Explicit Congestion Notification) on this queue.
                 ECN implies RED.

     The cbq scheduler supports an additional option:

     borrow      The queue can borrow bandwidth from the parent.

     The hfsc scheduler supports some additional options:

     realtime <sc>
                 The minimum required bandwidth for the queue.

     upperlimit <sc>
                 The maximum allowed bandwidth for the queue.

     linkshare <sc>
                 The bandwidth share of a backlogged queue.

     <sc> is an acronym for service curve.

     The format for service curve specifications is (m1, d, m2). m2 controls
     the bandwidth assigned to the queue. m1 and d are optional and can be
     used to control the initial bandwidth assignment. For the first d mil-
     liseconds the queue gets the bandwidth given as m1, afterwards the value
     given in m2.

     Furthermore, with cbq and hfsc, child queues can be specified as in an
     altq declaration, thus building a tree of queues using a part of their
     parent's bandwidth.

     Packets can be assigned to queues based on filter rules by using the
     queue keyword. Normally only one queue is specified; when a second one is
     specified it will instead be used for packets which have a TOS of
     lowdelay and for TCP ACKs with no data payload.

     To continue the previous example, the examples below would specify the
     four referenced queues, plus a few child queues. Interactive ssh(1) ses-
     sions get priority over bulk transfers like scp(1) and sftp(1). The
     queues may then be referenced by filtering rules (see PACKET FILTERING
     below).

     queue std bandwidth 10% cbq(default)
     queue http bandwidth 60% priority 2 cbq(borrow red) \
           { employees, developers }
     queue  developers bandwidth 75% cbq(borrow)
     queue  employees bandwidth 15%
     queue mail bandwidth 10% priority 0 cbq(borrow ecn)
     queue ssh bandwidth 20% cbq(borrow) { ssh_interactive, ssh_bulk }
     queue  ssh_interactive bandwidth 50% priority 7 cbq(borrow)
     queue  ssh_bulk bandwidth 50% priority 0 cbq(borrow)

     block return out on dc0 inet all queue std
     pass out on dc0 inet proto tcp from $developerhosts to any port 80 \
           keep state queue developers
     pass out on dc0 inet proto tcp from $employeehosts to any port 80 \
           keep state queue employees
     pass out on dc0 inet proto tcp from any to any port 22 \
           keep state queue(ssh_bulk, ssh_interactive)
     pass out on dc0 inet proto tcp from any to any port 25 \
           keep state queue mail

TRANSLATION

     Translation rules modify either the source or destination address of the
     packets associated with a stateful connection. A stateful connection is
     automatically created to track packets matching such a rule as long as
     they are not blocked by the filtering section of pf.conf. The translation
     engine modifies the specified address and/or port in the packet, recalcu-
     lates IP, TCP and UDP checksums as necessary, and passes it to the packet
     filter for evaluation.

     Since translation occurs before filtering the filter engine will see
     packets as they look after any addresses and ports have been translated.
     Filter rules will therefore have to filter based on the translated ad-
     dress and port number. Packets that match a translation rule are only au-
     tomatically passed if the pass modifier is given, otherwise they are
     still subject to block and pass rules.

     The state entry created permits pf(4) to keep track of the original ad-
     dress for traffic associated with that state and correctly direct return
     traffic for that connection.

     Various types of translation are possible with pf:

     binat
           A binat rule specifies a bidirectional mapping between an external
           IP netblock and an internal IP netblock.

     nat   A nat rule specifies that IP addresses are to be changed as the
           packet traverses the given interface. This technique allows one or
           more IP addresses on the translating host to support network traff-
           ic for a larger range of machines on an "inside" network. Although
           in theory any IP address can be used on the inside, it is strongly
           recommended that one of the address ranges defined by RFC 1918 be
           used. These netblocks are:

           10.0.0.0 - 10.255.255.255 (all of net 10, i.e., 10/8)
           172.16.0.0 - 172.31.255.255 (i.e., 172.16/12)
           192.168.0.0 - 192.168.255.255 (i.e., 192.168/16)

     rdr   The packet is redirected to another destination and possibly a dif-
           ferent port. rdr rules can optionally specify port ranges instead
           of single ports. rdr ... port 2000:2999 -> ... port 4000 redirects
           ports 2000 to 2999 (inclusive) to port 4000. rdr ... port 2000:2999
           -> ... port 4000:* redirects port 2000 to 4000, 2001 to 4001, ...,
           2999 to 4999.

     In addition to modifying the address, some translation rules may modify
     source or destination ports for tcp(4) or udp(4) connections; implicitly
     in the case of nat rules and explicitly in the case of rdr rules. Port
     numbers are never translated with a binat rule.

     For each packet processed by the translator, the translation rules are
     evaluated in sequential order, from first to last. The first matching
     rule decides what action is taken.

     The no option prefixed to a translation rule causes packets to remain un-
     translated, much in the same way as drop quick works in the packet filter
     (see below). If no rule matches the packet it is passed to the filter en-
     gine unmodified.

     Translation rules apply only to packets that pass through the specified
     interface, and if no interface is specified, translation is applied to
     packets on all interfaces. For instance, redirecting port 80 on an exter-
     nal interface to an internal web server will only work for connections
     originating from the outside. Connections to the address of the external
     interface from local hosts will not be redirected, since such packets do
     not actually pass through the external interface. Redirections cannot re-
     flect packets back through the interface they arrive on, they can only be
     redirected to hosts connected to different interfaces or to the firewall
     itself.

     Note that redirecting external incoming connections to the loopback ad-
     dress, as in

           rdr on ne3 inet proto tcp to port 8025 -> 127.0.0.1 port 25

     will effectively allow an external host to connect to daemons bound sole-
     ly to the loopback address, circumventing the traditional blocking of
     such connections on a real interface. Unless this effect is desired, any
     of the local non-loopback addresses should be used as redirection target
     instead, which allows external connections only to daemons bound to this
     address or not bound to any address.

     See TRANSLATION EXAMPLES below.

PACKET FILTERING

     pf(4) has the ability to block and pass packets based on attributes of
     their layer 3 (see ip(4) and ip6(4)) and layer 4 (see icmp(4), icmp6(4),
     tcp(4), udp(4)) headers. In addition, packets may also be assigned to
     queues for the purpose of bandwidth control.

     For each packet processed by the packet filter, the filter rules are
     evaluated in sequential order, from first to last. The last matching rule
     decides what action is taken.

     The following actions can be used in the filter:

     block
           The packet is blocked. There are a number of ways in which a block
           rule can behave when blocking a packet. The default behaviour is to
           drop packets silently, however this can be overridden or made ex-
           plicit either globally, by setting the block-policy option, or on a
           per-rule basis with one of the following options:

           drop  The packet is silently dropped.
           return-rst
                 This applies only to tcp(4) packets, and issues a TCP RST
                 which closes the connection.
           return-icmp
           return-icmp6
                 This causes ICMP messages to be returned for packets which
                 match the rule. By default this is an ICMP UNREACHABLE mes-
                 sage, however this can be overridden by specifying a message
                 as a code or number.
           return
                 This causes a TCP RST to be returned for tcp(4) packets and
                 an ICMP UNREACHABLE for UDP and other packets.

           Options returning ICMP packets currently have no effect if pf(4)
           operates on a bridge(4), as the code to support this feature has
           not yet been implemented.

     pass  The packet is passed.

     If no rule matches the packet, the default action is pass.

     To block everything by default and only pass packets that match explicit
     rules, one uses

           block all

     as the first filter rule.

     See FILTER EXAMPLES below.

PARAMETERS

     The rule parameters specify the packets to which a rule applies. A packet
     always comes in on, or goes out through, one interface. Most parameters
     are optional. If a parameter is specified, the rule only applies to pack-
     ets with matching attributes. Certain parameters can be expressed as
     lists, in which case pfctl(8) generates all needed rule combinations.

     in or out
           This rule applies to incoming or outgoing packets. If neither in
           nor out are specified, the rule will match packets in both direc-
           tions.

     log   In addition to the action specified, a log message is generated.
           All packets for that connection are logged, unless the keep state,
           modulate state or synproxy state options are specified, in which
           case only the packet that establishes the state is logged. (See
           keep state, modulate state and synproxy state below). The logged
           packets are sent to the pflog(4) interface. This interface is moni-
           tored by the pflogd(8) logging daemon, which dumps the logged pack-
           ets to the file /var/log/pflog in pcap(3) binary format.

     log (all)
           Used with keep state, modulate state or synproxy state rules to
           force logging of all packets for a connection. As with log, packets
           are logged to pflog(4).

     quick
           If a packet matches a rule which has the quick option set, this
           rule is considered the last matching rule, and evaluation of subse-
           quent rules is skipped.

     on <interface>
           This rule applies only to packets coming in on, or going out
           through, this particular interface or interface group.

     <af>  This rule applies only to packets of this address family. Supported
           values are inet and inet6.

     proto <protocol>
           This rule applies only to packets of this protocol. Common proto-
           cols are icmp(4), icmp6(4), tcp(4), and udp(4). For a list of all
           the protocol name to number mappings used by pfctl(8), see the file
           /etc/protocols.

     from <source> port <source> os <source> to <dest> port <dest>
           This rule applies only to packets with the specified source and
           destination addresses and ports.

           Addresses can be specified in CIDR notation (matching netblocks),
           as symbolic host names or interface names, or as any of the follow-
           ing keywords:

           any             Any address.
           route <label>   Any address whose associated route has label
                           <label>. See route(4) and route(8).
           no-route        Any address which is not currently routable.
           <table>         Any address that matches the given table.

           Interface names can have modifiers appended:

           :network      Translates to the network(s) attached to the inter-
                         face.
           :broadcast    Translates to the interface's broadcast address(es).
           :peer         Translates to the point to point interface's peer
                         address(es).
           :0            Do not include interface aliases.

           Host names may also have the :0 option appended to restrict the
           name resolution to the first of each v4 and v6 address found.

           Host name resolution and interface to address translation are done
           at ruleset load-time. When the address of an interface (or host
           name) changes (under DHCP or PPP, for instance), the ruleset must
           be reloaded for the change to be reflected in the kernel.

           Ports can be specified either by number or by name. For example,
           port 80 can be specified as www. For a list of all port name to
           number mappings used by pfctl(8), see the file /etc/services.

           Ports and ranges of ports are specified by using these operators:

                 =       (equal)
                 !=      (unequal)
                 <       (less than)
                 <=      (less than or equal)
                 >       (greater than)
                 >=      (greater than or equal)
                 :       (range including boundaries)
                 ><      (range excluding boundaries)
                 <>      (except range)

           ><, <> and : are binary operators (they take two arguments). For
           instance:

           port 2000:2004
                       means 'all ports >= 2000 and <= 2004', hence ports
                       2000, 2001, 2002, 2003 and 2004.

           port 2000 >< 2004
                       means 'all ports > 2000 and < 2004', hence ports 2001,
                       2002 and 2003.

           port 2000 <> 2004
                       means 'all ports < 2000 or > 2004', hence ports 1-1999
                       and 2005-65535.

           The operating system of the source host can be specified in the
           case of TCP rules with the OS modifier. See the OPERATING SYSTEM
           FINGERPRINTING section for more information.

           The host, port and OS specifications are optional, as in the fol-
           lowing examples:

                 pass in all
                 pass in from any to any
                 pass in proto tcp from any port <= 1024 to any
                 pass in proto tcp from any to any port 25
                 pass in proto tcp from 10.0.0.0/8 port > 1024 \
                       to ! 10.1.2.3 port != ssh
                 pass in proto tcp from any os "OpenBSD" flags S/SA
                 pass in proto tcp from route "DTAG"

     all   This is equivalent to "from any to any".

     group <group>
           Similar to user, this rule only applies to packets of sockets owned
           by the specified group.

     user <user>
           This rule only applies to packets of sockets owned by the specified
           user. For outgoing connections initiated from the firewall, this is
           the user that opened the connection. For incoming connections to
           the firewall itself, this is the user that listens on the destina-
           tion port. For forwarded connections, where the firewall is not a
           connection endpoint, the user and group are unknown.

           All packets, both outgoing and incoming, of one connection are as-
           sociated with the same user and group. Only TCP and UDP packets can
           be associated with users; for other protocols these parameters are
           ignored.

           User and group refer to the effective (as opposed to the real) IDs,
           in case the socket is created by a setuid/setgid process. User and
           group IDs are stored when a socket is created; when a process
           creates a listening socket as root (for instance, by binding to a
           privileged port) and subsequently changes to another user ID (to
           drop privileges), the credentials will remain root.

           User and group IDs can be specified as either numbers or names. The
           syntax is similar to the one for ports. The value unknown matches
           packets of forwarded connections. unknown can only be used with the
           operators = and !=. Other constructs like user >= unknown are in-
           valid. Forwarded packets with unknown user and group ID match only
           rules that explicitly compare against unknown with the operators =
           or !=. For instance user >= 0 does not match forwarded packets. The
           following example allows only selected users to open outgoing con-
           nections:

                 block out proto { tcp, udp } all
                 pass  out proto { tcp, udp } all \
                       user { < 1000, dhartmei } keep state

     flags <a>/<b> | /<b>
           This rule only applies to TCP packets that have the flags <a> set
           out of set <b>. Flags not specified in <b> are ignored. The flags
           are: (F)IN, (S)YN, (R)ST, (P)USH, (A)CK, (U)RG, (E)CE, and C(W)R.

           flags S/S   Flag SYN is set. The other flags are ignored.

           flags S/SA  Out of SYN and ACK, exactly SYN may be set. SYN,
                       SYN+PSH and SYN+RST match, but SYN+ACK, ACK and ACK+RST
                       do not. This is more restrictive than the previous ex-
                       ample.

           flags /SFRA
                       If the first set is not specified, it defaults to none.
                       All of SYN, FIN, RST and ACK must be unset.

     icmp-type <type> code <code>

     icmp6-type <type> code <code>
           This rule only applies to ICMP or ICMPv6 packets with the specified
           type and code. Text names for ICMP types and codes are listed in
           icmp(4) and icmp6(4). This parameter is only valid for rules that
           cover protocols ICMP or ICMP6. The protocol and the ICMP type indi-
           cator (icmp-type or icmp6-type) must match.

     allow-opts
           By default, packets which contain IP options are blocked. When
           allow-opts is specified for a pass rule, packets that pass the
           filter based on that rule (last matching) do so even if they con-
           tain IP options. For packets that match state, the rule that ini-
           tially created the state is used. The implicit pass rule that is
           used when a packet does not match any rules does not allow IP op-
           tions.

     label <string>
           Adds a label (name) to the rule, which can be used to identify the
           rule. For instance, pfctl -s labels shows per-rule statistics for
           rules that have labels.

           The following macros can be used in labels:

                 $if       The interface.
                 $srcaddr  The source IP address.
                 $dstaddr  The destination IP address.
                 $srcport  The source port specification.
                 $dstport  The destination port specification.
                 $proto    The protocol name.
                 $nr       The rule number.

           For example:

                 ips = "{ 1.2.3.4, 1.2.3.5 }"
                 pass in proto tcp from any to $ips \
                       port > 1023 label "$dstaddr:$dstport"

           expands to

                 pass in inet proto tcp from any to 1.2.3.4 \
                       port > 1023 label "1.2.3.4:>1023"
                 pass in inet proto tcp from any to 1.2.3.5 \
                       port > 1023 label "1.2.3.5:>1023"

           The macro expansion for the label directive occurs only at confi-
           guration file parse time, not during runtime.

     queue <queue> | (<queue>, <queue>)
           Packets matching this rule will be assigned to the specified queue.
           If two queues are given, packets which have a tos of lowdelay and
           TCP ACKs with no data payload will be assigned to the second one.
           See QUEUEING for setup details.

           For example:

                 pass in proto tcp to port 25 queue mail
                 pass in proto tcp to port 22 queue(ssh_bulk, ssh_prio)

     tag <string>
           Packets matching this rule will be tagged with the specified
           string. The tag acts as an internal marker that can be used to
           identify these packets later on. This can be used, for example, to
           provide trust between interfaces and to determine if packets have
           been processed by translation rules. Tags are "sticky", meaning
           that the packet will be tagged even if the rule is not the last
           matching rule. Further matching rules can replace the tag with a
           new one but will not remove a previously applied tag. A packet is
           only ever assigned one tag at a time. Packet tagging can be done
           during nat, rdr, or binat rules in addition to filter rules. Tags
           take the same macros as labels (see above).

     tagged <string>
           Used with filter or translation rules to specify that packets must
           already be tagged with the given tag in order to match the rule.
           Inverse tag matching can also be done by specifying the ! operator
           before the tagged keyword.

     probability <number>
           A probability attribute can be attached to a rule, with a value set
           between 0 and 1, bounds not included. In that case, the rule will
           be honoured using the given probability value only. For example,
           the following rule will drop 20% of incoming ICMP packets:

                 block in proto icmp probability 20%

ROUTING

     If a packet matches a rule with a route option set, the packet filter
     will route the packet according to the type of route option. When such a
     rule creates state, the route option is also applied to all packets
     matching the same connection.

     fastroute
           The fastroute option does a normal route lookup to find the next
           hop for the packet.

     route-to
           The route-to option routes the packet to the specified interface
           with an optional address for the next hop. When a route-to rule
           creates state, only packets that pass in the same direction as the
           filter rule specifies will be routed in this way. Packets passing
           in the opposite direction (replies) are not affected and are routed
           normally.

     reply-to
           The reply-to option is similar to route-to, but routes packets that
           pass in the opposite direction (replies) to the specified inter-
           face. Opposite direction is only defined in the context of a state
           entry, and reply-to is useful only in rules that create state. It
           can be used on systems with multiple external connections to route
           all outgoing packets of a connection through the interface the in-
           coming connection arrived through (symmetric routing enforcement).

     dup-to
           The dup-to option creates a duplicate of the packet and routes it
           like route-to. The original packet gets routed as it normally
           would.

POOL OPTIONS

     For nat and rdr rules, (as well as for the route-to, reply-to and dup-to
     rule options) for which there is a single redirection address which has a
     subnet mask smaller than 32 for IPv4 or 128 for IPv6 (more than one IP
     address), a variety of different methods for assigning this address can
     be used:

     bitmask
           The bitmask option applies the network portion of the redirection
           address to the address to be modified (source with nat, destination
           with rdr).

     random
           The random option selects an address at random within the defined
           block of addresses.

     source-hash
           The source-hash option uses a hash of the source address to deter-
           mine the redirection address, ensuring that the redirection address
           is always the same for a given source. An optional key can be
           specified after this keyword either in hex or as a string; by de-
           fault pfctl(8) randomly generates a key for source-hash every time
           the ruleset is reloaded.

     round-robin
           The round-robin option loops through the redirection address(es).

           When more than one redirection address is specified, round-robin is
           the only permitted pool type.

     static-port
           With nat rules, the static-port option prevents pf(4) from modify-
           ing the source port on TCP and UDP packets.

     Additionally, the sticky-address option can be specified to help ensure
     that multiple connections from the same source are mapped to the same
     redirection address. This option can be used with the random and round-
     robin pool options. Note that by default these associations are destroyed
     as soon as there are no longer states which refer to them; in order to
     make the mappings last beyond the lifetime of the states, increase the
     global options with set timeout source-track See STATEFUL TRACKING
     OPTIONS for more ways to control the source tracking.

STATEFUL INSPECTION

     pf(4) is a stateful packet filter, which means it can track the state of
     a connection. Instead of passing all traffic to port 25, for instance, it
     is possible to pass only the initial packet, and then begin to keep
     state. Subsequent traffic will flow because the filter is aware of the
     connection.

     If a packet matches a pass ... keep state rule, the filter creates a
     state for this connection and automatically lets pass all subsequent
     packets of that connection.

     Before any rules are evaluated, the filter checks whether the packet
     matches any state. If it does, the packet is passed without evaluation of
     any rules.

     States are removed after the connection is closed or has timed out.

     This has several advantages. Comparing a packet to a state involves
     checking its sequence numbers. If the sequence numbers are outside the
     narrow windows of expected values, the packet is dropped. This prevents
     spoofing attacks, such as when an attacker sends packets with a fake
     source address/port but does not know the connection's sequence numbers.

     Also, looking up states is usually faster than evaluating rules. If there
     are 50 rules, all of them are evaluated sequentially in O(n). Even with
     50000 states, only 16 comparisons are needed to match a state, since
     states are stored in a binary search tree that allows searches in O(log2
     n).

     For instance:

           block all
           pass out proto tcp from any to any flags S/SA keep state
           pass in  proto tcp from any to any port 25 flags S/SA keep state

     This ruleset blocks everything by default. Only outgoing connections and
     incoming connections to port 25 are allowed. The initial packet of each
     connection has the SYN flag set, will be passed and creates state. All
     further packets of these connections are passed if they match a state.

     By default, packets coming in and out of any interface can match a state,
     but it is also possible to change that behaviour by assigning states to a
     single interface.

     The default policy is specified by the state-policy global option, but
     this can be adjusted on a per-rule basis by adding one of the if-bound or
     floating keywords to the keep state option.

     Specifying flags S/SA restricts state creation to the initial SYN packet
     of the TCP handshake. One can also be less restrictive, and allow state
     creation from intermediate (non-SYN) packets. This will cause pf(4) to
     synchronize to existing connections, for instance if one flushes the
     state table.

     For UDP, which is stateless by nature, keep state will create state as
     well. UDP packets are matched to states using only host addresses and
     ports.

     ICMP messages fall into two categories: ICMP error messages, which always
     refer to a TCP or UDP packet, are matched against the referred to connec-
     tion. If one keeps state on a TCP connection, and an ICMP source quench
     message referring to this TCP connection arrives, it will be matched to
     the right state and get passed.

     For ICMP queries, keep state creates an ICMP state, and pf(4) knows how
     to match ICMP replies to states. For example,

           pass out inet proto icmp all icmp-type echoreq keep state

     allows echo requests (such as those created by ping(8)) out, creates
     state, and matches incoming echo replies correctly to states.

     Note: nat, binat and rdr rules implicitly create state for connections.

STATE MODULATION

     Much of the security derived from TCP is attributable to how well the in-
     itial sequence numbers (ISNs) are chosen. Some popular stack implementa-
     tions choose very poor ISNs and thus are normally susceptible to ISN
     prediction exploits. By applying a modulate state rule to a TCP connec-
     tion, pf(4) will create a high quality random sequence number for each
     connection endpoint.

     The modulate state directive implicitly keeps state on the rule and is
     only applicable to TCP connections.

     For instance:

           block all
           pass out proto tcp from any to any modulate state
           pass in  proto tcp from any to any port 25 flags S/SA modulate state

     There are two caveats associated with state modulation: A modulate state
     rule can not be applied to a pre-existing but unmodulated connection.
     Such an application would desynchronize TCP's strict sequencing between
     the two endpoints. Instead, pf(4) will treat the modulate state modifier
     as a keep state modifier and the pre-existing connection will be inferred
     without the protection conferred by modulation.

     The other caveat affects currently modulated states when the state table
     is lost (firewall reboot, flushing the state table, etc...). pf(4) will
     not be able to infer a connection again after the state table flushes the
     connection's modulator. When the state is lost, the connection may be
     left dangling until the respective endpoints time out the connection. It
     is possible on a fast local network for the endpoints to start an ACK
     storm while trying to resynchronize after the loss of the modulator. Us-
     ing a flags S/SA modifier on modulate state rules between fast networks
     is suggested to prevent ACK storms.

     Note that alternative methods are available to prevent loss of the state
     table and allow for firewall failover. See carp(4) and pfsync(4) for
     further information.

SYN PROXY

     By default, pf(4) passes packets that are part of a tcp(4) handshake
     between the endpoints. The synproxy state option can be used to cause
     pf(4) itself to complete the handshake with the active endpoint, perform
     a handshake with the passive endpoint, and then forward packets between
     the endpoints.

     No packets are sent to the passive endpoint before the active endpoint
     has completed the handshake, hence so-called SYN floods with spoofed
     source addresses will not reach the passive endpoint, as the sender can't
     complete the handshake.

     The proxy is transparent to both endpoints, they each see a single con-
     nection from/to the other endpoint. pf(4) chooses random initial sequence
     numbers for both handshakes. Once the handshakes are completed, the se-
     quence number modulators (see previous section) are used to translate
     further packets of the connection. Hence, synproxy state includes
     modulate state and keep state.

     Rules with synproxy will not work if pf(4) operates on a bridge(4).

     Example:

           pass in proto tcp from any to any port www flags S/SA synproxy state

STATEFUL TRACKING OPTIONS

     All three of keep state, modulate state and synproxy state support the
     following options:

     max <number>
           Limits the number of concurrent states the rule may create. When
           this limit is reached, further packets matching the rule that would
           create state are dropped, until existing states time out.
     no-sync
           Prevent state changes for states created by this rule from appear-
           ing on the pfsync(4) interface.
     <timeout> <seconds>
           Changes the timeout values used for states created by this rule.
           For a list of all valid timeout names, see OPTIONS above.

     Multiple options can be specified, separated by commas:

           pass in proto tcp from any to any \
                 port www flags S/SA keep state \
                 (max 100, source-track rule, max-src-nodes 75, \
                 max-src-states 3, tcp.established 60, tcp.closing 5)

     When the source-track keyword is specified, the number of states per
     source IP is tracked.

     source-track rule
           The maximum number of states created by this rule is limited by the
           rule's max-src-nodes and max-src-states options. Only state entries
           created by this particular rule count toward the rule's limits.
     source-track global
           The number of states created by all rules that use this option is
           limited. Each rule can specify different max-src-nodes and max-
           src-states options, however state entries created by any partici-
           pating rule count towards each individual rule's limits.

     The following limits can be set:

     max-src-nodes <number>
           Limits the maximum number of source addresses which can simultane-
           ously have state table entries.
     max-src-states <number>
           Limits the maximum number of simultaneous state entries that a sin-
           gle source address can create with this rule.

     For stateful TCP connections, limits on established connections (connec-
     tions which have completed the TCP 3-way handshake) can also be enforced
     per source IP.

     max-src-conn <number>
           Limits the maximum number of simultaneous TCP connections which
           have completed the 3-way handshake that a single host can make.
     max-src-conn-rate <number> / <seconds>
           Limit the rate of new connections over a time interval. The connec-
           tion rate is an approximation calculated as a moving average.

     Because the 3-way handshake ensures that the source address is not being
     spoofed, more aggressive action can be taken based on these limits. With
     the overload <table> state option, source IP addresses which hit either
     of the limits on established connections will be added to the named
     table. This table can be used in the ruleset to block further activity
     from the offending host, redirect it to a tarpit process, or restrict its
     bandwidth.

     The optional flush keyword kills all states created by the matching rule
     which originate from the host which exceeds these limits. The global
     modifier to the flush command kills all states originating from the of-
     fending host, regardless of which rule created the state.

     For example, the following rules will protect the webserver against hosts
     making more than 100 connections in 10 seconds. Any host which connects
     faster than this rate will have its address added to the <bad_hosts>
     table and have all states originating from it flushed. Any new packets
     arriving from this host will be dropped unconditionally by the block
     rule.

           block quick from <bad_hosts>
           pass in on $ext_if proto tcp to $webserver port www flags S/SA keep state \
                   (max-src-conn-rate 100/10, overload <bad_hosts> flush global)

OPERATING SYSTEM FINGERPRINTING

     Passive OS Fingerprinting is a mechanism to inspect nuances of a TCP
     connection's initial SYN packet and guess at the host's operating system.
     Unfortunately these nuances are easily spoofed by an attacker so the
     fingerprint is not useful in making security decisions. But the finger-
     print is typically accurate enough to make policy decisions upon.

     The fingerprints may be specified by operating system class, by version,
     or by subtype/patchlevel. The class of an operating system is typically
     the vendor or genre and would be OpenBSD for the pf(4) firewall itself.
     The version of the oldest available OpenBSD release on the main ftp site
     would be 2.6 and the fingerprint would be written

           "OpenBSD 2.6"

     The subtype of an operating system is typically used to describe the
     patchlevel if that patch led to changes in the TCP stack behavior. In the
     case of OpenBSD, the only subtype is for a fingerprint that was normal-
     ized by the no-df scrub option and would be specified as

           "OpenBSD 3.3 no-df"

     Fingerprints for most popular operating systems are provided by pf.os(5).
     Once pf(4) is running, a complete list of known operating system finger-
     prints may be listed by running:

           # pfctl -so

     Filter rules can enforce policy at any level of operating system specifi-
     cation assuming a fingerprint is present. Policy could limit traffic to
     approved operating systems or even ban traffic from hosts that aren't at
     the latest service pack.

     The unknown class can also be used as the fingerprint which will match
     packets for which no operating system fingerprint is known.

     Examples:

           pass  out proto tcp from any os OpenBSD keep state
           block out proto tcp from any os Doors
           block out proto tcp from any os "Doors PT"
           block out proto tcp from any os "Doors PT SP3"
           block out from any os "unknown"
           pass on lo0 proto tcp from any os "OpenBSD 3.3 lo0" keep state

     Operating system fingerprinting is limited only to the TCP SYN packet.
     This means that it will not work on other protocols and will not match a
     currently established connection.

     Caveat: operating system fingerprints are occasionally wrong. There are
     three problems: an attacker can trivially craft his packets to appear as
     any operating system he chooses; an operating system patch could change
     the stack behavior and no fingerprints will match it until the database
     is updated; and multiple operating systems may have the same fingerprint.

FRAGMENT HANDLING

     The size of IP datagrams (packets) can be significantly larger than the
     maximum transmission unit (MTU) of the network. In cases when it is
     necessary or more efficient to send such large packets, the large packet
     will be fragmented into many smaller packets that will each fit onto the
     wire. Unfortunately for a firewalling device, only the first logical
     fragment will contain the necessary header information for the subproto-
     col that allows pf(4) to filter on things such as TCP ports or to perform
     NAT.

     Besides the use of scrub rules as described in TRAFFIC NORMALIZATION
     above, there are three options for handling fragments in the packet
     filter.

     One alternative is to filter individual fragments with filter rules. If
     no scrub rule applies to a fragment, it is passed to the filter. Filter
     rules with matching IP header parameters decide whether the fragment is
     passed or blocked, in the same way as complete packets are filtered.
     Without reassembly, fragments can only be filtered based on IP header
     fields (source/destination address, protocol), since subprotocol header
     fields are not available (TCP/UDP port numbers, ICMP code/type). The
     fragment option can be used to restrict filter rules to apply only to
     fragments, but not complete packets. Filter rules without the fragment
     option still apply to fragments, if they only specify IP header fields.
     For instance, the rule

           pass in proto tcp from any to any port 80

     never applies to a fragment, even if the fragment is part of a TCP packet
     with destination port 80, because without reassembly this information is
     not available for each fragment. This also means that fragments cannot
     create new or match existing state table entries, which makes stateful
     filtering and address translation (NAT, redirection) for fragments impos-
     sible.

     It's also possible to reassemble only certain fragments by specifying
     source or destination addresses or protocols as parameters in scrub
     rules.

     In most cases, the benefits of reassembly outweigh the additional memory
     cost, and it's recommended to use scrub rules to reassemble all fragments
     via the fragment reassemble modifier.

     The memory allocated for fragment caching can be limited using pfctl(8).
     Once this limit is reached, fragments that would have to be cached are
     dropped until other entries time out. The timeout value can also be ad-
     justed.

     Currently, only IPv4 fragments are supported and IPv6 fragments are
     blocked unconditionally.

ANCHORS

     Besides the main ruleset, pfctl(8) can load rulesets into anchor attach-
     ment points. An anchor is a container that can hold rules, address
     tables, and other anchors.

     An anchor has a name which specifies the path where pfctl(8) can be used
     to access the anchor to perform operations on it, such as attaching child
     anchors to it or loading rules into it. Anchors may be nested, with com-
     ponents separated by '/' characters, similar to how file system hierar-
     chies are laid out. The main ruleset is actually the default anchor, so
     filter and translation rules, for example, may also be contained in any
     anchor.

     An anchor can reference another anchor attachment point using the follow-
     ing kinds of rules:

     nat-anchor <name>
           Evaluates the nat rules in the specified anchor.

     rdr-anchor <name>
           Evaluates the rdr rules in the specified anchor.

     binat-anchor <name>
           Evaluates the binat rules in the specified anchor.

     anchor <name>
           Evaluates the filter rules in the specified anchor.

     load anchor <name> from <file>
           Loads the rules from the specified file into the anchor name.

     When evaluation of the main ruleset reaches an anchor rule, pf(4) will
     proceed to evaluate all rules specified in that anchor.

     Matching filter and translation rules in anchors with the quick option
     are final and abort the evaluation of the rules in other anchors and the
     main ruleset.

     anchor rules are evaluated relative to the anchor in which they are con-
     tained. For example, all anchor rules specified in the main ruleset will
     reference anchor attachment points underneath the main ruleset, and
     anchor rules specified in a file loaded from a load anchor rule will be
     attached under that anchor point.

     Rules may be contained in anchor attachment points which do not contain
     any rules when the main ruleset is loaded, and later such anchors can be
     manipulated through pfctl(8) without reloading the main ruleset or other
     anchors. For example,

           ext_if = "kue0"
           block on $ext_if all
           anchor spam
           pass out on $ext_if all keep state
           pass in on $ext_if proto tcp from any \
                 to $ext_if port smtp keep state

     blocks all packets on the external interface by default, then evaluates
     all rules in the anchor named "spam", and finally passes all outgoing
     connections and incoming connections to port 25.

           # echo "block in quick from 1.2.3.4 to any" | \
                 pfctl -a spam -f -

     This loads a single rule into the anchor, which blocks all packets from a
     specific address.

     The anchor can also be populated by adding a load anchor rule after the
     anchor rule:

           anchor spam
           load anchor spam from "/etc/pf-spam.conf"

     When pfctl(8) loads pf.conf, it will also load all the rules from the
     file /etc/pf-spam.conf into the anchor.

     Optionally, anchor rules can specify the parameter's direction, inter-
     face, address family, protocol and source/destination address/port using
     the same syntax as filter rules. When parameters are used, the anchor
     rule is only evaluated for matching packets. This allows conditional
     evaluation of anchors, like:

           block on $ext_if all
           anchor spam proto tcp from any to any port smtp
           pass out on $ext_if all keep state
           pass in on $ext_if proto tcp from any to $ext_if port smtp keep state

     The rules inside anchor spam are only evaluated for tcp packets with des-
     tination port 25. Hence,

           # echo "block in quick from 1.2.3.4 to any" | \
                 pfctl -a spam -f -

     will only block connections from 1.2.3.4 to port 25.

     Anchors may end with the asterisk ('*') character, which signifies that
     all anchors attached at that point should be evaluated in the alphabeti-
     cal ordering of their anchor name. For example,

           anchor "spam/*"

     will evaluate each rule in each anchor attached to the spam anchor. Note
     that it will only evaluate anchors that are directly attached to the spam
     anchor, and will not descend to evaluate anchors recursively.

     Since anchors are evaluated relative to the anchor in which they are con-
     tained, there is a mechanism for accessing the parent and ancestor an-
     chors of a given anchor. Similar to file system path name resolution, if
     the sequence ".." appears as an anchor path component, the parent anchor
     of the current anchor in the path evaluation at that point will become
     the new current anchor. As an example, consider the following:

           # echo ' anchor "spam/allowed" ' | pfctl -f -
           # echo -e ' anchor "../banned" \n pass' | \
                 pfctl -a spam/allowed -f -

     Evaluation of the main ruleset will lead into the spam/allowed anchor,
     which will evaluate the rules in the spam/banned anchor, if any, before
     finally evaluating the pass rule.

     Since the parser specification for anchor names is a string, any refer-
     ence to an anchor name containing solidus ('/') characters will require
     double quote ('"') characters around the anchor name.

TRANSLATION EXAMPLES

     This example maps incoming requests on port 80 to port 8080, on which a
     daemon is running (because, for example, it is not run as root, and
     therefore lacks permission to bind to port 80).

     # use a macro for the interface name, so it can be changed easily
     ext_if = "ne3"

     # map daemon on 8080 to appear to be on 80
     rdr on $ext_if proto tcp from any to any port 80 -> 127.0.0.1 port 8080

     If the pass modifier is given, packets matching the translation rule are
     passed without inspecting the filter rules:

     rdr pass on $ext_if proto tcp from any to any port 80 -> 127.0.0.1 \
           port 8080

     In the example below, vlan12 is configured as 192.168.168.1; the machine
     translates all packets coming from 192.168.168.0/24 to 204.92.77.111 when
     they are going out any interface except vlan12. This has the net effect
     of making traffic from the 192.168.168.0/24 network appear as though it
     is the Internet routable address 204.92.77.111 to nodes behind any inter-
     face on the router except for the nodes on vlan12. (Thus, 192.168.168.1
     can talk to the 192.168.168.0/24 nodes.)

     nat on ! vlan12 from 192.168.168.0/24 to any -> 204.92.77.111

     In the example below, the machine sits between a fake internal
     144.19.74.* network, and a routable external IP of 204.92.77.100. The no
     nat rule excludes protocol AH from being translated.

     # NO NAT
     no nat on $ext_if proto ah from 144.19.74.0/24 to any
     nat on $ext_if from 144.19.74.0/24 to any -> 204.92.77.100

     In the example below, packets bound for one specific server, as well as
     those generated by the sysadmins are not proxied; all other connections
     are.

     # NO RDR
     no rdr on $int_if proto { tcp, udp } from any to $server port 80
     no rdr on $int_if proto { tcp, udp } from $sysadmins to any port 80
     rdr on $int_if proto { tcp, udp } from any to any port 80 -> 127.0.0.1 \
           port 80

     This longer example uses both a NAT and a redirection. The external in-
     terface has the address 157.161.48.183. On the internal interface, we are
     running ftp-proxy(8), listening for outbound ftp sessions captured to
     port 8021.

     # NAT
     # Translate outgoing packets' source addresses (any protocol).
     # In this case, any address but the gateway's external address is mapped.
     nat on $ext_if inet from ! ($ext_if) -> $ext_if

     # NAT PROXYING
     # Map outgoing packets' source port to an assigned proxy port instead of
     # an arbitrary port.
     # In this case, proxy outgoing isakmp with port 500 on the gateway.
     nat on $ext_if inet proto udp from any port = isakmp -> $ext_if \
           port 500

     # BINAT
     # Translate outgoing packets' source address (any protocol).
     # Translate incoming packets' destination address to an internal machine
     # (bidirectional).
     binat on $ext_if from 10.1.2.150 to any -> $ext_if

     # RDR
     # Translate incoming packets' destination addresses.
     # As an example, redirect a TCP and UDP port to an internal machine.
     rdr on $ext_if inet proto tcp from any to $ext_if port 8080 \
           -> 10.1.2.151 port 22
     rdr on $ext_if inet proto udp from any to $ext_if port 8080 \
           -> 10.1.2.151 port 53

     # RDR
     # Translate outgoing ftp control connections to send them to localhost
     # for proxying with ftp-proxy(8) running on port 8021.
     rdr on $int_if proto tcp from any to any port 21 -> 127.0.0.1 port 8021

     In this example, a NAT gateway is set up to translate internal addresses
     using a pool of public addresses (192.0.2.16/28) and to redirect incoming
     web server connections to a group of web servers on the internal network.

     # NAT LOAD BALANCE
     # Translate outgoing packets' source addresses using an address pool.
     # A given source address is always translated to the same pool address by
     # using the source-hash keyword.
     nat on $ext_if inet from any to any -> 192.0.2.16/28 source-hash

     # RDR ROUND ROBIN
     # Translate incoming web server connections to a group of web servers on
     # the internal network.
     rdr on $ext_if proto tcp from any to any port 80 \
           -> { 10.1.2.155, 10.1.2.160, 10.1.2.161 } round-robin

FILTER EXAMPLES

     # The external interface is kue0
     # (157.161.48.183, the only routable address)
     # and the private network is 10.0.0.0/8, for which we are doing NAT.

     # use a macro for the interface name, so it can be changed easily
     ext_if = "kue0"

     # normalize all incoming traffic
     scrub in on $ext_if all fragment reassemble

     # block and log everything by default
     block return log on $ext_if all

     # block anything coming from source we have no back routes for
     block in from no-route to any

     # block and log outgoing packets that do not have our address as source,
     # they are either spoofed or something is misconfigured (NAT disabled,
     # for instance), we want to be nice and do not send out garbage.
     block out log quick on $ext_if from ! 157.161.48.183 to any

     # silently drop broadcasts (cable modem noise)
     block in quick on $ext_if from any to 255.255.255.255

     # block and log incoming packets from reserved address space and invalid
     # addresses, they are either spoofed or misconfigured, we cannot reply to
     # them anyway (hence, no return-rst).
     block in log quick on $ext_if from { 10.0.0.0/8, 172.16.0.0/12, \
           192.168.0.0/16, 255.255.255.255/32 } to any

     # ICMP

     # pass out/in certain ICMP queries and keep state (ping)
     # state matching is done on host addresses and ICMP id (not type/code),
     # so replies (like 0/0 for 8/0) will match queries
     # ICMP error messages (which always refer to a TCP/UDP packet) are
     # handled by the TCP/UDP states
     pass on $ext_if inet proto icmp all icmp-type 8 code 0 keep state

     # UDP

     # pass out all UDP connections and keep state
     pass out on $ext_if proto udp all keep state

     # pass in certain UDP connections and keep state (DNS)
     pass in on $ext_if proto udp from any to any port domain keep state

     # TCP

     # pass out all TCP connections and modulate state
     pass out on $ext_if proto tcp all modulate state

     # pass in certain TCP connections and keep state (SSH, SMTP, DNS, IDENT)
     pass in on $ext_if proto tcp from any to any port { ssh, smtp, domain, \
           auth } flags S/SA keep state

     # pass in data mode connections for ftp-proxy running on this host.
     # (see ftp-proxy(8) for details)
     pass in on $ext_if proto tcp from any to 157.161.48.183 port >= 49152 \
           flags S/SA keep state

     # Do not allow Windows 9x SMTP connections since they are typically
     # a viral worm. Alternately we could limit these OSes to 1 connection each.
     block in on $ext_if proto tcp from any os {"Windows 95", "Windows 98"} \
           to any port smtp

     # IPv6
     # pass in/out all IPv6 traffic: note that we have to enable this in two
     # different ways, on both our physical interface and our tunnel
     pass quick on gif0 inet6 keep state
     pass quick on $ext_if proto ipv6 keep state

     # Packet Tagging

     # three interfaces: $int_if, $ext_if, and $wifi_if (wireless). NAT is
     # being done on $ext_if for all outgoing packets. tag packets in on
     # $int_if and pass those tagged packets out on $ext_if.  all other
     # outgoing packets (i.e., packets from the wireless network) are only
     # permitted to access port 80.

     pass in on $int_if from any to any tag INTNET keep state
     pass in on $wifi_if from any to any keep state

     block out on $ext_if from any to any
     pass out quick on $ext_if tagged INTNET keep state
     pass out on $ext_if proto tcp from any to any port 80 keep state

     # tag incoming packets as they are redirected to spamd(8). use the tag
     # to pass those packets through the packet filter.

     rdr on $ext_if inet proto tcp from <spammers> to port smtp \
             tag SPAMD -> 127.0.0.1 port spamd

     block in on $ext_if
     pass in on $ext_if inet proto tcp tagged SPAMD keep state

GRAMMAR

     Syntax for pf.conf in BNF:

     line           = ( option | pf-rule | nat-rule | binat-rule | rdr-rule |
                      altq-rule | queue-rule | anchor-rule |
                      trans-anchors | load-anchor | table-rule )

     option         = "set" ( [ "timeout" ( timeout | "{" timeout-list "}" ) ] |
                      [ "optimization" [ "default" | "normal" |
                      "high-latency" | "satellite" |
                      "aggressive" | "conservative" ] ]
                      [ "limit" ( limit-item | "{" limit-list "}" ) ] |
                      [ "loginterface" ( interface-name | "none" ) ] |
                      [ "block-policy" ( "drop" | "return" ) ] |
                      [ "state-policy" ( "if-bound" | "floating" ) ]
                      [ "require-order" ( "yes" | "no" ) ]
                      [ "fingerprints" filename ] |
                      [ "debug" ( "none" | "urgent" | "misc" | "loud" ) ] )

     pf-rule        = action [ ( "in" | "out" ) ]
                      [ "log" [ "(" logopts ")"] ] [ "quick" ]
                      [ "on" ifspec ] [ route ] [ af ] [ protospec ]
                      hosts [ filteropt-list ]

     logopts        = logopt [ "," logopts ]
     logopt         = "all" | "user"

     filteropt-list = filteropt-list filteropt | filteropt
     filteropt      = user | group | flags | icmp-type | icmp6-type | tos |
                      ( "keep" | "modulate" | "synproxy" ) "state"
                      [ "(" state-opts ")" ] |
                      "fragment" | "no-df" | "min-ttl" number |
                      "max-mss" number | "random-id" | "reassemble tcp" |
                      fragmentation | "allow-opts" |
                      "label" string | "tag" string | [ ! ] "tagged" string |
                      "queue" ( string | "(" string [ [ "," ] string ] ")" ) |
                      "probability" number"%"

     nat-rule       = [ "no" ] "nat" [ "pass" [ "log" [ "(" logopts ")" ] ] ]
                      [ "on" ifspec ] [ af ]
                      [ protospec ] hosts [ "tag" string ] [ "tagged" string ]
                      [ "->" ( redirhost | "{" redirhost-list "}" )
                      [ portspec ] [ pooltype ] [ "static-port" ] ]

     binat-rule     = [ "no" ] "binat" [ "pass" [ "log" [ "(" logopts ")" ] ] ]
                      [ "on" interface-name ] [ af ]
                      [ "proto" ( proto-name | proto-number ) ]
                      "from" address [ "/" mask-bits ] "to" ipspec
                      [ "tag" string ] [ "tagged" string ]
                      [ "->" address [ "/" mask-bits ] ]

     rdr-rule       = [ "no" ] "rdr" [ "pass" [ "log" [ "(" logopts ")" ] ] ]
                      [ "on" ifspec ] [ af ]
                      [ protospec ] hosts [ "tag" string ] [ "tagged" string ]
                      [ "->" ( redirhost | "{" redirhost-list "}" )
                      [ portspec ] [ pooltype ] ]

     table-rule     = "table" "<" string ">" [ tableopts-list ]
     tableopts-list = tableopts-list tableopts | tableopts
     tableopts      = "persist" | "const" | "file" string |
                      "{" [ tableaddr-list ] "}"
     tableaddr-list = tableaddr-list [ "," ] tableaddr-spec | tableaddr-spec
     tableaddr-spec = [ "!" ] tableaddr [ "/" mask-bits ]
     tableaddr      = hostname | ipv4-dotted-quad | ipv6-coloned-hex |
                      interface-name | "self"

     altq-rule      = "altq on" interface-name queueopts-list
                      "queue" subqueue
     queue-rule     = "queue" string [ "on" interface-name ] queueopts-list
                      subqueue

     anchor-rule    = "anchor" string [ ( "in" | "out" ) ] [ "on" ifspec ]
                      [ af ] [ protospec ] [ hosts ]

     trans-anchors  = ( "nat-anchor" | "rdr-anchor" | "binat-anchor" ) string
                      [ "on" ifspec ] [ af ] [ "proto" ] [ protospec ] [ hosts ]

     load-anchor    = "load anchor" string "from" filename

     queueopts-list = queueopts-list queueopts | queueopts
     queueopts      = [ "bandwidth" bandwidth-spec ] |
                      [ "qlimit" number ] | [ "tbrsize" number ] |
                      [ "priority" number ] | [ schedulers ]
     schedulers     = ( cbq-def | priq-def | hfsc-def )
     bandwidth-spec = "number" ( "b" | "Kib" | "Mib" | "Gib" | "%" )

     action         = "pass" | "block" [ return ] | "fuck-off" | [ "no" ] "scrub"
     return         = "drop" | "return" | "return-rst" [ "( ttl" number ")" ] |
                      "return-icmp" [ "(" icmpcode ["," icmp6code ] ")" ] |
                      "return-icmp6" [ "(" icmp6code ")" ]
     icmpcode       = ( icmp-code-name | icmp-code-number )
     icmp6code      = ( icmp6-code-name | icmp6-code-number )

     ifspec         = ( [ "!" ] interface-name ) | "{" interface-list "}"
     interface-list = [ "!" ] interface-name [ [ "," ] interface-list ]
     route          = "fastroute" |
                      ( "route-to" | "reply-to" | "dup-to" )
                      ( routehost | "{" routehost-list "}" )
                      [ pooltype ]
     af             = "inet" | "inet6"

     protospec      = "proto" ( proto-name | proto-number |
                      "{" proto-list "}" )
     proto-list     = ( proto-name | proto-number ) [ [ "," ] proto-list ]

     hosts          = "all" |
                      "from" ( "any" | "no-route" | "self" | host |
                      "{" host-list "}" | "route" string ) [ port ] [ os ]
                      "to"   ( "any" | "no-route" | "self" | host |
                      "{" host-list "}" | "route" string ) [ port ]

     ipspec         = "any" | host | "{" host-list "}"
     host           = [ "!" ] ( address [ "/" mask-bits ] | "<" string ">" )
     redirhost      = address [ "/" mask-bits ]
     routehost      = ( interface-name [ address [ "/" mask-bits ] ] )
     address        = ( interface-name | hostname |
                      ipv4-dotted-quad | ipv6-coloned-hex )
     host-list      = host [ [ "," ] host-list ]
     redirhost-list = redirhost [ [ "," ] redirhost-list ]
     routehost-list = routehost [ [ "," ] routehost-list ]

     port           = "port" ( unary-op | binary-op | "{" op-list "}" )
     portspec       = "port" ( number | name ) [ ":" ( "*" | number | name ) ]
     os             = "os"  ( os-name | "{" os-list "}" )
     user           = "user" ( unary-op | binary-op | "{" op-list "}" )
     group          = "group" ( unary-op | binary-op | "{" op-list "}" )

     unary-op       = [ "=" | "!=" | "<" | "<=" | ">" | ">=" ]
                      ( name | number )
     binary-op      = number ( "<>" | "><" | ":" ) number
     op-list        = ( unary-op | binary-op ) [ [ "," ] op-list ]

     os-name        = operating-system-name
     os-list        = os-name [ [ "," ] os-list ]

     flags          = "flags" [ flag-set ] "/" flag-set
     flag-set       = [ "F" ] [ "S" ] [ "R" ] [ "P" ] [ "A" ] [ "U" ] [ "E" ]
                      [ "W" ]

     icmp-type      = "icmp-type" ( icmp-type-code | "{" icmp-list "}" )
     icmp6-type     = "icmp6-type" ( icmp-type-code | "{" icmp-list "}" )
     icmp-type-code = ( icmp-type-name | icmp-type-number )
                      [ "code" ( icmp-code-name | icmp-code-number ) ]
     icmp-list      = icmp-type-code [ [ "," ] icmp-list ]

     tos            = "tos" ( "lowdelay" | "throughput" | "reliability" |
                      [ "0x" ] number )

     state-opts     = state-opt [ [ "," ] state-opts ]
     state-opt      = ( "max" number | "no-sync" | timeout |
                      "source-track" [ ( "rule" | "global" ) ] |
                      "max-src-nodes" number | "max-src-states" number |
                      "max-src-conn" number |
                      "max-src-conn-rate" number "/" number |
                      "overload" "<" string ">" [ "flush" ] |
                      "if-bound" | "floating" )

     fragmentation  = [ "fragment reassemble" | "fragment crop" |
                      "fragment drop-ovl" ]

     timeout-list   = timeout [ [ "," ] timeout-list ]
     timeout        = ( "tcp.first" | "tcp.opening" | "tcp.established" |
                      "tcp.closing" | "tcp.finwait" | "tcp.closed" |
                      "udp.first" | "udp.single" | "udp.multiple" |
                      "icmp.first" | "icmp.error" |
                      "other.first" | "other.single" | "other.multiple" |
                      "frag" | "interval" | "src.track" |
                      "adaptive.start" | "adaptive.end" ) number

     limit-list     = limit-item [ [ "," ] limit-list ]
     limit-item     = ( "states" | "frags" | "src-nodes" ) number

     pooltype       = ( "bitmask" | "random" |
                      "source-hash" [ ( hex-key | string-key ) ] |
                      "round-robin" ) [ sticky-address ]

     subqueue       = string | "{" queue-list "}"
     queue-list     = string [ [ "," ] string ]
     cbq-def        = "cbq" [ "(" cbq-opt [ [ "," ] cbq-opt ] ")" ]
     priq-def       = "priq" [ "(" priq-opt [ [ "," ] priq-opt ] ")" ]
     hfsc-def       = "hfsc" [ "(" hfsc-opt [ [ "," ] hfsc-opt ] ")" ]
     cbq-opt        = ( "default" | "borrow" | "red" | "ecn" | "rio" )
     priq-opt       = ( "default" | "red" | "ecn" | "rio" )
     hfsc-opt       = ( "default" | "red" | "ecn" | "rio" |
                      linkshare-sc | realtime-sc | upperlimit-sc )
     linkshare-sc   = "linkshare" sc-spec
     realtime-sc    = "realtime" sc-spec
     upperlimit-sc  = "upperlimit" sc-spec
     sc-spec        = ( bandwidth-spec |
                      "(" bandwidth-spec number bandwidth-spec ")" )

FILES

     /etc/hosts      Host name database.
     /etc/pf.conf    Default location of the ruleset file.
     /etc/pf.os      Default location of OS fingerprints.
     /etc/protocols  Protocol name database.
     /etc/services   Service name database.
     /usr/share/pf   Example rulesets.

SEE ALSO

     carp(4), icmp(4), icmp6(4), ip(4), ip6(4), pf(4), pfsync(4), route(4),
     tcp(4), udp(4), hosts(5), pf.os(5), protocols(5), services(5), ftpproxy(8),
     pfctl(8), pflogd(8), route(8)

HISTORY

     The pf.conf file format first appeared in OpenBSD 3.0.

MirOS BSD #10-current         November 19, 2002                             29

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