PF.CONF(5)

NAME
pf.conf - packet filter configuration file

DESCRIPTION
The NetBSD version of PF is obsolete, and its use is strongly discouraged. Use npf(7) instead.

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 flexibility of rules with large numbers of source or destination addresses.

Options
    Options tune the behaviour of the packet filtering engine.

Traffic Normalization (e.g. scrub)
    Traffic normalization protects internal machines against inconsistencies 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
    Packet filtering provides rule-based blocking 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 default pfctl(8) enforces this order (see set require-order below).

MACROS
Macros can be defined that will later be expanded 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
    pass in  on $ext_if proto tcp from any to any port 25

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 expansion).

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 contents of a pre-existing table defined without a list of addresses 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 = 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 interface 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 improve 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 connection is closed. Some hosts (notably web servers on Solaris) send TCP packets even after closing the connection. Increasing tcp.finwait (and possibly tcp.closing) can prevent blocking 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 immediately. This value is used to define the scale factor, it should not actually be reached (set a lower state limit, see below).

    Adaptive timeouts are enabled by default, with an adaptive.start value equal to 60% of the state limit, and an adaptive.end value equal to 120% of the state limit. They can be disabled by setting both adaptive.start and adaptive.end to 0.

    The adaptive timeout 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 pass rules which do not specify no state) 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. Using

        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 src.track options) to 2000. Using

        set limit tables 1000
        set limit table-entries 100000

    sets limits on the memory pools used by tables. The first limits the number of tables that can exist to 1000. The second limits the overall number of addresses that can be stored in tables to 100000.

    Various limits can be combined on a single line:

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

set ruleset-optimization
    none
        Disable the ruleset optimizer.
    basic
        Enable basic ruleset optimization. This is the default behaviour. Basic ruleset optimization does four things to improve the performance of ruleset evaluations:

        1.  remove duplicate rules
        2.  remove rules that are a subset of another rule
        3.  combine multiple rules into a table when advantageous
        4.  re-order the rules to improve evaluation performance

    profile
        Uses the currently loaded ruleset as a feedback profile to tailor the ordering of quick rules to actual network traffic.

    It is important to note that the ruleset optimizer will modify the ruleset to improve performance. A side effect of the ruleset modification is that per-rule accounting statistics will have different meanings than before. If per-rule accounting is important for billing purposes or whatnot, either the ruleset optimizer should not be used or a label field should be added to all of the accounting rules to act as optimization barriers.

    Optimization can also be set as a command-line argument to pfctl(8), overriding the settings in pf.conf.

set optimization
    Optimize state timeouts for one of the following network environments:

    normal
        A normal network environment. Suitable for almost all networks.
    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 increased 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 default).

    For example:

        set state-policy if-bound

set hostid
    The 32-bit hostid identifies this firewall's state table entries to other firewalls in a pfsync(4) failover cluster. By default the hostid is set to a pseudo-random value, however it may be desirable to manually configure it, for example to more easily identify the source of state table entries.

        set hostid 1

    The hostid may be specified in either decimal or hexadecimal.

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 enforcement.

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 ruleset 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 disabled, 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 deleterious 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 compensate 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 fragments is the additional memory cost. But the full reassembly method is the only method that currently works with NAT. This is the default behavior of a scrub rule if no fragmentation modifier is supplied.

fragment crop
    The default fragment reassembly method is expensive, hence the option to crop is provided. In this case, pf(4) will track the fragments 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 reassembly 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 performs 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 before 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 packet 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 observing 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 connection to wrap its 32-bit sequence space. In such an occurrence, 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 timestamp 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) artificially 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 exclude 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 absolute value or as a percentage of the interface bandwidth. When using an absolute value, the suffixes b, Kb, Mb, and Gb are used to represent bits, kilobits, megabits, and gigabits per second, respectively. The value must not exceed the interface bandwidth. If bandwidth is not specified, the interface bandwidth is used (but take note that some interfaces do not know their bandwidth, or can adapt their bandwidth rates).

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 5Mb 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 following 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 running RED two times more than RIO would achieve the same effect. 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 milliseconds 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) sessions 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 \
          queue developers
    pass out on dc0 inet proto tcp from $employeehosts to any port 80 \
          queue employees
    pass out on dc0 inet proto tcp from any to any port 22 \
          queue(ssh_bulk, ssh_interactive)
    pass out on dc0 inet proto tcp from any to any port 25 \
          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, recalculates 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 address and port number. Packets that match a translation rule are only automatically 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 address 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 traffic 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 different 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.

Evaluation order of the translation rules is dependent on the type of the translation rules and of the direction of a packet. binat rules are always evaluated first. Then either the rdr rules are evaluated on an inbound packet or the nat rules on an outbound packet. Rules of the same type are evaluated in the same order in which they appear in the ruleset. The first matching rule decides what action is taken.

The no option prefixed to a translation rule causes packets to remain untranslated, 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 engine 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 external 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 reflect 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 address, as in

    rdr on ne3 inet proto tcp to port spamd - 127.0.0.1 port smtp

will effectively allow an external host to connect to daemons bound solely 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. If no rule matches the packet, the default action is to pass the packet.

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 explicit 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 message, 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.

    The simplest mechanism to block everything by default and only pass packets that match explicit rules is specify a first filter rule of:

        block all

pass
    The packet is passed; state is created unless the no state option is specified.

By default pf(4) filters packets statefully; the first time a packet matches a pass rule, a state entry is created; for subsequent packets the filter checks whether the packet matches any state. If it does, the packet is passed without evaluation of any rules. After the connection is closed or times out, the state entry is automatically removed.

This has several advantages. For TCP connections, comparing a packet to a state involves checking its sequence numbers, as well as TCP timestamps if a scrub reassemble tcp rule applies to the connection. If these values 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. Similarly, pf(4) knows how to match ICMP replies to states. For example,

    pass out inet proto icmp all icmp-type echoreq

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

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).

Furthermore, correct handling of ICMP error messages is critical to many protocols, particularly TCP. pf(4) matches ICMP error messages to the correct connection, checks them against connection parameters, and passes them if appropriate. For example if an ICMP source quench message referring to a stateful TCP connection arrives, it will be matched to the state and get passed.

Finally, state tracking is required for nat, binat and rdr rules, in order to track address and port translations and reverse the translation on returning packets.

pf(4) will also create state for other protocols which are effectively stateless by nature. UDP packets are matched to states using only host addresses and ports, and other protocols are matched to states using only the host addresses.

If stateless filtering of individual packets is desired, the no state keyword can be used to specify that state will not be created if this is the last matching rule. A number of parameters can also be set to affect how pf(4) handles state tracking. See STATEFUL TRACKING OPTIONS below for further details.

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 packets 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 directions.

log In addition to the action specified, a log message is generated. Only the packet that establishes the state is logged, unless the no state option is specified. The logged packets are sent to a pflog(4) interface, by default pflog0. This interface is monitored by the pflogd(8) logging daemon, which dumps the logged packets to the file /var/log/pflog in pcap(3) binary format.

log (all)
    Used to force logging of all packets for a connection. This is not necessary when no state is explicitly specified. As with log, packets are logged to pflog(4).

log (user)
    Logs the UNIX user ID of the user that owns the socket and the PID of the process that has the socket open where the packet is sourced from or destined to (depending on which socket is local). This is in addition to the normal information logged.

log (to <interface>)
    Send logs to the specified pflog(4) interface instead of pflog0.

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

on <interface>
    This rule applies only to packets coming in on, or going out through, this particular interface or interface group. For more information on interface groups, see the group keyword in ifconfig(8).

<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 protocols 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, interface names or interface group names, or as any of the following 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.
    urpf-failed
        Any source address that fails a unicast reverse path forwarding (URPF) check, i.e. packets coming in on an interface other than that which holds the route back to the packet's source address.
    <table>
        Any address that matches the given table.

    Interface names and interface group names can have modifiers appended:

    :network
        Translates to the network(s) attached to the interface.
    :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. Surrounding the interface name (and optional modifiers) in parentheses changes this behaviour. When the interface name is surrounded by parentheses, the rule is automatically updated whenever the interface changes its address. The ruleset does not need to be reloaded. This is especially useful with nat.

    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 following 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"
        pass in proto tcp from route "DTAG"

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

group <group>
    This functionality is not supported in this version of NetBSD.

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 destination 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 associated 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 invalid. 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 connections:

        block out proto { tcp, udp } all
        pass  out proto { tcp, udp } all user {  1000, dhartmei }

flags <a> /<b> /<b> any
    This rule only applies to TCP packets that have the flags <a> set out of set <b>. Flags not specified in <b> are ignored. For stateful connections, the default is flags S/SA. To indicate that flags should not be checked at all, specify flags any. 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
        This is the default setting for stateful connections. 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 example.

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

    Because flags S/SA is applied by default (unless no state is specified), only the initial SYN packet of a TCP handshake will create a state for a TCP connection. It is possible to be less restrictive, and allow state creation from intermediate (non-SYN) packets, by specifying flags any. This will cause pf(4) to synchronize to existing connections, for instance if one flushes the state table. However, states created from such intermediate packets may be missing connection details such as the TCP window scaling factor. States which modify the packet flow, such as those affected by nat, binat or rdr rules, modulate or synproxy state options, or scrubbed with reassemble tcp will also not be recoverable from intermediate packets. Such connections will stall and time out.

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 indicator ( icmp-type or icmp6-type) must match.

tos <string> <number>
    This rule applies to packets with the specified TOS bits set. TOS may be given as one of lowdelay, throughput, reliability, or as either hex or decimal.

    For example, the following rules are identical:

        pass all tos lowdelay
        pass all tos 0x10
        pass all tos 16

allow-opts
    By default, IPv4 packets with IP options or IPv6 packets with routing extension headers 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 contain IP options or routing extension headers. For packets that match state, the rule that initially created the state is used. The implicit pass rule that is used when a packet does not match any rules does not allow IP options.

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 configuration 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.

rtable <number>
    Used to select an alternate routing table for the routing lookup. Only effective before the route lookup happened, i.e. when filtering inbound.

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 interface. 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 incoming 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 determine 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 default 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 modifying 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 src.track. See STATEFUL TRACKING OPTIONS for more ways to control the source tracking.

STATE MODULATION
Much of the security derived from TCP is attributable to how well the initial sequence numbers (ISNs) are chosen. Some popular stack implementations choose very poor ISNs and thus are normally susceptible to ISN prediction exploits. By applying a modulate state rule to a TCP connection, 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/SFRA modulate state

Note that modulated connections will not recover 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. The default flags settings (or a more strict equivalent) should be used on modulate state rules 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 connection from/to the other endpoint. pf(4) chooses random initial sequence numbers for both handshakes. Once the handshakes are completed, the sequence number modulators (see previous section) are used to translate further packets of the connection. synproxy state includes modulate 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 synproxy state

STATEFUL TRACKING OPTIONS
A number of options related to stateful tracking can be applied on a per-rule basis. keep state, modulate state and synproxy state support these options, and keep state must be specified explicitly to apply options to a rule.

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.
<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 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 participating 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 simultaneously have state table entries.
max-src-states <number>
    Limits the maximum number of simultaneous state entries that a single source address can create with this rule.

For stateful TCP connections, limits on established connections (connections 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 connection 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 offending 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 keep state \
    	(max-src-conn-rate 100/10, overload bad_hosts flush global)

You can adjust the state policy on individual nat and rdr translation rules by adding a keyword if-bound, group-bound or floating at the end of the rule. For example, a rule such as this,

    nat on sip0 from 10/8 to ! 10/8 -> 192.168.1.4/32 if-bound

will create states that only match packets on sip0.

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 fingerprint 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 normalized 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 fingerprints may be listed by running:

    # pfctl -so

Filter rules can enforce policy at any level of operating system specification 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
    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"

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.

BLOCKING SPOOFED TRAFFIC
"Spoofing" is the faking of IP addresses, typically for malicious purposes. The antispoof directive expands to a set of filter rules which will block all traffic with a source IP from the network(s) directly connected to the specified interface(s) from entering the system through any other interface.

For example, the line

    antispoof for lo0

expands to

    block drop in on ! lo0 inet from 127.0.0.1/8 to any
    block drop in on ! lo0 inet6 from ::1 to any

For non-loopback interfaces, there are additional rules to block incoming packets with a source IP address identical to the interface's IP(s). For example, assuming the interface wi0 had an IP address of 10.0.0.1 and a netmask of 255.255.255.0, the line

    antispoof for wi0 inet

expands to

    block drop in on ! wi0 inet from 10.0.0.0/24 to any
    block drop in inet from 10.0.0.1 to any

Caveat: Rules created by the antispoof directive interfere with packets sent over loopback interfaces to local addresses. One should pass these explicitly.

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 subprotocol 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 impossible.

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 adjusted.

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

ANCHORS
Besides the main ruleset, pfctl(8) can load rulesets into anchor attachment 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 components separated by ‘/’ characters, similar to how file system hierarchies 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 following 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 marked with the quick option are final and abort the evaluation of the rules in other anchors and the main ruleset. If the anchor itself is marked with the quick option, ruleset evaluation will terminate when the anchor is exited if the packet is matched by any rule within the anchor.

anchor rules are evaluated relative to the anchor in which they are contained. 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
    pass in on $ext_if proto tcp from any \
          to $ext_if port smtp

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, interface, 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
    pass in on $ext_if proto tcp from any to $ext_if port smtp

The rules inside anchor spam are only evaluated for tcp packets with destination 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 alphabetical 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 contained, there is a mechanism for accessing the parent and ancestor anchors 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.

Filter rule anchors can also be loaded inline in the ruleset within a brace ('{' '}') delimited block. Brace delimited blocks may contain rules or other brace-delimited blocks. When anchors are loaded this way the anchor name becomes optional.

    anchor "external" on egress {
    	block
    	anchor out {
    		pass proto tcp from any to port { 25, 80, 443 }
    	}
    	pass in proto tcp to any port 22
    }

Since the parser specification for anchor names is a string, any reference 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 interface 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 interface has the address 157.161.48.183. On localhost, we are running ftp-proxy(8), waiting for FTP sessions to be redirected to it. The three mandatory anchors for ftp-proxy(8) are omitted from this example; see the ftp-proxy(8) manpage.

    # 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) to any - ($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 $ext_if inet proto udp from any port = isakmp to any - ($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 packets whose ingress interface does not match the one in
    # the route back to their source address
    block in from urpf-failed 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
    # UDP
    # pass out all UDP connections and keep state
    pass out on $ext_if proto udp all
    # pass in certain UDP connections and keep state (DNS)
    pass in on $ext_if proto udp from any to any port domain
    # 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 }
    # 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
    pass quick on $ext_if proto ipv6
    # 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
    pass in on $wifi_if from any to any
    block out on $ext_if from any to any
    pass out quick on $ext_if tagged INTNET
    pass out on $ext_if proto tcp from any to any port 80
    # 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

GRAMMAR
Syntax for pf.conf in BNF:

    line           = ( option | pf-rule | nat-rule | binat-rule | rdr-rule |
                     antispoof-rule | altq-rule | queue-rule | trans-anchors |
    		 anchor-rule | anchor-close | load-anchor | table-rule | )
    option         = "set" ( [ "timeout" ( timeout | "{" timeout-list "}" ) ] |
    		 [ "ruleset-optimization" [ "none" | "basic" | "profile" ]] |
                     [ "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 ] |
                     [ "skip on" ifspec ] |
                     [ "debug" ( "none" | "urgent" | "misc" | "loud" ) ] )
    pf-rule        = action [ ( "in" | "out" ) ]
                     [ "log" [ "(" logopts ")"] ] [ "quick" ]
                     [ "on" ifspec ] [ "fastroute" | route ] [ af ] [ protospec ]
                     hosts [ filteropt-list ]
    logopts        = logopt [ "," logopts ]
    logopt         = "all" | "user" | "to" interface-name
    filteropt-list = filteropt-list filteropt | filteropt
    filteropt      = user | group | flags | icmp-type | icmp6-type | tos |
                     ( "no" | "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 ] ")" ) |
                     "rtable" number | "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" ] ]
                     [ ( "if-bound" | "group-bound" | "floating" ) ]
    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 ] ]
                     [ ( "if-bound" | "group-bound" | "floating" ) ]
    antispoof-rule = "antispoof" [ "log" ] [ "quick" ]
                     "for" ifspec [ af ] [ "label" string ]
    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 | ifspec | "self" |
                     ipv4-dotted-quad | ipv6-coloned-hex
    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 ] [ "{" ]
    anchor-close   = "}"
    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" | "Kb" | "Mb" | "Gb" | "%" )
    action         = "pass" | "block" [ return ] | [ "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-group ) ) |
                     "{" interface-list "}"
    interface-list = [ "!" ] ( interface-name | interface-group )
                     [ [ "," ] interface-list ]
    route          = ( "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" | "urpf-failed" | "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 | interface-group |
                     "(" ( interface-name | interface-group ) ")" |
                     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 "}" )
    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 | "any" )
    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 | 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/examples/pf
    Example rulesets.

SEE ALSO
carp(4), icmp(4), icmp6(4), ip(4), ip6(4), pf(4), route(4), tcp(4), udp(4), hosts(5), pf.os(5), protocols(5), services(5), ftp-proxy(8), pfctl(8), pflogd(8), route(8)

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