Manual Pages  — PF.CONF

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).

Comments can be put anywhere in the file using a hash mark ('#'), and extend to the end of the current line.

Additional configuration files can be included with the include keyword, for example:

include "/etc/pf/sub.filter.conf"

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 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.
counters
	The counters flag enables per-address packet and byte counters which can be displayed with pfctl(8). Note that this feature carries significant memory overhead for large tables.

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 amp;"/etc/spammersamp;" file amp;"/etc/openrelaysamp;"
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.

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.

SCTP timeout are handled similar to TCP, but with its own set of states:

sctp.first
	The state after the first packet.
sctp.opening
	The state before the destination host ever sends a packet.
sctp.established
	The fully established state.
sctp.closing
	The state after the first SHUTDOWN chunk has been sent.
sctp.closed
	The state after SHUTDOWN_ACK has been exchanged and the connection is closed.

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 or interface group. 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 zone(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: remove duplicate rules remove rules that are a subset of another rule combine multiple rules into a table when advantageous 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

For example:

set block-policy return

For example:

set fail-policy return

For example:

set state-policy if-bound

set syncookies adaptive (start 25%, end 12%)

set state-defaults no-sync

set hostid 1

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 primarily affects 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 5Mbps 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 ( <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

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 smtp -> 127.0.0.1 port spamd

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.

For each packet processed by the packet filter, the filter rules are evaluated in sequential order, from first to last. For block and pass , the last matching rule decides what action is taken. For match , rules are evaulated every time they match; the pass/block state of a packet remains unchanged. 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, an SCTP ABORT for SCTP and an ICMP UNREACHABLE for UDP and other packets.

Options returning ICMP packets currently have no effect if pf(4) operates on a if_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

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.

Ranges of addresses are specified by using the '-' operator. For instance: "10.1.1.10 - 10.1.1.12" means all addresses from 10.1.1.10 to 10.1.1.12, hence addresses 10.1.1.10, 10.1.1.11, and 10.1.1.12.

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 non-link-local 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 'amp;:' 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"

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

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.

code <code>
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.
| <number> This rule applies to packets with the specified TOS bits set. TOS may be given as one of critical, inetcontrol, lowdelay, netcontrol, throughput, reliability, or one of the DiffServ Code Points: ef, va, af11 ... af43, cs0 ... cs7; 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 = amp;"{ 1.2.3.4, 1.2.3.5 }amp;"
pass in proto tcp from any to $ips \
      port > 1023 label amp;"$dstaddr:$dstportamp;"

expands to

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

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

ridentifier <number>
	Add an identifier (number) to the rule, which can be used to correlate the rule to pflog entries, even after ruleset updates.
|( <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)
set prio priority | (priority , priority)
	Packets matching this rule will be assigned a specific queueing priority. Priorities are assigned as integers 0 through 7. If the packet is transmitted on a vlan(4) interface, the queueing priority will be written as the priority code point in the 802.1Q VLAN header. If two priorities are given, packets which have a TOS of lowdelay and TCP ACKs with no data payload will be assigned to the second one. For example: pass in proto tcp to port 25 set prio 2 pass in proto tcp to port 22 set prio (2, 5)
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, translation or scrub 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 !amp; 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.
port <port> Used to redirect packets to a local socket bound to host and port. The packets will not be modified, so getsockname(2) on the socket will return the original destination address of the packet.
divert-reply
	Used to receive replies for sockets that are bound to addresses which are not local to the machine. See setsockopt(2) for information on how to bind these sockets.
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%
prio <number>
	Only match packets which have the given queueing priority assigned.

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.

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

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

    amp;"OpenBSD 2.6amp;"

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

    amp;"OpenBSD 3.3 no-dfamp;"

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.

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.

When forwarding reassembled IPv6 packets, pf refragments them with the original maximum fragment size. This allows the sender to determine the optimal fragment size by path MTU discovery.

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.
<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 = amp;"kue0amp;"
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 amp;"block in quick from 1.2.3.4 to anyamp;" amp;| \
      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 packet filtering parameters 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 amp;"block in quick from 1.2.3.4 to any" amp;| \
      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 $ext_if {
        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 '/' characters will require double quote ('amp;"') characters around the anchor name.

# use a macro for the interface name, so it can be changed easily
ext_if = amp;"ne3amp;"

# 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

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

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

# 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

# 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