Manpages

NAME

pf.conf — packet filter configuration file

DESCRIPTION

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

STATEMENT ORDER

There are seven types of statements in pf.conf:

Macros

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

Tables

Tables provide a mechanism for increasing the performance and 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

Stateful and stateless 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

Much like cpp(1) or m4(1), 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 keep state
pass in on $ext_if proto tcp from any to any port 25 keep state

TABLES

Tables are named structures which can hold a collection of addresses and networks. Lookups against tables in pf(4) are relatively fast, making a single rule with tables much more efficient, in terms of processor usage and memory consumption, than a large number of rules which differ only in IP address (either created explicitly or automatically by rule 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(7) = 2.

For example,

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

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

# pfctl -t badhosts -Tadd 204.92.77.111

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

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

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

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

For example:

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

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

set loginterface

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

# pfctl -s info

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

set loginterface dc0

One can disable the loginterface using:

set loginterface none

set limit

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

For example,

set limit states 20000

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

set limit frags 20000

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

set limit src-nodes 2000

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

These can be combined:

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

set optimization

Optimize the engine for one of the following network environments:

normal

A normal network environment. Suitable for almost all 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.

group-bound

States are bound to interface group (i.e. ppp)

floating

States can match packets on any interfaces (the default).

For example:

set state-policy if-bound

set require-order

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

timeout 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.

For example,

scrub in on $ext_if all fragment reassemble

QUEUEING/ALTQ

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.

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.

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.

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. 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 priority 7
queue ssh_bulk priority 0

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

TRANSLATION

Translation rules modify either the source or destination address of the packets associated with a stateful connection. A stateful connection is automatically created to track packets matching such a rule as long as they are not blocked by the filtering section of pf.conf. The translation engine modifies the specified address and/or port in the packet, 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.

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

The no option prefixed to a translation rule causes packets to remain 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 8025 -> 127.0.0.1 port 25

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.

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 packets have no effect if pf(4) operates on a bridge(4).

pass

The packet is passed.

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

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

block all

as the first filter rule.

See FILTER EXAMPLES below.

PARAMETERS

The rule parameters specify the packets to which a rule applies. A packet always comes in on, or goes out through, one interface. Most parameters are optional. If a parameter is specified, the rule only applies to 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. All packets for that connection are logged, unless the keep state, modulate state or synproxy state options are specified, in which case only the packet that establishes the state is logged. (See keep state, modulate state and synproxy state below). The logged packets are sent to the pflog(4) interface. This interface is 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 with keep state, modulate state or synproxy state rules to force logging of all packets for a connection. As with log, packets are logged to pflog(4).

quick

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

on <interface>

This rule applies only to packets coming in on, or going out through, this particular interface. It is also possible to simply give the interface driver name, like ppp or fxp, to make the rule match packets flowing through a group of interfaces.

<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 or interface names, or as any of the following keywords:

any

Any address.

no-route

Any address which is not currently routable.

<table>

Any address that matches the given table.

Interface names can have modifiers appended:

:network

Translates to the network(s) attached to the 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)

><, <> 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" flags S/SA

all

This is equivalent to "from any to any".

group <group>

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

The use of group or user in debug.mpsafenet=1 environments may result in a deadlock. Please see the BUGS section for details.

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 } keep state

flags <a>/<b> | /<b>

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

flags S/S

Flag SYN is set. The other flags are ignored.

flags S/SA

Out of SYN and ACK, exactly SYN may be set. SYN, SYN+PSH and SYN+RST match, but SYN+ACK, ACK and ACK+RST do not. This is more restrictive than the previous example.

flags /SFRA

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

icmp-type <type> code <code>

icmp6-type <type> code <code>

This rule only applies to ICMP or ICMPv6 packets with the specified type and code. 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.

allow-opts

By default, packets which contain IP options are blocked. When allow-opts is specified for a pass rule, packets that pass the filter based on that rule (last matching) do so even if they contain IP options. 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/ALTQ 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. pass rules that use the tag keyword must also use keep state, modulate state or synproxy state. 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 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.

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 route-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 source-track See STATEFUL TRACKING OPTIONS for more ways to control the source tracking.

STATEFUL INSPECTION

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

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

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

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

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

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

For instance:

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

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

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

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

pass out on ppp from any to 10.12/16 keep state (group-bound)

A state created on ppp0 would match packets an all PPP interfaces, but not packets flowing through fxp0 or any other interface.

Keeping rules floating is the more flexible option when the firewall is in a dynamic routing environment. However, this has some security implications since a state created by one trusted network could allow potentially hostile packets coming in from other interfaces.

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

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

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

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

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

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

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

STATE MODULATION

Much of the security derived from TCP is attributable to how well the 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/SA modulate state

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

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

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. Hence, synproxy state includes modulate state and keep state.

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

Example:

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

STATEFUL TRACKING OPTIONS

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

max <number>

Limits the number of concurrent states the rule may create. When this limit is reached, further packets matching the rule that would create state are dropped, until existing states time out.

no-sync

Prevent state changes for states created by this rule from appearing on the pfsync(4) interface.

<timeout> <seconds>

Changes the timeout values used for states created by this rule.

When the source-track keyword is specified, the number of states per source IP is tracked. The following limits can be set:

max-src-nodes

Limits the maximum number of source addresses which can simultaneously have state table entries.

max-src-states

Limits the maximum number of simultaneous state entries that a single source address can create with this rule.

For a list of all valid timeout names, see OPTIONS above.

Multiple options can be specified, separated by commas:

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

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

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

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

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 AND NAMED RULESETS

Besides the main ruleset, pfctl(8) can load named rulesets into anchor attachment points. An anchor contains a list of named rulesets. An anchor has a name which specifies where pfctl(8) can be used to attach sub-rulesets. A named ruleset contains filter and translation rules, like the main ruleset. The main ruleset can reference anchor attachment points using the following kinds of rules:

nat-anchor <name>

Evaluates the nat rules of all named rulesets in the specified anchor.

rdr-anchor <name>

Evaluates the rdr rules of all named rulesets in the specified anchor.

binat-anchor <name>

Evaluates the binat rules of all named rulesets in the specified anchor.

anchor <name>

Evaluates the filter rules of all named rulesets in the specified anchor.

load anchor <name>:<ruleset> from <file>

Loads the rules from the specified file into the named ruleset <ruleset> attached to the anchor <name>.

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

Matching filter rules in named rulesets with the quick option and matching translation rules are final and abort the evaluation of both the rules in the anchor and the main ruleset.

Only the main ruleset can contain anchor rules.

When an anchor contains more than one named ruleset, they are evaluated in the alphabetical order of their names.

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

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

blocks all packets on the external interface by default, then evaluates all rulesets 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:manual -f -

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

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

anchor spam
load anchor spam:manual 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 named ruleset.

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 named rulesets, like:

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

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

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

will only block connections from 1.2.3.4 to port 25.

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 the internal interface, we are running ftp-proxy(8), listening for outbound ftp sessions captured to port 8021.

# NAT
# Translate outgoing packets’ source addresses (any protocol).
# In this case, any address but the gateway’s external address is mapped.
nat on $ext_if inet from ! ($ext_if) 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 and log outgoing packets that do not have our address as source,
# they are either spoofed or something is misconfigured (NAT disabled,
# for instance), we want to be nice and do not send out garbage.
block out log quick on $ext_if from ! 157.161.48.183 to any

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

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

# ICMP

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

# UDP

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

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

# TCP

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

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

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

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

# Packet Tagging

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

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

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

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

rdr on $ext_if inet proto tcp from <spammers> to port smtp \

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

GRAMMAR

Syntax for pf.conf in BNF:

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

option = "set" ( [ "timeout" ( timeout | "{" timeout-list "}" ) ] |
[ "optimization" [ "default" | "normal" |
"high-latency" | "satellite" |
"aggressive" | "conservative" ] ]
[ "limit" ( limit-item | "{" limit-list "}" ) ] |
[ "loginterface" ( interface-name | "none" ) ] |
[ "block-policy" ( "drop" | "return" ) ] |

"floating" ) ]
[ "require-order" ( "yes" | "no" ) ]
[ "fingerprints" filename ] |
[ "debug" ( "none" | "urgent" | "misc" | "loud" ) ] )

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

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

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

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

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

antispoof-rule = "antispoof" [ "log" ] [ "quick" ]
"for" ( interface-name | "{" interface-list "}" )
[ 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 | ipv4-dotted-quad | ipv6-coloned-hex |
interface-name | "self"

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

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

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

load-anchor = "load anchor" anchorname:rulesetname "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 ] | "scrub"
return = "drop" | "return" | "return-rst" [ "( ttl" number ")" ] |
"return-icmp" [ "(" icmpcode ["," icmp6code ] ")" ] |
"return-icmp6" [ "(" icmp6code ")" ]
icmpcode = ( icmp-code-name | icmp-code-number )
icmp6code = ( icmp6-code-name | icmp6-code-number )

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

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

hosts = "all" |
"from" ( "any" | "no-route" | "self" | host |
"{" host-list "}" ) [ port ] [ os ]
"to" ( "any" | "no-route" | "self" | host |
"{" host-list "}" ) [ 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-name ")" | hostname |
ipv4-dotted-quad | ipv6-coloned-hex )
host-list = host [ [ "," ] host-list ]
redirhost-list = redirhost [ [ "," ] redirhost-list ]
routehost-list = routehost [ [ "," ] routehost-list ]

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

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

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

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

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

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

state-opts = state-opt [ [ "," ] state-opts ]
state-opt = ( "max" number | "no-sync" | timeout |
"source-track" [ ( "rule" | "global" ) ] |

"if-bound" | "group-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.

BUGS

Due to a lock order reversal (LOR) with the socket layer, the use of the group and user filter parameter in conjuction with a Giant-free netstack can result in a deadlock. If you have to use group or user you must set debug.mpsafenet to ’’0’’ from the loader(8), for the moment. This workaround will still produce the LOR, but Giant will protect from the deadlock.

SEE ALSO

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

HISTORY

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

BSD October 3, 2004 BSD