pf.conf(5)

NAME

pf.conf - packet filter configuration file

DESCRIPTION

The pf(4) packet filter modifies, drops or passes packets

according to
rules or definitions specified in pf.conf.

STATEMENT ORDER

There are seven types of statements in pf.conf:

Macros

User-defined variables may be defined and used later,

simplifying
the configuration file. Macros must be defined before

they are
referenced in pf.conf.

Tables

Tables provide a mechanism for increasing the perfor

mance and flexibility of rules with large numbers of source or des

tination
addresses.

Options

Options tune the behaviour of the packet filtering en

gine.

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 state

ments should
be grouped and appear in pf.conf in the order shown above,

as this
matches the operation of the underlying packet filtering en

gine. 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 ta

bles. 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 speci

fied 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 address

es, 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 com

mand.

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 sec

onds to live
for the connection will be updated to that of the

proto.modifier
which corresponds to the connection state. Each pack

et 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 connec

tion. 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 re

sponse 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 in

terface 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 pack

et 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 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 al

most 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 pack

ets, 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 inter

faces (the

default).

For example:

set state-policy if-bound

set require-order

By default pfctl(8) enforces an ordering of the state

ment 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 sys

tems 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 ac

tive ruleset
are inconsistent until the new ruleset finishes load

ing.

For example:

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

set skip on <ifspec>

List interfaces for which packets should not be fil

tered. Packets
passing in or out on such interfaces are passed as if

pf was disabled, i.e. pf does not process them in any way. This

can be useful on loopback and other virtual interfaces, when

packet filtering
is not desired and can have unexpected effects. For

example:

set skip on lo0

set debug

Set the debug level to one of the following:

none Don't generate debug messages.
urgent Generate debug messages only for serious

errors.
misc Generate debug messages for various er

rors.
loud Generate debug messages for common con

ditions.

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 pack

et. 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 frag

ments 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 val

ues to compensate for predictable values generated by many hosts.

This option
only applies to packets that are not fragmented after

the optional
fragment reassembly.

fragment reassemble

Using scrub rules, fragments can be reassembled by

normalization.
In this case, fragments are buffered until they form a

complete
packet, and only the completed packet is passed on to

the filter.
The advantage is that filter rules have to deal only

with complete
packets, and can ignore fragments. The drawback of

caching fragments is the additional memory cost. But the full re

assembly
method is the only method that currently works with

NAT. This is
the default behavior of a scrub rule if no fragmenta

tion 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. Dupli

cate 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, frag

ments are not
buffered, they are passed as soon as they are re

ceived. The
fragment crop reassembly mechanism does not yet work

with NAT.

fragment drop-ovl

This option is similar to the fragment crop modifier

except that
all overlapping or duplicate fragments will be

dropped, and all
further corresponding fragments will be dropped as

well.

reassemble tcp

Statefully normalizes TCP connections. scrub

reassemble tcp rules
may not have the direction (in/out) specified.

reassemble tcp performs the following normalizations:

ttl Neither side of the connection is allowed to

reduce their

IP TTL. An attacker may send a packet such

that it
reaches the firewall, affects the firewall

state, and
expires before reaching the destination host.

reassemble
tcp will raise the TTL of all packets back up

to the highest value seen on the connection.

timestamp modulation

Modern TCP stacks will send a timestamp on

every TCP
packet and echo the other endpoint's times

tamp 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 spoof

ing TCP packets
into a connection requires knowing or guess

ing valid
timestamps. Timestamps merely need to be

monotonically
increasing and not derived off a guessable

base time.
reassemble tcp will cause scrub to modulate

the TCP timestamps with a random number.

extended PAWS checks

There is a problem with TCP on long fat

pipes, in that a
packet might get delayed for longer than it

takes the connection to wrap its 32-bit sequence space.

In such an
occurrence, the old packet would be indistin

guishable from
a new packet and would be accepted as such.

The solution
to this is called PAWS: Protection Against

Wrapped
Sequence numbers. It protects against it by

making sure
the timestamp on each packet does not go

backwards.
reassemble tcp also makes sure the timestamp

on the packet
does not go forward more than the RFC allows.

By doing
this, pf(4) artificially extends the security

of TCP
sequence numbers by 10 to 18 bits when the

host uses
appropriately randomized timestamps, since a

blind
attacker would have to guess the timestamp as

well.

For example,

scrub in on $ext_if all fragment reassemble

The no option prefixed to a scrub rule causes matching pack

ets to remain
unscrubbed, much in the same way as drop quick works in the

packet filter
(see below). This mechanism should be used when it is nec

essary to
exclude specific packets from broader scrub rules.

QUEUEING/ALTQ

The ALTQ system is currently not available in the GENERIC

kernel nor as
loadable modules. In order to use the herein after called

queueing
options one has to use a custom built kernel. Please refer

to altq(4) to
learn about the related kernel options.

Packets can be assigned to queues for the purpose of band

width 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 struc

tured 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. Pack

ets in the
queue with the highest priority are processed first.

hfsc Hierarchical Fair Service Curve. Queues attached to

an interface

build a tree, thus each queue can have further child

queues. Each
queue can have a priority and a bandwidth assigned.

Priority
mainly controls the time packets take to get sent out,

while
bandwidth has primarily effects on throughput. hfsc

supports both
link-sharing and guaranteed real-time services. It

employs a service curve based QoS model, and its unique feature is

an ability to
decouple delay and bandwidth allocation.

The interfaces on which queueing should be activated are de

clared using
the altq on declaration. altq on has the following key

words:

<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 Pri

ority 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 speci

fied 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 regu

lator. 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 asso

ciated 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 declara

tion. The following keywords can be used:

on <interface>

Specifies the interface the queue operates on. If not

given, it
operates on all matching interfaces.

bandwidth <bw>

Specifies the maximum bitrate to be processed by the

queue. This
value must not exceed the value of the parent queue

and can be
specified as an absolute value or a percentage of the

parent
queue's bandwidth. If not specified, defaults to 100%

of the parent queue's bandwidth. The priq scheduler does not

support bandwidth specification.

priority <level>

Between queues a priority level can be set. For cbq

and hfsc, the
range is 0 to 7 and for priq, the range is 0 to 15.

The default
for all is 1. Priq queues with a higher priority are

always served
first. Cbq and Hfsc queues with a higher priority are

preferred in
the case of overload.

qlimit <limit>

The maximum number of packets held in the queue. The

default is
50.

The scheduler can get additional parameters with

<scheduler>(
<parameters> ). Parameters are as follows:

default Packets not matched by another queue are as

signed to this

one. Exactly one default queue is required.

red Enable RED (Random Early Detection) on this

queue. RED drops

packets with a probability proportional to the

average queue
length.

rio Enables RIO on this queue. RIO is RED with

IN/OUT, thus run

ning RED two times more than RIO would achieve

the same
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, after

wards the value
given in m2.

Furthermore, with cbq and hfsc, child queues can be speci

fied 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. Interac

tive 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 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, other

wise 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" net

work.
Although in theory any IP address can be used on the

inside, it is
strongly recommended that one of the address ranges

defined by RFC
1918 be used. These netblocks are:

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

rdr The packet is redirected to another destination and

possibly a dif

ferent port. rdr rules can optionally specify port

ranges instead
of single ports. rdr ... port 2000:2999 -> ... port

4000 redirects
ports 2000 to 2999 (inclusive) to port 4000. rdr ...

port
2000:2999 -> ... port 4000:* redirects port 2000 to

4000, 2001 to
4001, ..., 2999 to 4999.

In addition to modifying the address, some translation rules

may modify
source or destination ports for tcp(4) or udp(4) connec

tions; 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 ad

dress 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 dae

mons bound
solely to the loopback address, circumventing the tradition

al blocking of
such connections on a real interface. Unless this effect is

desired, any
of the local non-loopback addresses should be used as redi

rection 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 at

tributes 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 overrid

den 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 UN

REACHABLE message, however this can be overridden by specify

ing a message
as a code or number.

return

This causes a TCP RST to be returned for tcp(4)

packets and
an ICMP UNREACHABLE for UDP and other packets.

Options returning ICMP packets currently have no ef

fect if pf(4)
operates on a bridge(4), as the code to support this

feature has
not yet been implemented.

pass The packet is passed.

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

To block everything by default and only pass packets that

match explicit
rules, one uses

block all

as the first filter rule.

See FILTER EXAMPLES below.

PARAMETERS

The rule parameters specify the packets to which a rule ap

plies. A
packet always comes in on, or goes out through, one inter

face. Most
parameters are optional. If a parameter is specified, the

rule only
applies to packets with matching attributes. Certain param

eters 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 speci

fied, 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 in

terface 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 evalua

tion of subsequent rules is skipped.

on <interface>

This rule applies only to packets coming in on, or go

ing out
through, this particular interface. It is also possi

ble 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 fam

ily. Sup

ported 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.
route <label> Any address whose associated route has

label

<label>. See route(4) and route(8).

no-route Any address which is not currently

routable.
<table> Any address that matches the given

table.

Interface names can have modifiers appended:

:network Translates to the network(s) attached to

the inter

face.

:broadcast Translates to the interface's broadcast

address(es).
:peer Translates to the point to point inter

face's peer

address(es).

:0 Do not include interface aliases.

Host names may also have the :0 option appended to re

strict the
name resolution to the first of each v4 and v6 address

found.

Host name resolution and interface to address transla

tion are done
at ruleset load-time. When the address of an inter

face (or host
name) changes (under DHCP or PPP, for instance), the

ruleset must
be reloaded for the change to be reflected in the ker

nel. 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 whenev

er the interface changes its address. The ruleset does not need

to be
reloaded. This is especially useful with nat.

Ports can be specified either by number or by name.

For example,
port 80 can be specified as www. For a list of all

port name to
number mappings used by pfctl(8), see the file

/etc/services.

Ports and ranges of ports are specified by using these

operators:

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

><, <> and : are binary operators (they take two argu

ments). 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 speci

fied 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
pass in proto tcp from route "DTAG"

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

group <group>

Similar to user, this rule only applies to packets of

sockets owned
by the specified group.

The use of group or user in debug.mpsafenet=1 environ

ments 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 con

nection 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 pro

cess. 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 ig

nored. 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 ig

nored.

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 de

faults to none.
All of SYN, FIN, RST and ACK must be un

set.

icmp-type <type> code <code>

icmp6-type <type> code <code>

This rule only applies to ICMP or ICMPv6 packets with

the specified
type and code. Text names for ICMP types and codes

are listed in
icmp(4) and icmp6(4). This parameter is only valid

for rules that
cover protocols ICMP or ICMP6. The protocol and the

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

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 on

ly 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 or translation rules to specify that

packets must
already be tagged with the given tag in order to match

the rule.
Inverse tag matching can also be done by specifying

the ! operator
before the tagged keyword.

probability <number>

A probability attribute can be attached to a rule,

with a value set
between 0 and 1, bounds not included. In that case,

the rule will
be honoured using the given probability value only.

For example,
the following rule will drop 20% of incoming ICMP

packets:

block in proto icmp probability 20%

ROUTING

If a packet matches a rule with a route option set, the

packet filter
will route the packet according to the type of route option.

When such a
rule creates state, the route option is also applied to all

packets
matching the same connection.

fastroute

The fastroute option does a normal route lookup to

find the next
hop for the packet.

route-to

The route-to option routes the packet to the specified

interface
with an optional address for the next hop. When a

route-to rule
creates state, only packets that pass in the same di

rection 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 speci

fied interface. Opposite direction is only defined in the con

text of a state
entry, and reply-to is useful only in rules that cre

ate state. It
can be used on systems with multiple external connec

tions to route
all outgoing packets of a connection through the in

terface 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 ad

dress 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 ad

dress to determine the redirection address, ensuring that the redi

rection 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 asso

ciations 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 track

ing.

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 be

gin 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 se

quence 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 assign

ing 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 in

terfaces, 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 ini

tial 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 cre

ate state as
well. UDP packets are matched to states using only host ad

dresses and
ports.

ICMP messages fall into two categories: ICMP error messages,

which always
refer to a TCP or UDP packet, are matched against the re

ferred 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 suscep

tible to ISN
prediction exploits. By applying a modulate state rule to a

TCP connection, pf(4) will create a high quality random sequence num

ber 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 se

quencing 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 connec

tion 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 end

point, perform
a handshake with the passive endpoint, and then forward

packets between
the endpoints.

No packets are sent to the passive endpoint before the ac

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

above.

Multiple options can be specified, separated by commas:

pass in proto tcp from any to any

port www flags S/SA keep state (max 100, source

track rule, max-src-nodes 75, max-src-states 3,

tcp.established 60, tcp.closing 5)

When the source-track keyword is specified, the number of

states per
source IP is tracked.

source-track rule

The maximum number of states created by this rule is

limited by the
rule's max-src-nodes and max-src-state options. Only

state entries
created by this particular rule count toward the

rule's limits.

source-track global

The number of states created by all rules that use

this option is
limited. Each rule can specify different

max-src-nodes and
max-src-states options, however state entries created

by any participating rule count towards each individual rule's

limits.

The following limits can be set:

max-src-nodes <number>

Limits the maximum number of source addresses which

can simultaneously have state table entries.

max-src-states <number>

Limits the maximum number of simultaneous state en

tries that a single source address can create with this rule.

For stateful TCP connections, limits on established connec

tions (connections which have completed the TCP 3-way handshake) can also

be enforced
per source IP.

max-src-conn <number>

Limits the maximum number of simultaneous TCP connec

tions which
have completed the 3-way handshake that a single host

can make.

max-src-conn-rate <number> / <seconds>

Limit the rate of new connections over a time inter

val. The connection rate is an approximation calculated as a mov

ing average.

Because the 3-way handshake ensures that the source address

is not being
spoofed, more aggressive action can be taken based on these

limits. With
the overload <table> state option, source IP addresses which

hit either
of the limits on established connections will be added to

the named
table. This table can be used in the ruleset to block fur

ther activity
from the offending host, redirect it to a tarpit process, or

restrict its
bandwidth.

The optional flush keyword kills all states created by the

matching rule
which originate from the host which exceeds these limits.

The global
modifier to the flush command kills all states originating

from the
offending host, regardless of which rule created the state.

For example, the following rules will protect the webserver

against hosts
making more than 100 connections in 10 seconds. Any host

which connects
faster than this rate will have its address added to the

<bad_hosts>
table and have all states originating from it flushed. Any

new packets
arriving from this host will be dropped unconditionally by

the block
rule.

block quick from <bad_hosts>
pass in on $ext_if proto tcp to $webserver port www

flags S/SA keep state (max-src-conn-rate

100/10, overload <bad_hosts> flush global)

OPERATING SYSTEM FINGERPRINTING

Passive OS Fingerprinting is a mechanism to inspect nuances

of a TCP connection's initial SYN packet and guess at the host's operat

ing system.
Unfortunately these nuances are easily spoofed by an attack

er so the fingerprint is not useful in making security decisions. But

the fingerprint
is typically accurate enough to make policy decisions upon.

The fingerprints may be specified by operating system class,

by version,
or by subtype/patchlevel. The class of an operating system

is typically
the vendor or genre and would be OpenBSD for the pf(4) fire

wall 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 de

scribe the
patchlevel if that patch led to changes in the TCP stack be

havior. 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 lim

it 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 ma

licious 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 sys

tem 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 inter

face'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 fil

ter 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 subpro

tocol header
fields are not available (TCP/UDP port numbers, ICMP

code/type). The
fragment option can be used to restrict filter rules to ap

ply 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 frag

ments 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 addi

tional 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 us

ing 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 frag

ments are
blocked unconditionally.

ANCHORS

Besides the main ruleset, pfctl(8) can load rulesets into

anchor attachment points. An anchor is a container that can hold rules,

address
tables, and other anchors.

An anchor has a name which specifies the path where pfctl(8)

can be used
to access the anchor to perform operations on it, such as

attaching child
anchors to it or loading rules into it. Anchors may be

nested, with components separated by `/' characters, similar to how file

system hierarchies are laid out. The main ruleset is actually the de

fault anchor, so
filter and translation rules, for example, may also be con

tained in any
anchor.

An anchor can reference another anchor attachment point us

ing the following kinds of rules:

nat-anchor <name>

Evaluates the nat rules in the specified anchor.

rdr-anchor <name>

Evaluates the rdr rules in the specified anchor.

binat-anchor <name>

Evaluates the binat rules in the specified anchor.

anchor <name>

Evaluates the filter rules in the specified anchor.

load anchor <name> from <file>

Loads the rules from the specified file into the an

chor name.

When evaluation of the main ruleset reaches an anchor rule,

pf(4) will
proceed to evaluate all rules specified in that anchor.

Matching filter and translation rules in anchors with the

quick option
are final and abort the evaluation of the rules in other an

chors and the
main ruleset.

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 rule

set, and
anchor rules specified in a file loaded from a load anchor

rule will be
attached under that anchor point.

Rules may be contained in anchor attachment points which do

not contain
any rules when the main ruleset is loaded, and later such

anchors can be
manipulated through pfctl(8) without reloading the main

ruleset or other
anchors. For example,

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

to $ext_if port smtp keep state

blocks all packets on the external interface by default,

then evaluates
all rules in the anchor named "spam", and finally passes all

outgoing
connections and incoming connections to port 25.

# echo "block in quick from 1.2.3.4 to any"

pfctl -a spam -f

This loads a single rule into the anchor, which blocks all

packets from a
specific address.

The anchor can also be populated by adding a load anchor

rule after the
anchor rule:

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

When pfctl(8) loads pf.conf, it will also load all the rules

from the
file /etc/pf-spam.conf into the anchor.

Optionally, anchor rules can specify the parameter's direc

tion, interface, address family, protocol and source/destination ad

dress/port using
the same syntax as filter rules. When parameters are used,

the anchor
rule is only evaluated for matching packets. This allows

conditional
evaluation of anchors, like:

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

smtp keep state

The rules inside anchor spam are only evaluated for tcp

packets with destination port 25. Hence,

# echo "block in quick from 1.2.3.4 to any"

pfctl -a spam -f

will only block connections from 1.2.3.4 to port 25.

Anchors may end with the asterisk (`*') character, which

signifies that
all anchors attached at that point should be evaluated in

the alphabetical ordering of their anchor name. For example,

anchor "spam/*"

will evaluate each rule in each anchor attached to the spam

anchor. Note
that it will only evaluate anchors that are directly at

tached to the spam
anchor, and will not descend to evaluate anchors recursive

ly.

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" pass'

pfctl -a spam/allowed -f

Evaluation of the main ruleset will lead into the spam/al

lowed anchor,
which will evaluate the rules in the spam/banned anchor, if

any, before
finally evaluating the pass rule.

Since the parser specification for anchor names is a string,

any reference to an anchor name containing solidus (`/') characters

will require
double quote (`"') characters around the anchor name.

TRANSLATION EXAMPLES

This example maps incoming requests on port 80 to port 8080,

on which a
daemon is running (because, for example, it is not run as

root, and
therefore lacks permission to bind to port 80).

# use a macro for the interface name, so it can be changed

easily
ext_if = "ne3"

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

127.0.0.1 port 8080

If the pass modifier is given, packets matching the transla

tion 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 be

hind 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 inter

nal
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 ses

sions captured to
port 8021.

# NAT
# Translate outgoing packets' source addresses (any proto

col).
# In this case, any address but the gateway's external ad

dress 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 in

ternal 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 inter

nal 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 in

ternal network.

# NAT LOAD BALANCE
# Translate outgoing packets' source addresses using an ad

dress 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 ad

dress 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 pack

et) 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 (wire

less). NAT is
# being done on $ext_if for all outgoing packets. tag pack

ets in on
# $int_if and pass those tagged packets out on $ext_if. all

other
# outgoing packets (i.e., packets from the wireless network)

are only
# permitted to access port 80.

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

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

state

# tag incoming packets as they are redirected to spamd(8).

use the tag
# to pass those packets through the packet filter.

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

tag SPAMD -> 127.0.0.1 port spamd

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

GRAMMAR

Syntax for pf.conf in BNF:

line = ( option | pf-rule | nat-rule | binat-rule

| rdr-rule

antispoof-rule | altq-rule | queue-rule

anchor-rule
trans-anchors | load-anchors | table-rule )

option = "set" ( [ "timeout" ( timeout | "{" time

out-list "}" ) ]

[ "optimization" [ "default" | "normal"
"high-latency" | "satellite"
"aggressive" | "conservative" ] ]
[ "limit" ( limit-item | "{" limit-list "}"

) ]
[ "loginterface" ( interface-name | "none"

) ]
[ "block-policy" ( "drop" | "return" ) ]
[ "state-policy" ( "if-bound" | "group

bound"
"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 ] [ proto

spec ]
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" | "reassem

ble tcp"
fragmentation | "allow-opts"
"label" string | "tag" string | [ ! ]

"tagged" string
"queue" ( string | "(" string [ [ "," ]

string ] ")" )
"probability" number"%"

nat-rule = [ "no" ] "nat" [ "pass" ] [ "on" ifspec ] [

af ]

[ protospec ] hosts [ "tag" string ] [

"tagged" string ]
[ "->" ( redirhost | "{" redirhost-list "}"

)
[ portspec ] [ pooltype ] [ "static-port" ]

]

binat-rule = [ "no" ] "binat" [ "pass" ] [ "on" inter

face-name ]

[ af ] [ "proto" ( proto-name | proto-num

ber ) ]
"from" address [ "/" mask-bits ] "to" ip

spec
[ "tag" string ] [ "tagged" string ]
[ "->" address [ "/" mask-bits ] ]

rdr-rule = [ "no" ] "rdr" [ "pass" ] [ "on" ifspec ] [

af ]

[ protospec ] hosts [ "tag" string ] [

"tagged" 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-an

chor" ) string

[ "on" ifspec ] [ af ] [ "proto" ] [ proto

spec ] [ hosts ]

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

queueopts-list = queueopts-list queueopts | queueopts
queueopts = [ "bandwidth" bandwidth-spec ]

[ "qlimit" number ] | [ "tbrsize" number ]
[ "priority" number ] | [ schedulers ]

schedulers = ( cbq-def | priq-def | hfsc-def )
bandwidth-spec = "number" ( "b" | "Kb" | "Mb" | "Gb" | "%" )

action = "pass" | "block" [ return ] | [ "no" ]

"scrub"
return = "drop" | "return" | "return-rst" [ "( ttl"

number ")" ]

"return-icmp" [ "(" icmpcode ["," icmp6code

] ")" ]
"return-icmp6" [ "(" icmp6code ")" ]

icmpcode = ( icmp-code-name | icmp-code-number )
icmp6code = ( icmp6-code-name | icmp6-code-number )

ifspec = ( [ "!" ] interface-name ) | "{" interface

list "}"
interface-list = [ "!" ] interface-name [ [ "," ] interface

list ]
route = "fastroute"

( "route-to" | "reply-to" | "dup-to" )
( routehost | "{" routehost-list "}" )
[ pooltype ]

af = "inet" | "inet6"

protospec = "proto" ( proto-name | proto-number

"{" proto-list "}" )

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

proto-list ]

hosts = "all"

"from" ( "any" | "no-route" | "self" | host
"{" host-list "}" | "route" string ) [ port

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

]

ipspec = "any" | host | "{" host-list "}"
host = [ "!" ] ( address [ "/" mask-bits ] | "<"

string ">" )
redirhost = address [ "/" mask-bits ]
routehost = ( interface-name [ address [ "/" mask-bits

] ] )
address = ( interface-name | "(" 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 ) [ ":" ( "*" | num

ber | 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-num

ber ) ]

icmp-list = icmp-type-code [ [ "," ] icmp-list ]

tos = "tos" ( "lowdelay" | "throughput" | "relia

bility"

[ "0x" ] number )

state-opts = state-opt [ [ "," ] state-opts ]
state-opt = ( "max" number | "no-sync" | timeout

"source-track" [ ( "rule" | "global" ) ]
"max-src-nodes" number | "max-src-states"

number
"max-src-conn" number
"max-src-conn-rate" number "/" number
"overload" "<" string ">" [ "flush" ]
"if-bound" | "group-bound" | "floating" )

fragmentation = [ "fragment reassemble" | "fragment crop"

"fragment drop-ovl" ]

timeout-list = timeout [ [ "," ] timeout-list ]
timeout = ( "tcp.first" | "tcp.opening" | "tcp.estab

lished"

"tcp.closing" | "tcp.finwait"

"tcp.closed"
"udp.first" | "udp.single" | "udp.multiple"
"icmp.first" | "icmp.error"
"other.first" | "other.single" | "oth

er.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 pro

tect from the
deadlock.

SEE ALSO

altq(4), icmp(4), icmp6(4), ip(4), ip6(4), pf(4), pfsync(4),

route(4),
tcp(4), udp(4), hosts(5), pf.os(5), protocols(5), ser

vices(5),
ftp-proxy(8), pfctl(8), pflogd(8), route(8)

HISTORY

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

BSD February 7, 2005

BSD