xref: /openbsd/share/man/man5/pf.conf.5 (revision d89ec533)

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.Dd $Mdocdate: November 1 2021 $
.Dt PF.CONF 5
.Os
.Sh NAME
.Nm pf.conf
.Nd packet filter configuration file
.Sh DESCRIPTION
The
.Xr pf 4
packet filter modifies, drops, or passes packets according to rules or
definitions specified in
.Nm .
.Pp
This is an overview of the sections in this manual page:
.Bl -inset
.It Sx PACKET FILTERING
including network address translation (NAT).
.It Sx OPTIONS
globally tune the behaviour of the packet filtering engine.
.It Sx QUEUEING
provides rule-based bandwidth and traffic control.
.It Sx TABLES
provide a method for dealing with large numbers of addresses.
.It Sx ANCHORS
are containers for rules and tables.
.It Sx STATEFUL FILTERING
tracks packets by state.
.It Sx TRAFFIC NORMALISATION
includes scrub, fragment handling, and blocking spoofed traffic.
.It Sx OPERATING SYSTEM FINGERPRINTING
is a method for detecting a host's operating system.
.It Sx EXAMPLES
provides some example rulesets.
.It Sx GRAMMAR
provides a complete BNF grammar reference.
.El
.Pp
The current line can be extended over multiple lines using a backslash
.Pq Sq \e .
Comments can be put anywhere in the file using a hash mark
.Pq Sq # ,
and extend to the end of the current line.
Care should be taken when commenting out multi-line text:
the comment is effective until the end of the entire block.
.Pp
Argument names not beginning with a letter, digit, or underscore
must be quoted.
.Pp
Additional configuration files can be included with the
.Ic include
keyword, for example:
.Bd -literal -offset indent
include "/etc/pf/sub.filter.conf"
.Ed
.Pp
Macros can be defined that will later be expanded in context.
Macro names must start with a letter, digit, or underscore,
and may contain any of those characters.
Macro names may not be reserved words (for example
.Ic pass ,
.Cm in ,
.Cm out ) .
Macros are not expanded inside quotes.
.Pp
For example:
.Bd -literal -offset indent
ext_if = "kue0"
all_ifs = "{" $ext_if lo0 "}"
pass out on $ext_if from any to any
pass in  on $ext_if proto tcp from any to any port 25
.Ed
.Sh PACKET FILTERING
.Xr pf 4
has the ability to
.Ic block ,
.Ic pass ,
and
.Ic match
packets based on attributes of their layer 3
and layer 4 headers.
Filter rules determine which of these actions are taken;
filter parameters specify the packets to which a rule applies.
.Pp
Each time a packet processed by the packet filter comes in on or
goes out through an interface, the filter rules are evaluated in
sequential order, from first to last.
For
.Ic block
and
.Ic pass ,
the last matching rule decides what action is taken;
if no rule matches the packet, the default action is to pass
the packet without creating a state.
For
.Ic match ,
rules are evaluated every time they match;
the pass/block state of a packet remains unchanged.
.Pp
Most parameters are optional.
If a parameter is specified, the rule only applies to packets with
matching attributes.
The matching for some parameters can be inverted with the
.Cm !\&
operator.
Certain parameters can be expressed as lists, in which case
.Xr pfctl 8
generates all needed rule combinations.
.Pp
By default
.Xr pf 4
filters packets statefully:
the first time a packet matches a
.Ic pass
rule, a state entry is created.
The packet filter examines each packet to see if it matches an existing state.
If it does, the packet is passed without evaluation of any rules.
After the connection is closed or times out, the state entry is automatically
removed.
.Pp
The following actions can be used in the filter:
.Bl -tag -width Ds
.It Ic block
The packet is blocked.
There are a number of ways in which a
.Ic block
rule can behave when blocking a packet.
The default behaviour is to
.Cm drop
packets silently, however this can be overridden or made
explicit either globally, by setting the
.Cm block-policy
option, or on a per-rule basis with one of the following options:
.Pp
.Bl -tag -width return-icmp6 -compact
.It Cm drop
The packet is silently dropped.
.It Cm return
This causes a TCP RST to be returned for TCP packets
and an ICMP UNREACHABLE for other types of packets.
.It Cm return-icmp
.It Cm 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.
.It Cm return-rst
This applies only to TCP packets,
and issues a TCP RST which closes the connection.
An optional parameter,
.Cm ttl ,
may be given with a TTL value.
.El
.Pp
Options returning ICMP packets currently have no effect if
.Xr pf 4
operates on a
.Xr bridge 4 ,
as the code to support this feature has not yet been implemented.
.Pp
The simplest mechanism to block everything by default and only pass
packets that match explicit rules is specify a first filter rule of:
.Pp
.Dl block all
.It Ic match
The packet is matched.
This mechanism is used to provide fine grained filtering
without altering the block/pass state of a packet.
.Ic match
rules differ from
.Ic block
and
.Ic pass
rules in that parameters are set every time a packet matches the
rule, not only on the last matching rule.
For the following parameters,
this means that the parameter effectively becomes
.Dq sticky
until explicitly overridden:
.Cm nat-to ,
.Cm binat-to ,
.Cm rdr-to ,
.Cm queue ,
.Cm rtable ,
and
.Cm scrub .
.Pp
.Cm log
is different still,
in that the action happens every time a rule matches
i.e. a single packet can get logged more than once.
.It Ic pass
The packet is passed;
state is created unless the
.Cm no state
option is specified.
.El
.Pp
The following parameters can be used in the filter:
.Bl -tag -width Ds
.It Cm in No or Cm out
A packet always comes in on, or goes out through, one interface.
.Cm in
and
.Cm out
apply to incoming and outgoing packets;
if neither are specified,
the rule will match packets in both directions.
.It Cm log Pq Cm all | matches | to Ar interface | Cm user
In addition to any action specified,
log the packet.
Only the packet that establishes the state is logged,
unless the
.Cm no state
option is specified.
The logged packets are sent to a
.Xr pflog 4
interface, by default
.Pa pflog0 ;
pflog0 is monitored by the
.Xr pflogd 8
logging daemon which logs to the file
.Pa /var/log/pflog
in pcap binary format.
.Pp
The keywords
.Cm all , matches , to ,
and
.Cm user
are optional and can be combined using commas,
but must be enclosed in parentheses if given.
.Pp
Use
.Cm all
to force logging of all packets for a connection.
This is not necessary when
.Cm no state
is explicitly specified.
.Pp
If
.Cm matches
is specified,
it logs the packet on all subsequent matching rules.
It is often combined with
.Cm to Ar interface
to avoid adding noise to the default log file.
.Pp
The keyword
.Cm user
logs the UID and PID of the
socket on the local host used to send or receive a packet,
in addition to the normal information.
.Pp
To specify a logging interface other than
.Pa pflog0 ,
use the syntax
.Cm to Ar interface .
.It Cm quick
If a packet matches a rule which has the
.Cm quick
option set, this rule
is considered the last matching rule, and evaluation of subsequent rules
is skipped.
.It Cm on Ar interface | Cm any
This rule applies only to packets coming in on, or going out through, this
particular interface or interface group.
For more information on interface groups,
see the
.Ic group
keyword in
.Xr ifconfig 8 .
.Cm any
will match any existing interface except loopback ones.
.It Cm on rdomain Ar number
This rule applies only to packets coming in on, or going out through, this
particular routing domain.
.It Cm inet | inet6
This rule applies only to packets of this address family.
.It Cm proto Ar protocol
This rule applies only to packets of this protocol.
Common protocols are ICMP, ICMP6, TCP, and UDP.
For a list of all the protocol name to number mappings used by
.Xr pfctl 8 ,
see the file
.Pa /etc/protocols .
.It Xo
.Cm from Ar source
.Cm port Ar source
.Cm os Ar source
.Cm to Ar dest
.Cm port Ar dest
.Xc
This rule applies only to packets with the specified source and destination
addresses and ports.
.Pp
Addresses can be specified in CIDR notation (matching netblocks), as
symbolic host names, interface names or interface group names, or as any
of the following keywords:
.Pp
.Bl -tag -width urpf-failed -compact
.It Cm any
Any address.
.It Cm no-route
Any address which is not currently routable.
.It Cm route Ar label
Any address matching the given
.Xr route 8
.Ar label .
.It Cm self
Expands to all addresses assigned to all interfaces.
.It Pf < Ar table Ns >
Any address matching the given
.Ar table .
.It Cm urpf-failed
Any source address that fails a unicast reverse path forwarding (URPF)
check, i.e. packets coming in on an interface other than that which holds
the route back to the packet's source address.
.El
.Pp
Ranges of addresses are specified using the
.Sq -
operator.
For instance:
.Dq 10.1.1.10 - 10.1.1.12
means all addresses from 10.1.1.10 to 10.1.1.12,
hence addresses 10.1.1.10, 10.1.1.11, and 10.1.1.12.
.Pp
Interface names, interface group names, and
.Cm self
can have modifiers appended:
.Pp
.Bl -tag -width :broadcast -compact
.It Cm :0
Do not include interface aliases.
.It Cm :broadcast
Translates to the interface's broadcast address(es).
.It Cm :network
Translates to the network(s) attached to the interface.
.It Cm :peer
Translates to the point-to-point interface's peer address(es).
.El
.Pp
Host names may also have the
.Cm :0
modifier appended to restrict the name resolution to the first of each
v4 and v6 address found.
.Pp
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.
.Pp
Ports can be specified either by number or by name.
For example, port 80 can be specified as
.Cm www .
For a list of all port name to number mappings used by
.Xr pfctl 8 ,
see the file
.Pa /etc/services .
.Pp
Ports and ranges of ports are specified using these operators:
.Bd -literal -offset indent
=	(equal)
!=	(unequal)
<	(less than)
<=	(less than or equal)
>	(greater than)
>=	(greater than or equal)
:	(range including boundaries)
><	(range excluding boundaries)
<>	(except range)
.Ed
.Pp
.Sq >< ,
.Sq <>
and
.Sq \&:
are binary operators (they take two arguments).
For instance:
.Bl -tag -width Ds
.It Li port 2000:2004
means
.Sq all ports \(>= 2000 and \(<= 2004 ,
hence ports 2000, 2001, 2002, 2003, and 2004.
.It Li port 2000 >< 2004
means
.Sq all ports > 2000 and < 2004 ,
hence ports 2001, 2002, and 2003.
.It Li port 2000 <> 2004
means
.Sq all ports < 2000 or > 2004 ,
hence ports 1\(en1999 and 2005\(en65535.
.El
.Pp
The operating system of the source host can be specified in the case of TCP
rules with the
.Cm os
modifier.
See the
.Sx OPERATING SYSTEM FINGERPRINTING
section for more information.
.Pp
The
.Cm host ,
.Cm port ,
and
.Cm os
specifications are optional, as in the following examples:
.Bd -literal -offset indent
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 \e
      to ! 10.1.2.3 port != ssh
pass in proto tcp from any os "OpenBSD"
pass in proto tcp from route "DTAG"
.Ed
.El
.Pp
The following additional parameters can be used in the filter:
.Pp
.Bl -tag -width Ds -compact
.It Cm all
This is equivalent to
.Ql from any to any .
.Pp
.It Cm allow-opts
By default, packets with IPv4 options or IPv6 hop-by-hop or destination
options header are blocked.
When
.Cm allow-opts
is specified for a
.Ic pass
rule, packets that pass the filter based on that rule (last matching)
do so even if they contain 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 or option headers.
Note that IPv6 packets with type 0 routing headers are always dropped.
.Pp
.It Cm divert-packet port Ar port
Used to send matching packets to
.Xr divert 4
sockets bound to port
.Ar port .
If the default option of fragment reassembly is enabled, scrubbing with
.Cm reassemble tcp
is also enabled for
.Cm divert-packet
rules.
.Pp
.It Cm divert-reply
Used to receive replies for sockets that are bound to addresses
which are not local to the machine.
See
.Xr setsockopt 2
for information on how to bind these sockets.
.Pp
.It Cm divert-to Ar host Cm port Ar port
Used to redirect packets to a local socket bound to
.Ar host
and
.Ar port .
The packets will not be modified, so
.Xr getsockname 2
on the socket will return the original destination address of the packet.
.Pp
.It Cm flags Ar a Ns / Ns Ar b | Cm any
This rule only applies to TCP packets that have the flags
.Ar a
set out of set
.Ar b .
Flags not specified in
.Ar b
are ignored.
For stateful connections, the default is
.Cm flags S/SA .
To indicate that flags should not be checked at all, specify
.Cm flags any .
The flags are: (F)IN, (S)YN, (R)ST, (P)USH, (A)CK, (U)RG, (E)CE, and C(W)R.
.Bl -tag -width "flags /SFRA"
.It Cm flags S/S
Flag SYN is set.
The other flags are ignored.
.It Cm flags S/SA
This is the default setting for stateful connections.
Out of SYN and ACK, exactly SYN may be set.
SYN, SYN+PSH, and SYN+RST match, but SYN+ACK, ACK, and ACK+RST do not.
This is more restrictive than the previous example.
.It Cm flags /SFRA
If the first set is not specified, it defaults to none.
All of SYN, FIN, RST, and ACK must be unset.
.El
.Pp
Because
.Cm flags S/SA
is applied by default (unless
.Cm no state
is specified), only the initial SYN packet of a TCP handshake will create
a state for a TCP connection.
It is possible to be less restrictive, and allow state creation from
intermediate
.Pq non-SYN
packets, by specifying
.Cm flags any .
This will cause
.Xr pf 4
to synchronize to existing connections, for instance
if one flushes the state table.
However, states created from such intermediate packets may be missing
connection details such as the TCP window scaling factor.
States which modify the packet flow, such as those affected by
.Cm af-to ,
.Cm modulate state ,
.Cm nat-to ,
.Cm rdr-to ,
or
.Cm synproxy state
options, or scrubbed with
.Cm reassemble tcp ,
will also not be recoverable from intermediate packets.
Such connections will stall and time out.
.Pp
.It Cm group Ar group
Similar to
.Cm user ,
this rule only applies to packets of sockets owned by the specified
.Ar group .
.Pp
.It Cm icmp-type Ar type Op Cm code Ar code
.It Cm icmp6-type Ar type Op Cm code Ar code
This rule only applies to ICMP or ICMP6 packets with the specified type
and code.
Text names for ICMP types and codes are listed in
.Xr icmp 4
and
.Xr icmp6 4 .
The protocol and the ICMP type indicator
.Po
.Cm icmp-type
or
.Cm icmp6-type
.Pc
must match.
.Pp
.It Cm label Ar string
Adds a label to the rule, which can be used to identify the rule.
For instance,
.Ql pfctl -s labels
shows per-rule statistics for rules that have labels.
.Pp
The following macros can be used in labels:
.Pp
.Bl -tag -width "$srcaddrXXX" -compact -offset indent
.It Va $dstaddr
The destination IP address.
.It Va $dstport
The destination port specification.
.It Va $if
The interface.
.It Va $nr
The rule number.
.It Va $proto
The protocol name.
.It Va $srcaddr
The source IP address.
.It Va $srcport
The source port specification.
.El
.Pp
For example:
.Bd -literal -offset indent -compact
ips = "{ 1.2.3.4, 1.2.3.5 }"
pass in proto tcp from any to $ips \e
      port > 1023 label "$dstaddr:$dstport"
.Ed
.Pp
Expands to:
.Bd -literal -offset indent -compact
pass in inet proto tcp from any to 1.2.3.4 \e
      port > 1023 label "1.2.3.4:>1023"
pass in inet proto tcp from any to 1.2.3.5 \e
      port > 1023 label "1.2.3.5:>1023"
.Ed
.Pp
The macro expansion for the
.Cm label
directive occurs only at configuration file parse time, not during runtime.
.Pp
.It Cm max-pkt-rate Ar number Ns / Ns Ar seconds
Measure the rate of packets matching the rule and states created by it.
When the specified rate is exceeded, the rule stops matching.
Only packets in the direction in which the state was created are considered,
so that typically requests are counted and replies are not.
For example,
to pass up to 100 ICMP packets per 10 seconds:
.Bd -literal -offset indent
block in proto icmp
pass in proto icmp max-pkt-rate 100/10
.Ed
.Pp
When the rate is exceeded, all ICMP is blocked until the rate falls below
100 per 10 seconds again.
.Pp
.It Cm once
Creates a one shot rule that will remove itself from an active ruleset after
the first match.
In case this is the only rule in the anchor, the anchor will be destroyed
automatically after the rule is matched.
.Pp
.It Cm probability Ar number Ns %
A probability attribute can be attached to a rule,
with a value set between 0 and 100%,
in which case the rule is honoured using the given probability value.
For example, the following rule will drop 20% of incoming ICMP packets:
.Pp
.Dl block in proto icmp probability 20%
.Pp
.It Cm prio Ar number
Only match packets which have the given queueing priority assigned.
.Pp
.It Oo Cm \&! Oc Ns Cm received-on Ar interface
Only match packets which were received on the specified
.Cm interface
(or interface group).
.Cm any
will match any existing interface except loopback ones.
.Pp
.It Cm rtable Ar number
Used to select an alternate routing table for the routing lookup.
Only effective before the route lookup happened, i.e. when filtering inbound.
.Pp
.It Cm set delay Ar milliseconds
Packets matching this rule will be delayed at the outbound interface by the
given number of milliseconds.
.Pp
.It Cm set prio Ar priority | Pq Ar priority , priority
Packets matching this rule will be assigned a specific queueing priority.
Priorities are assigned as integers 0 through 7,
with a default priority of 3.
If the packet is transmitted on a
.Xr vlan 4
interface, the queueing priority will also be written as the priority
code point in the 802.1Q VLAN header.
If two priorities are given, TCP ACKs with no data payload and packets
which have a TOS of
.Cm lowdelay
will be assigned to the second one.
Packets with a higher priority number are processed first,
and packets with the same priority are processed
in the order in which they are received.
.Pp
For example:
.Bd -literal -offset indent
pass in proto tcp to port 25 set prio 2
pass in proto tcp to port 22 set prio (2, 5)
.Ed
.Pp
The interface priority queues accessed by the
.Cm set prio
keyword are always enabled and do not require any additional
configuration, unlike the queues described below and in the
.Sx QUEUEING
section.
.Pp
.It Cm set queue Ar queue | Pq Ar queue , queue
Packets matching this rule will be assigned to the specified
.Ar queue .
If two queues are given, packets which have a TOS of
.Cm lowdelay
and TCP ACKs with no data payload will be assigned to the second one.
See
.Sx QUEUEING
for setup details.
.Pp
For example:
.Bd -literal -offset indent
pass in proto tcp to port 25 set queue mail
pass in proto tcp to port 22 set queue(ssh_bulk, ssh_prio)
.Ed
.Pp
.It Cm set tos Ar string | number
Enforces a TOS for matching packets.
.Ar string
may be one of
.Cm critical ,
.Cm inetcontrol ,
.Cm lowdelay ,
.Cm netcontrol ,
.Cm throughput ,
.Cm reliability ,
or one of the DiffServ Code Points:
.Cm ef ,
.Cm af11 No ... Cm af43 ,
.Cm cs0 No ... Cm cs7 ;
.Ar number
may be either a hex or decimal number.
.Pp
.It Cm tag Ar string
Packets matching this rule will be tagged with the specified
.Ar 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
.Dq 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.
Tags take the same macros as labels (see above).
.Pp
.It Oo Cm \&! Oc Ns Cm tagged Ar string
Used with filter or translation rules
to specify that packets must already
be tagged with the given
.Ar string
in order to match the rule.
.Pp
.It Cm tos Ar string | number
This rule applies to packets with the specified TOS bits set.
.Ar string
may be one of
.Cm critical ,
.Cm inetcontrol ,
.Cm lowdelay ,
.Cm netcontrol ,
.Cm throughput ,
.Cm reliability ,
or one of the DiffServ Code Points:
.Cm ef ,
.Cm af11 No ... Cm af43 ,
.Cm cs0 No ... Cm cs7 ;
.Ar number
may be either a hex or decimal number.
.Pp
For example, the following rules are identical:
.Bd -literal -offset indent
pass all tos lowdelay
pass all tos 0x10
pass all tos 16
.Ed
.Pp
.It Cm user Ar user
This rule only applies to packets of sockets owned by the specified
.Ar 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.
.Pp
When listening sockets are bound to the wildcard address,
.Xr pf 4
cannot determine if a connection is destined for the firewall itself.
To avoid false matches on just the destination port, combine a
.Cm user
rule with source or destination address
.Cm self .
.Pp
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.
.Pp
The
.Ar user
and
.Ar group
arguments 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.
.Pp
User and group IDs can be specified as either numbers or names.
The syntax is similar to the one for ports.
The following example allows only selected users to open outgoing
connections:
.Bd -literal -offset indent
block out proto tcp all
pass  out proto tcp from self user { < 1000, dhartmei }
.Ed
.Pp
The example below permits users with uid between 1000 and 1500
to open connections:
.Bd -literal -offset indent
block out proto tcp all
pass  out proto tcp from self user { 999 >< 1501 }
.Ed
.Pp
The
.Sq \&:
operator, which works for port number matching, does not work for
.Cm user
and
.Cm group
match.
.El
.Ss Translation
Translation options modify either the source or destination address and
port of the packets associated with a stateful connection.
.Xr pf 4
modifies the specified address and/or port in the packet and recalculates
IP, TCP, and UDP checksums as necessary.
.Pp
If specified on a
.Ic match
rule, subsequent rules will see packets as they look
after any addresses and ports have been translated.
These rules will therefore have to filter based on the translated
address and port number.
.Pp
The state entry created permits
.Xr pf 4
to keep track of the original address for traffic associated with that state
and correctly direct return traffic for that connection.
.Pp
Different types of translation are possible with pf:
.Bl -tag -width binat-to
.It Cm af-to
Translation between different address families (NAT64) is handled
using
.Cm af-to
rules.
Because address family translation overrides the routing table, it's
only possible to use
.Cm af-to
on inbound rules, and a source address for the resulting translation
must always be specified.
.Pp
The optional second argument is the host or subnet the original
addresses are translated into for the destination.
The lowest bits of the original destination address form the host
part of the new destination address according to the specified subnet.
It is possible to embed a complete IPv4 address into an IPv6 address
using a network prefix of /96 or smaller.
.Pp
When a destination address is not specified it is assumed that the host
part is 32-bit long.
For IPv6 to IPv4 translation this would mean using only the lower 32
bits of the original IPv6 destination address.
For IPv4 to IPv6 translation the destination subnet defaults to the
subnet of the new IPv6 source address with a prefix length of /96.
See RFC 6052 Section 2.2 for details on how the prefix determines the
destination address encoding.
.Pp
For example, the following rules are identical:
.Bd -literal -offset indent
pass in inet af-to inet6 from 2001:db8::1 to 2001:db8::/96
pass in inet af-to inet6 from 2001:db8::1
.Ed
.Pp
In the above example the matching IPv4 packets will be modified to
have a source address of 2001:db8::1 and a destination address will
get prefixed with 2001:db8::/96, e.g. 198.51.100.100 will be
translated to 2001:db8::c633:6464.
.Pp
In the reverse case the following rules are identical:
.Bd -literal -offset indent
pass in inet6 from any to 64:ff9b::/96 af-to inet \e
	from 198.51.100.1 to 0.0.0.0/0
pass in inet6 from any to 64:ff9b::/96 af-to inet \e
	from 198.51.100.1
.Ed
.Pp
The destination IPv4 address is assumed to be embedded inside the
original IPv6 destination address, e.g. 64:ff9b::c633:6464 will be
translated to 198.51.100.100.
.Pp
The current implementation will only extract IPv4 addresses from the
IPv6 addresses with a prefix length of /96 and greater.
.It Cm binat-to
A
.Cm binat-to
rule specifies a bidirectional mapping between an external IP
netblock and an internal IP netblock.
It expands to an outbound
.Cm nat-to
rule and an inbound
.Cm rdr-to
rule.
.It Cm nat-to
A
.Cm nat-to
option 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
.Dq 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.
Those netblocks are:
.Bd -literal -offset indent
10.0.0.0 \(en 10.255.255.255 (all of net 10, i.e. 10/8)
172.16.0.0 \(en 172.31.255.255 (i.e. 172.16/12)
192.168.0.0 \(en 192.168.255.255 (i.e. 192.168/16)
.Ed
.Pp
.Cm nat-to
is usually applied outbound.
If applied inbound, nat-to to a local IP address is not supported.
.It Cm rdr-to
The packet is redirected to another destination and possibly a
different port.
.Cm rdr-to
can optionally specify port ranges instead of single ports.
For instance:
.Bl -tag -width Ds
.It match in ... port 2000:2999 rdr-to ... port 4000
redirects ports 2000 to 2999 (inclusive) to port 4000.
.It match in ... port 2000:2999 rdr-to ... port 4000:*
redirects port 2000 to 4000, port 2001 to 4001, ..., port 2999 to 4999.
.El
.Pp
.Cm rdr-to
is usually applied inbound.
If applied outbound, rdr-to to a local IP address is not supported.
.El
.Pp
In addition to modifying the address, some translation rules may modify
source or destination ports for TCP or UDP connections;
implicitly in the case of
.Cm nat-to
options and explicitly in the case of
.Cm rdr-to
ones.
Port numbers are never translated with a
.Cm binat-to
rule.
.Pp
Translation options 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.
.Pp
However packets may be redirected to hosts connected to the interface the
packet arrived on by using redirection with NAT.
For example:
.Bd -literal -offset indent
pass in on $int_if proto tcp from $int_net to $ext_if port 80 \e
	rdr-to $server
pass out on $int_if proto tcp to $server port 80 \e
	received-on $int_if nat-to $int_if
.Ed
.Pp
Note that redirecting external incoming connections to the loopback address
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.
For example:
.Bd -literal -offset indent
pass in on egress proto tcp from any to any port smtp \e
	rdr-to 127.0.0.1 port spamd
.Ed
.Pp
Unless this effect is desired, any of the local non-loopback addresses
should be used instead as the redirection target, which allows external
connections only to daemons bound to this address or not bound to
any address.
.Pp
For
.Cm af-to ,
.Cm nat-to
and
.Cm rdr-to
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:
.Bl -tag -width xxxx
.It Cm bitmask
The
.Cm bitmask
option applies the network portion of the redirection address to the address
to be modified (source with
.Cm nat-to ,
destination with
.Cm rdr-to ) .
.It Cm least-states Op Cm sticky-address
The
.Cm least-states
option selects the address with the least active states from
a given address pool and considers given weights
associated with address(es).
Weights can be specified between 1 and 65535.
Addresses with higher weights are selected more often.
.Pp
.Cm sticky-address
can be specified to ensure that multiple connections from the
same source are mapped to the same redirection address.
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
.Ic set Cm timeout src.track .
.It Cm random Op Cm sticky-address
The
.Cm random
option selects an address at random within the defined block of addresses.
.Cm sticky-address
is as described above.
.It Cm round-robin Op Cm sticky-address
The
.Cm round-robin
option loops through the redirection address(es) and considers given weights
associated with address(es).
Weights can be specified between 1 and 65535.
Addresses with higher weights are selected more often.
.Cm sticky-address
is as described above.
.It Cm source-hash Oo Ar key Oc Op Cm sticky-address
The
.Cm 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
.Ar key
can be specified after this keyword either in hex or as a string;
by default
.Xr pfctl 8
randomly generates a key for source-hash every time the
ruleset is reloaded.
.Cm sticky-address
is as described above.
.It Cm static-port
With
.Cm nat-to
rules, the
.Cm static-port
option prevents
.Xr pf 4
from modifying the source port on TCP and UDP packets.
.El
.Pp
When more than one redirection address or a table is specified,
.Cm bitmask
is not permitted as a pool type.
.Ss Routing
If a packet matches a rule with one of the following route options 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.
.Bl -tag -width route-to
.It Cm dup-to
The
.Cm dup-to
option creates a duplicate of the packet and routes it like
.Cm route-to .
The original packet gets routed as it normally would.
.It Cm reply-to
The
.Cm reply-to
option is similar to
.Cm route-to ,
but routes packets that pass in the opposite direction (replies) to the
specified address.
Opposite direction is only defined in the context of a state entry, and
.Cm reply-to
is useful only in rules that create state.
It can be used on systems with multiple paths to the internet to ensure
that replies to an incoming network connection to a particular address
are sent using the path associated with that address (symmetric routing
enforcement).
.It Cm route-to
The
.Cm route-to
option routes the packet to the specified destination address instead
of the destination address in the packet header.
When a
.Cm 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.
.El
.Pp
For the
.Cm dup-to ,
.Cm reply-to ,
and
.Cm route-to
route 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),
the methods
.Cm least-states ,
.Cm random ,
.Cm round-robin ,
and
.Cm source-hash ,
as described above,
can be used.
.Sh OPTIONS
.Xr pf 4
may be tuned for various situations using the
.Ic set
command.
.Bl -tag -width Ds
.It Ic set Cm block-policy drop | return
The
.Cm block-policy
option sets the default behaviour for the packet
.Ic block
action:
.Pp
.Bl -tag -width return -compact
.It Cm drop
Packet is silently dropped.
.It Cm 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.
.El
.Pp
The default value is
.Cm drop .
.It Ic set Cm debug Ar level
Set the debug
.Ar level ,
which limits the severity of log messages printed by
.Xr pf 4 .
This should be a keyword from the following ordered list
(highest to lowest):
.Cm emerg ,
.Cm alert ,
.Cm crit ,
.Cm err ,
.Cm warning ,
.Cm notice ,
.Cm info ,
and
.Cm debug .
These keywords correspond to the similar (LOG_) values specified to the
.Xr syslog 3
library routine.
The default value is
.Cm err .
.It Cm set Cm fingerprints Ar filename
Load fingerprints of known operating systems from the given
.Ar filename .
By default fingerprints of known operating systems are automatically
loaded from
.Xr pf.os 5 ,
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.
The default location for fingerprints is
.Pa /etc/pf.os .
.It Ic set Cm hostid Ar number
The 32-bit hostid
.Ar number
identifies this firewall's state table entries to other firewalls
in a
.Xr pfsync 4
failover cluster.
By default the hostid is set to a pseudo-random value, however it may be
desirable to manually configure it, for example to more easily identify the
source of state table entries.
The hostid may be specified in either decimal or hexadecimal.
.It Ic set Cm limit Ar limit-item number
Sets hard limits on the memory pools used by the packet filter.
See
.Xr pool 9
for an explanation of memory pools.
.Pp
For example,
to set the maximum number of entries in the memory pool used by state table
entries (generated by
.Ic pass
rules which do not specify
.Cm no state )
to 20000:
.Pp
.Dl set limit states 20000
.Pp
To set the maximum number of entries in the memory pool used for fragment
reassembly to 2000:
.Pp
.Dl set limit frags 2000
.Pp
This maximum may not exceed, and should be well below, the maximum number
of mbuf clusters
.Pq sysctl kern.maxclusters
in the system.
.Pp
To set the maximum number of entries in the memory pool used for tracking
source IP addresses (generated by the
.Cm sticky-address
and
.Cm src.track
options) to 2000:
.Pp
.Dl set limit src-nodes 2000
.Pp
To set limits on the memory pools used by tables:
.Bd -literal -offset indent
set limit tables 1000
set limit table-entries 100000
.Ed
.Pp
The first limits the number of tables that can exist to 1000.
The second limits the overall number of addresses that can be stored
in tables to 100000.
.Pp
Various limits can be combined on a single line:
.Bd -literal -offset indent
set limit { states 20000, frags 2000, src-nodes 2000 }
.Ed
.Pp
.Xr pf 4
has the following defaults:
.Bl -column table-entries PFR_KENTRY_HIWAT_SMALL platform_dependent
.It states Ta Dv PFSTATE_HIWAT Ta Pq 100000
.It tables Ta Dv PFR_KTABLE_HIWAT Ta Pq 1000
.It table-entries Ta Dv PFR_KENTRY_HIWAT Ta Pq 200000
.It table-entries Ta Dv PFR_KENTRY_HIWAT_SMALL Ta Pq 100000
.It frags Ta Dv NMBCLUSTERS Ns /32 Ta Pq platform dependent
.El
.Pp
.Dv NMBCLUSTERS
defines the total number of packets which can exist in-system at any one time.
Refer to
.In machine/param.h
for the platform-specific value.
.It Ic set Cm loginterface Ar interface | Cm none
Enable collection of packet and byte count statistics for the given
interface or interface group.
These statistics can be viewed using:
.Pp
.Dl # pfctl -s info
.Pp
In this example
.Xr pf 4
collects statistics on the interface named dc0:
.Pp
.Dl set loginterface dc0
.Pp
One can disable the loginterface using:
.Pp
.Dl set loginterface none
.Pp
The default value is
.Cm none .
.It Ic set Cm optimization Ar environment
Optimize state timeouts for one of the following network environments:
.Pp
.Bl -tag -width Ds -compact
.It Cm aggressive
Aggressively expire connections.
This can greatly reduce the memory usage of the firewall at the cost of
dropping idle connections early.
.It Cm 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.
.It Cm high-latency
A high-latency environment (such as a satellite connection).
.It Cm normal
A normal network environment.
Suitable for almost all networks.
.It Cm satellite
Alias for
.Cm high-latency .
.El
.Pp
The default value is
.Cm normal .
.It Ic set Cm reassemble yes | no Op Cm no-df
The
.Cm reassemble
option is used to enable or disable the reassembly of fragmented packets,
and can be set to
.Cm yes
(the default) or
.Cm no .
If
.Cm no-df
is also specified, fragments with the
.Dq dont-fragment
bit set are reassembled too,
instead of being dropped;
the reassembled packet will have the
.Dq dont-fragment
bit cleared.
The default value is
.Cm yes .
.It Ic set Cm ruleset-optimization Ar level
.Bl -tag -width profile -compact
.It Cm basic
Enable basic ruleset optimization.
This is the default behaviour.
Basic ruleset optimization does four things to improve the
performance of ruleset evaluations:
.Pp
.Bl -enum -compact
.It
remove duplicate rules
.It
remove rules that are a subset of another rule
.It
combine multiple rules into a table when advantageous
.It
reorder the rules to improve evaluation performance
.El
.Pp
.It Cm none
Disable the ruleset optimizer.
.It Cm profile
Uses the currently loaded ruleset as a feedback profile to tailor the
ordering of
.Cm quick
rules to actual network traffic.
.El
.Pp
It is important to note that the ruleset optimizer will modify the ruleset
to improve performance.
A side effect of the ruleset modification is that per-rule accounting
statistics will have different meanings than before.
If per-rule accounting is important for billing purposes or whatnot,
either the ruleset optimizer should not be used or a label field should
be added to all of the accounting rules to act as optimization barriers.
.Pp
Optimization can also be set as a command-line argument to
.Xr pfctl 8 ,
overriding the settings in
.Nm .
.It Ic set Cm skip on Ar ifspec
List interfaces for which packets should not be filtered.
Packets passing in or out on such interfaces are passed as if pf was
disabled, i.e. pf does not process them in any way.
This can be useful on loopback and other virtual interfaces, when
packet filtering is not desired and can have unexpected effects.
.Ar ifspec
is only evaluated when the ruleset is loaded; interfaces created
later will not be skipped.
PF filters traffic on all interfaces by default.
.It Ic set Cm state-defaults Ar state-option , ...
The
.Cm state-defaults
option sets the state options for states created from rules
without an explicit
.Cm keep state .
For example:
.Pp
.Dl set state-defaults pflow, no-sync
.It Ic set Cm state-policy if-bound | floating
The
.Cm state-policy
option sets the default behaviour for states:
.Pp
.Bl -tag -width if-bound -compact
.It Cm if-bound
States are bound to an interface.
.It Cm floating
States can match packets on any interfaces (the default).
.El
.It Ic set Cm syncookies never | always | adaptive
When
.Cm syncookies
are active, pf will answer each and every incoming TCP SYN with a
syncookie SYNACK, without allocating any resources.
Upon reception of the client's ACK in response to the syncookie
SYNACK, pf will evaluate the ruleset and create state if the ruleset
permits it, complete the three way handshake with the target host,
and continue the connection with synproxy in place.
This allows pf to be resilient against large synflood attacks,
which could otherwise exhaust the state table.
Due to the blind answers to each and every SYN,
syncookies share the caveats of synproxy:
seemingly accepting connections that will be dropped later on.
.Pp
.Bl -tag -width adaptive -compact
.It Cm never
pf will never send syncookie SYNACKs (the default).
.It Cm always
pf will always send syncookie SYNACKs.
.It Cm adaptive
pf will enable syncookie mode when a given percentage of the state table
is used up by half-open TCP connections, such as those that saw the initial
SYN but didn't finish the three way handshake.
The thresholds for entering and leaving syncookie mode can be specified using:
.Bd -literal -offset indent
set syncookies adaptive (start 25%, end 12%)
.Ed
.El
.It Ic set Cm timeout Ar variable value
.Bl -tag -width "src.track" -compact
.It Cm frag
Seconds before an unassembled fragment is expired (60 by default).
.It Cm interval
Interval between purging expired states and fragments (10 seconds by default).
.It Cm src.track
Length of time to retain a source tracking entry after the last state
expires (0 by default, which means there is no global limit.
The value is defined by the rule which creates the state.).
.El
.Pp
When a packet matches a stateful connection, the seconds to live for the
connection will be updated to that of the
protocol and 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.
.Pp
.Bl -tag -width Ds -compact
.It Cm tcp.closed Pq 90 seconds by default
The state after one endpoint sends an RST.
.It Cm tcp.closing Pq 900 seconds by default
The state after the first FIN has been sent.
.It Cm tcp.established Pq 24 hours by default
The fully established state.
.It Cm tcp.finwait Pq 45 seconds by default
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
.Cm tcp.finwait
(and possibly
.Cm tcp.closing )
can prevent blocking of such packets.
.It Cm tcp.first Pq 120 seconds by default
The state after the first packet.
.It Cm tcp.opening Pq 30 seconds by default
The state after the second packet but before both endpoints have
acknowledged the connection.
.El
.Pp
ICMP and UDP are handled in a fashion similar to TCP, but with a much more
limited set of states:
.Pp
.Bl -tag -width Ds -compact
.It Cm icmp.error Pq 10 seconds by default
The state after an ICMP error came back in response to an ICMP packet.
.It Cm icmp.first Pq 20 seconds by default
The state after the first packet.
.It Cm udp.first Pq 60 seconds by default
The state after the first packet.
.It Cm udp.multiple Pq 60 seconds by default
The state if both hosts have sent packets.
.It Cm udp.single Pq 30 seconds by default
The state if the source host sends more than one packet but the destination
host has never sent one back.
.El
.Pp
Other protocols are handled similarly to UDP:
.Pp
.Bl -tag -width xxxx -compact
.It Cm other.first Pq 60 seconds by default
.It Cm other.multiple Pq 60 seconds by default
.It Cm other.single Pq 30 seconds by default
.El
.Pp
Timeout values can be reduced adaptively as the number of state table
entries grows.
.Pp
.Bl -tag -width Ds -compact
.It Cm adaptive.start Pq 60000 states by default
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).
.It Cm adaptive.end Pq 120000 states by default
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).
.El
.Pp
Adaptive timeouts are enabled by default, with an adaptive.start value
equal to 60% of the state limit, and an adaptive.end value equal to
120% of the state limit.
They can be disabled by setting both adaptive.start and adaptive.end to 0.
.Pp
The adaptive timeout values can be defined both globally and for each rule.
When used on a per-rule basis, the values relate to the number of
states created by the rule, otherwise to the total number of
states.
.Pp
For example:
.Bd -literal -offset indent
set timeout tcp.first 120
set timeout tcp.established 86400
set timeout { adaptive.start 60000, adaptive.end 120000 }
set limit states 100000
.Ed
.Pp
With 9000 state table entries, the timeout values are scaled to 50%
(tcp.first 60, tcp.established 43200).
.El
.Pp
.Dq pfctl -F Reset
restores default values for the following options: debug, all limit options,
loginterface, reassemble, skip, syncookies, all timeouts.
.Sh QUEUEING
Packets can be assigned to queues for the purpose of bandwidth
control.
At least one declaration is required to configure queues, and later
any packet filtering rule can reference the defined queues by name.
When filtering, the last referenced
.Ar queue
name is where any passed packets will be queued, while for
blocked packets it specifies where any resulting ICMP or TCP RST
packets should be queued.
If the referenced queue does not exist on the outgoing interface,
the default queue for that interface is used.
Queues attached to an interface build a tree,
thus each queue can have further child queues.
Only leaf queues, i.e. queues without children, can be used to assign
packets to.
The root queue must specifically reference an interface, all other queues
pick up the interfaces they should be created on from their parent queues.
.Pp
In the following example, a queue named std is created on the interface em0,
with 3 child queues ssh, mail and http:
.Bd -literal -offset indent
queue std on em0 bandwidth 100M
queue ssh parent std bandwidth 10M
queue mail parent std bandwidth 10M
queue http parent std bandwidth 80M default
.Ed
.Pp
The specified bandwidth is the target bandwidth, every queue can receive
more bandwidth as long as the parent still has some available.
The maximum bandwidth that should be assigned to a given queue can be limited
using the
.Cm max
keyword.
If a limitation isn't imposed on the root queue, borrowing can result in
saturating the bandwidth of the outgoing interface.
Similarly, a minimum (reserved) bandwidth can be specified:
.Pp
.Dl queue ssh parent std bandwidth 10M min 5M max 25M
.Pp
For each of these 3 bandwidth specifications an additional burst bandwidth and
time can be specified:
.Pp
.Dl queue ssh parent std bandwidth 10M burst 90M for 100ms
.Pp
All
.Cm bandwidth
values are specified as bits per second or using the suffixes
.Cm K ,
.Cm M ,
and
.Cm G
to represent kilobits, megabits, and gigabits per second, respectively.
The value must not exceed the interface bandwidth.
.Pp
If multiple connections are assigned the same queue, they're not guaranteed
to share the queue bandwidth fairly.
An alternative flow queue manager can be used to achieve fair sharing by
indicating how many simultaneous states are expected with a
.Cm flows
option, unless a minimum bandwidth has been specified as well.
.Pp
When packets are classified by the stateful inspection engine, a flow
identifier is assigned to all packets belonging to the state,
thus limiting the number of individual flows that can be recognized
by the resolution of a flow identifier.
The current implementation is able to classify traffic into 32767 distinct
flows.
However, efficient fair sharing is observed even with a much smaller number
of flows.
For example on a 10Mbit/s DSL or a cable modem uplink, the following simple
configuration can be used:
.Bd -literal -offset 4n
queue outq on em0 bandwidth 9M max 9M flows 1024 qlimit 1024 \e
      default
.Ed
.Pp
It's important to specify the upper bound within 90-95% of the expected
bandwidth and raise the default queue limit.
.Pp
If a
.Cm flows
option appears without a
.Cm bandwidth
specification, the flow queue manager is selected as the queueing discipline
for the corresponding interface acting as a default queue for all outgoing
packets.
In such a scenario, a queueing hierarchy is not supported.
.Pp
In addition to the bandwidth and flow specifications, queues support the
following options:
.Bl -tag -width xxxx
.It Cm default
Packets not matched by another queue are assigned to this queue.
Exactly one default queue per interface is required.
.It Cm on Ar interface
Specifies the interface the queue operates on.
If not given, it operates on all matching interfaces.
.It Cm parent Ar name
Defines which parent queue the queue should be attached to.
Mandatory for all queues except root queues.
The parent queue must exist.
.It Cm quantum Ar size
Specifies the quantum of service for the flow queue manager.
The lower the quantum size the more advantage is given to streams of smaller
packets at the expense of bulk transfers.
The default value is set to the configured Maximum Transmission Unit (MTU)
of the specified interface.
.It Cm qlimit Ar limit
The maximum number of packets held in the queue.
The default is 50.
.El
.Pp
Packets can be assigned to queues based on filter rules by using the
.Cm queue
keyword.
Normally only one
.Ar queue
is specified; when a second one is specified it will instead be used for
packets which have a TOS of
.Cm lowdelay
and for TCP ACKs with no data payload.
.Pp
To continue the previous example, the examples below would specify the
four referenced
queues, plus a few child queues.
Interactive
.Xr ssh 1
sessions get a queue with a minimum bandwidth;
.Xr scp 1
and
.Xr sftp 1
bulk transfers go to a separate queue.
The queues are then referenced by filtering rules.
.Bd -literal -offset 4n
queue rootq on em0 bandwidth 100M max 100M
queue http parent rootq bandwidth 60M burst 90M for 100ms
queue  developers parent http bandwidth 45M
queue  employees parent http bandwidth 15M
queue mail parent rootq bandwidth 10M
queue ssh parent rootq bandwidth 20M
queue  ssh_interactive parent ssh bandwidth 10M min 5M
queue  ssh_bulk parent ssh bandwidth 10M
queue std parent rootq bandwidth 20M default

block return out on em0 inet all set queue std
pass out on em0 inet proto tcp from $developerhosts to any port 80 \e
      set queue developers
pass out on em0 inet proto tcp from $employeehosts to any port 80 \e
      set queue employees
pass out on em0 inet proto tcp from any to any port 22 \e
      set queue(ssh_bulk, ssh_interactive)
pass out on em0 inet proto tcp from any to any port 25 \e
      set queue mail
.Ed
.Sh TABLES
Tables are named structures which can hold a collection of addresses and
networks.
Lookups against tables in
.Xr 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).
.Pp
Tables can be used as the source or destination of filter
or translation rules.
They can also be used for the redirect address of
.Cm nat-to
and
.Cm rdr-to
and in the routing options of filter rules, but not for
.Cm bitmask
pools.
.Pp
Tables can be defined with any of the following
.Xr pfctl 8
mechanisms.
As with macros, reserved words may not be used as table names.
.Bl -tag -width "manually"
.It manually
Persistent tables can be manually created with the
.Cm add
or
.Cm replace
option of
.Xr pfctl 8 ,
before or after the ruleset has been loaded.
.It Nm
Table definitions can be placed directly in this file and loaded at the
same time as other rules are loaded, atomically.
Table definitions inside
.Nm
use the
.Ic 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
.Nm
is loaded.
A table initialized with the empty list,
.Li { } ,
will be cleared on load.
.El
.Pp
Tables may be defined with the following attributes:
.Bl -tag -width counters
.It Cm const
The
.Cm const
flag prevents the user from altering the contents of the table once it
has been created.
Without that flag,
.Xr pfctl 8
can be used to add or remove addresses from the table at any time, even
when running with
.Xr securelevel 7
= 2.
.It Cm counters
The
.Cm counters
flag enables per-address packet and byte counters, which can be displayed with
.Xr pfctl 8 .
.It Cm persist
The
.Cm 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.
.El
.Pp
This example creates a table called
.Dq private ,
to hold RFC 1918 private network blocks,
and a table called
.Dq badhosts ,
which is initially empty.
A filter rule is set up to block all traffic coming from addresses listed in
either table:
.Bd -literal -offset indent
table <private> const { 10/8, 172.16/12, 192.168/16 }
table <badhosts> persist
block on fxp0 from { <private>, <badhosts> } to any
.Ed
.Pp
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 the following:
.Pp
.Dl # pfctl -t badhosts -Tadd 204.92.77.111
.Pp
A table can also be initialized with an address list specified in one or more
external files, using the following syntax:
.Bd -literal -offset indent
table <spam> persist file "/etc/spammers" file "/etc/openrelays"
block on fxp0 from <spam> to any
.Ed
.Pp
The files
.Pa /etc/spammers
and
.Pa /etc/openrelays
list IP addresses, one per line.
Any lines beginning with a
.Sq #
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,
.Em all
resulting IPv4 and IPv6 addresses are placed into the table.
IP addresses can also be entered in a table by specifying a valid interface
name, a valid interface group, or the
.Cm self
keyword, in which case all addresses assigned to the interface(s) will be
added to the table.
.Sh ANCHORS
Besides the main ruleset,
.Nm
can specify anchor attachment points.
An anchor is a container that can hold rules,
address tables, and other anchors.
When evaluation of the main ruleset reaches an
.Ic anchor
rule,
.Xr pf 4
will proceed to evaluate all rules specified in that anchor.
.Pp
The following example 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:
.Bd -literal -offset indent
ext_if = "kue0"
block on $ext_if all
anchor spam
pass out on $ext_if all
pass in on $ext_if proto tcp from any to $ext_if port smtp
.Ed
.Pp
Anchors can be manipulated through
.Xr pfctl 8
without reloading the main ruleset or other anchors.
This loads a single rule into the anchor,
which blocks all packets from a specific address:
.Bd -literal -offset indent
# echo "block in quick from 1.2.3.4 to any" | pfctl -a spam -f -
.Ed
.Pp
The anchor can also be populated by adding a
.Ic load anchor
rule after the anchor rule.
When
.Xr pfctl 8
loads
.Nm ,
it will also load all the rules from the file
.Pa /etc/pf-spam.conf
into the anchor.
.Bd -literal -offset indent
anchor spam
load anchor spam from "/etc/pf-spam.conf"
.Ed
.Pp
An anchor rule can also contain a filter ruleset
in a brace-delimited block.
In that case, no separate loading of rules into the anchor
is required.
Brace delimited blocks may contain rules or other brace-delimited blocks.
When an anchor is populated this way the anchor name becomes optional.
Since the parser specification for anchor names is a string,
double quote characters
.Pq Sq \&"
should be placed around the anchor name.
.Bd -literal -offset indent
anchor "external" on egress {
	block
	anchor out {
		pass proto tcp from any to port { 25, 80, 443 }
	}
	pass in proto tcp to any port 22
}
.Ed
.Pp
Anchor rules can also specify packet filtering parameters
using the same syntax as filter rules.
When parameters are used,
the anchor rule is only evaluated for matching packets.
This allows conditional evaluation of anchors, like:
.Bd -literal -offset indent
block on $ext_if all
anchor spam proto tcp from any to any port smtp
pass out on $ext_if all
pass in on $ext_if proto tcp from any to $ext_if port smtp
.Ed
.Pp
The rules inside anchor "spam" are only evaluated
for TCP packets with destination port 25.
Hence, the following
will only block connections from 1.2.3.4 to port 25:
.Bd -literal -offset indent
# echo "block in quick from 1.2.3.4 to any" | pfctl -a spam -f -
.Ed
.Pp
Matching filter and translation rules marked with the
.Cm quick
option are final and abort the evaluation of the rules in other
anchors and the main ruleset.
If the anchor itself is marked with the
.Cm quick
option,
ruleset evaluation will terminate when the anchor is exited if the packet is
matched by any rule within the anchor.
.Pp
An anchor references other anchor attachment points
using the following syntax:
.Bl -tag -width xxxx
.It Ic anchor Ar name
Evaluates the filter rules in the specified anchor.
.El
.Pp
An anchor has a name which specifies the path where
.Xr 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
.Sq /
characters, similar to how file system hierarchies are laid out.
The main ruleset is actually the default anchor, so filter and
translation rules, for example, may also be contained in any anchor.
.Pp
Anchor rules are evaluated relative to the anchor in which they are contained.
For example,
all anchor rules specified in the main ruleset will reference
anchor attachment points underneath the main ruleset,
and anchor rules specified in a file loaded from a
.Ic load anchor
rule will be attached under that anchor point.
.Pp
Anchors may end with the asterisk
.Pq Sq *
character, which signifies that all anchors attached at that point
should be evaluated in the alphabetical ordering of their anchor name.
For example,
the following
will evaluate each rule in each anchor attached to the "spam" anchor:
.Bd -literal -offset indent
anchor "spam/*"
.Ed
.Pp
Note that it will only evaluate anchors that are directly attached to the
"spam" anchor, and will not descend to evaluate anchors recursively.
.Pp
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
.Sq ..
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:
.Bd -literal -offset indent
# printf 'anchor "spam/allowed"\en' | pfctl -f -
# printf 'anchor "../banned"\enpass\en' | pfctl -a spam/allowed -f -
.Ed
.Pp
Evaluation of the main ruleset will lead into the
spam/allowed anchor, which will evaluate the rules in the
spam/banned anchor, if any, before finally evaluating the
.Ic pass
rule.
.Sh STATEFUL FILTERING
.Xr pf 4
filters packets statefully,
which has several advantages.
For TCP connections, comparing a packet to a state involves checking
its sequence numbers, as well as TCP timestamps if a rule using the
.Cm reassemble tcp
parameter applies to the connection.
If these values are outside the narrow windows of expected
values, the packet is dropped.
This prevents spoofing attacks, such as when an attacker sends packets with
a fake source address/port but does not know the connection's sequence
numbers.
Similarly,
.Xr pf 4
knows how to match ICMP replies to states.
For example,
to allow echo requests (such as those created by
.Xr ping 8 )
out statefully and match incoming echo replies correctly to states:
.Pp
.Dl pass out inet proto icmp all icmp-type echoreq
.Pp
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).
.Pp
Furthermore, correct handling of ICMP error messages is critical to
many protocols, particularly TCP.
.Xr pf 4
matches ICMP error messages to the correct connection, checks them against
connection parameters, and passes them if appropriate.
For example if an ICMP source quench message referring to a stateful TCP
connection arrives, it will be matched to the state and get passed.
.Pp
Finally, state tracking is required for
.Cm nat-to
and
.Cm rdr-to
options, in order to track address and port translations and reverse the
translation on returning packets.
.Pp
.Xr pf 4
will also create state for other protocols which are effectively stateless by
nature.
UDP packets are matched to states using only host addresses and ports,
and other protocols are matched to states using only the host addresses.
.Pp
If stateless filtering of individual packets is desired,
the
.Cm no state
keyword can be used to specify that state will not be created
if this is the last matching rule.
Note that packets which match neither block nor pass rules,
and thus are passed by default,
are effectively passed as if
.Cm no state
had been specified.
.Pp
A number of parameters can also be set to affect how
.Xr pf 4
handles state tracking,
as detailed below.
.Ss 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
.Em very
poor ISNs and thus are normally susceptible to ISN prediction exploits.
By applying a
.Cm modulate state
rule to a TCP connection,
.Xr pf 4
will create a high quality random sequence number for each connection
endpoint.
.Pp
The
.Cm modulate state
directive implicitly keeps state on the rule and is
only applicable to TCP connections.
.Pp
For instance:
.Bd -literal -offset indent
block all
pass out proto tcp from any to any modulate state
pass in  proto tcp from any to any port 25 flags S/SFRA \e
      modulate state
.Ed
.Pp
Note that modulated connections will not recover when the state table
is lost (firewall reboot, flushing the state table, etc.).
.Xr pf 4
will not be able to infer a connection again after the state table flushes
the connection's modulator.
When the state is lost, the connection may be left dangling until the
respective endpoints time out the connection.
It is possible on a fast local network for the endpoints to start an ACK
storm while trying to resynchronize after the loss of the modulator.
The default
.Cm flags
settings (or a more strict equivalent) should be used on
.Cm modulate state
rules to prevent ACK storms.
.Pp
Note that alternative methods are available
to prevent loss of the state table
and allow for firewall failover.
See
.Xr carp 4
and
.Xr pfsync 4
for further information.
.Ss SYN Proxy
By default,
.Xr pf 4
passes packets that are part of a
TCP handshake between the endpoints.
The
.Cm synproxy state
option can be used to cause
.Xr 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.
.Pp
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.
.Pp
The proxy is transparent to both endpoints; they each see a single
connection from/to the other endpoint.
.Xr 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.
.Cm synproxy state
includes
.Cm modulate state .
.Pp
Rules with
.Cm synproxy state
will not work if
.Xr pf 4
operates on a
.Xr bridge 4 .
Also they act on incoming SYN packets only.
.Pp
Example:
.Bd -literal -offset indent
pass in proto tcp from any to any port www synproxy state
.Ed
.Ss Stateful Tracking Options
A number of options related to stateful tracking can be applied on a
per-rule basis.
One of
.Cm keep state ,
.Cm modulate state ,
or
.Cm synproxy state
must be specified explicitly to apply these options to a rule.
.Pp
.Bl -tag -width xxxx -compact
.It Cm floating
States can match packets on any interfaces
(the opposite of
.Cm if-bound ) .
This is the default.
.It Cm if-bound
States are bound to an interface
(the opposite of
.Cm floating ) .
.It Cm max Ar number
Limits the number of concurrent states the rule may create.
When this limit is reached, further packets that would create
state are dropped until existing states time out.
.It Cm no-sync
Prevent state changes for states created by this rule from appearing on the
.Xr pfsync 4
interface.
.It Cm pflow
States created by this rule are exported on the
.Xr pflow 4
interface.
.It Cm sloppy
Uses a sloppy TCP connection tracker that does not check sequence
numbers at all, which makes insertion and ICMP teardown attacks way
easier.
This is intended to be used in situations where one does not see all
packets of a connection, e.g. in asymmetric routing situations.
It cannot be used with
.Cm modulate state
or
.Cm synproxy state .
.It Ar timeout seconds
Changes the
.Ar timeout
values used for states created by this rule.
For a list of all valid
.Ar timeout
names, see
.Sx OPTIONS
above.
.El
.Pp
Multiple options can be specified, separated by commas:
.Bd -literal -offset indent
pass in proto tcp from any to any \e
      port www keep state \e
      (max 100, source-track rule, max-src-nodes 75, \e
      max-src-states 3, tcp.established 60, tcp.closing 5)
.Ed
.Pp
When the
.Cm source-track
keyword is specified, the number of states per source IP is tracked.
.Pp
.Bl -tag -width xxxx -compact
.It Cm source-track global
The number of states created by all rules that use this option is limited.
Each rule can specify different
.Cm max-src-nodes
and
.Cm max-src-states
options, however state entries created by any participating rule count towards
each individual rule's limits.
.It Cm source-track rule
The maximum number of states created by this rule is limited by the rule's
.Cm max-src-nodes
and
.Cm max-src-states
options.
Only state entries created by this particular rule count toward the rule's
limits.
.El
.Pp
The following limits can be set:
.Pp
.Bl -tag -width xxxx -compact
.It Cm max-src-nodes Ar number
Limits the maximum number of source addresses which can simultaneously
have state table entries.
.It Cm max-src-states Ar number
Limits the maximum number of simultaneous state entries that a single
source address can create with this rule.
.El
.Pp
For stateful TCP connections, limits on established connections (connections
which have completed the TCP 3-way handshake) can also be enforced
per source IP.
.Pp
.Bl -tag -width xxxx -compact
.It Cm max-src-conn Ar number
Limits the maximum number of simultaneous TCP connections which have
completed the 3-way handshake that a single host can make.
.It Cm max-src-conn-rate Ar number Ns / Ns Ar seconds
Limit the rate of new connections over a time interval.
The connection rate is an approximation calculated as a moving average.
.El
.Pp
When one of these limits is reached, further packets that would create
state are dropped until existing states time out.
.Pp
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
.Cm overload Pf < Ar table Ns >
state option, source IP addresses which hit either of the limits on
established connections will be added to the named
.Ar table .
This table can be used in the ruleset to block further activity from
the offending host, redirect it to a tarpit process, or restrict its
bandwidth.
.Pp
The optional
.Cm flush
keyword kills all states created by the matching rule which originate
from the host which exceeds these limits.
The
.Cm global
modifier to the
.Cm flush
command kills all states originating from the
offending host, regardless of which rule created the state.
.Pp
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.
.Bd -literal -offset indent
block quick from <bad_hosts>
pass in on $ext_if proto tcp to $webserver port www keep state \e
      (max-src-conn-rate 100/10, overload <bad_hosts> flush global)
.Ed
.Sh TRAFFIC NORMALISATION
Traffic normalisation is a broad umbrella term
for aspects of the packet filter which deal with
verifying packets, packet fragments, spoof traffic,
and other irregularities.
.Ss Scrub
Scrub involves sanitising packet content in such a way
that there are no ambiguities in packet interpretation on the receiving side.
It is invoked with the
.Cm scrub
option, added to regular rules.
.Pp
Parameters are specified enclosed in parentheses.
At least one of the following parameters must be specified:
.Bl -tag -width xxxx
.It Cm max-mss Ar number
Enforces a maximum segment size (MSS) for matching TCP packets.
.It Cm min-ttl Ar number
Enforces a minimum TTL for matching IP packets.
.It Cm no-df
Clears the
.Dq dont-fragment
bit from a matching IPv4 packet.
Some operating systems have NFS implementations
which are known to generate fragmented packets with the
.Dq dont-fragment
bit set.
.Xr pf 4
will drop such fragmented
.Dq dont-fragment
packets unless
.Cm no-df
is specified.
.Pp
Unfortunately some operating systems also generate their
.Dq dont-fragment
packets with a zero IP identification field.
Clearing the
.Dq dont-fragment
bit on packets with a zero IP ID may cause deleterious results if an
upstream router later fragments the packet.
Using
.Cm random-id
is recommended in combination with
.Cm no-df
to ensure unique IP identifiers.
.It Cm random-id
Replaces the IPv4 identification field with random values to compensate
for predictable values generated by many hosts.
This option only applies to packets that are not fragmented
after the optional fragment reassembly.
.It Cm reassemble tcp
Statefully normalises TCP connections.
.Cm reassemble tcp
performs the following normalisations:
.Bl -ohang
.It 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.
.Cm reassemble tcp
will raise the TTL of all packets back up to the highest value seen on
the connection.
.It Timestamp Modulation
Modern TCP stacks will send a timestamp on every TCP packet and echo
the other endpoint's timestamp back to them.
Many operating systems will merely start the timestamp at zero when
first booted, and increment it several times a second.
The uptime of the host can be deduced by reading the timestamp and multiplying
by a constant.
Also observing several different timestamps can be used to count hosts
behind a NAT device.
And spoofing TCP packets into a connection requires knowing or guessing
valid timestamps.
Timestamps merely need to be monotonically increasing and not derived off a
guessable base time.
.Cm reassemble tcp
will cause
.Cm scrub
to modulate the TCP timestamps with a random number.
.It Extended PAWS Checks
There is a problem with TCP on long fat pipes, in that a packet might get
delayed for longer than it takes the connection to wrap its 32-bit sequence
space.
In such an occurrence, the old packet would be indistinguishable from a
new packet and would be accepted as such.
The solution to this is called PAWS: Protection Against Wrapped Sequence
numbers.
It protects against it by making sure the timestamp on each packet does
not go backwards.
.Cm reassemble tcp
also makes sure the timestamp on the packet does not go forward more
than the RFC allows.
By doing this,
.Xr 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.
.El
.El
.Pp
For example:
.Pp
.Dl match in all scrub (no-df random-id max-mss 1440)
.Ss 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
.Xr pf 4
to filter on things such as TCP ports or to perform NAT.
.Pp
One alternative is to filter individual fragments with filter rules.
If packet reassembly is turned off, 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
.Cm fragment
option can be used to restrict filter rules to apply only to
fragments, but not complete packets.
Filter rules without the
.Cm fragment
option still apply to fragments, if they only specify IP header fields.
For instance:
.Bd -literal -offset indent
pass in proto tcp from any to any port 80
.Ed
.Pp
The rule above 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.
.Pp
In most cases, the benefits of reassembly outweigh the additional
memory cost,
so reassembly is on by default.
.Pp
The memory allocated for fragment caching can be limited using
.Xr 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.
.Pp
When forwarding reassembled IPv6 packets, pf refragments them with
the original maximum fragment size.
This allows the sender to determine the optimal fragment size by
path MTU discovery.
.Ss Blocking Spoofed Traffic
Spoofing is the faking of IP addresses,
typically for malicious purposes.
The
.Ic 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.
.Pp
For example:
.Dl antispoof for lo0
.Pp
Expands to:
.Bd -literal -offset indent -compact
block drop in on ! lo0 inet from 127.0.0.1/8 to any
block drop in on ! lo0 inet6 from ::1 to any
.Ed
.Pp
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:
.Pp
.Dl antispoof for wi0 inet
.Pp
Expands to:
.Bd -literal -offset indent -compact
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
.Ed
.Pp
Caveat: Rules created by the
.Ic antispoof
directive interfere with packets sent over loopback interfaces
to local addresses.
One should pass these explicitly.
.Sh 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.
.Pp
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
.Ox
for the
.Xr pf 4
firewall itself.
The version of the oldest available
.Ox
release on the main FTP site
would be 2.6 and the fingerprint would be written as:
.Pp
.Dl \&"OpenBSD 2.6\&"
.Pp
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
.Ox ,
the only subtype is for a fingerprint that was
normalised by the
.Cm no-df
scrub option and would be specified as:
.Pp
.Dl \&"OpenBSD 3.3 no-df\&"
.Pp
Fingerprints for most popular operating systems are provided by
.Xr pf.os 5 .
Once
.Xr pf 4
is running, a complete list of known operating system fingerprints may
be listed by running:
.Pp
.Dl # pfctl -so
.Pp
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.
.Pp
The
.Cm unknown
class can also be used as the fingerprint which will match packets for
which no operating system fingerprint is known.
.Pp
Examples:
.Bd -literal -offset indent
pass  out proto tcp from any os OpenBSD
block out proto tcp from any os Doors
block out proto tcp from any os "Doors PT"
block out proto tcp from any os "Doors PT SP3"
block out from any os "unknown"
pass on lo0 proto tcp from any os "OpenBSD 3.3 lo0"
.Ed
.Pp
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.
.Pp
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.
.Sh EXAMPLES
In this example,
the external interface is
.Pa kue0 .
We use a macro for the interface name, so it can be changed easily.
All incoming traffic is "normalised",
and everything is blocked and logged by default.
.Bd -literal -offset 4n
ext_if = "kue0"
match in all scrub (no-df max-mss 1440)
block return log on $ext_if all
.Ed
.Pp
Here we specifically block packets we don't want:
anything coming from source we have no back routes for;
packets whose ingress interface does not match the one in
the route back to their source address;
anything that does not have our address (157.161.48.183) as source;
broadcasts (cable modem noise);
and anything from reserved address space or invalid addresses.
.Bd -literal -offset 4n
block in from no-route to any
block in from urpf-failed to any
block out log quick on $ext_if from ! 157.161.48.183 to any
block in quick on $ext_if from any to 255.255.255.255
block in log quick on $ext_if from { 10.0.0.0/8, 172.16.0.0/12, \e
    192.168.0.0/16, 255.255.255.255/32 } to any
.Ed
.Pp
For ICMP,
pass out/in ping queries.
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.
.Bd -literal -offset 4n
pass on $ext_if inet proto icmp all icmp-type 8 code 0
.Ed
.Pp
For UDP,
pass out all UDP connections.
DNS connections are passed in.
.Bd -literal -offset 4n
pass out on $ext_if proto udp all
pass in on $ext_if proto udp from any to any port domain
.Ed
.Pp
For TCP,
pass out all TCP connections and modulate state.
SSH, SMTP, DNS, and IDENT connections are passed in.
We do not allow Windows 9x SMTP connections since they are typically
a viral worm.
.Bd -literal -offset 4n
pass out on $ext_if proto tcp all modulate state
pass in on $ext_if proto tcp from any to any \e
    port { ssh, smtp, domain, auth }
block in on $ext_if proto tcp from any \e
    os { "Windows 95", "Windows 98" } to any port smtp
.Ed
.Pp
Here we pass in/out all IPv6 traffic:
note that we have to enable this in two different ways,
on both our physical interface and our tunnel.
.Bd -literal -offset 4n
pass quick on gif0 inet6
pass quick on $ext_if proto ipv6
.Ed
.Pp
This example illustrates packet tagging.
There are three interfaces: $int_if, $ext_if, and $wifi_if (wireless).
NAT is being done on $ext_if for all outgoing packets.
Packets in on $int_if are tagged and passed out on $ext_if.
All other outgoing packets
(i.e. packets from the wireless network)
are only permitted to access port 80.
.Bd -literal -offset 4n
pass in on $int_if from any to any tag INTNET
pass in on $wifi_if from any to any

block out on $ext_if from any to any
pass out quick on $ext_if tagged INTNET
pass out on $ext_if proto tcp from any to any port 80
.Ed
.Pp
In this example,
we tag incoming packets as they are redirected to
.Xr spamd 8 .
The tag is used to pass those packets through the packet filter.
.Bd -literal -offset 4n
match in on $ext_if inet proto tcp from <spammers> to port smtp \e
     tag SPAMD rdr-to 127.0.0.1 port spamd

block in on $ext_if
pass in on $ext_if inet proto tcp tagged SPAMD
.Ed
.Pp
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).
.Bd -literal -offset 4n
match in on $ext_if proto tcp from any to any port 80 \e
      rdr-to 127.0.0.1 port 8080
.Ed
.Pp
If a
.Ic pass
rule is used with the
.Cm quick
modifier, packets matching the translation rule are passed without
inspecting subsequent filter rules.
.Bd -literal -offset 4n
pass in quick on $ext_if proto tcp from any to any port 80 \e
      rdr-to 127.0.0.1 port 8080
.Ed
.Pp
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.
.Bd -literal -offset 4n
match out on ! vlan12 from 192.168.168.0/24 to any nat-to 204.92.77.111
.Ed
.Pp
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 last rule excludes protocol AH from being translated.
.Bd -literal -offset 4n
pass out on $ext_if from 144.19.74.0/24 nat-to 204.92.77.100
pass out on $ext_if proto ah from 144.19.74.0/24
.Ed
.Pp
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.
.Bd -literal -offset 4n
pass in on $int_if proto { tcp, udp } from any to any port 80 \e
      rdr-to 127.0.0.1 port 80
pass in on $int_if proto { tcp, udp } from any to $server port 80
pass in on $int_if proto { tcp, udp } from $sysadmins to any port 80
.Ed
.Pp
This example maps 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.
.Bd -literal -offset 4n
match out on $ext_if inet proto udp from any port isakmp to any \e
    nat-to ($ext_if) port 500
.Ed
.Pp
One more example uses
.Cm rdr-to
to redirect a TCP and UDP port to an internal machine.
.Bd -literal -offset 4n
match in on $ext_if inet proto tcp from any to ($ext_if) port 8080 \e
      rdr-to 10.1.2.151 port 22
match in on $ext_if inet proto udp from any to ($ext_if) port 8080 \e
      rdr-to 10.1.2.151 port 53
.Ed
.Pp
In this example, a NAT gateway is set up to translate internal addresses
using a pool of public addresses (192.0.2.16/28).
A given source address is always translated to the same pool address by
using the
.Cm source-hash
keyword.
The gateway also translates incoming web server connections
to a group of web servers on the internal network.
.Bd -literal -offset 4n
match out on $ext_if inet from any to any nat-to 192.0.2.16/28 \e
    source-hash
match in  on $ext_if proto tcp from any to any port 80 \e
    rdr-to { 10.1.2.155 weight 2, 10.1.2.160 weight 1, \e
             10.1.2.161 weight 8 } round-robin
.Ed
.Pp
The bidirectional address translation example uses a single
.Cm binat-to
rule that expands to a
.Cm nat-to
and an
.Cm rdr-to
rule.
.Bd -literal -offset 4n
pass on $ext_if from 10.1.2.120 to any binat-to 192.0.2.17
.Ed
.Pp
The previous example is identical to the following set of rules:
.Bd -literal -offset 4n
pass out on $ext_if inet from 10.1.2.120 to any \e
      nat-to 192.0.2.17 static-port
pass in on $ext_if inet from any to 192.0.2.17 rdr-to 10.1.2.120
.Ed
.Pp
In the example below, a router handling both address families
translates an internal IPv4 subnet to IPv6 using the well-known
64:ff9b::/96 prefix:
.Bd -literal -offset 4n
pass in on $v4_if inet af-to inet6 from ($v6_if) to 64:ff9b::/96
.Ed
.Pp
Paired with the example above, the example below can be used on
another router handling both address families to translate back
to IPv4:
.Bd -literal -offset 4n
pass in on $v6_if inet6 to 64:ff9b::/96 af-to inet from ($v4_if)
.Ed
.Sh GRAMMAR
Syntax for
.Nm
in BNF:
.Bd -literal
line           = ( option | pf-rule |
                 antispoof-rule | queue-rule | anchor-rule |
                 anchor-close | load-anchor | table-rule | include )

option         = "set" ( [ "timeout" ( timeout | "{" timeout-list "}" ) ] |
                 [ "ruleset-optimization" [ "none" | "basic" |
                 "profile" ] ] |
                 [ "optimization" [ "default" | "normal" | "high-latency" |
                 "satellite" | "aggressive" | "conservative" ] ]
                 [ "limit" ( limit-item | "{" limit-list "}" ) ] |
                 [ "loginterface" ( interface-name | "none" ) ] |
                 [ "block-policy" ( "drop" | "return" ) ] |
                 [ "state-policy" ( "if-bound" | "floating" ) ]
                 [ "state-defaults" state-opts ]
                 [ "fingerprints" filename ] |
                 [ "skip on" ifspec ] |
                 [ "debug" ( "emerg" | "alert" | "crit" | "err" |
                 "warning" | "notice" | "info" | "debug" ) ] |
                 [ "reassemble" ( "yes" | "no" ) [ "no-df" ] ] )

pf-rule        = action [ ( "in" | "out" ) ]
                 [ "log" [ "(" logopts ")"] ] [ "quick" ]
                 [ "on" ( ifspec | "rdomain" number ) ] [ af ]
                 [ protospec ] [ hosts ] [ filteropts ]

logopts        = logopt [ [ "," ] logopts ]
logopt         = "all" | "matches" | "user" | "to" interface-name

filteropts     = filteropt [ [ "," ] filteropts ]
filteropt      = user | group | flags | icmp-type | icmp6-type |
                 "tos" tos |
                 ( "no" | "keep" | "modulate" | "synproxy" ) "state"
                 [ "(" state-opts ")" ] | "scrub" "(" scrubopts ")" |
                 "fragment" | "allow-opts" | "once" |
                 "divert-packet" "port" port | "divert-reply" |
                 "divert-to" host "port" port |
                 "label" string | "tag" string | [ "!" ] "tagged" string |
                 "max-pkt-rate" number "/" seconds |
                 "set delay" number |
                 "set prio" ( number | "(" number [ [ "," ] number ] ")" ) |
                 "set queue" ( string | "(" string [ [ "," ] string ] ")" ) |
                 "rtable" number | "probability" number"%" | "prio" number |
                 "af-to" af "from" ( redirhost | "{" redirhost-list "}" )
                 [ "to" ( redirhost | "{" redirhost-list "}" ) ] |
                 "binat-to" ( redirhost | "{" redirhost-list "}" )
                 [ portspec ] [ pooltype ] |
                 "rdr-to" ( redirhost | "{" redirhost-list "}" )
                 [ portspec ] [ pooltype ] |
                 "nat-to" ( redirhost | "{" redirhost-list "}" )
                 [ portspec ] [ pooltype ] [ "static-port" ] |
                 [ route ] | [ "set tos" tos ] |
                 [ [ "!" ] "received-on" ( interface-name | interface-group ) ]

scrubopts      = scrubopt [ [ "," ] scrubopts ]
scrubopt       = "no-df" | "min-ttl" number | "max-mss" number |
                 "reassemble tcp" | "random-id"

antispoof-rule = "antispoof" [ "log" ] [ "quick" ]
                 "for" ifspec [ af ] [ "label" string ]

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

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

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

anchor-close   = "}"

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

queueopts-list = queueopts-list queueopts | queueopts
queueopts      = ([ "bandwidth" bandwidth ] | [ "min" bandwidth ] |
                 [ "max" bandwidth ] | [ "parent" string ] |
                 [ "default" ]) |
                 ([ "flows" number ] | [ "quantum" number ]) |
                 [ "qlimit" number ]

bandwidth      = bandwidth-spec [ "burst" bandwidth-spec "for" number "ms" ]
bandwidth-spec = number ( "" | "K" | "M" | "G" )

action         = "pass" | "match" | "block" [ return ]
return         = "drop" | "return" |
                 "return-rst" [ "(" "ttl" number ")" ] |
                 "return-icmp" [ "(" icmpcode [ [ "," ] icmp6code ] ")" ] |
                 "return-icmp6" [ "(" icmp6code ")" ]
icmpcode       = ( icmp-code-name | icmp-code-number )
icmp6code      = ( icmp6-code-name | icmp6-code-number )

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

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

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

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

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 | "any" )
flag-set       = [ "F" ] [ "S" ] [ "R" ] [ "P" ] [ "A" ] [ "U" ] [ "E" ]
                 [ "W" ]

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

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

state-opts     = state-opt [ [ "," ] state-opts ]
state-opt      = ( "max" number | "no-sync" | timeout | "sloppy" |
                 "pflow" | "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" [ "global" ] ] |
                 "if-bound" | "floating" )

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" | "tables" |
                 "table-entries" ) number

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

include        = "include" filename
.Ed
.Sh FILES
.Bl -tag -width /etc/examples/pf.conf -compact
.It Pa /etc/hosts
Host name database.
.It Pa /etc/pf.conf
Default location of the ruleset file.
.It Pa /etc/examples/pf.conf
Example ruleset file.
.It Pa /etc/pf.os
Default location of OS fingerprints.
.It Pa /etc/protocols
Protocol name database.
.It Pa /etc/services
Service name database.
.El
.Sh SEE ALSO
.Xr pf 4 ,
.Xr pflow 4 ,
.Xr pfsync 4 ,
.Xr pf.os 5 ,
.Xr pfctl 8 ,
.Xr pflogd 8
.Sh HISTORY
The
.Nm
file format first appeared in
.Ox 3.0 .