PF.CONF(5) OpenBSD Programmer's Manual PF.CONF(5)NAMEpf.conf - packet filter configuration file
DESCRIPTION
The pf(4) packet filter modifies, drops, or passes packets according to
rules or definitions specified in pf.conf.
This is an overview of the sections in this manual page:
Packet Filtering
Packet filtering, including network address translation (NAT).
Options
Global options tune the behaviour of the packet filtering engine.
Queueing
Queueing provides rule-based bandwidth control.
Tables
Tables provide a method for dealing with large numbers of addresses.
Anchors
Anchors are containers for rules and tables.
Stateful Filtering
Stateful filtering tracks packets by state.
Traffic Normalisation
Including scrub, fragment handling, and blocking spoofed traffic.
Operating System Fingerprinting
A method for detecting a host's operating system.
Examples
Some example rulesets.
Comments can be put anywhere in the file using a hash mark (`#'), and
extend to the end of the current line. Additional configuration files
can be included with the include keyword, for example:
include "/etc/pf/sub.filter.conf"
Macros can be defined that will later be expanded in context. Macro
names must start with a letter, and may contain letters, digits and
underscores. Macro names may not be reserved words (for example pass,
in, out). Macros are not expanded inside quotes.
For example:
ext_if = "kue0"
all_ifs = "{" $ext_if lo0 "}"
pass out on $ext_if from any to any
pass in on $ext_if proto tcp from any to any port 25
PACKET FILTERINGpf(4) has the ability to block, pass, and 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.
For each packet processed by the packet filter, the filter rules are
evaluated in sequential order, from first to last. For block and pass,
the last matching rule decides what action is taken; if no rule matches
the packet, the default action is to pass the packet. For match, rules
are evaluated every time they match; the pass/block state of a packet
remains unchanged.
Most parameters are optional. If a parameter is specified, the rule only
applies to packets with matching attributes. Certain parameters can be
expressed as lists, in which case pfctl(8) generates all needed rule
combinations.
By default pf(4) filters packets statefully: the first time a packet
matches a 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.
The following actions can be used in the filter:
block
The packet is blocked. There are a number of ways in which a block
rule can behave when blocking a packet. The default behaviour is
to drop packets silently, however this can be overridden or made
explicit either globally, by setting the block-policy option, or on
a per-rule basis with one of the following options:
drop The packet is silently dropped.
return This causes a TCP RST to be returned for TCP
packets and an ICMP UNREACHABLE for other types of
packets.
return-icmp
return-icmp6 This causes ICMP messages to be returned for
packets which match the rule. By default this is
an ICMP UNREACHABLE message, however this can be
overridden by specifying a message as a code or
number.
return-rst This applies only to TCP packets, and issues a TCP
RST which closes the connection. An optional
parameter, ttl, may be given with a TTL value.
Options returning ICMP packets currently have no effect if pf(4)
operates on a bridge(4), as the code to support this feature has
not yet been implemented.
The simplest mechanism to block everything by default and only pass
packets that match explicit rules is specify a first filter rule
of:
block all
match
The packet is matched. This mechanism is used to provide fine
grained filtering without altering the block/pass state of a
packet. match rules differ from block and 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 ``sticky'' until
explicitly overridden: nat-to, binat-to, rdr-to, queue, rtable, and
scrub.
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.
pass The packet is passed; state is created unless the no state option
is specified.
The following parameters can be used in the filter:
in or out
A packet always comes in on, or goes out through, one interface.
in and out apply to incoming and outgoing packets; if neither are
specified, the rule will match packets in both directions.
log In addition to the action specified, a log message is generated.
Only the packet that establishes the state is logged, unless the
no state option is specified. The logged packets are sent to a
pflog(4) interface, by default pflog0. This interface is
monitored by the pflogd(8) logging daemon, which dumps the logged
packets to the file /var/log/pflog in pcap(3) binary format.
log (all)
Used to force logging of all packets for a connection. This is
not necessary when no state is explicitly specified. As with
log, packets are logged to pflog(4).
log (matches)
Used to force logging of this packet on all subsequent matching
rules.
log (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.
log (to <interface>)
Send logs to the specified pflog(4) interface instead of pflog0.
quick If a packet matches a rule which has the quick option set, this
rule is considered the last matching rule, and evaluation of
subsequent rules is skipped.
on <interface>
This rule applies only to packets coming in on, or going out
through, this particular interface or interface group. For more
information on interface groups, see the group keyword in
ifconfig(8).
<af> This rule applies only to packets of this address family.
Supported values are inet and inet6.
proto <protocol>
This rule applies only to packets of this protocol. Common
protocols are ICMP, ICMP6, TCP, and UDP. For a list of all the
protocol name to number mappings used by pfctl(8), see the file
/etc/protocols.
from <source> port <source> os <source> to <dest> port <dest>
This rule applies only to packets with the specified source and
destination addresses and ports.
Addresses can be specified in CIDR notation (matching netblocks),
as symbolic host names, interface names or interface group names,
or as any of the following keywords:
any Any address.
no-route Any address which is not currently routable.
route <label> Any address matching the given route(8) label.
<table> Any address matching the given table.
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.
Ranges of addresses are specified using the `-' operator. For
instance: ``10.1.1.10 - 10.1.1.12'' means all addresses from
10.1.1.10 to 10.1.1.12, hence addresses 10.1.1.10, 10.1.1.11, and
10.1.1.12.
Interface names and interface group names can have modifiers
appended:
:0 Do not include interface aliases.
:broadcast Translates to the interface's broadcast
address(es).
:network Translates to the network(s) attached to the
interface.
:peer Translates to the point-to-point interface's peer
address(es).
Host names may also have the :0 option appended to restrict the
name resolution to the first of each v4 and v6 address found.
Host name resolution and interface to address translation are
done at ruleset load-time. When the address of an interface (or
host name) changes (under DHCP or PPP, for instance), the ruleset
must be reloaded for the change to be reflected in the kernel.
Surrounding the interface name (and optional modifiers) in
parentheses changes this behaviour. When the interface name is
surrounded by parentheses, the rule is automatically updated
whenever the interface changes its address. The ruleset does not
need to be reloaded. This is especially useful with nat.
Ports can be specified either by number or by name. For example,
port 80 can be specified as www. For a list of all port name to
number mappings used by pfctl(8), see the file /etc/services.
Ports and ranges of ports are specified using these operators:
= (equal)
!= (unequal)
< (less than)
<= (less than or equal)
> (greater than)
>= (greater than or equal)
: (range including boundaries)
>< (range excluding boundaries)
<> (except range)
`><', `<>' and `:' are binary operators (they take two
arguments). For instance:
port 2000:2004
means `all ports >= 2000 and <= 2004', hence ports 2000,
2001, 2002, 2003, and 2004.
port 2000 >< 2004
means `all ports > 2000 and < 2004', hence ports 2001,
2002, and 2003.
port 2000 <> 2004
means `all ports < 2000 or > 2004', hence ports 1-1999
and 2005-65535.
The operating system of the source host can be specified in the
case of TCP rules with the os modifier. See the OPERATING SYSTEM
FINGERPRINTING section for more information.
The host, port, and OS specifications are optional, as in the
following examples:
pass in all
pass in from any to any
pass in proto tcp from any port <= 1024 to any
pass in proto tcp from any to any port 25
pass in proto tcp from 10.0.0.0/8 port > 1024 \
to ! 10.1.2.3 port != ssh
pass in proto tcp from any os "OpenBSD"
pass in proto tcp from route "DTAG"
The following additional parameters can be used in the filter:
all This is equivalent to "from any to any".
allow-opts
By default, IPv4 packets with IP options or IPv6 packets with
routing extension headers are blocked. When allow-opts is
specified for a pass rule, packets that pass the filter based on
that rule (last matching) do so even if they contain IP options
or routing extension headers. For packets that match state, the
rule that initially created the state is used. The implicit pass
rule that is used when a packet does not match any rules does not
allow IP options.
divert-packet port <port>
Used to send matching packets to divert(4) sockets bound to port
port. If the default option of fragment reassembly is enabled,
scrubbing with reassemble tcp is also enabled for divert-packet
rules.
divert-reply
Used to receive replies for sockets that are bound to addresses
which are not local to the machine. See setsockopt(2) for
information on how to bind these sockets.
divert-to <host> port <port>
Used to redirect packets to a local socket bound to host and
port. The packets will not be modified, so getsockname(2) on the
socket will return the original destination address of the
packet.
flags <a> /<b> | any
This rule only applies to TCP packets that have the flags <a> set
out of set <b>. Flags not specified in <b> are ignored. For
stateful connections, the default is flags S/SA. To indicate
that flags should not be checked at all, specify flags any. The
flags are: (F)IN, (S)YN, (R)ST, (P)USH, (A)CK, (U)RG, (E)CE, and
C(W)R.
flags S/S Flag SYN is set. The other flags are ignored.
flags S/SA This is the default setting for stateful connections.
Out of SYN and ACK, exactly SYN may be set. SYN,
SYN+PSH, and SYN+RST match, but SYN+ACK, ACK, and
ACK+RST do not. This is more restrictive than the
previous example.
flags /SFRA
If the first set is not specified, it defaults to
none. All of SYN, FIN, RST, and ACK must be unset.
Because flags S/SA is applied by default (unless no state is
specified), only the initial SYN packet of a TCP handshake will
create a state for a TCP connection. It is possible to be less
restrictive, and allow state creation from intermediate (non-SYN)
packets, by specifying flags any. This will cause pf(4) to
synchronize to existing connections, for instance if one flushes
the state table. However, states created from such intermediate
packets may be missing connection details such as the TCP window
scaling factor. States which modify the packet flow, such as
those affected by modulate, nat-to, rdr-to, or synproxy state
options, or scrubbed with reassemble tcp, will also not be
recoverable from intermediate packets. Such connections will
stall and time out.
group <group>
Similar to user, this rule only applies to packets of sockets
owned by the specified group.
icmp-type <type> code <code>
icmp6-type <type> code <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 icmp(4) and icmp6(4). The protocol and the ICMP type
indicator (icmp-type or icmp6-type) must match.
label <string>
Adds a label to the rule, which can be used to identify the rule.
For instance, ``pfctl -s labels'' shows per-rule statistics for
rules that have labels.
The following macros can be used in labels:
$dstaddr The destination IP address.
$dstport The destination port specification.
$if The interface.
$nr The rule number.
$proto The protocol name.
$srcaddr The source IP address.
$srcport The source port specification.
For example:
ips = "{ 1.2.3.4, 1.2.3.5 }"
pass in proto tcp from any to $ips \
port > 1023 label "$dstaddr:$dstport"
Expands to:
pass in inet proto tcp from any to 1.2.3.4 \
port > 1023 label "1.2.3.4:>1023"
pass in inet proto tcp from any to 1.2.3.5 \
port > 1023 label "1.2.3.5:>1023"
The macro expansion for the label directive occurs only at
configuration file parse time, not during runtime.
probability <number>
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:
block in proto icmp probability 20%
queue <queue> | (<queue>, <queue>)
Packets matching this rule will be assigned to the specified
queue. If two queues are given, packets which have a TOS of
lowdelay and TCP ACKs with no data payload will be assigned to
the second one. See QUEUEING for setup details.
For example:
pass in proto tcp to port 25 queue mail
pass in proto tcp to port 22 queue(ssh_bulk, ssh_prio)
received-on <interface>
Only match packets which were received on the specified interface
(or interface group).
rtable <number>
Used to select an alternate routing table for the routing lookup.
Only effective before the route lookup happened, i.e. when
filtering inbound.
tag <string>
Packets matching this rule will be tagged with the specified
string. The tag acts as an internal marker that can be used to
identify these packets later on. This can be used, for example,
to provide trust between interfaces and to determine if packets
have been processed by translation rules. Tags are "sticky",
meaning that the packet will be tagged even if the rule is not
the last matching rule. Further matching rules can replace the
tag with a new one but will not remove a previously applied tag.
A packet is only ever assigned one tag at a time. Tags take the
same macros as labels (see above).
tagged <string>
Used with filter or translation rules to specify that packets
must already be tagged with the given tag in order to match the
rule. Inverse tag matching can also be done by specifying the !
operator before the tagged keyword.
tos <string> | <number>
This rule applies to packets with the specified TOS bits set.
string may be one of critical, inetcontrol, lowdelay, netcontrol,
throughput, reliability, or one of the DiffServ Code Points: ef,
af11 ... af43, cs0 ... cs7; number may be either a hex or decimal
number.
For example, the following rules are identical:
pass all tos lowdelay
pass all tos 0x10
pass all tos 16
user <user>
This rule only applies to packets of sockets owned by the
specified user. For outgoing connections initiated from the
firewall, this is the user that opened the connection. For
incoming connections to the firewall itself, this is the user
that listens on the destination port. For forwarded connections,
where the firewall is not a connection endpoint, the user and
group are unknown.
All packets, both outgoing and incoming, of one connection are
associated with the same user and group. Only TCP and UDP
packets can be associated with users; for other protocols these
parameters are ignored.
User and group refer to the effective (as opposed to the real)
IDs, in case the socket is created by a setuid/setgid process.
User and group IDs are stored when a socket is created; when a
process creates a listening socket as root (for instance, by
binding to a privileged port) and subsequently changes to another
user ID (to drop privileges), the credentials will remain root.
User and group IDs can be specified as either numbers or names.
The syntax is similar to the one for ports. The value unknown
matches packets of forwarded connections. unknown can only be
used with the operators = and !=. Other constructs like user >=
unknown are invalid. Forwarded packets with unknown user and
group ID match only rules that explicitly compare unknown with
the operators = or !=. For instance user >= 0 does not match
forwarded packets. The following example allows only selected
users to open outgoing connections:
block out proto { tcp, udp } all
pass out proto { tcp, udp } all user { < 1000, dhartmei }
Translation
Translation options modify either the source or destination address and
port of the packets associated with a stateful connection. pf(4)
modifies the specified address and/or port in the packet and recalculates
IP, TCP, and UDP checksums as necessary.
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.
The state entry created permits pf(4) to keep track of the original
address for traffic associated with that state and correctly direct
return traffic for that connection.
Different types of translation are possible with pf:
binat-to A binat-to rule specifies a bidirectional mapping between an
external IP netblock and an internal IP netblock. It expands
to an outbound nat-to rule and an inbound rdr-to rule.
nat-to A 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
"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:
10.0.0.0 - 10.255.255.255 (all of net 10, i.e. 10/8)
172.16.0.0 - 172.31.255.255 (i.e. 172.16/12)
192.168.0.0 - 192.168.255.255 (i.e. 192.168/16)
nat-to is usually applied outbound. If applied inbound, nat-to
to a local IP address is not supported.
rdr-to The packet is redirected to another destination and possibly a
different port. rdr-to can optionally specify port ranges
instead of single ports. For instance:
match in ... port 2000:2999 rdr-to ... port 4000
redirects ports 2000 to 2999 (inclusive) to port 4000.
match in ... port 2000:2999 rdr-to ... port 4000:*
redirects port 2000 to 4000, port 2001 to 4001, ...,
port 2999 to 4999.
rdr-to is usually applied inbound. If applied outbound, rdr-to
to a local IP address is not supported.
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 nat-to options and explicitly in the case of rdr-to ones. Port
numbers are never translated with a binat-to rule.
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.
However packets may be redirected to hosts connected to the interface the
packet arrived on by using redirection with NAT. For example:
pass in on $int_if proto tcp from $int_net to $ext_if port 80 \
rdr-to $server
pass out on $int_if proto tcp to $server port 80 \
received-on $int_if nat-to $int_if
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:
pass in on egress proto tcp from any to any port smtp \
rdr-to 127.0.0.1 port spamd
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.
For nat-to and 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:
bitmask
The bitmask option applies the network portion of the redirection
address to the address to be modified (source with nat-to,
destination with rdr-to).
random [sticky-address]
The random option selects an address at random within the defined
block of addresses.
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 set
timeout src.track.
round-robin [sticky-address]
The round-robin option loops through the redirection address(es).
sticky-address is as described above.
When more than one redirection address is specified, round-robin is
the only permitted pool type.
source-hash [key]
The source-hash option uses a hash of the source address to
determine the redirection address, ensuring that the redirection
address is always the same for a given source. An optional key can
be specified after this keyword either in hex or as a string; by
default pfctl(8) randomly generates a key for source-hash every
time the ruleset is reloaded.
static-port
With nat rules, the static-port option prevents pf(4) from
modifying the source port on TCP and UDP packets.
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.
dup-to
The dup-to option creates a duplicate of the packet and routes it
like route-to. The original packet gets routed as it normally
would.
fastroute
The fastroute option does a normal route lookup to find the next
hop for the packet.
reply-to
The reply-to option is similar to route-to, but routes packets that
pass in the opposite direction (replies) to the specified
interface. Opposite direction is only defined in the context of a
state entry, and reply-to is useful only in rules that create
state. It can be used on systems with multiple external
connections to route all outgoing packets of a connection through
the interface the incoming connection arrived through (symmetric
routing enforcement).
route-to
The route-to option routes the packet to the specified interface
with an optional address for the next hop. When a route-to rule
creates state, only packets that pass in the same direction as the
filter rule specifies will be routed in this way. Packets passing
in the opposite direction (replies) are not affected and are routed
normally.
For the dup-to, reply-to, and 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 random,
round-robin, and source-hash, as described above, can be used.
OPTIONSpf(4) may be tuned for various situations using the set command.
set block-policy
The block-policy option sets the default behaviour for the packet
block action:
drop Packet is silently dropped.
return A TCP RST is returned for blocked TCP packets, an ICMP
UNREACHABLE is returned for blocked UDP packets, and
all other packets are silently dropped.
set debug
Set the debug level, which limits the severity of log messages
printed by pf(4). This should be a keyword from the following
ordered list (highest to lowest): emerg, alert, crit, err,
warning, notice, info, and debug. The last keyword, debug, must
be quoted. These keywords correspond to the similar (LOG_)
values specified to the syslog(3) library routine.
set fingerprints
Load fingerprints of known operating systems from the given
filename. By default fingerprints of known operating systems are
automatically loaded from pf.os(5), 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.
set hostid
The 32-bit hostid identifies this firewall's state table entries
to other firewalls in a pfsync(4) failover cluster. By default
the hostid is set to a pseudo-random value, however it may be
desirable to manually configure it, for example to more easily
identify the source of state table entries. The hostid may be
specified in either decimal or hexadecimal.
set limit
Sets hard limits on the memory pools used by the packet filter.
See pool(9) for an explanation of memory pools.
For example, to set the maximum number of entries in the memory
pool used by state table entries (generated by pass rules which
do not specify no state) to 20000:
set limit states 20000
To set the maximum number of entries in the memory pool used for
fragment reassembly to 20000:
set limit frags 20000
To set the maximum number of entries in the memory pool used for
tracking source IP addresses (generated by the sticky-address and
src.track options) to 2000:
set limit src-nodes 2000
To set limits on the memory pools used by tables:
set limit tables 1000
set limit table-entries 100000
The first limits the number of tables that can exist to 1000.
The second limits the overall number of addresses that can be
stored in tables to 100000.
Various limits can be combined on a single line:
set limit { states 20000, frags 20000, src-nodes 2000 }
set loginterface
Enable collection of packet and byte count statistics for the
given interface or interface group. These statistics can be
viewed using:
# pfctl -s info
In this example pf(4) collects statistics on the interface named
dc0:
set loginterface dc0
One can disable the loginterface using:
set loginterface none
set optimization
Optimize state timeouts for one of the following network
environments:
aggressive
Aggressively expire connections. This can greatly reduce
the memory usage of the firewall at the cost of dropping
idle connections early.
conservative
Extremely conservative settings. Avoid dropping
legitimate connections at the expense of greater memory
utilization (possibly much greater on a busy network) and
slightly increased processor utilization.
high-latency
A high-latency environment (such as a satellite
connection).
normal A normal network environment. Suitable for almost all
networks.
satellite
Alias for high-latency.
set reassemble
The reassemble option is used to enable or disable the reassembly
of fragmented packets, and can be set to yes (the default) or no.
If no-df is also specified, fragments with the dont-fragment bit
set are reassembled too, instead of being dropped; the
reassembled packet will have the dont-fragment bit cleared.
set require-order
If set to yes, pfctl(8) will enforce that statement types in the
ruleset are listed in the following order, to match the operation
of the underlying packet filtering engine: options, queueing,
filtering. This option is disabled by default.
set ruleset-optimization
basic Enable basic ruleset optimization. This is the default
behaviour. Basic ruleset optimization does four things
to improve the performance of ruleset evaluations:
1. remove duplicate rules
2. remove rules that are a subset of another rule
3. combine multiple rules into a table when
advantageous
4. re-order the rules to improve evaluation
performance
none Disable the ruleset optimizer.
profile Uses the currently loaded ruleset as a feedback profile
to tailor the ordering of quick rules to actual network
traffic.
It is important to note that the ruleset optimizer will modify
the ruleset to improve performance. A side effect of the ruleset
modification is that per-rule accounting statistics will have
different meanings than before. If per-rule accounting is
important for billing purposes or whatnot, either the ruleset
optimizer should not be used or a label field should be added to
all of the accounting rules to act as optimization barriers.
Optimization can also be set as a command-line argument to
pfctl(8), overriding the settings in pf.conf.
set skip on <ifspec>
List interfaces for which packets should not be filtered.
Packets passing in or out on such interfaces are passed as if pf
was disabled, i.e. pf does not process them in any way. This can
be useful on loopback and other virtual interfaces, when packet
filtering is not desired and can have unexpected effects.
set state-defaults
The state-defaults option sets the state options for states
created from rules without an explicit keep state. For example:
set state-defaults pflow, no-sync
set state-policy
The state-policy option sets the default behaviour for states:
if-bound States are bound to an interface.
floating States can match packets on any interfaces (the
default).
set timeout
frag Seconds before an unassembled fragment is expired.
interval Interval between purging expired states and fragments.
src.track Length of time to retain a source tracking entry after
the last state expires.
When a packet matches a stateful connection, the seconds to live
for the connection will be updated to that of the 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.
tcp.closed
The state after one endpoint sends an RST.
tcp.closing
The state after the first FIN has been sent.
tcp.established
The fully established state.
tcp.finwait
The state after both FINs have been exchanged and the
connection is closed. Some hosts (notably web servers on
Solaris) send TCP packets even after closing the
connection. Increasing tcp.finwait (and possibly
tcp.closing) can prevent blocking of such packets.
tcp.first
The state after the first packet.
tcp.opening
The state before the destination host ever sends a
packet.
ICMP and UDP are handled in a fashion similar to TCP, but with a
much more limited set of states:
icmp.error
The state after an ICMP error came back in response to an
ICMP packet.
icmp.first
The state after the first packet.
udp.first
The state after the first packet.
udp.multiple
The state if both hosts have sent packets.
udp.single
The state if the source host sends more than one packet
but the destination host has never sent one back.
Other protocols are handled similarly to UDP:
other.first
other.multiple
other.single
Timeout values can be reduced adaptively as the number of state
table entries grows.
adaptive.end
When reaching this number of state entries, all timeout
values become zero, effectively purging all state entries
immediately. This value is used to define the scale
factor; it should not actually be reached (set a lower
state limit, see below).
adaptive.start
When the number of state entries exceeds this value,
adaptive scaling begins. All timeout values are scaled
linearly with factor (adaptive.end - number of states) /
(adaptive.end - adaptive.start).
Adaptive timeouts are enabled by default, with an adaptive.start
value equal to 60% of the state limit, and an adaptive.end value
equal to 120% of the state limit. They can be disabled by
setting both adaptive.start and adaptive.end to 0.
The adaptive timeout values can be defined both globally and for
each rule. When used on a per-rule basis, the values relate to
the number of states created by the rule, otherwise to the total
number of states.
For example:
set timeout tcp.first 120
set timeout tcp.established 86400
set timeout { adaptive.start 6000, adaptive.end 12000 }
set limit states 10000
With 9000 state table entries, the timeout values are scaled to
50% (tcp.first 60, tcp.established 43200).
QUEUEING
Packets can be assigned to queues for the purpose of bandwidth control.
At least two declarations are required to configure queues, and later any
packet filtering rule can reference the defined queues by name. During
the filtering component of pf.conf, the last referenced queue name is
where any packets from pass rules will be queued, while for block rules
it specifies where any resulting ICMP or TCP RST packets should be
queued. The scheduler defines the algorithm used to decide which packets
get delayed, dropped, or sent out immediately. There are three
schedulers currently supported:
cbq Class Based Queueing. Queues attached to an interface build a
tree, thus each queue can have further child queues. Each queue
can have a priority and a bandwidth assigned. Priority mainly
controls the time packets take to get sent out, while bandwidth has
primarily effects on throughput. cbq achieves both partitioning
and sharing of link bandwidth by hierarchically structured classes.
Each class has its own queue and is assigned its share of
bandwidth. A child class can borrow bandwidth from its parent
class as long as excess bandwidth is available (see the option
borrow, below).
hfsc Hierarchical Fair Service Curve. Queues attached to an interface
build a tree, thus each queue can have further child queues. Each
queue can have a priority and a bandwidth assigned. Priority
mainly controls the time packets take to get sent out, while
bandwidth primarily affects throughput. hfsc supports both link-
sharing and guaranteed real-time services. It employs a service
curve based QoS model, and its unique feature is an ability to
decouple delay and bandwidth allocation.
priq Priority Queueing. Queues are flat attached to the interface, thus
queues cannot have further child queues. Each queue has a unique
priority assigned, ranging from 0 to 15. Packets in the queue with
the highest priority are processed first.
The interfaces on which queueing should be activated are declared using
the altq on declaration. altq on has the following keywords:
<interface>
Queueing is enabled on the named interface.
<scheduler>
Specifies which queueing scheduler to use.
bandwidth <bw>
The maximum bitrate for all queues on an interface may be specified
using the bandwidth keyword. The value can be specified as an
absolute value or as a percentage of the interface bandwidth. When
using an absolute value, the suffixes b, Kb, Mb, and Gb are used to
represent bits, kilobits, megabits, and gigabits per second,
respectively. The value must not exceed the interface bandwidth.
If bandwidth is not specified, the interface bandwidth is used (but
take note that some interfaces do not know their bandwidth, or can
adapt their bandwidth rates).
qlimit <limit>
The maximum number of packets held in the queue. The default is
50.
tbrsize <size>
Adjusts the size, in bytes, of the token bucket regulator. If not
specified, heuristics based on the interface bandwidth are used to
determine the size.
queue <list>
Defines a list of subqueues to create on an interface.
In the following example, the interface dc0 should queue up to 5Mbps in
four second-level queues using Class Based Queueing. Those four queues
will be shown in a later example.
altq on dc0 cbq bandwidth 5Mb queue { std, http, mail, ssh }
Once interfaces are activated for queueing using the altq directive, a
sequence of queue directives may be defined. The name associated with a
queue must match a queue defined in the altq directive or, except for the
priq scheduler, in a parent queue declaration. The following keywords
can be used:
on <interface>
Specifies the interface the queue operates on. If not given, it
operates on all matching interfaces.
bandwidth <bw>
Specifies the maximum bitrate to be processed by the queue. This
value must not exceed the value of the parent queue and can be
specified as an absolute value or a percentage of the parent
queue's bandwidth. If not specified, defaults to 100% of the
parent queue's bandwidth. The priq scheduler does not support
bandwidth specification.
priority <level>
Between queues a priority level can be set. For cbq and hfsc, the
range is 0 to 7 and for priq, the range is 0 to 15. The default
for all is 1. priq queues with a higher priority are always served
first. cbq and hfsc queues with a higher priority are preferred in
the case of overload.
qlimit <limit>
The maximum number of packets held in the queue. The default is
50.
The scheduler can specify additional parameters using the format
scheduler(parameters). The parameters are:
default Packets not matched by another queue are assigned to this
one. Exactly one default queue is required.
ecn Enables Explicit Congestion Notification (ECN) on this queue.
ECN implies RED.
red Enables Random Early Detection (RED) on this queue. RED
drops packets with a probability proportional to the average
queue length.
The cbq scheduler supports an additional option:
borrow The queue can borrow bandwidth from the parent.
The hfsc scheduler supports some additional options:
linkshare <sc> The bandwidth share of a backlogged queue.
realtime <sc> The minimum required bandwidth for the queue.
upperlimit <sc> The maximum allowed bandwidth for the queue.
<sc> is an abbreviation for service curve.
The format for service curve specifications is (m1, d, m2). m2 controls
the bandwidth assigned to the queue. m1 and d are optional and can be
used to control the initial bandwidth assignment. For the first d
milliseconds the queue gets the bandwidth given as m1, afterwards the
value given in m2.
Furthermore, with cbq and hfsc, child queues can be specified as in an
altq declaration, thus building a tree of queues using a part of their
parent's bandwidth.
Packets can be assigned to queues based on filter rules by using the
queue keyword. Normally only one queue is specified; when a second one
is specified it will instead be used for packets which have a TOS of
lowdelay and for TCP ACKs with no data payload.
To continue the previous example, the examples below would specify the
four referenced queues, plus a few child queues. Interactive ssh(1)
sessions get priority over bulk transfers like scp(1) and sftp(1). The
queues may then be referenced by filtering rules (see PACKET FILTERING,
above).
queue std bandwidth 10% cbq(default)
queue http bandwidth 60% priority 2 cbq(borrow red) \
{ employees, developers }
queue developers bandwidth 75% cbq(borrow)
queue employees bandwidth 15%
queue mail bandwidth 10% priority 0 cbq(borrow ecn)
queue ssh bandwidth 20% cbq(borrow) { ssh_interactive, ssh_bulk }
queue ssh_interactive bandwidth 50% priority 7 cbq(borrow)
queue ssh_bulk bandwidth 50% priority 0 cbq(borrow)
block return out on dc0 inet all queue std
pass out on dc0 inet proto tcp from $developerhosts to any port 80 \
queue developers
pass out on dc0 inet proto tcp from $employeehosts to any port 80 \
queue employees
pass out on dc0 inet proto tcp from any to any port 22 \
queue(ssh_bulk, ssh_interactive)
pass out on dc0 inet proto tcp from any to any port 25 \
queue mail
TABLES
Tables are named structures which can hold a collection of addresses and
networks. Lookups against tables in pf(4) are relatively fast, making a
single rule with tables much more efficient, in terms of processor usage
and memory consumption, than a large number of rules which differ only in
IP address (either created explicitly or automatically by rule
expansion).
Tables can be used as the source or destination of filter or translation
rules. They can also be used for the redirect address of nat-to and
rdr-to and in the routing options of filter rules, but only for
round-robin pools.
Tables can be defined with any of the following pfctl(8) mechanisms. As
with macros, reserved words may not be used as table names.
manually Persistent tables can be manually created with the add or
replace option of pfctl(8), before or after the ruleset has
been loaded.
pf.conf Table definitions can be placed directly in this file and
loaded at the same time as other rules are loaded, atomically.
Table definitions inside pf.conf use the table statement, and
are especially useful to define non-persistent tables. The
contents of a pre-existing table defined without a list of
addresses to initialize it is not altered when pf.conf is
loaded. A table initialized with the empty list, { }, will be
cleared on load.
Tables may be defined with the following attributes:
const The const flag prevents the user from altering the contents of
the table once it has been created. Without that flag, pfctl(8)
can be used to add or remove addresses from the table at any
time, even when running with securelevel(7) = 2.
counters
The counters flag enables per-address packet and byte counters,
which can be displayed with pfctl(8).
persist The persist flag forces the kernel to keep the table even when
no rules refer to it. If the flag is not set, the kernel will
automatically remove the table when the last rule referring to
it is flushed.
This example creates a table called private, to hold RFC 1918 private
network blocks, and a table called badhosts, which is initially empty. A
filter rule is set up to block all traffic coming from addresses listed
in either table:
table <private> const { 10/8, 172.16/12, 192.168/16 }
table <badhosts> persist
block on fxp0 from { <private>, <badhosts> } to any
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:
# pfctl -t badhosts -Tadd 204.92.77.111
A table can also be initialized with an address list specified in one or
more external files, using the following syntax:
table <spam> persist file "/etc/spammers" file "/etc/openrelays"
block on fxp0 from <spam> to any
The files /etc/spammers and /etc/openrelays list IP addresses, one per
line. Any lines beginning with a `#' are treated as comments and
ignored. In addition to being specified by IP address, hosts may also be
specified by their hostname. When the resolver is called to add a
hostname to a table, all resulting IPv4 and IPv6 addresses are placed
into the table. IP addresses can also be entered in a table by
specifying a valid interface name, a valid interface group, or the self
keyword, in which case all addresses assigned to the interface(s) will be
added to the table.
ANCHORS
Besides the main ruleset, pf.conf 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 anchor rule,
pf(4) will proceed to evaluate all rules specified in that anchor.
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:
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
Anchors can be manipulated through 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:
# echo "block in quick from 1.2.3.4 to any" | pfctl -a spam -f -
The anchor can also be populated by adding a load anchor rule after the
anchor rule. When pfctl(8) loads pf.conf, it will also load all the
rules from the file /etc/pf-spam.conf into the anchor.
anchor spam
load anchor spam from "/etc/pf-spam.conf"
Filter rule anchors can also be loaded inline in the ruleset within a
brace-delimited block. Brace delimited blocks may contain rules or other
brace-delimited blocks. When anchors are loaded this way the anchor name
becomes optional. Since the parser specification for anchor names is a
string, double quote characters (`"') should be placed around the anchor
name.
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
}
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:
block on $ext_if all
anchor spam proto tcp from any to any port smtp
pass out on $ext_if all
pass in on $ext_if proto tcp from any to $ext_if port smtp
The rules inside anchor "spam" are only evaluated for TCP packets with
destination port 25. Hence, the following will only block connections
from 1.2.3.4 to port 25:
# echo "block in quick from 1.2.3.4 to any" | pfctl -a spam -f -
Matching filter and translation rules marked with the quick option are
final and abort the evaluation of the rules in other anchors and the main
ruleset. If the anchor itself is marked with the quick option, ruleset
evaluation will terminate when the anchor is exited if the packet is
matched by any rule within the anchor.
An anchor references other anchor attachment points using the following
syntax:
anchor <name>
Evaluates the filter rules in the specified anchor.
An anchor has a name which specifies the path where pfctl(8) can be used
to access the anchor to perform operations on it, such as attaching child
anchors to it or loading rules into it. Anchors may be nested, with
components separated by `/' characters, similar to how file system
hierarchies are laid out. The main ruleset is actually the default
anchor, so filter and translation rules, for example, may also be
contained in any anchor.
Anchor rules are evaluated relative to the anchor in which they are
contained. For example, all anchor rules specified in the main ruleset
will reference anchor attachment points underneath the main ruleset, and
anchor rules specified in a file loaded from a load anchor rule will be
attached under that anchor point.
Anchors may end with the asterisk (`*') character, which signifies that
all anchors attached at that point should be evaluated in the
alphabetical ordering of their anchor name. For example, the following
will evaluate each rule in each anchor attached to the "spam" anchor:
anchor "spam/*"
Note that it will only evaluate anchors that are directly attached to the
"spam" anchor, and will not descend to evaluate anchors recursively.
Since anchors are evaluated relative to the anchor in which they are
contained, there is a mechanism for accessing the parent and ancestor
anchors of a given anchor. Similar to file system path name resolution,
if the sequence `..' appears as an anchor path component, the parent
anchor of the current anchor in the path evaluation at that point will
become the new current anchor. As an example, consider the following:
# printf 'anchor "spam/allowed"\n' | pfctl -f -
# printf 'anchor "../banned"\npass\n' | pfctl -a spam/allowed -f -
Evaluation of the main ruleset will lead into the spam/allowed anchor,
which will evaluate the rules in the spam/banned anchor, if any, before
finally evaluating the pass rule.
STATEFUL FILTERINGpf(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 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, pf(4) knows how to match ICMP replies to states. For example,
to allow echo requests (such as those created by ping(8)) out statefully
and match incoming echo replies correctly to states:
pass out inet proto icmp all icmp-type echoreq
Also, looking up states is usually faster than evaluating rules. If
there are 50 rules, all of them are evaluated sequentially in O(n). Even
with 50000 states, only 16 comparisons are needed to match a state, since
states are stored in a binary search tree that allows searches in O(log2
n).
Furthermore, correct handling of ICMP error messages is critical to many
protocols, particularly TCP. pf(4) matches ICMP error messages to the
correct connection, checks them against connection parameters, and passes
them if appropriate. For example if an ICMP source quench message
referring to a stateful TCP connection arrives, it will be matched to the
state and get passed.
Finally, state tracking is required for nat-to and rdr-to options, in
order to track address and port translations and reverse the translation
on returning packets.
pf(4) will also create state for other protocols which are effectively
stateless by nature. UDP packets are matched to states using only host
addresses and ports, and other protocols are matched to states using only
the host addresses.
If stateless filtering of individual packets is desired, the no state
keyword can be used to specify that state will not be created if this is
the last matching rule. Note that packets which match neither block nor
pass rules, and thus are passed by default, are effectively passed as if
no state had been specified.
A number of parameters can also be set to affect how pf(4) handles state
tracking, as detailed below.
State Modulation
Much of the security derived from TCP is attributable to how well the
initial sequence numbers (ISNs) are chosen. Some popular stack
implementations choose very poor ISNs and thus are normally susceptible
to ISN prediction exploits. By applying a modulate state rule to a TCP
connection, pf(4) will create a high quality random sequence number for
each connection endpoint.
The modulate state directive implicitly keeps state on the rule and is
only applicable to TCP connections.
For instance:
block all
pass out proto tcp from any to any modulate state
pass in proto tcp from any to any port 25 flags S/SFRA \
modulate state
Note that modulated connections will not recover when the state table is
lost (firewall reboot, flushing the state table, etc.). pf(4) will not
be able to infer a connection again after the state table flushes the
connection's modulator. When the state is lost, the connection may be
left dangling until the respective endpoints time out the connection. It
is possible on a fast local network for the endpoints to start an ACK
storm while trying to resynchronize after the loss of the modulator. The
default flags settings (or a more strict equivalent) should be used on
modulate state rules to prevent ACK storms.
Note that alternative methods are available to prevent loss of the state
table and allow for firewall failover. See carp(4) and pfsync(4) for
further information.
SYN Proxy
By default, pf(4) passes packets that are part of a TCP handshake between
the endpoints. The synproxy state option can be used to cause pf(4)
itself to complete the handshake with the active endpoint, perform a
handshake with the passive endpoint, and then forward packets between the
endpoints.
No packets are sent to the passive endpoint before the active endpoint
has completed the handshake, hence so-called SYN floods with spoofed
source addresses will not reach the passive endpoint, as the sender can't
complete the handshake.
The proxy is transparent to both endpoints; they each see a single
connection from/to the other endpoint. pf(4) chooses random initial
sequence numbers for both handshakes. Once the handshakes are completed,
the sequence number modulators (see previous section) are used to
translate further packets of the connection. synproxy state includes
modulate state.
Rules with synproxy will not work if pf(4) operates on a bridge(4).
Example:
pass in proto tcp from any to any port www synproxy state
Stateful Tracking Options
A number of options related to stateful tracking can be applied on a per-
rule basis. One of keep state, modulate state, or synproxy state must be
specified explicitly to apply these options to a rule.
floating
States can match packets on any interfaces (the opposite of
if-bound). This is the default.
if-bound
States are bound to an interface (the opposite of floating).
max <number>
Limits the number of concurrent states the rule may create. When
this limit is reached, further packets that would create state will
not match this rule until existing states time out.
no-sync
Prevent state changes for states created by this rule from
appearing on the pfsync(4) interface.
pflow
States created by this rule are exported on the pflow(4) interface.
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 modulate or synproxy state.
<timeout> <seconds>
Changes the timeout values used for states created by this rule.
For a list of all valid timeout names, see OPTIONS above.
Multiple options can be specified, separated by commas:
pass in proto tcp from any to any \
port www keep state \
(max 100, source-track rule, max-src-nodes 75, \
max-src-states 3, tcp.established 60, tcp.closing 5)
When the source-track keyword is specified, the number of states per
source IP is tracked.
source-track global
The number of states created by all rules that use this option is
limited. Each rule can specify different max-src-nodes and
max-src-states options, however state entries created by any
participating rule count towards each individual rule's limits.
source-track rule
The maximum number of states created by this rule is limited by the
rule's max-src-nodes and max-src-states options. Only state
entries created by this particular rule count toward the rule's
limits.
The following limits can be set:
max-src-nodes <number>
Limits the maximum number of source addresses which can
simultaneously have state table entries.
max-src-states <number>
Limits the maximum number of simultaneous state entries that a
single source address can create with this rule.
For stateful TCP connections, limits on established connections
(connections which have completed the TCP 3-way handshake) can also be
enforced per source IP.
max-src-conn <number>
Limits the maximum number of simultaneous TCP connections which
have completed the 3-way handshake that a single host can make.
max-src-conn-rate <number> / <seconds>
Limit the rate of new connections over a time interval. The
connection rate is an approximation calculated as a moving average.
Because the 3-way handshake ensures that the source address is not being
spoofed, more aggressive action can be taken based on these limits. With
the overload <table> state option, source IP addresses which hit either
of the limits on established connections will be added to the named
table. This table can be used in the ruleset to block further activity
from the offending host, redirect it to a tarpit process, or restrict its
bandwidth.
The optional flush keyword kills all states created by the matching rule
which originate from the host which exceeds these limits. The global
modifier to the flush command kills all states originating from the
offending host, regardless of which rule created the state.
For example, the following rules will protect the webserver against hosts
making more than 100 connections in 10 seconds. Any host which connects
faster than this rate will have its address added to the <bad_hosts>
table and have all states originating from it flushed. Any new packets
arriving from this host will be dropped unconditionally by the block
rule.
block quick from <bad_hosts>
pass in on $ext_if proto tcp to $webserver port www keep state \
(max-src-conn-rate 100/10, overload <bad_hosts> flush global)
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.
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 scrub option, added to regular rules.
Parameters are specified enclosed in parentheses. At least one of the
following parameters must be specified:
max-mss <number>
Enforces a maximum segment size (MSS) for matching TCP packets.
min-ttl <number>
Enforces a minimum TTL for matching IP packets.
no-df
Clears the dont-fragment bit from a matching IPv4 packet. Some
operating systems have NFS implementations which are known to
generate fragmented packets with the dont-fragment bit set. pf(4)
will drop such fragmented dont-fragment packets unless no-df is
specified.
Unfortunately some operating systems also generate their
dont-fragment packets with a zero IP identification field.
Clearing the dont-fragment bit on packets with a zero IP ID may
cause deleterious results if an upstream router later fragments the
packet. Using random-id is recommended in combination with no-df
to ensure unique IP identifiers.
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.
reassemble tcp
Statefully normalises TCP connections. reassemble tcp performs the
following normalisations:
TTL
Neither side of the connection is allowed to reduce their IP TTL.
An attacker may send a packet such that it reaches the firewall,
affects the firewall state, and expires before reaching the
destination host. reassemble tcp will raise the TTL of all packets
back up to the highest value seen on the connection.
Timestamp Modulation
Modern TCP stacks will send a timestamp on every TCP packet and
echo the other endpoint's timestamp back to them. Many operating
systems will merely start the timestamp at zero when first booted,
and increment it several times a second. The uptime of the host
can be deduced by reading the timestamp and multiplying by a
constant. Also observing several different timestamps can be used
to count hosts behind a NAT device. And spoofing TCP packets into
a connection requires knowing or guessing valid timestamps.
Timestamps merely need to be monotonically increasing and not
derived off a guessable base time. reassemble tcp will cause scrub
to modulate the TCP timestamps with a random number.
Extended PAWS Checks
There is a problem with TCP on long fat pipes, in that a packet
might get delayed for longer than it takes the connection to wrap
its 32-bit sequence space. In such an occurrence, the old packet
would be indistinguishable from a new packet and would be accepted
as such. The solution to this is called PAWS: Protection Against
Wrapped Sequence numbers. It protects against it by making sure
the timestamp on each packet does not go backwards. reassemble tcp
also makes sure the timestamp on the packet does not go forward
more than the RFC allows. By doing this, pf(4) artificially
extends the security of TCP sequence numbers by 10 to 18 bits when
the host uses appropriately randomized timestamps, since a blind
attacker would have to guess the timestamp as well.
set-tos <string> | <number>
Enforces a TOS for matching IPv4 packets. string may be one of
lowdelay, throughput, or reliability; number may be either a hex or
decimal number.
For example:
match in all scrub (no-df max-mss 1440)
Fragment Handling
The size of IP datagrams (packets) can be significantly larger than the
maximum transmission unit (MTU) of the network. In cases when it is
necessary or more efficient to send such large packets, the large packet
will be fragmented into many smaller packets that will each fit onto the
wire. Unfortunately for a firewalling device, only the first logical
fragment will contain the necessary header information for the
subprotocol that allows pf(4) to filter on things such as TCP ports or to
perform NAT.
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
fragment option can be used to restrict filter rules to apply only to
fragments, but not complete packets. Filter rules without the fragment
option still apply to fragments, if they only specify IP header fields.
For instance:
pass in proto tcp from any to any port 80
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.
In most cases, the benefits of reassembly outweigh the additional memory
cost, so reassembly is on by default.
The memory allocated for fragment caching can be limited using pfctl(8).
Once this limit is reached, fragments that would have to be cached are
dropped until other entries time out. The timeout value can also be
adjusted.
Currently, only IPv4 fragments are supported and IPv6 fragments are
blocked unconditionally.
Blocking Spoofed Traffic
Spoofing is the faking of IP addresses, typically for malicious purposes.
The antispoof directive expands to a set of filter rules which will block
all traffic with a source IP from the network(s) directly connected to
the specified interface(s) from entering the system through any other
interface.
For example:
antispoof for lo0
Expands to:
block drop in on ! lo0 inet from 127.0.0.1/8 to any
block drop in on ! lo0 inet6 from ::1 to any
For non-loopback interfaces, there are additional rules to block incoming
packets with a source IP address identical to the interface's IP(s). For
example, assuming the interface wi0 had an IP address of 10.0.0.1 and a
netmask of 255.255.255.0:
antispoof for wi0 inet
Expands to:
block drop in on ! wi0 inet from 10.0.0.0/24 to any
block drop in inet from 10.0.0.1 to any
Caveat: Rules created by the antispoof directive interfere with packets
sent over loopback interfaces to local addresses. One should pass these
explicitly.
OPERATING SYSTEM FINGERPRINTING
Passive OS fingerprinting is a mechanism to inspect nuances of a TCP
connection's initial SYN packet and guess at the host's operating system.
Unfortunately these nuances are easily spoofed by an attacker so the
fingerprint is not useful in making security decisions. But the
fingerprint is typically accurate enough to make policy decisions upon.
The fingerprints may be specified by operating system class, by version,
or by subtype/patchlevel. The class of an operating system is typically
the vendor or genre and would be OpenBSD for the pf(4) firewall itself.
The version of the oldest available OpenBSD release on the main FTP site
would be 2.6 and the fingerprint would be written as:
"OpenBSD 2.6"
The subtype of an operating system is typically used to describe the
patchlevel if that patch led to changes in the TCP stack behavior. In
the case of OpenBSD, the only subtype is for a fingerprint that was
normalised by the no-df scrub option and would be specified as:
"OpenBSD 3.3 no-df"
Fingerprints for most popular operating systems are provided by pf.os(5).
Once pf(4) is running, a complete list of known operating system
fingerprints may be listed by running:
# pfctl -so
Filter rules can enforce policy at any level of operating system
specification assuming a fingerprint is present. Policy could limit
traffic to approved operating systems or even ban traffic from hosts that
aren't at the latest service pack.
The unknown class can also be used as the fingerprint which will match
packets for which no operating system fingerprint is known.
Examples:
pass out proto tcp from any os OpenBSD
block out proto tcp from any os Doors
block out proto tcp from any os "Doors PT"
block out proto tcp from any os "Doors PT SP3"
block out from any os "unknown"
pass on lo0 proto tcp from any os "OpenBSD 3.3 lo0"
Operating system fingerprinting is limited only to the TCP SYN packet.
This means that it will not work on other protocols and will not match a
currently established connection.
Caveat: operating system fingerprints are occasionally wrong. There are
three problems: an attacker can trivially craft his packets to appear as
any operating system he chooses; an operating system patch could change
the stack behavior and no fingerprints will match it until the database
is updated; and multiple operating systems may have the same fingerprint.
EXAMPLES
In this example, the external interface is 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.
ext_if = "kue0"
match in all scrub (no-df max-mss 1440)
block return log on $ext_if all
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.
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, \
192.168.0.0/16, 255.255.255.255/32 } to any
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.
pass on $ext_if inet proto icmp all icmp-type 8 code 0
For UDP, pass out all UDP connections. DNS connections are passed in.
pass out on $ext_if proto udp all
pass in on $ext_if proto udp from any to any port domain
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.
pass out on $ext_if proto tcp all modulate state
pass in on $ext_if proto tcp from any to any \
port { ssh, smtp, domain, auth }
block in on $ext_if proto tcp from any \
os { "Windows 95", "Windows 98" } to any port smtp
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.
pass quick on gif0 inet6
pass quick on $ext_if proto ipv6
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.
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
In this example, we tag incoming packets as they are redirected to
spamd(8). The tag is used to pass those packets through the packet
filter.
match in on $ext_if inet proto tcp from <spammers> to port smtp \
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
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).
match in on $ext_if proto tcp from any to any port 80 \
rdr-to 127.0.0.1 port 8080
If a pass rule is used with the quick modifier, packets matching the
translation rule are passed without inspecting subsequent filter rules.
pass in quick on $ext_if proto tcp from any to any port 80 \
rdr-to 127.0.0.1 port 8080
In the example below, vlan12 is configured as 192.168.168.1; the machine
translates all packets coming from 192.168.168.0/24 to 204.92.77.111 when
they are going out any interface except vlan12. This has the net effect
of making traffic from the 192.168.168.0/24 network appear as though it
is the Internet routable address 204.92.77.111 to nodes behind any
interface on the router except for the nodes on vlan12. Thus,
192.168.168.1 can talk to the 192.168.168.0/24 nodes.
match out on ! vlan12 from 192.168.168.0/24 to any nat-to 204.92.77.111
In the example below, the machine sits between a fake internal
144.19.74.* network, and a routable external IP of 204.92.77.100. The
last rule excludes protocol AH from being translated.
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
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.
pass in on $int_if proto { tcp, udp } from any to any port 80 \
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
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.
match out on $ext_if inet proto udp from any port isakmp to any \
nat-to ($ext_if) port 500
One more example uses rdr-to to redirect a TCP and UDP port to an
internal machine.
match in on $ext_if inet proto tcp from any to ($ext_if) port 8080 \
rdr-to 10.1.2.151 port 22
match in on $ext_if inet proto udp from any to ($ext_if) port 8080 \
rdr-to 10.1.2.151 port 53
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 source-hash
keyword. The gateway also translates incoming web server connections to
a group of web servers on the internal network.
match out on $ext_if inet from any to any nat-to 192.0.2.16/28 \
source-hash
match in on $ext_if proto tcp from any to any port 80 \
rdr-to { 10.1.2.155, 10.1.2.160, 10.1.2.161 } round-robin
The bidirectional address translation example uses a single binat-to rule
that expands to a nat-to and an rdr-to rule.
pass on $ext_if from 10.1.2.120 to any binat-to 192.0.2.17
The previous example is identical to the following set of rules:
pass out on $ext_if inet from 10.1.2.120 to any \
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
GRAMMAR
Syntax for pf.conf in BNF:
line = ( option | pf-rule |
antispoof-rule | altq-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 ]
[ "require-order" ( "yes" | "no" ) ]
[ "fingerprints" filename ] |
[ "skip on" ifspec ] |
[ "debug" ( "none" | "urgent" | "misc" | "loud" ) ] |
[ "reassemble" ( "yes" | "no" ) [ "no-df" ] ] )
pf-rule = action [ ( "in" | "out" ) ]
[ "log" [ "(" logopts ")"] ] [ "quick" ]
[ "on" ifspec ] [ 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" |
"divert-packet" "port" port | "divert-reply" |
"divert-to" host "port" port |
"label" string | "tag" string | [ ! ] "tagged" string |
"queue" ( string | "(" string [ [ "," ] string ] ")" ) |
"rtable" number | "probability" number"%" |
"binat-to" ( redirhost | "{" redirhost-list "}" )
[ portspec ] [ pooltype ] |
"rdr-to" ( redirhost | "{" redirhost-list "}" )
[ portspec ] [ pooltype ] |
"nat-to" ( redirhost | "{" redirhost-list "}" )
[ portspec ] [ pooltype ] [ "static-port" ] |
[ "fastroute" | route ] |
[ "received-on" ( interface-name | interface-group ) ]
scrubopts = scrubopt [ [ "," ] scrubopts ]
scrubopt = "no-df" | "min-ttl" number | "max-mss" number |
"set-tos" tos | "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
altq-rule = "altq on" interface-name queueopts-list
"queue" subqueue
queue-rule = "queue" string [ "on" interface-name ] queueopts-list
subqueue
anchor-rule = "anchor" [ string ] [ ( "in" | "out" ) ] [ "on" ifspec ]
[ af ] [ protospec ] [ hosts ] [ filteropt-list ] [ "{" ]
anchor-close = "}"
load-anchor = "load anchor" string "from" filename
queueopts-list = queueopts-list queueopts | queueopts
queueopts = [ "bandwidth" bandwidth-spec ] |
[ "qlimit" number ] | [ "tbrsize" number ] |
[ "priority" number ] | [ schedulers ]
schedulers = ( cbq-def | priq-def | hfsc-def )
bandwidth-spec = "number" ( "b" | "Kb" | "Mb" | "Gb" | "%" )
action = "pass" | "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" )
( routehost | "{" routehost-list "}" )
[ pooltype ]
af = "inet" | "inet6"
protospec = "proto" ( proto-name | proto-number |
"{" proto-list "}" )
proto-list = ( proto-name | proto-number ) [ [ "," ] proto-list ]
hosts = "all" |
"from" ( "any" | "no-route" | "urpf-failed" | "self" |
host | "{" host-list "}" | "route" string ) [ port ]
[ os ]
"to" ( "any" | "no-route" | "self" | host |
"{" host-list "}" | "route" string ) [ port ]
ipspec = "any" | host | "{" host-list "}"
host = [ "!" ] ( address [ "/" mask-bits ] | "<" string ">" )
redirhost = address [ "/" mask-bits ]
routehost = host | host "@" interface-name |
"(" interface-name [ address [ "/" mask-bits ] ] ")"
address = ( interface-name | interface-group |
"(" ( interface-name | interface-group ) ")" |
hostname | ipv4-dotted-quad | ipv6-coloned-hex )
host-list = host [ [ "," ] host-list ]
redirhost-list = redirhost [ [ "," ] redirhost-list ]
routehost-list = routehost [ [ "," ] routehost-list ]
port = "port" ( unary-op | binary-op | "{" op-list "}" )
portspec = "port" ( number | name ) [ ":" ( "*" | number | name ) ]
os = "os" ( os-name | "{" os-list "}" )
user = "user" ( unary-op | binary-op | "{" op-list "}" )
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" ] |
"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" ) number
pooltype = ( "bitmask" | "random" |
"source-hash" [ ( hex-key | string-key ) ] |
"round-robin" ) [ sticky-address ]
subqueue = string | "{" queue-list "}"
queue-list = string [ [ "," ] string ]
cbq-def = "cbq" [ "(" cbq-opt [ [ "," ] cbq-opt ] ")" ]
priq-def = "priq" [ "(" priq-opt [ [ "," ] priq-opt ] ")" ]
hfsc-def = "hfsc" [ "(" hfsc-opt [ [ "," ] hfsc-opt ] ")" ]
cbq-opt = ( "default" | "borrow" | "red" | "ecn" | "rio" )
priq-opt = ( "default" | "red" | "ecn" | "rio" )
hfsc-opt = ( "default" | "red" | "ecn" | "rio" |
linkshare-sc | realtime-sc | upperlimit-sc )
linkshare-sc = "linkshare" sc-spec
realtime-sc = "realtime" sc-spec
upperlimit-sc = "upperlimit" sc-spec
sc-spec = ( bandwidth-spec |
"(" bandwidth-spec number bandwidth-spec ")" )
include = "include" filename
FILES
/etc/hosts Host name database.
/etc/pf.conf Default location of the ruleset file.
/etc/pf.os Default location of OS fingerprints.
/etc/protocols Protocol name database.
/etc/services Service name database.
SEE ALSOpf(4), pflow(4), pfsync(4), pf.os(5), pfctl(8), pflogd(8)HISTORY
The pf.conf file format first appeared in OpenBSD 3.0.
OpenBSD 4.9 February 1, 2011 OpenBSD 4.9
