path: root/share/man/man5/pf.conf.5

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.Dd July 2, 2002
.Dt PF.CONF 5
.Os
.Sh NAME
.Nm pf.conf
.Nd filtering and translation (NAT) rules file for the
packet filter
.Sh DESCRIPTION
The
.Xr pf 4
packet filter drops, passes and modifies packets according to the
rules defined in this file.
Filter rules are used to selectively pass traffic while translation
rules specify which addresses are to be mapped and which are to be
redirected.
For each packet inspected by the filter, the set of rules is evaluated
from top to bottom, and the last matching rule decides what action is
performed.
For each packet inspected by the translator, the set of rules is evaluated
from top to bottom, and the first matching rule decides what action is
performed.
In short: filters are last match, nat is first match.
Rules must be in order: scrub, nat, filter.

.Sh GRAMMAR
Syntax for filter rules in BNF:
.Bd -literal
option         = set ( [ "timeout" ( timeout | "{" timeout-list "}" ) ] |
                       [ "optimization" [ "default" | "normal" | 
				"high-latency" | "satellite" | 
				"aggressive" | "conservative" ] ]
                       [ "limit" ( limit | "{" limit-list "}" ) ] |
		       [ "loginterface" interface-name ] ) .
rule           = action ( "in" | "out" )
                 [ "log" | "log-all" ] [ "quick" ]
                 [ "on" ( interface-name | "{" interface-list "}" ) ]
                 [ route ] [ af ]
                 [ "proto" ( proto-name | proto-number |
                             "{" proto-list "}" ) ]
                 hosts
                 [ user ] [ group ] [ flags ]
                 [ icmp-type | ipv6-icmp-type ]
                 [ ( "keep" | "modulate" ) "state" [ "(" state-opts ")" ] ]
                 [ "fragment" ] [ "no-df" ] [ "min-ttl" number ]
                 [ "max-mss" number ] [ "allow-opts" ]
                 [ "label" string ] .

action         = "pass" | "block" [ return ] | "scrub" .
return         = "return-rst" [ "(" "ttl" number ")" ] |
                 "return-icmp"
                     [ "(" ( icmp-code-name | icmp-code-number ) ")" ] |
                 "return-icmp6"
                     [ "(" ( icmp-code-name | icmp-code-number ) ")" ] .

interface-list = interface-name [ "," interface-list ] .
route          = "fastroute" |
                 "route-to" "(" interface-name address ")" |
                 "dup-to" interface-name
                 "route-to" "(" interface-name address ")" |
                 "dup-to" interface-name
af	       = "inet" | "inet6" .
proto-list     = ( proto-name | proto-number ) [ "," proto-list ] .

hosts          = "all" |
                 "from" ( "any" | "no-route" | host | "{" host-list "}" )
                     [ port ]
                 "to"   ( "any" | "no-route" | host | "{" host-list "}" )
                     [ port ] .

host           = [ "!" ] address [ "/" mask-bits ] .
address        = ( interface-name | "(" interface-name ")" | host-name |
                   ipv4-dotted-quad | ipv6-coloned-hex ) .
host-list      = host [ "," host-list ] .

port           = "port" ( unary-op | binary-op | "{" op-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 ] .

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

icmp-type      = "icmp-type" ( icmp-type-code | "{" icmp-list "}" ) . 
ipv6-icmp-type = "ipv6-icmp-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 ] . 

state-opts     = state-opt [ "," state-opts ] .
state-opt      = ( "max" seconds ) | ( timeout seconds ) .

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.multiple" ) seconds .
seconds        = number .

limit-list     = limit [ "," limit-list ] . 
limit          = ( "states" | "frags" ) number .
.Ed
.Pp
Syntax for translation rules in BNF:
.Bd -literal
rule      = [ "no" ] ( nat_rule | binat_rule | rdr_rule ) .

nat_rule  = "nat" "on" [ "!" ] ifname [ protospec ] hosts
            [ "->" address [ portspec ] ] .

binat_rule = "binat" "on" ifname [ protospec ] "from" address
             "to" ipspec [ "->" address ] .

rdr_rule  = "rdr" "on" [ "!" ] ifname [ protospec ] "from" ipspec
            "to" ipspec [ portspec ] [ "->" address [ portspec ] ] .

protospec = "proto" ( number | "tcp" | "udp" | "icmp" ) .

ipspec    = "any" | host | "{" host-list "}" .

portspec  = "port" ( number | name ) [ ":" ( "*" | number | name ) ] .

hosts     = "all" |
            "from" ( "any" | host | "{" host-list "}" ) [ port ]
            "to"   ( "any" | host | "{" host-list "}" ) [ port ] .

host      = [ "!" ] address [ "/" mask-bits ] .
address   = ( interface-name | "(" interface-name ")" | host-name |
              ipv4-dotted-quad | ipv6-coloned-hex ) .
host-list = host [ "," host-list ] .

port      = "port" ( unary-op | binary-op | "{" op-list "}" ) .
unary-op  = [ "=" | "!=" | "<" | "<=" | ">" | ">=" ]
            ( name | number ) .
binary-op = number ( "<>" | "><" ) number .
op-list   = ( unary-op | binary-op ) [ "," op-list ] .
.Ed
.Sh FILTER RULES
While filter rules are typically manipulated using
.Xr pfctl 8
other utilities may be written using the
.Xr ioctl 2
interface described in
.Xr pf 4 .
Filter and NAT rules are loaded from a text file into the kernel using
.Pp
.Cm # pfctl -f file
.Pp
which replaces the active rule set with the new one.
To load only the filter rules from a file, one would use the command
.Pp
.Cm # pfctl -R -f file
.Pp
To load only the NAT rules from a file, one would use the command
.Pp
.Cm # pfctl -N -f file
.Pp
To load only the options from a file, one would use the command
.Pp
.Cm # pfctl -O -f file
.Pp
The active filter rule set can be displayed using
.Pp
.Cm # pfctl -s r
.Pp
The active translation rule set can be displayed using
.Pp
.Cm # pfctl -s n
.Pp
The active options can be displayed using pfctl as well:
.Pp
.Cm # pfctl -s t
.Pp
shows the current timeouts.
.Pp
.Cm # pfctl -s m
.Pp
shows the current limits.
.Pp
For each packet processed by the packet filter, the filter rules are
evaluated in sequential order, from first to last.
Each rule either matches the packet or doesn't.
The last matching rule decides what action is taken.
.Pp
If no rule matches the packet, the default action is
.Em pass .
.Pp
To block everything by default and only pass packets
that match explicit rules, one uses
.Bd -literal
.Cm block in all
.Cm block out all
.Ed
.Pp
as the first two rules.
.Pp
For each packet processed by the translator, the translation rules are
evaluated in sequential order, from first to last.
Each rule either matches the packet or doesn't.
The first matching rule decides what action is taken.
.Pp
If no rule matches the packet, the default action is to pass the packet
up to the filter unmodified.
It should be noted that all translations of packets occur before
the filters are applied.
Hence, rules for redirected packets should specify the address and port
after translation.
Note that all translation rules apply only to packets that pass through
the specified interface.
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 can't 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.
.Sh OPTIONS
.Ss timeout
.Bl -tag -width interval -compact
.It Em interval
Interval between purging expired states and fragments.
.It Em frag
Seconds before an unassembled fragment is expired.
.El
.Pp
When a packet matches a stateful connection, the seconds to live of the
connection will be updated to that of the proto.modifier which corresponds
to the connection state.
Each packet which matches this state will reset the TTL.
Tuning these values may improve the performance of the
firewall at the risk of dropping valid idled connections.
.Pp
.Bl -tag -width "tcp.established " -compact
.It Em tcp.first
The state after the first packet.
.It Em tcp.opening
The state before the destination host ever sends a packet.
.It Em tcp.established
The fully established state.
.It Em tcp.closing
The state after the first FIN has been sent.
.It Em 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.
.It Em tcp.closed
The state after one endpoint sends a RST.
.El
.Pp
ICMP and UDP are handled in a similar fashion to TCP but with a much more
limited set of states:
.Pp
.Bl -tag -width "udp.multiple " -compact
.It Em udp.first
The state after the first packet.
.It Em udp.single
The state if the source host sends more than one packet but the destination
host has never sent one back.
.It Em udp.multiple
The state if both hosts have sent packets.
.It Em icmp.first
The state after the first packet.
.It Em icmp.error
The state after an icmp error came back in response to an icmp packet.
.El
.Pp
Other protocols are handled similarly to UDP:
.Pp
.Bl -tag -width "other.multiple " -compact
.It Em other.first
.It Em other.single
.It Em other.multiple
.El
.Pp
Example:
.Bd -literal
    set timeout tcp.established 3600
    set timeout { tcp.opening 30, tcp.closing 900 }
.Ed
.Ss loginterface
Enable collection of packet and byte count statistics for the given interface.
These statistics can be viewed using
.Bd -literal
    # pfctl -s info
.Ed
.Pp
In this example pf is told to collect statistics on the interface named dc0:
.Bd -literal
    set loginterface dc0
.Ed
.Pp
One can unset the loginterface using
.Bd -literal
    set loginterface none
.Ed
.Pp
.Ss limit
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,
.Bd -literal
    set limit states 20000
.Ed
.Pp
sets the maximum number of entries in the memory pool used by state table
entries (generated by 'keep state' rules) to 20000.
.Bd -literal
    set limit frags 20000
.Ed
.Pp
set the maximum number of entries in the memory pool used for fragment
reassemble (generated by 'scrub' rules) to 20000.
.Pp
These can be combined:
.Bd -literal
    set limit { states 20000, frags 20000 }
.Ed
.Ss optimization
Optimize the engine to one of the following network topographies or
environments:
.Bl -tag -width "O high-latency " -compact
.It Em default
A normal network environment.
Suitable for almost all networks.
.It Em normal
Alias for
.Em default
.It Em high-latency
A high-latency environment (such as a satellite connection)
.It Em satellite
Alias for
.Em high-latency
.It Em aggressive
Aggressively expire connections when they are likely no longer valid.
This can greatly reduce the memory usage of the firewall at the cost of
dropping idle connections early.
.It Em conservative
Extremely conservative settings.
Pains will be taken to avoid dropping legitimate connections at the
expense of greater memory utilization (possibly much greater on a busy
network) and slightly increased processor utilization.
.El
Example:
.Bd -literal
    set optimization aggressive
.Ed
.Sh ACTIONS
.Bl -tag -width Fl
.It Em block
The packet is blocked.
Optionally, the filter can return a TCP RST or ICMP UNREACHABLE packet
to the sender, where applicable.
Returning ICMP packets can have
an ICMP code set by number or name, TCP RST can have a TTL set.
.It Em pass
The packet is passed.
.It Em scrub
The packet is run through normalization/defragmentation.
Scrub rules are not considered last matching rules.
IPv6 packets are not defragmented.
.It Em binat
A
.Em binat
rule specifies a bidirectional mapping between an external IP address
and an internal IP address.
.It Em nat
A
.Em nat
rule specifies that IP addresses are to be changed as the packet
traverses the given interface.
This technique allows a single IP address
on the translating host to support network traffic for a larger range of
machines on an "inside" network.
Although in theory any IP address can be used on the inside, it is strongly
recommended that one of the address ranges defined by RFC 1918 be used.
These netblocks are:
.Bd -literal
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)
.Ed
.It Em rdr
The packet is redirected to another destination and possibly a
different port.
.Em rdr
rules can optionally specify port ranges instead of single ports.
\'rdr ... port 2000:2999 -> ... port 4000\' redirects ports 2000 to 2999
(including port 2000 and 2999) to the same port 4000.
\'rdr ... port 2000:2999 -> ... port 4000:*\' redirects port 2000 to 4000,
2001 to 4001, ..., 2999 to 4999.
.El
.Sh LOGGING
.Bl -tag -width Fl
.It Em log
In addition to the action specified, a log message is generated.
.It Em log-all
Used with 
.Sq keep state
or
.Sq modulate state
rules.
Not only the packet that creates state is logged, but all packets of
the connection.
.El
.Pp
The logged packets are sent to the
.Em pflog0
interface.
This interface is monitored by the
.Xr pflogd 8
logging daemon which dumps the logged packets to the file
.Em /var/log/pflog
in
.Xr tcpdump 8
binary format.
The log files can be read using tcpdump:
.Bd -literal
.Cm # tcpdump -n -e -ttt -r /var/log/pflog
.Ed
.Sh QUICK
If a packet matches a rule which has the 
.Sq quick
option set, this rule
is considered the last matching rule, and evaluation of subsequent rules
is skipped.
.Sh NO
The
.Sq no
option is to a NAT rule what the
.Sq quick
option is to a filter rule.
This option causes matching packets to remain untranslated.
.Sh ROUTING
If a packet matches a rule with a route option set, the packet filter will
route the packet according to the type of route option.
.Ss fastroute
The
.Em fastroute
option does a normal route lookup to find the next hop for the packet.
.Ss route-to
The
.Em route-to
option routes the packet to the specified interface with an optional address
for the next hop.
.Ss dup-to
The
.Em dup-to
option creates a duplicate of the packet and routes it like
.Em route-to.
The original packet gets routed as it normally would.
.Sh PARAMETERS
The rule parameters specify for what packets a rule applies.
A packet always comes in on or goes out through one interface.
Most parameters are optional.
If a parameter is specified, the rule only applies to packets with
matching attributes.
Certain parameters can be expressed as lists, in which case
.Em pfctl
generates all needed rule combinations.
.Ss in or out
The rule applies to incoming or outgoing packets.
Either
.Em in
or
.Em out
must be specified.
To cover both directions, two rules are needed.
.Ss on <interface>
The rule applies only to packets coming in on or going out through this
particular interface.
.Ss <af>
The rule applies only to packets of this address family.
Supported values are inet and inet6.
.Ss proto <protocol>
The rule applies only to packets of this protocol.
Common protocols used here are tcp, udp, icmp and ipv6-icmp.
.Ss from <source> port <source> to <dest> port <dest>
The rule applies only to packets with the specified source and destination
addresses/ports.
.Pp
Addresses can be specified in CIDR notation (matching netblocks), as
symbolic host names or interface names, or as any of the following keywords:
.Bl -tag -width no-route -compact
.It Em any
means any address;
.It Em no-route
means any address which is not currently routable.
.El
.Pp
Host name resolution and interface to address translation are done at
rule set load-time. 
When the address of an interface (or host name) changes (by DHCP or PPP,
for instance), the rule set must be reloaded for the change to be reflected
in the kernel.
Interface names surrounded by parentheses cause an automatic update of
the rule whenever the referenced interface changes its address.
Reloading the rule set is not required in this case.
.Pp
Ports can be specified using these operators
.Bd -literal
    = (equal), != (unequal), < (lesser), <= (lesser or equal), > (greater),
    >= (greater or equal), >< (range) and <> (except range).
.Ed
.Pp
>< and <> are binary operators (they take two arguments), and the range
doesn't include the limits, for instance:
.Bl -tag -width Fl
.It Em port 2000 >< 2004
means 
.Sq all ports > 2000 and < 2004 ,
hence ports 2001, 2002 and 2003.
.It Em port 2000 <> 2004
means 
.Sq all ports < 2000 or > 2004 ,
hence ports 1-1999 and 2005-65535.
.El
.Pp
The host and port specifications are optional, as the following examples
show:
.Bd -literal
    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 != 22
.Ed
.Ss user <user> group <group>
The rule only applies to packets of sockets owned by the specified user
and group.
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 isn't a connection endpoint,
the user and group are
.Em unknown .
.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, for other protocols
these parameters are ignored.
.Pp
User and group refer to the effective (as opposed to the real) IDs, in
case the socket is created by a setuid/setgid process.
Note that user and group IDs are stored when a socket is created;
when a process creates a listening socket as root (for instance, because
it wants to bind to a privileged port) and subsequently sets another
user ID (to drop privileges), the socket's uid remains root.
.Pp
User and group IDs can be specified as either numbers or names, the
syntax is similar to the one for ports.
The value
.Em unknown
matches packets of forwarded connections.
.Em unknown
can only be used with operators = and !=, other constructs
like 'user >= unknown' are invalid.
Forwarded packets with unknown user and group ID match only rules
that explicitely compare against
.Em unknown
with operator = or !=, for instance 'user >= 0' does not match
forwarded packets.
The following example allows only selected users to open outgoing
connections:
.Bd -literal
    block out proto { tcp, udp } all
    pass  out proto { tcp, udp } all user { < 1000, dhartmei } keep state
.Ed
.Ss flags <a> | <a>/<b> | /<b>
The rule only applies to TCP packets that have the flags <a> set
out of set <b>.
Flags not specified in <b> are ignored.
If <b> is not set, all flags are specified.
The flags are: (F)IN, (S)YN, (R)ST, (P)USH, (A)CK, (U)RG, (E)CE, and C(W)R.
.Bl -tag -width Fl
.It Em flags S/S
Flag SYN is set.
The other flags are ignored.
.It Em flags S/SA
Of SYN and ACK, exactly SYN is set.
SYN, SYN+PSH, SYN+RST match, but SYN+ACK, ACK and ACK+RST don't.
This is more restrictive than the previous example.
.It Em flags S
If the second set is not specified, it defaults to FSRPAUEW.
Hence, only packets with SYN set and all other flags unset match this 
rule.
This is more restrictive than the previous example.
.It Em flags /SFRA
If the first set is not specified, it defaults to none.
All of SYN, FIN, RST and ACK must be unset.
.El
.Ss icmp-type <type> code <code> and ipv6-icmp-type <type> code <code>
The rule only applies to ICMP or ICMPv6 packets with the specified type
and code.
This parameter is only valid for rules that cover protocols icmp or
ipv6-icmp.
The protocol and the icmp type indicator (icmp-type or ipv6-icmp-type)
must match.
.Ss allow-opts
By default, packets which contain IP options are blocked.
When
.Em allow-opts
is specified for a
.Em pass
rule, packets that pass the filter based on that rule (last matching)
do so even if they contain IP options.
For packets that match state, the rule that initially created the
state is used.
The implicit
.Em pass
rule that is used when a packet doesn't match any rules does not
allow IP options.
.Ss label <string>
Adds a label (name) to the rule, which can be used to identify the rule.
For instance,
.Em 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 $srcaddr -compact -offset indent
.It Em $srcaddr
the source IP address.
.It Em $dstaddr
the destination IP address.
.It Em $srcport
the source port specification.
.It Em $dstport
the destination port specification.
.It Em $proto
the protocol name.
.It Em $nr
the rule number.
.El
.Pp
Example:
.Bd -literal
    ips = "{ 1.2.3.4, 1.2.3.5 }"
    pass in proto tcp from any to $ips port >1023 label "$dstaddr:$dstport"
.Ed
.Pp
expands to
.Bd -literal
    pass in proto tcp from any to 1.2.3.4 port >1023 label "1.2.3.4:>1023"
    pass in proto tcp from any to 1.2.3.5 port >1023 label "1.2.3.5:>1023"
.Ed
.Pp
Note that evaluation takes place at parse time.
.Sh MACROS
.Em pfctl
supports macro definition and expansion like:
.Bd -literal
    ext_if = "kue0"
    pass out on $ext_if           from any to any         keep state
    pass in  on $ext_if proto tcp from any to any port 25 keep state
.Ed
.Pp
Macro names must start with a letter and may contain letters, digits
and underscores.
Macros are not expanded recursively.
.Sh STATEFUL INSPECTION
.Em pf
is a stateful packet filter, which means it can track the state of
a connection.
Instead of passing all traffic to port 25, for instance, one can pass
only the initial packet and keep state.
.Pp
If a packet matches a pass ... keep state rule, the filter creates
a state for this connection and automatically lets pass all following
packets of that connection.
.Pp
Before any rules are evaluated, the filter checks whether the packet
matches any state.
If it does, the packet is passed without evaluation of any rules.
.Pp
States are removed after the connection is closed or has timed out.
.Pp
This has several advantages.
Comparing a packet to a state involves checking its sequence numbers.
If the sequence numbers are outside the narrow windows of expected
values, the packet is dropped.
This prevents spoofing attacks, where the attacker sends packets with
a fake source address/port but doesn't know the connection's sequence
numbers.
.Pp
Also, looking up states is usually faster than evaluating rules.
If one has 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
For instance:
.Bd -literal
    block out all
    block in  all
    pass out proto tcp from any to any         flags S/SA keep state
    pass in  proto tcp from any to any port 25 flags S/SA keep state
.Ed
.Pp
This rule set blocks everything by default.
Only outgoing connections and incoming connection to port 25 are allowed.
The inital packet of each connection has the SYN flag set, will be passed
and creates state.
All further packets of these connections are passed if they match a state.
.Pp
Specifying flags S/SA restricts state creation to the initial SYN
packet of the TCP handshake.
One can also be less restrictive, and allow state creation from
intermediate 
.Pq non-SYN
packets.
This will cause
.Em pf
to synchronize to existing connections, for instance
if one flushes the state table.
.Pp
For UDP, which is stateless by nature, keep state will create state
as well.
UDP packets are matched to states using only host addresses and ports.
.Pp
ICMP messages fall in two categories: ICMP error messages, which always
refer to a TCP or UDP packet, are matched against the refered to connection.
If one keeps state on a TCP connection, and an ICMP source quench message
referring to this TCP connection arrives, it will be matched to the right
state and get passed.
.Pp
For ICMP queries, keep state creates an ICMP state, and
.Em pf
knows how to match ICMP replies to states.
For example
.Bd -literal
    pass out inet proto icmp all icmp-type echoreq keep state
.Ed
.Pp
lets echo requests 
.Pq pings
out, creates state, and matches incoming echo replies correctly to states.
.Pp
Note: nat/rdr rules implicitly create state for connections.
.Sh 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
.Cm very
poor ISNs and thus are normally susceptible to ISN prediction exploits.
By applying a "modulate state" rule to a TCP connection, 
.Em pf
will create a high quality random sequence number for each connection
endpoint.
.Pp
The "modulate state" directive implicitly keeps state on the rule and is
only applicable to TCP connections.
.Pp
For instance:
.Bd -literal
    block out all
    block in  all
    pass out proto tcp from any to any                    modulate state
    pass in  proto tcp from any to any port 25 flags S/SA modulate state
.Ed
.Pp
Caveat:  If
.Em pf
picks up an already established connection
.Po
the firewall was rebooted, the state table was flushed, ...
.Pc
it will not be able to safely modulate the state of that connection.
.Em pf
will fall back and operate as if "keep state" was specified instead.
Without this fallback, modulation would cause each host to
think that the other end had somehow lost sync.
.Pp
Caveat:  If the state table is flushed or the firewall is rebooted,
currently modulated connections can not be continued or picked
up again by the firewall.
State modulation causes the firewall to phase
shift the sequencing of each side of a connection
.Po
add a random number to each side.
.Pc
Both sides of the connection will notice, that its peer has suddenly
shifted its sequence by a random amount. 
Neither side
will be able to recover and the connection will stall and eventually close.
.Sh STATE OPTIONS
Both "keep state" and "modulate state" support the following options:
.Bl -tag -width timeout_seconds -compact
.It Em max number
Limits the number of concurrent states the rule may create.
When this limit is reached, further packets matching the rule that would
create state are dropped, until existing states time out.
.It Em timeout seconds
Changes the timeout values used for states created by this rule.
For a list of all valid timeout names, see
.Xr pfctl 8 .
.El
.Pp
Multiple options can be specified, separated by commas:
.Bd -literal
    pass in proto tcp from any to any port www flags S/SA \\
      keep state (max 100, tcp.established 60, tcp.closing 5)
.Ed
.Sh NORMALIZATION
Packet normalization is invoked via the
.Pa scrub
directive.
Normalization is used to sanitize packet content in such
a way that there are no ambiguities in packet interpretation on
the receiving side.
.Pp
The normalizer does full IP fragment reassembly to prevent attacks
that confuse intrusion detection systems by sending overlapping
IP fragments.
.Ss no-df
Clears the
.Pa dont-fragment
bit from a matching ip packet.
.Ss min-ttl <number>
Enforces a minimum ttl for matching ip packets.
.Ss max-mss <number>
Enforces a maximum mss for matching tcp packets.
.Pp
Normalization occurs before filtering, scrub rules and pass/block
rules are evaluated independently.
Hence, their relative position in the rule set is not relevant,
and packets can't be blocked before normalization.
.Sh FRAGMENT HANDLING
IP datagrams (packets) can have a size of up to 65335 bytes.
Most network links, however, have a maximum transmission unit (MTU)
that is significantly lower (1500 bytes is common).
When an IP packet's size exceeds the MTU of the interface it has to
be sent out through, the packet is fragmented.
In general, a fragment only contains an IP header, which is sufficient
for the receiver to reassemble the complete packet.
The headers of subprotocols like TCP, UDP or ICMP are only data payload
on IP level, and such headers are not part of all fragments of a packet.
It's even possible that no fragment contains a complete subprotocol
header, because that header is split among fragments.
.Pp
There are two options for handling fragments in the packet filter:
.Pp
Using scrub rules, fragments can be reassembled by normalization.
In this case, fragments are cached until they form a complete
packet, and only complete packets are passed on to the filter.
The advantage is that filter rules have to deal only with complete
packets, and can ignore fragments.
The drawback of caching fragments is the additional memory cost.
.Pp
The alternative is to filter individual fragments with filter rules.
If no scrub rule applies to a fragment, it is passed to the filter.
Filter rules with matching IP header parameters decide whether the
fragment is passed or blocked, in the same way as complete packets
are filtered.
Without reassembly, fragments can only be filtered based on IP header
fields (source/destination address, protocol), since subprotocol header
fields are not available (TCP/UDP port numbers, ICMP code/type).
The
.Pa fragment
option can be used to restrict filter rules to apply only to
fragments but not complete packets.
Filter rules without the
.Pa fragment
option still apply to fragments, if they only specify IP header fields.
For instance, the rule 'pass in proto tcp from any to any port 80' never
applies to a fragment, even if the fragment is part of a TCP packet with
destination port 80, because without reassembly, this information is not
available for each fragment.
This also means that fragments can't create new or match existing
state table entries, which makes stateful filtering and address
translations (NAT, redirection) for fragments impossible.
.Pp
It's also possible to reassemble only certain fragments by specifying
source or destination addresses or protocols as parameters in scrub
rules.
.Pp
In most cases, the benefits of reassembly outweigh the additional
memory cost, and it's recommended to use scrub rules to reassemble
all fragments.
.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
Currently, only IPv4 fragments are supported and IPv6 fragments
are blocked unconditionally.
.Sh FILTER EXAMPLES
.Bd -literal
# The external interface is kue0
# (157.161.48.183, the only routable address)
# and the private network is 10.0.0.0/8, for which we are doing NAT.

# use a macro for the interface name, so it can be changed easily
ext_if = "kue0"

# normalize all incoming traffic
scrub in on $ext_if all

# block and log everything by default
block             out log on $ext_if           all
block             in  log on $ext_if           all
block return-rst  out log on $ext_if proto tcp all
block return-rst  in  log on $ext_if proto tcp all
block return-icmp out log on $ext_if proto udp all
block return-icmp in  log on $ext_if proto udp all

# block anything coming from source we have no back routes for
block in from no-route to any

# block and log outgoing packets that don't have our address as source,
# they are either spoofed or something is misconfigured (NAT disabled,
# for instance), we want to be nice and don't send out garbage.
block out log quick on $ext_if from ! 157.161.48.183 to any

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

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

# ICMP

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

# UDP

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

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

# TCP

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

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

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

.Ed
.SH NAT EXAMPLES
This example maps incoming requests on port 80 to port 8080, on
which Apache Tomcat is running (say Tomcat is not run as root,
therefore lacks permission to bind to port 80).
.Bd -literal
# map tomcat on 8080 to appear to be on 80
rdr on ne3 proto tcp from any to any port 80 -> 127.0.0.1 port 8080
.Ed
.Pp
In the example below, vlan12 is configured for the 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 routeable 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
nat on ! vlan12 from 192.168.168.0/24 to any -> 204.92.77.111
.Ed
.Pp
In the example below, fxp1 is the outside interface; the machine sits between a
fake internal 144.19.74.* network, and a routable external IP of 204.92.77.100.
The "no nat" rule excludes protocol AH from being translated.
.Bd -literal
#NO NAT
no nat on fxp1 proto ah from 144.19.74.0/24 to any
nat on fxp1 from 144.19.74.0/24 to any -> 204.92.77.100
.Ed
.Pp
In the example below, fxp0 is the internal interface.
Packets bound
for one specific server, as well as those generated by the sysadmins
are not proxied, all other connections are.
.Bd -literal
# NO RDR
no rdr on fxp0 from any        to $server port 80
no rdr on fxp0 from $sysadmins to any     port 80
   rdr on fxp0 from any        to any     port 80 -> 127.0.0.1 port 80
.Ed
.Pp
This longer example uses both a NAT and a redirection.
Interface kue0 is the outside interface, and its external address is
157.161.48.183.
Interface fxp0 is the inside interface, and we are running
.Xr ftp-proxy 8
listening for outbound ftp sessions captured to port 8081.
.Bd -literal
# NAT
# translate outgoing packets' source addresses (any protocol)
# in this case, any address but the gateway's external address is mapped
nat on kue0 inet from ! (kue0) to any -> (kue0)

# NAT PROXYING
# map outgoing packets' source port to an assigned proxy port instead of
# an arbitrary port
# in this case, proxy outgoing isakmp with port 500 on the gateway
nat on kue0 inet proto udp from any port = isakmp to any -> (kue0) \\
	port 500

# BINAT
# translate outgoing packets' source address (any protocol)
# translate incoming packets' destination address to an internal machine
# (bidirectional)
binat on kue0 from 10.1.2.150 to any -> (kue0)

# RDR
# translate incoming packets' destination addresses
# as an example, redirect a TCP and UDP port to an internal machine
rdr on kue0 inet proto tcp from any to (kue0) port 8080 -> 10.1.2.151 \\
	port 22
rdr on kue0 inet proto udp from any to (kue0) port 8080 -> 10.1.2.151 \\
	port 53

# RDR
# translate outgoing ftp control connections to send them to localhost
# for proxying with ftp-proxy(8) running on port 8081
rdr on fxp0 proto tcp from any to any port 21 -> 127.0.0.1 port 8081
.Ed
.Sh FILES
.Bl -tag -width "/etc/pf.conf" -compact
.It Pa /etc/hosts
.It Pa /etc/pf.conf
.It Pa /etc/protocols
.It Pa /etc/services
.El
.Sh SEE ALSO
.Xr pf 4 ,
.Xr hosts 5 ,
.Xr protocols 5 ,
.Xr services 5 ,
.Xr ftp-proxy 8 ,
.Xr pfctl 8 ,
.Xr pflogd 8
.Sh HISTORY
The
.Nm
file format appeared in
.Ox 3.0 .