freebsd-skq/sbin/ipfw/ipfw.8

.\"
.\" $FreeBSD$
.\"
.Dd July 27, 2010
.Dt IPFW 8
.Os
.Sh NAME
.Nm ipfw
.Nd User interface for firewall, traffic shaper, packet scheduler,
in-kernel NAT.
.Sh SYNOPSIS
.Ss FIREWALL CONFIGURATION
.Nm
.Op Fl cq
.Cm add
.Ar rule
.Nm
.Op Fl acdefnNStT
.Op Cm set Ar N
.Brq Cm list | show
.Op Ar rule | first-last ...
.Nm
.Op Fl f | q
.Op Cm set Ar N
.Cm flush
.Nm
.Op Fl q
.Op Cm set Ar N
.Brq Cm delete | zero | resetlog
.Op Ar number ...
.Pp
.Nm
.Cm set Oo Cm disable Ar number ... Oc Op Cm enable Ar number ...
.Nm
.Cm set move
.Op Cm rule
.Ar number Cm to Ar number
.Nm
.Cm set swap Ar number number
.Nm
.Cm set show
.Ss SYSCTL SHORTCUTS
.Pp
.Nm
.Cm enable
.Brq Cm firewall | altq | one_pass | debug | verbose | dyn_keepalive
.Nm
.Cm disable
.Brq Cm firewall | altq | one_pass | debug | verbose | dyn_keepalive
.Pp
.Ss LOOKUP TABLES
.Nm
.Cm table Ar number Cm add Ar addr Ns Oo / Ns Ar masklen Oc Op Ar value
.Nm
.Cm table Ar number Cm delete Ar addr Ns Op / Ns Ar masklen
.Nm
.Cm table
.Brq Ar number | all
.Cm flush
.Nm
.Cm table
.Brq Ar number | all
.Cm list
.Pp
.Ss DUMMYNET CONFIGURATION (TRAFFIC SHAPER AND PACKET SCHEDULER)
.Nm
.Brq Cm pipe | queue | sched
.Ar number
.Cm config
.Ar config-options
.Nm
.Op Fl s Op Ar field
.Brq Cm pipe | queue | sched
.Brq Cm delete | list | show
.Op Ar number ...
.Pp
.Ss IN-KERNEL NAT
.Nm
.Op Fl q
.Cm nat
.Ar number
.Cm config
.Ar config-options
.Pp
.Nm
.Op Fl cfnNqS
.Oo
.Fl p Ar preproc
.Oo
.Ar preproc-flags
.Oc
.Oc
.Ar pathname
.Sh DESCRIPTION
The
.Nm
utility is the user interface for controlling the
.Xr ipfw 4
firewall, the
.Xr dummynet 4
traffic shaper/packet scheduler, and the
in-kernel NAT services.
.Pp
A firewall configuration, or
.Em ruleset ,
is made of a list of
.Em rules
numbered from 1 to 65535.
Packets are passed to the firewall
from a number of different places in the protocol stack
(depending on the source and destination of the packet,
it is possible for the firewall to be
invoked multiple times on the same packet).
The packet passed to the firewall is compared
against each of the rules in the
.Em ruleset ,
in rule-number order
(multiple rules with the same number are permitted, in which case
they are processed in order of insertion).
When a match is found, the action corresponding to the
matching rule is performed.
.Pp
Depending on the action and certain system settings, packets
can be reinjected into the firewall at some rule after the
matching one for further processing.
.Pp
A ruleset always includes a
.Em default
rule (numbered 65535) which cannot be modified or deleted,
and matches all packets.
The action associated with the
.Em default
rule can be either
.Cm deny
or
.Cm allow
depending on how the kernel is configured.
.Pp
If the ruleset includes one or more rules with the
.Cm keep-state
or
.Cm limit
option,
the firewall will have a
.Em stateful
behaviour, i.e., upon a match it will create
.Em dynamic rules ,
i.e. rules that match packets with the same 5-tuple
(protocol, source and destination addresses and ports)
as the packet which caused their creation.
Dynamic rules, which have a limited lifetime, are checked
at the first occurrence of a
.Cm check-state ,
.Cm keep-state
or
.Cm limit
rule, and are typically used to open the firewall on-demand to
legitimate traffic only.
See the
.Sx STATEFUL FIREWALL
and
.Sx EXAMPLES
Sections below for more information on the stateful behaviour of
.Nm .
.Pp
All rules (including dynamic ones) have a few associated counters:
a packet count, a byte count, a log count and a timestamp
indicating the time of the last match.
Counters can be displayed or reset with
.Nm
commands.
.Pp
Each rule belongs to one of 32 different
.Em sets
, and there are
.Nm
commands to atomically manipulate sets, such as enable,
disable, swap sets, move all rules in a set to another
one, delete all rules in a set.
These can be useful to
install temporary configurations, or to test them.
See Section
.Sx SETS OF RULES
for more information on
.Em sets .
.Pp
Rules can be added with the
.Cm add
command; deleted individually or in groups with the
.Cm delete
command, and globally (except those in set 31) with the
.Cm flush
command; displayed, optionally with the content of the
counters, using the
.Cm show
and
.Cm list
commands.
Finally, counters can be reset with the
.Cm zero
and
.Cm resetlog
commands.
.Pp
.Ss COMMAND OPTIONS
The following general options are available when invoking
.Nm :
.Bl -tag -width indent
.It Fl a
Show counter values when listing rules.
The
.Cm show
command implies this option.
.It Fl b
Only show the action and the comment, not the body of a rule.
Implies
.Fl c .
.It Fl c
When entering or showing rules, print them in compact form,
i.e., omitting the "ip from any to any" string
when this does not carry any additional information.
.It Fl d
When listing, show dynamic rules in addition to static ones.
.It Fl e
When listing and
.Fl d
is specified, also show expired dynamic rules.
.It Fl f
Do not ask for confirmation for commands that can cause problems
if misused,
.No i.e. Cm flush .
If there is no tty associated with the process, this is implied.
.It Fl i
When listing a table (see the
.Sx LOOKUP TABLES
section below for more information on lookup tables), format values
as IP addresses. By default, values are shown as integers.
.It Fl n
Only check syntax of the command strings, without actually passing
them to the kernel.
.It Fl N
Try to resolve addresses and service names in output.
.It Fl q
Be quiet when executing the
.Cm add ,
.Cm nat ,
.Cm zero ,
.Cm resetlog
or
.Cm flush
commands;
(implies
.Fl f ) .
This is useful when updating rulesets by executing multiple
.Nm
commands in a script
(e.g.,
.Ql sh\ /etc/rc.firewall ) ,
or by processing a file with many
.Nm
rules across a remote login session.
It also stops a table add or delete
from failing if the entry already exists or is not present.
.Pp
The reason why this option may be important is that
for some of these actions,
.Nm
may print a message; if the action results in blocking the
traffic to the remote client,
the remote login session will be closed
and the rest of the ruleset will not be processed.
Access to the console would then be required to recover.
.It Fl S
When listing rules, show the
.Em set
each rule belongs to.
If this flag is not specified, disabled rules will not be
listed.
.It Fl s Op Ar field
When listing pipes, sort according to one of the four
counters (total or current packets or bytes).
.It Fl t
When listing, show last match timestamp converted with ctime().
.It Fl T
When listing, show last match timestamp as seconds from the epoch.
This form can be more convenient for postprocessing by scripts.
.El
.Pp
.Ss LIST OF RULES AND PREPROCESSING
To ease configuration, rules can be put into a file which is
processed using
.Nm
as shown in the last synopsis line.
An absolute
.Ar pathname
must be used.
The file will be read line by line and applied as arguments to the
.Nm
utility.
.Pp
Optionally, a preprocessor can be specified using
.Fl p Ar preproc
where
.Ar pathname
is to be piped through.
Useful preprocessors include
.Xr cpp 1
and
.Xr m4 1 .
If
.Ar preproc
does not start with a slash
.Pq Ql /
as its first character, the usual
.Ev PATH
name search is performed.
Care should be taken with this in environments where not all
file systems are mounted (yet) by the time
.Nm
is being run (e.g.\& when they are mounted over NFS).
Once
.Fl p
has been specified, any additional arguments are passed on to the preprocessor
for interpretation.
This allows for flexible configuration files (like conditionalizing
them on the local hostname) and the use of macros to centralize
frequently required arguments like IP addresses.
.Pp
.Ss TRAFFIC SHAPER CONFIGURATION
The
.Nm
.Cm pipe , queue
and
.Cm sched
commands are used to configure the traffic shaper and packet scheduler.
See the
.Sx TRAFFIC SHAPER (DUMMYNET) CONFIGURATION
Section below for details.
.Pp
If the world and the kernel get out of sync the
.Nm
ABI may break, preventing you from being able to add any rules.
This can
adversely effect the booting process.
You can use
.Nm
.Cm disable
.Cm firewall
to temporarily disable the firewall to regain access to the network,
allowing you to fix the problem.
.Sh PACKET FLOW
A packet is checked against the active ruleset in multiple places
in the protocol stack, under control of several sysctl variables.
These places and variables are shown below, and it is important to
have this picture in mind in order to design a correct ruleset.
.Bd -literal -offset indent
       ^    to upper layers    V
       |                       |
       +----------->-----------+
       ^                       V
 [ip(6)_input]           [ip(6)_output]     net.inet(6).ip(6).fw.enable=1
       |                       |
       ^                       V
 [ether_demux]        [ether_output_frame]  net.link.ether.ipfw=1
       |                       |
       +-->--[bdg_forward]-->--+            net.link.bridge.ipfw=1
       ^                       V
       |      to devices       |
.Ed
.Pp
The number of
times the same packet goes through the firewall can
vary between 0 and 4 depending on packet source and
destination, and system configuration.
.Pp
Note that as packets flow through the stack, headers can be
stripped or added to it, and so they may or may not be available
for inspection.
E.g., incoming packets will include the MAC header when
.Nm
is invoked from
.Cm ether_demux() ,
but the same packets will have the MAC header stripped off when
.Nm
is invoked from
.Cm ip_input()
or
.Cm ip6_input() .
.Pp
Also note that each packet is always checked against the complete ruleset,
irrespective of the place where the check occurs, or the source of the packet.
If a rule contains some match patterns or actions which are not valid
for the place of invocation (e.g.\& trying to match a MAC header within
.Cm ip_input
or
.Cm ip6_input ),
the match pattern will not match, but a
.Cm not
operator in front of such patterns
.Em will
cause the pattern to
.Em always
match on those packets.
It is thus the responsibility of
the programmer, if necessary, to write a suitable ruleset to
differentiate among the possible places.
.Cm skipto
rules can be useful here, as an example:
.Bd -literal -offset indent
# packets from ether_demux or bdg_forward
ipfw add 10 skipto 1000 all from any to any layer2 in
# packets from ip_input
ipfw add 10 skipto 2000 all from any to any not layer2 in
# packets from ip_output
ipfw add 10 skipto 3000 all from any to any not layer2 out
# packets from ether_output_frame
ipfw add 10 skipto 4000 all from any to any layer2 out
.Ed
.Pp
(yes, at the moment there is no way to differentiate between
ether_demux and bdg_forward).
.Sh SYNTAX
In general, each keyword or argument must be provided as
a separate command line argument, with no leading or trailing
spaces.
Keywords are case-sensitive, whereas arguments may
or may not be case-sensitive depending on their nature
(e.g.\& uid's are, hostnames are not).
.Pp
Some arguments (e.g. port or address lists) are comma-separated
lists of values.
In this case, spaces after commas ',' are allowed to make
the line more readable.
You can also put the entire
command (including flags) into a single argument.
E.g., the following forms are equivalent:
.Bd -literal -offset indent
ipfw -q add deny src-ip 10.0.0.0/24,127.0.0.1/8
ipfw -q add deny src-ip 10.0.0.0/24, 127.0.0.1/8
ipfw "-q add deny src-ip 10.0.0.0/24, 127.0.0.1/8"
.Ed
.Sh RULE FORMAT
The format of firewall rules is the following:
.Bd -ragged -offset indent
.Bk -words
.Op Ar rule_number
.Op Cm set Ar set_number
.Op Cm prob Ar match_probability
.Ar action
.Op Cm log Op Cm logamount Ar number
.Op Cm altq Ar queue
.Oo
.Bro Cm tag | untag
.Brc Ar number
.Oc
.Ar body
.Ek
.Ed
.Pp
where the body of the rule specifies which information is used
for filtering packets, among the following:
.Pp
.Bl -tag -width "Source and dest. addresses and ports" -offset XXX -compact
.It Layer-2 header fields
When available
.It IPv4 and IPv6 Protocol
TCP, UDP, ICMP, etc.
.It Source and dest. addresses and ports
.It Direction
See Section
.Sx PACKET FLOW
.It Transmit and receive interface
By name or address
.It Misc. IP header fields
Version, type of service, datagram length, identification,
fragment flag (non-zero IP offset),
Time To Live
.It IP options
.It IPv6 Extension headers
Fragmentation, Hop-by-Hop options,
Routing Headers, Source routing rthdr0, Mobile IPv6 rthdr2, IPSec options.
.It IPv6 Flow-ID
.It Misc. TCP header fields
TCP flags (SYN, FIN, ACK, RST, etc.),
sequence number, acknowledgment number,
window
.It TCP options
.It ICMP types
for ICMP packets
.It ICMP6 types
for ICMP6 packets
.It User/group ID
When the packet can be associated with a local socket.
.It Divert status
Whether a packet came from a divert socket (e.g.,
.Xr natd 8 ) .
.It Fib annotation state
Whether a packet has been tagged for using a specific FIB (routing table)
in future forwarding decisions.
.El
.Pp
Note that some of the above information, e.g.\& source MAC or IP addresses and
TCP/UDP ports, can be easily spoofed, so filtering on those fields
alone might not guarantee the desired results.
.Bl -tag -width indent
.It Ar rule_number
Each rule is associated with a
.Ar rule_number
in the range 1..65535, with the latter reserved for the
.Em default
rule.
Rules are checked sequentially by rule number.
Multiple rules can have the same number, in which case they are
checked (and listed) according to the order in which they have
been added.
If a rule is entered without specifying a number, the kernel will
assign one in such a way that the rule becomes the last one
before the
.Em default
rule.
Automatic rule numbers are assigned by incrementing the last
non-default rule number by the value of the sysctl variable
.Ar net.inet.ip.fw.autoinc_step
which defaults to 100.
If this is not possible (e.g.\& because we would go beyond the
maximum allowed rule number), the number of the last
non-default value is used instead.
.It Cm set Ar set_number
Each rule is associated with a
.Ar set_number
in the range 0..31.
Sets can be individually disabled and enabled, so this parameter
is of fundamental importance for atomic ruleset manipulation.
It can be also used to simplify deletion of groups of rules.
If a rule is entered without specifying a set number,
set 0 will be used.
.br
Set 31 is special in that it cannot be disabled,
and rules in set 31 are not deleted by the
.Nm ipfw flush
command (but you can delete them with the
.Nm ipfw delete set 31
command).
Set 31 is also used for the
.Em default
rule.
.It Cm prob Ar match_probability
A match is only declared with the specified probability
(floating point number between 0 and 1).
This can be useful for a number of applications such as
random packet drop or
(in conjunction with
.Nm dummynet )
to simulate the effect of multiple paths leading to out-of-order
packet delivery.
.Pp
Note: this condition is checked before any other condition, including
ones such as keep-state or check-state which might have side effects.
.It Cm log Op Cm logamount Ar number
Packets matching a rule with the
.Cm log
keyword will be made available for logging in two ways:
if the sysctl variable
.Va net.inet.ip.fw.verbose
is set to 0 (default), one can use
.Xr bpf 4
attached to the
.Xr ipfw0
pseudo interface. There is no overhead if no 
.Xr bpf
is attached to the pseudo interface.
.Pp
If
.Va net.inet.ip.fw.verbose
is set to 1, packets will be logged to
.Xr syslogd 8
with a
.Dv LOG_SECURITY
facility up to a maximum of
.Cm logamount
packets.
If no
.Cm logamount
is specified, the limit is taken from the sysctl variable
.Va net.inet.ip.fw.verbose_limit .
In both cases, a value of 0 means unlimited logging.
.Pp
Once the limit is reached, logging can be re-enabled by
clearing the logging counter or the packet counter for that entry, see the
.Cm resetlog
command.
.Pp
Note: logging is done after all other packet matching conditions
have been successfully verified, and before performing the final
action (accept, deny, etc.) on the packet.
.It Cm tag Ar number
When a packet matches a rule with the
.Cm tag
keyword, the numeric tag for the given
.Ar number
in the range 1..65534 will be attached to the packet.
The tag acts as an internal marker (it is not sent out over
the wire) that can be used to identify these packets later on.
This can be used, for example, to provide trust between interfaces
and to start doing policy-based filtering.
A packet can have multiple tags at the same time.
Tags are "sticky", meaning once a tag is applied to a packet by a
matching rule it exists until explicit removal.
Tags are kept with the packet everywhere within the kernel, but are
lost when packet leaves the kernel, for example, on transmitting
packet out to the network or sending packet to a
.Xr divert 4
socket.
.Pp
To check for previously applied tags, use the
.Cm tagged
rule option.
To delete previously applied tag, use the
.Cm untag
keyword.
.Pp
Note: since tags are kept with the packet everywhere in kernelspace,
they can be set and unset anywhere in the kernel network subsystem
(using the
.Xr mbuf_tags 9
facility), not only by means of the
.Xr ipfw 4
.Cm tag
and
.Cm untag
keywords.
For example, there can be a specialized
.Xr netgraph 4
node doing traffic analyzing and tagging for later inspecting
in firewall.
.It Cm untag Ar number
When a packet matches a rule with the
.Cm untag
keyword, the tag with the number
.Ar number
is searched among the tags attached to this packet and,
if found, removed from it.
Other tags bound to packet, if present, are left untouched.
.It Cm altq Ar queue
When a packet matches a rule with the
.Cm altq
keyword, the ALTQ identifier for the given
.Ar queue
(see
.Xr altq 4 )
will be attached.
Note that this ALTQ tag is only meaningful for packets going "out" of IPFW,
and not being rejected or going to divert sockets.
Note that if there is insufficient memory at the time the packet is
processed, it will not be tagged, so it is wise to make your ALTQ
"default" queue policy account for this.
If multiple
.Cm altq
rules match a single packet, only the first one adds the ALTQ classification
tag.
In doing so, traffic may be shaped by using
.Cm count Cm altq Ar queue
rules for classification early in the ruleset, then later applying
the filtering decision.
For example,
.Cm check-state
and
.Cm keep-state
rules may come later and provide the actual filtering decisions in
addition to the fallback ALTQ tag.
.Pp
You must run
.Xr pfctl 8
to set up the queues before IPFW will be able to look them up by name,
and if the ALTQ disciplines are rearranged, the rules in containing the
queue identifiers in the kernel will likely have gone stale and need
to be reloaded.
Stale queue identifiers will probably result in misclassification.
.Pp
All system ALTQ processing can be turned on or off via
.Nm
.Cm enable Ar altq
and
.Nm
.Cm disable Ar altq .
The usage of
.Va net.inet.ip.fw.one_pass
is irrelevant to ALTQ traffic shaping, as the actual rule action is followed
always after adding an ALTQ tag.
.El
.Ss RULE ACTIONS
A rule can be associated with one of the following actions, which
will be executed when the packet matches the body of the rule.
.Bl -tag -width indent
.It Cm allow | accept | pass | permit
Allow packets that match rule.
The search terminates.
.It Cm check-state
Checks the packet against the dynamic ruleset.
If a match is found, execute the action associated with
the rule which generated this dynamic rule, otherwise
move to the next rule.
.br
.Cm Check-state
rules do not have a body.
If no
.Cm check-state
rule is found, the dynamic ruleset is checked at the first
.Cm keep-state
or
.Cm limit
rule.
.It Cm count
Update counters for all packets that match rule.
The search continues with the next rule.
.It Cm deny | drop
Discard packets that match this rule.
The search terminates.
.It Cm divert Ar port
Divert packets that match this rule to the
.Xr divert 4
socket bound to port
.Ar port .
The search terminates.
.It Cm fwd | forward Ar ipaddr | tablearg Ns Op , Ns Ar port
Change the next-hop on matching packets to
.Ar ipaddr ,
which can be an IP address or a host name.
The next hop can also be supplied by the last table
looked up for the packet by using the
.Cm tablearg
keyword instead of an explicit address.
The search terminates if this rule matches.
.Pp
If
.Ar ipaddr
is a local address, then matching packets will be forwarded to
.Ar port
(or the port number in the packet if one is not specified in the rule)
on the local machine.
.br
If
.Ar ipaddr
is not a local address, then the port number
(if specified) is ignored, and the packet will be
forwarded to the remote address, using the route as found in
the local routing table for that IP.
.br
A
.Ar fwd
rule will not match layer-2 packets (those received
on ether_input, ether_output, or bridged).
.br
The
.Cm fwd
action does not change the contents of the packet at all.
In particular, the destination address remains unmodified, so
packets forwarded to another system will usually be rejected by that system
unless there is a matching rule on that system to capture them.
For packets forwarded locally,
the local address of the socket will be
set to the original destination address of the packet.
This makes the
.Xr netstat 1
entry look rather weird but is intended for
use with transparent proxy servers.
.Pp
To enable
.Cm fwd
a custom kernel needs to be compiled with the option
.Cd "options IPFIREWALL_FORWARD" .
.It Cm nat Ar nat_nr
Pass packet to a
nat instance
(for network address translation, address redirect, etc.):
see the
.Sx NETWORK ADDRESS TRANSLATION (NAT)
Section for further information.
.It Cm pipe Ar pipe_nr
Pass packet to a
.Nm dummynet
.Dq pipe
(for bandwidth limitation, delay, etc.).
See the
.Sx TRAFFIC SHAPER (DUMMYNET) CONFIGURATION
Section for further information.
The search terminates; however, on exit from the pipe and if
the
.Xr sysctl 8
variable
.Va net.inet.ip.fw.one_pass
is not set, the packet is passed again to the firewall code
starting from the next rule.
.It Cm queue Ar queue_nr
Pass packet to a
.Nm dummynet
.Dq queue
(for bandwidth limitation using WF2Q+).
.It Cm reject
(Deprecated).
Synonym for
.Cm unreach host .
.It Cm reset
Discard packets that match this rule, and if the
packet is a TCP packet, try to send a TCP reset (RST) notice.
The search terminates.
.It Cm reset6
Discard packets that match this rule, and if the
packet is a TCP packet, try to send a TCP reset (RST) notice.
The search terminates.
.It Cm skipto Ar number | tablearg
Skip all subsequent rules numbered less than
.Ar number .
The search continues with the first rule numbered
.Ar number
or higher.
It is possible to use the 
.Cm tablearg
keyword with a skipto for a 
.Em computed
skipto, but care should be used, as no destination caching
is possible in this case so the rules are always walked to find it,
starting from the 
.Cm skipto .
.It Cm tee Ar port
Send a copy of packets matching this rule to the
.Xr divert 4
socket bound to port
.Ar port .
The search continues with the next rule.
.It Cm unreach Ar code
Discard packets that match this rule, and try to send an ICMP
unreachable notice with code
.Ar code ,
where
.Ar code
is a number from 0 to 255, or one of these aliases:
.Cm net , host , protocol , port ,
.Cm needfrag , srcfail , net-unknown , host-unknown ,
.Cm isolated , net-prohib , host-prohib , tosnet ,
.Cm toshost , filter-prohib , host-precedence
or
.Cm precedence-cutoff .
The search terminates.
.It Cm unreach6 Ar code
Discard packets that match this rule, and try to send an ICMPv6
unreachable notice with code
.Ar code ,
where
.Ar code
is a number from 0, 1, 3 or 4, or one of these aliases:
.Cm no-route, admin-prohib, address
or
.Cm port .
The search terminates.
.It Cm netgraph Ar cookie
Divert packet into netgraph with given
.Ar cookie .
The search terminates.
If packet is later returned from netgraph it is either
accepted or continues with the next rule, depending on
.Va net.inet.ip.fw.one_pass
sysctl variable.
.It Cm ngtee Ar cookie
A copy of packet is diverted into netgraph, original
packet continues with the next rule.
See
.Xr ng_ipfw 4
for more information on
.Cm netgraph
and
.Cm ngtee
actions.
.It Cm setfib Ar fibnum
The packet is tagged so as to use the FIB (routing table)
.Ar fibnum
in any subsequent forwarding decisions.
Initially this is limited to the values 0 through 15, see
.Xr setfib 1 .
Processing continues at the next rule.
.It Cm reass
Queue and reassemble ip fragments.
If the packet is not fragmented, counters are updated and processing continues with the next rule.
If the packet is the last logical fragment, the packet is reassembled and, if
.Va net.inet.ip.fw.one_pass
is set to 0, processing continues with the next rule, else packet is allowed to pass and search terminates.
If the packet is a fragment in the middle, it is consumed and processing stops immediately.
.Pp
Fragments handling can be tuned via
.Va net.inet.ip.maxfragpackets
and
.Va net.inet.ip.maxfragsperpacket
which limit, respectively, the maximum number of processable fragments (default: 800) and
the maximum number of fragments per packet (default: 16).
.Pp
NOTA BENE: since fragments do not contain port numbers, they should be avoided with the
.Nm reass
rule.
Alternatively, direction-based (like 
.Nm in
/
.Nm out
) and source-based (like
.Nm via
) match patterns can be used to select fragments.
.Pp
Usually a simple rule like:
.Bd -literal -offset indent
# reassemble incoming fragments
ipfw add reass all from any to any in
.Ed
.Pp
is all you need at the beginning of your ruleset.
.El
.Ss RULE BODY
The body of a rule contains zero or more patterns (such as
specific source and destination addresses or ports,
protocol options, incoming or outgoing interfaces, etc.)
that the packet must match in order to be recognised.
In general, the patterns are connected by (implicit)
.Cm and
operators -- i.e., all must match in order for the
rule to match.
Individual patterns can be prefixed by the
.Cm not
operator to reverse the result of the match, as in
.Pp
.Dl "ipfw add 100 allow ip from not 1.2.3.4 to any"
.Pp
Additionally, sets of alternative match patterns
.Pq Em or-blocks
can be constructed by putting the patterns in
lists enclosed between parentheses ( ) or braces { }, and
using the
.Cm or
operator as follows:
.Pp
.Dl "ipfw add 100 allow ip from { x or not y or z } to any"
.Pp
Only one level of parentheses is allowed.
Beware that most shells have special meanings for parentheses
or braces, so it is advisable to put a backslash \\ in front of them
to prevent such interpretations.
.Pp
The body of a rule must in general include a source and destination
address specifier.
The keyword
.Ar any
can be used in various places to specify that the content of
a required field is irrelevant.
.Pp
The rule body has the following format:
.Bd -ragged -offset indent
.Op Ar proto Cm from Ar src Cm to Ar dst
.Op Ar options
.Ed
.Pp
The first part (proto from src to dst) is for backward
compatibility with earlier versions of
.Fx .
In modern
.Fx
any match pattern (including MAC headers, IP protocols,
addresses and ports) can be specified in the
.Ar options
section.
.Pp
Rule fields have the following meaning:
.Bl -tag -width indent
.It Ar proto : protocol | Cm { Ar protocol Cm or ... }
.It Ar protocol : Oo Cm not Oc Ar protocol-name | protocol-number
An IP protocol specified by number or name
(for a complete list see
.Pa /etc/protocols ) ,
or one of the following keywords:
.Bl -tag -width indent
.It Cm ip4 | ipv4
Matches IPv4 packets.
.It Cm ip6 | ipv6
Matches IPv6 packets.
.It Cm ip | all
Matches any packet.
.El
.Pp
The
.Cm ipv6
in
.Cm proto
option will be treated as inner protocol.
And, the
.Cm ipv4
is not available in
.Cm proto
option.
.Pp
The
.Cm { Ar protocol Cm or ... }
format (an
.Em or-block )
is provided for convenience only but its use is deprecated.
.It Ar src No and Ar dst : Bro Cm addr | Cm { Ar addr Cm or ... } Brc Op Oo Cm not Oc Ar ports
An address (or a list, see below)
optionally followed by
.Ar ports
specifiers.
.Pp
The second format
.Em ( or-block
with multiple addresses) is provided for convenience only and
its use is discouraged.
.It Ar addr : Oo Cm not Oc Bro
.Cm any | me | me6 |
.Cm table Ns Pq Ar number Ns Op , Ns Ar value
.Ar | addr-list | addr-set
.Brc
.Bl -tag -width indent
.It Cm any
matches any IP address.
.It Cm me
matches any IP address configured on an interface in the system.
.It Cm me6
matches any IPv6 address configured on an interface in the system.
The address list is evaluated at the time the packet is
analysed.
.It Cm table Ns Pq Ar number Ns Op , Ns Ar value
Matches any IPv4 address for which an entry exists in the lookup table
.Ar number .
If an optional 32-bit unsigned
.Ar value
is also specified, an entry will match only if it has this value.
See the
.Sx LOOKUP TABLES
section below for more information on lookup tables.
.El
.It Ar addr-list : ip-addr Ns Op Ns , Ns Ar addr-list
.It Ar ip-addr :
A host or subnet address specified in one of the following ways:
.Bl -tag -width indent
.It Ar numeric-ip | hostname
Matches a single IPv4 address, specified as dotted-quad or a hostname.
Hostnames are resolved at the time the rule is added to the firewall list.
.It Ar addr Ns / Ns Ar masklen
Matches all addresses with base
.Ar addr
(specified as an IP address, a network number, or a hostname)
and mask width of
.Cm masklen
bits.
As an example, 1.2.3.4/25 or 1.2.3.0/25 will match
all IP numbers from 1.2.3.0 to 1.2.3.127 .
.It Ar addr Ns : Ns Ar mask
Matches all addresses with base
.Ar addr
(specified as an IP address, a network number, or a hostname)
and the mask of
.Ar mask ,
specified as a dotted quad.
As an example, 1.2.3.4:255.0.255.0 or 1.0.3.0:255.0.255.0 will match
1.*.3.*.
This form is advised only for non-contiguous
masks.
It is better to resort to the
.Ar addr Ns / Ns Ar masklen
format for contiguous masks, which is more compact and less
error-prone.
.El
.It Ar addr-set : addr Ns Oo Ns / Ns Ar masklen Oc Ns Cm { Ns Ar list Ns Cm }
.It Ar list : Bro Ar num | num-num Brc Ns Op Ns , Ns Ar list
Matches all addresses with base address
.Ar addr
(specified as an IP address, a network number, or a hostname)
and whose last byte is in the list between braces { } .
Note that there must be no spaces between braces and
numbers (spaces after commas are allowed).
Elements of the list can be specified as single entries
or ranges.
The
.Ar masklen
field is used to limit the size of the set of addresses,
and can have any value between 24 and 32.
If not specified,
it will be assumed as 24.
.br
This format is particularly useful to handle sparse address sets
within a single rule.
Because the matching occurs using a
bitmask, it takes constant time and dramatically reduces
the complexity of rulesets.
.br
As an example, an address specified as 1.2.3.4/24{128,35-55,89}
or 1.2.3.0/24{128,35-55,89}
will match the following IP addresses:
.br
1.2.3.128, 1.2.3.35 to 1.2.3.55, 1.2.3.89 .
.It Ar addr6-list : ip6-addr Ns Op Ns , Ns Ar addr6-list
.It Ar ip6-addr :
A host or subnet specified one of the following ways:
.Bl -tag -width indent
.It Ar numeric-ip | hostname
Matches a single IPv6 address as allowed by
.Xr inet_pton 3
or a hostname.
Hostnames are resolved at the time the rule is added to the firewall
list.
.It Ar addr Ns / Ns Ar masklen
Matches all IPv6 addresses with base
.Ar addr
(specified as allowed by
.Xr inet_pton
or a hostname)
and mask width of
.Cm masklen
bits.
.El
.Pp
No support for sets of IPv6 addresses is provided because IPv6 addresses
are typically random past the initial prefix.
.It Ar ports : Bro Ar port | port Ns \&- Ns Ar port Ns Brc Ns Op , Ns Ar ports
For protocols which support port numbers (such as TCP and UDP), optional
.Cm ports
may be specified as one or more ports or port ranges, separated
by commas but no spaces, and an optional
.Cm not
operator.
The
.Ql \&-
notation specifies a range of ports (including boundaries).
.Pp
Service names (from
.Pa /etc/services )
may be used instead of numeric port values.
The length of the port list is limited to 30 ports or ranges,
though one can specify larger ranges by using an
.Em or-block
in the
.Cm options
section of the rule.
.Pp
A backslash
.Pq Ql \e
can be used to escape the dash
.Pq Ql -
character in a service name (from a shell, the backslash must be
typed twice to avoid the shell itself interpreting it as an escape
character).
.Pp
.Dl "ipfw add count tcp from any ftp\e\e-data-ftp to any"
.Pp
Fragmented packets which have a non-zero offset (i.e., not the first
fragment) will never match a rule which has one or more port
specifications.
See the
.Cm frag
option for details on matching fragmented packets.
.El
.Ss RULE OPTIONS (MATCH PATTERNS)
Additional match patterns can be used within
rules.
Zero or more of these so-called
.Em options
can be present in a rule, optionally prefixed by the
.Cm not
operand, and possibly grouped into
.Em or-blocks .
.Pp
The following match patterns can be used (listed in alphabetical order):
.Bl -tag -width indent
.It Cm // this is a comment.
Inserts the specified text as a comment in the rule.
Everything following // is considered as a comment and stored in the rule.
You can have comment-only rules, which are listed as having a
.Cm count
action followed by the comment.
.It Cm bridged
Alias for
.Cm layer2 .
.It Cm diverted
Matches only packets generated by a divert socket.
.It Cm diverted-loopback
Matches only packets coming from a divert socket back into the IP stack
input for delivery.
.It Cm diverted-output
Matches only packets going from a divert socket back outward to the IP
stack output for delivery.
.It Cm dst-ip Ar ip-address
Matches IPv4 packets whose destination IP is one of the address(es)
specified as argument.
.It Bro Cm dst-ip6 | dst-ipv6 Brc Ar ip6-address
Matches IPv6 packets whose destination IP is one of the address(es)
specified as argument.
.It Cm dst-port Ar ports
Matches IP packets whose destination port is one of the port(s)
specified as argument.
.It Cm established
Matches TCP packets that have the RST or ACK bits set.
.It Cm ext6hdr Ar header
Matches IPv6 packets containing the extended header given by
.Ar header .
Supported headers are:
.Pp
Fragment,
.Pq Cm frag ,
Hop-to-hop options
.Pq Cm hopopt ,
any type of Routing Header
.Pq Cm route ,
Source routing Routing Header Type 0
.Pq Cm rthdr0 ,
Mobile IPv6 Routing Header Type 2
.Pq Cm rthdr2 ,
Destination options
.Pq Cm dstopt ,
IPSec authentication headers
.Pq Cm ah ,
and IPsec encapsulated security payload headers
.Pq Cm esp .
.It Cm fib Ar fibnum
Matches a packet that has been tagged to use
the given FIB (routing table) number.
.It Cm flow-id Ar labels
Matches IPv6 packets containing any of the flow labels given in
.Ar labels .
.Ar labels
is a comma separated list of numeric flow labels.
.It Cm frag
Matches packets that are fragments and not the first
fragment of an IP datagram.
Note that these packets will not have
the next protocol header (e.g.\& TCP, UDP) so options that look into
these headers cannot match.
.It Cm gid Ar group
Matches all TCP or UDP packets sent by or received for a
.Ar group .
A
.Ar group
may be specified by name or number.
.It Cm jail Ar prisonID
Matches all TCP or UDP packets sent by or received for the
jail whos prison ID is
.Ar prisonID .
.It Cm icmptypes Ar types
Matches ICMP packets whose ICMP type is in the list
.Ar types .
The list may be specified as any combination of
individual types (numeric) separated by commas.
.Em Ranges are not allowed .
The supported ICMP types are:
.Pp
echo reply
.Pq Cm 0 ,
destination unreachable
.Pq Cm 3 ,
source quench
.Pq Cm 4 ,
redirect
.Pq Cm 5 ,
echo request
.Pq Cm 8 ,
router advertisement
.Pq Cm 9 ,
router solicitation
.Pq Cm 10 ,
time-to-live exceeded
.Pq Cm 11 ,
IP header bad
.Pq Cm 12 ,
timestamp request
.Pq Cm 13 ,
timestamp reply
.Pq Cm 14 ,
information request
.Pq Cm 15 ,
information reply
.Pq Cm 16 ,
address mask request
.Pq Cm 17
and address mask reply
.Pq Cm 18 .
.It Cm icmp6types Ar types
Matches ICMP6 packets whose ICMP6 type is in the list of
.Ar types .
The list may be specified as any combination of
individual types (numeric) separated by commas.
.Em Ranges are not allowed .
.It Cm in | out
Matches incoming or outgoing packets, respectively.
.Cm in
and
.Cm out
are mutually exclusive (in fact,
.Cm out
is implemented as
.Cm not in Ns No ).
.It Cm ipid Ar id-list
Matches IPv4 packets whose
.Cm ip_id
field has value included in
.Ar id-list ,
which is either a single value or a list of values or ranges
specified in the same way as
.Ar ports .
.It Cm iplen Ar len-list
Matches IP packets whose total length, including header and data, is
in the set
.Ar len-list ,
which is either a single value or a list of values or ranges
specified in the same way as
.Ar ports .
.It Cm ipoptions Ar spec
Matches packets whose IPv4 header contains the comma separated list of
options specified in
.Ar spec .
The supported IP options are:
.Pp
.Cm ssrr
(strict source route),
.Cm lsrr
(loose source route),
.Cm rr
(record packet route) and
.Cm ts
(timestamp).
The absence of a particular option may be denoted
with a
.Ql \&! .
.It Cm ipprecedence Ar precedence
Matches IPv4 packets whose precedence field is equal to
.Ar precedence .
.It Cm ipsec
Matches packets that have IPSEC history associated with them
(i.e., the packet comes encapsulated in IPSEC, the kernel
has IPSEC support and IPSEC_FILTERTUNNEL option, and can correctly
decapsulate it).
.Pp
Note that specifying
.Cm ipsec
is different from specifying
.Cm proto Ar ipsec
as the latter will only look at the specific IP protocol field,
irrespective of IPSEC kernel support and the validity of the IPSEC data.
.Pp
Further note that this flag is silently ignored in kernels without
IPSEC support.
It does not affect rule processing when given and the
rules are handled as if with no
.Cm ipsec
flag.
.It Cm iptos Ar spec
Matches IPv4 packets whose
.Cm tos
field contains the comma separated list of
service types specified in
.Ar spec .
The supported IP types of service are:
.Pp
.Cm lowdelay
.Pq Dv IPTOS_LOWDELAY ,
.Cm throughput
.Pq Dv IPTOS_THROUGHPUT ,
.Cm reliability
.Pq Dv IPTOS_RELIABILITY ,
.Cm mincost
.Pq Dv IPTOS_MINCOST ,
.Cm congestion
.Pq Dv IPTOS_ECN_CE .
The absence of a particular type may be denoted
with a
.Ql \&! .
.It Cm ipttl Ar ttl-list
Matches IPv4 packets whose time to live is included in
.Ar ttl-list ,
which is either a single value or a list of values or ranges
specified in the same way as
.Ar ports .
.It Cm ipversion Ar ver
Matches IP packets whose IP version field is
.Ar ver .
.It Cm keep-state
Upon a match, the firewall will create a dynamic rule, whose
default behaviour is to match bidirectional traffic between
source and destination IP/port using the same protocol.
The rule has a limited lifetime (controlled by a set of
.Xr sysctl 8
variables), and the lifetime is refreshed every time a matching
packet is found.
.It Cm layer2
Matches only layer2 packets, i.e., those passed to
.Nm
from ether_demux() and ether_output_frame().
.It Cm limit Bro Cm src-addr | src-port | dst-addr | dst-port Brc Ar N
The firewall will only allow
.Ar N
connections with the same
set of parameters as specified in the rule.
One or more
of source and destination addresses and ports can be
specified.
Currently,
only IPv4 flows are supported.
.It Cm lookup Bro Cm dst-ip | dst-port | src-ip | src-port | uid | jail Brc Ar N
Search an entry in lookup table
.Ar N
that matches the field specified as argument.
If not found, the match fails.
Otherwise, the match succeeds and
.Cm tablearg
is set to the value extracted from the table.
.Pp
This option can be useful to quickly dispatch traffic based on
certain packet fields.
See the
.Sx LOOKUP TABLES
section below for more information on lookup tables.
.It Cm { MAC | mac } Ar dst-mac src-mac
Match packets with a given
.Ar dst-mac
and
.Ar src-mac
addresses, specified as the
.Cm any
keyword (matching any MAC address), or six groups of hex digits
separated by colons,
and optionally followed by a mask indicating the significant bits.
The mask may be specified using either of the following methods:
.Bl -enum -width indent
.It
A slash
.Pq /
followed by the number of significant bits.
For example, an address with 33 significant bits could be specified as:
.Pp
.Dl "MAC 10:20:30:40:50:60/33 any"
.Pp
.It
An ampersand
.Pq &
followed by a bitmask specified as six groups of hex digits separated
by colons.
For example, an address in which the last 16 bits are significant could
be specified as:
.Pp
.Dl "MAC 10:20:30:40:50:60&00:00:00:00:ff:ff any"
.Pp
Note that the ampersand character has a special meaning in many shells
and should generally be escaped.
.Pp
.El
Note that the order of MAC addresses (destination first,
source second) is
the same as on the wire, but the opposite of the one used for
IP addresses.
.It Cm mac-type Ar mac-type
Matches packets whose Ethernet Type field
corresponds to one of those specified as argument.
.Ar mac-type
is specified in the same way as
.Cm port numbers
(i.e., one or more comma-separated single values or ranges).
You can use symbolic names for known values such as
.Em vlan , ipv4, ipv6 .
Values can be entered as decimal or hexadecimal (if prefixed by 0x),
and they are always printed as hexadecimal (unless the
.Cm -N
option is used, in which case symbolic resolution will be attempted).
.It Cm proto Ar protocol
Matches packets with the corresponding IP protocol.
.It Cm recv | xmit | via Brq Ar ifX | Ar if Ns Cm * | Ar ipno | Ar any
Matches packets received, transmitted or going through,
respectively, the interface specified by exact name
.Ns No ( Ar ifX Ns No ),
by device name
.Ns No ( Ar if Ns Ar * Ns No ),
by IP address, or through some interface.
.Pp
The
.Cm via
keyword causes the interface to always be checked.
If
.Cm recv
or
.Cm xmit
is used instead of
.Cm via ,
then only the receive or transmit interface (respectively)
is checked.
By specifying both, it is possible to match packets based on
both receive and transmit interface, e.g.:
.Pp
.Dl "ipfw add deny ip from any to any out recv ed0 xmit ed1"
.Pp
The
.Cm recv
interface can be tested on either incoming or outgoing packets,
while the
.Cm xmit
interface can only be tested on outgoing packets.
So
.Cm out
is required (and
.Cm in
is invalid) whenever
.Cm xmit
is used.
.Pp
A packet might not have a receive or transmit interface: packets
originating from the local host have no receive interface,
while packets destined for the local host have no transmit
interface.
.It Cm setup
Matches TCP packets that have the SYN bit set but no ACK bit.
This is the short form of
.Dq Li tcpflags\ syn,!ack .
.It Cm sockarg
Matches packets that are associated to a local socket and
for which the SO_USER_COOKIE socket option has been set
to a non-zero value. As a side effect, the value of the
option is made available as
.Cm tablearg
value, which in turn can be used as
.Cm skipto
or
.Cm pipe
number.
.It Cm src-ip Ar ip-address
Matches IPv4 packets whose source IP is one of the address(es)
specified as an argument.
.It Cm src-ip6 Ar ip6-address
Matches IPv6 packets whose source IP is one of the address(es)
specified as an argument.
.It Cm src-port Ar ports
Matches IP packets whose source port is one of the port(s)
specified as argument.
.It Cm tagged Ar tag-list
Matches packets whose tags are included in
.Ar tag-list ,
which is either a single value or a list of values or ranges
specified in the same way as
.Ar ports .
Tags can be applied to the packet using
.Cm tag
rule action parameter (see it's description for details on tags).
.It Cm tcpack Ar ack
TCP packets only.
Match if the TCP header acknowledgment number field is set to
.Ar ack .
.It Cm tcpdatalen Ar tcpdatalen-list
Matches TCP packets whose length of TCP data is
.Ar tcpdatalen-list ,
which is either a single value or a list of values or ranges
specified in the same way as
.Ar ports .
.It Cm tcpflags Ar spec
TCP packets only.
Match if the TCP header contains the comma separated list of
flags specified in
.Ar spec .
The supported TCP flags are:
.Pp
.Cm fin ,
.Cm syn ,
.Cm rst ,
.Cm psh ,
.Cm ack
and
.Cm urg .
The absence of a particular flag may be denoted
with a
.Ql \&! .
A rule which contains a
.Cm tcpflags
specification can never match a fragmented packet which has
a non-zero offset.
See the
.Cm frag
option for details on matching fragmented packets.
.It Cm tcpseq Ar seq
TCP packets only.
Match if the TCP header sequence number field is set to
.Ar seq .
.It Cm tcpwin Ar win
TCP packets only.
Match if the TCP header window field is set to
.Ar win .
.It Cm tcpoptions Ar spec
TCP packets only.
Match if the TCP header contains the comma separated list of
options specified in
.Ar spec .
The supported TCP options are:
.Pp
.Cm mss
(maximum segment size),
.Cm window
(tcp window advertisement),
.Cm sack
(selective ack),
.Cm ts
(rfc1323 timestamp) and
.Cm cc
(rfc1644 t/tcp connection count).
The absence of a particular option may be denoted
with a
.Ql \&! .
.It Cm uid Ar user
Match all TCP or UDP packets sent by or received for a
.Ar user .
A
.Ar user
may be matched by name or identification number.
.It Cm verrevpath
For incoming packets,
a routing table lookup is done on the packet's source address.
If the interface on which the packet entered the system matches the
outgoing interface for the route,
the packet matches.
If the interfaces do not match up,
the packet does not match.
All outgoing packets or packets with no incoming interface match.
.Pp
The name and functionality of the option is intentionally similar to
the Cisco IOS command:
.Pp
.Dl ip verify unicast reverse-path
.Pp
This option can be used to make anti-spoofing rules to reject all
packets with source addresses not from this interface.
See also the option
.Cm antispoof .
.It Cm versrcreach
For incoming packets,
a routing table lookup is done on the packet's source address.
If a route to the source address exists, but not the default route
or a blackhole/reject route, the packet matches.
Otherwise, the packet does not match.
All outgoing packets match.
.Pp
The name and functionality of the option is intentionally similar to
the Cisco IOS command:
.Pp
.Dl ip verify unicast source reachable-via any
.Pp
This option can be used to make anti-spoofing rules to reject all
packets whose source address is unreachable.
.It Cm antispoof
For incoming packets, the packet's source address is checked if it
belongs to a directly connected network.
If the network is directly connected, then the interface the packet
came on in is compared to the interface the network is connected to.
When incoming interface and directly connected interface are not the
same, the packet does not match.
Otherwise, the packet does match.
All outgoing packets match.
.Pp
This option can be used to make anti-spoofing rules to reject all
packets that pretend to be from a directly connected network but do
not come in through that interface.
This option is similar to but more restricted than
.Cm verrevpath
because it engages only on packets with source addresses of directly
connected networks instead of all source addresses.
.El
.Sh LOOKUP TABLES
Lookup tables are useful to handle large sparse sets of
addresses or other search keys (e.g. ports, jail IDs).
In the rest of this section we will use the term ``address''
to mean any unsigned value of up to 32-bit.
There may be up to 128 different lookup tables, numbered 0 to 127.
.Pp
Each entry is represented by an
.Ar addr Ns Op / Ns Ar masklen
and will match all addresses with base
.Ar addr
(specified as an IP address, a hostname or an unsigned integer)
and mask width of
.Ar masklen
bits.
If
.Ar masklen
is not specified, it defaults to 32.
When looking up an IP address in a table, the most specific
entry will match.
Associated with each entry is a 32-bit unsigned
.Ar value ,
which can optionally be checked by a rule matching code.
When adding an entry, if
.Ar value
is not specified, it defaults to 0.
.Pp
An entry can be added to a table
.Pq Cm add ,
or removed from a table
.Pq Cm delete .
A table can be examined
.Pq Cm list
or flushed
.Pq Cm flush .
.Pp
Internally, each table is stored in a Radix tree, the same way as
the routing table (see
.Xr route 4 ) .
.Pp
Lookup tables currently support only ports, jail IDs and IPv4 addresses.
.Pp
The
.Cm tablearg
feature provides the ability to use a value, looked up in the table, as
the argument for a rule action, action parameter or rule option.
This can significantly reduce number of rules in some configurations.
If two tables are used in a rule, the result of the second (destination)
is used.
The
.Cm tablearg
argument can be used with the following actions:
.Cm nat, pipe , queue, divert, tee, netgraph, ngtee, fwd, skipto
action parameters:
.Cm tag, untag,
rule options:
.Cm limit, tagged.
.Pp
When used with
.Cm fwd
it is possible to supply table entries with values
that are in the form of IP addresses or hostnames.
See the
.Sx EXAMPLES
Section for example usage of tables and the tablearg keyword.
.Pp
When used with the
.Cm skipto
action, the user should be aware that the code will walk the ruleset
up to a rule equal to, or past, the given number, and should therefore try keep the
ruleset compact between the skipto and the target rules. 
.Sh SETS OF RULES
Each rule belongs to one of 32 different
.Em sets
, numbered 0 to 31.
Set 31 is reserved for the default rule.
.Pp
By default, rules are put in set 0, unless you use the
.Cm set N
attribute when entering a new rule.
Sets can be individually and atomically enabled or disabled,
so this mechanism permits an easy way to store multiple configurations
of the firewall and quickly (and atomically) switch between them.
The command to enable/disable sets is
.Bd -ragged -offset indent
.Nm
.Cm set Oo Cm disable Ar number ... Oc Op Cm enable Ar number ...
.Ed
.Pp
where multiple
.Cm enable
or
.Cm disable
sections can be specified.
Command execution is atomic on all the sets specified in the command.
By default, all sets are enabled.
.Pp
When you disable a set, its rules behave as if they do not exist
in the firewall configuration, with only one exception:
.Bd -ragged -offset indent
dynamic rules created from a rule before it had been disabled
will still be active until they expire.
In order to delete
dynamic rules you have to explicitly delete the parent rule
which generated them.
.Ed
.Pp
The set number of rules can be changed with the command
.Bd -ragged -offset indent
.Nm
.Cm set move
.Brq Cm rule Ar rule-number | old-set
.Cm to Ar new-set
.Ed
.Pp
Also, you can atomically swap two rulesets with the command
.Bd -ragged -offset indent
.Nm
.Cm set swap Ar first-set second-set
.Ed
.Pp
See the
.Sx EXAMPLES
Section on some possible uses of sets of rules.
.Sh STATEFUL FIREWALL
Stateful operation is a way for the firewall to dynamically
create rules for specific flows when packets that
match a given pattern are detected.
Support for stateful
operation comes through the
.Cm check-state , keep-state
and
.Cm limit
options of
.Nm rules .
.Pp
Dynamic rules are created when a packet matches a
.Cm keep-state
or
.Cm limit
rule, causing the creation of a
.Em dynamic
rule which will match all and only packets with
a given
.Em protocol
between a
.Em src-ip/src-port dst-ip/dst-port
pair of addresses
.Em ( src
and
.Em dst
are used here only to denote the initial match addresses, but they
are completely equivalent afterwards).
Dynamic rules will be checked at the first
.Cm check-state, keep-state
or
.Cm limit
occurrence, and the action performed upon a match will be the same
as in the parent rule.
.Pp
Note that no additional attributes other than protocol and IP addresses
and ports are checked on dynamic rules.
.Pp
The typical use of dynamic rules is to keep a closed firewall configuration,
but let the first TCP SYN packet from the inside network install a
dynamic rule for the flow so that packets belonging to that session
will be allowed through the firewall:
.Pp
.Dl "ipfw add check-state"
.Dl "ipfw add allow tcp from my-subnet to any setup keep-state"
.Dl "ipfw add deny tcp from any to any"
.Pp
A similar approach can be used for UDP, where an UDP packet coming
from the inside will install a dynamic rule to let the response through
the firewall:
.Pp
.Dl "ipfw add check-state"
.Dl "ipfw add allow udp from my-subnet to any keep-state"
.Dl "ipfw add deny udp from any to any"
.Pp
Dynamic rules expire after some time, which depends on the status
of the flow and the setting of some
.Cm sysctl
variables.
See Section
.Sx SYSCTL VARIABLES
for more details.
For TCP sessions, dynamic rules can be instructed to periodically
send keepalive packets to refresh the state of the rule when it is
about to expire.
.Pp
See Section
.Sx EXAMPLES
for more examples on how to use dynamic rules.
.Sh TRAFFIC SHAPER (DUMMYNET) CONFIGURATION
.Nm
is also the user interface for the
.Nm dummynet
traffic shaper, packet scheduler and network emulator, a subsystem that
can artificially queue, delay or drop packets
emulating the behaviour of certain network links
or queueing systems.
.Pp
.Nm dummynet
operates by first using the firewall to select packets
using any match pattern that can be used in
.Nm
rules.
Matching packets are then passed to either of two
different objects, which implement the traffic regulation:
.Bl -hang -offset XXXX
.It Em pipe
A
.Em pipe
emulates a
.Em link
with given bandwidth and propagation delay,
driven by a FIFO scheduler and a single queue with programmable
queue size and packet loss rate.
Packets are appended to the queue as they come out from
.Nm ipfw ,
and then transferred in FIFO order to the link at the desired rate.
.It Em queue
A
.Em queue
is an abstraction used to implement packet scheduling
using one of several packet scheduling algorithms.
Packets sent to a
.Em queue
are first grouped into flows according to a mask on the 5-tuple.
Flows are then passed to the scheduler associated to the
.Em queue ,
and each flow uses scheduling parameters (weight and others)
as configured in the
.Em queue
itself.
A scheduler in turn is connected to an emulated link,
and arbitrates the link's bandwidth among backlogged flows according to
weights and to the features of the scheduling algorithm in use.
.El
.Pp
In practice,
.Em pipes
can be used to set hard limits to the bandwidth that a flow can use, whereas
.Em queues
can be used to determine how different flows share the available bandwidth.
.Pp
A graphical representation of the binding of queues,
flows, schedulers and links is below.
.Bd -literal -offset indent
                 (flow_mask|sched_mask)  sched_mask
         +---------+   weight Wx  +-------------+
         |         |->-[flow]-->--|             |-+
    -->--| QUEUE x |   ...        |             | |
         |         |->-[flow]-->--| SCHEDuler N | |
         +---------+              |             | |
             ...                  |             +--[LINK N]-->--
         +---------+   weight Wy  |             | +--[LINK N]-->--
         |         |->-[flow]-->--|             | |
    -->--| QUEUE y |   ...        |             | |
         |         |->-[flow]-->--|             | |
         +---------+              +-------------+ |
                                    +-------------+
.Ed
It is important to understand the role of the SCHED_MASK
and FLOW_MASK, which are configured through the commands
.Dl "ipfw sched N config mask SCHED_MASK ..."
and
.Dl "ipfw queue X config mask FLOW_MASK ..." .
.Pp
The SCHED_MASK is used to assign flows to one or more
scheduler instances, one for each
value of the packet's 5-tuple after applying SCHED_MASK.
As an example, using ``src-ip 0xffffff00'' creates one instance
for each /24 destination subnet.
.Pp
The FLOW_MASK, together with the SCHED_MASK, is used to split
packets into flows. As an example, using
``src-ip 0x000000ff''
together with the previous SCHED_MASK makes a flow for
each individual source address. In turn, flows for each /24
subnet will be sent to the same scheduler instance.
.Pp
The above diagram holds even for the
.Em pipe
case, with the only restriction that a
.Em pipe
only supports a SCHED_MASK, and forces the use of a FIFO
scheduler (these are for backward compatibility reasons;
in fact, internally, a
.Nm dummynet's
pipe is implemented exactly as above).
.Pp
There are two modes of
.Nm dummynet
operation:
.Dq normal
and
.Dq fast .
The
.Dq normal
mode tries to emulate a real link: the
.Nm dummynet
scheduler ensures that the packet will not leave the pipe faster than it
would on the real link with a given bandwidth.
The
.Dq fast
mode allows certain packets to bypass the
.Nm dummynet
scheduler (if packet flow does not exceed pipe's bandwidth).
This is the reason why the
.Dq fast
mode requires less CPU cycles per packet (on average) and packet latency
can be significantly lower in comparison to a real link with the same
bandwidth.
The default mode is
.Dq normal .
The
.Dq fast
mode can be enabled by setting the
.Va net.inet.ip.dummynet.io_fast
.Xr sysctl 8
variable to a non-zero value.
.Pp
.Ss PIPE, QUEUE AND SCHEDULER CONFIGURATION
The
.Em pipe ,
.Em queue
and
.Em scheduler
configuration commands are the following:
.Bd -ragged -offset indent
.Cm pipe Ar number Cm config Ar pipe-configuration
.Pp
.Cm queue Ar number Cm config Ar queue-configuration
.Pp
.Cm sched Ar number Cm config Ar sched-configuration
.Ed
.Pp
The following parameters can be configured for a pipe:
.Pp
.Bl -tag -width indent -compact
.It Cm bw Ar bandwidth | device
Bandwidth, measured in
.Sm off
.Op Cm K | M
.Brq Cm bit/s | Byte/s .
.Sm on
.Pp
A value of 0 (default) means unlimited bandwidth.
The unit must immediately follow the number, as in
.Pp
.Dl "ipfw pipe 1 config bw 300Kbit/s"
.Pp
If a device name is specified instead of a numeric value, as in
.Pp
.Dl "ipfw pipe 1 config bw tun0"
.Pp
then the transmit clock is supplied by the specified device.
At the moment only the
.Xr tun 4
device supports this
functionality, for use in conjunction with
.Xr ppp 8 .
.Pp
.It Cm delay Ar ms-delay
Propagation delay, measured in milliseconds.
The value is rounded to the next multiple of the clock tick
(typically 10ms, but it is a good practice to run kernels
with
.Dq "options HZ=1000"
to reduce
the granularity to 1ms or less).
The default value is 0, meaning no delay.
.Pp
.It Cm burst Ar size
If the data to be sent exceeds the pipe's bandwidth limit
(and the pipe was previously idle), up to
.Ar size
bytes of data are allowed to bypass the
.Nm dummynet
scheduler, and will be sent as fast as the physical link allows.
Any additional data will be transmitted at the rate specified
by the
.Nm pipe
bandwidth.
The burst size depends on how long the pipe has been idle;
the effective burst size is calculated as follows:
MAX(
.Ar size
,
.Nm bw
* pipe_idle_time).
.Pp
.It Cm profile Ar filename
A file specifying the additional overhead incurred in the transmission
of a packet on the link.
.Pp
Some link types introduce extra delays in the transmission
of a packet, e.g. because of MAC level framing, contention on
the use of the channel, MAC level retransmissions and so on.
From our point of view, the channel is effectively unavailable
for this extra time, which is constant or variable depending
on the link type. Additionally, packets may be dropped after this
time (e.g. on a wireless link after too many retransmissions).
We can model the additional delay with an empirical curve
that represents its distribution.
.Bd -literal -offset indent
      cumulative probability
      1.0 ^
          |
      L   +-- loss-level          x
          |                 ******
          |                *
          |           *****
          |          *
          |        **
          |       *
          +-------*------------------->
                      delay
.Ed
The empirical curve may have both vertical and horizontal lines.
Vertical lines represent constant delay for a range of
probabilities.
Horizontal lines correspond to a discontinuity in the delay
distribution: the pipe will use the largest delay for a
given probability.
.Pp
The file format is the following, with whitespace acting as
a separator and '#' indicating the beginning a comment:
.Bl -tag -width indent
.It Cm name Ar identifier
optional name (listed by "ipfw pipe show")
to identify the delay distribution;
.It Cm bw Ar value
the bandwidth used for the pipe.
If not specified here, it must be present
explicitly as a configuration parameter for the pipe;
.It Cm loss-level Ar L
the probability above which packets are lost.
(0.0 <= L <= 1.0, default 1.0 i.e. no loss);
.It Cm samples Ar N
the number of samples used in the internal
representation of the curve (2..1024; default 100);
.It Cm "delay prob" | "prob delay"
One of these two lines is mandatory and defines
the format of the following lines with data points.
.It Ar XXX Ar YYY
2 or more lines representing points in the curve,
with either delay or probability first, according
to the chosen format.
The unit for delay is milliseconds.
Data points do not need to be sorted.
Also, the number of actual lines can be different
from the value of the "samples" parameter:
.Nm
utility will sort and interpolate
the curve as needed.
.El
.Pp
Example of a profile file:
.Bd -literal -offset indent
name    bla_bla_bla
samples 100
loss-level    0.86
prob    delay
0       200	# minimum overhead is 200ms
0.5     200
0.5     300
0.8     1000
0.9     1300
1       1300
#configuration file end
.Ed
.El
.Pp
The following parameters can be configured for a queue:
.Pp
.Bl -tag -width indent -compact
.It Cm pipe Ar pipe_nr
Connects a queue to the specified pipe.
Multiple queues (with the same or different weights) can be connected to
the same pipe, which specifies the aggregate rate for the set of queues.
.Pp
.It Cm weight Ar weight
Specifies the weight to be used for flows matching this queue.
The weight must be in the range 1..100, and defaults to 1.
.El
.Pp
The following parameters can be configured for a scheduler:
.Pp
.Bl -tag -width indent -compact
.It Cm type Ar {fifo | wf2qp | rr | qfq}
specifies the scheduling algorithm to use.
.Bl -tag -width indent -compact
.It cm fifo
is just a FIFO scheduler (which means that all packets
are stored in the same queue as they arrive to the scheduler).
FIFO has O(1) per-packet time complexity, with very low
constants (estimate 60-80ns on a 2Ghz desktop machine)
but gives no service guarantees.
.It Cm wf2qp
implements the WF2Q+ algorithm, which is a Weighted Fair Queueing
algorithm which permits flows to share bandwidth according to
their weights. Note that weights are not priorities; even a flow
with a minuscule weight will never starve.
WF2Q+ has O(log N) per-packet processing cost, where N is the number
of flows, and is the default algorithm used by previous versions
dummynet's queues.
.It Cm rr
implements the Deficit Round Robin algorithm, which has O(1) processing
costs (roughly, 100-150ns per packet)
and permits bandwidth allocation according to weights, but
with poor service guarantees.
.It Cm qfq
implements the QFQ algorithm, which is a very fast variant of
WF2Q+, with similar service guarantees and O(1) processing
costs (roughly, 200-250ns per packet).
.El
.El
.Pp
In addition to the type, all parameters allowed for a pipe can also
be specified for a scheduler.
.Pp
Finally, the following parameters can be configured for both
pipes and queues:
.Pp
.Bl -tag -width XXXX -compact
.It Cm buckets Ar hash-table-size
Specifies the size of the hash table used for storing the
various queues.
Default value is 64 controlled by the
.Xr sysctl 8
variable
.Va net.inet.ip.dummynet.hash_size ,
allowed range is 16 to 65536.
.Pp
.It Cm mask Ar mask-specifier
Packets sent to a given pipe or queue by an
.Nm
rule can be further classified into multiple flows, each of which is then
sent to a different
.Em dynamic
pipe or queue.
A flow identifier is constructed by masking the IP addresses,
ports and protocol types as specified with the
.Cm mask
options in the configuration of the pipe or queue.
For each different flow identifier, a new pipe or queue is created
with the same parameters as the original object, and matching packets
are sent to it.
.Pp
Thus, when
.Em dynamic pipes
are used, each flow will get the same bandwidth as defined by the pipe,
whereas when
.Em dynamic queues
are used, each flow will share the parent's pipe bandwidth evenly
with other flows generated by the same queue (note that other queues
with different weights might be connected to the same pipe).
.br
Available mask specifiers are a combination of one or more of the following:
.Pp
.Cm dst-ip Ar mask ,
.Cm dst-ip6 Ar mask ,
.Cm src-ip Ar mask ,
.Cm src-ip6 Ar mask ,
.Cm dst-port Ar mask ,
.Cm src-port Ar mask ,
.Cm flow-id Ar mask ,
.Cm proto Ar mask
or
.Cm all ,
.Pp
where the latter means all bits in all fields are significant.
.Pp
.It Cm noerror
When a packet is dropped by a
.Nm dummynet
queue or pipe, the error
is normally reported to the caller routine in the kernel, in the
same way as it happens when a device queue fills up.
Setting this
option reports the packet as successfully delivered, which can be
needed for some experimental setups where you want to simulate
loss or congestion at a remote router.
.Pp
.It Cm plr Ar packet-loss-rate
Packet loss rate.
Argument
.Ar packet-loss-rate
is a floating-point number between 0 and 1, with 0 meaning no
loss, 1 meaning 100% loss.
The loss rate is internally represented on 31 bits.
.Pp
.It Cm queue Brq Ar slots | size Ns Cm Kbytes
Queue size, in
.Ar slots
or
.Cm KBytes .
Default value is 50 slots, which
is the typical queue size for Ethernet devices.
Note that for slow speed links you should keep the queue
size short or your traffic might be affected by a significant
queueing delay.
E.g., 50 max-sized ethernet packets (1500 bytes) mean 600Kbit
or 20s of queue on a 30Kbit/s pipe.
Even worse effects can result if you get packets from an
interface with a much larger MTU, e.g.\& the loopback interface
with its 16KB packets.
The
.Xr sysctl 8
variables
.Em net.inet.ip.dummynet.pipe_byte_limit
and
.Em net.inet.ip.dummynet.pipe_slot_limit
control the maximum lengths that can be specified.
.Pp
.It Cm red | gred Ar w_q Ns / Ns Ar min_th Ns / Ns Ar max_th Ns / Ns Ar max_p
Make use of the RED (Random Early Detection) queue management algorithm.
.Ar w_q
and
.Ar max_p
are floating
point numbers between 0 and 1 (0 not included), while
.Ar min_th
and
.Ar max_th
are integer numbers specifying thresholds for queue management
(thresholds are computed in bytes if the queue has been defined
in bytes, in slots otherwise).
The
.Nm dummynet
also supports the gentle RED variant (gred).
Three
.Xr sysctl 8
variables can be used to control the RED behaviour:
.Bl -tag -width indent
.It Va net.inet.ip.dummynet.red_lookup_depth
specifies the accuracy in computing the average queue
when the link is idle (defaults to 256, must be greater than zero)
.It Va net.inet.ip.dummynet.red_avg_pkt_size
specifies the expected average packet size (defaults to 512, must be
greater than zero)
.It Va net.inet.ip.dummynet.red_max_pkt_size
specifies the expected maximum packet size, only used when queue
thresholds are in bytes (defaults to 1500, must be greater than zero).
.El
.El
.Pp
When used with IPv6 data,
.Nm dummynet
currently has several limitations.
Information necessary to route link-local packets to an
interface is not available after processing by
.Nm dummynet
so those packets are dropped in the output path.
Care should be taken to ensure that link-local packets are not passed to
.Nm dummynet .
.Sh CHECKLIST
Here are some important points to consider when designing your
rules:
.Bl -bullet
.It
Remember that you filter both packets going
.Cm in
and
.Cm out .
Most connections need packets going in both directions.
.It
Remember to test very carefully.
It is a good idea to be near the console when doing this.
If you cannot be near the console,
use an auto-recovery script such as the one in
.Pa /usr/share/examples/ipfw/change_rules.sh .
.It
Do not forget the loopback interface.
.El
.Sh FINE POINTS
.Bl -bullet
.It
There are circumstances where fragmented datagrams are unconditionally
dropped.
TCP packets are dropped if they do not contain at least 20 bytes of
TCP header, UDP packets are dropped if they do not contain a full 8
byte UDP header, and ICMP packets are dropped if they do not contain
4 bytes of ICMP header, enough to specify the ICMP type, code, and
checksum.
These packets are simply logged as
.Dq pullup failed
since there may not be enough good data in the packet to produce a
meaningful log entry.
.It
Another type of packet is unconditionally dropped, a TCP packet with a
fragment offset of one.
This is a valid packet, but it only has one use, to try
to circumvent firewalls.
When logging is enabled, these packets are
reported as being dropped by rule -1.
.It
If you are logged in over a network, loading the
.Xr kld 4
version of
.Nm
is probably not as straightforward as you would think.
The following command line is recommended:
.Bd -literal -offset indent
kldload ipfw && \e
ipfw add 32000 allow ip from any to any
.Ed
.Pp
Along the same lines, doing an
.Bd -literal -offset indent
ipfw flush
.Ed
.Pp
in similar surroundings is also a bad idea.
.It
The
.Nm
filter list may not be modified if the system security level
is set to 3 or higher
(see
.Xr init 8
for information on system security levels).
.El
.Sh PACKET DIVERSION
A
.Xr divert 4
socket bound to the specified port will receive all packets
diverted to that port.
If no socket is bound to the destination port, or if the divert module is
not loaded, or if the kernel was not compiled with divert socket support,
the packets are dropped.
.Sh NETWORK ADDRESS TRANSLATION (NAT)
.Pp
.Nm
support in-kernel NAT using the kernel version of
.Xr libalias 3 .
.Pp
The nat configuration command is the following:
.Bd -ragged -offset indent
.Bk -words
.Cm nat 
.Ar nat_number 
.Cm config 
.Ar nat-configuration
.Ek
.Ed
.Pp
The following parameters can be configured:
.Bl -tag -width indent
.It Cm ip Ar ip_address
Define an ip address to use for aliasing.
.It Cm if Ar nic
Use ip address of NIC for aliasing, dynamically changing
it if NIC's ip address changes.
.It Cm log
Enable logging on this nat instance.
.It Cm deny_in
Deny any incoming connection from outside world.
.It Cm same_ports
Try to leave the alias port numbers unchanged from
the actual local port numbers.
.It Cm unreg_only
Traffic on the local network not originating from an
unregistered address spaces will be ignored.
.It Cm reset
Reset table of the packet aliasing engine on address change.
.It Cm reverse
Reverse the way libalias handles aliasing.
.It Cm proxy_only
Obey transparent proxy rules only, packet aliasing is not performed.
.El
.Pp
To let the packet continue after being (de)aliased, set the sysctl variable
.Va net.inet.ip.fw.one_pass 
to 0.
For more information about aliasing modes, refer to
.Xr libalias 3 .
See Section
.Sx EXAMPLES
for some examples about nat usage.
.Ss REDIRECT AND LSNAT SUPPORT IN IPFW
Redirect and LSNAT support follow closely the syntax used in
.Xr natd 8 . 
See Section
.Sx EXAMPLES
for some examples on how to do redirect and lsnat.
.Ss SCTP NAT SUPPORT
SCTP nat can be configured in a similar manner to TCP through the
.Nm
command line tool.
The main difference is that 
.Nm sctp nat 
does not do port translation.
Since the local and global side ports will be the same,
there is no need to specify both.
Ports are redirected as follows:
.Bd -ragged -offset indent
.Bk -words
.Cm nat 
.Ar nat_number 
.Cm config if
.Ar nic
.Cm redirect_port sctp
.Ar ip_address [,addr_list] {[port | port-port] [,ports]}
.Ek
.Ed
.Pp
Most
.Nm sctp nat
configuration can be done in real-time through the
.Xr sysctl 8
interface.
All may be changed dynamically, though the hash_table size will only
change for new
.Nm nat
instances.
See
.Sx SYSCTL VARIABLES 
for more info.
.Sh SYSCTL VARIABLES
A set of
.Xr sysctl 8
variables controls the behaviour of the firewall and
associated modules
.Pq Nm dummynet , bridge , sctp nat .
These are shown below together with their default value
(but always check with the
.Xr sysctl 8
command what value is actually in use) and meaning:
.Bl -tag -width indent
.It Va net.inet.ip.alias.sctp.accept_global_ootb_addip: No 0
Defines how the 
.Nm nat 
responds to receipt of global OOTB ASCONF-AddIP:
.Bl -tag -width indent
.It Cm 0
No response (unless a partially matching association exists -
ports and vtags match but global address does not)
.It Cm 1
.Nm nat 
will accept and process all OOTB global AddIP messages.
.El
.Pp
Option 1 should never be selected as this forms a security risk.
An attacker can
establish multiple fake associations by sending AddIP messages.
.It Va net.inet.ip.alias.sctp.chunk_proc_limit: No 5
Defines the maximum number of chunks in an SCTP packet that will be parsed for a
packet that matches an existing association.
This value is enforced to be greater or equal than 
.Cm net.inet.ip.alias.sctp.initialising_chunk_proc_limit . 
A high value is
a DoS risk yet setting too low a value may result in important control chunks in
the packet not being located and parsed.
.It Va net.inet.ip.alias.sctp.error_on_ootb: No 1
Defines when the
.Nm nat 
responds to any Out-of-the-Blue (OOTB) packets with ErrorM packets.
An OOTB packet is a packet that arrives with no existing association
registered in the
.Nm nat 
and is not an INIT or ASCONF-AddIP packet:
.Bl -tag -width indent
.It Cm 0
ErrorM is never sent in response to OOTB packets.
.It Cm 1
ErrorM is only sent to OOTB packets received on the local side.
.It Cm 2
ErrorM is sent to the local side and on the global side ONLY if there is a
partial match (ports and vtags match but the source global IP does not).
This value is only useful if the 
.Nm nat 
is tracking global IP addresses.
.It Cm 3
ErrorM is sent in response to all OOTB packets on both the local and global side
(DoS risk).
.El
.Pp
At the moment the default is 0, since the ErrorM packet is not yet
supported by most SCTP stacks.
When it is supported, and if not tracking
global addresses, we recommend setting this value to 1 to allow
multi-homed local hosts to function with the 
.Nm nat .
To track global addresses, we recommend setting this value to 2 to
allow global hosts to be informed when they need to (re)send an
ASCONF-AddIP.
Value 3 should never be chosen (except for debugging) as the
.Nm nat 
will respond to all OOTB global packets (a DoS risk).
.It Va net.inet.ip.alias.sctp.hashtable_size: No 2003
Size of hash tables used for 
.Nm nat 
lookups (100 < prime_number > 1000001).
This value sets the 
.Nm hash table 
size for any future created 
.Nm nat
instance and therefore must be set prior to creating a 
.Nm nat 
instance.
The table sizes may be changed to suit specific needs.
If there will be few
concurrent associations, and memory is scarce, you may make these smaller.
If there will be many thousands (or millions) of concurrent associations, you
should make these larger.
A prime number is best for the table size.
The sysctl
update function will adjust your input value to the next highest prime number.
.It Va net.inet.ip.alias.sctp.holddown_time:  No 0
Hold association in table for this many seconds after receiving a
SHUTDOWN-COMPLETE.
This allows endpoints to correct shutdown gracefully if a
shutdown_complete is lost and retransmissions are required.
.It Va net.inet.ip.alias.sctp.init_timer: No 15
Timeout value while waiting for (INIT-ACK|AddIP-ACK).
This value cannot be 0.
.It Va net.inet.ip.alias.sctp.initialising_chunk_proc_limit: No 2
Defines the maximum number of chunks in an SCTP packet that will be parsed when
no existing association exists that matches that packet.
Ideally this packet
will only be an INIT or ASCONF-AddIP packet.
A higher value may become a DoS
risk as malformed packets can consume processing resources.
.It Va net.inet.ip.alias.sctp.param_proc_limit: No 25
Defines the maximum number of parameters within a chunk that will be parsed in a
packet.
As for other similar sysctl variables, larger values pose a DoS risk.
.It Va net.inet.ip.alias.sctp.log_level: No 0 
Level of detail in the system log messages (0 \- minimal, 1 \- event,
2 \- info, 3 \- detail, 4 \- debug, 5 \- max debug). May be a good
option in high loss environments.
.It Va net.inet.ip.alias.sctp.shutdown_time: No 15
Timeout value while waiting for SHUTDOWN-COMPLETE.
This value cannot be 0.
.It Va net.inet.ip.alias.sctp.track_global_addresses: No 0
Enables/disables global IP address tracking within the 
.Nm nat 
and places an
upper limit on the number of addresses tracked for each association:
.Bl -tag -width indent
.It Cm 0
Global tracking is disabled
.It Cm >1
Enables tracking, the maximum number of addresses tracked for each
association is limited to this value
.El
.Pp
This variable is fully dynamic, the new value will be adopted for all newly
arriving associations, existing associations are treated as they were previously.
Global tracking will decrease the number of collisions within the 
.Nm nat 
at a cost
of increased processing load, memory usage, complexity, and possible 
.Nm nat 
state
problems in complex networks with multiple 
.Nm nats . 
We recommend not tracking
global IP addresses, this will still result in a fully functional 
.Nm nat .
.It Va net.inet.ip.alias.sctp.up_timer: No 300
Timeout value to keep an association up with no traffic.
This value cannot be 0.
.It Va net.inet.ip.dummynet.expire : No 1
Lazily delete dynamic pipes/queue once they have no pending traffic.
You can disable this by setting the variable to 0, in which case
the pipes/queues will only be deleted when the threshold is reached.
.It Va net.inet.ip.dummynet.hash_size : No 64
Default size of the hash table used for dynamic pipes/queues.
This value is used when no
.Cm buckets
option is specified when configuring a pipe/queue.
.It Va net.inet.ip.dummynet.io_fast : No 0
If set to a non-zero value,
the
.Dq fast
mode of
.Nm dummynet
operation (see above) is enabled.
.It Va net.inet.ip.dummynet.io_pkt
Number of packets passed to
.Nm dummynet .
.It Va net.inet.ip.dummynet.io_pkt_drop
Number of packets dropped by
.Nm dummynet .
.It Va net.inet.ip.dummynet.io_pkt_fast
Number of packets bypassed by the
.Nm dummynet
scheduler.
.It Va net.inet.ip.dummynet.max_chain_len : No 16
Target value for the maximum number of pipes/queues in a hash bucket.
The product
.Cm max_chain_len*hash_size
is used to determine the threshold over which empty pipes/queues
will be expired even when
.Cm net.inet.ip.dummynet.expire=0 .
.It Va net.inet.ip.dummynet.red_lookup_depth : No 256
.It Va net.inet.ip.dummynet.red_avg_pkt_size : No 512
.It Va net.inet.ip.dummynet.red_max_pkt_size : No 1500
Parameters used in the computations of the drop probability
for the RED algorithm.
.It Va net.inet.ip.dummynet.pipe_byte_limit : No 1048576
.It Va net.inet.ip.dummynet.pipe_slot_limit : No 100
The maximum queue size that can be specified in bytes or packets.
These limits prevent accidental exhaustion of resources such as mbufs.
If you raise these limits,
you should make sure the system is configured so that sufficient resources
are available.
.It Va net.inet.ip.fw.autoinc_step : No 100
Delta between rule numbers when auto-generating them.
The value must be in the range 1..1000.
.It Va net.inet.ip.fw.curr_dyn_buckets : Va net.inet.ip.fw.dyn_buckets
The current number of buckets in the hash table for dynamic rules
(readonly).
.It Va net.inet.ip.fw.debug : No 1
Controls debugging messages produced by
.Nm .
.It Va net.inet.ip.fw.default_rule : No 65535
The default rule number (read-only).
By the design of
.Nm , the default rule is the last one, so its number
can also serve as the highest number allowed for a rule.
.It Va net.inet.ip.fw.dyn_buckets : No 256
The number of buckets in the hash table for dynamic rules.
Must be a power of 2, up to 65536.
It only takes effect when all dynamic rules have expired, so you
are advised to use a
.Cm flush
command to make sure that the hash table is resized.
.It Va net.inet.ip.fw.dyn_count : No 3
Current number of dynamic rules
(read-only).
.It Va net.inet.ip.fw.dyn_keepalive : No 1
Enables generation of keepalive packets for
.Cm keep-state
rules on TCP sessions.
A keepalive is generated to both
sides of the connection every 5 seconds for the last 20
seconds of the lifetime of the rule.
.It Va net.inet.ip.fw.dyn_max : No 8192
Maximum number of dynamic rules.
When you hit this limit, no more dynamic rules can be
installed until old ones expire.
.It Va net.inet.ip.fw.dyn_ack_lifetime : No 300
.It Va net.inet.ip.fw.dyn_syn_lifetime : No 20
.It Va net.inet.ip.fw.dyn_fin_lifetime : No 1
.It Va net.inet.ip.fw.dyn_rst_lifetime : No 1
.It Va net.inet.ip.fw.dyn_udp_lifetime : No 5
.It Va net.inet.ip.fw.dyn_short_lifetime : No 30
These variables control the lifetime, in seconds, of dynamic
rules.
Upon the initial SYN exchange the lifetime is kept short,
then increased after both SYN have been seen, then decreased
again during the final FIN exchange or when a RST is received.
Both
.Em dyn_fin_lifetime
and
.Em dyn_rst_lifetime
must be strictly lower than 5 seconds, the period of
repetition of keepalives.
The firewall enforces that.
.It Va net.inet.ip.fw.enable : No 1
Enables the firewall.
Setting this variable to 0 lets you run your machine without
firewall even if compiled in.
.It Va net.inet6.ip6.fw.enable : No 1
provides the same functionality as above for the IPv6 case.
.It Va net.inet.ip.fw.one_pass : No 1
When set, the packet exiting from the
.Nm dummynet
pipe or from
.Xr ng_ipfw 4
node is not passed though the firewall again.
Otherwise, after an action, the packet is
reinjected into the firewall at the next rule.
.It Va net.inet.ip.fw.tables_max : No 128
Maximum number of tables (read-only).
.It Va net.inet.ip.fw.verbose : No 1
Enables verbose messages.
.It Va net.inet.ip.fw.verbose_limit : No 0
Limits the number of messages produced by a verbose firewall.
.It Va net.inet6.ip6.fw.deny_unknown_exthdrs : No 1
If enabled packets with unknown IPv6 Extension Headers will be denied.
.It Va net.link.ether.ipfw : No 0
Controls whether layer-2 packets are passed to
.Nm .
Default is no.
.It Va net.link.bridge.ipfw : No 0
Controls whether bridged packets are passed to
.Nm .
Default is no.
.El
.Pp
.Sh EXAMPLES
There are far too many possible uses of
.Nm
so this Section will only give a small set of examples.
.Pp
.Ss BASIC PACKET FILTERING
This command adds an entry which denies all tcp packets from
.Em cracker.evil.org
to the telnet port of
.Em wolf.tambov.su
from being forwarded by the host:
.Pp
.Dl "ipfw add deny tcp from cracker.evil.org to wolf.tambov.su telnet"
.Pp
This one disallows any connection from the entire cracker's
network to my host:
.Pp
.Dl "ipfw add deny ip from 123.45.67.0/24 to my.host.org"
.Pp
A first and efficient way to limit access (not using dynamic rules)
is the use of the following rules:
.Pp
.Dl "ipfw add allow tcp from any to any established"
.Dl "ipfw add allow tcp from net1 portlist1 to net2 portlist2 setup"
.Dl "ipfw add allow tcp from net3 portlist3 to net3 portlist3 setup"
.Dl "..."
.Dl "ipfw add deny tcp from any to any"
.Pp
The first rule will be a quick match for normal TCP packets,
but it will not match the initial SYN packet, which will be
matched by the
.Cm setup
rules only for selected source/destination pairs.
All other SYN packets will be rejected by the final
.Cm deny
rule.
.Pp
If you administer one or more subnets, you can take advantage
of the address sets and or-blocks and write extremely
compact rulesets which selectively enable services to blocks
of clients, as below:
.Pp
.Dl "goodguys=\*q{ 10.1.2.0/24{20,35,66,18} or 10.2.3.0/28{6,3,11} }\*q"
.Dl "badguys=\*q10.1.2.0/24{8,38,60}\*q"
.Dl ""
.Dl "ipfw add allow ip from ${goodguys} to any"
.Dl "ipfw add deny ip from ${badguys} to any"
.Dl "... normal policies ..."
.Pp
The
.Cm verrevpath
option could be used to do automated anti-spoofing by adding the
following to the top of a ruleset:
.Pp
.Dl "ipfw add deny ip from any to any not verrevpath in"
.Pp
This rule drops all incoming packets that appear to be coming to the
system on the wrong interface.
For example, a packet with a source
address belonging to a host on a protected internal network would be
dropped if it tried to enter the system from an external interface.
.Pp
The
.Cm antispoof
option could be used to do similar but more restricted anti-spoofing
by adding the following to the top of a ruleset:
.Pp
.Dl "ipfw add deny ip from any to any not antispoof in"
.Pp
This rule drops all incoming packets that appear to be coming from another
directly connected system but on the wrong interface.
For example, a packet with a source address of
.Li 192.168.0.0/24 ,
configured on
.Li fxp0 ,
but coming in on
.Li fxp1
would be dropped.
.Ss DYNAMIC RULES
In order to protect a site from flood attacks involving fake
TCP packets, it is safer to use dynamic rules:
.Pp
.Dl "ipfw add check-state"
.Dl "ipfw add deny tcp from any to any established"
.Dl "ipfw add allow tcp from my-net to any setup keep-state"
.Pp
This will let the firewall install dynamic rules only for
those connection which start with a regular SYN packet coming
from the inside of our network.
Dynamic rules are checked when encountering the first
.Cm check-state
or
.Cm keep-state
rule.
A
.Cm check-state
rule should usually be placed near the beginning of the
ruleset to minimize the amount of work scanning the ruleset.
Your mileage may vary.
.Pp
To limit the number of connections a user can open
you can use the following type of rules:
.Pp
.Dl "ipfw add allow tcp from my-net/24 to any setup limit src-addr 10"
.Dl "ipfw add allow tcp from any to me setup limit src-addr 4"
.Pp
The former (assuming it runs on a gateway) will allow each host
on a /24 network to open at most 10 TCP connections.
The latter can be placed on a server to make sure that a single
client does not use more than 4 simultaneous connections.
.Pp
.Em BEWARE :
stateful rules can be subject to denial-of-service attacks
by a SYN-flood which opens a huge number of dynamic rules.
The effects of such attacks can be partially limited by
acting on a set of
.Xr sysctl 8
variables which control the operation of the firewall.
.Pp
Here is a good usage of the
.Cm list
command to see accounting records and timestamp information:
.Pp
.Dl ipfw -at list
.Pp
or in short form without timestamps:
.Pp
.Dl ipfw -a list
.Pp
which is equivalent to:
.Pp
.Dl ipfw show
.Pp
Next rule diverts all incoming packets from 192.168.2.0/24
to divert port 5000:
.Pp
.Dl ipfw divert 5000 ip from 192.168.2.0/24 to any in
.Pp
.Ss TRAFFIC SHAPING
The following rules show some of the applications of
.Nm
and
.Nm dummynet
for simulations and the like.
.Pp
This rule drops random incoming packets with a probability
of 5%:
.Pp
.Dl "ipfw add prob 0.05 deny ip from any to any in"
.Pp
A similar effect can be achieved making use of
.Nm dummynet
pipes:
.Pp
.Dl "ipfw add pipe 10 ip from any to any"
.Dl "ipfw pipe 10 config plr 0.05"
.Pp
We can use pipes to artificially limit bandwidth, e.g.\& on a
machine acting as a router, if we want to limit traffic from
local clients on 192.168.2.0/24 we do:
.Pp
.Dl "ipfw add pipe 1 ip from 192.168.2.0/24 to any out"
.Dl "ipfw pipe 1 config bw 300Kbit/s queue 50KBytes"
.Pp
note that we use the
.Cm out
modifier so that the rule is not used twice.
Remember in fact that
.Nm
rules are checked both on incoming and outgoing packets.
.Pp
Should we want to simulate a bidirectional link with bandwidth
limitations, the correct way is the following:
.Pp
.Dl "ipfw add pipe 1 ip from any to any out"
.Dl "ipfw add pipe 2 ip from any to any in"
.Dl "ipfw pipe 1 config bw 64Kbit/s queue 10Kbytes"
.Dl "ipfw pipe 2 config bw 64Kbit/s queue 10Kbytes"
.Pp
The above can be very useful, e.g.\& if you want to see how
your fancy Web page will look for a residential user who
is connected only through a slow link.
You should not use only one pipe for both directions, unless
you want to simulate a half-duplex medium (e.g.\& AppleTalk,
Ethernet, IRDA).
It is not necessary that both pipes have the same configuration,
so we can also simulate asymmetric links.
.Pp
Should we want to verify network performance with the RED queue
management algorithm:
.Pp
.Dl "ipfw add pipe 1 ip from any to any"
.Dl "ipfw pipe 1 config bw 500Kbit/s queue 100 red 0.002/30/80/0.1"
.Pp
Another typical application of the traffic shaper is to
introduce some delay in the communication.
This can significantly affect applications which do a lot of Remote
Procedure Calls, and where the round-trip-time of the
connection often becomes a limiting factor much more than
bandwidth:
.Pp
.Dl "ipfw add pipe 1 ip from any to any out"
.Dl "ipfw add pipe 2 ip from any to any in"
.Dl "ipfw pipe 1 config delay 250ms bw 1Mbit/s"
.Dl "ipfw pipe 2 config delay 250ms bw 1Mbit/s"
.Pp
Per-flow queueing can be useful for a variety of purposes.
A very simple one is counting traffic:
.Pp
.Dl "ipfw add pipe 1 tcp from any to any"
.Dl "ipfw add pipe 1 udp from any to any"
.Dl "ipfw add pipe 1 ip from any to any"
.Dl "ipfw pipe 1 config mask all"
.Pp
The above set of rules will create queues (and collect
statistics) for all traffic.
Because the pipes have no limitations, the only effect is
collecting statistics.
Note that we need 3 rules, not just the last one, because
when
.Nm
tries to match IP packets it will not consider ports, so we
would not see connections on separate ports as different
ones.
.Pp
A more sophisticated example is limiting the outbound traffic
on a net with per-host limits, rather than per-network limits:
.Pp
.Dl "ipfw add pipe 1 ip from 192.168.2.0/24 to any out"
.Dl "ipfw add pipe 2 ip from any to 192.168.2.0/24 in"
.Dl "ipfw pipe 1 config mask src-ip 0x000000ff bw 200Kbit/s queue 20Kbytes"
.Dl "ipfw pipe 2 config mask dst-ip 0x000000ff bw 200Kbit/s queue 20Kbytes"
.Ss LOOKUP TABLES
In the following example, we need to create several traffic bandwidth
classes and we need different hosts/networks to fall into different classes.
We create one pipe for each class and configure them accordingly.
Then we create a single table and fill it with IP subnets and addresses.
For each subnet/host we set the argument equal to the number of the pipe
that it should use.
Then we classify traffic using a single rule:
.Pp
.Dl "ipfw pipe 1 config bw 1000Kbyte/s"
.Dl "ipfw pipe 4 config bw 4000Kbyte/s"
.Dl "..."
.Dl "ipfw table 1 add 192.168.2.0/24 1"
.Dl "ipfw table 1 add 192.168.0.0/27 4"
.Dl "ipfw table 1 add 192.168.0.2 1"
.Dl "..."
.Dl "ipfw add pipe tablearg ip from table(1) to any"
.Pp
Using the
.Cm fwd
action, the table entries may include hostnames and IP addresses.
.Pp
.Dl "ipfw table 1 add 192.168.2.0/24 10.23.2.1"
.Dl "ipfw table 1 add 192.168.0.0/27 router1.dmz"
.Dl "..."
.Dl "ipfw add 100 fwd tablearg ip from any to table(1)"
.Ss SETS OF RULES
To add a set of rules atomically, e.g.\& set 18:
.Pp
.Dl "ipfw set disable 18"
.Dl "ipfw add NN set 18 ...         # repeat as needed"
.Dl "ipfw set enable 18"
.Pp
To delete a set of rules atomically the command is simply:
.Pp
.Dl "ipfw delete set 18"
.Pp
To test a ruleset and disable it and regain control if something goes wrong:
.Pp
.Dl "ipfw set disable 18"
.Dl "ipfw add NN set 18 ...         # repeat as needed"
.Dl "ipfw set enable 18; echo done; sleep 30 && ipfw set disable 18"
.Pp
Here if everything goes well, you press control-C before the "sleep"
terminates, and your ruleset will be left active.
Otherwise, e.g.\& if
you cannot access your box, the ruleset will be disabled after
the sleep terminates thus restoring the previous situation.
.Pp
To show rules of the specific set:
.Pp
.Dl "ipfw set 18 show"
.Pp
To show rules of the disabled set:
.Pp
.Dl "ipfw -S set 18 show"
.Pp
To clear a specific rule counters of the specific set:
.Pp
.Dl "ipfw set 18 zero NN"
.Pp
To delete a specific rule of the specific set:
.Pp
.Dl "ipfw set 18 delete NN"
.Ss NAT, REDIRECT AND LSNAT
First redirect all the traffic to nat instance 123:
.Pp
.Dl "ipfw add nat 123 all from any to any"
.Pp
Then to configure nat instance 123 to alias all the outgoing traffic with ip
192.168.0.123, blocking all incoming connections, trying to keep
same ports on both sides, clearing aliasing table on address change 
and keeping a log of traffic/link statistics:
.Pp
.Dl "ipfw nat 123 config ip 192.168.0.123 log deny_in reset same_ports"
.Pp
Or to change address of instance 123, aliasing table will be cleared (see
reset option):
.Pp
.Dl "ipfw nat 123 config ip 10.0.0.1"
.Pp
To see configuration of nat instance 123:
.Pp
.Dl "ipfw nat 123 show config"
.Pp
To show logs of all the instances in range 111-999:
.Pp
.Dl "ipfw nat 111-999 show"
.Pp
To see configurations of all instances:
.Pp
.Dl "ipfw nat show config"
.Pp
Or a redirect rule with mixed modes could looks like:
.Pp
.Dl "ipfw nat 123 config redirect_addr 10.0.0.1 10.0.0.66"
.Dl "			 redirect_port tcp 192.168.0.1:80 500"
.Dl "			 redirect_proto udp 192.168.1.43 192.168.1.1"
.Dl "			 redirect_addr 192.168.0.10,192.168.0.11"
.Dl "			 	    10.0.0.100	# LSNAT"
.Dl "			 redirect_port tcp 192.168.0.1:80,192.168.0.10:22" 
.Dl "			 	    500		# LSNAT"
.Pp
or it could be split in:
.Pp
.Dl "ipfw nat 1 config redirect_addr 10.0.0.1 10.0.0.66"
.Dl "ipfw nat 2 config redirect_port tcp 192.168.0.1:80 500"
.Dl "ipfw nat 3 config redirect_proto udp 192.168.1.43 192.168.1.1"
.Dl "ipfw nat 4 config redirect_addr 192.168.0.10,192.168.0.11,192.168.0.12" 
.Dl "				         10.0.0.100"
.Dl "ipfw nat 5 config redirect_port tcp"
.Dl "			192.168.0.1:80,192.168.0.10:22,192.168.0.20:25 500"
.Pp
.Sh SEE ALSO
.Xr cpp 1 ,
.Xr m4 1 ,
.Xr altq 4 ,
.Xr divert 4 ,
.Xr dummynet 4 ,
.Xr if_bridge 4 ,
.Xr ip 4 ,
.Xr ipfirewall 4 ,
.Xr ng_ipfw 4 ,
.Xr protocols 5 ,
.Xr services 5 ,
.Xr init 8 ,
.Xr kldload 8 ,
.Xr reboot 8 ,
.Xr sysctl 8 ,
.Xr syslogd 8
.Sh HISTORY
The
.Nm
utility first appeared in
.Fx 2.0 .
.Nm dummynet
was introduced in
.Fx 2.2.8 .
Stateful extensions were introduced in
.Fx 4.0 .
.Nm ipfw2
was introduced in Summer 2002.
.Sh AUTHORS
.An Ugen J. S. Antsilevich ,
.An Poul-Henning Kamp ,
.An Alex Nash ,
.An Archie Cobbs ,
.An Luigi Rizzo .
.Pp
.An -nosplit
API based upon code written by
.An Daniel Boulet
for BSDI.
.Pp
Dummynet has been introduced by Luigi Rizzo in 1997-1998.
.Pp
Some early work (1999-2000) on the
.Nm dummynet
traffic shaper supported by Akamba Corp.
.Pp
The ipfw core (ipfw2) has been completely redesigned and
reimplemented by Luigi Rizzo in summer 2002. Further
actions and
options have been added by various developer over the years.
.Pp
.An -nosplit
In-kernel NAT support written by
.An Paolo Pisati Aq piso@FreeBSD.org
as part of a Summer of Code 2005 project.
.Pp
SCTP
.Nm nat
support has been developed by
.An The Centre for Advanced Internet Architectures (CAIA) Aq http://www.caia.swin.edu.au .
The primary developers and maintainers are David Hayes and Jason But.
For further information visit:
.Aq http://www.caia.swin.edu.au/urp/SONATA
.Pp
Delay profiles have been developed by Alessandro Cerri and
Luigi Rizzo, supported by the
European Commission within Projects Onelab and Onelab2.
.Sh BUGS
The syntax has grown over the years and sometimes it might be confusing.
Unfortunately, backward compatibility prevents cleaning up mistakes
made in the definition of the syntax.
.Pp
.Em !!! WARNING !!!
.Pp
Misconfiguring the firewall can put your computer in an unusable state,
possibly shutting down network services and requiring console access to
regain control of it.
.Pp
Incoming packet fragments diverted by
.Cm divert
are reassembled before delivery to the socket.
The action used on those packet is the one from the
rule which matches the first fragment of the packet.
.Pp
Packets diverted to userland, and then reinserted by a userland process
may lose various packet attributes.
The packet source interface name
will be preserved if it is shorter than 8 bytes and the userland process
saves and reuses the sockaddr_in
(as does
.Xr natd 8 ) ;
otherwise, it may be lost.
If a packet is reinserted in this manner, later rules may be incorrectly
applied, making the order of
.Cm divert
rules in the rule sequence very important.
.Pp
Dummynet drops all packets with IPv6 link-local addresses.
.Pp
Rules using
.Cm uid
or
.Cm gid
may not behave as expected.
In particular, incoming SYN packets may
have no uid or gid associated with them since they do not yet belong
to a TCP connection, and the uid/gid associated with a packet may not
be as expected if the associated process calls
.Xr setuid 2
or similar system calls.
.Pp
Rule syntax is subject to the command line environment and some patterns
may need to be escaped with the backslash character
or quoted appropriately.
.Pp
Due to the architecture of 
.Xr libalias 3 , 
ipfw nat is not compatible with the TCP segmentation offloading (TSO).
Thus, to reliably nat your network traffic, please disable TSO
on your NICs using
.Xr ifconfig 8 .
.Pp
ICMP error messages are not implicitly matched by dynamic rules
for the respective conversations.
To avoid failures of network error detection and path MTU discovery,
ICMP error messages may need to be allowed explicitly through static
rules.