freebsd-skq/sbin/ipfw/ipfw.8

.\"
.\" $FreeBSD$
.\"
.Dd May 31, 2001
.Dt IPFW 8
.Os
.Sh NAME
.Nm ipfw
.Nd IP firewall and traffic shaper control program
.Sh SYNOPSIS
.Nm
.Op Fl q
.Cm add
.Ar rule
.Nm
.Op Fl q
.Cm delete
.Op Cm set
.Op Ar number ...
.Nm
.Op Fl adeftNS
.Brq Cm list | show
.Op Ar number ...
.Nm
.Op Fl f | q
.Cm flush
.Nm
.Op Fl q
.Brq Cm zero | resetlog
.Op Cm set
.Op Ar number ...
.Nm
.Op Fl q
.Brq Cm disable | enable
.Cm set
.Op Ar number ...
.Nm
.Cm show sets
.Pp
.Nm
.Brq Cm pipe | queue
.Ar number
.Cm config
.Ar config-options
.Nm
.Op Fl s Op Ar field
.Brq Cm pipe | queue
.Brq Cm delete | list | show
.Op Ar number ...
.Pp
.Nm
.Op Fl q
.Oo
.Fl p Ar preproc
.Oo Fl D
.Ar macro Ns Op = Ns Ar value
.Oc
.Op Fl U Ar macro
.Oc
.Ar pathname
.Sh DESCRIPTION
The
.Nm
utility is the user interface for controlling the
.Xr ipfw 4
firewall and the
.Xr dummynet 4
traffic shaper in
.Fx .
.Pp
An
.Nm
configuration, or
.Em ruleset ,
is made of a list of
.Em rules
numbered from 1 to 65535.
Packets are passed to
.Nm
in a number of different places in the protocol stack
(depending on the source and destination of the packet,
it is possible that
.Nm
is invoked multiple times on the same packet).
The packet passed to the firewall is compared
against each of the rules in the firewall
.Em ruleset .
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
An
.Nm
ruleset always includes a
.Em default
rule (numbered 65535) which cannot be modified,
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, then
.Nm
assumes a
.Em stateful
behaviour, i.e. upon a match it will create dynamic rules matching
the exact parameters (addresses and ports) of the matching packet.
.Pp
These 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 RULE FORMAT
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
Rules can be added with the
.Cm add
command; deleted individually or in groups with the
.Cm delete
command, and globally 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
The following options are available:
.Bl -tag -width indent
.It Fl a
While listing, show counter values.
The
.Cm show
command just implies this option.
.It Fl d
While listing, show dynamic rules in addition to static ones.
.It Fl e
While listing, if the
.Fl d
option was specified, also show expired dynamic rules.
.It Fl f
Don't ask for confirmation for commands that can cause problems
if misused,
.No i.e. Cm flush .
.Em Note ,
if there is no tty associated with the process, this is implied.
.It Fl N
Try to resolve addresses and service names in output.
.It Fl q
While
.Cm add Ns ing ,
.Cm zero Ns ing ,
.Cm resetlog Ns ging
or
.Cm flush Ns ing ,
be quiet about actions
(implies
.Fl f ) .
This is useful for adjusting rules by executing multiple
.Nm
commands in a script
(e.g.,
.Ql sh\ /etc/rc.firewall ) ,
or by processing a file of many
.Nm
rules,
across a remote login session.
If a
.Cm flush
is performed in normal (verbose) mode (with the default kernel
configuration), it prints a message.
Because all rules are flushed, the message might not be delivered
to the login session, causing the remote login session to be closed
and the remainder of the ruleset is not processed.
Access to the console would then be required to recover.
.It Fl S
While listing rules, show the set each rule belongs to.
If this flag is not specified, disabled rules will not be
listed.
.It Fl s Op Ar field
While listing pipes, sort according to one of the four
counters (total and current packets or bytes).
.It Fl t
While listing, show last match timestamp.
.El
.Pp
To ease configuration, rules can be put into a file which is
processed using
.Nm
as shown in the first 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
doesn't 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
filesystems are mounted (yet) by the time
.Nm
is being run (e.g. when they are mounted over NFS).
Once
.Fl p
has been specified, optional
.Fl D
and
.Fl U
specifications can follow and will be passed on to the preprocessor.
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
The
.Nm
.Cm pipe
and
.Cm queue
commands are used to configure the traffic shaper, as shown in the
.Sx TRAFFIC SHAPER CONFIGURATION
section below.
.Sh PACKET FLOW
.Nm
can be invoked from multiple places in the protocol stack,
under control of several system parameters,
and it is important to understand when this occurs in order to
design a proper ruleset. The places where
.Nm
is invoked are listed below, together with the sysctl variables
which control its invocation.
.Bd -literal -offset indent
      ^	    to upper layers   V
      |                       |
      +----------->-----------+
      ^                       V
 [ip_input]              [ip_output]   net.inet.ip.fw.enable=1
      |                       |
      ^                       V
[ether_demux]    [ether_output_frame]  net.link.ether.ipfw=1
      |                       |
      +-->--[bdg_forward]-->--+        net.link.ether.bridge_ipfw=1
      ^                       V
      |      to devices       |
.Ed
.Pp
As can be noted from the above picture, the number of
times the same packet goes through the firewall can
vary between 0 and 4 depending o packet source and
destination, and system configuration.
In each of these places, the packet is passed to
.Nm
with all (and only) the fields that belong to that level.
That is, 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() .
.br
The complete ruleset is always used,
irrespective of the place where
.Nm
is invoked, or the source of the packet.
If a rule contains some match patterns or actions which are not valid
for the place of invokation (e.g. trying
to match a MAC header when
.Nm
is called from
.Cm ip_input()
) the rule will simply not match. It is thus 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 RULE FORMAT
The format of
.Nm
rules is the following:
.Bd -ragged -offset indent
.Op Ar rule_number
.Op Cm set Ar set_number
.Op Cm prob Ar match_probability
.br
.Ar "   " action
.Op Cm log Op Cm logamount Ar number
.Ar body
.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 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 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 User/group ID
When the packet can be associate to a local socket.
.El
.Pp
Note that some of the above information, e.g. source MAC or IP addresses and
TCP/UDP ports, could easily be 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 same number of the last
non-default value is used instead.
.It Ar set_number
Each rule is associated to a
.Ar set_number
in the range 0..31, with the latter reserved for the
.Em default
rule.
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.
.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
.Xr dummynet 4 )
to simulate the effect of multiple paths leading to out-of-order
packet delivery.
.It Cm log Op Cm logamount Ar number
When a packet matches a rule with the
.Cm log
keyword, a message will be
logged to
.Xr syslogd 8
with a
.Dv LOG_SECURITY
facility.
The logging only occurs if the sysctl variable
.Em net.inet.ip.fw.verbose
is set to 1
(which is the default when the kernel is compiled with
.Dv IPFIREWALL_VERBOSE
) and the number of packets logged so far for that
particular rule does not exceed ther
.Cm logamount
parameter.
If no
.Cm logamount
is specified, the limit is taken from the sysctl variable
.Em net.inet.ip.fw.verbose_limit .
In both cases, a value of 0
removes the logging limit.
.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
.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 Ns Op , Ns Ar port
Change the next-hop on matching packets to
.Ar ipaddr ,
which can be an IP address in dotted quad or a host name.
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.
.It Cm pipe Ar pipe_nr
Pass packet to a
.Xr dummynet 4
.Dq pipe
(for bandwidth limitation, delay, etc.).
See the
.Sx TRAFFIC SHAPER CONFIGURATION
section for further information.
The search terminates; however, on exit from the pipe and if
the
.Xr sysctl 8
variable
.Em 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
.Xr dummynet 4
.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 skipto Ar number
Skip all subsequent rules numbered less than
.Ar number .
The search continues with the first rule numbered
.Ar number
or higher.
.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 terminates and the original packet is accepted
(but see section
.Sx BUGS
below).
.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.
.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)
.Em and
connectives -- i.e. all must match in order for the
rule to match.
Individual patterns can be prefixed by the
.Em not
keyword 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 (
.Em or-blocks
) can be constructed by putting the patterns in
lists enclosed between parentheses ( ) or braces { }, and
using
.Cm or
connectives 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 \\ in front of them.
.Pp
The body of a rule must in general comprise a source and destination
addres specifier.
The keyword
.Ar any
can be used in various places to specify that the content of
a required field is irrelevant.
.Pp
The general rule body format is one of the following:
.Bd -ragged -offset indent
.Ar proto
.Cm from Ar src
.Cm to Ar dst
.Op Ar options
.br
.Cm MAC Ar dst-mac src-mac mac-type
.Op Ar options
.Ed
.Pp
where fields have the following meaning:
.Bl -tag -width indent
.It Ar proto
An IP protocol specified by number or name (for a complete
list see
.Pa /etc/protocols ) .
The
.Cm ip
or
.Cm all
keywords mean any protocol will match.
.It Ar src No and Ar dst :
A single
.Ar ip address
, or an
.Em or-block
containing one or more of them,
optionally followed by
.Em port numbers.
followed by a set of port numbers.
.It Ar ip address :
An address (or set of addresses) specified in one of the following
ways, optionally preceded by a
.Cm not
operator:
.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.
The address list is evaluated at the time the packet is
analysed.
.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 a dotted quad or a hostname)
and mask width of
.Cm masklen
bits.
As an example, 1.2.3.4/25 will match
all IP numbers from 1.2.3.0 to 1.2.3.127 .
.It Ar addr Ns / Ns Ar masklen Ns Cm { Ns Ar num,num,... Ns Cm }
Matches all addresses with base address
.Ar addr
(specified as a dotted quad or a hostname)
and whose last byte is in the list between braces { } .
Note that there must be no spaces between braces, commas and
numbers.
The
.Ar masklen
field is used to limit the size of the set of addresses,
and can have any value between 24 and 32.
.br
As an example, an address specified as 1.2.3.4/24{128,35,55,89}
will match the following IP addresses:
.br
1.2.3.128 1.2.3.35 1.2.3.55 1.2.3.89 .
.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.
.El
.It Cm port numbers
With 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:
.Bd -ragged -offset indent
.Op Cm not
.Brq Ar port | port Ns \&- Ns Ar port Ns
.Op , Ns Ar ...
.Ed
.Pp
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 14 ports or ranges,
though you can also use port ranges within an
.Em or-block
to build essentially unlimited lists:
.Pp
.Dl "ipfw add allow tcp from any { 1-20,30-50 or 500-600 } to any"
.Pp
.Pp
A backslash
.Pq Ql \e
can be used to escape the dash
.Pq Ql -
character in a service name:
.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
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
Note that there is an ambiguity in the syntax: in a rule of
the form
.Pp
.Dl "ipfw add allow ip from any to any { in or layer2 }"
.Pp
the or-block could contain either port lists or options.
To remove the ambiguity, one should specify a destination
port, which can be done by either using the keyword
.Cm any
or an empty or-block
.Cm { }
e.g.:
.Pp
.Dl "ipfw add allow ip from any to any any { in or layer2 }"
.Pp
The following options are available:
.Bl -tag -width indent
.It Cm bridged
Matches only bridged packets.
.It Cm established
TCP packets only.
Match packets that have the RST or ACK bits set.
.It Cm frag
Match if the packet is a fragment and this is not the first
fragment of the datagram.
.Cm frag
may not be used in conjunction with either
.Cm tcpflags
or TCP/UDP port specifications.
.It Cm gid Ar group
Match all TCP or UDP packets sent by or received for a
.Ar group .
A
.Ar group
may be matched by name or identification number.
.It Cm icmptypes Ar types
ICMP packets only.
Match if the ICMP type is in the list
.Ar types .
The list may be specified as any combination of ranges or
individual types separated by commas.
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 in | out
Only match incoming or outgoing packets, respectively.
.Cm in
and
.Cm out
are mutually exclusive (in fact,
.Cm out
is implemented as
.Cm not in
).
.It Cm ipid Ar id
Match if the identification of IP datagram is
.Ar id .
.It Cm iplen Ar len
Match if the total length of a packet, including header and data, is
.Ar len
bytes.
.It Cm ipoptions Ar spec
Match if the IP 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
Match if the numeric value of IP datagram's precedence is equal to
.Ar precedence .
.It Cm iptos Ar spec
Match if the IP header 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_CE .
The absence of a particular type may be denoted
with a
.Ql \&! .
.It Cm ipttl Ar ttl
Match if the time to live of IP datagram is
.Ar ttl .
.It Cm ipversion Ar ver
Match if the IP header version is
.Ar ver .
.It Cm keep-state
Upon a match, the firewall will create a dynamic rule, whose
default behaviour is to matching 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.
.It Cm recv | xmit | via Brq Ar ifX | Ar if Ns Cm * | Ar ipno | Ar any
Packet must be received, transmitted or be going through,
respectively, the interface specified by exact name (
.Ar ifX
), by device name (
.Ar if Ns Cm *
), 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 may 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
TCP packets only.
Match packets that have the SYN bit set but no ACK bit.
This is the short form of
.Dq Li tcpflags\ syn,!ack .
.It Cm tcpack Ar ack
TCP packets only.
Match if the TCP header acknowledgment number field is set to
.Ar ack .
.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.
.El
.Sh STATEFUL FIREWALL
To be completed.
.Sh TRAFFIC SHAPER CONFIGURATION
The
.Nm
utility is also the user interface for the
.Xr dummynet 4
traffic shaper.
The shaper operates by dividing packets into
.Em flows
according to a user-specified mask on different fields
of the IP header.
Packets belonging to the same flow are then passed to two
different objects, named
.Em pipe
or
.Em queue .
.Pp
A
.Em pipe
emulates a link with given bandwidth, propagation delay,
queue size and packet loss rate.
Packets transit through the pipe according to its parameters.
.Pp
A
.Em queue
is an abstraction used to implement the WF2Q+ (Worst-case Fair Weighted Fair Queueing) policy.
The queue associates to each flow a weight and a reference pipe.
Then, all flows linked to the same pipe are scheduled at the
rate fixed by the pipe according to the WF2Q+ policy.
.Pp
The
.Nm
pipe configuration format is the following:
.Bd -ragged -offset indent
.Cm pipe Ar number Cm config Ar pipe-configuration
.Ed
.Pp
The
.Nm
queue configuration format is the following:
.Bd -ragged -offset indent
.Cm queue Ar number Cm config Ar queue-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 follow immediately 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, 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).
Default value is 0, meaning no delay.
.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 (usually
with 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
Finally, the following parameters can be configured for both
pipes and queues:
.Pp
.Bl -tag -width indent -compact
.Pp
.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
.Em net.inet.ip.dummynet.hash_size ,
allowed range is 16 to 1024.
.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 effect can result if you get packets from an
interface with a much larger MTU, e.g. the loopback interface
with its 16KB packets.
.Pp
.It Cm mask Ar mask-specifier
The
.Xr dummynet 4
lets you to create per-flow queues.
A flow identifier is constructed by masking the IP addresses,
ports and protocol types as specified in the pipe configuration.
Packets with the same identifier after masking fall into the
same queue.
Available mask specifiers are a combination of the following:
.Cm dst-ip Ar mask ,
.Cm src-ip Ar mask ,
.Cm dst-port Ar mask ,
.Cm src-port Ar mask ,
.Cm proto Ar mask
or
.Cm all ,
where the latter means all bits in all fields are significant.
When used within a
.Ar pipe
configuration, each flow is assigned a rate equal
to the rate of the pipe.
When used within a
.Ar queue
configuration, each flow is assigned a weight equal to the
weight of the queue, and all flows insisting on the same pipe
share bandwidth proportionally to their weight.
.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 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
.Xr dummynet 4
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 Em 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 Em net.inet.ip.dummynet.red_avg_pkt_size
specifies the expected average packet size (defaults to 512, must be
greater than zero)
.It Em 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
.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
Don't 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.
I recommend the following command line:
.Bd -literal -offset indent
kldload /modules/ipfw.ko && \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 kernel
wasn't compiled with divert socket support, the packets are
dropped.
.Sh SYSCTL VARIABLES
A set of
.Xr sysctl 8
variables controls the behaviour of the firewall.
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 Em net.inet.ip.fw.debug : No 1
Controls debugging messages produced by
.Nm .
.It Em net.inet.ip.fw.one_pass : No 1
When set, the packet exiting from the
.Xr dummynet 4
pipe is not passed though the firewall again.
Otherwise, after a pipe action, the packet is
reinjected into the firewall at the next rule.
.It Em net.inet.ip.fw.verbose : No 1
Enables verbose messages.
.It Em 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 Em net.inet.ip.fw.verbose_limit : No 0
Limits the number of messages produced by a verbose firewall.
.It Em net.inet.ip.fw.dyn_buckets : No 256
.It Em net.inet.ip.fw.curr_dyn_buckets : No 256
The configured and current size of the hash table used to
hold dynamic rules.
This must be a power of 2.
The table can only be resized when empty, so in order to
resize it on the fly you will probably have to
.Cm flush
and reload the ruleset.
.It Em net.inet.ip.fw.dyn_count : No 3
Current number of dynamic rules
(read-only).
.It Em net.inet.ip.fw.dyn_max : No 1000
Maximum number of dynamic rules.
When you hit this limit, no more dynamic rules can be
installed until old ones expire.
.It Em net.inet.ip.fw.dyn_ack_lifetime : No 300
.It Em net.inet.ip.fw.dyn_syn_lifetime : No 20
.It Em net.inet.ip.fw.dyn_fin_lifetime : No 1
.It Em net.inet.ip.fw.dyn_rst_lifetime : No 1
.It Em net.inet.ip.fw.dyn_udp_lifetime : No 5
.It Em 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
.It Em net.link.ether.ipfw : No 0
Controls whether layer-2 packets are passed to
.Nm .
Default is no.
.It Em net.link.ether.bridge_ipfw : No 0
Controls whether bridged packets are passed to
.Nm .
Default is no.
.El
.Sh EXAMPLES
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 crackers
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
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 be usually 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
The following rules show some of the applications of
.Nm
and
.Xr dummynet 4
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 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 like 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 which
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 like 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 affect a lot 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"
.Sh SEE ALSO
.Xr cpp 1 ,
.Xr m4 1 ,
.Xr bridge 4 ,
.Xr divert 4 ,
.Xr dummynet 4 ,
.Xr ip 4 ,
.Xr ipfirewall 4 ,
.Xr protocols 5 ,
.Xr services 5 ,
.Xr init 8 ,
.Xr kldload 8 ,
.Xr reboot 8 ,
.Xr sysctl 8 ,
.Xr syslogd 8
.Rs
.%A "S. Floyd"
.%A "V. Jacobson"
.%T "Random Early Detection gateways for Congestion Avoidance"
.%D "August 1993"
.Re
.Rs
.%A "B. Braden"
.%A "D. Clark"
.%A "J. Crowcroft"
.%A "B. Davie"
.%A "S. Deering"
.%A "D. Estrin"
.%A "S. Floyd"
.%A "V. Jacobson"
.%A "G. Minshall"
.%A "C. Partridge"
.%A "L. Peterson"
.%A "K. Ramakrishnan"
.%A "S. Shenker"
.%A "J. Wroclawski"
.%A "L. Zhang"
.%T "Recommendations on Queue Management and Congestion Avoidance in the Internet"
.%D "April 1998"
.%O "RFC 2309"
.Re
.Sh BUGS
The syntax has grown over the years and it is not very clean.
.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 to it.
.Pp
Incoming packet fragments diverted by
.Cm divert
or
.Cm tee
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 that match a
.Cm tee
rule should not be immediately accepted, but should continue
going through the rule list.
This may be fixed in a later version.
.Pp
Packets diverted to userland, and then reinserted by a userland process
(such as
.Xr natd 8 )
will lose various packet attributes, including their source interface.
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.
.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
Work on
.Xr dummynet 4
traffic shaper supported by Akamba Corp.
.Sh HISTORY
The
.Nm
utility first appeared in
.Fx 2.0 .
.Xr dummynet 4
was introduced in
.Fx 2.2.8 .
Stateful extensions were introduced in
.Fx 4.0 .