ipfw.8 revision 95858 

 $FreeBSD: head/sbin/ipfw/ipfw.8 95858 2002-05-01 06:29:16Z cjc $

.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
.Oo
.Fl p Ar preproc
.Oo Fl D
.Ar macro Ns Op = Ns Ar value
.Oc
.Op Fl U Ar macro
.Oc
.Ar pathname
.Nm
.Op Fl f | q
.Cm flush
.Nm
.Op Fl q
rq Cm zero | resetlog | delete .Op Ar number ...
.Nm
.Op Fl s Op Ar field
.Op Fl adeftN
rq Cm list | show .Op Ar number ...
.Nm
.Op Fl q
.Cm add
.Op Ar number
.Ar rule-body
.Nm
.Cm pipe
.Ar number
.Cm config
.Ar pipe-config-options
.Nm
.Cm pipe
rq Cm delete | list | show .Op Ar number ...
.Nm
.Cm queue
.Ar number
.Cm config
.Ar queue-config-options
.Nm
.Cm queue
rq Cm delete | list | show .Op Ar number ...
.Sh DESCRIPTION
.Nm
is the user interface for controlling the
.Xr ipfirewall 4
and the
.Xr dummynet 4
traffic shaper in
.Fx .

p
A firewall configuration is made of a list of numbered rules,
which is scanned for each incoming or outgoing IP packet
until a match is found and
the relevant action is performed.
Depending on the action and certain system settings, packets
can be reinjected into the firewall at the rule after the
matching one for further processing.
All rules apply to all interfaces, so it is responsibility
of the system administrator to write the ruleset in such a
way as to minimize the number of checks.

p
A configuration always includes a
.Em DEFAULT
rule (numbered 65535) which cannot be modified,
and matches all packets.
The action associated with the default rule can be either
.Cm deny
or
.Cm allow
depending on how the kernel is configured.

p
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.

p
These dynamic rules, which have a limited lifetime, are checked
at the first occurrence of a
.Cm check-state
or
.Cm keep-state
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 .

p
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.

p
Rules can be added with the
.Cm add
command; deleted individually 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.

p
The following options are available:
l -tag -width indent t Fl a While listing, show counter values.
The
.Cm show
command just implies this option.
t Fl d While listing, show dynamic rules in addition to static ones.
t Fl e While listing, if the
.Fl d
option was specified, also show expired dynamic rules.
t 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.
t 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 cannot be delivered
to the login session.
This causes the remote login session to be closed and the
remainder of the ruleset is not processed.
Access to the console is required to recover.
t Fl t While listing, show last match timestamp.
t Fl N Try to resolve addresses and service names in output.
t Fl s Op Ar field While listing pipes, sort according to one of the four
counters (total and current packets or bytes).
.El

p
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.

p
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

q 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.

p
The
.Nm
.Cm pipe
commands are used to configure the traffic shaper, as shown in the
.Sx TRAFFIC SHAPER CONFIGURATION
section below.
.Sh RULE FORMAT
The
.Nm
rule format is the following:
d -ragged .Op Cm prob Ar match_probability
.Ar action
.Op Cm log Op Cm logamount Ar number
.Ar proto
.Cm from Ar src
.Cm to Ar dst
.Op Ar interface-spec
.Op Ar options
.Ed

p
Each packet can be filtered based on the following information that is
associated with it:

p
l -tag -width "Source and destination IP address" -offset indent -compact t Protocol (TCP, UDP, ICMP, etc.)
t Source and destination IP address (possibly masked)
t Source and destination port (lists, ranges or masks)
t Direction (incoming or outgoing)
t Transmit and receive interface (by name or address)
t IP version t IP type of service t IP datagram length t IP identification t IP fragment flag (non-zero IP offset)
t IP time to live t IP options t TCP sequence number t TCP acknowledgment number t TCP flags (SYN, FIN, ACK, RST, etc.)
t TCP window t TCP options t ICMP types (for ICMP packets)
t User/group ID of the socket associated with the packet .El

p
Note that it may be dangerous to filter on the source IP
address or source TCP/UDP port because either or both could
easily be spoofed.
l -tag -width indent t 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.
t Ar action : l -tag -width indent t Cm allow Allow packets that match rule.
The search terminates.
Aliases are
.Cm pass ,
.Cm permit
and
.Cm accept .
t Cm deny Discard packets that match this rule.
The search terminates.
.Cm drop
is an alias for
.Cm deny .
t Cm reject (Deprecated).
Discard packets that match this rule, and try to send an ICMP
host unreachable notice.
The search terminates.
t 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.
t Cm reset TCP packets only.
Discard packets that match this rule, and try to send a TCP
reset (RST) notice.
The search terminates.
t Cm count Update counters for all packets that match rule.
The search continues with the next rule.
t Cm check-state Checks the packet against the dynamic ruleset.
If a match is found then the search terminates, otherwise
we move to the next rule.
If no
.Cm check-state
rule is found, the dynamic ruleset is checked at the first
.Cm keep-state
rule.
t Cm divert Ar port Divert packets that match this rule to the
.Xr divert 4
socket bound to port
.Ar port .
The search terminates.
t 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).
t Cm fwd 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.
If
.Ar ipaddr
is not a directly-reachable address, the route as found in
the local routing table for that IP is used instead.
If
.Ar ipaddr
is a local address, then on a packet matching a
.Cm fwd
rule,
it will be diverted to
.Ar port
on the local machine, keeping the local address of the socket
set to the original IP address the packet was destined for.
This makes the 
.Xr netstat 1
entry look rather weird but is intended for
use with transparent proxy servers.
If the IP is not a local address then the port number
(if specified) is ignored.
This will also map addresses when packets are
generated locally.
The search terminates if this rule matches.
If the port number is not given then the port number in the
packet is used, so that a packet for an external machine port
Y would be forwarded to local port Y.
The kernel must have been compiled with the
.Dv IPFIREWALL_FORWARD
option.
Bridging interferes with forwarding of packets not destined
to the local system as they bypass
.Fn ip_input
and
.Fn ip_output
where forwarding is implemented.
The
.Cm fwd
action does not change the contents of the packet at all 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.
t 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.
t Cm queue Ar queue_nr Pass packet to a
.Xr dummynet 4
.Dq queue
(for bandwidth limitation using WF2Q).
t 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.
.El
t Cm log Op Cm logamount Ar number If the kernel was compiled with
.Dv IPFIREWALL_VERBOSE ,
then 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.
.Em Note :
by default, they are appended to the

a /var/log/security
file (see
.Xr syslog.conf 5 ) .
If the kernel was compiled with the
.Dv IPFIREWALL_VERBOSE_LIMIT
option, then by default logging will cease after the number
of packets specified by the option are received for that
particular chain entry, and
.Em net.inet.ip.fw.verbose_limit
will be set to that number.
However, if
.Cm logamount Ar number
is used, that
.Ar number
will be the logging limit rather than
.Em net.inet.ip.fw.verbose_limit ,
where the value
.Dq 0
removes the logging limit.
Logging may then be re-enabled by clearing the logging counter
or the packet counter for that entry.

p
Console logging and the log limit are adjustable dynamically
through the
.Xr sysctl 8
interface in the MIB base of
.Em net.inet.ip.fw .
t Ar proto An IP protocol specified by number or name (for a complete
list see

a /etc/protocols ) .
The
.Cm ip
or
.Cm all
keywords mean any protocol will match.
t Ar src No and Ar dst : .Cm any | me | Op Cm not
.Aq Ar address Ns / Ns Ar mask
.Op Ar ports

p
Specifying
.Cm any
makes the rule match any IP address.

p
Specifying
.Cm me
makes the rule match any IP address configured on an interface in the system.

p
The
.Aq Ar address Ns / Ns Ar mask
may be specified as:
l -tag -width "ipno/bits" t Ar ipno An IP number of the form 1.2.3.4.
Only this exact IP number will match the rule.
t Ar ipno Ns / Ns Ar bits An IP number with a mask width of the form 1.2.3.4/24.
In this case all IP numbers from 1.2.3.0 to 1.2.3.255 will match.
t Ar ipno Ns : Ns Ar mask An IP number with a mask of the form 1.2.3.4:255.255.240.0.
In this case all IP numbers from 1.2.0.0 to 1.2.15.255 will match.
.El

p
The sense of the match can be inverted by preceding an address with the
.Cm not
modifier, causing all other addresses to be matched instead.
This does not affect the selection of port numbers.

p
With the TCP and UDP protocols, optional
.Em ports
may be specified as:
d -ragged -offset indent .Sm off
rq Ar port | port No - Ar port | port : mask .Op , Ar port Op , Ar ...
.Sm on
.Ed

p
The
.Ql -
notation specifies a range of ports (including boundaries).

p
The
.Ql :
notation specifies a port and a mask, a match is declared if
the port number in the packet matches the one in the rule,
limited to the bits which are set in the mask.

p
Service names (from

a /etc/services )
may be used instead of numeric port values.
A range may only be specified as the first value, and the
length of the port list is limited to
.Dv IP_FW_MAX_PORTS
ports (as defined in

a /usr/src/sys/netinet/ip_fw.h ) .
A backslash

q Ql \e
can be used to escape the dash

q Ql -
character in a service name:

p
.Dl "ipfw add count tcp from any ftp\e\e-data-ftp to any"

p
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.
t Ar interface-spec Some combinations of the following specifiers are allowed:
l -tag -width "via ipno" t Cm in Only match incoming packets.
t Cm out Only match outgoing packets.
t Cm via Ar ifX Packet must be going through interface
.Ar ifX .
t Cm via Ar if Ns Cm * Packet must be going through interface
.Ar ifX ,
where
.Ar X
is any unit number.
t Cm via any Packet must be going through
.Em some
interface.
t Cm via Ar ipno Packet must be going through the interface having IP address
.Ar ipno .
.El

p
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.:

p
.Dl "ipfw add 100 deny ip from any to any out recv ed0 xmit ed1"

p
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.
Specifying
.Cm via
together with
.Cm xmit
or
.Cm recv
is invalid.

p
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.
t Ar options : l -tag -width indent t 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.
t 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.
t Cm bridged Matches only bridged packets.
This can be useful for multicast or broadcast traffic, which
would otherwise pass through the firewall twice: once during
bridging, and a second time when the packet is delivered to
the local stack.

p
Apart from a small performance penalty, this would be a problem
when using
.Em pipes
because the same packet would be accounted for twice in terms
of bandwidth, queue occupation, and also counters.
t Cm ipversion Ar ver Match if the IP header version is
.Ar ver .
t Cm ipprecedence Ar precedence Match if the numeric value of IP datagram's precedence is equal to
.Ar precedence .
t 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:

p
.Cm lowdelay

q Dv IPTOS_LOWDELAY ,
.Cm throughput

q Dv IPTOS_THROUGHPUT ,
.Cm reliability

q Dv IPTOS_RELIABILITY ,
.Cm mincost

q Dv IPTOS_MINCOST ,
.Cm congestion

q Dv IPTOS_CE .
The absence of a particular type may be denoted
with a
.Ql ! .
t Cm iplen Ar len Match if the total length of a packet, including header and data, is
.Ar len
bytes.
t Cm ipid Ar id Match if the identification of IP datagram is
.Ar id .
t 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.
t Cm ipttl Ar ttl Match if the time to live of IP datagram is
.Ar ttl .
t 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:

p
.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 ! .
t Cm tcpseq Ar seq TCP packets only.
Match if the TCP header sequence number field is set to
.Ar seq .
t Cm tcpack Ar ack TCP packets only.
Match if the TCP header acknowledgment number field is set to
.Ar ack .
t 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:

p
.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.
t Cm established TCP packets only.
Match packets that have the RST or ACK bits set.
t 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 .
t Cm tcpwin Ar win TCP packets only.
Match if the TCP header window field is set to
.Ar win .
t 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:

p
.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 ! .
t 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:

p
echo reply

q Cm 0 ,
destination unreachable

q Cm 3 ,
source quench

q Cm 4 ,
redirect

q Cm 5 ,
echo request

q Cm 8 ,
router advertisement

q Cm 9 ,
router solicitation

q Cm 10 ,
time-to-live exceeded

q Cm 11 ,
IP header bad

q Cm 12 ,
timestamp request

q Cm 13 ,
timestamp reply

q Cm 14 ,
information request

q Cm 15 ,
information reply

q Cm 16 ,
address mask request

q Cm 17
and address mask reply

q Cm 18 .
t 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.
t 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.
.El
.El
.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 .

p
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.

p
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.

p
The
.Nm
pipe configuration format is the following:
d -ragged .Cm pipe Ar number Cm config
.Op Cm bw Ar bandwidth | device
.Op Cm delay Ar ms-delay
.Oo
.Cm queue
rq Ar slots | size .Oc
.Op Cm plr Ar loss-probability
.Op Cm mask Ar mask-specifier
.Op Cm buckets Ar hash-table-size
.Oo
.Cm red | gred
.Sm off
.Ar w_q No / Ar min_th No / Ar max_th No / Ar max_p
.Sm on
.Oc
.Ed

p
The
.Nm
queue configuration format is the following:
d -ragged .Cm queue Ar number Cm config
.Op Cm pipe Ar pipe_nr
.Op Cm weight Ar weight
.Oo
.Cm queue
rq Ar slots | size .Oc
.Op Cm plr Ar loss-probability
.Op Cm mask Ar mask-specifier
.Op Cm buckets Ar hash-table-size
.Oo
.Cm red | gred
.Sm off
.Ar w_q No / Ar min_th No / Ar max_th No / Ar max_p
.Sm on
.Oc
.Ed

p
The following parameters can be configured for a pipe:
l -tag -width indent t Cm bw Ar bandwidth | device Bandwidth, measured in
.Sm off
.Op Cm K | M
rq Cm bit/s | Byte/s . .Sm on

p
A value of 0 (default) means unlimited bandwidth.
The unit must follow immediately the number, as in

p
.Dl "ipfw pipe 1 config bw 300Kbit/s queue 50KBytes"

p
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 .
t 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.
t 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.
t 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.
t 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.
t 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.
t 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.
t 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.
t 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:
l -tag -width indent t 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)
t Em net.inet.ip.dummynet.red_avg_pkt_size specifies the expected average packet size (defaults to 512, must be
greater than zero)
t 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:
l -bullet t Remember that you filter both packets going
.Cm in
and
.Cm out .
Most connections need packets going in both directions.
t 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

a /usr/share/examples/ipfw/change_rules.sh .
t Don't forget the loopback interface.
.El
.Sh FINE POINTS
l -bullet t 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.
t 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.
t 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:
d -literal -offset indent kldload /modules/ipfw.ko && \e
ipfw add 32000 allow ip from any to any
.Ed

p
Along the same lines, doing an
d -literal -offset indent ipfw flush
.Ed

p
in similar surroundings is also a bad idea.
t 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:
l -tag -width indent t Em net.inet.ip.fw.debug : No 1 Controls debugging messages produced by
.Nm .
t 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.
t Em net.inet.ip.fw.verbose : No 1 Enables verbose messages.
t 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.
t Em net.inet.ip.fw.verbose_limit : No 0 Limits the number of messages produced by a verbose firewall.
t Em net.inet.ip.fw.dyn_buckets : No 256 t 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.
t Em net.inet.ip.fw.dyn_count : No 3 Current number of dynamic rules
(read-only).
t 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.
t Em net.inet.ip.fw.dyn_ack_lifetime : No 300 t Em net.inet.ip.fw.dyn_syn_lifetime : No 20 t Em net.inet.ip.fw.dyn_fin_lifetime : No 1 t Em net.inet.ip.fw.dyn_rst_lifetime : No 1 t Em net.inet.ip.fw.dyn_udp_lifetime : No 5 t 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
.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:

p
.Dl "ipfw add deny tcp from cracker.evil.org to wolf.tambov.su telnet"

p
This one disallows any connection from the entire crackers
network to my host:

p
.Dl "ipfw add deny ip from 123.45.67.0/24 to my.host.org"

p
A first and efficient way to limit access (not using dynamic rules)
is the use of the following rules:

p
.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"

p
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.

p
In order to protect a site from flood attacks involving fake
TCP packets, it is safer to use dynamic rules:

p
.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"

p
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.

p
To limit the number of connections a user can open
you can use the following type of rules:

p
.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"

p
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.

p
.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.

p
Here is a good usage of the
.Cm list
command to see accounting records and timestamp information:

p
.Dl ipfw -at list

p
or in short form without timestamps:

p
.Dl ipfw -a list

p
which is equivalent to:

p
.Dl ipfw show

p
Next rule diverts all incoming packets from 192.168.2.0/24
to divert port 5000:

p
.Dl ipfw divert 5000 ip from 192.168.2.0/24 to any in

p
The following rules show some of the applications of
.Nm
and
.Xr dummynet 4
for simulations and the like.

p
This rule drops random incoming packets with a probability
of 5%:

p
.Dl "ipfw add prob 0.05 deny ip from any to any in"

p
A similar effect can be achieved making use of dummynet pipes:

p
.Dl "ipfw add pipe 10 ip from any to any"
.Dl "ipfw pipe 10 config plr 0.05"

p
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:

p
.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"

p
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.

p
Should we like to simulate a bidirectional link with bandwidth
limitations, the correct way is the following:

p
.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"

p
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.

p
Should we like to verify network performance with the RED queue
management algorithm:

p
.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"

p
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:

p
.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"

p
Per-flow queueing can be useful for a variety of purposes.
A very simple one is counting traffic:

p
.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"

p
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.

p
A more sophisticated example is limiting the outbound traffic
on a net with per-host limits, rather than per-network limits:

p
.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 IMPLEMENTATION NOTES
The number of times a packet is processed by
.Nm
varies \(em basically,
.Nm
is invoked every time the kernel functions
.Fn ip_input ,
.Fn ip_output
and
.Fn bdg_forward
are invoked.
This means that packets are processed once for connections having
only one endpoint on the local host, twice for connections with
both endpoints on the local host, or for packet routed by the host
(acting as a gateway), and once for packets bridged by the host
(acting as a bridge).
.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
.Sh BUGS
The syntax has grown over the years and it is not very clean.

p
.Em WARNING!!WARNING!!WARNING!!WARNING!!WARNING!!WARNING!!WARNING!!

p
This program can put your computer in rather unusable state.
When using it for the first time, work on the console of the
computer, and do
.Em NOT
do anything you don't understand.

p
When manipulating/adding chain entries, service and protocol names
are not accepted.

p
Incoming packet fragments diverted by
.Cm divert
or
.Cm tee
are reassembled before delivery to the socket.

p
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.

p
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 .

p
.An -nosplit
API based upon code written by
.An Daniel Boulet
for BSDI.

p
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 .