sbin/ipfw/ipfw.8

.\"
.\" $FreeBSD$
.\"
.Dd February 16, 2000
.Dt IPFW 8
.Os FreeBSD
.Sh NAME
.Nm ipfw
.Nd controlling utility for IP firewall and traffic shaper
.Sh SYNOPSIS
.Nm ipfw
.Op Fl q
.Oo
.Fl p Ar preproc
.Oo Fl D
.Sm off
.Ar macro
.Op = Ar value
.Sm on
.Oc
.Op Fl U Ar macro
.Oc
.Ar file
.Nm ipfw
.Op Fl f | q
.Cm flush
.Nm ipfw
.Op Fl q
.Es \&{ \&}
.En Cm zero | resetlog | delete
.Op Ar number ...
.Nm ipfw
.Op Fl s Op Ar field
.Op Fl aftN
.Es \&{ \&}
.En Cm list | show
.Op Ar number ...
.Nm ipfw
.Op Fl q
.Cm add
.Op Ar number
.Ar rule-body
.Nm ipfw
.Cm pipe
.Ar number
.Cm config
.Ar pipe-config-options
.Nm ipfw
.Cm pipe
.Es \&{ \&}
.En 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 .
.Pp
Each incoming or outgoing packet is passed through the
.Nm
rules.
If host is acting as a gateway, packets forwarded by
the gateway are processed by
.Nm
twice.
In case a host is acting as a bridge, packets forwarded by
the bridge are processed by
.Nm
once.
.Pp
A firewall configuration is made of a list of numbered rules,
which is scanned for each 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.
.Pp
A configuration always includes a
.Em DEFAULT
rule (numbered 65535) which cannot be modified by the programmer
and always matches packets.
The action associated with the 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
option, then
.Nm
assumes a
.Em stateful
behaviour, i.e. upon a match 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
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 ipfw .
.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 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.
See also the
.Cm show
command.
.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 q
While
.Cm add Ns ing ,
.Cm zero Ns ing ,
.Cm resetlog Ns ging
or
.Cm flush Ns ing ,
be quiet about actions
.Po
implies
.Fl f
.Pc .
This is useful for adjusting rules by executing multiple
.Nm
commands in a script
.Po
e.g.,
.Ql sh\ /etc/rc.firewall
.Pc ,
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.
.It Fl t
While listing, show last match timestamp.
.It Fl N
Try to resolve addresses and service names in output.
.It Fl s Op Ar field
While listing pipes, sort according to one of the four
counters (total and current packets or bytes).
.El
.Pp
To ease configuration, rules can be put into a file which is
processed using
.Nm
as shown in the first synopsis line.
The
.Ar 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 file
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
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:
.Bd -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
.Pp
Each packet can be filtered based on the following information that is
associated with it:
.Pp
.Bl -tag -width "Source and destination IP address" -offset indent -compact
.It Transmit and receive interface
(by name or address)
.It Direction
(incoming or outgoing)
.It Source and destination IP address
(possibly masked)
.It Protocol
(TCP, UDP, ICMP, etc.)
.It Source and destination port
(lists, ranges or masks)
.It TCP flags
.It IP fragment flag
.It IP options
.It ICMP types
.It User/group ID of the socket associated with the packet
.El
.Pp
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.
.Bl -tag -width indent
.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
.Po
in conjunction with
.Xr dummynet 4
.Pc
to simulate the effect of multiple paths leading to out-of-order
packet delivery.
.It Ar action :
.Bl -tag -width indent
.It Cm allow
Allow packets that match rule.
The search terminates.
Aliases are
.Cm pass ,
.Cm permit
and
.Cm accept .
.It Cm deny
Discard packets that match this rule.
The search terminates.
.Cm drop
is an alias for
.Cm deny .
.It Cm reject
.Pq Deprecated .
Discard packets that match this rule, and try to send an ICMP
host unreachable notice.
The search terminates.
.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 reset
TCP packets only.
Discard packets that match this rule, and try to send a TCP
reset (RST) notice.
The search terminates.
.It Cm count
Update counters for all packets that match rule.
The search continues with the next rule.
.It 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.
.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 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
.Po
but see section
.Sx BUGS
below
.Pc .
.It Cm fwd Ar ipaddr Ns Xo
.Op , Ns Ar port
.Xc
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 entering the system
from a remote host 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 is intended for use with transparent proxy servers.
If the IP is not a local address then the port number
(if specified) is ignored and the rule only applies to packets
leaving the system.
This will also map addresses to local ports 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.
.It Cm pipe Ar pipe_nr
Pass packet to a
.Xr dummynet 4
.Dq pipe
(for bandwidth limitation, delay, etc.).
See the
.Xr dummynet 4
manpage 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 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
.It 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 printed on the console.
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.
However, if
.Cm logamount Ar number
is used, that
.Ar number
will be the default logging limit rather than
.Dv IPFIREWALL_VERBOSE_LIMIT .
Logging may then be re-enabled by clearing the logging counter
or the packet counter for that entry.
.Pp
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 .
.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 :
.Aq Ar address Ns / Ns Ar mask
.Op Ar ports
.Pp
The
.Aq Ar address Ns / Ns Ar mask
may be specified as:
.Bl -tag -width indent
.It Ar ipno
An IP number of the form 1.2.3.4.
Only this exact IP number will match the rule.
.It 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.
.It 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
.Pp
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.
.Pp
With the TCP and UDP protocols, optional
.Em ports
may be specified as:
.Bd -ragged -offset indent
.Sm off
.Eo \&{
.Ar port |
.Ar port No \&- Ar port |
.Ar port : mask
.Ec \&} Op , Ar port Op , Ar ...
.Sm on
.Ed
.Pp
The
.Ql \&-
notation specifies a range of ports (including boundaries).
.Pp
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.
.Pp
Service names (from
.Pa /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
.Pa /usr/src/sys/netinet/ip_fw.h ) .
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.
.It Ar interface-spec
Some combinations of the following specifiers are allowed:
.Bl -tag -width "via ipno"
.It Cm in
Only match incoming packets.
.It Cm out
Only match outgoing packets.
.It Cm via Ar ifX
Packet must be going through interface
.Ar ifX .
.It Cm via Ar if Ns Cm *
Packet must be going through interface
.Ar ifX ,
where
.Ar X
is any unit number.
.It Cm via any
Packet must be going through
.Em some
interface.
.It Cm via Ar ipno
Packet must be going through the interface having IP address
.Ar ipno .
.El
.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 the only 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 100 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.
Specifying
.Cm via
together with
.Cm xmit
or
.Cm recv
is invalid.
.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 Ar options :
.Bl -tag -width indent
.It Cm keep-state Op Ar method
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.
.Pp
The actual behaviour can be modified by specifying a different
.Ar method ,
although at the moment only the default one is specified.
.It 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.
.Pp
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.
.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 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 established
TCP packets only.
Match packets that have the RST or ACK bits set.
.It Cm setup
TCP packets only.
Match packets that have the SYN bit set but no ACK bit.
.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 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 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 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 passing packets to objects called
.Em pipes ,
which emulate a link with given bandwidth, propagation delay,
queue size and packet loss rate.
The
.Nm
pipe configuration format is the following:
.Bd -ragged
.Cm pipe Ar number Cm config
.Op Cm bw Ar bandwidth
.Oo
.Cm queue
.Es \&{ \&}
.En Ar slots | size
.Oc
.Op Cm delay Ar ms-delay
.Op Cm plr Ar loss-probability
.Op Cm mask Ar mask-specifier
.Op Cm buckets Ar hash-table-size
.Ed
.Pp
The following parameters can be configured for a pipe:
.Bl -tag -width indent
.It Cm bw Ar bandwidth
Bandwidth, measured in
.Sm off
.Oo
.Cm K | M
.Oc Eo \&{
.Cm bit/s | Byte/s
.Ec \&} .
.Sm on
.Pp
A value of 0 (default) means unlimited bandwidth.
The unit must follow immediately the number, as in
.Dl "ipfw pipe 1 config bw 300Kbit/s queue 50KBytes"
.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.
.It Cm queue Xo
.Es \&{ \&}
.En Ar slots | size Ns Cm Kbytes
.Xc
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.
.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.
.It Cm mask Ar mask-specifier
The
.Xr dummynet 4
allows you to generate per-flow queues using a single pipe
specification.
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.
.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.
.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.
.It
Don't forget the loopback interface.
.El
.Sh FINE POINTS
There is one kind of packet that the firewall will always
discard, that is an IP fragment with a fragment offset of
one.
This is a valid packet, but it only has one use, to try
to circumvent firewalls.
.Pp
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.
.Pp
The
.Nm
filter list may not be modified if the system security level
is set to 3 or higher
.Po
see
.Xr init 8
for information on system security levels
.Pc .
.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 and
meaning:
.Bl -tag -width indent
.It Em net.inet.ip.fw.debug : No 1
Controls debugging messages produced by
.Nm ipfw .
.It Em net.inet.ip.fw.one_pass : No 1
When set, permits only one pass through the firewall.
Otherwise, after a pipe or divert action, the packet is
reinjected in the firewall starting from 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
.Pq 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 20
.It Em net.inet.ip.fw.dyn_rst_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
.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
.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
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
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 .
.Sh BUGS
.Pp
The syntax has grown over the years and it is not very clean.
.Pp
.Em WARNING!!WARNING!!WARNING!!WARNING!!WARNING!!WARNING!!WARNING!!
.Pp
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.
.Pp
When manipulating/adding chain entries, service and protocol names
are not accepted.
.Pp
Incoming packet fragments diverted by
.Cm divert
or
.Cm tee
are reassembled before delivery to the socket.
.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.
.Sh AUTHORS
.An Ugen J. S. Antsilevich ,
.An Poul-Henning Kamp ,
.An Alex Nash ,
.An Archie Cobbs ,
.An Luigi Rizzo .
.Pp
API based upon code written by
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 .