freebsd-dev/sbin/ipfw/ipfw.8

.\"
.\" $FreeBSD$
.\"
.Dd July 20, 1996
.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
.Op Fl D Ar macro Ns Op Ns =value
.Op Fl U Ar macro
.Oc
.Ar file
.Nm ipfw
.Oo
.Fl f
|
.Fl q
.Oc
flush
.Nm ipfw
.Oo
.Fl q
.Oc
{zero|resetlog|delete}
.Op Ar number ...
.Nm ipfw
.Op Fl s Op field
.Op Fl aftN
{list|show}
.Op Ar number ...
.Nm ipfw
.Oo
.Fl q
.Oc
add
.Op Ar number
.Ar rule-body 
.Nm ipfw
pipe
.Ar number
config
.Ar pipe-config-options
.Nm ipfw
pipe {delete|list|show}
.Op Ar number ...
.Sh DESCRIPTION
.Nm
is the user interface for controlling the IPFW firewall and
.Nm dummynet
traffic shaper in FreeBSD.
.Pp
Each incoming or outgoing packet is passed through the
.Nm
rules. In case a 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 are
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 sysadmin to write the ruleset in such
a way to minimize the number of checks.
.Pp
A configuration always includes a
.Ar 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
.Ar deny
or
.Ar allow
depending on how the kernel is configured.
.Pp
If the ruleset includes one or more rules with the
.Ar keep-state
option, then
.Nm ipfw
assumes a
.Nm 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
.Ar check-state
or
.Ar keep-state
rule, and are typically
used to open the firewall on-demand to legitimate traffic
only. See the
.Xr options
and
.Xr EXAMPLES
sections for more information on the stateful behaviour of
.Nm ipfw
.Pp
All rules (including dynamic ones)
have a few associated counters: a packet count and
a byte count, a log count, and a timestamp indicating the time
of the last match. Counters can be visualized or reset with
.Nm
commands.
.Pp
Rules can be added with the
.Ar add
command; deleted individually with the
.Ar delete
command, and globally with the
.Ar flush
command; visualized, optionally with the content of
the counters, using the
.Ar show
and
.Ar list
commands. Finally, counters can be reset with the
.Ar zero
and
.Ar resetlog
commands.
.Pp
The following options are available:
.Bl -tag -width indent
.It Fl a
While listing, show counter values.  See also 
.Dq show
command.
.It Fl f
Don't ask for confirmation for commands that can cause problems if misused
(i.e. flush).
.Ar Note ,
if there is no tty associated with the process, this is implied.
.It Fl q
While adding, zeroing, resetlogging or flushing, be quiet about actions (implies
.Fl f Ns ).
This is useful for adjusting rules by executing multiple
.Nm
commands in a script
.Po
e.g.,
.Sq sh /etc/rc.firewall
.Pc ,
or by processing a file of many
.Nm 
rules,
across a remote login session.  If a 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 field
While listing pipes, sort according 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
command.
.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 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. since 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
.Ar pipe
commands are used to configure the traffic shaper, as shown in
the ``TRAFFIC SHAPER CONFIGURATION'' section below.
.Pp
.Sh RULE FORMAT
The
.Nm
rule format is the following
.Pp
.Op prob Ar match_probability
.Ar action
.Op log Op Ar logamount Ar number
.Ar proto
from
.Ar src
to
.Ar dst
.Op interface-spec
.Op Ar options
.Pp
Each packet can be filtered based on the following information that is
associated with it:
.Pp
.Bl -tag -offset indent -compact -width xxxx
.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 may be dangerous to filter on the source IP address or
source TCP/UDP port because either or both could easily be spoofed.
.Pp
.Ar prob match_probability
.Bd -ragged -offset flag
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.
.Ed
.Pp
.Ar action :
.Bl -hang -offset flag -width 1234567890123456
.It Ar allow
Allow packets that match rule.
The search terminates. Aliases are
.Ar pass ,
.Ar permit ,
and
.Ar accept .
.It Ar deny
Discard packets that match this rule.
The search terminates.
.Ar Drop
is an alias for
.Ar deny .
.It Ar reject
(Deprecated.) Discard packets that match this rule, and try to send an ICMP
host unreachable notice.
The search terminates.
.It Ar unreach 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 zero to 255, or one of these aliases:
.Ar net ,
.Ar host ,
.Ar protocol ,
.Ar port ,
.Ar needfrag ,
.Ar srcfail ,
.Ar net-unknown ,
.Ar host-unknown ,
.Ar isolated ,
.Ar net-prohib ,
.Ar host-prohib ,
.Ar tosnet ,
.Ar toshost ,
.Ar filter-prohib ,
.Ar host-precedence ,
or
.Ar precedence-cutoff .
The search terminates.
.It Ar reset
TCP packets only. Discard packets that match this rule,
and try to send a TCP reset
.Pq RST
notice.
The search terminates.
.It Ar count
Update counters for all packets that match rule.
The search continues with the next rule.
.It Ar 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
.Ar check-state
rule is found, the dynamic ruleset is checked at the first
.Ar keep-state
rule.
.It Ar divert port
Divert packets that match this rule to the
.Xr divert 4
socket bound to port
.Ar port .
The search terminates.
.It Ar tee 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 BUGS below).
.It Ar fwd ipaddr Op ,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 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 options IPFIREWALL_FORWARD.
.It Ar pipe pipe_nr
Pass packet to a
.Xr dummynet 4
``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 sysctl variable
net.inet.ip.fw.one_pass is not set, the packet is passed again to
the firewall code starting from the next rule.
.It Ar skipto number
Skip all subsequent rules numbered less than
.Ar number .
The search continues with the first rule numbered
.Ar number
or higher.
.El
.Pp
.Ar log Op Ar logamount Ar number
.Bd -ragged -offset flag
If the kernel was compiled with
.Dv IPFIREWALL_VERBOSE ,
then when a packet matches a rule with the
.Ar 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
.Ar 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
.Dv net.inet.ip.fw .
.Ed
.Pp
.Ar proto :
.Bd -ragged -offset flag
An IP protocol specified by number or name (see
.Pa /etc/protocols
for a complete list).
The
.Ar ip
or
.Ar all
keywords mean any protocol will match.
.Ed
.Pp
.Ar src 
and
.Ar dst :
.Bd -ragged -offset flag
.Ar <address/mask> Op Ar ports
.Pp
The
.Em <address/mask>
may be specified as:
.Pp
.Bl -hang -offset 0n -width 1234567890123456
.It Ar ipno
An ipnumber of the form 1.2.3.4.
Only this exact ip number match the rule.
.It Ar ipno/bits
An ipnumber 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:mask
An ipnumber 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
.Dq 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:
.Pp
.Bl -hang -offset flag
.It Ns {port|port-port|port:mask} Ns Op ,port Ns Op ,...
.El
.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
(as defined in 
.Pa /usr/src/sys/netinet/ip_fw.h )
ports.
A
.Ql \e
can be used to escape the
.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
.Ar frag
option for details on matching fragmented packets.
.Pp
.Ed
.Ar interface-spec :
.Pp
.Bd -ragged -offset flag
Some combinations of the following specifiers are allowed:
.Bl -hang -offset 0n -width 1234567890123456
.It Ar in
Only match incoming packets.
.It Ar out
Only match outgoing packets.
.It Ar via ifX
Packet must be going through interface
.Ar ifX.
.It Ar via if*
Packet must be going through interface
.Ar ifX ,
where X is any unit number.
.It Ar via any
Packet must be going through
.Em some
interface.
.It Ar via ipno
Packet must be going through the interface having IP address
.Ar ipno .
.El
.Pp
The
.Ar via
keyword causes the interface to always be checked.
If
.Ar recv
or
.Ar xmit
is used instead of
.Ar 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
.Ar recv
interface can be tested on either incoming or outgoing packets, while the
.Ar xmit
interface can only be tested on outgoing packets. So
.Ar out
is required (and
.Ar in
invalid) whenever
.Ar xmit
is used. Specifying
.Ar via
together with
.Ar xmit
or
.Ar 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.
.Ed
.Pp
.Ar options :
.Bl -hang -offset flag -width 1234567890123456
.It keep-state Op 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
.Nm sysctl
variables), and the lifetime is refreshed every time a matching packet
is found.
.Pp
The actual behaviour can be modified by specifying a different
.Op method ,
although at the moment only the default one is specified.
.It 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
.Ar pipes
because the same packet would be accounted for twice
in terms of bandwidth, queue occupation, and also counters.
.It frag
Match if the packet is a fragment and this is not the first fragment
of the datagram.
.Ar frag
may not be used in conjunction with either
.Ar tcpflags
or TCP/UDP port specifications.
.It 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
.Ar ssrr 
(strict source route),
.Ar lsrr 
(loose source route),
.Ar rr 
(record packet route), and
.Ar ts 
(timestamp).
The absence of a particular option may be denoted
with a
.Dq ! .
.It established
Match packets that have the RST or ACK bits set.
TCP packets only.
.It setup
Match packets that have the SYN bit set but no ACK bit.
TCP packets only.
.It tcpflags Ar spec
Match if the TCP header contains the comma separated list of
flags specified in
.Ar spec .
The supported TCP flags are:
.Pp
.Ar fin ,
.Ar syn ,
.Ar rst ,
.Ar psh ,
.Ar ack ,
and
.Ar urg .
The absence of a particular flag may be denoted
with a
.Dq ! .
A rule which contains a
.Ar tcpflags
specification can never match a fragmented packet which has
a non-zero offset.  See the
.Ar frag
option for details on matching fragmented packets.
.It icmptypes Ar types
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 Ar 0 ,
destination unreachable
.Pq Ar 3 ,
source quench
.Pq Ar 4 ,
redirect
.Pq Ar 5 ,
echo request
.Pq Ar 8 ,
router advertisement
.Pq Ar 9 ,
router solicitation
.Pq Ar 10 ,
time-to-live exceeded
.Pq Ar 11 ,
IP header bad
.Pq Ar 12 ,
timestamp request
.Pq Ar 13 ,timestamp reply
.Pq Ar 14 ,
information request
.Pq Ar 15 ,
information reply
.Pq Ar 16 ,
address mask request
.Pq Ar 17 ,
and address mask reply
.Pq Ar 18
.It Ar uid 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 Ar gid 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
.Sh TRAFFIC SHAPER CONFIGURATION
Ipfw is also the user interface for the
.Xr dummynet 4
traffic shaper.
The shaper operates by passing packets to objects called
.Ar pipes ,
which emulates a link with given bandwidth, propagation delay,
queue size and packet loss rate.
The
.Nm
pipe configuration format is the following
.Pp
.Ar pipe number config
.Op bw Ar bandwidth
.Op queue Ar {slots|size}
.Op delay Ar delay-ms
.Op plr Ar loss-probability
.Op mask Ar {all | {dst-ip|src-ip|dst-port|src-port|proto} bitmask}
.Op buckets Ar hash-table-size
.Pp
The following parameters can be configured for a pipe:
.Bl -hang -offset flag -width 1234567890
.It bw Ar bandwidth
Bandwidth, measured in
.Ar [K|M]{bit/s|Byte/s} .
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 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
good practice to run kernels with "options HZ=1000" to reduce
the granularity to 1ms or less). Default value is 0, meaning
no delay.
.It queue Ar {slots|size}
queue size, in slots or 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 plr packet-loss-rate
packet loss rate. NN is a floating-point number, with 0 meaning
no loss, 1 means 100% loss. The loss rate is internally represented
on 31 bits.
.It mask Ar mask-specifier
dummynet 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 ID after masking fall
into the same queue. Available mask specifiers are a combination
of the following:
.Ar dst-ip mask , src-ip mask ,
.Ar dst-port mask , src-port mask ,
.Ar proto mask
or
.Ar all
where the latter means all bits in all fields are significant.
.It buckets Ar NN
Specifies the size of the hash table used for storing the various queues.
Default value is 64 controlled by the sysctl variable
.Ar 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 -hang -offset flag 
.It 
Remember that you filter both packets going in and 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 KLD version of
.Nm
is probably not as straightforward as you would think.
I recommend this command line:
.Bd -literal -offset center
kldload /modules/ipfw.ko && \e
ipfw add 32000 allow all from any to any
.Ed
.Pp
Along the same lines, doing an
.Bd -literal -offset center
ipfw flush
.Ed
.Pp
in similar surroundings is also a bad idea.
.Pp
The IP 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 divert socket bound to the specified port will receive all packets diverted
to that port; see
.Xr divert 4 .
If no socket is bound to the destination port, or if the kernel
wasn't compiled with divert socket support, the packets are dropped.
.Pp
.Sh SYSCTL VARIABLES
A set of
.Nm sysctl
variables controls the behaviour of the firewall. These are shown
below together with their default value and meaning
.Bl -tag -offset flag -width 1234567890
.It "net.inet.ip.fw.debug: 1"
Controls debugging messages produced by ipfw.
.It "net.inet.ip.fw.one_pass: 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 net.inet.ip.fw.verbose: 1
Enables verbose messages.
.It net.inet.ip.fw.enable: 1
Enables the firewall. Setting this variable to 0 lets you run your
machine without firewall even if compiled in.
.It net.inet.ip.fw.verbose_limit: 0
Limits the number of messages produced by a verbose firewall.
.It net.inet.ip.fw.dyn_buckets: 256
.It net.inet.ip.fw.curr_dyn_buckets: 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
.Ar flush
and reload the ruleset.
.It net.inet.ip.fw.dyn_count: 3
(readonly) current number of dynamic rules.
.It net.inet.ip.fw.dyn_max: 1000
Maximum number of dynamic rules. When you hit this limit,
no more dynamic rules can be installed until old ones expire.
.It net.inet.ip.fw.dyn_ack_lifetime: 300
.It net.inet.ip.fw.dyn_syn_lifetime: 20
.It net.inet.ip.fw.dyn_fin_lifetime: 20
.It net.inet.ip.fw.dyn_rst_lifetime: 5
.It net.inet.ip.fw.dyn_short_lifetime: 30
These variable 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 23
.Pp 
This one disallows any connection from the entire crackers network to
my host:
.Pp
.Dl ipfw add deny all 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
.Ar setup
rules only for selected source/destination pairs.
All other SYN packets will be rejected by the final
.Ar 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
.Ar check-state
or
.Ar keep-state
rule. A
.Ar 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
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
.Nm sysctl
variables which control the operation of the firewall.
.Pp
There is a number of sysctl variables which controls the
.Pp
Here is a good usage of the
.Ar list
command to see accounting records
and timestamp information:
.Pp
.Dl ipfw -at l
.Pp
or in short form without timestamps:
.Pp
.Dl ipfw -a l
.Pp
This rule diverts all incoming packets from 192.168.2.0/24 to divert port 5000:
.Pp
.Dl ipfw divert 5000 all from 192.168.2.0/24 to any in
.Pp
The following rules show some of the applications of ipfw and
dummynet for simulations and the like.
.Pp
This rule drops random 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
.Ql out
specifier so that the rule is not used twice. Remember in fact
that ipfw 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 ipfw 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 divert 4 ,
.Xr dummynet 4 ,
.Xr bridge 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
.Ar divert
or
.Ar tee
are reassembled before delivery to the socket.
.Pp
Packets that match a
.Ar 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 dummynet traffic shaper supported by Akamba Corp.
.Sh HISTORY
.Nm Ipfw
first appeared in
.Fx 2.0 .
.Nm dummynet
was introduced in
.Fx 2.2.8 .
Stateful extensions were introduced in
.Fx 4.0-RELEASE