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IPFW(8) 		FreeBSD System Manager's Manual 	       IPFW(8)

     ipfw - controlling utility for IP firewall and traffic shaper

     ipfw [-q] [-p preproc [-D macro[=value]] [-U macro]] pathname
     ipfw [-f | -q] flush
     ipfw [-q] {zero | resetlog | delete} [number ...]
     ipfw [-s [field]] [-aftN] {list | show} [number ...]
     ipfw [-q] add [number] rule-body
     ipfw pipe number config pipe-config-options
     ipfw pipe {delete | list | show} [number ...]
     ipfw queue number config queue-config-options
     ipfw queue {delete | list | show} [number ...]

     ipfw is the user interface for controlling the ipfirewall(4) and the dum­
     mynet(4) traffic shaper in FreeBSD.

     Each incoming or outgoing packet is passed through the ipfw rules.  If
     host is acting as a gateway, packets forwarded by the gateway are pro­
     cessed by ipfw twice.  In case a host is acting as a bridge, packets for­
     warded by the bridge are processed by ipfw once.

     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 re­
     sponsibility of the system administrator to write the ruleset in such a
     way as to minimize the number of checks.

     A configuration always includes a DEFAULT rule (numbered 65535) which
     cannot be modified by the programmer and always matches packets.  The ac­
     tion associated with the default rule can be either deny or allow depend­
     ing on how the kernel is configured.

     If the ruleset includes one or more rules with the keep-state option,
     then ipfw assumes a stateful behaviour, i.e. upon a match will create dy­
     namic rules matching the exact parameters (addresses and ports) of the
     matching packet.

     These dynamic rules, which have a limited lifetime, are checked at the
     first occurrence of a check-state or keep-state rule, and are typically
     used to open the firewall on-demand to legitimate traffic only.  See the
     RULE FORMAT and EXAMPLES sections below for more information on the
     stateful behaviour of ipfw.

     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 ipfw

     Rules can be added with the add command; deleted individually with the
     delete command, and globally with the flush command; displayed, optional­
     ly with the content of the counters, using the show and list commands.
     Finally, counters can be reset with the zero and resetlog commands.

     The following options are available:

     -a      While listing, show counter values.  See also the show command.

     -f      Don't ask for confirmation for commands that can cause problems
	     if misused, i.e. flush. Note, if there is no tty associated with
	     the process, this is implied.

     -q      While adding, zeroing, resetlogging or flushing, be quiet about
	     actions (implies -f). This is useful for adjusting rules by exe­
	     cuting multiple ipfw commands in a script (e.g.,
	     `sh /etc/rc.firewall'), or by processing a file of many ipfw
	     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.

     -t      While listing, show last match timestamp.

     -N      Try to resolve addresses and service names in output.

     -s [field]
	     While listing pipes, sort according to one of the four counters
	     (total and current packets or bytes).

     To ease configuration, rules can be put into a file which is processed
     using ipfw as shown in the first synopsis line.  An absolute pathname
     must be used.  The file will be read line by line and applied as argu­
     ments to the ipfw utility.

     Optionally, a preprocessor can be specified using -p preproc where
     pathname is to be piped through.  Useful preprocessors include cpp(1) and
     m4(1).  If preproc doesn't start with a slash (`/') as its first charac­
     ter, the usual PATH name search is performed.  Care should be taken with
     this in environments where not all filesystems are mounted (yet) by the
     time ipfw is being run (e.g. when they are mounted over NFS).  Once -p
     has been specified, optional -D and -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.

     The ipfw pipe commands are used to configure the traffic shaper, as shown
     in the TRAFFIC SHAPER CONFIGURATION section below.

     The ipfw rule format is the following:

     [prob match_probability] action [log [logamount number]] proto from src
     to dst [interface-spec] [options]

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

	   Transmit and receive interface     (by name or address)
	   Direction			      (incoming or outgoing)
	   Source and destination IP address  (possibly masked)
	   Protocol			      (TCP, UDP, ICMP, etc.)
	   Source and destination port	      (lists, ranges or masks)
	   TCP flags
	   IP fragment flag
	   IP options
	   ICMP types
	   User/group ID of the socket associated with the packet

     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.

     prob 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 dummynet(4)) to simulate the effect of multiple paths lead­
	     ing to out-of-order packet delivery.


	     allow   Allow packets that match rule.  The search terminates.
		     Aliases are pass, permit and accept.

	     deny    Discard packets that match this rule.  The search termi­
		     nates.  drop is an alias for deny.

	     reject  (Deprecated). Discard packets that match this rule, and
		     try to send an ICMP host unreachable notice.  The search

	     unreach code
		     Discard packets that match this rule, and try to send an
		     ICMP unreachable notice with code code, where code is a
		     number from 0 to 255, or one of these aliases: net, host,
		     protocol, port, needfrag, srcfail, net-unknown, host-
		     unknown, isolated, net-prohib, host-prohib, tosnet,
		     toshost, filter-prohib, host-precedence or precedence-
		     cutoff. The search terminates.

	     reset   TCP packets only.	Discard packets that match this rule,
		     and try to send a TCP reset (RST) notice.	The search

	     count   Update counters for all packets that match rule.  The
		     search continues with the next rule.

		     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 check-state rule is found,
		     the dynamic ruleset is checked at the first keep-state

	     divert port
		     Divert packets that match this rule to the divert(4)
		     socket bound to port port. The search terminates.

	     tee port
		     Send a copy of packets matching this rule to the di­
		     vert(4) socket bound to port port. The search terminates
		     and the original packet is accepted (but see section BUGS

	     fwd ipaddr[,port]
		     Change the next-hop on matching packets to ipaddr, which
		     can be an IP address in dotted quad or a host name.  If
		     ipaddr is not a directly-reachable address, the route as
		     found in the local routing table for that IP is used in­
		     stead.  If ipaddr is a local address, then on a packet
		     entering the system from a remote host it will be divert­
		     ed to port on the local machine, keeping the local ad­
		     dress 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 ad­
		     dress 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

	     pipe pipe_nr
		     Pass packet to a dummynet(4) ``pipe'' (for bandwidth lim­
		     itation, delay, etc.).  See the TRAFFIC SHAPER
		     CONFIGURATION section for further information.  The
		     search terminates; however, on exit from the pipe and if
		     the sysctl(8) variable net.inet.ip.fw.one_pass is not
		     set, the packet is passed again to the firewall code
		     starting from the next rule.

	     queue queue_nr
		     Pass packet to a dummynet(4) ``queue'' (for bandwidth
		     limitation using WF2Q).

	     skipto number
		     Skip all subsequent rules numbered less than number. The
		     search continues with the first rule numbered number or

     log [logamount number]
	     If the kernel was compiled with IPFIREWALL_VERBOSE, then when a
	     packet matches a rule with the log keyword a message will be
	     printed on the console.  If the kernel was compiled with the
	     IPFIREWALL_VERBOSE_LIMIT option, then by default logging will
	     cease after the number of packets specified by the option are re­
	     ceived for that particular chain entry, and
	     net.inet.ip.fw.verbose_limit will be set to that number.  Howev­
	     er, if logamount number is used, that number will be the logging
	     limit rather than net.inet.ip.fw.verbose_limit, where the value
	     ``0'' removes the logging limit.  Logging may then be re-enabled
	     by clearing the logging counter or the packet counter for that

	     Console logging and the log limit are adjustable dynamically
	     through the sysctl(8) interface in the MIB base of

     proto   An IP protocol specified by number or name (for a complete list
	     see /etc/protocols). The ip or all keywords mean any protocol
	     will match.

     src and dst:
	     <address/mask> [ports]

	     The <address/mask> may be specified as:

	     ipno    An IP number of the form	Only this exact IP
		     number will match the rule.

		     An IP number with a mask width of the form
		     In this case all IP numbers from to
		     will match.

		     An IP number with a mask of the form  In this case all IP numbers from to will match.

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

	     With the TCP and UDP protocols, optional ports may be specified


	     The `-' notation specifies a range of ports (including bound­

	     The `:' 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.

	     Service names (from /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 IP_FW_MAX_PORTS
	     ports (as defined in /usr/src/sys/netinet/ip_fw.h). A backslash
	     (`\') can be used to escape the dash (`-') character in a service

		   ipfw add count tcp from any ftp\\-data-ftp to any

	     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 frag option for details on matching
	     fragmented packets.

	     Some combinations of the following specifiers are allowed:

	     in        Only match incoming packets.

	     out       Only match outgoing packets.

	     via ifX   Packet must be going through interface ifX.

	     via if*   Packet must be going through interface ifX, where X is
		       any unit number.

	     via any   Packet must be going through some interface.

	     via ipno  Packet must be going through the interface having IP
		       address ipno.

	     The via keyword causes the interface to always be checked.  If
	     recv or xmit is used instead of 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.:

		   ipfw add 100 deny ip from any to any out recv ed0 xmit ed1

	     The recv interface can be tested on either incoming or outgoing
	     packets, while the xmit interface can only be tested on outgoing
	     packets.  So out is required (and in is invalid) whenever xmit is
	     used.  Specifying via together with xmit or recv is invalid.

	     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.


	     keep-state [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 (con­
		     trolled by a set of sysctl(8) variables), and the life­
		     time is refreshed every time a matching packet is found.

		     The actual behaviour can be modified by specifying a dif­
		     ferent method, although at the moment only the default
		     one is specified.

		     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 lo­
		     cal stack.

		     Apart from a small performance penalty, this would be a
		     problem when using pipes because the same packet would be
		     accounted for twice in terms of bandwidth, queue occupa­
		     tion, and also counters.

	     frag    Match if the packet is a fragment and this is not the
		     first fragment of the datagram.  frag may not be used in
		     conjunction with either tcpflags or TCP/UDP port specifi­

	     ipoptions spec
		     Match if the IP header contains the comma separated list
		     of options specified in spec. The supported IP options

		     ssrr (strict source route), lsrr (loose source route), rr
		     (record packet route) and ts (timestamp).	The absence of
		     a particular option may be denoted with a `!'.

	     tcpoptions spec
		     Match if the TCP header contains the comma separated list
		     of options specified in spec. The supported TCP options

		     mss (maximum segment size), window (tcp window advertise­
		     ment), sack (selective ack), ts (rfc1323 timestamp) and
		     cc (rfc1644 t/tcp connection count).  The absence of a
		     particular option may be denoted with a `!'.

		     TCP packets only.	Match packets that have the RST or ACK
		     bits set.

	     setup   TCP packets only.	Match packets that have the SYN bit
		     set but no ACK bit.

	     tcpflags spec
		     TCP packets only.	Match if the TCP header contains the
		     comma separated list of flags specified in spec. The sup­
		     ported TCP flags are:

		     fin, syn, rst, psh, ack and urg. The absence of a partic­
		     ular flag may be denoted with a `!'. A rule which con­
		     tains a tcpflags specification can never match a frag­
		     mented packet which has a non-zero offset.  See the frag
		     option for details on matching fragmented packets.

	     icmptypes types
		     ICMP packets only.  Match if the ICMP type is in the list
		     types. The list may be specified as any combination of
		     ranges or individual types separated by commas.  The sup­
		     ported ICMP types are:

		     echo reply (0), destination unreachable (3), source
		     quench (4), redirect (5), echo request (8), router adver­
		     tisement (9), router solicitation (10), time-to-live ex­
		     ceeded (11), IP header bad (12), timestamp request (13),
		     timestamp reply (14), information request (15), informa­
		     tion reply (16), address mask request (17) and address
		     mask reply (18).

	     uid user
		     Match all TCP or UDP packets sent by or received for a
		     user. A user may be matched by name or identification

	     gid group
		     Match all TCP or UDP packets sent by or received for a
		     group. A group may be matched by name or identification

     The ipfw utility is also the user interface for the dummynet(4) traffic
     shaper.  The shaper operates by dividing packets into flows according to
     a user-specified mask on different fields of the IP header.  Packets be­
     longing to the same flow are then passed to two different objects, named
     pipe or queue.

     A pipe emulates a link with given bandwidth, propagation delay, queue
     size and packet loss rate.  Packets transit through the pipe according to
     its parameters.

     A queue is an abstraction used to implement the WF2Q+ 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 ac­
     cording to the WF2Q+ policy.

     The ipfw pipe configuration format is the following:

     pipe number config [bw bandwidth | device] [delay ms-delay] [queue {slots
     | size}] [plr loss-probability] [mask mask-specifier] [buckets
     hash-table-size] [red | gred w_q/min_th/max_th/max_p]

     The ipfw queue configuration format is the following:

     queue number config [pipe pipe_nr] [weight weight] [queue {slots | size}]
     [plr loss-probability] [mask mask-specifier] [buckets hash-table-size]
     [red | gred w_q/min_th/max_th/max_p]

     The following parameters can be configured for a pipe:

     bw bandwidth | device
	     Bandwidth, measured in [K|M]{bit/s|Byte/s}.

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

		   ipfw pipe 1 config bw 300Kbit/s queue 50KBytes

	     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 tun(4) device supports this functionality, for
	     use in conjunction with ppp(8).

     delay ms-delay
	     Propagation delay, measured in milliseconds.  The value is round­
	     ed to the next multiple of the clock tick (typically 10ms, but it
	     is a good practice to run kernels with ``options HZ=1000'' to re­
	     duce the granularity to 1ms or less).  Default value is 0, mean­
	     ing no delay.

     queue {slots | sizeKbytes}
	     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.

     plr packet-loss-rate
	     Packet loss rate.	Argument 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.

     mask mask-specifier
	     The dummynet(4) lets you to create per-flow queues.  A flow iden­
	     tifier is constructed by masking the IP addresses, ports and pro­
	     tocol 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:
	     dst-ip mask, src-ip mask, dst-port mask, src-port mask, proto
	     mask or all, where the latter means all bits in all fields are
	     significant.  When used within a pipe configuration, each flow is
	     assigned a rate equal to the rate of the pipe.  When used within
	     a 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.

     buckets hash-table-size
	     Specifies the size of the hash table used for storing the various
	     queues.  Default value is 64 controlled by the sysctl(8) variable
	     net.inet.ip.dummynet.hash_size, allowed range is 16 to 1024.

     pipe 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.

     weight 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.

     red | gred w_q/min_th/max_th/max_p
	     Make use of the RED queue management algorithm.  w_q and max_p
	     are floating point numbers between 0 and 1 (0 not included),
	     while min_th and 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 dum­
	     mynet(4) also supports the gentle RED variant (gred).  Three
	     sysctl(8) variables can be used to control the RED behaviour:

		     specifies the accuracy in computing the average queue
		     when the link is idle (defaults to 256, must be greater
		     than zero)

		     specifies the expected average packet size (defaults to
		     512, must be greater than zero)

		     specifies the expected maximum packet size, only used
		     when queue thresholds are in bytes (defaults to 1500,
		     must be greater than zero).

     Here are some important points to consider when designing your rules:

     ·	 Remember that you filter both packets going in and out. Most connec­
	 tions need packets going in both directions.

     ·	 Remember to test very carefully.  It is a good idea to be near the
	 console when doing this.

     ·	 Don't forget the loopback interface.

     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.

     If you are logged in over a network, loading the kld(4) version of ipfw
     is probably not as straightforward as you would think.  I recommend the
     following command line:

	   kldload /modules/ipfw.ko && \
	   ipfw add 32000 allow ip from any to any

     Along the same lines, doing an

	   ipfw flush

     in similar surroundings is also a bad idea.

     The ipfw filter list may not be modified if the system security level is
     set to 3 or higher (see init(8) for information on system security

     A 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

     A set of sysctl(8) variables controls the behaviour of the firewall.
     These are shown below together with their default value and meaning:

     net.inet.ip.fw.debug: 1
	     Controls debugging messages produced by ipfw.

     net.inet.ip.fw.one_pass: 1
	     When set, the packet exiting from the dummynet(4) pipe is not
	     passed though the firewall again.	Otherwise, after a pipe ac­
	     tion, the packet is reinjected into the firewall at the next

     net.inet.ip.fw.verbose: 1
	     Enables verbose messages.

     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.

     net.inet.ip.fw.verbose_limit: 0
	     Limits the number of messages produced by a verbose firewall.

     net.inet.ip.fw.dyn_buckets: 256

     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 flush and reload the ruleset.

     net.inet.ip.fw.dyn_count: 3
	     Current number of dynamic rules (read-only).

     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.

     net.inet.ip.fw.dyn_ack_lifetime: 300

     net.inet.ip.fw.dyn_syn_lifetime: 20

     net.inet.ip.fw.dyn_fin_lifetime: 20

     net.inet.ip.fw.dyn_rst_lifetime: 5

     net.inet.ip.fw.dyn_short_lifetime: 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

     This command adds an entry which denies all tcp packets from to the telnet port of from being forward­
     ed by the host:

	   ipfw add deny tcp from to telnet

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

	   ipfw add deny ip from to

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

	   ipfw add allow tcp from any to any established
	   ipfw add allow tcp from net1 portlist1 to net2 portlist2 setup
	   ipfw add allow tcp from net3 portlist3 to net3 portlist3 setup
	   ipfw add deny tcp from any to any

     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 setup
     rules only for selected source/destination pairs.	All other SYN packets
     will be rejected by the final deny rule.

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

	   ipfw add check-state
	   ipfw add deny tcp from any to any established
	   ipfw add allow tcp from my-net to any setup keep-state

     This will let the firewall install dynamic rules only for those connec­
     tion which start with a regular SYN packet coming from the inside of our
     network.  Dynamic rules are checked when encountering the first check-
     state or keep-state rule.	A 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.

     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 sysctl(8)
     variables which control the operation of the firewall.

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

	   ipfw -at list

     or in short form without timestamps:

	   ipfw -a list

     Next rule diverts all incoming packets from to divert port

	   ipfw divert 5000 ip from to any in

     The following rules show some of the applications of ipfw and dummynet(4)
     for simulations and the like.

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

	   ipfw add prob 0.05 deny ip from any to any in

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

	   ipfw add pipe 10 ip from any to any
	   ipfw pipe 10 config plr 0.05

     We can use pipes to artificially limit bandwidth, e.g. on a machine act­
     ing as a router, if we want to limit traffic from local clients on we do:

	   ipfw add pipe 1 ip from to any out
	   ipfw pipe 1 config bw 300Kbit/s queue 50KBytes

     note that we use the out modifier so that the rule is not used twice.
     Remember in fact that ipfw rules are checked both on incoming and outgo­
     ing packets.

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

	   ipfw add pipe 1 ip from any to any out
	   ipfw add pipe 2 ip from any to any in
	   ipfw pipe 1 config bw 64Kbit/s queue 10Kbytes
	   ipfw pipe 2 config bw 64Kbit/s queue 10Kbytes

     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, IR­
     DA).  It is not necessary that both pipes have the same configuration, so
     we can also simulate asymmetric links.

     Should we like to verify network performance with the RED queue manage­
     ment algorithm:

	   ipfw add pipe 1 ip from any to any
	   ipfw pipe 1 config bw 500Kbit/s queue 100 red 0.002/30/80/0.1

     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:

	   ipfw add pipe 1 ip from any to any out
	   ipfw add pipe 2 ip from any to any in
	   ipfw pipe 1 config delay 250ms bw 1Mbit/s
	   ipfw pipe 2 config delay 250ms bw 1Mbit/s

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

	   ipfw add pipe 1 tcp from any to any
	   ipfw add pipe 1 udp from any to any
	   ipfw add pipe 1 ip from any to any
	   ipfw pipe 1 config mask all

     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.

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

	   ipfw add pipe 1 ip from to any out
	   ipfw add pipe 2 ip from any to in
	   ipfw pipe 1 config mask src-ip 0x000000ff bw 200Kbit/s queue
	   ipfw pipe 2 config mask dst-ip 0x000000ff bw 200Kbit/s queue

     cpp(1),  m4(1),  bridge(4),  divert(4),  dummynet(4),  ip(4),
     ipfirewall(4),  protocols(5),  services(5),  init(8),  kldload(8),
     reboot(8),  sysctl(8),  syslogd(8).

     The syntax has grown over the years and it is not very clean.


     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 NOT do
     anything you don't understand.

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

     Incoming packet fragments diverted by divert or tee are reassembled be­
     fore delivery to the socket.

     Packets that match a tee rule should not be immediately accepted, but
     should continue going through the rule list.  This may be fixed in a lat­
     er version.

     Ugen J. S. Antsilevich,
     Poul-Henning Kamp,
     Alex Nash,
     Archie Cobbs,
     Luigi Rizzo.

     API based upon code written by Daniel Boulet for BSDI.

     Work on dummynet(4) traffic shaper supported by Akamba Corp.

     The ipfw utility first appeared in FreeBSD 2.0.  dummynet(4) was intro­
     duced in FreeBSD 2.2.8.  Stateful extensions were introduced in FreeBSD

FreeBSD 		       February 16, 2000			    13


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