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dnctl(8)                  BSD System Manager's Manual                 dnctl(8)


     dnctl -- Traffic shaper control program


     dnctl [-anqs] {list | show}
     dnctl [-f | -q] flush
     dnctl [-q] {delete} [number ...]
     dnctl {pipe | queue} number config config-options
     dnctl [-s [field]] {pipe | queue} {delete | list | show} [number ...]
     dnctl [-nq] [-p preproc [preproc-flags]] pathname


     The dnctl utility is the user interface for controlling the dummynet(4)
     traffic shaper.

     dummynet operates by first using a packet filter to classify packets and
     divide them into flows, using any match pattern that can be used in dnctl
     rules.  Depending on local policies, a flow can contain packets for a
     single TCP connection, or from/to a given host, or entire subnet, or a
     protocol type, etc.

     Packets belonging to the same flow are then passed to either of two dif-
     ferent objects, which implement the traffic regulation:

         pipe    A pipe emulates a link with given bandwidth, propagation
                 delay, queue size and packet loss rate.  Packets are queued
                 in front of the pipe as they come out from the classifier,
                 and then transferred to the pipe according to the pipe's

         queue   A queue is an abstraction used to implement the WF2Q+ (Worst-
                 case Fair Weighted Fair Queueing) policy, which is an effi-
                 cient variant of the WFQ policy.
                 The queue associates a weight and a reference pipe to each
                 flow, and then all backlogged (i.e., with packets queued)
                 flows linked to the same pipe share the pipe's bandwidth pro-
                 portionally to their weights.  Note that weights are not pri-
                 orities; a flow with a lower weight is still guaranteed to
                 get its fraction of the bandwidth even if a flow with a
                 higher weight is permanently backlogged.

     In practice, pipes can be used to set hard limits to the bandwidth that a
     flow can use, whereas queues can be used to determine how different flow
     share the available bandwidth.

     The pipe and queue configuration commands are the following:

           pipe number config pipe-configuration

           queue number config queue-configuration

     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
             immediately follow the number, as in

                   dnctl pipe 1 config bw 300Kbit/s

             If a device name is specified instead of a numeric value, as in

                   dnctl pipe 1 config bw tun0

             then the transmit clock is supplied by the specified device.  At
             the moment no device supports this functionality.

     delay 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 ``options HZ=1000''
             to reduce the granularity to 1ms or less).  Default value is 0,
             meaning no delay.

     The following parameters can be configured for a queue:

     pipe pipe_nr
             Connects a queue to the specified pipe.  Multiple queues (with
             the same or 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.

     Finally, the following parameters can be configured for both pipes and

     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 65536.

     mask mask-specifier
           Packets sent to a given pipe or queue by an dnctl rule can be fur-
           ther classified into multiple flows, each of which is then sent to
           a different dynamic pipe or queue.  A flow identifier is con-
           structed by masking the IP addresses, ports and protocol types as
           specified with the mask options in the configuration of the pipe or
           queue.  For each different flow identifier, a new pipe or queue is
           created with the same parameters as the original object, and match-
           ing packets are sent to it.

           Thus, when dynamic pipes are used, each flow will get the same
           bandwidth as defined by the pipe, whereas when dynamic queues are
           used, each flow will share the parent's pipe bandwidth evenly with
           other flows generated by the same queue (note that other queues
           with different weights might be connected to the same pipe).
           Available mask specifiers are a combination of one or more of the

           dst-ip mask, dst-ip6 mask, src-ip mask, src-ip6 mask, dst-port
           mask, src-port mask, proto mask or all,

           where the latter means all bits in all fields are significant.

           When a packet is dropped by a dummynet queue or pipe, the error is
           normally reported to the caller routine in the kernel, in the same
           way as it happens when a device queue fills up. Setting this option
           reports the packet as successfully delivered, which can be needed
           for some experimental setups where you want to simulate loss or
           congestion at a remote router.

     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.

     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 inter-
           face with its 16KB packets.

     red | gred w_q/min_th/max_th/max_p
           Make use of the RED (Random Early Detection) queue management algo-
           rithm.  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 specify-
           ing thresholds for queue management (thresholds are computed in
           bytes if the queue has been defined in bytes, in slots otherwise).
           The dummynet(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

                   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).

     The following options are available:

     -a      While listing, show counter values.  The show command just
             implies this option.

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

     -h      Displays a short help.

     -n      Only check syntax of the command strings, without actually pass-
             ing them to the kernel.

     -q      While adding, zeroing, resetlogging or flushing, be quiet about
             actions (implies -f).  This is useful for adjusting rules by exe-
             cuting multiple dnctl commands in a script or by processing a
             file of many dnctl 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 might not be delivered to the login ses-
             sion, causing the remote login session to be closed and the
             remainder of the ruleset to not be processed.  Access to the con-
             sole would then be required to recover.

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

     -v      Be verbose.

     To ease configuration, rules can be put into a file which is processed
     using dnctl as shown in the last synopsis line.  An absolute pathname
     must be used.  The file will be read line by line and applied as argu-
     ments to the dnctl 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 file systems are mounted (yet) by the
     time dnctl is being run (e.g. when they are mounted over NFS).  Once -p
     has been specified, any additional arguments as passed on to the pre-
     processor for interpretation.  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.


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

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

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

     o   Don't forget the loopback interface.


     A set of sysctl(8) variables controls the behaviour of the dummynet mod-
     ule.  These are shown below together with their default value (but always
     check with the sysctl(8) command what value is actually in use) and mean-

     net.inet.ip.dummynet.expire: 1
             Lazily delete dynamic pipes/queue once they have no pending traf-
             fic.  You can disable this by setting the variable to 0, in which
             case the pipes/queues will only be deleted when the threshold is

     net.inet.ip.dummynet.hash_size: 64
             Default size of the hash table used for dynamic pipes/queues.
             This value is used when no buckets option is specified when con-
             figuring a pipe/queue.

     net.inet.ip.dummynet.max_chain_len: 16
             Target value for the maximum number of pipes/queues in a hash
             bucket.  The product max_chain_len*hash_size is used to determine
             the threshold over which empty pipes/queues will be expired even
             when net.inet.ip.dummynet.expire=0.

     net.inet.ip.dummynet.red_lookup_depth: 256

     net.inet.ip.dummynet.red_avg_pkt_size: 512

     net.inet.ip.dummynet.red_max_pkt_size: 1500
             Parameters used in the computations of the drop probability for
             the RED algorithm.


     The following rules show some of the applications of for simulations and
     the like by using dummynet rules in pf.conf(8) configuration files.

     To drop random incoming IPv4 and IPv6 ICMP packets with a probability of
     5%, create a pipe:
           dnctl pipe 10 config plr 0.05

     and add these rules in your pf.conf file:
           dummynet in inet proto icmp all pipe 10
           dummynet in inet6 proto ipv6-icmp all pipe 10

     Should we want to simulate a bidirectional link with bandwidth limita-
     tions, the correct way is to create a pipe for each direction:
           dnctl pipe 1 config bw 14Kbit/s queue 10Kbytes
           dnctl pipe 2 config bw 1Kbit/s queue 10Kbytes

     and add these rules in your pf.conf file:
           dummynet in all pipe 1
           dummynet out all pipe 2

     The above can be very useful, e.g. if you want to see how your fancy Web
     page will look for a residential user who 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,

     Note that with the above rules the pipes receive traffic for both the
     IPv4 and IPv6 protocols.

     Should we want to verify network performance with the RED queue manage-
     ment algorithm, create this pipe:
           dnctl pipe 1 config bw 500Kbit/s queue 100 red 0.002/30/80/0.1

     and then add these rules to you pf.conf file:
           dummynet all pipe 1

     Another typical application of the traffic shaper is to introduce some
     delay in the communication.  This can significantly affect 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 band-
           dnctl pipe 1 config delay 250ms bw 1Mbit/s
           dnctl pipe 2 config delay 250ms bw 1Mbit/s

     and add these rules in your pf.conf file:
           dummynet in all pipe 1
           dummynet out all pipe 2

     Per-flow queueing can be useful for a variety of purposes.  A very simple
     one is counting traffic:
           dnctl pipe 1 config mask all

     and add these statements in your pf.conf file:
           dummynet in quick proto tcp all pipe 1
           dummynet out quick proto tcp all pipe 1
           dummynet in quick proto udp all pipe 1
           dummynet out quick proto udp all pipe 1
           dummynet in quick all pipe 1
           dummynet out quick all pipe 1

     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 six rules, not just the last
     two one, because when dnctl tries to match IP packets it will not con-
     sider ports, so we would not see connections on separate ports as differ-
     ent ones.


     cpp(1), dummynet(4), m4(1), ip(4), pfctl(8), pf.conf(5), protocols(5),
     services(5), sysctl(8)


     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 dnctl utility first appeared in FreeBSD 2.0.  dummynet(4) was intro-
     duced in FreeBSD 2.2.8.  Stateful extensions were introduced in
     FreeBSD 4.0.

Darwin                          August 13, 2002                         Darwin

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