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DUMMYNET(4) Device Drivers Manual DUMMYNET(4)


dummynettraffic shaper, bandwidth manager and delay emulator


The dummynet system facility permits the control of traffic going through the various network interfaces, by applying bandwidth and queue size limitations, implementing different scheduling and queue management policies, and emulating delays and losses.

The user interface for dummynet is implemented by the ipfw(8) utility, so please refer to the ipfw(8) manpage for a complete description of the dummynet capabilities and how to use it.

Kernel Options

The following options in the kernel configuration file are related to dummynet operation:

enable ipfirewall (required for dummynet)
enable firewall output
limit firewall output
enable dummynet operation
set the timer granularity

Generally, the following options are required:

options DUMMYNET
options HZ=1000		# strongly recommended

Additionally, one may want to increase the number of mbuf clusters (used to store network packets) according to the sum of the bandwidth-delay products and queue sizes of all configured pipes.


setsockopt(2), if_bridge(4), ip(4), ipfw(8), sysctl(8)


The dummynet facility was initially implemented as a testing tool for TCP congestion control by Luigi Rizzo <>, as described on ACM Computer Communication Review, Jan.97 issue. Later it has been modified to work at the IP and bridging levels, integrated with the ipfw(4) packet filter, and extended to support multiple queueing and scheduling policies.

October 28, 2002 Debian