IP multicast routing daemon


mrouted [-c config_file] [-d [debug_level]] [-p] 

Runs on:



-c config_file]
Specify a configuration file (default is /etc/mrouted.conf).
-d [debug_level]
Specify debug level (default is 0).
Start mrouted in a non-pruning mode.


The mrouted utility is an implementation of the Distance-Vector Multicast Routing Protocol (DVMRP) (RFC 1075 specifies an earlier version of this protocol). The utility maintains topological knowledge via a distance-vector routing protocol (like RIP, described in RFC 1058), where it implements a multicast datagram forwarding algorithm called Reverse Path Multicasting.

The mrouted utility forwards a multicast datagram along a shortest (reverse) path tree rooted at the subnet on which the datagram originates. The multicast delivery tree may be thought of as a broadcast delivery tree that has been pruned back so that it doesn't extend beyond those subnetworks that have members of the destination group. Hence, datagrams aren't forwarded along those branches which have no listeners of the multicast group. The IP time-to-live of a multicast datagram can be used to limit the range of multicast datagrams.

In order to support multicasting among subnets that are separated by (unicast) routers that don't support IP multicasting, mrouted includes support for “tunnels”, which are virtual point-to-point links between pairs of mrouteds located anywhere in an internet. IP multicast packets are encapsulated for transmission through tunnels, so that they look like normal unicast datagrams to intervening routers and subnets. The encapsulation is added on entry to a tunnel, and stripped off on exit from a tunnel. By default, the packets are encapsulated using the IP-in-IP protocol (IP protocol number 4). Older versions of mrouted tunnel using IP source routing, which puts a heavy load on some types of routers.


This version doesn't support IP source route tunneling.

The tunneling mechanism allows mrouted to establish a virtual internet, for the purpose of multicasting only, which is independent of the physical internet, and which may span multiple Autonomous Systems. This capability is intended for experimental support of internet multicasting only, pending widespread support for multicast routing by the regular (unicast) routers. The mrouted utility suffers from the well-known scaling problems of any distance-vector routing protocol, and doesn't (yet) support hierarchical multicast routing.

The mrouted utility handles multicast routing only; there may or may not be unicast routing software running on the same machine as mrouted. With the use of tunnels, it isn't necessary for mrouted to have access to more than one physical subnet in order to perform multicast forwarding.


If -d isn't specified, or if the debug level is specified as 0, mrouted detaches from the invoking terminal. Otherwise, it remains attached to the invoking terminal and responsive to signals from that terminal. If -d is specified with no argument, the debug level defaults to 2. Regardless of the debug level, mrouted always writes warning and error messages to the system log daemon. Nonzero debug levels have the following effects:

Level 1
All messages that are sent to the system log are also printed to stderr. In order to capture the log messages, you need to have syslogd running.
Level 2
All level 1 messages plus notifications of significant events are printed to stderr.
Level 3
All level 2 messages plus notifications of all packet arrivals and departures are printed to stderr.

At startup, mrouted writes its pid to the file /var/run/

Configuring mrouted

The mrouted utility automatically configures itself to forward on all multicast-capable interfaces, that is, interfaces that have the IFF_MULTICAST flag set (excluding the loopback “interface”), and it finds other mrouteds directly reachable via those interfaces. To override the default configuration, or to add tunnel links to other mrouteds, configuration commands may be placed in /etc/mrouted.conf (or an alternative file, specified by the -c option). There are five types of configuration commands:

phyint local-addr [disable] [metric m]
[threshold t] [rate_limit b]
[boundary (boundary-name|scoped-addr/mask-len)]
[altnet network/mask-len]
This command can be used to disable multicast routing on the physical interface identified by local IP address local-addr, or to associate a nondefault metric or threshold with the specified physical interface. The local IP address local-addr may be replaced by the interface name (e.g le0). If a phyint is attached to multiple IP subnets, describe each additional subnet with the altnet keyword. The phyint configuration commands must precede tunnel commands.
tunnel local-addr remote-addr [metric m]
[threshold t] [rate_limit b]
[boundary (boundary-name|scoped-addr/mask-len)]
This command can be used to establish a tunnel link between local IP address local-addr and remote IP address remote-addr, and to associate a nondefault metric or threshold with that tunnel. The local IP address local-addr may be replaced by the interface name (e.g. le0). The remote IP address remote-addr may be replaced by a hostname, if and only if the hostname has a single IP address associated with it. The tunnel must be set up in the mrouted.conf files of both routers before it can be used.
cache_lifetime ct
The time (in seconds) that a cached multicast route stays in kernel before timing out (the default is 300). The value of this entry should lie between 300 (5 min) and 86400 (1 day).
pruning off/on
Allow mrouted to act as a non-pruning router (default mode is pruning-enabled). It's also possible to start mrouted in a non-pruning mode using the -p option on the command line. It's expected that a router would be configured in this manner for test purposes only.
name boundary-name scoped-addr/mask-len
You may assign names to boundaries with this keyword, to make configuration easier. The boundary option on phyint or tunnel commands can accept either a name or a boundary.

The file format is free-form; whitespace (including newlines) isn't significant.

Configuration command options

The boundary and altnet options may be specified as many times as necessary.

The “cost” associated with sending a datagram on the given interface or tunnel; it may be used to influence the choice of routes (default is 1). Metrics should be kept as small as possible, because mrouted cannot route along paths with a sum of metrics greater than 31.
The minimum IP time-to-live required for a multicast datagram to be forwarded to the given interface or tunnel (default is 1). It's used to control the scope of multicast datagrams. (The TTL of forwarded packets is only compared to the threshold, it isn't decremented by the threshold.) Every multicast router decrements the TTL by 1.

In general, all mrouteds connected to a particular subnet or tunnel should use the same metric and threshold for that subnet or tunnel.

Allows the network administrator to specify a certain bandwidth in Kbits/second that would be allocated to multicast traffic. It defaults to 500Kbps on tunnels, and 0 (unlimited) on physical interfaces.
Allows an interface to be configured as an administrative boundary for the specified scoped address. Packets belonging to this address won't be forwarded on a scoped interface. The boundary option accepts either a name or a boundary spec.

The mrouted utility won't initiate execution if it has fewer than two enabled virtual interfaces (vifs), where a vif is either a physical multicast-capable interface or a tunnel. It'll log a warning if all of its vifs are tunnels; such an mrouted configuration would be better replaced by more direct tunnels (i.e. eliminate the middle man).


The mrouted utility responds to the following signals:

Restarts mrouted. The configuration file is reread every time this signal is evoked.
Terminates execution gracefully (i.e. by sending goodbye messages to all neighboring routers).
Same as SIGINT.
Dumps the internal routing tables to /var/tmp/mrouted.dump.
Dumps the internal cache tables to /var/tmp/mrouted.cache.
Dumps the internal routing tables to stderr (only if mrouted was invoked with a nonzero debug level).

For convenience in sending signals, on startup, mrouted writes its pid to /var/run/


This is an example configuration for a mythical multicast router at a big school.

# mrouted.conf example

# Name our boundaries to make it easier.

name LOCAL
name EE

# le1 is our gateway to compsci, don't forward our
# local groups to them.

phyint le1 boundary EE

# le2 is our interface on the classroom net, it has four
# different length subnets on it. Note that you can use 
# either an ip address or an interface name.

phyint boundary EE altnet
altnet altnet

# atm0 is our ATM interface, which doesn't properly
# support multicasting.

phyint atm0 disable

# This is an internal tunnel to another EE subnet
# Remove the default tunnel rate limit, since this
# tunnel is over Ethernets

tunnel metric 1 threshold 1
rate_limit 0

# This is our tunnel to the outside world.
# Careful with those boundaries, Eugene.

tunnel metric 1 threshold 32
boundary LOCAL boundary EE

Routing tables

The routing tables look like this:

Virtual Interface Table
 Vif  Local-Address                   Metric  Thresh  Flags
  0      subnet: 36.2         1       1    querier
                   pkts in: 3456
                  pkts out: 2322323

  1     subnet: 36.11        1       1    querier
                   pkts in: 345
                  pkts out: 3456

  2      tunnel:    3       1
                     peers: (2.2)
                boundaries: 239.0.1
                          : 239.1.2
                   pkts in: 34545433
                  pkts out: 234342

  3      tunnel:    3       16

Multicast Routing Table (1136 entries)
 Origin-Subnet    From-Gateway    Metric Tmr In-Vif  Out-Vifs
 36.2                                1    45    0    1* 2  3*
 36.8             4    15    2    0* 1* 3*
 36.11                               1    20    1    0* 2  3*

In the above example, there are four Vifs connecting to two subnets and two tunnels. The vif 3 tunnel isn't in use (no peer address). The vif 0 and vif 1 subnets have some groups present; tunnels never have any groups. This instance of mrouted is the one responsible for sending periodic group membership queries on the vif 0 and vif 1 subnets, as indicated by the “querier” flags. The list of boundaries indicate the scoped addresses on that interface. A count of the number of incoming and outgoing packets is also shown at each interface.

Associated with each subnet from which a multicast datagram can originate is the address of the previous hop router (unless the subnet is directly-connected), the metric of the path back to the origin, the amount of time since we last received an update for this subnet, the incoming vif for multicasts from that origin, and a list of outgoing vifs. A * means that the outgoing vif is connected to a leaf of the broadcast tree rooted at the origin, and a multicast datagram from that origin is forwarded on that outgoing vif only if there are members of the destination group on that leaf.

The mrouted utility also maintains a copy of the kernel forwarding cache table. Entries are created and deleted by mrouted.

The cache tables look like this:

Multicast Routing Cache Table (147 entries)
 Origin             Mcast-group    CTmr  Age Ptmr IVif Forwvifs
 13.2.116/22    3m   2m    -  0    1
 138.96.48/21    5m   2m    -  0    1
 128.9.160/20    3m   2m    -  0    1
 198.106.194/24    9m   28s  9m  0P

Each entry is characterized by the origin subnet number and mask and the destination multicast group.

A “>” as the first character on an additional line is printed for each source on the subnet. There can be many sources in one subnet.
The time since this cache entry was originally created. Since cache entries get refreshed if traffic is flowing, routing entries can grow very old.
Indicates the lifetime of the entry. The entry is deleted from the cache table when the timer decrements to zero.
Shows the interfaces along which datagrams belonging to the source-group are forwarded. A p indicates that no datagrams are being forwarded along that interface. An unlisted interface is a leaf subnet with no members of the particular group on that subnet. A b on an interface indicates that it's a boundary interface, i.e. traffic won't be forwarded on the scoped address on that interface.
Indicates the incoming vif for multicast packets from that origin. Each router also keeps a record of the number of prunes received from neighboring routers for a particular source and group. If there are no members of a multicast group on any downward link of the multicast tree for a subnet, a prune message is sent to the upstream router. They're indicated by a P after the vif number.
The amount of time until the upstream prune times out, or simply a dash if no prune was sent.


Default configuration file for mrouted.
At startup, mrouted writes its pid to this file.
The SIGUSR1 signal causes mrouted to dump the internal routing tables to this file.
The SIGUSR2 signal causes mrouted to dump the internal cache tables to this file.

Contributing author:

Steve Deering, Ajit Thyagarajan, Bill Fenner.


The mrouted program is COPYRIGHT 1989 by The Board of Trustees of Leland Stanford Junior University; for licensing information, see the Third Party License Terms List at

See also:


Based on RFC 1075, which specifies an earlier version of DVMRP.

DVMRP is described, along with other multicast routing algorithms, in the paper Multicast Routing in Internet-works and Extended LANs by S. Deering, in the Proceedings of the ACM SIGCOMM '88 Conference.