mrouted - IP multicast routing daemon
The command is an implementation of the Distance-Vector Multicast Routing Protocol (DVMRP), an earlier version of which is specified in RFC
1075. It maintains topological knowledge via a distance-vector routing protocol (like RIP, described in RFC 1058), upon which it imple-
ments a multicast datagram-forwarding algorithm called Reverse Path Multicasting.
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 does not extend beyond those subnetworks
that have members of the destination group. Hence, datagrams are not forwarded along those branches which have no listeners of the multi-
cast 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 do not support IP multicasting, includes sup-
port for "tunnels", which are virtual point-to-point links between pairs of 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).
The tunnelling mechanism allows 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.
handles multicast routing only; there may or may not be unicast routing software running on the same machine as With the use of tunnels, it
is not necessary for to have access to more than one physical subnet in order to perform multicast forwarding.
If the option is not specified or if the debug level is specified as 0, detaches from the invoking terminal. Otherwise, it remains
attached to the invoking terminal and responsive to signals from that terminal. If is specified with no argument, the debug level defaults
to 2. Regardless of the debug level, always writes warning and error messages to the system log demon. Nonzero debug levels have the fol-
level 1 all messages are also printed to
level 2 all level 1 messages plus notifications of "significant" events are printed to
level 3 all level 2 messages plus notifications of all packet arrivals and departures are printed to
Upon startup, writes its pid to the file
automatically configures itself to forward on all multicast-capable interfaces (i.e., interfaces that have the IFF_MULTICAST flag set,
excluding the loopback "interface"). finds other directly reachable via those interfaces. To override the default configuration or to add
tunnel links to other configuration commands may be placed in (or an alternative file, specified by the option). There are four types of
phyint <local-addr> [disable] [metric <m>]
[threshold <t>] [rate_limit <b>]
tunnel <local-addr> <remote-addr> [metric <m>]
[threshold <t>] [rate_limit <b>]
name <boundary-name> <scoped-addr>/<mask-len>
The file format is free-form; white space (including newlines) is not significant. The boundary and altnet options may be specified as
many times as necessary.
The 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 (such as ). If is attached to multiple IP subnets, describe each additional subnet with the altnet option. commands must precede
The command can be used to establish a tunnel link between local IP address <local-addr> and remote IP address <remote-addr>, and to asso-
ciate a nondefault metric or threshold with that tunnel. The local IP address <local-addr> may be replaced by the interface name (such as
The remote IP address <remote-addr> may be replaced by a host name, if and only if the host name has a single IP address associated with
it. The tunnel must be set up in the files of both routers before it can be used.
is a value that determines the amount of time that a cached multicast route stays in kernel before timing out. The value of this entry
should lie between 300 (5 min) and 86400 (1 day). It defaults to 300.
The command is provided for to act as a nonpruning router. It is also possible to start in a nonpruning mode using the option on the com-
mand line. It is expected that a router would be configured in this manner for test purposes only. The default mode is pruning enabled.
You may assign names to boundaries to make configuration easier with the command. The boundary option on or commands can accept either a
name or a boundary.
The metric option is the "cost" associated with sending a datagram on the given interface or tunnel; it may be used to influence the choice
of routes. The metric defaults to 1. Metrics should be kept as small as possible because cannot route along paths with a sum of metrics
greater than 31.
The threshold is the minimum IP time-to-live required for a multicast datagram to be forwarded to the given interface or tunnel. It is
used to control the scope of multicast datagrams. (The TTL of forwarded packets is only compared to the threshold; it is not decremented
by the threshold. Every multicast router decrements the TTL by 1.) The default threshold is 1.
In general, all connected to a particular subnet or tunnel should use the same metric and threshold for that subnet or tunnel.
The rate_limit option allows the network administrator to specify a certain bandwidth in Kbits/second which would be allocated to multicast
traffic. It defaults to 500Kbps on tunnels and 0 (unlimited) on physical interfaces.
The boundary option allows an interface to be configured as an administrative boundary for the specified scoped address. Packets belonging
to this address will not be forwarded on a scoped interface. The boundary option accepts either a name or a boundary spec.
will not initiate execution if it has fewer than two enabled vifs (virtual interface), where a vif is either a physical multicast-capable
interface or a tunnel. It will log a warning if all of its vifs are tunnels; such an configuration would be better replaced by more direct
This is an example configuration for a multicast router at a large school.
# mrouted.conf example
# Name our boundaries to make it easier
name LOCAL 220.127.116.11/16
name EE 18.104.22.168/16
# lan1 is our gateway to compsci, don't forward our
# local groups to them
phyint lan1 boundary EE
# lan2 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 172.16.12.38 boundary EE altnet 172.16.15.0/26
altnet 172.16.15.128/26 altnet 172.16.48.0/24
# 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 192.168.5.4 192.168.55.101 metric 1 threshold 1
# This is our tunnel to the outside world.
# Careful with those boundaries, Eugene.
tunnel 192.168.5.4 10.11.12.13 metric 1 threshold 32
boundary LOCAL boundary EE
responds to the following signals:
HUP restarts The configuration file is reread every time this signal is evoked.
INT terminates execution gracefully (i.e., by sending good-bye messages to all neighboring routers).
TERM same as INT
USR1 dumps the internal routing tables to
USR2 dumps the internal cache tables to
QUIT dumps the internal routing tables to (only if was invoked with a nonzero debug level).
For convenience in sending signals, writes its pid to upon startup.
The routing tables look like this:
Virtual Interface Table
Vif Local-Address Metric Thresh Flags
0 22.214.171.124 subnet: 36.2 1 1 querier
pkts in: 3456
pkts out: 2322323
1 126.96.36.199 subnet: 36.11 1 1 querier
pkts in: 345
pkts out: 3456
2 188.8.131.52 tunnel: 184.108.40.206 3 1
peers: 220.127.116.11 (2.2)
pkts in: 34545433
pkts out: 234342
3 18.104.22.168 tunnel: 22.214.171.124 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*
36.8 126.96.36.199 4 15 2 0* 1* 3*
36.11 1 20 1 0* 2 3*
In this example, there are four vifs connecting to two subnets and two tunnels. The vif 3 tunnel is not in use (no peer address). The vif
0 and vif 1 subnets have some groups present; tunnels never have any groups. This instance of 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 an update was received for this subnet, the incom-
ing vif for multicasts from that origin, and a list of outgoing vifs. The asterisk ( ) indicates that the outgoing vif is connected to a
leaf of the broadcast tree rooted at the origin, and a multicast datagram from that origin will be forwarded on that outgoing vif only if
there are members of the destination group on that leaf.
The command also maintains a copy of the kernel forwarding cache table. Entries are created and deleted by
The cache tables look like this:
Multicast Routing Cache Table (147 entries)
Origin Mcast-group CTmr Age Ptmr IVif Forwvifs
13.2.116/22 188.8.131.52 3m 2m - 0 1
138.96.48/21 184.108.40.206 5m 2m - 0 1
128.9.160/20 220.127.116.11 3m 2m - 0 1
198.106.194/24 18.104.22.168 9m 28s 9m 0P
Each entry is characterized by the origin subnet number, mask, and the destination multicast group. The CTmr field indicates the lifetime
of the entry. The entry is deleted from the cache table when the timer decrements to zero. The Age field is the time since this cache
entry was originally created. Since cache entries get refreshed if traffic is flowing, routing entries can grow very old. The Ptmr field
is simply a dash if no prune was sent upstream, or the amount of time until the upstream prune will time out. The Ivif field indicates the
incoming vif for multicast packets from that origin. Each router also maintains 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 are indicated by a after the vif number. The Forwvifs field shows the inter-
faces along which datagrams belonging to the source-group are forwarded. A 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 on an interface indicates
that it is a boundary interface; that is, traffic will not be forwarded on the scoped address on that interface. An additional line with a
as the first character is printed for each source on the subnet. Note that there can be many sources in one subnet.
was developed by Steve Deering, Ajit Thyagarajan, Bill Fenner.
DVMRP is described, along with other multicast routing algorithms, in the paper "Multicast Routing in Internetworks and Extended LANs" by
S. Deering, in the