mrouted - IP multicast routing daemon
/etc/mrouted [-p] [-c config_file] [-d [debug_level]]
-p Specifies that mrouted is to start as a nonpruning router. This can
also be specified in the configuration file (see the Configuration File
Specifies an alternative configuration file instead of
Specifies the debugging level (default is 2). If the -d option is not
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.
Regardless of the debug level, mrouted always writes warning and error
messages to the system log demon. Nonzero debug levels have the fol-
1 Prints all syslog messages to stderr.
2 Prints all level 1 messages and notifications of significant events
3 Prints all level 2 messages and notifications of all packet
arrivals and departures to stderr.
The mrouted program is an implementation of the Distance-Vector Multicast
Routing Protocol (DVMRP), an earlier version of which is specified in
RFC1075. The mrouted program maintains topological knowledge using a
distance-vector routing protocol (like Routing Information Protocol (RIP),
described in RFC1058), upon which it implements a multicast datagram for-
warding algorithm called Reverse Path Multicasting.
The mrouted program 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 subnet-
works that have members of the destination group. Hence, datagrams are not
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 (uni-
cast) routers that do not support IP multicasting, mrouted includes support
for tunnels, which are virtual point-to-point links between pairs of
mrouted programs 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 encapsu-
lation is added on entry to a tunnel, and stripped off on exit from a tun-
nel. By default, the packets are encapsulated using the IP-in-IP protocol
(IP protocol number 4). Older versions of mrouted tunnels use IP source
routing, which puts a heavy load on some types of routers. This version
does not support IP source route tunneling.
The tunneling mechanism allows mrouted to establish a virtual internet, for
the purpose of multicasting only, that is independent of the physical
internet and can 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 program suffers from the well-known scaling problems of any
distance-vector routing protocol, and does not support hierarchical multi-
The mrouted program 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 is not necessary for mrouted to have access to more than
one physical subnet in order to perform multicast forwarding.
Upon startup, mrouted writes its PID to the /var/run/mrouted.pid file.
The mrouted program automatically configures itself to forward on all
multicast-capable interfaces (all interfaces except the loopback interface
that have the IFF_MULTICAST flag set), and it finds other mrouted programs
directly reachable through those interfaces. To override the default con-
figuration or to add tunnel links to other mrouted programs, place confi-
guration commands in the /etc/mrouted.conf file (or an alternative file
specified by the -c option). The syntax of the valid configuration commands
are as follows:
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; whitespace (including newlines) is not signi-
ficant. Specify the boundary and altnet options as many times as neces-
A description of each command is as follows:
Disables multicast routing on the physical interface identified by the
local IP address local-addr, or associates a nondefault metric or
threshold with the specified physical interface. The local IP address,
local-addr, may be replaced by the interface name (for example, le0).
If a phyint command is attached to multiple IP subnets, describe each
additional subnet with the altnet keyword. The phyint commands must
precede tunnel commands.
Establishes a tunnel link between the local IP address local-addr and
the remote IP address remote-addr, and associates a nondefault metric
or threshold with that tunnel. The tunnel must be set up in the
mrouted.conf files of both routers before it can be used.
Specifies the amount of time that a cached multicast route stays in the
kernel before timing out. The value of this entry can be between 300
(5 minutes) and 86400 (1 day). The default is 300.
Specifies that mrouted is to act as a nonpruning router. This can also
be done when you start mrouted by specifying the -p option on the com-
mand line. It is expected that a router will be configured in this
manner for testing purposes only. The default mode is pruning enabled.
Assigns names to boundaries in order to ease configuration. The boun-
dary option on phyint or tunnel commands can accept either a name or a
The metric 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
mrouted 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
In general, all mrouted programs 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 kilobits per second that would be allocated to multicast
traffic. It defaults to 500 Kb/s on tunnels; 0 (unlimited) on physical
The boundary option allows an interface to be configured as an administra-
tive boundary for the specified scoped address. Packets belonging to this
address are not forwarded on a scoped interface. The boundary option
accepts either a name or a boundary specification.
The mrouted program does not initiate execution if it has fewer than two
enabled virtual interfaces (vifs); a vif can be either a physical
multicast-capable interface or a tunnel. If all vifs are tunnels, mrouted
logs a warning; such mrouted configurations should be replaced by more
Sample Configuration File
The following is a sample configuration file for a fictitious multicast
router at a large academic institution:
# mrouted.conf example
# Name our boundaries to make it easier
name LOCAL 184.108.40.206/16
name EE 220.127.116.11/16
# le1 is our gateway to compsci, do not 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 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 does not 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
The following is a sample routing table:
Virtual Interface Table
Vif Local-Address Metric Thresh Flags
0 18.104.22.168 subnet: 36.2 1 1 querier
pkts in: 3456
pkts out: 2322323
1 22.214.171.124 subnet: 36.11 1 1 querier
pkts in: 345
pkts out: 3456
2 126.96.36.199 tunnel: 188.8.131.52 3 1
peers: 184.108.40.206 (2.2)
pkts in: 34545433
pkts out: 234342
3 220.127.116.11 tunnel: 18.104.22.168 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 22.214.171.124 4 15 2 0* 1* 3*
36.11 1 20 1 0* 2 3*
In the previous 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 mrouted sends periodic group membership queries on the vif
0 and vif 1 subnets, as indicated by the querier flags. The list of boun-
daries indicates 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 mul-
ticasts from that origin, and a list of outgoing vifs. An asterisk (*)
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 will be
forwarded on that outgoing vif only if there are members of the destination
group on that leaf.
The mrouted program also maintains a copy of the kernel forwarding cache
table. Entries are created and deleted by mrouted.
The following is a sample cache table:
Multicast Routing Cache Table (147 entries)
Origin Mcast-group CTmr Age Ptmr IVif Forwvifs
13.2.116/22 126.96.36.199 3m 2m - 0 1
138.96.48/21 188.8.131.52 5m 2m - 0 1
128.9.160/20 184.108.40.206 3m 2m - 0 1
198.106.194/24 220.127.116.11 9m 28s 9m 0P
Each entry is characterized by the origin subnet number and mask and the
destination multicast group. A description of the remaining fields is as
CTmr Indicates the lifetime of the entry. The entry is deleted from the
cache table when the timer decrements to zero.
Age Indicates the time since this cache entry was originally created.
Since cache entries get refreshed if traffic is flowing, routing
entries can grow very old.
Ptmr Indicates the amount of time until the upstream prune times out.
This is a dash (-) if no prune was sent upstream.
IVif Indicates the incoming vif for multicast packets from that origin.
Each router also maintains a record of the number of prunes
received from neighbouring 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 "P" after the vif
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 is a boundary interface;
traffic is not 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 one subnet can contain many sources.
The mrouted program responds to the following signals:
HUP Restarts mrouted. The configuration file is reread every time this
signal is invoked.
INT Sends good-bye messages to all neighboring routers and terminates
TERM Same as INT.
USR1 Dumps the internal routing tables to the /var/tmp/mrouted.dump
USR2 Dumps the internal cache tables to the /var/tmp/mrouted.cache file.
QUIT Dumps the internal routing tables to stderr (only if mrouted was
invoked with a nonzero debug level).
For convenience in sending signals, mrouted writes its PID to the
/var/run/mrouted.pid file upon startup.
Specifies the default configuration file.
Specifies the mrouted PID file.
Specifies the default dump file.
Specifies the default cache file.
Commands: map-mbone(8), mrinfo(8), mtrace(8)
DVMRP is described, along with other multicast routing algorithms, in the
paper Multicast Routing in Internetworks and Extended LANs by S. Deering,
in the Proceedings of the ACM SIGCOMM '88 Conference
Steve Deering, Ajit Thyagarajan, Bill Fenner