IP multicast routing daemon
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 implements a multicast datagram forwarding algorithm called Reverse Path
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
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 do not support IP multicasting,
includes support for
"tunnels", which are virtual point-to-point links between pairs of
daemons 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
tunnel 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
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.
from the well-known scaling problems of any distance-vector routing protocol,
and does not support hierarchical multicast routing.
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
to have access to
more than one physical subnet in order to perform multicast forwarding.
This program follows the usual UNIX command line syntax, with long options
starting with two dashes (`--'). The options are as follows:
- Print a help message and exit.
- If an interface in /etc/mrouted.conf
does not exist yet, print a warning and continue. Useful with VPN, PPP and
other dynamic interfaces. However,
mrouted must still be restarted to
start listening on such interfaces, if they did not exist when
mrouted was started.
mrouted default behavior and
assume all interfaces are disabled unless explicitly enabled with
phyint enable in
- Wait for DELAY seconds before applying the routes. This delay enables to
exchange routes before starting to forward multicast packets and therefore
eliminate transient problems at startup, at the cost of a momentary black
hole. Defaults to 10 seconds.
- Run in foreground, do not detach from the calling terminal.
- Specify an alternative configuration file, default
- By default,
mrouted detaches from the
invoking terminal. If this option is specified,
mrouted it runs in foreground of the
starting terminal and responds to signals. If
-d is given with no argument, the debug
level defaults to
igmp, cache, interface,
groups, prunes, routes and peers.
Regardless of the debug level,
always writes warning and error messages to the system log daemon. Debug
levels have the following effects:
- Debug inbound/outbout packets
- Pruning operations, or pruned routes
- Routing messages
- Detailed routing information
- Neighbor gossip
- Debug routing cache
- Debug timeouts
- Show interface (VIF) debug messages
- Debug group memberships
- Multicast traceroute information
- Debug IGMP messages
- Debug ICMP messages
- Debug RSRR messages
mrouted in a non-pruning mode.
This was previously used in routers for test purposes only. However, this
is no longer supported and this option is only kept for compatibility
- Show state of VIFs and multicast routing tables. This command sends
SIGUSR1 to a running mrouted, waits for the dump file to be updated, and
then displays the result on stdout.
In many cases you do not need to configure
. It configures itself automatically
to forward multicast on all multicast-capable interfaces, i.e., interfaces
that have the IFF_MULTICAST flag set, excluding the loopback interface. It
locates other DVMRP capable routers directly reachable via those interfaces.
- will not start with less than two enabled virtual interfaces (VIFs). A VIF
is either a physical multicast-capable interface or a tunnel.
- will log a warning if all of its VIFs are tunnels; such a configuration is
likely better replaced by more direct (GRE) tunnels (i.e. eliminate the
To override the default settings, for example to to add tunnel links to other
DVMRP routers, configuration commands may be placed in
. There are five types of
The file format is free-form: whitespace (including newlines) is not
option to all
commands can accept either a name or a network boundary; the
options may be specified as many
times as necessary.
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.
option assigns names to boundaries
to make configuration easier.
command can be used to disable
multicast routing (or enable if
started with all interfaces disabled) on the physical interface identified by
local IP address local-addr
, or to associate
a non-default 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 an interface is attached to multiple IP
subnets, describe each additional subnet with the
commands must precede tunnel
command is provided for
to act as a non-pruning router.
This is no longer supported and the configuration option is only kept for
command can be used to establish a
tunnel link between local IP address
and remote IP address
, and to associate a non-default
metric or threshold with that tunnel. The local IP address
may be replaced by the interface
name (e.g. le0). The remote IP address
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 mrouted.conf files of both routers before it can
- 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.
- 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.
- allows the network administrator to specify a certain bandwidth in kbps
which would be allocated to multicast traffic. It defaults to 500 kbps on
tunnels, and 0 (unlimited) on physical interfaces.
- 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 DVMRP routers connected to a particular subnet or tunnel should
use the same metric and threshold for that subnet or tunnel.
This is an example configuration for a mythical multicast router at a big
# mrouted.conf example
# Name our boundaries to make it easier.
name LOCAL 126.96.36.199/16
name EE 188.8.131.52/16
# 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 172.16.12.38 boundary EE
# 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 rate_limit 0
# 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:
mrouted. The configuration
file is reread when SIGHUP is received.
- Terminates execution gracefully, i.e., by sending good-bye messages to all
- The same as INT.
- Dumps the internal routing tables to
- Dumps the internal cache tables to
- Dumps the internal routing tables to stderr (only if
mrouted was invoked with a non-zero
writes its process
ID to /var/run/mrouted.pid
when it has
completed its start up and is ready to receive signals.
- Main configuration file.
- Internal routing table, created and updated on SIGUSR1
- Internal cache table, created and updated on SIGUSR2
- Pidfile (re)created by
when it has started up and is ready to receive commands.
- Holds active IPv4 multicast routes (Linux).
- Holds the IPv4 virtual interfaces used by the active multicast routing
The routing table looks like this:
Virtual Interface Table
Vif Local-Address Metric Thresh Flags
0 184.108.40.206 subnet: 36.2 1 1 querier
pkts in: 3456
pkts out: 2322323
1 220.127.116.11 subnet: 36.11 1 1 querier
pkts in: 345
pkts out: 3456
2 18.104.22.168 tunnel: 22.214.171.124 3 1
peers: 126.96.36.199 (2.2)
pkts in: 34545433
pkts out: 234342
3 188.8.131.52 tunnel: 184.108.40.206 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*
36.8 220.127.116.11 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 we last
received an update for this subnet, the incoming VIF for multicasts from that
origin, and a list of outgoing VIFs. "*" 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.
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 18.104.22.168 3m 2m - 0 1
138.96.48/21 22.214.171.124 5m 2m - 0 1
128.9.160/20 126.96.36.199 3m 2m - 0 1
198.106.194/24 188.8.131.52 9m 28s 9m 0P
Each entry is characterized by the origin subnet number and 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 "P" after the VIF number.
The Forwvifs field 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, i.e.
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.
Multicast Routing in Internetworks and Extended LANs,
Proceedings of the ACM SIGCOMM '88 Conference.
. With contributions by many others.