mrouted - IP multicast routing daemon
mrouted [-c config_file] [-d debug_level] [-p]
mrouted 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
(like RIP, described in RFC 1058), upon which it implements
datagram forwarding algorithm called Reverse Path Multicasting.
mrouted forwards a multicast datagram along a shortest (reverse) path
tree rooted at the subnet on which the datagram originates.
delivery tree may be thought of as a broadcast delivery
has been pruned back so that it does not extend beyond those
that have members of the destination group. Hence, datagrams are not
forwarded along those branches which have no listeners of
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,
support for "tunnels", which are virtual point-to-point
pairs of mrouted daemons located anywhere in an internet.
packets are encapsulated for transmission through tunnels,
so that they
look like normal unicast datagrams to intervening routers
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-inIP
protocol (IP protocol number 4). Older versions of
mrouted tunnel use
IP source routing, which puts a heavy load on some types of
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, 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
routers. mrouted suffers from the well-known scaling
any distance-vector routing protocol, and does not (yet)
mrouted handles multicast routing only; there may or may not
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.
If no -d option is given, or if the debug level is specified
mrouted detaches from the invoking terminal. Otherwise, it
to the invoking terminal and responsive to signals
from that terminal.
If -d is given with no argument, the debug level defaults to 2.
Regardless of the debug level, mrouted always writes warning
messages to the system log daemon. Non-zero debug levels
have the following
1 All syslog'ed messages are also printed to
2 All level 1 messages plus notifications of
events are printed to stderr.
3 All level 2 messages plus notifications of all
and departures are printed to stderr.
Upon startup, mrouted writes its pid to the file
mrouted automatically configures itself to forward on all
interfaces, i.e., interfaces that have the IFF_MULTICAST
(excluding the loopback "interface"), and it finds other
reachable via those interfaces. To override the default
or to add tunnel links to other mrouted, 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>]
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 significant.
The boundary and altnet options may be specified
as many times
The phyint command can be used to disable multicast routing
on the physical
interface identified by local IP address <local-addr>,
or to associate
a non-default metric or threshold with the specified
The local IP address <local-addr> may be replaced by
name (e.g., le0). If a phyint is attached to multiple IP
each additional subnet with the altnet keyword.
must precede tunnel commands.
The tunnel command can be used to establish a tunnel link
IP address <local-addr> and remote IP address <remote-addr>,
and to associate
a non-default metric or threshold with that tunnel.
The local IP
address <local-addr> may be replaced by the interface name
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
The tunnel must be set up in the mrouted.conf files of both
it can be used.
The cache_lifetime 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).
The pruning option is provided for mrouted to act as a nonpruning
router. It is also possible to start mrouted in a non-pruning mode using
the -p option on the command line. It is expected that a
router would be
configured in this manner for test purposes only. The default mode is
You may assign names to boundaries to make configuration
easier with the
name keyword. The boundary option on phyint or tunnel commands can accept
either a name or a boundary.
The metric is the "cost" associated with sending a datagram
on the given
interface or tunnel; it may be used to influence the choice
The metric defaults to 1. Metrics should be kept as small
because mrouted cannot route along paths with a sum of metrics greater
The threshold is the minimum IP time-to-live required for a
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
Every multicast router decrements the TTL by 1.) The
threshold is 1.
In general, all mrouted connected to a particular subnet or
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
traffic. It defaults to 500Kbps on tunnels, and 0 (unlimited) on physical
The boundary option allows an interface to be configured as
boundary for the specified scoped address. Packets
this address will not be forwarded on a scoped interface.
option accepts either a name or a boundary spec.
mrouted will not initiate execution if it has fewer than two
vifs, where a vif (virtual interface) is either a physical
interface or a tunnel. It will log a warning if all
of its vifs
are tunnels; such an mrouted configuration would be better
more direct tunnels (i.e., eliminate the middle man).
EXAMPLE CONFIGURATION [Toc] [Back]
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 220.127.116.11/16
name EE 18.104.22.168/16
# 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 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
mrouted responds to the following signals:
HUP restarts mrouted. The configuration file is reread
every time this
signal is evoked.
INT terminates execution gracefully (i.e., by sending
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 stderr (only if
invoked with a non-zero debug level).
For convenience in sending signals, mrouted writes its pid
/var/run/mrouted.pid 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
pkts in: 3456
pkts out: 2322323
1 126.96.36.199 subnet: 36.11 1 1
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
groups. This instance of mrouted is the one responsible for
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 no. of incoming
packets is also shown at each interface.
Associated with each subnet from which a multicast datagram
is the address of the previous hop router (unless the subnet
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
on that outgoing vif only if there are members of the destination group
on that leaf.
mrouted also maintains a copy of the kernel forwarding cache
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
13.2.116/22 188.8.131.52 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 and
mask and the
destination multicast group. The 'CTmr' field indicates the
the entry. The entry is deleted from the cache table when
decrements to zero. The 'Age' field is the time since this
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
for multicast packets from that origin. Each router also
record of the number of prunes received from neighboring
routers for a
particular source and group. If there are no members of a
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
datagrams belonging to the source-group are forwarded. A
that no datagrams are being forwarded along that interface.
interface is a leaf subnet with no members of the particular
that subnet. A "b" on an interface indicates that it is a
i.e., traffic will not be forwarded on the scoped
that interface. An additional line with a ">" as the first
printed for each source on the subnet. Note that there can
sources in one subnet.
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
OpenBSD 3.6 May 8, 1995
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