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mrouted - IP multicast routing daemon
mrouted [ -p ] [ -c config_file ] [ -d [ debug_level ]]
Mrouted is an implementation of the Distance-Vector Multi-
cast Routing Protocol (DVMRP), an earlier version of which
is specified in RFC-1075. It maintains topological knowl-
edge via a distance-vector routing protocol (like RIP,
described in RFC-1058), upon which it implements a multi-
cast datagram forwarding algorithm called Reverse Path
Mrouted 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 sub-
networks 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 mul-
ticasting, 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 does not sup-
port IP source route tunnelling.
The tunnelling 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. Mrouted suffers from the
well-known scaling problems of any distance-vector routing
protocol, and does not (yet) support hierarchical multi-
Mrouted 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.
If no "-d" option is given, or if the debug level is spec-
ified 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
given with no argument, the debug level defaults to 2.
Regardless of the debug level, mrouted always writes warn-
ing and error messages to the system log demon. Non-zero
debug levels have the following effects:
all syslog'ed messages are also printed to stderr.
all level 1 messages plus notifications of "signif-
icant" events are printed to stderr.
all level 2 messages plus notifications of all
packet arrivals and departures are printed to
Upon startup, mrouted writes its pid to the file
Mrouted automatically configures itself to forward on all
multicast-capable interfaces, i.e., interfaces that have
the IFF_MULTICAST flag set (excluding the loopback "inter-
face"), 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 mrouted.conf (or an alternative
file, specified by the "-c" option). There are four 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 new-
lines) is not significant. The boundary and altnet
options may be specified as many times as necessary.
The phyint command can be used to disable multicast rout-
ing 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 a phyint is attached to mul-
tiple IP subnets, describe each additional subnet with the
altnet keyword. Phyint commands must precede tunnel com-
The tunnel 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 non-default met-
ric 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 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 be
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). It defaults to 300.
The pruning <off/on> option is provided for mrouted to act
as a non-pruning 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 con-
figured 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 name keyword. The boundary option on
phyint or tunnel commands can accept either a name or a
The metric is the "cost" associated with sending a data-
gram 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 inter-
face or tunnel. It is used to control the scope of multi-
cast 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 mrouteds 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.
Mrouted will not initiate execution if it has fewer than
two enabled vifs, where a vif (virtual interface) is
either a physical multicast-capable interface or a tunnel.
It will 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).
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 184.108.40.206/16
name EE 220.127.116.11/16
# 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
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 stderr (only
if mrouted was invoked with a non-zero debug
For convenience in sending signals, mrouted writes its pid
to /var/run/mrouted.pid upon startup.
The routing tables look like this:
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 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 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 inter-
face. A count of the no. of incoming and outgoing packets
is also shown at each interface.
Associated with each subnet from which a multicast data-
gram 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.
Mrouted 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
13.2.116/22 126.96.36.199 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. 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 data-
grams are being forwarded along that interface. An
unlisted interface is a leaf subnet with are 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 inter-
face. An additional line with a ">" as the first charac-
ter is printed for each source on the subnet. Note that
there can be many sources in one subnet.
mrinfo(8), mtrace(8), map-mbone(8)
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.
Steve Deering, Ajit Thyagarajan, Bill Fenner
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.
(Corrections, notes, and links courtesy of RocketAware.com)
FreeBSD Sources for mrouted(8)
OpenBSD sources for mrouted(8)
Up to: Multicast - Implementation of multicast routing for communications and networking
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