mrouted - IP multicast routing daemon
/usr/sbin/mrouted [-p] [-c config_file] [-d debug_level]
The mrouted 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 implements a multicast 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. 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, 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).
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.
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
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. If -d is specified with no argument, the debug
level defaults to 2. Regardless of the debug level, mrouted always
writes warning and error messages to the system log demon. Non-zero
debug levels have the following effects:
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level 1 all syslog messages are also printed to stderr.
level 2 all level 1 messages plus notifications of "significant"
events are printed to stderr.
level 3 all level 2 messages plus notifications of all packet
arrivals and departures are printed to stderr.
Upon startup, mrouted writes its pid to the file /var/tmp/mrouted.pid.
mrouted automatically configures itself to forward on all multicast-
capable interfaces (i.e., interfaces that have the IFF_MULTICAST flag
set, excluding the loopback "interface"). mrouted 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 /etc/mrouted.conf (or an alternative file,
specified by the -c option). There are four types of configuration
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 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
physical interface. The local IP address <local-addr> may be replaced
by the interface name (such as lan0 ). If phyint is attached to
multiple IP subnets, describe each additional subnet with the altnet
option. phyint commands must precede tunnel commands.
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 metric or threshold with that tunnel. The
local IP address <local-addr> may be replaced by the interface name
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(such as lan0 ). 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 used.
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 command 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 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 name command. The boundary option on phyint or tunnel 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 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 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 (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 mrouted configuration would be better
replaced by more direct tunnels.
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This is an example configuration for a multicast router at a large
# mrouted.conf example
# Name our boundaries to make it easier
name LOCAL 126.96.36.199/16
name EE 188.8.131.52/16
# 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
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 /usr/tmp/mrouted.dump.
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USR2 dumps the internal cache tables to /usr/tmp/mrouted.cache.
QUIT dumps the internal routing tables to stderr (only if mrouted
was invoked with a non-zero debug level).
For convenience in sending signals, mrouted writes its pid to
/var/tmp/mrouted.pid upon startup.
The routing tables look 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 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 interface. A count of the number of incoming
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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
incoming 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 mrouted command 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 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, 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; 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.
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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.
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