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MROUTED(8) System Manager's Manual MROUTED(8)

mroutedIP multicast routing daemon

mrouted [-p] [-c config_file] [-d [debug_level]]

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 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 mrouted 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 mrouted tunnel use IP source routing, which puts a heavy load on some types of routers. This version does not support 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 multicast routing.

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.

The options are as follows:

Specify an alternative configuration file, instead of the default mrouted.conf.
By default, mrouted detaches from the invoking terminal. If this option is specified, mrouted 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 warning and error messages to the system log daemon. Debug levels have the following effects:

Detach from the invoking terminal.
All syslog(3) messages are also printed to stderr.
All level 1 messages plus notifications of "significant" events are printed to stderr.
All level 2 messages plus notifications of all packet arrivals and departures are printed to stderr.
Start mrouted in a non-pruning mode. It is expected that a router would be configured in this manner for test purposes only. The default mode is pruning enabled.

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"), and it finds other mrouted directly reachable via those interfaces. To override the default configuration, or to add tunnel links to other mrouted, configuration commands may be placed in /etc/mrouted.conf. There are five types of configuration commands:

The file format is free-form: whitespace (including newlines) is not significant. The boundary option can accept either a name or a boundary; the boundary and altnet options may be specified as many times as necessary.

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 name option assigns names to boundaries to make configuration easier.

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 (e.g. le0). If a phyint is attached to multiple IP subnets, describe each additional subnet with the altnet keyword. Phyint commands must precede tunnel commands.

The pruning option is provided for mrouted to act as a non-pruning router.

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 (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 used.

boundary 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.

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.

rate_limit 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.

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 mrouted connected to a particular subnet or tunnel should use the same metric and threshold for that subnet or tunnel.

mrouted will not initiate execution if it has fewer than two enabled virtual interfaces (vifs), 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 (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
name EE
# 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 boundary EE altnet
	altnet altnet
# 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 metric 1 threshold 1
	rate_limit 0
# This is our tunnel to the outside world.
# Careful with those boundaries, Eugene.
tunnel metric 1 threshold 32
	boundary LOCAL boundary EE

mrouted responds to the following signals:

Restarts mrouted. The configuration file is reread every time this signal is evoked.
Terminates execution gracefully (i.e. by sending good-bye messages to all neighboring routers).
The same as INT.
Dumps the internal routing tables to /var/tmp/mrouted.dump.
Dumps the internal cache tables to /var/tmp/mrouted.cache.
Dumps the internal routing tables to stderr (only if mrouted was invoked with a non-zero debug level).


The routing tables look like this:

Virtual Interface Table
 Vif  Local-Address                    Metric  Thresh  Flags
  0      subnet: 36.2          1       1    querier
                   pkts in: 3456
                  pkts out: 2322323

  1     subnet: 36.11         1       1    querier
                   pkts in: 345
                  pkts out: 3456

  2      tunnel:     3       1
                     peers: (2.2)
                boundaries: 239.0.1
                          : 239.1.2
                   pkts in: 34545433
                  pkts out: 234342

  3	    tunnel:	  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            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 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.

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     3m   2m    -  0    1
 138.96.48/21     5m   2m    -  0    1
 128.9.160/20     3m   2m    -  0    1
 198.106.194/24     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.

map-mbone(8), mrinfo(8), mtrace(8)

S. Deering, Multicast Routing in Internetworks and Extended LANs, Proceedings of the ACM SIGCOMM '88 Conference.

Steve Deering, Ajit Thyagarajan, Bill Fenner

February 10, 2020 OpenBSD-7.3