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mrouted - IP multicast routing daemon


See Also


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


The mrouted utility 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.

The mrouted utility 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 multicast routers 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. The packets are encapsulated using the IP-in-IP protocol (IP protocol number 4). Older versions of mrouted tunneled using 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. The mrouted utility suffers from the well-known scaling problems of any distance-vector routing protocol, and does not (yet) support hierarchical multicast routing.

The mrouted utility 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 following options are available:

-c config_file
Specify an alternative file for configuration commands. Default is /etc/mrouted.conf.
-d [debug_level]
If no -d option is given, 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. Regardless of the debug level, mrouted always writes warning and error messages to the system log daemon. The -debug-level argument is a comma-separated list of any of the following:
Display the type, source and destination of all packets sent or received.
Display more information about prunes sent or received.
Display more information about routing update packets sent or received.
Display routing updates in excruciating detail. This is generally way too much information.
Display information about neighbor discovery.
Display insertions, deletions and refreshes of entries in the kernel forwarding cache.
Debug timeouts and periodic processes.
Display information about interfaces and their configuration.
Display information about group memberships on physical interfaces.
Display information about multicast traceroute requests passing through this router.
"igmp" Display IGMP operation including group membership and querier election.
"icmp" Monitor ICMP handling.
"rsrr" Monitor RSRR operation.

Upon startup, mrouted writes its pid to the file /var/run/mrouted.pid.


The mrouted utility 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 DVMRP routers directly reachable via those interfaces. To override the default configuration, or to add tunnel links to other multicast routers, configuration commands may be placed in /etc/mrouted.conf (or an alternative file, specified by the -c option).

The file format is free-form; whitespace (including newlines) is not significant. The file begins with commands that apply to ’s overall operation or set defaults.

cache_lifetime secs
Specifies, in seconds, the lifetime of a multicast forwarding cache entry in the kernel. Multicast forwarding cache entries in the kernel are checked every secs seconds, and are refreshed if the source is still active or deleted if not. Care should be taken when setting this value, as a low value can keep the kernel cache small at the cost of "thrashing" the cache for periodic senders, but high values can cause the kernel cache to grow unacceptably large. The default is 300 seconds (5 minutes).
prune_lifetime secs
Specifies, in seconds, the average lifetime of prunes that are sent towards parents. The actual lifetimes will be randomized in the range [.5secs,1.5secs]. The default is 7200 (2 hours). Smaller values cause less state to be kept both at this router and the parent, at the cost of more frequent broadcasts. However, some routers (e.g. mrouted <3.3 and all currently known versions of cisco’s IOS) do not use the DVMRP generation ID to determine that a neighbor has rebooted. Prunes sent towards these neighbors should be kept short, in order to shorten the time to recover from a reboot. For use in this situation, the prune_lifetime keyword may be specified on an interface as described below.
The mrouted utility uses a DVMRP optimization to prevent having to keep individual routing tables for each neighbor; part of this optimization is that mrouted assumes that it is the forwarder for each of its attached subnets on startup. This can cause duplicates for a short period (approximately one full route report interval), since both the router that just started up and the proper forwarder will be forwarding traffic. This behavior can be turned off with the noflood keyword; mrouted will not assume that it is the forwarder on startup. Turning on noflood can cause black holes on restart, which will generally last approximately one full route report interval. The noflood keyword can also be specified on individual interfaces.
rexmit_prunes [on|off]
Default is to retransmit prunes on all point-to-point interfaces (including tunnels) but no multi-access interfaces. This option may be used to make the default on (or off) for all interfaces. The rexmit_prunes keyword can also be specified on individual interfaces.
name "boundary-name scoped-addr/mask-len"
Associates boundary-name with the boundary described by scoped-addr/mask-len, to help make interface configurations more readable and reduce repetition in the configuration file.

The second section of the configuration file, which may optionally be empty, describes options that apply to physical interfaces.

phyint "local-addr|ifname"
The phyint command does nothing by itself; it is simply a place holder which interface-specific commands may follow. An interface address or name may be specified.
Disables multicast forwarding on this interface. By default, mrouted discovers all locally attached multicast capable interfaces and forwards on all of them.
netmask netmask
If the kernel’s netmask does not accurately reflect the subnet (e.g. you are using proxy-ARP in lieu of IP subnetting), use the netmask command to describe the real netmask.
altnet network/mask-len
If a phyint is attached to multiple IP subnets, describe each additional subnet with the altnet keyword. This command may be specified multiple times to describe multiple subnets.
If there are any IGMPv1 routers on the phyint, use the igmpv1 keyword to force mrouted into IGMPv1 mode. All routers on the phyint must use the same version of IGMP.
Force mrouted to ignore other routers on this interface. mrouted will never send or accept neighbor probes or route reports on this interface.

In addition, the common vif commands described later may all be used on a phyint.

The third section of the configuration file, also optional, describes the configuration of any DVMRP tunnels this router might have.

tunnel "local-addr|ifname" "remote-addr|remote-hostname"
This command establishes a DVMRP tunnel between this host (on the interface described by local-addr or ifname) and a remote host (identified by remote-addr or remote-hostname). A remote hostname may only be used if it maps to a single IP address. A tunnel must be configured on both routers before it can be used.

Be careful that the unicast route to the remote address goes out the interface specified by the "local-addr|ifname" argument. Some UNIX kernels rewrite the source address of ’s packets on their way out to contain the address of the transmission interface. This is best assured via a static host route.

The common vif commands described below may all be used on tunnels or phyints.

metric m
The metric is the "cost" associated with receiving 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 DVMRP cannot route along paths with a sum of metrics greater than 31.
advert_metric m
The advert_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 advert_metric defaults to 0. Note that the effective metric of a link is one end’s metric plus the other end’s advert_metric.
threshold t
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 exactly 1.) The default threshold is 1.

In general, all multicast routers connected to a particular subnet or tunnel should use the same metric and threshold for that subnet or tunnel.

rate_limit r
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 0 (unlimited).
boundary "boundary-name|scoped-addr/mask-len"
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. This command may be specified several times on an interface in order to describe multiple boundaries.
No packets will be sent on this link or tunnel until we hear from the other end. This is useful for the "server" end of a tunnel that goes over a dial-on-demand link; configure the "server" end as passive and it will not send its periodic probes until it hears one from the other side, so will not keep the link up. If this option is specified on both ends of a tunnel, the tunnel will never come up.
As described above, but only applicable to this interface/tunnel.
prune_lifetime secs
As described above, but only applicable to this interface/tunnel.
rexmit_prunes "[on|off]"
As described above, but only applicable to this interface/tunnel. Recall that prune retransmission defaults to on for point-to-point links and tunnels, and to off for multi-access links.
By default, mrouted refuses to peer with DVMRP neighbors that do not claim to support pruning. This option allows such peerings on this interface.
A specialized case of route filtering; no route learned from an interface marked "notransit" will be advertised on another interface marked "notransit". Marking only a single interface "notransit" has no meaning.
accept|deny "(route/mask-len [exact])+" [bidir]
The accept and deny commands allow rudimentary route filtering. The accept command causes mrouted to accept only the listed routes on the configured interface; the deny command causes mrouted to accept all but the listed routes. Only one of accept or deny commands may be used on a given interface.

The list of routes follows the accept or deny keyword. If the keyword exact follows a route, then only that route is matched; otherwise, that route and any more specific route is matched. For example, deny 0/0 denys all routes, while deny 0/0 exact denys only the default route. The default route may also be specified with the default keyword.

The bidir keyword enables bidirectional route filtering; the filter will be applied to routes on both output and input. Without the bidir keyword, accept and deny filters are only applied on input. Poison reverse routes are never filtered out.

The mrouted utility 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
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


The mrouted utility responds to the following signals:
HUP Restarts mrouted. The configuration file is reread every time this signal is evoked.
INT Terminate execution gracefully (i.e., by sending good-bye messages to all neighboring routers).
TERM Same as INT.
USR1 Dump the internal routing tables to /var/tmp/mrouted.dump.
USR2 Dump the internal cache tables to /var/tmp/mrouted.cache.
QUIT Dump 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/run/mrouted.pid upon startup.




The routing tables look like this:

Virtual Interface Table
Vif Local-AddressMetric Thresh Flags
0 subnet: 36.2/16 1 1 querier
pkts in: 3456
pkts out: 2322323

1 36.11/16 1 1 querier
pkts in: 345
pkts out: 3456

2 tunnel: 1
peers: (3.255)
boundaries: 239.0.1/24
: 239.1.2/24
pkts in: 34545433
pkts out: 234342

3 tunnel: 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 15 2 0* 1* 3*
36.111 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 no. 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.

The mrouted utility 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-groupCTmr Age Ptmr IVif Forwvifs
13.2.116/22 2m - 0 1
138.96.48/21 2m - 0 1
128.9.160/20 2m - 0 1
198.106.194/24224.2.135.1909m 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 (or refreshed, if traffic is flowing) 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. An additional line with a "<" as the first character is printed describing any prunes received from downstream dependent neighbors for this subnet and group.


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

Created by Blin Media, 2008-2013