IP multicast routing daemon
mrouted [-c config_file] [-d [debug_level]] [-p]
The mrouted utility is an implementation of the Distance-Vector Multicast Routing Protocol (DVMRP) (RFC 1075 specifies an earlier version of this protocol). The utility maintains topological knowledge via a distance-vector routing protocol (like RIP, described in RFC 1058), where 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 doesn't extend beyond those subnetworks that have members of the destination group. Hence, datagrams aren't 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 don't 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). Older versions of mrouted tunnel using IP source routing, which puts a heavy load on some types of routers.
This version doesn't 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 regular (unicast) routers. The mrouted utility suffers from the well-known scaling problems of any distance-vector routing protocol, and doesn't (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 isn't necessary for mrouted to have access to more than one physical subnet in order to perform multicast forwarding.
If -d isn't 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 daemon. Nonzero debug levels have the following effects:
At 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, that is, interfaces that have the IFF_MULTICAST flag set (excluding the loopback "interface"), 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 /etc/mrouted.conf (or an alternative file, specified by the -c option). There are five types of configuration commands:
The file format is free-form; whitespace (including newlines) isn't significant.
The boundary and altnet options may be specified as many times as necessary.
In general, all mrouteds connected to a particular subnet or tunnel should use the same metric and threshold for that subnet or tunnel.
The mrouted utility won't 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'll 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).
The mrouted utility responds to the following signals:
For convenience in sending signals, on startup, mrouted writes its pid to /var/run/mrouted.pid.
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 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 rate_limit 0 # 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
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 querier groups: 126.96.36.199 188.8.131.52 pkts in: 3456 pkts out: 2322323 1 184.108.40.206 subnet: 36.11 1 1 querier groups: 220.127.116.11 18.104.22.168 22.214.171.124 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) boundaries: 239.0.1 : 239.1.2 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 the above example, there are four Vifs connecting to two subnets and two tunnels. The vif 3 tunnel isn't 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. A * 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 is 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-group CTmr Age Ptmr IVif Forwvifs 13.2.116/22 126.96.36.199 3m 2m - 0 1 >188.8.131.52 >184.108.40.206 138.96.48/21 220.127.116.11 5m 2m - 0 1 >18.104.22.168 128.9.160/20 22.214.171.124 3m 2m - 0 1 >126.96.36.199 198.106.194/24 188.8.131.52 9m 28s 9m 0P >184.108.40.206
Each entry is characterized by the origin subnet number and mask and the destination multicast group.
Steve Deering, Ajit Thyagarajan, Bill Fenner.
The mrouted program is COPYRIGHT 1989 by The Board of Trustees of Leland Stanford Junior University; for the copyright notice and license wording, see mrouted in the appendix Third-Party Copyright Notices.
RFC 1075 specifies an earlier version of DVMRP.
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.