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MROUTED(8) DragonFly System Manager's Manual MROUTED(8)
mrouted -- 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), 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 origi-
nates. 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 physi-
cal internet, and which may span multiple Autonomous Systems. This capa-
bility 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 scal-
ing 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:
Specify an alternative file for configuration commands. Default
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 sig-
nals from that terminal. Regardless of the debug level, mrouted
always writes warning and error messages to the system log dae-
mon. The -debug-level argument is a comma-separated list of any
of the following:
packet 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.
cache Display insertions, deletions and refreshes of entries in
the kernel forwarding cache.
Debug timeouts and periodic processes.
Display information about interfaces and their configura-
Display information about group memberships on physical
Display information about multicast traceroute requests
passing through this router.
igmp Display IGMP operation including group membership and
icmp Monitor ICMP handling.
rsrr Monitor RSRR operation.
Upon startup, mrouted writes its pid to the file
The mrouted utility automatically configures itself to forward on all
multicast-capable interfaces, i.e., interfaces that have the IFF_MULTI-
CAST 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 alterna-
tive file, specified by the -c option).
The file format is free-form; whitespace (including newlines) is not sig-
nificant. The file begins with commands that apply to mrouted's overall
operation or set defaults.
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 unac-
ceptably large. The default is 300 seconds (5 minutes).
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. How-
ever, 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
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 dupli-
cates 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 approxi-
mately one full route report interval. The noflood keyword can
also be specified on individual interfaces.
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
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.
The phyint command does nothing by itself; it is simply a place
holder which interface-specific commands may follow. An inter-
face address or name may be specified.
Disables multicast forwarding on this interface. By default,
mrouted discovers all locally attached multicast capable inter-
faces and forwards on all of them.
If the kernel's netmask does not accurately reflect the subnet
(e.g. you're using proxy-ARP in lieu of IP subnetting), use the
netmask command to describe the real netmask.
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.
igmpv1 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
In addition, the common vif commands described later may all be used on a
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 host-
name may only be used if it maps to a single IP address. A tun-
nel 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 mrouted's packets on
their way out to contain the address of the transmission inter-
face. This is best assured via a static host route.
The common vif commands described below may all be used on tunnels or
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.
The advert_metric is the "cost" associated with sending a data-
gram on the given interface or tunnel; it may be used to influ-
ence 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.
The threshold is the minimum IP time-to-live required for a mul-
ticast 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 sub-
net 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 0 (unlimited).
The boundary option allows an interface to be configured as an
administrative boundary for the specified scoped address. Pack-
ets belonging to this address will not be forwarded on a scoped
interface. The boundary option accepts either a name or a bound-
ary 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.
As described above, but only applicable to this interface/tunnel.
As described above, but only applicable to this interface/tunnel.
Recall that prune retransmission defaults to on on point-to-point
links and tunnels, and off on 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 inter-
face 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 fil-
ter 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 multi-
cast-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
# mrouted.conf example
# Name our boundaries to make it easier
name LOCAL 126.96.36.199/16
name EE 188.8.131.52/16
# lnc1 is our gateway to compsci, don't forward our
# local groups to them
phyint lnc1 boundary EE
# lnc2 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
# 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
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 mes-
sages 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-Address Metric Thresh Flags
0 184.108.40.206 subnet: 36.2/16 1 1 querier
pkts in: 3456
pkts out: 2322323
1 220.127.116.11 subnet: 36.11/16 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 (3.255)
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 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-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 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 describ-
ing any prunes received from downstream dependent neighbors for this sub-
net and group.
map-mbone(8), mrinfo(8), mtrace(8)
DVMRP is described, along with other multicast routing algorithms, in the
paper "Multicast Routing in Internetworks and Extended LANs" by S. Deer-
ing, in the Proceedings of the ACM SIGCOMM '88 Conference.
Steve Deering, Ajit Thyagarajan, and Bill Fenner.
DragonFly 3.9 May 8, 1995 DragonFly 3.9