Static multicast routing for UNIX
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SMCRoute - A static multicast routing daemon

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Table of Contents


SMCRoute is a UNIX/Linux tool to manage and monitor multicast routes. It supports both IPv4 and IPv6 multicast routing.

SMCRoute can be used as an alternative to dynamic multicast routers like mrouted or pimd in setups where static multicast routes should be maintained and/or no proper IGMP or MLD signaling exists.

Multicast routes exist in the UNIX kernel as long as a multicast routing daemon runs. On Linux, multiple multicast routers can run simultaneously using different multicast routing tables.


  • Configuration file support, /etc/smcroute.conf
  • Support for restarting and reloading the .conf on SIGHUP
  • Source-less on-demand routing, a.k.a. (*,G) based static routing
  • Optional built-in mrdisc support, RFC4286
  • Support for multiple routing tables on Linux
  • Client with built-in support to show routes and joined groups
  • Interface wildcard matching, eth+ matches eth0, eth15


smcrouted [-nNhsv] [-c SEC] [-d SEC] [-e CMD] [-f CONF] [-l LVL] [-p USER:GROUP] [-t ID]
smcroutectl [-Fkhv] [COMMAND] [⟨add | rem⟩ ⟨ROUTE⟩] [⟨join | leave⟩ ⟨GROUP⟩]

To set multicast routes and join groups you must first start the daemon, which needs root privileges, or CAP_NET_ADMIN. Use smcrouted -n to run the daemon in the foreground, as required by modern init daemons like systemd and Finit.

By default smcrouted reads /etc/smcroute.conf, which can look something like this:

mgroup from eth0 group
mgroup from eth0 group source
mroute from eth0 group source to eth1 eth2

The first line means "Join multicast group on interface eth0". Useful if eth0 is not directly connected to the source, but to a LAN with switches with IGMP snooping. Joining the group opens up multicast for that group towards eth0. Only 20 groups can be joined, for large setups investigate enabling multicast router ports in the switches, or possibly use a dynamic multicast routing protocol.

The second mgroup is for source specific group join, i.e. the host specifies that it wants packets from and no other source.

The third mroute line is the actual layer-3 routing entry. Here we say that multicast data originating from on eth0 to the multicast group should be forwarded to interfaces eth1 and eth2.

Note: To test the above you can use ping from another device. The multicast should be visible as long as your IP# matches the source above and you ping -- REMEMBER TO SET THE TTL >1

ping -I eth0 -t 2

The TTL is what usually bites people first trying out multicast routing. Most TCP/IP stacks default to a TTL of 1 for multicast frames, e.g. ping above requires -t 2, or greater. This limitation is intentional and reduces the risk of someone accidentally flooding multicast. Remember, multicast behaves like broadcast unless limited.

The TTL should preferably be set on the sender side, e.g. the camera, but can also be modified in the firewall on a router. Be careful though because the TTL is the only thing that helps prevent routing loops! On Linux the following iptables command can be used to change the TTL:

iptables -t mangle -A PREROUTING -i eth0 -d -j TTL --ttl-inc 1

Some commands, like this one, must usually be run with root privileges or the correct set of capabilities.

Action Scripts

smcrouted -e /path/to/script

With -e CMD a user script or command can be called when smcrouted receives a SIGHUP or installs a multicast route to the kernel. This is useful if you, for instance, also run a NAT firewall and need to flush connection tracking after installing a multicast route.

Many Interfaces

smcrouted -N

With the -N command line option SMCRoute does not prepare all system interfaces for multicast routing. Very useful if your system has a lot of interfaces but only a select few are required for multicast routing. Use the following in /etc/smcroute.conf to enable interfaces:

phyint eth0 enable
phyint eth1 enable
phyint eth2 enable

It is possible to use any interface that supports the MULTICAST flag.

Multiple Routing Tables

On Linux it is possible to run multiple multicast routing daemons due to its support for multiple multicast routing tables. In such setups it may be useful to change the default identity of SMCRoute:

smcrouted -I mrt1 -t 1
smcrouted -I mrt2 -t 2

The -I NAME option alters the default syslog name, config file, PID file, and client socket file name used. In the first instance above, smcrouted will use:

  • /etc/mrt1.conf
  • /var/run/
  • /var/run/mrt1.sock

and syslog messages will use the mrt1 identity as well. Remember to use the same -I NAME also to smcroutectl.

Client Tool

SMCRoute also has a client interface to interact with the daemon:

smcroutectl join eth0
smcroutectl add  eth0 eth1 eth2

If the daemon runs with a different identity the client needs to be called using the same identity:

smcrouted   -I mrt
smcroutectl -I mrt show

There are more commands. See the man page or the online help for details:

smcroutectl help

Note: Root privileges are required by default for smcroutectl due to the IPC socket permissions.


Multicast often originates from different sources but usually not at the same time. For a more generic setup, and to reduce the number of rules required, it is possible to set (*,G) IPv4 multicast routes.

Example smcroute.conf:

phyint eth0 enable mrdisc
phyint eth1 enable
phyint eth1 enable

mgroup from eth0 group
mroute from eth0 group to eth1 eth2

or, from the command line:

# smcroutectl join eth0
# smcroutectl add  eth0 eth1 eth2

Also, see the smcrouted -c SEC option for periodic flushing of learned (*,G) rules, including the automatic blocking of unknown multicast, and the smcroutectl flush command.

Another experimental feature is multicast router discovery, mrdisc, described in RFC4286. This feature is disabled by default, enable with configure --enable-mrdisc. When enabled it periodically sends out an IGMP message on inbound interfaces¹ to alert switches to open up multicast in that direction. Not many managed switches have support for this yet.

¹ Notice the mrdisc flag to the above phyint eth0 directive, which is missing for eth1 and eth2.

Build & Install

SMCRoute should in theory work on any UNIX like operating system which supports the BSD MROUTING API. Both Linux and FreeBSD are tested on a regular basis.

Linux Requirements

On Linux the following kernel config is required:

CONFIG_IP_MROUTE_MULTIPLE_TABLES=y       # For multiple routing tables
CONFIG_IPV6_MROUTE_MULTIPLE_TABLES=y     # For multiple routing tables

*BSD Requirements

On *BSD the following kernel config is required:

options    MROUTING    # Multicast routing
options    PIM         # pimd extensions used for (*,G) support

General Requirements

Check the list of multicast capable interfaces:

cat /proc/net/dev_mcast

or look for interfaces with the MULTICAST flag in the output from:


Some interfaces have the MULTICAST flag disabled by default, like lo and greN. Usually this flag can be enabled administratively.

Configure & Build

The GNU Configure & Build system use /usr/local as the default install prefix. In many cases this is useful, but this means the configuration files and cache files will also use that same prefix. Most users have come to expect those files in /etc/ and /var/run/ and configure has a few useful options that are recommended to use:

./configure --prefix=/usr --sysconfdir=/etc --localstatedir=/var
make -j5
sudo make install-strip

Privilege Separation

As of SMCRoute v2.2 support for privilege separation using the libcap library was added. It is used to drop full root privileges at startup, retaining only CAP_NET_ADMIN for managing the multicast routes.

The build system searches for the libcap library and header file(s). Bith libcap-dev and pkg-config are required.

Note: Although support is automatically detected, the build system will issue a warning if libcap is missing. This can be silenced with configure --without-libcap

Integration with systemd

For systemd integration libsystemd-dev and pkg-config are required. When the unit file is installed, systemctl can be used to enable and start smcrouted:

$ sudo systemctl enable smcroute.service
$ sudo systemctl start smcroute.service

Check that it started properly by inspecting the system log, or:

$ sudo systemctl status smcroute.service

Static Build

Some people want to build statically, to do this with autoconf add the following LDFLAGS= after the configure script. You may also need to add LIBS=..., which will depend on your particular system:

./configure LDFLAGS="-static" ...

Building from GIT

The configure script and the files are generated and not stored in GIT. So if you checkout the sources from GitHub you first need to generated these files using ./

Origin & References

SMCRoute is maintained collaboratively at GitHub. Bug reports, feature requests, patches/pull requests, and documentation fixes are most welcome. The project was previously hosted and maintained by Debian at Alioth and before that by Carsten Schill, the original author.