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 - Daemon startup options support,
/etc/default/smcroute - Support for restarting and reloading the
.confonSIGHUP - 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+matcheseth0, 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 is started with -n -s options, i.e. the daemon runs
in the foreground and uses syslog.
By default smcrouted reads /etc/default/smcroute if the file exists. More
startup options could be specified in this file. For example, to raise log level
for debug purpose:
SMCROUTED_OPTS=-l debug
By default smcrouted reads /etc/smcroute.conf, which can look
something like this:
mgroup from eth0 group 225.1.2.3
mgroup from eth0 group 225.1.2.3 source 192.168.1.42
mroute from eth0 group 225.1.2.3 source 192.168.1.42 to eth1 eth2
The first line means "Join multicast group 225.1.2.3 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. See below Caveat for limitations.
The second mgroup is for source specific group join, i.e. the host
specifies that it wants packets from 192.168.1.42 and no other source.
The third mroute line is the actual layer-3 routing entry. Here we
say that multicast data originating from 192.168.1.42 on eth0 to the
multicast group 225.1.2.3 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 225.1.2.3 -- REMEMBER TO SET THE TTL >1
ping -I eth0 -t 2 225.1.2.3
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 225.1.2.3 -j TTL --ttl-inc 1
Some commands, like this one, must usually be run with root privileges or the correct set of capabilities.
On some platforms there is a limit of 20 groups per socket. This stems
from a limit in BSD UNIX, which also affects Linux. The setting that
controls this is IP_MAX_MEMBERSHIPTS, defined in the system header
file netinet/in.h. Linux users can tweak this with the following
/proc setting:
echo 40 > /proc/sys/net/ipv4/igmp_max_memberships
For large setups it is recommended to investigate enabling multicast router ports in the switches, either statically or by enabling support for multicast router discovery, RFC 4286, or possibly use a dynamic multicast routing protocol.
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.
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.
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/mrt1.pid/var/run/mrt1.sock
and syslog messages will use the mrt1 identity as well. Remember to
use the same -I NAME also to smcroutectl.
SMCRoute also has a client interface to interact with the daemon:
smcroutectl join eth0 225.1.2.3
smcroutectl add eth0 192.168.1.42 225.1.2.3 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 225.1.2.3
mroute from eth0 group 225.1.2.3 to eth1 eth2
or, from the command line:
# smcroutectl join eth0 225.1.2.3
# smcroutectl add eth0 225.1.2.3 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.
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.
On Linux the following kernel config is required:
CONFIG_IP_MROUTE=y
CONFIG_IP_PIMSM_V1=y
CONFIG_IP_PIMSM_V2=y
CONFIG_IP_MROUTE_MULTIPLE_TABLES=y # For multiple routing tables
CONFIG_IPV6_MROUTE_MULTIPLE_TABLES=y # For multiple routing tables
On *BSD the following kernel config is required:
options MROUTING # Multicast routing
options PIM # pimd extensions used for (*,G) support
Check the list of multicast capable interfaces:
cat /proc/net/dev_mcast
or look for interfaces with the MULTICAST flag in the output from:
ifconfig
Some interfaces have the MULTICAST flag disabled by default, like lo
and greN. Usually this flag can be enabled administratively.
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, cache, and PID files will also use that prefix. Most users have
come to expect those files in /etc/ and /var/ and configure has a
few useful options that are recommended to use. For SMCRoute you may
want to use something like this:
./configure --prefix=/usr --sysconfdir=/etc --runstatedir=/var/run
make -j5
sudo make install-strip
Usually your system reserves /usr for native pacakges, so most users
drop --prefix, installing to /usr/local, or use --prefix=/opt.
Note: On some systems --runstatedir may not be available in the
configure script, try --localstatedir=/var instead.
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
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
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" ...
The configure script and the Makefile.in files are generated and not
stored in GIT. So if you checkout the sources from GitHub you first
need to generated these files using ./autogen.sh.
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.