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troglobit
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Static multicast routing for UNIX

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SMCRoute - A static multicast routing daemon

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Table of Contents

About

SMCRoute is a static multicast routing daemon providing fine grained control over the multicast forwarding cache (MFC) in the UNIX kernel. Both IPv4 and IPv6 are fully supported.

SMCRoute can be used as an alternative to dynamic multicast routers like mrouted, pimd, or pim6sd 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.

The full documentation of SMCRoute is available in the manual pages, see smcrouted(8), smcroutectl(8), and smcroute.conf(5).

Features

All features, except mrdisc, are supported for both IPv4 and IPv6. Please note, some features may not be available on systems other than Linux. E.g., FreeBSD does not have SSM group join support.

  • Configuration file support,
    /etc/smcroute.conf
  • Configuration snippet include support,
    /etc/smcroute.d/*.conf
  • Daemon startup options support,
    /etc/default/smcroute
  • Support for seamless reloading of the configuration on
    SIGHUP
  • Source-less on-demand routing, a.k.a. wildcard
    (*,G)
    based static routing, including support for
    (*,G/LEN)
    and
    (S/LEN,G/LEN)
  • Optional built-in mrdisc support for IPv4, RFC4286
  • Support for multiple routing tables on Linux
  • Client to add/remove routes, join/leave groups, and built-in support to show both routes and joined groups
  • Interface wildcard matching,
    eth+
    matches
    eth0, eth15

Note:

smcroutectl
can be used to freely modify the runtime state of
smcrouted
, but any changes made (routes/groups) are lost when the configuration is reloaded. This is by design.

Usage

smcrouted [-nNhsv] [-c SEC] [-d SEC] [-e CMD] [-f FILE] [-i NAME]
          [-l LVL] [-p USER:GROUP] [-P FILE] [-t ID] [-u FILE]

smcroutectl [-dptv] [-i NAME] [-u FILE] [COMMAND] smcroutectl ⟨kill | reload⟩ smcroutectl ⟨add | rem⟩ ⟨ROUTE⟩ smcroutectl ⟨join | leave⟩ ⟨GROUP⟩ smcroutectl show [ routes | groups]

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.

When started from systemd,

smcrouted
rusn with the
-n -s
options, i.e. supervised in the foreground and uses syslog for logging output. The default log level is
INFO
, this can be adjusted using the file
/etc/default/smcroute
:
SMCROUTED_OPTS=-l debug

When configured with

--sysconfdir=/etc
, like most Linux distributions do,
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.

Caveat

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

smcrouted
probes this at runtime by attempting to join as many groups as possible (as have been requested), when the kernel accepts no further joins on a socket,
smcrouted
opens a new one.

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.

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.

Note, however, that depending on the UNIX kernel in use, you may have to have an interface address set, in the relevant address family, and the interface may likely also have to be

UP
.

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/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
.

Client Tool

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.

Wildcard Routes

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)
multicast routes for both IPv4 and IPv6. Variants include
(*,G/LEN)
and
(S/LEN,G/LEN
. These wildcard routes are used as "templates" to match against and install proper
(S,G)
routes when the kernel informs
smcrouted
of inbound multicast from new sources.

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.

Multicast Router Discovery

Another interesting 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.

Note: mrdisc only works on Linux due to

SO_BINDTODEVICE
.

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=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

*BSD Requirements

On *BSD the following kernel config is required:

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

FreeBSD support module loading,

kldload(8)
, edit
/boot/loader.conf
:
ip_mroute_load="yes"
ip_mroute6_load="yes"

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:
ifconfig

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, 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.

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). Both
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
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
.

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.

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