SMCROUTED(8) (smm)
SMCROUTED(8) System Manager's Manual (smm) SMCROUTED(8)

SMCRoute, a static multicast router

smcrouted [
] [
-c SEC
] [
-d SEC
] [
-e CMD
] [
] [
] [
] [
-l LVL
] [
-m SEC
] [
] [
] [
-t ID
] [

smcrouted 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.
smcrouted can be used as an alternative to dynamic multicast daemons like mrouted(8), pimd(8) or pim6sd(8) in situations where static multicast routes should be maintained and/or no proper IGMP or MLD signaling exists.
Multicast routes exist in the UNIX kernel only as long as a multicast routing daemon is running. On Linux, multiple multicast routers can run simultaneously using different multicast routing tables. To run smcrouted and, mrouted at the same time, set the former to use a routing table other than the default (0).
smcrouted modifies the kernel routing table and needs either full superuser rights, or CAP_NET_ADMIN on Linux. This also applies to the friendly control tool smcroutectl(8).

Be careful when creating multicast routes. You can easily flood your networks by inadvertently creating routing loops. Either direct loops listing an inbound interface also as an outbound, or indirect loops by going through other routers.

The following command line options are available:
Flush unused dynamic (*,G) multicast routes every SEC seconds.
This option is intended for systems with topology changes, i.e., when inbound multicast may change both interface and source IP address. E.g. in a setup with at least two VRRP routers. If there is no way of detecting such a topology change this option makes sure to periodically flush all dynamically learned multicast routes so that traffic may resume. Flushing of a specific route only occurs if it was unused during the last flush interval, i.e. there was no traffic matching it. This avoids toggling between different inbound interfaces if traffic arrives on several interfaces simultaneously. In this case, the first selected inbound interface is retained until traffic on it ceases.
Default is 60 sec, set to 0 to disable. See also the smcroutectl flush command, which can be called manually on topology changes.
Daemon startup delay. Delays the probe of interfaces and parsing of the configuration file. Note, the PID file is also not created, since the daemon is not ready yet.
This command line option, although useful in some use-cases, is fragile. It is almost always better to rely on an init or process supervisor that handles dependencies properly, like finit(8), which can wait for interfaces to come up and files to be created before starting a service.
Specify external script or command to be called when smcrouted has loaded/reloaded all static multicast routes from the configuration file, or when a source-less (ANY) rule has been installed.
Alternate configuration file, default /etc/smcroute.conf
Check configuration file syntax, use -l LEVEL to increase verbosity. Returns non-zero on error.
Show summary of command line options and exit.
Set daemon identity. Used to create unique PID, IPC socket, and configuration file names, as well as set the syslog identity. E.g., -I foo would make smcrouted look for /etc/foo.conf, write its PID to /var/run/ and create an IPC socket for smcroutectl in /var/run/foo.sock.
For smcroutectl the same option can be used to select the proper smcrouted instance to send IPC to.
This option is required for both daemon and client when running multiple smcrouted instances, using multiple routing tables, on Linux.
Set log level: none, err, notice, info, debug. Default is notice.
Modify Multicast Router Discovery (mrdisc) announcement interval. Default 20 sec. This option is only available when smcrouted is built with mrdisc support (Linux, and IPv4, only). RFC4286.
Run daemon in foreground, do not detach from controlling terminal
By default smcrouted enables multicast routing on all available, and multicast capable, interfaces in the system. These interfaces are enumerated as VIFs, virtual interfaces, of which most UNIX systems have a very limited amount, usually 32. This daemon option inverts the behavior so no interfaces are enabled by default. Useful on systems with many interfaces, where multicast routing only makes use of a few.
The config file setting phyint IFNAME enable is required to enable the required interfaces.
Drop root privileges to USER:GROUP after start and retain CAP_NET_ADMIN capabilities only. The :GROUP is optional. This option is only available when smcrouted is built with libcap support.
Set PID file name, and optionally full path, in case you need to override the default identity, or the identity set with -i NAME. Regardless, setting this option overrides all others, but it is recommended to use the ident option instead.
Let daemon log to syslog, default unless running in foreground.
Set multicast routing table ID. Remember to also create routing rules directing packets to the table. This example uses routing table ID 123:
ip mrule add iif eth0 lookup 123 
ip mrule add oif eth0 lookup 123
Note: Only available on Linux.
UNIX domain socket path, used for the IPC between smcrouted and smcroutectl. Use this to override the default socket path, derived from the daemon identity, -i NAME. This option can be useful when overriding the identity is not sufficient, e.g. for testing. The default depends on how smcrouted is configured at build time, see FILES.
Show program version and support information.
The -e CMD option is useful if you want to trigger other processes to start when smcrouted has completed installing dynamic multicast routes from (*,G) rules in /etc/smcroute.conf, or when a source-less (ANY) route, a.k.a (*,G) multicast rule, from /etc/smcroute.conf. is matched and installed. For instance, calling conntrack on Linux to flush firewall connection tracking when NAT:ing multicast.
The script CMD is called with an argument reload or install to let the script know if it is called on SIGHUP/startup, or when a (*,G) rule is matched and installed. In the latter case smcrouted also sets two environment variables: source, and group. Beware that these environment variables are unconditionally overwritten by smcrouted and can thus not be used to pass information to the script from outside of smcrouted.

When smcrouted starts up it scans for available network interfaces that have the MULTICAST flag set. Provided the -N flag is not set, each interface is enumerated as a virtual interface (VIF) which is what the kernel's multicast routing stack uses. The enumeration process on some operating systems also require each interface to have an IP address, but Linux and FreeBSD systems only require the ifindex and the MULTICAST flag. If the interface does not yet exist when smcrouted starts, the -d SEC flag can be used to delay startup. Otherwise smcrouted needs to be reloaded (e.g., using SIGHUP) when a new interface has been added to the system.
Since VIFs are a limited resource, most operating systems only support 32 in total, the administrator may need to declare which interfaces to use for multicast routing using the /etc/smcroute.conf phyint directive. It is recommended to always start smcrouted with the -N flag, disabling VIF creation by default, and then selectively enable each of the interfaces you are going to route between. See smcroute.conf(5) for more information.

Because multicast inherently is broadcast there is an obvious need to limit. On a LAN this is usually managed automatically by bridges (switches) with built-in multicast snooping (IGMP and MLD). Between LANs there is also the need to scope multicast, often the same multicast groups are used for different purposes on different LANs. This must be managed by administrators, at least three options exist:
The traditional way of "raising walls" between zones. The outbound interfaces of routers are given a TTL threshold greater than the hop it represents. The default TTL threshold is 1. Managing the routers is a lot easier than adjusting the TTL value of each multicast sender. The only real downside to this is that it scales poorly with the number of routers and it affects all multicast traversing the router's interfaces.
This is one of the current best practices, defining boundaries for sets of multicast groups instead of limiting all multicast (as TTL scoping does). In the case of smcrouted this is left to the administrator to manage. See mrouted(8), and mrouted.conf(5), for more details.
Some sort of filtering mechanism, e.g., firewall (Linux netfilter) or low-level filter (Linux tc or eBPF) that may even have some hardware offloading support (TCAM). The firewall is likely the most common since it is also often used to set up SNAT or 1:1 NAT (Linux netmap).

A multicast route is defined by an input interface IFNAME, the sender's unicast IP address SOURCE, which is optional, the multicast group GROUP and a list of, at least one, output interface IFNAME [IFNAME ...].
mroute from eth0                  group  to eth1 eth2 
mroute from eth0 source   group  to eth1 eth2 
mroute from eth0                  group  ff2e::42  to eth1 eth2 
mroute from eth0 source 2001:3::1 group  ff2e::43  to eth1 eth2
The sender address and multicast group must both be either IPv4 or IPv6 addresses.
The output interfaces are not needed when removing routes using the smcroutectl remove command. The first three parameters are sufficient to identify the source of the multicast route.
The intended purpose of smcrouted is to aid in situations where dynamic multicast routing does not work properly. However, a dynamic multicast routing protocol is in nearly all cases the preferred solution. The reason for this is their ability to translate Layer-3 signaling to Layer-2 and vice versa (IGMP or MLD).
Note: the optional source address multicast routes are not installed in the kernel multicast forwarding cache (MFC) by smcrouted. Instead, it dynamically installs new routes to the kernel MFC, matching the group and inbound interface, when the kernel notifies smcrouted using "upcalls" called NOCACHE messages. This feature was grafted onto smcrouted from mrouted(8), and may not work as intended in all use-cases.

smcrouted is capable of simple group join and leave by sending commands to the kernel. The kernel then handles sending Layer-2 IGMP/MLD join and leave frames as needed. This can be used for testing but is also useful sometimes to open up multicast from the sender if located on a LAN with switches equipped with IGMP/MLD Snooping. Such devices will prevent forwarding of multicast unless an IGMP/MLD capable router or multicast client is located on the same physical port as you run smcrouted on. However, this feature of smcrouted is only intended as a workaround. Some platforms impose a limit on the maximum number of groups that can be joined, some of these systems can be tuned to increase this limit. For bigger installations it is strongly recommended to instead address the root cause, e.g. enable multicast router ports on intermediate switches, either statically or by enabling the multicast router discovery feature of smcrouted.
To emulate a multicast client using smcrouted you use the join and leave commands to issue join and leave commands for a given multicast group on a given interface IFNAME. The GROUP may be given in an IPv4 or IPv6 address format.
The command is passed to the daemon that passes it to the kernel. The kernel then tries to join the multicast group GROUP on interface IFNAME by starting IGMP, or MLD for IPv6 group address, signaling on the given interface. This signaling may be received by routers/switches connected on that network supporting IGMP/MLD multicast signaling and, in turn, start forwarding the requested multicast stream eventually reach your desired interface.

When running multiple smcrouted instances, using the -t ID command line flag, one per routing table on Linux, it is required to use the -i NAME option to both daemon and client. This because the name of the IPC socket used for communicating is composed from the identity.

The most common problem when attempting to route multicast is the TTL. Always start by verifying that the TTL of your multicast stream is not set to 1, because the router decrements the TTL of an IP frame before routing it. Test your setup using ping(8) or iperf(1). Either of which is capable of creating multicast traffic with an adjustable TTL. Iperf in particular is useful since it can act both as a multicast source (sender) and a multicast sink (receiver). For more advanced IP multicast testing the mcjoin(1) tool can be used.

A lot of extra information is sent under the daemon facility and the debug priority to the syslog daemon. Use ‘smcrouted -s -l debug’ to enable.

For convenience in sending signals, smcrouted writes its process ID to /var/run/ upon startup, unless the -p FILE or -i NAME options are used to change the identity or file name used. The following signals are supported:
Tell smcrouted to reload its configuration file and activate the changes.
Terminates execution gracefully.
The same as INT.

Optional configuration file for smcrouted. Defined interfaces to use, groups to join, and routes to set when starting, or reloading smcrouted on SIGHUP. Like the PID file, the name of the configuration file may be different depending on command line options given to the daemon. Most notably, -I IDENT defines the full suite of files used by the smcrouted daemon. See smcroute.conf(5) for details.
Optional configuration directory, path defined by convention only, actual configuration directory, or file(s) to include, defined by /etc/smcroute.conf. See smcroute.conf(5) for details.
Default PID file (re)created by smcrouted when it has started up and is ready to receive commands. See also the -i NAME or -P FILE options which can change the default name.
IPC socket created by smcrouted for use by smcroutectl. Same caveats apply to this file as the previous two, command line options -i NAME and -S FILE to the daemon can be used to change the socket file name.
Linux specific, holds active IPv4 multicast routes.
Linux specific, holds the IPv4 virtual interfaces used by the active multicast routing daemon.
Linux specific, holds active IPv6 multicast routes.
Linux specific, holds the IPv6 virtual interfaces used by the active multicast routing daemon.
Linux specific, holds active IGMP ASM (*,G) joins.
Linux specific, holds active MLD ASM (*,G) joins.
Linux specific, holds active IGMP SSM (S,G) joins.
Linux specific, holds active MLD SSM (S,G) joins.
Linux specific tuning of max IGMP ASM (*,G) per socket, default 20.
Linux specific tuning of max IGMP SSM (S,G) per socket, default 10.
BSD systems may consult the netstat(1) tool for stats on virtual multicast interface tables and multicast forwarding caches, and VIF/MIF allocation, as well as the ifmcstat(8) tool for querying group membership.

smcrouted leverages BSD sysexits.h exit codes (64-78), which process supervisors like systemd(1) and finit(8) understands. The following table details what codes are used for and how to interpret them.
Status Symbolic Name Description
0 EX_OK Success
64 EX_USAGE Invalid command line option, or missing argument
69 EX_UNAVAILABLE Multicast routing socket (or table) already in use
79 EX_SOFTWARE Internal error, bug in smcrouted
71 EX_OSERR Failed fork(), daemon(), getifaddrs(), malloc(), etc.
76 EX_PROTOCOL Kernel does not seem to support multicast routing
77 EX_NOPERM Not enough permissions to run
78 EX_CONFIG Parse error in configuration file

smcroute.conf(5), smcroutectl(8), mrouted(8), pimd(8), pim6sd(8), ping(8), mcjoin(1), iperf(1)

SMCRoute was originally created by Carsten Schill <>. Initial IPv6 support by Todd Hayton <>. Initial FreeBSD support by Micha Lenk <>.
SMCRoute is currently maintained by Joachim Wiberg <>, and Micha Lenk <> at GitHub.
November 28, 2021 Debian