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


smcrouteSMCRoute, a static multicast router


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

smcroutectl [-dtv] [-I NAME] [COMMAND]

smcroutectl ⟨help | flush | kill | restart | version⟩

smcroutectl ⟨show⟩ [groups | routes]

smcroutectl ⟨add  |   rem⟩ ⟨IFNAME⟩ [SOURCE] GROUP[/LEN] IFNAME [IFNAME ...]

smcroutectl ⟨join | leave⟩ ⟨IFNAME⟩ [SOURCE] GROUP[/LEN]


smcroute is both a daemon and command line tool to manipulate the multicast routing table of a UNIX kernel. It supports both IPv4 and IPv6 multicast routing. smcroute can be used as an alternative to dynamic multicast routers like mrouted or pimd 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 as long as a multicast routing daemon is running. On Linux, multiple multicast routers can be used simultaneously with 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 smcroutectl.


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 smcrouted commands are available:
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.
Alternate configuration file, default /etc/smcroute.conf
-c SEC
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.
-d SEC
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.
-e CMD
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.
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.
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.
-t ID
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.
Show program version.
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.


The IFNAME argument in the below smcroutectl commands is the interface name, or an interface wildcard of the form eth+, which matches eth0, eth10, etc. Wildcards are available for inbound interfaces. The following commands are available:
Add a multicast route to the kernel routing cache so that multicast packets received on the network interface IFNAME originating from the IP address SOURCE and sent to the multicast group address GROUP will be forwarded to the outbound network interfaces OUTIFNAME [OUTIFNAME ...]. The interfaces provided as INIFNAME and OUTIFNAME can be any multicast capable network interface as listed by 'ifconfig' or 'ip link list' (incl. tunnel interfaces), including loopback.
To add a (*,G) route, either leave SOURCE out completely or set it to, and if you want to specify a range, set GROUP/LEN, e.g.
Remove a kernel multicast route.
Flush dynamic (*,G) multicast routes now. Similar to how -c SEC works in the daemon, this client command initiates an immediate flush of all dynamically set (*,G) routes. Useful when a topology change has been detected and need to be propagated to smcrouted.
Join a multicast group, with optional prefix length (IPv4 only), on a given interface. The source address is optional, but if given a source specific (SSM) join is performed.
Leave a multicast group, with optional prefix length (IPv4 only), on a given interface. As with the join command, above, the source address is optional.
help [cmd]
Print a usage information message.
Stop (kill) running daemon.
Tell daemon to restart and reload its configuration file. Same as SIGHUP.
show [groups|routes]
Show joined multicast groups or multicast routes, defaults to show routes. Can be combined with the -d option to get details for each multicast route.
Show program version.
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 ...].
The sender's address and the multicast group must both be either IPv4 or IPv6 addresses.
The output interfaces are not needed when removing routes using the remove command. The first three parameters are sufficient to identify the source of the multicast route.
The intended purpose of smcroute 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).
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 smcroute 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.
Note: when running multiple smcrouted instances, 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.


smcrouted supports reading and setting up multicast routes from a config file. The default location is /etc/smcroute.conf, but this can be overridden using the -f FILE command line option.
The IFNAME argument below is the interface name, or an interface wildcard of the form eth+, which matches eth0, eth10, etc. Wildcards are available for inbound interfaces.
# smcroute.conf example 
# The configuration file supports joining multicast groups, to use 
# Layer-2 signaling so that switches and routers open up multicast 
# traffic to your interfaces.  Leave is not supported, remove the 
# mgroup and SIGHUP your daemon, or send a specific leave command. 
# NOTE: Use of the mgroup command should be avoided if possible. 
#       Instead configure "router ports" or similar on the switches 
#       or bridges on your LAN.  This to have them direct all the 
#       multicast to your router, or direct select groups they have 
#       such capabilities.  Usually MAC multicast filters exist. 
#       The UNIX kernel usually limits the number of multicast groups 
#       a socket/client can join.  On Linux, 20 groups can be joined 
#       by default, but this can be changed with sysctl: 
#           sysctl -w net.ipv4.igmp_max_memberships=30 
# Similarly supported is setting mroutes.  Removing mroutes is not 
# supported, remove/comment out the mroute from the .conf file, or 
# send a remove command with smcroutectl. 
# Syntax: 
#   phyint IFNAME <enable|disable> [mrdisc] [ttl-threshold <1-255>] 
#   mgroup from IFNAME [source ADDRESS] group MCGROUP[/LEN] 
#   mroute from IFNAME [source ADDRESS] group MCGROUP[/LEN] to IFNAME [IFNAME ...] 
# This example disables the creation of a multicast VIF for WiFi 
# interface wlan0.  The kernel (at least Linux) sets the ALLMULTI 
# flag for all interfaces that have a VIF enabled.  Hence, it can 
# cause quite a bit of unnecessary traffic to reach the CPU if too 
# many interfaces have a VIF (or MIF in IPv6 lingo).  Only enable 
# interfaces required for inbound and outbound traffic. 
# phyint wlan0 disable 
phyint eth0 enable ttl-threshold 11 
phyint eth1 enable ttl-threshold 3 
phyint eth2 enable ttl-threshold 5 
phyint virbr0 enable ttl-threshold 5 
# The following example instructs the kernel to join the multicast 
# group on interface eth0.  Followed by setting up an 
# mroute of the same multicast stream, but from the explicit sender 
# on the eth0 network and forward to eth1 and eth2. 
mgroup from eth0                     group 
mroute from eth0 source group to eth1 eth2 
# Similar example, but using source-specific group join 
mgroup from virbr0 source group 
mroute from virbr0 source group to eth0 
# Here we allow routing of multicast to group from ANY 
# source coming in from interface eth0 and forward to eth1 and eth2. 
# NOTE: Routing from ANY source is currently only available for IPv4 
#       multicast. 
mgroup from eth0 group 
mroute from eth0 group to eth1 eth2 
# The previous is an example of the (*,G) support.  Such rules cause 
# SMCRoute to dynamically add multicast routes to the kernel when the 
# first frame of a stream reaches the router.  It is also possible to 
# specify a range of such rules, again, note that this currently only 
# works for IPv4.  Also, it is not possible to set a range of groups 
# to join atm. 
mroute from eth0 group to eth1 eth2
Fairly simple. As usual, to identify the origin of the inbound multicast we need the IFNAME, the sender's IP address and, of course, the multicast group address, MCGROUP. The last argument is a list of outbound interfaces.
The source address is optional for IPv4 multicast routes. If omitted it defaults to (INADDR_ANY) and will cause smcrouted to dynamically add new routes, matching the group and inbound interface, to the kernel. This is an experimental feature which may not work as intended, in particular not with 1:1 NAT.
Following the UNIX tradition the file format supports comments starting at the beginning of the line using a hash sign. It is untested to have comments at the end of a line, but should work.
When starting up in debug mode, smcrouted logs the success of parsing each line and setting up a route.


The current version compiles and runs fine on Linux kernel version 2.4, 2.6 and 3.0. Known limits:
Multicast routes
Depends on the kernel, more than 200, probably more than 1000
Multicast group memberships
Max. 20, see caveat above


smcrouted responds to the following signals:
Restart and reload the configuration file. All existing multicast routes and groups are dropped.
Terminates execution gracefully.
The same as INT.
For convenience in sending signals, smcrouted writes its process ID to /var/run/ upon startup.


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 omping(8) tool can be used.


Routes to be set when starting, or restarting 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.
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 to the daemon can change the file names.
Holds active IPv4 multicast routes.
Holds the IPv4 virtual interfaces used by the active multicast routing daemon.
Holds active IPv6 multicast routes.
Holds the IPv6 virtual interfaces used by the active multicast routing daemon.
Holds active IGMP joins.
Holds active MLD joins.
Linux specific tuning of max IGMP groups/socket, default: 20


mrouted(8), pimd(8), omping(8), ping(8), mcjoin(1), iperf(1)


SMCRoute was created by Carsten Schill <>. IPv6 support by Todd Hayton <>. FreeBSD support by Micha Lenk <>.
smcrouted is maintained by Joachim Wiberg <>, Todd Hayton <>, Micha Lenk <> and Julien BLACHE <> at


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.
May 6, 2017 Debian