--- name: "smcrouted" section: "8" platform: "linux" package: "smcroute" version: "2.5.7-1" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/smcrouted" --- # smcrouted(8) SMCRoute, a static multicast router ## NAME **smcrouted** – SMCRoute, a static multicast router ## SYNOPSIS **smcrouted** \[**-nNhsv**\] \[**-c** *SEC*\] \[**-d** *SEC*\] \[**-e** *CMD*\] \[**-f** *FILE*\] \[**-F** *FILE*\] \[**-i** *NAME*\] \[**-l** *LVL*\] \[**-m** *SEC*\] \[**-p** *USER:GROUP*\] \[**-P** *FILE*\] \[**-t** *ID*\] \[**-u** *FILE*\] ## DESCRIPTION **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)](/mrouted/8.md), [pimd(8)](/pimd/8.md) 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)](/smcroutectl/8.md). ### Warning 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. ## OPTIONS The following command line options are available: - **-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)](/finit/8.md), 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. - **-f** *FILE* Alternate configuration file, default */etc/smcroute.conf* - **-F** *FILE* Check configuration file syntax, use **-l** *LEVEL* to increase verbosity. Returns non-zero on error. - **-h** Show summary of command line options and exit. - **-i** *NAME* 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/foo.pid** 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. - **-l** *LEVEL* Set log level: none, err, notice, info, debug. Default is notice. - **-m** *SEC* 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. - **-n** Run daemon in foreground, do not detach from controlling terminal - **-N** 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. - **-p** *USER* \[:GROUP\] 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. - **-P** *FILE* 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. - **-s** 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. - **-u** *FILE* 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. - **-v** 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**. ## OPERATION ### Introduction 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)](/smcroute.conf/5.md) for more information. ### Multicast Scoping 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: - **TTL** **scoping** 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. - **Administrative** **scoping** **(RFC2365)** 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)](/mrouted/8.md), and mrouted.conf(5), for more details. - **Filtering** 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). ### Multicast Routes 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 225.1.2.3 to eth1 eth2 mroute from eth0 source 1.2.3.4 group 225.3.2.1 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)](/mrouted/8.md), and may not work as intended in all use-cases. ### Multicast Groups **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. ### Multiple Daemon Instances 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. ## DEBUGGING 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)](/ping/8.md) or [iperf(1)](/iperf/1.md). 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. ### Note 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. ## SIGNALS For convenience in sending signals, **smcrouted** writes its process ID to */var/run/smcroute.pid* 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: - **HUP** Tell **smcrouted** to reload its configuration file and activate the changes. - **INT** Terminates execution gracefully. - **TERM** The same as INT. ## FILES - */etc/smcroute.conf* 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)](/smcroute.conf/5.md) for details. - */etc/smcroute.d/\*.conf* 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)](/smcroute.conf/5.md) for details. - */var/run/smcroute.pid* 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. - */var/run/smcroute.sock* 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. - */proc/net/ip\_mr\_cache* Linux specific, holds active IPv4 multicast routes. - */proc/net/ip\_mr\_vif* Linux specific, holds the IPv4 virtual interfaces used by the active multicast routing daemon. - */proc/net/ip6\_mr\_cache* Linux specific, holds active IPv6 multicast routes. - */proc/net/ip6\_mr\_vif* Linux specific, holds the IPv6 virtual interfaces used by the active multicast routing daemon. - */proc/net/igmp* Linux specific, holds active IGMP ASM (\*,G) joins. - */proc/net/igmp6* Linux specific, holds active MLD ASM (\*,G) joins. - */proc/net/mcfilter* Linux specific, holds active IGMP SSM (S,G) joins. - */proc/net/mcfilter6* Linux specific, holds active MLD SSM (S,G) joins. - */proc/sys/net/ipv4/igmp\_max\_memberships* Linux specific tuning of max IGMP ASM (\*,G) per socket, default 20. - */proc/sys/net/ipv4/igmp\_max\_msf* Linux specific tuning of max IGMP SSM (S,G) per socket, default 10. BSD systems may consult the [netstat(1)](/netstat/1.md) tool for stats on virtual multicast interface tables and multicast forwarding caches, and VIF/MIF allocation, as well as the [ifmcstat(8)](/ifmcstat/8.md) tool for querying group membership. ## EXIT STATUS **smcrouted** leverages BSD *sysexits.h* exit codes (64-78), which process supervisors like [systemd(1)](/systemd/1.md) and [finit(8)](/finit/8.md) 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 | | 71 | EX\_OSERR | Failed | | 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 | ## SEE ALSO [smcroute.conf(5)](/smcroute.conf/5.md), [smcroutectl(8)](/smcroutectl/8.md), [mrouted(8)](/mrouted/8.md), [pimd(8)](/pimd/8.md), pim6sd(8), [ping(8)](/ping/8.md), mcjoin(1), [iperf(1)](/iperf/1.md) ## AUTHORS SMCRoute was originally created by Carsten Schill ⟨*carsten@cschill.de*⟩. Initial IPv6 support by Todd Hayton ⟨*todd.hayton@gmail.com*⟩. Initial FreeBSD support by Micha Lenk ⟨*micha@debian.org*⟩. SMCRoute is currently maintained by Joachim Wiberg ⟨*troglobit@gmail.com*⟩, and Micha Lenk ⟨*micha@debian.org*⟩ at *GitHub:* **https://github.com/troglobit/smcroute**.