NAME

mcd —

SYNOPSIS

mcd	[ -hnsv ] [ -f FILE ] [ -i NAME ] [ -l LEVEL ] [ -p FILE ] [ -u FILE ]

DESCRIPTION

mcd is a multicast querier for the Linux bridge. It runs one IGMP and one MLD querier on every interface enabled in mcd.conf(5), either the bridge itself or a VLAN interface on top of it. The bridge has a querier of its own, but it can only send proxy queries, from 0.0.0.0 or ::, and has no version fallback. Some end devices ignore proxy queries, and without a querier IGMP and MLD snooping times out and multicast floods the LAN. mcd fills that gap:

• IGMP queries are sent from the interface address, MLD queries from its link-local address, as RFC 2710 and RFC 3810 require.
• Querier election per RFC 2236, 3376, 2710 and 3810, the lowest address wins. When another querier wins, mcd backs off and takes over again if it goes silent for the router-timeout.
• On assuming the querier role, mcd sends a startup burst of robustness queries, query-interval / 4 apart, RFC 3376 §8.6 and §8.7.
• Starts at IGMPv3 and MLDv2 and falls back to IGMPv2, IGMPv1 or MLDv1 for as long as older hosts are heard.
• Proxy mode, a backup querier that always loses the election. An interface without an address is always in proxy mode.
• Raw VLAN mode, a querier on one VLAN of a VLAN filtering bridge with no VLAN interface on top.
• Static groups, joined in the kernel so the bridge always forwards them to itself.

Interfaces, links and addresses are followed with netlink, so an interface that appears late or gets its address from DHCP needs no restart. Use mctl(8) to inspect a running mcd.

OPTIONS

-f, --config=FILE

Configuration file, default derived from the identity, see -i and FILES.

-h, --help

Show usage, including the default paths, and exit.

-i, --ident=NAME

Identity, default mcd. Sets the syslog tag and the default names of the configuration, PID and IPC socket files. Use one identity per instance to run several, and the same with mctl(8).

-l, --loglevel=LEVEL

Log level, one of none, err, notice, info or debug. Default notice.

-n, --foreground

Run in the foreground, log to stderr.

-p, --pidfile=FILE

PID file, default derived from the identity. The file is touched when a reload completes.

-s, --syslog

Log to syslog, the default unless -n is given.

-u, --ipc=FILE

IPC socket used by mctl(8), default derived from the identity.

-v, --version

Show version and exit.

SIGNALS

HUP

Reload the configuration file and probe all interfaces again. Static group memberships and packet sockets that are still configured are kept, so a reload causes no traffic on the wire. The PID file is touched when done.

INT, TERM

Exit. Static group memberships are dropped, the kernel sends the leaves.

ENVIRONMENT

MCD_SOCK

IPC socket path, overridden by -u.

FILES

/etc/mcd.conf

Configuration file, see mcd.conf(5).

/run/mcd.pid

PID file.

/run/mcd.sock

IPC socket.

The names follow the identity, so -i foo uses /etc/foo.conf and so on. The directories are set at build time.

SEE ALSO

mcd.conf(5), bridge(8), mctl(8)

STANDARDS

Internet Group Management Protocol, Version 2, RFC 2236. Internet Group Management Protocol, Version 3, RFC 3376. Multicast Listener Discovery (MLD) for IPv6, RFC 2710. Multicast Listener Discovery Version 2 (MLDv2) for IPv6, RFC 3810. Lightweight IGMPv3 and MLDv2 Protocols, RFC 5790.

HISTORY

mcd began in 2021 as querierd, written by Joachim Wiberg at Westermo with DNA strands from mrouted. It gave the Linux bridge in WeOS 5 a querier that industrial end devices would actually answer. Westermo's last release of querierd was v0.10, in May 2023. In 2024 its original author adopted it as mcd. Version 2.0 replaced the last of the mrouted code, and version 3.0 added the MLD querier. Today mcd is the multicast querier in Infix OS.

AUTHORS

mcd is maintained by Joachim Wiberg, Tobias Waldekranz and the KernelKit team. KernelKit is the home of Infix OS, and the place where Open Source projects from industrial networking are picked up and carried on. Jacques de Laval added proxy mode to querierd at Westermo and made four of its releases. The configuration parser goes back to mrouted by Bill Fenner.