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Bridge Multicast Querier Helper

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This daemon is a querier helper for the Linux bridge. Currently IGMP (IPv4) is supported, MLD (IPv6) querier support is planned.

The daemon comes with a little helper tool called mctl which can be used to check the status of IGMP per interface, but also to dump the bridge's MDB in a more human-friendly format.

Please note, for collecting status, mcd requires the tools jq and iproute2 (ip and bridge) to be installed, their BusyBox equivalents are not sufficient.

Usage

For controlling mcd from another application, use the basic IPC support that mctl employs:

echo "help" |socat - UNIX-CONNECT:/run/mcd.sock

See mcd -h for help, e.g. to customize the IPC path.

Configuration

By default mcd is passive on all interfaces. Use the following settings to enable and tweak the defaults. There is no way to configure different IGMP/MLD settings per interface at the moment, only protocol version.

# /etc/mcd.conf syntax
global-query-interval [1-1024]            # default: 125 sec
global-response-interval [1-1024]         # default: 10 sec
global-last-member-interval [1-1024]      # default: 1
robustness [2-10]                         # default: 2
router-timeout [10-1024]                  # default: 255 sec

iface IFNAME [enable] [vlan [1-4094]]     # default: disable
      [proxy-mode] [igmpv2 | igmpv3]
      [query-interval [1-1024]]

include /path/to.d/*.conf

Description:

  • global-query-interval, query-interval: the interval between IGMP/MLD queries, when elected as querier for a LAN
  • global-response-interval: max response time to a query. Can be used to control the burstiness of IGMP/MLD traffic
  • global-last-member-interval: time between group specific queries, the robustness setting controls the number of queries sent
  • robustness: controls the tolerance to loss of replies from end devices and the loss of elected queriers (above)
  • router-timeout: also known as "other querier present interval", controls the timer used to detect when an elected querier stops sending queries. When the timer expires mcd will initiate a query. The default, when this is unset (commented out) is calculated based on: robustness * query-interval + query-response-interval / 2. Setting this to any value overrides the RFC algorithm, which may be necessary in some scenarios, it is however strongly recommended to leave this setting commented out!
  • include: include a single file, or a glob expression matching files in a directory: /etc/mc.d/*.conf

All interfaces in the system are probed at start (and SIGHUP), to enable mcd to act as a querier, use:

iface IFNAME enable

This enables standard querier mode, using the interface's address as the source IP in IGMP queries, igmpv3 is default but mcd knows how to fall back to IGMPv2 automatically if it detects legacy devices on the LAN. To force IGMPv2 operation from start, set only the igmpv2 flag.

If mcd should never participate in querier elections, only act as a LAN backup querier, set the proxy-mode flag. It ensures mcd always uses the special source address 0.0.0.0 and is therefore guaranteed to never win an election.

Note: unless mcd has an IP address it will operate as if set to proxy-mode. If the interface has no address, or does not yet exist when mcd starts up, mcd will adjust automatically since it listens to Linux NETLINK events.

mcd supports acting as a querier on VLAN filtering bridges that do not have any "upper" interface. In this raw mode of operating, mcd will always be in proxy-mode. To activate:

iface br0 vlan 42 enable