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Bridge 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 querierctl 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. There is a blog post describing how to set the bridge up and use querierd

For controlling querierd from another application, use the basic IPC support that querierctl employs:

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

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

Configuration

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

iface IFNAME [enable] [proxy-queries] [igmpv2 | igmpv3]   # default: disable

Description:

  • query-interval: the interval between IGMP/MLD queries, when elected as querier for a LAN
  • query-response-interval: max response time to a query. Can be used to control the burstiness of IGMP/MLD traffic
  • query-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 querierd 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!

Note: the daemon needs an address on interfaces to operate, it is expected that querierd runs on top of a bridge. Also, currently the daemon does not react automatically to IP address changes, so it needs to be SIGHUP'ed to use any new interface or address.

Motivation

The Linux bridge recently gained support for per-VLAN IGMP/MLD snooping. It stands fine on its own legs, with lots of new per-VLAN settings. But since it lives in the bridge, it cannot know about any VLAN interfaces that may (or may not) sit on top of the bridge. There are naming issues (br0.1 vs vlan1), and the fact that an interface can have multiple IP addresses assigned to it. Therefore, the current bridge implementation in br_multicast.c can only act as a proxy querier, i.e., for IGMP this means it can only send queries (per-VLAN) with source IP 0.0.0.0.

To understand why this might be a problem there are two things to consider:

  1. Some networks don't have a (dynamic) multicast router¹. Usually the multicast router is the IGMP/MLD querier for a LAN, but some LANs consist only of (industrial) switches that try their best to limit the spread of multicast² on low-capacity links
  2. Some end-devices discard queries with source IP 0.0.0.0. This is of course wrong, but good luck telling a PLC vendor they should change their embedded firmware of an aging product, or even the customer site to upgrade their locked-down system with a brand new firmware -- these people don't like change and sometimes because it comes with drawn-out re-certification processes

So, proxy queries are allowed per RFC, but that might not work with some end-devices, and we can't disable proxy-query because some end-devices don't do gratuitous join/leave. Hence, we need a service to provide us with an IGMP/MLD querier that can (at least) use one of the IP address on the interfaces we have on top of the bridge.

Other limitations to be aware of in br_multicast.c (currently) is the lack of IGMP/MLD version fallback. I.e., per RFC, the LAN should start out as IGMPv3 and when an end-device shows up with an IGMPv2 join then the whole LAN should fall back to to IGMPv2, with a timer to return to IGMPv3 if that end-device stops transmitting (IGMP control frames). Of course, optimizations can be done here, e.g., only falling back on the given port, or not tracking (the timer) per individual end-devices. It is worth remembering, that the IGMP standard was written for old hubbed networks, in those days everyone could see a single IGMPv2 message and everyone could potentially confuse that end-device with a newer version.

¹ E.g., Quagga/Frr PIM, or little mrouted/pimd/pimd-dense/pim6sd
² remember, multicast acts like broadcast if it's not limited (by IGMP or MLD), so disabling IGMP/MLD snooping is not an option in these networks, they are what actually keep most of these sites afloat.

Origin & References

This project is based on the mrouted project, with DNA strands also from the pimd project. It should be quite easy to also add MLD/MLDv2 querier functionality from the pim6sd project, because they all share a the same ancestor (mrouted).

The project is maintained by Westermo Network Technologies, and due to its origin, licensed under the same license as mrouted.