RFC: IGMP / multicast proxy

[ispyisail]

IGMP / multicast proxy — implementation plan

Why

IPTV services from some ISPs deliver channels over multicast, which doesn't
cross a router boundary without an IGMP proxy relaying group membership
between WAN and LAN. Without it, IPTV set-top boxes behind Gargoyle simply
don't receive video. This is a narrow, well-understood feature — OpenWrt
already ships igmpproxy/mcproxy; the gap is Gargoyle exposing it.

Approach

Wrap the existing OpenWrt igmpproxy package: enable it, point its "upstream"
interface at WAN and "downstream" at LAN (and any VLAN the admin designates
for IPTV, since some ISPs require multicast on a specific VLAN — see the LAN
VLAN Manager), and expose the couple of settings that actually vary in
practice (which interface is upstream, whether to also proxy on a second LAN
VLAN).

Config / UI sketch

config igmp_proxy 'global'
    option enabled '0'
    option upstream 'wan'
    list downstream 'lan'

A single settings block, likely folded into the existing Basic or Advanced
page rather than a standalone page — this is a one-screen feature.

Phases

1. Package dependency + init script wiring (igmpproxy config generation from
   the UCI block above).
2. UI: enable toggle + upstream/downstream interface pickers, reusing the
   interface list already surfaced by the VLAN Manager.

Test notes

• A vnet case joining a multicast group on the LAN side and confirming
  traffic actually proxies from a WAN-side multicast source — the meaningful
  test here is proxying real multicast, not just that the config file was
  written correctly.

Risks / scope notes

• Small, self-contained; no interaction with safe_apply (doesn't touch
  interfaces the admin manages the router through) or Device Groups.