To have multicast support in Jails it is required to allow group addresses to be configured for the jail.
In reality this is impossible as multicast groups are not always a local decision.
It also disallows joining the same multicast group within multiple jails.
The tiny patch allows IPv6 multicast adresses to be used within jails without special configuration.
It is used in production since more than one year, but considered more as an example than a complete patch.
A similar check should also be done for IPv4 and maybe placed under a sysctl or jail option which is disabled by default.
This change was worked out together with Aron Schlesinger <firstname.lastname@example.org>.
Over to maintainer(s).
ae, would you be interested in looking at this too?
It would be great if multicast worked in jails. A standard use case is multimedia/plexmediaserver which is often deployed in jails (eg, FreeNAS) but this breaks DLNA streaming.