[avahi] Code review request on PR #306

[Philip Prindeville]

Hi,

I was reviewing the logic where avahi-daemon requires the multicast bit (IFF_MULTICAST) be set on the interface of point-to-point tunnels, where a point-to-point connection is by definition unicast (in that the packets can only ever go to a single recipient, i.e. the node at the other end of the tunnel).

Seems it’s previously been an issue:

https://lists.freedesktop.org/archives/avahi/2020-May/thread.html#2538

Anyway, the change is trivial: 

https://github.com/lathiat/avahi/pull/306

It’s a relatively simple change I was hoping to get reviewed.  If the interface is:

(1) loopback OR
(2) multicast OR
(3) point-to-point AND allow-point-to-point is enabled

then the interface should be used.

I’ve run both the previous and the current version of the PR in production with GRE-over-IPsec tunnels and both work in those cases.

This patch is useful when you’re using an IPsec manager that doesn’t allow you to customize tunnel bring-up through scripts where you can simply flag the tunnel as “ip link set multicast on dev $IFNAME”… but that, as the original issue suggests, is the wrong mindset.  Requiring the multicast flag (i.e. “group delivery” capable) on a point-to-point interface (which is inherently limited to a “group of one”, i.e. not a “group” at all) is thinking about this wrong.

I have a few scenarios where I’m using mDNS in VPN scenarios (where the tunneling protocol of choice is ESP) and this should work transparently.

Thanks,

-Philip