[avahi] Code review request on PR #306

[Philip Prindeville]

Still hoping for a review of this relatively trivial PR.


> On Sep 26, 2020, at 11:24 AM, Philip Prindeville <philipp_subx at redfish-solutions.com> wrote:
> 
> 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
> 
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