[avahi] Is CONFIG_IP_MULTICAST needed for proper avahi-daemon operation ?

[Guillermo Rodriguez Garcia]

Hi all,

I was recently setting up avahi-daemon on a Linux system that uses a
4.9.11 kernel and noticed that avahi failed to advertise services on
IPv4, but had no such problems with IPv6. Looking through the logs I
found this:

Oct 26 08:19:51 avahi-daemon[280]: Joining mDNS multicast group on
interface eth0.IPv4 with address 192.168.0.125.
Oct 26 08:19:51 avahi-daemon[280]: IP_ADD_MEMBERSHIP failed: No buffer
space available

which is apparently due to /proc/sys/net/ipv4/igmp_max_memberships being 0.

Indeed, it looks like there was a change in kernel 4.6 [1] that causes
igmp_max_memberships to be 0 if CONFIG_IP_MULTICAST is not enabled.
Apparently this has been later reverted [2].

Enabling CONFIG_IP_MULTICAST in the offending kernel fixes the problem
obviously but I have noticed that even if CONFIG_IP_MULTICAST is not
enabled in the kernel, I can just do echo 20 >
/proc/sys/net/ipv4/igmp_max_memberships, and with that avahi-daemon
can already advertise services over IPv4.

So the question is: Is CONFIG_IP_MULTICAST needed for proper operation
of avahi-daemon ? What are the possible implications if
CONFIG_IP_MULTICAST is _not_ enabled?

 [1]: https://bugzilla.kernel.org/show_bug.cgi?id=119251
 [2]: https://lkml.org/lkml/2017/8/11/839

Thank you,

Guillermo Rodriguez Garcia
guille.rodriguez at gmail.com

-- 
Guillermo Rodriguez Garcia
guille.rodriguez at gmail.com