Proposal to export mdns and llmnr enabled interfaces

[Petr Menšík <pemensik redhat com>]

Hi!

I would like to propose improvement with mdns (and possible llmnr) 
resolution. Current Fedora and Ubuntu contains mdns4_minimal in 
/etc/nsswitch.conf. Which means any name.local gets resolved by mdns on 
every interface and always.

But network manager has configuration for mdns resolution on each 
connection. I know it targets primary systemd-resolved, but I think it 
could export those information in a simple way for mdns nss plugin.

For example into file /run/mdns.interfaces, which would change only on 
each connection change. It would be simple text file, containing on each 
line interface name followed by a list of supported address families.

Current defaults in distribution resolve only over IPv4. I don't see a 
reason for that, so I would enable also IPv6 resolution on any 
connection, which does not set ipv6.method to disabled. As long as it 
has link-local IPv6 address, mdns might work. But could be restricted to 
connections having public IPv6 address eventually.

nss-mdns plugin has separate mdns4_minimal (resolve over IPv4 only), 
mdns6_minimal (resolve over IPv6 only) and mdns_minimal (resolve over 
both). If it would be modified to read /run/mdns.interfaces before each 
query, it could just use single version and provide dynamic behaviour, 
while keeping simple logic in nss plugin.

I would like to have similar possibility also for LLMNR protocol, which 
si very similar. But does not have any nss plugin in current 
distributions. I would like to make one eventually.

I would like to have simple way to allow or restrict multicast 
resolution on some networks, like public transport or conferences. Where 
I don't trust other devices, so I don't want to ask them for names.

What would you think?

The overhead in NM seems minimal, yet it would allow good cooperation 
with the system name resolution. Similar configuration could be also 
provided by different service, like systemd-networkd or any other.

What do you think about such change?

Regards,
Petr