Re: [musl] mDNS in musl

[Rich Felker <dalias@libc.org>]

On Thu, Mar 07, 2024 at 08:47:20PM -0800, David Schinazi wrote:
> Thanks. How would you feel about the following potential configuration
> design?
> * Add a new configuration option "send_mdns_unicast"
> * When true, use the current behavior
> * When false, send the query on all non-loopback non-p2p interfaces
> * Have send_mdns_unicast default to false
> 
> I was thinking through how to pick interfaces, looked up what other mDNS
> libraries do, and pretty much all of them don't allow configuring
> interfaces, whereas Avahi exposes allow-interfaces and deny-interfaces. I'm
> leaning towards not making this configurable to reduce complexity. I think
> that anyone interested in that level of config is probably using Avahi
> anyway.
> 
> Additionally this design has two nice properties: the default behavior is
> RFC-compliant, and it means that for my use-case I don't need to change the
> config file, which was a big part of my motivation for doing this inside of
> musl in the first place :-)

As discussed in this thread, I don't think so. The biggest problems I
initially brought up were increased information leakage in the default
configuration and inability to control where the traffic goes when you
do want it on. The above proposal just reverts to the initial, except
for providing a way to opt-out.

For the most part, mDNS is very much a "home user, personal device on
trusted network" thing. Not only do you not want it to default on
because a lot of systems will be network servers on networks where
it's not meaningful (and can be a weakness that aids attackers in
lateral movement), but you also don't want it on when connected to
public wifi. For example if you have an open browser tab to
http://mything.local, and migrate to an untrusted network (with your
laptop, tablet, phone, whatever), now your browser will be leaking
private data (likely at least session auth tokens, maybe more) to
whoever answers the mDNS query for mything.local.

Windows has a setting when you add wifi networks for whether they're
treated as private/trusted or public. I would guess it controls
whether mDNS is used, among other things like SMB scanning or
whatever. The same really belongs in a network configurator for
Linux-based personal devices.

Fortunately, I think an approach where you opt-in particular
interfaces/source-addresses, rather than send everywhere by default,
has lower implementation cost and complexity on top of being the safe
thing to do. So none of the above should be taken as a "no" for the
functionality, just a no for "on by default and send everywhere".

Regarding untrusted networks, one thing I hadn't considered yet is
that a network configurator probably needs a way to setup resolv.conf
such that .local queries temp-fail rather than perma-fail (as they
would if you just sent the query to public dns) to use during certain
race windows while switching networks. IOW "send .local queries to
configured nameservers" and "treat .local specially but with an empty
list of interfaces to send to" should be distinct configurations.

Rich