Re: [musl] mDNS in musl

[Rich Felker <dalias@libc.org>]

On Fri, Mar 08, 2024 at 11:15:52AM -0800, David Schinazi wrote:
> On Fri, Mar 8, 2024 at 5:30 AM Rich Felker <dalias@libc.org> wrote:
> 
> > 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.
> 
> That's not quite right. The security properties of mDNS and DNS are the
> same. DNS is inherently insecure, regardless of unicast vs multicast. If
> I'm on a coffee shop Wi-Fi, all my DNS queries are sent in the clear to
> whatever IP address the DHCP server gave me.

That's not the case. Connections to non-mDNS hosts are authenticated
by TLS with certificates issued on the basis of ownership of the
domain name. That's not possible with mDNS hostnames, so they'll
either be no-TLS or self-signed certs. That's why the above attack is
possible. It was also possible with normal DNS in the bad old days of
http://, but that time is long gone.

> So the stack has to deal with
> the fact that any DNS response can be spoofed.

That's also not possible with DNSSEC, but only helps if you're
validating it.

> The most widely used
> solution is TLS: a successful DNS hijack can prevent you from accessing a
> TLS service, but can't impersonate it. That's true of both mDNS and regular
> unicast DNS. As an example, all Apple devices have mDNS enabled on all
> interfaces, with no security impact - the features that rely on it
> (AirDrop, AirPlay, contact sharing, etc) all use mTLS to ensure they're
> talking to the right device regardless of the correctness of DNS. (Printing
> remains completely insecure, but that's also independent of DNS - your
> coffee shop Wi-Fi access point can attack you at the IP layer too). One
> might think that DNSSEC could save us here, but it doesn't. DNSSEC was
> unfortunately built with a fundamental design flaw: it requires you to
> trust all resolvers on the path, including recursive resolvers. So even if
> you ask for DNSSEC validation of the DNS records for www.example.com, your
> coffee shop DNS recursive resolver can tell you "I checked, and example.com
> does not support DNSSEC, here's the IP address for www.example.com though"
> and you have to accept it.

This is a completely false but somehow persistent myth about DNSSEC.
You cannot lie that a zone does not support DNSSEC. The only way to
claim a zone does not support DNSSEC is with a signature chain from
the DNS root proving the nonexistence of the DS records for the
delegation. Without that, the reply is BOGUS and will be ignored as if
there was no reply at all.

> > 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".
> >
> 
> And that's totally fair. If your argument is that "defaults should reflect
> the previous behavior", then I can't argue. That said, I disagree that this
> is measurably safer.

Per above, your disagreement seems to be based at least in part on
misinformation.

> > 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.
> 
> Yeah, caching negative results in DNS has been a tricky thing from the
> start. You probably could hack something by installing a fake SOA record
> for .local. in your recursive resolver running on localhost. But the
> RFC-compliant answer is for stub resolvers to treat it specially and know
> that those often never get an answer (musl doesn't cache DNS results so in
> a way we're avoiding this problem altogether at the stub resolver).

The problem here is not about caching, just about clients using a
response. You want a task (like a browser with open tabs) trying to
contact the site to get a tempfail rather than NxDomain which might
make it stop trying. But you probably want NxDomain if mDNS has been
disabled entirely, so that every .local lookup doesn't hang 5 seconds
or whatever before saying "inconclusive".

Rich