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Message-ID: <CAPDSy+6YyWBqy87PN1FCUA6zKX5xpan0r5R4+cFa4GQny+w3kg@mail.gmail.com>
Date: Fri, 22 Mar 2024 10:10:29 +1000
From: David Schinazi <dschinazi.ietf@...il.com>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: mDNS in musl
On Fri, Mar 22, 2024 at 3:46 AM Luca Barbato <lu_zero@...too.org> wrote:
>
> Keep in mind that distributions do and will change the default behavior
> of software components as they see fit anyway so if your use-case is
> having a desktop oriented distribution using musl support mDNS out of
> box, having the feature default off in musl would not make impossible
> having the distro default having it on.
>
That's a good point. My use case is a docker container with Alpine Linux.
It has a single interface and /etc/resolv.conf contains a single DNS name
server on the local network.
lu
>
> PS: which are the stakeholders contacted while the relevant standards
> brought in such hazardous default?
These RFCs went through the IETF Standards Track process, so the entire
IETF community was consulted when this was finalized around 2011-2012.
I'd like to understand why you think this is hazardous though. mDNS only
applies to host names under .local - those names are not covered by DNSSEC,
and therefore any queries for them are always sent completely insecure.
Sending those queries over the wire to the configured DNS resolver has very
similar security properties to sending them over the wire as multicast.
On Fri, Mar 22, 2024 at 5:35 AM Rich Felker <dalias@...c.org> wrote:
> On Thu, Mar 21, 2024 at 11:50:21PM +1000, David Schinazi wrote:
> > My apologies, it wasn't clear to me that these options had been formally
> > rejected,
>
> I don't like to use harsh words like "rejected", but from the
> beginning my part in this conversation has been about what properties
> an mDNS implementation for musl's stub resolver would need to have as
> preconditions to make it acceptable for inclusion. My hope was that,
> out of bringing the topic to the IETF meeting, we'd have some ideas
> for recommended forms for the necessary configuration controls that
> might be suitable for multiple implementations to adopt and aid in
> interoperability, not for you to come back using it as an
> appeal-to-authority to disregard the issues already raised.
>
My apologies, this wasn't intended as an appeal to authority. And to
highlight what I mentioned in my previous email, if you have unique
deployment constraints it's perfectly reasonable to do other things like
change default behavior. I was confirming what the specifications say - you
don't have to follow them. I wasn't trying to disregard the previous
points, please see my emails from March 9 that discuss security and threat
models. I think the preconditions you've laid out stem from a view of the
security properties of DNS that don't match my understanding. We're
operating with different assumptions and I don't think we've reached a
point of understanding where the difference lies.
> I'm not familiar with the musl process. Unfortunately, the choice
> > of off-by-default doesn't follow the relevant standards and, more
> > importantly for me, doesn't solve the use case that got me interested in
>
> Along with what Luca noted, the relevant standards are about how a
> system behaves, not about the mechanisms of a particular part of that
> system.
>
Not really, no. RFC 6762 Section 22.1 paragraph 3 applies to "Name
resolution APIs and libraries", not to operating systems. But to be clear
it's a SHOULD, so not doing anything is not in violation of the spec per se.
> working on this. Your choice is reasonable, and I respect it, but given
> > that it no longer solves an issue for me, I won't be able to spend time
> > writing code for this. If you do end up moving forward with mDNS, I'm
> happy
> > to help answer any questions about the mDNS specifications if you find
> that
> > helpful.
>
> That's fine. I'm not in any hurry to make decisions or write code
> here, but I'd still be happy to hear from others who want this to
> happen for fleshing out whether it makes sense and if so, how the
> configuration should work.
>
>
>
> > Best of luck, and thank you for all the work you do.
> > David
> >
> > On Thu, Mar 21, 2024 at 10:07 PM Rich Felker <dalias@...c.org> wrote:
> >
> > > On Thu, Mar 21, 2024 at 07:21:05PM +1000, David Schinazi wrote:
> > > > Hi,
> > > >
> > > > Earlier today at IETF, I discussed this topic with Stuart Cheshire,
> the
> > > > creator of mDNS. From his perspective, implementing a simpler option
> in
> > > > musl makes a lot of sense. Even though querying mDNS on a single
> > > interface
> > > > is not comprehensive, it'll work for the majority of uses while
> > > minimizing
> > > > implementation complexity. Using the UDP connect() trick to find the
> > > > interface corresponding to the configured resolver and then sending
> > > > multicast only on that interface will work and provide reasonable
> > > security
> > > > properties. He recommends using the IP_MULTICAST_IF /
> IPV6_MULTICAST_IF
> > > > socket options to select an interface, as that's what mDNSResponder
> does
> > > on
> > > > Linux. Additionally, he feels strongly that this should be enabled by
> > > > default, since the whole point of zero-configuration networking was
> for
> > > > things to work without requiring user configuration.
> > >
> > > Again, most of these choices are *not workable*. They have already
> > > been rejected.
> > >
> > > If you want this to happen, let's please work on something that has
> > > not been rejected.
> > >
> > > 1. Why on-by-default was rejected:
> > >
> > > musl is not only or even mostly used in a desktop user configuration
> > > where mDNS makes sense. It's used in lots of places where silently
> > > starting (after upgrade) to query other devices on a network and
> > > accepting answers from them is unexpected and hostile behavior. Yes,
> > > "on by default" makes sense on an end-user desktop system connected to
> > > a *private* network. This would be a default of the particular OS
> > > using musl and its network configurator, not of musl itself. (And
> > > AFAIK mDNS is not even "on my default" on Windows unless the connected
> > > network is marked as private.)
> > >
> > > 2. Why deciding what network to query based on the interface of the
> > > configured resolver is rejected:
> > >
> > > In a proper DNSSEC-validating setup, the configured resolver is
> > > 127.0.0.1 or ::1. This would disable mDNS entirely, forcing you to
> > > essentially *switch DNSSEC off if you want mDNS*. It was already
> > > explained why this is very bad.
> > >
> > > Single-interface query has not been rejected, but I don't see any
> > > reason to limit mDNS to a single interface. That doesn't make it
> > > particularly simpler or anything. If you're able to select the
> > > interface, it's just as easy to allow selecting a reasonable number of
> > > interfaces.
> > >
> > > The particular implementation mechanisms we've discussed, including
> > > possibly identifying the interface(s) to send to via where a
> > > particular address would be routed, seem overall good.
> > >
> > >
> > > > On Sat, Mar 9, 2024 at 9:44 AM David Schinazi <dschinazi.ietf@...il.
> > > .com>
> > > > wrote:
> > > >
> > > > >
> > > > >
> > > > > On Fri, Mar 8, 2024 at 2:54 PM Rich Felker <dalias@...c.org>
> wrote:
> > > > >
> > > > >> On Fri, Mar 08, 2024 at 01:55:18PM -0800, David Schinazi wrote:
> > > > >> > On Fri, Mar 8, 2024 at 12:31 PM Rich Felker <dalias@...c.org>
> > > wrote:
> > > > >> >
> > > > >> > > 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@...c.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.
> > > > >> >
> > > > >> > Apologies for being pedantic, but that's not true. The ability
> to
> > > get
> > > > >> TLS
> > > > >> > certificates for a domain name that you own is a property of the
> > > WebPKI,
> > > > >> > not a property of TLS. What you wrote is true, but only in the
> > > context
> > > > >> of a
> > > > >> > Web browser with an unmodified root certificate store. The
> features
> > > I
> > > > >> > mentioned above don't use the WebPKI, they have a separate root
> of
> > > > >> trust.
> > > > >> > For example, some of those Apple features exchange TLS
> certificates
> > > via
> > > > >> an
> > > > >> > out-of-band mechanism such as Apple trusted servers. Another
> > > example is
> > > > >> the
> > > > >> > Apple Watch: when you first pair a new Apple Watch with an
> iPhone,
> > > they
> > > > >> > exchange ed25519 public keys. Then any time the watch wants to
> > > transfer
> > > > >> a
> > > > >> > large file to/from the phone, it'll connect to Wi-Fi, use mDNS
> to
> > > find
> > > > >> the
> > > > >> > phone, and set up an IKEv2/IPsec tunnel that then protects the
> > > exchange.
> > > > >> > It's resilient to any attacks at the mDNS level.
> > > > >> >
> > > > >> > You're absolutely right that the security of Web requests using
> > > local
> > > > >> > connectivity is completely broken by the lack of WebPKI
> > > certificates for
> > > > >> > those. But sending the DNS query over multicast as opposed to
> > > > >> unencrypted
> > > > >> > unicast to an untrusted DNS server doesn't change the security
> > > > >> properties.
> > > > >> > In your example above, the open tab to http://mything.local
> will
> > > send
> > > > >> that
> > > > >> > query to the recursive resolver - and if that's the one
> received by
> > > DHCP
> > > > >> > then that server can reply with its own address and receive your
> > > auth
> > > > >> > tokens. One potential fix here is to configure your resolv.conf
> to
> > > > >> > localhost and then apply policy in that local resolver. But in
> > > practice,
> > > > >> > application developers don't rely on security at that layer,
> they
> > > assume
> > > > >> > that DNS is unsafe and implement encryption in userspace with
> some
> > > out
> > > > >> of
> > > > >> > band trust mechanism.
> > > > >>
> > > > >> My specific example was http://mything.local in a web browser,
> which
> > > > >> is the way you access lots of mDNS-enabled things in the absence
> of a
> > > > >> specific software ecosystem like Apple's. Since we're talking
> about
> > > > >> musl which would be running on Linux or a Linux-syscall-compatible
> > > > >> environment, without Apple apps, I think that's the main way
> anyone
> > > > >> would be using hypothetical mDNS support. And indeed this is the
> way
> > > > >> you access many printers, 3D printers, IP cameras, etc.
> > > > >>
> > > > >
> > > > > I have multiple services at home that use HTTP and mDNS to
> communicate
> > > > > with. But they're built knowing that unencrypted HTTP is unsafe.
> For
> > > > > example, one of my servers doesn't have any authentication - my
> browser
> > > > > just uses unauthenticated GETs, POSTs and WebSockets. If I leave
> the
> > > tab
> > > > > open and go to a coffee shop, my browser might send that GET to a
> > > server I
> > > > > don't trust but that request won't carry any sensitive information.
> > > Another
> > > > > of my servers uses TLS with self-signed certs, so every time I
> want to
> > > > > communicate with it, I need to click through my browser's "this is
> > > unsafe"
> > > > > interstitial to get to the page. If I switch networks, the browser
> will
> > > > > send me the warning again and I'll know not to click through when
> I'm
> > > not
> > > > > at home. In both of those cases, the security is handled (or not
> > > handled at
> > > > > all) at the application layer.
> > > > >
> > > > > Maybe at some point we'll have a good framework for authenticating
> > > > >> this kind of usage with certificates (probably certificate
> pinning on
> > > > >> first use, with good UX, is the only easy solution),
> > > > >
> > > > >
> > > > > Trust on first use works, or even better there are emerging
> solutions
> > > that
> > > > > leverage codes printed on devices and PAKEs so that a device on the
> > > > > untrusted network can't even hijack the first connection without
> having
> > > > > access to that code. The leading one for home automation is Matter
> [1].
> > > > > Coincidentally, it also leverages mDNS for discovery, and doesn't
> rely
> > > on
> > > > > security at the DNS level.
> > > > >
> > > > > [1] https://csa-iot.org/all-solutions/matter/
> > > > >
> > > > > but at present,
> > > > >> mDNS devices on the .local zone get accessed with plain http://
> all
> > > > >> the time, and this means it's unsafe to do mDNS on
> > > > >> public/untrusted/hostile networks.
> > > > >>
> > > > >
> > > > > The notion of something being "unsafe" (and security in general) is
> > > > > predicated on the existence of a threat model. It's unsafe to use
> > > > > unencrypted HTTP to your bank when your threat model includes
> someone
> > > on
> > > > > the coffee shop Wi-Fi trying to steal your bank credentials.
> > > Conversely,
> > > > > it's safe for me to print to this coffee shop printer if my threat
> > > model
> > > > > assumes that I'm ok with the owner of the coffee shop seeing my
> > > document.
> > > > > Another example is Chromecast which also uses mDNS: from Chrome on
> a
> > > Linux
> > > > > laptop, I can cast YouTube videos to the TV in this coffee shop.
> That's
> > > > > safe because I trust the network with the YouTube link I'm telling
> the
> > > TV
> > > > > to play. mDNS is not in and of itself safe or unsafe. It converts
> > > > > names into addresses, and what you do with those addresses can
> > > potentially
> > > > > be unsafe.
> > > > >
> > > > > That doesn't mean that every single use of mDNS on untrusted
> networks
> > > is
> > > > > safe. If someone builds a web page that sends valuable secrets over
> > > > > unencrypted HTTP to a .local name, then you have a security
> problem.
> > > But my
> > > > > point is that this security problem needs to be solved at the
> > > application
> > > > > layer and not at the DNS layer. That said, I agree that having a
> way to
> > > > > disable mDNS on a machine is a good idea, because there probably
> are
> > > users
> > > > > out there that are stuck with applications that for some reason
> > > decided to
> > > > > rely on DNS being secure.
> > > > >
> > > > > In terms of the tradeoff between usability and security, the
> default
> > > to me
> > > > > lies with default-enabling mDNS on all interfaces as Apple and
> Avahi
> > > do.
> > > > > But this tradeoff is between two metrics that can't be quantified
> one
> > > > > against the other for all possible uses, so I totally understand if
> > > your
> > > > > opinion for musl is that the tradeoff there is different than in
> other
> > > > > situations. You know your users better than I do.
> > > > >
> > > > > > > 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.
> > > > >> >
> > > > >> > I was talking about the case where the recursive resolver does
> the
> > > > >> > validation, which is what's deployed in practice today. What you
> > > wrote
> > > > >> is
> > > > >> > only true if the client does the DNSSEC validation itself. Most
> > > clients
> > > > >> > don't do that today, because too many domains are just
> > > misconfigured and
> > > > >> > broken. Eric Rescorla (the editor of the TLS RFCs) wrote a great
> > > blog
> > > > >> post
> > > > >> > about this:
> > > > >>
> > > > >> The consensus of folks in the stub resolver space (at least
> glibc+musl
> > > > >> and I would assume the BSDs as well) is that the way you do DNSSEC
> > > > >> validation is by having a validating caching proxy or full
> recursive
> > > > >> resolver on localhost. Doing validation in the stub resolver is
> not
> > > > >> viable because it may be static-linked, where it would not be
> able to
> > > > >> be updated with new algorithms, root-of-trust, etc.
> > > > >
> > > > >
> > > > > No disagreement there. By "client" I meant the client device as a
> > > whole,
> > > > > and by "recursive resolver" I meant "the DNS server you got from
> DHCP".
> > > > > Running a DNSSEC-validating recursive resolver on the client device
> > > falls
> > > > > into what I meant by "if the client does the DNSSEC validation
> itself".
> > > > > Sorry for being unclear.
> > > > >
> > > > >
> > > > >> This is one of the
> > > > >> reasons our go-to response for new functionality wanted in the
> stub
> > > > >> resolver is "do it in a nameserver on localhost" -- because you
> > > > >> already need that to do DNSSEC.
> > > > >>
> > > > >
> > > > > That makes sense. I wasn't working with the assumption that DNSSEC
> was
> > > a
> > > > > requirement.
> > > > >
> > > > > It really did not sound like you were talking about trusting the
> > > > >> recursive, though. You called it a "fundamental design flaw",
> which it
> > > > >> is not, and said it requires you to "trust all resolvers on the
> path",
> > > > >> which it does not. It only requires you to trust the immediate
> > > > >> resolver you are interacting with (and not even that if you put
> the
> > > > >> validation in the stub resolver, but there are good reasons not
> to do
> > > > >> that, as above). A pure-proxying server that relies on upstream
> > > > >> recursives can do full DNSSEC validation. Dnsmasq is a canonical
> > > > >> example. I believe systemd-resolvd also does it.
> > > > >>
> > > > >
> > > > > That's fair, and I apologize for overstating my point. I absolutely
> > > agree
> > > > > that if you run a validating recursive resolver locally, then the
> > > attack I
> > > > > described isn't possible. When DNSSEC was designed, it was
> intended to
> > > be
> > > > > deployed in the model I described, where the validating recursive
> > > resolver
> > > > > is not on-device. And that's how it is still mostly deployed today
> > > because
> > > > > almost all general-purpose client devices do not validate locally.
> My
> > > > > mental model is very focused around consumer devices where folks
> buy
> > > them
> > > > > and use them without ever changing default settings. That might be
> a
> > > > > portion of musl users, but you clearly also have advanced users
> that do
> > > > > things differently.
> > > > >
> > > > > > > > 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".
> > > > >> >
> > > > >> > I'm assuming that by tempfail you mean EAI_AGAIN. The two
> browsers
> > > that
> > > > >> > I've written code in don't use that (Chrome just treats it the
> same
> > > as a
> > > > >> > resolution failure and will automatically refresh the tab on a
> > > network
> > > > >> > change; Safari doesn't use getaddrinfo and instead relies on an
> > > > >> > asynchronous DNS API that adds results as they come in - I wrote
> > > that
> > > > >> > algorithm up in RFC 8305). All that said, synchronous blocking
> APIs
> > > like
> > > > >> > getaddrinfo need to eventually return even if no one replies, so
> > > > >> EAI_AGAIN
> > > > >> > makes sense in that case - whereas if .local is blocked by
> policy
> > > then
> > > > >> > immediately returning EAI_NONAME is best.
> > > > >>
> > > > >> Right. Even if applications don't currently distinguish them well,
> > > > >> returning EAI_AGAIN vs EAI_NONAME is meaningful and enables them
> to do
> > > > >> the right thing.
> > > > >>
> > > > >
> > > > > Agreed.
> > > > >
> > > > > Thinking back to our discussion about whether to disable mDNS when
> the
> > > > > resolver is on localhost. I still agree that from an ergonomics
> > > > > perspective, using configs to mean multiple things isn't great. But
> > > > > focusing just on the security properties for a second: if
> resolv.conf
> > > is
> > > > > configured to an IP address that is routed over a given
> non-loopback
> > > > > interface, the current status quo is to send the .local query
> unsecured
> > > > > over that interface. So if we were to, in that specific scenario,
> > > instead
> > > > > send the query over multicast, but only on that interface - then we
> > > > > wouldn't measurably change the security properties of the system.
> In
> > > > > practice there is a slight difference where now you can be
> attacked by
> > > any
> > > > > device on the network as opposed to only by the router on that
> > > network, but
> > > > > I'd argue that there's no meaningful threat model that
> distinguishes
> > > > > between those two attacks. So that would be a safe default option.
> But
> > > > > again, your points about least surprise are still valid, so if you
> > > object
> > > > > to that on those grounds I can't disagree.
> > > > >
> > > > > David
> > > > >
> > >
>
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