Re: resolv.conf ordering

[Rich Felker <dalias@libc.org>]

On Tue, Sep 15, 2015 at 03:25:20AM +0000, Jameel Al-Aziz wrote:
> I'm sure this has been brought up before, but just thought I'd reach out
> see if there's a solution.
> 
> I use musl on Alpine via Docker. I encountered issues today where DNS
> wasn't resolving the way we expect in our images. I finally managed to
> trace it down to musl's resolver (
> http://wiki.musl-libc.org/wiki/Functional_differences_from_glibc#Name_Resolver_.2F_DNS
> ).
> 
> We configure resolv.conf with three DNS servers: Consul DNS, AWS VPC DNS,
> Google DNS. It turns out that the AWS VPC DNS is the fastest to respond and
> therefore causes results to fail even though they can be served via Consul
> DNS. Putting aside that the musl resolver logic breaks convention (which
> many people rely on), it seems that in this case it is more unpredictable
> than simply following the order.
> 
> The host DNS is Consul, and while we could just setup Consul with
> recursors, we run the risk of failing to resolve anything if Consul fails.
> Setting up a local caching DNS is also overkill (we're in Docker
> containers).
> 
> Is there no way to force musl to follow the order of nameservers in
> resolv.conf? Or even if not, to allow musl to accept the first successful
> response instead of failing on the first response? It seems to me that we
> have to give up reliability for predictability, which is not what this
> feature was intended to do from my understanding.
> 
> Any help on this matter would be greatly appreciated!

Someone else raised this question on our IRC channel a week or two
ago, and in short, the answer is no. Basically this setup does not
make sense, even if you do have a resolver (glibc) that does do
ordered fallback:

- If you expect to sometimes need the second or third nameserver for
  queries the first cannot answer, then you're going to have terrible
  performance (multi-second delay before falling back to the second
  one).

- Unless all the nameservers agree on the records they're serving (in
  which case you wouldn't care about order), your query results will
  be unstable/inconsistent when the first server fails to respond. The
  typical result is that you will wrongly get NxDomain instead of a
  failed/timed-out query.

The second issue is really the motivation for what musl is doing: musl
is assuming that all the nameservers have consistent records, because
if they didn't, actual positive/negative results would be affected by
transient failures rather than transient failures being reported to
the calling program. This is a serious class of robustness (and
possibly security, since DoS can translate into false results)
failure.

If you really need to union inconsistent records from multiple
nameservers, the right way to do this is with a dns proxy/cache.
Assuming no _conflicting_ positive responses, it would need to do
something like forward positive responses as soon as it has at least
one positive response from upstream, but only forward negative
responses once it has a negative response from _all_ upstream sources.
Of course these are the constraints to do it "right"/robustly. If all
you want is something that works at least as well as glibc is working
for you now, dnsmasq is probably sufficient.

The conversation about all this on IRC was actually quite interesting.
We have a no-public-logging policy so there are not logs posted
anywhere, but if you're interested in more of what was discussed I
could try to summarize it or see if the people involved would be ok
with sharing a log excerpt.

Rich