Docker embedded DNS server

[kooba]

I'm having issue with docker container name resolution when using dnspython. Issue seems to be only there when search is set in /etc/resolv.conf

Docker embedded DNS server allows unlinked containers to be resolved just by their name.

I've created gist that can be used to reproduce the problem:
https://gist.github.com/kooba/c464f7e33056cb0ad37edcbe43bb0da0

This can be run with docker-compose up

Expected results in stdout are:

### socket example ###
172.19.0.2
### dnspython example ###
172.19.0.2

getting:

### socket example ###
172.19.0.2
### dnspython example ###
Traceback (most recent call last):
  File "/var/tmp/test.py", line 19, in <module>
    answers = resolver.query(service_url)
  File "/usr/local/lib/python3.4/site-packages/dns/resolver.py", line 1051, in query
    raise NXDOMAIN(qnames=qnames_to_try, responses=nxdomain_responses)
dns.resolver.NXDOMAIN: The DNS query name does not exist: test-service.foo.example.com.

Any help would be greatly appreciated.

[MartinBasti]

Hello, what do you have in /etc/resolv.conf? dnspython asks directly DNS servers listed there, could you please check first with using dig command if test-service.foo.example.com. is resolvable?

Socket uses configuration from /etc/nsswitch.conf, i.e. host files, dns, myhostname, etc..

[kooba]

Hi /etc/resolv.conf is:

search foo.example.com
nameserver 127.0.0.11
options ndots:0

test-service.foo.example.com is not resolvable but I expected that test-service will be first looked up in 127.0.0.11 which would resolve it. I guess test-service is not treated as fully qualified domain and search is used instead?

The workaround I was able to implement was to add . after the url so test-service. would be treated as a fully qualified domain. That fixed it. Just not sure if that's desired behaviour.

[rthalley]

That's the normal desired behavior when there is a search list.