ndots issue on daemon config for services

[fcrisciani]

If the dns options is specified in the daemon.json like:

{
  "debug" : true,
  "dns-opts" : [
    "ndots:1"
  ],
  "experimental" : true
}

If a service is launched on a network like:
docker service create --network net1 alpine top

the resolv.conf results in:

options ndots: 2 ndots:0

Looks like the daemon config is ignored and the ndots:0 is appended blindly

[joeabbey]

Looks like ndots is almost entirely ignored. In sandbox_dns_unix.go, the number of dots is parsed only for validation. As you mentioned it is appended blindly, see sandbox_dns_unix.go:384:
dnsOptionsList = append(dnsOptionsList, resOptions...)
This probably works for forwarding unsupported options on... but seems like it would be easy to "merge" in the dnsOpts loop above.

Moreover, ndots is treated (by libnetwork) as a boolean setting which changes resolver behavior.

This was introduced in e51268a:
e51268a

[joeabbey]

Hmm.. looks like no test case was added into that commit either

[fcrisciani]

@joeabbey I created a patch, and will have to add some tests on top. Feel free to take a look and review it