Define docker hostname/DNS underscore behavior

[mostolog]

Hi

As discussed in moby/moby#27447 and suggested by @thaJeztah I file this issue in order to ask @docker for a clear statement on using underscores in container/services hostnames.

According to http://stackoverflow.com/a/2183140:

• DNS spec allows underscores
• Underscores aren't valid in hostnames
• Docker compose/stack deploy/services append _X to names

With moby/swarmkit#1242 in mind, it would be great to know how underscores should be considered: Should docker binaries use "-" instead of "_" for compose/stack deploy/services instead? Might docker allow underscores in hostname but emit a warning if there are any?

Regards.

[thaJeztah]

Keep in mind that my reference to this repository was for image distribution (e.g. docker pull _foo_bar.example.com:5000/repo/image:tag), which is separate from service names.

We removed the validation on hostnames for containers (moby/moby#28990), but hostnames are local to the container, so not the name under which containers can be "discovered".

[mostolog]

@thaJeztah thanks for the clarification.
Have in mind hostname -f is used to return FQDN in relation with those issues. so I thought it worth it to mention.

[stevvooe]

@mostolog The grammar for image names does not allow _.

As far as moby/swarmkit#1242 is concerned, _ should only ever be used for DNS names that correspond to service records, as is required by DNS.

If compose/stack/deploy are pushing names that end up with _ in container hostnames, that bug needs to be fixed immediately, as it is very wrong.

[mostolog]

@stevvooe should I report? Would you?

[stevvooe]

@mostolog I am not sure. Perhaps, the compose project would be the right place.

I am closing this issue, since we will not be adding support for underscores.