Edit: My assertion about setting domainname may be suspect; see phemmer's comment and discussion below. The issue of erroneously passing the FQDN to syscall.Sethostname remains however.

Environment

vagrant@vagrant-ubuntu-utopic-64:~$ docker --version
Docker version 1.7.0, build 0baf609

Problem

Docker accepts a fully qualified domain name as the argument to --hostname, and models separate Hostname and Domainname fields in runconfig.Config; however once inside libcontainer only Hostname is modelled, and this erroneously includes the domain name which is then fed into syscall.Sethostname with the following results:

vagrant@vagrant-ubuntu-utopic-64:~$ docker run --hostname test.example.com -ti ubuntu
root@test:/# hostname
test.example.com
root@test:/# domainname
(none)

This is not correct; the value passed to syscall.Sethostname should be the unqualified hostname, and the remainder to syscall.Setdomainname. This would yield the expected output:

vagrant@vagrant-ubuntu-utopic-64:~$ docker run --hostname test.example.com -ti ubuntu
root@test:/# hostname
test
root@test:/# domainname
example.com

Currently, libcontainer does not invoke syscall.Setdomainname at all, leaving the domain name parameter of the UTS namespace uninitialised.

Docker is also incorrectly generating an /etc/hostname file with the fully qualified name - again this should be the unqualified name, and the FQDN set by aliasing in /etc/hosts.

It could also be argued that --hostname should be renamed to --fqdn, or changed so that it admits only an unqualified name and companioned by a new --domainname option to set the remainder.

Finally, the error message is uninformative:

docker run --hostname 012345678901234567890123456789012345678901234567890123456789.example.com -ti ubuntu
Error response from daemon: Cannot start container c1bb14c2a10b2341c185aed804ab6f625ca7a1164ea90ea4fd37ea56b6e2bed0: [8] System error: invalid argument

Consequences

• UTS namepsace not correctly initialised
• The underlying sethostname system call limits the hostname to 64 characters. Because Docker is calling it with the fully qualified name, the entire FQDN is limited to 64 characters (long container name cause breakage when deriving hostname weaveworks/weave#1006)