hostname -f says "hostname: the specified hostname is invalid". The hostname (FQDN) specified in /etc/hostname is also listed in /etc/hosts and is pingable. No actual DNS server yet of the host knows. Is this a reason of the error reported or something else? (the OS is Ubuntu 10.04 Server).

Update: registering the host at a DNS server (so that the name can be successfully resolved via Internet) did not help.

  • 1
    Well... what's the contents of /etc/hostname or do we need to guess that too? – rodjek Jun 29 '10 at 5:06
  • Please share the contents of your /etc/hostname and /etc/hosts. Maybe the hostname is in fact invalid. – David Jun 30 '10 at 7:34

From this thread on Ubuntu Forums.

Apparently the underscore is an illegal character for hostnames. I had one in my hostname, and it gave me your failure message. Maybe you're getting something similar.


Check the configuration of:


host.conf should have "order hosts, bind" and nsswitch.conf should have "hosts: files dns" at a minimum. Possibly you've got something out of whack that's searching only DNS and not your local files?

  • clearly not the problem. – user1686 Jun 29 '10 at 21:51
  • Al that's there. – Ivan Jun 30 '10 at 6:50
  • solved the problem for me, Host \209\149something.sld.tld returned servfail - it looked ok though, osx terminal injecting funny things again ;) – ZaphodB Apr 8 '11 at 22:03

/etc/hostname should only contain the hostname and not a full FQDN.

  • Tried this. No difference. – Ivan Jun 30 '10 at 6:49

hostname -f relies on DNS working.

What does:

host `cat /etc/hostname`



You can edit your hostname at /etc/hostname or directly from the CLI like this:

echo "<desired_hostname>" > /etc/hostname hostname -F /etc/hostname

For systems that support it (such as Ubuntu 18.04+), you can also use hostnamectl like this:

hostnamectl set-hostname <desired_hostname>

Then reboot or login/logout and you should see the latest hostname, regardless of illegal characters like underscores.

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