Ubuntu – “Unable to resolve hostname” – how to cleanly change hostname without rebooting

hostnamelinuxUbuntu

At work, I work on a small website using Ubuntu servers hosted on Amazon EC2 instances. We have two webservers, and at any given time one of them is 'live' and one is 'testing', but when we've got our new features or bugfixes working on the 'testing' server, we run a script that uses the AWS API to swap the IP addresses assigned to the two boxes so that the domains being served by the two servers are swapped.

Consequently, at any given moment, one of these servers can be either our 'live' server or our 'testing' server, but these are not permanent states.

To avoid confusion when SSHing into either of these servers, especially if some silly person (like me) leaves an SSH session open for a long time and gets confused about which server they're on, I want to modify the hostnames of the two servers so that the prompt and title in the terminal window will read ubuntu@ourwebsiteLIVE or ubuntu@ourwebsiteTESTING depending upon whether the server is currently live or testing.

I know I can do this by running hostname ourwebsiteLIVE etc. in our IP-swapping script, and have already implemented this. It works, but now every time I sudo anything on either of our servers it prints e.g.

sudo: unable to resolve host ourwebsiteLIVE

which doesn't seem to break anything, but is fairly irritating. I figure it also might end up breaking stuff in future if I try to chain commands that pipe output to each other, so I guess it's technically not just an aesthetics issue.

All the solutions I've seen to the 'unable to resolve hostname' message involve modifying /etc/hosts or some similar file and then rebooting the server. I can see that this would be fine for many use cases, but is far from ideal for us since we're modifying the hostname routinely and programmatically; we don't want to have to wait for a reboot every time we run our swap script.

Is there a way I can make the error message go away without having to reboot?

Best Answer

I would suggest using nss-myhostname. It is a nss plugin which just always resolves your current hostname, so there is no need to modify /etc/hosts.

As the issue you are describing is just a resolver issue you can fix it by having a proper DNS setup, i.e. ourwebsiteLIVE.$DOMAIN just resolves to the ip of the machine.

Related Solutions

Ssh – Unable to resolve hostname

So we know hostname returns woofy. But that name can't be resolved to an IP address.

The short answer is that you need to add an entry for woofy in /etc/hosts. Make it resolve to 127.0.0.1. Or if your system is IPv6-capable, ::1.

Keep a backup of the previous version of /etc/hosts in case you make a mistake (I use the handy etckeeper package for this, but if you prefer it ye olde way, you can use a manual backup or even RCS).

The long answer is that how hostnames are resolved to IP addresses is controlled by a set of configuration files which vary slightly between Unix variants. You can configure your Unix system to resolve host names by hosts file (/etc/hosts will work on almost any Unix system) or by DNS (systems which have direct IP reachability to the Internet will always do this). There are other alternatives too, mostly less widely used (including LDAP and NIS/NIS+). See the Wikipedia article about the Name Service Switch for more context on this.

Edit: if this still causes a DNS lookup, the problem is probably that your Name Service Switch configuration consults DNS before /etc/hosts so the change to /etc/hosts has no effect. Try looking at /etc/nsswitch.conf (how the NSS is configured varies between operating systems).

Ubuntu – Unable to set hostname for use within network

If your DHCP server supports it you might want to try and have your client send the desired hostname that it wants. Add the following to the file /etc/dhcp/dhclient.conf:

send host-name 'your-hostname-here';

If you want to send a fully qualified domain name (fqdn) - myhost.mydomain.com instead of just myhost you need to add these lines too:

send fqdn.fqdn "myhost.mydomain.com.";
send fqdn.encoded on;
send fqdn.server-update off;
also request fqdn, dhcp6.fqdn;

Edit #1

The OP was asked to try the following commands and report back:

dig <hostname> @<router ip>

The OP reported that this worked so it was determined to try adding the router's IP explicitly to his dhclient.conf file.

Edit #2

It was suggested to try adding the following to the /etc/dhcp/dhclient.conf file:

prepend domain-name-servers 192.168.2.1;

Edit #3

Given you're able to now ping servers using your router's DNS server when you added 192.168.2.1, but not the internet, I would suggest you add some external DNS servers as well using the above prepend option like so:

prepend domain-name-servers 192.168.2.1, 8.8.8.8, 8.8.4.4;

This will add your router as a DNS resolver along with Google's DNS servers.