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We have a server on the company network named serverA, and if I type ssh serverA, it will connect my PC to this server, even though there is no entry for this server in my /etc/hosts, but I used to have another PC where I do have to set this up manually in /etc/hosts. So, how does ssh discover hostnames on the network, and how can I make sure it works? (Both these machines I mentioned are running Kubuntu Linux, the older one runs 12.10, the newer one runs 13.04.)

I have the following content in /etc/resolv.conf:

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
search mycompany.com

Processes listening on port 53:

sudo ss -ulnp | grep 53
#UNCONN     0      0                         *:5353                     *:*      users:(("avahi-daemon",937,12))
#UNCONN     0      0                              *:*      users:(("dnsmasq",1781,4))
#UNCONN     0      0                        :::5353                    :::*      users:(("avahi-daemon",937,13))
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Does you network have a DNS server? Does it have an entry for serverA ? – Hennes Jul 22 '13 at 12:40
@Hennes I don't really know, but I have posted the resolve.conf above, could you have a look? Thanks. – qed Jul 22 '13 at 13:01
You have this line nameserver which tells the resolver to try to translate hostnames to IP addresses. The IP of the nameserver starts with 127, and 127/8 points back to your own host. So you probably are running a nameserver on your own computer. – Hennes Jul 22 '13 at 13:04
What process is listening locally on your box on port 53? sudo ss -ulnp | grep 53 – Matt Jul 22 '13 at 13:15
@mindthemonkey Please see the edited post. – qed Jul 22 '13 at 15:38
up vote 2 down vote accepted

This happens when the server is within the DNS search domain you have configured.

For example, my current search domain is example.com:

$ grep ^search /etc/resolv.conf
search example.com

I can now do the following transparently:

$ ping foo.example.com
PING foo.example.com ( 56(84) bytes of data.
$ ping foo
PING foo.example.com ( 56(84) bytes of data.

Search domains allow automatic translation between the machine's name and the fully qualified domain name (FQDN).

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My resolve.conf has a line search example.com, so what's the difference between search and domain? – qed Jul 22 '13 at 13:07
Just a note, domain example.com configures the local domain which the search domain defaults to. The search domain can additionally be configured with search example.com someextradomain.com – Matt Jul 22 '13 at 13:09
@mindthemonkey - Thanks, changed. – Chris Down Jul 22 '13 at 13:38

The OpenSSH client uses the standard DNS resolver provided by your operating system to resolve host names to an IP address.

The hosts config in /etc/nsswitch.conf configures what should be looked at to resolve hosts and and in what order.

Normally most lookups will be completed by the dns resolver which is configured in /etc/resolv.conf. You probably need to contact the person who administers the nameserver noted in /etc/resolv.conf and ask them to configure the hostname/IP for you, which in your case of appears to be yourself. If you have a dynamic DHCP IP address this might not be possible depending on your setup.

If mdns or Multicast DNS is what enables your working host, your new host might be on a different subnet or not broadcasting it's name.

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