Names and Addresses

From a name to an address

You type a name like example.com, but the network needs an IP address. Turning one into the other is name resolution, and Linux tries two sources: the local file /etc/hosts first, then DNS servers (listed in /etc/resolv.conf). getent hosts NAME does a lookup the same way a program would:

cat /etc/hosts
getent hosts localhost

In the terminal

student@vulntech:~$ cat /etc/hosts
127.0.0.1	vulntech localhost
::1		vulntech localhost
student@vulntech:~$ getent hosts localhost
::1             vulntech localhost

Adding a name in /etc/hosts

/etc/hosts maps names to addresses by hand — useful for naming machines on a small network. Each line is an address then the names for it. Add one, then look it up:

echo "127.0.0.1  myserver" | sudo tee -a /etc/hosts
getent hosts myserver

In the terminal

student@vulntech:~$ echo "127.0.0.1  myserver" | sudo tee -a /etc/hosts
[sudo] password for student:
127.0.0.1  myserver
student@vulntech:~$ getent hosts myserver
127.0.0.1       myserver

Asking a DNS server: dig

For names not in /etc/hosts, a DNS server answers. dig queries one directly; @SERVER picks which. Here a small DNS server is running on this machine that answers for shop.lab, and dig +short shows just the address it returns:

dig +short @127.0.0.1 shop.lab
nslookup shop.lab 127.0.0.1

In the terminal

student@vulntech:~$ dig +short @127.0.0.1 shop.lab
10.20.30.40
student@vulntech:~$ nslookup shop.lab 127.0.0.1
Server:		127.0.0.1
Address:	127.0.0.1#53

Name:	shop.lab
Address: 10.20.30.40
** server can't find shop.lab: REFUSED

Common mistake: Order matters: /etc/hosts wins over DNS. A stray line there can send a name to the wrong address and override what DNS says — a classic cause of “it works on my machine”.

Remember: Name resolution turns a name into an IP: /etc/hosts first, then DNS (/etc/resolv.conf). Look up with getent hosts NAME; query a DNS server directly with dig @SERVER NAME.