How DNS works on Linux

How DNS works on Linux

Whenever we contact a server by its name whether by ping, browsing for example, this is what happens.

Ping www.google.com

http://www.google.com

The resolver’s libraries get invoked (Resolvers are just C libraries, which look up IP addresses and map them to names). These libraries read the configuration files /etc/hosts.conf (older Linux) or /etc/nsswitch.conf (newer Linux) each time they are called.

Depending on how these libraries - /etc/hosts.conf and /etc/nsswitch.conf - are configured, the resolvers will choose to use /etc/hosts FILE, DNS or NIS for mapping IPs to names.

My Linux servers use the new glibc, so the configuration file they will check is the /etc/nsswitch.confm see a snippet of this file below:

# hosts: db files nisplus nis dns

hosts: files dns

 

This is telling us:

Check first /etc/hosts file. If the address is not found on the /etc/hosts file, check DNS.

If DNS is requested, another configuration file gets read: /etc/resolv.conf. The /etc/resolv.conf should be set with the nameservers for the network. See an example below:

 

# cat /etc/resolv.conf

domain adlinux.int

search adlinux.int

nameserver 192.168.1.22

 

The resolver’s libraries will extract the ‘nameserver’ from the resolv.conf file and query them, to map www.google.com to its IP address.

 

The nameserver in turn, if authoritative for the DNS zone where www.google.com resides, will look the name up in its own database and return Google’s IP. If the nameserver is not authoritative for the zone, which is most likely, then it will return another IP for another nameserver, which can resolve .com.

 

Then the .com nameservers will be queried and they will look up in their own databases, and return an IP address for the authoritative nameserver for google.com domain.

 

Another query will be initiated to the google.com authoritative DNS server, and this time, it will look in its own database and find an IP address which matches a host named www.

Guys, this is the first version of this post. I will try and correct the mistakes and improve it.
This should give you a good idea how names are resolved on Linux.

I hope it will be useful to you

by Renato de Oliveira