BSNL DataOne provides software for connecting to Linux. However I found that you can setup much easier and without using their custom software.

First you must know that the DSL modem from BSNL can be directly setup to connect automatically, provide firewall, DHCP and even NAT to allow multiple computers to connect to the internet. So effectively you can have several computers connected to a hub / switch to which the DSL modem connects and then all these computers will be able to connect to the internet by setting the DSL modem as DHCP server in their settings. However in my experience, as the connection is often unreliable, it is better to use software on your computer to establish the link. It allows more control over the connection and faster reconnects after disconnection with little changes to configuration. The solution is simple.

Run adsl-setup. Most likely it is already installed on your system. Otherwise you can also install it on yum enabled distributions like Fedora Core simply by:
yum install rp-pppoe
It asks questions like your login name and password (provided by BSNL), your ethernet interface (choose the default provided - eth0; works for most cases) etc. After the setup you should preferably reboot (or restart the network - service network restart) and you will find your DSL connection up and running.

You can also manually bring up DSL connection by:
ifup ppp0

You can bring it down by:
ifdown ppp0

Note: Unless you have multiple DSL connections, ppp0 is a safe bet.