DNS Setup

• In order to be able to use host names (www.yahoo.com) rather then IP addresses (216.115.105.2), we need to let our system know which DNS servers it should use.
  
  
• The system may be given a set of DNS servers to query, in a specific order. If querying the first times out, the second will be queried, etc.
  
  
• In addition, it is possible to specify our default domain. If we try to access host using a partial domain name, the system will automatically try to append this default domain when doing lookups.
  
  For example, if our default domain is 'isp.co.il' and we try to access a machine named 'mail', the system will first try looking for the host 'mail', and on failure will try locating the host 'mail.isp.co.il'.
  
  
• Defining these details is done by editing the '/etc/resolv.conf' file. Here is an example of such a file:
  
  

       domain actcom.co.il
       nameserver 192.117.112.194
       nameserver 192.114.47.52
       

• For more info: man 5 resolver
  
  

• In addition to DNS definitions, we may hard-code IP addresses of various systems in a file on the system. This file is '/etc/hosts'.
• When the system performs a name lookup operation, it normally searches the hosts file first, and only if the info is not found there, it consults a DNS.
• Usually there is a special address defined in the hosts file, as follows:
  
  

       127.0.0.1               localhost
       

  this special address (127.0.0.1) is used to transfer data internally inside the machine, e.g. when using a web browser running on the machine, to connect to a web server running on that same machine.
  
  
• For frequently used addresses, we may place their information in the hosts file, to gain fast name lookup.
• If we do that, however, we must check periodically that the IP address we wrote in the hosts file, has not changed.