The Alias Command

You can use the alias command for two purposes

1) You need to Destination NAT of one destination address to another   

• In dnat, the PIX changes the destination IP of an application call from one IP address to another IP address.

• This process is used when you want the actual application call from the internal client to the server in a perimeter (dmz) network by its external IP address. This does not "doctor" the DNS replies.

  alias(inside) 99.99.99.99 192.168.100.10 255.255.255.255
  
  !--- This sets up the Destination NAT. In this example the DNS reply is not
  !--- doctored by the PIX because the external address (99.99.99.99) does not
  !--- match the foreign IP address in the alias command (the second IP).
  !--- But the call is "dnat-ed" because the destination address
  !--- in the call matches the dnat IP address in the alias command (the first IP).

2) You can use it to perforn DNS doctoring of DNS repliedfrom an external DNS Server

• In DNS Doctoring, the PIX changes the DNS response from a DNS server to be a different IP address than the DNS server actually answered for a given name.

This process is used when you want the actual application call from the internal client to connect to an internal server by its internal IP address.

• alias (inside) 10.10.10.10 99.99.99.99 255.255.255.255
  
  !--- This command sets up DNS Doctoring. It is initiated from the clients in
  !--- the "inside" network. It watches for DNS replies that contain
  !--- 99.99.99.99. Then it replaces the 99.99.99.99 address with the 10.10.10.10
  !--- address in the "DNS reply" sent to the client PC.

Source: Cisco Corp