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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  IP Network Address Translation (NAT) Protocol

IP NAT Unidirectional (Traditional/Outbound) Operation 1 2 3 IP NAT Port-Based ("Overloaded") Operation: Network Address Port Translation (NAPT) / Port Address Translation (PAT)

Facilitating Inbound NAT Using DNS

There only two methods to resolve the hidden address problem. One is to use static mapping for devices like servers on the inside network that need to be accessed from the outside. When static mapping is employed, the global address of the device that is using the static mapping will be publicly known, which solves the “where do I send my request to” problem.

The other solution is to make use of the TCP/IP Domain Name System (DNS). As explained in detail in the section on DNS, this protocol allows requests to be sent as names instead of IP addresses; the DNS server translates these names to their corresponding addresses. It is possible to integrate DNS and NAT so they work together. This process is described in RFC 2694, DNS extensions to Network Address Translators (DNS_ALG).

In this technique, an outside device can in fact make use of dynamic mapping. The basic process (highly simplified) is as follows:

1. The outside device sends a DNS request using the name of the device on the inside network it wishes to reach. For example, it might be “www.ilikenat.com”.
   
   
2. The DNS server for the internal network resolves the “www.ilikenat.com” name into an inside local address for the device that corresponds to this DNS entry.
   
   
3. The inside local address is passed to NAT and used to create a dynamic mapping between the inside local address of the server being accessed from the outside, and an inside global address. This mapping is put into the NAT router's translation table.
   
   
4. When the DNS server sends back the name resolution, it tells the outside device not the inside local (private) address of the server being sought, but the inside global (public) address mapped in the previous step.

IP NAT Unidirectional (Traditional/Outbound) Operation 1 2 3 IP NAT Port-Based ("Overloaded") Operation: Network Address Port Translation (NAPT) / Port Address Translation (PAT)