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IP NAT Bidirectional (Two-Way/Inbound) Operation
(Page 2 of 3)
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
In this technique, an outside device
can in fact make use of dynamic mapping. The basic process (highly simplified)
is as follows:
- 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.
- The DNS server for the internal network
the www.ilikenat.com name into an inside local address
for the device that corresponds to this DNS entry.
- 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
- 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.
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Version 3.0 - Version Date: September 20, 2005
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