NAT with DNS mapping allows an internal host to access an internal server on the same private network by using the domain name of the internal server when the DNS server is on the public network.
NAT with DNS mapping must operate with the NAT Server feature.
Figure 55: NAT with DNS mapping
As shown in Figure 55, NAT with DNS mapping works as follows:
The host sends a DNS request containing the domain name of the internal Web server.
Upon receiving the DNS response, the NAT device performs a DNS mapping lookup by using the domain name in the response. A DNS mapping for NAT maps the domain name to the public IP address, public port number, and the protocol type for the internal server.
If a match is found, the NAT continues to compare the public address, public port number, and the protocol type with the NAT Server configuration. The NAT Server configuration maps the public IP address and port number to the private IP address and port number for the internal server.
If a match is found, NAT translates the public IP address in the response into the private IP address of the Web server.
The internal host receives the DNS response, and obtains the private IP address of the Web server.
DNS mapping can also be used by DNS ALG. The DNS reply from the external DNS server contains only the domain name and public IP address of the internal server in the payload. The NAT interface might have multiple internal servers configured with the same public IP address but different private IP addresses. DNS ALG might find an incorrect internal server by using only the public IP address. If a DNS mapping is configured, DNS ALG can obtain the public IP address, public port number, and protocol type of the internal server by using the domain name. Then it can find the correct internal server by using the public IP address, public port number, and protocol type of the internal server.
