Dns And Nat - Cisco PIX 500 Series Configuration Manual

Chapter 17 Configuring NAT

DNS and NAT

You might need to configure the security appliance to modify DNS replies by replacing the address in
the reply with an address that matches the NAT configuration. You can configure DNS modification
when you configure each translation.
For example, a DNS server is accessible from the outside interface. A server, ftp.cisco.com, is on the
inside interface. You configure the security appliance to statically translate the ftp.cisco.com real address
(10.1.3.14) to a mapped address (209.165.201.10) that is visible on the outside network (see
Figure
inside users who have access to ftp.cisco.com using the real address receive the real address from the
DNS server, and not the mapped address.
When an inside host sends a DNS request for the address of ftp.cisco.com, the DNS server replies with
the mapped address (209.165.201.10). The security appliance refers to the static statement for the inside
server and translates the address inside the DNS reply to 10.1.3.14. If you do not enable DNS reply
modification, then the inside host attempts to send traffic to 209.165.201.10 instead of accessing
ftp.cisco.com directly.
Figure 17-12
3
DNS Reply Modification
209.165.201.10
See the following command for this example:
hostname(config)# static (inside,outside) 209.165.201.10 10.1.3.14 netmask 255.255.255.255
dns
OL-12172-03
17-12). In this case, you want to enable DNS reply modification on this static statement so that
DNS Reply Modification
1
DNS Query
ftp.cisco.com?
2
DNS Reply
209.165.201.10
10.1.3.14
4
DNS Reply
10.1.3.14
DNS Server
Outside
Security
Appliance
Inside
ftp.cisco.com
User
10.1.3.14
Static Translation
on Outside to:
209.165.201.10
5
FTP Request
10.1.3.14
Cisco Security Appliance Command Line Configuration Guide
NAT Overview
17-15