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47-4
Cisco ASA 5500 Series Configuration Guide using ASDM
Chapter 47 Configuring Inspection of Basic Internet Protocols
DNS Inspection
Figure 47-2 DNS Rewrite with Three NAT Zones
In Figure 47-2, a web server, server.example.com, has the real address 192.168.100.10 on the DMZ
interface of the ASA. A web client with the IP address 10.10.10.25 is on the inside interface and a public
DNS server is on the outside interface. The site NAT policies are as follows:
• The outside DNS server holds the authoritative address record for server.example.com.
• Hosts on the outside network can contact the web server with the domain name server.example.com
through the outside DNS server or with the IP address 209.165.200.5.
• Clients on the inside network can access the web server with the domain name server.example.com
through the outside DNS server or with the IP address 192.168.100.10.
When a host or client on any interface accesses the DMZ web server, it queries the public DNS server
for the A-record of server.example.com. The DNS server returns the A-record showing that
server.example.com binds to address 209.165.200.5.
When a web client on the outside network attempts to access http://server.example.com, the sequence of
events is as follows:
1. The host running the web client sends the DNS server a request for the IP address of
server.example.com.
2. The DNS server responds with the IP address 209.165.200.225 in the reply.
3. The web client sends its HTTP request to 209.165.200.225.
4. The packet from the outside host reaches the ASA at the outside interface.
5. The static rule translates the address 209.165.200.225 to 192.168.100.10 and the ASA directs the
packet to the web server on the DMZ.
When a web client on the inside network attempts to access http://server.example.com, the sequence of
events is as follows:
1. The host running the web client sends the DNS server a request for the IP address of
server.example.com.
2. The DNS server responds with the IP address 209.165.200.225 in the reply.
132407
Web client
10.10.10.25
Web server
192.168.100.10
DNS server
erver.example.com IN A 209.165.200.5
Security
appliance
Outside
DMZ
192.168.100.1
10.10.10.1
Inside
99.99.99.2