25-15
Cisco Security Appliance Command Line Configuration Guide
OL-12172-03
Chapter 25 Configuring Application Layer Protocol Inspection
DNS Inspection
DNS Rewrite performs two functions:
•
Translating a public address (the routable or “mapped” address) in a DNS reply to a private address
(the “real” address) when the DNS client is on a private interface.
•
Translating a private address to a public address when the DNS client is on the public interface.
In
Figure 25-1
, the DNS server resides on the external (ISP) network The real address of the server
(192.168.100.1) has been mapped using the
static
command to the ISP-assigned address
(209.165.200.5). When a web client on the inside interface attempts to access the web server with the
URL http://server.example.com, the host running the web client sends a DNS request to the DNS server
to resolve the IP address of the web server. The security appliance translates the non-routable source
address in the IP header and forwards the request to the ISP network on its outside interface. When the
DNS reply is returned, the security appliance applies address translation not only to the destination
address, but also to the embedded IP address of the web server, which is contained in the A-record in the
DNS reply. As a result, the web client on the inside network gets the correct address for connecting to
the web server on the inside network. For configuration instructions for scenarios similar to this one, see
the
“Configuring DNS Rewrite with Two NAT Zones” section on page 25-16
.
Figure 25-1
Translating the Address in a DNS Reply (DNS Rewrite)
DNS rewrite also works if the client making the DNS request is on a DMZ network and the DNS server
is on an inside interface. For an illustration and configuration instructions for this scenario, see the
“DNS
Rewrite with Three NAT Zones” section on page 25-17
.
Configuring DNS Rewrite
You configure DNS rewrite using the
alias
,
static
, or
nat
commands. The
alias
and
static
command can
be used interchangeably; however, we recommend using the
static
command for new deployments
because it is more precise and unambiguous. Also, DNS rewrite is optional when using the
static
command.
This section describes how to use the
alias
and
static
commands to configure DNS rewrite. It provides
configuration procedures for using the
static
command in a simple scenario and in a more complex
scenario. Using the
nat
command is similar to using the
static
command except that DNS Rewrite is
based on dynamic translation instead of a static mapping.
This section includes the following topics:
•
Using the Static Command for DNS Rewrite, page 25-16
•
Using the Static Command for DNS Rewrite, page 25-16
132406
Web server
server.example.com
192.168.100.1
Web client
http://server.example.com
192.168.100.2
ISP Internet
DNS server
server.example.com IN A 209.165.200.5
Security appliance
192.168.100.1IN A 209.165.200.5
Содержание 500 Series
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Страница 45: ...P A R T 1 Getting Started and General Information ...
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Страница 277: ...P A R T 2 Configuring the Firewall ...
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Страница 354: ...17 38 Cisco Security Appliance Command Line Configuration Guide OL 12172 03 Chapter 17 Configuring NAT NAT Examples ...
Страница 561: ...P A R T 3 Configuring VPN ...
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Страница 891: ...P A R T 4 System Administration ...
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