Cisco Systems and the ASA Services Module Manual De Usuario

A single connection is created for multiple DNS sessions, as long as they are between the same two 
hosts, and the sessions have the same 5-tuple (source/destination IP address, source/destination port, and 
protocol). DNS identification is tracked by app_id, and the idle timer for each app_id runs 
independently. Because the app_id expires independently, a legitimate DNS response can only pass 
through the ASA within a limited period of time and there is no resource build-up. However, if you enter 
the show conn command, you will see the idle timer of a DNS connection being reset by a new DNS 
session. This is due to the nature of the shared DNS connection and is by design.

DNS inspection is enabled by default. You can customize DNS inspection to perform many tasks:

Translate the DNS record based on the NAT configuration. For more information, see the 

.

Enforce message length, domain-name length, and label length.

Verify the integrity of the domain-name referred to by the pointer if compression pointers are 
encountered in the DNS message.

Check to see if a compression pointer loop exists.

Inspect packets based on the DNS header, type, class and more.

DNS inspection is enabled by default, using the preset_dns_map inspection class map:

The maximum DNS message length is 512 bytes.

The maximum client DNS message length is automatically set to match the Resource Record.

DNS Guard is enabled, so the ASA tears down the DNS session associated with a DNS query as 
soon as the DNS reply is forwarded by the ASA. The ASA also monitors the message exchange to 
ensure that the ID of the DNS reply matches the ID of the DNS query.

Translation of the DNS record based on the NAT configuration is enabled.

Protocol enforcement is enabled, which enables DNS message format check, including domain 
name length of no more than 255 characters, label length of 63 characters, compression, and looped 
pointer check.

See the following default DNS inspection commands:

class-map inspection_default

 match default-inspection-traffic