Cisco ASA 5505 - Manual (Page 883)

43-7
Cisco ASA 5500 Series Configuration Guide using the CLI
Chapter 43 Configuring Inspection of Basic Internet Protocols
DNS Inspection
where domain-qualified-hostname is the hostname with a domain suffix, as in server.example.com. The
period after the hostname is important. mapped-address is the translated IP address of the web server.
The following example configures the ASA for the scenario shown in Figure 43-2 . It assumes DNS
inspection is already enabled.
hostname(config)# object network obj-192.168.100.10-01
hostname(config-network-object)# host 192.168.100.10
hostname(config-network-object)# nat (dmz,outside) static 209.165.200.225 dns
hostname(config)# access-list 101 permit tcp any host 209.165.200.225 eq www
hostname(config)# access-group 101 in interface outside
This configuration requires the following A-record on the DNS server:
server.example.com. IN A 209.165.200.225
Configuring a DNS Inspection Policy Map for Additional Inspection Control
DNS application inspection supports DNS message controls that provide protection against DNS
spoofing and cache poisoning. User configurable rules allow filtering based on DNS header, domain
name, resource record type and class. Zone transfer can be restricted between servers with this function,
for example.
The Recursion Desired and Recursion Available flags in the DNS header can be masked to protect a
public server from attack if that server only supports a particular internal zone. In addition, DNS
randomization can be enabled avoid spoofing and cache poisoning of servers that either do not support
randomization, or utilize a weak pseudo random number generator. Limiting the domain names that can
be queried also restricts the domain names which can be queried, which protects the public server
further.
A configurable DNS mismatch alert can be used as notification if an excessive number of mismatching
DNS responses are received, which could indicate a cache poisoning attack. In addition, a configurable
check to enforce a Transaction Signature be attached to all DNS messages is also supported.
To specify actions when a message violates a parameter, create a DNS inspection policy map. You can
then apply the inspection policy map when you enable DNS inspection.
To create a DNS inspection policy map, perform the following steps:
Step 1 (Optional) Add one or more regular expressions for use in traffic matching commands according to the
“Creating a Regular Expression” section on page 13-12 . See the types of text you can match in the match
commands described in Step 3 .
Step 2 (Optional) Create one or more regular expression class maps to group regular expressions according to
the
“Creating a Regular Expression Class Map” section on page 13-15 .
Step 3 (Optional) Create a DNS inspection class map by performing the following steps.
A class map groups multiple traffic matches. Traffic must match all of the match commands to match
the class map. You can alternatively identify match commands directly in the policy map. The difference
between creating a class map and defining the traffic match directly in the inspection policy map is that
the class map lets you create more complex match criteria, and you can reuse class maps.
To specify traffic that should not match the class map, use the match not command. For example, if the
match not command specifies the string “example.com,” then any traffic that includes “example.com”
does not match the class map.