13-4
Cisco ASA Series Firewall CLI Configuration Guide
Chapter 13 Inspection of Basic Internet Protocols
DNS Inspection
• match [not] header-flag [eq] {f_name [f_name...] | f_value}—Matches the DNS flag. The
f_name argument is the DNS flag name, one of the following: AA (Authoritative Answer), QR
(Query), RA (Recursion Available), RD (Recursion Desired), TC (Truncation). The f_value
argument is the 16-bit value in hex starting with 0x, from 0x0 to 0xffff. The eq keyword
specifies an exact match (match all); without the eq keyword, the packet only needs to match
one of the specified headers (match any). For example, match header-flag AA QR.
• match [not] dns-type {eq {t_name | t_value} | range t_value1 t_value2}—Matches the DNS
type. The t_name argument is the DNS type name, one of the following: A (IPv4 address),
AXFR (full zone transfer), CNAME (canonical name), IXFR (incremental zone transfer), NS
(authoritative name server), SOA (start of a zone of authority) or TSIG (transaction signature).
The t_value arguments are arbitrary values in the DNS type field (0-65535). The range keyword
specifies a range, and the eq keyword specifies an exact match. For example: match dns-type
eq A.
• match [not] dns-class {eq {in | c_value} | range c_value1 c_value2}—Matches the DNS class.
The class is either in (for Internet) or c_value, an arbitrary value from 0 to 65535 in the DNS
class field. The range keyword specifies a range, and the eq keyword specifies an exact match.
For example: match dns-class eq in.
• match [not] {question | resource-record {answer | authority | additional}}—Matches a DNS
question or resource record. The question keyword specifies the question portion of a DNS
message. The resource-record keyword specifies one of these sections of the resource record:
answer, authority, or additional. For example: match resource-record answer.
• match [not] domain-name regex {regex_name | class class_name}—Matches the DNS
message domain name list against the specified regular expression or regular expression class.
d. Enter exit to leave class map configuration mode.
Step 2 Create a DNS inspection policy map, enter the following command:
hostname(config)# policy-map type inspect dns policy_map_name
hostname(config-pmap)#
Where the policy_map_name is the name of the policy map. The CLI enters policy-map configuration
mode.
Step 3 (Optional) To add a description to the policy map, enter the following command:
hostname(config-pmap)# description string
Step 4 To apply actions to matching traffic, perform the following steps.
a. Specify the traffic on which you want to perform actions using one of the following methods:
• If you created a DNS class map, specify it by entering the following command:
hostname(config-pmap)# class class_map_name
hostname(config-pmap-c)#
• Specify traffic directly in the policy map using one of the match commands described for DNS
class maps. If you use a match not command, then any traffic that does not match the criterion
in the match not command has the action applied.
b. Specify the action you want to perform on the matching traffic by entering the following command:
hostname(config-pmap-c)# {drop [log] | drop-connection [log]|
enforce-tsig {[drop] [log]} | mask [log] | log}
Not all options are available for each match or class command. See the CLI help or the command
reference for the exact options available.
The drop keyword drops all packets that match.
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