1-2
Cisco ASA Series CLI Configuration Guide
Chapter 1 Configuring Inspection of Basic Internet Protocols
DNS Inspection
•
Configuring DNS Inspection, page 1-8
•
Monitoring DNS Inspection, page 1-9
Information About DNS Inspection
•
General Information About DNS, page 1-2
•
DNS Inspection Actions, page 1-2
General Information About DNS
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 Actions
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.
Default Settings for DNS Inspection
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
Summary of Contents for 5505 - ASA Firewall Edition Bundle
Page 28: ...Glossary GL 24 Cisco ASA Series CLI Configuration Guide ...
Page 61: ...P A R T 1 Getting Started with the ASA ...
Page 62: ......
Page 219: ...P A R T 2 Configuring High Availability and Scalability ...
Page 220: ......
Page 403: ...P A R T 2 Configuring Interfaces ...
Page 404: ......
Page 499: ...P A R T 2 Configuring Basic Settings ...
Page 500: ......
Page 533: ...P A R T 2 Configuring Objects and Access Lists ...
Page 534: ......
Page 601: ...P A R T 2 Configuring IP Routing ...
Page 602: ......
Page 745: ...P A R T 2 Configuring Network Address Translation ...
Page 746: ......
Page 845: ...P A R T 2 Configuring AAA Servers and the Local Database ...
Page 846: ......
Page 981: ...P A R T 2 Configuring Access Control ...
Page 982: ......
Page 1061: ...P A R T 2 Configuring Service Policies Using the Modular Policy Framework ...
Page 1062: ......
Page 1093: ...P A R T 2 Configuring Application Inspection ...
Page 1094: ......
Page 1191: ...P A R T 2 Configuring Unified Communications ...
Page 1192: ......
Page 1333: ...P A R T 2 Configuring Connection Settings and QoS ...
Page 1334: ......
Page 1379: ...P A R T 2 Configuring Advanced Network Protection ...
Page 1380: ......
Page 1475: ...P A R T 2 Configuring Modules ...
Page 1476: ......
Page 1549: ...P A R T 2 Configuring VPN ...
Page 1550: ......
Page 1965: ...P A R T 2 Configuring Logging SNMP and Smart Call Home ...
Page 1966: ......
Page 2059: ...P A R T 2 System Administration ...
Page 2060: ......
Page 2098: ...1 8 Cisco ASA Series CLI Configuration Guide Chapter 1 Troubleshooting Viewing the Coredump ...
Page 2099: ...P A R T 2 Reference ...
Page 2100: ......