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Snort FAQ



Steven Sturges ssturges@sourcefire.com

Documentation last update 2006-08-25

== Overview ==

The DNS preprocessor decodes DNS Responses and can detect the following exploits: DNS Client RData Overflow, Obsolete Record Types, and Experimental Record Types.

DNS looks are DNS Response traffic over UDP and TCP and it requires Stream preprocessor to be enabled for TCP decoding.

== Configuration ==

By default, all alerts are disabled and the preprocessor checks traffic on port 53.

The available configuration options are described below:

  • ports { port[, port] .. }*

This option specifies the source ports that the DNS preprocessor should inspect traffic.

  • enable_obsolete_types *

Alert on Obsolete (per RFC 1035) Record Types

  • enable_experimental_types *

Alert on Experimental (per RFC 1035) Record Types

  • enable_rdata_overflow *

Check for DNS Client RData Overflow

== Example/Default Configuration ==

Looks for traffic on DNS server port 53. Check for the DNS Client RData overflow vulnerability. Do not alert on obsolete or experimental RData record types.

preprocessor dns: ports { 53 } \ enable_rdata_overflow

== Alerts == The DNS preprocessor uses generator ID 131 and can produce the following alerts:

SID Description — ———– 1 Obsolete DNS RData Type 2 Experimental DNS RData Type 3 Client RData TXT Overflow

== Conclusion ==

The DNS preprocessor does nothing if none of the 3 vulnerabilities it checks for are enabled. It will not operate on TCP sessions picked up midstream, and it will cease operation on a session if it loses state because of missing data (dropped packets).