Use the fixup protocol dns command to specify the maximum DNS packet length. DNS requires application inspection so that DNS queries are not subject to the generic UDP handling based on activity timeouts. Instead, UDP connections associated with DNS queries and responses are torn down as soon as a reply to a DNS query has been received.
The port assignment for the Domain Name System (DNS) is not configurable.
Set the maximum length for the DNS fixup as shown in the following example:
pixfirewall(config)# fixup protocol dns maximum-length 1500
pixfirewall(config)# show fixup protocol dns
fixup protocol dns maximum length 1500
Note The PIX Firewall drops DNS packets sent to UDP port 53 that are larger than the configured maximum length. The default value is 512 bytes. A syslog message will be generated when a DNS packet is dropped.
The no fixup protocol dns command disables the DNS fixup. The clear fixup protocol dns resets the DNS fixup to its default settings (512 byte maximum packet length).
Note If the DNS fixup is disabled, the A-record is not NATed and the DNS ID is not matched in requests and responses. By disabling the DNS fixup, the maximum length check on UDP DNS packets can be bypassed and packets greater than the maximum length configured will be permited.
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