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Cisco Security Appliance Command Line Configuration Guide
OL-12172-03
Chapter 25 Configuring Application Layer Protocol Inspection
DNS Inspection
Step 4
If DNS inspection is disabled or if you want to change the maximum DNS packet length, configure DNS
inspection. DNS application inspection is enabled by default with a maximum DNS packet length of 512
bytes. For configuration instructions, see the
“Configuring Application Inspection” section on
page 25-5
.
Step 5
On the public DNS server, add an A-record for the web server, such as:
domain-qualified-hostname
. IN A
mapped-address
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 security appliance for the scenario shown in
Figure 25-2
. It
assumes DNS inspection is already enabled.
hostname(config)#
static (dmz,outside) 209.165.200.225 192.168.100.10 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
Verifying and Monitoring DNS Inspection
To view information about the current DNS connections, enter the following command:
hostname#
show conn
For connections using a DNS server, the source port of the connection may be replaced by the IP address
of DNS server in the show conn command output.
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 security
appliance within a limited period of time and there is no resource build-up. However, when you enter the
show conn
command, you 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.
To display the statistics for DNS application inspection, enter the
show service-policy
command. The
following is sample output from the
show service-policy
command:
hostname#
show service-policy
Interface outside:
Service-policy: sample_policy
Class-map: dns_port
Inspect: dns maximum-length 1500, packet 0, drop 0, reset-drop 0
Содержание 500 Series
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Страница 45: ...P A R T 1 Getting Started and General Information ...
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Страница 277: ...P A R T 2 Configuring the Firewall ...
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Страница 561: ...P A R T 3 Configuring VPN ...
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