Cisco Cisco Email Security Appliance C160 Mode D'Emploi

The appliance attempts to perform a “double DNS lookup” on all remote hosts connecting to a listener 
for the purposes of sending or receiving email. [That is: the system acquires and verifies the validity of 
the remote host's IP address by performing a double DNS lookup. This consists of a reverse DNS (PTR) 
lookup on the IP address of the connecting host, followed by a forward DNS (A) lookup on the results 
of the PTR lookup. The system then checks that the results of the A lookup match the results of the PTR 
lookup. If the results do not match, or if an A record does not exist, the system only uses the IP address 
to match entries in the Host Access Table (HAT).] This particular timeout period applies only to this 
lookup and is not related to the general DNS timeout discussed in 

The default value is 20 seconds. You can disable the reverse DNS lookup timeout globally across all 
listeners by entering ‘0’ as the number of seconds. 

If the value is set to 0 seconds, the reverse DNS lookup is not attempted, and instead the standard timeout 
response is returned immediately. This also prevents the appliance from delivering mail to domains that 
require TLS-verified connections if the receiving host’s certificate has a common name (CN) that maps 
to the host’s IP lookup.

Occasionally, an alert may be generated with the message “Failed to bootstrap the DNS cache” when an 
appliance is rebooted. The messages means that the system was unable to contact its primary DNS 
servers, which can happen at boot time if the DNS subsystem comes online before network connectivity 
is established. If this message appears at other times, it could indicate network issues or that the DNS 
configuration is not pointing to a valid server.

The Clear Cache button from the GUI, or the d

nsflush

 command (for more information about the 

dnsflush

 command, see the Cisco AsyncOS CLI Reference Guide), clears all information in the DNS 

cache. You may choose to use this feature when changes have been made to your local DNS system. The 
command takes place immediately and may cause a temporary performance degradation while the cache 
is repopulated.

Select Network > DNS.

Click Edit Settings. 

Select whether to use the Internet’s root DNS servers or your own internal DNS server or the Internet’s 
root DNS servers and specify alternate DNS servers. 

If you want to use your own DNS server(s) enter the server ID and click Add Row. Repeat this for each 
server. When entering your own DNS servers, specify a priority as well. For more information, see 

.

If you want to specify alternate DNS servers for certain domains, enter the domain and the alternate DNS 
server IP address. Click Add Row to add additional domains.