Cisco Cisco Content Security Management Appliance M1070 Mode D'Emploi

You can configure the Domain Name System (DNS) settings for your content security appliance through 
the Management Appliance > Network > DNS page in the GUI, or via the 

dnsconfig

 command. 

You can configure the following settings:

Whether to use the Internet’s DNS servers or your own, and which server(s) to use

Which interface to use for DNS traffic

The number of seconds to wait before timing out a reverse DNS lookup

Clearing the DNS cache

AsyncOS can use the Internet root DNS servers, your own DNS servers, or the Internet root DNS servers 
and authoritative DNS servers that you specify. When using the Internet root servers, you may specify 
alternate servers to use for specific domains. Because an alternate DNS server applies to a single domain, 
it must be authoritative (provide definitive DNS records) for that domain.

AsyncOS supports “splitting” DNS servers when not using the Internet’s DNS servers. If you are using 
your own internal server, you can also specify exception domains and associated DNS servers.

When setting up “split DNS,” you should set up the in-addr.arpa (PTR) entries as well. For example, if 
you want to redirect “.eng” queries to the nameserver 1.2.3.4 and all the .eng entries are in the 172.16 
network, then you should specify “eng,16.172.in-addr.arpa” as the domains in the split DNS 
configuration.

For each DNS server that you enter, you can specify a numeric priority. AsyncOS attempts to use the 
DNS server with the priority closest to 0. If that DNS server is not responding, AsyncOS attempts to use 
the server at the next priority. If you specify multiple entries for DNS servers with the same priority, the 
system randomizes the list of DNS servers at that priority every time it performs a query. The system 
then waits a short amount of time for the first query to expire or “time out” and then a slightly longer 
amount of time for the second, and so on. The amount of time depends on the exact total number of DNS 
servers and priorities that have been configured. The timeout length is the same for all IP addresses at 
any particular priority. The first priority gets the shortest timeout; each subsequent priority gets a longer 
timeout. Further, the timeout period is roughly 60 seconds. If you have one priority, the timeout for each 
server at that priority is 60 seconds. If you have two priorities, the timeout for each server at the first 
priority is 15 seconds, and each server at the second priority is 45 seconds. For three priorities, the 
timeouts are 5, 10, 45. 

For example, suppose you configure four DNS servers, with two of them at priority 0, one at priority 1, 
and one at priority 2:

0

1.2.3.4, 1.2.3.5

5, 5

1

1.2.3.6

10

2

1.2.3.7

45