Cisco Cisco Content Security Management Appliance M1070 ユーザーガイド
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AsyncOS 8.1 for Cisco Content Security Management User Guide
Chapter 14 Common Administrative Tasks
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Clearing the DNS cache
Specifying DNS Servers
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.
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.
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.
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.
Multiple Entries and Priority
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.
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:
and one at priority 2:
AsyncOS randomly chooses between the two servers at priority 0. If one of the priority 0 servers is down,
the other is used. If both of the priority 0 servers are down, the priority 1 server (1.2.3.6) is used, and
then, finally, the priority 2 (1.2.3.7) server.
the other is used. If both of the priority 0 servers are down, the priority 1 server (1.2.3.6) is used, and
then, finally, the priority 2 (1.2.3.7) server.
The timeout period is the same for both priority 0 servers, longer for the priority 1 server, and longer still
for the priority 2 server.
for the priority 2 server.
Using the Internet Root Servers
The AsyncOS DNS resolver is designed to accommodate the large number of simultaneous DNS
connections required for high-performance email delivery.
connections required for high-performance email delivery.
Table 14-5
Example of DNS Servers, Priorities, and Timeout Intervals
Priority
Server(s)
Timeout (Seconds)
0
1.2.3.4, 1.2.3.5
5, 5
1
1.2.3.6
10
2
1.2.3.7
45