Cisco Cisco Web Security Appliance S170 사용자 가이드

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For example, four DNS servers with two configured at priority 0, one at priority 1, 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 priority 0 servers are down, the priority 1 server 
(1.2.3.6) is used, and 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.

If an alert with the message “Failed to bootstrap the DNS cache” is generated when an 
appliance is rebooted, it means that the system was unable to contact its primary DNS 
servers. This 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.

You can use the Clear DNS Cache button on Network > DNS page, or the 

dnsflush

 

command to clear all information in the DNS cache when changes have been made to your 
local DNS system. Using this command might cause a temporary performance degradation 
while the cache is repopulated.

To edit DNS Settings: 

1. Navigate to the Network > DNS page.

2. Click Edit Settings. The Edit DNS page appears.

Table 21-5 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