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client
The figure above is an application of DNS SERVER. Under some circumstances, the client PC doesn't know
the real DNS SERVER, and points to the switch instead. The switch plays the role of a DNS SERVER in two
steps: Enable the global DNS SERVER function, configure the IP address of the real DNS server. After the
DNS SERVER function is globally enabled, the switch will look up its local cache when receiving a DNS
request from a client PC. If there is a domain needed by the local client, it will directly answer the client's
request; otherwise, the switch will relay the request to the real DNS server, pass the reply from the DNS
Server to the client and record the domain and its IP address for a faster lookup in the future.
Switch configuration for DNS CLIENT:
Switch(config)# ip domain-lookup
Switch(config)# dns-server 219.240.250.101
Switch(config)# dns-server 2001::1
Switch#ping host www.sina.com.cn
Switch#traceroute host www.sina.com.cn
Switch#telnet host www.sina.com.cn
Switch configuration for DNS SERVER:
Switch(config)# ip domain-lookup
Switch(config)# dns-server 219.240.250.101
Switch(config)# dns-server 2001::1
Switch(config)# ip dns server
65.4 DNS Troubleshooting
In configuring and using DNS, the DNS may fail due to reasons such as physical connection failure or wrong
configurations. The user should ensure the following:
First make sure good condition of the TACACS+ server physical connection;
Second all interface and link protocols are in the UP state (use "show interface" command);
SWITCH
Figure 65-2: DNS SERVER typical environment
65-64
DNS SERVER
IP:219.240.250.101
IPv6:2001::1
INTERNET
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Troubleshooting
