3Com Switch 7750 Configuration Guide Guide

774 CHAPTER 72: DNS CONFIGURATION
Figure 198 Dynamic DNS resolution
Figure 198 shows the relationship between the user program, DNS Client and DNS
Server.
The resolver and cache compose the DNS Client. The user program runs on the
same machine as the DNS client, while the DNS Server and the DNS Client must
run on different machines.
Dynamic DNS resolution allows the DNS Client to store the latest name-to-address
mappings in the dynamic domain name cache. So there is no need to send a
request to the DNS Server for the same domain next time. The DNS Client removes
aged mappings from the cache, so as to obtain updated mappings from the DNS
Server. The setting on the DNS Server determines the aging time, and the DNS
Client gets the information from DNS messages.
DNS suffix list
The DNS Client normally holds a DNS suffix list where you can define some
domain name suffixes. It is used when the name to be resolved is not complete.
The resolver can use the list to supply the missing part. For example, you can
configure a suffix "com" in the list, and users only need to input "aabbcc" to get
the IP address of aabbcc.com, for the resolver will automatically add the suffix and
delimiter before passing the name to the DNS Server.
When a user input a domain name:
■ If there is no dot in the domain name, such as "aabbcc", the resolver will
consider this as a host name and add a suffix to the name before performing
DNS lookup. If all the suffixes in the DNS suffix list have been tried but no DNS
lookup succeeds, the resolver will use the original name (such as aabbcc) for a
DNS lookup.
■ If there is a dot in the domain name, such as "www.aabbcc", the resolver will
first use this domain name to perform DNS lookup before trying any other
suffix.
■ If there is a dot at the end of the domain name, such as "aabbcc.", the resolver
will remove the dot and use the remaining part of the name (aabbcc) to
perform DNS lookup. If the lookup fails, the resolver adds a suffix to the name
and performs another DNS lookup; this proceeds until a DNS lookup succeeds
or all the suffixes in the list have been tried.
Currently, the Switch 7750 Family Ethernet switches support both static and
dynamic domain name resolution on the DNS Client.
User program Res olv er
Cac he
Request
Response
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DNS Ser v e
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DNS Client
Request
Response
User program Res olv er
Cac he
Request
Response
Save Read
DNS Ser v e
r
DNS Client
Request
Response