3Com Switch 4800G 24-Port Configuration Manual page 972

972 C 78: DNS C
HAPTER ONFIGURATION
4 The DNS client returns the resolution result to the application after receiving a
response from the DNS server.
Figure 289 Dynamic domain name resolution
Request
User
program
Response
Figure 289 shows the relationship between the user program, DNS client, and DNS
server.
The resolver and cache comprise the DNS client. The user program and DNS client
can run on the same device or different devices, while the DNS server and the DNS
client usually run on different devices.
Dynamic domain name resolution allows the DNS client to store latest mappings
between domain names and IP addresses in the dynamic domain name cache.
There is no need to send a request to the DNS server for a repeated query next
time. The aged mappings are removed from the cache after some time, and latest
entries are required from the DNS server. The DNS server decides how long a
mapping is valid, and the DNS client gets the aging information from DNS
messages.
DNS suffixes
The DNS client normally holds a list of suffixes which can be defined by users. It is
used when the name to be resolved is incomplete. The resolver can supply the
missing part. For example, a user can configure com as the suffix for aabbcc.com.
The user only needs to type aabbcc to get the IP address of aabbcc.com. The
resolver can add the suffix and delimiter before passing the name to the DNS
server.
If there is no dot in the domain name (for example, aabbcc), the resolver will
■
consider this a host name and add a DNS suffix before query. If no match is
found after all the configured suffixes are used respectively, the original domain
name (for example, aabbcc) is used for query.
If there is a dot in the domain name (for example, www.aabbcc), the resolver
■
will directly use this domain name for query. If the query fails, the resolver adds
a DNS suffix for another query.
If the dot is at the end of the domain name (for example, aabbcc.com.), the
■
resolver will consider it a fully qualified domain name (FQDN) and return the
query result, successful or failed. Hence, the dot "." at the end of the domain
name is called the terminating symbol.
Currently, the device supports static and dynamic DNS services.
Request
Resolver
Response
Save
Read
Cache
DNS client
DNS server