3Com MSR 50 Series Configuration Manual page 610

610 C 38: DNS C
HAPTER ONFIGURATION
4 The DNS client returns the resolution result to the application after receiving a
response from the DNS server.
Figure 176 Dynamic domain name resolution
Request
User
program
Response
Figure 176 shows the relationship between 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 machine or different machines, while the DNS server and the
DNS client usually must run on different machines.
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 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 as a host name and add a DNS suffix before query. The original
domain name (for example, aabbcc) is used if the query fails.
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 as a fully qualified domain name (FQDN) and return the
query result, success or a failure. 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