H3C S5820X Series Configuration Manual page 97

Figure 11-1 Dynamic domain name resolution
Figure 11-1 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 on different devices. The DNS server and the DNS client usually run on different
devices.
The DNS client stores 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 a given time, and latest entries are
required from the DNS server. The DNS server decides how long a mapping is valid, and the DNS
client obtains 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 a query. If no match is found after all the configured suffixes
are used respectively, the original domain name (for example, aabbcc) is used for a query.
If there is a dot in the domain name (for example, www.aabbcc), the resolver will directly use this
domain name for a 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.
If an alias is configured for a domain name on the DNS server, the device can resolve the alias into the
IP address of the host.
11-2