H3C S5100-SI Series Operation Manual page 859

Operation Manual – DNS
H3C S5100-SI/EI Series Ethernet Switches
resolution procedure is as follows:
1) A user program sends a name query to the resolver in the DNS client.
2) The DNS resolver looks up the local domain name cache for a match. If a match is
found, it sends the corresponding IP address back. If not, it sends the query to the
DNS server.
3) The DNS server looks up its DNS database for a match. If no match is found, it
sends a query to a higher-level DNS server. This process continues until a result,
success or failure, is returned.
4) The DNS client performs the next operation according to the result.
Figure 1-1 Dynamic domain name resolution
Figure 1-1
The resolver and cache comprise the DNS client. The user program and DNS client run
on the same device, 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 name and IP address in the dynamic domain name cache of the DNS client.
There is no need to send a request to the DNS server for a repeated query request 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.
II. 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 not complete. The resolver can supply the missing
part (automatic domain name addition). 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.
shows the relationship between user program, DNS client, and DNS server.
1-2
Chapter 1 DNS Configuration