H3C Technologies H3C S5560 Series Switches - Manual (Page 98)

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Configuring DNS
Overview
Domain Name System (DNS) is a distributed database used by TCP/IP applications to translate domain
names into IP addresses. The domain name-to-IP address mapping is called a DNS entry.
DNS services can be static or dynamic. After a user specifies a name, the device checks the static name
resolution table for an IP address. If no IP address is available, it contacts the DNS server for dynamic
name resolution, which takes more time than static name resolution. To improve efficiency, you can put
frequently queried name-to-IP address mappings in the local static name resolution table.
Static domain name resolution
Static domain name resolution means manually creating mappings between domain names and IP
addresses. For example, you can create a static DNS mapping for a device so that you can Telnet to the
device by using the domain name.
Dynamic domain name resolution
Resolution process
1. A user program sends a name query to the resolver of the DNS client.
2. The DNS resolver looks up the local domain name cache for a match. If the resolver finds a match,
it sends the corresponding IP address back. If not, it sends a query to the DNS server.
3. The DNS server looks up the corresponding IP address of the domain name in its DNS database.
If no match is found, the server sends a query to other DNS servers. This process continues until a
result, whether successful or not, is returned.
4. After receiving a response from the DNS server, the DNS client returns the resolution result to the
user program.
Figure 30 shows the relationship between the user program, DNS client, and DNS server.
The DNS client includes the resolver and cache. The user program and DNS client can run on the same
device or different devices, but the DNS server and the DNS client usually run on different devices.