H3C Technologies H3C SecPath F1000-E - Manual (Page 236)

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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 corresponding IP addresses. With DNS, you can use easy-to-remember domain names in
some applications and let the DNS server translate them into correct IP addresses.
There are two types of DNS services, static and dynamic. After a user specifies a name, the firewall
checks the local 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. Therefore,
some frequently queried name-to-IP address mappings are stored in the local static name resolution table
to improve efficiency.
Static domain name resolution
Configuring static domain name resolution is to set up mappings between domain names and IP
addresses manually. IP addresses of the corresponding domain names can be found in the static domain
resolution table when you use applications such as telnet.
Dynamic domain name resolution
Resolving procedure
Dynamic domain name resolution is implemented by querying the DNS server. The resolution procedure
is as follows:
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 a match is found, 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, it sends a query to a higher level DNS server. This process continues until a
result, whether successful or not, is returned.
4. The DNS client returns the resolution result to the application after receiving a response from the
DNS server.