When configuring DNS, go to these sections
for information you are interested in:
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DNS Overview
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Configuring Static Domain Name Resolution
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Configuring Dynamic Domain Name Resolution
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Displaying and Maintaining DNS
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Troubleshooting DNS Configuration
1.1 DNS Overview
Domain name system (DNS) is a mechanism
used for TCP/IP applications such as Telnet to convert Internet addresses in mnemonic
form into the equivalent numeric IP addresses.
There are two types of DNS services, static
and dynamic. Each time the DNS Server receives a name query it checks its
static database before looking up the dynamic database. Reduction of the
searching time in the dynamic database would increase efficiency. Some
frequently used addresses can be put in the static database.
1.1.1 Static Domain Name Resolution
The static domain name resolution manually
sets up mappings between names and IP addresses. IP addresses of the corresponding
names can be found in the static domain name resolution database for
applications.
1.1.2 Dynamic Domain Name Resolution
I. Resolving procedure
The relationships of the user program, DNS
Client and DNS Server are shown in Figure 1-1.
1)
A user program sends a name query to the resolver
in the DNS Client.
2)
The DNS resolver looks up its cache for a match.
If one 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 its database for a
match. If no match is found, it sends a query to its parent DNS Server. If the
parent DNS Server does not have the information, it sends the query to yet
another server. This process continues until a result is found, either
successful or fail.
4)
The DNS Client performs the next operation
according to the result.
Figure 1-1 Dynamic domain name resolution
The resolver and cache comprise the DNS
Client. The user program can run on the same machine as the DNS Client, while
the DNS Server and the DNS Client must run on different machines.
Dynamic domain name resolution allows the
DNS Client to store latest mappings between name and IP address in the dynamic domain
name cache. There is no need to send a request to the DNS Server for the same
mapping 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 the DNS
messages.
II. DNS suffixes
The DNS Client normally holds a list of
suffixes which can be defined by the users. It is used when the name to be
resolved is not complete. 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.
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If there is no dot in the domain name, such as aabbcc,
the resolver will consider this as a host name and add the suffix before
processing. The original name such as aabbcc is used if all DNS lookups fail.
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If there is a dot in the domain name, such as www.aabbcc,
the resolver will use this domain name to do DNS lookup first before adding any
suffix.
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If the dot is at the end of the domain name,
such as “aabbcc.com.”, the resolver will consider this as a fully
qualified domain name and return the result whether it is a success or a failure.
Hence, the dot (.) is called the terminating symbol.
Currently, the device supports static and
dynamic domain name services on the DNS Client.
1.2 Configuring Static Domain Name Resolution
Table 1-1 Configure static domain name
resolution:
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Operation
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Command
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Description
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Enter system view
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system-view
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—
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Create a hostname to IP address mapping
entry
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ip host
hostname ip-address
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Required
No IP address is assigned to the host
name by default.
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The last IP address
you assigned to the host name can overwrite the old one if there is any.
You may create up
to 50 entries for the domain name resolution.
1.3 Configuring Dynamic Domain Name Resolution
Table 1-2 Configure dynamic domain name
resolution:
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Operation
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Command
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Description
|
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Enter the system view
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system-view
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—
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Enable dynamic domain name resolution
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dns resolve
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Required
Disabled by default
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Configure an IP address to the DNS Server
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dns server ip-address
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Required
No IP address is assigned by default.
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Configure DNS suffixes
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dns domain
domain-name
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Optional
No DNS suffix by default
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You may configure
up to 6 DNS Servers and 10 DNS suffixes.
I. Network requirements
As shown in Figure 1-2, a device is used as a DNS Client with dynamic domain name
resolution to visit host 1 with IP address 3.1.1.1/16. The DNS Server has IP
address 2.1.1.2/16. The DNS suffixes are com and net.
II. Network diagram
Figure 1-2 Network diagram for dynamic domain name resolution
III. Configuration procedure
Before doing the
following configuration, make sure the route between the device and host 1 is
reachable, and configurations are done on both devices. The IP address of each
interface is shown on Figure
1-2. Make sure the DNS Server works well and has a mapping between host
1 and IP address 3.1.1.1/16.
# Enable dynamic domain name resolution.
<H3C> system-view
[H3C] dns resolve
# Configure IP address 2.1.1.2 to the DNS
Server
[H3C] dns server 2.1.1.2
# Configure net as the DNS suffix
[H3C] dns domain net
# Configure com as the DNS suffix
[H3C] dns domain com
Ping host 1 to verify the configuration and
the corresponding IP address should be 3.1.1.1.
1.4 Displaying and Maintaining DNS
After the above configuration, you can execute
the display command in any view to view the DNS configuration information
to verify the configuration effect. And you can execute the reset command
to clear the information stored in the caching memory of dynamic domain name
resolution.
Table 1-3 Display and maintain DNS
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Operation
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Command…
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Description
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Display static DNS list
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display ip host
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Available in any view
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Display the DNS Server information
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display dns server [ dynamic ]
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Display the DNS suffixes
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display dns domain [ dynamic ]
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Display the caching information of
dynamic domain name resolution
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display dns dynamic-host
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Display the DNS resolution result
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nslookup type { ptr ip-address | a domain-name
}
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Available in any view
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Reset the caching memory of dynamic domain
name resolution
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reset dns dynamic-host
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Available in user view
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1.5 Troubleshooting DNS Configuration
I. Symptom
After enabling the dynamic domain name
resolution, the user cannot get the IP address or the IP address is incorrect.
II. Solution
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Use the display dns dynamic-host command
to check that the specified domain name is in the cache.
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If there is no defined domain name, check that
dynamic domain name resolution is enabled and the DNS Client can communicate
with the DNS Server.
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If the specified domain name is in the cache,
but the IP address is wrong, make sure the DNS Client has the correct IP
address of the DNS Server.
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Check the mapping list is correct on the DNS Server.
