Table of Contents

Chapter 1 DNS Configuration. 1-1

1.1 DNS Overview. 1-1

1.1.1 Static Domain Name Resolution. 1-1

1.1.2 Dynamic Domain Name Resolution. 1-1

1.2 Configuring Static Domain Name Resolution. 1-3

1.3 Configuring Dynamic Domain Name Resolution. 1-3

1.3.1 Configuration Procedure. 1-3

1.3.2 DNS Configuration Example. 1-4

1.4 Displaying and Maintaining DNS. 1-5

1.5 Troubleshooting DNS. 1-5

 

Chapter 1  DNS Configuration

1.1  DNS Overview

Domain name system (DNS) is a mechanism used for TCP/IP applications to provide domain name-to-IP address translation. With DNS, you can use memorizable and meaningful domain names in some applications and let the DNS server resolve it into correct 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 DNS database before looking up the dynamic DNS database. Reduction of the searching time in the dynamic DNS database would increase efficiency. Some frequently used addresses can be put in the static DNS database.

1.1.1  Static Domain Name Resolution

The static domain name resolution means manually setting up mappings between domain names and IP addresses. IP addresses of the corresponding domain names can be found in the static DNS database for applications.

1.1.2  Dynamic Domain Name Resolution

I. Resolution 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 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 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 shows the relationship between user program, NDS Client, and DNS Server.

The resolver and cache comprise the DNS Client. The user program and DNS Client can run on the same machine or different machines, while the DNS Server and the DNS Client usually 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 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. 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.

l           If there is no dot in the domain name, such as aabbcc, the resolver will consider this as a host name and add a DNS suffix before processing. The original name such as aabbcc is used if all DNS lookups fail.

l           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. If the lookup fails, the resolver adds a DNS suffix for another lookup.

l           If a 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, success or failure. Hence, the dot (.) is called the terminating symbol.

Currently, the device supports static and dynamic DNS services.

1.2  Configuring Static Domain Name Resolution

Table 1-1 Configure static domain name resolution:

Operation	Command	Description
Enter system view	system-view	—
Configure a mapping between a host name and an IP address	ip host hostname ip-address	Required No IP address is assigned to a host name by default.

 

 

1.3  Configuring Dynamic Domain Name Resolution

1.3.1  Configuration Procedure

Table 1-2 Configure dynamic domain name resolution:

Operation	Command	Description
Enter the system view	system-view	—
Enable dynamic domain name resolution	dns resolve	Required Disabled by default
Configure an IP address for the DNS Server	dns server ip-address	Required No IP address is configured for the DNS server by default.
Configure DNS suffixes	dns domain domain-name	Optional No DNS suffix is configured by default

 

 

1.3.2  DNS Configuration Example

I. Network requirements

As shown in Figure 1-2, the switch serving as a DNS Client uses the dynamic domain name resolution feature to access the host with the domain name being host1 and the IP address being 3.1.1.1/16. The DNS Server has the 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

 

 

# Enable dynamic domain name resolution.

<H3C> system-view

[H3C] dns resolve

# Configure the IP address 2.1.1.2 for 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

Execute the ping host1 command on the switch to verify that the communication between the switch and the host is normal and that the corresponding IP address is 3.1.1.1.

1.4  Displaying and Maintaining DNS

After the above configuration, you can execute the display command in any view to display the DNS configuration information to verify the configuration effect. You can execute the reset command in user view to clear the information stored in the dynamic domain name resolution cache.

Table 1-3 Display and maintain DNS

Operation	Command…	Description
Display static DNS database	display ip host	Available in any view
Display the DNS Server information	display dns server [ dynamic ]
Display the DNS suffixes	display dns domain [ dynamic ]
Display the information in the dynamic domain name cache	display dns dynamic-host
Display the DNS resolution result	nslookup type { ptr ip-address | a domain-name }	Available in any view
Clear the information in the dynamic domain name cache	reset dns dynamic-host	Available in user view

 

1.5  Troubleshooting DNS

I. Symptom

After enabling the dynamic domain name resolution, the user cannot get the correct IP address.

II. Solution

l           Use the display dns dynamic-host command to check that the specified domain name is in the cache.

l           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.

l           If the specified domain name exists in the cache but the IP address is incorrect, check that the DNS Client has the correct IP address of the DNS Server.

l           Check that the mapping between the domain name and IP address is correct on the DNS Server.