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. 1-2

1.1.3 DNS Proxy. 1-3

1.2 Configuring the DNS Client 1-4

1.2.1 Configuring Static Domain Name Resolution. 1-4

1.2.2 Configuring Dynamic Domain Name Resolution. 1-5

1.3 Configuring the DNS Proxy. 1-5

1.4 Displaying and Maintaining DNS. 1-6

1.5 DNS Configuration Examples. 1-6

1.5.1 Static Domain Name Resolution Configuration Example. 1-6

1.5.2 Dynamic Domain Name Resolution Configuration Example. 1-7

1.5.3 DNS Proxy Configuration Example. 1-10

1.6 Troubleshooting DNS Configuration. 1-12

 

Chapter 1  DNS Configuration

When configuring DNS, go to these sections for information you are interested in:

l           DNS Overview

l           Configuring the DNS Client

l           Configuring the DNS Proxy

l           Displaying and Maintaining DNS

l           DNS Configuration Examples

l           Troubleshooting DNS Configuration

 

 

1.1  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 device 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.

1.1.1  Static Domain Name Resolution

The static domain name resolution means setting up mappings between domain names and IP addresses. IP addresses of the corresponding domain names can be found in the static domain resolution table when you use applications such as telnet.

1.1.2  Dynamic Domain Name Resolution

I. 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.

Figure 1-1 Dynamic domain name resolution

Figure 1-1 shows the relationship between the user program, DNS client, and DNS server.

The resolver and cache comprise the DNS client. The user program and DNS client can run on the same device or different devices, 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 domain names and IP addresses in the dynamic domain name cache. There is no need to send a request to the DNS server for a repeated query 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 aging 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 incomplete. 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 (for example, aabbcc), the resolver will consider this a host name and add a DNS suffix before query. If no match is found after all the configured suffixes are used respectively, the original domain name (for example, aabbcc) is used for query.

l           If there is a dot in the domain name (for example, www.aabbcc), the resolver will directly use this domain name for query. If the query fails, the resolver adds a DNS suffix for another query.

l           If the dot is at the end of the domain name (for example, aabbcc.com.), the resolver will consider it a fully qualified domain name (FQDN) and return the query result, successful or failed. Hence, the dot “.” at the end of the domain name is called the terminating symbol.

Currently, the device supports static and dynamic DNS services.

 

 

1.1.3  DNS Proxy

I. Introduction to DNS proxy

A DNS proxy forwards DNS requests and replies between DNS clients and a DNS server.

As shown in Figure 1-2, a DNS client sends a DNS request to the DNS proxy, which forwards the request to the designated DNS server, and conveys the reply from the DNS server to the client.

The DNS proxy simplifies network management. When the DNS server address is changed, you only need to change the configuration on the DNS proxy instead of on each DNS client.

Figure 1-2 DNS proxy networking application

II. Operation of a DNS proxy

1)         A DNS client considers the DNS proxy as the DNS server, and sends a DNS request to the DNS proxy, that is, the destination address of the request is the IP address of the DNS proxy.

2)         The DNS proxy searches the local static domain name resolution table after receiving the request. If the requested information exists in the table, the DNS proxy returns a DNS reply to the client.

3)         If the requested information does not exist in the static domain name resolution table, the DNS proxy sends the request to the designated DNS server for domain name resolution.

4)         After receiving a reply from the DNS server, the DNS proxy forwards the reply to the DNS client.

1.2  Configuring the DNS Client

1.2.1  Configuring Static Domain Name Resolution

Follow these steps to configure static domain name resolution:

To do…	Use the command…	Remarks
Enter system view	system-view	––
Configure a mapping between a host name and IP address in the static name resolution table	ip host hostname ip-address	Required Not configured by default.

 

 

1.2.2  Configuring Dynamic Domain Name Resolution

Follow these steps to configure dynamic domain name resolution:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Enable dynamic domain name resolution	dns resolve	Required Disabled by default.
Specify a DNS server	dns server ip-address	Required Not specified by default
Configure a domain name suffix	dns domain domain-name	Optional Not configured by default

 

 

1.3  Configuring the DNS Proxy

Follow these steps to configure the DNS proxy:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Enable DNS proxy	dns proxy enable	Required Disabled by default.

 

1.4  Displaying and Maintaining DNS

To do…	Use the command…	Remarks
Display the static domain name resolution table	display ip host	Available in any view
Display DNS server information	display dns server [ dynamic ]
Display domain name suffixes	display dns domain [ dynamic ]	Available in any view
Display the information of the dynamic domain name cache	display dns dynamic-host
Display the DNS proxy table	display dns proxy table
Clear the information of the dynamic domain name cache	reset dns dynamic-host	Available in user view

 

1.5  DNS Configuration Examples

1.5.1  Static Domain Name Resolution Configuration Example

I. Network requirements

Switch uses the static domain name resolution to access Host with IP address 10.1.1.2 through domain name host.com.

II. Network diagram

Figure 1-3 Network diagram for static domain name resolution

III. Configuration procedure

# Configure a mapping between host name host.com and IP address 10.1.1.2.

<Sysname> system-view

[Sysname] ip host host.com 10.1.1.2

# Execute the ping host.com command to verify that the Switch can use the static domain name resolution to get the IP address 10.1.1.2 corresponding to host.com.

[Sysname] ping host.com

  PING host.com (10.1.1.2):

  56  data bytes, press CTRL_C to break

    Reply from 10.1.1.2: bytes=56 Sequence=1 ttl=128 time=2 ms

    Reply from 10.1.1.2: bytes=56 Sequence=2 ttl=128 time=2 ms

    Reply from 10.1.1.2: bytes=56 Sequence=3 ttl=128 time=2 ms

    Reply from 10.1.1.2: bytes=56 Sequence=4 ttl=128 time=2 ms

    Reply from 10.1.1.2: bytes=56 Sequence=5 ttl=128 time=2 ms

 

  --- host.com ping statistics ---

    5 packet(s) transmitted

    5 packet(s) received

    0.00% packet loss

    round-trip min/avg/max = 2/2/2 ms

1.5.2  Dynamic Domain Name Resolution Configuration Example

I. Network requirements

l           The IP address of the DNS server is 2.1.1.2/16 and the name suffix is com.

l           Switch serving as a DNS client uses the dynamic domain name resolution and the suffix to access the host with the domain name host.com and the IP address 3.1.1.1/16.

II. Network diagram

Figure 1-4 Network diagram for dynamic domain name resolution

III. Configuration procedure

 

 

1)         Configure the DNS server

# Enter DNS server configuration page.

Select Start > Programs > Administrative Tools > DNS.

# Create zone com.

In Figure 1-5, right click Forward Lookup Zones, select New zone, and then follow the instructions to create a new zone.

Figure 1-5 Create a zone

# Create a mapping between the host name and IP address.

Figure 1-6 Add a host

In Figure 1-6, right click zone com, and then select New Host to bring up a dialog box as shown in Figure 1-7. Enter host name host and IP address 3.1.1.1.

Figure 1-7 Add a mapping between domain name and IP address

2)         Configure the DNS client

# Enable dynamic domain name resolution.

<Sysname> system-view

[Sysname] dns resolve

# Specify the DNS server 2.1.1.2.

[Sysname] dns server 2.1.1.2

# Configure com as the name suffix.

[Sysname] dns domain com

3)         Configuration verification

# Execute the ping host command on the device to verify that the communication between the device and the host is normal and that the corresponding destination IP address is 3.1.1.1.

[Sysname] ping host

Trying DNS resolve, press CTRL_C to break

 Trying DNS server (2.1.1.2)

  PING host.com (3.1.1.1):

  56  data bytes, press CTRL_C to break

    Reply from 3.1.1.1: bytes=56 Sequence=1 ttl=126 time=3 ms

    Reply from 3.1.1.1: bytes=56 Sequence=2 ttl=126 time=1 ms

    Reply from 3.1.1.1: bytes=56 Sequence=3 ttl=126 time=1 ms

    Reply from 3.1.1.1: bytes=56 Sequence=4 ttl=126 time=1 ms

    Reply from 3.1.1.1: bytes=56 Sequence=5 ttl=126 time=1 ms

 

   --- host.com ping statistics ---

    5 packet(s) transmitted

    5 packet(s) received

    0.00% packet loss

    round-trip min/avg/max = 1/1/3 ms

1.5.3  DNS Proxy Configuration Example

I. Network requirements

l           Specify Switch A as the DNS server of Switch B (the DNS client).

l           Switch A acts as a DNS proxy. The IP address of the real DNS server is 4.1.1.1.

l           Switch B implements domain name resolution through Switch A.

II. Network diagram

Figure 1-8 Network diagram for DNS proxy

III. Configuration procedure

 

 

1)         Configure the DNS server

This configuration may vary with different DNS servers. When a Windows 2000 server acts as the DNS server, refer to Dynamic Domain Name Resolution Configuration Example for related configuration information.

2)         Configure the DNS proxy

# Specify the DNS server 4.1.1.1.

<SwitchA> system-view

[SwitchA] dns server 4.1.1.1

# Enable DNS proxy.

[SwitchA] dns proxy enable

3)         Configure the DNS client

# Enable the domain name resolution function.

<SwitchB> system-view

[SwitchB] dns resolve

# Specify the DNS server 2.1.1.2.

[SwitchB] dns server 2.1.1.2

4)         Configuration verification

# Execute the ping host.com command on Switch B to verify that the host can be pinged after the host’s IP address 3.1.1.1 is resolved.

[SwitchB] ping host.com

Trying DNS resolve, press CTRL_C to break

 Trying DNS server (2.1.1.2)

  PING host.com (3.1.1.1):

  56  data bytes, press CTRL_C to break

    Reply from 3.1.1.1: bytes=56 Sequence=1 ttl=126 time=3 ms

    Reply from 3.1.1.1: bytes=56 Sequence=2 ttl=126 time=1 ms

    Reply from 3.1.1.1: bytes=56 Sequence=3 ttl=126 time=1 ms

    Reply from 3.1.1.1: bytes=56 Sequence=4 ttl=126 time=1 ms

    Reply from 3.1.1.1: bytes=56 Sequence=5 ttl=126 time=1 ms

 

  --- host.com ping statistics ---

    5 packet(s) transmitted

    5 packet(s) received

    0.00% packet loss

    round-trip min/avg/max = 1/1/3 ms

1.6  Troubleshooting DNS Configuration

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 verify 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 is in the cache, but the IP address is incorrect, check that the DNS client has the correct IP address of the DNS server.

l           Verify the mapping between the domain name and IP address is correct on the DNS server.