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.

DNS services can be static or 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. 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 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.

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 a higher level DNS server. 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 application.

Figure 1 Dynamic domain name resolution

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

The DNS client is made up of 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.

Dynamic domain name resolution allows the DNS client to store latest mappings between domain names and IP addresses in the dynamic domain name cache. The DNS client does not 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.

DNS suffixes

The DNS client holds a list of suffixes which the user sets. 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 obtain the IP address of aabbcc.com because the resolver adds the suffix and delimiter before passing the name to the DNS server.

· If there is no dot in the domain name (for example, aabbcc), the resolver considers this a host name and adds 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.

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

· If the dot is at the end of the domain name (for example, aabbcc.com.), the resolver considers it a fully qualified domain name (FQDN) and returns the query result, successful or failed. The dot (.) at the end of the domain name is considered a terminating symbol.

The device supports static and dynamic DNS client services.

NOTE:

If an alias is configured for a domain name on the DNS server, the device can resolve the alias into the IP address of the host.

DNS proxy

Introduction to DNS proxy

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

As shown in Figure 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 can change the configuration on only the DNS proxy instead of on each DNS client.

Figure 2 DNS proxy networking application

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. 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 and dynamic domain name resolution table after receiving the request. If the requested information is found, the DNS proxy returns a DNS reply to the client.

3. If the requested information is not found, 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 records the IP address-to-domain name mapping and forwards the reply to the DNS client.

NOTE:

With no DNS server or route to a DNS server specified, the DNS proxy does not forward DNS requests, or answer the requests from the DNS clients.

DNS spoofing

Figure 3 Application of DNS spoofing

DNS spoofing is applied to the dial-up network, as shown in Figure 3.

· The device connects to the PSTN/ISDN network through a dial-up interface and triggers the establishment of a dial-up connection only when packets are to be forwarded through the dial-up interface.

· The device serves as a DNS proxy and is specified as a DNS server on the hosts. After the dial-up connection is established through the dial-up interface, the device dynamically obtains the DNS server address through DHCP or other autoconfiguration mechanisms.

Without DNS spoofing enabled, the device forwards the DNS requests received from the hosts to the DNS server, if it cannot find a match in the local domain name resolution table. However, without any dial-up connection established, the device cannot obtain the DNS server address and cannot forward or answer the requests from the clients. The domain name cannot be resolved and no traffic triggers the establishment of a dial-up connection.

DNS spoofing can solve the problem. DNS spoofing enables the device to reply the DNS client with a configured IP address when the device does not have a DNS server address or route to a DNS server. Subsequent packets sent by the DNS client trigger the establishment of a dial-up connection with the network.

In the network of Figure 3, a host accesses the HTTP server in following these steps.

1. The host sends a DNS request to the device to resolve the domain name of the HTTP server into an IP address.

2. Upon receiving the request, the device searches the local static and dynamic DNS entries for a match. If no match is found and the device does know the DNS server address, the device spoofs the host by replying a configured IP address. The TTL of the DNS reply is 0. Note that the device must have a route to the IP address with the dial-up interface as the outgoing interface.

3. Upon receiving the reply, the host sends an HTTP request to the replied IP address.

4. When forwarding the HTTP request through the dial-up interface, the device establishes a dial-up connection with the network and dynamically obtains the DNS server address through DHCP or other autoconfiguration mechanisms.

5. When the DNS reply ages out, the host sends a DNS request to the device again.

6. Then the device operates the same as a DNS proxy. For more information, see “Operation of a DNS proxy.”

7. After obtaining the IP address of the HTTP server, the host can access the HTTP server.

NOTE:

Because the IP address configured with DNS spoofing is not the actual IP address of the requested domain name, the TTL of the DNS reply is set to 0 to prevent the DNS client from generating incorrect domain name-to-IP address mappings.

Configuring the IPv4 DNS client

Configuring static domain name resolution

Configuring static domain name resolution refers to specifying the mappings between host names and IPv4 addresses. Static domain name resolution allows applications such as Telnet to contact hosts by using host names instead of IPv4 addresses.

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 an IPv4 address	ip host hostname ip-address	Required Not configured by default.

To do…

Use the command…

Remarks

Enter system view

system-view

––

Configure a mapping between a host name and an IPv4 address

ip host hostname ip-address

Required

Not configured by default.

NOTE:

· The IPv4 address you last assign to the host name will overwrite the previous one if there is any.

· You may create up to 50 static mappings between domain names and IPv4 addresses.

Configuring dynamic domain name resolution

To send DNS queries to a correct server for resolution, dynamic domain name resolution needs to be enabled and a DNS server needs to be configured.

In addition, you can configure a DNS suffix that the system will automatically add to the provided domain name for 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	System view	dns server ip-address	Required Not specified by default.
	Interface view	interface interface-type interface-number
		dns server ip-address
		quit
Configure a DNS suffix		dns domain domain-name	Optional Not configured by default. Only the provided domain name is resolved.

NOTE:

· You can configure up to six DNS servers, including those with IPv6 addresses, in system view and on all interfaces of a device.

· A DNS server configured in system view has a higher priority than one configured in interface view. A DNS server configured earlier has a higher priority than one configured later in the same view. A DNS server manually configured has a higher priority than one dynamically obtained through DHCP. A name query request is first sent to the DNS server that has the highest priority. If no reply is received, it is sent to the DNS server that has the second highest priority, and thus in turn.

· You can specify up to ten DNS suffixes.

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.
Specify a DNS server	System view	dns server ip-address	Required Configure the DNS server in at least one view. No DNS server is specified by default.
	Interface view	interface interface-type interface-number
	Interface view	dns server ip-address

NOTE:

You can specify multiple DNS servers by using the dns server command repeatedly. Upon receiving a name query request from a client, the DNS proxy forwards the request to the DNS server that has the highest priority. If having not received a reply, it forwards to the request to a DNS server that has the second highest priority, and thus in turn.

Configuring DNS spoofing

Configuration prerequisites

DNS spoofing is effective only when:

· The DNS proxy is enabled on the device.

· No DNS server or route to any DNS server is specified on the device.

Configuration procedure

Follow these steps to configure DNS spoofing:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Enable DNS spoofing and specify the translated IP address	dns spoofing ip-address	Required Disabled by default.

To do…

Use the command…

Remarks

Enter system view

system-view

—

Enable DNS spoofing and specify the translated IP address

dns spoofing ip-address

Required

Disabled by default.

Displaying and maintaining IPv4 DNS

To do…	Use the command…	Remarks
Display the static IPv4 domain name resolution table	display ip host [ \| { begin \| exclude \| include } regular-expression ]	Available in any view
Display IPv4 DNS server information	display dns server [ dynamic ] [ \| { begin \| exclude \| include } regular-expression ]	Available in any view
Display DNS suffixes	display dns domain [ dynamic ] [ \| { begin \| exclude \| include } regular-expression ]	Available in any view
Display the information of the dynamic IPv4 domain name cache	display dns host ip [ \| { begin \| exclude \| include } regular-expression ]	Available in any view
Clear the information of the dynamic IPv4 domain name cache	reset dns host ip	Available in user view

IPv4 DNS configuration examples

Static domain name resolution configuration example

Network requirements

As shown in Figure 4, the device wants to access the host by using an easy-to-remember domain name rather than an IP address.

Configure static domain name resolution on the device so that the device can use the domain name host.com to access the host whose IP address is 10.1.1.2.

Figure 4 Network diagram for static domain name resolution

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

# Use the ping host.com command to verify that the device can use static domain name resolution to resolve domain name host.com into IP address 10.1.1.2.

[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=1 ms

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

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

--- host.com ping statistics ---

5 packet(s) transmitted

5 packet(s) received

0.00% packet loss

round-trip min/avg/max = 1/2/4 ms

Dynamic domain name resolution configuration example

Network requirements

As shown in Figure 5, the device wants to access the host by using an easy-to-remember domain name rather than an IP address, and to request the DNS server on the network for an IP address by using dynamic domain name resolution. The IP address of the DNS server is 2.1.1.2/16 and the DNS server has a com domain, which stores the mapping between domain name host and IP address 3.1.1.1/16.

Configure dynamic domain name resolution and the domain name suffix com on the device that serves as a DNS client so that the device can use domain name host to access the host with the domain name host.com and the IP address 3.1.1.1/16.

Figure 5 Network diagram for dynamic domain name resolution

Configuration procedure

NOTE:

· Before performing the following configuration, make sure that the device and the host are accessible to each another via available routes, and the IP addresses of the interfaces are configured as shown Figure 5.

· This configuration may vary with different DNS servers. The following configuration is performed on a Windows server 2000 PC.

1. Configure the DNS server

# Enter the DNS server configuration page.

Select Start > Programs > Administrative Tools > DNS.

# Create zone com.

As shown in Figure 6, right click Forward Lookup Zones, select New zone, and then follow the instructions to create a new zone named com.

Figure 6 Create a zone

# Create a mapping between host name and IP address.

Figure 7 Add a host

In Figure 7, right click zone com, and then select New Host to bring up a dialog box as shown in Figure 8. Enter host name host and IP address 3.1.1.1.

Figure 8 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

# Use 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

DNS proxy configuration example

Network requirements

When the IP address of the DNS server changes, you must configure the new IP address of the DNS server on each device on the LAN. To simplify network management, you can use the DNS proxy function.

As shown in Figure 9:

· Specify Device A as the DNS server of Device B (the DNS client). Device A acts as a DNS proxy. The IP address of the real DNS server is 4.1.1.1.

· Configure the IP address of the DNS proxy on Device B. DNS requests of Device B are forwarded to the real DNS server through the DNS proxy.

Figure 9 Network diagram for DNS proxy

Configuration procedure

NOTE:

Before performing the following configuration, assume that Device A, the DNS server, and the host are reachable to each other and the IP addresses of the interfaces are configured as shown in Figure 9.

1. Configure the DNS server

This configuration may vary with different DNS servers. When a Windows server 2000 PC acts as the DNS server, see “Dynamic domain name resolution configuration example” for related configuration information.

2. Configure the DNS proxy

# Specify the DNS server 4.1.1.1.

<DeviceA> system-view

[DeviceA] dns server 4.1.1.1

# Enable DNS proxy.

[DeviceA] dns proxy enable

3. Configure the DNS client

# Enable the domain name resolution function.

<DeviceB> system-view

[DeviceB] dns resolve

# Specify the DNS server 2.1.1.2.

[DeviceB] dns server 2.1.1.2

4. Configuration verification

# Execute the ping host.com command on Device B 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.

[DeviceB] 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

Troubleshooting IPv4 DNS configuration

Symptom

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

Solution

· Use the display dns host ip command to verify that the specified domain name is in the cache.

· If the specified domain name does not exist, check that dynamic domain name resolution is enabled and the DNS client can communicate with the DNS server.

· 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.

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

IPv6 DNS configuration

This chapter includes these sections:

· Introduction to IPv6 DNS

· Configuring the IPv6 DNS client

· Displaying and maintaining IPv6 DNS

· IPv6 DNS configuration examples

NOTE:

· The term "switch" or "device" in this chapter refers to the switching engine on a WX3000E wireless switch.

· The WX3000E series comprises WX3024E and WX3010E wireless switches.

· The port numbers in this chapter are for illustration only.

Introduction to IPv6 DNS

IPv6 Domain Name System (DNS) is responsible for translating domain names into IPv6 addresses. Like IPv4 DNS, IPv6 DNS includes static domain name resolution and dynamic domain name resolution. The functions and implementations of the two types of domain name resolution are the same as those of IPv4 DNS. For more information, see the chapter “IPv4 DNS configuration.”

Configuring the IPv6 DNS client

Configuring static domain name resolution

Configuring static domain name resolution refers to specifying the mappings between host names and IPv6 addresses. Static domain name resolution allows applications such as Telnet to contact hosts by using host names instead of IPv6 addresses.

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 an IPv6 address	ipv6 host hostname ipv6-address	Required Not configured by default.

To do…

Use the command…

Remarks

Enter system view

system-view

—

Configure a mapping between a host name and an IPv6 address

ipv6 host hostname ipv6-address

Required

Not configured by default.

NOTE:

· A host name can be mapped to one IPv6 address only. If you map a host name to different IPv6 addresses, the last configuration takes effect.

· You can configure up to 50 mappings between domain name and IPv6 address on a device.

Configuring dynamic domain name resolution

To send DNS queries to a correct server for resolution, dynamic domain name resolution needs to be enabled and a DNS server needs to be configured.

In addition, you can configure a DNS suffix that the system will automatically add to the provided domain name for 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 ipv6 ipv6-address [ interface-type interface-number ]	Required Not specified by default. If the IPv6 address of a DNS server is a link-local address, you need to specify the interface-type and interface-number arguments.
Configure a DNS suffix	dns domain domain-name	Optional Not configured by default. Only the provided domain name is resolved.

NOTE:

· For more information about the dns resolve and dns domain commands, see the chapter “IPv4 DNS configuration”.

· You can configure up to six DNS servers, including those with IPv4 addresses on a device.

· You can specify up to ten DNS suffixes on a device.

Displaying and maintaining IPv6 DNS

To do…	Use the command…	Remarks
Display the static IPv6 domain name resolution table	display ipv6 host [ \| { begin \| exclude \| include } regular-expression ]	Available in any view
Display IPv6 DNS server information	display dns ipv6 server [ dynamic ] [ \| { begin \| exclude \| include } regular-expression ]	Available in any view
Display DNS suffixes	display dns domain [ dynamic ] [ \| { begin \| exclude \| include } regular-expression ]	Available in any view
Display the information of dynamic IPv6 domain name cache	display dns host ipv6 [ \| { begin \| exclude \| include } regular-expression ]	Available in any view
Clear the information of dynamic IPv6 domain name cache	reset dns host ipv6	Available in user view

IPv6 DNS configuration examples

Static domain name resolution configuration example

Network requirements

As shown in Figure 10, the device wants to access the host by using an easy-to-remember domain name rather than an IPv6 address. Configure static domain name resolution on the device so that the device can use the domain name host.com to access the host whose IPv6 address is 1::2.

Figure 10 Network diagram for static domain name resolution

Configuration procedure

# Configure a mapping between host name host.com and IPv6 address 1::2.

<Device> system-view

[Device] ipv6 host host.com 1::2

# Enable IPv6 packet forwarding.

[Device] ipv6

# Use the ping ipv6 host.com command to verify that the device can use static domain name resolution to resolve domain name host.com into IPv6 address 1::2.

[Device] ping ipv6 host.com

PING host.com (1::2):

56 data bytes, press CTRL_C to break

Reply from 1::2

bytes=56 Sequence=1 hop limit=128 time = 3 ms

Reply from 1::2

bytes=56 Sequence=2 hop limit=128 time = 1 ms

Reply from 1::2

bytes=56 Sequence=3 hop limit=128 time = 1 ms

Reply from 1::2

bytes=56 Sequence=4 hop limit=128 time = 2 ms

Reply from 1::2

bytes=56 Sequence=5 hop limit=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 = 1/1/3 ms

Dynamic domain name resolution configuration example

Network requirements

As shown in Figure 11, the device wants to access the host by using an easy-to-remember domain name rather than an IPv6 address. The IPv6 address of the DNS server is 2::2/64 and the server has a com domain, which stores the mapping between domain name host and IPv6 address 1::1/64.

Figure 11 Network diagram of dynamic domain name resolution

Configuration procedure

NOTE:

· Before performing the following configuration, make sure that the device and the host are accessible to each another via available routes, and the IPv6 addresses of the interfaces are configured as shown Figure 11.

· This configuration may vary with DNS servers. The following configuration is performed on a PC running Windows Server 2003. Make sure that the DNS server supports the IPv6 DNS function so that the server can process IPv6 DNS packets, and the interfaces of the DNS server can forward IPv6 packets.

1. Configure the DNS server

# Enter the DNS server configuration page.

Select Start > Programs > Administrative Tools > DNS.

# Create zone com.

As shown in Figure 12, right click Forward Lookup Zones, select New zone, and then follow the instructions to create a new zone named com.

Figure 12 Create a zone

1-1

# Create a mapping between the host name and the IPv6 address.

As shown in Figure 13, right click zone com.

Figure 13 Create a record

2-2

In Figure 13, select Other New Records to bring up a dialog box as shown in Figure 14. Select IPv6 Host (AAA) as the resource record type.

Figure 14 Select the resource record type

As shown in Figure 15, type host name host and IPv6 address 1::1, and then click OK.

Figure 15 Add a mapping between domain name and IPv6 address

2. Configure the DNS client

# Enable dynamic domain name resolution.

<Device> system-view

[Device] dns resolve

# Specify the DNS server 2::2.

[Device] dns server ipv6 2::2

# Configure com as the DNS suffix.

[Device] dns domain com

3. Configuration verification

# Use the ping ipv6 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 1::1.

[Device] ping ipv6 host

Trying DNS resolve, press CTRL_C to break

Trying DNS server (2::2)

PING host.com (1::1):

56 data bytes, press CTRL_C to break

Reply from 1::1

bytes=56 Sequence=1 hop limit=126 time = 2 ms

Reply from 1::1

bytes=56 Sequence=2 hop limit=126 time = 1 ms

Reply from 1::1

bytes=56 Sequence=3 hop limit=126 time = 1 ms

Reply from 1::1

bytes=56 Sequence=4 hop limit=126 time = 1 ms

Reply from 1::1

bytes=56 Sequence=5 hop limit=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 = 1/1/2 ms

03-Layer 3 Configuration Guide

DNS overview

Static domain name resolution

Dynamic domain name resolution

Resolution process

DNS suffixes

DNS proxy

Introduction to DNS proxy

Configuring dynamic domain name resolution

Configuring the DNS proxy

IPv4 DNS configuration examples

Network requirements

Configuration procedure

Network requirements

Configuration procedure

Network requirements

Configuration procedure

Symptom

Solution

Network requirements

Configuration procedure

Network requirements

Configuration procedure

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