H3C S3610 Series Operating instructions

Operation Manual – DNS

H3C S3610&S5510 Series Ethernet Switches Table of Contents

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 Domain Name Resolution ..............................................................................1-3

1.2.1 Configuring Static Domain Name Resolution..........................................................1-3

1.2.2 Configuring Dynamic Domain Name Resolution.....................................................1-3

1.3 Displaying and Maintaining DNS.......................................................................................1-4

1.4 DNS Configuration Examples............................................................................................1-4

1.4.1 Static Domain Name Resolution Configuration Example........................................1-4

1.4.2 Dynamic Domain Name Resolution Configuration Example ..................................1-5

1.5 Troubleshooting DNS Configuration..................................................................................1-9

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Chapter 1 DNS Configuration

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

z DNS Overview

z Configuring Dynamic Domain Name Resolution

z Displaying and Maintaining DNS

z DNS Configuration Examples

z Troubleshooting DNS Configuration

 Note:

This document only covers IPv4 DNS configurations. For introduction to IPv6 DNS

configurations, refer to IPv6 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:

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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 success or failure, 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 machine or different devices, while the DNS server and the DNS

client usually must 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.

z 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

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after all the configured suffixes are used respectively, the original domain name

(for example, aabbcc) is used for query.

z 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.

z 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, a success or failure. Hence, the dot “.” at the end of the domain name is

called the terminating symbol.

Currently, the device supports static and dynamic DNS 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.

1.2 Configuring Domain Name Resolution

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.

 Note:

The IP 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 IP addresses.

1.2.2 Configuring Dynamic Domain Name Resolution

Follow these steps to configure dynamic domain name resolution:

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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

 Note:

You may configure up to six DNS servers and ten DNS suffixes.

1.3 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 ]

Available in any view

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

Available in any view

Clear the information of

the dynamic domain

name cache

reset dns dynamic-host

Available in user view

1.4 DNS Configuration Examples

1.4.1 Static Domain Name Resolution Configuration Example

I. Network requirements

Device uses the static domain name resolution to access Host with IP address 10.1.1.2

through domain name host.com.

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II. Network diagram

10.1.1.1/24

10.1.1.2/24

host.com

Switch Host

Figure 1-2 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 device 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=255 time=1 ms

Reply from 10.1.1.2: bytes=56 Sequence=2 ttl=255 time=4 ms

Reply from 10.1.1.2: bytes=56 Sequence=3 ttl=255 time=3 ms

Reply from 10.1.1.2: bytes=56 Sequence=4 ttl=255 time=2 ms

Reply from 10.1.1.2: bytes=56 Sequence=5 ttl=255 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

1.4.2 Dynamic Domain Name Resolution Configuration Example

I. Network requirements

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

z Device serving as a DNS client uses the dynamic domain name resolution and the

suffix to access the host with the domain name being host.com and the IP address

3.1.1.1/16.

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II. Network diagram

2.1.1.2/16

2.1.1.1/16

DNS server

Device

DNS client

1.1.1.1/16

3.1.1.1/16

host.com

IP network

Host

Figure 1-3 Network diagram for dynamic domain name resolution

III. Configuration procedure

 Note:

z Before performing the following configuration, make sure that there is a route

between the device and the host, and configurations are done on both the device

and the host. For the IP addresses of the interfaces, see

Figure 1-3.

z This configuration may vary with different DNS servers. The following configuration

is performed on Windows 2000 server.

1) Configure the DNS server

# Enter DNS server configuration page.

Select Start > Programs > Administrative Tools > DNS.

# Create zone com.

Figure 1-4, right click Forward Lookup Zones, select New zone, and then follow

the instructions to create a new zone.

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Figure 1-4 Create a zone

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

Figure 1-5 Add a host

Figure 1-5, right click zone com, and then select New Host to bring up a dialog box

as shown in

Figure 1-6. Enter host name host and IP address 3.1.1.1.

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Figure 1-6 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

# Configure IP address 2.1.1.2 of the DNS server

[Sysname] dns server 2.1.1.2

# Configure com as the name suffix

[Sysname] dns domain com

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=255 time=3 ms

Reply from 3.1.1.1: bytes=56 Sequence=2 ttl=255 time=1 ms

Reply from 3.1.1.1: bytes=56 Sequence=3 ttl=255 time=1 ms

Reply from 3.1.1.1: bytes=56 Sequence=4 ttl=255 time=1 ms

Reply from 3.1.1.1: bytes=56 Sequence=5 ttl=255 time=1 ms

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--- 3.1.1.1 ping statistics ---

5 packet(s) transmitted

5 packet(s) received

0.00% packet loss

round-trip min/avg/max = 1/1/3 ms

--- host.com ping statistics ---

5 packet(s) transmitted

0 packet(s) received

100.00% packet loss

1.5 Troubleshooting DNS Configuration

I. Symptom

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

address.

II. Solution

z Use the display dns dynamic-host command to verify that the specified domain

name is in the cache.

z 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.

z 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.

z Verify the mapping between the domain name and IP address is correct on the

DNS server.

H3C S3610 Series, S5510 Series Operating instructions

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