H3C S3610 Series User manual

Operation Manual – DNS
H3C S3610&S5510 Series Ethernet Switches Table of Contents
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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:
zDNS Overview
zConfiguring Dynamic Domain Name Resolution
zDisplaying and Maintaining DNS
zDNS Configuration Examples
zTroubleshooting 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.
zIf 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.
zIf 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.
zIf 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 IPaddress 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
zThe IP address of the DNS server is 2.1.1.2/16 and the name suffix is com.
zDevice serving as a DNS client uses thedynamic 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:
zBefore 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.
zThis 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.
In 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
In 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
zUse the display dns dynamic-host command to verify that the specified domain
name is in the cache.
zIf there is no defined domain name, check that dynamic domain name resolution is
enabled and the DNS client can communicate with the DNS server.
zIf 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.
zVerify the mapping between the domain name and IP address is correct on the
DNS server.