1.1 DNS Overview
Domain name system (DNS) is a distributed
database system that provides domain name-to-IP address mappings for TCP/IP
applications. With DNS, users using IP applications can directly use meaningful
easy-to-remember domain names, which will be resolved and mapped to
corresponding IP addresses by DNS servers.
There are two types of
DNS resolution, static and dynamic. When a name
query is received, the static resolution is first performed to check the static
DNS list. If the static resolution fails, the dynamic resolution is performed
to look up the dynamic DNS list. Because dynamic resolution needs the
participating of DNS server and may spend some time, you can put some commonly
used domain names in the static DNS list to increase the resolution efficiency.
1.1.1 Static DNS
Resolution
With static DNS
resolution, you can manually configure some name-to-address mappings in the
static DNS list, and the system will search the static list for corresponding
IP addresses when users use domain names with some applications (such as
telnet).
1.1.2 Dynamic DNS
Resolution
I. Resolving
procedure
The procedure of
dynamic DNS resolution 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 DNS cache
for a match. If a match is found, it returns the corresponding IP address to
the user program. If not, it sends a query to the DNS Server.
3)
The DNS Server looks up its database for a
match. If no match is found, it sends a query to its parent DNS Server. If the
parent DNS Server does not have the information, it sends the query to another
server. This process continues until a result (either successful or failed) is
found. Finally, the resolution result is returned to the DNS Client.
4)
The DNS Client performs the next operation
according to the result.
Figure 1-1 Dynamic DNS resolution
Figure
1-1 shows the relationship between the user program, DNS Client and DNS
Server.
The resolver and cache
compose the DNS Client. The user program can run on the same machine as the DNS
client, while the DNS Server and the DNS Client must run on different machines.
Dynamic DNS resolution
allows the DNS Client to store the latest name-to-address mappings in the
dynamic domain name cache. So there is no need to send a request to the DNS
Server for the same domain next time. The DNS Client removes aged mappings from
the cache, so as to obtain updated mappings from the DNS Server. The setting on
the DNS Server determines the aging time, and the DNS Client gets the
information from DNS messages.
II. DNS
suffix list
The DNS Client normally
holds a DNS suffix list where you can define some domain name suffixes. It is
used when the name to be resolved is not complete. The resolver can use the
list to supply the missing part. For example, you can configure a suffix
"com" in the list, and users only need to input "aabbcc" to
get the IP address of aabbcc.com, for the resolver will automatically add the
suffix and delimiter before passing the name to the DNS Server.
When a user input a
domain name:
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
suffix to the name before performing DNS lookup. If all the suffixes in the DNS
suffix list have been tried but no DNS lookup succeeds, the resolver will use
the original name (such as aabbcc) for a DNS lookup.
l
If there is a dot in the domain name, such as
“www.aabbcc”, the resolver will first use this domain name to
perform DNS lookup before trying any other suffix.
l
If there is a dot at the end of the domain name,
such as “aabbcc.”, the resolver will remove the dot and use the
remaining part of the name (aabbcc) to perform DNS lookup. If the lookup fails,
the resolver adds a suffix to the name and performs another DNS lookup; this
proceeds until a DNS lookup succeeds or all the suffixes in the list have been
tried.
Currently, the S7500
series Ethernet switches support both static and dynamic domain name resolution
on the DNS Client.
1.2 Configuring
Static DNS Resolution
Table 1-1
Configure static DNS resolution
|
Operation
|
Command
|
Description
|
|
Enter system view
|
system-view
|
—
|
|
Add a
hostname-to-address mapping entry
|
ip host hostname ip-address
|
Required
There is no entry in
the static DNS list by default.
|
As
one hostname can mapped to only one IP address, when you add multiple
hostname-to-address mapping entries with the same hostname, only the last one
will be valid.
You
can add up to 50 entries for static DNS resolution.
1.3 Configuring Dynamic DNS Resolution
Table 1-2
Configure dynamic DNS
resolution
|
Operation
|
Command
|
Description
|
|
Enter system view
|
system-view
|
—
|
|
Enable dynamic DNS
resolution
|
dns resolve
|
Required
This function is
disabled by default.
|
|
Configure a DNS
server IP address
|
dns server ip-address
|
Required
No DNS server IP
address is configured by default.
|
|
Configure a DNS suffix
|
dns domain domain-name
|
Optional
No DNS suffix is
configured by default.
|
You
can configure up to 6 DNS servers and 10 DNS suffixes.
I. Network
requirements
As shown in Figure 1-2, an S7506 is used as a DNS client with dynamic DNS
resolution. It allows you to visit host 1 with IP address 3.1.1.1/16. The DNS
server IP address is 2.1.1.2/16. The DNS suffixes "com" and
"net" are configured.
II. Network
diagram
Figure 1-2 Network
diagram for dynamic DNS
resolution
III. Configuration
procedure
Before
doing the following configuration, suppose the route between the S7506 and host
1 is reachable, the DNS server works normally, and a mapping entry from host 1
to IP address 3.1.1.1/16 exists on the DNS server.
# Enable dynamic DNS
resolution.
<H3C>
system-view
[H3C] dns resolve
# Configure the DNS
server IP address 2.1.1.2.
[H3C] dns server
2.1.1.2
# Configure net as a
DNS suffix.
[H3C] dns domain
net
# Configure com as a
DNS suffix.
[H3C] dns domain
com
Ping host 1 on the
S7505 to verify the configuration and the corresponding IP address (it should
be 3.1.1.1).
1.4 Displaying and Maintaining DNS
After the above
configuration, you can execute the display command in any view to view
the DNS configuration and running information to verify your configuration. And
you can execute the reset command to clear the dynamic DNS cache.
Table 1-3
Display and maintain DNS
|
Operation
|
Command
|
Description
|
|
Display
static DNS list information
|
display
ip host
|
You
can execute the display command in any view.
|
|
Display DNS server
information
|
display dns server [ dynamic ]
|
|
Display DNS suffix
list information
|
display dns domain
|
|
Display dynamic DNS
cache information
|
display dns
dynamic-host
|
|
Clear the dynamic DNS
cache.
|
reset dns
dynamic-host
|
Execute the reset
command in user view.
|
1.5 Troubleshooting
DNS Configuration
I. Symptom
Dynamic DNS resolution
is enabled, but the user cannot get the correct IP address from a domain name.
II. Solution
l
Use the display dns dynamic-host command
to check if the specified domain name is in the cache.
l
If the specified domain name is in the cache,
but the IP address is wrong, ensure that the DNS Client has the correct IP
address of the DNS Server.
l
If the specified domain name is not in the
cache, ensure that dynamic DNS resolution is enabled, the DNS Client can
normally communicate with the DNS Server, and the DNS Server works normally.
l
Check the DNS mapping list is correct on the DNS
Server.
