Using DNS

DNS Overview

The DNS client software allows users on your network to use a domain name, rather than an IP address, when running Ping, Telnet, or Traceroute.

In the DNS client services configuration, you set the address of DNS servers that you want the router to query for the IP address of a host name. You can also have a hostname-to-IP address mapping table in your IP configuration that you set up using the add hostname command.

How DNS Client Works

The DNS software uses the following process to query DNS servers.

: 1. A user runs Telnet to the host name.
: 2. The router software checks for the name in your IP host table.
: 3. If the name is not in the host table, the router sends a query to the DNS server. If you have multiple DNS servers configured, the router sends the query to the DNS server that most recently answered one of the router's query requests.
: 4. If the DNS server does not respond, the DNS client sends a query to the next DNS server in your configuration.
: 5. If the DNS server responds that it cannot resolve the IP address of the host, and if you have set up domain names in your DNS configuration, the DNS client sends another query. In this query, the router appends the first domain name in your DNS configuration.
: 6. If the DNS server still cannot resolve the IP address, the router appends the next domain in your configuration can sends another query to the DNS server.

The router continues this process with each DNS server in its configuration until a server successfully resolves the IP address.

Displaying the DNS Client Prompts

To display the DNS client configuration prompt, do the following:

*config
Config>protocol ip
Internet protocol user configuration
IP config>dns
DNS Client Config>

To display the DNS client monitoring prompt, enter the following command at the IP> prompt:

*monitor
Monitor>protocol ip
IP>dns
DNS Client>

Sample DNS Configuration

This section outlines the steps needed to configure DNS client on your router.

1. Enable DNS client.

DNS Client Config> enable

2. Set the address of on or more DNS servers.

DNS Client Config> set server += 128.185.1.1
DNS Client Config> set server += 128.185.20.10

3. Optionally add domain names to your configuration.

DNS Client Config> set domain_name +=nxnetworks

4. Restart the router for the new configuration to take effect.

DNS Client Config> exit
IP config>exit
Config> <CTRL-P>
*restart
Are you sure you want to restart the gateway?(Yes or [No]):y

How This Configuration Works

Using the sample configuration, the following steps shows the process DNS uses to resolve host names.

: 1. A user runs Telnet to the host name merckx.
: 2. The DNS client software checks for the name merckx in its IP host table, and the name is not in the table.
: 3. The DNS client sends a query for the name merckx to DNS server 128.185.1.1.
: 4. The DNS server sends a response saying that it cannot resolve the host name.
: 5. The DNS client sends another query for the name merckx.nxnetworks to DNS server 128.185.1.1.
: 6. The DNS server is able to resolve the name and sends the IP address to the DNS client software.

DNS Commands

This section describes the DNS commands.

Press Space after you type a command to display the available options for each command. Enter help for information about using the command line interface.

[C] means the command is available at the DNS Client Config> prompt.

[M] means the command is available at the DNS Client> prompt.

Table 1 DNS Client Commands

Command Function

Disable [C] [M] Disables DNS client on the router.

Enable [C] [M] Enables DNS client on the router.

Exit [C] [M] Returns to the previous prompt level.

List [C] [M] Displays your DNS client configuration.

Revert [M] Restores the configuration saved in permanent memory.

Save [M] Saves changes you make at the DNS Client> prompt to permanent memory.

Set [C] [M] Sets the DNS client timeout and the IP address of the DNS server.

Table 1 DNS Client Commands
Command	Function
Disable [C] [M]	Disables DNS client on the router.
Enable [C] [M]	Enables DNS client on the router.
Exit [C] [M]	Returns to the previous prompt level.
List [C] [M]	Displays your DNS client configuration.
Revert [M]	Restores the configuration saved in permanent memory.
Save [M]	Saves changes you make at the `DNS Client>` prompt to permanent memory.
Set [C] [M]	Sets the DNS client timeout and the IP address of the DNS server.

Disable [C] [M]

Disables DNS client on the router.

Syntax: disable

Example: disable

Enable [C] [M]

Enables DNS client on the router.

Syntax: enable

Example: enable

Exit [C] [M]

Returns to the previous prompt level.

Syntax: exit

Example: exit

List [C] [M]

Displays your DNS client configuration.

Syntax: list

Example: list

DNS Client Status:   Enabled
Timeout, in seconds: 15
DNS Server location: 128.185.1.1
                     128.185.20.10
Domain name(s):      nxnetworks

Revert [M]

If you make configuration changes at the DNS Client> prompt, this command restores the saved configuration.

Syntax: revert

Example: revert

Save [M]

Saves changes you make at the DNS Client> prompt to permanent memory so they are still present after you restart the router.

Syntax: save

Example: save

Set [C] [M]

Sets the DNS client timeout and the IP address of the DNS server.

Syntax: set

timeout = #-of-seconds
server += IP address
server -= IP address
domain_name +=
domain_name -=

timeout

Sets DNS client timeout timer, in seconds. If the DNS server does not respond to DNS client queries within this timeout period, the DNS client stops sending requests to the server. If you have multiple servers configured, DNS sends queries to the next server in your list.

The default setting is 15 seconds. The range is 1 to 3600 seconds.

Example: set timeout = 600

server

Sets the IP address of the DNS server to use for DNS lookups. The software directs DNS queries to the DNS server in your configuration that most recently responded to a DNS query. If that server does not respond or cannot resolve a host name, the DNS software sends queries to the next server in your configuration.

Example: set server = 192.168.3.3

Enter . . . To . . .

+= IP address Add a DNS server to the DNS configuration. The new server goes to the end of the list.

-= IP address Detach a server from an interface.

Enter . . .	To . . .
+= IP address	Add a DNS server to the DNS configuration. The new server goes to the end of the list.
-= IP address	Detach a server from an interface.

domain_name

Optionally, you can add domain names to your DNS configuration. If a DNS server cannot resolve an IP address using just the host name, the router appends a domain name to the host name and resends the DNS query to the host. If the server still cannot resolve the name, the router appends the next domain name in your configuration to the host name and sends another query to the server. It repeats this process until the server successfully resolves a host name, or the software has sent a query with each domain name in your configuration appended to the host name.

Enter . . . To . . .

+= domain_name Add a domain name to the DNS configuration. The new domain name goes to the end of the list.

-= domain_name Detach domain name from an interface.

Enter . . .	To . . .
+= domain_name	Add a domain name to the DNS configuration. The new domain name goes to the end of the list.
-= domain_name	Detach domain name from an interface.