DNS Commands

Domain Name System (DNS) is a hierarchical distributed naming system used to translate resource names to IP addresses. DNS makes it possible to assign domain names to groups of Internet resources and users in a meaningful way, independent of each entity's physical location.

RCP100 implements the standard DNS facilities necessary to connect to Internet. It also features a DNS proxy. The proxy forwards the requests to a configured DNS server and maintains a cache entry for each resolved DNS query. The cached entries are used to speed up future queries. This offloads remote DNS servers and reduces response time for DNS lookups.

RCP proxy forwarding DNS requests and responses
RCP proxy forwarding DNS requests and responses.


DNS lookups speed up with cached DNS entries
DNS lookups speed up with cached DNS entries.


Each cache entry has associated a time-to-live (TTL) timer. RCP100 will remove the entry once the timer expires. The maximum value for TTL is hard-coded to 15 minutes. RCP100 also allows to manually add and remove entries from the cache.

Another typical usage for DNS proxy cache is to implement what is generally called a "split-horizon DNS". In a split DNS configuration, resources such as web servers are available under different IP addresses for the people on internal and external networks. This is mainly the case with servers placed in DMZ. The DNS global entry for such a server will report an external address unreachable from the internal network. The problem is solved by adding a static host entry in DNS proxy cache. Static proxy entries can also be used to assign name to hosts located on internal networks.


Table Of Contents

clear ip host
ip dns server
hostname
ip domain-name
ip host
ip name-server
show ip dns statistics
show ip host
Example

clear ip host

Clear all DNS cached entries.

clear ip host


ip dns server

Enable DNS proxy cache.

ip dns server [rate-limit <limit>]
no ip dns server

where:

rate-limit - optional rate limit of packets forwarded to servers

no form of the command disables the proxy.


hostname

Set router hostname.

hostname <name>
no hostname

no form of the command restores the default rcp.

Example:

rcp(config)#hostname lab1
lab1(config)#no hostname
rcp(config)#

Note: the hostname is reflected in the command prompt.


ip domain-name

Configure the default domain name RCP uses to resolve unqualified host names (names without a dotted-decimal domain suffix).

ip domain-name <name>
no ip domain-name

no form of the command removes the configuration.

Example:

rcp(config)#ip domain-name mycompany.com
rcp(config)#

ip host

Map a name to an IP address. If DNS proxy is enabled, an entry is added to proxy cache.

ip host <name> <IP address>
no ip host <name>

no form of the command removes the mapping.

Example:

rcp(config)#ip host www.mycompany.com 10.200.0.70
rcp(config)#

ip name-server

Configure an external DNS server to forward the queries. Two such servers can be configured.

ip name-server <IP address>
no ip name-server <IP address>

no form of the command removes the server.

Example:

rcp(config)#ip name-server 8.8.8.8
rcp(config)#

show ip dns statistics

Show DNS proxy cache statistics.

show ip dns statistics

Example:

rcp#show ip dns statistics 
DNS queries: 4759 packets
DNS answers: 4734 packets
DNS cached answers: 1266 packets
RR types:
	A: 509 packets
	CNAME: 1858 packets
	PTR: 3 packets
	MX: 0 packets
	AAAA (IPv6): 36 packets
Request queue 0
rcp#

show ip host

Show the proxy cache.

show ip host

Example:

rcp#show ip host 
Name                                            Address            TTL
mirror.us.leaseweb.net                          108.59.10.97       357
sourceforge.net                                 216.34.181.60      342
fonts.googleapis.com                            72.14.204.95       134
chi-10g-1-mirror.fastsoft.net                   208.78.26.135      150
mirror.us.as6453.net                            209.58.29.74       106
a.fsdn.com                                      184.50.243.172     12
html5center.sourceforge.net                     216.34.181.150     899
geek.net                                        216.34.181.202     899
www.google.com                                  173.194.73.103     213
themes.googleusercontent.com                    74.125.226.203     217
sourceforge.jp                                  202.221.179.21     899
twitter.com                                     199.59.150.7       23
rcp#

Example


Split-horizon DNS
Split-horizon DNS


RCP100 is connecting the office network (interface eth0), the server network (eth1) and the firewall (eth2).

interface ethernet eth0
  ip address 10.10.0.1/16
  ip mtu 1500
  no shutdown
!
interface ethernet eth1
  ip address 10.20.0.1/16
  ip mtu 1500
  no shutdown
!
interface ethernet eth2
  ip address 10.30.0.10/16
  ip mtu 1500
  no shutdown
!

The master DNS server is provided by the Internet service provider (8.8.8.8 in this example), and the company is known by the outside world as mycompany.com.

ip name-server 8.8.8.8
ip domain-name mycompany.com

Enabling DNS proxy cache allows the office users to access the company web server at 10.200.0.70 as www.mycompany.com, and the servers in the server room as srv1.mycompany.com and srv2.mycompany.com. Short names such as www, srv1, srv2 entered in the web browser will also be translated to the correct IP address.

ip dns server
ip host www.mycompany.com 10.200.0.70
ip host srv1.mycompany.com 10.20.0.50
ip host srv2.mycompany.com 10.20.0.51

The relevant DNS configuration is as follows:

rcp(config)#show configuration dns 
hostname rcp
ip name-server 8.8.8.8
ip domain-name mycompany.com
ip host www.mycompany.com 10.200.0.70
ip host srv1.mycompany.com 10.20.0.50
ip host srv2.mycompany.com 10.20.0.51
ip dns server
!
rcp(config)#