Configuring DNS

Topics on this page: Assigning DNS Addresses | Configuring DNS Relay

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Domain Name System (DNS) servers map the user-friendly domain names that users type into their Web browsers (e.g., "yahoo.com") to the equivalent numerical IP addresses that are used for Internet routing. 

When a PC user types a domain name into a browser, the PC must first send a request to a DNS server to obtain the equivalent IP address. The DNS server will attempt to look up the domain name in its own database, and will communicate with higher-level DNS servers when the name cannot be found locally. When the address is found, it is sent back to the requesting PC and is referenced in IP packets for the remainder of the communication.

Assigning DNS Addresses to PCs

Multiple DNS addresses are useful to provide alternatives when one of the servers is down or is encountering heavy traffic. ISPs typically provide primary and secondary DNS addresses, and may provide additional addresses. Your LAN PCs learn these DNS addresses in one of the following ways:

  • Statically: If your ISP provides you with their DNS server addresses, you can assign the addresses to each PC by modifying the PCs' IP properties.
  • Dynamically from a DHCP pool: You can configure the DHCP Server feature on the ADSL/Ethernet router and create an address pool that specify the DNS addresses to be distributed to the PCs.

In either case, you can specify the actual addresses of the ISP's DNS servers (on the PC or in the DHCP pool), or you can specify the address of the LAN interface on the ADSL/Ethernet router (e.g., 192.168.1.1). When you specify the LAN port IP address, the device performs DNS relay, as described in the following section.

If you specify the actual DNS addresses on the PCs or in the DHCP pool, the DNS relay feature is not used.

 

Configuring DNS Relay

When you specify the device's LAN interface IP address as the DNS address, then the ADSL/Ethernet automatically performs DNS relay; i.e., because the device itself is not a DNS server, it forwards domain name lookup requests that it receives from LAN computers to a DNS server at the ISP. It then relays the DNS server's response to the PC. 

When performing DNS relay, the device must maintain the IP addresses of the DNS servers it contacts. It can learn these addresses in either or both of the following ways:

  • Learned through PPP: If the device uses a PPP connection to the ISP, the primary and secondary DNS addresses can be learned via the PPP protocol. To use this method, the "Use DNS" checkbox must be selected in the PPP interface properties. (You cannot change this property by modifying an existing PPP interface; you must delete the interface and recreate it with the new setting.)
    Using this option is advantageous in that you will not need to reconfigure the PCs or the ADSL/Ethernet router if the ISP changes their DNS addresses.
  • Configured on the ADSL/Ethernet router: You can use the device's DNS feature to specify the ISP's DNS addresses. If the device also uses a PPP interface with the "Use DNS" property enabled, then these configured addresses can be used in addition to the two addresses learned through PPP. If "Use DNS" is not enabled, or if a protocol other than PPP is used (such as EoA), then these configured addresses will be used as the primary and secondary DNS addresses. 

Follow these steps to configure DNS relay:

  1. Configure the LAN PCs to use the ADSL/Ethernet router's LAN IP address as their DNS server address using either of the following methods:
    • Assigning the LAN IP address statically to each PC.
    • Inputting the LAN IP address or the address 0.0.0.0 as the DNS address in a DHCP server pool used by the PCs.
  1. If using a PPP connection to the ISP, configure it to "Use DNS" so that the DNS server addresses it learns are used for DNS relay. 

    --OR--
    If not using a PPP connection (or if you want to specify DNS addresses in addition to those learned through PPP), configure the DNS addresses on the ADSL/Ethernet router as follows:
a.  Type the IP address of the DNS server in 
     an empty row and click Add. You can enter up to 
     two addresses.
b.  Click the DNS Relay Poll Status check box if you
     want the software to send regular test messages to
     the DNS servers to ensure that they remain up. 
     If none of the specified DNS servers respond
     (including any acquired by PPP, which do not
     display in the table), then an alert will display in
     the System Log window.
    
     You can specify the interval in minutes between
     each DNS poll message in the DNS Relay Poll
     Timeout text box.
c.  Click the Enable radio button, and then click
     Save Changes.

NOTE: If want your changes to be permanent, be sure to Write Settings to Flash.

DNS addresses that are assigned to LAN PCs prior to enabling DNS relay will remain in effect until the PC is rebooted. DNS relay will only take effect when a PC's DNS address is the LAN IP address.

Similarly, if after enabling DNS relay, you specify a DNS address (other than the LAN IP address) in a DHCP pool or statically on a PC, then that address will be used instead of the DNS relay address.