DSL-500G ADSL Router User’s Guide
DNS
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.
Figure 26. DNS Configuration
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 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 port on the Router (e.g., 10.1.1.1). When you specify the LAN port IP address, the device performs DNS relay.
Configuring DNS Relay
When you specify the device's LAN port IP address as the DNS address, then the Router 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 provides the advantage that you will not need to reconfigure the PCs or the Router if the ISP changes their DNS addresses.
•Configured on the 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 will 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.
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