In system DNS, you define the list of name servers that the vRouter can use to resolve host names to IP addresses. This list is created by using the system name-server command. (The system name-server command is described in Ciena Vyatta Network OS Basic System Configuration Guide; for your convenience, an example of system DNS is provided in Configuring access to a name server.)

Originally, DNS mappings were statically specified in “zone files,” which were periodically loaded onto DNS servers. These zone files worked reasonably well at a time when most hosts were configured with static IP addresses. However, since the 1990s, many network endpoints have been assigned IP addresses using dynamic protocols such as DHCP. Until 1997, devices with DHCP-assigned IP addresses essentially could not participate in the DNS system.

In 1997, the Internet Engineering Task Force (IETF) published RFC 2136, Dynamic Updates in the Domain Name System , describing the dynamic DNS update protocol. Dynamic DNS (DDNS) provides a mechanism for DNS entries to be established and removed dynamically. Devices using dynamic DNS can notify a domain name server in real time of changes to host name, IP address, or other DNS-related information.

This feature is particularly useful for systems in which a dynamic IP address is provided by the ISP. Whenever the IP address changes, the vRouter updates a DDNS service provider with the change. The DDNS provider is responsible for propagating this change to other DNS servers. The vRouter supports a number of DDNS providers.

In many environments that use consumer-level ISP connections, the ISP both assigns the client router with its IP address and notifies the client router of the DNS server to use. In many cases, the IP address of the DNS server itself is assigned through DHCP and changes periodically; the ISP notifies the client router of the change in DNS server IP address through periodic updates. This makes it problematic to statically configure a DNS server IP address on the DHCP server of the client router for its LAN clients.

In cases like these, the vRouter can use DNS forwarding (also called DNS relay) to maintain connectivity between hosts on its network and the DNS server of the ISP.

When DNS forwarding is used, the client router offers its own client-side IP address (which is static) as the DNS server address to the hosts on its network, so that all client DNS requests are made to the client-side address of the client router. When DNS requests are made, the client router forwards them to the ISP DNS server; answers are directed back to the client router and forwarded through to the client hosts. If the ISP changes the address of its DNS server, the client router simply records the new address of the server. The server address remains unchanged from the perspective of the LAN clients.

Another advantage to DNS forwarding is that DNS requests are cached in the vRouter (until either the time-to-live value in the DNS record expires or the cache fills). Subsequent requests for a cached entry are responded to locally, with a corresponding reduction in WAN traffic.