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DNS Name Server Address Overview
When a client attempts to access a domain—for example, www.example.com—a request is sent to a Domain Name System (DNS) name server. The name server stores information that correlates domain names with IP addresses; the IP address is used to reach the requested domain. In response to the client request, the name server looks up the IP address for the domain—192.0.43.10 for www.example.com—and returns it to the client.
In your network configuration, you must configure the address of one or more name servers locally on the router or on your RADIUS server. The local configuration supports the following subscriber types:
- DHCPv4 or DHCPv6
- IP over Ethernet (VLAN)
- Terminated PPPoE (IPv4 or IPv6)
- Tunneled PPPoE (IPv4 or IPv6)
You can configure the name server addresses at different levels of granularity: globally (per routing instance), per access profile, or, for DHCP only, per address pool. You can configure more than one name server in a routing instance or access profile by repeating the statement for each address.
Because you can configure name server addresses at more than one level, the address returned to the client is determined by the order of preference among the levels. The preference depends on the client type.
- For DHCP subscribers, the preference in descending order
is
RADIUS > DHCP address pool > access profile > global
- For non-DHCP subscribers, the preference in descending
order is
RADIUS > access profile > global
According to the preference order, a name server address configured in RADIUS is preferred by all subscriber types over all other configuration levels. For all subscriber types, the global name server address is used only when no other name server addresses are configured. When a name server address is configured only in a DHCP address pool, then no address is available to non-DHCP subscribers.
When you configure multiple addresses for a name server, the order in which you configure them determines the preference within that configuration. The preference according to configuration level supersedes this ordering.
For IPv4 name server addresses, you can use either of two statements to configure the address. Addresses configured with the domain-name-server-inet statement take precedence over addresses configured with the domain-name-server statement.
There is no restriction on the number of DNS name server addresses that you can configure. For DHCP subscribers, all the addresses are sent in DHCP messages. However, only two addresses—determined by preference order—are sent to PPP subscribers.
All changes in these locally configured DNS name servers affect only new subscribers that subsequently log in. Existing subscribers are not affected by the changes.
