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Understanding Recursive DNS Servers for IPv6

To access any location on the Internet, the domain name system (DNS) server plays a pivotal role in resolving the domain name into its associated IP address. The DNS resolution service can also be provided by the DHCP server. The routing protocol process (rpd) of routers generates router advertisements to facilitate IPv6 hosts in autoconfiguration and in learning network information. For IPv6 stateless autoconfiguration, DNS configuration is provided by router advertisements. The router advertisement-based DNS configuration is useful in networks where an IPv6 host’s address is autoconfigured through an IPv6 stateless address and where there is no existing DHCPv6 infrastructure.

Depending on their configuration, DNS servers can be classified into the following types:

  • Recursive domain name system

  • Nonrecursive domain name system

DNS servers can resolve either recursive or nonrecursive queries. For a recursive query by a DNS client, the DNS server returns either the IP address associated with the domain name or an error. A recursive query does not return a referral. For a nonrecursive query, the DNS server returns the IP address of the domain name or an error or a referral to another DNS server which might have the resolution of the query.

For IPv6 hosts, a maximum of three recursive DNS server addresses can be configured along with their respective lifetimes. The default value of the lifetime of the configured recursive DNS server addresses is 1800 seconds. The configured IPv6 host uses the specified recursive DNS server address for DNS resolution where the IPv6 host’s address is autoconfigured through an IPv6 stateless address and where there is no DHCPv6 infrastructure available.


The recursive DNS server configuration is included in the router advertisement packet, which is a part of the Neighbor Discovery Protocol (NDP). In general, in an unsecured deployment scenario, an attacker could send a router advertisement with a fraudulent recursive DNS server address, misleading the IPv6 host into contacting an unintended DNS server for DNS name resolution. These attacks are similar to neighbor discovery attacks and attacks against unauthenticated DHCP. We recommend using the Secure Neighbor Discovery (SEND) protocol as a security mechanism for neighbor discovery to allow all the neighbor discovery options including the recursive DNS server options to be automatically included in the signatures.

For more information about configuring the SEND protocol, see