Description of the DHCPv6 option to support the connection-specific suffix search list for IPv6 in Windows Vista Service Pack 1

The Dynamic Host Configuration Protocol for Internet Protocol version 6 (DHCPv6) option to support the connection-specific suffix search list for IPv6 is included in Windows Vista Service Pack 1 (SP1). The connection-specific suffix search list for IPv6 provides a reliable, flexible, and seamless name resolution experience in environments that consist of many domains.

In IPv4, Group Policy is used to enforce a suffix search list for all adapters. However, a suffix search list cannot be configured for each network adapter in IPv4. The DHCPv6 option to support a connection-specific suffix search list for IPv6 enables dynamic configuration of a suffix search list by using a DHCPv6 server. This option is also designed to fine-tune the configuration for each network adapter.

The Internet Engineering Task Force (IETF) standards body added the DHCPv6 option to support the connection-specific suffix search list for IPv6. The IETF standard is documented in Request for Comments (RFC) 3646. To view RFC 3646, visit the following RFC Web site: The DHCPv6 option to support the connection-specific suffix search list for IPv6 can be used. This option autoconfigures client computers so that they use a set of suffixes to complete Domain Name System (DNS) searches. The client computers do not have to be joined to a domain.

This DHCPv6 option is useful in environments that use single-label name resolution. Single-label name resolution refers to a scenario in which only the first label, the computer's host name, is used to resolve a name. Typically, single-label name resolution scenarios use suffixes that are configured on a client computer. The suffixes are appended to the single-label name on the client computer before name resolution is completed by using a DNS server. If a computer is not joined to a domain, the computer cannot be configured to automatically obtain a list of suffixes to use for DNS name resolution. This may be an issue in virtual private network (VPN) scenarios in which a client computer tries to connect to a corporate resource. In this scenario, a suffix search list is more suitable than a single connection-specific suffix because large organizations typically use many domains and forests.

If Windows Internet Name Service (WINS) and IPv4 are available, WINS can be used to resolve single-label name queries across a deployment. However, WINS will eventually become a legacy component, and it will be removed from future products. The connection-specific suffix search list for IPv6 lets a DHCPv6 server autoconfigure client computers whether they are joined to a domain or not. To do this, the DHCPv6 server uses a set of suffixes to complete DNS searches, especially searches for single-label names. A suffix search list works better than a single connection-specific suffix.

Use the connection-specific suffix search list to resolve connection issues

Single-label name resolution may not be completed as expected when the following configurations are used:

• Only IPv6 is used on the network, and WINS is not used.
• IPv4 and IPv6 are used on the network, and WINS is not used.
• Only IPv4 is used on the network, and WINS is not used.

For example, single-label name resolution may not finish in the following scenarios:

• Home computers use a VPN connection to connect to a corporate network.
• Guest portable computers use a corporate network to access resources. For example, these portable computers may belong to a vendor or to a consultant.
• Computers are not joined to the domain. However, they are on the network.

The connection-specific suffix search list is used to avoid issues that may occur in these scenarios. By default, only single-label names are appended with suffixes if the single-label names are not qualified.

When a computer that is running the DNS Client is restarted, the DNS Client performs a routine check for configuration changes. (These checks occur every 15 minutes.) If the DNS Client detects a configuration change, each network adapter is assigned suffixes as follows:

• If the local connection-specific search list is defined, it is assigned as the connection-specific search list. Generally, the local configuration that is manually set on a computer always overrides the configuration that is provided by DHCP.
• If the local connection-specific search list is not defined and if the DHCP connection-specific search list is defined, the DHCP connection-specific search list is assigned as the connection-specific search list.
• If the local connection-specific search list is not defined and if the DHCP connection-specific search list is not defined, a connection-specific domain suffix search list is constructed.

If any duplicate Primary Domain Name (PDN) suffixes are listed in the suffix search list, the duplicates are removed. However, if devolved (partial) suffixes that are derived from the PDN are listed in the suffix search list, these suffixes are not removed. For example, the PDN may be City.Corp.Company_Name.com. If the suffix search list contains city.corp.Company_Name.com, it is removed. However, if Corp.Company_Name.com is listed, it is not removed.

When a name is queried, the following process occurs.

Note Group Policy settings take precedence over local, manually configured suffixes or over DHCP-configured suffixes.

1. If a Group Policy suffix search list is defined for all adapters, the following process occurs:
   1. The PDN is appended to the name.
   2. The suffixes in the Group Policy suffix search list are appended in the order in which they are listed.
   3. A devolved PDN is appended to the name.
2. If a Group Policy suffix search list is not defined for all adapters, the PDN is appended to the name for all adapters.
3. If a connection-specific suffix search list is defined for an adapter, the suffixes are appended to the adapter name in the order in which they are listed.
4. If the connection-specific suffix search list is not defined, the connection-specific domain suffix is appended to the adapter name.
5. The devolved suffixes that are derived from the PDN for all adapters are appended to the adapter names.