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SRV Resource Records

When a Windows 2000–based domain controller starts up, the Net Logon service uses dynamic updates to register SRV resource records in the DNS database, as described in "A DNS RR for specifying the location of services (DNS SRV)." For more information about this draft, see the Internet Engineering Task Force (IETF) link on the Web Resources page at http://windows.microsoft.com/windows2000/reskit/webresources . Follow the links to Internet Drafts, and then use a keyword search.

The SRV record is used to map the name of a service (in this case, the LDAP service) to the DNS computer name of a server that offers that service. In a Windows 2000 network, an LDAP resource record locates a domain controller.

A workstation that is logging on to a Windows 2000 domain queries DNS for SRV records in the general form:

_Service ._ Protocol . DnsDomainName

Active Directory servers offer the LDAP service over the TCP protocol; therefore, clients find an LDAP server by querying DNS for a record of the form:

_ldap._tcp. DnsDomainName

note-icon Note

The service and protocol strings require an underscore ( _ ) prefix to prevent potential collisions with existing names in the namespace

_msdcs Subdomain

There are possible implementations of LDAP servers other than Windows 2000–based domain controllers. There are also possible implementations of LDAP directory services that employ Global Catalog servers but are not servers that are running Windows 2000. To facilitate locating Windows 2000–based domain controllers, in addition to the standard _ Service ._ Protocol . DnsDomainName format, the Net Logon service registers SRV records that identify the well-known server-type pseudonyms "dc" (domain controller), "gc" (Global Catalog), "pdc" (primary domain controller, and "domains" (globally unique identifier, or GUID) as prefixes in the _msdcs subdomain. This Microsoft-specific subdomain allows location of domain controllers that have Windows 2000–specific roles in the domain or forest, as well as the location by GUID when a domain has been renamed. To accommodate locating domain controllers by server type or by GUID (abbreviated "dctype"), Windows 2000–based domain controllers register SRV records in the following form:

_ Service ._ Protocol . DcType ._msdcs. DnsDomainName

The addition of the _msdcs subdomain means that two sets of DNS names can be used to find an LDAP server: DnsDomainName is used to find an LDAP server or Kerberos server that is running TCP (or, in the case of a Kerberos server, either TCP or the User Datagram Protocol [UDP]), and the subdomain _msdcs. DnsDomainName is used to find an LDAP server that is running TCP and also functioning in a particular Windows 2000 role. The name "_msdcs" is reserved for locating domain controllers. The single keyword "_msdcs" was chosen to avoid cluttering the DNS namespace unnecessarily. Other constant, well-known names (pdc, dc, and gc) were kept short to avoid exceeding the maximum length of DnsDomainNam e.

SRV Records Registered by Net Logon

The list that follows provides the definitions of the names associated with registered SRV records. It also describes the lookup criteria supported by each record and the checks performed by Net Logon as each record is registered. Text in bold type denotes constant record components; text in italic type denotes variable names.

In the descriptions of registered SRV records, DnsDomainName refers to the DNS domain name that is used during creation of the domain controller when the domain tree is joined or created (that is, while the computer is running the Active Directory Installation Wizard). DnsForestName refers to the DNS domain name of the forest root domain.

The following is a list of the owner names of the SRV records that are registered by Net Logon. An owner name is the name of the DNS node to which the resource record pertains.

_ldap._tcp. DnsDomainName .

Allows a client to locate a server that is running the LDAP service in the domain named by DnsDomainName . The server is not necessarily a domain controller — that is, the only assumption that can be made about the server is that it supports the LDAP application programming interface (API). All Windows 2000 Server–based domain controllers register this SRV record (for example, _ldap._tcp.reskit.com.).

_ldap._tcp. SiteName ._sites. DnsDomainName .

Allows a client to locate a server that is running the LDAP service in the domain named in DnsDomainName in the site named by SiteName . SiteName is the relative distinguished name of the site object that is stored in the Configuration container in Active Directory. The server is not necessarily a domain controller. All Windows 2000 Server–based domain controllers register this SRV record (for example, _ldap._tcp.charlotte._sites.reskit.com.).

_ldap._tcp.dc._msdcs. DnsDomainName .

Allows a client to locate a domain controller (dc) of the domain named by DnsDomainName . All Windows 2000 Server–based domain controllers register this SRV record.

_ldap._tcp. SiteName ._sites.dc._msdcs. DnsDomainName .

Allows a client to locate a domain controller for the domain named by DnsDomainName and in the site named by SiteName . All Windows 2000 Server–based domain controllers register this SRV record.

_ldap._tcp.pdc._msdcs. DnsDomainName .

Allows a client to locate the server that is acting as the primary domain controller (also known as a "PDC") in the mixed-mode domain named in DnsDomainName . Only the PDC emulator master of the domain (the Windows 2000–based domain controller that advertises itself as the primary domain controller to computers that need a primary domain controller) registers this SRV record.

_ldap._tcp.gc._msdcs. DnsForestName .

Allows a client to locate a Global Catalog (gc) server for this forest. Only domain controllers that are functioning as Global Catalog servers for the forest named in DnsForestName register this SRV record (for example, _ldap._tcp.gc._msdcs.reskit.com.).

_ldap._tcp. SiteName ._sites.gc._msdcs. DnsForestName .

Allows a client to locate a Global Catalog (gc) server for this forest in the site named in SiteName . Only domain controllers that are serving as Global Catalog servers for the forest named in DnsForestName register this SRV record (for example, _ldap._tcp.charlotte._sites.gc._msdcs.reskit.com.).

_ gc._tcp. DnsForestName .

Allows a client to locate a Global Catalog (gc) server for this domain. The server is not necessarily a domain controller. Only a server that is running the LDAP service and functioning as the Global Catalog server for the forest named in DnsForestName registers this SRV record (for example, _gc._tcp.reskit.com.).

note-icon Note

In Windows 2000, a Global Catalog server is a domain controller. Other non-Windows 2000 implementations of directory services can also register servers as Global Catalog servers.

_ gc._tcp. SiteName ._sites. DnsForestName .

Allows a client to locate a Global Catalog (gc) server for this forest in the site named in SiteName . The server is not necessarily a domain controller. Only a server that is running the LDAP service and functioning as the Global Catalog server for the forest named in DnsForestName registers this SRV record (for example, _gc._tcp.charlotte._sites.reskit.com.).

_ldap._tcp. DomainGuid .domains._msdcs. DnsForestName .

Allows a client to locate a domain controller in a domain on the basis of its GUID. A GUID is a 128-bit number that is automatically generated for referencing objects in Active Directory — in this case, the domain object. This operation is expected to be infrequent; it occurs only when the DnsDomainName of the domain has changed, the DnsForestName is known, and DnsForestName has not also been renamed (for example, _ldap._tcp.4f904480-7c78-11cf-b057-00aa006b4f8f.domains._msdcs.reskit.com.). All domain controllers register this SRV record.

_kerberos._tcp. DnsDomainName .

Allows a client to locate a server that is running the Kerberos KDC service for the domain that is named in DnsDomainName . The server is not necessarily a domain controller. All Windows 2000 Server–based domain controllers that are running an RFC 1510–compliant Kerberos KDC service register this SRV record.

_kerberos._udp. DnsDomainName .

Same as _ kerberos ._ tcp. DnsDomainName , except that UDP is implied.

_kerberos._tcp. SiteName ._sites. DnsDomainName .

Allows a client to locate a server that is running the Kerberos KDC service for the domain that is named in DnsDomainName and is also in the site named in SiteName . The server is not necessarily a domain controller. All Windows 2000 Server–based domain controllers that are running an RFC 1510–compliant Kerberos KDC service register this SRV record.

_kerberos._tcp.dc._msdcs. DnsDomainName .

Allows a client to locate a domain controller that is running the Windows 2000 implementation of the Kerberos KDC service for the domain named in DnsDomainName . All Windows 2000 Server–based domain controllers that are running the KDC service (that is, that implement a public key extension to the Kerberos v5 protocol Authentication Service Exchange subprotocol) register this SRV record.

_kerberos.tcp. SiteName ._sites.dc._msdcs. DnsDomainName .

Allows a client to locate a domain controller that is running the Windows 2000 implementation of the Kerberos KDC service for the domain that is named in DnsDomainName and that is also in the site named in SiteName . All Windows 2000 Server–based domain controllers that are running the KDC service (that is, that implement a public key extension to the Kerberos protocol Authentication Service Exchange subprotocol) register this SRV record.

_kpasswd._tcp . DnsDomainName .

Allows a client to locate a Kerberos Password Change server for the domain. All servers that provide the Kerberos Password Change service (which includes all Windows 2000–based domain controllers) register this name. This server at least conforms to "Kerberos Change Password Protocol." (For more information about this draft, see the Microsoft Platform SDK link on the Web Resources page at http://windows.microsoft.com/windows2000/reskit/webresources. Use a keyword search to locate the draft.) The server is not necessarily a domain controller. All Windows 2000 Server–based domain controllers that are running an RFC 1510–compliant Kerberos KDC service register this SRV record.

_kpasswd._udp . DnsDomainName .

Same as _ kpasswd ._ tcp. DnsDomainName , except that UDP is implied.

If multiple domain controllers have the same criteria, multiple records exist with the same owner name. A client that is looking for a domain controller with specific criteria would receive all the applicable records from the DNS server. The client would pick one of the returned records to select a domain controller, as described in "A DNS RR for specifying the location of services (DNS SRV)." For more information about this draft, see the Internet Engineering Task Force (IETF) link on the Web Resources page at http://windows.microsoft.com/windows2000/reskit/webresources . Follow the links to Internet Drafts, and then use a keyword search.

For information about the Kerberos v5 authentication protocol and Kerberos subprotocol extensions, see "Authentication" in this book.

Host Records for Non-SRV-Aware Clients

Net Logon registers the following DNS A records for the use of LDAP clients that do not support DNS SRV records (that is, that are "non-SRV-aware"). The Locator does not use these records.

The following owner names of A (host) records are registered by Net Logon:

DnsDomainName .

Allows a non-SRV-aware client to locate any domain controller in the domain by looking up an A record. A name in this form is returned to the LDAP client through an LDAP referral. (For more information about LDAP referrals, see "LDAP Referrals" later in this chapter.) A non-SRV-aware client looks up the name; an SRV-aware client looks up the appropriate SRV resource record.

gc._msdcs. DnsForestName .

Allows a non-SRV-aware client to locate any Global Catalog server in the forest by looking up an A record. A name in this form is returned to the LDAP client through an LDAP referral. A non-SRV-aware client looks up this name; an SRV-aware client looks up the appropriate SRV resource record.

Net Logon also registers a DNS CNAME (alias) record for use by Active Directory replication The Locator does not use this record.

The owner name of the CNAME record is:

DsaGuid ._msdcs. DnsForestName .

Allows a client to locate any domain controller in the forest by looking up an A record. The only information that is known about the domain controller is the GUID of the directory system agent (also known as the "DSA") object for the domain controller and the name of the forest in which the domain controller is located. This record is used to facilitate renaming a domain controller.

Other SRV Record Content

The following information is also included in an SRV record:

Priority    The priority of the server. Clients attempt to contact the server with the lowest priority.

Weight    A load-balancing mechanism that is used when selecting a target host from those that have the same priority. Clients randomly choose SRV records that specify target hosts to be contacted, with probability proportional to the weight

Port Number    The port where the server is listening for this service.

Target    The fully qualified domain name of the host computer.

The following example illustrates the combined information that is contained in A resource records and SRV resource records. A domain controller named Phoenix in the domain reskit.com has an IP address of 157.55.81.157. It registers the following A records and SRV records with DNS:

phoenix.reskit.com A 157.55.81.157

_ldap._tcp.reskit.com SRV 0 0 389 phoenix.reskit.com

_kerberos._tcp.reskit.com SRV 0 0 88 phoenix.reskit.com

_ldap._tcp.dc._msdcs.reskit.com SRV 0 0 389 phoenix.reskit.com

_kerberos._tcp.dc._msdcs.reskit.com SRV 0 0 88 phoenix.reskit.com.

When the appropriate SRV records and A records are in place, a DNS lookup of _ldap._tcp.dc._msdcs.reskit.com returns the names and addresses of all domain controllers in the domain.

For more information about A records, SRV records, DNS, and dynamic updates, see "Introduction to DNS" and "Windows 2000 DNS" in the TCP/IP Core Networking Guide .

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