how dns support for active directory works
TRANSCRIPT
How DNS Support for Active Directory Works
Updated: March 28, 2003
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2, Windows Server 2008, Windows Server 2008 R2
How DNS Support for Active Directory Works
In this section
DNS Support for Active Directory Architecture
DNS Physical Structure in Support of Active Directory
DNS Support for Active Directory Processes and Interactions
Network Ports Used by DNS in Support of Active Directory
Related Information
Active Directory uses DNS as its domain controller location mechanism and leverages the namespace design of DNS in the design of Active Directory domain names. As a result, DNS is positioned within the discoverability and logical structure components of Active Directory technology components.
Typically, a Windows Server 2003 DNS namespace is deployed to mirror an Active Directory forest and domain infrastructure. In such a deployment, a partition of the DNS namespace is set aside for Active Directory, where a DNS domain name such as corp.contoso.com is used support the Active Directory forest root domain, and then subdomains of this name are created to suit additional Active Directory domains as needed.
DNS Support for Active Directory Architecture
Active Directory is dependent on DNS as a domain controller location mechanism and uses DNS domain naming conventions in the architecture of Active Directory domains. There are three components in the dependency of Active Directory on DNS:
Domain controller locator (Locator)
Active Directory domain names in DNS
Active Directory DNS objects
DNS Support for Active Directory Components
Component Description
Domain controller locator (Locator)
The Windows Server 2003 domain controller locator, implemented in the Net Logon service, enables a client to locate a domain controller. The component contains the DNS–compatible and the Windows NT
4.0–compatible locators that provide interoperability in a mixed Windows Server 2003– and Windows NT 4.0–based environment.
Active Directory domain names in DNS
Every Windows Server 2003 Active Directory domain has a DNS domain name (for example, contoso.com), and every Windows Server 2003–based computer has a DNS name (for example, win2kserver.contoso.com). Architecturally, domains and computers are represented both as objects in Active Directory and as nodes in DNS.Because DNS domains and Active Directory domains share identical domain names, it is easy to confuse their roles. The two namespaces, although typically sharing an identical domain structure, store different data and, therefore, manage different objects:
DNS stores zones and resource records, and Active Directory
stores domains and domain objects. Both systems use a
database to resolve names.
DNS resolves domain names and computer names to resource
records through requests received by DNS servers as DNS
queries to the DNS database.
Active Directory resolves domain object names to object records
through requests that are received by domain controllers either
as LDAP search requests or as modify requests to the Active
Directory database.
Thus, the Active Directory domain computer account object is in a different namespace from the DNS host record that represents the same computer in the DNS zone.
Active Directory DNS objects
When DNS data is stored in Active Directory, each DNS zone is an Active Directory container object (class dnsZone). The dnsZone object contains a DNS node object (class dnsNode) for every unique name within that zone. These unique names include the variations assigned to a specific host computer when it functions, for example, as a primary domain controller or as a global catalog server. The dnsNode object has a dnsRecord multivalue attribute that contains a value for every resource record that is associated with an object’s name.For more information about Active Directory DNS objects, see How DNS Works.
Active Directory and DNS Domain NamesActive Directory domains have two types of names: DNS names and NetBIOS names. In general, both names are visible to end users. The DNS names of Active Directory domains include two parts, a prefix and a suffix. The DNS prefix is the first label in the DNS name of the domain. The suffix is the name of the Active Directory forest root domain.
When a Windows NT 4.0 master user domain (MUD) is upgraded, the decision is made whether or not to use the current NetBIOS name of the domain as a DNS prefix. If the name is appropriate to represent the organizational structure of the enterprise and satisfies the
prefix naming rules in the table below, the name is typically kept. In this case, the NetBIOS name of the domain is the same as the DNS prefix of the domain.
Registered DNS Name Prefix Rules
Rule Explanation
Select a prefix that is not likely to become outdated.
Avoid names such as a business line or operating system that might change in the future. Geographical names are recommended.
Select a prefix that includes Internet standard characters only.
A-Z, a-z, 0-9, and (-), but not entirely numeric.
Include 15 characters or less in the prefix.
If you choose a prefix length of 15 characters or less, then the NetBIOS name is the same as the prefix.
If the current NetBIOS name of the domain is inappropriate to represent the organizational structure of the enterprise or if the current name fails to satisfy the prefix naming rules, a new prefix is used. In this case, the NetBIOS name of the domain will be different from the DNS prefix of the domain.
For each new Active Directory domain, a prefix that is appropriate for the organizational structure of the enterprise and that satisfies prefix naming rules is used. Typically, the NetBIOS name of the domain will be the same as the name of the DNS prefix.
The Active Directory forest root domain is also the name of the Active Directory forest. The forest root name is a DNS name that consists of a prefix and a suffix in the form of prefix.suffix. For example, an organization might have the forest root name corp.contoso.com. In this example, corp is the prefix and contoso.com is the suffix.
The suffix is selected from a list of existing DNS names on the network. For the prefix, a new name that has not been used on the network previously is selected. By attaching a new prefix to an existing suffix, a unique namespace is created. Creating a new namespace for Active Directory ensures that any existing DNS infrastructure does not need to be modified to accommodate Active Directory.
Typically, the DNS names that are used in the Active Directory namespace are registered with an Internet authority. Only registered names are guaranteed to be globally unique. If two organizations register the same DNS domain name, or if an organization merges with, acquires, or is acquired by another company that uses the same DNS name, then the two infrastructures cannot interact with one another.
Note
Using unregistered suffixes is not recommended. Using single label names, such
as .local, is not supported.
DNS Physical Structure in Support of Active Directory
The physical structure of Active Directory information in DNS is represented in DNS zones and resource records, which, in turn, are typically stored in Active Directory as Active Directory–integrated DNS zones. The DNS zones that support Active Directory domains can
also be stored in standard, file-based, DNS zones. In addition, the DNS dynamic update protocol is utilized by Active Directory in order to make the registration of domain controller DNS resource records automatic.
The following diagram illustrates the physical storage of Active Directory domain information in DNS zones hosted on domain controllers, and in standard DNS zones hosted on member servers.
DNS Physical Structure in Support of Active Directory
The following table describes the physical components in the Active Directory/DNS physical structure.
DNS Physical Structure in Support of Active Directory Components
Active Directory/DNS Physical Structure Component Description
Active Directory/DNS physical structure requirements
The DNS server used to support Active Directory must support the SRV resource record and, ideally, the DNS dynamic update protocol. If the DNS server does not support the DNS dynamic update protocol, the SRV resource records required to locate Active Directory domain controllers can be added to DNS manually.
_msdcs DNS subdomain
The Microsoft-specific subdomain enables location of domain controllers that have specific roles in the Active Directory domain or forest. Resource records for the DNS root domain of a new Active Directory forest are stored in a _msdcs zone
instead of a subdomain, and that zone is stored in the forest-wide application directory partition.
Domain controller SRV resource records registered in DNS
There are multiple DNS SRV resource records registered on behalf of domain controllers. A description of each resource record is below.
DNS application directory partitions
In Windows Server 2003, the Active Directory–integrated DNS zones that support Active Directory domains can be stored in Active Directory application directory partitions, which is a new feature of Windows Server 2003. By default, the DNS root domain of a new Active Directory forest is stored in the domain-wide application directory partition for the forest root domain.
DNS Support for Active Directory Physical Structure RequirementsIn order for a DNS server to be able to support Active Directory, the server is required to support the service (SRV) resource record type and the dynamic update protocol, as described in the RFC 2136. Active Directory uses DNS as the location mechanism for domain controllers, enabling computers on the network to obtain IP addresses of domain controllers. During the installation of Active Directory, the service (SRV) and address (A) resource records are dynamically registered in DNS. Both types of records are necessary for the functionality of the domain controller locator (Locator) mechanism.
To find domain controllers in a domain or forest, a client queries DNS for the SRV and A DNS resource records of the domain controller. The resource records provide the client with the names and IP addresses of the domain controllers. In this context, the SRV and A resource records are referred to as Locator DNS resource records.
When a domain controller is added to a forest, a DNS zone hosted on a DNS server is updated with the Locator DNS resource records for that domain controller. For this reason, the DNS zone must allow dynamic updates (RFC 2136), and the DNS server hosting that zone must support the SRV resource records (RFC 2782) to advertise the Active Directory directory service.
At the very least, the DNS server must support the SRV resource record; but the SRV resource records can be added to DNS manually. After installing Active Directory, these records can be found on the domain controller in the following location: systemroot\System32\Config\Netlogon.dns.
Note
The configuration of reverse lookup zones is not based on the Windows Server 2003
domain structure; instead, it is based on the range of IP addresses assigned to a
company. If a company is assigned B class IP addresses such as 172.56.X.Y., then a
reverse lookup zone of 56.172.in-addr.arpa. will be created. It might contain
delegations to some other domains such as 1.56.172.in-addr.arpa., 2.56.172.in-
addr.arpa., etc. It is also possible to configure classless reverse lookup zones as
described in the RFC Best Current Practice paper “Classless IN_ADDR.ARPA
delegation.”
_msdcs Subdomain
To facilitate locating Windows Server 2003–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 enables location of domain controllers that have Windows Server 2003–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 Server 2003-based domain controllers register SRV records in the following form:
_Service._Protocol.DcType. _msdcs. DnsDomainName
Note
There are also site–specific DNS resource records that use a slightly different format.
For more information about site discovery, see “Domain Controller Location in the
Closest Site” later in this document.
The subdomain _msdcs.DnsDomainName is used to find an LDAP server that is running TCP and also functioning in a particular Windows Server role. The name _msdcsis reserved for locating domain controllers and Kerberos servers. The single keyword _msdcswas 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 allowed for a DNS name.
Domain Controller SRV Resource RecordsWhen a Windows Server 2003–based domain controller starts up, the Net Logon service uses dynamic updates to register SRV and A resource records in the DNS database, as described in Internet Engineering Task Force (IETF) RFC 2782, “A DNS RR for specifying the location of services (DNS SRV).” Windows Server 2003 also uses secure dynamic update using the GSS-TSIG algorithm, as described in RFC 3645, “Generic Security Service Algorithm for Secret Key Transaction Authentication for DNS (GSS-TSIG).”
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 Server 2003 network, an LDAP resource record locates a domain controller.
When a workstation logs on to a Windows Server 2003 domain, it queries DNS for SRV records in this general form:
_Service ._ Protocol . DnsDomainName
Because Active Directory servers offer the LDAP service over the TCP protocol, clients find an LDAP server by querying DNS for a record of the form:
_ldap._tcp. DnsDomainName
Note
The service and protocol strings require an underscore (_) prefix to prevent potential
collisions with existing names in the namespace.
Active Directory site-specific resource records are also used during logon. For more information, see “Domain Controller Location in the Closest Site” later in this document.
SRV Records Registered by Net LogonThe list in the following table 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.
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, when the computer runs the Active Directory Installation Wizard). DnsForestName refers to the DNS domain name of the forest root domain.
An owner name is the name of the DNS node to which a resource record pertains. These resource records are used by domain controller Locator. The following table lists the owner names of the SRV records that are registered by Net Logon.
SRV Records That Are Registered by Net Logon
SRV Resource Record Description
_ldap._tcp. DnsDomainName . Enables a client to locate a server that is running the LDAP service in the domain named 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 Server 2003–based domain controllers register this SRV record (for example, _ldap._tcp.contoso.com.).
_ldap._tcp. SiteName . _sites. DnsDomainName .
Enables a client to locate a server that is running the LDAP service in the domain named DnsDomainName in the site named SiteName. SiteName is the relative distinguished name of the site object that is stored in the Configuration container in Active Directory. All Windows Server 2003–based domain controllers register this SRV record (for example, _ldap._tcp.charlotte._sites.contoso.com.).
_ldap._tcp.dc._msdcs. DnsDomainName .
Enables a client to locate a domain controller (dc) of the domain named DnsDomainName. All Windows Server 2003–based domain controllers register this SRV record.
_ldap._tcp. SiteName . _sites.dc._msdcs. DnsDomainName .
Enables a client to locate a domain controller for the domain named DnsDomainName and in the site named SiteName. All Windows Server 2003–based domain controllers register this SRV record.
_ldap._tcp.pdc._msdcs. DnsDomainName .
Enables a client to locate the server that is acting as the primary domain controller (PDC) in the mixed-mode domain named DnsDomainName.
Only the PDC emulator master of the domain (the Windows Server 2003–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 .
Enables a client to locate a global catalog (gc) server for this forest. Only domain controllers that are functioning as gc servers for the forest named in DnsForestName register this SRV record (for example, _ldap._tcp.gc._msdcs.contoso.com.).
_ldap._tcp. SiteName . _sites.gc._msdcs. DnsForestName .
Enables 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 gc servers for the forest named in DnsForestName register this SRV record (for example, _ldap._tcp.charlotte._sites.gc._msdcs.contoso.com.).
_gc._tcp.DnsForestName. Enables 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 GC server for the forest named DnsForestName registers this SRV record (for example, _gc._tcp.contoso.com.). In Windows Server 2003, a GC server is a domain controller. Other implementations of directory services (that are not Windows Server 2003 implementations) can also register servers as GC servers.
_gc._tcp.SiteName. _sites.DnsForestName.
Enables a client to locate a global catalog (gc) server for this forest in the site named SiteName. The server is not necessarily a domain controller. Only a server that is running the LDAP service and functioning as the GC server for the forest named DnsForestName registers this SRV record (for example, _gc._tcp.charlotte._sites.contoso.com.).
_ldap._tcp. DomainGuid . domains._msdcs. DnsForestName .
Enables 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.contoso.com.).All domain controllers register this SRV record.
_kerberos._tcp. DnsDomainName .
Enables 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 Server 2003–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 .
Enables a client to locate a server that is running the Kerberos KDC service for the domain that is named DnsDomainName and is also in the site named SiteName. The server is not necessarily a domain controller. All Windows Server 2003–based domain controllers that are running an RFC 1510–compliant Kerberos KDC service register this SRV record.
_kerberos._tcp.dc._msdcs. DnsDomainName .
Enables a client to locate a domain controller that is running the Windows Server 2003 implementation of the Kerberos KDC service for the domain named in DnsDomainName. All Windows Server 2003–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 .
Enables a client to locate a domain controller that is running the Windows Server 2003 implementation of the Kerberos KDC service for the domain that is named DnsDomainName and that is also in the site named SiteName. All Windows Server 2003–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.
_kpasswd._tcp.DnsDomainName. Enables 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 Server 2003–based domain controllers) register this name. This server must at least conform to the Kerberos Change Password Protocol. (For more information about this draft, see the Microsoft Platform SDK.) The server is not necessarily a domain controller. All Windows Server 2003–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.
In addition to the SRV records listed in the table above, Net Logon also registers a DNS alias (CNAME) record for use by Active Directory replication, in the format:
DsaGuid._msdcs.DnsForestName. The Locator does not use this record. This record enables 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 (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.
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 RFC 2782.
Other SRV Record ContentThe following information is also included in an SRV record.
SRV Record Field Description
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 algorithm by which clients interpret and select among SRV resource records is defined in RFC 2782, “A DNS RR for specifying the location of services (DNS SRV).”
Host Records for Non-SRV-Aware ClientsNet Logon registers the following DNS A records for the use of LDAP clients that do not support DNS SRV records (that is, clients that are non-SRV-aware). Locator does not use these records.
Host (A) Records Registered by Net Logon
Host (A) Resource Record Description
DnsDomainName . Enables 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. A non-SRV-aware client looks up the name; an SRV-aware client looks up the appropriate SRV resource record.
gc._msdcs. DnsForestName .
Enables 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.
Example of Registered Resource RecordsThe 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 contoso.com has an IP address of 157.55.81.157. It registers the following A records and SRV records with DNS:
Copy Code phoenix.contoso.com A 157.55.81.157_ldap._tcp.contoso.com SRV 0 0 389 phoenix.contoso.com_kerberos._tcp.contoso.com SRV 0 0 88 phoenix.contoso.com_ldap._tcp.dc._msdcs.contoso.com SRV 0 0 389 phoenix.contoso.com_kerberos._tcp.dc._msdcs.contoso.com SRV 0 0 88 phoenix.contoso.com.When the appropriate SRV records and A records are in place, a DNS query for _ldap._tcp.dc._msdcs.contoso.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 “DNS Protocol” and “DNS Physical Structure” in How DNS Works.
DNS Application Directory PartitionsDNS zones stored in Active Directory replicate to Active Directory domain controllers according to different replication scopes. In Windows 2000 Server, a DNS zone stored in Active Directory is replicated to all domain controllers in the Active Directory domain. Windows Server 2003 has added application directory partitions, which enable the DNS zone to be stored in different replication scopes. The following table describes all of the replication scopes available to a DNS zone stored in Active Directory.
Windows Server 2003 Active Directory Storage Options
Active Directory Storage Option Replication Scope
Domain partition Active Directory domain partition for each domain in the forest. DNS zones stored in this partition are replicated to all domain controllers in the domain. This is the only Active Directory storage option for DNS zones that are replicated to domain controllers running Windows 2000 Server.
Forest-wide DNS application directory partition
DNS application directory partition for the entire forest. DNS zones stored in this application directory partition are replicated to all DNS servers running on domain controllers in the forest. This DNS application directory partition is created when you install the DNS Server service on the first Windows Server 2003 domain controller in the forest.
Domain-wide DNS application directory partition
DNS application directory partition for each domain in the forest. DNS zones stored in this application directory partition are replicated to all DNS servers running on domain controllers in the domain. For the forest root domain, this DNS application directory
partition is created when you first install the DNS Server service on a Windows Server 2003 domain controller in the forest.For each new domain in the forest (child domain), this DNS application directory partition is created when you first install the DNS Server service on a Windows Server 2003 domain controller for the new domain.
Custom DNS application directory partition
DNS application directory partition for any domain controller that is enlisted in its replication scope. This type of DNS application directory partition does not exist by default and must be created. DNS zones stored in this application directory partition are replicated to all DNS servers running on domain controller that enlist in the partition.
As stated earlier, the Locator DNS resource records for an Active Directory domain are stored in the _msdcs.DnsDomainName subdomain for the Active Directory domain. In Windows Server 2003, when the DNS root domain of a new Active Directory forest is created on a Windows Server 2003 domain controller, a DNS zone is automatically created for the _msdcs.DnsForestName and stored in the forest-wide DNS application directory partition, which replicates to all Windows Server 2003 domain controllers in the forest running the Windows Server 2003 DNS Server service.
Note
DNS zones stored in application directory partitions cannot be accessed by Windows
2000 Server domain controllers.
DNS Support for Active Directory Processes and Interactions
When a Windows Server 2003 member server is promoted to an Active Directory domain controller by installing Active Directory, the Net Logon service registers the DNS resource records necessary for network hosts and services to be able to locate the domain controller on the network. When network hosts and services attempt to perform an operation (such as joining a domain, for example) that requires an Active Directory domain controller, the Locator mechanism is used to locate the domain controller through DNS. The following table describes the processes and interactions involved in the registration and location of domain controllers in DNS.
Active Directory and DNS Processes and Interactions
Process or Interaction Description
Domain controller DNS name registration
The Net Logon service registers DNS resource records on behalf of the Active Directory domain controller in the DNS zone with the same name as the Active Directory domain hosted by the domain controller.
DNS delegation, forwarders
DNS delegation resource records are created in the zone that is a parent of the zone supporting the Active Directory domain. The delegation enables the DNS name of the domain controller to be resolved downward from the root of the DNS hierarchy.DNS forwarders are another DNS feature that enable domain
controller location, and are commonly used for an Active Directory client in one domain to locate a domain controller in another domain.
DNS domain controller location
Network hosts and services use the DNS Locator mechanism to locate domain controllers in the Active Directory forest.
Domain Controller Name RegistrationEvery Windows Server 2003–based domain controller registers two types of names at startup:
A DNS domain name with the DNS service (for example, noam.contoso.com).
A NetBIOS name with Windows Internet Name Service (WINS) or another transport-
specific service (for example, noam).
When a user starts a computer and logs on to a domain, the computer must do one of two things:
If the name of the logon domain is a DNS name, the computer must query DNS to
find a domain controller with which to authenticate.
If the name of the logon domain is a NetBIOS name, the computer must send a
mailslot message to find a domain controller for the specified domain.
After the computer has found a domain controller, the information is cached so that a new query is not required for subsequent logon sessions.
DNS Domain Name RegistrationActive Directory supports dynamic registration of domain controller addresses in DNS. After Active Directory has been installed during domain controller creation, the Net Logon service dynamically creates records in the DNS database that are used to locate the server. Dynamic update (described in RFC 2136) is a recent addition to the DNS standard; this addition to the standard defines a protocol for dynamically updating a DNS server with new or changed resource record values. Before the advent of this new protocol, administrators had to manually create the records that are stored on DNS servers. The implementation of DNS server that is included with Windows Server 2003 supports dynamic updates, as does the Berkeley Internet Name Domain (BIND) version 8.x implementation of DNS.
Note
By default, the Windows Server 2003 DNS Server service running on a domain
controller is configured to accept secure dynamic update only.
Every Windows Server 2003–based domain controller dynamically registers SRV records in DNS. The SRV records enable servers to be located by service type (for example, LDAP) and protocol (for example, TCP). Because domain controllers are LDAP servers that communicate over TCP, SRV records can be used to find the DNS computer names of domain controllers. In addition to registering LDAP-specific SRV records, Net Logon also registers Kerberos v5 authentication protocol–specific SRV records to enable locating servers that run the Kerberos Key Distribution Center (KDC) service.
Every Windows Server 2003–based domain controller also dynamically registers a single host (A) resource record that contains the name of the domain (DnsDomainName) where the domain controller is and the IP address of the domain controller. The host (A) resource record makes it possible for clients that do not recognize service (SRV) resource records to locate a domain controller by means of a generic host lookup.
NetBIOS Domain Name RegistrationA domain controller registers its NetBIOS name (DomainName[1C]) by broadcasting or directing a NetBIOS name registration request to a NetBIOS name server, such as a WINS server. Registering the NetBIOS name makes it possible for Windows-based clients that are not DNS-enabled to find the domain controllers that are running Windows Server 2003, Windows 2000, Windows NT 4.0, or Windows NT 3.51. In this case, the client finds the domain controller by sending a Net Logon mailslot request that is based on the NetBIOS domain name.
Note
NetBIOS recognizes domain controllers by the [1C] registration.
DNS Delegation, ForwardersTo fully support an Active Directory domain, the DNS infrastructure must have the DNS delegations that are necessary to enable name resolution during domain controller location. When an Active Directory domain is created, a DNS delegation entry must exist in the DNS zone that is the parent of the zone supporting the Active Directory domain. The delegation enables the name of the domain controller hosting the domain to be resolved by any host or service in the DNS namespace. The delegation resource records must be added by the network administrator who administers the DNS server hosting the parent zone. Alternately, the parent zone could host the DNS resource records for the domain name and, in this case, the delegation is unnecessary.
For example, if there is a DNS zone supporting the domain corp.contoso.com, then a delegation for this name must exist in the parent zone contoso.com. When a DNS query for the name is sent to the root of the DNS namespace, delegations can be followed until the DNS server hosting the zone for corp.contoso.com is identified. Without this delegation, only those network hosts and services configured with the IP address of the DNS server hosting the zone for corp.contoso.com will be able to resolve its DNS name. All other network hosts and services will be unable to resolve the name, and the domain controller will not be available to them for such Active Directory operations as joining a domain, logging on to a domain, or searching Active Directory.
When the Active Directory domain name is specified in the Active Directory Installation Wizard, the wizard reads the delegation entry in DNS, prompts the user to install the DNS Server service locally and configures the corresponding DNS zone automatically. Also, if the computer is already configured with a preferred DNS server, the wizard will configure a forwarder on the DNS Server service with the IP address of the prior preferred DNS server.
Forwarders are also commonly configured on the DNS server hosting a zone in support of an Active Directory domain. Forwarders are used by a DNS server to forward queries for a domain name about which it has no local data. If an Active Directory client in one domain needs to access a resource in another Active Directory domain, it will need to locate a domain controller in that domain. If the DNS server used by that client is unable to locate a domain controller for the other domain using its local data, it can forward the client request to another DNS server, such as the DNS server that hosts the zone for the Active Directory forest root domain.
In summary, delegation enables name resolution in a descending direction from the root of the hierarchy, and forwarders enable name resolution in a ascending direction toward the root of the hierarchy.
Windows Server 2003 introduces an enhancement to forwarding called conditional forwarders. When you use conditional forwarding, you can configure your DNS servers to forward queries to different servers based on the domain name specified in the query. This eliminates steps in the standard forwarding chain and reduces network traffic. When conditional forwarding is applied, the DNS server hosting a zone in support of the one Active Directory domain can forward queries to DNS servers hosting zones in support of the Active Directory domain name specified in the client query.
Although root hints (resource records that list the DNS servers hosting the DNS root zone) can also be used to facilitate domain controller location in place of forwarders, DNS deployments commonly use forwarders for remote domain controller location to reduce the complexity of administering root hints for both internal and Internet resolution.
For more information about delegation, forwarders, and root hints, see “How DNS Works.”
Domain Controller LocatorThe domain controller locator (Locator) algorithm consists of two main parts:
Locator finds which domain controllers are registered with a DNS server.
Locator submits a DNS query to the DNS server to locate a domain controller in the
specified domain.
After this query is resolved, an LDAP User Datagram Protocol (UDP) lookup is sent to one or more of the domain controllers listed in the response to the DNS query to ensure their availability. Finally, the Net Logon service caches the discovered domain controller to aid in resolving future requests.
Domain Controller Locator ProcessEach Windows Server 2003–based domain controller registers its DNS domain name on the DNS server and registers its NetBIOS name by using a transport-specific mechanism (for example, WINS). Thus, a DNS client locates a domain controller by querying DNS, and a NetBIOS client locates a domain controller by querying the appropriate transport-specific name service. Because the code for the DNS-compatible Locator and the Windows NT 4.0–compatible Locator is shared, both DNS clients and NetBIOS clients are supported.
The process that the Locator follows can be summarized as follows:
1. On the client (the computer that is locating the domain controller), the Locator is
initiated as a remote procedure call (RPC) to the local Net Logon service. The Locator
API (DsGetDcName) is implemented by the Net Logon service.
2. The client collects the information that is needed to select a domain controller and
passes the information to the Net Logon service by using the DsGetDcName API.
3. The Net Logon service on the client uses the collected information to look up a
domain controller for the specified domain in one of two ways:
For a DNS name, Net Logon queries DNS by using the DNS-compatible Locator
— that is, DsGetDcName calls DnsQuery to read the SRV records and
A records from DNS after it appends an appropriate string to the front of the
domain name that specifies the SRV record.
For a single label name, Net Logon performs domain controller discovery by
using the Windows NT 4.0–compatible Locator — that is, by using the
transport-specific mechanism (for example, WINS).
Note
In Windows NT 4.0 and earlier, “discovery” is a process for locating a domain controllerfor authentication in either the primary domain or a trusted domain.
1. The Net Logon service sends a datagram to the discovered domain controllers that
register the name. For NetBIOS domain names, the datagram is implemented as a
mailslot message. For DNS domain names, the datagram is implemented as an LDAP
UDP search.
2. Each available domain controller responds to the datagram to indicate that it is
currently operational and then returns the information to DsGetDcName.
3. The Net Logon service returns the information to the client from the domain
controller that responds first.
4. The Net Logon service caches the domain controller information so that it is not
necessary to repeat the discovery process for subsequent requests. Caching this
information encourages the consistent use of the same domain controller and, thus, a
consistent view of Active Directory.
Domain Controller Location in the Closest SiteDuring a search for a domain controller, the Locator attempts to find a domain controller in the site closest to the client. When the domain that is being sought is a Windows Server 2003 domain, the domain controller uses the information stored in Active Directory to determine the closest site. When the domain being sought is a Windows NT 4.0 domain, domain controller discovery occurs when the client starts and uses the first domain controller that it finds.
Each Windows Server 2003–based domain controller registers DNS records that indicate the site where the domain controller is located. The site name (the relative distinguished name of the site object in Active Directory) is registered in several records so that the various roles the domain controller might perform (for example, global catalog server or Kerberos server) can be associated with the domain controller’s site. When DNS is used, the Locator searches first for a site-specific DNS record before it begins to search for a DNS record that is not site-specific (thereby preferentially locating a domain controller in that site).
A client computer stores its own site information in the registry, but the computer is not necessarily located physically in the site associated with its IP address. For example, a portable computer that was moved to a new location could contact a domain controller in its home site, which is not the site to which the computer is currently connected. In this situation, the domain controller looks up the client site on the basis of the client IP address by comparing the address to the sites that are identified in Active Directory, and then returns the name of the site that is closest to the client. The client then updates the information in the registry.
The domain controller stores site information for the entire forest in the Configuration container. The domain controller uses the site information to check the IP address of the client computer against the list of subnets in the forest. In this way, the domain controller ascertains the name of the site in which the client is assumed to be located, or the site that is the closest match, and returns this information to the client.
Active Directory Site and Subnet ObjectsA site is a collection of subnets that have high-speed connections. In Active Directory, a site is defined by a site object in the cn=Sites,cn=Configuration,dc=ForestRootDomain container. A subnet is an addressed segment within a site and is represented by an object in the cn=Subnets,cn=Sites,cn=Configuration,dc=ForestRootDomain container.
The site in which a domain controller is located is identified in the Configuration container by the domain controller object that is located within the cn=Servers container beneath the site object for a particular site. A domain controller can identify the site of a client by using the subnet object in the Sites container. Each subnet object has a siteObject property (“attribute”) that links it to a site object; the value of the siteObject property is the distinguished name of the site object. This link enables a domain controller to identify clients that have an IP address in the specified subnet as being in the specified site.
Subnet names in Active Directory take the form “network/bits masked” (for example, the subnet object 172.16.72.0/22 has a subnet of 172.16.72.0 and a 22-bit subnet mask). If this subnet had a siteObject property value that contained the distinguished name of the Seattle site object, all IP addresses in the 172.16.72.0/22 subnet would be considered to be in the Seattle site. The siteObject property is a single value, which implies that a single subnet maps to a single site. However, multiple subnet objects can be linked to the same site object. The directory administrator manually creates subnet objects and, hence, the siteObject property value.
The Configuration container (including all of the site and subnet objects in it) is replicated to all domain controllers in the forest. Therefore, any domain controller in the forest can identify the site in which a client is located, compare it to the site in which the domain controller is located, and indicate to the client whether that domain controller’s site is the closest site to the client.
For more information about networks, subnets, and subnet masks, see “How TCP/IP Works.”
IP Addresses Mapped to Site NamesDuring Net Logon startup, the Net Logon service on each domain controller enumerates the site objects in the Configuration container. Net Logon on each domain controller is also notified of any changes made to the site objects. Net Logon uses the site information to build an in-memory structure that is used to map IP addresses to site names.
When a client that is searching for a domain controller receives the list of domain controller IP addresses from DNS, the client begins querying the domain controllers in turn to find out which domain controller is available and appropriate. Active Directory intercepts the query,
which contains the IP address of the client, and passes it to Net Logon on the domain controller. Net Logon looks up the client IP address in its subnet-to-site mapping table by finding the subnet object that most closely matches the client IP address and then returns the following information:
The name of the site in which the client is located, or the site that most closely
matches the client IP address.
The name of the site in which the current domain controller is located.
A bit that indicates whether the found domain controller is located (bit is set) or not
located (bit is not set) in the site closest to the client.
The domain controller returns the information to the client. The response also contains various other pieces of information that describe the domain controller.
The client inspects the information to determine whether to try to find a better domain controller. The decision is made as follows:
If the returned domain controller is in the closest site (the returned bit is set), the
client uses that domain controller.
If the client has already tried to find a domain controller in the site in which the
domain controller claims the client is located, the client uses that domain controller.
If the domain controller is not in the closest site, the client updates its site
information and sends a new DNS query to find a new domain controller in the site. If
the second query is successful, the client uses the new domain controller. If the
second query fails, the client uses the original domain controller.
If the domain that is being queried by a computer is the same as the domain to which
the computer is joined, the site in which the computer resides (as reported by a
domain controller) is stored in the computer registry. The client stores this site name
in the DynamicSiteName registry entry in HKEY_LOCAL_MACHINE\SYSTEM\
CurrentControlSet\Services\ Netlogon\Parameters. Therefore, the DsGetSiteName API
returns the site in which the computer is located.
You can override the dynamically determined value using the registry, but you should never change dynamically determined values. To override the dynamic site name, add the SiteName entry with the REG_SZ data type in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Netlogon\Parameters. When a value is present for the SiteName entry, the DynamicSiteName entry is not used.
Note
Editing the registry directly can have serious, unexpected consequences that can
prevent the system from starting and require that you reinstall Windows Server 2003.
There are programs available in Control Panel or Microsoft Management Console
(MMC) for performing most administrative tasks. These programs provide safeguards
that prevent you from entering conflicting settings or settings that are likely to
degrade performance or damage your system. Registry editors bypass the standard
safeguards that are provided by these administrative tools. Modifying the registry is
recommended only when no administrative tool is available. Before you make
changes to the registry, it is recommended that you back up any valuable data on
the computer. For instructions about how to edit registry entries, see Help for the
registry editor that you are using.
If the domain being located is the same as the domain to which the computer is joined and the computer has not physically moved to a different site since the last query, the dynamically determined site name in the registry is the actual site in which the computer is located. Thus, the client finds a domain controller in the correct site without having to retry the operation. On the other hand, if the site name in the registry is not the current site of the computer (for example, if the computer is portable), the domain controller location process serves to update the site information in the registry.
Automatic Site CoverageThere is not necessarily a domain controller in every site. For various reasons, it is possible that no domain controller exists for a particular domain at the local site. By default, each domain controller checks all sites in the forest and then checks the replication cost matrix. A domain controller advertises itself (registers a site-related SRV record in DNS) in any site that does not have a domain controller for that domain and for which its site has the lowest-cost connections. This process ensures that every site has a domain controller that is defined by default for every domain in the forest, even if a site does not contain a domain controller for that domain. The domain controllers that are published in DNS are those from the closest site (as defined by the replication topology).
For example, given one domain and three sites, a domain controller for that domain might be located in two of the sites, but there might be no domain controller for the domain in the third site. Replication to the domain that does not have a domain controller in the third site might be too expensive in terms of cost or replication latency. To ensure that a domain controller can be located in the site closest to a client computer, if not the same site, Windows Server 2003 automatically attempts to register a domain controller in every site. The algorithm that is used to accomplish automatic site coverage determines how one site can cover another site when no domain controller exists in the second site.
Determining Site Coverage on the Basis of Site-link CostSite coverage is determined according to site-link costs, and domain controllers register themselves in sites accordingly. For example, given one domain and sites A, B, and C, site A has no domain controllers for the domain. If a client in site A attempts to locate a domain controller, which domain controller should be returned? The answer depends on which site covers site A for the domain.
In the example, a site link exists between site A and both of the other sites — that is, the connections between domain controllers in site A, site B, and site C are configured for replication over site links in Active Directory Sites and Services. Costs are associated with site links based on the expense of transferring data over the connections. The administrator
uses the speed of the connection between sites to assign a cost to the communication link, and replication uses the cost to establish the least expensive route for replication traffic.
Site A and site B are connected by site link AB. Site A and site C are connected by site link AC, with the following costs:
Site link AB cost = 50.
Site link AC cost = 100.
The link between site A and site C has a much higher cost than the link between site A and site B. The administrator configured this cost based on the expensive Integrated Services Digital Network (ISDN) line that connects site A and site C, and the administrator would prefer that resources in site B be used when possible. The site coverage algorithm (described in the next sub-section) ensures that a domain controller in site B registers itself as a domain controller for site A. In this way, clients in Site A that are looking for a domain controller find one from site B, instead of possibly finding one from site C.
Site Coverage AlgorithmDuring registration of SRV records in DNS, the following algorithm determines if the domain controllers should register site SRV records to designate themselves as preferred domain controllers in Active Directory sites where no domain controller exists for a particular domain.
For every domain controller in the forest, this procedure is followed:
Build a list of target sites — sites that have no domain controllers for this domain (the
domain of the current domain controller).
Build a list of candidate sites — sites that have domain controllers for this domain.
For every target site, follow these steps:
1. Build a list of candidate sites of which this domain is a member. (If none, do
nothing.)
2. Of these, build a list of sites that have the lowest site link cost to the target
site. (If none, do nothing.)
3. If more than one, break ties (reduce this list to one candidate site) by
choosing the site with the largest number of domain controllers.
4. If more than one, break ties by choosing the site that is first alphabetically.
5. Register target-site-specific SRV records for the domain controllers for this
domain in the selected site.
Cache Time-out and Closest SiteIf a domain member computer requests a domain controller while all domain controllers in its site are offline, the Locator necessarily returns a domain controller in a different site. The
location of this domain controller is stored in the client cache. The cache lifetime is controlled by the CloseSiteTimeout entry in the registry.
In addition, the domain controller performs authentication, and a secure channel is set up. On subsequent location attempts, the lifetime of the cache and the lifetime of the secure channel are secondary to the location of a domain controller in the closest site.
If the domain controller that is stored in the client cache is not in a site that is close to the client, Net Logon attempts to find a close domain controller when either of the following events occurs:
An interactive logon process uses pass-through authentication on the secure channel.
The value in the CloseSiteTimeout registry entry has elapsed since the last attempt,
and any other attempt is made to use the secure channel (for example, pass-through
authentication of network logons).
Thus, Net Logon attempts to find a close domain controller only on demand. The default value of the CloseSiteTimeout period is 15 minutes; the maximum value is 49 days; the minimum value is 60 seconds. The implications of these settings are:
If the time-out value is too large, a client never tries to find a close domain controller
if there is not one available at startup.
If the time-out value is too small, secure channel traffic is unnecessarily slowed down
by discovery attempts.
Clients with No Apparent SiteSometimes the client pings a domain controller and the client IP address cannot be found in the subnet-to-site mapping table. In this case, the domain controller returns a NULL site name, and the client uses the returned domain controller.
Types of LocatorsOn the basis of parameters passed to Net Logon in the DsGetDcName API, the process of locating a domain controller proceeds in one of two ways:
The DNS-compatible Locator is used if the domain name passed to DsGetDcName is a
DNS-compatible name. The Net Logon service on the client looks up the name in DNS
(by calling DnsQuery) after it appends an appropriate string to the front of the
domain name. The DNS service supports a query for determining the set of domain
controllers. If the client site name is known, the client DNS query specifies the site.
DNS returns the IP addresses of domain controllers that match the DNS query. The
client Net Logon service sends an LDAP UDP message to one or more of the domain
controllers that have been returned by DNS in order to determine whether any of the
specified domain controllers are running and support the specified domain.
The Windows NT 4.0–compatible Locator is used if the domain name passed to
DsGetDcName is a NetBIOS name. The Net Logon service on the client sends a
transport-specific logon request query to locate a domain controller in a particular
domain and then sends a mailslot message to one or more of the domain controllers
to determine whether any of the domain controllers it found are running and support
the specified domain.
DNS Client Service Configuration ElementsThe DNS configuration on Active Directory client computers follows a specific computer naming scheme and specifies how these clients will locate DNS servers. The following table lists the configurations for DNS configuration elements.
DNS Configuration for Client Computers
DNS Configuration Element Configuration
Computer naming Use default naming. When a Windows 2000, Windows XP, or Windows Server 2003 computer joins a domain, the computer assigns itself a primary DNS name comprised of the host name of the computer and the name of the domain.
Client resolver configuration
Configure client computers to point to any DNS server on the network.
Note
Active Directory clients and domain controllers can dynamically register their DNS
names using a DNS server that is not authoritative for the DNS name of the Active
Directory domain. The DNS server used by these clients will refer the registration to
the DNS server that is authoritative for the name of the Active Directory domain.
A computer might have an existing DNS name if the organization previously statically registered the computer in DNS or the organization previously deployed an integrated DHCP solution. If client computers already have a registered DNS name, when the domain to which they are joined is upgraded to Windows Server 2003 Active Directory, the client computers will have two names: the existing DNS name, and the new primary name.
Clients can be located by either name. Any existing DNS, DHCP, or integrated DNS/DHCP solution is left intact. The new primary names are created automatically and updated by means of dynamic update. The old names are cleaned up automatically by means of DNS scavenging.
To take advantage of Kerberos authentication when connecting to a server running Windows 2000, Windows XP, or Windows Server 2003, the client must connect to the server by using the primary name.
Network Ports Used by DNS in Support of Active Directory
The network ports used by DNS are documented in the DNS Technical Reference. For more information, see “Network Ports used by DNS” in How DNS Works.
Related Information
The following resource contains additional information that is relevant to this section.
How DNS Works
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DNS Records Explained with Examples
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DNS (Domain Name System), is the service which translates between Internet names and Internet addresses.Internet names are the names which we use to refer to hosts on the Internet, such as www.debianhelp.co.uk.Internet addresses are the numbers which routers use to move traffic across the Internet, such as 211.1.13.115 and
What are DNS Records ?
DNS records or Zone files are used for mapping URLs to an IPs. Located on servers called the DNS servers, these records are typically the connection of your website with the outside world. Requests for your website are forwarded to your DNS servers and then get pointed to the WebServers that serve the website or to Email servers that handle the incoming email.
Different Types of DNS Records With Syntax and Examples
Types of DNS Records
AAAAACNAMEMX
PTRNSSOASRVTXTNAPTR
The above DNS records are mostly used in all DNS Configurations. Now we will see each one with examples.
A Record
An A record or address record.
Address Record, assigns an IP address to a domain or subdomain name. When the domain name system was designed it was recommended that no two A records refer to the same IP address.
Suppose you have the somedomain.tld domain and want to assign 10.10.0.1 IP address to your web server, then you should create an A record with "www.somedomain.tld" as Fully Qualified Domain Name and "10.10.0.1" in the value field.
From now on, all the requests for www.somedomain.tld will be sent to a server with that IP.
Basically is useful to use an A record when you have subdomains residing on various systems.
Usefultip: you might use a "*.somedomain.tld" A record to allow WHATEVER.somedomain.tld to be resolved to your IP, though a wildcard CNAME record is often better than a wildcard A record.
Example of A Record with Syntax
example.com. IN A 69.9.64.11
Where
IN indicates Internet
A indicates the Address record.
The above example indicate that the IP Address for the domain example.com is 69.9.64.11
AAAA Record
An AAAA record or IPv6 address record maps a hostname to a 128-bit IPv6 address.
The regular DNS Address resource record is defined for a 32-bit IPv4 address, so a new one was created to allow a domain name to be associated with a 128-bit IPv6 address. The four “A”s (“AAAA”) are a mnemonic to indicate that the IPv6 address is four times the size of the IPv4 address. The AAAA record
is structured in very much the same way as the A record in both binary and master file formats; it is just much larger. The DNS resource record Type value for AAAA is 28.
Example of AAAA Record with Syntax
The AAAA record is to help transition and coexistence between IPv4 and IPv6 networks.An IPv4 nameserver can provide IPv6 addresses:
linux aaaa 3ffe:1900:4545:2:02d0:09ff:fef7:6d2c
CNAME Record
A CNAME record or canonical name record makes one domain name an alias of another. The aliased domain gets all the subdomains and DNS records of the original.
You should use a CNAME record whenever you want associate a new subdomain to an already existing A record; i.e. you can make "www.somedomain.tld" to "somedomain.tld", which should already have been assigned an IP with an A record.
This allows you to have as many subdomains as you wish without having to specify the IP for every record. Use a CNAME if you have more services pointing to the same IP. This way you will have to update only one record in the convenience of a change of IP address.
Example of a CNAME record: "stuff.everybox.com CNAME www.everybox.com" where 'www.everybox.com' is an A record listing an IP address, and 'stuff.everybox.com' points to 'www.everybox.com'. It will NOT allow you to foward a domain to a specific web page. Use a webhop for that. Port numbers can be changed with webhops, as well; CNAMEs cannot change the HTTP default of 80 to any other port number.
Do not use CNAME defined hostnames in MX records. For example, this is not recommended
Example Of CNAME With syntax
mail.example.com IN CNAME mail.example.net
where
IN indicates Internet
CNAME indicates CNAME record.
MX Record
An MX record or mail exchange record maps a domain name to a list of mail exchange servers for that domain.
Example with MX Record Syntax - Single mail servers
mydomain.com. 14400 IN MX 0 mydomain.com.
The MX record shows that all emails @ mydomain.com should be routed to the mail server at mydomain.com. The DNS record shows that mydomain.com is located at 26.34.9.14. This means that email meant for [email protected] will be routed to the email server at 26.34.9.14. This finishes the task of the MX record. The email server on that server then takes over, collects the email and then proceeds to distribute it to the user ``test''.
It is important that there be a dot(``.'') after the domain name in the MX record. If the dot is absent, it routes to ``mydomain.com.mydomain.com''. The number 0, indicates Preferance number. Mail is always routed to the server which has the lowest Preferance number. If there is only one mail server, it is safe to mark it 0.
Using Multiple mail servers
If you want to use multiple mail servers you have to use MX record preferences.The MX record preference values indicate which mail server to use and in which order to try them when they fail or don't respond. A larger preference number is less preferred. Thus, a mail exchanger with a preference of zero (0) is always preferred over all other mail exchangers. Setting preference values to equal numbers makes mail servers equally preferred.
Example with MX Record Syntax - Multiple mail servers
mydomain.com. 14400 IN MX 0 mydomain.com.mydomain.com. 14400 IN MX 30 server2.mydomain.com
You can have unlimited MX entries for Fallback or backup purpose.If all the MX records are equal Preference numbers, the client simply attempts all equal Preference servers in random order, and then goes to MX record with the next highest Preference number.
PTR Record
A PTR record or pointer record maps an IPv4 address to the canonical name for that host. Setting up a PTR record for a hostname in the in-addr.arpa domain that corresponds to an IP address implements reverse DNS lookup for that address. For example www.name.net has the IP address 122.0.3.16, but a PTR record maps 16.3.0.122.in-addr.arpa.
Example of PTR Record with syntax
16.3.0.122.in-addr.arpa. IN PTR name.net
Here as you see the IP Address is reversed and added with in-addr.arpa and this has come to the left side while the actual domain name has gone to right side of IN PTR.
This is mostly used as a security and an anti-spam measure wherein most of the webservers or the email servers do a reverse DNS lookup to check if the host is actually coming from where it claims to come from. It is always advisable to have a proper reverse DNS record (PTR) is been setup for your servers especially when you are running a mail / smtp server.
NS Record
An NS record or name server record maps a domain name to a list of DNS servers authoritative for that domain. Delegations depend on NS records.
NS Record Name Server Record which indicates the Authoritative Name Servers for a particular Domain. The NS records of the Authoritative Name Server for any given Domain will be listed on the Parent Server. These are called as the Delegation Records as these records on the Parent Server indicates the delegation of the domain to the Authoritative servers.
The NS record will also be listed in the Zone records of the Authoritative Name Server itself. These records are called as the Authoritative Records.
The NS records found on the Parent Server should match the NS records on the Authoritative Server as well. However, you can have NS records listed on the Authoritative server that is not listed in the Parent Server. This arrangement is normally used to configure Stealth Name Servers.
Example of NS Record With syntax
example.com. IN NS ns1.live.secure.com.
where
IN indicates the Internet
NS indicates the type of record which Name Server record
The above indicates that the ns1.live.secure.com is the authoritative server for the domain example.com
SOA Record
An SOA record or start of authority record specifies the DNS server providing authoritative information about an Internet domain, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone.
An SOA(State of Authority) Record is the most essential part of a Zone file. The SOA record is a way for the Domain Administrator to give out simple information about the domain like, how often it is updated, when it was last updated, when to check back for more info, what is the admins email address and so on. A Zone file can contain only one SOA Record.
A properly optimized and updated SOA record can reduce bandwidth between nameservers, increase the speed of website access and ensure the site is alive even when the primary DNS server is down.
Example of SOA Record with syntax
Here is the SOA record. Notice the starting bracket ``(``. This has to be on the same line, otherwise the record gets broken.
; name TTL class rr Nameserver email-address
mydomain.com. 14400 IN SOA ns.mynameserver.com. root.ns.mynameserver.com. (2004123001 ; Serial number86000 ; Refresh rate in seconds7200 ; Update Retry in seconds3600000 ; Expiry in seconds600 ; minimum in seconds )
name - mydomain.com is the main name in this zone.
TTL - 14400 - TTL defines the duration in seconds that the record may be cached by client side programs. If it is set as 0, it indicates that the record should not be cached. The range is defined to be between 0 to 2147483647 (close to 68 years !) .
Class - IN - The class shows the type of record. IN equates to Internet. Other options are all historic. So as long as your DNS is on the Internet or Intranet, you must use IN.
Nameserver - ns.nameserver.com. - The nameserver is the server which holds the zone files. It can be either an external server in which case, the entire domain name must be specified followed by a dot. In case it is defined in this zone file, then it can be written as ``ns'' .
Email address - root.ns.nameserver.com. - This is the email of the domain name administrator. Now, this is really confusing, because people expect an @ to be in an email address. However in this case, email is sent to [email protected], but written as root.ns.nameserver.com . And yes, remember to put the dot behind the domain name.
Serial number - 2004123001 - This is a sort of a revision numbering system to show the changes made to the DNS Zone. This number has to increment , whenever any change is made to the Zone file. The standard convention is to use the date of update YYYYMMDDnn, where nn is a revision number in case more than one updates are done in a day. So if the first update done today would be 2005301200 and second update would be 2005301201.
Refresh - 86000 - This is time(in seconds) when the slave DNS server will refresh from the master. This value represents how often a secondary will poll the primary server to see if the serial number for the zone has increased (so it knows to request a new copy of the data for the zone). It can be written as ``23h88M'' indicating 23 hours and 88 minutes. If you have a regular Internet server, you can keep it between 6 to 24 hours.
Retry - 7200 - Now assume that a slave tried to contact the master server and failed to contact it because it was down. The Retry value (time in seconds) will tell it when to get back. This value is not very important and can be a fraction of the refresh value.
Expiry - 3600000 - This is the time (in seconds) that a slave server will keep a cached zone file as valid, if it can't contact the primary server. If this value were set to say 2 weeks ( in seconds), what it means is that a slave would still be able to give out domain information from its cached zone file for 2 weeks, without anyone knowing the difference. The recommended value is between 2 to 4 weeks.
Minimum - 600 - This is the default time(in seconds) that the slave servers should cache the Zone file. This is the most important time field in the SOA Record. If your DNS information keeps changing, keep it down to a day or less. Otherwise if your DNS record doesn't change regularly, step it up between 1 to 5 days. The benefit of keeping this value high, is that your website speeds increase drastically as a result of reduced lookups. Caching servers around the globe would cache your records and this improves site performance.
SRV Record
The theory behind SRV is that given a known domain name e.g. example.com, a given service e.g. web (http) which runs on tcp in this case, a DNS query may be issued to find the host name that provides such on behalf of the domain - and which may or may not be within the domain.
Example of SRV Record with syntax
srvce.prot.name ttl class rr pri weight port target_http._tcp.example.com. IN SRV 0 5 80 www.example.com.
srvce
Defines the symbolic service name (see IANA port-numbers) prepended with a '_' (underscore). Case insensitive. Common values are:
_http - web service _ftp - file transfer service _ldap - LDAP service
prot
Defines the protocol name (see IANA service-names) prepended with a '_' (underscore). Case insensitive. Common values are
_tcp - TCP protocol _udp - UDP protocol
name
Incomprehensible description in RFC 2782. Leaving the entry blank (without a dot) will substitute the current zone root (the $ORIGIN), or you can explicitly add it as in the above _http._tcp.example.com. (with a dot).
ttl
Standard TTL parameter. For more information about TTL values.
pri
The relative Priority of this service (range 0 - 65535). Lowest is highest priority.
weight
Used when more than one service with same priority. A 16 bit unsigned integer in the range 0 - 65535. The value 0 indicates no weighting should be applied. If the weight is 1 or greater it is a relative number in which the highest is most frequently delivered i.e. given two SRV records both with Priority = 0, one with weight = 1 the other weight = 6, the one with weight 6 will have its RR delivered first 6 times out of 7 by the name server.
port
Normally the port number assigned to the symbolic service but does this is not a requirement e.g. it is permissible to define a _http service with a port number of 8100 rather than the more normal port 80.
target
The name of the host that will provide this service. Does not have to be in the same zone (domain).
TXT Record
A TXT record allows an administrator to insert arbitrary text into a DNS record. For example, this record is used to implement the Sender Policy Framework specification.
Example of TXT Record with syntax
SPF domains have to publish at least two directives: a version identifier and a default mechanism.
mydomain.com. TXT "v=spf1 -all"
This is the simplest possible SPF record: it means your domain mydomain.com never sends mail.
It makes sense to do this when a domain is only used for web services and doesn't do email.
MX servers send mail, designate them.
mydomain.com. TXT "v=spf1 mx -all"
Let's pretend mydomain.com has two MX servers, mx01 and mx02. They would both be allowed to send mail from mydomain.com.
other machines in the domain also send mail, designate them.
mydomain.com. TXT "v=spf1 mx ptr -all"
This designates all the hosts whose PTR hostname match mydomain.com.
any other machines not in the domain also send mail from that domain, designate them.
mydomain.com. TXT "v=spf1 a:mydomain.com mx ptr -all"
mydomain.com's IP address doesn't show up in its list of MX servers. So we add an "a" mechanism to the directive set to match it.
mydomain.com. TXT "v=spf1 a mx ptr -all"
This is shorthand for the same thing.
Each of your mail servers should have an SPF record also.When your mail servers create a bounce message, they will send it using a blank envelope sender: <>. When an SPF MTA sees a blank envelope sender, it will perform the lookup using the HELO domain name instead. These records take care of that scenario.
amx.mail.net. TXT "v=spf1 a -all"mx.mail.net. TXT "v=spf1 a -all"
NAPTR Record
NAPTR records (NAPTR stands for "Naming Authority Pointer") are a newer type of DNS record that support regular expression based rewriting.
Example of NAPTR Record with syntax
$ORIGIN 3.8.0.0.6.9.2.3.6.1.4.4.e164.arpa.
NAPTR 10 100 "u" "E2U+sip" "!^.*$!sip:[email protected]!" . NAPTR 10 101 "u" "E2U+h323" "!^.*$!h323:[email protected]!" . NAPTR 10 102 "u" "E2U+msg" "!^.*$!mailto:[email protected]!" .
This record set maps the phone number +441632960083 onto three possible identically ordered URIs, with a preference for SIP, then H323, and finally email. In each case, the regular expression matches the full AUS (^.$), and replaces it with a URI (e.g., sip:[email protected]). As this is a terminal record, this URI is returned to the client.Though most NAPTR records replace the full AUS, it is possible for the regular expression to back-reference part of the AUS, to grab an extension number, say:
$ORIGIN 0.6.9.2.3.6.1.4.4.e164.arpa. *
NAPTR 10 100 "u" "E2U+sip""!^+441632960(.*)$!sip:\[email protected]!" .
Once the client has the URI it must be resolved using DNS, but this is no longer part of the DDDS algorithm..
wildcard DNS record
A wildcard DNS record is a record in a DNS zone file that will match all requests for non-existent domain names, i.e. domain names for which there are no records at all.
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DNS Zones Overview
A DNS zone is the contiguous portion of the DNS domain name space over which a DNS server has authority, or is authoritative. A zone is a portion of a namespace . it is not a domain. A domain is a branch of the DNS namespace. A DNS zone can contain one or more contiguous domains. A DNS server can be authoritative for multiple DNS zones. A noncontiguous namespace cannot be a DNS zone.
A zone contains the resource records for all of the names within the particular zone. Zone files are used if DNS data is not integrated with Active Directory. The zone files contain the DNS database resource records which define the zone. If DNS and Active Directory are integrated, then DNS data is stored in Active Directory.
The different types of zones used in Windows Server 2003 DNS are listed below:
Primary zone
Secondary zone
Active Directory-integrated zone
Reverse lookup zone
Stub zone
A primary zone is the only zone type that can be edited or updated because the data in the zone is the original source of the data for all domains in the zone. Updates made to the primary zone are made by the DNS server that is authoritative for the specific primary zone. You can also back up data from a primary zone to a secondary zone.
A secondary zone is a read-only copy of the zone that was copied from the master server during zone transfer. In fact, a secondary zone can only be updated through zone transfer.
An Active Directory-integrated zone is a zone that stores its data in Active Directory. DNS zone files are not needed. This type of zone is an authoritative primary zone. Zone data of an Active Directory-integrated zone is replicated during the Active Directory replication process. Active Directory-integrated zones also enjoy the security features of Active Directory.
A reverse lookup zone is an authoritative DNS zone. These zones are mainly used to resolve IP addresses to resource names on the network. A reverse lookup zone can be either of the following zones:
Primary zone
Secondary zone
Active Directory-integrated zone
A stub zone is a new Windows Server 2003 feature. Stub zones only contain those resource records necessary to identify the authoritative DNS servers for the master zone. Stub zones therefore contain only a copy of a zone, and are used to resolve recursive queries and iterative queries:
Iterative queries: The DNS server provides the best answer it can. This can be:
o The resolved name
o A referral to a different DNS server
Recursive queries: The DNS server has to reply with the requested information, or with an error. The DNS server cannot provide a referral to a different DNS server
Stub zones contain the following information:
Start of Authority (SOA) resource records of the zone.
Resource records that list the authoritative DNS servers of the zone
Glue address (A) resource records that are necessary for contacting the authoritative servers of the zone.
Zone delegation occurs when you assign authority over portions of the DNS namespace to subdomains of the DNS namespace. You should delegate a zone under the following circumstances:
You want to delegate administration of a DNS domain to a department or branch of your organization.
You want to improve performance and fault tolerance of your DNS environment . you can distribute DNS database management and maintenance between several DNS servers.
Understanding DNS Zone Transfer
A zone transfer can be defined as the process that occurs to copy the resource records of a zone on the primary DNS server to secondary DNS servers. Zone transfer enables a secondary DNS server to continue handling queries if the primary DNS server fails. A secondary DNS server can also transfer it zone data to other secondary DNS servers, who are beneath it in the DNS hierarchy. In this case, the secondary DNS server is regarded as the master DNS server to the other secondary servers.
The zone transfer methods are:
Full transfer: When you configure a secondary DNS server for a zone, and start the secondary DNS server, the secondary DNS server requests a full copy of the zone from the primary DNS server. A full transfer is performed of all the zone information. Full zone transfers tend to be resource intensive. This disadvantage of full transfers has led to the development of incremental zone transfers.
Incremental zone transfer: With an incremental zone transfer, only those resource records that have since changed in a zone are transferred to the secondary DNS servers. During zone transfer, the DNS databases on the primary DNS server and the secondary DNS server are compared to determine whether there are differences in the DNS data. If the DNS data of the primary and secondary DNS servers are the same, zone transfer does not take place. If the DNS data of the two servers are different, transfer of the delta resource records starts. This occurs when the serial number on the primary DNS server database is higher than that of secondary DNS server.s serial number. For incremental zone transfer to occur, the primary DNS server has to record incremental changes to its DNS database. Incremental zone transfers require less bandwidth than full zone transfers.
Active Directory transfers: These zone transfers occur when Active Directory-integrated zones are replicated to the domain controllers in a domain. Replication occurs through Active Directory replication.
DNS Notify is a mechanism that enables a primary DNS server to inform secondary DNS servers when its database has been updated. DNS Notify informs the secondary DNS servers when they need to initiate a zone transfer so that the updates of the primary DNS server can be replicated to them. When a secondary DNS server receives the notification from the primary DNS server, it can start an incremental zone transfer or a full zone transfer to pull zone changes from the primary DNS servers.
Understanding DNS Resource Records (RRs)
The DNS database contains resource records (entries) that are used to resolve name resolution queries sent to the DNS server. Each DNS server contains the resource records (RRs) it needs to respond to name resolution queries for the portion of the DNS namespace for which it is authoritative. There are different types of resource records.
A few of the commonly used resource records (RR) and their associated functions are described in the Table.
Resource Records Type Name Function
A Host recordContains the IP address of a specific host, and maps the FQDN to this 32-bit IPv4 addresses.
AAAA IPv6 address record Ties a FQDN to an IPv6 128-bit address.
AFSDB Andrews files systemAssociates a DNS domain name to a server subtype: an AFS version 3 volume or an authenticated name server using DCE/NCA
ATMA Asynchronous Transfer Mode addressAssociates a DNS domain name to the ATM address of the atm_address field.
CNAME Canonical Name / Alias name Ties an alias to its associated domain name.
HINFO Host info record Indicates the CPU and OS type for a particular host.
ISDN ISDN info record Ties a FQDN to an associated ISDN telephone number
KEY Public key resource recordContains the public key for zones that can use DNS Security Extensions (DNSSEC).
MB Mailbox name recordMaps the domain mail server name to the mail server.s host name
MG Mail group record Ties th domain mailing group to mailbox resource records
MINFO Mailbox info record Associates a mailbox for an individual that maintains it.
MR Mailbox renamed record Maps an older mailbox name to its new mailbox name.
MX Mail exchange recordProvides routing for messages to mail servers and backup servers.
NS Name server recordProvides a list of the authoritative servers for a domain. Also provides the authoritative DNS server for delegated subdomains.
NXT Next resource recordIndicates those resource record types that exist for a name. Specifies the resource record in the zone.
OPT Option resource record A pseudo-resource record which provides extended DNS
functionality.
PTR Pointer resource recordPoints to a different resource record, and is used for reverse lookups to point to A type resource records.
RT Route through recordProvides routing information for hosts that do not have a WAN address.
SIG Signature resource record Stores the digital signature for an RR set.
SOA Start of Authority resource recordThis resource record contains zone information for determining the name of the primary DNS server for the zone. The SOA record stores other zone property information, such as version information.
SRV Service locator recordUsed by Active directory to locate domain controllers, LDAP servers, and global catalog servers.
TXT Text record Maps a DNS name to descriptive text.
X25 X.25 info recordMaps a DNS address to the public switched data network (PSDN) address number.
While there are various resource records that contain different information or data, there are a few required fields that each particular resource record has to contain:
Owner; the DNS domain that contains the resource record
TTL (Time to Live); indicates the time duration that DNS servers can cache resource record information, prior to discarding the information. This is however an optional resource records field.
Class; is another optional resource records field. Class types were used in earlier implementations of the DNS naming system, and are no longer used these days.
Type; indicates the type of information contained by the resource record.
Record-Specific Data; a variable length field that further defines the function of the resource. The format of the field is determined by Class and Type.
Delegation records and glue records can also be added to a zone. These records are used to delegate a subdomain into a separate zone.
Delegation records: These are Name Space (NS) resource records in a parent zone. The delegation record specifies the parent zone as being authoritative for the delegated zones.
Glue records: These are A type resource records for the DNS server who is authoritative for delegated zone.
The more important resource records are discussed now. This includes the following:
Start of Authority (SOA), Name Server (NS), Host (A), Alias (CNAME), Mail exchanger (MX), Pointer (PTR), Service location (SRV)
Start of Authority (SOA) Resource Record
This is the first record in the DNS database file. The SOA record includes information on the zone property information, such as of the primary DNS server for the zone, and version information.
The fields located within the SOA record are listed below:
Source host; the host for who the DNS database file is maintained
Contact e-mail; e-mail address for the individual who is responsible for the database file.
Serial number; the version number of the database.
Refresh time; the time that a secondary DNS server waits, while determining whether database updates have been made, that have to be replicated via zone transfer.
Retry time; the time for which a secondary DNS server waits before attempting a failed zone transfer again.
Expiration time; the time for which a secondary DNS server will continue to attempt to download zone information. Old zone information is discarded when this limit is reached.
Time to live; the time that the particular DNS server can cache resource records from the DNS database file.
Name Server (NS) Resource Record
The Name Server (NS) resource record provides a list of the authoritative DNS servers for a domain, as well authoritative DNS server for any delegated subdomains. Each zone must have one (or more) NS resource records at the zone root. The NS resource record indicates the primary and secondary DNS servers for the zone defined in the SOA resource record. This in turn enables other DNS servers to look up names in the domain.
Host (A) Resource Record
The host (A) resource record contains the IP address of a specific host, and maps the FQDN to this 32-bit IPv4 addresses. Host (A) resource records basically associates the domain names of computers (FQDNs) or hosts names to their associated IP addresses. Because a host (A) resource record statically associates a host name to a specific IP address, you can manually add these records to zones if you have machines who have statically assigned IP addresses.
The methods which are used to add host (A) resource records to zones are:
Manually add these records, using the DNS management console.
You can use the Dnscmd tool at the command line to add host (A) resource records.
TCP/IP client computers running Windows 2000, Windows XP or Windows Server 2003 use the DHCP Client service to both register their names, and update their host (A) resource records.
Alias (CNAME) Resource Record
Alias (CNAME) resource records ties an alias name to its associated domain name. Alias (CNAME) resource records are referred to as canonical names. By using canonical names, you can hide network information from the clients who connect to your network. Alias (CNAME) resource records should be used when you have to rename a host that is defined in a host (A) resource record in the identical zone.
Mail exchanger (MX) Resource Record
The mail exchanger (MX) resource record provides routing for messages to mail servers and backup servers. The mail MX resource record provides information on which mail servers processes e-mail for the particular domain name. E-mail applications therefore mostly utilize MX resource records.
A mail exchanger (MX) resource record has the following parameters:
Priority
Mail server
The mail exchanger (MX) resource record enables your DNS server to work with e-mail addresses where no specific mail server is defined. A DNS domain can have multiple MX records. MX resource records can therefore also be used to provide failover to different mail servers when the primary server specified is unavailable. In this case, a server preference value is added to indicate the priority of a server in the list. Lower server preference values specify higher preference.
Pointer (PTR) Resource Record
The pointer (PTR) resource record points to a different resource record, and is used for reverse lookups to point to A resource records. Reverse lookups resolve IP addresses to host names or FQDNs.
You can add PTR resource records to zones through the following methods:
Manually add these records, using the DNS management console.
You can use the Dnscmd tool at the command line to add PTR resource records.
Service (SRV) Resource Records
Service (SRV) resource records are typically used by Active directory to locate domain controllers, LDAP servers, and global catalog servers. The SRV records define the location of specific services in a domain. They associate the location of a service such as a domain controller or global catalog server; with details on how the particular service can be contacted.
The fields of the service (SRV) resource record are explained below:
Service name
The protocol used
The domain name associated with the SRV records.
The port number for the particular service
The Time to Live value
The class
The priority and weight.
The target specifying the FQDN of the particular host supporting the service
The Zone Database Files
If you are not using Active Directory-integrated zones, the specific zone database files that are used for zone data are:
Domain Name file: When new A type resource records are added to the domain, they are stored in this file. When a zone is created, the Domain Name file contains the following:
o A SOA resource record for the domain
o A NS resource record that indicates the name of the DNS server that was created.
Reverse Lookup file: This database file contains information on a reverse lookup zone.
Cache file: This file contains a listing of the names and addresses of root name servers that are needed for resolving names which are external to the authoritative domains.
Boot file: This file controls the startup behavior of the DNS server. The boot file supports the commands listed below:
o Directory command; this command defines the location of the other files specified in the Boot file.
o Primary command; defines the domain for which this particular DNS server has authority.
o Secondary; specifies a domain as being a secondary domain.
o Cache command; this command defines the list of root hints used for contacting DNS servers for the root domain
Planning DNS Zone Implementations
When you divide the up the DNS namespace, DNS zones are created. Breaking up the namespace into zones enables DNS to more efficiently manage available bandwidth usage, which in turn improves DNS performance.
When determining how to break up the DNS zones, a few considerations you should include are listed below:
DNS traffic patterns: You can use the System Monitor tool to examine DNS performance counters, and to obtain DNS server statistics.
Network link speed: The types of network links that exist between your DNS servers should be determined when you plan the zones for your environment.
Whether full DNS servers or caching-only DNS servers are being used also affects how you break up DNS zones
The main zone types used in Windows Server 2003 DNS environments are primary zones and Active Directory-integrated zones. The question on whether to implement primary zones or Active Directory-integrated zones; would be determined by the DNS design requirements of your environment.
Both primary zones and secondary zones are standard DNS zones that use zone files. The main difference between primary zones and secondary zones is that primary zones can be updated. Secondary zones contain read-only copies of zone data. A secondary DNS zone can only be updated through DNS zone transfer. Secondary DNS
zones are usually implemented to provide fault tolerance for your DNS server environment.
An Active Directory-integrated zone can be defined as an improved version of a primary DNS zone because it can use multi-master replication and the security features of Active Directory. The zone data of Active Directory-integrated zones are stored in Active Directory. Active Directory-integrated zones are authoritative primary zones.
A few advantages that Active Directory-integrated zone implementations have oer standard primary zone implementations are:
Active Directory replication is faster, which means that the time needed to transfer zone data between zones is far less.
The Active Directory replication topology is used for Active Directory replication, and for Active Directory-integrated zone replication. There is no longer a need for DNS replication when DNS and Active Directory are integrated.
Active Directory-integrated zones can enjoy the security features of Active Directory.
The need to manage your Active Directory domains and DNS namespaces as separate entities is eliminated. This in turn reduces administrative overhead.
When DNS and Active Directory are integrated; the Active Directory-integrated zones are replicated, and stored on any new domain controllers automatically. Synchronization takes place automatically when new domain controllers are deployed.
The mechanism that DNS utilizes to forward a query that one DNS server cannot resolve, to another DNS server is called DNS forwarding. DNS forwarders are the DNS servers used to forward DNS queries for different DNS namespace to those DNS servers who can answer the query. A DNS server is configured as a DNS forwarder when you configure the other DNS servers to direct any unresolved queries to a specific DNS server. Creating DNS forwarders can improve name resolution efficiency.
Windows Server 2003 DNS introduces a new feature, called conditional forwarding. With conditional forwarding, you create conditional forwarders within your environment that will forward DNS queries based on the specific domain names being requested in the query. This differs from DNS forwarders where the standard DNS resolution path to the root was used to resolve the query. A conditional forwarder can only forward queries for domains that are defined in the particular conditional forwarders list. The query is passed to the default DNS forwarder if there are
no entries in the forwarders list for the specific domain queried.
When conditional forwarders are configured, the process to resolve domain names is illustrated below:
1. A client sends a query to the DNS server for name resolution.
2. The DNS server checks its DNS database file to determine whether it can resolve the query with its zone data.
3. The DNS server also checks its DNS server cache to resolve the request.
4. If the DNS server is not configured to use forwarding, the server uses recursion to attempt to resolve the query.
5. If the DNS server is configured to forward the query for a specific domain name to a DNS forwarder, the DNS server then forwards the query to the IP address of its configured DNS forwarder.
A few considerations for configuring forwarders for your DNS environment are:
You should only implement the DNS forwarders that are necessary for your environment. You should refrain from creating loads of forwarders for your internal DNS servers.
You should avoid chaining your DNS servers together in a forwarding configuration.
To avoid the DNS forwarder turning into a bottleneck, do not configure one external DNS forwarder for all your internal DNS servers.
How to create a new zone
1. Click Start, Administrative Tools, and then click DNS to open the DNS console.
2. Expand the Forward Lookup Zones folder
3. Select the Forward Lookup Zones folder.
4. From the Action menu, select New Zone.
5. The New Zone Wizard initiates.
6. On the initial page of the Wizard, click Next.
7. On the Zone Type page, ensure that the Primary Zone. Creates A Copy Of A Zone That Can Be Updated Directly On This Server option is selected. This option is by default selected.
8. Uncheck the Store The Zone In Active Directory (Available Only If DNS Server Is A Domain Controller) checkbox. Click Next.
9. On the Zone Name page, enter the correct name for the zone in he Zone Name textbox. Click Next.
10. On the Zone File page, ensure that the default option, Create A New File With This File Name is selected. Click Next.
11. On the Dynamic Update page, ensure that the Do Not Allow Dynamic Updates. Dynamic Updates Of Resource Records Are Not Accepted By This Zone. You Must Update These Records Manually option is selected. Click Next.
12. The Completing The New Zone Wizard page is displayed next.
13. Click Finish to create the new zone.
How to create subdomains
1. Click Start, Administrative Tools, and then click DNS to open the DNS console.
2. In the console tree, select the appropriate zone.
3. From the Action menu, select New Domain.
4. The DNS Domain dialog box opens.
5. Enter the name for new subdomain.
6. Click OK to create the new subdomain.
How to create a reverse lookup zone
1. Click Start, Administrative Tools, and the select DNS to open the DNS console.
2. Select the appropriate DNS server in the console tree.
3. Right-click the DNS server, and then select New Zone from the shortcut menu.
4. The New Zone Wizard starts.
5. Click Next on the first page of the New Zone Wizard.
6. On the Zone Type page, ensure that the Primary Zone option is selected. Click Next.
7. On the following page, select the Reverse lookup zone option. Click Next.
8. Enter the IP network in the Network ID box, for the domain name that you are creating this new reverse lookup zone for. Click Next.
9. Accept the default zone file name. Click Next.
10. On the Dynamic Update page, select the Allow both nonsecure and secure dynamic updates option, and then click Next.
11. .The Completing The New Zone Wizard page is displayed next.
12. Click Finish to create the new reverse lookup zone.
How to create a stub zone
1. Click Start, Administrative Tools, and then click DNS to open the DNS console.
2. Expand the Forward Lookup Zones folder
3. Select the Forward Lookup Zones folder.
4. From the Action menu, select New Zone.
5. The New Zone Wizard initiates.
6. On the initial page of the Wizard, click Next.
7. On the Zone Type page, select the Stub Zone option.
8. Uncheck the Store The Zone In Active Directory (Available Only If DNS Server Is A Domain Controller) checkbox. Click Next.
9. On the Zone Name page, enter the name for the new stub zone in the Zone Name textbox, and then click Next.
10. Accept the default setting on the Zone file page. Click Next.
11. On the Master DNS Servers page, enter the IP address of the master server in the Address text box. Click Next.
12. On the Completing The New Zone Wizard page, click Finish.
How to add resource records to zones
1. Click Start, Administrative Tools, and then click DNS to open the DNS console.
2. In the console tree, select the zone that you want to add resource records to.
3. From the Action menu, select the resource record type that you want to add to the zone. The options are:
o New Host (A)
o New Alias (CNAME)
o New Mail Exchanger (MX)
o Other New Records
4. Select the New Host (A) option.
5. The New Host dialog box opens.
6. In the Name (Use Parent Domain Name If Blank) textbox, enter the name of the new host.
7. When you specify the name of the new host, the resulting FQDN is displayed in the Fully qualified domain name (FQDN) textbox.
8. In the IP Address box, enter the address for the new host.
9. If you want to create an associated pointer (PTR) record, enable the checkbox.
10. Click the Add Host button.
11. The new host (A) resource record is added to the particular zone.
12. A message box is displayed, verifying that the new host (A) resource record was successfully created for the zone.
13. Click OK.
14. Click Done to close the New Host dialog box./li>
How to create a zone delegation
1. Click Start, Administrative Tools, and then select DNS to open the DNS console.
2. Right-click your subdomain in the console tree, and then select New Delegation from the shortcut menu.
3. The New Delegation Wizard initiates.
4. Click Next on the first page of the New Delegation Wizard.
5. When the Delegated Domain Name page opens, provide a delegated domain name, and then click Next.
6. On the Name Servers page, click the Add button to provide the names and the IP addresses of your DNS servers that should host the delegation
7. On the Name Servers page, click Next.
8. Click Finish.
How to enable dynamic updates for a zone
1. Click Start, Administrative Tools, and the select DNS to open the DNS console.
2. Right-click the zone you want to work with in the console tree, and then select Properties from the shortcut menu.
3. When the Zone Properties dialog box opens, on the General tab, select Yes in the Allow Dynamic Updates list box.
4. Click OK.
How to configure a zone to use WINS for name resolution
You can configure your forward lookup zone to use WINS for name resolution in instances where the queried name is not found in the DNS namespace.
1. Click Start, Administrative Tools, and the select DNS to open the DNS console.
2. In the console tree, proceed to expand your DNS server node, and then expand the Forward Lookup Zones folder.
3. Locate and right-click the zone which you want to configure and then select Properties from the shortcut menu.
4. When the Zone Properties dialog box opens, click the WINS tab.
5. Enable the Use WINS Forward Lookup checkbox.
6. Type the WINS server IP address. Click Add, and then click OK.
7. On the General tab, select Yes in the Allow Dynamic Updates list box.