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Name Resolution in Active Directory

Windows 2000 prefers the Domain Name System (DNS) as its main name resolution method. DNS is an integral part of Active Directory and must be installed on your network. Active Directory clients use DNS servers to locate Active Directory domain controllers as well as other services on the network.

Explaining DNS fully would require a book in itself; therefore, this section covers only the specifics.

Naming Standards

Your naming convention should follow the Internet standard character set permitted for use in DNS host naming. Standard characters, which are defined in RFC 1123, include all uppercase letters (A–Z), lowercase letters (a–z), numbers (0–9), and the hyphen (-). If you implemented NetBIOS with more unconventional names, existing computer names might not conform to the DNS naming standard. If this is the case, consider revising your computer names.


Note - You cannot change the NetBIOS name of the machine after upgrading to Windows 2000. You must edit the Registry because there is no GUI tool to make this change at the present time.


To ease the transition from NetBIOS names to DNS domain names, the Windows 2000 DNS service includes support for extended ASCII and Unicode characters.


Note - ASCII and Unicode character support can be used only in a pure Windows 2000 network environment because most other DNS client software is based on RFC 1123, the specification that standardizes Internet host naming requirements.


If a non-standard DNS domain name is entered during Windows 2000 setup, a warning message appears recommending the use of a standard DNS name.

In earlier versions of Windows NT, a NetBIOS name was used to identify a Windows computer on the network. In Windows 2000, a computer can be identified by

  • Its NetBIOS computer name, which is optional and used for NT 4.0 interoperability. For example, dc1.

  • The fully qualified domain name (FQDN) for the computer. For example, dc1.kevinkocis.com.

  • Its primary (or default) Windows 2000 name, which would also be dc1 for this example.

The full computer name is a combination of both the computer name and the DNS domain name for the computer. The DNS domain name for the computer is part of the system properties for the computer and is not related to any specifically installed networking components.

The NetBIOS computer name is implemented to ensure interoperability between NetBIOS and DNS naming in Windows 2000. The value of this parameter, which is not required in a pure Windows 2000 environment, is derived from the first 15 characters of the DNS full computer name.

When the full computer name is a combination of the computer name and the DNS domain name for the computer, the impact of renaming and making the transition from a NetBIOS namespace to a DNS namespace can be minimal. Users continue to focus on the short computer name. If this name is 15 or fewer characters, you can keep the name identical to the NetBIOS computer name. You can then also assign a DNS domain name for each computer by using remote administration tools.

Name Restrictions

Different DNS implementations impose different character and length restrictions. Table 6.1 shows the restrictions for each implementation.

Table 6.1 DNS Name Restrictions

Restriction

Standard DNS (Including Windows NT 4.0)

DNS in Windows 2000

NetBIOS

Characters

Supports RFC 1123, which permits A to Z, a to z, 0 to 9, and the hyphen (-)

Several different configurations are possible, as described at the end of this section.

Unicode characters, numbers, whitespace, symbols: !@# $ % ^ & ' )(. - _ { } ~

Fully qualified domain name length

63 bytes per label and 255 bytes for an FQDN

63 bytes per label and 255 bytes for an FQDN; domain controllers are limited to 155 bytes for an FQDN.

15 bytes



Note - Although you can create long, complex DNS names, creating shorter, user-friendly names is recommended.


Microsoft has proposed that the DNS name specification be readjusted to accommodate a larger character set: UTF-character encoding, which is a superset of ASCII and a translation of the UCS (also known as Unicode) character encoding. The UTF-character set includes characters from most languages.

You can configure the Windows 2000 DNS server to allow or disallow the use of UTF-characters on your Windows 2000 server on a per-server basis with the DNS console. From the Advanced tab of the server properties page, set Name Checking to one of the following:

  • Strict RFC (ANSI)—Allows A to Z, a to z, the hyphen (-), the asterisk (*) as a first label; and the underscore (_) as the first character in a label.

  • Non RFC (ANSI)—Allows all Strict RFC (ANSI) characters as well as placement of the underscore (_) anywhere in a name.

  • Multibyte (UTF-)—Allows all Non RFC (ANSI) characters, as well as UTF-8 characters.

  • Any character—Allows any character, including UTF-8 characters.

DNS Server Roles

For Active Directory to function properly, DNS servers must maintain high availability. To ensure this, you might not want to rely only on AD domain controllers for DNS. You should provide at least a primary and a secondary name server per domain. This way, you can load balance between servers and provide quicker access and redundancy. Clients should be configured to query both a primary and secondary DNS server. You can configure this with DHCP, which is covered in the next chapter.


Note - Clients use an application called a resolver to query DNS servers.


Let's take a closer look at the various DNS server roles.

Primary Name Server

The primary name server is the original source of address data (zone files for zone transfers) for the domain. It is also the Start of Authority (SOA) for the domain and holds the master files. There can be only one primary name server per DNS zone.

Secondary Name Server

The secondary name server contains authoritative address data for the domain, which it receives from the primary server in the form of zone transfers. Secondary name servers relieve the primary name server's load by answering queries. As you learned earlier, this server provides redundancy and load balancing for the domain.

Forwarders and Slaves

When a DNS server receives a query, it first searches its local zones and cache. If it does not find the requested information and is not authoritative for the requested information, it queries other servers to resolve the request.

To solve this problem, DNS allows for the use of forwarders, which are DNS servers that provide forwarding of offsite queries for other DNS servers. A forwarder is basically a designated machine for handling queries. It waits for a response, and if it doesn't receive one, it searches for an answer itself.

A slave performs similarly to the forwarder, except that it does not attempt to resolve the address itself. When a slave receives a DNS query that it cannot resolve through its own zones, it passes the query to a forwarder. If the forwarder cannot resolve the request, the slave returns a query failure to its requestor. Slaves do not resolve queries on their own.

Caching-Only Servers

All DNS servers perform caching when they receive information from other servers and store the information for a certain amount of time. This capability enhances DNS resolution and reduces traffic associated with DNS queries.

Caching-only servers simply perform queries and cache the answers, therefore they do not generate zone transfer network traffic because they do not contain any zones.

Caching-only servers are ideal for companies with an Internet service provider (ISP) providing primary DNS services.

Resource Records

Resource records (RRs) are sets of information in the DNS database for processing client queries. Each DNS server hosts the resource records it needs to handle authoritative queries.


Note - A DNS server is authoritative for a contiguous portion of the DNS namespace if it contains information about that portion of the namespace.


In DNS, resource records are represented in binary form for queries and replies. Resource records are represented as text entries in Active Directory database files.

Resource Record Types

Different types of resource records can be used to provide DNS-based data about computers on a TCP/IP network. This section describes the following resource records:

  • SOA

  • NS

  • A

  • PTR

  • CNAME

  • MX

  • SRV

SOA Resource Records

Every zone contains a Start of Authority (SOA) resource record at the beginning of the zone. They contain general zone information, including the authoritative DNS server for the zone and various expiration parameters.

NS Resource Records

The name server (NS) resource record indicates the servers authoritative for the zone, including primary and secondary servers specified in the SOA resource record. Every zone must contain at least one NS record at the zone root.

A Resource Records

The address (A) resource record maps an FQDN or host name to an IP address, so clients can obtain an IP address for an FQDN.

PTR Records

The pointer (PTR) resource record performs tasks opposite from those performed by the A resource record. This record maps an IP address to an FQDN or host name, and is used in reverse lookup zones.

CNAME Resource Records

The canonical name (CNAME) resource record creates an alias (synonymous name) for the specified FQDN or host name. CNAME records can hide the implementation details of your network from the clients that connect to it. This can be useful in a proxy situation where you want to hide the actual proxy FQDN from your user population.


Note - According to RFC 2181, each alias can have only one canonical name.


MX Resource Records

The mail exchange (MX) resource record specifies a mail exchange server for a DNS domain name. A mail exchange server is a host that either processes or forwards mail for the DNS domain name.


Note - Only mail exchange servers use MX records.


You can have multiple MX resource records for multiple mail exchange servers in a domain and assign them different weight preferences. The lower-weighted mail server will be contacted first.

SRV Records

Service (SRV) resource records specify the location of the servers for a specific service, protocol, and DNS domain.

The format of an SRV record is as follows:

_Service._Proto.Name TTL Class SRV  Priority  Weight  Port  Target
  • The Service field specifies the name of the service, such as http or telnet.

  • The Proto field specifies the protocol, which is usually TCP or UDP.

  • The Name field specifies the DNS domain.

  • The TTL field is the Time to Live (optional).

  • The Class field often represented by IN (for Internet). This field is optional.

  • The Priority field specifies the priority of the host (a number from 0 to 65,535). The host with the lowest number has priority.

  • The Weight field is used for load-balancing when hosts have the same priority (a number from 0 to 65,535).

  • The Port field shows the port of the service on this host (a number from 0 to 65,535).

  • The Target field shows the FQDN for the host providing the service.

If a computer needs to locate a Web server in the kevinkocis.com DNS domain, the client sends the following query:

_http._tcp.http://www.kevinkocis.com

The DNS server replies with the SRV records listed earlier. The client then chooses between servers by looking at their priority values.


Note - If the priority values are the same but the weight values are different, the client would randomly choose a Web server, except that the server with the highest weight value would have a higher probability of being chosen.


Next, the client requests the A record for web1.kevinkocis.com, and the DNS server sends the A record. Finally, the client attempts to contact the Web server.

Zones and Zone Files

A zone is a contiguous portion of the DNS namespace hosted on a specific domain node. A zone is a portion of the DNS namespace generally stored in a file, and can contain multiple domains. A domain can be subdivided into several partitions or zones, which can be controlled by a separate DNS server. Using the zone, the DNS server answers queries about hosts in its zone, and is authoritative for that zone.

Zones fall into three classifications:

  • Standard primary

  • Standard secondary

  • Active Directory–integrated

These zone types are created in the DNS Wizard, and are stored either in files or in Active Directory. A primary zone is the copy of the zone to which the updates are made. It is stored in a text file. A secondary zone is a read-only copy of the zone that is replicated from a master server, and is also a text file.

Standard primary and secondary zones are stored as zone files on the server's hard drive. Some secondary servers store them in memory and perform a zone transfer whenever they are restarted. Active Directory–integrated zones are stored and replicated in the Active Directory.


Note -

Only one server can manage the primary zone for each DNS domain. You cannot configure two different servers to manage the same primary zones.


There is one exception, however: Multiple computers can manage Windows 2000 Active Directory–integrated zones. You can configure a single DNS server to manage one zone or multiple zones, depending on your needs. You can create multiple zones to distribute administrative tasks to different groups and provide efficient data distribution. You can also store the same zone on multiple servers to provide load balancing and fault tolerance.

Lookup Zones

Lookup zones store the information required to resolve host names and IP addresses within the domain. Depending on which variable you have (host name or address), you'll use forward or reverse lookup zones, respectively.

Forward Lookup Zone

Forward lookup zones contain information needed to resolve names within the DNS domain. They must include SOA and NS records and can include any type of resource record except the PTR resource record. With most queries, the client supplies a name and requests the IP address that corresponds to that name. This type of query is typically described as a forward lookup.

Reverse Lookup Zone

Reverse lookup zones contain information needed to perform reverse lookups. They usually include SOA, NS, PTR, and CNAME records.

If you already have the IP address but need the client name, you would use a reverse lookup.

Dynamic DNS and Zone Transfers

Windows 2000 supports both dynamic and secure dynamic updates. With dynamic updates, clients can automatically send updates to the domain's authoritative name server. The authoritative name server then verifies that certain prerequisites have been met. If the prerequisites have been met, the authoritative name server makes the change.


Note - Both clients and servers can send dynamic updates.


Dynamic update provides the following benefits:

  • Reduces administrative overhead by allowing clients (including DHCP clients), to dynamically register A and PTR resource records with a primary server.

  • Allows DHCP servers to register A and PTR resource records on behalf of DHCP clients (Windows NT and 9x clients).

  • Allows domain controllers to dynamically register their SRV records.

Secure dynamic update works like dynamic update, but requires authentication to update the dnsZone and dnsNode objects. See Figure 3.4 for setting dynamic updates.

Figure 3.4
Setting dynamic updates.

Secure dynamic update allows you to protect zones and resource records from being modified without authorization and enables you to specify exactly which users and groups can modify zones and resource records.


Note - Although any primary zone can be configured for dynamic update, only Active Directory–integrated zones can be configured for secure dynamic update. If you disable secure dynamic update, the client cannot perform updates on zones that have been configured for secure dynamic update.


By default, the dynamic update client automatically deregisters name–to–IP address mappings whenever the DHCP lease expires. You can configure the client not to register its name and IP address in DNS. If you configure the client not to automatically register name–to–IP address mappings, the DHCP server is running Windows 2000, and it is configured to register DNS resource records on behalf of clients that are running versions of Windows earlier than Windows 2000, the DHCP server attempts to update the mappings instead.

To prevent the client from registering name–to–IP address mappings, follow these steps:

  1. Double-click the Network icon in Control Panel.

  2. Right-click the icon for the connection on which you want to disable registration of name–to–IP address mappings, and then click Properties.

  3. Click Internet Protocol (TCP/IP), and then click Properties.

  4. Click Advanced, and then click the DNS tab.

  5. Clear the Register This Connection's Address in DNS check box.

You can force a re-registration by using the command-line tool Ipconfig. For Windows 2000–based clients, type the following at the command prompt:

ipconfig /registerdns

For Windows NT 4.0–based clients, type the following:

ipconfig /release
ipconfig /renew

For Microsoft Windows 9x–based clients, type the following:

winipcfg /renew

Zone Transfer

Zone changes made to a master server must be replicated to all the secondary servers for that zone. This process is called a zone transfer. Traditionally, only one form of zone transfer—known as full zone transfer (AXFR)—was available. Active Directory incorporates a new type of zone transfer, the incremental zone transfer (IXFR). Let's take a closer look at these zone transfers, as shown in Figure 3.5.

Figure 3.5
The zone transfer options.

Full Zone Transfer

In a full zone transfer, the zone's master server transmits the entire zone database to the secondary server for that zone. Secondary servers initiate full zone transfers using the following process:

  1. The secondary server polls the master server at the time interval set in the Refresh field of the State Of Authority (SOA) resource record tab.

  2. The master server responds with the SOA resource record.

  3. The secondary server compares the serial numbers of the SOA records. If the master server's serial number for the zone is higher than the secondary server's serial number, its zone database is out of date, and the secondary server sends an AXFR request (a request for a full zone transfer).

  4. The master server then sends the full zone database to the secondary server.

If the master server for the zone does not respond to polling by the secondary server, the secondary server continues to poll based on the interval specified in the Retry field of the SOA resource record. If there is still no answer after the interval specified in the Expire field since the last successful zone transfer, it discards its zone and stops responding to client requests.


Note - Name servers running versions of Berkeley Internet Name Domain (BIND) earlier than 4.9.4 can send and receive only one resource record per message during a full zone transfer. Windows 2000 and later versions of BIND can send and receive multiple resource records per message. This capability improves the performance of full zone transfers. For more information about BIND, visit the Internet Software Consortium Web site at http://isc.org/.


Incremental Transfer

Full zone transfers can affect network bandwidth. Because of this, a new standard was defined, which is called the incremental zone transfer (IXFR).

Incremental zone transfer works much the same as full zone transfer except that it transfers only the modified parts of the zone. In this situation, if a zone transfer is required, it sends an incremental zone transfer (IXFR) query instead of a full zone transfer (AXFR) query, requesting that the master server for the zone perform an incremental zone transfer.

The master server sends the oldest updates first and the newest updates last to the secondary server. When it receives an incremental zone transfer, the secondary server creates a new version of the zone and begins replacing its resource records with the updated resource records, starting with the oldest updates. After all the updates have been made, the secondary server replaces its old version of the zone with the new version of the zone.

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