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Configuring DNS

Several configuration issues need to be considered when setting up DNS. These include the following:

  • Root servers

  • Forward lookup zones

  • Reverse lookup zones

  • Resource records

  • Dynamic DNS

Root Servers

When you initially launch the DNS Microsoft Management Console (MMC) snap-in after installing the DNS service, a configuration wizard opens. You initially have the option of configuring your server as a root server. Root servers on the Internet are authoritative for the entire DNS namespace. Obviously, you would not be able to create a root server that is authoritative for the entire Internet, so you should only create a root server if your network is not connected to the Internet. If your LAN is not connected to the Internet and you create a root server, the root server is authoritative for any namespace you create in the AD forest.

Forward Lookup Zones

For DNS services to work, at least one forward lookup zone must be configured on your server. The reason is that forward lookup zones are what enable forward lookup queries, the standard method of name resolution in DNS, to work. Forward lookup zones allow computers to resolve host names to IP addresses.

To create a forward lookup zone, right-click Forward Lookup Zones in the DNS MMC console and click New Zone. A configuration wizard launches.

The first choice you have to make when configuring a new zone is what type of zone it will be. The choices are as follows:

  • Active Directory integrated

  • Standard primary

  • Standard secondary

The zones correspond with the types of name servers discussed previously in this chapter. If you have not installed AD yet (which isn't required for DNS), the AD integrated option will be grayed out. In many cases, it is easier to configure DNS before installing AD, then create the zones as primary and secondary zones, and finally convert them to AD integrated zones after installing AD. The reason is that installing both at the same time requires configuring two major services and significant network changes simultaneously. Instead, you should take on one task at a time, which makes troubleshooting much easier in the event of problems.

Unless you have a need to communicate with non-Windows 2000 DNS servers, you should use AD integrated zones whenever possible.

CAUTION

You need to know when to use one type of AD integrated zone versus another.

Reverse Lookup Zones

A reverse lookup zone is not required for DNS services to function; however, you will want to create a reverse lookup zone to allow reverse lookup queries to function. Without a reverse lookup zone, troubleshooting tools such as NSLOOKUP that can resolve host names from IP addresses cannot work. Whereas forward lookup zones allow computers to resolve host names to IP addresses, reverse lookup zones allow computers to resolve IP addresses to host names. As with forward lookup zones, you have the option of creating AD integrated, standard primary, or standard secondary zones.

Unlike naming a forward lookup zone, you name a reverse lookup zone by its IP address. You can either type your network ID into the first field and watch the reverse lookup zone name automatically be created for you, or you can choose to type in the reverse lookup zone name into the second field following RFC conventions. The information text between the network ID and reverse lookup zone name fields describes how to name a reverse lookup zone.

As with forward lookup zones, if you are creating an AD integrated zone, you are done after supplying the zone name. With standard primary and standard secondary zones, you have to also supply the zone filename, which defaults to adding a .dns extension onto the end of your zone name. With a standard secondary zone, you have to list the IP addresses of the primary DNS servers with which the secondary zone should communicate.

With your zones configured and DNS services functioning, let's look at the entries, known as resource records, you'll find within the server.

Resource Records

Resource records (RRs) are the basic units of information within DNS used to resolve all DNS queries. When the Windows 2000 DNS service starts up, a number of records are registered at the server.

Several common RRs are used with Windows 2000 DNS:

  • Start of Authority (SOA)

  • Name Server (NS)

  • Address (A)

  • Pointer (PTR)

  • Mail Exchanger (MX)

  • Service (SRV)

  • Canonical Name (CNAME)

Start of Authority Record (SOA)

The SOA record is contained at the beginning of every zone, both forward lookup and reverse lookup zones. It defines a number of details for the zone, including the following:

  • Time-to-live (TTL)—The amount of time a record is considered valid in the DNS database. Higher values reduce network bandwidth utilization but increase the possibility of outdated information existing.

  • Authoritative server—Shows the primary DNS server that is authoritative for the zone.

  • Responsible person—Shows the e-mail address of the person who administers the zone.

  • Serial number—Shows the serial number of the zone. Remember that the serial number is incremented whenever an update is made and that secondary servers use serial numbers to determine whether their copy of the zone database is out of date.

  • Refresh—Shows how often secondary servers check to see whether their zone database files need updating.

  • Retry—Shows how long a secondary server will wait after sending an AXFR (full zone transfer for standard zones) or IXFR (incremental zone transfer for standard zones) request before resending the request.

  • Expire—Shows how long after a zone transfer that a secondary server will respond to zone queries before discarding the zone as invalid because of no communication with the primary server.

  • Minimum TTL—Shows the minimum TTL a resource record will use if it does not explicitly state a TTL value.

Name Server (NS) Records

NS records show all servers that are authoritative for a zone, both primary and secondary servers for the zone specified in the SOA record, and primary name servers for any delegated zones.

Address (A) Records

The most basic entry in DNS, the A record maps the FQDN of a host to its IP address. When a client sends a standard forward lookup name-resolution query, the server uses A records to resolve the name.

Pointer (PTR) Records

The opposite of A records, PTR records provide reverse lookup services for DNS. That is, a PTR record maps an IP address to an FQDN. When a reverse lookup query is sent to a DNS server, such as through the NSLOOKUP utility, PTR records are consulted to resolve the address. PTR records are mapped in reverse lookup zones, which are in the in-addr.arpa zone.

Mail Exchanger (MX) Records

MX records designate a mail exchanging server for a DNS zone, which is a host that processes or forwards e-mail. In addition to the standard Owner, Class, and Type fields, MX records also support a fourth field: Mail Server Priority. This field is used when you have multiple mail servers in your domain, and mail exchangers with lower values are "preferred" over mail exchangers with higher values when determining which server to use to process an email message.

Service (SRV) Records

SRV records allow you to specify the location of servers providing a particular service, such as Web servers. You can create SRV records to identify hosts in the zone that provide a service, and then a resolver can find the A record of a service to resolve the name.

If you are having trouble with name-resolution services, where clients cannot successfully contact DCs, ensure that the appropriate SRV records exist for the DCs on the network.

Canonical Name (CNAME) Records

A CNAME record creates an alias for a specified host. This type of record is used most commonly to hide implementation details of your network. For example, say you have a Web server running at http://www.mycorp.com. The server that the Web site is running on might really be server1.mycorp.com. You don't want users to have to use the real server name, and you want the flexibility of being able to move the Web site to a newer, faster server in the future as traffic grows, without having to change the address of your Web site from server1.mycorp.com to server2.mycorp.com. CNAME provides the ability to alias the host name so that problems like this do not occur.

DDNS

DDNS, defined in RFC 2136, is used in Windows 2000 in conjunction with DHCP. When a Windows 2000 client boots up and receives addressing information from DHCP, it can register itself with DNS, automatically adding the requisite resource records.

Dynamic Updates

DDNS is enabled/disabled through zone properties. To do so, open the DNS MMC console from the Start menu, right-click your forward or reverse lookup zone, and then click Properties. On the General property sheet is the Allow Dynamic Updates? option. The choices are Yes, No, and Only Secure Updates. When DDNS is enabled, DHCP manages the resource records for DHCP clients. When a DHCP lease expires, the DHCP service cleans up the A and PTR records from DNS. By default, a dynamic update is performed from Windows 2000 clients every 24 hours, or immediately when one of the following occurs:

  • A DHCP-obtained IP address is renewed or a new lease is obtained.

  • A static IP address is added or removed from a computer.

  • The TCP/IP configuration of a client changes.

  • A Plug and Play event occurs on a client, such as the installation of a new network adapter.

Secure Updates

In addition to enabling dynamic updates, you can configure Windows 2000 DNS to perform only secure updates. As just mentioned, this is enabled by selecting Only Secure Updates on the General property sheet of the desired zone. So what do secure updates do?

Once you have enabled secure updates, you can control which users and computers can register themselves into DNS. By default, all members of the Authenticated Users security group are granted permission, although this can be changed as necessary through access control lists (ACLs) by clicking the Security tab on a zone's property sheet.

A dynamic update in general requires that the client computers be capable of registering an FQDN. Windows 2000 clients are capable of this; Windows NT and 9X clients are not. As we've previously discussed, Windows 2000 DHCP can act as a proxy for these legacy clients, because DHCP can register them in DNS on their behalf. The reason the FQDN consideration is important is that once you've enabled secure updates, you can reserve FQDNs in DNS so that only certain users can use them. To do this, create a new host record in the DNS console with the desired FQDN. Then, on the Security tab, configure the ACL so that only the desired user can update the resource records associated with the particular FQDN.

Note that all resource records for a single FQDN share a single ACL. In other words, if you create an A record for workstation1.inside-corner.com and then create an MX record for the same host, there is only one ACL for the two records. This would be true even if you added additional records for the same host.

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