Host Address A and Pointer PTR Records

Host Address (A) records contain the name of a host and its IPv4 address. Any computer that has multiple network interfaces or IP addresses should have multiple address records. Pointer (PTR) records enable reverse lookups by creating a pointer that maps an IP address to a host name.

You do not need to create A and PTR records for hosts that use dynamic DNS. These records are created automatically. For hosts that don't use dynamic DNS, you can create a new host entry with A and PTR records by completing the following steps:

1 In the DNS console, expand the node for the primary name server, and then expand the related Forward Lookup Zones folder. Right-click the domain to which you want to add the records, and then choose New Host (A). This displays the dialog box shown in Figure 27-16.

Figure 27-16. Create a host record.

Figure 27-16. Create a host record.

2 Type the host name, such as corpsrv17, and then type the IP address, such as 192.168.15.22.

3 If a reverse lookup zone has been created for the domain and you want to create a PTR record for this host, select the Create Associated Pointer (PTR) Record option.

Note If you are working with an Active Directory-integrated zone, you have the option of allowing any authenticated client with the designated host name to update the record. To enable this, select Allow Any Authenticated User To Update DNS Records With The Same Owner Name. This is a nonsecure dynamic update where only the client host name is checked.

4 Click Add Host. Repeat this process as necessary to add other hosts.

5 Click Done when you're finished.

If you opt not to create a PTR record when you create an A record, you can create the PTR later as necessary. In the DNS console, expand the node for the primary name server, and then expand the related Reverse Lookup Zones folder. Right-click the subnet to which you want to add the record, and then choose New Pointer (PTR). This displays the dialog box shown in Figure 27-17. Type the Host IP Number for the designated subnet, such as 206, and then type the FQDN for the host, such as corpsvr05.cpandl.com. Click OK.

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Figure 27-17. Create a PTR record.

Inside Out

Using round robin for load balancing

If a host name has multiple A records associated with it, the DNS Server service will use round robin for load balancing. With round robin, the DNS server cycles between the A records so that queries are routed proportionally to the various IP addresses that are configured. Here's how round robin works: Say that your organization's Web server gets a ton of hits—so much that the single Web server you've set up can't handle the load anymore.

To spread the workload, you configure three machines, one with the IP address 192.168.12.18, one with the IP address 192.168.12.19, and one with the IP address 192.168.12.20. On the DNS server, you configure a separate A record for each IP address, but use the same host name: www.cpandl.com. This tells the DNS server to use round robin to balance the incoming requests proportionally. As requests come in, DNS will respond in a fixed circular fashion with an IP address. For a series of requests, the first user might be directed to 192.168.12.18, the next user to 192.168.12.19, and the next user to 192.168.12.20. The next time around the order will be the same so the fourth user is directed to 192.168.12.18, the next user to 192.168.12.19, and the next user to 192.168.12.20. As you can see, with three servers, each server will get approximately one-third of the incoming requests and hopefully about one-third of the workload as well.

Round robin isn't meant to be a replacement for clustering technologies, but it is an easy and fast way to get basic load balancing. Support for round robin is enabled by default. If you have to disable round robin, type dnscmd ServerName /config /roundrobin 0. To enable round robin again later, type dnscmd ServerName /config /roundrobin 1. In both cases, ServerName is the name or IP address of the DNS server you want to configure.

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