What is the forward lookup DNS Zone created on a DNS installed on a domain controller ? When I DCPromo`ed the server a zone with the same name of my primary zone but with a _ at the beginning was created on the dns server. Why ? What does it do ? What is it good for ?
Active Directory uses DNS to do a lot of its magic:
I'm not familiar with the specific record you're describing, but AD creates a metric ton of them on a new domain and I don't know them all.
The _msdcs zone that is created hosts AD specific DNS records. All the records that a DC would register are visible in the netlogon.dns file of each DC. See http://support.microsoft.com/kb/816587
Starting with Windows Server 2003 what would have been a subdomain is now hosted as its own zone. Each record has a specific purpose it serves. For example the _ldap._tcp.dc._msdcs.domain.com is used for locating a DC by a client's netlogon service. Its also called a DCLocator record. There are similar records for locating a KDC for kerberos, a GC and also site specific records to request a service from a specific site.
These _msdcs zones and records are critical for AD to function. Please ensure all domain members including DCs always point to one or more DNS servers that host these zones.