This is one of the topics which took time for me to search and understand in detail as not much good articles are there which explains _msdcs and its role. I would like to point out a few things on _msdcs, which was collected from few blogs.
_msdcs is a Microsoft specific subdomain in the DNS. It contains the AD forestwide locator (SRV) records which are required by the clients to find AD DS resources available in that forest.
It is recommended to install the DNS server role when running the AD DS installation wizard (dcpromo.exe), if you are creating the first DC in a forest. When DNS is installed on a new forest on a windows server, the Active Directory installation wizard (Dcpromo.exe) creates and delegates a forest root domain DNS zone in your forest. It also creates the zone for the _msdcs subdomain of the root domain of the forest. Then it makes that zone to replicate to all DCs in that forest.
This _msdcs subdomain enables location of domain controllers that have Windows Server specific roles in the domain or forest, as well as the location by GUID when a domain has been renamed. To help locating domain controllers by server type or GUID, Windows Server based domain controllers register SRV records in the following form:
These SRV resource records map the name of the service that the domain controller provides to the DNS name for that domain.
The Net Logon service running on the domain controller uses dynamic updates to register SRV resource records in the _msdcs subdomain in the DNS database. SRV records registers information of resources such as Global Catalog (GC), Domain Controllers (DC), Primary Domain Controller (PDC), Globally unique Identifier (GUID) etc. These well-known names were kept short to avoid exceeding the maximum length allowed for a DNS name. Even the name _msdcs was shortened to avoid usage of more space in the DNS namespace.