How Domain controllers are located in windows

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How do servers locate a domain controller in a Network?

One of the first major tasks a domain member computer has to do when it starts is to locate a domain controller. Generally, this task requires the use of a Domain Name System (DNS) server, which contains records for each domain controller in the domain, and the Locator, a remote procedure call to the computer’s local Netlogon service.

Starting Up

When the computer starts, its Netlogon service starts automatically (in the default configuration). Th is service implements the DsGetDcName application programming interface (API), which is used to locate a domain controller.

The computer  begins  by collecting  a number  of pieces of information that  will be used  to  locate a domain controller. This information includes the client’s local IP address, which is used to determine the client’s Active Directory site membership, the desired domain name, and a DNS server address.

Finding the Domain Controllers

Netlogon then queries the configured DNS server. Netlogon retrieves the service resource (SRV) records and host (A) records from DNS that correspond to the domain controllers for the desired domain. The general form for the queried SRV records is _service._protocol.domainname, where service is the domain service, protocol is the TCP/IP protocol, and domainname is the desired Active Directory fully qualified domain name (FQDN). For example, because Active Directory is a Lightweight Directory Access Protocol (LDAP)-compliant directory service,  clients query for  _ldap._tcp.domainname  (or  _ldap._tcp.dc._msdcs.domainname when  locating  the nearest domain controller).

Each domain controller in a domain will register its host name with the SRV record, so the client’s query results will be a list of domain controller host names. The client also retrieves the associated A records, providing the client with the IP address of every domain controller in the domain. The client then sends an LDAP search query,  via  the  User  Datagram  Protocol  (UDP),  to  each  domain  controller.  Each  domain  controller  then responds, indicating that it is operational. The Netlogon service caches all of this information so that finding a domain controller in the future won’t require a repeat of this initial process. Instead, the service can simply refer to its cache to find another domain controller.

Selecting a Domain Controller

After the client locates a domain controller, the client uses LDAP to access Active Directory on a domain controller, preferably one in the client’s own subnet. The domain contro ller uses the client’s IP address to identify the client’s Active Directory site. If the domain controller is not in the closest site, then the domain controller returns the name of the client’s site, and the client tries to find a domain controller in tha t site by querying DNS. If the client has already attempted to find a domain controller in that site, then the client will continue using the current, nonoptima domain controller. Once the client finds a domain controller it likes, it caches that domain controller’s information, and the client will continue to use that domain controller for future contacts (unless the domain controller becomes unavailable).

For more details on troubleshooting please refer below KB article

http://support.microsoft.com/kb/247811

 

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Dependency of Active Directory on DNS

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DNS Records that are required for proper functionality of Active Directory

 

We know that DNS servers serves more than resolving Name to IP and IP into Name. It is core protocol or you can say daddy of all protocols over a network.

In this article I have tried to visualize and explain all the core records of DNS without which Active Directory cannot function properly.

Here are the list of all core SRV, A and C-Name records that are used by Active Directory and Domain clients.

Please Note: The Red marked records in below table are used by Non-SRV-Aware Clients

 

Mnemonic Type DNS Record Requirements
  1. PDC
SRV _ldap._tcp.pdc._msdcs.<DnsDomainName> One per domain
  1. GC
SRV _ldap._tcp.gc._msdcs.<DnsForestName> At least one per forest
  1. KDC
SRV _kerberos._tcp.dc._msdcs.<DnsDomainName> At least one per domain
  1. DC
SRV _ldap._tcp.dc._msdcs.<DnsDomainName> At least one per domain
A <DomainControllerFQDN> One per domain controller (domain controllers that have multiple IP addresses can have more than one A resource record)
  1. GcIpAddress
A gc._msdcs.<DnsForestName> At least one per forest
  1. DsaCname
CNAME <DsaGUID>._msdcs.<DnsForestName> One per domain controller

 

Below I have mentioned the location of all these important records, with their properties and NSLOOKUP commands to verify if the record exists correctly or not. I have taken screenshot from a single domain lab, on default site i.e. my domain itself represent the forest. So results may vary if you explore these in big infrastructure.

 

 

In the records Properties window, you will notice below few fields:

Priority-    The priority of the server. Clients attempt to contact the server with the lowest priority.

Weight –   A load-balancing mechanism that is used when selecting a target host from those that have the same priority. Clients randomly choose SRV records that specify target hosts to be contacted, with probability proportional to the weight

Port Number-    The port where the server is listening for this service.

Target –   The fully qualified domain name of the host computer.

 

 

 

Host Records for SRV-Aware Clients

  1. PDC Record – _ldap._tcp.pdc._msdcs.<DnsDomainName>

Allows a client to locate the server that is acting as the primary domain controller (also known as a “PDC”) in the mixed-mode domain named in DnsDomainName . Only the PDC emulator master of the domain registers this SRV record.

 

 

 

  1. GC Record – _ldap._tcp.gc._msdcs.<DnsForestName>

Allows a client to locate a Global Catalog (gc) server for this forest. Only domain controllers that are functioning as Global Catalog servers for the forest named in DnsForestName register this SRV record.

 

 

  1. KDC Record – _kerberos._tcp.dc._msdcs.<DnsDomainName>

Allows a client to locate a domain controller that is running the Windows implementation of the Kerberos KDC service for the domain named in DnsDomainName . All Windows Server–based domain controllers that are running the KDC service (that is, that implement a public key extension to the Kerberos v5 protocol Authentication Service Exchange subprotocol) register this SRV record.

 

 

  1. DC Record – _ldap._tcp.dc._msdcs.<DnsDomainName>

Allows a client to locate a domain controller (dc) of the domain named by DnsDomainName . All Windows Server–based domain controllers register this SRV record.

 

 

  1. Domain FQDN A Record  – <DomainControllerFQDN>

This record helps to locate the domain controllers IP address in a domain.

 

Host Records for Non-SRV-Aware Clients

 

  1. GC IP Address – gc._msdcs.<DnsForestName>

Allows a non-SRV-aware client to locate any Global Catalog server in the forest by looking up an A record. A name in this form is returned to the LDAP client through an LDAP referral. A non-SRV-aware client looks up this name; an SRV-aware client looks up the appropriate SRV resource record.

Net Logon also registers a DNS CNAME (alias) record for use by Active Directory replication The Locator does not use this record.

 

 

  1. DsaCname Record – <DsaGUID>._msdcs.<DnsForestName>

Allows a client to locate any domain controller in the forest by looking up an A record. The only information that is known about the domain controller is the GUID of the directory system agent (also known as the “DSA”) object for the domain controller and the name of the forest in which the domain controller is located. This record is used to facilitate renaming a domain controller.

 

To know more about SRV records in DNS , please refer to below article

http://technet.microsoft.com/en-us/library/cc961719.aspx

 

 

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How to Kill SQL Threads or Kill User Sessions

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How to Kill SQL Threads or Kill User Sessions

Method-1

This is one of best and quickest way to Kill All Threads or Kill All User Sessions or Kill All Sessions

Sometime you may get error while restoring a database that action cannot be performed as user’s sessions are active on the database. If it is important to restore and killing database user’s session won’t harm much, then you can use below script. This will temporally change the access mode to single user and kill sessions, and then revert it to multi user mode.

Note: This script will kill all users session associated with a database. In this script I have demonstrated AdventureWorks database.

USE master;

GO

ALTER DATABASE AdventureWorks

SET SINGLE_USER

WITH ROLLBACK IMMEDIATE;

ALTER DATABASE AdventureWorks

SET MULTI_USER;

GO


Method-2

Let’s say you want to kill all processes by a particular user or account, then you can use below script to find the related spid for all the processes initiated by user or account. Here my account is HABIB\Administrator

SELECT spid from master..sysprocesses

WHERE loginame = ‘HABIB\Administrator’


Once you get the spid details, you can simply execute KILL command against those processes.

Command is KILL <spid>

 

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