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Domain controller SRV records
Step 4 of 6 in How domain controllers work · video 2:26
What you will learn
- Why clients use SRV records in DNS to find a domain controller
- How to read an SRV record name such as _ldap._tcp.adatum.com
- How sites decide which domain controllers a client is offered
- How to re-register missing SRV and host records
About this lesson
Clients find domain controllers through DNS service location (SRV) records. At startup the Net Logon service on each domain controller registers records describing the services it offers, and clients query them when a user signs in. This lesson explains the naming pattern _service._protocol.domain, with _ldap._tcp.adatum.com as the example, and shows how AD DS sites shape the answer: a client first gets domain controllers in its own site, then, if the Try Next Closest Site Group Policy setting is enabled, domain controllers from the next closest site, excluding RODCs, and otherwise a random list from the whole domain. It gives advice on defining sites around network quality, such as keeping a branch on an unreliable WAN link separate from the data centre. It ends with repair steps: restart the Net Logon service to re-register missing SRV records, and run ipconfig /registerdns to re-register the host (A) record.
Check yourself
1. Which service registers a domain controller's SRV records in DNS?
The Net Logon service running on that domain controller. It registers the records dynamically when the domain controller starts, so clients can locate it without knowing its name.
2. Which DNS name does a client query to find an LDAP server in the adatum.com domain?
_ldap._tcp.adatum.com. AD DS SRV records follow the pattern _service._protocol.domain, so the service (LDAP) comes first, then the protocol (TCP), then the domain name.
3. No domain controller is available in a client's site and Try Next Closest Site is enabled. What does the DNS lookup return?
A list of domain controllers from the next closest site, without any RODCs. Only if there is no domain controller there either does the client get a random list from across the domain.
4. A domain controller's SRV records are missing from DNS and its host (A) record is also wrong. What do you run?
Restart the Net Logon service on it, for example with Restart-Service Netlogon, which re-registers the SRV records only. Then run ipconfig /registerdns to re-register the host record, as you would on any computer.
Go deeper
This lesson comes from the course below - with the full set of lessons, demonstrations and practice.