SRV Record for NTP? In *MY* Active Directory?

Howdy fellow IT goons. I am probably not going to talk about Powershell today... but no promises.

Good ole' RFC 2782, the great fireside reading that it is, spells out the concept behind DNS SRV records and using them to locate services within a domain. The Microsoft article "How DNS Support for Active Directory Works", which is also more than just a heart-warming story but is also required reading if you're a Windows admin, mentions that Active Directory is pretty much, more or less, compliant with the aforementioned RFC:

"When a domain controller is added to a forest, a DNS zone hosted on a DNS server is updated with the Locator DNS resource records for that domain controller. For this reason, the DNS zone must allow dynamic updates (RFC 2136), and the DNS server hosting that zone must support the SRV resource records (RFC 2782) to advertise the Active Directory directory service."

It goes _<service>._<protocol>.domain.com, so if I wanted to locate LDAP services in a domain I'd issue a DNS query for _ldap._tcp.domain.com, or if I wanted to find Kerberos service I'd do _kerberos._tcp.domain.com.

But no one ever said that Active Directory uses every type of SRV there is by default. Not even close. Take NTP, Network Time Protocol, as an example.  Given the above logic I might issue a DNS query for _ntp._udp.domain.com, searching for NTP time service in that domain. Assuming I'm in a Microsoft Active Directory domain, odds are that I will not find it.

An SRV record is not created by default for the NTP service.  This is because Windows clients connecting to an AD domain already know to use domain controllers for time service in an AD domain, and the domain controllers already have their own SRV records, so separate NTP records would be redundant and unnecessary.

In fact, the only Microsoft-centric scenario I know of where the SRV record _ntp._udp.domain.com comes in to play is smart phones and devices using Microsoft Office Communicator or Lync - and even then it's optional since they'll fail back to time.windows.com if the SRV record is not found. You can find those examples here and here. If you know of any other situations where Windows-based applications use such an SRV record, please let me know.

But maybe you have a heterogeneous IT environment and you may want to add these records for yourself in order to support Unix/Linux clients and their applications that are making such DNS queries.  It's very easy:

  • Open the DNS Manager console/MMC snap-in.
  • Drill down into your Forward Lookup Zones.
  • Locate the _udp subdomain, since the NTP service operates over the UDP protocol.
  • You should see a list of _kerberos and _kpasswd SRV records there already, that represent the domain controllers currently in your domain.
  • Right-click in that white space and choose "Other New Records..."
  • Select "Service Location (SRV)" from the list.
  • Configure your new record like this screenshot:

SRV record

Mind the underscores, and notice the trailing period at the end of your domain name. You will probably want to add one of these for each domain controller you have, and you can play around with the weights and priorities however you like. NTP uses port 123 of course. There will be some options in the drop down list that they give you as examples. Don't confuse it with _nntp, unless you host the News Network Transfer service in your domain too.

Comments (2) -

Hi,
I am trying to setup Lync phones and this seems to be an important record to add.

I can not see _ntp as a selectable option in the drop down list in Windows DNS Manager. We have Windows 2008 R2 SP1. _nntp shows like you mentioned along with _ftp and others but no _ntp.

Is having _ntp missing normal? Do I just type in the values as shown and cick OK or is something broken in my DNS that I need to fix first?

Hey there,

Correct, _ntp is not in the default drop down list. You are meant to type it in, free-form.

Comments are closed