DC locator / DC Failover

Posted on 2009-05-10
Last Modified: 2012-09-10
Have some questions about the DC locator process and the netlogon cache.

So heres the background. We have three DCs in one of our active directory Sites. Upon logon all clients and servers will locate a DC via the netlogon service. So lets say an application server (appserver1) chooses as its DC that its going to authenticate with. Once the netlong service on chooses it will cache that information and all subsequent authentication events will go through We had some hardware issues on that forced us to bring it down (had to replace some bad memory that kept crashing the server). I figured since AD/DNS load balanced/fault tolerant, that is would try what's in it's cache and would notice was unavailable and do a DNS lookup find SRV records and find a new domain controller to do its authentication&say it would find

Well, once we took down our monitoring tool reported that many services were down. We quickly brought up and all services came back up.

So i got to reading.  I found everything under the sun.  Some links say that the netlogon service will NOT go and discover a new DC (, < this link says that clients will purge their cache only if the client has cached a DC that is not local to their a link that says "yes, clients will rediscover" (

Here's a good link for an advicate of "yes, clients will failover" (  - see section on domain Controller stick Stickiness

I have a long thread posted on Mark Minasi's Forum: if anyone isinterested.  

Basically i'm just tryin to get to the bottom of this.  If servers do failover, then why does our monitoring tool say services are down when we bounce a Domain Controller?  What kinds of checks does a client do to see if a DC is actually servicing authentication requests?  Is it a ping only, some kind of LDAP query.....this artical says that "yes clients will failover...." but doesn't go into what kinds of checks the client does to verify 

We use What's UP to monitor our servers which uses a service account  (domain account) which is an admin on all servers.  What's UP uses this account to get into WMI to test the services.  When we bounce a DC, What's up is saying that some services are down but not all.  We have a 5 min time laps between when serivces are down and when an e-mail will be sent, and when we bounce a DC, we get e-mails saying services are down.  

Any help would be MUCH appreciated,
Question by:esbfern
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Expert Comment

ID: 24349696
Is holding all the FSMO roles?  If so, that's possibly the reason appserver1 can't find it.  Is the ONLY DNS server in your infrastructure?  DNS is required for a machine to find the DC.  No DNS = No SRV records = Nothing working.

Author Comment

ID: 24350066
Ah yes failed to mention that part.  

FSMO: does not hold FSMO roles (we only have 3 FSMO roles as we are a child domain).  Further, we have no downlevel clients, thus our clients can authenticate to all DC's.  Just out of curiousity, what reason would holding FSMO roles on and downing cause appserver1 to not be able to find it?  

DNS: does house DNS and depending on where our Memberserver / workstations are geographicaly located, would determine if would be the server / workstationi Primary DNS server or its secondary.  When we recieve e-mails that services are down after is rebooted, the e-mails say that member servers in all locations (where is the primary DNS server and where is secondary) are down, just not in locations where is primary.  

I also failed to mention this.  While we are recieving e-mails about services being down, i am not 100% positive that the service is actually down (plan to test this durring the next maintenance window).  With that said, on those servers we recieve e-mails saying the "server service" is down amongst other services, i can check the security log and the account our monitoring tool is using, is failing to authenticate.  This is what make me think the member server isn't going out and finding a 2nd domain controller to do it's authentication.  Rather is continuing to try the DC in it's cache, which i bet is  

To prove this durring the next maintenance window i'm gonig to enable netlogon debugging as this will allow me to see which DC my client/member servers are caching.  



Expert Comment

ID: 24350758
Try making the other DC's in that site global catalog servers
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Author Comment

ID: 24353489
We have two DC's in the site that are Global Catalog Servers.  

Author Comment

ID: 24353573
Having one, two, or zero GC's in the same site shouldn't make a difference because our DC's are the most up to date / authoritative information wise for our Child Domain.  GC's just store limited attributes about all records in other domains.  Having two DNS servers should be all that is required.    

Expert Comment

ID: 24354058
In a native-mode domain, a Global Catalog server is a requirement for logging on to the domain. For this reason, it is advisable to have at least one Global Catalog server in a site. If a Global Catalog is not available in a site and there is another Global Catalog server in a remote site, the server in the remote site can be used for the logon process. If no Global Catalog is available in any site, the logon process proceeds with cached logon information

Accepted Solution

esbfern earned 0 total points
ID: 25849786
Turned out that when clients try and discover if a DC is down, it's a little bit more than a ping, but not much.  So should a DC take a few minutes to shutdown, a client may continue to keep trying to use the DC that is in it's cache.  we created a shutdown scrip that turns on the windows firewall to stop all traffic from the DC.  We then created one for a start up script that turns it back on.  Once implamenting this, all alerts have stoped.

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