Setting SPN causes Credential Prompts on SharePoint Site

I’ll keep this post short. So the other day noticed after doing some audits that some sites were not using kerberos, even though the SharePoint Web Application Auth Providers settings were already configured to use Kerberos. Which in most cases will always fall back to the less secure NTLM auth method. Sure enough the SPN was not configured for the service.

So in a test environment…

SETSPN -S HTTP/SPSite domain\webappserviceaccount

To my dismay when I attempted to access the site I was present with a credential prompt, entering my creds did auth succeed, but it shouldn’t have prompted for creds considering all requirements for Kerberos to work is there, and if that failed fall back to NTLM. In either case the SSO part is usually handled by the internet security settings on the client machines. Since these are all managed by company based GPOs. I know they were in fact good as nothing there has changed, and the site was working fine before setting the SPN.

Googling this I only a couple examples of this, like here I attempted a reboot and that failed. Since it was test I could start over again, and verified the only change was the setting of the SPN which caused this to happen, even though everyone is stating it’s not related. It this case it def was.

The only solution I found from my testing was to:

  1. Go to the Web Application in CA
  2. Highlight the problematic Site, click on Auth Providers in the Ribbon
  3. Click default (claims Auth)
  4. Switch it back to NTLM. (Watch Front End server resources spike as IIS is reconfigured) (Can’t remember if reboot was required here)
  5. after it’s done access the site. Ensure no prompts for creds and SSO works as intended.
  6. ensure SPN exists and is proper.
  7. Reset the Site Auth providers setting back to Kerberos. (Again reboot may be required)
  8. Access the site, SSO (no prompt for creds after already logged on) and Kerberos (klist shows a TGTicket) should work as expected.

When I went to implement this in production I figured it was less risky to just set the auth provider to NTLM before even setting the SPN thus there should be no point in time where it prompts for credentials for the end user. Despair ensues…

So set auth to NTLM.. Prompts for creds (wait what…), even worse enter creds many times and site will not load… WAT!?!

In a panic I call my superior, he wants to look through the log, but there show no major indicators (event viewer) I mention my usual quick n easy first thing to try… yeah, reboot! Sure enough a reboot resolved the issue. Not sure why that happened but conitue on as above from this point and sure enough got both SSO and kerberos working as intended.

Let this be a friendly reminder that even though you test stuff in test, even the slightest change in your procedure can have devastating consequences. Hope this post was insightful for someone.