Kernel-mode Authentication: 401 errors when accessing site from remote machines
- by CJM
I have several Classic ASP sites that use Integrated Windows Authentication and Kerberos delegation.
They work OK on the live servers (recently moved to a Server 2008/IIS7 servers), but do not work fully on my development PC or my development server. The IIS on both machines were configured through an IIS web deployment tool package which was exported from an old machine; the deployment didn't work perfectly, and I had to tinker a bit to get the sites working.
When accessing the apps locally on either machine, they work fine; when accessing from another machine, the user is prompted by a username/password dialog, and regardless of what you enter, ultimately it results in a 401 (Unauthorised) error.
I've tried comparing the configuration of these machines against similar live servers (that all work fine), and they seem generally comparable (given that none of the live servers are yet on IIS7.5 (Windows 7/Server 2008 R2).
These applications run in a common application pool which uses a special domain user as it's identity - this user has similar permissions on the live and development machines. On IIS6 platforms, to enable kerberos delegation, I needed to set up some SPNs for this user, and they are still in place (even though I don't believe they are needed any longer for IIS7+ due to kernel-mode authentication),
Furthermore, this account is enabled for Kerberos delegation in Active Directory, as is each machine I am dealing with.
I'm considering the possibility that the deployment might have made changes/failed to make changes to the IIS configuration thus causing this problem. Perhaps a complete rebuild (minus another web deployment attempt) would solve the problem, but I'd rather fix (thus understand) the current problem.
Any ideas so far?
I've just had another attempt at fixing this issue, and I've made some progress, but I don't have a complete fix...yet.
I've discovered that if I access the sites via IP address (than via NetBIOS name), I get the same dialog, except that it accepts my credentials and thus the application works - not quite a fix, but a useful step.
More interestingly, I discovered that if I disable Kernel-mode authentication (in IIS Manager Website Authentication Advanced Settings), the applications work perfectly. My foggy understanding is that this is effectively working in the pre-IIS7 way. A reasonable short-term solution, but consider the following explicit advice from IIS on this issue:
By default, IIS enables kernel-mode authentication, which may improve authentication performance and prevent authentication problems with application pools configured to use a custom identity. As a best practice, do not disable this setting if Kerberos authentication is used in your environment and the application pool is configured to use a custom identity.
Clearly, this is not the way my applications should be working. So what is the issue?
