Many people consider configuring Kerberos authentication and making it work as a daunting task. One of the reasons for this is different configuration elements involved in the process.
One such important configuration is setting the appropriate SPN.
There are three important elements that needs to be considered while setting the Kerberos SPN for our application. I have listed them below:
1.How users will browse the application?
Users can browse the application with machine name or with a custom domain name. In most of the cases browsing with machine name does not need an SPN registered. But there are few exceptions.
2.The application pool identity
3.How server is going to decrypt the Kerberos token forwarded by the client.
There are two important properties under
system.webServer/security/authentication/windowsAuthentication
- useAppPoolCredentials: When we have useAppPoolCredentials set to true, server will decrypt the Kerberos traffic using application pool identity.
- useKernelMode: When we have useKernelMode set to true, server will decrypt the Kerberos traffic using machine account.
Even with good understanding of Kerberos workflow and above-mentioned elements, sometimes people get confused on what SPN to set.
I have included the below table, which can be a quick reference for the SPN’s needed for different combinations of host name and application pool identities.
|
URL
|
Use Kernel Mode
|
Use App pool Credentials
|
Application pool Identity
|
SPN requirement
|
|
Browsed with machine name
|
True
|
False
|
Machine account
|
No additional SPN’s are needed
HOST SPN will be sufficient
|
|
Browsed with machine name
|
True
|
False
|
Custom domain account
|
No additional SPN’s are needed
HOST SPN will be sufficient
|
|
Browsed with machine name
|
False
|
True
|
Machine account
|
No additional SPN’s are needed
HOST SPN will be sufficient
|
|
Browsed with machine name
|
False
|
True
|
Custom domain account
|
Setspn –a HTTP/<machine name> <custom account name>
|
|
Browsed with machine name
|
True
|
True
|
Machine account
|
No additional SPN’s are needed
HOST SPN will be sufficient
|
|
Browsed with machine name
|
True
|
True
|
Custom domain account
|
Setspn –a HTTP/<machine name> <custom account name>
|
|
Browsed with custom host name
|
True
|
False
|
Machine account
|
Setspn –a HTTP/<custom host name> <machine name>
|
|
Browsed with custom host name
|
True
|
False
|
Custom domain account
|
Setspn –a HTTP/<custom host name> <machine name>
|
|
Browsed with custom host name
|
False
|
True
|
Machine account
|
Setspn –a HTTP/<custom host name> <machine name>
|
|
Browsed with custom host name
|
False
|
True
|
Custom domain account
|
Setspn –a HTTP/<custom host name> <custom account name>
|
|
Browsed with custom host name
|
True
|
True
|
Machine account
|
Setspn –a HTTP/<custom host name> <machine name>
|
|
Browsed with custom host name
|
True
|
True
|
Custom domain account
|
Setspn –a HTTP/<custom host name> <custom account name>
|
|
Browsed with machine name
|
False
|
False
|
Machine account
|
No additional SPN’s are needed
HOST SPN will be sufficient
|
|
Browsed with machine name
|
False
|
False
|
Custom domain account
|
Setspn –a HTTP/<machine name> <custom account name>
|
|
Browsed with custom host name
|
False
|
False
|
Machine account
|
Setspn –a HTTP/<custom host name> <machine name>
|
|
Browsed with custom host name
|
False
|
False
|
Custom domain account
|
Setspn –a HTTP/<custom host name> <custom account name>
|
Hope this helps.
