Orchestrating Kerberos Authentication .... SPN Cheat Sheet

When it comes to orchestrating Kerberos authentication on IIS websites most people get it wrong when the question of Service Principal Names (SPN) comes up. Microsoft PSS gets a huge number of such issues.

When you set a SPN you are telling the Key Distribution Centre (KDC) that a service of this type and this name is being provided by this account and you can issue Kerberos tickets for it.

Here I have put together a table of the most common configuration scenarios and the SPNs that are required. I am assuming the netbios name of your server is alpha and its FQDN is alpha.example.com. I am also assuming that you will use the setspn tool to set a particular spn.

Scenario I

Website Application Pool Identity : Preconfigured Identity (Network Service/Local Service/Local System)

Website accessed with URL : http://alpha/

Theoritically you will need the two spns below. But you do not have to set them as the netbiosname account will already have the HOST/ SPN which is  a superset.

setspn -A HTTP/alpha alpha

setspn -A HTTP/alpha.example.com alpha

You do not have to explicitly set these as the alpha account already has a SPN HOST/alpha by default.

Scenario II

Website Application Pool Identity : Preconfigured Identity (Network Service/Local Service/Local System)

Website accessed with URL : http://www.example.com

setspn -A HTTP/www.example.com alpha

Note : This URL entry has to be an Host (A) Record in the DNS and not a CNAME.

Scenario III

Website Application Pool Identity : CustomDomainIdentity

Website accessed with URL : http://alpha/

setspn -A HTTP/alpha CustomDomainIdentity

setspn -A HTTP/alpha.example.com CustomDomainIdentity

Scenario IV

Website Application Pool Identity : CustomDomainIdentity

Website accessed with URL : http://www.example.com

setspn -A HTTP/www.example.com CustomDomainIdentity

Scenario V

Load Balanced Website on servers alpha and beta

Website Application Pool Identity : CustomDomainIdentity(Mandatory)

Website accessed with URL : http://www.example.com

setspn -A HTTP/www.example.com CustomDomainIdentity

If the website is accessed with individual netbios names : http://alpha/ http://beta

setspn -A HTTP/alpha CustomDomainIdentity

setspn -A HTTP/alpha.example.com CustomDomainIdentity

setspn -A HTTP/beta CustomDomainIdentity

setspn - A HTTP/beta.example.com CustomDomainIdentity

Note : The custom domain identity running the application pool has to be added to the IIS_WPG group.

Remember : Internet Explorer treats URLs with a dot in it as an Internet Address. You have to explicitly tell IE to treat it as a Local Intranet website by adding it to the Local Intranet Zone.

 

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