Question
Answer and Explanation
The error message "The target principal name is incorrect" typically indicates that the service principal name (SPN) used for a particular service or application doesn't match the expected or registered SPN. SPNs are identifiers used by Kerberos authentication to associate a service instance with a logon account, which might be a user account, a machine account, or a built-in service account. When an SPN is incorrect, the authentication process fails, usually causing applications to fail to communicate or users being unable to authenticate.
Here is a breakdown of why this issue occurs and how to troubleshoot it:
1. Understanding SPNs:
- An SPN is a unique identifier for a service instance, constructed in a specific format like SERVICECLASS/HOSTNAME:PORT or SERVICECLASS/FQDN:PORT (where FQDN is fully qualified domain name). Examples are `HTTP/webserver.example.com` or `MSSQLSvc/dbserver.example.com:1433`. The SERVICECLASS identifies what type of service this is, for example 'HTTP' or 'MSSQLSvc'.
2. Mismatched SPNs:
- The most common reason for the error is that the client tries to access a service using an incorrect or non-registered SPN, usually after the domain/name of service was changed. This can occur for many different reasons. For example:
- If the client is trying to connect to an instance named ‘service.example.com’, but SPN is only configured with 'service', you'd face this error.
- When applications are using outdated configurations. - An application uses incorrect syntax to define SPNs, sometimes with extra forward or backslashes (\\ or //)
- When you are trying to connect to service using incorrect/outdated credentials, but you expect your connection to work using Kerberos/Negotiate. This might happen when for example you use one username/password to check for data availability (to check network reachability etc), but for production traffic your system expects kerberos authorization.
3. Incorrect Service Configuration:
- The application server itself might have incorrect SPN registrations that don't match its configuration.
4. Firewall/DNS Issues:
- Sometimes DNS resolution errors lead to services being addressed via the wrong name (an alias rather than actual name, which won't resolve using the service's account), causing Kerberos not to find any SPN associated with server.
5. Kerberos Related Configuration problems
- The local Kerberos client/system configuration might have related issues: e.g. it is trying to retrieve ticket from an old, unavailable domain, misconfigured 'realm' etc.
To resolve the "target principal name is incorrect" error, the most basic approaches you should pursue include:
- Correct SPN Configuration
- Re-register SPN using setspn
- Clean your system DNS records and use proper host names (e.g. avoid aliased servers for crucial system services), correct DNS domain mapping for each hostname
- Review firewall config in case some communication might get intercepted at OS or Network Layer.
To avoid such issues, always verify SPN entries during service installation or configuration and ensure to check the client systems when errors of this type show up. Regularly validating DNS and networking rules that would change your host names resolution is good practice too.