Kerberos•Benefits •Delegated Authentication •Interoperability •More Efficient Authentication •Mutual Authentication•Reasons to Use •Need Auditing at the Data Sources •Data Sources contain Row Level Security •Otherwise, DO NOT USE IT!
Access Web Site
Kerberos•IIS – Chatty by default (make sure you do this!) •IIS6 – See MS KB 917557 •IIS7 – See MS KB 954873
Troubleshooting Tools•Knowledge•SetSPN•Windows 2008 ADUC or ADSIEdit•Windows Security Logs and IIS Logs•Klist•Netmon/Wireshark and Fiddler•IIS7 Failed Request Tracing•Kerberos Logging •Event Logging and/or Debug Logs
Why So ManyStupid Settings?
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For 2010 & 2013• Uses Protocol Transition (Domain limited) (Constrained Only) • Excel Services • Visio Services • PerformancePoint • InfoPath Form Services• Does NOT Use Protocol Transition (Forest limited) (Unconstrained or Constrained) • SQL Reporting Services • BCS • Access Services • Project Server• Doesn’t usually require Kerberos • PowerPivot for SharePoint Server
References•Ken Schaefer’s Multi-Part Kerberos Blog Posts:http://www.adopenstatic.com/cs/blogs/ken/archive/2006/10/20/512.aspx•What Is Kerberos Authentication?http://technet.microsoft.com/en-us/library/cc780469%28WS.10%29.aspx•How the Kerberos Version 5 Authentication ProtocolWorkshttp://technet.microsoft.com/en-us/library/cc772815%28WS.10%29.aspx•Explained: Windows Authentication in ASP.NET 2.0http://msdn.microsoft.com/en-us/library/ff647076.aspx
References•Kerberos Authentication Tools and Settingshttp://technet.microsoft.com/en-us/library/cc738673%28WS.10%29.aspx•How To: Use Protocol Transition and ConstrainedDelegation in ASP.NET 2.0http://msdn.microsoft.com/en-us/library/ff649317.aspx•Spence Harbar’s Bloghttp://www.harbar.net
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•Kerberos is an open authentication protocol. Kerberos v5was invented in 1993 at MIT.•Authentication is the process of proving your identity to aremote system. • Your identity is who you are, and authentication is the process of proving that. In many systems your identity is your username, and you use a secret shared between you and the remote system (a password) to prove that your identity.•User password is encrypted as the user key. User key isstored in credentials cache. Once the logon session key isreceived, the user key is discarded.•Service password is encrypted as the service key.•KDCs are found through a DNS query. Service registeredin DNS by DCs.
•Showing detail behind what is happening inside of KDCbut for day-to-day, use can just remember KDC•Another reason for simplification: encryption uponencryption upon encryption…just remember it is encrypted•This is a Windows-centric Kerberos presentation•Load balanced solutions need service account•All web applications hosted using the same SPN have tobe hosted with the same account•Use A records, not CNAME records
•Terms•Key Distribution Center (KDC) – In Windows AD, KDClives on domain controllers (DC), KDCs share a long termkey across all DCs.•KDC security account database – In Windows, it is ActiveDirectory•Authorization Service (AS) – part of the KDC•Ticket Granting Service (TGS) – part of the KDC•Ticket Granting Ticket (TGT) - A users initial ticket fromthe authentication service, used to request service tickets,and meant only for use by the ticket granting service.Keeps the user from having to enter password each time aticket is requested.
Tickets•Ticket Granting Ticket (TGT) •A users initial ticket from the authentication service •Used to request service tickets •Meant only for use by the ticket-granting service. •Service ticket for the KDC (service class = krbtgt)•Service Ticket •Enables the ticket-granting service (TGS) to safely transport the requesters credentials to the target server or service.
Tools•Knowledge•SetSPN•Windows Security Logs•Windows 2008 ADUC or ADSIEdit•Kerbtray or Klist•Netmon and Fiddler•IIS Logs and IIS7 Failed Request Tracing•LDP•Kerberos Logging •Event Logging and/or Debug Logs
•Troubleshooting • Have user logon and logoff if they don’t regularly: TGTs are only renewable for so long and then they expire (7 day default), then password has to be re- entered. • Remember that authenticators contain the current time. Check for time sync issues.
•Common Issues • Missing SPN • Duplicate SPN • SPN assigned to wrong service account • Times are out of sync • Client TGT expired (7 days) • IE and non-default ports
•Request TGT (Remember there is even more complexity)1. User (client) logs into workstation entering their password.2. Client builds an authentication service request containing the user’s username (KPN), the SPN of the TGS, and encrypts the current time using the user’s password as an authenticator.3. Client sends these three items to the KDC.4. KDC get user’s password from AD, decrypts time and verifies it is valid.5. AS generates a logon session key and encrypts with the user’s password. AS generates a service ticket which contains a logon session key and the user’s KPN encrypted with the AS shared key. This is a special service ticket called a Ticket Granting Ticket (TGT).
•Request TGT (Remember there is even more complexity)6. KDC sends both to the client.7. Client decrypts logon session key using its password and stores the logon session key in cache. The client stores the TGT in cache.
•Access Service (Remember there is even more complexity)1. User (client) encrypts the current time using the logon session key in cache creating an authenticator and sends the authenticator, the user’s KPN, the name of the target service (SPN), and the TGT to the TGS.2. TGS decrypts the TGT using its shared key to access the logon session key. The logon session key is used to decrypt the authenticator and confirms the time is valid.3. TGS extracts the user’s KPN from the TGT. TGS generates a service session key and encrypts the service session key using the logon session key. TGS uses server session key to generate service ticket and encrypts it using service’s password.4. TGS sends service session key and the service ticket to the client.
•Access Service (Remember there is even more complexity)5. Client decrypts service session key using cached logon session key, adds current time (as well as other items), and encrypts with the service session key to create an authenticator.6. Client sends ticket and authenticator to remote server which runs service.7. Service decrypts service ticket accessing the server session key and the KPN. Using the service session key, the service decrypts the authenticator and confirms the current time is valid. A Windows access token is generated8. (Optional) If client requests mutual authentication, service encrypts current time using the service session key creating an authenticator and sends to the client.9. Clients decrypts authenticator and validates time.
Troubleshooting Tools• Patience – Test methodically and• Knowledge - Know your Forests, Domains, Trusts, Functional Levels…get a basic lay of the land.• Always test from a different machine than the web server or domain controller!• SetSPN• Windows Security Logs• Windows 2008 ADUC• Kerbtray• Netmon and Fiddler• IIS Logs and IIS7 Failed Request Tracing• Kerberos Logging • Event Logging and/or Debug Logs
Common Issues that break Kerberos • Times are out of sync – authenticators contain current time • Missing SPN • Duplicate SPN • SPN assigned to wrong service account • IIS Providers are incorrect (For IIS 5 or 6, see http://support.microsoft.com/kb/215383) • IIS 7 – remember Kernel mode authentication and check settings • Client TGT expired (7 days expiration – have user logon and logoff, no reboot required) • IE and non-default ports