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DelegatedAuthentication

Sample app showing delegated authentication

Setup

Domain Controller

You need a domain controller and it should probably have DNS configured. Domain should be contoso.com.

App Host

You should run these apps on a separate application host. You should also set up DNS entries to map the SPNs below. Assuming an IP 10.0.0.3:

  • service.contoso.com => 10.0.0.3
  • delegated.contoso.com => 10.0.0.3

These should be A records and not CNAMEs.

Service Accounts

App Service Account

  • Username: appserviceacct
  • Password: P@ssw0rd!
  • SPN (servicePrincipalName): host/service.contoso.com
  • delegate to (msDS-AllowedToDelegateTo): host/delegated.contoso.com

Delegate Service Account

  • Username: appdelegateacct
  • Password: P@ssw0rd!
  • SPN (servicePrincipalName): host/delegated.contoso.com

S4U Proxy (Protocol Transition)

  • Username: s4u

App Host Permissions

The application needs to run with the SeImpersonatePrivilege privilege. Normally you host this sort of process as a Windows Service (e.g. via IIS), which inherits this permission from the service control. In this case you need to assign this privilege since this sample scenario doesn't run as a service. You can do this a few ways.

  • Add the contoso\hostserviceacct account to the local administrators group. Don't do that.
  • Assign the privilege via Local Security Policy. Use the "Impersonate a Client After Authentication" policy and add the contoso\hostserviceacct account.
  • Apply via group policy. Maybe. This means this account would have this permission on every host, and that's bad.
  • Run it as a service. This is preferred in the long run.

You'll note that running this sample indicates the delegated app receives an identification token. This means the app cannot then further delegate to other apps via 3rd or 4th hop. You can also include the contoso\appdelegateacct in the SeImpersonatePrivilege for further effect.

Running the Apps

The service and delegated apps need to run as the service accounts created previously. This is most easily done using the PSExec tool. As it turns out, therunas tool is not good to use in this case because it tends to uss restricted tokens, which complicates troubleshooting.

You need to start the apps in the following order:

  1. PsExec.exe -u contoso\hostserviceacct -p P@ssw0rd! -h -i -d c:\sample\serverapp.exe
  2. PsExec.exe -u contoso\appdelegateacct -p P@ssw0rd! -h -i -d c:\sample\delegatedapp.exe
  3. clientapp.exe host.contoso.com 5555 delegated.contoso.com 5655

Note that you may have to open the 5555 and 5655 ports on the firewall.

S4U Impersonation

You can attempt S4U/Proxy impersonation, AKA "I've just got a username but wan't to impersonate a user for delegation" by passing a 5th parameter to the client app.

App Behavior

The apps work in a client => service => backend relay. The client will use the current user's identity and authenticate to the service app. Once the service has authenticated the user, it will forward the original request to the delegated backend app using the current impersonated identity.

You should see log messages in the console output indicating the various steps of the process. Note that the client is set up to wait 10 seconds and then constantly poll the server with messages.

The communications mechanism is a modified implementation found in the NSSPI sample. Don't use it as a basis for delegated communications. It's ...wonky.

Debugging

You can run all three apps locally without any parameters and it will route accordingly. This will inherently work every time because its the same user running each application so there's no security boundary being crossed. Note that the client is set up to delay for 10 seconds so the other two apps have time to spin up.

Dependencies