4.3 S4U2proxy Example

The following figure depicts a service obtaining a service ticket on behalf of a client to another service, a proxy service. The ticket-granting service (TGS) is the TGS for both the service and the proxy service. It is also assumed that service has already authenticated to the Key Distribution Center (KDC) and has obtained a ticket-granting ticket (TGT) to the TGS.

Figure 6: S4U2proxy Example

In step 1, Service 1 is attempting to obtain a service ticket to Service 2 on behalf of the user. Service 1 sends the KRB_TGS_REQ message with the user's service ticket for Service 1 as an additional ticket in the request. Service 1 also sets the CNAME-IN-ADDL-TKT flag in the kdc-options in the request.

The TGS makes sure the forwardable flag is set in the additional-ticket and uses its local policy to determine if Service 1 is allowed to obtain a service ticket on behalf of a user to Service 2. If these conditions are met, the TGS crafts the KRB_TGS_REP message to return a service ticket. This response will contain the cname field of the user that is taken from the additional-ticket, instead of using the cname of Service 1. The forwardable flag will be set in the service ticket. The authorization data in the service ticket will be copied from the service ticket passed to the TGS in the additional-tickets field.

In step 3, Service 1 uses the service ticket from step 2 to contact Service 2. The service ticket will contain the user's name as the cname field. Step 4 shows the KRB_AP_REP message from Service 2 to Service 1 in response to the KRB_AP_REQ message, as described in step 3.<25>