At times it may be necessary for a principal to allow a service to perform an operation on its behalf. The service must be able to take on the identity of the client, but only for a particular purpose. A principal can allow a service to take on the principal's identity for a particular purpose by granting it a proxy.
The PROXIABLE flag in a ticket is normally only interpreted by the ticket-granting service. It can be ignored by application servers. When set, this flag tells the ticket-granting server that it is OK to issue a new ticket (but not a ticket-granting ticket) with a different network address based on this ticket. This flag is set by default.
This flag allows a client to pass a proxy to a server to perform a remote request on its behalf, e.g., a print service client can give the print server a proxy to access the client's files on a particular file server in order to satisfy a print request.
In order to complicate the use of stolen credentials, Kerberos tickets are usually valid from only those network addresses specifically included in the ticket (It is permissible to request or issue tickets with no network addresses specified, but we do not recommend it). For this reason, a client wishing to grant a proxy must request a new ticket valid for the network address of the service to be granted the proxy.
The PROXY flag is set in a ticket by the TGS when it issues a proxy ticket. Application servers may check this flag and require additional authentication from the agent presenting the proxy in order to provide an audit trail.