`udp_preference_limit` is a `krb5.conf` setting to control how a KDC-REQ message is sent. If the size of the message is greater than the value of `udp_preference_limit`, TCP will be used; otherwise UDP will be used. Before this change, setting it to zero is ignored and the message is sent with UDP. With this change, setting it to zero forces the message to be sent with TCP. This behavior is consistent with other Kerberos 5 vendors.