rdesktop builds the target SPN using servername argument. If an ip adress is passed as server name a corresponding target SPN will be constructed TERMSRV/<ipaddress> which is not a valid SPN.
We should add some IP detection of servername argument and make an reverse lookup for a better chance of correct SPN format providing a better end user experience.
For now, CredSSP+kerberos does not work if rdesktop connects to ip address.
rdesktop (and associated tools) is being removed from the ThinLinc product.