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The rexecd daemon listens for service requests at the port indicated in the exec service specification; see services(4). When a service request is received, the following protocol is initiated: The server reads characters from the socket up to a null (`\0') byte. The resultant string is interpreted as an ASCII number, base 10. If the number received in step 1 is nonzero, it is interpreted as the port number of a secondary stream to be used for the stderr. A second connection is then created to the specified port on the client's machine. A null-terminated username of at most 16 bytes is retrieved on the initial socket. A null-terminated, unencrypted password of at most 80 bytes is retrieved on the initial socket. A null-terminated command to be passed to a shell is retrieved on the initial socket. The length of the command is limited by the upper bound on the size of the system's argument list. The rexecd server then validates the user as is done at login time and, if the authentication was successful, changes to the user's home directory, and establishes the user and group protections for the user. If any of these steps fail, the connection is aborted with a diagnostic message returned. A null byte is returned on the initial socket and the command line is passed to the normal login shell of the user. The shell inherits the network connections established by rexecd.
IndicatingLogin incorrect as opposed to Password incorrect is a security breach that allows people to probe a system for users with null passwords.