RSHD(8) UNIX Programmer's Manual RSHD(8) NAME rshd - remote shell server SYNOPSIS rshd [-aln] DESCRIPTION _R_s_h_d is the server for the _r_c_m_d(3) routine and, conse- quently, for the _r_s_h(1) program. The server provides remote execution facilities with authentication based on privileged port numbers from trusted hosts. _R_s_h_d listens for service requests at the port indicated in the ``cmd'' service specification; see _s_e_r_v_i_c_e_s(5). When a service request is received the following protocol is ini- tiated: 1) The server checks the client's source port. If the port is not in the range 512-1023, the server aborts the connection. 2) The server reads characters from the socket up to a null (`\0') byte. The resultant string is interpreted as an ASCII number, base 10. 3) If the number received in step 2 is non-zero, 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. The source port of this second connection is also in the range 512-1023. 4) The server checks the client's source address and requests the corresponding host name (see _g_e_t_h_o_s_t_- _b_y_a_d_d_r(3), _h_o_s_t_s(5) and _n_a_m_e_d(8)). If the hostname cannot be determined, the dot-notation representation of the host address is used. If the hostname is in the same domain as the server (according to the last two components of the domain name), or if the -a option is given, the addresses for the hostname are requested, verifying that the name and address correspond. If address verification fails, the connection is aborted with the message, ``Host address mismatch.'' 5) A null terminated user name of at most 16 characters is retrieved on the initial socket. This user name is interpreted as the user identity on the client's machine. 6) A null terminated user name of at most 16 characters is retrieved on the initial socket. This user name is interpreted as a user identity to use on the server's Printed 11/24/99 September 11, 1989 1 RSHD(8) UNIX Programmer's Manual RSHD(8) machine. 7) 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. 8) _R_s_h_d then validates the user using _r_u_s_e_r_o_k(3), which uses the file ``/etc/hosts.equiv'' and the ``.rhosts'' file found in the user's home directory. The -l option prevents _r_u_s_e_r_o_k(3) from doing any validation based on the user's ``.rhosts'' file, unless the user is the superuser. 9) 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 esta- blished by _r_s_h_d. Transport-level keepalive messages are enabled unless the -n option is present. The use of keepalive messages allows sessions to be timed out if the client crashes or becomes unreachable. DIAGNOSTICS Except for the last one listed below, all diagnostic mes- sages are returned on the initial socket, after which any network connections are closed. An error is indicated by a leading byte with a value of 1 (0 is returned in step 9 above upon successful completion of all the steps prior to the execution of the login shell). ``locuser too long'' The name of the user on the client's machine is longer than 16 characters. ``remuser too long'' The name of the user on the remote machine is longer than 16 characters. ``command too long '' The command line passed exceeds the size of the argument list (as configured into the system). ``Login incorrect.'' No password file entry for the user name existed. ``No remote directory.'' The _c_h_d_i_r command to the home directory failed. ``Permission denied.'' The authentication procedure described above failed. Printed 11/24/99 September 11, 1989 2 RSHD(8) UNIX Programmer's Manual RSHD(8) ``Can't make pipe.'' The pipe needed for the stderr, wasn't created. ``Can't fork; try again.'' A _f_o_r_k by the server failed. ``: ...'' The user's login shell could not be started. This message is returned on the connection associated with the stderr, and is not preceded by a flag byte. SEE ALSO rsh(1), rcmd(3), ruserok(3) BUGS The authentication procedure used here assumes the integrity of each client machine and the connecting medium. This is insecure, but is useful in an ``open'' environment. A facility to allow all data exchanges to be encrypted should be present. A more extensible protocol (such as Telnet) should be used. Printed 11/24/99 September 11, 1989 3