RSHD(8C)

NAME
rshd, in.rshd - remote shell server

SYNOPSIS
/usr/etc/in.rshd host.port

DESCRIPTION
rshd is the server for the rcmd(3N) routine and, consequently, for the rsh(1C) program. The server provides remote execution facilities with authentication based on privileged port numbers.

rshd is invoked by inetd(8C) each time a shell service is requested, and executes the following protocol:

•   The server checks the client's source port. If the port is not in the range 512-1023, the server aborts the connection. The clients host address (in hex) and port number (in decimal) are the argument passed to rshd.

•   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 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.

•   The server checks the client's source address. If the address is associated with a host for which no corresponding entry exists in the host name data base (see hosts(5)), the server aborts the connection.

•   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 machine.

•   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.

•   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.

•   rshd then validates the user according to the following steps. The remote user name is looked up in the password file and a chdir is performed to the user's home directory. If the lookup or fails, the connection is terminated. If the chdir fails, it does a chdir to / (root). If the user is not the super-user, (user ID 0), the file /etc/hosts.equiv is consulted for a list of hosts considered “equivalent”. If the client's host name is present in this file, the authentication is considered successful. If the lookup fails, or the user is the super-user, then the file .rhosts in the home directory of the remote user is checked for the machine name and identity of the user on the client's machine. If this lookup fails, the connection is terminated.

•   A null byte is returned on the connection associated with the stderr and the command line is passed to the normal login shell of the user. The shell inherits the network connections established by rshd.

FILES
/etc/hosts.equiv

SEE ALSO
rsh(1C), rcmd(3N), syslogd(8)

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.

DIAGNOSTICS

The following diagnostic messages are returned on the connection associated with the stderr, 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 command execution).

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).

Hostname for your address unknown.
    No entry in the host name database existed for the client's machine.

Login incorrect.
    No password file entry for the user name existed.

Permission denied.
    The authentication procedure described above failed.

Can't make pipe.
    The pipe needed for the stderr, was not created.

Try again.
    A fork by the server failed.

/usr/bin/sh: ...
    The user's login shell could not be started.

In addition, daemon's status messages and internal diagnostics are logged to the appropriate system log using the syslogd(8) facility.