rshd - remote shell server


rshd [-alnL]


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

The following options are supported by rshd(1):

Ask host name for verification.
Prevent any authentication based on the user's .rhosts file.
Disable keep-alive messages.
Log successful accesses very verbosely.

The rshd(1) server listens for service requests at the port indicated in the "cmd" service specification. When a service request is received, the following protocol is initiated:

  1. The server checks the client's source port. If the port is not in the range 512-1023, the server ends 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 computer. 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 gethostbyaddr(3)). If the host name cannot be determined, the dot-notation representation of the host address is used. If the host name 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 host name are requested, verifying that the name and address correspond. If address verification fails, the connection is ended with the message, "Host address mismatch."
  5. A null terminated user name of at most 128 characters is retrieved on the initial socket. This user name is interpreted as the user identity on the client computer.
  6. A null terminated user name of at most 128 characters is retrieved on the initial socket. This user name is interpreted as a user identity to use on the server computer.
  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. The rshd(1) utility then validates the user using iruserok(3), which uses the file /etc/hosts.equiv and the .rhosts file found in the user's home directory. If the daemon is not running as the SYSTEM user, the password of the user account used to run the daemon must have been previously stored using the regpwd(1) utility. The -l option prevents ruserok(3) from doing any validation based on the user's .rhosts file, unless the user is the superuser. If the iruserok(3) function reports that the address might have been spoofed, the connection is refused.

    The file /etc/nologinexists.

    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 rshd(1).

Transport-level keep-alive messages are enabled unless the -n option is present. The use of keep-alive messages allows sessions to be timed out if the client crashes or becomes unreachable.

The -L option causes all successful accesses to be logged to syslogd(1) as messages.

Interix authentication

On traditional systems, a user can run rsh(1) from a system identified in the user's .rhosts file without providing a password. This is because on a traditional system, the rshd(1) program is run as the superuser, and can take any login identity. On Interix, this is not possible. Instead, the user must run the regpwd(1) utility on th host to store the user's password before running rsh.


Except for the last diagnostic message listed in this section, all diagnostic messages 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 10 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 computer is longer than 128 characters.
Ruser too long.
The name of the user on the remote computer is longer than 128 characters.
Command too long.
The command line passed exceeds the size of the argument list (as configured into the system).
Remote directory.
The chdir(1) command to the home directory failed.
Permission denied.
The authentication procedure described above failed or there is no password file entry for the specified user.
Can't make pipe.
The pipe needed for the stderr, was not created.
Can't fork; try again.
A fork(1) by the server failed.
<shellname>: ...
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.


The rsh(1) and rshd(1) programs make use of the following files, if they exist:

On the target system, this file contains the names of systems and users allowed to login. See rlogind(1) for more information.
On the target system, this file can contain the names of systems (and user names) allowed to login.


The authentication procedure used here assumes the integrity of each client computer 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.