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24 rshd: Remote shell server

The rshd server is the server for the rcmd routine and, consequently, for the rsh (see rsh: Remote shell) program. The server provides remote execution facilities with authentication based on privileged port numbers from trusted hosts. The rshd 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 aborts the connection. However, this condition is not applied for Kerberized service.
2. The server reads characters from the socket up to a NUL (‘\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. 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 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. Rshd then validates the user using ruserok, which uses the file /etc/hosts.equiv and the .rhosts file found in the user’s home directory. The -l option prevents ruserok from doing any validation based on the user’s .rhosts file, unless the user is the superuser.
9. If the file /etc/nologin exists and the user is not the superuser, the connection is closed.
10. 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.
11. 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.
12. The -L option causes all successful accesses to be logged to syslogd (see syslogd: system service logging faclity) as ‘auth.info’ messages.

See ruserok in The GNU C Library Reference Manual, for details.

• Invoking
• Diagnostics