Remote shell daemon
Note:
You must be root or have the right abilities to start this daemon.
Options:
- -a
- Request the address for the hostname, and verify whether the address and name correspond.
- -l
- Unless the user is the superuser, prevent any validation based on the user's .rhosts file.
- -n
- Disable transport-level keepalive messages.
Description:
The rshd daemon is the server for the
rcmd()
function and, consequently, for the
rsh
utility. The daemon provides remote execution
facilities with authentication based on privileged port numbers from trusted hosts.
The rshd daemon is started when
inetd
receives a service request at the port indicated by the cmd entry
(inetd listens for service requests specified in the
inetd.conf
file at a port defined in the
services file).
Note that the descriptions in the default inetd.conf file are
commented out; uncomment the ones that you want to use.
The following protocol is initiated:
- The server checks the client's source port.
If the port isn't in the range 512–1023, the server aborts the connection.
- 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 2 is nonzero, it's interpreted as the port number of a secondary stream to be
used for standard error.
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 and requests the corresponding hostname (see
gethostbyaddr(),
the
/etc/hosts
file, and
named).
If the hostname can't 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
address for the hostname is requested, and the server
verifies whether the address and name correspond. If address
verification fails, the connection is aborted with the
message, Host address mismatch.
- A NULL-terminated username of at most 16 characters is
retrieved on the initial socket. This username is
interpreted as the user ID on the client's machine.
- A NULL-terminated username of at most 16 characters is
retrieved on the initial socket. This username is
interpreted as a user ID to use on the server'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 boundary on the size of the system's argument list.
- The rshd server then validates the user by means of
ruserok(),
which uses the file /etc/hosts.equiv and the file
.rhosts 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.
- 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.
Transport-level keepalive messages, which allow sessions to
be timed out if the client crashes or becomes unreachable,
are enabled unless the -n option is given.
Diagnostics
Except for the last one listed below, 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 9 above upon successful completion of all
the steps prior to the execution of the login shell).
- Can't fork; try again.
- A fork() by the server failed.
- Can't make pipe.
- The pipe needed for standard error wasn't created.
- Command too long.
- The command line passed is > the size of the argument list (as configured into the system).
- Locuser too long.
- The name of the user on the client's machine is > 16 characters.
- Login incorrect.
- No password file entry for the username existed.
- Permission denied.
- The authentication procedure described above failed.
- Remote directory.
- The chdir command to the home directory failed.
- Ruser too long.
- The name of the user on the remote machine is > 16 characters.
- <shellname>: ...
- The user's login shell couldn't be started.
This message, which is returned on the connection associated with standard error, isn't preceded by a flag byte.
Caveats:
The authentication procedure used here assumes the integrity of each client machine and the connecting medium.
Though insecure, this procedure is useful in an open environment.