NAME¶
rshd —
remote shell server
SYNOPSIS¶
DESCRIPTION¶
The
rshd 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
rshd server listens for service requests at the port
indicated in the ``cmd'' service specification; see
services(5). When a service request is received the
following protocol is initiated:
- The server checks the client's source port. If the port is
not 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 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 and requests
the corresponding host name (see gethostbyaddr(3),
hosts(5) and named(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.''
- 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 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 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 using
PAM, which uses the file
/etc/hosts.equiv and the .rhosts file
found in the user's home directory. Due to the way the protocol was
designed, it is not possible to exchange information other than those
specified here for authentication. Thus PAM authentication modules such as
pam_unix should not be used with this service.
Also note that the design of the .rhosts system is
COMPLETELY INSECURE except on a carefully firewalled private network.
Under all other circumstances, rshd should be disabled
entirely.
- 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 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 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).
- Locuser too
long.
- The name of the user on the client's machine is longer than
16 characters.
- Ruser 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).
- Remote
directory.
- The chdir command to the home directory
failed.
- Permission
denied.
- The authentication procedure described above failed, or the
user requested did not exist. (These conditions are intentionally
conflated.)
- Can't make
pipe.
- The pipe needed for the stderr, wasn't
created.
- Can't fork; try
again.
- A fork 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.
SEE ALSO¶
rsh(1),
rcmd(3),
pam(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.
A more extensible protocol (such as Telnet) should be used.