NAME
rshd - remote shell server

SYNOPSIS
rshd [-aln]

DESCRIPTION
Rshd 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.

Rshd 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:

1) The server checks the client’s source port. If the port is
not in the range 512-1023, the server aborts the connection.

2) The server reads characters from the socket up to a null
(‘ ’) 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 (see gethostbyname(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."

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 ruserpass(3)*, which uses
the file "/etc/hosts.equiv" and the ".rhosts" file if
found in the user’s home directory. The -l option prevents
ruserpass(3)* from doing any validation based on the user’s
".rhosts" file, unless the user is the superuser.

9) 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.

"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).

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

"No remote directory."
The chdir command to the home directory failed.

"Permission denied."
The authentication procedure described above failed.

"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.