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.\"     from: @(#)rshd.8	8.1 (Berkeley) 6/4/93
.\"
.Dd January 22, 2000
.Dt RSHD 8
.Os
.Sh NAME
.Nm rshd
.Nd remote shell server
.Sh SYNOPSIS
.Nm
.Op Fl alnL
.Sh DESCRIPTION
The
.Nm
server is the server for the
.Xr rcmd 3
routine and, consequently, for the
.Xr rsh 1
program.
The server provides remote execution facilities
with authentication based on privileged port numbers from trusted hosts.
.Pp
The
.Nm
server listens for service requests at the port indicated in
the ``cmd'' service specification; see
.Xr services 5 .
When a service request is received the following protocol
is initiated:
.Bl -enum
.It
The server checks the client's source port.
If the port is not in the range 512-1023, the server
aborts the connection.
.It
The server reads characters from the socket up
to a null (`\e0') byte.
The resultant string is interpreted as an
.Tn ASCII
number, base 10.
.It
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
.Em 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.
.It
The server checks the client's source address
and requests the corresponding host name (see
.Xr gethostbyaddr 3 ,
.Xr hosts 5
and
.Xr 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
.Fl 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.''
.It
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
.Em client Ns 's
machine.
.It
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
.Sy server Ns 's
machine.
.It
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.
.It
.Nm
then validates the user using
.Xr ruserok 3 ,
which uses the file
.Pa /etc/hosts.equiv
and the
.Pa .rhosts
file found in the user's home directory.
The
.Fl l
option prevents
.Xr ruserok 3
from doing any validation based on the user's ``.rhosts'' file,
unless the user is the superuser.
.It
If the file
.Pa /etc/nologin
exists and the user is not the superuser,
the connection is closed.
.It
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
.Nm "" .
.El
.Pp
Transport-level keepalive messages are enabled unless the
.Fl n
option is present.
The use of keepalive messages allows sessions to be timed out
if the client crashes or becomes unreachable.
.Pp
The
.Fl L
option causes all successful accesses to be logged to
.Xr syslogd 8
as
.Li auth.info
messages.
.Sh 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 10 above upon successful completion
of all the steps prior to the execution of the login shell).
.Bl -tag -width indent
.It Sy Locuser too long.
The name of the user on the client's machine is
longer than 16 characters.
.It Sy Ruser too long.
The name of the user on the remote machine is
longer than 16 characters.
.It Sy Command too long  .
The command line passed exceeds the size of the argument
list (as configured into the system).
.It Sy Login incorrect.
No password file entry for the user name existed.
.It Sy Remote directory.
The
.Xr chdir 2
to the home directory failed.
.It Sy Permission denied.
The authentication procedure described above failed.
.It Sy Can't make pipe.
The pipe needed for the
.Em stderr ,
wasn't created.
.It Sy Can't fork; try again.
A
.Xr fork 2
by the server failed.
.It Sy \*[Lt]shellname\*[Gt]: ...
The user's login shell could not be started.
This message is returned on the connection associated with the
.Em stderr ,
and is not preceded by a flag byte.
.El
.Sh SEE ALSO
.Xr rsh 1 ,
.Xr rcmd 3 ,
.Xr ruserok 3 ,
.Xr login.conf 5
.Sh 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.
.Pp
A facility to allow all data exchanges to be encrypted should be
present.
.Pp
A more extensible protocol (such as Telnet) should be used.
.Pp
.Nm
intentionally rejects accesses from IPv4 mapped address on top of
.Dv AF_INET6
socket, since IPv4 mapped address complicates
host-address based authentication.
If you would like to accept connections from IPv4 peers, you will need to run
.Nm
on top of
.Dv AF_INET
socket, not
.Dv AF_INET6
socket.