rshd - remote shell server
rshd [-aklnL]
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 options are as follows:
-a Ask hostname for verification.
-k Use Kerberos authentication.
-l Prevent any authentication based on the user's
``.rhosts'' file,
unless the user is logging in as the superuser.
-n Disable keep-alive messages.
-L Log successful accesses very verbosely.
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:
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 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.''.
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(3), which
uses the file
/etc/hosts.equiv and the .rhosts file found in the user's home directory.
The -l option prevents ruserok(3) 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.
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.
The -L option causes all successful accesses to be logged to
syslogd(8)
as auth.info messages.
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).
Locuser too long. [Toc] [Back]
The name of the user on the client's machine is
longer than 16
characters.
Ruser too long. [Toc] [Back]
The name of the user on the remote machine is longer
than 16
characters.
Command too long. [Toc] [Back]
The command line passed exceeds the size of the argument list (as
configured into the system).
Remote directory. [Toc] [Back]
The chdir(2) command to the home directory failed.
Permission denied. [Toc] [Back]
The authentication procedure described above failed
or there is
no password file entry for the specified user.
Can't make pipe.
The pipe needed for the stderr, wasn't created.
Can't fork; try again.
A fork(2) 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.
rsh(1), rcmd(3), ruserok(3)
The authentication procedure used here assumes the integrity
of each
client machine and the connecting medium. This is insecure,
so ssh(1) or
kerberos (see `info heimdal') should be used instead.
rshd does not currently support encryption of the datastream
when Kerberos
authentication is used.
A more extensible protocol (such as ssh(1)) should be used.
OpenBSD 3.6 June 4, 1993
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