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rshd(8)
NAME
rshd - The remote shell server daemon
SYNOPSIS
rshd [-aelnxK]
OPTIONS
-a The addresses for the hostname are requested, verifying that the name
and address correspond.
-e Causes rshd to check for the /etc/nologin_hostname and /etc/nologin
files. If either exists, rshd prints its contents and exits.
-l Prevents the ruserok command from doing any validation based on the
user's .rhosts file, unless the user is the root user.
-n Disables transport-level, keep-alive messages.
-x Encrypts the data transmitted between the local host and the remote
host. This option requires that the local and remote hosts be
configured to use Kerberos authentication in the same or trusting
Kerberos realms.
If the rshd daemon is started with the -x option, only connections
initiated with the -x option from a remote host will be accepted. All
communications between the two hosts will be encrypted.
-K Specifies that only Kerberos authenticated connections will be
accepted. This option requires that the local and remote hosts be
configured to use Kerberos authentication in the same or trusting
Kerberos realms.
If the rshd daemon is started with the -K option, only connections
initiated from a host in the same or trusting Kerberos domain will be
accepted. All communications between the two hosts will be encrypted.
DESCRIPTION
The rshd daemon is the server for the rcmd(3) routine and for the rsh(1)
program. The server provides remote execution facilities with
authentication based on privileged port numbers from trusted hosts.
The rshd daemon listens for service requests at the port indicated in the
cmd service specification; see services(4). 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 to 1023, the server aborts the connection.
2. The server reads bytes from the socket up to a null (`\0') byte. The
resultant string is interpreted as an ASCII number, base 10.
3. If the number received in step 2 is nonzero, it is interpreted as the
port number of a secondary stream to be used for the stderr option. 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 to 1023.
4. The server checks the client's source address and requests the
corresponding hostname (see gethostbyaddr(3), hosts(4), 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 username of at most 16 bytes is retrieved on the
initial socket. This username is interpreted as the user identity on
the client 's machine.
6. A null-terminated username of at most 16 bytes is retrieved on the
initial socket. This username 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. The rshd daemon then validates the user. The way in which the rshd
daemon authenticates a user and transmits data depends on if the local
and remote hosts are using a basic connection or a secure connection
(Kerberos). Basic and secure connections provide user authentication,
however a secure connection also provides client and server
authentication, data encryption, data integrity, and nonrepudiation.
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, keep-alive messages are enabled unless the -n option is
present. The use of keep-alive messages allows sessions to be timed out if
the client crashes or becomes unreachable.
Basic Connection
A basic connection is one where the rshd daemon 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.
If rshd was started with the -e option, the user is not the superuser, and
either the /etc/nologin_hostname or /etc/nologin file exists, rshd prints
the contents of the first file found and aborts the connection. If the
file has a zero length, rshd prints a logins disabled message.
Secure Connection
A secure connection is one where the rshd daemon authenticates a user by
using Kerberos. Kerberos is a client/server application that authenticate
the client, server, and user; encrypt data; and ensure data integrity and
nonrepudiation. See your system administrator to determine if your system
is running Kerberos. See Security Administration for more information about
Kerberos.
Kerberos authenticates by using secret-key cryptography and tickets between
Kerberos clients and Kerberos server in the same or trusting Kerberos
realms. Once authenticated by Kerberos, users receive a Kerberos Ticket
Granting Ticket (TGT). Users with a valid TGT are not prompted for a
username or password when the remote host is in the same or trusting
Kerberos realm.
Alternatively, you can configure the rsh, rlogin, and rcp commands and
applications that use the rcmd function to automatically use a Secure Shell
connection by enabling the Secure Shell EnforceSecureRutils keyword in the
/etc/ssh2/ssh2_config file or in a user's $HOME/.ssh2/ssh2_config file.
When the EnforceSecureRutils keyword is enabled:
· The rsh command can use only host-based authentication to authenticate
users. Secure Shell host-based authentication uses the .rhosts file as
described in Basic Connection, but also requires additional
configuration as described in Security Administration.
· The sshd daemon runs and spawns the srcmd child process; the rshd and
rlogind daemons do not run.
See Security Administration for more information about Secure Shell
authentication and the EnforceSecureRutils keyword.
After it is determined that Secure Shell will be used, all authentication
and communication between the client and server will use the Secure Shell
connection. A connection is not established if a user cannot be
authenticated.
DIAGNOSTICS
Except for the last diagnostic message listed, 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 username existed.
Logins disabled.
The server is currently not accepting connections.
No remote directory.
The chdir command to the home directory failed.
Permission denied.
The authentication procedure previously described failed.
Can't make pipe.
The pipe needed for the stderr option, but it was not 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 option, and is not
preceded by a option byte.
Kerberos is not installed.
An attempt was made to start rshd using the -K flag without first
configuring the system as part of a Kerberos realm or domain.
FILES
/usr/sbin/rshd
Specifies the command path
/etc/nologin
Stops logins. In a cluster, there is also /etc/nologin_hostname.
/etc/ssh2/ssh2_config
Specifies Secure Shell client configuration information.
SEE ALSO
Commands: kinit(1), kdestroy(1), klist(1), rcp(1), rlogin(1), rsh(1),
ssh2(1)
Functions: rcmd(3), ruserok(3)
Files: hosts.equiv(4), rhosts(4), ssh2_config(4)
Guides: Security Administration
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