rsh

, remsh

, remote_shell

- remote shell

Synopsis

rsh [-n] [-a] [-K] [-PN | -PO] [-x] [-f | -F] [-l username] 
     [-k realm] hostname command

rsh hostname [-n] [-a] [-K] [-PN | -PO] [-x] [-f | -F] 
     [-l username] [-k realm] command

remsh [-n] [-a] [-K] [-PN | -PO] [-x] [-f | -F] [-l username] 
     [-k realm] hostname command

remsh hostname [-n] [-a] [-K] [-PN | -PO] [-x] [-f | -F] 
     [-l username] [-k realm] command

 hostname [-n] [-a] [-PN | -PO] [-x] [-f | -F] 
     [-l username] [-k realm] command

Description

The rsh utility connects to the specified hostname and executes the specified command. rsh copies its standard input to the remote command, the standard output of the remote command to its standard output, and the standard error of the remote command to its standard error. Interrupt, quit, and terminate signals are propagated to the remote command. rsh normally terminates when the remote command does.

The user can opt for a secure session of rsh which uses Kerberos V5 for authentication. Encryption of the network session traffic is also possible. The rsh session can be kerberized using any of the following Kerberos specific options: -a, -PN or -PO, -x, -f or -F, and -k realm. Some of these options (-a, -x, -PN or -PO, and -f or -F) can also be specified in the [appdefaults] section of krb5.conf(4). The usage of these options and the expected behavior is discussed in the OPTIONS section below. If Kerberos authentication is used, authorization to the account is controlled by rules in krb5_auth_rules(5). If this authorization fails, fallback to normal rsh using rhosts occurs only if the -PO option is used explicitly on the command line or is specified in krb5.conf(4). Also, the -PN or -PO, -x, -f or -F, and -k realm options are just supersets of the -a option.

If you omit command, instead of executing a single command, rsh logs you in on the remote host using rlogin(1).

rsh does not return the exit status code of command.

Shell metacharacters which are not quoted are interpreted on the local machine, while quoted metacharacters are interpreted on the remote machine. See EXAMPLES.

If there is no locale setting in the initialization file of the login shell (.cshrc, . . .) for a particular user, rsh always executes the command in the “C” locale instead of using the default locale of the remote machine.

The command is sent unencrypted to the remote system. All subsequent network session traffic is encrypted. See -x.

Options

The following options are supported:

-a

Explicitly enable Kerberos authentication and trusts the .k5login file for access-control. If the authorization check by in.rshd(1M) on the server-side succeeds and if the .k5login file permits access, the user is allowed to carry out the command.

-f

Forward a copy of the local credentials (Kerberos Ticket Granting Ticket) to the remote system. This is a non-forwardable ticket granting ticket. Forward a ticket granting ticket if you need to authenticate yourself to other Kerberized network services on the remote host. An example would be if your home directory on the remote host is NFS mounted by way of Kerberos V5. If your local credentials are not forwarded in this case, you cannot access your home directory. This option is mutually exclusive with the -F option.

-F

Forward a forwardable copy of the local credentials (Kerberos Ticket Granting Ticket) to the remote system. The -F option provides a superset of the functionality offered by the -f option. For example, with the -f option, if, after you connected to the remote host, your remote command attempted to invoke /usr/bin/ftp, /usr/bin/telnet, /usr/bin/rlogin, or /usr/bin/rsh, with the -f or -F options, the attempt would fail. Thus, you would be unable to push your single network sign on trust beyond one system. This option is mutually exclusive with the -f option.

-k realm

Causes rsh to obtain tickets for the remote host in realm instead of the remote host's realm as determined by krb5.conf(4).

-K

This option explicitly disables Kerberos authentication. It can be used to override the autologin variable in krb5.conf(4).

-l username

Uses username as the remote username instead of your local username. In the absence of this option, the remote username is the same as your local username.

-n

Redirect the input of rsh to /dev/null. You sometimes need this option to avoid unfortunate interactions between rsh and the shell which invokes it. For example, if you are running rsh and invoke a rsh in the background without redirecting its input away from the terminal, it blocks even if no reads are posted by the remote command. The -n option prevents this.

-PO

-PN

Explicitly request new (-PN) or old (-PO) version of the Kerberos “rcmd” protocol. The new protocol avoids many security problems prevalant in the old one and is regarded much more secure, but is not interoperable with older (MIT/SEAM) servers. The new protocol is used by default, unless explicitly specified using these options or through krb5.conf(4). If Kerberos authorization fails when using the old “rcmd” protocol, there is fallback to regular, non-kerberized rsh. This is not the case when the new, more secure “rcmd” protocol is used.

-x

Cause the network session traffic to be encrypted. See DESCRIPTION.

The type of remote shell (sh, rsh, or other) is determined by the user's entry in the file /etc/passwd on the remote system.

Operands

The following operand is supported:

command

The command to be executed on the specified hostname.

Usage

See largefile(5) for the description of the behavior of rsh and remsh when encountering files greater than or equal to 2 Gbyte ( 2³¹ bytes).

The rsh and remsh commands are IPv6–enabled. See ip6(7P). IPv6 is not currently supported with Kerberos V5 authentication.

Hostnames are given in the hosts database, which can be contained in the /etc/hosts file, the Internet domain name database, or both. Each host has one official name (the first name in the database entry) and optionally one or more nicknames. Official hostnames or nicknames can be given as hostname.

If the name of the file from which rsh is executed is anything other than rsh, rsh takes this name as its hostname argument. This allows you to create a symbolic link to rsh in the name of a host which, when executed, invokes a remote shell on that host. By creating a directory and populating it with symbolic links in the names of commonly used hosts, then including the directory in your shell's search path, you can run rsh by typing hostname to your shell.

If rsh is invoked with the basename remsh, rsh checks for the existence of the file /usr/bin/remsh. If this file exists, rsh behaves as if remsh is an alias for rsh. If /usr/bin/remsh does not exist, rsh behaves as if remsh is a host name.

For the kerberized rsh session, each user can have a private authorization list in a file .k5login in their home directory. Each line in this file should contain a Kerberos principal name of the form principal/instance@realm. If there is a ~/.k5login file, then access is granted to the account if and only if the originater user is authenticated to one of the principals named in the ~/.k5login file. Otherwise, the originating user is granted access to the account if and only if the authenticated principal name of the user can be mapped to the local account name using the authenticated-principal-name → local-user-name mapping rules. The .k5login file (for access control) comes into play only when Kerberos authentication is being done.

For the non-secure rsh session, each remote machine can have a file named /etc/hosts.equiv containing a list of trusted hostnames with which it shares usernames. Users with the same username on both the local and remote machine can run rsh from the machines listed in the remote machine's /etc/hosts.equiv file. Individual users can set up a similar private equivalence list with the file .rhosts in their home directories. Each line in this file contains two names: a hostname and a username separated by a space. The entry permits the user named username who is logged into hostname to use rsh to access the remote machine as the remote user. If the name of the local host is not found in the /etc/hosts.equiv file on the remote machine, and the local username and hostname are not found in the remote user's .rhosts file, then the access is denied. The hostnames listed in the /etc/hosts.equiv and .rhosts files must be the official hostnames listed in the hosts database; nicknames can not be used in either of these files.

You cannot log in using rsh as a trusted user from a trusted hostname if the trusted user account is locked.

rsh does not prompt for a password if access is denied on the remote machine unless the command argument is omitted.

Examples

Example 1 Using rsh to Append Files

The following command appends the remote file lizard.file from the machine called lizard to the file called example.file on the machine called example:

example% rsh lizard cat lizard.file >> example.file

The following command appends the file lizard.file on the machine called lizard to the file lizard.file2 which also resides on the machine called lizard:

example% rsh lizard cat lizard.file ">>" lizard.file2

Exit Status

The following exit values are returned:

0

Successful completion.

1

An error occurred.

Files

/etc/hosts

Internet host table

/etc/hosts.equiv

Trusted remote hosts and users

/etc/passwd

System password file

$HOME/.k5login

File containing Kerberos principals that are allowed access

/etc/krb5/krb5.conf

Kerberos configuration file

Attributes

See attributes(5) for descriptions of the following attributes:

See Also

on(1), rlogin(1), ssh(1), telnet(1), vi(1), in.rshd(1M), hosts(4), hosts.equiv(4), krb5.conf(4), attributes(5), krb5_auth_rules(5), largefile(5), ip6(7P)

Notes

When a system is listed in hosts.equiv, its security must be as good as local security. One insecure system listed in hosts.equiv can compromise the security of the entire system.

You cannot run an interactive command (such as vi(1)). Use rlogin if you wish to do this.

Stop signals stop the local rsh process only. This is arguably wrong, but currently hard to fix for reasons too complicated to explain here.

The current local environment is not passed to the remote shell.

Sometimes the -n option is needed for reasons that are less than obvious. For example, the command:

example% rsh somehost dd if=/dev/nrmt0 bs=20b | tar xvpBf -

puts your shell into a strange state. Evidently, the tar process terminates before the rsh process. The rsh command then tries to write into the ``broken pipe'' and, instead of terminating neatly, proceeds to compete with your shell for its standard input. Invoking rsh with the -n option avoids such incidents.

This bug occurs only when rsh is at the beginning of a pipeline and is not reading standard input. Do not use the -n option if rsh actually needs to read standard input. For example:

example% tar cf - . | rsh sundial dd of=/dev/rmt0 obs=20b

does not produce the bug. If you were to use the -n option in a case like this, rsh would incorrectly read from /dev/null instead of from the pipe.

For most purposes, ssh(1) is preferred over rsh.