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Berkeley “r” commands (rlogin, rsh)

This group of network services goes under the names login and exec in /etc/services. They are an alternative method of providing remote access to your machine. Time does not allow for an in-depth discussion of these commands, but a host of security considerations are involved in using them. Make sure they are disabled in /etc/inetd.conf unless you intend to allow them. The well-known port numbers for these services are 512 and 513. The daemons are called in.rexecd and in.rlogind.

The rlogin and rsh commands are the generic members of the Berkeley r family of programs that allow remote access to a host. To complement these, a small number of specific remote commands were also defined. These are essentially remote versions of some of the more common UNIX functions instead of the command being applied to only one system, however, it is used between two systems or across all systems on a TCP/IP network.
All of these commands are based on rlogin the same way that rsh is. They work in the same way, but instead of opening up a session or passing a user-specified command to the remote host, they execute a particular function. The following are the most common of these remote commands and briefly, how each is used:
  1. Remote Copy (rcp): This is the remote version of the UNIX copy (cp) command; it allows a file to be copied between the local host and the remote host, or between two remote hosts. The usual syntax is basically the same as the regular cp command, but the source and/or destination is specified as being on a remote host.
  2. The rcp command can be used in a manner similar to FTP, but is much simpler and less capable. Or to put it another way: rcp is to FTP, what rlogin is to Telnet.
  3. Remote Uptime (ruptime): The UNIX command uptime is used to display how long a computer has been running since it was last booted, along with information related to its current load. ruptime is the remote version of this command; it displays the current status of each machine on the network (up or down), how long each up machine has been up since its last boot, and its load statistics.
  4. Remote Who (rwho): This is the remote version of the who command. Where who shows all the users logged on to the host where it is run, rwho shows all users logged on to all machines on the network.
The ruptime and rwho commands both rely on the presence of the rwhod (remote who daemon) running in the background on networked machines. These processes routinely share information with each other about host uptime and who is logged on to each system, so it can be quickly displayed when either ruptime or rwho is run.
On some operating systems, other remote commands may also be implemented. As with rlogin and rsh, security issues may apply to these commands, and there may be efficiency concerns with others (such as rwho). For these reasons, on many networks these commands are no longer used.

Linux System Administration