Remote shell

rsh runs a command, or a shell, on another computer. rcp copies files between computers. rshd is the daemon on the other computer. They are named for Berkeley’s r-commands because they have what those had: no passwords and no encryption.

Anyone whose network card reaches a computer that runs rshd gets a shell on it. Leave it off where that matters. It is off until you enable it.

Setting up

On the computer you want to log in to, which needs a network card or a WLAN card:

$ hostname lab
$ rc rshd enable
$ rc rshd start

hostname lab names the computer and keeps the name in /etc/hostname (the boot script reads it). rc rshd enable starts the daemon at every boot.

On any computer whose cards reach it:

$ rsh lab
$ rsh lab ls /
$ cat notes | rsh lab 'cat > /notes'

For the tutorial, see Two computers.

Naming a host

A host is its name (hostname), or a card’s id (shown by lsdev -i), whole or as much of its start as names one computer. A name given in full wins over an id’s start. If no name has been set, the name is the first network card’s address.

rsh finds a host by broadcasting a probe on UDP port 514. Each rshd answers with its name. It asks up to three times, as ARP does, because a busy computer may miss one.

A command

With a command, the other computer runs sh -c on it, on pipes. Its input is rsh’s standard input (none with -n), its output and its errors come back apart, and rsh ends as the command did: with its exit status, or 128 plus the signal’s number if a signal ended it.

With no command, rsh logs in: a shell on a pty on the other computer, as big as your terminal. Your terminal goes raw, so every key goes there as it is typed, Ctrl+C and the arrows included. A change to your terminal’s size goes there too. ~. at the start of a line disconnects, and ~~ sends one ~.

rcp

rcp is rsh underneath, as BSD’s was: on the other computer, cat reads or writes each file, mkdir -p makes directories, and find lists a remote directory’s tree.

$ rcp /notes lab:/notes
$ rcp lab:/log /tmp/
$ rcp -r /projects lab:/backup/

A name with host:path is remote; a name with no colon, or with a / before its colon, is local. Both ends being remote isn’t supported.

How it works

One TCP connection to port 514, framed in both directions, so output, errors and the exit status stay apart. (BSD’s used a second connection for errors, and never sent the exit status.) A frame is a kind byte, then its data with a 2-byte length.

rshd runs each connection as rshd -s with the connection as its standard input, as inetd ran BSD’s. A login runs on a pty (sys.openpty): the pty does the terminal’s work (echo, line editing, Ctrl+C) in the kernel, so rshd only relays. When rsh goes away, a login’s line hangs up (SIGHUP to its shell), and a command’s input ends.