I need an automated program on machine "seq" to rsync to machine "four" as user "miseq". To maintain security, I only want to allow this automated process to run rsync - nothing else.

Steps:

On "seq", run
```
ssh-keygen -t rsa
```
enter NO passphrase - just hit return both times. Do NOT use "id_rsa" as the name of the private key file - name it something like "id_rsa.seq2four". Note that this also creates the file id_rsa.pub - you will need the line inside this file for the next step on "four".
On "four", create or append to the file ~.ssh/authorized_keys the single line key that was in id_rsa.pub on "seq" generated in step 1, or use "ssh-copy-id -i id_rsa.seq2four <user>@<four>.
Create an executable shell script on "four" that contains this simple script - let's call it "~/bin/validate-rsync-ssh.sh":

From: http://blog.jolexa.net/2011/02/tip-single-purpose-password-less-ssh-key/

#!/bin/bash
case "$SSH_ORIGINAL_COMMAND" in
	rsync\ --server*)
		# uncomment for debug
		# echo "$(date +%Y%m%d): $SSH_ORIGINAL_COMMAND" >> /var/log/ssh-cmd.log
		$SSH_ORIGINAL_COMMAND
		;;
	# debug
	testconnect)
		echo "You successfully connected to $(hostname)"
		;;
	*)
		echo "Sorry, command '$SSH_ORIGINAL_COMMAND' is not allowed"
		exit 1
		;;
esac

Don't forget to make this file executable (chmod +x validate-rsync-ssh.sh).

Pre-pend the text: command="~/bin/validate-rsync-ssh.sh" to your ssh-rsa key in the file ~/.ssh/authorized_keys, with a space between this and the text "ssh-rsa".
Now test everything by doing this command back on "seq":
```
ssh -i ~/.ssh/id_rsa.seq2four miq@four testconnect
```
This should give you the message from your "validate-rsync-ssh.sh" script, "You successfully connected to four". Commands other than "testconnect" should give you the, "Sorry, command... is not allowed" error message.
Now try your rsync from "seq" to "four" - it should work smoothly:
```
rsync -avP -e 'ssh -i /home/me/.ssh/id_rsa.seq2four' localfiles.txt miq@four:RemoteDir
```
Note that the path to your "id_rsa.seq2four" must be absolute - the shell and rsync get confused about who's expanding what when if you try using variables or "~".

Other notes:
ssh is VERY picky about the permissions of the .ssh directory on "four" - they MUST be:
```
miseq@four:~/.ssh$ ls -la
drwx------  2 miq group 4096 2013-07-25 10:36 .
```
IN ADDITION - the .ssh directory should be tight:
```
chmod 700 ~/.ssh
ls -ld .ssh:
drwx------ 2 miq group 4096 Nov 26 17:25 .ssh
```
AND your home dir must be at least 775:

chmod 775 ~
ls -ld ~
drwxrwsr-x 22 miq group 4096 Nov 26 17:25 /home/miq

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