After an update to DSM 4.2 rutabaga no longer allowed rsync backups, failing with:

sh: rsync: not found
rsync: connection unexpectedly closed (0 bytes received so far) [Receiver]
rsync error: remote command not found (code 127) at io.c(605) [Receiver=3.0.9]

After much wailing and gnashing of teeth we discovered that non-interactive users do not have /usr/syno/bin in their path (it *is* in their path if they shell in to the NAS, so they can run rsync *from* the NAS when shell'd in).

So, that's an easy fix, says us: add a symlink to /usr/syno/bin/rsync in a logical spot that *is* in a non-interactive path, like /usr/bin.

Problem: admin user cannot su root (error message = su: must be suid to work properly), so cannot create symlink.

Answer: TURN ON TELNET AND LOG IN AS ROOT USING THE WORST POSSIBLE METHOD!!! Then, you make the symlink and turn off telnet - quick!