Dropbear is a relatively small open source SSH 2 server. It runs on a variety of POSIX-based platforms.
The perfect environment for this server could be a little low-end system like your mobile phone or BeagleBone.
apt-get install dropbear
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NO_START=0 DROPBEAR_PORT=22 # Alternative dropbear login banner DROPBEAR_BANNER="/opt/dropbear-banner"