Dropbear SSH

"Dropbear is a relatively small SSH server and client. It runs on a variety of POSIX-based platforms. Dropbear is open source software, distributed under a MIT-style license. Dropbear is particularly useful for "embedded"-type Linux (or other Unix) systems, such as wireless routers.

https://matt.ucc.asn.au/dropbear/dropbear.html

The features include:

• A small memory footprint suitable for memory-constrained environments – Dropbear can compile to a 110kB statically linked binary with uClibc on x86 (only minimal options selected)
• Dropbear server implements X11 forwarding, and authentication-agent forwarding for OpenSSH clients
• Can run from inetd or standalone
• Compatible with OpenSSH ~/.ssh/authorized_keys public key authentication
• The server, client, keygen, and key converter can be compiled into a single binary (like busybox)
• Features can easily be disabled when compiling to save space
• Multi-hop mode uses SSH TCP forwarding to tunnel through multiple SSH hosts in a single command. dbclient user1@hop1,user2@hop2,destination