This is Dropbear, a smallish SSH server and client. https://matt.ucc.asn.au/dropbear/dropbear.html INSTALL has compilation instructions. MULTI has instructions on making a multi-purpose binary (ie a single binary which performs multiple tasks, to save disk space) SMALL has some tips on creating small binaries. A mirror of the Dropbear website and tarballs is available at https://dropbear.nl/mirror/ Please contact me if you have any questions/bugs found/features/ideas/comments etc :) There is also a mailing list http://lists.ucc.gu.uwa.edu.au/mailman/listinfo/dropbear Matt Johnston matt@ucc.asn.au In the absence of detailed documentation, some notes follow: ============================================================================ Server public key auth: You can use ~/.ssh/authorized_keys in the same way as with OpenSSH, just put the key entries in that file. They should be of the form: ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIEAwVa6M6cGVmUcLl2cFzkxEoJd06Ub4bVDsYrWvXhvUV+ZAM9uGuewZBDoAqNKJxoIn0Hyd0Nk/yU99UVv6NWV/5YSHtnf35LKds56j7cuzoQpFIdjNwdxAN0PCET/MG8qyskG/2IE2DPNIaJ3Wy+Ws4IZEgdJgPlTYUBWWtCWOGc= someone@hostname You must make sure that ~/.ssh, and the key file, are only writable by the user. Beware of editors that split the key into multiple lines. Dropbear supports some options for authorized_keys entries, see the manpage. ============================================================================ Client public key auth: Dropbear can do public key auth as a client, but you will have to convert OpenSSH style keys to Dropbear format, or use dropbearkey to create them. If you have an OpenSSH-style private key ~/.ssh/id_rsa, you need to do: dropbearconvert openssh dropbear ~/.ssh/id_rsa ~/.ssh/id_rsa.db dbclient -i ~/.ssh/id_rsa.db <hostname> Dropbear does not support encrypted hostkeys though can connect to ssh-agent. ============================================================================ If you want to get the public-key portion of a Dropbear private key, look at dropbearkey's '-y' option. ============================================================================ To run the server, you need to generate server keys, this is one-off: ./dropbearkey -t rsa -f dropbear_rsa_host_key ./dropbearkey -t dss -f dropbear_dss_host_key ./dropbearkey -t ecdsa -f dropbear_ecdsa_host_key ./dropbearkey -t ed25519 -f dropbear_ed25519_host_key or alternatively convert OpenSSH keys to Dropbear: ./dropbearconvert openssh dropbear /etc/ssh/ssh_host_dsa_key dropbear_dss_host_key You can also get Dropbear to create keys when the first connection is made - this is preferable to generating keys when the system boots. Make sure /etc/dropbear/ exists and then pass '-R' to the dropbear server. ============================================================================ If the server is run as non-root, you most likely won't be able to allocate a pty, and you cannot login as any user other than that running the daemon (obviously). Shadow passwords will also be unusable as non-root. ============================================================================ The Dropbear distribution includes a standalone version of OpenSSH's scp program. You can compile it with "make scp", you may want to change the path of the ssh binary, specified by _PATH_SSH_PROGRAM in options.h . By default the progress meter isn't compiled in to save space, you can enable it by adding 'SCPPROGRESS=1' to the make commandline.