tinyssh

Introduction

• tinysshd is a minimalistic SSH server which implements only a subset of SSHv2 features.
• tinysshd supports only secure cryptography (minimum 128-bit security, protected against cache-timing attacks)
• tinysshd doesn't implement older crypto (such as RSA, DSA, HMAC-MD5, HMAC-SHA1, 3DES, RC4, ...)
• tinysshd doesn't implement unsafe features (such as password or hostbased authentication)
• tinysshd doesn't have features such: SSH1 protocol, compression, port forwarding, agent forwarding, X11 forwarding ...
• tinysshd uses fixed-size buffers for SSH packet processing. Authorization path resolution uses dynamically allocated memory, allocation failure rejects authorization.

Important change since version 20260906

Starting with version 20260906, TinySSH rejects all client-supplied environment variables (env channel requests) for security reasons. This may affect configurations using OpenSSH SendEnv or SetEnv; any required variables must be set on the server instead.

Supported SSH algorithms

As of version 20260906, TinySSH supports the following SSHv2 algorithms:

TinySSH also supports the OpenSSH strict key-exchange extension [email protected].

The curve25519-sha256 algorithms can be disabled with -S. The post-quantum [email protected] algorithm can be disabled with -P.

Project timeline

• experimental: 2014.01.01 - 2014.12.31 (experimentation)
• alpha(updated): 2015.01.01 - 2017.12.31 (not ready for production use, ready for testing)
• beta(updated): 2018.01.01 - ????.??.?? (ready for production use)
• stable: expected ????.??.?? - (ready for production use - including post-quantum crypto)

Current release (20260906)

• has 74260 words of code
• beta release

How-to compile and install

make
sudo make install

How-to run

       mkdir -p /etc/tinyssh
       tinysshd-makekey /etc/tinyssh/sshkeydir

       TCPSERVER
              tcpserver -HRDl0 0.0.0.0 22 /usr/sbin/tinysshd -v /etc/tinyssh/sshkeydir &

       BUSYBOX
              busybox tcpsvd 0 22 tinysshd -v /etc/tinyssh/sshkeydir &

       INETD
           /etc/inetd.conf:
               ssh stream tcp nowait root /usr/sbin/tinysshd tinysshd -l -v /etc/tinyssh/sshkeydir

       SYSTEMD
           tinysshd.socket:
               [Unit]
               Description=TinySSH server socket
               ConditionPathExists=!/etc/tinyssh/disable_tinysshd

               [Socket]
               ListenStream=22
               Accept=yes

               [Install]
               WantedBy=sockets.target

           [email protected]:
               [Unit]
               Description=Tiny SSH server
               After=network.target auditd.service

               [Service]
               ExecStartPre=-/usr/sbin/tinysshd-makekey -q /etc/tinyssh/sshkeydir
               EnvironmentFile=-/etc/default/tinysshd
               ExecStart=/usr/sbin/tinysshd $TINYSSHDOPTS -- /etc/tinyssh/sshkeydir
               KillMode=process
               SuccessExitStatus=111
               StandardInput=socket
               StandardError=journal

               [Install]
               WantedBy=multi-user.target