OpenSSH

OpenSSH is a popular suite of software utilities implementing Secure Shell (SSH) protocol. OpenSSH includes the ability to set up a TCP secured channel and it is widely use as a replacement for not secured telnet and secure replacement of file transfers such as rcp and ftp. OpenSSH offers a great number of features including ssh session multiplexing. ^[1]^[2]

The OpenSSH suite includes the following command-line utilities and daemons:

ssh, ssh client and TCP secure replacement for rlogin, rsh and telnet to allow shell access to a remote machine.
scp, a replacement for rcp
sftp, a replacement for ftp to copy files between computers
sshd, the SSH server daemon which allows shell access and file transfers to a remote machine.
ssh-keygen, a tool to inspect and generate the RSA, DSA and Elliptic Curve keys that are used for user and host authentication
ssh-keyscan, which scans a list of hosts and collects their public keys
ssh-agent and ssh-add, utilities to ease authentication by holding keys ready and avoid the need to enter passphrases every time they are used
ssh-copy-id, copy local keys to remote machine.

Readings

Config

Client: /etc/ssh/ssh_config or ~./config
Server: /etc/ssh/sshd_config

ssh clients

OpenSSH includes an ssh client:ssh. Others clients are available such us PuTTY, mosh, paramiko and autossh^[3].

autossh^[4] main feature not include in OpenSSH ssh client is the capability to monitor an ssh connection and restart it if necessary.

Loop waiting to connect to server: AUTOSSH_POLL=5 AUTOSSH_GATETIME=0 autossh -M 0 -o ServerAliveInterval=5 -o ServerAliveCountMax=1 YOUR_SERVER_NAME_OR_IP

Ssh clients in Linux are frequently executed inside a terminal or using any kind of terminal multiplexer such as tmux or screen.

Activities

Basic

Install OpenSSH: apt install openssh-server
Convert a PuTTY ssh key format to Openssh format, you can follow the following instructions http://www.codeblocq.com/2016/05/Convert-a-putty-ppk-key-to-a-pem-file-on-OSX/, https://stackoverflow.com/questions/3475069/use-ppk-file-in-mac-terminal-to-connect-to-remote-connection-over-ssh
Open a reverse ssh tunnel, follow the following instructions https://www.howtoforge.com/reverse-ssh-tunneling
Configure OpenSSH to reuse ssh connections (ControlMaster)
Generate a public Key from a private Key: ^[5]ssh-keygen -f ~/.ssh/id_rsa -y > ~/.ssh/id_rsa.pub (example for RSA keys but can be applied to other key types)
Configure OpenSSH to allow Public-key authentication (authorized_keys)^[6]
Activate SSH on macOS: sudo systemsetup -setremotelogin on
Activate OpenSSH on Windows (Windows Server 2019 or Windows 10):^[7]

Intermediate

Learn about different client connection options, such us: -oBatchMode=yes or -o ConnectTimeout=2^[8]
Connect to remote server temporarily turning off host key checking, (security implications): ssh -oStrictHostKeyChecking=no SERVER_NAME