Editing OpenSSH

'''[[wikipedia: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]].
<ref>https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Multiplexing</ref><ref>https://stackoverflow.com/questions/20410252/how-to-reuse-an-ssh-connection</ref> 


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

== Readings ==
* [[Wikibooks: OpenSSH]]
* [[OpenSSH changelog]]

== Config ==
* Client: <code>[[/etc/ssh/ssh_config]]</code> or <code>~./[[config]]</code>
* Server: <code>[[/etc/ssh/sshd_config]]</code>

== ssh clients ==
OpenSSH includes an ssh client:<code>[[ssh]]</code>. Others clients are available such us <code>[[PuTTY]]</code>, <code>mosh</code>, <code>paramiko</code> and <code>autossh</code><ref>https://linux.die.net/man/1/autossh</ref>.

<code>autossh</code><ref>https://linux.die.net/man/1/autossh</ref> 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: <code>AUTOSSH_POLL=5 AUTOSSH_GATETIME=0 [[autossh]] -M 0 -o ServerAliveInterval=5 -o ServerAliveCountMax=1 YOUR_SERVER_NAME_OR_IP</code>

Ssh clients in Linux are frequently executed inside a terminal or using any kind of terminal multiplexer such as <code>[[tmux]]</code> or <code>[[screen]]</code>.

== Activities ==
=== Basic ===
* Install OpenSSH: <code>[[apt install]] openssh-server</code>
* ''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]] (<code>ControlMaster</code>)
* Generate a public Key from a private Key: <ref>https://serverfault.com/questions/52285/create-a-public-ssh-key-from-the-private-key</ref><code>[[ssh-keygen]] -f ~/.ssh/id_rsa -y > ~/.ssh/id_rsa.pub</code> (example for [[RSA]] keys but can be applied to other key types)
* [[Configure OpenSSH to allow Public-key authentication]] (<code>authorized_keys</code>)<ref>https://www.digitalocean.com/community/tutorials/how-to-configure-ssh-key-based-authentication-on-a-linux-server</ref>
* [[Activate SSH on macOS]]: <code>sudo [[systemsetup]] -setremotelogin on</code>
* [[Activate OpenSSH on Windows]] ([[Windows Server 2019]] or [[Windows 10]]):<ref>https://docs.microsoft.com/en-us/windows-server/administration/openssh/openssh_install_firstuse</ref>

=== Intermediate ===
* Learn about different client connection options, such us: <code>-oBatchMode=yes</code> or <code>-o ConnectTimeout=2</code><ref>https://linux.die.net/man/1/ssh</ref>
* Connect to remote server temporarily turning off [[host key checking]], (security implications): <code>ssh -oStrictHostKeyChecking=no SERVER_NAME</code>

=== Advanced ===
# Read ssh documentation about multiplexing https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Multiplexing and its implementation details: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.mux?annotate=HEAD
# Configure ssh session [[multiplexing]]
# Use <code>[[ProxyJump]]</code> directive to connect using a "Jump Server"<ref>https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Proxies_and_Jump_Hosts#Passing_Through_One_or_More_Gateways_Using_ProxyJump</ref>
# Run a shell script on a remote machine using ssh: <code>ssh root@MachineB 'bash -s' < local_script.sh</code><ref>https://stackoverflow.com/a/2732991</ref>. See also: [[parallel]]
# Read https://github.com/openssh/openssh-portable source code
# Read [[OpenSSH changelog]]

== Related terms ==
* [[MAC (message authentication code)]]
* [[Damien Miller]]
* [[Key Revocation Lists (KRL)]]
* [[AWS EC2 Instance Connect]] (Jun 2019)

== See also ==
* [[Telnet]] (deprecated use), [[netcat]]
* {{OpenSSH}}
* <code>sslh</code><ref>https://github.com/yrutschle/sslh</ref> Applicative Protocol Multiplexer (e.g. share SSH and HTTPS on the same port)
* <code>sshpass</code> (<code>brew install http://git.io/sshpass.rb</code>)
* <code>conch</code> client written in python
* [[openssl]]
* {{fail2ban}}
* {{security}}

{{CC license}}
Original source: https://en.wikiversity.org/wiki/OpenSSH

[[Category:Networking]]
[[Category:Server administration]]
[[Category:Server software]]
[[Category:Cryptography]]