SSH

SSH (Secure SHell) is an encrypted terminal program that replaces the classic telnet tool on Unix-like operating systems.

In addition to remote terminal access provided by the main ssh binary, the SSH suite of programs has grown to include other tools such as scp (in very common use) and sftp (less common).

Originally, SSH was not free. However, today the most popular and de-facto standard version of SSH is OpenBSD's OpenSSH, which comes pre-installed on Gentoo.

Server
The OpenSSH server can be controlled like any other OpenRC-managed service, with /etc/init.d/ssh start, /etc/init.d/ssh stop, and /etc/init.d/ssh restart.

Note that active connections to the server remain unaffected when issuing /etc/init.d/ssh restart, whereas running /etc/init.d/ssh stop followed by /etc/init.d/ssh start may cause disconnection.

Server configuration
The SSH server is usually configured in the /etc/ssh/sshd_config file, though it is also possible to perform further configuration in OpenRC's /etc/conf.d/sshd, including changing the location of the former. For detailed information on how to configure the server, use man sshd_config.

Client configuration
The ssh client and related programs (scp, sftp, etc.) are configured in the /etc/ssh/ssh_config file. For more information on how to configure these programs, man ssh_config.

Troubleshooting
There are 3 different levels of debug modes. -v Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple -v options increase the verbosity. The maximum is 3.

debug1 mode:

debug2 mode:

debug3 mode:

Death of Long-lived Connections
Many internet access devices perform network address translation (NAT), a process that enables devices on a private network such as that typically found in a home or business place to access foreign networks, such as the internet, despite only having a single IP address on that network. Unfortunately, not all NAT devices are created equal, and some of them incorrectly close long-lived, occasional-use TCP connections such as those used by SSH. This is generally observable as a sudden inability to interact with the remote server, though the ssh client program has not exited.

In order to resolve the issue, OpenSSH clients and servers can be configured to send a 'keep alive', or invisible message aimed at maintaining and confirming the live status of the link.
 * To enable keep alive for all clients connecting to your local server, set ClientAliveInterval 30 (or some other value, in seconds) within the /etc/ssh/sshd_config file.
 * To enable keep alive for all servers connected to by your local client, set ServerAliveInterval 30 (or some other value, in seconds) within the /etc/ssh/ssh_config file.

External resources

 * OpenSSH official page