Setting up a SSH server

Overview

This article provides a short overview of SSH on Alpine Linux.

Also see Secure Shell (Wikipedia).

Note: This article describes two popular SSH implementations: OpenSSH and Dropbear. Either can be installed using the setup-sshd script, or by following the below instructions.

OpenSSH

OpenSSH is a popular SSH implementation for remote encrypted login to a machine. OpenSSH defines sshd as the daemon, and ssh as the client program.

The openssh package provides OpenSSH on Alpine Linux.

Installation

Install the openssh package:

apk add openssh

Note: To use the ACF-frontend for openssh, install acf-openssh instead (assuming that you have the setup-acf script).

Also see Alpine Linux package management .

Service commands

Enable the sshd daemon so that it starts at boot:

rc-update add sshd

List services to verify sshd is enabled:

rc-status

Start the sshd service immediately and create configuration files:

/etc/init.d/sshd start

Note: If you are running from RAM, ensure you save your settings using the 'lbu ci' command as necessary. See Alpine local backup.

Also see Alpine Linux Init System.

Fine tuning

You may wish to change the default configuration. This section describes some of the configuration options as examples, however it is by no means an exhaustive list. See the manual for full details.

The fine-tuning is done by editing /etc/ssh/sshd_config. Any line starting with "#" will be ignored by sshd.

UseDNS no   # By setting this to no, connection speed can increase.
PasswordAuthentication no  # Do not allow password authentication.

Other configuration options are shown in /etc/ssh/sshd_config. The file includes comments that explain many of the options.

Firewalling and Port Changes

By default, sshd will communicate on TCP port 22.

Sometimes 22/tcp is blocked by a firewall over which you have no control. Changing the Port option to an unused port number in /etc/ssh/sshd_config may be useful in this situation.

Port 443   # Use whatever port number that fits your needs

Note: Ensure the port you wish to use is not already in use by running netstat -lnp on the machine running sshd.

Restart sshd after making modifications to the configuration file:

/etc/init.d/sshd restart

Note: If you are running from RAM, ensure you save your settings using the 'lbu ci' command as necessary. See Alpine local backup.

Dropbear

Dropbear is another open source SSH implementation. Install it through the Alpine setup scripts, or manually with:

apk add dropbear

Start it:

rc-service dropbear start

Add it to the default runlevel:

rc-update add dropbear

Use the following command to check all available server options:

dropbear -h

The config file is located at /etc/conf.d/dropbear

dropbear also includes an SSH client which in its simplest form can be used like this:

dbclient host.example.com

dbclient x.x.x.x

(where x.x.x.x is the IP address of the remote machine).

Use dbclient -h to see all available options.

Further Reading

OpenSSH (openssh.com)
OpenSSH (wikipedia.org)