OpenSSH Server

Introduction

This section of the Ubuntu Server Guide introduces a powerful collection of tools for the remote control of networked computers and transfer of data between networked computers, called OpenSSH. You will also learn about some of the configuration settings possible with the OpenSSH server application and how to change them on your Ubuntu system.

OpenSSH is a freely available version of the Secure Shell (SSH) protocol family of tools for remotely controlling a computer or transferring files between computers. Traditional tools used to accomplish these functions, such as telnet or rcp, are insecure and transmit the user's password in cleartext when used. OpenSSH provides a server daemon and client tools to facilitate secure, encrypted remote control and file transfer operations, effectively replacing the legacy tools.

The OpenSSH server component, sshd, listens continuously for client connections from any of the client tools. When a connection request occurs, sshd sets up the correct connection depending on the type of client tool connecting. For example, if the remote computer is connecting with the ssh client application, the OpenSSH server sets up a remote control session after authentication. If a remote user connects to an OpenSSH server with scp, the OpenSSH server daemon initiates a secure copy of files between the server and client after authentication. OpenSSH can use many authentication methods, including plain password, public key, and Kerberos tickets.

Installation

Installation of the OpenSSH client and server applications is simple. To install the OpenSSH client applications on your Ubuntu system, use this command at a terminal prompt:

sudo apt-get install openssh-client

To install the OpenSSH server application, and related support files, use this command at a terminal prompt:

sudo apt-get install openssh-server

The openssh-server package can also be selected to install during the Server Edition installation process.

Configuration

You may configure the default behavior of the OpenSSH server application, sshd, by editing the file /etc/ssh/sshd_config. For information about the configuration directives used in this file, you may view the appropriate manual page with the following command, issued at a terminal prompt:

man sshd_config

There are many directives in the sshd configuration file controlling such things as communication settings and authentication modes. The following are examples of configuration directives that can be changed by editing the /etc/ssh/sshd_config file.


Prior to editing the configuration file, you should make a copy of the original file and protect it from writing so you will have the original settings as a reference and to reuse as necessary. Copy the `/etc/ssh/sshd_config` file and protect it from writing with the following commands, issued at a terminal prompt:

Prior to editing the configuration file, you should make a copy of the original file and protect it from writing so you will have the original settings as a reference and to reuse as necessary.

Copy the /etc/ssh/sshd_config file and protect it from writing with the following commands, issued at a terminal prompt:

sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.original
sudo chmod a-w /etc/ssh/sshd_config.original

The following are examples of configuration directives you may change:

To set your OpenSSH to listen on TCP port 2222 instead of the default TCP port 22, change the Port directive as such:

Port 2222
To have sshd allow public key-based login credentials, simply add or modify the line:

PubkeyAuthentication yes

In the /etc/ssh/sshd_config file, or if already present, ensure the line is not commented out.
To make your OpenSSH server display the contents of the /etc/issue.net file as a pre-login banner, simply add or modify the line:

Banner /etc/issue.net

In the /etc/ssh/sshd_config file.

After making changes to the /etc/ssh/sshd_config file, save the file, and restart the sshd server application to effect the changes using the following command at a terminal prompt:

sudo /etc/init.d/ssh restart


Many other configuration directives for sshd are available for changing the server application's behavior to fit your needs. Be advised, however, if your only method of access to a server is ssh, and you make a mistake in configuring sshd via the `/etc/ssh/sshd_config` file, you may find you are locked out of the server upon restarting it, or that the sshd server refuses to start due to an incorrect configuration directive, so be extra careful when editing this file on a remote server.

Many other configuration directives for sshd are available for changing the server application's behavior to fit your needs. Be advised, however, if your only method of access to a server is ssh, and you make a mistake in configuring sshd via the /etc/ssh/sshd_config file, you may find you are locked out of the server upon restarting it, or that the sshd server refuses to start due to an incorrect configuration directive, so be extra careful when editing this file on a remote server.

SSH Keys

SSH keys allow authentication between two hosts without the need of a password. SSH key authentication uses two keys a private key and a public key.

To generate the keys, from a terminal prompt enter:

ssh-keygen -t dsa

This will generate the keys using a DSA authentication identity of the user. During the process you will be prompted for a password. Simply hit Enter when prompted to create the key.

By default the public key is saved in the file ~/.ssh/id_dsa.pub, while ~/.ssh/id_dsa is the private key. Now copy the id_dsa.pub file to the remote host and append it to ~/.ssh/authorized_keys by entering:

ssh-copy-id username@remotehost

Finally, double check the permissions on the authorized_keys file, only the authenticated user should have read and write permissions. If the permissions are not correct change them by:

chmod 600 .ssh/authorized_keys

You should now be able to SSH to the host without being prompted for a password.