Configuring Ubuntu 10.x Remote Access using SSH

SSH is a TCP/IP service that provides a secure mechanism for remotely logging into one system over either a local network or the internet into another system. SSH also provides the ability to transfer files between remote systems. When a user logs into a remote system using SSH, they receive a command prompt allowing them to enter commands on the remote system as if they were sitting at the remote system and had opened a terminal session.

In this chapter we will cover the steps necessary to configure an Ubuntu system to accept SSH connections. This involves installing the SSH server on the local system and configuring the firewall to allow SSH connections.

In order for a system to accept SSH connections the system must first be running the SSH server. By default, Ubuntu does not install the SSH server so the first step is to ensure that the server is installed. This can be performed using either the Ubuntu Software Center tool or the apt-get command-line tool.

To install using the Ubuntu Software Center, select the Applications desktop menu and then click on Ubuntu Software Manager. Enter your password when prompted to do so. Using the search box in the top right hand corner, search for openssh-server. After the search completes, you will see openssh-server in the package list. Simply select this item and click on the Install button to initiate the installation process.

To install from the command line, begin by opening a terminal window by selecting the Applications menu and selecting Terminal from the Accessories menu. In the terminal window enter the following command and press enter to execute it:

sudo apt-get install openssh-server

The installation process will download the SSH server, install it and start the service running in the background. You may now attempt to connect from a remote system (see below for details on how to do this). If you receive a "connection refused" message when you try to connect you may need to configure the firewall to allow SSH connections to be established to this system.

If you are using a firewall to protect your system you will need to allow SSH connections before you be able to connect from a remote system. If you have configured your firewall using the Firestarter tool you will need to set up an incoming connection policy to allow connections to the SSH service. Configuring Firestarter is covered in detail in Using Firestarter to Configure an Ubuntu 10.x Firewall.

SSH can be used to log into your system from a remote system. It is also possible to test that the SSH server is running and accessible from the local machine. SSH connections are established using the ssh client utility.

To connect from your local machine back to itself use the following command:

ssh -l username ipaddresss

Where username is the name of the user you wish to log in as and ipaddress is the IP address of your system. You can also substitute the hostname of the system in place of the IP address. If you do not know the IP address run the ipconfig command in a terminal window. This will output information similar to:

eth0      Link encap:Ethernet  HWaddr 00:13:72:0B:14:57
          inet addr:192.168.2.21  Bcast:192.168.2.255  Mask:255.255.255.0
          inet6 addr: fe80::213:72ff:fe0b:1457/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:4261067 errors:0 dropped:0 overruns:0 frame:0
          TX packets:4409081 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100
          RX bytes:2068332349 (1.9 GiB)  TX bytes:2408187471 (2.2 GiB)
          Base address:0xcce0 Memory:fe3e0000-fe400000

In the above output the IP address is shown as inet addr:, in this case 192.168.2.21. Similarly, the hostname may be obtained by running the hostname tool at a terminal command prompt.

To connect from a remote system perform the same steps above using either the IP address or host name of the remote host to which you connect. Enter your password when prompted and you will find yourself logged into the remote system.

The SSH service provides a mechanism for copying files to and from a remote system. Copying is performed using the scp utility. To copy a file to a directory on a remote system, execute the following command:

scp myfile.txt [email protected]:/home/demo

Where myfile.txt is the name of the file to be uploaded to the remote system, username is the name of user account to be used to log into the remote system, 192.168.2.1 is replaced by the real IP address or hostname of the system and /home/demo represents the directory into which the file should be copied.

The above file could similarly be copied from the remote system to the local system as follows:

scp [email protected]:/home/demo/myfile.txt .

The above command will copy the remote file to the current directory on the local system.

Having configured the system to run the SSH server we can now look at how to disable it. As mentioned previously, the SSH server runs in the background as a service. In order to disable SSH we need to turn off the SSH service. This can be achieved using the invoke-rc.d command line tool in a terminal window as follows:

sudo invoke-rc.d ssh stop
 * Stopping OpenBSD Secure Shell server sshd     [ OK ]