Beginning SSH on Ubuntu

Installing OpenSSH

The Ubuntu (and MacOS X) flavor of SSH is called OpenSSH, a free, open-source implementation of the ssh protocol. It consists of two basic components, an openssh-client and an openssh-server. SSH clients communicate with SSH servers over encrypted network connections.

The openssh-client software should already be installed by default on Ubuntu. If you want to be able to accept SSH connections as well as request them, you’ll need the server software as well. The easiest way to ensure you have both is simply to run:

sudo apt-get install openssh-client openssh-server

Using SSH to Log into a Remote Computer

Once OpenSSH is installed, you can login to a remote SSH server by using the ssh command:

ssh remoteuser@remotebox

where remoteuser is the username of the remote account you’re trying to access, and remotebox is the remote server’s hostname or IP address.

For example, if you know that your Kubuntu desktop box (now running openssh-server) has a user account named joebanks and that the IP address of that computer on your private LAN is 192.168.0.12, you could login remotely to that account from your Linux/Mac laptop by typing:

ssh joebanks@192.168.0.12

If you’re unsure of a computer’s local IP address, try running ifconfig on that machine. This will display the status of the active network interfaces, and the local IP address of that device will be listed after inet addr. It will most likely be in the form of 192.168.0.xx.

Or, if you’ve got a web hosting account that allows shell access (e.g., DreamHost) with a domain name like cooldomain.com, your syntax might look like:

ssh joebanks@cooldomain.com

Now, the first time an SSH client encounters a new remote server, it will report that it’s never seen the machine before, by printing the following message:

The authenticity of host 'remotebox (192.168.0.12)' can't be established.
RSA key fingerprint is 53:b4:ad:c8:51:17:99:4b:c9:08:ac:c1:b6:05:71:9b.
Are you sure you want to continue connecting (yes/no)?

This is just an extra security measure to ensure that you’re actually connecting to the machine you think you are. If you type yes (the most common response), you’ll see the following:

Warning: Permanently added 'remotebox' (RSA) to the list of known hosts.

Subsequent login attempts to this machine will omit the warning message. You’ll then be asked for the remoteuser’s password:

remoteuser@remotebox's password:

And after correctly entering it, like magic, you’ll be logged into the remote machine, and instead of your local machine’s command prompt, you’ll see the following:

remoteuser@remotebox:~$

And, voila! You can execute commands on the remote machine just as you would on your local box. To close the connection to the remote server, type exit, or use Ctrl-D.

Copying Files

To transfer files and directories from your local machine to the remote server and vice-versa, you’ll use SSH’s “secure copy” command, or scp. To copy a single file from your local machine to the server, use the following syntax:

scp file.txt remoteuser@remotebox:/directory

where file.txt is the name of a file in the current directory of your local machine, remoteuser@remotebox is the username and hostname or IP address of the server (just like in the above ssh examples, and /directory is the directory path on the server where you want your file copied.

For example, if you want to copy the local file.txt to the /home/joebanks/docs directory on the server you logged into above, you’ll run the following command from a local terminal session:

scp file.txt joebanks@192.168.0.12:/home/joebanks/docs



You can be as verbose as you want with the local and remote filenames and directories, even changing the filename in the process, like so:

scp ~/docs/oldfile.txt joebanks@192.168.0.12:/home/joebanks/docs/newfile.txt

To copy a file from the server to your local machine, use the following syntax:

scp remoteuser@remotebox:file.txt /local/directory

where remoteuser@remotebox is the username and hostname or IP address of the server, file.txt is a file in the /home/remoteuser directory, and /local/directory is the local directory path into which the file will be copied.

Again, you can be as verbose as necessary, for example:

scp joebanks@192.168.0.12:~/docs/newfile.txt /home/joe/downloads

Copying Directories

To copy an entire directory (and all of its contents) from the local machine to the remote server, use the recursive -r switch, like so:

scp -r /local/directory remoteuser@remotebox:/remote/directory

where /local/directory is the path to the local directory you want copied, and /remote/directory is the remote directory into which you want the directory to be copied.

To copy an entire directory (and all of its contents) from the remote server to the local machine, use the following:

scp -r remoteuser@remotebox:/remote/directory /local/directory

where /remote/directory is the path to the remote directory you want copied, and /local/directory is the local directory into which you want the directory to be copied.