Configuring an OpenSSH Client

Using the ssh Command

The ssh command is a secure replacement for the rlogin, rsh, and telnet commands. It allows you to log in to a remote machine as well as execute commands on a remote machine.

Logging in to a remote machine with ssh is similar to using telnet. To log in to a remote machine named penguin.example.net, type the following command at a shell prompt:

ssh penguin.example.net

The first time you ssh to a remote machine, you will see a message similar to the following:

The authenticity of host 'penguin.example.net' can't be established.
DSA key fingerprint is 94:68:3a:3a:bc:f3:9a:9b:01:5d:b3:07:38:e2:11:0c.
Are you sure you want to continue connecting (yes/no)?

Type yes to continue. This will add the server to your list of known hosts (~/.ssh/known_hosts) as seen in the following message:

Warning: Permanently added 'penguin.example.net' (RSA) to the list of known hosts.

Next, you will see a prompt asking for your password for the remote machine. After entering your password, you will be at a shell prompt for the remote machine. If you do not specify a username the username that you are logged in as on the local client machine is passed to the remote machine. If you want to specify a different username, use the following command:

ssh username@penguin.example.net

You can also use the syntax ssh -l username penguin.example.net.

The ssh command can be used to execute a command on the remote machine without logging in to a shell prompt. The syntax is ssh hostname command. For example, if you want to execute the command ls /usr/share/doc on the remote machine penguin.example.net, type the following command at a shell prompt:

ssh penguin.example.net ls /usr/share/doc

After you enter the correct password, the contents of the remote directory /usr/share/doc will be displayed, and you will return to your local shell prompt.

Using the scp Command

The scp command can be used to transfer files between machines over a secure, encrypted connection. It is similar to rcp.

The general syntax to transfer a local file to a remote system is as follows:

scp <localfile> username@tohostname:<remotefile>

The <localfile> specifies the source including path to the file, such as /var/log/maillog. The <remotefile> specifies the destination, which can be a new filename such as /tmp/hostname-maillog. For the remote system, if you do not have a preceding /, the path will be relative to the home directory of username, typically /home/username/.

To transfer the local file shadowman to the home directory of your account on penguin.example.net, type the following at a shell prompt (replace username with your username):

scp shadowman username@penguin.example.net:shadowman

This will transfer the local file shadowman to /home/username/shadowman on penguin.example.net. Alternately, you can leave off the final shadowman in the scp command.

The general syntax to transfer a remote file to the local system is as follows:

scp username@tohostname:<remotefile> <newlocalfile>

The <remotefile> specifies the source including path, and <newlocalfile> specifies the destination including path.

Multiple files can be specified as the source files. For example, to transfer the contents of the directory downloads/ to an existing directory called uploads/ on the remote machine penguin.example.net, type the following at a shell prompt:

scp downloads/* username@penguin.example.net:uploads/

Using the sftp Command

The sftp utility can be used to open a secure, interactive FTP session. It is similar to ftp except that it uses a secure, encrypted connection. The general syntax is sftp username@hostname.com. Once authenticated, you can use a set of commands similar to those used by FTP. Refer to the sftp man page for a list of these commands. To read the man page, execute the command man sftp at a shell prompt. The sftp utility is only available in OpenSSH version 2.5.0p1 and higher.