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SSH Commands

Practically every Unix and Linux system includes the ssh command. This command is used to start the SSH (client) program that enables secure connection to the SSH server on a remote machine. The ssh command is used from logging into the remote machine, trasferring files between the two machines, and for executing commands on the remote machine.

SSH Command in Unix

The ssh command replaces telnet, 'rlogin',and 'rsh' commands and provides a secure encrypted connection between two hosts over an insecure network. This connection can also be used to tunnel the communications of other applications. For example, it is a common practise to forward X11 connections over and SSH connection.

When a user connects to a remote host with the ssh command, he/she must prove his/her identity to the remote machine. There are several supported methods for this depending on the protocol version used.

Using the SSH Command

Using the ssh command to log into a remote machine is very simple. To log in to a remote computer called sample.ssh.com, type the following command at a shell prompt:

ssh sample.ssh.com

If this is the first time you use ssh to connect to this remote machine, you will see a message like:

The authenticity of host 'sample.ssh.com' cannot be established.
DSA key fingerprint is 04:48:30:31:b0:f3:5a:9b:01:9d:b3:a7:38:e2:b1:0c.
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 'sample.ssh.com' (DSA) to the list of known hosts.

Each server has a host key, and the above question related to verifying and saving the host key, so that next time you connect to the server, it can verify that it actually is the same server.

Once the server connection has been established, the user is authenticated. Typically, it asks for a password. However, many other authentication methods are also supported, such as public key authentication, which allows automatic logins without passwords. For some servers, you may be required to type in a one-time password generated by a special hardware token.

Once authentication has been accepted, you will be at the shell prompt for the remote machine.

Using a Different User Name

It is also possible to use a different username at the remote machine by entering the command as:

ssh alternative-username@sample.ssh.com

The above can also be expressed with the syntax:

ssh -l alternative-username sample.ssh.com

Executing Commands on the Server

The ssh command is often also used to remotely execute commands on the remote machine without logging in to a shell prompt. The syntax for this is:

ssh hostname command

For example, to execute the command:

ls /tmp/doc 

on host sample.ssh.com, type the following command at a shell prompt:

ssh sample.ssh.com  ls /tmp/doc

After authenticating to the remote server, the contents of the remote directory will be displayed, and you will return to your local shell prompt.

SSH Command Line Options

The most important command-line options for the OpenSSH client are:

-1 Use protocol version 1 only.

-2 Use protocol version 2 only.

-4 Use IPv4 addresses only.

-6 Use IPv6 addresses only.

-A Enable forwarding of the authentication agent connection.

-a Disable forwarding of the authentication agent connection.

-C Use data compression

-c cipher_spec Selects the cipher specification for encrypting the session.

-D [bind_address:]port Dynamic application-level port forwarding. This allocates a socket to listen to port on the local side. When a connection is made to this port, the connection is forwarded over the secure channel, and the application protocol is then used to determine where to connect to from the remote machine.

-E log_file Append debug logs to log_file instead of standard error.

-F configfile Specifies a per-user configuration file. The default for the per-user configuration file is ~/.ssh/config.

-g Allows remote hosts to connect to local forwarded ports.

-i identity_file A file from which the identity key (private key) for public key authentication is read.

-J [user@]host[:port] Connect to the target host by first making a ssh connection to the pjump host[(/iam/jump-host) and then establishing a TCP forwarding to the ultimate destination from there.

-l login_name Specifies the user to log in as on the remote machine.

-p port Port to connect to on the remote host.

-q Quiet mode.

-V Display the version number.

-v Verbose mode.

-X Enables X11 forwarding.

-x Disables X11 forwarding.

SSH Configuration File

The ssh program can be configured from either the command line with the options described above, or from the configuration file.

For more information see the page on the SSH configuration file

For public key authentication, you may want to set up an authorized_keys file using the ssh-keygen and ssh-copy-id commands.