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SSH command usage, options, and configuration in Linux/Unix

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This page is about the OpenSSH ssh command on Unix/Linux or the Mac terminal. For general information about SSH and other implementations, see the SSH protocol home page page.

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

SSH Command in Linux

The ssh command provides a secure encrypted connection between two hosts over an insecure network. This connection can also be used for terminal access, tệp tin transfers, and for tunneling other applications. Graphical X11 applications can also be lập cập securely over SSH from a remote location.

Other SSH Commands

There are other SSH commands besides the client ssh. Each has its own page.

  • ssh-keygen - creates a key pair for public key authentication

  • ssh-copy-id - configures a public key as authorized on a server

  • ssh-agent - agent to tướng hold private key for single sign-on

  • ssh-add - tool to tướng add a key to tướng the agent

  • scp - tệp tin transfer client with RCP-like command interface

  • sftp - tệp tin transfer client with FTP-like command interface

  • sshd - OpenSSH server

Using the Linux client

Linux typically uses the OpenSSH client. The ssh command to tướng log into a remote machine is very simple. To log in to tướng 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 tướng connect to tướng 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 tướng continue connecting (yes/no)?

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

Warning: Permanently added 'sample.ssh.com' (DSA) to tướng the list of known hosts.

Each server has a host key, and the above question related to tướng verifying and saving the host key, sánh that next time you connect to tướng 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. For some servers, you may be required to tướng 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.

Specifying a different user name

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

ssh [email protected]

The above can also be expressed with the syntax:

ssh -l alternative-username sample.ssh.com

Executing remote commands on the server

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

ssh hostname command

For example, to tướng 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 tướng the remote server, the contents of the remote directory will be displayed, and you will return to tướng your local shell prompt. -x Disables X11 forwarding.

SSH client configuration file

The ssh command reads its configuration from the SSH client configuration tệp tin ~/.ssh/config. For more information, see the page on SSH client configuration tệp tin.

Configuring public key authentication

To configure passwordless public key authentication, you may want to tướng create an SSH key and phối up an authorized_keys tệp tin. See the pages on ssh-keygen and ssh-copy-id for more information.

Configuring port forwarding

Command-line options can be used to tướng phối up port forwarding. Local fowarding means that a local port (at the client computer) is tunneled to tướng an IP address and port from the server. Remote forwarding means that a remote port (at the server computer) is forwarded to tướng a given IP address and port from the client machine. See the page on configuring port forwarding on how to tướng configure them.

OpenSSH also supports forwarding Unix domain name sockets and IP packets from a tunnel device to tướng establish a VPN (Virtual Private Network).

SSH command line options

Some of 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 tướng listen to tướng port on the local side. When a connection is made to tướng this port, the connection is forwarded over the secure channel, and the application protocol is then used to tướng determine where to tướng connect to tướng from the remote machine.

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

-F configfile Specifies a per-user configuration tệp tin. The mặc định for the per-user configuration tệp tin is ~/.ssh/config.

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

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

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

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

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

-q Quiet mode.

-V Display the version number.

-v Verbose mode.

-X Enables X11 forwarding.

A little history

SSH replaced several older commands and protocols in Unix and Linux the 1990s. The include telnet, rlogin, and rsh.

SSH runs at TCP/IP port 22. This is right between ftp and telnet, which are đôi mươi years older. Read the story of how SSH got port 22.

The following đoạn Clip summarizes how and why SSH was originally developed.

SSH history with Tatu Ylonen