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Command Library

NAME

ssh - OpenSSH SSH client (remote login program)  

SYNOPSIS

ssh [-l login_name ] hostname | user@hostname [command ]


ssh [-afgknqstvxACNTX1246 ] [-b bind_address ] [-c cipher_spec ] [-e escape_char ] [-i identity_file ] [-l login_name ] [-m mac_spec ] [-o option ] [-p port ] [-F configfile ] [-L port host hostport ] [-R port host hostport ] [-D port ] hostname | user@hostname [command ]  

DESCRIPTION

ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine. It is intended to replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. X11 connections and arbitrary TCP/IP ports can also be forwarded over the secure channel.

ssh connects and logs into the specified hostname The user must prove his/her identity to the remote machine using one of several methods depending on the protocol version used:

 

SSH protocol version 1

First, if the machine the user logs in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the remote machine, and the user names are the same on both sides, the user is immediately permitted to log in. Second, if .rhosts or .shosts exists in the user's home directory on the remote machine and contains a line containing the name of the client machine and the name of the user on that machine, the user is permitted to log in. This form of authentication alone is normally not allowed by the server because it is not secure.

The second authentication method is the rhosts or hosts.equiv method combined with RSA-based host authentication. It means that if the login would be permitted by $HOME/.rhosts $HOME/.shosts /etc/hosts.equiv or /etc/ssh/shosts.equiv and if additionally the server can verify the client's host key (see /etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts in the Sx FILES section), only then login is permitted. This authentication method closes security holes due to IP spoofing, DNS spoofing and routing spoofing. [Note to the administrator: /etc/hosts.equiv $HOME/.rhosts and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is desired.]

As a third authentication method, ssh supports RSA based authentication. The scheme is based on public-key cryptography: there are cryptosystems where encryption and decryption are done using separate keys, and it is not possible to derive the decryption key from the encryption key. RSA is one such system. The idea is that each user creates a public/private key pair for authentication purposes. The server knows the public key, and only the user knows the private key. The file $HOME/.ssh/authorized_keys lists the public keys that are permitted for logging in. When the user logs in, the ssh program tells the server which key pair it would like to use for authentication. The server checks if this key is permitted, and if so, sends the user (actually the ssh program running on behalf of the user) a challenge, a random number, encrypted by the user's public key. The challenge can only be decrypted using the proper private key. The user's client then decrypts the challenge using the private key, proving that he/she knows the private key but without disclosing it to the server.

ssh implements the RSA authentication protocol automatically. The user creates his/her RSA key pair by running ssh-keygen1. This stores the private key in $HOME/.ssh/identity and the public key in $HOME/.ssh/identity.pub in the user's home directory. The user should then copy the identity.pub to $HOME/.ssh/authorized_keys in his/her home directory on the remote machine (the authorized_keys file corresponds to the conventional $HOME/.rhosts file, and has one key per line, though the lines can be very long). After this, the user can log in without giving the password. RSA authentication is much more secure than rhosts authentication.

The most convenient way to use RSA authentication may be with an authentication agent. See ssh-agent1 for more information.

If other authentication methods fail, ssh prompts the user for a password. The password is sent to the remote host for checking; however, since all communications are encrypted, the password cannot be seen by someone listening on the network.

 

SSH protocol version 2

When a user connects using protocol version 2 similar authentication methods are available. Using the default values for PreferredAuthentications the client will try to authenticate first using the hostbased method; if this method fails public key authentication is attempted, and finally if this method fails keyboard-interactive and password authentication are tried.

The public key method is similar to RSA authentication described in the previous section and allows the RSA or DSA algorithm to be used: The client uses his private key, $HOME/.ssh/id_dsa or $HOME/.ssh/id_rsa to sign the session identifier and sends the result to the server. The server checks whether the matching public key is listed in $HOME/.ssh/authorized_keys and grants access if both the key is found and the signature is correct. The session identifier is derived from a shared Diffie-Hellman value and is only known to the client and the server.

If public key authentication fails or is not available a password can be sent encrypted to the remote host for proving the user's identity.

Additionally, ssh supports hostbased or challenge response authentication.

Protocol 2 provides additional mechanisms for confidentiality (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) and integrity (hmac-md5, hmac-sha1). Note that protocol 1 lacks a strong mechanism for ensuring the integrity of the connection.

 

Login session and remote execution

When the user's identity has been accepted by the server, the server either executes the given command, or logs into the machine and gives the user a normal shell on the remote machine. All communication with the remote command or shell will be automatically encrypted.

If a pseudo-terminal has been allocated (normal login session), the user may use the escape characters noted below.

If no pseudo tty has been allocated, the session is transparent and can be used to reliably transfer binary data. On most systems, setting the escape character to ``none'' will also make the session transparent even if a tty is used.

The session terminates when the command or shell on the remote machine exits and all X11 and TCP/IP connections have been closed. The exit status of the remote program is returned as the exit status of ssh

 

Escape Characters

When a pseudo terminal has been requested, ssh supports a number of functions through the use of an escape character.

A single tilde character can be sent as ~~ or by following the tilde by a character other than those described below. The escape character must always follow a newline to be interpreted as special. The escape character can be changed in configuration files using the EscapeChar configuration directive or on the command line by the -e option.

The supported escapes (assuming the default `~' ) are:

~.
Disconnect
~^Z
Background ssh
~#
List forwarded connections
~&
Background ssh at logout when waiting for forwarded connection / X11 sessions to terminate
~?
Display a list of escape characters
~C
Open command line (only useful for adding port forwardings using the -L and -R options)
~R
Request rekeying of the connection (only useful for SSH protocol version 2 and if the peer supports it)

 

X11 and TCP forwarding

If the ForwardX11 variable is set to ``yes'' (or, see the description of the -X and -x options described later) and the user is using X11 (the DISPLAY environment variable is set), the connection to the X11 display is automatically forwarded to the remote side in such a way that any X11 programs started from the shell (or command) will go through the encrypted channel, and the connection to the real X server will be made from the local machine. The user should not manually set DISPLAY Forwarding of X11 connections can be configured on the command line or in configuration files.

The DISPLAY value set by ssh will point to the server machine, but with a display number greater than zero. This is normal, and happens because ssh creates a ``proxy'' X server on the server machine for forwarding the connections over the encrypted channel.

ssh will also automatically set up Xauthority data on the server machine. For this purpose, it will generate a random authorization cookie, store it in Xauthority on the server, and verify that any forwarded connections carry this cookie and replace it by the real cookie when the connection is opened. The real authentication cookie is never sent to the server machine (and no cookies are sent in the plain).

If the ForwardAgent variable is set to ``yes'' (or, see the description of the -A and -a options described later) and the user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side.

Forwarding of arbitrary TCP/IP connections over the secure channel can be specified either on the command line or in a configuration file. One possible application of TCP/IP forwarding is a secure connection to an electronic purse; another is going through firewalls.

 

Server authentication

ssh automatically maintains and checks a database containing identifications for all hosts it has ever been used with. Host keys are stored in $HOME/.ssh/known_hosts in the user's home directory. Additionally, the file /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any new hosts are automatically added to the user's file. If a host's identification ever changes, ssh warns about this and disables password authentication to prevent a trojan horse from getting the user's password. Another purpose of this mechanism is to prevent man-in-the-middle attacks which could otherwise be used to circumvent the encryption. The StrictHostKeyChecking option can be used to prevent logins to machines whose host key is not known or has changed.

The options are as follows:

-a
Disables forwarding of the authentication agent connection.
-A
Enables forwarding of the authentication agent connection. This can also be specified on a per-host basis in a configuration file.

Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the agent's Unix-domain socket) can access the local agent through the forwarded connection. An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent.

-b bind_address
Specify the interface to transmit from on machines with multiple interfaces or aliased addresses.
-c blowfish|3des|des
Selects the cipher to use for encrypting the session. 3des is used by default. It is believed to be secure. 3des (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. blowfish is a fast block cipher, it appears very secure and is much faster than 3des des is only supported in the ssh client for interoperability with legacy protocol 1 implementations that do not support the 3des cipher. Its use is strongly discouraged due to cryptographic weaknesses.
-c cipher_spec
Additionally, for protocol version 2 a comma-separated list of ciphers can be specified in order of preference. See Ciphers for more information.
-e ch|^ch|none
Sets the escape character for sessions with a pty (default: `~' ) . The escape character is only recognized at the beginning of a line. The escape character followed by a dot (`.' ) closes the connection, followed by control-Z suspends the connection, and followed by itself sends the escape character once. Setting the character to ``none'' disables any escapes and makes the session fully transparent.
-f
Requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the background. This implies -n The recommended way to start X11 programs at a remote site is with something like ssh -f host xterm
-g
Allows remote hosts to connect to local forwarded ports.
-i identity_file
Selects a file from which the identity (private key) for RSA or DSA authentication is read. The default is $HOME/.ssh/identity for protocol version 1, and $HOME/.ssh/id_rsa and $HOME/.ssh/id_dsa for protocol version 2. Identity files may also be specified on a per-host basis in the configuration file. It is possible to have multiple -i options (and multiple identities specified in configuration files).
-I smartcard_device
Specifies which smartcard device to use. The argument is the device ssh should use to communicate with a smartcard used for storing the user's private RSA key.
-k
Disables forwarding of Kerberos tickets and AFS tokens. This may also be specified on a per-host basis in the configuration file.
-l login_name
Specifies the user to log in as on the remote machine. This also may be specified on a per-host basis in the configuration file.
-m mac_spec
Additionally, for protocol version 2 a comma-separated list of MAC (message authentication code) algorithms can be specified in order of preference. See the MACs keyword for more information.
-n
Redirects stdin from /dev/null (actually, prevents reading from stdin). This must be used when ssh is run in the background. A common trick is to use this to run X11 programs on a remote machine. For example, ssh -n shadows.cs.hut.fi emacs will start an emacs on shadows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted channel. The ssh program will be put in the background. (This does not work if ssh needs to ask for a password or passphrase; see also the -f option.)
-N
Do not execute a remote command. This is useful for just forwarding ports (protocol version 2 only).
-o option
Can be used to give options in the format used in the configuration file. This is useful for specifying options for which there is no separate command-line flag.
-p port
Port to connect to on the remote host. This can be specified on a per-host basis in the configuration file.
-q
Quiet mode. Causes all warning and diagnostic messages to be suppressed.
-s
May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use of SSH as a secure transport for other applications (eg. sftp). The subsystem is specified as the remote command.
-t
Force pseudo-tty allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, e.g., when implementing menu services. Multiple -t options force tty allocation, even if ssh has no local tty.
-T
Disable pseudo-tty allocation.
-v
Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple -v options increases the verbosity. Maximum is 3.
-x
Disables X11 forwarding.
-X
Enables X11 forwarding. This can also be specified on a per-host basis in a configuration file.

X11 forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the user's X authorization database) can access the local X11 display through the forwarded connection. An attacker may then be able to perform activities such as keystroke monitoring.

-C
Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11 and TCP/IP connections). The compression algorithm is the same used by gzip(1), and the ``level'' can be controlled by the CompressionLevel option for protocol version 1. Compression is desirable on modem lines and other slow connections, but will only slow down things on fast networks. The default value can be set on a host-by-host basis in the configuration files; see the Compression option.
-F configfile
Specifies an alternative per-user configuration file. If a configuration file is given on the command line, the system-wide configuration file (/etc/ssh/ssh_config ) will be ignored. The default for the per-user configuration file is $HOME/.ssh/config
-L port:host:hostport
Specifies that the given port on the local (client) host is to be forwarded to the given host and port on the remote side. This works by allocating a socket to listen to port on the local side, and whenever a connection is made to this port, the connection is forwarded over the secure channel, and a connection is made to host port hostport from the remote machine. Port forwardings can also be specified in the configuration file. Only root can forward privileged ports. IPv6 addresses can be specified with an alternative syntax: port/host/hostport
-R port:host:hostport
Specifies that the given port on the remote (server) host is to be forwarded to the given host and port on the local side. This works by allocating a socket to listen to port on the remote side, and whenever a connection is made to this port, the connection is forwarded over the secure channel, and a connection is made to host port hostport from the local machine. Port forwardings can also be specified in the configuration file. Privileged ports can be forwarded only when logging in as root on the remote machine. IPv6 addresses can be specified with an alternative syntax: port/host/hostport
-D port
Specifies a local ``dynamic'' application-level port forwarding. This works by allocating a socket to listen to port on the local side, and whenever 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. Currently the SOCKS4 protocol is supported, and ssh will act as a SOCKS4 server. Only root can forward privileged ports. Dynamic port forwardings can also be specified in the configuration file.
-1
Forces ssh to try protocol version 1 only.
-2
Forces ssh to try protocol version 2 only.
-4
Forces ssh to use IPv4 addresses only.
-6
Forces ssh to use IPv6 addresses only.

 

CONFIGURATION FILES

ssh may additionally obtain configuration data from a per-user configuration file and a system-wide configuration file. The file format and configuration options are described in ssh_config5.  

SEE ALSO

rsh(1), scp(1), sftp(1), ssh-add1, ssh-agent1, ssh-keygen1, telnet(1), ssh_config5, ssh-keysign8, sshd(8)
T. Ylonen T. Kivinen M. Saarinen T. Rinne S. Lehtinen "SSH Protocol Architecture" draft-ietf-secsh-architecture-12.txt January 2002 work in progress material


Important: Use the man command (% man) to see how a command is used on your particular computer.

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