ssh connects and logs into the specified hostname (with optional user name). The user must prove his/her identity to the remote machine using one of several methods depending on the protocol version used (see below).

If command is specified, it is executed on the remote host instead of a login shell.

The options are as follows:

-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.

-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.

-a

Disables forwarding of the authentication agent connection.

-b bind_address

Use bind_address on the local machine as the source address of the connection. Only useful on systems with more than one address.

-C

Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11 and TCP 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.

-c cipher_spec

Selects the cipher specification for encrypting the session.

Protocol version 1 allows specification of a single cipher. The supported values are ``3des'' ``blowfish'' and ``des'' 3des (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. It is believed to be secure. 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. The default is ``3des''

For protocol version 2, cipher_spec is a comma-separated list of ciphers listed in order of preference. The supported ciphers are: 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr, aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, blowfish-cbc, and cast128-cbc. The default is:

aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
aes192-ctr,aes256-ctr

-D [bind_address : ] port

Specifies a local ``dynamic'' application-level port forwarding. This works by allocating a socket to listen to port on the local side, optionally bound to the specified bind_address 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 and SOCKS5 protocols are supported, and ssh will act as a SOCKS server. Only root can forward privileged ports. Dynamic port forwardings can also be specified in the configuration file.

IPv6 addresses can be specified with an alternative syntax: [bind_address / ] port or by enclosing the address in square brackets. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address of ``localhost'' indicates that the listening port be bound for local use only, while an empty address or `*' indicates that the port should be available from all interfaces.

-e escape_char

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 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 ~/.ssh/config

-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 smartcard_device

Specify the device ssh should use to communicate with a smartcard used for storing the user's private RSA key. This option is only available if support for smartcard devices is compiled in (default is no support).

-i identity_file

Selects a file from which the identity (private key) for RSA or DSA authentication is read. The default is ~/.ssh/identity for protocol version 1, and ~/.ssh/id_rsa and ~/.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).

-k

Disables forwarding (delegation) of GSSAPI credentials to the server.

-L [bind_address : ] 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, optionally bound to the specified bind_address 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. IPv6 addresses can be specified with an alternative syntax: [bind_address / ] port / host / hostport or by enclosing the address in square brackets. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address of ``localhost'' indicates that the listening port be bound for local use only, while an empty address or `*' indicates that the port should be available from all interfaces.

-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

Places the ssh client into ``master'' mode for connection sharing. Multiple -M options places ssh into ``master'' mode with confirmation required before slave connections are accepted. Refer to the description of ControlMaster in ssh_config5 for details.

-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

Do not execute a remote command. This is useful for just forwarding ports (protocol version 2 only).

-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.)

-O ctl_cmd

Control an active connection multiplexing master process. When the -O option is specified, the ctl_cmd argument is interpreted and passed to the master process. Valid commands are: ``check'' (check that the master process is running) and ``exit'' (request the master to exit).

-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. For full details of the options listed below, and their possible values, see ssh_config5.

AddressFamily

BatchMode

BindAddress

ChallengeResponseAuthentication

CheckHostIP

Cipher

Ciphers

ClearAllForwardings

Compression

CompressionLevel

ConnectionAttempts

ConnectTimeout

ControlMaster

ControlPath

DynamicForward

EscapeChar

ForwardAgent

ForwardX11

ForwardX11Trusted

GatewayPorts

GlobalKnownHostsFile

GSSAPIAuthentication

GSSAPIDelegateCredentials

HashKnownHosts

Host

HostbasedAuthentication

HostKeyAlgorithms

HostKeyAlias

HostName

IdentityFile

IdentitiesOnly

KbdInteractiveDevices

LocalCommand

LocalForward

LogLevel

MACs

NoHostAuthenticationForLocalhost

NumberOfPasswordPrompts

PasswordAuthentication

PermitLocalCommand

Port

PreferredAuthentications

Protocol

ProxyCommand

PubkeyAuthentication

RekeyLimit

RemoteForward

RhostsRSAAuthentication

RSAAuthentication

SendEnv

ServerAliveInterval

ServerAliveCountMax

SmartcardDevice

StrictHostKeyChecking

TCPKeepAlive

Tunnel

TunnelDevice

UsePrivilegedPort

User

UserKnownHostsFile

VerifyHostKeyDNS

XAuthLocation

-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.

-R [bind_address : ] 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 by enclosing the address in square braces or using an alternative syntax: [bind_address / ] host / port / hostport .

By default, the listening socket on the server will be bound to the loopback interface only. This may be overriden by specifying a bind_address An empty bind_address or the address `*' , indicates that the remote socket should listen on all interfaces. Specifying a remote bind_address will only succeed if the server's GatewayPorts option is enabled (see sshd_config5).

-S ctl_path

Specifies the location of a control socket for connection sharing. Refer to the description of ControlPath and ControlMaster in ssh_config5 for details.

-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(1)). The subsystem is specified as the remote command.

-T

Disable pseudo-tty allocation.

-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.

-V

Display the version number and exit.

-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 increase the verbosity. The maximum is 3.

-w tunnel : tunnel

Requests a tun(4) device on the client (first tunnel arg) and server (second tunnel arg). The devices may be specified by numerical ID or the keyword ``any'' which uses the next available tunnel device. See also the Tunnel directive in ssh_config5.

-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.

For this reason, X11 forwarding is subjected to X11 SECURITY extension restrictions by default. Please refer to the ssh -Y option and the ForwardX11Trusted directive in ssh_config5 for more information.

-x

Disables X11 forwarding.

-Y

Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY extension controls.

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.

ssh exits with the exit status of the remote command or with 255 if an error occurred.

The methods available for authentication are: host-based authentication, public key authentication, challenge-response authentication, and password authentication. Authentication methods are tried in the order specified above, though protocol 2 has a configuration option to change the default order: PreferredAuthentications

Host-based authentication works as follows: 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, or if the files ~/.rhosts or ~/.shosts exist in the user's home directory on the remote machine and contain a line containing the name of the client machine and the name of the user on that machine, the user is considered for login. Additionally, the server must be able to verify the client's host key (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts below) for login to be permitted. This authentication method closes security holes due to IP spoofing, DNS spoofing, and routing spoofing. [Note to the administrator: /etc/hosts.equiv ~/.rhosts and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is desired.]

Public key authentication works as follows: The scheme is based on public-key cryptography, using cryptosystems where encryption and decryption are done using separate keys, and it is unfeasible to derive the decryption key from the encryption key. 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. ssh implements public key authentication protocol automatically, using either the RSA or DSA algorithms. Protocol 1 is restricted to using only RSA keys, but protocol 2 may use either. The Sx HISTORY section of ssl(8) contains a brief discussion of the two algorithms.

The file ~/.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 client proves that it has access to the private key and the server checks that the corresponding public key is authorized to accept the account.

The user creates his/her key pair by running ssh-keygen1. This stores the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The user should then copy the public key to ~/.ssh/authorized_keys in his/her home directory on the remote machine. The authorized_keys file corresponds to the conventional ~/.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.

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

Challenge-response authentication works as follows: The server sends an arbitrary Qq challenge text, and prompts for a response. Protocol 2 allows multiple challenges and responses; protocol 1 is restricted to just one challenge/response. Examples of challenge-response authentication include BSD Authentication (see login.conf5) and PAM (some non-OpenBSD systems).

Finally, 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 automatically maintains and checks a database containing identification for all hosts it has ever been used with. Host keys are stored in ~/.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 server spoofing or man-in-the-middle attacks, which could otherwise be used to circumvent the encryption. The StrictHostKeyChecking option can be used to control logins to machines whose host key is not known or has changed.

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 connections have been closed.

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 .

~#

List forwarded connections.

~&

Background ssh at logout when waiting for forwarded connection / X11 sessions to terminate.

~?

Display a list of escape characters.

~B

Send a BREAK to the remote system (only useful for SSH protocol version 2 and if the peer supports it).

~C

Open command line. Currently this allows the addition of port forwardings using the -L and -R options (see above). It also allows the cancellation of existing remote port-forwardings using -KR hostport ! command allows the user to execute a local command if the PermitLocalCommand option is enabled in ssh_config5. Basic help is available, using the -h option.

~R

Request rekeying of the connection (only useful for SSH protocol version 2 and if the peer supports it).

In the example below, we look at encrypting communication between an IRC client and server, even though the IRC server does not directly support encrypted communications. This works as follows: the user connects to the remote host using , specifying a port to be used to forward connections to the remote server. After that it is possible to start the service which is to be encrypted on the client machine, connecting to the same local port, and ssh will encrypt and forward the connection.

The following example tunnels an IRC session from client machine ``127.0.0.1'' (localhost) to remote server ``server.example.com''

$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
$ irc -c '#users' -p 1234 pinky 127.0.0.1

This tunnels a connection to IRC server ``server.example.com'' joining channel ``#users'' nickname ``pinky'' using port 1234. It doesn't matter which port is used, as long as it's greater than 1023 (remember, only root can open sockets on privileged ports) and doesn't conflict with any ports already in use. The connection is forwarded to port 6667 on the remote server, since that's the standard port for IRC services.

The -f option backgrounds ssh and the remote command ``sleep 10'' is specified to allow an amount of time (10 seconds, in the example) to start the service which is to be tunnelled. If no connections are made within the time specified, ssh will exit.

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 above) and the user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side.

$ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key

If the fingerprint is already known, it can be matched and verified, and the key can be accepted. If the fingerprint is unknown, an alternative method of verification is available: SSH fingerprints verified by DNS. An additional resource record (RR), SSHFP, is added to a zonefile and the connecting client is able to match the fingerprint with that of the key presented.

In this example, we are connecting a client to a server, ``host.example.com'' The SSHFP resource records should first be added to the zonefile for host.example.com:

$ ssh-keygen -f /etc/ssh/ssh_host_rsa_key.pub -r host.example.com.
$ ssh-keygen -f /etc/ssh/ssh_host_dsa_key.pub -r host.example.com.

The output lines will have to be added to the zonefile. To check that the zone is answering fingerprint queries:

$ dig -t SSHFP host.example.com

Finally the client connects:

$ ssh -o "VerifyHostKeyDNS ask" host.example.com
[...]
Matching host key fingerprint found in DNS.
Are you sure you want to continue connecting (yes/no)?

See the VerifyHostKeyDNS option in ssh_config5 for more information.

The following example would connect client network 10.0.50.0/24 with remote network 10.0.99.0/24, provided that the SSH server running on the gateway to the remote network, at 192.168.1.15, allows it:

# ssh -f -w 0:1 192.168.1.15 true
# ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252

Client access may be more finely tuned via the /root/.ssh/authorized_keys file (see below) and the PermitRootLogin server option. The following entry would permit connections on the first tun(4) device from user ``jane'' and on the second device from user ``john'' if PermitRootLogin is set to ``forced-commands-only''

tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
tunnel="2",command="sh /etc/netstart tun1" ssh-rsa ... john

Since a SSH-based setup entails a fair amount of overhead, it may be more suited to temporary setups, such as for wireless VPNs. More permanent VPNs are better provided by tools such as ipsecctl(8) and isakmpd(8).

DISPLAY

The DISPLAY variable indicates the location of the X11 server. It is automatically set by ssh to point to a value of the form ``hostname:n'' where ``hostname'' indicates the host where the shell runs, and `n' is an integer 1. ssh uses this special value to forward X11 connections over the secure channel. The user should normally not set DISPLAY explicitly, as that will render the X11 connection insecure (and will require the user to manually copy any required authorization cookies).

HOME

Set to the path of the user's home directory.

LOGNAME

Synonym for USER set for compatibility with systems that use this variable.

MAIL

Set to the path of the user's mailbox.

PATH

Set to the default PATH as specified when compiling .

SSH_ASKPASS

If ssh needs a passphrase, it will read the passphrase from the current terminal if it was run from a terminal. If ssh does not have a terminal associated with it but DISPLAY and SSH_ASKPASS are set, it will execute the program specified by SSH_ASKPASS and open an X11 window to read the passphrase. This is particularly useful when calling ssh from a .xsession or related script. (Note that on some machines it may be necessary to redirect the input from /dev/null to make this work.)

SSH_AUTH_SOCK

Identifies the path of a UNIX socket used to communicate with the agent.

SSH_CONNECTION

Identifies the client and server ends of the connection. The variable contains four space-separated values: client IP address, client port number, server IP address, and server port number.

SSH_ORIGINAL_COMMAND

This variable contains the original command line if a forced command is executed. It can be used to extract the original arguments.

SSH_TTY

This is set to the name of the tty (path to the device) associated with the current shell or command. If the current session has no tty, this variable is not set.

TZ

This variable is set to indicate the present time zone if it was set when the daemon was started (i.e., the daemon passes the value on to new connections).

USER

Set to the name of the user logging in.

Additionally, ssh reads ~/.ssh/environment and adds lines of the format ``VARNAME=value'' to the environment if the file exists and users are allowed to change their environment. For more information, see the PermitUserEnvironment option in sshd_config5.

~/.rhosts

This file is used for host-based authentication (see above). On some machines this file may need to be world-readable if the user's home directory is on an NFS partition, because sshd(8) reads it as root. Additionally, this file must be owned by the user, and must not have write permissions for anyone else. The recommended permission for most machines is read/write for the user, and not accessible by others.

~/.shosts

This file is used in exactly the same way as .rhosts but allows host-based authentication without permitting login with rlogin/rsh.

~/.ssh/authorized_keys

Lists the public keys (RSA/DSA) that can be used for logging in as this user. The format of this file is described in the sshd(8) manual page. This file is not highly sensitive, but the recommended permissions are read/write for the user, and not accessible by others.

~/.ssh/config

This is the per-user configuration file. The file format and configuration options are described in ssh_config5. Because of the potential for abuse, this file must have strict permissions: read/write for the user, and not accessible by others.

~/.ssh/environment

Contains additional definitions for environment variables; see Sx ENVIRONMENT , above.

~/.ssh/identity

~/.ssh/id_dsa

~/.ssh/id_rsa

Contains the private key for authentication. These files contain sensitive data and should be readable by the user but not accessible by others (read/write/execute). ssh will simply ignore a private key file if it is accessible by others. It is possible to specify a passphrase when generating the key which will be used to encrypt the sensitive part of this file using 3DES.

~/.ssh/identity.pub

~/.ssh/id_dsa.pub

~/.ssh/id_rsa.pub

Contains the public key for authentication. These files are not sensitive and can (but need not) be readable by anyone.

~/.ssh/known_hosts

Contains a list of host keys for all hosts the user has logged into that are not already in the systemwide list of known host keys. See sshd(8) for further details of the format of this file.

~/.ssh/rc

Commands in this file are executed by ssh when the user logs in, just before the user's shell (or command) is started. See the sshd(8) manual page for more information.

/etc/hosts.equiv

This file is for host-based authentication (see above). It should only be writable by root.

/etc/ssh/shosts.equiv

This file is used in exactly the same way as hosts.equiv but allows host-based authentication without permitting login with rlogin/rsh.

/etc/ssh/ssh_config

Systemwide configuration file. The file format and configuration options are described in ssh_config5.

/etc/ssh/ssh_host_key

/etc/ssh/ssh_host_dsa_key

/etc/ssh/ssh_host_rsa_key

These three files contain the private parts of the host keys and are used for host-based authentication. If protocol version 1 is used, ssh must be setuid root, since the host key is readable only by root. For protocol version 2, ssh uses ssh-keysign8 to access the host keys, eliminating the requirement that ssh be setuid root when host-based authentication is used. By default ssh is not setuid root.

/etc/ssh/ssh_known_hosts

Systemwide list of known host keys. This file should be prepared by the system administrator to contain the public host keys of all machines in the organization. It should be world-readable. See sshd(8) for further details of the format of this file.

/etc/ssh/sshrc

Commands in this file are executed by ssh when the user logs in, just before the user's shell (or command) is started. See the sshd(8) manual page for more information.

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