SSH(1)

NAME
ssh - OpenSSH secure shell client (remote login program)

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

ssh [-afgknqtvxCPX] [-c blowfish | 3des] [-e escape_char] [-i identity_file] [-l login_name] [-o option] [-p port] [-L host: port: hostport] [-R host: port: hostport] [hostname | user@hostname] [command]

DESCRIPTION
ssh (Secure Shell) 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.

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 (and primary) 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 .rhosts, .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 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, .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-keygen(1). This stores the private key in .ssh/identity and the public key in .ssh/identity.pub in the user's home directory. The user should then copy the identity.pub 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. 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-agent(1) 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.

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 can disconnect with ~., and suspend ssh with ~^Z. All forwarded connections can be listed with ~# and if the session blocks waiting for forwarded X11 or TCP/IP connections to terminate, it can be backgrounded with ~& (this should not be used while the user shell is active, as it can cause the shell to hang). All available escapes can be listed with ~?.

A single tilde character can be sent as ~~ (or by following the tilde by a character other than those described above). The escape character must always follow a newline to be interpreted as special. The escape character can be changed in configuration files or on the command line.

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 in on the remote machine exists 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.

If 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 user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side unless disabled on command line or in a configuration file.

Forwarding of arbitrary TCP/IP connections over the secure channel can be specified either on 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 trough firewalls.

ssh automatically maintains and checks a database containing RSA-based identifications for all hosts it has ever been used with. The database is 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 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 (see below) can be used to prevent logins to machines whose host key is not known or has changed.

OPTIONS

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

-c blowfish|3des
    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. It is presumably more secure than the des cipher which is no longer supported in ssh. blowfish is a fast block cipher, it appears very secure and is much faster than 3des.

-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.

-i identity_file
    Selects the file from which the identity (private key) for RSA authentication is read. Default is .ssh/identity in the user's home directory. 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).

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

-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 may also be specified on a per-host basis in the configuration file.

-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 in 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 option
    Can be used to give options in the format used in the config file. This is useful for specifying options for which there is no separate command-line flag. The option has the same format as a line in the configuration file.

-p port
    Port to connect to on the remote host. This can be specified on a per-host basis in the configuration file.

-P  Use a non-privileged port for outgoing connections. This can be used if your firewall does not permit connections from privileged ports. Note that this option turns of RhostsAuthentication and RhostsRSAAuthentication.

-q  Quiet mode. Causes all warning and diagnostic messages to be suppressed. Only fatal errors are displayed.

-t  Force pseudo-tty allocation. This can be used to execute arbitary screen-based programs on a remote machine, which can be very useful e.g. when implementing menu services.

-v  Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. The verbose mode is also used to display skey(1) challenges, if the user entered "s/key" as password.

-x  Disables X11 forwarding. This can also be specified on a per-host basis in a configuration file.

-X  Enables X11 forwarding.

-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, and the “level” can be controlled by the CompressionLevel option (see below). 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 Compress option below.

-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:hostport from the remote machine. Port forwardings can also be specified in the configuration file. Only root can forward privileged ports.

-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: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.

CONFIGURATION FILES
ssh obtains configuration data from the following sources (in this order): command line options, user's configuration file ($HOME/.ssh/config), and system-wide configuration file (/etc/ssh/ssh_config). For each parameter, the first obtained value will be used. The configuration files contain sections bracketed by "Host" specifications, and that section is only applied for hosts that match one of the patterns given in the specification. The matched host name is the one given on the command line.

Since the first obtained value for each parameter is used, more host-specific declarations should be given near the beginning of the file, and general defaults at the end.

The configuration file has the following format:

Empty lines and lines starting with ‘#’ are comments.

Otherwise a line is of the format “keyword arguments”. The possible keywords and their meanings are as follows (note that the configuration files are case-sensitive):

Host
    Restricts the following declarations (up to the next Host keyword) to be only for those hosts that match one of the patterns given after the keyword. ‘*’ and ‘’? can be used as wildcards in the patterns. A single ‘*’ as a pattern can be used to provide global defaults for all hosts. The host is the hostname argument given on the command line (i.e., the name is not converted to a canonicalized host name before matching).

AFSTokenPassing
    Specifies whether to pass AFS tokens to remote host. The argument to this keyword must be “yes” or “no”.

BatchMode
    If set to “yes”, passphrase/password querying will be disabled. This option is useful in scripts and other batch jobs where you have no user to supply the password. The argument must be “yes” or “no”.

Cipher
    Specifies the cipher to use for encrypting the session. Currently, “blowfish”, and “3des” are supported. The default is “3des”.

Compression
    Specifies whether to use compression. The argument must be “yes” or “no”.

CompressionLevel
    Specifies the compression level to use if compression is enable. The argument must be an integer from 1 (fast) to 9 (slow, best). The default level is 6, which is good for most applications. The meaning of the values is the same as in GNU GZIP.

ConnectionAttempts
    Specifies the number of tries (one per second) to make before falling back to rsh or exiting. The argument must be an integer. This may be useful in scripts if the connection sometimes fails.

EscapeChar
    Sets the escape character (default: ‘~’). The escape character can also be set on the command line. The argument should be a single character, ‘^’ followed by a letter, or “none” to disable the escape character entirely (making the connection transparent for binary data).

FallBackToRsh
    Specifies that if connecting via ssh fails due to a connection refused error (there is no sshd(8) listening on the remote host), rsh(1) should automatically be used instead (after a suitable warning about the session being unencrypted). The argument must be “yes” or “no”.

ForwardAgent
    Specifies whether the connection to the authentication agent (if any) will be forwarded to the remote machine. The argument must be “yes” or “no”.

ForwardX11
    Specifies whether X11 connections will be automatically redirected over the secure channel and DISPLAY set. The argument must be “yes” or “no”.

GatewayPorts
    Specifies whether remote hosts are allowed to connect to local forwarded ports. The argument must be “yes” or “no”. The default is “no”.

GlobalKnownHostsFile
    Specifies a file to use instead of /etc/ssh/ssh_known_hosts.

HostName
    Specifies the real host name to log into. This can be used to specify nicnames or abbreviations for hosts. Default is the name given on the command line. Numeric IP addresses are also permitted (both on the command line and in HostName specifications).

IdentityFile
    Specifies the file from which the user's RSA authentication identity is read (default .ssh/identity in the user's home directory). Additionally, any identities represented by the authentication agent will be used for authentication. The file name may use the tilde syntax to refer to a user's home directory. It is possible to have multiple identity files specified in configuration files; all these identities will be tried in sequence.

KeepAlive
    Specifies whether the system should send keepalive messages to the other side. If they are sent, death of the connection or crash of one of the machines will be properly noticed. However, this means that connections will die if the route is down temporarily, and some people find it annoying.

    The default is “yes” (to send keepalives), and the client will notice if the network goes down or the remote host dies. This is important in scripts, and many users want it too.

    To disable keepalives, the value should be set to “no” in both the server and the client configuration files.

KerberosAuthentication
    Specifies whether Kerberos authentication will be used. The argument to this keyword must be “yes” or “no”.

KerberosTgtPassing
    Specifies whether a Kerberos TGT will be forwarded to the server. This will only work if the Kerberos server is actually an AFS kaserver. The argument to this keyword must be “yes” or “no”.

LocalForward
    Specifies that a TCP/IP port on the local machine be forwarded over the secure channel to given host:port from the remote machine. The first argument must be a port number, and the second must be host:port. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the root can forward privileged ports.

PasswordAuthentication
    Specifies whether to use password authentication. The argument to this keyword must be “yes” or “no”.

LogLevel
    Gives the verbosity level that is used when logging messages from ssh. The possible values are: QUIET, FATAL, ERROR, INFO, CHAT and DEBUG. The default is INFO.

NumberOfPasswordPrompts
    Specifies the number of password prompts before giving up. The argument to this keyword must be an integer. Default is 3.

Port
    Specifies the port number to connect on the remote host. Default is 22.

ProxyCommand
    Specifies the command to use to connect to the server. The command string extends to the end of the line, and is executed with /bin/sh. In the command string, %h will be substituted by the host name to connect and %p by the port. The command can be basically anything, and should read from its stdin and write to its stdout. It should eventually connect an sshd(8) server running on some machine, or execute sshd -i somewhere. Host key management will be done using the HostName of the host being connected (defaulting to the name typed by the user). Note that CheckHostIP is not available for connects with a proxy command.

RemoteForward
    Specifies that a TCP/IP port on the remote machine be forwarded over the secure channel to given host:port from the local machine. The first argument must be a port number, and the second must be host:port. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the root can forward privileged ports.

RhostsAuthentication
    Specifies whether to try rhosts based authentication. Note that this declaration only affects the client side and has no effect whatsoever on security. Disabling rhosts authentication may reduce authentication time on slow connections when rhosts authentication is not used. Most servers do not permit RhostsAuthentication because it is not secure (see RhostsRSAAuthentication). The argument to this keyword must be “yes” or “no”.

RhostsRSAAuthentication
    Specifies whether to try rhosts based authentication with RSA host authentication. This is the primary authentication method for most sites. The argument must be “yes” or “no”.

RSAAuthentication
    Specifies whether to try RSA authentication. The argument to this keyword must be “yes” or “no”. RSA authentication will only be attempted if the identity file exists, or an authentication agent is running.

SkeyAuthentication
    Specifies whether to use skey(1) authentication. The argument to this keyword must be “yes” or “no”. The default is “no”.

CheckHostIP
    If this flag is set to “yes”, ssh will additionally check the host ip address in the known_hosts file. This allows ssh to detect if a host key changed due to DNS spoofing. If the option is set to “no”, the check will not be executed.

StrictHostKeyChecking
    If this flag is set to “yes”, ssh ssh will never automatically add host keys to the $HOME/.ssh/known_hosts file, and refuses to connect hosts whose host key has changed. This provides maximum protection against trojan horse attacks. However, it can be somewhat annoying if you don't have good /etc/ssh/ssh_known_hosts files installed and frequently connect new hosts. Basically this option forces the user to manually add any new hosts. Normally this option is disabled, and new hosts will automatically be added to the known host files. The host keys of known hosts will be verified automatically in either case. The argument must be “yes” or “no”.

User
    Specifies the user to log in as. This can be useful if you have a different user name in different machines. This saves the trouble of having to remember to give the user name on the command line.

UserKnownHostsFile
    Specifies a file to use instead of $HOME/.ssh/known_hosts.

UsePrivilegedPort
    Specifies whether to use a privileged port for outgoing connections. The argument must be “yes” or “no”. The default is “yes”. Note that setting this option to “no” turns of RhostsAuthentication and RhostsRSAAuthentication.

UseRsh
    Specifies that rlogin/rsh should be used for this host. It is possible that the host does not at all support the ssh protocol. This causes ssh to immediately exec rsh(1). All other options (except HostName) are ignored if this has been specified. The argument must be “yes” or “no”.

ENVIRONMENT
ssh will normally set the following environment variables:

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 point the user's mailbox.

PATH
    Set to the default PATH, as specified when compiling ssh.

SSH_AUTH_SOCK
    indicates the path of a unix-domain socket used to communicate with the agent.

SSH_CLIENT
    Identifies the client end of the connection. The variable contains three space-separated values: client ip-address, client port number, and server port number.

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

USER
    Set to the name of the user logging in.

Additionally, ssh reads $HOME/.ssh/environment, and adds lines of the format “VARNAME=value” to the environment.

FILES

$HOME/.ssh/known_hosts
    Records host keys for all hosts the user has logged into (that are not in /etc/ssh/ssh_known_hosts). See sshd(8).

$HOME/.ssh/random_seed
    Used for seeding the random number generator. This file contains sensitive data and should read/write for the user and not accessible for others. This file is created the first time the program is run and updated automatically. The user should never need to read or modify this file.

$HOME/.ssh/identity
    Contains the RSA authentication identity of the user. This file contains sensitive data and should be readable by the user but not accessible by others (read/write/execute). Note that ssh ignores this file if it is accessible by others. It is possible to specify a passphrase when generating the key; the passphrase will be used to encrypt the sensitive part of this file using 3DES.

$HOME/.ssh/identity.pub
    Contains the public key for authentication (public part of the identity file in human-readable form). The contents of this file should be added to $HOME/.ssh/authorized_keys on all machines where you wish to log in using RSA authentication. This file is not sensitive and can (but need not) be readable by anyone. This file is never used automatically and is not necessary; it is only provided for the convenience of the user.

$HOME/.ssh/config
    This is the per-user configuration file. The format of this file is described above. This file is used by the ssh client. This file does not usually contain any sensitive information, but the recommended permissions are read/write for the user, and not accessible by others.

$HOME/.ssh/authorized_keys
    Lists the RSA keys that can be used for logging in as this user. The format of this file is described in the sshd(8) manual page. In the simplest form the format is the same as the .pub identity files (that is, each line contains the number of bits in modulus, public exponent, modulus, and comment fields, separated by spaces). This file is not highly sensitive, but the recommended permissions are read/write for the user, and not accessible by others.

/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. This file should be world-readable. This file contains public keys, one per line, in the following format (fields separated by spaces): system name, number of bits in modulus, public exponent, modulus, and optional comment field. When different names are used for the same machine, all such names should be listed, separated by commas. The format is described on the sshd(8) manual page.

    The canonical system name (as returned by name servers) is used by sshd(8) to verify the client host when logging in; other names are needed because ssh does not convert the user-supplied name to a canonical name before checking the key, because someone with access to the name servers would then be able to fool host authentication.

/etc/ssh/ssh_config
    Systemwide configuration file. This file provides defaults for those values that are not specified in the user's configuration file, and for those users who do not have a configuration file. This file must be world-readable.

$HOME/.rhosts
    This file is used in .rhosts authentication to list the host/user pairs that are permitted to log in. (Note that this file is also used by rlogin and rsh, which makes using this file insecure.) Each line of the file contains a host name (in the canonical form returned by name servers), and then a user name on that host, separated by a space. One some machines this file may need to be world-readable if the user's home directory is on a 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.

    Note that by default sshd(8) will be installed so that it requires successful RSA host authentication before permitting .rhosts authentication. If your server machine does not have the client's host key in /etc/ssh/ssh_known_hosts, you can store it in $HOME/.ssh/known_hosts. The easiest way to do this is to connect back to the client from the server machine using ssh; this will automatically add the host key inxi $HOME/.ssh/known_hosts.

$HOME/.shosts
    This file is used exactly the same way as .rhosts. The purpose for having this file is to be able to use rhosts authentication with ssh without permitting login with rlogin(1) or rsh(1).

/etc/hosts.equiv
    This file is used during .rhosts authentication. It contains canonical hosts names, one per line (the full format is described on the sshd(8) manual page). If the client host is found in this file, login is automatically permitted provided client and server user names are the same. Additionally, successful RSA host authentication is normally required. This file should only be writable by root.

/etc/ssh/shosts.equiv
    This file is processed exactly as /etc/hosts.equiv. This file may be useful to permit logins using ssh but not using rsh/rlogin.

/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.

$HOME/.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.

libcrypto.so.X.1
    A version of this library which includes support for the RSA algorithm is required for proper operation.

AUTHOR
Tatu Ylonen <ylo@cs.hut.fi>

Issues can be found from the SSH WWW home page:

    http://www.cs.hut.fi/ssh

OpenSSH is a derivative of the original (free) ssh 1.2.12 release, but with bugs removed and newer features re-added. Rapidly after the 1.2.12 release, newer versions bore successively more restrictive licenses. This version of OpenSSH

•   has all components of a restrictive nature (ie. patents, see ssl(8)) directly removed from the source code; any licensed or patented components are chosen from external libraries.

•   has been updated to support ssh protocol 1.5.

•   contains added support for kerberos(8) authentication and ticket passing.

•   supports one-time password authentication with skey(1).

The libraries described in ssl(8) are required for proper operation.

OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt, and Dug Song.

SEE ALSO
rlogin(1), rsh(1), scp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), telnet(1), sshd(8), ssl(8)