SSH Man Page

NAME

ssh - OpenSSH SSH client (remote login program)

SYNOPSIS

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

ssh [-afgknqstvxACNPTX1246] [-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/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/shosts.equiv, and if additionally the server can verify the client's host key (see /etc/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, $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-keygen(1). 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-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.

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

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

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_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.

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.

-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.

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

-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.

-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 (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 Compression option below.

-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_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 obtains configuration data from the following sources in the following order: command line options, user's configuration file ($HOME/.ssh/config), and system-wide configuration file (/etc/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''. Configuration options may be separated by whitespace or optional whitespace and exactly one `='; the latter format is useful to avoid the need to quote whites pace when specifying configuration options using the ssh, scp and sftp -o option.

The possible keywords and their meanings are as follows (note that keywords are case-insensitive and arguments 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''. This option applies to protocol version 1 only.

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

BindAddress Specify the interface to transmit from on machines with multiple interfaces or aliased addresses. Note that this option does not work if

UsePrivilegedPort is set to ``yes''.

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. The default is ``yes''.

Cipher Specifies the cipher to use for encrypting the session in protocol version 1. Currently, ``blowfish'', ``3des'', and ``des'' are supported. 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''.

Ciphers Specifies the ciphers allowed for protocol version 2 in order of preference. Multiple ciphers must be comma-separated. The default is ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, aes192-cbc,aes256-cbc''

ClearAllForwardings Specifies that all local, remote and dynamic port forwardings specified in the configuration files or on the command line be cleared. This option is primarily useful when used from the ssh command line to clear port forwardings set in configuration files, and is automatically set by scp(1) and sftp(1). The argument must be ``yes'' or ``no''. The default is ``no''.

Compression Specifies whether to use compression. The argument must be ``yes'' or ``no''. The default is ``no''.

CompressionLevel Specifies the compression level to use if compression is enabled. 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 gzip(1). Note that this option applies to protocol version 1 only.

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. The default is 1.

DynamicForward Specifies that a TCP/IP port on the local machine be forwarded over the secure channel, and the application protocol is then used to determine where to connect to from the remote machine. The argument must be a port number. Currently the SOCKS4 protocol is supported, and ssh will act as a SOCKS4 server. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports.

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''. The default is ``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''. The default is ``no''.

ForwardX11 Specifies whether X11 connections will be automatically redirected over the secure channel and DISPLAY set. The argument must be ``yes'' or ``no''. The default is ``no''.

GatewayPorts Specifies whether remote hosts are allowed to connect to local forwarded ports. By default, ssh binds local port forwardings to the loopback addresss. This prevents other remote hosts from connecting to forwarded ports.

GatewayPorts can be used to specify that ssh should bind local port forwardings to the wildcard address, thus allowing remote hosts to connect to forwarded ports. The argument must be ``yes'' or ``no''. The default is ``no''.

GlobalKnownHostsFile Specifies a file to use for the global host key database instead of /etc/ssh_known_hosts.

HostbasedAuthentication Specifies whether to try rhosts based authentication with public key authentication. The argument must be ``yes'' or ``no''. The default is ``no''. This option applies to protocol version 2 only and is similar to RhostsRSAAuthentication.

HostKeyAlgorithms Specifies the protocol version 2 host key algorithms that the client wants to use in order of preference. The default for this option is: ``ssh-rsa,ssh-dss''.

HostKeyAlias Specifies an alias that should be used instead of the real host name when looking up or saving the host key in the host key database files. This option is useful for tunneling ssh connections or for multiple servers running on a single host.

HostName Specifies the real host name to log into. This can be used to specify nicknames 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 a file from which the user's RSA or DSA authentication identity 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. 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 TCP 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''.

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 the specified host and port from the remote machine. The first argument must be a port number, and the second must be host:port. IPv6 addresses can be specified with an alternative syntax: host/port. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports.

LogLevel Gives the verbosity level that is used when logging messages from ssh. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify higher levels of verbose output.

MACs Specifies the MAC (message authentication code) algorithms in order of preference. The MAC algorithm is used in protocol version 2 for data integrity protection. Multiple algorithms must be comma-separated. The default is ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96''.

NoHostAuthenticationForLocalhost This option can be used if the home directory is shared across machines. In this case localhost will refer to a different machine on each of the machines and the user will get many warnings about changed host keys. However, this option disables host authentication for localhost. The argument to this keyword must be ``yes'' or ``no''. The default is to check the host key for localhost.

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

PasswordAuthentication Specifies whether to use password authentication. The argument to this keyword must be ``yes'' or ``no''. The default is ``yes''.

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

PreferredAuthentications Specifies the order in which the client should try protocol 2 authentication methods. This allows a client to prefer one method (e.g. keyboard-interactive) over another method (e.g. password) The default for this option is: ``hostbased,publickey,keyboard-interactive,password''.

Protocol Specifies the protocol versions ssh should support in order of preference. The possible values are ``1'' and ``2''. Multiple versions must be comma-separated. The default is ``2,1''. This means that ssh tries version 2 and falls back to version 1 if version 2 is not available.

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 standard input and write to its standard output. 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.

PubkeyAuthentication Specifies whether to try public key authentication. The argument to this keyword must be ``yes'' or ``no''. The default is ``yes''. This option applies to protocol version 2 only.

RemoteForward Specifies that a TCP/IP port on the remote machine be forwarded over the secure channel to the specified host and port from the local machine. The first argument must be a port number, and the second must be host:port. IPv6 addresses can be specified with an alternative syntax: host/port. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser 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''. The default is ``yes''. This option applies to protocol version 1 only.

RhostsRSAAuthentication Specifies whether to try rhosts based authentication with RSA host authentication. The argument must be ``yes'' or ``no''. The default is ``yes''. This option applies to protocol version 1 only.

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. The default is ``yes''. Note that this option applies to protocol version 1 only.

ChallengeResponseAuthentication Specifies whether to use challenge response authentication. The argument to this keyword must be ``yes'' or ``no''. The default is ``yes''.

SmartcardDevice Specifies which smartcard device to use. The argument to this keyword is the device ssh should use to communicate with a smartcard used for storing the user's private RSA key. By default, no device is specified and smartcard support is not activated.

StrictHostKeyChecking If this flag is set to ``yes'', ssh will never automatically add host keys to the $HOME/.ssh/known_hosts file, and refuses to connect to hosts whose host key has changed. This provides maximum protection against trojan horse attacks, however, can be annoying when the /etc/ssh_known_hosts file is poorly maintained, or connections to new hosts are frequently made. This option forces the user to manually add all new hosts. If this flag is set to ``no'', ssh will automatically add new host keys to the user known hosts files. If this flag is set to ``ask'', new host keys will be added to the user known host files only after the user has confirmed that is what they really want to do, and ssh will refuse to connect to hosts whose host key has changed. The host keys of known hosts will be verified automatically in all cases. The argument must be ``yes'', ``no'' or ``ask''. The default is ``ask''.

UsePrivilegedPort Specifies whether to use a privileged port for outgoing connections. The argument must be ``yes'' or ``no''. The default is ``no''. Note that this option must be set to ``yes'' if RhostsAuthentication and RhostsRSAAuthentication authentications are needed with older servers.

User Specifies the user to log in as. This can be useful when a different user name is used on 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 for the user host key database instead of $HOME/.ssh/known_hosts

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 execute rsh(1). All other options (except HostName) are ignored if this has been specified. The argument must be ``yes'' or ``no''.

XAuthLocation Specifies the location of the xauth(1) program. The default is undefined.

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 the path of the user's mailbox.

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

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-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_ORIGINAL_COMMAND The 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 The timezone variable is set to indicate the present timezone 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 $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_known_hosts. See sshd(8).

$HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
Contains the authentication identity of the user. They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively. These files contain sensitive data and should be readable by the user but not accessible by others (read/write/execute). Note that ssh ignores a private key 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, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub Contains the public key for authentication (public part of the identity file in human-readable form). The contents of the $HOME/.ssh/identity.pub file should be added to $HOME/.ssh/authorized_keys on all machines where the user wishes to log in using protocol version 1 RSA authentication. The contents of the $HOME/.ssh/id_dsa.pub and $HOME/.ssh/id_rsa.pub file should be added to $HOME/.ssh/authorized_keys on all machines where the user wishes to log in using protocol version 2 DSA/RSA authentication. These files are not sensitive and can (but need not) be readable by anyone. These files are never used automatically and are not necessary; they are 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 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. In the simplest form the format is the same as the .pub identity files. This file is not highly sensitive, but the recommended permissions are read/write for the user, and not accessible by others.

/etc/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 worldreadable. This file contains public keys, one per line, in the following format (fields separated by spaces): system name, public key 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_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.

/etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
These three files contain the private parts of the host keys and are used for RhostsRSAAuthentication and HostbasedAuthentication. Since they are readable only by root ssh must be setuid root if these authentication methods are desired.

$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. On 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 the server machine does not have the client's host key in /etc/ssh_known_hosts, it can be stored 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 to $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/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/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.

$HOME/.ssh/environment
Contains additional definitions for environment variables, see section ENVIRONMENT above.

DIAGNOSTICS

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

AUTHORS

OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.

SEE ALSO

scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1)

Information Technology and Engineering Computer Services (ITECS)
College of Engineering, North Carolina State University, Raleigh, NC 27695
Comments to eoshelp@ncsu.edu. URL: http://www.eos.ncsu.edu/

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