Provided by: openssh-client_7.2p2-4_amd64 bug

NAME

     ssh — OpenSSH SSH client (remote login program)

SYNOPSIS

     ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port]
         [-E log_file] [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
         [-L address] [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port]
         [-Q query_option] [-R address] [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]]
         [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 provide secure encrypted communications between two
     untrusted hosts over an insecure network.  X11 connections, arbitrary TCP ports and
     UNIX-domain sockets can also be forwarded over the secure channel.

     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 (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, TCP and UNIX-domain 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”.  For protocol version 2, cipher_spec is a comma-
             separated list of ciphers listed in order of preference.  See the Ciphers keyword in
             ssh_config(5) for more information.

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

     -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.

             If the ExitOnForwardFailure configuration option is set to “yes”, then a client
             started with -f will wait for all remote port forwards to be successfully
             established before placing itself in the background.

     -G      Causes ssh to print its configuration after evaluating Host and Match blocks and
             exit.

     -g      Allows remote hosts to connect to local forwarded ports.  If used on a multiplexed
             connection, then this option must be specified on the master process.

     -I pkcs11
             Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11
             token providing the user's private RSA key.

     -i identity_file
             Selects a file from which the identity (private key) for public key authentication
             is read.  The default is ~/.ssh/identity for protocol version 1, and ~/.ssh/id_dsa,
             ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and ~/.ssh/id_rsa 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).  If no certificates have been explicitly specified by the
             CertificateFile directive, ssh will also try to load certificate information from
             the filename obtained by appending -cert.pub to identity filenames.

     -K      Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
             credentials to the server.

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

     -L [bind_address:]port:host:hostport
     -L [bind_address:]port:remote_socket
     -L local_socket:host:hostport
     -L local_socket:remote_socket
             Specifies that connections to the given TCP port or Unix socket on the local
             (client) host are to be forwarded to the given host and port, or Unix socket, on the
             remote side.  This works by allocating a socket to listen to either a TCP port on
             the local side, optionally bound to the specified bind_address, or to a Unix socket.
             Whenever a connection is made to the local port or socket, the connection is
             forwarded over the secure channel, and a connection is made to either host port
             hostport, or the Unix socket remote_socket, from the remote machine.

             Port forwardings can also be specified in the configuration file.  Only the
             superuser can forward privileged ports.  IPv6 addresses can be specified by
             enclosing the address in square brackets.

             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_config(5) for details.

     -m mac_spec
             A comma-separated list of MAC (message authentication code) algorithms, 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.

     -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), “forward”
             (request forwardings without command execution), “cancel” (cancel forwardings),
             “exit” (request the master to exit), and “stop” (request the master to stop
             accepting further multiplexing requests).

     -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_config(5).

                   AddKeysToAgent
                   AddressFamily
                   BatchMode
                   BindAddress
                   CanonicalDomains
                   CanonicalizeFallbackLocal
                   CanonicalizeHostname
                   CanonicalizeMaxDots
                   CanonicalizePermittedCNAMEs
                   CertificateFile
                   ChallengeResponseAuthentication
                   CheckHostIP
                   Cipher
                   Ciphers
                   ClearAllForwardings
                   Compression
                   CompressionLevel
                   ConnectionAttempts
                   ConnectTimeout
                   ControlMaster
                   ControlPath
                   ControlPersist
                   DynamicForward
                   EscapeChar
                   ExitOnForwardFailure
                   FingerprintHash
                   ForwardAgent
                   ForwardX11
                   ForwardX11Timeout
                   ForwardX11Trusted
                   GatewayPorts
                   GlobalKnownHostsFile
                   GSSAPIAuthentication
                   GSSAPIDelegateCredentials
                   HashKnownHosts
                   Host
                   HostbasedAuthentication
                   HostbasedKeyTypes
                   HostKeyAlgorithms
                   HostKeyAlias
                   HostName
                   IdentityFile
                   IdentitiesOnly
                   IPQoS
                   KbdInteractiveAuthentication
                   KbdInteractiveDevices
                   KexAlgorithms
                   LocalCommand
                   LocalForward
                   LogLevel
                   MACs
                   Match
                   NoHostAuthenticationForLocalhost
                   NumberOfPasswordPrompts
                   PasswordAuthentication
                   PermitLocalCommand
                   PKCS11Provider
                   Port
                   PreferredAuthentications
                   Protocol
                   ProxyCommand
                   ProxyUseFdpass
                   PubkeyAcceptedKeyTypes
                   PubkeyAuthentication
                   RekeyLimit
                   RemoteForward
                   RequestTTY
                   RhostsRSAAuthentication
                   RSAAuthentication
                   SendEnv
                   ServerAliveInterval
                   ServerAliveCountMax
                   StreamLocalBindMask
                   StreamLocalBindUnlink
                   StrictHostKeyChecking
                   TCPKeepAlive
                   Tunnel
                   TunnelDevice
                   UpdateHostKeys
                   UsePrivilegedPort
                   User
                   UserKnownHostsFile
                   VerifyHostKeyDNS
                   VisualHostKey
                   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 query_option
             Queries ssh for the algorithms supported for the specified version 2.  The available
             features are: cipher (supported symmetric ciphers), cipher-auth (supported symmetric
             ciphers that support authenticated encryption), mac (supported message integrity
             codes), kex (key exchange algorithms), key (key types), key-cert (certificate key
             types), key-plain (non-certificate key types), and protocol-version (supported SSH
             protocol versions).

     -q      Quiet mode.  Causes most warning and diagnostic messages to be suppressed.

     -R [bind_address:]port:host:hostport
     -R [bind_address:]port:local_socket
     -R remote_socket:host:hostport
     -R remote_socket:local_socket
             Specifies that connections to the given TCP port or Unix socket on the remote
             (server) host are to be forwarded to the given host and port, or Unix socket, on the
             local side.  This works by allocating a socket to listen to either a TCP port or to
             a Unix socket on the remote side.  Whenever a connection is made to this port or
             Unix socket, the connection is forwarded over the secure channel, and a connection
             is made to either host port hostport, or local_socket, 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 brackets.

             By default, TCP listening sockets on the server will be bound to the loopback
             interface only.  This may be overridden 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_config(5)).

             If the port argument is ‘0’, the listen port will be dynamically allocated on the
             server and reported to the client at run time.  When used together with -O forward
             the allocated port will be printed to the standard output.

     -S ctl_path
             Specifies the location of a control socket for connection sharing, or the string
             “none” to disable connection sharing.  Refer to the description of ControlPath and
             ControlMaster in ssh_config(5) for details.

     -s      May be used to request invocation of a subsystem on the remote system.  Subsystems
             facilitate the use of SSH as a secure transport for other applications (e.g.
             sftp(1)).  The subsystem is specified as the remote command.

     -T      Disable pseudo-terminal allocation.

     -t      Force pseudo-terminal 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 host:port
             Requests that standard input and output on the client be forwarded to host on port
             over the secure channel.  Implies -N, -T, ExitOnForwardFailure and
             ClearAllForwardings.

     -w local_tun[:remote_tun]
             Requests tunnel device forwarding with the specified tun(4) devices between the
             client (local_tun) and the server (remote_tun).

             The devices may be specified by numerical ID or the keyword “any”, which uses the
             next available tunnel device.  If remote_tun is not specified, it defaults to “any”.
             See also the Tunnel and TunnelDevice directives in ssh_config(5).  If the Tunnel
             directive is unset, it is set to the default tunnel mode, which is “point-to-point”.

     -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_config(5) for more information.

             (Debian-specific: X11 forwarding is not subjected to X11 SECURITY extension
             restrictions by default, because too many programs currently crash in this mode.
             Set the ForwardX11Trusted option to “no” to restore the upstream behaviour.  This
             may change in future depending on client-side improvements.)

     -x      Disables X11 forwarding.

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

             (Debian-specific: This option does nothing in the default configuration: it is
             equivalent to “ForwardX11Trusted yes”, which is the default as described above.  Set
             the ForwardX11Trusted option to “no” to restore the upstream behaviour.  This may
             change in future depending on client-side improvements.)

     -y      Send log information using the syslog(3) system module.  By default this information
             is sent to stderr.

     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_config(5).

AUTHENTICATION

     The OpenSSH SSH client supports SSH protocols 1 and 2.  The default is to use protocol 2
     only, though this can be changed via the Protocol option in ssh_config(5) or the -1 and -2
     options (see above).  Protocol 1 should not be used and is only offered to support legacy
     devices.  It suffers from a number of cryptographic weaknesses and doesn't support many of
     the advanced features available for protocol 2.

     The methods available for authentication are: GSSAPI-based authentication, host-based
     authentication, public key authentication, challenge-response authentication, and password
     authentication.  Authentication methods are tried in the order specified above, though
     PreferredAuthentications can be used to change the default order.

     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 one of the DSA, ECDSA, Ed25519 or RSA algorithms.  The HISTORY
     section of ssl(8) (on non-OpenBSD systems, see
     http://www.openbsd.org/cgi-bin/man.cgi?query=ssl&sektion=8#HISTORY) contains a brief
     discussion of the DSA and RSA 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-keygen(1).  This stores the private key in
     ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
     ~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa (RSA) and stores the public key in
     ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
     ~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (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.

     A variation on public key authentication is available in the form of certificate
     authentication: instead of a set of public/private keys, signed certificates are used.  This
     has the advantage that a single trusted certification authority can be used in place of many
     public/private keys.  See the CERTIFICATES section of ssh-keygen(1) for more information.

     The most convenient way to use public key or certificate authentication may be with an
     authentication agent.  See ssh-agent(1) and (optionally) the AddKeysToAgent directive in
     ssh_config(5) for more information.

     Challenge-response authentication works as follows: The server sends an arbitrary
     "challenge" text, and prompts for a response.  Examples of challenge-response authentication
     include BSD Authentication (see login.conf(5)) 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 in a non-interactive session or, if no command has been specified, logs into
     the machine and gives the user a normal shell as an interactive session.  All communication
     with the remote command or shell will be automatically encrypted.

     If an interactive session is requested ssh by default will only request a pseudo-terminal
     (pty) for interactive sessions when the client has one.  The flags -T and -t can be used to
     override this behaviour.

     If a pseudo-terminal has been allocated the user may use the escape characters noted below.

     If no pseudo-terminal 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.

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.

     ~B      Send a BREAK to the remote system (only useful if the peer supports it).

     ~C      Open command line.  Currently this allows the addition of port forwardings using the
             -L, -R and -D options (see above).  It also allows the cancellation of existing
             port-forwardings with -KL[bind_address:]port for local, -KR[bind_address:]port for
             remote and -KD[bind_address:]port for dynamic port-forwardings.  !command allows the
             user to execute a local command if the PermitLocalCommand option is enabled in
             ssh_config(5).  Basic help is available, using the -h option.

     ~R      Request rekeying of the connection (only useful if the peer supports it).

     ~V      Decrease the verbosity (LogLevel) when errors are being written to stderr.

     ~v      Increase the verbosity (LogLevel) when errors are being written to stderr.

TCP FORWARDING

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

     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 ssh, 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.

X11 FORWARDING

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

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

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

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

VERIFYING HOST KEYS

     When connecting to a server for the first time, a fingerprint of the server's public key is
     presented to the user (unless the option StrictHostKeyChecking has been disabled).
     Fingerprints can be determined using ssh-keygen(1):

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

     If the fingerprint is already known, it can be matched and the key can be accepted or
     rejected.  If only legacy (MD5) fingerprints for the server are available, the ssh-keygen(1)
     -E option may be used to downgrade the fingerprint algorithm to match.

     Because of the difficulty of comparing host keys just by looking at fingerprint strings,
     there is also support to compare host keys visually, using random art.  By setting the
     VisualHostKey option to “yes”, a small ASCII graphic gets displayed on every login to a
     server, no matter if the session itself is interactive or not.  By learning the pattern a
     known server produces, a user can easily find out that the host key has changed when a
     completely different pattern is displayed.  Because these patterns are not unambiguous
     however, a pattern that looks similar to the pattern remembered only gives a good
     probability that the host key is the same, not guaranteed proof.

     To get a listing of the fingerprints along with their random art for all known hosts, the
     following command line can be used:

           $ ssh-keygen -lv -f ~/.ssh/known_hosts

     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 -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_config(5) for more information.

SSH-BASED VIRTUAL PRIVATE NETWORKS

     ssh contains support for Virtual Private Network (VPN) tunnelling using the tun(4) network
     pseudo-device, allowing two networks to be joined securely.  The sshd_config(5)
     configuration option PermitTunnel controls whether the server supports this, and at what
     level (layer 2 or 3 traffic).

     The following example would connect client network 10.0.50.0/24 with remote network
     10.0.99.0/24 using a point-to-point connection from 10.1.1.1 to 10.1.1.2, provided that the
     SSH server running on the gateway to the remote network, at 192.168.1.15, allows it.

     On the client:

           # ssh -f -w 0:1 192.168.1.15 true
           # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
           # route add 10.0.99.0/24 10.1.1.2

     On the server:

           # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
           # route add 10.0.50.0/24 10.1.1.1

     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
     tun(4) device 1 from user “jane” and on tun device 2 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 tun2" ssh-rsa ... john

     Since an 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).

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_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_config(5).

FILES

     ~/.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/
             This directory is the default location for all user-specific configuration and
             authentication information.  There is no general requirement to keep the entire
             contents of this directory secret, but the recommended permissions are
             read/write/execute for the user, and not accessible by others.

     ~/.ssh/authorized_keys
             Lists the public keys (DSA, ECDSA, Ed25519, RSA) 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_config(5).  Because of the potential for abuse, this file must
             have strict permissions: read/write for the user, and not writable by others.  It
             may be group-writable provided that the group in question contains only the user.

     ~/.ssh/environment
             Contains additional definitions for environment variables; see ENVIRONMENT, above.

     ~/.ssh/identity
     ~/.ssh/id_dsa
     ~/.ssh/id_ecdsa
     ~/.ssh/id_ed25519
     ~/.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_ecdsa.pub
     ~/.ssh/id_ed25519.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_config(5).

     /etc/ssh/ssh_host_key
     /etc/ssh/ssh_host_dsa_key
     /etc/ssh/ssh_host_ecdsa_key
     /etc/ssh/ssh_host_ed25519_key
     /etc/ssh/ssh_host_rsa_key
             These files contain the private parts of the host keys and are used for host-based
             authentication.

     /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.

EXIT STATUS

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

SEE ALSO

     scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-argv0(1), ssh-keygen(1), ssh-keyscan(1),
     tun(4), ssh_config(5), ssh-keysign(8), sshd(8)

STANDARDS

     S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250,
     January 2006.

     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture, RFC 4251, January
     2006.

     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol, RFC 4252, January
     2006.

     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, January
     2006.

     T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC 4254, January
     2006.

     J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell (SSH) Key
     Fingerprints, RFC 4255, January 2006.

     F. Cusack and M. Forssen, Generic Message Exchange Authentication for the Secure Shell
     Protocol (SSH), RFC 4256, January 2006.

     J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break Extension, RFC
     4335, January 2006.

     M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport Layer Encryption
     Modes, RFC 4344, January 2006.

     B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol, RFC
     4345, January 2006.

     M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for the Secure Shell
     (SSH) Transport Layer Protocol, RFC 4419, March 2006.

     J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File Format, RFC 4716,
     November 2006.

     D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the Secure Shell Transport
     Layer, RFC 5656, December 2009.

     A. Perrig and D. Song, Hash Visualization: a New Technique to improve Real-World Security,
     1999, International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99).

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.