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+SSH(1) OpenBSD Reference Manual SSH(1)
+
+NAME
+ ssh - OpenSSH SSH client (remote login program)
+
+SYNOPSIS
+ ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
+ [-D [bind_address:]port] [-e escape_char] [-F configfile] [-I pkcs11]
+ [-i identity_file] [-L [bind_address:]port:host:hostport]
+ [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port]
+ [-R [bind_address:]port:host:hostport] [-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 replace rlogin
+ and rsh, and provide secure encrypted communications between two
+ untrusted hosts over an insecure network. X11 connections and arbitrary
+ TCP ports 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 depending on the protocol version used (see
+ below).
+
+ If command is specified, it is executed on the remote host instead of a
+ login shell.
+
+ The options are as follows:
+
+ -1 Forces ssh to try protocol version 1 only.
+
+ -2 Forces ssh to try protocol version 2 only.
+
+ -4 Forces ssh to use IPv4 addresses only.
+
+ -6 Forces ssh to use IPv6 addresses only.
+
+ -A Enables forwarding of the authentication agent connection. This
+ can also be specified on a per-host basis in a configuration
+ file.
+
+ Agent forwarding should be enabled with caution. Users with the
+ ability to bypass file permissions on the remote host (for the
+ agent's UNIX-domain socket) can access the local agent through
+ the forwarded connection. An attacker cannot obtain key material
+ from the agent, however they can perform operations on the keys
+ that enable them to authenticate using the identities loaded into
+ the agent.
+
+ -a Disables forwarding of the authentication agent connection.
+
+ -b bind_address
+ Use bind_address on the local machine as the source address of
+ the connection. Only useful on systems with more than one
+ address.
+
+ -C Requests compression of all data (including stdin, stdout,
+ stderr, and data for forwarded X11 and TCP connections). The
+ compression algorithm is the same used by gzip(1), and the
+ ``level'' can be controlled by the CompressionLevel option for
+ protocol version 1. Compression is desirable on modem lines and
+ other slow connections, but will only slow down things on fast
+ networks. The default value can be set on a host-by-host basis
+ in the configuration files; see the Compression option.
+
+ -c cipher_spec
+ Selects the cipher specification for encrypting the session.
+
+ Protocol version 1 allows specification of a single cipher. The
+ supported values are ``3des'', ``blowfish'', and ``des''. 3des
+ (triple-des) is an encrypt-decrypt-encrypt triple with three
+ different keys. It is believed to be secure. blowfish is a fast
+ block cipher; it appears very secure and is much faster than
+ 3des. des is only supported in the ssh client for
+ interoperability with legacy protocol 1 implementations that do
+ not support the 3des cipher. Its use is strongly discouraged due
+ to cryptographic weaknesses. The default is ``3des''.
+
+ For protocol version 2, cipher_spec is a comma-separated list of
+ ciphers listed in order of preference. 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 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 Allows remote hosts to connect to local forwarded ports.
+
+ -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 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). 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
+ Specifies that the given port on the local (client) host is to be
+ forwarded to the given host and port on the remote side. This
+ works by allocating a socket to listen to port on the local side,
+ optionally bound to the specified bind_address. Whenever a
+ connection is made to this port, the connection is forwarded over
+ the secure channel, and a connection is made to host port
+ hostport from the remote machine. Port forwardings can also be
+ specified in the configuration file. IPv6 addresses can be
+ specified by enclosing the address in square brackets. Only the
+ superuser can forward privileged ports. By default, the local
+ port is bound in accordance with the GatewayPorts setting.
+ However, an explicit bind_address may be used to bind the
+ connection to a specific address. The bind_address of
+ ``localhost'' indicates that the listening port be bound for
+ local use only, while an empty address or `*' indicates that the
+ port should be available from all interfaces.
+
+ -l login_name
+ Specifies the user to log in as on the remote machine. This also
+ may be specified on a per-host basis in the configuration file.
+
+ -M Places the ssh client into ``master'' mode for connection
+ sharing. Multiple -M options places ssh into ``master'' mode
+ with confirmation required before slave connections are accepted.
+ Refer to the description of ControlMaster in ssh_config(5) for
+ details.
+
+ -m mac_spec
+ Additionally, for protocol version 2 a comma-separated list of
+ MAC (message authentication code) algorithms can be specified in
+ order of preference. See the MACs keyword for more information.
+
+ -N Do not execute a remote command. This is useful for just
+ forwarding ports (protocol version 2 only).
+
+ -n Redirects stdin from /dev/null (actually, prevents reading from
+ stdin). This must be used when ssh is run in the background. A
+ common trick is to use this to run X11 programs on a remote
+ machine. For example, ssh -n shadows.cs.hut.fi emacs & will
+ start an emacs on shadows.cs.hut.fi, and the X11 connection will
+ be automatically forwarded over an encrypted channel. The ssh
+ program will be put in the background. (This does not work if
+ ssh needs to ask for a password or passphrase; see also the -f
+ option.)
+
+ -O ctl_cmd
+ Control an active connection multiplexing master process. When
+ the -O option is specified, the ctl_cmd argument is interpreted
+ and passed to the master process. Valid commands are: ``check''
+ (check that the master process is running), ``forward'' (request
+ forwardings without command execution), ``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).
+
+ AddressFamily
+ BatchMode
+ BindAddress
+ ChallengeResponseAuthentication
+ CheckHostIP
+ Cipher
+ Ciphers
+ ClearAllForwardings
+ Compression
+ CompressionLevel
+ ConnectionAttempts
+ ConnectTimeout
+ ControlMaster
+ ControlPath
+ DynamicForward
+ EscapeChar
+ ExitOnForwardFailure
+ ForwardAgent
+ ForwardX11
+ ForwardX11Trusted
+ GatewayPorts
+ GlobalKnownHostsFile
+ GSSAPIAuthentication
+ GSSAPIDelegateCredentials
+ HashKnownHosts
+ Host
+ HostbasedAuthentication
+ HostKeyAlgorithms
+ HostKeyAlias
+ HostName
+ IdentityFile
+ IdentitiesOnly
+ IPQoS
+ KbdInteractiveDevices
+ KexAlgorithms
+ LocalCommand
+ LocalForward
+ LogLevel
+ MACs
+ NoHostAuthenticationForLocalhost
+ NumberOfPasswordPrompts
+ PasswordAuthentication
+ PermitLocalCommand
+ PKCS11Provider
+ Port
+ PreferredAuthentications
+ Protocol
+ ProxyCommand
+ PubkeyAuthentication
+ RekeyLimit
+ RemoteForward
+ RequestTTY
+ RhostsRSAAuthentication
+ RSAAuthentication
+ SendEnv
+ ServerAliveInterval
+ ServerAliveCountMax
+ StrictHostKeyChecking
+ TCPKeepAlive
+ Tunnel
+ TunnelDevice
+ 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 Quiet mode. Causes most warning and diagnostic messages to be
+ suppressed.
+
+ -R [bind_address:]port:host:hostport
+ Specifies that the given port on the remote (server) host is to
+ be forwarded to the given host and port on the local side. This
+ works by allocating a socket to listen to port on the remote
+ side, and whenever a connection is made to this port, the
+ connection is forwarded over the secure channel, and a connection
+ is made to host port hostport from the local machine.
+
+ Port forwardings can also be specified in the configuration file.
+ Privileged ports can be forwarded only when logging in as root on
+ the remote machine. IPv6 addresses can be specified by enclosing
+ the address in square braces.
+
+ By default, the listening socket 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 are a feature of the SSH2 protocol which
+ facilitate the use of SSH as a secure transport for other
+ applications (eg. sftp(1)). The subsystem is specified as the
+ remote command.
+
+ -T Disable pseudo-tty allocation.
+
+ -t Force pseudo-tty allocation. This can be used to execute
+ arbitrary screen-based programs on a remote machine, which can be
+ very useful, e.g. when implementing menu services. Multiple -t
+ options force tty allocation, even if ssh has no local tty.
+
+ -V Display the version number and exit.
+
+ -v Verbose mode. Causes ssh to print debugging messages about its
+ progress. This is helpful in debugging connection,
+ authentication, and configuration problems. Multiple -v options
+ increase the verbosity. The maximum is 3.
+
+ -W 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 and works with
+ Protocol version 2 only.
+
+ -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.
+
+ -x Disables X11 forwarding.
+
+ -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
+ subjected to the X11 SECURITY extension controls.
+
+ -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). Both protocols
+ support similar authentication methods, but protocol 2 is the default
+ since it provides additional mechanisms for confidentiality (the traffic
+ is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
+ integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64,
+ hmac-ripemd160). Protocol 1 lacks a strong mechanism for ensuring the
+ integrity of the connection.
+
+ 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
+ protocol 2 has a configuration option to change the default order:
+ PreferredAuthentications.
+
+ Host-based authentication works as follows: If the machine the user logs
+ in from is listed in /etc/hosts.equiv or /etc/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 or RSA algorithms. Protocol 1 is restricted to
+ using only RSA keys, but protocol 2 may use any. The HISTORY section of
+ ssl(8) 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 (protocol
+ 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), or ~/.ssh/id_rsa (protocol 2
+ RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1),
+ ~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (protocol 2
+ ECDSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home
+ directory. The user should then copy the public key to
+ ~/.ssh/authorized_keys in his/her home directory on the remote machine.
+ The authorized_keys file corresponds to the conventional ~/.rhosts file,
+ and has one key per line, though the lines can be very long. After this,
+ the user can log in without giving the password.
+
+ 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) for more
+ information.
+
+ Challenge-response authentication works as follows: The server sends an
+ arbitrary "challenge" text, and prompts for a response. Protocol 2
+ allows multiple challenges and responses; protocol 1 is restricted to
+ just one challenge/response. Examples of challenge-response
+ authentication include BSD Authentication (see login.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, or logs into the machine and gives the
+ user a normal shell on the remote machine. All communication with the
+ remote command or shell will be automatically encrypted.
+
+ If a pseudo-terminal has been allocated (normal login session), the user
+ may use the escape characters noted below.
+
+ If no pseudo-tty has been allocated, the session is transparent and can
+ be used to reliably transfer binary data. On most systems, setting the
+ escape character to ``none'' will also make the session transparent even
+ if a tty is used.
+
+ The session terminates when the command or shell on the remote machine
+ exits and all X11 and TCP connections have been closed.
+
+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 for SSH protocol
+ version 2 and if the peer supports it).
+
+ ~C Open command line. Currently this allows the addition of port
+ forwardings using the -L, -R and -D options (see above). It also
+ allows the cancellation of existing remote port-forwardings using
+ -KR[bind_address:]port. !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 for SSH protocol
+ version 2 and if the peer supports it).
+
+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. Because of the difficulty of comparing host keys
+ just by looking at hex 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/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 accessible by others.
+
+ ~/.ssh/environment
+ Contains additional definitions for environment variables; see
+ ENVIRONMENT, above.
+
+ ~/.ssh/identity
+ ~/.ssh/id_dsa
+ ~/.ssh/id_ecdsa
+ ~/.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_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/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_rsa_key
+ These three files contain the private parts of the host keys and
+ are used for host-based authentication. If protocol version 1 is
+ used, ssh must be setuid root, since the host key is readable
+ only by root. For protocol version 2, ssh uses ssh-keysign(8) to
+ access the host keys, eliminating the requirement that ssh be
+ setuid root when host-based authentication is used. By default
+ ssh is not setuid root.
+
+ /etc/ssh/ssh_known_hosts
+ Systemwide list of known host keys. This file should be prepared
+ by the system administrator to contain the public host keys of
+ all machines in the organization. It should be world-readable.
+ See sshd(8) for further details of the format of this file.
+
+ /etc/ssh/sshrc
+ Commands in this file are executed by ssh when the user logs in,
+ just before the user's shell (or command) is started. See the
+ sshd(8) manual page for more information.
+
+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-keygen(1), ssh-keyscan(1),
+ tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8)
+
+ The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
+
+ The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
+
+ The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
+
+ The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
+
+ The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
+
+ Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
+ 4255, 2006.
+
+ Generic Message Exchange Authentication for the Secure Shell Protocol
+ (SSH), RFC 4256, 2006.
+
+ The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
+
+ The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
+
+ Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer
+ Protocol, RFC 4345, 2006.
+
+ Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
+ Protocol, RFC 4419, 2006.
+
+ The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
+
+ Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer,
+ RFC 5656, 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.
+
+OpenBSD 5.0 August 2, 2011 OpenBSD 5.0