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Annotation of src/usr.bin/ssh/ssh.1, Revision 1.322

1.1       deraadt     1: .\"
                      2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
                      3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
                      4: .\"                    All rights reserved
                      5: .\"
1.59      deraadt     6: .\" As far as I am concerned, the code I have written for this software
                      7: .\" can be used freely for any purpose.  Any derived versions of this
                      8: .\" software must be clearly marked as such, and if the derived work is
                      9: .\" incompatible with the protocol description in the RFC file, it must be
                     10: .\" called by a name other than "ssh" or "Secure Shell".
                     11: .\"
1.93      deraadt    12: .\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
                     13: .\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
                     14: .\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
1.59      deraadt    15: .\"
                     16: .\" Redistribution and use in source and binary forms, with or without
                     17: .\" modification, are permitted provided that the following conditions
                     18: .\" are met:
                     19: .\" 1. Redistributions of source code must retain the above copyright
                     20: .\"    notice, this list of conditions and the following disclaimer.
                     21: .\" 2. Redistributions in binary form must reproduce the above copyright
                     22: .\"    notice, this list of conditions and the following disclaimer in the
                     23: .\"    documentation and/or other materials provided with the distribution.
1.1       deraadt    24: .\"
1.59      deraadt    25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
                     26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
                     27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
                     28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
                     29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
                     30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
                     31: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
                     32: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
                     33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
                     34: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1       deraadt    35: .\"
1.322   ! markus     36: .\" $OpenBSD: ssh.1,v 1.321 2011/08/26 01:45:15 djm Exp $
        !            37: .Dd $Mdocdate: August 26 2011 $
1.2       deraadt    38: .Dt SSH 1
                     39: .Os
                     40: .Sh NAME
                     41: .Nm ssh
1.96      deraadt    42: .Nd OpenSSH SSH client (remote login program)
1.2       deraadt    43: .Sh SYNOPSIS
                     44: .Nm ssh
1.306     jmc        45: .Bk -words
1.278     djm        46: .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
1.108     markus     47: .Op Fl b Ar bind_address
1.51      markus     48: .Op Fl c Ar cipher_spec
1.306     jmc        49: .Op Fl D Oo Ar bind_address : Oc Ns Ar port
1.2       deraadt    50: .Op Fl e Ar escape_char
1.176     jmc        51: .Op Fl F Ar configfile
1.292     jmc        52: .Op Fl I Ar pkcs11
1.2       deraadt    53: .Op Fl i Ar identity_file
1.306     jmc        54: .Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
1.176     jmc        55: .Op Fl l Ar login_name
                     56: .Op Fl m Ar mac_spec
1.198     djm        57: .Op Fl O Ar ctl_cmd
1.176     jmc        58: .Op Fl o Ar option
                     59: .Op Fl p Ar port
1.306     jmc        60: .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
1.198     djm        61: .Op Fl S Ar ctl_path
1.290     dtucker    62: .Op Fl W Ar host : Ns Ar port
1.306     jmc        63: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.176     jmc        64: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2       deraadt    65: .Op Ar command
1.306     jmc        66: .Ek
1.44      aaron      67: .Sh DESCRIPTION
1.2       deraadt    68: .Nm
1.96      deraadt    69: (SSH client) is a program for logging into a remote machine and for
1.40      aaron      70: executing commands on a remote machine.
1.176     jmc        71: It is intended to replace rlogin and rsh,
                     72: and provide secure encrypted communications between
1.40      aaron      73: two untrusted hosts over an insecure network.
1.247     jmc        74: X11 connections and arbitrary TCP ports
1.176     jmc        75: can also be forwarded over the secure channel.
1.2       deraadt    76: .Pp
                     77: .Nm
1.44      aaron      78: connects and logs into the specified
1.176     jmc        79: .Ar hostname
                     80: (with optional
                     81: .Ar user
                     82: name).
1.1       deraadt    83: The user must prove
1.49      markus     84: his/her identity to the remote machine using one of several methods
1.221     jmc        85: depending on the protocol version used (see below).
1.49      markus     86: .Pp
1.176     jmc        87: If
                     88: .Ar command
                     89: is specified,
1.219     jmc        90: it is executed on the remote host instead of a login shell.
1.2       deraadt    91: .Pp
1.218     jmc        92: The options are as follows:
                     93: .Bl -tag -width Ds
                     94: .It Fl 1
                     95: Forces
1.2       deraadt    96: .Nm
1.218     jmc        97: to try protocol version 1 only.
                     98: .It Fl 2
                     99: Forces
1.2       deraadt   100: .Nm
1.218     jmc       101: to try protocol version 2 only.
                    102: .It Fl 4
                    103: Forces
1.2       deraadt   104: .Nm
1.218     jmc       105: to use IPv4 addresses only.
                    106: .It Fl 6
                    107: Forces
1.2       deraadt   108: .Nm
1.218     jmc       109: to use IPv6 addresses only.
                    110: .It Fl A
                    111: Enables forwarding of the authentication agent connection.
                    112: This can also be specified on a per-host basis in a configuration file.
1.2       deraadt   113: .Pp
1.218     jmc       114: Agent forwarding should be enabled with caution.
                    115: Users with the ability to bypass file permissions on the remote host
1.285     sobrado   116: (for the agent's
1.286     sobrado   117: .Ux Ns -domain
                    118: socket) can access the local agent through the forwarded connection.
1.218     jmc       119: An attacker cannot obtain key material from the agent,
                    120: however they can perform operations on the keys that enable them to
                    121: authenticate using the identities loaded into the agent.
                    122: .It Fl a
                    123: Disables forwarding of the authentication agent connection.
                    124: .It Fl b Ar bind_address
                    125: Use
                    126: .Ar bind_address
                    127: on the local machine as the source address
                    128: of the connection.
                    129: Only useful on systems with more than one address.
                    130: .It Fl C
                    131: Requests compression of all data (including stdin, stdout, stderr, and
1.247     jmc       132: data for forwarded X11 and TCP connections).
1.218     jmc       133: The compression algorithm is the same used by
                    134: .Xr gzip 1 ,
                    135: and the
                    136: .Dq level
                    137: can be controlled by the
                    138: .Cm CompressionLevel
                    139: option for protocol version 1.
                    140: Compression is desirable on modem lines and other
                    141: slow connections, but will only slow down things on fast networks.
                    142: The default value can be set on a host-by-host basis in the
                    143: configuration files; see the
                    144: .Cm Compression
                    145: option.
                    146: .It Fl c Ar cipher_spec
                    147: Selects the cipher specification for encrypting the session.
1.2       deraadt   148: .Pp
1.218     jmc       149: Protocol version 1 allows specification of a single cipher.
                    150: The supported values are
                    151: .Dq 3des ,
1.220     jmc       152: .Dq blowfish ,
1.218     jmc       153: and
                    154: .Dq des .
                    155: .Ar 3des
                    156: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
                    157: It is believed to be secure.
                    158: .Ar blowfish
                    159: is a fast block cipher; it appears very secure and is much faster than
                    160: .Ar 3des .
                    161: .Ar des
                    162: is only supported in the
1.2       deraadt   163: .Nm
1.218     jmc       164: client for interoperability with legacy protocol 1 implementations
                    165: that do not support the
                    166: .Ar 3des
                    167: cipher.
                    168: Its use is strongly discouraged due to cryptographic weaknesses.
                    169: The default is
                    170: .Dq 3des .
1.49      markus    171: .Pp
1.230     jmc       172: For protocol version 2,
1.218     jmc       173: .Ar cipher_spec
                    174: is a comma-separated list of ciphers
                    175: listed in order of preference.
1.283     jmc       176: See the
                    177: .Cm Ciphers
1.307     dtucker   178: keyword in
                    179: .Xr ssh_config 5
                    180: for more information.
1.218     jmc       181: .It Fl D Xo
                    182: .Sm off
                    183: .Oo Ar bind_address : Oc
                    184: .Ar port
                    185: .Sm on
                    186: .Xc
                    187: Specifies a local
                    188: .Dq dynamic
                    189: application-level port forwarding.
                    190: This works by allocating a socket to listen to
                    191: .Ar port
                    192: on the local side, optionally bound to the specified
                    193: .Ar bind_address .
                    194: Whenever a connection is made to this port, the
                    195: connection is forwarded over the secure channel, and the application
                    196: protocol is then used to determine where to connect to from the
                    197: remote machine.
                    198: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107     markus    199: .Nm
1.218     jmc       200: will act as a SOCKS server.
                    201: Only root can forward privileged ports.
                    202: Dynamic port forwardings can also be specified in the configuration file.
1.49      markus    203: .Pp
1.308     djm       204: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218     jmc       205: Only the superuser can forward privileged ports.
                    206: By default, the local port is bound in accordance with the
                    207: .Cm GatewayPorts
                    208: setting.
                    209: However, an explicit
                    210: .Ar bind_address
                    211: may be used to bind the connection to a specific address.
                    212: The
                    213: .Ar bind_address
                    214: of
                    215: .Dq localhost
                    216: indicates that the listening port be bound for local use only, while an
                    217: empty address or
                    218: .Sq *
                    219: indicates that the port should be available from all interfaces.
1.229     jmc       220: .It Fl e Ar escape_char
1.218     jmc       221: Sets the escape character for sessions with a pty (default:
                    222: .Ql ~ ) .
                    223: The escape character is only recognized at the beginning of a line.
                    224: The escape character followed by a dot
                    225: .Pq Ql \&.
                    226: closes the connection;
                    227: followed by control-Z suspends the connection;
                    228: and followed by itself sends the escape character once.
                    229: Setting the character to
1.2       deraadt   230: .Dq none
1.218     jmc       231: disables any escapes and makes the session fully transparent.
                    232: .It Fl F Ar configfile
                    233: Specifies an alternative per-user configuration file.
                    234: If a configuration file is given on the command line,
                    235: the system-wide configuration file
                    236: .Pq Pa /etc/ssh/ssh_config
                    237: will be ignored.
                    238: The default for the per-user configuration file is
                    239: .Pa ~/.ssh/config .
                    240: .It Fl f
                    241: Requests
                    242: .Nm
                    243: to go to background just before command execution.
                    244: This is useful if
1.176     jmc       245: .Nm
1.218     jmc       246: is going to ask for passwords or passphrases, but the user
                    247: wants it in the background.
                    248: This implies
                    249: .Fl n .
                    250: The recommended way to start X11 programs at a remote site is with
                    251: something like
                    252: .Ic ssh -f host xterm .
1.277     djm       253: .Pp
                    254: If the
                    255: .Cm ExitOnForwardFailure
                    256: configuration option is set to
                    257: .Dq yes ,
                    258: then a client started with
                    259: .Fl f
                    260: will wait for all remote port forwards to be successfully established
                    261: before placing itself in the background.
1.218     jmc       262: .It Fl g
                    263: Allows remote hosts to connect to local forwarded ports.
1.291     markus    264: .It Fl I Ar pkcs11
1.294     jmc       265: Specify the PKCS#11 shared library
1.176     jmc       266: .Nm
1.293     markus    267: should use to communicate with a PKCS#11 token providing the user's
1.218     jmc       268: private RSA key.
                    269: .It Fl i Ar identity_file
                    270: Selects a file from which the identity (private key) for
1.310     djm       271: public key authentication is read.
1.218     jmc       272: The default is
                    273: .Pa ~/.ssh/identity
                    274: for protocol version 1, and
1.310     djm       275: .Pa ~/.ssh/id_dsa ,
                    276: .Pa ~/.ssh/id_ecdsa
                    277: and
1.218     jmc       278: .Pa ~/.ssh/id_rsa
                    279: for protocol version 2.
                    280: Identity files may also be specified on
                    281: a per-host basis in the configuration file.
                    282: It is possible to have multiple
                    283: .Fl i
                    284: options (and multiple identities specified in
                    285: configuration files).
1.302     djm       286: .Nm
                    287: will also try to load certificate information from the filename obtained
                    288: by appending
                    289: .Pa -cert.pub
                    290: to identity filenames.
1.269     djm       291: .It Fl K
                    292: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
                    293: credentials to the server.
1.218     jmc       294: .It Fl k
                    295: Disables forwarding (delegation) of GSSAPI credentials to the server.
                    296: .It Fl L Xo
                    297: .Sm off
                    298: .Oo Ar bind_address : Oc
                    299: .Ar port : host : hostport
                    300: .Sm on
                    301: .Xc
                    302: Specifies that the given port on the local (client) host is to be
                    303: forwarded to the given host and port on the remote side.
                    304: This works by allocating a socket to listen to
                    305: .Ar port
                    306: on the local side, optionally bound to the specified
                    307: .Ar bind_address .
                    308: Whenever a connection is made to this port, the
                    309: connection is forwarded over the secure channel, and a connection is
                    310: made to
                    311: .Ar host
                    312: port
                    313: .Ar hostport
                    314: from the remote machine.
                    315: Port forwardings can also be specified in the configuration file.
1.308     djm       316: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218     jmc       317: Only the superuser can forward privileged ports.
                    318: By default, the local port is bound in accordance with the
                    319: .Cm GatewayPorts
                    320: setting.
                    321: However, an explicit
                    322: .Ar bind_address
                    323: may be used to bind the connection to a specific address.
1.2       deraadt   324: The
1.218     jmc       325: .Ar bind_address
                    326: of
                    327: .Dq localhost
                    328: indicates that the listening port be bound for local use only, while an
                    329: empty address or
                    330: .Sq *
                    331: indicates that the port should be available from all interfaces.
                    332: .It Fl l Ar login_name
                    333: Specifies the user to log in as on the remote machine.
                    334: This also may be specified on a per-host basis in the configuration file.
                    335: .It Fl M
                    336: Places the
                    337: .Nm
                    338: client into
                    339: .Dq master
                    340: mode for connection sharing.
1.231     stevesk   341: Multiple
                    342: .Fl M
                    343: options places
                    344: .Nm
                    345: into
                    346: .Dq master
                    347: mode with confirmation required before slave connections are accepted.
1.218     jmc       348: Refer to the description of
                    349: .Cm ControlMaster
                    350: in
                    351: .Xr ssh_config 5
                    352: for details.
                    353: .It Fl m Ar mac_spec
                    354: Additionally, for protocol version 2 a comma-separated list of MAC
                    355: (message authentication code) algorithms can
                    356: be specified in order of preference.
                    357: See the
                    358: .Cm MACs
                    359: keyword for more information.
                    360: .It Fl N
                    361: Do not execute a remote command.
                    362: This is useful for just forwarding ports
                    363: (protocol version 2 only).
                    364: .It Fl n
                    365: Redirects stdin from
                    366: .Pa /dev/null
                    367: (actually, prevents reading from stdin).
                    368: This must be used when
1.2       deraadt   369: .Nm
1.218     jmc       370: is run in the background.
                    371: A common trick is to use this to run X11 programs on a remote machine.
                    372: For example,
                    373: .Ic ssh -n shadows.cs.hut.fi emacs &
                    374: will start an emacs on shadows.cs.hut.fi, and the X11
                    375: connection will be automatically forwarded over an encrypted channel.
                    376: The
1.2       deraadt   377: .Nm
1.218     jmc       378: program will be put in the background.
                    379: (This does not work if
1.2       deraadt   380: .Nm
1.218     jmc       381: needs to ask for a password or passphrase; see also the
                    382: .Fl f
                    383: option.)
                    384: .It Fl O Ar ctl_cmd
                    385: Control an active connection multiplexing master process.
                    386: When the
                    387: .Fl O
                    388: option is specified, the
                    389: .Ar ctl_cmd
                    390: argument is interpreted and passed to the master process.
                    391: Valid commands are:
                    392: .Dq check
1.305     markus    393: (check that the master process is running),
                    394: .Dq forward
1.318     jmc       395: (request forwardings without command execution),
1.218     jmc       396: .Dq exit
1.318     jmc       397: (request the master to exit), and
1.317     djm       398: .Dq stop
                    399: (request the master to stop accepting further multiplexing requests).
1.218     jmc       400: .It Fl o Ar option
                    401: Can be used to give options in the format used in the configuration file.
                    402: This is useful for specifying options for which there is no separate
                    403: command-line flag.
                    404: For full details of the options listed below, and their possible values, see
                    405: .Xr ssh_config 5 .
1.2       deraadt   406: .Pp
1.218     jmc       407: .Bl -tag -width Ds -offset indent -compact
                    408: .It AddressFamily
                    409: .It BatchMode
                    410: .It BindAddress
                    411: .It ChallengeResponseAuthentication
                    412: .It CheckHostIP
                    413: .It Cipher
                    414: .It Ciphers
                    415: .It ClearAllForwardings
                    416: .It Compression
                    417: .It CompressionLevel
                    418: .It ConnectionAttempts
                    419: .It ConnectTimeout
                    420: .It ControlMaster
                    421: .It ControlPath
1.321     djm       422: .It ControlPersist
1.218     jmc       423: .It DynamicForward
                    424: .It EscapeChar
1.263     markus    425: .It ExitOnForwardFailure
1.218     jmc       426: .It ForwardAgent
                    427: .It ForwardX11
1.321     djm       428: .It ForwardX11Timeout
1.218     jmc       429: .It ForwardX11Trusted
                    430: .It GatewayPorts
                    431: .It GlobalKnownHostsFile
                    432: .It GSSAPIAuthentication
                    433: .It GSSAPIDelegateCredentials
                    434: .It HashKnownHosts
                    435: .It Host
                    436: .It HostbasedAuthentication
                    437: .It HostKeyAlgorithms
                    438: .It HostKeyAlias
                    439: .It HostName
                    440: .It IdentityFile
                    441: .It IdentitiesOnly
1.316     jmc       442: .It IPQoS
1.321     djm       443: .It KbdInteractiveAuthentication
1.218     jmc       444: .It KbdInteractiveDevices
1.314     jmc       445: .It KexAlgorithms
1.218     jmc       446: .It LocalCommand
                    447: .It LocalForward
                    448: .It LogLevel
                    449: .It MACs
                    450: .It NoHostAuthenticationForLocalhost
                    451: .It NumberOfPasswordPrompts
                    452: .It PasswordAuthentication
                    453: .It PermitLocalCommand
1.291     markus    454: .It PKCS11Provider
1.218     jmc       455: .It Port
                    456: .It PreferredAuthentications
                    457: .It Protocol
                    458: .It ProxyCommand
                    459: .It PubkeyAuthentication
1.251     dtucker   460: .It RekeyLimit
1.218     jmc       461: .It RemoteForward
1.319     jmc       462: .It RequestTTY
1.218     jmc       463: .It RhostsRSAAuthentication
                    464: .It RSAAuthentication
                    465: .It SendEnv
                    466: .It ServerAliveInterval
                    467: .It ServerAliveCountMax
                    468: .It StrictHostKeyChecking
                    469: .It TCPKeepAlive
                    470: .It Tunnel
                    471: .It TunnelDevice
                    472: .It UsePrivilegedPort
                    473: .It User
                    474: .It UserKnownHostsFile
                    475: .It VerifyHostKeyDNS
1.276     jmc       476: .It VisualHostKey
1.218     jmc       477: .It XAuthLocation
                    478: .El
                    479: .It Fl p Ar port
                    480: Port to connect to on the remote host.
                    481: This can be specified on a
                    482: per-host basis in the configuration file.
                    483: .It Fl q
                    484: Quiet mode.
1.271     djm       485: Causes most warning and diagnostic messages to be suppressed.
1.218     jmc       486: .It Fl R Xo
                    487: .Sm off
                    488: .Oo Ar bind_address : Oc
                    489: .Ar port : host : hostport
                    490: .Sm on
                    491: .Xc
                    492: Specifies that the given port on the remote (server) host is to be
                    493: forwarded to the given host and port on the local side.
                    494: This works by allocating a socket to listen to
                    495: .Ar port
                    496: on the remote side, and whenever a connection is made to this port, the
                    497: connection is forwarded over the secure channel, and a connection is
                    498: made to
                    499: .Ar host
                    500: port
                    501: .Ar hostport
                    502: from the local machine.
1.2       deraadt   503: .Pp
1.218     jmc       504: Port forwardings can also be specified in the configuration file.
                    505: Privileged ports can be forwarded only when
                    506: logging in as root on the remote machine.
1.308     djm       507: IPv6 addresses can be specified by enclosing the address in square braces.
1.194     jakob     508: .Pp
1.218     jmc       509: By default, the listening socket on the server will be bound to the loopback
                    510: interface only.
1.280     tobias    511: This may be overridden by specifying a
1.218     jmc       512: .Ar bind_address .
                    513: An empty
                    514: .Ar bind_address ,
                    515: or the address
                    516: .Ql * ,
                    517: indicates that the remote socket should listen on all interfaces.
                    518: Specifying a remote
                    519: .Ar bind_address
                    520: will only succeed if the server's
                    521: .Cm GatewayPorts
                    522: option is enabled (see
                    523: .Xr sshd_config 5 ) .
1.281     djm       524: .Pp
                    525: If the
                    526: .Ar port
                    527: argument is
1.282     djm       528: .Ql 0 ,
1.281     djm       529: the listen port will be dynamically allocated on the server and reported
                    530: to the client at run time.
1.305     markus    531: When used together with
                    532: .Ic -O forward
                    533: the allocated port will be printed to the standard output.
1.218     jmc       534: .It Fl S Ar ctl_path
1.304     jmc       535: Specifies the location of a control socket for connection sharing,
1.303     djm       536: or the string
                    537: .Dq none
                    538: to disable connection sharing.
1.218     jmc       539: Refer to the description of
                    540: .Cm ControlPath
                    541: and
                    542: .Cm ControlMaster
                    543: in
                    544: .Xr ssh_config 5
                    545: for details.
                    546: .It Fl s
                    547: May be used to request invocation of a subsystem on the remote system.
                    548: Subsystems are a feature of the SSH2 protocol which facilitate the use
                    549: of SSH as a secure transport for other applications (eg.\&
                    550: .Xr sftp 1 ) .
                    551: The subsystem is specified as the remote command.
                    552: .It Fl T
                    553: Disable pseudo-tty allocation.
                    554: .It Fl t
                    555: Force pseudo-tty allocation.
                    556: This can be used to execute arbitrary
                    557: screen-based programs on a remote machine, which can be very useful,
1.257     jmc       558: e.g. when implementing menu services.
1.218     jmc       559: Multiple
                    560: .Fl t
                    561: options force tty allocation, even if
1.194     jakob     562: .Nm
1.218     jmc       563: has no local tty.
                    564: .It Fl V
                    565: Display the version number and exit.
                    566: .It Fl v
                    567: Verbose mode.
                    568: Causes
1.176     jmc       569: .Nm
1.218     jmc       570: to print debugging messages about its progress.
                    571: This is helpful in
                    572: debugging connection, authentication, and configuration problems.
                    573: Multiple
                    574: .Fl v
                    575: options increase the verbosity.
                    576: The maximum is 3.
1.290     dtucker   577: .It Fl W Ar host : Ns Ar port
                    578: Requests that standard input and output on the client be forwarded to
                    579: .Ar host
                    580: on
                    581: .Ar port
                    582: over the secure channel.
                    583: Implies
                    584: .Fl N ,
                    585: .Fl T ,
                    586: .Cm ExitOnForwardFailure
                    587: and
                    588: .Cm ClearAllForwardings
                    589: and works with Protocol version 2 only.
1.261     stevesk   590: .It Fl w Xo
                    591: .Ar local_tun Ns Op : Ns Ar remote_tun
                    592: .Xc
                    593: Requests
                    594: tunnel
                    595: device forwarding with the specified
1.218     jmc       596: .Xr tun 4
1.261     stevesk   597: devices between the client
                    598: .Pq Ar local_tun
                    599: and the server
                    600: .Pq Ar remote_tun .
                    601: .Pp
1.228     jmc       602: The devices may be specified by numerical ID or the keyword
                    603: .Dq any ,
                    604: which uses the next available tunnel device.
1.261     stevesk   605: If
                    606: .Ar remote_tun
                    607: is not specified, it defaults to
                    608: .Dq any .
1.228     jmc       609: See also the
1.218     jmc       610: .Cm Tunnel
1.261     stevesk   611: and
                    612: .Cm TunnelDevice
                    613: directives in
1.218     jmc       614: .Xr ssh_config 5 .
1.261     stevesk   615: If the
                    616: .Cm Tunnel
                    617: directive is unset, it is set to the default tunnel mode, which is
                    618: .Dq point-to-point .
1.218     jmc       619: .It Fl X
                    620: Enables X11 forwarding.
1.54      markus    621: This can also be specified on a per-host basis in a configuration file.
1.165     stevesk   622: .Pp
1.218     jmc       623: X11 forwarding should be enabled with caution.
1.168     jmc       624: Users with the ability to bypass file permissions on the remote host
1.218     jmc       625: (for the user's X authorization database)
                    626: can access the local X11 display through the forwarded connection.
                    627: An attacker may then be able to perform activities such as keystroke monitoring.
                    628: .Pp
                    629: For this reason, X11 forwarding is subjected to X11 SECURITY extension
                    630: restrictions by default.
                    631: Please refer to the
                    632: .Nm
                    633: .Fl Y
                    634: option and the
                    635: .Cm ForwardX11Trusted
                    636: directive in
                    637: .Xr ssh_config 5
                    638: for more information.
                    639: .It Fl x
                    640: Disables X11 forwarding.
                    641: .It Fl Y
                    642: Enables trusted X11 forwarding.
                    643: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
                    644: controls.
1.278     djm       645: .It Fl y
                    646: Send log information using the
                    647: .Xr syslog 3
                    648: system module.
                    649: By default this information is sent to stderr.
1.218     jmc       650: .El
1.224     jmc       651: .Pp
                    652: .Nm
                    653: may additionally obtain configuration data from
                    654: a per-user configuration file and a system-wide configuration file.
                    655: The file format and configuration options are described in
                    656: .Xr ssh_config 5 .
1.222     jmc       657: .Sh AUTHENTICATION
1.249     jmc       658: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284     jmc       659: The default is to use protocol 2 only,
                    660: though this can be changed via the
1.222     jmc       661: .Cm Protocol
                    662: option in
1.284     jmc       663: .Xr ssh_config 5
                    664: or the
1.222     jmc       665: .Fl 1
                    666: and
                    667: .Fl 2
                    668: options (see above).
                    669: Both protocols support similar authentication methods,
1.284     jmc       670: but protocol 2 is the default since
1.222     jmc       671: it provides additional mechanisms for confidentiality
                    672: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320     djm       673: and integrity (hmac-md5, hmac-sha1,
                    674: hmac-sha2-256, hmac-sha2-512,
                    675: umac-64, hmac-ripemd160).
1.222     jmc       676: Protocol 1 lacks a strong mechanism for ensuring the
                    677: integrity of the connection.
                    678: .Pp
                    679: The methods available for authentication are:
1.260     jmc       680: GSSAPI-based authentication,
1.222     jmc       681: host-based authentication,
                    682: public key authentication,
                    683: challenge-response authentication,
                    684: and password authentication.
                    685: Authentication methods are tried in the order specified above,
                    686: though protocol 2 has a configuration option to change the default order:
                    687: .Cm PreferredAuthentications .
                    688: .Pp
                    689: Host-based authentication works as follows:
1.218     jmc       690: If the machine the user logs in from is listed in
                    691: .Pa /etc/hosts.equiv
                    692: or
                    693: .Pa /etc/shosts.equiv
                    694: on the remote machine, and the user names are
                    695: the same on both sides, or if the files
                    696: .Pa ~/.rhosts
                    697: or
                    698: .Pa ~/.shosts
                    699: exist in the user's home directory on the
                    700: remote machine and contain a line containing the name of the client
                    701: machine and the name of the user on that machine, the user is
1.222     jmc       702: considered for login.
                    703: Additionally, the server
                    704: .Em must
                    705: be able to verify the client's
                    706: host key (see the description of
1.218     jmc       707: .Pa /etc/ssh/ssh_known_hosts
1.189     dtucker   708: and
1.222     jmc       709: .Pa ~/.ssh/known_hosts ,
                    710: below)
                    711: for login to be permitted.
1.218     jmc       712: This authentication method closes security holes due to IP
1.222     jmc       713: spoofing, DNS spoofing, and routing spoofing.
1.218     jmc       714: [Note to the administrator:
                    715: .Pa /etc/hosts.equiv ,
                    716: .Pa ~/.rhosts ,
                    717: and the rlogin/rsh protocol in general, are inherently insecure and should be
                    718: disabled if security is desired.]
1.189     dtucker   719: .Pp
1.222     jmc       720: Public key authentication works as follows:
                    721: The scheme is based on public-key cryptography,
                    722: using cryptosystems
                    723: where encryption and decryption are done using separate keys,
                    724: and it is unfeasible to derive the decryption key from the encryption key.
1.218     jmc       725: The idea is that each user creates a public/private
                    726: key pair for authentication purposes.
                    727: The server knows the public key, and only the user knows the private key.
1.222     jmc       728: .Nm
                    729: implements public key authentication protocol automatically,
1.310     djm       730: using one of the DSA, ECDSA or RSA algorithms.
1.222     jmc       731: Protocol 1 is restricted to using only RSA keys,
1.310     djm       732: but protocol 2 may use any.
1.222     jmc       733: The
                    734: .Sx HISTORY
                    735: section of
                    736: .Xr ssl 8
1.311     jmc       737: contains a brief discussion of the DSA and RSA algorithms.
1.210     djm       738: .Pp
1.218     jmc       739: The file
                    740: .Pa ~/.ssh/authorized_keys
                    741: lists the public keys that are permitted for logging in.
                    742: When the user logs in, the
1.2       deraadt   743: .Nm
1.218     jmc       744: program tells the server which key pair it would like to use for
                    745: authentication.
1.222     jmc       746: The client proves that it has access to the private key
                    747: and the server checks that the corresponding public key
                    748: is authorized to accept the account.
1.218     jmc       749: .Pp
1.222     jmc       750: The user creates his/her key pair by running
1.218     jmc       751: .Xr ssh-keygen 1 .
                    752: This stores the private key in
1.207     djm       753: .Pa ~/.ssh/identity
1.222     jmc       754: (protocol 1),
                    755: .Pa ~/.ssh/id_dsa
                    756: (protocol 2 DSA),
1.310     djm       757: .Pa ~/.ssh/id_ecdsa
                    758: (protocol 2 ECDSA),
1.222     jmc       759: or
                    760: .Pa ~/.ssh/id_rsa
                    761: (protocol 2 RSA)
1.218     jmc       762: and stores the public key in
                    763: .Pa ~/.ssh/identity.pub
1.222     jmc       764: (protocol 1),
                    765: .Pa ~/.ssh/id_dsa.pub
                    766: (protocol 2 DSA),
1.310     djm       767: .Pa ~/.ssh/id_ecdsa.pub
                    768: (protocol 2 ECDSA),
1.222     jmc       769: or
                    770: .Pa ~/.ssh/id_rsa.pub
                    771: (protocol 2 RSA)
1.218     jmc       772: in the user's home directory.
1.222     jmc       773: The user should then copy the public key
1.218     jmc       774: to
                    775: .Pa ~/.ssh/authorized_keys
1.222     jmc       776: in his/her home directory on the remote machine.
                    777: The
1.218     jmc       778: .Pa authorized_keys
                    779: file corresponds to the conventional
                    780: .Pa ~/.rhosts
                    781: file, and has one key
1.222     jmc       782: per line, though the lines can be very long.
1.218     jmc       783: After this, the user can log in without giving the password.
                    784: .Pp
1.301     jmc       785: A variation on public key authentication
                    786: is available in the form of certificate authentication:
                    787: instead of a set of public/private keys,
                    788: signed certificates are used.
                    789: This has the advantage that a single trusted certification authority
                    790: can be used in place of many public/private keys.
                    791: See the
                    792: .Sx CERTIFICATES
                    793: section of
                    794: .Xr ssh-keygen 1
                    795: for more information.
                    796: .Pp
                    797: The most convenient way to use public key or certificate authentication
                    798: may be with an authentication agent.
1.218     jmc       799: See
                    800: .Xr ssh-agent 1
                    801: for more information.
                    802: .Pp
1.222     jmc       803: Challenge-response authentication works as follows:
                    804: The server sends an arbitrary
                    805: .Qq challenge
                    806: text, and prompts for a response.
                    807: Protocol 2 allows multiple challenges and responses;
                    808: protocol 1 is restricted to just one challenge/response.
                    809: Examples of challenge-response authentication include
                    810: BSD Authentication (see
                    811: .Xr login.conf 5 )
                    812: and PAM (some non-OpenBSD systems).
                    813: .Pp
                    814: Finally, if other authentication methods fail,
1.218     jmc       815: .Nm
                    816: prompts the user for a password.
                    817: The password is sent to the remote
                    818: host for checking; however, since all communications are encrypted,
                    819: the password cannot be seen by someone listening on the network.
1.232     jmc       820: .Pp
                    821: .Nm
                    822: automatically maintains and checks a database containing
                    823: identification for all hosts it has ever been used with.
                    824: Host keys are stored in
                    825: .Pa ~/.ssh/known_hosts
                    826: in the user's home directory.
                    827: Additionally, the file
                    828: .Pa /etc/ssh/ssh_known_hosts
                    829: is automatically checked for known hosts.
                    830: Any new hosts are automatically added to the user's file.
                    831: If a host's identification ever changes,
                    832: .Nm
                    833: warns about this and disables password authentication to prevent
                    834: server spoofing or man-in-the-middle attacks,
                    835: which could otherwise be used to circumvent the encryption.
                    836: The
                    837: .Cm StrictHostKeyChecking
                    838: option can be used to control logins to machines whose
                    839: host key is not known or has changed.
                    840: .Pp
1.218     jmc       841: When the user's identity has been accepted by the server, the server
                    842: either executes the given command, or logs into the machine and gives
                    843: the user a normal shell on the remote machine.
                    844: All communication with
                    845: the remote command or shell will be automatically encrypted.
                    846: .Pp
                    847: If a pseudo-terminal has been allocated (normal login session), the
                    848: user may use the escape characters noted below.
                    849: .Pp
                    850: If no pseudo-tty has been allocated,
                    851: the session is transparent and can be used to reliably transfer binary data.
                    852: On most systems, setting the escape character to
                    853: .Dq none
                    854: will also make the session transparent even if a tty is used.
                    855: .Pp
                    856: The session terminates when the command or shell on the remote
1.247     jmc       857: machine exits and all X11 and TCP connections have been closed.
1.223     jmc       858: .Sh ESCAPE CHARACTERS
1.218     jmc       859: When a pseudo-terminal has been requested,
1.2       deraadt   860: .Nm
1.218     jmc       861: supports a number of functions through the use of an escape character.
                    862: .Pp
                    863: A single tilde character can be sent as
                    864: .Ic ~~
                    865: or by following the tilde by a character other than those described below.
                    866: The escape character must always follow a newline to be interpreted as
                    867: special.
                    868: The escape character can be changed in configuration files using the
                    869: .Cm EscapeChar
                    870: configuration directive or on the command line by the
                    871: .Fl e
                    872: option.
                    873: .Pp
                    874: The supported escapes (assuming the default
                    875: .Ql ~ )
                    876: are:
                    877: .Bl -tag -width Ds
                    878: .It Cm ~.
                    879: Disconnect.
                    880: .It Cm ~^Z
                    881: Background
1.234     jmc       882: .Nm .
1.218     jmc       883: .It Cm ~#
                    884: List forwarded connections.
                    885: .It Cm ~&
                    886: Background
1.2       deraadt   887: .Nm
1.218     jmc       888: at logout when waiting for forwarded connection / X11 sessions to terminate.
                    889: .It Cm ~?
                    890: Display a list of escape characters.
                    891: .It Cm ~B
                    892: Send a BREAK to the remote system
                    893: (only useful for SSH protocol version 2 and if the peer supports it).
                    894: .It Cm ~C
                    895: Open command line.
                    896: Currently this allows the addition of port forwardings using the
1.279     stevesk   897: .Fl L ,
                    898: .Fl R
1.218     jmc       899: and
1.279     stevesk   900: .Fl D
1.225     jmc       901: options (see above).
1.322   ! markus    902: It also allows the cancellation of existing port-forwardings
        !           903: with
1.262     stevesk   904: .Sm off
1.322   ! markus    905: .Fl KL Oo Ar bind_address : Oc Ar port
1.262     stevesk   906: .Sm on
1.322   ! markus    907: for local,
        !           908: .Sm off
        !           909: .Fl KR Oo Ar bind_address : Oc Ar port
        !           910: .Sm on
        !           911: for remote and
        !           912: .Sm off
        !           913: .Fl KD Oo Ar bind_address : Oc Ar port
        !           914: .Sm on
        !           915: for dynamic port-forwardings.
1.218     jmc       916: .Ic !\& Ns Ar command
                    917: allows the user to execute a local command if the
                    918: .Ic PermitLocalCommand
                    919: option is enabled in
1.176     jmc       920: .Xr ssh_config 5 .
1.218     jmc       921: Basic help is available, using the
                    922: .Fl h
                    923: option.
                    924: .It Cm ~R
                    925: Request rekeying of the connection
                    926: (only useful for SSH protocol version 2 and if the peer supports it).
1.176     jmc       927: .El
1.246     jmc       928: .Sh TCP FORWARDING
                    929: Forwarding of arbitrary TCP connections over the secure channel can
                    930: be specified either on the command line or in a configuration file.
                    931: One possible application of TCP forwarding is a secure connection to a
                    932: mail server; another is going through firewalls.
                    933: .Pp
                    934: In the example below, we look at encrypting communication between
                    935: an IRC client and server, even though the IRC server does not directly
                    936: support encrypted communications.
                    937: This works as follows:
                    938: the user connects to the remote host using
                    939: .Nm ,
                    940: specifying a port to be used to forward connections
                    941: to the remote server.
                    942: After that it is possible to start the service which is to be encrypted
                    943: on the client machine,
                    944: connecting to the same local port,
                    945: and
                    946: .Nm
                    947: will encrypt and forward the connection.
                    948: .Pp
                    949: The following example tunnels an IRC session from client machine
                    950: .Dq 127.0.0.1
                    951: (localhost)
                    952: to remote server
                    953: .Dq server.example.com :
                    954: .Bd -literal -offset 4n
                    955: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
                    956: $ irc -c '#users' -p 1234 pinky 127.0.0.1
                    957: .Ed
                    958: .Pp
                    959: This tunnels a connection to IRC server
                    960: .Dq server.example.com ,
                    961: joining channel
                    962: .Dq #users ,
                    963: nickname
                    964: .Dq pinky ,
                    965: using port 1234.
                    966: It doesn't matter which port is used,
                    967: as long as it's greater than 1023
                    968: (remember, only root can open sockets on privileged ports)
                    969: and doesn't conflict with any ports already in use.
                    970: The connection is forwarded to port 6667 on the remote server,
                    971: since that's the standard port for IRC services.
                    972: .Pp
                    973: The
                    974: .Fl f
                    975: option backgrounds
                    976: .Nm
                    977: and the remote command
                    978: .Dq sleep 10
                    979: is specified to allow an amount of time
                    980: (10 seconds, in the example)
                    981: to start the service which is to be tunnelled.
                    982: If no connections are made within the time specified,
                    983: .Nm
                    984: will exit.
                    985: .Sh X11 FORWARDING
1.218     jmc       986: If the
                    987: .Cm ForwardX11
                    988: variable is set to
                    989: .Dq yes
                    990: (or see the description of the
1.227     jmc       991: .Fl X ,
                    992: .Fl x ,
1.218     jmc       993: and
1.227     jmc       994: .Fl Y
1.226     jmc       995: options above)
1.218     jmc       996: and the user is using X11 (the
                    997: .Ev DISPLAY
                    998: environment variable is set), the connection to the X11 display is
                    999: automatically forwarded to the remote side in such a way that any X11
                   1000: programs started from the shell (or command) will go through the
                   1001: encrypted channel, and the connection to the real X server will be made
1.176     jmc      1002: from the local machine.
1.218     jmc      1003: The user should not manually set
                   1004: .Ev DISPLAY .
                   1005: Forwarding of X11 connections can be
                   1006: configured on the command line or in configuration files.
                   1007: .Pp
                   1008: The
                   1009: .Ev DISPLAY
                   1010: value set by
                   1011: .Nm
                   1012: will point to the server machine, but with a display number greater than zero.
                   1013: This is normal, and happens because
                   1014: .Nm
                   1015: creates a
                   1016: .Dq proxy
                   1017: X server on the server machine for forwarding the
                   1018: connections over the encrypted channel.
1.200     djm      1019: .Pp
1.218     jmc      1020: .Nm
                   1021: will also automatically set up Xauthority data on the server machine.
                   1022: For this purpose, it will generate a random authorization cookie,
                   1023: store it in Xauthority on the server, and verify that any forwarded
                   1024: connections carry this cookie and replace it by the real cookie when
                   1025: the connection is opened.
                   1026: The real authentication cookie is never
                   1027: sent to the server machine (and no cookies are sent in the plain).
1.200     djm      1028: .Pp
1.218     jmc      1029: If the
                   1030: .Cm ForwardAgent
                   1031: variable is set to
                   1032: .Dq yes
                   1033: (or see the description of the
                   1034: .Fl A
1.191     djm      1035: and
1.218     jmc      1036: .Fl a
1.226     jmc      1037: options above) and
1.218     jmc      1038: the user is using an authentication agent, the connection to the agent
                   1039: is automatically forwarded to the remote side.
1.252     jmc      1040: .Sh VERIFYING HOST KEYS
                   1041: When connecting to a server for the first time,
                   1042: a fingerprint of the server's public key is presented to the user
                   1043: (unless the option
                   1044: .Cm StrictHostKeyChecking
                   1045: has been disabled).
                   1046: Fingerprints can be determined using
                   1047: .Xr ssh-keygen 1 :
                   1048: .Pp
                   1049: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
                   1050: .Pp
1.274     grunk    1051: If the fingerprint is already known, it can be matched
                   1052: and the key can be accepted or rejected.
                   1053: Because of the difficulty of comparing host keys
                   1054: just by looking at hex strings,
                   1055: there is also support to compare host keys visually,
                   1056: using
                   1057: .Em random art .
                   1058: By setting the
1.275     grunk    1059: .Cm VisualHostKey
1.274     grunk    1060: option to
1.275     grunk    1061: .Dq yes ,
1.274     grunk    1062: a small ASCII graphic gets displayed on every login to a server, no matter
                   1063: if the session itself is interactive or not.
                   1064: By learning the pattern a known server produces, a user can easily
                   1065: find out that the host key has changed when a completely different pattern
                   1066: is displayed.
                   1067: Because these patterns are not unambiguous however, a pattern that looks
                   1068: similar to the pattern remembered only gives a good probability that the
                   1069: host key is the same, not guaranteed proof.
                   1070: .Pp
                   1071: To get a listing of the fingerprints along with their random art for
                   1072: all known hosts, the following command line can be used:
                   1073: .Pp
                   1074: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
                   1075: .Pp
1.252     jmc      1076: If the fingerprint is unknown,
                   1077: an alternative method of verification is available:
                   1078: SSH fingerprints verified by DNS.
                   1079: An additional resource record (RR),
                   1080: SSHFP,
                   1081: is added to a zonefile
                   1082: and the connecting client is able to match the fingerprint
                   1083: with that of the key presented.
                   1084: .Pp
                   1085: In this example, we are connecting a client to a server,
                   1086: .Dq host.example.com .
                   1087: The SSHFP resource records should first be added to the zonefile for
                   1088: host.example.com:
                   1089: .Bd -literal -offset indent
1.259     jakob    1090: $ ssh-keygen -r host.example.com.
1.252     jmc      1091: .Ed
                   1092: .Pp
                   1093: The output lines will have to be added to the zonefile.
                   1094: To check that the zone is answering fingerprint queries:
                   1095: .Pp
                   1096: .Dl $ dig -t SSHFP host.example.com
                   1097: .Pp
                   1098: Finally the client connects:
                   1099: .Bd -literal -offset indent
                   1100: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
                   1101: [...]
                   1102: Matching host key fingerprint found in DNS.
                   1103: Are you sure you want to continue connecting (yes/no)?
                   1104: .Ed
                   1105: .Pp
                   1106: See the
                   1107: .Cm VerifyHostKeyDNS
                   1108: option in
                   1109: .Xr ssh_config 5
                   1110: for more information.
1.250     jmc      1111: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
                   1112: .Nm
                   1113: contains support for Virtual Private Network (VPN) tunnelling
                   1114: using the
                   1115: .Xr tun 4
                   1116: network pseudo-device,
                   1117: allowing two networks to be joined securely.
                   1118: The
                   1119: .Xr sshd_config 5
                   1120: configuration option
                   1121: .Cm PermitTunnel
                   1122: controls whether the server supports this,
                   1123: and at what level (layer 2 or 3 traffic).
                   1124: .Pp
                   1125: The following example would connect client network 10.0.50.0/24
1.265     otto     1126: with remote network 10.0.99.0/24 using a point-to-point connection
                   1127: from 10.1.1.1 to 10.1.1.2,
                   1128: provided that the SSH server running on the gateway to the remote network,
                   1129: at 192.168.1.15, allows it.
                   1130: .Pp
                   1131: On the client:
1.250     jmc      1132: .Bd -literal -offset indent
                   1133: # ssh -f -w 0:1 192.168.1.15 true
1.265     otto     1134: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
                   1135: # route add 10.0.99.0/24 10.1.1.2
                   1136: .Ed
                   1137: .Pp
                   1138: On the server:
                   1139: .Bd -literal -offset indent
                   1140: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
                   1141: # route add 10.0.50.0/24 10.1.1.1
1.250     jmc      1142: .Ed
                   1143: .Pp
                   1144: Client access may be more finely tuned via the
                   1145: .Pa /root/.ssh/authorized_keys
                   1146: file (see below) and the
                   1147: .Cm PermitRootLogin
                   1148: server option.
1.255     jmc      1149: The following entry would permit connections on
1.250     jmc      1150: .Xr tun 4
1.255     jmc      1151: device 1 from user
1.250     jmc      1152: .Dq jane
1.255     jmc      1153: and on tun device 2 from user
1.250     jmc      1154: .Dq john ,
                   1155: if
                   1156: .Cm PermitRootLogin
                   1157: is set to
                   1158: .Dq forced-commands-only :
                   1159: .Bd -literal -offset 2n
                   1160: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254     msf      1161: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250     jmc      1162: .Ed
                   1163: .Pp
1.264     ray      1164: Since an SSH-based setup entails a fair amount of overhead,
1.250     jmc      1165: it may be more suited to temporary setups,
                   1166: such as for wireless VPNs.
                   1167: More permanent VPNs are better provided by tools such as
                   1168: .Xr ipsecctl 8
                   1169: and
                   1170: .Xr isakmpd 8 .
1.2       deraadt  1171: .Sh ENVIRONMENT
                   1172: .Nm
1.1       deraadt  1173: will normally set the following environment variables:
1.237     jmc      1174: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2       deraadt  1175: .It Ev DISPLAY
                   1176: The
                   1177: .Ev DISPLAY
1.40      aaron    1178: variable indicates the location of the X11 server.
1.44      aaron    1179: It is automatically set by
1.2       deraadt  1180: .Nm
                   1181: to point to a value of the form
1.233     jmc      1182: .Dq hostname:n ,
                   1183: where
                   1184: .Dq hostname
                   1185: indicates the host where the shell runs, and
                   1186: .Sq n
                   1187: is an integer \*(Ge 1.
1.40      aaron    1188: .Nm
                   1189: uses this special value to forward X11 connections over the secure
                   1190: channel.
1.107     markus   1191: The user should normally not set
                   1192: .Ev DISPLAY
                   1193: explicitly, as that
1.1       deraadt  1194: will render the X11 connection insecure (and will require the user to
                   1195: manually copy any required authorization cookies).
1.2       deraadt  1196: .It Ev HOME
1.1       deraadt  1197: Set to the path of the user's home directory.
1.2       deraadt  1198: .It Ev LOGNAME
                   1199: Synonym for
1.12      aaron    1200: .Ev USER ;
                   1201: set for compatibility with systems that use this variable.
1.2       deraadt  1202: .It Ev MAIL
1.129     stevesk  1203: Set to the path of the user's mailbox.
1.40      aaron    1204: .It Ev PATH
1.2       deraadt  1205: Set to the default
                   1206: .Ev PATH ,
                   1207: as specified when compiling
1.234     jmc      1208: .Nm .
1.118     markus   1209: .It Ev SSH_ASKPASS
                   1210: If
                   1211: .Nm
                   1212: needs a passphrase, it will read the passphrase from the current
                   1213: terminal if it was run from a terminal.
                   1214: If
                   1215: .Nm
                   1216: does not have a terminal associated with it but
                   1217: .Ev DISPLAY
                   1218: and
                   1219: .Ev SSH_ASKPASS
                   1220: are set, it will execute the program specified by
                   1221: .Ev SSH_ASKPASS
                   1222: and open an X11 window to read the passphrase.
                   1223: This is particularly useful when calling
                   1224: .Nm
                   1225: from a
1.196     jmc      1226: .Pa .xsession
1.118     markus   1227: or related script.
                   1228: (Note that on some machines it
                   1229: may be necessary to redirect the input from
                   1230: .Pa /dev/null
                   1231: to make this work.)
1.18      markus   1232: .It Ev SSH_AUTH_SOCK
1.233     jmc      1233: Identifies the path of a
                   1234: .Ux Ns -domain
                   1235: socket used to communicate with the agent.
1.166     stevesk  1236: .It Ev SSH_CONNECTION
                   1237: Identifies the client and server ends of the connection.
1.40      aaron    1238: The variable contains
1.233     jmc      1239: four space-separated values: client IP address, client port number,
                   1240: server IP address, and server port number.
1.73      markus   1241: .It Ev SSH_ORIGINAL_COMMAND
1.233     jmc      1242: This variable contains the original command line if a forced command
1.73      markus   1243: is executed.
                   1244: It can be used to extract the original arguments.
1.2       deraadt  1245: .It Ev SSH_TTY
1.1       deraadt  1246: This is set to the name of the tty (path to the device) associated
1.40      aaron    1247: with the current shell or command.
                   1248: If the current session has no tty,
1.1       deraadt  1249: this variable is not set.
1.2       deraadt  1250: .It Ev TZ
1.214     jmc      1251: This variable is set to indicate the present time zone if it
1.257     jmc      1252: was set when the daemon was started (i.e. the daemon passes the value
1.1       deraadt  1253: on to new connections).
1.2       deraadt  1254: .It Ev USER
1.1       deraadt  1255: Set to the name of the user logging in.
1.2       deraadt  1256: .El
                   1257: .Pp
1.44      aaron    1258: Additionally,
1.2       deraadt  1259: .Nm
1.44      aaron    1260: reads
1.207     djm      1261: .Pa ~/.ssh/environment ,
1.2       deraadt  1262: and adds lines of the format
                   1263: .Dq VARNAME=value
1.233     jmc      1264: to the environment if the file exists and users are allowed to
1.161     marc     1265: change their environment.
1.176     jmc      1266: For more information, see the
1.161     marc     1267: .Cm PermitUserEnvironment
1.162     stevesk  1268: option in
1.161     marc     1269: .Xr sshd_config 5 .
1.2       deraadt  1270: .Sh FILES
1.236     jmc      1271: .Bl -tag -width Ds -compact
1.309     jmc      1272: .It Pa ~/.rhosts
1.240     jmc      1273: This file is used for host-based authentication (see above).
1.92      markus   1274: On some machines this file may need to be
1.240     jmc      1275: world-readable if the user's home directory is on an NFS partition,
1.1       deraadt  1276: because
1.2       deraadt  1277: .Xr sshd 8
1.40      aaron    1278: reads it as root.
                   1279: Additionally, this file must be owned by the user,
                   1280: and must not have write permissions for anyone else.
                   1281: The recommended
1.1       deraadt  1282: permission for most machines is read/write for the user, and not
                   1283: accessible by others.
1.2       deraadt  1284: .Pp
1.309     jmc      1285: .It Pa ~/.shosts
1.240     jmc      1286: This file is used in exactly the same way as
                   1287: .Pa .rhosts ,
                   1288: but allows host-based authentication without permitting login with
                   1289: rlogin/rsh.
1.272     mcbride  1290: .Pp
1.309     jmc      1291: .It Pa ~/.ssh/
1.272     mcbride  1292: This directory is the default location for all user-specific configuration
                   1293: and authentication information.
                   1294: There is no general requirement to keep the entire contents of this directory
                   1295: secret, but the recommended permissions are read/write/execute for the user,
                   1296: and not accessible by others.
1.236     jmc      1297: .Pp
1.309     jmc      1298: .It Pa ~/.ssh/authorized_keys
1.310     djm      1299: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
                   1300: this user.
1.238     jmc      1301: The format of this file is described in the
                   1302: .Xr sshd 8
                   1303: manual page.
                   1304: This file is not highly sensitive, but the recommended
                   1305: permissions are read/write for the user, and not accessible by others.
                   1306: .Pp
1.309     jmc      1307: .It Pa ~/.ssh/config
1.238     jmc      1308: This is the per-user configuration file.
                   1309: The file format and configuration options are described in
                   1310: .Xr ssh_config 5 .
                   1311: Because of the potential for abuse, this file must have strict permissions:
                   1312: read/write for the user, and not accessible by others.
                   1313: .Pp
1.309     jmc      1314: .It Pa ~/.ssh/environment
1.239     jmc      1315: Contains additional definitions for environment variables; see
                   1316: .Sx ENVIRONMENT ,
1.238     jmc      1317: above.
                   1318: .Pp
1.309     jmc      1319: .It Pa ~/.ssh/identity
                   1320: .It Pa ~/.ssh/id_dsa
1.310     djm      1321: .It Pa ~/.ssh/id_ecdsa
1.309     jmc      1322: .It Pa ~/.ssh/id_rsa
1.238     jmc      1323: Contains the private key for authentication.
                   1324: These files
                   1325: contain sensitive data and should be readable by the user but not
                   1326: accessible by others (read/write/execute).
                   1327: .Nm
                   1328: will simply ignore a private key file if it is accessible by others.
                   1329: It is possible to specify a passphrase when
                   1330: generating the key which will be used to encrypt the
                   1331: sensitive part of this file using 3DES.
                   1332: .Pp
1.309     jmc      1333: .It Pa ~/.ssh/identity.pub
                   1334: .It Pa ~/.ssh/id_dsa.pub
1.310     djm      1335: .It Pa ~/.ssh/id_ecdsa.pub
1.309     jmc      1336: .It Pa ~/.ssh/id_rsa.pub
1.238     jmc      1337: Contains the public key for authentication.
                   1338: These files are not
                   1339: sensitive and can (but need not) be readable by anyone.
                   1340: .Pp
1.309     jmc      1341: .It Pa ~/.ssh/known_hosts
1.244     jmc      1342: Contains a list of host keys for all hosts the user has logged into
                   1343: that are not already in the systemwide list of known host keys.
1.238     jmc      1344: See
1.244     jmc      1345: .Xr sshd 8
                   1346: for further details of the format of this file.
1.238     jmc      1347: .Pp
1.309     jmc      1348: .It Pa ~/.ssh/rc
1.238     jmc      1349: Commands in this file are executed by
                   1350: .Nm
1.245     jmc      1351: when the user logs in, just before the user's shell (or command) is
1.238     jmc      1352: started.
                   1353: See the
                   1354: .Xr sshd 8
                   1355: manual page for more information.
                   1356: .Pp
1.309     jmc      1357: .It Pa /etc/hosts.equiv
1.240     jmc      1358: This file is for host-based authentication (see above).
                   1359: It should only be writable by root.
1.236     jmc      1360: .Pp
1.309     jmc      1361: .It Pa /etc/shosts.equiv
1.240     jmc      1362: This file is used in exactly the same way as
                   1363: .Pa hosts.equiv ,
                   1364: but allows host-based authentication without permitting login with
                   1365: rlogin/rsh.
1.236     jmc      1366: .Pp
1.238     jmc      1367: .It Pa /etc/ssh/ssh_config
                   1368: Systemwide configuration file.
                   1369: The file format and configuration options are described in
                   1370: .Xr ssh_config 5 .
                   1371: .Pp
1.309     jmc      1372: .It Pa /etc/ssh/ssh_host_key
                   1373: .It Pa /etc/ssh/ssh_host_dsa_key
1.310     djm      1374: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.309     jmc      1375: .It Pa /etc/ssh/ssh_host_rsa_key
1.238     jmc      1376: These three files contain the private parts of the host keys
1.245     jmc      1377: and are used for host-based authentication.
                   1378: If protocol version 1 is used,
1.238     jmc      1379: .Nm
                   1380: must be setuid root, since the host key is readable only by root.
                   1381: For protocol version 2,
                   1382: .Nm
                   1383: uses
                   1384: .Xr ssh-keysign 8
1.245     jmc      1385: to access the host keys,
                   1386: eliminating the requirement that
1.238     jmc      1387: .Nm
1.245     jmc      1388: be setuid root when host-based authentication is used.
1.238     jmc      1389: By default
1.2       deraadt  1390: .Nm
1.238     jmc      1391: is not setuid root.
                   1392: .Pp
1.309     jmc      1393: .It Pa /etc/ssh/ssh_known_hosts
1.238     jmc      1394: Systemwide list of known host keys.
                   1395: This file should be prepared by the
                   1396: system administrator to contain the public host keys of all machines in the
                   1397: organization.
1.244     jmc      1398: It should be world-readable.
                   1399: See
1.238     jmc      1400: .Xr sshd 8
1.244     jmc      1401: for further details of the format of this file.
1.236     jmc      1402: .Pp
1.309     jmc      1403: .It Pa /etc/ssh/sshrc
1.1       deraadt  1404: Commands in this file are executed by
1.2       deraadt  1405: .Nm
1.245     jmc      1406: when the user logs in, just before the user's shell (or command) is started.
1.44      aaron    1407: See the
1.2       deraadt  1408: .Xr sshd 8
1.1       deraadt  1409: manual page for more information.
1.58      itojun   1410: .El
1.312     jmc      1411: .Sh EXIT STATUS
                   1412: .Nm
                   1413: exits with the exit status of the remote command or with 255
                   1414: if an error occurred.
1.2       deraadt  1415: .Sh SEE ALSO
                   1416: .Xr scp 1 ,
1.83      djm      1417: .Xr sftp 1 ,
1.2       deraadt  1418: .Xr ssh-add 1 ,
                   1419: .Xr ssh-agent 1 ,
                   1420: .Xr ssh-keygen 1 ,
1.242     jmc      1421: .Xr ssh-keyscan 1 ,
1.250     jmc      1422: .Xr tun 4 ,
1.176     jmc      1423: .Xr hosts.equiv 5 ,
1.159     stevesk  1424: .Xr ssh_config 5 ,
1.160     naddy    1425: .Xr ssh-keysign 8 ,
1.87      itojun   1426: .Xr sshd 8
1.106     markus   1427: .Rs
1.256     jmc      1428: .%R RFC 4250
                   1429: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
                   1430: .%D 2006
                   1431: .Re
                   1432: .Rs
                   1433: .%R RFC 4251
                   1434: .%T "The Secure Shell (SSH) Protocol Architecture"
                   1435: .%D 2006
                   1436: .Re
                   1437: .Rs
                   1438: .%R RFC 4252
                   1439: .%T "The Secure Shell (SSH) Authentication Protocol"
                   1440: .%D 2006
                   1441: .Re
                   1442: .Rs
                   1443: .%R RFC 4253
                   1444: .%T "The Secure Shell (SSH) Transport Layer Protocol"
                   1445: .%D 2006
                   1446: .Re
                   1447: .Rs
                   1448: .%R RFC 4254
                   1449: .%T "The Secure Shell (SSH) Connection Protocol"
                   1450: .%D 2006
                   1451: .Re
                   1452: .Rs
                   1453: .%R RFC 4255
                   1454: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
                   1455: .%D 2006
                   1456: .Re
                   1457: .Rs
                   1458: .%R RFC 4256
                   1459: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
                   1460: .%D 2006
                   1461: .Re
                   1462: .Rs
                   1463: .%R RFC 4335
                   1464: .%T "The Secure Shell (SSH) Session Channel Break Extension"
                   1465: .%D 2006
                   1466: .Re
                   1467: .Rs
                   1468: .%R RFC 4344
                   1469: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
                   1470: .%D 2006
                   1471: .Re
                   1472: .Rs
                   1473: .%R RFC 4345
                   1474: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258     djm      1475: .%D 2006
                   1476: .Re
                   1477: .Rs
                   1478: .%R RFC 4419
                   1479: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266     markus   1480: .%D 2006
                   1481: .Re
                   1482: .Rs
                   1483: .%R RFC 4716
                   1484: .%T "The Secure Shell (SSH) Public Key File Format"
1.256     jmc      1485: .%D 2006
1.313     djm      1486: .Re
                   1487: .Rs
                   1488: .%R RFC 5656
                   1489: .%T "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer"
                   1490: .%D 2009
1.274     grunk    1491: .Re
                   1492: .Rs
                   1493: .%T "Hash Visualization: a New Technique to improve Real-World Security"
                   1494: .%A A. Perrig
                   1495: .%A D. Song
                   1496: .%D 1999
                   1497: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106     markus   1498: .Re
1.173     jmc      1499: .Sh AUTHORS
                   1500: OpenSSH is a derivative of the original and free
                   1501: ssh 1.2.12 release by Tatu Ylonen.
                   1502: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
                   1503: Theo de Raadt and Dug Song
                   1504: removed many bugs, re-added newer features and
                   1505: created OpenSSH.
                   1506: Markus Friedl contributed the support for SSH
                   1507: protocol versions 1.5 and 2.0.