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Annotation of src/usr.bin/ssh/ssh.1, Revision 1.379

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: .\"
1.379   ! jmc        36: .\" $OpenBSD: ssh.1,v 1.378 2017/05/02 13:44:51 jmc Exp $
        !            37: .Dd $Mdocdate: May 2 2017 $
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
1.377     djm        46: .Op Fl 46AaCfGgKkMNnqsTtVvXxYy
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
1.331     dtucker    50: .Op Fl E Ar log_file
1.2       deraadt    51: .Op Fl e Ar escape_char
1.176     jmc        52: .Op Fl F Ar configfile
1.292     jmc        53: .Op Fl I Ar pkcs11
1.2       deraadt    54: .Op Fl i Ar identity_file
1.376     jmc        55: .Op Fl J Oo Ar user Ns @ Oc Ns Ar host Ns Op : Ns Ar port
1.360     millert    56: .Op Fl L Ar address
1.176     jmc        57: .Op Fl l Ar login_name
                     58: .Op Fl m Ar mac_spec
1.198     djm        59: .Op Fl O Ar ctl_cmd
1.176     jmc        60: .Op Fl o Ar option
                     61: .Op Fl p Ar port
1.362     djm        62: .Op Fl Q Ar query_option
1.360     millert    63: .Op Fl R Ar address
1.198     djm        64: .Op Fl S Ar ctl_path
1.290     dtucker    65: .Op Fl W Ar host : Ns Ar port
1.306     jmc        66: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.176     jmc        67: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2       deraadt    68: .Op Ar command
1.306     jmc        69: .Ek
1.44      aaron      70: .Sh DESCRIPTION
1.2       deraadt    71: .Nm
1.96      deraadt    72: (SSH client) is a program for logging into a remote machine and for
1.40      aaron      73: executing commands on a remote machine.
1.365     mmcc       74: It is intended to provide secure encrypted communications between
1.40      aaron      75: two untrusted hosts over an insecure network.
1.348     millert    76: X11 connections, arbitrary TCP ports and
                     77: .Ux Ns -domain
                     78: sockets can also be forwarded over the secure channel.
1.2       deraadt    79: .Pp
                     80: .Nm
1.44      aaron      81: connects and logs into the specified
1.176     jmc        82: .Ar hostname
                     83: (with optional
                     84: .Ar user
                     85: name).
1.1       deraadt    86: The user must prove
1.49      markus     87: his/her identity to the remote machine using one of several methods
1.365     mmcc       88: (see below).
1.49      markus     89: .Pp
1.176     jmc        90: If
                     91: .Ar command
                     92: is specified,
1.219     jmc        93: it is executed on the remote host instead of a login shell.
1.2       deraadt    94: .Pp
1.218     jmc        95: The options are as follows:
1.361     millert    96: .Pp
1.360     millert    97: .Bl -tag -width Ds -compact
1.218     jmc        98: .It Fl 4
                     99: Forces
1.2       deraadt   100: .Nm
1.218     jmc       101: to use IPv4 addresses only.
1.360     millert   102: .Pp
1.218     jmc       103: .It Fl 6
                    104: Forces
1.2       deraadt   105: .Nm
1.218     jmc       106: to use IPv6 addresses only.
1.360     millert   107: .Pp
1.218     jmc       108: .It Fl A
                    109: Enables forwarding of the authentication agent connection.
                    110: This can also be specified on a per-host basis in a configuration file.
1.2       deraadt   111: .Pp
1.218     jmc       112: Agent forwarding should be enabled with caution.
                    113: Users with the ability to bypass file permissions on the remote host
1.285     sobrado   114: (for the agent's
1.286     sobrado   115: .Ux Ns -domain
                    116: socket) can access the local agent through the forwarded connection.
1.218     jmc       117: An attacker cannot obtain key material from the agent,
                    118: however they can perform operations on the keys that enable them to
                    119: authenticate using the identities loaded into the agent.
1.360     millert   120: .Pp
1.218     jmc       121: .It Fl a
                    122: Disables forwarding of the authentication agent connection.
1.360     millert   123: .Pp
1.218     jmc       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.
1.360     millert   130: .Pp
1.218     jmc       131: .It Fl C
                    132: Requests compression of all data (including stdin, stdout, stderr, and
1.348     millert   133: data for forwarded X11, TCP and
                    134: .Ux Ns -domain
                    135: connections).
1.218     jmc       136: The compression algorithm is the same used by
1.377     djm       137: .Xr gzip 1 .
1.218     jmc       138: Compression is desirable on modem lines and other
                    139: slow connections, but will only slow down things on fast networks.
                    140: The default value can be set on a host-by-host basis in the
                    141: configuration files; see the
                    142: .Cm Compression
                    143: option.
1.360     millert   144: .Pp
1.218     jmc       145: .It Fl c Ar cipher_spec
                    146: Selects the cipher specification for encrypting the session.
                    147: .Ar cipher_spec
                    148: is a comma-separated list of ciphers
                    149: listed in order of preference.
1.283     jmc       150: See the
                    151: .Cm Ciphers
1.307     dtucker   152: keyword in
                    153: .Xr ssh_config 5
                    154: for more information.
1.360     millert   155: .Pp
1.218     jmc       156: .It Fl D Xo
                    157: .Sm off
                    158: .Oo Ar bind_address : Oc
                    159: .Ar port
                    160: .Sm on
                    161: .Xc
                    162: Specifies a local
                    163: .Dq dynamic
                    164: application-level port forwarding.
                    165: This works by allocating a socket to listen to
                    166: .Ar port
                    167: on the local side, optionally bound to the specified
                    168: .Ar bind_address .
                    169: Whenever a connection is made to this port, the
                    170: connection is forwarded over the secure channel, and the application
                    171: protocol is then used to determine where to connect to from the
                    172: remote machine.
                    173: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107     markus    174: .Nm
1.218     jmc       175: will act as a SOCKS server.
                    176: Only root can forward privileged ports.
                    177: Dynamic port forwardings can also be specified in the configuration file.
1.49      markus    178: .Pp
1.308     djm       179: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218     jmc       180: Only the superuser can forward privileged ports.
                    181: By default, the local port is bound in accordance with the
                    182: .Cm GatewayPorts
                    183: setting.
                    184: However, an explicit
                    185: .Ar bind_address
                    186: may be used to bind the connection to a specific address.
                    187: The
                    188: .Ar bind_address
                    189: of
                    190: .Dq localhost
                    191: indicates that the listening port be bound for local use only, while an
                    192: empty address or
                    193: .Sq *
                    194: indicates that the port should be available from all interfaces.
1.360     millert   195: .Pp
1.331     dtucker   196: .It Fl E Ar log_file
                    197: Append debug logs to
                    198: .Ar log_file
                    199: instead of standard error.
1.360     millert   200: .Pp
1.229     jmc       201: .It Fl e Ar escape_char
1.218     jmc       202: Sets the escape character for sessions with a pty (default:
                    203: .Ql ~ ) .
                    204: The escape character is only recognized at the beginning of a line.
                    205: The escape character followed by a dot
                    206: .Pq Ql \&.
                    207: closes the connection;
                    208: followed by control-Z suspends the connection;
                    209: and followed by itself sends the escape character once.
                    210: Setting the character to
1.2       deraadt   211: .Dq none
1.218     jmc       212: disables any escapes and makes the session fully transparent.
1.360     millert   213: .Pp
1.218     jmc       214: .It Fl F Ar configfile
                    215: Specifies an alternative per-user configuration file.
                    216: If a configuration file is given on the command line,
                    217: the system-wide configuration file
                    218: .Pq Pa /etc/ssh/ssh_config
                    219: will be ignored.
                    220: The default for the per-user configuration file is
                    221: .Pa ~/.ssh/config .
1.360     millert   222: .Pp
1.218     jmc       223: .It Fl f
                    224: Requests
                    225: .Nm
                    226: to go to background just before command execution.
                    227: This is useful if
1.176     jmc       228: .Nm
1.218     jmc       229: is going to ask for passwords or passphrases, but the user
                    230: wants it in the background.
                    231: This implies
                    232: .Fl n .
                    233: The recommended way to start X11 programs at a remote site is with
                    234: something like
                    235: .Ic ssh -f host xterm .
1.277     djm       236: .Pp
                    237: If the
                    238: .Cm ExitOnForwardFailure
                    239: configuration option is set to
                    240: .Dq yes ,
                    241: then a client started with
                    242: .Fl f
                    243: will wait for all remote port forwards to be successfully established
                    244: before placing itself in the background.
1.360     millert   245: .Pp
1.350     djm       246: .It Fl G
                    247: Causes
                    248: .Nm
                    249: to print its configuration after evaluating
                    250: .Cm Host
                    251: and
                    252: .Cm Match
                    253: blocks and exit.
1.360     millert   254: .Pp
1.218     jmc       255: .It Fl g
                    256: Allows remote hosts to connect to local forwarded ports.
1.346     djm       257: If used on a multiplexed connection, then this option must be specified
                    258: on the master process.
1.360     millert   259: .Pp
1.291     markus    260: .It Fl I Ar pkcs11
1.294     jmc       261: Specify the PKCS#11 shared library
1.176     jmc       262: .Nm
1.293     markus    263: should use to communicate with a PKCS#11 token providing the user's
1.218     jmc       264: private RSA key.
1.360     millert   265: .Pp
1.218     jmc       266: .It Fl i Ar identity_file
                    267: Selects a file from which the identity (private key) for
1.310     djm       268: public key authentication is read.
1.218     jmc       269: The default is
1.310     djm       270: .Pa ~/.ssh/id_dsa ,
1.343     naddy     271: .Pa ~/.ssh/id_ecdsa ,
                    272: .Pa ~/.ssh/id_ed25519
1.310     djm       273: and
1.218     jmc       274: .Pa ~/.ssh/id_rsa
                    275: for protocol version 2.
                    276: Identity files may also be specified on
                    277: a per-host basis in the configuration file.
                    278: It is possible to have multiple
                    279: .Fl i
                    280: options (and multiple identities specified in
                    281: configuration files).
1.364     jmc       282: If no certificates have been explicitly specified by the
1.363     djm       283: .Cm CertificateFile
                    284: directive,
1.302     djm       285: .Nm
                    286: will also try to load certificate information from the filename obtained
                    287: by appending
                    288: .Pa -cert.pub
                    289: to identity filenames.
1.360     millert   290: .Pp
1.375     djm       291: .It Fl J Xo
                    292: .Sm off
1.376     jmc       293: .Op Ar user No @
                    294: .Ar host
                    295: .Op : Ar port
1.375     djm       296: .Sm on
                    297: .Xc
                    298: Connect to the target host by first making a
                    299: .Nm
1.376     jmc       300: connection to the jump
                    301: .Ar host
                    302: and then establishing a TCP forwarding to the ultimate destination from
1.375     djm       303: there.
                    304: Multiple jump hops may be specified separated by comma characters.
                    305: This is a shortcut to specify a
                    306: .Cm ProxyJump
                    307: configuration directive.
                    308: .Pp
1.269     djm       309: .It Fl K
                    310: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
                    311: credentials to the server.
1.360     millert   312: .Pp
1.218     jmc       313: .It Fl k
                    314: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360     millert   315: .Pp
1.218     jmc       316: .It Fl L Xo
                    317: .Sm off
                    318: .Oo Ar bind_address : Oc
                    319: .Ar port : host : hostport
                    320: .Sm on
                    321: .Xc
1.360     millert   322: .It Fl L Xo
                    323: .Sm off
                    324: .Oo Ar bind_address : Oc
                    325: .Ar port : remote_socket
                    326: .Sm on
                    327: .Xc
                    328: .It Fl L Xo
                    329: .Sm off
                    330: .Ar local_socket : host : hostport
                    331: .Sm on
                    332: .Xc
                    333: .It Fl L Xo
                    334: .Sm off
                    335: .Ar local_socket : remote_socket
                    336: .Sm on
                    337: .Xc
                    338: Specifies that connections to the given TCP port or Unix socket on the local
                    339: (client) host are to be forwarded to the given host and port, or Unix socket,
                    340: on the remote side.
                    341: This works by allocating a socket to listen to either a TCP
1.218     jmc       342: .Ar port
                    343: on the local side, optionally bound to the specified
1.360     millert   344: .Ar bind_address ,
                    345: or to a Unix socket.
                    346: Whenever a connection is made to the local port or socket, the
1.218     jmc       347: connection is forwarded over the secure channel, and a connection is
1.360     millert   348: made to either
1.218     jmc       349: .Ar host
                    350: port
1.360     millert   351: .Ar hostport ,
                    352: or the Unix socket
                    353: .Ar remote_socket ,
1.218     jmc       354: from the remote machine.
1.360     millert   355: .Pp
1.218     jmc       356: Port forwardings can also be specified in the configuration file.
1.360     millert   357: Only the superuser can forward privileged ports.
1.308     djm       358: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360     millert   359: .Pp
1.218     jmc       360: By default, the local port is bound in accordance with the
                    361: .Cm GatewayPorts
                    362: setting.
                    363: However, an explicit
                    364: .Ar bind_address
                    365: may be used to bind the connection to a specific address.
1.2       deraadt   366: The
1.218     jmc       367: .Ar bind_address
                    368: of
                    369: .Dq localhost
                    370: indicates that the listening port be bound for local use only, while an
                    371: empty address or
                    372: .Sq *
                    373: indicates that the port should be available from all interfaces.
1.360     millert   374: .Pp
1.218     jmc       375: .It Fl l Ar login_name
                    376: Specifies the user to log in as on the remote machine.
                    377: This also may be specified on a per-host basis in the configuration file.
1.360     millert   378: .Pp
1.218     jmc       379: .It Fl M
                    380: Places the
                    381: .Nm
                    382: client into
                    383: .Dq master
                    384: mode for connection sharing.
1.231     stevesk   385: Multiple
                    386: .Fl M
                    387: options places
                    388: .Nm
                    389: into
                    390: .Dq master
                    391: mode with confirmation required before slave connections are accepted.
1.218     jmc       392: Refer to the description of
                    393: .Cm ControlMaster
                    394: in
                    395: .Xr ssh_config 5
                    396: for details.
1.360     millert   397: .Pp
1.218     jmc       398: .It Fl m Ar mac_spec
1.369     jmc       399: A comma-separated list of MAC (message authentication code) algorithms,
                    400: specified in order of preference.
1.218     jmc       401: See the
                    402: .Cm MACs
                    403: keyword for more information.
1.360     millert   404: .Pp
1.218     jmc       405: .It Fl N
                    406: Do not execute a remote command.
1.369     jmc       407: This is useful for just forwarding ports.
1.360     millert   408: .Pp
1.218     jmc       409: .It Fl n
                    410: Redirects stdin from
                    411: .Pa /dev/null
                    412: (actually, prevents reading from stdin).
                    413: This must be used when
1.2       deraadt   414: .Nm
1.218     jmc       415: is run in the background.
                    416: A common trick is to use this to run X11 programs on a remote machine.
                    417: For example,
                    418: .Ic ssh -n shadows.cs.hut.fi emacs &
                    419: will start an emacs on shadows.cs.hut.fi, and the X11
                    420: connection will be automatically forwarded over an encrypted channel.
                    421: The
1.2       deraadt   422: .Nm
1.218     jmc       423: program will be put in the background.
                    424: (This does not work if
1.2       deraadt   425: .Nm
1.218     jmc       426: needs to ask for a password or passphrase; see also the
                    427: .Fl f
                    428: option.)
1.360     millert   429: .Pp
1.218     jmc       430: .It Fl O Ar ctl_cmd
                    431: Control an active connection multiplexing master process.
                    432: When the
                    433: .Fl O
                    434: option is specified, the
                    435: .Ar ctl_cmd
                    436: argument is interpreted and passed to the master process.
                    437: Valid commands are:
                    438: .Dq check
1.305     markus    439: (check that the master process is running),
                    440: .Dq forward
1.318     jmc       441: (request forwardings without command execution),
1.323     okan      442: .Dq cancel
                    443: (cancel forwardings),
1.218     jmc       444: .Dq exit
1.318     jmc       445: (request the master to exit), and
1.317     djm       446: .Dq stop
                    447: (request the master to stop accepting further multiplexing requests).
1.360     millert   448: .Pp
1.218     jmc       449: .It Fl o Ar option
                    450: Can be used to give options in the format used in the configuration file.
                    451: This is useful for specifying options for which there is no separate
                    452: command-line flag.
                    453: For full details of the options listed below, and their possible values, see
                    454: .Xr ssh_config 5 .
1.2       deraadt   455: .Pp
1.218     jmc       456: .Bl -tag -width Ds -offset indent -compact
1.366     jcs       457: .It AddKeysToAgent
1.218     jmc       458: .It AddressFamily
                    459: .It BatchMode
                    460: .It BindAddress
1.338     djm       461: .It CanonicalDomains
1.339     djm       462: .It CanonicalizeFallbackLocal
                    463: .It CanonicalizeHostname
                    464: .It CanonicalizeMaxDots
                    465: .It CanonicalizePermittedCNAMEs
1.363     djm       466: .It CertificateFile
1.218     jmc       467: .It ChallengeResponseAuthentication
                    468: .It CheckHostIP
                    469: .It Ciphers
                    470: .It ClearAllForwardings
                    471: .It Compression
                    472: .It ConnectTimeout
                    473: .It ControlMaster
                    474: .It ControlPath
1.321     djm       475: .It ControlPersist
1.218     jmc       476: .It DynamicForward
                    477: .It EscapeChar
1.263     markus    478: .It ExitOnForwardFailure
1.353     jmc       479: .It FingerprintHash
1.218     jmc       480: .It ForwardAgent
                    481: .It ForwardX11
1.321     djm       482: .It ForwardX11Timeout
1.218     jmc       483: .It ForwardX11Trusted
                    484: .It GatewayPorts
                    485: .It GlobalKnownHostsFile
                    486: .It GSSAPIAuthentication
                    487: .It GSSAPIDelegateCredentials
                    488: .It HashKnownHosts
                    489: .It Host
                    490: .It HostbasedAuthentication
1.355     djm       491: .It HostbasedKeyTypes
1.218     jmc       492: .It HostKeyAlgorithms
                    493: .It HostKeyAlias
                    494: .It HostName
1.374     jmc       495: .It IdentitiesOnly
1.371     markus    496: .It IdentityAgent
1.218     jmc       497: .It IdentityFile
1.370     djm       498: .It Include
1.316     jmc       499: .It IPQoS
1.321     djm       500: .It KbdInteractiveAuthentication
1.218     jmc       501: .It KbdInteractiveDevices
1.314     jmc       502: .It KexAlgorithms
1.218     jmc       503: .It LocalCommand
                    504: .It LocalForward
                    505: .It LogLevel
                    506: .It MACs
1.337     jmc       507: .It Match
1.218     jmc       508: .It NoHostAuthenticationForLocalhost
                    509: .It NumberOfPasswordPrompts
                    510: .It PasswordAuthentication
                    511: .It PermitLocalCommand
1.291     markus    512: .It PKCS11Provider
1.218     jmc       513: .It Port
                    514: .It PreferredAuthentications
                    515: .It ProxyCommand
1.375     djm       516: .It ProxyJump
1.336     jmc       517: .It ProxyUseFdpass
1.359     markus    518: .It PubkeyAcceptedKeyTypes
1.218     jmc       519: .It PubkeyAuthentication
1.251     dtucker   520: .It RekeyLimit
1.218     jmc       521: .It RemoteForward
1.319     jmc       522: .It RequestTTY
1.218     jmc       523: .It SendEnv
                    524: .It ServerAliveInterval
                    525: .It ServerAliveCountMax
1.347     jmc       526: .It StreamLocalBindMask
                    527: .It StreamLocalBindUnlink
1.218     jmc       528: .It StrictHostKeyChecking
                    529: .It TCPKeepAlive
                    530: .It Tunnel
                    531: .It TunnelDevice
1.354     djm       532: .It UpdateHostKeys
1.218     jmc       533: .It UsePrivilegedPort
                    534: .It User
                    535: .It UserKnownHostsFile
                    536: .It VerifyHostKeyDNS
1.276     jmc       537: .It VisualHostKey
1.218     jmc       538: .It XAuthLocation
                    539: .El
1.360     millert   540: .Pp
1.218     jmc       541: .It Fl p Ar port
                    542: Port to connect to on the remote host.
                    543: This can be specified on a
                    544: per-host basis in the configuration file.
1.360     millert   545: .Pp
1.362     djm       546: .It Fl Q Ar query_option
1.332     djm       547: Queries
                    548: .Nm
1.341     deraadt   549: for the algorithms supported for the specified version 2.
                    550: The available features are:
                    551: .Ar cipher
1.332     djm       552: (supported symmetric ciphers),
1.341     deraadt   553: .Ar cipher-auth
1.340     djm       554: (supported symmetric ciphers that support authenticated encryption),
1.341     deraadt   555: .Ar mac
1.332     djm       556: (supported message integrity codes),
1.341     deraadt   557: .Ar kex
1.332     djm       558: (key exchange algorithms),
1.341     deraadt   559: .Ar key
1.362     djm       560: (key types),
                    561: .Ar key-cert
                    562: (certificate key types),
                    563: .Ar key-plain
                    564: (non-certificate key types), and
1.356     djm       565: .Ar protocol-version
                    566: (supported SSH protocol versions).
1.360     millert   567: .Pp
1.218     jmc       568: .It Fl q
                    569: Quiet mode.
1.271     djm       570: Causes most warning and diagnostic messages to be suppressed.
1.360     millert   571: .Pp
1.218     jmc       572: .It Fl R Xo
                    573: .Sm off
                    574: .Oo Ar bind_address : Oc
                    575: .Ar port : host : hostport
                    576: .Sm on
                    577: .Xc
1.360     millert   578: .It Fl R Xo
                    579: .Sm off
                    580: .Oo Ar bind_address : Oc
                    581: .Ar port : local_socket
                    582: .Sm on
                    583: .Xc
                    584: .It Fl R Xo
                    585: .Sm off
                    586: .Ar remote_socket : host : hostport
                    587: .Sm on
                    588: .Xc
                    589: .It Fl R Xo
                    590: .Sm off
                    591: .Ar remote_socket : local_socket
                    592: .Sm on
                    593: .Xc
                    594: Specifies that connections to the given TCP port or Unix socket on the remote
                    595: (server) host are to be forwarded to the given host and port, or Unix socket,
                    596: on the local side.
                    597: This works by allocating a socket to listen to either a TCP
1.218     jmc       598: .Ar port
1.360     millert   599: or to a Unix socket on the remote side.
                    600: Whenever a connection is made to this port or Unix socket, the
                    601: connection is forwarded over the secure channel, and a connection
                    602: is made to either
1.218     jmc       603: .Ar host
                    604: port
1.360     millert   605: .Ar hostport ,
                    606: or
                    607: .Ar local_socket ,
1.218     jmc       608: from the local machine.
1.2       deraadt   609: .Pp
1.218     jmc       610: Port forwardings can also be specified in the configuration file.
                    611: Privileged ports can be forwarded only when
                    612: logging in as root on the remote machine.
1.324     jmc       613: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194     jakob     614: .Pp
1.360     millert   615: By default, TCP listening sockets on the server will be bound to the loopback
1.218     jmc       616: interface only.
1.280     tobias    617: This may be overridden by specifying a
1.218     jmc       618: .Ar bind_address .
                    619: An empty
                    620: .Ar bind_address ,
                    621: or the address
                    622: .Ql * ,
                    623: indicates that the remote socket should listen on all interfaces.
                    624: Specifying a remote
                    625: .Ar bind_address
                    626: will only succeed if the server's
                    627: .Cm GatewayPorts
                    628: option is enabled (see
                    629: .Xr sshd_config 5 ) .
1.281     djm       630: .Pp
                    631: If the
                    632: .Ar port
                    633: argument is
1.282     djm       634: .Ql 0 ,
1.281     djm       635: the listen port will be dynamically allocated on the server and reported
                    636: to the client at run time.
1.305     markus    637: When used together with
                    638: .Ic -O forward
                    639: the allocated port will be printed to the standard output.
1.360     millert   640: .Pp
1.218     jmc       641: .It Fl S Ar ctl_path
1.304     jmc       642: Specifies the location of a control socket for connection sharing,
1.303     djm       643: or the string
                    644: .Dq none
                    645: to disable connection sharing.
1.218     jmc       646: Refer to the description of
                    647: .Cm ControlPath
                    648: and
                    649: .Cm ControlMaster
                    650: in
                    651: .Xr ssh_config 5
                    652: for details.
1.360     millert   653: .Pp
1.218     jmc       654: .It Fl s
                    655: May be used to request invocation of a subsystem on the remote system.
1.369     jmc       656: Subsystems facilitate the use of SSH
                    657: as a secure transport for other applications (e.g.\&
1.218     jmc       658: .Xr sftp 1 ) .
                    659: The subsystem is specified as the remote command.
1.360     millert   660: .Pp
1.218     jmc       661: .It Fl T
1.357     dtucker   662: Disable pseudo-terminal allocation.
1.360     millert   663: .Pp
1.218     jmc       664: .It Fl t
1.357     dtucker   665: Force pseudo-terminal allocation.
1.218     jmc       666: This can be used to execute arbitrary
                    667: screen-based programs on a remote machine, which can be very useful,
1.257     jmc       668: e.g. when implementing menu services.
1.218     jmc       669: Multiple
                    670: .Fl t
                    671: options force tty allocation, even if
1.194     jakob     672: .Nm
1.218     jmc       673: has no local tty.
1.360     millert   674: .Pp
1.218     jmc       675: .It Fl V
                    676: Display the version number and exit.
1.360     millert   677: .Pp
1.218     jmc       678: .It Fl v
                    679: Verbose mode.
                    680: Causes
1.176     jmc       681: .Nm
1.218     jmc       682: to print debugging messages about its progress.
                    683: This is helpful in
                    684: debugging connection, authentication, and configuration problems.
                    685: Multiple
                    686: .Fl v
                    687: options increase the verbosity.
                    688: The maximum is 3.
1.360     millert   689: .Pp
1.290     dtucker   690: .It Fl W Ar host : Ns Ar port
                    691: Requests that standard input and output on the client be forwarded to
                    692: .Ar host
                    693: on
                    694: .Ar port
                    695: over the secure channel.
                    696: Implies
                    697: .Fl N ,
                    698: .Fl T ,
                    699: .Cm ExitOnForwardFailure
                    700: and
1.373     jmc       701: .Cm ClearAllForwardings ,
                    702: though these can be overridden in the configuration file or using
1.372     dtucker   703: .Fl o
                    704: command line options.
1.360     millert   705: .Pp
1.261     stevesk   706: .It Fl w Xo
                    707: .Ar local_tun Ns Op : Ns Ar remote_tun
                    708: .Xc
                    709: Requests
                    710: tunnel
                    711: device forwarding with the specified
1.218     jmc       712: .Xr tun 4
1.261     stevesk   713: devices between the client
                    714: .Pq Ar local_tun
                    715: and the server
                    716: .Pq Ar remote_tun .
                    717: .Pp
1.228     jmc       718: The devices may be specified by numerical ID or the keyword
                    719: .Dq any ,
                    720: which uses the next available tunnel device.
1.261     stevesk   721: If
                    722: .Ar remote_tun
                    723: is not specified, it defaults to
                    724: .Dq any .
1.228     jmc       725: See also the
1.218     jmc       726: .Cm Tunnel
1.261     stevesk   727: and
                    728: .Cm TunnelDevice
                    729: directives in
1.218     jmc       730: .Xr ssh_config 5 .
1.261     stevesk   731: If the
                    732: .Cm Tunnel
                    733: directive is unset, it is set to the default tunnel mode, which is
                    734: .Dq point-to-point .
1.360     millert   735: .Pp
1.218     jmc       736: .It Fl X
                    737: Enables X11 forwarding.
1.54      markus    738: This can also be specified on a per-host basis in a configuration file.
1.165     stevesk   739: .Pp
1.218     jmc       740: X11 forwarding should be enabled with caution.
1.168     jmc       741: Users with the ability to bypass file permissions on the remote host
1.218     jmc       742: (for the user's X authorization database)
                    743: can access the local X11 display through the forwarded connection.
                    744: An attacker may then be able to perform activities such as keystroke monitoring.
                    745: .Pp
                    746: For this reason, X11 forwarding is subjected to X11 SECURITY extension
                    747: restrictions by default.
                    748: Please refer to the
                    749: .Nm
                    750: .Fl Y
                    751: option and the
                    752: .Cm ForwardX11Trusted
                    753: directive in
                    754: .Xr ssh_config 5
                    755: for more information.
1.360     millert   756: .Pp
1.218     jmc       757: .It Fl x
                    758: Disables X11 forwarding.
1.360     millert   759: .Pp
1.218     jmc       760: .It Fl Y
                    761: Enables trusted X11 forwarding.
                    762: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
                    763: controls.
1.360     millert   764: .Pp
1.278     djm       765: .It Fl y
                    766: Send log information using the
                    767: .Xr syslog 3
                    768: system module.
                    769: By default this information is sent to stderr.
1.218     jmc       770: .El
1.224     jmc       771: .Pp
                    772: .Nm
                    773: may additionally obtain configuration data from
                    774: a per-user configuration file and a system-wide configuration file.
                    775: The file format and configuration options are described in
                    776: .Xr ssh_config 5 .
1.222     jmc       777: .Sh AUTHENTICATION
1.377     djm       778: The OpenSSH SSH client supports SSH protocol 2.
1.222     jmc       779: .Pp
                    780: The methods available for authentication are:
1.260     jmc       781: GSSAPI-based authentication,
1.222     jmc       782: host-based authentication,
                    783: public key authentication,
                    784: challenge-response authentication,
                    785: and password authentication.
                    786: Authentication methods are tried in the order specified above,
1.369     jmc       787: though
                    788: .Cm PreferredAuthentications
                    789: can be used to change the default order.
1.222     jmc       790: .Pp
                    791: Host-based authentication works as follows:
1.218     jmc       792: If the machine the user logs in from is listed in
                    793: .Pa /etc/hosts.equiv
                    794: or
                    795: .Pa /etc/shosts.equiv
                    796: on the remote machine, and the user names are
                    797: the same on both sides, or if the files
                    798: .Pa ~/.rhosts
                    799: or
                    800: .Pa ~/.shosts
                    801: exist in the user's home directory on the
                    802: remote machine and contain a line containing the name of the client
                    803: machine and the name of the user on that machine, the user is
1.222     jmc       804: considered for login.
                    805: Additionally, the server
                    806: .Em must
                    807: be able to verify the client's
                    808: host key (see the description of
1.218     jmc       809: .Pa /etc/ssh/ssh_known_hosts
1.189     dtucker   810: and
1.222     jmc       811: .Pa ~/.ssh/known_hosts ,
                    812: below)
                    813: for login to be permitted.
1.218     jmc       814: This authentication method closes security holes due to IP
1.222     jmc       815: spoofing, DNS spoofing, and routing spoofing.
1.218     jmc       816: [Note to the administrator:
                    817: .Pa /etc/hosts.equiv ,
                    818: .Pa ~/.rhosts ,
                    819: and the rlogin/rsh protocol in general, are inherently insecure and should be
                    820: disabled if security is desired.]
1.189     dtucker   821: .Pp
1.222     jmc       822: Public key authentication works as follows:
                    823: The scheme is based on public-key cryptography,
                    824: using cryptosystems
                    825: where encryption and decryption are done using separate keys,
                    826: and it is unfeasible to derive the decryption key from the encryption key.
1.218     jmc       827: The idea is that each user creates a public/private
                    828: key pair for authentication purposes.
                    829: The server knows the public key, and only the user knows the private key.
1.222     jmc       830: .Nm
                    831: implements public key authentication protocol automatically,
1.349     sobrado   832: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333     jmc       833: The HISTORY section of
1.222     jmc       834: .Xr ssl 8
1.311     jmc       835: contains a brief discussion of the DSA and RSA algorithms.
1.210     djm       836: .Pp
1.218     jmc       837: The file
                    838: .Pa ~/.ssh/authorized_keys
                    839: lists the public keys that are permitted for logging in.
                    840: When the user logs in, the
1.2       deraadt   841: .Nm
1.218     jmc       842: program tells the server which key pair it would like to use for
                    843: authentication.
1.222     jmc       844: The client proves that it has access to the private key
                    845: and the server checks that the corresponding public key
                    846: is authorized to accept the account.
1.218     jmc       847: .Pp
1.222     jmc       848: The user creates his/her key pair by running
1.218     jmc       849: .Xr ssh-keygen 1 .
                    850: This stores the private key in
1.222     jmc       851: .Pa ~/.ssh/id_dsa
1.369     jmc       852: (DSA),
1.310     djm       853: .Pa ~/.ssh/id_ecdsa
1.369     jmc       854: (ECDSA),
1.343     naddy     855: .Pa ~/.ssh/id_ed25519
1.369     jmc       856: (Ed25519),
1.222     jmc       857: or
                    858: .Pa ~/.ssh/id_rsa
1.369     jmc       859: (RSA)
1.218     jmc       860: and stores the public key in
1.222     jmc       861: .Pa ~/.ssh/id_dsa.pub
1.369     jmc       862: (DSA),
1.310     djm       863: .Pa ~/.ssh/id_ecdsa.pub
1.369     jmc       864: (ECDSA),
1.343     naddy     865: .Pa ~/.ssh/id_ed25519.pub
1.369     jmc       866: (Ed25519),
1.222     jmc       867: or
                    868: .Pa ~/.ssh/id_rsa.pub
1.369     jmc       869: (RSA)
1.218     jmc       870: in the user's home directory.
1.222     jmc       871: The user should then copy the public key
1.218     jmc       872: to
                    873: .Pa ~/.ssh/authorized_keys
1.222     jmc       874: in his/her home directory on the remote machine.
                    875: The
1.218     jmc       876: .Pa authorized_keys
                    877: file corresponds to the conventional
                    878: .Pa ~/.rhosts
                    879: file, and has one key
1.222     jmc       880: per line, though the lines can be very long.
1.218     jmc       881: After this, the user can log in without giving the password.
                    882: .Pp
1.301     jmc       883: A variation on public key authentication
                    884: is available in the form of certificate authentication:
                    885: instead of a set of public/private keys,
                    886: signed certificates are used.
                    887: This has the advantage that a single trusted certification authority
                    888: can be used in place of many public/private keys.
1.333     jmc       889: See the CERTIFICATES section of
1.301     jmc       890: .Xr ssh-keygen 1
                    891: for more information.
                    892: .Pp
                    893: The most convenient way to use public key or certificate authentication
                    894: may be with an authentication agent.
1.218     jmc       895: See
                    896: .Xr ssh-agent 1
1.366     jcs       897: and (optionally) the
                    898: .Cm AddKeysToAgent
                    899: directive in
                    900: .Xr ssh_config 5
1.218     jmc       901: for more information.
                    902: .Pp
1.222     jmc       903: Challenge-response authentication works as follows:
                    904: The server sends an arbitrary
                    905: .Qq challenge
                    906: text, and prompts for a response.
                    907: Examples of challenge-response authentication include
1.335     jmc       908: .Bx
                    909: Authentication (see
1.222     jmc       910: .Xr login.conf 5 )
1.335     jmc       911: and PAM (some
                    912: .Pf non- Ox
                    913: systems).
1.222     jmc       914: .Pp
                    915: Finally, if other authentication methods fail,
1.218     jmc       916: .Nm
                    917: prompts the user for a password.
                    918: The password is sent to the remote
                    919: host for checking; however, since all communications are encrypted,
                    920: the password cannot be seen by someone listening on the network.
1.232     jmc       921: .Pp
                    922: .Nm
                    923: automatically maintains and checks a database containing
                    924: identification for all hosts it has ever been used with.
                    925: Host keys are stored in
                    926: .Pa ~/.ssh/known_hosts
                    927: in the user's home directory.
                    928: Additionally, the file
                    929: .Pa /etc/ssh/ssh_known_hosts
                    930: is automatically checked for known hosts.
                    931: Any new hosts are automatically added to the user's file.
                    932: If a host's identification ever changes,
                    933: .Nm
                    934: warns about this and disables password authentication to prevent
                    935: server spoofing or man-in-the-middle attacks,
                    936: which could otherwise be used to circumvent the encryption.
                    937: The
                    938: .Cm StrictHostKeyChecking
                    939: option can be used to control logins to machines whose
                    940: host key is not known or has changed.
                    941: .Pp
1.218     jmc       942: When the user's identity has been accepted by the server, the server
1.357     dtucker   943: either executes the given command in a non-interactive session or,
                    944: if no command has been specified, logs into the machine and gives
                    945: the user a normal shell as an interactive session.
1.218     jmc       946: All communication with
                    947: the remote command or shell will be automatically encrypted.
                    948: .Pp
1.357     dtucker   949: If an interactive session is requested
                    950: .Nm
                    951: by default will only request a pseudo-terminal (pty) for interactive
                    952: sessions when the client has one.
                    953: The flags
                    954: .Fl T
                    955: and
                    956: .Fl t
                    957: can be used to override this behaviour.
                    958: .Pp
                    959: If a pseudo-terminal has been allocated the
1.218     jmc       960: user may use the escape characters noted below.
                    961: .Pp
1.357     dtucker   962: If no pseudo-terminal has been allocated,
1.218     jmc       963: the session is transparent and can be used to reliably transfer binary data.
                    964: On most systems, setting the escape character to
                    965: .Dq none
                    966: will also make the session transparent even if a tty is used.
                    967: .Pp
                    968: The session terminates when the command or shell on the remote
1.247     jmc       969: machine exits and all X11 and TCP connections have been closed.
1.223     jmc       970: .Sh ESCAPE CHARACTERS
1.218     jmc       971: When a pseudo-terminal has been requested,
1.2       deraadt   972: .Nm
1.218     jmc       973: supports a number of functions through the use of an escape character.
                    974: .Pp
                    975: A single tilde character can be sent as
                    976: .Ic ~~
                    977: or by following the tilde by a character other than those described below.
                    978: The escape character must always follow a newline to be interpreted as
                    979: special.
                    980: The escape character can be changed in configuration files using the
                    981: .Cm EscapeChar
                    982: configuration directive or on the command line by the
                    983: .Fl e
                    984: option.
                    985: .Pp
                    986: The supported escapes (assuming the default
                    987: .Ql ~ )
                    988: are:
                    989: .Bl -tag -width Ds
                    990: .It Cm ~.
                    991: Disconnect.
                    992: .It Cm ~^Z
                    993: Background
1.234     jmc       994: .Nm .
1.218     jmc       995: .It Cm ~#
                    996: List forwarded connections.
                    997: .It Cm ~&
                    998: Background
1.2       deraadt   999: .Nm
1.218     jmc      1000: at logout when waiting for forwarded connection / X11 sessions to terminate.
                   1001: .It Cm ~?
                   1002: Display a list of escape characters.
                   1003: .It Cm ~B
                   1004: Send a BREAK to the remote system
1.369     jmc      1005: (only useful if the peer supports it).
1.218     jmc      1006: .It Cm ~C
                   1007: Open command line.
                   1008: Currently this allows the addition of port forwardings using the
1.279     stevesk  1009: .Fl L ,
                   1010: .Fl R
1.218     jmc      1011: and
1.279     stevesk  1012: .Fl D
1.225     jmc      1013: options (see above).
1.322     markus   1014: It also allows the cancellation of existing port-forwardings
                   1015: with
1.262     stevesk  1016: .Sm off
1.322     markus   1017: .Fl KL Oo Ar bind_address : Oc Ar port
1.262     stevesk  1018: .Sm on
1.322     markus   1019: for local,
                   1020: .Sm off
                   1021: .Fl KR Oo Ar bind_address : Oc Ar port
                   1022: .Sm on
                   1023: for remote and
                   1024: .Sm off
                   1025: .Fl KD Oo Ar bind_address : Oc Ar port
                   1026: .Sm on
                   1027: for dynamic port-forwardings.
1.218     jmc      1028: .Ic !\& Ns Ar command
                   1029: allows the user to execute a local command if the
                   1030: .Ic PermitLocalCommand
                   1031: option is enabled in
1.176     jmc      1032: .Xr ssh_config 5 .
1.218     jmc      1033: Basic help is available, using the
                   1034: .Fl h
                   1035: option.
                   1036: .It Cm ~R
                   1037: Request rekeying of the connection
1.369     jmc      1038: (only useful if the peer supports it).
1.327     dtucker  1039: .It Cm ~V
                   1040: Decrease the verbosity
                   1041: .Pq Ic LogLevel
                   1042: when errors are being written to stderr.
                   1043: .It Cm ~v
1.328     jmc      1044: Increase the verbosity
1.327     dtucker  1045: .Pq Ic LogLevel
                   1046: when errors are being written to stderr.
1.176     jmc      1047: .El
1.246     jmc      1048: .Sh TCP FORWARDING
                   1049: Forwarding of arbitrary TCP connections over the secure channel can
                   1050: be specified either on the command line or in a configuration file.
                   1051: One possible application of TCP forwarding is a secure connection to a
                   1052: mail server; another is going through firewalls.
                   1053: .Pp
                   1054: In the example below, we look at encrypting communication between
                   1055: an IRC client and server, even though the IRC server does not directly
                   1056: support encrypted communications.
                   1057: This works as follows:
                   1058: the user connects to the remote host using
                   1059: .Nm ,
                   1060: specifying a port to be used to forward connections
                   1061: to the remote server.
                   1062: After that it is possible to start the service which is to be encrypted
                   1063: on the client machine,
                   1064: connecting to the same local port,
                   1065: and
                   1066: .Nm
                   1067: will encrypt and forward the connection.
                   1068: .Pp
                   1069: The following example tunnels an IRC session from client machine
                   1070: .Dq 127.0.0.1
                   1071: (localhost)
                   1072: to remote server
                   1073: .Dq server.example.com :
                   1074: .Bd -literal -offset 4n
                   1075: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
                   1076: $ irc -c '#users' -p 1234 pinky 127.0.0.1
                   1077: .Ed
                   1078: .Pp
                   1079: This tunnels a connection to IRC server
                   1080: .Dq server.example.com ,
                   1081: joining channel
                   1082: .Dq #users ,
                   1083: nickname
                   1084: .Dq pinky ,
                   1085: using port 1234.
                   1086: It doesn't matter which port is used,
                   1087: as long as it's greater than 1023
                   1088: (remember, only root can open sockets on privileged ports)
                   1089: and doesn't conflict with any ports already in use.
                   1090: The connection is forwarded to port 6667 on the remote server,
                   1091: since that's the standard port for IRC services.
                   1092: .Pp
                   1093: The
                   1094: .Fl f
                   1095: option backgrounds
                   1096: .Nm
                   1097: and the remote command
                   1098: .Dq sleep 10
                   1099: is specified to allow an amount of time
                   1100: (10 seconds, in the example)
                   1101: to start the service which is to be tunnelled.
                   1102: If no connections are made within the time specified,
                   1103: .Nm
                   1104: will exit.
                   1105: .Sh X11 FORWARDING
1.218     jmc      1106: If the
                   1107: .Cm ForwardX11
                   1108: variable is set to
                   1109: .Dq yes
                   1110: (or see the description of the
1.227     jmc      1111: .Fl X ,
                   1112: .Fl x ,
1.218     jmc      1113: and
1.227     jmc      1114: .Fl Y
1.226     jmc      1115: options above)
1.218     jmc      1116: and the user is using X11 (the
                   1117: .Ev DISPLAY
                   1118: environment variable is set), the connection to the X11 display is
                   1119: automatically forwarded to the remote side in such a way that any X11
                   1120: programs started from the shell (or command) will go through the
                   1121: encrypted channel, and the connection to the real X server will be made
1.176     jmc      1122: from the local machine.
1.218     jmc      1123: The user should not manually set
                   1124: .Ev DISPLAY .
                   1125: Forwarding of X11 connections can be
                   1126: configured on the command line or in configuration files.
                   1127: .Pp
                   1128: The
                   1129: .Ev DISPLAY
                   1130: value set by
                   1131: .Nm
                   1132: will point to the server machine, but with a display number greater than zero.
                   1133: This is normal, and happens because
                   1134: .Nm
                   1135: creates a
                   1136: .Dq proxy
                   1137: X server on the server machine for forwarding the
                   1138: connections over the encrypted channel.
1.200     djm      1139: .Pp
1.218     jmc      1140: .Nm
                   1141: will also automatically set up Xauthority data on the server machine.
                   1142: For this purpose, it will generate a random authorization cookie,
                   1143: store it in Xauthority on the server, and verify that any forwarded
                   1144: connections carry this cookie and replace it by the real cookie when
                   1145: the connection is opened.
                   1146: The real authentication cookie is never
                   1147: sent to the server machine (and no cookies are sent in the plain).
1.200     djm      1148: .Pp
1.218     jmc      1149: If the
                   1150: .Cm ForwardAgent
                   1151: variable is set to
                   1152: .Dq yes
                   1153: (or see the description of the
                   1154: .Fl A
1.191     djm      1155: and
1.218     jmc      1156: .Fl a
1.226     jmc      1157: options above) and
1.218     jmc      1158: the user is using an authentication agent, the connection to the agent
                   1159: is automatically forwarded to the remote side.
1.252     jmc      1160: .Sh VERIFYING HOST KEYS
                   1161: When connecting to a server for the first time,
                   1162: a fingerprint of the server's public key is presented to the user
                   1163: (unless the option
                   1164: .Cm StrictHostKeyChecking
                   1165: has been disabled).
                   1166: Fingerprints can be determined using
                   1167: .Xr ssh-keygen 1 :
                   1168: .Pp
                   1169: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
                   1170: .Pp
1.274     grunk    1171: If the fingerprint is already known, it can be matched
                   1172: and the key can be accepted or rejected.
1.358     djm      1173: If only legacy (MD5) fingerprints for the server are available, the
                   1174: .Xr ssh-keygen 1
                   1175: .Fl E
                   1176: option may be used to downgrade the fingerprint algorithm to match.
                   1177: .Pp
1.274     grunk    1178: Because of the difficulty of comparing host keys
1.352     djm      1179: just by looking at fingerprint strings,
1.274     grunk    1180: there is also support to compare host keys visually,
                   1181: using
                   1182: .Em random art .
                   1183: By setting the
1.275     grunk    1184: .Cm VisualHostKey
1.274     grunk    1185: option to
1.275     grunk    1186: .Dq yes ,
1.274     grunk    1187: a small ASCII graphic gets displayed on every login to a server, no matter
                   1188: if the session itself is interactive or not.
                   1189: By learning the pattern a known server produces, a user can easily
                   1190: find out that the host key has changed when a completely different pattern
                   1191: is displayed.
                   1192: Because these patterns are not unambiguous however, a pattern that looks
                   1193: similar to the pattern remembered only gives a good probability that the
                   1194: host key is the same, not guaranteed proof.
                   1195: .Pp
                   1196: To get a listing of the fingerprints along with their random art for
                   1197: all known hosts, the following command line can be used:
                   1198: .Pp
                   1199: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
                   1200: .Pp
1.252     jmc      1201: If the fingerprint is unknown,
                   1202: an alternative method of verification is available:
                   1203: SSH fingerprints verified by DNS.
                   1204: An additional resource record (RR),
                   1205: SSHFP,
                   1206: is added to a zonefile
                   1207: and the connecting client is able to match the fingerprint
                   1208: with that of the key presented.
                   1209: .Pp
                   1210: In this example, we are connecting a client to a server,
                   1211: .Dq host.example.com .
                   1212: The SSHFP resource records should first be added to the zonefile for
                   1213: host.example.com:
                   1214: .Bd -literal -offset indent
1.259     jakob    1215: $ ssh-keygen -r host.example.com.
1.252     jmc      1216: .Ed
                   1217: .Pp
                   1218: The output lines will have to be added to the zonefile.
                   1219: To check that the zone is answering fingerprint queries:
                   1220: .Pp
                   1221: .Dl $ dig -t SSHFP host.example.com
                   1222: .Pp
                   1223: Finally the client connects:
                   1224: .Bd -literal -offset indent
                   1225: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
                   1226: [...]
                   1227: Matching host key fingerprint found in DNS.
                   1228: Are you sure you want to continue connecting (yes/no)?
                   1229: .Ed
                   1230: .Pp
                   1231: See the
                   1232: .Cm VerifyHostKeyDNS
                   1233: option in
                   1234: .Xr ssh_config 5
                   1235: for more information.
1.250     jmc      1236: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
                   1237: .Nm
                   1238: contains support for Virtual Private Network (VPN) tunnelling
                   1239: using the
                   1240: .Xr tun 4
                   1241: network pseudo-device,
                   1242: allowing two networks to be joined securely.
                   1243: The
                   1244: .Xr sshd_config 5
                   1245: configuration option
                   1246: .Cm PermitTunnel
                   1247: controls whether the server supports this,
                   1248: and at what level (layer 2 or 3 traffic).
                   1249: .Pp
                   1250: The following example would connect client network 10.0.50.0/24
1.265     otto     1251: with remote network 10.0.99.0/24 using a point-to-point connection
                   1252: from 10.1.1.1 to 10.1.1.2,
                   1253: provided that the SSH server running on the gateway to the remote network,
                   1254: at 192.168.1.15, allows it.
                   1255: .Pp
                   1256: On the client:
1.250     jmc      1257: .Bd -literal -offset indent
                   1258: # ssh -f -w 0:1 192.168.1.15 true
1.265     otto     1259: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
                   1260: # route add 10.0.99.0/24 10.1.1.2
                   1261: .Ed
                   1262: .Pp
                   1263: On the server:
                   1264: .Bd -literal -offset indent
                   1265: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
                   1266: # route add 10.0.50.0/24 10.1.1.1
1.250     jmc      1267: .Ed
                   1268: .Pp
                   1269: Client access may be more finely tuned via the
                   1270: .Pa /root/.ssh/authorized_keys
                   1271: file (see below) and the
                   1272: .Cm PermitRootLogin
                   1273: server option.
1.255     jmc      1274: The following entry would permit connections on
1.250     jmc      1275: .Xr tun 4
1.255     jmc      1276: device 1 from user
1.250     jmc      1277: .Dq jane
1.255     jmc      1278: and on tun device 2 from user
1.250     jmc      1279: .Dq john ,
                   1280: if
                   1281: .Cm PermitRootLogin
                   1282: is set to
                   1283: .Dq forced-commands-only :
                   1284: .Bd -literal -offset 2n
                   1285: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254     msf      1286: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250     jmc      1287: .Ed
                   1288: .Pp
1.264     ray      1289: Since an SSH-based setup entails a fair amount of overhead,
1.250     jmc      1290: it may be more suited to temporary setups,
                   1291: such as for wireless VPNs.
                   1292: More permanent VPNs are better provided by tools such as
                   1293: .Xr ipsecctl 8
                   1294: and
                   1295: .Xr isakmpd 8 .
1.2       deraadt  1296: .Sh ENVIRONMENT
                   1297: .Nm
1.1       deraadt  1298: will normally set the following environment variables:
1.237     jmc      1299: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2       deraadt  1300: .It Ev DISPLAY
                   1301: The
                   1302: .Ev DISPLAY
1.40      aaron    1303: variable indicates the location of the X11 server.
1.44      aaron    1304: It is automatically set by
1.2       deraadt  1305: .Nm
                   1306: to point to a value of the form
1.233     jmc      1307: .Dq hostname:n ,
                   1308: where
                   1309: .Dq hostname
                   1310: indicates the host where the shell runs, and
                   1311: .Sq n
                   1312: is an integer \*(Ge 1.
1.40      aaron    1313: .Nm
                   1314: uses this special value to forward X11 connections over the secure
                   1315: channel.
1.107     markus   1316: The user should normally not set
                   1317: .Ev DISPLAY
                   1318: explicitly, as that
1.1       deraadt  1319: will render the X11 connection insecure (and will require the user to
                   1320: manually copy any required authorization cookies).
1.2       deraadt  1321: .It Ev HOME
1.1       deraadt  1322: Set to the path of the user's home directory.
1.2       deraadt  1323: .It Ev LOGNAME
                   1324: Synonym for
1.12      aaron    1325: .Ev USER ;
                   1326: set for compatibility with systems that use this variable.
1.2       deraadt  1327: .It Ev MAIL
1.129     stevesk  1328: Set to the path of the user's mailbox.
1.40      aaron    1329: .It Ev PATH
1.2       deraadt  1330: Set to the default
                   1331: .Ev PATH ,
                   1332: as specified when compiling
1.234     jmc      1333: .Nm .
1.118     markus   1334: .It Ev SSH_ASKPASS
                   1335: If
                   1336: .Nm
                   1337: needs a passphrase, it will read the passphrase from the current
                   1338: terminal if it was run from a terminal.
                   1339: If
                   1340: .Nm
                   1341: does not have a terminal associated with it but
                   1342: .Ev DISPLAY
                   1343: and
                   1344: .Ev SSH_ASKPASS
                   1345: are set, it will execute the program specified by
                   1346: .Ev SSH_ASKPASS
                   1347: and open an X11 window to read the passphrase.
                   1348: This is particularly useful when calling
                   1349: .Nm
                   1350: from a
1.196     jmc      1351: .Pa .xsession
1.118     markus   1352: or related script.
                   1353: (Note that on some machines it
                   1354: may be necessary to redirect the input from
                   1355: .Pa /dev/null
                   1356: to make this work.)
1.18      markus   1357: .It Ev SSH_AUTH_SOCK
1.233     jmc      1358: Identifies the path of a
                   1359: .Ux Ns -domain
                   1360: socket used to communicate with the agent.
1.166     stevesk  1361: .It Ev SSH_CONNECTION
                   1362: Identifies the client and server ends of the connection.
1.40      aaron    1363: The variable contains
1.233     jmc      1364: four space-separated values: client IP address, client port number,
                   1365: server IP address, and server port number.
1.73      markus   1366: .It Ev SSH_ORIGINAL_COMMAND
1.233     jmc      1367: This variable contains the original command line if a forced command
1.73      markus   1368: is executed.
                   1369: It can be used to extract the original arguments.
1.2       deraadt  1370: .It Ev SSH_TTY
1.1       deraadt  1371: This is set to the name of the tty (path to the device) associated
1.40      aaron    1372: with the current shell or command.
                   1373: If the current session has no tty,
1.1       deraadt  1374: this variable is not set.
1.2       deraadt  1375: .It Ev TZ
1.214     jmc      1376: This variable is set to indicate the present time zone if it
1.257     jmc      1377: was set when the daemon was started (i.e. the daemon passes the value
1.1       deraadt  1378: on to new connections).
1.2       deraadt  1379: .It Ev USER
1.1       deraadt  1380: Set to the name of the user logging in.
1.2       deraadt  1381: .El
                   1382: .Pp
1.44      aaron    1383: Additionally,
1.2       deraadt  1384: .Nm
1.44      aaron    1385: reads
1.207     djm      1386: .Pa ~/.ssh/environment ,
1.2       deraadt  1387: and adds lines of the format
                   1388: .Dq VARNAME=value
1.233     jmc      1389: to the environment if the file exists and users are allowed to
1.161     marc     1390: change their environment.
1.176     jmc      1391: For more information, see the
1.161     marc     1392: .Cm PermitUserEnvironment
1.162     stevesk  1393: option in
1.161     marc     1394: .Xr sshd_config 5 .
1.2       deraadt  1395: .Sh FILES
1.236     jmc      1396: .Bl -tag -width Ds -compact
1.309     jmc      1397: .It Pa ~/.rhosts
1.240     jmc      1398: This file is used for host-based authentication (see above).
1.92      markus   1399: On some machines this file may need to be
1.240     jmc      1400: world-readable if the user's home directory is on an NFS partition,
1.1       deraadt  1401: because
1.2       deraadt  1402: .Xr sshd 8
1.40      aaron    1403: reads it as root.
                   1404: Additionally, this file must be owned by the user,
                   1405: and must not have write permissions for anyone else.
                   1406: The recommended
1.1       deraadt  1407: permission for most machines is read/write for the user, and not
                   1408: accessible by others.
1.2       deraadt  1409: .Pp
1.309     jmc      1410: .It Pa ~/.shosts
1.240     jmc      1411: This file is used in exactly the same way as
                   1412: .Pa .rhosts ,
                   1413: but allows host-based authentication without permitting login with
                   1414: rlogin/rsh.
1.272     mcbride  1415: .Pp
1.309     jmc      1416: .It Pa ~/.ssh/
1.272     mcbride  1417: This directory is the default location for all user-specific configuration
                   1418: and authentication information.
                   1419: There is no general requirement to keep the entire contents of this directory
                   1420: secret, but the recommended permissions are read/write/execute for the user,
                   1421: and not accessible by others.
1.236     jmc      1422: .Pp
1.309     jmc      1423: .It Pa ~/.ssh/authorized_keys
1.349     sobrado  1424: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343     naddy    1425: that can be used for logging in as this user.
1.238     jmc      1426: The format of this file is described in the
                   1427: .Xr sshd 8
                   1428: manual page.
                   1429: This file is not highly sensitive, but the recommended
                   1430: permissions are read/write for the user, and not accessible by others.
                   1431: .Pp
1.309     jmc      1432: .It Pa ~/.ssh/config
1.238     jmc      1433: This is the per-user configuration file.
                   1434: The file format and configuration options are described in
                   1435: .Xr ssh_config 5 .
                   1436: Because of the potential for abuse, this file must have strict permissions:
1.334     djm      1437: read/write for the user, and not writable by others.
1.238     jmc      1438: .Pp
1.309     jmc      1439: .It Pa ~/.ssh/environment
1.239     jmc      1440: Contains additional definitions for environment variables; see
                   1441: .Sx ENVIRONMENT ,
1.238     jmc      1442: above.
                   1443: .Pp
1.309     jmc      1444: .It Pa ~/.ssh/id_dsa
1.310     djm      1445: .It Pa ~/.ssh/id_ecdsa
1.343     naddy    1446: .It Pa ~/.ssh/id_ed25519
1.309     jmc      1447: .It Pa ~/.ssh/id_rsa
1.238     jmc      1448: Contains the private key for authentication.
                   1449: These files
                   1450: contain sensitive data and should be readable by the user but not
                   1451: accessible by others (read/write/execute).
                   1452: .Nm
                   1453: will simply ignore a private key file if it is accessible by others.
                   1454: It is possible to specify a passphrase when
                   1455: generating the key which will be used to encrypt the
                   1456: sensitive part of this file using 3DES.
                   1457: .Pp
1.309     jmc      1458: .It Pa ~/.ssh/id_dsa.pub
1.310     djm      1459: .It Pa ~/.ssh/id_ecdsa.pub
1.343     naddy    1460: .It Pa ~/.ssh/id_ed25519.pub
1.309     jmc      1461: .It Pa ~/.ssh/id_rsa.pub
1.238     jmc      1462: Contains the public key for authentication.
                   1463: These files are not
                   1464: sensitive and can (but need not) be readable by anyone.
                   1465: .Pp
1.309     jmc      1466: .It Pa ~/.ssh/known_hosts
1.244     jmc      1467: Contains a list of host keys for all hosts the user has logged into
                   1468: that are not already in the systemwide list of known host keys.
1.238     jmc      1469: See
1.244     jmc      1470: .Xr sshd 8
                   1471: for further details of the format of this file.
1.238     jmc      1472: .Pp
1.309     jmc      1473: .It Pa ~/.ssh/rc
1.238     jmc      1474: Commands in this file are executed by
                   1475: .Nm
1.245     jmc      1476: when the user logs in, just before the user's shell (or command) is
1.238     jmc      1477: started.
                   1478: See the
                   1479: .Xr sshd 8
                   1480: manual page for more information.
                   1481: .Pp
1.309     jmc      1482: .It Pa /etc/hosts.equiv
1.240     jmc      1483: This file is for host-based authentication (see above).
                   1484: It should only be writable by root.
1.236     jmc      1485: .Pp
1.309     jmc      1486: .It Pa /etc/shosts.equiv
1.240     jmc      1487: This file is used in exactly the same way as
                   1488: .Pa hosts.equiv ,
                   1489: but allows host-based authentication without permitting login with
                   1490: rlogin/rsh.
1.236     jmc      1491: .Pp
1.238     jmc      1492: .It Pa /etc/ssh/ssh_config
                   1493: Systemwide configuration file.
                   1494: The file format and configuration options are described in
                   1495: .Xr ssh_config 5 .
                   1496: .Pp
1.309     jmc      1497: .It Pa /etc/ssh/ssh_host_key
                   1498: .It Pa /etc/ssh/ssh_host_dsa_key
1.310     djm      1499: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343     naddy    1500: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309     jmc      1501: .It Pa /etc/ssh/ssh_host_rsa_key
1.325     dtucker  1502: These files contain the private parts of the host keys
1.245     jmc      1503: and are used for host-based authentication.
1.238     jmc      1504: .Pp
1.309     jmc      1505: .It Pa /etc/ssh/ssh_known_hosts
1.238     jmc      1506: Systemwide list of known host keys.
                   1507: This file should be prepared by the
                   1508: system administrator to contain the public host keys of all machines in the
                   1509: organization.
1.244     jmc      1510: It should be world-readable.
                   1511: See
1.238     jmc      1512: .Xr sshd 8
1.244     jmc      1513: for further details of the format of this file.
1.236     jmc      1514: .Pp
1.309     jmc      1515: .It Pa /etc/ssh/sshrc
1.1       deraadt  1516: Commands in this file are executed by
1.2       deraadt  1517: .Nm
1.245     jmc      1518: when the user logs in, just before the user's shell (or command) is started.
1.44      aaron    1519: See the
1.2       deraadt  1520: .Xr sshd 8
1.1       deraadt  1521: manual page for more information.
1.58      itojun   1522: .El
1.312     jmc      1523: .Sh EXIT STATUS
                   1524: .Nm
                   1525: exits with the exit status of the remote command or with 255
                   1526: if an error occurred.
1.2       deraadt  1527: .Sh SEE ALSO
                   1528: .Xr scp 1 ,
1.83      djm      1529: .Xr sftp 1 ,
1.2       deraadt  1530: .Xr ssh-add 1 ,
                   1531: .Xr ssh-agent 1 ,
                   1532: .Xr ssh-keygen 1 ,
1.242     jmc      1533: .Xr ssh-keyscan 1 ,
1.250     jmc      1534: .Xr tun 4 ,
1.159     stevesk  1535: .Xr ssh_config 5 ,
1.160     naddy    1536: .Xr ssh-keysign 8 ,
1.87      itojun   1537: .Xr sshd 8
1.329     jmc      1538: .Sh STANDARDS
1.106     markus   1539: .Rs
1.329     jmc      1540: .%A S. Lehtinen
                   1541: .%A C. Lonvick
                   1542: .%D January 2006
1.256     jmc      1543: .%R RFC 4250
1.329     jmc      1544: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256     jmc      1545: .Re
1.329     jmc      1546: .Pp
1.256     jmc      1547: .Rs
1.329     jmc      1548: .%A T. Ylonen
                   1549: .%A C. Lonvick
                   1550: .%D January 2006
1.256     jmc      1551: .%R RFC 4251
1.329     jmc      1552: .%T The Secure Shell (SSH) Protocol Architecture
1.256     jmc      1553: .Re
1.329     jmc      1554: .Pp
1.256     jmc      1555: .Rs
1.329     jmc      1556: .%A T. Ylonen
                   1557: .%A C. Lonvick
                   1558: .%D January 2006
1.256     jmc      1559: .%R RFC 4252
1.329     jmc      1560: .%T The Secure Shell (SSH) Authentication Protocol
1.256     jmc      1561: .Re
1.329     jmc      1562: .Pp
1.256     jmc      1563: .Rs
1.329     jmc      1564: .%A T. Ylonen
                   1565: .%A C. Lonvick
                   1566: .%D January 2006
1.256     jmc      1567: .%R RFC 4253
1.329     jmc      1568: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256     jmc      1569: .Re
1.329     jmc      1570: .Pp
1.256     jmc      1571: .Rs
1.329     jmc      1572: .%A T. Ylonen
                   1573: .%A C. Lonvick
                   1574: .%D January 2006
1.256     jmc      1575: .%R RFC 4254
1.329     jmc      1576: .%T The Secure Shell (SSH) Connection Protocol
1.256     jmc      1577: .Re
1.329     jmc      1578: .Pp
1.256     jmc      1579: .Rs
1.329     jmc      1580: .%A J. Schlyter
                   1581: .%A W. Griffin
                   1582: .%D January 2006
1.256     jmc      1583: .%R RFC 4255
1.329     jmc      1584: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256     jmc      1585: .Re
1.329     jmc      1586: .Pp
1.256     jmc      1587: .Rs
1.329     jmc      1588: .%A F. Cusack
                   1589: .%A M. Forssen
                   1590: .%D January 2006
1.256     jmc      1591: .%R RFC 4256
1.329     jmc      1592: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256     jmc      1593: .Re
1.329     jmc      1594: .Pp
1.256     jmc      1595: .Rs
1.329     jmc      1596: .%A J. Galbraith
                   1597: .%A P. Remaker
                   1598: .%D January 2006
1.256     jmc      1599: .%R RFC 4335
1.329     jmc      1600: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256     jmc      1601: .Re
1.329     jmc      1602: .Pp
1.256     jmc      1603: .Rs
1.329     jmc      1604: .%A M. Bellare
                   1605: .%A T. Kohno
                   1606: .%A C. Namprempre
                   1607: .%D January 2006
1.256     jmc      1608: .%R RFC 4344
1.329     jmc      1609: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256     jmc      1610: .Re
1.329     jmc      1611: .Pp
1.256     jmc      1612: .Rs
1.329     jmc      1613: .%A B. Harris
                   1614: .%D January 2006
1.256     jmc      1615: .%R RFC 4345
1.329     jmc      1616: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258     djm      1617: .Re
1.329     jmc      1618: .Pp
1.258     djm      1619: .Rs
1.329     jmc      1620: .%A M. Friedl
                   1621: .%A N. Provos
                   1622: .%A W. Simpson
                   1623: .%D March 2006
1.258     djm      1624: .%R RFC 4419
1.329     jmc      1625: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266     markus   1626: .Re
1.329     jmc      1627: .Pp
1.266     markus   1628: .Rs
1.329     jmc      1629: .%A J. Galbraith
                   1630: .%A R. Thayer
                   1631: .%D November 2006
1.266     markus   1632: .%R RFC 4716
1.329     jmc      1633: .%T The Secure Shell (SSH) Public Key File Format
1.313     djm      1634: .Re
1.329     jmc      1635: .Pp
1.313     djm      1636: .Rs
1.329     jmc      1637: .%A D. Stebila
                   1638: .%A J. Green
                   1639: .%D December 2009
1.313     djm      1640: .%R RFC 5656
1.329     jmc      1641: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274     grunk    1642: .Re
1.329     jmc      1643: .Pp
1.274     grunk    1644: .Rs
                   1645: .%A A. Perrig
                   1646: .%A D. Song
                   1647: .%D 1999
1.329     jmc      1648: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
                   1649: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106     markus   1650: .Re
1.173     jmc      1651: .Sh AUTHORS
                   1652: OpenSSH is a derivative of the original and free
                   1653: ssh 1.2.12 release by Tatu Ylonen.
                   1654: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
                   1655: Theo de Raadt and Dug Song
                   1656: removed many bugs, re-added newer features and
                   1657: created OpenSSH.
                   1658: Markus Friedl contributed the support for SSH
                   1659: protocol versions 1.5 and 2.0.