Annotation of src/usr.bin/ssh/ssh.1, Revision 1.391
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: .\"
1.391 ! jmc 36: .\" $OpenBSD: ssh.1,v 1.390 2018/02/23 02:34:33 djm Exp $
! 37: .Dd $Mdocdate: February 23 2018 $
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
1.377 djm 45: .Op Fl 46AaCfGgKkMNnqsTtVvXxYy
1.390 djm 46: .Op Fl B Ar bind_interface
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
1.386 millert 55: .Op Fl J Ar destination
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
1.386 millert 67: .Ar destination
1.2 deraadt 68: .Op Ar command
1.44 aaron 69: .Sh DESCRIPTION
1.2 deraadt 70: .Nm
1.96 deraadt 71: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 72: executing commands on a remote machine.
1.365 mmcc 73: It is intended to provide secure encrypted communications between
1.40 aaron 74: two untrusted hosts over an insecure network.
1.348 millert 75: X11 connections, arbitrary TCP ports and
76: .Ux Ns -domain
77: sockets can also be forwarded over the secure channel.
1.2 deraadt 78: .Pp
79: .Nm
1.44 aaron 80: connects and logs into the specified
1.388 jmc 81: .Ar destination ,
1.386 millert 82: which may be specified as either
1.388 jmc 83: .Sm off
84: .Oo user @ Oc hostname
85: .Sm on
86: or a URI of the form
87: .Sm off
88: .No ssh:// Oo user @ Oc hostname Op : port .
89: .Sm on
1.1 deraadt 90: The user must prove
1.49 markus 91: his/her identity to the remote machine using one of several methods
1.365 mmcc 92: (see below).
1.49 markus 93: .Pp
1.386 millert 94: If a
1.176 jmc 95: .Ar command
96: is specified,
1.219 jmc 97: it is executed on the remote host instead of a login shell.
1.2 deraadt 98: .Pp
1.218 jmc 99: The options are as follows:
1.361 millert 100: .Pp
1.360 millert 101: .Bl -tag -width Ds -compact
1.218 jmc 102: .It Fl 4
103: Forces
1.2 deraadt 104: .Nm
1.218 jmc 105: to use IPv4 addresses only.
1.360 millert 106: .Pp
1.218 jmc 107: .It Fl 6
108: Forces
1.2 deraadt 109: .Nm
1.218 jmc 110: to use IPv6 addresses only.
1.360 millert 111: .Pp
1.218 jmc 112: .It Fl A
113: Enables forwarding of the authentication agent connection.
114: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 115: .Pp
1.218 jmc 116: Agent forwarding should be enabled with caution.
117: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 118: (for the agent's
1.286 sobrado 119: .Ux Ns -domain
120: socket) can access the local agent through the forwarded connection.
1.218 jmc 121: An attacker cannot obtain key material from the agent,
122: however they can perform operations on the keys that enable them to
123: authenticate using the identities loaded into the agent.
1.360 millert 124: .Pp
1.218 jmc 125: .It Fl a
126: Disables forwarding of the authentication agent connection.
1.390 djm 127: .Pp
1.391 ! jmc 128: .It Fl B Ar bind_interface
1.390 djm 129: Bind to the address of
1.391 ! jmc 130: .Ar bind_interface
1.390 djm 131: before attempting to connect to the destination host.
132: This is only useful on systems with more than one address.
1.360 millert 133: .Pp
1.218 jmc 134: .It Fl b Ar bind_address
135: Use
136: .Ar bind_address
137: on the local machine as the source address
138: of the connection.
139: Only useful on systems with more than one address.
1.360 millert 140: .Pp
1.218 jmc 141: .It Fl C
142: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 143: data for forwarded X11, TCP and
144: .Ux Ns -domain
145: connections).
1.218 jmc 146: The compression algorithm is the same used by
1.377 djm 147: .Xr gzip 1 .
1.218 jmc 148: Compression is desirable on modem lines and other
149: slow connections, but will only slow down things on fast networks.
150: The default value can be set on a host-by-host basis in the
151: configuration files; see the
152: .Cm Compression
153: option.
1.360 millert 154: .Pp
1.218 jmc 155: .It Fl c Ar cipher_spec
156: Selects the cipher specification for encrypting the session.
157: .Ar cipher_spec
158: is a comma-separated list of ciphers
159: listed in order of preference.
1.283 jmc 160: See the
161: .Cm Ciphers
1.307 dtucker 162: keyword in
163: .Xr ssh_config 5
164: for more information.
1.360 millert 165: .Pp
1.218 jmc 166: .It Fl D Xo
167: .Sm off
168: .Oo Ar bind_address : Oc
169: .Ar port
170: .Sm on
171: .Xc
172: Specifies a local
173: .Dq dynamic
174: application-level port forwarding.
175: This works by allocating a socket to listen to
176: .Ar port
177: on the local side, optionally bound to the specified
178: .Ar bind_address .
179: Whenever a connection is made to this port, the
180: connection is forwarded over the secure channel, and the application
181: protocol is then used to determine where to connect to from the
182: remote machine.
183: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 184: .Nm
1.218 jmc 185: will act as a SOCKS server.
186: Only root can forward privileged ports.
187: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 188: .Pp
1.308 djm 189: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 190: Only the superuser can forward privileged ports.
191: By default, the local port is bound in accordance with the
192: .Cm GatewayPorts
193: setting.
194: However, an explicit
195: .Ar bind_address
196: may be used to bind the connection to a specific address.
197: The
198: .Ar bind_address
199: of
200: .Dq localhost
201: indicates that the listening port be bound for local use only, while an
202: empty address or
203: .Sq *
204: indicates that the port should be available from all interfaces.
1.360 millert 205: .Pp
1.331 dtucker 206: .It Fl E Ar log_file
207: Append debug logs to
208: .Ar log_file
209: instead of standard error.
1.360 millert 210: .Pp
1.229 jmc 211: .It Fl e Ar escape_char
1.218 jmc 212: Sets the escape character for sessions with a pty (default:
213: .Ql ~ ) .
214: The escape character is only recognized at the beginning of a line.
215: The escape character followed by a dot
216: .Pq Ql \&.
217: closes the connection;
218: followed by control-Z suspends the connection;
219: and followed by itself sends the escape character once.
220: Setting the character to
1.2 deraadt 221: .Dq none
1.218 jmc 222: disables any escapes and makes the session fully transparent.
1.360 millert 223: .Pp
1.218 jmc 224: .It Fl F Ar configfile
225: Specifies an alternative per-user configuration file.
226: If a configuration file is given on the command line,
227: the system-wide configuration file
228: .Pq Pa /etc/ssh/ssh_config
229: will be ignored.
230: The default for the per-user configuration file is
231: .Pa ~/.ssh/config .
1.360 millert 232: .Pp
1.218 jmc 233: .It Fl f
234: Requests
235: .Nm
236: to go to background just before command execution.
237: This is useful if
1.176 jmc 238: .Nm
1.218 jmc 239: is going to ask for passwords or passphrases, but the user
240: wants it in the background.
241: This implies
242: .Fl n .
243: The recommended way to start X11 programs at a remote site is with
244: something like
245: .Ic ssh -f host xterm .
1.277 djm 246: .Pp
247: If the
248: .Cm ExitOnForwardFailure
249: configuration option is set to
250: .Dq yes ,
251: then a client started with
252: .Fl f
253: will wait for all remote port forwards to be successfully established
254: before placing itself in the background.
1.360 millert 255: .Pp
1.350 djm 256: .It Fl G
257: Causes
258: .Nm
259: to print its configuration after evaluating
260: .Cm Host
261: and
262: .Cm Match
263: blocks and exit.
1.360 millert 264: .Pp
1.218 jmc 265: .It Fl g
266: Allows remote hosts to connect to local forwarded ports.
1.346 djm 267: If used on a multiplexed connection, then this option must be specified
268: on the master process.
1.360 millert 269: .Pp
1.291 markus 270: .It Fl I Ar pkcs11
1.294 jmc 271: Specify the PKCS#11 shared library
1.176 jmc 272: .Nm
1.293 markus 273: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 274: private RSA key.
1.360 millert 275: .Pp
1.218 jmc 276: .It Fl i Ar identity_file
277: Selects a file from which the identity (private key) for
1.310 djm 278: public key authentication is read.
1.218 jmc 279: The default is
1.310 djm 280: .Pa ~/.ssh/id_dsa ,
1.343 naddy 281: .Pa ~/.ssh/id_ecdsa ,
282: .Pa ~/.ssh/id_ed25519
1.310 djm 283: and
1.381 naddy 284: .Pa ~/.ssh/id_rsa .
1.218 jmc 285: Identity files may also be specified on
286: a per-host basis in the configuration file.
287: It is possible to have multiple
288: .Fl i
289: options (and multiple identities specified in
290: configuration files).
1.364 jmc 291: If no certificates have been explicitly specified by the
1.363 djm 292: .Cm CertificateFile
293: directive,
1.302 djm 294: .Nm
295: will also try to load certificate information from the filename obtained
296: by appending
297: .Pa -cert.pub
298: to identity filenames.
1.360 millert 299: .Pp
1.386 millert 300: .It Fl J Ar destination
1.375 djm 301: Connect to the target host by first making a
302: .Nm
1.386 millert 303: connection to the jump host described by
304: .Ar destination
1.376 jmc 305: and then establishing a TCP forwarding to the ultimate destination from
1.375 djm 306: there.
307: Multiple jump hops may be specified separated by comma characters.
308: This is a shortcut to specify a
309: .Cm ProxyJump
310: configuration directive.
311: .Pp
1.269 djm 312: .It Fl K
313: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
314: credentials to the server.
1.360 millert 315: .Pp
1.218 jmc 316: .It Fl k
317: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 318: .Pp
1.218 jmc 319: .It Fl L Xo
320: .Sm off
321: .Oo Ar bind_address : Oc
322: .Ar port : host : hostport
323: .Sm on
324: .Xc
1.360 millert 325: .It Fl L Xo
326: .Sm off
327: .Oo Ar bind_address : Oc
328: .Ar port : remote_socket
329: .Sm on
330: .Xc
331: .It Fl L Xo
332: .Sm off
333: .Ar local_socket : host : hostport
334: .Sm on
335: .Xc
336: .It Fl L Xo
337: .Sm off
338: .Ar local_socket : remote_socket
339: .Sm on
340: .Xc
341: Specifies that connections to the given TCP port or Unix socket on the local
342: (client) host are to be forwarded to the given host and port, or Unix socket,
343: on the remote side.
344: This works by allocating a socket to listen to either a TCP
1.218 jmc 345: .Ar port
346: on the local side, optionally bound to the specified
1.360 millert 347: .Ar bind_address ,
348: or to a Unix socket.
349: Whenever a connection is made to the local port or socket, the
1.218 jmc 350: connection is forwarded over the secure channel, and a connection is
1.360 millert 351: made to either
1.218 jmc 352: .Ar host
353: port
1.360 millert 354: .Ar hostport ,
355: or the Unix socket
356: .Ar remote_socket ,
1.218 jmc 357: from the remote machine.
1.360 millert 358: .Pp
1.218 jmc 359: Port forwardings can also be specified in the configuration file.
1.360 millert 360: Only the superuser can forward privileged ports.
1.308 djm 361: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 362: .Pp
1.218 jmc 363: By default, the local port is bound in accordance with the
364: .Cm GatewayPorts
365: setting.
366: However, an explicit
367: .Ar bind_address
368: may be used to bind the connection to a specific address.
1.2 deraadt 369: The
1.218 jmc 370: .Ar bind_address
371: of
372: .Dq localhost
373: indicates that the listening port be bound for local use only, while an
374: empty address or
375: .Sq *
376: indicates that the port should be available from all interfaces.
1.360 millert 377: .Pp
1.218 jmc 378: .It Fl l Ar login_name
379: Specifies the user to log in as on the remote machine.
380: This also may be specified on a per-host basis in the configuration file.
1.360 millert 381: .Pp
1.218 jmc 382: .It Fl M
383: Places the
384: .Nm
385: client into
386: .Dq master
387: mode for connection sharing.
1.231 stevesk 388: Multiple
389: .Fl M
390: options places
391: .Nm
392: into
393: .Dq master
394: mode with confirmation required before slave connections are accepted.
1.218 jmc 395: Refer to the description of
396: .Cm ControlMaster
397: in
398: .Xr ssh_config 5
399: for details.
1.360 millert 400: .Pp
1.218 jmc 401: .It Fl m Ar mac_spec
1.369 jmc 402: A comma-separated list of MAC (message authentication code) algorithms,
403: specified in order of preference.
1.218 jmc 404: See the
405: .Cm MACs
406: keyword for more information.
1.360 millert 407: .Pp
1.218 jmc 408: .It Fl N
409: Do not execute a remote command.
1.369 jmc 410: This is useful for just forwarding ports.
1.360 millert 411: .Pp
1.218 jmc 412: .It Fl n
413: Redirects stdin from
414: .Pa /dev/null
415: (actually, prevents reading from stdin).
416: This must be used when
1.2 deraadt 417: .Nm
1.218 jmc 418: is run in the background.
419: A common trick is to use this to run X11 programs on a remote machine.
420: For example,
421: .Ic ssh -n shadows.cs.hut.fi emacs &
422: will start an emacs on shadows.cs.hut.fi, and the X11
423: connection will be automatically forwarded over an encrypted channel.
424: The
1.2 deraadt 425: .Nm
1.218 jmc 426: program will be put in the background.
427: (This does not work if
1.2 deraadt 428: .Nm
1.218 jmc 429: needs to ask for a password or passphrase; see also the
430: .Fl f
431: option.)
1.360 millert 432: .Pp
1.218 jmc 433: .It Fl O Ar ctl_cmd
434: Control an active connection multiplexing master process.
435: When the
436: .Fl O
437: option is specified, the
438: .Ar ctl_cmd
439: argument is interpreted and passed to the master process.
440: Valid commands are:
441: .Dq check
1.305 markus 442: (check that the master process is running),
443: .Dq forward
1.318 jmc 444: (request forwardings without command execution),
1.323 okan 445: .Dq cancel
446: (cancel forwardings),
1.218 jmc 447: .Dq exit
1.318 jmc 448: (request the master to exit), and
1.317 djm 449: .Dq stop
450: (request the master to stop accepting further multiplexing requests).
1.360 millert 451: .Pp
1.218 jmc 452: .It Fl o Ar option
453: Can be used to give options in the format used in the configuration file.
454: This is useful for specifying options for which there is no separate
455: command-line flag.
456: For full details of the options listed below, and their possible values, see
457: .Xr ssh_config 5 .
1.2 deraadt 458: .Pp
1.218 jmc 459: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 460: .It AddKeysToAgent
1.218 jmc 461: .It AddressFamily
462: .It BatchMode
463: .It BindAddress
1.338 djm 464: .It CanonicalDomains
1.339 djm 465: .It CanonicalizeFallbackLocal
466: .It CanonicalizeHostname
467: .It CanonicalizeMaxDots
468: .It CanonicalizePermittedCNAMEs
1.363 djm 469: .It CertificateFile
1.218 jmc 470: .It ChallengeResponseAuthentication
471: .It CheckHostIP
472: .It Ciphers
473: .It ClearAllForwardings
474: .It Compression
1.380 naddy 475: .It ConnectionAttempts
1.218 jmc 476: .It ConnectTimeout
477: .It ControlMaster
478: .It ControlPath
1.321 djm 479: .It ControlPersist
1.218 jmc 480: .It DynamicForward
481: .It EscapeChar
1.263 markus 482: .It ExitOnForwardFailure
1.353 jmc 483: .It FingerprintHash
1.218 jmc 484: .It ForwardAgent
485: .It ForwardX11
1.321 djm 486: .It ForwardX11Timeout
1.218 jmc 487: .It ForwardX11Trusted
488: .It GatewayPorts
489: .It GlobalKnownHostsFile
490: .It GSSAPIAuthentication
491: .It GSSAPIDelegateCredentials
492: .It HashKnownHosts
493: .It Host
494: .It HostbasedAuthentication
1.355 djm 495: .It HostbasedKeyTypes
1.218 jmc 496: .It HostKeyAlgorithms
497: .It HostKeyAlias
498: .It HostName
1.374 jmc 499: .It IdentitiesOnly
1.371 markus 500: .It IdentityAgent
1.218 jmc 501: .It IdentityFile
1.370 djm 502: .It Include
1.316 jmc 503: .It IPQoS
1.321 djm 504: .It KbdInteractiveAuthentication
1.218 jmc 505: .It KbdInteractiveDevices
1.314 jmc 506: .It KexAlgorithms
1.218 jmc 507: .It LocalCommand
508: .It LocalForward
509: .It LogLevel
510: .It MACs
1.337 jmc 511: .It Match
1.218 jmc 512: .It NoHostAuthenticationForLocalhost
513: .It NumberOfPasswordPrompts
514: .It PasswordAuthentication
515: .It PermitLocalCommand
1.291 markus 516: .It PKCS11Provider
1.218 jmc 517: .It Port
518: .It PreferredAuthentications
519: .It ProxyCommand
1.375 djm 520: .It ProxyJump
1.336 jmc 521: .It ProxyUseFdpass
1.359 markus 522: .It PubkeyAcceptedKeyTypes
1.218 jmc 523: .It PubkeyAuthentication
1.251 dtucker 524: .It RekeyLimit
1.382 bluhm 525: .It RemoteCommand
1.218 jmc 526: .It RemoteForward
1.319 jmc 527: .It RequestTTY
1.218 jmc 528: .It SendEnv
529: .It ServerAliveInterval
530: .It ServerAliveCountMax
1.347 jmc 531: .It StreamLocalBindMask
532: .It StreamLocalBindUnlink
1.218 jmc 533: .It StrictHostKeyChecking
534: .It TCPKeepAlive
535: .It Tunnel
536: .It TunnelDevice
1.354 djm 537: .It UpdateHostKeys
1.218 jmc 538: .It UsePrivilegedPort
539: .It User
540: .It UserKnownHostsFile
541: .It VerifyHostKeyDNS
1.276 jmc 542: .It VisualHostKey
1.218 jmc 543: .It XAuthLocation
544: .El
1.360 millert 545: .Pp
1.218 jmc 546: .It Fl p Ar port
547: Port to connect to on the remote host.
548: This can be specified on a
549: per-host basis in the configuration file.
1.360 millert 550: .Pp
1.362 djm 551: .It Fl Q Ar query_option
1.332 djm 552: Queries
553: .Nm
1.341 deraadt 554: for the algorithms supported for the specified version 2.
555: The available features are:
556: .Ar cipher
1.332 djm 557: (supported symmetric ciphers),
1.341 deraadt 558: .Ar cipher-auth
1.340 djm 559: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 560: .Ar mac
1.332 djm 561: (supported message integrity codes),
1.341 deraadt 562: .Ar kex
1.332 djm 563: (key exchange algorithms),
1.341 deraadt 564: .Ar key
1.362 djm 565: (key types),
566: .Ar key-cert
567: (certificate key types),
568: .Ar key-plain
569: (non-certificate key types), and
1.356 djm 570: .Ar protocol-version
571: (supported SSH protocol versions).
1.360 millert 572: .Pp
1.218 jmc 573: .It Fl q
574: Quiet mode.
1.271 djm 575: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 576: .Pp
1.218 jmc 577: .It Fl R Xo
578: .Sm off
579: .Oo Ar bind_address : Oc
580: .Ar port : host : hostport
581: .Sm on
582: .Xc
1.360 millert 583: .It Fl R Xo
584: .Sm off
585: .Oo Ar bind_address : Oc
586: .Ar port : local_socket
587: .Sm on
588: .Xc
589: .It Fl R Xo
590: .Sm off
591: .Ar remote_socket : host : hostport
592: .Sm on
593: .Xc
594: .It Fl R Xo
595: .Sm off
596: .Ar remote_socket : local_socket
597: .Sm on
598: .Xc
1.384 markus 599: .It Fl R Xo
600: .Sm off
601: .Oo Ar bind_address : Oc
602: .Ar port
603: .Sm on
604: .Xc
1.360 millert 605: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 606: (server) host are to be forwarded to the local side.
607: .Pp
1.360 millert 608: This works by allocating a socket to listen to either a TCP
1.218 jmc 609: .Ar port
1.360 millert 610: or to a Unix socket on the remote side.
611: Whenever a connection is made to this port or Unix socket, the
612: connection is forwarded over the secure channel, and a connection
1.384 markus 613: is made from the local machine to either an explicit destination specified by
1.218 jmc 614: .Ar host
615: port
1.360 millert 616: .Ar hostport ,
617: or
618: .Ar local_socket ,
1.384 markus 619: or, if no explicit destination was specified,
620: .Nm
621: will act as a SOCKS 4/5 proxy and forward connections to the destinations
622: requested by the remote SOCKS client.
1.2 deraadt 623: .Pp
1.218 jmc 624: Port forwardings can also be specified in the configuration file.
625: Privileged ports can be forwarded only when
626: logging in as root on the remote machine.
1.324 jmc 627: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 628: .Pp
1.360 millert 629: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 630: interface only.
1.280 tobias 631: This may be overridden by specifying a
1.218 jmc 632: .Ar bind_address .
633: An empty
634: .Ar bind_address ,
635: or the address
636: .Ql * ,
637: indicates that the remote socket should listen on all interfaces.
638: Specifying a remote
639: .Ar bind_address
640: will only succeed if the server's
641: .Cm GatewayPorts
642: option is enabled (see
643: .Xr sshd_config 5 ) .
1.281 djm 644: .Pp
645: If the
646: .Ar port
647: argument is
1.282 djm 648: .Ql 0 ,
1.281 djm 649: the listen port will be dynamically allocated on the server and reported
650: to the client at run time.
1.305 markus 651: When used together with
652: .Ic -O forward
653: the allocated port will be printed to the standard output.
1.360 millert 654: .Pp
1.218 jmc 655: .It Fl S Ar ctl_path
1.304 jmc 656: Specifies the location of a control socket for connection sharing,
1.303 djm 657: or the string
658: .Dq none
659: to disable connection sharing.
1.218 jmc 660: Refer to the description of
661: .Cm ControlPath
662: and
663: .Cm ControlMaster
664: in
665: .Xr ssh_config 5
666: for details.
1.360 millert 667: .Pp
1.218 jmc 668: .It Fl s
669: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 670: Subsystems facilitate the use of SSH
671: as a secure transport for other applications (e.g.\&
1.218 jmc 672: .Xr sftp 1 ) .
673: The subsystem is specified as the remote command.
1.360 millert 674: .Pp
1.218 jmc 675: .It Fl T
1.357 dtucker 676: Disable pseudo-terminal allocation.
1.360 millert 677: .Pp
1.218 jmc 678: .It Fl t
1.357 dtucker 679: Force pseudo-terminal allocation.
1.218 jmc 680: This can be used to execute arbitrary
681: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 682: e.g. when implementing menu services.
1.218 jmc 683: Multiple
684: .Fl t
685: options force tty allocation, even if
1.194 jakob 686: .Nm
1.218 jmc 687: has no local tty.
1.360 millert 688: .Pp
1.218 jmc 689: .It Fl V
690: Display the version number and exit.
1.360 millert 691: .Pp
1.218 jmc 692: .It Fl v
693: Verbose mode.
694: Causes
1.176 jmc 695: .Nm
1.218 jmc 696: to print debugging messages about its progress.
697: This is helpful in
698: debugging connection, authentication, and configuration problems.
699: Multiple
700: .Fl v
701: options increase the verbosity.
702: The maximum is 3.
1.360 millert 703: .Pp
1.290 dtucker 704: .It Fl W Ar host : Ns Ar port
705: Requests that standard input and output on the client be forwarded to
706: .Ar host
707: on
708: .Ar port
709: over the secure channel.
710: Implies
711: .Fl N ,
712: .Fl T ,
713: .Cm ExitOnForwardFailure
714: and
1.373 jmc 715: .Cm ClearAllForwardings ,
716: though these can be overridden in the configuration file or using
1.372 dtucker 717: .Fl o
718: command line options.
1.360 millert 719: .Pp
1.261 stevesk 720: .It Fl w Xo
721: .Ar local_tun Ns Op : Ns Ar remote_tun
722: .Xc
723: Requests
724: tunnel
725: device forwarding with the specified
1.218 jmc 726: .Xr tun 4
1.261 stevesk 727: devices between the client
728: .Pq Ar local_tun
729: and the server
730: .Pq Ar remote_tun .
731: .Pp
1.228 jmc 732: The devices may be specified by numerical ID or the keyword
733: .Dq any ,
734: which uses the next available tunnel device.
1.261 stevesk 735: If
736: .Ar remote_tun
737: is not specified, it defaults to
738: .Dq any .
1.228 jmc 739: See also the
1.218 jmc 740: .Cm Tunnel
1.261 stevesk 741: and
742: .Cm TunnelDevice
743: directives in
1.218 jmc 744: .Xr ssh_config 5 .
1.261 stevesk 745: If the
746: .Cm Tunnel
747: directive is unset, it is set to the default tunnel mode, which is
748: .Dq point-to-point .
1.360 millert 749: .Pp
1.218 jmc 750: .It Fl X
751: Enables X11 forwarding.
1.54 markus 752: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 753: .Pp
1.218 jmc 754: X11 forwarding should be enabled with caution.
1.168 jmc 755: Users with the ability to bypass file permissions on the remote host
1.218 jmc 756: (for the user's X authorization database)
757: can access the local X11 display through the forwarded connection.
758: An attacker may then be able to perform activities such as keystroke monitoring.
759: .Pp
760: For this reason, X11 forwarding is subjected to X11 SECURITY extension
761: restrictions by default.
762: Please refer to the
763: .Nm
764: .Fl Y
765: option and the
766: .Cm ForwardX11Trusted
767: directive in
768: .Xr ssh_config 5
769: for more information.
1.360 millert 770: .Pp
1.218 jmc 771: .It Fl x
772: Disables X11 forwarding.
1.360 millert 773: .Pp
1.218 jmc 774: .It Fl Y
775: Enables trusted X11 forwarding.
776: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
777: controls.
1.360 millert 778: .Pp
1.278 djm 779: .It Fl y
780: Send log information using the
781: .Xr syslog 3
782: system module.
783: By default this information is sent to stderr.
1.218 jmc 784: .El
1.224 jmc 785: .Pp
786: .Nm
787: may additionally obtain configuration data from
788: a per-user configuration file and a system-wide configuration file.
789: The file format and configuration options are described in
790: .Xr ssh_config 5 .
1.222 jmc 791: .Sh AUTHENTICATION
1.377 djm 792: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 793: .Pp
794: The methods available for authentication are:
1.260 jmc 795: GSSAPI-based authentication,
1.222 jmc 796: host-based authentication,
797: public key authentication,
798: challenge-response authentication,
799: and password authentication.
800: Authentication methods are tried in the order specified above,
1.369 jmc 801: though
802: .Cm PreferredAuthentications
803: can be used to change the default order.
1.222 jmc 804: .Pp
805: Host-based authentication works as follows:
1.218 jmc 806: If the machine the user logs in from is listed in
807: .Pa /etc/hosts.equiv
808: or
809: .Pa /etc/shosts.equiv
810: on the remote machine, and the user names are
811: the same on both sides, or if the files
812: .Pa ~/.rhosts
813: or
814: .Pa ~/.shosts
815: exist in the user's home directory on the
816: remote machine and contain a line containing the name of the client
817: machine and the name of the user on that machine, the user is
1.222 jmc 818: considered for login.
819: Additionally, the server
820: .Em must
821: be able to verify the client's
822: host key (see the description of
1.218 jmc 823: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 824: and
1.222 jmc 825: .Pa ~/.ssh/known_hosts ,
826: below)
827: for login to be permitted.
1.218 jmc 828: This authentication method closes security holes due to IP
1.222 jmc 829: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 830: [Note to the administrator:
831: .Pa /etc/hosts.equiv ,
832: .Pa ~/.rhosts ,
833: and the rlogin/rsh protocol in general, are inherently insecure and should be
834: disabled if security is desired.]
1.189 dtucker 835: .Pp
1.222 jmc 836: Public key authentication works as follows:
837: The scheme is based on public-key cryptography,
838: using cryptosystems
839: where encryption and decryption are done using separate keys,
840: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 841: The idea is that each user creates a public/private
842: key pair for authentication purposes.
843: The server knows the public key, and only the user knows the private key.
1.222 jmc 844: .Nm
845: implements public key authentication protocol automatically,
1.349 sobrado 846: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 847: The HISTORY section of
1.222 jmc 848: .Xr ssl 8
1.311 jmc 849: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 850: .Pp
1.218 jmc 851: The file
852: .Pa ~/.ssh/authorized_keys
853: lists the public keys that are permitted for logging in.
854: When the user logs in, the
1.2 deraadt 855: .Nm
1.218 jmc 856: program tells the server which key pair it would like to use for
857: authentication.
1.222 jmc 858: The client proves that it has access to the private key
859: and the server checks that the corresponding public key
860: is authorized to accept the account.
1.383 djm 861: .Pp
862: The server may inform the client of errors that prevented public key
863: authentication from succeeding after authentication completes using a
864: different method.
865: These may be viewed by increasing the
866: .Cm LogLevel
867: to
868: .Cm DEBUG
869: or higher (e.g. by using the
870: .Fl v
871: flag).
1.218 jmc 872: .Pp
1.222 jmc 873: The user creates his/her key pair by running
1.218 jmc 874: .Xr ssh-keygen 1 .
875: This stores the private key in
1.222 jmc 876: .Pa ~/.ssh/id_dsa
1.369 jmc 877: (DSA),
1.310 djm 878: .Pa ~/.ssh/id_ecdsa
1.369 jmc 879: (ECDSA),
1.343 naddy 880: .Pa ~/.ssh/id_ed25519
1.369 jmc 881: (Ed25519),
1.222 jmc 882: or
883: .Pa ~/.ssh/id_rsa
1.369 jmc 884: (RSA)
1.218 jmc 885: and stores the public key in
1.222 jmc 886: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 887: (DSA),
1.310 djm 888: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 889: (ECDSA),
1.343 naddy 890: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 891: (Ed25519),
1.222 jmc 892: or
893: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 894: (RSA)
1.218 jmc 895: in the user's home directory.
1.222 jmc 896: The user should then copy the public key
1.218 jmc 897: to
898: .Pa ~/.ssh/authorized_keys
1.222 jmc 899: in his/her home directory on the remote machine.
900: The
1.218 jmc 901: .Pa authorized_keys
902: file corresponds to the conventional
903: .Pa ~/.rhosts
904: file, and has one key
1.222 jmc 905: per line, though the lines can be very long.
1.218 jmc 906: After this, the user can log in without giving the password.
907: .Pp
1.301 jmc 908: A variation on public key authentication
909: is available in the form of certificate authentication:
910: instead of a set of public/private keys,
911: signed certificates are used.
912: This has the advantage that a single trusted certification authority
913: can be used in place of many public/private keys.
1.333 jmc 914: See the CERTIFICATES section of
1.301 jmc 915: .Xr ssh-keygen 1
916: for more information.
917: .Pp
918: The most convenient way to use public key or certificate authentication
919: may be with an authentication agent.
1.218 jmc 920: See
921: .Xr ssh-agent 1
1.366 jcs 922: and (optionally) the
923: .Cm AddKeysToAgent
924: directive in
925: .Xr ssh_config 5
1.218 jmc 926: for more information.
927: .Pp
1.222 jmc 928: Challenge-response authentication works as follows:
929: The server sends an arbitrary
930: .Qq challenge
931: text, and prompts for a response.
932: Examples of challenge-response authentication include
1.335 jmc 933: .Bx
934: Authentication (see
1.222 jmc 935: .Xr login.conf 5 )
1.335 jmc 936: and PAM (some
937: .Pf non- Ox
938: systems).
1.222 jmc 939: .Pp
940: Finally, if other authentication methods fail,
1.218 jmc 941: .Nm
942: prompts the user for a password.
943: The password is sent to the remote
944: host for checking; however, since all communications are encrypted,
945: the password cannot be seen by someone listening on the network.
1.232 jmc 946: .Pp
947: .Nm
948: automatically maintains and checks a database containing
949: identification for all hosts it has ever been used with.
950: Host keys are stored in
951: .Pa ~/.ssh/known_hosts
952: in the user's home directory.
953: Additionally, the file
954: .Pa /etc/ssh/ssh_known_hosts
955: is automatically checked for known hosts.
956: Any new hosts are automatically added to the user's file.
957: If a host's identification ever changes,
958: .Nm
959: warns about this and disables password authentication to prevent
960: server spoofing or man-in-the-middle attacks,
961: which could otherwise be used to circumvent the encryption.
962: The
963: .Cm StrictHostKeyChecking
964: option can be used to control logins to machines whose
965: host key is not known or has changed.
966: .Pp
1.218 jmc 967: When the user's identity has been accepted by the server, the server
1.357 dtucker 968: either executes the given command in a non-interactive session or,
969: if no command has been specified, logs into the machine and gives
970: the user a normal shell as an interactive session.
1.218 jmc 971: All communication with
972: the remote command or shell will be automatically encrypted.
973: .Pp
1.357 dtucker 974: If an interactive session is requested
975: .Nm
976: by default will only request a pseudo-terminal (pty) for interactive
977: sessions when the client has one.
978: The flags
979: .Fl T
980: and
981: .Fl t
982: can be used to override this behaviour.
983: .Pp
984: If a pseudo-terminal has been allocated the
1.218 jmc 985: user may use the escape characters noted below.
986: .Pp
1.357 dtucker 987: If no pseudo-terminal has been allocated,
1.218 jmc 988: the session is transparent and can be used to reliably transfer binary data.
989: On most systems, setting the escape character to
990: .Dq none
991: will also make the session transparent even if a tty is used.
992: .Pp
993: The session terminates when the command or shell on the remote
1.247 jmc 994: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 995: .Sh ESCAPE CHARACTERS
1.218 jmc 996: When a pseudo-terminal has been requested,
1.2 deraadt 997: .Nm
1.218 jmc 998: supports a number of functions through the use of an escape character.
999: .Pp
1000: A single tilde character can be sent as
1001: .Ic ~~
1002: or by following the tilde by a character other than those described below.
1003: The escape character must always follow a newline to be interpreted as
1004: special.
1005: The escape character can be changed in configuration files using the
1006: .Cm EscapeChar
1007: configuration directive or on the command line by the
1008: .Fl e
1009: option.
1010: .Pp
1011: The supported escapes (assuming the default
1012: .Ql ~ )
1013: are:
1014: .Bl -tag -width Ds
1015: .It Cm ~.
1016: Disconnect.
1017: .It Cm ~^Z
1018: Background
1.234 jmc 1019: .Nm .
1.218 jmc 1020: .It Cm ~#
1021: List forwarded connections.
1022: .It Cm ~&
1023: Background
1.2 deraadt 1024: .Nm
1.218 jmc 1025: at logout when waiting for forwarded connection / X11 sessions to terminate.
1026: .It Cm ~?
1027: Display a list of escape characters.
1028: .It Cm ~B
1029: Send a BREAK to the remote system
1.369 jmc 1030: (only useful if the peer supports it).
1.218 jmc 1031: .It Cm ~C
1032: Open command line.
1033: Currently this allows the addition of port forwardings using the
1.279 stevesk 1034: .Fl L ,
1035: .Fl R
1.218 jmc 1036: and
1.279 stevesk 1037: .Fl D
1.225 jmc 1038: options (see above).
1.322 markus 1039: It also allows the cancellation of existing port-forwardings
1040: with
1.262 stevesk 1041: .Sm off
1.322 markus 1042: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1043: .Sm on
1.322 markus 1044: for local,
1045: .Sm off
1046: .Fl KR Oo Ar bind_address : Oc Ar port
1047: .Sm on
1048: for remote and
1049: .Sm off
1050: .Fl KD Oo Ar bind_address : Oc Ar port
1051: .Sm on
1052: for dynamic port-forwardings.
1.218 jmc 1053: .Ic !\& Ns Ar command
1054: allows the user to execute a local command if the
1055: .Ic PermitLocalCommand
1056: option is enabled in
1.176 jmc 1057: .Xr ssh_config 5 .
1.218 jmc 1058: Basic help is available, using the
1059: .Fl h
1060: option.
1061: .It Cm ~R
1062: Request rekeying of the connection
1.369 jmc 1063: (only useful if the peer supports it).
1.327 dtucker 1064: .It Cm ~V
1065: Decrease the verbosity
1066: .Pq Ic LogLevel
1067: when errors are being written to stderr.
1068: .It Cm ~v
1.328 jmc 1069: Increase the verbosity
1.327 dtucker 1070: .Pq Ic LogLevel
1071: when errors are being written to stderr.
1.176 jmc 1072: .El
1.246 jmc 1073: .Sh TCP FORWARDING
1074: Forwarding of arbitrary TCP connections over the secure channel can
1075: be specified either on the command line or in a configuration file.
1076: One possible application of TCP forwarding is a secure connection to a
1077: mail server; another is going through firewalls.
1078: .Pp
1079: In the example below, we look at encrypting communication between
1080: an IRC client and server, even though the IRC server does not directly
1081: support encrypted communications.
1082: This works as follows:
1083: the user connects to the remote host using
1084: .Nm ,
1085: specifying a port to be used to forward connections
1086: to the remote server.
1087: After that it is possible to start the service which is to be encrypted
1088: on the client machine,
1089: connecting to the same local port,
1090: and
1091: .Nm
1092: will encrypt and forward the connection.
1093: .Pp
1094: The following example tunnels an IRC session from client machine
1095: .Dq 127.0.0.1
1096: (localhost)
1097: to remote server
1098: .Dq server.example.com :
1099: .Bd -literal -offset 4n
1100: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1101: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1102: .Ed
1103: .Pp
1104: This tunnels a connection to IRC server
1105: .Dq server.example.com ,
1106: joining channel
1107: .Dq #users ,
1108: nickname
1109: .Dq pinky ,
1110: using port 1234.
1111: It doesn't matter which port is used,
1112: as long as it's greater than 1023
1113: (remember, only root can open sockets on privileged ports)
1114: and doesn't conflict with any ports already in use.
1115: The connection is forwarded to port 6667 on the remote server,
1116: since that's the standard port for IRC services.
1117: .Pp
1118: The
1119: .Fl f
1120: option backgrounds
1121: .Nm
1122: and the remote command
1123: .Dq sleep 10
1124: is specified to allow an amount of time
1125: (10 seconds, in the example)
1126: to start the service which is to be tunnelled.
1127: If no connections are made within the time specified,
1128: .Nm
1129: will exit.
1130: .Sh X11 FORWARDING
1.218 jmc 1131: If the
1132: .Cm ForwardX11
1133: variable is set to
1134: .Dq yes
1135: (or see the description of the
1.227 jmc 1136: .Fl X ,
1137: .Fl x ,
1.218 jmc 1138: and
1.227 jmc 1139: .Fl Y
1.226 jmc 1140: options above)
1.218 jmc 1141: and the user is using X11 (the
1142: .Ev DISPLAY
1143: environment variable is set), the connection to the X11 display is
1144: automatically forwarded to the remote side in such a way that any X11
1145: programs started from the shell (or command) will go through the
1146: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1147: from the local machine.
1.218 jmc 1148: The user should not manually set
1149: .Ev DISPLAY .
1150: Forwarding of X11 connections can be
1151: configured on the command line or in configuration files.
1152: .Pp
1153: The
1154: .Ev DISPLAY
1155: value set by
1156: .Nm
1157: will point to the server machine, but with a display number greater than zero.
1158: This is normal, and happens because
1159: .Nm
1160: creates a
1161: .Dq proxy
1162: X server on the server machine for forwarding the
1163: connections over the encrypted channel.
1.200 djm 1164: .Pp
1.218 jmc 1165: .Nm
1166: will also automatically set up Xauthority data on the server machine.
1167: For this purpose, it will generate a random authorization cookie,
1168: store it in Xauthority on the server, and verify that any forwarded
1169: connections carry this cookie and replace it by the real cookie when
1170: the connection is opened.
1171: The real authentication cookie is never
1172: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1173: .Pp
1.218 jmc 1174: If the
1175: .Cm ForwardAgent
1176: variable is set to
1177: .Dq yes
1178: (or see the description of the
1179: .Fl A
1.191 djm 1180: and
1.218 jmc 1181: .Fl a
1.226 jmc 1182: options above) and
1.218 jmc 1183: the user is using an authentication agent, the connection to the agent
1184: is automatically forwarded to the remote side.
1.252 jmc 1185: .Sh VERIFYING HOST KEYS
1186: When connecting to a server for the first time,
1187: a fingerprint of the server's public key is presented to the user
1188: (unless the option
1189: .Cm StrictHostKeyChecking
1190: has been disabled).
1191: Fingerprints can be determined using
1192: .Xr ssh-keygen 1 :
1193: .Pp
1194: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1195: .Pp
1.274 grunk 1196: If the fingerprint is already known, it can be matched
1197: and the key can be accepted or rejected.
1.358 djm 1198: If only legacy (MD5) fingerprints for the server are available, the
1199: .Xr ssh-keygen 1
1200: .Fl E
1201: option may be used to downgrade the fingerprint algorithm to match.
1202: .Pp
1.274 grunk 1203: Because of the difficulty of comparing host keys
1.352 djm 1204: just by looking at fingerprint strings,
1.274 grunk 1205: there is also support to compare host keys visually,
1206: using
1207: .Em random art .
1208: By setting the
1.275 grunk 1209: .Cm VisualHostKey
1.274 grunk 1210: option to
1.275 grunk 1211: .Dq yes ,
1.274 grunk 1212: a small ASCII graphic gets displayed on every login to a server, no matter
1213: if the session itself is interactive or not.
1214: By learning the pattern a known server produces, a user can easily
1215: find out that the host key has changed when a completely different pattern
1216: is displayed.
1217: Because these patterns are not unambiguous however, a pattern that looks
1218: similar to the pattern remembered only gives a good probability that the
1219: host key is the same, not guaranteed proof.
1220: .Pp
1221: To get a listing of the fingerprints along with their random art for
1222: all known hosts, the following command line can be used:
1223: .Pp
1224: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1225: .Pp
1.252 jmc 1226: If the fingerprint is unknown,
1227: an alternative method of verification is available:
1228: SSH fingerprints verified by DNS.
1229: An additional resource record (RR),
1230: SSHFP,
1231: is added to a zonefile
1232: and the connecting client is able to match the fingerprint
1233: with that of the key presented.
1234: .Pp
1235: In this example, we are connecting a client to a server,
1236: .Dq host.example.com .
1237: The SSHFP resource records should first be added to the zonefile for
1238: host.example.com:
1239: .Bd -literal -offset indent
1.259 jakob 1240: $ ssh-keygen -r host.example.com.
1.252 jmc 1241: .Ed
1242: .Pp
1243: The output lines will have to be added to the zonefile.
1244: To check that the zone is answering fingerprint queries:
1245: .Pp
1246: .Dl $ dig -t SSHFP host.example.com
1247: .Pp
1248: Finally the client connects:
1249: .Bd -literal -offset indent
1250: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1251: [...]
1252: Matching host key fingerprint found in DNS.
1253: Are you sure you want to continue connecting (yes/no)?
1254: .Ed
1255: .Pp
1256: See the
1257: .Cm VerifyHostKeyDNS
1258: option in
1259: .Xr ssh_config 5
1260: for more information.
1.250 jmc 1261: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1262: .Nm
1263: contains support for Virtual Private Network (VPN) tunnelling
1264: using the
1265: .Xr tun 4
1266: network pseudo-device,
1267: allowing two networks to be joined securely.
1268: The
1269: .Xr sshd_config 5
1270: configuration option
1271: .Cm PermitTunnel
1272: controls whether the server supports this,
1273: and at what level (layer 2 or 3 traffic).
1274: .Pp
1275: The following example would connect client network 10.0.50.0/24
1.265 otto 1276: with remote network 10.0.99.0/24 using a point-to-point connection
1277: from 10.1.1.1 to 10.1.1.2,
1278: provided that the SSH server running on the gateway to the remote network,
1279: at 192.168.1.15, allows it.
1280: .Pp
1281: On the client:
1.250 jmc 1282: .Bd -literal -offset indent
1283: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1284: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1285: # route add 10.0.99.0/24 10.1.1.2
1286: .Ed
1287: .Pp
1288: On the server:
1289: .Bd -literal -offset indent
1290: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1291: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1292: .Ed
1293: .Pp
1294: Client access may be more finely tuned via the
1295: .Pa /root/.ssh/authorized_keys
1296: file (see below) and the
1297: .Cm PermitRootLogin
1298: server option.
1.255 jmc 1299: The following entry would permit connections on
1.250 jmc 1300: .Xr tun 4
1.255 jmc 1301: device 1 from user
1.250 jmc 1302: .Dq jane
1.255 jmc 1303: and on tun device 2 from user
1.250 jmc 1304: .Dq john ,
1305: if
1306: .Cm PermitRootLogin
1307: is set to
1308: .Dq forced-commands-only :
1309: .Bd -literal -offset 2n
1310: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1311: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1312: .Ed
1313: .Pp
1.264 ray 1314: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1315: it may be more suited to temporary setups,
1316: such as for wireless VPNs.
1317: More permanent VPNs are better provided by tools such as
1318: .Xr ipsecctl 8
1319: and
1320: .Xr isakmpd 8 .
1.2 deraadt 1321: .Sh ENVIRONMENT
1322: .Nm
1.1 deraadt 1323: will normally set the following environment variables:
1.237 jmc 1324: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1325: .It Ev DISPLAY
1326: The
1327: .Ev DISPLAY
1.40 aaron 1328: variable indicates the location of the X11 server.
1.44 aaron 1329: It is automatically set by
1.2 deraadt 1330: .Nm
1331: to point to a value of the form
1.233 jmc 1332: .Dq hostname:n ,
1333: where
1334: .Dq hostname
1335: indicates the host where the shell runs, and
1336: .Sq n
1337: is an integer \*(Ge 1.
1.40 aaron 1338: .Nm
1339: uses this special value to forward X11 connections over the secure
1340: channel.
1.107 markus 1341: The user should normally not set
1342: .Ev DISPLAY
1343: explicitly, as that
1.1 deraadt 1344: will render the X11 connection insecure (and will require the user to
1345: manually copy any required authorization cookies).
1.2 deraadt 1346: .It Ev HOME
1.1 deraadt 1347: Set to the path of the user's home directory.
1.2 deraadt 1348: .It Ev LOGNAME
1349: Synonym for
1.12 aaron 1350: .Ev USER ;
1351: set for compatibility with systems that use this variable.
1.2 deraadt 1352: .It Ev MAIL
1.129 stevesk 1353: Set to the path of the user's mailbox.
1.40 aaron 1354: .It Ev PATH
1.2 deraadt 1355: Set to the default
1356: .Ev PATH ,
1357: as specified when compiling
1.234 jmc 1358: .Nm .
1.118 markus 1359: .It Ev SSH_ASKPASS
1360: If
1361: .Nm
1362: needs a passphrase, it will read the passphrase from the current
1363: terminal if it was run from a terminal.
1364: If
1365: .Nm
1366: does not have a terminal associated with it but
1367: .Ev DISPLAY
1368: and
1369: .Ev SSH_ASKPASS
1370: are set, it will execute the program specified by
1371: .Ev SSH_ASKPASS
1372: and open an X11 window to read the passphrase.
1373: This is particularly useful when calling
1374: .Nm
1375: from a
1.196 jmc 1376: .Pa .xsession
1.118 markus 1377: or related script.
1378: (Note that on some machines it
1379: may be necessary to redirect the input from
1380: .Pa /dev/null
1381: to make this work.)
1.18 markus 1382: .It Ev SSH_AUTH_SOCK
1.233 jmc 1383: Identifies the path of a
1384: .Ux Ns -domain
1385: socket used to communicate with the agent.
1.166 stevesk 1386: .It Ev SSH_CONNECTION
1387: Identifies the client and server ends of the connection.
1.40 aaron 1388: The variable contains
1.233 jmc 1389: four space-separated values: client IP address, client port number,
1390: server IP address, and server port number.
1.73 markus 1391: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1392: This variable contains the original command line if a forced command
1.73 markus 1393: is executed.
1394: It can be used to extract the original arguments.
1.2 deraadt 1395: .It Ev SSH_TTY
1.1 deraadt 1396: This is set to the name of the tty (path to the device) associated
1.40 aaron 1397: with the current shell or command.
1398: If the current session has no tty,
1.1 deraadt 1399: this variable is not set.
1.387 djm 1400: .It Ev SSH_TUNNEL
1401: Optionally set by
1402: .Xr sshd 8
1403: to contain the interface names assigned if tunnel forwarding was
1404: requested by the client.
1.385 djm 1405: .It Ev SSH_USER_AUTH
1406: Optionally set by
1407: .Xr sshd 8 ,
1408: this variable may contain a pathname to a file that lists the authentication
1409: methods successfully used when the session was established, including any
1410: public keys that were used.
1.2 deraadt 1411: .It Ev TZ
1.214 jmc 1412: This variable is set to indicate the present time zone if it
1.257 jmc 1413: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1414: on to new connections).
1.2 deraadt 1415: .It Ev USER
1.1 deraadt 1416: Set to the name of the user logging in.
1.2 deraadt 1417: .El
1418: .Pp
1.44 aaron 1419: Additionally,
1.2 deraadt 1420: .Nm
1.44 aaron 1421: reads
1.207 djm 1422: .Pa ~/.ssh/environment ,
1.2 deraadt 1423: and adds lines of the format
1424: .Dq VARNAME=value
1.233 jmc 1425: to the environment if the file exists and users are allowed to
1.161 marc 1426: change their environment.
1.176 jmc 1427: For more information, see the
1.161 marc 1428: .Cm PermitUserEnvironment
1.162 stevesk 1429: option in
1.161 marc 1430: .Xr sshd_config 5 .
1.2 deraadt 1431: .Sh FILES
1.236 jmc 1432: .Bl -tag -width Ds -compact
1.309 jmc 1433: .It Pa ~/.rhosts
1.240 jmc 1434: This file is used for host-based authentication (see above).
1.92 markus 1435: On some machines this file may need to be
1.240 jmc 1436: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1437: because
1.2 deraadt 1438: .Xr sshd 8
1.40 aaron 1439: reads it as root.
1440: Additionally, this file must be owned by the user,
1441: and must not have write permissions for anyone else.
1442: The recommended
1.1 deraadt 1443: permission for most machines is read/write for the user, and not
1444: accessible by others.
1.2 deraadt 1445: .Pp
1.309 jmc 1446: .It Pa ~/.shosts
1.240 jmc 1447: This file is used in exactly the same way as
1448: .Pa .rhosts ,
1449: but allows host-based authentication without permitting login with
1450: rlogin/rsh.
1.272 mcbride 1451: .Pp
1.309 jmc 1452: .It Pa ~/.ssh/
1.272 mcbride 1453: This directory is the default location for all user-specific configuration
1454: and authentication information.
1455: There is no general requirement to keep the entire contents of this directory
1456: secret, but the recommended permissions are read/write/execute for the user,
1457: and not accessible by others.
1.236 jmc 1458: .Pp
1.309 jmc 1459: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1460: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1461: that can be used for logging in as this user.
1.238 jmc 1462: The format of this file is described in the
1463: .Xr sshd 8
1464: manual page.
1465: This file is not highly sensitive, but the recommended
1466: permissions are read/write for the user, and not accessible by others.
1467: .Pp
1.309 jmc 1468: .It Pa ~/.ssh/config
1.238 jmc 1469: This is the per-user configuration file.
1470: The file format and configuration options are described in
1471: .Xr ssh_config 5 .
1472: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1473: read/write for the user, and not writable by others.
1.238 jmc 1474: .Pp
1.309 jmc 1475: .It Pa ~/.ssh/environment
1.239 jmc 1476: Contains additional definitions for environment variables; see
1477: .Sx ENVIRONMENT ,
1.238 jmc 1478: above.
1479: .Pp
1.309 jmc 1480: .It Pa ~/.ssh/id_dsa
1.310 djm 1481: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1482: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1483: .It Pa ~/.ssh/id_rsa
1.238 jmc 1484: Contains the private key for authentication.
1485: These files
1486: contain sensitive data and should be readable by the user but not
1487: accessible by others (read/write/execute).
1488: .Nm
1489: will simply ignore a private key file if it is accessible by others.
1490: It is possible to specify a passphrase when
1491: generating the key which will be used to encrypt the
1.389 djm 1492: sensitive part of this file using AES-128.
1.238 jmc 1493: .Pp
1.309 jmc 1494: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1495: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1496: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1497: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1498: Contains the public key for authentication.
1499: These files are not
1500: sensitive and can (but need not) be readable by anyone.
1501: .Pp
1.309 jmc 1502: .It Pa ~/.ssh/known_hosts
1.244 jmc 1503: Contains a list of host keys for all hosts the user has logged into
1504: that are not already in the systemwide list of known host keys.
1.238 jmc 1505: See
1.244 jmc 1506: .Xr sshd 8
1507: for further details of the format of this file.
1.238 jmc 1508: .Pp
1.309 jmc 1509: .It Pa ~/.ssh/rc
1.238 jmc 1510: Commands in this file are executed by
1511: .Nm
1.245 jmc 1512: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1513: started.
1514: See the
1515: .Xr sshd 8
1516: manual page for more information.
1517: .Pp
1.309 jmc 1518: .It Pa /etc/hosts.equiv
1.240 jmc 1519: This file is for host-based authentication (see above).
1520: It should only be writable by root.
1.236 jmc 1521: .Pp
1.309 jmc 1522: .It Pa /etc/shosts.equiv
1.240 jmc 1523: This file is used in exactly the same way as
1524: .Pa hosts.equiv ,
1525: but allows host-based authentication without permitting login with
1526: rlogin/rsh.
1.236 jmc 1527: .Pp
1.238 jmc 1528: .It Pa /etc/ssh/ssh_config
1529: Systemwide configuration file.
1530: The file format and configuration options are described in
1531: .Xr ssh_config 5 .
1532: .Pp
1.309 jmc 1533: .It Pa /etc/ssh/ssh_host_key
1534: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1535: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1536: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1537: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1538: These files contain the private parts of the host keys
1.245 jmc 1539: and are used for host-based authentication.
1.238 jmc 1540: .Pp
1.309 jmc 1541: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1542: Systemwide list of known host keys.
1543: This file should be prepared by the
1544: system administrator to contain the public host keys of all machines in the
1545: organization.
1.244 jmc 1546: It should be world-readable.
1547: See
1.238 jmc 1548: .Xr sshd 8
1.244 jmc 1549: for further details of the format of this file.
1.236 jmc 1550: .Pp
1.309 jmc 1551: .It Pa /etc/ssh/sshrc
1.1 deraadt 1552: Commands in this file are executed by
1.2 deraadt 1553: .Nm
1.245 jmc 1554: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1555: See the
1.2 deraadt 1556: .Xr sshd 8
1.1 deraadt 1557: manual page for more information.
1.58 itojun 1558: .El
1.312 jmc 1559: .Sh EXIT STATUS
1560: .Nm
1561: exits with the exit status of the remote command or with 255
1562: if an error occurred.
1.2 deraadt 1563: .Sh SEE ALSO
1564: .Xr scp 1 ,
1.83 djm 1565: .Xr sftp 1 ,
1.2 deraadt 1566: .Xr ssh-add 1 ,
1567: .Xr ssh-agent 1 ,
1568: .Xr ssh-keygen 1 ,
1.242 jmc 1569: .Xr ssh-keyscan 1 ,
1.250 jmc 1570: .Xr tun 4 ,
1.159 stevesk 1571: .Xr ssh_config 5 ,
1.160 naddy 1572: .Xr ssh-keysign 8 ,
1.87 itojun 1573: .Xr sshd 8
1.329 jmc 1574: .Sh STANDARDS
1.106 markus 1575: .Rs
1.329 jmc 1576: .%A S. Lehtinen
1577: .%A C. Lonvick
1578: .%D January 2006
1.256 jmc 1579: .%R RFC 4250
1.329 jmc 1580: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1581: .Re
1.329 jmc 1582: .Pp
1.256 jmc 1583: .Rs
1.329 jmc 1584: .%A T. Ylonen
1585: .%A C. Lonvick
1586: .%D January 2006
1.256 jmc 1587: .%R RFC 4251
1.329 jmc 1588: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1589: .Re
1.329 jmc 1590: .Pp
1.256 jmc 1591: .Rs
1.329 jmc 1592: .%A T. Ylonen
1593: .%A C. Lonvick
1594: .%D January 2006
1.256 jmc 1595: .%R RFC 4252
1.329 jmc 1596: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1597: .Re
1.329 jmc 1598: .Pp
1.256 jmc 1599: .Rs
1.329 jmc 1600: .%A T. Ylonen
1601: .%A C. Lonvick
1602: .%D January 2006
1.256 jmc 1603: .%R RFC 4253
1.329 jmc 1604: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1605: .Re
1.329 jmc 1606: .Pp
1.256 jmc 1607: .Rs
1.329 jmc 1608: .%A T. Ylonen
1609: .%A C. Lonvick
1610: .%D January 2006
1.256 jmc 1611: .%R RFC 4254
1.329 jmc 1612: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1613: .Re
1.329 jmc 1614: .Pp
1.256 jmc 1615: .Rs
1.329 jmc 1616: .%A J. Schlyter
1617: .%A W. Griffin
1618: .%D January 2006
1.256 jmc 1619: .%R RFC 4255
1.329 jmc 1620: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1621: .Re
1.329 jmc 1622: .Pp
1.256 jmc 1623: .Rs
1.329 jmc 1624: .%A F. Cusack
1625: .%A M. Forssen
1626: .%D January 2006
1.256 jmc 1627: .%R RFC 4256
1.329 jmc 1628: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1629: .Re
1.329 jmc 1630: .Pp
1.256 jmc 1631: .Rs
1.329 jmc 1632: .%A J. Galbraith
1633: .%A P. Remaker
1634: .%D January 2006
1.256 jmc 1635: .%R RFC 4335
1.329 jmc 1636: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1637: .Re
1.329 jmc 1638: .Pp
1.256 jmc 1639: .Rs
1.329 jmc 1640: .%A M. Bellare
1641: .%A T. Kohno
1642: .%A C. Namprempre
1643: .%D January 2006
1.256 jmc 1644: .%R RFC 4344
1.329 jmc 1645: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1646: .Re
1.329 jmc 1647: .Pp
1.256 jmc 1648: .Rs
1.329 jmc 1649: .%A B. Harris
1650: .%D January 2006
1.256 jmc 1651: .%R RFC 4345
1.329 jmc 1652: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1653: .Re
1.329 jmc 1654: .Pp
1.258 djm 1655: .Rs
1.329 jmc 1656: .%A M. Friedl
1657: .%A N. Provos
1658: .%A W. Simpson
1659: .%D March 2006
1.258 djm 1660: .%R RFC 4419
1.329 jmc 1661: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1662: .Re
1.329 jmc 1663: .Pp
1.266 markus 1664: .Rs
1.329 jmc 1665: .%A J. Galbraith
1666: .%A R. Thayer
1667: .%D November 2006
1.266 markus 1668: .%R RFC 4716
1.329 jmc 1669: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1670: .Re
1.329 jmc 1671: .Pp
1.313 djm 1672: .Rs
1.329 jmc 1673: .%A D. Stebila
1674: .%A J. Green
1675: .%D December 2009
1.313 djm 1676: .%R RFC 5656
1.329 jmc 1677: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1678: .Re
1.329 jmc 1679: .Pp
1.274 grunk 1680: .Rs
1681: .%A A. Perrig
1682: .%A D. Song
1683: .%D 1999
1.329 jmc 1684: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1685: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1686: .Re
1.173 jmc 1687: .Sh AUTHORS
1688: OpenSSH is a derivative of the original and free
1689: ssh 1.2.12 release by Tatu Ylonen.
1690: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1691: Theo de Raadt and Dug Song
1692: removed many bugs, re-added newer features and
1693: created OpenSSH.
1694: Markus Friedl contributed the support for SSH
1695: protocol versions 1.5 and 2.0.
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