Annotation of src/usr.bin/ssh/ssh.1, Revision 1.393
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: .\"
1.393 ! djm 36: .\" $OpenBSD: ssh.1,v 1.392 2018/04/06 03:34:27 djm Exp $
! 37: .Dd $Mdocdate: April 6 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
1.316 jmc 502: .It IPQoS
1.321 djm 503: .It KbdInteractiveAuthentication
1.218 jmc 504: .It KbdInteractiveDevices
1.314 jmc 505: .It KexAlgorithms
1.218 jmc 506: .It LocalCommand
507: .It LocalForward
508: .It LogLevel
509: .It MACs
1.337 jmc 510: .It Match
1.218 jmc 511: .It NoHostAuthenticationForLocalhost
512: .It NumberOfPasswordPrompts
513: .It PasswordAuthentication
514: .It PermitLocalCommand
1.291 markus 515: .It PKCS11Provider
1.218 jmc 516: .It Port
517: .It PreferredAuthentications
518: .It ProxyCommand
1.375 djm 519: .It ProxyJump
1.336 jmc 520: .It ProxyUseFdpass
1.359 markus 521: .It PubkeyAcceptedKeyTypes
1.218 jmc 522: .It PubkeyAuthentication
1.251 dtucker 523: .It RekeyLimit
1.382 bluhm 524: .It RemoteCommand
1.218 jmc 525: .It RemoteForward
1.319 jmc 526: .It RequestTTY
1.218 jmc 527: .It SendEnv
528: .It ServerAliveInterval
529: .It ServerAliveCountMax
1.347 jmc 530: .It StreamLocalBindMask
531: .It StreamLocalBindUnlink
1.218 jmc 532: .It StrictHostKeyChecking
533: .It TCPKeepAlive
534: .It Tunnel
535: .It TunnelDevice
1.354 djm 536: .It UpdateHostKeys
1.218 jmc 537: .It UsePrivilegedPort
538: .It User
539: .It UserKnownHostsFile
540: .It VerifyHostKeyDNS
1.276 jmc 541: .It VisualHostKey
1.218 jmc 542: .It XAuthLocation
543: .El
1.360 millert 544: .Pp
1.218 jmc 545: .It Fl p Ar port
546: Port to connect to on the remote host.
547: This can be specified on a
548: per-host basis in the configuration file.
1.360 millert 549: .Pp
1.362 djm 550: .It Fl Q Ar query_option
1.332 djm 551: Queries
552: .Nm
1.341 deraadt 553: for the algorithms supported for the specified version 2.
554: The available features are:
555: .Ar cipher
1.332 djm 556: (supported symmetric ciphers),
1.341 deraadt 557: .Ar cipher-auth
1.340 djm 558: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 559: .Ar mac
1.332 djm 560: (supported message integrity codes),
1.341 deraadt 561: .Ar kex
1.332 djm 562: (key exchange algorithms),
1.341 deraadt 563: .Ar key
1.362 djm 564: (key types),
565: .Ar key-cert
566: (certificate key types),
567: .Ar key-plain
568: (non-certificate key types), and
1.356 djm 569: .Ar protocol-version
570: (supported SSH protocol versions).
1.360 millert 571: .Pp
1.218 jmc 572: .It Fl q
573: Quiet mode.
1.271 djm 574: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 575: .Pp
1.218 jmc 576: .It Fl R Xo
577: .Sm off
578: .Oo Ar bind_address : Oc
579: .Ar port : host : hostport
580: .Sm on
581: .Xc
1.360 millert 582: .It Fl R Xo
583: .Sm off
584: .Oo Ar bind_address : Oc
585: .Ar port : local_socket
586: .Sm on
587: .Xc
588: .It Fl R Xo
589: .Sm off
590: .Ar remote_socket : host : hostport
591: .Sm on
592: .Xc
593: .It Fl R Xo
594: .Sm off
595: .Ar remote_socket : local_socket
596: .Sm on
597: .Xc
1.384 markus 598: .It Fl R Xo
599: .Sm off
600: .Oo Ar bind_address : Oc
601: .Ar port
602: .Sm on
603: .Xc
1.360 millert 604: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 605: (server) host are to be forwarded to the local side.
606: .Pp
1.360 millert 607: This works by allocating a socket to listen to either a TCP
1.218 jmc 608: .Ar port
1.360 millert 609: or to a Unix socket on the remote side.
610: Whenever a connection is made to this port or Unix socket, the
611: connection is forwarded over the secure channel, and a connection
1.384 markus 612: is made from the local machine to either an explicit destination specified by
1.218 jmc 613: .Ar host
614: port
1.360 millert 615: .Ar hostport ,
616: or
617: .Ar local_socket ,
1.384 markus 618: or, if no explicit destination was specified,
619: .Nm
620: will act as a SOCKS 4/5 proxy and forward connections to the destinations
621: requested by the remote SOCKS client.
1.2 deraadt 622: .Pp
1.218 jmc 623: Port forwardings can also be specified in the configuration file.
624: Privileged ports can be forwarded only when
625: logging in as root on the remote machine.
1.324 jmc 626: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 627: .Pp
1.360 millert 628: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 629: interface only.
1.280 tobias 630: This may be overridden by specifying a
1.218 jmc 631: .Ar bind_address .
632: An empty
633: .Ar bind_address ,
634: or the address
635: .Ql * ,
636: indicates that the remote socket should listen on all interfaces.
637: Specifying a remote
638: .Ar bind_address
639: will only succeed if the server's
640: .Cm GatewayPorts
641: option is enabled (see
642: .Xr sshd_config 5 ) .
1.281 djm 643: .Pp
644: If the
645: .Ar port
646: argument is
1.282 djm 647: .Ql 0 ,
1.281 djm 648: the listen port will be dynamically allocated on the server and reported
649: to the client at run time.
1.305 markus 650: When used together with
651: .Ic -O forward
652: the allocated port will be printed to the standard output.
1.360 millert 653: .Pp
1.218 jmc 654: .It Fl S Ar ctl_path
1.304 jmc 655: Specifies the location of a control socket for connection sharing,
1.303 djm 656: or the string
657: .Dq none
658: to disable connection sharing.
1.218 jmc 659: Refer to the description of
660: .Cm ControlPath
661: and
662: .Cm ControlMaster
663: in
664: .Xr ssh_config 5
665: for details.
1.360 millert 666: .Pp
1.218 jmc 667: .It Fl s
668: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 669: Subsystems facilitate the use of SSH
670: as a secure transport for other applications (e.g.\&
1.218 jmc 671: .Xr sftp 1 ) .
672: The subsystem is specified as the remote command.
1.360 millert 673: .Pp
1.218 jmc 674: .It Fl T
1.357 dtucker 675: Disable pseudo-terminal allocation.
1.360 millert 676: .Pp
1.218 jmc 677: .It Fl t
1.357 dtucker 678: Force pseudo-terminal allocation.
1.218 jmc 679: This can be used to execute arbitrary
680: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 681: e.g. when implementing menu services.
1.218 jmc 682: Multiple
683: .Fl t
684: options force tty allocation, even if
1.194 jakob 685: .Nm
1.218 jmc 686: has no local tty.
1.360 millert 687: .Pp
1.218 jmc 688: .It Fl V
689: Display the version number and exit.
1.360 millert 690: .Pp
1.218 jmc 691: .It Fl v
692: Verbose mode.
693: Causes
1.176 jmc 694: .Nm
1.218 jmc 695: to print debugging messages about its progress.
696: This is helpful in
697: debugging connection, authentication, and configuration problems.
698: Multiple
699: .Fl v
700: options increase the verbosity.
701: The maximum is 3.
1.360 millert 702: .Pp
1.290 dtucker 703: .It Fl W Ar host : Ns Ar port
704: Requests that standard input and output on the client be forwarded to
705: .Ar host
706: on
707: .Ar port
708: over the secure channel.
709: Implies
710: .Fl N ,
711: .Fl T ,
712: .Cm ExitOnForwardFailure
713: and
1.373 jmc 714: .Cm ClearAllForwardings ,
715: though these can be overridden in the configuration file or using
1.372 dtucker 716: .Fl o
717: command line options.
1.360 millert 718: .Pp
1.261 stevesk 719: .It Fl w Xo
720: .Ar local_tun Ns Op : Ns Ar remote_tun
721: .Xc
722: Requests
723: tunnel
724: device forwarding with the specified
1.218 jmc 725: .Xr tun 4
1.261 stevesk 726: devices between the client
727: .Pq Ar local_tun
728: and the server
729: .Pq Ar remote_tun .
730: .Pp
1.228 jmc 731: The devices may be specified by numerical ID or the keyword
732: .Dq any ,
733: which uses the next available tunnel device.
1.261 stevesk 734: If
735: .Ar remote_tun
736: is not specified, it defaults to
737: .Dq any .
1.228 jmc 738: See also the
1.218 jmc 739: .Cm Tunnel
1.261 stevesk 740: and
741: .Cm TunnelDevice
742: directives in
1.218 jmc 743: .Xr ssh_config 5 .
1.393 ! djm 744: .Pp
1.261 stevesk 745: If the
746: .Cm Tunnel
1.393 ! djm 747: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 748: .Dq point-to-point .
1.393 ! djm 749: If a different
! 750: .Cm Tunnel
! 751: forwarding mode it desired, then it should be specified before
! 752: .Fl w .
1.360 millert 753: .Pp
1.218 jmc 754: .It Fl X
755: Enables X11 forwarding.
1.54 markus 756: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 757: .Pp
1.218 jmc 758: X11 forwarding should be enabled with caution.
1.168 jmc 759: Users with the ability to bypass file permissions on the remote host
1.218 jmc 760: (for the user's X authorization database)
761: can access the local X11 display through the forwarded connection.
762: An attacker may then be able to perform activities such as keystroke monitoring.
763: .Pp
764: For this reason, X11 forwarding is subjected to X11 SECURITY extension
765: restrictions by default.
766: Please refer to the
767: .Nm
768: .Fl Y
769: option and the
770: .Cm ForwardX11Trusted
771: directive in
772: .Xr ssh_config 5
773: for more information.
1.360 millert 774: .Pp
1.218 jmc 775: .It Fl x
776: Disables X11 forwarding.
1.360 millert 777: .Pp
1.218 jmc 778: .It Fl Y
779: Enables trusted X11 forwarding.
780: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
781: controls.
1.360 millert 782: .Pp
1.278 djm 783: .It Fl y
784: Send log information using the
785: .Xr syslog 3
786: system module.
787: By default this information is sent to stderr.
1.218 jmc 788: .El
1.224 jmc 789: .Pp
790: .Nm
791: may additionally obtain configuration data from
792: a per-user configuration file and a system-wide configuration file.
793: The file format and configuration options are described in
794: .Xr ssh_config 5 .
1.222 jmc 795: .Sh AUTHENTICATION
1.377 djm 796: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 797: .Pp
798: The methods available for authentication are:
1.260 jmc 799: GSSAPI-based authentication,
1.222 jmc 800: host-based authentication,
801: public key authentication,
802: challenge-response authentication,
803: and password authentication.
804: Authentication methods are tried in the order specified above,
1.369 jmc 805: though
806: .Cm PreferredAuthentications
807: can be used to change the default order.
1.222 jmc 808: .Pp
809: Host-based authentication works as follows:
1.218 jmc 810: If the machine the user logs in from is listed in
811: .Pa /etc/hosts.equiv
812: or
813: .Pa /etc/shosts.equiv
814: on the remote machine, and the user names are
815: the same on both sides, or if the files
816: .Pa ~/.rhosts
817: or
818: .Pa ~/.shosts
819: exist in the user's home directory on the
820: remote machine and contain a line containing the name of the client
821: machine and the name of the user on that machine, the user is
1.222 jmc 822: considered for login.
823: Additionally, the server
824: .Em must
825: be able to verify the client's
826: host key (see the description of
1.218 jmc 827: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 828: and
1.222 jmc 829: .Pa ~/.ssh/known_hosts ,
830: below)
831: for login to be permitted.
1.218 jmc 832: This authentication method closes security holes due to IP
1.222 jmc 833: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 834: [Note to the administrator:
835: .Pa /etc/hosts.equiv ,
836: .Pa ~/.rhosts ,
837: and the rlogin/rsh protocol in general, are inherently insecure and should be
838: disabled if security is desired.]
1.189 dtucker 839: .Pp
1.222 jmc 840: Public key authentication works as follows:
841: The scheme is based on public-key cryptography,
842: using cryptosystems
843: where encryption and decryption are done using separate keys,
844: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 845: The idea is that each user creates a public/private
846: key pair for authentication purposes.
847: The server knows the public key, and only the user knows the private key.
1.222 jmc 848: .Nm
849: implements public key authentication protocol automatically,
1.349 sobrado 850: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 851: The HISTORY section of
1.222 jmc 852: .Xr ssl 8
1.311 jmc 853: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 854: .Pp
1.218 jmc 855: The file
856: .Pa ~/.ssh/authorized_keys
857: lists the public keys that are permitted for logging in.
858: When the user logs in, the
1.2 deraadt 859: .Nm
1.218 jmc 860: program tells the server which key pair it would like to use for
861: authentication.
1.222 jmc 862: The client proves that it has access to the private key
863: and the server checks that the corresponding public key
864: is authorized to accept the account.
1.383 djm 865: .Pp
866: The server may inform the client of errors that prevented public key
867: authentication from succeeding after authentication completes using a
868: different method.
869: These may be viewed by increasing the
870: .Cm LogLevel
871: to
872: .Cm DEBUG
873: or higher (e.g. by using the
874: .Fl v
875: flag).
1.218 jmc 876: .Pp
1.222 jmc 877: The user creates his/her key pair by running
1.218 jmc 878: .Xr ssh-keygen 1 .
879: This stores the private key in
1.222 jmc 880: .Pa ~/.ssh/id_dsa
1.369 jmc 881: (DSA),
1.310 djm 882: .Pa ~/.ssh/id_ecdsa
1.369 jmc 883: (ECDSA),
1.343 naddy 884: .Pa ~/.ssh/id_ed25519
1.369 jmc 885: (Ed25519),
1.222 jmc 886: or
887: .Pa ~/.ssh/id_rsa
1.369 jmc 888: (RSA)
1.218 jmc 889: and stores the public key in
1.222 jmc 890: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 891: (DSA),
1.310 djm 892: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 893: (ECDSA),
1.343 naddy 894: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 895: (Ed25519),
1.222 jmc 896: or
897: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 898: (RSA)
1.218 jmc 899: in the user's home directory.
1.222 jmc 900: The user should then copy the public key
1.218 jmc 901: to
902: .Pa ~/.ssh/authorized_keys
1.222 jmc 903: in his/her home directory on the remote machine.
904: The
1.218 jmc 905: .Pa authorized_keys
906: file corresponds to the conventional
907: .Pa ~/.rhosts
908: file, and has one key
1.222 jmc 909: per line, though the lines can be very long.
1.218 jmc 910: After this, the user can log in without giving the password.
911: .Pp
1.301 jmc 912: A variation on public key authentication
913: is available in the form of certificate authentication:
914: instead of a set of public/private keys,
915: signed certificates are used.
916: This has the advantage that a single trusted certification authority
917: can be used in place of many public/private keys.
1.333 jmc 918: See the CERTIFICATES section of
1.301 jmc 919: .Xr ssh-keygen 1
920: for more information.
921: .Pp
922: The most convenient way to use public key or certificate authentication
923: may be with an authentication agent.
1.218 jmc 924: See
925: .Xr ssh-agent 1
1.366 jcs 926: and (optionally) the
927: .Cm AddKeysToAgent
928: directive in
929: .Xr ssh_config 5
1.218 jmc 930: for more information.
931: .Pp
1.222 jmc 932: Challenge-response authentication works as follows:
933: The server sends an arbitrary
934: .Qq challenge
935: text, and prompts for a response.
936: Examples of challenge-response authentication include
1.335 jmc 937: .Bx
938: Authentication (see
1.222 jmc 939: .Xr login.conf 5 )
1.335 jmc 940: and PAM (some
941: .Pf non- Ox
942: systems).
1.222 jmc 943: .Pp
944: Finally, if other authentication methods fail,
1.218 jmc 945: .Nm
946: prompts the user for a password.
947: The password is sent to the remote
948: host for checking; however, since all communications are encrypted,
949: the password cannot be seen by someone listening on the network.
1.232 jmc 950: .Pp
951: .Nm
952: automatically maintains and checks a database containing
953: identification for all hosts it has ever been used with.
954: Host keys are stored in
955: .Pa ~/.ssh/known_hosts
956: in the user's home directory.
957: Additionally, the file
958: .Pa /etc/ssh/ssh_known_hosts
959: is automatically checked for known hosts.
960: Any new hosts are automatically added to the user's file.
961: If a host's identification ever changes,
962: .Nm
963: warns about this and disables password authentication to prevent
964: server spoofing or man-in-the-middle attacks,
965: which could otherwise be used to circumvent the encryption.
966: The
967: .Cm StrictHostKeyChecking
968: option can be used to control logins to machines whose
969: host key is not known or has changed.
970: .Pp
1.218 jmc 971: When the user's identity has been accepted by the server, the server
1.357 dtucker 972: either executes the given command in a non-interactive session or,
973: if no command has been specified, logs into the machine and gives
974: the user a normal shell as an interactive session.
1.218 jmc 975: All communication with
976: the remote command or shell will be automatically encrypted.
977: .Pp
1.357 dtucker 978: If an interactive session is requested
979: .Nm
980: by default will only request a pseudo-terminal (pty) for interactive
981: sessions when the client has one.
982: The flags
983: .Fl T
984: and
985: .Fl t
986: can be used to override this behaviour.
987: .Pp
988: If a pseudo-terminal has been allocated the
1.218 jmc 989: user may use the escape characters noted below.
990: .Pp
1.357 dtucker 991: If no pseudo-terminal has been allocated,
1.218 jmc 992: the session is transparent and can be used to reliably transfer binary data.
993: On most systems, setting the escape character to
994: .Dq none
995: will also make the session transparent even if a tty is used.
996: .Pp
997: The session terminates when the command or shell on the remote
1.247 jmc 998: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 999: .Sh ESCAPE CHARACTERS
1.218 jmc 1000: When a pseudo-terminal has been requested,
1.2 deraadt 1001: .Nm
1.218 jmc 1002: supports a number of functions through the use of an escape character.
1003: .Pp
1004: A single tilde character can be sent as
1005: .Ic ~~
1006: or by following the tilde by a character other than those described below.
1007: The escape character must always follow a newline to be interpreted as
1008: special.
1009: The escape character can be changed in configuration files using the
1010: .Cm EscapeChar
1011: configuration directive or on the command line by the
1012: .Fl e
1013: option.
1014: .Pp
1015: The supported escapes (assuming the default
1016: .Ql ~ )
1017: are:
1018: .Bl -tag -width Ds
1019: .It Cm ~.
1020: Disconnect.
1021: .It Cm ~^Z
1022: Background
1.234 jmc 1023: .Nm .
1.218 jmc 1024: .It Cm ~#
1025: List forwarded connections.
1026: .It Cm ~&
1027: Background
1.2 deraadt 1028: .Nm
1.218 jmc 1029: at logout when waiting for forwarded connection / X11 sessions to terminate.
1030: .It Cm ~?
1031: Display a list of escape characters.
1032: .It Cm ~B
1033: Send a BREAK to the remote system
1.369 jmc 1034: (only useful if the peer supports it).
1.218 jmc 1035: .It Cm ~C
1036: Open command line.
1037: Currently this allows the addition of port forwardings using the
1.279 stevesk 1038: .Fl L ,
1039: .Fl R
1.218 jmc 1040: and
1.279 stevesk 1041: .Fl D
1.225 jmc 1042: options (see above).
1.322 markus 1043: It also allows the cancellation of existing port-forwardings
1044: with
1.262 stevesk 1045: .Sm off
1.322 markus 1046: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1047: .Sm on
1.322 markus 1048: for local,
1049: .Sm off
1050: .Fl KR Oo Ar bind_address : Oc Ar port
1051: .Sm on
1052: for remote and
1053: .Sm off
1054: .Fl KD Oo Ar bind_address : Oc Ar port
1055: .Sm on
1056: for dynamic port-forwardings.
1.218 jmc 1057: .Ic !\& Ns Ar command
1058: allows the user to execute a local command if the
1059: .Ic PermitLocalCommand
1060: option is enabled in
1.176 jmc 1061: .Xr ssh_config 5 .
1.218 jmc 1062: Basic help is available, using the
1063: .Fl h
1064: option.
1065: .It Cm ~R
1066: Request rekeying of the connection
1.369 jmc 1067: (only useful if the peer supports it).
1.327 dtucker 1068: .It Cm ~V
1069: Decrease the verbosity
1070: .Pq Ic LogLevel
1071: when errors are being written to stderr.
1072: .It Cm ~v
1.328 jmc 1073: Increase the verbosity
1.327 dtucker 1074: .Pq Ic LogLevel
1075: when errors are being written to stderr.
1.176 jmc 1076: .El
1.246 jmc 1077: .Sh TCP FORWARDING
1078: Forwarding of arbitrary TCP connections over the secure channel can
1079: be specified either on the command line or in a configuration file.
1080: One possible application of TCP forwarding is a secure connection to a
1081: mail server; another is going through firewalls.
1082: .Pp
1083: In the example below, we look at encrypting communication between
1084: an IRC client and server, even though the IRC server does not directly
1085: support encrypted communications.
1086: This works as follows:
1087: the user connects to the remote host using
1088: .Nm ,
1089: specifying a port to be used to forward connections
1090: to the remote server.
1091: After that it is possible to start the service which is to be encrypted
1092: on the client machine,
1093: connecting to the same local port,
1094: and
1095: .Nm
1096: will encrypt and forward the connection.
1097: .Pp
1098: The following example tunnels an IRC session from client machine
1099: .Dq 127.0.0.1
1100: (localhost)
1101: to remote server
1102: .Dq server.example.com :
1103: .Bd -literal -offset 4n
1104: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1105: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1106: .Ed
1107: .Pp
1108: This tunnels a connection to IRC server
1109: .Dq server.example.com ,
1110: joining channel
1111: .Dq #users ,
1112: nickname
1113: .Dq pinky ,
1114: using port 1234.
1115: It doesn't matter which port is used,
1116: as long as it's greater than 1023
1117: (remember, only root can open sockets on privileged ports)
1118: and doesn't conflict with any ports already in use.
1119: The connection is forwarded to port 6667 on the remote server,
1120: since that's the standard port for IRC services.
1121: .Pp
1122: The
1123: .Fl f
1124: option backgrounds
1125: .Nm
1126: and the remote command
1127: .Dq sleep 10
1128: is specified to allow an amount of time
1129: (10 seconds, in the example)
1130: to start the service which is to be tunnelled.
1131: If no connections are made within the time specified,
1132: .Nm
1133: will exit.
1134: .Sh X11 FORWARDING
1.218 jmc 1135: If the
1136: .Cm ForwardX11
1137: variable is set to
1138: .Dq yes
1139: (or see the description of the
1.227 jmc 1140: .Fl X ,
1141: .Fl x ,
1.218 jmc 1142: and
1.227 jmc 1143: .Fl Y
1.226 jmc 1144: options above)
1.218 jmc 1145: and the user is using X11 (the
1146: .Ev DISPLAY
1147: environment variable is set), the connection to the X11 display is
1148: automatically forwarded to the remote side in such a way that any X11
1149: programs started from the shell (or command) will go through the
1150: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1151: from the local machine.
1.218 jmc 1152: The user should not manually set
1153: .Ev DISPLAY .
1154: Forwarding of X11 connections can be
1155: configured on the command line or in configuration files.
1156: .Pp
1157: The
1158: .Ev DISPLAY
1159: value set by
1160: .Nm
1161: will point to the server machine, but with a display number greater than zero.
1162: This is normal, and happens because
1163: .Nm
1164: creates a
1165: .Dq proxy
1166: X server on the server machine for forwarding the
1167: connections over the encrypted channel.
1.200 djm 1168: .Pp
1.218 jmc 1169: .Nm
1170: will also automatically set up Xauthority data on the server machine.
1171: For this purpose, it will generate a random authorization cookie,
1172: store it in Xauthority on the server, and verify that any forwarded
1173: connections carry this cookie and replace it by the real cookie when
1174: the connection is opened.
1175: The real authentication cookie is never
1176: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1177: .Pp
1.218 jmc 1178: If the
1179: .Cm ForwardAgent
1180: variable is set to
1181: .Dq yes
1182: (or see the description of the
1183: .Fl A
1.191 djm 1184: and
1.218 jmc 1185: .Fl a
1.226 jmc 1186: options above) and
1.218 jmc 1187: the user is using an authentication agent, the connection to the agent
1188: is automatically forwarded to the remote side.
1.252 jmc 1189: .Sh VERIFYING HOST KEYS
1190: When connecting to a server for the first time,
1191: a fingerprint of the server's public key is presented to the user
1192: (unless the option
1193: .Cm StrictHostKeyChecking
1194: has been disabled).
1195: Fingerprints can be determined using
1196: .Xr ssh-keygen 1 :
1197: .Pp
1198: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1199: .Pp
1.274 grunk 1200: If the fingerprint is already known, it can be matched
1201: and the key can be accepted or rejected.
1.358 djm 1202: If only legacy (MD5) fingerprints for the server are available, the
1203: .Xr ssh-keygen 1
1204: .Fl E
1205: option may be used to downgrade the fingerprint algorithm to match.
1206: .Pp
1.274 grunk 1207: Because of the difficulty of comparing host keys
1.352 djm 1208: just by looking at fingerprint strings,
1.274 grunk 1209: there is also support to compare host keys visually,
1210: using
1211: .Em random art .
1212: By setting the
1.275 grunk 1213: .Cm VisualHostKey
1.274 grunk 1214: option to
1.275 grunk 1215: .Dq yes ,
1.274 grunk 1216: a small ASCII graphic gets displayed on every login to a server, no matter
1217: if the session itself is interactive or not.
1218: By learning the pattern a known server produces, a user can easily
1219: find out that the host key has changed when a completely different pattern
1220: is displayed.
1221: Because these patterns are not unambiguous however, a pattern that looks
1222: similar to the pattern remembered only gives a good probability that the
1223: host key is the same, not guaranteed proof.
1224: .Pp
1225: To get a listing of the fingerprints along with their random art for
1226: all known hosts, the following command line can be used:
1227: .Pp
1228: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1229: .Pp
1.252 jmc 1230: If the fingerprint is unknown,
1231: an alternative method of verification is available:
1232: SSH fingerprints verified by DNS.
1233: An additional resource record (RR),
1234: SSHFP,
1235: is added to a zonefile
1236: and the connecting client is able to match the fingerprint
1237: with that of the key presented.
1238: .Pp
1239: In this example, we are connecting a client to a server,
1240: .Dq host.example.com .
1241: The SSHFP resource records should first be added to the zonefile for
1242: host.example.com:
1243: .Bd -literal -offset indent
1.259 jakob 1244: $ ssh-keygen -r host.example.com.
1.252 jmc 1245: .Ed
1246: .Pp
1247: The output lines will have to be added to the zonefile.
1248: To check that the zone is answering fingerprint queries:
1249: .Pp
1250: .Dl $ dig -t SSHFP host.example.com
1251: .Pp
1252: Finally the client connects:
1253: .Bd -literal -offset indent
1254: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1255: [...]
1256: Matching host key fingerprint found in DNS.
1257: Are you sure you want to continue connecting (yes/no)?
1258: .Ed
1259: .Pp
1260: See the
1261: .Cm VerifyHostKeyDNS
1262: option in
1263: .Xr ssh_config 5
1264: for more information.
1.250 jmc 1265: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1266: .Nm
1267: contains support for Virtual Private Network (VPN) tunnelling
1268: using the
1269: .Xr tun 4
1270: network pseudo-device,
1271: allowing two networks to be joined securely.
1272: The
1273: .Xr sshd_config 5
1274: configuration option
1275: .Cm PermitTunnel
1276: controls whether the server supports this,
1277: and at what level (layer 2 or 3 traffic).
1278: .Pp
1279: The following example would connect client network 10.0.50.0/24
1.265 otto 1280: with remote network 10.0.99.0/24 using a point-to-point connection
1281: from 10.1.1.1 to 10.1.1.2,
1282: provided that the SSH server running on the gateway to the remote network,
1283: at 192.168.1.15, allows it.
1284: .Pp
1285: On the client:
1.250 jmc 1286: .Bd -literal -offset indent
1287: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1288: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1289: # route add 10.0.99.0/24 10.1.1.2
1290: .Ed
1291: .Pp
1292: On the server:
1293: .Bd -literal -offset indent
1294: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1295: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1296: .Ed
1297: .Pp
1298: Client access may be more finely tuned via the
1299: .Pa /root/.ssh/authorized_keys
1300: file (see below) and the
1301: .Cm PermitRootLogin
1302: server option.
1.255 jmc 1303: The following entry would permit connections on
1.250 jmc 1304: .Xr tun 4
1.255 jmc 1305: device 1 from user
1.250 jmc 1306: .Dq jane
1.255 jmc 1307: and on tun device 2 from user
1.250 jmc 1308: .Dq john ,
1309: if
1310: .Cm PermitRootLogin
1311: is set to
1312: .Dq forced-commands-only :
1313: .Bd -literal -offset 2n
1314: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1315: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1316: .Ed
1317: .Pp
1.264 ray 1318: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1319: it may be more suited to temporary setups,
1320: such as for wireless VPNs.
1321: More permanent VPNs are better provided by tools such as
1322: .Xr ipsecctl 8
1323: and
1324: .Xr isakmpd 8 .
1.2 deraadt 1325: .Sh ENVIRONMENT
1326: .Nm
1.1 deraadt 1327: will normally set the following environment variables:
1.237 jmc 1328: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1329: .It Ev DISPLAY
1330: The
1331: .Ev DISPLAY
1.40 aaron 1332: variable indicates the location of the X11 server.
1.44 aaron 1333: It is automatically set by
1.2 deraadt 1334: .Nm
1335: to point to a value of the form
1.233 jmc 1336: .Dq hostname:n ,
1337: where
1338: .Dq hostname
1339: indicates the host where the shell runs, and
1340: .Sq n
1341: is an integer \*(Ge 1.
1.40 aaron 1342: .Nm
1343: uses this special value to forward X11 connections over the secure
1344: channel.
1.107 markus 1345: The user should normally not set
1346: .Ev DISPLAY
1347: explicitly, as that
1.1 deraadt 1348: will render the X11 connection insecure (and will require the user to
1349: manually copy any required authorization cookies).
1.2 deraadt 1350: .It Ev HOME
1.1 deraadt 1351: Set to the path of the user's home directory.
1.2 deraadt 1352: .It Ev LOGNAME
1353: Synonym for
1.12 aaron 1354: .Ev USER ;
1355: set for compatibility with systems that use this variable.
1.2 deraadt 1356: .It Ev MAIL
1.129 stevesk 1357: Set to the path of the user's mailbox.
1.40 aaron 1358: .It Ev PATH
1.2 deraadt 1359: Set to the default
1360: .Ev PATH ,
1361: as specified when compiling
1.234 jmc 1362: .Nm .
1.118 markus 1363: .It Ev SSH_ASKPASS
1364: If
1365: .Nm
1366: needs a passphrase, it will read the passphrase from the current
1367: terminal if it was run from a terminal.
1368: If
1369: .Nm
1370: does not have a terminal associated with it but
1371: .Ev DISPLAY
1372: and
1373: .Ev SSH_ASKPASS
1374: are set, it will execute the program specified by
1375: .Ev SSH_ASKPASS
1376: and open an X11 window to read the passphrase.
1377: This is particularly useful when calling
1378: .Nm
1379: from a
1.196 jmc 1380: .Pa .xsession
1.118 markus 1381: or related script.
1382: (Note that on some machines it
1383: may be necessary to redirect the input from
1384: .Pa /dev/null
1385: to make this work.)
1.18 markus 1386: .It Ev SSH_AUTH_SOCK
1.233 jmc 1387: Identifies the path of a
1388: .Ux Ns -domain
1389: socket used to communicate with the agent.
1.166 stevesk 1390: .It Ev SSH_CONNECTION
1391: Identifies the client and server ends of the connection.
1.40 aaron 1392: The variable contains
1.233 jmc 1393: four space-separated values: client IP address, client port number,
1394: server IP address, and server port number.
1.73 markus 1395: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1396: This variable contains the original command line if a forced command
1.73 markus 1397: is executed.
1398: It can be used to extract the original arguments.
1.2 deraadt 1399: .It Ev SSH_TTY
1.1 deraadt 1400: This is set to the name of the tty (path to the device) associated
1.40 aaron 1401: with the current shell or command.
1402: If the current session has no tty,
1.1 deraadt 1403: this variable is not set.
1.387 djm 1404: .It Ev SSH_TUNNEL
1405: Optionally set by
1406: .Xr sshd 8
1407: to contain the interface names assigned if tunnel forwarding was
1408: requested by the client.
1.385 djm 1409: .It Ev SSH_USER_AUTH
1410: Optionally set by
1411: .Xr sshd 8 ,
1412: this variable may contain a pathname to a file that lists the authentication
1413: methods successfully used when the session was established, including any
1414: public keys that were used.
1.2 deraadt 1415: .It Ev TZ
1.214 jmc 1416: This variable is set to indicate the present time zone if it
1.257 jmc 1417: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1418: on to new connections).
1.2 deraadt 1419: .It Ev USER
1.1 deraadt 1420: Set to the name of the user logging in.
1.2 deraadt 1421: .El
1422: .Pp
1.44 aaron 1423: Additionally,
1.2 deraadt 1424: .Nm
1.44 aaron 1425: reads
1.207 djm 1426: .Pa ~/.ssh/environment ,
1.2 deraadt 1427: and adds lines of the format
1428: .Dq VARNAME=value
1.233 jmc 1429: to the environment if the file exists and users are allowed to
1.161 marc 1430: change their environment.
1.176 jmc 1431: For more information, see the
1.161 marc 1432: .Cm PermitUserEnvironment
1.162 stevesk 1433: option in
1.161 marc 1434: .Xr sshd_config 5 .
1.2 deraadt 1435: .Sh FILES
1.236 jmc 1436: .Bl -tag -width Ds -compact
1.309 jmc 1437: .It Pa ~/.rhosts
1.240 jmc 1438: This file is used for host-based authentication (see above).
1.92 markus 1439: On some machines this file may need to be
1.240 jmc 1440: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1441: because
1.2 deraadt 1442: .Xr sshd 8
1.40 aaron 1443: reads it as root.
1444: Additionally, this file must be owned by the user,
1445: and must not have write permissions for anyone else.
1446: The recommended
1.1 deraadt 1447: permission for most machines is read/write for the user, and not
1448: accessible by others.
1.2 deraadt 1449: .Pp
1.309 jmc 1450: .It Pa ~/.shosts
1.240 jmc 1451: This file is used in exactly the same way as
1452: .Pa .rhosts ,
1453: but allows host-based authentication without permitting login with
1454: rlogin/rsh.
1.272 mcbride 1455: .Pp
1.309 jmc 1456: .It Pa ~/.ssh/
1.272 mcbride 1457: This directory is the default location for all user-specific configuration
1458: and authentication information.
1459: There is no general requirement to keep the entire contents of this directory
1460: secret, but the recommended permissions are read/write/execute for the user,
1461: and not accessible by others.
1.236 jmc 1462: .Pp
1.309 jmc 1463: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1464: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1465: that can be used for logging in as this user.
1.238 jmc 1466: The format of this file is described in the
1467: .Xr sshd 8
1468: manual page.
1469: This file is not highly sensitive, but the recommended
1470: permissions are read/write for the user, and not accessible by others.
1471: .Pp
1.309 jmc 1472: .It Pa ~/.ssh/config
1.238 jmc 1473: This is the per-user configuration file.
1474: The file format and configuration options are described in
1475: .Xr ssh_config 5 .
1476: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1477: read/write for the user, and not writable by others.
1.238 jmc 1478: .Pp
1.309 jmc 1479: .It Pa ~/.ssh/environment
1.239 jmc 1480: Contains additional definitions for environment variables; see
1481: .Sx ENVIRONMENT ,
1.238 jmc 1482: above.
1483: .Pp
1.309 jmc 1484: .It Pa ~/.ssh/id_dsa
1.310 djm 1485: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1486: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1487: .It Pa ~/.ssh/id_rsa
1.238 jmc 1488: Contains the private key for authentication.
1489: These files
1490: contain sensitive data and should be readable by the user but not
1491: accessible by others (read/write/execute).
1492: .Nm
1493: will simply ignore a private key file if it is accessible by others.
1494: It is possible to specify a passphrase when
1495: generating the key which will be used to encrypt the
1.389 djm 1496: sensitive part of this file using AES-128.
1.238 jmc 1497: .Pp
1.309 jmc 1498: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1499: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1500: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1501: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1502: Contains the public key for authentication.
1503: These files are not
1504: sensitive and can (but need not) be readable by anyone.
1505: .Pp
1.309 jmc 1506: .It Pa ~/.ssh/known_hosts
1.244 jmc 1507: Contains a list of host keys for all hosts the user has logged into
1508: that are not already in the systemwide list of known host keys.
1.238 jmc 1509: See
1.244 jmc 1510: .Xr sshd 8
1511: for further details of the format of this file.
1.238 jmc 1512: .Pp
1.309 jmc 1513: .It Pa ~/.ssh/rc
1.238 jmc 1514: Commands in this file are executed by
1515: .Nm
1.245 jmc 1516: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1517: started.
1518: See the
1519: .Xr sshd 8
1520: manual page for more information.
1521: .Pp
1.309 jmc 1522: .It Pa /etc/hosts.equiv
1.240 jmc 1523: This file is for host-based authentication (see above).
1524: It should only be writable by root.
1.236 jmc 1525: .Pp
1.309 jmc 1526: .It Pa /etc/shosts.equiv
1.240 jmc 1527: This file is used in exactly the same way as
1528: .Pa hosts.equiv ,
1529: but allows host-based authentication without permitting login with
1530: rlogin/rsh.
1.236 jmc 1531: .Pp
1.238 jmc 1532: .It Pa /etc/ssh/ssh_config
1533: Systemwide configuration file.
1534: The file format and configuration options are described in
1535: .Xr ssh_config 5 .
1536: .Pp
1.309 jmc 1537: .It Pa /etc/ssh/ssh_host_key
1538: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1539: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1540: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1541: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1542: These files contain the private parts of the host keys
1.245 jmc 1543: and are used for host-based authentication.
1.238 jmc 1544: .Pp
1.309 jmc 1545: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1546: Systemwide list of known host keys.
1547: This file should be prepared by the
1548: system administrator to contain the public host keys of all machines in the
1549: organization.
1.244 jmc 1550: It should be world-readable.
1551: See
1.238 jmc 1552: .Xr sshd 8
1.244 jmc 1553: for further details of the format of this file.
1.236 jmc 1554: .Pp
1.309 jmc 1555: .It Pa /etc/ssh/sshrc
1.1 deraadt 1556: Commands in this file are executed by
1.2 deraadt 1557: .Nm
1.245 jmc 1558: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1559: See the
1.2 deraadt 1560: .Xr sshd 8
1.1 deraadt 1561: manual page for more information.
1.58 itojun 1562: .El
1.312 jmc 1563: .Sh EXIT STATUS
1564: .Nm
1565: exits with the exit status of the remote command or with 255
1566: if an error occurred.
1.2 deraadt 1567: .Sh SEE ALSO
1568: .Xr scp 1 ,
1.83 djm 1569: .Xr sftp 1 ,
1.2 deraadt 1570: .Xr ssh-add 1 ,
1571: .Xr ssh-agent 1 ,
1572: .Xr ssh-keygen 1 ,
1.242 jmc 1573: .Xr ssh-keyscan 1 ,
1.250 jmc 1574: .Xr tun 4 ,
1.159 stevesk 1575: .Xr ssh_config 5 ,
1.160 naddy 1576: .Xr ssh-keysign 8 ,
1.87 itojun 1577: .Xr sshd 8
1.329 jmc 1578: .Sh STANDARDS
1.106 markus 1579: .Rs
1.329 jmc 1580: .%A S. Lehtinen
1581: .%A C. Lonvick
1582: .%D January 2006
1.256 jmc 1583: .%R RFC 4250
1.329 jmc 1584: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1585: .Re
1.329 jmc 1586: .Pp
1.256 jmc 1587: .Rs
1.329 jmc 1588: .%A T. Ylonen
1589: .%A C. Lonvick
1590: .%D January 2006
1.256 jmc 1591: .%R RFC 4251
1.329 jmc 1592: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1593: .Re
1.329 jmc 1594: .Pp
1.256 jmc 1595: .Rs
1.329 jmc 1596: .%A T. Ylonen
1597: .%A C. Lonvick
1598: .%D January 2006
1.256 jmc 1599: .%R RFC 4252
1.329 jmc 1600: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1601: .Re
1.329 jmc 1602: .Pp
1.256 jmc 1603: .Rs
1.329 jmc 1604: .%A T. Ylonen
1605: .%A C. Lonvick
1606: .%D January 2006
1.256 jmc 1607: .%R RFC 4253
1.329 jmc 1608: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1609: .Re
1.329 jmc 1610: .Pp
1.256 jmc 1611: .Rs
1.329 jmc 1612: .%A T. Ylonen
1613: .%A C. Lonvick
1614: .%D January 2006
1.256 jmc 1615: .%R RFC 4254
1.329 jmc 1616: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1617: .Re
1.329 jmc 1618: .Pp
1.256 jmc 1619: .Rs
1.329 jmc 1620: .%A J. Schlyter
1621: .%A W. Griffin
1622: .%D January 2006
1.256 jmc 1623: .%R RFC 4255
1.329 jmc 1624: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1625: .Re
1.329 jmc 1626: .Pp
1.256 jmc 1627: .Rs
1.329 jmc 1628: .%A F. Cusack
1629: .%A M. Forssen
1630: .%D January 2006
1.256 jmc 1631: .%R RFC 4256
1.329 jmc 1632: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1633: .Re
1.329 jmc 1634: .Pp
1.256 jmc 1635: .Rs
1.329 jmc 1636: .%A J. Galbraith
1637: .%A P. Remaker
1638: .%D January 2006
1.256 jmc 1639: .%R RFC 4335
1.329 jmc 1640: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1641: .Re
1.329 jmc 1642: .Pp
1.256 jmc 1643: .Rs
1.329 jmc 1644: .%A M. Bellare
1645: .%A T. Kohno
1646: .%A C. Namprempre
1647: .%D January 2006
1.256 jmc 1648: .%R RFC 4344
1.329 jmc 1649: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1650: .Re
1.329 jmc 1651: .Pp
1.256 jmc 1652: .Rs
1.329 jmc 1653: .%A B. Harris
1654: .%D January 2006
1.256 jmc 1655: .%R RFC 4345
1.329 jmc 1656: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1657: .Re
1.329 jmc 1658: .Pp
1.258 djm 1659: .Rs
1.329 jmc 1660: .%A M. Friedl
1661: .%A N. Provos
1662: .%A W. Simpson
1663: .%D March 2006
1.258 djm 1664: .%R RFC 4419
1.329 jmc 1665: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1666: .Re
1.329 jmc 1667: .Pp
1.266 markus 1668: .Rs
1.329 jmc 1669: .%A J. Galbraith
1670: .%A R. Thayer
1671: .%D November 2006
1.266 markus 1672: .%R RFC 4716
1.329 jmc 1673: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1674: .Re
1.329 jmc 1675: .Pp
1.313 djm 1676: .Rs
1.329 jmc 1677: .%A D. Stebila
1678: .%A J. Green
1679: .%D December 2009
1.313 djm 1680: .%R RFC 5656
1.329 jmc 1681: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1682: .Re
1.329 jmc 1683: .Pp
1.274 grunk 1684: .Rs
1685: .%A A. Perrig
1686: .%A D. Song
1687: .%D 1999
1.329 jmc 1688: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1689: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1690: .Re
1.173 jmc 1691: .Sh AUTHORS
1692: OpenSSH is a derivative of the original and free
1693: ssh 1.2.12 release by Tatu Ylonen.
1694: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1695: Theo de Raadt and Dug Song
1696: removed many bugs, re-added newer features and
1697: created OpenSSH.
1698: Markus Friedl contributed the support for SSH
1699: protocol versions 1.5 and 2.0.
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