Annotation of src/usr.bin/ssh/ssh.1, Revision 1.395
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: .\"
1.395 ! jmc 36: .\" $OpenBSD: ssh.1,v 1.394 2018/06/09 03:01:12 djm Exp $
! 37: .Dd $Mdocdate: June 9 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
1.395 ! jmc 530: .It SetEnv
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 .
1.393 djm 745: .Pp
1.261 stevesk 746: If the
747: .Cm Tunnel
1.393 djm 748: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 749: .Dq point-to-point .
1.393 djm 750: If a different
751: .Cm Tunnel
752: forwarding mode it desired, then it should be specified before
753: .Fl w .
1.360 millert 754: .Pp
1.218 jmc 755: .It Fl X
756: Enables X11 forwarding.
1.54 markus 757: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 758: .Pp
1.218 jmc 759: X11 forwarding should be enabled with caution.
1.168 jmc 760: Users with the ability to bypass file permissions on the remote host
1.218 jmc 761: (for the user's X authorization database)
762: can access the local X11 display through the forwarded connection.
763: An attacker may then be able to perform activities such as keystroke monitoring.
764: .Pp
765: For this reason, X11 forwarding is subjected to X11 SECURITY extension
766: restrictions by default.
767: Please refer to the
768: .Nm
769: .Fl Y
770: option and the
771: .Cm ForwardX11Trusted
772: directive in
773: .Xr ssh_config 5
774: for more information.
1.360 millert 775: .Pp
1.218 jmc 776: .It Fl x
777: Disables X11 forwarding.
1.360 millert 778: .Pp
1.218 jmc 779: .It Fl Y
780: Enables trusted X11 forwarding.
781: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
782: controls.
1.360 millert 783: .Pp
1.278 djm 784: .It Fl y
785: Send log information using the
786: .Xr syslog 3
787: system module.
788: By default this information is sent to stderr.
1.218 jmc 789: .El
1.224 jmc 790: .Pp
791: .Nm
792: may additionally obtain configuration data from
793: a per-user configuration file and a system-wide configuration file.
794: The file format and configuration options are described in
795: .Xr ssh_config 5 .
1.222 jmc 796: .Sh AUTHENTICATION
1.377 djm 797: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 798: .Pp
799: The methods available for authentication are:
1.260 jmc 800: GSSAPI-based authentication,
1.222 jmc 801: host-based authentication,
802: public key authentication,
803: challenge-response authentication,
804: and password authentication.
805: Authentication methods are tried in the order specified above,
1.369 jmc 806: though
807: .Cm PreferredAuthentications
808: can be used to change the default order.
1.222 jmc 809: .Pp
810: Host-based authentication works as follows:
1.218 jmc 811: If the machine the user logs in from is listed in
812: .Pa /etc/hosts.equiv
813: or
814: .Pa /etc/shosts.equiv
815: on the remote machine, and the user names are
816: the same on both sides, or if the files
817: .Pa ~/.rhosts
818: or
819: .Pa ~/.shosts
820: exist in the user's home directory on the
821: remote machine and contain a line containing the name of the client
822: machine and the name of the user on that machine, the user is
1.222 jmc 823: considered for login.
824: Additionally, the server
825: .Em must
826: be able to verify the client's
827: host key (see the description of
1.218 jmc 828: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 829: and
1.222 jmc 830: .Pa ~/.ssh/known_hosts ,
831: below)
832: for login to be permitted.
1.218 jmc 833: This authentication method closes security holes due to IP
1.222 jmc 834: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 835: [Note to the administrator:
836: .Pa /etc/hosts.equiv ,
837: .Pa ~/.rhosts ,
838: and the rlogin/rsh protocol in general, are inherently insecure and should be
839: disabled if security is desired.]
1.189 dtucker 840: .Pp
1.222 jmc 841: Public key authentication works as follows:
842: The scheme is based on public-key cryptography,
843: using cryptosystems
844: where encryption and decryption are done using separate keys,
845: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 846: The idea is that each user creates a public/private
847: key pair for authentication purposes.
848: The server knows the public key, and only the user knows the private key.
1.222 jmc 849: .Nm
850: implements public key authentication protocol automatically,
1.349 sobrado 851: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 852: The HISTORY section of
1.222 jmc 853: .Xr ssl 8
1.311 jmc 854: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 855: .Pp
1.218 jmc 856: The file
857: .Pa ~/.ssh/authorized_keys
858: lists the public keys that are permitted for logging in.
859: When the user logs in, the
1.2 deraadt 860: .Nm
1.218 jmc 861: program tells the server which key pair it would like to use for
862: authentication.
1.222 jmc 863: The client proves that it has access to the private key
864: and the server checks that the corresponding public key
865: is authorized to accept the account.
1.383 djm 866: .Pp
867: The server may inform the client of errors that prevented public key
868: authentication from succeeding after authentication completes using a
869: different method.
870: These may be viewed by increasing the
871: .Cm LogLevel
872: to
873: .Cm DEBUG
874: or higher (e.g. by using the
875: .Fl v
876: flag).
1.218 jmc 877: .Pp
1.222 jmc 878: The user creates his/her key pair by running
1.218 jmc 879: .Xr ssh-keygen 1 .
880: This stores the private key in
1.222 jmc 881: .Pa ~/.ssh/id_dsa
1.369 jmc 882: (DSA),
1.310 djm 883: .Pa ~/.ssh/id_ecdsa
1.369 jmc 884: (ECDSA),
1.343 naddy 885: .Pa ~/.ssh/id_ed25519
1.369 jmc 886: (Ed25519),
1.222 jmc 887: or
888: .Pa ~/.ssh/id_rsa
1.369 jmc 889: (RSA)
1.218 jmc 890: and stores the public key in
1.222 jmc 891: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 892: (DSA),
1.310 djm 893: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 894: (ECDSA),
1.343 naddy 895: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 896: (Ed25519),
1.222 jmc 897: or
898: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 899: (RSA)
1.218 jmc 900: in the user's home directory.
1.222 jmc 901: The user should then copy the public key
1.218 jmc 902: to
903: .Pa ~/.ssh/authorized_keys
1.222 jmc 904: in his/her home directory on the remote machine.
905: The
1.218 jmc 906: .Pa authorized_keys
907: file corresponds to the conventional
908: .Pa ~/.rhosts
909: file, and has one key
1.222 jmc 910: per line, though the lines can be very long.
1.218 jmc 911: After this, the user can log in without giving the password.
912: .Pp
1.301 jmc 913: A variation on public key authentication
914: is available in the form of certificate authentication:
915: instead of a set of public/private keys,
916: signed certificates are used.
917: This has the advantage that a single trusted certification authority
918: can be used in place of many public/private keys.
1.333 jmc 919: See the CERTIFICATES section of
1.301 jmc 920: .Xr ssh-keygen 1
921: for more information.
922: .Pp
923: The most convenient way to use public key or certificate authentication
924: may be with an authentication agent.
1.218 jmc 925: See
926: .Xr ssh-agent 1
1.366 jcs 927: and (optionally) the
928: .Cm AddKeysToAgent
929: directive in
930: .Xr ssh_config 5
1.218 jmc 931: for more information.
932: .Pp
1.222 jmc 933: Challenge-response authentication works as follows:
934: The server sends an arbitrary
935: .Qq challenge
936: text, and prompts for a response.
937: Examples of challenge-response authentication include
1.335 jmc 938: .Bx
939: Authentication (see
1.222 jmc 940: .Xr login.conf 5 )
1.335 jmc 941: and PAM (some
942: .Pf non- Ox
943: systems).
1.222 jmc 944: .Pp
945: Finally, if other authentication methods fail,
1.218 jmc 946: .Nm
947: prompts the user for a password.
948: The password is sent to the remote
949: host for checking; however, since all communications are encrypted,
950: the password cannot be seen by someone listening on the network.
1.232 jmc 951: .Pp
952: .Nm
953: automatically maintains and checks a database containing
954: identification for all hosts it has ever been used with.
955: Host keys are stored in
956: .Pa ~/.ssh/known_hosts
957: in the user's home directory.
958: Additionally, the file
959: .Pa /etc/ssh/ssh_known_hosts
960: is automatically checked for known hosts.
961: Any new hosts are automatically added to the user's file.
962: If a host's identification ever changes,
963: .Nm
964: warns about this and disables password authentication to prevent
965: server spoofing or man-in-the-middle attacks,
966: which could otherwise be used to circumvent the encryption.
967: The
968: .Cm StrictHostKeyChecking
969: option can be used to control logins to machines whose
970: host key is not known or has changed.
971: .Pp
1.218 jmc 972: When the user's identity has been accepted by the server, the server
1.357 dtucker 973: either executes the given command in a non-interactive session or,
974: if no command has been specified, logs into the machine and gives
975: the user a normal shell as an interactive session.
1.218 jmc 976: All communication with
977: the remote command or shell will be automatically encrypted.
978: .Pp
1.357 dtucker 979: If an interactive session is requested
980: .Nm
981: by default will only request a pseudo-terminal (pty) for interactive
982: sessions when the client has one.
983: The flags
984: .Fl T
985: and
986: .Fl t
987: can be used to override this behaviour.
988: .Pp
989: If a pseudo-terminal has been allocated the
1.218 jmc 990: user may use the escape characters noted below.
991: .Pp
1.357 dtucker 992: If no pseudo-terminal has been allocated,
1.218 jmc 993: the session is transparent and can be used to reliably transfer binary data.
994: On most systems, setting the escape character to
995: .Dq none
996: will also make the session transparent even if a tty is used.
997: .Pp
998: The session terminates when the command or shell on the remote
1.247 jmc 999: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1000: .Sh ESCAPE CHARACTERS
1.218 jmc 1001: When a pseudo-terminal has been requested,
1.2 deraadt 1002: .Nm
1.218 jmc 1003: supports a number of functions through the use of an escape character.
1004: .Pp
1005: A single tilde character can be sent as
1006: .Ic ~~
1007: or by following the tilde by a character other than those described below.
1008: The escape character must always follow a newline to be interpreted as
1009: special.
1010: The escape character can be changed in configuration files using the
1011: .Cm EscapeChar
1012: configuration directive or on the command line by the
1013: .Fl e
1014: option.
1015: .Pp
1016: The supported escapes (assuming the default
1017: .Ql ~ )
1018: are:
1019: .Bl -tag -width Ds
1020: .It Cm ~.
1021: Disconnect.
1022: .It Cm ~^Z
1023: Background
1.234 jmc 1024: .Nm .
1.218 jmc 1025: .It Cm ~#
1026: List forwarded connections.
1027: .It Cm ~&
1028: Background
1.2 deraadt 1029: .Nm
1.218 jmc 1030: at logout when waiting for forwarded connection / X11 sessions to terminate.
1031: .It Cm ~?
1032: Display a list of escape characters.
1033: .It Cm ~B
1034: Send a BREAK to the remote system
1.369 jmc 1035: (only useful if the peer supports it).
1.218 jmc 1036: .It Cm ~C
1037: Open command line.
1038: Currently this allows the addition of port forwardings using the
1.279 stevesk 1039: .Fl L ,
1040: .Fl R
1.218 jmc 1041: and
1.279 stevesk 1042: .Fl D
1.225 jmc 1043: options (see above).
1.322 markus 1044: It also allows the cancellation of existing port-forwardings
1045: with
1.262 stevesk 1046: .Sm off
1.322 markus 1047: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1048: .Sm on
1.322 markus 1049: for local,
1050: .Sm off
1051: .Fl KR Oo Ar bind_address : Oc Ar port
1052: .Sm on
1053: for remote and
1054: .Sm off
1055: .Fl KD Oo Ar bind_address : Oc Ar port
1056: .Sm on
1057: for dynamic port-forwardings.
1.218 jmc 1058: .Ic !\& Ns Ar command
1059: allows the user to execute a local command if the
1060: .Ic PermitLocalCommand
1061: option is enabled in
1.176 jmc 1062: .Xr ssh_config 5 .
1.218 jmc 1063: Basic help is available, using the
1064: .Fl h
1065: option.
1066: .It Cm ~R
1067: Request rekeying of the connection
1.369 jmc 1068: (only useful if the peer supports it).
1.327 dtucker 1069: .It Cm ~V
1070: Decrease the verbosity
1071: .Pq Ic LogLevel
1072: when errors are being written to stderr.
1073: .It Cm ~v
1.328 jmc 1074: Increase the verbosity
1.327 dtucker 1075: .Pq Ic LogLevel
1076: when errors are being written to stderr.
1.176 jmc 1077: .El
1.246 jmc 1078: .Sh TCP FORWARDING
1079: Forwarding of arbitrary TCP connections over the secure channel can
1080: be specified either on the command line or in a configuration file.
1081: One possible application of TCP forwarding is a secure connection to a
1082: mail server; another is going through firewalls.
1083: .Pp
1084: In the example below, we look at encrypting communication between
1085: an IRC client and server, even though the IRC server does not directly
1086: support encrypted communications.
1087: This works as follows:
1088: the user connects to the remote host using
1089: .Nm ,
1090: specifying a port to be used to forward connections
1091: to the remote server.
1092: After that it is possible to start the service which is to be encrypted
1093: on the client machine,
1094: connecting to the same local port,
1095: and
1096: .Nm
1097: will encrypt and forward the connection.
1098: .Pp
1099: The following example tunnels an IRC session from client machine
1100: .Dq 127.0.0.1
1101: (localhost)
1102: to remote server
1103: .Dq server.example.com :
1104: .Bd -literal -offset 4n
1105: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1106: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1107: .Ed
1108: .Pp
1109: This tunnels a connection to IRC server
1110: .Dq server.example.com ,
1111: joining channel
1112: .Dq #users ,
1113: nickname
1114: .Dq pinky ,
1115: using port 1234.
1116: It doesn't matter which port is used,
1117: as long as it's greater than 1023
1118: (remember, only root can open sockets on privileged ports)
1119: and doesn't conflict with any ports already in use.
1120: The connection is forwarded to port 6667 on the remote server,
1121: since that's the standard port for IRC services.
1122: .Pp
1123: The
1124: .Fl f
1125: option backgrounds
1126: .Nm
1127: and the remote command
1128: .Dq sleep 10
1129: is specified to allow an amount of time
1130: (10 seconds, in the example)
1131: to start the service which is to be tunnelled.
1132: If no connections are made within the time specified,
1133: .Nm
1134: will exit.
1135: .Sh X11 FORWARDING
1.218 jmc 1136: If the
1137: .Cm ForwardX11
1138: variable is set to
1139: .Dq yes
1140: (or see the description of the
1.227 jmc 1141: .Fl X ,
1142: .Fl x ,
1.218 jmc 1143: and
1.227 jmc 1144: .Fl Y
1.226 jmc 1145: options above)
1.218 jmc 1146: and the user is using X11 (the
1147: .Ev DISPLAY
1148: environment variable is set), the connection to the X11 display is
1149: automatically forwarded to the remote side in such a way that any X11
1150: programs started from the shell (or command) will go through the
1151: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1152: from the local machine.
1.218 jmc 1153: The user should not manually set
1154: .Ev DISPLAY .
1155: Forwarding of X11 connections can be
1156: configured on the command line or in configuration files.
1157: .Pp
1158: The
1159: .Ev DISPLAY
1160: value set by
1161: .Nm
1162: will point to the server machine, but with a display number greater than zero.
1163: This is normal, and happens because
1164: .Nm
1165: creates a
1166: .Dq proxy
1167: X server on the server machine for forwarding the
1168: connections over the encrypted channel.
1.200 djm 1169: .Pp
1.218 jmc 1170: .Nm
1171: will also automatically set up Xauthority data on the server machine.
1172: For this purpose, it will generate a random authorization cookie,
1173: store it in Xauthority on the server, and verify that any forwarded
1174: connections carry this cookie and replace it by the real cookie when
1175: the connection is opened.
1176: The real authentication cookie is never
1177: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1178: .Pp
1.218 jmc 1179: If the
1180: .Cm ForwardAgent
1181: variable is set to
1182: .Dq yes
1183: (or see the description of the
1184: .Fl A
1.191 djm 1185: and
1.218 jmc 1186: .Fl a
1.226 jmc 1187: options above) and
1.218 jmc 1188: the user is using an authentication agent, the connection to the agent
1189: is automatically forwarded to the remote side.
1.252 jmc 1190: .Sh VERIFYING HOST KEYS
1191: When connecting to a server for the first time,
1192: a fingerprint of the server's public key is presented to the user
1193: (unless the option
1194: .Cm StrictHostKeyChecking
1195: has been disabled).
1196: Fingerprints can be determined using
1197: .Xr ssh-keygen 1 :
1198: .Pp
1199: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1200: .Pp
1.274 grunk 1201: If the fingerprint is already known, it can be matched
1202: and the key can be accepted or rejected.
1.358 djm 1203: If only legacy (MD5) fingerprints for the server are available, the
1204: .Xr ssh-keygen 1
1205: .Fl E
1206: option may be used to downgrade the fingerprint algorithm to match.
1207: .Pp
1.274 grunk 1208: Because of the difficulty of comparing host keys
1.352 djm 1209: just by looking at fingerprint strings,
1.274 grunk 1210: there is also support to compare host keys visually,
1211: using
1212: .Em random art .
1213: By setting the
1.275 grunk 1214: .Cm VisualHostKey
1.274 grunk 1215: option to
1.275 grunk 1216: .Dq yes ,
1.274 grunk 1217: a small ASCII graphic gets displayed on every login to a server, no matter
1218: if the session itself is interactive or not.
1219: By learning the pattern a known server produces, a user can easily
1220: find out that the host key has changed when a completely different pattern
1221: is displayed.
1222: Because these patterns are not unambiguous however, a pattern that looks
1223: similar to the pattern remembered only gives a good probability that the
1224: host key is the same, not guaranteed proof.
1225: .Pp
1226: To get a listing of the fingerprints along with their random art for
1227: all known hosts, the following command line can be used:
1228: .Pp
1229: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1230: .Pp
1.252 jmc 1231: If the fingerprint is unknown,
1232: an alternative method of verification is available:
1233: SSH fingerprints verified by DNS.
1234: An additional resource record (RR),
1235: SSHFP,
1236: is added to a zonefile
1237: and the connecting client is able to match the fingerprint
1238: with that of the key presented.
1239: .Pp
1240: In this example, we are connecting a client to a server,
1241: .Dq host.example.com .
1242: The SSHFP resource records should first be added to the zonefile for
1243: host.example.com:
1244: .Bd -literal -offset indent
1.259 jakob 1245: $ ssh-keygen -r host.example.com.
1.252 jmc 1246: .Ed
1247: .Pp
1248: The output lines will have to be added to the zonefile.
1249: To check that the zone is answering fingerprint queries:
1250: .Pp
1251: .Dl $ dig -t SSHFP host.example.com
1252: .Pp
1253: Finally the client connects:
1254: .Bd -literal -offset indent
1255: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1256: [...]
1257: Matching host key fingerprint found in DNS.
1258: Are you sure you want to continue connecting (yes/no)?
1259: .Ed
1260: .Pp
1261: See the
1262: .Cm VerifyHostKeyDNS
1263: option in
1264: .Xr ssh_config 5
1265: for more information.
1.250 jmc 1266: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1267: .Nm
1268: contains support for Virtual Private Network (VPN) tunnelling
1269: using the
1270: .Xr tun 4
1271: network pseudo-device,
1272: allowing two networks to be joined securely.
1273: The
1274: .Xr sshd_config 5
1275: configuration option
1276: .Cm PermitTunnel
1277: controls whether the server supports this,
1278: and at what level (layer 2 or 3 traffic).
1279: .Pp
1280: The following example would connect client network 10.0.50.0/24
1.265 otto 1281: with remote network 10.0.99.0/24 using a point-to-point connection
1282: from 10.1.1.1 to 10.1.1.2,
1283: provided that the SSH server running on the gateway to the remote network,
1284: at 192.168.1.15, allows it.
1285: .Pp
1286: On the client:
1.250 jmc 1287: .Bd -literal -offset indent
1288: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1289: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1290: # route add 10.0.99.0/24 10.1.1.2
1291: .Ed
1292: .Pp
1293: On the server:
1294: .Bd -literal -offset indent
1295: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1296: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1297: .Ed
1298: .Pp
1299: Client access may be more finely tuned via the
1300: .Pa /root/.ssh/authorized_keys
1301: file (see below) and the
1302: .Cm PermitRootLogin
1303: server option.
1.255 jmc 1304: The following entry would permit connections on
1.250 jmc 1305: .Xr tun 4
1.255 jmc 1306: device 1 from user
1.250 jmc 1307: .Dq jane
1.255 jmc 1308: and on tun device 2 from user
1.250 jmc 1309: .Dq john ,
1310: if
1311: .Cm PermitRootLogin
1312: is set to
1313: .Dq forced-commands-only :
1314: .Bd -literal -offset 2n
1315: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1316: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1317: .Ed
1318: .Pp
1.264 ray 1319: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1320: it may be more suited to temporary setups,
1321: such as for wireless VPNs.
1322: More permanent VPNs are better provided by tools such as
1323: .Xr ipsecctl 8
1324: and
1325: .Xr isakmpd 8 .
1.2 deraadt 1326: .Sh ENVIRONMENT
1327: .Nm
1.1 deraadt 1328: will normally set the following environment variables:
1.237 jmc 1329: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1330: .It Ev DISPLAY
1331: The
1332: .Ev DISPLAY
1.40 aaron 1333: variable indicates the location of the X11 server.
1.44 aaron 1334: It is automatically set by
1.2 deraadt 1335: .Nm
1336: to point to a value of the form
1.233 jmc 1337: .Dq hostname:n ,
1338: where
1339: .Dq hostname
1340: indicates the host where the shell runs, and
1341: .Sq n
1342: is an integer \*(Ge 1.
1.40 aaron 1343: .Nm
1344: uses this special value to forward X11 connections over the secure
1345: channel.
1.107 markus 1346: The user should normally not set
1347: .Ev DISPLAY
1348: explicitly, as that
1.1 deraadt 1349: will render the X11 connection insecure (and will require the user to
1350: manually copy any required authorization cookies).
1.2 deraadt 1351: .It Ev HOME
1.1 deraadt 1352: Set to the path of the user's home directory.
1.2 deraadt 1353: .It Ev LOGNAME
1354: Synonym for
1.12 aaron 1355: .Ev USER ;
1356: set for compatibility with systems that use this variable.
1.2 deraadt 1357: .It Ev MAIL
1.129 stevesk 1358: Set to the path of the user's mailbox.
1.40 aaron 1359: .It Ev PATH
1.2 deraadt 1360: Set to the default
1361: .Ev PATH ,
1362: as specified when compiling
1.234 jmc 1363: .Nm .
1.118 markus 1364: .It Ev SSH_ASKPASS
1365: If
1366: .Nm
1367: needs a passphrase, it will read the passphrase from the current
1368: terminal if it was run from a terminal.
1369: If
1370: .Nm
1371: does not have a terminal associated with it but
1372: .Ev DISPLAY
1373: and
1374: .Ev SSH_ASKPASS
1375: are set, it will execute the program specified by
1376: .Ev SSH_ASKPASS
1377: and open an X11 window to read the passphrase.
1378: This is particularly useful when calling
1379: .Nm
1380: from a
1.196 jmc 1381: .Pa .xsession
1.118 markus 1382: or related script.
1383: (Note that on some machines it
1384: may be necessary to redirect the input from
1385: .Pa /dev/null
1386: to make this work.)
1.18 markus 1387: .It Ev SSH_AUTH_SOCK
1.233 jmc 1388: Identifies the path of a
1389: .Ux Ns -domain
1390: socket used to communicate with the agent.
1.166 stevesk 1391: .It Ev SSH_CONNECTION
1392: Identifies the client and server ends of the connection.
1.40 aaron 1393: The variable contains
1.233 jmc 1394: four space-separated values: client IP address, client port number,
1395: server IP address, and server port number.
1.73 markus 1396: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1397: This variable contains the original command line if a forced command
1.73 markus 1398: is executed.
1399: It can be used to extract the original arguments.
1.2 deraadt 1400: .It Ev SSH_TTY
1.1 deraadt 1401: This is set to the name of the tty (path to the device) associated
1.40 aaron 1402: with the current shell or command.
1403: If the current session has no tty,
1.1 deraadt 1404: this variable is not set.
1.387 djm 1405: .It Ev SSH_TUNNEL
1406: Optionally set by
1407: .Xr sshd 8
1408: to contain the interface names assigned if tunnel forwarding was
1409: requested by the client.
1.385 djm 1410: .It Ev SSH_USER_AUTH
1411: Optionally set by
1412: .Xr sshd 8 ,
1413: this variable may contain a pathname to a file that lists the authentication
1414: methods successfully used when the session was established, including any
1415: public keys that were used.
1.2 deraadt 1416: .It Ev TZ
1.214 jmc 1417: This variable is set to indicate the present time zone if it
1.257 jmc 1418: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1419: on to new connections).
1.2 deraadt 1420: .It Ev USER
1.1 deraadt 1421: Set to the name of the user logging in.
1.2 deraadt 1422: .El
1423: .Pp
1.44 aaron 1424: Additionally,
1.2 deraadt 1425: .Nm
1.44 aaron 1426: reads
1.207 djm 1427: .Pa ~/.ssh/environment ,
1.2 deraadt 1428: and adds lines of the format
1429: .Dq VARNAME=value
1.233 jmc 1430: to the environment if the file exists and users are allowed to
1.161 marc 1431: change their environment.
1.176 jmc 1432: For more information, see the
1.161 marc 1433: .Cm PermitUserEnvironment
1.162 stevesk 1434: option in
1.161 marc 1435: .Xr sshd_config 5 .
1.2 deraadt 1436: .Sh FILES
1.236 jmc 1437: .Bl -tag -width Ds -compact
1.309 jmc 1438: .It Pa ~/.rhosts
1.240 jmc 1439: This file is used for host-based authentication (see above).
1.92 markus 1440: On some machines this file may need to be
1.240 jmc 1441: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1442: because
1.2 deraadt 1443: .Xr sshd 8
1.40 aaron 1444: reads it as root.
1445: Additionally, this file must be owned by the user,
1446: and must not have write permissions for anyone else.
1447: The recommended
1.1 deraadt 1448: permission for most machines is read/write for the user, and not
1449: accessible by others.
1.2 deraadt 1450: .Pp
1.309 jmc 1451: .It Pa ~/.shosts
1.240 jmc 1452: This file is used in exactly the same way as
1453: .Pa .rhosts ,
1454: but allows host-based authentication without permitting login with
1455: rlogin/rsh.
1.272 mcbride 1456: .Pp
1.309 jmc 1457: .It Pa ~/.ssh/
1.272 mcbride 1458: This directory is the default location for all user-specific configuration
1459: and authentication information.
1460: There is no general requirement to keep the entire contents of this directory
1461: secret, but the recommended permissions are read/write/execute for the user,
1462: and not accessible by others.
1.236 jmc 1463: .Pp
1.309 jmc 1464: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1465: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1466: that can be used for logging in as this user.
1.238 jmc 1467: The format of this file is described in the
1468: .Xr sshd 8
1469: manual page.
1470: This file is not highly sensitive, but the recommended
1471: permissions are read/write for the user, and not accessible by others.
1472: .Pp
1.309 jmc 1473: .It Pa ~/.ssh/config
1.238 jmc 1474: This is the per-user configuration file.
1475: The file format and configuration options are described in
1476: .Xr ssh_config 5 .
1477: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1478: read/write for the user, and not writable by others.
1.238 jmc 1479: .Pp
1.309 jmc 1480: .It Pa ~/.ssh/environment
1.239 jmc 1481: Contains additional definitions for environment variables; see
1482: .Sx ENVIRONMENT ,
1.238 jmc 1483: above.
1484: .Pp
1.309 jmc 1485: .It Pa ~/.ssh/id_dsa
1.310 djm 1486: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1487: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1488: .It Pa ~/.ssh/id_rsa
1.238 jmc 1489: Contains the private key for authentication.
1490: These files
1491: contain sensitive data and should be readable by the user but not
1492: accessible by others (read/write/execute).
1493: .Nm
1494: will simply ignore a private key file if it is accessible by others.
1495: It is possible to specify a passphrase when
1496: generating the key which will be used to encrypt the
1.389 djm 1497: sensitive part of this file using AES-128.
1.238 jmc 1498: .Pp
1.309 jmc 1499: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1500: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1501: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1502: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1503: Contains the public key for authentication.
1504: These files are not
1505: sensitive and can (but need not) be readable by anyone.
1506: .Pp
1.309 jmc 1507: .It Pa ~/.ssh/known_hosts
1.244 jmc 1508: Contains a list of host keys for all hosts the user has logged into
1509: that are not already in the systemwide list of known host keys.
1.238 jmc 1510: See
1.244 jmc 1511: .Xr sshd 8
1512: for further details of the format of this file.
1.238 jmc 1513: .Pp
1.309 jmc 1514: .It Pa ~/.ssh/rc
1.238 jmc 1515: Commands in this file are executed by
1516: .Nm
1.245 jmc 1517: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1518: started.
1519: See the
1520: .Xr sshd 8
1521: manual page for more information.
1522: .Pp
1.309 jmc 1523: .It Pa /etc/hosts.equiv
1.240 jmc 1524: This file is for host-based authentication (see above).
1525: It should only be writable by root.
1.236 jmc 1526: .Pp
1.309 jmc 1527: .It Pa /etc/shosts.equiv
1.240 jmc 1528: This file is used in exactly the same way as
1529: .Pa hosts.equiv ,
1530: but allows host-based authentication without permitting login with
1531: rlogin/rsh.
1.236 jmc 1532: .Pp
1.238 jmc 1533: .It Pa /etc/ssh/ssh_config
1534: Systemwide configuration file.
1535: The file format and configuration options are described in
1536: .Xr ssh_config 5 .
1537: .Pp
1.309 jmc 1538: .It Pa /etc/ssh/ssh_host_key
1539: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1540: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1541: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1542: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1543: These files contain the private parts of the host keys
1.245 jmc 1544: and are used for host-based authentication.
1.238 jmc 1545: .Pp
1.309 jmc 1546: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1547: Systemwide list of known host keys.
1548: This file should be prepared by the
1549: system administrator to contain the public host keys of all machines in the
1550: organization.
1.244 jmc 1551: It should be world-readable.
1552: See
1.238 jmc 1553: .Xr sshd 8
1.244 jmc 1554: for further details of the format of this file.
1.236 jmc 1555: .Pp
1.309 jmc 1556: .It Pa /etc/ssh/sshrc
1.1 deraadt 1557: Commands in this file are executed by
1.2 deraadt 1558: .Nm
1.245 jmc 1559: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1560: See the
1.2 deraadt 1561: .Xr sshd 8
1.1 deraadt 1562: manual page for more information.
1.58 itojun 1563: .El
1.312 jmc 1564: .Sh EXIT STATUS
1565: .Nm
1566: exits with the exit status of the remote command or with 255
1567: if an error occurred.
1.2 deraadt 1568: .Sh SEE ALSO
1569: .Xr scp 1 ,
1.83 djm 1570: .Xr sftp 1 ,
1.2 deraadt 1571: .Xr ssh-add 1 ,
1572: .Xr ssh-agent 1 ,
1573: .Xr ssh-keygen 1 ,
1.242 jmc 1574: .Xr ssh-keyscan 1 ,
1.250 jmc 1575: .Xr tun 4 ,
1.159 stevesk 1576: .Xr ssh_config 5 ,
1.160 naddy 1577: .Xr ssh-keysign 8 ,
1.87 itojun 1578: .Xr sshd 8
1.329 jmc 1579: .Sh STANDARDS
1.106 markus 1580: .Rs
1.329 jmc 1581: .%A S. Lehtinen
1582: .%A C. Lonvick
1583: .%D January 2006
1.256 jmc 1584: .%R RFC 4250
1.329 jmc 1585: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1586: .Re
1.329 jmc 1587: .Pp
1.256 jmc 1588: .Rs
1.329 jmc 1589: .%A T. Ylonen
1590: .%A C. Lonvick
1591: .%D January 2006
1.256 jmc 1592: .%R RFC 4251
1.329 jmc 1593: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1594: .Re
1.329 jmc 1595: .Pp
1.256 jmc 1596: .Rs
1.329 jmc 1597: .%A T. Ylonen
1598: .%A C. Lonvick
1599: .%D January 2006
1.256 jmc 1600: .%R RFC 4252
1.329 jmc 1601: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1602: .Re
1.329 jmc 1603: .Pp
1.256 jmc 1604: .Rs
1.329 jmc 1605: .%A T. Ylonen
1606: .%A C. Lonvick
1607: .%D January 2006
1.256 jmc 1608: .%R RFC 4253
1.329 jmc 1609: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1610: .Re
1.329 jmc 1611: .Pp
1.256 jmc 1612: .Rs
1.329 jmc 1613: .%A T. Ylonen
1614: .%A C. Lonvick
1615: .%D January 2006
1.256 jmc 1616: .%R RFC 4254
1.329 jmc 1617: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1618: .Re
1.329 jmc 1619: .Pp
1.256 jmc 1620: .Rs
1.329 jmc 1621: .%A J. Schlyter
1622: .%A W. Griffin
1623: .%D January 2006
1.256 jmc 1624: .%R RFC 4255
1.329 jmc 1625: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1626: .Re
1.329 jmc 1627: .Pp
1.256 jmc 1628: .Rs
1.329 jmc 1629: .%A F. Cusack
1630: .%A M. Forssen
1631: .%D January 2006
1.256 jmc 1632: .%R RFC 4256
1.329 jmc 1633: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1634: .Re
1.329 jmc 1635: .Pp
1.256 jmc 1636: .Rs
1.329 jmc 1637: .%A J. Galbraith
1638: .%A P. Remaker
1639: .%D January 2006
1.256 jmc 1640: .%R RFC 4335
1.329 jmc 1641: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1642: .Re
1.329 jmc 1643: .Pp
1.256 jmc 1644: .Rs
1.329 jmc 1645: .%A M. Bellare
1646: .%A T. Kohno
1647: .%A C. Namprempre
1648: .%D January 2006
1.256 jmc 1649: .%R RFC 4344
1.329 jmc 1650: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1651: .Re
1.329 jmc 1652: .Pp
1.256 jmc 1653: .Rs
1.329 jmc 1654: .%A B. Harris
1655: .%D January 2006
1.256 jmc 1656: .%R RFC 4345
1.329 jmc 1657: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1658: .Re
1.329 jmc 1659: .Pp
1.258 djm 1660: .Rs
1.329 jmc 1661: .%A M. Friedl
1662: .%A N. Provos
1663: .%A W. Simpson
1664: .%D March 2006
1.258 djm 1665: .%R RFC 4419
1.329 jmc 1666: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1667: .Re
1.329 jmc 1668: .Pp
1.266 markus 1669: .Rs
1.329 jmc 1670: .%A J. Galbraith
1671: .%A R. Thayer
1672: .%D November 2006
1.266 markus 1673: .%R RFC 4716
1.329 jmc 1674: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1675: .Re
1.329 jmc 1676: .Pp
1.313 djm 1677: .Rs
1.329 jmc 1678: .%A D. Stebila
1679: .%A J. Green
1680: .%D December 2009
1.313 djm 1681: .%R RFC 5656
1.329 jmc 1682: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1683: .Re
1.329 jmc 1684: .Pp
1.274 grunk 1685: .Rs
1686: .%A A. Perrig
1687: .%A D. Song
1688: .%D 1999
1.329 jmc 1689: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1690: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1691: .Re
1.173 jmc 1692: .Sh AUTHORS
1693: OpenSSH is a derivative of the original and free
1694: ssh 1.2.12 release by Tatu Ylonen.
1695: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1696: Theo de Raadt and Dug Song
1697: removed many bugs, re-added newer features and
1698: created OpenSSH.
1699: Markus Friedl contributed the support for SSH
1700: protocol versions 1.5 and 2.0.
![[BACK]](/icons/back.gif){kind=icon}
Return to ssh.1 CVS log ![[TXT]](/icons/text.gif){kind=icon}
![[DIR]](/icons/dir.gif){kind=icon}
Up to [local] / src / usr.bin / ssh