Annotation of src/usr.bin/ssh/ssh.1, Revision 1.392
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: .\"
1.392 ! djm 36: .\" $OpenBSD: ssh.1,v 1.391 2018/02/23 07:38:09 jmc Exp $
1.391 jmc 37: .Dd $Mdocdate: February 23 2018 $
1.2 deraadt 38: .Dt SSH 1
39: .Os
40: .Sh NAME
41: .Nm ssh
1.96 deraadt 42: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm ssh
1.377 djm 45: .Op Fl 46AaCfGgKkMNnqsTtVvXxYy
1.390 djm 46: .Op Fl B Ar bind_interface
1.108 markus 47: .Op Fl b Ar bind_address
1.51 markus 48: .Op Fl c Ar cipher_spec
1.306 jmc 49: .Op Fl D Oo Ar bind_address : Oc Ns Ar port
1.331 dtucker 50: .Op Fl E Ar log_file
1.2 deraadt 51: .Op Fl e Ar escape_char
1.176 jmc 52: .Op Fl F Ar configfile
1.292 jmc 53: .Op Fl I Ar pkcs11
1.2 deraadt 54: .Op Fl i Ar identity_file
1.386 millert 55: .Op Fl J Ar destination
1.360 millert 56: .Op Fl L Ar address
1.176 jmc 57: .Op Fl l Ar login_name
58: .Op Fl m Ar mac_spec
1.198 djm 59: .Op Fl O Ar ctl_cmd
1.176 jmc 60: .Op Fl o Ar option
61: .Op Fl p Ar port
1.362 djm 62: .Op Fl Q Ar query_option
1.360 millert 63: .Op Fl R Ar address
1.198 djm 64: .Op Fl S Ar ctl_path
1.290 dtucker 65: .Op Fl W Ar host : Ns Ar port
1.306 jmc 66: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.386 millert 67: .Ar destination
1.2 deraadt 68: .Op Ar command
1.44 aaron 69: .Sh DESCRIPTION
1.2 deraadt 70: .Nm
1.96 deraadt 71: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 72: executing commands on a remote machine.
1.365 mmcc 73: It is intended to provide secure encrypted communications between
1.40 aaron 74: two untrusted hosts over an insecure network.
1.348 millert 75: X11 connections, arbitrary TCP ports and
76: .Ux Ns -domain
77: sockets can also be forwarded over the secure channel.
1.2 deraadt 78: .Pp
79: .Nm
1.44 aaron 80: connects and logs into the specified
1.388 jmc 81: .Ar destination ,
1.386 millert 82: which may be specified as either
1.388 jmc 83: .Sm off
84: .Oo user @ Oc hostname
85: .Sm on
86: or a URI of the form
87: .Sm off
88: .No ssh:// Oo user @ Oc hostname Op : port .
89: .Sm on
1.1 deraadt 90: The user must prove
1.49 markus 91: his/her identity to the remote machine using one of several methods
1.365 mmcc 92: (see below).
1.49 markus 93: .Pp
1.386 millert 94: If a
1.176 jmc 95: .Ar command
96: is specified,
1.219 jmc 97: it is executed on the remote host instead of a login shell.
1.2 deraadt 98: .Pp
1.218 jmc 99: The options are as follows:
1.361 millert 100: .Pp
1.360 millert 101: .Bl -tag -width Ds -compact
1.218 jmc 102: .It Fl 4
103: Forces
1.2 deraadt 104: .Nm
1.218 jmc 105: to use IPv4 addresses only.
1.360 millert 106: .Pp
1.218 jmc 107: .It Fl 6
108: Forces
1.2 deraadt 109: .Nm
1.218 jmc 110: to use IPv6 addresses only.
1.360 millert 111: .Pp
1.218 jmc 112: .It Fl A
113: Enables forwarding of the authentication agent connection.
114: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 115: .Pp
1.218 jmc 116: Agent forwarding should be enabled with caution.
117: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 118: (for the agent's
1.286 sobrado 119: .Ux Ns -domain
120: socket) can access the local agent through the forwarded connection.
1.218 jmc 121: An attacker cannot obtain key material from the agent,
122: however they can perform operations on the keys that enable them to
123: authenticate using the identities loaded into the agent.
1.360 millert 124: .Pp
1.218 jmc 125: .It Fl a
126: Disables forwarding of the authentication agent connection.
1.390 djm 127: .Pp
1.391 jmc 128: .It Fl B Ar bind_interface
1.390 djm 129: Bind to the address of
1.391 jmc 130: .Ar bind_interface
1.390 djm 131: before attempting to connect to the destination host.
132: This is only useful on systems with more than one address.
1.360 millert 133: .Pp
1.218 jmc 134: .It Fl b Ar bind_address
135: Use
136: .Ar bind_address
137: on the local machine as the source address
138: of the connection.
139: Only useful on systems with more than one address.
1.360 millert 140: .Pp
1.218 jmc 141: .It Fl C
142: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 143: data for forwarded X11, TCP and
144: .Ux Ns -domain
145: connections).
1.218 jmc 146: The compression algorithm is the same used by
1.377 djm 147: .Xr gzip 1 .
1.218 jmc 148: Compression is desirable on modem lines and other
149: slow connections, but will only slow down things on fast networks.
150: The default value can be set on a host-by-host basis in the
151: configuration files; see the
152: .Cm Compression
153: option.
1.360 millert 154: .Pp
1.218 jmc 155: .It Fl c Ar cipher_spec
156: Selects the cipher specification for encrypting the session.
157: .Ar cipher_spec
158: is a comma-separated list of ciphers
159: listed in order of preference.
1.283 jmc 160: See the
161: .Cm Ciphers
1.307 dtucker 162: keyword in
163: .Xr ssh_config 5
164: for more information.
1.360 millert 165: .Pp
1.218 jmc 166: .It Fl D Xo
167: .Sm off
168: .Oo Ar bind_address : Oc
169: .Ar port
170: .Sm on
171: .Xc
172: Specifies a local
173: .Dq dynamic
174: application-level port forwarding.
175: This works by allocating a socket to listen to
176: .Ar port
177: on the local side, optionally bound to the specified
178: .Ar bind_address .
179: Whenever a connection is made to this port, the
180: connection is forwarded over the secure channel, and the application
181: protocol is then used to determine where to connect to from the
182: remote machine.
183: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 184: .Nm
1.218 jmc 185: will act as a SOCKS server.
186: Only root can forward privileged ports.
187: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 188: .Pp
1.308 djm 189: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 190: Only the superuser can forward privileged ports.
191: By default, the local port is bound in accordance with the
192: .Cm GatewayPorts
193: setting.
194: However, an explicit
195: .Ar bind_address
196: may be used to bind the connection to a specific address.
197: The
198: .Ar bind_address
199: of
200: .Dq localhost
201: indicates that the listening port be bound for local use only, while an
202: empty address or
203: .Sq *
204: indicates that the port should be available from all interfaces.
1.360 millert 205: .Pp
1.331 dtucker 206: .It Fl E Ar log_file
207: Append debug logs to
208: .Ar log_file
209: instead of standard error.
1.360 millert 210: .Pp
1.229 jmc 211: .It Fl e Ar escape_char
1.218 jmc 212: Sets the escape character for sessions with a pty (default:
213: .Ql ~ ) .
214: The escape character is only recognized at the beginning of a line.
215: The escape character followed by a dot
216: .Pq Ql \&.
217: closes the connection;
218: followed by control-Z suspends the connection;
219: and followed by itself sends the escape character once.
220: Setting the character to
1.2 deraadt 221: .Dq none
1.218 jmc 222: disables any escapes and makes the session fully transparent.
1.360 millert 223: .Pp
1.218 jmc 224: .It Fl F Ar configfile
225: Specifies an alternative per-user configuration file.
226: If a configuration file is given on the command line,
227: the system-wide configuration file
228: .Pq Pa /etc/ssh/ssh_config
229: will be ignored.
230: The default for the per-user configuration file is
231: .Pa ~/.ssh/config .
1.360 millert 232: .Pp
1.218 jmc 233: .It Fl f
234: Requests
235: .Nm
236: to go to background just before command execution.
237: This is useful if
1.176 jmc 238: .Nm
1.218 jmc 239: is going to ask for passwords or passphrases, but the user
240: wants it in the background.
241: This implies
242: .Fl n .
243: The recommended way to start X11 programs at a remote site is with
244: something like
245: .Ic ssh -f host xterm .
1.277 djm 246: .Pp
247: If the
248: .Cm ExitOnForwardFailure
249: configuration option is set to
250: .Dq yes ,
251: then a client started with
252: .Fl f
253: will wait for all remote port forwards to be successfully established
254: before placing itself in the background.
1.360 millert 255: .Pp
1.350 djm 256: .It Fl G
257: Causes
258: .Nm
259: to print its configuration after evaluating
260: .Cm Host
261: and
262: .Cm Match
263: blocks and exit.
1.360 millert 264: .Pp
1.218 jmc 265: .It Fl g
266: Allows remote hosts to connect to local forwarded ports.
1.346 djm 267: If used on a multiplexed connection, then this option must be specified
268: on the master process.
1.360 millert 269: .Pp
1.291 markus 270: .It Fl I Ar pkcs11
1.294 jmc 271: Specify the PKCS#11 shared library
1.176 jmc 272: .Nm
1.293 markus 273: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 274: private RSA key.
1.360 millert 275: .Pp
1.218 jmc 276: .It Fl i Ar identity_file
277: Selects a file from which the identity (private key) for
1.310 djm 278: public key authentication is read.
1.218 jmc 279: The default is
1.310 djm 280: .Pa ~/.ssh/id_dsa ,
1.343 naddy 281: .Pa ~/.ssh/id_ecdsa ,
282: .Pa ~/.ssh/id_ed25519
1.310 djm 283: and
1.381 naddy 284: .Pa ~/.ssh/id_rsa .
1.218 jmc 285: Identity files may also be specified on
286: a per-host basis in the configuration file.
287: It is possible to have multiple
288: .Fl i
289: options (and multiple identities specified in
290: configuration files).
1.364 jmc 291: If no certificates have been explicitly specified by the
1.363 djm 292: .Cm CertificateFile
293: directive,
1.302 djm 294: .Nm
295: will also try to load certificate information from the filename obtained
296: by appending
297: .Pa -cert.pub
298: to identity filenames.
1.360 millert 299: .Pp
1.386 millert 300: .It Fl J Ar destination
1.375 djm 301: Connect to the target host by first making a
302: .Nm
1.386 millert 303: connection to the jump host described by
304: .Ar destination
1.376 jmc 305: and then establishing a TCP forwarding to the ultimate destination from
1.375 djm 306: there.
307: Multiple jump hops may be specified separated by comma characters.
308: This is a shortcut to specify a
309: .Cm ProxyJump
310: configuration directive.
311: .Pp
1.269 djm 312: .It Fl K
313: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
314: credentials to the server.
1.360 millert 315: .Pp
1.218 jmc 316: .It Fl k
317: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 318: .Pp
1.218 jmc 319: .It Fl L Xo
320: .Sm off
321: .Oo Ar bind_address : Oc
322: .Ar port : host : hostport
323: .Sm on
324: .Xc
1.360 millert 325: .It Fl L Xo
326: .Sm off
327: .Oo Ar bind_address : Oc
328: .Ar port : remote_socket
329: .Sm on
330: .Xc
331: .It Fl L Xo
332: .Sm off
333: .Ar local_socket : host : hostport
334: .Sm on
335: .Xc
336: .It Fl L Xo
337: .Sm off
338: .Ar local_socket : remote_socket
339: .Sm on
340: .Xc
341: Specifies that connections to the given TCP port or Unix socket on the local
342: (client) host are to be forwarded to the given host and port, or Unix socket,
343: on the remote side.
344: This works by allocating a socket to listen to either a TCP
1.218 jmc 345: .Ar port
346: on the local side, optionally bound to the specified
1.360 millert 347: .Ar bind_address ,
348: or to a Unix socket.
349: Whenever a connection is made to the local port or socket, the
1.218 jmc 350: connection is forwarded over the secure channel, and a connection is
1.360 millert 351: made to either
1.218 jmc 352: .Ar host
353: port
1.360 millert 354: .Ar hostport ,
355: or the Unix socket
356: .Ar remote_socket ,
1.218 jmc 357: from the remote machine.
1.360 millert 358: .Pp
1.218 jmc 359: Port forwardings can also be specified in the configuration file.
1.360 millert 360: Only the superuser can forward privileged ports.
1.308 djm 361: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 362: .Pp
1.218 jmc 363: By default, the local port is bound in accordance with the
364: .Cm GatewayPorts
365: setting.
366: However, an explicit
367: .Ar bind_address
368: may be used to bind the connection to a specific address.
1.2 deraadt 369: The
1.218 jmc 370: .Ar bind_address
371: of
372: .Dq localhost
373: indicates that the listening port be bound for local use only, while an
374: empty address or
375: .Sq *
376: indicates that the port should be available from all interfaces.
1.360 millert 377: .Pp
1.218 jmc 378: .It Fl l Ar login_name
379: Specifies the user to log in as on the remote machine.
380: This also may be specified on a per-host basis in the configuration file.
1.360 millert 381: .Pp
1.218 jmc 382: .It Fl M
383: Places the
384: .Nm
385: client into
386: .Dq master
387: mode for connection sharing.
1.231 stevesk 388: Multiple
389: .Fl M
390: options places
391: .Nm
392: into
393: .Dq master
394: mode with confirmation required before slave connections are accepted.
1.218 jmc 395: Refer to the description of
396: .Cm ControlMaster
397: in
398: .Xr ssh_config 5
399: for details.
1.360 millert 400: .Pp
1.218 jmc 401: .It Fl m Ar mac_spec
1.369 jmc 402: A comma-separated list of MAC (message authentication code) algorithms,
403: specified in order of preference.
1.218 jmc 404: See the
405: .Cm MACs
406: keyword for more information.
1.360 millert 407: .Pp
1.218 jmc 408: .It Fl N
409: Do not execute a remote command.
1.369 jmc 410: This is useful for just forwarding ports.
1.360 millert 411: .Pp
1.218 jmc 412: .It Fl n
413: Redirects stdin from
414: .Pa /dev/null
415: (actually, prevents reading from stdin).
416: This must be used when
1.2 deraadt 417: .Nm
1.218 jmc 418: is run in the background.
419: A common trick is to use this to run X11 programs on a remote machine.
420: For example,
421: .Ic ssh -n shadows.cs.hut.fi emacs &
422: will start an emacs on shadows.cs.hut.fi, and the X11
423: connection will be automatically forwarded over an encrypted channel.
424: The
1.2 deraadt 425: .Nm
1.218 jmc 426: program will be put in the background.
427: (This does not work if
1.2 deraadt 428: .Nm
1.218 jmc 429: needs to ask for a password or passphrase; see also the
430: .Fl f
431: option.)
1.360 millert 432: .Pp
1.218 jmc 433: .It Fl O Ar ctl_cmd
434: Control an active connection multiplexing master process.
435: When the
436: .Fl O
437: option is specified, the
438: .Ar ctl_cmd
439: argument is interpreted and passed to the master process.
440: Valid commands are:
441: .Dq check
1.305 markus 442: (check that the master process is running),
443: .Dq forward
1.318 jmc 444: (request forwardings without command execution),
1.323 okan 445: .Dq cancel
446: (cancel forwardings),
1.218 jmc 447: .Dq exit
1.318 jmc 448: (request the master to exit), and
1.317 djm 449: .Dq stop
450: (request the master to stop accepting further multiplexing requests).
1.360 millert 451: .Pp
1.218 jmc 452: .It Fl o Ar option
453: Can be used to give options in the format used in the configuration file.
454: This is useful for specifying options for which there is no separate
455: command-line flag.
456: For full details of the options listed below, and their possible values, see
457: .Xr ssh_config 5 .
1.2 deraadt 458: .Pp
1.218 jmc 459: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 460: .It AddKeysToAgent
1.218 jmc 461: .It AddressFamily
462: .It BatchMode
463: .It BindAddress
1.338 djm 464: .It CanonicalDomains
1.339 djm 465: .It CanonicalizeFallbackLocal
466: .It CanonicalizeHostname
467: .It CanonicalizeMaxDots
468: .It CanonicalizePermittedCNAMEs
1.363 djm 469: .It CertificateFile
1.218 jmc 470: .It ChallengeResponseAuthentication
471: .It CheckHostIP
472: .It Ciphers
473: .It ClearAllForwardings
474: .It Compression
1.380 naddy 475: .It ConnectionAttempts
1.218 jmc 476: .It ConnectTimeout
477: .It ControlMaster
478: .It ControlPath
1.321 djm 479: .It ControlPersist
1.218 jmc 480: .It DynamicForward
481: .It EscapeChar
1.263 markus 482: .It ExitOnForwardFailure
1.353 jmc 483: .It FingerprintHash
1.218 jmc 484: .It ForwardAgent
485: .It ForwardX11
1.321 djm 486: .It ForwardX11Timeout
1.218 jmc 487: .It ForwardX11Trusted
488: .It GatewayPorts
489: .It GlobalKnownHostsFile
490: .It GSSAPIAuthentication
491: .It GSSAPIDelegateCredentials
492: .It HashKnownHosts
493: .It Host
494: .It HostbasedAuthentication
1.355 djm 495: .It HostbasedKeyTypes
1.218 jmc 496: .It HostKeyAlgorithms
497: .It HostKeyAlias
498: .It HostName
1.374 jmc 499: .It IdentitiesOnly
1.371 markus 500: .It IdentityAgent
1.218 jmc 501: .It IdentityFile
1.316 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 .
1.261 stevesk 744: If the
745: .Cm Tunnel
746: directive is unset, it is set to the default tunnel mode, which is
747: .Dq point-to-point .
1.360 millert 748: .Pp
1.218 jmc 749: .It Fl X
750: Enables X11 forwarding.
1.54 markus 751: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 752: .Pp
1.218 jmc 753: X11 forwarding should be enabled with caution.
1.168 jmc 754: Users with the ability to bypass file permissions on the remote host
1.218 jmc 755: (for the user's X authorization database)
756: can access the local X11 display through the forwarded connection.
757: An attacker may then be able to perform activities such as keystroke monitoring.
758: .Pp
759: For this reason, X11 forwarding is subjected to X11 SECURITY extension
760: restrictions by default.
761: Please refer to the
762: .Nm
763: .Fl Y
764: option and the
765: .Cm ForwardX11Trusted
766: directive in
767: .Xr ssh_config 5
768: for more information.
1.360 millert 769: .Pp
1.218 jmc 770: .It Fl x
771: Disables X11 forwarding.
1.360 millert 772: .Pp
1.218 jmc 773: .It Fl Y
774: Enables trusted X11 forwarding.
775: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
776: controls.
1.360 millert 777: .Pp
1.278 djm 778: .It Fl y
779: Send log information using the
780: .Xr syslog 3
781: system module.
782: By default this information is sent to stderr.
1.218 jmc 783: .El
1.224 jmc 784: .Pp
785: .Nm
786: may additionally obtain configuration data from
787: a per-user configuration file and a system-wide configuration file.
788: The file format and configuration options are described in
789: .Xr ssh_config 5 .
1.222 jmc 790: .Sh AUTHENTICATION
1.377 djm 791: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 792: .Pp
793: The methods available for authentication are:
1.260 jmc 794: GSSAPI-based authentication,
1.222 jmc 795: host-based authentication,
796: public key authentication,
797: challenge-response authentication,
798: and password authentication.
799: Authentication methods are tried in the order specified above,
1.369 jmc 800: though
801: .Cm PreferredAuthentications
802: can be used to change the default order.
1.222 jmc 803: .Pp
804: Host-based authentication works as follows:
1.218 jmc 805: If the machine the user logs in from is listed in
806: .Pa /etc/hosts.equiv
807: or
808: .Pa /etc/shosts.equiv
809: on the remote machine, and the user names are
810: the same on both sides, or if the files
811: .Pa ~/.rhosts
812: or
813: .Pa ~/.shosts
814: exist in the user's home directory on the
815: remote machine and contain a line containing the name of the client
816: machine and the name of the user on that machine, the user is
1.222 jmc 817: considered for login.
818: Additionally, the server
819: .Em must
820: be able to verify the client's
821: host key (see the description of
1.218 jmc 822: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 823: and
1.222 jmc 824: .Pa ~/.ssh/known_hosts ,
825: below)
826: for login to be permitted.
1.218 jmc 827: This authentication method closes security holes due to IP
1.222 jmc 828: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 829: [Note to the administrator:
830: .Pa /etc/hosts.equiv ,
831: .Pa ~/.rhosts ,
832: and the rlogin/rsh protocol in general, are inherently insecure and should be
833: disabled if security is desired.]
1.189 dtucker 834: .Pp
1.222 jmc 835: Public key authentication works as follows:
836: The scheme is based on public-key cryptography,
837: using cryptosystems
838: where encryption and decryption are done using separate keys,
839: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 840: The idea is that each user creates a public/private
841: key pair for authentication purposes.
842: The server knows the public key, and only the user knows the private key.
1.222 jmc 843: .Nm
844: implements public key authentication protocol automatically,
1.349 sobrado 845: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 846: The HISTORY section of
1.222 jmc 847: .Xr ssl 8
1.311 jmc 848: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 849: .Pp
1.218 jmc 850: The file
851: .Pa ~/.ssh/authorized_keys
852: lists the public keys that are permitted for logging in.
853: When the user logs in, the
1.2 deraadt 854: .Nm
1.218 jmc 855: program tells the server which key pair it would like to use for
856: authentication.
1.222 jmc 857: The client proves that it has access to the private key
858: and the server checks that the corresponding public key
859: is authorized to accept the account.
1.383 djm 860: .Pp
861: The server may inform the client of errors that prevented public key
862: authentication from succeeding after authentication completes using a
863: different method.
864: These may be viewed by increasing the
865: .Cm LogLevel
866: to
867: .Cm DEBUG
868: or higher (e.g. by using the
869: .Fl v
870: flag).
1.218 jmc 871: .Pp
1.222 jmc 872: The user creates his/her key pair by running
1.218 jmc 873: .Xr ssh-keygen 1 .
874: This stores the private key in
1.222 jmc 875: .Pa ~/.ssh/id_dsa
1.369 jmc 876: (DSA),
1.310 djm 877: .Pa ~/.ssh/id_ecdsa
1.369 jmc 878: (ECDSA),
1.343 naddy 879: .Pa ~/.ssh/id_ed25519
1.369 jmc 880: (Ed25519),
1.222 jmc 881: or
882: .Pa ~/.ssh/id_rsa
1.369 jmc 883: (RSA)
1.218 jmc 884: and stores the public key in
1.222 jmc 885: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 886: (DSA),
1.310 djm 887: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 888: (ECDSA),
1.343 naddy 889: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 890: (Ed25519),
1.222 jmc 891: or
892: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 893: (RSA)
1.218 jmc 894: in the user's home directory.
1.222 jmc 895: The user should then copy the public key
1.218 jmc 896: to
897: .Pa ~/.ssh/authorized_keys
1.222 jmc 898: in his/her home directory on the remote machine.
899: The
1.218 jmc 900: .Pa authorized_keys
901: file corresponds to the conventional
902: .Pa ~/.rhosts
903: file, and has one key
1.222 jmc 904: per line, though the lines can be very long.
1.218 jmc 905: After this, the user can log in without giving the password.
906: .Pp
1.301 jmc 907: A variation on public key authentication
908: is available in the form of certificate authentication:
909: instead of a set of public/private keys,
910: signed certificates are used.
911: This has the advantage that a single trusted certification authority
912: can be used in place of many public/private keys.
1.333 jmc 913: See the CERTIFICATES section of
1.301 jmc 914: .Xr ssh-keygen 1
915: for more information.
916: .Pp
917: The most convenient way to use public key or certificate authentication
918: may be with an authentication agent.
1.218 jmc 919: See
920: .Xr ssh-agent 1
1.366 jcs 921: and (optionally) the
922: .Cm AddKeysToAgent
923: directive in
924: .Xr ssh_config 5
1.218 jmc 925: for more information.
926: .Pp
1.222 jmc 927: Challenge-response authentication works as follows:
928: The server sends an arbitrary
929: .Qq challenge
930: text, and prompts for a response.
931: Examples of challenge-response authentication include
1.335 jmc 932: .Bx
933: Authentication (see
1.222 jmc 934: .Xr login.conf 5 )
1.335 jmc 935: and PAM (some
936: .Pf non- Ox
937: systems).
1.222 jmc 938: .Pp
939: Finally, if other authentication methods fail,
1.218 jmc 940: .Nm
941: prompts the user for a password.
942: The password is sent to the remote
943: host for checking; however, since all communications are encrypted,
944: the password cannot be seen by someone listening on the network.
1.232 jmc 945: .Pp
946: .Nm
947: automatically maintains and checks a database containing
948: identification for all hosts it has ever been used with.
949: Host keys are stored in
950: .Pa ~/.ssh/known_hosts
951: in the user's home directory.
952: Additionally, the file
953: .Pa /etc/ssh/ssh_known_hosts
954: is automatically checked for known hosts.
955: Any new hosts are automatically added to the user's file.
956: If a host's identification ever changes,
957: .Nm
958: warns about this and disables password authentication to prevent
959: server spoofing or man-in-the-middle attacks,
960: which could otherwise be used to circumvent the encryption.
961: The
962: .Cm StrictHostKeyChecking
963: option can be used to control logins to machines whose
964: host key is not known or has changed.
965: .Pp
1.218 jmc 966: When the user's identity has been accepted by the server, the server
1.357 dtucker 967: either executes the given command in a non-interactive session or,
968: if no command has been specified, logs into the machine and gives
969: the user a normal shell as an interactive session.
1.218 jmc 970: All communication with
971: the remote command or shell will be automatically encrypted.
972: .Pp
1.357 dtucker 973: If an interactive session is requested
974: .Nm
975: by default will only request a pseudo-terminal (pty) for interactive
976: sessions when the client has one.
977: The flags
978: .Fl T
979: and
980: .Fl t
981: can be used to override this behaviour.
982: .Pp
983: If a pseudo-terminal has been allocated the
1.218 jmc 984: user may use the escape characters noted below.
985: .Pp
1.357 dtucker 986: If no pseudo-terminal has been allocated,
1.218 jmc 987: the session is transparent and can be used to reliably transfer binary data.
988: On most systems, setting the escape character to
989: .Dq none
990: will also make the session transparent even if a tty is used.
991: .Pp
992: The session terminates when the command or shell on the remote
1.247 jmc 993: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 994: .Sh ESCAPE CHARACTERS
1.218 jmc 995: When a pseudo-terminal has been requested,
1.2 deraadt 996: .Nm
1.218 jmc 997: supports a number of functions through the use of an escape character.
998: .Pp
999: A single tilde character can be sent as
1000: .Ic ~~
1001: or by following the tilde by a character other than those described below.
1002: The escape character must always follow a newline to be interpreted as
1003: special.
1004: The escape character can be changed in configuration files using the
1005: .Cm EscapeChar
1006: configuration directive or on the command line by the
1007: .Fl e
1008: option.
1009: .Pp
1010: The supported escapes (assuming the default
1011: .Ql ~ )
1012: are:
1013: .Bl -tag -width Ds
1014: .It Cm ~.
1015: Disconnect.
1016: .It Cm ~^Z
1017: Background
1.234 jmc 1018: .Nm .
1.218 jmc 1019: .It Cm ~#
1020: List forwarded connections.
1021: .It Cm ~&
1022: Background
1.2 deraadt 1023: .Nm
1.218 jmc 1024: at logout when waiting for forwarded connection / X11 sessions to terminate.
1025: .It Cm ~?
1026: Display a list of escape characters.
1027: .It Cm ~B
1028: Send a BREAK to the remote system
1.369 jmc 1029: (only useful if the peer supports it).
1.218 jmc 1030: .It Cm ~C
1031: Open command line.
1032: Currently this allows the addition of port forwardings using the
1.279 stevesk 1033: .Fl L ,
1034: .Fl R
1.218 jmc 1035: and
1.279 stevesk 1036: .Fl D
1.225 jmc 1037: options (see above).
1.322 markus 1038: It also allows the cancellation of existing port-forwardings
1039: with
1.262 stevesk 1040: .Sm off
1.322 markus 1041: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1042: .Sm on
1.322 markus 1043: for local,
1044: .Sm off
1045: .Fl KR Oo Ar bind_address : Oc Ar port
1046: .Sm on
1047: for remote and
1048: .Sm off
1049: .Fl KD Oo Ar bind_address : Oc Ar port
1050: .Sm on
1051: for dynamic port-forwardings.
1.218 jmc 1052: .Ic !\& Ns Ar command
1053: allows the user to execute a local command if the
1054: .Ic PermitLocalCommand
1055: option is enabled in
1.176 jmc 1056: .Xr ssh_config 5 .
1.218 jmc 1057: Basic help is available, using the
1058: .Fl h
1059: option.
1060: .It Cm ~R
1061: Request rekeying of the connection
1.369 jmc 1062: (only useful if the peer supports it).
1.327 dtucker 1063: .It Cm ~V
1064: Decrease the verbosity
1065: .Pq Ic LogLevel
1066: when errors are being written to stderr.
1067: .It Cm ~v
1.328 jmc 1068: Increase the verbosity
1.327 dtucker 1069: .Pq Ic LogLevel
1070: when errors are being written to stderr.
1.176 jmc 1071: .El
1.246 jmc 1072: .Sh TCP FORWARDING
1073: Forwarding of arbitrary TCP connections over the secure channel can
1074: be specified either on the command line or in a configuration file.
1075: One possible application of TCP forwarding is a secure connection to a
1076: mail server; another is going through firewalls.
1077: .Pp
1078: In the example below, we look at encrypting communication between
1079: an IRC client and server, even though the IRC server does not directly
1080: support encrypted communications.
1081: This works as follows:
1082: the user connects to the remote host using
1083: .Nm ,
1084: specifying a port to be used to forward connections
1085: to the remote server.
1086: After that it is possible to start the service which is to be encrypted
1087: on the client machine,
1088: connecting to the same local port,
1089: and
1090: .Nm
1091: will encrypt and forward the connection.
1092: .Pp
1093: The following example tunnels an IRC session from client machine
1094: .Dq 127.0.0.1
1095: (localhost)
1096: to remote server
1097: .Dq server.example.com :
1098: .Bd -literal -offset 4n
1099: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1100: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1101: .Ed
1102: .Pp
1103: This tunnels a connection to IRC server
1104: .Dq server.example.com ,
1105: joining channel
1106: .Dq #users ,
1107: nickname
1108: .Dq pinky ,
1109: using port 1234.
1110: It doesn't matter which port is used,
1111: as long as it's greater than 1023
1112: (remember, only root can open sockets on privileged ports)
1113: and doesn't conflict with any ports already in use.
1114: The connection is forwarded to port 6667 on the remote server,
1115: since that's the standard port for IRC services.
1116: .Pp
1117: The
1118: .Fl f
1119: option backgrounds
1120: .Nm
1121: and the remote command
1122: .Dq sleep 10
1123: is specified to allow an amount of time
1124: (10 seconds, in the example)
1125: to start the service which is to be tunnelled.
1126: If no connections are made within the time specified,
1127: .Nm
1128: will exit.
1129: .Sh X11 FORWARDING
1.218 jmc 1130: If the
1131: .Cm ForwardX11
1132: variable is set to
1133: .Dq yes
1134: (or see the description of the
1.227 jmc 1135: .Fl X ,
1136: .Fl x ,
1.218 jmc 1137: and
1.227 jmc 1138: .Fl Y
1.226 jmc 1139: options above)
1.218 jmc 1140: and the user is using X11 (the
1141: .Ev DISPLAY
1142: environment variable is set), the connection to the X11 display is
1143: automatically forwarded to the remote side in such a way that any X11
1144: programs started from the shell (or command) will go through the
1145: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1146: from the local machine.
1.218 jmc 1147: The user should not manually set
1148: .Ev DISPLAY .
1149: Forwarding of X11 connections can be
1150: configured on the command line or in configuration files.
1151: .Pp
1152: The
1153: .Ev DISPLAY
1154: value set by
1155: .Nm
1156: will point to the server machine, but with a display number greater than zero.
1157: This is normal, and happens because
1158: .Nm
1159: creates a
1160: .Dq proxy
1161: X server on the server machine for forwarding the
1162: connections over the encrypted channel.
1.200 djm 1163: .Pp
1.218 jmc 1164: .Nm
1165: will also automatically set up Xauthority data on the server machine.
1166: For this purpose, it will generate a random authorization cookie,
1167: store it in Xauthority on the server, and verify that any forwarded
1168: connections carry this cookie and replace it by the real cookie when
1169: the connection is opened.
1170: The real authentication cookie is never
1171: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1172: .Pp
1.218 jmc 1173: If the
1174: .Cm ForwardAgent
1175: variable is set to
1176: .Dq yes
1177: (or see the description of the
1178: .Fl A
1.191 djm 1179: and
1.218 jmc 1180: .Fl a
1.226 jmc 1181: options above) and
1.218 jmc 1182: the user is using an authentication agent, the connection to the agent
1183: is automatically forwarded to the remote side.
1.252 jmc 1184: .Sh VERIFYING HOST KEYS
1185: When connecting to a server for the first time,
1186: a fingerprint of the server's public key is presented to the user
1187: (unless the option
1188: .Cm StrictHostKeyChecking
1189: has been disabled).
1190: Fingerprints can be determined using
1191: .Xr ssh-keygen 1 :
1192: .Pp
1193: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1194: .Pp
1.274 grunk 1195: If the fingerprint is already known, it can be matched
1196: and the key can be accepted or rejected.
1.358 djm 1197: If only legacy (MD5) fingerprints for the server are available, the
1198: .Xr ssh-keygen 1
1199: .Fl E
1200: option may be used to downgrade the fingerprint algorithm to match.
1201: .Pp
1.274 grunk 1202: Because of the difficulty of comparing host keys
1.352 djm 1203: just by looking at fingerprint strings,
1.274 grunk 1204: there is also support to compare host keys visually,
1205: using
1206: .Em random art .
1207: By setting the
1.275 grunk 1208: .Cm VisualHostKey
1.274 grunk 1209: option to
1.275 grunk 1210: .Dq yes ,
1.274 grunk 1211: a small ASCII graphic gets displayed on every login to a server, no matter
1212: if the session itself is interactive or not.
1213: By learning the pattern a known server produces, a user can easily
1214: find out that the host key has changed when a completely different pattern
1215: is displayed.
1216: Because these patterns are not unambiguous however, a pattern that looks
1217: similar to the pattern remembered only gives a good probability that the
1218: host key is the same, not guaranteed proof.
1219: .Pp
1220: To get a listing of the fingerprints along with their random art for
1221: all known hosts, the following command line can be used:
1222: .Pp
1223: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1224: .Pp
1.252 jmc 1225: If the fingerprint is unknown,
1226: an alternative method of verification is available:
1227: SSH fingerprints verified by DNS.
1228: An additional resource record (RR),
1229: SSHFP,
1230: is added to a zonefile
1231: and the connecting client is able to match the fingerprint
1232: with that of the key presented.
1233: .Pp
1234: In this example, we are connecting a client to a server,
1235: .Dq host.example.com .
1236: The SSHFP resource records should first be added to the zonefile for
1237: host.example.com:
1238: .Bd -literal -offset indent
1.259 jakob 1239: $ ssh-keygen -r host.example.com.
1.252 jmc 1240: .Ed
1241: .Pp
1242: The output lines will have to be added to the zonefile.
1243: To check that the zone is answering fingerprint queries:
1244: .Pp
1245: .Dl $ dig -t SSHFP host.example.com
1246: .Pp
1247: Finally the client connects:
1248: .Bd -literal -offset indent
1249: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1250: [...]
1251: Matching host key fingerprint found in DNS.
1252: Are you sure you want to continue connecting (yes/no)?
1253: .Ed
1254: .Pp
1255: See the
1256: .Cm VerifyHostKeyDNS
1257: option in
1258: .Xr ssh_config 5
1259: for more information.
1.250 jmc 1260: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1261: .Nm
1262: contains support for Virtual Private Network (VPN) tunnelling
1263: using the
1264: .Xr tun 4
1265: network pseudo-device,
1266: allowing two networks to be joined securely.
1267: The
1268: .Xr sshd_config 5
1269: configuration option
1270: .Cm PermitTunnel
1271: controls whether the server supports this,
1272: and at what level (layer 2 or 3 traffic).
1273: .Pp
1274: The following example would connect client network 10.0.50.0/24
1.265 otto 1275: with remote network 10.0.99.0/24 using a point-to-point connection
1276: from 10.1.1.1 to 10.1.1.2,
1277: provided that the SSH server running on the gateway to the remote network,
1278: at 192.168.1.15, allows it.
1279: .Pp
1280: On the client:
1.250 jmc 1281: .Bd -literal -offset indent
1282: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1283: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1284: # route add 10.0.99.0/24 10.1.1.2
1285: .Ed
1286: .Pp
1287: On the server:
1288: .Bd -literal -offset indent
1289: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1290: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1291: .Ed
1292: .Pp
1293: Client access may be more finely tuned via the
1294: .Pa /root/.ssh/authorized_keys
1295: file (see below) and the
1296: .Cm PermitRootLogin
1297: server option.
1.255 jmc 1298: The following entry would permit connections on
1.250 jmc 1299: .Xr tun 4
1.255 jmc 1300: device 1 from user
1.250 jmc 1301: .Dq jane
1.255 jmc 1302: and on tun device 2 from user
1.250 jmc 1303: .Dq john ,
1304: if
1305: .Cm PermitRootLogin
1306: is set to
1307: .Dq forced-commands-only :
1308: .Bd -literal -offset 2n
1309: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1310: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1311: .Ed
1312: .Pp
1.264 ray 1313: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1314: it may be more suited to temporary setups,
1315: such as for wireless VPNs.
1316: More permanent VPNs are better provided by tools such as
1317: .Xr ipsecctl 8
1318: and
1319: .Xr isakmpd 8 .
1.2 deraadt 1320: .Sh ENVIRONMENT
1321: .Nm
1.1 deraadt 1322: will normally set the following environment variables:
1.237 jmc 1323: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1324: .It Ev DISPLAY
1325: The
1326: .Ev DISPLAY
1.40 aaron 1327: variable indicates the location of the X11 server.
1.44 aaron 1328: It is automatically set by
1.2 deraadt 1329: .Nm
1330: to point to a value of the form
1.233 jmc 1331: .Dq hostname:n ,
1332: where
1333: .Dq hostname
1334: indicates the host where the shell runs, and
1335: .Sq n
1336: is an integer \*(Ge 1.
1.40 aaron 1337: .Nm
1338: uses this special value to forward X11 connections over the secure
1339: channel.
1.107 markus 1340: The user should normally not set
1341: .Ev DISPLAY
1342: explicitly, as that
1.1 deraadt 1343: will render the X11 connection insecure (and will require the user to
1344: manually copy any required authorization cookies).
1.2 deraadt 1345: .It Ev HOME
1.1 deraadt 1346: Set to the path of the user's home directory.
1.2 deraadt 1347: .It Ev LOGNAME
1348: Synonym for
1.12 aaron 1349: .Ev USER ;
1350: set for compatibility with systems that use this variable.
1.2 deraadt 1351: .It Ev MAIL
1.129 stevesk 1352: Set to the path of the user's mailbox.
1.40 aaron 1353: .It Ev PATH
1.2 deraadt 1354: Set to the default
1355: .Ev PATH ,
1356: as specified when compiling
1.234 jmc 1357: .Nm .
1.118 markus 1358: .It Ev SSH_ASKPASS
1359: If
1360: .Nm
1361: needs a passphrase, it will read the passphrase from the current
1362: terminal if it was run from a terminal.
1363: If
1364: .Nm
1365: does not have a terminal associated with it but
1366: .Ev DISPLAY
1367: and
1368: .Ev SSH_ASKPASS
1369: are set, it will execute the program specified by
1370: .Ev SSH_ASKPASS
1371: and open an X11 window to read the passphrase.
1372: This is particularly useful when calling
1373: .Nm
1374: from a
1.196 jmc 1375: .Pa .xsession
1.118 markus 1376: or related script.
1377: (Note that on some machines it
1378: may be necessary to redirect the input from
1379: .Pa /dev/null
1380: to make this work.)
1.18 markus 1381: .It Ev SSH_AUTH_SOCK
1.233 jmc 1382: Identifies the path of a
1383: .Ux Ns -domain
1384: socket used to communicate with the agent.
1.166 stevesk 1385: .It Ev SSH_CONNECTION
1386: Identifies the client and server ends of the connection.
1.40 aaron 1387: The variable contains
1.233 jmc 1388: four space-separated values: client IP address, client port number,
1389: server IP address, and server port number.
1.73 markus 1390: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1391: This variable contains the original command line if a forced command
1.73 markus 1392: is executed.
1393: It can be used to extract the original arguments.
1.2 deraadt 1394: .It Ev SSH_TTY
1.1 deraadt 1395: This is set to the name of the tty (path to the device) associated
1.40 aaron 1396: with the current shell or command.
1397: If the current session has no tty,
1.1 deraadt 1398: this variable is not set.
1.387 djm 1399: .It Ev SSH_TUNNEL
1400: Optionally set by
1401: .Xr sshd 8
1402: to contain the interface names assigned if tunnel forwarding was
1403: requested by the client.
1.385 djm 1404: .It Ev SSH_USER_AUTH
1405: Optionally set by
1406: .Xr sshd 8 ,
1407: this variable may contain a pathname to a file that lists the authentication
1408: methods successfully used when the session was established, including any
1409: public keys that were used.
1.2 deraadt 1410: .It Ev TZ
1.214 jmc 1411: This variable is set to indicate the present time zone if it
1.257 jmc 1412: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1413: on to new connections).
1.2 deraadt 1414: .It Ev USER
1.1 deraadt 1415: Set to the name of the user logging in.
1.2 deraadt 1416: .El
1417: .Pp
1.44 aaron 1418: Additionally,
1.2 deraadt 1419: .Nm
1.44 aaron 1420: reads
1.207 djm 1421: .Pa ~/.ssh/environment ,
1.2 deraadt 1422: and adds lines of the format
1423: .Dq VARNAME=value
1.233 jmc 1424: to the environment if the file exists and users are allowed to
1.161 marc 1425: change their environment.
1.176 jmc 1426: For more information, see the
1.161 marc 1427: .Cm PermitUserEnvironment
1.162 stevesk 1428: option in
1.161 marc 1429: .Xr sshd_config 5 .
1.2 deraadt 1430: .Sh FILES
1.236 jmc 1431: .Bl -tag -width Ds -compact
1.309 jmc 1432: .It Pa ~/.rhosts
1.240 jmc 1433: This file is used for host-based authentication (see above).
1.92 markus 1434: On some machines this file may need to be
1.240 jmc 1435: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1436: because
1.2 deraadt 1437: .Xr sshd 8
1.40 aaron 1438: reads it as root.
1439: Additionally, this file must be owned by the user,
1440: and must not have write permissions for anyone else.
1441: The recommended
1.1 deraadt 1442: permission for most machines is read/write for the user, and not
1443: accessible by others.
1.2 deraadt 1444: .Pp
1.309 jmc 1445: .It Pa ~/.shosts
1.240 jmc 1446: This file is used in exactly the same way as
1447: .Pa .rhosts ,
1448: but allows host-based authentication without permitting login with
1449: rlogin/rsh.
1.272 mcbride 1450: .Pp
1.309 jmc 1451: .It Pa ~/.ssh/
1.272 mcbride 1452: This directory is the default location for all user-specific configuration
1453: and authentication information.
1454: There is no general requirement to keep the entire contents of this directory
1455: secret, but the recommended permissions are read/write/execute for the user,
1456: and not accessible by others.
1.236 jmc 1457: .Pp
1.309 jmc 1458: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1459: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1460: that can be used for logging in as this user.
1.238 jmc 1461: The format of this file is described in the
1462: .Xr sshd 8
1463: manual page.
1464: This file is not highly sensitive, but the recommended
1465: permissions are read/write for the user, and not accessible by others.
1466: .Pp
1.309 jmc 1467: .It Pa ~/.ssh/config
1.238 jmc 1468: This is the per-user configuration file.
1469: The file format and configuration options are described in
1470: .Xr ssh_config 5 .
1471: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1472: read/write for the user, and not writable by others.
1.238 jmc 1473: .Pp
1.309 jmc 1474: .It Pa ~/.ssh/environment
1.239 jmc 1475: Contains additional definitions for environment variables; see
1476: .Sx ENVIRONMENT ,
1.238 jmc 1477: above.
1478: .Pp
1.309 jmc 1479: .It Pa ~/.ssh/id_dsa
1.310 djm 1480: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1481: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1482: .It Pa ~/.ssh/id_rsa
1.238 jmc 1483: Contains the private key for authentication.
1484: These files
1485: contain sensitive data and should be readable by the user but not
1486: accessible by others (read/write/execute).
1487: .Nm
1488: will simply ignore a private key file if it is accessible by others.
1489: It is possible to specify a passphrase when
1490: generating the key which will be used to encrypt the
1.389 djm 1491: sensitive part of this file using AES-128.
1.238 jmc 1492: .Pp
1.309 jmc 1493: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1494: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1495: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1496: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1497: Contains the public key for authentication.
1498: These files are not
1499: sensitive and can (but need not) be readable by anyone.
1500: .Pp
1.309 jmc 1501: .It Pa ~/.ssh/known_hosts
1.244 jmc 1502: Contains a list of host keys for all hosts the user has logged into
1503: that are not already in the systemwide list of known host keys.
1.238 jmc 1504: See
1.244 jmc 1505: .Xr sshd 8
1506: for further details of the format of this file.
1.238 jmc 1507: .Pp
1.309 jmc 1508: .It Pa ~/.ssh/rc
1.238 jmc 1509: Commands in this file are executed by
1510: .Nm
1.245 jmc 1511: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1512: started.
1513: See the
1514: .Xr sshd 8
1515: manual page for more information.
1516: .Pp
1.309 jmc 1517: .It Pa /etc/hosts.equiv
1.240 jmc 1518: This file is for host-based authentication (see above).
1519: It should only be writable by root.
1.236 jmc 1520: .Pp
1.309 jmc 1521: .It Pa /etc/shosts.equiv
1.240 jmc 1522: This file is used in exactly the same way as
1523: .Pa hosts.equiv ,
1524: but allows host-based authentication without permitting login with
1525: rlogin/rsh.
1.236 jmc 1526: .Pp
1.238 jmc 1527: .It Pa /etc/ssh/ssh_config
1528: Systemwide configuration file.
1529: The file format and configuration options are described in
1530: .Xr ssh_config 5 .
1531: .Pp
1.309 jmc 1532: .It Pa /etc/ssh/ssh_host_key
1533: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1534: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1535: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1536: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1537: These files contain the private parts of the host keys
1.245 jmc 1538: and are used for host-based authentication.
1.238 jmc 1539: .Pp
1.309 jmc 1540: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1541: Systemwide list of known host keys.
1542: This file should be prepared by the
1543: system administrator to contain the public host keys of all machines in the
1544: organization.
1.244 jmc 1545: It should be world-readable.
1546: See
1.238 jmc 1547: .Xr sshd 8
1.244 jmc 1548: for further details of the format of this file.
1.236 jmc 1549: .Pp
1.309 jmc 1550: .It Pa /etc/ssh/sshrc
1.1 deraadt 1551: Commands in this file are executed by
1.2 deraadt 1552: .Nm
1.245 jmc 1553: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1554: See the
1.2 deraadt 1555: .Xr sshd 8
1.1 deraadt 1556: manual page for more information.
1.58 itojun 1557: .El
1.312 jmc 1558: .Sh EXIT STATUS
1559: .Nm
1560: exits with the exit status of the remote command or with 255
1561: if an error occurred.
1.2 deraadt 1562: .Sh SEE ALSO
1563: .Xr scp 1 ,
1.83 djm 1564: .Xr sftp 1 ,
1.2 deraadt 1565: .Xr ssh-add 1 ,
1566: .Xr ssh-agent 1 ,
1567: .Xr ssh-keygen 1 ,
1.242 jmc 1568: .Xr ssh-keyscan 1 ,
1.250 jmc 1569: .Xr tun 4 ,
1.159 stevesk 1570: .Xr ssh_config 5 ,
1.160 naddy 1571: .Xr ssh-keysign 8 ,
1.87 itojun 1572: .Xr sshd 8
1.329 jmc 1573: .Sh STANDARDS
1.106 markus 1574: .Rs
1.329 jmc 1575: .%A S. Lehtinen
1576: .%A C. Lonvick
1577: .%D January 2006
1.256 jmc 1578: .%R RFC 4250
1.329 jmc 1579: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1580: .Re
1.329 jmc 1581: .Pp
1.256 jmc 1582: .Rs
1.329 jmc 1583: .%A T. Ylonen
1584: .%A C. Lonvick
1585: .%D January 2006
1.256 jmc 1586: .%R RFC 4251
1.329 jmc 1587: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1588: .Re
1.329 jmc 1589: .Pp
1.256 jmc 1590: .Rs
1.329 jmc 1591: .%A T. Ylonen
1592: .%A C. Lonvick
1593: .%D January 2006
1.256 jmc 1594: .%R RFC 4252
1.329 jmc 1595: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1596: .Re
1.329 jmc 1597: .Pp
1.256 jmc 1598: .Rs
1.329 jmc 1599: .%A T. Ylonen
1600: .%A C. Lonvick
1601: .%D January 2006
1.256 jmc 1602: .%R RFC 4253
1.329 jmc 1603: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1604: .Re
1.329 jmc 1605: .Pp
1.256 jmc 1606: .Rs
1.329 jmc 1607: .%A T. Ylonen
1608: .%A C. Lonvick
1609: .%D January 2006
1.256 jmc 1610: .%R RFC 4254
1.329 jmc 1611: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1612: .Re
1.329 jmc 1613: .Pp
1.256 jmc 1614: .Rs
1.329 jmc 1615: .%A J. Schlyter
1616: .%A W. Griffin
1617: .%D January 2006
1.256 jmc 1618: .%R RFC 4255
1.329 jmc 1619: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1620: .Re
1.329 jmc 1621: .Pp
1.256 jmc 1622: .Rs
1.329 jmc 1623: .%A F. Cusack
1624: .%A M. Forssen
1625: .%D January 2006
1.256 jmc 1626: .%R RFC 4256
1.329 jmc 1627: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1628: .Re
1.329 jmc 1629: .Pp
1.256 jmc 1630: .Rs
1.329 jmc 1631: .%A J. Galbraith
1632: .%A P. Remaker
1633: .%D January 2006
1.256 jmc 1634: .%R RFC 4335
1.329 jmc 1635: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1636: .Re
1.329 jmc 1637: .Pp
1.256 jmc 1638: .Rs
1.329 jmc 1639: .%A M. Bellare
1640: .%A T. Kohno
1641: .%A C. Namprempre
1642: .%D January 2006
1.256 jmc 1643: .%R RFC 4344
1.329 jmc 1644: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1645: .Re
1.329 jmc 1646: .Pp
1.256 jmc 1647: .Rs
1.329 jmc 1648: .%A B. Harris
1649: .%D January 2006
1.256 jmc 1650: .%R RFC 4345
1.329 jmc 1651: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1652: .Re
1.329 jmc 1653: .Pp
1.258 djm 1654: .Rs
1.329 jmc 1655: .%A M. Friedl
1656: .%A N. Provos
1657: .%A W. Simpson
1658: .%D March 2006
1.258 djm 1659: .%R RFC 4419
1.329 jmc 1660: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1661: .Re
1.329 jmc 1662: .Pp
1.266 markus 1663: .Rs
1.329 jmc 1664: .%A J. Galbraith
1665: .%A R. Thayer
1666: .%D November 2006
1.266 markus 1667: .%R RFC 4716
1.329 jmc 1668: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1669: .Re
1.329 jmc 1670: .Pp
1.313 djm 1671: .Rs
1.329 jmc 1672: .%A D. Stebila
1673: .%A J. Green
1674: .%D December 2009
1.313 djm 1675: .%R RFC 5656
1.329 jmc 1676: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1677: .Re
1.329 jmc 1678: .Pp
1.274 grunk 1679: .Rs
1680: .%A A. Perrig
1681: .%A D. Song
1682: .%D 1999
1.329 jmc 1683: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1684: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1685: .Re
1.173 jmc 1686: .Sh AUTHORS
1687: OpenSSH is a derivative of the original and free
1688: ssh 1.2.12 release by Tatu Ylonen.
1689: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1690: Theo de Raadt and Dug Song
1691: removed many bugs, re-added newer features and
1692: created OpenSSH.
1693: Markus Friedl contributed the support for SSH
1694: protocol versions 1.5 and 2.0.