Annotation of src/usr.bin/ssh/ssh.1, Revision 1.399
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: .\"
1.399 ! jmc 36: .\" $OpenBSD: ssh.1,v 1.398 2018/09/12 01:30:10 djm Exp $
! 37: .Dd $Mdocdate: September 12 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
1.397 djm 394: mode but with confirmation required using
395: .Xr ssh-askpass 1
396: before each operation that changes the multiplexing state
397: (e.g. opening a new session).
1.218 jmc 398: Refer to the description of
399: .Cm ControlMaster
400: in
401: .Xr ssh_config 5
402: for details.
1.360 millert 403: .Pp
1.218 jmc 404: .It Fl m Ar mac_spec
1.369 jmc 405: A comma-separated list of MAC (message authentication code) algorithms,
406: specified in order of preference.
1.218 jmc 407: See the
408: .Cm MACs
409: keyword for more information.
1.360 millert 410: .Pp
1.218 jmc 411: .It Fl N
412: Do not execute a remote command.
1.369 jmc 413: This is useful for just forwarding ports.
1.360 millert 414: .Pp
1.218 jmc 415: .It Fl n
416: Redirects stdin from
417: .Pa /dev/null
418: (actually, prevents reading from stdin).
419: This must be used when
1.2 deraadt 420: .Nm
1.218 jmc 421: is run in the background.
422: A common trick is to use this to run X11 programs on a remote machine.
423: For example,
424: .Ic ssh -n shadows.cs.hut.fi emacs &
425: will start an emacs on shadows.cs.hut.fi, and the X11
426: connection will be automatically forwarded over an encrypted channel.
427: The
1.2 deraadt 428: .Nm
1.218 jmc 429: program will be put in the background.
430: (This does not work if
1.2 deraadt 431: .Nm
1.218 jmc 432: needs to ask for a password or passphrase; see also the
433: .Fl f
434: option.)
1.360 millert 435: .Pp
1.218 jmc 436: .It Fl O Ar ctl_cmd
437: Control an active connection multiplexing master process.
438: When the
439: .Fl O
440: option is specified, the
441: .Ar ctl_cmd
442: argument is interpreted and passed to the master process.
443: Valid commands are:
444: .Dq check
1.305 markus 445: (check that the master process is running),
446: .Dq forward
1.318 jmc 447: (request forwardings without command execution),
1.323 okan 448: .Dq cancel
449: (cancel forwardings),
1.218 jmc 450: .Dq exit
1.318 jmc 451: (request the master to exit), and
1.317 djm 452: .Dq stop
453: (request the master to stop accepting further multiplexing requests).
1.360 millert 454: .Pp
1.218 jmc 455: .It Fl o Ar option
456: Can be used to give options in the format used in the configuration file.
457: This is useful for specifying options for which there is no separate
458: command-line flag.
459: For full details of the options listed below, and their possible values, see
460: .Xr ssh_config 5 .
1.2 deraadt 461: .Pp
1.218 jmc 462: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 463: .It AddKeysToAgent
1.218 jmc 464: .It AddressFamily
465: .It BatchMode
466: .It BindAddress
1.338 djm 467: .It CanonicalDomains
1.339 djm 468: .It CanonicalizeFallbackLocal
469: .It CanonicalizeHostname
470: .It CanonicalizeMaxDots
471: .It CanonicalizePermittedCNAMEs
1.399 ! jmc 472: .It CASignatureAlgorithms
1.363 djm 473: .It CertificateFile
1.218 jmc 474: .It ChallengeResponseAuthentication
475: .It CheckHostIP
476: .It Ciphers
477: .It ClearAllForwardings
478: .It Compression
1.380 naddy 479: .It ConnectionAttempts
1.218 jmc 480: .It ConnectTimeout
481: .It ControlMaster
482: .It ControlPath
1.321 djm 483: .It ControlPersist
1.218 jmc 484: .It DynamicForward
485: .It EscapeChar
1.263 markus 486: .It ExitOnForwardFailure
1.353 jmc 487: .It FingerprintHash
1.218 jmc 488: .It ForwardAgent
489: .It ForwardX11
1.321 djm 490: .It ForwardX11Timeout
1.218 jmc 491: .It ForwardX11Trusted
492: .It GatewayPorts
493: .It GlobalKnownHostsFile
494: .It GSSAPIAuthentication
495: .It GSSAPIDelegateCredentials
496: .It HashKnownHosts
497: .It Host
498: .It HostbasedAuthentication
1.355 djm 499: .It HostbasedKeyTypes
1.218 jmc 500: .It HostKeyAlgorithms
501: .It HostKeyAlias
502: .It HostName
1.374 jmc 503: .It IdentitiesOnly
1.371 markus 504: .It IdentityAgent
1.218 jmc 505: .It IdentityFile
1.316 jmc 506: .It IPQoS
1.321 djm 507: .It KbdInteractiveAuthentication
1.218 jmc 508: .It KbdInteractiveDevices
1.314 jmc 509: .It KexAlgorithms
1.218 jmc 510: .It LocalCommand
511: .It LocalForward
512: .It LogLevel
513: .It MACs
1.337 jmc 514: .It Match
1.218 jmc 515: .It NoHostAuthenticationForLocalhost
516: .It NumberOfPasswordPrompts
517: .It PasswordAuthentication
518: .It PermitLocalCommand
1.291 markus 519: .It PKCS11Provider
1.218 jmc 520: .It Port
521: .It PreferredAuthentications
522: .It ProxyCommand
1.375 djm 523: .It ProxyJump
1.336 jmc 524: .It ProxyUseFdpass
1.359 markus 525: .It PubkeyAcceptedKeyTypes
1.218 jmc 526: .It PubkeyAuthentication
1.251 dtucker 527: .It RekeyLimit
1.382 bluhm 528: .It RemoteCommand
1.218 jmc 529: .It RemoteForward
1.319 jmc 530: .It RequestTTY
1.218 jmc 531: .It SendEnv
532: .It ServerAliveInterval
533: .It ServerAliveCountMax
1.395 jmc 534: .It SetEnv
1.347 jmc 535: .It StreamLocalBindMask
536: .It StreamLocalBindUnlink
1.218 jmc 537: .It StrictHostKeyChecking
538: .It TCPKeepAlive
539: .It Tunnel
540: .It TunnelDevice
1.354 djm 541: .It UpdateHostKeys
1.218 jmc 542: .It User
543: .It UserKnownHostsFile
544: .It VerifyHostKeyDNS
1.276 jmc 545: .It VisualHostKey
1.218 jmc 546: .It XAuthLocation
547: .El
1.360 millert 548: .Pp
1.218 jmc 549: .It Fl p Ar port
550: Port to connect to on the remote host.
551: This can be specified on a
552: per-host basis in the configuration file.
1.360 millert 553: .Pp
1.362 djm 554: .It Fl Q Ar query_option
1.332 djm 555: Queries
556: .Nm
1.341 deraadt 557: for the algorithms supported for the specified version 2.
558: The available features are:
559: .Ar cipher
1.332 djm 560: (supported symmetric ciphers),
1.341 deraadt 561: .Ar cipher-auth
1.340 djm 562: (supported symmetric ciphers that support authenticated encryption),
1.398 djm 563: .Ar help
564: (supported query terms for use with the
565: .Fl Q
566: flag),
1.341 deraadt 567: .Ar mac
1.332 djm 568: (supported message integrity codes),
1.341 deraadt 569: .Ar kex
1.332 djm 570: (key exchange algorithms),
1.341 deraadt 571: .Ar key
1.362 djm 572: (key types),
573: .Ar key-cert
574: (certificate key types),
575: .Ar key-plain
1.398 djm 576: (non-certificate key types),
1.356 djm 577: .Ar protocol-version
1.398 djm 578: (supported SSH protocol versions), and
579: .Ar sig
580: (supported signature algorithms).
1.360 millert 581: .Pp
1.218 jmc 582: .It Fl q
583: Quiet mode.
1.271 djm 584: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 585: .Pp
1.218 jmc 586: .It Fl R Xo
587: .Sm off
588: .Oo Ar bind_address : Oc
589: .Ar port : host : hostport
590: .Sm on
591: .Xc
1.360 millert 592: .It Fl R Xo
593: .Sm off
594: .Oo Ar bind_address : Oc
595: .Ar port : local_socket
596: .Sm on
597: .Xc
598: .It Fl R Xo
599: .Sm off
600: .Ar remote_socket : host : hostport
601: .Sm on
602: .Xc
603: .It Fl R Xo
604: .Sm off
605: .Ar remote_socket : local_socket
606: .Sm on
607: .Xc
1.384 markus 608: .It Fl R Xo
609: .Sm off
610: .Oo Ar bind_address : Oc
611: .Ar port
612: .Sm on
613: .Xc
1.360 millert 614: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 615: (server) host are to be forwarded to the local side.
616: .Pp
1.360 millert 617: This works by allocating a socket to listen to either a TCP
1.218 jmc 618: .Ar port
1.360 millert 619: or to a Unix socket on the remote side.
620: Whenever a connection is made to this port or Unix socket, the
621: connection is forwarded over the secure channel, and a connection
1.384 markus 622: is made from the local machine to either an explicit destination specified by
1.218 jmc 623: .Ar host
624: port
1.360 millert 625: .Ar hostport ,
626: or
627: .Ar local_socket ,
1.384 markus 628: or, if no explicit destination was specified,
629: .Nm
630: will act as a SOCKS 4/5 proxy and forward connections to the destinations
631: requested by the remote SOCKS client.
1.2 deraadt 632: .Pp
1.218 jmc 633: Port forwardings can also be specified in the configuration file.
634: Privileged ports can be forwarded only when
635: logging in as root on the remote machine.
1.324 jmc 636: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 637: .Pp
1.360 millert 638: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 639: interface only.
1.280 tobias 640: This may be overridden by specifying a
1.218 jmc 641: .Ar bind_address .
642: An empty
643: .Ar bind_address ,
644: or the address
645: .Ql * ,
646: indicates that the remote socket should listen on all interfaces.
647: Specifying a remote
648: .Ar bind_address
649: will only succeed if the server's
650: .Cm GatewayPorts
651: option is enabled (see
652: .Xr sshd_config 5 ) .
1.281 djm 653: .Pp
654: If the
655: .Ar port
656: argument is
1.282 djm 657: .Ql 0 ,
1.281 djm 658: the listen port will be dynamically allocated on the server and reported
659: to the client at run time.
1.305 markus 660: When used together with
661: .Ic -O forward
662: the allocated port will be printed to the standard output.
1.360 millert 663: .Pp
1.218 jmc 664: .It Fl S Ar ctl_path
1.304 jmc 665: Specifies the location of a control socket for connection sharing,
1.303 djm 666: or the string
667: .Dq none
668: to disable connection sharing.
1.218 jmc 669: Refer to the description of
670: .Cm ControlPath
671: and
672: .Cm ControlMaster
673: in
674: .Xr ssh_config 5
675: for details.
1.360 millert 676: .Pp
1.218 jmc 677: .It Fl s
678: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 679: Subsystems facilitate the use of SSH
680: as a secure transport for other applications (e.g.\&
1.218 jmc 681: .Xr sftp 1 ) .
682: The subsystem is specified as the remote command.
1.360 millert 683: .Pp
1.218 jmc 684: .It Fl T
1.357 dtucker 685: Disable pseudo-terminal allocation.
1.360 millert 686: .Pp
1.218 jmc 687: .It Fl t
1.357 dtucker 688: Force pseudo-terminal allocation.
1.218 jmc 689: This can be used to execute arbitrary
690: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 691: e.g. when implementing menu services.
1.218 jmc 692: Multiple
693: .Fl t
694: options force tty allocation, even if
1.194 jakob 695: .Nm
1.218 jmc 696: has no local tty.
1.360 millert 697: .Pp
1.218 jmc 698: .It Fl V
699: Display the version number and exit.
1.360 millert 700: .Pp
1.218 jmc 701: .It Fl v
702: Verbose mode.
703: Causes
1.176 jmc 704: .Nm
1.218 jmc 705: to print debugging messages about its progress.
706: This is helpful in
707: debugging connection, authentication, and configuration problems.
708: Multiple
709: .Fl v
710: options increase the verbosity.
711: The maximum is 3.
1.360 millert 712: .Pp
1.290 dtucker 713: .It Fl W Ar host : Ns Ar port
714: Requests that standard input and output on the client be forwarded to
715: .Ar host
716: on
717: .Ar port
718: over the secure channel.
719: Implies
720: .Fl N ,
721: .Fl T ,
722: .Cm ExitOnForwardFailure
723: and
1.373 jmc 724: .Cm ClearAllForwardings ,
725: though these can be overridden in the configuration file or using
1.372 dtucker 726: .Fl o
727: command line options.
1.360 millert 728: .Pp
1.261 stevesk 729: .It Fl w Xo
730: .Ar local_tun Ns Op : Ns Ar remote_tun
731: .Xc
732: Requests
733: tunnel
734: device forwarding with the specified
1.218 jmc 735: .Xr tun 4
1.261 stevesk 736: devices between the client
737: .Pq Ar local_tun
738: and the server
739: .Pq Ar remote_tun .
740: .Pp
1.228 jmc 741: The devices may be specified by numerical ID or the keyword
742: .Dq any ,
743: which uses the next available tunnel device.
1.261 stevesk 744: If
745: .Ar remote_tun
746: is not specified, it defaults to
747: .Dq any .
1.228 jmc 748: See also the
1.218 jmc 749: .Cm Tunnel
1.261 stevesk 750: and
751: .Cm TunnelDevice
752: directives in
1.218 jmc 753: .Xr ssh_config 5 .
1.393 djm 754: .Pp
1.261 stevesk 755: If the
756: .Cm Tunnel
1.393 djm 757: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 758: .Dq point-to-point .
1.393 djm 759: If a different
760: .Cm Tunnel
761: forwarding mode it desired, then it should be specified before
762: .Fl w .
1.360 millert 763: .Pp
1.218 jmc 764: .It Fl X
765: Enables X11 forwarding.
1.54 markus 766: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 767: .Pp
1.218 jmc 768: X11 forwarding should be enabled with caution.
1.168 jmc 769: Users with the ability to bypass file permissions on the remote host
1.218 jmc 770: (for the user's X authorization database)
771: can access the local X11 display through the forwarded connection.
772: An attacker may then be able to perform activities such as keystroke monitoring.
773: .Pp
774: For this reason, X11 forwarding is subjected to X11 SECURITY extension
775: restrictions by default.
776: Please refer to the
777: .Nm
778: .Fl Y
779: option and the
780: .Cm ForwardX11Trusted
781: directive in
782: .Xr ssh_config 5
783: for more information.
1.360 millert 784: .Pp
1.218 jmc 785: .It Fl x
786: Disables X11 forwarding.
1.360 millert 787: .Pp
1.218 jmc 788: .It Fl Y
789: Enables trusted X11 forwarding.
790: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
791: controls.
1.360 millert 792: .Pp
1.278 djm 793: .It Fl y
794: Send log information using the
795: .Xr syslog 3
796: system module.
797: By default this information is sent to stderr.
1.218 jmc 798: .El
1.224 jmc 799: .Pp
800: .Nm
801: may additionally obtain configuration data from
802: a per-user configuration file and a system-wide configuration file.
803: The file format and configuration options are described in
804: .Xr ssh_config 5 .
1.222 jmc 805: .Sh AUTHENTICATION
1.377 djm 806: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 807: .Pp
808: The methods available for authentication are:
1.260 jmc 809: GSSAPI-based authentication,
1.222 jmc 810: host-based authentication,
811: public key authentication,
812: challenge-response authentication,
813: and password authentication.
814: Authentication methods are tried in the order specified above,
1.369 jmc 815: though
816: .Cm PreferredAuthentications
817: can be used to change the default order.
1.222 jmc 818: .Pp
819: Host-based authentication works as follows:
1.218 jmc 820: If the machine the user logs in from is listed in
821: .Pa /etc/hosts.equiv
822: or
823: .Pa /etc/shosts.equiv
824: on the remote machine, and the user names are
825: the same on both sides, or if the files
826: .Pa ~/.rhosts
827: or
828: .Pa ~/.shosts
829: exist in the user's home directory on the
830: remote machine and contain a line containing the name of the client
831: machine and the name of the user on that machine, the user is
1.222 jmc 832: considered for login.
833: Additionally, the server
834: .Em must
835: be able to verify the client's
836: host key (see the description of
1.218 jmc 837: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 838: and
1.222 jmc 839: .Pa ~/.ssh/known_hosts ,
840: below)
841: for login to be permitted.
1.218 jmc 842: This authentication method closes security holes due to IP
1.222 jmc 843: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 844: [Note to the administrator:
845: .Pa /etc/hosts.equiv ,
846: .Pa ~/.rhosts ,
847: and the rlogin/rsh protocol in general, are inherently insecure and should be
848: disabled if security is desired.]
1.189 dtucker 849: .Pp
1.222 jmc 850: Public key authentication works as follows:
851: The scheme is based on public-key cryptography,
852: using cryptosystems
853: where encryption and decryption are done using separate keys,
854: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 855: The idea is that each user creates a public/private
856: key pair for authentication purposes.
857: The server knows the public key, and only the user knows the private key.
1.222 jmc 858: .Nm
859: implements public key authentication protocol automatically,
1.349 sobrado 860: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 861: The HISTORY section of
1.222 jmc 862: .Xr ssl 8
1.311 jmc 863: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 864: .Pp
1.218 jmc 865: The file
866: .Pa ~/.ssh/authorized_keys
867: lists the public keys that are permitted for logging in.
868: When the user logs in, the
1.2 deraadt 869: .Nm
1.218 jmc 870: program tells the server which key pair it would like to use for
871: authentication.
1.222 jmc 872: The client proves that it has access to the private key
873: and the server checks that the corresponding public key
874: is authorized to accept the account.
1.383 djm 875: .Pp
876: The server may inform the client of errors that prevented public key
877: authentication from succeeding after authentication completes using a
878: different method.
879: These may be viewed by increasing the
880: .Cm LogLevel
881: to
882: .Cm DEBUG
883: or higher (e.g. by using the
884: .Fl v
885: flag).
1.218 jmc 886: .Pp
1.222 jmc 887: The user creates his/her key pair by running
1.218 jmc 888: .Xr ssh-keygen 1 .
889: This stores the private key in
1.222 jmc 890: .Pa ~/.ssh/id_dsa
1.369 jmc 891: (DSA),
1.310 djm 892: .Pa ~/.ssh/id_ecdsa
1.369 jmc 893: (ECDSA),
1.343 naddy 894: .Pa ~/.ssh/id_ed25519
1.369 jmc 895: (Ed25519),
1.222 jmc 896: or
897: .Pa ~/.ssh/id_rsa
1.369 jmc 898: (RSA)
1.218 jmc 899: and stores the public key in
1.222 jmc 900: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 901: (DSA),
1.310 djm 902: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 903: (ECDSA),
1.343 naddy 904: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 905: (Ed25519),
1.222 jmc 906: or
907: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 908: (RSA)
1.218 jmc 909: in the user's home directory.
1.222 jmc 910: The user should then copy the public key
1.218 jmc 911: to
912: .Pa ~/.ssh/authorized_keys
1.222 jmc 913: in his/her home directory on the remote machine.
914: The
1.218 jmc 915: .Pa authorized_keys
916: file corresponds to the conventional
917: .Pa ~/.rhosts
918: file, and has one key
1.222 jmc 919: per line, though the lines can be very long.
1.218 jmc 920: After this, the user can log in without giving the password.
921: .Pp
1.301 jmc 922: A variation on public key authentication
923: is available in the form of certificate authentication:
924: instead of a set of public/private keys,
925: signed certificates are used.
926: This has the advantage that a single trusted certification authority
927: can be used in place of many public/private keys.
1.333 jmc 928: See the CERTIFICATES section of
1.301 jmc 929: .Xr ssh-keygen 1
930: for more information.
931: .Pp
932: The most convenient way to use public key or certificate authentication
933: may be with an authentication agent.
1.218 jmc 934: See
935: .Xr ssh-agent 1
1.366 jcs 936: and (optionally) the
937: .Cm AddKeysToAgent
938: directive in
939: .Xr ssh_config 5
1.218 jmc 940: for more information.
941: .Pp
1.222 jmc 942: Challenge-response authentication works as follows:
943: The server sends an arbitrary
944: .Qq challenge
945: text, and prompts for a response.
946: Examples of challenge-response authentication include
1.335 jmc 947: .Bx
948: Authentication (see
1.222 jmc 949: .Xr login.conf 5 )
1.335 jmc 950: and PAM (some
951: .Pf non- Ox
952: systems).
1.222 jmc 953: .Pp
954: Finally, if other authentication methods fail,
1.218 jmc 955: .Nm
956: prompts the user for a password.
957: The password is sent to the remote
958: host for checking; however, since all communications are encrypted,
959: the password cannot be seen by someone listening on the network.
1.232 jmc 960: .Pp
961: .Nm
962: automatically maintains and checks a database containing
963: identification for all hosts it has ever been used with.
964: Host keys are stored in
965: .Pa ~/.ssh/known_hosts
966: in the user's home directory.
967: Additionally, the file
968: .Pa /etc/ssh/ssh_known_hosts
969: is automatically checked for known hosts.
970: Any new hosts are automatically added to the user's file.
971: If a host's identification ever changes,
972: .Nm
973: warns about this and disables password authentication to prevent
974: server spoofing or man-in-the-middle attacks,
975: which could otherwise be used to circumvent the encryption.
976: The
977: .Cm StrictHostKeyChecking
978: option can be used to control logins to machines whose
979: host key is not known or has changed.
980: .Pp
1.218 jmc 981: When the user's identity has been accepted by the server, the server
1.357 dtucker 982: either executes the given command in a non-interactive session or,
983: if no command has been specified, logs into the machine and gives
984: the user a normal shell as an interactive session.
1.218 jmc 985: All communication with
986: the remote command or shell will be automatically encrypted.
987: .Pp
1.357 dtucker 988: If an interactive session is requested
989: .Nm
990: by default will only request a pseudo-terminal (pty) for interactive
991: sessions when the client has one.
992: The flags
993: .Fl T
994: and
995: .Fl t
996: can be used to override this behaviour.
997: .Pp
998: If a pseudo-terminal has been allocated the
1.218 jmc 999: user may use the escape characters noted below.
1000: .Pp
1.357 dtucker 1001: If no pseudo-terminal has been allocated,
1.218 jmc 1002: the session is transparent and can be used to reliably transfer binary data.
1003: On most systems, setting the escape character to
1004: .Dq none
1005: will also make the session transparent even if a tty is used.
1006: .Pp
1007: The session terminates when the command or shell on the remote
1.247 jmc 1008: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1009: .Sh ESCAPE CHARACTERS
1.218 jmc 1010: When a pseudo-terminal has been requested,
1.2 deraadt 1011: .Nm
1.218 jmc 1012: supports a number of functions through the use of an escape character.
1013: .Pp
1014: A single tilde character can be sent as
1015: .Ic ~~
1016: or by following the tilde by a character other than those described below.
1017: The escape character must always follow a newline to be interpreted as
1018: special.
1019: The escape character can be changed in configuration files using the
1020: .Cm EscapeChar
1021: configuration directive or on the command line by the
1022: .Fl e
1023: option.
1024: .Pp
1025: The supported escapes (assuming the default
1026: .Ql ~ )
1027: are:
1028: .Bl -tag -width Ds
1029: .It Cm ~.
1030: Disconnect.
1031: .It Cm ~^Z
1032: Background
1.234 jmc 1033: .Nm .
1.218 jmc 1034: .It Cm ~#
1035: List forwarded connections.
1036: .It Cm ~&
1037: Background
1.2 deraadt 1038: .Nm
1.218 jmc 1039: at logout when waiting for forwarded connection / X11 sessions to terminate.
1040: .It Cm ~?
1041: Display a list of escape characters.
1042: .It Cm ~B
1043: Send a BREAK to the remote system
1.369 jmc 1044: (only useful if the peer supports it).
1.218 jmc 1045: .It Cm ~C
1046: Open command line.
1047: Currently this allows the addition of port forwardings using the
1.279 stevesk 1048: .Fl L ,
1049: .Fl R
1.218 jmc 1050: and
1.279 stevesk 1051: .Fl D
1.225 jmc 1052: options (see above).
1.322 markus 1053: It also allows the cancellation of existing port-forwardings
1054: with
1.262 stevesk 1055: .Sm off
1.322 markus 1056: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1057: .Sm on
1.322 markus 1058: for local,
1059: .Sm off
1060: .Fl KR Oo Ar bind_address : Oc Ar port
1061: .Sm on
1062: for remote and
1063: .Sm off
1064: .Fl KD Oo Ar bind_address : Oc Ar port
1065: .Sm on
1066: for dynamic port-forwardings.
1.218 jmc 1067: .Ic !\& Ns Ar command
1068: allows the user to execute a local command if the
1069: .Ic PermitLocalCommand
1070: option is enabled in
1.176 jmc 1071: .Xr ssh_config 5 .
1.218 jmc 1072: Basic help is available, using the
1073: .Fl h
1074: option.
1075: .It Cm ~R
1076: Request rekeying of the connection
1.369 jmc 1077: (only useful if the peer supports it).
1.327 dtucker 1078: .It Cm ~V
1079: Decrease the verbosity
1080: .Pq Ic LogLevel
1081: when errors are being written to stderr.
1082: .It Cm ~v
1.328 jmc 1083: Increase the verbosity
1.327 dtucker 1084: .Pq Ic LogLevel
1085: when errors are being written to stderr.
1.176 jmc 1086: .El
1.246 jmc 1087: .Sh TCP FORWARDING
1088: Forwarding of arbitrary TCP connections over the secure channel can
1089: be specified either on the command line or in a configuration file.
1090: One possible application of TCP forwarding is a secure connection to a
1091: mail server; another is going through firewalls.
1092: .Pp
1093: In the example below, we look at encrypting communication between
1094: an IRC client and server, even though the IRC server does not directly
1095: support encrypted communications.
1096: This works as follows:
1097: the user connects to the remote host using
1098: .Nm ,
1099: specifying a port to be used to forward connections
1100: to the remote server.
1101: After that it is possible to start the service which is to be encrypted
1102: on the client machine,
1103: connecting to the same local port,
1104: and
1105: .Nm
1106: will encrypt and forward the connection.
1107: .Pp
1108: The following example tunnels an IRC session from client machine
1109: .Dq 127.0.0.1
1110: (localhost)
1111: to remote server
1112: .Dq server.example.com :
1113: .Bd -literal -offset 4n
1114: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1115: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1116: .Ed
1117: .Pp
1118: This tunnels a connection to IRC server
1119: .Dq server.example.com ,
1120: joining channel
1121: .Dq #users ,
1122: nickname
1123: .Dq pinky ,
1124: using port 1234.
1125: It doesn't matter which port is used,
1126: as long as it's greater than 1023
1127: (remember, only root can open sockets on privileged ports)
1128: and doesn't conflict with any ports already in use.
1129: The connection is forwarded to port 6667 on the remote server,
1130: since that's the standard port for IRC services.
1131: .Pp
1132: The
1133: .Fl f
1134: option backgrounds
1135: .Nm
1136: and the remote command
1137: .Dq sleep 10
1138: is specified to allow an amount of time
1139: (10 seconds, in the example)
1140: to start the service which is to be tunnelled.
1141: If no connections are made within the time specified,
1142: .Nm
1143: will exit.
1144: .Sh X11 FORWARDING
1.218 jmc 1145: If the
1146: .Cm ForwardX11
1147: variable is set to
1148: .Dq yes
1149: (or see the description of the
1.227 jmc 1150: .Fl X ,
1151: .Fl x ,
1.218 jmc 1152: and
1.227 jmc 1153: .Fl Y
1.226 jmc 1154: options above)
1.218 jmc 1155: and the user is using X11 (the
1156: .Ev DISPLAY
1157: environment variable is set), the connection to the X11 display is
1158: automatically forwarded to the remote side in such a way that any X11
1159: programs started from the shell (or command) will go through the
1160: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1161: from the local machine.
1.218 jmc 1162: The user should not manually set
1163: .Ev DISPLAY .
1164: Forwarding of X11 connections can be
1165: configured on the command line or in configuration files.
1166: .Pp
1167: The
1168: .Ev DISPLAY
1169: value set by
1170: .Nm
1171: will point to the server machine, but with a display number greater than zero.
1172: This is normal, and happens because
1173: .Nm
1174: creates a
1175: .Dq proxy
1176: X server on the server machine for forwarding the
1177: connections over the encrypted channel.
1.200 djm 1178: .Pp
1.218 jmc 1179: .Nm
1180: will also automatically set up Xauthority data on the server machine.
1181: For this purpose, it will generate a random authorization cookie,
1182: store it in Xauthority on the server, and verify that any forwarded
1183: connections carry this cookie and replace it by the real cookie when
1184: the connection is opened.
1185: The real authentication cookie is never
1186: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1187: .Pp
1.218 jmc 1188: If the
1189: .Cm ForwardAgent
1190: variable is set to
1191: .Dq yes
1192: (or see the description of the
1193: .Fl A
1.191 djm 1194: and
1.218 jmc 1195: .Fl a
1.226 jmc 1196: options above) and
1.218 jmc 1197: the user is using an authentication agent, the connection to the agent
1198: is automatically forwarded to the remote side.
1.252 jmc 1199: .Sh VERIFYING HOST KEYS
1200: When connecting to a server for the first time,
1201: a fingerprint of the server's public key is presented to the user
1202: (unless the option
1203: .Cm StrictHostKeyChecking
1204: has been disabled).
1205: Fingerprints can be determined using
1206: .Xr ssh-keygen 1 :
1207: .Pp
1208: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1209: .Pp
1.274 grunk 1210: If the fingerprint is already known, it can be matched
1211: and the key can be accepted or rejected.
1.358 djm 1212: If only legacy (MD5) fingerprints for the server are available, the
1213: .Xr ssh-keygen 1
1214: .Fl E
1215: option may be used to downgrade the fingerprint algorithm to match.
1216: .Pp
1.274 grunk 1217: Because of the difficulty of comparing host keys
1.352 djm 1218: just by looking at fingerprint strings,
1.274 grunk 1219: there is also support to compare host keys visually,
1220: using
1221: .Em random art .
1222: By setting the
1.275 grunk 1223: .Cm VisualHostKey
1.274 grunk 1224: option to
1.275 grunk 1225: .Dq yes ,
1.274 grunk 1226: a small ASCII graphic gets displayed on every login to a server, no matter
1227: if the session itself is interactive or not.
1228: By learning the pattern a known server produces, a user can easily
1229: find out that the host key has changed when a completely different pattern
1230: is displayed.
1231: Because these patterns are not unambiguous however, a pattern that looks
1232: similar to the pattern remembered only gives a good probability that the
1233: host key is the same, not guaranteed proof.
1234: .Pp
1235: To get a listing of the fingerprints along with their random art for
1236: all known hosts, the following command line can be used:
1237: .Pp
1238: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1239: .Pp
1.252 jmc 1240: If the fingerprint is unknown,
1241: an alternative method of verification is available:
1242: SSH fingerprints verified by DNS.
1243: An additional resource record (RR),
1244: SSHFP,
1245: is added to a zonefile
1246: and the connecting client is able to match the fingerprint
1247: with that of the key presented.
1248: .Pp
1249: In this example, we are connecting a client to a server,
1250: .Dq host.example.com .
1251: The SSHFP resource records should first be added to the zonefile for
1252: host.example.com:
1253: .Bd -literal -offset indent
1.259 jakob 1254: $ ssh-keygen -r host.example.com.
1.252 jmc 1255: .Ed
1256: .Pp
1257: The output lines will have to be added to the zonefile.
1258: To check that the zone is answering fingerprint queries:
1259: .Pp
1260: .Dl $ dig -t SSHFP host.example.com
1261: .Pp
1262: Finally the client connects:
1263: .Bd -literal -offset indent
1264: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1265: [...]
1266: Matching host key fingerprint found in DNS.
1267: Are you sure you want to continue connecting (yes/no)?
1268: .Ed
1269: .Pp
1270: See the
1271: .Cm VerifyHostKeyDNS
1272: option in
1273: .Xr ssh_config 5
1274: for more information.
1.250 jmc 1275: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1276: .Nm
1277: contains support for Virtual Private Network (VPN) tunnelling
1278: using the
1279: .Xr tun 4
1280: network pseudo-device,
1281: allowing two networks to be joined securely.
1282: The
1283: .Xr sshd_config 5
1284: configuration option
1285: .Cm PermitTunnel
1286: controls whether the server supports this,
1287: and at what level (layer 2 or 3 traffic).
1288: .Pp
1289: The following example would connect client network 10.0.50.0/24
1.265 otto 1290: with remote network 10.0.99.0/24 using a point-to-point connection
1291: from 10.1.1.1 to 10.1.1.2,
1292: provided that the SSH server running on the gateway to the remote network,
1293: at 192.168.1.15, allows it.
1294: .Pp
1295: On the client:
1.250 jmc 1296: .Bd -literal -offset indent
1297: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1298: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1299: # route add 10.0.99.0/24 10.1.1.2
1300: .Ed
1301: .Pp
1302: On the server:
1303: .Bd -literal -offset indent
1304: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1305: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1306: .Ed
1307: .Pp
1308: Client access may be more finely tuned via the
1309: .Pa /root/.ssh/authorized_keys
1310: file (see below) and the
1311: .Cm PermitRootLogin
1312: server option.
1.255 jmc 1313: The following entry would permit connections on
1.250 jmc 1314: .Xr tun 4
1.255 jmc 1315: device 1 from user
1.250 jmc 1316: .Dq jane
1.255 jmc 1317: and on tun device 2 from user
1.250 jmc 1318: .Dq john ,
1319: if
1320: .Cm PermitRootLogin
1321: is set to
1322: .Dq forced-commands-only :
1323: .Bd -literal -offset 2n
1324: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1325: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1326: .Ed
1327: .Pp
1.264 ray 1328: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1329: it may be more suited to temporary setups,
1330: such as for wireless VPNs.
1331: More permanent VPNs are better provided by tools such as
1332: .Xr ipsecctl 8
1333: and
1334: .Xr isakmpd 8 .
1.2 deraadt 1335: .Sh ENVIRONMENT
1336: .Nm
1.1 deraadt 1337: will normally set the following environment variables:
1.237 jmc 1338: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1339: .It Ev DISPLAY
1340: The
1341: .Ev DISPLAY
1.40 aaron 1342: variable indicates the location of the X11 server.
1.44 aaron 1343: It is automatically set by
1.2 deraadt 1344: .Nm
1345: to point to a value of the form
1.233 jmc 1346: .Dq hostname:n ,
1347: where
1348: .Dq hostname
1349: indicates the host where the shell runs, and
1350: .Sq n
1351: is an integer \*(Ge 1.
1.40 aaron 1352: .Nm
1353: uses this special value to forward X11 connections over the secure
1354: channel.
1.107 markus 1355: The user should normally not set
1356: .Ev DISPLAY
1357: explicitly, as that
1.1 deraadt 1358: will render the X11 connection insecure (and will require the user to
1359: manually copy any required authorization cookies).
1.2 deraadt 1360: .It Ev HOME
1.1 deraadt 1361: Set to the path of the user's home directory.
1.2 deraadt 1362: .It Ev LOGNAME
1363: Synonym for
1.12 aaron 1364: .Ev USER ;
1365: set for compatibility with systems that use this variable.
1.2 deraadt 1366: .It Ev MAIL
1.129 stevesk 1367: Set to the path of the user's mailbox.
1.40 aaron 1368: .It Ev PATH
1.2 deraadt 1369: Set to the default
1370: .Ev PATH ,
1371: as specified when compiling
1.234 jmc 1372: .Nm .
1.118 markus 1373: .It Ev SSH_ASKPASS
1374: If
1375: .Nm
1376: needs a passphrase, it will read the passphrase from the current
1377: terminal if it was run from a terminal.
1378: If
1379: .Nm
1380: does not have a terminal associated with it but
1381: .Ev DISPLAY
1382: and
1383: .Ev SSH_ASKPASS
1384: are set, it will execute the program specified by
1385: .Ev SSH_ASKPASS
1386: and open an X11 window to read the passphrase.
1387: This is particularly useful when calling
1388: .Nm
1389: from a
1.196 jmc 1390: .Pa .xsession
1.118 markus 1391: or related script.
1392: (Note that on some machines it
1393: may be necessary to redirect the input from
1394: .Pa /dev/null
1395: to make this work.)
1.18 markus 1396: .It Ev SSH_AUTH_SOCK
1.233 jmc 1397: Identifies the path of a
1398: .Ux Ns -domain
1399: socket used to communicate with the agent.
1.166 stevesk 1400: .It Ev SSH_CONNECTION
1401: Identifies the client and server ends of the connection.
1.40 aaron 1402: The variable contains
1.233 jmc 1403: four space-separated values: client IP address, client port number,
1404: server IP address, and server port number.
1.73 markus 1405: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1406: This variable contains the original command line if a forced command
1.73 markus 1407: is executed.
1408: It can be used to extract the original arguments.
1.2 deraadt 1409: .It Ev SSH_TTY
1.1 deraadt 1410: This is set to the name of the tty (path to the device) associated
1.40 aaron 1411: with the current shell or command.
1412: If the current session has no tty,
1.1 deraadt 1413: this variable is not set.
1.387 djm 1414: .It Ev SSH_TUNNEL
1415: Optionally set by
1416: .Xr sshd 8
1417: to contain the interface names assigned if tunnel forwarding was
1418: requested by the client.
1.385 djm 1419: .It Ev SSH_USER_AUTH
1420: Optionally set by
1421: .Xr sshd 8 ,
1422: this variable may contain a pathname to a file that lists the authentication
1423: methods successfully used when the session was established, including any
1424: public keys that were used.
1.2 deraadt 1425: .It Ev TZ
1.214 jmc 1426: This variable is set to indicate the present time zone if it
1.257 jmc 1427: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1428: on to new connections).
1.2 deraadt 1429: .It Ev USER
1.1 deraadt 1430: Set to the name of the user logging in.
1.2 deraadt 1431: .El
1432: .Pp
1.44 aaron 1433: Additionally,
1.2 deraadt 1434: .Nm
1.44 aaron 1435: reads
1.207 djm 1436: .Pa ~/.ssh/environment ,
1.2 deraadt 1437: and adds lines of the format
1438: .Dq VARNAME=value
1.233 jmc 1439: to the environment if the file exists and users are allowed to
1.161 marc 1440: change their environment.
1.176 jmc 1441: For more information, see the
1.161 marc 1442: .Cm PermitUserEnvironment
1.162 stevesk 1443: option in
1.161 marc 1444: .Xr sshd_config 5 .
1.2 deraadt 1445: .Sh FILES
1.236 jmc 1446: .Bl -tag -width Ds -compact
1.309 jmc 1447: .It Pa ~/.rhosts
1.240 jmc 1448: This file is used for host-based authentication (see above).
1.92 markus 1449: On some machines this file may need to be
1.240 jmc 1450: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1451: because
1.2 deraadt 1452: .Xr sshd 8
1.40 aaron 1453: reads it as root.
1454: Additionally, this file must be owned by the user,
1455: and must not have write permissions for anyone else.
1456: The recommended
1.1 deraadt 1457: permission for most machines is read/write for the user, and not
1458: accessible by others.
1.2 deraadt 1459: .Pp
1.309 jmc 1460: .It Pa ~/.shosts
1.240 jmc 1461: This file is used in exactly the same way as
1462: .Pa .rhosts ,
1463: but allows host-based authentication without permitting login with
1464: rlogin/rsh.
1.272 mcbride 1465: .Pp
1.309 jmc 1466: .It Pa ~/.ssh/
1.272 mcbride 1467: This directory is the default location for all user-specific configuration
1468: and authentication information.
1469: There is no general requirement to keep the entire contents of this directory
1470: secret, but the recommended permissions are read/write/execute for the user,
1471: and not accessible by others.
1.236 jmc 1472: .Pp
1.309 jmc 1473: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1474: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1475: that can be used for logging in as this user.
1.238 jmc 1476: The format of this file is described in the
1477: .Xr sshd 8
1478: manual page.
1479: This file is not highly sensitive, but the recommended
1480: permissions are read/write for the user, and not accessible by others.
1481: .Pp
1.309 jmc 1482: .It Pa ~/.ssh/config
1.238 jmc 1483: This is the per-user configuration file.
1484: The file format and configuration options are described in
1485: .Xr ssh_config 5 .
1486: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1487: read/write for the user, and not writable by others.
1.238 jmc 1488: .Pp
1.309 jmc 1489: .It Pa ~/.ssh/environment
1.239 jmc 1490: Contains additional definitions for environment variables; see
1491: .Sx ENVIRONMENT ,
1.238 jmc 1492: above.
1493: .Pp
1.309 jmc 1494: .It Pa ~/.ssh/id_dsa
1.310 djm 1495: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1496: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1497: .It Pa ~/.ssh/id_rsa
1.238 jmc 1498: Contains the private key for authentication.
1499: These files
1500: contain sensitive data and should be readable by the user but not
1501: accessible by others (read/write/execute).
1502: .Nm
1503: will simply ignore a private key file if it is accessible by others.
1504: It is possible to specify a passphrase when
1505: generating the key which will be used to encrypt the
1.389 djm 1506: sensitive part of this file using AES-128.
1.238 jmc 1507: .Pp
1.309 jmc 1508: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1509: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1510: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1511: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1512: Contains the public key for authentication.
1513: These files are not
1514: sensitive and can (but need not) be readable by anyone.
1515: .Pp
1.309 jmc 1516: .It Pa ~/.ssh/known_hosts
1.244 jmc 1517: Contains a list of host keys for all hosts the user has logged into
1518: that are not already in the systemwide list of known host keys.
1.238 jmc 1519: See
1.244 jmc 1520: .Xr sshd 8
1521: for further details of the format of this file.
1.238 jmc 1522: .Pp
1.309 jmc 1523: .It Pa ~/.ssh/rc
1.238 jmc 1524: Commands in this file are executed by
1525: .Nm
1.245 jmc 1526: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1527: started.
1528: See the
1529: .Xr sshd 8
1530: manual page for more information.
1531: .Pp
1.309 jmc 1532: .It Pa /etc/hosts.equiv
1.240 jmc 1533: This file is for host-based authentication (see above).
1534: It should only be writable by root.
1.236 jmc 1535: .Pp
1.309 jmc 1536: .It Pa /etc/shosts.equiv
1.240 jmc 1537: This file is used in exactly the same way as
1538: .Pa hosts.equiv ,
1539: but allows host-based authentication without permitting login with
1540: rlogin/rsh.
1.236 jmc 1541: .Pp
1.238 jmc 1542: .It Pa /etc/ssh/ssh_config
1543: Systemwide configuration file.
1544: The file format and configuration options are described in
1545: .Xr ssh_config 5 .
1546: .Pp
1.309 jmc 1547: .It Pa /etc/ssh/ssh_host_key
1548: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1549: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1550: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1551: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1552: These files contain the private parts of the host keys
1.245 jmc 1553: and are used for host-based authentication.
1.238 jmc 1554: .Pp
1.309 jmc 1555: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1556: Systemwide list of known host keys.
1557: This file should be prepared by the
1558: system administrator to contain the public host keys of all machines in the
1559: organization.
1.244 jmc 1560: It should be world-readable.
1561: See
1.238 jmc 1562: .Xr sshd 8
1.244 jmc 1563: for further details of the format of this file.
1.236 jmc 1564: .Pp
1.309 jmc 1565: .It Pa /etc/ssh/sshrc
1.1 deraadt 1566: Commands in this file are executed by
1.2 deraadt 1567: .Nm
1.245 jmc 1568: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1569: See the
1.2 deraadt 1570: .Xr sshd 8
1.1 deraadt 1571: manual page for more information.
1.58 itojun 1572: .El
1.312 jmc 1573: .Sh EXIT STATUS
1574: .Nm
1575: exits with the exit status of the remote command or with 255
1576: if an error occurred.
1.2 deraadt 1577: .Sh SEE ALSO
1578: .Xr scp 1 ,
1.83 djm 1579: .Xr sftp 1 ,
1.2 deraadt 1580: .Xr ssh-add 1 ,
1581: .Xr ssh-agent 1 ,
1582: .Xr ssh-keygen 1 ,
1.242 jmc 1583: .Xr ssh-keyscan 1 ,
1.250 jmc 1584: .Xr tun 4 ,
1.159 stevesk 1585: .Xr ssh_config 5 ,
1.160 naddy 1586: .Xr ssh-keysign 8 ,
1.87 itojun 1587: .Xr sshd 8
1.329 jmc 1588: .Sh STANDARDS
1.106 markus 1589: .Rs
1.329 jmc 1590: .%A S. Lehtinen
1591: .%A C. Lonvick
1592: .%D January 2006
1.256 jmc 1593: .%R RFC 4250
1.329 jmc 1594: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1595: .Re
1.329 jmc 1596: .Pp
1.256 jmc 1597: .Rs
1.329 jmc 1598: .%A T. Ylonen
1599: .%A C. Lonvick
1600: .%D January 2006
1.256 jmc 1601: .%R RFC 4251
1.329 jmc 1602: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1603: .Re
1.329 jmc 1604: .Pp
1.256 jmc 1605: .Rs
1.329 jmc 1606: .%A T. Ylonen
1607: .%A C. Lonvick
1608: .%D January 2006
1.256 jmc 1609: .%R RFC 4252
1.329 jmc 1610: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1611: .Re
1.329 jmc 1612: .Pp
1.256 jmc 1613: .Rs
1.329 jmc 1614: .%A T. Ylonen
1615: .%A C. Lonvick
1616: .%D January 2006
1.256 jmc 1617: .%R RFC 4253
1.329 jmc 1618: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1619: .Re
1.329 jmc 1620: .Pp
1.256 jmc 1621: .Rs
1.329 jmc 1622: .%A T. Ylonen
1623: .%A C. Lonvick
1624: .%D January 2006
1.256 jmc 1625: .%R RFC 4254
1.329 jmc 1626: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1627: .Re
1.329 jmc 1628: .Pp
1.256 jmc 1629: .Rs
1.329 jmc 1630: .%A J. Schlyter
1631: .%A W. Griffin
1632: .%D January 2006
1.256 jmc 1633: .%R RFC 4255
1.329 jmc 1634: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1635: .Re
1.329 jmc 1636: .Pp
1.256 jmc 1637: .Rs
1.329 jmc 1638: .%A F. Cusack
1639: .%A M. Forssen
1640: .%D January 2006
1.256 jmc 1641: .%R RFC 4256
1.329 jmc 1642: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1643: .Re
1.329 jmc 1644: .Pp
1.256 jmc 1645: .Rs
1.329 jmc 1646: .%A J. Galbraith
1647: .%A P. Remaker
1648: .%D January 2006
1.256 jmc 1649: .%R RFC 4335
1.329 jmc 1650: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1651: .Re
1.329 jmc 1652: .Pp
1.256 jmc 1653: .Rs
1.329 jmc 1654: .%A M. Bellare
1655: .%A T. Kohno
1656: .%A C. Namprempre
1657: .%D January 2006
1.256 jmc 1658: .%R RFC 4344
1.329 jmc 1659: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1660: .Re
1.329 jmc 1661: .Pp
1.256 jmc 1662: .Rs
1.329 jmc 1663: .%A B. Harris
1664: .%D January 2006
1.256 jmc 1665: .%R RFC 4345
1.329 jmc 1666: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1667: .Re
1.329 jmc 1668: .Pp
1.258 djm 1669: .Rs
1.329 jmc 1670: .%A M. Friedl
1671: .%A N. Provos
1672: .%A W. Simpson
1673: .%D March 2006
1.258 djm 1674: .%R RFC 4419
1.329 jmc 1675: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1676: .Re
1.329 jmc 1677: .Pp
1.266 markus 1678: .Rs
1.329 jmc 1679: .%A J. Galbraith
1680: .%A R. Thayer
1681: .%D November 2006
1.266 markus 1682: .%R RFC 4716
1.329 jmc 1683: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1684: .Re
1.329 jmc 1685: .Pp
1.313 djm 1686: .Rs
1.329 jmc 1687: .%A D. Stebila
1688: .%A J. Green
1689: .%D December 2009
1.313 djm 1690: .%R RFC 5656
1.329 jmc 1691: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1692: .Re
1.329 jmc 1693: .Pp
1.274 grunk 1694: .Rs
1695: .%A A. Perrig
1696: .%A D. Song
1697: .%D 1999
1.329 jmc 1698: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1699: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1700: .Re
1.173 jmc 1701: .Sh AUTHORS
1702: OpenSSH is a derivative of the original and free
1703: ssh 1.2.12 release by Tatu Ylonen.
1704: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1705: Theo de Raadt and Dug Song
1706: removed many bugs, re-added newer features and
1707: created OpenSSH.
1708: Markus Friedl contributed the support for SSH
1709: protocol versions 1.5 and 2.0.
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