Annotation of src/usr.bin/ssh/ssh.1, Revision 1.386
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: .\"
1.386 ! millert 36: .\" $OpenBSD: ssh.1,v 1.385 2017/10/13 06:45:18 djm Exp $
! 37: .Dd $Mdocdate: October 13 2017 $
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
1.306 jmc 45: .Bk -words
1.377 djm 46: .Op Fl 46AaCfGgKkMNnqsTtVvXxYy
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
1.306 jmc 69: .Ek
1.44 aaron 70: .Sh DESCRIPTION
1.2 deraadt 71: .Nm
1.96 deraadt 72: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 73: executing commands on a remote machine.
1.365 mmcc 74: It is intended to provide secure encrypted communications between
1.40 aaron 75: two untrusted hosts over an insecure network.
1.348 millert 76: X11 connections, arbitrary TCP ports and
77: .Ux Ns -domain
78: sockets can also be forwarded over the secure channel.
1.2 deraadt 79: .Pp
80: .Nm
1.44 aaron 81: connects and logs into the specified
1.386 ! millert 82: .Ar destination
! 83: which may be specified as either
! 84: .Oo Ar user Ns @ Oc Ns Ar hostname
! 85: where the
1.176 jmc 86: .Ar user
1.386 ! millert 87: is optional, or an ssh URI of the form
! 88: .No ssh:// Ns Oo Ar user Ns @ Oc Ns Ar hostname Ns Oo : Ns Ar port Oc
! 89: where the
! 90: .Ar user
! 91: and
! 92: .Ar port
! 93: are optional.
1.1 deraadt 94: The user must prove
1.49 markus 95: his/her identity to the remote machine using one of several methods
1.365 mmcc 96: (see below).
1.49 markus 97: .Pp
1.386 ! millert 98: If a
1.176 jmc 99: .Ar command
100: is specified,
1.219 jmc 101: it is executed on the remote host instead of a login shell.
1.2 deraadt 102: .Pp
1.218 jmc 103: The options are as follows:
1.361 millert 104: .Pp
1.360 millert 105: .Bl -tag -width Ds -compact
1.218 jmc 106: .It Fl 4
107: Forces
1.2 deraadt 108: .Nm
1.218 jmc 109: to use IPv4 addresses only.
1.360 millert 110: .Pp
1.218 jmc 111: .It Fl 6
112: Forces
1.2 deraadt 113: .Nm
1.218 jmc 114: to use IPv6 addresses only.
1.360 millert 115: .Pp
1.218 jmc 116: .It Fl A
117: Enables forwarding of the authentication agent connection.
118: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 119: .Pp
1.218 jmc 120: Agent forwarding should be enabled with caution.
121: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 122: (for the agent's
1.286 sobrado 123: .Ux Ns -domain
124: socket) can access the local agent through the forwarded connection.
1.218 jmc 125: An attacker cannot obtain key material from the agent,
126: however they can perform operations on the keys that enable them to
127: authenticate using the identities loaded into the agent.
1.360 millert 128: .Pp
1.218 jmc 129: .It Fl a
130: Disables forwarding of the authentication agent connection.
1.360 millert 131: .Pp
1.218 jmc 132: .It Fl b Ar bind_address
133: Use
134: .Ar bind_address
135: on the local machine as the source address
136: of the connection.
137: Only useful on systems with more than one address.
1.360 millert 138: .Pp
1.218 jmc 139: .It Fl C
140: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 141: data for forwarded X11, TCP and
142: .Ux Ns -domain
143: connections).
1.218 jmc 144: The compression algorithm is the same used by
1.377 djm 145: .Xr gzip 1 .
1.218 jmc 146: Compression is desirable on modem lines and other
147: slow connections, but will only slow down things on fast networks.
148: The default value can be set on a host-by-host basis in the
149: configuration files; see the
150: .Cm Compression
151: option.
1.360 millert 152: .Pp
1.218 jmc 153: .It Fl c Ar cipher_spec
154: Selects the cipher specification for encrypting the session.
155: .Ar cipher_spec
156: is a comma-separated list of ciphers
157: listed in order of preference.
1.283 jmc 158: See the
159: .Cm Ciphers
1.307 dtucker 160: keyword in
161: .Xr ssh_config 5
162: for more information.
1.360 millert 163: .Pp
1.218 jmc 164: .It Fl D Xo
165: .Sm off
166: .Oo Ar bind_address : Oc
167: .Ar port
168: .Sm on
169: .Xc
170: Specifies a local
171: .Dq dynamic
172: application-level port forwarding.
173: This works by allocating a socket to listen to
174: .Ar port
175: on the local side, optionally bound to the specified
176: .Ar bind_address .
177: Whenever a connection is made to this port, the
178: connection is forwarded over the secure channel, and the application
179: protocol is then used to determine where to connect to from the
180: remote machine.
181: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 182: .Nm
1.218 jmc 183: will act as a SOCKS server.
184: Only root can forward privileged ports.
185: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 186: .Pp
1.308 djm 187: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 188: Only the superuser can forward privileged ports.
189: By default, the local port is bound in accordance with the
190: .Cm GatewayPorts
191: setting.
192: However, an explicit
193: .Ar bind_address
194: may be used to bind the connection to a specific address.
195: The
196: .Ar bind_address
197: of
198: .Dq localhost
199: indicates that the listening port be bound for local use only, while an
200: empty address or
201: .Sq *
202: indicates that the port should be available from all interfaces.
1.360 millert 203: .Pp
1.331 dtucker 204: .It Fl E Ar log_file
205: Append debug logs to
206: .Ar log_file
207: instead of standard error.
1.360 millert 208: .Pp
1.229 jmc 209: .It Fl e Ar escape_char
1.218 jmc 210: Sets the escape character for sessions with a pty (default:
211: .Ql ~ ) .
212: The escape character is only recognized at the beginning of a line.
213: The escape character followed by a dot
214: .Pq Ql \&.
215: closes the connection;
216: followed by control-Z suspends the connection;
217: and followed by itself sends the escape character once.
218: Setting the character to
1.2 deraadt 219: .Dq none
1.218 jmc 220: disables any escapes and makes the session fully transparent.
1.360 millert 221: .Pp
1.218 jmc 222: .It Fl F Ar configfile
223: Specifies an alternative per-user configuration file.
224: If a configuration file is given on the command line,
225: the system-wide configuration file
226: .Pq Pa /etc/ssh/ssh_config
227: will be ignored.
228: The default for the per-user configuration file is
229: .Pa ~/.ssh/config .
1.360 millert 230: .Pp
1.218 jmc 231: .It Fl f
232: Requests
233: .Nm
234: to go to background just before command execution.
235: This is useful if
1.176 jmc 236: .Nm
1.218 jmc 237: is going to ask for passwords or passphrases, but the user
238: wants it in the background.
239: This implies
240: .Fl n .
241: The recommended way to start X11 programs at a remote site is with
242: something like
243: .Ic ssh -f host xterm .
1.277 djm 244: .Pp
245: If the
246: .Cm ExitOnForwardFailure
247: configuration option is set to
248: .Dq yes ,
249: then a client started with
250: .Fl f
251: will wait for all remote port forwards to be successfully established
252: before placing itself in the background.
1.360 millert 253: .Pp
1.350 djm 254: .It Fl G
255: Causes
256: .Nm
257: to print its configuration after evaluating
258: .Cm Host
259: and
260: .Cm Match
261: blocks and exit.
1.360 millert 262: .Pp
1.218 jmc 263: .It Fl g
264: Allows remote hosts to connect to local forwarded ports.
1.346 djm 265: If used on a multiplexed connection, then this option must be specified
266: on the master process.
1.360 millert 267: .Pp
1.291 markus 268: .It Fl I Ar pkcs11
1.294 jmc 269: Specify the PKCS#11 shared library
1.176 jmc 270: .Nm
1.293 markus 271: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 272: private RSA key.
1.360 millert 273: .Pp
1.218 jmc 274: .It Fl i Ar identity_file
275: Selects a file from which the identity (private key) for
1.310 djm 276: public key authentication is read.
1.218 jmc 277: The default is
1.310 djm 278: .Pa ~/.ssh/id_dsa ,
1.343 naddy 279: .Pa ~/.ssh/id_ecdsa ,
280: .Pa ~/.ssh/id_ed25519
1.310 djm 281: and
1.381 naddy 282: .Pa ~/.ssh/id_rsa .
1.218 jmc 283: Identity files may also be specified on
284: a per-host basis in the configuration file.
285: It is possible to have multiple
286: .Fl i
287: options (and multiple identities specified in
288: configuration files).
1.364 jmc 289: If no certificates have been explicitly specified by the
1.363 djm 290: .Cm CertificateFile
291: directive,
1.302 djm 292: .Nm
293: will also try to load certificate information from the filename obtained
294: by appending
295: .Pa -cert.pub
296: to identity filenames.
1.360 millert 297: .Pp
1.386 ! millert 298: .It Fl J Ar destination
1.375 djm 299: Connect to the target host by first making a
300: .Nm
1.386 ! millert 301: connection to the jump host described by
! 302: .Ar destination
1.376 jmc 303: and then establishing a TCP forwarding to the ultimate destination from
1.375 djm 304: there.
305: Multiple jump hops may be specified separated by comma characters.
306: This is a shortcut to specify a
307: .Cm ProxyJump
308: configuration directive.
309: .Pp
1.269 djm 310: .It Fl K
311: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
312: credentials to the server.
1.360 millert 313: .Pp
1.218 jmc 314: .It Fl k
315: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 316: .Pp
1.218 jmc 317: .It Fl L Xo
318: .Sm off
319: .Oo Ar bind_address : Oc
320: .Ar port : host : hostport
321: .Sm on
322: .Xc
1.360 millert 323: .It Fl L Xo
324: .Sm off
325: .Oo Ar bind_address : Oc
326: .Ar port : remote_socket
327: .Sm on
328: .Xc
329: .It Fl L Xo
330: .Sm off
331: .Ar local_socket : host : hostport
332: .Sm on
333: .Xc
334: .It Fl L Xo
335: .Sm off
336: .Ar local_socket : remote_socket
337: .Sm on
338: .Xc
339: Specifies that connections to the given TCP port or Unix socket on the local
340: (client) host are to be forwarded to the given host and port, or Unix socket,
341: on the remote side.
342: This works by allocating a socket to listen to either a TCP
1.218 jmc 343: .Ar port
344: on the local side, optionally bound to the specified
1.360 millert 345: .Ar bind_address ,
346: or to a Unix socket.
347: Whenever a connection is made to the local port or socket, the
1.218 jmc 348: connection is forwarded over the secure channel, and a connection is
1.360 millert 349: made to either
1.218 jmc 350: .Ar host
351: port
1.360 millert 352: .Ar hostport ,
353: or the Unix socket
354: .Ar remote_socket ,
1.218 jmc 355: from the remote machine.
1.360 millert 356: .Pp
1.218 jmc 357: Port forwardings can also be specified in the configuration file.
1.360 millert 358: Only the superuser can forward privileged ports.
1.308 djm 359: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 360: .Pp
1.218 jmc 361: By default, the local port is bound in accordance with the
362: .Cm GatewayPorts
363: setting.
364: However, an explicit
365: .Ar bind_address
366: may be used to bind the connection to a specific address.
1.2 deraadt 367: The
1.218 jmc 368: .Ar bind_address
369: of
370: .Dq localhost
371: indicates that the listening port be bound for local use only, while an
372: empty address or
373: .Sq *
374: indicates that the port should be available from all interfaces.
1.360 millert 375: .Pp
1.218 jmc 376: .It Fl l Ar login_name
377: Specifies the user to log in as on the remote machine.
378: This also may be specified on a per-host basis in the configuration file.
1.360 millert 379: .Pp
1.218 jmc 380: .It Fl M
381: Places the
382: .Nm
383: client into
384: .Dq master
385: mode for connection sharing.
1.231 stevesk 386: Multiple
387: .Fl M
388: options places
389: .Nm
390: into
391: .Dq master
392: mode with confirmation required before slave connections are accepted.
1.218 jmc 393: Refer to the description of
394: .Cm ControlMaster
395: in
396: .Xr ssh_config 5
397: for details.
1.360 millert 398: .Pp
1.218 jmc 399: .It Fl m Ar mac_spec
1.369 jmc 400: A comma-separated list of MAC (message authentication code) algorithms,
401: specified in order of preference.
1.218 jmc 402: See the
403: .Cm MACs
404: keyword for more information.
1.360 millert 405: .Pp
1.218 jmc 406: .It Fl N
407: Do not execute a remote command.
1.369 jmc 408: This is useful for just forwarding ports.
1.360 millert 409: .Pp
1.218 jmc 410: .It Fl n
411: Redirects stdin from
412: .Pa /dev/null
413: (actually, prevents reading from stdin).
414: This must be used when
1.2 deraadt 415: .Nm
1.218 jmc 416: is run in the background.
417: A common trick is to use this to run X11 programs on a remote machine.
418: For example,
419: .Ic ssh -n shadows.cs.hut.fi emacs &
420: will start an emacs on shadows.cs.hut.fi, and the X11
421: connection will be automatically forwarded over an encrypted channel.
422: The
1.2 deraadt 423: .Nm
1.218 jmc 424: program will be put in the background.
425: (This does not work if
1.2 deraadt 426: .Nm
1.218 jmc 427: needs to ask for a password or passphrase; see also the
428: .Fl f
429: option.)
1.360 millert 430: .Pp
1.218 jmc 431: .It Fl O Ar ctl_cmd
432: Control an active connection multiplexing master process.
433: When the
434: .Fl O
435: option is specified, the
436: .Ar ctl_cmd
437: argument is interpreted and passed to the master process.
438: Valid commands are:
439: .Dq check
1.305 markus 440: (check that the master process is running),
441: .Dq forward
1.318 jmc 442: (request forwardings without command execution),
1.323 okan 443: .Dq cancel
444: (cancel forwardings),
1.218 jmc 445: .Dq exit
1.318 jmc 446: (request the master to exit), and
1.317 djm 447: .Dq stop
448: (request the master to stop accepting further multiplexing requests).
1.360 millert 449: .Pp
1.218 jmc 450: .It Fl o Ar option
451: Can be used to give options in the format used in the configuration file.
452: This is useful for specifying options for which there is no separate
453: command-line flag.
454: For full details of the options listed below, and their possible values, see
455: .Xr ssh_config 5 .
1.2 deraadt 456: .Pp
1.218 jmc 457: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 458: .It AddKeysToAgent
1.218 jmc 459: .It AddressFamily
460: .It BatchMode
461: .It BindAddress
1.338 djm 462: .It CanonicalDomains
1.339 djm 463: .It CanonicalizeFallbackLocal
464: .It CanonicalizeHostname
465: .It CanonicalizeMaxDots
466: .It CanonicalizePermittedCNAMEs
1.363 djm 467: .It CertificateFile
1.218 jmc 468: .It ChallengeResponseAuthentication
469: .It CheckHostIP
470: .It Ciphers
471: .It ClearAllForwardings
472: .It Compression
1.380 naddy 473: .It ConnectionAttempts
1.218 jmc 474: .It ConnectTimeout
475: .It ControlMaster
476: .It ControlPath
1.321 djm 477: .It ControlPersist
1.218 jmc 478: .It DynamicForward
479: .It EscapeChar
1.263 markus 480: .It ExitOnForwardFailure
1.353 jmc 481: .It FingerprintHash
1.218 jmc 482: .It ForwardAgent
483: .It ForwardX11
1.321 djm 484: .It ForwardX11Timeout
1.218 jmc 485: .It ForwardX11Trusted
486: .It GatewayPorts
487: .It GlobalKnownHostsFile
488: .It GSSAPIAuthentication
489: .It GSSAPIDelegateCredentials
490: .It HashKnownHosts
491: .It Host
492: .It HostbasedAuthentication
1.355 djm 493: .It HostbasedKeyTypes
1.218 jmc 494: .It HostKeyAlgorithms
495: .It HostKeyAlias
496: .It HostName
1.374 jmc 497: .It IdentitiesOnly
1.371 markus 498: .It IdentityAgent
1.218 jmc 499: .It IdentityFile
1.370 djm 500: .It Include
1.316 jmc 501: .It IPQoS
1.321 djm 502: .It KbdInteractiveAuthentication
1.218 jmc 503: .It KbdInteractiveDevices
1.314 jmc 504: .It KexAlgorithms
1.218 jmc 505: .It LocalCommand
506: .It LocalForward
507: .It LogLevel
508: .It MACs
1.337 jmc 509: .It Match
1.218 jmc 510: .It NoHostAuthenticationForLocalhost
511: .It NumberOfPasswordPrompts
512: .It PasswordAuthentication
513: .It PermitLocalCommand
1.291 markus 514: .It PKCS11Provider
1.218 jmc 515: .It Port
516: .It PreferredAuthentications
517: .It ProxyCommand
1.375 djm 518: .It ProxyJump
1.336 jmc 519: .It ProxyUseFdpass
1.359 markus 520: .It PubkeyAcceptedKeyTypes
1.218 jmc 521: .It PubkeyAuthentication
1.251 dtucker 522: .It RekeyLimit
1.382 bluhm 523: .It RemoteCommand
1.218 jmc 524: .It RemoteForward
1.319 jmc 525: .It RequestTTY
1.218 jmc 526: .It SendEnv
527: .It ServerAliveInterval
528: .It ServerAliveCountMax
1.347 jmc 529: .It StreamLocalBindMask
530: .It StreamLocalBindUnlink
1.218 jmc 531: .It StrictHostKeyChecking
532: .It TCPKeepAlive
533: .It Tunnel
534: .It TunnelDevice
1.354 djm 535: .It UpdateHostKeys
1.218 jmc 536: .It UsePrivilegedPort
537: .It User
538: .It UserKnownHostsFile
539: .It VerifyHostKeyDNS
1.276 jmc 540: .It VisualHostKey
1.218 jmc 541: .It XAuthLocation
542: .El
1.360 millert 543: .Pp
1.218 jmc 544: .It Fl p Ar port
545: Port to connect to on the remote host.
546: This can be specified on a
547: per-host basis in the configuration file.
1.360 millert 548: .Pp
1.362 djm 549: .It Fl Q Ar query_option
1.332 djm 550: Queries
551: .Nm
1.341 deraadt 552: for the algorithms supported for the specified version 2.
553: The available features are:
554: .Ar cipher
1.332 djm 555: (supported symmetric ciphers),
1.341 deraadt 556: .Ar cipher-auth
1.340 djm 557: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 558: .Ar mac
1.332 djm 559: (supported message integrity codes),
1.341 deraadt 560: .Ar kex
1.332 djm 561: (key exchange algorithms),
1.341 deraadt 562: .Ar key
1.362 djm 563: (key types),
564: .Ar key-cert
565: (certificate key types),
566: .Ar key-plain
567: (non-certificate key types), and
1.356 djm 568: .Ar protocol-version
569: (supported SSH protocol versions).
1.360 millert 570: .Pp
1.218 jmc 571: .It Fl q
572: Quiet mode.
1.271 djm 573: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 574: .Pp
1.218 jmc 575: .It Fl R Xo
576: .Sm off
577: .Oo Ar bind_address : Oc
578: .Ar port : host : hostport
579: .Sm on
580: .Xc
1.360 millert 581: .It Fl R Xo
582: .Sm off
583: .Oo Ar bind_address : Oc
584: .Ar port : local_socket
585: .Sm on
586: .Xc
587: .It Fl R Xo
588: .Sm off
589: .Ar remote_socket : host : hostport
590: .Sm on
591: .Xc
592: .It Fl R Xo
593: .Sm off
594: .Ar remote_socket : local_socket
595: .Sm on
596: .Xc
1.384 markus 597: .It Fl R Xo
598: .Sm off
599: .Oo Ar bind_address : Oc
600: .Ar port
601: .Sm on
602: .Xc
1.360 millert 603: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 604: (server) host are to be forwarded to the local side.
605: .Pp
1.360 millert 606: This works by allocating a socket to listen to either a TCP
1.218 jmc 607: .Ar port
1.360 millert 608: or to a Unix socket on the remote side.
609: Whenever a connection is made to this port or Unix socket, the
610: connection is forwarded over the secure channel, and a connection
1.384 markus 611: is made from the local machine to either an explicit destination specified by
1.218 jmc 612: .Ar host
613: port
1.360 millert 614: .Ar hostport ,
615: or
616: .Ar local_socket ,
1.384 markus 617: or, if no explicit destination was specified,
618: .Nm
619: will act as a SOCKS 4/5 proxy and forward connections to the destinations
620: requested by the remote SOCKS client.
1.2 deraadt 621: .Pp
1.218 jmc 622: Port forwardings can also be specified in the configuration file.
623: Privileged ports can be forwarded only when
624: logging in as root on the remote machine.
1.324 jmc 625: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 626: .Pp
1.360 millert 627: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 628: interface only.
1.280 tobias 629: This may be overridden by specifying a
1.218 jmc 630: .Ar bind_address .
631: An empty
632: .Ar bind_address ,
633: or the address
634: .Ql * ,
635: indicates that the remote socket should listen on all interfaces.
636: Specifying a remote
637: .Ar bind_address
638: will only succeed if the server's
639: .Cm GatewayPorts
640: option is enabled (see
641: .Xr sshd_config 5 ) .
1.281 djm 642: .Pp
643: If the
644: .Ar port
645: argument is
1.282 djm 646: .Ql 0 ,
1.281 djm 647: the listen port will be dynamically allocated on the server and reported
648: to the client at run time.
1.305 markus 649: When used together with
650: .Ic -O forward
651: the allocated port will be printed to the standard output.
1.360 millert 652: .Pp
1.218 jmc 653: .It Fl S Ar ctl_path
1.304 jmc 654: Specifies the location of a control socket for connection sharing,
1.303 djm 655: or the string
656: .Dq none
657: to disable connection sharing.
1.218 jmc 658: Refer to the description of
659: .Cm ControlPath
660: and
661: .Cm ControlMaster
662: in
663: .Xr ssh_config 5
664: for details.
1.360 millert 665: .Pp
1.218 jmc 666: .It Fl s
667: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 668: Subsystems facilitate the use of SSH
669: as a secure transport for other applications (e.g.\&
1.218 jmc 670: .Xr sftp 1 ) .
671: The subsystem is specified as the remote command.
1.360 millert 672: .Pp
1.218 jmc 673: .It Fl T
1.357 dtucker 674: Disable pseudo-terminal allocation.
1.360 millert 675: .Pp
1.218 jmc 676: .It Fl t
1.357 dtucker 677: Force pseudo-terminal allocation.
1.218 jmc 678: This can be used to execute arbitrary
679: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 680: e.g. when implementing menu services.
1.218 jmc 681: Multiple
682: .Fl t
683: options force tty allocation, even if
1.194 jakob 684: .Nm
1.218 jmc 685: has no local tty.
1.360 millert 686: .Pp
1.218 jmc 687: .It Fl V
688: Display the version number and exit.
1.360 millert 689: .Pp
1.218 jmc 690: .It Fl v
691: Verbose mode.
692: Causes
1.176 jmc 693: .Nm
1.218 jmc 694: to print debugging messages about its progress.
695: This is helpful in
696: debugging connection, authentication, and configuration problems.
697: Multiple
698: .Fl v
699: options increase the verbosity.
700: The maximum is 3.
1.360 millert 701: .Pp
1.290 dtucker 702: .It Fl W Ar host : Ns Ar port
703: Requests that standard input and output on the client be forwarded to
704: .Ar host
705: on
706: .Ar port
707: over the secure channel.
708: Implies
709: .Fl N ,
710: .Fl T ,
711: .Cm ExitOnForwardFailure
712: and
1.373 jmc 713: .Cm ClearAllForwardings ,
714: though these can be overridden in the configuration file or using
1.372 dtucker 715: .Fl o
716: command line options.
1.360 millert 717: .Pp
1.261 stevesk 718: .It Fl w Xo
719: .Ar local_tun Ns Op : Ns Ar remote_tun
720: .Xc
721: Requests
722: tunnel
723: device forwarding with the specified
1.218 jmc 724: .Xr tun 4
1.261 stevesk 725: devices between the client
726: .Pq Ar local_tun
727: and the server
728: .Pq Ar remote_tun .
729: .Pp
1.228 jmc 730: The devices may be specified by numerical ID or the keyword
731: .Dq any ,
732: which uses the next available tunnel device.
1.261 stevesk 733: If
734: .Ar remote_tun
735: is not specified, it defaults to
736: .Dq any .
1.228 jmc 737: See also the
1.218 jmc 738: .Cm Tunnel
1.261 stevesk 739: and
740: .Cm TunnelDevice
741: directives in
1.218 jmc 742: .Xr ssh_config 5 .
1.261 stevesk 743: If the
744: .Cm Tunnel
745: directive is unset, it is set to the default tunnel mode, which is
746: .Dq point-to-point .
1.360 millert 747: .Pp
1.218 jmc 748: .It Fl X
749: Enables X11 forwarding.
1.54 markus 750: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 751: .Pp
1.218 jmc 752: X11 forwarding should be enabled with caution.
1.168 jmc 753: Users with the ability to bypass file permissions on the remote host
1.218 jmc 754: (for the user's X authorization database)
755: can access the local X11 display through the forwarded connection.
756: An attacker may then be able to perform activities such as keystroke monitoring.
757: .Pp
758: For this reason, X11 forwarding is subjected to X11 SECURITY extension
759: restrictions by default.
760: Please refer to the
761: .Nm
762: .Fl Y
763: option and the
764: .Cm ForwardX11Trusted
765: directive in
766: .Xr ssh_config 5
767: for more information.
1.360 millert 768: .Pp
1.218 jmc 769: .It Fl x
770: Disables X11 forwarding.
1.360 millert 771: .Pp
1.218 jmc 772: .It Fl Y
773: Enables trusted X11 forwarding.
774: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
775: controls.
1.360 millert 776: .Pp
1.278 djm 777: .It Fl y
778: Send log information using the
779: .Xr syslog 3
780: system module.
781: By default this information is sent to stderr.
1.218 jmc 782: .El
1.224 jmc 783: .Pp
784: .Nm
785: may additionally obtain configuration data from
786: a per-user configuration file and a system-wide configuration file.
787: The file format and configuration options are described in
788: .Xr ssh_config 5 .
1.222 jmc 789: .Sh AUTHENTICATION
1.377 djm 790: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 791: .Pp
792: The methods available for authentication are:
1.260 jmc 793: GSSAPI-based authentication,
1.222 jmc 794: host-based authentication,
795: public key authentication,
796: challenge-response authentication,
797: and password authentication.
798: Authentication methods are tried in the order specified above,
1.369 jmc 799: though
800: .Cm PreferredAuthentications
801: can be used to change the default order.
1.222 jmc 802: .Pp
803: Host-based authentication works as follows:
1.218 jmc 804: If the machine the user logs in from is listed in
805: .Pa /etc/hosts.equiv
806: or
807: .Pa /etc/shosts.equiv
808: on the remote machine, and the user names are
809: the same on both sides, or if the files
810: .Pa ~/.rhosts
811: or
812: .Pa ~/.shosts
813: exist in the user's home directory on the
814: remote machine and contain a line containing the name of the client
815: machine and the name of the user on that machine, the user is
1.222 jmc 816: considered for login.
817: Additionally, the server
818: .Em must
819: be able to verify the client's
820: host key (see the description of
1.218 jmc 821: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 822: and
1.222 jmc 823: .Pa ~/.ssh/known_hosts ,
824: below)
825: for login to be permitted.
1.218 jmc 826: This authentication method closes security holes due to IP
1.222 jmc 827: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 828: [Note to the administrator:
829: .Pa /etc/hosts.equiv ,
830: .Pa ~/.rhosts ,
831: and the rlogin/rsh protocol in general, are inherently insecure and should be
832: disabled if security is desired.]
1.189 dtucker 833: .Pp
1.222 jmc 834: Public key authentication works as follows:
835: The scheme is based on public-key cryptography,
836: using cryptosystems
837: where encryption and decryption are done using separate keys,
838: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 839: The idea is that each user creates a public/private
840: key pair for authentication purposes.
841: The server knows the public key, and only the user knows the private key.
1.222 jmc 842: .Nm
843: implements public key authentication protocol automatically,
1.349 sobrado 844: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 845: The HISTORY section of
1.222 jmc 846: .Xr ssl 8
1.311 jmc 847: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 848: .Pp
1.218 jmc 849: The file
850: .Pa ~/.ssh/authorized_keys
851: lists the public keys that are permitted for logging in.
852: When the user logs in, the
1.2 deraadt 853: .Nm
1.218 jmc 854: program tells the server which key pair it would like to use for
855: authentication.
1.222 jmc 856: The client proves that it has access to the private key
857: and the server checks that the corresponding public key
858: is authorized to accept the account.
1.383 djm 859: .Pp
860: The server may inform the client of errors that prevented public key
861: authentication from succeeding after authentication completes using a
862: different method.
863: These may be viewed by increasing the
864: .Cm LogLevel
865: to
866: .Cm DEBUG
867: or higher (e.g. by using the
868: .Fl v
869: flag).
1.218 jmc 870: .Pp
1.222 jmc 871: The user creates his/her key pair by running
1.218 jmc 872: .Xr ssh-keygen 1 .
873: This stores the private key in
1.222 jmc 874: .Pa ~/.ssh/id_dsa
1.369 jmc 875: (DSA),
1.310 djm 876: .Pa ~/.ssh/id_ecdsa
1.369 jmc 877: (ECDSA),
1.343 naddy 878: .Pa ~/.ssh/id_ed25519
1.369 jmc 879: (Ed25519),
1.222 jmc 880: or
881: .Pa ~/.ssh/id_rsa
1.369 jmc 882: (RSA)
1.218 jmc 883: and stores the public key in
1.222 jmc 884: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 885: (DSA),
1.310 djm 886: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 887: (ECDSA),
1.343 naddy 888: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 889: (Ed25519),
1.222 jmc 890: or
891: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 892: (RSA)
1.218 jmc 893: in the user's home directory.
1.222 jmc 894: The user should then copy the public key
1.218 jmc 895: to
896: .Pa ~/.ssh/authorized_keys
1.222 jmc 897: in his/her home directory on the remote machine.
898: The
1.218 jmc 899: .Pa authorized_keys
900: file corresponds to the conventional
901: .Pa ~/.rhosts
902: file, and has one key
1.222 jmc 903: per line, though the lines can be very long.
1.218 jmc 904: After this, the user can log in without giving the password.
905: .Pp
1.301 jmc 906: A variation on public key authentication
907: is available in the form of certificate authentication:
908: instead of a set of public/private keys,
909: signed certificates are used.
910: This has the advantage that a single trusted certification authority
911: can be used in place of many public/private keys.
1.333 jmc 912: See the CERTIFICATES section of
1.301 jmc 913: .Xr ssh-keygen 1
914: for more information.
915: .Pp
916: The most convenient way to use public key or certificate authentication
917: may be with an authentication agent.
1.218 jmc 918: See
919: .Xr ssh-agent 1
1.366 jcs 920: and (optionally) the
921: .Cm AddKeysToAgent
922: directive in
923: .Xr ssh_config 5
1.218 jmc 924: for more information.
925: .Pp
1.222 jmc 926: Challenge-response authentication works as follows:
927: The server sends an arbitrary
928: .Qq challenge
929: text, and prompts for a response.
930: Examples of challenge-response authentication include
1.335 jmc 931: .Bx
932: Authentication (see
1.222 jmc 933: .Xr login.conf 5 )
1.335 jmc 934: and PAM (some
935: .Pf non- Ox
936: systems).
1.222 jmc 937: .Pp
938: Finally, if other authentication methods fail,
1.218 jmc 939: .Nm
940: prompts the user for a password.
941: The password is sent to the remote
942: host for checking; however, since all communications are encrypted,
943: the password cannot be seen by someone listening on the network.
1.232 jmc 944: .Pp
945: .Nm
946: automatically maintains and checks a database containing
947: identification for all hosts it has ever been used with.
948: Host keys are stored in
949: .Pa ~/.ssh/known_hosts
950: in the user's home directory.
951: Additionally, the file
952: .Pa /etc/ssh/ssh_known_hosts
953: is automatically checked for known hosts.
954: Any new hosts are automatically added to the user's file.
955: If a host's identification ever changes,
956: .Nm
957: warns about this and disables password authentication to prevent
958: server spoofing or man-in-the-middle attacks,
959: which could otherwise be used to circumvent the encryption.
960: The
961: .Cm StrictHostKeyChecking
962: option can be used to control logins to machines whose
963: host key is not known or has changed.
964: .Pp
1.218 jmc 965: When the user's identity has been accepted by the server, the server
1.357 dtucker 966: either executes the given command in a non-interactive session or,
967: if no command has been specified, logs into the machine and gives
968: the user a normal shell as an interactive session.
1.218 jmc 969: All communication with
970: the remote command or shell will be automatically encrypted.
971: .Pp
1.357 dtucker 972: If an interactive session is requested
973: .Nm
974: by default will only request a pseudo-terminal (pty) for interactive
975: sessions when the client has one.
976: The flags
977: .Fl T
978: and
979: .Fl t
980: can be used to override this behaviour.
981: .Pp
982: If a pseudo-terminal has been allocated the
1.218 jmc 983: user may use the escape characters noted below.
984: .Pp
1.357 dtucker 985: If no pseudo-terminal has been allocated,
1.218 jmc 986: the session is transparent and can be used to reliably transfer binary data.
987: On most systems, setting the escape character to
988: .Dq none
989: will also make the session transparent even if a tty is used.
990: .Pp
991: The session terminates when the command or shell on the remote
1.247 jmc 992: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 993: .Sh ESCAPE CHARACTERS
1.218 jmc 994: When a pseudo-terminal has been requested,
1.2 deraadt 995: .Nm
1.218 jmc 996: supports a number of functions through the use of an escape character.
997: .Pp
998: A single tilde character can be sent as
999: .Ic ~~
1000: or by following the tilde by a character other than those described below.
1001: The escape character must always follow a newline to be interpreted as
1002: special.
1003: The escape character can be changed in configuration files using the
1004: .Cm EscapeChar
1005: configuration directive or on the command line by the
1006: .Fl e
1007: option.
1008: .Pp
1009: The supported escapes (assuming the default
1010: .Ql ~ )
1011: are:
1012: .Bl -tag -width Ds
1013: .It Cm ~.
1014: Disconnect.
1015: .It Cm ~^Z
1016: Background
1.234 jmc 1017: .Nm .
1.218 jmc 1018: .It Cm ~#
1019: List forwarded connections.
1020: .It Cm ~&
1021: Background
1.2 deraadt 1022: .Nm
1.218 jmc 1023: at logout when waiting for forwarded connection / X11 sessions to terminate.
1024: .It Cm ~?
1025: Display a list of escape characters.
1026: .It Cm ~B
1027: Send a BREAK to the remote system
1.369 jmc 1028: (only useful if the peer supports it).
1.218 jmc 1029: .It Cm ~C
1030: Open command line.
1031: Currently this allows the addition of port forwardings using the
1.279 stevesk 1032: .Fl L ,
1033: .Fl R
1.218 jmc 1034: and
1.279 stevesk 1035: .Fl D
1.225 jmc 1036: options (see above).
1.322 markus 1037: It also allows the cancellation of existing port-forwardings
1038: with
1.262 stevesk 1039: .Sm off
1.322 markus 1040: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1041: .Sm on
1.322 markus 1042: for local,
1043: .Sm off
1044: .Fl KR Oo Ar bind_address : Oc Ar port
1045: .Sm on
1046: for remote and
1047: .Sm off
1048: .Fl KD Oo Ar bind_address : Oc Ar port
1049: .Sm on
1050: for dynamic port-forwardings.
1.218 jmc 1051: .Ic !\& Ns Ar command
1052: allows the user to execute a local command if the
1053: .Ic PermitLocalCommand
1054: option is enabled in
1.176 jmc 1055: .Xr ssh_config 5 .
1.218 jmc 1056: Basic help is available, using the
1057: .Fl h
1058: option.
1059: .It Cm ~R
1060: Request rekeying of the connection
1.369 jmc 1061: (only useful if the peer supports it).
1.327 dtucker 1062: .It Cm ~V
1063: Decrease the verbosity
1064: .Pq Ic LogLevel
1065: when errors are being written to stderr.
1066: .It Cm ~v
1.328 jmc 1067: Increase the verbosity
1.327 dtucker 1068: .Pq Ic LogLevel
1069: when errors are being written to stderr.
1.176 jmc 1070: .El
1.246 jmc 1071: .Sh TCP FORWARDING
1072: Forwarding of arbitrary TCP connections over the secure channel can
1073: be specified either on the command line or in a configuration file.
1074: One possible application of TCP forwarding is a secure connection to a
1075: mail server; another is going through firewalls.
1076: .Pp
1077: In the example below, we look at encrypting communication between
1078: an IRC client and server, even though the IRC server does not directly
1079: support encrypted communications.
1080: This works as follows:
1081: the user connects to the remote host using
1082: .Nm ,
1083: specifying a port to be used to forward connections
1084: to the remote server.
1085: After that it is possible to start the service which is to be encrypted
1086: on the client machine,
1087: connecting to the same local port,
1088: and
1089: .Nm
1090: will encrypt and forward the connection.
1091: .Pp
1092: The following example tunnels an IRC session from client machine
1093: .Dq 127.0.0.1
1094: (localhost)
1095: to remote server
1096: .Dq server.example.com :
1097: .Bd -literal -offset 4n
1098: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1099: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1100: .Ed
1101: .Pp
1102: This tunnels a connection to IRC server
1103: .Dq server.example.com ,
1104: joining channel
1105: .Dq #users ,
1106: nickname
1107: .Dq pinky ,
1108: using port 1234.
1109: It doesn't matter which port is used,
1110: as long as it's greater than 1023
1111: (remember, only root can open sockets on privileged ports)
1112: and doesn't conflict with any ports already in use.
1113: The connection is forwarded to port 6667 on the remote server,
1114: since that's the standard port for IRC services.
1115: .Pp
1116: The
1117: .Fl f
1118: option backgrounds
1119: .Nm
1120: and the remote command
1121: .Dq sleep 10
1122: is specified to allow an amount of time
1123: (10 seconds, in the example)
1124: to start the service which is to be tunnelled.
1125: If no connections are made within the time specified,
1126: .Nm
1127: will exit.
1128: .Sh X11 FORWARDING
1.218 jmc 1129: If the
1130: .Cm ForwardX11
1131: variable is set to
1132: .Dq yes
1133: (or see the description of the
1.227 jmc 1134: .Fl X ,
1135: .Fl x ,
1.218 jmc 1136: and
1.227 jmc 1137: .Fl Y
1.226 jmc 1138: options above)
1.218 jmc 1139: and the user is using X11 (the
1140: .Ev DISPLAY
1141: environment variable is set), the connection to the X11 display is
1142: automatically forwarded to the remote side in such a way that any X11
1143: programs started from the shell (or command) will go through the
1144: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1145: from the local machine.
1.218 jmc 1146: The user should not manually set
1147: .Ev DISPLAY .
1148: Forwarding of X11 connections can be
1149: configured on the command line or in configuration files.
1150: .Pp
1151: The
1152: .Ev DISPLAY
1153: value set by
1154: .Nm
1155: will point to the server machine, but with a display number greater than zero.
1156: This is normal, and happens because
1157: .Nm
1158: creates a
1159: .Dq proxy
1160: X server on the server machine for forwarding the
1161: connections over the encrypted channel.
1.200 djm 1162: .Pp
1.218 jmc 1163: .Nm
1164: will also automatically set up Xauthority data on the server machine.
1165: For this purpose, it will generate a random authorization cookie,
1166: store it in Xauthority on the server, and verify that any forwarded
1167: connections carry this cookie and replace it by the real cookie when
1168: the connection is opened.
1169: The real authentication cookie is never
1170: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1171: .Pp
1.218 jmc 1172: If the
1173: .Cm ForwardAgent
1174: variable is set to
1175: .Dq yes
1176: (or see the description of the
1177: .Fl A
1.191 djm 1178: and
1.218 jmc 1179: .Fl a
1.226 jmc 1180: options above) and
1.218 jmc 1181: the user is using an authentication agent, the connection to the agent
1182: is automatically forwarded to the remote side.
1.252 jmc 1183: .Sh VERIFYING HOST KEYS
1184: When connecting to a server for the first time,
1185: a fingerprint of the server's public key is presented to the user
1186: (unless the option
1187: .Cm StrictHostKeyChecking
1188: has been disabled).
1189: Fingerprints can be determined using
1190: .Xr ssh-keygen 1 :
1191: .Pp
1192: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1193: .Pp
1.274 grunk 1194: If the fingerprint is already known, it can be matched
1195: and the key can be accepted or rejected.
1.358 djm 1196: If only legacy (MD5) fingerprints for the server are available, the
1197: .Xr ssh-keygen 1
1198: .Fl E
1199: option may be used to downgrade the fingerprint algorithm to match.
1200: .Pp
1.274 grunk 1201: Because of the difficulty of comparing host keys
1.352 djm 1202: just by looking at fingerprint strings,
1.274 grunk 1203: there is also support to compare host keys visually,
1204: using
1205: .Em random art .
1206: By setting the
1.275 grunk 1207: .Cm VisualHostKey
1.274 grunk 1208: option to
1.275 grunk 1209: .Dq yes ,
1.274 grunk 1210: a small ASCII graphic gets displayed on every login to a server, no matter
1211: if the session itself is interactive or not.
1212: By learning the pattern a known server produces, a user can easily
1213: find out that the host key has changed when a completely different pattern
1214: is displayed.
1215: Because these patterns are not unambiguous however, a pattern that looks
1216: similar to the pattern remembered only gives a good probability that the
1217: host key is the same, not guaranteed proof.
1218: .Pp
1219: To get a listing of the fingerprints along with their random art for
1220: all known hosts, the following command line can be used:
1221: .Pp
1222: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1223: .Pp
1.252 jmc 1224: If the fingerprint is unknown,
1225: an alternative method of verification is available:
1226: SSH fingerprints verified by DNS.
1227: An additional resource record (RR),
1228: SSHFP,
1229: is added to a zonefile
1230: and the connecting client is able to match the fingerprint
1231: with that of the key presented.
1232: .Pp
1233: In this example, we are connecting a client to a server,
1234: .Dq host.example.com .
1235: The SSHFP resource records should first be added to the zonefile for
1236: host.example.com:
1237: .Bd -literal -offset indent
1.259 jakob 1238: $ ssh-keygen -r host.example.com.
1.252 jmc 1239: .Ed
1240: .Pp
1241: The output lines will have to be added to the zonefile.
1242: To check that the zone is answering fingerprint queries:
1243: .Pp
1244: .Dl $ dig -t SSHFP host.example.com
1245: .Pp
1246: Finally the client connects:
1247: .Bd -literal -offset indent
1248: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1249: [...]
1250: Matching host key fingerprint found in DNS.
1251: Are you sure you want to continue connecting (yes/no)?
1252: .Ed
1253: .Pp
1254: See the
1255: .Cm VerifyHostKeyDNS
1256: option in
1257: .Xr ssh_config 5
1258: for more information.
1.250 jmc 1259: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1260: .Nm
1261: contains support for Virtual Private Network (VPN) tunnelling
1262: using the
1263: .Xr tun 4
1264: network pseudo-device,
1265: allowing two networks to be joined securely.
1266: The
1267: .Xr sshd_config 5
1268: configuration option
1269: .Cm PermitTunnel
1270: controls whether the server supports this,
1271: and at what level (layer 2 or 3 traffic).
1272: .Pp
1273: The following example would connect client network 10.0.50.0/24
1.265 otto 1274: with remote network 10.0.99.0/24 using a point-to-point connection
1275: from 10.1.1.1 to 10.1.1.2,
1276: provided that the SSH server running on the gateway to the remote network,
1277: at 192.168.1.15, allows it.
1278: .Pp
1279: On the client:
1.250 jmc 1280: .Bd -literal -offset indent
1281: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1282: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1283: # route add 10.0.99.0/24 10.1.1.2
1284: .Ed
1285: .Pp
1286: On the server:
1287: .Bd -literal -offset indent
1288: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1289: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1290: .Ed
1291: .Pp
1292: Client access may be more finely tuned via the
1293: .Pa /root/.ssh/authorized_keys
1294: file (see below) and the
1295: .Cm PermitRootLogin
1296: server option.
1.255 jmc 1297: The following entry would permit connections on
1.250 jmc 1298: .Xr tun 4
1.255 jmc 1299: device 1 from user
1.250 jmc 1300: .Dq jane
1.255 jmc 1301: and on tun device 2 from user
1.250 jmc 1302: .Dq john ,
1303: if
1304: .Cm PermitRootLogin
1305: is set to
1306: .Dq forced-commands-only :
1307: .Bd -literal -offset 2n
1308: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1309: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1310: .Ed
1311: .Pp
1.264 ray 1312: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1313: it may be more suited to temporary setups,
1314: such as for wireless VPNs.
1315: More permanent VPNs are better provided by tools such as
1316: .Xr ipsecctl 8
1317: and
1318: .Xr isakmpd 8 .
1.2 deraadt 1319: .Sh ENVIRONMENT
1320: .Nm
1.1 deraadt 1321: will normally set the following environment variables:
1.237 jmc 1322: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1323: .It Ev DISPLAY
1324: The
1325: .Ev DISPLAY
1.40 aaron 1326: variable indicates the location of the X11 server.
1.44 aaron 1327: It is automatically set by
1.2 deraadt 1328: .Nm
1329: to point to a value of the form
1.233 jmc 1330: .Dq hostname:n ,
1331: where
1332: .Dq hostname
1333: indicates the host where the shell runs, and
1334: .Sq n
1335: is an integer \*(Ge 1.
1.40 aaron 1336: .Nm
1337: uses this special value to forward X11 connections over the secure
1338: channel.
1.107 markus 1339: The user should normally not set
1340: .Ev DISPLAY
1341: explicitly, as that
1.1 deraadt 1342: will render the X11 connection insecure (and will require the user to
1343: manually copy any required authorization cookies).
1.2 deraadt 1344: .It Ev HOME
1.1 deraadt 1345: Set to the path of the user's home directory.
1.2 deraadt 1346: .It Ev LOGNAME
1347: Synonym for
1.12 aaron 1348: .Ev USER ;
1349: set for compatibility with systems that use this variable.
1.2 deraadt 1350: .It Ev MAIL
1.129 stevesk 1351: Set to the path of the user's mailbox.
1.40 aaron 1352: .It Ev PATH
1.2 deraadt 1353: Set to the default
1354: .Ev PATH ,
1355: as specified when compiling
1.234 jmc 1356: .Nm .
1.118 markus 1357: .It Ev SSH_ASKPASS
1358: If
1359: .Nm
1360: needs a passphrase, it will read the passphrase from the current
1361: terminal if it was run from a terminal.
1362: If
1363: .Nm
1364: does not have a terminal associated with it but
1365: .Ev DISPLAY
1366: and
1367: .Ev SSH_ASKPASS
1368: are set, it will execute the program specified by
1369: .Ev SSH_ASKPASS
1370: and open an X11 window to read the passphrase.
1371: This is particularly useful when calling
1372: .Nm
1373: from a
1.196 jmc 1374: .Pa .xsession
1.118 markus 1375: or related script.
1376: (Note that on some machines it
1377: may be necessary to redirect the input from
1378: .Pa /dev/null
1379: to make this work.)
1.18 markus 1380: .It Ev SSH_AUTH_SOCK
1.233 jmc 1381: Identifies the path of a
1382: .Ux Ns -domain
1383: socket used to communicate with the agent.
1.166 stevesk 1384: .It Ev SSH_CONNECTION
1385: Identifies the client and server ends of the connection.
1.40 aaron 1386: The variable contains
1.233 jmc 1387: four space-separated values: client IP address, client port number,
1388: server IP address, and server port number.
1.73 markus 1389: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1390: This variable contains the original command line if a forced command
1.73 markus 1391: is executed.
1392: It can be used to extract the original arguments.
1.2 deraadt 1393: .It Ev SSH_TTY
1.1 deraadt 1394: This is set to the name of the tty (path to the device) associated
1.40 aaron 1395: with the current shell or command.
1396: If the current session has no tty,
1.1 deraadt 1397: this variable is not set.
1.385 djm 1398: .It Ev SSH_USER_AUTH
1399: Optionally set by
1400: .Xr sshd 8 ,
1401: this variable may contain a pathname to a file that lists the authentication
1402: methods successfully used when the session was established, including any
1403: public keys that were used.
1.2 deraadt 1404: .It Ev TZ
1.214 jmc 1405: This variable is set to indicate the present time zone if it
1.257 jmc 1406: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1407: on to new connections).
1.2 deraadt 1408: .It Ev USER
1.1 deraadt 1409: Set to the name of the user logging in.
1.2 deraadt 1410: .El
1411: .Pp
1.44 aaron 1412: Additionally,
1.2 deraadt 1413: .Nm
1.44 aaron 1414: reads
1.207 djm 1415: .Pa ~/.ssh/environment ,
1.2 deraadt 1416: and adds lines of the format
1417: .Dq VARNAME=value
1.233 jmc 1418: to the environment if the file exists and users are allowed to
1.161 marc 1419: change their environment.
1.176 jmc 1420: For more information, see the
1.161 marc 1421: .Cm PermitUserEnvironment
1.162 stevesk 1422: option in
1.161 marc 1423: .Xr sshd_config 5 .
1.2 deraadt 1424: .Sh FILES
1.236 jmc 1425: .Bl -tag -width Ds -compact
1.309 jmc 1426: .It Pa ~/.rhosts
1.240 jmc 1427: This file is used for host-based authentication (see above).
1.92 markus 1428: On some machines this file may need to be
1.240 jmc 1429: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1430: because
1.2 deraadt 1431: .Xr sshd 8
1.40 aaron 1432: reads it as root.
1433: Additionally, this file must be owned by the user,
1434: and must not have write permissions for anyone else.
1435: The recommended
1.1 deraadt 1436: permission for most machines is read/write for the user, and not
1437: accessible by others.
1.2 deraadt 1438: .Pp
1.309 jmc 1439: .It Pa ~/.shosts
1.240 jmc 1440: This file is used in exactly the same way as
1441: .Pa .rhosts ,
1442: but allows host-based authentication without permitting login with
1443: rlogin/rsh.
1.272 mcbride 1444: .Pp
1.309 jmc 1445: .It Pa ~/.ssh/
1.272 mcbride 1446: This directory is the default location for all user-specific configuration
1447: and authentication information.
1448: There is no general requirement to keep the entire contents of this directory
1449: secret, but the recommended permissions are read/write/execute for the user,
1450: and not accessible by others.
1.236 jmc 1451: .Pp
1.309 jmc 1452: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1453: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1454: that can be used for logging in as this user.
1.238 jmc 1455: The format of this file is described in the
1456: .Xr sshd 8
1457: manual page.
1458: This file is not highly sensitive, but the recommended
1459: permissions are read/write for the user, and not accessible by others.
1460: .Pp
1.309 jmc 1461: .It Pa ~/.ssh/config
1.238 jmc 1462: This is the per-user configuration file.
1463: The file format and configuration options are described in
1464: .Xr ssh_config 5 .
1465: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1466: read/write for the user, and not writable by others.
1.238 jmc 1467: .Pp
1.309 jmc 1468: .It Pa ~/.ssh/environment
1.239 jmc 1469: Contains additional definitions for environment variables; see
1470: .Sx ENVIRONMENT ,
1.238 jmc 1471: above.
1472: .Pp
1.309 jmc 1473: .It Pa ~/.ssh/id_dsa
1.310 djm 1474: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1475: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1476: .It Pa ~/.ssh/id_rsa
1.238 jmc 1477: Contains the private key for authentication.
1478: These files
1479: contain sensitive data and should be readable by the user but not
1480: accessible by others (read/write/execute).
1481: .Nm
1482: will simply ignore a private key file if it is accessible by others.
1483: It is possible to specify a passphrase when
1484: generating the key which will be used to encrypt the
1485: sensitive part of this file using 3DES.
1486: .Pp
1.309 jmc 1487: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1488: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1489: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1490: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1491: Contains the public key for authentication.
1492: These files are not
1493: sensitive and can (but need not) be readable by anyone.
1494: .Pp
1.309 jmc 1495: .It Pa ~/.ssh/known_hosts
1.244 jmc 1496: Contains a list of host keys for all hosts the user has logged into
1497: that are not already in the systemwide list of known host keys.
1.238 jmc 1498: See
1.244 jmc 1499: .Xr sshd 8
1500: for further details of the format of this file.
1.238 jmc 1501: .Pp
1.309 jmc 1502: .It Pa ~/.ssh/rc
1.238 jmc 1503: Commands in this file are executed by
1504: .Nm
1.245 jmc 1505: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1506: started.
1507: See the
1508: .Xr sshd 8
1509: manual page for more information.
1510: .Pp
1.309 jmc 1511: .It Pa /etc/hosts.equiv
1.240 jmc 1512: This file is for host-based authentication (see above).
1513: It should only be writable by root.
1.236 jmc 1514: .Pp
1.309 jmc 1515: .It Pa /etc/shosts.equiv
1.240 jmc 1516: This file is used in exactly the same way as
1517: .Pa hosts.equiv ,
1518: but allows host-based authentication without permitting login with
1519: rlogin/rsh.
1.236 jmc 1520: .Pp
1.238 jmc 1521: .It Pa /etc/ssh/ssh_config
1522: Systemwide configuration file.
1523: The file format and configuration options are described in
1524: .Xr ssh_config 5 .
1525: .Pp
1.309 jmc 1526: .It Pa /etc/ssh/ssh_host_key
1527: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1528: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1529: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1530: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1531: These files contain the private parts of the host keys
1.245 jmc 1532: and are used for host-based authentication.
1.238 jmc 1533: .Pp
1.309 jmc 1534: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1535: Systemwide list of known host keys.
1536: This file should be prepared by the
1537: system administrator to contain the public host keys of all machines in the
1538: organization.
1.244 jmc 1539: It should be world-readable.
1540: See
1.238 jmc 1541: .Xr sshd 8
1.244 jmc 1542: for further details of the format of this file.
1.236 jmc 1543: .Pp
1.309 jmc 1544: .It Pa /etc/ssh/sshrc
1.1 deraadt 1545: Commands in this file are executed by
1.2 deraadt 1546: .Nm
1.245 jmc 1547: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1548: See the
1.2 deraadt 1549: .Xr sshd 8
1.1 deraadt 1550: manual page for more information.
1.58 itojun 1551: .El
1.312 jmc 1552: .Sh EXIT STATUS
1553: .Nm
1554: exits with the exit status of the remote command or with 255
1555: if an error occurred.
1.2 deraadt 1556: .Sh SEE ALSO
1557: .Xr scp 1 ,
1.83 djm 1558: .Xr sftp 1 ,
1.2 deraadt 1559: .Xr ssh-add 1 ,
1560: .Xr ssh-agent 1 ,
1561: .Xr ssh-keygen 1 ,
1.242 jmc 1562: .Xr ssh-keyscan 1 ,
1.250 jmc 1563: .Xr tun 4 ,
1.159 stevesk 1564: .Xr ssh_config 5 ,
1.160 naddy 1565: .Xr ssh-keysign 8 ,
1.87 itojun 1566: .Xr sshd 8
1.329 jmc 1567: .Sh STANDARDS
1.106 markus 1568: .Rs
1.329 jmc 1569: .%A S. Lehtinen
1570: .%A C. Lonvick
1571: .%D January 2006
1.256 jmc 1572: .%R RFC 4250
1.329 jmc 1573: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1574: .Re
1.329 jmc 1575: .Pp
1.256 jmc 1576: .Rs
1.329 jmc 1577: .%A T. Ylonen
1578: .%A C. Lonvick
1579: .%D January 2006
1.256 jmc 1580: .%R RFC 4251
1.329 jmc 1581: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1582: .Re
1.329 jmc 1583: .Pp
1.256 jmc 1584: .Rs
1.329 jmc 1585: .%A T. Ylonen
1586: .%A C. Lonvick
1587: .%D January 2006
1.256 jmc 1588: .%R RFC 4252
1.329 jmc 1589: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1590: .Re
1.329 jmc 1591: .Pp
1.256 jmc 1592: .Rs
1.329 jmc 1593: .%A T. Ylonen
1594: .%A C. Lonvick
1595: .%D January 2006
1.256 jmc 1596: .%R RFC 4253
1.329 jmc 1597: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1598: .Re
1.329 jmc 1599: .Pp
1.256 jmc 1600: .Rs
1.329 jmc 1601: .%A T. Ylonen
1602: .%A C. Lonvick
1603: .%D January 2006
1.256 jmc 1604: .%R RFC 4254
1.329 jmc 1605: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1606: .Re
1.329 jmc 1607: .Pp
1.256 jmc 1608: .Rs
1.329 jmc 1609: .%A J. Schlyter
1610: .%A W. Griffin
1611: .%D January 2006
1.256 jmc 1612: .%R RFC 4255
1.329 jmc 1613: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1614: .Re
1.329 jmc 1615: .Pp
1.256 jmc 1616: .Rs
1.329 jmc 1617: .%A F. Cusack
1618: .%A M. Forssen
1619: .%D January 2006
1.256 jmc 1620: .%R RFC 4256
1.329 jmc 1621: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1622: .Re
1.329 jmc 1623: .Pp
1.256 jmc 1624: .Rs
1.329 jmc 1625: .%A J. Galbraith
1626: .%A P. Remaker
1627: .%D January 2006
1.256 jmc 1628: .%R RFC 4335
1.329 jmc 1629: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1630: .Re
1.329 jmc 1631: .Pp
1.256 jmc 1632: .Rs
1.329 jmc 1633: .%A M. Bellare
1634: .%A T. Kohno
1635: .%A C. Namprempre
1636: .%D January 2006
1.256 jmc 1637: .%R RFC 4344
1.329 jmc 1638: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1639: .Re
1.329 jmc 1640: .Pp
1.256 jmc 1641: .Rs
1.329 jmc 1642: .%A B. Harris
1643: .%D January 2006
1.256 jmc 1644: .%R RFC 4345
1.329 jmc 1645: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1646: .Re
1.329 jmc 1647: .Pp
1.258 djm 1648: .Rs
1.329 jmc 1649: .%A M. Friedl
1650: .%A N. Provos
1651: .%A W. Simpson
1652: .%D March 2006
1.258 djm 1653: .%R RFC 4419
1.329 jmc 1654: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1655: .Re
1.329 jmc 1656: .Pp
1.266 markus 1657: .Rs
1.329 jmc 1658: .%A J. Galbraith
1659: .%A R. Thayer
1660: .%D November 2006
1.266 markus 1661: .%R RFC 4716
1.329 jmc 1662: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1663: .Re
1.329 jmc 1664: .Pp
1.313 djm 1665: .Rs
1.329 jmc 1666: .%A D. Stebila
1667: .%A J. Green
1668: .%D December 2009
1.313 djm 1669: .%R RFC 5656
1.329 jmc 1670: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1671: .Re
1.329 jmc 1672: .Pp
1.274 grunk 1673: .Rs
1674: .%A A. Perrig
1675: .%A D. Song
1676: .%D 1999
1.329 jmc 1677: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1678: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1679: .Re
1.173 jmc 1680: .Sh AUTHORS
1681: OpenSSH is a derivative of the original and free
1682: ssh 1.2.12 release by Tatu Ylonen.
1683: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1684: Theo de Raadt and Dug Song
1685: removed many bugs, re-added newer features and
1686: created OpenSSH.
1687: Markus Friedl contributed the support for SSH
1688: protocol versions 1.5 and 2.0.
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