Annotation of src/usr.bin/ssh/ssh.1, Revision 1.403
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: .\"
1.403 ! jmc 36: .\" $OpenBSD: ssh.1,v 1.402 2019/03/16 19:14:21 jmc Exp $
! 37: .Dd $Mdocdate: March 16 2019 $
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
1.401 naddy 273: should use to communicate with a PKCS#11 token providing keys for user
274: authentication.
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.
1.400 djm 311: Note that configuration directives supplied on the command-line generally
312: apply to the destination host and not any specified jump hosts.
313: Use
314: .Pa ~/.ssh/config
315: to specify configuration for jump hosts.
1.375 djm 316: .Pp
1.269 djm 317: .It Fl K
318: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
319: credentials to the server.
1.360 millert 320: .Pp
1.218 jmc 321: .It Fl k
322: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 323: .Pp
1.218 jmc 324: .It Fl L Xo
325: .Sm off
326: .Oo Ar bind_address : Oc
327: .Ar port : host : hostport
328: .Sm on
329: .Xc
1.360 millert 330: .It Fl L Xo
331: .Sm off
332: .Oo Ar bind_address : Oc
333: .Ar port : remote_socket
334: .Sm on
335: .Xc
336: .It Fl L Xo
337: .Sm off
338: .Ar local_socket : host : hostport
339: .Sm on
340: .Xc
341: .It Fl L Xo
342: .Sm off
343: .Ar local_socket : remote_socket
344: .Sm on
345: .Xc
346: Specifies that connections to the given TCP port or Unix socket on the local
347: (client) host are to be forwarded to the given host and port, or Unix socket,
348: on the remote side.
349: This works by allocating a socket to listen to either a TCP
1.218 jmc 350: .Ar port
351: on the local side, optionally bound to the specified
1.360 millert 352: .Ar bind_address ,
353: or to a Unix socket.
354: Whenever a connection is made to the local port or socket, the
1.218 jmc 355: connection is forwarded over the secure channel, and a connection is
1.360 millert 356: made to either
1.218 jmc 357: .Ar host
358: port
1.360 millert 359: .Ar hostport ,
360: or the Unix socket
361: .Ar remote_socket ,
1.218 jmc 362: from the remote machine.
1.360 millert 363: .Pp
1.218 jmc 364: Port forwardings can also be specified in the configuration file.
1.360 millert 365: Only the superuser can forward privileged ports.
1.308 djm 366: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 367: .Pp
1.218 jmc 368: By default, the local port is bound in accordance with the
369: .Cm GatewayPorts
370: setting.
371: However, an explicit
372: .Ar bind_address
373: may be used to bind the connection to a specific address.
1.2 deraadt 374: The
1.218 jmc 375: .Ar bind_address
376: of
377: .Dq localhost
378: indicates that the listening port be bound for local use only, while an
379: empty address or
380: .Sq *
381: indicates that the port should be available from all interfaces.
1.360 millert 382: .Pp
1.218 jmc 383: .It Fl l Ar login_name
384: Specifies the user to log in as on the remote machine.
385: This also may be specified on a per-host basis in the configuration file.
1.360 millert 386: .Pp
1.218 jmc 387: .It Fl M
388: Places the
389: .Nm
390: client into
391: .Dq master
392: mode for connection sharing.
1.231 stevesk 393: Multiple
394: .Fl M
395: options places
396: .Nm
397: into
398: .Dq master
1.397 djm 399: mode but with confirmation required using
400: .Xr ssh-askpass 1
401: before each operation that changes the multiplexing state
402: (e.g. opening a new session).
1.218 jmc 403: Refer to the description of
404: .Cm ControlMaster
405: in
406: .Xr ssh_config 5
407: for details.
1.360 millert 408: .Pp
1.218 jmc 409: .It Fl m Ar mac_spec
1.369 jmc 410: A comma-separated list of MAC (message authentication code) algorithms,
411: specified in order of preference.
1.218 jmc 412: See the
413: .Cm MACs
414: keyword for more information.
1.360 millert 415: .Pp
1.218 jmc 416: .It Fl N
417: Do not execute a remote command.
1.369 jmc 418: This is useful for just forwarding ports.
1.360 millert 419: .Pp
1.218 jmc 420: .It Fl n
421: Redirects stdin from
422: .Pa /dev/null
423: (actually, prevents reading from stdin).
424: This must be used when
1.2 deraadt 425: .Nm
1.218 jmc 426: is run in the background.
427: A common trick is to use this to run X11 programs on a remote machine.
428: For example,
429: .Ic ssh -n shadows.cs.hut.fi emacs &
430: will start an emacs on shadows.cs.hut.fi, and the X11
431: connection will be automatically forwarded over an encrypted channel.
432: The
1.2 deraadt 433: .Nm
1.218 jmc 434: program will be put in the background.
435: (This does not work if
1.2 deraadt 436: .Nm
1.218 jmc 437: needs to ask for a password or passphrase; see also the
438: .Fl f
439: option.)
1.360 millert 440: .Pp
1.218 jmc 441: .It Fl O Ar ctl_cmd
442: Control an active connection multiplexing master process.
443: When the
444: .Fl O
445: option is specified, the
446: .Ar ctl_cmd
447: argument is interpreted and passed to the master process.
448: Valid commands are:
449: .Dq check
1.305 markus 450: (check that the master process is running),
451: .Dq forward
1.318 jmc 452: (request forwardings without command execution),
1.323 okan 453: .Dq cancel
454: (cancel forwardings),
1.218 jmc 455: .Dq exit
1.318 jmc 456: (request the master to exit), and
1.317 djm 457: .Dq stop
458: (request the master to stop accepting further multiplexing requests).
1.360 millert 459: .Pp
1.218 jmc 460: .It Fl o Ar option
461: Can be used to give options in the format used in the configuration file.
462: This is useful for specifying options for which there is no separate
463: command-line flag.
464: For full details of the options listed below, and their possible values, see
465: .Xr ssh_config 5 .
1.2 deraadt 466: .Pp
1.218 jmc 467: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 468: .It AddKeysToAgent
1.218 jmc 469: .It AddressFamily
470: .It BatchMode
471: .It BindAddress
1.338 djm 472: .It CanonicalDomains
1.339 djm 473: .It CanonicalizeFallbackLocal
474: .It CanonicalizeHostname
475: .It CanonicalizeMaxDots
476: .It CanonicalizePermittedCNAMEs
1.399 jmc 477: .It CASignatureAlgorithms
1.363 djm 478: .It CertificateFile
1.218 jmc 479: .It ChallengeResponseAuthentication
480: .It CheckHostIP
481: .It Ciphers
482: .It ClearAllForwardings
483: .It Compression
1.380 naddy 484: .It ConnectionAttempts
1.218 jmc 485: .It ConnectTimeout
486: .It ControlMaster
487: .It ControlPath
1.321 djm 488: .It ControlPersist
1.218 jmc 489: .It DynamicForward
490: .It EscapeChar
1.263 markus 491: .It ExitOnForwardFailure
1.353 jmc 492: .It FingerprintHash
1.218 jmc 493: .It ForwardAgent
494: .It ForwardX11
1.321 djm 495: .It ForwardX11Timeout
1.218 jmc 496: .It ForwardX11Trusted
497: .It GatewayPorts
498: .It GlobalKnownHostsFile
499: .It GSSAPIAuthentication
500: .It GSSAPIDelegateCredentials
501: .It HashKnownHosts
502: .It Host
503: .It HostbasedAuthentication
1.355 djm 504: .It HostbasedKeyTypes
1.218 jmc 505: .It HostKeyAlgorithms
506: .It HostKeyAlias
1.403 ! jmc 507: .It Hostname
1.374 jmc 508: .It IdentitiesOnly
1.371 markus 509: .It IdentityAgent
1.218 jmc 510: .It IdentityFile
1.316 jmc 511: .It IPQoS
1.321 djm 512: .It KbdInteractiveAuthentication
1.218 jmc 513: .It KbdInteractiveDevices
1.314 jmc 514: .It KexAlgorithms
1.218 jmc 515: .It LocalCommand
516: .It LocalForward
517: .It LogLevel
518: .It MACs
1.337 jmc 519: .It Match
1.218 jmc 520: .It NoHostAuthenticationForLocalhost
521: .It NumberOfPasswordPrompts
522: .It PasswordAuthentication
523: .It PermitLocalCommand
1.291 markus 524: .It PKCS11Provider
1.218 jmc 525: .It Port
526: .It PreferredAuthentications
527: .It ProxyCommand
1.375 djm 528: .It ProxyJump
1.336 jmc 529: .It ProxyUseFdpass
1.359 markus 530: .It PubkeyAcceptedKeyTypes
1.218 jmc 531: .It PubkeyAuthentication
1.251 dtucker 532: .It RekeyLimit
1.382 bluhm 533: .It RemoteCommand
1.218 jmc 534: .It RemoteForward
1.319 jmc 535: .It RequestTTY
1.218 jmc 536: .It SendEnv
537: .It ServerAliveInterval
538: .It ServerAliveCountMax
1.395 jmc 539: .It SetEnv
1.347 jmc 540: .It StreamLocalBindMask
541: .It StreamLocalBindUnlink
1.218 jmc 542: .It StrictHostKeyChecking
543: .It TCPKeepAlive
544: .It Tunnel
545: .It TunnelDevice
1.354 djm 546: .It UpdateHostKeys
1.218 jmc 547: .It User
548: .It UserKnownHostsFile
549: .It VerifyHostKeyDNS
1.276 jmc 550: .It VisualHostKey
1.218 jmc 551: .It XAuthLocation
552: .El
1.360 millert 553: .Pp
1.218 jmc 554: .It Fl p Ar port
555: Port to connect to on the remote host.
556: This can be specified on a
557: per-host basis in the configuration file.
1.360 millert 558: .Pp
1.362 djm 559: .It Fl Q Ar query_option
1.332 djm 560: Queries
561: .Nm
1.341 deraadt 562: for the algorithms supported for the specified version 2.
563: The available features are:
564: .Ar cipher
1.332 djm 565: (supported symmetric ciphers),
1.341 deraadt 566: .Ar cipher-auth
1.340 djm 567: (supported symmetric ciphers that support authenticated encryption),
1.398 djm 568: .Ar help
569: (supported query terms for use with the
570: .Fl Q
571: flag),
1.341 deraadt 572: .Ar mac
1.332 djm 573: (supported message integrity codes),
1.341 deraadt 574: .Ar kex
1.332 djm 575: (key exchange algorithms),
1.341 deraadt 576: .Ar key
1.362 djm 577: (key types),
578: .Ar key-cert
579: (certificate key types),
580: .Ar key-plain
1.398 djm 581: (non-certificate key types),
1.356 djm 582: .Ar protocol-version
1.398 djm 583: (supported SSH protocol versions), and
584: .Ar sig
585: (supported signature algorithms).
1.360 millert 586: .Pp
1.218 jmc 587: .It Fl q
588: Quiet mode.
1.271 djm 589: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 590: .Pp
1.218 jmc 591: .It Fl R Xo
592: .Sm off
593: .Oo Ar bind_address : Oc
594: .Ar port : host : hostport
595: .Sm on
596: .Xc
1.360 millert 597: .It Fl R Xo
598: .Sm off
599: .Oo Ar bind_address : Oc
600: .Ar port : local_socket
601: .Sm on
602: .Xc
603: .It Fl R Xo
604: .Sm off
605: .Ar remote_socket : host : hostport
606: .Sm on
607: .Xc
608: .It Fl R Xo
609: .Sm off
610: .Ar remote_socket : local_socket
611: .Sm on
612: .Xc
1.384 markus 613: .It Fl R Xo
614: .Sm off
615: .Oo Ar bind_address : Oc
616: .Ar port
617: .Sm on
618: .Xc
1.360 millert 619: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 620: (server) host are to be forwarded to the local side.
621: .Pp
1.360 millert 622: This works by allocating a socket to listen to either a TCP
1.218 jmc 623: .Ar port
1.360 millert 624: or to a Unix socket on the remote side.
625: Whenever a connection is made to this port or Unix socket, the
626: connection is forwarded over the secure channel, and a connection
1.384 markus 627: is made from the local machine to either an explicit destination specified by
1.218 jmc 628: .Ar host
629: port
1.360 millert 630: .Ar hostport ,
631: or
632: .Ar local_socket ,
1.384 markus 633: or, if no explicit destination was specified,
634: .Nm
635: will act as a SOCKS 4/5 proxy and forward connections to the destinations
636: requested by the remote SOCKS client.
1.2 deraadt 637: .Pp
1.218 jmc 638: Port forwardings can also be specified in the configuration file.
639: Privileged ports can be forwarded only when
640: logging in as root on the remote machine.
1.324 jmc 641: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 642: .Pp
1.360 millert 643: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 644: interface only.
1.280 tobias 645: This may be overridden by specifying a
1.218 jmc 646: .Ar bind_address .
647: An empty
648: .Ar bind_address ,
649: or the address
650: .Ql * ,
651: indicates that the remote socket should listen on all interfaces.
652: Specifying a remote
653: .Ar bind_address
654: will only succeed if the server's
655: .Cm GatewayPorts
656: option is enabled (see
657: .Xr sshd_config 5 ) .
1.281 djm 658: .Pp
659: If the
660: .Ar port
661: argument is
1.282 djm 662: .Ql 0 ,
1.281 djm 663: the listen port will be dynamically allocated on the server and reported
664: to the client at run time.
1.305 markus 665: When used together with
666: .Ic -O forward
667: the allocated port will be printed to the standard output.
1.360 millert 668: .Pp
1.218 jmc 669: .It Fl S Ar ctl_path
1.304 jmc 670: Specifies the location of a control socket for connection sharing,
1.303 djm 671: or the string
672: .Dq none
673: to disable connection sharing.
1.218 jmc 674: Refer to the description of
675: .Cm ControlPath
676: and
677: .Cm ControlMaster
678: in
679: .Xr ssh_config 5
680: for details.
1.360 millert 681: .Pp
1.218 jmc 682: .It Fl s
683: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 684: Subsystems facilitate the use of SSH
685: as a secure transport for other applications (e.g.\&
1.218 jmc 686: .Xr sftp 1 ) .
687: The subsystem is specified as the remote command.
1.360 millert 688: .Pp
1.218 jmc 689: .It Fl T
1.357 dtucker 690: Disable pseudo-terminal allocation.
1.360 millert 691: .Pp
1.218 jmc 692: .It Fl t
1.357 dtucker 693: Force pseudo-terminal allocation.
1.218 jmc 694: This can be used to execute arbitrary
695: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 696: e.g. when implementing menu services.
1.218 jmc 697: Multiple
698: .Fl t
699: options force tty allocation, even if
1.194 jakob 700: .Nm
1.218 jmc 701: has no local tty.
1.360 millert 702: .Pp
1.218 jmc 703: .It Fl V
704: Display the version number and exit.
1.360 millert 705: .Pp
1.218 jmc 706: .It Fl v
707: Verbose mode.
708: Causes
1.176 jmc 709: .Nm
1.218 jmc 710: to print debugging messages about its progress.
711: This is helpful in
712: debugging connection, authentication, and configuration problems.
713: Multiple
714: .Fl v
715: options increase the verbosity.
716: The maximum is 3.
1.360 millert 717: .Pp
1.290 dtucker 718: .It Fl W Ar host : Ns Ar port
719: Requests that standard input and output on the client be forwarded to
720: .Ar host
721: on
722: .Ar port
723: over the secure channel.
724: Implies
725: .Fl N ,
726: .Fl T ,
727: .Cm ExitOnForwardFailure
728: and
1.373 jmc 729: .Cm ClearAllForwardings ,
730: though these can be overridden in the configuration file or using
1.372 dtucker 731: .Fl o
732: command line options.
1.360 millert 733: .Pp
1.261 stevesk 734: .It Fl w Xo
735: .Ar local_tun Ns Op : Ns Ar remote_tun
736: .Xc
737: Requests
738: tunnel
739: device forwarding with the specified
1.218 jmc 740: .Xr tun 4
1.261 stevesk 741: devices between the client
742: .Pq Ar local_tun
743: and the server
744: .Pq Ar remote_tun .
745: .Pp
1.228 jmc 746: The devices may be specified by numerical ID or the keyword
747: .Dq any ,
748: which uses the next available tunnel device.
1.261 stevesk 749: If
750: .Ar remote_tun
751: is not specified, it defaults to
752: .Dq any .
1.228 jmc 753: See also the
1.218 jmc 754: .Cm Tunnel
1.261 stevesk 755: and
756: .Cm TunnelDevice
757: directives in
1.218 jmc 758: .Xr ssh_config 5 .
1.393 djm 759: .Pp
1.261 stevesk 760: If the
761: .Cm Tunnel
1.393 djm 762: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 763: .Dq point-to-point .
1.393 djm 764: If a different
765: .Cm Tunnel
766: forwarding mode it desired, then it should be specified before
767: .Fl w .
1.360 millert 768: .Pp
1.218 jmc 769: .It Fl X
770: Enables X11 forwarding.
1.54 markus 771: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 772: .Pp
1.218 jmc 773: X11 forwarding should be enabled with caution.
1.168 jmc 774: Users with the ability to bypass file permissions on the remote host
1.218 jmc 775: (for the user's X authorization database)
776: can access the local X11 display through the forwarded connection.
777: An attacker may then be able to perform activities such as keystroke monitoring.
778: .Pp
779: For this reason, X11 forwarding is subjected to X11 SECURITY extension
780: restrictions by default.
781: Please refer to the
782: .Nm
783: .Fl Y
784: option and the
785: .Cm ForwardX11Trusted
786: directive in
787: .Xr ssh_config 5
788: for more information.
1.360 millert 789: .Pp
1.218 jmc 790: .It Fl x
791: Disables X11 forwarding.
1.360 millert 792: .Pp
1.218 jmc 793: .It Fl Y
794: Enables trusted X11 forwarding.
795: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
796: controls.
1.360 millert 797: .Pp
1.278 djm 798: .It Fl y
799: Send log information using the
800: .Xr syslog 3
801: system module.
802: By default this information is sent to stderr.
1.218 jmc 803: .El
1.224 jmc 804: .Pp
805: .Nm
806: may additionally obtain configuration data from
807: a per-user configuration file and a system-wide configuration file.
808: The file format and configuration options are described in
809: .Xr ssh_config 5 .
1.222 jmc 810: .Sh AUTHENTICATION
1.377 djm 811: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 812: .Pp
813: The methods available for authentication are:
1.260 jmc 814: GSSAPI-based authentication,
1.222 jmc 815: host-based authentication,
816: public key authentication,
817: challenge-response authentication,
818: and password authentication.
819: Authentication methods are tried in the order specified above,
1.369 jmc 820: though
821: .Cm PreferredAuthentications
822: can be used to change the default order.
1.222 jmc 823: .Pp
824: Host-based authentication works as follows:
1.218 jmc 825: If the machine the user logs in from is listed in
826: .Pa /etc/hosts.equiv
827: or
828: .Pa /etc/shosts.equiv
829: on the remote machine, and the user names are
830: the same on both sides, or if the files
831: .Pa ~/.rhosts
832: or
833: .Pa ~/.shosts
834: exist in the user's home directory on the
835: remote machine and contain a line containing the name of the client
836: machine and the name of the user on that machine, the user is
1.222 jmc 837: considered for login.
838: Additionally, the server
839: .Em must
840: be able to verify the client's
841: host key (see the description of
1.218 jmc 842: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 843: and
1.222 jmc 844: .Pa ~/.ssh/known_hosts ,
845: below)
846: for login to be permitted.
1.218 jmc 847: This authentication method closes security holes due to IP
1.222 jmc 848: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 849: [Note to the administrator:
850: .Pa /etc/hosts.equiv ,
851: .Pa ~/.rhosts ,
852: and the rlogin/rsh protocol in general, are inherently insecure and should be
853: disabled if security is desired.]
1.189 dtucker 854: .Pp
1.222 jmc 855: Public key authentication works as follows:
856: The scheme is based on public-key cryptography,
857: using cryptosystems
858: where encryption and decryption are done using separate keys,
859: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 860: The idea is that each user creates a public/private
861: key pair for authentication purposes.
862: The server knows the public key, and only the user knows the private key.
1.222 jmc 863: .Nm
864: implements public key authentication protocol automatically,
1.349 sobrado 865: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 866: The HISTORY section of
1.222 jmc 867: .Xr ssl 8
1.311 jmc 868: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 869: .Pp
1.218 jmc 870: The file
871: .Pa ~/.ssh/authorized_keys
872: lists the public keys that are permitted for logging in.
873: When the user logs in, the
1.2 deraadt 874: .Nm
1.218 jmc 875: program tells the server which key pair it would like to use for
876: authentication.
1.222 jmc 877: The client proves that it has access to the private key
878: and the server checks that the corresponding public key
879: is authorized to accept the account.
1.383 djm 880: .Pp
881: The server may inform the client of errors that prevented public key
882: authentication from succeeding after authentication completes using a
883: different method.
884: These may be viewed by increasing the
885: .Cm LogLevel
886: to
887: .Cm DEBUG
888: or higher (e.g. by using the
889: .Fl v
890: flag).
1.218 jmc 891: .Pp
1.222 jmc 892: The user creates his/her key pair by running
1.218 jmc 893: .Xr ssh-keygen 1 .
894: This stores the private key in
1.222 jmc 895: .Pa ~/.ssh/id_dsa
1.369 jmc 896: (DSA),
1.310 djm 897: .Pa ~/.ssh/id_ecdsa
1.369 jmc 898: (ECDSA),
1.343 naddy 899: .Pa ~/.ssh/id_ed25519
1.369 jmc 900: (Ed25519),
1.222 jmc 901: or
902: .Pa ~/.ssh/id_rsa
1.369 jmc 903: (RSA)
1.218 jmc 904: and stores the public key in
1.222 jmc 905: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 906: (DSA),
1.310 djm 907: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 908: (ECDSA),
1.343 naddy 909: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 910: (Ed25519),
1.222 jmc 911: or
912: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 913: (RSA)
1.218 jmc 914: in the user's home directory.
1.222 jmc 915: The user should then copy the public key
1.218 jmc 916: to
917: .Pa ~/.ssh/authorized_keys
1.222 jmc 918: in his/her home directory on the remote machine.
919: The
1.218 jmc 920: .Pa authorized_keys
921: file corresponds to the conventional
922: .Pa ~/.rhosts
923: file, and has one key
1.222 jmc 924: per line, though the lines can be very long.
1.218 jmc 925: After this, the user can log in without giving the password.
926: .Pp
1.301 jmc 927: A variation on public key authentication
928: is available in the form of certificate authentication:
929: instead of a set of public/private keys,
930: signed certificates are used.
931: This has the advantage that a single trusted certification authority
932: can be used in place of many public/private keys.
1.333 jmc 933: See the CERTIFICATES section of
1.301 jmc 934: .Xr ssh-keygen 1
935: for more information.
936: .Pp
937: The most convenient way to use public key or certificate authentication
938: may be with an authentication agent.
1.218 jmc 939: See
940: .Xr ssh-agent 1
1.366 jcs 941: and (optionally) the
942: .Cm AddKeysToAgent
943: directive in
944: .Xr ssh_config 5
1.218 jmc 945: for more information.
946: .Pp
1.222 jmc 947: Challenge-response authentication works as follows:
948: The server sends an arbitrary
949: .Qq challenge
950: text, and prompts for a response.
951: Examples of challenge-response authentication include
1.335 jmc 952: .Bx
953: Authentication (see
1.222 jmc 954: .Xr login.conf 5 )
1.335 jmc 955: and PAM (some
956: .Pf non- Ox
957: systems).
1.222 jmc 958: .Pp
959: Finally, if other authentication methods fail,
1.218 jmc 960: .Nm
961: prompts the user for a password.
962: The password is sent to the remote
963: host for checking; however, since all communications are encrypted,
964: the password cannot be seen by someone listening on the network.
1.232 jmc 965: .Pp
966: .Nm
967: automatically maintains and checks a database containing
968: identification for all hosts it has ever been used with.
969: Host keys are stored in
970: .Pa ~/.ssh/known_hosts
971: in the user's home directory.
972: Additionally, the file
973: .Pa /etc/ssh/ssh_known_hosts
974: is automatically checked for known hosts.
975: Any new hosts are automatically added to the user's file.
976: If a host's identification ever changes,
977: .Nm
978: warns about this and disables password authentication to prevent
979: server spoofing or man-in-the-middle attacks,
980: which could otherwise be used to circumvent the encryption.
981: The
982: .Cm StrictHostKeyChecking
983: option can be used to control logins to machines whose
984: host key is not known or has changed.
985: .Pp
1.218 jmc 986: When the user's identity has been accepted by the server, the server
1.357 dtucker 987: either executes the given command in a non-interactive session or,
988: if no command has been specified, logs into the machine and gives
989: the user a normal shell as an interactive session.
1.218 jmc 990: All communication with
991: the remote command or shell will be automatically encrypted.
992: .Pp
1.357 dtucker 993: If an interactive session is requested
994: .Nm
995: by default will only request a pseudo-terminal (pty) for interactive
996: sessions when the client has one.
997: The flags
998: .Fl T
999: and
1000: .Fl t
1001: can be used to override this behaviour.
1002: .Pp
1003: If a pseudo-terminal has been allocated the
1.218 jmc 1004: user may use the escape characters noted below.
1005: .Pp
1.357 dtucker 1006: If no pseudo-terminal has been allocated,
1.218 jmc 1007: the session is transparent and can be used to reliably transfer binary data.
1008: On most systems, setting the escape character to
1009: .Dq none
1010: will also make the session transparent even if a tty is used.
1011: .Pp
1012: The session terminates when the command or shell on the remote
1.247 jmc 1013: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1014: .Sh ESCAPE CHARACTERS
1.218 jmc 1015: When a pseudo-terminal has been requested,
1.2 deraadt 1016: .Nm
1.218 jmc 1017: supports a number of functions through the use of an escape character.
1018: .Pp
1019: A single tilde character can be sent as
1020: .Ic ~~
1021: or by following the tilde by a character other than those described below.
1022: The escape character must always follow a newline to be interpreted as
1023: special.
1024: The escape character can be changed in configuration files using the
1025: .Cm EscapeChar
1026: configuration directive or on the command line by the
1027: .Fl e
1028: option.
1029: .Pp
1030: The supported escapes (assuming the default
1031: .Ql ~ )
1032: are:
1033: .Bl -tag -width Ds
1034: .It Cm ~.
1035: Disconnect.
1036: .It Cm ~^Z
1037: Background
1.234 jmc 1038: .Nm .
1.218 jmc 1039: .It Cm ~#
1040: List forwarded connections.
1041: .It Cm ~&
1042: Background
1.2 deraadt 1043: .Nm
1.218 jmc 1044: at logout when waiting for forwarded connection / X11 sessions to terminate.
1045: .It Cm ~?
1046: Display a list of escape characters.
1047: .It Cm ~B
1048: Send a BREAK to the remote system
1.369 jmc 1049: (only useful if the peer supports it).
1.218 jmc 1050: .It Cm ~C
1051: Open command line.
1052: Currently this allows the addition of port forwardings using the
1.279 stevesk 1053: .Fl L ,
1054: .Fl R
1.218 jmc 1055: and
1.279 stevesk 1056: .Fl D
1.225 jmc 1057: options (see above).
1.322 markus 1058: It also allows the cancellation of existing port-forwardings
1059: with
1.262 stevesk 1060: .Sm off
1.322 markus 1061: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1062: .Sm on
1.322 markus 1063: for local,
1064: .Sm off
1065: .Fl KR Oo Ar bind_address : Oc Ar port
1066: .Sm on
1067: for remote and
1068: .Sm off
1069: .Fl KD Oo Ar bind_address : Oc Ar port
1070: .Sm on
1071: for dynamic port-forwardings.
1.218 jmc 1072: .Ic !\& Ns Ar command
1073: allows the user to execute a local command if the
1074: .Ic PermitLocalCommand
1075: option is enabled in
1.176 jmc 1076: .Xr ssh_config 5 .
1.218 jmc 1077: Basic help is available, using the
1078: .Fl h
1079: option.
1080: .It Cm ~R
1081: Request rekeying of the connection
1.369 jmc 1082: (only useful if the peer supports it).
1.327 dtucker 1083: .It Cm ~V
1084: Decrease the verbosity
1085: .Pq Ic LogLevel
1086: when errors are being written to stderr.
1087: .It Cm ~v
1.328 jmc 1088: Increase the verbosity
1.327 dtucker 1089: .Pq Ic LogLevel
1090: when errors are being written to stderr.
1.176 jmc 1091: .El
1.246 jmc 1092: .Sh TCP FORWARDING
1.402 jmc 1093: Forwarding of arbitrary TCP connections over a secure channel
1094: can be specified either on the command line or in a configuration file.
1.246 jmc 1095: One possible application of TCP forwarding is a secure connection to a
1096: mail server; another is going through firewalls.
1097: .Pp
1.402 jmc 1098: In the example below, we look at encrypting communication for an IRC client,
1099: even though the IRC server it connects to does not directly
1100: support encrypted communication.
1.246 jmc 1101: This works as follows:
1102: the user connects to the remote host using
1103: .Nm ,
1.402 jmc 1104: specifying the ports to be used to forward the connection.
1105: After that it is possible to start the program locally,
1.246 jmc 1106: and
1107: .Nm
1.402 jmc 1108: will encrypt and forward the connection to the remote server.
1.246 jmc 1109: .Pp
1.402 jmc 1110: The following example tunnels an IRC session from the client
1111: to an IRC server at
1.246 jmc 1112: .Dq server.example.com ,
1113: joining channel
1114: .Dq #users ,
1115: nickname
1116: .Dq pinky ,
1.402 jmc 1117: using the standard IRC port, 6667:
1118: .Bd -literal -offset 4n
1119: $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1120: $ irc -c '#users' pinky IRC/127.0.0.1
1121: .Ed
1.246 jmc 1122: .Pp
1123: The
1124: .Fl f
1125: option backgrounds
1126: .Nm
1127: and the remote command
1128: .Dq sleep 10
1129: is specified to allow an amount of time
1130: (10 seconds, in the example)
1.402 jmc 1131: to start the program which is going to use the tunnel.
1.246 jmc 1132: If no connections are made within the time specified,
1133: .Nm
1134: will exit.
1135: .Sh X11 FORWARDING
1.218 jmc 1136: If the
1137: .Cm ForwardX11
1138: variable is set to
1139: .Dq yes
1140: (or see the description of the
1.227 jmc 1141: .Fl X ,
1142: .Fl x ,
1.218 jmc 1143: and
1.227 jmc 1144: .Fl Y
1.226 jmc 1145: options above)
1.218 jmc 1146: and the user is using X11 (the
1147: .Ev DISPLAY
1148: environment variable is set), the connection to the X11 display is
1149: automatically forwarded to the remote side in such a way that any X11
1150: programs started from the shell (or command) will go through the
1151: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1152: from the local machine.
1.218 jmc 1153: The user should not manually set
1154: .Ev DISPLAY .
1155: Forwarding of X11 connections can be
1156: configured on the command line or in configuration files.
1157: .Pp
1158: The
1159: .Ev DISPLAY
1160: value set by
1161: .Nm
1162: will point to the server machine, but with a display number greater than zero.
1163: This is normal, and happens because
1164: .Nm
1165: creates a
1166: .Dq proxy
1167: X server on the server machine for forwarding the
1168: connections over the encrypted channel.
1.200 djm 1169: .Pp
1.218 jmc 1170: .Nm
1171: will also automatically set up Xauthority data on the server machine.
1172: For this purpose, it will generate a random authorization cookie,
1173: store it in Xauthority on the server, and verify that any forwarded
1174: connections carry this cookie and replace it by the real cookie when
1175: the connection is opened.
1176: The real authentication cookie is never
1177: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1178: .Pp
1.218 jmc 1179: If the
1180: .Cm ForwardAgent
1181: variable is set to
1182: .Dq yes
1183: (or see the description of the
1184: .Fl A
1.191 djm 1185: and
1.218 jmc 1186: .Fl a
1.226 jmc 1187: options above) and
1.218 jmc 1188: the user is using an authentication agent, the connection to the agent
1189: is automatically forwarded to the remote side.
1.252 jmc 1190: .Sh VERIFYING HOST KEYS
1191: When connecting to a server for the first time,
1192: a fingerprint of the server's public key is presented to the user
1193: (unless the option
1194: .Cm StrictHostKeyChecking
1195: has been disabled).
1196: Fingerprints can be determined using
1197: .Xr ssh-keygen 1 :
1198: .Pp
1199: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1200: .Pp
1.274 grunk 1201: If the fingerprint is already known, it can be matched
1202: and the key can be accepted or rejected.
1.358 djm 1203: If only legacy (MD5) fingerprints for the server are available, the
1204: .Xr ssh-keygen 1
1205: .Fl E
1206: option may be used to downgrade the fingerprint algorithm to match.
1207: .Pp
1.274 grunk 1208: Because of the difficulty of comparing host keys
1.352 djm 1209: just by looking at fingerprint strings,
1.274 grunk 1210: there is also support to compare host keys visually,
1211: using
1212: .Em random art .
1213: By setting the
1.275 grunk 1214: .Cm VisualHostKey
1.274 grunk 1215: option to
1.275 grunk 1216: .Dq yes ,
1.274 grunk 1217: a small ASCII graphic gets displayed on every login to a server, no matter
1218: if the session itself is interactive or not.
1219: By learning the pattern a known server produces, a user can easily
1220: find out that the host key has changed when a completely different pattern
1221: is displayed.
1222: Because these patterns are not unambiguous however, a pattern that looks
1223: similar to the pattern remembered only gives a good probability that the
1224: host key is the same, not guaranteed proof.
1225: .Pp
1226: To get a listing of the fingerprints along with their random art for
1227: all known hosts, the following command line can be used:
1228: .Pp
1229: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1230: .Pp
1.252 jmc 1231: If the fingerprint is unknown,
1232: an alternative method of verification is available:
1233: SSH fingerprints verified by DNS.
1234: An additional resource record (RR),
1235: SSHFP,
1236: is added to a zonefile
1237: and the connecting client is able to match the fingerprint
1238: with that of the key presented.
1239: .Pp
1240: In this example, we are connecting a client to a server,
1241: .Dq host.example.com .
1242: The SSHFP resource records should first be added to the zonefile for
1243: host.example.com:
1244: .Bd -literal -offset indent
1.259 jakob 1245: $ ssh-keygen -r host.example.com.
1.252 jmc 1246: .Ed
1247: .Pp
1248: The output lines will have to be added to the zonefile.
1249: To check that the zone is answering fingerprint queries:
1250: .Pp
1251: .Dl $ dig -t SSHFP host.example.com
1252: .Pp
1253: Finally the client connects:
1254: .Bd -literal -offset indent
1255: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1256: [...]
1257: Matching host key fingerprint found in DNS.
1258: Are you sure you want to continue connecting (yes/no)?
1259: .Ed
1260: .Pp
1261: See the
1262: .Cm VerifyHostKeyDNS
1263: option in
1264: .Xr ssh_config 5
1265: for more information.
1.250 jmc 1266: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1267: .Nm
1268: contains support for Virtual Private Network (VPN) tunnelling
1269: using the
1270: .Xr tun 4
1271: network pseudo-device,
1272: allowing two networks to be joined securely.
1273: The
1274: .Xr sshd_config 5
1275: configuration option
1276: .Cm PermitTunnel
1277: controls whether the server supports this,
1278: and at what level (layer 2 or 3 traffic).
1279: .Pp
1280: The following example would connect client network 10.0.50.0/24
1.265 otto 1281: with remote network 10.0.99.0/24 using a point-to-point connection
1282: from 10.1.1.1 to 10.1.1.2,
1283: provided that the SSH server running on the gateway to the remote network,
1284: at 192.168.1.15, allows it.
1285: .Pp
1286: On the client:
1.250 jmc 1287: .Bd -literal -offset indent
1288: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1289: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1290: # route add 10.0.99.0/24 10.1.1.2
1291: .Ed
1292: .Pp
1293: On the server:
1294: .Bd -literal -offset indent
1295: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1296: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1297: .Ed
1298: .Pp
1299: Client access may be more finely tuned via the
1300: .Pa /root/.ssh/authorized_keys
1301: file (see below) and the
1302: .Cm PermitRootLogin
1303: server option.
1.255 jmc 1304: The following entry would permit connections on
1.250 jmc 1305: .Xr tun 4
1.255 jmc 1306: device 1 from user
1.250 jmc 1307: .Dq jane
1.255 jmc 1308: and on tun device 2 from user
1.250 jmc 1309: .Dq john ,
1310: if
1311: .Cm PermitRootLogin
1312: is set to
1313: .Dq forced-commands-only :
1314: .Bd -literal -offset 2n
1315: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1316: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1317: .Ed
1318: .Pp
1.264 ray 1319: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1320: it may be more suited to temporary setups,
1321: such as for wireless VPNs.
1322: More permanent VPNs are better provided by tools such as
1323: .Xr ipsecctl 8
1324: and
1325: .Xr isakmpd 8 .
1.2 deraadt 1326: .Sh ENVIRONMENT
1327: .Nm
1.1 deraadt 1328: will normally set the following environment variables:
1.237 jmc 1329: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1330: .It Ev DISPLAY
1331: The
1332: .Ev DISPLAY
1.40 aaron 1333: variable indicates the location of the X11 server.
1.44 aaron 1334: It is automatically set by
1.2 deraadt 1335: .Nm
1336: to point to a value of the form
1.233 jmc 1337: .Dq hostname:n ,
1338: where
1339: .Dq hostname
1340: indicates the host where the shell runs, and
1341: .Sq n
1342: is an integer \*(Ge 1.
1.40 aaron 1343: .Nm
1344: uses this special value to forward X11 connections over the secure
1345: channel.
1.107 markus 1346: The user should normally not set
1347: .Ev DISPLAY
1348: explicitly, as that
1.1 deraadt 1349: will render the X11 connection insecure (and will require the user to
1350: manually copy any required authorization cookies).
1.2 deraadt 1351: .It Ev HOME
1.1 deraadt 1352: Set to the path of the user's home directory.
1.2 deraadt 1353: .It Ev LOGNAME
1354: Synonym for
1.12 aaron 1355: .Ev USER ;
1356: set for compatibility with systems that use this variable.
1.2 deraadt 1357: .It Ev MAIL
1.129 stevesk 1358: Set to the path of the user's mailbox.
1.40 aaron 1359: .It Ev PATH
1.2 deraadt 1360: Set to the default
1361: .Ev PATH ,
1362: as specified when compiling
1.234 jmc 1363: .Nm .
1.118 markus 1364: .It Ev SSH_ASKPASS
1365: If
1366: .Nm
1367: needs a passphrase, it will read the passphrase from the current
1368: terminal if it was run from a terminal.
1369: If
1370: .Nm
1371: does not have a terminal associated with it but
1372: .Ev DISPLAY
1373: and
1374: .Ev SSH_ASKPASS
1375: are set, it will execute the program specified by
1376: .Ev SSH_ASKPASS
1377: and open an X11 window to read the passphrase.
1378: This is particularly useful when calling
1379: .Nm
1380: from a
1.196 jmc 1381: .Pa .xsession
1.118 markus 1382: or related script.
1383: (Note that on some machines it
1384: may be necessary to redirect the input from
1385: .Pa /dev/null
1386: to make this work.)
1.18 markus 1387: .It Ev SSH_AUTH_SOCK
1.233 jmc 1388: Identifies the path of a
1389: .Ux Ns -domain
1390: socket used to communicate with the agent.
1.166 stevesk 1391: .It Ev SSH_CONNECTION
1392: Identifies the client and server ends of the connection.
1.40 aaron 1393: The variable contains
1.233 jmc 1394: four space-separated values: client IP address, client port number,
1395: server IP address, and server port number.
1.73 markus 1396: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1397: This variable contains the original command line if a forced command
1.73 markus 1398: is executed.
1399: It can be used to extract the original arguments.
1.2 deraadt 1400: .It Ev SSH_TTY
1.1 deraadt 1401: This is set to the name of the tty (path to the device) associated
1.40 aaron 1402: with the current shell or command.
1403: If the current session has no tty,
1.1 deraadt 1404: this variable is not set.
1.387 djm 1405: .It Ev SSH_TUNNEL
1406: Optionally set by
1407: .Xr sshd 8
1408: to contain the interface names assigned if tunnel forwarding was
1409: requested by the client.
1.385 djm 1410: .It Ev SSH_USER_AUTH
1411: Optionally set by
1412: .Xr sshd 8 ,
1413: this variable may contain a pathname to a file that lists the authentication
1414: methods successfully used when the session was established, including any
1415: public keys that were used.
1.2 deraadt 1416: .It Ev TZ
1.214 jmc 1417: This variable is set to indicate the present time zone if it
1.257 jmc 1418: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1419: on to new connections).
1.2 deraadt 1420: .It Ev USER
1.1 deraadt 1421: Set to the name of the user logging in.
1.2 deraadt 1422: .El
1423: .Pp
1.44 aaron 1424: Additionally,
1.2 deraadt 1425: .Nm
1.44 aaron 1426: reads
1.207 djm 1427: .Pa ~/.ssh/environment ,
1.2 deraadt 1428: and adds lines of the format
1429: .Dq VARNAME=value
1.233 jmc 1430: to the environment if the file exists and users are allowed to
1.161 marc 1431: change their environment.
1.176 jmc 1432: For more information, see the
1.161 marc 1433: .Cm PermitUserEnvironment
1.162 stevesk 1434: option in
1.161 marc 1435: .Xr sshd_config 5 .
1.2 deraadt 1436: .Sh FILES
1.236 jmc 1437: .Bl -tag -width Ds -compact
1.309 jmc 1438: .It Pa ~/.rhosts
1.240 jmc 1439: This file is used for host-based authentication (see above).
1.92 markus 1440: On some machines this file may need to be
1.240 jmc 1441: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1442: because
1.2 deraadt 1443: .Xr sshd 8
1.40 aaron 1444: reads it as root.
1445: Additionally, this file must be owned by the user,
1446: and must not have write permissions for anyone else.
1447: The recommended
1.1 deraadt 1448: permission for most machines is read/write for the user, and not
1449: accessible by others.
1.2 deraadt 1450: .Pp
1.309 jmc 1451: .It Pa ~/.shosts
1.240 jmc 1452: This file is used in exactly the same way as
1453: .Pa .rhosts ,
1454: but allows host-based authentication without permitting login with
1455: rlogin/rsh.
1.272 mcbride 1456: .Pp
1.309 jmc 1457: .It Pa ~/.ssh/
1.272 mcbride 1458: This directory is the default location for all user-specific configuration
1459: and authentication information.
1460: There is no general requirement to keep the entire contents of this directory
1461: secret, but the recommended permissions are read/write/execute for the user,
1462: and not accessible by others.
1.236 jmc 1463: .Pp
1.309 jmc 1464: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1465: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1466: that can be used for logging in as this user.
1.238 jmc 1467: The format of this file is described in the
1468: .Xr sshd 8
1469: manual page.
1470: This file is not highly sensitive, but the recommended
1471: permissions are read/write for the user, and not accessible by others.
1472: .Pp
1.309 jmc 1473: .It Pa ~/.ssh/config
1.238 jmc 1474: This is the per-user configuration file.
1475: The file format and configuration options are described in
1476: .Xr ssh_config 5 .
1477: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1478: read/write for the user, and not writable by others.
1.238 jmc 1479: .Pp
1.309 jmc 1480: .It Pa ~/.ssh/environment
1.239 jmc 1481: Contains additional definitions for environment variables; see
1482: .Sx ENVIRONMENT ,
1.238 jmc 1483: above.
1484: .Pp
1.309 jmc 1485: .It Pa ~/.ssh/id_dsa
1.310 djm 1486: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1487: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1488: .It Pa ~/.ssh/id_rsa
1.238 jmc 1489: Contains the private key for authentication.
1490: These files
1491: contain sensitive data and should be readable by the user but not
1492: accessible by others (read/write/execute).
1493: .Nm
1494: will simply ignore a private key file if it is accessible by others.
1495: It is possible to specify a passphrase when
1496: generating the key which will be used to encrypt the
1.389 djm 1497: sensitive part of this file using AES-128.
1.238 jmc 1498: .Pp
1.309 jmc 1499: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1500: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1501: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1502: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1503: Contains the public key for authentication.
1504: These files are not
1505: sensitive and can (but need not) be readable by anyone.
1506: .Pp
1.309 jmc 1507: .It Pa ~/.ssh/known_hosts
1.244 jmc 1508: Contains a list of host keys for all hosts the user has logged into
1509: that are not already in the systemwide list of known host keys.
1.238 jmc 1510: See
1.244 jmc 1511: .Xr sshd 8
1512: for further details of the format of this file.
1.238 jmc 1513: .Pp
1.309 jmc 1514: .It Pa ~/.ssh/rc
1.238 jmc 1515: Commands in this file are executed by
1516: .Nm
1.245 jmc 1517: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1518: started.
1519: See the
1520: .Xr sshd 8
1521: manual page for more information.
1522: .Pp
1.309 jmc 1523: .It Pa /etc/hosts.equiv
1.240 jmc 1524: This file is for host-based authentication (see above).
1525: It should only be writable by root.
1.236 jmc 1526: .Pp
1.309 jmc 1527: .It Pa /etc/shosts.equiv
1.240 jmc 1528: This file is used in exactly the same way as
1529: .Pa hosts.equiv ,
1530: but allows host-based authentication without permitting login with
1531: rlogin/rsh.
1.236 jmc 1532: .Pp
1.238 jmc 1533: .It Pa /etc/ssh/ssh_config
1534: Systemwide configuration file.
1535: The file format and configuration options are described in
1536: .Xr ssh_config 5 .
1537: .Pp
1.309 jmc 1538: .It Pa /etc/ssh/ssh_host_key
1539: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1540: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1541: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1542: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1543: These files contain the private parts of the host keys
1.245 jmc 1544: and are used for host-based authentication.
1.238 jmc 1545: .Pp
1.309 jmc 1546: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1547: Systemwide list of known host keys.
1548: This file should be prepared by the
1549: system administrator to contain the public host keys of all machines in the
1550: organization.
1.244 jmc 1551: It should be world-readable.
1552: See
1.238 jmc 1553: .Xr sshd 8
1.244 jmc 1554: for further details of the format of this file.
1.236 jmc 1555: .Pp
1.309 jmc 1556: .It Pa /etc/ssh/sshrc
1.1 deraadt 1557: Commands in this file are executed by
1.2 deraadt 1558: .Nm
1.245 jmc 1559: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1560: See the
1.2 deraadt 1561: .Xr sshd 8
1.1 deraadt 1562: manual page for more information.
1.58 itojun 1563: .El
1.312 jmc 1564: .Sh EXIT STATUS
1565: .Nm
1566: exits with the exit status of the remote command or with 255
1567: if an error occurred.
1.2 deraadt 1568: .Sh SEE ALSO
1569: .Xr scp 1 ,
1.83 djm 1570: .Xr sftp 1 ,
1.2 deraadt 1571: .Xr ssh-add 1 ,
1572: .Xr ssh-agent 1 ,
1573: .Xr ssh-keygen 1 ,
1.242 jmc 1574: .Xr ssh-keyscan 1 ,
1.250 jmc 1575: .Xr tun 4 ,
1.159 stevesk 1576: .Xr ssh_config 5 ,
1.160 naddy 1577: .Xr ssh-keysign 8 ,
1.87 itojun 1578: .Xr sshd 8
1.329 jmc 1579: .Sh STANDARDS
1.106 markus 1580: .Rs
1.329 jmc 1581: .%A S. Lehtinen
1582: .%A C. Lonvick
1583: .%D January 2006
1.256 jmc 1584: .%R RFC 4250
1.329 jmc 1585: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1586: .Re
1.329 jmc 1587: .Pp
1.256 jmc 1588: .Rs
1.329 jmc 1589: .%A T. Ylonen
1590: .%A C. Lonvick
1591: .%D January 2006
1.256 jmc 1592: .%R RFC 4251
1.329 jmc 1593: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1594: .Re
1.329 jmc 1595: .Pp
1.256 jmc 1596: .Rs
1.329 jmc 1597: .%A T. Ylonen
1598: .%A C. Lonvick
1599: .%D January 2006
1.256 jmc 1600: .%R RFC 4252
1.329 jmc 1601: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1602: .Re
1.329 jmc 1603: .Pp
1.256 jmc 1604: .Rs
1.329 jmc 1605: .%A T. Ylonen
1606: .%A C. Lonvick
1607: .%D January 2006
1.256 jmc 1608: .%R RFC 4253
1.329 jmc 1609: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1610: .Re
1.329 jmc 1611: .Pp
1.256 jmc 1612: .Rs
1.329 jmc 1613: .%A T. Ylonen
1614: .%A C. Lonvick
1615: .%D January 2006
1.256 jmc 1616: .%R RFC 4254
1.329 jmc 1617: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1618: .Re
1.329 jmc 1619: .Pp
1.256 jmc 1620: .Rs
1.329 jmc 1621: .%A J. Schlyter
1622: .%A W. Griffin
1623: .%D January 2006
1.256 jmc 1624: .%R RFC 4255
1.329 jmc 1625: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1626: .Re
1.329 jmc 1627: .Pp
1.256 jmc 1628: .Rs
1.329 jmc 1629: .%A F. Cusack
1630: .%A M. Forssen
1631: .%D January 2006
1.256 jmc 1632: .%R RFC 4256
1.329 jmc 1633: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1634: .Re
1.329 jmc 1635: .Pp
1.256 jmc 1636: .Rs
1.329 jmc 1637: .%A J. Galbraith
1638: .%A P. Remaker
1639: .%D January 2006
1.256 jmc 1640: .%R RFC 4335
1.329 jmc 1641: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1642: .Re
1.329 jmc 1643: .Pp
1.256 jmc 1644: .Rs
1.329 jmc 1645: .%A M. Bellare
1646: .%A T. Kohno
1647: .%A C. Namprempre
1648: .%D January 2006
1.256 jmc 1649: .%R RFC 4344
1.329 jmc 1650: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1651: .Re
1.329 jmc 1652: .Pp
1.256 jmc 1653: .Rs
1.329 jmc 1654: .%A B. Harris
1655: .%D January 2006
1.256 jmc 1656: .%R RFC 4345
1.329 jmc 1657: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1658: .Re
1.329 jmc 1659: .Pp
1.258 djm 1660: .Rs
1.329 jmc 1661: .%A M. Friedl
1662: .%A N. Provos
1663: .%A W. Simpson
1664: .%D March 2006
1.258 djm 1665: .%R RFC 4419
1.329 jmc 1666: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1667: .Re
1.329 jmc 1668: .Pp
1.266 markus 1669: .Rs
1.329 jmc 1670: .%A J. Galbraith
1671: .%A R. Thayer
1672: .%D November 2006
1.266 markus 1673: .%R RFC 4716
1.329 jmc 1674: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1675: .Re
1.329 jmc 1676: .Pp
1.313 djm 1677: .Rs
1.329 jmc 1678: .%A D. Stebila
1679: .%A J. Green
1680: .%D December 2009
1.313 djm 1681: .%R RFC 5656
1.329 jmc 1682: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1683: .Re
1.329 jmc 1684: .Pp
1.274 grunk 1685: .Rs
1686: .%A A. Perrig
1687: .%A D. Song
1688: .%D 1999
1.329 jmc 1689: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1690: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1691: .Re
1.173 jmc 1692: .Sh AUTHORS
1693: OpenSSH is a derivative of the original and free
1694: ssh 1.2.12 release by Tatu Ylonen.
1695: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1696: Theo de Raadt and Dug Song
1697: removed many bugs, re-added newer features and
1698: created OpenSSH.
1699: Markus Friedl contributed the support for SSH
1700: protocol versions 1.5 and 2.0.