Annotation of src/usr.bin/ssh/ssh.1, Revision 1.389
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: .\"
1.389 ! djm 36: .\" $OpenBSD: ssh.1,v 1.388 2017/10/25 06:19:46 jmc Exp $
! 37: .Dd $Mdocdate: October 25 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
1.377 djm 45: .Op Fl 46AaCfGgKkMNnqsTtVvXxYy
1.108 markus 46: .Op Fl b Ar bind_address
1.51 markus 47: .Op Fl c Ar cipher_spec
1.306 jmc 48: .Op Fl D Oo Ar bind_address : Oc Ns Ar port
1.331 dtucker 49: .Op Fl E Ar log_file
1.2 deraadt 50: .Op Fl e Ar escape_char
1.176 jmc 51: .Op Fl F Ar configfile
1.292 jmc 52: .Op Fl I Ar pkcs11
1.2 deraadt 53: .Op Fl i Ar identity_file
1.386 millert 54: .Op Fl J Ar destination
1.360 millert 55: .Op Fl L Ar address
1.176 jmc 56: .Op Fl l Ar login_name
57: .Op Fl m Ar mac_spec
1.198 djm 58: .Op Fl O Ar ctl_cmd
1.176 jmc 59: .Op Fl o Ar option
60: .Op Fl p Ar port
1.362 djm 61: .Op Fl Q Ar query_option
1.360 millert 62: .Op Fl R Ar address
1.198 djm 63: .Op Fl S Ar ctl_path
1.290 dtucker 64: .Op Fl W Ar host : Ns Ar port
1.306 jmc 65: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.386 millert 66: .Ar destination
1.2 deraadt 67: .Op Ar command
1.44 aaron 68: .Sh DESCRIPTION
1.2 deraadt 69: .Nm
1.96 deraadt 70: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 71: executing commands on a remote machine.
1.365 mmcc 72: It is intended to provide secure encrypted communications between
1.40 aaron 73: two untrusted hosts over an insecure network.
1.348 millert 74: X11 connections, arbitrary TCP ports and
75: .Ux Ns -domain
76: sockets can also be forwarded over the secure channel.
1.2 deraadt 77: .Pp
78: .Nm
1.44 aaron 79: connects and logs into the specified
1.388 jmc 80: .Ar destination ,
1.386 millert 81: which may be specified as either
1.388 jmc 82: .Sm off
83: .Oo user @ Oc hostname
84: .Sm on
85: or a URI of the form
86: .Sm off
87: .No ssh:// Oo user @ Oc hostname Op : port .
88: .Sm on
1.1 deraadt 89: The user must prove
1.49 markus 90: his/her identity to the remote machine using one of several methods
1.365 mmcc 91: (see below).
1.49 markus 92: .Pp
1.386 millert 93: If a
1.176 jmc 94: .Ar command
95: is specified,
1.219 jmc 96: it is executed on the remote host instead of a login shell.
1.2 deraadt 97: .Pp
1.218 jmc 98: The options are as follows:
1.361 millert 99: .Pp
1.360 millert 100: .Bl -tag -width Ds -compact
1.218 jmc 101: .It Fl 4
102: Forces
1.2 deraadt 103: .Nm
1.218 jmc 104: to use IPv4 addresses only.
1.360 millert 105: .Pp
1.218 jmc 106: .It Fl 6
107: Forces
1.2 deraadt 108: .Nm
1.218 jmc 109: to use IPv6 addresses only.
1.360 millert 110: .Pp
1.218 jmc 111: .It Fl A
112: Enables forwarding of the authentication agent connection.
113: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 114: .Pp
1.218 jmc 115: Agent forwarding should be enabled with caution.
116: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 117: (for the agent's
1.286 sobrado 118: .Ux Ns -domain
119: socket) can access the local agent through the forwarded connection.
1.218 jmc 120: An attacker cannot obtain key material from the agent,
121: however they can perform operations on the keys that enable them to
122: authenticate using the identities loaded into the agent.
1.360 millert 123: .Pp
1.218 jmc 124: .It Fl a
125: Disables forwarding of the authentication agent connection.
1.360 millert 126: .Pp
1.218 jmc 127: .It Fl b Ar bind_address
128: Use
129: .Ar bind_address
130: on the local machine as the source address
131: of the connection.
132: Only useful on systems with more than one address.
1.360 millert 133: .Pp
1.218 jmc 134: .It Fl C
135: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 136: data for forwarded X11, TCP and
137: .Ux Ns -domain
138: connections).
1.218 jmc 139: The compression algorithm is the same used by
1.377 djm 140: .Xr gzip 1 .
1.218 jmc 141: Compression is desirable on modem lines and other
142: slow connections, but will only slow down things on fast networks.
143: The default value can be set on a host-by-host basis in the
144: configuration files; see the
145: .Cm Compression
146: option.
1.360 millert 147: .Pp
1.218 jmc 148: .It Fl c Ar cipher_spec
149: Selects the cipher specification for encrypting the session.
150: .Ar cipher_spec
151: is a comma-separated list of ciphers
152: listed in order of preference.
1.283 jmc 153: See the
154: .Cm Ciphers
1.307 dtucker 155: keyword in
156: .Xr ssh_config 5
157: for more information.
1.360 millert 158: .Pp
1.218 jmc 159: .It Fl D Xo
160: .Sm off
161: .Oo Ar bind_address : Oc
162: .Ar port
163: .Sm on
164: .Xc
165: Specifies a local
166: .Dq dynamic
167: application-level port forwarding.
168: This works by allocating a socket to listen to
169: .Ar port
170: on the local side, optionally bound to the specified
171: .Ar bind_address .
172: Whenever a connection is made to this port, the
173: connection is forwarded over the secure channel, and the application
174: protocol is then used to determine where to connect to from the
175: remote machine.
176: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 177: .Nm
1.218 jmc 178: will act as a SOCKS server.
179: Only root can forward privileged ports.
180: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 181: .Pp
1.308 djm 182: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 183: Only the superuser can forward privileged ports.
184: By default, the local port is bound in accordance with the
185: .Cm GatewayPorts
186: setting.
187: However, an explicit
188: .Ar bind_address
189: may be used to bind the connection to a specific address.
190: The
191: .Ar bind_address
192: of
193: .Dq localhost
194: indicates that the listening port be bound for local use only, while an
195: empty address or
196: .Sq *
197: indicates that the port should be available from all interfaces.
1.360 millert 198: .Pp
1.331 dtucker 199: .It Fl E Ar log_file
200: Append debug logs to
201: .Ar log_file
202: instead of standard error.
1.360 millert 203: .Pp
1.229 jmc 204: .It Fl e Ar escape_char
1.218 jmc 205: Sets the escape character for sessions with a pty (default:
206: .Ql ~ ) .
207: The escape character is only recognized at the beginning of a line.
208: The escape character followed by a dot
209: .Pq Ql \&.
210: closes the connection;
211: followed by control-Z suspends the connection;
212: and followed by itself sends the escape character once.
213: Setting the character to
1.2 deraadt 214: .Dq none
1.218 jmc 215: disables any escapes and makes the session fully transparent.
1.360 millert 216: .Pp
1.218 jmc 217: .It Fl F Ar configfile
218: Specifies an alternative per-user configuration file.
219: If a configuration file is given on the command line,
220: the system-wide configuration file
221: .Pq Pa /etc/ssh/ssh_config
222: will be ignored.
223: The default for the per-user configuration file is
224: .Pa ~/.ssh/config .
1.360 millert 225: .Pp
1.218 jmc 226: .It Fl f
227: Requests
228: .Nm
229: to go to background just before command execution.
230: This is useful if
1.176 jmc 231: .Nm
1.218 jmc 232: is going to ask for passwords or passphrases, but the user
233: wants it in the background.
234: This implies
235: .Fl n .
236: The recommended way to start X11 programs at a remote site is with
237: something like
238: .Ic ssh -f host xterm .
1.277 djm 239: .Pp
240: If the
241: .Cm ExitOnForwardFailure
242: configuration option is set to
243: .Dq yes ,
244: then a client started with
245: .Fl f
246: will wait for all remote port forwards to be successfully established
247: before placing itself in the background.
1.360 millert 248: .Pp
1.350 djm 249: .It Fl G
250: Causes
251: .Nm
252: to print its configuration after evaluating
253: .Cm Host
254: and
255: .Cm Match
256: blocks and exit.
1.360 millert 257: .Pp
1.218 jmc 258: .It Fl g
259: Allows remote hosts to connect to local forwarded ports.
1.346 djm 260: If used on a multiplexed connection, then this option must be specified
261: on the master process.
1.360 millert 262: .Pp
1.291 markus 263: .It Fl I Ar pkcs11
1.294 jmc 264: Specify the PKCS#11 shared library
1.176 jmc 265: .Nm
1.293 markus 266: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 267: private RSA key.
1.360 millert 268: .Pp
1.218 jmc 269: .It Fl i Ar identity_file
270: Selects a file from which the identity (private key) for
1.310 djm 271: public key authentication is read.
1.218 jmc 272: The default is
1.310 djm 273: .Pa ~/.ssh/id_dsa ,
1.343 naddy 274: .Pa ~/.ssh/id_ecdsa ,
275: .Pa ~/.ssh/id_ed25519
1.310 djm 276: and
1.381 naddy 277: .Pa ~/.ssh/id_rsa .
1.218 jmc 278: Identity files may also be specified on
279: a per-host basis in the configuration file.
280: It is possible to have multiple
281: .Fl i
282: options (and multiple identities specified in
283: configuration files).
1.364 jmc 284: If no certificates have been explicitly specified by the
1.363 djm 285: .Cm CertificateFile
286: directive,
1.302 djm 287: .Nm
288: will also try to load certificate information from the filename obtained
289: by appending
290: .Pa -cert.pub
291: to identity filenames.
1.360 millert 292: .Pp
1.386 millert 293: .It Fl J Ar destination
1.375 djm 294: Connect to the target host by first making a
295: .Nm
1.386 millert 296: connection to the jump host described by
297: .Ar destination
1.376 jmc 298: and then establishing a TCP forwarding to the ultimate destination from
1.375 djm 299: there.
300: Multiple jump hops may be specified separated by comma characters.
301: This is a shortcut to specify a
302: .Cm ProxyJump
303: configuration directive.
304: .Pp
1.269 djm 305: .It Fl K
306: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
307: credentials to the server.
1.360 millert 308: .Pp
1.218 jmc 309: .It Fl k
310: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 311: .Pp
1.218 jmc 312: .It Fl L Xo
313: .Sm off
314: .Oo Ar bind_address : Oc
315: .Ar port : host : hostport
316: .Sm on
317: .Xc
1.360 millert 318: .It Fl L Xo
319: .Sm off
320: .Oo Ar bind_address : Oc
321: .Ar port : remote_socket
322: .Sm on
323: .Xc
324: .It Fl L Xo
325: .Sm off
326: .Ar local_socket : host : hostport
327: .Sm on
328: .Xc
329: .It Fl L Xo
330: .Sm off
331: .Ar local_socket : remote_socket
332: .Sm on
333: .Xc
334: Specifies that connections to the given TCP port or Unix socket on the local
335: (client) host are to be forwarded to the given host and port, or Unix socket,
336: on the remote side.
337: This works by allocating a socket to listen to either a TCP
1.218 jmc 338: .Ar port
339: on the local side, optionally bound to the specified
1.360 millert 340: .Ar bind_address ,
341: or to a Unix socket.
342: Whenever a connection is made to the local port or socket, the
1.218 jmc 343: connection is forwarded over the secure channel, and a connection is
1.360 millert 344: made to either
1.218 jmc 345: .Ar host
346: port
1.360 millert 347: .Ar hostport ,
348: or the Unix socket
349: .Ar remote_socket ,
1.218 jmc 350: from the remote machine.
1.360 millert 351: .Pp
1.218 jmc 352: Port forwardings can also be specified in the configuration file.
1.360 millert 353: Only the superuser can forward privileged ports.
1.308 djm 354: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 355: .Pp
1.218 jmc 356: By default, the local port is bound in accordance with the
357: .Cm GatewayPorts
358: setting.
359: However, an explicit
360: .Ar bind_address
361: may be used to bind the connection to a specific address.
1.2 deraadt 362: The
1.218 jmc 363: .Ar bind_address
364: of
365: .Dq localhost
366: indicates that the listening port be bound for local use only, while an
367: empty address or
368: .Sq *
369: indicates that the port should be available from all interfaces.
1.360 millert 370: .Pp
1.218 jmc 371: .It Fl l Ar login_name
372: Specifies the user to log in as on the remote machine.
373: This also may be specified on a per-host basis in the configuration file.
1.360 millert 374: .Pp
1.218 jmc 375: .It Fl M
376: Places the
377: .Nm
378: client into
379: .Dq master
380: mode for connection sharing.
1.231 stevesk 381: Multiple
382: .Fl M
383: options places
384: .Nm
385: into
386: .Dq master
387: mode with confirmation required before slave connections are accepted.
1.218 jmc 388: Refer to the description of
389: .Cm ControlMaster
390: in
391: .Xr ssh_config 5
392: for details.
1.360 millert 393: .Pp
1.218 jmc 394: .It Fl m Ar mac_spec
1.369 jmc 395: A comma-separated list of MAC (message authentication code) algorithms,
396: specified in order of preference.
1.218 jmc 397: See the
398: .Cm MACs
399: keyword for more information.
1.360 millert 400: .Pp
1.218 jmc 401: .It Fl N
402: Do not execute a remote command.
1.369 jmc 403: This is useful for just forwarding ports.
1.360 millert 404: .Pp
1.218 jmc 405: .It Fl n
406: Redirects stdin from
407: .Pa /dev/null
408: (actually, prevents reading from stdin).
409: This must be used when
1.2 deraadt 410: .Nm
1.218 jmc 411: is run in the background.
412: A common trick is to use this to run X11 programs on a remote machine.
413: For example,
414: .Ic ssh -n shadows.cs.hut.fi emacs &
415: will start an emacs on shadows.cs.hut.fi, and the X11
416: connection will be automatically forwarded over an encrypted channel.
417: The
1.2 deraadt 418: .Nm
1.218 jmc 419: program will be put in the background.
420: (This does not work if
1.2 deraadt 421: .Nm
1.218 jmc 422: needs to ask for a password or passphrase; see also the
423: .Fl f
424: option.)
1.360 millert 425: .Pp
1.218 jmc 426: .It Fl O Ar ctl_cmd
427: Control an active connection multiplexing master process.
428: When the
429: .Fl O
430: option is specified, the
431: .Ar ctl_cmd
432: argument is interpreted and passed to the master process.
433: Valid commands are:
434: .Dq check
1.305 markus 435: (check that the master process is running),
436: .Dq forward
1.318 jmc 437: (request forwardings without command execution),
1.323 okan 438: .Dq cancel
439: (cancel forwardings),
1.218 jmc 440: .Dq exit
1.318 jmc 441: (request the master to exit), and
1.317 djm 442: .Dq stop
443: (request the master to stop accepting further multiplexing requests).
1.360 millert 444: .Pp
1.218 jmc 445: .It Fl o Ar option
446: Can be used to give options in the format used in the configuration file.
447: This is useful for specifying options for which there is no separate
448: command-line flag.
449: For full details of the options listed below, and their possible values, see
450: .Xr ssh_config 5 .
1.2 deraadt 451: .Pp
1.218 jmc 452: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 453: .It AddKeysToAgent
1.218 jmc 454: .It AddressFamily
455: .It BatchMode
456: .It BindAddress
1.338 djm 457: .It CanonicalDomains
1.339 djm 458: .It CanonicalizeFallbackLocal
459: .It CanonicalizeHostname
460: .It CanonicalizeMaxDots
461: .It CanonicalizePermittedCNAMEs
1.363 djm 462: .It CertificateFile
1.218 jmc 463: .It ChallengeResponseAuthentication
464: .It CheckHostIP
465: .It Ciphers
466: .It ClearAllForwardings
467: .It Compression
1.380 naddy 468: .It ConnectionAttempts
1.218 jmc 469: .It ConnectTimeout
470: .It ControlMaster
471: .It ControlPath
1.321 djm 472: .It ControlPersist
1.218 jmc 473: .It DynamicForward
474: .It EscapeChar
1.263 markus 475: .It ExitOnForwardFailure
1.353 jmc 476: .It FingerprintHash
1.218 jmc 477: .It ForwardAgent
478: .It ForwardX11
1.321 djm 479: .It ForwardX11Timeout
1.218 jmc 480: .It ForwardX11Trusted
481: .It GatewayPorts
482: .It GlobalKnownHostsFile
483: .It GSSAPIAuthentication
484: .It GSSAPIDelegateCredentials
485: .It HashKnownHosts
486: .It Host
487: .It HostbasedAuthentication
1.355 djm 488: .It HostbasedKeyTypes
1.218 jmc 489: .It HostKeyAlgorithms
490: .It HostKeyAlias
491: .It HostName
1.374 jmc 492: .It IdentitiesOnly
1.371 markus 493: .It IdentityAgent
1.218 jmc 494: .It IdentityFile
1.370 djm 495: .It Include
1.316 jmc 496: .It IPQoS
1.321 djm 497: .It KbdInteractiveAuthentication
1.218 jmc 498: .It KbdInteractiveDevices
1.314 jmc 499: .It KexAlgorithms
1.218 jmc 500: .It LocalCommand
501: .It LocalForward
502: .It LogLevel
503: .It MACs
1.337 jmc 504: .It Match
1.218 jmc 505: .It NoHostAuthenticationForLocalhost
506: .It NumberOfPasswordPrompts
507: .It PasswordAuthentication
508: .It PermitLocalCommand
1.291 markus 509: .It PKCS11Provider
1.218 jmc 510: .It Port
511: .It PreferredAuthentications
512: .It ProxyCommand
1.375 djm 513: .It ProxyJump
1.336 jmc 514: .It ProxyUseFdpass
1.359 markus 515: .It PubkeyAcceptedKeyTypes
1.218 jmc 516: .It PubkeyAuthentication
1.251 dtucker 517: .It RekeyLimit
1.382 bluhm 518: .It RemoteCommand
1.218 jmc 519: .It RemoteForward
1.319 jmc 520: .It RequestTTY
1.218 jmc 521: .It SendEnv
522: .It ServerAliveInterval
523: .It ServerAliveCountMax
1.347 jmc 524: .It StreamLocalBindMask
525: .It StreamLocalBindUnlink
1.218 jmc 526: .It StrictHostKeyChecking
527: .It TCPKeepAlive
528: .It Tunnel
529: .It TunnelDevice
1.354 djm 530: .It UpdateHostKeys
1.218 jmc 531: .It UsePrivilegedPort
532: .It User
533: .It UserKnownHostsFile
534: .It VerifyHostKeyDNS
1.276 jmc 535: .It VisualHostKey
1.218 jmc 536: .It XAuthLocation
537: .El
1.360 millert 538: .Pp
1.218 jmc 539: .It Fl p Ar port
540: Port to connect to on the remote host.
541: This can be specified on a
542: per-host basis in the configuration file.
1.360 millert 543: .Pp
1.362 djm 544: .It Fl Q Ar query_option
1.332 djm 545: Queries
546: .Nm
1.341 deraadt 547: for the algorithms supported for the specified version 2.
548: The available features are:
549: .Ar cipher
1.332 djm 550: (supported symmetric ciphers),
1.341 deraadt 551: .Ar cipher-auth
1.340 djm 552: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 553: .Ar mac
1.332 djm 554: (supported message integrity codes),
1.341 deraadt 555: .Ar kex
1.332 djm 556: (key exchange algorithms),
1.341 deraadt 557: .Ar key
1.362 djm 558: (key types),
559: .Ar key-cert
560: (certificate key types),
561: .Ar key-plain
562: (non-certificate key types), and
1.356 djm 563: .Ar protocol-version
564: (supported SSH protocol versions).
1.360 millert 565: .Pp
1.218 jmc 566: .It Fl q
567: Quiet mode.
1.271 djm 568: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 569: .Pp
1.218 jmc 570: .It Fl R Xo
571: .Sm off
572: .Oo Ar bind_address : Oc
573: .Ar port : host : hostport
574: .Sm on
575: .Xc
1.360 millert 576: .It Fl R Xo
577: .Sm off
578: .Oo Ar bind_address : Oc
579: .Ar port : local_socket
580: .Sm on
581: .Xc
582: .It Fl R Xo
583: .Sm off
584: .Ar remote_socket : host : hostport
585: .Sm on
586: .Xc
587: .It Fl R Xo
588: .Sm off
589: .Ar remote_socket : local_socket
590: .Sm on
591: .Xc
1.384 markus 592: .It Fl R Xo
593: .Sm off
594: .Oo Ar bind_address : Oc
595: .Ar port
596: .Sm on
597: .Xc
1.360 millert 598: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 599: (server) host are to be forwarded to the local side.
600: .Pp
1.360 millert 601: This works by allocating a socket to listen to either a TCP
1.218 jmc 602: .Ar port
1.360 millert 603: or to a Unix socket on the remote side.
604: Whenever a connection is made to this port or Unix socket, the
605: connection is forwarded over the secure channel, and a connection
1.384 markus 606: is made from the local machine to either an explicit destination specified by
1.218 jmc 607: .Ar host
608: port
1.360 millert 609: .Ar hostport ,
610: or
611: .Ar local_socket ,
1.384 markus 612: or, if no explicit destination was specified,
613: .Nm
614: will act as a SOCKS 4/5 proxy and forward connections to the destinations
615: requested by the remote SOCKS client.
1.2 deraadt 616: .Pp
1.218 jmc 617: Port forwardings can also be specified in the configuration file.
618: Privileged ports can be forwarded only when
619: logging in as root on the remote machine.
1.324 jmc 620: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 621: .Pp
1.360 millert 622: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 623: interface only.
1.280 tobias 624: This may be overridden by specifying a
1.218 jmc 625: .Ar bind_address .
626: An empty
627: .Ar bind_address ,
628: or the address
629: .Ql * ,
630: indicates that the remote socket should listen on all interfaces.
631: Specifying a remote
632: .Ar bind_address
633: will only succeed if the server's
634: .Cm GatewayPorts
635: option is enabled (see
636: .Xr sshd_config 5 ) .
1.281 djm 637: .Pp
638: If the
639: .Ar port
640: argument is
1.282 djm 641: .Ql 0 ,
1.281 djm 642: the listen port will be dynamically allocated on the server and reported
643: to the client at run time.
1.305 markus 644: When used together with
645: .Ic -O forward
646: the allocated port will be printed to the standard output.
1.360 millert 647: .Pp
1.218 jmc 648: .It Fl S Ar ctl_path
1.304 jmc 649: Specifies the location of a control socket for connection sharing,
1.303 djm 650: or the string
651: .Dq none
652: to disable connection sharing.
1.218 jmc 653: Refer to the description of
654: .Cm ControlPath
655: and
656: .Cm ControlMaster
657: in
658: .Xr ssh_config 5
659: for details.
1.360 millert 660: .Pp
1.218 jmc 661: .It Fl s
662: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 663: Subsystems facilitate the use of SSH
664: as a secure transport for other applications (e.g.\&
1.218 jmc 665: .Xr sftp 1 ) .
666: The subsystem is specified as the remote command.
1.360 millert 667: .Pp
1.218 jmc 668: .It Fl T
1.357 dtucker 669: Disable pseudo-terminal allocation.
1.360 millert 670: .Pp
1.218 jmc 671: .It Fl t
1.357 dtucker 672: Force pseudo-terminal allocation.
1.218 jmc 673: This can be used to execute arbitrary
674: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 675: e.g. when implementing menu services.
1.218 jmc 676: Multiple
677: .Fl t
678: options force tty allocation, even if
1.194 jakob 679: .Nm
1.218 jmc 680: has no local tty.
1.360 millert 681: .Pp
1.218 jmc 682: .It Fl V
683: Display the version number and exit.
1.360 millert 684: .Pp
1.218 jmc 685: .It Fl v
686: Verbose mode.
687: Causes
1.176 jmc 688: .Nm
1.218 jmc 689: to print debugging messages about its progress.
690: This is helpful in
691: debugging connection, authentication, and configuration problems.
692: Multiple
693: .Fl v
694: options increase the verbosity.
695: The maximum is 3.
1.360 millert 696: .Pp
1.290 dtucker 697: .It Fl W Ar host : Ns Ar port
698: Requests that standard input and output on the client be forwarded to
699: .Ar host
700: on
701: .Ar port
702: over the secure channel.
703: Implies
704: .Fl N ,
705: .Fl T ,
706: .Cm ExitOnForwardFailure
707: and
1.373 jmc 708: .Cm ClearAllForwardings ,
709: though these can be overridden in the configuration file or using
1.372 dtucker 710: .Fl o
711: command line options.
1.360 millert 712: .Pp
1.261 stevesk 713: .It Fl w Xo
714: .Ar local_tun Ns Op : Ns Ar remote_tun
715: .Xc
716: Requests
717: tunnel
718: device forwarding with the specified
1.218 jmc 719: .Xr tun 4
1.261 stevesk 720: devices between the client
721: .Pq Ar local_tun
722: and the server
723: .Pq Ar remote_tun .
724: .Pp
1.228 jmc 725: The devices may be specified by numerical ID or the keyword
726: .Dq any ,
727: which uses the next available tunnel device.
1.261 stevesk 728: If
729: .Ar remote_tun
730: is not specified, it defaults to
731: .Dq any .
1.228 jmc 732: See also the
1.218 jmc 733: .Cm Tunnel
1.261 stevesk 734: and
735: .Cm TunnelDevice
736: directives in
1.218 jmc 737: .Xr ssh_config 5 .
1.261 stevesk 738: If the
739: .Cm Tunnel
740: directive is unset, it is set to the default tunnel mode, which is
741: .Dq point-to-point .
1.360 millert 742: .Pp
1.218 jmc 743: .It Fl X
744: Enables X11 forwarding.
1.54 markus 745: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 746: .Pp
1.218 jmc 747: X11 forwarding should be enabled with caution.
1.168 jmc 748: Users with the ability to bypass file permissions on the remote host
1.218 jmc 749: (for the user's X authorization database)
750: can access the local X11 display through the forwarded connection.
751: An attacker may then be able to perform activities such as keystroke monitoring.
752: .Pp
753: For this reason, X11 forwarding is subjected to X11 SECURITY extension
754: restrictions by default.
755: Please refer to the
756: .Nm
757: .Fl Y
758: option and the
759: .Cm ForwardX11Trusted
760: directive in
761: .Xr ssh_config 5
762: for more information.
1.360 millert 763: .Pp
1.218 jmc 764: .It Fl x
765: Disables X11 forwarding.
1.360 millert 766: .Pp
1.218 jmc 767: .It Fl Y
768: Enables trusted X11 forwarding.
769: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
770: controls.
1.360 millert 771: .Pp
1.278 djm 772: .It Fl y
773: Send log information using the
774: .Xr syslog 3
775: system module.
776: By default this information is sent to stderr.
1.218 jmc 777: .El
1.224 jmc 778: .Pp
779: .Nm
780: may additionally obtain configuration data from
781: a per-user configuration file and a system-wide configuration file.
782: The file format and configuration options are described in
783: .Xr ssh_config 5 .
1.222 jmc 784: .Sh AUTHENTICATION
1.377 djm 785: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 786: .Pp
787: The methods available for authentication are:
1.260 jmc 788: GSSAPI-based authentication,
1.222 jmc 789: host-based authentication,
790: public key authentication,
791: challenge-response authentication,
792: and password authentication.
793: Authentication methods are tried in the order specified above,
1.369 jmc 794: though
795: .Cm PreferredAuthentications
796: can be used to change the default order.
1.222 jmc 797: .Pp
798: Host-based authentication works as follows:
1.218 jmc 799: If the machine the user logs in from is listed in
800: .Pa /etc/hosts.equiv
801: or
802: .Pa /etc/shosts.equiv
803: on the remote machine, and the user names are
804: the same on both sides, or if the files
805: .Pa ~/.rhosts
806: or
807: .Pa ~/.shosts
808: exist in the user's home directory on the
809: remote machine and contain a line containing the name of the client
810: machine and the name of the user on that machine, the user is
1.222 jmc 811: considered for login.
812: Additionally, the server
813: .Em must
814: be able to verify the client's
815: host key (see the description of
1.218 jmc 816: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 817: and
1.222 jmc 818: .Pa ~/.ssh/known_hosts ,
819: below)
820: for login to be permitted.
1.218 jmc 821: This authentication method closes security holes due to IP
1.222 jmc 822: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 823: [Note to the administrator:
824: .Pa /etc/hosts.equiv ,
825: .Pa ~/.rhosts ,
826: and the rlogin/rsh protocol in general, are inherently insecure and should be
827: disabled if security is desired.]
1.189 dtucker 828: .Pp
1.222 jmc 829: Public key authentication works as follows:
830: The scheme is based on public-key cryptography,
831: using cryptosystems
832: where encryption and decryption are done using separate keys,
833: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 834: The idea is that each user creates a public/private
835: key pair for authentication purposes.
836: The server knows the public key, and only the user knows the private key.
1.222 jmc 837: .Nm
838: implements public key authentication protocol automatically,
1.349 sobrado 839: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 840: The HISTORY section of
1.222 jmc 841: .Xr ssl 8
1.311 jmc 842: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 843: .Pp
1.218 jmc 844: The file
845: .Pa ~/.ssh/authorized_keys
846: lists the public keys that are permitted for logging in.
847: When the user logs in, the
1.2 deraadt 848: .Nm
1.218 jmc 849: program tells the server which key pair it would like to use for
850: authentication.
1.222 jmc 851: The client proves that it has access to the private key
852: and the server checks that the corresponding public key
853: is authorized to accept the account.
1.383 djm 854: .Pp
855: The server may inform the client of errors that prevented public key
856: authentication from succeeding after authentication completes using a
857: different method.
858: These may be viewed by increasing the
859: .Cm LogLevel
860: to
861: .Cm DEBUG
862: or higher (e.g. by using the
863: .Fl v
864: flag).
1.218 jmc 865: .Pp
1.222 jmc 866: The user creates his/her key pair by running
1.218 jmc 867: .Xr ssh-keygen 1 .
868: This stores the private key in
1.222 jmc 869: .Pa ~/.ssh/id_dsa
1.369 jmc 870: (DSA),
1.310 djm 871: .Pa ~/.ssh/id_ecdsa
1.369 jmc 872: (ECDSA),
1.343 naddy 873: .Pa ~/.ssh/id_ed25519
1.369 jmc 874: (Ed25519),
1.222 jmc 875: or
876: .Pa ~/.ssh/id_rsa
1.369 jmc 877: (RSA)
1.218 jmc 878: and stores the public key in
1.222 jmc 879: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 880: (DSA),
1.310 djm 881: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 882: (ECDSA),
1.343 naddy 883: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 884: (Ed25519),
1.222 jmc 885: or
886: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 887: (RSA)
1.218 jmc 888: in the user's home directory.
1.222 jmc 889: The user should then copy the public key
1.218 jmc 890: to
891: .Pa ~/.ssh/authorized_keys
1.222 jmc 892: in his/her home directory on the remote machine.
893: The
1.218 jmc 894: .Pa authorized_keys
895: file corresponds to the conventional
896: .Pa ~/.rhosts
897: file, and has one key
1.222 jmc 898: per line, though the lines can be very long.
1.218 jmc 899: After this, the user can log in without giving the password.
900: .Pp
1.301 jmc 901: A variation on public key authentication
902: is available in the form of certificate authentication:
903: instead of a set of public/private keys,
904: signed certificates are used.
905: This has the advantage that a single trusted certification authority
906: can be used in place of many public/private keys.
1.333 jmc 907: See the CERTIFICATES section of
1.301 jmc 908: .Xr ssh-keygen 1
909: for more information.
910: .Pp
911: The most convenient way to use public key or certificate authentication
912: may be with an authentication agent.
1.218 jmc 913: See
914: .Xr ssh-agent 1
1.366 jcs 915: and (optionally) the
916: .Cm AddKeysToAgent
917: directive in
918: .Xr ssh_config 5
1.218 jmc 919: for more information.
920: .Pp
1.222 jmc 921: Challenge-response authentication works as follows:
922: The server sends an arbitrary
923: .Qq challenge
924: text, and prompts for a response.
925: Examples of challenge-response authentication include
1.335 jmc 926: .Bx
927: Authentication (see
1.222 jmc 928: .Xr login.conf 5 )
1.335 jmc 929: and PAM (some
930: .Pf non- Ox
931: systems).
1.222 jmc 932: .Pp
933: Finally, if other authentication methods fail,
1.218 jmc 934: .Nm
935: prompts the user for a password.
936: The password is sent to the remote
937: host for checking; however, since all communications are encrypted,
938: the password cannot be seen by someone listening on the network.
1.232 jmc 939: .Pp
940: .Nm
941: automatically maintains and checks a database containing
942: identification for all hosts it has ever been used with.
943: Host keys are stored in
944: .Pa ~/.ssh/known_hosts
945: in the user's home directory.
946: Additionally, the file
947: .Pa /etc/ssh/ssh_known_hosts
948: is automatically checked for known hosts.
949: Any new hosts are automatically added to the user's file.
950: If a host's identification ever changes,
951: .Nm
952: warns about this and disables password authentication to prevent
953: server spoofing or man-in-the-middle attacks,
954: which could otherwise be used to circumvent the encryption.
955: The
956: .Cm StrictHostKeyChecking
957: option can be used to control logins to machines whose
958: host key is not known or has changed.
959: .Pp
1.218 jmc 960: When the user's identity has been accepted by the server, the server
1.357 dtucker 961: either executes the given command in a non-interactive session or,
962: if no command has been specified, logs into the machine and gives
963: the user a normal shell as an interactive session.
1.218 jmc 964: All communication with
965: the remote command or shell will be automatically encrypted.
966: .Pp
1.357 dtucker 967: If an interactive session is requested
968: .Nm
969: by default will only request a pseudo-terminal (pty) for interactive
970: sessions when the client has one.
971: The flags
972: .Fl T
973: and
974: .Fl t
975: can be used to override this behaviour.
976: .Pp
977: If a pseudo-terminal has been allocated the
1.218 jmc 978: user may use the escape characters noted below.
979: .Pp
1.357 dtucker 980: If no pseudo-terminal has been allocated,
1.218 jmc 981: the session is transparent and can be used to reliably transfer binary data.
982: On most systems, setting the escape character to
983: .Dq none
984: will also make the session transparent even if a tty is used.
985: .Pp
986: The session terminates when the command or shell on the remote
1.247 jmc 987: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 988: .Sh ESCAPE CHARACTERS
1.218 jmc 989: When a pseudo-terminal has been requested,
1.2 deraadt 990: .Nm
1.218 jmc 991: supports a number of functions through the use of an escape character.
992: .Pp
993: A single tilde character can be sent as
994: .Ic ~~
995: or by following the tilde by a character other than those described below.
996: The escape character must always follow a newline to be interpreted as
997: special.
998: The escape character can be changed in configuration files using the
999: .Cm EscapeChar
1000: configuration directive or on the command line by the
1001: .Fl e
1002: option.
1003: .Pp
1004: The supported escapes (assuming the default
1005: .Ql ~ )
1006: are:
1007: .Bl -tag -width Ds
1008: .It Cm ~.
1009: Disconnect.
1010: .It Cm ~^Z
1011: Background
1.234 jmc 1012: .Nm .
1.218 jmc 1013: .It Cm ~#
1014: List forwarded connections.
1015: .It Cm ~&
1016: Background
1.2 deraadt 1017: .Nm
1.218 jmc 1018: at logout when waiting for forwarded connection / X11 sessions to terminate.
1019: .It Cm ~?
1020: Display a list of escape characters.
1021: .It Cm ~B
1022: Send a BREAK to the remote system
1.369 jmc 1023: (only useful if the peer supports it).
1.218 jmc 1024: .It Cm ~C
1025: Open command line.
1026: Currently this allows the addition of port forwardings using the
1.279 stevesk 1027: .Fl L ,
1028: .Fl R
1.218 jmc 1029: and
1.279 stevesk 1030: .Fl D
1.225 jmc 1031: options (see above).
1.322 markus 1032: It also allows the cancellation of existing port-forwardings
1033: with
1.262 stevesk 1034: .Sm off
1.322 markus 1035: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1036: .Sm on
1.322 markus 1037: for local,
1038: .Sm off
1039: .Fl KR Oo Ar bind_address : Oc Ar port
1040: .Sm on
1041: for remote and
1042: .Sm off
1043: .Fl KD Oo Ar bind_address : Oc Ar port
1044: .Sm on
1045: for dynamic port-forwardings.
1.218 jmc 1046: .Ic !\& Ns Ar command
1047: allows the user to execute a local command if the
1048: .Ic PermitLocalCommand
1049: option is enabled in
1.176 jmc 1050: .Xr ssh_config 5 .
1.218 jmc 1051: Basic help is available, using the
1052: .Fl h
1053: option.
1054: .It Cm ~R
1055: Request rekeying of the connection
1.369 jmc 1056: (only useful if the peer supports it).
1.327 dtucker 1057: .It Cm ~V
1058: Decrease the verbosity
1059: .Pq Ic LogLevel
1060: when errors are being written to stderr.
1061: .It Cm ~v
1.328 jmc 1062: Increase the verbosity
1.327 dtucker 1063: .Pq Ic LogLevel
1064: when errors are being written to stderr.
1.176 jmc 1065: .El
1.246 jmc 1066: .Sh TCP FORWARDING
1067: Forwarding of arbitrary TCP connections over the secure channel can
1068: be specified either on the command line or in a configuration file.
1069: One possible application of TCP forwarding is a secure connection to a
1070: mail server; another is going through firewalls.
1071: .Pp
1072: In the example below, we look at encrypting communication between
1073: an IRC client and server, even though the IRC server does not directly
1074: support encrypted communications.
1075: This works as follows:
1076: the user connects to the remote host using
1077: .Nm ,
1078: specifying a port to be used to forward connections
1079: to the remote server.
1080: After that it is possible to start the service which is to be encrypted
1081: on the client machine,
1082: connecting to the same local port,
1083: and
1084: .Nm
1085: will encrypt and forward the connection.
1086: .Pp
1087: The following example tunnels an IRC session from client machine
1088: .Dq 127.0.0.1
1089: (localhost)
1090: to remote server
1091: .Dq server.example.com :
1092: .Bd -literal -offset 4n
1093: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1094: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1095: .Ed
1096: .Pp
1097: This tunnels a connection to IRC server
1098: .Dq server.example.com ,
1099: joining channel
1100: .Dq #users ,
1101: nickname
1102: .Dq pinky ,
1103: using port 1234.
1104: It doesn't matter which port is used,
1105: as long as it's greater than 1023
1106: (remember, only root can open sockets on privileged ports)
1107: and doesn't conflict with any ports already in use.
1108: The connection is forwarded to port 6667 on the remote server,
1109: since that's the standard port for IRC services.
1110: .Pp
1111: The
1112: .Fl f
1113: option backgrounds
1114: .Nm
1115: and the remote command
1116: .Dq sleep 10
1117: is specified to allow an amount of time
1118: (10 seconds, in the example)
1119: to start the service which is to be tunnelled.
1120: If no connections are made within the time specified,
1121: .Nm
1122: will exit.
1123: .Sh X11 FORWARDING
1.218 jmc 1124: If the
1125: .Cm ForwardX11
1126: variable is set to
1127: .Dq yes
1128: (or see the description of the
1.227 jmc 1129: .Fl X ,
1130: .Fl x ,
1.218 jmc 1131: and
1.227 jmc 1132: .Fl Y
1.226 jmc 1133: options above)
1.218 jmc 1134: and the user is using X11 (the
1135: .Ev DISPLAY
1136: environment variable is set), the connection to the X11 display is
1137: automatically forwarded to the remote side in such a way that any X11
1138: programs started from the shell (or command) will go through the
1139: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1140: from the local machine.
1.218 jmc 1141: The user should not manually set
1142: .Ev DISPLAY .
1143: Forwarding of X11 connections can be
1144: configured on the command line or in configuration files.
1145: .Pp
1146: The
1147: .Ev DISPLAY
1148: value set by
1149: .Nm
1150: will point to the server machine, but with a display number greater than zero.
1151: This is normal, and happens because
1152: .Nm
1153: creates a
1154: .Dq proxy
1155: X server on the server machine for forwarding the
1156: connections over the encrypted channel.
1.200 djm 1157: .Pp
1.218 jmc 1158: .Nm
1159: will also automatically set up Xauthority data on the server machine.
1160: For this purpose, it will generate a random authorization cookie,
1161: store it in Xauthority on the server, and verify that any forwarded
1162: connections carry this cookie and replace it by the real cookie when
1163: the connection is opened.
1164: The real authentication cookie is never
1165: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1166: .Pp
1.218 jmc 1167: If the
1168: .Cm ForwardAgent
1169: variable is set to
1170: .Dq yes
1171: (or see the description of the
1172: .Fl A
1.191 djm 1173: and
1.218 jmc 1174: .Fl a
1.226 jmc 1175: options above) and
1.218 jmc 1176: the user is using an authentication agent, the connection to the agent
1177: is automatically forwarded to the remote side.
1.252 jmc 1178: .Sh VERIFYING HOST KEYS
1179: When connecting to a server for the first time,
1180: a fingerprint of the server's public key is presented to the user
1181: (unless the option
1182: .Cm StrictHostKeyChecking
1183: has been disabled).
1184: Fingerprints can be determined using
1185: .Xr ssh-keygen 1 :
1186: .Pp
1187: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1188: .Pp
1.274 grunk 1189: If the fingerprint is already known, it can be matched
1190: and the key can be accepted or rejected.
1.358 djm 1191: If only legacy (MD5) fingerprints for the server are available, the
1192: .Xr ssh-keygen 1
1193: .Fl E
1194: option may be used to downgrade the fingerprint algorithm to match.
1195: .Pp
1.274 grunk 1196: Because of the difficulty of comparing host keys
1.352 djm 1197: just by looking at fingerprint strings,
1.274 grunk 1198: there is also support to compare host keys visually,
1199: using
1200: .Em random art .
1201: By setting the
1.275 grunk 1202: .Cm VisualHostKey
1.274 grunk 1203: option to
1.275 grunk 1204: .Dq yes ,
1.274 grunk 1205: a small ASCII graphic gets displayed on every login to a server, no matter
1206: if the session itself is interactive or not.
1207: By learning the pattern a known server produces, a user can easily
1208: find out that the host key has changed when a completely different pattern
1209: is displayed.
1210: Because these patterns are not unambiguous however, a pattern that looks
1211: similar to the pattern remembered only gives a good probability that the
1212: host key is the same, not guaranteed proof.
1213: .Pp
1214: To get a listing of the fingerprints along with their random art for
1215: all known hosts, the following command line can be used:
1216: .Pp
1217: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1218: .Pp
1.252 jmc 1219: If the fingerprint is unknown,
1220: an alternative method of verification is available:
1221: SSH fingerprints verified by DNS.
1222: An additional resource record (RR),
1223: SSHFP,
1224: is added to a zonefile
1225: and the connecting client is able to match the fingerprint
1226: with that of the key presented.
1227: .Pp
1228: In this example, we are connecting a client to a server,
1229: .Dq host.example.com .
1230: The SSHFP resource records should first be added to the zonefile for
1231: host.example.com:
1232: .Bd -literal -offset indent
1.259 jakob 1233: $ ssh-keygen -r host.example.com.
1.252 jmc 1234: .Ed
1235: .Pp
1236: The output lines will have to be added to the zonefile.
1237: To check that the zone is answering fingerprint queries:
1238: .Pp
1239: .Dl $ dig -t SSHFP host.example.com
1240: .Pp
1241: Finally the client connects:
1242: .Bd -literal -offset indent
1243: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1244: [...]
1245: Matching host key fingerprint found in DNS.
1246: Are you sure you want to continue connecting (yes/no)?
1247: .Ed
1248: .Pp
1249: See the
1250: .Cm VerifyHostKeyDNS
1251: option in
1252: .Xr ssh_config 5
1253: for more information.
1.250 jmc 1254: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1255: .Nm
1256: contains support for Virtual Private Network (VPN) tunnelling
1257: using the
1258: .Xr tun 4
1259: network pseudo-device,
1260: allowing two networks to be joined securely.
1261: The
1262: .Xr sshd_config 5
1263: configuration option
1264: .Cm PermitTunnel
1265: controls whether the server supports this,
1266: and at what level (layer 2 or 3 traffic).
1267: .Pp
1268: The following example would connect client network 10.0.50.0/24
1.265 otto 1269: with remote network 10.0.99.0/24 using a point-to-point connection
1270: from 10.1.1.1 to 10.1.1.2,
1271: provided that the SSH server running on the gateway to the remote network,
1272: at 192.168.1.15, allows it.
1273: .Pp
1274: On the client:
1.250 jmc 1275: .Bd -literal -offset indent
1276: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1277: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1278: # route add 10.0.99.0/24 10.1.1.2
1279: .Ed
1280: .Pp
1281: On the server:
1282: .Bd -literal -offset indent
1283: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1284: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1285: .Ed
1286: .Pp
1287: Client access may be more finely tuned via the
1288: .Pa /root/.ssh/authorized_keys
1289: file (see below) and the
1290: .Cm PermitRootLogin
1291: server option.
1.255 jmc 1292: The following entry would permit connections on
1.250 jmc 1293: .Xr tun 4
1.255 jmc 1294: device 1 from user
1.250 jmc 1295: .Dq jane
1.255 jmc 1296: and on tun device 2 from user
1.250 jmc 1297: .Dq john ,
1298: if
1299: .Cm PermitRootLogin
1300: is set to
1301: .Dq forced-commands-only :
1302: .Bd -literal -offset 2n
1303: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1304: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1305: .Ed
1306: .Pp
1.264 ray 1307: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1308: it may be more suited to temporary setups,
1309: such as for wireless VPNs.
1310: More permanent VPNs are better provided by tools such as
1311: .Xr ipsecctl 8
1312: and
1313: .Xr isakmpd 8 .
1.2 deraadt 1314: .Sh ENVIRONMENT
1315: .Nm
1.1 deraadt 1316: will normally set the following environment variables:
1.237 jmc 1317: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1318: .It Ev DISPLAY
1319: The
1320: .Ev DISPLAY
1.40 aaron 1321: variable indicates the location of the X11 server.
1.44 aaron 1322: It is automatically set by
1.2 deraadt 1323: .Nm
1324: to point to a value of the form
1.233 jmc 1325: .Dq hostname:n ,
1326: where
1327: .Dq hostname
1328: indicates the host where the shell runs, and
1329: .Sq n
1330: is an integer \*(Ge 1.
1.40 aaron 1331: .Nm
1332: uses this special value to forward X11 connections over the secure
1333: channel.
1.107 markus 1334: The user should normally not set
1335: .Ev DISPLAY
1336: explicitly, as that
1.1 deraadt 1337: will render the X11 connection insecure (and will require the user to
1338: manually copy any required authorization cookies).
1.2 deraadt 1339: .It Ev HOME
1.1 deraadt 1340: Set to the path of the user's home directory.
1.2 deraadt 1341: .It Ev LOGNAME
1342: Synonym for
1.12 aaron 1343: .Ev USER ;
1344: set for compatibility with systems that use this variable.
1.2 deraadt 1345: .It Ev MAIL
1.129 stevesk 1346: Set to the path of the user's mailbox.
1.40 aaron 1347: .It Ev PATH
1.2 deraadt 1348: Set to the default
1349: .Ev PATH ,
1350: as specified when compiling
1.234 jmc 1351: .Nm .
1.118 markus 1352: .It Ev SSH_ASKPASS
1353: If
1354: .Nm
1355: needs a passphrase, it will read the passphrase from the current
1356: terminal if it was run from a terminal.
1357: If
1358: .Nm
1359: does not have a terminal associated with it but
1360: .Ev DISPLAY
1361: and
1362: .Ev SSH_ASKPASS
1363: are set, it will execute the program specified by
1364: .Ev SSH_ASKPASS
1365: and open an X11 window to read the passphrase.
1366: This is particularly useful when calling
1367: .Nm
1368: from a
1.196 jmc 1369: .Pa .xsession
1.118 markus 1370: or related script.
1371: (Note that on some machines it
1372: may be necessary to redirect the input from
1373: .Pa /dev/null
1374: to make this work.)
1.18 markus 1375: .It Ev SSH_AUTH_SOCK
1.233 jmc 1376: Identifies the path of a
1377: .Ux Ns -domain
1378: socket used to communicate with the agent.
1.166 stevesk 1379: .It Ev SSH_CONNECTION
1380: Identifies the client and server ends of the connection.
1.40 aaron 1381: The variable contains
1.233 jmc 1382: four space-separated values: client IP address, client port number,
1383: server IP address, and server port number.
1.73 markus 1384: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1385: This variable contains the original command line if a forced command
1.73 markus 1386: is executed.
1387: It can be used to extract the original arguments.
1.2 deraadt 1388: .It Ev SSH_TTY
1.1 deraadt 1389: This is set to the name of the tty (path to the device) associated
1.40 aaron 1390: with the current shell or command.
1391: If the current session has no tty,
1.1 deraadt 1392: this variable is not set.
1.387 djm 1393: .It Ev SSH_TUNNEL
1394: Optionally set by
1395: .Xr sshd 8
1396: to contain the interface names assigned if tunnel forwarding was
1397: requested by the client.
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
1.389 ! djm 1485: sensitive part of this file using AES-128.
1.238 jmc 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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