Annotation of src/usr.bin/ssh/ssh.1, Revision 1.370
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: .\"
1.370 ! djm 36: .\" $OpenBSD: ssh.1,v 1.369 2016/02/17 07:38:19 jmc Exp $
! 37: .Dd $Mdocdate: February 17 2016 $
1.2 deraadt 38: .Dt SSH 1
39: .Os
40: .Sh NAME
41: .Nm ssh
1.96 deraadt 42: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm ssh
1.306 jmc 45: .Bk -words
1.351 jmc 46: .Op Fl 1246AaCfGgKkMNnqsTtVvXxYy
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
1.360 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
1.176 jmc 66: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 67: .Op Ar command
1.306 jmc 68: .Ek
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
1.176 jmc 81: .Ar hostname
82: (with optional
83: .Ar user
84: name).
1.1 deraadt 85: The user must prove
1.49 markus 86: his/her identity to the remote machine using one of several methods
1.365 mmcc 87: (see below).
1.49 markus 88: .Pp
1.176 jmc 89: If
90: .Ar command
91: is specified,
1.219 jmc 92: it is executed on the remote host instead of a login shell.
1.2 deraadt 93: .Pp
1.218 jmc 94: The options are as follows:
1.361 millert 95: .Pp
1.360 millert 96: .Bl -tag -width Ds -compact
1.218 jmc 97: .It Fl 1
98: Forces
1.2 deraadt 99: .Nm
1.218 jmc 100: to try protocol version 1 only.
1.360 millert 101: .Pp
1.218 jmc 102: .It Fl 2
103: Forces
1.2 deraadt 104: .Nm
1.218 jmc 105: to try protocol version 2 only.
1.360 millert 106: .Pp
1.218 jmc 107: .It Fl 4
108: Forces
1.2 deraadt 109: .Nm
1.218 jmc 110: to use IPv4 addresses only.
1.360 millert 111: .Pp
1.218 jmc 112: .It Fl 6
113: Forces
1.2 deraadt 114: .Nm
1.218 jmc 115: to use IPv6 addresses only.
1.360 millert 116: .Pp
1.218 jmc 117: .It Fl A
118: Enables forwarding of the authentication agent connection.
119: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 120: .Pp
1.218 jmc 121: Agent forwarding should be enabled with caution.
122: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 123: (for the agent's
1.286 sobrado 124: .Ux Ns -domain
125: socket) can access the local agent through the forwarded connection.
1.218 jmc 126: An attacker cannot obtain key material from the agent,
127: however they can perform operations on the keys that enable them to
128: authenticate using the identities loaded into the agent.
1.360 millert 129: .Pp
1.218 jmc 130: .It Fl a
131: Disables forwarding of the authentication agent connection.
1.360 millert 132: .Pp
1.218 jmc 133: .It Fl b Ar bind_address
134: Use
135: .Ar bind_address
136: on the local machine as the source address
137: of the connection.
138: Only useful on systems with more than one address.
1.360 millert 139: .Pp
1.218 jmc 140: .It Fl C
141: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 142: data for forwarded X11, TCP and
143: .Ux Ns -domain
144: connections).
1.218 jmc 145: The compression algorithm is the same used by
146: .Xr gzip 1 ,
147: and the
148: .Dq level
149: can be controlled by the
150: .Cm CompressionLevel
151: option for protocol version 1.
152: Compression is desirable on modem lines and other
153: slow connections, but will only slow down things on fast networks.
154: The default value can be set on a host-by-host basis in the
155: configuration files; see the
156: .Cm Compression
157: option.
1.360 millert 158: .Pp
1.218 jmc 159: .It Fl c Ar cipher_spec
160: Selects the cipher specification for encrypting the session.
1.2 deraadt 161: .Pp
1.218 jmc 162: Protocol version 1 allows specification of a single cipher.
163: The supported values are
164: .Dq 3des ,
1.220 jmc 165: .Dq blowfish ,
1.218 jmc 166: and
167: .Dq des .
1.230 jmc 168: For protocol version 2,
1.218 jmc 169: .Ar cipher_spec
170: is a comma-separated list of ciphers
171: listed in order of preference.
1.283 jmc 172: See the
173: .Cm Ciphers
1.307 dtucker 174: keyword in
175: .Xr ssh_config 5
176: for more information.
1.360 millert 177: .Pp
1.218 jmc 178: .It Fl D Xo
179: .Sm off
180: .Oo Ar bind_address : Oc
181: .Ar port
182: .Sm on
183: .Xc
184: Specifies a local
185: .Dq dynamic
186: application-level port forwarding.
187: This works by allocating a socket to listen to
188: .Ar port
189: on the local side, optionally bound to the specified
190: .Ar bind_address .
191: Whenever a connection is made to this port, the
192: connection is forwarded over the secure channel, and the application
193: protocol is then used to determine where to connect to from the
194: remote machine.
195: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 196: .Nm
1.218 jmc 197: will act as a SOCKS server.
198: Only root can forward privileged ports.
199: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 200: .Pp
1.308 djm 201: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 202: Only the superuser can forward privileged ports.
203: By default, the local port is bound in accordance with the
204: .Cm GatewayPorts
205: setting.
206: However, an explicit
207: .Ar bind_address
208: may be used to bind the connection to a specific address.
209: The
210: .Ar bind_address
211: of
212: .Dq localhost
213: indicates that the listening port be bound for local use only, while an
214: empty address or
215: .Sq *
216: indicates that the port should be available from all interfaces.
1.360 millert 217: .Pp
1.331 dtucker 218: .It Fl E Ar log_file
219: Append debug logs to
220: .Ar log_file
221: instead of standard error.
1.360 millert 222: .Pp
1.229 jmc 223: .It Fl e Ar escape_char
1.218 jmc 224: Sets the escape character for sessions with a pty (default:
225: .Ql ~ ) .
226: The escape character is only recognized at the beginning of a line.
227: The escape character followed by a dot
228: .Pq Ql \&.
229: closes the connection;
230: followed by control-Z suspends the connection;
231: and followed by itself sends the escape character once.
232: Setting the character to
1.2 deraadt 233: .Dq none
1.218 jmc 234: disables any escapes and makes the session fully transparent.
1.360 millert 235: .Pp
1.218 jmc 236: .It Fl F Ar configfile
237: Specifies an alternative per-user configuration file.
238: If a configuration file is given on the command line,
239: the system-wide configuration file
240: .Pq Pa /etc/ssh/ssh_config
241: will be ignored.
242: The default for the per-user configuration file is
243: .Pa ~/.ssh/config .
1.360 millert 244: .Pp
1.218 jmc 245: .It Fl f
246: Requests
247: .Nm
248: to go to background just before command execution.
249: This is useful if
1.176 jmc 250: .Nm
1.218 jmc 251: is going to ask for passwords or passphrases, but the user
252: wants it in the background.
253: This implies
254: .Fl n .
255: The recommended way to start X11 programs at a remote site is with
256: something like
257: .Ic ssh -f host xterm .
1.277 djm 258: .Pp
259: If the
260: .Cm ExitOnForwardFailure
261: configuration option is set to
262: .Dq yes ,
263: then a client started with
264: .Fl f
265: will wait for all remote port forwards to be successfully established
266: before placing itself in the background.
1.360 millert 267: .Pp
1.350 djm 268: .It Fl G
269: Causes
270: .Nm
271: to print its configuration after evaluating
272: .Cm Host
273: and
274: .Cm Match
275: blocks and exit.
1.360 millert 276: .Pp
1.218 jmc 277: .It Fl g
278: Allows remote hosts to connect to local forwarded ports.
1.346 djm 279: If used on a multiplexed connection, then this option must be specified
280: on the master process.
1.360 millert 281: .Pp
1.291 markus 282: .It Fl I Ar pkcs11
1.294 jmc 283: Specify the PKCS#11 shared library
1.176 jmc 284: .Nm
1.293 markus 285: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 286: private RSA key.
1.360 millert 287: .Pp
1.218 jmc 288: .It Fl i Ar identity_file
289: Selects a file from which the identity (private key) for
1.310 djm 290: public key authentication is read.
1.218 jmc 291: The default is
292: .Pa ~/.ssh/identity
293: for protocol version 1, and
1.310 djm 294: .Pa ~/.ssh/id_dsa ,
1.343 naddy 295: .Pa ~/.ssh/id_ecdsa ,
296: .Pa ~/.ssh/id_ed25519
1.310 djm 297: and
1.218 jmc 298: .Pa ~/.ssh/id_rsa
299: for protocol version 2.
300: Identity files may also be specified on
301: a per-host basis in the configuration file.
302: It is possible to have multiple
303: .Fl i
304: options (and multiple identities specified in
305: configuration files).
1.364 jmc 306: If no certificates have been explicitly specified by the
1.363 djm 307: .Cm CertificateFile
308: directive,
1.302 djm 309: .Nm
310: will also try to load certificate information from the filename obtained
311: by appending
312: .Pa -cert.pub
313: to identity filenames.
1.360 millert 314: .Pp
1.269 djm 315: .It Fl K
316: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
317: credentials to the server.
1.360 millert 318: .Pp
1.218 jmc 319: .It Fl k
320: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 321: .Pp
1.218 jmc 322: .It Fl L Xo
323: .Sm off
324: .Oo Ar bind_address : Oc
325: .Ar port : host : hostport
326: .Sm on
327: .Xc
1.360 millert 328: .It Fl L Xo
329: .Sm off
330: .Oo Ar bind_address : Oc
331: .Ar port : remote_socket
332: .Sm on
333: .Xc
334: .It Fl L Xo
335: .Sm off
336: .Ar local_socket : host : hostport
337: .Sm on
338: .Xc
339: .It Fl L Xo
340: .Sm off
341: .Ar local_socket : remote_socket
342: .Sm on
343: .Xc
344: Specifies that connections to the given TCP port or Unix socket on the local
345: (client) host are to be forwarded to the given host and port, or Unix socket,
346: on the remote side.
347: This works by allocating a socket to listen to either a TCP
1.218 jmc 348: .Ar port
349: on the local side, optionally bound to the specified
1.360 millert 350: .Ar bind_address ,
351: or to a Unix socket.
352: Whenever a connection is made to the local port or socket, the
1.218 jmc 353: connection is forwarded over the secure channel, and a connection is
1.360 millert 354: made to either
1.218 jmc 355: .Ar host
356: port
1.360 millert 357: .Ar hostport ,
358: or the Unix socket
359: .Ar remote_socket ,
1.218 jmc 360: from the remote machine.
1.360 millert 361: .Pp
1.218 jmc 362: Port forwardings can also be specified in the configuration file.
1.360 millert 363: Only the superuser can forward privileged ports.
1.308 djm 364: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 365: .Pp
1.218 jmc 366: By default, the local port is bound in accordance with the
367: .Cm GatewayPorts
368: setting.
369: However, an explicit
370: .Ar bind_address
371: may be used to bind the connection to a specific address.
1.2 deraadt 372: The
1.218 jmc 373: .Ar bind_address
374: of
375: .Dq localhost
376: indicates that the listening port be bound for local use only, while an
377: empty address or
378: .Sq *
379: indicates that the port should be available from all interfaces.
1.360 millert 380: .Pp
1.218 jmc 381: .It Fl l Ar login_name
382: Specifies the user to log in as on the remote machine.
383: This also may be specified on a per-host basis in the configuration file.
1.360 millert 384: .Pp
1.218 jmc 385: .It Fl M
386: Places the
387: .Nm
388: client into
389: .Dq master
390: mode for connection sharing.
1.231 stevesk 391: Multiple
392: .Fl M
393: options places
394: .Nm
395: into
396: .Dq master
397: mode with confirmation required before slave connections are accepted.
1.218 jmc 398: Refer to the description of
399: .Cm ControlMaster
400: in
401: .Xr ssh_config 5
402: for details.
1.360 millert 403: .Pp
1.218 jmc 404: .It Fl m Ar mac_spec
1.369 jmc 405: A comma-separated list of MAC (message authentication code) algorithms,
406: specified in order of preference.
1.218 jmc 407: See the
408: .Cm MACs
409: keyword for more information.
1.360 millert 410: .Pp
1.218 jmc 411: .It Fl N
412: Do not execute a remote command.
1.369 jmc 413: This is useful for just forwarding ports.
1.360 millert 414: .Pp
1.218 jmc 415: .It Fl n
416: Redirects stdin from
417: .Pa /dev/null
418: (actually, prevents reading from stdin).
419: This must be used when
1.2 deraadt 420: .Nm
1.218 jmc 421: is run in the background.
422: A common trick is to use this to run X11 programs on a remote machine.
423: For example,
424: .Ic ssh -n shadows.cs.hut.fi emacs &
425: will start an emacs on shadows.cs.hut.fi, and the X11
426: connection will be automatically forwarded over an encrypted channel.
427: The
1.2 deraadt 428: .Nm
1.218 jmc 429: program will be put in the background.
430: (This does not work if
1.2 deraadt 431: .Nm
1.218 jmc 432: needs to ask for a password or passphrase; see also the
433: .Fl f
434: option.)
1.360 millert 435: .Pp
1.218 jmc 436: .It Fl O Ar ctl_cmd
437: Control an active connection multiplexing master process.
438: When the
439: .Fl O
440: option is specified, the
441: .Ar ctl_cmd
442: argument is interpreted and passed to the master process.
443: Valid commands are:
444: .Dq check
1.305 markus 445: (check that the master process is running),
446: .Dq forward
1.318 jmc 447: (request forwardings without command execution),
1.323 okan 448: .Dq cancel
449: (cancel forwardings),
1.218 jmc 450: .Dq exit
1.318 jmc 451: (request the master to exit), and
1.317 djm 452: .Dq stop
453: (request the master to stop accepting further multiplexing requests).
1.360 millert 454: .Pp
1.218 jmc 455: .It Fl o Ar option
456: Can be used to give options in the format used in the configuration file.
457: This is useful for specifying options for which there is no separate
458: command-line flag.
459: For full details of the options listed below, and their possible values, see
460: .Xr ssh_config 5 .
1.2 deraadt 461: .Pp
1.218 jmc 462: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 463: .It AddKeysToAgent
1.218 jmc 464: .It AddressFamily
465: .It BatchMode
466: .It BindAddress
1.338 djm 467: .It CanonicalDomains
1.339 djm 468: .It CanonicalizeFallbackLocal
469: .It CanonicalizeHostname
470: .It CanonicalizeMaxDots
471: .It CanonicalizePermittedCNAMEs
1.363 djm 472: .It CertificateFile
1.218 jmc 473: .It ChallengeResponseAuthentication
474: .It CheckHostIP
475: .It Cipher
476: .It Ciphers
477: .It ClearAllForwardings
478: .It Compression
479: .It CompressionLevel
480: .It ConnectionAttempts
481: .It ConnectTimeout
482: .It ControlMaster
483: .It ControlPath
1.321 djm 484: .It ControlPersist
1.218 jmc 485: .It DynamicForward
486: .It EscapeChar
1.263 markus 487: .It ExitOnForwardFailure
1.353 jmc 488: .It FingerprintHash
1.218 jmc 489: .It ForwardAgent
490: .It ForwardX11
1.321 djm 491: .It ForwardX11Timeout
1.218 jmc 492: .It ForwardX11Trusted
493: .It GatewayPorts
494: .It GlobalKnownHostsFile
495: .It GSSAPIAuthentication
496: .It GSSAPIDelegateCredentials
497: .It HashKnownHosts
498: .It Host
499: .It HostbasedAuthentication
1.355 djm 500: .It HostbasedKeyTypes
1.218 jmc 501: .It HostKeyAlgorithms
502: .It HostKeyAlias
503: .It HostName
504: .It IdentityFile
505: .It IdentitiesOnly
1.370 ! djm 506: .It Include
1.316 jmc 507: .It IPQoS
1.321 djm 508: .It KbdInteractiveAuthentication
1.218 jmc 509: .It KbdInteractiveDevices
1.314 jmc 510: .It KexAlgorithms
1.218 jmc 511: .It LocalCommand
512: .It LocalForward
513: .It LogLevel
514: .It MACs
1.337 jmc 515: .It Match
1.218 jmc 516: .It NoHostAuthenticationForLocalhost
517: .It NumberOfPasswordPrompts
518: .It PasswordAuthentication
519: .It PermitLocalCommand
1.291 markus 520: .It PKCS11Provider
1.218 jmc 521: .It Port
522: .It PreferredAuthentications
523: .It Protocol
524: .It ProxyCommand
1.336 jmc 525: .It ProxyUseFdpass
1.359 markus 526: .It PubkeyAcceptedKeyTypes
1.218 jmc 527: .It PubkeyAuthentication
1.251 dtucker 528: .It RekeyLimit
1.218 jmc 529: .It RemoteForward
1.319 jmc 530: .It RequestTTY
1.218 jmc 531: .It RhostsRSAAuthentication
532: .It RSAAuthentication
533: .It SendEnv
534: .It ServerAliveInterval
535: .It ServerAliveCountMax
1.347 jmc 536: .It StreamLocalBindMask
537: .It StreamLocalBindUnlink
1.218 jmc 538: .It StrictHostKeyChecking
539: .It TCPKeepAlive
540: .It Tunnel
541: .It TunnelDevice
1.354 djm 542: .It UpdateHostKeys
1.218 jmc 543: .It UsePrivilegedPort
544: .It User
545: .It UserKnownHostsFile
546: .It VerifyHostKeyDNS
1.276 jmc 547: .It VisualHostKey
1.218 jmc 548: .It XAuthLocation
549: .El
1.360 millert 550: .Pp
1.218 jmc 551: .It Fl p Ar port
552: Port to connect to on the remote host.
553: This can be specified on a
554: per-host basis in the configuration file.
1.360 millert 555: .Pp
1.362 djm 556: .It Fl Q Ar query_option
1.332 djm 557: Queries
558: .Nm
1.341 deraadt 559: for the algorithms supported for the specified version 2.
560: The available features are:
561: .Ar cipher
1.332 djm 562: (supported symmetric ciphers),
1.341 deraadt 563: .Ar cipher-auth
1.340 djm 564: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 565: .Ar mac
1.332 djm 566: (supported message integrity codes),
1.341 deraadt 567: .Ar kex
1.332 djm 568: (key exchange algorithms),
1.341 deraadt 569: .Ar key
1.362 djm 570: (key types),
571: .Ar key-cert
572: (certificate key types),
573: .Ar key-plain
574: (non-certificate key types), and
1.356 djm 575: .Ar protocol-version
576: (supported SSH protocol versions).
1.360 millert 577: .Pp
1.218 jmc 578: .It Fl q
579: Quiet mode.
1.271 djm 580: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 581: .Pp
1.218 jmc 582: .It Fl R Xo
583: .Sm off
584: .Oo Ar bind_address : Oc
585: .Ar port : host : hostport
586: .Sm on
587: .Xc
1.360 millert 588: .It Fl R Xo
589: .Sm off
590: .Oo Ar bind_address : Oc
591: .Ar port : local_socket
592: .Sm on
593: .Xc
594: .It Fl R Xo
595: .Sm off
596: .Ar remote_socket : host : hostport
597: .Sm on
598: .Xc
599: .It Fl R Xo
600: .Sm off
601: .Ar remote_socket : local_socket
602: .Sm on
603: .Xc
604: Specifies that connections to the given TCP port or Unix socket on the remote
605: (server) host are to be forwarded to the given host and port, or Unix socket,
606: on the local side.
607: This works by allocating a socket to listen to either a TCP
1.218 jmc 608: .Ar port
1.360 millert 609: or to a Unix socket on the remote side.
610: Whenever a connection is made to this port or Unix socket, the
611: connection is forwarded over the secure channel, and a connection
612: is made to either
1.218 jmc 613: .Ar host
614: port
1.360 millert 615: .Ar hostport ,
616: or
617: .Ar local_socket ,
1.218 jmc 618: from the local machine.
1.2 deraadt 619: .Pp
1.218 jmc 620: Port forwardings can also be specified in the configuration file.
621: Privileged ports can be forwarded only when
622: logging in as root on the remote machine.
1.324 jmc 623: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 624: .Pp
1.360 millert 625: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 626: interface only.
1.280 tobias 627: This may be overridden by specifying a
1.218 jmc 628: .Ar bind_address .
629: An empty
630: .Ar bind_address ,
631: or the address
632: .Ql * ,
633: indicates that the remote socket should listen on all interfaces.
634: Specifying a remote
635: .Ar bind_address
636: will only succeed if the server's
637: .Cm GatewayPorts
638: option is enabled (see
639: .Xr sshd_config 5 ) .
1.281 djm 640: .Pp
641: If the
642: .Ar port
643: argument is
1.282 djm 644: .Ql 0 ,
1.281 djm 645: the listen port will be dynamically allocated on the server and reported
646: to the client at run time.
1.305 markus 647: When used together with
648: .Ic -O forward
649: the allocated port will be printed to the standard output.
1.360 millert 650: .Pp
1.218 jmc 651: .It Fl S Ar ctl_path
1.304 jmc 652: Specifies the location of a control socket for connection sharing,
1.303 djm 653: or the string
654: .Dq none
655: to disable connection sharing.
1.218 jmc 656: Refer to the description of
657: .Cm ControlPath
658: and
659: .Cm ControlMaster
660: in
661: .Xr ssh_config 5
662: for details.
1.360 millert 663: .Pp
1.218 jmc 664: .It Fl s
665: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 666: Subsystems facilitate the use of SSH
667: as a secure transport for other applications (e.g.\&
1.218 jmc 668: .Xr sftp 1 ) .
669: The subsystem is specified as the remote command.
1.360 millert 670: .Pp
1.218 jmc 671: .It Fl T
1.357 dtucker 672: Disable pseudo-terminal allocation.
1.360 millert 673: .Pp
1.218 jmc 674: .It Fl t
1.357 dtucker 675: Force pseudo-terminal allocation.
1.218 jmc 676: This can be used to execute arbitrary
677: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 678: e.g. when implementing menu services.
1.218 jmc 679: Multiple
680: .Fl t
681: options force tty allocation, even if
1.194 jakob 682: .Nm
1.218 jmc 683: has no local tty.
1.360 millert 684: .Pp
1.218 jmc 685: .It Fl V
686: Display the version number and exit.
1.360 millert 687: .Pp
1.218 jmc 688: .It Fl v
689: Verbose mode.
690: Causes
1.176 jmc 691: .Nm
1.218 jmc 692: to print debugging messages about its progress.
693: This is helpful in
694: debugging connection, authentication, and configuration problems.
695: Multiple
696: .Fl v
697: options increase the verbosity.
698: The maximum is 3.
1.360 millert 699: .Pp
1.290 dtucker 700: .It Fl W Ar host : Ns Ar port
701: Requests that standard input and output on the client be forwarded to
702: .Ar host
703: on
704: .Ar port
705: over the secure channel.
706: Implies
707: .Fl N ,
708: .Fl T ,
709: .Cm ExitOnForwardFailure
710: and
1.326 dtucker 711: .Cm ClearAllForwardings .
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
1.249 jmc 785: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 786: The default is to use protocol 2 only,
787: though this can be changed via the
1.222 jmc 788: .Cm Protocol
789: option in
1.284 jmc 790: .Xr ssh_config 5
791: or the
1.222 jmc 792: .Fl 1
793: and
794: .Fl 2
795: options (see above).
1.369 jmc 796: Protocol 1 should not be used
797: and is only offered to support legacy devices.
798: It suffers from a number of cryptographic weaknesses
799: and doesn't support many of the advanced features available for protocol 2.
1.222 jmc 800: .Pp
801: The methods available for authentication are:
1.260 jmc 802: GSSAPI-based authentication,
1.222 jmc 803: host-based authentication,
804: public key authentication,
805: challenge-response authentication,
806: and password authentication.
807: Authentication methods are tried in the order specified above,
1.369 jmc 808: though
809: .Cm PreferredAuthentications
810: can be used to change the default order.
1.222 jmc 811: .Pp
812: Host-based authentication works as follows:
1.218 jmc 813: If the machine the user logs in from is listed in
814: .Pa /etc/hosts.equiv
815: or
816: .Pa /etc/shosts.equiv
817: on the remote machine, and the user names are
818: the same on both sides, or if the files
819: .Pa ~/.rhosts
820: or
821: .Pa ~/.shosts
822: exist in the user's home directory on the
823: remote machine and contain a line containing the name of the client
824: machine and the name of the user on that machine, the user is
1.222 jmc 825: considered for login.
826: Additionally, the server
827: .Em must
828: be able to verify the client's
829: host key (see the description of
1.218 jmc 830: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 831: and
1.222 jmc 832: .Pa ~/.ssh/known_hosts ,
833: below)
834: for login to be permitted.
1.218 jmc 835: This authentication method closes security holes due to IP
1.222 jmc 836: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 837: [Note to the administrator:
838: .Pa /etc/hosts.equiv ,
839: .Pa ~/.rhosts ,
840: and the rlogin/rsh protocol in general, are inherently insecure and should be
841: disabled if security is desired.]
1.189 dtucker 842: .Pp
1.222 jmc 843: Public key authentication works as follows:
844: The scheme is based on public-key cryptography,
845: using cryptosystems
846: where encryption and decryption are done using separate keys,
847: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 848: The idea is that each user creates a public/private
849: key pair for authentication purposes.
850: The server knows the public key, and only the user knows the private key.
1.222 jmc 851: .Nm
852: implements public key authentication protocol automatically,
1.349 sobrado 853: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 854: The HISTORY section of
1.222 jmc 855: .Xr ssl 8
1.311 jmc 856: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 857: .Pp
1.218 jmc 858: The file
859: .Pa ~/.ssh/authorized_keys
860: lists the public keys that are permitted for logging in.
861: When the user logs in, the
1.2 deraadt 862: .Nm
1.218 jmc 863: program tells the server which key pair it would like to use for
864: authentication.
1.222 jmc 865: The client proves that it has access to the private key
866: and the server checks that the corresponding public key
867: is authorized to accept the account.
1.218 jmc 868: .Pp
1.222 jmc 869: The user creates his/her key pair by running
1.218 jmc 870: .Xr ssh-keygen 1 .
871: This stores the private key in
1.207 djm 872: .Pa ~/.ssh/identity
1.222 jmc 873: (protocol 1),
874: .Pa ~/.ssh/id_dsa
1.369 jmc 875: (DSA),
1.310 djm 876: .Pa ~/.ssh/id_ecdsa
1.369 jmc 877: (ECDSA),
1.343 naddy 878: .Pa ~/.ssh/id_ed25519
1.369 jmc 879: (Ed25519),
1.222 jmc 880: or
881: .Pa ~/.ssh/id_rsa
1.369 jmc 882: (RSA)
1.218 jmc 883: and stores the public key in
884: .Pa ~/.ssh/identity.pub
1.222 jmc 885: (protocol 1),
886: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 887: (DSA),
1.310 djm 888: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 889: (ECDSA),
1.343 naddy 890: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 891: (Ed25519),
1.222 jmc 892: or
893: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 894: (RSA)
1.218 jmc 895: in the user's home directory.
1.222 jmc 896: The user should then copy the public key
1.218 jmc 897: to
898: .Pa ~/.ssh/authorized_keys
1.222 jmc 899: in his/her home directory on the remote machine.
900: The
1.218 jmc 901: .Pa authorized_keys
902: file corresponds to the conventional
903: .Pa ~/.rhosts
904: file, and has one key
1.222 jmc 905: per line, though the lines can be very long.
1.218 jmc 906: After this, the user can log in without giving the password.
907: .Pp
1.301 jmc 908: A variation on public key authentication
909: is available in the form of certificate authentication:
910: instead of a set of public/private keys,
911: signed certificates are used.
912: This has the advantage that a single trusted certification authority
913: can be used in place of many public/private keys.
1.333 jmc 914: See the CERTIFICATES section of
1.301 jmc 915: .Xr ssh-keygen 1
916: for more information.
917: .Pp
918: The most convenient way to use public key or certificate authentication
919: may be with an authentication agent.
1.218 jmc 920: See
921: .Xr ssh-agent 1
1.366 jcs 922: and (optionally) the
923: .Cm AddKeysToAgent
924: directive in
925: .Xr ssh_config 5
1.218 jmc 926: for more information.
927: .Pp
1.222 jmc 928: Challenge-response authentication works as follows:
929: The server sends an arbitrary
930: .Qq challenge
931: text, and prompts for a response.
932: Examples of challenge-response authentication include
1.335 jmc 933: .Bx
934: Authentication (see
1.222 jmc 935: .Xr login.conf 5 )
1.335 jmc 936: and PAM (some
937: .Pf non- Ox
938: systems).
1.222 jmc 939: .Pp
940: Finally, if other authentication methods fail,
1.218 jmc 941: .Nm
942: prompts the user for a password.
943: The password is sent to the remote
944: host for checking; however, since all communications are encrypted,
945: the password cannot be seen by someone listening on the network.
1.232 jmc 946: .Pp
947: .Nm
948: automatically maintains and checks a database containing
949: identification for all hosts it has ever been used with.
950: Host keys are stored in
951: .Pa ~/.ssh/known_hosts
952: in the user's home directory.
953: Additionally, the file
954: .Pa /etc/ssh/ssh_known_hosts
955: is automatically checked for known hosts.
956: Any new hosts are automatically added to the user's file.
957: If a host's identification ever changes,
958: .Nm
959: warns about this and disables password authentication to prevent
960: server spoofing or man-in-the-middle attacks,
961: which could otherwise be used to circumvent the encryption.
962: The
963: .Cm StrictHostKeyChecking
964: option can be used to control logins to machines whose
965: host key is not known or has changed.
966: .Pp
1.218 jmc 967: When the user's identity has been accepted by the server, the server
1.357 dtucker 968: either executes the given command in a non-interactive session or,
969: if no command has been specified, logs into the machine and gives
970: the user a normal shell as an interactive session.
1.218 jmc 971: All communication with
972: the remote command or shell will be automatically encrypted.
973: .Pp
1.357 dtucker 974: If an interactive session is requested
975: .Nm
976: by default will only request a pseudo-terminal (pty) for interactive
977: sessions when the client has one.
978: The flags
979: .Fl T
980: and
981: .Fl t
982: can be used to override this behaviour.
983: .Pp
984: If a pseudo-terminal has been allocated the
1.218 jmc 985: user may use the escape characters noted below.
986: .Pp
1.357 dtucker 987: If no pseudo-terminal has been allocated,
1.218 jmc 988: the session is transparent and can be used to reliably transfer binary data.
989: On most systems, setting the escape character to
990: .Dq none
991: will also make the session transparent even if a tty is used.
992: .Pp
993: The session terminates when the command or shell on the remote
1.247 jmc 994: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 995: .Sh ESCAPE CHARACTERS
1.218 jmc 996: When a pseudo-terminal has been requested,
1.2 deraadt 997: .Nm
1.218 jmc 998: supports a number of functions through the use of an escape character.
999: .Pp
1000: A single tilde character can be sent as
1001: .Ic ~~
1002: or by following the tilde by a character other than those described below.
1003: The escape character must always follow a newline to be interpreted as
1004: special.
1005: The escape character can be changed in configuration files using the
1006: .Cm EscapeChar
1007: configuration directive or on the command line by the
1008: .Fl e
1009: option.
1010: .Pp
1011: The supported escapes (assuming the default
1012: .Ql ~ )
1013: are:
1014: .Bl -tag -width Ds
1015: .It Cm ~.
1016: Disconnect.
1017: .It Cm ~^Z
1018: Background
1.234 jmc 1019: .Nm .
1.218 jmc 1020: .It Cm ~#
1021: List forwarded connections.
1022: .It Cm ~&
1023: Background
1.2 deraadt 1024: .Nm
1.218 jmc 1025: at logout when waiting for forwarded connection / X11 sessions to terminate.
1026: .It Cm ~?
1027: Display a list of escape characters.
1028: .It Cm ~B
1029: Send a BREAK to the remote system
1.369 jmc 1030: (only useful if the peer supports it).
1.218 jmc 1031: .It Cm ~C
1032: Open command line.
1033: Currently this allows the addition of port forwardings using the
1.279 stevesk 1034: .Fl L ,
1035: .Fl R
1.218 jmc 1036: and
1.279 stevesk 1037: .Fl D
1.225 jmc 1038: options (see above).
1.322 markus 1039: It also allows the cancellation of existing port-forwardings
1040: with
1.262 stevesk 1041: .Sm off
1.322 markus 1042: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1043: .Sm on
1.322 markus 1044: for local,
1045: .Sm off
1046: .Fl KR Oo Ar bind_address : Oc Ar port
1047: .Sm on
1048: for remote and
1049: .Sm off
1050: .Fl KD Oo Ar bind_address : Oc Ar port
1051: .Sm on
1052: for dynamic port-forwardings.
1.218 jmc 1053: .Ic !\& Ns Ar command
1054: allows the user to execute a local command if the
1055: .Ic PermitLocalCommand
1056: option is enabled in
1.176 jmc 1057: .Xr ssh_config 5 .
1.218 jmc 1058: Basic help is available, using the
1059: .Fl h
1060: option.
1061: .It Cm ~R
1062: Request rekeying of the connection
1.369 jmc 1063: (only useful if the peer supports it).
1.327 dtucker 1064: .It Cm ~V
1065: Decrease the verbosity
1066: .Pq Ic LogLevel
1067: when errors are being written to stderr.
1068: .It Cm ~v
1.328 jmc 1069: Increase the verbosity
1.327 dtucker 1070: .Pq Ic LogLevel
1071: when errors are being written to stderr.
1.176 jmc 1072: .El
1.246 jmc 1073: .Sh TCP FORWARDING
1074: Forwarding of arbitrary TCP connections over the secure channel can
1075: be specified either on the command line or in a configuration file.
1076: One possible application of TCP forwarding is a secure connection to a
1077: mail server; another is going through firewalls.
1078: .Pp
1079: In the example below, we look at encrypting communication between
1080: an IRC client and server, even though the IRC server does not directly
1081: support encrypted communications.
1082: This works as follows:
1083: the user connects to the remote host using
1084: .Nm ,
1085: specifying a port to be used to forward connections
1086: to the remote server.
1087: After that it is possible to start the service which is to be encrypted
1088: on the client machine,
1089: connecting to the same local port,
1090: and
1091: .Nm
1092: will encrypt and forward the connection.
1093: .Pp
1094: The following example tunnels an IRC session from client machine
1095: .Dq 127.0.0.1
1096: (localhost)
1097: to remote server
1098: .Dq server.example.com :
1099: .Bd -literal -offset 4n
1100: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1101: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1102: .Ed
1103: .Pp
1104: This tunnels a connection to IRC server
1105: .Dq server.example.com ,
1106: joining channel
1107: .Dq #users ,
1108: nickname
1109: .Dq pinky ,
1110: using port 1234.
1111: It doesn't matter which port is used,
1112: as long as it's greater than 1023
1113: (remember, only root can open sockets on privileged ports)
1114: and doesn't conflict with any ports already in use.
1115: The connection is forwarded to port 6667 on the remote server,
1116: since that's the standard port for IRC services.
1117: .Pp
1118: The
1119: .Fl f
1120: option backgrounds
1121: .Nm
1122: and the remote command
1123: .Dq sleep 10
1124: is specified to allow an amount of time
1125: (10 seconds, in the example)
1126: to start the service which is to be tunnelled.
1127: If no connections are made within the time specified,
1128: .Nm
1129: will exit.
1130: .Sh X11 FORWARDING
1.218 jmc 1131: If the
1132: .Cm ForwardX11
1133: variable is set to
1134: .Dq yes
1135: (or see the description of the
1.227 jmc 1136: .Fl X ,
1137: .Fl x ,
1.218 jmc 1138: and
1.227 jmc 1139: .Fl Y
1.226 jmc 1140: options above)
1.218 jmc 1141: and the user is using X11 (the
1142: .Ev DISPLAY
1143: environment variable is set), the connection to the X11 display is
1144: automatically forwarded to the remote side in such a way that any X11
1145: programs started from the shell (or command) will go through the
1146: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1147: from the local machine.
1.218 jmc 1148: The user should not manually set
1149: .Ev DISPLAY .
1150: Forwarding of X11 connections can be
1151: configured on the command line or in configuration files.
1152: .Pp
1153: The
1154: .Ev DISPLAY
1155: value set by
1156: .Nm
1157: will point to the server machine, but with a display number greater than zero.
1158: This is normal, and happens because
1159: .Nm
1160: creates a
1161: .Dq proxy
1162: X server on the server machine for forwarding the
1163: connections over the encrypted channel.
1.200 djm 1164: .Pp
1.218 jmc 1165: .Nm
1166: will also automatically set up Xauthority data on the server machine.
1167: For this purpose, it will generate a random authorization cookie,
1168: store it in Xauthority on the server, and verify that any forwarded
1169: connections carry this cookie and replace it by the real cookie when
1170: the connection is opened.
1171: The real authentication cookie is never
1172: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1173: .Pp
1.218 jmc 1174: If the
1175: .Cm ForwardAgent
1176: variable is set to
1177: .Dq yes
1178: (or see the description of the
1179: .Fl A
1.191 djm 1180: and
1.218 jmc 1181: .Fl a
1.226 jmc 1182: options above) and
1.218 jmc 1183: the user is using an authentication agent, the connection to the agent
1184: is automatically forwarded to the remote side.
1.252 jmc 1185: .Sh VERIFYING HOST KEYS
1186: When connecting to a server for the first time,
1187: a fingerprint of the server's public key is presented to the user
1188: (unless the option
1189: .Cm StrictHostKeyChecking
1190: has been disabled).
1191: Fingerprints can be determined using
1192: .Xr ssh-keygen 1 :
1193: .Pp
1194: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1195: .Pp
1.274 grunk 1196: If the fingerprint is already known, it can be matched
1197: and the key can be accepted or rejected.
1.358 djm 1198: If only legacy (MD5) fingerprints for the server are available, the
1199: .Xr ssh-keygen 1
1200: .Fl E
1201: option may be used to downgrade the fingerprint algorithm to match.
1202: .Pp
1.274 grunk 1203: Because of the difficulty of comparing host keys
1.352 djm 1204: just by looking at fingerprint strings,
1.274 grunk 1205: there is also support to compare host keys visually,
1206: using
1207: .Em random art .
1208: By setting the
1.275 grunk 1209: .Cm VisualHostKey
1.274 grunk 1210: option to
1.275 grunk 1211: .Dq yes ,
1.274 grunk 1212: a small ASCII graphic gets displayed on every login to a server, no matter
1213: if the session itself is interactive or not.
1214: By learning the pattern a known server produces, a user can easily
1215: find out that the host key has changed when a completely different pattern
1216: is displayed.
1217: Because these patterns are not unambiguous however, a pattern that looks
1218: similar to the pattern remembered only gives a good probability that the
1219: host key is the same, not guaranteed proof.
1220: .Pp
1221: To get a listing of the fingerprints along with their random art for
1222: all known hosts, the following command line can be used:
1223: .Pp
1224: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1225: .Pp
1.252 jmc 1226: If the fingerprint is unknown,
1227: an alternative method of verification is available:
1228: SSH fingerprints verified by DNS.
1229: An additional resource record (RR),
1230: SSHFP,
1231: is added to a zonefile
1232: and the connecting client is able to match the fingerprint
1233: with that of the key presented.
1234: .Pp
1235: In this example, we are connecting a client to a server,
1236: .Dq host.example.com .
1237: The SSHFP resource records should first be added to the zonefile for
1238: host.example.com:
1239: .Bd -literal -offset indent
1.259 jakob 1240: $ ssh-keygen -r host.example.com.
1.252 jmc 1241: .Ed
1242: .Pp
1243: The output lines will have to be added to the zonefile.
1244: To check that the zone is answering fingerprint queries:
1245: .Pp
1246: .Dl $ dig -t SSHFP host.example.com
1247: .Pp
1248: Finally the client connects:
1249: .Bd -literal -offset indent
1250: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1251: [...]
1252: Matching host key fingerprint found in DNS.
1253: Are you sure you want to continue connecting (yes/no)?
1254: .Ed
1255: .Pp
1256: See the
1257: .Cm VerifyHostKeyDNS
1258: option in
1259: .Xr ssh_config 5
1260: for more information.
1.250 jmc 1261: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1262: .Nm
1263: contains support for Virtual Private Network (VPN) tunnelling
1264: using the
1265: .Xr tun 4
1266: network pseudo-device,
1267: allowing two networks to be joined securely.
1268: The
1269: .Xr sshd_config 5
1270: configuration option
1271: .Cm PermitTunnel
1272: controls whether the server supports this,
1273: and at what level (layer 2 or 3 traffic).
1274: .Pp
1275: The following example would connect client network 10.0.50.0/24
1.265 otto 1276: with remote network 10.0.99.0/24 using a point-to-point connection
1277: from 10.1.1.1 to 10.1.1.2,
1278: provided that the SSH server running on the gateway to the remote network,
1279: at 192.168.1.15, allows it.
1280: .Pp
1281: On the client:
1.250 jmc 1282: .Bd -literal -offset indent
1283: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1284: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1285: # route add 10.0.99.0/24 10.1.1.2
1286: .Ed
1287: .Pp
1288: On the server:
1289: .Bd -literal -offset indent
1290: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1291: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1292: .Ed
1293: .Pp
1294: Client access may be more finely tuned via the
1295: .Pa /root/.ssh/authorized_keys
1296: file (see below) and the
1297: .Cm PermitRootLogin
1298: server option.
1.255 jmc 1299: The following entry would permit connections on
1.250 jmc 1300: .Xr tun 4
1.255 jmc 1301: device 1 from user
1.250 jmc 1302: .Dq jane
1.255 jmc 1303: and on tun device 2 from user
1.250 jmc 1304: .Dq john ,
1305: if
1306: .Cm PermitRootLogin
1307: is set to
1308: .Dq forced-commands-only :
1309: .Bd -literal -offset 2n
1310: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1311: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1312: .Ed
1313: .Pp
1.264 ray 1314: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1315: it may be more suited to temporary setups,
1316: such as for wireless VPNs.
1317: More permanent VPNs are better provided by tools such as
1318: .Xr ipsecctl 8
1319: and
1320: .Xr isakmpd 8 .
1.2 deraadt 1321: .Sh ENVIRONMENT
1322: .Nm
1.1 deraadt 1323: will normally set the following environment variables:
1.237 jmc 1324: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1325: .It Ev DISPLAY
1326: The
1327: .Ev DISPLAY
1.40 aaron 1328: variable indicates the location of the X11 server.
1.44 aaron 1329: It is automatically set by
1.2 deraadt 1330: .Nm
1331: to point to a value of the form
1.233 jmc 1332: .Dq hostname:n ,
1333: where
1334: .Dq hostname
1335: indicates the host where the shell runs, and
1336: .Sq n
1337: is an integer \*(Ge 1.
1.40 aaron 1338: .Nm
1339: uses this special value to forward X11 connections over the secure
1340: channel.
1.107 markus 1341: The user should normally not set
1342: .Ev DISPLAY
1343: explicitly, as that
1.1 deraadt 1344: will render the X11 connection insecure (and will require the user to
1345: manually copy any required authorization cookies).
1.2 deraadt 1346: .It Ev HOME
1.1 deraadt 1347: Set to the path of the user's home directory.
1.2 deraadt 1348: .It Ev LOGNAME
1349: Synonym for
1.12 aaron 1350: .Ev USER ;
1351: set for compatibility with systems that use this variable.
1.2 deraadt 1352: .It Ev MAIL
1.129 stevesk 1353: Set to the path of the user's mailbox.
1.40 aaron 1354: .It Ev PATH
1.2 deraadt 1355: Set to the default
1356: .Ev PATH ,
1357: as specified when compiling
1.234 jmc 1358: .Nm .
1.118 markus 1359: .It Ev SSH_ASKPASS
1360: If
1361: .Nm
1362: needs a passphrase, it will read the passphrase from the current
1363: terminal if it was run from a terminal.
1364: If
1365: .Nm
1366: does not have a terminal associated with it but
1367: .Ev DISPLAY
1368: and
1369: .Ev SSH_ASKPASS
1370: are set, it will execute the program specified by
1371: .Ev SSH_ASKPASS
1372: and open an X11 window to read the passphrase.
1373: This is particularly useful when calling
1374: .Nm
1375: from a
1.196 jmc 1376: .Pa .xsession
1.118 markus 1377: or related script.
1378: (Note that on some machines it
1379: may be necessary to redirect the input from
1380: .Pa /dev/null
1381: to make this work.)
1.18 markus 1382: .It Ev SSH_AUTH_SOCK
1.233 jmc 1383: Identifies the path of a
1384: .Ux Ns -domain
1385: socket used to communicate with the agent.
1.166 stevesk 1386: .It Ev SSH_CONNECTION
1387: Identifies the client and server ends of the connection.
1.40 aaron 1388: The variable contains
1.233 jmc 1389: four space-separated values: client IP address, client port number,
1390: server IP address, and server port number.
1.73 markus 1391: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1392: This variable contains the original command line if a forced command
1.73 markus 1393: is executed.
1394: It can be used to extract the original arguments.
1.2 deraadt 1395: .It Ev SSH_TTY
1.1 deraadt 1396: This is set to the name of the tty (path to the device) associated
1.40 aaron 1397: with the current shell or command.
1398: If the current session has no tty,
1.1 deraadt 1399: this variable is not set.
1.2 deraadt 1400: .It Ev TZ
1.214 jmc 1401: This variable is set to indicate the present time zone if it
1.257 jmc 1402: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1403: on to new connections).
1.2 deraadt 1404: .It Ev USER
1.1 deraadt 1405: Set to the name of the user logging in.
1.2 deraadt 1406: .El
1407: .Pp
1.44 aaron 1408: Additionally,
1.2 deraadt 1409: .Nm
1.44 aaron 1410: reads
1.207 djm 1411: .Pa ~/.ssh/environment ,
1.2 deraadt 1412: and adds lines of the format
1413: .Dq VARNAME=value
1.233 jmc 1414: to the environment if the file exists and users are allowed to
1.161 marc 1415: change their environment.
1.176 jmc 1416: For more information, see the
1.161 marc 1417: .Cm PermitUserEnvironment
1.162 stevesk 1418: option in
1.161 marc 1419: .Xr sshd_config 5 .
1.2 deraadt 1420: .Sh FILES
1.236 jmc 1421: .Bl -tag -width Ds -compact
1.309 jmc 1422: .It Pa ~/.rhosts
1.240 jmc 1423: This file is used for host-based authentication (see above).
1.92 markus 1424: On some machines this file may need to be
1.240 jmc 1425: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1426: because
1.2 deraadt 1427: .Xr sshd 8
1.40 aaron 1428: reads it as root.
1429: Additionally, this file must be owned by the user,
1430: and must not have write permissions for anyone else.
1431: The recommended
1.1 deraadt 1432: permission for most machines is read/write for the user, and not
1433: accessible by others.
1.2 deraadt 1434: .Pp
1.309 jmc 1435: .It Pa ~/.shosts
1.240 jmc 1436: This file is used in exactly the same way as
1437: .Pa .rhosts ,
1438: but allows host-based authentication without permitting login with
1439: rlogin/rsh.
1.272 mcbride 1440: .Pp
1.309 jmc 1441: .It Pa ~/.ssh/
1.272 mcbride 1442: This directory is the default location for all user-specific configuration
1443: and authentication information.
1444: There is no general requirement to keep the entire contents of this directory
1445: secret, but the recommended permissions are read/write/execute for the user,
1446: and not accessible by others.
1.236 jmc 1447: .Pp
1.309 jmc 1448: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1449: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1450: that can be used for logging in as this user.
1.238 jmc 1451: The format of this file is described in the
1452: .Xr sshd 8
1453: manual page.
1454: This file is not highly sensitive, but the recommended
1455: permissions are read/write for the user, and not accessible by others.
1456: .Pp
1.309 jmc 1457: .It Pa ~/.ssh/config
1.238 jmc 1458: This is the per-user configuration file.
1459: The file format and configuration options are described in
1460: .Xr ssh_config 5 .
1461: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1462: read/write for the user, and not writable by others.
1.238 jmc 1463: .Pp
1.309 jmc 1464: .It Pa ~/.ssh/environment
1.239 jmc 1465: Contains additional definitions for environment variables; see
1466: .Sx ENVIRONMENT ,
1.238 jmc 1467: above.
1468: .Pp
1.309 jmc 1469: .It Pa ~/.ssh/identity
1470: .It Pa ~/.ssh/id_dsa
1.310 djm 1471: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1472: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1473: .It Pa ~/.ssh/id_rsa
1.238 jmc 1474: Contains the private key for authentication.
1475: These files
1476: contain sensitive data and should be readable by the user but not
1477: accessible by others (read/write/execute).
1478: .Nm
1479: will simply ignore a private key file if it is accessible by others.
1480: It is possible to specify a passphrase when
1481: generating the key which will be used to encrypt the
1482: sensitive part of this file using 3DES.
1483: .Pp
1.309 jmc 1484: .It Pa ~/.ssh/identity.pub
1485: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1486: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1487: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1488: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1489: Contains the public key for authentication.
1490: These files are not
1491: sensitive and can (but need not) be readable by anyone.
1492: .Pp
1.309 jmc 1493: .It Pa ~/.ssh/known_hosts
1.244 jmc 1494: Contains a list of host keys for all hosts the user has logged into
1495: that are not already in the systemwide list of known host keys.
1.238 jmc 1496: See
1.244 jmc 1497: .Xr sshd 8
1498: for further details of the format of this file.
1.238 jmc 1499: .Pp
1.309 jmc 1500: .It Pa ~/.ssh/rc
1.238 jmc 1501: Commands in this file are executed by
1502: .Nm
1.245 jmc 1503: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1504: started.
1505: See the
1506: .Xr sshd 8
1507: manual page for more information.
1508: .Pp
1.309 jmc 1509: .It Pa /etc/hosts.equiv
1.240 jmc 1510: This file is for host-based authentication (see above).
1511: It should only be writable by root.
1.236 jmc 1512: .Pp
1.309 jmc 1513: .It Pa /etc/shosts.equiv
1.240 jmc 1514: This file is used in exactly the same way as
1515: .Pa hosts.equiv ,
1516: but allows host-based authentication without permitting login with
1517: rlogin/rsh.
1.236 jmc 1518: .Pp
1.238 jmc 1519: .It Pa /etc/ssh/ssh_config
1520: Systemwide configuration file.
1521: The file format and configuration options are described in
1522: .Xr ssh_config 5 .
1523: .Pp
1.309 jmc 1524: .It Pa /etc/ssh/ssh_host_key
1525: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1526: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1527: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1528: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1529: These files contain the private parts of the host keys
1.245 jmc 1530: and are used for host-based authentication.
1.238 jmc 1531: .Pp
1.309 jmc 1532: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1533: Systemwide list of known host keys.
1534: This file should be prepared by the
1535: system administrator to contain the public host keys of all machines in the
1536: organization.
1.244 jmc 1537: It should be world-readable.
1538: See
1.238 jmc 1539: .Xr sshd 8
1.244 jmc 1540: for further details of the format of this file.
1.236 jmc 1541: .Pp
1.309 jmc 1542: .It Pa /etc/ssh/sshrc
1.1 deraadt 1543: Commands in this file are executed by
1.2 deraadt 1544: .Nm
1.245 jmc 1545: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1546: See the
1.2 deraadt 1547: .Xr sshd 8
1.1 deraadt 1548: manual page for more information.
1.58 itojun 1549: .El
1.312 jmc 1550: .Sh EXIT STATUS
1551: .Nm
1552: exits with the exit status of the remote command or with 255
1553: if an error occurred.
1.2 deraadt 1554: .Sh SEE ALSO
1555: .Xr scp 1 ,
1.83 djm 1556: .Xr sftp 1 ,
1.2 deraadt 1557: .Xr ssh-add 1 ,
1558: .Xr ssh-agent 1 ,
1559: .Xr ssh-keygen 1 ,
1.242 jmc 1560: .Xr ssh-keyscan 1 ,
1.250 jmc 1561: .Xr tun 4 ,
1.159 stevesk 1562: .Xr ssh_config 5 ,
1.160 naddy 1563: .Xr ssh-keysign 8 ,
1.87 itojun 1564: .Xr sshd 8
1.329 jmc 1565: .Sh STANDARDS
1.106 markus 1566: .Rs
1.329 jmc 1567: .%A S. Lehtinen
1568: .%A C. Lonvick
1569: .%D January 2006
1.256 jmc 1570: .%R RFC 4250
1.329 jmc 1571: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1572: .Re
1.329 jmc 1573: .Pp
1.256 jmc 1574: .Rs
1.329 jmc 1575: .%A T. Ylonen
1576: .%A C. Lonvick
1577: .%D January 2006
1.256 jmc 1578: .%R RFC 4251
1.329 jmc 1579: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1580: .Re
1.329 jmc 1581: .Pp
1.256 jmc 1582: .Rs
1.329 jmc 1583: .%A T. Ylonen
1584: .%A C. Lonvick
1585: .%D January 2006
1.256 jmc 1586: .%R RFC 4252
1.329 jmc 1587: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1588: .Re
1.329 jmc 1589: .Pp
1.256 jmc 1590: .Rs
1.329 jmc 1591: .%A T. Ylonen
1592: .%A C. Lonvick
1593: .%D January 2006
1.256 jmc 1594: .%R RFC 4253
1.329 jmc 1595: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1596: .Re
1.329 jmc 1597: .Pp
1.256 jmc 1598: .Rs
1.329 jmc 1599: .%A T. Ylonen
1600: .%A C. Lonvick
1601: .%D January 2006
1.256 jmc 1602: .%R RFC 4254
1.329 jmc 1603: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1604: .Re
1.329 jmc 1605: .Pp
1.256 jmc 1606: .Rs
1.329 jmc 1607: .%A J. Schlyter
1608: .%A W. Griffin
1609: .%D January 2006
1.256 jmc 1610: .%R RFC 4255
1.329 jmc 1611: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1612: .Re
1.329 jmc 1613: .Pp
1.256 jmc 1614: .Rs
1.329 jmc 1615: .%A F. Cusack
1616: .%A M. Forssen
1617: .%D January 2006
1.256 jmc 1618: .%R RFC 4256
1.329 jmc 1619: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1620: .Re
1.329 jmc 1621: .Pp
1.256 jmc 1622: .Rs
1.329 jmc 1623: .%A J. Galbraith
1624: .%A P. Remaker
1625: .%D January 2006
1.256 jmc 1626: .%R RFC 4335
1.329 jmc 1627: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1628: .Re
1.329 jmc 1629: .Pp
1.256 jmc 1630: .Rs
1.329 jmc 1631: .%A M. Bellare
1632: .%A T. Kohno
1633: .%A C. Namprempre
1634: .%D January 2006
1.256 jmc 1635: .%R RFC 4344
1.329 jmc 1636: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1637: .Re
1.329 jmc 1638: .Pp
1.256 jmc 1639: .Rs
1.329 jmc 1640: .%A B. Harris
1641: .%D January 2006
1.256 jmc 1642: .%R RFC 4345
1.329 jmc 1643: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1644: .Re
1.329 jmc 1645: .Pp
1.258 djm 1646: .Rs
1.329 jmc 1647: .%A M. Friedl
1648: .%A N. Provos
1649: .%A W. Simpson
1650: .%D March 2006
1.258 djm 1651: .%R RFC 4419
1.329 jmc 1652: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1653: .Re
1.329 jmc 1654: .Pp
1.266 markus 1655: .Rs
1.329 jmc 1656: .%A J. Galbraith
1657: .%A R. Thayer
1658: .%D November 2006
1.266 markus 1659: .%R RFC 4716
1.329 jmc 1660: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1661: .Re
1.329 jmc 1662: .Pp
1.313 djm 1663: .Rs
1.329 jmc 1664: .%A D. Stebila
1665: .%A J. Green
1666: .%D December 2009
1.313 djm 1667: .%R RFC 5656
1.329 jmc 1668: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1669: .Re
1.329 jmc 1670: .Pp
1.274 grunk 1671: .Rs
1672: .%A A. Perrig
1673: .%A D. Song
1674: .%D 1999
1.329 jmc 1675: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1676: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1677: .Re
1.173 jmc 1678: .Sh AUTHORS
1679: OpenSSH is a derivative of the original and free
1680: ssh 1.2.12 release by Tatu Ylonen.
1681: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1682: Theo de Raadt and Dug Song
1683: removed many bugs, re-added newer features and
1684: created OpenSSH.
1685: Markus Friedl contributed the support for SSH
1686: protocol versions 1.5 and 2.0.
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