Annotation of src/usr.bin/ssh/ssh.1, Revision 1.286
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
1.59 deraadt 7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
1.93 deraadt 13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.59 deraadt 16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 25: .\"
1.59 deraadt 26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.286 ! sobrado 37: .\" $OpenBSD: ssh.1,v 1.285 2009/10/22 12:35:53 sobrado Exp $
! 38: .Dd $Mdocdate: October 22 2009 $
1.2 deraadt 39: .Dt SSH 1
40: .Os
41: .Sh NAME
42: .Nm ssh
1.96 deraadt 43: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm ssh
1.278 djm 46: .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
1.108 markus 47: .Op Fl b Ar bind_address
1.51 markus 48: .Op Fl c Ar cipher_spec
1.210 djm 49: .Oo Fl D\ \&
50: .Sm off
51: .Oo Ar bind_address : Oc
52: .Ar port
53: .Sm on
54: .Oc
1.2 deraadt 55: .Op Fl e Ar escape_char
1.176 jmc 56: .Op Fl F Ar configfile
1.211 jmc 57: .Bk -words
1.2 deraadt 58: .Op Fl i Ar identity_file
1.211 jmc 59: .Ek
1.202 jmc 60: .Oo Fl L\ \&
1.12 aaron 61: .Sm off
1.200 djm 62: .Oo Ar bind_address : Oc
1.202 jmc 63: .Ar port : host : hostport
1.12 aaron 64: .Sm on
65: .Oc
1.211 jmc 66: .Bk -words
1.176 jmc 67: .Op Fl l Ar login_name
1.211 jmc 68: .Ek
1.176 jmc 69: .Op Fl m Ar mac_spec
1.198 djm 70: .Op Fl O Ar ctl_cmd
1.176 jmc 71: .Op Fl o Ar option
72: .Op Fl p Ar port
1.202 jmc 73: .Oo Fl R\ \&
1.12 aaron 74: .Sm off
1.200 djm 75: .Oo Ar bind_address : Oc
1.202 jmc 76: .Ar port : host : hostport
1.12 aaron 77: .Sm on
78: .Oc
1.198 djm 79: .Op Fl S Ar ctl_path
1.216 jmc 80: .Bk -words
1.261 stevesk 81: .Oo Fl w Ar local_tun Ns
82: .Op : Ns Ar remote_tun Oc
1.176 jmc 83: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 84: .Op Ar command
1.216 jmc 85: .Ek
1.44 aaron 86: .Sh DESCRIPTION
1.2 deraadt 87: .Nm
1.96 deraadt 88: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 89: executing commands on a remote machine.
1.176 jmc 90: It is intended to replace rlogin and rsh,
91: and provide secure encrypted communications between
1.40 aaron 92: two untrusted hosts over an insecure network.
1.247 jmc 93: X11 connections and arbitrary TCP ports
1.176 jmc 94: can also be forwarded over the secure channel.
1.2 deraadt 95: .Pp
96: .Nm
1.44 aaron 97: connects and logs into the specified
1.176 jmc 98: .Ar hostname
99: (with optional
100: .Ar user
101: name).
1.1 deraadt 102: The user must prove
1.49 markus 103: his/her identity to the remote machine using one of several methods
1.221 jmc 104: depending on the protocol version used (see below).
1.49 markus 105: .Pp
1.176 jmc 106: If
107: .Ar command
108: is specified,
1.219 jmc 109: it is executed on the remote host instead of a login shell.
1.2 deraadt 110: .Pp
1.218 jmc 111: The options are as follows:
112: .Bl -tag -width Ds
113: .It Fl 1
114: Forces
1.2 deraadt 115: .Nm
1.218 jmc 116: to try protocol version 1 only.
117: .It Fl 2
118: Forces
1.2 deraadt 119: .Nm
1.218 jmc 120: to try protocol version 2 only.
121: .It Fl 4
122: Forces
1.2 deraadt 123: .Nm
1.218 jmc 124: to use IPv4 addresses only.
125: .It Fl 6
126: Forces
1.2 deraadt 127: .Nm
1.218 jmc 128: to use IPv6 addresses only.
129: .It Fl A
130: Enables forwarding of the authentication agent connection.
131: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 132: .Pp
1.218 jmc 133: Agent forwarding should be enabled with caution.
134: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 135: (for the agent's
1.286 ! sobrado 136: .Ux Ns -domain
! 137: socket) can access the local agent through the forwarded connection.
1.218 jmc 138: An attacker cannot obtain key material from the agent,
139: however they can perform operations on the keys that enable them to
140: authenticate using the identities loaded into the agent.
141: .It Fl a
142: Disables forwarding of the authentication agent connection.
143: .It Fl b Ar bind_address
144: Use
145: .Ar bind_address
146: on the local machine as the source address
147: of the connection.
148: Only useful on systems with more than one address.
149: .It Fl C
150: Requests compression of all data (including stdin, stdout, stderr, and
1.247 jmc 151: data for forwarded X11 and TCP connections).
1.218 jmc 152: The compression algorithm is the same used by
153: .Xr gzip 1 ,
154: and the
155: .Dq level
156: can be controlled by the
157: .Cm CompressionLevel
158: option for protocol version 1.
159: Compression is desirable on modem lines and other
160: slow connections, but will only slow down things on fast networks.
161: The default value can be set on a host-by-host basis in the
162: configuration files; see the
163: .Cm Compression
164: option.
165: .It Fl c Ar cipher_spec
166: Selects the cipher specification for encrypting the session.
1.2 deraadt 167: .Pp
1.218 jmc 168: Protocol version 1 allows specification of a single cipher.
169: The supported values are
170: .Dq 3des ,
1.220 jmc 171: .Dq blowfish ,
1.218 jmc 172: and
173: .Dq des .
174: .Ar 3des
175: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
176: It is believed to be secure.
177: .Ar blowfish
178: is a fast block cipher; it appears very secure and is much faster than
179: .Ar 3des .
180: .Ar des
181: is only supported in the
1.2 deraadt 182: .Nm
1.218 jmc 183: client for interoperability with legacy protocol 1 implementations
184: that do not support the
185: .Ar 3des
186: cipher.
187: Its use is strongly discouraged due to cryptographic weaknesses.
188: The default is
189: .Dq 3des .
1.49 markus 190: .Pp
1.230 jmc 191: For protocol version 2,
1.218 jmc 192: .Ar cipher_spec
193: is a comma-separated list of ciphers
194: listed in order of preference.
1.283 jmc 195: See the
196: .Cm Ciphers
197: keyword for more information.
1.218 jmc 198: .It Fl D Xo
199: .Sm off
200: .Oo Ar bind_address : Oc
201: .Ar port
202: .Sm on
203: .Xc
204: Specifies a local
205: .Dq dynamic
206: application-level port forwarding.
207: This works by allocating a socket to listen to
208: .Ar port
209: on the local side, optionally bound to the specified
210: .Ar bind_address .
211: Whenever a connection is made to this port, the
212: connection is forwarded over the secure channel, and the application
213: protocol is then used to determine where to connect to from the
214: remote machine.
215: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 216: .Nm
1.218 jmc 217: will act as a SOCKS server.
218: Only root can forward privileged ports.
219: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 220: .Pp
1.218 jmc 221: IPv6 addresses can be specified with an alternative syntax:
222: .Sm off
223: .Xo
224: .Op Ar bind_address No /
225: .Ar port
226: .Xc
227: .Sm on
228: or by enclosing the address in square brackets.
229: Only the superuser can forward privileged ports.
230: By default, the local port is bound in accordance with the
231: .Cm GatewayPorts
232: setting.
233: However, an explicit
234: .Ar bind_address
235: may be used to bind the connection to a specific address.
236: The
237: .Ar bind_address
238: of
239: .Dq localhost
240: indicates that the listening port be bound for local use only, while an
241: empty address or
242: .Sq *
243: indicates that the port should be available from all interfaces.
1.229 jmc 244: .It Fl e Ar escape_char
1.218 jmc 245: Sets the escape character for sessions with a pty (default:
246: .Ql ~ ) .
247: The escape character is only recognized at the beginning of a line.
248: The escape character followed by a dot
249: .Pq Ql \&.
250: closes the connection;
251: followed by control-Z suspends the connection;
252: and followed by itself sends the escape character once.
253: Setting the character to
1.2 deraadt 254: .Dq none
1.218 jmc 255: disables any escapes and makes the session fully transparent.
256: .It Fl F Ar configfile
257: Specifies an alternative per-user configuration file.
258: If a configuration file is given on the command line,
259: the system-wide configuration file
260: .Pq Pa /etc/ssh/ssh_config
261: will be ignored.
262: The default for the per-user configuration file is
263: .Pa ~/.ssh/config .
264: .It Fl f
265: Requests
266: .Nm
267: to go to background just before command execution.
268: This is useful if
1.176 jmc 269: .Nm
1.218 jmc 270: is going to ask for passwords or passphrases, but the user
271: wants it in the background.
272: This implies
273: .Fl n .
274: The recommended way to start X11 programs at a remote site is with
275: something like
276: .Ic ssh -f host xterm .
1.277 djm 277: .Pp
278: If the
279: .Cm ExitOnForwardFailure
280: configuration option is set to
281: .Dq yes ,
282: then a client started with
283: .Fl f
284: will wait for all remote port forwards to be successfully established
285: before placing itself in the background.
1.218 jmc 286: .It Fl g
287: Allows remote hosts to connect to local forwarded ports.
288: .It Fl I Ar smartcard_device
1.229 jmc 289: Specify the device
1.176 jmc 290: .Nm
1.218 jmc 291: should use to communicate with a smartcard used for storing the user's
292: private RSA key.
1.229 jmc 293: This option is only available if support for smartcard devices
294: is compiled in (default is no support).
1.218 jmc 295: .It Fl i Ar identity_file
296: Selects a file from which the identity (private key) for
297: RSA or DSA authentication is read.
298: The default is
299: .Pa ~/.ssh/identity
300: for protocol version 1, and
301: .Pa ~/.ssh/id_rsa
1.149 jakob 302: and
1.218 jmc 303: .Pa ~/.ssh/id_dsa
304: for protocol version 2.
305: Identity files may also be specified on
306: a per-host basis in the configuration file.
307: It is possible to have multiple
308: .Fl i
309: options (and multiple identities specified in
310: configuration files).
1.269 djm 311: .It Fl K
312: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
313: credentials to the server.
1.218 jmc 314: .It Fl k
315: Disables forwarding (delegation) of GSSAPI credentials to the server.
316: .It Fl L Xo
317: .Sm off
318: .Oo Ar bind_address : Oc
319: .Ar port : host : hostport
320: .Sm on
321: .Xc
322: Specifies that the given port on the local (client) host is to be
323: forwarded to the given host and port on the remote side.
324: This works by allocating a socket to listen to
325: .Ar port
326: on the local side, optionally bound to the specified
327: .Ar bind_address .
328: Whenever a connection is made to this port, the
329: connection is forwarded over the secure channel, and a connection is
330: made to
331: .Ar host
332: port
333: .Ar hostport
334: from the remote machine.
335: Port forwardings can also be specified in the configuration file.
336: IPv6 addresses can be specified with an alternative syntax:
337: .Sm off
338: .Xo
339: .Op Ar bind_address No /
340: .Ar port No / Ar host No /
341: .Ar hostport
342: .Xc
343: .Sm on
344: or by enclosing the address in square brackets.
345: Only the superuser can forward privileged ports.
346: By default, the local port is bound in accordance with the
347: .Cm GatewayPorts
348: setting.
349: However, an explicit
350: .Ar bind_address
351: may be used to bind the connection to a specific address.
1.2 deraadt 352: The
1.218 jmc 353: .Ar bind_address
354: of
355: .Dq localhost
356: indicates that the listening port be bound for local use only, while an
357: empty address or
358: .Sq *
359: indicates that the port should be available from all interfaces.
360: .It Fl l Ar login_name
361: Specifies the user to log in as on the remote machine.
362: This also may be specified on a per-host basis in the configuration file.
363: .It Fl M
364: Places the
365: .Nm
366: client into
367: .Dq master
368: mode for connection sharing.
1.231 stevesk 369: Multiple
370: .Fl M
371: options places
372: .Nm
373: into
374: .Dq master
375: mode with confirmation required before slave connections are accepted.
1.218 jmc 376: Refer to the description of
377: .Cm ControlMaster
378: in
379: .Xr ssh_config 5
380: for details.
381: .It Fl m Ar mac_spec
382: Additionally, for protocol version 2 a comma-separated list of MAC
383: (message authentication code) algorithms can
384: be specified in order of preference.
385: See the
386: .Cm MACs
387: keyword for more information.
388: .It Fl N
389: Do not execute a remote command.
390: This is useful for just forwarding ports
391: (protocol version 2 only).
392: .It Fl n
393: Redirects stdin from
394: .Pa /dev/null
395: (actually, prevents reading from stdin).
396: This must be used when
1.2 deraadt 397: .Nm
1.218 jmc 398: is run in the background.
399: A common trick is to use this to run X11 programs on a remote machine.
400: For example,
401: .Ic ssh -n shadows.cs.hut.fi emacs &
402: will start an emacs on shadows.cs.hut.fi, and the X11
403: connection will be automatically forwarded over an encrypted channel.
404: The
1.2 deraadt 405: .Nm
1.218 jmc 406: program will be put in the background.
407: (This does not work if
1.2 deraadt 408: .Nm
1.218 jmc 409: needs to ask for a password or passphrase; see also the
410: .Fl f
411: option.)
412: .It Fl O Ar ctl_cmd
413: Control an active connection multiplexing master process.
414: When the
415: .Fl O
416: option is specified, the
417: .Ar ctl_cmd
418: argument is interpreted and passed to the master process.
419: Valid commands are:
420: .Dq check
421: (check that the master process is running) and
422: .Dq exit
423: (request the master to exit).
424: .It Fl o Ar option
425: Can be used to give options in the format used in the configuration file.
426: This is useful for specifying options for which there is no separate
427: command-line flag.
428: For full details of the options listed below, and their possible values, see
429: .Xr ssh_config 5 .
1.2 deraadt 430: .Pp
1.218 jmc 431: .Bl -tag -width Ds -offset indent -compact
432: .It AddressFamily
433: .It BatchMode
434: .It BindAddress
435: .It ChallengeResponseAuthentication
436: .It CheckHostIP
437: .It Cipher
438: .It Ciphers
439: .It ClearAllForwardings
440: .It Compression
441: .It CompressionLevel
442: .It ConnectionAttempts
443: .It ConnectTimeout
444: .It ControlMaster
445: .It ControlPath
446: .It DynamicForward
447: .It EscapeChar
1.263 markus 448: .It ExitOnForwardFailure
1.218 jmc 449: .It ForwardAgent
450: .It ForwardX11
451: .It ForwardX11Trusted
452: .It GatewayPorts
453: .It GlobalKnownHostsFile
454: .It GSSAPIAuthentication
455: .It GSSAPIDelegateCredentials
456: .It HashKnownHosts
457: .It Host
458: .It HostbasedAuthentication
459: .It HostKeyAlgorithms
460: .It HostKeyAlias
461: .It HostName
462: .It IdentityFile
463: .It IdentitiesOnly
464: .It KbdInteractiveDevices
465: .It LocalCommand
466: .It LocalForward
467: .It LogLevel
468: .It MACs
469: .It NoHostAuthenticationForLocalhost
470: .It NumberOfPasswordPrompts
471: .It PasswordAuthentication
472: .It PermitLocalCommand
473: .It Port
474: .It PreferredAuthentications
475: .It Protocol
476: .It ProxyCommand
477: .It PubkeyAuthentication
1.251 dtucker 478: .It RekeyLimit
1.218 jmc 479: .It RemoteForward
480: .It RhostsRSAAuthentication
481: .It RSAAuthentication
482: .It SendEnv
483: .It ServerAliveInterval
484: .It ServerAliveCountMax
485: .It SmartcardDevice
486: .It StrictHostKeyChecking
487: .It TCPKeepAlive
488: .It Tunnel
489: .It TunnelDevice
490: .It UsePrivilegedPort
491: .It User
492: .It UserKnownHostsFile
493: .It VerifyHostKeyDNS
1.276 jmc 494: .It VisualHostKey
1.218 jmc 495: .It XAuthLocation
496: .El
497: .It Fl p Ar port
498: Port to connect to on the remote host.
499: This can be specified on a
500: per-host basis in the configuration file.
501: .It Fl q
502: Quiet mode.
1.271 djm 503: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 504: .It Fl R Xo
505: .Sm off
506: .Oo Ar bind_address : Oc
507: .Ar port : host : hostport
508: .Sm on
509: .Xc
510: Specifies that the given port on the remote (server) host is to be
511: forwarded to the given host and port on the local side.
512: This works by allocating a socket to listen to
513: .Ar port
514: on the remote side, and whenever a connection is made to this port, the
515: connection is forwarded over the secure channel, and a connection is
516: made to
517: .Ar host
518: port
519: .Ar hostport
520: from the local machine.
1.2 deraadt 521: .Pp
1.218 jmc 522: Port forwardings can also be specified in the configuration file.
523: Privileged ports can be forwarded only when
524: logging in as root on the remote machine.
525: IPv6 addresses can be specified by enclosing the address in square braces or
526: using an alternative syntax:
527: .Sm off
528: .Xo
529: .Op Ar bind_address No /
530: .Ar host No / Ar port No /
531: .Ar hostport
532: .Xc .
533: .Sm on
1.194 jakob 534: .Pp
1.218 jmc 535: By default, the listening socket on the server will be bound to the loopback
536: interface only.
1.280 tobias 537: This may be overridden by specifying a
1.218 jmc 538: .Ar bind_address .
539: An empty
540: .Ar bind_address ,
541: or the address
542: .Ql * ,
543: indicates that the remote socket should listen on all interfaces.
544: Specifying a remote
545: .Ar bind_address
546: will only succeed if the server's
547: .Cm GatewayPorts
548: option is enabled (see
549: .Xr sshd_config 5 ) .
1.281 djm 550: .Pp
551: If the
552: .Ar port
553: argument is
1.282 djm 554: .Ql 0 ,
1.281 djm 555: the listen port will be dynamically allocated on the server and reported
556: to the client at run time.
1.218 jmc 557: .It Fl S Ar ctl_path
558: Specifies the location of a control socket for connection sharing.
559: Refer to the description of
560: .Cm ControlPath
561: and
562: .Cm ControlMaster
563: in
564: .Xr ssh_config 5
565: for details.
566: .It Fl s
567: May be used to request invocation of a subsystem on the remote system.
568: Subsystems are a feature of the SSH2 protocol which facilitate the use
569: of SSH as a secure transport for other applications (eg.\&
570: .Xr sftp 1 ) .
571: The subsystem is specified as the remote command.
572: .It Fl T
573: Disable pseudo-tty allocation.
574: .It Fl t
575: Force pseudo-tty allocation.
576: This can be used to execute arbitrary
577: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 578: e.g. when implementing menu services.
1.218 jmc 579: Multiple
580: .Fl t
581: options force tty allocation, even if
1.194 jakob 582: .Nm
1.218 jmc 583: has no local tty.
584: .It Fl V
585: Display the version number and exit.
586: .It Fl v
587: Verbose mode.
588: Causes
1.176 jmc 589: .Nm
1.218 jmc 590: to print debugging messages about its progress.
591: This is helpful in
592: debugging connection, authentication, and configuration problems.
593: Multiple
594: .Fl v
595: options increase the verbosity.
596: The maximum is 3.
1.261 stevesk 597: .It Fl w Xo
598: .Ar local_tun Ns Op : Ns Ar remote_tun
599: .Xc
600: Requests
601: tunnel
602: device forwarding with the specified
1.218 jmc 603: .Xr tun 4
1.261 stevesk 604: devices between the client
605: .Pq Ar local_tun
606: and the server
607: .Pq Ar remote_tun .
608: .Pp
1.228 jmc 609: The devices may be specified by numerical ID or the keyword
610: .Dq any ,
611: which uses the next available tunnel device.
1.261 stevesk 612: If
613: .Ar remote_tun
614: is not specified, it defaults to
615: .Dq any .
1.228 jmc 616: See also the
1.218 jmc 617: .Cm Tunnel
1.261 stevesk 618: and
619: .Cm TunnelDevice
620: directives in
1.218 jmc 621: .Xr ssh_config 5 .
1.261 stevesk 622: If the
623: .Cm Tunnel
624: directive is unset, it is set to the default tunnel mode, which is
625: .Dq point-to-point .
1.218 jmc 626: .It Fl X
627: Enables X11 forwarding.
1.54 markus 628: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 629: .Pp
1.218 jmc 630: X11 forwarding should be enabled with caution.
1.168 jmc 631: Users with the ability to bypass file permissions on the remote host
1.218 jmc 632: (for the user's X authorization database)
633: can access the local X11 display through the forwarded connection.
634: An attacker may then be able to perform activities such as keystroke monitoring.
635: .Pp
636: For this reason, X11 forwarding is subjected to X11 SECURITY extension
637: restrictions by default.
638: Please refer to the
639: .Nm
640: .Fl Y
641: option and the
642: .Cm ForwardX11Trusted
643: directive in
644: .Xr ssh_config 5
645: for more information.
646: .It Fl x
647: Disables X11 forwarding.
648: .It Fl Y
649: Enables trusted X11 forwarding.
650: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
651: controls.
1.278 djm 652: .It Fl y
653: Send log information using the
654: .Xr syslog 3
655: system module.
656: By default this information is sent to stderr.
1.218 jmc 657: .El
1.224 jmc 658: .Pp
659: .Nm
660: may additionally obtain configuration data from
661: a per-user configuration file and a system-wide configuration file.
662: The file format and configuration options are described in
663: .Xr ssh_config 5 .
664: .Pp
665: .Nm
666: exits with the exit status of the remote command or with 255
667: if an error occurred.
1.222 jmc 668: .Sh AUTHENTICATION
1.249 jmc 669: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 670: The default is to use protocol 2 only,
671: though this can be changed via the
1.222 jmc 672: .Cm Protocol
673: option in
1.284 jmc 674: .Xr ssh_config 5
675: or the
1.222 jmc 676: .Fl 1
677: and
678: .Fl 2
679: options (see above).
680: Both protocols support similar authentication methods,
1.284 jmc 681: but protocol 2 is the default since
1.222 jmc 682: it provides additional mechanisms for confidentiality
683: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 684: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 685: Protocol 1 lacks a strong mechanism for ensuring the
686: integrity of the connection.
687: .Pp
688: The methods available for authentication are:
1.260 jmc 689: GSSAPI-based authentication,
1.222 jmc 690: host-based authentication,
691: public key authentication,
692: challenge-response authentication,
693: and password authentication.
694: Authentication methods are tried in the order specified above,
695: though protocol 2 has a configuration option to change the default order:
696: .Cm PreferredAuthentications .
697: .Pp
698: Host-based authentication works as follows:
1.218 jmc 699: If the machine the user logs in from is listed in
700: .Pa /etc/hosts.equiv
701: or
702: .Pa /etc/shosts.equiv
703: on the remote machine, and the user names are
704: the same on both sides, or if the files
705: .Pa ~/.rhosts
706: or
707: .Pa ~/.shosts
708: exist in the user's home directory on the
709: remote machine and contain a line containing the name of the client
710: machine and the name of the user on that machine, the user is
1.222 jmc 711: considered for login.
712: Additionally, the server
713: .Em must
714: be able to verify the client's
715: host key (see the description of
1.218 jmc 716: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 717: and
1.222 jmc 718: .Pa ~/.ssh/known_hosts ,
719: below)
720: for login to be permitted.
1.218 jmc 721: This authentication method closes security holes due to IP
1.222 jmc 722: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 723: [Note to the administrator:
724: .Pa /etc/hosts.equiv ,
725: .Pa ~/.rhosts ,
726: and the rlogin/rsh protocol in general, are inherently insecure and should be
727: disabled if security is desired.]
1.189 dtucker 728: .Pp
1.222 jmc 729: Public key authentication works as follows:
730: The scheme is based on public-key cryptography,
731: using cryptosystems
732: where encryption and decryption are done using separate keys,
733: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 734: The idea is that each user creates a public/private
735: key pair for authentication purposes.
736: The server knows the public key, and only the user knows the private key.
1.222 jmc 737: .Nm
738: implements public key authentication protocol automatically,
739: using either the RSA or DSA algorithms.
740: Protocol 1 is restricted to using only RSA keys,
741: but protocol 2 may use either.
742: The
743: .Sx HISTORY
744: section of
745: .Xr ssl 8
746: contains a brief discussion of the two algorithms.
1.210 djm 747: .Pp
1.218 jmc 748: The file
749: .Pa ~/.ssh/authorized_keys
750: lists the public keys that are permitted for logging in.
751: When the user logs in, the
1.2 deraadt 752: .Nm
1.218 jmc 753: program tells the server which key pair it would like to use for
754: authentication.
1.222 jmc 755: The client proves that it has access to the private key
756: and the server checks that the corresponding public key
757: is authorized to accept the account.
1.218 jmc 758: .Pp
1.222 jmc 759: The user creates his/her key pair by running
1.218 jmc 760: .Xr ssh-keygen 1 .
761: This stores the private key in
1.207 djm 762: .Pa ~/.ssh/identity
1.222 jmc 763: (protocol 1),
764: .Pa ~/.ssh/id_dsa
765: (protocol 2 DSA),
766: or
767: .Pa ~/.ssh/id_rsa
768: (protocol 2 RSA)
1.218 jmc 769: and stores the public key in
770: .Pa ~/.ssh/identity.pub
1.222 jmc 771: (protocol 1),
772: .Pa ~/.ssh/id_dsa.pub
773: (protocol 2 DSA),
774: or
775: .Pa ~/.ssh/id_rsa.pub
776: (protocol 2 RSA)
1.218 jmc 777: in the user's home directory.
1.222 jmc 778: The user should then copy the public key
1.218 jmc 779: to
780: .Pa ~/.ssh/authorized_keys
1.222 jmc 781: in his/her home directory on the remote machine.
782: The
1.218 jmc 783: .Pa authorized_keys
784: file corresponds to the conventional
785: .Pa ~/.rhosts
786: file, and has one key
1.222 jmc 787: per line, though the lines can be very long.
1.218 jmc 788: After this, the user can log in without giving the password.
789: .Pp
1.222 jmc 790: The most convenient way to use public key authentication may be with an
1.218 jmc 791: authentication agent.
792: See
793: .Xr ssh-agent 1
794: for more information.
795: .Pp
1.222 jmc 796: Challenge-response authentication works as follows:
797: The server sends an arbitrary
798: .Qq challenge
799: text, and prompts for a response.
800: Protocol 2 allows multiple challenges and responses;
801: protocol 1 is restricted to just one challenge/response.
802: Examples of challenge-response authentication include
803: BSD Authentication (see
804: .Xr login.conf 5 )
805: and PAM (some non-OpenBSD systems).
806: .Pp
807: Finally, if other authentication methods fail,
1.218 jmc 808: .Nm
809: prompts the user for a password.
810: The password is sent to the remote
811: host for checking; however, since all communications are encrypted,
812: the password cannot be seen by someone listening on the network.
1.232 jmc 813: .Pp
814: .Nm
815: automatically maintains and checks a database containing
816: identification for all hosts it has ever been used with.
817: Host keys are stored in
818: .Pa ~/.ssh/known_hosts
819: in the user's home directory.
820: Additionally, the file
821: .Pa /etc/ssh/ssh_known_hosts
822: is automatically checked for known hosts.
823: Any new hosts are automatically added to the user's file.
824: If a host's identification ever changes,
825: .Nm
826: warns about this and disables password authentication to prevent
827: server spoofing or man-in-the-middle attacks,
828: which could otherwise be used to circumvent the encryption.
829: The
830: .Cm StrictHostKeyChecking
831: option can be used to control logins to machines whose
832: host key is not known or has changed.
833: .Pp
1.218 jmc 834: When the user's identity has been accepted by the server, the server
835: either executes the given command, or logs into the machine and gives
836: the user a normal shell on the remote machine.
837: All communication with
838: the remote command or shell will be automatically encrypted.
839: .Pp
840: If a pseudo-terminal has been allocated (normal login session), the
841: user may use the escape characters noted below.
842: .Pp
843: If no pseudo-tty has been allocated,
844: the session is transparent and can be used to reliably transfer binary data.
845: On most systems, setting the escape character to
846: .Dq none
847: will also make the session transparent even if a tty is used.
848: .Pp
849: The session terminates when the command or shell on the remote
1.247 jmc 850: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 851: .Sh ESCAPE CHARACTERS
1.218 jmc 852: When a pseudo-terminal has been requested,
1.2 deraadt 853: .Nm
1.218 jmc 854: supports a number of functions through the use of an escape character.
855: .Pp
856: A single tilde character can be sent as
857: .Ic ~~
858: or by following the tilde by a character other than those described below.
859: The escape character must always follow a newline to be interpreted as
860: special.
861: The escape character can be changed in configuration files using the
862: .Cm EscapeChar
863: configuration directive or on the command line by the
864: .Fl e
865: option.
866: .Pp
867: The supported escapes (assuming the default
868: .Ql ~ )
869: are:
870: .Bl -tag -width Ds
871: .It Cm ~.
872: Disconnect.
873: .It Cm ~^Z
874: Background
1.234 jmc 875: .Nm .
1.218 jmc 876: .It Cm ~#
877: List forwarded connections.
878: .It Cm ~&
879: Background
1.2 deraadt 880: .Nm
1.218 jmc 881: at logout when waiting for forwarded connection / X11 sessions to terminate.
882: .It Cm ~?
883: Display a list of escape characters.
884: .It Cm ~B
885: Send a BREAK to the remote system
886: (only useful for SSH protocol version 2 and if the peer supports it).
887: .It Cm ~C
888: Open command line.
889: Currently this allows the addition of port forwardings using the
1.279 stevesk 890: .Fl L ,
891: .Fl R
1.218 jmc 892: and
1.279 stevesk 893: .Fl D
1.225 jmc 894: options (see above).
1.218 jmc 895: It also allows the cancellation of existing remote port-forwardings
896: using
1.262 stevesk 897: .Sm off
898: .Fl KR Oo Ar bind_address : Oc Ar port .
899: .Sm on
1.218 jmc 900: .Ic !\& Ns Ar command
901: allows the user to execute a local command if the
902: .Ic PermitLocalCommand
903: option is enabled in
1.176 jmc 904: .Xr ssh_config 5 .
1.218 jmc 905: Basic help is available, using the
906: .Fl h
907: option.
908: .It Cm ~R
909: Request rekeying of the connection
910: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 911: .El
1.246 jmc 912: .Sh TCP FORWARDING
913: Forwarding of arbitrary TCP connections over the secure channel can
914: be specified either on the command line or in a configuration file.
915: One possible application of TCP forwarding is a secure connection to a
916: mail server; another is going through firewalls.
917: .Pp
918: In the example below, we look at encrypting communication between
919: an IRC client and server, even though the IRC server does not directly
920: support encrypted communications.
921: This works as follows:
922: the user connects to the remote host using
923: .Nm ,
924: specifying a port to be used to forward connections
925: to the remote server.
926: After that it is possible to start the service which is to be encrypted
927: on the client machine,
928: connecting to the same local port,
929: and
930: .Nm
931: will encrypt and forward the connection.
932: .Pp
933: The following example tunnels an IRC session from client machine
934: .Dq 127.0.0.1
935: (localhost)
936: to remote server
937: .Dq server.example.com :
938: .Bd -literal -offset 4n
939: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
940: $ irc -c '#users' -p 1234 pinky 127.0.0.1
941: .Ed
942: .Pp
943: This tunnels a connection to IRC server
944: .Dq server.example.com ,
945: joining channel
946: .Dq #users ,
947: nickname
948: .Dq pinky ,
949: using port 1234.
950: It doesn't matter which port is used,
951: as long as it's greater than 1023
952: (remember, only root can open sockets on privileged ports)
953: and doesn't conflict with any ports already in use.
954: The connection is forwarded to port 6667 on the remote server,
955: since that's the standard port for IRC services.
956: .Pp
957: The
958: .Fl f
959: option backgrounds
960: .Nm
961: and the remote command
962: .Dq sleep 10
963: is specified to allow an amount of time
964: (10 seconds, in the example)
965: to start the service which is to be tunnelled.
966: If no connections are made within the time specified,
967: .Nm
968: will exit.
969: .Sh X11 FORWARDING
1.218 jmc 970: If the
971: .Cm ForwardX11
972: variable is set to
973: .Dq yes
974: (or see the description of the
1.227 jmc 975: .Fl X ,
976: .Fl x ,
1.218 jmc 977: and
1.227 jmc 978: .Fl Y
1.226 jmc 979: options above)
1.218 jmc 980: and the user is using X11 (the
981: .Ev DISPLAY
982: environment variable is set), the connection to the X11 display is
983: automatically forwarded to the remote side in such a way that any X11
984: programs started from the shell (or command) will go through the
985: encrypted channel, and the connection to the real X server will be made
1.176 jmc 986: from the local machine.
1.218 jmc 987: The user should not manually set
988: .Ev DISPLAY .
989: Forwarding of X11 connections can be
990: configured on the command line or in configuration files.
991: .Pp
992: The
993: .Ev DISPLAY
994: value set by
995: .Nm
996: will point to the server machine, but with a display number greater than zero.
997: This is normal, and happens because
998: .Nm
999: creates a
1000: .Dq proxy
1001: X server on the server machine for forwarding the
1002: connections over the encrypted channel.
1.200 djm 1003: .Pp
1.218 jmc 1004: .Nm
1005: will also automatically set up Xauthority data on the server machine.
1006: For this purpose, it will generate a random authorization cookie,
1007: store it in Xauthority on the server, and verify that any forwarded
1008: connections carry this cookie and replace it by the real cookie when
1009: the connection is opened.
1010: The real authentication cookie is never
1011: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1012: .Pp
1.218 jmc 1013: If the
1014: .Cm ForwardAgent
1015: variable is set to
1016: .Dq yes
1017: (or see the description of the
1018: .Fl A
1.191 djm 1019: and
1.218 jmc 1020: .Fl a
1.226 jmc 1021: options above) and
1.218 jmc 1022: the user is using an authentication agent, the connection to the agent
1023: is automatically forwarded to the remote side.
1.252 jmc 1024: .Sh VERIFYING HOST KEYS
1025: When connecting to a server for the first time,
1026: a fingerprint of the server's public key is presented to the user
1027: (unless the option
1028: .Cm StrictHostKeyChecking
1029: has been disabled).
1030: Fingerprints can be determined using
1031: .Xr ssh-keygen 1 :
1032: .Pp
1033: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1034: .Pp
1.274 grunk 1035: If the fingerprint is already known, it can be matched
1036: and the key can be accepted or rejected.
1037: Because of the difficulty of comparing host keys
1038: just by looking at hex strings,
1039: there is also support to compare host keys visually,
1040: using
1041: .Em random art .
1042: By setting the
1.275 grunk 1043: .Cm VisualHostKey
1.274 grunk 1044: option to
1.275 grunk 1045: .Dq yes ,
1.274 grunk 1046: a small ASCII graphic gets displayed on every login to a server, no matter
1047: if the session itself is interactive or not.
1048: By learning the pattern a known server produces, a user can easily
1049: find out that the host key has changed when a completely different pattern
1050: is displayed.
1051: Because these patterns are not unambiguous however, a pattern that looks
1052: similar to the pattern remembered only gives a good probability that the
1053: host key is the same, not guaranteed proof.
1054: .Pp
1055: To get a listing of the fingerprints along with their random art for
1056: all known hosts, the following command line can be used:
1057: .Pp
1058: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1059: .Pp
1.252 jmc 1060: If the fingerprint is unknown,
1061: an alternative method of verification is available:
1062: SSH fingerprints verified by DNS.
1063: An additional resource record (RR),
1064: SSHFP,
1065: is added to a zonefile
1066: and the connecting client is able to match the fingerprint
1067: with that of the key presented.
1068: .Pp
1069: In this example, we are connecting a client to a server,
1070: .Dq host.example.com .
1071: The SSHFP resource records should first be added to the zonefile for
1072: host.example.com:
1073: .Bd -literal -offset indent
1.259 jakob 1074: $ ssh-keygen -r host.example.com.
1.252 jmc 1075: .Ed
1076: .Pp
1077: The output lines will have to be added to the zonefile.
1078: To check that the zone is answering fingerprint queries:
1079: .Pp
1080: .Dl $ dig -t SSHFP host.example.com
1081: .Pp
1082: Finally the client connects:
1083: .Bd -literal -offset indent
1084: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1085: [...]
1086: Matching host key fingerprint found in DNS.
1087: Are you sure you want to continue connecting (yes/no)?
1088: .Ed
1089: .Pp
1090: See the
1091: .Cm VerifyHostKeyDNS
1092: option in
1093: .Xr ssh_config 5
1094: for more information.
1.250 jmc 1095: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1096: .Nm
1097: contains support for Virtual Private Network (VPN) tunnelling
1098: using the
1099: .Xr tun 4
1100: network pseudo-device,
1101: allowing two networks to be joined securely.
1102: The
1103: .Xr sshd_config 5
1104: configuration option
1105: .Cm PermitTunnel
1106: controls whether the server supports this,
1107: and at what level (layer 2 or 3 traffic).
1108: .Pp
1109: The following example would connect client network 10.0.50.0/24
1.265 otto 1110: with remote network 10.0.99.0/24 using a point-to-point connection
1111: from 10.1.1.1 to 10.1.1.2,
1112: provided that the SSH server running on the gateway to the remote network,
1113: at 192.168.1.15, allows it.
1114: .Pp
1115: On the client:
1.250 jmc 1116: .Bd -literal -offset indent
1117: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1118: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1119: # route add 10.0.99.0/24 10.1.1.2
1120: .Ed
1121: .Pp
1122: On the server:
1123: .Bd -literal -offset indent
1124: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1125: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1126: .Ed
1127: .Pp
1128: Client access may be more finely tuned via the
1129: .Pa /root/.ssh/authorized_keys
1130: file (see below) and the
1131: .Cm PermitRootLogin
1132: server option.
1.255 jmc 1133: The following entry would permit connections on
1.250 jmc 1134: .Xr tun 4
1.255 jmc 1135: device 1 from user
1.250 jmc 1136: .Dq jane
1.255 jmc 1137: and on tun device 2 from user
1.250 jmc 1138: .Dq john ,
1139: if
1140: .Cm PermitRootLogin
1141: is set to
1142: .Dq forced-commands-only :
1143: .Bd -literal -offset 2n
1144: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1145: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1146: .Ed
1147: .Pp
1.264 ray 1148: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1149: it may be more suited to temporary setups,
1150: such as for wireless VPNs.
1151: More permanent VPNs are better provided by tools such as
1152: .Xr ipsecctl 8
1153: and
1154: .Xr isakmpd 8 .
1.2 deraadt 1155: .Sh ENVIRONMENT
1156: .Nm
1.1 deraadt 1157: will normally set the following environment variables:
1.237 jmc 1158: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1159: .It Ev DISPLAY
1160: The
1161: .Ev DISPLAY
1.40 aaron 1162: variable indicates the location of the X11 server.
1.44 aaron 1163: It is automatically set by
1.2 deraadt 1164: .Nm
1165: to point to a value of the form
1.233 jmc 1166: .Dq hostname:n ,
1167: where
1168: .Dq hostname
1169: indicates the host where the shell runs, and
1170: .Sq n
1171: is an integer \*(Ge 1.
1.40 aaron 1172: .Nm
1173: uses this special value to forward X11 connections over the secure
1174: channel.
1.107 markus 1175: The user should normally not set
1176: .Ev DISPLAY
1177: explicitly, as that
1.1 deraadt 1178: will render the X11 connection insecure (and will require the user to
1179: manually copy any required authorization cookies).
1.2 deraadt 1180: .It Ev HOME
1.1 deraadt 1181: Set to the path of the user's home directory.
1.2 deraadt 1182: .It Ev LOGNAME
1183: Synonym for
1.12 aaron 1184: .Ev USER ;
1185: set for compatibility with systems that use this variable.
1.2 deraadt 1186: .It Ev MAIL
1.129 stevesk 1187: Set to the path of the user's mailbox.
1.40 aaron 1188: .It Ev PATH
1.2 deraadt 1189: Set to the default
1190: .Ev PATH ,
1191: as specified when compiling
1.234 jmc 1192: .Nm .
1.118 markus 1193: .It Ev SSH_ASKPASS
1194: If
1195: .Nm
1196: needs a passphrase, it will read the passphrase from the current
1197: terminal if it was run from a terminal.
1198: If
1199: .Nm
1200: does not have a terminal associated with it but
1201: .Ev DISPLAY
1202: and
1203: .Ev SSH_ASKPASS
1204: are set, it will execute the program specified by
1205: .Ev SSH_ASKPASS
1206: and open an X11 window to read the passphrase.
1207: This is particularly useful when calling
1208: .Nm
1209: from a
1.196 jmc 1210: .Pa .xsession
1.118 markus 1211: or related script.
1212: (Note that on some machines it
1213: may be necessary to redirect the input from
1214: .Pa /dev/null
1215: to make this work.)
1.18 markus 1216: .It Ev SSH_AUTH_SOCK
1.233 jmc 1217: Identifies the path of a
1218: .Ux Ns -domain
1219: socket used to communicate with the agent.
1.166 stevesk 1220: .It Ev SSH_CONNECTION
1221: Identifies the client and server ends of the connection.
1.40 aaron 1222: The variable contains
1.233 jmc 1223: four space-separated values: client IP address, client port number,
1224: server IP address, and server port number.
1.73 markus 1225: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1226: This variable contains the original command line if a forced command
1.73 markus 1227: is executed.
1228: It can be used to extract the original arguments.
1.2 deraadt 1229: .It Ev SSH_TTY
1.1 deraadt 1230: This is set to the name of the tty (path to the device) associated
1.40 aaron 1231: with the current shell or command.
1232: If the current session has no tty,
1.1 deraadt 1233: this variable is not set.
1.2 deraadt 1234: .It Ev TZ
1.214 jmc 1235: This variable is set to indicate the present time zone if it
1.257 jmc 1236: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1237: on to new connections).
1.2 deraadt 1238: .It Ev USER
1.1 deraadt 1239: Set to the name of the user logging in.
1.2 deraadt 1240: .El
1241: .Pp
1.44 aaron 1242: Additionally,
1.2 deraadt 1243: .Nm
1.44 aaron 1244: reads
1.207 djm 1245: .Pa ~/.ssh/environment ,
1.2 deraadt 1246: and adds lines of the format
1247: .Dq VARNAME=value
1.233 jmc 1248: to the environment if the file exists and users are allowed to
1.161 marc 1249: change their environment.
1.176 jmc 1250: For more information, see the
1.161 marc 1251: .Cm PermitUserEnvironment
1.162 stevesk 1252: option in
1.161 marc 1253: .Xr sshd_config 5 .
1.2 deraadt 1254: .Sh FILES
1.236 jmc 1255: .Bl -tag -width Ds -compact
1256: .It ~/.rhosts
1.240 jmc 1257: This file is used for host-based authentication (see above).
1.92 markus 1258: On some machines this file may need to be
1.240 jmc 1259: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1260: because
1.2 deraadt 1261: .Xr sshd 8
1.40 aaron 1262: reads it as root.
1263: Additionally, this file must be owned by the user,
1264: and must not have write permissions for anyone else.
1265: The recommended
1.1 deraadt 1266: permission for most machines is read/write for the user, and not
1267: accessible by others.
1.2 deraadt 1268: .Pp
1.236 jmc 1269: .It ~/.shosts
1.240 jmc 1270: This file is used in exactly the same way as
1271: .Pa .rhosts ,
1272: but allows host-based authentication without permitting login with
1273: rlogin/rsh.
1.272 mcbride 1274: .Pp
1275: .It ~/.ssh/
1276: This directory is the default location for all user-specific configuration
1277: and authentication information.
1278: There is no general requirement to keep the entire contents of this directory
1279: secret, but the recommended permissions are read/write/execute for the user,
1280: and not accessible by others.
1.236 jmc 1281: .Pp
1.238 jmc 1282: .It ~/.ssh/authorized_keys
1283: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1284: The format of this file is described in the
1285: .Xr sshd 8
1286: manual page.
1287: This file is not highly sensitive, but the recommended
1288: permissions are read/write for the user, and not accessible by others.
1289: .Pp
1290: .It ~/.ssh/config
1291: This is the per-user configuration file.
1292: The file format and configuration options are described in
1293: .Xr ssh_config 5 .
1294: Because of the potential for abuse, this file must have strict permissions:
1295: read/write for the user, and not accessible by others.
1296: .Pp
1297: .It ~/.ssh/environment
1.239 jmc 1298: Contains additional definitions for environment variables; see
1299: .Sx ENVIRONMENT ,
1.238 jmc 1300: above.
1301: .Pp
1302: .It ~/.ssh/identity
1303: .It ~/.ssh/id_dsa
1304: .It ~/.ssh/id_rsa
1305: Contains the private key for authentication.
1306: These files
1307: contain sensitive data and should be readable by the user but not
1308: accessible by others (read/write/execute).
1309: .Nm
1310: will simply ignore a private key file if it is accessible by others.
1311: It is possible to specify a passphrase when
1312: generating the key which will be used to encrypt the
1313: sensitive part of this file using 3DES.
1314: .Pp
1315: .It ~/.ssh/identity.pub
1316: .It ~/.ssh/id_dsa.pub
1317: .It ~/.ssh/id_rsa.pub
1318: Contains the public key for authentication.
1319: These files are not
1320: sensitive and can (but need not) be readable by anyone.
1321: .Pp
1322: .It ~/.ssh/known_hosts
1.244 jmc 1323: Contains a list of host keys for all hosts the user has logged into
1324: that are not already in the systemwide list of known host keys.
1.238 jmc 1325: See
1.244 jmc 1326: .Xr sshd 8
1327: for further details of the format of this file.
1.238 jmc 1328: .Pp
1329: .It ~/.ssh/rc
1330: Commands in this file are executed by
1331: .Nm
1.245 jmc 1332: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1333: started.
1334: See the
1335: .Xr sshd 8
1336: manual page for more information.
1337: .Pp
1.236 jmc 1338: .It /etc/hosts.equiv
1.240 jmc 1339: This file is for host-based authentication (see above).
1340: It should only be writable by root.
1.236 jmc 1341: .Pp
1342: .It /etc/shosts.equiv
1.240 jmc 1343: This file is used in exactly the same way as
1344: .Pa hosts.equiv ,
1345: but allows host-based authentication without permitting login with
1346: rlogin/rsh.
1.236 jmc 1347: .Pp
1.238 jmc 1348: .It Pa /etc/ssh/ssh_config
1349: Systemwide configuration file.
1350: The file format and configuration options are described in
1351: .Xr ssh_config 5 .
1352: .Pp
1353: .It /etc/ssh/ssh_host_key
1354: .It /etc/ssh/ssh_host_dsa_key
1355: .It /etc/ssh/ssh_host_rsa_key
1356: These three files contain the private parts of the host keys
1.245 jmc 1357: and are used for host-based authentication.
1358: If protocol version 1 is used,
1.238 jmc 1359: .Nm
1360: must be setuid root, since the host key is readable only by root.
1361: For protocol version 2,
1362: .Nm
1363: uses
1364: .Xr ssh-keysign 8
1.245 jmc 1365: to access the host keys,
1366: eliminating the requirement that
1.238 jmc 1367: .Nm
1.245 jmc 1368: be setuid root when host-based authentication is used.
1.238 jmc 1369: By default
1.2 deraadt 1370: .Nm
1.238 jmc 1371: is not setuid root.
1372: .Pp
1373: .It /etc/ssh/ssh_known_hosts
1374: Systemwide list of known host keys.
1375: This file should be prepared by the
1376: system administrator to contain the public host keys of all machines in the
1377: organization.
1.244 jmc 1378: It should be world-readable.
1379: See
1.238 jmc 1380: .Xr sshd 8
1.244 jmc 1381: for further details of the format of this file.
1.236 jmc 1382: .Pp
1.238 jmc 1383: .It /etc/ssh/sshrc
1.1 deraadt 1384: Commands in this file are executed by
1.2 deraadt 1385: .Nm
1.245 jmc 1386: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1387: See the
1.2 deraadt 1388: .Xr sshd 8
1.1 deraadt 1389: manual page for more information.
1.58 itojun 1390: .El
1.2 deraadt 1391: .Sh SEE ALSO
1392: .Xr scp 1 ,
1.83 djm 1393: .Xr sftp 1 ,
1.2 deraadt 1394: .Xr ssh-add 1 ,
1395: .Xr ssh-agent 1 ,
1396: .Xr ssh-keygen 1 ,
1.242 jmc 1397: .Xr ssh-keyscan 1 ,
1.250 jmc 1398: .Xr tun 4 ,
1.176 jmc 1399: .Xr hosts.equiv 5 ,
1.159 stevesk 1400: .Xr ssh_config 5 ,
1.160 naddy 1401: .Xr ssh-keysign 8 ,
1.87 itojun 1402: .Xr sshd 8
1.106 markus 1403: .Rs
1.256 jmc 1404: .%R RFC 4250
1405: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1406: .%D 2006
1407: .Re
1408: .Rs
1409: .%R RFC 4251
1410: .%T "The Secure Shell (SSH) Protocol Architecture"
1411: .%D 2006
1412: .Re
1413: .Rs
1414: .%R RFC 4252
1415: .%T "The Secure Shell (SSH) Authentication Protocol"
1416: .%D 2006
1417: .Re
1418: .Rs
1419: .%R RFC 4253
1420: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1421: .%D 2006
1422: .Re
1423: .Rs
1424: .%R RFC 4254
1425: .%T "The Secure Shell (SSH) Connection Protocol"
1426: .%D 2006
1427: .Re
1428: .Rs
1429: .%R RFC 4255
1430: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1431: .%D 2006
1432: .Re
1433: .Rs
1434: .%R RFC 4256
1435: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1436: .%D 2006
1437: .Re
1438: .Rs
1439: .%R RFC 4335
1440: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1441: .%D 2006
1442: .Re
1443: .Rs
1444: .%R RFC 4344
1445: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1446: .%D 2006
1447: .Re
1448: .Rs
1449: .%R RFC 4345
1450: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1451: .%D 2006
1452: .Re
1453: .Rs
1454: .%R RFC 4419
1455: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1456: .%D 2006
1457: .Re
1458: .Rs
1459: .%R RFC 4716
1460: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1461: .%D 2006
1.274 grunk 1462: .Re
1463: .Rs
1464: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1465: .%A A. Perrig
1466: .%A D. Song
1467: .%D 1999
1468: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1469: .Re
1.173 jmc 1470: .Sh AUTHORS
1471: OpenSSH is a derivative of the original and free
1472: ssh 1.2.12 release by Tatu Ylonen.
1473: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1474: Theo de Raadt and Dug Song
1475: removed many bugs, re-added newer features and
1476: created OpenSSH.
1477: Markus Friedl contributed the support for SSH
1478: protocol versions 1.5 and 2.0.
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