Annotation of src/usr.bin/ssh/ssh.1, Revision 1.290
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: .\"
1.290 ! dtucker 37: .\" $OpenBSD: ssh.1,v 1.289 2010/01/09 23:04:13 dtucker Exp $
! 38: .Dd $Mdocdate: January 9 2010 $
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
1.290 ! dtucker 80: .Op Fl W Ar host : Ns Ar port
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
1.44 aaron 85: .Sh DESCRIPTION
1.2 deraadt 86: .Nm
1.96 deraadt 87: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 88: executing commands on a remote machine.
1.176 jmc 89: It is intended to replace rlogin and rsh,
90: and provide secure encrypted communications between
1.40 aaron 91: two untrusted hosts over an insecure network.
1.247 jmc 92: X11 connections and arbitrary TCP ports
1.176 jmc 93: can also be forwarded over the secure channel.
1.2 deraadt 94: .Pp
95: .Nm
1.44 aaron 96: connects and logs into the specified
1.176 jmc 97: .Ar hostname
98: (with optional
99: .Ar user
100: name).
1.1 deraadt 101: The user must prove
1.49 markus 102: his/her identity to the remote machine using one of several methods
1.221 jmc 103: depending on the protocol version used (see below).
1.49 markus 104: .Pp
1.176 jmc 105: If
106: .Ar command
107: is specified,
1.219 jmc 108: it is executed on the remote host instead of a login shell.
1.2 deraadt 109: .Pp
1.218 jmc 110: The options are as follows:
111: .Bl -tag -width Ds
112: .It Fl 1
113: Forces
1.2 deraadt 114: .Nm
1.218 jmc 115: to try protocol version 1 only.
116: .It Fl 2
117: Forces
1.2 deraadt 118: .Nm
1.218 jmc 119: to try protocol version 2 only.
120: .It Fl 4
121: Forces
1.2 deraadt 122: .Nm
1.218 jmc 123: to use IPv4 addresses only.
124: .It Fl 6
125: Forces
1.2 deraadt 126: .Nm
1.218 jmc 127: to use IPv6 addresses only.
128: .It Fl A
129: Enables forwarding of the authentication agent connection.
130: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 131: .Pp
1.218 jmc 132: Agent forwarding should be enabled with caution.
133: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 134: (for the agent's
1.286 sobrado 135: .Ux Ns -domain
136: socket) can access the local agent through the forwarded connection.
1.218 jmc 137: An attacker cannot obtain key material from the agent,
138: however they can perform operations on the keys that enable them to
139: authenticate using the identities loaded into the agent.
140: .It Fl a
141: Disables forwarding of the authentication agent connection.
142: .It Fl b Ar bind_address
143: Use
144: .Ar bind_address
145: on the local machine as the source address
146: of the connection.
147: Only useful on systems with more than one address.
148: .It Fl C
149: Requests compression of all data (including stdin, stdout, stderr, and
1.247 jmc 150: data for forwarded X11 and TCP connections).
1.218 jmc 151: The compression algorithm is the same used by
152: .Xr gzip 1 ,
153: and the
154: .Dq level
155: can be controlled by the
156: .Cm CompressionLevel
157: option for protocol version 1.
158: Compression is desirable on modem lines and other
159: slow connections, but will only slow down things on fast networks.
160: The default value can be set on a host-by-host basis in the
161: configuration files; see the
162: .Cm Compression
163: option.
164: .It Fl c Ar cipher_spec
165: Selects the cipher specification for encrypting the session.
1.2 deraadt 166: .Pp
1.218 jmc 167: Protocol version 1 allows specification of a single cipher.
168: The supported values are
169: .Dq 3des ,
1.220 jmc 170: .Dq blowfish ,
1.218 jmc 171: and
172: .Dq des .
173: .Ar 3des
174: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
175: It is believed to be secure.
176: .Ar blowfish
177: is a fast block cipher; it appears very secure and is much faster than
178: .Ar 3des .
179: .Ar des
180: is only supported in the
1.2 deraadt 181: .Nm
1.218 jmc 182: client for interoperability with legacy protocol 1 implementations
183: that do not support the
184: .Ar 3des
185: cipher.
186: Its use is strongly discouraged due to cryptographic weaknesses.
187: The default is
188: .Dq 3des .
1.49 markus 189: .Pp
1.230 jmc 190: For protocol version 2,
1.218 jmc 191: .Ar cipher_spec
192: is a comma-separated list of ciphers
193: listed in order of preference.
1.283 jmc 194: See the
195: .Cm Ciphers
196: keyword for more information.
1.218 jmc 197: .It Fl D Xo
198: .Sm off
199: .Oo Ar bind_address : Oc
200: .Ar port
201: .Sm on
202: .Xc
203: Specifies a local
204: .Dq dynamic
205: application-level port forwarding.
206: This works by allocating a socket to listen to
207: .Ar port
208: on the local side, optionally bound to the specified
209: .Ar bind_address .
210: Whenever a connection is made to this port, the
211: connection is forwarded over the secure channel, and the application
212: protocol is then used to determine where to connect to from the
213: remote machine.
214: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 215: .Nm
1.218 jmc 216: will act as a SOCKS server.
217: Only root can forward privileged ports.
218: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 219: .Pp
1.218 jmc 220: IPv6 addresses can be specified with an alternative syntax:
221: .Sm off
222: .Xo
223: .Op Ar bind_address No /
224: .Ar port
225: .Xc
226: .Sm on
227: or by enclosing the address in square brackets.
228: Only the superuser can forward privileged ports.
229: By default, the local port is bound in accordance with the
230: .Cm GatewayPorts
231: setting.
232: However, an explicit
233: .Ar bind_address
234: may be used to bind the connection to a specific address.
235: The
236: .Ar bind_address
237: of
238: .Dq localhost
239: indicates that the listening port be bound for local use only, while an
240: empty address or
241: .Sq *
242: indicates that the port should be available from all interfaces.
1.229 jmc 243: .It Fl e Ar escape_char
1.218 jmc 244: Sets the escape character for sessions with a pty (default:
245: .Ql ~ ) .
246: The escape character is only recognized at the beginning of a line.
247: The escape character followed by a dot
248: .Pq Ql \&.
249: closes the connection;
250: followed by control-Z suspends the connection;
251: and followed by itself sends the escape character once.
252: Setting the character to
1.2 deraadt 253: .Dq none
1.218 jmc 254: disables any escapes and makes the session fully transparent.
255: .It Fl F Ar configfile
256: Specifies an alternative per-user configuration file.
257: If a configuration file is given on the command line,
258: the system-wide configuration file
259: .Pq Pa /etc/ssh/ssh_config
260: will be ignored.
261: The default for the per-user configuration file is
262: .Pa ~/.ssh/config .
263: .It Fl f
264: Requests
265: .Nm
266: to go to background just before command execution.
267: This is useful if
1.176 jmc 268: .Nm
1.218 jmc 269: is going to ask for passwords or passphrases, but the user
270: wants it in the background.
271: This implies
272: .Fl n .
273: The recommended way to start X11 programs at a remote site is with
274: something like
275: .Ic ssh -f host xterm .
1.277 djm 276: .Pp
277: If the
278: .Cm ExitOnForwardFailure
279: configuration option is set to
280: .Dq yes ,
281: then a client started with
282: .Fl f
283: will wait for all remote port forwards to be successfully established
284: before placing itself in the background.
1.218 jmc 285: .It Fl g
286: Allows remote hosts to connect to local forwarded ports.
287: .It Fl I Ar smartcard_device
1.229 jmc 288: Specify the device
1.176 jmc 289: .Nm
1.218 jmc 290: should use to communicate with a smartcard used for storing the user's
291: private RSA key.
1.229 jmc 292: This option is only available if support for smartcard devices
293: is compiled in (default is no support).
1.218 jmc 294: .It Fl i Ar identity_file
295: Selects a file from which the identity (private key) for
296: RSA or DSA authentication is read.
297: The default is
298: .Pa ~/.ssh/identity
299: for protocol version 1, and
300: .Pa ~/.ssh/id_rsa
1.149 jakob 301: and
1.218 jmc 302: .Pa ~/.ssh/id_dsa
303: for protocol version 2.
304: Identity files may also be specified on
305: a per-host basis in the configuration file.
306: It is possible to have multiple
307: .Fl i
308: options (and multiple identities specified in
309: configuration files).
1.269 djm 310: .It Fl K
311: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
312: credentials to the server.
1.218 jmc 313: .It Fl k
314: Disables forwarding (delegation) of GSSAPI credentials to the server.
315: .It Fl L Xo
316: .Sm off
317: .Oo Ar bind_address : Oc
318: .Ar port : host : hostport
319: .Sm on
320: .Xc
321: Specifies that the given port on the local (client) host is to be
322: forwarded to the given host and port on the remote side.
323: This works by allocating a socket to listen to
324: .Ar port
325: on the local side, optionally bound to the specified
326: .Ar bind_address .
327: Whenever a connection is made to this port, the
328: connection is forwarded over the secure channel, and a connection is
329: made to
330: .Ar host
331: port
332: .Ar hostport
333: from the remote machine.
334: Port forwardings can also be specified in the configuration file.
335: IPv6 addresses can be specified with an alternative syntax:
336: .Sm off
337: .Xo
338: .Op Ar bind_address No /
339: .Ar port No / Ar host No /
340: .Ar hostport
341: .Xc
342: .Sm on
343: or by enclosing the address in square brackets.
344: Only the superuser can forward privileged ports.
345: By default, the local port is bound in accordance with the
346: .Cm GatewayPorts
347: setting.
348: However, an explicit
349: .Ar bind_address
350: may be used to bind the connection to a specific address.
1.2 deraadt 351: The
1.218 jmc 352: .Ar bind_address
353: of
354: .Dq localhost
355: indicates that the listening port be bound for local use only, while an
356: empty address or
357: .Sq *
358: indicates that the port should be available from all interfaces.
359: .It Fl l Ar login_name
360: Specifies the user to log in as on the remote machine.
361: This also may be specified on a per-host basis in the configuration file.
362: .It Fl M
363: Places the
364: .Nm
365: client into
366: .Dq master
367: mode for connection sharing.
1.231 stevesk 368: Multiple
369: .Fl M
370: options places
371: .Nm
372: into
373: .Dq master
374: mode with confirmation required before slave connections are accepted.
1.218 jmc 375: Refer to the description of
376: .Cm ControlMaster
377: in
378: .Xr ssh_config 5
379: for details.
380: .It Fl m Ar mac_spec
381: Additionally, for protocol version 2 a comma-separated list of MAC
382: (message authentication code) algorithms can
383: be specified in order of preference.
384: See the
385: .Cm MACs
386: keyword for more information.
387: .It Fl N
388: Do not execute a remote command.
389: This is useful for just forwarding ports
390: (protocol version 2 only).
391: .It Fl n
392: Redirects stdin from
393: .Pa /dev/null
394: (actually, prevents reading from stdin).
395: This must be used when
1.2 deraadt 396: .Nm
1.218 jmc 397: is run in the background.
398: A common trick is to use this to run X11 programs on a remote machine.
399: For example,
400: .Ic ssh -n shadows.cs.hut.fi emacs &
401: will start an emacs on shadows.cs.hut.fi, and the X11
402: connection will be automatically forwarded over an encrypted channel.
403: The
1.2 deraadt 404: .Nm
1.218 jmc 405: program will be put in the background.
406: (This does not work if
1.2 deraadt 407: .Nm
1.218 jmc 408: needs to ask for a password or passphrase; see also the
409: .Fl f
410: option.)
411: .It Fl O Ar ctl_cmd
412: Control an active connection multiplexing master process.
413: When the
414: .Fl O
415: option is specified, the
416: .Ar ctl_cmd
417: argument is interpreted and passed to the master process.
418: Valid commands are:
419: .Dq check
420: (check that the master process is running) and
421: .Dq exit
422: (request the master to exit).
423: .It Fl o Ar option
424: Can be used to give options in the format used in the configuration file.
425: This is useful for specifying options for which there is no separate
426: command-line flag.
427: For full details of the options listed below, and their possible values, see
428: .Xr ssh_config 5 .
1.2 deraadt 429: .Pp
1.218 jmc 430: .Bl -tag -width Ds -offset indent -compact
431: .It AddressFamily
432: .It BatchMode
433: .It BindAddress
434: .It ChallengeResponseAuthentication
435: .It CheckHostIP
436: .It Cipher
437: .It Ciphers
438: .It ClearAllForwardings
439: .It Compression
440: .It CompressionLevel
441: .It ConnectionAttempts
442: .It ConnectTimeout
443: .It ControlMaster
444: .It ControlPath
445: .It DynamicForward
446: .It EscapeChar
1.263 markus 447: .It ExitOnForwardFailure
1.218 jmc 448: .It ForwardAgent
449: .It ForwardX11
450: .It ForwardX11Trusted
451: .It GatewayPorts
452: .It GlobalKnownHostsFile
453: .It GSSAPIAuthentication
454: .It GSSAPIDelegateCredentials
455: .It HashKnownHosts
456: .It Host
457: .It HostbasedAuthentication
458: .It HostKeyAlgorithms
459: .It HostKeyAlias
460: .It HostName
461: .It IdentityFile
462: .It IdentitiesOnly
463: .It KbdInteractiveDevices
464: .It LocalCommand
465: .It LocalForward
466: .It LogLevel
467: .It MACs
468: .It NoHostAuthenticationForLocalhost
469: .It NumberOfPasswordPrompts
470: .It PasswordAuthentication
471: .It PermitLocalCommand
472: .It Port
473: .It PreferredAuthentications
474: .It Protocol
475: .It ProxyCommand
476: .It PubkeyAuthentication
1.251 dtucker 477: .It RekeyLimit
1.218 jmc 478: .It RemoteForward
479: .It RhostsRSAAuthentication
480: .It RSAAuthentication
481: .It SendEnv
482: .It ServerAliveInterval
483: .It ServerAliveCountMax
484: .It SmartcardDevice
485: .It StrictHostKeyChecking
486: .It TCPKeepAlive
487: .It Tunnel
488: .It TunnelDevice
489: .It UsePrivilegedPort
490: .It User
491: .It UserKnownHostsFile
492: .It VerifyHostKeyDNS
1.276 jmc 493: .It VisualHostKey
1.218 jmc 494: .It XAuthLocation
495: .El
496: .It Fl p Ar port
497: Port to connect to on the remote host.
498: This can be specified on a
499: per-host basis in the configuration file.
500: .It Fl q
501: Quiet mode.
1.271 djm 502: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 503: .It Fl R Xo
504: .Sm off
505: .Oo Ar bind_address : Oc
506: .Ar port : host : hostport
507: .Sm on
508: .Xc
509: Specifies that the given port on the remote (server) host is to be
510: forwarded to the given host and port on the local side.
511: This works by allocating a socket to listen to
512: .Ar port
513: on the remote side, and whenever a connection is made to this port, the
514: connection is forwarded over the secure channel, and a connection is
515: made to
516: .Ar host
517: port
518: .Ar hostport
519: from the local machine.
1.2 deraadt 520: .Pp
1.218 jmc 521: Port forwardings can also be specified in the configuration file.
522: Privileged ports can be forwarded only when
523: logging in as root on the remote machine.
524: IPv6 addresses can be specified by enclosing the address in square braces or
525: using an alternative syntax:
526: .Sm off
527: .Xo
528: .Op Ar bind_address No /
529: .Ar host No / Ar port No /
530: .Ar hostport
531: .Xc .
532: .Sm on
1.194 jakob 533: .Pp
1.218 jmc 534: By default, the listening socket on the server will be bound to the loopback
535: interface only.
1.280 tobias 536: This may be overridden by specifying a
1.218 jmc 537: .Ar bind_address .
538: An empty
539: .Ar bind_address ,
540: or the address
541: .Ql * ,
542: indicates that the remote socket should listen on all interfaces.
543: Specifying a remote
544: .Ar bind_address
545: will only succeed if the server's
546: .Cm GatewayPorts
547: option is enabled (see
548: .Xr sshd_config 5 ) .
1.281 djm 549: .Pp
550: If the
551: .Ar port
552: argument is
1.282 djm 553: .Ql 0 ,
1.281 djm 554: the listen port will be dynamically allocated on the server and reported
555: to the client at run time.
1.218 jmc 556: .It Fl S Ar ctl_path
557: Specifies the location of a control socket for connection sharing.
558: Refer to the description of
559: .Cm ControlPath
560: and
561: .Cm ControlMaster
562: in
563: .Xr ssh_config 5
564: for details.
565: .It Fl s
566: May be used to request invocation of a subsystem on the remote system.
567: Subsystems are a feature of the SSH2 protocol which facilitate the use
568: of SSH as a secure transport for other applications (eg.\&
569: .Xr sftp 1 ) .
570: The subsystem is specified as the remote command.
571: .It Fl T
572: Disable pseudo-tty allocation.
573: .It Fl t
574: Force pseudo-tty allocation.
575: This can be used to execute arbitrary
576: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 577: e.g. when implementing menu services.
1.218 jmc 578: Multiple
579: .Fl t
580: options force tty allocation, even if
1.194 jakob 581: .Nm
1.218 jmc 582: has no local tty.
583: .It Fl V
584: Display the version number and exit.
585: .It Fl v
586: Verbose mode.
587: Causes
1.176 jmc 588: .Nm
1.218 jmc 589: to print debugging messages about its progress.
590: This is helpful in
591: debugging connection, authentication, and configuration problems.
592: Multiple
593: .Fl v
594: options increase the verbosity.
595: The maximum is 3.
1.290 ! dtucker 596: .It Fl W Ar host : Ns Ar port
! 597: Requests that standard input and output on the client be forwarded to
! 598: .Ar host
! 599: on
! 600: .Ar port
! 601: over the secure channel.
! 602: Implies
! 603: .Fl N ,
! 604: .Fl T ,
! 605: .Cm ExitOnForwardFailure
! 606: and
! 607: .Cm ClearAllForwardings
! 608: and works with Protocol version 2 only.
1.261 stevesk 609: .It Fl w Xo
610: .Ar local_tun Ns Op : Ns Ar remote_tun
611: .Xc
612: Requests
613: tunnel
614: device forwarding with the specified
1.218 jmc 615: .Xr tun 4
1.261 stevesk 616: devices between the client
617: .Pq Ar local_tun
618: and the server
619: .Pq Ar remote_tun .
620: .Pp
1.228 jmc 621: The devices may be specified by numerical ID or the keyword
622: .Dq any ,
623: which uses the next available tunnel device.
1.261 stevesk 624: If
625: .Ar remote_tun
626: is not specified, it defaults to
627: .Dq any .
1.228 jmc 628: See also the
1.218 jmc 629: .Cm Tunnel
1.261 stevesk 630: and
631: .Cm TunnelDevice
632: directives in
1.218 jmc 633: .Xr ssh_config 5 .
1.261 stevesk 634: If the
635: .Cm Tunnel
636: directive is unset, it is set to the default tunnel mode, which is
637: .Dq point-to-point .
1.218 jmc 638: .It Fl X
639: Enables X11 forwarding.
1.54 markus 640: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 641: .Pp
1.218 jmc 642: X11 forwarding should be enabled with caution.
1.168 jmc 643: Users with the ability to bypass file permissions on the remote host
1.218 jmc 644: (for the user's X authorization database)
645: can access the local X11 display through the forwarded connection.
646: An attacker may then be able to perform activities such as keystroke monitoring.
647: .Pp
648: For this reason, X11 forwarding is subjected to X11 SECURITY extension
649: restrictions by default.
650: Please refer to the
651: .Nm
652: .Fl Y
653: option and the
654: .Cm ForwardX11Trusted
655: directive in
656: .Xr ssh_config 5
657: for more information.
658: .It Fl x
659: Disables X11 forwarding.
660: .It Fl Y
661: Enables trusted X11 forwarding.
662: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
663: controls.
1.278 djm 664: .It Fl y
665: Send log information using the
666: .Xr syslog 3
667: system module.
668: By default this information is sent to stderr.
1.218 jmc 669: .El
1.224 jmc 670: .Pp
671: .Nm
672: may additionally obtain configuration data from
673: a per-user configuration file and a system-wide configuration file.
674: The file format and configuration options are described in
675: .Xr ssh_config 5 .
676: .Pp
677: .Nm
678: exits with the exit status of the remote command or with 255
679: if an error occurred.
1.222 jmc 680: .Sh AUTHENTICATION
1.249 jmc 681: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 682: The default is to use protocol 2 only,
683: though this can be changed via the
1.222 jmc 684: .Cm Protocol
685: option in
1.284 jmc 686: .Xr ssh_config 5
687: or the
1.222 jmc 688: .Fl 1
689: and
690: .Fl 2
691: options (see above).
692: Both protocols support similar authentication methods,
1.284 jmc 693: but protocol 2 is the default since
1.222 jmc 694: it provides additional mechanisms for confidentiality
695: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 696: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 697: Protocol 1 lacks a strong mechanism for ensuring the
698: integrity of the connection.
699: .Pp
700: The methods available for authentication are:
1.260 jmc 701: GSSAPI-based authentication,
1.222 jmc 702: host-based authentication,
703: public key authentication,
704: challenge-response authentication,
705: and password authentication.
706: Authentication methods are tried in the order specified above,
707: though protocol 2 has a configuration option to change the default order:
708: .Cm PreferredAuthentications .
709: .Pp
710: Host-based authentication works as follows:
1.218 jmc 711: If the machine the user logs in from is listed in
712: .Pa /etc/hosts.equiv
713: or
714: .Pa /etc/shosts.equiv
715: on the remote machine, and the user names are
716: the same on both sides, or if the files
717: .Pa ~/.rhosts
718: or
719: .Pa ~/.shosts
720: exist in the user's home directory on the
721: remote machine and contain a line containing the name of the client
722: machine and the name of the user on that machine, the user is
1.222 jmc 723: considered for login.
724: Additionally, the server
725: .Em must
726: be able to verify the client's
727: host key (see the description of
1.218 jmc 728: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 729: and
1.222 jmc 730: .Pa ~/.ssh/known_hosts ,
731: below)
732: for login to be permitted.
1.218 jmc 733: This authentication method closes security holes due to IP
1.222 jmc 734: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 735: [Note to the administrator:
736: .Pa /etc/hosts.equiv ,
737: .Pa ~/.rhosts ,
738: and the rlogin/rsh protocol in general, are inherently insecure and should be
739: disabled if security is desired.]
1.189 dtucker 740: .Pp
1.222 jmc 741: Public key authentication works as follows:
742: The scheme is based on public-key cryptography,
743: using cryptosystems
744: where encryption and decryption are done using separate keys,
745: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 746: The idea is that each user creates a public/private
747: key pair for authentication purposes.
748: The server knows the public key, and only the user knows the private key.
1.222 jmc 749: .Nm
750: implements public key authentication protocol automatically,
751: using either the RSA or DSA algorithms.
752: Protocol 1 is restricted to using only RSA keys,
753: but protocol 2 may use either.
754: The
755: .Sx HISTORY
756: section of
757: .Xr ssl 8
758: contains a brief discussion of the two algorithms.
1.210 djm 759: .Pp
1.218 jmc 760: The file
761: .Pa ~/.ssh/authorized_keys
762: lists the public keys that are permitted for logging in.
763: When the user logs in, the
1.2 deraadt 764: .Nm
1.218 jmc 765: program tells the server which key pair it would like to use for
766: authentication.
1.222 jmc 767: The client proves that it has access to the private key
768: and the server checks that the corresponding public key
769: is authorized to accept the account.
1.218 jmc 770: .Pp
1.222 jmc 771: The user creates his/her key pair by running
1.218 jmc 772: .Xr ssh-keygen 1 .
773: This stores the private key in
1.207 djm 774: .Pa ~/.ssh/identity
1.222 jmc 775: (protocol 1),
776: .Pa ~/.ssh/id_dsa
777: (protocol 2 DSA),
778: or
779: .Pa ~/.ssh/id_rsa
780: (protocol 2 RSA)
1.218 jmc 781: and stores the public key in
782: .Pa ~/.ssh/identity.pub
1.222 jmc 783: (protocol 1),
784: .Pa ~/.ssh/id_dsa.pub
785: (protocol 2 DSA),
786: or
787: .Pa ~/.ssh/id_rsa.pub
788: (protocol 2 RSA)
1.218 jmc 789: in the user's home directory.
1.222 jmc 790: The user should then copy the public key
1.218 jmc 791: to
792: .Pa ~/.ssh/authorized_keys
1.222 jmc 793: in his/her home directory on the remote machine.
794: The
1.218 jmc 795: .Pa authorized_keys
796: file corresponds to the conventional
797: .Pa ~/.rhosts
798: file, and has one key
1.222 jmc 799: per line, though the lines can be very long.
1.218 jmc 800: After this, the user can log in without giving the password.
801: .Pp
1.222 jmc 802: The most convenient way to use public key authentication may be with an
1.218 jmc 803: authentication agent.
804: See
805: .Xr ssh-agent 1
806: for more information.
807: .Pp
1.222 jmc 808: Challenge-response authentication works as follows:
809: The server sends an arbitrary
810: .Qq challenge
811: text, and prompts for a response.
812: Protocol 2 allows multiple challenges and responses;
813: protocol 1 is restricted to just one challenge/response.
814: Examples of challenge-response authentication include
815: BSD Authentication (see
816: .Xr login.conf 5 )
817: and PAM (some non-OpenBSD systems).
818: .Pp
819: Finally, if other authentication methods fail,
1.218 jmc 820: .Nm
821: prompts the user for a password.
822: The password is sent to the remote
823: host for checking; however, since all communications are encrypted,
824: the password cannot be seen by someone listening on the network.
1.232 jmc 825: .Pp
826: .Nm
827: automatically maintains and checks a database containing
828: identification for all hosts it has ever been used with.
829: Host keys are stored in
830: .Pa ~/.ssh/known_hosts
831: in the user's home directory.
832: Additionally, the file
833: .Pa /etc/ssh/ssh_known_hosts
834: is automatically checked for known hosts.
835: Any new hosts are automatically added to the user's file.
836: If a host's identification ever changes,
837: .Nm
838: warns about this and disables password authentication to prevent
839: server spoofing or man-in-the-middle attacks,
840: which could otherwise be used to circumvent the encryption.
841: The
842: .Cm StrictHostKeyChecking
843: option can be used to control logins to machines whose
844: host key is not known or has changed.
845: .Pp
1.218 jmc 846: When the user's identity has been accepted by the server, the server
847: either executes the given command, or logs into the machine and gives
848: the user a normal shell on the remote machine.
849: All communication with
850: the remote command or shell will be automatically encrypted.
851: .Pp
852: If a pseudo-terminal has been allocated (normal login session), the
853: user may use the escape characters noted below.
854: .Pp
855: If no pseudo-tty has been allocated,
856: the session is transparent and can be used to reliably transfer binary data.
857: On most systems, setting the escape character to
858: .Dq none
859: will also make the session transparent even if a tty is used.
860: .Pp
861: The session terminates when the command or shell on the remote
1.247 jmc 862: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 863: .Sh ESCAPE CHARACTERS
1.218 jmc 864: When a pseudo-terminal has been requested,
1.2 deraadt 865: .Nm
1.218 jmc 866: supports a number of functions through the use of an escape character.
867: .Pp
868: A single tilde character can be sent as
869: .Ic ~~
870: or by following the tilde by a character other than those described below.
871: The escape character must always follow a newline to be interpreted as
872: special.
873: The escape character can be changed in configuration files using the
874: .Cm EscapeChar
875: configuration directive or on the command line by the
876: .Fl e
877: option.
878: .Pp
879: The supported escapes (assuming the default
880: .Ql ~ )
881: are:
882: .Bl -tag -width Ds
883: .It Cm ~.
884: Disconnect.
885: .It Cm ~^Z
886: Background
1.234 jmc 887: .Nm .
1.218 jmc 888: .It Cm ~#
889: List forwarded connections.
890: .It Cm ~&
891: Background
1.2 deraadt 892: .Nm
1.218 jmc 893: at logout when waiting for forwarded connection / X11 sessions to terminate.
894: .It Cm ~?
895: Display a list of escape characters.
896: .It Cm ~B
897: Send a BREAK to the remote system
898: (only useful for SSH protocol version 2 and if the peer supports it).
899: .It Cm ~C
900: Open command line.
901: Currently this allows the addition of port forwardings using the
1.279 stevesk 902: .Fl L ,
903: .Fl R
1.218 jmc 904: and
1.279 stevesk 905: .Fl D
1.225 jmc 906: options (see above).
1.218 jmc 907: It also allows the cancellation of existing remote port-forwardings
908: using
1.262 stevesk 909: .Sm off
910: .Fl KR Oo Ar bind_address : Oc Ar port .
911: .Sm on
1.218 jmc 912: .Ic !\& Ns Ar command
913: allows the user to execute a local command if the
914: .Ic PermitLocalCommand
915: option is enabled in
1.176 jmc 916: .Xr ssh_config 5 .
1.218 jmc 917: Basic help is available, using the
918: .Fl h
919: option.
920: .It Cm ~R
921: Request rekeying of the connection
922: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 923: .El
1.246 jmc 924: .Sh TCP FORWARDING
925: Forwarding of arbitrary TCP connections over the secure channel can
926: be specified either on the command line or in a configuration file.
927: One possible application of TCP forwarding is a secure connection to a
928: mail server; another is going through firewalls.
929: .Pp
930: In the example below, we look at encrypting communication between
931: an IRC client and server, even though the IRC server does not directly
932: support encrypted communications.
933: This works as follows:
934: the user connects to the remote host using
935: .Nm ,
936: specifying a port to be used to forward connections
937: to the remote server.
938: After that it is possible to start the service which is to be encrypted
939: on the client machine,
940: connecting to the same local port,
941: and
942: .Nm
943: will encrypt and forward the connection.
944: .Pp
945: The following example tunnels an IRC session from client machine
946: .Dq 127.0.0.1
947: (localhost)
948: to remote server
949: .Dq server.example.com :
950: .Bd -literal -offset 4n
951: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
952: $ irc -c '#users' -p 1234 pinky 127.0.0.1
953: .Ed
954: .Pp
955: This tunnels a connection to IRC server
956: .Dq server.example.com ,
957: joining channel
958: .Dq #users ,
959: nickname
960: .Dq pinky ,
961: using port 1234.
962: It doesn't matter which port is used,
963: as long as it's greater than 1023
964: (remember, only root can open sockets on privileged ports)
965: and doesn't conflict with any ports already in use.
966: The connection is forwarded to port 6667 on the remote server,
967: since that's the standard port for IRC services.
968: .Pp
969: The
970: .Fl f
971: option backgrounds
972: .Nm
973: and the remote command
974: .Dq sleep 10
975: is specified to allow an amount of time
976: (10 seconds, in the example)
977: to start the service which is to be tunnelled.
978: If no connections are made within the time specified,
979: .Nm
980: will exit.
981: .Sh X11 FORWARDING
1.218 jmc 982: If the
983: .Cm ForwardX11
984: variable is set to
985: .Dq yes
986: (or see the description of the
1.227 jmc 987: .Fl X ,
988: .Fl x ,
1.218 jmc 989: and
1.227 jmc 990: .Fl Y
1.226 jmc 991: options above)
1.218 jmc 992: and the user is using X11 (the
993: .Ev DISPLAY
994: environment variable is set), the connection to the X11 display is
995: automatically forwarded to the remote side in such a way that any X11
996: programs started from the shell (or command) will go through the
997: encrypted channel, and the connection to the real X server will be made
1.176 jmc 998: from the local machine.
1.218 jmc 999: The user should not manually set
1000: .Ev DISPLAY .
1001: Forwarding of X11 connections can be
1002: configured on the command line or in configuration files.
1003: .Pp
1004: The
1005: .Ev DISPLAY
1006: value set by
1007: .Nm
1008: will point to the server machine, but with a display number greater than zero.
1009: This is normal, and happens because
1010: .Nm
1011: creates a
1012: .Dq proxy
1013: X server on the server machine for forwarding the
1014: connections over the encrypted channel.
1.200 djm 1015: .Pp
1.218 jmc 1016: .Nm
1017: will also automatically set up Xauthority data on the server machine.
1018: For this purpose, it will generate a random authorization cookie,
1019: store it in Xauthority on the server, and verify that any forwarded
1020: connections carry this cookie and replace it by the real cookie when
1021: the connection is opened.
1022: The real authentication cookie is never
1023: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1024: .Pp
1.218 jmc 1025: If the
1026: .Cm ForwardAgent
1027: variable is set to
1028: .Dq yes
1029: (or see the description of the
1030: .Fl A
1.191 djm 1031: and
1.218 jmc 1032: .Fl a
1.226 jmc 1033: options above) and
1.218 jmc 1034: the user is using an authentication agent, the connection to the agent
1035: is automatically forwarded to the remote side.
1.252 jmc 1036: .Sh VERIFYING HOST KEYS
1037: When connecting to a server for the first time,
1038: a fingerprint of the server's public key is presented to the user
1039: (unless the option
1040: .Cm StrictHostKeyChecking
1041: has been disabled).
1042: Fingerprints can be determined using
1043: .Xr ssh-keygen 1 :
1044: .Pp
1045: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1046: .Pp
1.274 grunk 1047: If the fingerprint is already known, it can be matched
1048: and the key can be accepted or rejected.
1049: Because of the difficulty of comparing host keys
1050: just by looking at hex strings,
1051: there is also support to compare host keys visually,
1052: using
1053: .Em random art .
1054: By setting the
1.275 grunk 1055: .Cm VisualHostKey
1.274 grunk 1056: option to
1.275 grunk 1057: .Dq yes ,
1.274 grunk 1058: a small ASCII graphic gets displayed on every login to a server, no matter
1059: if the session itself is interactive or not.
1060: By learning the pattern a known server produces, a user can easily
1061: find out that the host key has changed when a completely different pattern
1062: is displayed.
1063: Because these patterns are not unambiguous however, a pattern that looks
1064: similar to the pattern remembered only gives a good probability that the
1065: host key is the same, not guaranteed proof.
1066: .Pp
1067: To get a listing of the fingerprints along with their random art for
1068: all known hosts, the following command line can be used:
1069: .Pp
1070: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1071: .Pp
1.252 jmc 1072: If the fingerprint is unknown,
1073: an alternative method of verification is available:
1074: SSH fingerprints verified by DNS.
1075: An additional resource record (RR),
1076: SSHFP,
1077: is added to a zonefile
1078: and the connecting client is able to match the fingerprint
1079: with that of the key presented.
1080: .Pp
1081: In this example, we are connecting a client to a server,
1082: .Dq host.example.com .
1083: The SSHFP resource records should first be added to the zonefile for
1084: host.example.com:
1085: .Bd -literal -offset indent
1.259 jakob 1086: $ ssh-keygen -r host.example.com.
1.252 jmc 1087: .Ed
1088: .Pp
1089: The output lines will have to be added to the zonefile.
1090: To check that the zone is answering fingerprint queries:
1091: .Pp
1092: .Dl $ dig -t SSHFP host.example.com
1093: .Pp
1094: Finally the client connects:
1095: .Bd -literal -offset indent
1096: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1097: [...]
1098: Matching host key fingerprint found in DNS.
1099: Are you sure you want to continue connecting (yes/no)?
1100: .Ed
1101: .Pp
1102: See the
1103: .Cm VerifyHostKeyDNS
1104: option in
1105: .Xr ssh_config 5
1106: for more information.
1.250 jmc 1107: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1108: .Nm
1109: contains support for Virtual Private Network (VPN) tunnelling
1110: using the
1111: .Xr tun 4
1112: network pseudo-device,
1113: allowing two networks to be joined securely.
1114: The
1115: .Xr sshd_config 5
1116: configuration option
1117: .Cm PermitTunnel
1118: controls whether the server supports this,
1119: and at what level (layer 2 or 3 traffic).
1120: .Pp
1121: The following example would connect client network 10.0.50.0/24
1.265 otto 1122: with remote network 10.0.99.0/24 using a point-to-point connection
1123: from 10.1.1.1 to 10.1.1.2,
1124: provided that the SSH server running on the gateway to the remote network,
1125: at 192.168.1.15, allows it.
1126: .Pp
1127: On the client:
1.250 jmc 1128: .Bd -literal -offset indent
1129: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1130: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1131: # route add 10.0.99.0/24 10.1.1.2
1132: .Ed
1133: .Pp
1134: On the server:
1135: .Bd -literal -offset indent
1136: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1137: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1138: .Ed
1139: .Pp
1140: Client access may be more finely tuned via the
1141: .Pa /root/.ssh/authorized_keys
1142: file (see below) and the
1143: .Cm PermitRootLogin
1144: server option.
1.255 jmc 1145: The following entry would permit connections on
1.250 jmc 1146: .Xr tun 4
1.255 jmc 1147: device 1 from user
1.250 jmc 1148: .Dq jane
1.255 jmc 1149: and on tun device 2 from user
1.250 jmc 1150: .Dq john ,
1151: if
1152: .Cm PermitRootLogin
1153: is set to
1154: .Dq forced-commands-only :
1155: .Bd -literal -offset 2n
1156: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1157: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1158: .Ed
1159: .Pp
1.264 ray 1160: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1161: it may be more suited to temporary setups,
1162: such as for wireless VPNs.
1163: More permanent VPNs are better provided by tools such as
1164: .Xr ipsecctl 8
1165: and
1166: .Xr isakmpd 8 .
1.2 deraadt 1167: .Sh ENVIRONMENT
1168: .Nm
1.1 deraadt 1169: will normally set the following environment variables:
1.237 jmc 1170: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1171: .It Ev DISPLAY
1172: The
1173: .Ev DISPLAY
1.40 aaron 1174: variable indicates the location of the X11 server.
1.44 aaron 1175: It is automatically set by
1.2 deraadt 1176: .Nm
1177: to point to a value of the form
1.233 jmc 1178: .Dq hostname:n ,
1179: where
1180: .Dq hostname
1181: indicates the host where the shell runs, and
1182: .Sq n
1183: is an integer \*(Ge 1.
1.40 aaron 1184: .Nm
1185: uses this special value to forward X11 connections over the secure
1186: channel.
1.107 markus 1187: The user should normally not set
1188: .Ev DISPLAY
1189: explicitly, as that
1.1 deraadt 1190: will render the X11 connection insecure (and will require the user to
1191: manually copy any required authorization cookies).
1.2 deraadt 1192: .It Ev HOME
1.1 deraadt 1193: Set to the path of the user's home directory.
1.2 deraadt 1194: .It Ev LOGNAME
1195: Synonym for
1.12 aaron 1196: .Ev USER ;
1197: set for compatibility with systems that use this variable.
1.2 deraadt 1198: .It Ev MAIL
1.129 stevesk 1199: Set to the path of the user's mailbox.
1.40 aaron 1200: .It Ev PATH
1.2 deraadt 1201: Set to the default
1202: .Ev PATH ,
1203: as specified when compiling
1.234 jmc 1204: .Nm .
1.118 markus 1205: .It Ev SSH_ASKPASS
1206: If
1207: .Nm
1208: needs a passphrase, it will read the passphrase from the current
1209: terminal if it was run from a terminal.
1210: If
1211: .Nm
1212: does not have a terminal associated with it but
1213: .Ev DISPLAY
1214: and
1215: .Ev SSH_ASKPASS
1216: are set, it will execute the program specified by
1217: .Ev SSH_ASKPASS
1218: and open an X11 window to read the passphrase.
1219: This is particularly useful when calling
1220: .Nm
1221: from a
1.196 jmc 1222: .Pa .xsession
1.118 markus 1223: or related script.
1224: (Note that on some machines it
1225: may be necessary to redirect the input from
1226: .Pa /dev/null
1227: to make this work.)
1.18 markus 1228: .It Ev SSH_AUTH_SOCK
1.233 jmc 1229: Identifies the path of a
1230: .Ux Ns -domain
1231: socket used to communicate with the agent.
1.166 stevesk 1232: .It Ev SSH_CONNECTION
1233: Identifies the client and server ends of the connection.
1.40 aaron 1234: The variable contains
1.233 jmc 1235: four space-separated values: client IP address, client port number,
1236: server IP address, and server port number.
1.73 markus 1237: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1238: This variable contains the original command line if a forced command
1.73 markus 1239: is executed.
1240: It can be used to extract the original arguments.
1.2 deraadt 1241: .It Ev SSH_TTY
1.1 deraadt 1242: This is set to the name of the tty (path to the device) associated
1.40 aaron 1243: with the current shell or command.
1244: If the current session has no tty,
1.1 deraadt 1245: this variable is not set.
1.2 deraadt 1246: .It Ev TZ
1.214 jmc 1247: This variable is set to indicate the present time zone if it
1.257 jmc 1248: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1249: on to new connections).
1.2 deraadt 1250: .It Ev USER
1.1 deraadt 1251: Set to the name of the user logging in.
1.2 deraadt 1252: .El
1253: .Pp
1.44 aaron 1254: Additionally,
1.2 deraadt 1255: .Nm
1.44 aaron 1256: reads
1.207 djm 1257: .Pa ~/.ssh/environment ,
1.2 deraadt 1258: and adds lines of the format
1259: .Dq VARNAME=value
1.233 jmc 1260: to the environment if the file exists and users are allowed to
1.161 marc 1261: change their environment.
1.176 jmc 1262: For more information, see the
1.161 marc 1263: .Cm PermitUserEnvironment
1.162 stevesk 1264: option in
1.161 marc 1265: .Xr sshd_config 5 .
1.2 deraadt 1266: .Sh FILES
1.236 jmc 1267: .Bl -tag -width Ds -compact
1268: .It ~/.rhosts
1.240 jmc 1269: This file is used for host-based authentication (see above).
1.92 markus 1270: On some machines this file may need to be
1.240 jmc 1271: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1272: because
1.2 deraadt 1273: .Xr sshd 8
1.40 aaron 1274: reads it as root.
1275: Additionally, this file must be owned by the user,
1276: and must not have write permissions for anyone else.
1277: The recommended
1.1 deraadt 1278: permission for most machines is read/write for the user, and not
1279: accessible by others.
1.2 deraadt 1280: .Pp
1.236 jmc 1281: .It ~/.shosts
1.240 jmc 1282: This file is used in exactly the same way as
1283: .Pa .rhosts ,
1284: but allows host-based authentication without permitting login with
1285: rlogin/rsh.
1.272 mcbride 1286: .Pp
1287: .It ~/.ssh/
1288: This directory is the default location for all user-specific configuration
1289: and authentication information.
1290: There is no general requirement to keep the entire contents of this directory
1291: secret, but the recommended permissions are read/write/execute for the user,
1292: and not accessible by others.
1.236 jmc 1293: .Pp
1.238 jmc 1294: .It ~/.ssh/authorized_keys
1295: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1296: The format of this file is described in the
1297: .Xr sshd 8
1298: manual page.
1299: This file is not highly sensitive, but the recommended
1300: permissions are read/write for the user, and not accessible by others.
1301: .Pp
1302: .It ~/.ssh/config
1303: This is the per-user configuration file.
1304: The file format and configuration options are described in
1305: .Xr ssh_config 5 .
1306: Because of the potential for abuse, this file must have strict permissions:
1307: read/write for the user, and not accessible by others.
1308: .Pp
1309: .It ~/.ssh/environment
1.239 jmc 1310: Contains additional definitions for environment variables; see
1311: .Sx ENVIRONMENT ,
1.238 jmc 1312: above.
1313: .Pp
1314: .It ~/.ssh/identity
1315: .It ~/.ssh/id_dsa
1316: .It ~/.ssh/id_rsa
1317: Contains the private key for authentication.
1318: These files
1319: contain sensitive data and should be readable by the user but not
1320: accessible by others (read/write/execute).
1321: .Nm
1322: will simply ignore a private key file if it is accessible by others.
1323: It is possible to specify a passphrase when
1324: generating the key which will be used to encrypt the
1325: sensitive part of this file using 3DES.
1326: .Pp
1327: .It ~/.ssh/identity.pub
1328: .It ~/.ssh/id_dsa.pub
1329: .It ~/.ssh/id_rsa.pub
1330: Contains the public key for authentication.
1331: These files are not
1332: sensitive and can (but need not) be readable by anyone.
1333: .Pp
1334: .It ~/.ssh/known_hosts
1.244 jmc 1335: Contains a list of host keys for all hosts the user has logged into
1336: that are not already in the systemwide list of known host keys.
1.238 jmc 1337: See
1.244 jmc 1338: .Xr sshd 8
1339: for further details of the format of this file.
1.238 jmc 1340: .Pp
1341: .It ~/.ssh/rc
1342: Commands in this file are executed by
1343: .Nm
1.245 jmc 1344: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1345: started.
1346: See the
1347: .Xr sshd 8
1348: manual page for more information.
1349: .Pp
1.236 jmc 1350: .It /etc/hosts.equiv
1.240 jmc 1351: This file is for host-based authentication (see above).
1352: It should only be writable by root.
1.236 jmc 1353: .Pp
1354: .It /etc/shosts.equiv
1.240 jmc 1355: This file is used in exactly the same way as
1356: .Pa hosts.equiv ,
1357: but allows host-based authentication without permitting login with
1358: rlogin/rsh.
1.236 jmc 1359: .Pp
1.238 jmc 1360: .It Pa /etc/ssh/ssh_config
1361: Systemwide configuration file.
1362: The file format and configuration options are described in
1363: .Xr ssh_config 5 .
1364: .Pp
1365: .It /etc/ssh/ssh_host_key
1366: .It /etc/ssh/ssh_host_dsa_key
1367: .It /etc/ssh/ssh_host_rsa_key
1368: These three files contain the private parts of the host keys
1.245 jmc 1369: and are used for host-based authentication.
1370: If protocol version 1 is used,
1.238 jmc 1371: .Nm
1372: must be setuid root, since the host key is readable only by root.
1373: For protocol version 2,
1374: .Nm
1375: uses
1376: .Xr ssh-keysign 8
1.245 jmc 1377: to access the host keys,
1378: eliminating the requirement that
1.238 jmc 1379: .Nm
1.245 jmc 1380: be setuid root when host-based authentication is used.
1.238 jmc 1381: By default
1.2 deraadt 1382: .Nm
1.238 jmc 1383: is not setuid root.
1384: .Pp
1385: .It /etc/ssh/ssh_known_hosts
1386: Systemwide list of known host keys.
1387: This file should be prepared by the
1388: system administrator to contain the public host keys of all machines in the
1389: organization.
1.244 jmc 1390: It should be world-readable.
1391: See
1.238 jmc 1392: .Xr sshd 8
1.244 jmc 1393: for further details of the format of this file.
1.236 jmc 1394: .Pp
1.238 jmc 1395: .It /etc/ssh/sshrc
1.1 deraadt 1396: Commands in this file are executed by
1.2 deraadt 1397: .Nm
1.245 jmc 1398: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1399: See the
1.2 deraadt 1400: .Xr sshd 8
1.1 deraadt 1401: manual page for more information.
1.58 itojun 1402: .El
1.2 deraadt 1403: .Sh SEE ALSO
1404: .Xr scp 1 ,
1.83 djm 1405: .Xr sftp 1 ,
1.2 deraadt 1406: .Xr ssh-add 1 ,
1407: .Xr ssh-agent 1 ,
1408: .Xr ssh-keygen 1 ,
1.242 jmc 1409: .Xr ssh-keyscan 1 ,
1.250 jmc 1410: .Xr tun 4 ,
1.176 jmc 1411: .Xr hosts.equiv 5 ,
1.159 stevesk 1412: .Xr ssh_config 5 ,
1.160 naddy 1413: .Xr ssh-keysign 8 ,
1.87 itojun 1414: .Xr sshd 8
1.106 markus 1415: .Rs
1.256 jmc 1416: .%R RFC 4250
1417: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1418: .%D 2006
1419: .Re
1420: .Rs
1421: .%R RFC 4251
1422: .%T "The Secure Shell (SSH) Protocol Architecture"
1423: .%D 2006
1424: .Re
1425: .Rs
1426: .%R RFC 4252
1427: .%T "The Secure Shell (SSH) Authentication Protocol"
1428: .%D 2006
1429: .Re
1430: .Rs
1431: .%R RFC 4253
1432: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1433: .%D 2006
1434: .Re
1435: .Rs
1436: .%R RFC 4254
1437: .%T "The Secure Shell (SSH) Connection Protocol"
1438: .%D 2006
1439: .Re
1440: .Rs
1441: .%R RFC 4255
1442: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1443: .%D 2006
1444: .Re
1445: .Rs
1446: .%R RFC 4256
1447: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1448: .%D 2006
1449: .Re
1450: .Rs
1451: .%R RFC 4335
1452: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1453: .%D 2006
1454: .Re
1455: .Rs
1456: .%R RFC 4344
1457: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1458: .%D 2006
1459: .Re
1460: .Rs
1461: .%R RFC 4345
1462: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1463: .%D 2006
1464: .Re
1465: .Rs
1466: .%R RFC 4419
1467: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1468: .%D 2006
1469: .Re
1470: .Rs
1471: .%R RFC 4716
1472: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1473: .%D 2006
1.274 grunk 1474: .Re
1475: .Rs
1476: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1477: .%A A. Perrig
1478: .%A D. Song
1479: .%D 1999
1480: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1481: .Re
1.173 jmc 1482: .Sh AUTHORS
1483: OpenSSH is a derivative of the original and free
1484: ssh 1.2.12 release by Tatu Ylonen.
1485: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1486: Theo de Raadt and Dug Song
1487: removed many bugs, re-added newer features and
1488: created OpenSSH.
1489: Markus Friedl contributed the support for SSH
1490: protocol versions 1.5 and 2.0.
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