Annotation of src/usr.bin/ssh/ssh.1, Revision 1.276
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: .\"
1.276 ! jmc 37: .\" $OpenBSD: ssh.1,v 1.275 2008/06/26 11:46:31 grunk Exp $
! 38: .Dd $Mdocdate: June 26 2008 $
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
1.270 jmc 46: .Op Fl 1246AaCfgKkMNnqsTtVvXxY
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
135: (for the agent's Unix-domain socket)
136: can access the local agent through the forwarded connection.
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.
1.230 jmc 194: The supported ciphers are:
195: 3des-cbc,
196: aes128-cbc,
197: aes192-cbc,
198: aes256-cbc,
199: aes128-ctr,
200: aes192-ctr,
201: aes256-ctr,
202: arcfour128,
203: arcfour256,
204: arcfour,
205: blowfish-cbc,
1.218 jmc 206: and
1.230 jmc 207: cast128-cbc.
1.220 jmc 208: The default is:
209: .Bd -literal -offset indent
210: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
211: arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
212: aes192-ctr,aes256-ctr
1.218 jmc 213: .Ed
214: .It Fl D Xo
215: .Sm off
216: .Oo Ar bind_address : Oc
217: .Ar port
218: .Sm on
219: .Xc
220: Specifies a local
221: .Dq dynamic
222: application-level port forwarding.
223: This works by allocating a socket to listen to
224: .Ar port
225: on the local side, optionally bound to the specified
226: .Ar bind_address .
227: Whenever a connection is made to this port, the
228: connection is forwarded over the secure channel, and the application
229: protocol is then used to determine where to connect to from the
230: remote machine.
231: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 232: .Nm
1.218 jmc 233: will act as a SOCKS server.
234: Only root can forward privileged ports.
235: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 236: .Pp
1.218 jmc 237: IPv6 addresses can be specified with an alternative syntax:
238: .Sm off
239: .Xo
240: .Op Ar bind_address No /
241: .Ar port
242: .Xc
243: .Sm on
244: or by enclosing the address in square brackets.
245: Only the superuser can forward privileged ports.
246: By default, the local port is bound in accordance with the
247: .Cm GatewayPorts
248: setting.
249: However, an explicit
250: .Ar bind_address
251: may be used to bind the connection to a specific address.
252: The
253: .Ar bind_address
254: of
255: .Dq localhost
256: indicates that the listening port be bound for local use only, while an
257: empty address or
258: .Sq *
259: indicates that the port should be available from all interfaces.
1.229 jmc 260: .It Fl e Ar escape_char
1.218 jmc 261: Sets the escape character for sessions with a pty (default:
262: .Ql ~ ) .
263: The escape character is only recognized at the beginning of a line.
264: The escape character followed by a dot
265: .Pq Ql \&.
266: closes the connection;
267: followed by control-Z suspends the connection;
268: and followed by itself sends the escape character once.
269: Setting the character to
1.2 deraadt 270: .Dq none
1.218 jmc 271: disables any escapes and makes the session fully transparent.
272: .It Fl F Ar configfile
273: Specifies an alternative per-user configuration file.
274: If a configuration file is given on the command line,
275: the system-wide configuration file
276: .Pq Pa /etc/ssh/ssh_config
277: will be ignored.
278: The default for the per-user configuration file is
279: .Pa ~/.ssh/config .
280: .It Fl f
281: Requests
282: .Nm
283: to go to background just before command execution.
284: This is useful if
1.176 jmc 285: .Nm
1.218 jmc 286: is going to ask for passwords or passphrases, but the user
287: wants it in the background.
288: This implies
289: .Fl n .
290: The recommended way to start X11 programs at a remote site is with
291: something like
292: .Ic ssh -f host xterm .
293: .It Fl g
294: Allows remote hosts to connect to local forwarded ports.
295: .It Fl I Ar smartcard_device
1.229 jmc 296: Specify the device
1.176 jmc 297: .Nm
1.218 jmc 298: should use to communicate with a smartcard used for storing the user's
299: private RSA key.
1.229 jmc 300: This option is only available if support for smartcard devices
301: is compiled in (default is no support).
1.218 jmc 302: .It Fl i Ar identity_file
303: Selects a file from which the identity (private key) for
304: RSA or DSA authentication is read.
305: The default is
306: .Pa ~/.ssh/identity
307: for protocol version 1, and
308: .Pa ~/.ssh/id_rsa
1.149 jakob 309: and
1.218 jmc 310: .Pa ~/.ssh/id_dsa
311: for protocol version 2.
312: Identity files may also be specified on
313: a per-host basis in the configuration file.
314: It is possible to have multiple
315: .Fl i
316: options (and multiple identities specified in
317: configuration files).
1.269 djm 318: .It Fl K
319: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
320: credentials to the server.
1.218 jmc 321: .It Fl k
322: Disables forwarding (delegation) of GSSAPI credentials to the server.
323: .It Fl L Xo
324: .Sm off
325: .Oo Ar bind_address : Oc
326: .Ar port : host : hostport
327: .Sm on
328: .Xc
329: Specifies that the given port on the local (client) host is to be
330: forwarded to the given host and port on the remote side.
331: This works by allocating a socket to listen to
332: .Ar port
333: on the local side, optionally bound to the specified
334: .Ar bind_address .
335: Whenever a connection is made to this port, the
336: connection is forwarded over the secure channel, and a connection is
337: made to
338: .Ar host
339: port
340: .Ar hostport
341: from the remote machine.
342: Port forwardings can also be specified in the configuration file.
343: IPv6 addresses can be specified with an alternative syntax:
344: .Sm off
345: .Xo
346: .Op Ar bind_address No /
347: .Ar port No / Ar host No /
348: .Ar hostport
349: .Xc
350: .Sm on
351: or by enclosing the address in square brackets.
352: Only the superuser can forward privileged ports.
353: By default, the local port is bound in accordance with the
354: .Cm GatewayPorts
355: setting.
356: However, an explicit
357: .Ar bind_address
358: may be used to bind the connection to a specific address.
1.2 deraadt 359: The
1.218 jmc 360: .Ar bind_address
361: of
362: .Dq localhost
363: indicates that the listening port be bound for local use only, while an
364: empty address or
365: .Sq *
366: indicates that the port should be available from all interfaces.
367: .It Fl l Ar login_name
368: Specifies the user to log in as on the remote machine.
369: This also may be specified on a per-host basis in the configuration file.
370: .It Fl M
371: Places the
372: .Nm
373: client into
374: .Dq master
375: mode for connection sharing.
1.231 stevesk 376: Multiple
377: .Fl M
378: options places
379: .Nm
380: into
381: .Dq master
382: mode with confirmation required before slave connections are accepted.
1.218 jmc 383: Refer to the description of
384: .Cm ControlMaster
385: in
386: .Xr ssh_config 5
387: for details.
388: .It Fl m Ar mac_spec
389: Additionally, for protocol version 2 a comma-separated list of MAC
390: (message authentication code) algorithms can
391: be specified in order of preference.
392: See the
393: .Cm MACs
394: keyword for more information.
395: .It Fl N
396: Do not execute a remote command.
397: This is useful for just forwarding ports
398: (protocol version 2 only).
399: .It Fl n
400: Redirects stdin from
401: .Pa /dev/null
402: (actually, prevents reading from stdin).
403: This must be used when
1.2 deraadt 404: .Nm
1.218 jmc 405: is run in the background.
406: A common trick is to use this to run X11 programs on a remote machine.
407: For example,
408: .Ic ssh -n shadows.cs.hut.fi emacs &
409: will start an emacs on shadows.cs.hut.fi, and the X11
410: connection will be automatically forwarded over an encrypted channel.
411: The
1.2 deraadt 412: .Nm
1.218 jmc 413: program will be put in the background.
414: (This does not work if
1.2 deraadt 415: .Nm
1.218 jmc 416: needs to ask for a password or passphrase; see also the
417: .Fl f
418: option.)
419: .It Fl O Ar ctl_cmd
420: Control an active connection multiplexing master process.
421: When the
422: .Fl O
423: option is specified, the
424: .Ar ctl_cmd
425: argument is interpreted and passed to the master process.
426: Valid commands are:
427: .Dq check
428: (check that the master process is running) and
429: .Dq exit
430: (request the master to exit).
431: .It Fl o Ar option
432: Can be used to give options in the format used in the configuration file.
433: This is useful for specifying options for which there is no separate
434: command-line flag.
435: For full details of the options listed below, and their possible values, see
436: .Xr ssh_config 5 .
1.2 deraadt 437: .Pp
1.218 jmc 438: .Bl -tag -width Ds -offset indent -compact
439: .It AddressFamily
440: .It BatchMode
441: .It BindAddress
442: .It ChallengeResponseAuthentication
443: .It CheckHostIP
444: .It Cipher
445: .It Ciphers
446: .It ClearAllForwardings
447: .It Compression
448: .It CompressionLevel
449: .It ConnectionAttempts
450: .It ConnectTimeout
451: .It ControlMaster
452: .It ControlPath
453: .It DynamicForward
454: .It EscapeChar
1.263 markus 455: .It ExitOnForwardFailure
1.218 jmc 456: .It ForwardAgent
457: .It ForwardX11
458: .It ForwardX11Trusted
459: .It GatewayPorts
460: .It GlobalKnownHostsFile
461: .It GSSAPIAuthentication
462: .It GSSAPIDelegateCredentials
463: .It HashKnownHosts
464: .It Host
465: .It HostbasedAuthentication
466: .It HostKeyAlgorithms
467: .It HostKeyAlias
468: .It HostName
469: .It IdentityFile
470: .It IdentitiesOnly
471: .It KbdInteractiveDevices
472: .It LocalCommand
473: .It LocalForward
474: .It LogLevel
475: .It MACs
476: .It NoHostAuthenticationForLocalhost
477: .It NumberOfPasswordPrompts
478: .It PasswordAuthentication
479: .It PermitLocalCommand
480: .It Port
481: .It PreferredAuthentications
482: .It Protocol
483: .It ProxyCommand
484: .It PubkeyAuthentication
1.251 dtucker 485: .It RekeyLimit
1.218 jmc 486: .It RemoteForward
487: .It RhostsRSAAuthentication
488: .It RSAAuthentication
489: .It SendEnv
490: .It ServerAliveInterval
491: .It ServerAliveCountMax
492: .It SmartcardDevice
493: .It StrictHostKeyChecking
494: .It TCPKeepAlive
495: .It Tunnel
496: .It TunnelDevice
497: .It UsePrivilegedPort
498: .It User
499: .It UserKnownHostsFile
500: .It VerifyHostKeyDNS
1.276 ! jmc 501: .It VisualHostKey
1.218 jmc 502: .It XAuthLocation
503: .El
504: .It Fl p Ar port
505: Port to connect to on the remote host.
506: This can be specified on a
507: per-host basis in the configuration file.
508: .It Fl q
509: Quiet mode.
1.271 djm 510: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 511: .It Fl R Xo
512: .Sm off
513: .Oo Ar bind_address : Oc
514: .Ar port : host : hostport
515: .Sm on
516: .Xc
517: Specifies that the given port on the remote (server) host is to be
518: forwarded to the given host and port on the local side.
519: This works by allocating a socket to listen to
520: .Ar port
521: on the remote side, and whenever a connection is made to this port, the
522: connection is forwarded over the secure channel, and a connection is
523: made to
524: .Ar host
525: port
526: .Ar hostport
527: from the local machine.
1.2 deraadt 528: .Pp
1.218 jmc 529: Port forwardings can also be specified in the configuration file.
530: Privileged ports can be forwarded only when
531: logging in as root on the remote machine.
532: IPv6 addresses can be specified by enclosing the address in square braces or
533: using an alternative syntax:
534: .Sm off
535: .Xo
536: .Op Ar bind_address No /
537: .Ar host No / Ar port No /
538: .Ar hostport
539: .Xc .
540: .Sm on
1.194 jakob 541: .Pp
1.218 jmc 542: By default, the listening socket on the server will be bound to the loopback
543: interface only.
544: This may be overriden by specifying a
545: .Ar bind_address .
546: An empty
547: .Ar bind_address ,
548: or the address
549: .Ql * ,
550: indicates that the remote socket should listen on all interfaces.
551: Specifying a remote
552: .Ar bind_address
553: will only succeed if the server's
554: .Cm GatewayPorts
555: option is enabled (see
556: .Xr sshd_config 5 ) .
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.
652: .El
1.224 jmc 653: .Pp
654: .Nm
655: may additionally obtain configuration data from
656: a per-user configuration file and a system-wide configuration file.
657: The file format and configuration options are described in
658: .Xr ssh_config 5 .
659: .Pp
660: .Nm
661: exits with the exit status of the remote command or with 255
662: if an error occurred.
1.222 jmc 663: .Sh AUTHENTICATION
1.249 jmc 664: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.222 jmc 665: Protocol 2 is the default, with
666: .Nm
667: falling back to protocol 1 if it detects protocol 2 is unsupported.
668: These settings may be altered using the
669: .Cm Protocol
670: option in
671: .Xr ssh_config 5 ,
672: or enforced using the
673: .Fl 1
674: and
675: .Fl 2
676: options (see above).
677: Both protocols support similar authentication methods,
678: but protocol 2 is preferred since
679: it provides additional mechanisms for confidentiality
680: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 681: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 682: Protocol 1 lacks a strong mechanism for ensuring the
683: integrity of the connection.
684: .Pp
685: The methods available for authentication are:
1.260 jmc 686: GSSAPI-based authentication,
1.222 jmc 687: host-based authentication,
688: public key authentication,
689: challenge-response authentication,
690: and password authentication.
691: Authentication methods are tried in the order specified above,
692: though protocol 2 has a configuration option to change the default order:
693: .Cm PreferredAuthentications .
694: .Pp
695: Host-based authentication works as follows:
1.218 jmc 696: If the machine the user logs in from is listed in
697: .Pa /etc/hosts.equiv
698: or
699: .Pa /etc/shosts.equiv
700: on the remote machine, and the user names are
701: the same on both sides, or if the files
702: .Pa ~/.rhosts
703: or
704: .Pa ~/.shosts
705: exist in the user's home directory on the
706: remote machine and contain a line containing the name of the client
707: machine and the name of the user on that machine, the user is
1.222 jmc 708: considered for login.
709: Additionally, the server
710: .Em must
711: be able to verify the client's
712: host key (see the description of
1.218 jmc 713: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 714: and
1.222 jmc 715: .Pa ~/.ssh/known_hosts ,
716: below)
717: for login to be permitted.
1.218 jmc 718: This authentication method closes security holes due to IP
1.222 jmc 719: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 720: [Note to the administrator:
721: .Pa /etc/hosts.equiv ,
722: .Pa ~/.rhosts ,
723: and the rlogin/rsh protocol in general, are inherently insecure and should be
724: disabled if security is desired.]
1.189 dtucker 725: .Pp
1.222 jmc 726: Public key authentication works as follows:
727: The scheme is based on public-key cryptography,
728: using cryptosystems
729: where encryption and decryption are done using separate keys,
730: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 731: The idea is that each user creates a public/private
732: key pair for authentication purposes.
733: The server knows the public key, and only the user knows the private key.
1.222 jmc 734: .Nm
735: implements public key authentication protocol automatically,
736: using either the RSA or DSA algorithms.
737: Protocol 1 is restricted to using only RSA keys,
738: but protocol 2 may use either.
739: The
740: .Sx HISTORY
741: section of
742: .Xr ssl 8
743: contains a brief discussion of the two algorithms.
1.210 djm 744: .Pp
1.218 jmc 745: The file
746: .Pa ~/.ssh/authorized_keys
747: lists the public keys that are permitted for logging in.
748: When the user logs in, the
1.2 deraadt 749: .Nm
1.218 jmc 750: program tells the server which key pair it would like to use for
751: authentication.
1.222 jmc 752: The client proves that it has access to the private key
753: and the server checks that the corresponding public key
754: is authorized to accept the account.
1.218 jmc 755: .Pp
1.222 jmc 756: The user creates his/her key pair by running
1.218 jmc 757: .Xr ssh-keygen 1 .
758: This stores the private key in
1.207 djm 759: .Pa ~/.ssh/identity
1.222 jmc 760: (protocol 1),
761: .Pa ~/.ssh/id_dsa
762: (protocol 2 DSA),
763: or
764: .Pa ~/.ssh/id_rsa
765: (protocol 2 RSA)
1.218 jmc 766: and stores the public key in
767: .Pa ~/.ssh/identity.pub
1.222 jmc 768: (protocol 1),
769: .Pa ~/.ssh/id_dsa.pub
770: (protocol 2 DSA),
771: or
772: .Pa ~/.ssh/id_rsa.pub
773: (protocol 2 RSA)
1.218 jmc 774: in the user's home directory.
1.222 jmc 775: The user should then copy the public key
1.218 jmc 776: to
777: .Pa ~/.ssh/authorized_keys
1.222 jmc 778: in his/her home directory on the remote machine.
779: The
1.218 jmc 780: .Pa authorized_keys
781: file corresponds to the conventional
782: .Pa ~/.rhosts
783: file, and has one key
1.222 jmc 784: per line, though the lines can be very long.
1.218 jmc 785: After this, the user can log in without giving the password.
786: .Pp
1.222 jmc 787: The most convenient way to use public key authentication may be with an
1.218 jmc 788: authentication agent.
789: See
790: .Xr ssh-agent 1
791: for more information.
792: .Pp
1.222 jmc 793: Challenge-response authentication works as follows:
794: The server sends an arbitrary
795: .Qq challenge
796: text, and prompts for a response.
797: Protocol 2 allows multiple challenges and responses;
798: protocol 1 is restricted to just one challenge/response.
799: Examples of challenge-response authentication include
800: BSD Authentication (see
801: .Xr login.conf 5 )
802: and PAM (some non-OpenBSD systems).
803: .Pp
804: Finally, if other authentication methods fail,
1.218 jmc 805: .Nm
806: prompts the user for a password.
807: The password is sent to the remote
808: host for checking; however, since all communications are encrypted,
809: the password cannot be seen by someone listening on the network.
1.232 jmc 810: .Pp
811: .Nm
812: automatically maintains and checks a database containing
813: identification for all hosts it has ever been used with.
814: Host keys are stored in
815: .Pa ~/.ssh/known_hosts
816: in the user's home directory.
817: Additionally, the file
818: .Pa /etc/ssh/ssh_known_hosts
819: is automatically checked for known hosts.
820: Any new hosts are automatically added to the user's file.
821: If a host's identification ever changes,
822: .Nm
823: warns about this and disables password authentication to prevent
824: server spoofing or man-in-the-middle attacks,
825: which could otherwise be used to circumvent the encryption.
826: The
827: .Cm StrictHostKeyChecking
828: option can be used to control logins to machines whose
829: host key is not known or has changed.
830: .Pp
1.218 jmc 831: When the user's identity has been accepted by the server, the server
832: either executes the given command, or logs into the machine and gives
833: the user a normal shell on the remote machine.
834: All communication with
835: the remote command or shell will be automatically encrypted.
836: .Pp
837: If a pseudo-terminal has been allocated (normal login session), the
838: user may use the escape characters noted below.
839: .Pp
840: If no pseudo-tty has been allocated,
841: the session is transparent and can be used to reliably transfer binary data.
842: On most systems, setting the escape character to
843: .Dq none
844: will also make the session transparent even if a tty is used.
845: .Pp
846: The session terminates when the command or shell on the remote
1.247 jmc 847: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 848: .Sh ESCAPE CHARACTERS
1.218 jmc 849: When a pseudo-terminal has been requested,
1.2 deraadt 850: .Nm
1.218 jmc 851: supports a number of functions through the use of an escape character.
852: .Pp
853: A single tilde character can be sent as
854: .Ic ~~
855: or by following the tilde by a character other than those described below.
856: The escape character must always follow a newline to be interpreted as
857: special.
858: The escape character can be changed in configuration files using the
859: .Cm EscapeChar
860: configuration directive or on the command line by the
861: .Fl e
862: option.
863: .Pp
864: The supported escapes (assuming the default
865: .Ql ~ )
866: are:
867: .Bl -tag -width Ds
868: .It Cm ~.
869: Disconnect.
870: .It Cm ~^Z
871: Background
1.234 jmc 872: .Nm .
1.218 jmc 873: .It Cm ~#
874: List forwarded connections.
875: .It Cm ~&
876: Background
1.2 deraadt 877: .Nm
1.218 jmc 878: at logout when waiting for forwarded connection / X11 sessions to terminate.
879: .It Cm ~?
880: Display a list of escape characters.
881: .It Cm ~B
882: Send a BREAK to the remote system
883: (only useful for SSH protocol version 2 and if the peer supports it).
884: .It Cm ~C
885: Open command line.
886: Currently this allows the addition of port forwardings using the
887: .Fl L
888: and
889: .Fl R
1.225 jmc 890: options (see above).
1.218 jmc 891: It also allows the cancellation of existing remote port-forwardings
892: using
1.262 stevesk 893: .Sm off
894: .Fl KR Oo Ar bind_address : Oc Ar port .
895: .Sm on
1.218 jmc 896: .Ic !\& Ns Ar command
897: allows the user to execute a local command if the
898: .Ic PermitLocalCommand
899: option is enabled in
1.176 jmc 900: .Xr ssh_config 5 .
1.218 jmc 901: Basic help is available, using the
902: .Fl h
903: option.
904: .It Cm ~R
905: Request rekeying of the connection
906: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 907: .El
1.246 jmc 908: .Sh TCP FORWARDING
909: Forwarding of arbitrary TCP connections over the secure channel can
910: be specified either on the command line or in a configuration file.
911: One possible application of TCP forwarding is a secure connection to a
912: mail server; another is going through firewalls.
913: .Pp
914: In the example below, we look at encrypting communication between
915: an IRC client and server, even though the IRC server does not directly
916: support encrypted communications.
917: This works as follows:
918: the user connects to the remote host using
919: .Nm ,
920: specifying a port to be used to forward connections
921: to the remote server.
922: After that it is possible to start the service which is to be encrypted
923: on the client machine,
924: connecting to the same local port,
925: and
926: .Nm
927: will encrypt and forward the connection.
928: .Pp
929: The following example tunnels an IRC session from client machine
930: .Dq 127.0.0.1
931: (localhost)
932: to remote server
933: .Dq server.example.com :
934: .Bd -literal -offset 4n
935: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
936: $ irc -c '#users' -p 1234 pinky 127.0.0.1
937: .Ed
938: .Pp
939: This tunnels a connection to IRC server
940: .Dq server.example.com ,
941: joining channel
942: .Dq #users ,
943: nickname
944: .Dq pinky ,
945: using port 1234.
946: It doesn't matter which port is used,
947: as long as it's greater than 1023
948: (remember, only root can open sockets on privileged ports)
949: and doesn't conflict with any ports already in use.
950: The connection is forwarded to port 6667 on the remote server,
951: since that's the standard port for IRC services.
952: .Pp
953: The
954: .Fl f
955: option backgrounds
956: .Nm
957: and the remote command
958: .Dq sleep 10
959: is specified to allow an amount of time
960: (10 seconds, in the example)
961: to start the service which is to be tunnelled.
962: If no connections are made within the time specified,
963: .Nm
964: will exit.
965: .Sh X11 FORWARDING
1.218 jmc 966: If the
967: .Cm ForwardX11
968: variable is set to
969: .Dq yes
970: (or see the description of the
1.227 jmc 971: .Fl X ,
972: .Fl x ,
1.218 jmc 973: and
1.227 jmc 974: .Fl Y
1.226 jmc 975: options above)
1.218 jmc 976: and the user is using X11 (the
977: .Ev DISPLAY
978: environment variable is set), the connection to the X11 display is
979: automatically forwarded to the remote side in such a way that any X11
980: programs started from the shell (or command) will go through the
981: encrypted channel, and the connection to the real X server will be made
1.176 jmc 982: from the local machine.
1.218 jmc 983: The user should not manually set
984: .Ev DISPLAY .
985: Forwarding of X11 connections can be
986: configured on the command line or in configuration files.
987: .Pp
988: The
989: .Ev DISPLAY
990: value set by
991: .Nm
992: will point to the server machine, but with a display number greater than zero.
993: This is normal, and happens because
994: .Nm
995: creates a
996: .Dq proxy
997: X server on the server machine for forwarding the
998: connections over the encrypted channel.
1.200 djm 999: .Pp
1.218 jmc 1000: .Nm
1001: will also automatically set up Xauthority data on the server machine.
1002: For this purpose, it will generate a random authorization cookie,
1003: store it in Xauthority on the server, and verify that any forwarded
1004: connections carry this cookie and replace it by the real cookie when
1005: the connection is opened.
1006: The real authentication cookie is never
1007: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1008: .Pp
1.218 jmc 1009: If the
1010: .Cm ForwardAgent
1011: variable is set to
1012: .Dq yes
1013: (or see the description of the
1014: .Fl A
1.191 djm 1015: and
1.218 jmc 1016: .Fl a
1.226 jmc 1017: options above) and
1.218 jmc 1018: the user is using an authentication agent, the connection to the agent
1019: is automatically forwarded to the remote side.
1.252 jmc 1020: .Sh VERIFYING HOST KEYS
1021: When connecting to a server for the first time,
1022: a fingerprint of the server's public key is presented to the user
1023: (unless the option
1024: .Cm StrictHostKeyChecking
1025: has been disabled).
1026: Fingerprints can be determined using
1027: .Xr ssh-keygen 1 :
1028: .Pp
1029: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1030: .Pp
1.274 grunk 1031: If the fingerprint is already known, it can be matched
1032: and the key can be accepted or rejected.
1033: Because of the difficulty of comparing host keys
1034: just by looking at hex strings,
1035: there is also support to compare host keys visually,
1036: using
1037: .Em random art .
1038: By setting the
1.275 grunk 1039: .Cm VisualHostKey
1.274 grunk 1040: option to
1.275 grunk 1041: .Dq yes ,
1.274 grunk 1042: a small ASCII graphic gets displayed on every login to a server, no matter
1043: if the session itself is interactive or not.
1044: By learning the pattern a known server produces, a user can easily
1045: find out that the host key has changed when a completely different pattern
1046: is displayed.
1047: Because these patterns are not unambiguous however, a pattern that looks
1048: similar to the pattern remembered only gives a good probability that the
1049: host key is the same, not guaranteed proof.
1050: .Pp
1051: To get a listing of the fingerprints along with their random art for
1052: all known hosts, the following command line can be used:
1053: .Pp
1054: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1055: .Pp
1.252 jmc 1056: If the fingerprint is unknown,
1057: an alternative method of verification is available:
1058: SSH fingerprints verified by DNS.
1059: An additional resource record (RR),
1060: SSHFP,
1061: is added to a zonefile
1062: and the connecting client is able to match the fingerprint
1063: with that of the key presented.
1064: .Pp
1065: In this example, we are connecting a client to a server,
1066: .Dq host.example.com .
1067: The SSHFP resource records should first be added to the zonefile for
1068: host.example.com:
1069: .Bd -literal -offset indent
1.259 jakob 1070: $ ssh-keygen -r host.example.com.
1.252 jmc 1071: .Ed
1072: .Pp
1073: The output lines will have to be added to the zonefile.
1074: To check that the zone is answering fingerprint queries:
1075: .Pp
1076: .Dl $ dig -t SSHFP host.example.com
1077: .Pp
1078: Finally the client connects:
1079: .Bd -literal -offset indent
1080: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1081: [...]
1082: Matching host key fingerprint found in DNS.
1083: Are you sure you want to continue connecting (yes/no)?
1084: .Ed
1085: .Pp
1086: See the
1087: .Cm VerifyHostKeyDNS
1088: option in
1089: .Xr ssh_config 5
1090: for more information.
1.250 jmc 1091: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1092: .Nm
1093: contains support for Virtual Private Network (VPN) tunnelling
1094: using the
1095: .Xr tun 4
1096: network pseudo-device,
1097: allowing two networks to be joined securely.
1098: The
1099: .Xr sshd_config 5
1100: configuration option
1101: .Cm PermitTunnel
1102: controls whether the server supports this,
1103: and at what level (layer 2 or 3 traffic).
1104: .Pp
1105: The following example would connect client network 10.0.50.0/24
1.265 otto 1106: with remote network 10.0.99.0/24 using a point-to-point connection
1107: from 10.1.1.1 to 10.1.1.2,
1108: provided that the SSH server running on the gateway to the remote network,
1109: at 192.168.1.15, allows it.
1110: .Pp
1111: On the client:
1.250 jmc 1112: .Bd -literal -offset indent
1113: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1114: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1115: # route add 10.0.99.0/24 10.1.1.2
1116: .Ed
1117: .Pp
1118: On the server:
1119: .Bd -literal -offset indent
1120: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1121: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1122: .Ed
1123: .Pp
1124: Client access may be more finely tuned via the
1125: .Pa /root/.ssh/authorized_keys
1126: file (see below) and the
1127: .Cm PermitRootLogin
1128: server option.
1.255 jmc 1129: The following entry would permit connections on
1.250 jmc 1130: .Xr tun 4
1.255 jmc 1131: device 1 from user
1.250 jmc 1132: .Dq jane
1.255 jmc 1133: and on tun device 2 from user
1.250 jmc 1134: .Dq john ,
1135: if
1136: .Cm PermitRootLogin
1137: is set to
1138: .Dq forced-commands-only :
1139: .Bd -literal -offset 2n
1140: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1141: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1142: .Ed
1143: .Pp
1.264 ray 1144: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1145: it may be more suited to temporary setups,
1146: such as for wireless VPNs.
1147: More permanent VPNs are better provided by tools such as
1148: .Xr ipsecctl 8
1149: and
1150: .Xr isakmpd 8 .
1.2 deraadt 1151: .Sh ENVIRONMENT
1152: .Nm
1.1 deraadt 1153: will normally set the following environment variables:
1.237 jmc 1154: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1155: .It Ev DISPLAY
1156: The
1157: .Ev DISPLAY
1.40 aaron 1158: variable indicates the location of the X11 server.
1.44 aaron 1159: It is automatically set by
1.2 deraadt 1160: .Nm
1161: to point to a value of the form
1.233 jmc 1162: .Dq hostname:n ,
1163: where
1164: .Dq hostname
1165: indicates the host where the shell runs, and
1166: .Sq n
1167: is an integer \*(Ge 1.
1.40 aaron 1168: .Nm
1169: uses this special value to forward X11 connections over the secure
1170: channel.
1.107 markus 1171: The user should normally not set
1172: .Ev DISPLAY
1173: explicitly, as that
1.1 deraadt 1174: will render the X11 connection insecure (and will require the user to
1175: manually copy any required authorization cookies).
1.2 deraadt 1176: .It Ev HOME
1.1 deraadt 1177: Set to the path of the user's home directory.
1.2 deraadt 1178: .It Ev LOGNAME
1179: Synonym for
1.12 aaron 1180: .Ev USER ;
1181: set for compatibility with systems that use this variable.
1.2 deraadt 1182: .It Ev MAIL
1.129 stevesk 1183: Set to the path of the user's mailbox.
1.40 aaron 1184: .It Ev PATH
1.2 deraadt 1185: Set to the default
1186: .Ev PATH ,
1187: as specified when compiling
1.234 jmc 1188: .Nm .
1.118 markus 1189: .It Ev SSH_ASKPASS
1190: If
1191: .Nm
1192: needs a passphrase, it will read the passphrase from the current
1193: terminal if it was run from a terminal.
1194: If
1195: .Nm
1196: does not have a terminal associated with it but
1197: .Ev DISPLAY
1198: and
1199: .Ev SSH_ASKPASS
1200: are set, it will execute the program specified by
1201: .Ev SSH_ASKPASS
1202: and open an X11 window to read the passphrase.
1203: This is particularly useful when calling
1204: .Nm
1205: from a
1.196 jmc 1206: .Pa .xsession
1.118 markus 1207: or related script.
1208: (Note that on some machines it
1209: may be necessary to redirect the input from
1210: .Pa /dev/null
1211: to make this work.)
1.18 markus 1212: .It Ev SSH_AUTH_SOCK
1.233 jmc 1213: Identifies the path of a
1214: .Ux Ns -domain
1215: socket used to communicate with the agent.
1.166 stevesk 1216: .It Ev SSH_CONNECTION
1217: Identifies the client and server ends of the connection.
1.40 aaron 1218: The variable contains
1.233 jmc 1219: four space-separated values: client IP address, client port number,
1220: server IP address, and server port number.
1.73 markus 1221: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1222: This variable contains the original command line if a forced command
1.73 markus 1223: is executed.
1224: It can be used to extract the original arguments.
1.2 deraadt 1225: .It Ev SSH_TTY
1.1 deraadt 1226: This is set to the name of the tty (path to the device) associated
1.40 aaron 1227: with the current shell or command.
1228: If the current session has no tty,
1.1 deraadt 1229: this variable is not set.
1.2 deraadt 1230: .It Ev TZ
1.214 jmc 1231: This variable is set to indicate the present time zone if it
1.257 jmc 1232: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1233: on to new connections).
1.2 deraadt 1234: .It Ev USER
1.1 deraadt 1235: Set to the name of the user logging in.
1.2 deraadt 1236: .El
1237: .Pp
1.44 aaron 1238: Additionally,
1.2 deraadt 1239: .Nm
1.44 aaron 1240: reads
1.207 djm 1241: .Pa ~/.ssh/environment ,
1.2 deraadt 1242: and adds lines of the format
1243: .Dq VARNAME=value
1.233 jmc 1244: to the environment if the file exists and users are allowed to
1.161 marc 1245: change their environment.
1.176 jmc 1246: For more information, see the
1.161 marc 1247: .Cm PermitUserEnvironment
1.162 stevesk 1248: option in
1.161 marc 1249: .Xr sshd_config 5 .
1.2 deraadt 1250: .Sh FILES
1.236 jmc 1251: .Bl -tag -width Ds -compact
1252: .It ~/.rhosts
1.240 jmc 1253: This file is used for host-based authentication (see above).
1.92 markus 1254: On some machines this file may need to be
1.240 jmc 1255: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1256: because
1.2 deraadt 1257: .Xr sshd 8
1.40 aaron 1258: reads it as root.
1259: Additionally, this file must be owned by the user,
1260: and must not have write permissions for anyone else.
1261: The recommended
1.1 deraadt 1262: permission for most machines is read/write for the user, and not
1263: accessible by others.
1.2 deraadt 1264: .Pp
1.236 jmc 1265: .It ~/.shosts
1.240 jmc 1266: This file is used in exactly the same way as
1267: .Pa .rhosts ,
1268: but allows host-based authentication without permitting login with
1269: rlogin/rsh.
1.272 mcbride 1270: .Pp
1271: .It ~/.ssh/
1272: This directory is the default location for all user-specific configuration
1273: and authentication information.
1274: There is no general requirement to keep the entire contents of this directory
1275: secret, but the recommended permissions are read/write/execute for the user,
1276: and not accessible by others.
1.236 jmc 1277: .Pp
1.238 jmc 1278: .It ~/.ssh/authorized_keys
1279: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1280: The format of this file is described in the
1281: .Xr sshd 8
1282: manual page.
1283: This file is not highly sensitive, but the recommended
1284: permissions are read/write for the user, and not accessible by others.
1285: .Pp
1286: .It ~/.ssh/config
1287: This is the per-user configuration file.
1288: The file format and configuration options are described in
1289: .Xr ssh_config 5 .
1290: Because of the potential for abuse, this file must have strict permissions:
1291: read/write for the user, and not accessible by others.
1292: .Pp
1293: .It ~/.ssh/environment
1.239 jmc 1294: Contains additional definitions for environment variables; see
1295: .Sx ENVIRONMENT ,
1.238 jmc 1296: above.
1297: .Pp
1298: .It ~/.ssh/identity
1299: .It ~/.ssh/id_dsa
1300: .It ~/.ssh/id_rsa
1301: Contains the private key for authentication.
1302: These files
1303: contain sensitive data and should be readable by the user but not
1304: accessible by others (read/write/execute).
1305: .Nm
1306: will simply ignore a private key file if it is accessible by others.
1307: It is possible to specify a passphrase when
1308: generating the key which will be used to encrypt the
1309: sensitive part of this file using 3DES.
1310: .Pp
1311: .It ~/.ssh/identity.pub
1312: .It ~/.ssh/id_dsa.pub
1313: .It ~/.ssh/id_rsa.pub
1314: Contains the public key for authentication.
1315: These files are not
1316: sensitive and can (but need not) be readable by anyone.
1317: .Pp
1318: .It ~/.ssh/known_hosts
1.244 jmc 1319: Contains a list of host keys for all hosts the user has logged into
1320: that are not already in the systemwide list of known host keys.
1.238 jmc 1321: See
1.244 jmc 1322: .Xr sshd 8
1323: for further details of the format of this file.
1.238 jmc 1324: .Pp
1325: .It ~/.ssh/rc
1326: Commands in this file are executed by
1327: .Nm
1.245 jmc 1328: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1329: started.
1330: See the
1331: .Xr sshd 8
1332: manual page for more information.
1333: .Pp
1.236 jmc 1334: .It /etc/hosts.equiv
1.240 jmc 1335: This file is for host-based authentication (see above).
1336: It should only be writable by root.
1.236 jmc 1337: .Pp
1338: .It /etc/shosts.equiv
1.240 jmc 1339: This file is used in exactly the same way as
1340: .Pa hosts.equiv ,
1341: but allows host-based authentication without permitting login with
1342: rlogin/rsh.
1.236 jmc 1343: .Pp
1.238 jmc 1344: .It Pa /etc/ssh/ssh_config
1345: Systemwide configuration file.
1346: The file format and configuration options are described in
1347: .Xr ssh_config 5 .
1348: .Pp
1349: .It /etc/ssh/ssh_host_key
1350: .It /etc/ssh/ssh_host_dsa_key
1351: .It /etc/ssh/ssh_host_rsa_key
1352: These three files contain the private parts of the host keys
1.245 jmc 1353: and are used for host-based authentication.
1354: If protocol version 1 is used,
1.238 jmc 1355: .Nm
1356: must be setuid root, since the host key is readable only by root.
1357: For protocol version 2,
1358: .Nm
1359: uses
1360: .Xr ssh-keysign 8
1.245 jmc 1361: to access the host keys,
1362: eliminating the requirement that
1.238 jmc 1363: .Nm
1.245 jmc 1364: be setuid root when host-based authentication is used.
1.238 jmc 1365: By default
1.2 deraadt 1366: .Nm
1.238 jmc 1367: is not setuid root.
1368: .Pp
1369: .It /etc/ssh/ssh_known_hosts
1370: Systemwide list of known host keys.
1371: This file should be prepared by the
1372: system administrator to contain the public host keys of all machines in the
1373: organization.
1.244 jmc 1374: It should be world-readable.
1375: See
1.238 jmc 1376: .Xr sshd 8
1.244 jmc 1377: for further details of the format of this file.
1.236 jmc 1378: .Pp
1.238 jmc 1379: .It /etc/ssh/sshrc
1.1 deraadt 1380: Commands in this file are executed by
1.2 deraadt 1381: .Nm
1.245 jmc 1382: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1383: See the
1.2 deraadt 1384: .Xr sshd 8
1.1 deraadt 1385: manual page for more information.
1.58 itojun 1386: .El
1.2 deraadt 1387: .Sh SEE ALSO
1388: .Xr scp 1 ,
1.83 djm 1389: .Xr sftp 1 ,
1.2 deraadt 1390: .Xr ssh-add 1 ,
1391: .Xr ssh-agent 1 ,
1392: .Xr ssh-keygen 1 ,
1.242 jmc 1393: .Xr ssh-keyscan 1 ,
1.250 jmc 1394: .Xr tun 4 ,
1.176 jmc 1395: .Xr hosts.equiv 5 ,
1.159 stevesk 1396: .Xr ssh_config 5 ,
1.160 naddy 1397: .Xr ssh-keysign 8 ,
1.87 itojun 1398: .Xr sshd 8
1.106 markus 1399: .Rs
1.256 jmc 1400: .%R RFC 4250
1401: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1402: .%D 2006
1403: .Re
1404: .Rs
1405: .%R RFC 4251
1406: .%T "The Secure Shell (SSH) Protocol Architecture"
1407: .%D 2006
1408: .Re
1409: .Rs
1410: .%R RFC 4252
1411: .%T "The Secure Shell (SSH) Authentication Protocol"
1412: .%D 2006
1413: .Re
1414: .Rs
1415: .%R RFC 4253
1416: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1417: .%D 2006
1418: .Re
1419: .Rs
1420: .%R RFC 4254
1421: .%T "The Secure Shell (SSH) Connection Protocol"
1422: .%D 2006
1423: .Re
1424: .Rs
1425: .%R RFC 4255
1426: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1427: .%D 2006
1428: .Re
1429: .Rs
1430: .%R RFC 4256
1431: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1432: .%D 2006
1433: .Re
1434: .Rs
1435: .%R RFC 4335
1436: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1437: .%D 2006
1438: .Re
1439: .Rs
1440: .%R RFC 4344
1441: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1442: .%D 2006
1443: .Re
1444: .Rs
1445: .%R RFC 4345
1446: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1447: .%D 2006
1448: .Re
1449: .Rs
1450: .%R RFC 4419
1451: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1452: .%D 2006
1453: .Re
1454: .Rs
1455: .%R RFC 4716
1456: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1457: .%D 2006
1.274 grunk 1458: .Re
1459: .Rs
1460: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1461: .%A A. Perrig
1462: .%A D. Song
1463: .%D 1999
1464: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1465: .Re
1.173 jmc 1466: .Sh AUTHORS
1467: OpenSSH is a derivative of the original and free
1468: ssh 1.2.12 release by Tatu Ylonen.
1469: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1470: Theo de Raadt and Dug Song
1471: removed many bugs, re-added newer features and
1472: created OpenSSH.
1473: Markus Friedl contributed the support for SSH
1474: protocol versions 1.5 and 2.0.
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