Annotation of src/usr.bin/ssh/ssh.1, Revision 1.281
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: .\"
1.281 ! djm 37: .\" $OpenBSD: ssh.1,v 1.280 2008/11/09 12:34:47 tobias Exp $
! 38: .Dd $Mdocdate: November 9 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
1.278 djm 46: .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
1.108 markus 47: .Op Fl b Ar bind_address
1.51 markus 48: .Op Fl c Ar cipher_spec
1.210 djm 49: .Oo Fl D\ \&
50: .Sm off
51: .Oo Ar bind_address : Oc
52: .Ar port
53: .Sm on
54: .Oc
1.2 deraadt 55: .Op Fl e Ar escape_char
1.176 jmc 56: .Op Fl F Ar configfile
1.211 jmc 57: .Bk -words
1.2 deraadt 58: .Op Fl i Ar identity_file
1.211 jmc 59: .Ek
1.202 jmc 60: .Oo Fl L\ \&
1.12 aaron 61: .Sm off
1.200 djm 62: .Oo Ar bind_address : Oc
1.202 jmc 63: .Ar port : host : hostport
1.12 aaron 64: .Sm on
65: .Oc
1.211 jmc 66: .Bk -words
1.176 jmc 67: .Op Fl l Ar login_name
1.211 jmc 68: .Ek
1.176 jmc 69: .Op Fl m Ar mac_spec
1.198 djm 70: .Op Fl O Ar ctl_cmd
1.176 jmc 71: .Op Fl o Ar option
72: .Op Fl p Ar port
1.202 jmc 73: .Oo Fl R\ \&
1.12 aaron 74: .Sm off
1.200 djm 75: .Oo Ar bind_address : Oc
1.202 jmc 76: .Ar port : host : hostport
1.12 aaron 77: .Sm on
78: .Oc
1.198 djm 79: .Op Fl S Ar ctl_path
1.216 jmc 80: .Bk -words
1.261 stevesk 81: .Oo Fl w Ar local_tun Ns
82: .Op : Ns Ar remote_tun Oc
1.176 jmc 83: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 84: .Op Ar command
1.216 jmc 85: .Ek
1.44 aaron 86: .Sh DESCRIPTION
1.2 deraadt 87: .Nm
1.96 deraadt 88: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 89: executing commands on a remote machine.
1.176 jmc 90: It is intended to replace rlogin and rsh,
91: and provide secure encrypted communications between
1.40 aaron 92: two untrusted hosts over an insecure network.
1.247 jmc 93: X11 connections and arbitrary TCP ports
1.176 jmc 94: can also be forwarded over the secure channel.
1.2 deraadt 95: .Pp
96: .Nm
1.44 aaron 97: connects and logs into the specified
1.176 jmc 98: .Ar hostname
99: (with optional
100: .Ar user
101: name).
1.1 deraadt 102: The user must prove
1.49 markus 103: his/her identity to the remote machine using one of several methods
1.221 jmc 104: depending on the protocol version used (see below).
1.49 markus 105: .Pp
1.176 jmc 106: If
107: .Ar command
108: is specified,
1.219 jmc 109: it is executed on the remote host instead of a login shell.
1.2 deraadt 110: .Pp
1.218 jmc 111: The options are as follows:
112: .Bl -tag -width Ds
113: .It Fl 1
114: Forces
1.2 deraadt 115: .Nm
1.218 jmc 116: to try protocol version 1 only.
117: .It Fl 2
118: Forces
1.2 deraadt 119: .Nm
1.218 jmc 120: to try protocol version 2 only.
121: .It Fl 4
122: Forces
1.2 deraadt 123: .Nm
1.218 jmc 124: to use IPv4 addresses only.
125: .It Fl 6
126: Forces
1.2 deraadt 127: .Nm
1.218 jmc 128: to use IPv6 addresses only.
129: .It Fl A
130: Enables forwarding of the authentication agent connection.
131: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 132: .Pp
1.218 jmc 133: Agent forwarding should be enabled with caution.
134: Users with the ability to bypass file permissions on the remote host
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 .
1.277 djm 293: .Pp
294: If the
295: .Cm ExitOnForwardFailure
296: configuration option is set to
297: .Dq yes ,
298: then a client started with
299: .Fl f
300: will wait for all remote port forwards to be successfully established
301: before placing itself in the background.
1.218 jmc 302: .It Fl g
303: Allows remote hosts to connect to local forwarded ports.
304: .It Fl I Ar smartcard_device
1.229 jmc 305: Specify the device
1.176 jmc 306: .Nm
1.218 jmc 307: should use to communicate with a smartcard used for storing the user's
308: private RSA key.
1.229 jmc 309: This option is only available if support for smartcard devices
310: is compiled in (default is no support).
1.218 jmc 311: .It Fl i Ar identity_file
312: Selects a file from which the identity (private key) for
313: RSA or DSA authentication is read.
314: The default is
315: .Pa ~/.ssh/identity
316: for protocol version 1, and
317: .Pa ~/.ssh/id_rsa
1.149 jakob 318: and
1.218 jmc 319: .Pa ~/.ssh/id_dsa
320: for protocol version 2.
321: Identity files may also be specified on
322: a per-host basis in the configuration file.
323: It is possible to have multiple
324: .Fl i
325: options (and multiple identities specified in
326: configuration files).
1.269 djm 327: .It Fl K
328: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
329: credentials to the server.
1.218 jmc 330: .It Fl k
331: Disables forwarding (delegation) of GSSAPI credentials to the server.
332: .It Fl L Xo
333: .Sm off
334: .Oo Ar bind_address : Oc
335: .Ar port : host : hostport
336: .Sm on
337: .Xc
338: Specifies that the given port on the local (client) host is to be
339: forwarded to the given host and port on the remote side.
340: This works by allocating a socket to listen to
341: .Ar port
342: on the local side, optionally bound to the specified
343: .Ar bind_address .
344: Whenever a connection is made to this port, the
345: connection is forwarded over the secure channel, and a connection is
346: made to
347: .Ar host
348: port
349: .Ar hostport
350: from the remote machine.
351: Port forwardings can also be specified in the configuration file.
352: IPv6 addresses can be specified with an alternative syntax:
353: .Sm off
354: .Xo
355: .Op Ar bind_address No /
356: .Ar port No / Ar host No /
357: .Ar hostport
358: .Xc
359: .Sm on
360: or by enclosing the address in square brackets.
361: Only the superuser can forward privileged ports.
362: By default, the local port is bound in accordance with the
363: .Cm GatewayPorts
364: setting.
365: However, an explicit
366: .Ar bind_address
367: may be used to bind the connection to a specific address.
1.2 deraadt 368: The
1.218 jmc 369: .Ar bind_address
370: of
371: .Dq localhost
372: indicates that the listening port be bound for local use only, while an
373: empty address or
374: .Sq *
375: indicates that the port should be available from all interfaces.
376: .It Fl l Ar login_name
377: Specifies the user to log in as on the remote machine.
378: This also may be specified on a per-host basis in the configuration file.
379: .It Fl M
380: Places the
381: .Nm
382: client into
383: .Dq master
384: mode for connection sharing.
1.231 stevesk 385: Multiple
386: .Fl M
387: options places
388: .Nm
389: into
390: .Dq master
391: mode with confirmation required before slave connections are accepted.
1.218 jmc 392: Refer to the description of
393: .Cm ControlMaster
394: in
395: .Xr ssh_config 5
396: for details.
397: .It Fl m Ar mac_spec
398: Additionally, for protocol version 2 a comma-separated list of MAC
399: (message authentication code) algorithms can
400: be specified in order of preference.
401: See the
402: .Cm MACs
403: keyword for more information.
404: .It Fl N
405: Do not execute a remote command.
406: This is useful for just forwarding ports
407: (protocol version 2 only).
408: .It Fl n
409: Redirects stdin from
410: .Pa /dev/null
411: (actually, prevents reading from stdin).
412: This must be used when
1.2 deraadt 413: .Nm
1.218 jmc 414: is run in the background.
415: A common trick is to use this to run X11 programs on a remote machine.
416: For example,
417: .Ic ssh -n shadows.cs.hut.fi emacs &
418: will start an emacs on shadows.cs.hut.fi, and the X11
419: connection will be automatically forwarded over an encrypted channel.
420: The
1.2 deraadt 421: .Nm
1.218 jmc 422: program will be put in the background.
423: (This does not work if
1.2 deraadt 424: .Nm
1.218 jmc 425: needs to ask for a password or passphrase; see also the
426: .Fl f
427: option.)
428: .It Fl O Ar ctl_cmd
429: Control an active connection multiplexing master process.
430: When the
431: .Fl O
432: option is specified, the
433: .Ar ctl_cmd
434: argument is interpreted and passed to the master process.
435: Valid commands are:
436: .Dq check
437: (check that the master process is running) and
438: .Dq exit
439: (request the master to exit).
440: .It Fl o Ar option
441: Can be used to give options in the format used in the configuration file.
442: This is useful for specifying options for which there is no separate
443: command-line flag.
444: For full details of the options listed below, and their possible values, see
445: .Xr ssh_config 5 .
1.2 deraadt 446: .Pp
1.218 jmc 447: .Bl -tag -width Ds -offset indent -compact
448: .It AddressFamily
449: .It BatchMode
450: .It BindAddress
451: .It ChallengeResponseAuthentication
452: .It CheckHostIP
453: .It Cipher
454: .It Ciphers
455: .It ClearAllForwardings
456: .It Compression
457: .It CompressionLevel
458: .It ConnectionAttempts
459: .It ConnectTimeout
460: .It ControlMaster
461: .It ControlPath
462: .It DynamicForward
463: .It EscapeChar
1.263 markus 464: .It ExitOnForwardFailure
1.218 jmc 465: .It ForwardAgent
466: .It ForwardX11
467: .It ForwardX11Trusted
468: .It GatewayPorts
469: .It GlobalKnownHostsFile
470: .It GSSAPIAuthentication
471: .It GSSAPIDelegateCredentials
472: .It HashKnownHosts
473: .It Host
474: .It HostbasedAuthentication
475: .It HostKeyAlgorithms
476: .It HostKeyAlias
477: .It HostName
478: .It IdentityFile
479: .It IdentitiesOnly
480: .It KbdInteractiveDevices
481: .It LocalCommand
482: .It LocalForward
483: .It LogLevel
484: .It MACs
485: .It NoHostAuthenticationForLocalhost
486: .It NumberOfPasswordPrompts
487: .It PasswordAuthentication
488: .It PermitLocalCommand
489: .It Port
490: .It PreferredAuthentications
491: .It Protocol
492: .It ProxyCommand
493: .It PubkeyAuthentication
1.251 dtucker 494: .It RekeyLimit
1.218 jmc 495: .It RemoteForward
496: .It RhostsRSAAuthentication
497: .It RSAAuthentication
498: .It SendEnv
499: .It ServerAliveInterval
500: .It ServerAliveCountMax
501: .It SmartcardDevice
502: .It StrictHostKeyChecking
503: .It TCPKeepAlive
504: .It Tunnel
505: .It TunnelDevice
506: .It UsePrivilegedPort
507: .It User
508: .It UserKnownHostsFile
509: .It VerifyHostKeyDNS
1.276 jmc 510: .It VisualHostKey
1.218 jmc 511: .It XAuthLocation
512: .El
513: .It Fl p Ar port
514: Port to connect to on the remote host.
515: This can be specified on a
516: per-host basis in the configuration file.
517: .It Fl q
518: Quiet mode.
1.271 djm 519: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 520: .It Fl R Xo
521: .Sm off
522: .Oo Ar bind_address : Oc
523: .Ar port : host : hostport
524: .Sm on
525: .Xc
526: Specifies that the given port on the remote (server) host is to be
527: forwarded to the given host and port on the local side.
528: This works by allocating a socket to listen to
529: .Ar port
530: on the remote side, and whenever a connection is made to this port, the
531: connection is forwarded over the secure channel, and a connection is
532: made to
533: .Ar host
534: port
535: .Ar hostport
536: from the local machine.
1.2 deraadt 537: .Pp
1.218 jmc 538: Port forwardings can also be specified in the configuration file.
539: Privileged ports can be forwarded only when
540: logging in as root on the remote machine.
541: IPv6 addresses can be specified by enclosing the address in square braces or
542: using an alternative syntax:
543: .Sm off
544: .Xo
545: .Op Ar bind_address No /
546: .Ar host No / Ar port No /
547: .Ar hostport
548: .Xc .
549: .Sm on
1.194 jakob 550: .Pp
1.218 jmc 551: By default, the listening socket on the server will be bound to the loopback
552: interface only.
1.280 tobias 553: This may be overridden by specifying a
1.218 jmc 554: .Ar bind_address .
555: An empty
556: .Ar bind_address ,
557: or the address
558: .Ql * ,
559: indicates that the remote socket should listen on all interfaces.
560: Specifying a remote
561: .Ar bind_address
562: will only succeed if the server's
563: .Cm GatewayPorts
564: option is enabled (see
565: .Xr sshd_config 5 ) .
1.281 ! djm 566: .Pp
! 567: If the
! 568: .Ar port
! 569: argument is
! 570: .Dq 0 ,
! 571: the listen port will be dynamically allocated on the server and reported
! 572: to the client at run time.
1.218 jmc 573: .It Fl S Ar ctl_path
574: Specifies the location of a control socket for connection sharing.
575: Refer to the description of
576: .Cm ControlPath
577: and
578: .Cm ControlMaster
579: in
580: .Xr ssh_config 5
581: for details.
582: .It Fl s
583: May be used to request invocation of a subsystem on the remote system.
584: Subsystems are a feature of the SSH2 protocol which facilitate the use
585: of SSH as a secure transport for other applications (eg.\&
586: .Xr sftp 1 ) .
587: The subsystem is specified as the remote command.
588: .It Fl T
589: Disable pseudo-tty allocation.
590: .It Fl t
591: Force pseudo-tty allocation.
592: This can be used to execute arbitrary
593: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 594: e.g. when implementing menu services.
1.218 jmc 595: Multiple
596: .Fl t
597: options force tty allocation, even if
1.194 jakob 598: .Nm
1.218 jmc 599: has no local tty.
600: .It Fl V
601: Display the version number and exit.
602: .It Fl v
603: Verbose mode.
604: Causes
1.176 jmc 605: .Nm
1.218 jmc 606: to print debugging messages about its progress.
607: This is helpful in
608: debugging connection, authentication, and configuration problems.
609: Multiple
610: .Fl v
611: options increase the verbosity.
612: The maximum is 3.
1.261 stevesk 613: .It Fl w Xo
614: .Ar local_tun Ns Op : Ns Ar remote_tun
615: .Xc
616: Requests
617: tunnel
618: device forwarding with the specified
1.218 jmc 619: .Xr tun 4
1.261 stevesk 620: devices between the client
621: .Pq Ar local_tun
622: and the server
623: .Pq Ar remote_tun .
624: .Pp
1.228 jmc 625: The devices may be specified by numerical ID or the keyword
626: .Dq any ,
627: which uses the next available tunnel device.
1.261 stevesk 628: If
629: .Ar remote_tun
630: is not specified, it defaults to
631: .Dq any .
1.228 jmc 632: See also the
1.218 jmc 633: .Cm Tunnel
1.261 stevesk 634: and
635: .Cm TunnelDevice
636: directives in
1.218 jmc 637: .Xr ssh_config 5 .
1.261 stevesk 638: If the
639: .Cm Tunnel
640: directive is unset, it is set to the default tunnel mode, which is
641: .Dq point-to-point .
1.218 jmc 642: .It Fl X
643: Enables X11 forwarding.
1.54 markus 644: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 645: .Pp
1.218 jmc 646: X11 forwarding should be enabled with caution.
1.168 jmc 647: Users with the ability to bypass file permissions on the remote host
1.218 jmc 648: (for the user's X authorization database)
649: can access the local X11 display through the forwarded connection.
650: An attacker may then be able to perform activities such as keystroke monitoring.
651: .Pp
652: For this reason, X11 forwarding is subjected to X11 SECURITY extension
653: restrictions by default.
654: Please refer to the
655: .Nm
656: .Fl Y
657: option and the
658: .Cm ForwardX11Trusted
659: directive in
660: .Xr ssh_config 5
661: for more information.
662: .It Fl x
663: Disables X11 forwarding.
664: .It Fl Y
665: Enables trusted X11 forwarding.
666: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
667: controls.
1.278 djm 668: .It Fl y
669: Send log information using the
670: .Xr syslog 3
671: system module.
672: By default this information is sent to stderr.
1.218 jmc 673: .El
1.224 jmc 674: .Pp
675: .Nm
676: may additionally obtain configuration data from
677: a per-user configuration file and a system-wide configuration file.
678: The file format and configuration options are described in
679: .Xr ssh_config 5 .
680: .Pp
681: .Nm
682: exits with the exit status of the remote command or with 255
683: if an error occurred.
1.222 jmc 684: .Sh AUTHENTICATION
1.249 jmc 685: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.222 jmc 686: Protocol 2 is the default, with
687: .Nm
688: falling back to protocol 1 if it detects protocol 2 is unsupported.
689: These settings may be altered using the
690: .Cm Protocol
691: option in
692: .Xr ssh_config 5 ,
693: or enforced using the
694: .Fl 1
695: and
696: .Fl 2
697: options (see above).
698: Both protocols support similar authentication methods,
699: but protocol 2 is preferred since
700: it provides additional mechanisms for confidentiality
701: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 702: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 703: Protocol 1 lacks a strong mechanism for ensuring the
704: integrity of the connection.
705: .Pp
706: The methods available for authentication are:
1.260 jmc 707: GSSAPI-based authentication,
1.222 jmc 708: host-based authentication,
709: public key authentication,
710: challenge-response authentication,
711: and password authentication.
712: Authentication methods are tried in the order specified above,
713: though protocol 2 has a configuration option to change the default order:
714: .Cm PreferredAuthentications .
715: .Pp
716: Host-based authentication works as follows:
1.218 jmc 717: If the machine the user logs in from is listed in
718: .Pa /etc/hosts.equiv
719: or
720: .Pa /etc/shosts.equiv
721: on the remote machine, and the user names are
722: the same on both sides, or if the files
723: .Pa ~/.rhosts
724: or
725: .Pa ~/.shosts
726: exist in the user's home directory on the
727: remote machine and contain a line containing the name of the client
728: machine and the name of the user on that machine, the user is
1.222 jmc 729: considered for login.
730: Additionally, the server
731: .Em must
732: be able to verify the client's
733: host key (see the description of
1.218 jmc 734: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 735: and
1.222 jmc 736: .Pa ~/.ssh/known_hosts ,
737: below)
738: for login to be permitted.
1.218 jmc 739: This authentication method closes security holes due to IP
1.222 jmc 740: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 741: [Note to the administrator:
742: .Pa /etc/hosts.equiv ,
743: .Pa ~/.rhosts ,
744: and the rlogin/rsh protocol in general, are inherently insecure and should be
745: disabled if security is desired.]
1.189 dtucker 746: .Pp
1.222 jmc 747: Public key authentication works as follows:
748: The scheme is based on public-key cryptography,
749: using cryptosystems
750: where encryption and decryption are done using separate keys,
751: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 752: The idea is that each user creates a public/private
753: key pair for authentication purposes.
754: The server knows the public key, and only the user knows the private key.
1.222 jmc 755: .Nm
756: implements public key authentication protocol automatically,
757: using either the RSA or DSA algorithms.
758: Protocol 1 is restricted to using only RSA keys,
759: but protocol 2 may use either.
760: The
761: .Sx HISTORY
762: section of
763: .Xr ssl 8
764: contains a brief discussion of the two algorithms.
1.210 djm 765: .Pp
1.218 jmc 766: The file
767: .Pa ~/.ssh/authorized_keys
768: lists the public keys that are permitted for logging in.
769: When the user logs in, the
1.2 deraadt 770: .Nm
1.218 jmc 771: program tells the server which key pair it would like to use for
772: authentication.
1.222 jmc 773: The client proves that it has access to the private key
774: and the server checks that the corresponding public key
775: is authorized to accept the account.
1.218 jmc 776: .Pp
1.222 jmc 777: The user creates his/her key pair by running
1.218 jmc 778: .Xr ssh-keygen 1 .
779: This stores the private key in
1.207 djm 780: .Pa ~/.ssh/identity
1.222 jmc 781: (protocol 1),
782: .Pa ~/.ssh/id_dsa
783: (protocol 2 DSA),
784: or
785: .Pa ~/.ssh/id_rsa
786: (protocol 2 RSA)
1.218 jmc 787: and stores the public key in
788: .Pa ~/.ssh/identity.pub
1.222 jmc 789: (protocol 1),
790: .Pa ~/.ssh/id_dsa.pub
791: (protocol 2 DSA),
792: or
793: .Pa ~/.ssh/id_rsa.pub
794: (protocol 2 RSA)
1.218 jmc 795: in the user's home directory.
1.222 jmc 796: The user should then copy the public key
1.218 jmc 797: to
798: .Pa ~/.ssh/authorized_keys
1.222 jmc 799: in his/her home directory on the remote machine.
800: The
1.218 jmc 801: .Pa authorized_keys
802: file corresponds to the conventional
803: .Pa ~/.rhosts
804: file, and has one key
1.222 jmc 805: per line, though the lines can be very long.
1.218 jmc 806: After this, the user can log in without giving the password.
807: .Pp
1.222 jmc 808: The most convenient way to use public key authentication may be with an
1.218 jmc 809: authentication agent.
810: See
811: .Xr ssh-agent 1
812: for more information.
813: .Pp
1.222 jmc 814: Challenge-response authentication works as follows:
815: The server sends an arbitrary
816: .Qq challenge
817: text, and prompts for a response.
818: Protocol 2 allows multiple challenges and responses;
819: protocol 1 is restricted to just one challenge/response.
820: Examples of challenge-response authentication include
821: BSD Authentication (see
822: .Xr login.conf 5 )
823: and PAM (some non-OpenBSD systems).
824: .Pp
825: Finally, if other authentication methods fail,
1.218 jmc 826: .Nm
827: prompts the user for a password.
828: The password is sent to the remote
829: host for checking; however, since all communications are encrypted,
830: the password cannot be seen by someone listening on the network.
1.232 jmc 831: .Pp
832: .Nm
833: automatically maintains and checks a database containing
834: identification for all hosts it has ever been used with.
835: Host keys are stored in
836: .Pa ~/.ssh/known_hosts
837: in the user's home directory.
838: Additionally, the file
839: .Pa /etc/ssh/ssh_known_hosts
840: is automatically checked for known hosts.
841: Any new hosts are automatically added to the user's file.
842: If a host's identification ever changes,
843: .Nm
844: warns about this and disables password authentication to prevent
845: server spoofing or man-in-the-middle attacks,
846: which could otherwise be used to circumvent the encryption.
847: The
848: .Cm StrictHostKeyChecking
849: option can be used to control logins to machines whose
850: host key is not known or has changed.
851: .Pp
1.218 jmc 852: When the user's identity has been accepted by the server, the server
853: either executes the given command, or logs into the machine and gives
854: the user a normal shell on the remote machine.
855: All communication with
856: the remote command or shell will be automatically encrypted.
857: .Pp
858: If a pseudo-terminal has been allocated (normal login session), the
859: user may use the escape characters noted below.
860: .Pp
861: If no pseudo-tty has been allocated,
862: the session is transparent and can be used to reliably transfer binary data.
863: On most systems, setting the escape character to
864: .Dq none
865: will also make the session transparent even if a tty is used.
866: .Pp
867: The session terminates when the command or shell on the remote
1.247 jmc 868: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 869: .Sh ESCAPE CHARACTERS
1.218 jmc 870: When a pseudo-terminal has been requested,
1.2 deraadt 871: .Nm
1.218 jmc 872: supports a number of functions through the use of an escape character.
873: .Pp
874: A single tilde character can be sent as
875: .Ic ~~
876: or by following the tilde by a character other than those described below.
877: The escape character must always follow a newline to be interpreted as
878: special.
879: The escape character can be changed in configuration files using the
880: .Cm EscapeChar
881: configuration directive or on the command line by the
882: .Fl e
883: option.
884: .Pp
885: The supported escapes (assuming the default
886: .Ql ~ )
887: are:
888: .Bl -tag -width Ds
889: .It Cm ~.
890: Disconnect.
891: .It Cm ~^Z
892: Background
1.234 jmc 893: .Nm .
1.218 jmc 894: .It Cm ~#
895: List forwarded connections.
896: .It Cm ~&
897: Background
1.2 deraadt 898: .Nm
1.218 jmc 899: at logout when waiting for forwarded connection / X11 sessions to terminate.
900: .It Cm ~?
901: Display a list of escape characters.
902: .It Cm ~B
903: Send a BREAK to the remote system
904: (only useful for SSH protocol version 2 and if the peer supports it).
905: .It Cm ~C
906: Open command line.
907: Currently this allows the addition of port forwardings using the
1.279 stevesk 908: .Fl L ,
909: .Fl R
1.218 jmc 910: and
1.279 stevesk 911: .Fl D
1.225 jmc 912: options (see above).
1.218 jmc 913: It also allows the cancellation of existing remote port-forwardings
914: using
1.262 stevesk 915: .Sm off
916: .Fl KR Oo Ar bind_address : Oc Ar port .
917: .Sm on
1.218 jmc 918: .Ic !\& Ns Ar command
919: allows the user to execute a local command if the
920: .Ic PermitLocalCommand
921: option is enabled in
1.176 jmc 922: .Xr ssh_config 5 .
1.218 jmc 923: Basic help is available, using the
924: .Fl h
925: option.
926: .It Cm ~R
927: Request rekeying of the connection
928: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 929: .El
1.246 jmc 930: .Sh TCP FORWARDING
931: Forwarding of arbitrary TCP connections over the secure channel can
932: be specified either on the command line or in a configuration file.
933: One possible application of TCP forwarding is a secure connection to a
934: mail server; another is going through firewalls.
935: .Pp
936: In the example below, we look at encrypting communication between
937: an IRC client and server, even though the IRC server does not directly
938: support encrypted communications.
939: This works as follows:
940: the user connects to the remote host using
941: .Nm ,
942: specifying a port to be used to forward connections
943: to the remote server.
944: After that it is possible to start the service which is to be encrypted
945: on the client machine,
946: connecting to the same local port,
947: and
948: .Nm
949: will encrypt and forward the connection.
950: .Pp
951: The following example tunnels an IRC session from client machine
952: .Dq 127.0.0.1
953: (localhost)
954: to remote server
955: .Dq server.example.com :
956: .Bd -literal -offset 4n
957: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
958: $ irc -c '#users' -p 1234 pinky 127.0.0.1
959: .Ed
960: .Pp
961: This tunnels a connection to IRC server
962: .Dq server.example.com ,
963: joining channel
964: .Dq #users ,
965: nickname
966: .Dq pinky ,
967: using port 1234.
968: It doesn't matter which port is used,
969: as long as it's greater than 1023
970: (remember, only root can open sockets on privileged ports)
971: and doesn't conflict with any ports already in use.
972: The connection is forwarded to port 6667 on the remote server,
973: since that's the standard port for IRC services.
974: .Pp
975: The
976: .Fl f
977: option backgrounds
978: .Nm
979: and the remote command
980: .Dq sleep 10
981: is specified to allow an amount of time
982: (10 seconds, in the example)
983: to start the service which is to be tunnelled.
984: If no connections are made within the time specified,
985: .Nm
986: will exit.
987: .Sh X11 FORWARDING
1.218 jmc 988: If the
989: .Cm ForwardX11
990: variable is set to
991: .Dq yes
992: (or see the description of the
1.227 jmc 993: .Fl X ,
994: .Fl x ,
1.218 jmc 995: and
1.227 jmc 996: .Fl Y
1.226 jmc 997: options above)
1.218 jmc 998: and the user is using X11 (the
999: .Ev DISPLAY
1000: environment variable is set), the connection to the X11 display is
1001: automatically forwarded to the remote side in such a way that any X11
1002: programs started from the shell (or command) will go through the
1003: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1004: from the local machine.
1.218 jmc 1005: The user should not manually set
1006: .Ev DISPLAY .
1007: Forwarding of X11 connections can be
1008: configured on the command line or in configuration files.
1009: .Pp
1010: The
1011: .Ev DISPLAY
1012: value set by
1013: .Nm
1014: will point to the server machine, but with a display number greater than zero.
1015: This is normal, and happens because
1016: .Nm
1017: creates a
1018: .Dq proxy
1019: X server on the server machine for forwarding the
1020: connections over the encrypted channel.
1.200 djm 1021: .Pp
1.218 jmc 1022: .Nm
1023: will also automatically set up Xauthority data on the server machine.
1024: For this purpose, it will generate a random authorization cookie,
1025: store it in Xauthority on the server, and verify that any forwarded
1026: connections carry this cookie and replace it by the real cookie when
1027: the connection is opened.
1028: The real authentication cookie is never
1029: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1030: .Pp
1.218 jmc 1031: If the
1032: .Cm ForwardAgent
1033: variable is set to
1034: .Dq yes
1035: (or see the description of the
1036: .Fl A
1.191 djm 1037: and
1.218 jmc 1038: .Fl a
1.226 jmc 1039: options above) and
1.218 jmc 1040: the user is using an authentication agent, the connection to the agent
1041: is automatically forwarded to the remote side.
1.252 jmc 1042: .Sh VERIFYING HOST KEYS
1043: When connecting to a server for the first time,
1044: a fingerprint of the server's public key is presented to the user
1045: (unless the option
1046: .Cm StrictHostKeyChecking
1047: has been disabled).
1048: Fingerprints can be determined using
1049: .Xr ssh-keygen 1 :
1050: .Pp
1051: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1052: .Pp
1.274 grunk 1053: If the fingerprint is already known, it can be matched
1054: and the key can be accepted or rejected.
1055: Because of the difficulty of comparing host keys
1056: just by looking at hex strings,
1057: there is also support to compare host keys visually,
1058: using
1059: .Em random art .
1060: By setting the
1.275 grunk 1061: .Cm VisualHostKey
1.274 grunk 1062: option to
1.275 grunk 1063: .Dq yes ,
1.274 grunk 1064: a small ASCII graphic gets displayed on every login to a server, no matter
1065: if the session itself is interactive or not.
1066: By learning the pattern a known server produces, a user can easily
1067: find out that the host key has changed when a completely different pattern
1068: is displayed.
1069: Because these patterns are not unambiguous however, a pattern that looks
1070: similar to the pattern remembered only gives a good probability that the
1071: host key is the same, not guaranteed proof.
1072: .Pp
1073: To get a listing of the fingerprints along with their random art for
1074: all known hosts, the following command line can be used:
1075: .Pp
1076: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1077: .Pp
1.252 jmc 1078: If the fingerprint is unknown,
1079: an alternative method of verification is available:
1080: SSH fingerprints verified by DNS.
1081: An additional resource record (RR),
1082: SSHFP,
1083: is added to a zonefile
1084: and the connecting client is able to match the fingerprint
1085: with that of the key presented.
1086: .Pp
1087: In this example, we are connecting a client to a server,
1088: .Dq host.example.com .
1089: The SSHFP resource records should first be added to the zonefile for
1090: host.example.com:
1091: .Bd -literal -offset indent
1.259 jakob 1092: $ ssh-keygen -r host.example.com.
1.252 jmc 1093: .Ed
1094: .Pp
1095: The output lines will have to be added to the zonefile.
1096: To check that the zone is answering fingerprint queries:
1097: .Pp
1098: .Dl $ dig -t SSHFP host.example.com
1099: .Pp
1100: Finally the client connects:
1101: .Bd -literal -offset indent
1102: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1103: [...]
1104: Matching host key fingerprint found in DNS.
1105: Are you sure you want to continue connecting (yes/no)?
1106: .Ed
1107: .Pp
1108: See the
1109: .Cm VerifyHostKeyDNS
1110: option in
1111: .Xr ssh_config 5
1112: for more information.
1.250 jmc 1113: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1114: .Nm
1115: contains support for Virtual Private Network (VPN) tunnelling
1116: using the
1117: .Xr tun 4
1118: network pseudo-device,
1119: allowing two networks to be joined securely.
1120: The
1121: .Xr sshd_config 5
1122: configuration option
1123: .Cm PermitTunnel
1124: controls whether the server supports this,
1125: and at what level (layer 2 or 3 traffic).
1126: .Pp
1127: The following example would connect client network 10.0.50.0/24
1.265 otto 1128: with remote network 10.0.99.0/24 using a point-to-point connection
1129: from 10.1.1.1 to 10.1.1.2,
1130: provided that the SSH server running on the gateway to the remote network,
1131: at 192.168.1.15, allows it.
1132: .Pp
1133: On the client:
1.250 jmc 1134: .Bd -literal -offset indent
1135: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1136: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1137: # route add 10.0.99.0/24 10.1.1.2
1138: .Ed
1139: .Pp
1140: On the server:
1141: .Bd -literal -offset indent
1142: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1143: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1144: .Ed
1145: .Pp
1146: Client access may be more finely tuned via the
1147: .Pa /root/.ssh/authorized_keys
1148: file (see below) and the
1149: .Cm PermitRootLogin
1150: server option.
1.255 jmc 1151: The following entry would permit connections on
1.250 jmc 1152: .Xr tun 4
1.255 jmc 1153: device 1 from user
1.250 jmc 1154: .Dq jane
1.255 jmc 1155: and on tun device 2 from user
1.250 jmc 1156: .Dq john ,
1157: if
1158: .Cm PermitRootLogin
1159: is set to
1160: .Dq forced-commands-only :
1161: .Bd -literal -offset 2n
1162: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1163: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1164: .Ed
1165: .Pp
1.264 ray 1166: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1167: it may be more suited to temporary setups,
1168: such as for wireless VPNs.
1169: More permanent VPNs are better provided by tools such as
1170: .Xr ipsecctl 8
1171: and
1172: .Xr isakmpd 8 .
1.2 deraadt 1173: .Sh ENVIRONMENT
1174: .Nm
1.1 deraadt 1175: will normally set the following environment variables:
1.237 jmc 1176: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1177: .It Ev DISPLAY
1178: The
1179: .Ev DISPLAY
1.40 aaron 1180: variable indicates the location of the X11 server.
1.44 aaron 1181: It is automatically set by
1.2 deraadt 1182: .Nm
1183: to point to a value of the form
1.233 jmc 1184: .Dq hostname:n ,
1185: where
1186: .Dq hostname
1187: indicates the host where the shell runs, and
1188: .Sq n
1189: is an integer \*(Ge 1.
1.40 aaron 1190: .Nm
1191: uses this special value to forward X11 connections over the secure
1192: channel.
1.107 markus 1193: The user should normally not set
1194: .Ev DISPLAY
1195: explicitly, as that
1.1 deraadt 1196: will render the X11 connection insecure (and will require the user to
1197: manually copy any required authorization cookies).
1.2 deraadt 1198: .It Ev HOME
1.1 deraadt 1199: Set to the path of the user's home directory.
1.2 deraadt 1200: .It Ev LOGNAME
1201: Synonym for
1.12 aaron 1202: .Ev USER ;
1203: set for compatibility with systems that use this variable.
1.2 deraadt 1204: .It Ev MAIL
1.129 stevesk 1205: Set to the path of the user's mailbox.
1.40 aaron 1206: .It Ev PATH
1.2 deraadt 1207: Set to the default
1208: .Ev PATH ,
1209: as specified when compiling
1.234 jmc 1210: .Nm .
1.118 markus 1211: .It Ev SSH_ASKPASS
1212: If
1213: .Nm
1214: needs a passphrase, it will read the passphrase from the current
1215: terminal if it was run from a terminal.
1216: If
1217: .Nm
1218: does not have a terminal associated with it but
1219: .Ev DISPLAY
1220: and
1221: .Ev SSH_ASKPASS
1222: are set, it will execute the program specified by
1223: .Ev SSH_ASKPASS
1224: and open an X11 window to read the passphrase.
1225: This is particularly useful when calling
1226: .Nm
1227: from a
1.196 jmc 1228: .Pa .xsession
1.118 markus 1229: or related script.
1230: (Note that on some machines it
1231: may be necessary to redirect the input from
1232: .Pa /dev/null
1233: to make this work.)
1.18 markus 1234: .It Ev SSH_AUTH_SOCK
1.233 jmc 1235: Identifies the path of a
1236: .Ux Ns -domain
1237: socket used to communicate with the agent.
1.166 stevesk 1238: .It Ev SSH_CONNECTION
1239: Identifies the client and server ends of the connection.
1.40 aaron 1240: The variable contains
1.233 jmc 1241: four space-separated values: client IP address, client port number,
1242: server IP address, and server port number.
1.73 markus 1243: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1244: This variable contains the original command line if a forced command
1.73 markus 1245: is executed.
1246: It can be used to extract the original arguments.
1.2 deraadt 1247: .It Ev SSH_TTY
1.1 deraadt 1248: This is set to the name of the tty (path to the device) associated
1.40 aaron 1249: with the current shell or command.
1250: If the current session has no tty,
1.1 deraadt 1251: this variable is not set.
1.2 deraadt 1252: .It Ev TZ
1.214 jmc 1253: This variable is set to indicate the present time zone if it
1.257 jmc 1254: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1255: on to new connections).
1.2 deraadt 1256: .It Ev USER
1.1 deraadt 1257: Set to the name of the user logging in.
1.2 deraadt 1258: .El
1259: .Pp
1.44 aaron 1260: Additionally,
1.2 deraadt 1261: .Nm
1.44 aaron 1262: reads
1.207 djm 1263: .Pa ~/.ssh/environment ,
1.2 deraadt 1264: and adds lines of the format
1265: .Dq VARNAME=value
1.233 jmc 1266: to the environment if the file exists and users are allowed to
1.161 marc 1267: change their environment.
1.176 jmc 1268: For more information, see the
1.161 marc 1269: .Cm PermitUserEnvironment
1.162 stevesk 1270: option in
1.161 marc 1271: .Xr sshd_config 5 .
1.2 deraadt 1272: .Sh FILES
1.236 jmc 1273: .Bl -tag -width Ds -compact
1274: .It ~/.rhosts
1.240 jmc 1275: This file is used for host-based authentication (see above).
1.92 markus 1276: On some machines this file may need to be
1.240 jmc 1277: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1278: because
1.2 deraadt 1279: .Xr sshd 8
1.40 aaron 1280: reads it as root.
1281: Additionally, this file must be owned by the user,
1282: and must not have write permissions for anyone else.
1283: The recommended
1.1 deraadt 1284: permission for most machines is read/write for the user, and not
1285: accessible by others.
1.2 deraadt 1286: .Pp
1.236 jmc 1287: .It ~/.shosts
1.240 jmc 1288: This file is used in exactly the same way as
1289: .Pa .rhosts ,
1290: but allows host-based authentication without permitting login with
1291: rlogin/rsh.
1.272 mcbride 1292: .Pp
1293: .It ~/.ssh/
1294: This directory is the default location for all user-specific configuration
1295: and authentication information.
1296: There is no general requirement to keep the entire contents of this directory
1297: secret, but the recommended permissions are read/write/execute for the user,
1298: and not accessible by others.
1.236 jmc 1299: .Pp
1.238 jmc 1300: .It ~/.ssh/authorized_keys
1301: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1302: The format of this file is described in the
1303: .Xr sshd 8
1304: manual page.
1305: This file is not highly sensitive, but the recommended
1306: permissions are read/write for the user, and not accessible by others.
1307: .Pp
1308: .It ~/.ssh/config
1309: This is the per-user configuration file.
1310: The file format and configuration options are described in
1311: .Xr ssh_config 5 .
1312: Because of the potential for abuse, this file must have strict permissions:
1313: read/write for the user, and not accessible by others.
1314: .Pp
1315: .It ~/.ssh/environment
1.239 jmc 1316: Contains additional definitions for environment variables; see
1317: .Sx ENVIRONMENT ,
1.238 jmc 1318: above.
1319: .Pp
1320: .It ~/.ssh/identity
1321: .It ~/.ssh/id_dsa
1322: .It ~/.ssh/id_rsa
1323: Contains the private key for authentication.
1324: These files
1325: contain sensitive data and should be readable by the user but not
1326: accessible by others (read/write/execute).
1327: .Nm
1328: will simply ignore a private key file if it is accessible by others.
1329: It is possible to specify a passphrase when
1330: generating the key which will be used to encrypt the
1331: sensitive part of this file using 3DES.
1332: .Pp
1333: .It ~/.ssh/identity.pub
1334: .It ~/.ssh/id_dsa.pub
1335: .It ~/.ssh/id_rsa.pub
1336: Contains the public key for authentication.
1337: These files are not
1338: sensitive and can (but need not) be readable by anyone.
1339: .Pp
1340: .It ~/.ssh/known_hosts
1.244 jmc 1341: Contains a list of host keys for all hosts the user has logged into
1342: that are not already in the systemwide list of known host keys.
1.238 jmc 1343: See
1.244 jmc 1344: .Xr sshd 8
1345: for further details of the format of this file.
1.238 jmc 1346: .Pp
1347: .It ~/.ssh/rc
1348: Commands in this file are executed by
1349: .Nm
1.245 jmc 1350: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1351: started.
1352: See the
1353: .Xr sshd 8
1354: manual page for more information.
1355: .Pp
1.236 jmc 1356: .It /etc/hosts.equiv
1.240 jmc 1357: This file is for host-based authentication (see above).
1358: It should only be writable by root.
1.236 jmc 1359: .Pp
1360: .It /etc/shosts.equiv
1.240 jmc 1361: This file is used in exactly the same way as
1362: .Pa hosts.equiv ,
1363: but allows host-based authentication without permitting login with
1364: rlogin/rsh.
1.236 jmc 1365: .Pp
1.238 jmc 1366: .It Pa /etc/ssh/ssh_config
1367: Systemwide configuration file.
1368: The file format and configuration options are described in
1369: .Xr ssh_config 5 .
1370: .Pp
1371: .It /etc/ssh/ssh_host_key
1372: .It /etc/ssh/ssh_host_dsa_key
1373: .It /etc/ssh/ssh_host_rsa_key
1374: These three files contain the private parts of the host keys
1.245 jmc 1375: and are used for host-based authentication.
1376: If protocol version 1 is used,
1.238 jmc 1377: .Nm
1378: must be setuid root, since the host key is readable only by root.
1379: For protocol version 2,
1380: .Nm
1381: uses
1382: .Xr ssh-keysign 8
1.245 jmc 1383: to access the host keys,
1384: eliminating the requirement that
1.238 jmc 1385: .Nm
1.245 jmc 1386: be setuid root when host-based authentication is used.
1.238 jmc 1387: By default
1.2 deraadt 1388: .Nm
1.238 jmc 1389: is not setuid root.
1390: .Pp
1391: .It /etc/ssh/ssh_known_hosts
1392: Systemwide list of known host keys.
1393: This file should be prepared by the
1394: system administrator to contain the public host keys of all machines in the
1395: organization.
1.244 jmc 1396: It should be world-readable.
1397: See
1.238 jmc 1398: .Xr sshd 8
1.244 jmc 1399: for further details of the format of this file.
1.236 jmc 1400: .Pp
1.238 jmc 1401: .It /etc/ssh/sshrc
1.1 deraadt 1402: Commands in this file are executed by
1.2 deraadt 1403: .Nm
1.245 jmc 1404: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1405: See the
1.2 deraadt 1406: .Xr sshd 8
1.1 deraadt 1407: manual page for more information.
1.58 itojun 1408: .El
1.2 deraadt 1409: .Sh SEE ALSO
1410: .Xr scp 1 ,
1.83 djm 1411: .Xr sftp 1 ,
1.2 deraadt 1412: .Xr ssh-add 1 ,
1413: .Xr ssh-agent 1 ,
1414: .Xr ssh-keygen 1 ,
1.242 jmc 1415: .Xr ssh-keyscan 1 ,
1.250 jmc 1416: .Xr tun 4 ,
1.176 jmc 1417: .Xr hosts.equiv 5 ,
1.159 stevesk 1418: .Xr ssh_config 5 ,
1.160 naddy 1419: .Xr ssh-keysign 8 ,
1.87 itojun 1420: .Xr sshd 8
1.106 markus 1421: .Rs
1.256 jmc 1422: .%R RFC 4250
1423: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1424: .%D 2006
1425: .Re
1426: .Rs
1427: .%R RFC 4251
1428: .%T "The Secure Shell (SSH) Protocol Architecture"
1429: .%D 2006
1430: .Re
1431: .Rs
1432: .%R RFC 4252
1433: .%T "The Secure Shell (SSH) Authentication Protocol"
1434: .%D 2006
1435: .Re
1436: .Rs
1437: .%R RFC 4253
1438: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1439: .%D 2006
1440: .Re
1441: .Rs
1442: .%R RFC 4254
1443: .%T "The Secure Shell (SSH) Connection Protocol"
1444: .%D 2006
1445: .Re
1446: .Rs
1447: .%R RFC 4255
1448: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1449: .%D 2006
1450: .Re
1451: .Rs
1452: .%R RFC 4256
1453: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1454: .%D 2006
1455: .Re
1456: .Rs
1457: .%R RFC 4335
1458: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1459: .%D 2006
1460: .Re
1461: .Rs
1462: .%R RFC 4344
1463: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1464: .%D 2006
1465: .Re
1466: .Rs
1467: .%R RFC 4345
1468: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1469: .%D 2006
1470: .Re
1471: .Rs
1472: .%R RFC 4419
1473: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1474: .%D 2006
1475: .Re
1476: .Rs
1477: .%R RFC 4716
1478: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1479: .%D 2006
1.274 grunk 1480: .Re
1481: .Rs
1482: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1483: .%A A. Perrig
1484: .%A D. Song
1485: .%D 1999
1486: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1487: .Re
1.173 jmc 1488: .Sh AUTHORS
1489: OpenSSH is a derivative of the original and free
1490: ssh 1.2.12 release by Tatu Ylonen.
1491: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1492: Theo de Raadt and Dug Song
1493: removed many bugs, re-added newer features and
1494: created OpenSSH.
1495: Markus Friedl contributed the support for SSH
1496: protocol versions 1.5 and 2.0.
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