Annotation of src/usr.bin/ssh/ssh.1, Revision 1.278
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: .\"
1.278 ! djm 37: .\" $OpenBSD: ssh.1,v 1.277 2008/07/02 13:47:39 djm Exp $
! 38: .Dd $Mdocdate: July 2 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.
553: This may be overriden by specifying a
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 ) .
566: .It Fl S Ar ctl_path
567: Specifies the location of a control socket for connection sharing.
568: Refer to the description of
569: .Cm ControlPath
570: and
571: .Cm ControlMaster
572: in
573: .Xr ssh_config 5
574: for details.
575: .It Fl s
576: May be used to request invocation of a subsystem on the remote system.
577: Subsystems are a feature of the SSH2 protocol which facilitate the use
578: of SSH as a secure transport for other applications (eg.\&
579: .Xr sftp 1 ) .
580: The subsystem is specified as the remote command.
581: .It Fl T
582: Disable pseudo-tty allocation.
583: .It Fl t
584: Force pseudo-tty allocation.
585: This can be used to execute arbitrary
586: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 587: e.g. when implementing menu services.
1.218 jmc 588: Multiple
589: .Fl t
590: options force tty allocation, even if
1.194 jakob 591: .Nm
1.218 jmc 592: has no local tty.
593: .It Fl V
594: Display the version number and exit.
595: .It Fl v
596: Verbose mode.
597: Causes
1.176 jmc 598: .Nm
1.218 jmc 599: to print debugging messages about its progress.
600: This is helpful in
601: debugging connection, authentication, and configuration problems.
602: Multiple
603: .Fl v
604: options increase the verbosity.
605: The maximum is 3.
1.261 stevesk 606: .It Fl w Xo
607: .Ar local_tun Ns Op : Ns Ar remote_tun
608: .Xc
609: Requests
610: tunnel
611: device forwarding with the specified
1.218 jmc 612: .Xr tun 4
1.261 stevesk 613: devices between the client
614: .Pq Ar local_tun
615: and the server
616: .Pq Ar remote_tun .
617: .Pp
1.228 jmc 618: The devices may be specified by numerical ID or the keyword
619: .Dq any ,
620: which uses the next available tunnel device.
1.261 stevesk 621: If
622: .Ar remote_tun
623: is not specified, it defaults to
624: .Dq any .
1.228 jmc 625: See also the
1.218 jmc 626: .Cm Tunnel
1.261 stevesk 627: and
628: .Cm TunnelDevice
629: directives in
1.218 jmc 630: .Xr ssh_config 5 .
1.261 stevesk 631: If the
632: .Cm Tunnel
633: directive is unset, it is set to the default tunnel mode, which is
634: .Dq point-to-point .
1.218 jmc 635: .It Fl X
636: Enables X11 forwarding.
1.54 markus 637: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 638: .Pp
1.218 jmc 639: X11 forwarding should be enabled with caution.
1.168 jmc 640: Users with the ability to bypass file permissions on the remote host
1.218 jmc 641: (for the user's X authorization database)
642: can access the local X11 display through the forwarded connection.
643: An attacker may then be able to perform activities such as keystroke monitoring.
644: .Pp
645: For this reason, X11 forwarding is subjected to X11 SECURITY extension
646: restrictions by default.
647: Please refer to the
648: .Nm
649: .Fl Y
650: option and the
651: .Cm ForwardX11Trusted
652: directive in
653: .Xr ssh_config 5
654: for more information.
655: .It Fl x
656: Disables X11 forwarding.
657: .It Fl Y
658: Enables trusted X11 forwarding.
659: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
660: controls.
1.278 ! djm 661: .It Fl y
! 662: Send log information using the
! 663: .Xr syslog 3
! 664: system module.
! 665: By default this information is sent to stderr.
1.218 jmc 666: .El
1.224 jmc 667: .Pp
668: .Nm
669: may additionally obtain configuration data from
670: a per-user configuration file and a system-wide configuration file.
671: The file format and configuration options are described in
672: .Xr ssh_config 5 .
673: .Pp
674: .Nm
675: exits with the exit status of the remote command or with 255
676: if an error occurred.
1.222 jmc 677: .Sh AUTHENTICATION
1.249 jmc 678: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.222 jmc 679: Protocol 2 is the default, with
680: .Nm
681: falling back to protocol 1 if it detects protocol 2 is unsupported.
682: These settings may be altered using the
683: .Cm Protocol
684: option in
685: .Xr ssh_config 5 ,
686: or enforced using the
687: .Fl 1
688: and
689: .Fl 2
690: options (see above).
691: Both protocols support similar authentication methods,
692: but protocol 2 is preferred since
693: it provides additional mechanisms for confidentiality
694: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 695: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 696: Protocol 1 lacks a strong mechanism for ensuring the
697: integrity of the connection.
698: .Pp
699: The methods available for authentication are:
1.260 jmc 700: GSSAPI-based authentication,
1.222 jmc 701: host-based authentication,
702: public key authentication,
703: challenge-response authentication,
704: and password authentication.
705: Authentication methods are tried in the order specified above,
706: though protocol 2 has a configuration option to change the default order:
707: .Cm PreferredAuthentications .
708: .Pp
709: Host-based authentication works as follows:
1.218 jmc 710: If the machine the user logs in from is listed in
711: .Pa /etc/hosts.equiv
712: or
713: .Pa /etc/shosts.equiv
714: on the remote machine, and the user names are
715: the same on both sides, or if the files
716: .Pa ~/.rhosts
717: or
718: .Pa ~/.shosts
719: exist in the user's home directory on the
720: remote machine and contain a line containing the name of the client
721: machine and the name of the user on that machine, the user is
1.222 jmc 722: considered for login.
723: Additionally, the server
724: .Em must
725: be able to verify the client's
726: host key (see the description of
1.218 jmc 727: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 728: and
1.222 jmc 729: .Pa ~/.ssh/known_hosts ,
730: below)
731: for login to be permitted.
1.218 jmc 732: This authentication method closes security holes due to IP
1.222 jmc 733: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 734: [Note to the administrator:
735: .Pa /etc/hosts.equiv ,
736: .Pa ~/.rhosts ,
737: and the rlogin/rsh protocol in general, are inherently insecure and should be
738: disabled if security is desired.]
1.189 dtucker 739: .Pp
1.222 jmc 740: Public key authentication works as follows:
741: The scheme is based on public-key cryptography,
742: using cryptosystems
743: where encryption and decryption are done using separate keys,
744: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 745: The idea is that each user creates a public/private
746: key pair for authentication purposes.
747: The server knows the public key, and only the user knows the private key.
1.222 jmc 748: .Nm
749: implements public key authentication protocol automatically,
750: using either the RSA or DSA algorithms.
751: Protocol 1 is restricted to using only RSA keys,
752: but protocol 2 may use either.
753: The
754: .Sx HISTORY
755: section of
756: .Xr ssl 8
757: contains a brief discussion of the two algorithms.
1.210 djm 758: .Pp
1.218 jmc 759: The file
760: .Pa ~/.ssh/authorized_keys
761: lists the public keys that are permitted for logging in.
762: When the user logs in, the
1.2 deraadt 763: .Nm
1.218 jmc 764: program tells the server which key pair it would like to use for
765: authentication.
1.222 jmc 766: The client proves that it has access to the private key
767: and the server checks that the corresponding public key
768: is authorized to accept the account.
1.218 jmc 769: .Pp
1.222 jmc 770: The user creates his/her key pair by running
1.218 jmc 771: .Xr ssh-keygen 1 .
772: This stores the private key in
1.207 djm 773: .Pa ~/.ssh/identity
1.222 jmc 774: (protocol 1),
775: .Pa ~/.ssh/id_dsa
776: (protocol 2 DSA),
777: or
778: .Pa ~/.ssh/id_rsa
779: (protocol 2 RSA)
1.218 jmc 780: and stores the public key in
781: .Pa ~/.ssh/identity.pub
1.222 jmc 782: (protocol 1),
783: .Pa ~/.ssh/id_dsa.pub
784: (protocol 2 DSA),
785: or
786: .Pa ~/.ssh/id_rsa.pub
787: (protocol 2 RSA)
1.218 jmc 788: in the user's home directory.
1.222 jmc 789: The user should then copy the public key
1.218 jmc 790: to
791: .Pa ~/.ssh/authorized_keys
1.222 jmc 792: in his/her home directory on the remote machine.
793: The
1.218 jmc 794: .Pa authorized_keys
795: file corresponds to the conventional
796: .Pa ~/.rhosts
797: file, and has one key
1.222 jmc 798: per line, though the lines can be very long.
1.218 jmc 799: After this, the user can log in without giving the password.
800: .Pp
1.222 jmc 801: The most convenient way to use public key authentication may be with an
1.218 jmc 802: authentication agent.
803: See
804: .Xr ssh-agent 1
805: for more information.
806: .Pp
1.222 jmc 807: Challenge-response authentication works as follows:
808: The server sends an arbitrary
809: .Qq challenge
810: text, and prompts for a response.
811: Protocol 2 allows multiple challenges and responses;
812: protocol 1 is restricted to just one challenge/response.
813: Examples of challenge-response authentication include
814: BSD Authentication (see
815: .Xr login.conf 5 )
816: and PAM (some non-OpenBSD systems).
817: .Pp
818: Finally, if other authentication methods fail,
1.218 jmc 819: .Nm
820: prompts the user for a password.
821: The password is sent to the remote
822: host for checking; however, since all communications are encrypted,
823: the password cannot be seen by someone listening on the network.
1.232 jmc 824: .Pp
825: .Nm
826: automatically maintains and checks a database containing
827: identification for all hosts it has ever been used with.
828: Host keys are stored in
829: .Pa ~/.ssh/known_hosts
830: in the user's home directory.
831: Additionally, the file
832: .Pa /etc/ssh/ssh_known_hosts
833: is automatically checked for known hosts.
834: Any new hosts are automatically added to the user's file.
835: If a host's identification ever changes,
836: .Nm
837: warns about this and disables password authentication to prevent
838: server spoofing or man-in-the-middle attacks,
839: which could otherwise be used to circumvent the encryption.
840: The
841: .Cm StrictHostKeyChecking
842: option can be used to control logins to machines whose
843: host key is not known or has changed.
844: .Pp
1.218 jmc 845: When the user's identity has been accepted by the server, the server
846: either executes the given command, or logs into the machine and gives
847: the user a normal shell on the remote machine.
848: All communication with
849: the remote command or shell will be automatically encrypted.
850: .Pp
851: If a pseudo-terminal has been allocated (normal login session), the
852: user may use the escape characters noted below.
853: .Pp
854: If no pseudo-tty has been allocated,
855: the session is transparent and can be used to reliably transfer binary data.
856: On most systems, setting the escape character to
857: .Dq none
858: will also make the session transparent even if a tty is used.
859: .Pp
860: The session terminates when the command or shell on the remote
1.247 jmc 861: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 862: .Sh ESCAPE CHARACTERS
1.218 jmc 863: When a pseudo-terminal has been requested,
1.2 deraadt 864: .Nm
1.218 jmc 865: supports a number of functions through the use of an escape character.
866: .Pp
867: A single tilde character can be sent as
868: .Ic ~~
869: or by following the tilde by a character other than those described below.
870: The escape character must always follow a newline to be interpreted as
871: special.
872: The escape character can be changed in configuration files using the
873: .Cm EscapeChar
874: configuration directive or on the command line by the
875: .Fl e
876: option.
877: .Pp
878: The supported escapes (assuming the default
879: .Ql ~ )
880: are:
881: .Bl -tag -width Ds
882: .It Cm ~.
883: Disconnect.
884: .It Cm ~^Z
885: Background
1.234 jmc 886: .Nm .
1.218 jmc 887: .It Cm ~#
888: List forwarded connections.
889: .It Cm ~&
890: Background
1.2 deraadt 891: .Nm
1.218 jmc 892: at logout when waiting for forwarded connection / X11 sessions to terminate.
893: .It Cm ~?
894: Display a list of escape characters.
895: .It Cm ~B
896: Send a BREAK to the remote system
897: (only useful for SSH protocol version 2 and if the peer supports it).
898: .It Cm ~C
899: Open command line.
900: Currently this allows the addition of port forwardings using the
901: .Fl L
902: and
903: .Fl R
1.225 jmc 904: options (see above).
1.218 jmc 905: It also allows the cancellation of existing remote port-forwardings
906: using
1.262 stevesk 907: .Sm off
908: .Fl KR Oo Ar bind_address : Oc Ar port .
909: .Sm on
1.218 jmc 910: .Ic !\& Ns Ar command
911: allows the user to execute a local command if the
912: .Ic PermitLocalCommand
913: option is enabled in
1.176 jmc 914: .Xr ssh_config 5 .
1.218 jmc 915: Basic help is available, using the
916: .Fl h
917: option.
918: .It Cm ~R
919: Request rekeying of the connection
920: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 921: .El
1.246 jmc 922: .Sh TCP FORWARDING
923: Forwarding of arbitrary TCP connections over the secure channel can
924: be specified either on the command line or in a configuration file.
925: One possible application of TCP forwarding is a secure connection to a
926: mail server; another is going through firewalls.
927: .Pp
928: In the example below, we look at encrypting communication between
929: an IRC client and server, even though the IRC server does not directly
930: support encrypted communications.
931: This works as follows:
932: the user connects to the remote host using
933: .Nm ,
934: specifying a port to be used to forward connections
935: to the remote server.
936: After that it is possible to start the service which is to be encrypted
937: on the client machine,
938: connecting to the same local port,
939: and
940: .Nm
941: will encrypt and forward the connection.
942: .Pp
943: The following example tunnels an IRC session from client machine
944: .Dq 127.0.0.1
945: (localhost)
946: to remote server
947: .Dq server.example.com :
948: .Bd -literal -offset 4n
949: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
950: $ irc -c '#users' -p 1234 pinky 127.0.0.1
951: .Ed
952: .Pp
953: This tunnels a connection to IRC server
954: .Dq server.example.com ,
955: joining channel
956: .Dq #users ,
957: nickname
958: .Dq pinky ,
959: using port 1234.
960: It doesn't matter which port is used,
961: as long as it's greater than 1023
962: (remember, only root can open sockets on privileged ports)
963: and doesn't conflict with any ports already in use.
964: The connection is forwarded to port 6667 on the remote server,
965: since that's the standard port for IRC services.
966: .Pp
967: The
968: .Fl f
969: option backgrounds
970: .Nm
971: and the remote command
972: .Dq sleep 10
973: is specified to allow an amount of time
974: (10 seconds, in the example)
975: to start the service which is to be tunnelled.
976: If no connections are made within the time specified,
977: .Nm
978: will exit.
979: .Sh X11 FORWARDING
1.218 jmc 980: If the
981: .Cm ForwardX11
982: variable is set to
983: .Dq yes
984: (or see the description of the
1.227 jmc 985: .Fl X ,
986: .Fl x ,
1.218 jmc 987: and
1.227 jmc 988: .Fl Y
1.226 jmc 989: options above)
1.218 jmc 990: and the user is using X11 (the
991: .Ev DISPLAY
992: environment variable is set), the connection to the X11 display is
993: automatically forwarded to the remote side in such a way that any X11
994: programs started from the shell (or command) will go through the
995: encrypted channel, and the connection to the real X server will be made
1.176 jmc 996: from the local machine.
1.218 jmc 997: The user should not manually set
998: .Ev DISPLAY .
999: Forwarding of X11 connections can be
1000: configured on the command line or in configuration files.
1001: .Pp
1002: The
1003: .Ev DISPLAY
1004: value set by
1005: .Nm
1006: will point to the server machine, but with a display number greater than zero.
1007: This is normal, and happens because
1008: .Nm
1009: creates a
1010: .Dq proxy
1011: X server on the server machine for forwarding the
1012: connections over the encrypted channel.
1.200 djm 1013: .Pp
1.218 jmc 1014: .Nm
1015: will also automatically set up Xauthority data on the server machine.
1016: For this purpose, it will generate a random authorization cookie,
1017: store it in Xauthority on the server, and verify that any forwarded
1018: connections carry this cookie and replace it by the real cookie when
1019: the connection is opened.
1020: The real authentication cookie is never
1021: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1022: .Pp
1.218 jmc 1023: If the
1024: .Cm ForwardAgent
1025: variable is set to
1026: .Dq yes
1027: (or see the description of the
1028: .Fl A
1.191 djm 1029: and
1.218 jmc 1030: .Fl a
1.226 jmc 1031: options above) and
1.218 jmc 1032: the user is using an authentication agent, the connection to the agent
1033: is automatically forwarded to the remote side.
1.252 jmc 1034: .Sh VERIFYING HOST KEYS
1035: When connecting to a server for the first time,
1036: a fingerprint of the server's public key is presented to the user
1037: (unless the option
1038: .Cm StrictHostKeyChecking
1039: has been disabled).
1040: Fingerprints can be determined using
1041: .Xr ssh-keygen 1 :
1042: .Pp
1043: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1044: .Pp
1.274 grunk 1045: If the fingerprint is already known, it can be matched
1046: and the key can be accepted or rejected.
1047: Because of the difficulty of comparing host keys
1048: just by looking at hex strings,
1049: there is also support to compare host keys visually,
1050: using
1051: .Em random art .
1052: By setting the
1.275 grunk 1053: .Cm VisualHostKey
1.274 grunk 1054: option to
1.275 grunk 1055: .Dq yes ,
1.274 grunk 1056: a small ASCII graphic gets displayed on every login to a server, no matter
1057: if the session itself is interactive or not.
1058: By learning the pattern a known server produces, a user can easily
1059: find out that the host key has changed when a completely different pattern
1060: is displayed.
1061: Because these patterns are not unambiguous however, a pattern that looks
1062: similar to the pattern remembered only gives a good probability that the
1063: host key is the same, not guaranteed proof.
1064: .Pp
1065: To get a listing of the fingerprints along with their random art for
1066: all known hosts, the following command line can be used:
1067: .Pp
1068: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1069: .Pp
1.252 jmc 1070: If the fingerprint is unknown,
1071: an alternative method of verification is available:
1072: SSH fingerprints verified by DNS.
1073: An additional resource record (RR),
1074: SSHFP,
1075: is added to a zonefile
1076: and the connecting client is able to match the fingerprint
1077: with that of the key presented.
1078: .Pp
1079: In this example, we are connecting a client to a server,
1080: .Dq host.example.com .
1081: The SSHFP resource records should first be added to the zonefile for
1082: host.example.com:
1083: .Bd -literal -offset indent
1.259 jakob 1084: $ ssh-keygen -r host.example.com.
1.252 jmc 1085: .Ed
1086: .Pp
1087: The output lines will have to be added to the zonefile.
1088: To check that the zone is answering fingerprint queries:
1089: .Pp
1090: .Dl $ dig -t SSHFP host.example.com
1091: .Pp
1092: Finally the client connects:
1093: .Bd -literal -offset indent
1094: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1095: [...]
1096: Matching host key fingerprint found in DNS.
1097: Are you sure you want to continue connecting (yes/no)?
1098: .Ed
1099: .Pp
1100: See the
1101: .Cm VerifyHostKeyDNS
1102: option in
1103: .Xr ssh_config 5
1104: for more information.
1.250 jmc 1105: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1106: .Nm
1107: contains support for Virtual Private Network (VPN) tunnelling
1108: using the
1109: .Xr tun 4
1110: network pseudo-device,
1111: allowing two networks to be joined securely.
1112: The
1113: .Xr sshd_config 5
1114: configuration option
1115: .Cm PermitTunnel
1116: controls whether the server supports this,
1117: and at what level (layer 2 or 3 traffic).
1118: .Pp
1119: The following example would connect client network 10.0.50.0/24
1.265 otto 1120: with remote network 10.0.99.0/24 using a point-to-point connection
1121: from 10.1.1.1 to 10.1.1.2,
1122: provided that the SSH server running on the gateway to the remote network,
1123: at 192.168.1.15, allows it.
1124: .Pp
1125: On the client:
1.250 jmc 1126: .Bd -literal -offset indent
1127: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1128: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1129: # route add 10.0.99.0/24 10.1.1.2
1130: .Ed
1131: .Pp
1132: On the server:
1133: .Bd -literal -offset indent
1134: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1135: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1136: .Ed
1137: .Pp
1138: Client access may be more finely tuned via the
1139: .Pa /root/.ssh/authorized_keys
1140: file (see below) and the
1141: .Cm PermitRootLogin
1142: server option.
1.255 jmc 1143: The following entry would permit connections on
1.250 jmc 1144: .Xr tun 4
1.255 jmc 1145: device 1 from user
1.250 jmc 1146: .Dq jane
1.255 jmc 1147: and on tun device 2 from user
1.250 jmc 1148: .Dq john ,
1149: if
1150: .Cm PermitRootLogin
1151: is set to
1152: .Dq forced-commands-only :
1153: .Bd -literal -offset 2n
1154: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1155: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1156: .Ed
1157: .Pp
1.264 ray 1158: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1159: it may be more suited to temporary setups,
1160: such as for wireless VPNs.
1161: More permanent VPNs are better provided by tools such as
1162: .Xr ipsecctl 8
1163: and
1164: .Xr isakmpd 8 .
1.2 deraadt 1165: .Sh ENVIRONMENT
1166: .Nm
1.1 deraadt 1167: will normally set the following environment variables:
1.237 jmc 1168: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1169: .It Ev DISPLAY
1170: The
1171: .Ev DISPLAY
1.40 aaron 1172: variable indicates the location of the X11 server.
1.44 aaron 1173: It is automatically set by
1.2 deraadt 1174: .Nm
1175: to point to a value of the form
1.233 jmc 1176: .Dq hostname:n ,
1177: where
1178: .Dq hostname
1179: indicates the host where the shell runs, and
1180: .Sq n
1181: is an integer \*(Ge 1.
1.40 aaron 1182: .Nm
1183: uses this special value to forward X11 connections over the secure
1184: channel.
1.107 markus 1185: The user should normally not set
1186: .Ev DISPLAY
1187: explicitly, as that
1.1 deraadt 1188: will render the X11 connection insecure (and will require the user to
1189: manually copy any required authorization cookies).
1.2 deraadt 1190: .It Ev HOME
1.1 deraadt 1191: Set to the path of the user's home directory.
1.2 deraadt 1192: .It Ev LOGNAME
1193: Synonym for
1.12 aaron 1194: .Ev USER ;
1195: set for compatibility with systems that use this variable.
1.2 deraadt 1196: .It Ev MAIL
1.129 stevesk 1197: Set to the path of the user's mailbox.
1.40 aaron 1198: .It Ev PATH
1.2 deraadt 1199: Set to the default
1200: .Ev PATH ,
1201: as specified when compiling
1.234 jmc 1202: .Nm .
1.118 markus 1203: .It Ev SSH_ASKPASS
1204: If
1205: .Nm
1206: needs a passphrase, it will read the passphrase from the current
1207: terminal if it was run from a terminal.
1208: If
1209: .Nm
1210: does not have a terminal associated with it but
1211: .Ev DISPLAY
1212: and
1213: .Ev SSH_ASKPASS
1214: are set, it will execute the program specified by
1215: .Ev SSH_ASKPASS
1216: and open an X11 window to read the passphrase.
1217: This is particularly useful when calling
1218: .Nm
1219: from a
1.196 jmc 1220: .Pa .xsession
1.118 markus 1221: or related script.
1222: (Note that on some machines it
1223: may be necessary to redirect the input from
1224: .Pa /dev/null
1225: to make this work.)
1.18 markus 1226: .It Ev SSH_AUTH_SOCK
1.233 jmc 1227: Identifies the path of a
1228: .Ux Ns -domain
1229: socket used to communicate with the agent.
1.166 stevesk 1230: .It Ev SSH_CONNECTION
1231: Identifies the client and server ends of the connection.
1.40 aaron 1232: The variable contains
1.233 jmc 1233: four space-separated values: client IP address, client port number,
1234: server IP address, and server port number.
1.73 markus 1235: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1236: This variable contains the original command line if a forced command
1.73 markus 1237: is executed.
1238: It can be used to extract the original arguments.
1.2 deraadt 1239: .It Ev SSH_TTY
1.1 deraadt 1240: This is set to the name of the tty (path to the device) associated
1.40 aaron 1241: with the current shell or command.
1242: If the current session has no tty,
1.1 deraadt 1243: this variable is not set.
1.2 deraadt 1244: .It Ev TZ
1.214 jmc 1245: This variable is set to indicate the present time zone if it
1.257 jmc 1246: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1247: on to new connections).
1.2 deraadt 1248: .It Ev USER
1.1 deraadt 1249: Set to the name of the user logging in.
1.2 deraadt 1250: .El
1251: .Pp
1.44 aaron 1252: Additionally,
1.2 deraadt 1253: .Nm
1.44 aaron 1254: reads
1.207 djm 1255: .Pa ~/.ssh/environment ,
1.2 deraadt 1256: and adds lines of the format
1257: .Dq VARNAME=value
1.233 jmc 1258: to the environment if the file exists and users are allowed to
1.161 marc 1259: change their environment.
1.176 jmc 1260: For more information, see the
1.161 marc 1261: .Cm PermitUserEnvironment
1.162 stevesk 1262: option in
1.161 marc 1263: .Xr sshd_config 5 .
1.2 deraadt 1264: .Sh FILES
1.236 jmc 1265: .Bl -tag -width Ds -compact
1266: .It ~/.rhosts
1.240 jmc 1267: This file is used for host-based authentication (see above).
1.92 markus 1268: On some machines this file may need to be
1.240 jmc 1269: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1270: because
1.2 deraadt 1271: .Xr sshd 8
1.40 aaron 1272: reads it as root.
1273: Additionally, this file must be owned by the user,
1274: and must not have write permissions for anyone else.
1275: The recommended
1.1 deraadt 1276: permission for most machines is read/write for the user, and not
1277: accessible by others.
1.2 deraadt 1278: .Pp
1.236 jmc 1279: .It ~/.shosts
1.240 jmc 1280: This file is used in exactly the same way as
1281: .Pa .rhosts ,
1282: but allows host-based authentication without permitting login with
1283: rlogin/rsh.
1.272 mcbride 1284: .Pp
1285: .It ~/.ssh/
1286: This directory is the default location for all user-specific configuration
1287: and authentication information.
1288: There is no general requirement to keep the entire contents of this directory
1289: secret, but the recommended permissions are read/write/execute for the user,
1290: and not accessible by others.
1.236 jmc 1291: .Pp
1.238 jmc 1292: .It ~/.ssh/authorized_keys
1293: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1294: The format of this file is described in the
1295: .Xr sshd 8
1296: manual page.
1297: This file is not highly sensitive, but the recommended
1298: permissions are read/write for the user, and not accessible by others.
1299: .Pp
1300: .It ~/.ssh/config
1301: This is the per-user configuration file.
1302: The file format and configuration options are described in
1303: .Xr ssh_config 5 .
1304: Because of the potential for abuse, this file must have strict permissions:
1305: read/write for the user, and not accessible by others.
1306: .Pp
1307: .It ~/.ssh/environment
1.239 jmc 1308: Contains additional definitions for environment variables; see
1309: .Sx ENVIRONMENT ,
1.238 jmc 1310: above.
1311: .Pp
1312: .It ~/.ssh/identity
1313: .It ~/.ssh/id_dsa
1314: .It ~/.ssh/id_rsa
1315: Contains the private key for authentication.
1316: These files
1317: contain sensitive data and should be readable by the user but not
1318: accessible by others (read/write/execute).
1319: .Nm
1320: will simply ignore a private key file if it is accessible by others.
1321: It is possible to specify a passphrase when
1322: generating the key which will be used to encrypt the
1323: sensitive part of this file using 3DES.
1324: .Pp
1325: .It ~/.ssh/identity.pub
1326: .It ~/.ssh/id_dsa.pub
1327: .It ~/.ssh/id_rsa.pub
1328: Contains the public key for authentication.
1329: These files are not
1330: sensitive and can (but need not) be readable by anyone.
1331: .Pp
1332: .It ~/.ssh/known_hosts
1.244 jmc 1333: Contains a list of host keys for all hosts the user has logged into
1334: that are not already in the systemwide list of known host keys.
1.238 jmc 1335: See
1.244 jmc 1336: .Xr sshd 8
1337: for further details of the format of this file.
1.238 jmc 1338: .Pp
1339: .It ~/.ssh/rc
1340: Commands in this file are executed by
1341: .Nm
1.245 jmc 1342: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1343: started.
1344: See the
1345: .Xr sshd 8
1346: manual page for more information.
1347: .Pp
1.236 jmc 1348: .It /etc/hosts.equiv
1.240 jmc 1349: This file is for host-based authentication (see above).
1350: It should only be writable by root.
1.236 jmc 1351: .Pp
1352: .It /etc/shosts.equiv
1.240 jmc 1353: This file is used in exactly the same way as
1354: .Pa hosts.equiv ,
1355: but allows host-based authentication without permitting login with
1356: rlogin/rsh.
1.236 jmc 1357: .Pp
1.238 jmc 1358: .It Pa /etc/ssh/ssh_config
1359: Systemwide configuration file.
1360: The file format and configuration options are described in
1361: .Xr ssh_config 5 .
1362: .Pp
1363: .It /etc/ssh/ssh_host_key
1364: .It /etc/ssh/ssh_host_dsa_key
1365: .It /etc/ssh/ssh_host_rsa_key
1366: These three files contain the private parts of the host keys
1.245 jmc 1367: and are used for host-based authentication.
1368: If protocol version 1 is used,
1.238 jmc 1369: .Nm
1370: must be setuid root, since the host key is readable only by root.
1371: For protocol version 2,
1372: .Nm
1373: uses
1374: .Xr ssh-keysign 8
1.245 jmc 1375: to access the host keys,
1376: eliminating the requirement that
1.238 jmc 1377: .Nm
1.245 jmc 1378: be setuid root when host-based authentication is used.
1.238 jmc 1379: By default
1.2 deraadt 1380: .Nm
1.238 jmc 1381: is not setuid root.
1382: .Pp
1383: .It /etc/ssh/ssh_known_hosts
1384: Systemwide list of known host keys.
1385: This file should be prepared by the
1386: system administrator to contain the public host keys of all machines in the
1387: organization.
1.244 jmc 1388: It should be world-readable.
1389: See
1.238 jmc 1390: .Xr sshd 8
1.244 jmc 1391: for further details of the format of this file.
1.236 jmc 1392: .Pp
1.238 jmc 1393: .It /etc/ssh/sshrc
1.1 deraadt 1394: Commands in this file are executed by
1.2 deraadt 1395: .Nm
1.245 jmc 1396: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1397: See the
1.2 deraadt 1398: .Xr sshd 8
1.1 deraadt 1399: manual page for more information.
1.58 itojun 1400: .El
1.2 deraadt 1401: .Sh SEE ALSO
1402: .Xr scp 1 ,
1.83 djm 1403: .Xr sftp 1 ,
1.2 deraadt 1404: .Xr ssh-add 1 ,
1405: .Xr ssh-agent 1 ,
1406: .Xr ssh-keygen 1 ,
1.242 jmc 1407: .Xr ssh-keyscan 1 ,
1.250 jmc 1408: .Xr tun 4 ,
1.176 jmc 1409: .Xr hosts.equiv 5 ,
1.159 stevesk 1410: .Xr ssh_config 5 ,
1.160 naddy 1411: .Xr ssh-keysign 8 ,
1.87 itojun 1412: .Xr sshd 8
1.106 markus 1413: .Rs
1.256 jmc 1414: .%R RFC 4250
1415: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1416: .%D 2006
1417: .Re
1418: .Rs
1419: .%R RFC 4251
1420: .%T "The Secure Shell (SSH) Protocol Architecture"
1421: .%D 2006
1422: .Re
1423: .Rs
1424: .%R RFC 4252
1425: .%T "The Secure Shell (SSH) Authentication Protocol"
1426: .%D 2006
1427: .Re
1428: .Rs
1429: .%R RFC 4253
1430: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1431: .%D 2006
1432: .Re
1433: .Rs
1434: .%R RFC 4254
1435: .%T "The Secure Shell (SSH) Connection Protocol"
1436: .%D 2006
1437: .Re
1438: .Rs
1439: .%R RFC 4255
1440: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1441: .%D 2006
1442: .Re
1443: .Rs
1444: .%R RFC 4256
1445: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1446: .%D 2006
1447: .Re
1448: .Rs
1449: .%R RFC 4335
1450: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1451: .%D 2006
1452: .Re
1453: .Rs
1454: .%R RFC 4344
1455: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1456: .%D 2006
1457: .Re
1458: .Rs
1459: .%R RFC 4345
1460: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1461: .%D 2006
1462: .Re
1463: .Rs
1464: .%R RFC 4419
1465: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1466: .%D 2006
1467: .Re
1468: .Rs
1469: .%R RFC 4716
1470: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1471: .%D 2006
1.274 grunk 1472: .Re
1473: .Rs
1474: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1475: .%A A. Perrig
1476: .%A D. Song
1477: .%D 1999
1478: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1479: .Re
1.173 jmc 1480: .Sh AUTHORS
1481: OpenSSH is a derivative of the original and free
1482: ssh 1.2.12 release by Tatu Ylonen.
1483: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1484: Theo de Raadt and Dug Song
1485: removed many bugs, re-added newer features and
1486: created OpenSSH.
1487: Markus Friedl contributed the support for SSH
1488: protocol versions 1.5 and 2.0.