Annotation of src/usr.bin/ssh/ssh.1, Revision 1.301
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: .\"
1.301 ! jmc 37: .\" $OpenBSD: ssh.1,v 1.300 2010/03/05 06:50:34 jmc Exp $
! 38: .Dd $Mdocdate: March 5 2010 $
1.2 deraadt 39: .Dt SSH 1
40: .Os
41: .Sh NAME
42: .Nm ssh
1.96 deraadt 43: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm ssh
1.278 djm 46: .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
1.108 markus 47: .Op Fl b Ar bind_address
1.51 markus 48: .Op Fl c Ar cipher_spec
1.210 djm 49: .Oo Fl D\ \&
50: .Sm off
51: .Oo Ar bind_address : Oc
52: .Ar port
53: .Sm on
54: .Oc
1.2 deraadt 55: .Op Fl e Ar escape_char
1.176 jmc 56: .Op Fl F Ar configfile
1.292 jmc 57: .Op Fl I Ar pkcs11
1.211 jmc 58: .Bk -words
1.2 deraadt 59: .Op Fl i Ar identity_file
1.211 jmc 60: .Ek
1.202 jmc 61: .Oo Fl L\ \&
1.12 aaron 62: .Sm off
1.200 djm 63: .Oo Ar bind_address : Oc
1.202 jmc 64: .Ar port : host : hostport
1.12 aaron 65: .Sm on
66: .Oc
1.211 jmc 67: .Bk -words
1.176 jmc 68: .Op Fl l Ar login_name
1.211 jmc 69: .Ek
1.176 jmc 70: .Op Fl m Ar mac_spec
1.198 djm 71: .Op Fl O Ar ctl_cmd
1.176 jmc 72: .Op Fl o Ar option
73: .Op Fl p Ar port
1.202 jmc 74: .Oo Fl R\ \&
1.12 aaron 75: .Sm off
1.200 djm 76: .Oo Ar bind_address : Oc
1.202 jmc 77: .Ar port : host : hostport
1.12 aaron 78: .Sm on
79: .Oc
1.198 djm 80: .Op Fl S Ar ctl_path
1.290 dtucker 81: .Op Fl W Ar host : Ns Ar port
1.261 stevesk 82: .Oo Fl w Ar local_tun Ns
83: .Op : Ns Ar remote_tun Oc
1.176 jmc 84: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 85: .Op Ar command
1.44 aaron 86: .Sh DESCRIPTION
1.2 deraadt 87: .Nm
1.96 deraadt 88: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 89: executing commands on a remote machine.
1.176 jmc 90: It is intended to replace rlogin and rsh,
91: and provide secure encrypted communications between
1.40 aaron 92: two untrusted hosts over an insecure network.
1.247 jmc 93: X11 connections and arbitrary TCP ports
1.176 jmc 94: can also be forwarded over the secure channel.
1.2 deraadt 95: .Pp
96: .Nm
1.44 aaron 97: connects and logs into the specified
1.176 jmc 98: .Ar hostname
99: (with optional
100: .Ar user
101: name).
1.1 deraadt 102: The user must prove
1.49 markus 103: his/her identity to the remote machine using one of several methods
1.221 jmc 104: depending on the protocol version used (see below).
1.49 markus 105: .Pp
1.176 jmc 106: If
107: .Ar command
108: is specified,
1.219 jmc 109: it is executed on the remote host instead of a login shell.
1.2 deraadt 110: .Pp
1.218 jmc 111: The options are as follows:
112: .Bl -tag -width Ds
113: .It Fl 1
114: Forces
1.2 deraadt 115: .Nm
1.218 jmc 116: to try protocol version 1 only.
117: .It Fl 2
118: Forces
1.2 deraadt 119: .Nm
1.218 jmc 120: to try protocol version 2 only.
121: .It Fl 4
122: Forces
1.2 deraadt 123: .Nm
1.218 jmc 124: to use IPv4 addresses only.
125: .It Fl 6
126: Forces
1.2 deraadt 127: .Nm
1.218 jmc 128: to use IPv6 addresses only.
129: .It Fl A
130: Enables forwarding of the authentication agent connection.
131: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 132: .Pp
1.218 jmc 133: Agent forwarding should be enabled with caution.
134: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 135: (for the agent's
1.286 sobrado 136: .Ux Ns -domain
137: socket) can access the local agent through the forwarded connection.
1.218 jmc 138: An attacker cannot obtain key material from the agent,
139: however they can perform operations on the keys that enable them to
140: authenticate using the identities loaded into the agent.
141: .It Fl a
142: Disables forwarding of the authentication agent connection.
143: .It Fl b Ar bind_address
144: Use
145: .Ar bind_address
146: on the local machine as the source address
147: of the connection.
148: Only useful on systems with more than one address.
149: .It Fl C
150: Requests compression of all data (including stdin, stdout, stderr, and
1.247 jmc 151: data for forwarded X11 and TCP connections).
1.218 jmc 152: The compression algorithm is the same used by
153: .Xr gzip 1 ,
154: and the
155: .Dq level
156: can be controlled by the
157: .Cm CompressionLevel
158: option for protocol version 1.
159: Compression is desirable on modem lines and other
160: slow connections, but will only slow down things on fast networks.
161: The default value can be set on a host-by-host basis in the
162: configuration files; see the
163: .Cm Compression
164: option.
165: .It Fl c Ar cipher_spec
166: Selects the cipher specification for encrypting the session.
1.2 deraadt 167: .Pp
1.218 jmc 168: Protocol version 1 allows specification of a single cipher.
169: The supported values are
170: .Dq 3des ,
1.220 jmc 171: .Dq blowfish ,
1.218 jmc 172: and
173: .Dq des .
174: .Ar 3des
175: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
176: It is believed to be secure.
177: .Ar blowfish
178: is a fast block cipher; it appears very secure and is much faster than
179: .Ar 3des .
180: .Ar des
181: is only supported in the
1.2 deraadt 182: .Nm
1.218 jmc 183: client for interoperability with legacy protocol 1 implementations
184: that do not support the
185: .Ar 3des
186: cipher.
187: Its use is strongly discouraged due to cryptographic weaknesses.
188: The default is
189: .Dq 3des .
1.49 markus 190: .Pp
1.230 jmc 191: For protocol version 2,
1.218 jmc 192: .Ar cipher_spec
193: is a comma-separated list of ciphers
194: listed in order of preference.
1.283 jmc 195: See the
196: .Cm Ciphers
197: keyword for more information.
1.218 jmc 198: .It Fl D Xo
199: .Sm off
200: .Oo Ar bind_address : Oc
201: .Ar port
202: .Sm on
203: .Xc
204: Specifies a local
205: .Dq dynamic
206: application-level port forwarding.
207: This works by allocating a socket to listen to
208: .Ar port
209: on the local side, optionally bound to the specified
210: .Ar bind_address .
211: Whenever a connection is made to this port, the
212: connection is forwarded over the secure channel, and the application
213: protocol is then used to determine where to connect to from the
214: remote machine.
215: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 216: .Nm
1.218 jmc 217: will act as a SOCKS server.
218: Only root can forward privileged ports.
219: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 220: .Pp
1.218 jmc 221: IPv6 addresses can be specified with an alternative syntax:
222: .Sm off
223: .Xo
224: .Op Ar bind_address No /
225: .Ar port
226: .Xc
227: .Sm on
228: or by enclosing the address in square brackets.
229: Only the superuser can forward privileged ports.
230: By default, the local port is bound in accordance with the
231: .Cm GatewayPorts
232: setting.
233: However, an explicit
234: .Ar bind_address
235: may be used to bind the connection to a specific address.
236: The
237: .Ar bind_address
238: of
239: .Dq localhost
240: indicates that the listening port be bound for local use only, while an
241: empty address or
242: .Sq *
243: indicates that the port should be available from all interfaces.
1.229 jmc 244: .It Fl e Ar escape_char
1.218 jmc 245: Sets the escape character for sessions with a pty (default:
246: .Ql ~ ) .
247: The escape character is only recognized at the beginning of a line.
248: The escape character followed by a dot
249: .Pq Ql \&.
250: closes the connection;
251: followed by control-Z suspends the connection;
252: and followed by itself sends the escape character once.
253: Setting the character to
1.2 deraadt 254: .Dq none
1.218 jmc 255: disables any escapes and makes the session fully transparent.
256: .It Fl F Ar configfile
257: Specifies an alternative per-user configuration file.
258: If a configuration file is given on the command line,
259: the system-wide configuration file
260: .Pq Pa /etc/ssh/ssh_config
261: will be ignored.
262: The default for the per-user configuration file is
263: .Pa ~/.ssh/config .
264: .It Fl f
265: Requests
266: .Nm
267: to go to background just before command execution.
268: This is useful if
1.176 jmc 269: .Nm
1.218 jmc 270: is going to ask for passwords or passphrases, but the user
271: wants it in the background.
272: This implies
273: .Fl n .
274: The recommended way to start X11 programs at a remote site is with
275: something like
276: .Ic ssh -f host xterm .
1.277 djm 277: .Pp
278: If the
279: .Cm ExitOnForwardFailure
280: configuration option is set to
281: .Dq yes ,
282: then a client started with
283: .Fl f
284: will wait for all remote port forwards to be successfully established
285: before placing itself in the background.
1.218 jmc 286: .It Fl g
287: Allows remote hosts to connect to local forwarded ports.
1.291 markus 288: .It Fl I Ar pkcs11
1.294 jmc 289: Specify the PKCS#11 shared library
1.176 jmc 290: .Nm
1.293 markus 291: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 292: private RSA key.
293: .It Fl i Ar identity_file
294: Selects a file from which the identity (private key) for
295: RSA or DSA authentication is read.
296: The default is
297: .Pa ~/.ssh/identity
298: for protocol version 1, and
299: .Pa ~/.ssh/id_rsa
1.149 jakob 300: and
1.218 jmc 301: .Pa ~/.ssh/id_dsa
302: for protocol version 2.
303: Identity files may also be specified on
304: a per-host basis in the configuration file.
305: It is possible to have multiple
306: .Fl i
307: options (and multiple identities specified in
308: configuration files).
1.269 djm 309: .It Fl K
310: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
311: credentials to the server.
1.218 jmc 312: .It Fl k
313: Disables forwarding (delegation) of GSSAPI credentials to the server.
314: .It Fl L Xo
315: .Sm off
316: .Oo Ar bind_address : Oc
317: .Ar port : host : hostport
318: .Sm on
319: .Xc
320: Specifies that the given port on the local (client) host is to be
321: forwarded to the given host and port on the remote side.
322: This works by allocating a socket to listen to
323: .Ar port
324: on the local side, optionally bound to the specified
325: .Ar bind_address .
326: Whenever a connection is made to this port, the
327: connection is forwarded over the secure channel, and a connection is
328: made to
329: .Ar host
330: port
331: .Ar hostport
332: from the remote machine.
333: Port forwardings can also be specified in the configuration file.
334: IPv6 addresses can be specified with an alternative syntax:
335: .Sm off
336: .Xo
337: .Op Ar bind_address No /
338: .Ar port No / Ar host No /
339: .Ar hostport
340: .Xc
341: .Sm on
342: or by enclosing the address in square brackets.
343: Only the superuser can forward privileged ports.
344: By default, the local port is bound in accordance with the
345: .Cm GatewayPorts
346: setting.
347: However, an explicit
348: .Ar bind_address
349: may be used to bind the connection to a specific address.
1.2 deraadt 350: The
1.218 jmc 351: .Ar bind_address
352: of
353: .Dq localhost
354: indicates that the listening port be bound for local use only, while an
355: empty address or
356: .Sq *
357: indicates that the port should be available from all interfaces.
358: .It Fl l Ar login_name
359: Specifies the user to log in as on the remote machine.
360: This also may be specified on a per-host basis in the configuration file.
361: .It Fl M
362: Places the
363: .Nm
364: client into
365: .Dq master
366: mode for connection sharing.
1.231 stevesk 367: Multiple
368: .Fl M
369: options places
370: .Nm
371: into
372: .Dq master
373: mode with confirmation required before slave connections are accepted.
1.218 jmc 374: Refer to the description of
375: .Cm ControlMaster
376: in
377: .Xr ssh_config 5
378: for details.
379: .It Fl m Ar mac_spec
380: Additionally, for protocol version 2 a comma-separated list of MAC
381: (message authentication code) algorithms can
382: be specified in order of preference.
383: See the
384: .Cm MACs
385: keyword for more information.
386: .It Fl N
387: Do not execute a remote command.
388: This is useful for just forwarding ports
389: (protocol version 2 only).
390: .It Fl n
391: Redirects stdin from
392: .Pa /dev/null
393: (actually, prevents reading from stdin).
394: This must be used when
1.2 deraadt 395: .Nm
1.218 jmc 396: is run in the background.
397: A common trick is to use this to run X11 programs on a remote machine.
398: For example,
399: .Ic ssh -n shadows.cs.hut.fi emacs &
400: will start an emacs on shadows.cs.hut.fi, and the X11
401: connection will be automatically forwarded over an encrypted channel.
402: The
1.2 deraadt 403: .Nm
1.218 jmc 404: program will be put in the background.
405: (This does not work if
1.2 deraadt 406: .Nm
1.218 jmc 407: needs to ask for a password or passphrase; see also the
408: .Fl f
409: option.)
410: .It Fl O Ar ctl_cmd
411: Control an active connection multiplexing master process.
412: When the
413: .Fl O
414: option is specified, the
415: .Ar ctl_cmd
416: argument is interpreted and passed to the master process.
417: Valid commands are:
418: .Dq check
419: (check that the master process is running) and
420: .Dq exit
421: (request the master to exit).
422: .It Fl o Ar option
423: Can be used to give options in the format used in the configuration file.
424: This is useful for specifying options for which there is no separate
425: command-line flag.
426: For full details of the options listed below, and their possible values, see
427: .Xr ssh_config 5 .
1.2 deraadt 428: .Pp
1.218 jmc 429: .Bl -tag -width Ds -offset indent -compact
430: .It AddressFamily
431: .It BatchMode
432: .It BindAddress
433: .It ChallengeResponseAuthentication
434: .It CheckHostIP
435: .It Cipher
436: .It Ciphers
437: .It ClearAllForwardings
438: .It Compression
439: .It CompressionLevel
440: .It ConnectionAttempts
441: .It ConnectTimeout
442: .It ControlMaster
443: .It ControlPath
444: .It DynamicForward
445: .It EscapeChar
1.263 markus 446: .It ExitOnForwardFailure
1.218 jmc 447: .It ForwardAgent
448: .It ForwardX11
449: .It ForwardX11Trusted
450: .It GatewayPorts
451: .It GlobalKnownHostsFile
452: .It GSSAPIAuthentication
453: .It GSSAPIDelegateCredentials
454: .It HashKnownHosts
455: .It Host
456: .It HostbasedAuthentication
457: .It HostKeyAlgorithms
458: .It HostKeyAlias
459: .It HostName
460: .It IdentityFile
461: .It IdentitiesOnly
462: .It KbdInteractiveDevices
463: .It LocalCommand
464: .It LocalForward
465: .It LogLevel
466: .It MACs
467: .It NoHostAuthenticationForLocalhost
468: .It NumberOfPasswordPrompts
469: .It PasswordAuthentication
470: .It PermitLocalCommand
1.291 markus 471: .It PKCS11Provider
1.218 jmc 472: .It Port
473: .It PreferredAuthentications
474: .It Protocol
475: .It ProxyCommand
476: .It PubkeyAuthentication
1.251 dtucker 477: .It RekeyLimit
1.218 jmc 478: .It RemoteForward
479: .It RhostsRSAAuthentication
480: .It RSAAuthentication
481: .It SendEnv
482: .It ServerAliveInterval
483: .It ServerAliveCountMax
484: .It StrictHostKeyChecking
485: .It TCPKeepAlive
486: .It Tunnel
487: .It TunnelDevice
488: .It UsePrivilegedPort
489: .It User
490: .It UserKnownHostsFile
491: .It VerifyHostKeyDNS
1.276 jmc 492: .It VisualHostKey
1.218 jmc 493: .It XAuthLocation
494: .El
495: .It Fl p Ar port
496: Port to connect to on the remote host.
497: This can be specified on a
498: per-host basis in the configuration file.
499: .It Fl q
500: Quiet mode.
1.271 djm 501: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 502: .It Fl R Xo
503: .Sm off
504: .Oo Ar bind_address : Oc
505: .Ar port : host : hostport
506: .Sm on
507: .Xc
508: Specifies that the given port on the remote (server) host is to be
509: forwarded to the given host and port on the local side.
510: This works by allocating a socket to listen to
511: .Ar port
512: on the remote side, and whenever a connection is made to this port, the
513: connection is forwarded over the secure channel, and a connection is
514: made to
515: .Ar host
516: port
517: .Ar hostport
518: from the local machine.
1.2 deraadt 519: .Pp
1.218 jmc 520: Port forwardings can also be specified in the configuration file.
521: Privileged ports can be forwarded only when
522: logging in as root on the remote machine.
523: IPv6 addresses can be specified by enclosing the address in square braces or
524: using an alternative syntax:
525: .Sm off
526: .Xo
527: .Op Ar bind_address No /
528: .Ar host No / Ar port No /
529: .Ar hostport
530: .Xc .
531: .Sm on
1.194 jakob 532: .Pp
1.218 jmc 533: By default, the listening socket on the server will be bound to the loopback
534: interface only.
1.280 tobias 535: This may be overridden by specifying a
1.218 jmc 536: .Ar bind_address .
537: An empty
538: .Ar bind_address ,
539: or the address
540: .Ql * ,
541: indicates that the remote socket should listen on all interfaces.
542: Specifying a remote
543: .Ar bind_address
544: will only succeed if the server's
545: .Cm GatewayPorts
546: option is enabled (see
547: .Xr sshd_config 5 ) .
1.281 djm 548: .Pp
549: If the
550: .Ar port
551: argument is
1.282 djm 552: .Ql 0 ,
1.281 djm 553: the listen port will be dynamically allocated on the server and reported
554: to the client at run time.
1.218 jmc 555: .It Fl S Ar ctl_path
556: Specifies the location of a control socket for connection sharing.
557: Refer to the description of
558: .Cm ControlPath
559: and
560: .Cm ControlMaster
561: in
562: .Xr ssh_config 5
563: for details.
564: .It Fl s
565: May be used to request invocation of a subsystem on the remote system.
566: Subsystems are a feature of the SSH2 protocol which facilitate the use
567: of SSH as a secure transport for other applications (eg.\&
568: .Xr sftp 1 ) .
569: The subsystem is specified as the remote command.
570: .It Fl T
571: Disable pseudo-tty allocation.
572: .It Fl t
573: Force pseudo-tty allocation.
574: This can be used to execute arbitrary
575: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 576: e.g. when implementing menu services.
1.218 jmc 577: Multiple
578: .Fl t
579: options force tty allocation, even if
1.194 jakob 580: .Nm
1.218 jmc 581: has no local tty.
582: .It Fl V
583: Display the version number and exit.
584: .It Fl v
585: Verbose mode.
586: Causes
1.176 jmc 587: .Nm
1.218 jmc 588: to print debugging messages about its progress.
589: This is helpful in
590: debugging connection, authentication, and configuration problems.
591: Multiple
592: .Fl v
593: options increase the verbosity.
594: The maximum is 3.
1.290 dtucker 595: .It Fl W Ar host : Ns Ar port
596: Requests that standard input and output on the client be forwarded to
597: .Ar host
598: on
599: .Ar port
600: over the secure channel.
601: Implies
602: .Fl N ,
603: .Fl T ,
604: .Cm ExitOnForwardFailure
605: and
606: .Cm ClearAllForwardings
607: and works with Protocol version 2 only.
1.261 stevesk 608: .It Fl w Xo
609: .Ar local_tun Ns Op : Ns Ar remote_tun
610: .Xc
611: Requests
612: tunnel
613: device forwarding with the specified
1.218 jmc 614: .Xr tun 4
1.261 stevesk 615: devices between the client
616: .Pq Ar local_tun
617: and the server
618: .Pq Ar remote_tun .
619: .Pp
1.228 jmc 620: The devices may be specified by numerical ID or the keyword
621: .Dq any ,
622: which uses the next available tunnel device.
1.261 stevesk 623: If
624: .Ar remote_tun
625: is not specified, it defaults to
626: .Dq any .
1.228 jmc 627: See also the
1.218 jmc 628: .Cm Tunnel
1.261 stevesk 629: and
630: .Cm TunnelDevice
631: directives in
1.218 jmc 632: .Xr ssh_config 5 .
1.261 stevesk 633: If the
634: .Cm Tunnel
635: directive is unset, it is set to the default tunnel mode, which is
636: .Dq point-to-point .
1.218 jmc 637: .It Fl X
638: Enables X11 forwarding.
1.54 markus 639: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 640: .Pp
1.218 jmc 641: X11 forwarding should be enabled with caution.
1.168 jmc 642: Users with the ability to bypass file permissions on the remote host
1.218 jmc 643: (for the user's X authorization database)
644: can access the local X11 display through the forwarded connection.
645: An attacker may then be able to perform activities such as keystroke monitoring.
646: .Pp
647: For this reason, X11 forwarding is subjected to X11 SECURITY extension
648: restrictions by default.
649: Please refer to the
650: .Nm
651: .Fl Y
652: option and the
653: .Cm ForwardX11Trusted
654: directive in
655: .Xr ssh_config 5
656: for more information.
657: .It Fl x
658: Disables X11 forwarding.
659: .It Fl Y
660: Enables trusted X11 forwarding.
661: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
662: controls.
1.278 djm 663: .It Fl y
664: Send log information using the
665: .Xr syslog 3
666: system module.
667: By default this information is sent to stderr.
1.218 jmc 668: .El
1.224 jmc 669: .Pp
670: .Nm
671: may additionally obtain configuration data from
672: a per-user configuration file and a system-wide configuration file.
673: The file format and configuration options are described in
674: .Xr ssh_config 5 .
675: .Pp
676: .Nm
677: exits with the exit status of the remote command or with 255
678: if an error occurred.
1.222 jmc 679: .Sh AUTHENTICATION
1.249 jmc 680: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 681: The default is to use protocol 2 only,
682: though this can be changed via the
1.222 jmc 683: .Cm Protocol
684: option in
1.284 jmc 685: .Xr ssh_config 5
686: or the
1.222 jmc 687: .Fl 1
688: and
689: .Fl 2
690: options (see above).
691: Both protocols support similar authentication methods,
1.284 jmc 692: but protocol 2 is the default since
1.222 jmc 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
1.301 ! jmc 801: A variation on public key authentication
! 802: is available in the form of certificate authentication:
! 803: instead of a set of public/private keys,
! 804: signed certificates are used.
! 805: This has the advantage that a single trusted certification authority
! 806: can be used in place of many public/private keys.
! 807: See the
! 808: .Sx CERTIFICATES
! 809: section of
! 810: .Xr ssh-keygen 1
! 811: for more information.
! 812: .Pp
! 813: The most convenient way to use public key or certificate authentication
! 814: may be with an authentication agent.
1.218 jmc 815: See
816: .Xr ssh-agent 1
817: for more information.
818: .Pp
1.222 jmc 819: Challenge-response authentication works as follows:
820: The server sends an arbitrary
821: .Qq challenge
822: text, and prompts for a response.
823: Protocol 2 allows multiple challenges and responses;
824: protocol 1 is restricted to just one challenge/response.
825: Examples of challenge-response authentication include
826: BSD Authentication (see
827: .Xr login.conf 5 )
828: and PAM (some non-OpenBSD systems).
829: .Pp
830: Finally, if other authentication methods fail,
1.218 jmc 831: .Nm
832: prompts the user for a password.
833: The password is sent to the remote
834: host for checking; however, since all communications are encrypted,
835: the password cannot be seen by someone listening on the network.
1.232 jmc 836: .Pp
837: .Nm
838: automatically maintains and checks a database containing
839: identification for all hosts it has ever been used with.
840: Host keys are stored in
841: .Pa ~/.ssh/known_hosts
842: in the user's home directory.
843: Additionally, the file
844: .Pa /etc/ssh/ssh_known_hosts
845: is automatically checked for known hosts.
846: Any new hosts are automatically added to the user's file.
847: If a host's identification ever changes,
848: .Nm
849: warns about this and disables password authentication to prevent
850: server spoofing or man-in-the-middle attacks,
851: which could otherwise be used to circumvent the encryption.
852: The
853: .Cm StrictHostKeyChecking
854: option can be used to control logins to machines whose
855: host key is not known or has changed.
856: .Pp
1.218 jmc 857: When the user's identity has been accepted by the server, the server
858: either executes the given command, or logs into the machine and gives
859: the user a normal shell on the remote machine.
860: All communication with
861: the remote command or shell will be automatically encrypted.
862: .Pp
863: If a pseudo-terminal has been allocated (normal login session), the
864: user may use the escape characters noted below.
865: .Pp
866: If no pseudo-tty has been allocated,
867: the session is transparent and can be used to reliably transfer binary data.
868: On most systems, setting the escape character to
869: .Dq none
870: will also make the session transparent even if a tty is used.
871: .Pp
872: The session terminates when the command or shell on the remote
1.247 jmc 873: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 874: .Sh ESCAPE CHARACTERS
1.218 jmc 875: When a pseudo-terminal has been requested,
1.2 deraadt 876: .Nm
1.218 jmc 877: supports a number of functions through the use of an escape character.
878: .Pp
879: A single tilde character can be sent as
880: .Ic ~~
881: or by following the tilde by a character other than those described below.
882: The escape character must always follow a newline to be interpreted as
883: special.
884: The escape character can be changed in configuration files using the
885: .Cm EscapeChar
886: configuration directive or on the command line by the
887: .Fl e
888: option.
889: .Pp
890: The supported escapes (assuming the default
891: .Ql ~ )
892: are:
893: .Bl -tag -width Ds
894: .It Cm ~.
895: Disconnect.
896: .It Cm ~^Z
897: Background
1.234 jmc 898: .Nm .
1.218 jmc 899: .It Cm ~#
900: List forwarded connections.
901: .It Cm ~&
902: Background
1.2 deraadt 903: .Nm
1.218 jmc 904: at logout when waiting for forwarded connection / X11 sessions to terminate.
905: .It Cm ~?
906: Display a list of escape characters.
907: .It Cm ~B
908: Send a BREAK to the remote system
909: (only useful for SSH protocol version 2 and if the peer supports it).
910: .It Cm ~C
911: Open command line.
912: Currently this allows the addition of port forwardings using the
1.279 stevesk 913: .Fl L ,
914: .Fl R
1.218 jmc 915: and
1.279 stevesk 916: .Fl D
1.225 jmc 917: options (see above).
1.218 jmc 918: It also allows the cancellation of existing remote port-forwardings
919: using
1.262 stevesk 920: .Sm off
921: .Fl KR Oo Ar bind_address : Oc Ar port .
922: .Sm on
1.218 jmc 923: .Ic !\& Ns Ar command
924: allows the user to execute a local command if the
925: .Ic PermitLocalCommand
926: option is enabled in
1.176 jmc 927: .Xr ssh_config 5 .
1.218 jmc 928: Basic help is available, using the
929: .Fl h
930: option.
931: .It Cm ~R
932: Request rekeying of the connection
933: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 934: .El
1.246 jmc 935: .Sh TCP FORWARDING
936: Forwarding of arbitrary TCP connections over the secure channel can
937: be specified either on the command line or in a configuration file.
938: One possible application of TCP forwarding is a secure connection to a
939: mail server; another is going through firewalls.
940: .Pp
941: In the example below, we look at encrypting communication between
942: an IRC client and server, even though the IRC server does not directly
943: support encrypted communications.
944: This works as follows:
945: the user connects to the remote host using
946: .Nm ,
947: specifying a port to be used to forward connections
948: to the remote server.
949: After that it is possible to start the service which is to be encrypted
950: on the client machine,
951: connecting to the same local port,
952: and
953: .Nm
954: will encrypt and forward the connection.
955: .Pp
956: The following example tunnels an IRC session from client machine
957: .Dq 127.0.0.1
958: (localhost)
959: to remote server
960: .Dq server.example.com :
961: .Bd -literal -offset 4n
962: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
963: $ irc -c '#users' -p 1234 pinky 127.0.0.1
964: .Ed
965: .Pp
966: This tunnels a connection to IRC server
967: .Dq server.example.com ,
968: joining channel
969: .Dq #users ,
970: nickname
971: .Dq pinky ,
972: using port 1234.
973: It doesn't matter which port is used,
974: as long as it's greater than 1023
975: (remember, only root can open sockets on privileged ports)
976: and doesn't conflict with any ports already in use.
977: The connection is forwarded to port 6667 on the remote server,
978: since that's the standard port for IRC services.
979: .Pp
980: The
981: .Fl f
982: option backgrounds
983: .Nm
984: and the remote command
985: .Dq sleep 10
986: is specified to allow an amount of time
987: (10 seconds, in the example)
988: to start the service which is to be tunnelled.
989: If no connections are made within the time specified,
990: .Nm
991: will exit.
992: .Sh X11 FORWARDING
1.218 jmc 993: If the
994: .Cm ForwardX11
995: variable is set to
996: .Dq yes
997: (or see the description of the
1.227 jmc 998: .Fl X ,
999: .Fl x ,
1.218 jmc 1000: and
1.227 jmc 1001: .Fl Y
1.226 jmc 1002: options above)
1.218 jmc 1003: and the user is using X11 (the
1004: .Ev DISPLAY
1005: environment variable is set), the connection to the X11 display is
1006: automatically forwarded to the remote side in such a way that any X11
1007: programs started from the shell (or command) will go through the
1008: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1009: from the local machine.
1.218 jmc 1010: The user should not manually set
1011: .Ev DISPLAY .
1012: Forwarding of X11 connections can be
1013: configured on the command line or in configuration files.
1014: .Pp
1015: The
1016: .Ev DISPLAY
1017: value set by
1018: .Nm
1019: will point to the server machine, but with a display number greater than zero.
1020: This is normal, and happens because
1021: .Nm
1022: creates a
1023: .Dq proxy
1024: X server on the server machine for forwarding the
1025: connections over the encrypted channel.
1.200 djm 1026: .Pp
1.218 jmc 1027: .Nm
1028: will also automatically set up Xauthority data on the server machine.
1029: For this purpose, it will generate a random authorization cookie,
1030: store it in Xauthority on the server, and verify that any forwarded
1031: connections carry this cookie and replace it by the real cookie when
1032: the connection is opened.
1033: The real authentication cookie is never
1034: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1035: .Pp
1.218 jmc 1036: If the
1037: .Cm ForwardAgent
1038: variable is set to
1039: .Dq yes
1040: (or see the description of the
1041: .Fl A
1.191 djm 1042: and
1.218 jmc 1043: .Fl a
1.226 jmc 1044: options above) and
1.218 jmc 1045: the user is using an authentication agent, the connection to the agent
1046: is automatically forwarded to the remote side.
1.252 jmc 1047: .Sh VERIFYING HOST KEYS
1048: When connecting to a server for the first time,
1049: a fingerprint of the server's public key is presented to the user
1050: (unless the option
1051: .Cm StrictHostKeyChecking
1052: has been disabled).
1053: Fingerprints can be determined using
1054: .Xr ssh-keygen 1 :
1055: .Pp
1056: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1057: .Pp
1.274 grunk 1058: If the fingerprint is already known, it can be matched
1059: and the key can be accepted or rejected.
1060: Because of the difficulty of comparing host keys
1061: just by looking at hex strings,
1062: there is also support to compare host keys visually,
1063: using
1064: .Em random art .
1065: By setting the
1.275 grunk 1066: .Cm VisualHostKey
1.274 grunk 1067: option to
1.275 grunk 1068: .Dq yes ,
1.274 grunk 1069: a small ASCII graphic gets displayed on every login to a server, no matter
1070: if the session itself is interactive or not.
1071: By learning the pattern a known server produces, a user can easily
1072: find out that the host key has changed when a completely different pattern
1073: is displayed.
1074: Because these patterns are not unambiguous however, a pattern that looks
1075: similar to the pattern remembered only gives a good probability that the
1076: host key is the same, not guaranteed proof.
1077: .Pp
1078: To get a listing of the fingerprints along with their random art for
1079: all known hosts, the following command line can be used:
1080: .Pp
1081: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1082: .Pp
1.252 jmc 1083: If the fingerprint is unknown,
1084: an alternative method of verification is available:
1085: SSH fingerprints verified by DNS.
1086: An additional resource record (RR),
1087: SSHFP,
1088: is added to a zonefile
1089: and the connecting client is able to match the fingerprint
1090: with that of the key presented.
1091: .Pp
1092: In this example, we are connecting a client to a server,
1093: .Dq host.example.com .
1094: The SSHFP resource records should first be added to the zonefile for
1095: host.example.com:
1096: .Bd -literal -offset indent
1.259 jakob 1097: $ ssh-keygen -r host.example.com.
1.252 jmc 1098: .Ed
1099: .Pp
1100: The output lines will have to be added to the zonefile.
1101: To check that the zone is answering fingerprint queries:
1102: .Pp
1103: .Dl $ dig -t SSHFP host.example.com
1104: .Pp
1105: Finally the client connects:
1106: .Bd -literal -offset indent
1107: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1108: [...]
1109: Matching host key fingerprint found in DNS.
1110: Are you sure you want to continue connecting (yes/no)?
1111: .Ed
1112: .Pp
1113: See the
1114: .Cm VerifyHostKeyDNS
1115: option in
1116: .Xr ssh_config 5
1117: for more information.
1.250 jmc 1118: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1119: .Nm
1120: contains support for Virtual Private Network (VPN) tunnelling
1121: using the
1122: .Xr tun 4
1123: network pseudo-device,
1124: allowing two networks to be joined securely.
1125: The
1126: .Xr sshd_config 5
1127: configuration option
1128: .Cm PermitTunnel
1129: controls whether the server supports this,
1130: and at what level (layer 2 or 3 traffic).
1131: .Pp
1132: The following example would connect client network 10.0.50.0/24
1.265 otto 1133: with remote network 10.0.99.0/24 using a point-to-point connection
1134: from 10.1.1.1 to 10.1.1.2,
1135: provided that the SSH server running on the gateway to the remote network,
1136: at 192.168.1.15, allows it.
1137: .Pp
1138: On the client:
1.250 jmc 1139: .Bd -literal -offset indent
1140: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1141: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1142: # route add 10.0.99.0/24 10.1.1.2
1143: .Ed
1144: .Pp
1145: On the server:
1146: .Bd -literal -offset indent
1147: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1148: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1149: .Ed
1150: .Pp
1151: Client access may be more finely tuned via the
1152: .Pa /root/.ssh/authorized_keys
1153: file (see below) and the
1154: .Cm PermitRootLogin
1155: server option.
1.255 jmc 1156: The following entry would permit connections on
1.250 jmc 1157: .Xr tun 4
1.255 jmc 1158: device 1 from user
1.250 jmc 1159: .Dq jane
1.255 jmc 1160: and on tun device 2 from user
1.250 jmc 1161: .Dq john ,
1162: if
1163: .Cm PermitRootLogin
1164: is set to
1165: .Dq forced-commands-only :
1166: .Bd -literal -offset 2n
1167: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1168: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1169: .Ed
1170: .Pp
1.264 ray 1171: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1172: it may be more suited to temporary setups,
1173: such as for wireless VPNs.
1174: More permanent VPNs are better provided by tools such as
1175: .Xr ipsecctl 8
1176: and
1177: .Xr isakmpd 8 .
1.2 deraadt 1178: .Sh ENVIRONMENT
1179: .Nm
1.1 deraadt 1180: will normally set the following environment variables:
1.237 jmc 1181: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1182: .It Ev DISPLAY
1183: The
1184: .Ev DISPLAY
1.40 aaron 1185: variable indicates the location of the X11 server.
1.44 aaron 1186: It is automatically set by
1.2 deraadt 1187: .Nm
1188: to point to a value of the form
1.233 jmc 1189: .Dq hostname:n ,
1190: where
1191: .Dq hostname
1192: indicates the host where the shell runs, and
1193: .Sq n
1194: is an integer \*(Ge 1.
1.40 aaron 1195: .Nm
1196: uses this special value to forward X11 connections over the secure
1197: channel.
1.107 markus 1198: The user should normally not set
1199: .Ev DISPLAY
1200: explicitly, as that
1.1 deraadt 1201: will render the X11 connection insecure (and will require the user to
1202: manually copy any required authorization cookies).
1.2 deraadt 1203: .It Ev HOME
1.1 deraadt 1204: Set to the path of the user's home directory.
1.2 deraadt 1205: .It Ev LOGNAME
1206: Synonym for
1.12 aaron 1207: .Ev USER ;
1208: set for compatibility with systems that use this variable.
1.2 deraadt 1209: .It Ev MAIL
1.129 stevesk 1210: Set to the path of the user's mailbox.
1.40 aaron 1211: .It Ev PATH
1.2 deraadt 1212: Set to the default
1213: .Ev PATH ,
1214: as specified when compiling
1.234 jmc 1215: .Nm .
1.118 markus 1216: .It Ev SSH_ASKPASS
1217: If
1218: .Nm
1219: needs a passphrase, it will read the passphrase from the current
1220: terminal if it was run from a terminal.
1221: If
1222: .Nm
1223: does not have a terminal associated with it but
1224: .Ev DISPLAY
1225: and
1226: .Ev SSH_ASKPASS
1227: are set, it will execute the program specified by
1228: .Ev SSH_ASKPASS
1229: and open an X11 window to read the passphrase.
1230: This is particularly useful when calling
1231: .Nm
1232: from a
1.196 jmc 1233: .Pa .xsession
1.118 markus 1234: or related script.
1235: (Note that on some machines it
1236: may be necessary to redirect the input from
1237: .Pa /dev/null
1238: to make this work.)
1.18 markus 1239: .It Ev SSH_AUTH_SOCK
1.233 jmc 1240: Identifies the path of a
1241: .Ux Ns -domain
1242: socket used to communicate with the agent.
1.166 stevesk 1243: .It Ev SSH_CONNECTION
1244: Identifies the client and server ends of the connection.
1.40 aaron 1245: The variable contains
1.233 jmc 1246: four space-separated values: client IP address, client port number,
1247: server IP address, and server port number.
1.73 markus 1248: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1249: This variable contains the original command line if a forced command
1.73 markus 1250: is executed.
1251: It can be used to extract the original arguments.
1.2 deraadt 1252: .It Ev SSH_TTY
1.1 deraadt 1253: This is set to the name of the tty (path to the device) associated
1.40 aaron 1254: with the current shell or command.
1255: If the current session has no tty,
1.1 deraadt 1256: this variable is not set.
1.2 deraadt 1257: .It Ev TZ
1.214 jmc 1258: This variable is set to indicate the present time zone if it
1.257 jmc 1259: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1260: on to new connections).
1.2 deraadt 1261: .It Ev USER
1.1 deraadt 1262: Set to the name of the user logging in.
1.2 deraadt 1263: .El
1264: .Pp
1.44 aaron 1265: Additionally,
1.2 deraadt 1266: .Nm
1.44 aaron 1267: reads
1.207 djm 1268: .Pa ~/.ssh/environment ,
1.2 deraadt 1269: and adds lines of the format
1270: .Dq VARNAME=value
1.233 jmc 1271: to the environment if the file exists and users are allowed to
1.161 marc 1272: change their environment.
1.176 jmc 1273: For more information, see the
1.161 marc 1274: .Cm PermitUserEnvironment
1.162 stevesk 1275: option in
1.161 marc 1276: .Xr sshd_config 5 .
1.2 deraadt 1277: .Sh FILES
1.236 jmc 1278: .Bl -tag -width Ds -compact
1279: .It ~/.rhosts
1.240 jmc 1280: This file is used for host-based authentication (see above).
1.92 markus 1281: On some machines this file may need to be
1.240 jmc 1282: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1283: because
1.2 deraadt 1284: .Xr sshd 8
1.40 aaron 1285: reads it as root.
1286: Additionally, this file must be owned by the user,
1287: and must not have write permissions for anyone else.
1288: The recommended
1.1 deraadt 1289: permission for most machines is read/write for the user, and not
1290: accessible by others.
1.2 deraadt 1291: .Pp
1.236 jmc 1292: .It ~/.shosts
1.240 jmc 1293: This file is used in exactly the same way as
1294: .Pa .rhosts ,
1295: but allows host-based authentication without permitting login with
1296: rlogin/rsh.
1.272 mcbride 1297: .Pp
1298: .It ~/.ssh/
1299: This directory is the default location for all user-specific configuration
1300: and authentication information.
1301: There is no general requirement to keep the entire contents of this directory
1302: secret, but the recommended permissions are read/write/execute for the user,
1303: and not accessible by others.
1.236 jmc 1304: .Pp
1.238 jmc 1305: .It ~/.ssh/authorized_keys
1306: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1307: The format of this file is described in the
1308: .Xr sshd 8
1309: manual page.
1310: This file is not highly sensitive, but the recommended
1311: permissions are read/write for the user, and not accessible by others.
1312: .Pp
1313: .It ~/.ssh/config
1314: This is the per-user configuration file.
1315: The file format and configuration options are described in
1316: .Xr ssh_config 5 .
1317: Because of the potential for abuse, this file must have strict permissions:
1318: read/write for the user, and not accessible by others.
1319: .Pp
1320: .It ~/.ssh/environment
1.239 jmc 1321: Contains additional definitions for environment variables; see
1322: .Sx ENVIRONMENT ,
1.238 jmc 1323: above.
1324: .Pp
1325: .It ~/.ssh/identity
1326: .It ~/.ssh/id_dsa
1327: .It ~/.ssh/id_rsa
1328: Contains the private key for authentication.
1329: These files
1330: contain sensitive data and should be readable by the user but not
1331: accessible by others (read/write/execute).
1332: .Nm
1333: will simply ignore a private key file if it is accessible by others.
1334: It is possible to specify a passphrase when
1335: generating the key which will be used to encrypt the
1336: sensitive part of this file using 3DES.
1337: .Pp
1338: .It ~/.ssh/identity.pub
1339: .It ~/.ssh/id_dsa.pub
1340: .It ~/.ssh/id_rsa.pub
1341: Contains the public key for authentication.
1342: These files are not
1343: sensitive and can (but need not) be readable by anyone.
1344: .Pp
1345: .It ~/.ssh/known_hosts
1.244 jmc 1346: Contains a list of host keys for all hosts the user has logged into
1347: that are not already in the systemwide list of known host keys.
1.238 jmc 1348: See
1.244 jmc 1349: .Xr sshd 8
1350: for further details of the format of this file.
1.238 jmc 1351: .Pp
1352: .It ~/.ssh/rc
1353: Commands in this file are executed by
1354: .Nm
1.245 jmc 1355: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1356: started.
1357: See the
1358: .Xr sshd 8
1359: manual page for more information.
1360: .Pp
1.236 jmc 1361: .It /etc/hosts.equiv
1.240 jmc 1362: This file is for host-based authentication (see above).
1363: It should only be writable by root.
1.236 jmc 1364: .Pp
1365: .It /etc/shosts.equiv
1.240 jmc 1366: This file is used in exactly the same way as
1367: .Pa hosts.equiv ,
1368: but allows host-based authentication without permitting login with
1369: rlogin/rsh.
1.236 jmc 1370: .Pp
1.238 jmc 1371: .It Pa /etc/ssh/ssh_config
1372: Systemwide configuration file.
1373: The file format and configuration options are described in
1374: .Xr ssh_config 5 .
1375: .Pp
1376: .It /etc/ssh/ssh_host_key
1377: .It /etc/ssh/ssh_host_dsa_key
1378: .It /etc/ssh/ssh_host_rsa_key
1379: These three files contain the private parts of the host keys
1.245 jmc 1380: and are used for host-based authentication.
1381: If protocol version 1 is used,
1.238 jmc 1382: .Nm
1383: must be setuid root, since the host key is readable only by root.
1384: For protocol version 2,
1385: .Nm
1386: uses
1387: .Xr ssh-keysign 8
1.245 jmc 1388: to access the host keys,
1389: eliminating the requirement that
1.238 jmc 1390: .Nm
1.245 jmc 1391: be setuid root when host-based authentication is used.
1.238 jmc 1392: By default
1.2 deraadt 1393: .Nm
1.238 jmc 1394: is not setuid root.
1395: .Pp
1396: .It /etc/ssh/ssh_known_hosts
1397: Systemwide list of known host keys.
1398: This file should be prepared by the
1399: system administrator to contain the public host keys of all machines in the
1400: organization.
1.244 jmc 1401: It should be world-readable.
1402: See
1.238 jmc 1403: .Xr sshd 8
1.244 jmc 1404: for further details of the format of this file.
1.236 jmc 1405: .Pp
1.238 jmc 1406: .It /etc/ssh/sshrc
1.1 deraadt 1407: Commands in this file are executed by
1.2 deraadt 1408: .Nm
1.245 jmc 1409: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1410: See the
1.2 deraadt 1411: .Xr sshd 8
1.1 deraadt 1412: manual page for more information.
1.58 itojun 1413: .El
1.2 deraadt 1414: .Sh SEE ALSO
1415: .Xr scp 1 ,
1.83 djm 1416: .Xr sftp 1 ,
1.2 deraadt 1417: .Xr ssh-add 1 ,
1418: .Xr ssh-agent 1 ,
1419: .Xr ssh-keygen 1 ,
1.242 jmc 1420: .Xr ssh-keyscan 1 ,
1.250 jmc 1421: .Xr tun 4 ,
1.176 jmc 1422: .Xr hosts.equiv 5 ,
1.159 stevesk 1423: .Xr ssh_config 5 ,
1.160 naddy 1424: .Xr ssh-keysign 8 ,
1.87 itojun 1425: .Xr sshd 8
1.106 markus 1426: .Rs
1.256 jmc 1427: .%R RFC 4250
1428: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1429: .%D 2006
1430: .Re
1431: .Rs
1432: .%R RFC 4251
1433: .%T "The Secure Shell (SSH) Protocol Architecture"
1434: .%D 2006
1435: .Re
1436: .Rs
1437: .%R RFC 4252
1438: .%T "The Secure Shell (SSH) Authentication Protocol"
1439: .%D 2006
1440: .Re
1441: .Rs
1442: .%R RFC 4253
1443: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1444: .%D 2006
1445: .Re
1446: .Rs
1447: .%R RFC 4254
1448: .%T "The Secure Shell (SSH) Connection Protocol"
1449: .%D 2006
1450: .Re
1451: .Rs
1452: .%R RFC 4255
1453: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1454: .%D 2006
1455: .Re
1456: .Rs
1457: .%R RFC 4256
1458: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1459: .%D 2006
1460: .Re
1461: .Rs
1462: .%R RFC 4335
1463: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1464: .%D 2006
1465: .Re
1466: .Rs
1467: .%R RFC 4344
1468: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1469: .%D 2006
1470: .Re
1471: .Rs
1472: .%R RFC 4345
1473: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1474: .%D 2006
1475: .Re
1476: .Rs
1477: .%R RFC 4419
1478: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1479: .%D 2006
1480: .Re
1481: .Rs
1482: .%R RFC 4716
1483: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1484: .%D 2006
1.274 grunk 1485: .Re
1486: .Rs
1487: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1488: .%A A. Perrig
1489: .%A D. Song
1490: .%D 1999
1491: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1492: .Re
1.173 jmc 1493: .Sh AUTHORS
1494: OpenSSH is a derivative of the original and free
1495: ssh 1.2.12 release by Tatu Ylonen.
1496: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1497: Theo de Raadt and Dug Song
1498: removed many bugs, re-added newer features and
1499: created OpenSSH.
1500: Markus Friedl contributed the support for SSH
1501: protocol versions 1.5 and 2.0.
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