Annotation of src/usr.bin/ssh/ssh.1, Revision 1.302
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: .\"
1.302 ! djm 37: .\" $OpenBSD: ssh.1,v 1.301 2010/03/05 08:31:20 jmc Exp $
1.301 jmc 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).
1.302 ! djm 309: .Nm
! 310: will also try to load certificate information from the filename obtained
! 311: by appending
! 312: .Pa -cert.pub
! 313: to identity filenames.
1.269 djm 314: .It Fl K
315: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
316: credentials to the server.
1.218 jmc 317: .It Fl k
318: Disables forwarding (delegation) of GSSAPI credentials to the server.
319: .It Fl L Xo
320: .Sm off
321: .Oo Ar bind_address : Oc
322: .Ar port : host : hostport
323: .Sm on
324: .Xc
325: Specifies that the given port on the local (client) host is to be
326: forwarded to the given host and port on the remote side.
327: This works by allocating a socket to listen to
328: .Ar port
329: on the local side, optionally bound to the specified
330: .Ar bind_address .
331: Whenever a connection is made to this port, the
332: connection is forwarded over the secure channel, and a connection is
333: made to
334: .Ar host
335: port
336: .Ar hostport
337: from the remote machine.
338: Port forwardings can also be specified in the configuration file.
339: IPv6 addresses can be specified with an alternative syntax:
340: .Sm off
341: .Xo
342: .Op Ar bind_address No /
343: .Ar port No / Ar host No /
344: .Ar hostport
345: .Xc
346: .Sm on
347: or by enclosing the address in square brackets.
348: Only the superuser can forward privileged ports.
349: By default, the local port is bound in accordance with the
350: .Cm GatewayPorts
351: setting.
352: However, an explicit
353: .Ar bind_address
354: may be used to bind the connection to a specific address.
1.2 deraadt 355: The
1.218 jmc 356: .Ar bind_address
357: of
358: .Dq localhost
359: indicates that the listening port be bound for local use only, while an
360: empty address or
361: .Sq *
362: indicates that the port should be available from all interfaces.
363: .It Fl l Ar login_name
364: Specifies the user to log in as on the remote machine.
365: This also may be specified on a per-host basis in the configuration file.
366: .It Fl M
367: Places the
368: .Nm
369: client into
370: .Dq master
371: mode for connection sharing.
1.231 stevesk 372: Multiple
373: .Fl M
374: options places
375: .Nm
376: into
377: .Dq master
378: mode with confirmation required before slave connections are accepted.
1.218 jmc 379: Refer to the description of
380: .Cm ControlMaster
381: in
382: .Xr ssh_config 5
383: for details.
384: .It Fl m Ar mac_spec
385: Additionally, for protocol version 2 a comma-separated list of MAC
386: (message authentication code) algorithms can
387: be specified in order of preference.
388: See the
389: .Cm MACs
390: keyword for more information.
391: .It Fl N
392: Do not execute a remote command.
393: This is useful for just forwarding ports
394: (protocol version 2 only).
395: .It Fl n
396: Redirects stdin from
397: .Pa /dev/null
398: (actually, prevents reading from stdin).
399: This must be used when
1.2 deraadt 400: .Nm
1.218 jmc 401: is run in the background.
402: A common trick is to use this to run X11 programs on a remote machine.
403: For example,
404: .Ic ssh -n shadows.cs.hut.fi emacs &
405: will start an emacs on shadows.cs.hut.fi, and the X11
406: connection will be automatically forwarded over an encrypted channel.
407: The
1.2 deraadt 408: .Nm
1.218 jmc 409: program will be put in the background.
410: (This does not work if
1.2 deraadt 411: .Nm
1.218 jmc 412: needs to ask for a password or passphrase; see also the
413: .Fl f
414: option.)
415: .It Fl O Ar ctl_cmd
416: Control an active connection multiplexing master process.
417: When the
418: .Fl O
419: option is specified, the
420: .Ar ctl_cmd
421: argument is interpreted and passed to the master process.
422: Valid commands are:
423: .Dq check
424: (check that the master process is running) and
425: .Dq exit
426: (request the master to exit).
427: .It Fl o Ar option
428: Can be used to give options in the format used in the configuration file.
429: This is useful for specifying options for which there is no separate
430: command-line flag.
431: For full details of the options listed below, and their possible values, see
432: .Xr ssh_config 5 .
1.2 deraadt 433: .Pp
1.218 jmc 434: .Bl -tag -width Ds -offset indent -compact
435: .It AddressFamily
436: .It BatchMode
437: .It BindAddress
438: .It ChallengeResponseAuthentication
439: .It CheckHostIP
440: .It Cipher
441: .It Ciphers
442: .It ClearAllForwardings
443: .It Compression
444: .It CompressionLevel
445: .It ConnectionAttempts
446: .It ConnectTimeout
447: .It ControlMaster
448: .It ControlPath
449: .It DynamicForward
450: .It EscapeChar
1.263 markus 451: .It ExitOnForwardFailure
1.218 jmc 452: .It ForwardAgent
453: .It ForwardX11
454: .It ForwardX11Trusted
455: .It GatewayPorts
456: .It GlobalKnownHostsFile
457: .It GSSAPIAuthentication
458: .It GSSAPIDelegateCredentials
459: .It HashKnownHosts
460: .It Host
461: .It HostbasedAuthentication
462: .It HostKeyAlgorithms
463: .It HostKeyAlias
464: .It HostName
465: .It IdentityFile
466: .It IdentitiesOnly
467: .It KbdInteractiveDevices
468: .It LocalCommand
469: .It LocalForward
470: .It LogLevel
471: .It MACs
472: .It NoHostAuthenticationForLocalhost
473: .It NumberOfPasswordPrompts
474: .It PasswordAuthentication
475: .It PermitLocalCommand
1.291 markus 476: .It PKCS11Provider
1.218 jmc 477: .It Port
478: .It PreferredAuthentications
479: .It Protocol
480: .It ProxyCommand
481: .It PubkeyAuthentication
1.251 dtucker 482: .It RekeyLimit
1.218 jmc 483: .It RemoteForward
484: .It RhostsRSAAuthentication
485: .It RSAAuthentication
486: .It SendEnv
487: .It ServerAliveInterval
488: .It ServerAliveCountMax
489: .It StrictHostKeyChecking
490: .It TCPKeepAlive
491: .It Tunnel
492: .It TunnelDevice
493: .It UsePrivilegedPort
494: .It User
495: .It UserKnownHostsFile
496: .It VerifyHostKeyDNS
1.276 jmc 497: .It VisualHostKey
1.218 jmc 498: .It XAuthLocation
499: .El
500: .It Fl p Ar port
501: Port to connect to on the remote host.
502: This can be specified on a
503: per-host basis in the configuration file.
504: .It Fl q
505: Quiet mode.
1.271 djm 506: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 507: .It Fl R Xo
508: .Sm off
509: .Oo Ar bind_address : Oc
510: .Ar port : host : hostport
511: .Sm on
512: .Xc
513: Specifies that the given port on the remote (server) host is to be
514: forwarded to the given host and port on the local side.
515: This works by allocating a socket to listen to
516: .Ar port
517: on the remote side, and whenever a connection is made to this port, the
518: connection is forwarded over the secure channel, and a connection is
519: made to
520: .Ar host
521: port
522: .Ar hostport
523: from the local machine.
1.2 deraadt 524: .Pp
1.218 jmc 525: Port forwardings can also be specified in the configuration file.
526: Privileged ports can be forwarded only when
527: logging in as root on the remote machine.
528: IPv6 addresses can be specified by enclosing the address in square braces or
529: using an alternative syntax:
530: .Sm off
531: .Xo
532: .Op Ar bind_address No /
533: .Ar host No / Ar port No /
534: .Ar hostport
535: .Xc .
536: .Sm on
1.194 jakob 537: .Pp
1.218 jmc 538: By default, the listening socket on the server will be bound to the loopback
539: interface only.
1.280 tobias 540: This may be overridden by specifying a
1.218 jmc 541: .Ar bind_address .
542: An empty
543: .Ar bind_address ,
544: or the address
545: .Ql * ,
546: indicates that the remote socket should listen on all interfaces.
547: Specifying a remote
548: .Ar bind_address
549: will only succeed if the server's
550: .Cm GatewayPorts
551: option is enabled (see
552: .Xr sshd_config 5 ) .
1.281 djm 553: .Pp
554: If the
555: .Ar port
556: argument is
1.282 djm 557: .Ql 0 ,
1.281 djm 558: the listen port will be dynamically allocated on the server and reported
559: to the client at run time.
1.218 jmc 560: .It Fl S Ar ctl_path
561: Specifies the location of a control socket for connection sharing.
562: Refer to the description of
563: .Cm ControlPath
564: and
565: .Cm ControlMaster
566: in
567: .Xr ssh_config 5
568: for details.
569: .It Fl s
570: May be used to request invocation of a subsystem on the remote system.
571: Subsystems are a feature of the SSH2 protocol which facilitate the use
572: of SSH as a secure transport for other applications (eg.\&
573: .Xr sftp 1 ) .
574: The subsystem is specified as the remote command.
575: .It Fl T
576: Disable pseudo-tty allocation.
577: .It Fl t
578: Force pseudo-tty allocation.
579: This can be used to execute arbitrary
580: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 581: e.g. when implementing menu services.
1.218 jmc 582: Multiple
583: .Fl t
584: options force tty allocation, even if
1.194 jakob 585: .Nm
1.218 jmc 586: has no local tty.
587: .It Fl V
588: Display the version number and exit.
589: .It Fl v
590: Verbose mode.
591: Causes
1.176 jmc 592: .Nm
1.218 jmc 593: to print debugging messages about its progress.
594: This is helpful in
595: debugging connection, authentication, and configuration problems.
596: Multiple
597: .Fl v
598: options increase the verbosity.
599: The maximum is 3.
1.290 dtucker 600: .It Fl W Ar host : Ns Ar port
601: Requests that standard input and output on the client be forwarded to
602: .Ar host
603: on
604: .Ar port
605: over the secure channel.
606: Implies
607: .Fl N ,
608: .Fl T ,
609: .Cm ExitOnForwardFailure
610: and
611: .Cm ClearAllForwardings
612: and works with Protocol version 2 only.
1.261 stevesk 613: .It Fl w Xo
614: .Ar local_tun Ns Op : Ns Ar remote_tun
615: .Xc
616: Requests
617: tunnel
618: device forwarding with the specified
1.218 jmc 619: .Xr tun 4
1.261 stevesk 620: devices between the client
621: .Pq Ar local_tun
622: and the server
623: .Pq Ar remote_tun .
624: .Pp
1.228 jmc 625: The devices may be specified by numerical ID or the keyword
626: .Dq any ,
627: which uses the next available tunnel device.
1.261 stevesk 628: If
629: .Ar remote_tun
630: is not specified, it defaults to
631: .Dq any .
1.228 jmc 632: See also the
1.218 jmc 633: .Cm Tunnel
1.261 stevesk 634: and
635: .Cm TunnelDevice
636: directives in
1.218 jmc 637: .Xr ssh_config 5 .
1.261 stevesk 638: If the
639: .Cm Tunnel
640: directive is unset, it is set to the default tunnel mode, which is
641: .Dq point-to-point .
1.218 jmc 642: .It Fl X
643: Enables X11 forwarding.
1.54 markus 644: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 645: .Pp
1.218 jmc 646: X11 forwarding should be enabled with caution.
1.168 jmc 647: Users with the ability to bypass file permissions on the remote host
1.218 jmc 648: (for the user's X authorization database)
649: can access the local X11 display through the forwarded connection.
650: An attacker may then be able to perform activities such as keystroke monitoring.
651: .Pp
652: For this reason, X11 forwarding is subjected to X11 SECURITY extension
653: restrictions by default.
654: Please refer to the
655: .Nm
656: .Fl Y
657: option and the
658: .Cm ForwardX11Trusted
659: directive in
660: .Xr ssh_config 5
661: for more information.
662: .It Fl x
663: Disables X11 forwarding.
664: .It Fl Y
665: Enables trusted X11 forwarding.
666: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
667: controls.
1.278 djm 668: .It Fl y
669: Send log information using the
670: .Xr syslog 3
671: system module.
672: By default this information is sent to stderr.
1.218 jmc 673: .El
1.224 jmc 674: .Pp
675: .Nm
676: may additionally obtain configuration data from
677: a per-user configuration file and a system-wide configuration file.
678: The file format and configuration options are described in
679: .Xr ssh_config 5 .
680: .Pp
681: .Nm
682: exits with the exit status of the remote command or with 255
683: if an error occurred.
1.222 jmc 684: .Sh AUTHENTICATION
1.249 jmc 685: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 686: The default is to use protocol 2 only,
687: though this can be changed via the
1.222 jmc 688: .Cm Protocol
689: option in
1.284 jmc 690: .Xr ssh_config 5
691: or the
1.222 jmc 692: .Fl 1
693: and
694: .Fl 2
695: options (see above).
696: Both protocols support similar authentication methods,
1.284 jmc 697: but protocol 2 is the default since
1.222 jmc 698: it provides additional mechanisms for confidentiality
699: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 700: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 701: Protocol 1 lacks a strong mechanism for ensuring the
702: integrity of the connection.
703: .Pp
704: The methods available for authentication are:
1.260 jmc 705: GSSAPI-based authentication,
1.222 jmc 706: host-based authentication,
707: public key authentication,
708: challenge-response authentication,
709: and password authentication.
710: Authentication methods are tried in the order specified above,
711: though protocol 2 has a configuration option to change the default order:
712: .Cm PreferredAuthentications .
713: .Pp
714: Host-based authentication works as follows:
1.218 jmc 715: If the machine the user logs in from is listed in
716: .Pa /etc/hosts.equiv
717: or
718: .Pa /etc/shosts.equiv
719: on the remote machine, and the user names are
720: the same on both sides, or if the files
721: .Pa ~/.rhosts
722: or
723: .Pa ~/.shosts
724: exist in the user's home directory on the
725: remote machine and contain a line containing the name of the client
726: machine and the name of the user on that machine, the user is
1.222 jmc 727: considered for login.
728: Additionally, the server
729: .Em must
730: be able to verify the client's
731: host key (see the description of
1.218 jmc 732: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 733: and
1.222 jmc 734: .Pa ~/.ssh/known_hosts ,
735: below)
736: for login to be permitted.
1.218 jmc 737: This authentication method closes security holes due to IP
1.222 jmc 738: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 739: [Note to the administrator:
740: .Pa /etc/hosts.equiv ,
741: .Pa ~/.rhosts ,
742: and the rlogin/rsh protocol in general, are inherently insecure and should be
743: disabled if security is desired.]
1.189 dtucker 744: .Pp
1.222 jmc 745: Public key authentication works as follows:
746: The scheme is based on public-key cryptography,
747: using cryptosystems
748: where encryption and decryption are done using separate keys,
749: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 750: The idea is that each user creates a public/private
751: key pair for authentication purposes.
752: The server knows the public key, and only the user knows the private key.
1.222 jmc 753: .Nm
754: implements public key authentication protocol automatically,
755: using either the RSA or DSA algorithms.
756: Protocol 1 is restricted to using only RSA keys,
757: but protocol 2 may use either.
758: The
759: .Sx HISTORY
760: section of
761: .Xr ssl 8
762: contains a brief discussion of the two algorithms.
1.210 djm 763: .Pp
1.218 jmc 764: The file
765: .Pa ~/.ssh/authorized_keys
766: lists the public keys that are permitted for logging in.
767: When the user logs in, the
1.2 deraadt 768: .Nm
1.218 jmc 769: program tells the server which key pair it would like to use for
770: authentication.
1.222 jmc 771: The client proves that it has access to the private key
772: and the server checks that the corresponding public key
773: is authorized to accept the account.
1.218 jmc 774: .Pp
1.222 jmc 775: The user creates his/her key pair by running
1.218 jmc 776: .Xr ssh-keygen 1 .
777: This stores the private key in
1.207 djm 778: .Pa ~/.ssh/identity
1.222 jmc 779: (protocol 1),
780: .Pa ~/.ssh/id_dsa
781: (protocol 2 DSA),
782: or
783: .Pa ~/.ssh/id_rsa
784: (protocol 2 RSA)
1.218 jmc 785: and stores the public key in
786: .Pa ~/.ssh/identity.pub
1.222 jmc 787: (protocol 1),
788: .Pa ~/.ssh/id_dsa.pub
789: (protocol 2 DSA),
790: or
791: .Pa ~/.ssh/id_rsa.pub
792: (protocol 2 RSA)
1.218 jmc 793: in the user's home directory.
1.222 jmc 794: The user should then copy the public key
1.218 jmc 795: to
796: .Pa ~/.ssh/authorized_keys
1.222 jmc 797: in his/her home directory on the remote machine.
798: The
1.218 jmc 799: .Pa authorized_keys
800: file corresponds to the conventional
801: .Pa ~/.rhosts
802: file, and has one key
1.222 jmc 803: per line, though the lines can be very long.
1.218 jmc 804: After this, the user can log in without giving the password.
805: .Pp
1.301 jmc 806: A variation on public key authentication
807: is available in the form of certificate authentication:
808: instead of a set of public/private keys,
809: signed certificates are used.
810: This has the advantage that a single trusted certification authority
811: can be used in place of many public/private keys.
812: See the
813: .Sx CERTIFICATES
814: section of
815: .Xr ssh-keygen 1
816: for more information.
817: .Pp
818: The most convenient way to use public key or certificate authentication
819: may be with an authentication agent.
1.218 jmc 820: See
821: .Xr ssh-agent 1
822: for more information.
823: .Pp
1.222 jmc 824: Challenge-response authentication works as follows:
825: The server sends an arbitrary
826: .Qq challenge
827: text, and prompts for a response.
828: Protocol 2 allows multiple challenges and responses;
829: protocol 1 is restricted to just one challenge/response.
830: Examples of challenge-response authentication include
831: BSD Authentication (see
832: .Xr login.conf 5 )
833: and PAM (some non-OpenBSD systems).
834: .Pp
835: Finally, if other authentication methods fail,
1.218 jmc 836: .Nm
837: prompts the user for a password.
838: The password is sent to the remote
839: host for checking; however, since all communications are encrypted,
840: the password cannot be seen by someone listening on the network.
1.232 jmc 841: .Pp
842: .Nm
843: automatically maintains and checks a database containing
844: identification for all hosts it has ever been used with.
845: Host keys are stored in
846: .Pa ~/.ssh/known_hosts
847: in the user's home directory.
848: Additionally, the file
849: .Pa /etc/ssh/ssh_known_hosts
850: is automatically checked for known hosts.
851: Any new hosts are automatically added to the user's file.
852: If a host's identification ever changes,
853: .Nm
854: warns about this and disables password authentication to prevent
855: server spoofing or man-in-the-middle attacks,
856: which could otherwise be used to circumvent the encryption.
857: The
858: .Cm StrictHostKeyChecking
859: option can be used to control logins to machines whose
860: host key is not known or has changed.
861: .Pp
1.218 jmc 862: When the user's identity has been accepted by the server, the server
863: either executes the given command, or logs into the machine and gives
864: the user a normal shell on the remote machine.
865: All communication with
866: the remote command or shell will be automatically encrypted.
867: .Pp
868: If a pseudo-terminal has been allocated (normal login session), the
869: user may use the escape characters noted below.
870: .Pp
871: If no pseudo-tty has been allocated,
872: the session is transparent and can be used to reliably transfer binary data.
873: On most systems, setting the escape character to
874: .Dq none
875: will also make the session transparent even if a tty is used.
876: .Pp
877: The session terminates when the command or shell on the remote
1.247 jmc 878: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 879: .Sh ESCAPE CHARACTERS
1.218 jmc 880: When a pseudo-terminal has been requested,
1.2 deraadt 881: .Nm
1.218 jmc 882: supports a number of functions through the use of an escape character.
883: .Pp
884: A single tilde character can be sent as
885: .Ic ~~
886: or by following the tilde by a character other than those described below.
887: The escape character must always follow a newline to be interpreted as
888: special.
889: The escape character can be changed in configuration files using the
890: .Cm EscapeChar
891: configuration directive or on the command line by the
892: .Fl e
893: option.
894: .Pp
895: The supported escapes (assuming the default
896: .Ql ~ )
897: are:
898: .Bl -tag -width Ds
899: .It Cm ~.
900: Disconnect.
901: .It Cm ~^Z
902: Background
1.234 jmc 903: .Nm .
1.218 jmc 904: .It Cm ~#
905: List forwarded connections.
906: .It Cm ~&
907: Background
1.2 deraadt 908: .Nm
1.218 jmc 909: at logout when waiting for forwarded connection / X11 sessions to terminate.
910: .It Cm ~?
911: Display a list of escape characters.
912: .It Cm ~B
913: Send a BREAK to the remote system
914: (only useful for SSH protocol version 2 and if the peer supports it).
915: .It Cm ~C
916: Open command line.
917: Currently this allows the addition of port forwardings using the
1.279 stevesk 918: .Fl L ,
919: .Fl R
1.218 jmc 920: and
1.279 stevesk 921: .Fl D
1.225 jmc 922: options (see above).
1.218 jmc 923: It also allows the cancellation of existing remote port-forwardings
924: using
1.262 stevesk 925: .Sm off
926: .Fl KR Oo Ar bind_address : Oc Ar port .
927: .Sm on
1.218 jmc 928: .Ic !\& Ns Ar command
929: allows the user to execute a local command if the
930: .Ic PermitLocalCommand
931: option is enabled in
1.176 jmc 932: .Xr ssh_config 5 .
1.218 jmc 933: Basic help is available, using the
934: .Fl h
935: option.
936: .It Cm ~R
937: Request rekeying of the connection
938: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 939: .El
1.246 jmc 940: .Sh TCP FORWARDING
941: Forwarding of arbitrary TCP connections over the secure channel can
942: be specified either on the command line or in a configuration file.
943: One possible application of TCP forwarding is a secure connection to a
944: mail server; another is going through firewalls.
945: .Pp
946: In the example below, we look at encrypting communication between
947: an IRC client and server, even though the IRC server does not directly
948: support encrypted communications.
949: This works as follows:
950: the user connects to the remote host using
951: .Nm ,
952: specifying a port to be used to forward connections
953: to the remote server.
954: After that it is possible to start the service which is to be encrypted
955: on the client machine,
956: connecting to the same local port,
957: and
958: .Nm
959: will encrypt and forward the connection.
960: .Pp
961: The following example tunnels an IRC session from client machine
962: .Dq 127.0.0.1
963: (localhost)
964: to remote server
965: .Dq server.example.com :
966: .Bd -literal -offset 4n
967: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
968: $ irc -c '#users' -p 1234 pinky 127.0.0.1
969: .Ed
970: .Pp
971: This tunnels a connection to IRC server
972: .Dq server.example.com ,
973: joining channel
974: .Dq #users ,
975: nickname
976: .Dq pinky ,
977: using port 1234.
978: It doesn't matter which port is used,
979: as long as it's greater than 1023
980: (remember, only root can open sockets on privileged ports)
981: and doesn't conflict with any ports already in use.
982: The connection is forwarded to port 6667 on the remote server,
983: since that's the standard port for IRC services.
984: .Pp
985: The
986: .Fl f
987: option backgrounds
988: .Nm
989: and the remote command
990: .Dq sleep 10
991: is specified to allow an amount of time
992: (10 seconds, in the example)
993: to start the service which is to be tunnelled.
994: If no connections are made within the time specified,
995: .Nm
996: will exit.
997: .Sh X11 FORWARDING
1.218 jmc 998: If the
999: .Cm ForwardX11
1000: variable is set to
1001: .Dq yes
1002: (or see the description of the
1.227 jmc 1003: .Fl X ,
1004: .Fl x ,
1.218 jmc 1005: and
1.227 jmc 1006: .Fl Y
1.226 jmc 1007: options above)
1.218 jmc 1008: and the user is using X11 (the
1009: .Ev DISPLAY
1010: environment variable is set), the connection to the X11 display is
1011: automatically forwarded to the remote side in such a way that any X11
1012: programs started from the shell (or command) will go through the
1013: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1014: from the local machine.
1.218 jmc 1015: The user should not manually set
1016: .Ev DISPLAY .
1017: Forwarding of X11 connections can be
1018: configured on the command line or in configuration files.
1019: .Pp
1020: The
1021: .Ev DISPLAY
1022: value set by
1023: .Nm
1024: will point to the server machine, but with a display number greater than zero.
1025: This is normal, and happens because
1026: .Nm
1027: creates a
1028: .Dq proxy
1029: X server on the server machine for forwarding the
1030: connections over the encrypted channel.
1.200 djm 1031: .Pp
1.218 jmc 1032: .Nm
1033: will also automatically set up Xauthority data on the server machine.
1034: For this purpose, it will generate a random authorization cookie,
1035: store it in Xauthority on the server, and verify that any forwarded
1036: connections carry this cookie and replace it by the real cookie when
1037: the connection is opened.
1038: The real authentication cookie is never
1039: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1040: .Pp
1.218 jmc 1041: If the
1042: .Cm ForwardAgent
1043: variable is set to
1044: .Dq yes
1045: (or see the description of the
1046: .Fl A
1.191 djm 1047: and
1.218 jmc 1048: .Fl a
1.226 jmc 1049: options above) and
1.218 jmc 1050: the user is using an authentication agent, the connection to the agent
1051: is automatically forwarded to the remote side.
1.252 jmc 1052: .Sh VERIFYING HOST KEYS
1053: When connecting to a server for the first time,
1054: a fingerprint of the server's public key is presented to the user
1055: (unless the option
1056: .Cm StrictHostKeyChecking
1057: has been disabled).
1058: Fingerprints can be determined using
1059: .Xr ssh-keygen 1 :
1060: .Pp
1061: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1062: .Pp
1.274 grunk 1063: If the fingerprint is already known, it can be matched
1064: and the key can be accepted or rejected.
1065: Because of the difficulty of comparing host keys
1066: just by looking at hex strings,
1067: there is also support to compare host keys visually,
1068: using
1069: .Em random art .
1070: By setting the
1.275 grunk 1071: .Cm VisualHostKey
1.274 grunk 1072: option to
1.275 grunk 1073: .Dq yes ,
1.274 grunk 1074: a small ASCII graphic gets displayed on every login to a server, no matter
1075: if the session itself is interactive or not.
1076: By learning the pattern a known server produces, a user can easily
1077: find out that the host key has changed when a completely different pattern
1078: is displayed.
1079: Because these patterns are not unambiguous however, a pattern that looks
1080: similar to the pattern remembered only gives a good probability that the
1081: host key is the same, not guaranteed proof.
1082: .Pp
1083: To get a listing of the fingerprints along with their random art for
1084: all known hosts, the following command line can be used:
1085: .Pp
1086: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1087: .Pp
1.252 jmc 1088: If the fingerprint is unknown,
1089: an alternative method of verification is available:
1090: SSH fingerprints verified by DNS.
1091: An additional resource record (RR),
1092: SSHFP,
1093: is added to a zonefile
1094: and the connecting client is able to match the fingerprint
1095: with that of the key presented.
1096: .Pp
1097: In this example, we are connecting a client to a server,
1098: .Dq host.example.com .
1099: The SSHFP resource records should first be added to the zonefile for
1100: host.example.com:
1101: .Bd -literal -offset indent
1.259 jakob 1102: $ ssh-keygen -r host.example.com.
1.252 jmc 1103: .Ed
1104: .Pp
1105: The output lines will have to be added to the zonefile.
1106: To check that the zone is answering fingerprint queries:
1107: .Pp
1108: .Dl $ dig -t SSHFP host.example.com
1109: .Pp
1110: Finally the client connects:
1111: .Bd -literal -offset indent
1112: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1113: [...]
1114: Matching host key fingerprint found in DNS.
1115: Are you sure you want to continue connecting (yes/no)?
1116: .Ed
1117: .Pp
1118: See the
1119: .Cm VerifyHostKeyDNS
1120: option in
1121: .Xr ssh_config 5
1122: for more information.
1.250 jmc 1123: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1124: .Nm
1125: contains support for Virtual Private Network (VPN) tunnelling
1126: using the
1127: .Xr tun 4
1128: network pseudo-device,
1129: allowing two networks to be joined securely.
1130: The
1131: .Xr sshd_config 5
1132: configuration option
1133: .Cm PermitTunnel
1134: controls whether the server supports this,
1135: and at what level (layer 2 or 3 traffic).
1136: .Pp
1137: The following example would connect client network 10.0.50.0/24
1.265 otto 1138: with remote network 10.0.99.0/24 using a point-to-point connection
1139: from 10.1.1.1 to 10.1.1.2,
1140: provided that the SSH server running on the gateway to the remote network,
1141: at 192.168.1.15, allows it.
1142: .Pp
1143: On the client:
1.250 jmc 1144: .Bd -literal -offset indent
1145: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1146: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1147: # route add 10.0.99.0/24 10.1.1.2
1148: .Ed
1149: .Pp
1150: On the server:
1151: .Bd -literal -offset indent
1152: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1153: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1154: .Ed
1155: .Pp
1156: Client access may be more finely tuned via the
1157: .Pa /root/.ssh/authorized_keys
1158: file (see below) and the
1159: .Cm PermitRootLogin
1160: server option.
1.255 jmc 1161: The following entry would permit connections on
1.250 jmc 1162: .Xr tun 4
1.255 jmc 1163: device 1 from user
1.250 jmc 1164: .Dq jane
1.255 jmc 1165: and on tun device 2 from user
1.250 jmc 1166: .Dq john ,
1167: if
1168: .Cm PermitRootLogin
1169: is set to
1170: .Dq forced-commands-only :
1171: .Bd -literal -offset 2n
1172: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1173: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1174: .Ed
1175: .Pp
1.264 ray 1176: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1177: it may be more suited to temporary setups,
1178: such as for wireless VPNs.
1179: More permanent VPNs are better provided by tools such as
1180: .Xr ipsecctl 8
1181: and
1182: .Xr isakmpd 8 .
1.2 deraadt 1183: .Sh ENVIRONMENT
1184: .Nm
1.1 deraadt 1185: will normally set the following environment variables:
1.237 jmc 1186: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1187: .It Ev DISPLAY
1188: The
1189: .Ev DISPLAY
1.40 aaron 1190: variable indicates the location of the X11 server.
1.44 aaron 1191: It is automatically set by
1.2 deraadt 1192: .Nm
1193: to point to a value of the form
1.233 jmc 1194: .Dq hostname:n ,
1195: where
1196: .Dq hostname
1197: indicates the host where the shell runs, and
1198: .Sq n
1199: is an integer \*(Ge 1.
1.40 aaron 1200: .Nm
1201: uses this special value to forward X11 connections over the secure
1202: channel.
1.107 markus 1203: The user should normally not set
1204: .Ev DISPLAY
1205: explicitly, as that
1.1 deraadt 1206: will render the X11 connection insecure (and will require the user to
1207: manually copy any required authorization cookies).
1.2 deraadt 1208: .It Ev HOME
1.1 deraadt 1209: Set to the path of the user's home directory.
1.2 deraadt 1210: .It Ev LOGNAME
1211: Synonym for
1.12 aaron 1212: .Ev USER ;
1213: set for compatibility with systems that use this variable.
1.2 deraadt 1214: .It Ev MAIL
1.129 stevesk 1215: Set to the path of the user's mailbox.
1.40 aaron 1216: .It Ev PATH
1.2 deraadt 1217: Set to the default
1218: .Ev PATH ,
1219: as specified when compiling
1.234 jmc 1220: .Nm .
1.118 markus 1221: .It Ev SSH_ASKPASS
1222: If
1223: .Nm
1224: needs a passphrase, it will read the passphrase from the current
1225: terminal if it was run from a terminal.
1226: If
1227: .Nm
1228: does not have a terminal associated with it but
1229: .Ev DISPLAY
1230: and
1231: .Ev SSH_ASKPASS
1232: are set, it will execute the program specified by
1233: .Ev SSH_ASKPASS
1234: and open an X11 window to read the passphrase.
1235: This is particularly useful when calling
1236: .Nm
1237: from a
1.196 jmc 1238: .Pa .xsession
1.118 markus 1239: or related script.
1240: (Note that on some machines it
1241: may be necessary to redirect the input from
1242: .Pa /dev/null
1243: to make this work.)
1.18 markus 1244: .It Ev SSH_AUTH_SOCK
1.233 jmc 1245: Identifies the path of a
1246: .Ux Ns -domain
1247: socket used to communicate with the agent.
1.166 stevesk 1248: .It Ev SSH_CONNECTION
1249: Identifies the client and server ends of the connection.
1.40 aaron 1250: The variable contains
1.233 jmc 1251: four space-separated values: client IP address, client port number,
1252: server IP address, and server port number.
1.73 markus 1253: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1254: This variable contains the original command line if a forced command
1.73 markus 1255: is executed.
1256: It can be used to extract the original arguments.
1.2 deraadt 1257: .It Ev SSH_TTY
1.1 deraadt 1258: This is set to the name of the tty (path to the device) associated
1.40 aaron 1259: with the current shell or command.
1260: If the current session has no tty,
1.1 deraadt 1261: this variable is not set.
1.2 deraadt 1262: .It Ev TZ
1.214 jmc 1263: This variable is set to indicate the present time zone if it
1.257 jmc 1264: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1265: on to new connections).
1.2 deraadt 1266: .It Ev USER
1.1 deraadt 1267: Set to the name of the user logging in.
1.2 deraadt 1268: .El
1269: .Pp
1.44 aaron 1270: Additionally,
1.2 deraadt 1271: .Nm
1.44 aaron 1272: reads
1.207 djm 1273: .Pa ~/.ssh/environment ,
1.2 deraadt 1274: and adds lines of the format
1275: .Dq VARNAME=value
1.233 jmc 1276: to the environment if the file exists and users are allowed to
1.161 marc 1277: change their environment.
1.176 jmc 1278: For more information, see the
1.161 marc 1279: .Cm PermitUserEnvironment
1.162 stevesk 1280: option in
1.161 marc 1281: .Xr sshd_config 5 .
1.2 deraadt 1282: .Sh FILES
1.236 jmc 1283: .Bl -tag -width Ds -compact
1284: .It ~/.rhosts
1.240 jmc 1285: This file is used for host-based authentication (see above).
1.92 markus 1286: On some machines this file may need to be
1.240 jmc 1287: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1288: because
1.2 deraadt 1289: .Xr sshd 8
1.40 aaron 1290: reads it as root.
1291: Additionally, this file must be owned by the user,
1292: and must not have write permissions for anyone else.
1293: The recommended
1.1 deraadt 1294: permission for most machines is read/write for the user, and not
1295: accessible by others.
1.2 deraadt 1296: .Pp
1.236 jmc 1297: .It ~/.shosts
1.240 jmc 1298: This file is used in exactly the same way as
1299: .Pa .rhosts ,
1300: but allows host-based authentication without permitting login with
1301: rlogin/rsh.
1.272 mcbride 1302: .Pp
1303: .It ~/.ssh/
1304: This directory is the default location for all user-specific configuration
1305: and authentication information.
1306: There is no general requirement to keep the entire contents of this directory
1307: secret, but the recommended permissions are read/write/execute for the user,
1308: and not accessible by others.
1.236 jmc 1309: .Pp
1.238 jmc 1310: .It ~/.ssh/authorized_keys
1311: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1312: The format of this file is described in the
1313: .Xr sshd 8
1314: manual page.
1315: This file is not highly sensitive, but the recommended
1316: permissions are read/write for the user, and not accessible by others.
1317: .Pp
1318: .It ~/.ssh/config
1319: This is the per-user configuration file.
1320: The file format and configuration options are described in
1321: .Xr ssh_config 5 .
1322: Because of the potential for abuse, this file must have strict permissions:
1323: read/write for the user, and not accessible by others.
1324: .Pp
1325: .It ~/.ssh/environment
1.239 jmc 1326: Contains additional definitions for environment variables; see
1327: .Sx ENVIRONMENT ,
1.238 jmc 1328: above.
1329: .Pp
1330: .It ~/.ssh/identity
1331: .It ~/.ssh/id_dsa
1332: .It ~/.ssh/id_rsa
1333: Contains the private key for authentication.
1334: These files
1335: contain sensitive data and should be readable by the user but not
1336: accessible by others (read/write/execute).
1337: .Nm
1338: will simply ignore a private key file if it is accessible by others.
1339: It is possible to specify a passphrase when
1340: generating the key which will be used to encrypt the
1341: sensitive part of this file using 3DES.
1342: .Pp
1343: .It ~/.ssh/identity.pub
1344: .It ~/.ssh/id_dsa.pub
1345: .It ~/.ssh/id_rsa.pub
1346: Contains the public key for authentication.
1347: These files are not
1348: sensitive and can (but need not) be readable by anyone.
1349: .Pp
1350: .It ~/.ssh/known_hosts
1.244 jmc 1351: Contains a list of host keys for all hosts the user has logged into
1352: that are not already in the systemwide list of known host keys.
1.238 jmc 1353: See
1.244 jmc 1354: .Xr sshd 8
1355: for further details of the format of this file.
1.238 jmc 1356: .Pp
1357: .It ~/.ssh/rc
1358: Commands in this file are executed by
1359: .Nm
1.245 jmc 1360: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1361: started.
1362: See the
1363: .Xr sshd 8
1364: manual page for more information.
1365: .Pp
1.236 jmc 1366: .It /etc/hosts.equiv
1.240 jmc 1367: This file is for host-based authentication (see above).
1368: It should only be writable by root.
1.236 jmc 1369: .Pp
1370: .It /etc/shosts.equiv
1.240 jmc 1371: This file is used in exactly the same way as
1372: .Pa hosts.equiv ,
1373: but allows host-based authentication without permitting login with
1374: rlogin/rsh.
1.236 jmc 1375: .Pp
1.238 jmc 1376: .It Pa /etc/ssh/ssh_config
1377: Systemwide configuration file.
1378: The file format and configuration options are described in
1379: .Xr ssh_config 5 .
1380: .Pp
1381: .It /etc/ssh/ssh_host_key
1382: .It /etc/ssh/ssh_host_dsa_key
1383: .It /etc/ssh/ssh_host_rsa_key
1384: These three files contain the private parts of the host keys
1.245 jmc 1385: and are used for host-based authentication.
1386: If protocol version 1 is used,
1.238 jmc 1387: .Nm
1388: must be setuid root, since the host key is readable only by root.
1389: For protocol version 2,
1390: .Nm
1391: uses
1392: .Xr ssh-keysign 8
1.245 jmc 1393: to access the host keys,
1394: eliminating the requirement that
1.238 jmc 1395: .Nm
1.245 jmc 1396: be setuid root when host-based authentication is used.
1.238 jmc 1397: By default
1.2 deraadt 1398: .Nm
1.238 jmc 1399: is not setuid root.
1400: .Pp
1401: .It /etc/ssh/ssh_known_hosts
1402: Systemwide list of known host keys.
1403: This file should be prepared by the
1404: system administrator to contain the public host keys of all machines in the
1405: organization.
1.244 jmc 1406: It should be world-readable.
1407: See
1.238 jmc 1408: .Xr sshd 8
1.244 jmc 1409: for further details of the format of this file.
1.236 jmc 1410: .Pp
1.238 jmc 1411: .It /etc/ssh/sshrc
1.1 deraadt 1412: Commands in this file are executed by
1.2 deraadt 1413: .Nm
1.245 jmc 1414: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1415: See the
1.2 deraadt 1416: .Xr sshd 8
1.1 deraadt 1417: manual page for more information.
1.58 itojun 1418: .El
1.2 deraadt 1419: .Sh SEE ALSO
1420: .Xr scp 1 ,
1.83 djm 1421: .Xr sftp 1 ,
1.2 deraadt 1422: .Xr ssh-add 1 ,
1423: .Xr ssh-agent 1 ,
1424: .Xr ssh-keygen 1 ,
1.242 jmc 1425: .Xr ssh-keyscan 1 ,
1.250 jmc 1426: .Xr tun 4 ,
1.176 jmc 1427: .Xr hosts.equiv 5 ,
1.159 stevesk 1428: .Xr ssh_config 5 ,
1.160 naddy 1429: .Xr ssh-keysign 8 ,
1.87 itojun 1430: .Xr sshd 8
1.106 markus 1431: .Rs
1.256 jmc 1432: .%R RFC 4250
1433: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1434: .%D 2006
1435: .Re
1436: .Rs
1437: .%R RFC 4251
1438: .%T "The Secure Shell (SSH) Protocol Architecture"
1439: .%D 2006
1440: .Re
1441: .Rs
1442: .%R RFC 4252
1443: .%T "The Secure Shell (SSH) Authentication Protocol"
1444: .%D 2006
1445: .Re
1446: .Rs
1447: .%R RFC 4253
1448: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1449: .%D 2006
1450: .Re
1451: .Rs
1452: .%R RFC 4254
1453: .%T "The Secure Shell (SSH) Connection Protocol"
1454: .%D 2006
1455: .Re
1456: .Rs
1457: .%R RFC 4255
1458: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1459: .%D 2006
1460: .Re
1461: .Rs
1462: .%R RFC 4256
1463: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1464: .%D 2006
1465: .Re
1466: .Rs
1467: .%R RFC 4335
1468: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1469: .%D 2006
1470: .Re
1471: .Rs
1472: .%R RFC 4344
1473: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1474: .%D 2006
1475: .Re
1476: .Rs
1477: .%R RFC 4345
1478: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1479: .%D 2006
1480: .Re
1481: .Rs
1482: .%R RFC 4419
1483: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1484: .%D 2006
1485: .Re
1486: .Rs
1487: .%R RFC 4716
1488: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1489: .%D 2006
1.274 grunk 1490: .Re
1491: .Rs
1492: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1493: .%A A. Perrig
1494: .%A D. Song
1495: .%D 1999
1496: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1497: .Re
1.173 jmc 1498: .Sh AUTHORS
1499: OpenSSH is a derivative of the original and free
1500: ssh 1.2.12 release by Tatu Ylonen.
1501: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1502: Theo de Raadt and Dug Song
1503: removed many bugs, re-added newer features and
1504: created OpenSSH.
1505: Markus Friedl contributed the support for SSH
1506: protocol versions 1.5 and 2.0.
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