Annotation of src/usr.bin/ssh/ssh.1, Revision 1.309
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: .\"
1.309 ! jmc 37: .\" $OpenBSD: ssh.1,v 1.308 2010/08/04 05:37:01 djm Exp $
! 38: .Dd $Mdocdate: August 4 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
1.306 jmc 46: .Bk -words
1.278 djm 47: .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
1.108 markus 48: .Op Fl b Ar bind_address
1.51 markus 49: .Op Fl c Ar cipher_spec
1.306 jmc 50: .Op Fl D Oo Ar bind_address : Oc Ns Ar port
1.2 deraadt 51: .Op Fl e Ar escape_char
1.176 jmc 52: .Op Fl F Ar configfile
1.292 jmc 53: .Op Fl I Ar pkcs11
1.2 deraadt 54: .Op Fl i Ar identity_file
1.306 jmc 55: .Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
1.176 jmc 56: .Op Fl l Ar login_name
57: .Op Fl m Ar mac_spec
1.198 djm 58: .Op Fl O Ar ctl_cmd
1.176 jmc 59: .Op Fl o Ar option
60: .Op Fl p Ar port
1.306 jmc 61: .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
1.198 djm 62: .Op Fl S Ar ctl_path
1.290 dtucker 63: .Op Fl W Ar host : Ns Ar port
1.306 jmc 64: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.176 jmc 65: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 66: .Op Ar command
1.306 jmc 67: .Ek
1.44 aaron 68: .Sh DESCRIPTION
1.2 deraadt 69: .Nm
1.96 deraadt 70: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 71: executing commands on a remote machine.
1.176 jmc 72: It is intended to replace rlogin and rsh,
73: and provide secure encrypted communications between
1.40 aaron 74: two untrusted hosts over an insecure network.
1.247 jmc 75: X11 connections and arbitrary TCP ports
1.176 jmc 76: can also be forwarded over the secure channel.
1.2 deraadt 77: .Pp
78: .Nm
1.44 aaron 79: connects and logs into the specified
1.176 jmc 80: .Ar hostname
81: (with optional
82: .Ar user
83: name).
1.1 deraadt 84: The user must prove
1.49 markus 85: his/her identity to the remote machine using one of several methods
1.221 jmc 86: depending on the protocol version used (see below).
1.49 markus 87: .Pp
1.176 jmc 88: If
89: .Ar command
90: is specified,
1.219 jmc 91: it is executed on the remote host instead of a login shell.
1.2 deraadt 92: .Pp
1.218 jmc 93: The options are as follows:
94: .Bl -tag -width Ds
95: .It Fl 1
96: Forces
1.2 deraadt 97: .Nm
1.218 jmc 98: to try protocol version 1 only.
99: .It Fl 2
100: Forces
1.2 deraadt 101: .Nm
1.218 jmc 102: to try protocol version 2 only.
103: .It Fl 4
104: Forces
1.2 deraadt 105: .Nm
1.218 jmc 106: to use IPv4 addresses only.
107: .It Fl 6
108: Forces
1.2 deraadt 109: .Nm
1.218 jmc 110: to use IPv6 addresses only.
111: .It Fl A
112: Enables forwarding of the authentication agent connection.
113: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 114: .Pp
1.218 jmc 115: Agent forwarding should be enabled with caution.
116: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 117: (for the agent's
1.286 sobrado 118: .Ux Ns -domain
119: socket) can access the local agent through the forwarded connection.
1.218 jmc 120: An attacker cannot obtain key material from the agent,
121: however they can perform operations on the keys that enable them to
122: authenticate using the identities loaded into the agent.
123: .It Fl a
124: Disables forwarding of the authentication agent connection.
125: .It Fl b Ar bind_address
126: Use
127: .Ar bind_address
128: on the local machine as the source address
129: of the connection.
130: Only useful on systems with more than one address.
131: .It Fl C
132: Requests compression of all data (including stdin, stdout, stderr, and
1.247 jmc 133: data for forwarded X11 and TCP connections).
1.218 jmc 134: The compression algorithm is the same used by
135: .Xr gzip 1 ,
136: and the
137: .Dq level
138: can be controlled by the
139: .Cm CompressionLevel
140: option for protocol version 1.
141: Compression is desirable on modem lines and other
142: slow connections, but will only slow down things on fast networks.
143: The default value can be set on a host-by-host basis in the
144: configuration files; see the
145: .Cm Compression
146: option.
147: .It Fl c Ar cipher_spec
148: Selects the cipher specification for encrypting the session.
1.2 deraadt 149: .Pp
1.218 jmc 150: Protocol version 1 allows specification of a single cipher.
151: The supported values are
152: .Dq 3des ,
1.220 jmc 153: .Dq blowfish ,
1.218 jmc 154: and
155: .Dq des .
156: .Ar 3des
157: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
158: It is believed to be secure.
159: .Ar blowfish
160: is a fast block cipher; it appears very secure and is much faster than
161: .Ar 3des .
162: .Ar des
163: is only supported in the
1.2 deraadt 164: .Nm
1.218 jmc 165: client for interoperability with legacy protocol 1 implementations
166: that do not support the
167: .Ar 3des
168: cipher.
169: Its use is strongly discouraged due to cryptographic weaknesses.
170: The default is
171: .Dq 3des .
1.49 markus 172: .Pp
1.230 jmc 173: For protocol version 2,
1.218 jmc 174: .Ar cipher_spec
175: is a comma-separated list of ciphers
176: listed in order of preference.
1.283 jmc 177: See the
178: .Cm Ciphers
1.307 dtucker 179: keyword in
180: .Xr ssh_config 5
181: for more information.
1.218 jmc 182: .It Fl D Xo
183: .Sm off
184: .Oo Ar bind_address : Oc
185: .Ar port
186: .Sm on
187: .Xc
188: Specifies a local
189: .Dq dynamic
190: application-level port forwarding.
191: This works by allocating a socket to listen to
192: .Ar port
193: on the local side, optionally bound to the specified
194: .Ar bind_address .
195: Whenever a connection is made to this port, the
196: connection is forwarded over the secure channel, and the application
197: protocol is then used to determine where to connect to from the
198: remote machine.
199: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 200: .Nm
1.218 jmc 201: will act as a SOCKS server.
202: Only root can forward privileged ports.
203: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 204: .Pp
1.308 djm 205: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 206: Only the superuser can forward privileged ports.
207: By default, the local port is bound in accordance with the
208: .Cm GatewayPorts
209: setting.
210: However, an explicit
211: .Ar bind_address
212: may be used to bind the connection to a specific address.
213: The
214: .Ar bind_address
215: of
216: .Dq localhost
217: indicates that the listening port be bound for local use only, while an
218: empty address or
219: .Sq *
220: indicates that the port should be available from all interfaces.
1.229 jmc 221: .It Fl e Ar escape_char
1.218 jmc 222: Sets the escape character for sessions with a pty (default:
223: .Ql ~ ) .
224: The escape character is only recognized at the beginning of a line.
225: The escape character followed by a dot
226: .Pq Ql \&.
227: closes the connection;
228: followed by control-Z suspends the connection;
229: and followed by itself sends the escape character once.
230: Setting the character to
1.2 deraadt 231: .Dq none
1.218 jmc 232: disables any escapes and makes the session fully transparent.
233: .It Fl F Ar configfile
234: Specifies an alternative per-user configuration file.
235: If a configuration file is given on the command line,
236: the system-wide configuration file
237: .Pq Pa /etc/ssh/ssh_config
238: will be ignored.
239: The default for the per-user configuration file is
240: .Pa ~/.ssh/config .
241: .It Fl f
242: Requests
243: .Nm
244: to go to background just before command execution.
245: This is useful if
1.176 jmc 246: .Nm
1.218 jmc 247: is going to ask for passwords or passphrases, but the user
248: wants it in the background.
249: This implies
250: .Fl n .
251: The recommended way to start X11 programs at a remote site is with
252: something like
253: .Ic ssh -f host xterm .
1.277 djm 254: .Pp
255: If the
256: .Cm ExitOnForwardFailure
257: configuration option is set to
258: .Dq yes ,
259: then a client started with
260: .Fl f
261: will wait for all remote port forwards to be successfully established
262: before placing itself in the background.
1.218 jmc 263: .It Fl g
264: Allows remote hosts to connect to local forwarded ports.
1.291 markus 265: .It Fl I Ar pkcs11
1.294 jmc 266: Specify the PKCS#11 shared library
1.176 jmc 267: .Nm
1.293 markus 268: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 269: private RSA key.
270: .It Fl i Ar identity_file
271: Selects a file from which the identity (private key) for
272: RSA or DSA authentication is read.
273: The default is
274: .Pa ~/.ssh/identity
275: for protocol version 1, and
276: .Pa ~/.ssh/id_rsa
1.149 jakob 277: and
1.218 jmc 278: .Pa ~/.ssh/id_dsa
279: for protocol version 2.
280: Identity files may also be specified on
281: a per-host basis in the configuration file.
282: It is possible to have multiple
283: .Fl i
284: options (and multiple identities specified in
285: configuration files).
1.302 djm 286: .Nm
287: will also try to load certificate information from the filename obtained
288: by appending
289: .Pa -cert.pub
290: to identity filenames.
1.269 djm 291: .It Fl K
292: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
293: credentials to the server.
1.218 jmc 294: .It Fl k
295: Disables forwarding (delegation) of GSSAPI credentials to the server.
296: .It Fl L Xo
297: .Sm off
298: .Oo Ar bind_address : Oc
299: .Ar port : host : hostport
300: .Sm on
301: .Xc
302: Specifies that the given port on the local (client) host is to be
303: forwarded to the given host and port on the remote side.
304: This works by allocating a socket to listen to
305: .Ar port
306: on the local side, optionally bound to the specified
307: .Ar bind_address .
308: Whenever a connection is made to this port, the
309: connection is forwarded over the secure channel, and a connection is
310: made to
311: .Ar host
312: port
313: .Ar hostport
314: from the remote machine.
315: Port forwardings can also be specified in the configuration file.
1.308 djm 316: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 317: Only the superuser can forward privileged ports.
318: By default, the local port is bound in accordance with the
319: .Cm GatewayPorts
320: setting.
321: However, an explicit
322: .Ar bind_address
323: may be used to bind the connection to a specific address.
1.2 deraadt 324: The
1.218 jmc 325: .Ar bind_address
326: of
327: .Dq localhost
328: indicates that the listening port be bound for local use only, while an
329: empty address or
330: .Sq *
331: indicates that the port should be available from all interfaces.
332: .It Fl l Ar login_name
333: Specifies the user to log in as on the remote machine.
334: This also may be specified on a per-host basis in the configuration file.
335: .It Fl M
336: Places the
337: .Nm
338: client into
339: .Dq master
340: mode for connection sharing.
1.231 stevesk 341: Multiple
342: .Fl M
343: options places
344: .Nm
345: into
346: .Dq master
347: mode with confirmation required before slave connections are accepted.
1.218 jmc 348: Refer to the description of
349: .Cm ControlMaster
350: in
351: .Xr ssh_config 5
352: for details.
353: .It Fl m Ar mac_spec
354: Additionally, for protocol version 2 a comma-separated list of MAC
355: (message authentication code) algorithms can
356: be specified in order of preference.
357: See the
358: .Cm MACs
359: keyword for more information.
360: .It Fl N
361: Do not execute a remote command.
362: This is useful for just forwarding ports
363: (protocol version 2 only).
364: .It Fl n
365: Redirects stdin from
366: .Pa /dev/null
367: (actually, prevents reading from stdin).
368: This must be used when
1.2 deraadt 369: .Nm
1.218 jmc 370: is run in the background.
371: A common trick is to use this to run X11 programs on a remote machine.
372: For example,
373: .Ic ssh -n shadows.cs.hut.fi emacs &
374: will start an emacs on shadows.cs.hut.fi, and the X11
375: connection will be automatically forwarded over an encrypted channel.
376: The
1.2 deraadt 377: .Nm
1.218 jmc 378: program will be put in the background.
379: (This does not work if
1.2 deraadt 380: .Nm
1.218 jmc 381: needs to ask for a password or passphrase; see also the
382: .Fl f
383: option.)
384: .It Fl O Ar ctl_cmd
385: Control an active connection multiplexing master process.
386: When the
387: .Fl O
388: option is specified, the
389: .Ar ctl_cmd
390: argument is interpreted and passed to the master process.
391: Valid commands are:
392: .Dq check
1.305 markus 393: (check that the master process is running),
394: .Dq forward
395: (request forwardings without command execution) and
1.218 jmc 396: .Dq exit
397: (request the master to exit).
398: .It Fl o Ar option
399: Can be used to give options in the format used in the configuration file.
400: This is useful for specifying options for which there is no separate
401: command-line flag.
402: For full details of the options listed below, and their possible values, see
403: .Xr ssh_config 5 .
1.2 deraadt 404: .Pp
1.218 jmc 405: .Bl -tag -width Ds -offset indent -compact
406: .It AddressFamily
407: .It BatchMode
408: .It BindAddress
409: .It ChallengeResponseAuthentication
410: .It CheckHostIP
411: .It Cipher
412: .It Ciphers
413: .It ClearAllForwardings
414: .It Compression
415: .It CompressionLevel
416: .It ConnectionAttempts
417: .It ConnectTimeout
418: .It ControlMaster
419: .It ControlPath
420: .It DynamicForward
421: .It EscapeChar
1.263 markus 422: .It ExitOnForwardFailure
1.218 jmc 423: .It ForwardAgent
424: .It ForwardX11
425: .It ForwardX11Trusted
426: .It GatewayPorts
427: .It GlobalKnownHostsFile
428: .It GSSAPIAuthentication
429: .It GSSAPIDelegateCredentials
430: .It HashKnownHosts
431: .It Host
432: .It HostbasedAuthentication
433: .It HostKeyAlgorithms
434: .It HostKeyAlias
435: .It HostName
436: .It IdentityFile
437: .It IdentitiesOnly
438: .It KbdInteractiveDevices
439: .It LocalCommand
440: .It LocalForward
441: .It LogLevel
442: .It MACs
443: .It NoHostAuthenticationForLocalhost
444: .It NumberOfPasswordPrompts
445: .It PasswordAuthentication
446: .It PermitLocalCommand
1.291 markus 447: .It PKCS11Provider
1.218 jmc 448: .It Port
449: .It PreferredAuthentications
450: .It Protocol
451: .It ProxyCommand
452: .It PubkeyAuthentication
1.251 dtucker 453: .It RekeyLimit
1.218 jmc 454: .It RemoteForward
455: .It RhostsRSAAuthentication
456: .It RSAAuthentication
457: .It SendEnv
458: .It ServerAliveInterval
459: .It ServerAliveCountMax
460: .It StrictHostKeyChecking
461: .It TCPKeepAlive
462: .It Tunnel
463: .It TunnelDevice
464: .It UsePrivilegedPort
465: .It User
466: .It UserKnownHostsFile
467: .It VerifyHostKeyDNS
1.276 jmc 468: .It VisualHostKey
1.218 jmc 469: .It XAuthLocation
470: .El
471: .It Fl p Ar port
472: Port to connect to on the remote host.
473: This can be specified on a
474: per-host basis in the configuration file.
475: .It Fl q
476: Quiet mode.
1.271 djm 477: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 478: .It Fl R Xo
479: .Sm off
480: .Oo Ar bind_address : Oc
481: .Ar port : host : hostport
482: .Sm on
483: .Xc
484: Specifies that the given port on the remote (server) host is to be
485: forwarded to the given host and port on the local side.
486: This works by allocating a socket to listen to
487: .Ar port
488: on the remote side, and whenever a connection is made to this port, the
489: connection is forwarded over the secure channel, and a connection is
490: made to
491: .Ar host
492: port
493: .Ar hostport
494: from the local machine.
1.2 deraadt 495: .Pp
1.218 jmc 496: Port forwardings can also be specified in the configuration file.
497: Privileged ports can be forwarded only when
498: logging in as root on the remote machine.
1.308 djm 499: IPv6 addresses can be specified by enclosing the address in square braces.
1.194 jakob 500: .Pp
1.218 jmc 501: By default, the listening socket on the server will be bound to the loopback
502: interface only.
1.280 tobias 503: This may be overridden by specifying a
1.218 jmc 504: .Ar bind_address .
505: An empty
506: .Ar bind_address ,
507: or the address
508: .Ql * ,
509: indicates that the remote socket should listen on all interfaces.
510: Specifying a remote
511: .Ar bind_address
512: will only succeed if the server's
513: .Cm GatewayPorts
514: option is enabled (see
515: .Xr sshd_config 5 ) .
1.281 djm 516: .Pp
517: If the
518: .Ar port
519: argument is
1.282 djm 520: .Ql 0 ,
1.281 djm 521: the listen port will be dynamically allocated on the server and reported
522: to the client at run time.
1.305 markus 523: When used together with
524: .Ic -O forward
525: the allocated port will be printed to the standard output.
1.218 jmc 526: .It Fl S Ar ctl_path
1.304 jmc 527: Specifies the location of a control socket for connection sharing,
1.303 djm 528: or the string
529: .Dq none
530: to disable connection sharing.
1.218 jmc 531: Refer to the description of
532: .Cm ControlPath
533: and
534: .Cm ControlMaster
535: in
536: .Xr ssh_config 5
537: for details.
538: .It Fl s
539: May be used to request invocation of a subsystem on the remote system.
540: Subsystems are a feature of the SSH2 protocol which facilitate the use
541: of SSH as a secure transport for other applications (eg.\&
542: .Xr sftp 1 ) .
543: The subsystem is specified as the remote command.
544: .It Fl T
545: Disable pseudo-tty allocation.
546: .It Fl t
547: Force pseudo-tty allocation.
548: This can be used to execute arbitrary
549: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 550: e.g. when implementing menu services.
1.218 jmc 551: Multiple
552: .Fl t
553: options force tty allocation, even if
1.194 jakob 554: .Nm
1.218 jmc 555: has no local tty.
556: .It Fl V
557: Display the version number and exit.
558: .It Fl v
559: Verbose mode.
560: Causes
1.176 jmc 561: .Nm
1.218 jmc 562: to print debugging messages about its progress.
563: This is helpful in
564: debugging connection, authentication, and configuration problems.
565: Multiple
566: .Fl v
567: options increase the verbosity.
568: The maximum is 3.
1.290 dtucker 569: .It Fl W Ar host : Ns Ar port
570: Requests that standard input and output on the client be forwarded to
571: .Ar host
572: on
573: .Ar port
574: over the secure channel.
575: Implies
576: .Fl N ,
577: .Fl T ,
578: .Cm ExitOnForwardFailure
579: and
580: .Cm ClearAllForwardings
581: and works with Protocol version 2 only.
1.261 stevesk 582: .It Fl w Xo
583: .Ar local_tun Ns Op : Ns Ar remote_tun
584: .Xc
585: Requests
586: tunnel
587: device forwarding with the specified
1.218 jmc 588: .Xr tun 4
1.261 stevesk 589: devices between the client
590: .Pq Ar local_tun
591: and the server
592: .Pq Ar remote_tun .
593: .Pp
1.228 jmc 594: The devices may be specified by numerical ID or the keyword
595: .Dq any ,
596: which uses the next available tunnel device.
1.261 stevesk 597: If
598: .Ar remote_tun
599: is not specified, it defaults to
600: .Dq any .
1.228 jmc 601: See also the
1.218 jmc 602: .Cm Tunnel
1.261 stevesk 603: and
604: .Cm TunnelDevice
605: directives in
1.218 jmc 606: .Xr ssh_config 5 .
1.261 stevesk 607: If the
608: .Cm Tunnel
609: directive is unset, it is set to the default tunnel mode, which is
610: .Dq point-to-point .
1.218 jmc 611: .It Fl X
612: Enables X11 forwarding.
1.54 markus 613: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 614: .Pp
1.218 jmc 615: X11 forwarding should be enabled with caution.
1.168 jmc 616: Users with the ability to bypass file permissions on the remote host
1.218 jmc 617: (for the user's X authorization database)
618: can access the local X11 display through the forwarded connection.
619: An attacker may then be able to perform activities such as keystroke monitoring.
620: .Pp
621: For this reason, X11 forwarding is subjected to X11 SECURITY extension
622: restrictions by default.
623: Please refer to the
624: .Nm
625: .Fl Y
626: option and the
627: .Cm ForwardX11Trusted
628: directive in
629: .Xr ssh_config 5
630: for more information.
631: .It Fl x
632: Disables X11 forwarding.
633: .It Fl Y
634: Enables trusted X11 forwarding.
635: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
636: controls.
1.278 djm 637: .It Fl y
638: Send log information using the
639: .Xr syslog 3
640: system module.
641: By default this information is sent to stderr.
1.218 jmc 642: .El
1.224 jmc 643: .Pp
644: .Nm
645: may additionally obtain configuration data from
646: a per-user configuration file and a system-wide configuration file.
647: The file format and configuration options are described in
648: .Xr ssh_config 5 .
649: .Pp
650: .Nm
651: exits with the exit status of the remote command or with 255
652: if an error occurred.
1.222 jmc 653: .Sh AUTHENTICATION
1.249 jmc 654: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 655: The default is to use protocol 2 only,
656: though this can be changed via the
1.222 jmc 657: .Cm Protocol
658: option in
1.284 jmc 659: .Xr ssh_config 5
660: or the
1.222 jmc 661: .Fl 1
662: and
663: .Fl 2
664: options (see above).
665: Both protocols support similar authentication methods,
1.284 jmc 666: but protocol 2 is the default since
1.222 jmc 667: it provides additional mechanisms for confidentiality
668: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 669: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 670: Protocol 1 lacks a strong mechanism for ensuring the
671: integrity of the connection.
672: .Pp
673: The methods available for authentication are:
1.260 jmc 674: GSSAPI-based authentication,
1.222 jmc 675: host-based authentication,
676: public key authentication,
677: challenge-response authentication,
678: and password authentication.
679: Authentication methods are tried in the order specified above,
680: though protocol 2 has a configuration option to change the default order:
681: .Cm PreferredAuthentications .
682: .Pp
683: Host-based authentication works as follows:
1.218 jmc 684: If the machine the user logs in from is listed in
685: .Pa /etc/hosts.equiv
686: or
687: .Pa /etc/shosts.equiv
688: on the remote machine, and the user names are
689: the same on both sides, or if the files
690: .Pa ~/.rhosts
691: or
692: .Pa ~/.shosts
693: exist in the user's home directory on the
694: remote machine and contain a line containing the name of the client
695: machine and the name of the user on that machine, the user is
1.222 jmc 696: considered for login.
697: Additionally, the server
698: .Em must
699: be able to verify the client's
700: host key (see the description of
1.218 jmc 701: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 702: and
1.222 jmc 703: .Pa ~/.ssh/known_hosts ,
704: below)
705: for login to be permitted.
1.218 jmc 706: This authentication method closes security holes due to IP
1.222 jmc 707: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 708: [Note to the administrator:
709: .Pa /etc/hosts.equiv ,
710: .Pa ~/.rhosts ,
711: and the rlogin/rsh protocol in general, are inherently insecure and should be
712: disabled if security is desired.]
1.189 dtucker 713: .Pp
1.222 jmc 714: Public key authentication works as follows:
715: The scheme is based on public-key cryptography,
716: using cryptosystems
717: where encryption and decryption are done using separate keys,
718: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 719: The idea is that each user creates a public/private
720: key pair for authentication purposes.
721: The server knows the public key, and only the user knows the private key.
1.222 jmc 722: .Nm
723: implements public key authentication protocol automatically,
724: using either the RSA or DSA algorithms.
725: Protocol 1 is restricted to using only RSA keys,
726: but protocol 2 may use either.
727: The
728: .Sx HISTORY
729: section of
730: .Xr ssl 8
731: contains a brief discussion of the two algorithms.
1.210 djm 732: .Pp
1.218 jmc 733: The file
734: .Pa ~/.ssh/authorized_keys
735: lists the public keys that are permitted for logging in.
736: When the user logs in, the
1.2 deraadt 737: .Nm
1.218 jmc 738: program tells the server which key pair it would like to use for
739: authentication.
1.222 jmc 740: The client proves that it has access to the private key
741: and the server checks that the corresponding public key
742: is authorized to accept the account.
1.218 jmc 743: .Pp
1.222 jmc 744: The user creates his/her key pair by running
1.218 jmc 745: .Xr ssh-keygen 1 .
746: This stores the private key in
1.207 djm 747: .Pa ~/.ssh/identity
1.222 jmc 748: (protocol 1),
749: .Pa ~/.ssh/id_dsa
750: (protocol 2 DSA),
751: or
752: .Pa ~/.ssh/id_rsa
753: (protocol 2 RSA)
1.218 jmc 754: and stores the public key in
755: .Pa ~/.ssh/identity.pub
1.222 jmc 756: (protocol 1),
757: .Pa ~/.ssh/id_dsa.pub
758: (protocol 2 DSA),
759: or
760: .Pa ~/.ssh/id_rsa.pub
761: (protocol 2 RSA)
1.218 jmc 762: in the user's home directory.
1.222 jmc 763: The user should then copy the public key
1.218 jmc 764: to
765: .Pa ~/.ssh/authorized_keys
1.222 jmc 766: in his/her home directory on the remote machine.
767: The
1.218 jmc 768: .Pa authorized_keys
769: file corresponds to the conventional
770: .Pa ~/.rhosts
771: file, and has one key
1.222 jmc 772: per line, though the lines can be very long.
1.218 jmc 773: After this, the user can log in without giving the password.
774: .Pp
1.301 jmc 775: A variation on public key authentication
776: is available in the form of certificate authentication:
777: instead of a set of public/private keys,
778: signed certificates are used.
779: This has the advantage that a single trusted certification authority
780: can be used in place of many public/private keys.
781: See the
782: .Sx CERTIFICATES
783: section of
784: .Xr ssh-keygen 1
785: for more information.
786: .Pp
787: The most convenient way to use public key or certificate authentication
788: may be with an authentication agent.
1.218 jmc 789: See
790: .Xr ssh-agent 1
791: for more information.
792: .Pp
1.222 jmc 793: Challenge-response authentication works as follows:
794: The server sends an arbitrary
795: .Qq challenge
796: text, and prompts for a response.
797: Protocol 2 allows multiple challenges and responses;
798: protocol 1 is restricted to just one challenge/response.
799: Examples of challenge-response authentication include
800: BSD Authentication (see
801: .Xr login.conf 5 )
802: and PAM (some non-OpenBSD systems).
803: .Pp
804: Finally, if other authentication methods fail,
1.218 jmc 805: .Nm
806: prompts the user for a password.
807: The password is sent to the remote
808: host for checking; however, since all communications are encrypted,
809: the password cannot be seen by someone listening on the network.
1.232 jmc 810: .Pp
811: .Nm
812: automatically maintains and checks a database containing
813: identification for all hosts it has ever been used with.
814: Host keys are stored in
815: .Pa ~/.ssh/known_hosts
816: in the user's home directory.
817: Additionally, the file
818: .Pa /etc/ssh/ssh_known_hosts
819: is automatically checked for known hosts.
820: Any new hosts are automatically added to the user's file.
821: If a host's identification ever changes,
822: .Nm
823: warns about this and disables password authentication to prevent
824: server spoofing or man-in-the-middle attacks,
825: which could otherwise be used to circumvent the encryption.
826: The
827: .Cm StrictHostKeyChecking
828: option can be used to control logins to machines whose
829: host key is not known or has changed.
830: .Pp
1.218 jmc 831: When the user's identity has been accepted by the server, the server
832: either executes the given command, or logs into the machine and gives
833: the user a normal shell on the remote machine.
834: All communication with
835: the remote command or shell will be automatically encrypted.
836: .Pp
837: If a pseudo-terminal has been allocated (normal login session), the
838: user may use the escape characters noted below.
839: .Pp
840: If no pseudo-tty has been allocated,
841: the session is transparent and can be used to reliably transfer binary data.
842: On most systems, setting the escape character to
843: .Dq none
844: will also make the session transparent even if a tty is used.
845: .Pp
846: The session terminates when the command or shell on the remote
1.247 jmc 847: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 848: .Sh ESCAPE CHARACTERS
1.218 jmc 849: When a pseudo-terminal has been requested,
1.2 deraadt 850: .Nm
1.218 jmc 851: supports a number of functions through the use of an escape character.
852: .Pp
853: A single tilde character can be sent as
854: .Ic ~~
855: or by following the tilde by a character other than those described below.
856: The escape character must always follow a newline to be interpreted as
857: special.
858: The escape character can be changed in configuration files using the
859: .Cm EscapeChar
860: configuration directive or on the command line by the
861: .Fl e
862: option.
863: .Pp
864: The supported escapes (assuming the default
865: .Ql ~ )
866: are:
867: .Bl -tag -width Ds
868: .It Cm ~.
869: Disconnect.
870: .It Cm ~^Z
871: Background
1.234 jmc 872: .Nm .
1.218 jmc 873: .It Cm ~#
874: List forwarded connections.
875: .It Cm ~&
876: Background
1.2 deraadt 877: .Nm
1.218 jmc 878: at logout when waiting for forwarded connection / X11 sessions to terminate.
879: .It Cm ~?
880: Display a list of escape characters.
881: .It Cm ~B
882: Send a BREAK to the remote system
883: (only useful for SSH protocol version 2 and if the peer supports it).
884: .It Cm ~C
885: Open command line.
886: Currently this allows the addition of port forwardings using the
1.279 stevesk 887: .Fl L ,
888: .Fl R
1.218 jmc 889: and
1.279 stevesk 890: .Fl D
1.225 jmc 891: options (see above).
1.218 jmc 892: It also allows the cancellation of existing remote port-forwardings
893: using
1.262 stevesk 894: .Sm off
895: .Fl KR Oo Ar bind_address : Oc Ar port .
896: .Sm on
1.218 jmc 897: .Ic !\& Ns Ar command
898: allows the user to execute a local command if the
899: .Ic PermitLocalCommand
900: option is enabled in
1.176 jmc 901: .Xr ssh_config 5 .
1.218 jmc 902: Basic help is available, using the
903: .Fl h
904: option.
905: .It Cm ~R
906: Request rekeying of the connection
907: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 908: .El
1.246 jmc 909: .Sh TCP FORWARDING
910: Forwarding of arbitrary TCP connections over the secure channel can
911: be specified either on the command line or in a configuration file.
912: One possible application of TCP forwarding is a secure connection to a
913: mail server; another is going through firewalls.
914: .Pp
915: In the example below, we look at encrypting communication between
916: an IRC client and server, even though the IRC server does not directly
917: support encrypted communications.
918: This works as follows:
919: the user connects to the remote host using
920: .Nm ,
921: specifying a port to be used to forward connections
922: to the remote server.
923: After that it is possible to start the service which is to be encrypted
924: on the client machine,
925: connecting to the same local port,
926: and
927: .Nm
928: will encrypt and forward the connection.
929: .Pp
930: The following example tunnels an IRC session from client machine
931: .Dq 127.0.0.1
932: (localhost)
933: to remote server
934: .Dq server.example.com :
935: .Bd -literal -offset 4n
936: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
937: $ irc -c '#users' -p 1234 pinky 127.0.0.1
938: .Ed
939: .Pp
940: This tunnels a connection to IRC server
941: .Dq server.example.com ,
942: joining channel
943: .Dq #users ,
944: nickname
945: .Dq pinky ,
946: using port 1234.
947: It doesn't matter which port is used,
948: as long as it's greater than 1023
949: (remember, only root can open sockets on privileged ports)
950: and doesn't conflict with any ports already in use.
951: The connection is forwarded to port 6667 on the remote server,
952: since that's the standard port for IRC services.
953: .Pp
954: The
955: .Fl f
956: option backgrounds
957: .Nm
958: and the remote command
959: .Dq sleep 10
960: is specified to allow an amount of time
961: (10 seconds, in the example)
962: to start the service which is to be tunnelled.
963: If no connections are made within the time specified,
964: .Nm
965: will exit.
966: .Sh X11 FORWARDING
1.218 jmc 967: If the
968: .Cm ForwardX11
969: variable is set to
970: .Dq yes
971: (or see the description of the
1.227 jmc 972: .Fl X ,
973: .Fl x ,
1.218 jmc 974: and
1.227 jmc 975: .Fl Y
1.226 jmc 976: options above)
1.218 jmc 977: and the user is using X11 (the
978: .Ev DISPLAY
979: environment variable is set), the connection to the X11 display is
980: automatically forwarded to the remote side in such a way that any X11
981: programs started from the shell (or command) will go through the
982: encrypted channel, and the connection to the real X server will be made
1.176 jmc 983: from the local machine.
1.218 jmc 984: The user should not manually set
985: .Ev DISPLAY .
986: Forwarding of X11 connections can be
987: configured on the command line or in configuration files.
988: .Pp
989: The
990: .Ev DISPLAY
991: value set by
992: .Nm
993: will point to the server machine, but with a display number greater than zero.
994: This is normal, and happens because
995: .Nm
996: creates a
997: .Dq proxy
998: X server on the server machine for forwarding the
999: connections over the encrypted channel.
1.200 djm 1000: .Pp
1.218 jmc 1001: .Nm
1002: will also automatically set up Xauthority data on the server machine.
1003: For this purpose, it will generate a random authorization cookie,
1004: store it in Xauthority on the server, and verify that any forwarded
1005: connections carry this cookie and replace it by the real cookie when
1006: the connection is opened.
1007: The real authentication cookie is never
1008: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1009: .Pp
1.218 jmc 1010: If the
1011: .Cm ForwardAgent
1012: variable is set to
1013: .Dq yes
1014: (or see the description of the
1015: .Fl A
1.191 djm 1016: and
1.218 jmc 1017: .Fl a
1.226 jmc 1018: options above) and
1.218 jmc 1019: the user is using an authentication agent, the connection to the agent
1020: is automatically forwarded to the remote side.
1.252 jmc 1021: .Sh VERIFYING HOST KEYS
1022: When connecting to a server for the first time,
1023: a fingerprint of the server's public key is presented to the user
1024: (unless the option
1025: .Cm StrictHostKeyChecking
1026: has been disabled).
1027: Fingerprints can be determined using
1028: .Xr ssh-keygen 1 :
1029: .Pp
1030: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1031: .Pp
1.274 grunk 1032: If the fingerprint is already known, it can be matched
1033: and the key can be accepted or rejected.
1034: Because of the difficulty of comparing host keys
1035: just by looking at hex strings,
1036: there is also support to compare host keys visually,
1037: using
1038: .Em random art .
1039: By setting the
1.275 grunk 1040: .Cm VisualHostKey
1.274 grunk 1041: option to
1.275 grunk 1042: .Dq yes ,
1.274 grunk 1043: a small ASCII graphic gets displayed on every login to a server, no matter
1044: if the session itself is interactive or not.
1045: By learning the pattern a known server produces, a user can easily
1046: find out that the host key has changed when a completely different pattern
1047: is displayed.
1048: Because these patterns are not unambiguous however, a pattern that looks
1049: similar to the pattern remembered only gives a good probability that the
1050: host key is the same, not guaranteed proof.
1051: .Pp
1052: To get a listing of the fingerprints along with their random art for
1053: all known hosts, the following command line can be used:
1054: .Pp
1055: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1056: .Pp
1.252 jmc 1057: If the fingerprint is unknown,
1058: an alternative method of verification is available:
1059: SSH fingerprints verified by DNS.
1060: An additional resource record (RR),
1061: SSHFP,
1062: is added to a zonefile
1063: and the connecting client is able to match the fingerprint
1064: with that of the key presented.
1065: .Pp
1066: In this example, we are connecting a client to a server,
1067: .Dq host.example.com .
1068: The SSHFP resource records should first be added to the zonefile for
1069: host.example.com:
1070: .Bd -literal -offset indent
1.259 jakob 1071: $ ssh-keygen -r host.example.com.
1.252 jmc 1072: .Ed
1073: .Pp
1074: The output lines will have to be added to the zonefile.
1075: To check that the zone is answering fingerprint queries:
1076: .Pp
1077: .Dl $ dig -t SSHFP host.example.com
1078: .Pp
1079: Finally the client connects:
1080: .Bd -literal -offset indent
1081: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1082: [...]
1083: Matching host key fingerprint found in DNS.
1084: Are you sure you want to continue connecting (yes/no)?
1085: .Ed
1086: .Pp
1087: See the
1088: .Cm VerifyHostKeyDNS
1089: option in
1090: .Xr ssh_config 5
1091: for more information.
1.250 jmc 1092: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1093: .Nm
1094: contains support for Virtual Private Network (VPN) tunnelling
1095: using the
1096: .Xr tun 4
1097: network pseudo-device,
1098: allowing two networks to be joined securely.
1099: The
1100: .Xr sshd_config 5
1101: configuration option
1102: .Cm PermitTunnel
1103: controls whether the server supports this,
1104: and at what level (layer 2 or 3 traffic).
1105: .Pp
1106: The following example would connect client network 10.0.50.0/24
1.265 otto 1107: with remote network 10.0.99.0/24 using a point-to-point connection
1108: from 10.1.1.1 to 10.1.1.2,
1109: provided that the SSH server running on the gateway to the remote network,
1110: at 192.168.1.15, allows it.
1111: .Pp
1112: On the client:
1.250 jmc 1113: .Bd -literal -offset indent
1114: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1115: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1116: # route add 10.0.99.0/24 10.1.1.2
1117: .Ed
1118: .Pp
1119: On the server:
1120: .Bd -literal -offset indent
1121: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1122: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1123: .Ed
1124: .Pp
1125: Client access may be more finely tuned via the
1126: .Pa /root/.ssh/authorized_keys
1127: file (see below) and the
1128: .Cm PermitRootLogin
1129: server option.
1.255 jmc 1130: The following entry would permit connections on
1.250 jmc 1131: .Xr tun 4
1.255 jmc 1132: device 1 from user
1.250 jmc 1133: .Dq jane
1.255 jmc 1134: and on tun device 2 from user
1.250 jmc 1135: .Dq john ,
1136: if
1137: .Cm PermitRootLogin
1138: is set to
1139: .Dq forced-commands-only :
1140: .Bd -literal -offset 2n
1141: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1142: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1143: .Ed
1144: .Pp
1.264 ray 1145: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1146: it may be more suited to temporary setups,
1147: such as for wireless VPNs.
1148: More permanent VPNs are better provided by tools such as
1149: .Xr ipsecctl 8
1150: and
1151: .Xr isakmpd 8 .
1.2 deraadt 1152: .Sh ENVIRONMENT
1153: .Nm
1.1 deraadt 1154: will normally set the following environment variables:
1.237 jmc 1155: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1156: .It Ev DISPLAY
1157: The
1158: .Ev DISPLAY
1.40 aaron 1159: variable indicates the location of the X11 server.
1.44 aaron 1160: It is automatically set by
1.2 deraadt 1161: .Nm
1162: to point to a value of the form
1.233 jmc 1163: .Dq hostname:n ,
1164: where
1165: .Dq hostname
1166: indicates the host where the shell runs, and
1167: .Sq n
1168: is an integer \*(Ge 1.
1.40 aaron 1169: .Nm
1170: uses this special value to forward X11 connections over the secure
1171: channel.
1.107 markus 1172: The user should normally not set
1173: .Ev DISPLAY
1174: explicitly, as that
1.1 deraadt 1175: will render the X11 connection insecure (and will require the user to
1176: manually copy any required authorization cookies).
1.2 deraadt 1177: .It Ev HOME
1.1 deraadt 1178: Set to the path of the user's home directory.
1.2 deraadt 1179: .It Ev LOGNAME
1180: Synonym for
1.12 aaron 1181: .Ev USER ;
1182: set for compatibility with systems that use this variable.
1.2 deraadt 1183: .It Ev MAIL
1.129 stevesk 1184: Set to the path of the user's mailbox.
1.40 aaron 1185: .It Ev PATH
1.2 deraadt 1186: Set to the default
1187: .Ev PATH ,
1188: as specified when compiling
1.234 jmc 1189: .Nm .
1.118 markus 1190: .It Ev SSH_ASKPASS
1191: If
1192: .Nm
1193: needs a passphrase, it will read the passphrase from the current
1194: terminal if it was run from a terminal.
1195: If
1196: .Nm
1197: does not have a terminal associated with it but
1198: .Ev DISPLAY
1199: and
1200: .Ev SSH_ASKPASS
1201: are set, it will execute the program specified by
1202: .Ev SSH_ASKPASS
1203: and open an X11 window to read the passphrase.
1204: This is particularly useful when calling
1205: .Nm
1206: from a
1.196 jmc 1207: .Pa .xsession
1.118 markus 1208: or related script.
1209: (Note that on some machines it
1210: may be necessary to redirect the input from
1211: .Pa /dev/null
1212: to make this work.)
1.18 markus 1213: .It Ev SSH_AUTH_SOCK
1.233 jmc 1214: Identifies the path of a
1215: .Ux Ns -domain
1216: socket used to communicate with the agent.
1.166 stevesk 1217: .It Ev SSH_CONNECTION
1218: Identifies the client and server ends of the connection.
1.40 aaron 1219: The variable contains
1.233 jmc 1220: four space-separated values: client IP address, client port number,
1221: server IP address, and server port number.
1.73 markus 1222: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1223: This variable contains the original command line if a forced command
1.73 markus 1224: is executed.
1225: It can be used to extract the original arguments.
1.2 deraadt 1226: .It Ev SSH_TTY
1.1 deraadt 1227: This is set to the name of the tty (path to the device) associated
1.40 aaron 1228: with the current shell or command.
1229: If the current session has no tty,
1.1 deraadt 1230: this variable is not set.
1.2 deraadt 1231: .It Ev TZ
1.214 jmc 1232: This variable is set to indicate the present time zone if it
1.257 jmc 1233: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1234: on to new connections).
1.2 deraadt 1235: .It Ev USER
1.1 deraadt 1236: Set to the name of the user logging in.
1.2 deraadt 1237: .El
1238: .Pp
1.44 aaron 1239: Additionally,
1.2 deraadt 1240: .Nm
1.44 aaron 1241: reads
1.207 djm 1242: .Pa ~/.ssh/environment ,
1.2 deraadt 1243: and adds lines of the format
1244: .Dq VARNAME=value
1.233 jmc 1245: to the environment if the file exists and users are allowed to
1.161 marc 1246: change their environment.
1.176 jmc 1247: For more information, see the
1.161 marc 1248: .Cm PermitUserEnvironment
1.162 stevesk 1249: option in
1.161 marc 1250: .Xr sshd_config 5 .
1.2 deraadt 1251: .Sh FILES
1.236 jmc 1252: .Bl -tag -width Ds -compact
1.309 ! jmc 1253: .It Pa ~/.rhosts
1.240 jmc 1254: This file is used for host-based authentication (see above).
1.92 markus 1255: On some machines this file may need to be
1.240 jmc 1256: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1257: because
1.2 deraadt 1258: .Xr sshd 8
1.40 aaron 1259: reads it as root.
1260: Additionally, this file must be owned by the user,
1261: and must not have write permissions for anyone else.
1262: The recommended
1.1 deraadt 1263: permission for most machines is read/write for the user, and not
1264: accessible by others.
1.2 deraadt 1265: .Pp
1.309 ! jmc 1266: .It Pa ~/.shosts
1.240 jmc 1267: This file is used in exactly the same way as
1268: .Pa .rhosts ,
1269: but allows host-based authentication without permitting login with
1270: rlogin/rsh.
1.272 mcbride 1271: .Pp
1.309 ! jmc 1272: .It Pa ~/.ssh/
1.272 mcbride 1273: This directory is the default location for all user-specific configuration
1274: and authentication information.
1275: There is no general requirement to keep the entire contents of this directory
1276: secret, but the recommended permissions are read/write/execute for the user,
1277: and not accessible by others.
1.236 jmc 1278: .Pp
1.309 ! jmc 1279: .It Pa ~/.ssh/authorized_keys
1.238 jmc 1280: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1281: The format of this file is described in the
1282: .Xr sshd 8
1283: manual page.
1284: This file is not highly sensitive, but the recommended
1285: permissions are read/write for the user, and not accessible by others.
1286: .Pp
1.309 ! jmc 1287: .It Pa ~/.ssh/config
1.238 jmc 1288: This is the per-user configuration file.
1289: The file format and configuration options are described in
1290: .Xr ssh_config 5 .
1291: Because of the potential for abuse, this file must have strict permissions:
1292: read/write for the user, and not accessible by others.
1293: .Pp
1.309 ! jmc 1294: .It Pa ~/.ssh/environment
1.239 jmc 1295: Contains additional definitions for environment variables; see
1296: .Sx ENVIRONMENT ,
1.238 jmc 1297: above.
1298: .Pp
1.309 ! jmc 1299: .It Pa ~/.ssh/identity
! 1300: .It Pa ~/.ssh/id_dsa
! 1301: .It Pa ~/.ssh/id_rsa
1.238 jmc 1302: Contains the private key for authentication.
1303: These files
1304: contain sensitive data and should be readable by the user but not
1305: accessible by others (read/write/execute).
1306: .Nm
1307: will simply ignore a private key file if it is accessible by others.
1308: It is possible to specify a passphrase when
1309: generating the key which will be used to encrypt the
1310: sensitive part of this file using 3DES.
1311: .Pp
1.309 ! jmc 1312: .It Pa ~/.ssh/identity.pub
! 1313: .It Pa ~/.ssh/id_dsa.pub
! 1314: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1315: Contains the public key for authentication.
1316: These files are not
1317: sensitive and can (but need not) be readable by anyone.
1318: .Pp
1.309 ! jmc 1319: .It Pa ~/.ssh/known_hosts
1.244 jmc 1320: Contains a list of host keys for all hosts the user has logged into
1321: that are not already in the systemwide list of known host keys.
1.238 jmc 1322: See
1.244 jmc 1323: .Xr sshd 8
1324: for further details of the format of this file.
1.238 jmc 1325: .Pp
1.309 ! jmc 1326: .It Pa ~/.ssh/rc
1.238 jmc 1327: Commands in this file are executed by
1328: .Nm
1.245 jmc 1329: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1330: started.
1331: See the
1332: .Xr sshd 8
1333: manual page for more information.
1334: .Pp
1.309 ! jmc 1335: .It Pa /etc/hosts.equiv
1.240 jmc 1336: This file is for host-based authentication (see above).
1337: It should only be writable by root.
1.236 jmc 1338: .Pp
1.309 ! jmc 1339: .It Pa /etc/shosts.equiv
1.240 jmc 1340: This file is used in exactly the same way as
1341: .Pa hosts.equiv ,
1342: but allows host-based authentication without permitting login with
1343: rlogin/rsh.
1.236 jmc 1344: .Pp
1.238 jmc 1345: .It Pa /etc/ssh/ssh_config
1346: Systemwide configuration file.
1347: The file format and configuration options are described in
1348: .Xr ssh_config 5 .
1349: .Pp
1.309 ! jmc 1350: .It Pa /etc/ssh/ssh_host_key
! 1351: .It Pa /etc/ssh/ssh_host_dsa_key
! 1352: .It Pa /etc/ssh/ssh_host_rsa_key
1.238 jmc 1353: These three files contain the private parts of the host keys
1.245 jmc 1354: and are used for host-based authentication.
1355: If protocol version 1 is used,
1.238 jmc 1356: .Nm
1357: must be setuid root, since the host key is readable only by root.
1358: For protocol version 2,
1359: .Nm
1360: uses
1361: .Xr ssh-keysign 8
1.245 jmc 1362: to access the host keys,
1363: eliminating the requirement that
1.238 jmc 1364: .Nm
1.245 jmc 1365: be setuid root when host-based authentication is used.
1.238 jmc 1366: By default
1.2 deraadt 1367: .Nm
1.238 jmc 1368: is not setuid root.
1369: .Pp
1.309 ! jmc 1370: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1371: Systemwide list of known host keys.
1372: This file should be prepared by the
1373: system administrator to contain the public host keys of all machines in the
1374: organization.
1.244 jmc 1375: It should be world-readable.
1376: See
1.238 jmc 1377: .Xr sshd 8
1.244 jmc 1378: for further details of the format of this file.
1.236 jmc 1379: .Pp
1.309 ! jmc 1380: .It Pa /etc/ssh/sshrc
1.1 deraadt 1381: Commands in this file are executed by
1.2 deraadt 1382: .Nm
1.245 jmc 1383: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1384: See the
1.2 deraadt 1385: .Xr sshd 8
1.1 deraadt 1386: manual page for more information.
1.58 itojun 1387: .El
1.2 deraadt 1388: .Sh SEE ALSO
1389: .Xr scp 1 ,
1.83 djm 1390: .Xr sftp 1 ,
1.2 deraadt 1391: .Xr ssh-add 1 ,
1392: .Xr ssh-agent 1 ,
1393: .Xr ssh-keygen 1 ,
1.242 jmc 1394: .Xr ssh-keyscan 1 ,
1.250 jmc 1395: .Xr tun 4 ,
1.176 jmc 1396: .Xr hosts.equiv 5 ,
1.159 stevesk 1397: .Xr ssh_config 5 ,
1.160 naddy 1398: .Xr ssh-keysign 8 ,
1.87 itojun 1399: .Xr sshd 8
1.106 markus 1400: .Rs
1.256 jmc 1401: .%R RFC 4250
1402: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1403: .%D 2006
1404: .Re
1405: .Rs
1406: .%R RFC 4251
1407: .%T "The Secure Shell (SSH) Protocol Architecture"
1408: .%D 2006
1409: .Re
1410: .Rs
1411: .%R RFC 4252
1412: .%T "The Secure Shell (SSH) Authentication Protocol"
1413: .%D 2006
1414: .Re
1415: .Rs
1416: .%R RFC 4253
1417: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1418: .%D 2006
1419: .Re
1420: .Rs
1421: .%R RFC 4254
1422: .%T "The Secure Shell (SSH) Connection Protocol"
1423: .%D 2006
1424: .Re
1425: .Rs
1426: .%R RFC 4255
1427: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1428: .%D 2006
1429: .Re
1430: .Rs
1431: .%R RFC 4256
1432: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1433: .%D 2006
1434: .Re
1435: .Rs
1436: .%R RFC 4335
1437: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1438: .%D 2006
1439: .Re
1440: .Rs
1441: .%R RFC 4344
1442: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1443: .%D 2006
1444: .Re
1445: .Rs
1446: .%R RFC 4345
1447: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1448: .%D 2006
1449: .Re
1450: .Rs
1451: .%R RFC 4419
1452: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1453: .%D 2006
1454: .Re
1455: .Rs
1456: .%R RFC 4716
1457: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1458: .%D 2006
1.274 grunk 1459: .Re
1460: .Rs
1461: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1462: .%A A. Perrig
1463: .%A D. Song
1464: .%D 1999
1465: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1466: .Re
1.173 jmc 1467: .Sh AUTHORS
1468: OpenSSH is a derivative of the original and free
1469: ssh 1.2.12 release by Tatu Ylonen.
1470: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1471: Theo de Raadt and Dug Song
1472: removed many bugs, re-added newer features and
1473: created OpenSSH.
1474: Markus Friedl contributed the support for SSH
1475: protocol versions 1.5 and 2.0.
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