Annotation of src/usr.bin/ssh/ssh.1, Revision 1.310
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: .\"
1.310 ! djm 37: .\" $OpenBSD: ssh.1,v 1.309 2010/08/08 19:36:30 jmc Exp $
! 38: .Dd $Mdocdate: August 8 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
1.310 ! djm 272: public key authentication is read.
1.218 jmc 273: The default is
274: .Pa ~/.ssh/identity
275: for protocol version 1, and
1.310 ! djm 276: .Pa ~/.ssh/id_dsa ,
! 277: .Pa ~/.ssh/id_ecdsa
! 278: and
1.218 jmc 279: .Pa ~/.ssh/id_rsa
280: for protocol version 2.
281: Identity files may also be specified on
282: a per-host basis in the configuration file.
283: It is possible to have multiple
284: .Fl i
285: options (and multiple identities specified in
286: configuration files).
1.302 djm 287: .Nm
288: will also try to load certificate information from the filename obtained
289: by appending
290: .Pa -cert.pub
291: to identity filenames.
1.269 djm 292: .It Fl K
293: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
294: credentials to the server.
1.218 jmc 295: .It Fl k
296: Disables forwarding (delegation) of GSSAPI credentials to the server.
297: .It Fl L Xo
298: .Sm off
299: .Oo Ar bind_address : Oc
300: .Ar port : host : hostport
301: .Sm on
302: .Xc
303: Specifies that the given port on the local (client) host is to be
304: forwarded to the given host and port on the remote side.
305: This works by allocating a socket to listen to
306: .Ar port
307: on the local side, optionally bound to the specified
308: .Ar bind_address .
309: Whenever a connection is made to this port, the
310: connection is forwarded over the secure channel, and a connection is
311: made to
312: .Ar host
313: port
314: .Ar hostport
315: from the remote machine.
316: Port forwardings can also be specified in the configuration file.
1.308 djm 317: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 318: Only the superuser can forward privileged ports.
319: By default, the local port is bound in accordance with the
320: .Cm GatewayPorts
321: setting.
322: However, an explicit
323: .Ar bind_address
324: may be used to bind the connection to a specific address.
1.2 deraadt 325: The
1.218 jmc 326: .Ar bind_address
327: of
328: .Dq localhost
329: indicates that the listening port be bound for local use only, while an
330: empty address or
331: .Sq *
332: indicates that the port should be available from all interfaces.
333: .It Fl l Ar login_name
334: Specifies the user to log in as on the remote machine.
335: This also may be specified on a per-host basis in the configuration file.
336: .It Fl M
337: Places the
338: .Nm
339: client into
340: .Dq master
341: mode for connection sharing.
1.231 stevesk 342: Multiple
343: .Fl M
344: options places
345: .Nm
346: into
347: .Dq master
348: mode with confirmation required before slave connections are accepted.
1.218 jmc 349: Refer to the description of
350: .Cm ControlMaster
351: in
352: .Xr ssh_config 5
353: for details.
354: .It Fl m Ar mac_spec
355: Additionally, for protocol version 2 a comma-separated list of MAC
356: (message authentication code) algorithms can
357: be specified in order of preference.
358: See the
359: .Cm MACs
360: keyword for more information.
361: .It Fl N
362: Do not execute a remote command.
363: This is useful for just forwarding ports
364: (protocol version 2 only).
365: .It Fl n
366: Redirects stdin from
367: .Pa /dev/null
368: (actually, prevents reading from stdin).
369: This must be used when
1.2 deraadt 370: .Nm
1.218 jmc 371: is run in the background.
372: A common trick is to use this to run X11 programs on a remote machine.
373: For example,
374: .Ic ssh -n shadows.cs.hut.fi emacs &
375: will start an emacs on shadows.cs.hut.fi, and the X11
376: connection will be automatically forwarded over an encrypted channel.
377: The
1.2 deraadt 378: .Nm
1.218 jmc 379: program will be put in the background.
380: (This does not work if
1.2 deraadt 381: .Nm
1.218 jmc 382: needs to ask for a password or passphrase; see also the
383: .Fl f
384: option.)
385: .It Fl O Ar ctl_cmd
386: Control an active connection multiplexing master process.
387: When the
388: .Fl O
389: option is specified, the
390: .Ar ctl_cmd
391: argument is interpreted and passed to the master process.
392: Valid commands are:
393: .Dq check
1.305 markus 394: (check that the master process is running),
395: .Dq forward
396: (request forwardings without command execution) and
1.218 jmc 397: .Dq exit
398: (request the master to exit).
399: .It Fl o Ar option
400: Can be used to give options in the format used in the configuration file.
401: This is useful for specifying options for which there is no separate
402: command-line flag.
403: For full details of the options listed below, and their possible values, see
404: .Xr ssh_config 5 .
1.2 deraadt 405: .Pp
1.218 jmc 406: .Bl -tag -width Ds -offset indent -compact
407: .It AddressFamily
408: .It BatchMode
409: .It BindAddress
410: .It ChallengeResponseAuthentication
411: .It CheckHostIP
412: .It Cipher
413: .It Ciphers
414: .It ClearAllForwardings
415: .It Compression
416: .It CompressionLevel
417: .It ConnectionAttempts
418: .It ConnectTimeout
419: .It ControlMaster
420: .It ControlPath
421: .It DynamicForward
422: .It EscapeChar
1.263 markus 423: .It ExitOnForwardFailure
1.218 jmc 424: .It ForwardAgent
425: .It ForwardX11
426: .It ForwardX11Trusted
427: .It GatewayPorts
428: .It GlobalKnownHostsFile
429: .It GSSAPIAuthentication
430: .It GSSAPIDelegateCredentials
431: .It HashKnownHosts
432: .It Host
433: .It HostbasedAuthentication
434: .It HostKeyAlgorithms
435: .It HostKeyAlias
436: .It HostName
437: .It IdentityFile
438: .It IdentitiesOnly
439: .It KbdInteractiveDevices
440: .It LocalCommand
441: .It LocalForward
442: .It LogLevel
443: .It MACs
444: .It NoHostAuthenticationForLocalhost
445: .It NumberOfPasswordPrompts
446: .It PasswordAuthentication
447: .It PermitLocalCommand
1.291 markus 448: .It PKCS11Provider
1.218 jmc 449: .It Port
450: .It PreferredAuthentications
451: .It Protocol
452: .It ProxyCommand
453: .It PubkeyAuthentication
1.251 dtucker 454: .It RekeyLimit
1.218 jmc 455: .It RemoteForward
456: .It RhostsRSAAuthentication
457: .It RSAAuthentication
458: .It SendEnv
459: .It ServerAliveInterval
460: .It ServerAliveCountMax
461: .It StrictHostKeyChecking
462: .It TCPKeepAlive
463: .It Tunnel
464: .It TunnelDevice
465: .It UsePrivilegedPort
466: .It User
467: .It UserKnownHostsFile
468: .It VerifyHostKeyDNS
1.276 jmc 469: .It VisualHostKey
1.218 jmc 470: .It XAuthLocation
471: .El
472: .It Fl p Ar port
473: Port to connect to on the remote host.
474: This can be specified on a
475: per-host basis in the configuration file.
476: .It Fl q
477: Quiet mode.
1.271 djm 478: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 479: .It Fl R Xo
480: .Sm off
481: .Oo Ar bind_address : Oc
482: .Ar port : host : hostport
483: .Sm on
484: .Xc
485: Specifies that the given port on the remote (server) host is to be
486: forwarded to the given host and port on the local side.
487: This works by allocating a socket to listen to
488: .Ar port
489: on the remote side, and whenever a connection is made to this port, the
490: connection is forwarded over the secure channel, and a connection is
491: made to
492: .Ar host
493: port
494: .Ar hostport
495: from the local machine.
1.2 deraadt 496: .Pp
1.218 jmc 497: Port forwardings can also be specified in the configuration file.
498: Privileged ports can be forwarded only when
499: logging in as root on the remote machine.
1.308 djm 500: IPv6 addresses can be specified by enclosing the address in square braces.
1.194 jakob 501: .Pp
1.218 jmc 502: By default, the listening socket on the server will be bound to the loopback
503: interface only.
1.280 tobias 504: This may be overridden by specifying a
1.218 jmc 505: .Ar bind_address .
506: An empty
507: .Ar bind_address ,
508: or the address
509: .Ql * ,
510: indicates that the remote socket should listen on all interfaces.
511: Specifying a remote
512: .Ar bind_address
513: will only succeed if the server's
514: .Cm GatewayPorts
515: option is enabled (see
516: .Xr sshd_config 5 ) .
1.281 djm 517: .Pp
518: If the
519: .Ar port
520: argument is
1.282 djm 521: .Ql 0 ,
1.281 djm 522: the listen port will be dynamically allocated on the server and reported
523: to the client at run time.
1.305 markus 524: When used together with
525: .Ic -O forward
526: the allocated port will be printed to the standard output.
1.218 jmc 527: .It Fl S Ar ctl_path
1.304 jmc 528: Specifies the location of a control socket for connection sharing,
1.303 djm 529: or the string
530: .Dq none
531: to disable connection sharing.
1.218 jmc 532: Refer to the description of
533: .Cm ControlPath
534: and
535: .Cm ControlMaster
536: in
537: .Xr ssh_config 5
538: for details.
539: .It Fl s
540: May be used to request invocation of a subsystem on the remote system.
541: Subsystems are a feature of the SSH2 protocol which facilitate the use
542: of SSH as a secure transport for other applications (eg.\&
543: .Xr sftp 1 ) .
544: The subsystem is specified as the remote command.
545: .It Fl T
546: Disable pseudo-tty allocation.
547: .It Fl t
548: Force pseudo-tty allocation.
549: This can be used to execute arbitrary
550: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 551: e.g. when implementing menu services.
1.218 jmc 552: Multiple
553: .Fl t
554: options force tty allocation, even if
1.194 jakob 555: .Nm
1.218 jmc 556: has no local tty.
557: .It Fl V
558: Display the version number and exit.
559: .It Fl v
560: Verbose mode.
561: Causes
1.176 jmc 562: .Nm
1.218 jmc 563: to print debugging messages about its progress.
564: This is helpful in
565: debugging connection, authentication, and configuration problems.
566: Multiple
567: .Fl v
568: options increase the verbosity.
569: The maximum is 3.
1.290 dtucker 570: .It Fl W Ar host : Ns Ar port
571: Requests that standard input and output on the client be forwarded to
572: .Ar host
573: on
574: .Ar port
575: over the secure channel.
576: Implies
577: .Fl N ,
578: .Fl T ,
579: .Cm ExitOnForwardFailure
580: and
581: .Cm ClearAllForwardings
582: and works with Protocol version 2 only.
1.261 stevesk 583: .It Fl w Xo
584: .Ar local_tun Ns Op : Ns Ar remote_tun
585: .Xc
586: Requests
587: tunnel
588: device forwarding with the specified
1.218 jmc 589: .Xr tun 4
1.261 stevesk 590: devices between the client
591: .Pq Ar local_tun
592: and the server
593: .Pq Ar remote_tun .
594: .Pp
1.228 jmc 595: The devices may be specified by numerical ID or the keyword
596: .Dq any ,
597: which uses the next available tunnel device.
1.261 stevesk 598: If
599: .Ar remote_tun
600: is not specified, it defaults to
601: .Dq any .
1.228 jmc 602: See also the
1.218 jmc 603: .Cm Tunnel
1.261 stevesk 604: and
605: .Cm TunnelDevice
606: directives in
1.218 jmc 607: .Xr ssh_config 5 .
1.261 stevesk 608: If the
609: .Cm Tunnel
610: directive is unset, it is set to the default tunnel mode, which is
611: .Dq point-to-point .
1.218 jmc 612: .It Fl X
613: Enables X11 forwarding.
1.54 markus 614: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 615: .Pp
1.218 jmc 616: X11 forwarding should be enabled with caution.
1.168 jmc 617: Users with the ability to bypass file permissions on the remote host
1.218 jmc 618: (for the user's X authorization database)
619: can access the local X11 display through the forwarded connection.
620: An attacker may then be able to perform activities such as keystroke monitoring.
621: .Pp
622: For this reason, X11 forwarding is subjected to X11 SECURITY extension
623: restrictions by default.
624: Please refer to the
625: .Nm
626: .Fl Y
627: option and the
628: .Cm ForwardX11Trusted
629: directive in
630: .Xr ssh_config 5
631: for more information.
632: .It Fl x
633: Disables X11 forwarding.
634: .It Fl Y
635: Enables trusted X11 forwarding.
636: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
637: controls.
1.278 djm 638: .It Fl y
639: Send log information using the
640: .Xr syslog 3
641: system module.
642: By default this information is sent to stderr.
1.218 jmc 643: .El
1.224 jmc 644: .Pp
645: .Nm
646: may additionally obtain configuration data from
647: a per-user configuration file and a system-wide configuration file.
648: The file format and configuration options are described in
649: .Xr ssh_config 5 .
650: .Pp
651: .Nm
652: exits with the exit status of the remote command or with 255
653: if an error occurred.
1.222 jmc 654: .Sh AUTHENTICATION
1.249 jmc 655: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 656: The default is to use protocol 2 only,
657: though this can be changed via the
1.222 jmc 658: .Cm Protocol
659: option in
1.284 jmc 660: .Xr ssh_config 5
661: or the
1.222 jmc 662: .Fl 1
663: and
664: .Fl 2
665: options (see above).
666: Both protocols support similar authentication methods,
1.284 jmc 667: but protocol 2 is the default since
1.222 jmc 668: it provides additional mechanisms for confidentiality
669: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 670: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 671: Protocol 1 lacks a strong mechanism for ensuring the
672: integrity of the connection.
673: .Pp
674: The methods available for authentication are:
1.260 jmc 675: GSSAPI-based authentication,
1.222 jmc 676: host-based authentication,
677: public key authentication,
678: challenge-response authentication,
679: and password authentication.
680: Authentication methods are tried in the order specified above,
681: though protocol 2 has a configuration option to change the default order:
682: .Cm PreferredAuthentications .
683: .Pp
684: Host-based authentication works as follows:
1.218 jmc 685: If the machine the user logs in from is listed in
686: .Pa /etc/hosts.equiv
687: or
688: .Pa /etc/shosts.equiv
689: on the remote machine, and the user names are
690: the same on both sides, or if the files
691: .Pa ~/.rhosts
692: or
693: .Pa ~/.shosts
694: exist in the user's home directory on the
695: remote machine and contain a line containing the name of the client
696: machine and the name of the user on that machine, the user is
1.222 jmc 697: considered for login.
698: Additionally, the server
699: .Em must
700: be able to verify the client's
701: host key (see the description of
1.218 jmc 702: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 703: and
1.222 jmc 704: .Pa ~/.ssh/known_hosts ,
705: below)
706: for login to be permitted.
1.218 jmc 707: This authentication method closes security holes due to IP
1.222 jmc 708: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 709: [Note to the administrator:
710: .Pa /etc/hosts.equiv ,
711: .Pa ~/.rhosts ,
712: and the rlogin/rsh protocol in general, are inherently insecure and should be
713: disabled if security is desired.]
1.189 dtucker 714: .Pp
1.222 jmc 715: Public key authentication works as follows:
716: The scheme is based on public-key cryptography,
717: using cryptosystems
718: where encryption and decryption are done using separate keys,
719: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 720: The idea is that each user creates a public/private
721: key pair for authentication purposes.
722: The server knows the public key, and only the user knows the private key.
1.222 jmc 723: .Nm
724: implements public key authentication protocol automatically,
1.310 ! djm 725: using one of the DSA, ECDSA or RSA algorithms.
1.222 jmc 726: Protocol 1 is restricted to using only RSA keys,
1.310 ! djm 727: but protocol 2 may use any.
1.222 jmc 728: The
729: .Sx HISTORY
730: section of
731: .Xr ssl 8
732: contains a brief discussion of the two algorithms.
1.210 djm 733: .Pp
1.218 jmc 734: The file
735: .Pa ~/.ssh/authorized_keys
736: lists the public keys that are permitted for logging in.
737: When the user logs in, the
1.2 deraadt 738: .Nm
1.218 jmc 739: program tells the server which key pair it would like to use for
740: authentication.
1.222 jmc 741: The client proves that it has access to the private key
742: and the server checks that the corresponding public key
743: is authorized to accept the account.
1.218 jmc 744: .Pp
1.222 jmc 745: The user creates his/her key pair by running
1.218 jmc 746: .Xr ssh-keygen 1 .
747: This stores the private key in
1.207 djm 748: .Pa ~/.ssh/identity
1.222 jmc 749: (protocol 1),
750: .Pa ~/.ssh/id_dsa
751: (protocol 2 DSA),
1.310 ! djm 752: .Pa ~/.ssh/id_ecdsa
! 753: (protocol 2 ECDSA),
1.222 jmc 754: or
755: .Pa ~/.ssh/id_rsa
756: (protocol 2 RSA)
1.218 jmc 757: and stores the public key in
758: .Pa ~/.ssh/identity.pub
1.222 jmc 759: (protocol 1),
760: .Pa ~/.ssh/id_dsa.pub
761: (protocol 2 DSA),
1.310 ! djm 762: .Pa ~/.ssh/id_ecdsa.pub
! 763: (protocol 2 ECDSA),
1.222 jmc 764: or
765: .Pa ~/.ssh/id_rsa.pub
766: (protocol 2 RSA)
1.218 jmc 767: in the user's home directory.
1.222 jmc 768: The user should then copy the public key
1.218 jmc 769: to
770: .Pa ~/.ssh/authorized_keys
1.222 jmc 771: in his/her home directory on the remote machine.
772: The
1.218 jmc 773: .Pa authorized_keys
774: file corresponds to the conventional
775: .Pa ~/.rhosts
776: file, and has one key
1.222 jmc 777: per line, though the lines can be very long.
1.218 jmc 778: After this, the user can log in without giving the password.
779: .Pp
1.301 jmc 780: A variation on public key authentication
781: is available in the form of certificate authentication:
782: instead of a set of public/private keys,
783: signed certificates are used.
784: This has the advantage that a single trusted certification authority
785: can be used in place of many public/private keys.
786: See the
787: .Sx CERTIFICATES
788: section of
789: .Xr ssh-keygen 1
790: for more information.
791: .Pp
792: The most convenient way to use public key or certificate authentication
793: may be with an authentication agent.
1.218 jmc 794: See
795: .Xr ssh-agent 1
796: for more information.
797: .Pp
1.222 jmc 798: Challenge-response authentication works as follows:
799: The server sends an arbitrary
800: .Qq challenge
801: text, and prompts for a response.
802: Protocol 2 allows multiple challenges and responses;
803: protocol 1 is restricted to just one challenge/response.
804: Examples of challenge-response authentication include
805: BSD Authentication (see
806: .Xr login.conf 5 )
807: and PAM (some non-OpenBSD systems).
808: .Pp
809: Finally, if other authentication methods fail,
1.218 jmc 810: .Nm
811: prompts the user for a password.
812: The password is sent to the remote
813: host for checking; however, since all communications are encrypted,
814: the password cannot be seen by someone listening on the network.
1.232 jmc 815: .Pp
816: .Nm
817: automatically maintains and checks a database containing
818: identification for all hosts it has ever been used with.
819: Host keys are stored in
820: .Pa ~/.ssh/known_hosts
821: in the user's home directory.
822: Additionally, the file
823: .Pa /etc/ssh/ssh_known_hosts
824: is automatically checked for known hosts.
825: Any new hosts are automatically added to the user's file.
826: If a host's identification ever changes,
827: .Nm
828: warns about this and disables password authentication to prevent
829: server spoofing or man-in-the-middle attacks,
830: which could otherwise be used to circumvent the encryption.
831: The
832: .Cm StrictHostKeyChecking
833: option can be used to control logins to machines whose
834: host key is not known or has changed.
835: .Pp
1.218 jmc 836: When the user's identity has been accepted by the server, the server
837: either executes the given command, or logs into the machine and gives
838: the user a normal shell on the remote machine.
839: All communication with
840: the remote command or shell will be automatically encrypted.
841: .Pp
842: If a pseudo-terminal has been allocated (normal login session), the
843: user may use the escape characters noted below.
844: .Pp
845: If no pseudo-tty has been allocated,
846: the session is transparent and can be used to reliably transfer binary data.
847: On most systems, setting the escape character to
848: .Dq none
849: will also make the session transparent even if a tty is used.
850: .Pp
851: The session terminates when the command or shell on the remote
1.247 jmc 852: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 853: .Sh ESCAPE CHARACTERS
1.218 jmc 854: When a pseudo-terminal has been requested,
1.2 deraadt 855: .Nm
1.218 jmc 856: supports a number of functions through the use of an escape character.
857: .Pp
858: A single tilde character can be sent as
859: .Ic ~~
860: or by following the tilde by a character other than those described below.
861: The escape character must always follow a newline to be interpreted as
862: special.
863: The escape character can be changed in configuration files using the
864: .Cm EscapeChar
865: configuration directive or on the command line by the
866: .Fl e
867: option.
868: .Pp
869: The supported escapes (assuming the default
870: .Ql ~ )
871: are:
872: .Bl -tag -width Ds
873: .It Cm ~.
874: Disconnect.
875: .It Cm ~^Z
876: Background
1.234 jmc 877: .Nm .
1.218 jmc 878: .It Cm ~#
879: List forwarded connections.
880: .It Cm ~&
881: Background
1.2 deraadt 882: .Nm
1.218 jmc 883: at logout when waiting for forwarded connection / X11 sessions to terminate.
884: .It Cm ~?
885: Display a list of escape characters.
886: .It Cm ~B
887: Send a BREAK to the remote system
888: (only useful for SSH protocol version 2 and if the peer supports it).
889: .It Cm ~C
890: Open command line.
891: Currently this allows the addition of port forwardings using the
1.279 stevesk 892: .Fl L ,
893: .Fl R
1.218 jmc 894: and
1.279 stevesk 895: .Fl D
1.225 jmc 896: options (see above).
1.218 jmc 897: It also allows the cancellation of existing remote port-forwardings
898: using
1.262 stevesk 899: .Sm off
900: .Fl KR Oo Ar bind_address : Oc Ar port .
901: .Sm on
1.218 jmc 902: .Ic !\& Ns Ar command
903: allows the user to execute a local command if the
904: .Ic PermitLocalCommand
905: option is enabled in
1.176 jmc 906: .Xr ssh_config 5 .
1.218 jmc 907: Basic help is available, using the
908: .Fl h
909: option.
910: .It Cm ~R
911: Request rekeying of the connection
912: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 913: .El
1.246 jmc 914: .Sh TCP FORWARDING
915: Forwarding of arbitrary TCP connections over the secure channel can
916: be specified either on the command line or in a configuration file.
917: One possible application of TCP forwarding is a secure connection to a
918: mail server; another is going through firewalls.
919: .Pp
920: In the example below, we look at encrypting communication between
921: an IRC client and server, even though the IRC server does not directly
922: support encrypted communications.
923: This works as follows:
924: the user connects to the remote host using
925: .Nm ,
926: specifying a port to be used to forward connections
927: to the remote server.
928: After that it is possible to start the service which is to be encrypted
929: on the client machine,
930: connecting to the same local port,
931: and
932: .Nm
933: will encrypt and forward the connection.
934: .Pp
935: The following example tunnels an IRC session from client machine
936: .Dq 127.0.0.1
937: (localhost)
938: to remote server
939: .Dq server.example.com :
940: .Bd -literal -offset 4n
941: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
942: $ irc -c '#users' -p 1234 pinky 127.0.0.1
943: .Ed
944: .Pp
945: This tunnels a connection to IRC server
946: .Dq server.example.com ,
947: joining channel
948: .Dq #users ,
949: nickname
950: .Dq pinky ,
951: using port 1234.
952: It doesn't matter which port is used,
953: as long as it's greater than 1023
954: (remember, only root can open sockets on privileged ports)
955: and doesn't conflict with any ports already in use.
956: The connection is forwarded to port 6667 on the remote server,
957: since that's the standard port for IRC services.
958: .Pp
959: The
960: .Fl f
961: option backgrounds
962: .Nm
963: and the remote command
964: .Dq sleep 10
965: is specified to allow an amount of time
966: (10 seconds, in the example)
967: to start the service which is to be tunnelled.
968: If no connections are made within the time specified,
969: .Nm
970: will exit.
971: .Sh X11 FORWARDING
1.218 jmc 972: If the
973: .Cm ForwardX11
974: variable is set to
975: .Dq yes
976: (or see the description of the
1.227 jmc 977: .Fl X ,
978: .Fl x ,
1.218 jmc 979: and
1.227 jmc 980: .Fl Y
1.226 jmc 981: options above)
1.218 jmc 982: and the user is using X11 (the
983: .Ev DISPLAY
984: environment variable is set), the connection to the X11 display is
985: automatically forwarded to the remote side in such a way that any X11
986: programs started from the shell (or command) will go through the
987: encrypted channel, and the connection to the real X server will be made
1.176 jmc 988: from the local machine.
1.218 jmc 989: The user should not manually set
990: .Ev DISPLAY .
991: Forwarding of X11 connections can be
992: configured on the command line or in configuration files.
993: .Pp
994: The
995: .Ev DISPLAY
996: value set by
997: .Nm
998: will point to the server machine, but with a display number greater than zero.
999: This is normal, and happens because
1000: .Nm
1001: creates a
1002: .Dq proxy
1003: X server on the server machine for forwarding the
1004: connections over the encrypted channel.
1.200 djm 1005: .Pp
1.218 jmc 1006: .Nm
1007: will also automatically set up Xauthority data on the server machine.
1008: For this purpose, it will generate a random authorization cookie,
1009: store it in Xauthority on the server, and verify that any forwarded
1010: connections carry this cookie and replace it by the real cookie when
1011: the connection is opened.
1012: The real authentication cookie is never
1013: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1014: .Pp
1.218 jmc 1015: If the
1016: .Cm ForwardAgent
1017: variable is set to
1018: .Dq yes
1019: (or see the description of the
1020: .Fl A
1.191 djm 1021: and
1.218 jmc 1022: .Fl a
1.226 jmc 1023: options above) and
1.218 jmc 1024: the user is using an authentication agent, the connection to the agent
1025: is automatically forwarded to the remote side.
1.252 jmc 1026: .Sh VERIFYING HOST KEYS
1027: When connecting to a server for the first time,
1028: a fingerprint of the server's public key is presented to the user
1029: (unless the option
1030: .Cm StrictHostKeyChecking
1031: has been disabled).
1032: Fingerprints can be determined using
1033: .Xr ssh-keygen 1 :
1034: .Pp
1035: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1036: .Pp
1.274 grunk 1037: If the fingerprint is already known, it can be matched
1038: and the key can be accepted or rejected.
1039: Because of the difficulty of comparing host keys
1040: just by looking at hex strings,
1041: there is also support to compare host keys visually,
1042: using
1043: .Em random art .
1044: By setting the
1.275 grunk 1045: .Cm VisualHostKey
1.274 grunk 1046: option to
1.275 grunk 1047: .Dq yes ,
1.274 grunk 1048: a small ASCII graphic gets displayed on every login to a server, no matter
1049: if the session itself is interactive or not.
1050: By learning the pattern a known server produces, a user can easily
1051: find out that the host key has changed when a completely different pattern
1052: is displayed.
1053: Because these patterns are not unambiguous however, a pattern that looks
1054: similar to the pattern remembered only gives a good probability that the
1055: host key is the same, not guaranteed proof.
1056: .Pp
1057: To get a listing of the fingerprints along with their random art for
1058: all known hosts, the following command line can be used:
1059: .Pp
1060: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1061: .Pp
1.252 jmc 1062: If the fingerprint is unknown,
1063: an alternative method of verification is available:
1064: SSH fingerprints verified by DNS.
1065: An additional resource record (RR),
1066: SSHFP,
1067: is added to a zonefile
1068: and the connecting client is able to match the fingerprint
1069: with that of the key presented.
1070: .Pp
1071: In this example, we are connecting a client to a server,
1072: .Dq host.example.com .
1073: The SSHFP resource records should first be added to the zonefile for
1074: host.example.com:
1075: .Bd -literal -offset indent
1.259 jakob 1076: $ ssh-keygen -r host.example.com.
1.252 jmc 1077: .Ed
1078: .Pp
1079: The output lines will have to be added to the zonefile.
1080: To check that the zone is answering fingerprint queries:
1081: .Pp
1082: .Dl $ dig -t SSHFP host.example.com
1083: .Pp
1084: Finally the client connects:
1085: .Bd -literal -offset indent
1086: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1087: [...]
1088: Matching host key fingerprint found in DNS.
1089: Are you sure you want to continue connecting (yes/no)?
1090: .Ed
1091: .Pp
1092: See the
1093: .Cm VerifyHostKeyDNS
1094: option in
1095: .Xr ssh_config 5
1096: for more information.
1.250 jmc 1097: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1098: .Nm
1099: contains support for Virtual Private Network (VPN) tunnelling
1100: using the
1101: .Xr tun 4
1102: network pseudo-device,
1103: allowing two networks to be joined securely.
1104: The
1105: .Xr sshd_config 5
1106: configuration option
1107: .Cm PermitTunnel
1108: controls whether the server supports this,
1109: and at what level (layer 2 or 3 traffic).
1110: .Pp
1111: The following example would connect client network 10.0.50.0/24
1.265 otto 1112: with remote network 10.0.99.0/24 using a point-to-point connection
1113: from 10.1.1.1 to 10.1.1.2,
1114: provided that the SSH server running on the gateway to the remote network,
1115: at 192.168.1.15, allows it.
1116: .Pp
1117: On the client:
1.250 jmc 1118: .Bd -literal -offset indent
1119: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1120: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1121: # route add 10.0.99.0/24 10.1.1.2
1122: .Ed
1123: .Pp
1124: On the server:
1125: .Bd -literal -offset indent
1126: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1127: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1128: .Ed
1129: .Pp
1130: Client access may be more finely tuned via the
1131: .Pa /root/.ssh/authorized_keys
1132: file (see below) and the
1133: .Cm PermitRootLogin
1134: server option.
1.255 jmc 1135: The following entry would permit connections on
1.250 jmc 1136: .Xr tun 4
1.255 jmc 1137: device 1 from user
1.250 jmc 1138: .Dq jane
1.255 jmc 1139: and on tun device 2 from user
1.250 jmc 1140: .Dq john ,
1141: if
1142: .Cm PermitRootLogin
1143: is set to
1144: .Dq forced-commands-only :
1145: .Bd -literal -offset 2n
1146: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1147: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1148: .Ed
1149: .Pp
1.264 ray 1150: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1151: it may be more suited to temporary setups,
1152: such as for wireless VPNs.
1153: More permanent VPNs are better provided by tools such as
1154: .Xr ipsecctl 8
1155: and
1156: .Xr isakmpd 8 .
1.2 deraadt 1157: .Sh ENVIRONMENT
1158: .Nm
1.1 deraadt 1159: will normally set the following environment variables:
1.237 jmc 1160: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1161: .It Ev DISPLAY
1162: The
1163: .Ev DISPLAY
1.40 aaron 1164: variable indicates the location of the X11 server.
1.44 aaron 1165: It is automatically set by
1.2 deraadt 1166: .Nm
1167: to point to a value of the form
1.233 jmc 1168: .Dq hostname:n ,
1169: where
1170: .Dq hostname
1171: indicates the host where the shell runs, and
1172: .Sq n
1173: is an integer \*(Ge 1.
1.40 aaron 1174: .Nm
1175: uses this special value to forward X11 connections over the secure
1176: channel.
1.107 markus 1177: The user should normally not set
1178: .Ev DISPLAY
1179: explicitly, as that
1.1 deraadt 1180: will render the X11 connection insecure (and will require the user to
1181: manually copy any required authorization cookies).
1.2 deraadt 1182: .It Ev HOME
1.1 deraadt 1183: Set to the path of the user's home directory.
1.2 deraadt 1184: .It Ev LOGNAME
1185: Synonym for
1.12 aaron 1186: .Ev USER ;
1187: set for compatibility with systems that use this variable.
1.2 deraadt 1188: .It Ev MAIL
1.129 stevesk 1189: Set to the path of the user's mailbox.
1.40 aaron 1190: .It Ev PATH
1.2 deraadt 1191: Set to the default
1192: .Ev PATH ,
1193: as specified when compiling
1.234 jmc 1194: .Nm .
1.118 markus 1195: .It Ev SSH_ASKPASS
1196: If
1197: .Nm
1198: needs a passphrase, it will read the passphrase from the current
1199: terminal if it was run from a terminal.
1200: If
1201: .Nm
1202: does not have a terminal associated with it but
1203: .Ev DISPLAY
1204: and
1205: .Ev SSH_ASKPASS
1206: are set, it will execute the program specified by
1207: .Ev SSH_ASKPASS
1208: and open an X11 window to read the passphrase.
1209: This is particularly useful when calling
1210: .Nm
1211: from a
1.196 jmc 1212: .Pa .xsession
1.118 markus 1213: or related script.
1214: (Note that on some machines it
1215: may be necessary to redirect the input from
1216: .Pa /dev/null
1217: to make this work.)
1.18 markus 1218: .It Ev SSH_AUTH_SOCK
1.233 jmc 1219: Identifies the path of a
1220: .Ux Ns -domain
1221: socket used to communicate with the agent.
1.166 stevesk 1222: .It Ev SSH_CONNECTION
1223: Identifies the client and server ends of the connection.
1.40 aaron 1224: The variable contains
1.233 jmc 1225: four space-separated values: client IP address, client port number,
1226: server IP address, and server port number.
1.73 markus 1227: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1228: This variable contains the original command line if a forced command
1.73 markus 1229: is executed.
1230: It can be used to extract the original arguments.
1.2 deraadt 1231: .It Ev SSH_TTY
1.1 deraadt 1232: This is set to the name of the tty (path to the device) associated
1.40 aaron 1233: with the current shell or command.
1234: If the current session has no tty,
1.1 deraadt 1235: this variable is not set.
1.2 deraadt 1236: .It Ev TZ
1.214 jmc 1237: This variable is set to indicate the present time zone if it
1.257 jmc 1238: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1239: on to new connections).
1.2 deraadt 1240: .It Ev USER
1.1 deraadt 1241: Set to the name of the user logging in.
1.2 deraadt 1242: .El
1243: .Pp
1.44 aaron 1244: Additionally,
1.2 deraadt 1245: .Nm
1.44 aaron 1246: reads
1.207 djm 1247: .Pa ~/.ssh/environment ,
1.2 deraadt 1248: and adds lines of the format
1249: .Dq VARNAME=value
1.233 jmc 1250: to the environment if the file exists and users are allowed to
1.161 marc 1251: change their environment.
1.176 jmc 1252: For more information, see the
1.161 marc 1253: .Cm PermitUserEnvironment
1.162 stevesk 1254: option in
1.161 marc 1255: .Xr sshd_config 5 .
1.2 deraadt 1256: .Sh FILES
1.236 jmc 1257: .Bl -tag -width Ds -compact
1.309 jmc 1258: .It Pa ~/.rhosts
1.240 jmc 1259: This file is used for host-based authentication (see above).
1.92 markus 1260: On some machines this file may need to be
1.240 jmc 1261: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1262: because
1.2 deraadt 1263: .Xr sshd 8
1.40 aaron 1264: reads it as root.
1265: Additionally, this file must be owned by the user,
1266: and must not have write permissions for anyone else.
1267: The recommended
1.1 deraadt 1268: permission for most machines is read/write for the user, and not
1269: accessible by others.
1.2 deraadt 1270: .Pp
1.309 jmc 1271: .It Pa ~/.shosts
1.240 jmc 1272: This file is used in exactly the same way as
1273: .Pa .rhosts ,
1274: but allows host-based authentication without permitting login with
1275: rlogin/rsh.
1.272 mcbride 1276: .Pp
1.309 jmc 1277: .It Pa ~/.ssh/
1.272 mcbride 1278: This directory is the default location for all user-specific configuration
1279: and authentication information.
1280: There is no general requirement to keep the entire contents of this directory
1281: secret, but the recommended permissions are read/write/execute for the user,
1282: and not accessible by others.
1.236 jmc 1283: .Pp
1.309 jmc 1284: .It Pa ~/.ssh/authorized_keys
1.310 ! djm 1285: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
! 1286: this user.
1.238 jmc 1287: The format of this file is described in the
1288: .Xr sshd 8
1289: manual page.
1290: This file is not highly sensitive, but the recommended
1291: permissions are read/write for the user, and not accessible by others.
1292: .Pp
1.309 jmc 1293: .It Pa ~/.ssh/config
1.238 jmc 1294: This is the per-user configuration file.
1295: The file format and configuration options are described in
1296: .Xr ssh_config 5 .
1297: Because of the potential for abuse, this file must have strict permissions:
1298: read/write for the user, and not accessible by others.
1299: .Pp
1.309 jmc 1300: .It Pa ~/.ssh/environment
1.239 jmc 1301: Contains additional definitions for environment variables; see
1302: .Sx ENVIRONMENT ,
1.238 jmc 1303: above.
1304: .Pp
1.309 jmc 1305: .It Pa ~/.ssh/identity
1306: .It Pa ~/.ssh/id_dsa
1.310 ! djm 1307: .It Pa ~/.ssh/id_ecdsa
1.309 jmc 1308: .It Pa ~/.ssh/id_rsa
1.238 jmc 1309: Contains the private key for authentication.
1310: These files
1311: contain sensitive data and should be readable by the user but not
1312: accessible by others (read/write/execute).
1313: .Nm
1314: will simply ignore a private key file if it is accessible by others.
1315: It is possible to specify a passphrase when
1316: generating the key which will be used to encrypt the
1317: sensitive part of this file using 3DES.
1318: .Pp
1.309 jmc 1319: .It Pa ~/.ssh/identity.pub
1320: .It Pa ~/.ssh/id_dsa.pub
1.310 ! djm 1321: .It Pa ~/.ssh/id_ecdsa.pub
1.309 jmc 1322: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1323: Contains the public key for authentication.
1324: These files are not
1325: sensitive and can (but need not) be readable by anyone.
1326: .Pp
1.309 jmc 1327: .It Pa ~/.ssh/known_hosts
1.244 jmc 1328: Contains a list of host keys for all hosts the user has logged into
1329: that are not already in the systemwide list of known host keys.
1.238 jmc 1330: See
1.244 jmc 1331: .Xr sshd 8
1332: for further details of the format of this file.
1.238 jmc 1333: .Pp
1.309 jmc 1334: .It Pa ~/.ssh/rc
1.238 jmc 1335: Commands in this file are executed by
1336: .Nm
1.245 jmc 1337: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1338: started.
1339: See the
1340: .Xr sshd 8
1341: manual page for more information.
1342: .Pp
1.309 jmc 1343: .It Pa /etc/hosts.equiv
1.240 jmc 1344: This file is for host-based authentication (see above).
1345: It should only be writable by root.
1.236 jmc 1346: .Pp
1.309 jmc 1347: .It Pa /etc/shosts.equiv
1.240 jmc 1348: This file is used in exactly the same way as
1349: .Pa hosts.equiv ,
1350: but allows host-based authentication without permitting login with
1351: rlogin/rsh.
1.236 jmc 1352: .Pp
1.238 jmc 1353: .It Pa /etc/ssh/ssh_config
1354: Systemwide configuration file.
1355: The file format and configuration options are described in
1356: .Xr ssh_config 5 .
1357: .Pp
1.309 jmc 1358: .It Pa /etc/ssh/ssh_host_key
1359: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 ! djm 1360: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.309 jmc 1361: .It Pa /etc/ssh/ssh_host_rsa_key
1.238 jmc 1362: These three files contain the private parts of the host keys
1.245 jmc 1363: and are used for host-based authentication.
1364: If protocol version 1 is used,
1.238 jmc 1365: .Nm
1366: must be setuid root, since the host key is readable only by root.
1367: For protocol version 2,
1368: .Nm
1369: uses
1370: .Xr ssh-keysign 8
1.245 jmc 1371: to access the host keys,
1372: eliminating the requirement that
1.238 jmc 1373: .Nm
1.245 jmc 1374: be setuid root when host-based authentication is used.
1.238 jmc 1375: By default
1.2 deraadt 1376: .Nm
1.238 jmc 1377: is not setuid root.
1378: .Pp
1.309 jmc 1379: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1380: Systemwide list of known host keys.
1381: This file should be prepared by the
1382: system administrator to contain the public host keys of all machines in the
1383: organization.
1.244 jmc 1384: It should be world-readable.
1385: See
1.238 jmc 1386: .Xr sshd 8
1.244 jmc 1387: for further details of the format of this file.
1.236 jmc 1388: .Pp
1.309 jmc 1389: .It Pa /etc/ssh/sshrc
1.1 deraadt 1390: Commands in this file are executed by
1.2 deraadt 1391: .Nm
1.245 jmc 1392: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1393: See the
1.2 deraadt 1394: .Xr sshd 8
1.1 deraadt 1395: manual page for more information.
1.58 itojun 1396: .El
1.2 deraadt 1397: .Sh SEE ALSO
1398: .Xr scp 1 ,
1.83 djm 1399: .Xr sftp 1 ,
1.2 deraadt 1400: .Xr ssh-add 1 ,
1401: .Xr ssh-agent 1 ,
1402: .Xr ssh-keygen 1 ,
1.242 jmc 1403: .Xr ssh-keyscan 1 ,
1.250 jmc 1404: .Xr tun 4 ,
1.176 jmc 1405: .Xr hosts.equiv 5 ,
1.159 stevesk 1406: .Xr ssh_config 5 ,
1.160 naddy 1407: .Xr ssh-keysign 8 ,
1.87 itojun 1408: .Xr sshd 8
1.106 markus 1409: .Rs
1.256 jmc 1410: .%R RFC 4250
1411: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1412: .%D 2006
1413: .Re
1414: .Rs
1415: .%R RFC 4251
1416: .%T "The Secure Shell (SSH) Protocol Architecture"
1417: .%D 2006
1418: .Re
1419: .Rs
1420: .%R RFC 4252
1421: .%T "The Secure Shell (SSH) Authentication Protocol"
1422: .%D 2006
1423: .Re
1424: .Rs
1425: .%R RFC 4253
1426: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1427: .%D 2006
1428: .Re
1429: .Rs
1430: .%R RFC 4254
1431: .%T "The Secure Shell (SSH) Connection Protocol"
1432: .%D 2006
1433: .Re
1434: .Rs
1435: .%R RFC 4255
1436: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1437: .%D 2006
1438: .Re
1439: .Rs
1440: .%R RFC 4256
1441: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1442: .%D 2006
1443: .Re
1444: .Rs
1445: .%R RFC 4335
1446: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1447: .%D 2006
1448: .Re
1449: .Rs
1450: .%R RFC 4344
1451: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1452: .%D 2006
1453: .Re
1454: .Rs
1455: .%R RFC 4345
1456: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1457: .%D 2006
1458: .Re
1459: .Rs
1460: .%R RFC 4419
1461: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1462: .%D 2006
1463: .Re
1464: .Rs
1465: .%R RFC 4716
1466: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1467: .%D 2006
1.274 grunk 1468: .Re
1469: .Rs
1470: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1471: .%A A. Perrig
1472: .%A D. Song
1473: .%D 1999
1474: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1475: .Re
1.173 jmc 1476: .Sh AUTHORS
1477: OpenSSH is a derivative of the original and free
1478: ssh 1.2.12 release by Tatu Ylonen.
1479: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1480: Theo de Raadt and Dug Song
1481: removed many bugs, re-added newer features and
1482: created OpenSSH.
1483: Markus Friedl contributed the support for SSH
1484: protocol versions 1.5 and 2.0.
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