Annotation of src/usr.bin/ssh/ssh.1, Revision 1.313
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: .\"
1.313 ! djm 37: .\" $OpenBSD: ssh.1,v 1.312 2010/09/04 09:38:34 jmc Exp $
! 38: .Dd $Mdocdate: September 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
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 .
1.222 jmc 650: .Sh AUTHENTICATION
1.249 jmc 651: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 652: The default is to use protocol 2 only,
653: though this can be changed via the
1.222 jmc 654: .Cm Protocol
655: option in
1.284 jmc 656: .Xr ssh_config 5
657: or the
1.222 jmc 658: .Fl 1
659: and
660: .Fl 2
661: options (see above).
662: Both protocols support similar authentication methods,
1.284 jmc 663: but protocol 2 is the default since
1.222 jmc 664: it provides additional mechanisms for confidentiality
665: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 666: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 667: Protocol 1 lacks a strong mechanism for ensuring the
668: integrity of the connection.
669: .Pp
670: The methods available for authentication are:
1.260 jmc 671: GSSAPI-based authentication,
1.222 jmc 672: host-based authentication,
673: public key authentication,
674: challenge-response authentication,
675: and password authentication.
676: Authentication methods are tried in the order specified above,
677: though protocol 2 has a configuration option to change the default order:
678: .Cm PreferredAuthentications .
679: .Pp
680: Host-based authentication works as follows:
1.218 jmc 681: If the machine the user logs in from is listed in
682: .Pa /etc/hosts.equiv
683: or
684: .Pa /etc/shosts.equiv
685: on the remote machine, and the user names are
686: the same on both sides, or if the files
687: .Pa ~/.rhosts
688: or
689: .Pa ~/.shosts
690: exist in the user's home directory on the
691: remote machine and contain a line containing the name of the client
692: machine and the name of the user on that machine, the user is
1.222 jmc 693: considered for login.
694: Additionally, the server
695: .Em must
696: be able to verify the client's
697: host key (see the description of
1.218 jmc 698: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 699: and
1.222 jmc 700: .Pa ~/.ssh/known_hosts ,
701: below)
702: for login to be permitted.
1.218 jmc 703: This authentication method closes security holes due to IP
1.222 jmc 704: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 705: [Note to the administrator:
706: .Pa /etc/hosts.equiv ,
707: .Pa ~/.rhosts ,
708: and the rlogin/rsh protocol in general, are inherently insecure and should be
709: disabled if security is desired.]
1.189 dtucker 710: .Pp
1.222 jmc 711: Public key authentication works as follows:
712: The scheme is based on public-key cryptography,
713: using cryptosystems
714: where encryption and decryption are done using separate keys,
715: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 716: The idea is that each user creates a public/private
717: key pair for authentication purposes.
718: The server knows the public key, and only the user knows the private key.
1.222 jmc 719: .Nm
720: implements public key authentication protocol automatically,
1.310 djm 721: using one of the DSA, ECDSA or RSA algorithms.
1.222 jmc 722: Protocol 1 is restricted to using only RSA keys,
1.310 djm 723: but protocol 2 may use any.
1.222 jmc 724: The
725: .Sx HISTORY
726: section of
727: .Xr ssl 8
1.311 jmc 728: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 729: .Pp
1.218 jmc 730: The file
731: .Pa ~/.ssh/authorized_keys
732: lists the public keys that are permitted for logging in.
733: When the user logs in, the
1.2 deraadt 734: .Nm
1.218 jmc 735: program tells the server which key pair it would like to use for
736: authentication.
1.222 jmc 737: The client proves that it has access to the private key
738: and the server checks that the corresponding public key
739: is authorized to accept the account.
1.218 jmc 740: .Pp
1.222 jmc 741: The user creates his/her key pair by running
1.218 jmc 742: .Xr ssh-keygen 1 .
743: This stores the private key in
1.207 djm 744: .Pa ~/.ssh/identity
1.222 jmc 745: (protocol 1),
746: .Pa ~/.ssh/id_dsa
747: (protocol 2 DSA),
1.310 djm 748: .Pa ~/.ssh/id_ecdsa
749: (protocol 2 ECDSA),
1.222 jmc 750: or
751: .Pa ~/.ssh/id_rsa
752: (protocol 2 RSA)
1.218 jmc 753: and stores the public key in
754: .Pa ~/.ssh/identity.pub
1.222 jmc 755: (protocol 1),
756: .Pa ~/.ssh/id_dsa.pub
757: (protocol 2 DSA),
1.310 djm 758: .Pa ~/.ssh/id_ecdsa.pub
759: (protocol 2 ECDSA),
1.222 jmc 760: or
761: .Pa ~/.ssh/id_rsa.pub
762: (protocol 2 RSA)
1.218 jmc 763: in the user's home directory.
1.222 jmc 764: The user should then copy the public key
1.218 jmc 765: to
766: .Pa ~/.ssh/authorized_keys
1.222 jmc 767: in his/her home directory on the remote machine.
768: The
1.218 jmc 769: .Pa authorized_keys
770: file corresponds to the conventional
771: .Pa ~/.rhosts
772: file, and has one key
1.222 jmc 773: per line, though the lines can be very long.
1.218 jmc 774: After this, the user can log in without giving the password.
775: .Pp
1.301 jmc 776: A variation on public key authentication
777: is available in the form of certificate authentication:
778: instead of a set of public/private keys,
779: signed certificates are used.
780: This has the advantage that a single trusted certification authority
781: can be used in place of many public/private keys.
782: See the
783: .Sx CERTIFICATES
784: section of
785: .Xr ssh-keygen 1
786: for more information.
787: .Pp
788: The most convenient way to use public key or certificate authentication
789: may be with an authentication agent.
1.218 jmc 790: See
791: .Xr ssh-agent 1
792: for more information.
793: .Pp
1.222 jmc 794: Challenge-response authentication works as follows:
795: The server sends an arbitrary
796: .Qq challenge
797: text, and prompts for a response.
798: Protocol 2 allows multiple challenges and responses;
799: protocol 1 is restricted to just one challenge/response.
800: Examples of challenge-response authentication include
801: BSD Authentication (see
802: .Xr login.conf 5 )
803: and PAM (some non-OpenBSD systems).
804: .Pp
805: Finally, if other authentication methods fail,
1.218 jmc 806: .Nm
807: prompts the user for a password.
808: The password is sent to the remote
809: host for checking; however, since all communications are encrypted,
810: the password cannot be seen by someone listening on the network.
1.232 jmc 811: .Pp
812: .Nm
813: automatically maintains and checks a database containing
814: identification for all hosts it has ever been used with.
815: Host keys are stored in
816: .Pa ~/.ssh/known_hosts
817: in the user's home directory.
818: Additionally, the file
819: .Pa /etc/ssh/ssh_known_hosts
820: is automatically checked for known hosts.
821: Any new hosts are automatically added to the user's file.
822: If a host's identification ever changes,
823: .Nm
824: warns about this and disables password authentication to prevent
825: server spoofing or man-in-the-middle attacks,
826: which could otherwise be used to circumvent the encryption.
827: The
828: .Cm StrictHostKeyChecking
829: option can be used to control logins to machines whose
830: host key is not known or has changed.
831: .Pp
1.218 jmc 832: When the user's identity has been accepted by the server, the server
833: either executes the given command, or logs into the machine and gives
834: the user a normal shell on the remote machine.
835: All communication with
836: the remote command or shell will be automatically encrypted.
837: .Pp
838: If a pseudo-terminal has been allocated (normal login session), the
839: user may use the escape characters noted below.
840: .Pp
841: If no pseudo-tty has been allocated,
842: the session is transparent and can be used to reliably transfer binary data.
843: On most systems, setting the escape character to
844: .Dq none
845: will also make the session transparent even if a tty is used.
846: .Pp
847: The session terminates when the command or shell on the remote
1.247 jmc 848: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 849: .Sh ESCAPE CHARACTERS
1.218 jmc 850: When a pseudo-terminal has been requested,
1.2 deraadt 851: .Nm
1.218 jmc 852: supports a number of functions through the use of an escape character.
853: .Pp
854: A single tilde character can be sent as
855: .Ic ~~
856: or by following the tilde by a character other than those described below.
857: The escape character must always follow a newline to be interpreted as
858: special.
859: The escape character can be changed in configuration files using the
860: .Cm EscapeChar
861: configuration directive or on the command line by the
862: .Fl e
863: option.
864: .Pp
865: The supported escapes (assuming the default
866: .Ql ~ )
867: are:
868: .Bl -tag -width Ds
869: .It Cm ~.
870: Disconnect.
871: .It Cm ~^Z
872: Background
1.234 jmc 873: .Nm .
1.218 jmc 874: .It Cm ~#
875: List forwarded connections.
876: .It Cm ~&
877: Background
1.2 deraadt 878: .Nm
1.218 jmc 879: at logout when waiting for forwarded connection / X11 sessions to terminate.
880: .It Cm ~?
881: Display a list of escape characters.
882: .It Cm ~B
883: Send a BREAK to the remote system
884: (only useful for SSH protocol version 2 and if the peer supports it).
885: .It Cm ~C
886: Open command line.
887: Currently this allows the addition of port forwardings using the
1.279 stevesk 888: .Fl L ,
889: .Fl R
1.218 jmc 890: and
1.279 stevesk 891: .Fl D
1.225 jmc 892: options (see above).
1.218 jmc 893: It also allows the cancellation of existing remote port-forwardings
894: using
1.262 stevesk 895: .Sm off
896: .Fl KR Oo Ar bind_address : Oc Ar port .
897: .Sm on
1.218 jmc 898: .Ic !\& Ns Ar command
899: allows the user to execute a local command if the
900: .Ic PermitLocalCommand
901: option is enabled in
1.176 jmc 902: .Xr ssh_config 5 .
1.218 jmc 903: Basic help is available, using the
904: .Fl h
905: option.
906: .It Cm ~R
907: Request rekeying of the connection
908: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 909: .El
1.246 jmc 910: .Sh TCP FORWARDING
911: Forwarding of arbitrary TCP connections over the secure channel can
912: be specified either on the command line or in a configuration file.
913: One possible application of TCP forwarding is a secure connection to a
914: mail server; another is going through firewalls.
915: .Pp
916: In the example below, we look at encrypting communication between
917: an IRC client and server, even though the IRC server does not directly
918: support encrypted communications.
919: This works as follows:
920: the user connects to the remote host using
921: .Nm ,
922: specifying a port to be used to forward connections
923: to the remote server.
924: After that it is possible to start the service which is to be encrypted
925: on the client machine,
926: connecting to the same local port,
927: and
928: .Nm
929: will encrypt and forward the connection.
930: .Pp
931: The following example tunnels an IRC session from client machine
932: .Dq 127.0.0.1
933: (localhost)
934: to remote server
935: .Dq server.example.com :
936: .Bd -literal -offset 4n
937: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
938: $ irc -c '#users' -p 1234 pinky 127.0.0.1
939: .Ed
940: .Pp
941: This tunnels a connection to IRC server
942: .Dq server.example.com ,
943: joining channel
944: .Dq #users ,
945: nickname
946: .Dq pinky ,
947: using port 1234.
948: It doesn't matter which port is used,
949: as long as it's greater than 1023
950: (remember, only root can open sockets on privileged ports)
951: and doesn't conflict with any ports already in use.
952: The connection is forwarded to port 6667 on the remote server,
953: since that's the standard port for IRC services.
954: .Pp
955: The
956: .Fl f
957: option backgrounds
958: .Nm
959: and the remote command
960: .Dq sleep 10
961: is specified to allow an amount of time
962: (10 seconds, in the example)
963: to start the service which is to be tunnelled.
964: If no connections are made within the time specified,
965: .Nm
966: will exit.
967: .Sh X11 FORWARDING
1.218 jmc 968: If the
969: .Cm ForwardX11
970: variable is set to
971: .Dq yes
972: (or see the description of the
1.227 jmc 973: .Fl X ,
974: .Fl x ,
1.218 jmc 975: and
1.227 jmc 976: .Fl Y
1.226 jmc 977: options above)
1.218 jmc 978: and the user is using X11 (the
979: .Ev DISPLAY
980: environment variable is set), the connection to the X11 display is
981: automatically forwarded to the remote side in such a way that any X11
982: programs started from the shell (or command) will go through the
983: encrypted channel, and the connection to the real X server will be made
1.176 jmc 984: from the local machine.
1.218 jmc 985: The user should not manually set
986: .Ev DISPLAY .
987: Forwarding of X11 connections can be
988: configured on the command line or in configuration files.
989: .Pp
990: The
991: .Ev DISPLAY
992: value set by
993: .Nm
994: will point to the server machine, but with a display number greater than zero.
995: This is normal, and happens because
996: .Nm
997: creates a
998: .Dq proxy
999: X server on the server machine for forwarding the
1000: connections over the encrypted channel.
1.200 djm 1001: .Pp
1.218 jmc 1002: .Nm
1003: will also automatically set up Xauthority data on the server machine.
1004: For this purpose, it will generate a random authorization cookie,
1005: store it in Xauthority on the server, and verify that any forwarded
1006: connections carry this cookie and replace it by the real cookie when
1007: the connection is opened.
1008: The real authentication cookie is never
1009: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1010: .Pp
1.218 jmc 1011: If the
1012: .Cm ForwardAgent
1013: variable is set to
1014: .Dq yes
1015: (or see the description of the
1016: .Fl A
1.191 djm 1017: and
1.218 jmc 1018: .Fl a
1.226 jmc 1019: options above) and
1.218 jmc 1020: the user is using an authentication agent, the connection to the agent
1021: is automatically forwarded to the remote side.
1.252 jmc 1022: .Sh VERIFYING HOST KEYS
1023: When connecting to a server for the first time,
1024: a fingerprint of the server's public key is presented to the user
1025: (unless the option
1026: .Cm StrictHostKeyChecking
1027: has been disabled).
1028: Fingerprints can be determined using
1029: .Xr ssh-keygen 1 :
1030: .Pp
1031: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1032: .Pp
1.274 grunk 1033: If the fingerprint is already known, it can be matched
1034: and the key can be accepted or rejected.
1035: Because of the difficulty of comparing host keys
1036: just by looking at hex strings,
1037: there is also support to compare host keys visually,
1038: using
1039: .Em random art .
1040: By setting the
1.275 grunk 1041: .Cm VisualHostKey
1.274 grunk 1042: option to
1.275 grunk 1043: .Dq yes ,
1.274 grunk 1044: a small ASCII graphic gets displayed on every login to a server, no matter
1045: if the session itself is interactive or not.
1046: By learning the pattern a known server produces, a user can easily
1047: find out that the host key has changed when a completely different pattern
1048: is displayed.
1049: Because these patterns are not unambiguous however, a pattern that looks
1050: similar to the pattern remembered only gives a good probability that the
1051: host key is the same, not guaranteed proof.
1052: .Pp
1053: To get a listing of the fingerprints along with their random art for
1054: all known hosts, the following command line can be used:
1055: .Pp
1056: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1057: .Pp
1.252 jmc 1058: If the fingerprint is unknown,
1059: an alternative method of verification is available:
1060: SSH fingerprints verified by DNS.
1061: An additional resource record (RR),
1062: SSHFP,
1063: is added to a zonefile
1064: and the connecting client is able to match the fingerprint
1065: with that of the key presented.
1066: .Pp
1067: In this example, we are connecting a client to a server,
1068: .Dq host.example.com .
1069: The SSHFP resource records should first be added to the zonefile for
1070: host.example.com:
1071: .Bd -literal -offset indent
1.259 jakob 1072: $ ssh-keygen -r host.example.com.
1.252 jmc 1073: .Ed
1074: .Pp
1075: The output lines will have to be added to the zonefile.
1076: To check that the zone is answering fingerprint queries:
1077: .Pp
1078: .Dl $ dig -t SSHFP host.example.com
1079: .Pp
1080: Finally the client connects:
1081: .Bd -literal -offset indent
1082: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1083: [...]
1084: Matching host key fingerprint found in DNS.
1085: Are you sure you want to continue connecting (yes/no)?
1086: .Ed
1087: .Pp
1088: See the
1089: .Cm VerifyHostKeyDNS
1090: option in
1091: .Xr ssh_config 5
1092: for more information.
1.250 jmc 1093: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1094: .Nm
1095: contains support for Virtual Private Network (VPN) tunnelling
1096: using the
1097: .Xr tun 4
1098: network pseudo-device,
1099: allowing two networks to be joined securely.
1100: The
1101: .Xr sshd_config 5
1102: configuration option
1103: .Cm PermitTunnel
1104: controls whether the server supports this,
1105: and at what level (layer 2 or 3 traffic).
1106: .Pp
1107: The following example would connect client network 10.0.50.0/24
1.265 otto 1108: with remote network 10.0.99.0/24 using a point-to-point connection
1109: from 10.1.1.1 to 10.1.1.2,
1110: provided that the SSH server running on the gateway to the remote network,
1111: at 192.168.1.15, allows it.
1112: .Pp
1113: On the client:
1.250 jmc 1114: .Bd -literal -offset indent
1115: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1116: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1117: # route add 10.0.99.0/24 10.1.1.2
1118: .Ed
1119: .Pp
1120: On the server:
1121: .Bd -literal -offset indent
1122: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1123: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1124: .Ed
1125: .Pp
1126: Client access may be more finely tuned via the
1127: .Pa /root/.ssh/authorized_keys
1128: file (see below) and the
1129: .Cm PermitRootLogin
1130: server option.
1.255 jmc 1131: The following entry would permit connections on
1.250 jmc 1132: .Xr tun 4
1.255 jmc 1133: device 1 from user
1.250 jmc 1134: .Dq jane
1.255 jmc 1135: and on tun device 2 from user
1.250 jmc 1136: .Dq john ,
1137: if
1138: .Cm PermitRootLogin
1139: is set to
1140: .Dq forced-commands-only :
1141: .Bd -literal -offset 2n
1142: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1143: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1144: .Ed
1145: .Pp
1.264 ray 1146: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1147: it may be more suited to temporary setups,
1148: such as for wireless VPNs.
1149: More permanent VPNs are better provided by tools such as
1150: .Xr ipsecctl 8
1151: and
1152: .Xr isakmpd 8 .
1.2 deraadt 1153: .Sh ENVIRONMENT
1154: .Nm
1.1 deraadt 1155: will normally set the following environment variables:
1.237 jmc 1156: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1157: .It Ev DISPLAY
1158: The
1159: .Ev DISPLAY
1.40 aaron 1160: variable indicates the location of the X11 server.
1.44 aaron 1161: It is automatically set by
1.2 deraadt 1162: .Nm
1163: to point to a value of the form
1.233 jmc 1164: .Dq hostname:n ,
1165: where
1166: .Dq hostname
1167: indicates the host where the shell runs, and
1168: .Sq n
1169: is an integer \*(Ge 1.
1.40 aaron 1170: .Nm
1171: uses this special value to forward X11 connections over the secure
1172: channel.
1.107 markus 1173: The user should normally not set
1174: .Ev DISPLAY
1175: explicitly, as that
1.1 deraadt 1176: will render the X11 connection insecure (and will require the user to
1177: manually copy any required authorization cookies).
1.2 deraadt 1178: .It Ev HOME
1.1 deraadt 1179: Set to the path of the user's home directory.
1.2 deraadt 1180: .It Ev LOGNAME
1181: Synonym for
1.12 aaron 1182: .Ev USER ;
1183: set for compatibility with systems that use this variable.
1.2 deraadt 1184: .It Ev MAIL
1.129 stevesk 1185: Set to the path of the user's mailbox.
1.40 aaron 1186: .It Ev PATH
1.2 deraadt 1187: Set to the default
1188: .Ev PATH ,
1189: as specified when compiling
1.234 jmc 1190: .Nm .
1.118 markus 1191: .It Ev SSH_ASKPASS
1192: If
1193: .Nm
1194: needs a passphrase, it will read the passphrase from the current
1195: terminal if it was run from a terminal.
1196: If
1197: .Nm
1198: does not have a terminal associated with it but
1199: .Ev DISPLAY
1200: and
1201: .Ev SSH_ASKPASS
1202: are set, it will execute the program specified by
1203: .Ev SSH_ASKPASS
1204: and open an X11 window to read the passphrase.
1205: This is particularly useful when calling
1206: .Nm
1207: from a
1.196 jmc 1208: .Pa .xsession
1.118 markus 1209: or related script.
1210: (Note that on some machines it
1211: may be necessary to redirect the input from
1212: .Pa /dev/null
1213: to make this work.)
1.18 markus 1214: .It Ev SSH_AUTH_SOCK
1.233 jmc 1215: Identifies the path of a
1216: .Ux Ns -domain
1217: socket used to communicate with the agent.
1.166 stevesk 1218: .It Ev SSH_CONNECTION
1219: Identifies the client and server ends of the connection.
1.40 aaron 1220: The variable contains
1.233 jmc 1221: four space-separated values: client IP address, client port number,
1222: server IP address, and server port number.
1.73 markus 1223: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1224: This variable contains the original command line if a forced command
1.73 markus 1225: is executed.
1226: It can be used to extract the original arguments.
1.2 deraadt 1227: .It Ev SSH_TTY
1.1 deraadt 1228: This is set to the name of the tty (path to the device) associated
1.40 aaron 1229: with the current shell or command.
1230: If the current session has no tty,
1.1 deraadt 1231: this variable is not set.
1.2 deraadt 1232: .It Ev TZ
1.214 jmc 1233: This variable is set to indicate the present time zone if it
1.257 jmc 1234: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1235: on to new connections).
1.2 deraadt 1236: .It Ev USER
1.1 deraadt 1237: Set to the name of the user logging in.
1.2 deraadt 1238: .El
1239: .Pp
1.44 aaron 1240: Additionally,
1.2 deraadt 1241: .Nm
1.44 aaron 1242: reads
1.207 djm 1243: .Pa ~/.ssh/environment ,
1.2 deraadt 1244: and adds lines of the format
1245: .Dq VARNAME=value
1.233 jmc 1246: to the environment if the file exists and users are allowed to
1.161 marc 1247: change their environment.
1.176 jmc 1248: For more information, see the
1.161 marc 1249: .Cm PermitUserEnvironment
1.162 stevesk 1250: option in
1.161 marc 1251: .Xr sshd_config 5 .
1.2 deraadt 1252: .Sh FILES
1.236 jmc 1253: .Bl -tag -width Ds -compact
1.309 jmc 1254: .It Pa ~/.rhosts
1.240 jmc 1255: This file is used for host-based authentication (see above).
1.92 markus 1256: On some machines this file may need to be
1.240 jmc 1257: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1258: because
1.2 deraadt 1259: .Xr sshd 8
1.40 aaron 1260: reads it as root.
1261: Additionally, this file must be owned by the user,
1262: and must not have write permissions for anyone else.
1263: The recommended
1.1 deraadt 1264: permission for most machines is read/write for the user, and not
1265: accessible by others.
1.2 deraadt 1266: .Pp
1.309 jmc 1267: .It Pa ~/.shosts
1.240 jmc 1268: This file is used in exactly the same way as
1269: .Pa .rhosts ,
1270: but allows host-based authentication without permitting login with
1271: rlogin/rsh.
1.272 mcbride 1272: .Pp
1.309 jmc 1273: .It Pa ~/.ssh/
1.272 mcbride 1274: This directory is the default location for all user-specific configuration
1275: and authentication information.
1276: There is no general requirement to keep the entire contents of this directory
1277: secret, but the recommended permissions are read/write/execute for the user,
1278: and not accessible by others.
1.236 jmc 1279: .Pp
1.309 jmc 1280: .It Pa ~/.ssh/authorized_keys
1.310 djm 1281: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1282: this user.
1.238 jmc 1283: The format of this file is described in the
1284: .Xr sshd 8
1285: manual page.
1286: This file is not highly sensitive, but the recommended
1287: permissions are read/write for the user, and not accessible by others.
1288: .Pp
1.309 jmc 1289: .It Pa ~/.ssh/config
1.238 jmc 1290: This is the per-user configuration file.
1291: The file format and configuration options are described in
1292: .Xr ssh_config 5 .
1293: Because of the potential for abuse, this file must have strict permissions:
1294: read/write for the user, and not accessible by others.
1295: .Pp
1.309 jmc 1296: .It Pa ~/.ssh/environment
1.239 jmc 1297: Contains additional definitions for environment variables; see
1298: .Sx ENVIRONMENT ,
1.238 jmc 1299: above.
1300: .Pp
1.309 jmc 1301: .It Pa ~/.ssh/identity
1302: .It Pa ~/.ssh/id_dsa
1.310 djm 1303: .It Pa ~/.ssh/id_ecdsa
1.309 jmc 1304: .It Pa ~/.ssh/id_rsa
1.238 jmc 1305: Contains the private key for authentication.
1306: These files
1307: contain sensitive data and should be readable by the user but not
1308: accessible by others (read/write/execute).
1309: .Nm
1310: will simply ignore a private key file if it is accessible by others.
1311: It is possible to specify a passphrase when
1312: generating the key which will be used to encrypt the
1313: sensitive part of this file using 3DES.
1314: .Pp
1.309 jmc 1315: .It Pa ~/.ssh/identity.pub
1316: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1317: .It Pa ~/.ssh/id_ecdsa.pub
1.309 jmc 1318: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1319: Contains the public key for authentication.
1320: These files are not
1321: sensitive and can (but need not) be readable by anyone.
1322: .Pp
1.309 jmc 1323: .It Pa ~/.ssh/known_hosts
1.244 jmc 1324: Contains a list of host keys for all hosts the user has logged into
1325: that are not already in the systemwide list of known host keys.
1.238 jmc 1326: See
1.244 jmc 1327: .Xr sshd 8
1328: for further details of the format of this file.
1.238 jmc 1329: .Pp
1.309 jmc 1330: .It Pa ~/.ssh/rc
1.238 jmc 1331: Commands in this file are executed by
1332: .Nm
1.245 jmc 1333: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1334: started.
1335: See the
1336: .Xr sshd 8
1337: manual page for more information.
1338: .Pp
1.309 jmc 1339: .It Pa /etc/hosts.equiv
1.240 jmc 1340: This file is for host-based authentication (see above).
1341: It should only be writable by root.
1.236 jmc 1342: .Pp
1.309 jmc 1343: .It Pa /etc/shosts.equiv
1.240 jmc 1344: This file is used in exactly the same way as
1345: .Pa hosts.equiv ,
1346: but allows host-based authentication without permitting login with
1347: rlogin/rsh.
1.236 jmc 1348: .Pp
1.238 jmc 1349: .It Pa /etc/ssh/ssh_config
1350: Systemwide configuration file.
1351: The file format and configuration options are described in
1352: .Xr ssh_config 5 .
1353: .Pp
1.309 jmc 1354: .It Pa /etc/ssh/ssh_host_key
1355: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1356: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.309 jmc 1357: .It Pa /etc/ssh/ssh_host_rsa_key
1.238 jmc 1358: These three files contain the private parts of the host keys
1.245 jmc 1359: and are used for host-based authentication.
1360: If protocol version 1 is used,
1.238 jmc 1361: .Nm
1362: must be setuid root, since the host key is readable only by root.
1363: For protocol version 2,
1364: .Nm
1365: uses
1366: .Xr ssh-keysign 8
1.245 jmc 1367: to access the host keys,
1368: eliminating the requirement that
1.238 jmc 1369: .Nm
1.245 jmc 1370: be setuid root when host-based authentication is used.
1.238 jmc 1371: By default
1.2 deraadt 1372: .Nm
1.238 jmc 1373: is not setuid root.
1374: .Pp
1.309 jmc 1375: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1376: Systemwide list of known host keys.
1377: This file should be prepared by the
1378: system administrator to contain the public host keys of all machines in the
1379: organization.
1.244 jmc 1380: It should be world-readable.
1381: See
1.238 jmc 1382: .Xr sshd 8
1.244 jmc 1383: for further details of the format of this file.
1.236 jmc 1384: .Pp
1.309 jmc 1385: .It Pa /etc/ssh/sshrc
1.1 deraadt 1386: Commands in this file are executed by
1.2 deraadt 1387: .Nm
1.245 jmc 1388: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1389: See the
1.2 deraadt 1390: .Xr sshd 8
1.1 deraadt 1391: manual page for more information.
1.58 itojun 1392: .El
1.312 jmc 1393: .Sh EXIT STATUS
1394: .Nm
1395: exits with the exit status of the remote command or with 255
1396: if an error occurred.
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
1.313 ! djm 1468: .Re
! 1469: .Rs
! 1470: .%R RFC 5656
! 1471: .%T "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer"
! 1472: .%D 2009
1.274 grunk 1473: .Re
1474: .Rs
1475: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1476: .%A A. Perrig
1477: .%A D. Song
1478: .%D 1999
1479: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1480: .Re
1.173 jmc 1481: .Sh AUTHORS
1482: OpenSSH is a derivative of the original and free
1483: ssh 1.2.12 release by Tatu Ylonen.
1484: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1485: Theo de Raadt and Dug Song
1486: removed many bugs, re-added newer features and
1487: created OpenSSH.
1488: Markus Friedl contributed the support for SSH
1489: protocol versions 1.5 and 2.0.
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