Annotation of src/usr.bin/ssh/ssh.1, Revision 1.335
1.1 deraadt 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
1.59 deraadt 6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
1.93 deraadt 12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.59 deraadt 15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 24: .\"
1.59 deraadt 25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 35: .\"
1.335 ! jmc 36: .\" $OpenBSD: ssh.1,v 1.334 2013/07/18 01:12:26 djm Exp $
! 37: .Dd $Mdocdate: July 18 2013 $
1.2 deraadt 38: .Dt SSH 1
39: .Os
40: .Sh NAME
41: .Nm ssh
1.96 deraadt 42: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm ssh
1.306 jmc 45: .Bk -words
1.278 djm 46: .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
1.108 markus 47: .Op Fl b Ar bind_address
1.51 markus 48: .Op Fl c Ar cipher_spec
1.306 jmc 49: .Op Fl D Oo Ar bind_address : Oc Ns Ar port
1.331 dtucker 50: .Op Fl E Ar log_file
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
1.332 djm 68: .Nm
69: .Fl Q Ar protocol_feature
1.44 aaron 70: .Sh DESCRIPTION
1.2 deraadt 71: .Nm
1.96 deraadt 72: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 73: executing commands on a remote machine.
1.176 jmc 74: It is intended to replace rlogin and rsh,
75: and provide secure encrypted communications between
1.40 aaron 76: two untrusted hosts over an insecure network.
1.247 jmc 77: X11 connections and arbitrary TCP ports
1.176 jmc 78: can also be forwarded over the secure channel.
1.2 deraadt 79: .Pp
80: .Nm
1.44 aaron 81: connects and logs into the specified
1.176 jmc 82: .Ar hostname
83: (with optional
84: .Ar user
85: name).
1.1 deraadt 86: The user must prove
1.49 markus 87: his/her identity to the remote machine using one of several methods
1.221 jmc 88: depending on the protocol version used (see below).
1.49 markus 89: .Pp
1.176 jmc 90: If
91: .Ar command
92: is specified,
1.219 jmc 93: it is executed on the remote host instead of a login shell.
1.2 deraadt 94: .Pp
1.218 jmc 95: The options are as follows:
96: .Bl -tag -width Ds
97: .It Fl 1
98: Forces
1.2 deraadt 99: .Nm
1.218 jmc 100: to try protocol version 1 only.
101: .It Fl 2
102: Forces
1.2 deraadt 103: .Nm
1.218 jmc 104: to try protocol version 2 only.
105: .It Fl 4
106: Forces
1.2 deraadt 107: .Nm
1.218 jmc 108: to use IPv4 addresses only.
109: .It Fl 6
110: Forces
1.2 deraadt 111: .Nm
1.218 jmc 112: to use IPv6 addresses only.
113: .It Fl A
114: Enables forwarding of the authentication agent connection.
115: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 116: .Pp
1.218 jmc 117: Agent forwarding should be enabled with caution.
118: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 119: (for the agent's
1.286 sobrado 120: .Ux Ns -domain
121: socket) can access the local agent through the forwarded connection.
1.218 jmc 122: An attacker cannot obtain key material from the agent,
123: however they can perform operations on the keys that enable them to
124: authenticate using the identities loaded into the agent.
125: .It Fl a
126: Disables forwarding of the authentication agent connection.
127: .It Fl b Ar bind_address
128: Use
129: .Ar bind_address
130: on the local machine as the source address
131: of the connection.
132: Only useful on systems with more than one address.
133: .It Fl C
134: Requests compression of all data (including stdin, stdout, stderr, and
1.247 jmc 135: data for forwarded X11 and TCP connections).
1.218 jmc 136: The compression algorithm is the same used by
137: .Xr gzip 1 ,
138: and the
139: .Dq level
140: can be controlled by the
141: .Cm CompressionLevel
142: option for protocol version 1.
143: Compression is desirable on modem lines and other
144: slow connections, but will only slow down things on fast networks.
145: The default value can be set on a host-by-host basis in the
146: configuration files; see the
147: .Cm Compression
148: option.
149: .It Fl c Ar cipher_spec
150: Selects the cipher specification for encrypting the session.
1.2 deraadt 151: .Pp
1.218 jmc 152: Protocol version 1 allows specification of a single cipher.
153: The supported values are
154: .Dq 3des ,
1.220 jmc 155: .Dq blowfish ,
1.218 jmc 156: and
157: .Dq des .
158: .Ar 3des
159: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
160: It is believed to be secure.
161: .Ar blowfish
162: is a fast block cipher; it appears very secure and is much faster than
163: .Ar 3des .
164: .Ar des
165: is only supported in the
1.2 deraadt 166: .Nm
1.218 jmc 167: client for interoperability with legacy protocol 1 implementations
168: that do not support the
169: .Ar 3des
170: cipher.
171: Its use is strongly discouraged due to cryptographic weaknesses.
172: The default is
173: .Dq 3des .
1.49 markus 174: .Pp
1.230 jmc 175: For protocol version 2,
1.218 jmc 176: .Ar cipher_spec
177: is a comma-separated list of ciphers
178: listed in order of preference.
1.283 jmc 179: See the
180: .Cm Ciphers
1.307 dtucker 181: keyword in
182: .Xr ssh_config 5
183: for more information.
1.218 jmc 184: .It Fl D Xo
185: .Sm off
186: .Oo Ar bind_address : Oc
187: .Ar port
188: .Sm on
189: .Xc
190: Specifies a local
191: .Dq dynamic
192: application-level port forwarding.
193: This works by allocating a socket to listen to
194: .Ar port
195: on the local side, optionally bound to the specified
196: .Ar bind_address .
197: Whenever a connection is made to this port, the
198: connection is forwarded over the secure channel, and the application
199: protocol is then used to determine where to connect to from the
200: remote machine.
201: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 202: .Nm
1.218 jmc 203: will act as a SOCKS server.
204: Only root can forward privileged ports.
205: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 206: .Pp
1.308 djm 207: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 208: Only the superuser can forward privileged ports.
209: By default, the local port is bound in accordance with the
210: .Cm GatewayPorts
211: setting.
212: However, an explicit
213: .Ar bind_address
214: may be used to bind the connection to a specific address.
215: The
216: .Ar bind_address
217: of
218: .Dq localhost
219: indicates that the listening port be bound for local use only, while an
220: empty address or
221: .Sq *
222: indicates that the port should be available from all interfaces.
1.331 dtucker 223: .It Fl E Ar log_file
224: Append debug logs to
225: .Ar log_file
226: instead of standard error.
1.229 jmc 227: .It Fl e Ar escape_char
1.218 jmc 228: Sets the escape character for sessions with a pty (default:
229: .Ql ~ ) .
230: The escape character is only recognized at the beginning of a line.
231: The escape character followed by a dot
232: .Pq Ql \&.
233: closes the connection;
234: followed by control-Z suspends the connection;
235: and followed by itself sends the escape character once.
236: Setting the character to
1.2 deraadt 237: .Dq none
1.218 jmc 238: disables any escapes and makes the session fully transparent.
239: .It Fl F Ar configfile
240: Specifies an alternative per-user configuration file.
241: If a configuration file is given on the command line,
242: the system-wide configuration file
243: .Pq Pa /etc/ssh/ssh_config
244: will be ignored.
245: The default for the per-user configuration file is
246: .Pa ~/.ssh/config .
247: .It Fl f
248: Requests
249: .Nm
250: to go to background just before command execution.
251: This is useful if
1.176 jmc 252: .Nm
1.218 jmc 253: is going to ask for passwords or passphrases, but the user
254: wants it in the background.
255: This implies
256: .Fl n .
257: The recommended way to start X11 programs at a remote site is with
258: something like
259: .Ic ssh -f host xterm .
1.277 djm 260: .Pp
261: If the
262: .Cm ExitOnForwardFailure
263: configuration option is set to
264: .Dq yes ,
265: then a client started with
266: .Fl f
267: will wait for all remote port forwards to be successfully established
268: before placing itself in the background.
1.218 jmc 269: .It Fl g
270: Allows remote hosts to connect to local forwarded ports.
1.291 markus 271: .It Fl I Ar pkcs11
1.294 jmc 272: Specify the PKCS#11 shared library
1.176 jmc 273: .Nm
1.293 markus 274: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 275: private RSA key.
276: .It Fl i Ar identity_file
277: Selects a file from which the identity (private key) for
1.310 djm 278: public key authentication is read.
1.218 jmc 279: The default is
280: .Pa ~/.ssh/identity
281: for protocol version 1, and
1.310 djm 282: .Pa ~/.ssh/id_dsa ,
283: .Pa ~/.ssh/id_ecdsa
284: and
1.218 jmc 285: .Pa ~/.ssh/id_rsa
286: for protocol version 2.
287: Identity files may also be specified on
288: a per-host basis in the configuration file.
289: It is possible to have multiple
290: .Fl i
291: options (and multiple identities specified in
292: configuration files).
1.302 djm 293: .Nm
294: will also try to load certificate information from the filename obtained
295: by appending
296: .Pa -cert.pub
297: to identity filenames.
1.269 djm 298: .It Fl K
299: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
300: credentials to the server.
1.218 jmc 301: .It Fl k
302: Disables forwarding (delegation) of GSSAPI credentials to the server.
303: .It Fl L Xo
304: .Sm off
305: .Oo Ar bind_address : Oc
306: .Ar port : host : hostport
307: .Sm on
308: .Xc
309: Specifies that the given port on the local (client) host is to be
310: forwarded to the given host and port on the remote side.
311: This works by allocating a socket to listen to
312: .Ar port
313: on the local side, optionally bound to the specified
314: .Ar bind_address .
315: Whenever a connection is made to this port, the
316: connection is forwarded over the secure channel, and a connection is
317: made to
318: .Ar host
319: port
320: .Ar hostport
321: from the remote machine.
322: Port forwardings can also be specified in the configuration file.
1.308 djm 323: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 324: Only the superuser can forward privileged ports.
325: By default, the local port is bound in accordance with the
326: .Cm GatewayPorts
327: setting.
328: However, an explicit
329: .Ar bind_address
330: may be used to bind the connection to a specific address.
1.2 deraadt 331: The
1.218 jmc 332: .Ar bind_address
333: of
334: .Dq localhost
335: indicates that the listening port be bound for local use only, while an
336: empty address or
337: .Sq *
338: indicates that the port should be available from all interfaces.
339: .It Fl l Ar login_name
340: Specifies the user to log in as on the remote machine.
341: This also may be specified on a per-host basis in the configuration file.
342: .It Fl M
343: Places the
344: .Nm
345: client into
346: .Dq master
347: mode for connection sharing.
1.231 stevesk 348: Multiple
349: .Fl M
350: options places
351: .Nm
352: into
353: .Dq master
354: mode with confirmation required before slave connections are accepted.
1.218 jmc 355: Refer to the description of
356: .Cm ControlMaster
357: in
358: .Xr ssh_config 5
359: for details.
360: .It Fl m Ar mac_spec
361: Additionally, for protocol version 2 a comma-separated list of MAC
362: (message authentication code) algorithms can
363: be specified in order of preference.
364: See the
365: .Cm MACs
366: keyword for more information.
367: .It Fl N
368: Do not execute a remote command.
369: This is useful for just forwarding ports
370: (protocol version 2 only).
371: .It Fl n
372: Redirects stdin from
373: .Pa /dev/null
374: (actually, prevents reading from stdin).
375: This must be used when
1.2 deraadt 376: .Nm
1.218 jmc 377: is run in the background.
378: A common trick is to use this to run X11 programs on a remote machine.
379: For example,
380: .Ic ssh -n shadows.cs.hut.fi emacs &
381: will start an emacs on shadows.cs.hut.fi, and the X11
382: connection will be automatically forwarded over an encrypted channel.
383: The
1.2 deraadt 384: .Nm
1.218 jmc 385: program will be put in the background.
386: (This does not work if
1.2 deraadt 387: .Nm
1.218 jmc 388: needs to ask for a password or passphrase; see also the
389: .Fl f
390: option.)
391: .It Fl O Ar ctl_cmd
392: Control an active connection multiplexing master process.
393: When the
394: .Fl O
395: option is specified, the
396: .Ar ctl_cmd
397: argument is interpreted and passed to the master process.
398: Valid commands are:
399: .Dq check
1.305 markus 400: (check that the master process is running),
401: .Dq forward
1.318 jmc 402: (request forwardings without command execution),
1.323 okan 403: .Dq cancel
404: (cancel forwardings),
1.218 jmc 405: .Dq exit
1.318 jmc 406: (request the master to exit), and
1.317 djm 407: .Dq stop
408: (request the master to stop accepting further multiplexing requests).
1.218 jmc 409: .It Fl o Ar option
410: Can be used to give options in the format used in the configuration file.
411: This is useful for specifying options for which there is no separate
412: command-line flag.
413: For full details of the options listed below, and their possible values, see
414: .Xr ssh_config 5 .
1.2 deraadt 415: .Pp
1.218 jmc 416: .Bl -tag -width Ds -offset indent -compact
417: .It AddressFamily
418: .It BatchMode
419: .It BindAddress
420: .It ChallengeResponseAuthentication
421: .It CheckHostIP
422: .It Cipher
423: .It Ciphers
424: .It ClearAllForwardings
425: .It Compression
426: .It CompressionLevel
427: .It ConnectionAttempts
428: .It ConnectTimeout
429: .It ControlMaster
430: .It ControlPath
1.321 djm 431: .It ControlPersist
1.218 jmc 432: .It DynamicForward
433: .It EscapeChar
1.263 markus 434: .It ExitOnForwardFailure
1.218 jmc 435: .It ForwardAgent
436: .It ForwardX11
1.321 djm 437: .It ForwardX11Timeout
1.218 jmc 438: .It ForwardX11Trusted
439: .It GatewayPorts
440: .It GlobalKnownHostsFile
441: .It GSSAPIAuthentication
442: .It GSSAPIDelegateCredentials
443: .It HashKnownHosts
444: .It Host
445: .It HostbasedAuthentication
446: .It HostKeyAlgorithms
447: .It HostKeyAlias
448: .It HostName
449: .It IdentityFile
450: .It IdentitiesOnly
1.316 jmc 451: .It IPQoS
1.321 djm 452: .It KbdInteractiveAuthentication
1.218 jmc 453: .It KbdInteractiveDevices
1.314 jmc 454: .It KexAlgorithms
1.218 jmc 455: .It LocalCommand
456: .It LocalForward
457: .It LogLevel
458: .It MACs
459: .It NoHostAuthenticationForLocalhost
460: .It NumberOfPasswordPrompts
461: .It PasswordAuthentication
462: .It PermitLocalCommand
1.291 markus 463: .It PKCS11Provider
1.218 jmc 464: .It Port
465: .It PreferredAuthentications
466: .It Protocol
467: .It ProxyCommand
468: .It PubkeyAuthentication
1.251 dtucker 469: .It RekeyLimit
1.218 jmc 470: .It RemoteForward
1.319 jmc 471: .It RequestTTY
1.218 jmc 472: .It RhostsRSAAuthentication
473: .It RSAAuthentication
474: .It SendEnv
475: .It ServerAliveInterval
476: .It ServerAliveCountMax
477: .It StrictHostKeyChecking
478: .It TCPKeepAlive
479: .It Tunnel
480: .It TunnelDevice
481: .It UsePrivilegedPort
482: .It User
483: .It UserKnownHostsFile
484: .It VerifyHostKeyDNS
1.276 jmc 485: .It VisualHostKey
1.218 jmc 486: .It XAuthLocation
487: .El
488: .It Fl p Ar port
489: Port to connect to on the remote host.
490: This can be specified on a
491: per-host basis in the configuration file.
1.332 djm 492: .It Fl Q Ar protocol_feature
493: Queries
494: .Nm
495: for the algorithms supported for the specified version 2
496: .Ar protocol_feature .
497: The queriable features are:
498: .Dq cipher
499: (supported symmetric ciphers),
500: .Dq MAC
501: (supported message integrity codes),
502: .Dq KEX
503: (key exchange algorithms),
504: .Dq key
505: (key types).
506: Protocol features are treated case-insensitively.
1.218 jmc 507: .It Fl q
508: Quiet mode.
1.271 djm 509: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 510: .It Fl R Xo
511: .Sm off
512: .Oo Ar bind_address : Oc
513: .Ar port : host : hostport
514: .Sm on
515: .Xc
516: Specifies that the given port on the remote (server) host is to be
517: forwarded to the given host and port on the local side.
518: This works by allocating a socket to listen to
519: .Ar port
520: on the remote side, and whenever a connection is made to this port, the
521: connection is forwarded over the secure channel, and a connection is
522: made to
523: .Ar host
524: port
525: .Ar hostport
526: from the local machine.
1.2 deraadt 527: .Pp
1.218 jmc 528: Port forwardings can also be specified in the configuration file.
529: Privileged ports can be forwarded only when
530: logging in as root on the remote machine.
1.324 jmc 531: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 532: .Pp
1.218 jmc 533: By default, the listening socket on the server will be bound to the loopback
534: interface only.
1.280 tobias 535: This may be overridden by specifying a
1.218 jmc 536: .Ar bind_address .
537: An empty
538: .Ar bind_address ,
539: or the address
540: .Ql * ,
541: indicates that the remote socket should listen on all interfaces.
542: Specifying a remote
543: .Ar bind_address
544: will only succeed if the server's
545: .Cm GatewayPorts
546: option is enabled (see
547: .Xr sshd_config 5 ) .
1.281 djm 548: .Pp
549: If the
550: .Ar port
551: argument is
1.282 djm 552: .Ql 0 ,
1.281 djm 553: the listen port will be dynamically allocated on the server and reported
554: to the client at run time.
1.305 markus 555: When used together with
556: .Ic -O forward
557: the allocated port will be printed to the standard output.
1.218 jmc 558: .It Fl S Ar ctl_path
1.304 jmc 559: Specifies the location of a control socket for connection sharing,
1.303 djm 560: or the string
561: .Dq none
562: to disable connection sharing.
1.218 jmc 563: Refer to the description of
564: .Cm ControlPath
565: and
566: .Cm ControlMaster
567: in
568: .Xr ssh_config 5
569: for details.
570: .It Fl s
571: May be used to request invocation of a subsystem on the remote system.
572: Subsystems are a feature of the SSH2 protocol which facilitate the use
573: of SSH as a secure transport for other applications (eg.\&
574: .Xr sftp 1 ) .
575: The subsystem is specified as the remote command.
576: .It Fl T
577: Disable pseudo-tty allocation.
578: .It Fl t
579: Force pseudo-tty allocation.
580: This can be used to execute arbitrary
581: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 582: e.g. when implementing menu services.
1.218 jmc 583: Multiple
584: .Fl t
585: options force tty allocation, even if
1.194 jakob 586: .Nm
1.218 jmc 587: has no local tty.
588: .It Fl V
589: Display the version number and exit.
590: .It Fl v
591: Verbose mode.
592: Causes
1.176 jmc 593: .Nm
1.218 jmc 594: to print debugging messages about its progress.
595: This is helpful in
596: debugging connection, authentication, and configuration problems.
597: Multiple
598: .Fl v
599: options increase the verbosity.
600: The maximum is 3.
1.290 dtucker 601: .It Fl W Ar host : Ns Ar port
602: Requests that standard input and output on the client be forwarded to
603: .Ar host
604: on
605: .Ar port
606: over the secure channel.
607: Implies
608: .Fl N ,
609: .Fl T ,
610: .Cm ExitOnForwardFailure
611: and
1.326 dtucker 612: .Cm ClearAllForwardings .
613: Works with Protocol version 2 only.
1.261 stevesk 614: .It Fl w Xo
615: .Ar local_tun Ns Op : Ns Ar remote_tun
616: .Xc
617: Requests
618: tunnel
619: device forwarding with the specified
1.218 jmc 620: .Xr tun 4
1.261 stevesk 621: devices between the client
622: .Pq Ar local_tun
623: and the server
624: .Pq Ar remote_tun .
625: .Pp
1.228 jmc 626: The devices may be specified by numerical ID or the keyword
627: .Dq any ,
628: which uses the next available tunnel device.
1.261 stevesk 629: If
630: .Ar remote_tun
631: is not specified, it defaults to
632: .Dq any .
1.228 jmc 633: See also the
1.218 jmc 634: .Cm Tunnel
1.261 stevesk 635: and
636: .Cm TunnelDevice
637: directives in
1.218 jmc 638: .Xr ssh_config 5 .
1.261 stevesk 639: If the
640: .Cm Tunnel
641: directive is unset, it is set to the default tunnel mode, which is
642: .Dq point-to-point .
1.218 jmc 643: .It Fl X
644: Enables X11 forwarding.
1.54 markus 645: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 646: .Pp
1.218 jmc 647: X11 forwarding should be enabled with caution.
1.168 jmc 648: Users with the ability to bypass file permissions on the remote host
1.218 jmc 649: (for the user's X authorization database)
650: can access the local X11 display through the forwarded connection.
651: An attacker may then be able to perform activities such as keystroke monitoring.
652: .Pp
653: For this reason, X11 forwarding is subjected to X11 SECURITY extension
654: restrictions by default.
655: Please refer to the
656: .Nm
657: .Fl Y
658: option and the
659: .Cm ForwardX11Trusted
660: directive in
661: .Xr ssh_config 5
662: for more information.
663: .It Fl x
664: Disables X11 forwarding.
665: .It Fl Y
666: Enables trusted X11 forwarding.
667: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
668: controls.
1.278 djm 669: .It Fl y
670: Send log information using the
671: .Xr syslog 3
672: system module.
673: By default this information is sent to stderr.
1.218 jmc 674: .El
1.224 jmc 675: .Pp
676: .Nm
677: may additionally obtain configuration data from
678: a per-user configuration file and a system-wide configuration file.
679: The file format and configuration options are described in
680: .Xr ssh_config 5 .
1.222 jmc 681: .Sh AUTHENTICATION
1.249 jmc 682: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 683: The default is to use protocol 2 only,
684: though this can be changed via the
1.222 jmc 685: .Cm Protocol
686: option in
1.284 jmc 687: .Xr ssh_config 5
688: or the
1.222 jmc 689: .Fl 1
690: and
691: .Fl 2
692: options (see above).
693: Both protocols support similar authentication methods,
1.284 jmc 694: but protocol 2 is the default since
1.222 jmc 695: it provides additional mechanisms for confidentiality
696: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 697: and integrity (hmac-md5, hmac-sha1,
698: hmac-sha2-256, hmac-sha2-512,
1.330 markus 699: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 700: Protocol 1 lacks a strong mechanism for ensuring the
701: integrity of the connection.
702: .Pp
703: The methods available for authentication are:
1.260 jmc 704: GSSAPI-based authentication,
1.222 jmc 705: host-based authentication,
706: public key authentication,
707: challenge-response authentication,
708: and password authentication.
709: Authentication methods are tried in the order specified above,
710: though protocol 2 has a configuration option to change the default order:
711: .Cm PreferredAuthentications .
712: .Pp
713: Host-based authentication works as follows:
1.218 jmc 714: If the machine the user logs in from is listed in
715: .Pa /etc/hosts.equiv
716: or
717: .Pa /etc/shosts.equiv
718: on the remote machine, and the user names are
719: the same on both sides, or if the files
720: .Pa ~/.rhosts
721: or
722: .Pa ~/.shosts
723: exist in the user's home directory on the
724: remote machine and contain a line containing the name of the client
725: machine and the name of the user on that machine, the user is
1.222 jmc 726: considered for login.
727: Additionally, the server
728: .Em must
729: be able to verify the client's
730: host key (see the description of
1.218 jmc 731: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 732: and
1.222 jmc 733: .Pa ~/.ssh/known_hosts ,
734: below)
735: for login to be permitted.
1.218 jmc 736: This authentication method closes security holes due to IP
1.222 jmc 737: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 738: [Note to the administrator:
739: .Pa /etc/hosts.equiv ,
740: .Pa ~/.rhosts ,
741: and the rlogin/rsh protocol in general, are inherently insecure and should be
742: disabled if security is desired.]
1.189 dtucker 743: .Pp
1.222 jmc 744: Public key authentication works as follows:
745: The scheme is based on public-key cryptography,
746: using cryptosystems
747: where encryption and decryption are done using separate keys,
748: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 749: The idea is that each user creates a public/private
750: key pair for authentication purposes.
751: The server knows the public key, and only the user knows the private key.
1.222 jmc 752: .Nm
753: implements public key authentication protocol automatically,
1.310 djm 754: using one of the DSA, ECDSA or RSA algorithms.
1.222 jmc 755: Protocol 1 is restricted to using only RSA keys,
1.310 djm 756: but protocol 2 may use any.
1.333 jmc 757: The HISTORY section of
1.222 jmc 758: .Xr ssl 8
1.311 jmc 759: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 760: .Pp
1.218 jmc 761: The file
762: .Pa ~/.ssh/authorized_keys
763: lists the public keys that are permitted for logging in.
764: When the user logs in, the
1.2 deraadt 765: .Nm
1.218 jmc 766: program tells the server which key pair it would like to use for
767: authentication.
1.222 jmc 768: The client proves that it has access to the private key
769: and the server checks that the corresponding public key
770: is authorized to accept the account.
1.218 jmc 771: .Pp
1.222 jmc 772: The user creates his/her key pair by running
1.218 jmc 773: .Xr ssh-keygen 1 .
774: This stores the private key in
1.207 djm 775: .Pa ~/.ssh/identity
1.222 jmc 776: (protocol 1),
777: .Pa ~/.ssh/id_dsa
778: (protocol 2 DSA),
1.310 djm 779: .Pa ~/.ssh/id_ecdsa
780: (protocol 2 ECDSA),
1.222 jmc 781: or
782: .Pa ~/.ssh/id_rsa
783: (protocol 2 RSA)
1.218 jmc 784: and stores the public key in
785: .Pa ~/.ssh/identity.pub
1.222 jmc 786: (protocol 1),
787: .Pa ~/.ssh/id_dsa.pub
788: (protocol 2 DSA),
1.310 djm 789: .Pa ~/.ssh/id_ecdsa.pub
790: (protocol 2 ECDSA),
1.222 jmc 791: or
792: .Pa ~/.ssh/id_rsa.pub
793: (protocol 2 RSA)
1.218 jmc 794: in the user's home directory.
1.222 jmc 795: The user should then copy the public key
1.218 jmc 796: to
797: .Pa ~/.ssh/authorized_keys
1.222 jmc 798: in his/her home directory on the remote machine.
799: The
1.218 jmc 800: .Pa authorized_keys
801: file corresponds to the conventional
802: .Pa ~/.rhosts
803: file, and has one key
1.222 jmc 804: per line, though the lines can be very long.
1.218 jmc 805: After this, the user can log in without giving the password.
806: .Pp
1.301 jmc 807: A variation on public key authentication
808: is available in the form of certificate authentication:
809: instead of a set of public/private keys,
810: signed certificates are used.
811: This has the advantage that a single trusted certification authority
812: can be used in place of many public/private keys.
1.333 jmc 813: See the CERTIFICATES section of
1.301 jmc 814: .Xr ssh-keygen 1
815: for more information.
816: .Pp
817: The most convenient way to use public key or certificate authentication
818: may be with an authentication agent.
1.218 jmc 819: See
820: .Xr ssh-agent 1
821: for more information.
822: .Pp
1.222 jmc 823: Challenge-response authentication works as follows:
824: The server sends an arbitrary
825: .Qq challenge
826: text, and prompts for a response.
827: Protocol 2 allows multiple challenges and responses;
828: protocol 1 is restricted to just one challenge/response.
829: Examples of challenge-response authentication include
1.335 ! jmc 830: .Bx
! 831: Authentication (see
1.222 jmc 832: .Xr login.conf 5 )
1.335 ! jmc 833: and PAM (some
! 834: .Pf non- Ox
! 835: systems).
1.222 jmc 836: .Pp
837: Finally, if other authentication methods fail,
1.218 jmc 838: .Nm
839: prompts the user for a password.
840: The password is sent to the remote
841: host for checking; however, since all communications are encrypted,
842: the password cannot be seen by someone listening on the network.
1.232 jmc 843: .Pp
844: .Nm
845: automatically maintains and checks a database containing
846: identification for all hosts it has ever been used with.
847: Host keys are stored in
848: .Pa ~/.ssh/known_hosts
849: in the user's home directory.
850: Additionally, the file
851: .Pa /etc/ssh/ssh_known_hosts
852: is automatically checked for known hosts.
853: Any new hosts are automatically added to the user's file.
854: If a host's identification ever changes,
855: .Nm
856: warns about this and disables password authentication to prevent
857: server spoofing or man-in-the-middle attacks,
858: which could otherwise be used to circumvent the encryption.
859: The
860: .Cm StrictHostKeyChecking
861: option can be used to control logins to machines whose
862: host key is not known or has changed.
863: .Pp
1.218 jmc 864: When the user's identity has been accepted by the server, the server
865: either executes the given command, or logs into the machine and gives
866: the user a normal shell on the remote machine.
867: All communication with
868: the remote command or shell will be automatically encrypted.
869: .Pp
870: If a pseudo-terminal has been allocated (normal login session), the
871: user may use the escape characters noted below.
872: .Pp
873: If no pseudo-tty has been allocated,
874: the session is transparent and can be used to reliably transfer binary data.
875: On most systems, setting the escape character to
876: .Dq none
877: will also make the session transparent even if a tty is used.
878: .Pp
879: The session terminates when the command or shell on the remote
1.247 jmc 880: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 881: .Sh ESCAPE CHARACTERS
1.218 jmc 882: When a pseudo-terminal has been requested,
1.2 deraadt 883: .Nm
1.218 jmc 884: supports a number of functions through the use of an escape character.
885: .Pp
886: A single tilde character can be sent as
887: .Ic ~~
888: or by following the tilde by a character other than those described below.
889: The escape character must always follow a newline to be interpreted as
890: special.
891: The escape character can be changed in configuration files using the
892: .Cm EscapeChar
893: configuration directive or on the command line by the
894: .Fl e
895: option.
896: .Pp
897: The supported escapes (assuming the default
898: .Ql ~ )
899: are:
900: .Bl -tag -width Ds
901: .It Cm ~.
902: Disconnect.
903: .It Cm ~^Z
904: Background
1.234 jmc 905: .Nm .
1.218 jmc 906: .It Cm ~#
907: List forwarded connections.
908: .It Cm ~&
909: Background
1.2 deraadt 910: .Nm
1.218 jmc 911: at logout when waiting for forwarded connection / X11 sessions to terminate.
912: .It Cm ~?
913: Display a list of escape characters.
914: .It Cm ~B
915: Send a BREAK to the remote system
916: (only useful for SSH protocol version 2 and if the peer supports it).
917: .It Cm ~C
918: Open command line.
919: Currently this allows the addition of port forwardings using the
1.279 stevesk 920: .Fl L ,
921: .Fl R
1.218 jmc 922: and
1.279 stevesk 923: .Fl D
1.225 jmc 924: options (see above).
1.322 markus 925: It also allows the cancellation of existing port-forwardings
926: with
1.262 stevesk 927: .Sm off
1.322 markus 928: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 929: .Sm on
1.322 markus 930: for local,
931: .Sm off
932: .Fl KR Oo Ar bind_address : Oc Ar port
933: .Sm on
934: for remote and
935: .Sm off
936: .Fl KD Oo Ar bind_address : Oc Ar port
937: .Sm on
938: for dynamic port-forwardings.
1.218 jmc 939: .Ic !\& Ns Ar command
940: allows the user to execute a local command if the
941: .Ic PermitLocalCommand
942: option is enabled in
1.176 jmc 943: .Xr ssh_config 5 .
1.218 jmc 944: Basic help is available, using the
945: .Fl h
946: option.
947: .It Cm ~R
948: Request rekeying of the connection
949: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 950: .It Cm ~V
951: Decrease the verbosity
952: .Pq Ic LogLevel
953: when errors are being written to stderr.
954: .It Cm ~v
1.328 jmc 955: Increase the verbosity
1.327 dtucker 956: .Pq Ic LogLevel
957: when errors are being written to stderr.
1.176 jmc 958: .El
1.246 jmc 959: .Sh TCP FORWARDING
960: Forwarding of arbitrary TCP connections over the secure channel can
961: be specified either on the command line or in a configuration file.
962: One possible application of TCP forwarding is a secure connection to a
963: mail server; another is going through firewalls.
964: .Pp
965: In the example below, we look at encrypting communication between
966: an IRC client and server, even though the IRC server does not directly
967: support encrypted communications.
968: This works as follows:
969: the user connects to the remote host using
970: .Nm ,
971: specifying a port to be used to forward connections
972: to the remote server.
973: After that it is possible to start the service which is to be encrypted
974: on the client machine,
975: connecting to the same local port,
976: and
977: .Nm
978: will encrypt and forward the connection.
979: .Pp
980: The following example tunnels an IRC session from client machine
981: .Dq 127.0.0.1
982: (localhost)
983: to remote server
984: .Dq server.example.com :
985: .Bd -literal -offset 4n
986: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
987: $ irc -c '#users' -p 1234 pinky 127.0.0.1
988: .Ed
989: .Pp
990: This tunnels a connection to IRC server
991: .Dq server.example.com ,
992: joining channel
993: .Dq #users ,
994: nickname
995: .Dq pinky ,
996: using port 1234.
997: It doesn't matter which port is used,
998: as long as it's greater than 1023
999: (remember, only root can open sockets on privileged ports)
1000: and doesn't conflict with any ports already in use.
1001: The connection is forwarded to port 6667 on the remote server,
1002: since that's the standard port for IRC services.
1003: .Pp
1004: The
1005: .Fl f
1006: option backgrounds
1007: .Nm
1008: and the remote command
1009: .Dq sleep 10
1010: is specified to allow an amount of time
1011: (10 seconds, in the example)
1012: to start the service which is to be tunnelled.
1013: If no connections are made within the time specified,
1014: .Nm
1015: will exit.
1016: .Sh X11 FORWARDING
1.218 jmc 1017: If the
1018: .Cm ForwardX11
1019: variable is set to
1020: .Dq yes
1021: (or see the description of the
1.227 jmc 1022: .Fl X ,
1023: .Fl x ,
1.218 jmc 1024: and
1.227 jmc 1025: .Fl Y
1.226 jmc 1026: options above)
1.218 jmc 1027: and the user is using X11 (the
1028: .Ev DISPLAY
1029: environment variable is set), the connection to the X11 display is
1030: automatically forwarded to the remote side in such a way that any X11
1031: programs started from the shell (or command) will go through the
1032: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1033: from the local machine.
1.218 jmc 1034: The user should not manually set
1035: .Ev DISPLAY .
1036: Forwarding of X11 connections can be
1037: configured on the command line or in configuration files.
1038: .Pp
1039: The
1040: .Ev DISPLAY
1041: value set by
1042: .Nm
1043: will point to the server machine, but with a display number greater than zero.
1044: This is normal, and happens because
1045: .Nm
1046: creates a
1047: .Dq proxy
1048: X server on the server machine for forwarding the
1049: connections over the encrypted channel.
1.200 djm 1050: .Pp
1.218 jmc 1051: .Nm
1052: will also automatically set up Xauthority data on the server machine.
1053: For this purpose, it will generate a random authorization cookie,
1054: store it in Xauthority on the server, and verify that any forwarded
1055: connections carry this cookie and replace it by the real cookie when
1056: the connection is opened.
1057: The real authentication cookie is never
1058: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1059: .Pp
1.218 jmc 1060: If the
1061: .Cm ForwardAgent
1062: variable is set to
1063: .Dq yes
1064: (or see the description of the
1065: .Fl A
1.191 djm 1066: and
1.218 jmc 1067: .Fl a
1.226 jmc 1068: options above) and
1.218 jmc 1069: the user is using an authentication agent, the connection to the agent
1070: is automatically forwarded to the remote side.
1.252 jmc 1071: .Sh VERIFYING HOST KEYS
1072: When connecting to a server for the first time,
1073: a fingerprint of the server's public key is presented to the user
1074: (unless the option
1075: .Cm StrictHostKeyChecking
1076: has been disabled).
1077: Fingerprints can be determined using
1078: .Xr ssh-keygen 1 :
1079: .Pp
1080: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1081: .Pp
1.274 grunk 1082: If the fingerprint is already known, it can be matched
1083: and the key can be accepted or rejected.
1084: Because of the difficulty of comparing host keys
1085: just by looking at hex strings,
1086: there is also support to compare host keys visually,
1087: using
1088: .Em random art .
1089: By setting the
1.275 grunk 1090: .Cm VisualHostKey
1.274 grunk 1091: option to
1.275 grunk 1092: .Dq yes ,
1.274 grunk 1093: a small ASCII graphic gets displayed on every login to a server, no matter
1094: if the session itself is interactive or not.
1095: By learning the pattern a known server produces, a user can easily
1096: find out that the host key has changed when a completely different pattern
1097: is displayed.
1098: Because these patterns are not unambiguous however, a pattern that looks
1099: similar to the pattern remembered only gives a good probability that the
1100: host key is the same, not guaranteed proof.
1101: .Pp
1102: To get a listing of the fingerprints along with their random art for
1103: all known hosts, the following command line can be used:
1104: .Pp
1105: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1106: .Pp
1.252 jmc 1107: If the fingerprint is unknown,
1108: an alternative method of verification is available:
1109: SSH fingerprints verified by DNS.
1110: An additional resource record (RR),
1111: SSHFP,
1112: is added to a zonefile
1113: and the connecting client is able to match the fingerprint
1114: with that of the key presented.
1115: .Pp
1116: In this example, we are connecting a client to a server,
1117: .Dq host.example.com .
1118: The SSHFP resource records should first be added to the zonefile for
1119: host.example.com:
1120: .Bd -literal -offset indent
1.259 jakob 1121: $ ssh-keygen -r host.example.com.
1.252 jmc 1122: .Ed
1123: .Pp
1124: The output lines will have to be added to the zonefile.
1125: To check that the zone is answering fingerprint queries:
1126: .Pp
1127: .Dl $ dig -t SSHFP host.example.com
1128: .Pp
1129: Finally the client connects:
1130: .Bd -literal -offset indent
1131: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1132: [...]
1133: Matching host key fingerprint found in DNS.
1134: Are you sure you want to continue connecting (yes/no)?
1135: .Ed
1136: .Pp
1137: See the
1138: .Cm VerifyHostKeyDNS
1139: option in
1140: .Xr ssh_config 5
1141: for more information.
1.250 jmc 1142: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1143: .Nm
1144: contains support for Virtual Private Network (VPN) tunnelling
1145: using the
1146: .Xr tun 4
1147: network pseudo-device,
1148: allowing two networks to be joined securely.
1149: The
1150: .Xr sshd_config 5
1151: configuration option
1152: .Cm PermitTunnel
1153: controls whether the server supports this,
1154: and at what level (layer 2 or 3 traffic).
1155: .Pp
1156: The following example would connect client network 10.0.50.0/24
1.265 otto 1157: with remote network 10.0.99.0/24 using a point-to-point connection
1158: from 10.1.1.1 to 10.1.1.2,
1159: provided that the SSH server running on the gateway to the remote network,
1160: at 192.168.1.15, allows it.
1161: .Pp
1162: On the client:
1.250 jmc 1163: .Bd -literal -offset indent
1164: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1165: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1166: # route add 10.0.99.0/24 10.1.1.2
1167: .Ed
1168: .Pp
1169: On the server:
1170: .Bd -literal -offset indent
1171: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1172: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1173: .Ed
1174: .Pp
1175: Client access may be more finely tuned via the
1176: .Pa /root/.ssh/authorized_keys
1177: file (see below) and the
1178: .Cm PermitRootLogin
1179: server option.
1.255 jmc 1180: The following entry would permit connections on
1.250 jmc 1181: .Xr tun 4
1.255 jmc 1182: device 1 from user
1.250 jmc 1183: .Dq jane
1.255 jmc 1184: and on tun device 2 from user
1.250 jmc 1185: .Dq john ,
1186: if
1187: .Cm PermitRootLogin
1188: is set to
1189: .Dq forced-commands-only :
1190: .Bd -literal -offset 2n
1191: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1192: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1193: .Ed
1194: .Pp
1.264 ray 1195: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1196: it may be more suited to temporary setups,
1197: such as for wireless VPNs.
1198: More permanent VPNs are better provided by tools such as
1199: .Xr ipsecctl 8
1200: and
1201: .Xr isakmpd 8 .
1.2 deraadt 1202: .Sh ENVIRONMENT
1203: .Nm
1.1 deraadt 1204: will normally set the following environment variables:
1.237 jmc 1205: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1206: .It Ev DISPLAY
1207: The
1208: .Ev DISPLAY
1.40 aaron 1209: variable indicates the location of the X11 server.
1.44 aaron 1210: It is automatically set by
1.2 deraadt 1211: .Nm
1212: to point to a value of the form
1.233 jmc 1213: .Dq hostname:n ,
1214: where
1215: .Dq hostname
1216: indicates the host where the shell runs, and
1217: .Sq n
1218: is an integer \*(Ge 1.
1.40 aaron 1219: .Nm
1220: uses this special value to forward X11 connections over the secure
1221: channel.
1.107 markus 1222: The user should normally not set
1223: .Ev DISPLAY
1224: explicitly, as that
1.1 deraadt 1225: will render the X11 connection insecure (and will require the user to
1226: manually copy any required authorization cookies).
1.2 deraadt 1227: .It Ev HOME
1.1 deraadt 1228: Set to the path of the user's home directory.
1.2 deraadt 1229: .It Ev LOGNAME
1230: Synonym for
1.12 aaron 1231: .Ev USER ;
1232: set for compatibility with systems that use this variable.
1.2 deraadt 1233: .It Ev MAIL
1.129 stevesk 1234: Set to the path of the user's mailbox.
1.40 aaron 1235: .It Ev PATH
1.2 deraadt 1236: Set to the default
1237: .Ev PATH ,
1238: as specified when compiling
1.234 jmc 1239: .Nm .
1.118 markus 1240: .It Ev SSH_ASKPASS
1241: If
1242: .Nm
1243: needs a passphrase, it will read the passphrase from the current
1244: terminal if it was run from a terminal.
1245: If
1246: .Nm
1247: does not have a terminal associated with it but
1248: .Ev DISPLAY
1249: and
1250: .Ev SSH_ASKPASS
1251: are set, it will execute the program specified by
1252: .Ev SSH_ASKPASS
1253: and open an X11 window to read the passphrase.
1254: This is particularly useful when calling
1255: .Nm
1256: from a
1.196 jmc 1257: .Pa .xsession
1.118 markus 1258: or related script.
1259: (Note that on some machines it
1260: may be necessary to redirect the input from
1261: .Pa /dev/null
1262: to make this work.)
1.18 markus 1263: .It Ev SSH_AUTH_SOCK
1.233 jmc 1264: Identifies the path of a
1265: .Ux Ns -domain
1266: socket used to communicate with the agent.
1.166 stevesk 1267: .It Ev SSH_CONNECTION
1268: Identifies the client and server ends of the connection.
1.40 aaron 1269: The variable contains
1.233 jmc 1270: four space-separated values: client IP address, client port number,
1271: server IP address, and server port number.
1.73 markus 1272: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1273: This variable contains the original command line if a forced command
1.73 markus 1274: is executed.
1275: It can be used to extract the original arguments.
1.2 deraadt 1276: .It Ev SSH_TTY
1.1 deraadt 1277: This is set to the name of the tty (path to the device) associated
1.40 aaron 1278: with the current shell or command.
1279: If the current session has no tty,
1.1 deraadt 1280: this variable is not set.
1.2 deraadt 1281: .It Ev TZ
1.214 jmc 1282: This variable is set to indicate the present time zone if it
1.257 jmc 1283: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1284: on to new connections).
1.2 deraadt 1285: .It Ev USER
1.1 deraadt 1286: Set to the name of the user logging in.
1.2 deraadt 1287: .El
1288: .Pp
1.44 aaron 1289: Additionally,
1.2 deraadt 1290: .Nm
1.44 aaron 1291: reads
1.207 djm 1292: .Pa ~/.ssh/environment ,
1.2 deraadt 1293: and adds lines of the format
1294: .Dq VARNAME=value
1.233 jmc 1295: to the environment if the file exists and users are allowed to
1.161 marc 1296: change their environment.
1.176 jmc 1297: For more information, see the
1.161 marc 1298: .Cm PermitUserEnvironment
1.162 stevesk 1299: option in
1.161 marc 1300: .Xr sshd_config 5 .
1.2 deraadt 1301: .Sh FILES
1.236 jmc 1302: .Bl -tag -width Ds -compact
1.309 jmc 1303: .It Pa ~/.rhosts
1.240 jmc 1304: This file is used for host-based authentication (see above).
1.92 markus 1305: On some machines this file may need to be
1.240 jmc 1306: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1307: because
1.2 deraadt 1308: .Xr sshd 8
1.40 aaron 1309: reads it as root.
1310: Additionally, this file must be owned by the user,
1311: and must not have write permissions for anyone else.
1312: The recommended
1.1 deraadt 1313: permission for most machines is read/write for the user, and not
1314: accessible by others.
1.2 deraadt 1315: .Pp
1.309 jmc 1316: .It Pa ~/.shosts
1.240 jmc 1317: This file is used in exactly the same way as
1318: .Pa .rhosts ,
1319: but allows host-based authentication without permitting login with
1320: rlogin/rsh.
1.272 mcbride 1321: .Pp
1.309 jmc 1322: .It Pa ~/.ssh/
1.272 mcbride 1323: This directory is the default location for all user-specific configuration
1324: and authentication information.
1325: There is no general requirement to keep the entire contents of this directory
1326: secret, but the recommended permissions are read/write/execute for the user,
1327: and not accessible by others.
1.236 jmc 1328: .Pp
1.309 jmc 1329: .It Pa ~/.ssh/authorized_keys
1.310 djm 1330: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1331: this user.
1.238 jmc 1332: The format of this file is described in the
1333: .Xr sshd 8
1334: manual page.
1335: This file is not highly sensitive, but the recommended
1336: permissions are read/write for the user, and not accessible by others.
1337: .Pp
1.309 jmc 1338: .It Pa ~/.ssh/config
1.238 jmc 1339: This is the per-user configuration file.
1340: The file format and configuration options are described in
1341: .Xr ssh_config 5 .
1342: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1343: read/write for the user, and not writable by others.
1.238 jmc 1344: .Pp
1.309 jmc 1345: .It Pa ~/.ssh/environment
1.239 jmc 1346: Contains additional definitions for environment variables; see
1347: .Sx ENVIRONMENT ,
1.238 jmc 1348: above.
1349: .Pp
1.309 jmc 1350: .It Pa ~/.ssh/identity
1351: .It Pa ~/.ssh/id_dsa
1.310 djm 1352: .It Pa ~/.ssh/id_ecdsa
1.309 jmc 1353: .It Pa ~/.ssh/id_rsa
1.238 jmc 1354: Contains the private key for authentication.
1355: These files
1356: contain sensitive data and should be readable by the user but not
1357: accessible by others (read/write/execute).
1358: .Nm
1359: will simply ignore a private key file if it is accessible by others.
1360: It is possible to specify a passphrase when
1361: generating the key which will be used to encrypt the
1362: sensitive part of this file using 3DES.
1363: .Pp
1.309 jmc 1364: .It Pa ~/.ssh/identity.pub
1365: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1366: .It Pa ~/.ssh/id_ecdsa.pub
1.309 jmc 1367: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1368: Contains the public key for authentication.
1369: These files are not
1370: sensitive and can (but need not) be readable by anyone.
1371: .Pp
1.309 jmc 1372: .It Pa ~/.ssh/known_hosts
1.244 jmc 1373: Contains a list of host keys for all hosts the user has logged into
1374: that are not already in the systemwide list of known host keys.
1.238 jmc 1375: See
1.244 jmc 1376: .Xr sshd 8
1377: for further details of the format of this file.
1.238 jmc 1378: .Pp
1.309 jmc 1379: .It Pa ~/.ssh/rc
1.238 jmc 1380: Commands in this file are executed by
1381: .Nm
1.245 jmc 1382: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1383: started.
1384: See the
1385: .Xr sshd 8
1386: manual page for more information.
1387: .Pp
1.309 jmc 1388: .It Pa /etc/hosts.equiv
1.240 jmc 1389: This file is for host-based authentication (see above).
1390: It should only be writable by root.
1.236 jmc 1391: .Pp
1.309 jmc 1392: .It Pa /etc/shosts.equiv
1.240 jmc 1393: This file is used in exactly the same way as
1394: .Pa hosts.equiv ,
1395: but allows host-based authentication without permitting login with
1396: rlogin/rsh.
1.236 jmc 1397: .Pp
1.238 jmc 1398: .It Pa /etc/ssh/ssh_config
1399: Systemwide configuration file.
1400: The file format and configuration options are described in
1401: .Xr ssh_config 5 .
1402: .Pp
1.309 jmc 1403: .It Pa /etc/ssh/ssh_host_key
1404: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1405: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.309 jmc 1406: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1407: These files contain the private parts of the host keys
1.245 jmc 1408: and are used for host-based authentication.
1409: If protocol version 1 is used,
1.238 jmc 1410: .Nm
1411: must be setuid root, since the host key is readable only by root.
1412: For protocol version 2,
1413: .Nm
1414: uses
1415: .Xr ssh-keysign 8
1.245 jmc 1416: to access the host keys,
1417: eliminating the requirement that
1.238 jmc 1418: .Nm
1.245 jmc 1419: be setuid root when host-based authentication is used.
1.238 jmc 1420: By default
1.2 deraadt 1421: .Nm
1.238 jmc 1422: is not setuid root.
1423: .Pp
1.309 jmc 1424: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1425: Systemwide list of known host keys.
1426: This file should be prepared by the
1427: system administrator to contain the public host keys of all machines in the
1428: organization.
1.244 jmc 1429: It should be world-readable.
1430: See
1.238 jmc 1431: .Xr sshd 8
1.244 jmc 1432: for further details of the format of this file.
1.236 jmc 1433: .Pp
1.309 jmc 1434: .It Pa /etc/ssh/sshrc
1.1 deraadt 1435: Commands in this file are executed by
1.2 deraadt 1436: .Nm
1.245 jmc 1437: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1438: See the
1.2 deraadt 1439: .Xr sshd 8
1.1 deraadt 1440: manual page for more information.
1.58 itojun 1441: .El
1.312 jmc 1442: .Sh EXIT STATUS
1443: .Nm
1444: exits with the exit status of the remote command or with 255
1445: if an error occurred.
1.2 deraadt 1446: .Sh SEE ALSO
1447: .Xr scp 1 ,
1.83 djm 1448: .Xr sftp 1 ,
1.2 deraadt 1449: .Xr ssh-add 1 ,
1450: .Xr ssh-agent 1 ,
1451: .Xr ssh-keygen 1 ,
1.242 jmc 1452: .Xr ssh-keyscan 1 ,
1.250 jmc 1453: .Xr tun 4 ,
1.176 jmc 1454: .Xr hosts.equiv 5 ,
1.159 stevesk 1455: .Xr ssh_config 5 ,
1.160 naddy 1456: .Xr ssh-keysign 8 ,
1.87 itojun 1457: .Xr sshd 8
1.329 jmc 1458: .Sh STANDARDS
1.106 markus 1459: .Rs
1.329 jmc 1460: .%A S. Lehtinen
1461: .%A C. Lonvick
1462: .%D January 2006
1.256 jmc 1463: .%R RFC 4250
1.329 jmc 1464: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1465: .Re
1.329 jmc 1466: .Pp
1.256 jmc 1467: .Rs
1.329 jmc 1468: .%A T. Ylonen
1469: .%A C. Lonvick
1470: .%D January 2006
1.256 jmc 1471: .%R RFC 4251
1.329 jmc 1472: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1473: .Re
1.329 jmc 1474: .Pp
1.256 jmc 1475: .Rs
1.329 jmc 1476: .%A T. Ylonen
1477: .%A C. Lonvick
1478: .%D January 2006
1.256 jmc 1479: .%R RFC 4252
1.329 jmc 1480: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1481: .Re
1.329 jmc 1482: .Pp
1.256 jmc 1483: .Rs
1.329 jmc 1484: .%A T. Ylonen
1485: .%A C. Lonvick
1486: .%D January 2006
1.256 jmc 1487: .%R RFC 4253
1.329 jmc 1488: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1489: .Re
1.329 jmc 1490: .Pp
1.256 jmc 1491: .Rs
1.329 jmc 1492: .%A T. Ylonen
1493: .%A C. Lonvick
1494: .%D January 2006
1.256 jmc 1495: .%R RFC 4254
1.329 jmc 1496: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1497: .Re
1.329 jmc 1498: .Pp
1.256 jmc 1499: .Rs
1.329 jmc 1500: .%A J. Schlyter
1501: .%A W. Griffin
1502: .%D January 2006
1.256 jmc 1503: .%R RFC 4255
1.329 jmc 1504: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1505: .Re
1.329 jmc 1506: .Pp
1.256 jmc 1507: .Rs
1.329 jmc 1508: .%A F. Cusack
1509: .%A M. Forssen
1510: .%D January 2006
1.256 jmc 1511: .%R RFC 4256
1.329 jmc 1512: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1513: .Re
1.329 jmc 1514: .Pp
1.256 jmc 1515: .Rs
1.329 jmc 1516: .%A J. Galbraith
1517: .%A P. Remaker
1518: .%D January 2006
1.256 jmc 1519: .%R RFC 4335
1.329 jmc 1520: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1521: .Re
1.329 jmc 1522: .Pp
1.256 jmc 1523: .Rs
1.329 jmc 1524: .%A M. Bellare
1525: .%A T. Kohno
1526: .%A C. Namprempre
1527: .%D January 2006
1.256 jmc 1528: .%R RFC 4344
1.329 jmc 1529: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1530: .Re
1.329 jmc 1531: .Pp
1.256 jmc 1532: .Rs
1.329 jmc 1533: .%A B. Harris
1534: .%D January 2006
1.256 jmc 1535: .%R RFC 4345
1.329 jmc 1536: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1537: .Re
1.329 jmc 1538: .Pp
1.258 djm 1539: .Rs
1.329 jmc 1540: .%A M. Friedl
1541: .%A N. Provos
1542: .%A W. Simpson
1543: .%D March 2006
1.258 djm 1544: .%R RFC 4419
1.329 jmc 1545: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1546: .Re
1.329 jmc 1547: .Pp
1.266 markus 1548: .Rs
1.329 jmc 1549: .%A J. Galbraith
1550: .%A R. Thayer
1551: .%D November 2006
1.266 markus 1552: .%R RFC 4716
1.329 jmc 1553: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1554: .Re
1.329 jmc 1555: .Pp
1.313 djm 1556: .Rs
1.329 jmc 1557: .%A D. Stebila
1558: .%A J. Green
1559: .%D December 2009
1.313 djm 1560: .%R RFC 5656
1.329 jmc 1561: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1562: .Re
1.329 jmc 1563: .Pp
1.274 grunk 1564: .Rs
1565: .%A A. Perrig
1566: .%A D. Song
1567: .%D 1999
1.329 jmc 1568: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1569: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1570: .Re
1.173 jmc 1571: .Sh AUTHORS
1572: OpenSSH is a derivative of the original and free
1573: ssh 1.2.12 release by Tatu Ylonen.
1574: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1575: Theo de Raadt and Dug Song
1576: removed many bugs, re-added newer features and
1577: created OpenSSH.
1578: Markus Friedl contributed the support for SSH
1579: protocol versions 1.5 and 2.0.
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