Annotation of src/usr.bin/ssh/ssh.1, Revision 1.339
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: .\"
1.339 ! djm 36: .\" $OpenBSD: ssh.1,v 1.338 2013/10/16 02:31:46 djm Exp $
! 37: .Dd $Mdocdate: October 16 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
1.338 djm 420: .It CanonicalDomains
1.339 ! djm 421: .It CanonicalizeFallbackLocal
! 422: .It CanonicalizeHostname
! 423: .It CanonicalizeMaxDots
! 424: .It CanonicalizePermittedCNAMEs
1.218 jmc 425: .It ChallengeResponseAuthentication
426: .It CheckHostIP
427: .It Cipher
428: .It Ciphers
429: .It ClearAllForwardings
430: .It Compression
431: .It CompressionLevel
432: .It ConnectionAttempts
433: .It ConnectTimeout
434: .It ControlMaster
435: .It ControlPath
1.321 djm 436: .It ControlPersist
1.218 jmc 437: .It DynamicForward
438: .It EscapeChar
1.263 markus 439: .It ExitOnForwardFailure
1.218 jmc 440: .It ForwardAgent
441: .It ForwardX11
1.321 djm 442: .It ForwardX11Timeout
1.218 jmc 443: .It ForwardX11Trusted
444: .It GatewayPorts
445: .It GlobalKnownHostsFile
446: .It GSSAPIAuthentication
447: .It GSSAPIDelegateCredentials
448: .It HashKnownHosts
449: .It Host
450: .It HostbasedAuthentication
451: .It HostKeyAlgorithms
452: .It HostKeyAlias
453: .It HostName
454: .It IdentityFile
455: .It IdentitiesOnly
1.316 jmc 456: .It IPQoS
1.321 djm 457: .It KbdInteractiveAuthentication
1.218 jmc 458: .It KbdInteractiveDevices
1.314 jmc 459: .It KexAlgorithms
1.218 jmc 460: .It LocalCommand
461: .It LocalForward
462: .It LogLevel
463: .It MACs
1.337 jmc 464: .It Match
1.218 jmc 465: .It NoHostAuthenticationForLocalhost
466: .It NumberOfPasswordPrompts
467: .It PasswordAuthentication
468: .It PermitLocalCommand
1.291 markus 469: .It PKCS11Provider
1.218 jmc 470: .It Port
471: .It PreferredAuthentications
472: .It Protocol
473: .It ProxyCommand
1.336 jmc 474: .It ProxyUseFdpass
1.218 jmc 475: .It PubkeyAuthentication
1.251 dtucker 476: .It RekeyLimit
1.218 jmc 477: .It RemoteForward
1.319 jmc 478: .It RequestTTY
1.218 jmc 479: .It RhostsRSAAuthentication
480: .It RSAAuthentication
481: .It SendEnv
482: .It ServerAliveInterval
483: .It ServerAliveCountMax
484: .It StrictHostKeyChecking
485: .It TCPKeepAlive
486: .It Tunnel
487: .It TunnelDevice
488: .It UsePrivilegedPort
489: .It User
490: .It UserKnownHostsFile
491: .It VerifyHostKeyDNS
1.276 jmc 492: .It VisualHostKey
1.218 jmc 493: .It XAuthLocation
494: .El
495: .It Fl p Ar port
496: Port to connect to on the remote host.
497: This can be specified on a
498: per-host basis in the configuration file.
1.332 djm 499: .It Fl Q Ar protocol_feature
500: Queries
501: .Nm
502: for the algorithms supported for the specified version 2
503: .Ar protocol_feature .
504: The queriable features are:
505: .Dq cipher
506: (supported symmetric ciphers),
507: .Dq MAC
508: (supported message integrity codes),
509: .Dq KEX
510: (key exchange algorithms),
511: .Dq key
512: (key types).
513: Protocol features are treated case-insensitively.
1.218 jmc 514: .It Fl q
515: Quiet mode.
1.271 djm 516: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 517: .It Fl R Xo
518: .Sm off
519: .Oo Ar bind_address : Oc
520: .Ar port : host : hostport
521: .Sm on
522: .Xc
523: Specifies that the given port on the remote (server) host is to be
524: forwarded to the given host and port on the local side.
525: This works by allocating a socket to listen to
526: .Ar port
527: on the remote side, and whenever a connection is made to this port, the
528: connection is forwarded over the secure channel, and a connection is
529: made to
530: .Ar host
531: port
532: .Ar hostport
533: from the local machine.
1.2 deraadt 534: .Pp
1.218 jmc 535: Port forwardings can also be specified in the configuration file.
536: Privileged ports can be forwarded only when
537: logging in as root on the remote machine.
1.324 jmc 538: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 539: .Pp
1.218 jmc 540: By default, the listening socket on the server will be bound to the loopback
541: interface only.
1.280 tobias 542: This may be overridden by specifying a
1.218 jmc 543: .Ar bind_address .
544: An empty
545: .Ar bind_address ,
546: or the address
547: .Ql * ,
548: indicates that the remote socket should listen on all interfaces.
549: Specifying a remote
550: .Ar bind_address
551: will only succeed if the server's
552: .Cm GatewayPorts
553: option is enabled (see
554: .Xr sshd_config 5 ) .
1.281 djm 555: .Pp
556: If the
557: .Ar port
558: argument is
1.282 djm 559: .Ql 0 ,
1.281 djm 560: the listen port will be dynamically allocated on the server and reported
561: to the client at run time.
1.305 markus 562: When used together with
563: .Ic -O forward
564: the allocated port will be printed to the standard output.
1.218 jmc 565: .It Fl S Ar ctl_path
1.304 jmc 566: Specifies the location of a control socket for connection sharing,
1.303 djm 567: or the string
568: .Dq none
569: to disable connection sharing.
1.218 jmc 570: Refer to the description of
571: .Cm ControlPath
572: and
573: .Cm ControlMaster
574: in
575: .Xr ssh_config 5
576: for details.
577: .It Fl s
578: May be used to request invocation of a subsystem on the remote system.
579: Subsystems are a feature of the SSH2 protocol which facilitate the use
580: of SSH as a secure transport for other applications (eg.\&
581: .Xr sftp 1 ) .
582: The subsystem is specified as the remote command.
583: .It Fl T
584: Disable pseudo-tty allocation.
585: .It Fl t
586: Force pseudo-tty allocation.
587: This can be used to execute arbitrary
588: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 589: e.g. when implementing menu services.
1.218 jmc 590: Multiple
591: .Fl t
592: options force tty allocation, even if
1.194 jakob 593: .Nm
1.218 jmc 594: has no local tty.
595: .It Fl V
596: Display the version number and exit.
597: .It Fl v
598: Verbose mode.
599: Causes
1.176 jmc 600: .Nm
1.218 jmc 601: to print debugging messages about its progress.
602: This is helpful in
603: debugging connection, authentication, and configuration problems.
604: Multiple
605: .Fl v
606: options increase the verbosity.
607: The maximum is 3.
1.290 dtucker 608: .It Fl W Ar host : Ns Ar port
609: Requests that standard input and output on the client be forwarded to
610: .Ar host
611: on
612: .Ar port
613: over the secure channel.
614: Implies
615: .Fl N ,
616: .Fl T ,
617: .Cm ExitOnForwardFailure
618: and
1.326 dtucker 619: .Cm ClearAllForwardings .
620: Works with Protocol version 2 only.
1.261 stevesk 621: .It Fl w Xo
622: .Ar local_tun Ns Op : Ns Ar remote_tun
623: .Xc
624: Requests
625: tunnel
626: device forwarding with the specified
1.218 jmc 627: .Xr tun 4
1.261 stevesk 628: devices between the client
629: .Pq Ar local_tun
630: and the server
631: .Pq Ar remote_tun .
632: .Pp
1.228 jmc 633: The devices may be specified by numerical ID or the keyword
634: .Dq any ,
635: which uses the next available tunnel device.
1.261 stevesk 636: If
637: .Ar remote_tun
638: is not specified, it defaults to
639: .Dq any .
1.228 jmc 640: See also the
1.218 jmc 641: .Cm Tunnel
1.261 stevesk 642: and
643: .Cm TunnelDevice
644: directives in
1.218 jmc 645: .Xr ssh_config 5 .
1.261 stevesk 646: If the
647: .Cm Tunnel
648: directive is unset, it is set to the default tunnel mode, which is
649: .Dq point-to-point .
1.218 jmc 650: .It Fl X
651: Enables X11 forwarding.
1.54 markus 652: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 653: .Pp
1.218 jmc 654: X11 forwarding should be enabled with caution.
1.168 jmc 655: Users with the ability to bypass file permissions on the remote host
1.218 jmc 656: (for the user's X authorization database)
657: can access the local X11 display through the forwarded connection.
658: An attacker may then be able to perform activities such as keystroke monitoring.
659: .Pp
660: For this reason, X11 forwarding is subjected to X11 SECURITY extension
661: restrictions by default.
662: Please refer to the
663: .Nm
664: .Fl Y
665: option and the
666: .Cm ForwardX11Trusted
667: directive in
668: .Xr ssh_config 5
669: for more information.
670: .It Fl x
671: Disables X11 forwarding.
672: .It Fl Y
673: Enables trusted X11 forwarding.
674: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
675: controls.
1.278 djm 676: .It Fl y
677: Send log information using the
678: .Xr syslog 3
679: system module.
680: By default this information is sent to stderr.
1.218 jmc 681: .El
1.224 jmc 682: .Pp
683: .Nm
684: may additionally obtain configuration data from
685: a per-user configuration file and a system-wide configuration file.
686: The file format and configuration options are described in
687: .Xr ssh_config 5 .
1.222 jmc 688: .Sh AUTHENTICATION
1.249 jmc 689: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 690: The default is to use protocol 2 only,
691: though this can be changed via the
1.222 jmc 692: .Cm Protocol
693: option in
1.284 jmc 694: .Xr ssh_config 5
695: or the
1.222 jmc 696: .Fl 1
697: and
698: .Fl 2
699: options (see above).
700: Both protocols support similar authentication methods,
1.284 jmc 701: but protocol 2 is the default since
1.222 jmc 702: it provides additional mechanisms for confidentiality
703: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 704: and integrity (hmac-md5, hmac-sha1,
705: hmac-sha2-256, hmac-sha2-512,
1.330 markus 706: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 707: Protocol 1 lacks a strong mechanism for ensuring the
708: integrity of the connection.
709: .Pp
710: The methods available for authentication are:
1.260 jmc 711: GSSAPI-based authentication,
1.222 jmc 712: host-based authentication,
713: public key authentication,
714: challenge-response authentication,
715: and password authentication.
716: Authentication methods are tried in the order specified above,
717: though protocol 2 has a configuration option to change the default order:
718: .Cm PreferredAuthentications .
719: .Pp
720: Host-based authentication works as follows:
1.218 jmc 721: If the machine the user logs in from is listed in
722: .Pa /etc/hosts.equiv
723: or
724: .Pa /etc/shosts.equiv
725: on the remote machine, and the user names are
726: the same on both sides, or if the files
727: .Pa ~/.rhosts
728: or
729: .Pa ~/.shosts
730: exist in the user's home directory on the
731: remote machine and contain a line containing the name of the client
732: machine and the name of the user on that machine, the user is
1.222 jmc 733: considered for login.
734: Additionally, the server
735: .Em must
736: be able to verify the client's
737: host key (see the description of
1.218 jmc 738: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 739: and
1.222 jmc 740: .Pa ~/.ssh/known_hosts ,
741: below)
742: for login to be permitted.
1.218 jmc 743: This authentication method closes security holes due to IP
1.222 jmc 744: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 745: [Note to the administrator:
746: .Pa /etc/hosts.equiv ,
747: .Pa ~/.rhosts ,
748: and the rlogin/rsh protocol in general, are inherently insecure and should be
749: disabled if security is desired.]
1.189 dtucker 750: .Pp
1.222 jmc 751: Public key authentication works as follows:
752: The scheme is based on public-key cryptography,
753: using cryptosystems
754: where encryption and decryption are done using separate keys,
755: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 756: The idea is that each user creates a public/private
757: key pair for authentication purposes.
758: The server knows the public key, and only the user knows the private key.
1.222 jmc 759: .Nm
760: implements public key authentication protocol automatically,
1.310 djm 761: using one of the DSA, ECDSA or RSA algorithms.
1.222 jmc 762: Protocol 1 is restricted to using only RSA keys,
1.310 djm 763: but protocol 2 may use any.
1.333 jmc 764: The HISTORY section of
1.222 jmc 765: .Xr ssl 8
1.311 jmc 766: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 767: .Pp
1.218 jmc 768: The file
769: .Pa ~/.ssh/authorized_keys
770: lists the public keys that are permitted for logging in.
771: When the user logs in, the
1.2 deraadt 772: .Nm
1.218 jmc 773: program tells the server which key pair it would like to use for
774: authentication.
1.222 jmc 775: The client proves that it has access to the private key
776: and the server checks that the corresponding public key
777: is authorized to accept the account.
1.218 jmc 778: .Pp
1.222 jmc 779: The user creates his/her key pair by running
1.218 jmc 780: .Xr ssh-keygen 1 .
781: This stores the private key in
1.207 djm 782: .Pa ~/.ssh/identity
1.222 jmc 783: (protocol 1),
784: .Pa ~/.ssh/id_dsa
785: (protocol 2 DSA),
1.310 djm 786: .Pa ~/.ssh/id_ecdsa
787: (protocol 2 ECDSA),
1.222 jmc 788: or
789: .Pa ~/.ssh/id_rsa
790: (protocol 2 RSA)
1.218 jmc 791: and stores the public key in
792: .Pa ~/.ssh/identity.pub
1.222 jmc 793: (protocol 1),
794: .Pa ~/.ssh/id_dsa.pub
795: (protocol 2 DSA),
1.310 djm 796: .Pa ~/.ssh/id_ecdsa.pub
797: (protocol 2 ECDSA),
1.222 jmc 798: or
799: .Pa ~/.ssh/id_rsa.pub
800: (protocol 2 RSA)
1.218 jmc 801: in the user's home directory.
1.222 jmc 802: The user should then copy the public key
1.218 jmc 803: to
804: .Pa ~/.ssh/authorized_keys
1.222 jmc 805: in his/her home directory on the remote machine.
806: The
1.218 jmc 807: .Pa authorized_keys
808: file corresponds to the conventional
809: .Pa ~/.rhosts
810: file, and has one key
1.222 jmc 811: per line, though the lines can be very long.
1.218 jmc 812: After this, the user can log in without giving the password.
813: .Pp
1.301 jmc 814: A variation on public key authentication
815: is available in the form of certificate authentication:
816: instead of a set of public/private keys,
817: signed certificates are used.
818: This has the advantage that a single trusted certification authority
819: can be used in place of many public/private keys.
1.333 jmc 820: See the CERTIFICATES section of
1.301 jmc 821: .Xr ssh-keygen 1
822: for more information.
823: .Pp
824: The most convenient way to use public key or certificate authentication
825: may be with an authentication agent.
1.218 jmc 826: See
827: .Xr ssh-agent 1
828: for more information.
829: .Pp
1.222 jmc 830: Challenge-response authentication works as follows:
831: The server sends an arbitrary
832: .Qq challenge
833: text, and prompts for a response.
834: Protocol 2 allows multiple challenges and responses;
835: protocol 1 is restricted to just one challenge/response.
836: Examples of challenge-response authentication include
1.335 jmc 837: .Bx
838: Authentication (see
1.222 jmc 839: .Xr login.conf 5 )
1.335 jmc 840: and PAM (some
841: .Pf non- Ox
842: systems).
1.222 jmc 843: .Pp
844: Finally, if other authentication methods fail,
1.218 jmc 845: .Nm
846: prompts the user for a password.
847: The password is sent to the remote
848: host for checking; however, since all communications are encrypted,
849: the password cannot be seen by someone listening on the network.
1.232 jmc 850: .Pp
851: .Nm
852: automatically maintains and checks a database containing
853: identification for all hosts it has ever been used with.
854: Host keys are stored in
855: .Pa ~/.ssh/known_hosts
856: in the user's home directory.
857: Additionally, the file
858: .Pa /etc/ssh/ssh_known_hosts
859: is automatically checked for known hosts.
860: Any new hosts are automatically added to the user's file.
861: If a host's identification ever changes,
862: .Nm
863: warns about this and disables password authentication to prevent
864: server spoofing or man-in-the-middle attacks,
865: which could otherwise be used to circumvent the encryption.
866: The
867: .Cm StrictHostKeyChecking
868: option can be used to control logins to machines whose
869: host key is not known or has changed.
870: .Pp
1.218 jmc 871: When the user's identity has been accepted by the server, the server
872: either executes the given command, or logs into the machine and gives
873: the user a normal shell on the remote machine.
874: All communication with
875: the remote command or shell will be automatically encrypted.
876: .Pp
877: If a pseudo-terminal has been allocated (normal login session), the
878: user may use the escape characters noted below.
879: .Pp
880: If no pseudo-tty has been allocated,
881: the session is transparent and can be used to reliably transfer binary data.
882: On most systems, setting the escape character to
883: .Dq none
884: will also make the session transparent even if a tty is used.
885: .Pp
886: The session terminates when the command or shell on the remote
1.247 jmc 887: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 888: .Sh ESCAPE CHARACTERS
1.218 jmc 889: When a pseudo-terminal has been requested,
1.2 deraadt 890: .Nm
1.218 jmc 891: supports a number of functions through the use of an escape character.
892: .Pp
893: A single tilde character can be sent as
894: .Ic ~~
895: or by following the tilde by a character other than those described below.
896: The escape character must always follow a newline to be interpreted as
897: special.
898: The escape character can be changed in configuration files using the
899: .Cm EscapeChar
900: configuration directive or on the command line by the
901: .Fl e
902: option.
903: .Pp
904: The supported escapes (assuming the default
905: .Ql ~ )
906: are:
907: .Bl -tag -width Ds
908: .It Cm ~.
909: Disconnect.
910: .It Cm ~^Z
911: Background
1.234 jmc 912: .Nm .
1.218 jmc 913: .It Cm ~#
914: List forwarded connections.
915: .It Cm ~&
916: Background
1.2 deraadt 917: .Nm
1.218 jmc 918: at logout when waiting for forwarded connection / X11 sessions to terminate.
919: .It Cm ~?
920: Display a list of escape characters.
921: .It Cm ~B
922: Send a BREAK to the remote system
923: (only useful for SSH protocol version 2 and if the peer supports it).
924: .It Cm ~C
925: Open command line.
926: Currently this allows the addition of port forwardings using the
1.279 stevesk 927: .Fl L ,
928: .Fl R
1.218 jmc 929: and
1.279 stevesk 930: .Fl D
1.225 jmc 931: options (see above).
1.322 markus 932: It also allows the cancellation of existing port-forwardings
933: with
1.262 stevesk 934: .Sm off
1.322 markus 935: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 936: .Sm on
1.322 markus 937: for local,
938: .Sm off
939: .Fl KR Oo Ar bind_address : Oc Ar port
940: .Sm on
941: for remote and
942: .Sm off
943: .Fl KD Oo Ar bind_address : Oc Ar port
944: .Sm on
945: for dynamic port-forwardings.
1.218 jmc 946: .Ic !\& Ns Ar command
947: allows the user to execute a local command if the
948: .Ic PermitLocalCommand
949: option is enabled in
1.176 jmc 950: .Xr ssh_config 5 .
1.218 jmc 951: Basic help is available, using the
952: .Fl h
953: option.
954: .It Cm ~R
955: Request rekeying of the connection
956: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 957: .It Cm ~V
958: Decrease the verbosity
959: .Pq Ic LogLevel
960: when errors are being written to stderr.
961: .It Cm ~v
1.328 jmc 962: Increase the verbosity
1.327 dtucker 963: .Pq Ic LogLevel
964: when errors are being written to stderr.
1.176 jmc 965: .El
1.246 jmc 966: .Sh TCP FORWARDING
967: Forwarding of arbitrary TCP connections over the secure channel can
968: be specified either on the command line or in a configuration file.
969: One possible application of TCP forwarding is a secure connection to a
970: mail server; another is going through firewalls.
971: .Pp
972: In the example below, we look at encrypting communication between
973: an IRC client and server, even though the IRC server does not directly
974: support encrypted communications.
975: This works as follows:
976: the user connects to the remote host using
977: .Nm ,
978: specifying a port to be used to forward connections
979: to the remote server.
980: After that it is possible to start the service which is to be encrypted
981: on the client machine,
982: connecting to the same local port,
983: and
984: .Nm
985: will encrypt and forward the connection.
986: .Pp
987: The following example tunnels an IRC session from client machine
988: .Dq 127.0.0.1
989: (localhost)
990: to remote server
991: .Dq server.example.com :
992: .Bd -literal -offset 4n
993: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
994: $ irc -c '#users' -p 1234 pinky 127.0.0.1
995: .Ed
996: .Pp
997: This tunnels a connection to IRC server
998: .Dq server.example.com ,
999: joining channel
1000: .Dq #users ,
1001: nickname
1002: .Dq pinky ,
1003: using port 1234.
1004: It doesn't matter which port is used,
1005: as long as it's greater than 1023
1006: (remember, only root can open sockets on privileged ports)
1007: and doesn't conflict with any ports already in use.
1008: The connection is forwarded to port 6667 on the remote server,
1009: since that's the standard port for IRC services.
1010: .Pp
1011: The
1012: .Fl f
1013: option backgrounds
1014: .Nm
1015: and the remote command
1016: .Dq sleep 10
1017: is specified to allow an amount of time
1018: (10 seconds, in the example)
1019: to start the service which is to be tunnelled.
1020: If no connections are made within the time specified,
1021: .Nm
1022: will exit.
1023: .Sh X11 FORWARDING
1.218 jmc 1024: If the
1025: .Cm ForwardX11
1026: variable is set to
1027: .Dq yes
1028: (or see the description of the
1.227 jmc 1029: .Fl X ,
1030: .Fl x ,
1.218 jmc 1031: and
1.227 jmc 1032: .Fl Y
1.226 jmc 1033: options above)
1.218 jmc 1034: and the user is using X11 (the
1035: .Ev DISPLAY
1036: environment variable is set), the connection to the X11 display is
1037: automatically forwarded to the remote side in such a way that any X11
1038: programs started from the shell (or command) will go through the
1039: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1040: from the local machine.
1.218 jmc 1041: The user should not manually set
1042: .Ev DISPLAY .
1043: Forwarding of X11 connections can be
1044: configured on the command line or in configuration files.
1045: .Pp
1046: The
1047: .Ev DISPLAY
1048: value set by
1049: .Nm
1050: will point to the server machine, but with a display number greater than zero.
1051: This is normal, and happens because
1052: .Nm
1053: creates a
1054: .Dq proxy
1055: X server on the server machine for forwarding the
1056: connections over the encrypted channel.
1.200 djm 1057: .Pp
1.218 jmc 1058: .Nm
1059: will also automatically set up Xauthority data on the server machine.
1060: For this purpose, it will generate a random authorization cookie,
1061: store it in Xauthority on the server, and verify that any forwarded
1062: connections carry this cookie and replace it by the real cookie when
1063: the connection is opened.
1064: The real authentication cookie is never
1065: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1066: .Pp
1.218 jmc 1067: If the
1068: .Cm ForwardAgent
1069: variable is set to
1070: .Dq yes
1071: (or see the description of the
1072: .Fl A
1.191 djm 1073: and
1.218 jmc 1074: .Fl a
1.226 jmc 1075: options above) and
1.218 jmc 1076: the user is using an authentication agent, the connection to the agent
1077: is automatically forwarded to the remote side.
1.252 jmc 1078: .Sh VERIFYING HOST KEYS
1079: When connecting to a server for the first time,
1080: a fingerprint of the server's public key is presented to the user
1081: (unless the option
1082: .Cm StrictHostKeyChecking
1083: has been disabled).
1084: Fingerprints can be determined using
1085: .Xr ssh-keygen 1 :
1086: .Pp
1087: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1088: .Pp
1.274 grunk 1089: If the fingerprint is already known, it can be matched
1090: and the key can be accepted or rejected.
1091: Because of the difficulty of comparing host keys
1092: just by looking at hex strings,
1093: there is also support to compare host keys visually,
1094: using
1095: .Em random art .
1096: By setting the
1.275 grunk 1097: .Cm VisualHostKey
1.274 grunk 1098: option to
1.275 grunk 1099: .Dq yes ,
1.274 grunk 1100: a small ASCII graphic gets displayed on every login to a server, no matter
1101: if the session itself is interactive or not.
1102: By learning the pattern a known server produces, a user can easily
1103: find out that the host key has changed when a completely different pattern
1104: is displayed.
1105: Because these patterns are not unambiguous however, a pattern that looks
1106: similar to the pattern remembered only gives a good probability that the
1107: host key is the same, not guaranteed proof.
1108: .Pp
1109: To get a listing of the fingerprints along with their random art for
1110: all known hosts, the following command line can be used:
1111: .Pp
1112: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1113: .Pp
1.252 jmc 1114: If the fingerprint is unknown,
1115: an alternative method of verification is available:
1116: SSH fingerprints verified by DNS.
1117: An additional resource record (RR),
1118: SSHFP,
1119: is added to a zonefile
1120: and the connecting client is able to match the fingerprint
1121: with that of the key presented.
1122: .Pp
1123: In this example, we are connecting a client to a server,
1124: .Dq host.example.com .
1125: The SSHFP resource records should first be added to the zonefile for
1126: host.example.com:
1127: .Bd -literal -offset indent
1.259 jakob 1128: $ ssh-keygen -r host.example.com.
1.252 jmc 1129: .Ed
1130: .Pp
1131: The output lines will have to be added to the zonefile.
1132: To check that the zone is answering fingerprint queries:
1133: .Pp
1134: .Dl $ dig -t SSHFP host.example.com
1135: .Pp
1136: Finally the client connects:
1137: .Bd -literal -offset indent
1138: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1139: [...]
1140: Matching host key fingerprint found in DNS.
1141: Are you sure you want to continue connecting (yes/no)?
1142: .Ed
1143: .Pp
1144: See the
1145: .Cm VerifyHostKeyDNS
1146: option in
1147: .Xr ssh_config 5
1148: for more information.
1.250 jmc 1149: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1150: .Nm
1151: contains support for Virtual Private Network (VPN) tunnelling
1152: using the
1153: .Xr tun 4
1154: network pseudo-device,
1155: allowing two networks to be joined securely.
1156: The
1157: .Xr sshd_config 5
1158: configuration option
1159: .Cm PermitTunnel
1160: controls whether the server supports this,
1161: and at what level (layer 2 or 3 traffic).
1162: .Pp
1163: The following example would connect client network 10.0.50.0/24
1.265 otto 1164: with remote network 10.0.99.0/24 using a point-to-point connection
1165: from 10.1.1.1 to 10.1.1.2,
1166: provided that the SSH server running on the gateway to the remote network,
1167: at 192.168.1.15, allows it.
1168: .Pp
1169: On the client:
1.250 jmc 1170: .Bd -literal -offset indent
1171: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1172: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1173: # route add 10.0.99.0/24 10.1.1.2
1174: .Ed
1175: .Pp
1176: On the server:
1177: .Bd -literal -offset indent
1178: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1179: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1180: .Ed
1181: .Pp
1182: Client access may be more finely tuned via the
1183: .Pa /root/.ssh/authorized_keys
1184: file (see below) and the
1185: .Cm PermitRootLogin
1186: server option.
1.255 jmc 1187: The following entry would permit connections on
1.250 jmc 1188: .Xr tun 4
1.255 jmc 1189: device 1 from user
1.250 jmc 1190: .Dq jane
1.255 jmc 1191: and on tun device 2 from user
1.250 jmc 1192: .Dq john ,
1193: if
1194: .Cm PermitRootLogin
1195: is set to
1196: .Dq forced-commands-only :
1197: .Bd -literal -offset 2n
1198: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1199: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1200: .Ed
1201: .Pp
1.264 ray 1202: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1203: it may be more suited to temporary setups,
1204: such as for wireless VPNs.
1205: More permanent VPNs are better provided by tools such as
1206: .Xr ipsecctl 8
1207: and
1208: .Xr isakmpd 8 .
1.2 deraadt 1209: .Sh ENVIRONMENT
1210: .Nm
1.1 deraadt 1211: will normally set the following environment variables:
1.237 jmc 1212: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1213: .It Ev DISPLAY
1214: The
1215: .Ev DISPLAY
1.40 aaron 1216: variable indicates the location of the X11 server.
1.44 aaron 1217: It is automatically set by
1.2 deraadt 1218: .Nm
1219: to point to a value of the form
1.233 jmc 1220: .Dq hostname:n ,
1221: where
1222: .Dq hostname
1223: indicates the host where the shell runs, and
1224: .Sq n
1225: is an integer \*(Ge 1.
1.40 aaron 1226: .Nm
1227: uses this special value to forward X11 connections over the secure
1228: channel.
1.107 markus 1229: The user should normally not set
1230: .Ev DISPLAY
1231: explicitly, as that
1.1 deraadt 1232: will render the X11 connection insecure (and will require the user to
1233: manually copy any required authorization cookies).
1.2 deraadt 1234: .It Ev HOME
1.1 deraadt 1235: Set to the path of the user's home directory.
1.2 deraadt 1236: .It Ev LOGNAME
1237: Synonym for
1.12 aaron 1238: .Ev USER ;
1239: set for compatibility with systems that use this variable.
1.2 deraadt 1240: .It Ev MAIL
1.129 stevesk 1241: Set to the path of the user's mailbox.
1.40 aaron 1242: .It Ev PATH
1.2 deraadt 1243: Set to the default
1244: .Ev PATH ,
1245: as specified when compiling
1.234 jmc 1246: .Nm .
1.118 markus 1247: .It Ev SSH_ASKPASS
1248: If
1249: .Nm
1250: needs a passphrase, it will read the passphrase from the current
1251: terminal if it was run from a terminal.
1252: If
1253: .Nm
1254: does not have a terminal associated with it but
1255: .Ev DISPLAY
1256: and
1257: .Ev SSH_ASKPASS
1258: are set, it will execute the program specified by
1259: .Ev SSH_ASKPASS
1260: and open an X11 window to read the passphrase.
1261: This is particularly useful when calling
1262: .Nm
1263: from a
1.196 jmc 1264: .Pa .xsession
1.118 markus 1265: or related script.
1266: (Note that on some machines it
1267: may be necessary to redirect the input from
1268: .Pa /dev/null
1269: to make this work.)
1.18 markus 1270: .It Ev SSH_AUTH_SOCK
1.233 jmc 1271: Identifies the path of a
1272: .Ux Ns -domain
1273: socket used to communicate with the agent.
1.166 stevesk 1274: .It Ev SSH_CONNECTION
1275: Identifies the client and server ends of the connection.
1.40 aaron 1276: The variable contains
1.233 jmc 1277: four space-separated values: client IP address, client port number,
1278: server IP address, and server port number.
1.73 markus 1279: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1280: This variable contains the original command line if a forced command
1.73 markus 1281: is executed.
1282: It can be used to extract the original arguments.
1.2 deraadt 1283: .It Ev SSH_TTY
1.1 deraadt 1284: This is set to the name of the tty (path to the device) associated
1.40 aaron 1285: with the current shell or command.
1286: If the current session has no tty,
1.1 deraadt 1287: this variable is not set.
1.2 deraadt 1288: .It Ev TZ
1.214 jmc 1289: This variable is set to indicate the present time zone if it
1.257 jmc 1290: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1291: on to new connections).
1.2 deraadt 1292: .It Ev USER
1.1 deraadt 1293: Set to the name of the user logging in.
1.2 deraadt 1294: .El
1295: .Pp
1.44 aaron 1296: Additionally,
1.2 deraadt 1297: .Nm
1.44 aaron 1298: reads
1.207 djm 1299: .Pa ~/.ssh/environment ,
1.2 deraadt 1300: and adds lines of the format
1301: .Dq VARNAME=value
1.233 jmc 1302: to the environment if the file exists and users are allowed to
1.161 marc 1303: change their environment.
1.176 jmc 1304: For more information, see the
1.161 marc 1305: .Cm PermitUserEnvironment
1.162 stevesk 1306: option in
1.161 marc 1307: .Xr sshd_config 5 .
1.2 deraadt 1308: .Sh FILES
1.236 jmc 1309: .Bl -tag -width Ds -compact
1.309 jmc 1310: .It Pa ~/.rhosts
1.240 jmc 1311: This file is used for host-based authentication (see above).
1.92 markus 1312: On some machines this file may need to be
1.240 jmc 1313: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1314: because
1.2 deraadt 1315: .Xr sshd 8
1.40 aaron 1316: reads it as root.
1317: Additionally, this file must be owned by the user,
1318: and must not have write permissions for anyone else.
1319: The recommended
1.1 deraadt 1320: permission for most machines is read/write for the user, and not
1321: accessible by others.
1.2 deraadt 1322: .Pp
1.309 jmc 1323: .It Pa ~/.shosts
1.240 jmc 1324: This file is used in exactly the same way as
1325: .Pa .rhosts ,
1326: but allows host-based authentication without permitting login with
1327: rlogin/rsh.
1.272 mcbride 1328: .Pp
1.309 jmc 1329: .It Pa ~/.ssh/
1.272 mcbride 1330: This directory is the default location for all user-specific configuration
1331: and authentication information.
1332: There is no general requirement to keep the entire contents of this directory
1333: secret, but the recommended permissions are read/write/execute for the user,
1334: and not accessible by others.
1.236 jmc 1335: .Pp
1.309 jmc 1336: .It Pa ~/.ssh/authorized_keys
1.310 djm 1337: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1338: this user.
1.238 jmc 1339: The format of this file is described in the
1340: .Xr sshd 8
1341: manual page.
1342: This file is not highly sensitive, but the recommended
1343: permissions are read/write for the user, and not accessible by others.
1344: .Pp
1.309 jmc 1345: .It Pa ~/.ssh/config
1.238 jmc 1346: This is the per-user configuration file.
1347: The file format and configuration options are described in
1348: .Xr ssh_config 5 .
1349: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1350: read/write for the user, and not writable by others.
1.238 jmc 1351: .Pp
1.309 jmc 1352: .It Pa ~/.ssh/environment
1.239 jmc 1353: Contains additional definitions for environment variables; see
1354: .Sx ENVIRONMENT ,
1.238 jmc 1355: above.
1356: .Pp
1.309 jmc 1357: .It Pa ~/.ssh/identity
1358: .It Pa ~/.ssh/id_dsa
1.310 djm 1359: .It Pa ~/.ssh/id_ecdsa
1.309 jmc 1360: .It Pa ~/.ssh/id_rsa
1.238 jmc 1361: Contains the private key for authentication.
1362: These files
1363: contain sensitive data and should be readable by the user but not
1364: accessible by others (read/write/execute).
1365: .Nm
1366: will simply ignore a private key file if it is accessible by others.
1367: It is possible to specify a passphrase when
1368: generating the key which will be used to encrypt the
1369: sensitive part of this file using 3DES.
1370: .Pp
1.309 jmc 1371: .It Pa ~/.ssh/identity.pub
1372: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1373: .It Pa ~/.ssh/id_ecdsa.pub
1.309 jmc 1374: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1375: Contains the public key for authentication.
1376: These files are not
1377: sensitive and can (but need not) be readable by anyone.
1378: .Pp
1.309 jmc 1379: .It Pa ~/.ssh/known_hosts
1.244 jmc 1380: Contains a list of host keys for all hosts the user has logged into
1381: that are not already in the systemwide list of known host keys.
1.238 jmc 1382: See
1.244 jmc 1383: .Xr sshd 8
1384: for further details of the format of this file.
1.238 jmc 1385: .Pp
1.309 jmc 1386: .It Pa ~/.ssh/rc
1.238 jmc 1387: Commands in this file are executed by
1388: .Nm
1.245 jmc 1389: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1390: started.
1391: See the
1392: .Xr sshd 8
1393: manual page for more information.
1394: .Pp
1.309 jmc 1395: .It Pa /etc/hosts.equiv
1.240 jmc 1396: This file is for host-based authentication (see above).
1397: It should only be writable by root.
1.236 jmc 1398: .Pp
1.309 jmc 1399: .It Pa /etc/shosts.equiv
1.240 jmc 1400: This file is used in exactly the same way as
1401: .Pa hosts.equiv ,
1402: but allows host-based authentication without permitting login with
1403: rlogin/rsh.
1.236 jmc 1404: .Pp
1.238 jmc 1405: .It Pa /etc/ssh/ssh_config
1406: Systemwide configuration file.
1407: The file format and configuration options are described in
1408: .Xr ssh_config 5 .
1409: .Pp
1.309 jmc 1410: .It Pa /etc/ssh/ssh_host_key
1411: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1412: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.309 jmc 1413: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1414: These files contain the private parts of the host keys
1.245 jmc 1415: and are used for host-based authentication.
1416: If protocol version 1 is used,
1.238 jmc 1417: .Nm
1418: must be setuid root, since the host key is readable only by root.
1419: For protocol version 2,
1420: .Nm
1421: uses
1422: .Xr ssh-keysign 8
1.245 jmc 1423: to access the host keys,
1424: eliminating the requirement that
1.238 jmc 1425: .Nm
1.245 jmc 1426: be setuid root when host-based authentication is used.
1.238 jmc 1427: By default
1.2 deraadt 1428: .Nm
1.238 jmc 1429: is not setuid root.
1430: .Pp
1.309 jmc 1431: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1432: Systemwide list of known host keys.
1433: This file should be prepared by the
1434: system administrator to contain the public host keys of all machines in the
1435: organization.
1.244 jmc 1436: It should be world-readable.
1437: See
1.238 jmc 1438: .Xr sshd 8
1.244 jmc 1439: for further details of the format of this file.
1.236 jmc 1440: .Pp
1.309 jmc 1441: .It Pa /etc/ssh/sshrc
1.1 deraadt 1442: Commands in this file are executed by
1.2 deraadt 1443: .Nm
1.245 jmc 1444: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1445: See the
1.2 deraadt 1446: .Xr sshd 8
1.1 deraadt 1447: manual page for more information.
1.58 itojun 1448: .El
1.312 jmc 1449: .Sh EXIT STATUS
1450: .Nm
1451: exits with the exit status of the remote command or with 255
1452: if an error occurred.
1.2 deraadt 1453: .Sh SEE ALSO
1454: .Xr scp 1 ,
1.83 djm 1455: .Xr sftp 1 ,
1.2 deraadt 1456: .Xr ssh-add 1 ,
1457: .Xr ssh-agent 1 ,
1458: .Xr ssh-keygen 1 ,
1.242 jmc 1459: .Xr ssh-keyscan 1 ,
1.250 jmc 1460: .Xr tun 4 ,
1.176 jmc 1461: .Xr hosts.equiv 5 ,
1.159 stevesk 1462: .Xr ssh_config 5 ,
1.160 naddy 1463: .Xr ssh-keysign 8 ,
1.87 itojun 1464: .Xr sshd 8
1.329 jmc 1465: .Sh STANDARDS
1.106 markus 1466: .Rs
1.329 jmc 1467: .%A S. Lehtinen
1468: .%A C. Lonvick
1469: .%D January 2006
1.256 jmc 1470: .%R RFC 4250
1.329 jmc 1471: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1472: .Re
1.329 jmc 1473: .Pp
1.256 jmc 1474: .Rs
1.329 jmc 1475: .%A T. Ylonen
1476: .%A C. Lonvick
1477: .%D January 2006
1.256 jmc 1478: .%R RFC 4251
1.329 jmc 1479: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1480: .Re
1.329 jmc 1481: .Pp
1.256 jmc 1482: .Rs
1.329 jmc 1483: .%A T. Ylonen
1484: .%A C. Lonvick
1485: .%D January 2006
1.256 jmc 1486: .%R RFC 4252
1.329 jmc 1487: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1488: .Re
1.329 jmc 1489: .Pp
1.256 jmc 1490: .Rs
1.329 jmc 1491: .%A T. Ylonen
1492: .%A C. Lonvick
1493: .%D January 2006
1.256 jmc 1494: .%R RFC 4253
1.329 jmc 1495: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1496: .Re
1.329 jmc 1497: .Pp
1.256 jmc 1498: .Rs
1.329 jmc 1499: .%A T. Ylonen
1500: .%A C. Lonvick
1501: .%D January 2006
1.256 jmc 1502: .%R RFC 4254
1.329 jmc 1503: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1504: .Re
1.329 jmc 1505: .Pp
1.256 jmc 1506: .Rs
1.329 jmc 1507: .%A J. Schlyter
1508: .%A W. Griffin
1509: .%D January 2006
1.256 jmc 1510: .%R RFC 4255
1.329 jmc 1511: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1512: .Re
1.329 jmc 1513: .Pp
1.256 jmc 1514: .Rs
1.329 jmc 1515: .%A F. Cusack
1516: .%A M. Forssen
1517: .%D January 2006
1.256 jmc 1518: .%R RFC 4256
1.329 jmc 1519: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1520: .Re
1.329 jmc 1521: .Pp
1.256 jmc 1522: .Rs
1.329 jmc 1523: .%A J. Galbraith
1524: .%A P. Remaker
1525: .%D January 2006
1.256 jmc 1526: .%R RFC 4335
1.329 jmc 1527: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1528: .Re
1.329 jmc 1529: .Pp
1.256 jmc 1530: .Rs
1.329 jmc 1531: .%A M. Bellare
1532: .%A T. Kohno
1533: .%A C. Namprempre
1534: .%D January 2006
1.256 jmc 1535: .%R RFC 4344
1.329 jmc 1536: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1537: .Re
1.329 jmc 1538: .Pp
1.256 jmc 1539: .Rs
1.329 jmc 1540: .%A B. Harris
1541: .%D January 2006
1.256 jmc 1542: .%R RFC 4345
1.329 jmc 1543: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1544: .Re
1.329 jmc 1545: .Pp
1.258 djm 1546: .Rs
1.329 jmc 1547: .%A M. Friedl
1548: .%A N. Provos
1549: .%A W. Simpson
1550: .%D March 2006
1.258 djm 1551: .%R RFC 4419
1.329 jmc 1552: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1553: .Re
1.329 jmc 1554: .Pp
1.266 markus 1555: .Rs
1.329 jmc 1556: .%A J. Galbraith
1557: .%A R. Thayer
1558: .%D November 2006
1.266 markus 1559: .%R RFC 4716
1.329 jmc 1560: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1561: .Re
1.329 jmc 1562: .Pp
1.313 djm 1563: .Rs
1.329 jmc 1564: .%A D. Stebila
1565: .%A J. Green
1566: .%D December 2009
1.313 djm 1567: .%R RFC 5656
1.329 jmc 1568: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1569: .Re
1.329 jmc 1570: .Pp
1.274 grunk 1571: .Rs
1572: .%A A. Perrig
1573: .%A D. Song
1574: .%D 1999
1.329 jmc 1575: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1576: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1577: .Re
1.173 jmc 1578: .Sh AUTHORS
1579: OpenSSH is a derivative of the original and free
1580: ssh 1.2.12 release by Tatu Ylonen.
1581: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1582: Theo de Raadt and Dug Song
1583: removed many bugs, re-added newer features and
1584: created OpenSSH.
1585: Markus Friedl contributed the support for SSH
1586: protocol versions 1.5 and 2.0.
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