Annotation of src/usr.bin/ssh/ssh.1, Revision 1.350
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: .\"
1.350 ! djm 36: .\" $OpenBSD: ssh.1,v 1.349 2014/08/30 15:33:50 sobrado Exp $
! 37: .Dd $Mdocdate: August 30 2014 $
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
1.350 ! djm 46: .Op Fl 1246AaCfgGKkMNnqsTtVvXxYy
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
1.342 jmc 61: .Op Fl Q Cm cipher | cipher-auth | mac | kex | key
1.306 jmc 62: .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
1.198 djm 63: .Op Fl S Ar ctl_path
1.290 dtucker 64: .Op Fl W Ar host : Ns Ar port
1.306 jmc 65: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.176 jmc 66: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 67: .Op Ar command
1.306 jmc 68: .Ek
1.44 aaron 69: .Sh DESCRIPTION
1.2 deraadt 70: .Nm
1.96 deraadt 71: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 72: executing commands on a remote machine.
1.176 jmc 73: It is intended to replace rlogin and rsh,
74: and provide secure encrypted communications between
1.40 aaron 75: two untrusted hosts over an insecure network.
1.348 millert 76: X11 connections, arbitrary TCP ports and
77: .Ux Ns -domain
78: sockets 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
1.348 millert 135: data for forwarded X11, TCP and
136: .Ux Ns -domain
137: connections).
1.218 jmc 138: The compression algorithm is the same used by
139: .Xr gzip 1 ,
140: and the
141: .Dq level
142: can be controlled by the
143: .Cm CompressionLevel
144: option for protocol version 1.
145: Compression is desirable on modem lines and other
146: slow connections, but will only slow down things on fast networks.
147: The default value can be set on a host-by-host basis in the
148: configuration files; see the
149: .Cm Compression
150: option.
151: .It Fl c Ar cipher_spec
152: Selects the cipher specification for encrypting the session.
1.2 deraadt 153: .Pp
1.218 jmc 154: Protocol version 1 allows specification of a single cipher.
155: The supported values are
156: .Dq 3des ,
1.220 jmc 157: .Dq blowfish ,
1.218 jmc 158: and
159: .Dq des .
1.230 jmc 160: For protocol version 2,
1.218 jmc 161: .Ar cipher_spec
162: is a comma-separated list of ciphers
163: listed in order of preference.
1.283 jmc 164: See the
165: .Cm Ciphers
1.307 dtucker 166: keyword in
167: .Xr ssh_config 5
168: for more information.
1.218 jmc 169: .It Fl D Xo
170: .Sm off
171: .Oo Ar bind_address : Oc
172: .Ar port
173: .Sm on
174: .Xc
175: Specifies a local
176: .Dq dynamic
177: application-level port forwarding.
178: This works by allocating a socket to listen to
179: .Ar port
180: on the local side, optionally bound to the specified
181: .Ar bind_address .
182: Whenever a connection is made to this port, the
183: connection is forwarded over the secure channel, and the application
184: protocol is then used to determine where to connect to from the
185: remote machine.
186: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 187: .Nm
1.218 jmc 188: will act as a SOCKS server.
189: Only root can forward privileged ports.
190: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 191: .Pp
1.308 djm 192: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 193: Only the superuser can forward privileged ports.
194: By default, the local port is bound in accordance with the
195: .Cm GatewayPorts
196: setting.
197: However, an explicit
198: .Ar bind_address
199: may be used to bind the connection to a specific address.
200: The
201: .Ar bind_address
202: of
203: .Dq localhost
204: indicates that the listening port be bound for local use only, while an
205: empty address or
206: .Sq *
207: indicates that the port should be available from all interfaces.
1.331 dtucker 208: .It Fl E Ar log_file
209: Append debug logs to
210: .Ar log_file
211: instead of standard error.
1.229 jmc 212: .It Fl e Ar escape_char
1.218 jmc 213: Sets the escape character for sessions with a pty (default:
214: .Ql ~ ) .
215: The escape character is only recognized at the beginning of a line.
216: The escape character followed by a dot
217: .Pq Ql \&.
218: closes the connection;
219: followed by control-Z suspends the connection;
220: and followed by itself sends the escape character once.
221: Setting the character to
1.2 deraadt 222: .Dq none
1.218 jmc 223: disables any escapes and makes the session fully transparent.
224: .It Fl F Ar configfile
225: Specifies an alternative per-user configuration file.
226: If a configuration file is given on the command line,
227: the system-wide configuration file
228: .Pq Pa /etc/ssh/ssh_config
229: will be ignored.
230: The default for the per-user configuration file is
231: .Pa ~/.ssh/config .
232: .It Fl f
233: Requests
234: .Nm
235: to go to background just before command execution.
236: This is useful if
1.176 jmc 237: .Nm
1.218 jmc 238: is going to ask for passwords or passphrases, but the user
239: wants it in the background.
240: This implies
241: .Fl n .
242: The recommended way to start X11 programs at a remote site is with
243: something like
244: .Ic ssh -f host xterm .
1.277 djm 245: .Pp
246: If the
247: .Cm ExitOnForwardFailure
248: configuration option is set to
249: .Dq yes ,
250: then a client started with
251: .Fl f
252: will wait for all remote port forwards to be successfully established
253: before placing itself in the background.
1.350 ! djm 254: .It Fl G
! 255: Causes
! 256: .Nm
! 257: to print its configuration after evaluating
! 258: .Cm Host
! 259: and
! 260: .Cm Match
! 261: blocks and exit.
1.218 jmc 262: .It Fl g
263: Allows remote hosts to connect to local forwarded ports.
1.346 djm 264: If used on a multiplexed connection, then this option must be specified
265: on the master process.
1.291 markus 266: .It Fl I Ar pkcs11
1.294 jmc 267: Specify the PKCS#11 shared library
1.176 jmc 268: .Nm
1.293 markus 269: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 270: private RSA key.
271: .It Fl i Ar identity_file
272: Selects a file from which the identity (private key) for
1.310 djm 273: public key authentication is read.
1.218 jmc 274: The default is
275: .Pa ~/.ssh/identity
276: for protocol version 1, and
1.310 djm 277: .Pa ~/.ssh/id_dsa ,
1.343 naddy 278: .Pa ~/.ssh/id_ecdsa ,
279: .Pa ~/.ssh/id_ed25519
1.310 djm 280: and
1.218 jmc 281: .Pa ~/.ssh/id_rsa
282: for protocol version 2.
283: Identity files may also be specified on
284: a per-host basis in the configuration file.
285: It is possible to have multiple
286: .Fl i
287: options (and multiple identities specified in
288: configuration files).
1.302 djm 289: .Nm
290: will also try to load certificate information from the filename obtained
291: by appending
292: .Pa -cert.pub
293: to identity filenames.
1.269 djm 294: .It Fl K
295: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
296: credentials to the server.
1.218 jmc 297: .It Fl k
298: Disables forwarding (delegation) of GSSAPI credentials to the server.
299: .It Fl L Xo
300: .Sm off
301: .Oo Ar bind_address : Oc
302: .Ar port : host : hostport
303: .Sm on
304: .Xc
305: Specifies that the given port on the local (client) host is to be
306: forwarded to the given host and port on the remote side.
307: This works by allocating a socket to listen to
308: .Ar port
309: on the local side, optionally bound to the specified
310: .Ar bind_address .
311: Whenever a connection is made to this port, the
312: connection is forwarded over the secure channel, and a connection is
313: made to
314: .Ar host
315: port
316: .Ar hostport
317: from the remote machine.
318: Port forwardings can also be specified in the configuration file.
1.308 djm 319: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 320: Only the superuser can forward privileged ports.
321: By default, the local port is bound in accordance with the
322: .Cm GatewayPorts
323: setting.
324: However, an explicit
325: .Ar bind_address
326: may be used to bind the connection to a specific address.
1.2 deraadt 327: The
1.218 jmc 328: .Ar bind_address
329: of
330: .Dq localhost
331: indicates that the listening port be bound for local use only, while an
332: empty address or
333: .Sq *
334: indicates that the port should be available from all interfaces.
335: .It Fl l Ar login_name
336: Specifies the user to log in as on the remote machine.
337: This also may be specified on a per-host basis in the configuration file.
338: .It Fl M
339: Places the
340: .Nm
341: client into
342: .Dq master
343: mode for connection sharing.
1.231 stevesk 344: Multiple
345: .Fl M
346: options places
347: .Nm
348: into
349: .Dq master
350: mode with confirmation required before slave connections are accepted.
1.218 jmc 351: Refer to the description of
352: .Cm ControlMaster
353: in
354: .Xr ssh_config 5
355: for details.
356: .It Fl m Ar mac_spec
357: Additionally, for protocol version 2 a comma-separated list of MAC
358: (message authentication code) algorithms can
359: be specified in order of preference.
360: See the
361: .Cm MACs
362: keyword for more information.
363: .It Fl N
364: Do not execute a remote command.
365: This is useful for just forwarding ports
366: (protocol version 2 only).
367: .It Fl n
368: Redirects stdin from
369: .Pa /dev/null
370: (actually, prevents reading from stdin).
371: This must be used when
1.2 deraadt 372: .Nm
1.218 jmc 373: is run in the background.
374: A common trick is to use this to run X11 programs on a remote machine.
375: For example,
376: .Ic ssh -n shadows.cs.hut.fi emacs &
377: will start an emacs on shadows.cs.hut.fi, and the X11
378: connection will be automatically forwarded over an encrypted channel.
379: The
1.2 deraadt 380: .Nm
1.218 jmc 381: program will be put in the background.
382: (This does not work if
1.2 deraadt 383: .Nm
1.218 jmc 384: needs to ask for a password or passphrase; see also the
385: .Fl f
386: option.)
387: .It Fl O Ar ctl_cmd
388: Control an active connection multiplexing master process.
389: When the
390: .Fl O
391: option is specified, the
392: .Ar ctl_cmd
393: argument is interpreted and passed to the master process.
394: Valid commands are:
395: .Dq check
1.305 markus 396: (check that the master process is running),
397: .Dq forward
1.318 jmc 398: (request forwardings without command execution),
1.323 okan 399: .Dq cancel
400: (cancel forwardings),
1.218 jmc 401: .Dq exit
1.318 jmc 402: (request the master to exit), and
1.317 djm 403: .Dq stop
404: (request the master to stop accepting further multiplexing requests).
1.218 jmc 405: .It Fl o Ar option
406: Can be used to give options in the format used in the configuration file.
407: This is useful for specifying options for which there is no separate
408: command-line flag.
409: For full details of the options listed below, and their possible values, see
410: .Xr ssh_config 5 .
1.2 deraadt 411: .Pp
1.218 jmc 412: .Bl -tag -width Ds -offset indent -compact
413: .It AddressFamily
414: .It BatchMode
415: .It BindAddress
1.338 djm 416: .It CanonicalDomains
1.339 djm 417: .It CanonicalizeFallbackLocal
418: .It CanonicalizeHostname
419: .It CanonicalizeMaxDots
420: .It CanonicalizePermittedCNAMEs
1.218 jmc 421: .It ChallengeResponseAuthentication
422: .It CheckHostIP
423: .It Cipher
424: .It Ciphers
425: .It ClearAllForwardings
426: .It Compression
427: .It CompressionLevel
428: .It ConnectionAttempts
429: .It ConnectTimeout
430: .It ControlMaster
431: .It ControlPath
1.321 djm 432: .It ControlPersist
1.218 jmc 433: .It DynamicForward
434: .It EscapeChar
1.263 markus 435: .It ExitOnForwardFailure
1.218 jmc 436: .It ForwardAgent
437: .It ForwardX11
1.321 djm 438: .It ForwardX11Timeout
1.218 jmc 439: .It ForwardX11Trusted
440: .It GatewayPorts
441: .It GlobalKnownHostsFile
442: .It GSSAPIAuthentication
443: .It GSSAPIDelegateCredentials
444: .It HashKnownHosts
445: .It Host
446: .It HostbasedAuthentication
447: .It HostKeyAlgorithms
448: .It HostKeyAlias
449: .It HostName
450: .It IdentityFile
451: .It IdentitiesOnly
1.316 jmc 452: .It IPQoS
1.321 djm 453: .It KbdInteractiveAuthentication
1.218 jmc 454: .It KbdInteractiveDevices
1.314 jmc 455: .It KexAlgorithms
1.218 jmc 456: .It LocalCommand
457: .It LocalForward
458: .It LogLevel
459: .It MACs
1.337 jmc 460: .It Match
1.218 jmc 461: .It NoHostAuthenticationForLocalhost
462: .It NumberOfPasswordPrompts
463: .It PasswordAuthentication
464: .It PermitLocalCommand
1.291 markus 465: .It PKCS11Provider
1.218 jmc 466: .It Port
467: .It PreferredAuthentications
468: .It Protocol
469: .It ProxyCommand
1.336 jmc 470: .It ProxyUseFdpass
1.218 jmc 471: .It PubkeyAuthentication
1.251 dtucker 472: .It RekeyLimit
1.218 jmc 473: .It RemoteForward
1.319 jmc 474: .It RequestTTY
1.218 jmc 475: .It RhostsRSAAuthentication
476: .It RSAAuthentication
477: .It SendEnv
478: .It ServerAliveInterval
479: .It ServerAliveCountMax
1.347 jmc 480: .It StreamLocalBindMask
481: .It StreamLocalBindUnlink
1.218 jmc 482: .It StrictHostKeyChecking
483: .It TCPKeepAlive
484: .It Tunnel
485: .It TunnelDevice
486: .It UsePrivilegedPort
487: .It User
488: .It UserKnownHostsFile
489: .It VerifyHostKeyDNS
1.276 jmc 490: .It VisualHostKey
1.218 jmc 491: .It XAuthLocation
492: .El
493: .It Fl p Ar port
494: Port to connect to on the remote host.
495: This can be specified on a
496: per-host basis in the configuration file.
1.342 jmc 497: .It Fl Q Cm cipher | cipher-auth | mac | kex | key
1.332 djm 498: Queries
499: .Nm
1.341 deraadt 500: for the algorithms supported for the specified version 2.
501: The available features are:
502: .Ar cipher
1.332 djm 503: (supported symmetric ciphers),
1.341 deraadt 504: .Ar cipher-auth
1.340 djm 505: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 506: .Ar mac
1.332 djm 507: (supported message integrity codes),
1.341 deraadt 508: .Ar kex
1.332 djm 509: (key exchange algorithms),
1.341 deraadt 510: .Ar key
1.332 djm 511: (key types).
1.218 jmc 512: .It Fl q
513: Quiet mode.
1.271 djm 514: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 515: .It Fl R Xo
516: .Sm off
517: .Oo Ar bind_address : Oc
518: .Ar port : host : hostport
519: .Sm on
520: .Xc
521: Specifies that the given port on the remote (server) host is to be
522: forwarded to the given host and port on the local side.
523: This works by allocating a socket to listen to
524: .Ar port
525: on the remote side, and whenever a connection is made to this port, the
526: connection is forwarded over the secure channel, and a connection is
527: made to
528: .Ar host
529: port
530: .Ar hostport
531: from the local machine.
1.2 deraadt 532: .Pp
1.218 jmc 533: Port forwardings can also be specified in the configuration file.
534: Privileged ports can be forwarded only when
535: logging in as root on the remote machine.
1.324 jmc 536: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 537: .Pp
1.218 jmc 538: By default, the listening socket on the server will be bound to the loopback
539: interface only.
1.280 tobias 540: This may be overridden by specifying a
1.218 jmc 541: .Ar bind_address .
542: An empty
543: .Ar bind_address ,
544: or the address
545: .Ql * ,
546: indicates that the remote socket should listen on all interfaces.
547: Specifying a remote
548: .Ar bind_address
549: will only succeed if the server's
550: .Cm GatewayPorts
551: option is enabled (see
552: .Xr sshd_config 5 ) .
1.281 djm 553: .Pp
554: If the
555: .Ar port
556: argument is
1.282 djm 557: .Ql 0 ,
1.281 djm 558: the listen port will be dynamically allocated on the server and reported
559: to the client at run time.
1.305 markus 560: When used together with
561: .Ic -O forward
562: the allocated port will be printed to the standard output.
1.218 jmc 563: .It Fl S Ar ctl_path
1.304 jmc 564: Specifies the location of a control socket for connection sharing,
1.303 djm 565: or the string
566: .Dq none
567: to disable connection sharing.
1.218 jmc 568: Refer to the description of
569: .Cm ControlPath
570: and
571: .Cm ControlMaster
572: in
573: .Xr ssh_config 5
574: for details.
575: .It Fl s
576: May be used to request invocation of a subsystem on the remote system.
577: Subsystems are a feature of the SSH2 protocol which facilitate the use
578: of SSH as a secure transport for other applications (eg.\&
579: .Xr sftp 1 ) .
580: The subsystem is specified as the remote command.
581: .It Fl T
582: Disable pseudo-tty allocation.
583: .It Fl t
584: Force pseudo-tty allocation.
585: This can be used to execute arbitrary
586: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 587: e.g. when implementing menu services.
1.218 jmc 588: Multiple
589: .Fl t
590: options force tty allocation, even if
1.194 jakob 591: .Nm
1.218 jmc 592: has no local tty.
593: .It Fl V
594: Display the version number and exit.
595: .It Fl v
596: Verbose mode.
597: Causes
1.176 jmc 598: .Nm
1.218 jmc 599: to print debugging messages about its progress.
600: This is helpful in
601: debugging connection, authentication, and configuration problems.
602: Multiple
603: .Fl v
604: options increase the verbosity.
605: The maximum is 3.
1.290 dtucker 606: .It Fl W Ar host : Ns Ar port
607: Requests that standard input and output on the client be forwarded to
608: .Ar host
609: on
610: .Ar port
611: over the secure channel.
612: Implies
613: .Fl N ,
614: .Fl T ,
615: .Cm ExitOnForwardFailure
616: and
1.326 dtucker 617: .Cm ClearAllForwardings .
618: Works with Protocol version 2 only.
1.261 stevesk 619: .It Fl w Xo
620: .Ar local_tun Ns Op : Ns Ar remote_tun
621: .Xc
622: Requests
623: tunnel
624: device forwarding with the specified
1.218 jmc 625: .Xr tun 4
1.261 stevesk 626: devices between the client
627: .Pq Ar local_tun
628: and the server
629: .Pq Ar remote_tun .
630: .Pp
1.228 jmc 631: The devices may be specified by numerical ID or the keyword
632: .Dq any ,
633: which uses the next available tunnel device.
1.261 stevesk 634: If
635: .Ar remote_tun
636: is not specified, it defaults to
637: .Dq any .
1.228 jmc 638: See also the
1.218 jmc 639: .Cm Tunnel
1.261 stevesk 640: and
641: .Cm TunnelDevice
642: directives in
1.218 jmc 643: .Xr ssh_config 5 .
1.261 stevesk 644: If the
645: .Cm Tunnel
646: directive is unset, it is set to the default tunnel mode, which is
647: .Dq point-to-point .
1.218 jmc 648: .It Fl X
649: Enables X11 forwarding.
1.54 markus 650: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 651: .Pp
1.218 jmc 652: X11 forwarding should be enabled with caution.
1.168 jmc 653: Users with the ability to bypass file permissions on the remote host
1.218 jmc 654: (for the user's X authorization database)
655: can access the local X11 display through the forwarded connection.
656: An attacker may then be able to perform activities such as keystroke monitoring.
657: .Pp
658: For this reason, X11 forwarding is subjected to X11 SECURITY extension
659: restrictions by default.
660: Please refer to the
661: .Nm
662: .Fl Y
663: option and the
664: .Cm ForwardX11Trusted
665: directive in
666: .Xr ssh_config 5
667: for more information.
668: .It Fl x
669: Disables X11 forwarding.
670: .It Fl Y
671: Enables trusted X11 forwarding.
672: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
673: controls.
1.278 djm 674: .It Fl y
675: Send log information using the
676: .Xr syslog 3
677: system module.
678: By default this information is sent to stderr.
1.218 jmc 679: .El
1.224 jmc 680: .Pp
681: .Nm
682: may additionally obtain configuration data from
683: a per-user configuration file and a system-wide configuration file.
684: The file format and configuration options are described in
685: .Xr ssh_config 5 .
1.222 jmc 686: .Sh AUTHENTICATION
1.249 jmc 687: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 688: The default is to use protocol 2 only,
689: though this can be changed via the
1.222 jmc 690: .Cm Protocol
691: option in
1.284 jmc 692: .Xr ssh_config 5
693: or the
1.222 jmc 694: .Fl 1
695: and
696: .Fl 2
697: options (see above).
698: Both protocols support similar authentication methods,
1.284 jmc 699: but protocol 2 is the default since
1.222 jmc 700: it provides additional mechanisms for confidentiality
701: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 702: and integrity (hmac-md5, hmac-sha1,
703: hmac-sha2-256, hmac-sha2-512,
1.330 markus 704: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 705: Protocol 1 lacks a strong mechanism for ensuring the
706: integrity of the connection.
707: .Pp
708: The methods available for authentication are:
1.260 jmc 709: GSSAPI-based authentication,
1.222 jmc 710: host-based authentication,
711: public key authentication,
712: challenge-response authentication,
713: and password authentication.
714: Authentication methods are tried in the order specified above,
715: though protocol 2 has a configuration option to change the default order:
716: .Cm PreferredAuthentications .
717: .Pp
718: Host-based authentication works as follows:
1.218 jmc 719: If the machine the user logs in from is listed in
720: .Pa /etc/hosts.equiv
721: or
722: .Pa /etc/shosts.equiv
723: on the remote machine, and the user names are
724: the same on both sides, or if the files
725: .Pa ~/.rhosts
726: or
727: .Pa ~/.shosts
728: exist in the user's home directory on the
729: remote machine and contain a line containing the name of the client
730: machine and the name of the user on that machine, the user is
1.222 jmc 731: considered for login.
732: Additionally, the server
733: .Em must
734: be able to verify the client's
735: host key (see the description of
1.218 jmc 736: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 737: and
1.222 jmc 738: .Pa ~/.ssh/known_hosts ,
739: below)
740: for login to be permitted.
1.218 jmc 741: This authentication method closes security holes due to IP
1.222 jmc 742: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 743: [Note to the administrator:
744: .Pa /etc/hosts.equiv ,
745: .Pa ~/.rhosts ,
746: and the rlogin/rsh protocol in general, are inherently insecure and should be
747: disabled if security is desired.]
1.189 dtucker 748: .Pp
1.222 jmc 749: Public key authentication works as follows:
750: The scheme is based on public-key cryptography,
751: using cryptosystems
752: where encryption and decryption are done using separate keys,
753: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 754: The idea is that each user creates a public/private
755: key pair for authentication purposes.
756: The server knows the public key, and only the user knows the private key.
1.222 jmc 757: .Nm
758: implements public key authentication protocol automatically,
1.349 sobrado 759: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.222 jmc 760: Protocol 1 is restricted to using only RSA keys,
1.310 djm 761: but protocol 2 may use any.
1.333 jmc 762: The HISTORY section of
1.222 jmc 763: .Xr ssl 8
1.311 jmc 764: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 765: .Pp
1.218 jmc 766: The file
767: .Pa ~/.ssh/authorized_keys
768: lists the public keys that are permitted for logging in.
769: When the user logs in, the
1.2 deraadt 770: .Nm
1.218 jmc 771: program tells the server which key pair it would like to use for
772: authentication.
1.222 jmc 773: The client proves that it has access to the private key
774: and the server checks that the corresponding public key
775: is authorized to accept the account.
1.218 jmc 776: .Pp
1.222 jmc 777: The user creates his/her key pair by running
1.218 jmc 778: .Xr ssh-keygen 1 .
779: This stores the private key in
1.207 djm 780: .Pa ~/.ssh/identity
1.222 jmc 781: (protocol 1),
782: .Pa ~/.ssh/id_dsa
783: (protocol 2 DSA),
1.310 djm 784: .Pa ~/.ssh/id_ecdsa
785: (protocol 2 ECDSA),
1.343 naddy 786: .Pa ~/.ssh/id_ed25519
1.349 sobrado 787: (protocol 2 Ed25519),
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),
1.343 naddy 798: .Pa ~/.ssh/id_ed25519.pub
1.349 sobrado 799: (protocol 2 Ed25519),
1.222 jmc 800: or
801: .Pa ~/.ssh/id_rsa.pub
802: (protocol 2 RSA)
1.218 jmc 803: in the user's home directory.
1.222 jmc 804: The user should then copy the public key
1.218 jmc 805: to
806: .Pa ~/.ssh/authorized_keys
1.222 jmc 807: in his/her home directory on the remote machine.
808: The
1.218 jmc 809: .Pa authorized_keys
810: file corresponds to the conventional
811: .Pa ~/.rhosts
812: file, and has one key
1.222 jmc 813: per line, though the lines can be very long.
1.218 jmc 814: After this, the user can log in without giving the password.
815: .Pp
1.301 jmc 816: A variation on public key authentication
817: is available in the form of certificate authentication:
818: instead of a set of public/private keys,
819: signed certificates are used.
820: This has the advantage that a single trusted certification authority
821: can be used in place of many public/private keys.
1.333 jmc 822: See the CERTIFICATES section of
1.301 jmc 823: .Xr ssh-keygen 1
824: for more information.
825: .Pp
826: The most convenient way to use public key or certificate authentication
827: may be with an authentication agent.
1.218 jmc 828: See
829: .Xr ssh-agent 1
830: for more information.
831: .Pp
1.222 jmc 832: Challenge-response authentication works as follows:
833: The server sends an arbitrary
834: .Qq challenge
835: text, and prompts for a response.
836: Protocol 2 allows multiple challenges and responses;
837: protocol 1 is restricted to just one challenge/response.
838: Examples of challenge-response authentication include
1.335 jmc 839: .Bx
840: Authentication (see
1.222 jmc 841: .Xr login.conf 5 )
1.335 jmc 842: and PAM (some
843: .Pf non- Ox
844: systems).
1.222 jmc 845: .Pp
846: Finally, if other authentication methods fail,
1.218 jmc 847: .Nm
848: prompts the user for a password.
849: The password is sent to the remote
850: host for checking; however, since all communications are encrypted,
851: the password cannot be seen by someone listening on the network.
1.232 jmc 852: .Pp
853: .Nm
854: automatically maintains and checks a database containing
855: identification for all hosts it has ever been used with.
856: Host keys are stored in
857: .Pa ~/.ssh/known_hosts
858: in the user's home directory.
859: Additionally, the file
860: .Pa /etc/ssh/ssh_known_hosts
861: is automatically checked for known hosts.
862: Any new hosts are automatically added to the user's file.
863: If a host's identification ever changes,
864: .Nm
865: warns about this and disables password authentication to prevent
866: server spoofing or man-in-the-middle attacks,
867: which could otherwise be used to circumvent the encryption.
868: The
869: .Cm StrictHostKeyChecking
870: option can be used to control logins to machines whose
871: host key is not known or has changed.
872: .Pp
1.218 jmc 873: When the user's identity has been accepted by the server, the server
874: either executes the given command, or logs into the machine and gives
875: the user a normal shell on the remote machine.
876: All communication with
877: the remote command or shell will be automatically encrypted.
878: .Pp
879: If a pseudo-terminal has been allocated (normal login session), the
880: user may use the escape characters noted below.
881: .Pp
882: If no pseudo-tty has been allocated,
883: the session is transparent and can be used to reliably transfer binary data.
884: On most systems, setting the escape character to
885: .Dq none
886: will also make the session transparent even if a tty is used.
887: .Pp
888: The session terminates when the command or shell on the remote
1.247 jmc 889: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 890: .Sh ESCAPE CHARACTERS
1.218 jmc 891: When a pseudo-terminal has been requested,
1.2 deraadt 892: .Nm
1.218 jmc 893: supports a number of functions through the use of an escape character.
894: .Pp
895: A single tilde character can be sent as
896: .Ic ~~
897: or by following the tilde by a character other than those described below.
898: The escape character must always follow a newline to be interpreted as
899: special.
900: The escape character can be changed in configuration files using the
901: .Cm EscapeChar
902: configuration directive or on the command line by the
903: .Fl e
904: option.
905: .Pp
906: The supported escapes (assuming the default
907: .Ql ~ )
908: are:
909: .Bl -tag -width Ds
910: .It Cm ~.
911: Disconnect.
912: .It Cm ~^Z
913: Background
1.234 jmc 914: .Nm .
1.218 jmc 915: .It Cm ~#
916: List forwarded connections.
917: .It Cm ~&
918: Background
1.2 deraadt 919: .Nm
1.218 jmc 920: at logout when waiting for forwarded connection / X11 sessions to terminate.
921: .It Cm ~?
922: Display a list of escape characters.
923: .It Cm ~B
924: Send a BREAK to the remote system
925: (only useful for SSH protocol version 2 and if the peer supports it).
926: .It Cm ~C
927: Open command line.
928: Currently this allows the addition of port forwardings using the
1.279 stevesk 929: .Fl L ,
930: .Fl R
1.218 jmc 931: and
1.279 stevesk 932: .Fl D
1.225 jmc 933: options (see above).
1.322 markus 934: It also allows the cancellation of existing port-forwardings
935: with
1.262 stevesk 936: .Sm off
1.322 markus 937: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 938: .Sm on
1.322 markus 939: for local,
940: .Sm off
941: .Fl KR Oo Ar bind_address : Oc Ar port
942: .Sm on
943: for remote and
944: .Sm off
945: .Fl KD Oo Ar bind_address : Oc Ar port
946: .Sm on
947: for dynamic port-forwardings.
1.218 jmc 948: .Ic !\& Ns Ar command
949: allows the user to execute a local command if the
950: .Ic PermitLocalCommand
951: option is enabled in
1.176 jmc 952: .Xr ssh_config 5 .
1.218 jmc 953: Basic help is available, using the
954: .Fl h
955: option.
956: .It Cm ~R
957: Request rekeying of the connection
958: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 959: .It Cm ~V
960: Decrease the verbosity
961: .Pq Ic LogLevel
962: when errors are being written to stderr.
963: .It Cm ~v
1.328 jmc 964: Increase the verbosity
1.327 dtucker 965: .Pq Ic LogLevel
966: when errors are being written to stderr.
1.176 jmc 967: .El
1.246 jmc 968: .Sh TCP FORWARDING
969: Forwarding of arbitrary TCP connections over the secure channel can
970: be specified either on the command line or in a configuration file.
971: One possible application of TCP forwarding is a secure connection to a
972: mail server; another is going through firewalls.
973: .Pp
974: In the example below, we look at encrypting communication between
975: an IRC client and server, even though the IRC server does not directly
976: support encrypted communications.
977: This works as follows:
978: the user connects to the remote host using
979: .Nm ,
980: specifying a port to be used to forward connections
981: to the remote server.
982: After that it is possible to start the service which is to be encrypted
983: on the client machine,
984: connecting to the same local port,
985: and
986: .Nm
987: will encrypt and forward the connection.
988: .Pp
989: The following example tunnels an IRC session from client machine
990: .Dq 127.0.0.1
991: (localhost)
992: to remote server
993: .Dq server.example.com :
994: .Bd -literal -offset 4n
995: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
996: $ irc -c '#users' -p 1234 pinky 127.0.0.1
997: .Ed
998: .Pp
999: This tunnels a connection to IRC server
1000: .Dq server.example.com ,
1001: joining channel
1002: .Dq #users ,
1003: nickname
1004: .Dq pinky ,
1005: using port 1234.
1006: It doesn't matter which port is used,
1007: as long as it's greater than 1023
1008: (remember, only root can open sockets on privileged ports)
1009: and doesn't conflict with any ports already in use.
1010: The connection is forwarded to port 6667 on the remote server,
1011: since that's the standard port for IRC services.
1012: .Pp
1013: The
1014: .Fl f
1015: option backgrounds
1016: .Nm
1017: and the remote command
1018: .Dq sleep 10
1019: is specified to allow an amount of time
1020: (10 seconds, in the example)
1021: to start the service which is to be tunnelled.
1022: If no connections are made within the time specified,
1023: .Nm
1024: will exit.
1025: .Sh X11 FORWARDING
1.218 jmc 1026: If the
1027: .Cm ForwardX11
1028: variable is set to
1029: .Dq yes
1030: (or see the description of the
1.227 jmc 1031: .Fl X ,
1032: .Fl x ,
1.218 jmc 1033: and
1.227 jmc 1034: .Fl Y
1.226 jmc 1035: options above)
1.218 jmc 1036: and the user is using X11 (the
1037: .Ev DISPLAY
1038: environment variable is set), the connection to the X11 display is
1039: automatically forwarded to the remote side in such a way that any X11
1040: programs started from the shell (or command) will go through the
1041: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1042: from the local machine.
1.218 jmc 1043: The user should not manually set
1044: .Ev DISPLAY .
1045: Forwarding of X11 connections can be
1046: configured on the command line or in configuration files.
1047: .Pp
1048: The
1049: .Ev DISPLAY
1050: value set by
1051: .Nm
1052: will point to the server machine, but with a display number greater than zero.
1053: This is normal, and happens because
1054: .Nm
1055: creates a
1056: .Dq proxy
1057: X server on the server machine for forwarding the
1058: connections over the encrypted channel.
1.200 djm 1059: .Pp
1.218 jmc 1060: .Nm
1061: will also automatically set up Xauthority data on the server machine.
1062: For this purpose, it will generate a random authorization cookie,
1063: store it in Xauthority on the server, and verify that any forwarded
1064: connections carry this cookie and replace it by the real cookie when
1065: the connection is opened.
1066: The real authentication cookie is never
1067: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1068: .Pp
1.218 jmc 1069: If the
1070: .Cm ForwardAgent
1071: variable is set to
1072: .Dq yes
1073: (or see the description of the
1074: .Fl A
1.191 djm 1075: and
1.218 jmc 1076: .Fl a
1.226 jmc 1077: options above) and
1.218 jmc 1078: the user is using an authentication agent, the connection to the agent
1079: is automatically forwarded to the remote side.
1.252 jmc 1080: .Sh VERIFYING HOST KEYS
1081: When connecting to a server for the first time,
1082: a fingerprint of the server's public key is presented to the user
1083: (unless the option
1084: .Cm StrictHostKeyChecking
1085: has been disabled).
1086: Fingerprints can be determined using
1087: .Xr ssh-keygen 1 :
1088: .Pp
1089: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1090: .Pp
1.274 grunk 1091: If the fingerprint is already known, it can be matched
1092: and the key can be accepted or rejected.
1093: Because of the difficulty of comparing host keys
1094: just by looking at hex strings,
1095: there is also support to compare host keys visually,
1096: using
1097: .Em random art .
1098: By setting the
1.275 grunk 1099: .Cm VisualHostKey
1.274 grunk 1100: option to
1.275 grunk 1101: .Dq yes ,
1.274 grunk 1102: a small ASCII graphic gets displayed on every login to a server, no matter
1103: if the session itself is interactive or not.
1104: By learning the pattern a known server produces, a user can easily
1105: find out that the host key has changed when a completely different pattern
1106: is displayed.
1107: Because these patterns are not unambiguous however, a pattern that looks
1108: similar to the pattern remembered only gives a good probability that the
1109: host key is the same, not guaranteed proof.
1110: .Pp
1111: To get a listing of the fingerprints along with their random art for
1112: all known hosts, the following command line can be used:
1113: .Pp
1114: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1115: .Pp
1.252 jmc 1116: If the fingerprint is unknown,
1117: an alternative method of verification is available:
1118: SSH fingerprints verified by DNS.
1119: An additional resource record (RR),
1120: SSHFP,
1121: is added to a zonefile
1122: and the connecting client is able to match the fingerprint
1123: with that of the key presented.
1124: .Pp
1125: In this example, we are connecting a client to a server,
1126: .Dq host.example.com .
1127: The SSHFP resource records should first be added to the zonefile for
1128: host.example.com:
1129: .Bd -literal -offset indent
1.259 jakob 1130: $ ssh-keygen -r host.example.com.
1.252 jmc 1131: .Ed
1132: .Pp
1133: The output lines will have to be added to the zonefile.
1134: To check that the zone is answering fingerprint queries:
1135: .Pp
1136: .Dl $ dig -t SSHFP host.example.com
1137: .Pp
1138: Finally the client connects:
1139: .Bd -literal -offset indent
1140: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1141: [...]
1142: Matching host key fingerprint found in DNS.
1143: Are you sure you want to continue connecting (yes/no)?
1144: .Ed
1145: .Pp
1146: See the
1147: .Cm VerifyHostKeyDNS
1148: option in
1149: .Xr ssh_config 5
1150: for more information.
1.250 jmc 1151: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1152: .Nm
1153: contains support for Virtual Private Network (VPN) tunnelling
1154: using the
1155: .Xr tun 4
1156: network pseudo-device,
1157: allowing two networks to be joined securely.
1158: The
1159: .Xr sshd_config 5
1160: configuration option
1161: .Cm PermitTunnel
1162: controls whether the server supports this,
1163: and at what level (layer 2 or 3 traffic).
1164: .Pp
1165: The following example would connect client network 10.0.50.0/24
1.265 otto 1166: with remote network 10.0.99.0/24 using a point-to-point connection
1167: from 10.1.1.1 to 10.1.1.2,
1168: provided that the SSH server running on the gateway to the remote network,
1169: at 192.168.1.15, allows it.
1170: .Pp
1171: On the client:
1.250 jmc 1172: .Bd -literal -offset indent
1173: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1174: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1175: # route add 10.0.99.0/24 10.1.1.2
1176: .Ed
1177: .Pp
1178: On the server:
1179: .Bd -literal -offset indent
1180: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1181: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1182: .Ed
1183: .Pp
1184: Client access may be more finely tuned via the
1185: .Pa /root/.ssh/authorized_keys
1186: file (see below) and the
1187: .Cm PermitRootLogin
1188: server option.
1.255 jmc 1189: The following entry would permit connections on
1.250 jmc 1190: .Xr tun 4
1.255 jmc 1191: device 1 from user
1.250 jmc 1192: .Dq jane
1.255 jmc 1193: and on tun device 2 from user
1.250 jmc 1194: .Dq john ,
1195: if
1196: .Cm PermitRootLogin
1197: is set to
1198: .Dq forced-commands-only :
1199: .Bd -literal -offset 2n
1200: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1201: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1202: .Ed
1203: .Pp
1.264 ray 1204: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1205: it may be more suited to temporary setups,
1206: such as for wireless VPNs.
1207: More permanent VPNs are better provided by tools such as
1208: .Xr ipsecctl 8
1209: and
1210: .Xr isakmpd 8 .
1.2 deraadt 1211: .Sh ENVIRONMENT
1212: .Nm
1.1 deraadt 1213: will normally set the following environment variables:
1.237 jmc 1214: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1215: .It Ev DISPLAY
1216: The
1217: .Ev DISPLAY
1.40 aaron 1218: variable indicates the location of the X11 server.
1.44 aaron 1219: It is automatically set by
1.2 deraadt 1220: .Nm
1221: to point to a value of the form
1.233 jmc 1222: .Dq hostname:n ,
1223: where
1224: .Dq hostname
1225: indicates the host where the shell runs, and
1226: .Sq n
1227: is an integer \*(Ge 1.
1.40 aaron 1228: .Nm
1229: uses this special value to forward X11 connections over the secure
1230: channel.
1.107 markus 1231: The user should normally not set
1232: .Ev DISPLAY
1233: explicitly, as that
1.1 deraadt 1234: will render the X11 connection insecure (and will require the user to
1235: manually copy any required authorization cookies).
1.2 deraadt 1236: .It Ev HOME
1.1 deraadt 1237: Set to the path of the user's home directory.
1.2 deraadt 1238: .It Ev LOGNAME
1239: Synonym for
1.12 aaron 1240: .Ev USER ;
1241: set for compatibility with systems that use this variable.
1.2 deraadt 1242: .It Ev MAIL
1.129 stevesk 1243: Set to the path of the user's mailbox.
1.40 aaron 1244: .It Ev PATH
1.2 deraadt 1245: Set to the default
1246: .Ev PATH ,
1247: as specified when compiling
1.234 jmc 1248: .Nm .
1.118 markus 1249: .It Ev SSH_ASKPASS
1250: If
1251: .Nm
1252: needs a passphrase, it will read the passphrase from the current
1253: terminal if it was run from a terminal.
1254: If
1255: .Nm
1256: does not have a terminal associated with it but
1257: .Ev DISPLAY
1258: and
1259: .Ev SSH_ASKPASS
1260: are set, it will execute the program specified by
1261: .Ev SSH_ASKPASS
1262: and open an X11 window to read the passphrase.
1263: This is particularly useful when calling
1264: .Nm
1265: from a
1.196 jmc 1266: .Pa .xsession
1.118 markus 1267: or related script.
1268: (Note that on some machines it
1269: may be necessary to redirect the input from
1270: .Pa /dev/null
1271: to make this work.)
1.18 markus 1272: .It Ev SSH_AUTH_SOCK
1.233 jmc 1273: Identifies the path of a
1274: .Ux Ns -domain
1275: socket used to communicate with the agent.
1.166 stevesk 1276: .It Ev SSH_CONNECTION
1277: Identifies the client and server ends of the connection.
1.40 aaron 1278: The variable contains
1.233 jmc 1279: four space-separated values: client IP address, client port number,
1280: server IP address, and server port number.
1.73 markus 1281: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1282: This variable contains the original command line if a forced command
1.73 markus 1283: is executed.
1284: It can be used to extract the original arguments.
1.2 deraadt 1285: .It Ev SSH_TTY
1.1 deraadt 1286: This is set to the name of the tty (path to the device) associated
1.40 aaron 1287: with the current shell or command.
1288: If the current session has no tty,
1.1 deraadt 1289: this variable is not set.
1.2 deraadt 1290: .It Ev TZ
1.214 jmc 1291: This variable is set to indicate the present time zone if it
1.257 jmc 1292: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1293: on to new connections).
1.2 deraadt 1294: .It Ev USER
1.1 deraadt 1295: Set to the name of the user logging in.
1.2 deraadt 1296: .El
1297: .Pp
1.44 aaron 1298: Additionally,
1.2 deraadt 1299: .Nm
1.44 aaron 1300: reads
1.207 djm 1301: .Pa ~/.ssh/environment ,
1.2 deraadt 1302: and adds lines of the format
1303: .Dq VARNAME=value
1.233 jmc 1304: to the environment if the file exists and users are allowed to
1.161 marc 1305: change their environment.
1.176 jmc 1306: For more information, see the
1.161 marc 1307: .Cm PermitUserEnvironment
1.162 stevesk 1308: option in
1.161 marc 1309: .Xr sshd_config 5 .
1.2 deraadt 1310: .Sh FILES
1.236 jmc 1311: .Bl -tag -width Ds -compact
1.309 jmc 1312: .It Pa ~/.rhosts
1.240 jmc 1313: This file is used for host-based authentication (see above).
1.92 markus 1314: On some machines this file may need to be
1.240 jmc 1315: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1316: because
1.2 deraadt 1317: .Xr sshd 8
1.40 aaron 1318: reads it as root.
1319: Additionally, this file must be owned by the user,
1320: and must not have write permissions for anyone else.
1321: The recommended
1.1 deraadt 1322: permission for most machines is read/write for the user, and not
1323: accessible by others.
1.2 deraadt 1324: .Pp
1.309 jmc 1325: .It Pa ~/.shosts
1.240 jmc 1326: This file is used in exactly the same way as
1327: .Pa .rhosts ,
1328: but allows host-based authentication without permitting login with
1329: rlogin/rsh.
1.272 mcbride 1330: .Pp
1.309 jmc 1331: .It Pa ~/.ssh/
1.272 mcbride 1332: This directory is the default location for all user-specific configuration
1333: and authentication information.
1334: There is no general requirement to keep the entire contents of this directory
1335: secret, but the recommended permissions are read/write/execute for the user,
1336: and not accessible by others.
1.236 jmc 1337: .Pp
1.309 jmc 1338: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1339: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1340: that can be used for logging in as this user.
1.238 jmc 1341: The format of this file is described in the
1342: .Xr sshd 8
1343: manual page.
1344: This file is not highly sensitive, but the recommended
1345: permissions are read/write for the user, and not accessible by others.
1346: .Pp
1.309 jmc 1347: .It Pa ~/.ssh/config
1.238 jmc 1348: This is the per-user configuration file.
1349: The file format and configuration options are described in
1350: .Xr ssh_config 5 .
1351: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1352: read/write for the user, and not writable by others.
1.238 jmc 1353: .Pp
1.309 jmc 1354: .It Pa ~/.ssh/environment
1.239 jmc 1355: Contains additional definitions for environment variables; see
1356: .Sx ENVIRONMENT ,
1.238 jmc 1357: above.
1358: .Pp
1.309 jmc 1359: .It Pa ~/.ssh/identity
1360: .It Pa ~/.ssh/id_dsa
1.310 djm 1361: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1362: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1363: .It Pa ~/.ssh/id_rsa
1.238 jmc 1364: Contains the private key for authentication.
1365: These files
1366: contain sensitive data and should be readable by the user but not
1367: accessible by others (read/write/execute).
1368: .Nm
1369: will simply ignore a private key file if it is accessible by others.
1370: It is possible to specify a passphrase when
1371: generating the key which will be used to encrypt the
1372: sensitive part of this file using 3DES.
1373: .Pp
1.309 jmc 1374: .It Pa ~/.ssh/identity.pub
1375: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1376: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1377: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1378: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1379: Contains the public key for authentication.
1380: These files are not
1381: sensitive and can (but need not) be readable by anyone.
1382: .Pp
1.309 jmc 1383: .It Pa ~/.ssh/known_hosts
1.244 jmc 1384: Contains a list of host keys for all hosts the user has logged into
1385: that are not already in the systemwide list of known host keys.
1.238 jmc 1386: See
1.244 jmc 1387: .Xr sshd 8
1388: for further details of the format of this file.
1.238 jmc 1389: .Pp
1.309 jmc 1390: .It Pa ~/.ssh/rc
1.238 jmc 1391: Commands in this file are executed by
1392: .Nm
1.245 jmc 1393: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1394: started.
1395: See the
1396: .Xr sshd 8
1397: manual page for more information.
1398: .Pp
1.309 jmc 1399: .It Pa /etc/hosts.equiv
1.240 jmc 1400: This file is for host-based authentication (see above).
1401: It should only be writable by root.
1.236 jmc 1402: .Pp
1.309 jmc 1403: .It Pa /etc/shosts.equiv
1.240 jmc 1404: This file is used in exactly the same way as
1405: .Pa hosts.equiv ,
1406: but allows host-based authentication without permitting login with
1407: rlogin/rsh.
1.236 jmc 1408: .Pp
1.238 jmc 1409: .It Pa /etc/ssh/ssh_config
1410: Systemwide configuration file.
1411: The file format and configuration options are described in
1412: .Xr ssh_config 5 .
1413: .Pp
1.309 jmc 1414: .It Pa /etc/ssh/ssh_host_key
1415: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1416: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1417: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1418: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1419: These files contain the private parts of the host keys
1.245 jmc 1420: and are used for host-based authentication.
1421: If protocol version 1 is used,
1.238 jmc 1422: .Nm
1423: must be setuid root, since the host key is readable only by root.
1424: For protocol version 2,
1425: .Nm
1426: uses
1427: .Xr ssh-keysign 8
1.245 jmc 1428: to access the host keys,
1429: eliminating the requirement that
1.238 jmc 1430: .Nm
1.245 jmc 1431: be setuid root when host-based authentication is used.
1.238 jmc 1432: By default
1.2 deraadt 1433: .Nm
1.238 jmc 1434: is not setuid root.
1435: .Pp
1.309 jmc 1436: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1437: Systemwide list of known host keys.
1438: This file should be prepared by the
1439: system administrator to contain the public host keys of all machines in the
1440: organization.
1.244 jmc 1441: It should be world-readable.
1442: See
1.238 jmc 1443: .Xr sshd 8
1.244 jmc 1444: for further details of the format of this file.
1.236 jmc 1445: .Pp
1.309 jmc 1446: .It Pa /etc/ssh/sshrc
1.1 deraadt 1447: Commands in this file are executed by
1.2 deraadt 1448: .Nm
1.245 jmc 1449: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1450: See the
1.2 deraadt 1451: .Xr sshd 8
1.1 deraadt 1452: manual page for more information.
1.58 itojun 1453: .El
1.312 jmc 1454: .Sh EXIT STATUS
1455: .Nm
1456: exits with the exit status of the remote command or with 255
1457: if an error occurred.
1.2 deraadt 1458: .Sh SEE ALSO
1459: .Xr scp 1 ,
1.83 djm 1460: .Xr sftp 1 ,
1.2 deraadt 1461: .Xr ssh-add 1 ,
1462: .Xr ssh-agent 1 ,
1463: .Xr ssh-keygen 1 ,
1.242 jmc 1464: .Xr ssh-keyscan 1 ,
1.250 jmc 1465: .Xr tun 4 ,
1.159 stevesk 1466: .Xr ssh_config 5 ,
1.160 naddy 1467: .Xr ssh-keysign 8 ,
1.87 itojun 1468: .Xr sshd 8
1.329 jmc 1469: .Sh STANDARDS
1.106 markus 1470: .Rs
1.329 jmc 1471: .%A S. Lehtinen
1472: .%A C. Lonvick
1473: .%D January 2006
1.256 jmc 1474: .%R RFC 4250
1.329 jmc 1475: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1476: .Re
1.329 jmc 1477: .Pp
1.256 jmc 1478: .Rs
1.329 jmc 1479: .%A T. Ylonen
1480: .%A C. Lonvick
1481: .%D January 2006
1.256 jmc 1482: .%R RFC 4251
1.329 jmc 1483: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1484: .Re
1.329 jmc 1485: .Pp
1.256 jmc 1486: .Rs
1.329 jmc 1487: .%A T. Ylonen
1488: .%A C. Lonvick
1489: .%D January 2006
1.256 jmc 1490: .%R RFC 4252
1.329 jmc 1491: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1492: .Re
1.329 jmc 1493: .Pp
1.256 jmc 1494: .Rs
1.329 jmc 1495: .%A T. Ylonen
1496: .%A C. Lonvick
1497: .%D January 2006
1.256 jmc 1498: .%R RFC 4253
1.329 jmc 1499: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1500: .Re
1.329 jmc 1501: .Pp
1.256 jmc 1502: .Rs
1.329 jmc 1503: .%A T. Ylonen
1504: .%A C. Lonvick
1505: .%D January 2006
1.256 jmc 1506: .%R RFC 4254
1.329 jmc 1507: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1508: .Re
1.329 jmc 1509: .Pp
1.256 jmc 1510: .Rs
1.329 jmc 1511: .%A J. Schlyter
1512: .%A W. Griffin
1513: .%D January 2006
1.256 jmc 1514: .%R RFC 4255
1.329 jmc 1515: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1516: .Re
1.329 jmc 1517: .Pp
1.256 jmc 1518: .Rs
1.329 jmc 1519: .%A F. Cusack
1520: .%A M. Forssen
1521: .%D January 2006
1.256 jmc 1522: .%R RFC 4256
1.329 jmc 1523: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1524: .Re
1.329 jmc 1525: .Pp
1.256 jmc 1526: .Rs
1.329 jmc 1527: .%A J. Galbraith
1528: .%A P. Remaker
1529: .%D January 2006
1.256 jmc 1530: .%R RFC 4335
1.329 jmc 1531: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1532: .Re
1.329 jmc 1533: .Pp
1.256 jmc 1534: .Rs
1.329 jmc 1535: .%A M. Bellare
1536: .%A T. Kohno
1537: .%A C. Namprempre
1538: .%D January 2006
1.256 jmc 1539: .%R RFC 4344
1.329 jmc 1540: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1541: .Re
1.329 jmc 1542: .Pp
1.256 jmc 1543: .Rs
1.329 jmc 1544: .%A B. Harris
1545: .%D January 2006
1.256 jmc 1546: .%R RFC 4345
1.329 jmc 1547: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1548: .Re
1.329 jmc 1549: .Pp
1.258 djm 1550: .Rs
1.329 jmc 1551: .%A M. Friedl
1552: .%A N. Provos
1553: .%A W. Simpson
1554: .%D March 2006
1.258 djm 1555: .%R RFC 4419
1.329 jmc 1556: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1557: .Re
1.329 jmc 1558: .Pp
1.266 markus 1559: .Rs
1.329 jmc 1560: .%A J. Galbraith
1561: .%A R. Thayer
1562: .%D November 2006
1.266 markus 1563: .%R RFC 4716
1.329 jmc 1564: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1565: .Re
1.329 jmc 1566: .Pp
1.313 djm 1567: .Rs
1.329 jmc 1568: .%A D. Stebila
1569: .%A J. Green
1570: .%D December 2009
1.313 djm 1571: .%R RFC 5656
1.329 jmc 1572: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1573: .Re
1.329 jmc 1574: .Pp
1.274 grunk 1575: .Rs
1576: .%A A. Perrig
1577: .%A D. Song
1578: .%D 1999
1.329 jmc 1579: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1580: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1581: .Re
1.173 jmc 1582: .Sh AUTHORS
1583: OpenSSH is a derivative of the original and free
1584: ssh 1.2.12 release by Tatu Ylonen.
1585: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1586: Theo de Raadt and Dug Song
1587: removed many bugs, re-added newer features and
1588: created OpenSSH.
1589: Markus Friedl contributed the support for SSH
1590: protocol versions 1.5 and 2.0.
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