Annotation of src/usr.bin/ssh/ssh.1, Revision 1.359
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: .\"
1.359 ! markus 36: .\" $OpenBSD: ssh.1,v 1.358 2015/05/22 05:28:45 djm Exp $
! 37: .Dd $Mdocdate: May 22 2015 $
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
1.351 jmc 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
1.356 djm 61: .Op Fl Q Cm cipher | cipher-auth | mac | kex | key | protocol-version
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
1.353 jmc 436: .It FingerprintHash
1.218 jmc 437: .It ForwardAgent
438: .It ForwardX11
1.321 djm 439: .It ForwardX11Timeout
1.218 jmc 440: .It ForwardX11Trusted
441: .It GatewayPorts
442: .It GlobalKnownHostsFile
443: .It GSSAPIAuthentication
444: .It GSSAPIDelegateCredentials
445: .It HashKnownHosts
446: .It Host
447: .It HostbasedAuthentication
1.355 djm 448: .It HostbasedKeyTypes
1.218 jmc 449: .It HostKeyAlgorithms
450: .It HostKeyAlias
451: .It HostName
452: .It IdentityFile
453: .It IdentitiesOnly
1.316 jmc 454: .It IPQoS
1.321 djm 455: .It KbdInteractiveAuthentication
1.218 jmc 456: .It KbdInteractiveDevices
1.314 jmc 457: .It KexAlgorithms
1.218 jmc 458: .It LocalCommand
459: .It LocalForward
460: .It LogLevel
461: .It MACs
1.337 jmc 462: .It Match
1.218 jmc 463: .It NoHostAuthenticationForLocalhost
464: .It NumberOfPasswordPrompts
465: .It PasswordAuthentication
466: .It PermitLocalCommand
1.291 markus 467: .It PKCS11Provider
1.218 jmc 468: .It Port
469: .It PreferredAuthentications
470: .It Protocol
471: .It ProxyCommand
1.336 jmc 472: .It ProxyUseFdpass
1.359 ! markus 473: .It PubkeyAcceptedKeyTypes
1.218 jmc 474: .It PubkeyAuthentication
1.251 dtucker 475: .It RekeyLimit
1.218 jmc 476: .It RemoteForward
1.319 jmc 477: .It RequestTTY
1.218 jmc 478: .It RhostsRSAAuthentication
479: .It RSAAuthentication
480: .It SendEnv
481: .It ServerAliveInterval
482: .It ServerAliveCountMax
1.347 jmc 483: .It StreamLocalBindMask
484: .It StreamLocalBindUnlink
1.218 jmc 485: .It StrictHostKeyChecking
486: .It TCPKeepAlive
487: .It Tunnel
488: .It TunnelDevice
1.354 djm 489: .It UpdateHostKeys
1.218 jmc 490: .It UsePrivilegedPort
491: .It User
492: .It UserKnownHostsFile
493: .It VerifyHostKeyDNS
1.276 jmc 494: .It VisualHostKey
1.218 jmc 495: .It XAuthLocation
496: .El
497: .It Fl p Ar port
498: Port to connect to on the remote host.
499: This can be specified on a
500: per-host basis in the configuration file.
1.356 djm 501: .It Fl Q Cm cipher | cipher-auth | mac | kex | key | protocol-version
1.332 djm 502: Queries
503: .Nm
1.341 deraadt 504: for the algorithms supported for the specified version 2.
505: The available features are:
506: .Ar cipher
1.332 djm 507: (supported symmetric ciphers),
1.341 deraadt 508: .Ar cipher-auth
1.340 djm 509: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 510: .Ar mac
1.332 djm 511: (supported message integrity codes),
1.341 deraadt 512: .Ar kex
1.332 djm 513: (key exchange algorithms),
1.341 deraadt 514: .Ar key
1.356 djm 515: (key types) and
516: .Ar protocol-version
517: (supported SSH protocol versions).
1.218 jmc 518: .It Fl q
519: Quiet mode.
1.271 djm 520: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 521: .It Fl R Xo
522: .Sm off
523: .Oo Ar bind_address : Oc
524: .Ar port : host : hostport
525: .Sm on
526: .Xc
527: Specifies that the given port on the remote (server) host is to be
528: forwarded to the given host and port on the local side.
529: This works by allocating a socket to listen to
530: .Ar port
531: on the remote side, and whenever a connection is made to this port, the
532: connection is forwarded over the secure channel, and a connection is
533: made to
534: .Ar host
535: port
536: .Ar hostport
537: from the local machine.
1.2 deraadt 538: .Pp
1.218 jmc 539: Port forwardings can also be specified in the configuration file.
540: Privileged ports can be forwarded only when
541: logging in as root on the remote machine.
1.324 jmc 542: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 543: .Pp
1.218 jmc 544: By default, the listening socket on the server will be bound to the loopback
545: interface only.
1.280 tobias 546: This may be overridden by specifying a
1.218 jmc 547: .Ar bind_address .
548: An empty
549: .Ar bind_address ,
550: or the address
551: .Ql * ,
552: indicates that the remote socket should listen on all interfaces.
553: Specifying a remote
554: .Ar bind_address
555: will only succeed if the server's
556: .Cm GatewayPorts
557: option is enabled (see
558: .Xr sshd_config 5 ) .
1.281 djm 559: .Pp
560: If the
561: .Ar port
562: argument is
1.282 djm 563: .Ql 0 ,
1.281 djm 564: the listen port will be dynamically allocated on the server and reported
565: to the client at run time.
1.305 markus 566: When used together with
567: .Ic -O forward
568: the allocated port will be printed to the standard output.
1.218 jmc 569: .It Fl S Ar ctl_path
1.304 jmc 570: Specifies the location of a control socket for connection sharing,
1.303 djm 571: or the string
572: .Dq none
573: to disable connection sharing.
1.218 jmc 574: Refer to the description of
575: .Cm ControlPath
576: and
577: .Cm ControlMaster
578: in
579: .Xr ssh_config 5
580: for details.
581: .It Fl s
582: May be used to request invocation of a subsystem on the remote system.
583: Subsystems are a feature of the SSH2 protocol which facilitate the use
584: of SSH as a secure transport for other applications (eg.\&
585: .Xr sftp 1 ) .
586: The subsystem is specified as the remote command.
587: .It Fl T
1.357 dtucker 588: Disable pseudo-terminal allocation.
1.218 jmc 589: .It Fl t
1.357 dtucker 590: Force pseudo-terminal allocation.
1.218 jmc 591: This can be used to execute arbitrary
592: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 593: e.g. when implementing menu services.
1.218 jmc 594: Multiple
595: .Fl t
596: options force tty allocation, even if
1.194 jakob 597: .Nm
1.218 jmc 598: has no local tty.
599: .It Fl V
600: Display the version number and exit.
601: .It Fl v
602: Verbose mode.
603: Causes
1.176 jmc 604: .Nm
1.218 jmc 605: to print debugging messages about its progress.
606: This is helpful in
607: debugging connection, authentication, and configuration problems.
608: Multiple
609: .Fl v
610: options increase the verbosity.
611: The maximum is 3.
1.290 dtucker 612: .It Fl W Ar host : Ns Ar port
613: Requests that standard input and output on the client be forwarded to
614: .Ar host
615: on
616: .Ar port
617: over the secure channel.
618: Implies
619: .Fl N ,
620: .Fl T ,
621: .Cm ExitOnForwardFailure
622: and
1.326 dtucker 623: .Cm ClearAllForwardings .
624: Works with Protocol version 2 only.
1.261 stevesk 625: .It Fl w Xo
626: .Ar local_tun Ns Op : Ns Ar remote_tun
627: .Xc
628: Requests
629: tunnel
630: device forwarding with the specified
1.218 jmc 631: .Xr tun 4
1.261 stevesk 632: devices between the client
633: .Pq Ar local_tun
634: and the server
635: .Pq Ar remote_tun .
636: .Pp
1.228 jmc 637: The devices may be specified by numerical ID or the keyword
638: .Dq any ,
639: which uses the next available tunnel device.
1.261 stevesk 640: If
641: .Ar remote_tun
642: is not specified, it defaults to
643: .Dq any .
1.228 jmc 644: See also the
1.218 jmc 645: .Cm Tunnel
1.261 stevesk 646: and
647: .Cm TunnelDevice
648: directives in
1.218 jmc 649: .Xr ssh_config 5 .
1.261 stevesk 650: If the
651: .Cm Tunnel
652: directive is unset, it is set to the default tunnel mode, which is
653: .Dq point-to-point .
1.218 jmc 654: .It Fl X
655: Enables X11 forwarding.
1.54 markus 656: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 657: .Pp
1.218 jmc 658: X11 forwarding should be enabled with caution.
1.168 jmc 659: Users with the ability to bypass file permissions on the remote host
1.218 jmc 660: (for the user's X authorization database)
661: can access the local X11 display through the forwarded connection.
662: An attacker may then be able to perform activities such as keystroke monitoring.
663: .Pp
664: For this reason, X11 forwarding is subjected to X11 SECURITY extension
665: restrictions by default.
666: Please refer to the
667: .Nm
668: .Fl Y
669: option and the
670: .Cm ForwardX11Trusted
671: directive in
672: .Xr ssh_config 5
673: for more information.
674: .It Fl x
675: Disables X11 forwarding.
676: .It Fl Y
677: Enables trusted X11 forwarding.
678: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
679: controls.
1.278 djm 680: .It Fl y
681: Send log information using the
682: .Xr syslog 3
683: system module.
684: By default this information is sent to stderr.
1.218 jmc 685: .El
1.224 jmc 686: .Pp
687: .Nm
688: may additionally obtain configuration data from
689: a per-user configuration file and a system-wide configuration file.
690: The file format and configuration options are described in
691: .Xr ssh_config 5 .
1.222 jmc 692: .Sh AUTHENTICATION
1.249 jmc 693: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 694: The default is to use protocol 2 only,
695: though this can be changed via the
1.222 jmc 696: .Cm Protocol
697: option in
1.284 jmc 698: .Xr ssh_config 5
699: or the
1.222 jmc 700: .Fl 1
701: and
702: .Fl 2
703: options (see above).
704: Both protocols support similar authentication methods,
1.284 jmc 705: but protocol 2 is the default since
1.222 jmc 706: it provides additional mechanisms for confidentiality
707: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 708: and integrity (hmac-md5, hmac-sha1,
709: hmac-sha2-256, hmac-sha2-512,
1.330 markus 710: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 711: Protocol 1 lacks a strong mechanism for ensuring the
712: integrity of the connection.
713: .Pp
714: The methods available for authentication are:
1.260 jmc 715: GSSAPI-based authentication,
1.222 jmc 716: host-based authentication,
717: public key authentication,
718: challenge-response authentication,
719: and password authentication.
720: Authentication methods are tried in the order specified above,
721: though protocol 2 has a configuration option to change the default order:
722: .Cm PreferredAuthentications .
723: .Pp
724: Host-based authentication works as follows:
1.218 jmc 725: If the machine the user logs in from is listed in
726: .Pa /etc/hosts.equiv
727: or
728: .Pa /etc/shosts.equiv
729: on the remote machine, and the user names are
730: the same on both sides, or if the files
731: .Pa ~/.rhosts
732: or
733: .Pa ~/.shosts
734: exist in the user's home directory on the
735: remote machine and contain a line containing the name of the client
736: machine and the name of the user on that machine, the user is
1.222 jmc 737: considered for login.
738: Additionally, the server
739: .Em must
740: be able to verify the client's
741: host key (see the description of
1.218 jmc 742: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 743: and
1.222 jmc 744: .Pa ~/.ssh/known_hosts ,
745: below)
746: for login to be permitted.
1.218 jmc 747: This authentication method closes security holes due to IP
1.222 jmc 748: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 749: [Note to the administrator:
750: .Pa /etc/hosts.equiv ,
751: .Pa ~/.rhosts ,
752: and the rlogin/rsh protocol in general, are inherently insecure and should be
753: disabled if security is desired.]
1.189 dtucker 754: .Pp
1.222 jmc 755: Public key authentication works as follows:
756: The scheme is based on public-key cryptography,
757: using cryptosystems
758: where encryption and decryption are done using separate keys,
759: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 760: The idea is that each user creates a public/private
761: key pair for authentication purposes.
762: The server knows the public key, and only the user knows the private key.
1.222 jmc 763: .Nm
764: implements public key authentication protocol automatically,
1.349 sobrado 765: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.222 jmc 766: Protocol 1 is restricted to using only RSA keys,
1.310 djm 767: but protocol 2 may use any.
1.333 jmc 768: The HISTORY section of
1.222 jmc 769: .Xr ssl 8
1.311 jmc 770: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 771: .Pp
1.218 jmc 772: The file
773: .Pa ~/.ssh/authorized_keys
774: lists the public keys that are permitted for logging in.
775: When the user logs in, the
1.2 deraadt 776: .Nm
1.218 jmc 777: program tells the server which key pair it would like to use for
778: authentication.
1.222 jmc 779: The client proves that it has access to the private key
780: and the server checks that the corresponding public key
781: is authorized to accept the account.
1.218 jmc 782: .Pp
1.222 jmc 783: The user creates his/her key pair by running
1.218 jmc 784: .Xr ssh-keygen 1 .
785: This stores the private key in
1.207 djm 786: .Pa ~/.ssh/identity
1.222 jmc 787: (protocol 1),
788: .Pa ~/.ssh/id_dsa
789: (protocol 2 DSA),
1.310 djm 790: .Pa ~/.ssh/id_ecdsa
791: (protocol 2 ECDSA),
1.343 naddy 792: .Pa ~/.ssh/id_ed25519
1.349 sobrado 793: (protocol 2 Ed25519),
1.222 jmc 794: or
795: .Pa ~/.ssh/id_rsa
796: (protocol 2 RSA)
1.218 jmc 797: and stores the public key in
798: .Pa ~/.ssh/identity.pub
1.222 jmc 799: (protocol 1),
800: .Pa ~/.ssh/id_dsa.pub
801: (protocol 2 DSA),
1.310 djm 802: .Pa ~/.ssh/id_ecdsa.pub
803: (protocol 2 ECDSA),
1.343 naddy 804: .Pa ~/.ssh/id_ed25519.pub
1.349 sobrado 805: (protocol 2 Ed25519),
1.222 jmc 806: or
807: .Pa ~/.ssh/id_rsa.pub
808: (protocol 2 RSA)
1.218 jmc 809: in the user's home directory.
1.222 jmc 810: The user should then copy the public key
1.218 jmc 811: to
812: .Pa ~/.ssh/authorized_keys
1.222 jmc 813: in his/her home directory on the remote machine.
814: The
1.218 jmc 815: .Pa authorized_keys
816: file corresponds to the conventional
817: .Pa ~/.rhosts
818: file, and has one key
1.222 jmc 819: per line, though the lines can be very long.
1.218 jmc 820: After this, the user can log in without giving the password.
821: .Pp
1.301 jmc 822: A variation on public key authentication
823: is available in the form of certificate authentication:
824: instead of a set of public/private keys,
825: signed certificates are used.
826: This has the advantage that a single trusted certification authority
827: can be used in place of many public/private keys.
1.333 jmc 828: See the CERTIFICATES section of
1.301 jmc 829: .Xr ssh-keygen 1
830: for more information.
831: .Pp
832: The most convenient way to use public key or certificate authentication
833: may be with an authentication agent.
1.218 jmc 834: See
835: .Xr ssh-agent 1
836: for more information.
837: .Pp
1.222 jmc 838: Challenge-response authentication works as follows:
839: The server sends an arbitrary
840: .Qq challenge
841: text, and prompts for a response.
842: Protocol 2 allows multiple challenges and responses;
843: protocol 1 is restricted to just one challenge/response.
844: Examples of challenge-response authentication include
1.335 jmc 845: .Bx
846: Authentication (see
1.222 jmc 847: .Xr login.conf 5 )
1.335 jmc 848: and PAM (some
849: .Pf non- Ox
850: systems).
1.222 jmc 851: .Pp
852: Finally, if other authentication methods fail,
1.218 jmc 853: .Nm
854: prompts the user for a password.
855: The password is sent to the remote
856: host for checking; however, since all communications are encrypted,
857: the password cannot be seen by someone listening on the network.
1.232 jmc 858: .Pp
859: .Nm
860: automatically maintains and checks a database containing
861: identification for all hosts it has ever been used with.
862: Host keys are stored in
863: .Pa ~/.ssh/known_hosts
864: in the user's home directory.
865: Additionally, the file
866: .Pa /etc/ssh/ssh_known_hosts
867: is automatically checked for known hosts.
868: Any new hosts are automatically added to the user's file.
869: If a host's identification ever changes,
870: .Nm
871: warns about this and disables password authentication to prevent
872: server spoofing or man-in-the-middle attacks,
873: which could otherwise be used to circumvent the encryption.
874: The
875: .Cm StrictHostKeyChecking
876: option can be used to control logins to machines whose
877: host key is not known or has changed.
878: .Pp
1.218 jmc 879: When the user's identity has been accepted by the server, the server
1.357 dtucker 880: either executes the given command in a non-interactive session or,
881: if no command has been specified, logs into the machine and gives
882: the user a normal shell as an interactive session.
1.218 jmc 883: All communication with
884: the remote command or shell will be automatically encrypted.
885: .Pp
1.357 dtucker 886: If an interactive session is requested
887: .Nm
888: by default will only request a pseudo-terminal (pty) for interactive
889: sessions when the client has one.
890: The flags
891: .Fl T
892: and
893: .Fl t
894: can be used to override this behaviour.
895: .Pp
896: If a pseudo-terminal has been allocated the
1.218 jmc 897: user may use the escape characters noted below.
898: .Pp
1.357 dtucker 899: If no pseudo-terminal has been allocated,
1.218 jmc 900: the session is transparent and can be used to reliably transfer binary data.
901: On most systems, setting the escape character to
902: .Dq none
903: will also make the session transparent even if a tty is used.
904: .Pp
905: The session terminates when the command or shell on the remote
1.247 jmc 906: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 907: .Sh ESCAPE CHARACTERS
1.218 jmc 908: When a pseudo-terminal has been requested,
1.2 deraadt 909: .Nm
1.218 jmc 910: supports a number of functions through the use of an escape character.
911: .Pp
912: A single tilde character can be sent as
913: .Ic ~~
914: or by following the tilde by a character other than those described below.
915: The escape character must always follow a newline to be interpreted as
916: special.
917: The escape character can be changed in configuration files using the
918: .Cm EscapeChar
919: configuration directive or on the command line by the
920: .Fl e
921: option.
922: .Pp
923: The supported escapes (assuming the default
924: .Ql ~ )
925: are:
926: .Bl -tag -width Ds
927: .It Cm ~.
928: Disconnect.
929: .It Cm ~^Z
930: Background
1.234 jmc 931: .Nm .
1.218 jmc 932: .It Cm ~#
933: List forwarded connections.
934: .It Cm ~&
935: Background
1.2 deraadt 936: .Nm
1.218 jmc 937: at logout when waiting for forwarded connection / X11 sessions to terminate.
938: .It Cm ~?
939: Display a list of escape characters.
940: .It Cm ~B
941: Send a BREAK to the remote system
942: (only useful for SSH protocol version 2 and if the peer supports it).
943: .It Cm ~C
944: Open command line.
945: Currently this allows the addition of port forwardings using the
1.279 stevesk 946: .Fl L ,
947: .Fl R
1.218 jmc 948: and
1.279 stevesk 949: .Fl D
1.225 jmc 950: options (see above).
1.322 markus 951: It also allows the cancellation of existing port-forwardings
952: with
1.262 stevesk 953: .Sm off
1.322 markus 954: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 955: .Sm on
1.322 markus 956: for local,
957: .Sm off
958: .Fl KR Oo Ar bind_address : Oc Ar port
959: .Sm on
960: for remote and
961: .Sm off
962: .Fl KD Oo Ar bind_address : Oc Ar port
963: .Sm on
964: for dynamic port-forwardings.
1.218 jmc 965: .Ic !\& Ns Ar command
966: allows the user to execute a local command if the
967: .Ic PermitLocalCommand
968: option is enabled in
1.176 jmc 969: .Xr ssh_config 5 .
1.218 jmc 970: Basic help is available, using the
971: .Fl h
972: option.
973: .It Cm ~R
974: Request rekeying of the connection
975: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 976: .It Cm ~V
977: Decrease the verbosity
978: .Pq Ic LogLevel
979: when errors are being written to stderr.
980: .It Cm ~v
1.328 jmc 981: Increase the verbosity
1.327 dtucker 982: .Pq Ic LogLevel
983: when errors are being written to stderr.
1.176 jmc 984: .El
1.246 jmc 985: .Sh TCP FORWARDING
986: Forwarding of arbitrary TCP connections over the secure channel can
987: be specified either on the command line or in a configuration file.
988: One possible application of TCP forwarding is a secure connection to a
989: mail server; another is going through firewalls.
990: .Pp
991: In the example below, we look at encrypting communication between
992: an IRC client and server, even though the IRC server does not directly
993: support encrypted communications.
994: This works as follows:
995: the user connects to the remote host using
996: .Nm ,
997: specifying a port to be used to forward connections
998: to the remote server.
999: After that it is possible to start the service which is to be encrypted
1000: on the client machine,
1001: connecting to the same local port,
1002: and
1003: .Nm
1004: will encrypt and forward the connection.
1005: .Pp
1006: The following example tunnels an IRC session from client machine
1007: .Dq 127.0.0.1
1008: (localhost)
1009: to remote server
1010: .Dq server.example.com :
1011: .Bd -literal -offset 4n
1012: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1013: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1014: .Ed
1015: .Pp
1016: This tunnels a connection to IRC server
1017: .Dq server.example.com ,
1018: joining channel
1019: .Dq #users ,
1020: nickname
1021: .Dq pinky ,
1022: using port 1234.
1023: It doesn't matter which port is used,
1024: as long as it's greater than 1023
1025: (remember, only root can open sockets on privileged ports)
1026: and doesn't conflict with any ports already in use.
1027: The connection is forwarded to port 6667 on the remote server,
1028: since that's the standard port for IRC services.
1029: .Pp
1030: The
1031: .Fl f
1032: option backgrounds
1033: .Nm
1034: and the remote command
1035: .Dq sleep 10
1036: is specified to allow an amount of time
1037: (10 seconds, in the example)
1038: to start the service which is to be tunnelled.
1039: If no connections are made within the time specified,
1040: .Nm
1041: will exit.
1042: .Sh X11 FORWARDING
1.218 jmc 1043: If the
1044: .Cm ForwardX11
1045: variable is set to
1046: .Dq yes
1047: (or see the description of the
1.227 jmc 1048: .Fl X ,
1049: .Fl x ,
1.218 jmc 1050: and
1.227 jmc 1051: .Fl Y
1.226 jmc 1052: options above)
1.218 jmc 1053: and the user is using X11 (the
1054: .Ev DISPLAY
1055: environment variable is set), the connection to the X11 display is
1056: automatically forwarded to the remote side in such a way that any X11
1057: programs started from the shell (or command) will go through the
1058: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1059: from the local machine.
1.218 jmc 1060: The user should not manually set
1061: .Ev DISPLAY .
1062: Forwarding of X11 connections can be
1063: configured on the command line or in configuration files.
1064: .Pp
1065: The
1066: .Ev DISPLAY
1067: value set by
1068: .Nm
1069: will point to the server machine, but with a display number greater than zero.
1070: This is normal, and happens because
1071: .Nm
1072: creates a
1073: .Dq proxy
1074: X server on the server machine for forwarding the
1075: connections over the encrypted channel.
1.200 djm 1076: .Pp
1.218 jmc 1077: .Nm
1078: will also automatically set up Xauthority data on the server machine.
1079: For this purpose, it will generate a random authorization cookie,
1080: store it in Xauthority on the server, and verify that any forwarded
1081: connections carry this cookie and replace it by the real cookie when
1082: the connection is opened.
1083: The real authentication cookie is never
1084: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1085: .Pp
1.218 jmc 1086: If the
1087: .Cm ForwardAgent
1088: variable is set to
1089: .Dq yes
1090: (or see the description of the
1091: .Fl A
1.191 djm 1092: and
1.218 jmc 1093: .Fl a
1.226 jmc 1094: options above) and
1.218 jmc 1095: the user is using an authentication agent, the connection to the agent
1096: is automatically forwarded to the remote side.
1.252 jmc 1097: .Sh VERIFYING HOST KEYS
1098: When connecting to a server for the first time,
1099: a fingerprint of the server's public key is presented to the user
1100: (unless the option
1101: .Cm StrictHostKeyChecking
1102: has been disabled).
1103: Fingerprints can be determined using
1104: .Xr ssh-keygen 1 :
1105: .Pp
1106: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1107: .Pp
1.274 grunk 1108: If the fingerprint is already known, it can be matched
1109: and the key can be accepted or rejected.
1.358 djm 1110: If only legacy (MD5) fingerprints for the server are available, the
1111: .Xr ssh-keygen 1
1112: .Fl E
1113: option may be used to downgrade the fingerprint algorithm to match.
1114: .Pp
1.274 grunk 1115: Because of the difficulty of comparing host keys
1.352 djm 1116: just by looking at fingerprint strings,
1.274 grunk 1117: there is also support to compare host keys visually,
1118: using
1119: .Em random art .
1120: By setting the
1.275 grunk 1121: .Cm VisualHostKey
1.274 grunk 1122: option to
1.275 grunk 1123: .Dq yes ,
1.274 grunk 1124: a small ASCII graphic gets displayed on every login to a server, no matter
1125: if the session itself is interactive or not.
1126: By learning the pattern a known server produces, a user can easily
1127: find out that the host key has changed when a completely different pattern
1128: is displayed.
1129: Because these patterns are not unambiguous however, a pattern that looks
1130: similar to the pattern remembered only gives a good probability that the
1131: host key is the same, not guaranteed proof.
1132: .Pp
1133: To get a listing of the fingerprints along with their random art for
1134: all known hosts, the following command line can be used:
1135: .Pp
1136: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1137: .Pp
1.252 jmc 1138: If the fingerprint is unknown,
1139: an alternative method of verification is available:
1140: SSH fingerprints verified by DNS.
1141: An additional resource record (RR),
1142: SSHFP,
1143: is added to a zonefile
1144: and the connecting client is able to match the fingerprint
1145: with that of the key presented.
1146: .Pp
1147: In this example, we are connecting a client to a server,
1148: .Dq host.example.com .
1149: The SSHFP resource records should first be added to the zonefile for
1150: host.example.com:
1151: .Bd -literal -offset indent
1.259 jakob 1152: $ ssh-keygen -r host.example.com.
1.252 jmc 1153: .Ed
1154: .Pp
1155: The output lines will have to be added to the zonefile.
1156: To check that the zone is answering fingerprint queries:
1157: .Pp
1158: .Dl $ dig -t SSHFP host.example.com
1159: .Pp
1160: Finally the client connects:
1161: .Bd -literal -offset indent
1162: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1163: [...]
1164: Matching host key fingerprint found in DNS.
1165: Are you sure you want to continue connecting (yes/no)?
1166: .Ed
1167: .Pp
1168: See the
1169: .Cm VerifyHostKeyDNS
1170: option in
1171: .Xr ssh_config 5
1172: for more information.
1.250 jmc 1173: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1174: .Nm
1175: contains support for Virtual Private Network (VPN) tunnelling
1176: using the
1177: .Xr tun 4
1178: network pseudo-device,
1179: allowing two networks to be joined securely.
1180: The
1181: .Xr sshd_config 5
1182: configuration option
1183: .Cm PermitTunnel
1184: controls whether the server supports this,
1185: and at what level (layer 2 or 3 traffic).
1186: .Pp
1187: The following example would connect client network 10.0.50.0/24
1.265 otto 1188: with remote network 10.0.99.0/24 using a point-to-point connection
1189: from 10.1.1.1 to 10.1.1.2,
1190: provided that the SSH server running on the gateway to the remote network,
1191: at 192.168.1.15, allows it.
1192: .Pp
1193: On the client:
1.250 jmc 1194: .Bd -literal -offset indent
1195: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1196: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1197: # route add 10.0.99.0/24 10.1.1.2
1198: .Ed
1199: .Pp
1200: On the server:
1201: .Bd -literal -offset indent
1202: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1203: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1204: .Ed
1205: .Pp
1206: Client access may be more finely tuned via the
1207: .Pa /root/.ssh/authorized_keys
1208: file (see below) and the
1209: .Cm PermitRootLogin
1210: server option.
1.255 jmc 1211: The following entry would permit connections on
1.250 jmc 1212: .Xr tun 4
1.255 jmc 1213: device 1 from user
1.250 jmc 1214: .Dq jane
1.255 jmc 1215: and on tun device 2 from user
1.250 jmc 1216: .Dq john ,
1217: if
1218: .Cm PermitRootLogin
1219: is set to
1220: .Dq forced-commands-only :
1221: .Bd -literal -offset 2n
1222: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1223: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1224: .Ed
1225: .Pp
1.264 ray 1226: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1227: it may be more suited to temporary setups,
1228: such as for wireless VPNs.
1229: More permanent VPNs are better provided by tools such as
1230: .Xr ipsecctl 8
1231: and
1232: .Xr isakmpd 8 .
1.2 deraadt 1233: .Sh ENVIRONMENT
1234: .Nm
1.1 deraadt 1235: will normally set the following environment variables:
1.237 jmc 1236: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1237: .It Ev DISPLAY
1238: The
1239: .Ev DISPLAY
1.40 aaron 1240: variable indicates the location of the X11 server.
1.44 aaron 1241: It is automatically set by
1.2 deraadt 1242: .Nm
1243: to point to a value of the form
1.233 jmc 1244: .Dq hostname:n ,
1245: where
1246: .Dq hostname
1247: indicates the host where the shell runs, and
1248: .Sq n
1249: is an integer \*(Ge 1.
1.40 aaron 1250: .Nm
1251: uses this special value to forward X11 connections over the secure
1252: channel.
1.107 markus 1253: The user should normally not set
1254: .Ev DISPLAY
1255: explicitly, as that
1.1 deraadt 1256: will render the X11 connection insecure (and will require the user to
1257: manually copy any required authorization cookies).
1.2 deraadt 1258: .It Ev HOME
1.1 deraadt 1259: Set to the path of the user's home directory.
1.2 deraadt 1260: .It Ev LOGNAME
1261: Synonym for
1.12 aaron 1262: .Ev USER ;
1263: set for compatibility with systems that use this variable.
1.2 deraadt 1264: .It Ev MAIL
1.129 stevesk 1265: Set to the path of the user's mailbox.
1.40 aaron 1266: .It Ev PATH
1.2 deraadt 1267: Set to the default
1268: .Ev PATH ,
1269: as specified when compiling
1.234 jmc 1270: .Nm .
1.118 markus 1271: .It Ev SSH_ASKPASS
1272: If
1273: .Nm
1274: needs a passphrase, it will read the passphrase from the current
1275: terminal if it was run from a terminal.
1276: If
1277: .Nm
1278: does not have a terminal associated with it but
1279: .Ev DISPLAY
1280: and
1281: .Ev SSH_ASKPASS
1282: are set, it will execute the program specified by
1283: .Ev SSH_ASKPASS
1284: and open an X11 window to read the passphrase.
1285: This is particularly useful when calling
1286: .Nm
1287: from a
1.196 jmc 1288: .Pa .xsession
1.118 markus 1289: or related script.
1290: (Note that on some machines it
1291: may be necessary to redirect the input from
1292: .Pa /dev/null
1293: to make this work.)
1.18 markus 1294: .It Ev SSH_AUTH_SOCK
1.233 jmc 1295: Identifies the path of a
1296: .Ux Ns -domain
1297: socket used to communicate with the agent.
1.166 stevesk 1298: .It Ev SSH_CONNECTION
1299: Identifies the client and server ends of the connection.
1.40 aaron 1300: The variable contains
1.233 jmc 1301: four space-separated values: client IP address, client port number,
1302: server IP address, and server port number.
1.73 markus 1303: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1304: This variable contains the original command line if a forced command
1.73 markus 1305: is executed.
1306: It can be used to extract the original arguments.
1.2 deraadt 1307: .It Ev SSH_TTY
1.1 deraadt 1308: This is set to the name of the tty (path to the device) associated
1.40 aaron 1309: with the current shell or command.
1310: If the current session has no tty,
1.1 deraadt 1311: this variable is not set.
1.2 deraadt 1312: .It Ev TZ
1.214 jmc 1313: This variable is set to indicate the present time zone if it
1.257 jmc 1314: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1315: on to new connections).
1.2 deraadt 1316: .It Ev USER
1.1 deraadt 1317: Set to the name of the user logging in.
1.2 deraadt 1318: .El
1319: .Pp
1.44 aaron 1320: Additionally,
1.2 deraadt 1321: .Nm
1.44 aaron 1322: reads
1.207 djm 1323: .Pa ~/.ssh/environment ,
1.2 deraadt 1324: and adds lines of the format
1325: .Dq VARNAME=value
1.233 jmc 1326: to the environment if the file exists and users are allowed to
1.161 marc 1327: change their environment.
1.176 jmc 1328: For more information, see the
1.161 marc 1329: .Cm PermitUserEnvironment
1.162 stevesk 1330: option in
1.161 marc 1331: .Xr sshd_config 5 .
1.2 deraadt 1332: .Sh FILES
1.236 jmc 1333: .Bl -tag -width Ds -compact
1.309 jmc 1334: .It Pa ~/.rhosts
1.240 jmc 1335: This file is used for host-based authentication (see above).
1.92 markus 1336: On some machines this file may need to be
1.240 jmc 1337: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1338: because
1.2 deraadt 1339: .Xr sshd 8
1.40 aaron 1340: reads it as root.
1341: Additionally, this file must be owned by the user,
1342: and must not have write permissions for anyone else.
1343: The recommended
1.1 deraadt 1344: permission for most machines is read/write for the user, and not
1345: accessible by others.
1.2 deraadt 1346: .Pp
1.309 jmc 1347: .It Pa ~/.shosts
1.240 jmc 1348: This file is used in exactly the same way as
1349: .Pa .rhosts ,
1350: but allows host-based authentication without permitting login with
1351: rlogin/rsh.
1.272 mcbride 1352: .Pp
1.309 jmc 1353: .It Pa ~/.ssh/
1.272 mcbride 1354: This directory is the default location for all user-specific configuration
1355: and authentication information.
1356: There is no general requirement to keep the entire contents of this directory
1357: secret, but the recommended permissions are read/write/execute for the user,
1358: and not accessible by others.
1.236 jmc 1359: .Pp
1.309 jmc 1360: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1361: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1362: that can be used for logging in as this user.
1.238 jmc 1363: The format of this file is described in the
1364: .Xr sshd 8
1365: manual page.
1366: This file is not highly sensitive, but the recommended
1367: permissions are read/write for the user, and not accessible by others.
1368: .Pp
1.309 jmc 1369: .It Pa ~/.ssh/config
1.238 jmc 1370: This is the per-user configuration file.
1371: The file format and configuration options are described in
1372: .Xr ssh_config 5 .
1373: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1374: read/write for the user, and not writable by others.
1.238 jmc 1375: .Pp
1.309 jmc 1376: .It Pa ~/.ssh/environment
1.239 jmc 1377: Contains additional definitions for environment variables; see
1378: .Sx ENVIRONMENT ,
1.238 jmc 1379: above.
1380: .Pp
1.309 jmc 1381: .It Pa ~/.ssh/identity
1382: .It Pa ~/.ssh/id_dsa
1.310 djm 1383: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1384: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1385: .It Pa ~/.ssh/id_rsa
1.238 jmc 1386: Contains the private key for authentication.
1387: These files
1388: contain sensitive data and should be readable by the user but not
1389: accessible by others (read/write/execute).
1390: .Nm
1391: will simply ignore a private key file if it is accessible by others.
1392: It is possible to specify a passphrase when
1393: generating the key which will be used to encrypt the
1394: sensitive part of this file using 3DES.
1395: .Pp
1.309 jmc 1396: .It Pa ~/.ssh/identity.pub
1397: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1398: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1399: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1400: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1401: Contains the public key for authentication.
1402: These files are not
1403: sensitive and can (but need not) be readable by anyone.
1404: .Pp
1.309 jmc 1405: .It Pa ~/.ssh/known_hosts
1.244 jmc 1406: Contains a list of host keys for all hosts the user has logged into
1407: that are not already in the systemwide list of known host keys.
1.238 jmc 1408: See
1.244 jmc 1409: .Xr sshd 8
1410: for further details of the format of this file.
1.238 jmc 1411: .Pp
1.309 jmc 1412: .It Pa ~/.ssh/rc
1.238 jmc 1413: Commands in this file are executed by
1414: .Nm
1.245 jmc 1415: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1416: started.
1417: See the
1418: .Xr sshd 8
1419: manual page for more information.
1420: .Pp
1.309 jmc 1421: .It Pa /etc/hosts.equiv
1.240 jmc 1422: This file is for host-based authentication (see above).
1423: It should only be writable by root.
1.236 jmc 1424: .Pp
1.309 jmc 1425: .It Pa /etc/shosts.equiv
1.240 jmc 1426: This file is used in exactly the same way as
1427: .Pa hosts.equiv ,
1428: but allows host-based authentication without permitting login with
1429: rlogin/rsh.
1.236 jmc 1430: .Pp
1.238 jmc 1431: .It Pa /etc/ssh/ssh_config
1432: Systemwide configuration file.
1433: The file format and configuration options are described in
1434: .Xr ssh_config 5 .
1435: .Pp
1.309 jmc 1436: .It Pa /etc/ssh/ssh_host_key
1437: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1438: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1439: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1440: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1441: These files contain the private parts of the host keys
1.245 jmc 1442: and are used for host-based authentication.
1443: If protocol version 1 is used,
1.238 jmc 1444: .Nm
1445: must be setuid root, since the host key is readable only by root.
1446: For protocol version 2,
1447: .Nm
1448: uses
1449: .Xr ssh-keysign 8
1.245 jmc 1450: to access the host keys,
1451: eliminating the requirement that
1.238 jmc 1452: .Nm
1.245 jmc 1453: be setuid root when host-based authentication is used.
1.238 jmc 1454: By default
1.2 deraadt 1455: .Nm
1.238 jmc 1456: is not setuid root.
1457: .Pp
1.309 jmc 1458: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1459: Systemwide list of known host keys.
1460: This file should be prepared by the
1461: system administrator to contain the public host keys of all machines in the
1462: organization.
1.244 jmc 1463: It should be world-readable.
1464: See
1.238 jmc 1465: .Xr sshd 8
1.244 jmc 1466: for further details of the format of this file.
1.236 jmc 1467: .Pp
1.309 jmc 1468: .It Pa /etc/ssh/sshrc
1.1 deraadt 1469: Commands in this file are executed by
1.2 deraadt 1470: .Nm
1.245 jmc 1471: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1472: See the
1.2 deraadt 1473: .Xr sshd 8
1.1 deraadt 1474: manual page for more information.
1.58 itojun 1475: .El
1.312 jmc 1476: .Sh EXIT STATUS
1477: .Nm
1478: exits with the exit status of the remote command or with 255
1479: if an error occurred.
1.2 deraadt 1480: .Sh SEE ALSO
1481: .Xr scp 1 ,
1.83 djm 1482: .Xr sftp 1 ,
1.2 deraadt 1483: .Xr ssh-add 1 ,
1484: .Xr ssh-agent 1 ,
1485: .Xr ssh-keygen 1 ,
1.242 jmc 1486: .Xr ssh-keyscan 1 ,
1.250 jmc 1487: .Xr tun 4 ,
1.159 stevesk 1488: .Xr ssh_config 5 ,
1.160 naddy 1489: .Xr ssh-keysign 8 ,
1.87 itojun 1490: .Xr sshd 8
1.329 jmc 1491: .Sh STANDARDS
1.106 markus 1492: .Rs
1.329 jmc 1493: .%A S. Lehtinen
1494: .%A C. Lonvick
1495: .%D January 2006
1.256 jmc 1496: .%R RFC 4250
1.329 jmc 1497: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1498: .Re
1.329 jmc 1499: .Pp
1.256 jmc 1500: .Rs
1.329 jmc 1501: .%A T. Ylonen
1502: .%A C. Lonvick
1503: .%D January 2006
1.256 jmc 1504: .%R RFC 4251
1.329 jmc 1505: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1506: .Re
1.329 jmc 1507: .Pp
1.256 jmc 1508: .Rs
1.329 jmc 1509: .%A T. Ylonen
1510: .%A C. Lonvick
1511: .%D January 2006
1.256 jmc 1512: .%R RFC 4252
1.329 jmc 1513: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1514: .Re
1.329 jmc 1515: .Pp
1.256 jmc 1516: .Rs
1.329 jmc 1517: .%A T. Ylonen
1518: .%A C. Lonvick
1519: .%D January 2006
1.256 jmc 1520: .%R RFC 4253
1.329 jmc 1521: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1522: .Re
1.329 jmc 1523: .Pp
1.256 jmc 1524: .Rs
1.329 jmc 1525: .%A T. Ylonen
1526: .%A C. Lonvick
1527: .%D January 2006
1.256 jmc 1528: .%R RFC 4254
1.329 jmc 1529: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1530: .Re
1.329 jmc 1531: .Pp
1.256 jmc 1532: .Rs
1.329 jmc 1533: .%A J. Schlyter
1534: .%A W. Griffin
1535: .%D January 2006
1.256 jmc 1536: .%R RFC 4255
1.329 jmc 1537: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1538: .Re
1.329 jmc 1539: .Pp
1.256 jmc 1540: .Rs
1.329 jmc 1541: .%A F. Cusack
1542: .%A M. Forssen
1543: .%D January 2006
1.256 jmc 1544: .%R RFC 4256
1.329 jmc 1545: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1546: .Re
1.329 jmc 1547: .Pp
1.256 jmc 1548: .Rs
1.329 jmc 1549: .%A J. Galbraith
1550: .%A P. Remaker
1551: .%D January 2006
1.256 jmc 1552: .%R RFC 4335
1.329 jmc 1553: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1554: .Re
1.329 jmc 1555: .Pp
1.256 jmc 1556: .Rs
1.329 jmc 1557: .%A M. Bellare
1558: .%A T. Kohno
1559: .%A C. Namprempre
1560: .%D January 2006
1.256 jmc 1561: .%R RFC 4344
1.329 jmc 1562: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1563: .Re
1.329 jmc 1564: .Pp
1.256 jmc 1565: .Rs
1.329 jmc 1566: .%A B. Harris
1567: .%D January 2006
1.256 jmc 1568: .%R RFC 4345
1.329 jmc 1569: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1570: .Re
1.329 jmc 1571: .Pp
1.258 djm 1572: .Rs
1.329 jmc 1573: .%A M. Friedl
1574: .%A N. Provos
1575: .%A W. Simpson
1576: .%D March 2006
1.258 djm 1577: .%R RFC 4419
1.329 jmc 1578: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1579: .Re
1.329 jmc 1580: .Pp
1.266 markus 1581: .Rs
1.329 jmc 1582: .%A J. Galbraith
1583: .%A R. Thayer
1584: .%D November 2006
1.266 markus 1585: .%R RFC 4716
1.329 jmc 1586: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1587: .Re
1.329 jmc 1588: .Pp
1.313 djm 1589: .Rs
1.329 jmc 1590: .%A D. Stebila
1591: .%A J. Green
1592: .%D December 2009
1.313 djm 1593: .%R RFC 5656
1.329 jmc 1594: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1595: .Re
1.329 jmc 1596: .Pp
1.274 grunk 1597: .Rs
1598: .%A A. Perrig
1599: .%A D. Song
1600: .%D 1999
1.329 jmc 1601: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1602: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1603: .Re
1.173 jmc 1604: .Sh AUTHORS
1605: OpenSSH is a derivative of the original and free
1606: ssh 1.2.12 release by Tatu Ylonen.
1607: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1608: Theo de Raadt and Dug Song
1609: removed many bugs, re-added newer features and
1610: created OpenSSH.
1611: Markus Friedl contributed the support for SSH
1612: protocol versions 1.5 and 2.0.