Annotation of src/usr.bin/ssh/ssh.1, Revision 1.369
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: .\"
1.369 ! jmc 36: .\" $OpenBSD: ssh.1,v 1.368 2016/02/16 07:47:54 jmc Exp $
1.368 jmc 37: .Dd $Mdocdate: February 16 2016 $
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
1.360 millert 55: .Op Fl L Ar address
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
1.362 djm 61: .Op Fl Q Ar query_option
1.360 millert 62: .Op Fl R Ar address
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.
1.365 mmcc 73: It is intended to provide secure encrypted communications between
1.40 aaron 74: two untrusted hosts over an insecure network.
1.348 millert 75: X11 connections, arbitrary TCP ports and
76: .Ux Ns -domain
77: sockets can also be forwarded over the secure channel.
1.2 deraadt 78: .Pp
79: .Nm
1.44 aaron 80: connects and logs into the specified
1.176 jmc 81: .Ar hostname
82: (with optional
83: .Ar user
84: name).
1.1 deraadt 85: The user must prove
1.49 markus 86: his/her identity to the remote machine using one of several methods
1.365 mmcc 87: (see below).
1.49 markus 88: .Pp
1.176 jmc 89: If
90: .Ar command
91: is specified,
1.219 jmc 92: it is executed on the remote host instead of a login shell.
1.2 deraadt 93: .Pp
1.218 jmc 94: The options are as follows:
1.361 millert 95: .Pp
1.360 millert 96: .Bl -tag -width Ds -compact
1.218 jmc 97: .It Fl 1
98: Forces
1.2 deraadt 99: .Nm
1.218 jmc 100: to try protocol version 1 only.
1.360 millert 101: .Pp
1.218 jmc 102: .It Fl 2
103: Forces
1.2 deraadt 104: .Nm
1.218 jmc 105: to try protocol version 2 only.
1.360 millert 106: .Pp
1.218 jmc 107: .It Fl 4
108: Forces
1.2 deraadt 109: .Nm
1.218 jmc 110: to use IPv4 addresses only.
1.360 millert 111: .Pp
1.218 jmc 112: .It Fl 6
113: Forces
1.2 deraadt 114: .Nm
1.218 jmc 115: to use IPv6 addresses only.
1.360 millert 116: .Pp
1.218 jmc 117: .It Fl A
118: Enables forwarding of the authentication agent connection.
119: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 120: .Pp
1.218 jmc 121: Agent forwarding should be enabled with caution.
122: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 123: (for the agent's
1.286 sobrado 124: .Ux Ns -domain
125: socket) can access the local agent through the forwarded connection.
1.218 jmc 126: An attacker cannot obtain key material from the agent,
127: however they can perform operations on the keys that enable them to
128: authenticate using the identities loaded into the agent.
1.360 millert 129: .Pp
1.218 jmc 130: .It Fl a
131: Disables forwarding of the authentication agent connection.
1.360 millert 132: .Pp
1.218 jmc 133: .It Fl b Ar bind_address
134: Use
135: .Ar bind_address
136: on the local machine as the source address
137: of the connection.
138: Only useful on systems with more than one address.
1.360 millert 139: .Pp
1.218 jmc 140: .It Fl C
141: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 142: data for forwarded X11, TCP and
143: .Ux Ns -domain
144: connections).
1.218 jmc 145: The compression algorithm is the same used by
146: .Xr gzip 1 ,
147: and the
148: .Dq level
149: can be controlled by the
150: .Cm CompressionLevel
151: option for protocol version 1.
152: Compression is desirable on modem lines and other
153: slow connections, but will only slow down things on fast networks.
154: The default value can be set on a host-by-host basis in the
155: configuration files; see the
156: .Cm Compression
157: option.
1.360 millert 158: .Pp
1.218 jmc 159: .It Fl c Ar cipher_spec
160: Selects the cipher specification for encrypting the session.
1.2 deraadt 161: .Pp
1.218 jmc 162: Protocol version 1 allows specification of a single cipher.
163: The supported values are
164: .Dq 3des ,
1.220 jmc 165: .Dq blowfish ,
1.218 jmc 166: and
167: .Dq des .
1.230 jmc 168: For protocol version 2,
1.218 jmc 169: .Ar cipher_spec
170: is a comma-separated list of ciphers
171: listed in order of preference.
1.283 jmc 172: See the
173: .Cm Ciphers
1.307 dtucker 174: keyword in
175: .Xr ssh_config 5
176: for more information.
1.360 millert 177: .Pp
1.218 jmc 178: .It Fl D Xo
179: .Sm off
180: .Oo Ar bind_address : Oc
181: .Ar port
182: .Sm on
183: .Xc
184: Specifies a local
185: .Dq dynamic
186: application-level port forwarding.
187: This works by allocating a socket to listen to
188: .Ar port
189: on the local side, optionally bound to the specified
190: .Ar bind_address .
191: Whenever a connection is made to this port, the
192: connection is forwarded over the secure channel, and the application
193: protocol is then used to determine where to connect to from the
194: remote machine.
195: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 196: .Nm
1.218 jmc 197: will act as a SOCKS server.
198: Only root can forward privileged ports.
199: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 200: .Pp
1.308 djm 201: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 202: Only the superuser can forward privileged ports.
203: By default, the local port is bound in accordance with the
204: .Cm GatewayPorts
205: setting.
206: However, an explicit
207: .Ar bind_address
208: may be used to bind the connection to a specific address.
209: The
210: .Ar bind_address
211: of
212: .Dq localhost
213: indicates that the listening port be bound for local use only, while an
214: empty address or
215: .Sq *
216: indicates that the port should be available from all interfaces.
1.360 millert 217: .Pp
1.331 dtucker 218: .It Fl E Ar log_file
219: Append debug logs to
220: .Ar log_file
221: instead of standard error.
1.360 millert 222: .Pp
1.229 jmc 223: .It Fl e Ar escape_char
1.218 jmc 224: Sets the escape character for sessions with a pty (default:
225: .Ql ~ ) .
226: The escape character is only recognized at the beginning of a line.
227: The escape character followed by a dot
228: .Pq Ql \&.
229: closes the connection;
230: followed by control-Z suspends the connection;
231: and followed by itself sends the escape character once.
232: Setting the character to
1.2 deraadt 233: .Dq none
1.218 jmc 234: disables any escapes and makes the session fully transparent.
1.360 millert 235: .Pp
1.218 jmc 236: .It Fl F Ar configfile
237: Specifies an alternative per-user configuration file.
238: If a configuration file is given on the command line,
239: the system-wide configuration file
240: .Pq Pa /etc/ssh/ssh_config
241: will be ignored.
242: The default for the per-user configuration file is
243: .Pa ~/.ssh/config .
1.360 millert 244: .Pp
1.218 jmc 245: .It Fl f
246: Requests
247: .Nm
248: to go to background just before command execution.
249: This is useful if
1.176 jmc 250: .Nm
1.218 jmc 251: is going to ask for passwords or passphrases, but the user
252: wants it in the background.
253: This implies
254: .Fl n .
255: The recommended way to start X11 programs at a remote site is with
256: something like
257: .Ic ssh -f host xterm .
1.277 djm 258: .Pp
259: If the
260: .Cm ExitOnForwardFailure
261: configuration option is set to
262: .Dq yes ,
263: then a client started with
264: .Fl f
265: will wait for all remote port forwards to be successfully established
266: before placing itself in the background.
1.360 millert 267: .Pp
1.350 djm 268: .It Fl G
269: Causes
270: .Nm
271: to print its configuration after evaluating
272: .Cm Host
273: and
274: .Cm Match
275: blocks and exit.
1.360 millert 276: .Pp
1.218 jmc 277: .It Fl g
278: Allows remote hosts to connect to local forwarded ports.
1.346 djm 279: If used on a multiplexed connection, then this option must be specified
280: on the master process.
1.360 millert 281: .Pp
1.291 markus 282: .It Fl I Ar pkcs11
1.294 jmc 283: Specify the PKCS#11 shared library
1.176 jmc 284: .Nm
1.293 markus 285: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 286: private RSA key.
1.360 millert 287: .Pp
1.218 jmc 288: .It Fl i Ar identity_file
289: Selects a file from which the identity (private key) for
1.310 djm 290: public key authentication is read.
1.218 jmc 291: The default is
292: .Pa ~/.ssh/identity
293: for protocol version 1, and
1.310 djm 294: .Pa ~/.ssh/id_dsa ,
1.343 naddy 295: .Pa ~/.ssh/id_ecdsa ,
296: .Pa ~/.ssh/id_ed25519
1.310 djm 297: and
1.218 jmc 298: .Pa ~/.ssh/id_rsa
299: for protocol version 2.
300: Identity files may also be specified on
301: a per-host basis in the configuration file.
302: It is possible to have multiple
303: .Fl i
304: options (and multiple identities specified in
305: configuration files).
1.364 jmc 306: If no certificates have been explicitly specified by the
1.363 djm 307: .Cm CertificateFile
308: directive,
1.302 djm 309: .Nm
310: will also try to load certificate information from the filename obtained
311: by appending
312: .Pa -cert.pub
313: to identity filenames.
1.360 millert 314: .Pp
1.269 djm 315: .It Fl K
316: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
317: credentials to the server.
1.360 millert 318: .Pp
1.218 jmc 319: .It Fl k
320: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 321: .Pp
1.218 jmc 322: .It Fl L Xo
323: .Sm off
324: .Oo Ar bind_address : Oc
325: .Ar port : host : hostport
326: .Sm on
327: .Xc
1.360 millert 328: .It Fl L Xo
329: .Sm off
330: .Oo Ar bind_address : Oc
331: .Ar port : remote_socket
332: .Sm on
333: .Xc
334: .It Fl L Xo
335: .Sm off
336: .Ar local_socket : host : hostport
337: .Sm on
338: .Xc
339: .It Fl L Xo
340: .Sm off
341: .Ar local_socket : remote_socket
342: .Sm on
343: .Xc
344: Specifies that connections to the given TCP port or Unix socket on the local
345: (client) host are to be forwarded to the given host and port, or Unix socket,
346: on the remote side.
347: This works by allocating a socket to listen to either a TCP
1.218 jmc 348: .Ar port
349: on the local side, optionally bound to the specified
1.360 millert 350: .Ar bind_address ,
351: or to a Unix socket.
352: Whenever a connection is made to the local port or socket, the
1.218 jmc 353: connection is forwarded over the secure channel, and a connection is
1.360 millert 354: made to either
1.218 jmc 355: .Ar host
356: port
1.360 millert 357: .Ar hostport ,
358: or the Unix socket
359: .Ar remote_socket ,
1.218 jmc 360: from the remote machine.
1.360 millert 361: .Pp
1.218 jmc 362: Port forwardings can also be specified in the configuration file.
1.360 millert 363: Only the superuser can forward privileged ports.
1.308 djm 364: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 365: .Pp
1.218 jmc 366: By default, the local port is bound in accordance with the
367: .Cm GatewayPorts
368: setting.
369: However, an explicit
370: .Ar bind_address
371: may be used to bind the connection to a specific address.
1.2 deraadt 372: The
1.218 jmc 373: .Ar bind_address
374: of
375: .Dq localhost
376: indicates that the listening port be bound for local use only, while an
377: empty address or
378: .Sq *
379: indicates that the port should be available from all interfaces.
1.360 millert 380: .Pp
1.218 jmc 381: .It Fl l Ar login_name
382: Specifies the user to log in as on the remote machine.
383: This also may be specified on a per-host basis in the configuration file.
1.360 millert 384: .Pp
1.218 jmc 385: .It Fl M
386: Places the
387: .Nm
388: client into
389: .Dq master
390: mode for connection sharing.
1.231 stevesk 391: Multiple
392: .Fl M
393: options places
394: .Nm
395: into
396: .Dq master
397: mode with confirmation required before slave connections are accepted.
1.218 jmc 398: Refer to the description of
399: .Cm ControlMaster
400: in
401: .Xr ssh_config 5
402: for details.
1.360 millert 403: .Pp
1.218 jmc 404: .It Fl m Ar mac_spec
1.369 ! jmc 405: A comma-separated list of MAC (message authentication code) algorithms,
! 406: specified in order of preference.
1.218 jmc 407: See the
408: .Cm MACs
409: keyword for more information.
1.360 millert 410: .Pp
1.218 jmc 411: .It Fl N
412: Do not execute a remote command.
1.369 ! jmc 413: This is useful for just forwarding ports.
1.360 millert 414: .Pp
1.218 jmc 415: .It Fl n
416: Redirects stdin from
417: .Pa /dev/null
418: (actually, prevents reading from stdin).
419: This must be used when
1.2 deraadt 420: .Nm
1.218 jmc 421: is run in the background.
422: A common trick is to use this to run X11 programs on a remote machine.
423: For example,
424: .Ic ssh -n shadows.cs.hut.fi emacs &
425: will start an emacs on shadows.cs.hut.fi, and the X11
426: connection will be automatically forwarded over an encrypted channel.
427: The
1.2 deraadt 428: .Nm
1.218 jmc 429: program will be put in the background.
430: (This does not work if
1.2 deraadt 431: .Nm
1.218 jmc 432: needs to ask for a password or passphrase; see also the
433: .Fl f
434: option.)
1.360 millert 435: .Pp
1.218 jmc 436: .It Fl O Ar ctl_cmd
437: Control an active connection multiplexing master process.
438: When the
439: .Fl O
440: option is specified, the
441: .Ar ctl_cmd
442: argument is interpreted and passed to the master process.
443: Valid commands are:
444: .Dq check
1.305 markus 445: (check that the master process is running),
446: .Dq forward
1.318 jmc 447: (request forwardings without command execution),
1.323 okan 448: .Dq cancel
449: (cancel forwardings),
1.218 jmc 450: .Dq exit
1.318 jmc 451: (request the master to exit), and
1.317 djm 452: .Dq stop
453: (request the master to stop accepting further multiplexing requests).
1.360 millert 454: .Pp
1.218 jmc 455: .It Fl o Ar option
456: Can be used to give options in the format used in the configuration file.
457: This is useful for specifying options for which there is no separate
458: command-line flag.
459: For full details of the options listed below, and their possible values, see
460: .Xr ssh_config 5 .
1.2 deraadt 461: .Pp
1.218 jmc 462: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 463: .It AddKeysToAgent
1.218 jmc 464: .It AddressFamily
465: .It BatchMode
466: .It BindAddress
1.338 djm 467: .It CanonicalDomains
1.339 djm 468: .It CanonicalizeFallbackLocal
469: .It CanonicalizeHostname
470: .It CanonicalizeMaxDots
471: .It CanonicalizePermittedCNAMEs
1.363 djm 472: .It CertificateFile
1.218 jmc 473: .It ChallengeResponseAuthentication
474: .It CheckHostIP
475: .It Cipher
476: .It Ciphers
477: .It ClearAllForwardings
478: .It Compression
479: .It CompressionLevel
480: .It ConnectionAttempts
481: .It ConnectTimeout
482: .It ControlMaster
483: .It ControlPath
1.321 djm 484: .It ControlPersist
1.218 jmc 485: .It DynamicForward
486: .It EscapeChar
1.263 markus 487: .It ExitOnForwardFailure
1.353 jmc 488: .It FingerprintHash
1.218 jmc 489: .It ForwardAgent
490: .It ForwardX11
1.321 djm 491: .It ForwardX11Timeout
1.218 jmc 492: .It ForwardX11Trusted
493: .It GatewayPorts
494: .It GlobalKnownHostsFile
495: .It GSSAPIAuthentication
496: .It GSSAPIDelegateCredentials
497: .It HashKnownHosts
498: .It Host
499: .It HostbasedAuthentication
1.355 djm 500: .It HostbasedKeyTypes
1.218 jmc 501: .It HostKeyAlgorithms
502: .It HostKeyAlias
503: .It HostName
504: .It IdentityFile
505: .It IdentitiesOnly
1.316 jmc 506: .It IPQoS
1.321 djm 507: .It KbdInteractiveAuthentication
1.218 jmc 508: .It KbdInteractiveDevices
1.314 jmc 509: .It KexAlgorithms
1.218 jmc 510: .It LocalCommand
511: .It LocalForward
512: .It LogLevel
513: .It MACs
1.337 jmc 514: .It Match
1.218 jmc 515: .It NoHostAuthenticationForLocalhost
516: .It NumberOfPasswordPrompts
517: .It PasswordAuthentication
518: .It PermitLocalCommand
1.291 markus 519: .It PKCS11Provider
1.218 jmc 520: .It Port
521: .It PreferredAuthentications
522: .It Protocol
523: .It ProxyCommand
1.336 jmc 524: .It ProxyUseFdpass
1.359 markus 525: .It PubkeyAcceptedKeyTypes
1.218 jmc 526: .It PubkeyAuthentication
1.251 dtucker 527: .It RekeyLimit
1.218 jmc 528: .It RemoteForward
1.319 jmc 529: .It RequestTTY
1.218 jmc 530: .It RhostsRSAAuthentication
531: .It RSAAuthentication
532: .It SendEnv
533: .It ServerAliveInterval
534: .It ServerAliveCountMax
1.347 jmc 535: .It StreamLocalBindMask
536: .It StreamLocalBindUnlink
1.218 jmc 537: .It StrictHostKeyChecking
538: .It TCPKeepAlive
539: .It Tunnel
540: .It TunnelDevice
1.354 djm 541: .It UpdateHostKeys
1.218 jmc 542: .It UsePrivilegedPort
543: .It User
544: .It UserKnownHostsFile
545: .It VerifyHostKeyDNS
1.276 jmc 546: .It VisualHostKey
1.218 jmc 547: .It XAuthLocation
548: .El
1.360 millert 549: .Pp
1.218 jmc 550: .It Fl p Ar port
551: Port to connect to on the remote host.
552: This can be specified on a
553: per-host basis in the configuration file.
1.360 millert 554: .Pp
1.362 djm 555: .It Fl Q Ar query_option
1.332 djm 556: Queries
557: .Nm
1.341 deraadt 558: for the algorithms supported for the specified version 2.
559: The available features are:
560: .Ar cipher
1.332 djm 561: (supported symmetric ciphers),
1.341 deraadt 562: .Ar cipher-auth
1.340 djm 563: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 564: .Ar mac
1.332 djm 565: (supported message integrity codes),
1.341 deraadt 566: .Ar kex
1.332 djm 567: (key exchange algorithms),
1.341 deraadt 568: .Ar key
1.362 djm 569: (key types),
570: .Ar key-cert
571: (certificate key types),
572: .Ar key-plain
573: (non-certificate key types), and
1.356 djm 574: .Ar protocol-version
575: (supported SSH protocol versions).
1.360 millert 576: .Pp
1.218 jmc 577: .It Fl q
578: Quiet mode.
1.271 djm 579: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 580: .Pp
1.218 jmc 581: .It Fl R Xo
582: .Sm off
583: .Oo Ar bind_address : Oc
584: .Ar port : host : hostport
585: .Sm on
586: .Xc
1.360 millert 587: .It Fl R Xo
588: .Sm off
589: .Oo Ar bind_address : Oc
590: .Ar port : local_socket
591: .Sm on
592: .Xc
593: .It Fl R Xo
594: .Sm off
595: .Ar remote_socket : host : hostport
596: .Sm on
597: .Xc
598: .It Fl R Xo
599: .Sm off
600: .Ar remote_socket : local_socket
601: .Sm on
602: .Xc
603: Specifies that connections to the given TCP port or Unix socket on the remote
604: (server) host are to be forwarded to the given host and port, or Unix socket,
605: on the local side.
606: This works by allocating a socket to listen to either a TCP
1.218 jmc 607: .Ar port
1.360 millert 608: or to a Unix socket on the remote side.
609: Whenever a connection is made to this port or Unix socket, the
610: connection is forwarded over the secure channel, and a connection
611: is made to either
1.218 jmc 612: .Ar host
613: port
1.360 millert 614: .Ar hostport ,
615: or
616: .Ar local_socket ,
1.218 jmc 617: from the local machine.
1.2 deraadt 618: .Pp
1.218 jmc 619: Port forwardings can also be specified in the configuration file.
620: Privileged ports can be forwarded only when
621: logging in as root on the remote machine.
1.324 jmc 622: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 623: .Pp
1.360 millert 624: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 625: interface only.
1.280 tobias 626: This may be overridden by specifying a
1.218 jmc 627: .Ar bind_address .
628: An empty
629: .Ar bind_address ,
630: or the address
631: .Ql * ,
632: indicates that the remote socket should listen on all interfaces.
633: Specifying a remote
634: .Ar bind_address
635: will only succeed if the server's
636: .Cm GatewayPorts
637: option is enabled (see
638: .Xr sshd_config 5 ) .
1.281 djm 639: .Pp
640: If the
641: .Ar port
642: argument is
1.282 djm 643: .Ql 0 ,
1.281 djm 644: the listen port will be dynamically allocated on the server and reported
645: to the client at run time.
1.305 markus 646: When used together with
647: .Ic -O forward
648: the allocated port will be printed to the standard output.
1.360 millert 649: .Pp
1.218 jmc 650: .It Fl S Ar ctl_path
1.304 jmc 651: Specifies the location of a control socket for connection sharing,
1.303 djm 652: or the string
653: .Dq none
654: to disable connection sharing.
1.218 jmc 655: Refer to the description of
656: .Cm ControlPath
657: and
658: .Cm ControlMaster
659: in
660: .Xr ssh_config 5
661: for details.
1.360 millert 662: .Pp
1.218 jmc 663: .It Fl s
664: May be used to request invocation of a subsystem on the remote system.
1.369 ! jmc 665: Subsystems facilitate the use of SSH
! 666: as a secure transport for other applications (e.g.\&
1.218 jmc 667: .Xr sftp 1 ) .
668: The subsystem is specified as the remote command.
1.360 millert 669: .Pp
1.218 jmc 670: .It Fl T
1.357 dtucker 671: Disable pseudo-terminal allocation.
1.360 millert 672: .Pp
1.218 jmc 673: .It Fl t
1.357 dtucker 674: Force pseudo-terminal allocation.
1.218 jmc 675: This can be used to execute arbitrary
676: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 677: e.g. when implementing menu services.
1.218 jmc 678: Multiple
679: .Fl t
680: options force tty allocation, even if
1.194 jakob 681: .Nm
1.218 jmc 682: has no local tty.
1.360 millert 683: .Pp
1.218 jmc 684: .It Fl V
685: Display the version number and exit.
1.360 millert 686: .Pp
1.218 jmc 687: .It Fl v
688: Verbose mode.
689: Causes
1.176 jmc 690: .Nm
1.218 jmc 691: to print debugging messages about its progress.
692: This is helpful in
693: debugging connection, authentication, and configuration problems.
694: Multiple
695: .Fl v
696: options increase the verbosity.
697: The maximum is 3.
1.360 millert 698: .Pp
1.290 dtucker 699: .It Fl W Ar host : Ns Ar port
700: Requests that standard input and output on the client be forwarded to
701: .Ar host
702: on
703: .Ar port
704: over the secure channel.
705: Implies
706: .Fl N ,
707: .Fl T ,
708: .Cm ExitOnForwardFailure
709: and
1.326 dtucker 710: .Cm ClearAllForwardings .
1.360 millert 711: .Pp
1.261 stevesk 712: .It Fl w Xo
713: .Ar local_tun Ns Op : Ns Ar remote_tun
714: .Xc
715: Requests
716: tunnel
717: device forwarding with the specified
1.218 jmc 718: .Xr tun 4
1.261 stevesk 719: devices between the client
720: .Pq Ar local_tun
721: and the server
722: .Pq Ar remote_tun .
723: .Pp
1.228 jmc 724: The devices may be specified by numerical ID or the keyword
725: .Dq any ,
726: which uses the next available tunnel device.
1.261 stevesk 727: If
728: .Ar remote_tun
729: is not specified, it defaults to
730: .Dq any .
1.228 jmc 731: See also the
1.218 jmc 732: .Cm Tunnel
1.261 stevesk 733: and
734: .Cm TunnelDevice
735: directives in
1.218 jmc 736: .Xr ssh_config 5 .
1.261 stevesk 737: If the
738: .Cm Tunnel
739: directive is unset, it is set to the default tunnel mode, which is
740: .Dq point-to-point .
1.360 millert 741: .Pp
1.218 jmc 742: .It Fl X
743: Enables X11 forwarding.
1.54 markus 744: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 745: .Pp
1.218 jmc 746: X11 forwarding should be enabled with caution.
1.168 jmc 747: Users with the ability to bypass file permissions on the remote host
1.218 jmc 748: (for the user's X authorization database)
749: can access the local X11 display through the forwarded connection.
750: An attacker may then be able to perform activities such as keystroke monitoring.
751: .Pp
752: For this reason, X11 forwarding is subjected to X11 SECURITY extension
753: restrictions by default.
754: Please refer to the
755: .Nm
756: .Fl Y
757: option and the
758: .Cm ForwardX11Trusted
759: directive in
760: .Xr ssh_config 5
761: for more information.
1.360 millert 762: .Pp
1.218 jmc 763: .It Fl x
764: Disables X11 forwarding.
1.360 millert 765: .Pp
1.218 jmc 766: .It Fl Y
767: Enables trusted X11 forwarding.
768: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
769: controls.
1.360 millert 770: .Pp
1.278 djm 771: .It Fl y
772: Send log information using the
773: .Xr syslog 3
774: system module.
775: By default this information is sent to stderr.
1.218 jmc 776: .El
1.224 jmc 777: .Pp
778: .Nm
779: may additionally obtain configuration data from
780: a per-user configuration file and a system-wide configuration file.
781: The file format and configuration options are described in
782: .Xr ssh_config 5 .
1.222 jmc 783: .Sh AUTHENTICATION
1.249 jmc 784: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 785: The default is to use protocol 2 only,
786: though this can be changed via the
1.222 jmc 787: .Cm Protocol
788: option in
1.284 jmc 789: .Xr ssh_config 5
790: or the
1.222 jmc 791: .Fl 1
792: and
793: .Fl 2
794: options (see above).
1.369 ! jmc 795: Protocol 1 should not be used
! 796: and is only offered to support legacy devices.
! 797: It suffers from a number of cryptographic weaknesses
! 798: and doesn't support many of the advanced features available for protocol 2.
1.222 jmc 799: .Pp
800: The methods available for authentication are:
1.260 jmc 801: GSSAPI-based authentication,
1.222 jmc 802: host-based authentication,
803: public key authentication,
804: challenge-response authentication,
805: and password authentication.
806: Authentication methods are tried in the order specified above,
1.369 ! jmc 807: though
! 808: .Cm PreferredAuthentications
! 809: can be used to change the default order.
1.222 jmc 810: .Pp
811: Host-based authentication works as follows:
1.218 jmc 812: If the machine the user logs in from is listed in
813: .Pa /etc/hosts.equiv
814: or
815: .Pa /etc/shosts.equiv
816: on the remote machine, and the user names are
817: the same on both sides, or if the files
818: .Pa ~/.rhosts
819: or
820: .Pa ~/.shosts
821: exist in the user's home directory on the
822: remote machine and contain a line containing the name of the client
823: machine and the name of the user on that machine, the user is
1.222 jmc 824: considered for login.
825: Additionally, the server
826: .Em must
827: be able to verify the client's
828: host key (see the description of
1.218 jmc 829: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 830: and
1.222 jmc 831: .Pa ~/.ssh/known_hosts ,
832: below)
833: for login to be permitted.
1.218 jmc 834: This authentication method closes security holes due to IP
1.222 jmc 835: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 836: [Note to the administrator:
837: .Pa /etc/hosts.equiv ,
838: .Pa ~/.rhosts ,
839: and the rlogin/rsh protocol in general, are inherently insecure and should be
840: disabled if security is desired.]
1.189 dtucker 841: .Pp
1.222 jmc 842: Public key authentication works as follows:
843: The scheme is based on public-key cryptography,
844: using cryptosystems
845: where encryption and decryption are done using separate keys,
846: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 847: The idea is that each user creates a public/private
848: key pair for authentication purposes.
849: The server knows the public key, and only the user knows the private key.
1.222 jmc 850: .Nm
851: implements public key authentication protocol automatically,
1.349 sobrado 852: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 853: The HISTORY section of
1.222 jmc 854: .Xr ssl 8
1.311 jmc 855: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 856: .Pp
1.218 jmc 857: The file
858: .Pa ~/.ssh/authorized_keys
859: lists the public keys that are permitted for logging in.
860: When the user logs in, the
1.2 deraadt 861: .Nm
1.218 jmc 862: program tells the server which key pair it would like to use for
863: authentication.
1.222 jmc 864: The client proves that it has access to the private key
865: and the server checks that the corresponding public key
866: is authorized to accept the account.
1.218 jmc 867: .Pp
1.222 jmc 868: The user creates his/her key pair by running
1.218 jmc 869: .Xr ssh-keygen 1 .
870: This stores the private key in
1.207 djm 871: .Pa ~/.ssh/identity
1.222 jmc 872: (protocol 1),
873: .Pa ~/.ssh/id_dsa
1.369 ! jmc 874: (DSA),
1.310 djm 875: .Pa ~/.ssh/id_ecdsa
1.369 ! jmc 876: (ECDSA),
1.343 naddy 877: .Pa ~/.ssh/id_ed25519
1.369 ! jmc 878: (Ed25519),
1.222 jmc 879: or
880: .Pa ~/.ssh/id_rsa
1.369 ! jmc 881: (RSA)
1.218 jmc 882: and stores the public key in
883: .Pa ~/.ssh/identity.pub
1.222 jmc 884: (protocol 1),
885: .Pa ~/.ssh/id_dsa.pub
1.369 ! jmc 886: (DSA),
1.310 djm 887: .Pa ~/.ssh/id_ecdsa.pub
1.369 ! jmc 888: (ECDSA),
1.343 naddy 889: .Pa ~/.ssh/id_ed25519.pub
1.369 ! jmc 890: (Ed25519),
1.222 jmc 891: or
892: .Pa ~/.ssh/id_rsa.pub
1.369 ! jmc 893: (RSA)
1.218 jmc 894: in the user's home directory.
1.222 jmc 895: The user should then copy the public key
1.218 jmc 896: to
897: .Pa ~/.ssh/authorized_keys
1.222 jmc 898: in his/her home directory on the remote machine.
899: The
1.218 jmc 900: .Pa authorized_keys
901: file corresponds to the conventional
902: .Pa ~/.rhosts
903: file, and has one key
1.222 jmc 904: per line, though the lines can be very long.
1.218 jmc 905: After this, the user can log in without giving the password.
906: .Pp
1.301 jmc 907: A variation on public key authentication
908: is available in the form of certificate authentication:
909: instead of a set of public/private keys,
910: signed certificates are used.
911: This has the advantage that a single trusted certification authority
912: can be used in place of many public/private keys.
1.333 jmc 913: See the CERTIFICATES section of
1.301 jmc 914: .Xr ssh-keygen 1
915: for more information.
916: .Pp
917: The most convenient way to use public key or certificate authentication
918: may be with an authentication agent.
1.218 jmc 919: See
920: .Xr ssh-agent 1
1.366 jcs 921: and (optionally) the
922: .Cm AddKeysToAgent
923: directive in
924: .Xr ssh_config 5
1.218 jmc 925: for more information.
926: .Pp
1.222 jmc 927: Challenge-response authentication works as follows:
928: The server sends an arbitrary
929: .Qq challenge
930: text, and prompts for a response.
931: Examples of challenge-response authentication include
1.335 jmc 932: .Bx
933: Authentication (see
1.222 jmc 934: .Xr login.conf 5 )
1.335 jmc 935: and PAM (some
936: .Pf non- Ox
937: systems).
1.222 jmc 938: .Pp
939: Finally, if other authentication methods fail,
1.218 jmc 940: .Nm
941: prompts the user for a password.
942: The password is sent to the remote
943: host for checking; however, since all communications are encrypted,
944: the password cannot be seen by someone listening on the network.
1.232 jmc 945: .Pp
946: .Nm
947: automatically maintains and checks a database containing
948: identification for all hosts it has ever been used with.
949: Host keys are stored in
950: .Pa ~/.ssh/known_hosts
951: in the user's home directory.
952: Additionally, the file
953: .Pa /etc/ssh/ssh_known_hosts
954: is automatically checked for known hosts.
955: Any new hosts are automatically added to the user's file.
956: If a host's identification ever changes,
957: .Nm
958: warns about this and disables password authentication to prevent
959: server spoofing or man-in-the-middle attacks,
960: which could otherwise be used to circumvent the encryption.
961: The
962: .Cm StrictHostKeyChecking
963: option can be used to control logins to machines whose
964: host key is not known or has changed.
965: .Pp
1.218 jmc 966: When the user's identity has been accepted by the server, the server
1.357 dtucker 967: either executes the given command in a non-interactive session or,
968: if no command has been specified, logs into the machine and gives
969: the user a normal shell as an interactive session.
1.218 jmc 970: All communication with
971: the remote command or shell will be automatically encrypted.
972: .Pp
1.357 dtucker 973: If an interactive session is requested
974: .Nm
975: by default will only request a pseudo-terminal (pty) for interactive
976: sessions when the client has one.
977: The flags
978: .Fl T
979: and
980: .Fl t
981: can be used to override this behaviour.
982: .Pp
983: If a pseudo-terminal has been allocated the
1.218 jmc 984: user may use the escape characters noted below.
985: .Pp
1.357 dtucker 986: If no pseudo-terminal has been allocated,
1.218 jmc 987: the session is transparent and can be used to reliably transfer binary data.
988: On most systems, setting the escape character to
989: .Dq none
990: will also make the session transparent even if a tty is used.
991: .Pp
992: The session terminates when the command or shell on the remote
1.247 jmc 993: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 994: .Sh ESCAPE CHARACTERS
1.218 jmc 995: When a pseudo-terminal has been requested,
1.2 deraadt 996: .Nm
1.218 jmc 997: supports a number of functions through the use of an escape character.
998: .Pp
999: A single tilde character can be sent as
1000: .Ic ~~
1001: or by following the tilde by a character other than those described below.
1002: The escape character must always follow a newline to be interpreted as
1003: special.
1004: The escape character can be changed in configuration files using the
1005: .Cm EscapeChar
1006: configuration directive or on the command line by the
1007: .Fl e
1008: option.
1009: .Pp
1010: The supported escapes (assuming the default
1011: .Ql ~ )
1012: are:
1013: .Bl -tag -width Ds
1014: .It Cm ~.
1015: Disconnect.
1016: .It Cm ~^Z
1017: Background
1.234 jmc 1018: .Nm .
1.218 jmc 1019: .It Cm ~#
1020: List forwarded connections.
1021: .It Cm ~&
1022: Background
1.2 deraadt 1023: .Nm
1.218 jmc 1024: at logout when waiting for forwarded connection / X11 sessions to terminate.
1025: .It Cm ~?
1026: Display a list of escape characters.
1027: .It Cm ~B
1028: Send a BREAK to the remote system
1.369 ! jmc 1029: (only useful if the peer supports it).
1.218 jmc 1030: .It Cm ~C
1031: Open command line.
1032: Currently this allows the addition of port forwardings using the
1.279 stevesk 1033: .Fl L ,
1034: .Fl R
1.218 jmc 1035: and
1.279 stevesk 1036: .Fl D
1.225 jmc 1037: options (see above).
1.322 markus 1038: It also allows the cancellation of existing port-forwardings
1039: with
1.262 stevesk 1040: .Sm off
1.322 markus 1041: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1042: .Sm on
1.322 markus 1043: for local,
1044: .Sm off
1045: .Fl KR Oo Ar bind_address : Oc Ar port
1046: .Sm on
1047: for remote and
1048: .Sm off
1049: .Fl KD Oo Ar bind_address : Oc Ar port
1050: .Sm on
1051: for dynamic port-forwardings.
1.218 jmc 1052: .Ic !\& Ns Ar command
1053: allows the user to execute a local command if the
1054: .Ic PermitLocalCommand
1055: option is enabled in
1.176 jmc 1056: .Xr ssh_config 5 .
1.218 jmc 1057: Basic help is available, using the
1058: .Fl h
1059: option.
1060: .It Cm ~R
1061: Request rekeying of the connection
1.369 ! jmc 1062: (only useful if the peer supports it).
1.327 dtucker 1063: .It Cm ~V
1064: Decrease the verbosity
1065: .Pq Ic LogLevel
1066: when errors are being written to stderr.
1067: .It Cm ~v
1.328 jmc 1068: Increase the verbosity
1.327 dtucker 1069: .Pq Ic LogLevel
1070: when errors are being written to stderr.
1.176 jmc 1071: .El
1.246 jmc 1072: .Sh TCP FORWARDING
1073: Forwarding of arbitrary TCP connections over the secure channel can
1074: be specified either on the command line or in a configuration file.
1075: One possible application of TCP forwarding is a secure connection to a
1076: mail server; another is going through firewalls.
1077: .Pp
1078: In the example below, we look at encrypting communication between
1079: an IRC client and server, even though the IRC server does not directly
1080: support encrypted communications.
1081: This works as follows:
1082: the user connects to the remote host using
1083: .Nm ,
1084: specifying a port to be used to forward connections
1085: to the remote server.
1086: After that it is possible to start the service which is to be encrypted
1087: on the client machine,
1088: connecting to the same local port,
1089: and
1090: .Nm
1091: will encrypt and forward the connection.
1092: .Pp
1093: The following example tunnels an IRC session from client machine
1094: .Dq 127.0.0.1
1095: (localhost)
1096: to remote server
1097: .Dq server.example.com :
1098: .Bd -literal -offset 4n
1099: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1100: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1101: .Ed
1102: .Pp
1103: This tunnels a connection to IRC server
1104: .Dq server.example.com ,
1105: joining channel
1106: .Dq #users ,
1107: nickname
1108: .Dq pinky ,
1109: using port 1234.
1110: It doesn't matter which port is used,
1111: as long as it's greater than 1023
1112: (remember, only root can open sockets on privileged ports)
1113: and doesn't conflict with any ports already in use.
1114: The connection is forwarded to port 6667 on the remote server,
1115: since that's the standard port for IRC services.
1116: .Pp
1117: The
1118: .Fl f
1119: option backgrounds
1120: .Nm
1121: and the remote command
1122: .Dq sleep 10
1123: is specified to allow an amount of time
1124: (10 seconds, in the example)
1125: to start the service which is to be tunnelled.
1126: If no connections are made within the time specified,
1127: .Nm
1128: will exit.
1129: .Sh X11 FORWARDING
1.218 jmc 1130: If the
1131: .Cm ForwardX11
1132: variable is set to
1133: .Dq yes
1134: (or see the description of the
1.227 jmc 1135: .Fl X ,
1136: .Fl x ,
1.218 jmc 1137: and
1.227 jmc 1138: .Fl Y
1.226 jmc 1139: options above)
1.218 jmc 1140: and the user is using X11 (the
1141: .Ev DISPLAY
1142: environment variable is set), the connection to the X11 display is
1143: automatically forwarded to the remote side in such a way that any X11
1144: programs started from the shell (or command) will go through the
1145: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1146: from the local machine.
1.218 jmc 1147: The user should not manually set
1148: .Ev DISPLAY .
1149: Forwarding of X11 connections can be
1150: configured on the command line or in configuration files.
1151: .Pp
1152: The
1153: .Ev DISPLAY
1154: value set by
1155: .Nm
1156: will point to the server machine, but with a display number greater than zero.
1157: This is normal, and happens because
1158: .Nm
1159: creates a
1160: .Dq proxy
1161: X server on the server machine for forwarding the
1162: connections over the encrypted channel.
1.200 djm 1163: .Pp
1.218 jmc 1164: .Nm
1165: will also automatically set up Xauthority data on the server machine.
1166: For this purpose, it will generate a random authorization cookie,
1167: store it in Xauthority on the server, and verify that any forwarded
1168: connections carry this cookie and replace it by the real cookie when
1169: the connection is opened.
1170: The real authentication cookie is never
1171: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1172: .Pp
1.218 jmc 1173: If the
1174: .Cm ForwardAgent
1175: variable is set to
1176: .Dq yes
1177: (or see the description of the
1178: .Fl A
1.191 djm 1179: and
1.218 jmc 1180: .Fl a
1.226 jmc 1181: options above) and
1.218 jmc 1182: the user is using an authentication agent, the connection to the agent
1183: is automatically forwarded to the remote side.
1.252 jmc 1184: .Sh VERIFYING HOST KEYS
1185: When connecting to a server for the first time,
1186: a fingerprint of the server's public key is presented to the user
1187: (unless the option
1188: .Cm StrictHostKeyChecking
1189: has been disabled).
1190: Fingerprints can be determined using
1191: .Xr ssh-keygen 1 :
1192: .Pp
1193: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1194: .Pp
1.274 grunk 1195: If the fingerprint is already known, it can be matched
1196: and the key can be accepted or rejected.
1.358 djm 1197: If only legacy (MD5) fingerprints for the server are available, the
1198: .Xr ssh-keygen 1
1199: .Fl E
1200: option may be used to downgrade the fingerprint algorithm to match.
1201: .Pp
1.274 grunk 1202: Because of the difficulty of comparing host keys
1.352 djm 1203: just by looking at fingerprint strings,
1.274 grunk 1204: there is also support to compare host keys visually,
1205: using
1206: .Em random art .
1207: By setting the
1.275 grunk 1208: .Cm VisualHostKey
1.274 grunk 1209: option to
1.275 grunk 1210: .Dq yes ,
1.274 grunk 1211: a small ASCII graphic gets displayed on every login to a server, no matter
1212: if the session itself is interactive or not.
1213: By learning the pattern a known server produces, a user can easily
1214: find out that the host key has changed when a completely different pattern
1215: is displayed.
1216: Because these patterns are not unambiguous however, a pattern that looks
1217: similar to the pattern remembered only gives a good probability that the
1218: host key is the same, not guaranteed proof.
1219: .Pp
1220: To get a listing of the fingerprints along with their random art for
1221: all known hosts, the following command line can be used:
1222: .Pp
1223: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1224: .Pp
1.252 jmc 1225: If the fingerprint is unknown,
1226: an alternative method of verification is available:
1227: SSH fingerprints verified by DNS.
1228: An additional resource record (RR),
1229: SSHFP,
1230: is added to a zonefile
1231: and the connecting client is able to match the fingerprint
1232: with that of the key presented.
1233: .Pp
1234: In this example, we are connecting a client to a server,
1235: .Dq host.example.com .
1236: The SSHFP resource records should first be added to the zonefile for
1237: host.example.com:
1238: .Bd -literal -offset indent
1.259 jakob 1239: $ ssh-keygen -r host.example.com.
1.252 jmc 1240: .Ed
1241: .Pp
1242: The output lines will have to be added to the zonefile.
1243: To check that the zone is answering fingerprint queries:
1244: .Pp
1245: .Dl $ dig -t SSHFP host.example.com
1246: .Pp
1247: Finally the client connects:
1248: .Bd -literal -offset indent
1249: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1250: [...]
1251: Matching host key fingerprint found in DNS.
1252: Are you sure you want to continue connecting (yes/no)?
1253: .Ed
1254: .Pp
1255: See the
1256: .Cm VerifyHostKeyDNS
1257: option in
1258: .Xr ssh_config 5
1259: for more information.
1.250 jmc 1260: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1261: .Nm
1262: contains support for Virtual Private Network (VPN) tunnelling
1263: using the
1264: .Xr tun 4
1265: network pseudo-device,
1266: allowing two networks to be joined securely.
1267: The
1268: .Xr sshd_config 5
1269: configuration option
1270: .Cm PermitTunnel
1271: controls whether the server supports this,
1272: and at what level (layer 2 or 3 traffic).
1273: .Pp
1274: The following example would connect client network 10.0.50.0/24
1.265 otto 1275: with remote network 10.0.99.0/24 using a point-to-point connection
1276: from 10.1.1.1 to 10.1.1.2,
1277: provided that the SSH server running on the gateway to the remote network,
1278: at 192.168.1.15, allows it.
1279: .Pp
1280: On the client:
1.250 jmc 1281: .Bd -literal -offset indent
1282: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1283: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1284: # route add 10.0.99.0/24 10.1.1.2
1285: .Ed
1286: .Pp
1287: On the server:
1288: .Bd -literal -offset indent
1289: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1290: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1291: .Ed
1292: .Pp
1293: Client access may be more finely tuned via the
1294: .Pa /root/.ssh/authorized_keys
1295: file (see below) and the
1296: .Cm PermitRootLogin
1297: server option.
1.255 jmc 1298: The following entry would permit connections on
1.250 jmc 1299: .Xr tun 4
1.255 jmc 1300: device 1 from user
1.250 jmc 1301: .Dq jane
1.255 jmc 1302: and on tun device 2 from user
1.250 jmc 1303: .Dq john ,
1304: if
1305: .Cm PermitRootLogin
1306: is set to
1307: .Dq forced-commands-only :
1308: .Bd -literal -offset 2n
1309: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1310: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1311: .Ed
1312: .Pp
1.264 ray 1313: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1314: it may be more suited to temporary setups,
1315: such as for wireless VPNs.
1316: More permanent VPNs are better provided by tools such as
1317: .Xr ipsecctl 8
1318: and
1319: .Xr isakmpd 8 .
1.2 deraadt 1320: .Sh ENVIRONMENT
1321: .Nm
1.1 deraadt 1322: will normally set the following environment variables:
1.237 jmc 1323: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1324: .It Ev DISPLAY
1325: The
1326: .Ev DISPLAY
1.40 aaron 1327: variable indicates the location of the X11 server.
1.44 aaron 1328: It is automatically set by
1.2 deraadt 1329: .Nm
1330: to point to a value of the form
1.233 jmc 1331: .Dq hostname:n ,
1332: where
1333: .Dq hostname
1334: indicates the host where the shell runs, and
1335: .Sq n
1336: is an integer \*(Ge 1.
1.40 aaron 1337: .Nm
1338: uses this special value to forward X11 connections over the secure
1339: channel.
1.107 markus 1340: The user should normally not set
1341: .Ev DISPLAY
1342: explicitly, as that
1.1 deraadt 1343: will render the X11 connection insecure (and will require the user to
1344: manually copy any required authorization cookies).
1.2 deraadt 1345: .It Ev HOME
1.1 deraadt 1346: Set to the path of the user's home directory.
1.2 deraadt 1347: .It Ev LOGNAME
1348: Synonym for
1.12 aaron 1349: .Ev USER ;
1350: set for compatibility with systems that use this variable.
1.2 deraadt 1351: .It Ev MAIL
1.129 stevesk 1352: Set to the path of the user's mailbox.
1.40 aaron 1353: .It Ev PATH
1.2 deraadt 1354: Set to the default
1355: .Ev PATH ,
1356: as specified when compiling
1.234 jmc 1357: .Nm .
1.118 markus 1358: .It Ev SSH_ASKPASS
1359: If
1360: .Nm
1361: needs a passphrase, it will read the passphrase from the current
1362: terminal if it was run from a terminal.
1363: If
1364: .Nm
1365: does not have a terminal associated with it but
1366: .Ev DISPLAY
1367: and
1368: .Ev SSH_ASKPASS
1369: are set, it will execute the program specified by
1370: .Ev SSH_ASKPASS
1371: and open an X11 window to read the passphrase.
1372: This is particularly useful when calling
1373: .Nm
1374: from a
1.196 jmc 1375: .Pa .xsession
1.118 markus 1376: or related script.
1377: (Note that on some machines it
1378: may be necessary to redirect the input from
1379: .Pa /dev/null
1380: to make this work.)
1.18 markus 1381: .It Ev SSH_AUTH_SOCK
1.233 jmc 1382: Identifies the path of a
1383: .Ux Ns -domain
1384: socket used to communicate with the agent.
1.166 stevesk 1385: .It Ev SSH_CONNECTION
1386: Identifies the client and server ends of the connection.
1.40 aaron 1387: The variable contains
1.233 jmc 1388: four space-separated values: client IP address, client port number,
1389: server IP address, and server port number.
1.73 markus 1390: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1391: This variable contains the original command line if a forced command
1.73 markus 1392: is executed.
1393: It can be used to extract the original arguments.
1.2 deraadt 1394: .It Ev SSH_TTY
1.1 deraadt 1395: This is set to the name of the tty (path to the device) associated
1.40 aaron 1396: with the current shell or command.
1397: If the current session has no tty,
1.1 deraadt 1398: this variable is not set.
1.2 deraadt 1399: .It Ev TZ
1.214 jmc 1400: This variable is set to indicate the present time zone if it
1.257 jmc 1401: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1402: on to new connections).
1.2 deraadt 1403: .It Ev USER
1.1 deraadt 1404: Set to the name of the user logging in.
1.2 deraadt 1405: .El
1406: .Pp
1.44 aaron 1407: Additionally,
1.2 deraadt 1408: .Nm
1.44 aaron 1409: reads
1.207 djm 1410: .Pa ~/.ssh/environment ,
1.2 deraadt 1411: and adds lines of the format
1412: .Dq VARNAME=value
1.233 jmc 1413: to the environment if the file exists and users are allowed to
1.161 marc 1414: change their environment.
1.176 jmc 1415: For more information, see the
1.161 marc 1416: .Cm PermitUserEnvironment
1.162 stevesk 1417: option in
1.161 marc 1418: .Xr sshd_config 5 .
1.2 deraadt 1419: .Sh FILES
1.236 jmc 1420: .Bl -tag -width Ds -compact
1.309 jmc 1421: .It Pa ~/.rhosts
1.240 jmc 1422: This file is used for host-based authentication (see above).
1.92 markus 1423: On some machines this file may need to be
1.240 jmc 1424: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1425: because
1.2 deraadt 1426: .Xr sshd 8
1.40 aaron 1427: reads it as root.
1428: Additionally, this file must be owned by the user,
1429: and must not have write permissions for anyone else.
1430: The recommended
1.1 deraadt 1431: permission for most machines is read/write for the user, and not
1432: accessible by others.
1.2 deraadt 1433: .Pp
1.309 jmc 1434: .It Pa ~/.shosts
1.240 jmc 1435: This file is used in exactly the same way as
1436: .Pa .rhosts ,
1437: but allows host-based authentication without permitting login with
1438: rlogin/rsh.
1.272 mcbride 1439: .Pp
1.309 jmc 1440: .It Pa ~/.ssh/
1.272 mcbride 1441: This directory is the default location for all user-specific configuration
1442: and authentication information.
1443: There is no general requirement to keep the entire contents of this directory
1444: secret, but the recommended permissions are read/write/execute for the user,
1445: and not accessible by others.
1.236 jmc 1446: .Pp
1.309 jmc 1447: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1448: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1449: that can be used for logging in as this user.
1.238 jmc 1450: The format of this file is described in the
1451: .Xr sshd 8
1452: manual page.
1453: This file is not highly sensitive, but the recommended
1454: permissions are read/write for the user, and not accessible by others.
1455: .Pp
1.309 jmc 1456: .It Pa ~/.ssh/config
1.238 jmc 1457: This is the per-user configuration file.
1458: The file format and configuration options are described in
1459: .Xr ssh_config 5 .
1460: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1461: read/write for the user, and not writable by others.
1.238 jmc 1462: .Pp
1.309 jmc 1463: .It Pa ~/.ssh/environment
1.239 jmc 1464: Contains additional definitions for environment variables; see
1465: .Sx ENVIRONMENT ,
1.238 jmc 1466: above.
1467: .Pp
1.309 jmc 1468: .It Pa ~/.ssh/identity
1469: .It Pa ~/.ssh/id_dsa
1.310 djm 1470: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1471: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1472: .It Pa ~/.ssh/id_rsa
1.238 jmc 1473: Contains the private key for authentication.
1474: These files
1475: contain sensitive data and should be readable by the user but not
1476: accessible by others (read/write/execute).
1477: .Nm
1478: will simply ignore a private key file if it is accessible by others.
1479: It is possible to specify a passphrase when
1480: generating the key which will be used to encrypt the
1481: sensitive part of this file using 3DES.
1482: .Pp
1.309 jmc 1483: .It Pa ~/.ssh/identity.pub
1484: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1485: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1486: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1487: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1488: Contains the public key for authentication.
1489: These files are not
1490: sensitive and can (but need not) be readable by anyone.
1491: .Pp
1.309 jmc 1492: .It Pa ~/.ssh/known_hosts
1.244 jmc 1493: Contains a list of host keys for all hosts the user has logged into
1494: that are not already in the systemwide list of known host keys.
1.238 jmc 1495: See
1.244 jmc 1496: .Xr sshd 8
1497: for further details of the format of this file.
1.238 jmc 1498: .Pp
1.309 jmc 1499: .It Pa ~/.ssh/rc
1.238 jmc 1500: Commands in this file are executed by
1501: .Nm
1.245 jmc 1502: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1503: started.
1504: See the
1505: .Xr sshd 8
1506: manual page for more information.
1507: .Pp
1.309 jmc 1508: .It Pa /etc/hosts.equiv
1.240 jmc 1509: This file is for host-based authentication (see above).
1510: It should only be writable by root.
1.236 jmc 1511: .Pp
1.309 jmc 1512: .It Pa /etc/shosts.equiv
1.240 jmc 1513: This file is used in exactly the same way as
1514: .Pa hosts.equiv ,
1515: but allows host-based authentication without permitting login with
1516: rlogin/rsh.
1.236 jmc 1517: .Pp
1.238 jmc 1518: .It Pa /etc/ssh/ssh_config
1519: Systemwide configuration file.
1520: The file format and configuration options are described in
1521: .Xr ssh_config 5 .
1522: .Pp
1.309 jmc 1523: .It Pa /etc/ssh/ssh_host_key
1524: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1525: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1526: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1527: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1528: These files contain the private parts of the host keys
1.245 jmc 1529: and are used for host-based authentication.
1.238 jmc 1530: .Pp
1.309 jmc 1531: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1532: Systemwide list of known host keys.
1533: This file should be prepared by the
1534: system administrator to contain the public host keys of all machines in the
1535: organization.
1.244 jmc 1536: It should be world-readable.
1537: See
1.238 jmc 1538: .Xr sshd 8
1.244 jmc 1539: for further details of the format of this file.
1.236 jmc 1540: .Pp
1.309 jmc 1541: .It Pa /etc/ssh/sshrc
1.1 deraadt 1542: Commands in this file are executed by
1.2 deraadt 1543: .Nm
1.245 jmc 1544: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1545: See the
1.2 deraadt 1546: .Xr sshd 8
1.1 deraadt 1547: manual page for more information.
1.58 itojun 1548: .El
1.312 jmc 1549: .Sh EXIT STATUS
1550: .Nm
1551: exits with the exit status of the remote command or with 255
1552: if an error occurred.
1.2 deraadt 1553: .Sh SEE ALSO
1554: .Xr scp 1 ,
1.83 djm 1555: .Xr sftp 1 ,
1.2 deraadt 1556: .Xr ssh-add 1 ,
1557: .Xr ssh-agent 1 ,
1558: .Xr ssh-keygen 1 ,
1.242 jmc 1559: .Xr ssh-keyscan 1 ,
1.250 jmc 1560: .Xr tun 4 ,
1.159 stevesk 1561: .Xr ssh_config 5 ,
1.160 naddy 1562: .Xr ssh-keysign 8 ,
1.87 itojun 1563: .Xr sshd 8
1.329 jmc 1564: .Sh STANDARDS
1.106 markus 1565: .Rs
1.329 jmc 1566: .%A S. Lehtinen
1567: .%A C. Lonvick
1568: .%D January 2006
1.256 jmc 1569: .%R RFC 4250
1.329 jmc 1570: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1571: .Re
1.329 jmc 1572: .Pp
1.256 jmc 1573: .Rs
1.329 jmc 1574: .%A T. Ylonen
1575: .%A C. Lonvick
1576: .%D January 2006
1.256 jmc 1577: .%R RFC 4251
1.329 jmc 1578: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1579: .Re
1.329 jmc 1580: .Pp
1.256 jmc 1581: .Rs
1.329 jmc 1582: .%A T. Ylonen
1583: .%A C. Lonvick
1584: .%D January 2006
1.256 jmc 1585: .%R RFC 4252
1.329 jmc 1586: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1587: .Re
1.329 jmc 1588: .Pp
1.256 jmc 1589: .Rs
1.329 jmc 1590: .%A T. Ylonen
1591: .%A C. Lonvick
1592: .%D January 2006
1.256 jmc 1593: .%R RFC 4253
1.329 jmc 1594: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1595: .Re
1.329 jmc 1596: .Pp
1.256 jmc 1597: .Rs
1.329 jmc 1598: .%A T. Ylonen
1599: .%A C. Lonvick
1600: .%D January 2006
1.256 jmc 1601: .%R RFC 4254
1.329 jmc 1602: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1603: .Re
1.329 jmc 1604: .Pp
1.256 jmc 1605: .Rs
1.329 jmc 1606: .%A J. Schlyter
1607: .%A W. Griffin
1608: .%D January 2006
1.256 jmc 1609: .%R RFC 4255
1.329 jmc 1610: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1611: .Re
1.329 jmc 1612: .Pp
1.256 jmc 1613: .Rs
1.329 jmc 1614: .%A F. Cusack
1615: .%A M. Forssen
1616: .%D January 2006
1.256 jmc 1617: .%R RFC 4256
1.329 jmc 1618: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1619: .Re
1.329 jmc 1620: .Pp
1.256 jmc 1621: .Rs
1.329 jmc 1622: .%A J. Galbraith
1623: .%A P. Remaker
1624: .%D January 2006
1.256 jmc 1625: .%R RFC 4335
1.329 jmc 1626: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1627: .Re
1.329 jmc 1628: .Pp
1.256 jmc 1629: .Rs
1.329 jmc 1630: .%A M. Bellare
1631: .%A T. Kohno
1632: .%A C. Namprempre
1633: .%D January 2006
1.256 jmc 1634: .%R RFC 4344
1.329 jmc 1635: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1636: .Re
1.329 jmc 1637: .Pp
1.256 jmc 1638: .Rs
1.329 jmc 1639: .%A B. Harris
1640: .%D January 2006
1.256 jmc 1641: .%R RFC 4345
1.329 jmc 1642: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1643: .Re
1.329 jmc 1644: .Pp
1.258 djm 1645: .Rs
1.329 jmc 1646: .%A M. Friedl
1647: .%A N. Provos
1648: .%A W. Simpson
1649: .%D March 2006
1.258 djm 1650: .%R RFC 4419
1.329 jmc 1651: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1652: .Re
1.329 jmc 1653: .Pp
1.266 markus 1654: .Rs
1.329 jmc 1655: .%A J. Galbraith
1656: .%A R. Thayer
1657: .%D November 2006
1.266 markus 1658: .%R RFC 4716
1.329 jmc 1659: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1660: .Re
1.329 jmc 1661: .Pp
1.313 djm 1662: .Rs
1.329 jmc 1663: .%A D. Stebila
1664: .%A J. Green
1665: .%D December 2009
1.313 djm 1666: .%R RFC 5656
1.329 jmc 1667: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1668: .Re
1.329 jmc 1669: .Pp
1.274 grunk 1670: .Rs
1671: .%A A. Perrig
1672: .%A D. Song
1673: .%D 1999
1.329 jmc 1674: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1675: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1676: .Re
1.173 jmc 1677: .Sh AUTHORS
1678: OpenSSH is a derivative of the original and free
1679: ssh 1.2.12 release by Tatu Ylonen.
1680: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1681: Theo de Raadt and Dug Song
1682: removed many bugs, re-added newer features and
1683: created OpenSSH.
1684: Markus Friedl contributed the support for SSH
1685: protocol versions 1.5 and 2.0.
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