Annotation of src/usr.bin/ssh/ssh.1, Revision 1.360
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: .\"
1.360 ! millert 36: .\" $OpenBSD: ssh.1,v 1.359 2015/07/10 06:21:53 markus Exp $
! 37: .Dd $Mdocdate: July 10 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
1.360 ! 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
1.356 djm 61: .Op Fl Q Cm cipher | cipher-auth | mac | kex | key | protocol-version
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.
1.176 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:
1.360 ! millert 96: .Bl -tag -width Ds -compact
! 97: .Pp
1.218 jmc 98: .It Fl 1
99: Forces
1.2 deraadt 100: .Nm
1.218 jmc 101: to try protocol version 1 only.
1.360 ! millert 102: .Pp
1.218 jmc 103: .It Fl 2
104: Forces
1.2 deraadt 105: .Nm
1.218 jmc 106: to try protocol version 2 only.
1.360 ! millert 107: .Pp
1.218 jmc 108: .It Fl 4
109: Forces
1.2 deraadt 110: .Nm
1.218 jmc 111: to use IPv4 addresses only.
1.360 ! millert 112: .Pp
1.218 jmc 113: .It Fl 6
114: Forces
1.2 deraadt 115: .Nm
1.218 jmc 116: to use IPv6 addresses only.
1.360 ! millert 117: .Pp
1.218 jmc 118: .It Fl A
119: Enables forwarding of the authentication agent connection.
120: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 121: .Pp
1.218 jmc 122: Agent forwarding should be enabled with caution.
123: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 124: (for the agent's
1.286 sobrado 125: .Ux Ns -domain
126: socket) can access the local agent through the forwarded connection.
1.218 jmc 127: An attacker cannot obtain key material from the agent,
128: however they can perform operations on the keys that enable them to
129: authenticate using the identities loaded into the agent.
1.360 ! millert 130: .Pp
1.218 jmc 131: .It Fl a
132: Disables forwarding of the authentication agent connection.
1.360 ! millert 133: .Pp
1.218 jmc 134: .It Fl b Ar bind_address
135: Use
136: .Ar bind_address
137: on the local machine as the source address
138: of the connection.
139: Only useful on systems with more than one address.
1.360 ! millert 140: .Pp
1.218 jmc 141: .It Fl C
142: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 143: data for forwarded X11, TCP and
144: .Ux Ns -domain
145: connections).
1.218 jmc 146: The compression algorithm is the same used by
147: .Xr gzip 1 ,
148: and the
149: .Dq level
150: can be controlled by the
151: .Cm CompressionLevel
152: option for protocol version 1.
153: Compression is desirable on modem lines and other
154: slow connections, but will only slow down things on fast networks.
155: The default value can be set on a host-by-host basis in the
156: configuration files; see the
157: .Cm Compression
158: option.
1.360 ! millert 159: .Pp
1.218 jmc 160: .It Fl c Ar cipher_spec
161: Selects the cipher specification for encrypting the session.
1.2 deraadt 162: .Pp
1.218 jmc 163: Protocol version 1 allows specification of a single cipher.
164: The supported values are
165: .Dq 3des ,
1.220 jmc 166: .Dq blowfish ,
1.218 jmc 167: and
168: .Dq des .
1.230 jmc 169: For protocol version 2,
1.218 jmc 170: .Ar cipher_spec
171: is a comma-separated list of ciphers
172: listed in order of preference.
1.283 jmc 173: See the
174: .Cm Ciphers
1.307 dtucker 175: keyword in
176: .Xr ssh_config 5
177: for more information.
1.360 ! millert 178: .Pp
1.218 jmc 179: .It Fl D Xo
180: .Sm off
181: .Oo Ar bind_address : Oc
182: .Ar port
183: .Sm on
184: .Xc
185: Specifies a local
186: .Dq dynamic
187: application-level port forwarding.
188: This works by allocating a socket to listen to
189: .Ar port
190: on the local side, optionally bound to the specified
191: .Ar bind_address .
192: Whenever a connection is made to this port, the
193: connection is forwarded over the secure channel, and the application
194: protocol is then used to determine where to connect to from the
195: remote machine.
196: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 197: .Nm
1.218 jmc 198: will act as a SOCKS server.
199: Only root can forward privileged ports.
200: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 201: .Pp
1.308 djm 202: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 203: Only the superuser can forward privileged ports.
204: By default, the local port is bound in accordance with the
205: .Cm GatewayPorts
206: setting.
207: However, an explicit
208: .Ar bind_address
209: may be used to bind the connection to a specific address.
210: The
211: .Ar bind_address
212: of
213: .Dq localhost
214: indicates that the listening port be bound for local use only, while an
215: empty address or
216: .Sq *
217: indicates that the port should be available from all interfaces.
1.360 ! millert 218: .Pp
1.331 dtucker 219: .It Fl E Ar log_file
220: Append debug logs to
221: .Ar log_file
222: instead of standard error.
1.360 ! millert 223: .Pp
1.229 jmc 224: .It Fl e Ar escape_char
1.218 jmc 225: Sets the escape character for sessions with a pty (default:
226: .Ql ~ ) .
227: The escape character is only recognized at the beginning of a line.
228: The escape character followed by a dot
229: .Pq Ql \&.
230: closes the connection;
231: followed by control-Z suspends the connection;
232: and followed by itself sends the escape character once.
233: Setting the character to
1.2 deraadt 234: .Dq none
1.218 jmc 235: disables any escapes and makes the session fully transparent.
1.360 ! millert 236: .Pp
1.218 jmc 237: .It Fl F Ar configfile
238: Specifies an alternative per-user configuration file.
239: If a configuration file is given on the command line,
240: the system-wide configuration file
241: .Pq Pa /etc/ssh/ssh_config
242: will be ignored.
243: The default for the per-user configuration file is
244: .Pa ~/.ssh/config .
1.360 ! millert 245: .Pp
1.218 jmc 246: .It Fl f
247: Requests
248: .Nm
249: to go to background just before command execution.
250: This is useful if
1.176 jmc 251: .Nm
1.218 jmc 252: is going to ask for passwords or passphrases, but the user
253: wants it in the background.
254: This implies
255: .Fl n .
256: The recommended way to start X11 programs at a remote site is with
257: something like
258: .Ic ssh -f host xterm .
1.277 djm 259: .Pp
260: If the
261: .Cm ExitOnForwardFailure
262: configuration option is set to
263: .Dq yes ,
264: then a client started with
265: .Fl f
266: will wait for all remote port forwards to be successfully established
267: before placing itself in the background.
1.360 ! millert 268: .Pp
1.350 djm 269: .It Fl G
270: Causes
271: .Nm
272: to print its configuration after evaluating
273: .Cm Host
274: and
275: .Cm Match
276: blocks and exit.
1.360 ! millert 277: .Pp
1.218 jmc 278: .It Fl g
279: Allows remote hosts to connect to local forwarded ports.
1.346 djm 280: If used on a multiplexed connection, then this option must be specified
281: on the master process.
1.360 ! millert 282: .Pp
1.291 markus 283: .It Fl I Ar pkcs11
1.294 jmc 284: Specify the PKCS#11 shared library
1.176 jmc 285: .Nm
1.293 markus 286: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 287: private RSA key.
1.360 ! millert 288: .Pp
1.218 jmc 289: .It Fl i Ar identity_file
290: Selects a file from which the identity (private key) for
1.310 djm 291: public key authentication is read.
1.218 jmc 292: The default is
293: .Pa ~/.ssh/identity
294: for protocol version 1, and
1.310 djm 295: .Pa ~/.ssh/id_dsa ,
1.343 naddy 296: .Pa ~/.ssh/id_ecdsa ,
297: .Pa ~/.ssh/id_ed25519
1.310 djm 298: and
1.218 jmc 299: .Pa ~/.ssh/id_rsa
300: for protocol version 2.
301: Identity files may also be specified on
302: a per-host basis in the configuration file.
303: It is possible to have multiple
304: .Fl i
305: options (and multiple identities specified in
306: configuration files).
1.302 djm 307: .Nm
308: will also try to load certificate information from the filename obtained
309: by appending
310: .Pa -cert.pub
311: to identity filenames.
1.360 ! millert 312: .Pp
1.269 djm 313: .It Fl K
314: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
315: credentials to the server.
1.360 ! millert 316: .Pp
1.218 jmc 317: .It Fl k
318: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 ! millert 319: .Pp
1.218 jmc 320: .It Fl L Xo
321: .Sm off
322: .Oo Ar bind_address : Oc
323: .Ar port : host : hostport
324: .Sm on
325: .Xc
1.360 ! millert 326: .It Fl L Xo
! 327: .Sm off
! 328: .Oo Ar bind_address : Oc
! 329: .Ar port : remote_socket
! 330: .Sm on
! 331: .Xc
! 332: .It Fl L Xo
! 333: .Sm off
! 334: .Ar local_socket : host : hostport
! 335: .Sm on
! 336: .Xc
! 337: .It Fl L Xo
! 338: .Sm off
! 339: .Ar local_socket : remote_socket
! 340: .Sm on
! 341: .Xc
! 342: Specifies that connections to the given TCP port or Unix socket on the local
! 343: (client) host are to be forwarded to the given host and port, or Unix socket,
! 344: on the remote side.
! 345: This works by allocating a socket to listen to either a TCP
1.218 jmc 346: .Ar port
347: on the local side, optionally bound to the specified
1.360 ! millert 348: .Ar bind_address ,
! 349: or to a Unix socket.
! 350: Whenever a connection is made to the local port or socket, the
1.218 jmc 351: connection is forwarded over the secure channel, and a connection is
1.360 ! millert 352: made to either
1.218 jmc 353: .Ar host
354: port
1.360 ! millert 355: .Ar hostport ,
! 356: or the Unix socket
! 357: .Ar remote_socket ,
1.218 jmc 358: from the remote machine.
1.360 ! millert 359: .Pp
1.218 jmc 360: Port forwardings can also be specified in the configuration file.
1.360 ! millert 361: Only the superuser can forward privileged ports.
1.308 djm 362: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 ! millert 363: .Pp
1.218 jmc 364: By default, the local port is bound in accordance with the
365: .Cm GatewayPorts
366: setting.
367: However, an explicit
368: .Ar bind_address
369: may be used to bind the connection to a specific address.
1.2 deraadt 370: The
1.218 jmc 371: .Ar bind_address
372: of
373: .Dq localhost
374: indicates that the listening port be bound for local use only, while an
375: empty address or
376: .Sq *
377: indicates that the port should be available from all interfaces.
1.360 ! millert 378: .Pp
1.218 jmc 379: .It Fl l Ar login_name
380: Specifies the user to log in as on the remote machine.
381: This also may be specified on a per-host basis in the configuration file.
1.360 ! millert 382: .Pp
1.218 jmc 383: .It Fl M
384: Places the
385: .Nm
386: client into
387: .Dq master
388: mode for connection sharing.
1.231 stevesk 389: Multiple
390: .Fl M
391: options places
392: .Nm
393: into
394: .Dq master
395: mode with confirmation required before slave connections are accepted.
1.218 jmc 396: Refer to the description of
397: .Cm ControlMaster
398: in
399: .Xr ssh_config 5
400: for details.
1.360 ! millert 401: .Pp
1.218 jmc 402: .It Fl m Ar mac_spec
403: Additionally, for protocol version 2 a comma-separated list of MAC
404: (message authentication code) algorithms can
405: be specified in order of preference.
406: See the
407: .Cm MACs
408: keyword for more information.
1.360 ! millert 409: .Pp
1.218 jmc 410: .It Fl N
411: Do not execute a remote command.
412: This is useful for just forwarding ports
413: (protocol version 2 only).
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
463: .It AddressFamily
464: .It BatchMode
465: .It BindAddress
1.338 djm 466: .It CanonicalDomains
1.339 djm 467: .It CanonicalizeFallbackLocal
468: .It CanonicalizeHostname
469: .It CanonicalizeMaxDots
470: .It CanonicalizePermittedCNAMEs
1.218 jmc 471: .It ChallengeResponseAuthentication
472: .It CheckHostIP
473: .It Cipher
474: .It Ciphers
475: .It ClearAllForwardings
476: .It Compression
477: .It CompressionLevel
478: .It ConnectionAttempts
479: .It ConnectTimeout
480: .It ControlMaster
481: .It ControlPath
1.321 djm 482: .It ControlPersist
1.218 jmc 483: .It DynamicForward
484: .It EscapeChar
1.263 markus 485: .It ExitOnForwardFailure
1.353 jmc 486: .It FingerprintHash
1.218 jmc 487: .It ForwardAgent
488: .It ForwardX11
1.321 djm 489: .It ForwardX11Timeout
1.218 jmc 490: .It ForwardX11Trusted
491: .It GatewayPorts
492: .It GlobalKnownHostsFile
493: .It GSSAPIAuthentication
494: .It GSSAPIDelegateCredentials
495: .It HashKnownHosts
496: .It Host
497: .It HostbasedAuthentication
1.355 djm 498: .It HostbasedKeyTypes
1.218 jmc 499: .It HostKeyAlgorithms
500: .It HostKeyAlias
501: .It HostName
502: .It IdentityFile
503: .It IdentitiesOnly
1.316 jmc 504: .It IPQoS
1.321 djm 505: .It KbdInteractiveAuthentication
1.218 jmc 506: .It KbdInteractiveDevices
1.314 jmc 507: .It KexAlgorithms
1.218 jmc 508: .It LocalCommand
509: .It LocalForward
510: .It LogLevel
511: .It MACs
1.337 jmc 512: .It Match
1.218 jmc 513: .It NoHostAuthenticationForLocalhost
514: .It NumberOfPasswordPrompts
515: .It PasswordAuthentication
516: .It PermitLocalCommand
1.291 markus 517: .It PKCS11Provider
1.218 jmc 518: .It Port
519: .It PreferredAuthentications
520: .It Protocol
521: .It ProxyCommand
1.336 jmc 522: .It ProxyUseFdpass
1.359 markus 523: .It PubkeyAcceptedKeyTypes
1.218 jmc 524: .It PubkeyAuthentication
1.251 dtucker 525: .It RekeyLimit
1.218 jmc 526: .It RemoteForward
1.319 jmc 527: .It RequestTTY
1.218 jmc 528: .It RhostsRSAAuthentication
529: .It RSAAuthentication
530: .It SendEnv
531: .It ServerAliveInterval
532: .It ServerAliveCountMax
1.347 jmc 533: .It StreamLocalBindMask
534: .It StreamLocalBindUnlink
1.218 jmc 535: .It StrictHostKeyChecking
536: .It TCPKeepAlive
537: .It Tunnel
538: .It TunnelDevice
1.354 djm 539: .It UpdateHostKeys
1.218 jmc 540: .It UsePrivilegedPort
541: .It User
542: .It UserKnownHostsFile
543: .It VerifyHostKeyDNS
1.276 jmc 544: .It VisualHostKey
1.218 jmc 545: .It XAuthLocation
546: .El
1.360 ! millert 547: .Pp
1.218 jmc 548: .It Fl p Ar port
549: Port to connect to on the remote host.
550: This can be specified on a
551: per-host basis in the configuration file.
1.360 ! millert 552: .Pp
1.356 djm 553: .It Fl Q Cm cipher | cipher-auth | mac | kex | key | protocol-version
1.332 djm 554: Queries
555: .Nm
1.341 deraadt 556: for the algorithms supported for the specified version 2.
557: The available features are:
558: .Ar cipher
1.332 djm 559: (supported symmetric ciphers),
1.341 deraadt 560: .Ar cipher-auth
1.340 djm 561: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 562: .Ar mac
1.332 djm 563: (supported message integrity codes),
1.341 deraadt 564: .Ar kex
1.332 djm 565: (key exchange algorithms),
1.341 deraadt 566: .Ar key
1.356 djm 567: (key types) and
568: .Ar protocol-version
569: (supported SSH protocol versions).
1.360 ! millert 570: .Pp
1.218 jmc 571: .It Fl q
572: Quiet mode.
1.271 djm 573: Causes most warning and diagnostic messages to be suppressed.
1.360 ! millert 574: .Pp
1.218 jmc 575: .It Fl R Xo
576: .Sm off
577: .Oo Ar bind_address : Oc
578: .Ar port : host : hostport
579: .Sm on
580: .Xc
1.360 ! millert 581: .It Fl R Xo
! 582: .Sm off
! 583: .Oo Ar bind_address : Oc
! 584: .Ar port : local_socket
! 585: .Sm on
! 586: .Xc
! 587: .It Fl R Xo
! 588: .Sm off
! 589: .Ar remote_socket : host : hostport
! 590: .Sm on
! 591: .Xc
! 592: .It Fl R Xo
! 593: .Sm off
! 594: .Ar remote_socket : local_socket
! 595: .Sm on
! 596: .Xc
! 597: Specifies that connections to the given TCP port or Unix socket on the remote
! 598: (server) host are to be forwarded to the given host and port, or Unix socket,
! 599: on the local side.
! 600: This works by allocating a socket to listen to either a TCP
1.218 jmc 601: .Ar port
1.360 ! millert 602: or to a Unix socket on the remote side.
! 603: Whenever a connection is made to this port or Unix socket, the
! 604: connection is forwarded over the secure channel, and a connection
! 605: is made to either
1.218 jmc 606: .Ar host
607: port
1.360 ! millert 608: .Ar hostport ,
! 609: or
! 610: .Ar local_socket ,
1.218 jmc 611: from the local machine.
1.2 deraadt 612: .Pp
1.218 jmc 613: Port forwardings can also be specified in the configuration file.
614: Privileged ports can be forwarded only when
615: logging in as root on the remote machine.
1.324 jmc 616: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 617: .Pp
1.360 ! millert 618: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 619: interface only.
1.280 tobias 620: This may be overridden by specifying a
1.218 jmc 621: .Ar bind_address .
622: An empty
623: .Ar bind_address ,
624: or the address
625: .Ql * ,
626: indicates that the remote socket should listen on all interfaces.
627: Specifying a remote
628: .Ar bind_address
629: will only succeed if the server's
630: .Cm GatewayPorts
631: option is enabled (see
632: .Xr sshd_config 5 ) .
1.281 djm 633: .Pp
634: If the
635: .Ar port
636: argument is
1.282 djm 637: .Ql 0 ,
1.281 djm 638: the listen port will be dynamically allocated on the server and reported
639: to the client at run time.
1.305 markus 640: When used together with
641: .Ic -O forward
642: the allocated port will be printed to the standard output.
1.360 ! millert 643: .Pp
1.218 jmc 644: .It Fl S Ar ctl_path
1.304 jmc 645: Specifies the location of a control socket for connection sharing,
1.303 djm 646: or the string
647: .Dq none
648: to disable connection sharing.
1.218 jmc 649: Refer to the description of
650: .Cm ControlPath
651: and
652: .Cm ControlMaster
653: in
654: .Xr ssh_config 5
655: for details.
1.360 ! millert 656: .Pp
1.218 jmc 657: .It Fl s
658: May be used to request invocation of a subsystem on the remote system.
659: Subsystems are a feature of the SSH2 protocol which facilitate the use
660: of SSH as a secure transport for other applications (eg.\&
661: .Xr sftp 1 ) .
662: The subsystem is specified as the remote command.
1.360 ! millert 663: .Pp
1.218 jmc 664: .It Fl T
1.357 dtucker 665: Disable pseudo-terminal allocation.
1.360 ! millert 666: .Pp
1.218 jmc 667: .It Fl t
1.357 dtucker 668: Force pseudo-terminal allocation.
1.218 jmc 669: This can be used to execute arbitrary
670: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 671: e.g. when implementing menu services.
1.218 jmc 672: Multiple
673: .Fl t
674: options force tty allocation, even if
1.194 jakob 675: .Nm
1.218 jmc 676: has no local tty.
1.360 ! millert 677: .Pp
1.218 jmc 678: .It Fl V
679: Display the version number and exit.
1.360 ! millert 680: .Pp
1.218 jmc 681: .It Fl v
682: Verbose mode.
683: Causes
1.176 jmc 684: .Nm
1.218 jmc 685: to print debugging messages about its progress.
686: This is helpful in
687: debugging connection, authentication, and configuration problems.
688: Multiple
689: .Fl v
690: options increase the verbosity.
691: The maximum is 3.
1.360 ! millert 692: .Pp
1.290 dtucker 693: .It Fl W Ar host : Ns Ar port
694: Requests that standard input and output on the client be forwarded to
695: .Ar host
696: on
697: .Ar port
698: over the secure channel.
699: Implies
700: .Fl N ,
701: .Fl T ,
702: .Cm ExitOnForwardFailure
703: and
1.326 dtucker 704: .Cm ClearAllForwardings .
705: Works with Protocol version 2 only.
1.360 ! millert 706: .Pp
1.261 stevesk 707: .It Fl w Xo
708: .Ar local_tun Ns Op : Ns Ar remote_tun
709: .Xc
710: Requests
711: tunnel
712: device forwarding with the specified
1.218 jmc 713: .Xr tun 4
1.261 stevesk 714: devices between the client
715: .Pq Ar local_tun
716: and the server
717: .Pq Ar remote_tun .
718: .Pp
1.228 jmc 719: The devices may be specified by numerical ID or the keyword
720: .Dq any ,
721: which uses the next available tunnel device.
1.261 stevesk 722: If
723: .Ar remote_tun
724: is not specified, it defaults to
725: .Dq any .
1.228 jmc 726: See also the
1.218 jmc 727: .Cm Tunnel
1.261 stevesk 728: and
729: .Cm TunnelDevice
730: directives in
1.218 jmc 731: .Xr ssh_config 5 .
1.261 stevesk 732: If the
733: .Cm Tunnel
734: directive is unset, it is set to the default tunnel mode, which is
735: .Dq point-to-point .
1.360 ! millert 736: .Pp
1.218 jmc 737: .It Fl X
738: Enables X11 forwarding.
1.54 markus 739: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 740: .Pp
1.218 jmc 741: X11 forwarding should be enabled with caution.
1.168 jmc 742: Users with the ability to bypass file permissions on the remote host
1.218 jmc 743: (for the user's X authorization database)
744: can access the local X11 display through the forwarded connection.
745: An attacker may then be able to perform activities such as keystroke monitoring.
746: .Pp
747: For this reason, X11 forwarding is subjected to X11 SECURITY extension
748: restrictions by default.
749: Please refer to the
750: .Nm
751: .Fl Y
752: option and the
753: .Cm ForwardX11Trusted
754: directive in
755: .Xr ssh_config 5
756: for more information.
1.360 ! millert 757: .Pp
1.218 jmc 758: .It Fl x
759: Disables X11 forwarding.
1.360 ! millert 760: .Pp
1.218 jmc 761: .It Fl Y
762: Enables trusted X11 forwarding.
763: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
764: controls.
1.360 ! millert 765: .Pp
1.278 djm 766: .It Fl y
767: Send log information using the
768: .Xr syslog 3
769: system module.
770: By default this information is sent to stderr.
1.218 jmc 771: .El
1.224 jmc 772: .Pp
773: .Nm
774: may additionally obtain configuration data from
775: a per-user configuration file and a system-wide configuration file.
776: The file format and configuration options are described in
777: .Xr ssh_config 5 .
1.222 jmc 778: .Sh AUTHENTICATION
1.249 jmc 779: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 780: The default is to use protocol 2 only,
781: though this can be changed via the
1.222 jmc 782: .Cm Protocol
783: option in
1.284 jmc 784: .Xr ssh_config 5
785: or the
1.222 jmc 786: .Fl 1
787: and
788: .Fl 2
789: options (see above).
790: Both protocols support similar authentication methods,
1.284 jmc 791: but protocol 2 is the default since
1.222 jmc 792: it provides additional mechanisms for confidentiality
793: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 794: and integrity (hmac-md5, hmac-sha1,
795: hmac-sha2-256, hmac-sha2-512,
1.330 markus 796: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 797: Protocol 1 lacks a strong mechanism for ensuring the
798: integrity of the connection.
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,
807: though protocol 2 has a configuration option to change the default order:
808: .Cm PreferredAuthentications .
809: .Pp
810: Host-based authentication works as follows:
1.218 jmc 811: If the machine the user logs in from is listed in
812: .Pa /etc/hosts.equiv
813: or
814: .Pa /etc/shosts.equiv
815: on the remote machine, and the user names are
816: the same on both sides, or if the files
817: .Pa ~/.rhosts
818: or
819: .Pa ~/.shosts
820: exist in the user's home directory on the
821: remote machine and contain a line containing the name of the client
822: machine and the name of the user on that machine, the user is
1.222 jmc 823: considered for login.
824: Additionally, the server
825: .Em must
826: be able to verify the client's
827: host key (see the description of
1.218 jmc 828: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 829: and
1.222 jmc 830: .Pa ~/.ssh/known_hosts ,
831: below)
832: for login to be permitted.
1.218 jmc 833: This authentication method closes security holes due to IP
1.222 jmc 834: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 835: [Note to the administrator:
836: .Pa /etc/hosts.equiv ,
837: .Pa ~/.rhosts ,
838: and the rlogin/rsh protocol in general, are inherently insecure and should be
839: disabled if security is desired.]
1.189 dtucker 840: .Pp
1.222 jmc 841: Public key authentication works as follows:
842: The scheme is based on public-key cryptography,
843: using cryptosystems
844: where encryption and decryption are done using separate keys,
845: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 846: The idea is that each user creates a public/private
847: key pair for authentication purposes.
848: The server knows the public key, and only the user knows the private key.
1.222 jmc 849: .Nm
850: implements public key authentication protocol automatically,
1.349 sobrado 851: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.222 jmc 852: Protocol 1 is restricted to using only RSA keys,
1.310 djm 853: but protocol 2 may use any.
1.333 jmc 854: The HISTORY section of
1.222 jmc 855: .Xr ssl 8
1.311 jmc 856: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 857: .Pp
1.218 jmc 858: The file
859: .Pa ~/.ssh/authorized_keys
860: lists the public keys that are permitted for logging in.
861: When the user logs in, the
1.2 deraadt 862: .Nm
1.218 jmc 863: program tells the server which key pair it would like to use for
864: authentication.
1.222 jmc 865: The client proves that it has access to the private key
866: and the server checks that the corresponding public key
867: is authorized to accept the account.
1.218 jmc 868: .Pp
1.222 jmc 869: The user creates his/her key pair by running
1.218 jmc 870: .Xr ssh-keygen 1 .
871: This stores the private key in
1.207 djm 872: .Pa ~/.ssh/identity
1.222 jmc 873: (protocol 1),
874: .Pa ~/.ssh/id_dsa
875: (protocol 2 DSA),
1.310 djm 876: .Pa ~/.ssh/id_ecdsa
877: (protocol 2 ECDSA),
1.343 naddy 878: .Pa ~/.ssh/id_ed25519
1.349 sobrado 879: (protocol 2 Ed25519),
1.222 jmc 880: or
881: .Pa ~/.ssh/id_rsa
882: (protocol 2 RSA)
1.218 jmc 883: and stores the public key in
884: .Pa ~/.ssh/identity.pub
1.222 jmc 885: (protocol 1),
886: .Pa ~/.ssh/id_dsa.pub
887: (protocol 2 DSA),
1.310 djm 888: .Pa ~/.ssh/id_ecdsa.pub
889: (protocol 2 ECDSA),
1.343 naddy 890: .Pa ~/.ssh/id_ed25519.pub
1.349 sobrado 891: (protocol 2 Ed25519),
1.222 jmc 892: or
893: .Pa ~/.ssh/id_rsa.pub
894: (protocol 2 RSA)
1.218 jmc 895: in the user's home directory.
1.222 jmc 896: The user should then copy the public key
1.218 jmc 897: to
898: .Pa ~/.ssh/authorized_keys
1.222 jmc 899: in his/her home directory on the remote machine.
900: The
1.218 jmc 901: .Pa authorized_keys
902: file corresponds to the conventional
903: .Pa ~/.rhosts
904: file, and has one key
1.222 jmc 905: per line, though the lines can be very long.
1.218 jmc 906: After this, the user can log in without giving the password.
907: .Pp
1.301 jmc 908: A variation on public key authentication
909: is available in the form of certificate authentication:
910: instead of a set of public/private keys,
911: signed certificates are used.
912: This has the advantage that a single trusted certification authority
913: can be used in place of many public/private keys.
1.333 jmc 914: See the CERTIFICATES section of
1.301 jmc 915: .Xr ssh-keygen 1
916: for more information.
917: .Pp
918: The most convenient way to use public key or certificate authentication
919: may be with an authentication agent.
1.218 jmc 920: See
921: .Xr ssh-agent 1
922: for more information.
923: .Pp
1.222 jmc 924: Challenge-response authentication works as follows:
925: The server sends an arbitrary
926: .Qq challenge
927: text, and prompts for a response.
928: Protocol 2 allows multiple challenges and responses;
929: protocol 1 is restricted to just one challenge/response.
930: Examples of challenge-response authentication include
1.335 jmc 931: .Bx
932: Authentication (see
1.222 jmc 933: .Xr login.conf 5 )
1.335 jmc 934: and PAM (some
935: .Pf non- Ox
936: systems).
1.222 jmc 937: .Pp
938: Finally, if other authentication methods fail,
1.218 jmc 939: .Nm
940: prompts the user for a password.
941: The password is sent to the remote
942: host for checking; however, since all communications are encrypted,
943: the password cannot be seen by someone listening on the network.
1.232 jmc 944: .Pp
945: .Nm
946: automatically maintains and checks a database containing
947: identification for all hosts it has ever been used with.
948: Host keys are stored in
949: .Pa ~/.ssh/known_hosts
950: in the user's home directory.
951: Additionally, the file
952: .Pa /etc/ssh/ssh_known_hosts
953: is automatically checked for known hosts.
954: Any new hosts are automatically added to the user's file.
955: If a host's identification ever changes,
956: .Nm
957: warns about this and disables password authentication to prevent
958: server spoofing or man-in-the-middle attacks,
959: which could otherwise be used to circumvent the encryption.
960: The
961: .Cm StrictHostKeyChecking
962: option can be used to control logins to machines whose
963: host key is not known or has changed.
964: .Pp
1.218 jmc 965: When the user's identity has been accepted by the server, the server
1.357 dtucker 966: either executes the given command in a non-interactive session or,
967: if no command has been specified, logs into the machine and gives
968: the user a normal shell as an interactive session.
1.218 jmc 969: All communication with
970: the remote command or shell will be automatically encrypted.
971: .Pp
1.357 dtucker 972: If an interactive session is requested
973: .Nm
974: by default will only request a pseudo-terminal (pty) for interactive
975: sessions when the client has one.
976: The flags
977: .Fl T
978: and
979: .Fl t
980: can be used to override this behaviour.
981: .Pp
982: If a pseudo-terminal has been allocated the
1.218 jmc 983: user may use the escape characters noted below.
984: .Pp
1.357 dtucker 985: If no pseudo-terminal has been allocated,
1.218 jmc 986: the session is transparent and can be used to reliably transfer binary data.
987: On most systems, setting the escape character to
988: .Dq none
989: will also make the session transparent even if a tty is used.
990: .Pp
991: The session terminates when the command or shell on the remote
1.247 jmc 992: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 993: .Sh ESCAPE CHARACTERS
1.218 jmc 994: When a pseudo-terminal has been requested,
1.2 deraadt 995: .Nm
1.218 jmc 996: supports a number of functions through the use of an escape character.
997: .Pp
998: A single tilde character can be sent as
999: .Ic ~~
1000: or by following the tilde by a character other than those described below.
1001: The escape character must always follow a newline to be interpreted as
1002: special.
1003: The escape character can be changed in configuration files using the
1004: .Cm EscapeChar
1005: configuration directive or on the command line by the
1006: .Fl e
1007: option.
1008: .Pp
1009: The supported escapes (assuming the default
1010: .Ql ~ )
1011: are:
1012: .Bl -tag -width Ds
1013: .It Cm ~.
1014: Disconnect.
1015: .It Cm ~^Z
1016: Background
1.234 jmc 1017: .Nm .
1.218 jmc 1018: .It Cm ~#
1019: List forwarded connections.
1020: .It Cm ~&
1021: Background
1.2 deraadt 1022: .Nm
1.218 jmc 1023: at logout when waiting for forwarded connection / X11 sessions to terminate.
1024: .It Cm ~?
1025: Display a list of escape characters.
1026: .It Cm ~B
1027: Send a BREAK to the remote system
1028: (only useful for SSH protocol version 2 and if the peer supports it).
1029: .It Cm ~C
1030: Open command line.
1031: Currently this allows the addition of port forwardings using the
1.279 stevesk 1032: .Fl L ,
1033: .Fl R
1.218 jmc 1034: and
1.279 stevesk 1035: .Fl D
1.225 jmc 1036: options (see above).
1.322 markus 1037: It also allows the cancellation of existing port-forwardings
1038: with
1.262 stevesk 1039: .Sm off
1.322 markus 1040: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1041: .Sm on
1.322 markus 1042: for local,
1043: .Sm off
1044: .Fl KR Oo Ar bind_address : Oc Ar port
1045: .Sm on
1046: for remote and
1047: .Sm off
1048: .Fl KD Oo Ar bind_address : Oc Ar port
1049: .Sm on
1050: for dynamic port-forwardings.
1.218 jmc 1051: .Ic !\& Ns Ar command
1052: allows the user to execute a local command if the
1053: .Ic PermitLocalCommand
1054: option is enabled in
1.176 jmc 1055: .Xr ssh_config 5 .
1.218 jmc 1056: Basic help is available, using the
1057: .Fl h
1058: option.
1059: .It Cm ~R
1060: Request rekeying of the connection
1061: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 1062: .It Cm ~V
1063: Decrease the verbosity
1064: .Pq Ic LogLevel
1065: when errors are being written to stderr.
1066: .It Cm ~v
1.328 jmc 1067: Increase the verbosity
1.327 dtucker 1068: .Pq Ic LogLevel
1069: when errors are being written to stderr.
1.176 jmc 1070: .El
1.246 jmc 1071: .Sh TCP FORWARDING
1072: Forwarding of arbitrary TCP connections over the secure channel can
1073: be specified either on the command line or in a configuration file.
1074: One possible application of TCP forwarding is a secure connection to a
1075: mail server; another is going through firewalls.
1076: .Pp
1077: In the example below, we look at encrypting communication between
1078: an IRC client and server, even though the IRC server does not directly
1079: support encrypted communications.
1080: This works as follows:
1081: the user connects to the remote host using
1082: .Nm ,
1083: specifying a port to be used to forward connections
1084: to the remote server.
1085: After that it is possible to start the service which is to be encrypted
1086: on the client machine,
1087: connecting to the same local port,
1088: and
1089: .Nm
1090: will encrypt and forward the connection.
1091: .Pp
1092: The following example tunnels an IRC session from client machine
1093: .Dq 127.0.0.1
1094: (localhost)
1095: to remote server
1096: .Dq server.example.com :
1097: .Bd -literal -offset 4n
1098: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1099: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1100: .Ed
1101: .Pp
1102: This tunnels a connection to IRC server
1103: .Dq server.example.com ,
1104: joining channel
1105: .Dq #users ,
1106: nickname
1107: .Dq pinky ,
1108: using port 1234.
1109: It doesn't matter which port is used,
1110: as long as it's greater than 1023
1111: (remember, only root can open sockets on privileged ports)
1112: and doesn't conflict with any ports already in use.
1113: The connection is forwarded to port 6667 on the remote server,
1114: since that's the standard port for IRC services.
1115: .Pp
1116: The
1117: .Fl f
1118: option backgrounds
1119: .Nm
1120: and the remote command
1121: .Dq sleep 10
1122: is specified to allow an amount of time
1123: (10 seconds, in the example)
1124: to start the service which is to be tunnelled.
1125: If no connections are made within the time specified,
1126: .Nm
1127: will exit.
1128: .Sh X11 FORWARDING
1.218 jmc 1129: If the
1130: .Cm ForwardX11
1131: variable is set to
1132: .Dq yes
1133: (or see the description of the
1.227 jmc 1134: .Fl X ,
1135: .Fl x ,
1.218 jmc 1136: and
1.227 jmc 1137: .Fl Y
1.226 jmc 1138: options above)
1.218 jmc 1139: and the user is using X11 (the
1140: .Ev DISPLAY
1141: environment variable is set), the connection to the X11 display is
1142: automatically forwarded to the remote side in such a way that any X11
1143: programs started from the shell (or command) will go through the
1144: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1145: from the local machine.
1.218 jmc 1146: The user should not manually set
1147: .Ev DISPLAY .
1148: Forwarding of X11 connections can be
1149: configured on the command line or in configuration files.
1150: .Pp
1151: The
1152: .Ev DISPLAY
1153: value set by
1154: .Nm
1155: will point to the server machine, but with a display number greater than zero.
1156: This is normal, and happens because
1157: .Nm
1158: creates a
1159: .Dq proxy
1160: X server on the server machine for forwarding the
1161: connections over the encrypted channel.
1.200 djm 1162: .Pp
1.218 jmc 1163: .Nm
1164: will also automatically set up Xauthority data on the server machine.
1165: For this purpose, it will generate a random authorization cookie,
1166: store it in Xauthority on the server, and verify that any forwarded
1167: connections carry this cookie and replace it by the real cookie when
1168: the connection is opened.
1169: The real authentication cookie is never
1170: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1171: .Pp
1.218 jmc 1172: If the
1173: .Cm ForwardAgent
1174: variable is set to
1175: .Dq yes
1176: (or see the description of the
1177: .Fl A
1.191 djm 1178: and
1.218 jmc 1179: .Fl a
1.226 jmc 1180: options above) and
1.218 jmc 1181: the user is using an authentication agent, the connection to the agent
1182: is automatically forwarded to the remote side.
1.252 jmc 1183: .Sh VERIFYING HOST KEYS
1184: When connecting to a server for the first time,
1185: a fingerprint of the server's public key is presented to the user
1186: (unless the option
1187: .Cm StrictHostKeyChecking
1188: has been disabled).
1189: Fingerprints can be determined using
1190: .Xr ssh-keygen 1 :
1191: .Pp
1192: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1193: .Pp
1.274 grunk 1194: If the fingerprint is already known, it can be matched
1195: and the key can be accepted or rejected.
1.358 djm 1196: If only legacy (MD5) fingerprints for the server are available, the
1197: .Xr ssh-keygen 1
1198: .Fl E
1199: option may be used to downgrade the fingerprint algorithm to match.
1200: .Pp
1.274 grunk 1201: Because of the difficulty of comparing host keys
1.352 djm 1202: just by looking at fingerprint strings,
1.274 grunk 1203: there is also support to compare host keys visually,
1204: using
1205: .Em random art .
1206: By setting the
1.275 grunk 1207: .Cm VisualHostKey
1.274 grunk 1208: option to
1.275 grunk 1209: .Dq yes ,
1.274 grunk 1210: a small ASCII graphic gets displayed on every login to a server, no matter
1211: if the session itself is interactive or not.
1212: By learning the pattern a known server produces, a user can easily
1213: find out that the host key has changed when a completely different pattern
1214: is displayed.
1215: Because these patterns are not unambiguous however, a pattern that looks
1216: similar to the pattern remembered only gives a good probability that the
1217: host key is the same, not guaranteed proof.
1218: .Pp
1219: To get a listing of the fingerprints along with their random art for
1220: all known hosts, the following command line can be used:
1221: .Pp
1222: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1223: .Pp
1.252 jmc 1224: If the fingerprint is unknown,
1225: an alternative method of verification is available:
1226: SSH fingerprints verified by DNS.
1227: An additional resource record (RR),
1228: SSHFP,
1229: is added to a zonefile
1230: and the connecting client is able to match the fingerprint
1231: with that of the key presented.
1232: .Pp
1233: In this example, we are connecting a client to a server,
1234: .Dq host.example.com .
1235: The SSHFP resource records should first be added to the zonefile for
1236: host.example.com:
1237: .Bd -literal -offset indent
1.259 jakob 1238: $ ssh-keygen -r host.example.com.
1.252 jmc 1239: .Ed
1240: .Pp
1241: The output lines will have to be added to the zonefile.
1242: To check that the zone is answering fingerprint queries:
1243: .Pp
1244: .Dl $ dig -t SSHFP host.example.com
1245: .Pp
1246: Finally the client connects:
1247: .Bd -literal -offset indent
1248: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1249: [...]
1250: Matching host key fingerprint found in DNS.
1251: Are you sure you want to continue connecting (yes/no)?
1252: .Ed
1253: .Pp
1254: See the
1255: .Cm VerifyHostKeyDNS
1256: option in
1257: .Xr ssh_config 5
1258: for more information.
1.250 jmc 1259: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1260: .Nm
1261: contains support for Virtual Private Network (VPN) tunnelling
1262: using the
1263: .Xr tun 4
1264: network pseudo-device,
1265: allowing two networks to be joined securely.
1266: The
1267: .Xr sshd_config 5
1268: configuration option
1269: .Cm PermitTunnel
1270: controls whether the server supports this,
1271: and at what level (layer 2 or 3 traffic).
1272: .Pp
1273: The following example would connect client network 10.0.50.0/24
1.265 otto 1274: with remote network 10.0.99.0/24 using a point-to-point connection
1275: from 10.1.1.1 to 10.1.1.2,
1276: provided that the SSH server running on the gateway to the remote network,
1277: at 192.168.1.15, allows it.
1278: .Pp
1279: On the client:
1.250 jmc 1280: .Bd -literal -offset indent
1281: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1282: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1283: # route add 10.0.99.0/24 10.1.1.2
1284: .Ed
1285: .Pp
1286: On the server:
1287: .Bd -literal -offset indent
1288: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1289: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1290: .Ed
1291: .Pp
1292: Client access may be more finely tuned via the
1293: .Pa /root/.ssh/authorized_keys
1294: file (see below) and the
1295: .Cm PermitRootLogin
1296: server option.
1.255 jmc 1297: The following entry would permit connections on
1.250 jmc 1298: .Xr tun 4
1.255 jmc 1299: device 1 from user
1.250 jmc 1300: .Dq jane
1.255 jmc 1301: and on tun device 2 from user
1.250 jmc 1302: .Dq john ,
1303: if
1304: .Cm PermitRootLogin
1305: is set to
1306: .Dq forced-commands-only :
1307: .Bd -literal -offset 2n
1308: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1309: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1310: .Ed
1311: .Pp
1.264 ray 1312: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1313: it may be more suited to temporary setups,
1314: such as for wireless VPNs.
1315: More permanent VPNs are better provided by tools such as
1316: .Xr ipsecctl 8
1317: and
1318: .Xr isakmpd 8 .
1.2 deraadt 1319: .Sh ENVIRONMENT
1320: .Nm
1.1 deraadt 1321: will normally set the following environment variables:
1.237 jmc 1322: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1323: .It Ev DISPLAY
1324: The
1325: .Ev DISPLAY
1.40 aaron 1326: variable indicates the location of the X11 server.
1.44 aaron 1327: It is automatically set by
1.2 deraadt 1328: .Nm
1329: to point to a value of the form
1.233 jmc 1330: .Dq hostname:n ,
1331: where
1332: .Dq hostname
1333: indicates the host where the shell runs, and
1334: .Sq n
1335: is an integer \*(Ge 1.
1.40 aaron 1336: .Nm
1337: uses this special value to forward X11 connections over the secure
1338: channel.
1.107 markus 1339: The user should normally not set
1340: .Ev DISPLAY
1341: explicitly, as that
1.1 deraadt 1342: will render the X11 connection insecure (and will require the user to
1343: manually copy any required authorization cookies).
1.2 deraadt 1344: .It Ev HOME
1.1 deraadt 1345: Set to the path of the user's home directory.
1.2 deraadt 1346: .It Ev LOGNAME
1347: Synonym for
1.12 aaron 1348: .Ev USER ;
1349: set for compatibility with systems that use this variable.
1.2 deraadt 1350: .It Ev MAIL
1.129 stevesk 1351: Set to the path of the user's mailbox.
1.40 aaron 1352: .It Ev PATH
1.2 deraadt 1353: Set to the default
1354: .Ev PATH ,
1355: as specified when compiling
1.234 jmc 1356: .Nm .
1.118 markus 1357: .It Ev SSH_ASKPASS
1358: If
1359: .Nm
1360: needs a passphrase, it will read the passphrase from the current
1361: terminal if it was run from a terminal.
1362: If
1363: .Nm
1364: does not have a terminal associated with it but
1365: .Ev DISPLAY
1366: and
1367: .Ev SSH_ASKPASS
1368: are set, it will execute the program specified by
1369: .Ev SSH_ASKPASS
1370: and open an X11 window to read the passphrase.
1371: This is particularly useful when calling
1372: .Nm
1373: from a
1.196 jmc 1374: .Pa .xsession
1.118 markus 1375: or related script.
1376: (Note that on some machines it
1377: may be necessary to redirect the input from
1378: .Pa /dev/null
1379: to make this work.)
1.18 markus 1380: .It Ev SSH_AUTH_SOCK
1.233 jmc 1381: Identifies the path of a
1382: .Ux Ns -domain
1383: socket used to communicate with the agent.
1.166 stevesk 1384: .It Ev SSH_CONNECTION
1385: Identifies the client and server ends of the connection.
1.40 aaron 1386: The variable contains
1.233 jmc 1387: four space-separated values: client IP address, client port number,
1388: server IP address, and server port number.
1.73 markus 1389: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1390: This variable contains the original command line if a forced command
1.73 markus 1391: is executed.
1392: It can be used to extract the original arguments.
1.2 deraadt 1393: .It Ev SSH_TTY
1.1 deraadt 1394: This is set to the name of the tty (path to the device) associated
1.40 aaron 1395: with the current shell or command.
1396: If the current session has no tty,
1.1 deraadt 1397: this variable is not set.
1.2 deraadt 1398: .It Ev TZ
1.214 jmc 1399: This variable is set to indicate the present time zone if it
1.257 jmc 1400: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1401: on to new connections).
1.2 deraadt 1402: .It Ev USER
1.1 deraadt 1403: Set to the name of the user logging in.
1.2 deraadt 1404: .El
1405: .Pp
1.44 aaron 1406: Additionally,
1.2 deraadt 1407: .Nm
1.44 aaron 1408: reads
1.207 djm 1409: .Pa ~/.ssh/environment ,
1.2 deraadt 1410: and adds lines of the format
1411: .Dq VARNAME=value
1.233 jmc 1412: to the environment if the file exists and users are allowed to
1.161 marc 1413: change their environment.
1.176 jmc 1414: For more information, see the
1.161 marc 1415: .Cm PermitUserEnvironment
1.162 stevesk 1416: option in
1.161 marc 1417: .Xr sshd_config 5 .
1.2 deraadt 1418: .Sh FILES
1.236 jmc 1419: .Bl -tag -width Ds -compact
1.309 jmc 1420: .It Pa ~/.rhosts
1.240 jmc 1421: This file is used for host-based authentication (see above).
1.92 markus 1422: On some machines this file may need to be
1.240 jmc 1423: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1424: because
1.2 deraadt 1425: .Xr sshd 8
1.40 aaron 1426: reads it as root.
1427: Additionally, this file must be owned by the user,
1428: and must not have write permissions for anyone else.
1429: The recommended
1.1 deraadt 1430: permission for most machines is read/write for the user, and not
1431: accessible by others.
1.2 deraadt 1432: .Pp
1.309 jmc 1433: .It Pa ~/.shosts
1.240 jmc 1434: This file is used in exactly the same way as
1435: .Pa .rhosts ,
1436: but allows host-based authentication without permitting login with
1437: rlogin/rsh.
1.272 mcbride 1438: .Pp
1.309 jmc 1439: .It Pa ~/.ssh/
1.272 mcbride 1440: This directory is the default location for all user-specific configuration
1441: and authentication information.
1442: There is no general requirement to keep the entire contents of this directory
1443: secret, but the recommended permissions are read/write/execute for the user,
1444: and not accessible by others.
1.236 jmc 1445: .Pp
1.309 jmc 1446: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1447: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1448: that can be used for logging in as this user.
1.238 jmc 1449: The format of this file is described in the
1450: .Xr sshd 8
1451: manual page.
1452: This file is not highly sensitive, but the recommended
1453: permissions are read/write for the user, and not accessible by others.
1454: .Pp
1.309 jmc 1455: .It Pa ~/.ssh/config
1.238 jmc 1456: This is the per-user configuration file.
1457: The file format and configuration options are described in
1458: .Xr ssh_config 5 .
1459: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1460: read/write for the user, and not writable by others.
1.238 jmc 1461: .Pp
1.309 jmc 1462: .It Pa ~/.ssh/environment
1.239 jmc 1463: Contains additional definitions for environment variables; see
1464: .Sx ENVIRONMENT ,
1.238 jmc 1465: above.
1466: .Pp
1.309 jmc 1467: .It Pa ~/.ssh/identity
1468: .It Pa ~/.ssh/id_dsa
1.310 djm 1469: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1470: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1471: .It Pa ~/.ssh/id_rsa
1.238 jmc 1472: Contains the private key for authentication.
1473: These files
1474: contain sensitive data and should be readable by the user but not
1475: accessible by others (read/write/execute).
1476: .Nm
1477: will simply ignore a private key file if it is accessible by others.
1478: It is possible to specify a passphrase when
1479: generating the key which will be used to encrypt the
1480: sensitive part of this file using 3DES.
1481: .Pp
1.309 jmc 1482: .It Pa ~/.ssh/identity.pub
1483: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1484: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1485: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1486: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1487: Contains the public key for authentication.
1488: These files are not
1489: sensitive and can (but need not) be readable by anyone.
1490: .Pp
1.309 jmc 1491: .It Pa ~/.ssh/known_hosts
1.244 jmc 1492: Contains a list of host keys for all hosts the user has logged into
1493: that are not already in the systemwide list of known host keys.
1.238 jmc 1494: See
1.244 jmc 1495: .Xr sshd 8
1496: for further details of the format of this file.
1.238 jmc 1497: .Pp
1.309 jmc 1498: .It Pa ~/.ssh/rc
1.238 jmc 1499: Commands in this file are executed by
1500: .Nm
1.245 jmc 1501: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1502: started.
1503: See the
1504: .Xr sshd 8
1505: manual page for more information.
1506: .Pp
1.309 jmc 1507: .It Pa /etc/hosts.equiv
1.240 jmc 1508: This file is for host-based authentication (see above).
1509: It should only be writable by root.
1.236 jmc 1510: .Pp
1.309 jmc 1511: .It Pa /etc/shosts.equiv
1.240 jmc 1512: This file is used in exactly the same way as
1513: .Pa hosts.equiv ,
1514: but allows host-based authentication without permitting login with
1515: rlogin/rsh.
1.236 jmc 1516: .Pp
1.238 jmc 1517: .It Pa /etc/ssh/ssh_config
1518: Systemwide configuration file.
1519: The file format and configuration options are described in
1520: .Xr ssh_config 5 .
1521: .Pp
1.309 jmc 1522: .It Pa /etc/ssh/ssh_host_key
1523: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1524: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1525: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1526: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1527: These files contain the private parts of the host keys
1.245 jmc 1528: and are used for host-based authentication.
1529: If protocol version 1 is used,
1.238 jmc 1530: .Nm
1531: must be setuid root, since the host key is readable only by root.
1532: For protocol version 2,
1533: .Nm
1534: uses
1535: .Xr ssh-keysign 8
1.245 jmc 1536: to access the host keys,
1537: eliminating the requirement that
1.238 jmc 1538: .Nm
1.245 jmc 1539: be setuid root when host-based authentication is used.
1.238 jmc 1540: By default
1.2 deraadt 1541: .Nm
1.238 jmc 1542: is not setuid root.
1543: .Pp
1.309 jmc 1544: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1545: Systemwide list of known host keys.
1546: This file should be prepared by the
1547: system administrator to contain the public host keys of all machines in the
1548: organization.
1.244 jmc 1549: It should be world-readable.
1550: See
1.238 jmc 1551: .Xr sshd 8
1.244 jmc 1552: for further details of the format of this file.
1.236 jmc 1553: .Pp
1.309 jmc 1554: .It Pa /etc/ssh/sshrc
1.1 deraadt 1555: Commands in this file are executed by
1.2 deraadt 1556: .Nm
1.245 jmc 1557: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1558: See the
1.2 deraadt 1559: .Xr sshd 8
1.1 deraadt 1560: manual page for more information.
1.58 itojun 1561: .El
1.312 jmc 1562: .Sh EXIT STATUS
1563: .Nm
1564: exits with the exit status of the remote command or with 255
1565: if an error occurred.
1.2 deraadt 1566: .Sh SEE ALSO
1567: .Xr scp 1 ,
1.83 djm 1568: .Xr sftp 1 ,
1.2 deraadt 1569: .Xr ssh-add 1 ,
1570: .Xr ssh-agent 1 ,
1571: .Xr ssh-keygen 1 ,
1.242 jmc 1572: .Xr ssh-keyscan 1 ,
1.250 jmc 1573: .Xr tun 4 ,
1.159 stevesk 1574: .Xr ssh_config 5 ,
1.160 naddy 1575: .Xr ssh-keysign 8 ,
1.87 itojun 1576: .Xr sshd 8
1.329 jmc 1577: .Sh STANDARDS
1.106 markus 1578: .Rs
1.329 jmc 1579: .%A S. Lehtinen
1580: .%A C. Lonvick
1581: .%D January 2006
1.256 jmc 1582: .%R RFC 4250
1.329 jmc 1583: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1584: .Re
1.329 jmc 1585: .Pp
1.256 jmc 1586: .Rs
1.329 jmc 1587: .%A T. Ylonen
1588: .%A C. Lonvick
1589: .%D January 2006
1.256 jmc 1590: .%R RFC 4251
1.329 jmc 1591: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1592: .Re
1.329 jmc 1593: .Pp
1.256 jmc 1594: .Rs
1.329 jmc 1595: .%A T. Ylonen
1596: .%A C. Lonvick
1597: .%D January 2006
1.256 jmc 1598: .%R RFC 4252
1.329 jmc 1599: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1600: .Re
1.329 jmc 1601: .Pp
1.256 jmc 1602: .Rs
1.329 jmc 1603: .%A T. Ylonen
1604: .%A C. Lonvick
1605: .%D January 2006
1.256 jmc 1606: .%R RFC 4253
1.329 jmc 1607: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1608: .Re
1.329 jmc 1609: .Pp
1.256 jmc 1610: .Rs
1.329 jmc 1611: .%A T. Ylonen
1612: .%A C. Lonvick
1613: .%D January 2006
1.256 jmc 1614: .%R RFC 4254
1.329 jmc 1615: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1616: .Re
1.329 jmc 1617: .Pp
1.256 jmc 1618: .Rs
1.329 jmc 1619: .%A J. Schlyter
1620: .%A W. Griffin
1621: .%D January 2006
1.256 jmc 1622: .%R RFC 4255
1.329 jmc 1623: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1624: .Re
1.329 jmc 1625: .Pp
1.256 jmc 1626: .Rs
1.329 jmc 1627: .%A F. Cusack
1628: .%A M. Forssen
1629: .%D January 2006
1.256 jmc 1630: .%R RFC 4256
1.329 jmc 1631: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1632: .Re
1.329 jmc 1633: .Pp
1.256 jmc 1634: .Rs
1.329 jmc 1635: .%A J. Galbraith
1636: .%A P. Remaker
1637: .%D January 2006
1.256 jmc 1638: .%R RFC 4335
1.329 jmc 1639: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1640: .Re
1.329 jmc 1641: .Pp
1.256 jmc 1642: .Rs
1.329 jmc 1643: .%A M. Bellare
1644: .%A T. Kohno
1645: .%A C. Namprempre
1646: .%D January 2006
1.256 jmc 1647: .%R RFC 4344
1.329 jmc 1648: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1649: .Re
1.329 jmc 1650: .Pp
1.256 jmc 1651: .Rs
1.329 jmc 1652: .%A B. Harris
1653: .%D January 2006
1.256 jmc 1654: .%R RFC 4345
1.329 jmc 1655: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1656: .Re
1.329 jmc 1657: .Pp
1.258 djm 1658: .Rs
1.329 jmc 1659: .%A M. Friedl
1660: .%A N. Provos
1661: .%A W. Simpson
1662: .%D March 2006
1.258 djm 1663: .%R RFC 4419
1.329 jmc 1664: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1665: .Re
1.329 jmc 1666: .Pp
1.266 markus 1667: .Rs
1.329 jmc 1668: .%A J. Galbraith
1669: .%A R. Thayer
1670: .%D November 2006
1.266 markus 1671: .%R RFC 4716
1.329 jmc 1672: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1673: .Re
1.329 jmc 1674: .Pp
1.313 djm 1675: .Rs
1.329 jmc 1676: .%A D. Stebila
1677: .%A J. Green
1678: .%D December 2009
1.313 djm 1679: .%R RFC 5656
1.329 jmc 1680: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1681: .Re
1.329 jmc 1682: .Pp
1.274 grunk 1683: .Rs
1684: .%A A. Perrig
1685: .%A D. Song
1686: .%D 1999
1.329 jmc 1687: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1688: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1689: .Re
1.173 jmc 1690: .Sh AUTHORS
1691: OpenSSH is a derivative of the original and free
1692: ssh 1.2.12 release by Tatu Ylonen.
1693: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1694: Theo de Raadt and Dug Song
1695: removed many bugs, re-added newer features and
1696: created OpenSSH.
1697: Markus Friedl contributed the support for SSH
1698: protocol versions 1.5 and 2.0.
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