Annotation of src/usr.bin/ssh/ssh.1, Revision 1.142
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
1.59 deraadt 7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
1.93 deraadt 13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.59 deraadt 16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 25: .\"
1.59 deraadt 26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.142 ! stevesk 37: .\" $OpenBSD: ssh.1,v 1.141 2001/11/08 17:49:53 markus Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSH 1
40: .Os
41: .Sh NAME
42: .Nm ssh
1.96 deraadt 43: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm ssh
46: .Op Fl l Ar login_name
1.130 stevesk 47: .Ar hostname | user@hostname
1.2 deraadt 48: .Op Ar command
49: .Pp
50: .Nm ssh
1.86 jakob 51: .Op Fl afgknqstvxACNPTX1246
1.108 markus 52: .Op Fl b Ar bind_address
1.51 markus 53: .Op Fl c Ar cipher_spec
1.2 deraadt 54: .Op Fl e Ar escape_char
55: .Op Fl i Ar identity_file
56: .Op Fl l Ar login_name
1.91 markus 57: .Op Fl m Ar mac_spec
1.2 deraadt 58: .Op Fl o Ar option
59: .Op Fl p Ar port
1.132 markus 60: .Op Fl F Ar configfile
1.12 aaron 61: .Oo Fl L Xo
62: .Sm off
1.33 markus 63: .Ar port :
1.12 aaron 64: .Ar host :
65: .Ar hostport
66: .Sm on
67: .Xc
68: .Oc
69: .Oo Fl R Xo
70: .Sm off
1.33 markus 71: .Ar port :
1.12 aaron 72: .Ar host :
73: .Ar hostport
74: .Sm on
75: .Xc
76: .Oc
1.135 naddy 77: .Op Fl D Ar port
1.130 stevesk 78: .Ar hostname | user@hostname
1.2 deraadt 79: .Op Ar command
1.44 aaron 80: .Sh DESCRIPTION
1.2 deraadt 81: .Nm
1.96 deraadt 82: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 83: executing commands on a remote machine.
84: It is intended to replace
1.1 deraadt 85: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 86: two untrusted hosts over an insecure network.
87: X11 connections and
1.1 deraadt 88: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 89: .Pp
90: .Nm
1.44 aaron 91: connects and logs into the specified
1.2 deraadt 92: .Ar hostname .
1.1 deraadt 93: The user must prove
1.49 markus 94: his/her identity to the remote machine using one of several methods
95: depending on the protocol version used:
96: .Pp
97: .Ss SSH protocol version 1
1.2 deraadt 98: .Pp
1.1 deraadt 99: First, if the machine the user logs in from is listed in
1.2 deraadt 100: .Pa /etc/hosts.equiv
1.1 deraadt 101: or
1.2 deraadt 102: .Pa /etc/shosts.equiv
1.1 deraadt 103: on the remote machine, and the user names are
104: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 105: Second, if
1.2 deraadt 106: .Pa \&.rhosts
1.1 deraadt 107: or
1.2 deraadt 108: .Pa \&.shosts
1.1 deraadt 109: exists in the user's home directory on the
110: remote machine and contains a line containing the name of the client
111: machine and the name of the user on that machine, the user is
1.40 aaron 112: permitted to log in.
113: This form of authentication alone is normally not
1.1 deraadt 114: allowed by the server because it is not secure.
1.2 deraadt 115: .Pp
1.107 markus 116: The second authentication method is the
1.2 deraadt 117: .Pa rhosts
1.1 deraadt 118: or
1.2 deraadt 119: .Pa hosts.equiv
1.40 aaron 120: method combined with RSA-based host authentication.
121: It means that if the login would be permitted by
1.49 markus 122: .Pa $HOME/.rhosts ,
123: .Pa $HOME/.shosts ,
1.2 deraadt 124: .Pa /etc/hosts.equiv ,
1.1 deraadt 125: or
1.2 deraadt 126: .Pa /etc/shosts.equiv ,
1.11 deraadt 127: and if additionally the server can verify the client's
1.44 aaron 128: host key (see
1.2 deraadt 129: .Pa /etc/ssh_known_hosts
1.23 markus 130: and
131: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 132: in the
1.2 deraadt 133: .Sx FILES
1.40 aaron 134: section), only then login is permitted.
135: This authentication method closes security holes due to IP
136: spoofing, DNS spoofing and routing spoofing.
137: [Note to the administrator:
1.2 deraadt 138: .Pa /etc/hosts.equiv ,
1.49 markus 139: .Pa $HOME/.rhosts ,
1.1 deraadt 140: and the rlogin/rsh protocol in general, are inherently insecure and should be
141: disabled if security is desired.]
1.2 deraadt 142: .Pp
1.44 aaron 143: As a third authentication method,
1.2 deraadt 144: .Nm
1.1 deraadt 145: supports RSA based authentication.
146: The scheme is based on public-key cryptography: there are cryptosystems
147: where encryption and decryption are done using separate keys, and it
148: is not possible to derive the decryption key from the encryption key.
1.40 aaron 149: RSA is one such system.
1.44 aaron 150: The idea is that each user creates a public/private
1.40 aaron 151: key pair for authentication purposes.
152: The server knows the public key, and only the user knows the private key.
1.44 aaron 153: The file
1.2 deraadt 154: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 155: lists the public keys that are permitted for logging
1.40 aaron 156: in.
157: When the user logs in, the
1.2 deraadt 158: .Nm
1.1 deraadt 159: program tells the server which key pair it would like to use for
1.40 aaron 160: authentication.
161: The server checks if this key is permitted, and if
1.1 deraadt 162: so, sends the user (actually the
1.2 deraadt 163: .Nm
1.1 deraadt 164: program running on behalf of the user) a challenge, a random number,
1.40 aaron 165: encrypted by the user's public key.
166: The challenge can only be
167: decrypted using the proper private key.
168: The user's client then decrypts the
1.1 deraadt 169: challenge using the private key, proving that he/she knows the private
170: key but without disclosing it to the server.
1.2 deraadt 171: .Pp
172: .Nm
1.40 aaron 173: implements the RSA authentication protocol automatically.
174: The user creates his/her RSA key pair by running
1.2 deraadt 175: .Xr ssh-keygen 1 .
1.44 aaron 176: This stores the private key in
1.49 markus 177: .Pa $HOME/.ssh/identity
1.1 deraadt 178: and the public key in
1.49 markus 179: .Pa $HOME/.ssh/identity.pub
1.40 aaron 180: in the user's home directory.
181: The user should then copy the
1.2 deraadt 182: .Pa identity.pub
1.44 aaron 183: to
1.49 markus 184: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 185: in his/her home directory on the remote machine (the
1.2 deraadt 186: .Pa authorized_keys
1.44 aaron 187: file corresponds to the conventional
1.49 markus 188: .Pa $HOME/.rhosts
1.1 deraadt 189: file, and has one key
1.40 aaron 190: per line, though the lines can be very long).
191: After this, the user can log in without giving the password.
192: RSA authentication is much
1.1 deraadt 193: more secure than rhosts authentication.
1.2 deraadt 194: .Pp
1.1 deraadt 195: The most convenient way to use RSA authentication may be with an
1.40 aaron 196: authentication agent.
197: See
1.2 deraadt 198: .Xr ssh-agent 1
1.1 deraadt 199: for more information.
1.2 deraadt 200: .Pp
1.44 aaron 201: If other authentication methods fail,
1.2 deraadt 202: .Nm
1.40 aaron 203: prompts the user for a password.
204: The password is sent to the remote
1.1 deraadt 205: host for checking; however, since all communications are encrypted,
206: the password cannot be seen by someone listening on the network.
1.2 deraadt 207: .Pp
1.49 markus 208: .Ss SSH protocol version 2
209: .Pp
210: When a user connects using the protocol version 2
1.107 markus 211: different authentication methods are available.
212: Using the default values for
213: .Cm PreferredAuthentications ,
1.123 markus 214: the client will try to authenticate first using the hostbased method;
215: if this method fails public key authentication is attempted,
216: and finally if this method fails keyboard-interactive and
217: password authentication are tried.
1.49 markus 218: .Pp
219: The public key method is similar to RSA authentication described
1.107 markus 220: in the previous section and allows the RSA or DSA algorithm to be used:
1.102 itojun 221: The client uses his private key,
1.49 markus 222: .Pa $HOME/.ssh/id_dsa
1.102 itojun 223: or
224: .Pa $HOME/.ssh/id_rsa ,
1.49 markus 225: to sign the session identifier and sends the result to the server.
226: The server checks whether the matching public key is listed in
1.115 markus 227: .Pa $HOME/.ssh/authorized_keys
1.49 markus 228: and grants access if both the key is found and the signature is correct.
229: The session identifier is derived from a shared Diffie-Hellman value
230: and is only known to the client and the server.
231: .Pp
232: If public key authentication fails or is not available a password
233: can be sent encrypted to the remote host for proving the user's identity.
1.107 markus 234: .Pp
235: Additionally,
236: .Nm
237: supports hostbased or challenge response authentication.
1.49 markus 238: .Pp
239: Protocol 2 provides additional mechanisms for confidentiality
1.51 markus 240: (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
1.94 deraadt 241: and integrity (hmac-md5, hmac-sha1).
1.49 markus 242: Note that protocol 1 lacks a strong mechanism for ensuring the
243: integrity of the connection.
244: .Pp
245: .Ss Login session and remote execution
246: .Pp
1.1 deraadt 247: When the user's identity has been accepted by the server, the server
248: either executes the given command, or logs into the machine and gives
1.40 aaron 249: the user a normal shell on the remote machine.
250: All communication with
1.1 deraadt 251: the remote command or shell will be automatically encrypted.
1.2 deraadt 252: .Pp
1.1 deraadt 253: If a pseudo-terminal has been allocated (normal login session), the
1.104 djm 254: user may use the escape characters noted below.
1.2 deraadt 255: .Pp
1.1 deraadt 256: If no pseudo tty has been allocated, the
257: session is transparent and can be used to reliably transfer binary
1.40 aaron 258: data.
259: On most systems, setting the escape character to
1.2 deraadt 260: .Dq none
261: will also make the session transparent even if a tty is used.
262: .Pp
1.71 djm 263: The session terminates when the command or shell on the remote
1.92 markus 264: machine exits and all X11 and TCP/IP connections have been closed.
1.1 deraadt 265: The exit status of the remote program is returned as the exit status
266: of
1.2 deraadt 267: .Nm ssh .
1.104 djm 268: .Pp
269: .Ss Escape Characters
270: .Pp
271: When a pseudo terminal has been requested, ssh supports a number of functions
1.117 itojun 272: through the use of an escape character.
1.104 djm 273: .Pp
274: A single tilde character can be sent as
275: .Ic ~~
1.119 stevesk 276: or by following the tilde by a character other than those described below.
1.104 djm 277: The escape character must always follow a newline to be interpreted as
278: special.
279: The escape character can be changed in configuration files using the
280: .Cm EscapeChar
1.117 itojun 281: configuration directive or on the command line by the
1.104 djm 282: .Fl e
283: option.
284: .Pp
285: The supported escapes (assuming the default
286: .Ql ~ )
287: are:
288: .Bl -tag -width Ds
289: .It Cm ~.
290: Disconnect
291: .It Cm ~^Z
292: Background ssh
293: .It Cm ~#
294: List forwarded connections
295: .It Cm ~&
296: Background ssh at logout when waiting for forwarded connection / X11 sessions
1.140 markus 297: to terminate
1.104 djm 298: .It Cm ~?
299: Display a list of escape characters
300: .It Cm ~R
1.105 djm 301: Request rekeying of the connection (only useful for SSH protocol version 2
302: and if the peer supports it)
1.104 djm 303: .El
1.2 deraadt 304: .Pp
1.49 markus 305: .Ss X11 and TCP forwarding
306: .Pp
1.110 deraadt 307: If the
308: .Cm ForwardX11
309: variable is set to
310: .Dq yes
311: (or, see the description of the
312: .Fl X
313: and
314: .Fl x
315: options described later)
316: and the user is using X11 (the
1.2 deraadt 317: .Ev DISPLAY
1.1 deraadt 318: environment variable is set), the connection to the X11 display is
319: automatically forwarded to the remote side in such a way that any X11
320: programs started from the shell (or command) will go through the
321: encrypted channel, and the connection to the real X server will be made
1.40 aaron 322: from the local machine.
323: The user should not manually set
1.2 deraadt 324: .Ev DISPLAY .
1.1 deraadt 325: Forwarding of X11 connections can be
326: configured on the command line or in configuration files.
1.2 deraadt 327: .Pp
328: The
1.44 aaron 329: .Ev DISPLAY
1.2 deraadt 330: value set by
331: .Nm
1.1 deraadt 332: will point to the server machine, but with a display number greater
1.40 aaron 333: than zero.
334: This is normal, and happens because
1.2 deraadt 335: .Nm
336: creates a
337: .Dq proxy
338: X server on the server machine for forwarding the
1.1 deraadt 339: connections over the encrypted channel.
1.2 deraadt 340: .Pp
341: .Nm
1.1 deraadt 342: will also automatically set up Xauthority data on the server machine.
343: For this purpose, it will generate a random authorization cookie,
344: store it in Xauthority on the server, and verify that any forwarded
345: connections carry this cookie and replace it by the real cookie when
1.40 aaron 346: the connection is opened.
347: The real authentication cookie is never
1.1 deraadt 348: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 349: .Pp
1.1 deraadt 350: If the user is using an authentication agent, the connection to the agent
351: is automatically forwarded to the remote side unless disabled on
1.120 stevesk 352: the command line or in a configuration file.
1.2 deraadt 353: .Pp
1.1 deraadt 354: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.120 stevesk 355: be specified either on the command line or in a configuration file.
1.40 aaron 356: One possible application of TCP/IP forwarding is a secure connection to an
1.92 markus 357: electronic purse; another is going through firewalls.
1.2 deraadt 358: .Pp
1.49 markus 359: .Ss Server authentication
360: .Pp
1.2 deraadt 361: .Nm
1.49 markus 362: automatically maintains and checks a database containing
1.40 aaron 363: identifications for all hosts it has ever been used with.
1.116 markus 364: Host keys are stored in
1.49 markus 365: .Pa $HOME/.ssh/known_hosts
1.40 aaron 366: in the user's home directory.
1.116 markus 367: Additionally, the file
1.2 deraadt 368: .Pa /etc/ssh_known_hosts
1.116 markus 369: is automatically checked for known hosts.
1.40 aaron 370: Any new hosts are automatically added to the user's file.
371: If a host's identification
1.1 deraadt 372: ever changes,
1.2 deraadt 373: .Nm
1.1 deraadt 374: warns about this and disables password authentication to prevent a
1.40 aaron 375: trojan horse from getting the user's password.
376: Another purpose of
1.1 deraadt 377: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 378: otherwise be used to circumvent the encryption.
379: The
1.2 deraadt 380: .Cm StrictHostKeyChecking
1.1 deraadt 381: option (see below) can be used to prevent logins to machines whose
382: host key is not known or has changed.
1.65 aaron 383: .Pp
384: The options are as follows:
1.2 deraadt 385: .Bl -tag -width Ds
1.4 dugsong 386: .It Fl a
1.42 aaron 387: Disables forwarding of the authentication agent connection.
1.54 markus 388: .It Fl A
389: Enables forwarding of the authentication agent connection.
390: This can also be specified on a per-host basis in a configuration file.
1.108 markus 391: .It Fl b Ar bind_address
392: Specify the interface to transmit from on machines with multiple
393: interfaces or aliased addresses.
1.131 stevesk 394: .It Fl c Ar blowfish|3des|des
1.44 aaron 395: Selects the cipher to use for encrypting the session.
1.2 deraadt 396: .Ar 3des
1.40 aaron 397: is used by default.
1.44 aaron 398: It is believed to be secure.
1.5 deraadt 399: .Ar 3des
400: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
401: .Ar blowfish
402: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 403: .Ar 3des .
1.131 stevesk 404: .Ar des
405: is only supported in the
406: .Nm
407: client for interoperability with legacy protocol 1 implementations
408: that do not support the
409: .Ar 3des
410: cipher. Its use is strongly discouraged due to cryptographic
411: weaknesses.
1.90 markus 412: .It Fl c Ar cipher_spec
1.51 markus 413: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 414: be specified in order of preference.
1.90 markus 415: See
416: .Cm Ciphers
417: for more information.
1.2 deraadt 418: .It Fl e Ar ch|^ch|none
419: Sets the escape character for sessions with a pty (default:
420: .Ql ~ ) .
1.40 aaron 421: The escape character is only recognized at the beginning of a line.
422: The escape character followed by a dot
1.2 deraadt 423: .Pq Ql \&.
424: closes the connection, followed
1.1 deraadt 425: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 426: escape character once.
427: Setting the character to
1.2 deraadt 428: .Dq none
429: disables any escapes and makes the session fully transparent.
430: .It Fl f
431: Requests
432: .Nm
1.40 aaron 433: to go to background just before command execution.
434: This is useful if
1.2 deraadt 435: .Nm
436: is going to ask for passwords or passphrases, but the user
1.40 aaron 437: wants it in the background.
1.44 aaron 438: This implies
1.2 deraadt 439: .Fl n .
1.1 deraadt 440: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 441: something like
442: .Ic ssh -f host xterm .
1.34 markus 443: .It Fl g
444: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 445: .It Fl i Ar identity_file
1.44 aaron 446: Selects the file from which the identity (private key) for
1.68 markus 447: RSA or DSA authentication is read.
1.44 aaron 448: Default is
1.49 markus 449: .Pa $HOME/.ssh/identity
1.40 aaron 450: in the user's home directory.
451: Identity files may also be specified on
452: a per-host basis in the configuration file.
453: It is possible to have multiple
1.2 deraadt 454: .Fl i
455: options (and multiple identities specified in
1.1 deraadt 456: configuration files).
1.125 jakob 457: .It Fl I Ar smartcard_device
458: Specifies which smartcard device to use. The argument is
459: the device
460: .Nm
461: should use to communicate with a smartcard used for storing the user's
462: private RSA key.
1.2 deraadt 463: .It Fl k
1.42 aaron 464: Disables forwarding of Kerberos tickets and AFS tokens.
465: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 466: .It Fl l Ar login_name
1.40 aaron 467: Specifies the user to log in as on the remote machine.
468: This also may be specified on a per-host basis in the configuration file.
1.91 markus 469: .It Fl m Ar mac_spec
470: Additionally, for protocol version 2 a comma-separated list of MAC
471: (message authentication code) algorithms can
472: be specified in order of preference.
473: See the
474: .Cm MACs
475: keyword for more information.
1.2 deraadt 476: .It Fl n
477: Redirects stdin from
478: .Pa /dev/null
479: (actually, prevents reading from stdin).
1.1 deraadt 480: This must be used when
1.2 deraadt 481: .Nm
1.40 aaron 482: is run in the background.
483: A common trick is to use this to run X11 programs on a remote machine.
484: For example,
1.2 deraadt 485: .Ic ssh -n shadows.cs.hut.fi emacs &
486: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 487: connection will be automatically forwarded over an encrypted channel.
488: The
1.2 deraadt 489: .Nm
1.1 deraadt 490: program will be put in the background.
491: (This does not work if
1.2 deraadt 492: .Nm
493: needs to ask for a password or passphrase; see also the
494: .Fl f
495: option.)
1.53 markus 496: .It Fl N
497: Do not execute a remote command.
1.137 deraadt 498: This is useful for just forwarding ports
1.53 markus 499: (protocol version 2 only).
1.2 deraadt 500: .It Fl o Ar option
1.127 stevesk 501: Can be used to give options in the format used in the configuration file.
1.1 deraadt 502: This is useful for specifying options for which there is no separate
1.40 aaron 503: command-line flag.
1.2 deraadt 504: .It Fl p Ar port
1.40 aaron 505: Port to connect to on the remote host.
506: This can be specified on a
1.1 deraadt 507: per-host basis in the configuration file.
1.16 markus 508: .It Fl P
509: Use a non-privileged port for outgoing connections.
1.137 deraadt 510: This can be used if a firewall does
1.16 markus 511: not permit connections from privileged ports.
1.30 provos 512: Note that this option turns off
1.16 markus 513: .Cm RhostsAuthentication
514: and
1.72 markus 515: .Cm RhostsRSAAuthentication
516: for older servers.
1.2 deraadt 517: .It Fl q
1.40 aaron 518: Quiet mode.
519: Causes all warning and diagnostic messages to be suppressed.
520: Only fatal errors are displayed.
1.80 djm 521: .It Fl s
1.117 itojun 522: May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use
523: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 524: subsystem is specified as the remote command.
1.2 deraadt 525: .It Fl t
1.40 aaron 526: Force pseudo-tty allocation.
1.43 brad 527: This can be used to execute arbitrary
1.40 aaron 528: screen-based programs on a remote machine, which can be very useful,
529: e.g., when implementing menu services.
1.73 markus 530: Multiple
531: .Fl t
532: options force tty allocation, even if
533: .Nm
534: has no local tty.
1.53 markus 535: .It Fl T
1.69 markus 536: Disable pseudo-tty allocation.
1.2 deraadt 537: .It Fl v
1.40 aaron 538: Verbose mode.
539: Causes
1.2 deraadt 540: .Nm
1.40 aaron 541: to print debugging messages about its progress.
542: This is helpful in
1.1 deraadt 543: debugging connection, authentication, and configuration problems.
1.73 markus 544: Multiple
545: .Fl v
546: options increases the verbosity.
1.61 aaron 547: Maximum is 3.
1.2 deraadt 548: .It Fl x
1.40 aaron 549: Disables X11 forwarding.
1.2 deraadt 550: .It Fl X
1.1 deraadt 551: Enables X11 forwarding.
1.54 markus 552: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 553: .It Fl C
1.1 deraadt 554: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 555: data for forwarded X11 and TCP/IP connections).
556: The compression algorithm is the same used by
1.34 markus 557: .Xr gzip 1 ,
558: and the
1.2 deraadt 559: .Dq level
560: can be controlled by the
561: .Cm CompressionLevel
1.40 aaron 562: option (see below).
563: Compression is desirable on modem lines and other
1.1 deraadt 564: slow connections, but will only slow down things on fast networks.
565: The default value can be set on a host-by-host basis in the
566: configuration files; see the
1.121 pvalchev 567: .Cm Compression
1.1 deraadt 568: option below.
1.132 markus 569: .It Fl F Ar configfile
570: Specifies an alternative per-user configuration file.
571: If a configuration file is given on the command line,
572: the system-wide configuration file
573: .Pq Pa /etc/ssh_config
574: will be ignored.
575: The default for the per-user configuration file is
576: .Pa $HOME/.ssh/config .
1.2 deraadt 577: .It Fl L Ar port:host:hostport
1.1 deraadt 578: Specifies that the given port on the local (client) host is to be
1.40 aaron 579: forwarded to the given host and port on the remote side.
580: This works by allocating a socket to listen to
1.2 deraadt 581: .Ar port
1.1 deraadt 582: on the local side, and whenever a connection is made to this port, the
583: connection is forwarded over the secure channel, and a connection is
584: made to
1.32 markus 585: .Ar host
586: port
587: .Ar hostport
1.40 aaron 588: from the remote machine.
589: Port forwardings can also be specified in the configuration file.
590: Only root can forward privileged ports.
1.32 markus 591: IPv6 addresses can be specified with an alternative syntax:
592: .Ar port/host/hostport
1.2 deraadt 593: .It Fl R Ar port:host:hostport
1.1 deraadt 594: Specifies that the given port on the remote (server) host is to be
1.40 aaron 595: forwarded to the given host and port on the local side.
596: This works by allocating a socket to listen to
1.2 deraadt 597: .Ar port
1.1 deraadt 598: on the remote side, and whenever a connection is made to this port, the
599: connection is forwarded over the secure channel, and a connection is
600: made to
1.32 markus 601: .Ar host
602: port
603: .Ar hostport
1.40 aaron 604: from the local machine.
605: Port forwardings can also be specified in the configuration file.
606: Privileged ports can be forwarded only when
1.1 deraadt 607: logging in as root on the remote machine.
1.107 markus 608: IPv6 addresses can be specified with an alternative syntax:
609: .Ar port/host/hostport
1.133 stevesk 610: .It Fl D Ar port
611: Specifies a local
612: .Dq dynamic
613: application-level port forwarding.
614: This works by allocating a socket to listen to
615: .Ar port
616: on the local side, and whenever a connection is made to this port, the
617: connection is forwarded over the secure channel, and the application
618: protocol is then used to determine where to connect to from the
619: remote machine. Currently the SOCKS4 protocol is supported, and
620: .Nm
621: will act as a SOCKS4 server.
622: Only root can forward privileged ports.
623: Dynamic port forwardings can also be specified in the configuration file.
1.85 jakob 624: .It Fl 1
625: Forces
626: .Nm
627: to try protocol version 1 only.
1.46 markus 628: .It Fl 2
629: Forces
630: .Nm
1.50 markus 631: to try protocol version 2 only.
1.32 markus 632: .It Fl 4
633: Forces
634: .Nm
635: to use IPv4 addresses only.
636: .It Fl 6
637: Forces
638: .Nm
639: to use IPv6 addresses only.
1.2 deraadt 640: .El
641: .Sh CONFIGURATION FILES
642: .Nm
1.127 stevesk 643: obtains configuration data from the following sources in
644: the following order:
1.1 deraadt 645: command line options, user's configuration file
1.2 deraadt 646: .Pq Pa $HOME/.ssh/config ,
647: and system-wide configuration file
648: .Pq Pa /etc/ssh_config .
649: For each parameter, the first obtained value
1.40 aaron 650: will be used.
651: The configuration files contain sections bracketed by
652: .Dq Host
653: specifications, and that section is only applied for hosts that
654: match one of the patterns given in the specification.
655: The matched host name is the one given on the command line.
1.2 deraadt 656: .Pp
1.1 deraadt 657: Since the first obtained value for each parameter is used, more
658: host-specific declarations should be given near the beginning of the
659: file, and general defaults at the end.
1.2 deraadt 660: .Pp
1.1 deraadt 661: The configuration file has the following format:
1.2 deraadt 662: .Pp
663: Empty lines and lines starting with
664: .Ql #
665: are comments.
666: .Pp
667: Otherwise a line is of the format
668: .Dq keyword arguments .
1.127 stevesk 669: Configuration options may be separated by whitespace or
670: optional whitespace and exactly one
671: .Ql = ;
672: the latter format is useful to avoid the need to quote whitespace
673: when specifying configuration options using the
674: .Nm ssh ,
675: .Nm scp
676: and
677: .Nm sftp
678: .Fl o
679: option.
680: .Pp
1.2 deraadt 681: The possible
1.128 stevesk 682: keywords and their meanings are as follows (note that
683: keywords are case-insensitive and arguments are case-sensitive):
1.2 deraadt 684: .Bl -tag -width Ds
685: .It Cm Host
1.1 deraadt 686: Restricts the following declarations (up to the next
1.2 deraadt 687: .Cm Host
1.1 deraadt 688: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 689: given after the keyword.
690: .Ql \&*
691: and
692: .Ql ?
693: can be used as wildcards in the
1.40 aaron 694: patterns.
695: A single
1.2 deraadt 696: .Ql \&*
697: as a pattern can be used to provide global
1.40 aaron 698: defaults for all hosts.
699: The host is the
1.2 deraadt 700: .Ar hostname
1.1 deraadt 701: argument given on the command line (i.e., the name is not converted to
702: a canonicalized host name before matching).
1.2 deraadt 703: .It Cm AFSTokenPassing
1.42 aaron 704: Specifies whether to pass AFS tokens to remote host.
705: The argument to this keyword must be
1.2 deraadt 706: .Dq yes
707: or
708: .Dq no .
1.107 markus 709: This option applies to protocol version 1 only.
1.2 deraadt 710: .It Cm BatchMode
711: If set to
712: .Dq yes ,
1.40 aaron 713: passphrase/password querying will be disabled.
1.137 deraadt 714: This option is useful in scripts and other batch jobs where no user
715: is present to supply the password.
1.40 aaron 716: The argument must be
1.2 deraadt 717: .Dq yes
718: or
719: .Dq no .
1.100 stevesk 720: The default is
721: .Dq no .
1.108 markus 722: .It Cm BindAddress
723: Specify the interface to transmit from on machines with multiple
724: interfaces or aliased addresses.
725: Note that this option does not work if
726: .Cm UsePrivilegedPort
727: is set to
728: .Dq yes .
1.34 markus 729: .It Cm CheckHostIP
730: If this flag is set to
731: .Dq yes ,
1.100 stevesk 732: ssh will additionally check the host IP address in the
1.34 markus 733: .Pa known_hosts
1.42 aaron 734: file.
735: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 736: If the option is set to
737: .Dq no ,
738: the check will not be executed.
1.100 stevesk 739: The default is
740: .Dq yes .
1.2 deraadt 741: .It Cm Cipher
1.62 markus 742: Specifies the cipher to use for encrypting the session
1.64 markus 743: in protocol version 1.
1.40 aaron 744: Currently,
1.131 stevesk 745: .Dq blowfish ,
746: .Dq 3des ,
1.1 deraadt 747: and
1.131 stevesk 748: .Dq des
1.40 aaron 749: are supported.
1.131 stevesk 750: .Ar des
751: is only supported in the
752: .Nm
753: client for interoperability with legacy protocol 1 implementations
754: that do not support the
755: .Ar 3des
756: cipher. Its use is strongly discouraged due to cryptographic
757: weaknesses.
1.40 aaron 758: The default is
1.2 deraadt 759: .Dq 3des .
1.45 markus 760: .It Cm Ciphers
761: Specifies the ciphers allowed for protocol version 2
762: in order of preference.
763: Multiple ciphers must be comma-separated.
764: The default is
1.88 provos 765: .Pp
766: .Bd -literal
1.94 deraadt 767: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.107 markus 768: aes192-cbc,aes256-cbc''
1.88 provos 769: .Ed
1.138 stevesk 770: .It Cm ClearAllForwardings
771: Specifies that all local, remote and dynamic port forwardings
772: specified in the configuration files or on the command line be
773: cleared. This option is primarily useful when used from the
774: .Nm
775: command line to clear port forwardings set in
776: configuration files, and is automatically set by
777: .Xr scp 1
778: and
779: .Xr sftp 1 .
780: The argument must be
781: .Dq yes
782: or
783: .Dq no .
784: The default is
785: .Dq no .
1.2 deraadt 786: .It Cm Compression
1.40 aaron 787: Specifies whether to use compression.
788: The argument must be
1.2 deraadt 789: .Dq yes
790: or
791: .Dq no .
1.100 stevesk 792: The default is
793: .Dq no .
1.2 deraadt 794: .It Cm CompressionLevel
1.100 stevesk 795: Specifies the compression level to use if compression is enabled.
1.40 aaron 796: The argument must be an integer from 1 (fast) to 9 (slow, best).
797: The default level is 6, which is good for most applications.
798: The meaning of the values is the same as in
1.34 markus 799: .Xr gzip 1 .
1.107 markus 800: Note that this option applies to protocol version 1 only.
1.2 deraadt 801: .It Cm ConnectionAttempts
1.1 deraadt 802: Specifies the number of tries (one per second) to make before falling
1.40 aaron 803: back to rsh or exiting.
804: The argument must be an integer.
805: This may be useful in scripts if the connection sometimes fails.
1.124 markus 806: The default is 1.
1.133 stevesk 807: .It Cm DynamicForward
808: Specifies that a TCP/IP port on the local machine be forwarded
809: over the secure channel, and the application
810: protocol is then used to determine where to connect to from the
811: remote machine. The argument must be a port number.
812: Currently the SOCKS4 protocol is supported, and
813: .Nm
814: will act as a SOCKS4 server.
815: Multiple forwardings may be specified, and
816: additional forwardings can be given on the command line. Only
817: the superuser can forward privileged ports.
1.2 deraadt 818: .It Cm EscapeChar
819: Sets the escape character (default:
820: .Ql ~ ) .
821: The escape character can also
1.40 aaron 822: be set on the command line.
823: The argument should be a single character,
1.2 deraadt 824: .Ql ^
825: followed by a letter, or
826: .Dq none
827: to disable the escape
1.1 deraadt 828: character entirely (making the connection transparent for binary
829: data).
1.44 aaron 830: .It Cm FallBackToRsh
1.1 deraadt 831: Specifies that if connecting via
1.2 deraadt 832: .Nm
1.1 deraadt 833: fails due to a connection refused error (there is no
1.2 deraadt 834: .Xr sshd 8
1.44 aaron 835: listening on the remote host),
1.2 deraadt 836: .Xr rsh 1
1.1 deraadt 837: should automatically be used instead (after a suitable warning about
1.40 aaron 838: the session being unencrypted).
839: The argument must be
1.2 deraadt 840: .Dq yes
841: or
842: .Dq no .
1.100 stevesk 843: The default is
844: .Dq no .
1.2 deraadt 845: .It Cm ForwardAgent
1.1 deraadt 846: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 847: will be forwarded to the remote machine.
848: The argument must be
1.2 deraadt 849: .Dq yes
850: or
1.54 markus 851: .Dq no .
852: The default is
1.2 deraadt 853: .Dq no .
854: .It Cm ForwardX11
1.1 deraadt 855: Specifies whether X11 connections will be automatically redirected
1.44 aaron 856: over the secure channel and
1.2 deraadt 857: .Ev DISPLAY
1.40 aaron 858: set.
1.44 aaron 859: The argument must be
1.2 deraadt 860: .Dq yes
861: or
1.38 markus 862: .Dq no .
863: The default is
1.3 deraadt 864: .Dq no .
865: .It Cm GatewayPorts
866: Specifies whether remote hosts are allowed to connect to local
867: forwarded ports.
1.134 stevesk 868: By default,
869: .Nm
870: binds local port forwardings to the loopback addresss. This
871: prevents other remote hosts from connecting to forwarded ports.
872: .Cm GatewayPorts
873: can be used to specify that
874: .Nm
875: should bind local port forwardings to the wildcard address,
876: thus allowing remote hosts to connect to forwarded ports.
1.3 deraadt 877: The argument must be
878: .Dq yes
879: or
880: .Dq no .
881: The default is
1.2 deraadt 882: .Dq no .
883: .It Cm GlobalKnownHostsFile
1.116 markus 884: Specifies a file to use for the global
1.95 stevesk 885: host key database instead of
1.2 deraadt 886: .Pa /etc/ssh_known_hosts .
1.107 markus 887: .It Cm HostbasedAuthentication
888: Specifies whether to try rhosts based authentication with public key
889: authentication.
890: The argument must be
891: .Dq yes
892: or
893: .Dq no .
894: The default is
1.112 markus 895: .Dq no .
1.107 markus 896: This option applies to protocol version 2 only and
897: is similar to
898: .Cm RhostsRSAAuthentication .
899: .It Cm HostKeyAlgorithms
1.109 stevesk 900: Specifies the protocol version 2 host key algorithms
1.107 markus 901: that the client wants to use in order of preference.
902: The default for this option is:
903: .Dq ssh-rsa,ssh-dss
1.74 markus 904: .It Cm HostKeyAlias
905: Specifies an alias that should be used instead of the
906: real host name when looking up or saving the host key
1.107 markus 907: in the host key database files.
1.82 stevesk 908: This option is useful for tunneling ssh connections
1.137 deraadt 909: or for multiple servers running on a single host.
1.2 deraadt 910: .It Cm HostName
1.40 aaron 911: Specifies the real host name to log into.
912: This can be used to specify nicknames or abbreviations for hosts.
913: Default is the name given on the command line.
914: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 915: .Cm HostName
1.1 deraadt 916: specifications).
1.2 deraadt 917: .It Cm IdentityFile
1.107 markus 918: Specifies the file from which the user's RSA or DSA authentication identity
1.2 deraadt 919: is read (default
1.49 markus 920: .Pa $HOME/.ssh/identity
1.2 deraadt 921: in the user's home directory).
1.1 deraadt 922: Additionally, any identities represented by the authentication agent
1.40 aaron 923: will be used for authentication.
924: The file name may use the tilde
925: syntax to refer to a user's home directory.
926: It is possible to have
1.1 deraadt 927: multiple identity files specified in configuration files; all these
928: identities will be tried in sequence.
1.2 deraadt 929: .It Cm KeepAlive
1.1 deraadt 930: Specifies whether the system should send keepalive messages to the
1.40 aaron 931: other side.
932: If they are sent, death of the connection or crash of one
933: of the machines will be properly noticed.
934: However, this means that
1.1 deraadt 935: connections will die if the route is down temporarily, and some people
1.41 aaron 936: find it annoying.
1.2 deraadt 937: .Pp
938: The default is
939: .Dq yes
940: (to send keepalives), and the client will notice
1.40 aaron 941: if the network goes down or the remote host dies.
942: This is important in scripts, and many users want it too.
1.2 deraadt 943: .Pp
944: To disable keepalives, the value should be set to
945: .Dq no
946: in both the server and the client configuration files.
947: .It Cm KerberosAuthentication
1.42 aaron 948: Specifies whether Kerberos authentication will be used.
949: The argument to this keyword must be
1.4 dugsong 950: .Dq yes
951: or
952: .Dq no .
1.2 deraadt 953: .It Cm KerberosTgtPassing
1.42 aaron 954: Specifies whether a Kerberos TGT will be forwarded to the server.
955: This will only work if the Kerberos server is actually an AFS kaserver.
956: The argument to this keyword must be
1.4 dugsong 957: .Dq yes
958: or
959: .Dq no .
1.2 deraadt 960: .It Cm LocalForward
1.1 deraadt 961: Specifies that a TCP/IP port on the local machine be forwarded over
1.136 stevesk 962: the secure channel to the specified host and port from the remote machine.
1.40 aaron 963: The first argument must be a port number, and the second must be
1.136 stevesk 964: .Ar host:port .
965: IPv6 addresses can be specified with an alternative syntax:
966: .Ar host/port .
1.40 aaron 967: Multiple forwardings may be specified, and additional
968: forwardings can be given on the command line.
969: Only the superuser can forward privileged ports.
1.24 markus 970: .It Cm LogLevel
971: Gives the verbosity level that is used when logging messages from
972: .Nm ssh .
973: The possible values are:
1.142 ! stevesk 974: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
! 975: The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
! 976: and DEBUG3 each specify higher levels of verbose output.
1.91 markus 977: .It Cm MACs
1.117 itojun 978: Specifies the MAC (message authentication code) algorithms
1.91 markus 979: in order of preference.
980: The MAC algorithm is used in protocol version 2
981: for data integrity protection.
982: Multiple algorithms must be comma-separated.
983: The default is
1.113 markus 984: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.139 markus 985: .It Cm NoHostAuthenticationForLocalhost
986: This option can be used if the home directory is shared across machines.
987: In this case localhost will refer to a different machine on each of
988: the machines and the user will get many warnings about changed host keys.
989: However, this option disables host authentication for localhost.
990: The argument to this keyword must be
991: .Dq yes
992: or
993: .Dq no .
994: The default is to check the host key for localhost.
1.14 dugsong 995: .It Cm NumberOfPasswordPrompts
1.42 aaron 996: Specifies the number of password prompts before giving up.
997: The argument to this keyword must be an integer.
998: Default is 3.
1.34 markus 999: .It Cm PasswordAuthentication
1.40 aaron 1000: Specifies whether to use password authentication.
1001: The argument to this keyword must be
1.34 markus 1002: .Dq yes
1003: or
1004: .Dq no .
1.100 stevesk 1005: The default is
1006: .Dq yes .
1.2 deraadt 1007: .It Cm Port
1.40 aaron 1008: Specifies the port number to connect on the remote host.
1009: Default is 22.
1.99 djm 1010: .It Cm PreferredAuthentications
1.117 itojun 1011: Specifies the order in which the client should try protocol 2
1012: authentication methods. This allows a client to prefer one method (e.g.
1.99 djm 1013: .Cm keyboard-interactive )
1014: over another method (e.g.
1015: .Cm password )
1016: The default for this option is:
1.123 markus 1017: .Dq hostbased,publickey,keyboard-interactive,password
1.45 markus 1018: .It Cm Protocol
1019: Specifies the protocol versions
1020: .Nm
1021: should support in order of preference.
1022: The possible values are
1023: .Dq 1
1024: and
1025: .Dq 2 .
1026: Multiple versions must be comma-separated.
1027: The default is
1.101 markus 1028: .Dq 2,1 .
1.49 markus 1029: This means that
1030: .Nm
1.101 markus 1031: tries version 2 and falls back to version 1
1032: if version 2 is not available.
1.2 deraadt 1033: .It Cm ProxyCommand
1.40 aaron 1034: Specifies the command to use to connect to the server.
1035: The command
1036: string extends to the end of the line, and is executed with
1037: .Pa /bin/sh .
1038: In the command string,
1039: .Ql %h
1040: will be substituted by the host name to
1041: connect and
1042: .Ql %p
1043: by the port.
1044: The command can be basically anything,
1045: and should read from its standard input and write to its standard output.
1046: It should eventually connect an
1.2 deraadt 1047: .Xr sshd 8
1.1 deraadt 1048: server running on some machine, or execute
1.2 deraadt 1049: .Ic sshd -i
1.40 aaron 1050: somewhere.
1051: Host key management will be done using the
1.1 deraadt 1052: HostName of the host being connected (defaulting to the name typed by
1053: the user).
1.29 markus 1054: Note that
1055: .Cm CheckHostIP
1056: is not available for connects with a proxy command.
1.2 deraadt 1057: .Pp
1.107 markus 1058: .It Cm PubkeyAuthentication
1059: Specifies whether to try public key authentication.
1060: The argument to this keyword must be
1061: .Dq yes
1062: or
1063: .Dq no .
1064: The default is
1065: .Dq yes .
1066: This option applies to protocol version 2 only.
1.2 deraadt 1067: .It Cm RemoteForward
1.1 deraadt 1068: Specifies that a TCP/IP port on the remote machine be forwarded over
1.136 stevesk 1069: the secure channel to the specified host and port from the local machine.
1.40 aaron 1070: The first argument must be a port number, and the second must be
1.136 stevesk 1071: .Ar host:port .
1072: IPv6 addresses can be specified with an alternative syntax:
1073: .Ar host/port .
1.40 aaron 1074: Multiple forwardings may be specified, and additional
1075: forwardings can be given on the command line.
1076: Only the superuser can forward privileged ports.
1.2 deraadt 1077: .It Cm RhostsAuthentication
1.40 aaron 1078: Specifies whether to try rhosts based authentication.
1079: Note that this
1.1 deraadt 1080: declaration only affects the client side and has no effect whatsoever
1.40 aaron 1081: on security.
1082: Disabling rhosts authentication may reduce
1.1 deraadt 1083: authentication time on slow connections when rhosts authentication is
1.40 aaron 1084: not used.
1085: Most servers do not permit RhostsAuthentication because it
1.117 itojun 1086: is not secure (see
1.114 mpech 1087: .Cm RhostsRSAAuthentication ) .
1.40 aaron 1088: The argument to this keyword must be
1.2 deraadt 1089: .Dq yes
1090: or
1091: .Dq no .
1.100 stevesk 1092: The default is
1093: .Dq yes .
1.107 markus 1094: This option applies to protocol version 1 only.
1.2 deraadt 1095: .It Cm RhostsRSAAuthentication
1.1 deraadt 1096: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 1097: authentication.
1098: The argument must be
1.2 deraadt 1099: .Dq yes
1100: or
1101: .Dq no .
1.100 stevesk 1102: The default is
1103: .Dq yes .
1.107 markus 1104: This option applies to protocol version 1 only.
1.2 deraadt 1105: .It Cm RSAAuthentication
1.40 aaron 1106: Specifies whether to try RSA authentication.
1107: The argument to this keyword must be
1.2 deraadt 1108: .Dq yes
1109: or
1110: .Dq no .
1.1 deraadt 1111: RSA authentication will only be
1112: attempted if the identity file exists, or an authentication agent is
1113: running.
1.100 stevesk 1114: The default is
1115: .Dq yes .
1.50 markus 1116: Note that this option applies to protocol version 1 only.
1.81 markus 1117: .It Cm ChallengeResponseAuthentication
1118: Specifies whether to use challenge response authentication.
1.40 aaron 1119: The argument to this keyword must be
1.27 markus 1120: .Dq yes
1121: or
1122: .Dq no .
1123: The default is
1.122 markus 1124: .Dq yes .
1.125 jakob 1125: .It Cm SmartcardDevice
1126: Specifies which smartcard device to use. The argument to this keyword is
1127: the device
1128: .Nm
1129: should use to communicate with a smartcard used for storing the user's
1130: private RSA key. By default, no device is specified and smartcard support
1131: is not activated.
1.2 deraadt 1132: .It Cm StrictHostKeyChecking
1133: If this flag is set to
1.44 aaron 1134: .Dq yes ,
1.2 deraadt 1135: .Nm
1.79 stevesk 1136: will never automatically add host keys to the
1.2 deraadt 1137: .Pa $HOME/.ssh/known_hosts
1.116 markus 1138: file, and refuses to connect to hosts whose host key has changed.
1.137 deraadt 1139: This provides maximum protection against trojan horse attacks,
1140: however, can be annoying when the
1.2 deraadt 1141: .Pa /etc/ssh_known_hosts
1.137 deraadt 1142: file is poorly maintained, or connections to new hosts are
1143: frequently made.
1.79 stevesk 1144: This option forces the user to manually
1145: add all new hosts.
1146: If this flag is set to
1147: .Dq no ,
1148: .Nm
1149: will automatically add new host keys to the
1150: user known hosts files.
1151: If this flag is set to
1152: .Dq ask ,
1153: new host keys
1154: will be added to the user known host files only after the user
1155: has confirmed that is what they really want to do, and
1156: .Nm
1157: will refuse to connect to hosts whose host key has changed.
1.40 aaron 1158: The host keys of
1.79 stevesk 1159: known hosts will be verified automatically in all cases.
1.40 aaron 1160: The argument must be
1.79 stevesk 1161: .Dq yes ,
1162: .Dq no
1.2 deraadt 1163: or
1.79 stevesk 1164: .Dq ask .
1165: The default is
1166: .Dq ask .
1.16 markus 1167: .It Cm UsePrivilegedPort
1168: Specifies whether to use a privileged port for outgoing connections.
1169: The argument must be
1170: .Dq yes
1171: or
1172: .Dq no .
1173: The default is
1.98 markus 1174: .Dq no .
1.137 deraadt 1175: Note that this option must be set to
1.107 markus 1176: .Dq yes
1.137 deraadt 1177: if
1.16 markus 1178: .Cm RhostsAuthentication
1179: and
1.72 markus 1180: .Cm RhostsRSAAuthentication
1.137 deraadt 1181: authentications are needed with older servers.
1.34 markus 1182: .It Cm User
1.40 aaron 1183: Specifies the user to log in as.
1.137 deraadt 1184: This can be useful when a different user name is used on different machines.
1.40 aaron 1185: This saves the trouble of
1.34 markus 1186: having to remember to give the user name on the command line.
1187: .It Cm UserKnownHostsFile
1.116 markus 1188: Specifies a file to use for the user
1.95 stevesk 1189: host key database instead of
1.34 markus 1190: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 1191: .It Cm UseRsh
1.40 aaron 1192: Specifies that rlogin/rsh should be used for this host.
1193: It is possible that the host does not at all support the
1.2 deraadt 1194: .Nm
1.40 aaron 1195: protocol.
1196: This causes
1.2 deraadt 1197: .Nm
1.40 aaron 1198: to immediately execute
1.2 deraadt 1199: .Xr rsh 1 .
1.1 deraadt 1200: All other options (except
1.2 deraadt 1201: .Cm HostName )
1.40 aaron 1202: are ignored if this has been specified.
1203: The argument must be
1.2 deraadt 1204: .Dq yes
1205: or
1206: .Dq no .
1.55 markus 1207: .It Cm XAuthLocation
1208: Specifies the location of the
1209: .Xr xauth 1
1210: program.
1211: The default is
1212: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1213: .El
1.2 deraadt 1214: .Sh ENVIRONMENT
1215: .Nm
1.1 deraadt 1216: will normally set the following environment variables:
1.2 deraadt 1217: .Bl -tag -width Ds
1218: .It Ev DISPLAY
1219: The
1220: .Ev DISPLAY
1.40 aaron 1221: variable indicates the location of the X11 server.
1.44 aaron 1222: It is automatically set by
1.2 deraadt 1223: .Nm
1224: to point to a value of the form
1225: .Dq hostname:n
1226: where hostname indicates
1.40 aaron 1227: the host where the shell runs, and n is an integer >= 1.
1228: .Nm
1229: uses this special value to forward X11 connections over the secure
1230: channel.
1.107 markus 1231: The user should normally not set
1232: .Ev DISPLAY
1233: explicitly, as that
1.1 deraadt 1234: will render the X11 connection insecure (and will require the user to
1235: manually copy any required authorization cookies).
1.2 deraadt 1236: .It Ev HOME
1.1 deraadt 1237: Set to the path of the user's home directory.
1.2 deraadt 1238: .It Ev LOGNAME
1239: Synonym for
1.12 aaron 1240: .Ev USER ;
1241: set for compatibility with systems that use this variable.
1.2 deraadt 1242: .It Ev MAIL
1.129 stevesk 1243: Set to the path of the user's mailbox.
1.40 aaron 1244: .It Ev PATH
1.2 deraadt 1245: Set to the default
1246: .Ev PATH ,
1247: as specified when compiling
1.12 aaron 1248: .Nm ssh .
1.118 markus 1249: .It Ev SSH_ASKPASS
1250: If
1251: .Nm
1252: needs a passphrase, it will read the passphrase from the current
1253: terminal if it was run from a terminal.
1254: If
1255: .Nm
1256: does not have a terminal associated with it but
1257: .Ev DISPLAY
1258: and
1259: .Ev SSH_ASKPASS
1260: are set, it will execute the program specified by
1261: .Ev SSH_ASKPASS
1262: and open an X11 window to read the passphrase.
1263: This is particularly useful when calling
1264: .Nm
1265: from a
1266: .Pa .Xsession
1267: or related script.
1268: (Note that on some machines it
1269: may be necessary to redirect the input from
1270: .Pa /dev/null
1271: to make this work.)
1.18 markus 1272: .It Ev SSH_AUTH_SOCK
1.129 stevesk 1273: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 1274: agent.
1.2 deraadt 1275: .It Ev SSH_CLIENT
1.40 aaron 1276: Identifies the client end of the connection.
1277: The variable contains
1.1 deraadt 1278: three space-separated values: client ip-address, client port number,
1279: and server port number.
1.73 markus 1280: .It Ev SSH_ORIGINAL_COMMAND
1281: The variable contains the original command line if a forced command
1282: is executed.
1283: It can be used to extract the original arguments.
1.2 deraadt 1284: .It Ev SSH_TTY
1.1 deraadt 1285: This is set to the name of the tty (path to the device) associated
1.40 aaron 1286: with the current shell or command.
1287: If the current session has no tty,
1.1 deraadt 1288: this variable is not set.
1.2 deraadt 1289: .It Ev TZ
1.1 deraadt 1290: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1291: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1292: on to new connections).
1.2 deraadt 1293: .It Ev USER
1.1 deraadt 1294: Set to the name of the user logging in.
1.2 deraadt 1295: .El
1296: .Pp
1.44 aaron 1297: Additionally,
1.2 deraadt 1298: .Nm
1.44 aaron 1299: reads
1300: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1301: and adds lines of the format
1302: .Dq VARNAME=value
1.12 aaron 1303: to the environment.
1.2 deraadt 1304: .Sh FILES
1.36 markus 1305: .Bl -tag -width Ds
1.116 markus 1306: .It Pa $HOME/.ssh/known_hosts
1.129 stevesk 1307: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 1308: in
1.116 markus 1309: .Pa /etc/ssh_known_hosts .
1.2 deraadt 1310: See
1311: .Xr sshd 8 .
1.102 itojun 1312: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1313: Contains the authentication identity of the user.
1314: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 1315: These files
1316: contain sensitive data and should be readable by the user but not
1.15 markus 1317: accessible by others (read/write/execute).
1318: Note that
1319: .Nm
1.48 markus 1320: ignores a private key file if it is accessible by others.
1.15 markus 1321: It is possible to specify a passphrase when
1.1 deraadt 1322: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1323: sensitive part of this file using 3DES.
1.102 itojun 1324: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 1325: Contains the public key for authentication (public part of the
1.40 aaron 1326: identity file in human-readable form).
1.48 markus 1327: The contents of the
1328: .Pa $HOME/.ssh/identity.pub
1329: file should be added to
1.2 deraadt 1330: .Pa $HOME/.ssh/authorized_keys
1331: on all machines
1.137 deraadt 1332: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 1333: The contents of the
1334: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 1335: and
1336: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 1337: file should be added to
1.115 markus 1338: .Pa $HOME/.ssh/authorized_keys
1.48 markus 1339: on all machines
1.137 deraadt 1340: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 1341: These files are not
1.40 aaron 1342: sensitive and can (but need not) be readable by anyone.
1.48 markus 1343: These files are
1.84 markus 1344: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1345: the convenience of the user.
1.2 deraadt 1346: .It Pa $HOME/.ssh/config
1.40 aaron 1347: This is the per-user configuration file.
1348: The format of this file is described above.
1349: This file is used by the
1.2 deraadt 1350: .Nm
1.40 aaron 1351: client.
1352: This file does not usually contain any sensitive information,
1.1 deraadt 1353: but the recommended permissions are read/write for the user, and not
1354: accessible by others.
1.2 deraadt 1355: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 1356: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 1357: The format of this file is described in the
1.2 deraadt 1358: .Xr sshd 8
1.40 aaron 1359: manual page.
1.116 markus 1360: In the simplest form the format is the same as the .pub
1361: identity files.
1.48 markus 1362: This file is not highly sensitive, but the recommended
1363: permissions are read/write for the user, and not accessible by others.
1.116 markus 1364: .It Pa /etc/ssh_known_hosts
1.40 aaron 1365: Systemwide list of known host keys.
1.116 markus 1366: This file should be prepared by the
1.1 deraadt 1367: system administrator to contain the public host keys of all machines in the
1.40 aaron 1368: organization.
1369: This file should be world-readable.
1370: This file contains
1.1 deraadt 1371: public keys, one per line, in the following format (fields separated
1.116 markus 1372: by spaces): system name, public key and optional comment field.
1.40 aaron 1373: When different names are used
1.1 deraadt 1374: for the same machine, all such names should be listed, separated by
1.40 aaron 1375: commas.
1376: The format is described on the
1.2 deraadt 1377: .Xr sshd 8
1.1 deraadt 1378: manual page.
1.2 deraadt 1379: .Pp
1.1 deraadt 1380: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1381: .Xr sshd 8
1.1 deraadt 1382: to verify the client host when logging in; other names are needed because
1.2 deraadt 1383: .Nm
1.1 deraadt 1384: does not convert the user-supplied name to a canonical name before
1385: checking the key, because someone with access to the name servers
1386: would then be able to fool host authentication.
1.2 deraadt 1387: .It Pa /etc/ssh_config
1.40 aaron 1388: Systemwide configuration file.
1389: This file provides defaults for those
1.1 deraadt 1390: values that are not specified in the user's configuration file, and
1.40 aaron 1391: for those users who do not have a configuration file.
1392: This file must be world-readable.
1.141 markus 1393: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1394: These three files contain the private parts of the host keys
1395: and are used for
1396: .Cm RhostsRSAAuthentication
1397: and
1398: .Cm HostbasedAuthentication .
1399: Since they are readable only by root
1400: .Nm
1401: must be setuid root if these authentication methods are desired.
1.2 deraadt 1402: .It Pa $HOME/.rhosts
1403: This file is used in
1404: .Pa \&.rhosts
1405: authentication to list the
1.40 aaron 1406: host/user pairs that are permitted to log in.
1407: (Note that this file is
1.1 deraadt 1408: also used by rlogin and rsh, which makes using this file insecure.)
1409: Each line of the file contains a host name (in the canonical form
1410: returned by name servers), and then a user name on that host,
1.40 aaron 1411: separated by a space.
1.92 markus 1412: On some machines this file may need to be
1.1 deraadt 1413: world-readable if the user's home directory is on a NFS partition,
1414: because
1.2 deraadt 1415: .Xr sshd 8
1.40 aaron 1416: reads it as root.
1417: Additionally, this file must be owned by the user,
1418: and must not have write permissions for anyone else.
1419: The recommended
1.1 deraadt 1420: permission for most machines is read/write for the user, and not
1421: accessible by others.
1.2 deraadt 1422: .Pp
1.1 deraadt 1423: Note that by default
1.2 deraadt 1424: .Xr sshd 8
1.1 deraadt 1425: will be installed so that it requires successful RSA host
1.40 aaron 1426: authentication before permitting \s+2.\s0rhosts authentication.
1.137 deraadt 1427: If the server machine does not have the client's host key in
1.2 deraadt 1428: .Pa /etc/ssh_known_hosts ,
1.137 deraadt 1429: it can be stored in
1.2 deraadt 1430: .Pa $HOME/.ssh/known_hosts .
1431: The easiest way to do this is to
1.1 deraadt 1432: connect back to the client from the server machine using ssh; this
1.48 markus 1433: will automatically add the host key to
1.2 deraadt 1434: .Pa $HOME/.ssh/known_hosts .
1435: .It Pa $HOME/.shosts
1436: This file is used exactly the same way as
1437: .Pa \&.rhosts .
1438: The purpose for
1.1 deraadt 1439: having this file is to be able to use rhosts authentication with
1.2 deraadt 1440: .Nm
1441: without permitting login with
1442: .Xr rlogin 1
1443: or
1444: .Xr rsh 1 .
1445: .It Pa /etc/hosts.equiv
1446: This file is used during
1.40 aaron 1447: .Pa \&.rhosts authentication.
1448: It contains
1.1 deraadt 1449: canonical hosts names, one per line (the full format is described on
1450: the
1.2 deraadt 1451: .Xr sshd 8
1.40 aaron 1452: manual page).
1453: If the client host is found in this file, login is
1.1 deraadt 1454: automatically permitted provided client and server user names are the
1.40 aaron 1455: same.
1456: Additionally, successful RSA host authentication is normally
1457: required.
1458: This file should only be writable by root.
1.2 deraadt 1459: .It Pa /etc/shosts.equiv
1.44 aaron 1460: This file is processed exactly as
1.2 deraadt 1461: .Pa /etc/hosts.equiv .
1.1 deraadt 1462: This file may be useful to permit logins using
1.2 deraadt 1463: .Nm
1.1 deraadt 1464: but not using rsh/rlogin.
1.2 deraadt 1465: .It Pa /etc/sshrc
1.1 deraadt 1466: Commands in this file are executed by
1.2 deraadt 1467: .Nm
1.1 deraadt 1468: when the user logs in just before the user's shell (or command) is started.
1469: See the
1.2 deraadt 1470: .Xr sshd 8
1.1 deraadt 1471: manual page for more information.
1.2 deraadt 1472: .It Pa $HOME/.ssh/rc
1.1 deraadt 1473: Commands in this file are executed by
1.2 deraadt 1474: .Nm
1.1 deraadt 1475: when the user logs in just before the user's shell (or command) is
1476: started.
1.44 aaron 1477: See the
1.2 deraadt 1478: .Xr sshd 8
1.1 deraadt 1479: manual page for more information.
1.31 markus 1480: .It Pa $HOME/.ssh/environment
1481: Contains additional definitions for environment variables, see section
1482: .Sx ENVIRONMENT
1483: above.
1.58 itojun 1484: .El
1.67 aaron 1485: .Sh AUTHORS
1.78 markus 1486: OpenSSH is a derivative of the original and free
1487: ssh 1.2.12 release by Tatu Ylonen.
1488: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1489: Theo de Raadt and Dug Song
1490: removed many bugs, re-added newer features and
1491: created OpenSSH.
1492: Markus Friedl contributed the support for SSH
1493: protocol versions 1.5 and 2.0.
1.2 deraadt 1494: .Sh SEE ALSO
1495: .Xr rlogin 1 ,
1496: .Xr rsh 1 ,
1497: .Xr scp 1 ,
1.83 djm 1498: .Xr sftp 1 ,
1.2 deraadt 1499: .Xr ssh-add 1 ,
1500: .Xr ssh-agent 1 ,
1501: .Xr ssh-keygen 1 ,
1502: .Xr telnet 1 ,
1.87 itojun 1503: .Xr sshd 8
1.106 markus 1504: .Rs
1505: .%A T. Ylonen
1506: .%A T. Kivinen
1507: .%A M. Saarinen
1508: .%A T. Rinne
1509: .%A S. Lehtinen
1510: .%T "SSH Protocol Architecture"
1.126 markus 1511: .%N draft-ietf-secsh-architecture-09.txt
1512: .%D July 2001
1.106 markus 1513: .%O work in progress material
1514: .Re
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