Annotation of src/usr.bin/ssh/ssh.1, Revision 1.145
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,
32: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.145 ! markus 37: .\" $OpenBSD: ssh.1,v 1.144 2002/01/16 17:42:33 stevesk 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
1.145 ! markus 210: When a user connects using protocol version 2
! 211: similar authentication methods are available.
1.107 markus 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
1.144 stevesk 446: Selects a file from which the identity (private key) for
1.68 markus 447: RSA or DSA authentication is read.
1.144 stevesk 448: The default is
1.49 markus 449: .Pa $HOME/.ssh/identity
1.144 stevesk 450: for protocol version 1, and
451: .Pa $HOME/.ssh/id_rsa
452: and
453: .Pa $HOME/.ssh/id_dsa
454: for protocol version 2.
1.40 aaron 455: Identity files may also be specified on
456: a per-host basis in the configuration file.
457: It is possible to have multiple
1.2 deraadt 458: .Fl i
459: options (and multiple identities specified in
1.1 deraadt 460: configuration files).
1.125 jakob 461: .It Fl I Ar smartcard_device
462: Specifies which smartcard device to use. The argument is
463: the device
464: .Nm
465: should use to communicate with a smartcard used for storing the user's
466: private RSA key.
1.2 deraadt 467: .It Fl k
1.42 aaron 468: Disables forwarding of Kerberos tickets and AFS tokens.
469: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 470: .It Fl l Ar login_name
1.40 aaron 471: Specifies the user to log in as on the remote machine.
472: This also may be specified on a per-host basis in the configuration file.
1.91 markus 473: .It Fl m Ar mac_spec
474: Additionally, for protocol version 2 a comma-separated list of MAC
475: (message authentication code) algorithms can
476: be specified in order of preference.
477: See the
478: .Cm MACs
479: keyword for more information.
1.2 deraadt 480: .It Fl n
481: Redirects stdin from
482: .Pa /dev/null
483: (actually, prevents reading from stdin).
1.1 deraadt 484: This must be used when
1.2 deraadt 485: .Nm
1.40 aaron 486: is run in the background.
487: A common trick is to use this to run X11 programs on a remote machine.
488: For example,
1.2 deraadt 489: .Ic ssh -n shadows.cs.hut.fi emacs &
490: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 491: connection will be automatically forwarded over an encrypted channel.
492: The
1.2 deraadt 493: .Nm
1.1 deraadt 494: program will be put in the background.
495: (This does not work if
1.2 deraadt 496: .Nm
497: needs to ask for a password or passphrase; see also the
498: .Fl f
499: option.)
1.53 markus 500: .It Fl N
501: Do not execute a remote command.
1.137 deraadt 502: This is useful for just forwarding ports
1.53 markus 503: (protocol version 2 only).
1.2 deraadt 504: .It Fl o Ar option
1.127 stevesk 505: Can be used to give options in the format used in the configuration file.
1.1 deraadt 506: This is useful for specifying options for which there is no separate
1.40 aaron 507: command-line flag.
1.2 deraadt 508: .It Fl p Ar port
1.40 aaron 509: Port to connect to on the remote host.
510: This can be specified on a
1.1 deraadt 511: per-host basis in the configuration file.
1.16 markus 512: .It Fl P
513: Use a non-privileged port for outgoing connections.
1.137 deraadt 514: This can be used if a firewall does
1.16 markus 515: not permit connections from privileged ports.
1.30 provos 516: Note that this option turns off
1.16 markus 517: .Cm RhostsAuthentication
518: and
1.72 markus 519: .Cm RhostsRSAAuthentication
520: for older servers.
1.2 deraadt 521: .It Fl q
1.40 aaron 522: Quiet mode.
523: Causes all warning and diagnostic messages to be suppressed.
524: Only fatal errors are displayed.
1.80 djm 525: .It Fl s
1.117 itojun 526: 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
527: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 528: subsystem is specified as the remote command.
1.2 deraadt 529: .It Fl t
1.40 aaron 530: Force pseudo-tty allocation.
1.43 brad 531: This can be used to execute arbitrary
1.40 aaron 532: screen-based programs on a remote machine, which can be very useful,
533: e.g., when implementing menu services.
1.73 markus 534: Multiple
535: .Fl t
536: options force tty allocation, even if
537: .Nm
538: has no local tty.
1.53 markus 539: .It Fl T
1.69 markus 540: Disable pseudo-tty allocation.
1.2 deraadt 541: .It Fl v
1.40 aaron 542: Verbose mode.
543: Causes
1.2 deraadt 544: .Nm
1.40 aaron 545: to print debugging messages about its progress.
546: This is helpful in
1.1 deraadt 547: debugging connection, authentication, and configuration problems.
1.73 markus 548: Multiple
549: .Fl v
550: options increases the verbosity.
1.61 aaron 551: Maximum is 3.
1.2 deraadt 552: .It Fl x
1.40 aaron 553: Disables X11 forwarding.
1.2 deraadt 554: .It Fl X
1.1 deraadt 555: Enables X11 forwarding.
1.54 markus 556: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 557: .It Fl C
1.1 deraadt 558: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 559: data for forwarded X11 and TCP/IP connections).
560: The compression algorithm is the same used by
1.34 markus 561: .Xr gzip 1 ,
562: and the
1.2 deraadt 563: .Dq level
564: can be controlled by the
565: .Cm CompressionLevel
1.40 aaron 566: option (see below).
567: Compression is desirable on modem lines and other
1.1 deraadt 568: slow connections, but will only slow down things on fast networks.
569: The default value can be set on a host-by-host basis in the
570: configuration files; see the
1.121 pvalchev 571: .Cm Compression
1.1 deraadt 572: option below.
1.132 markus 573: .It Fl F Ar configfile
574: Specifies an alternative per-user configuration file.
575: If a configuration file is given on the command line,
576: the system-wide configuration file
577: .Pq Pa /etc/ssh_config
578: will be ignored.
579: The default for the per-user configuration file is
580: .Pa $HOME/.ssh/config .
1.2 deraadt 581: .It Fl L Ar port:host:hostport
1.1 deraadt 582: Specifies that the given port on the local (client) host is to be
1.40 aaron 583: forwarded to the given host and port on the remote side.
584: This works by allocating a socket to listen to
1.2 deraadt 585: .Ar port
1.1 deraadt 586: on the local side, and whenever a connection is made to this port, the
587: connection is forwarded over the secure channel, and a connection is
588: made to
1.32 markus 589: .Ar host
590: port
591: .Ar hostport
1.40 aaron 592: from the remote machine.
593: Port forwardings can also be specified in the configuration file.
594: Only root can forward privileged ports.
1.32 markus 595: IPv6 addresses can be specified with an alternative syntax:
596: .Ar port/host/hostport
1.2 deraadt 597: .It Fl R Ar port:host:hostport
1.1 deraadt 598: Specifies that the given port on the remote (server) host is to be
1.40 aaron 599: forwarded to the given host and port on the local side.
600: This works by allocating a socket to listen to
1.2 deraadt 601: .Ar port
1.1 deraadt 602: on the remote side, and whenever a connection is made to this port, the
603: connection is forwarded over the secure channel, and a connection is
604: made to
1.32 markus 605: .Ar host
606: port
607: .Ar hostport
1.40 aaron 608: from the local machine.
609: Port forwardings can also be specified in the configuration file.
610: Privileged ports can be forwarded only when
1.1 deraadt 611: logging in as root on the remote machine.
1.107 markus 612: IPv6 addresses can be specified with an alternative syntax:
613: .Ar port/host/hostport
1.133 stevesk 614: .It Fl D Ar port
615: Specifies a local
616: .Dq dynamic
617: application-level port forwarding.
618: This works by allocating a socket to listen to
619: .Ar port
620: on the local side, and whenever a connection is made to this port, the
621: connection is forwarded over the secure channel, and the application
622: protocol is then used to determine where to connect to from the
623: remote machine. Currently the SOCKS4 protocol is supported, and
624: .Nm
625: will act as a SOCKS4 server.
626: Only root can forward privileged ports.
627: Dynamic port forwardings can also be specified in the configuration file.
1.85 jakob 628: .It Fl 1
629: Forces
630: .Nm
631: to try protocol version 1 only.
1.46 markus 632: .It Fl 2
633: Forces
634: .Nm
1.50 markus 635: to try protocol version 2 only.
1.32 markus 636: .It Fl 4
637: Forces
638: .Nm
639: to use IPv4 addresses only.
640: .It Fl 6
641: Forces
642: .Nm
643: to use IPv6 addresses only.
1.2 deraadt 644: .El
645: .Sh CONFIGURATION FILES
646: .Nm
1.127 stevesk 647: obtains configuration data from the following sources in
648: the following order:
1.1 deraadt 649: command line options, user's configuration file
1.2 deraadt 650: .Pq Pa $HOME/.ssh/config ,
651: and system-wide configuration file
652: .Pq Pa /etc/ssh_config .
653: For each parameter, the first obtained value
1.40 aaron 654: will be used.
655: The configuration files contain sections bracketed by
656: .Dq Host
657: specifications, and that section is only applied for hosts that
658: match one of the patterns given in the specification.
659: The matched host name is the one given on the command line.
1.2 deraadt 660: .Pp
1.1 deraadt 661: Since the first obtained value for each parameter is used, more
662: host-specific declarations should be given near the beginning of the
663: file, and general defaults at the end.
1.2 deraadt 664: .Pp
1.1 deraadt 665: The configuration file has the following format:
1.2 deraadt 666: .Pp
667: Empty lines and lines starting with
668: .Ql #
669: are comments.
670: .Pp
671: Otherwise a line is of the format
672: .Dq keyword arguments .
1.127 stevesk 673: Configuration options may be separated by whitespace or
674: optional whitespace and exactly one
675: .Ql = ;
676: the latter format is useful to avoid the need to quote whitespace
677: when specifying configuration options using the
678: .Nm ssh ,
679: .Nm scp
680: and
681: .Nm sftp
682: .Fl o
683: option.
684: .Pp
1.2 deraadt 685: The possible
1.128 stevesk 686: keywords and their meanings are as follows (note that
687: keywords are case-insensitive and arguments are case-sensitive):
1.2 deraadt 688: .Bl -tag -width Ds
689: .It Cm Host
1.1 deraadt 690: Restricts the following declarations (up to the next
1.2 deraadt 691: .Cm Host
1.1 deraadt 692: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 693: given after the keyword.
694: .Ql \&*
695: and
696: .Ql ?
697: can be used as wildcards in the
1.40 aaron 698: patterns.
699: A single
1.2 deraadt 700: .Ql \&*
701: as a pattern can be used to provide global
1.40 aaron 702: defaults for all hosts.
703: The host is the
1.2 deraadt 704: .Ar hostname
1.1 deraadt 705: argument given on the command line (i.e., the name is not converted to
706: a canonicalized host name before matching).
1.2 deraadt 707: .It Cm AFSTokenPassing
1.42 aaron 708: Specifies whether to pass AFS tokens to remote host.
709: The argument to this keyword must be
1.2 deraadt 710: .Dq yes
711: or
712: .Dq no .
1.107 markus 713: This option applies to protocol version 1 only.
1.2 deraadt 714: .It Cm BatchMode
715: If set to
716: .Dq yes ,
1.40 aaron 717: passphrase/password querying will be disabled.
1.137 deraadt 718: This option is useful in scripts and other batch jobs where no user
719: is present to supply the password.
1.40 aaron 720: The argument must be
1.2 deraadt 721: .Dq yes
722: or
723: .Dq no .
1.100 stevesk 724: The default is
725: .Dq no .
1.108 markus 726: .It Cm BindAddress
727: Specify the interface to transmit from on machines with multiple
728: interfaces or aliased addresses.
729: Note that this option does not work if
730: .Cm UsePrivilegedPort
731: is set to
732: .Dq yes .
1.34 markus 733: .It Cm CheckHostIP
734: If this flag is set to
735: .Dq yes ,
1.100 stevesk 736: ssh will additionally check the host IP address in the
1.34 markus 737: .Pa known_hosts
1.42 aaron 738: file.
739: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 740: If the option is set to
741: .Dq no ,
742: the check will not be executed.
1.100 stevesk 743: The default is
744: .Dq yes .
1.2 deraadt 745: .It Cm Cipher
1.62 markus 746: Specifies the cipher to use for encrypting the session
1.64 markus 747: in protocol version 1.
1.40 aaron 748: Currently,
1.131 stevesk 749: .Dq blowfish ,
750: .Dq 3des ,
1.1 deraadt 751: and
1.131 stevesk 752: .Dq des
1.40 aaron 753: are supported.
1.131 stevesk 754: .Ar des
755: is only supported in the
756: .Nm
757: client for interoperability with legacy protocol 1 implementations
758: that do not support the
759: .Ar 3des
760: cipher. Its use is strongly discouraged due to cryptographic
761: weaknesses.
1.40 aaron 762: The default is
1.2 deraadt 763: .Dq 3des .
1.45 markus 764: .It Cm Ciphers
765: Specifies the ciphers allowed for protocol version 2
766: in order of preference.
767: Multiple ciphers must be comma-separated.
768: The default is
1.88 provos 769: .Pp
770: .Bd -literal
1.94 deraadt 771: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.107 markus 772: aes192-cbc,aes256-cbc''
1.88 provos 773: .Ed
1.138 stevesk 774: .It Cm ClearAllForwardings
775: Specifies that all local, remote and dynamic port forwardings
776: specified in the configuration files or on the command line be
777: cleared. This option is primarily useful when used from the
778: .Nm
779: command line to clear port forwardings set in
780: configuration files, and is automatically set by
781: .Xr scp 1
782: and
783: .Xr sftp 1 .
784: The argument must be
785: .Dq yes
786: or
787: .Dq no .
788: The default is
789: .Dq no .
1.2 deraadt 790: .It Cm Compression
1.40 aaron 791: Specifies whether to use compression.
792: The argument must be
1.2 deraadt 793: .Dq yes
794: or
795: .Dq no .
1.100 stevesk 796: The default is
797: .Dq no .
1.2 deraadt 798: .It Cm CompressionLevel
1.100 stevesk 799: Specifies the compression level to use if compression is enabled.
1.40 aaron 800: The argument must be an integer from 1 (fast) to 9 (slow, best).
801: The default level is 6, which is good for most applications.
802: The meaning of the values is the same as in
1.34 markus 803: .Xr gzip 1 .
1.107 markus 804: Note that this option applies to protocol version 1 only.
1.2 deraadt 805: .It Cm ConnectionAttempts
1.1 deraadt 806: Specifies the number of tries (one per second) to make before falling
1.40 aaron 807: back to rsh or exiting.
808: The argument must be an integer.
809: This may be useful in scripts if the connection sometimes fails.
1.124 markus 810: The default is 1.
1.133 stevesk 811: .It Cm DynamicForward
812: Specifies that a TCP/IP port on the local machine be forwarded
813: over the secure channel, and the application
814: protocol is then used to determine where to connect to from the
815: remote machine. The argument must be a port number.
816: Currently the SOCKS4 protocol is supported, and
817: .Nm
818: will act as a SOCKS4 server.
819: Multiple forwardings may be specified, and
820: additional forwardings can be given on the command line. Only
821: the superuser can forward privileged ports.
1.2 deraadt 822: .It Cm EscapeChar
823: Sets the escape character (default:
824: .Ql ~ ) .
825: The escape character can also
1.40 aaron 826: be set on the command line.
827: The argument should be a single character,
1.2 deraadt 828: .Ql ^
829: followed by a letter, or
830: .Dq none
831: to disable the escape
1.1 deraadt 832: character entirely (making the connection transparent for binary
833: data).
1.44 aaron 834: .It Cm FallBackToRsh
1.1 deraadt 835: Specifies that if connecting via
1.2 deraadt 836: .Nm
1.1 deraadt 837: fails due to a connection refused error (there is no
1.2 deraadt 838: .Xr sshd 8
1.44 aaron 839: listening on the remote host),
1.2 deraadt 840: .Xr rsh 1
1.1 deraadt 841: should automatically be used instead (after a suitable warning about
1.40 aaron 842: the session being unencrypted).
843: The argument must be
1.2 deraadt 844: .Dq yes
845: or
846: .Dq no .
1.100 stevesk 847: The default is
848: .Dq no .
1.2 deraadt 849: .It Cm ForwardAgent
1.1 deraadt 850: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 851: will be forwarded to the remote machine.
852: The argument must be
1.2 deraadt 853: .Dq yes
854: or
1.54 markus 855: .Dq no .
856: The default is
1.2 deraadt 857: .Dq no .
858: .It Cm ForwardX11
1.1 deraadt 859: Specifies whether X11 connections will be automatically redirected
1.44 aaron 860: over the secure channel and
1.2 deraadt 861: .Ev DISPLAY
1.40 aaron 862: set.
1.44 aaron 863: The argument must be
1.2 deraadt 864: .Dq yes
865: or
1.38 markus 866: .Dq no .
867: The default is
1.3 deraadt 868: .Dq no .
869: .It Cm GatewayPorts
870: Specifies whether remote hosts are allowed to connect to local
871: forwarded ports.
1.134 stevesk 872: By default,
873: .Nm
874: binds local port forwardings to the loopback addresss. This
875: prevents other remote hosts from connecting to forwarded ports.
876: .Cm GatewayPorts
877: can be used to specify that
878: .Nm
879: should bind local port forwardings to the wildcard address,
880: thus allowing remote hosts to connect to forwarded ports.
1.3 deraadt 881: The argument must be
882: .Dq yes
883: or
884: .Dq no .
885: The default is
1.2 deraadt 886: .Dq no .
887: .It Cm GlobalKnownHostsFile
1.116 markus 888: Specifies a file to use for the global
1.95 stevesk 889: host key database instead of
1.2 deraadt 890: .Pa /etc/ssh_known_hosts .
1.107 markus 891: .It Cm HostbasedAuthentication
892: Specifies whether to try rhosts based authentication with public key
893: authentication.
894: The argument must be
895: .Dq yes
896: or
897: .Dq no .
898: The default is
1.112 markus 899: .Dq no .
1.107 markus 900: This option applies to protocol version 2 only and
901: is similar to
902: .Cm RhostsRSAAuthentication .
903: .It Cm HostKeyAlgorithms
1.109 stevesk 904: Specifies the protocol version 2 host key algorithms
1.107 markus 905: that the client wants to use in order of preference.
906: The default for this option is:
1.143 stevesk 907: .Dq ssh-rsa,ssh-dss .
1.74 markus 908: .It Cm HostKeyAlias
909: Specifies an alias that should be used instead of the
910: real host name when looking up or saving the host key
1.107 markus 911: in the host key database files.
1.82 stevesk 912: This option is useful for tunneling ssh connections
1.137 deraadt 913: or for multiple servers running on a single host.
1.2 deraadt 914: .It Cm HostName
1.40 aaron 915: Specifies the real host name to log into.
916: This can be used to specify nicknames or abbreviations for hosts.
917: Default is the name given on the command line.
918: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 919: .Cm HostName
1.1 deraadt 920: specifications).
1.2 deraadt 921: .It Cm IdentityFile
1.144 stevesk 922: Specifies a file from which the user's RSA or DSA authentication identity
923: is read. The default is
1.49 markus 924: .Pa $HOME/.ssh/identity
1.144 stevesk 925: for protocol version 1, and
926: .Pa $HOME/.ssh/id_rsa
927: and
928: .Pa $HOME/.ssh/id_dsa
929: for protocol version 2.
1.1 deraadt 930: Additionally, any identities represented by the authentication agent
1.40 aaron 931: will be used for authentication.
932: The file name may use the tilde
933: syntax to refer to a user's home directory.
934: It is possible to have
1.1 deraadt 935: multiple identity files specified in configuration files; all these
936: identities will be tried in sequence.
1.2 deraadt 937: .It Cm KeepAlive
1.1 deraadt 938: Specifies whether the system should send keepalive messages to the
1.40 aaron 939: other side.
940: If they are sent, death of the connection or crash of one
941: of the machines will be properly noticed.
942: However, this means that
1.1 deraadt 943: connections will die if the route is down temporarily, and some people
1.41 aaron 944: find it annoying.
1.2 deraadt 945: .Pp
946: The default is
947: .Dq yes
948: (to send keepalives), and the client will notice
1.40 aaron 949: if the network goes down or the remote host dies.
950: This is important in scripts, and many users want it too.
1.2 deraadt 951: .Pp
952: To disable keepalives, the value should be set to
953: .Dq no
954: in both the server and the client configuration files.
955: .It Cm KerberosAuthentication
1.42 aaron 956: Specifies whether Kerberos authentication will be used.
957: The argument to this keyword must be
1.4 dugsong 958: .Dq yes
959: or
960: .Dq no .
1.2 deraadt 961: .It Cm KerberosTgtPassing
1.42 aaron 962: Specifies whether a Kerberos TGT will be forwarded to the server.
963: This will only work if the Kerberos server is actually an AFS kaserver.
964: The argument to this keyword must be
1.4 dugsong 965: .Dq yes
966: or
967: .Dq no .
1.2 deraadt 968: .It Cm LocalForward
1.1 deraadt 969: Specifies that a TCP/IP port on the local machine be forwarded over
1.136 stevesk 970: the secure channel to the specified host and port from the remote machine.
1.40 aaron 971: The first argument must be a port number, and the second must be
1.136 stevesk 972: .Ar host:port .
973: IPv6 addresses can be specified with an alternative syntax:
974: .Ar host/port .
1.40 aaron 975: Multiple forwardings may be specified, and additional
976: forwardings can be given on the command line.
977: Only the superuser can forward privileged ports.
1.24 markus 978: .It Cm LogLevel
979: Gives the verbosity level that is used when logging messages from
980: .Nm ssh .
981: The possible values are:
1.142 stevesk 982: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
983: The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
984: and DEBUG3 each specify higher levels of verbose output.
1.91 markus 985: .It Cm MACs
1.117 itojun 986: Specifies the MAC (message authentication code) algorithms
1.91 markus 987: in order of preference.
988: The MAC algorithm is used in protocol version 2
989: for data integrity protection.
990: Multiple algorithms must be comma-separated.
991: The default is
1.113 markus 992: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.139 markus 993: .It Cm NoHostAuthenticationForLocalhost
994: This option can be used if the home directory is shared across machines.
995: In this case localhost will refer to a different machine on each of
996: the machines and the user will get many warnings about changed host keys.
997: However, this option disables host authentication for localhost.
998: The argument to this keyword must be
999: .Dq yes
1000: or
1001: .Dq no .
1002: The default is to check the host key for localhost.
1.14 dugsong 1003: .It Cm NumberOfPasswordPrompts
1.42 aaron 1004: Specifies the number of password prompts before giving up.
1005: The argument to this keyword must be an integer.
1006: Default is 3.
1.34 markus 1007: .It Cm PasswordAuthentication
1.40 aaron 1008: Specifies whether to use password authentication.
1009: The argument to this keyword must be
1.34 markus 1010: .Dq yes
1011: or
1012: .Dq no .
1.100 stevesk 1013: The default is
1014: .Dq yes .
1.2 deraadt 1015: .It Cm Port
1.40 aaron 1016: Specifies the port number to connect on the remote host.
1017: Default is 22.
1.99 djm 1018: .It Cm PreferredAuthentications
1.117 itojun 1019: Specifies the order in which the client should try protocol 2
1020: authentication methods. This allows a client to prefer one method (e.g.
1.99 djm 1021: .Cm keyboard-interactive )
1022: over another method (e.g.
1023: .Cm password )
1024: The default for this option is:
1.143 stevesk 1025: .Dq hostbased,publickey,keyboard-interactive,password .
1.45 markus 1026: .It Cm Protocol
1027: Specifies the protocol versions
1028: .Nm
1029: should support in order of preference.
1030: The possible values are
1031: .Dq 1
1032: and
1033: .Dq 2 .
1034: Multiple versions must be comma-separated.
1035: The default is
1.101 markus 1036: .Dq 2,1 .
1.49 markus 1037: This means that
1038: .Nm
1.101 markus 1039: tries version 2 and falls back to version 1
1040: if version 2 is not available.
1.2 deraadt 1041: .It Cm ProxyCommand
1.40 aaron 1042: Specifies the command to use to connect to the server.
1043: The command
1044: string extends to the end of the line, and is executed with
1045: .Pa /bin/sh .
1046: In the command string,
1047: .Ql %h
1048: will be substituted by the host name to
1049: connect and
1050: .Ql %p
1051: by the port.
1052: The command can be basically anything,
1053: and should read from its standard input and write to its standard output.
1054: It should eventually connect an
1.2 deraadt 1055: .Xr sshd 8
1.1 deraadt 1056: server running on some machine, or execute
1.2 deraadt 1057: .Ic sshd -i
1.40 aaron 1058: somewhere.
1059: Host key management will be done using the
1.1 deraadt 1060: HostName of the host being connected (defaulting to the name typed by
1061: the user).
1.29 markus 1062: Note that
1063: .Cm CheckHostIP
1064: is not available for connects with a proxy command.
1.2 deraadt 1065: .Pp
1.107 markus 1066: .It Cm PubkeyAuthentication
1067: Specifies whether to try public key authentication.
1068: The argument to this keyword must be
1069: .Dq yes
1070: or
1071: .Dq no .
1072: The default is
1073: .Dq yes .
1074: This option applies to protocol version 2 only.
1.2 deraadt 1075: .It Cm RemoteForward
1.1 deraadt 1076: Specifies that a TCP/IP port on the remote machine be forwarded over
1.136 stevesk 1077: the secure channel to the specified host and port from the local machine.
1.40 aaron 1078: The first argument must be a port number, and the second must be
1.136 stevesk 1079: .Ar host:port .
1080: IPv6 addresses can be specified with an alternative syntax:
1081: .Ar host/port .
1.40 aaron 1082: Multiple forwardings may be specified, and additional
1083: forwardings can be given on the command line.
1084: Only the superuser can forward privileged ports.
1.2 deraadt 1085: .It Cm RhostsAuthentication
1.40 aaron 1086: Specifies whether to try rhosts based authentication.
1087: Note that this
1.1 deraadt 1088: declaration only affects the client side and has no effect whatsoever
1.40 aaron 1089: on security.
1090: Disabling rhosts authentication may reduce
1.1 deraadt 1091: authentication time on slow connections when rhosts authentication is
1.40 aaron 1092: not used.
1093: Most servers do not permit RhostsAuthentication because it
1.117 itojun 1094: is not secure (see
1.114 mpech 1095: .Cm RhostsRSAAuthentication ) .
1.40 aaron 1096: The argument to this keyword must be
1.2 deraadt 1097: .Dq yes
1098: or
1099: .Dq no .
1.100 stevesk 1100: The default is
1101: .Dq yes .
1.107 markus 1102: This option applies to protocol version 1 only.
1.2 deraadt 1103: .It Cm RhostsRSAAuthentication
1.1 deraadt 1104: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 1105: authentication.
1106: The argument must be
1.2 deraadt 1107: .Dq yes
1108: or
1109: .Dq no .
1.100 stevesk 1110: The default is
1111: .Dq yes .
1.107 markus 1112: This option applies to protocol version 1 only.
1.2 deraadt 1113: .It Cm RSAAuthentication
1.40 aaron 1114: Specifies whether to try RSA authentication.
1115: The argument to this keyword must be
1.2 deraadt 1116: .Dq yes
1117: or
1118: .Dq no .
1.1 deraadt 1119: RSA authentication will only be
1120: attempted if the identity file exists, or an authentication agent is
1121: running.
1.100 stevesk 1122: The default is
1123: .Dq yes .
1.50 markus 1124: Note that this option applies to protocol version 1 only.
1.81 markus 1125: .It Cm ChallengeResponseAuthentication
1126: Specifies whether to use challenge response authentication.
1.40 aaron 1127: The argument to this keyword must be
1.27 markus 1128: .Dq yes
1129: or
1130: .Dq no .
1131: The default is
1.122 markus 1132: .Dq yes .
1.125 jakob 1133: .It Cm SmartcardDevice
1134: Specifies which smartcard device to use. The argument to this keyword is
1135: the device
1136: .Nm
1137: should use to communicate with a smartcard used for storing the user's
1138: private RSA key. By default, no device is specified and smartcard support
1139: is not activated.
1.2 deraadt 1140: .It Cm StrictHostKeyChecking
1141: If this flag is set to
1.44 aaron 1142: .Dq yes ,
1.2 deraadt 1143: .Nm
1.79 stevesk 1144: will never automatically add host keys to the
1.2 deraadt 1145: .Pa $HOME/.ssh/known_hosts
1.116 markus 1146: file, and refuses to connect to hosts whose host key has changed.
1.137 deraadt 1147: This provides maximum protection against trojan horse attacks,
1148: however, can be annoying when the
1.2 deraadt 1149: .Pa /etc/ssh_known_hosts
1.137 deraadt 1150: file is poorly maintained, or connections to new hosts are
1151: frequently made.
1.79 stevesk 1152: This option forces the user to manually
1153: add all new hosts.
1154: If this flag is set to
1155: .Dq no ,
1156: .Nm
1157: will automatically add new host keys to the
1158: user known hosts files.
1159: If this flag is set to
1160: .Dq ask ,
1161: new host keys
1162: will be added to the user known host files only after the user
1163: has confirmed that is what they really want to do, and
1164: .Nm
1165: will refuse to connect to hosts whose host key has changed.
1.40 aaron 1166: The host keys of
1.79 stevesk 1167: known hosts will be verified automatically in all cases.
1.40 aaron 1168: The argument must be
1.79 stevesk 1169: .Dq yes ,
1170: .Dq no
1.2 deraadt 1171: or
1.79 stevesk 1172: .Dq ask .
1173: The default is
1174: .Dq ask .
1.16 markus 1175: .It Cm UsePrivilegedPort
1176: Specifies whether to use a privileged port for outgoing connections.
1177: The argument must be
1178: .Dq yes
1179: or
1180: .Dq no .
1181: The default is
1.98 markus 1182: .Dq no .
1.137 deraadt 1183: Note that this option must be set to
1.107 markus 1184: .Dq yes
1.137 deraadt 1185: if
1.16 markus 1186: .Cm RhostsAuthentication
1187: and
1.72 markus 1188: .Cm RhostsRSAAuthentication
1.137 deraadt 1189: authentications are needed with older servers.
1.34 markus 1190: .It Cm User
1.40 aaron 1191: Specifies the user to log in as.
1.137 deraadt 1192: This can be useful when a different user name is used on different machines.
1.40 aaron 1193: This saves the trouble of
1.34 markus 1194: having to remember to give the user name on the command line.
1195: .It Cm UserKnownHostsFile
1.116 markus 1196: Specifies a file to use for the user
1.95 stevesk 1197: host key database instead of
1.34 markus 1198: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 1199: .It Cm UseRsh
1.40 aaron 1200: Specifies that rlogin/rsh should be used for this host.
1201: It is possible that the host does not at all support the
1.2 deraadt 1202: .Nm
1.40 aaron 1203: protocol.
1204: This causes
1.2 deraadt 1205: .Nm
1.40 aaron 1206: to immediately execute
1.2 deraadt 1207: .Xr rsh 1 .
1.1 deraadt 1208: All other options (except
1.2 deraadt 1209: .Cm HostName )
1.40 aaron 1210: are ignored if this has been specified.
1211: The argument must be
1.2 deraadt 1212: .Dq yes
1213: or
1214: .Dq no .
1.55 markus 1215: .It Cm XAuthLocation
1216: Specifies the location of the
1217: .Xr xauth 1
1218: program.
1219: The default is
1220: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1221: .El
1.2 deraadt 1222: .Sh ENVIRONMENT
1223: .Nm
1.1 deraadt 1224: will normally set the following environment variables:
1.2 deraadt 1225: .Bl -tag -width Ds
1226: .It Ev DISPLAY
1227: The
1228: .Ev DISPLAY
1.40 aaron 1229: variable indicates the location of the X11 server.
1.44 aaron 1230: It is automatically set by
1.2 deraadt 1231: .Nm
1232: to point to a value of the form
1233: .Dq hostname:n
1234: where hostname indicates
1.40 aaron 1235: the host where the shell runs, and n is an integer >= 1.
1236: .Nm
1237: uses this special value to forward X11 connections over the secure
1238: channel.
1.107 markus 1239: The user should normally not set
1240: .Ev DISPLAY
1241: explicitly, as that
1.1 deraadt 1242: will render the X11 connection insecure (and will require the user to
1243: manually copy any required authorization cookies).
1.2 deraadt 1244: .It Ev HOME
1.1 deraadt 1245: Set to the path of the user's home directory.
1.2 deraadt 1246: .It Ev LOGNAME
1247: Synonym for
1.12 aaron 1248: .Ev USER ;
1249: set for compatibility with systems that use this variable.
1.2 deraadt 1250: .It Ev MAIL
1.129 stevesk 1251: Set to the path of the user's mailbox.
1.40 aaron 1252: .It Ev PATH
1.2 deraadt 1253: Set to the default
1254: .Ev PATH ,
1255: as specified when compiling
1.12 aaron 1256: .Nm ssh .
1.118 markus 1257: .It Ev SSH_ASKPASS
1258: If
1259: .Nm
1260: needs a passphrase, it will read the passphrase from the current
1261: terminal if it was run from a terminal.
1262: If
1263: .Nm
1264: does not have a terminal associated with it but
1265: .Ev DISPLAY
1266: and
1267: .Ev SSH_ASKPASS
1268: are set, it will execute the program specified by
1269: .Ev SSH_ASKPASS
1270: and open an X11 window to read the passphrase.
1271: This is particularly useful when calling
1272: .Nm
1273: from a
1274: .Pa .Xsession
1275: or related script.
1276: (Note that on some machines it
1277: may be necessary to redirect the input from
1278: .Pa /dev/null
1279: to make this work.)
1.18 markus 1280: .It Ev SSH_AUTH_SOCK
1.129 stevesk 1281: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 1282: agent.
1.2 deraadt 1283: .It Ev SSH_CLIENT
1.40 aaron 1284: Identifies the client end of the connection.
1285: The variable contains
1.1 deraadt 1286: three space-separated values: client ip-address, client port number,
1287: and server port number.
1.73 markus 1288: .It Ev SSH_ORIGINAL_COMMAND
1289: The variable contains the original command line if a forced command
1290: is executed.
1291: It can be used to extract the original arguments.
1.2 deraadt 1292: .It Ev SSH_TTY
1.1 deraadt 1293: This is set to the name of the tty (path to the device) associated
1.40 aaron 1294: with the current shell or command.
1295: If the current session has no tty,
1.1 deraadt 1296: this variable is not set.
1.2 deraadt 1297: .It Ev TZ
1.1 deraadt 1298: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1299: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1300: on to new connections).
1.2 deraadt 1301: .It Ev USER
1.1 deraadt 1302: Set to the name of the user logging in.
1.2 deraadt 1303: .El
1304: .Pp
1.44 aaron 1305: Additionally,
1.2 deraadt 1306: .Nm
1.44 aaron 1307: reads
1308: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1309: and adds lines of the format
1310: .Dq VARNAME=value
1.12 aaron 1311: to the environment.
1.2 deraadt 1312: .Sh FILES
1.36 markus 1313: .Bl -tag -width Ds
1.116 markus 1314: .It Pa $HOME/.ssh/known_hosts
1.129 stevesk 1315: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 1316: in
1.116 markus 1317: .Pa /etc/ssh_known_hosts .
1.2 deraadt 1318: See
1319: .Xr sshd 8 .
1.102 itojun 1320: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1321: Contains the authentication identity of the user.
1322: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 1323: These files
1324: contain sensitive data and should be readable by the user but not
1.15 markus 1325: accessible by others (read/write/execute).
1326: Note that
1327: .Nm
1.48 markus 1328: ignores a private key file if it is accessible by others.
1.15 markus 1329: It is possible to specify a passphrase when
1.1 deraadt 1330: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1331: sensitive part of this file using 3DES.
1.102 itojun 1332: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 1333: Contains the public key for authentication (public part of the
1.40 aaron 1334: identity file in human-readable form).
1.48 markus 1335: The contents of the
1336: .Pa $HOME/.ssh/identity.pub
1337: file should be added to
1.2 deraadt 1338: .Pa $HOME/.ssh/authorized_keys
1339: on all machines
1.137 deraadt 1340: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 1341: The contents of the
1342: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 1343: and
1344: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 1345: file should be added to
1.115 markus 1346: .Pa $HOME/.ssh/authorized_keys
1.48 markus 1347: on all machines
1.137 deraadt 1348: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 1349: These files are not
1.40 aaron 1350: sensitive and can (but need not) be readable by anyone.
1.48 markus 1351: These files are
1.84 markus 1352: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1353: the convenience of the user.
1.2 deraadt 1354: .It Pa $HOME/.ssh/config
1.40 aaron 1355: This is the per-user configuration file.
1356: The format of this file is described above.
1357: This file is used by the
1.2 deraadt 1358: .Nm
1.40 aaron 1359: client.
1360: This file does not usually contain any sensitive information,
1.1 deraadt 1361: but the recommended permissions are read/write for the user, and not
1362: accessible by others.
1.2 deraadt 1363: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 1364: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 1365: The format of this file is described in the
1.2 deraadt 1366: .Xr sshd 8
1.40 aaron 1367: manual page.
1.116 markus 1368: In the simplest form the format is the same as the .pub
1369: identity files.
1.48 markus 1370: This file is not highly sensitive, but the recommended
1371: permissions are read/write for the user, and not accessible by others.
1.116 markus 1372: .It Pa /etc/ssh_known_hosts
1.40 aaron 1373: Systemwide list of known host keys.
1.116 markus 1374: This file should be prepared by the
1.1 deraadt 1375: system administrator to contain the public host keys of all machines in the
1.40 aaron 1376: organization.
1377: This file should be world-readable.
1378: This file contains
1.1 deraadt 1379: public keys, one per line, in the following format (fields separated
1.116 markus 1380: by spaces): system name, public key and optional comment field.
1.40 aaron 1381: When different names are used
1.1 deraadt 1382: for the same machine, all such names should be listed, separated by
1.40 aaron 1383: commas.
1384: The format is described on the
1.2 deraadt 1385: .Xr sshd 8
1.1 deraadt 1386: manual page.
1.2 deraadt 1387: .Pp
1.1 deraadt 1388: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1389: .Xr sshd 8
1.1 deraadt 1390: to verify the client host when logging in; other names are needed because
1.2 deraadt 1391: .Nm
1.1 deraadt 1392: does not convert the user-supplied name to a canonical name before
1393: checking the key, because someone with access to the name servers
1394: would then be able to fool host authentication.
1.2 deraadt 1395: .It Pa /etc/ssh_config
1.40 aaron 1396: Systemwide configuration file.
1397: This file provides defaults for those
1.1 deraadt 1398: values that are not specified in the user's configuration file, and
1.40 aaron 1399: for those users who do not have a configuration file.
1400: This file must be world-readable.
1.141 markus 1401: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1402: These three files contain the private parts of the host keys
1403: and are used for
1404: .Cm RhostsRSAAuthentication
1405: and
1406: .Cm HostbasedAuthentication .
1407: Since they are readable only by root
1408: .Nm
1409: must be setuid root if these authentication methods are desired.
1.2 deraadt 1410: .It Pa $HOME/.rhosts
1411: This file is used in
1412: .Pa \&.rhosts
1413: authentication to list the
1.40 aaron 1414: host/user pairs that are permitted to log in.
1415: (Note that this file is
1.1 deraadt 1416: also used by rlogin and rsh, which makes using this file insecure.)
1417: Each line of the file contains a host name (in the canonical form
1418: returned by name servers), and then a user name on that host,
1.40 aaron 1419: separated by a space.
1.92 markus 1420: On some machines this file may need to be
1.1 deraadt 1421: world-readable if the user's home directory is on a NFS partition,
1422: because
1.2 deraadt 1423: .Xr sshd 8
1.40 aaron 1424: reads it as root.
1425: Additionally, this file must be owned by the user,
1426: and must not have write permissions for anyone else.
1427: The recommended
1.1 deraadt 1428: permission for most machines is read/write for the user, and not
1429: accessible by others.
1.2 deraadt 1430: .Pp
1.1 deraadt 1431: Note that by default
1.2 deraadt 1432: .Xr sshd 8
1.1 deraadt 1433: will be installed so that it requires successful RSA host
1.40 aaron 1434: authentication before permitting \s+2.\s0rhosts authentication.
1.137 deraadt 1435: If the server machine does not have the client's host key in
1.2 deraadt 1436: .Pa /etc/ssh_known_hosts ,
1.137 deraadt 1437: it can be stored in
1.2 deraadt 1438: .Pa $HOME/.ssh/known_hosts .
1439: The easiest way to do this is to
1.1 deraadt 1440: connect back to the client from the server machine using ssh; this
1.48 markus 1441: will automatically add the host key to
1.2 deraadt 1442: .Pa $HOME/.ssh/known_hosts .
1443: .It Pa $HOME/.shosts
1444: This file is used exactly the same way as
1445: .Pa \&.rhosts .
1446: The purpose for
1.1 deraadt 1447: having this file is to be able to use rhosts authentication with
1.2 deraadt 1448: .Nm
1449: without permitting login with
1450: .Xr rlogin 1
1451: or
1452: .Xr rsh 1 .
1453: .It Pa /etc/hosts.equiv
1454: This file is used during
1.40 aaron 1455: .Pa \&.rhosts authentication.
1456: It contains
1.1 deraadt 1457: canonical hosts names, one per line (the full format is described on
1458: the
1.2 deraadt 1459: .Xr sshd 8
1.40 aaron 1460: manual page).
1461: If the client host is found in this file, login is
1.1 deraadt 1462: automatically permitted provided client and server user names are the
1.40 aaron 1463: same.
1464: Additionally, successful RSA host authentication is normally
1465: required.
1466: This file should only be writable by root.
1.2 deraadt 1467: .It Pa /etc/shosts.equiv
1.44 aaron 1468: This file is processed exactly as
1.2 deraadt 1469: .Pa /etc/hosts.equiv .
1.1 deraadt 1470: This file may be useful to permit logins using
1.2 deraadt 1471: .Nm
1.1 deraadt 1472: but not using rsh/rlogin.
1.2 deraadt 1473: .It Pa /etc/sshrc
1.1 deraadt 1474: Commands in this file are executed by
1.2 deraadt 1475: .Nm
1.1 deraadt 1476: when the user logs in just before the user's shell (or command) is started.
1477: See the
1.2 deraadt 1478: .Xr sshd 8
1.1 deraadt 1479: manual page for more information.
1.2 deraadt 1480: .It Pa $HOME/.ssh/rc
1.1 deraadt 1481: Commands in this file are executed by
1.2 deraadt 1482: .Nm
1.1 deraadt 1483: when the user logs in just before the user's shell (or command) is
1484: started.
1.44 aaron 1485: See the
1.2 deraadt 1486: .Xr sshd 8
1.1 deraadt 1487: manual page for more information.
1.31 markus 1488: .It Pa $HOME/.ssh/environment
1489: Contains additional definitions for environment variables, see section
1490: .Sx ENVIRONMENT
1491: above.
1.58 itojun 1492: .El
1.145 ! markus 1493: .Sh DIAGNOSTICS
! 1494: .Nm
! 1495: exits with the exit status of the remote command or with 255
! 1496: if an error occurred.
1.67 aaron 1497: .Sh AUTHORS
1.78 markus 1498: OpenSSH is a derivative of the original and free
1499: ssh 1.2.12 release by Tatu Ylonen.
1500: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1501: Theo de Raadt and Dug Song
1502: removed many bugs, re-added newer features and
1503: created OpenSSH.
1504: Markus Friedl contributed the support for SSH
1505: protocol versions 1.5 and 2.0.
1.2 deraadt 1506: .Sh SEE ALSO
1507: .Xr rlogin 1 ,
1508: .Xr rsh 1 ,
1509: .Xr scp 1 ,
1.83 djm 1510: .Xr sftp 1 ,
1.2 deraadt 1511: .Xr ssh-add 1 ,
1512: .Xr ssh-agent 1 ,
1513: .Xr ssh-keygen 1 ,
1514: .Xr telnet 1 ,
1.87 itojun 1515: .Xr sshd 8
1.106 markus 1516: .Rs
1517: .%A T. Ylonen
1518: .%A T. Kivinen
1519: .%A M. Saarinen
1520: .%A T. Rinne
1521: .%A S. Lehtinen
1522: .%T "SSH Protocol Architecture"
1.126 markus 1523: .%N draft-ietf-secsh-architecture-09.txt
1524: .%D July 2001
1.106 markus 1525: .%O work in progress material
1526: .Re
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