Annotation of src/usr.bin/ssh/ssh.1, Revision 1.121
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
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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: .\"
1.121 ! pvalchev 37: .\" $OpenBSD: ssh.1,v 1.120 2001/07/20 18:41:51 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
1.5 deraadt 47: .Op 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
1.12 aaron 60: .Oo Fl L Xo
61: .Sm off
1.33 markus 62: .Ar port :
1.12 aaron 63: .Ar host :
64: .Ar hostport
65: .Sm on
66: .Xc
67: .Oc
68: .Oo Fl R Xo
69: .Sm off
1.33 markus 70: .Ar port :
1.12 aaron 71: .Ar host :
72: .Ar hostport
73: .Sm on
74: .Xc
75: .Oc
1.5 deraadt 76: .Op Ar hostname | user@hostname
1.2 deraadt 77: .Op Ar command
1.44 aaron 78: .Sh DESCRIPTION
1.2 deraadt 79: .Nm
1.96 deraadt 80: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 81: executing commands on a remote machine.
82: It is intended to replace
1.1 deraadt 83: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 84: two untrusted hosts over an insecure network.
85: X11 connections and
1.1 deraadt 86: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 87: .Pp
88: .Nm
1.44 aaron 89: connects and logs into the specified
1.2 deraadt 90: .Ar hostname .
1.1 deraadt 91: The user must prove
1.49 markus 92: his/her identity to the remote machine using one of several methods
93: depending on the protocol version used:
94: .Pp
95: .Ss SSH protocol version 1
1.2 deraadt 96: .Pp
1.1 deraadt 97: First, if the machine the user logs in from is listed in
1.2 deraadt 98: .Pa /etc/hosts.equiv
1.1 deraadt 99: or
1.2 deraadt 100: .Pa /etc/shosts.equiv
1.1 deraadt 101: on the remote machine, and the user names are
102: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 103: Second, if
1.2 deraadt 104: .Pa \&.rhosts
1.1 deraadt 105: or
1.2 deraadt 106: .Pa \&.shosts
1.1 deraadt 107: exists in the user's home directory on the
108: remote machine and contains a line containing the name of the client
109: machine and the name of the user on that machine, the user is
1.40 aaron 110: permitted to log in.
111: This form of authentication alone is normally not
1.1 deraadt 112: allowed by the server because it is not secure.
1.2 deraadt 113: .Pp
1.107 markus 114: The second authentication method is the
1.2 deraadt 115: .Pa rhosts
1.1 deraadt 116: or
1.2 deraadt 117: .Pa hosts.equiv
1.40 aaron 118: method combined with RSA-based host authentication.
119: It means that if the login would be permitted by
1.49 markus 120: .Pa $HOME/.rhosts ,
121: .Pa $HOME/.shosts ,
1.2 deraadt 122: .Pa /etc/hosts.equiv ,
1.1 deraadt 123: or
1.2 deraadt 124: .Pa /etc/shosts.equiv ,
1.11 deraadt 125: and if additionally the server can verify the client's
1.44 aaron 126: host key (see
1.2 deraadt 127: .Pa /etc/ssh_known_hosts
1.23 markus 128: and
129: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 130: in the
1.2 deraadt 131: .Sx FILES
1.40 aaron 132: section), only then login is permitted.
133: This authentication method closes security holes due to IP
134: spoofing, DNS spoofing and routing spoofing.
135: [Note to the administrator:
1.2 deraadt 136: .Pa /etc/hosts.equiv ,
1.49 markus 137: .Pa $HOME/.rhosts ,
1.1 deraadt 138: and the rlogin/rsh protocol in general, are inherently insecure and should be
139: disabled if security is desired.]
1.2 deraadt 140: .Pp
1.44 aaron 141: As a third authentication method,
1.2 deraadt 142: .Nm
1.1 deraadt 143: supports RSA based authentication.
144: The scheme is based on public-key cryptography: there are cryptosystems
145: where encryption and decryption are done using separate keys, and it
146: is not possible to derive the decryption key from the encryption key.
1.40 aaron 147: RSA is one such system.
1.44 aaron 148: The idea is that each user creates a public/private
1.40 aaron 149: key pair for authentication purposes.
150: The server knows the public key, and only the user knows the private key.
1.44 aaron 151: The file
1.2 deraadt 152: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 153: lists the public keys that are permitted for logging
1.40 aaron 154: in.
155: When the user logs in, the
1.2 deraadt 156: .Nm
1.1 deraadt 157: program tells the server which key pair it would like to use for
1.40 aaron 158: authentication.
159: The server checks if this key is permitted, and if
1.1 deraadt 160: so, sends the user (actually the
1.2 deraadt 161: .Nm
1.1 deraadt 162: program running on behalf of the user) a challenge, a random number,
1.40 aaron 163: encrypted by the user's public key.
164: The challenge can only be
165: decrypted using the proper private key.
166: The user's client then decrypts the
1.1 deraadt 167: challenge using the private key, proving that he/she knows the private
168: key but without disclosing it to the server.
1.2 deraadt 169: .Pp
170: .Nm
1.40 aaron 171: implements the RSA authentication protocol automatically.
172: The user creates his/her RSA key pair by running
1.2 deraadt 173: .Xr ssh-keygen 1 .
1.44 aaron 174: This stores the private key in
1.49 markus 175: .Pa $HOME/.ssh/identity
1.1 deraadt 176: and the public key in
1.49 markus 177: .Pa $HOME/.ssh/identity.pub
1.40 aaron 178: in the user's home directory.
179: The user should then copy the
1.2 deraadt 180: .Pa identity.pub
1.44 aaron 181: to
1.49 markus 182: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 183: in his/her home directory on the remote machine (the
1.2 deraadt 184: .Pa authorized_keys
1.44 aaron 185: file corresponds to the conventional
1.49 markus 186: .Pa $HOME/.rhosts
1.1 deraadt 187: file, and has one key
1.40 aaron 188: per line, though the lines can be very long).
189: After this, the user can log in without giving the password.
190: RSA authentication is much
1.1 deraadt 191: more secure than rhosts authentication.
1.2 deraadt 192: .Pp
1.1 deraadt 193: The most convenient way to use RSA authentication may be with an
1.40 aaron 194: authentication agent.
195: See
1.2 deraadt 196: .Xr ssh-agent 1
1.1 deraadt 197: for more information.
1.2 deraadt 198: .Pp
1.44 aaron 199: If other authentication methods fail,
1.2 deraadt 200: .Nm
1.40 aaron 201: prompts the user for a password.
202: The password is sent to the remote
1.1 deraadt 203: host for checking; however, since all communications are encrypted,
204: the password cannot be seen by someone listening on the network.
1.2 deraadt 205: .Pp
1.49 markus 206: .Ss SSH protocol version 2
207: .Pp
208: When a user connects using the protocol version 2
1.107 markus 209: different authentication methods are available.
210: Using the default values for
211: .Cm PreferredAuthentications ,
212: the client will try to authenticate first using the public key method;
213: if this method fails password authentication is attempted,
214: and finally if this method fails keyboard-interactive authentication
215: is attempted.
216: If this method fails password authentication is
217: 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
1.107 markus 297: to terminate (protocol version 1 only)
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.
1.6 deraadt 394: .It Fl c Ar blowfish|3des
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: It is presumably more secure than the
1.2 deraadt 402: .Ar des
1.64 markus 403: cipher which is no longer fully supported in
1.51 markus 404: .Nm ssh .
1.5 deraadt 405: .Ar blowfish
406: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 407: .Ar 3des .
1.90 markus 408: .It Fl c Ar cipher_spec
1.51 markus 409: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 410: be specified in order of preference.
1.90 markus 411: See
412: .Cm Ciphers
413: for more information.
1.2 deraadt 414: .It Fl e Ar ch|^ch|none
415: Sets the escape character for sessions with a pty (default:
416: .Ql ~ ) .
1.40 aaron 417: The escape character is only recognized at the beginning of a line.
418: The escape character followed by a dot
1.2 deraadt 419: .Pq Ql \&.
420: closes the connection, followed
1.1 deraadt 421: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 422: escape character once.
423: Setting the character to
1.2 deraadt 424: .Dq none
425: disables any escapes and makes the session fully transparent.
426: .It Fl f
427: Requests
428: .Nm
1.40 aaron 429: to go to background just before command execution.
430: This is useful if
1.2 deraadt 431: .Nm
432: is going to ask for passwords or passphrases, but the user
1.40 aaron 433: wants it in the background.
1.44 aaron 434: This implies
1.2 deraadt 435: .Fl n .
1.1 deraadt 436: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 437: something like
438: .Ic ssh -f host xterm .
1.34 markus 439: .It Fl g
440: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 441: .It Fl i Ar identity_file
1.44 aaron 442: Selects the file from which the identity (private key) for
1.68 markus 443: RSA or DSA authentication is read.
1.44 aaron 444: Default is
1.49 markus 445: .Pa $HOME/.ssh/identity
1.40 aaron 446: in the user's home directory.
447: Identity files may also be specified on
448: a per-host basis in the configuration file.
449: It is possible to have multiple
1.2 deraadt 450: .Fl i
451: options (and multiple identities specified in
1.1 deraadt 452: configuration files).
1.2 deraadt 453: .It Fl k
1.42 aaron 454: Disables forwarding of Kerberos tickets and AFS tokens.
455: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 456: .It Fl l Ar login_name
1.40 aaron 457: Specifies the user to log in as on the remote machine.
458: This also may be specified on a per-host basis in the configuration file.
1.91 markus 459: .It Fl m Ar mac_spec
460: Additionally, for protocol version 2 a comma-separated list of MAC
461: (message authentication code) algorithms can
462: be specified in order of preference.
463: See the
464: .Cm MACs
465: keyword for more information.
1.2 deraadt 466: .It Fl n
467: Redirects stdin from
468: .Pa /dev/null
469: (actually, prevents reading from stdin).
1.1 deraadt 470: This must be used when
1.2 deraadt 471: .Nm
1.40 aaron 472: is run in the background.
473: A common trick is to use this to run X11 programs on a remote machine.
474: For example,
1.2 deraadt 475: .Ic ssh -n shadows.cs.hut.fi emacs &
476: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 477: connection will be automatically forwarded over an encrypted channel.
478: The
1.2 deraadt 479: .Nm
1.1 deraadt 480: program will be put in the background.
481: (This does not work if
1.2 deraadt 482: .Nm
483: needs to ask for a password or passphrase; see also the
484: .Fl f
485: option.)
1.53 markus 486: .It Fl N
487: Do not execute a remote command.
1.70 markus 488: This is useful if you just want to forward ports
1.53 markus 489: (protocol version 2 only).
1.2 deraadt 490: .It Fl o Ar option
1.1 deraadt 491: Can be used to give options in the format used in the config file.
492: This is useful for specifying options for which there is no separate
1.40 aaron 493: command-line flag.
494: The option has the same format as a line in the configuration file.
1.2 deraadt 495: .It Fl p Ar port
1.40 aaron 496: Port to connect to on the remote host.
497: This can be specified on a
1.1 deraadt 498: per-host basis in the configuration file.
1.16 markus 499: .It Fl P
500: Use a non-privileged port for outgoing connections.
501: This can be used if your firewall does
502: not permit connections from privileged ports.
1.30 provos 503: Note that this option turns off
1.16 markus 504: .Cm RhostsAuthentication
505: and
1.72 markus 506: .Cm RhostsRSAAuthentication
507: for older servers.
1.2 deraadt 508: .It Fl q
1.40 aaron 509: Quiet mode.
510: Causes all warning and diagnostic messages to be suppressed.
511: Only fatal errors are displayed.
1.80 djm 512: .It Fl s
1.117 itojun 513: 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
514: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 515: subsystem is specified as the remote command.
1.2 deraadt 516: .It Fl t
1.40 aaron 517: Force pseudo-tty allocation.
1.43 brad 518: This can be used to execute arbitrary
1.40 aaron 519: screen-based programs on a remote machine, which can be very useful,
520: e.g., when implementing menu services.
1.73 markus 521: Multiple
522: .Fl t
523: options force tty allocation, even if
524: .Nm
525: has no local tty.
1.53 markus 526: .It Fl T
1.69 markus 527: Disable pseudo-tty allocation.
1.2 deraadt 528: .It Fl v
1.40 aaron 529: Verbose mode.
530: Causes
1.2 deraadt 531: .Nm
1.40 aaron 532: to print debugging messages about its progress.
533: This is helpful in
1.1 deraadt 534: debugging connection, authentication, and configuration problems.
1.73 markus 535: Multiple
536: .Fl v
537: options increases the verbosity.
1.61 aaron 538: Maximum is 3.
1.2 deraadt 539: .It Fl x
1.40 aaron 540: Disables X11 forwarding.
1.2 deraadt 541: .It Fl X
1.1 deraadt 542: Enables X11 forwarding.
1.54 markus 543: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 544: .It Fl C
1.1 deraadt 545: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 546: data for forwarded X11 and TCP/IP connections).
547: The compression algorithm is the same used by
1.34 markus 548: .Xr gzip 1 ,
549: and the
1.2 deraadt 550: .Dq level
551: can be controlled by the
552: .Cm CompressionLevel
1.40 aaron 553: option (see below).
554: Compression is desirable on modem lines and other
1.1 deraadt 555: slow connections, but will only slow down things on fast networks.
556: The default value can be set on a host-by-host basis in the
557: configuration files; see the
1.121 ! pvalchev 558: .Cm Compression
1.1 deraadt 559: option below.
1.2 deraadt 560: .It Fl L Ar port:host:hostport
1.1 deraadt 561: Specifies that the given port on the local (client) host is to be
1.40 aaron 562: forwarded to the given host and port on the remote side.
563: This works by allocating a socket to listen to
1.2 deraadt 564: .Ar port
1.1 deraadt 565: on the local side, and whenever a connection is made to this port, the
566: connection is forwarded over the secure channel, and a connection is
567: made to
1.32 markus 568: .Ar host
569: port
570: .Ar hostport
1.40 aaron 571: from the remote machine.
572: Port forwardings can also be specified in the configuration file.
573: Only root can forward privileged ports.
1.32 markus 574: IPv6 addresses can be specified with an alternative syntax:
575: .Ar port/host/hostport
1.2 deraadt 576: .It Fl R Ar port:host:hostport
1.1 deraadt 577: Specifies that the given port on the remote (server) host is to be
1.40 aaron 578: forwarded to the given host and port on the local side.
579: This works by allocating a socket to listen to
1.2 deraadt 580: .Ar port
1.1 deraadt 581: on the remote side, and whenever a connection is made to this port, the
582: connection is forwarded over the secure channel, and a connection is
583: made to
1.32 markus 584: .Ar host
585: port
586: .Ar hostport
1.40 aaron 587: from the local machine.
588: Port forwardings can also be specified in the configuration file.
589: Privileged ports can be forwarded only when
1.1 deraadt 590: logging in as root on the remote machine.
1.107 markus 591: IPv6 addresses can be specified with an alternative syntax:
592: .Ar port/host/hostport
1.85 jakob 593: .It Fl 1
594: Forces
595: .Nm
596: to try protocol version 1 only.
1.46 markus 597: .It Fl 2
598: Forces
599: .Nm
1.50 markus 600: to try protocol version 2 only.
1.32 markus 601: .It Fl 4
602: Forces
603: .Nm
604: to use IPv4 addresses only.
605: .It Fl 6
606: Forces
607: .Nm
608: to use IPv6 addresses only.
1.2 deraadt 609: .El
610: .Sh CONFIGURATION FILES
611: .Nm
1.1 deraadt 612: obtains configuration data from the following sources (in this order):
613: command line options, user's configuration file
1.2 deraadt 614: .Pq Pa $HOME/.ssh/config ,
615: and system-wide configuration file
616: .Pq Pa /etc/ssh_config .
617: For each parameter, the first obtained value
1.40 aaron 618: will be used.
619: The configuration files contain sections bracketed by
620: .Dq Host
621: specifications, and that section is only applied for hosts that
622: match one of the patterns given in the specification.
623: The matched host name is the one given on the command line.
1.2 deraadt 624: .Pp
1.1 deraadt 625: Since the first obtained value for each parameter is used, more
626: host-specific declarations should be given near the beginning of the
627: file, and general defaults at the end.
1.2 deraadt 628: .Pp
1.1 deraadt 629: The configuration file has the following format:
1.2 deraadt 630: .Pp
631: Empty lines and lines starting with
632: .Ql #
633: are comments.
634: .Pp
635: Otherwise a line is of the format
636: .Dq keyword arguments .
637: The possible
1.1 deraadt 638: keywords and their meanings are as follows (note that the
639: configuration files are case-sensitive):
1.2 deraadt 640: .Bl -tag -width Ds
641: .It Cm Host
1.1 deraadt 642: Restricts the following declarations (up to the next
1.2 deraadt 643: .Cm Host
1.1 deraadt 644: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 645: given after the keyword.
646: .Ql \&*
647: and
648: .Ql ?
649: can be used as wildcards in the
1.40 aaron 650: patterns.
651: A single
1.2 deraadt 652: .Ql \&*
653: as a pattern can be used to provide global
1.40 aaron 654: defaults for all hosts.
655: The host is the
1.2 deraadt 656: .Ar hostname
1.1 deraadt 657: argument given on the command line (i.e., the name is not converted to
658: a canonicalized host name before matching).
1.2 deraadt 659: .It Cm AFSTokenPassing
1.42 aaron 660: Specifies whether to pass AFS tokens to remote host.
661: The argument to this keyword must be
1.2 deraadt 662: .Dq yes
663: or
664: .Dq no .
1.107 markus 665: This option applies to protocol version 1 only.
1.2 deraadt 666: .It Cm BatchMode
667: If set to
668: .Dq yes ,
1.40 aaron 669: passphrase/password querying will be disabled.
670: This option is useful in scripts and other batch jobs where you have no
671: user to supply the password.
672: The argument must be
1.2 deraadt 673: .Dq yes
674: or
675: .Dq no .
1.100 stevesk 676: The default is
677: .Dq no .
1.108 markus 678: .It Cm BindAddress
679: Specify the interface to transmit from on machines with multiple
680: interfaces or aliased addresses.
681: Note that this option does not work if
682: .Cm UsePrivilegedPort
683: is set to
684: .Dq yes .
1.34 markus 685: .It Cm CheckHostIP
686: If this flag is set to
687: .Dq yes ,
1.100 stevesk 688: ssh will additionally check the host IP address in the
1.34 markus 689: .Pa known_hosts
1.42 aaron 690: file.
691: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 692: If the option is set to
693: .Dq no ,
694: the check will not be executed.
1.100 stevesk 695: The default is
696: .Dq yes .
1.2 deraadt 697: .It Cm Cipher
1.62 markus 698: Specifies the cipher to use for encrypting the session
1.64 markus 699: in protocol version 1.
1.40 aaron 700: Currently,
1.117 itojun 701: .Dq blowfish
1.1 deraadt 702: and
1.10 provos 703: .Dq 3des
1.40 aaron 704: are supported.
705: The default is
1.2 deraadt 706: .Dq 3des .
1.45 markus 707: .It Cm Ciphers
708: Specifies the ciphers allowed for protocol version 2
709: in order of preference.
710: Multiple ciphers must be comma-separated.
711: The default is
1.88 provos 712: .Pp
713: .Bd -literal
1.94 deraadt 714: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.107 markus 715: aes192-cbc,aes256-cbc''
1.88 provos 716: .Ed
1.2 deraadt 717: .It Cm Compression
1.40 aaron 718: Specifies whether to use compression.
719: The argument must be
1.2 deraadt 720: .Dq yes
721: or
722: .Dq no .
1.100 stevesk 723: The default is
724: .Dq no .
1.2 deraadt 725: .It Cm CompressionLevel
1.100 stevesk 726: Specifies the compression level to use if compression is enabled.
1.40 aaron 727: The argument must be an integer from 1 (fast) to 9 (slow, best).
728: The default level is 6, which is good for most applications.
729: The meaning of the values is the same as in
1.34 markus 730: .Xr gzip 1 .
1.107 markus 731: Note that this option applies to protocol version 1 only.
1.2 deraadt 732: .It Cm ConnectionAttempts
1.1 deraadt 733: Specifies the number of tries (one per second) to make before falling
1.40 aaron 734: back to rsh or exiting.
735: The argument must be an integer.
736: This may be useful in scripts if the connection sometimes fails.
1.100 stevesk 737: The default is 4.
1.2 deraadt 738: .It Cm EscapeChar
739: Sets the escape character (default:
740: .Ql ~ ) .
741: The escape character can also
1.40 aaron 742: be set on the command line.
743: The argument should be a single character,
1.2 deraadt 744: .Ql ^
745: followed by a letter, or
746: .Dq none
747: to disable the escape
1.1 deraadt 748: character entirely (making the connection transparent for binary
749: data).
1.44 aaron 750: .It Cm FallBackToRsh
1.1 deraadt 751: Specifies that if connecting via
1.2 deraadt 752: .Nm
1.1 deraadt 753: fails due to a connection refused error (there is no
1.2 deraadt 754: .Xr sshd 8
1.44 aaron 755: listening on the remote host),
1.2 deraadt 756: .Xr rsh 1
1.1 deraadt 757: should automatically be used instead (after a suitable warning about
1.40 aaron 758: the session being unencrypted).
759: The argument must be
1.2 deraadt 760: .Dq yes
761: or
762: .Dq no .
1.100 stevesk 763: The default is
764: .Dq no .
1.2 deraadt 765: .It Cm ForwardAgent
1.1 deraadt 766: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 767: will be forwarded to the remote machine.
768: The argument must be
1.2 deraadt 769: .Dq yes
770: or
1.54 markus 771: .Dq no .
772: The default is
1.2 deraadt 773: .Dq no .
774: .It Cm ForwardX11
1.1 deraadt 775: Specifies whether X11 connections will be automatically redirected
1.44 aaron 776: over the secure channel and
1.2 deraadt 777: .Ev DISPLAY
1.40 aaron 778: set.
1.44 aaron 779: The argument must be
1.2 deraadt 780: .Dq yes
781: or
1.38 markus 782: .Dq no .
783: The default is
1.3 deraadt 784: .Dq no .
785: .It Cm GatewayPorts
786: Specifies whether remote hosts are allowed to connect to local
787: forwarded ports.
788: The argument must be
789: .Dq yes
790: or
791: .Dq no .
792: The default is
1.2 deraadt 793: .Dq no .
794: .It Cm GlobalKnownHostsFile
1.116 markus 795: Specifies a file to use for the global
1.95 stevesk 796: host key database instead of
1.2 deraadt 797: .Pa /etc/ssh_known_hosts .
1.107 markus 798: .It Cm HostbasedAuthentication
799: Specifies whether to try rhosts based authentication with public key
800: authentication.
801: The argument must be
802: .Dq yes
803: or
804: .Dq no .
805: The default is
1.112 markus 806: .Dq no .
1.107 markus 807: This option applies to protocol version 2 only and
808: is similar to
809: .Cm RhostsRSAAuthentication .
810: .It Cm HostKeyAlgorithms
1.109 stevesk 811: Specifies the protocol version 2 host key algorithms
1.107 markus 812: that the client wants to use in order of preference.
813: The default for this option is:
814: .Dq ssh-rsa,ssh-dss
1.74 markus 815: .It Cm HostKeyAlias
816: Specifies an alias that should be used instead of the
817: real host name when looking up or saving the host key
1.107 markus 818: in the host key database files.
1.82 stevesk 819: This option is useful for tunneling ssh connections
1.74 markus 820: or if you have multiple servers running on a single host.
1.2 deraadt 821: .It Cm HostName
1.40 aaron 822: Specifies the real host name to log into.
823: This can be used to specify nicknames or abbreviations for hosts.
824: Default is the name given on the command line.
825: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 826: .Cm HostName
1.1 deraadt 827: specifications).
1.2 deraadt 828: .It Cm IdentityFile
1.107 markus 829: Specifies the file from which the user's RSA or DSA authentication identity
1.2 deraadt 830: is read (default
1.49 markus 831: .Pa $HOME/.ssh/identity
1.2 deraadt 832: in the user's home directory).
1.1 deraadt 833: Additionally, any identities represented by the authentication agent
1.40 aaron 834: will be used for authentication.
835: The file name may use the tilde
836: syntax to refer to a user's home directory.
837: It is possible to have
1.1 deraadt 838: multiple identity files specified in configuration files; all these
839: identities will be tried in sequence.
1.2 deraadt 840: .It Cm KeepAlive
1.1 deraadt 841: Specifies whether the system should send keepalive messages to the
1.40 aaron 842: other side.
843: If they are sent, death of the connection or crash of one
844: of the machines will be properly noticed.
845: However, this means that
1.1 deraadt 846: connections will die if the route is down temporarily, and some people
1.41 aaron 847: find it annoying.
1.2 deraadt 848: .Pp
849: The default is
850: .Dq yes
851: (to send keepalives), and the client will notice
1.40 aaron 852: if the network goes down or the remote host dies.
853: This is important in scripts, and many users want it too.
1.2 deraadt 854: .Pp
855: To disable keepalives, the value should be set to
856: .Dq no
857: in both the server and the client configuration files.
858: .It Cm KerberosAuthentication
1.42 aaron 859: Specifies whether Kerberos authentication will be used.
860: The argument to this keyword must be
1.4 dugsong 861: .Dq yes
862: or
863: .Dq no .
1.2 deraadt 864: .It Cm KerberosTgtPassing
1.42 aaron 865: Specifies whether a Kerberos TGT will be forwarded to the server.
866: This will only work if the Kerberos server is actually an AFS kaserver.
867: The argument to this keyword must be
1.4 dugsong 868: .Dq yes
869: or
870: .Dq no .
1.2 deraadt 871: .It Cm LocalForward
1.1 deraadt 872: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 873: the secure channel to given host:port from the remote machine.
874: The first argument must be a port number, and the second must be
875: host:port.
876: Multiple forwardings may be specified, and additional
877: forwardings can be given on the command line.
878: Only the superuser can forward privileged ports.
1.24 markus 879: .It Cm LogLevel
880: Gives the verbosity level that is used when logging messages from
881: .Nm ssh .
882: The possible values are:
1.77 markus 883: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
884: The default is INFO.
1.91 markus 885: .It Cm MACs
1.117 itojun 886: Specifies the MAC (message authentication code) algorithms
1.91 markus 887: in order of preference.
888: The MAC algorithm is used in protocol version 2
889: for data integrity protection.
890: Multiple algorithms must be comma-separated.
891: The default is
1.113 markus 892: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.14 dugsong 893: .It Cm NumberOfPasswordPrompts
1.42 aaron 894: Specifies the number of password prompts before giving up.
895: The argument to this keyword must be an integer.
896: Default is 3.
1.34 markus 897: .It Cm PasswordAuthentication
1.40 aaron 898: Specifies whether to use password authentication.
899: The argument to this keyword must be
1.34 markus 900: .Dq yes
901: or
902: .Dq no .
1.100 stevesk 903: The default is
904: .Dq yes .
1.2 deraadt 905: .It Cm Port
1.40 aaron 906: Specifies the port number to connect on the remote host.
907: Default is 22.
1.99 djm 908: .It Cm PreferredAuthentications
1.117 itojun 909: Specifies the order in which the client should try protocol 2
910: authentication methods. This allows a client to prefer one method (e.g.
1.99 djm 911: .Cm keyboard-interactive )
912: over another method (e.g.
913: .Cm password )
914: The default for this option is:
1.112 markus 915: .Dq publickey,hostbased,password,keyboard-interactive
1.45 markus 916: .It Cm Protocol
917: Specifies the protocol versions
918: .Nm
919: should support in order of preference.
920: The possible values are
921: .Dq 1
922: and
923: .Dq 2 .
924: Multiple versions must be comma-separated.
925: The default is
1.101 markus 926: .Dq 2,1 .
1.49 markus 927: This means that
928: .Nm
1.101 markus 929: tries version 2 and falls back to version 1
930: if version 2 is not available.
1.2 deraadt 931: .It Cm ProxyCommand
1.40 aaron 932: Specifies the command to use to connect to the server.
933: The command
934: string extends to the end of the line, and is executed with
935: .Pa /bin/sh .
936: In the command string,
937: .Ql %h
938: will be substituted by the host name to
939: connect and
940: .Ql %p
941: by the port.
942: The command can be basically anything,
943: and should read from its standard input and write to its standard output.
944: It should eventually connect an
1.2 deraadt 945: .Xr sshd 8
1.1 deraadt 946: server running on some machine, or execute
1.2 deraadt 947: .Ic sshd -i
1.40 aaron 948: somewhere.
949: Host key management will be done using the
1.1 deraadt 950: HostName of the host being connected (defaulting to the name typed by
951: the user).
1.29 markus 952: Note that
953: .Cm CheckHostIP
954: is not available for connects with a proxy command.
1.2 deraadt 955: .Pp
1.107 markus 956: .It Cm PubkeyAuthentication
957: Specifies whether to try public key authentication.
958: The argument to this keyword must be
959: .Dq yes
960: or
961: .Dq no .
962: The default is
963: .Dq yes .
964: This option applies to protocol version 2 only.
1.2 deraadt 965: .It Cm RemoteForward
1.1 deraadt 966: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 967: the secure channel to given host:port from the local machine.
968: The first argument must be a port number, and the second must be
969: host:port.
970: Multiple forwardings may be specified, and additional
971: forwardings can be given on the command line.
972: Only the superuser can forward privileged ports.
1.2 deraadt 973: .It Cm RhostsAuthentication
1.40 aaron 974: Specifies whether to try rhosts based authentication.
975: Note that this
1.1 deraadt 976: declaration only affects the client side and has no effect whatsoever
1.40 aaron 977: on security.
978: Disabling rhosts authentication may reduce
1.1 deraadt 979: authentication time on slow connections when rhosts authentication is
1.40 aaron 980: not used.
981: Most servers do not permit RhostsAuthentication because it
1.117 itojun 982: is not secure (see
1.114 mpech 983: .Cm RhostsRSAAuthentication ) .
1.40 aaron 984: The argument to this keyword must be
1.2 deraadt 985: .Dq yes
986: or
987: .Dq no .
1.100 stevesk 988: The default is
989: .Dq yes .
1.107 markus 990: This option applies to protocol version 1 only.
1.2 deraadt 991: .It Cm RhostsRSAAuthentication
1.1 deraadt 992: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 993: authentication.
994: The argument must be
1.2 deraadt 995: .Dq yes
996: or
997: .Dq no .
1.100 stevesk 998: The default is
999: .Dq yes .
1.107 markus 1000: This option applies to protocol version 1 only.
1.2 deraadt 1001: .It Cm RSAAuthentication
1.40 aaron 1002: Specifies whether to try RSA authentication.
1003: The argument to this keyword must be
1.2 deraadt 1004: .Dq yes
1005: or
1006: .Dq no .
1.1 deraadt 1007: RSA authentication will only be
1008: attempted if the identity file exists, or an authentication agent is
1009: running.
1.100 stevesk 1010: The default is
1011: .Dq yes .
1.50 markus 1012: Note that this option applies to protocol version 1 only.
1.81 markus 1013: .It Cm ChallengeResponseAuthentication
1014: Specifies whether to use challenge response authentication.
1015: Currently there is only support for
1.27 markus 1016: .Xr skey 1
1.40 aaron 1017: authentication.
1018: The argument to this keyword must be
1.27 markus 1019: .Dq yes
1020: or
1021: .Dq no .
1022: The default is
1023: .Dq no .
1.2 deraadt 1024: .It Cm StrictHostKeyChecking
1025: If this flag is set to
1.44 aaron 1026: .Dq yes ,
1.2 deraadt 1027: .Nm
1.79 stevesk 1028: will never automatically add host keys to the
1.2 deraadt 1029: .Pa $HOME/.ssh/known_hosts
1.116 markus 1030: file, and refuses to connect to hosts whose host key has changed.
1.40 aaron 1031: This provides maximum protection against trojan horse attacks.
1032: However, it can be somewhat annoying if you don't have good
1.2 deraadt 1033: .Pa /etc/ssh_known_hosts
1.1 deraadt 1034: files installed and frequently
1.79 stevesk 1035: connect to new hosts.
1036: This option forces the user to manually
1037: add all new hosts.
1038: If this flag is set to
1039: .Dq no ,
1040: .Nm
1041: will automatically add new host keys to the
1042: user known hosts files.
1043: If this flag is set to
1044: .Dq ask ,
1045: new host keys
1046: will be added to the user known host files only after the user
1047: has confirmed that is what they really want to do, and
1048: .Nm
1049: will refuse to connect to hosts whose host key has changed.
1.40 aaron 1050: The host keys of
1.79 stevesk 1051: known hosts will be verified automatically in all cases.
1.40 aaron 1052: The argument must be
1.79 stevesk 1053: .Dq yes ,
1054: .Dq no
1.2 deraadt 1055: or
1.79 stevesk 1056: .Dq ask .
1057: The default is
1058: .Dq ask .
1.16 markus 1059: .It Cm UsePrivilegedPort
1060: Specifies whether to use a privileged port for outgoing connections.
1061: The argument must be
1062: .Dq yes
1063: or
1064: .Dq no .
1065: The default is
1.98 markus 1066: .Dq no .
1.107 markus 1067: Note that you need to set this option to
1068: .Dq yes
1069: if you want to use
1.16 markus 1070: .Cm RhostsAuthentication
1071: and
1.72 markus 1072: .Cm RhostsRSAAuthentication
1.107 markus 1073: with older servers.
1.34 markus 1074: .It Cm User
1.40 aaron 1075: Specifies the user to log in as.
1076: This can be useful if you have a different user name on different machines.
1077: This saves the trouble of
1.34 markus 1078: having to remember to give the user name on the command line.
1079: .It Cm UserKnownHostsFile
1.116 markus 1080: Specifies a file to use for the user
1.95 stevesk 1081: host key database instead of
1.34 markus 1082: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 1083: .It Cm UseRsh
1.40 aaron 1084: Specifies that rlogin/rsh should be used for this host.
1085: It is possible that the host does not at all support the
1.2 deraadt 1086: .Nm
1.40 aaron 1087: protocol.
1088: This causes
1.2 deraadt 1089: .Nm
1.40 aaron 1090: to immediately execute
1.2 deraadt 1091: .Xr rsh 1 .
1.1 deraadt 1092: All other options (except
1.2 deraadt 1093: .Cm HostName )
1.40 aaron 1094: are ignored if this has been specified.
1095: The argument must be
1.2 deraadt 1096: .Dq yes
1097: or
1098: .Dq no .
1.55 markus 1099: .It Cm XAuthLocation
1100: Specifies the location of the
1101: .Xr xauth 1
1102: program.
1103: The default is
1104: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1105: .El
1.2 deraadt 1106: .Sh ENVIRONMENT
1107: .Nm
1.1 deraadt 1108: will normally set the following environment variables:
1.2 deraadt 1109: .Bl -tag -width Ds
1110: .It Ev DISPLAY
1111: The
1112: .Ev DISPLAY
1.40 aaron 1113: variable indicates the location of the X11 server.
1.44 aaron 1114: It is automatically set by
1.2 deraadt 1115: .Nm
1116: to point to a value of the form
1117: .Dq hostname:n
1118: where hostname indicates
1.40 aaron 1119: the host where the shell runs, and n is an integer >= 1.
1120: .Nm
1121: uses this special value to forward X11 connections over the secure
1122: channel.
1.107 markus 1123: The user should normally not set
1124: .Ev DISPLAY
1125: explicitly, as that
1.1 deraadt 1126: will render the X11 connection insecure (and will require the user to
1127: manually copy any required authorization cookies).
1.2 deraadt 1128: .It Ev HOME
1.1 deraadt 1129: Set to the path of the user's home directory.
1.2 deraadt 1130: .It Ev LOGNAME
1131: Synonym for
1.12 aaron 1132: .Ev USER ;
1133: set for compatibility with systems that use this variable.
1.2 deraadt 1134: .It Ev MAIL
1.1 deraadt 1135: Set to point the user's mailbox.
1.40 aaron 1136: .It Ev PATH
1.2 deraadt 1137: Set to the default
1138: .Ev PATH ,
1139: as specified when compiling
1.12 aaron 1140: .Nm ssh .
1.118 markus 1141: .It Ev SSH_ASKPASS
1142: If
1143: .Nm
1144: needs a passphrase, it will read the passphrase from the current
1145: terminal if it was run from a terminal.
1146: If
1147: .Nm
1148: does not have a terminal associated with it but
1149: .Ev DISPLAY
1150: and
1151: .Ev SSH_ASKPASS
1152: are set, it will execute the program specified by
1153: .Ev SSH_ASKPASS
1154: and open an X11 window to read the passphrase.
1155: This is particularly useful when calling
1156: .Nm
1157: from a
1158: .Pa .Xsession
1159: or related script.
1160: (Note that on some machines it
1161: may be necessary to redirect the input from
1162: .Pa /dev/null
1163: to make this work.)
1.18 markus 1164: .It Ev SSH_AUTH_SOCK
1.17 markus 1165: indicates the path of a unix-domain socket used to communicate with the
1166: agent.
1.2 deraadt 1167: .It Ev SSH_CLIENT
1.40 aaron 1168: Identifies the client end of the connection.
1169: The variable contains
1.1 deraadt 1170: three space-separated values: client ip-address, client port number,
1171: and server port number.
1.73 markus 1172: .It Ev SSH_ORIGINAL_COMMAND
1173: The variable contains the original command line if a forced command
1174: is executed.
1175: It can be used to extract the original arguments.
1.2 deraadt 1176: .It Ev SSH_TTY
1.1 deraadt 1177: This is set to the name of the tty (path to the device) associated
1.40 aaron 1178: with the current shell or command.
1179: If the current session has no tty,
1.1 deraadt 1180: this variable is not set.
1.2 deraadt 1181: .It Ev TZ
1.1 deraadt 1182: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1183: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1184: on to new connections).
1.2 deraadt 1185: .It Ev USER
1.1 deraadt 1186: Set to the name of the user logging in.
1.2 deraadt 1187: .El
1188: .Pp
1.44 aaron 1189: Additionally,
1.2 deraadt 1190: .Nm
1.44 aaron 1191: reads
1192: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1193: and adds lines of the format
1194: .Dq VARNAME=value
1.12 aaron 1195: to the environment.
1.2 deraadt 1196: .Sh FILES
1.36 markus 1197: .Bl -tag -width Ds
1.116 markus 1198: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 1199: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1200: in
1.116 markus 1201: .Pa /etc/ssh_known_hosts .
1.2 deraadt 1202: See
1203: .Xr sshd 8 .
1.102 itojun 1204: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1205: Contains the authentication identity of the user.
1206: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 1207: These files
1208: contain sensitive data and should be readable by the user but not
1.15 markus 1209: accessible by others (read/write/execute).
1210: Note that
1211: .Nm
1.48 markus 1212: ignores a private key file if it is accessible by others.
1.15 markus 1213: It is possible to specify a passphrase when
1.1 deraadt 1214: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1215: sensitive part of this file using 3DES.
1.102 itojun 1216: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 1217: Contains the public key for authentication (public part of the
1.40 aaron 1218: identity file in human-readable form).
1.48 markus 1219: The contents of the
1220: .Pa $HOME/.ssh/identity.pub
1221: file should be added to
1.2 deraadt 1222: .Pa $HOME/.ssh/authorized_keys
1223: on all machines
1.102 itojun 1224: where you wish to log in using protocol version 1 RSA authentication.
1.48 markus 1225: The contents of the
1226: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 1227: and
1228: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 1229: file should be added to
1.115 markus 1230: .Pa $HOME/.ssh/authorized_keys
1.48 markus 1231: on all machines
1.102 itojun 1232: where you wish to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 1233: These files are not
1.40 aaron 1234: sensitive and can (but need not) be readable by anyone.
1.48 markus 1235: These files are
1.84 markus 1236: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1237: the convenience of the user.
1.2 deraadt 1238: .It Pa $HOME/.ssh/config
1.40 aaron 1239: This is the per-user configuration file.
1240: The format of this file is described above.
1241: This file is used by the
1.2 deraadt 1242: .Nm
1.40 aaron 1243: client.
1244: This file does not usually contain any sensitive information,
1.1 deraadt 1245: but the recommended permissions are read/write for the user, and not
1246: accessible by others.
1.2 deraadt 1247: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 1248: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 1249: The format of this file is described in the
1.2 deraadt 1250: .Xr sshd 8
1.40 aaron 1251: manual page.
1.116 markus 1252: In the simplest form the format is the same as the .pub
1253: identity files.
1.48 markus 1254: This file is not highly sensitive, but the recommended
1255: permissions are read/write for the user, and not accessible by others.
1.116 markus 1256: .It Pa /etc/ssh_known_hosts
1.40 aaron 1257: Systemwide list of known host keys.
1.116 markus 1258: This file should be prepared by the
1.1 deraadt 1259: system administrator to contain the public host keys of all machines in the
1.40 aaron 1260: organization.
1261: This file should be world-readable.
1262: This file contains
1.1 deraadt 1263: public keys, one per line, in the following format (fields separated
1.116 markus 1264: by spaces): system name, public key and optional comment field.
1.40 aaron 1265: When different names are used
1.1 deraadt 1266: for the same machine, all such names should be listed, separated by
1.40 aaron 1267: commas.
1268: The format is described on the
1.2 deraadt 1269: .Xr sshd 8
1.1 deraadt 1270: manual page.
1.2 deraadt 1271: .Pp
1.1 deraadt 1272: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1273: .Xr sshd 8
1.1 deraadt 1274: to verify the client host when logging in; other names are needed because
1.2 deraadt 1275: .Nm
1.1 deraadt 1276: does not convert the user-supplied name to a canonical name before
1277: checking the key, because someone with access to the name servers
1278: would then be able to fool host authentication.
1.2 deraadt 1279: .It Pa /etc/ssh_config
1.40 aaron 1280: Systemwide configuration file.
1281: This file provides defaults for those
1.1 deraadt 1282: values that are not specified in the user's configuration file, and
1.40 aaron 1283: for those users who do not have a configuration file.
1284: This file must be world-readable.
1.2 deraadt 1285: .It Pa $HOME/.rhosts
1286: This file is used in
1287: .Pa \&.rhosts
1288: authentication to list the
1.40 aaron 1289: host/user pairs that are permitted to log in.
1290: (Note that this file is
1.1 deraadt 1291: also used by rlogin and rsh, which makes using this file insecure.)
1292: Each line of the file contains a host name (in the canonical form
1293: returned by name servers), and then a user name on that host,
1.40 aaron 1294: separated by a space.
1.92 markus 1295: On some machines this file may need to be
1.1 deraadt 1296: world-readable if the user's home directory is on a NFS partition,
1297: because
1.2 deraadt 1298: .Xr sshd 8
1.40 aaron 1299: reads it as root.
1300: Additionally, this file must be owned by the user,
1301: and must not have write permissions for anyone else.
1302: The recommended
1.1 deraadt 1303: permission for most machines is read/write for the user, and not
1304: accessible by others.
1.2 deraadt 1305: .Pp
1.1 deraadt 1306: Note that by default
1.2 deraadt 1307: .Xr sshd 8
1.1 deraadt 1308: will be installed so that it requires successful RSA host
1.40 aaron 1309: authentication before permitting \s+2.\s0rhosts authentication.
1310: If your server machine does not have the client's host key in
1.2 deraadt 1311: .Pa /etc/ssh_known_hosts ,
1312: you can store it in
1313: .Pa $HOME/.ssh/known_hosts .
1314: The easiest way to do this is to
1.1 deraadt 1315: connect back to the client from the server machine using ssh; this
1.48 markus 1316: will automatically add the host key to
1.2 deraadt 1317: .Pa $HOME/.ssh/known_hosts .
1318: .It Pa $HOME/.shosts
1319: This file is used exactly the same way as
1320: .Pa \&.rhosts .
1321: The purpose for
1.1 deraadt 1322: having this file is to be able to use rhosts authentication with
1.2 deraadt 1323: .Nm
1324: without permitting login with
1325: .Xr rlogin 1
1326: or
1327: .Xr rsh 1 .
1328: .It Pa /etc/hosts.equiv
1329: This file is used during
1.40 aaron 1330: .Pa \&.rhosts authentication.
1331: It contains
1.1 deraadt 1332: canonical hosts names, one per line (the full format is described on
1333: the
1.2 deraadt 1334: .Xr sshd 8
1.40 aaron 1335: manual page).
1336: If the client host is found in this file, login is
1.1 deraadt 1337: automatically permitted provided client and server user names are the
1.40 aaron 1338: same.
1339: Additionally, successful RSA host authentication is normally
1340: required.
1341: This file should only be writable by root.
1.2 deraadt 1342: .It Pa /etc/shosts.equiv
1.44 aaron 1343: This file is processed exactly as
1.2 deraadt 1344: .Pa /etc/hosts.equiv .
1.1 deraadt 1345: This file may be useful to permit logins using
1.2 deraadt 1346: .Nm
1.1 deraadt 1347: but not using rsh/rlogin.
1.2 deraadt 1348: .It Pa /etc/sshrc
1.1 deraadt 1349: Commands in this file are executed by
1.2 deraadt 1350: .Nm
1.1 deraadt 1351: when the user logs in just before the user's shell (or command) is started.
1352: See the
1.2 deraadt 1353: .Xr sshd 8
1.1 deraadt 1354: manual page for more information.
1.2 deraadt 1355: .It Pa $HOME/.ssh/rc
1.1 deraadt 1356: Commands in this file are executed by
1.2 deraadt 1357: .Nm
1.1 deraadt 1358: when the user logs in just before the user's shell (or command) is
1359: started.
1.44 aaron 1360: See the
1.2 deraadt 1361: .Xr sshd 8
1.1 deraadt 1362: manual page for more information.
1.31 markus 1363: .It Pa $HOME/.ssh/environment
1364: Contains additional definitions for environment variables, see section
1365: .Sx ENVIRONMENT
1366: above.
1.58 itojun 1367: .El
1.67 aaron 1368: .Sh AUTHORS
1.78 markus 1369: OpenSSH is a derivative of the original and free
1370: ssh 1.2.12 release by Tatu Ylonen.
1371: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1372: Theo de Raadt and Dug Song
1373: removed many bugs, re-added newer features and
1374: created OpenSSH.
1375: Markus Friedl contributed the support for SSH
1376: protocol versions 1.5 and 2.0.
1.2 deraadt 1377: .Sh SEE ALSO
1378: .Xr rlogin 1 ,
1379: .Xr rsh 1 ,
1380: .Xr scp 1 ,
1.83 djm 1381: .Xr sftp 1 ,
1.2 deraadt 1382: .Xr ssh-add 1 ,
1383: .Xr ssh-agent 1 ,
1384: .Xr ssh-keygen 1 ,
1385: .Xr telnet 1 ,
1.87 itojun 1386: .Xr sshd 8
1.106 markus 1387: .Rs
1388: .%A T. Ylonen
1389: .%A T. Kivinen
1390: .%A M. Saarinen
1391: .%A T. Rinne
1392: .%A S. Lehtinen
1393: .%T "SSH Protocol Architecture"
1394: .%N draft-ietf-secsh-architecture-07.txt
1395: .%D January 2001
1396: .%O work in progress material
1397: .Re
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