Annotation of src/usr.bin/ssh/ssh.1, Revision 1.110
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: .\"
1.110 ! deraadt 37: .\" $OpenBSD: ssh.1,v 1.109 2001/05/04 14:21:55 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
227: .Pa $HOME/.ssh/authorized_keys2
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
1.105 djm 272: through the use of an escape character.
1.104 djm 273: .Pp
274: A single tilde character can be sent as
275: .Ic ~~
276: (or by following the tilde by a character other than those described above).
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
281: configuration directive or on the command line by the
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
352: 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
1.40 aaron 355: be specified either on command line or in a configuration file.
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.
1.49 markus 364: RSA host keys are stored in
365: .Pa $HOME/.ssh/known_hosts
366: and
1.68 markus 367: host keys used in the protocol version 2 are stored in
1.49 markus 368: .Pa $HOME/.ssh/known_hosts2
1.40 aaron 369: in the user's home directory.
1.49 markus 370: Additionally, the files
1.2 deraadt 371: .Pa /etc/ssh_known_hosts
1.49 markus 372: and
373: .Pa /etc/ssh_known_hosts2
374: are automatically checked for known hosts.
1.40 aaron 375: Any new hosts are automatically added to the user's file.
376: If a host's identification
1.1 deraadt 377: ever changes,
1.2 deraadt 378: .Nm
1.1 deraadt 379: warns about this and disables password authentication to prevent a
1.40 aaron 380: trojan horse from getting the user's password.
381: Another purpose of
1.1 deraadt 382: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 383: otherwise be used to circumvent the encryption.
384: The
1.2 deraadt 385: .Cm StrictHostKeyChecking
1.1 deraadt 386: option (see below) can be used to prevent logins to machines whose
387: host key is not known or has changed.
1.65 aaron 388: .Pp
389: The options are as follows:
1.2 deraadt 390: .Bl -tag -width Ds
1.4 dugsong 391: .It Fl a
1.42 aaron 392: Disables forwarding of the authentication agent connection.
1.54 markus 393: .It Fl A
394: Enables forwarding of the authentication agent connection.
395: This can also be specified on a per-host basis in a configuration file.
1.108 markus 396: .It Fl b Ar bind_address
397: Specify the interface to transmit from on machines with multiple
398: interfaces or aliased addresses.
1.6 deraadt 399: .It Fl c Ar blowfish|3des
1.44 aaron 400: Selects the cipher to use for encrypting the session.
1.2 deraadt 401: .Ar 3des
1.40 aaron 402: is used by default.
1.44 aaron 403: It is believed to be secure.
1.5 deraadt 404: .Ar 3des
405: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
406: It is presumably more secure than the
1.2 deraadt 407: .Ar des
1.64 markus 408: cipher which is no longer fully supported in
1.51 markus 409: .Nm ssh .
1.5 deraadt 410: .Ar blowfish
411: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 412: .Ar 3des .
1.90 markus 413: .It Fl c Ar cipher_spec
1.51 markus 414: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 415: be specified in order of preference.
1.90 markus 416: See
417: .Cm Ciphers
418: for more information.
1.2 deraadt 419: .It Fl e Ar ch|^ch|none
420: Sets the escape character for sessions with a pty (default:
421: .Ql ~ ) .
1.40 aaron 422: The escape character is only recognized at the beginning of a line.
423: The escape character followed by a dot
1.2 deraadt 424: .Pq Ql \&.
425: closes the connection, followed
1.1 deraadt 426: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 427: escape character once.
428: Setting the character to
1.2 deraadt 429: .Dq none
430: disables any escapes and makes the session fully transparent.
431: .It Fl f
432: Requests
433: .Nm
1.40 aaron 434: to go to background just before command execution.
435: This is useful if
1.2 deraadt 436: .Nm
437: is going to ask for passwords or passphrases, but the user
1.40 aaron 438: wants it in the background.
1.44 aaron 439: This implies
1.2 deraadt 440: .Fl n .
1.1 deraadt 441: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 442: something like
443: .Ic ssh -f host xterm .
1.34 markus 444: .It Fl g
445: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 446: .It Fl i Ar identity_file
1.44 aaron 447: Selects the file from which the identity (private key) for
1.68 markus 448: RSA or DSA authentication is read.
1.44 aaron 449: Default is
1.49 markus 450: .Pa $HOME/.ssh/identity
1.40 aaron 451: in the user's home directory.
452: Identity files may also be specified on
453: a per-host basis in the configuration file.
454: It is possible to have multiple
1.2 deraadt 455: .Fl i
456: options (and multiple identities specified in
1.1 deraadt 457: configuration files).
1.2 deraadt 458: .It Fl k
1.42 aaron 459: Disables forwarding of Kerberos tickets and AFS tokens.
460: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 461: .It Fl l Ar login_name
1.40 aaron 462: Specifies the user to log in as on the remote machine.
463: This also may be specified on a per-host basis in the configuration file.
1.91 markus 464: .It Fl m Ar mac_spec
465: Additionally, for protocol version 2 a comma-separated list of MAC
466: (message authentication code) algorithms can
467: be specified in order of preference.
468: See the
469: .Cm MACs
470: keyword for more information.
1.2 deraadt 471: .It Fl n
472: Redirects stdin from
473: .Pa /dev/null
474: (actually, prevents reading from stdin).
1.1 deraadt 475: This must be used when
1.2 deraadt 476: .Nm
1.40 aaron 477: is run in the background.
478: A common trick is to use this to run X11 programs on a remote machine.
479: For example,
1.2 deraadt 480: .Ic ssh -n shadows.cs.hut.fi emacs &
481: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 482: connection will be automatically forwarded over an encrypted channel.
483: The
1.2 deraadt 484: .Nm
1.1 deraadt 485: program will be put in the background.
486: (This does not work if
1.2 deraadt 487: .Nm
488: needs to ask for a password or passphrase; see also the
489: .Fl f
490: option.)
1.53 markus 491: .It Fl N
492: Do not execute a remote command.
1.70 markus 493: This is useful if you just want to forward ports
1.53 markus 494: (protocol version 2 only).
1.2 deraadt 495: .It Fl o Ar option
1.1 deraadt 496: Can be used to give options in the format used in the config file.
497: This is useful for specifying options for which there is no separate
1.40 aaron 498: command-line flag.
499: The option has the same format as a line in the configuration file.
1.2 deraadt 500: .It Fl p Ar port
1.40 aaron 501: Port to connect to on the remote host.
502: This can be specified on a
1.1 deraadt 503: per-host basis in the configuration file.
1.16 markus 504: .It Fl P
505: Use a non-privileged port for outgoing connections.
506: This can be used if your firewall does
507: not permit connections from privileged ports.
1.30 provos 508: Note that this option turns off
1.16 markus 509: .Cm RhostsAuthentication
510: and
1.72 markus 511: .Cm RhostsRSAAuthentication
512: for older servers.
1.2 deraadt 513: .It Fl q
1.40 aaron 514: Quiet mode.
515: Causes all warning and diagnostic messages to be suppressed.
516: Only fatal errors are displayed.
1.80 djm 517: .It Fl s
518: 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
1.109 stevesk 519: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 520: subsystem is specified as the remote command.
1.2 deraadt 521: .It Fl t
1.40 aaron 522: Force pseudo-tty allocation.
1.43 brad 523: This can be used to execute arbitrary
1.40 aaron 524: screen-based programs on a remote machine, which can be very useful,
525: e.g., when implementing menu services.
1.73 markus 526: Multiple
527: .Fl t
528: options force tty allocation, even if
529: .Nm
530: has no local tty.
1.53 markus 531: .It Fl T
1.69 markus 532: Disable pseudo-tty allocation.
1.2 deraadt 533: .It Fl v
1.40 aaron 534: Verbose mode.
535: Causes
1.2 deraadt 536: .Nm
1.40 aaron 537: to print debugging messages about its progress.
538: This is helpful in
1.1 deraadt 539: debugging connection, authentication, and configuration problems.
1.73 markus 540: Multiple
541: .Fl v
542: options increases the verbosity.
1.61 aaron 543: Maximum is 3.
1.2 deraadt 544: .It Fl x
1.40 aaron 545: Disables X11 forwarding.
1.2 deraadt 546: .It Fl X
1.1 deraadt 547: Enables X11 forwarding.
1.54 markus 548: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 549: .It Fl C
1.1 deraadt 550: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 551: data for forwarded X11 and TCP/IP connections).
552: The compression algorithm is the same used by
1.34 markus 553: .Xr gzip 1 ,
554: and the
1.2 deraadt 555: .Dq level
556: can be controlled by the
557: .Cm CompressionLevel
1.40 aaron 558: option (see below).
559: Compression is desirable on modem lines and other
1.1 deraadt 560: slow connections, but will only slow down things on fast networks.
561: The default value can be set on a host-by-host basis in the
562: configuration files; see the
1.2 deraadt 563: .Cm Compress
1.1 deraadt 564: option below.
1.2 deraadt 565: .It Fl L Ar port:host:hostport
1.1 deraadt 566: Specifies that the given port on the local (client) host is to be
1.40 aaron 567: forwarded to the given host and port on the remote side.
568: This works by allocating a socket to listen to
1.2 deraadt 569: .Ar port
1.1 deraadt 570: on the local side, and whenever a connection is made to this port, the
571: connection is forwarded over the secure channel, and a connection is
572: made to
1.32 markus 573: .Ar host
574: port
575: .Ar hostport
1.40 aaron 576: from the remote machine.
577: Port forwardings can also be specified in the configuration file.
578: Only root can forward privileged ports.
1.32 markus 579: IPv6 addresses can be specified with an alternative syntax:
580: .Ar port/host/hostport
1.2 deraadt 581: .It Fl R Ar port:host:hostport
1.1 deraadt 582: Specifies that the given port on the remote (server) host is to be
1.40 aaron 583: forwarded to the given host and port on the local side.
584: This works by allocating a socket to listen to
1.2 deraadt 585: .Ar port
1.1 deraadt 586: on the remote 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 local machine.
593: Port forwardings can also be specified in the configuration file.
594: Privileged ports can be forwarded only when
1.1 deraadt 595: logging in as root on the remote machine.
1.107 markus 596: IPv6 addresses can be specified with an alternative syntax:
597: .Ar port/host/hostport
1.85 jakob 598: .It Fl 1
599: Forces
600: .Nm
601: to try protocol version 1 only.
1.46 markus 602: .It Fl 2
603: Forces
604: .Nm
1.50 markus 605: to try protocol version 2 only.
1.32 markus 606: .It Fl 4
607: Forces
608: .Nm
609: to use IPv4 addresses only.
610: .It Fl 6
611: Forces
612: .Nm
613: to use IPv6 addresses only.
1.2 deraadt 614: .El
615: .Sh CONFIGURATION FILES
616: .Nm
1.1 deraadt 617: obtains configuration data from the following sources (in this order):
618: command line options, user's configuration file
1.2 deraadt 619: .Pq Pa $HOME/.ssh/config ,
620: and system-wide configuration file
621: .Pq Pa /etc/ssh_config .
622: For each parameter, the first obtained value
1.40 aaron 623: will be used.
624: The configuration files contain sections bracketed by
625: .Dq Host
626: specifications, and that section is only applied for hosts that
627: match one of the patterns given in the specification.
628: The matched host name is the one given on the command line.
1.2 deraadt 629: .Pp
1.1 deraadt 630: Since the first obtained value for each parameter is used, more
631: host-specific declarations should be given near the beginning of the
632: file, and general defaults at the end.
1.2 deraadt 633: .Pp
1.1 deraadt 634: The configuration file has the following format:
1.2 deraadt 635: .Pp
636: Empty lines and lines starting with
637: .Ql #
638: are comments.
639: .Pp
640: Otherwise a line is of the format
641: .Dq keyword arguments .
642: The possible
1.1 deraadt 643: keywords and their meanings are as follows (note that the
644: configuration files are case-sensitive):
1.2 deraadt 645: .Bl -tag -width Ds
646: .It Cm Host
1.1 deraadt 647: Restricts the following declarations (up to the next
1.2 deraadt 648: .Cm Host
1.1 deraadt 649: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 650: given after the keyword.
651: .Ql \&*
652: and
653: .Ql ?
654: can be used as wildcards in the
1.40 aaron 655: patterns.
656: A single
1.2 deraadt 657: .Ql \&*
658: as a pattern can be used to provide global
1.40 aaron 659: defaults for all hosts.
660: The host is the
1.2 deraadt 661: .Ar hostname
1.1 deraadt 662: argument given on the command line (i.e., the name is not converted to
663: a canonicalized host name before matching).
1.2 deraadt 664: .It Cm AFSTokenPassing
1.42 aaron 665: Specifies whether to pass AFS tokens to remote host.
666: The argument to this keyword must be
1.2 deraadt 667: .Dq yes
668: or
669: .Dq no .
1.107 markus 670: This option applies to protocol version 1 only.
1.2 deraadt 671: .It Cm BatchMode
672: If set to
673: .Dq yes ,
1.40 aaron 674: passphrase/password querying will be disabled.
675: This option is useful in scripts and other batch jobs where you have no
676: user to supply the password.
677: The argument must be
1.2 deraadt 678: .Dq yes
679: or
680: .Dq no .
1.100 stevesk 681: The default is
682: .Dq no .
1.108 markus 683: .It Cm BindAddress
684: Specify the interface to transmit from on machines with multiple
685: interfaces or aliased addresses.
686: Note that this option does not work if
687: .Cm UsePrivilegedPort
688: is set to
689: .Dq yes .
1.34 markus 690: .It Cm CheckHostIP
691: If this flag is set to
692: .Dq yes ,
1.100 stevesk 693: ssh will additionally check the host IP address in the
1.34 markus 694: .Pa known_hosts
1.42 aaron 695: file.
696: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 697: If the option is set to
698: .Dq no ,
699: the check will not be executed.
1.100 stevesk 700: The default is
701: .Dq yes .
1.2 deraadt 702: .It Cm Cipher
1.62 markus 703: Specifies the cipher to use for encrypting the session
1.64 markus 704: in protocol version 1.
1.40 aaron 705: Currently,
1.64 markus 706: .Dq blowfish
1.1 deraadt 707: and
1.10 provos 708: .Dq 3des
1.40 aaron 709: are supported.
710: The default is
1.2 deraadt 711: .Dq 3des .
1.45 markus 712: .It Cm Ciphers
713: Specifies the ciphers allowed for protocol version 2
714: in order of preference.
715: Multiple ciphers must be comma-separated.
716: The default is
1.88 provos 717: .Pp
718: .Bd -literal
1.94 deraadt 719: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.107 markus 720: aes192-cbc,aes256-cbc''
1.88 provos 721: .Ed
1.2 deraadt 722: .It Cm Compression
1.40 aaron 723: Specifies whether to use compression.
724: The argument must be
1.2 deraadt 725: .Dq yes
726: or
727: .Dq no .
1.100 stevesk 728: The default is
729: .Dq no .
1.2 deraadt 730: .It Cm CompressionLevel
1.100 stevesk 731: Specifies the compression level to use if compression is enabled.
1.40 aaron 732: The argument must be an integer from 1 (fast) to 9 (slow, best).
733: The default level is 6, which is good for most applications.
734: The meaning of the values is the same as in
1.34 markus 735: .Xr gzip 1 .
1.107 markus 736: Note that this option applies to protocol version 1 only.
1.2 deraadt 737: .It Cm ConnectionAttempts
1.1 deraadt 738: Specifies the number of tries (one per second) to make before falling
1.40 aaron 739: back to rsh or exiting.
740: The argument must be an integer.
741: This may be useful in scripts if the connection sometimes fails.
1.100 stevesk 742: The default is 4.
1.2 deraadt 743: .It Cm EscapeChar
744: Sets the escape character (default:
745: .Ql ~ ) .
746: The escape character can also
1.40 aaron 747: be set on the command line.
748: The argument should be a single character,
1.2 deraadt 749: .Ql ^
750: followed by a letter, or
751: .Dq none
752: to disable the escape
1.1 deraadt 753: character entirely (making the connection transparent for binary
754: data).
1.44 aaron 755: .It Cm FallBackToRsh
1.1 deraadt 756: Specifies that if connecting via
1.2 deraadt 757: .Nm
1.1 deraadt 758: fails due to a connection refused error (there is no
1.2 deraadt 759: .Xr sshd 8
1.44 aaron 760: listening on the remote host),
1.2 deraadt 761: .Xr rsh 1
1.1 deraadt 762: should automatically be used instead (after a suitable warning about
1.40 aaron 763: the session being unencrypted).
764: The argument must be
1.2 deraadt 765: .Dq yes
766: or
767: .Dq no .
1.100 stevesk 768: The default is
769: .Dq no .
1.2 deraadt 770: .It Cm ForwardAgent
1.1 deraadt 771: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 772: will be forwarded to the remote machine.
773: The argument must be
1.2 deraadt 774: .Dq yes
775: or
1.54 markus 776: .Dq no .
777: The default is
1.2 deraadt 778: .Dq no .
779: .It Cm ForwardX11
1.1 deraadt 780: Specifies whether X11 connections will be automatically redirected
1.44 aaron 781: over the secure channel and
1.2 deraadt 782: .Ev DISPLAY
1.40 aaron 783: set.
1.44 aaron 784: The argument must be
1.2 deraadt 785: .Dq yes
786: or
1.38 markus 787: .Dq no .
788: The default is
1.3 deraadt 789: .Dq no .
790: .It Cm GatewayPorts
791: Specifies whether remote hosts are allowed to connect to local
792: forwarded ports.
793: The argument must be
794: .Dq yes
795: or
796: .Dq no .
797: The default is
1.2 deraadt 798: .Dq no .
799: .It Cm GlobalKnownHostsFile
1.95 stevesk 800: Specifies a file to use for the protocol version 1 global
801: host key database instead of
1.2 deraadt 802: .Pa /etc/ssh_known_hosts .
1.95 stevesk 803: .It Cm GlobalKnownHostsFile2
804: Specifies a file to use for the protocol version 2 global
805: host key database instead of
806: .Pa /etc/ssh_known_hosts2 .
1.107 markus 807: .It Cm HostbasedAuthentication
808: Specifies whether to try rhosts based authentication with public key
809: authentication.
810: The argument must be
811: .Dq yes
812: or
813: .Dq no .
814: The default is
815: .Dq yes .
816: This option applies to protocol version 2 only and
817: is similar to
818: .Cm RhostsRSAAuthentication .
819: .It Cm HostKeyAlgorithms
1.109 stevesk 820: Specifies the protocol version 2 host key algorithms
1.107 markus 821: that the client wants to use in order of preference.
822: The default for this option is:
823: .Dq ssh-rsa,ssh-dss
1.74 markus 824: .It Cm HostKeyAlias
825: Specifies an alias that should be used instead of the
826: real host name when looking up or saving the host key
1.107 markus 827: in the host key database files.
1.82 stevesk 828: This option is useful for tunneling ssh connections
1.74 markus 829: or if you have multiple servers running on a single host.
1.2 deraadt 830: .It Cm HostName
1.40 aaron 831: Specifies the real host name to log into.
832: This can be used to specify nicknames or abbreviations for hosts.
833: Default is the name given on the command line.
834: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 835: .Cm HostName
1.1 deraadt 836: specifications).
1.2 deraadt 837: .It Cm IdentityFile
1.107 markus 838: Specifies the file from which the user's RSA or DSA authentication identity
1.2 deraadt 839: is read (default
1.49 markus 840: .Pa $HOME/.ssh/identity
1.2 deraadt 841: in the user's home directory).
1.1 deraadt 842: Additionally, any identities represented by the authentication agent
1.40 aaron 843: will be used for authentication.
844: The file name may use the tilde
845: syntax to refer to a user's home directory.
846: It is possible to have
1.1 deraadt 847: multiple identity files specified in configuration files; all these
848: identities will be tried in sequence.
1.2 deraadt 849: .It Cm KeepAlive
1.1 deraadt 850: Specifies whether the system should send keepalive messages to the
1.40 aaron 851: other side.
852: If they are sent, death of the connection or crash of one
853: of the machines will be properly noticed.
854: However, this means that
1.1 deraadt 855: connections will die if the route is down temporarily, and some people
1.41 aaron 856: find it annoying.
1.2 deraadt 857: .Pp
858: The default is
859: .Dq yes
860: (to send keepalives), and the client will notice
1.40 aaron 861: if the network goes down or the remote host dies.
862: This is important in scripts, and many users want it too.
1.2 deraadt 863: .Pp
864: To disable keepalives, the value should be set to
865: .Dq no
866: in both the server and the client configuration files.
867: .It Cm KerberosAuthentication
1.42 aaron 868: Specifies whether Kerberos authentication will be used.
869: The argument to this keyword must be
1.4 dugsong 870: .Dq yes
871: or
872: .Dq no .
1.2 deraadt 873: .It Cm KerberosTgtPassing
1.42 aaron 874: Specifies whether a Kerberos TGT will be forwarded to the server.
875: This will only work if the Kerberos server is actually an AFS kaserver.
876: The argument to this keyword must be
1.4 dugsong 877: .Dq yes
878: or
879: .Dq no .
1.2 deraadt 880: .It Cm LocalForward
1.1 deraadt 881: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 882: the secure channel to given host:port from the remote machine.
883: The first argument must be a port number, and the second must be
884: host:port.
885: Multiple forwardings may be specified, and additional
886: forwardings can be given on the command line.
887: Only the superuser can forward privileged ports.
1.24 markus 888: .It Cm LogLevel
889: Gives the verbosity level that is used when logging messages from
890: .Nm ssh .
891: The possible values are:
1.77 markus 892: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
893: The default is INFO.
1.91 markus 894: .It Cm MACs
895: Specifies the MAC (message authentication code) algorithms
896: in order of preference.
897: The MAC algorithm is used in protocol version 2
898: for data integrity protection.
899: Multiple algorithms must be comma-separated.
900: The default is
901: .Pp
902: .Bd -literal
1.94 deraadt 903: ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
1.91 markus 904: hmac-sha1-96,hmac-md5-96''
905: .Ed
1.14 dugsong 906: .It Cm NumberOfPasswordPrompts
1.42 aaron 907: Specifies the number of password prompts before giving up.
908: The argument to this keyword must be an integer.
909: Default is 3.
1.34 markus 910: .It Cm PasswordAuthentication
1.40 aaron 911: Specifies whether to use password authentication.
912: The argument to this keyword must be
1.34 markus 913: .Dq yes
914: or
915: .Dq no .
1.100 stevesk 916: The default is
917: .Dq yes .
1.2 deraadt 918: .It Cm Port
1.40 aaron 919: Specifies the port number to connect on the remote host.
920: Default is 22.
1.99 djm 921: .It Cm PreferredAuthentications
922: Specifies the order in which the client should try protocol 2
923: authentication methods. This allows a client to prefer one method (e.g.
924: .Cm keyboard-interactive )
925: over another method (e.g.
926: .Cm password )
927: The default for this option is:
928: .Dq publickey, password, keyboard-interactive
1.45 markus 929: .It Cm Protocol
930: Specifies the protocol versions
931: .Nm
932: should support in order of preference.
933: The possible values are
934: .Dq 1
935: and
936: .Dq 2 .
937: Multiple versions must be comma-separated.
938: The default is
1.101 markus 939: .Dq 2,1 .
1.49 markus 940: This means that
941: .Nm
1.101 markus 942: tries version 2 and falls back to version 1
943: if version 2 is not available.
1.2 deraadt 944: .It Cm ProxyCommand
1.40 aaron 945: Specifies the command to use to connect to the server.
946: The command
947: string extends to the end of the line, and is executed with
948: .Pa /bin/sh .
949: In the command string,
950: .Ql %h
951: will be substituted by the host name to
952: connect and
953: .Ql %p
954: by the port.
955: The command can be basically anything,
956: and should read from its standard input and write to its standard output.
957: It should eventually connect an
1.2 deraadt 958: .Xr sshd 8
1.1 deraadt 959: server running on some machine, or execute
1.2 deraadt 960: .Ic sshd -i
1.40 aaron 961: somewhere.
962: Host key management will be done using the
1.1 deraadt 963: HostName of the host being connected (defaulting to the name typed by
964: the user).
1.29 markus 965: Note that
966: .Cm CheckHostIP
967: is not available for connects with a proxy command.
1.2 deraadt 968: .Pp
1.107 markus 969: .It Cm PubkeyAuthentication
970: Specifies whether to try public key authentication.
971: The argument to this keyword must be
972: .Dq yes
973: or
974: .Dq no .
975: The default is
976: .Dq yes .
977: This option applies to protocol version 2 only.
1.2 deraadt 978: .It Cm RemoteForward
1.1 deraadt 979: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 980: the secure channel to given host:port from the local machine.
981: The first argument must be a port number, and the second must be
982: host:port.
983: Multiple forwardings may be specified, and additional
984: forwardings can be given on the command line.
985: Only the superuser can forward privileged ports.
1.2 deraadt 986: .It Cm RhostsAuthentication
1.40 aaron 987: Specifies whether to try rhosts based authentication.
988: Note that this
1.1 deraadt 989: declaration only affects the client side and has no effect whatsoever
1.40 aaron 990: on security.
991: Disabling rhosts authentication may reduce
1.1 deraadt 992: authentication time on slow connections when rhosts authentication is
1.40 aaron 993: not used.
994: Most servers do not permit RhostsAuthentication because it
1.107 markus 995: is not secure (see
996: .Cm RhostsRSAAuthentication ).
1.40 aaron 997: The argument to this keyword must be
1.2 deraadt 998: .Dq yes
999: or
1000: .Dq no .
1.100 stevesk 1001: The default is
1002: .Dq yes .
1.107 markus 1003: This option applies to protocol version 1 only.
1.2 deraadt 1004: .It Cm RhostsRSAAuthentication
1.1 deraadt 1005: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 1006: authentication.
1007: The argument must be
1.2 deraadt 1008: .Dq yes
1009: or
1010: .Dq no .
1.100 stevesk 1011: The default is
1012: .Dq yes .
1.107 markus 1013: This option applies to protocol version 1 only.
1.2 deraadt 1014: .It Cm RSAAuthentication
1.40 aaron 1015: Specifies whether to try RSA authentication.
1016: The argument to this keyword must be
1.2 deraadt 1017: .Dq yes
1018: or
1019: .Dq no .
1.1 deraadt 1020: RSA authentication will only be
1021: attempted if the identity file exists, or an authentication agent is
1022: running.
1.100 stevesk 1023: The default is
1024: .Dq yes .
1.50 markus 1025: Note that this option applies to protocol version 1 only.
1.81 markus 1026: .It Cm ChallengeResponseAuthentication
1027: Specifies whether to use challenge response authentication.
1028: Currently there is only support for
1.27 markus 1029: .Xr skey 1
1.40 aaron 1030: authentication.
1031: The argument to this keyword must be
1.27 markus 1032: .Dq yes
1033: or
1034: .Dq no .
1035: The default is
1036: .Dq no .
1.2 deraadt 1037: .It Cm StrictHostKeyChecking
1038: If this flag is set to
1.44 aaron 1039: .Dq yes ,
1.2 deraadt 1040: .Nm
1.79 stevesk 1041: will never automatically add host keys to the
1.2 deraadt 1042: .Pa $HOME/.ssh/known_hosts
1.48 markus 1043: and
1044: .Pa $HOME/.ssh/known_hosts2
1.79 stevesk 1045: files, and refuses to connect to hosts whose host key has changed.
1.40 aaron 1046: This provides maximum protection against trojan horse attacks.
1047: However, it can be somewhat annoying if you don't have good
1.2 deraadt 1048: .Pa /etc/ssh_known_hosts
1.48 markus 1049: and
1050: .Pa /etc/ssh_known_hosts2
1.1 deraadt 1051: files installed and frequently
1.79 stevesk 1052: connect to new hosts.
1053: This option forces the user to manually
1054: add all new hosts.
1055: If this flag is set to
1056: .Dq no ,
1057: .Nm
1058: will automatically add new host keys to the
1059: user known hosts files.
1060: If this flag is set to
1061: .Dq ask ,
1062: new host keys
1063: will be added to the user known host files only after the user
1064: has confirmed that is what they really want to do, and
1065: .Nm
1066: will refuse to connect to hosts whose host key has changed.
1.40 aaron 1067: The host keys of
1.79 stevesk 1068: known hosts will be verified automatically in all cases.
1.40 aaron 1069: The argument must be
1.79 stevesk 1070: .Dq yes ,
1071: .Dq no
1.2 deraadt 1072: or
1.79 stevesk 1073: .Dq ask .
1074: The default is
1075: .Dq ask .
1.16 markus 1076: .It Cm UsePrivilegedPort
1077: Specifies whether to use a privileged port for outgoing connections.
1078: The argument must be
1079: .Dq yes
1080: or
1081: .Dq no .
1082: The default is
1.98 markus 1083: .Dq no .
1.107 markus 1084: Note that you need to set this option to
1085: .Dq yes
1086: if you want to use
1.16 markus 1087: .Cm RhostsAuthentication
1088: and
1.72 markus 1089: .Cm RhostsRSAAuthentication
1.107 markus 1090: with older servers.
1.34 markus 1091: .It Cm User
1.40 aaron 1092: Specifies the user to log in as.
1093: This can be useful if you have a different user name on different machines.
1094: This saves the trouble of
1.34 markus 1095: having to remember to give the user name on the command line.
1096: .It Cm UserKnownHostsFile
1.95 stevesk 1097: Specifies a file to use for the protocol version 1 user
1098: host key database instead of
1.34 markus 1099: .Pa $HOME/.ssh/known_hosts .
1.95 stevesk 1100: .It Cm UserKnownHostsFile2
1101: Specifies a file to use for the protocol version 2 user
1102: host key database instead of
1103: .Pa $HOME/.ssh/known_hosts2 .
1.2 deraadt 1104: .It Cm UseRsh
1.40 aaron 1105: Specifies that rlogin/rsh should be used for this host.
1106: It is possible that the host does not at all support the
1.2 deraadt 1107: .Nm
1.40 aaron 1108: protocol.
1109: This causes
1.2 deraadt 1110: .Nm
1.40 aaron 1111: to immediately execute
1.2 deraadt 1112: .Xr rsh 1 .
1.1 deraadt 1113: All other options (except
1.2 deraadt 1114: .Cm HostName )
1.40 aaron 1115: are ignored if this has been specified.
1116: The argument must be
1.2 deraadt 1117: .Dq yes
1118: or
1119: .Dq no .
1.55 markus 1120: .It Cm XAuthLocation
1121: Specifies the location of the
1122: .Xr xauth 1
1123: program.
1124: The default is
1125: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1126: .El
1.2 deraadt 1127: .Sh ENVIRONMENT
1128: .Nm
1.1 deraadt 1129: will normally set the following environment variables:
1.2 deraadt 1130: .Bl -tag -width Ds
1131: .It Ev DISPLAY
1132: The
1133: .Ev DISPLAY
1.40 aaron 1134: variable indicates the location of the X11 server.
1.44 aaron 1135: It is automatically set by
1.2 deraadt 1136: .Nm
1137: to point to a value of the form
1138: .Dq hostname:n
1139: where hostname indicates
1.40 aaron 1140: the host where the shell runs, and n is an integer >= 1.
1141: .Nm
1142: uses this special value to forward X11 connections over the secure
1143: channel.
1.107 markus 1144: The user should normally not set
1145: .Ev DISPLAY
1146: explicitly, as that
1.1 deraadt 1147: will render the X11 connection insecure (and will require the user to
1148: manually copy any required authorization cookies).
1.2 deraadt 1149: .It Ev HOME
1.1 deraadt 1150: Set to the path of the user's home directory.
1.2 deraadt 1151: .It Ev LOGNAME
1152: Synonym for
1.12 aaron 1153: .Ev USER ;
1154: set for compatibility with systems that use this variable.
1.2 deraadt 1155: .It Ev MAIL
1.1 deraadt 1156: Set to point the user's mailbox.
1.40 aaron 1157: .It Ev PATH
1.2 deraadt 1158: Set to the default
1159: .Ev PATH ,
1160: as specified when compiling
1.12 aaron 1161: .Nm ssh .
1.18 markus 1162: .It Ev SSH_AUTH_SOCK
1.17 markus 1163: indicates the path of a unix-domain socket used to communicate with the
1164: agent.
1.2 deraadt 1165: .It Ev SSH_CLIENT
1.40 aaron 1166: Identifies the client end of the connection.
1167: The variable contains
1.1 deraadt 1168: three space-separated values: client ip-address, client port number,
1169: and server port number.
1.73 markus 1170: .It Ev SSH_ORIGINAL_COMMAND
1171: The variable contains the original command line if a forced command
1172: is executed.
1173: It can be used to extract the original arguments.
1.2 deraadt 1174: .It Ev SSH_TTY
1.1 deraadt 1175: This is set to the name of the tty (path to the device) associated
1.40 aaron 1176: with the current shell or command.
1177: If the current session has no tty,
1.1 deraadt 1178: this variable is not set.
1.2 deraadt 1179: .It Ev TZ
1.1 deraadt 1180: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1181: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1182: on to new connections).
1.2 deraadt 1183: .It Ev USER
1.1 deraadt 1184: Set to the name of the user logging in.
1.2 deraadt 1185: .El
1186: .Pp
1.44 aaron 1187: Additionally,
1.2 deraadt 1188: .Nm
1.44 aaron 1189: reads
1190: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1191: and adds lines of the format
1192: .Dq VARNAME=value
1.12 aaron 1193: to the environment.
1.2 deraadt 1194: .Sh FILES
1.36 markus 1195: .Bl -tag -width Ds
1.95 stevesk 1196: .It Pa $HOME/.ssh/known_hosts, $HOME/.ssh/known_hosts2
1.1 deraadt 1197: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1198: in
1.95 stevesk 1199: .Pa /etc/ssh_known_hosts
1200: for protocol version 1 or
1201: .Pa /etc/ssh_known_hosts2
1202: for protocol version 2).
1.2 deraadt 1203: See
1204: .Xr sshd 8 .
1.102 itojun 1205: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1206: Contains the authentication identity of the user.
1207: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 1208: These files
1209: contain sensitive data and should be readable by the user but not
1.15 markus 1210: accessible by others (read/write/execute).
1211: Note that
1212: .Nm
1.48 markus 1213: ignores a private key file if it is accessible by others.
1.15 markus 1214: It is possible to specify a passphrase when
1.1 deraadt 1215: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1216: sensitive part of this file using 3DES.
1.102 itojun 1217: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 1218: Contains the public key for authentication (public part of the
1.40 aaron 1219: identity file in human-readable form).
1.48 markus 1220: The contents of the
1221: .Pa $HOME/.ssh/identity.pub
1222: file should be added to
1.2 deraadt 1223: .Pa $HOME/.ssh/authorized_keys
1224: on all machines
1.102 itojun 1225: where you wish to log in using protocol version 1 RSA authentication.
1.48 markus 1226: The contents of the
1227: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 1228: and
1229: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 1230: file should be added to
1231: .Pa $HOME/.ssh/authorized_keys2
1232: on all machines
1.102 itojun 1233: where you wish to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 1234: These files are not
1.40 aaron 1235: sensitive and can (but need not) be readable by anyone.
1.48 markus 1236: These files are
1.84 markus 1237: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1238: the convenience of the user.
1.2 deraadt 1239: .It Pa $HOME/.ssh/config
1.40 aaron 1240: This is the per-user configuration file.
1241: The format of this file is described above.
1242: This file is used by the
1.2 deraadt 1243: .Nm
1.40 aaron 1244: client.
1245: This file does not usually contain any sensitive information,
1.1 deraadt 1246: but the recommended permissions are read/write for the user, and not
1247: accessible by others.
1.2 deraadt 1248: .It Pa $HOME/.ssh/authorized_keys
1.40 aaron 1249: Lists the RSA keys that can be used for logging in as this user.
1250: The format of this file is described in the
1.2 deraadt 1251: .Xr sshd 8
1.40 aaron 1252: manual page.
1253: In the simplest form the format is the same as the .pub
1.1 deraadt 1254: identity files (that is, each line contains the number of bits in
1255: modulus, public exponent, modulus, and comment fields, separated by
1.40 aaron 1256: spaces).
1257: This file is not highly sensitive, but the recommended
1.1 deraadt 1258: permissions are read/write for the user, and not accessible by others.
1.48 markus 1259: .It Pa $HOME/.ssh/authorized_keys2
1.107 markus 1260: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.48 markus 1261: This file is not highly sensitive, but the recommended
1262: permissions are read/write for the user, and not accessible by others.
1263: .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1.40 aaron 1264: Systemwide list of known host keys.
1.48 markus 1265: .Pa /etc/ssh_known_hosts
1266: contains RSA and
1267: .Pa /etc/ssh_known_hosts2
1.107 markus 1268: contains RSA or DSA keys for protocol version 2.
1.48 markus 1269: These files should be prepared by the
1.1 deraadt 1270: system administrator to contain the public host keys of all machines in the
1.40 aaron 1271: organization.
1272: This file should be world-readable.
1273: This file contains
1.1 deraadt 1274: public keys, one per line, in the following format (fields separated
1275: by spaces): system name, number of bits in modulus, public exponent,
1.40 aaron 1276: modulus, and optional comment field.
1277: When different names are used
1.1 deraadt 1278: for the same machine, all such names should be listed, separated by
1.40 aaron 1279: commas.
1280: The format is described on the
1.2 deraadt 1281: .Xr sshd 8
1.1 deraadt 1282: manual page.
1.2 deraadt 1283: .Pp
1.1 deraadt 1284: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1285: .Xr sshd 8
1.1 deraadt 1286: to verify the client host when logging in; other names are needed because
1.2 deraadt 1287: .Nm
1.1 deraadt 1288: does not convert the user-supplied name to a canonical name before
1289: checking the key, because someone with access to the name servers
1290: would then be able to fool host authentication.
1.2 deraadt 1291: .It Pa /etc/ssh_config
1.40 aaron 1292: Systemwide configuration file.
1293: This file provides defaults for those
1.1 deraadt 1294: values that are not specified in the user's configuration file, and
1.40 aaron 1295: for those users who do not have a configuration file.
1296: This file must be world-readable.
1.2 deraadt 1297: .It Pa $HOME/.rhosts
1298: This file is used in
1299: .Pa \&.rhosts
1300: authentication to list the
1.40 aaron 1301: host/user pairs that are permitted to log in.
1302: (Note that this file is
1.1 deraadt 1303: also used by rlogin and rsh, which makes using this file insecure.)
1304: Each line of the file contains a host name (in the canonical form
1305: returned by name servers), and then a user name on that host,
1.40 aaron 1306: separated by a space.
1.92 markus 1307: On some machines this file may need to be
1.1 deraadt 1308: world-readable if the user's home directory is on a NFS partition,
1309: because
1.2 deraadt 1310: .Xr sshd 8
1.40 aaron 1311: reads it as root.
1312: Additionally, this file must be owned by the user,
1313: and must not have write permissions for anyone else.
1314: The recommended
1.1 deraadt 1315: permission for most machines is read/write for the user, and not
1316: accessible by others.
1.2 deraadt 1317: .Pp
1.1 deraadt 1318: Note that by default
1.2 deraadt 1319: .Xr sshd 8
1.1 deraadt 1320: will be installed so that it requires successful RSA host
1.40 aaron 1321: authentication before permitting \s+2.\s0rhosts authentication.
1322: If your server machine does not have the client's host key in
1.2 deraadt 1323: .Pa /etc/ssh_known_hosts ,
1324: you can store it in
1325: .Pa $HOME/.ssh/known_hosts .
1326: The easiest way to do this is to
1.1 deraadt 1327: connect back to the client from the server machine using ssh; this
1.48 markus 1328: will automatically add the host key to
1.2 deraadt 1329: .Pa $HOME/.ssh/known_hosts .
1330: .It Pa $HOME/.shosts
1331: This file is used exactly the same way as
1332: .Pa \&.rhosts .
1333: The purpose for
1.1 deraadt 1334: having this file is to be able to use rhosts authentication with
1.2 deraadt 1335: .Nm
1336: without permitting login with
1337: .Xr rlogin 1
1338: or
1339: .Xr rsh 1 .
1340: .It Pa /etc/hosts.equiv
1341: This file is used during
1.40 aaron 1342: .Pa \&.rhosts authentication.
1343: It contains
1.1 deraadt 1344: canonical hosts names, one per line (the full format is described on
1345: the
1.2 deraadt 1346: .Xr sshd 8
1.40 aaron 1347: manual page).
1348: If the client host is found in this file, login is
1.1 deraadt 1349: automatically permitted provided client and server user names are the
1.40 aaron 1350: same.
1351: Additionally, successful RSA host authentication is normally
1352: required.
1353: This file should only be writable by root.
1.2 deraadt 1354: .It Pa /etc/shosts.equiv
1.44 aaron 1355: This file is processed exactly as
1.2 deraadt 1356: .Pa /etc/hosts.equiv .
1.1 deraadt 1357: This file may be useful to permit logins using
1.2 deraadt 1358: .Nm
1.1 deraadt 1359: but not using rsh/rlogin.
1.2 deraadt 1360: .It Pa /etc/sshrc
1.1 deraadt 1361: Commands in this file are executed by
1.2 deraadt 1362: .Nm
1.1 deraadt 1363: when the user logs in just before the user's shell (or command) is started.
1364: See the
1.2 deraadt 1365: .Xr sshd 8
1.1 deraadt 1366: manual page for more information.
1.2 deraadt 1367: .It Pa $HOME/.ssh/rc
1.1 deraadt 1368: Commands in this file are executed by
1.2 deraadt 1369: .Nm
1.1 deraadt 1370: when the user logs in just before the user's shell (or command) is
1371: started.
1.44 aaron 1372: See the
1.2 deraadt 1373: .Xr sshd 8
1.1 deraadt 1374: manual page for more information.
1.31 markus 1375: .It Pa $HOME/.ssh/environment
1376: Contains additional definitions for environment variables, see section
1377: .Sx ENVIRONMENT
1378: above.
1.58 itojun 1379: .El
1.67 aaron 1380: .Sh AUTHORS
1.78 markus 1381: OpenSSH is a derivative of the original and free
1382: ssh 1.2.12 release by Tatu Ylonen.
1383: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1384: Theo de Raadt and Dug Song
1385: removed many bugs, re-added newer features and
1386: created OpenSSH.
1387: Markus Friedl contributed the support for SSH
1388: protocol versions 1.5 and 2.0.
1.2 deraadt 1389: .Sh SEE ALSO
1390: .Xr rlogin 1 ,
1391: .Xr rsh 1 ,
1392: .Xr scp 1 ,
1.83 djm 1393: .Xr sftp 1 ,
1.2 deraadt 1394: .Xr ssh-add 1 ,
1395: .Xr ssh-agent 1 ,
1396: .Xr ssh-keygen 1 ,
1397: .Xr telnet 1 ,
1.87 itojun 1398: .Xr sshd 8
1.106 markus 1399: .Rs
1400: .%A T. Ylonen
1401: .%A T. Kivinen
1402: .%A M. Saarinen
1403: .%A T. Rinne
1404: .%A S. Lehtinen
1405: .%T "SSH Protocol Architecture"
1406: .%N draft-ietf-secsh-architecture-07.txt
1407: .%D January 2001
1408: .%O work in progress material
1409: .Re
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