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