Annotation of src/usr.bin/ssh/ssh.1, Revision 1.127
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: .\"
1.127 ! stevesk 37: .\" $OpenBSD: ssh.1,v 1.126 2001/08/01 22:16:45 markus Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSH 1
40: .Os
41: .Sh NAME
42: .Nm ssh
1.96 deraadt 43: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm ssh
46: .Op Fl l Ar login_name
1.5 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 ,
1.123 markus 212: the client will try to authenticate first using the hostbased method;
213: if this method fails public key authentication is attempted,
214: and finally if this method fails keyboard-interactive and
215: password authentication are tried.
1.49 markus 216: .Pp
217: The public key method is similar to RSA authentication described
1.107 markus 218: in the previous section and allows the RSA or DSA algorithm to be used:
1.102 itojun 219: The client uses his private key,
1.49 markus 220: .Pa $HOME/.ssh/id_dsa
1.102 itojun 221: or
222: .Pa $HOME/.ssh/id_rsa ,
1.49 markus 223: to sign the session identifier and sends the result to the server.
224: The server checks whether the matching public key is listed in
1.115 markus 225: .Pa $HOME/.ssh/authorized_keys
1.49 markus 226: and grants access if both the key is found and the signature is correct.
227: The session identifier is derived from a shared Diffie-Hellman value
228: and is only known to the client and the server.
229: .Pp
230: If public key authentication fails or is not available a password
231: can be sent encrypted to the remote host for proving the user's identity.
1.107 markus 232: .Pp
233: Additionally,
234: .Nm
235: supports hostbased or challenge response authentication.
1.49 markus 236: .Pp
237: Protocol 2 provides additional mechanisms for confidentiality
1.51 markus 238: (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
1.94 deraadt 239: and integrity (hmac-md5, hmac-sha1).
1.49 markus 240: Note that protocol 1 lacks a strong mechanism for ensuring the
241: integrity of the connection.
242: .Pp
243: .Ss Login session and remote execution
244: .Pp
1.1 deraadt 245: When the user's identity has been accepted by the server, the server
246: either executes the given command, or logs into the machine and gives
1.40 aaron 247: the user a normal shell on the remote machine.
248: All communication with
1.1 deraadt 249: the remote command or shell will be automatically encrypted.
1.2 deraadt 250: .Pp
1.1 deraadt 251: If a pseudo-terminal has been allocated (normal login session), the
1.104 djm 252: user may use the escape characters noted below.
1.2 deraadt 253: .Pp
1.1 deraadt 254: If no pseudo tty has been allocated, the
255: session is transparent and can be used to reliably transfer binary
1.40 aaron 256: data.
257: On most systems, setting the escape character to
1.2 deraadt 258: .Dq none
259: will also make the session transparent even if a tty is used.
260: .Pp
1.71 djm 261: The session terminates when the command or shell on the remote
1.92 markus 262: machine exits and all X11 and TCP/IP connections have been closed.
1.1 deraadt 263: The exit status of the remote program is returned as the exit status
264: of
1.2 deraadt 265: .Nm ssh .
1.104 djm 266: .Pp
267: .Ss Escape Characters
268: .Pp
269: When a pseudo terminal has been requested, ssh supports a number of functions
1.117 itojun 270: through the use of an escape character.
1.104 djm 271: .Pp
272: A single tilde character can be sent as
273: .Ic ~~
1.119 stevesk 274: or by following the tilde by a character other than those described below.
1.104 djm 275: The escape character must always follow a newline to be interpreted as
276: special.
277: The escape character can be changed in configuration files using the
278: .Cm EscapeChar
1.117 itojun 279: configuration directive or on the command line by the
1.104 djm 280: .Fl e
281: option.
282: .Pp
283: The supported escapes (assuming the default
284: .Ql ~ )
285: are:
286: .Bl -tag -width Ds
287: .It Cm ~.
288: Disconnect
289: .It Cm ~^Z
290: Background ssh
291: .It Cm ~#
292: List forwarded connections
293: .It Cm ~&
294: Background ssh at logout when waiting for forwarded connection / X11 sessions
1.107 markus 295: to terminate (protocol version 1 only)
1.104 djm 296: .It Cm ~?
297: Display a list of escape characters
298: .It Cm ~R
1.105 djm 299: Request rekeying of the connection (only useful for SSH protocol version 2
300: and if the peer supports it)
1.104 djm 301: .El
1.2 deraadt 302: .Pp
1.49 markus 303: .Ss X11 and TCP forwarding
304: .Pp
1.110 deraadt 305: If the
306: .Cm ForwardX11
307: variable is set to
308: .Dq yes
309: (or, see the description of the
310: .Fl X
311: and
312: .Fl x
313: options described later)
314: and the user is using X11 (the
1.2 deraadt 315: .Ev DISPLAY
1.1 deraadt 316: environment variable is set), the connection to the X11 display is
317: automatically forwarded to the remote side in such a way that any X11
318: programs started from the shell (or command) will go through the
319: encrypted channel, and the connection to the real X server will be made
1.40 aaron 320: from the local machine.
321: The user should not manually set
1.2 deraadt 322: .Ev DISPLAY .
1.1 deraadt 323: Forwarding of X11 connections can be
324: configured on the command line or in configuration files.
1.2 deraadt 325: .Pp
326: The
1.44 aaron 327: .Ev DISPLAY
1.2 deraadt 328: value set by
329: .Nm
1.1 deraadt 330: will point to the server machine, but with a display number greater
1.40 aaron 331: than zero.
332: This is normal, and happens because
1.2 deraadt 333: .Nm
334: creates a
335: .Dq proxy
336: X server on the server machine for forwarding the
1.1 deraadt 337: connections over the encrypted channel.
1.2 deraadt 338: .Pp
339: .Nm
1.1 deraadt 340: will also automatically set up Xauthority data on the server machine.
341: For this purpose, it will generate a random authorization cookie,
342: store it in Xauthority on the server, and verify that any forwarded
343: connections carry this cookie and replace it by the real cookie when
1.40 aaron 344: the connection is opened.
345: The real authentication cookie is never
1.1 deraadt 346: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 347: .Pp
1.1 deraadt 348: If the user is using an authentication agent, the connection to the agent
349: is automatically forwarded to the remote side unless disabled on
1.120 stevesk 350: the command line or in a configuration file.
1.2 deraadt 351: .Pp
1.1 deraadt 352: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.120 stevesk 353: be specified either on the command line or in a configuration file.
1.40 aaron 354: One possible application of TCP/IP forwarding is a secure connection to an
1.92 markus 355: electronic purse; another is going through firewalls.
1.2 deraadt 356: .Pp
1.49 markus 357: .Ss Server authentication
358: .Pp
1.2 deraadt 359: .Nm
1.49 markus 360: automatically maintains and checks a database containing
1.40 aaron 361: identifications for all hosts it has ever been used with.
1.116 markus 362: Host keys are stored in
1.49 markus 363: .Pa $HOME/.ssh/known_hosts
1.40 aaron 364: in the user's home directory.
1.116 markus 365: Additionally, the file
1.2 deraadt 366: .Pa /etc/ssh_known_hosts
1.116 markus 367: is automatically checked for known hosts.
1.40 aaron 368: Any new hosts are automatically added to the user's file.
369: If a host's identification
1.1 deraadt 370: ever changes,
1.2 deraadt 371: .Nm
1.1 deraadt 372: warns about this and disables password authentication to prevent a
1.40 aaron 373: trojan horse from getting the user's password.
374: Another purpose of
1.1 deraadt 375: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 376: otherwise be used to circumvent the encryption.
377: The
1.2 deraadt 378: .Cm StrictHostKeyChecking
1.1 deraadt 379: option (see below) can be used to prevent logins to machines whose
380: host key is not known or has changed.
1.65 aaron 381: .Pp
382: The options are as follows:
1.2 deraadt 383: .Bl -tag -width Ds
1.4 dugsong 384: .It Fl a
1.42 aaron 385: Disables forwarding of the authentication agent connection.
1.54 markus 386: .It Fl A
387: Enables forwarding of the authentication agent connection.
388: This can also be specified on a per-host basis in a configuration file.
1.108 markus 389: .It Fl b Ar bind_address
390: Specify the interface to transmit from on machines with multiple
391: interfaces or aliased addresses.
1.6 deraadt 392: .It Fl c Ar blowfish|3des
1.44 aaron 393: Selects the cipher to use for encrypting the session.
1.2 deraadt 394: .Ar 3des
1.40 aaron 395: is used by default.
1.44 aaron 396: It is believed to be secure.
1.5 deraadt 397: .Ar 3des
398: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
399: It is presumably more secure than the
1.2 deraadt 400: .Ar des
1.64 markus 401: cipher which is no longer fully supported in
1.51 markus 402: .Nm ssh .
1.5 deraadt 403: .Ar blowfish
404: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 405: .Ar 3des .
1.90 markus 406: .It Fl c Ar cipher_spec
1.51 markus 407: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 408: be specified in order of preference.
1.90 markus 409: See
410: .Cm Ciphers
411: for more information.
1.2 deraadt 412: .It Fl e Ar ch|^ch|none
413: Sets the escape character for sessions with a pty (default:
414: .Ql ~ ) .
1.40 aaron 415: The escape character is only recognized at the beginning of a line.
416: The escape character followed by a dot
1.2 deraadt 417: .Pq Ql \&.
418: closes the connection, followed
1.1 deraadt 419: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 420: escape character once.
421: Setting the character to
1.2 deraadt 422: .Dq none
423: disables any escapes and makes the session fully transparent.
424: .It Fl f
425: Requests
426: .Nm
1.40 aaron 427: to go to background just before command execution.
428: This is useful if
1.2 deraadt 429: .Nm
430: is going to ask for passwords or passphrases, but the user
1.40 aaron 431: wants it in the background.
1.44 aaron 432: This implies
1.2 deraadt 433: .Fl n .
1.1 deraadt 434: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 435: something like
436: .Ic ssh -f host xterm .
1.34 markus 437: .It Fl g
438: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 439: .It Fl i Ar identity_file
1.44 aaron 440: Selects the file from which the identity (private key) for
1.68 markus 441: RSA or DSA authentication is read.
1.44 aaron 442: Default is
1.49 markus 443: .Pa $HOME/.ssh/identity
1.40 aaron 444: in the user's home directory.
445: Identity files may also be specified on
446: a per-host basis in the configuration file.
447: It is possible to have multiple
1.2 deraadt 448: .Fl i
449: options (and multiple identities specified in
1.1 deraadt 450: configuration files).
1.125 jakob 451: .It Fl I Ar smartcard_device
452: Specifies which smartcard device to use. The argument is
453: the device
454: .Nm
455: should use to communicate with a smartcard used for storing the user's
456: private RSA key.
1.2 deraadt 457: .It Fl k
1.42 aaron 458: Disables forwarding of Kerberos tickets and AFS tokens.
459: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 460: .It Fl l Ar login_name
1.40 aaron 461: Specifies the user to log in as on the remote machine.
462: This also may be specified on a per-host basis in the configuration file.
1.91 markus 463: .It Fl m Ar mac_spec
464: Additionally, for protocol version 2 a comma-separated list of MAC
465: (message authentication code) algorithms can
466: be specified in order of preference.
467: See the
468: .Cm MACs
469: keyword for more information.
1.2 deraadt 470: .It Fl n
471: Redirects stdin from
472: .Pa /dev/null
473: (actually, prevents reading from stdin).
1.1 deraadt 474: This must be used when
1.2 deraadt 475: .Nm
1.40 aaron 476: is run in the background.
477: A common trick is to use this to run X11 programs on a remote machine.
478: For example,
1.2 deraadt 479: .Ic ssh -n shadows.cs.hut.fi emacs &
480: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 481: connection will be automatically forwarded over an encrypted channel.
482: The
1.2 deraadt 483: .Nm
1.1 deraadt 484: program will be put in the background.
485: (This does not work if
1.2 deraadt 486: .Nm
487: needs to ask for a password or passphrase; see also the
488: .Fl f
489: option.)
1.53 markus 490: .It Fl N
491: Do not execute a remote command.
1.70 markus 492: This is useful if you just want to forward ports
1.53 markus 493: (protocol version 2 only).
1.2 deraadt 494: .It Fl o Ar option
1.127 ! stevesk 495: Can be used to give options in the format used in the configuration file.
1.1 deraadt 496: This is useful for specifying options for which there is no separate
1.40 aaron 497: command-line flag.
1.2 deraadt 498: .It Fl p Ar port
1.40 aaron 499: Port to connect to on the remote host.
500: This can be specified on a
1.1 deraadt 501: per-host basis in the configuration file.
1.16 markus 502: .It Fl P
503: Use a non-privileged port for outgoing connections.
504: This can be used if your firewall does
505: not permit connections from privileged ports.
1.30 provos 506: Note that this option turns off
1.16 markus 507: .Cm RhostsAuthentication
508: and
1.72 markus 509: .Cm RhostsRSAAuthentication
510: for older servers.
1.2 deraadt 511: .It Fl q
1.40 aaron 512: Quiet mode.
513: Causes all warning and diagnostic messages to be suppressed.
514: Only fatal errors are displayed.
1.80 djm 515: .It Fl s
1.117 itojun 516: 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
517: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 518: subsystem is specified as the remote command.
1.2 deraadt 519: .It Fl t
1.40 aaron 520: Force pseudo-tty allocation.
1.43 brad 521: This can be used to execute arbitrary
1.40 aaron 522: screen-based programs on a remote machine, which can be very useful,
523: e.g., when implementing menu services.
1.73 markus 524: Multiple
525: .Fl t
526: options force tty allocation, even if
527: .Nm
528: has no local tty.
1.53 markus 529: .It Fl T
1.69 markus 530: Disable pseudo-tty allocation.
1.2 deraadt 531: .It Fl v
1.40 aaron 532: Verbose mode.
533: Causes
1.2 deraadt 534: .Nm
1.40 aaron 535: to print debugging messages about its progress.
536: This is helpful in
1.1 deraadt 537: debugging connection, authentication, and configuration problems.
1.73 markus 538: Multiple
539: .Fl v
540: options increases the verbosity.
1.61 aaron 541: Maximum is 3.
1.2 deraadt 542: .It Fl x
1.40 aaron 543: Disables X11 forwarding.
1.2 deraadt 544: .It Fl X
1.1 deraadt 545: Enables X11 forwarding.
1.54 markus 546: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 547: .It Fl C
1.1 deraadt 548: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 549: data for forwarded X11 and TCP/IP connections).
550: The compression algorithm is the same used by
1.34 markus 551: .Xr gzip 1 ,
552: and the
1.2 deraadt 553: .Dq level
554: can be controlled by the
555: .Cm CompressionLevel
1.40 aaron 556: option (see below).
557: Compression is desirable on modem lines and other
1.1 deraadt 558: slow connections, but will only slow down things on fast networks.
559: The default value can be set on a host-by-host basis in the
560: configuration files; see the
1.121 pvalchev 561: .Cm Compression
1.1 deraadt 562: option below.
1.2 deraadt 563: .It Fl L Ar port:host:hostport
1.1 deraadt 564: Specifies that the given port on the local (client) host is to be
1.40 aaron 565: forwarded to the given host and port on the remote side.
566: This works by allocating a socket to listen to
1.2 deraadt 567: .Ar port
1.1 deraadt 568: on the local side, and whenever a connection is made to this port, the
569: connection is forwarded over the secure channel, and a connection is
570: made to
1.32 markus 571: .Ar host
572: port
573: .Ar hostport
1.40 aaron 574: from the remote machine.
575: Port forwardings can also be specified in the configuration file.
576: Only root can forward privileged ports.
1.32 markus 577: IPv6 addresses can be specified with an alternative syntax:
578: .Ar port/host/hostport
1.2 deraadt 579: .It Fl R Ar port:host:hostport
1.1 deraadt 580: Specifies that the given port on the remote (server) host is to be
1.40 aaron 581: forwarded to the given host and port on the local side.
582: This works by allocating a socket to listen to
1.2 deraadt 583: .Ar port
1.1 deraadt 584: on the remote side, and whenever a connection is made to this port, the
585: connection is forwarded over the secure channel, and a connection is
586: made to
1.32 markus 587: .Ar host
588: port
589: .Ar hostport
1.40 aaron 590: from the local machine.
591: Port forwardings can also be specified in the configuration file.
592: Privileged ports can be forwarded only when
1.1 deraadt 593: logging in as root on the remote machine.
1.107 markus 594: IPv6 addresses can be specified with an alternative syntax:
595: .Ar port/host/hostport
1.85 jakob 596: .It Fl 1
597: Forces
598: .Nm
599: to try protocol version 1 only.
1.46 markus 600: .It Fl 2
601: Forces
602: .Nm
1.50 markus 603: to try protocol version 2 only.
1.32 markus 604: .It Fl 4
605: Forces
606: .Nm
607: to use IPv4 addresses only.
608: .It Fl 6
609: Forces
610: .Nm
611: to use IPv6 addresses only.
1.2 deraadt 612: .El
613: .Sh CONFIGURATION FILES
614: .Nm
1.127 ! stevesk 615: obtains configuration data from the following sources in
! 616: the following order:
1.1 deraadt 617: command line options, user's configuration file
1.2 deraadt 618: .Pq Pa $HOME/.ssh/config ,
619: and system-wide configuration file
620: .Pq Pa /etc/ssh_config .
621: For each parameter, the first obtained value
1.40 aaron 622: will be used.
623: The configuration files contain sections bracketed by
624: .Dq Host
625: specifications, and that section is only applied for hosts that
626: match one of the patterns given in the specification.
627: The matched host name is the one given on the command line.
1.2 deraadt 628: .Pp
1.1 deraadt 629: Since the first obtained value for each parameter is used, more
630: host-specific declarations should be given near the beginning of the
631: file, and general defaults at the end.
1.2 deraadt 632: .Pp
1.1 deraadt 633: The configuration file has the following format:
1.2 deraadt 634: .Pp
635: Empty lines and lines starting with
636: .Ql #
637: are comments.
638: .Pp
639: Otherwise a line is of the format
640: .Dq keyword arguments .
1.127 ! stevesk 641: Configuration options may be separated by whitespace or
! 642: optional whitespace and exactly one
! 643: .Ql = ;
! 644: the latter format is useful to avoid the need to quote whitespace
! 645: when specifying configuration options using the
! 646: .Nm ssh ,
! 647: .Nm scp
! 648: and
! 649: .Nm sftp
! 650: .Fl o
! 651: option.
! 652: .Pp
1.2 deraadt 653: The possible
1.1 deraadt 654: keywords and their meanings are as follows (note that the
655: configuration files are case-sensitive):
1.2 deraadt 656: .Bl -tag -width Ds
657: .It Cm Host
1.1 deraadt 658: Restricts the following declarations (up to the next
1.2 deraadt 659: .Cm Host
1.1 deraadt 660: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 661: given after the keyword.
662: .Ql \&*
663: and
664: .Ql ?
665: can be used as wildcards in the
1.40 aaron 666: patterns.
667: A single
1.2 deraadt 668: .Ql \&*
669: as a pattern can be used to provide global
1.40 aaron 670: defaults for all hosts.
671: The host is the
1.2 deraadt 672: .Ar hostname
1.1 deraadt 673: argument given on the command line (i.e., the name is not converted to
674: a canonicalized host name before matching).
1.2 deraadt 675: .It Cm AFSTokenPassing
1.42 aaron 676: Specifies whether to pass AFS tokens to remote host.
677: The argument to this keyword must be
1.2 deraadt 678: .Dq yes
679: or
680: .Dq no .
1.107 markus 681: This option applies to protocol version 1 only.
1.2 deraadt 682: .It Cm BatchMode
683: If set to
684: .Dq yes ,
1.40 aaron 685: passphrase/password querying will be disabled.
686: This option is useful in scripts and other batch jobs where you have no
687: user to supply the password.
688: The argument must be
1.2 deraadt 689: .Dq yes
690: or
691: .Dq no .
1.100 stevesk 692: The default is
693: .Dq no .
1.108 markus 694: .It Cm BindAddress
695: Specify the interface to transmit from on machines with multiple
696: interfaces or aliased addresses.
697: Note that this option does not work if
698: .Cm UsePrivilegedPort
699: is set to
700: .Dq yes .
1.34 markus 701: .It Cm CheckHostIP
702: If this flag is set to
703: .Dq yes ,
1.100 stevesk 704: ssh will additionally check the host IP address in the
1.34 markus 705: .Pa known_hosts
1.42 aaron 706: file.
707: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 708: If the option is set to
709: .Dq no ,
710: the check will not be executed.
1.100 stevesk 711: The default is
712: .Dq yes .
1.2 deraadt 713: .It Cm Cipher
1.62 markus 714: Specifies the cipher to use for encrypting the session
1.64 markus 715: in protocol version 1.
1.40 aaron 716: Currently,
1.117 itojun 717: .Dq blowfish
1.1 deraadt 718: and
1.10 provos 719: .Dq 3des
1.40 aaron 720: are supported.
721: The default is
1.2 deraadt 722: .Dq 3des .
1.45 markus 723: .It Cm Ciphers
724: Specifies the ciphers allowed for protocol version 2
725: in order of preference.
726: Multiple ciphers must be comma-separated.
727: The default is
1.88 provos 728: .Pp
729: .Bd -literal
1.94 deraadt 730: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.107 markus 731: aes192-cbc,aes256-cbc''
1.88 provos 732: .Ed
1.2 deraadt 733: .It Cm Compression
1.40 aaron 734: Specifies whether to use compression.
735: The argument must be
1.2 deraadt 736: .Dq yes
737: or
738: .Dq no .
1.100 stevesk 739: The default is
740: .Dq no .
1.2 deraadt 741: .It Cm CompressionLevel
1.100 stevesk 742: Specifies the compression level to use if compression is enabled.
1.40 aaron 743: The argument must be an integer from 1 (fast) to 9 (slow, best).
744: The default level is 6, which is good for most applications.
745: The meaning of the values is the same as in
1.34 markus 746: .Xr gzip 1 .
1.107 markus 747: Note that this option applies to protocol version 1 only.
1.2 deraadt 748: .It Cm ConnectionAttempts
1.1 deraadt 749: Specifies the number of tries (one per second) to make before falling
1.40 aaron 750: back to rsh or exiting.
751: The argument must be an integer.
752: This may be useful in scripts if the connection sometimes fails.
1.124 markus 753: The default is 1.
1.2 deraadt 754: .It Cm EscapeChar
755: Sets the escape character (default:
756: .Ql ~ ) .
757: The escape character can also
1.40 aaron 758: be set on the command line.
759: The argument should be a single character,
1.2 deraadt 760: .Ql ^
761: followed by a letter, or
762: .Dq none
763: to disable the escape
1.1 deraadt 764: character entirely (making the connection transparent for binary
765: data).
1.44 aaron 766: .It Cm FallBackToRsh
1.1 deraadt 767: Specifies that if connecting via
1.2 deraadt 768: .Nm
1.1 deraadt 769: fails due to a connection refused error (there is no
1.2 deraadt 770: .Xr sshd 8
1.44 aaron 771: listening on the remote host),
1.2 deraadt 772: .Xr rsh 1
1.1 deraadt 773: should automatically be used instead (after a suitable warning about
1.40 aaron 774: the session being unencrypted).
775: The argument must be
1.2 deraadt 776: .Dq yes
777: or
778: .Dq no .
1.100 stevesk 779: The default is
780: .Dq no .
1.2 deraadt 781: .It Cm ForwardAgent
1.1 deraadt 782: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 783: will be forwarded to the remote machine.
784: The argument must be
1.2 deraadt 785: .Dq yes
786: or
1.54 markus 787: .Dq no .
788: The default is
1.2 deraadt 789: .Dq no .
790: .It Cm ForwardX11
1.1 deraadt 791: Specifies whether X11 connections will be automatically redirected
1.44 aaron 792: over the secure channel and
1.2 deraadt 793: .Ev DISPLAY
1.40 aaron 794: set.
1.44 aaron 795: The argument must be
1.2 deraadt 796: .Dq yes
797: or
1.38 markus 798: .Dq no .
799: The default is
1.3 deraadt 800: .Dq no .
801: .It Cm GatewayPorts
802: Specifies whether remote hosts are allowed to connect to local
803: forwarded ports.
804: The argument must be
805: .Dq yes
806: or
807: .Dq no .
808: The default is
1.2 deraadt 809: .Dq no .
810: .It Cm GlobalKnownHostsFile
1.116 markus 811: Specifies a file to use for the global
1.95 stevesk 812: host key database instead of
1.2 deraadt 813: .Pa /etc/ssh_known_hosts .
1.107 markus 814: .It Cm HostbasedAuthentication
815: Specifies whether to try rhosts based authentication with public key
816: authentication.
817: The argument must be
818: .Dq yes
819: or
820: .Dq no .
821: The default is
1.112 markus 822: .Dq no .
1.107 markus 823: This option applies to protocol version 2 only and
824: is similar to
825: .Cm RhostsRSAAuthentication .
826: .It Cm HostKeyAlgorithms
1.109 stevesk 827: Specifies the protocol version 2 host key algorithms
1.107 markus 828: that the client wants to use in order of preference.
829: The default for this option is:
830: .Dq ssh-rsa,ssh-dss
1.74 markus 831: .It Cm HostKeyAlias
832: Specifies an alias that should be used instead of the
833: real host name when looking up or saving the host key
1.107 markus 834: in the host key database files.
1.82 stevesk 835: This option is useful for tunneling ssh connections
1.74 markus 836: or if you have multiple servers running on a single host.
1.2 deraadt 837: .It Cm HostName
1.40 aaron 838: Specifies the real host name to log into.
839: This can be used to specify nicknames or abbreviations for hosts.
840: Default is the name given on the command line.
841: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 842: .Cm HostName
1.1 deraadt 843: specifications).
1.2 deraadt 844: .It Cm IdentityFile
1.107 markus 845: Specifies the file from which the user's RSA or DSA authentication identity
1.2 deraadt 846: is read (default
1.49 markus 847: .Pa $HOME/.ssh/identity
1.2 deraadt 848: in the user's home directory).
1.1 deraadt 849: Additionally, any identities represented by the authentication agent
1.40 aaron 850: will be used for authentication.
851: The file name may use the tilde
852: syntax to refer to a user's home directory.
853: It is possible to have
1.1 deraadt 854: multiple identity files specified in configuration files; all these
855: identities will be tried in sequence.
1.2 deraadt 856: .It Cm KeepAlive
1.1 deraadt 857: Specifies whether the system should send keepalive messages to the
1.40 aaron 858: other side.
859: If they are sent, death of the connection or crash of one
860: of the machines will be properly noticed.
861: However, this means that
1.1 deraadt 862: connections will die if the route is down temporarily, and some people
1.41 aaron 863: find it annoying.
1.2 deraadt 864: .Pp
865: The default is
866: .Dq yes
867: (to send keepalives), and the client will notice
1.40 aaron 868: if the network goes down or the remote host dies.
869: This is important in scripts, and many users want it too.
1.2 deraadt 870: .Pp
871: To disable keepalives, the value should be set to
872: .Dq no
873: in both the server and the client configuration files.
874: .It Cm KerberosAuthentication
1.42 aaron 875: Specifies whether Kerberos authentication will be used.
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 KerberosTgtPassing
1.42 aaron 881: Specifies whether a Kerberos TGT will be forwarded to the server.
882: This will only work if the Kerberos server is actually an AFS kaserver.
883: The argument to this keyword must be
1.4 dugsong 884: .Dq yes
885: or
886: .Dq no .
1.2 deraadt 887: .It Cm LocalForward
1.1 deraadt 888: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 889: the secure channel to given host:port from the remote machine.
890: The first argument must be a port number, and the second must be
891: host:port.
892: Multiple forwardings may be specified, and additional
893: forwardings can be given on the command line.
894: Only the superuser can forward privileged ports.
1.24 markus 895: .It Cm LogLevel
896: Gives the verbosity level that is used when logging messages from
897: .Nm ssh .
898: The possible values are:
1.77 markus 899: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
900: The default is INFO.
1.91 markus 901: .It Cm MACs
1.117 itojun 902: Specifies the MAC (message authentication code) algorithms
1.91 markus 903: in order of preference.
904: The MAC algorithm is used in protocol version 2
905: for data integrity protection.
906: Multiple algorithms must be comma-separated.
907: The default is
1.113 markus 908: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.14 dugsong 909: .It Cm NumberOfPasswordPrompts
1.42 aaron 910: Specifies the number of password prompts before giving up.
911: The argument to this keyword must be an integer.
912: Default is 3.
1.34 markus 913: .It Cm PasswordAuthentication
1.40 aaron 914: Specifies whether to use password authentication.
915: The argument to this keyword must be
1.34 markus 916: .Dq yes
917: or
918: .Dq no .
1.100 stevesk 919: The default is
920: .Dq yes .
1.2 deraadt 921: .It Cm Port
1.40 aaron 922: Specifies the port number to connect on the remote host.
923: Default is 22.
1.99 djm 924: .It Cm PreferredAuthentications
1.117 itojun 925: Specifies the order in which the client should try protocol 2
926: authentication methods. This allows a client to prefer one method (e.g.
1.99 djm 927: .Cm keyboard-interactive )
928: over another method (e.g.
929: .Cm password )
930: The default for this option is:
1.123 markus 931: .Dq hostbased,publickey,keyboard-interactive,password
1.45 markus 932: .It Cm Protocol
933: Specifies the protocol versions
934: .Nm
935: should support in order of preference.
936: The possible values are
937: .Dq 1
938: and
939: .Dq 2 .
940: Multiple versions must be comma-separated.
941: The default is
1.101 markus 942: .Dq 2,1 .
1.49 markus 943: This means that
944: .Nm
1.101 markus 945: tries version 2 and falls back to version 1
946: if version 2 is not available.
1.2 deraadt 947: .It Cm ProxyCommand
1.40 aaron 948: Specifies the command to use to connect to the server.
949: The command
950: string extends to the end of the line, and is executed with
951: .Pa /bin/sh .
952: In the command string,
953: .Ql %h
954: will be substituted by the host name to
955: connect and
956: .Ql %p
957: by the port.
958: The command can be basically anything,
959: and should read from its standard input and write to its standard output.
960: It should eventually connect an
1.2 deraadt 961: .Xr sshd 8
1.1 deraadt 962: server running on some machine, or execute
1.2 deraadt 963: .Ic sshd -i
1.40 aaron 964: somewhere.
965: Host key management will be done using the
1.1 deraadt 966: HostName of the host being connected (defaulting to the name typed by
967: the user).
1.29 markus 968: Note that
969: .Cm CheckHostIP
970: is not available for connects with a proxy command.
1.2 deraadt 971: .Pp
1.107 markus 972: .It Cm PubkeyAuthentication
973: Specifies whether to try public key authentication.
974: The argument to this keyword must be
975: .Dq yes
976: or
977: .Dq no .
978: The default is
979: .Dq yes .
980: This option applies to protocol version 2 only.
1.2 deraadt 981: .It Cm RemoteForward
1.1 deraadt 982: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 983: the secure channel to given host:port from the local machine.
984: The first argument must be a port number, and the second must be
985: host:port.
986: Multiple forwardings may be specified, and additional
987: forwardings can be given on the command line.
988: Only the superuser can forward privileged ports.
1.2 deraadt 989: .It Cm RhostsAuthentication
1.40 aaron 990: Specifies whether to try rhosts based authentication.
991: Note that this
1.1 deraadt 992: declaration only affects the client side and has no effect whatsoever
1.40 aaron 993: on security.
994: Disabling rhosts authentication may reduce
1.1 deraadt 995: authentication time on slow connections when rhosts authentication is
1.40 aaron 996: not used.
997: Most servers do not permit RhostsAuthentication because it
1.117 itojun 998: is not secure (see
1.114 mpech 999: .Cm RhostsRSAAuthentication ) .
1.40 aaron 1000: The argument to this keyword must be
1.2 deraadt 1001: .Dq yes
1002: or
1003: .Dq no .
1.100 stevesk 1004: The default is
1005: .Dq yes .
1.107 markus 1006: This option applies to protocol version 1 only.
1.2 deraadt 1007: .It Cm RhostsRSAAuthentication
1.1 deraadt 1008: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 1009: authentication.
1010: The argument must be
1.2 deraadt 1011: .Dq yes
1012: or
1013: .Dq no .
1.100 stevesk 1014: The default is
1015: .Dq yes .
1.107 markus 1016: This option applies to protocol version 1 only.
1.2 deraadt 1017: .It Cm RSAAuthentication
1.40 aaron 1018: Specifies whether to try RSA authentication.
1019: The argument to this keyword must be
1.2 deraadt 1020: .Dq yes
1021: or
1022: .Dq no .
1.1 deraadt 1023: RSA authentication will only be
1024: attempted if the identity file exists, or an authentication agent is
1025: running.
1.100 stevesk 1026: The default is
1027: .Dq yes .
1.50 markus 1028: Note that this option applies to protocol version 1 only.
1.81 markus 1029: .It Cm ChallengeResponseAuthentication
1030: Specifies whether to use challenge response authentication.
1.40 aaron 1031: The argument to this keyword must be
1.27 markus 1032: .Dq yes
1033: or
1034: .Dq no .
1035: The default is
1.122 markus 1036: .Dq yes .
1.125 jakob 1037: .It Cm SmartcardDevice
1038: Specifies which smartcard device to use. The argument to this keyword is
1039: the device
1040: .Nm
1041: should use to communicate with a smartcard used for storing the user's
1042: private RSA key. By default, no device is specified and smartcard support
1043: is not activated.
1.2 deraadt 1044: .It Cm StrictHostKeyChecking
1045: If this flag is set to
1.44 aaron 1046: .Dq yes ,
1.2 deraadt 1047: .Nm
1.79 stevesk 1048: will never automatically add host keys to the
1.2 deraadt 1049: .Pa $HOME/.ssh/known_hosts
1.116 markus 1050: file, and refuses to connect to hosts whose host key has changed.
1.40 aaron 1051: This provides maximum protection against trojan horse attacks.
1052: However, it can be somewhat annoying if you don't have good
1.2 deraadt 1053: .Pa /etc/ssh_known_hosts
1.1 deraadt 1054: files installed and frequently
1.79 stevesk 1055: connect to new hosts.
1056: This option forces the user to manually
1057: add all new hosts.
1058: If this flag is set to
1059: .Dq no ,
1060: .Nm
1061: will automatically add new host keys to the
1062: user known hosts files.
1063: If this flag is set to
1064: .Dq ask ,
1065: new host keys
1066: will be added to the user known host files only after the user
1067: has confirmed that is what they really want to do, and
1068: .Nm
1069: will refuse to connect to hosts whose host key has changed.
1.40 aaron 1070: The host keys of
1.79 stevesk 1071: known hosts will be verified automatically in all cases.
1.40 aaron 1072: The argument must be
1.79 stevesk 1073: .Dq yes ,
1074: .Dq no
1.2 deraadt 1075: or
1.79 stevesk 1076: .Dq ask .
1077: The default is
1078: .Dq ask .
1.16 markus 1079: .It Cm UsePrivilegedPort
1080: Specifies whether to use a privileged port for outgoing connections.
1081: The argument must be
1082: .Dq yes
1083: or
1084: .Dq no .
1085: The default is
1.98 markus 1086: .Dq no .
1.107 markus 1087: Note that you need to set this option to
1088: .Dq yes
1089: if you want to use
1.16 markus 1090: .Cm RhostsAuthentication
1091: and
1.72 markus 1092: .Cm RhostsRSAAuthentication
1.107 markus 1093: with older servers.
1.34 markus 1094: .It Cm User
1.40 aaron 1095: Specifies the user to log in as.
1096: This can be useful if you have a different user name on different machines.
1097: This saves the trouble of
1.34 markus 1098: having to remember to give the user name on the command line.
1099: .It Cm UserKnownHostsFile
1.116 markus 1100: Specifies a file to use for the user
1.95 stevesk 1101: host key database instead of
1.34 markus 1102: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 1103: .It Cm UseRsh
1.40 aaron 1104: Specifies that rlogin/rsh should be used for this host.
1105: It is possible that the host does not at all support the
1.2 deraadt 1106: .Nm
1.40 aaron 1107: protocol.
1108: This causes
1.2 deraadt 1109: .Nm
1.40 aaron 1110: to immediately execute
1.2 deraadt 1111: .Xr rsh 1 .
1.1 deraadt 1112: All other options (except
1.2 deraadt 1113: .Cm HostName )
1.40 aaron 1114: are ignored if this has been specified.
1115: The argument must be
1.2 deraadt 1116: .Dq yes
1117: or
1118: .Dq no .
1.55 markus 1119: .It Cm XAuthLocation
1120: Specifies the location of the
1121: .Xr xauth 1
1122: program.
1123: The default is
1124: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1125: .El
1.2 deraadt 1126: .Sh ENVIRONMENT
1127: .Nm
1.1 deraadt 1128: will normally set the following environment variables:
1.2 deraadt 1129: .Bl -tag -width Ds
1130: .It Ev DISPLAY
1131: The
1132: .Ev DISPLAY
1.40 aaron 1133: variable indicates the location of the X11 server.
1.44 aaron 1134: It is automatically set by
1.2 deraadt 1135: .Nm
1136: to point to a value of the form
1137: .Dq hostname:n
1138: where hostname indicates
1.40 aaron 1139: the host where the shell runs, and n is an integer >= 1.
1140: .Nm
1141: uses this special value to forward X11 connections over the secure
1142: channel.
1.107 markus 1143: The user should normally not set
1144: .Ev DISPLAY
1145: explicitly, as that
1.1 deraadt 1146: will render the X11 connection insecure (and will require the user to
1147: manually copy any required authorization cookies).
1.2 deraadt 1148: .It Ev HOME
1.1 deraadt 1149: Set to the path of the user's home directory.
1.2 deraadt 1150: .It Ev LOGNAME
1151: Synonym for
1.12 aaron 1152: .Ev USER ;
1153: set for compatibility with systems that use this variable.
1.2 deraadt 1154: .It Ev MAIL
1.1 deraadt 1155: Set to point the user's mailbox.
1.40 aaron 1156: .It Ev PATH
1.2 deraadt 1157: Set to the default
1158: .Ev PATH ,
1159: as specified when compiling
1.12 aaron 1160: .Nm ssh .
1.118 markus 1161: .It Ev SSH_ASKPASS
1162: If
1163: .Nm
1164: needs a passphrase, it will read the passphrase from the current
1165: terminal if it was run from a terminal.
1166: If
1167: .Nm
1168: does not have a terminal associated with it but
1169: .Ev DISPLAY
1170: and
1171: .Ev SSH_ASKPASS
1172: are set, it will execute the program specified by
1173: .Ev SSH_ASKPASS
1174: and open an X11 window to read the passphrase.
1175: This is particularly useful when calling
1176: .Nm
1177: from a
1178: .Pa .Xsession
1179: or related script.
1180: (Note that on some machines it
1181: may be necessary to redirect the input from
1182: .Pa /dev/null
1183: to make this work.)
1.18 markus 1184: .It Ev SSH_AUTH_SOCK
1.17 markus 1185: indicates the path of a unix-domain socket used to communicate with the
1186: agent.
1.2 deraadt 1187: .It Ev SSH_CLIENT
1.40 aaron 1188: Identifies the client end of the connection.
1189: The variable contains
1.1 deraadt 1190: three space-separated values: client ip-address, client port number,
1191: and server port number.
1.73 markus 1192: .It Ev SSH_ORIGINAL_COMMAND
1193: The variable contains the original command line if a forced command
1194: is executed.
1195: It can be used to extract the original arguments.
1.2 deraadt 1196: .It Ev SSH_TTY
1.1 deraadt 1197: This is set to the name of the tty (path to the device) associated
1.40 aaron 1198: with the current shell or command.
1199: If the current session has no tty,
1.1 deraadt 1200: this variable is not set.
1.2 deraadt 1201: .It Ev TZ
1.1 deraadt 1202: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1203: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1204: on to new connections).
1.2 deraadt 1205: .It Ev USER
1.1 deraadt 1206: Set to the name of the user logging in.
1.2 deraadt 1207: .El
1208: .Pp
1.44 aaron 1209: Additionally,
1.2 deraadt 1210: .Nm
1.44 aaron 1211: reads
1212: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1213: and adds lines of the format
1214: .Dq VARNAME=value
1.12 aaron 1215: to the environment.
1.2 deraadt 1216: .Sh FILES
1.36 markus 1217: .Bl -tag -width Ds
1.116 markus 1218: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 1219: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1220: in
1.116 markus 1221: .Pa /etc/ssh_known_hosts .
1.2 deraadt 1222: See
1223: .Xr sshd 8 .
1.102 itojun 1224: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1225: Contains the authentication identity of the user.
1226: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 1227: These files
1228: contain sensitive data and should be readable by the user but not
1.15 markus 1229: accessible by others (read/write/execute).
1230: Note that
1231: .Nm
1.48 markus 1232: ignores a private key file if it is accessible by others.
1.15 markus 1233: It is possible to specify a passphrase when
1.1 deraadt 1234: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1235: sensitive part of this file using 3DES.
1.102 itojun 1236: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 1237: Contains the public key for authentication (public part of the
1.40 aaron 1238: identity file in human-readable form).
1.48 markus 1239: The contents of the
1240: .Pa $HOME/.ssh/identity.pub
1241: file should be added to
1.2 deraadt 1242: .Pa $HOME/.ssh/authorized_keys
1243: on all machines
1.102 itojun 1244: where you wish to log in using protocol version 1 RSA authentication.
1.48 markus 1245: The contents of the
1246: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 1247: and
1248: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 1249: file should be added to
1.115 markus 1250: .Pa $HOME/.ssh/authorized_keys
1.48 markus 1251: on all machines
1.102 itojun 1252: where you wish to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 1253: These files are not
1.40 aaron 1254: sensitive and can (but need not) be readable by anyone.
1.48 markus 1255: These files are
1.84 markus 1256: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1257: the convenience of the user.
1.2 deraadt 1258: .It Pa $HOME/.ssh/config
1.40 aaron 1259: This is the per-user configuration file.
1260: The format of this file is described above.
1261: This file is used by the
1.2 deraadt 1262: .Nm
1.40 aaron 1263: client.
1264: This file does not usually contain any sensitive information,
1.1 deraadt 1265: but the recommended permissions are read/write for the user, and not
1266: accessible by others.
1.2 deraadt 1267: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 1268: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 1269: The format of this file is described in the
1.2 deraadt 1270: .Xr sshd 8
1.40 aaron 1271: manual page.
1.116 markus 1272: In the simplest form the format is the same as the .pub
1273: identity files.
1.48 markus 1274: This file is not highly sensitive, but the recommended
1275: permissions are read/write for the user, and not accessible by others.
1.116 markus 1276: .It Pa /etc/ssh_known_hosts
1.40 aaron 1277: Systemwide list of known host keys.
1.116 markus 1278: This file should be prepared by the
1.1 deraadt 1279: system administrator to contain the public host keys of all machines in the
1.40 aaron 1280: organization.
1281: This file should be world-readable.
1282: This file contains
1.1 deraadt 1283: public keys, one per line, in the following format (fields separated
1.116 markus 1284: by spaces): system name, public key and optional comment field.
1.40 aaron 1285: When different names are used
1.1 deraadt 1286: for the same machine, all such names should be listed, separated by
1.40 aaron 1287: commas.
1288: The format is described on the
1.2 deraadt 1289: .Xr sshd 8
1.1 deraadt 1290: manual page.
1.2 deraadt 1291: .Pp
1.1 deraadt 1292: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1293: .Xr sshd 8
1.1 deraadt 1294: to verify the client host when logging in; other names are needed because
1.2 deraadt 1295: .Nm
1.1 deraadt 1296: does not convert the user-supplied name to a canonical name before
1297: checking the key, because someone with access to the name servers
1298: would then be able to fool host authentication.
1.2 deraadt 1299: .It Pa /etc/ssh_config
1.40 aaron 1300: Systemwide configuration file.
1301: This file provides defaults for those
1.1 deraadt 1302: values that are not specified in the user's configuration file, and
1.40 aaron 1303: for those users who do not have a configuration file.
1304: This file must be world-readable.
1.2 deraadt 1305: .It Pa $HOME/.rhosts
1306: This file is used in
1307: .Pa \&.rhosts
1308: authentication to list the
1.40 aaron 1309: host/user pairs that are permitted to log in.
1310: (Note that this file is
1.1 deraadt 1311: also used by rlogin and rsh, which makes using this file insecure.)
1312: Each line of the file contains a host name (in the canonical form
1313: returned by name servers), and then a user name on that host,
1.40 aaron 1314: separated by a space.
1.92 markus 1315: On some machines this file may need to be
1.1 deraadt 1316: world-readable if the user's home directory is on a NFS partition,
1317: because
1.2 deraadt 1318: .Xr sshd 8
1.40 aaron 1319: reads it as root.
1320: Additionally, this file must be owned by the user,
1321: and must not have write permissions for anyone else.
1322: The recommended
1.1 deraadt 1323: permission for most machines is read/write for the user, and not
1324: accessible by others.
1.2 deraadt 1325: .Pp
1.1 deraadt 1326: Note that by default
1.2 deraadt 1327: .Xr sshd 8
1.1 deraadt 1328: will be installed so that it requires successful RSA host
1.40 aaron 1329: authentication before permitting \s+2.\s0rhosts authentication.
1330: If your server machine does not have the client's host key in
1.2 deraadt 1331: .Pa /etc/ssh_known_hosts ,
1332: you can store it in
1333: .Pa $HOME/.ssh/known_hosts .
1334: The easiest way to do this is to
1.1 deraadt 1335: connect back to the client from the server machine using ssh; this
1.48 markus 1336: will automatically add the host key to
1.2 deraadt 1337: .Pa $HOME/.ssh/known_hosts .
1338: .It Pa $HOME/.shosts
1339: This file is used exactly the same way as
1340: .Pa \&.rhosts .
1341: The purpose for
1.1 deraadt 1342: having this file is to be able to use rhosts authentication with
1.2 deraadt 1343: .Nm
1344: without permitting login with
1345: .Xr rlogin 1
1346: or
1347: .Xr rsh 1 .
1348: .It Pa /etc/hosts.equiv
1349: This file is used during
1.40 aaron 1350: .Pa \&.rhosts authentication.
1351: It contains
1.1 deraadt 1352: canonical hosts names, one per line (the full format is described on
1353: the
1.2 deraadt 1354: .Xr sshd 8
1.40 aaron 1355: manual page).
1356: If the client host is found in this file, login is
1.1 deraadt 1357: automatically permitted provided client and server user names are the
1.40 aaron 1358: same.
1359: Additionally, successful RSA host authentication is normally
1360: required.
1361: This file should only be writable by root.
1.2 deraadt 1362: .It Pa /etc/shosts.equiv
1.44 aaron 1363: This file is processed exactly as
1.2 deraadt 1364: .Pa /etc/hosts.equiv .
1.1 deraadt 1365: This file may be useful to permit logins using
1.2 deraadt 1366: .Nm
1.1 deraadt 1367: but not using rsh/rlogin.
1.2 deraadt 1368: .It Pa /etc/sshrc
1.1 deraadt 1369: Commands in this file are executed by
1.2 deraadt 1370: .Nm
1.1 deraadt 1371: when the user logs in just before the user's shell (or command) is started.
1372: See the
1.2 deraadt 1373: .Xr sshd 8
1.1 deraadt 1374: manual page for more information.
1.2 deraadt 1375: .It Pa $HOME/.ssh/rc
1.1 deraadt 1376: Commands in this file are executed by
1.2 deraadt 1377: .Nm
1.1 deraadt 1378: when the user logs in just before the user's shell (or command) is
1379: started.
1.44 aaron 1380: See the
1.2 deraadt 1381: .Xr sshd 8
1.1 deraadt 1382: manual page for more information.
1.31 markus 1383: .It Pa $HOME/.ssh/environment
1384: Contains additional definitions for environment variables, see section
1385: .Sx ENVIRONMENT
1386: above.
1.58 itojun 1387: .El
1.67 aaron 1388: .Sh AUTHORS
1.78 markus 1389: OpenSSH is a derivative of the original and free
1390: ssh 1.2.12 release by Tatu Ylonen.
1391: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1392: Theo de Raadt and Dug Song
1393: removed many bugs, re-added newer features and
1394: created OpenSSH.
1395: Markus Friedl contributed the support for SSH
1396: protocol versions 1.5 and 2.0.
1.2 deraadt 1397: .Sh SEE ALSO
1398: .Xr rlogin 1 ,
1399: .Xr rsh 1 ,
1400: .Xr scp 1 ,
1.83 djm 1401: .Xr sftp 1 ,
1.2 deraadt 1402: .Xr ssh-add 1 ,
1403: .Xr ssh-agent 1 ,
1404: .Xr ssh-keygen 1 ,
1405: .Xr telnet 1 ,
1.87 itojun 1406: .Xr sshd 8
1.106 markus 1407: .Rs
1408: .%A T. Ylonen
1409: .%A T. Kivinen
1410: .%A M. Saarinen
1411: .%A T. Rinne
1412: .%A S. Lehtinen
1413: .%T "SSH Protocol Architecture"
1.126 markus 1414: .%N draft-ietf-secsh-architecture-09.txt
1415: .%D July 2001
1.106 markus 1416: .%O work in progress material
1417: .Re
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