Annotation of src/usr.bin/ssh/ssh.1, Revision 1.126
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: .\"
1.126 ! markus 37: .\" $OpenBSD: ssh.1,v 1.125 2001/07/31 09:28:44 jakob 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
1.1 deraadt 495: Can be used to give options in the format used in the config file.
496: This is useful for specifying options for which there is no separate
1.40 aaron 497: command-line flag.
498: The option has the same format as a line in the configuration file.
1.2 deraadt 499: .It Fl p Ar port
1.40 aaron 500: Port to connect to on the remote host.
501: This can be specified on a
1.1 deraadt 502: per-host basis in the configuration file.
1.16 markus 503: .It Fl P
504: Use a non-privileged port for outgoing connections.
505: This can be used if your firewall does
506: not permit connections from privileged ports.
1.30 provos 507: Note that this option turns off
1.16 markus 508: .Cm RhostsAuthentication
509: and
1.72 markus 510: .Cm RhostsRSAAuthentication
511: for older servers.
1.2 deraadt 512: .It Fl q
1.40 aaron 513: Quiet mode.
514: Causes all warning and diagnostic messages to be suppressed.
515: Only fatal errors are displayed.
1.80 djm 516: .It Fl s
1.117 itojun 517: 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
518: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 519: subsystem is specified as the remote command.
1.2 deraadt 520: .It Fl t
1.40 aaron 521: Force pseudo-tty allocation.
1.43 brad 522: This can be used to execute arbitrary
1.40 aaron 523: screen-based programs on a remote machine, which can be very useful,
524: e.g., when implementing menu services.
1.73 markus 525: Multiple
526: .Fl t
527: options force tty allocation, even if
528: .Nm
529: has no local tty.
1.53 markus 530: .It Fl T
1.69 markus 531: Disable pseudo-tty allocation.
1.2 deraadt 532: .It Fl v
1.40 aaron 533: Verbose mode.
534: Causes
1.2 deraadt 535: .Nm
1.40 aaron 536: to print debugging messages about its progress.
537: This is helpful in
1.1 deraadt 538: debugging connection, authentication, and configuration problems.
1.73 markus 539: Multiple
540: .Fl v
541: options increases the verbosity.
1.61 aaron 542: Maximum is 3.
1.2 deraadt 543: .It Fl x
1.40 aaron 544: Disables X11 forwarding.
1.2 deraadt 545: .It Fl X
1.1 deraadt 546: Enables X11 forwarding.
1.54 markus 547: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 548: .It Fl C
1.1 deraadt 549: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 550: data for forwarded X11 and TCP/IP connections).
551: The compression algorithm is the same used by
1.34 markus 552: .Xr gzip 1 ,
553: and the
1.2 deraadt 554: .Dq level
555: can be controlled by the
556: .Cm CompressionLevel
1.40 aaron 557: option (see below).
558: Compression is desirable on modem lines and other
1.1 deraadt 559: slow connections, but will only slow down things on fast networks.
560: The default value can be set on a host-by-host basis in the
561: configuration files; see the
1.121 pvalchev 562: .Cm Compression
1.1 deraadt 563: option below.
1.2 deraadt 564: .It Fl L Ar port:host:hostport
1.1 deraadt 565: Specifies that the given port on the local (client) host is to be
1.40 aaron 566: forwarded to the given host and port on the remote side.
567: This works by allocating a socket to listen to
1.2 deraadt 568: .Ar port
1.1 deraadt 569: on the local side, and whenever a connection is made to this port, the
570: connection is forwarded over the secure channel, and a connection is
571: made to
1.32 markus 572: .Ar host
573: port
574: .Ar hostport
1.40 aaron 575: from the remote machine.
576: Port forwardings can also be specified in the configuration file.
577: Only root can forward privileged ports.
1.32 markus 578: IPv6 addresses can be specified with an alternative syntax:
579: .Ar port/host/hostport
1.2 deraadt 580: .It Fl R Ar port:host:hostport
1.1 deraadt 581: Specifies that the given port on the remote (server) host is to be
1.40 aaron 582: forwarded to the given host and port on the local side.
583: This works by allocating a socket to listen to
1.2 deraadt 584: .Ar port
1.1 deraadt 585: on the remote side, and whenever a connection is made to this port, the
586: connection is forwarded over the secure channel, and a connection is
587: made to
1.32 markus 588: .Ar host
589: port
590: .Ar hostport
1.40 aaron 591: from the local machine.
592: Port forwardings can also be specified in the configuration file.
593: Privileged ports can be forwarded only when
1.1 deraadt 594: logging in as root on the remote machine.
1.107 markus 595: IPv6 addresses can be specified with an alternative syntax:
596: .Ar port/host/hostport
1.85 jakob 597: .It Fl 1
598: Forces
599: .Nm
600: to try protocol version 1 only.
1.46 markus 601: .It Fl 2
602: Forces
603: .Nm
1.50 markus 604: to try protocol version 2 only.
1.32 markus 605: .It Fl 4
606: Forces
607: .Nm
608: to use IPv4 addresses only.
609: .It Fl 6
610: Forces
611: .Nm
612: to use IPv6 addresses only.
1.2 deraadt 613: .El
614: .Sh CONFIGURATION FILES
615: .Nm
1.1 deraadt 616: obtains configuration data from the following sources (in this order):
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 .
641: The possible
1.1 deraadt 642: keywords and their meanings are as follows (note that the
643: configuration files are case-sensitive):
1.2 deraadt 644: .Bl -tag -width Ds
645: .It Cm Host
1.1 deraadt 646: Restricts the following declarations (up to the next
1.2 deraadt 647: .Cm Host
1.1 deraadt 648: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 649: given after the keyword.
650: .Ql \&*
651: and
652: .Ql ?
653: can be used as wildcards in the
1.40 aaron 654: patterns.
655: A single
1.2 deraadt 656: .Ql \&*
657: as a pattern can be used to provide global
1.40 aaron 658: defaults for all hosts.
659: The host is the
1.2 deraadt 660: .Ar hostname
1.1 deraadt 661: argument given on the command line (i.e., the name is not converted to
662: a canonicalized host name before matching).
1.2 deraadt 663: .It Cm AFSTokenPassing
1.42 aaron 664: Specifies whether to pass AFS tokens to remote host.
665: The argument to this keyword must be
1.2 deraadt 666: .Dq yes
667: or
668: .Dq no .
1.107 markus 669: This option applies to protocol version 1 only.
1.2 deraadt 670: .It Cm BatchMode
671: If set to
672: .Dq yes ,
1.40 aaron 673: passphrase/password querying will be disabled.
674: This option is useful in scripts and other batch jobs where you have no
675: user to supply the password.
676: The argument must be
1.2 deraadt 677: .Dq yes
678: or
679: .Dq no .
1.100 stevesk 680: The default is
681: .Dq no .
1.108 markus 682: .It Cm BindAddress
683: Specify the interface to transmit from on machines with multiple
684: interfaces or aliased addresses.
685: Note that this option does not work if
686: .Cm UsePrivilegedPort
687: is set to
688: .Dq yes .
1.34 markus 689: .It Cm CheckHostIP
690: If this flag is set to
691: .Dq yes ,
1.100 stevesk 692: ssh will additionally check the host IP address in the
1.34 markus 693: .Pa known_hosts
1.42 aaron 694: file.
695: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 696: If the option is set to
697: .Dq no ,
698: the check will not be executed.
1.100 stevesk 699: The default is
700: .Dq yes .
1.2 deraadt 701: .It Cm Cipher
1.62 markus 702: Specifies the cipher to use for encrypting the session
1.64 markus 703: in protocol version 1.
1.40 aaron 704: Currently,
1.117 itojun 705: .Dq blowfish
1.1 deraadt 706: and
1.10 provos 707: .Dq 3des
1.40 aaron 708: are supported.
709: The default is
1.2 deraadt 710: .Dq 3des .
1.45 markus 711: .It Cm Ciphers
712: Specifies the ciphers allowed for protocol version 2
713: in order of preference.
714: Multiple ciphers must be comma-separated.
715: The default is
1.88 provos 716: .Pp
717: .Bd -literal
1.94 deraadt 718: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.107 markus 719: aes192-cbc,aes256-cbc''
1.88 provos 720: .Ed
1.2 deraadt 721: .It Cm Compression
1.40 aaron 722: Specifies whether to use compression.
723: The argument must be
1.2 deraadt 724: .Dq yes
725: or
726: .Dq no .
1.100 stevesk 727: The default is
728: .Dq no .
1.2 deraadt 729: .It Cm CompressionLevel
1.100 stevesk 730: Specifies the compression level to use if compression is enabled.
1.40 aaron 731: The argument must be an integer from 1 (fast) to 9 (slow, best).
732: The default level is 6, which is good for most applications.
733: The meaning of the values is the same as in
1.34 markus 734: .Xr gzip 1 .
1.107 markus 735: Note that this option applies to protocol version 1 only.
1.2 deraadt 736: .It Cm ConnectionAttempts
1.1 deraadt 737: Specifies the number of tries (one per second) to make before falling
1.40 aaron 738: back to rsh or exiting.
739: The argument must be an integer.
740: This may be useful in scripts if the connection sometimes fails.
1.124 markus 741: The default is 1.
1.2 deraadt 742: .It Cm EscapeChar
743: Sets the escape character (default:
744: .Ql ~ ) .
745: The escape character can also
1.40 aaron 746: be set on the command line.
747: The argument should be a single character,
1.2 deraadt 748: .Ql ^
749: followed by a letter, or
750: .Dq none
751: to disable the escape
1.1 deraadt 752: character entirely (making the connection transparent for binary
753: data).
1.44 aaron 754: .It Cm FallBackToRsh
1.1 deraadt 755: Specifies that if connecting via
1.2 deraadt 756: .Nm
1.1 deraadt 757: fails due to a connection refused error (there is no
1.2 deraadt 758: .Xr sshd 8
1.44 aaron 759: listening on the remote host),
1.2 deraadt 760: .Xr rsh 1
1.1 deraadt 761: should automatically be used instead (after a suitable warning about
1.40 aaron 762: the session being unencrypted).
763: The argument must be
1.2 deraadt 764: .Dq yes
765: or
766: .Dq no .
1.100 stevesk 767: The default is
768: .Dq no .
1.2 deraadt 769: .It Cm ForwardAgent
1.1 deraadt 770: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 771: will be forwarded to the remote machine.
772: The argument must be
1.2 deraadt 773: .Dq yes
774: or
1.54 markus 775: .Dq no .
776: The default is
1.2 deraadt 777: .Dq no .
778: .It Cm ForwardX11
1.1 deraadt 779: Specifies whether X11 connections will be automatically redirected
1.44 aaron 780: over the secure channel and
1.2 deraadt 781: .Ev DISPLAY
1.40 aaron 782: set.
1.44 aaron 783: The argument must be
1.2 deraadt 784: .Dq yes
785: or
1.38 markus 786: .Dq no .
787: The default is
1.3 deraadt 788: .Dq no .
789: .It Cm GatewayPorts
790: Specifies whether remote hosts are allowed to connect to local
791: forwarded ports.
792: The argument must be
793: .Dq yes
794: or
795: .Dq no .
796: The default is
1.2 deraadt 797: .Dq no .
798: .It Cm GlobalKnownHostsFile
1.116 markus 799: Specifies a file to use for the global
1.95 stevesk 800: host key database instead of
1.2 deraadt 801: .Pa /etc/ssh_known_hosts .
1.107 markus 802: .It Cm HostbasedAuthentication
803: Specifies whether to try rhosts based authentication with public key
804: authentication.
805: The argument must be
806: .Dq yes
807: or
808: .Dq no .
809: The default is
1.112 markus 810: .Dq no .
1.107 markus 811: This option applies to protocol version 2 only and
812: is similar to
813: .Cm RhostsRSAAuthentication .
814: .It Cm HostKeyAlgorithms
1.109 stevesk 815: Specifies the protocol version 2 host key algorithms
1.107 markus 816: that the client wants to use in order of preference.
817: The default for this option is:
818: .Dq ssh-rsa,ssh-dss
1.74 markus 819: .It Cm HostKeyAlias
820: Specifies an alias that should be used instead of the
821: real host name when looking up or saving the host key
1.107 markus 822: in the host key database files.
1.82 stevesk 823: This option is useful for tunneling ssh connections
1.74 markus 824: or if you have multiple servers running on a single host.
1.2 deraadt 825: .It Cm HostName
1.40 aaron 826: Specifies the real host name to log into.
827: This can be used to specify nicknames or abbreviations for hosts.
828: Default is the name given on the command line.
829: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 830: .Cm HostName
1.1 deraadt 831: specifications).
1.2 deraadt 832: .It Cm IdentityFile
1.107 markus 833: Specifies the file from which the user's RSA or DSA authentication identity
1.2 deraadt 834: is read (default
1.49 markus 835: .Pa $HOME/.ssh/identity
1.2 deraadt 836: in the user's home directory).
1.1 deraadt 837: Additionally, any identities represented by the authentication agent
1.40 aaron 838: will be used for authentication.
839: The file name may use the tilde
840: syntax to refer to a user's home directory.
841: It is possible to have
1.1 deraadt 842: multiple identity files specified in configuration files; all these
843: identities will be tried in sequence.
1.2 deraadt 844: .It Cm KeepAlive
1.1 deraadt 845: Specifies whether the system should send keepalive messages to the
1.40 aaron 846: other side.
847: If they are sent, death of the connection or crash of one
848: of the machines will be properly noticed.
849: However, this means that
1.1 deraadt 850: connections will die if the route is down temporarily, and some people
1.41 aaron 851: find it annoying.
1.2 deraadt 852: .Pp
853: The default is
854: .Dq yes
855: (to send keepalives), and the client will notice
1.40 aaron 856: if the network goes down or the remote host dies.
857: This is important in scripts, and many users want it too.
1.2 deraadt 858: .Pp
859: To disable keepalives, the value should be set to
860: .Dq no
861: in both the server and the client configuration files.
862: .It Cm KerberosAuthentication
1.42 aaron 863: Specifies whether Kerberos authentication will be used.
864: The argument to this keyword must be
1.4 dugsong 865: .Dq yes
866: or
867: .Dq no .
1.2 deraadt 868: .It Cm KerberosTgtPassing
1.42 aaron 869: Specifies whether a Kerberos TGT will be forwarded to the server.
870: This will only work if the Kerberos server is actually an AFS kaserver.
871: The argument to this keyword must be
1.4 dugsong 872: .Dq yes
873: or
874: .Dq no .
1.2 deraadt 875: .It Cm LocalForward
1.1 deraadt 876: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 877: the secure channel to given host:port from the remote machine.
878: The first argument must be a port number, and the second must be
879: host:port.
880: Multiple forwardings may be specified, and additional
881: forwardings can be given on the command line.
882: Only the superuser can forward privileged ports.
1.24 markus 883: .It Cm LogLevel
884: Gives the verbosity level that is used when logging messages from
885: .Nm ssh .
886: The possible values are:
1.77 markus 887: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
888: The default is INFO.
1.91 markus 889: .It Cm MACs
1.117 itojun 890: Specifies the MAC (message authentication code) algorithms
1.91 markus 891: in order of preference.
892: The MAC algorithm is used in protocol version 2
893: for data integrity protection.
894: Multiple algorithms must be comma-separated.
895: The default is
1.113 markus 896: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.14 dugsong 897: .It Cm NumberOfPasswordPrompts
1.42 aaron 898: Specifies the number of password prompts before giving up.
899: The argument to this keyword must be an integer.
900: Default is 3.
1.34 markus 901: .It Cm PasswordAuthentication
1.40 aaron 902: Specifies whether to use password authentication.
903: The argument to this keyword must be
1.34 markus 904: .Dq yes
905: or
906: .Dq no .
1.100 stevesk 907: The default is
908: .Dq yes .
1.2 deraadt 909: .It Cm Port
1.40 aaron 910: Specifies the port number to connect on the remote host.
911: Default is 22.
1.99 djm 912: .It Cm PreferredAuthentications
1.117 itojun 913: Specifies the order in which the client should try protocol 2
914: authentication methods. This allows a client to prefer one method (e.g.
1.99 djm 915: .Cm keyboard-interactive )
916: over another method (e.g.
917: .Cm password )
918: The default for this option is:
1.123 markus 919: .Dq hostbased,publickey,keyboard-interactive,password
1.45 markus 920: .It Cm Protocol
921: Specifies the protocol versions
922: .Nm
923: should support in order of preference.
924: The possible values are
925: .Dq 1
926: and
927: .Dq 2 .
928: Multiple versions must be comma-separated.
929: The default is
1.101 markus 930: .Dq 2,1 .
1.49 markus 931: This means that
932: .Nm
1.101 markus 933: tries version 2 and falls back to version 1
934: if version 2 is not available.
1.2 deraadt 935: .It Cm ProxyCommand
1.40 aaron 936: Specifies the command to use to connect to the server.
937: The command
938: string extends to the end of the line, and is executed with
939: .Pa /bin/sh .
940: In the command string,
941: .Ql %h
942: will be substituted by the host name to
943: connect and
944: .Ql %p
945: by the port.
946: The command can be basically anything,
947: and should read from its standard input and write to its standard output.
948: It should eventually connect an
1.2 deraadt 949: .Xr sshd 8
1.1 deraadt 950: server running on some machine, or execute
1.2 deraadt 951: .Ic sshd -i
1.40 aaron 952: somewhere.
953: Host key management will be done using the
1.1 deraadt 954: HostName of the host being connected (defaulting to the name typed by
955: the user).
1.29 markus 956: Note that
957: .Cm CheckHostIP
958: is not available for connects with a proxy command.
1.2 deraadt 959: .Pp
1.107 markus 960: .It Cm PubkeyAuthentication
961: Specifies whether to try public key authentication.
962: The argument to this keyword must be
963: .Dq yes
964: or
965: .Dq no .
966: The default is
967: .Dq yes .
968: This option applies to protocol version 2 only.
1.2 deraadt 969: .It Cm RemoteForward
1.1 deraadt 970: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 971: the secure channel to given host:port from the local machine.
972: The first argument must be a port number, and the second must be
973: host:port.
974: Multiple forwardings may be specified, and additional
975: forwardings can be given on the command line.
976: Only the superuser can forward privileged ports.
1.2 deraadt 977: .It Cm RhostsAuthentication
1.40 aaron 978: Specifies whether to try rhosts based authentication.
979: Note that this
1.1 deraadt 980: declaration only affects the client side and has no effect whatsoever
1.40 aaron 981: on security.
982: Disabling rhosts authentication may reduce
1.1 deraadt 983: authentication time on slow connections when rhosts authentication is
1.40 aaron 984: not used.
985: Most servers do not permit RhostsAuthentication because it
1.117 itojun 986: is not secure (see
1.114 mpech 987: .Cm RhostsRSAAuthentication ) .
1.40 aaron 988: The argument to this keyword must be
1.2 deraadt 989: .Dq yes
990: or
991: .Dq no .
1.100 stevesk 992: The default is
993: .Dq yes .
1.107 markus 994: This option applies to protocol version 1 only.
1.2 deraadt 995: .It Cm RhostsRSAAuthentication
1.1 deraadt 996: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 997: authentication.
998: The argument must be
1.2 deraadt 999: .Dq yes
1000: or
1001: .Dq no .
1.100 stevesk 1002: The default is
1003: .Dq yes .
1.107 markus 1004: This option applies to protocol version 1 only.
1.2 deraadt 1005: .It Cm RSAAuthentication
1.40 aaron 1006: Specifies whether to try RSA authentication.
1007: The argument to this keyword must be
1.2 deraadt 1008: .Dq yes
1009: or
1010: .Dq no .
1.1 deraadt 1011: RSA authentication will only be
1012: attempted if the identity file exists, or an authentication agent is
1013: running.
1.100 stevesk 1014: The default is
1015: .Dq yes .
1.50 markus 1016: Note that this option applies to protocol version 1 only.
1.81 markus 1017: .It Cm ChallengeResponseAuthentication
1018: Specifies whether to use challenge response authentication.
1.40 aaron 1019: The argument to this keyword must be
1.27 markus 1020: .Dq yes
1021: or
1022: .Dq no .
1023: The default is
1.122 markus 1024: .Dq yes .
1.125 jakob 1025: .It Cm SmartcardDevice
1026: Specifies which smartcard device to use. The argument to this keyword is
1027: the device
1028: .Nm
1029: should use to communicate with a smartcard used for storing the user's
1030: private RSA key. By default, no device is specified and smartcard support
1031: is not activated.
1.2 deraadt 1032: .It Cm StrictHostKeyChecking
1033: If this flag is set to
1.44 aaron 1034: .Dq yes ,
1.2 deraadt 1035: .Nm
1.79 stevesk 1036: will never automatically add host keys to the
1.2 deraadt 1037: .Pa $HOME/.ssh/known_hosts
1.116 markus 1038: file, and refuses to connect to hosts whose host key has changed.
1.40 aaron 1039: This provides maximum protection against trojan horse attacks.
1040: However, it can be somewhat annoying if you don't have good
1.2 deraadt 1041: .Pa /etc/ssh_known_hosts
1.1 deraadt 1042: files installed and frequently
1.79 stevesk 1043: connect to new hosts.
1044: This option forces the user to manually
1045: add all new hosts.
1046: If this flag is set to
1047: .Dq no ,
1048: .Nm
1049: will automatically add new host keys to the
1050: user known hosts files.
1051: If this flag is set to
1052: .Dq ask ,
1053: new host keys
1054: will be added to the user known host files only after the user
1055: has confirmed that is what they really want to do, and
1056: .Nm
1057: will refuse to connect to hosts whose host key has changed.
1.40 aaron 1058: The host keys of
1.79 stevesk 1059: known hosts will be verified automatically in all cases.
1.40 aaron 1060: The argument must be
1.79 stevesk 1061: .Dq yes ,
1062: .Dq no
1.2 deraadt 1063: or
1.79 stevesk 1064: .Dq ask .
1065: The default is
1066: .Dq ask .
1.16 markus 1067: .It Cm UsePrivilegedPort
1068: Specifies whether to use a privileged port for outgoing connections.
1069: The argument must be
1070: .Dq yes
1071: or
1072: .Dq no .
1073: The default is
1.98 markus 1074: .Dq no .
1.107 markus 1075: Note that you need to set this option to
1076: .Dq yes
1077: if you want to use
1.16 markus 1078: .Cm RhostsAuthentication
1079: and
1.72 markus 1080: .Cm RhostsRSAAuthentication
1.107 markus 1081: with older servers.
1.34 markus 1082: .It Cm User
1.40 aaron 1083: Specifies the user to log in as.
1084: This can be useful if you have a different user name on different machines.
1085: This saves the trouble of
1.34 markus 1086: having to remember to give the user name on the command line.
1087: .It Cm UserKnownHostsFile
1.116 markus 1088: Specifies a file to use for the user
1.95 stevesk 1089: host key database instead of
1.34 markus 1090: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 1091: .It Cm UseRsh
1.40 aaron 1092: Specifies that rlogin/rsh should be used for this host.
1093: It is possible that the host does not at all support the
1.2 deraadt 1094: .Nm
1.40 aaron 1095: protocol.
1096: This causes
1.2 deraadt 1097: .Nm
1.40 aaron 1098: to immediately execute
1.2 deraadt 1099: .Xr rsh 1 .
1.1 deraadt 1100: All other options (except
1.2 deraadt 1101: .Cm HostName )
1.40 aaron 1102: are ignored if this has been specified.
1103: The argument must be
1.2 deraadt 1104: .Dq yes
1105: or
1106: .Dq no .
1.55 markus 1107: .It Cm XAuthLocation
1108: Specifies the location of the
1109: .Xr xauth 1
1110: program.
1111: The default is
1112: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1113: .El
1.2 deraadt 1114: .Sh ENVIRONMENT
1115: .Nm
1.1 deraadt 1116: will normally set the following environment variables:
1.2 deraadt 1117: .Bl -tag -width Ds
1118: .It Ev DISPLAY
1119: The
1120: .Ev DISPLAY
1.40 aaron 1121: variable indicates the location of the X11 server.
1.44 aaron 1122: It is automatically set by
1.2 deraadt 1123: .Nm
1124: to point to a value of the form
1125: .Dq hostname:n
1126: where hostname indicates
1.40 aaron 1127: the host where the shell runs, and n is an integer >= 1.
1128: .Nm
1129: uses this special value to forward X11 connections over the secure
1130: channel.
1.107 markus 1131: The user should normally not set
1132: .Ev DISPLAY
1133: explicitly, as that
1.1 deraadt 1134: will render the X11 connection insecure (and will require the user to
1135: manually copy any required authorization cookies).
1.2 deraadt 1136: .It Ev HOME
1.1 deraadt 1137: Set to the path of the user's home directory.
1.2 deraadt 1138: .It Ev LOGNAME
1139: Synonym for
1.12 aaron 1140: .Ev USER ;
1141: set for compatibility with systems that use this variable.
1.2 deraadt 1142: .It Ev MAIL
1.1 deraadt 1143: Set to point the user's mailbox.
1.40 aaron 1144: .It Ev PATH
1.2 deraadt 1145: Set to the default
1146: .Ev PATH ,
1147: as specified when compiling
1.12 aaron 1148: .Nm ssh .
1.118 markus 1149: .It Ev SSH_ASKPASS
1150: If
1151: .Nm
1152: needs a passphrase, it will read the passphrase from the current
1153: terminal if it was run from a terminal.
1154: If
1155: .Nm
1156: does not have a terminal associated with it but
1157: .Ev DISPLAY
1158: and
1159: .Ev SSH_ASKPASS
1160: are set, it will execute the program specified by
1161: .Ev SSH_ASKPASS
1162: and open an X11 window to read the passphrase.
1163: This is particularly useful when calling
1164: .Nm
1165: from a
1166: .Pa .Xsession
1167: or related script.
1168: (Note that on some machines it
1169: may be necessary to redirect the input from
1170: .Pa /dev/null
1171: to make this work.)
1.18 markus 1172: .It Ev SSH_AUTH_SOCK
1.17 markus 1173: indicates the path of a unix-domain socket used to communicate with the
1174: agent.
1.2 deraadt 1175: .It Ev SSH_CLIENT
1.40 aaron 1176: Identifies the client end of the connection.
1177: The variable contains
1.1 deraadt 1178: three space-separated values: client ip-address, client port number,
1179: and server port number.
1.73 markus 1180: .It Ev SSH_ORIGINAL_COMMAND
1181: The variable contains the original command line if a forced command
1182: is executed.
1183: It can be used to extract the original arguments.
1.2 deraadt 1184: .It Ev SSH_TTY
1.1 deraadt 1185: This is set to the name of the tty (path to the device) associated
1.40 aaron 1186: with the current shell or command.
1187: If the current session has no tty,
1.1 deraadt 1188: this variable is not set.
1.2 deraadt 1189: .It Ev TZ
1.1 deraadt 1190: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1191: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1192: on to new connections).
1.2 deraadt 1193: .It Ev USER
1.1 deraadt 1194: Set to the name of the user logging in.
1.2 deraadt 1195: .El
1196: .Pp
1.44 aaron 1197: Additionally,
1.2 deraadt 1198: .Nm
1.44 aaron 1199: reads
1200: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1201: and adds lines of the format
1202: .Dq VARNAME=value
1.12 aaron 1203: to the environment.
1.2 deraadt 1204: .Sh FILES
1.36 markus 1205: .Bl -tag -width Ds
1.116 markus 1206: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 1207: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1208: in
1.116 markus 1209: .Pa /etc/ssh_known_hosts .
1.2 deraadt 1210: See
1211: .Xr sshd 8 .
1.102 itojun 1212: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1213: Contains the authentication identity of the user.
1214: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 1215: These files
1216: contain sensitive data and should be readable by the user but not
1.15 markus 1217: accessible by others (read/write/execute).
1218: Note that
1219: .Nm
1.48 markus 1220: ignores a private key file if it is accessible by others.
1.15 markus 1221: It is possible to specify a passphrase when
1.1 deraadt 1222: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1223: sensitive part of this file using 3DES.
1.102 itojun 1224: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 1225: Contains the public key for authentication (public part of the
1.40 aaron 1226: identity file in human-readable form).
1.48 markus 1227: The contents of the
1228: .Pa $HOME/.ssh/identity.pub
1229: file should be added to
1.2 deraadt 1230: .Pa $HOME/.ssh/authorized_keys
1231: on all machines
1.102 itojun 1232: where you wish to log in using protocol version 1 RSA authentication.
1.48 markus 1233: The contents of the
1234: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 1235: and
1236: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 1237: file should be added to
1.115 markus 1238: .Pa $HOME/.ssh/authorized_keys
1.48 markus 1239: on all machines
1.102 itojun 1240: where you wish to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 1241: These files are not
1.40 aaron 1242: sensitive and can (but need not) be readable by anyone.
1.48 markus 1243: These files are
1.84 markus 1244: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1245: the convenience of the user.
1.2 deraadt 1246: .It Pa $HOME/.ssh/config
1.40 aaron 1247: This is the per-user configuration file.
1248: The format of this file is described above.
1249: This file is used by the
1.2 deraadt 1250: .Nm
1.40 aaron 1251: client.
1252: This file does not usually contain any sensitive information,
1.1 deraadt 1253: but the recommended permissions are read/write for the user, and not
1254: accessible by others.
1.2 deraadt 1255: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 1256: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 1257: The format of this file is described in the
1.2 deraadt 1258: .Xr sshd 8
1.40 aaron 1259: manual page.
1.116 markus 1260: In the simplest form the format is the same as the .pub
1261: identity files.
1.48 markus 1262: This file is not highly sensitive, but the recommended
1263: permissions are read/write for the user, and not accessible by others.
1.116 markus 1264: .It Pa /etc/ssh_known_hosts
1.40 aaron 1265: Systemwide list of known host keys.
1.116 markus 1266: This file should be prepared by the
1.1 deraadt 1267: system administrator to contain the public host keys of all machines in the
1.40 aaron 1268: organization.
1269: This file should be world-readable.
1270: This file contains
1.1 deraadt 1271: public keys, one per line, in the following format (fields separated
1.116 markus 1272: by spaces): system name, public key and optional comment field.
1.40 aaron 1273: When different names are used
1.1 deraadt 1274: for the same machine, all such names should be listed, separated by
1.40 aaron 1275: commas.
1276: The format is described on the
1.2 deraadt 1277: .Xr sshd 8
1.1 deraadt 1278: manual page.
1.2 deraadt 1279: .Pp
1.1 deraadt 1280: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1281: .Xr sshd 8
1.1 deraadt 1282: to verify the client host when logging in; other names are needed because
1.2 deraadt 1283: .Nm
1.1 deraadt 1284: does not convert the user-supplied name to a canonical name before
1285: checking the key, because someone with access to the name servers
1286: would then be able to fool host authentication.
1.2 deraadt 1287: .It Pa /etc/ssh_config
1.40 aaron 1288: Systemwide configuration file.
1289: This file provides defaults for those
1.1 deraadt 1290: values that are not specified in the user's configuration file, and
1.40 aaron 1291: for those users who do not have a configuration file.
1292: This file must be world-readable.
1.2 deraadt 1293: .It Pa $HOME/.rhosts
1294: This file is used in
1295: .Pa \&.rhosts
1296: authentication to list the
1.40 aaron 1297: host/user pairs that are permitted to log in.
1298: (Note that this file is
1.1 deraadt 1299: also used by rlogin and rsh, which makes using this file insecure.)
1300: Each line of the file contains a host name (in the canonical form
1301: returned by name servers), and then a user name on that host,
1.40 aaron 1302: separated by a space.
1.92 markus 1303: On some machines this file may need to be
1.1 deraadt 1304: world-readable if the user's home directory is on a NFS partition,
1305: because
1.2 deraadt 1306: .Xr sshd 8
1.40 aaron 1307: reads it as root.
1308: Additionally, this file must be owned by the user,
1309: and must not have write permissions for anyone else.
1310: The recommended
1.1 deraadt 1311: permission for most machines is read/write for the user, and not
1312: accessible by others.
1.2 deraadt 1313: .Pp
1.1 deraadt 1314: Note that by default
1.2 deraadt 1315: .Xr sshd 8
1.1 deraadt 1316: will be installed so that it requires successful RSA host
1.40 aaron 1317: authentication before permitting \s+2.\s0rhosts authentication.
1318: If your server machine does not have the client's host key in
1.2 deraadt 1319: .Pa /etc/ssh_known_hosts ,
1320: you can store it in
1321: .Pa $HOME/.ssh/known_hosts .
1322: The easiest way to do this is to
1.1 deraadt 1323: connect back to the client from the server machine using ssh; this
1.48 markus 1324: will automatically add the host key to
1.2 deraadt 1325: .Pa $HOME/.ssh/known_hosts .
1326: .It Pa $HOME/.shosts
1327: This file is used exactly the same way as
1328: .Pa \&.rhosts .
1329: The purpose for
1.1 deraadt 1330: having this file is to be able to use rhosts authentication with
1.2 deraadt 1331: .Nm
1332: without permitting login with
1333: .Xr rlogin 1
1334: or
1335: .Xr rsh 1 .
1336: .It Pa /etc/hosts.equiv
1337: This file is used during
1.40 aaron 1338: .Pa \&.rhosts authentication.
1339: It contains
1.1 deraadt 1340: canonical hosts names, one per line (the full format is described on
1341: the
1.2 deraadt 1342: .Xr sshd 8
1.40 aaron 1343: manual page).
1344: If the client host is found in this file, login is
1.1 deraadt 1345: automatically permitted provided client and server user names are the
1.40 aaron 1346: same.
1347: Additionally, successful RSA host authentication is normally
1348: required.
1349: This file should only be writable by root.
1.2 deraadt 1350: .It Pa /etc/shosts.equiv
1.44 aaron 1351: This file is processed exactly as
1.2 deraadt 1352: .Pa /etc/hosts.equiv .
1.1 deraadt 1353: This file may be useful to permit logins using
1.2 deraadt 1354: .Nm
1.1 deraadt 1355: but not using rsh/rlogin.
1.2 deraadt 1356: .It Pa /etc/sshrc
1.1 deraadt 1357: Commands in this file are executed by
1.2 deraadt 1358: .Nm
1.1 deraadt 1359: when the user logs in just before the user's shell (or command) is started.
1360: See the
1.2 deraadt 1361: .Xr sshd 8
1.1 deraadt 1362: manual page for more information.
1.2 deraadt 1363: .It Pa $HOME/.ssh/rc
1.1 deraadt 1364: Commands in this file are executed by
1.2 deraadt 1365: .Nm
1.1 deraadt 1366: when the user logs in just before the user's shell (or command) is
1367: started.
1.44 aaron 1368: See the
1.2 deraadt 1369: .Xr sshd 8
1.1 deraadt 1370: manual page for more information.
1.31 markus 1371: .It Pa $HOME/.ssh/environment
1372: Contains additional definitions for environment variables, see section
1373: .Sx ENVIRONMENT
1374: above.
1.58 itojun 1375: .El
1.67 aaron 1376: .Sh AUTHORS
1.78 markus 1377: OpenSSH is a derivative of the original and free
1378: ssh 1.2.12 release by Tatu Ylonen.
1379: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1380: Theo de Raadt and Dug Song
1381: removed many bugs, re-added newer features and
1382: created OpenSSH.
1383: Markus Friedl contributed the support for SSH
1384: protocol versions 1.5 and 2.0.
1.2 deraadt 1385: .Sh SEE ALSO
1386: .Xr rlogin 1 ,
1387: .Xr rsh 1 ,
1388: .Xr scp 1 ,
1.83 djm 1389: .Xr sftp 1 ,
1.2 deraadt 1390: .Xr ssh-add 1 ,
1391: .Xr ssh-agent 1 ,
1392: .Xr ssh-keygen 1 ,
1393: .Xr telnet 1 ,
1.87 itojun 1394: .Xr sshd 8
1.106 markus 1395: .Rs
1396: .%A T. Ylonen
1397: .%A T. Kivinen
1398: .%A M. Saarinen
1399: .%A T. Rinne
1400: .%A S. Lehtinen
1401: .%T "SSH Protocol Architecture"
1.126 ! markus 1402: .%N draft-ietf-secsh-architecture-09.txt
! 1403: .%D July 2001
1.106 markus 1404: .%O work in progress material
1405: .Re
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