Annotation of src/usr.bin/ssh/ssh.1, Revision 1.98
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: .\"
1.98 ! markus 37: .\" $OpenBSD: ssh.1,v 1.97 2001/03/07 04:05:58 deraadt 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
1.51 markus 52: .Op Fl c Ar cipher_spec
1.2 deraadt 53: .Op Fl e Ar escape_char
54: .Op Fl i Ar identity_file
55: .Op Fl l Ar login_name
1.91 markus 56: .Op Fl m Ar mac_spec
1.2 deraadt 57: .Op Fl o Ar option
58: .Op Fl p Ar port
1.12 aaron 59: .Oo Fl L Xo
60: .Sm off
1.33 markus 61: .Ar port :
1.12 aaron 62: .Ar host :
63: .Ar hostport
64: .Sm on
65: .Xc
66: .Oc
67: .Oo Fl R Xo
68: .Sm off
1.33 markus 69: .Ar port :
1.12 aaron 70: .Ar host :
71: .Ar hostport
72: .Sm on
73: .Xc
74: .Oc
1.5 deraadt 75: .Op Ar hostname | user@hostname
1.2 deraadt 76: .Op Ar command
1.44 aaron 77: .Sh DESCRIPTION
1.2 deraadt 78: .Nm
1.96 deraadt 79: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 80: executing commands on a remote machine.
81: It is intended to replace
1.1 deraadt 82: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 83: two untrusted hosts over an insecure network.
84: X11 connections and
1.1 deraadt 85: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 86: .Pp
87: .Nm
1.44 aaron 88: connects and logs into the specified
1.2 deraadt 89: .Ar hostname .
1.1 deraadt 90: The user must prove
1.49 markus 91: his/her identity to the remote machine using one of several methods
92: depending on the protocol version used:
93: .Pp
94: .Ss SSH protocol version 1
1.2 deraadt 95: .Pp
1.1 deraadt 96: First, if the machine the user logs in from is listed in
1.2 deraadt 97: .Pa /etc/hosts.equiv
1.1 deraadt 98: or
1.2 deraadt 99: .Pa /etc/shosts.equiv
1.1 deraadt 100: on the remote machine, and the user names are
101: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 102: Second, if
1.2 deraadt 103: .Pa \&.rhosts
1.1 deraadt 104: or
1.2 deraadt 105: .Pa \&.shosts
1.1 deraadt 106: exists in the user's home directory on the
107: remote machine and contains a line containing the name of the client
108: machine and the name of the user on that machine, the user is
1.40 aaron 109: permitted to log in.
110: This form of authentication alone is normally not
1.1 deraadt 111: allowed by the server because it is not secure.
1.2 deraadt 112: .Pp
1.1 deraadt 113: The second (and primary) authentication method is the
1.2 deraadt 114: .Pa rhosts
1.1 deraadt 115: or
1.2 deraadt 116: .Pa hosts.equiv
1.40 aaron 117: method combined with RSA-based host authentication.
118: It means that if the login would be permitted by
1.49 markus 119: .Pa $HOME/.rhosts ,
120: .Pa $HOME/.shosts ,
1.2 deraadt 121: .Pa /etc/hosts.equiv ,
1.1 deraadt 122: or
1.2 deraadt 123: .Pa /etc/shosts.equiv ,
1.11 deraadt 124: and if additionally the server can verify the client's
1.44 aaron 125: host key (see
1.2 deraadt 126: .Pa /etc/ssh_known_hosts
1.23 markus 127: and
128: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 129: in the
1.2 deraadt 130: .Sx FILES
1.40 aaron 131: section), only then login is permitted.
132: This authentication method closes security holes due to IP
133: spoofing, DNS spoofing and routing spoofing.
134: [Note to the administrator:
1.2 deraadt 135: .Pa /etc/hosts.equiv ,
1.49 markus 136: .Pa $HOME/.rhosts ,
1.1 deraadt 137: and the rlogin/rsh protocol in general, are inherently insecure and should be
138: disabled if security is desired.]
1.2 deraadt 139: .Pp
1.44 aaron 140: As a third authentication method,
1.2 deraadt 141: .Nm
1.1 deraadt 142: supports RSA based authentication.
143: The scheme is based on public-key cryptography: there are cryptosystems
144: where encryption and decryption are done using separate keys, and it
145: is not possible to derive the decryption key from the encryption key.
1.40 aaron 146: RSA is one such system.
1.44 aaron 147: The idea is that each user creates a public/private
1.40 aaron 148: key pair for authentication purposes.
149: The server knows the public key, and only the user knows the private key.
1.44 aaron 150: The file
1.2 deraadt 151: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 152: lists the public keys that are permitted for logging
1.40 aaron 153: in.
154: When the user logs in, the
1.2 deraadt 155: .Nm
1.1 deraadt 156: program tells the server which key pair it would like to use for
1.40 aaron 157: authentication.
158: The server checks if this key is permitted, and if
1.1 deraadt 159: so, sends the user (actually the
1.2 deraadt 160: .Nm
1.1 deraadt 161: program running on behalf of the user) a challenge, a random number,
1.40 aaron 162: encrypted by the user's public key.
163: The challenge can only be
164: decrypted using the proper private key.
165: The user's client then decrypts the
1.1 deraadt 166: challenge using the private key, proving that he/she knows the private
167: key but without disclosing it to the server.
1.2 deraadt 168: .Pp
169: .Nm
1.40 aaron 170: implements the RSA authentication protocol automatically.
171: The user creates his/her RSA key pair by running
1.2 deraadt 172: .Xr ssh-keygen 1 .
1.44 aaron 173: This stores the private key in
1.49 markus 174: .Pa $HOME/.ssh/identity
1.1 deraadt 175: and the public key in
1.49 markus 176: .Pa $HOME/.ssh/identity.pub
1.40 aaron 177: in the user's home directory.
178: The user should then copy the
1.2 deraadt 179: .Pa identity.pub
1.44 aaron 180: to
1.49 markus 181: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 182: in his/her home directory on the remote machine (the
1.2 deraadt 183: .Pa authorized_keys
1.44 aaron 184: file corresponds to the conventional
1.49 markus 185: .Pa $HOME/.rhosts
1.1 deraadt 186: file, and has one key
1.40 aaron 187: per line, though the lines can be very long).
188: After this, the user can log in without giving the password.
189: RSA authentication is much
1.1 deraadt 190: more secure than rhosts authentication.
1.2 deraadt 191: .Pp
1.1 deraadt 192: The most convenient way to use RSA authentication may be with an
1.40 aaron 193: authentication agent.
194: See
1.2 deraadt 195: .Xr ssh-agent 1
1.1 deraadt 196: for more information.
1.2 deraadt 197: .Pp
1.44 aaron 198: If other authentication methods fail,
1.2 deraadt 199: .Nm
1.40 aaron 200: prompts the user for a password.
201: The password is sent to the remote
1.1 deraadt 202: host for checking; however, since all communications are encrypted,
203: the password cannot be seen by someone listening on the network.
1.2 deraadt 204: .Pp
1.49 markus 205: .Ss SSH protocol version 2
206: .Pp
207: When a user connects using the protocol version 2
208: different authentication methods are available:
209: At first, the client attempts to authenticate using the public key method.
210: If this method fails password authentication is tried.
211: .Pp
212: The public key method is similar to RSA authentication described
1.68 markus 213: in the previous section except that the DSA or RSA algorithm is used
214: instead.
215: The client uses his private key
1.49 markus 216: .Pa $HOME/.ssh/id_dsa
217: to sign the session identifier and sends the result to the server.
218: The server checks whether the matching public key is listed in
219: .Pa $HOME/.ssh/authorized_keys2
220: and grants access if both the key is found and the signature is correct.
221: The session identifier is derived from a shared Diffie-Hellman value
222: and is only known to the client and the server.
223: .Pp
224: If public key authentication fails or is not available a password
225: can be sent encrypted to the remote host for proving the user's identity.
226: This protocol 2 implementation does not yet support Kerberos or
227: S/Key authentication.
228: .Pp
229: Protocol 2 provides additional mechanisms for confidentiality
1.51 markus 230: (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
1.94 deraadt 231: and integrity (hmac-md5, hmac-sha1).
1.49 markus 232: Note that protocol 1 lacks a strong mechanism for ensuring the
233: integrity of the connection.
234: .Pp
235: .Ss Login session and remote execution
236: .Pp
1.1 deraadt 237: When the user's identity has been accepted by the server, the server
238: either executes the given command, or logs into the machine and gives
1.40 aaron 239: the user a normal shell on the remote machine.
240: All communication with
1.1 deraadt 241: the remote command or shell will be automatically encrypted.
1.2 deraadt 242: .Pp
1.1 deraadt 243: If a pseudo-terminal has been allocated (normal login session), the
1.2 deraadt 244: user can disconnect with
245: .Ic ~. ,
246: and suspend
247: .Nm
248: with
249: .Ic ~^Z .
250: All forwarded connections can be listed with
1.44 aaron 251: .Ic ~#
1.2 deraadt 252: and if
1.1 deraadt 253: the session blocks waiting for forwarded X11 or TCP/IP
1.2 deraadt 254: connections to terminate, it can be backgrounded with
255: .Ic ~&
256: (this should not be used while the user shell is active, as it can cause the
1.40 aaron 257: shell to hang).
258: All available escapes can be listed with
1.2 deraadt 259: .Ic ~? .
260: .Pp
261: A single tilde character can be sent as
262: .Ic ~~
263: (or by following the tilde by a character other than those described above).
1.1 deraadt 264: The escape character must always follow a newline to be interpreted as
1.40 aaron 265: special.
266: The escape character can be changed in configuration files
267: or on the command line.
1.2 deraadt 268: .Pp
1.1 deraadt 269: If no pseudo tty has been allocated, the
270: session is transparent and can be used to reliably transfer binary
1.40 aaron 271: data.
272: On most systems, setting the escape character to
1.2 deraadt 273: .Dq none
274: will also make the session transparent even if a tty is used.
275: .Pp
1.71 djm 276: The session terminates when the command or shell on the remote
1.92 markus 277: machine exits and all X11 and TCP/IP connections have been closed.
1.1 deraadt 278: The exit status of the remote program is returned as the exit status
279: of
1.2 deraadt 280: .Nm ssh .
281: .Pp
1.49 markus 282: .Ss X11 and TCP forwarding
283: .Pp
1.1 deraadt 284: If the user is using X11 (the
1.2 deraadt 285: .Ev DISPLAY
1.1 deraadt 286: environment variable is set), the connection to the X11 display is
287: automatically forwarded to the remote side in such a way that any X11
288: programs started from the shell (or command) will go through the
289: encrypted channel, and the connection to the real X server will be made
1.40 aaron 290: from the local machine.
291: The user should not manually set
1.2 deraadt 292: .Ev DISPLAY .
1.1 deraadt 293: Forwarding of X11 connections can be
294: configured on the command line or in configuration files.
1.2 deraadt 295: .Pp
296: The
1.44 aaron 297: .Ev DISPLAY
1.2 deraadt 298: value set by
299: .Nm
1.1 deraadt 300: will point to the server machine, but with a display number greater
1.40 aaron 301: than zero.
302: This is normal, and happens because
1.2 deraadt 303: .Nm
304: creates a
305: .Dq proxy
306: X server on the server machine for forwarding the
1.1 deraadt 307: connections over the encrypted channel.
1.2 deraadt 308: .Pp
309: .Nm
1.1 deraadt 310: will also automatically set up Xauthority data on the server machine.
311: For this purpose, it will generate a random authorization cookie,
312: store it in Xauthority on the server, and verify that any forwarded
313: connections carry this cookie and replace it by the real cookie when
1.40 aaron 314: the connection is opened.
315: The real authentication cookie is never
1.1 deraadt 316: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 317: .Pp
1.1 deraadt 318: If the user is using an authentication agent, the connection to the agent
319: is automatically forwarded to the remote side unless disabled on
320: command line or in a configuration file.
1.2 deraadt 321: .Pp
1.1 deraadt 322: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.40 aaron 323: be specified either on command line or in a configuration file.
324: One possible application of TCP/IP forwarding is a secure connection to an
1.92 markus 325: electronic purse; another is going through firewalls.
1.2 deraadt 326: .Pp
1.49 markus 327: .Ss Server authentication
328: .Pp
1.2 deraadt 329: .Nm
1.49 markus 330: automatically maintains and checks a database containing
1.40 aaron 331: identifications for all hosts it has ever been used with.
1.49 markus 332: RSA host keys are stored in
333: .Pa $HOME/.ssh/known_hosts
334: and
1.68 markus 335: host keys used in the protocol version 2 are stored in
1.49 markus 336: .Pa $HOME/.ssh/known_hosts2
1.40 aaron 337: in the user's home directory.
1.49 markus 338: Additionally, the files
1.2 deraadt 339: .Pa /etc/ssh_known_hosts
1.49 markus 340: and
341: .Pa /etc/ssh_known_hosts2
342: are automatically checked for known hosts.
1.40 aaron 343: Any new hosts are automatically added to the user's file.
344: If a host's identification
1.1 deraadt 345: ever changes,
1.2 deraadt 346: .Nm
1.1 deraadt 347: warns about this and disables password authentication to prevent a
1.40 aaron 348: trojan horse from getting the user's password.
349: Another purpose of
1.1 deraadt 350: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 351: otherwise be used to circumvent the encryption.
352: The
1.2 deraadt 353: .Cm StrictHostKeyChecking
1.1 deraadt 354: option (see below) can be used to prevent logins to machines whose
355: host key is not known or has changed.
1.65 aaron 356: .Pp
357: The options are as follows:
1.2 deraadt 358: .Bl -tag -width Ds
1.4 dugsong 359: .It Fl a
1.42 aaron 360: Disables forwarding of the authentication agent connection.
1.54 markus 361: .It Fl A
362: Enables forwarding of the authentication agent connection.
363: This can also be specified on a per-host basis in a configuration file.
1.6 deraadt 364: .It Fl c Ar blowfish|3des
1.44 aaron 365: Selects the cipher to use for encrypting the session.
1.2 deraadt 366: .Ar 3des
1.40 aaron 367: is used by default.
1.44 aaron 368: It is believed to be secure.
1.5 deraadt 369: .Ar 3des
370: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
371: It is presumably more secure than the
1.2 deraadt 372: .Ar des
1.64 markus 373: cipher which is no longer fully supported in
1.51 markus 374: .Nm ssh .
1.5 deraadt 375: .Ar blowfish
376: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 377: .Ar 3des .
1.90 markus 378: .It Fl c Ar cipher_spec
1.51 markus 379: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 380: be specified in order of preference.
1.90 markus 381: See
382: .Cm Ciphers
383: for more information.
1.2 deraadt 384: .It Fl e Ar ch|^ch|none
385: Sets the escape character for sessions with a pty (default:
386: .Ql ~ ) .
1.40 aaron 387: The escape character is only recognized at the beginning of a line.
388: The escape character followed by a dot
1.2 deraadt 389: .Pq Ql \&.
390: closes the connection, followed
1.1 deraadt 391: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 392: escape character once.
393: Setting the character to
1.2 deraadt 394: .Dq none
395: disables any escapes and makes the session fully transparent.
396: .It Fl f
397: Requests
398: .Nm
1.40 aaron 399: to go to background just before command execution.
400: This is useful if
1.2 deraadt 401: .Nm
402: is going to ask for passwords or passphrases, but the user
1.40 aaron 403: wants it in the background.
1.44 aaron 404: This implies
1.2 deraadt 405: .Fl n .
1.1 deraadt 406: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 407: something like
408: .Ic ssh -f host xterm .
1.34 markus 409: .It Fl g
410: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 411: .It Fl i Ar identity_file
1.44 aaron 412: Selects the file from which the identity (private key) for
1.68 markus 413: RSA or DSA authentication is read.
1.44 aaron 414: Default is
1.49 markus 415: .Pa $HOME/.ssh/identity
1.40 aaron 416: in the user's home directory.
417: Identity files may also be specified on
418: a per-host basis in the configuration file.
419: It is possible to have multiple
1.2 deraadt 420: .Fl i
421: options (and multiple identities specified in
1.1 deraadt 422: configuration files).
1.2 deraadt 423: .It Fl k
1.42 aaron 424: Disables forwarding of Kerberos tickets and AFS tokens.
425: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 426: .It Fl l Ar login_name
1.40 aaron 427: Specifies the user to log in as on the remote machine.
428: This also may be specified on a per-host basis in the configuration file.
1.91 markus 429: .It Fl m Ar mac_spec
430: Additionally, for protocol version 2 a comma-separated list of MAC
431: (message authentication code) algorithms can
432: be specified in order of preference.
433: See the
434: .Cm MACs
435: keyword for more information.
1.2 deraadt 436: .It Fl n
437: Redirects stdin from
438: .Pa /dev/null
439: (actually, prevents reading from stdin).
1.1 deraadt 440: This must be used when
1.2 deraadt 441: .Nm
1.40 aaron 442: is run in the background.
443: A common trick is to use this to run X11 programs on a remote machine.
444: For example,
1.2 deraadt 445: .Ic ssh -n shadows.cs.hut.fi emacs &
446: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 447: connection will be automatically forwarded over an encrypted channel.
448: The
1.2 deraadt 449: .Nm
1.1 deraadt 450: program will be put in the background.
451: (This does not work if
1.2 deraadt 452: .Nm
453: needs to ask for a password or passphrase; see also the
454: .Fl f
455: option.)
1.53 markus 456: .It Fl N
457: Do not execute a remote command.
1.70 markus 458: This is useful if you just want to forward ports
1.53 markus 459: (protocol version 2 only).
1.2 deraadt 460: .It Fl o Ar option
1.1 deraadt 461: Can be used to give options in the format used in the config file.
462: This is useful for specifying options for which there is no separate
1.40 aaron 463: command-line flag.
464: The option has the same format as a line in the configuration file.
1.2 deraadt 465: .It Fl p Ar port
1.40 aaron 466: Port to connect to on the remote host.
467: This can be specified on a
1.1 deraadt 468: per-host basis in the configuration file.
1.16 markus 469: .It Fl P
470: Use a non-privileged port for outgoing connections.
471: This can be used if your firewall does
472: not permit connections from privileged ports.
1.30 provos 473: Note that this option turns off
1.16 markus 474: .Cm RhostsAuthentication
475: and
1.72 markus 476: .Cm RhostsRSAAuthentication
477: for older servers.
1.2 deraadt 478: .It Fl q
1.40 aaron 479: Quiet mode.
480: Causes all warning and diagnostic messages to be suppressed.
481: Only fatal errors are displayed.
1.80 djm 482: .It Fl s
483: 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
484: of SSH as a secure transport for other application (eg. sftp). The
485: subsystem is specified as the remote command.
1.2 deraadt 486: .It Fl t
1.40 aaron 487: Force pseudo-tty allocation.
1.43 brad 488: This can be used to execute arbitrary
1.40 aaron 489: screen-based programs on a remote machine, which can be very useful,
490: e.g., when implementing menu services.
1.73 markus 491: Multiple
492: .Fl t
493: options force tty allocation, even if
494: .Nm
495: has no local tty.
1.53 markus 496: .It Fl T
1.69 markus 497: Disable pseudo-tty allocation.
1.2 deraadt 498: .It Fl v
1.40 aaron 499: Verbose mode.
500: Causes
1.2 deraadt 501: .Nm
1.40 aaron 502: to print debugging messages about its progress.
503: This is helpful in
1.1 deraadt 504: debugging connection, authentication, and configuration problems.
1.73 markus 505: Multiple
506: .Fl v
507: options increases the verbosity.
1.61 aaron 508: Maximum is 3.
1.2 deraadt 509: .It Fl x
1.40 aaron 510: Disables X11 forwarding.
1.2 deraadt 511: .It Fl X
1.1 deraadt 512: Enables X11 forwarding.
1.54 markus 513: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 514: .It Fl C
1.1 deraadt 515: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 516: data for forwarded X11 and TCP/IP connections).
517: The compression algorithm is the same used by
1.34 markus 518: .Xr gzip 1 ,
519: and the
1.2 deraadt 520: .Dq level
521: can be controlled by the
522: .Cm CompressionLevel
1.40 aaron 523: option (see below).
524: Compression is desirable on modem lines and other
1.1 deraadt 525: slow connections, but will only slow down things on fast networks.
526: The default value can be set on a host-by-host basis in the
527: configuration files; see the
1.2 deraadt 528: .Cm Compress
1.1 deraadt 529: option below.
1.2 deraadt 530: .It Fl L Ar port:host:hostport
1.1 deraadt 531: Specifies that the given port on the local (client) host is to be
1.40 aaron 532: forwarded to the given host and port on the remote side.
533: This works by allocating a socket to listen to
1.2 deraadt 534: .Ar port
1.1 deraadt 535: on the local side, and whenever a connection is made to this port, the
536: connection is forwarded over the secure channel, and a connection is
537: made to
1.32 markus 538: .Ar host
539: port
540: .Ar hostport
1.40 aaron 541: from the remote machine.
542: Port forwardings can also be specified in the configuration file.
543: Only root can forward privileged ports.
1.32 markus 544: IPv6 addresses can be specified with an alternative syntax:
545: .Ar port/host/hostport
1.2 deraadt 546: .It Fl R Ar port:host:hostport
1.1 deraadt 547: Specifies that the given port on the remote (server) host is to be
1.40 aaron 548: forwarded to the given host and port on the local side.
549: This works by allocating a socket to listen to
1.2 deraadt 550: .Ar port
1.1 deraadt 551: on the remote side, and whenever a connection is made to this port, the
552: connection is forwarded over the secure channel, and a connection is
553: made to
1.32 markus 554: .Ar host
555: port
556: .Ar hostport
1.40 aaron 557: from the local machine.
558: Port forwardings can also be specified in the configuration file.
559: Privileged ports can be forwarded only when
1.1 deraadt 560: logging in as root on the remote machine.
1.85 jakob 561: .It Fl 1
562: Forces
563: .Nm
564: to try protocol version 1 only.
1.46 markus 565: .It Fl 2
566: Forces
567: .Nm
1.50 markus 568: to try protocol version 2 only.
1.32 markus 569: .It Fl 4
570: Forces
571: .Nm
572: to use IPv4 addresses only.
573: .It Fl 6
574: Forces
575: .Nm
576: to use IPv6 addresses only.
1.2 deraadt 577: .El
578: .Sh CONFIGURATION FILES
579: .Nm
1.1 deraadt 580: obtains configuration data from the following sources (in this order):
581: command line options, user's configuration file
1.2 deraadt 582: .Pq Pa $HOME/.ssh/config ,
583: and system-wide configuration file
584: .Pq Pa /etc/ssh_config .
585: For each parameter, the first obtained value
1.40 aaron 586: will be used.
587: The configuration files contain sections bracketed by
588: .Dq Host
589: specifications, and that section is only applied for hosts that
590: match one of the patterns given in the specification.
591: The matched host name is the one given on the command line.
1.2 deraadt 592: .Pp
1.1 deraadt 593: Since the first obtained value for each parameter is used, more
594: host-specific declarations should be given near the beginning of the
595: file, and general defaults at the end.
1.2 deraadt 596: .Pp
1.1 deraadt 597: The configuration file has the following format:
1.2 deraadt 598: .Pp
599: Empty lines and lines starting with
600: .Ql #
601: are comments.
602: .Pp
603: Otherwise a line is of the format
604: .Dq keyword arguments .
605: The possible
1.1 deraadt 606: keywords and their meanings are as follows (note that the
607: configuration files are case-sensitive):
1.2 deraadt 608: .Bl -tag -width Ds
609: .It Cm Host
1.1 deraadt 610: Restricts the following declarations (up to the next
1.2 deraadt 611: .Cm Host
1.1 deraadt 612: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 613: given after the keyword.
614: .Ql \&*
615: and
616: .Ql ?
617: can be used as wildcards in the
1.40 aaron 618: patterns.
619: A single
1.2 deraadt 620: .Ql \&*
621: as a pattern can be used to provide global
1.40 aaron 622: defaults for all hosts.
623: The host is the
1.2 deraadt 624: .Ar hostname
1.1 deraadt 625: argument given on the command line (i.e., the name is not converted to
626: a canonicalized host name before matching).
1.2 deraadt 627: .It Cm AFSTokenPassing
1.42 aaron 628: Specifies whether to pass AFS tokens to remote host.
629: The argument to this keyword must be
1.2 deraadt 630: .Dq yes
631: or
632: .Dq no .
633: .It Cm BatchMode
634: If set to
635: .Dq yes ,
1.40 aaron 636: passphrase/password querying will be disabled.
637: This option is useful in scripts and other batch jobs where you have no
638: user to supply the password.
639: The argument must be
1.2 deraadt 640: .Dq yes
641: or
642: .Dq no .
1.34 markus 643: .It Cm CheckHostIP
644: If this flag is set to
645: .Dq yes ,
646: ssh will additionally check the host ip address in the
647: .Pa known_hosts
1.42 aaron 648: file.
649: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 650: If the option is set to
651: .Dq no ,
652: the check will not be executed.
1.2 deraadt 653: .It Cm Cipher
1.62 markus 654: Specifies the cipher to use for encrypting the session
1.64 markus 655: in protocol version 1.
1.40 aaron 656: Currently,
1.64 markus 657: .Dq blowfish
1.1 deraadt 658: and
1.10 provos 659: .Dq 3des
1.40 aaron 660: are supported.
661: The default is
1.2 deraadt 662: .Dq 3des .
1.45 markus 663: .It Cm Ciphers
664: Specifies the ciphers allowed for protocol version 2
665: in order of preference.
666: Multiple ciphers must be comma-separated.
667: The default is
1.88 provos 668: .Pp
669: .Bd -literal
1.94 deraadt 670: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.88 provos 671: aes192-cbc,aes256-cbc,rijndael128-cbc,rijndael192-cbc,
672: rijndael256-cbc,rijndael-cbc@lysator.liu.se''
673: .Ed
1.2 deraadt 674: .It Cm Compression
1.40 aaron 675: Specifies whether to use compression.
676: The argument must be
1.2 deraadt 677: .Dq yes
678: or
679: .Dq no .
680: .It Cm CompressionLevel
1.40 aaron 681: Specifies the compression level to use if compression is enable.
682: The argument must be an integer from 1 (fast) to 9 (slow, best).
683: The default level is 6, which is good for most applications.
684: The meaning of the values is the same as in
1.34 markus 685: .Xr gzip 1 .
1.2 deraadt 686: .It Cm ConnectionAttempts
1.1 deraadt 687: Specifies the number of tries (one per second) to make before falling
1.40 aaron 688: back to rsh or exiting.
689: The argument must be an integer.
690: This may be useful in scripts if the connection sometimes fails.
1.68 markus 691: .It Cm PubkeyAuthentication
692: Specifies whether to try public key authentication.
1.50 markus 693: The argument to this keyword must be
694: .Dq yes
695: or
696: .Dq no .
697: Note that this option applies to protocol version 2 only.
1.2 deraadt 698: .It Cm EscapeChar
699: Sets the escape character (default:
700: .Ql ~ ) .
701: The escape character can also
1.40 aaron 702: be set on the command line.
703: The argument should be a single character,
1.2 deraadt 704: .Ql ^
705: followed by a letter, or
706: .Dq none
707: to disable the escape
1.1 deraadt 708: character entirely (making the connection transparent for binary
709: data).
1.44 aaron 710: .It Cm FallBackToRsh
1.1 deraadt 711: Specifies that if connecting via
1.2 deraadt 712: .Nm
1.1 deraadt 713: fails due to a connection refused error (there is no
1.2 deraadt 714: .Xr sshd 8
1.44 aaron 715: listening on the remote host),
1.2 deraadt 716: .Xr rsh 1
1.1 deraadt 717: should automatically be used instead (after a suitable warning about
1.40 aaron 718: the session being unencrypted).
719: The argument must be
1.2 deraadt 720: .Dq yes
721: or
722: .Dq no .
723: .It Cm ForwardAgent
1.1 deraadt 724: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 725: will be forwarded to the remote machine.
726: The argument must be
1.2 deraadt 727: .Dq yes
728: or
1.54 markus 729: .Dq no .
730: The default is
1.2 deraadt 731: .Dq no .
732: .It Cm ForwardX11
1.1 deraadt 733: Specifies whether X11 connections will be automatically redirected
1.44 aaron 734: over the secure channel and
1.2 deraadt 735: .Ev DISPLAY
1.40 aaron 736: set.
1.44 aaron 737: The argument must be
1.2 deraadt 738: .Dq yes
739: or
1.38 markus 740: .Dq no .
741: The default is
1.3 deraadt 742: .Dq no .
743: .It Cm GatewayPorts
744: Specifies whether remote hosts are allowed to connect to local
745: forwarded ports.
746: The argument must be
747: .Dq yes
748: or
749: .Dq no .
750: The default is
1.2 deraadt 751: .Dq no .
752: .It Cm GlobalKnownHostsFile
1.95 stevesk 753: Specifies a file to use for the protocol version 1 global
754: host key database instead of
1.2 deraadt 755: .Pa /etc/ssh_known_hosts .
1.95 stevesk 756: .It Cm GlobalKnownHostsFile2
757: Specifies a file to use for the protocol version 2 global
758: host key database instead of
759: .Pa /etc/ssh_known_hosts2 .
1.74 markus 760: .It Cm HostKeyAlias
761: Specifies an alias that should be used instead of the
762: real host name when looking up or saving the host key
1.82 stevesk 763: in the known_hosts files.
764: This option is useful for tunneling ssh connections
1.74 markus 765: or if you have multiple servers running on a single host.
1.2 deraadt 766: .It Cm HostName
1.40 aaron 767: Specifies the real host name to log into.
768: This can be used to specify nicknames or abbreviations for hosts.
769: Default is the name given on the command line.
770: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 771: .Cm HostName
1.1 deraadt 772: specifications).
1.2 deraadt 773: .It Cm IdentityFile
1.1 deraadt 774: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 775: is read (default
1.49 markus 776: .Pa $HOME/.ssh/identity
1.2 deraadt 777: in the user's home directory).
1.1 deraadt 778: Additionally, any identities represented by the authentication agent
1.40 aaron 779: will be used for authentication.
780: The file name may use the tilde
781: syntax to refer to a user's home directory.
782: It is possible to have
1.1 deraadt 783: multiple identity files specified in configuration files; all these
784: identities will be tried in sequence.
1.2 deraadt 785: .It Cm KeepAlive
1.1 deraadt 786: Specifies whether the system should send keepalive messages to the
1.40 aaron 787: other side.
788: If they are sent, death of the connection or crash of one
789: of the machines will be properly noticed.
790: However, this means that
1.1 deraadt 791: connections will die if the route is down temporarily, and some people
1.41 aaron 792: find it annoying.
1.2 deraadt 793: .Pp
794: The default is
795: .Dq yes
796: (to send keepalives), and the client will notice
1.40 aaron 797: if the network goes down or the remote host dies.
798: This is important in scripts, and many users want it too.
1.2 deraadt 799: .Pp
800: To disable keepalives, the value should be set to
801: .Dq no
802: in both the server and the client configuration files.
803: .It Cm KerberosAuthentication
1.42 aaron 804: Specifies whether Kerberos authentication will be used.
805: The argument to this keyword must be
1.4 dugsong 806: .Dq yes
807: or
808: .Dq no .
1.2 deraadt 809: .It Cm KerberosTgtPassing
1.42 aaron 810: Specifies whether a Kerberos TGT will be forwarded to the server.
811: This will only work if the Kerberos server is actually an AFS kaserver.
812: The argument to this keyword must be
1.4 dugsong 813: .Dq yes
814: or
815: .Dq no .
1.2 deraadt 816: .It Cm LocalForward
1.1 deraadt 817: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 818: the secure channel to given host:port from the remote machine.
819: The first argument must be a port number, and the second must be
820: host:port.
821: Multiple forwardings may be specified, and additional
822: forwardings can be given on the command line.
823: Only the superuser can forward privileged ports.
1.24 markus 824: .It Cm LogLevel
825: Gives the verbosity level that is used when logging messages from
826: .Nm ssh .
827: The possible values are:
1.77 markus 828: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
829: The default is INFO.
1.91 markus 830: .It Cm MACs
831: Specifies the MAC (message authentication code) algorithms
832: in order of preference.
833: The MAC algorithm is used in protocol version 2
834: for data integrity protection.
835: Multiple algorithms must be comma-separated.
836: The default is
837: .Pp
838: .Bd -literal
1.94 deraadt 839: ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
1.91 markus 840: hmac-sha1-96,hmac-md5-96''
841: .Ed
1.14 dugsong 842: .It Cm NumberOfPasswordPrompts
1.42 aaron 843: Specifies the number of password prompts before giving up.
844: The argument to this keyword must be an integer.
845: Default is 3.
1.34 markus 846: .It Cm PasswordAuthentication
1.40 aaron 847: Specifies whether to use password authentication.
848: The argument to this keyword must be
1.34 markus 849: .Dq yes
850: or
851: .Dq no .
1.50 markus 852: Note that this option applies to both protocol version 1 and 2.
1.2 deraadt 853: .It Cm Port
1.40 aaron 854: Specifies the port number to connect on the remote host.
855: Default is 22.
1.45 markus 856: .It Cm Protocol
857: Specifies the protocol versions
858: .Nm
859: should support in order of preference.
860: The possible values are
861: .Dq 1
862: and
863: .Dq 2 .
864: Multiple versions must be comma-separated.
865: The default is
1.49 markus 866: .Dq 1,2 .
867: This means that
868: .Nm
869: tries version 1 and falls back to version 2
1.52 hugh 870: if version 1 is not available.
1.2 deraadt 871: .It Cm ProxyCommand
1.40 aaron 872: Specifies the command to use to connect to the server.
873: The command
874: string extends to the end of the line, and is executed with
875: .Pa /bin/sh .
876: In the command string,
877: .Ql %h
878: will be substituted by the host name to
879: connect and
880: .Ql %p
881: by the port.
882: The command can be basically anything,
883: and should read from its standard input and write to its standard output.
884: It should eventually connect an
1.2 deraadt 885: .Xr sshd 8
1.1 deraadt 886: server running on some machine, or execute
1.2 deraadt 887: .Ic sshd -i
1.40 aaron 888: somewhere.
889: Host key management will be done using the
1.1 deraadt 890: HostName of the host being connected (defaulting to the name typed by
891: the user).
1.29 markus 892: Note that
893: .Cm CheckHostIP
894: is not available for connects with a proxy command.
1.2 deraadt 895: .Pp
896: .It Cm RemoteForward
1.1 deraadt 897: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 898: the secure channel to given host:port from the local machine.
899: The first argument must be a port number, and the second must be
900: host:port.
901: Multiple forwardings may be specified, and additional
902: forwardings can be given on the command line.
903: Only the superuser can forward privileged ports.
1.2 deraadt 904: .It Cm RhostsAuthentication
1.40 aaron 905: Specifies whether to try rhosts based authentication.
906: Note that this
1.1 deraadt 907: declaration only affects the client side and has no effect whatsoever
1.40 aaron 908: on security.
909: Disabling rhosts authentication may reduce
1.1 deraadt 910: authentication time on slow connections when rhosts authentication is
1.40 aaron 911: not used.
912: Most servers do not permit RhostsAuthentication because it
913: is not secure (see RhostsRSAAuthentication).
914: The argument to this keyword must be
1.2 deraadt 915: .Dq yes
916: or
917: .Dq no .
918: .It Cm RhostsRSAAuthentication
1.1 deraadt 919: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 920: authentication.
921: This is the primary authentication method for most sites.
922: The argument must be
1.2 deraadt 923: .Dq yes
924: or
925: .Dq no .
926: .It Cm RSAAuthentication
1.40 aaron 927: Specifies whether to try RSA authentication.
928: The argument to this keyword must be
1.2 deraadt 929: .Dq yes
930: or
931: .Dq no .
1.1 deraadt 932: RSA authentication will only be
933: attempted if the identity file exists, or an authentication agent is
934: running.
1.50 markus 935: Note that this option applies to protocol version 1 only.
1.81 markus 936: .It Cm ChallengeResponseAuthentication
937: Specifies whether to use challenge response authentication.
938: Currently there is only support for
1.27 markus 939: .Xr skey 1
1.40 aaron 940: authentication.
941: The argument to this keyword must be
1.27 markus 942: .Dq yes
943: or
944: .Dq no .
945: The default is
946: .Dq no .
1.2 deraadt 947: .It Cm StrictHostKeyChecking
948: If this flag is set to
1.44 aaron 949: .Dq yes ,
1.2 deraadt 950: .Nm
1.79 stevesk 951: will never automatically add host keys to the
1.2 deraadt 952: .Pa $HOME/.ssh/known_hosts
1.48 markus 953: and
954: .Pa $HOME/.ssh/known_hosts2
1.79 stevesk 955: files, and refuses to connect to hosts whose host key has changed.
1.40 aaron 956: This provides maximum protection against trojan horse attacks.
957: However, it can be somewhat annoying if you don't have good
1.2 deraadt 958: .Pa /etc/ssh_known_hosts
1.48 markus 959: and
960: .Pa /etc/ssh_known_hosts2
1.1 deraadt 961: files installed and frequently
1.79 stevesk 962: connect to new hosts.
963: This option forces the user to manually
964: add all new hosts.
965: If this flag is set to
966: .Dq no ,
967: .Nm
968: will automatically add new host keys to the
969: user known hosts files.
970: If this flag is set to
971: .Dq ask ,
972: new host keys
973: will be added to the user known host files only after the user
974: has confirmed that is what they really want to do, and
975: .Nm
976: will refuse to connect to hosts whose host key has changed.
1.40 aaron 977: The host keys of
1.79 stevesk 978: known hosts will be verified automatically in all cases.
1.40 aaron 979: The argument must be
1.79 stevesk 980: .Dq yes ,
981: .Dq no
1.2 deraadt 982: or
1.79 stevesk 983: .Dq ask .
984: The default is
985: .Dq ask .
1.16 markus 986: .It Cm UsePrivilegedPort
987: Specifies whether to use a privileged port for outgoing connections.
988: The argument must be
989: .Dq yes
990: or
991: .Dq no .
992: The default is
1.98 ! markus 993: .Dq no .
1.16 markus 994: Note that setting this option to
995: .Dq no
1.30 provos 996: turns off
1.16 markus 997: .Cm RhostsAuthentication
998: and
1.72 markus 999: .Cm RhostsRSAAuthentication
1000: for older servers.
1.34 markus 1001: .It Cm User
1.40 aaron 1002: Specifies the user to log in as.
1003: This can be useful if you have a different user name on different machines.
1004: This saves the trouble of
1.34 markus 1005: having to remember to give the user name on the command line.
1006: .It Cm UserKnownHostsFile
1.95 stevesk 1007: Specifies a file to use for the protocol version 1 user
1008: host key database instead of
1.34 markus 1009: .Pa $HOME/.ssh/known_hosts .
1.95 stevesk 1010: .It Cm UserKnownHostsFile2
1011: Specifies a file to use for the protocol version 2 user
1012: host key database instead of
1013: .Pa $HOME/.ssh/known_hosts2 .
1.2 deraadt 1014: .It Cm UseRsh
1.40 aaron 1015: Specifies that rlogin/rsh should be used for this host.
1016: It is possible that the host does not at all support the
1.2 deraadt 1017: .Nm
1.40 aaron 1018: protocol.
1019: This causes
1.2 deraadt 1020: .Nm
1.40 aaron 1021: to immediately execute
1.2 deraadt 1022: .Xr rsh 1 .
1.1 deraadt 1023: All other options (except
1.2 deraadt 1024: .Cm HostName )
1.40 aaron 1025: are ignored if this has been specified.
1026: The argument must be
1.2 deraadt 1027: .Dq yes
1028: or
1029: .Dq no .
1.55 markus 1030: .It Cm XAuthLocation
1031: Specifies the location of the
1032: .Xr xauth 1
1033: program.
1034: The default is
1035: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1036: .El
1.2 deraadt 1037: .Sh ENVIRONMENT
1038: .Nm
1.1 deraadt 1039: will normally set the following environment variables:
1.2 deraadt 1040: .Bl -tag -width Ds
1041: .It Ev DISPLAY
1042: The
1043: .Ev DISPLAY
1.40 aaron 1044: variable indicates the location of the X11 server.
1.44 aaron 1045: It is automatically set by
1.2 deraadt 1046: .Nm
1047: to point to a value of the form
1048: .Dq hostname:n
1049: where hostname indicates
1.40 aaron 1050: the host where the shell runs, and n is an integer >= 1.
1051: .Nm
1052: uses this special value to forward X11 connections over the secure
1053: channel.
1054: The user should normally not set DISPLAY explicitly, as that
1.1 deraadt 1055: will render the X11 connection insecure (and will require the user to
1056: manually copy any required authorization cookies).
1.2 deraadt 1057: .It Ev HOME
1.1 deraadt 1058: Set to the path of the user's home directory.
1.2 deraadt 1059: .It Ev LOGNAME
1060: Synonym for
1.12 aaron 1061: .Ev USER ;
1062: set for compatibility with systems that use this variable.
1.2 deraadt 1063: .It Ev MAIL
1.1 deraadt 1064: Set to point the user's mailbox.
1.40 aaron 1065: .It Ev PATH
1.2 deraadt 1066: Set to the default
1067: .Ev PATH ,
1068: as specified when compiling
1.12 aaron 1069: .Nm ssh .
1.18 markus 1070: .It Ev SSH_AUTH_SOCK
1.17 markus 1071: indicates the path of a unix-domain socket used to communicate with the
1072: agent.
1.2 deraadt 1073: .It Ev SSH_CLIENT
1.40 aaron 1074: Identifies the client end of the connection.
1075: The variable contains
1.1 deraadt 1076: three space-separated values: client ip-address, client port number,
1077: and server port number.
1.73 markus 1078: .It Ev SSH_ORIGINAL_COMMAND
1079: The variable contains the original command line if a forced command
1080: is executed.
1081: It can be used to extract the original arguments.
1.2 deraadt 1082: .It Ev SSH_TTY
1.1 deraadt 1083: This is set to the name of the tty (path to the device) associated
1.40 aaron 1084: with the current shell or command.
1085: If the current session has no tty,
1.1 deraadt 1086: this variable is not set.
1.2 deraadt 1087: .It Ev TZ
1.1 deraadt 1088: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1089: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1090: on to new connections).
1.2 deraadt 1091: .It Ev USER
1.1 deraadt 1092: Set to the name of the user logging in.
1.2 deraadt 1093: .El
1094: .Pp
1.44 aaron 1095: Additionally,
1.2 deraadt 1096: .Nm
1.44 aaron 1097: reads
1098: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1099: and adds lines of the format
1100: .Dq VARNAME=value
1.12 aaron 1101: to the environment.
1.2 deraadt 1102: .Sh FILES
1.36 markus 1103: .Bl -tag -width Ds
1.95 stevesk 1104: .It Pa $HOME/.ssh/known_hosts, $HOME/.ssh/known_hosts2
1.1 deraadt 1105: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1106: in
1.95 stevesk 1107: .Pa /etc/ssh_known_hosts
1108: for protocol version 1 or
1109: .Pa /etc/ssh_known_hosts2
1110: for protocol version 2).
1.2 deraadt 1111: See
1112: .Xr sshd 8 .
1.48 markus 1113: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1114: Contains the RSA and the DSA authentication identity of the user.
1115: These files
1116: contain sensitive data and should be readable by the user but not
1.15 markus 1117: accessible by others (read/write/execute).
1118: Note that
1119: .Nm
1.48 markus 1120: ignores a private key file if it is accessible by others.
1.15 markus 1121: It is possible to specify a passphrase when
1.1 deraadt 1122: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1123: sensitive part of this file using 3DES.
1.48 markus 1124: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1.1 deraadt 1125: Contains the public key for authentication (public part of the
1.40 aaron 1126: identity file in human-readable form).
1.48 markus 1127: The contents of the
1128: .Pa $HOME/.ssh/identity.pub
1129: file should be added to
1.2 deraadt 1130: .Pa $HOME/.ssh/authorized_keys
1131: on all machines
1.40 aaron 1132: where you wish to log in using RSA authentication.
1.48 markus 1133: The contents of the
1134: .Pa $HOME/.ssh/id_dsa.pub
1135: file should be added to
1136: .Pa $HOME/.ssh/authorized_keys2
1137: on all machines
1138: where you wish to log in using DSA authentication.
1139: These files are not
1.40 aaron 1140: sensitive and can (but need not) be readable by anyone.
1.48 markus 1141: These files are
1.84 markus 1142: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1143: the convenience of the user.
1.2 deraadt 1144: .It Pa $HOME/.ssh/config
1.40 aaron 1145: This is the per-user configuration file.
1146: The format of this file is described above.
1147: This file is used by the
1.2 deraadt 1148: .Nm
1.40 aaron 1149: client.
1150: This file does not usually contain any sensitive information,
1.1 deraadt 1151: but the recommended permissions are read/write for the user, and not
1152: accessible by others.
1.2 deraadt 1153: .It Pa $HOME/.ssh/authorized_keys
1.40 aaron 1154: Lists the RSA keys that can be used for logging in as this user.
1155: The format of this file is described in the
1.2 deraadt 1156: .Xr sshd 8
1.40 aaron 1157: manual page.
1158: In the simplest form the format is the same as the .pub
1.1 deraadt 1159: identity files (that is, each line contains the number of bits in
1160: modulus, public exponent, modulus, and comment fields, separated by
1.40 aaron 1161: spaces).
1162: This file is not highly sensitive, but the recommended
1.1 deraadt 1163: permissions are read/write for the user, and not accessible by others.
1.48 markus 1164: .It Pa $HOME/.ssh/authorized_keys2
1.68 markus 1165: Lists the public keys (DSA/RSA) that can be used for logging in as this user.
1.48 markus 1166: This file is not highly sensitive, but the recommended
1167: permissions are read/write for the user, and not accessible by others.
1168: .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1.40 aaron 1169: Systemwide list of known host keys.
1.48 markus 1170: .Pa /etc/ssh_known_hosts
1171: contains RSA and
1172: .Pa /etc/ssh_known_hosts2
1.68 markus 1173: contains DSA or RSA keys for protocol version 2.
1.48 markus 1174: These files should be prepared by the
1.1 deraadt 1175: system administrator to contain the public host keys of all machines in the
1.40 aaron 1176: organization.
1177: This file should be world-readable.
1178: This file contains
1.1 deraadt 1179: public keys, one per line, in the following format (fields separated
1180: by spaces): system name, number of bits in modulus, public exponent,
1.40 aaron 1181: modulus, and optional comment field.
1182: When different names are used
1.1 deraadt 1183: for the same machine, all such names should be listed, separated by
1.40 aaron 1184: commas.
1185: The format is described on the
1.2 deraadt 1186: .Xr sshd 8
1.1 deraadt 1187: manual page.
1.2 deraadt 1188: .Pp
1.1 deraadt 1189: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1190: .Xr sshd 8
1.1 deraadt 1191: to verify the client host when logging in; other names are needed because
1.2 deraadt 1192: .Nm
1.1 deraadt 1193: does not convert the user-supplied name to a canonical name before
1194: checking the key, because someone with access to the name servers
1195: would then be able to fool host authentication.
1.2 deraadt 1196: .It Pa /etc/ssh_config
1.40 aaron 1197: Systemwide configuration file.
1198: This file provides defaults for those
1.1 deraadt 1199: values that are not specified in the user's configuration file, and
1.40 aaron 1200: for those users who do not have a configuration file.
1201: This file must be world-readable.
1.2 deraadt 1202: .It Pa $HOME/.rhosts
1203: This file is used in
1204: .Pa \&.rhosts
1205: authentication to list the
1.40 aaron 1206: host/user pairs that are permitted to log in.
1207: (Note that this file is
1.1 deraadt 1208: also used by rlogin and rsh, which makes using this file insecure.)
1209: Each line of the file contains a host name (in the canonical form
1210: returned by name servers), and then a user name on that host,
1.40 aaron 1211: separated by a space.
1.92 markus 1212: On some machines this file may need to be
1.1 deraadt 1213: world-readable if the user's home directory is on a NFS partition,
1214: because
1.2 deraadt 1215: .Xr sshd 8
1.40 aaron 1216: reads it as root.
1217: Additionally, this file must be owned by the user,
1218: and must not have write permissions for anyone else.
1219: The recommended
1.1 deraadt 1220: permission for most machines is read/write for the user, and not
1221: accessible by others.
1.2 deraadt 1222: .Pp
1.1 deraadt 1223: Note that by default
1.2 deraadt 1224: .Xr sshd 8
1.1 deraadt 1225: will be installed so that it requires successful RSA host
1.40 aaron 1226: authentication before permitting \s+2.\s0rhosts authentication.
1227: If your server machine does not have the client's host key in
1.2 deraadt 1228: .Pa /etc/ssh_known_hosts ,
1229: you can store it in
1230: .Pa $HOME/.ssh/known_hosts .
1231: The easiest way to do this is to
1.1 deraadt 1232: connect back to the client from the server machine using ssh; this
1.48 markus 1233: will automatically add the host key to
1.2 deraadt 1234: .Pa $HOME/.ssh/known_hosts .
1235: .It Pa $HOME/.shosts
1236: This file is used exactly the same way as
1237: .Pa \&.rhosts .
1238: The purpose for
1.1 deraadt 1239: having this file is to be able to use rhosts authentication with
1.2 deraadt 1240: .Nm
1241: without permitting login with
1242: .Xr rlogin 1
1243: or
1244: .Xr rsh 1 .
1245: .It Pa /etc/hosts.equiv
1246: This file is used during
1.40 aaron 1247: .Pa \&.rhosts authentication.
1248: It contains
1.1 deraadt 1249: canonical hosts names, one per line (the full format is described on
1250: the
1.2 deraadt 1251: .Xr sshd 8
1.40 aaron 1252: manual page).
1253: If the client host is found in this file, login is
1.1 deraadt 1254: automatically permitted provided client and server user names are the
1.40 aaron 1255: same.
1256: Additionally, successful RSA host authentication is normally
1257: required.
1258: This file should only be writable by root.
1.2 deraadt 1259: .It Pa /etc/shosts.equiv
1.44 aaron 1260: This file is processed exactly as
1.2 deraadt 1261: .Pa /etc/hosts.equiv .
1.1 deraadt 1262: This file may be useful to permit logins using
1.2 deraadt 1263: .Nm
1.1 deraadt 1264: but not using rsh/rlogin.
1.2 deraadt 1265: .It Pa /etc/sshrc
1.1 deraadt 1266: Commands in this file are executed by
1.2 deraadt 1267: .Nm
1.1 deraadt 1268: when the user logs in just before the user's shell (or command) is started.
1269: See the
1.2 deraadt 1270: .Xr sshd 8
1.1 deraadt 1271: manual page for more information.
1.2 deraadt 1272: .It Pa $HOME/.ssh/rc
1.1 deraadt 1273: Commands in this file are executed by
1.2 deraadt 1274: .Nm
1.1 deraadt 1275: when the user logs in just before the user's shell (or command) is
1276: started.
1.44 aaron 1277: See the
1.2 deraadt 1278: .Xr sshd 8
1.1 deraadt 1279: manual page for more information.
1.31 markus 1280: .It Pa $HOME/.ssh/environment
1281: Contains additional definitions for environment variables, see section
1282: .Sx ENVIRONMENT
1283: above.
1.58 itojun 1284: .El
1.67 aaron 1285: .Sh AUTHORS
1.78 markus 1286: OpenSSH is a derivative of the original and free
1287: ssh 1.2.12 release by Tatu Ylonen.
1288: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1289: Theo de Raadt and Dug Song
1290: removed many bugs, re-added newer features and
1291: created OpenSSH.
1292: Markus Friedl contributed the support for SSH
1293: protocol versions 1.5 and 2.0.
1.2 deraadt 1294: .Sh SEE ALSO
1295: .Xr rlogin 1 ,
1296: .Xr rsh 1 ,
1297: .Xr scp 1 ,
1.83 djm 1298: .Xr sftp 1 ,
1.2 deraadt 1299: .Xr ssh-add 1 ,
1300: .Xr ssh-agent 1 ,
1301: .Xr ssh-keygen 1 ,
1302: .Xr telnet 1 ,
1.87 itojun 1303: .Xr sshd 8
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