Annotation of src/usr.bin/ssh/ssh.1, Revision 1.61
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: .\"
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.
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: .\"
1.2 deraadt 37: .Dd September 25, 1999
38: .Dt SSH 1
39: .Os
40: .Sh NAME
41: .Nm ssh
1.20 provos 42: .Nd OpenSSH secure shell client (remote login program)
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm ssh
45: .Op Fl l Ar login_name
1.5 deraadt 46: .Op Ar hostname | user@hostname
1.2 deraadt 47: .Op Ar command
48: .Pp
49: .Nm ssh
1.54 markus 50: .Op Fl afgknqtvxACNPTX246
1.51 markus 51: .Op Fl c Ar cipher_spec
1.2 deraadt 52: .Op Fl e Ar escape_char
53: .Op Fl i Ar identity_file
54: .Op Fl l Ar login_name
55: .Op Fl o Ar option
56: .Op Fl p Ar port
1.12 aaron 57: .Oo Fl L Xo
58: .Sm off
1.33 markus 59: .Ar port :
1.12 aaron 60: .Ar host :
61: .Ar hostport
62: .Sm on
63: .Xc
64: .Oc
65: .Oo Fl R Xo
66: .Sm off
1.33 markus 67: .Ar port :
1.12 aaron 68: .Ar host :
69: .Ar hostport
70: .Sm on
71: .Xc
72: .Oc
1.5 deraadt 73: .Op Ar hostname | user@hostname
1.2 deraadt 74: .Op Ar command
1.44 aaron 75: .Sh DESCRIPTION
1.2 deraadt 76: .Nm
1.5 deraadt 77: (Secure Shell) is a program for logging into a remote machine and for
1.40 aaron 78: executing commands on a remote machine.
79: It is intended to replace
1.1 deraadt 80: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 81: two untrusted hosts over an insecure network.
82: X11 connections and
1.1 deraadt 83: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 84: .Pp
85: .Nm
1.44 aaron 86: connects and logs into the specified
1.2 deraadt 87: .Ar hostname .
1.1 deraadt 88: The user must prove
1.49 markus 89: his/her identity to the remote machine using one of several methods
90: depending on the protocol version used:
91: .Pp
92: .Ss SSH protocol version 1
1.2 deraadt 93: .Pp
1.1 deraadt 94: First, if the machine the user logs in from is listed in
1.2 deraadt 95: .Pa /etc/hosts.equiv
1.1 deraadt 96: or
1.2 deraadt 97: .Pa /etc/shosts.equiv
1.1 deraadt 98: on the remote machine, and the user names are
99: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 100: Second, if
1.2 deraadt 101: .Pa \&.rhosts
1.1 deraadt 102: or
1.2 deraadt 103: .Pa \&.shosts
1.1 deraadt 104: exists in the user's home directory on the
105: remote machine and contains a line containing the name of the client
106: machine and the name of the user on that machine, the user is
1.40 aaron 107: permitted to log in.
108: This form of authentication alone is normally not
1.1 deraadt 109: allowed by the server because it is not secure.
1.2 deraadt 110: .Pp
1.1 deraadt 111: The second (and primary) authentication method is the
1.2 deraadt 112: .Pa rhosts
1.1 deraadt 113: or
1.2 deraadt 114: .Pa hosts.equiv
1.40 aaron 115: method combined with RSA-based host authentication.
116: It means that if the login would be permitted by
1.49 markus 117: .Pa $HOME/.rhosts ,
118: .Pa $HOME/.shosts ,
1.2 deraadt 119: .Pa /etc/hosts.equiv ,
1.1 deraadt 120: or
1.2 deraadt 121: .Pa /etc/shosts.equiv ,
1.11 deraadt 122: and if additionally the server can verify the client's
1.44 aaron 123: host key (see
1.2 deraadt 124: .Pa /etc/ssh_known_hosts
1.23 markus 125: and
126: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 127: in the
1.2 deraadt 128: .Sx FILES
1.40 aaron 129: section), only then login is permitted.
130: This authentication method closes security holes due to IP
131: spoofing, DNS spoofing and routing spoofing.
132: [Note to the administrator:
1.2 deraadt 133: .Pa /etc/hosts.equiv ,
1.49 markus 134: .Pa $HOME/.rhosts ,
1.1 deraadt 135: and the rlogin/rsh protocol in general, are inherently insecure and should be
136: disabled if security is desired.]
1.2 deraadt 137: .Pp
1.44 aaron 138: As a third authentication method,
1.2 deraadt 139: .Nm
1.1 deraadt 140: supports RSA based authentication.
141: The scheme is based on public-key cryptography: there are cryptosystems
142: where encryption and decryption are done using separate keys, and it
143: is not possible to derive the decryption key from the encryption key.
1.40 aaron 144: RSA is one such system.
1.44 aaron 145: The idea is that each user creates a public/private
1.40 aaron 146: key pair for authentication purposes.
147: The server knows the public key, and only the user knows the private key.
1.44 aaron 148: The file
1.2 deraadt 149: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 150: lists the public keys that are permitted for logging
1.40 aaron 151: in.
152: When the user logs in, the
1.2 deraadt 153: .Nm
1.1 deraadt 154: program tells the server which key pair it would like to use for
1.40 aaron 155: authentication.
156: The server checks if this key is permitted, and if
1.1 deraadt 157: so, sends the user (actually the
1.2 deraadt 158: .Nm
1.1 deraadt 159: program running on behalf of the user) a challenge, a random number,
1.40 aaron 160: encrypted by the user's public key.
161: The challenge can only be
162: decrypted using the proper private key.
163: The user's client then decrypts the
1.1 deraadt 164: challenge using the private key, proving that he/she knows the private
165: key but without disclosing it to the server.
1.2 deraadt 166: .Pp
167: .Nm
1.40 aaron 168: implements the RSA authentication protocol automatically.
169: The user creates his/her RSA key pair by running
1.2 deraadt 170: .Xr ssh-keygen 1 .
1.44 aaron 171: This stores the private key in
1.49 markus 172: .Pa $HOME/.ssh/identity
1.1 deraadt 173: and the public key in
1.49 markus 174: .Pa $HOME/.ssh/identity.pub
1.40 aaron 175: in the user's home directory.
176: The user should then copy the
1.2 deraadt 177: .Pa identity.pub
1.44 aaron 178: to
1.49 markus 179: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 180: in his/her home directory on the remote machine (the
1.2 deraadt 181: .Pa authorized_keys
1.44 aaron 182: file corresponds to the conventional
1.49 markus 183: .Pa $HOME/.rhosts
1.1 deraadt 184: file, and has one key
1.40 aaron 185: per line, though the lines can be very long).
186: After this, the user can log in without giving the password.
187: RSA authentication is much
1.1 deraadt 188: more secure than rhosts authentication.
1.2 deraadt 189: .Pp
1.1 deraadt 190: The most convenient way to use RSA authentication may be with an
1.40 aaron 191: authentication agent.
192: See
1.2 deraadt 193: .Xr ssh-agent 1
1.1 deraadt 194: for more information.
1.2 deraadt 195: .Pp
1.44 aaron 196: If other authentication methods fail,
1.2 deraadt 197: .Nm
1.40 aaron 198: prompts the user for a password.
199: The password is sent to the remote
1.1 deraadt 200: host for checking; however, since all communications are encrypted,
201: the password cannot be seen by someone listening on the network.
1.2 deraadt 202: .Pp
1.49 markus 203: .Ss SSH protocol version 2
204: .Pp
205: When a user connects using the protocol version 2
206: different authentication methods are available:
207: At first, the client attempts to authenticate using the public key method.
208: If this method fails password authentication is tried.
209: .Pp
210: The public key method is similar to RSA authentication described
211: in the previous section except that the DSA algorithm is used
212: instead of the patented RSA algorithm.
213: The client uses his private DSA key
214: .Pa $HOME/.ssh/id_dsa
215: to sign the session identifier and sends the result to the server.
216: The server checks whether the matching public key is listed in
217: .Pa $HOME/.ssh/authorized_keys2
218: and grants access if both the key is found and the signature is correct.
219: The session identifier is derived from a shared Diffie-Hellman value
220: and is only known to the client and the server.
221: .Pp
222: If public key authentication fails or is not available a password
223: can be sent encrypted to the remote host for proving the user's identity.
224: This protocol 2 implementation does not yet support Kerberos or
225: S/Key authentication.
226: .Pp
227: Protocol 2 provides additional mechanisms for confidentiality
1.51 markus 228: (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
1.49 markus 229: and integrity (hmac-sha1, hmac-md5).
230: Note that protocol 1 lacks a strong mechanism for ensuring the
231: integrity of the connection.
232: .Pp
233: .Ss Login session and remote execution
234: .Pp
1.1 deraadt 235: When the user's identity has been accepted by the server, the server
236: either executes the given command, or logs into the machine and gives
1.40 aaron 237: the user a normal shell on the remote machine.
238: All communication with
1.1 deraadt 239: the remote command or shell will be automatically encrypted.
1.2 deraadt 240: .Pp
1.1 deraadt 241: If a pseudo-terminal has been allocated (normal login session), the
1.2 deraadt 242: user can disconnect with
243: .Ic ~. ,
244: and suspend
245: .Nm
246: with
247: .Ic ~^Z .
248: All forwarded connections can be listed with
1.44 aaron 249: .Ic ~#
1.2 deraadt 250: and if
1.1 deraadt 251: the session blocks waiting for forwarded X11 or TCP/IP
1.2 deraadt 252: connections to terminate, it can be backgrounded with
253: .Ic ~&
254: (this should not be used while the user shell is active, as it can cause the
1.40 aaron 255: shell to hang).
256: All available escapes can be listed with
1.2 deraadt 257: .Ic ~? .
258: .Pp
259: A single tilde character can be sent as
260: .Ic ~~
261: (or by following the tilde by a character other than those described above).
1.1 deraadt 262: The escape character must always follow a newline to be interpreted as
1.40 aaron 263: special.
264: The escape character can be changed in configuration files
265: or on the command line.
1.2 deraadt 266: .Pp
1.1 deraadt 267: If no pseudo tty has been allocated, the
268: session is transparent and can be used to reliably transfer binary
1.40 aaron 269: data.
270: On most systems, setting the escape character to
1.2 deraadt 271: .Dq none
272: will also make the session transparent even if a tty is used.
273: .Pp
1.1 deraadt 274: The session terminates when the command or shell in on the remote
275: machine exists and all X11 and TCP/IP connections have been closed.
276: The exit status of the remote program is returned as the exit status
277: of
1.2 deraadt 278: .Nm ssh .
279: .Pp
1.49 markus 280: .Ss X11 and TCP forwarding
281: .Pp
1.1 deraadt 282: If the user is using X11 (the
1.2 deraadt 283: .Ev DISPLAY
1.1 deraadt 284: environment variable is set), the connection to the X11 display is
285: automatically forwarded to the remote side in such a way that any X11
286: programs started from the shell (or command) will go through the
287: encrypted channel, and the connection to the real X server will be made
1.40 aaron 288: from the local machine.
289: The user should not manually set
1.2 deraadt 290: .Ev DISPLAY .
1.1 deraadt 291: Forwarding of X11 connections can be
292: configured on the command line or in configuration files.
1.2 deraadt 293: .Pp
294: The
1.44 aaron 295: .Ev DISPLAY
1.2 deraadt 296: value set by
297: .Nm
1.1 deraadt 298: will point to the server machine, but with a display number greater
1.40 aaron 299: than zero.
300: This is normal, and happens because
1.2 deraadt 301: .Nm
302: creates a
303: .Dq proxy
304: X server on the server machine for forwarding the
1.1 deraadt 305: connections over the encrypted channel.
1.2 deraadt 306: .Pp
307: .Nm
1.1 deraadt 308: will also automatically set up Xauthority data on the server machine.
309: For this purpose, it will generate a random authorization cookie,
310: store it in Xauthority on the server, and verify that any forwarded
311: connections carry this cookie and replace it by the real cookie when
1.40 aaron 312: the connection is opened.
313: The real authentication cookie is never
1.1 deraadt 314: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 315: .Pp
1.1 deraadt 316: If the user is using an authentication agent, the connection to the agent
317: is automatically forwarded to the remote side unless disabled on
318: command line or in a configuration file.
1.2 deraadt 319: .Pp
1.1 deraadt 320: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.40 aaron 321: be specified either on command line or in a configuration file.
322: One possible application of TCP/IP forwarding is a secure connection to an
1.1 deraadt 323: electronic purse; another is going trough firewalls.
1.2 deraadt 324: .Pp
1.49 markus 325: .Ss Server authentication
326: .Pp
1.2 deraadt 327: .Nm
1.49 markus 328: automatically maintains and checks a database containing
1.40 aaron 329: identifications for all hosts it has ever been used with.
1.49 markus 330: RSA host keys are stored in
331: .Pa $HOME/.ssh/known_hosts
332: and
333: DSA host keys are stored in
334: .Pa $HOME/.ssh/known_hosts2
1.40 aaron 335: in the user's home directory.
1.49 markus 336: Additionally, the files
1.2 deraadt 337: .Pa /etc/ssh_known_hosts
1.49 markus 338: and
339: .Pa /etc/ssh_known_hosts2
340: are automatically checked for known hosts.
1.40 aaron 341: Any new hosts are automatically added to the user's file.
342: If a host's identification
1.1 deraadt 343: ever changes,
1.2 deraadt 344: .Nm
1.1 deraadt 345: warns about this and disables password authentication to prevent a
1.40 aaron 346: trojan horse from getting the user's password.
347: Another purpose of
1.1 deraadt 348: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 349: otherwise be used to circumvent the encryption.
350: The
1.2 deraadt 351: .Cm StrictHostKeyChecking
1.1 deraadt 352: option (see below) can be used to prevent logins to machines whose
353: host key is not known or has changed.
1.2 deraadt 354: .Sh OPTIONS
355: .Bl -tag -width Ds
1.4 dugsong 356: .It Fl a
1.42 aaron 357: Disables forwarding of the authentication agent connection.
1.54 markus 358: .It Fl A
359: Enables forwarding of the authentication agent connection.
360: This can also be specified on a per-host basis in a configuration file.
1.6 deraadt 361: .It Fl c Ar blowfish|3des
1.44 aaron 362: Selects the cipher to use for encrypting the session.
1.2 deraadt 363: .Ar 3des
1.40 aaron 364: is used by default.
1.44 aaron 365: It is believed to be secure.
1.5 deraadt 366: .Ar 3des
367: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
368: It is presumably more secure than the
1.2 deraadt 369: .Ar des
1.51 markus 370: cipher which is no longer supported in
371: .Nm ssh .
1.5 deraadt 372: .Ar blowfish
373: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 374: .Ar 3des .
1.51 markus 375: .It Fl c Ar "3des-cbc,blowfish-cbc,arcfour,cast128-cbc"
376: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 ! aaron 377: be specified in order of preference.
! 378: Protocol version 2 supports 3DES, Blowfish, and CAST128 in CBC mode
! 379: and Arcfour.
1.2 deraadt 380: .It Fl e Ar ch|^ch|none
381: Sets the escape character for sessions with a pty (default:
382: .Ql ~ ) .
1.40 aaron 383: The escape character is only recognized at the beginning of a line.
384: The escape character followed by a dot
1.2 deraadt 385: .Pq Ql \&.
386: closes the connection, followed
1.1 deraadt 387: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 388: escape character once.
389: Setting the character to
1.2 deraadt 390: .Dq none
391: disables any escapes and makes the session fully transparent.
392: .It Fl f
393: Requests
394: .Nm
1.40 aaron 395: to go to background just before command execution.
396: This is useful if
1.2 deraadt 397: .Nm
398: is going to ask for passwords or passphrases, but the user
1.40 aaron 399: wants it in the background.
1.44 aaron 400: This implies
1.2 deraadt 401: .Fl n .
1.1 deraadt 402: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 403: something like
404: .Ic ssh -f host xterm .
1.34 markus 405: .It Fl g
406: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 407: .It Fl i Ar identity_file
1.44 aaron 408: Selects the file from which the identity (private key) for
1.40 aaron 409: RSA authentication is read.
1.44 aaron 410: Default is
1.49 markus 411: .Pa $HOME/.ssh/identity
1.40 aaron 412: in the user's home directory.
413: Identity files may also be specified on
414: a per-host basis in the configuration file.
415: It is possible to have multiple
1.2 deraadt 416: .Fl i
417: options (and multiple identities specified in
1.1 deraadt 418: configuration files).
1.2 deraadt 419: .It Fl k
1.42 aaron 420: Disables forwarding of Kerberos tickets and AFS tokens.
421: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 422: .It Fl l Ar login_name
1.40 aaron 423: Specifies the user to log in as on the remote machine.
424: This also may be specified on a per-host basis in the configuration file.
1.2 deraadt 425: .It Fl n
426: Redirects stdin from
427: .Pa /dev/null
428: (actually, prevents reading from stdin).
1.1 deraadt 429: This must be used when
1.2 deraadt 430: .Nm
1.40 aaron 431: is run in the background.
432: A common trick is to use this to run X11 programs on a remote machine.
433: For example,
1.2 deraadt 434: .Ic ssh -n shadows.cs.hut.fi emacs &
435: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 436: connection will be automatically forwarded over an encrypted channel.
437: The
1.2 deraadt 438: .Nm
1.1 deraadt 439: program will be put in the background.
440: (This does not work if
1.2 deraadt 441: .Nm
442: needs to ask for a password or passphrase; see also the
443: .Fl f
444: option.)
1.53 markus 445: .It Fl N
446: Do not execute a remote command.
447: This is usefull if you just want to forward ports
448: (protocol version 2 only).
1.2 deraadt 449: .It Fl o Ar option
1.1 deraadt 450: Can be used to give options in the format used in the config file.
451: This is useful for specifying options for which there is no separate
1.40 aaron 452: command-line flag.
453: The option has the same format as a line in the configuration file.
1.2 deraadt 454: .It Fl p Ar port
1.40 aaron 455: Port to connect to on the remote host.
456: This can be specified on a
1.1 deraadt 457: per-host basis in the configuration file.
1.16 markus 458: .It Fl P
459: Use a non-privileged port for outgoing connections.
460: This can be used if your firewall does
461: not permit connections from privileged ports.
1.30 provos 462: Note that this option turns off
1.16 markus 463: .Cm RhostsAuthentication
464: and
465: .Cm RhostsRSAAuthentication .
1.2 deraadt 466: .It Fl q
1.40 aaron 467: Quiet mode.
468: Causes all warning and diagnostic messages to be suppressed.
469: Only fatal errors are displayed.
1.2 deraadt 470: .It Fl t
1.40 aaron 471: Force pseudo-tty allocation.
1.43 brad 472: This can be used to execute arbitrary
1.40 aaron 473: screen-based programs on a remote machine, which can be very useful,
474: e.g., when implementing menu services.
1.53 markus 475: .It Fl T
476: Disable pseudo-tty allocation (protocol version 2 only).
1.2 deraadt 477: .It Fl v
1.40 aaron 478: Verbose mode.
479: Causes
1.2 deraadt 480: .Nm
1.40 aaron 481: to print debugging messages about its progress.
482: This is helpful in
1.1 deraadt 483: debugging connection, authentication, and configuration problems.
1.22 provos 484: The verbose mode is also used to display
485: .Xr skey 1
486: challenges, if the user entered "s/key" as password.
1.61 ! aaron 487: Multiple -v options increases the verbosity.
! 488: Maximum is 3.
1.2 deraadt 489: .It Fl x
1.40 aaron 490: Disables X11 forwarding.
1.2 deraadt 491: .It Fl X
1.1 deraadt 492: Enables X11 forwarding.
1.54 markus 493: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 494: .It Fl C
1.1 deraadt 495: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 496: data for forwarded X11 and TCP/IP connections).
497: The compression algorithm is the same used by
1.34 markus 498: .Xr gzip 1 ,
499: and the
1.2 deraadt 500: .Dq level
501: can be controlled by the
502: .Cm CompressionLevel
1.40 aaron 503: option (see below).
504: Compression is desirable on modem lines and other
1.1 deraadt 505: slow connections, but will only slow down things on fast networks.
506: The default value can be set on a host-by-host basis in the
507: configuration files; see the
1.2 deraadt 508: .Cm Compress
1.1 deraadt 509: option below.
1.2 deraadt 510: .It Fl L Ar port:host:hostport
1.1 deraadt 511: Specifies that the given port on the local (client) host is to be
1.40 aaron 512: forwarded to the given host and port on the remote side.
513: This works by allocating a socket to listen to
1.2 deraadt 514: .Ar port
1.1 deraadt 515: on the local side, and whenever a connection is made to this port, the
516: connection is forwarded over the secure channel, and a connection is
517: made to
1.32 markus 518: .Ar host
519: port
520: .Ar hostport
1.40 aaron 521: from the remote machine.
522: Port forwardings can also be specified in the configuration file.
523: Only root can forward privileged ports.
1.32 markus 524: IPv6 addresses can be specified with an alternative syntax:
525: .Ar port/host/hostport
1.2 deraadt 526: .It Fl R Ar port:host:hostport
1.1 deraadt 527: Specifies that the given port on the remote (server) host is to be
1.40 aaron 528: forwarded to the given host and port on the local side.
529: This works by allocating a socket to listen to
1.2 deraadt 530: .Ar port
1.1 deraadt 531: on the remote side, and whenever a connection is made to this port, the
532: connection is forwarded over the secure channel, and a connection is
533: made to
1.32 markus 534: .Ar host
535: port
536: .Ar hostport
1.40 aaron 537: from the local machine.
538: Port forwardings can also be specified in the configuration file.
539: Privileged ports can be forwarded only when
1.1 deraadt 540: logging in as root on the remote machine.
1.46 markus 541: .It Fl 2
542: Forces
543: .Nm
1.50 markus 544: to try protocol version 2 only.
1.32 markus 545: .It Fl 4
546: Forces
547: .Nm
548: to use IPv4 addresses only.
549: .It Fl 6
550: Forces
551: .Nm
552: to use IPv6 addresses only.
1.2 deraadt 553: .El
554: .Sh CONFIGURATION FILES
555: .Nm
1.1 deraadt 556: obtains configuration data from the following sources (in this order):
557: command line options, user's configuration file
1.2 deraadt 558: .Pq Pa $HOME/.ssh/config ,
559: and system-wide configuration file
560: .Pq Pa /etc/ssh_config .
561: For each parameter, the first obtained value
1.40 aaron 562: will be used.
563: The configuration files contain sections bracketed by
564: .Dq Host
565: specifications, and that section is only applied for hosts that
566: match one of the patterns given in the specification.
567: The matched host name is the one given on the command line.
1.2 deraadt 568: .Pp
1.1 deraadt 569: Since the first obtained value for each parameter is used, more
570: host-specific declarations should be given near the beginning of the
571: file, and general defaults at the end.
1.2 deraadt 572: .Pp
1.1 deraadt 573: The configuration file has the following format:
1.2 deraadt 574: .Pp
575: Empty lines and lines starting with
576: .Ql #
577: are comments.
578: .Pp
579: Otherwise a line is of the format
580: .Dq keyword arguments .
581: The possible
1.1 deraadt 582: keywords and their meanings are as follows (note that the
583: configuration files are case-sensitive):
1.2 deraadt 584: .Bl -tag -width Ds
585: .It Cm Host
1.1 deraadt 586: Restricts the following declarations (up to the next
1.2 deraadt 587: .Cm Host
1.1 deraadt 588: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 589: given after the keyword.
590: .Ql \&*
591: and
592: .Ql ?
593: can be used as wildcards in the
1.40 aaron 594: patterns.
595: A single
1.2 deraadt 596: .Ql \&*
597: as a pattern can be used to provide global
1.40 aaron 598: defaults for all hosts.
599: The host is the
1.2 deraadt 600: .Ar hostname
1.1 deraadt 601: argument given on the command line (i.e., the name is not converted to
602: a canonicalized host name before matching).
1.2 deraadt 603: .It Cm AFSTokenPassing
1.42 aaron 604: Specifies whether to pass AFS tokens to remote host.
605: The argument to this keyword must be
1.2 deraadt 606: .Dq yes
607: or
608: .Dq no .
609: .It Cm BatchMode
610: If set to
611: .Dq yes ,
1.40 aaron 612: passphrase/password querying will be disabled.
613: This option is useful in scripts and other batch jobs where you have no
614: user to supply the password.
615: The argument must be
1.2 deraadt 616: .Dq yes
617: or
618: .Dq no .
1.34 markus 619: .It Cm CheckHostIP
620: If this flag is set to
621: .Dq yes ,
622: ssh will additionally check the host ip address in the
623: .Pa known_hosts
1.42 aaron 624: file.
625: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 626: If the option is set to
627: .Dq no ,
628: the check will not be executed.
1.2 deraadt 629: .It Cm Cipher
1.40 aaron 630: Specifies the cipher to use for encrypting the session.
631: Currently,
1.2 deraadt 632: .Dq blowfish ,
1.1 deraadt 633: and
1.10 provos 634: .Dq 3des
1.40 aaron 635: are supported.
636: The default is
1.2 deraadt 637: .Dq 3des .
1.45 markus 638: .It Cm Ciphers
639: Specifies the ciphers allowed for protocol version 2
640: in order of preference.
641: Multiple ciphers must be comma-separated.
642: The default is
1.51 markus 643: .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
1.2 deraadt 644: .It Cm Compression
1.40 aaron 645: Specifies whether to use compression.
646: The argument must be
1.2 deraadt 647: .Dq yes
648: or
649: .Dq no .
650: .It Cm CompressionLevel
1.40 aaron 651: Specifies the compression level to use if compression is enable.
652: The argument must be an integer from 1 (fast) to 9 (slow, best).
653: The default level is 6, which is good for most applications.
654: The meaning of the values is the same as in
1.34 markus 655: .Xr gzip 1 .
1.2 deraadt 656: .It Cm ConnectionAttempts
1.1 deraadt 657: Specifies the number of tries (one per second) to make before falling
1.40 aaron 658: back to rsh or exiting.
659: The argument must be an integer.
660: This may be useful in scripts if the connection sometimes fails.
1.50 markus 661: .It Cm DSAAuthentication
662: Specifies whether to try DSA authentication.
663: The argument to this keyword must be
664: .Dq yes
665: or
666: .Dq no .
667: DSA authentication will only be
668: attempted if a DSA identity file exists.
669: Note that this option applies to protocol version 2 only.
1.2 deraadt 670: .It Cm EscapeChar
671: Sets the escape character (default:
672: .Ql ~ ) .
673: The escape character can also
1.40 aaron 674: be set on the command line.
675: The argument should be a single character,
1.2 deraadt 676: .Ql ^
677: followed by a letter, or
678: .Dq none
679: to disable the escape
1.1 deraadt 680: character entirely (making the connection transparent for binary
681: data).
1.44 aaron 682: .It Cm FallBackToRsh
1.1 deraadt 683: Specifies that if connecting via
1.2 deraadt 684: .Nm
1.1 deraadt 685: fails due to a connection refused error (there is no
1.2 deraadt 686: .Xr sshd 8
1.44 aaron 687: listening on the remote host),
1.2 deraadt 688: .Xr rsh 1
1.1 deraadt 689: should automatically be used instead (after a suitable warning about
1.40 aaron 690: the session being unencrypted).
691: The argument must be
1.2 deraadt 692: .Dq yes
693: or
694: .Dq no .
695: .It Cm ForwardAgent
1.1 deraadt 696: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 697: will be forwarded to the remote machine.
698: The argument must be
1.2 deraadt 699: .Dq yes
700: or
1.54 markus 701: .Dq no .
702: The default is
1.2 deraadt 703: .Dq no .
704: .It Cm ForwardX11
1.1 deraadt 705: Specifies whether X11 connections will be automatically redirected
1.44 aaron 706: over the secure channel and
1.2 deraadt 707: .Ev DISPLAY
1.40 aaron 708: set.
1.44 aaron 709: The argument must be
1.2 deraadt 710: .Dq yes
711: or
1.38 markus 712: .Dq no .
713: The default is
1.3 deraadt 714: .Dq no .
715: .It Cm GatewayPorts
716: Specifies whether remote hosts are allowed to connect to local
717: forwarded ports.
718: The argument must be
719: .Dq yes
720: or
721: .Dq no .
722: The default is
1.2 deraadt 723: .Dq no .
724: .It Cm GlobalKnownHostsFile
1.44 aaron 725: Specifies a file to use instead of
1.2 deraadt 726: .Pa /etc/ssh_known_hosts .
727: .It Cm HostName
1.40 aaron 728: Specifies the real host name to log into.
729: This can be used to specify nicknames or abbreviations for hosts.
730: Default is the name given on the command line.
731: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 732: .Cm HostName
1.1 deraadt 733: specifications).
1.2 deraadt 734: .It Cm IdentityFile
1.1 deraadt 735: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 736: is read (default
1.49 markus 737: .Pa $HOME/.ssh/identity
1.2 deraadt 738: in the user's home directory).
1.1 deraadt 739: Additionally, any identities represented by the authentication agent
1.40 aaron 740: will be used for authentication.
741: The file name may use the tilde
742: syntax to refer to a user's home directory.
743: It is possible to have
1.1 deraadt 744: multiple identity files specified in configuration files; all these
745: identities will be tried in sequence.
1.48 markus 746: .It Cm IdentityFile2
747: Specifies the file from which the user's DSA authentication identity
748: is read (default
1.49 markus 749: .Pa $HOME/.ssh/id_dsa
1.48 markus 750: in the user's home directory).
751: The file name may use the tilde
752: syntax to refer to a user's home directory.
753: It is possible to have
754: multiple identity files specified in configuration files; all these
755: identities will be tried in sequence.
1.2 deraadt 756: .It Cm KeepAlive
1.1 deraadt 757: Specifies whether the system should send keepalive messages to the
1.40 aaron 758: other side.
759: If they are sent, death of the connection or crash of one
760: of the machines will be properly noticed.
761: However, this means that
1.1 deraadt 762: connections will die if the route is down temporarily, and some people
1.41 aaron 763: find it annoying.
1.2 deraadt 764: .Pp
765: The default is
766: .Dq yes
767: (to send keepalives), and the client will notice
1.40 aaron 768: if the network goes down or the remote host dies.
769: This is important in scripts, and many users want it too.
1.2 deraadt 770: .Pp
771: To disable keepalives, the value should be set to
772: .Dq no
773: in both the server and the client configuration files.
774: .It Cm KerberosAuthentication
1.42 aaron 775: Specifies whether Kerberos authentication will be used.
776: The argument to this keyword must be
1.4 dugsong 777: .Dq yes
778: or
779: .Dq no .
1.2 deraadt 780: .It Cm KerberosTgtPassing
1.42 aaron 781: Specifies whether a Kerberos TGT will be forwarded to the server.
782: This will only work if the Kerberos server is actually an AFS kaserver.
783: The argument to this keyword must be
1.4 dugsong 784: .Dq yes
785: or
786: .Dq no .
1.2 deraadt 787: .It Cm LocalForward
1.1 deraadt 788: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 789: the secure channel to given host:port from the remote machine.
790: The first argument must be a port number, and the second must be
791: host:port.
792: Multiple forwardings may be specified, and additional
793: forwardings can be given on the command line.
794: Only the superuser can forward privileged ports.
1.24 markus 795: .It Cm LogLevel
796: Gives the verbosity level that is used when logging messages from
797: .Nm ssh .
798: The possible values are:
1.39 djm 799: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.24 markus 800: The default is INFO.
1.14 dugsong 801: .It Cm NumberOfPasswordPrompts
1.42 aaron 802: Specifies the number of password prompts before giving up.
803: The argument to this keyword must be an integer.
804: Default is 3.
1.34 markus 805: .It Cm PasswordAuthentication
1.40 aaron 806: Specifies whether to use password authentication.
807: The argument to this keyword must be
1.34 markus 808: .Dq yes
809: or
810: .Dq no .
1.50 markus 811: Note that this option applies to both protocol version 1 and 2.
1.2 deraadt 812: .It Cm Port
1.40 aaron 813: Specifies the port number to connect on the remote host.
814: Default is 22.
1.45 markus 815: .It Cm Protocol
816: Specifies the protocol versions
817: .Nm
818: should support in order of preference.
819: The possible values are
820: .Dq 1
821: and
822: .Dq 2 .
823: Multiple versions must be comma-separated.
824: The default is
1.49 markus 825: .Dq 1,2 .
826: This means that
827: .Nm
828: tries version 1 and falls back to version 2
1.52 hugh 829: if version 1 is not available.
1.2 deraadt 830: .It Cm ProxyCommand
1.40 aaron 831: Specifies the command to use to connect to the server.
832: The command
833: string extends to the end of the line, and is executed with
834: .Pa /bin/sh .
835: In the command string,
836: .Ql %h
837: will be substituted by the host name to
838: connect and
839: .Ql %p
840: by the port.
841: The command can be basically anything,
842: and should read from its standard input and write to its standard output.
843: It should eventually connect an
1.2 deraadt 844: .Xr sshd 8
1.1 deraadt 845: server running on some machine, or execute
1.2 deraadt 846: .Ic sshd -i
1.40 aaron 847: somewhere.
848: Host key management will be done using the
1.1 deraadt 849: HostName of the host being connected (defaulting to the name typed by
850: the user).
1.29 markus 851: Note that
852: .Cm CheckHostIP
853: is not available for connects with a proxy command.
1.2 deraadt 854: .Pp
855: .It Cm RemoteForward
1.1 deraadt 856: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 857: the secure channel to given host:port from the local machine.
858: The first argument must be a port number, and the second must be
859: host:port.
860: Multiple forwardings may be specified, and additional
861: forwardings can be given on the command line.
862: Only the superuser can forward privileged ports.
1.2 deraadt 863: .It Cm RhostsAuthentication
1.40 aaron 864: Specifies whether to try rhosts based authentication.
865: Note that this
1.1 deraadt 866: declaration only affects the client side and has no effect whatsoever
1.40 aaron 867: on security.
868: Disabling rhosts authentication may reduce
1.1 deraadt 869: authentication time on slow connections when rhosts authentication is
1.40 aaron 870: not used.
871: Most servers do not permit RhostsAuthentication because it
872: is not secure (see RhostsRSAAuthentication).
873: The argument to this keyword must be
1.2 deraadt 874: .Dq yes
875: or
876: .Dq no .
877: .It Cm RhostsRSAAuthentication
1.1 deraadt 878: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 879: authentication.
880: This is the primary authentication method for most sites.
881: The argument must be
1.2 deraadt 882: .Dq yes
883: or
884: .Dq no .
885: .It Cm RSAAuthentication
1.40 aaron 886: Specifies whether to try RSA authentication.
887: The argument to this keyword must be
1.2 deraadt 888: .Dq yes
889: or
890: .Dq no .
1.1 deraadt 891: RSA authentication will only be
892: attempted if the identity file exists, or an authentication agent is
893: running.
1.50 markus 894: Note that this option applies to protocol version 1 only.
1.27 markus 895: .It Cm SkeyAuthentication
896: Specifies whether to use
897: .Xr skey 1
1.40 aaron 898: authentication.
899: The argument to this keyword must be
1.27 markus 900: .Dq yes
901: or
902: .Dq no .
903: The default is
904: .Dq no .
1.2 deraadt 905: .It Cm StrictHostKeyChecking
906: If this flag is set to
1.44 aaron 907: .Dq yes ,
1.2 deraadt 908: .Nm
1.1 deraadt 909: ssh will never automatically add host keys to the
1.2 deraadt 910: .Pa $HOME/.ssh/known_hosts
1.48 markus 911: and
912: .Pa $HOME/.ssh/known_hosts2
913: files, and refuses to connect hosts whose host key has changed.
1.40 aaron 914: This provides maximum protection against trojan horse attacks.
915: However, it can be somewhat annoying if you don't have good
1.2 deraadt 916: .Pa /etc/ssh_known_hosts
1.48 markus 917: and
918: .Pa /etc/ssh_known_hosts2
1.1 deraadt 919: files installed and frequently
1.40 aaron 920: connect new hosts.
921: Basically this option forces the user to manually
922: add any new hosts.
923: Normally this option is disabled, and new hosts
924: will automatically be added to the known host files.
925: The host keys of
926: known hosts will be verified automatically in either case.
927: The argument must be
1.2 deraadt 928: .Dq yes
929: or
930: .Dq no .
1.16 markus 931: .It Cm UsePrivilegedPort
932: Specifies whether to use a privileged port for outgoing connections.
933: The argument must be
934: .Dq yes
935: or
936: .Dq no .
937: The default is
938: .Dq yes .
939: Note that setting this option to
940: .Dq no
1.30 provos 941: turns off
1.16 markus 942: .Cm RhostsAuthentication
943: and
944: .Cm RhostsRSAAuthentication .
1.34 markus 945: .It Cm User
1.40 aaron 946: Specifies the user to log in as.
947: This can be useful if you have a different user name on different machines.
948: This saves the trouble of
1.34 markus 949: having to remember to give the user name on the command line.
950: .It Cm UserKnownHostsFile
951: Specifies a file to use instead of
952: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 953: .It Cm UseRsh
1.40 aaron 954: Specifies that rlogin/rsh should be used for this host.
955: It is possible that the host does not at all support the
1.2 deraadt 956: .Nm
1.40 aaron 957: protocol.
958: This causes
1.2 deraadt 959: .Nm
1.40 aaron 960: to immediately execute
1.2 deraadt 961: .Xr rsh 1 .
1.1 deraadt 962: All other options (except
1.2 deraadt 963: .Cm HostName )
1.40 aaron 964: are ignored if this has been specified.
965: The argument must be
1.2 deraadt 966: .Dq yes
967: or
968: .Dq no .
1.55 markus 969: .It Cm XAuthLocation
970: Specifies the location of the
971: .Xr xauth 1
972: program.
973: The default is
974: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 975: .El
1.2 deraadt 976: .Sh ENVIRONMENT
977: .Nm
1.1 deraadt 978: will normally set the following environment variables:
1.2 deraadt 979: .Bl -tag -width Ds
980: .It Ev DISPLAY
981: The
982: .Ev DISPLAY
1.40 aaron 983: variable indicates the location of the X11 server.
1.44 aaron 984: It is automatically set by
1.2 deraadt 985: .Nm
986: to point to a value of the form
987: .Dq hostname:n
988: where hostname indicates
1.40 aaron 989: the host where the shell runs, and n is an integer >= 1.
990: .Nm
991: uses this special value to forward X11 connections over the secure
992: channel.
993: The user should normally not set DISPLAY explicitly, as that
1.1 deraadt 994: will render the X11 connection insecure (and will require the user to
995: manually copy any required authorization cookies).
1.2 deraadt 996: .It Ev HOME
1.1 deraadt 997: Set to the path of the user's home directory.
1.2 deraadt 998: .It Ev LOGNAME
999: Synonym for
1.12 aaron 1000: .Ev USER ;
1001: set for compatibility with systems that use this variable.
1.2 deraadt 1002: .It Ev MAIL
1.1 deraadt 1003: Set to point the user's mailbox.
1.40 aaron 1004: .It Ev PATH
1.2 deraadt 1005: Set to the default
1006: .Ev PATH ,
1007: as specified when compiling
1.12 aaron 1008: .Nm ssh .
1.18 markus 1009: .It Ev SSH_AUTH_SOCK
1.17 markus 1010: indicates the path of a unix-domain socket used to communicate with the
1011: agent.
1.2 deraadt 1012: .It Ev SSH_CLIENT
1.40 aaron 1013: Identifies the client end of the connection.
1014: The variable contains
1.1 deraadt 1015: three space-separated values: client ip-address, client port number,
1016: and server port number.
1.2 deraadt 1017: .It Ev SSH_TTY
1.1 deraadt 1018: This is set to the name of the tty (path to the device) associated
1.40 aaron 1019: with the current shell or command.
1020: If the current session has no tty,
1.1 deraadt 1021: this variable is not set.
1.2 deraadt 1022: .It Ev TZ
1.1 deraadt 1023: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1024: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1025: on to new connections).
1.2 deraadt 1026: .It Ev USER
1.1 deraadt 1027: Set to the name of the user logging in.
1.2 deraadt 1028: .El
1029: .Pp
1.44 aaron 1030: Additionally,
1.2 deraadt 1031: .Nm
1.44 aaron 1032: reads
1033: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1034: and adds lines of the format
1035: .Dq VARNAME=value
1.12 aaron 1036: to the environment.
1.2 deraadt 1037: .Sh FILES
1.36 markus 1038: .Bl -tag -width Ds
1.2 deraadt 1039: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 1040: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1041: in
1042: .Pa /etc/ssh_known_hosts ) .
1043: See
1044: .Xr sshd 8 .
1.48 markus 1045: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1046: Contains the RSA and the DSA authentication identity of the user.
1047: These files
1048: contain sensitive data and should be readable by the user but not
1.15 markus 1049: accessible by others (read/write/execute).
1050: Note that
1051: .Nm
1.48 markus 1052: ignores a private key file if it is accessible by others.
1.15 markus 1053: It is possible to specify a passphrase when
1.1 deraadt 1054: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1055: sensitive part of this file using 3DES.
1.48 markus 1056: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1.1 deraadt 1057: Contains the public key for authentication (public part of the
1.40 aaron 1058: identity file in human-readable form).
1.48 markus 1059: The contents of the
1060: .Pa $HOME/.ssh/identity.pub
1061: file should be added to
1.2 deraadt 1062: .Pa $HOME/.ssh/authorized_keys
1063: on all machines
1.40 aaron 1064: where you wish to log in using RSA authentication.
1.48 markus 1065: The contents of the
1066: .Pa $HOME/.ssh/id_dsa.pub
1067: file should be added to
1068: .Pa $HOME/.ssh/authorized_keys2
1069: on all machines
1070: where you wish to log in using DSA authentication.
1071: These files are not
1.40 aaron 1072: sensitive and can (but need not) be readable by anyone.
1.48 markus 1073: These files are
1074: never used automatically and are not necessary; they is only provided for
1.1 deraadt 1075: the convenience of the user.
1.2 deraadt 1076: .It Pa $HOME/.ssh/config
1.40 aaron 1077: This is the per-user configuration file.
1078: The format of this file is described above.
1079: This file is used by the
1.2 deraadt 1080: .Nm
1.40 aaron 1081: client.
1082: This file does not usually contain any sensitive information,
1.1 deraadt 1083: but the recommended permissions are read/write for the user, and not
1084: accessible by others.
1.2 deraadt 1085: .It Pa $HOME/.ssh/authorized_keys
1.40 aaron 1086: Lists the RSA keys that can be used for logging in as this user.
1087: The format of this file is described in the
1.2 deraadt 1088: .Xr sshd 8
1.40 aaron 1089: manual page.
1090: In the simplest form the format is the same as the .pub
1.1 deraadt 1091: identity files (that is, each line contains the number of bits in
1092: modulus, public exponent, modulus, and comment fields, separated by
1.40 aaron 1093: spaces).
1094: This file is not highly sensitive, but the recommended
1.1 deraadt 1095: permissions are read/write for the user, and not accessible by others.
1.48 markus 1096: .It Pa $HOME/.ssh/authorized_keys2
1097: Lists the DSA keys that can be used for logging in as this user.
1098: This file is not highly sensitive, but the recommended
1099: permissions are read/write for the user, and not accessible by others.
1100: .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1.40 aaron 1101: Systemwide list of known host keys.
1.48 markus 1102: .Pa /etc/ssh_known_hosts
1103: contains RSA and
1104: .Pa /etc/ssh_known_hosts2
1105: contains DSA keys.
1106: These files should be prepared by the
1.1 deraadt 1107: system administrator to contain the public host keys of all machines in the
1.40 aaron 1108: organization.
1109: This file should be world-readable.
1110: This file contains
1.1 deraadt 1111: public keys, one per line, in the following format (fields separated
1112: by spaces): system name, number of bits in modulus, public exponent,
1.40 aaron 1113: modulus, and optional comment field.
1114: When different names are used
1.1 deraadt 1115: for the same machine, all such names should be listed, separated by
1.40 aaron 1116: commas.
1117: The format is described on the
1.2 deraadt 1118: .Xr sshd 8
1.1 deraadt 1119: manual page.
1.2 deraadt 1120: .Pp
1.1 deraadt 1121: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1122: .Xr sshd 8
1.1 deraadt 1123: to verify the client host when logging in; other names are needed because
1.2 deraadt 1124: .Nm
1.1 deraadt 1125: does not convert the user-supplied name to a canonical name before
1126: checking the key, because someone with access to the name servers
1127: would then be able to fool host authentication.
1.2 deraadt 1128: .It Pa /etc/ssh_config
1.40 aaron 1129: Systemwide configuration file.
1130: This file provides defaults for those
1.1 deraadt 1131: values that are not specified in the user's configuration file, and
1.40 aaron 1132: for those users who do not have a configuration file.
1133: This file must be world-readable.
1.2 deraadt 1134: .It Pa $HOME/.rhosts
1135: This file is used in
1136: .Pa \&.rhosts
1137: authentication to list the
1.40 aaron 1138: host/user pairs that are permitted to log in.
1139: (Note that this file is
1.1 deraadt 1140: also used by rlogin and rsh, which makes using this file insecure.)
1141: Each line of the file contains a host name (in the canonical form
1142: returned by name servers), and then a user name on that host,
1.40 aaron 1143: separated by a space.
1144: One some machines this file may need to be
1.1 deraadt 1145: world-readable if the user's home directory is on a NFS partition,
1146: because
1.2 deraadt 1147: .Xr sshd 8
1.40 aaron 1148: reads it as root.
1149: Additionally, this file must be owned by the user,
1150: and must not have write permissions for anyone else.
1151: The recommended
1.1 deraadt 1152: permission for most machines is read/write for the user, and not
1153: accessible by others.
1.2 deraadt 1154: .Pp
1.1 deraadt 1155: Note that by default
1.2 deraadt 1156: .Xr sshd 8
1.1 deraadt 1157: will be installed so that it requires successful RSA host
1.40 aaron 1158: authentication before permitting \s+2.\s0rhosts authentication.
1159: If your server machine does not have the client's host key in
1.2 deraadt 1160: .Pa /etc/ssh_known_hosts ,
1161: you can store it in
1162: .Pa $HOME/.ssh/known_hosts .
1163: The easiest way to do this is to
1.1 deraadt 1164: connect back to the client from the server machine using ssh; this
1.48 markus 1165: will automatically add the host key to
1.2 deraadt 1166: .Pa $HOME/.ssh/known_hosts .
1167: .It Pa $HOME/.shosts
1168: This file is used exactly the same way as
1169: .Pa \&.rhosts .
1170: The purpose for
1.1 deraadt 1171: having this file is to be able to use rhosts authentication with
1.2 deraadt 1172: .Nm
1173: without permitting login with
1174: .Xr rlogin 1
1175: or
1176: .Xr rsh 1 .
1177: .It Pa /etc/hosts.equiv
1178: This file is used during
1.40 aaron 1179: .Pa \&.rhosts authentication.
1180: It contains
1.1 deraadt 1181: canonical hosts names, one per line (the full format is described on
1182: the
1.2 deraadt 1183: .Xr sshd 8
1.40 aaron 1184: manual page).
1185: If the client host is found in this file, login is
1.1 deraadt 1186: automatically permitted provided client and server user names are the
1.40 aaron 1187: same.
1188: Additionally, successful RSA host authentication is normally
1189: required.
1190: This file should only be writable by root.
1.2 deraadt 1191: .It Pa /etc/shosts.equiv
1.44 aaron 1192: This file is processed exactly as
1.2 deraadt 1193: .Pa /etc/hosts.equiv .
1.1 deraadt 1194: This file may be useful to permit logins using
1.2 deraadt 1195: .Nm
1.1 deraadt 1196: but not using rsh/rlogin.
1.2 deraadt 1197: .It Pa /etc/sshrc
1.1 deraadt 1198: Commands in this file are executed by
1.2 deraadt 1199: .Nm
1.1 deraadt 1200: when the user logs in just before the user's shell (or command) is started.
1201: See the
1.2 deraadt 1202: .Xr sshd 8
1.1 deraadt 1203: manual page for more information.
1.2 deraadt 1204: .It Pa $HOME/.ssh/rc
1.1 deraadt 1205: Commands in this file are executed by
1.2 deraadt 1206: .Nm
1.1 deraadt 1207: when the user logs in just before the user's shell (or command) is
1208: started.
1.44 aaron 1209: See the
1.2 deraadt 1210: .Xr sshd 8
1.1 deraadt 1211: manual page for more information.
1.31 markus 1212: .It Pa $HOME/.ssh/environment
1213: Contains additional definitions for environment variables, see section
1214: .Sx ENVIRONMENT
1215: above.
1.5 deraadt 1216: .It Pa libcrypto.so.X.1
1217: A version of this library which includes support for the RSA algorithm
1218: is required for proper operation.
1.58 itojun 1219: .El
1.2 deraadt 1220: .Sh AUTHOR
1.20 provos 1221: OpenSSH
1.37 deraadt 1222: is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1.40 aaron 1223: but with bugs removed and newer features re-added.
1224: Rapidly after the
1.37 deraadt 1225: 1.2.12 release, newer versions of the original ssh bore successively
1226: more restrictive licenses, and thus demand for a free version was born.
1.47 markus 1227: .Pp
1.37 deraadt 1228: This version of OpenSSH
1.20 provos 1229: .Bl -bullet
1230: .It
1.35 aaron 1231: has all components of a restrictive nature (i.e., patents, see
1.21 deraadt 1232: .Xr ssl 8 )
1233: directly removed from the source code; any licensed or patented components
1234: are chosen from
1235: external libraries.
1.20 provos 1236: .It
1.47 markus 1237: has been updated to support SSH protocol 1.5 and 2, making it compatible with
1238: all other SSH clients and servers.
1.20 provos 1239: .It
1.44 aaron 1240: contains added support for
1.20 provos 1241: .Xr kerberos 8
1242: authentication and ticket passing.
1243: .It
1.21 deraadt 1244: supports one-time password authentication with
1.20 provos 1245: .Xr skey 1 .
1246: .El
1.25 provos 1247: .Pp
1.26 aaron 1248: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1.25 provos 1249: Niels Provos, Theo de Raadt, and Dug Song.
1.47 markus 1250: .Pp
1251: The support for SSH protocol 2 was written by Markus Friedl.
1.2 deraadt 1252: .Sh SEE ALSO
1253: .Xr rlogin 1 ,
1254: .Xr rsh 1 ,
1255: .Xr scp 1 ,
1256: .Xr ssh-add 1 ,
1257: .Xr ssh-agent 1 ,
1258: .Xr ssh-keygen 1 ,
1259: .Xr telnet 1 ,
1.5 deraadt 1260: .Xr sshd 8 ,
1261: .Xr ssl 8
![[BACK]](/icons/back.gif){kind=icon}
Return to ssh.1 CVS log ![[TXT]](/icons/text.gif){kind=icon}
![[DIR]](/icons/dir.gif){kind=icon}
Up to [local] / src / usr.bin / ssh