Annotation of src/usr.bin/ssh/ssh.1, Revision 1.175.2.1
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: .\"
1.175.2.1! brad 37: .\" $OpenBSD: ssh.1,v 1.181 2003/12/16 15:49:51 markus Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSH 1
40: .Os
41: .Sh NAME
42: .Nm ssh
1.96 deraadt 43: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm ssh
1.175.2.1! brad 46: .Op Fl 1246AaCfgkNnqsTtVvXxY
1.108 markus 47: .Op Fl b Ar bind_address
1.51 markus 48: .Op Fl c Ar cipher_spec
1.175.2.1! brad 49: .Op Fl D Ar port
1.2 deraadt 50: .Op Fl e Ar escape_char
1.132 markus 51: .Op Fl F Ar configfile
1.175.2.1! brad 52: .Op Fl i Ar identity_file
! 53: .Bk -words
1.12 aaron 54: .Oo Fl L Xo
55: .Sm off
1.33 markus 56: .Ar port :
1.12 aaron 57: .Ar host :
58: .Ar hostport
59: .Sm on
60: .Xc
61: .Oc
1.168 jmc 62: .Ek
1.175.2.1! brad 63: .Op Fl l Ar login_name
! 64: .Op Fl m Ar mac_spec
! 65: .Op Fl o Ar option
1.168 jmc 66: .Bk -words
1.175.2.1! brad 67: .Op Fl p Ar port
! 68: .Ek
1.12 aaron 69: .Oo Fl R Xo
70: .Sm off
1.33 markus 71: .Ar port :
1.12 aaron 72: .Ar host :
73: .Ar hostport
74: .Sm on
75: .Xc
76: .Oc
1.175.2.1! brad 77: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 78: .Op Ar command
1.44 aaron 79: .Sh DESCRIPTION
1.2 deraadt 80: .Nm
1.96 deraadt 81: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 82: executing commands on a remote machine.
1.175.2.1! brad 83: It is intended to replace rlogin and rsh,
! 84: and provide secure encrypted communications between
1.40 aaron 85: two untrusted hosts over an insecure network.
1.175.2.1! brad 86: X11 connections and arbitrary TCP/IP ports
! 87: can also be forwarded over the secure channel.
1.2 deraadt 88: .Pp
89: .Nm
1.44 aaron 90: connects and logs into the specified
1.175.2.1! brad 91: .Ar hostname
! 92: (with optional
! 93: .Ar user
! 94: name).
1.1 deraadt 95: The user must prove
1.49 markus 96: his/her identity to the remote machine using one of several methods
1.175.2.1! brad 97: depending on the protocol version used.
1.49 markus 98: .Pp
1.175.2.1! brad 99: If
! 100: .Ar command
! 101: is specified,
! 102: .Ar command
! 103: is executed on the remote host instead of a login shell.
1.49 markus 104: .Ss SSH protocol version 1
1.1 deraadt 105: First, if the machine the user logs in from is listed in
1.2 deraadt 106: .Pa /etc/hosts.equiv
1.1 deraadt 107: or
1.2 deraadt 108: .Pa /etc/shosts.equiv
1.1 deraadt 109: on the remote machine, and the user names are
110: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 111: Second, if
1.175.2.1! brad 112: .Pa .rhosts
1.1 deraadt 113: or
1.175.2.1! brad 114: .Pa .shosts
1.1 deraadt 115: exists in the user's home directory on the
116: remote machine and contains a line containing the name of the client
117: machine and the name of the user on that machine, the user is
1.40 aaron 118: permitted to log in.
119: This form of authentication alone is normally not
1.1 deraadt 120: allowed by the server because it is not secure.
1.2 deraadt 121: .Pp
1.107 markus 122: The second authentication method is the
1.175.2.1! brad 123: .Em rhosts
1.1 deraadt 124: or
1.175.2.1! brad 125: .Em hosts.equiv
1.40 aaron 126: method combined with RSA-based host authentication.
127: It means that if the login would be permitted by
1.49 markus 128: .Pa $HOME/.rhosts ,
129: .Pa $HOME/.shosts ,
1.2 deraadt 130: .Pa /etc/hosts.equiv ,
1.1 deraadt 131: or
1.2 deraadt 132: .Pa /etc/shosts.equiv ,
1.11 deraadt 133: and if additionally the server can verify the client's
1.44 aaron 134: host key (see
1.147 deraadt 135: .Pa /etc/ssh/ssh_known_hosts
1.23 markus 136: and
137: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 138: in the
1.2 deraadt 139: .Sx FILES
1.175.2.1! brad 140: section), only then is login permitted.
1.40 aaron 141: This authentication method closes security holes due to IP
142: spoofing, DNS spoofing and routing spoofing.
143: [Note to the administrator:
1.2 deraadt 144: .Pa /etc/hosts.equiv ,
1.49 markus 145: .Pa $HOME/.rhosts ,
1.1 deraadt 146: and the rlogin/rsh protocol in general, are inherently insecure and should be
147: disabled if security is desired.]
1.2 deraadt 148: .Pp
1.44 aaron 149: As a third authentication method,
1.2 deraadt 150: .Nm
1.1 deraadt 151: supports RSA based authentication.
152: The scheme is based on public-key cryptography: there are cryptosystems
153: where encryption and decryption are done using separate keys, and it
154: is not possible to derive the decryption key from the encryption key.
1.40 aaron 155: RSA is one such system.
1.44 aaron 156: The idea is that each user creates a public/private
1.40 aaron 157: key pair for authentication purposes.
158: The server knows the public key, and only the user knows the private key.
1.175.2.1! brad 159: .Pp
1.44 aaron 160: The file
1.2 deraadt 161: .Pa $HOME/.ssh/authorized_keys
1.175.2.1! brad 162: lists the public keys that are permitted for logging in.
1.40 aaron 163: When the user logs in, the
1.2 deraadt 164: .Nm
1.1 deraadt 165: program tells the server which key pair it would like to use for
1.40 aaron 166: authentication.
1.175.2.1! brad 167: The server checks if this key is permitted, and if so,
! 168: sends the user (actually the
1.2 deraadt 169: .Nm
1.1 deraadt 170: program running on behalf of the user) a challenge, a random number,
1.40 aaron 171: encrypted by the user's public key.
1.175.2.1! brad 172: The challenge can only be decrypted using the proper private key.
! 173: The user's client then decrypts the challenge using the private key,
! 174: proving that he/she knows the private key
! 175: but without disclosing it to the server.
1.2 deraadt 176: .Pp
177: .Nm
1.40 aaron 178: implements the RSA authentication protocol automatically.
179: The user creates his/her RSA key pair by running
1.2 deraadt 180: .Xr ssh-keygen 1 .
1.44 aaron 181: This stores the private key in
1.49 markus 182: .Pa $HOME/.ssh/identity
1.175.2.1! brad 183: and stores the public key in
1.49 markus 184: .Pa $HOME/.ssh/identity.pub
1.40 aaron 185: in the user's home directory.
186: The user should then copy the
1.2 deraadt 187: .Pa identity.pub
1.44 aaron 188: to
1.49 markus 189: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 190: in his/her home directory on the remote machine (the
1.2 deraadt 191: .Pa authorized_keys
1.44 aaron 192: file corresponds to the conventional
1.49 markus 193: .Pa $HOME/.rhosts
1.1 deraadt 194: file, and has one key
1.40 aaron 195: per line, though the lines can be very long).
196: After this, the user can log in without giving the password.
1.175.2.1! brad 197: RSA authentication is much more secure than
! 198: .Em rhosts
! 199: authentication.
1.2 deraadt 200: .Pp
1.1 deraadt 201: The most convenient way to use RSA authentication may be with an
1.40 aaron 202: authentication agent.
203: See
1.2 deraadt 204: .Xr ssh-agent 1
1.1 deraadt 205: for more information.
1.2 deraadt 206: .Pp
1.44 aaron 207: If other authentication methods fail,
1.2 deraadt 208: .Nm
1.40 aaron 209: prompts the user for a password.
210: The password is sent to the remote
1.1 deraadt 211: host for checking; however, since all communications are encrypted,
212: the password cannot be seen by someone listening on the network.
1.49 markus 213: .Ss SSH protocol version 2
1.175.2.1! brad 214: When a user connects using protocol version 2,
1.145 markus 215: similar authentication methods are available.
1.107 markus 216: Using the default values for
217: .Cm PreferredAuthentications ,
1.123 markus 218: the client will try to authenticate first using the hostbased method;
1.175.2.1! brad 219: if this method fails, public key authentication is attempted,
! 220: and finally if this method fails, keyboard-interactive and
1.123 markus 221: password authentication are tried.
1.49 markus 222: .Pp
223: The public key method is similar to RSA authentication described
1.107 markus 224: in the previous section and allows the RSA or DSA algorithm to be used:
1.102 itojun 225: The client uses his private key,
1.49 markus 226: .Pa $HOME/.ssh/id_dsa
1.102 itojun 227: or
228: .Pa $HOME/.ssh/id_rsa ,
1.49 markus 229: to sign the session identifier and sends the result to the server.
230: The server checks whether the matching public key is listed in
1.115 markus 231: .Pa $HOME/.ssh/authorized_keys
1.49 markus 232: and grants access if both the key is found and the signature is correct.
233: The session identifier is derived from a shared Diffie-Hellman value
234: and is only known to the client and the server.
235: .Pp
1.175.2.1! brad 236: If public key authentication fails or is not available, a password
! 237: can be sent encrypted to the remote host to prove the user's identity.
1.107 markus 238: .Pp
239: Additionally,
240: .Nm
241: supports hostbased or challenge response authentication.
1.49 markus 242: .Pp
243: Protocol 2 provides additional mechanisms for confidentiality
1.51 markus 244: (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
1.94 deraadt 245: and integrity (hmac-md5, hmac-sha1).
1.49 markus 246: Note that protocol 1 lacks a strong mechanism for ensuring the
247: integrity of the connection.
248: .Ss Login session and remote execution
1.1 deraadt 249: When the user's identity has been accepted by the server, the server
250: either executes the given command, or logs into the machine and gives
1.40 aaron 251: the user a normal shell on the remote machine.
252: All communication with
1.1 deraadt 253: the remote command or shell will be automatically encrypted.
1.2 deraadt 254: .Pp
1.1 deraadt 255: If a pseudo-terminal has been allocated (normal login session), the
1.104 djm 256: user may use the escape characters noted below.
1.2 deraadt 257: .Pp
1.175.2.1! brad 258: If no pseudo-tty has been allocated,
! 259: the session is transparent and can be used to reliably transfer binary data.
1.40 aaron 260: On most systems, setting the escape character to
1.2 deraadt 261: .Dq none
262: will also make the session transparent even if a tty is used.
263: .Pp
1.71 djm 264: The session terminates when the command or shell on the remote
1.92 markus 265: machine exits and all X11 and TCP/IP connections have been closed.
1.175.2.1! brad 266: The exit status of the remote program is returned as the exit status of
1.2 deraadt 267: .Nm ssh .
1.104 djm 268: .Ss Escape Characters
1.175.2.1! brad 269: When a pseudo-terminal has been requested,
! 270: .Nm
! 271: supports a number of functions through the use of an escape character.
1.104 djm 272: .Pp
273: A single tilde character can be sent as
274: .Ic ~~
1.119 stevesk 275: or by following the tilde by a character other than those described below.
1.104 djm 276: The escape character must always follow a newline to be interpreted as
277: special.
278: The escape character can be changed in configuration files using the
279: .Cm EscapeChar
1.117 itojun 280: configuration directive or on the command line by the
1.104 djm 281: .Fl e
282: option.
283: .Pp
284: The supported escapes (assuming the default
285: .Ql ~ )
286: are:
287: .Bl -tag -width Ds
288: .It Cm ~.
1.175.2.1! brad 289: Disconnect.
1.104 djm 290: .It Cm ~^Z
1.175.2.1! brad 291: Background
! 292: .Nm ssh .
1.104 djm 293: .It Cm ~#
1.175.2.1! brad 294: List forwarded connections.
1.104 djm 295: .It Cm ~&
1.175.2.1! brad 296: Background
! 297: .Nm
! 298: at logout when waiting for forwarded connection / X11 sessions to terminate.
1.104 djm 299: .It Cm ~?
1.175.2.1! brad 300: Display a list of escape characters.
1.170 markus 301: .It Cm ~B
1.175.2.1! brad 302: Send a BREAK to the remote system
! 303: (only useful for SSH protocol version 2 and if the peer supports it).
1.149 jakob 304: .It Cm ~C
305: Open command line (only useful for adding port forwardings using the
306: .Fl L
307: and
308: .Fl R
1.175.2.1! brad 309: options).
1.104 djm 310: .It Cm ~R
1.175.2.1! brad 311: Request rekeying of the connection
! 312: (only useful for SSH protocol version 2 and if the peer supports it).
1.104 djm 313: .El
1.49 markus 314: .Ss X11 and TCP forwarding
1.110 deraadt 315: If the
316: .Cm ForwardX11
317: variable is set to
318: .Dq yes
1.175.2.1! brad 319: (or see the description of the
1.110 deraadt 320: .Fl X
321: and
322: .Fl x
323: options described later)
324: and the user is using X11 (the
1.2 deraadt 325: .Ev DISPLAY
1.1 deraadt 326: environment variable is set), the connection to the X11 display is
327: automatically forwarded to the remote side in such a way that any X11
328: programs started from the shell (or command) will go through the
329: encrypted channel, and the connection to the real X server will be made
1.40 aaron 330: from the local machine.
331: The user should not manually set
1.2 deraadt 332: .Ev DISPLAY .
1.1 deraadt 333: Forwarding of X11 connections can be
334: configured on the command line or in configuration files.
1.2 deraadt 335: .Pp
336: The
1.44 aaron 337: .Ev DISPLAY
1.2 deraadt 338: value set by
339: .Nm
1.175.2.1! brad 340: will point to the server machine, but with a display number greater than zero.
1.40 aaron 341: This is normal, and happens because
1.2 deraadt 342: .Nm
343: creates a
344: .Dq proxy
345: X server on the server machine for forwarding the
1.1 deraadt 346: connections over the encrypted channel.
1.2 deraadt 347: .Pp
348: .Nm
1.1 deraadt 349: will also automatically set up Xauthority data on the server machine.
350: For this purpose, it will generate a random authorization cookie,
351: store it in Xauthority on the server, and verify that any forwarded
352: connections carry this cookie and replace it by the real cookie when
1.40 aaron 353: the connection is opened.
354: The real authentication cookie is never
1.1 deraadt 355: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 356: .Pp
1.163 stevesk 357: If the
358: .Cm ForwardAgent
359: variable is set to
360: .Dq yes
1.175.2.1! brad 361: (or see the description of the
1.163 stevesk 362: .Fl A
363: and
364: .Fl a
1.168 jmc 365: options described later) and
1.163 stevesk 366: the user is using an authentication agent, the connection to the agent
367: is automatically forwarded to the remote side.
1.2 deraadt 368: .Pp
1.1 deraadt 369: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.120 stevesk 370: be specified either on the command line or in a configuration file.
1.40 aaron 371: One possible application of TCP/IP forwarding is a secure connection to an
1.92 markus 372: electronic purse; another is going through firewalls.
1.49 markus 373: .Ss Server authentication
1.2 deraadt 374: .Nm
1.49 markus 375: automatically maintains and checks a database containing
1.40 aaron 376: identifications for all hosts it has ever been used with.
1.116 markus 377: Host keys are stored in
1.49 markus 378: .Pa $HOME/.ssh/known_hosts
1.40 aaron 379: in the user's home directory.
1.116 markus 380: Additionally, the file
1.147 deraadt 381: .Pa /etc/ssh/ssh_known_hosts
1.116 markus 382: is automatically checked for known hosts.
1.40 aaron 383: Any new hosts are automatically added to the user's file.
1.175.2.1! brad 384: If a host's identification ever changes,
1.2 deraadt 385: .Nm
1.1 deraadt 386: warns about this and disables password authentication to prevent a
1.40 aaron 387: trojan horse from getting the user's password.
1.175.2.1! brad 388: Another purpose of this mechanism is to prevent man-in-the-middle attacks
! 389: which could otherwise be used to circumvent the encryption.
1.40 aaron 390: The
1.2 deraadt 391: .Cm StrictHostKeyChecking
1.158 stevesk 392: option can be used to prevent logins to machines whose
1.1 deraadt 393: host key is not known or has changed.
1.65 aaron 394: .Pp
395: The options are as follows:
1.2 deraadt 396: .Bl -tag -width Ds
1.175.2.1! brad 397: .It Fl 1
! 398: Forces
! 399: .Nm
! 400: to try protocol version 1 only.
! 401: .It Fl 2
! 402: Forces
! 403: .Nm
! 404: to try protocol version 2 only.
! 405: .It Fl 4
! 406: Forces
! 407: .Nm
! 408: to use IPv4 addresses only.
! 409: .It Fl 6
! 410: Forces
! 411: .Nm
! 412: to use IPv6 addresses only.
1.54 markus 413: .It Fl A
414: Enables forwarding of the authentication agent connection.
415: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 416: .Pp
1.168 jmc 417: Agent forwarding should be enabled with caution.
418: Users with the ability to bypass file permissions on the remote host
419: (for the agent's Unix-domain socket)
420: can access the local agent through the forwarded connection.
421: An attacker cannot obtain key material from the agent,
1.165 stevesk 422: however they can perform operations on the keys that enable them to
423: authenticate using the identities loaded into the agent.
1.175.2.1! brad 424: .It Fl a
! 425: Disables forwarding of the authentication agent connection.
1.108 markus 426: .It Fl b Ar bind_address
427: Specify the interface to transmit from on machines with multiple
428: interfaces or aliased addresses.
1.175.2.1! brad 429: .It Fl C
! 430: Requests compression of all data (including stdin, stdout, stderr, and
! 431: data for forwarded X11 and TCP/IP connections).
! 432: The compression algorithm is the same used by
! 433: .Xr gzip 1 ,
! 434: and the
! 435: .Dq level
! 436: can be controlled by the
! 437: .Cm CompressionLevel
! 438: option for protocol version 1.
! 439: Compression is desirable on modem lines and other
! 440: slow connections, but will only slow down things on fast networks.
! 441: The default value can be set on a host-by-host basis in the
! 442: configuration files; see the
! 443: .Cm Compression
! 444: option.
! 445: .It Fl c Ar blowfish | 3des | des
1.44 aaron 446: Selects the cipher to use for encrypting the session.
1.2 deraadt 447: .Ar 3des
1.40 aaron 448: is used by default.
1.44 aaron 449: It is believed to be secure.
1.5 deraadt 450: .Ar 3des
451: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
452: .Ar blowfish
1.175.2.1! brad 453: is a fast block cipher; it appears very secure and is much faster than
1.40 aaron 454: .Ar 3des .
1.131 stevesk 455: .Ar des
456: is only supported in the
457: .Nm
458: client for interoperability with legacy protocol 1 implementations
459: that do not support the
460: .Ar 3des
1.168 jmc 461: cipher.
462: Its use is strongly discouraged due to cryptographic weaknesses.
1.90 markus 463: .It Fl c Ar cipher_spec
1.51 markus 464: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 465: be specified in order of preference.
1.90 markus 466: See
467: .Cm Ciphers
468: for more information.
1.175.2.1! brad 469: .It Fl D Ar port
! 470: Specifies a local
! 471: .Dq dynamic
! 472: application-level port forwarding.
! 473: This works by allocating a socket to listen to
! 474: .Ar port
! 475: on the local side, and whenever a connection is made to this port, the
! 476: connection is forwarded over the secure channel, and the application
! 477: protocol is then used to determine where to connect to from the
! 478: remote machine.
! 479: Currently the SOCKS4 and SOCKS5 protocols are supported, and
! 480: .Nm
! 481: will act as a SOCKS server.
! 482: Only root can forward privileged ports.
! 483: Dynamic port forwardings can also be specified in the configuration file.
! 484: .It Fl e Ar ch | ^ch | none
1.2 deraadt 485: Sets the escape character for sessions with a pty (default:
486: .Ql ~ ) .
1.40 aaron 487: The escape character is only recognized at the beginning of a line.
488: The escape character followed by a dot
1.2 deraadt 489: .Pq Ql \&.
1.175.2.1! brad 490: closes the connection;
! 491: followed by control-Z suspends the connection;
! 492: and followed by itself sends the escape character once.
1.40 aaron 493: Setting the character to
1.2 deraadt 494: .Dq none
495: disables any escapes and makes the session fully transparent.
1.175.2.1! brad 496: .It Fl F Ar configfile
! 497: Specifies an alternative per-user configuration file.
! 498: If a configuration file is given on the command line,
! 499: the system-wide configuration file
! 500: .Pq Pa /etc/ssh/ssh_config
! 501: will be ignored.
! 502: The default for the per-user configuration file is
! 503: .Pa $HOME/.ssh/config .
1.2 deraadt 504: .It Fl f
505: Requests
506: .Nm
1.40 aaron 507: to go to background just before command execution.
508: This is useful if
1.2 deraadt 509: .Nm
510: is going to ask for passwords or passphrases, but the user
1.40 aaron 511: wants it in the background.
1.44 aaron 512: This implies
1.2 deraadt 513: .Fl n .
1.1 deraadt 514: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 515: something like
516: .Ic ssh -f host xterm .
1.34 markus 517: .It Fl g
518: Allows remote hosts to connect to local forwarded ports.
1.175.2.1! brad 519: .It Fl I Ar smartcard_device
! 520: Specifies which smartcard device to use.
! 521: The argument is the device
! 522: .Nm
! 523: should use to communicate with a smartcard used for storing the user's
! 524: private RSA key.
1.2 deraadt 525: .It Fl i Ar identity_file
1.144 stevesk 526: Selects a file from which the identity (private key) for
1.68 markus 527: RSA or DSA authentication is read.
1.144 stevesk 528: The default is
1.49 markus 529: .Pa $HOME/.ssh/identity
1.144 stevesk 530: for protocol version 1, and
531: .Pa $HOME/.ssh/id_rsa
532: and
533: .Pa $HOME/.ssh/id_dsa
534: for protocol version 2.
1.40 aaron 535: Identity files may also be specified on
536: a per-host basis in the configuration file.
537: It is possible to have multiple
1.2 deraadt 538: .Fl i
539: options (and multiple identities specified in
1.1 deraadt 540: configuration files).
1.2 deraadt 541: .It Fl k
1.175.2.1! brad 542: Disables forwarding (delegation) of GSSAPI credentials to the server.
! 543: .It Fl L Xo
! 544: .Sm off
! 545: .Ar port : host : hostport
! 546: .Sm on
! 547: .Xc
! 548: Specifies that the given port on the local (client) host is to be
! 549: forwarded to the given host and port on the remote side.
! 550: This works by allocating a socket to listen to
! 551: .Ar port
! 552: on the local side, and whenever a connection is made to this port, the
! 553: connection is forwarded over the secure channel, and a connection is
! 554: made to
! 555: .Ar host
! 556: port
! 557: .Ar hostport
! 558: from the remote machine.
! 559: Port forwardings can also be specified in the configuration file.
! 560: Only root can forward privileged ports.
! 561: IPv6 addresses can be specified with an alternative syntax:
! 562: .Sm off
! 563: .Xo
! 564: .Ar port No / Ar host No /
! 565: .Ar hostport .
! 566: .Xc
! 567: .Sm on
1.2 deraadt 568: .It Fl l Ar login_name
1.40 aaron 569: Specifies the user to log in as on the remote machine.
570: This also may be specified on a per-host basis in the configuration file.
1.91 markus 571: .It Fl m Ar mac_spec
572: Additionally, for protocol version 2 a comma-separated list of MAC
573: (message authentication code) algorithms can
574: be specified in order of preference.
575: See the
576: .Cm MACs
577: keyword for more information.
1.175.2.1! brad 578: .It Fl N
! 579: Do not execute a remote command.
! 580: This is useful for just forwarding ports
! 581: (protocol version 2 only).
1.2 deraadt 582: .It Fl n
583: Redirects stdin from
584: .Pa /dev/null
585: (actually, prevents reading from stdin).
1.1 deraadt 586: This must be used when
1.2 deraadt 587: .Nm
1.40 aaron 588: is run in the background.
589: A common trick is to use this to run X11 programs on a remote machine.
590: For example,
1.2 deraadt 591: .Ic ssh -n shadows.cs.hut.fi emacs &
592: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 593: connection will be automatically forwarded over an encrypted channel.
594: The
1.2 deraadt 595: .Nm
1.1 deraadt 596: program will be put in the background.
597: (This does not work if
1.2 deraadt 598: .Nm
599: needs to ask for a password or passphrase; see also the
600: .Fl f
601: option.)
602: .It Fl o Ar option
1.127 stevesk 603: Can be used to give options in the format used in the configuration file.
1.1 deraadt 604: This is useful for specifying options for which there is no separate
1.40 aaron 605: command-line flag.
1.175.2.1! brad 606: For full details of the options listed below, and their possible values, see
! 607: .Xr ssh_config 5 .
! 608: .Pp
! 609: .Bl -tag -width Ds -offset indent -compact
! 610: .It AddressFamily
! 611: .It BatchMode
! 612: .It BindAddress
! 613: .It ChallengeResponseAuthentication
! 614: .It CheckHostIP
! 615: .It Cipher
! 616: .It Ciphers
! 617: .It ClearAllForwardings
! 618: .It Compression
! 619: .It CompressionLevel
! 620: .It ConnectionAttempts
! 621: .It ConnectionTimeout
! 622: .It DynamicForward
! 623: .It EscapeChar
! 624: .It ForwardAgent
! 625: .It ForwardX11
! 626: .It ForwardX11Trusted
! 627: .It GatewayPorts
! 628: .It GlobalKnownHostsFile
! 629: .It GSSAPIAuthentication
! 630: .It GSSAPIDelegateCredentials
! 631: .It Host
! 632: .It HostbasedAuthentication
! 633: .It HostKeyAlgorithms
! 634: .It HostKeyAlias
! 635: .It HostName
! 636: .It IdentityFile
! 637: .It LocalForward
! 638: .It LogLevel
! 639: .It MACs
! 640: .It NoHostAuthenticationForLocalhost
! 641: .It NumberOfPasswordPrompts
! 642: .It PasswordAuthentication
! 643: .It Port
! 644: .It PreferredAuthentications
! 645: .It Protocol
! 646: .It ProxyCommand
! 647: .It PubkeyAuthentication
! 648: .It RemoteForward
! 649: .It RhostsRSAAuthentication
! 650: .It RSAAuthentication
! 651: .It ServerAliveInterval
! 652: .It ServerAliveCountMax
! 653: .It SmartcardDevice
! 654: .It StrictHostKeyChecking
! 655: .It TCPKeepAlive
! 656: .It UsePrivilegedPort
! 657: .It User
! 658: .It UserKnownHostsFile
! 659: .It VerifyHostKeyDNS
! 660: .It XAuthLocation
! 661: .El
1.2 deraadt 662: .It Fl p Ar port
1.40 aaron 663: Port to connect to on the remote host.
664: This can be specified on a
1.1 deraadt 665: per-host basis in the configuration file.
1.2 deraadt 666: .It Fl q
1.40 aaron 667: Quiet mode.
668: Causes all warning and diagnostic messages to be suppressed.
1.175.2.1! brad 669: .It Fl R Xo
! 670: .Sm off
! 671: .Ar port : host : hostport
! 672: .Sm on
! 673: .Xc
! 674: Specifies that the given port on the remote (server) host is to be
! 675: forwarded to the given host and port on the local side.
! 676: This works by allocating a socket to listen to
! 677: .Ar port
! 678: on the remote side, and whenever a connection is made to this port, the
! 679: connection is forwarded over the secure channel, and a connection is
! 680: made to
! 681: .Ar host
! 682: port
! 683: .Ar hostport
! 684: from the local machine.
! 685: Port forwardings can also be specified in the configuration file.
! 686: Privileged ports can be forwarded only when
! 687: logging in as root on the remote machine.
! 688: IPv6 addresses can be specified with an alternative syntax:
! 689: .Sm off
! 690: .Xo
! 691: .Ar port No / Ar host No /
! 692: .Ar hostport .
! 693: .Xc
! 694: .Sm on
1.80 djm 695: .It Fl s
1.172 jmc 696: May be used to request invocation of a subsystem on the remote system.
697: Subsystems are a feature of the SSH2 protocol which facilitate the use
1.175.2.1! brad 698: of SSH as a secure transport for other applications (eg.\&
! 699: .Xr sftp 1 ) .
1.172 jmc 700: The subsystem is specified as the remote command.
1.175.2.1! brad 701: .It Fl T
! 702: Disable pseudo-tty allocation.
1.2 deraadt 703: .It Fl t
1.40 aaron 704: Force pseudo-tty allocation.
1.43 brad 705: This can be used to execute arbitrary
1.40 aaron 706: screen-based programs on a remote machine, which can be very useful,
707: e.g., when implementing menu services.
1.73 markus 708: Multiple
709: .Fl t
710: options force tty allocation, even if
711: .Nm
712: has no local tty.
1.175.2.1! brad 713: .It Fl V
! 714: Display the version number and exit.
1.2 deraadt 715: .It Fl v
1.40 aaron 716: Verbose mode.
717: Causes
1.2 deraadt 718: .Nm
1.40 aaron 719: to print debugging messages about its progress.
720: This is helpful in
1.1 deraadt 721: debugging connection, authentication, and configuration problems.
1.73 markus 722: Multiple
723: .Fl v
1.169 naddy 724: options increase the verbosity.
725: The maximum is 3.
1.2 deraadt 726: .It Fl X
1.1 deraadt 727: Enables X11 forwarding.
1.54 markus 728: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 729: .Pp
1.168 jmc 730: X11 forwarding should be enabled with caution.
731: Users with the ability to bypass file permissions on the remote host
732: (for the user's X authorization database)
733: can access the local X11 display through the forwarded connection.
734: An attacker may then be able to perform activities such as keystroke monitoring.
1.175.2.1! brad 735: .It Fl x
! 736: Disables X11 forwarding.
! 737: .It Fl Y
! 738: Enables trusted X11 forwarding.
1.2 deraadt 739: .El
740: .Sh CONFIGURATION FILES
741: .Nm
1.158 stevesk 742: may additionally obtain configuration data from
743: a per-user configuration file and a system-wide configuration file.
744: The file format and configuration options are described in
745: .Xr ssh_config 5 .
1.2 deraadt 746: .Sh ENVIRONMENT
747: .Nm
1.1 deraadt 748: will normally set the following environment variables:
1.175.2.1! brad 749: .Bl -tag -width LOGNAME
1.2 deraadt 750: .It Ev DISPLAY
751: The
752: .Ev DISPLAY
1.40 aaron 753: variable indicates the location of the X11 server.
1.44 aaron 754: It is automatically set by
1.2 deraadt 755: .Nm
756: to point to a value of the form
757: .Dq hostname:n
758: where hostname indicates
1.175.2.1! brad 759: the host where the shell runs, and n is an integer \*(Ge 1.
1.40 aaron 760: .Nm
761: uses this special value to forward X11 connections over the secure
762: channel.
1.107 markus 763: The user should normally not set
764: .Ev DISPLAY
765: explicitly, as that
1.1 deraadt 766: will render the X11 connection insecure (and will require the user to
767: manually copy any required authorization cookies).
1.2 deraadt 768: .It Ev HOME
1.1 deraadt 769: Set to the path of the user's home directory.
1.2 deraadt 770: .It Ev LOGNAME
771: Synonym for
1.12 aaron 772: .Ev USER ;
773: set for compatibility with systems that use this variable.
1.2 deraadt 774: .It Ev MAIL
1.129 stevesk 775: Set to the path of the user's mailbox.
1.40 aaron 776: .It Ev PATH
1.2 deraadt 777: Set to the default
778: .Ev PATH ,
779: as specified when compiling
1.12 aaron 780: .Nm ssh .
1.118 markus 781: .It Ev SSH_ASKPASS
782: If
783: .Nm
784: needs a passphrase, it will read the passphrase from the current
785: terminal if it was run from a terminal.
786: If
787: .Nm
788: does not have a terminal associated with it but
789: .Ev DISPLAY
790: and
791: .Ev SSH_ASKPASS
792: are set, it will execute the program specified by
793: .Ev SSH_ASKPASS
794: and open an X11 window to read the passphrase.
795: This is particularly useful when calling
796: .Nm
797: from a
798: .Pa .Xsession
799: or related script.
800: (Note that on some machines it
801: may be necessary to redirect the input from
802: .Pa /dev/null
803: to make this work.)
1.18 markus 804: .It Ev SSH_AUTH_SOCK
1.129 stevesk 805: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 806: agent.
1.166 stevesk 807: .It Ev SSH_CONNECTION
808: Identifies the client and server ends of the connection.
1.40 aaron 809: The variable contains
1.166 stevesk 810: four space-separated values: client ip-address, client port number,
811: server ip-address and server port number.
1.73 markus 812: .It Ev SSH_ORIGINAL_COMMAND
813: The variable contains the original command line if a forced command
814: is executed.
815: It can be used to extract the original arguments.
1.2 deraadt 816: .It Ev SSH_TTY
1.1 deraadt 817: This is set to the name of the tty (path to the device) associated
1.40 aaron 818: with the current shell or command.
819: If the current session has no tty,
1.1 deraadt 820: this variable is not set.
1.2 deraadt 821: .It Ev TZ
1.1 deraadt 822: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 823: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 824: on to new connections).
1.2 deraadt 825: .It Ev USER
1.1 deraadt 826: Set to the name of the user logging in.
1.2 deraadt 827: .El
828: .Pp
1.44 aaron 829: Additionally,
1.2 deraadt 830: .Nm
1.44 aaron 831: reads
832: .Pa $HOME/.ssh/environment ,
1.2 deraadt 833: and adds lines of the format
834: .Dq VARNAME=value
1.161 marc 835: to the environment if the file exists and if users are allowed to
836: change their environment.
1.175.2.1! brad 837: For more information, see the
1.161 marc 838: .Cm PermitUserEnvironment
1.162 stevesk 839: option in
1.161 marc 840: .Xr sshd_config 5 .
1.2 deraadt 841: .Sh FILES
1.36 markus 842: .Bl -tag -width Ds
1.116 markus 843: .It Pa $HOME/.ssh/known_hosts
1.129 stevesk 844: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 845: in
1.147 deraadt 846: .Pa /etc/ssh/ssh_known_hosts .
1.2 deraadt 847: See
848: .Xr sshd 8 .
1.102 itojun 849: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
850: Contains the authentication identity of the user.
851: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 852: These files
853: contain sensitive data and should be readable by the user but not
1.15 markus 854: accessible by others (read/write/execute).
855: Note that
856: .Nm
1.48 markus 857: ignores a private key file if it is accessible by others.
1.15 markus 858: It is possible to specify a passphrase when
1.1 deraadt 859: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 860: sensitive part of this file using 3DES.
1.102 itojun 861: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 862: Contains the public key for authentication (public part of the
1.40 aaron 863: identity file in human-readable form).
1.48 markus 864: The contents of the
865: .Pa $HOME/.ssh/identity.pub
1.175.2.1! brad 866: file should be added to the file
1.2 deraadt 867: .Pa $HOME/.ssh/authorized_keys
868: on all machines
1.137 deraadt 869: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 870: The contents of the
871: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 872: and
873: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 874: file should be added to
1.115 markus 875: .Pa $HOME/.ssh/authorized_keys
1.48 markus 876: on all machines
1.137 deraadt 877: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 878: These files are not
1.40 aaron 879: sensitive and can (but need not) be readable by anyone.
1.48 markus 880: These files are
1.84 markus 881: never used automatically and are not necessary; they are only provided for
1.1 deraadt 882: the convenience of the user.
1.2 deraadt 883: .It Pa $HOME/.ssh/config
1.40 aaron 884: This is the per-user configuration file.
1.158 stevesk 885: The file format and configuration options are described in
886: .Xr ssh_config 5 .
1.2 deraadt 887: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 888: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 889: The format of this file is described in the
1.2 deraadt 890: .Xr sshd 8
1.40 aaron 891: manual page.
1.175.2.1! brad 892: In the simplest form the format is the same as the
! 893: .Pa .pub
1.116 markus 894: identity files.
1.48 markus 895: This file is not highly sensitive, but the recommended
896: permissions are read/write for the user, and not accessible by others.
1.147 deraadt 897: .It Pa /etc/ssh/ssh_known_hosts
1.40 aaron 898: Systemwide list of known host keys.
1.116 markus 899: This file should be prepared by the
1.1 deraadt 900: system administrator to contain the public host keys of all machines in the
1.40 aaron 901: organization.
902: This file should be world-readable.
903: This file contains
1.1 deraadt 904: public keys, one per line, in the following format (fields separated
1.116 markus 905: by spaces): system name, public key and optional comment field.
1.40 aaron 906: When different names are used
1.1 deraadt 907: for the same machine, all such names should be listed, separated by
1.40 aaron 908: commas.
1.175.2.1! brad 909: The format is described in the
1.2 deraadt 910: .Xr sshd 8
1.1 deraadt 911: manual page.
1.2 deraadt 912: .Pp
1.1 deraadt 913: The canonical system name (as returned by name servers) is used by
1.2 deraadt 914: .Xr sshd 8
1.1 deraadt 915: to verify the client host when logging in; other names are needed because
1.2 deraadt 916: .Nm
1.1 deraadt 917: does not convert the user-supplied name to a canonical name before
918: checking the key, because someone with access to the name servers
919: would then be able to fool host authentication.
1.147 deraadt 920: .It Pa /etc/ssh/ssh_config
1.40 aaron 921: Systemwide configuration file.
1.158 stevesk 922: The file format and configuration options are described in
923: .Xr ssh_config 5 .
1.147 deraadt 924: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.141 markus 925: These three files contain the private parts of the host keys
926: and are used for
927: .Cm RhostsRSAAuthentication
928: and
929: .Cm HostbasedAuthentication .
1.155 stevesk 930: If the protocol version 1
931: .Cm RhostsRSAAuthentication
1.157 deraadt 932: method is used,
1.155 stevesk 933: .Nm
934: must be setuid root, since the host key is readable only by root.
935: For protocol version 2,
936: .Nm
937: uses
938: .Xr ssh-keysign 8
939: to access the host keys for
940: .Cm HostbasedAuthentication .
941: This eliminates the requirement that
942: .Nm
943: be setuid root when that authentication method is used.
944: By default
1.141 markus 945: .Nm
1.155 stevesk 946: is not setuid root.
1.2 deraadt 947: .It Pa $HOME/.rhosts
948: This file is used in
1.175.2.1! brad 949: .Em rhosts
1.2 deraadt 950: authentication to list the
1.40 aaron 951: host/user pairs that are permitted to log in.
952: (Note that this file is
1.1 deraadt 953: also used by rlogin and rsh, which makes using this file insecure.)
954: Each line of the file contains a host name (in the canonical form
955: returned by name servers), and then a user name on that host,
1.40 aaron 956: separated by a space.
1.92 markus 957: On some machines this file may need to be
1.1 deraadt 958: world-readable if the user's home directory is on a NFS partition,
959: because
1.2 deraadt 960: .Xr sshd 8
1.40 aaron 961: reads it as root.
962: Additionally, this file must be owned by the user,
963: and must not have write permissions for anyone else.
964: The recommended
1.1 deraadt 965: permission for most machines is read/write for the user, and not
966: accessible by others.
1.2 deraadt 967: .Pp
1.1 deraadt 968: Note that by default
1.2 deraadt 969: .Xr sshd 8
1.1 deraadt 970: will be installed so that it requires successful RSA host
1.175.2.1! brad 971: authentication before permitting
! 972: .Em rhosts
! 973: authentication.
1.137 deraadt 974: If the server machine does not have the client's host key in
1.147 deraadt 975: .Pa /etc/ssh/ssh_known_hosts ,
1.137 deraadt 976: it can be stored in
1.2 deraadt 977: .Pa $HOME/.ssh/known_hosts .
978: The easiest way to do this is to
1.1 deraadt 979: connect back to the client from the server machine using ssh; this
1.48 markus 980: will automatically add the host key to
1.2 deraadt 981: .Pa $HOME/.ssh/known_hosts .
982: .It Pa $HOME/.shosts
983: This file is used exactly the same way as
1.175.2.1! brad 984: .Pa .rhosts .
1.2 deraadt 985: The purpose for
1.1 deraadt 986: having this file is to be able to use rhosts authentication with
1.2 deraadt 987: .Nm
988: without permitting login with
1.175.2.1! brad 989: .Xr rlogin
1.2 deraadt 990: or
991: .Xr rsh 1 .
992: .It Pa /etc/hosts.equiv
993: This file is used during
1.175.2.1! brad 994: .Em rhosts
1.173 jmc 995: authentication.
1.40 aaron 996: It contains
1.175.2.1! brad 997: canonical hosts names, one per line (the full format is described in the
1.2 deraadt 998: .Xr sshd 8
1.40 aaron 999: manual page).
1000: If the client host is found in this file, login is
1.1 deraadt 1001: automatically permitted provided client and server user names are the
1.40 aaron 1002: same.
1003: Additionally, successful RSA host authentication is normally
1004: required.
1005: This file should only be writable by root.
1.2 deraadt 1006: .It Pa /etc/shosts.equiv
1.44 aaron 1007: This file is processed exactly as
1.2 deraadt 1008: .Pa /etc/hosts.equiv .
1.1 deraadt 1009: This file may be useful to permit logins using
1.2 deraadt 1010: .Nm
1.1 deraadt 1011: but not using rsh/rlogin.
1.147 deraadt 1012: .It Pa /etc/ssh/sshrc
1.1 deraadt 1013: Commands in this file are executed by
1.2 deraadt 1014: .Nm
1.1 deraadt 1015: when the user logs in just before the user's shell (or command) is started.
1016: See the
1.2 deraadt 1017: .Xr sshd 8
1.1 deraadt 1018: manual page for more information.
1.2 deraadt 1019: .It Pa $HOME/.ssh/rc
1.1 deraadt 1020: Commands in this file are executed by
1.2 deraadt 1021: .Nm
1.1 deraadt 1022: when the user logs in just before the user's shell (or command) is
1023: started.
1.44 aaron 1024: See the
1.2 deraadt 1025: .Xr sshd 8
1.1 deraadt 1026: manual page for more information.
1.31 markus 1027: .It Pa $HOME/.ssh/environment
1028: Contains additional definitions for environment variables, see section
1029: .Sx ENVIRONMENT
1030: above.
1.58 itojun 1031: .El
1.145 markus 1032: .Sh DIAGNOSTICS
1033: .Nm
1034: exits with the exit status of the remote command or with 255
1035: if an error occurred.
1.2 deraadt 1036: .Sh SEE ALSO
1.175.2.1! brad 1037: .Xr gzip 1 ,
1.2 deraadt 1038: .Xr rsh 1 ,
1039: .Xr scp 1 ,
1.83 djm 1040: .Xr sftp 1 ,
1.2 deraadt 1041: .Xr ssh-add 1 ,
1042: .Xr ssh-agent 1 ,
1043: .Xr ssh-keygen 1 ,
1044: .Xr telnet 1 ,
1.175.2.1! brad 1045: .Xr hosts.equiv 5 ,
1.159 stevesk 1046: .Xr ssh_config 5 ,
1.160 naddy 1047: .Xr ssh-keysign 8 ,
1.87 itojun 1048: .Xr sshd 8
1.106 markus 1049: .Rs
1050: .%A T. Ylonen
1051: .%A T. Kivinen
1052: .%A M. Saarinen
1053: .%A T. Rinne
1054: .%A S. Lehtinen
1055: .%T "SSH Protocol Architecture"
1.150 markus 1056: .%N draft-ietf-secsh-architecture-12.txt
1057: .%D January 2002
1.106 markus 1058: .%O work in progress material
1059: .Re
1.173 jmc 1060: .Sh AUTHORS
1061: OpenSSH is a derivative of the original and free
1062: ssh 1.2.12 release by Tatu Ylonen.
1063: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1064: Theo de Raadt and Dug Song
1065: removed many bugs, re-added newer features and
1066: created OpenSSH.
1067: Markus Friedl contributed the support for SSH
1068: protocol versions 1.5 and 2.0.
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