Annotation of src/usr.bin/ssh/sshd.8, Revision 1.257
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: .\"
1.64 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: .\"
1.99 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.
1.64 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: .\"
1.64 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,
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35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.257 ! djm 37: .\" $OpenBSD: sshd.8,v 1.256 2010/05/07 11:30:30 djm Exp $
! 38: .Dd $Mdocdate: May 7 2010 $
1.2 deraadt 39: .Dt SSHD 8
40: .Os
41: .Sh NAME
42: .Nm sshd
1.120 markus 43: .Nd OpenSSH SSH daemon
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm sshd
1.194 jmc 46: .Bk -words
1.243 jmc 47: .Op Fl 46DdeiqTt
1.2 deraadt 48: .Op Fl b Ar bits
1.242 dtucker 49: .Op Fl C Ar connection_spec
1.251 djm 50: .Op Fl c Ar host_certificate_file
1.2 deraadt 51: .Op Fl f Ar config_file
52: .Op Fl g Ar login_grace_time
53: .Op Fl h Ar host_key_file
54: .Op Fl k Ar key_gen_time
1.156 markus 55: .Op Fl o Ar option
1.2 deraadt 56: .Op Fl p Ar port
1.61 markus 57: .Op Fl u Ar len
1.194 jmc 58: .Ek
1.40 aaron 59: .Sh DESCRIPTION
1.2 deraadt 60: .Nm
1.212 jmc 61: (OpenSSH Daemon) is the daemon program for
1.2 deraadt 62: .Xr ssh 1 .
1.235 jmc 63: Together these programs replace
64: .Xr rlogin 1
65: and
66: .Xr rsh 1 ,
67: and provide secure encrypted communications between two untrusted hosts
1.36 aaron 68: over an insecure network.
1.2 deraadt 69: .Pp
70: .Nm
1.212 jmc 71: listens for connections from clients.
1.40 aaron 72: It is normally started at boot from
1.2 deraadt 73: .Pa /etc/rc .
74: It forks a new
1.36 aaron 75: daemon for each incoming connection.
76: The forked daemons handle
1.1 deraadt 77: key exchange, encryption, authentication, command execution,
78: and data exchange.
1.2 deraadt 79: .Pp
80: .Nm
1.200 jmc 81: can be configured using command-line options or a configuration file
82: (by default
1.212 jmc 83: .Xr sshd_config 5 ) ;
84: command-line options override values specified in the
1.1 deraadt 85: configuration file.
1.25 markus 86: .Nm
87: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 88: .Dv SIGHUP ,
1.230 jmc 89: by executing itself with the name and options it was started with, e.g.\&
1.97 deraadt 90: .Pa /usr/sbin/sshd .
1.18 aaron 91: .Pp
92: The options are as follows:
1.2 deraadt 93: .Bl -tag -width Ds
1.200 jmc 94: .It Fl 4
95: Forces
96: .Nm
97: to use IPv4 addresses only.
98: .It Fl 6
99: Forces
100: .Nm
101: to use IPv6 addresses only.
1.2 deraadt 102: .It Fl b Ar bits
1.120 markus 103: Specifies the number of bits in the ephemeral protocol version 1
1.246 djm 104: server key (default 1024).
1.243 jmc 105: .It Fl C Ar connection_spec
106: Specify the connection parameters to use for the
107: .Fl T
108: extended test mode.
109: If provided, any
110: .Cm Match
111: directives in the configuration file
112: that would apply to the specified user, host, and address will be set before
113: the configuration is written to standard output.
114: The connection parameters are supplied as keyword=value pairs.
115: The keywords are
116: .Dq user ,
117: .Dq host ,
118: and
119: .Dq addr .
120: All are required and may be supplied in any order, either with multiple
121: .Fl C
122: options or as a comma-separated list.
1.252 jmc 123: .It Fl c Ar host_certificate_file
124: Specifies a path to a certificate file to identify
125: .Nm
126: during key exchange.
127: The certificate file must match a host key file specified using the
128: .Fl h
129: option or the
130: .Cm HostKey
131: configuration directive.
1.200 jmc 132: .It Fl D
133: When this option is specified,
134: .Nm
135: will not detach and does not become a daemon.
136: This allows easy monitoring of
137: .Nm sshd .
1.2 deraadt 138: .It Fl d
1.36 aaron 139: Debug mode.
1.250 djm 140: The server sends verbose debug output to standard error,
141: and does not put itself in the background.
1.36 aaron 142: The server also will not fork and will only process one connection.
143: This option is only intended for debugging for the server.
1.194 jmc 144: Multiple
145: .Fl d
146: options increase the debugging level.
1.67 aaron 147: Maximum is 3.
1.120 markus 148: .It Fl e
149: When this option is specified,
150: .Nm
151: will send the output to the standard error instead of the system log.
1.235 jmc 152: .It Fl f Ar config_file
1.36 aaron 153: Specifies the name of the configuration file.
154: The default is
1.167 deraadt 155: .Pa /etc/ssh/sshd_config .
1.16 markus 156: .Nm
157: refuses to start if there is no configuration file.
1.2 deraadt 158: .It Fl g Ar login_grace_time
1.1 deraadt 159: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 160: 120 seconds).
1.36 aaron 161: If the client fails to authenticate the user within
162: this many seconds, the server disconnects and exits.
163: A value of zero indicates no limit.
1.2 deraadt 164: .It Fl h Ar host_key_file
1.160 stevesk 165: Specifies a file from which a host key is read.
1.7 markus 166: This option must be given if
167: .Nm
168: is not run as root (as the normal
1.160 stevesk 169: host key files are normally not readable by anyone but root).
170: The default is
1.167 deraadt 171: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 172: for protocol version 1, and
1.167 deraadt 173: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 174: and
1.167 deraadt 175: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 176: for protocol version 2.
1.75 markus 177: It is possible to have multiple host key files for
1.120 markus 178: the different protocol versions and host key algorithms.
1.2 deraadt 179: .It Fl i
1.7 markus 180: Specifies that
181: .Nm
1.194 jmc 182: is being run from
183: .Xr inetd 8 .
1.7 markus 184: .Nm
185: is normally not run
1.1 deraadt 186: from inetd because it needs to generate the server key before it can
1.36 aaron 187: respond to the client, and this may take tens of seconds.
188: Clients would have to wait too long if the key was regenerated every time.
1.230 jmc 189: However, with small key sizes (e.g. 512) using
1.7 markus 190: .Nm
191: from inetd may
1.1 deraadt 192: be feasible.
1.2 deraadt 193: .It Fl k Ar key_gen_time
1.120 markus 194: Specifies how often the ephemeral protocol version 1 server key is
195: regenerated (default 3600 seconds, or one hour).
1.36 aaron 196: The motivation for regenerating the key fairly
1.200 jmc 197: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 198: it becomes impossible to recover the key for decrypting intercepted
199: communications even if the machine is cracked into or physically
1.36 aaron 200: seized.
201: A value of zero indicates that the key will never be regenerated.
1.156 markus 202: .It Fl o Ar option
203: Can be used to give options in the format used in the configuration file.
204: This is useful for specifying options for which there is no separate
205: command-line flag.
1.200 jmc 206: For full details of the options, and their values, see
207: .Xr sshd_config 5 .
1.2 deraadt 208: .It Fl p Ar port
1.1 deraadt 209: Specifies the port on which the server listens for connections
210: (default 22).
1.158 stevesk 211: Multiple port options are permitted.
1.210 stevesk 212: Ports specified in the configuration file with the
213: .Cm Port
214: option are ignored when a command-line port is specified.
215: Ports specified using the
216: .Cm ListenAddress
217: option override command-line ports.
1.2 deraadt 218: .It Fl q
1.36 aaron 219: Quiet mode.
220: Nothing is sent to the system log.
221: Normally the beginning,
1.1 deraadt 222: authentication, and termination of each connection is logged.
1.242 dtucker 223: .It Fl T
224: Extended test mode.
225: Check the validity of the configuration file, output the effective configuration
226: to stdout and then exit.
227: Optionally,
228: .Cm Match
229: rules may be applied by specifying the connection parameters using one or more
230: .Fl C
231: options.
1.243 jmc 232: .It Fl t
233: Test mode.
234: Only check the validity of the configuration file and sanity of the keys.
235: This is useful for updating
236: .Nm
237: reliably as configuration options may change.
1.61 markus 238: .It Fl u Ar len
239: This option is used to specify the size of the field
240: in the
241: .Li utmp
242: structure that holds the remote host name.
243: If the resolved host name is longer than
244: .Ar len ,
245: the dotted decimal value will be used instead.
246: This allows hosts with very long host names that
247: overflow this field to still be uniquely identified.
248: Specifying
249: .Fl u0
250: indicates that only dotted decimal addresses
251: should be put into the
252: .Pa utmp
253: file.
1.144 stevesk 254: .Fl u0
1.194 jmc 255: may also be used to prevent
1.144 stevesk 256: .Nm
257: from making DNS requests unless the authentication
258: mechanism or configuration requires it.
259: Authentication mechanisms that may require DNS include
260: .Cm RhostsRSAAuthentication ,
1.215 jmc 261: .Cm HostbasedAuthentication ,
1.144 stevesk 262: and using a
263: .Cm from="pattern-list"
264: option in a key file.
1.170 stevesk 265: Configuration options that require DNS include using a
266: USER@HOST pattern in
267: .Cm AllowUsers
268: or
269: .Cm DenyUsers .
1.2 deraadt 270: .El
1.214 jmc 271: .Sh AUTHENTICATION
272: The OpenSSH SSH daemon supports SSH protocols 1 and 2.
1.249 jmc 273: The default is to use protocol 2 only,
1.214 jmc 274: though this can be changed via the
275: .Cm Protocol
276: option in
277: .Xr sshd_config 5 .
278: Protocol 2 supports both RSA and DSA keys;
279: protocol 1 only supports RSA keys.
280: For both protocols,
281: each host has a host-specific key,
282: normally 2048 bits,
283: used to identify the host.
284: .Pp
285: Forward security for protocol 1 is provided through
286: an additional server key,
287: normally 768 bits,
288: generated when the server starts.
1.212 jmc 289: This key is normally regenerated every hour if it has been used, and
290: is never stored on disk.
291: Whenever a client connects, the daemon responds with its public
292: host and server keys.
293: The client compares the
294: RSA host key against its own database to verify that it has not changed.
295: The client then generates a 256-bit random number.
296: It encrypts this
297: random number using both the host key and the server key, and sends
298: the encrypted number to the server.
299: Both sides then use this
300: random number as a session key which is used to encrypt all further
301: communications in the session.
302: The rest of the session is encrypted
303: using a conventional cipher, currently Blowfish or 3DES, with 3DES
304: being used by default.
305: The client selects the encryption algorithm
306: to use from those offered by the server.
307: .Pp
1.214 jmc 308: For protocol 2,
309: forward security is provided through a Diffie-Hellman key agreement.
310: This key agreement results in a shared session key.
311: The rest of the session is encrypted using a symmetric cipher, currently
312: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
313: The client selects the encryption algorithm
314: to use from those offered by the server.
315: Additionally, session integrity is provided
316: through a cryptographic message authentication code
1.237 pvalchev 317: (hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160).
1.214 jmc 318: .Pp
319: Finally, the server and the client enter an authentication dialog.
1.212 jmc 320: The client tries to authenticate itself using
1.214 jmc 321: host-based authentication,
322: public key authentication,
323: challenge-response authentication,
324: or password authentication.
1.212 jmc 325: .Pp
326: If the client successfully authenticates itself, a dialog for
327: preparing the session is entered.
328: At this time the client may request
329: things like allocating a pseudo-tty, forwarding X11 connections,
330: forwarding TCP connections, or forwarding the authentication agent
331: connection over the secure channel.
332: .Pp
1.216 jmc 333: After this, the client either requests a shell or execution of a command.
1.212 jmc 334: The sides then enter session mode.
335: In this mode, either side may send
336: data at any time, and such data is forwarded to/from the shell or
337: command on the server side, and the user terminal in the client side.
338: .Pp
339: When the user program terminates and all forwarded X11 and other
340: connections have been closed, the server sends command exit status to
341: the client, and both sides exit.
1.2 deraadt 342: .Sh LOGIN PROCESS
1.1 deraadt 343: When a user successfully logs in,
1.2 deraadt 344: .Nm
1.1 deraadt 345: does the following:
1.2 deraadt 346: .Bl -enum -offset indent
347: .It
1.1 deraadt 348: If the login is on a tty, and no command has been specified,
1.40 aaron 349: prints last login time and
1.2 deraadt 350: .Pa /etc/motd
1.1 deraadt 351: (unless prevented in the configuration file or by
1.207 djm 352: .Pa ~/.hushlogin ;
1.2 deraadt 353: see the
1.40 aaron 354: .Sx FILES
1.2 deraadt 355: section).
356: .It
1.1 deraadt 357: If the login is on a tty, records login time.
1.2 deraadt 358: .It
359: Checks
360: .Pa /etc/nologin ;
361: if it exists, prints contents and quits
1.1 deraadt 362: (unless root).
1.2 deraadt 363: .It
1.1 deraadt 364: Changes to run with normal user privileges.
1.2 deraadt 365: .It
1.1 deraadt 366: Sets up basic environment.
1.2 deraadt 367: .It
1.200 jmc 368: Reads the file
1.207 djm 369: .Pa ~/.ssh/environment ,
1.200 jmc 370: if it exists, and users are allowed to change their environment.
1.188 stevesk 371: See the
1.187 marc 372: .Cm PermitUserEnvironment
1.188 stevesk 373: option in
1.187 marc 374: .Xr sshd_config 5 .
1.2 deraadt 375: .It
1.1 deraadt 376: Changes to user's home directory.
1.2 deraadt 377: .It
378: If
1.207 djm 379: .Pa ~/.ssh/rc
1.2 deraadt 380: exists, runs it; else if
1.168 deraadt 381: .Pa /etc/ssh/sshrc
1.2 deraadt 382: exists, runs
1.36 aaron 383: it; otherwise runs xauth.
384: The
1.2 deraadt 385: .Dq rc
386: files are given the X11
1.1 deraadt 387: authentication protocol and cookie in standard input.
1.226 jmc 388: See
389: .Sx SSHRC ,
390: below.
1.2 deraadt 391: .It
1.1 deraadt 392: Runs user's shell or command.
1.2 deraadt 393: .El
1.226 jmc 394: .Sh SSHRC
395: If the file
396: .Pa ~/.ssh/rc
397: exists,
398: .Xr sh 1
399: runs it after reading the
400: environment files but before starting the user's shell or command.
401: It must not produce any output on stdout; stderr must be used
402: instead.
403: If X11 forwarding is in use, it will receive the "proto cookie" pair in
404: its standard input (and
405: .Ev DISPLAY
406: in its environment).
407: The script must call
408: .Xr xauth 1
409: because
410: .Nm
411: will not run xauth automatically to add X11 cookies.
412: .Pp
413: The primary purpose of this file is to run any initialization routines
414: which may be needed before the user's home directory becomes
415: accessible; AFS is a particular example of such an environment.
416: .Pp
417: This file will probably contain some initialization code followed by
418: something similar to:
419: .Bd -literal -offset 3n
420: if read proto cookie && [ -n "$DISPLAY" ]; then
421: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
422: # X11UseLocalhost=yes
423: echo add unix:`echo $DISPLAY |
424: cut -c11-` $proto $cookie
425: else
426: # X11UseLocalhost=no
427: echo add $DISPLAY $proto $cookie
428: fi | xauth -q -
429: fi
430: .Ed
431: .Pp
432: If this file does not exist,
433: .Pa /etc/ssh/sshrc
434: is run, and if that
435: does not exist either, xauth is used to add the cookie.
1.2 deraadt 436: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 437: .Cm AuthorizedKeysFile
1.217 jmc 438: specifies the file containing public keys for
439: public key authentication;
440: if none is specified, the default is
441: .Pa ~/.ssh/authorized_keys .
1.36 aaron 442: Each line of the file contains one
1.2 deraadt 443: key (empty lines and lines starting with a
444: .Ql #
445: are ignored as
1.36 aaron 446: comments).
1.217 jmc 447: Protocol 1 public keys consist of the following space-separated fields:
448: options, bits, exponent, modulus, comment.
449: Protocol 2 public key consist of:
450: options, keytype, base64-encoded key, comment.
451: The options field is optional;
452: its presence is determined by whether the line starts
1.189 espie 453: with a number or not (the options field never starts with a number).
1.217 jmc 454: The bits, exponent, modulus, and comment fields give the RSA key for
1.75 markus 455: protocol version 1; the
1.1 deraadt 456: comment field is not used for anything (but may be convenient for the
457: user to identify the key).
1.75 markus 458: For protocol version 2 the keytype is
459: .Dq ssh-dss
460: or
461: .Dq ssh-rsa .
1.2 deraadt 462: .Pp
1.1 deraadt 463: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 464: (because of the size of the public key encoding) up to a limit of
465: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
466: keys up to 16 kilobits.
1.36 aaron 467: You don't want to type them in; instead, copy the
1.113 itojun 468: .Pa identity.pub ,
1.217 jmc 469: .Pa id_dsa.pub ,
1.75 markus 470: or the
1.113 itojun 471: .Pa id_rsa.pub
1.1 deraadt 472: file and edit it.
1.175 stevesk 473: .Pp
474: .Nm
475: enforces a minimum RSA key modulus size for protocol 1
476: and protocol 2 keys of 768 bits.
1.2 deraadt 477: .Pp
1.58 deraadt 478: The options (if present) consist of comma-separated option
1.36 aaron 479: specifications.
480: No spaces are permitted, except within double quotes.
1.141 stevesk 481: The following option specifications are supported (note
482: that option keywords are case-insensitive):
1.2 deraadt 483: .Bl -tag -width Ds
1.253 djm 484: .It Cm cert-authority
1.251 djm 485: Specifies that the listed key is a certification authority (CA) that is
486: trusted to validate signed certificates for user authentication.
487: .Pp
488: Certificates may encode access restrictions similar to these key options.
489: If both certificate restrictions and key options are present, the most
490: restrictive union of the two is applied.
1.2 deraadt 491: .It Cm command="command"
1.1 deraadt 492: Specifies that the command is executed whenever this key is used for
1.36 aaron 493: authentication.
494: The command supplied by the user (if any) is ignored.
1.148 markus 495: The command is run on a pty if the client requests a pty;
1.36 aaron 496: otherwise it is run without a tty.
1.194 jmc 497: If an 8-bit clean channel is required,
1.147 deraadt 498: one must not request a pty or should specify
1.89 markus 499: .Cm no-pty .
1.36 aaron 500: A quote may be included in the command by quoting it with a backslash.
501: This option might be useful
1.189 espie 502: to restrict certain public keys to perform just a specific operation.
1.36 aaron 503: An example might be a key that permits remote backups but nothing else.
1.211 jmc 504: Note that the client may specify TCP and/or X11
1.51 hugh 505: forwarding unless they are explicitly prohibited.
1.233 dtucker 506: The command originally supplied by the client is available in the
507: .Ev SSH_ORIGINAL_COMMAND
508: environment variable.
1.149 markus 509: Note that this option applies to shell, command or subsystem execution.
1.251 djm 510: Also note that this command may be superseded by either a
511: .Xr sshd_config 5
512: .Cm ForceCommand
513: directive or a command embedded in a certificate.
1.2 deraadt 514: .It Cm environment="NAME=value"
1.1 deraadt 515: Specifies that the string is to be added to the environment when
1.36 aaron 516: logging in using this key.
517: Environment variables set this way
518: override other default environment values.
519: Multiple options of this type are permitted.
1.188 stevesk 520: Environment processing is disabled by default and is
521: controlled via the
522: .Cm PermitUserEnvironment
523: option.
1.155 markus 524: This option is automatically disabled if
525: .Cm UseLogin
526: is enabled.
1.218 jmc 527: .It Cm from="pattern-list"
1.244 djm 528: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 529: name of the remote host or its IP address must be present in the
1.244 djm 530: comma-separated list of patterns.
1.229 jmc 531: See
532: .Sx PATTERNS
533: in
534: .Xr ssh_config 5
535: for more information on patterns.
1.244 djm 536: .Pp
537: In addition to the wildcard matching that may be applied to hostnames or
538: addresses, a
539: .Cm from
1.248 sobrado 540: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 541: .Pp
542: The purpose of this option is to optionally increase security: public key
543: authentication by itself does not trust the network or name servers or
544: anything (but the key); however, if somebody somehow steals the key, the key
545: permits an intruder to log in from anywhere in the world.
546: This additional option makes using a stolen key more difficult (name
547: servers and/or routers would have to be compromised in addition to
548: just the key).
1.218 jmc 549: .It Cm no-agent-forwarding
550: Forbids authentication agent forwarding when this key is used for
551: authentication.
1.2 deraadt 552: .It Cm no-port-forwarding
1.211 jmc 553: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 554: Any port forward requests by the client will return an error.
1.230 jmc 555: This might be used, e.g. in connection with the
1.2 deraadt 556: .Cm command
1.1 deraadt 557: option.
1.218 jmc 558: .It Cm no-pty
559: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 560: .It Cm no-user-rc
1.241 jmc 561: Disables execution of
1.240 djm 562: .Pa ~/.ssh/rc .
1.2 deraadt 563: .It Cm no-X11-forwarding
1.1 deraadt 564: Forbids X11 forwarding when this key is used for authentication.
565: Any X11 forward requests by the client will return an error.
1.107 djm 566: .It Cm permitopen="host:port"
1.133 itojun 567: Limit local
1.107 djm 568: .Li ``ssh -L''
1.111 stevesk 569: port forwarding such that it may only connect to the specified host and
1.146 stevesk 570: port.
1.257 ! djm 571: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 572: Multiple
1.107 djm 573: .Cm permitopen
1.197 jmc 574: options may be applied separated by commas.
575: No pattern matching is performed on the specified hostnames,
576: they must be literal domains or addresses.
1.256 djm 577: .It Cm principals="principals"
578: On a
579: .Cm cert-authority
580: line, specifies allowed principals for certificate authentication as a
581: comma-separated list.
582: At least one name from the list must appear in the certificate's
583: list of principals for the certificate to be accepted.
584: This option is ignored for keys that are not marked as trusted certificate
585: signers using the
586: .Cm cert-authority
587: option.
1.209 reyk 588: .It Cm tunnel="n"
589: Force a
590: .Xr tun 4
591: device on the server.
592: Without this option, the next available device will be used if
593: the client requests a tunnel.
1.2 deraadt 594: .El
595: .Pp
1.219 jmc 596: An example authorized_keys file:
1.222 jmc 597: .Bd -literal -offset 3n
1.219 jmc 598: # Comments allowed at start of line
599: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 600: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
601: AAAAB2...19Q== john@example.net
602: command="dump /home",no-pty,no-port-forwarding ssh-dss
603: AAAAC3...51R== example.net
604: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
605: AAAAB5...21S==
606: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
607: jane@example.net
1.219 jmc 608: .Ed
1.2 deraadt 609: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 610: The
1.194 jmc 611: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 612: and
1.207 djm 613: .Pa ~/.ssh/known_hosts
1.36 aaron 614: files contain host public keys for all known hosts.
615: The global file should
1.37 brad 616: be prepared by the administrator (optional), and the per-user file is
1.220 jmc 617: maintained automatically: whenever the user connects from an unknown host,
1.36 aaron 618: its key is added to the per-user file.
1.2 deraadt 619: .Pp
1.254 djm 620: Each line in these files contains the following fields: markers (optional),
621: hostnames, bits, exponent, modulus, comment.
1.36 aaron 622: The fields are separated by spaces.
1.2 deraadt 623: .Pp
1.254 djm 624: The marker is optional, but if it is present then it must be one of
625: .Dq @cert-authority ,
626: to indicate that the line contains a certification authority (CA) key,
627: or
628: .Dq @revoked ,
629: to indicate that the key contained on the line is revoked and must not ever
630: be accepted.
631: Only one marker should be used on a key line.
632: .Pp
1.196 david 633: Hostnames is a comma-separated list of patterns
1.220 jmc 634: .Pf ( Ql *
1.197 jmc 635: and
636: .Ql \&?
1.195 mouring 637: act as
1.1 deraadt 638: wildcards); each pattern in turn is matched against the canonical host
639: name (when authenticating a client) or against the user-supplied
1.36 aaron 640: name (when authenticating a server).
641: A pattern may also be preceded by
1.195 mouring 642: .Ql \&!
1.2 deraadt 643: to indicate negation: if the host name matches a negated
1.1 deraadt 644: pattern, it is not accepted (by that line) even if it matched another
645: pattern on the line.
1.231 dtucker 646: A hostname or address may optionally be enclosed within
647: .Ql \&[
648: and
649: .Ql \&]
650: brackets then followed by
651: .Ql \&:
1.232 jmc 652: and a non-standard port number.
1.2 deraadt 653: .Pp
1.205 djm 654: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 655: and addresses should the file's contents be disclosed.
656: Hashed hostnames start with a
657: .Ql |
1.205 djm 658: character.
659: Only one hashed hostname may appear on a single line and none of the above
660: negation or wildcard operators may be applied.
661: .Pp
1.49 markus 662: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.220 jmc 663: can be obtained, for example, from
1.167 deraadt 664: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 665: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 666: .Pp
667: Lines starting with
668: .Ql #
669: and empty lines are ignored as comments.
670: .Pp
1.1 deraadt 671: When performing host authentication, authentication is accepted if any
1.254 djm 672: matching line has the proper key; either one that matches exactly or,
673: if the server has presented a certificate for authentication, the key
674: of the certification authority that signed the certificate.
675: For a key to be trusted as a certification authority, it must use the
676: .Dq @cert-authority
677: marker described above.
678: .Pp
679: The known hosts file also provides a facility to mark keys as revoked,
680: for example when it is known that the associated private key has been
681: stolen.
682: Revoked keys are specified by including the
683: .Dq @revoked
684: marker at the beginning of the key line, and are never accepted for
685: authentication or as certification authorities, but instead will
686: produce a warning from
687: .Xr ssh 1
688: when they are encountered.
689: .Pp
690: It is permissible (but not
1.1 deraadt 691: recommended) to have several lines or different host keys for the same
1.36 aaron 692: names.
693: This will inevitably happen when short forms of host names
694: from different domains are put in the file.
695: It is possible
1.1 deraadt 696: that the files contain conflicting information; authentication is
697: accepted if valid information can be found from either file.
1.2 deraadt 698: .Pp
1.1 deraadt 699: Note that the lines in these files are typically hundreds of characters
700: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 701: Rather, generate them by a script,
702: .Xr ssh-keyscan 1
1.40 aaron 703: or by taking
1.167 deraadt 704: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 705: and adding the host names at the front.
1.254 djm 706: .Xr ssh-keygen 1
1.255 jmc 707: also offers some basic automated editing for
1.254 djm 708: .Pa ~/.ssh/known_hosts
709: including removing hosts matching a host name and converting all host
710: names to their hashed representations.
1.221 jmc 711: .Pp
712: An example ssh_known_hosts file:
713: .Bd -literal -offset 3n
714: # Comments allowed at start of line
715: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
716: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 717: # A hashed hostname
718: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
719: AAAA1234.....=
1.254 djm 720: # A revoked key
721: @revoked * ssh-rsa AAAAB5W...
722: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
723: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 724: .Ed
1.2 deraadt 725: .Sh FILES
1.223 jmc 726: .Bl -tag -width Ds -compact
727: .It ~/.hushlogin
1.204 jmc 728: This file is used to suppress printing the last login time and
729: .Pa /etc/motd ,
730: if
731: .Cm PrintLastLog
732: and
733: .Cm PrintMotd ,
734: respectively,
735: are enabled.
736: It does not suppress printing of the banner specified by
737: .Cm Banner .
1.223 jmc 738: .Pp
739: .It ~/.rhosts
1.225 jmc 740: This file is used for host-based authentication (see
741: .Xr ssh 1
742: for more information).
743: On some machines this file may need to be
744: world-readable if the user's home directory is on an NFS partition,
745: because
746: .Nm
747: reads it as root.
748: Additionally, this file must be owned by the user,
749: and must not have write permissions for anyone else.
750: The recommended
751: permission for most machines is read/write for the user, and not
1.1 deraadt 752: accessible by others.
1.2 deraadt 753: .Pp
1.223 jmc 754: .It ~/.shosts
1.225 jmc 755: This file is used in exactly the same way as
756: .Pa .rhosts ,
757: but allows host-based authentication without permitting login with
758: rlogin/rsh.
1.238 mcbride 759: .Pp
760: .It ~/.ssh/
761: This directory is the default location for all user-specific configuration
762: and authentication information.
763: There is no general requirement to keep the entire contents of this directory
764: secret, but the recommended permissions are read/write/execute for the user,
765: and not accessible by others.
1.2 deraadt 766: .Pp
1.223 jmc 767: .It ~/.ssh/authorized_keys
1.225 jmc 768: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.223 jmc 769: The format of this file is described above.
1.234 dtucker 770: The content of the file is not highly sensitive, but the recommended
1.225 jmc 771: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 772: .Pp
773: If this file, the
774: .Pa ~/.ssh
775: directory, or the user's home directory are writable
776: by other users, then the file could be modified or replaced by unauthorized
777: users.
778: In this case,
779: .Nm
780: will not allow it to be used unless the
781: .Cm StrictModes
782: option has been set to
783: .Dq no .
1.2 deraadt 784: .Pp
1.223 jmc 785: .It ~/.ssh/environment
1.36 aaron 786: This file is read into the environment at login (if it exists).
787: It can only contain empty lines, comment lines (that start with
1.2 deraadt 788: .Ql # ) ,
1.36 aaron 789: and assignment lines of the form name=value.
790: The file should be writable
1.6 aaron 791: only by the user; it need not be readable by anyone else.
1.188 stevesk 792: Environment processing is disabled by default and is
793: controlled via the
794: .Cm PermitUserEnvironment
795: option.
1.223 jmc 796: .Pp
797: .It ~/.ssh/known_hosts
1.225 jmc 798: Contains a list of host keys for all hosts the user has logged into
799: that are not already in the systemwide list of known host keys.
800: The format of this file is described above.
801: This file should be writable only by root/the owner and
1.223 jmc 802: can, but need not be, world-readable.
803: .Pp
804: .It ~/.ssh/rc
1.226 jmc 805: Contains initialization routines to be run before
806: the user's home directory becomes accessible.
1.1 deraadt 807: This file should be writable only by the user, and need not be
808: readable by anyone else.
1.223 jmc 809: .Pp
810: .It /etc/hosts.allow
811: .It /etc/hosts.deny
812: Access controls that should be enforced by tcp-wrappers are defined here.
813: Further details are described in
814: .Xr hosts_access 5 .
815: .Pp
816: .It /etc/hosts.equiv
1.227 jmc 817: This file is for host-based authentication (see
818: .Xr ssh 1 ) .
819: It should only be writable by root.
1.223 jmc 820: .Pp
821: .It /etc/moduli
822: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
823: The file format is described in
824: .Xr moduli 5 .
825: .Pp
826: .It /etc/motd
827: See
828: .Xr motd 5 .
829: .Pp
830: .It /etc/nologin
831: If this file exists,
832: .Nm
833: refuses to let anyone except root log in.
834: The contents of the file
835: are displayed to anyone trying to log in, and non-root connections are
836: refused.
837: The file should be world-readable.
838: .Pp
839: .It /etc/shosts.equiv
1.227 jmc 840: This file is used in exactly the same way as
841: .Pa hosts.equiv ,
842: but allows host-based authentication without permitting login with
843: rlogin/rsh.
1.225 jmc 844: .Pp
1.223 jmc 845: .It /etc/ssh/ssh_host_key
846: .It /etc/ssh/ssh_host_dsa_key
847: .It /etc/ssh/ssh_host_rsa_key
848: These three files contain the private parts of the host keys.
849: These files should only be owned by root, readable only by root, and not
850: accessible to others.
851: Note that
852: .Nm
1.228 jmc 853: does not start if these files are group/world-accessible.
1.223 jmc 854: .Pp
855: .It /etc/ssh/ssh_host_key.pub
856: .It /etc/ssh/ssh_host_dsa_key.pub
857: .It /etc/ssh/ssh_host_rsa_key.pub
858: These three files contain the public parts of the host keys.
859: These files should be world-readable but writable only by
860: root.
861: Their contents should match the respective private parts.
862: These files are not
863: really used for anything; they are provided for the convenience of
864: the user so their contents can be copied to known hosts files.
865: These files are created using
866: .Xr ssh-keygen 1 .
867: .Pp
1.235 jmc 868: .It /etc/ssh/ssh_known_hosts
869: Systemwide list of known host keys.
870: This file should be prepared by the
871: system administrator to contain the public host keys of all machines in the
872: organization.
873: The format of this file is described above.
874: This file should be writable only by root/the owner and
875: should be world-readable.
876: .Pp
1.223 jmc 877: .It /etc/ssh/sshd_config
878: Contains configuration data for
879: .Nm sshd .
880: The file format and configuration options are described in
881: .Xr sshd_config 5 .
882: .Pp
883: .It /etc/ssh/sshrc
1.226 jmc 884: Similar to
885: .Pa ~/.ssh/rc ,
886: it can be used to specify
1.36 aaron 887: machine-specific login-time initializations globally.
888: This file should be writable only by root, and should be world-readable.
1.223 jmc 889: .Pp
890: .It /var/empty
891: .Xr chroot 2
892: directory used by
893: .Nm
894: during privilege separation in the pre-authentication phase.
895: The directory should not contain any files and must be owned by root
896: and not group or world-writable.
897: .Pp
898: .It /var/run/sshd.pid
899: Contains the process ID of the
900: .Nm
901: listening for connections (if there are several daemons running
902: concurrently for different ports, this contains the process ID of the one
903: started last).
904: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 905: .El
1.2 deraadt 906: .Sh SEE ALSO
907: .Xr scp 1 ,
1.90 djm 908: .Xr sftp 1 ,
1.2 deraadt 909: .Xr ssh 1 ,
1.5 deraadt 910: .Xr ssh-add 1 ,
1.2 deraadt 911: .Xr ssh-agent 1 ,
1.5 deraadt 912: .Xr ssh-keygen 1 ,
1.235 jmc 913: .Xr ssh-keyscan 1 ,
1.200 jmc 914: .Xr chroot 2 ,
915: .Xr hosts_access 5 ,
1.136 markus 916: .Xr login.conf 5 ,
917: .Xr moduli 5 ,
1.184 stevesk 918: .Xr sshd_config 5 ,
1.200 jmc 919: .Xr inetd 8 ,
1.128 mpech 920: .Xr sftp-server 8
1.198 jmc 921: .Sh AUTHORS
922: OpenSSH is a derivative of the original and free
923: ssh 1.2.12 release by Tatu Ylonen.
924: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
925: Theo de Raadt and Dug Song
926: removed many bugs, re-added newer features and
927: created OpenSSH.
928: Markus Friedl contributed the support for SSH
929: protocol versions 1.5 and 2.0.
930: Niels Provos and Markus Friedl contributed support
931: for privilege separation.
1.216 jmc 932: .Sh CAVEATS
933: System security is not improved unless
934: .Nm rshd ,
935: .Nm rlogind ,
936: and
937: .Nm rexecd
938: are disabled (thus completely disabling
939: .Xr rlogin
940: and
941: .Xr rsh
942: into the machine).
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