Annotation of src/usr.bin/ssh/sshd.8, Revision 1.259
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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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: .\"
1.259 ! djm 37: .\" $OpenBSD: sshd.8,v 1.258 2010/08/08 19:36:30 jmc Exp $
! 38: .Dd $Mdocdate: August 8 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
1.259 ! djm 173: .Pa /etc/ssh/ssh_host_dsa_key ,
! 174: .Pa /etc/ssh/ssh_host_ecdsa_key
! 175: and
1.167 deraadt 176: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 177: for protocol version 2.
1.75 markus 178: It is possible to have multiple host key files for
1.120 markus 179: the different protocol versions and host key algorithms.
1.2 deraadt 180: .It Fl i
1.7 markus 181: Specifies that
182: .Nm
1.194 jmc 183: is being run from
184: .Xr inetd 8 .
1.7 markus 185: .Nm
186: is normally not run
1.1 deraadt 187: from inetd because it needs to generate the server key before it can
1.36 aaron 188: respond to the client, and this may take tens of seconds.
189: Clients would have to wait too long if the key was regenerated every time.
1.230 jmc 190: However, with small key sizes (e.g. 512) using
1.7 markus 191: .Nm
192: from inetd may
1.1 deraadt 193: be feasible.
1.2 deraadt 194: .It Fl k Ar key_gen_time
1.120 markus 195: Specifies how often the ephemeral protocol version 1 server key is
196: regenerated (default 3600 seconds, or one hour).
1.36 aaron 197: The motivation for regenerating the key fairly
1.200 jmc 198: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 199: it becomes impossible to recover the key for decrypting intercepted
200: communications even if the machine is cracked into or physically
1.36 aaron 201: seized.
202: A value of zero indicates that the key will never be regenerated.
1.156 markus 203: .It Fl o Ar option
204: Can be used to give options in the format used in the configuration file.
205: This is useful for specifying options for which there is no separate
206: command-line flag.
1.200 jmc 207: For full details of the options, and their values, see
208: .Xr sshd_config 5 .
1.2 deraadt 209: .It Fl p Ar port
1.1 deraadt 210: Specifies the port on which the server listens for connections
211: (default 22).
1.158 stevesk 212: Multiple port options are permitted.
1.210 stevesk 213: Ports specified in the configuration file with the
214: .Cm Port
215: option are ignored when a command-line port is specified.
216: Ports specified using the
217: .Cm ListenAddress
218: option override command-line ports.
1.2 deraadt 219: .It Fl q
1.36 aaron 220: Quiet mode.
221: Nothing is sent to the system log.
222: Normally the beginning,
1.1 deraadt 223: authentication, and termination of each connection is logged.
1.242 dtucker 224: .It Fl T
225: Extended test mode.
226: Check the validity of the configuration file, output the effective configuration
227: to stdout and then exit.
228: Optionally,
229: .Cm Match
230: rules may be applied by specifying the connection parameters using one or more
231: .Fl C
232: options.
1.243 jmc 233: .It Fl t
234: Test mode.
235: Only check the validity of the configuration file and sanity of the keys.
236: This is useful for updating
237: .Nm
238: reliably as configuration options may change.
1.61 markus 239: .It Fl u Ar len
240: This option is used to specify the size of the field
241: in the
242: .Li utmp
243: structure that holds the remote host name.
244: If the resolved host name is longer than
245: .Ar len ,
246: the dotted decimal value will be used instead.
247: This allows hosts with very long host names that
248: overflow this field to still be uniquely identified.
249: Specifying
250: .Fl u0
251: indicates that only dotted decimal addresses
252: should be put into the
253: .Pa utmp
254: file.
1.144 stevesk 255: .Fl u0
1.194 jmc 256: may also be used to prevent
1.144 stevesk 257: .Nm
258: from making DNS requests unless the authentication
259: mechanism or configuration requires it.
260: Authentication mechanisms that may require DNS include
261: .Cm RhostsRSAAuthentication ,
1.215 jmc 262: .Cm HostbasedAuthentication ,
1.144 stevesk 263: and using a
264: .Cm from="pattern-list"
265: option in a key file.
1.170 stevesk 266: Configuration options that require DNS include using a
267: USER@HOST pattern in
268: .Cm AllowUsers
269: or
270: .Cm DenyUsers .
1.2 deraadt 271: .El
1.214 jmc 272: .Sh AUTHENTICATION
273: The OpenSSH SSH daemon supports SSH protocols 1 and 2.
1.249 jmc 274: The default is to use protocol 2 only,
1.214 jmc 275: though this can be changed via the
276: .Cm Protocol
277: option in
278: .Xr sshd_config 5 .
1.259 ! djm 279: Protocol 2 supports DSA, ECDSA and RSA keys;
1.214 jmc 280: protocol 1 only supports RSA keys.
281: For both protocols,
282: each host has a host-specific key,
283: normally 2048 bits,
284: used to identify the host.
285: .Pp
286: Forward security for protocol 1 is provided through
287: an additional server key,
288: normally 768 bits,
289: generated when the server starts.
1.212 jmc 290: This key is normally regenerated every hour if it has been used, and
291: is never stored on disk.
292: Whenever a client connects, the daemon responds with its public
293: host and server keys.
294: The client compares the
295: RSA host key against its own database to verify that it has not changed.
296: The client then generates a 256-bit random number.
297: It encrypts this
298: random number using both the host key and the server key, and sends
299: the encrypted number to the server.
300: Both sides then use this
301: random number as a session key which is used to encrypt all further
302: communications in the session.
303: The rest of the session is encrypted
304: using a conventional cipher, currently Blowfish or 3DES, with 3DES
305: being used by default.
306: The client selects the encryption algorithm
307: to use from those offered by the server.
308: .Pp
1.214 jmc 309: For protocol 2,
310: forward security is provided through a Diffie-Hellman key agreement.
311: This key agreement results in a shared session key.
312: The rest of the session is encrypted using a symmetric cipher, currently
313: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
314: The client selects the encryption algorithm
315: to use from those offered by the server.
316: Additionally, session integrity is provided
317: through a cryptographic message authentication code
1.237 pvalchev 318: (hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160).
1.214 jmc 319: .Pp
320: Finally, the server and the client enter an authentication dialog.
1.212 jmc 321: The client tries to authenticate itself using
1.214 jmc 322: host-based authentication,
323: public key authentication,
324: challenge-response authentication,
325: or password authentication.
1.212 jmc 326: .Pp
327: If the client successfully authenticates itself, a dialog for
328: preparing the session is entered.
329: At this time the client may request
330: things like allocating a pseudo-tty, forwarding X11 connections,
331: forwarding TCP connections, or forwarding the authentication agent
332: connection over the secure channel.
333: .Pp
1.216 jmc 334: After this, the client either requests a shell or execution of a command.
1.212 jmc 335: The sides then enter session mode.
336: In this mode, either side may send
337: data at any time, and such data is forwarded to/from the shell or
338: command on the server side, and the user terminal in the client side.
339: .Pp
340: When the user program terminates and all forwarded X11 and other
341: connections have been closed, the server sends command exit status to
342: the client, and both sides exit.
1.2 deraadt 343: .Sh LOGIN PROCESS
1.1 deraadt 344: When a user successfully logs in,
1.2 deraadt 345: .Nm
1.1 deraadt 346: does the following:
1.2 deraadt 347: .Bl -enum -offset indent
348: .It
1.1 deraadt 349: If the login is on a tty, and no command has been specified,
1.40 aaron 350: prints last login time and
1.2 deraadt 351: .Pa /etc/motd
1.1 deraadt 352: (unless prevented in the configuration file or by
1.207 djm 353: .Pa ~/.hushlogin ;
1.2 deraadt 354: see the
1.40 aaron 355: .Sx FILES
1.2 deraadt 356: section).
357: .It
1.1 deraadt 358: If the login is on a tty, records login time.
1.2 deraadt 359: .It
360: Checks
361: .Pa /etc/nologin ;
362: if it exists, prints contents and quits
1.1 deraadt 363: (unless root).
1.2 deraadt 364: .It
1.1 deraadt 365: Changes to run with normal user privileges.
1.2 deraadt 366: .It
1.1 deraadt 367: Sets up basic environment.
1.2 deraadt 368: .It
1.200 jmc 369: Reads the file
1.207 djm 370: .Pa ~/.ssh/environment ,
1.200 jmc 371: if it exists, and users are allowed to change their environment.
1.188 stevesk 372: See the
1.187 marc 373: .Cm PermitUserEnvironment
1.188 stevesk 374: option in
1.187 marc 375: .Xr sshd_config 5 .
1.2 deraadt 376: .It
1.1 deraadt 377: Changes to user's home directory.
1.2 deraadt 378: .It
379: If
1.207 djm 380: .Pa ~/.ssh/rc
1.2 deraadt 381: exists, runs it; else if
1.168 deraadt 382: .Pa /etc/ssh/sshrc
1.2 deraadt 383: exists, runs
1.36 aaron 384: it; otherwise runs xauth.
385: The
1.2 deraadt 386: .Dq rc
387: files are given the X11
1.1 deraadt 388: authentication protocol and cookie in standard input.
1.226 jmc 389: See
390: .Sx SSHRC ,
391: below.
1.2 deraadt 392: .It
1.1 deraadt 393: Runs user's shell or command.
1.2 deraadt 394: .El
1.226 jmc 395: .Sh SSHRC
396: If the file
397: .Pa ~/.ssh/rc
398: exists,
399: .Xr sh 1
400: runs it after reading the
401: environment files but before starting the user's shell or command.
402: It must not produce any output on stdout; stderr must be used
403: instead.
404: If X11 forwarding is in use, it will receive the "proto cookie" pair in
405: its standard input (and
406: .Ev DISPLAY
407: in its environment).
408: The script must call
409: .Xr xauth 1
410: because
411: .Nm
412: will not run xauth automatically to add X11 cookies.
413: .Pp
414: The primary purpose of this file is to run any initialization routines
415: which may be needed before the user's home directory becomes
416: accessible; AFS is a particular example of such an environment.
417: .Pp
418: This file will probably contain some initialization code followed by
419: something similar to:
420: .Bd -literal -offset 3n
421: if read proto cookie && [ -n "$DISPLAY" ]; then
422: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
423: # X11UseLocalhost=yes
424: echo add unix:`echo $DISPLAY |
425: cut -c11-` $proto $cookie
426: else
427: # X11UseLocalhost=no
428: echo add $DISPLAY $proto $cookie
429: fi | xauth -q -
430: fi
431: .Ed
432: .Pp
433: If this file does not exist,
434: .Pa /etc/ssh/sshrc
435: is run, and if that
436: does not exist either, xauth is used to add the cookie.
1.2 deraadt 437: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 438: .Cm AuthorizedKeysFile
1.217 jmc 439: specifies the file containing public keys for
440: public key authentication;
441: if none is specified, the default is
442: .Pa ~/.ssh/authorized_keys .
1.36 aaron 443: Each line of the file contains one
1.2 deraadt 444: key (empty lines and lines starting with a
445: .Ql #
446: are ignored as
1.36 aaron 447: comments).
1.217 jmc 448: Protocol 1 public keys consist of the following space-separated fields:
449: options, bits, exponent, modulus, comment.
450: Protocol 2 public key consist of:
451: options, keytype, base64-encoded key, comment.
452: The options field is optional;
453: its presence is determined by whether the line starts
1.189 espie 454: with a number or not (the options field never starts with a number).
1.217 jmc 455: The bits, exponent, modulus, and comment fields give the RSA key for
1.75 markus 456: protocol version 1; the
1.1 deraadt 457: comment field is not used for anything (but may be convenient for the
458: user to identify the key).
1.75 markus 459: For protocol version 2 the keytype is
1.259 ! djm 460: .Dq ecdsa-sha2-nistp256 ,
! 461: .Dq ecdsa-sha2-nistp384 ,
! 462: .Dq ecdsa-sha2-nistp521 ,
1.75 markus 463: .Dq ssh-dss
464: or
465: .Dq ssh-rsa .
1.2 deraadt 466: .Pp
1.1 deraadt 467: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 468: (because of the size of the public key encoding) up to a limit of
469: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
470: keys up to 16 kilobits.
1.36 aaron 471: You don't want to type them in; instead, copy the
1.113 itojun 472: .Pa identity.pub ,
1.217 jmc 473: .Pa id_dsa.pub ,
1.259 ! djm 474: .Pa id_ecdsa.pub ,
1.75 markus 475: or the
1.113 itojun 476: .Pa id_rsa.pub
1.1 deraadt 477: file and edit it.
1.175 stevesk 478: .Pp
479: .Nm
480: enforces a minimum RSA key modulus size for protocol 1
481: and protocol 2 keys of 768 bits.
1.2 deraadt 482: .Pp
1.58 deraadt 483: The options (if present) consist of comma-separated option
1.36 aaron 484: specifications.
485: No spaces are permitted, except within double quotes.
1.141 stevesk 486: The following option specifications are supported (note
487: that option keywords are case-insensitive):
1.2 deraadt 488: .Bl -tag -width Ds
1.253 djm 489: .It Cm cert-authority
1.251 djm 490: Specifies that the listed key is a certification authority (CA) that is
491: trusted to validate signed certificates for user authentication.
492: .Pp
493: Certificates may encode access restrictions similar to these key options.
494: If both certificate restrictions and key options are present, the most
495: restrictive union of the two is applied.
1.2 deraadt 496: .It Cm command="command"
1.1 deraadt 497: Specifies that the command is executed whenever this key is used for
1.36 aaron 498: authentication.
499: The command supplied by the user (if any) is ignored.
1.148 markus 500: The command is run on a pty if the client requests a pty;
1.36 aaron 501: otherwise it is run without a tty.
1.194 jmc 502: If an 8-bit clean channel is required,
1.147 deraadt 503: one must not request a pty or should specify
1.89 markus 504: .Cm no-pty .
1.36 aaron 505: A quote may be included in the command by quoting it with a backslash.
506: This option might be useful
1.189 espie 507: to restrict certain public keys to perform just a specific operation.
1.36 aaron 508: An example might be a key that permits remote backups but nothing else.
1.211 jmc 509: Note that the client may specify TCP and/or X11
1.51 hugh 510: forwarding unless they are explicitly prohibited.
1.233 dtucker 511: The command originally supplied by the client is available in the
512: .Ev SSH_ORIGINAL_COMMAND
513: environment variable.
1.149 markus 514: Note that this option applies to shell, command or subsystem execution.
1.251 djm 515: Also note that this command may be superseded by either a
516: .Xr sshd_config 5
517: .Cm ForceCommand
518: directive or a command embedded in a certificate.
1.2 deraadt 519: .It Cm environment="NAME=value"
1.1 deraadt 520: Specifies that the string is to be added to the environment when
1.36 aaron 521: logging in using this key.
522: Environment variables set this way
523: override other default environment values.
524: Multiple options of this type are permitted.
1.188 stevesk 525: Environment processing is disabled by default and is
526: controlled via the
527: .Cm PermitUserEnvironment
528: option.
1.155 markus 529: This option is automatically disabled if
530: .Cm UseLogin
531: is enabled.
1.218 jmc 532: .It Cm from="pattern-list"
1.244 djm 533: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 534: name of the remote host or its IP address must be present in the
1.244 djm 535: comma-separated list of patterns.
1.229 jmc 536: See
537: .Sx PATTERNS
538: in
539: .Xr ssh_config 5
540: for more information on patterns.
1.244 djm 541: .Pp
542: In addition to the wildcard matching that may be applied to hostnames or
543: addresses, a
544: .Cm from
1.248 sobrado 545: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 546: .Pp
547: The purpose of this option is to optionally increase security: public key
548: authentication by itself does not trust the network or name servers or
549: anything (but the key); however, if somebody somehow steals the key, the key
550: permits an intruder to log in from anywhere in the world.
551: This additional option makes using a stolen key more difficult (name
552: servers and/or routers would have to be compromised in addition to
553: just the key).
1.218 jmc 554: .It Cm no-agent-forwarding
555: Forbids authentication agent forwarding when this key is used for
556: authentication.
1.2 deraadt 557: .It Cm no-port-forwarding
1.211 jmc 558: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 559: Any port forward requests by the client will return an error.
1.230 jmc 560: This might be used, e.g. in connection with the
1.2 deraadt 561: .Cm command
1.1 deraadt 562: option.
1.218 jmc 563: .It Cm no-pty
564: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 565: .It Cm no-user-rc
1.241 jmc 566: Disables execution of
1.240 djm 567: .Pa ~/.ssh/rc .
1.2 deraadt 568: .It Cm no-X11-forwarding
1.1 deraadt 569: Forbids X11 forwarding when this key is used for authentication.
570: Any X11 forward requests by the client will return an error.
1.107 djm 571: .It Cm permitopen="host:port"
1.133 itojun 572: Limit local
1.107 djm 573: .Li ``ssh -L''
1.111 stevesk 574: port forwarding such that it may only connect to the specified host and
1.146 stevesk 575: port.
1.257 djm 576: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 577: Multiple
1.107 djm 578: .Cm permitopen
1.197 jmc 579: options may be applied separated by commas.
580: No pattern matching is performed on the specified hostnames,
581: they must be literal domains or addresses.
1.256 djm 582: .It Cm principals="principals"
583: On a
584: .Cm cert-authority
585: line, specifies allowed principals for certificate authentication as a
586: comma-separated list.
587: At least one name from the list must appear in the certificate's
588: list of principals for the certificate to be accepted.
589: This option is ignored for keys that are not marked as trusted certificate
590: signers using the
591: .Cm cert-authority
592: option.
1.209 reyk 593: .It Cm tunnel="n"
594: Force a
595: .Xr tun 4
596: device on the server.
597: Without this option, the next available device will be used if
598: the client requests a tunnel.
1.2 deraadt 599: .El
600: .Pp
1.219 jmc 601: An example authorized_keys file:
1.222 jmc 602: .Bd -literal -offset 3n
1.219 jmc 603: # Comments allowed at start of line
604: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 605: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
606: AAAAB2...19Q== john@example.net
607: command="dump /home",no-pty,no-port-forwarding ssh-dss
608: AAAAC3...51R== example.net
609: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
610: AAAAB5...21S==
611: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
612: jane@example.net
1.219 jmc 613: .Ed
1.2 deraadt 614: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 615: The
1.194 jmc 616: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 617: and
1.207 djm 618: .Pa ~/.ssh/known_hosts
1.36 aaron 619: files contain host public keys for all known hosts.
620: The global file should
1.37 brad 621: be prepared by the administrator (optional), and the per-user file is
1.220 jmc 622: maintained automatically: whenever the user connects from an unknown host,
1.36 aaron 623: its key is added to the per-user file.
1.2 deraadt 624: .Pp
1.254 djm 625: Each line in these files contains the following fields: markers (optional),
626: hostnames, bits, exponent, modulus, comment.
1.36 aaron 627: The fields are separated by spaces.
1.2 deraadt 628: .Pp
1.254 djm 629: The marker is optional, but if it is present then it must be one of
630: .Dq @cert-authority ,
631: to indicate that the line contains a certification authority (CA) key,
632: or
633: .Dq @revoked ,
634: to indicate that the key contained on the line is revoked and must not ever
635: be accepted.
636: Only one marker should be used on a key line.
637: .Pp
1.196 david 638: Hostnames is a comma-separated list of patterns
1.220 jmc 639: .Pf ( Ql *
1.197 jmc 640: and
641: .Ql \&?
1.195 mouring 642: act as
1.1 deraadt 643: wildcards); each pattern in turn is matched against the canonical host
644: name (when authenticating a client) or against the user-supplied
1.36 aaron 645: name (when authenticating a server).
646: A pattern may also be preceded by
1.195 mouring 647: .Ql \&!
1.2 deraadt 648: to indicate negation: if the host name matches a negated
1.1 deraadt 649: pattern, it is not accepted (by that line) even if it matched another
650: pattern on the line.
1.231 dtucker 651: A hostname or address may optionally be enclosed within
652: .Ql \&[
653: and
654: .Ql \&]
655: brackets then followed by
656: .Ql \&:
1.232 jmc 657: and a non-standard port number.
1.2 deraadt 658: .Pp
1.205 djm 659: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 660: and addresses should the file's contents be disclosed.
661: Hashed hostnames start with a
662: .Ql |
1.205 djm 663: character.
664: Only one hashed hostname may appear on a single line and none of the above
665: negation or wildcard operators may be applied.
666: .Pp
1.49 markus 667: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.220 jmc 668: can be obtained, for example, from
1.167 deraadt 669: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 670: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 671: .Pp
672: Lines starting with
673: .Ql #
674: and empty lines are ignored as comments.
675: .Pp
1.1 deraadt 676: When performing host authentication, authentication is accepted if any
1.254 djm 677: matching line has the proper key; either one that matches exactly or,
678: if the server has presented a certificate for authentication, the key
679: of the certification authority that signed the certificate.
680: For a key to be trusted as a certification authority, it must use the
681: .Dq @cert-authority
682: marker described above.
683: .Pp
684: The known hosts file also provides a facility to mark keys as revoked,
685: for example when it is known that the associated private key has been
686: stolen.
687: Revoked keys are specified by including the
688: .Dq @revoked
689: marker at the beginning of the key line, and are never accepted for
690: authentication or as certification authorities, but instead will
691: produce a warning from
692: .Xr ssh 1
693: when they are encountered.
694: .Pp
695: It is permissible (but not
1.1 deraadt 696: recommended) to have several lines or different host keys for the same
1.36 aaron 697: names.
698: This will inevitably happen when short forms of host names
699: from different domains are put in the file.
700: It is possible
1.1 deraadt 701: that the files contain conflicting information; authentication is
702: accepted if valid information can be found from either file.
1.2 deraadt 703: .Pp
1.1 deraadt 704: Note that the lines in these files are typically hundreds of characters
705: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 706: Rather, generate them by a script,
707: .Xr ssh-keyscan 1
1.40 aaron 708: or by taking
1.167 deraadt 709: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 710: and adding the host names at the front.
1.254 djm 711: .Xr ssh-keygen 1
1.255 jmc 712: also offers some basic automated editing for
1.254 djm 713: .Pa ~/.ssh/known_hosts
714: including removing hosts matching a host name and converting all host
715: names to their hashed representations.
1.221 jmc 716: .Pp
717: An example ssh_known_hosts file:
718: .Bd -literal -offset 3n
719: # Comments allowed at start of line
720: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
721: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 722: # A hashed hostname
723: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
724: AAAA1234.....=
1.254 djm 725: # A revoked key
726: @revoked * ssh-rsa AAAAB5W...
727: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
728: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 729: .Ed
1.2 deraadt 730: .Sh FILES
1.223 jmc 731: .Bl -tag -width Ds -compact
1.258 jmc 732: .It Pa ~/.hushlogin
1.204 jmc 733: This file is used to suppress printing the last login time and
734: .Pa /etc/motd ,
735: if
736: .Cm PrintLastLog
737: and
738: .Cm PrintMotd ,
739: respectively,
740: are enabled.
741: It does not suppress printing of the banner specified by
742: .Cm Banner .
1.223 jmc 743: .Pp
1.258 jmc 744: .It Pa ~/.rhosts
1.225 jmc 745: This file is used for host-based authentication (see
746: .Xr ssh 1
747: for more information).
748: On some machines this file may need to be
749: world-readable if the user's home directory is on an NFS partition,
750: because
751: .Nm
752: reads it as root.
753: Additionally, this file must be owned by the user,
754: and must not have write permissions for anyone else.
755: The recommended
756: permission for most machines is read/write for the user, and not
1.1 deraadt 757: accessible by others.
1.2 deraadt 758: .Pp
1.258 jmc 759: .It Pa ~/.shosts
1.225 jmc 760: This file is used in exactly the same way as
761: .Pa .rhosts ,
762: but allows host-based authentication without permitting login with
763: rlogin/rsh.
1.238 mcbride 764: .Pp
1.258 jmc 765: .It Pa ~/.ssh/
1.238 mcbride 766: This directory is the default location for all user-specific configuration
767: and authentication information.
768: There is no general requirement to keep the entire contents of this directory
769: secret, but the recommended permissions are read/write/execute for the user,
770: and not accessible by others.
1.2 deraadt 771: .Pp
1.258 jmc 772: .It Pa ~/.ssh/authorized_keys
1.259 ! djm 773: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in
! 774: as this user.
1.223 jmc 775: The format of this file is described above.
1.234 dtucker 776: The content of the file is not highly sensitive, but the recommended
1.225 jmc 777: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 778: .Pp
779: If this file, the
780: .Pa ~/.ssh
781: directory, or the user's home directory are writable
782: by other users, then the file could be modified or replaced by unauthorized
783: users.
784: In this case,
785: .Nm
786: will not allow it to be used unless the
787: .Cm StrictModes
788: option has been set to
789: .Dq no .
1.2 deraadt 790: .Pp
1.258 jmc 791: .It Pa ~/.ssh/environment
1.36 aaron 792: This file is read into the environment at login (if it exists).
793: It can only contain empty lines, comment lines (that start with
1.2 deraadt 794: .Ql # ) ,
1.36 aaron 795: and assignment lines of the form name=value.
796: The file should be writable
1.6 aaron 797: only by the user; it need not be readable by anyone else.
1.188 stevesk 798: Environment processing is disabled by default and is
799: controlled via the
800: .Cm PermitUserEnvironment
801: option.
1.223 jmc 802: .Pp
1.258 jmc 803: .It Pa ~/.ssh/known_hosts
1.225 jmc 804: Contains a list of host keys for all hosts the user has logged into
805: that are not already in the systemwide list of known host keys.
806: The format of this file is described above.
807: This file should be writable only by root/the owner and
1.223 jmc 808: can, but need not be, world-readable.
809: .Pp
1.258 jmc 810: .It Pa ~/.ssh/rc
1.226 jmc 811: Contains initialization routines to be run before
812: the user's home directory becomes accessible.
1.1 deraadt 813: This file should be writable only by the user, and need not be
814: readable by anyone else.
1.223 jmc 815: .Pp
1.258 jmc 816: .It Pa /etc/hosts.allow
817: .It Pa /etc/hosts.deny
1.223 jmc 818: Access controls that should be enforced by tcp-wrappers are defined here.
819: Further details are described in
820: .Xr hosts_access 5 .
821: .Pp
1.258 jmc 822: .It Pa /etc/hosts.equiv
1.227 jmc 823: This file is for host-based authentication (see
824: .Xr ssh 1 ) .
825: It should only be writable by root.
1.223 jmc 826: .Pp
1.258 jmc 827: .It Pa /etc/moduli
1.223 jmc 828: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
829: The file format is described in
830: .Xr moduli 5 .
831: .Pp
1.258 jmc 832: .It Pa /etc/motd
1.223 jmc 833: See
834: .Xr motd 5 .
835: .Pp
1.258 jmc 836: .It Pa /etc/nologin
1.223 jmc 837: If this file exists,
838: .Nm
839: refuses to let anyone except root log in.
840: The contents of the file
841: are displayed to anyone trying to log in, and non-root connections are
842: refused.
843: The file should be world-readable.
844: .Pp
1.258 jmc 845: .It Pa /etc/shosts.equiv
1.227 jmc 846: This file is used in exactly the same way as
847: .Pa hosts.equiv ,
848: but allows host-based authentication without permitting login with
849: rlogin/rsh.
1.225 jmc 850: .Pp
1.258 jmc 851: .It Pa /etc/ssh/ssh_host_key
852: .It Pa /etc/ssh/ssh_host_dsa_key
1.259 ! djm 853: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.258 jmc 854: .It Pa /etc/ssh/ssh_host_rsa_key
1.223 jmc 855: These three files contain the private parts of the host keys.
856: These files should only be owned by root, readable only by root, and not
857: accessible to others.
858: Note that
859: .Nm
1.228 jmc 860: does not start if these files are group/world-accessible.
1.223 jmc 861: .Pp
1.258 jmc 862: .It Pa /etc/ssh/ssh_host_key.pub
863: .It Pa /etc/ssh/ssh_host_dsa_key.pub
1.259 ! djm 864: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.258 jmc 865: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.223 jmc 866: These three files contain the public parts of the host keys.
867: These files should be world-readable but writable only by
868: root.
869: Their contents should match the respective private parts.
870: These files are not
871: really used for anything; they are provided for the convenience of
872: the user so their contents can be copied to known hosts files.
873: These files are created using
874: .Xr ssh-keygen 1 .
875: .Pp
1.258 jmc 876: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 877: Systemwide list of known host keys.
878: This file should be prepared by the
879: system administrator to contain the public host keys of all machines in the
880: organization.
881: The format of this file is described above.
882: This file should be writable only by root/the owner and
883: should be world-readable.
884: .Pp
1.258 jmc 885: .It Pa /etc/ssh/sshd_config
1.223 jmc 886: Contains configuration data for
887: .Nm sshd .
888: The file format and configuration options are described in
889: .Xr sshd_config 5 .
890: .Pp
1.258 jmc 891: .It Pa /etc/ssh/sshrc
1.226 jmc 892: Similar to
893: .Pa ~/.ssh/rc ,
894: it can be used to specify
1.36 aaron 895: machine-specific login-time initializations globally.
896: This file should be writable only by root, and should be world-readable.
1.223 jmc 897: .Pp
1.258 jmc 898: .It Pa /var/empty
1.223 jmc 899: .Xr chroot 2
900: directory used by
901: .Nm
902: during privilege separation in the pre-authentication phase.
903: The directory should not contain any files and must be owned by root
904: and not group or world-writable.
905: .Pp
1.258 jmc 906: .It Pa /var/run/sshd.pid
1.223 jmc 907: Contains the process ID of the
908: .Nm
909: listening for connections (if there are several daemons running
910: concurrently for different ports, this contains the process ID of the one
911: started last).
912: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 913: .El
1.2 deraadt 914: .Sh SEE ALSO
915: .Xr scp 1 ,
1.90 djm 916: .Xr sftp 1 ,
1.2 deraadt 917: .Xr ssh 1 ,
1.5 deraadt 918: .Xr ssh-add 1 ,
1.2 deraadt 919: .Xr ssh-agent 1 ,
1.5 deraadt 920: .Xr ssh-keygen 1 ,
1.235 jmc 921: .Xr ssh-keyscan 1 ,
1.200 jmc 922: .Xr chroot 2 ,
923: .Xr hosts_access 5 ,
1.136 markus 924: .Xr login.conf 5 ,
925: .Xr moduli 5 ,
1.184 stevesk 926: .Xr sshd_config 5 ,
1.200 jmc 927: .Xr inetd 8 ,
1.128 mpech 928: .Xr sftp-server 8
1.198 jmc 929: .Sh AUTHORS
930: OpenSSH is a derivative of the original and free
931: ssh 1.2.12 release by Tatu Ylonen.
932: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
933: Theo de Raadt and Dug Song
934: removed many bugs, re-added newer features and
935: created OpenSSH.
936: Markus Friedl contributed the support for SSH
937: protocol versions 1.5 and 2.0.
938: Niels Provos and Markus Friedl contributed support
939: for privilege separation.
1.216 jmc 940: .Sh CAVEATS
941: System security is not improved unless
942: .Nm rshd ,
943: .Nm rlogind ,
944: and
945: .Nm rexecd
946: are disabled (thus completely disabling
947: .Xr rlogin
948: and
949: .Xr rsh
950: into the machine).
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