Annotation of src/usr.bin/ssh/sshd.8, Revision 1.284
1.1 deraadt 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
1.64 deraadt 6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
1.99 deraadt 12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.64 deraadt 15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 24: .\"
1.64 deraadt 25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 35: .\"
1.284 ! jmc 36: .\" $OpenBSD: sshd.8,v 1.283 2016/02/05 03:07:06 djm Exp $
! 37: .Dd $Mdocdate: February 5 2016 $
1.2 deraadt 38: .Dt SSHD 8
39: .Os
40: .Sh NAME
41: .Nm sshd
1.120 markus 42: .Nd OpenSSH SSH daemon
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm sshd
1.194 jmc 45: .Bk -words
1.243 jmc 46: .Op Fl 46DdeiqTt
1.2 deraadt 47: .Op Fl b Ar bits
1.242 dtucker 48: .Op Fl C Ar connection_spec
1.251 djm 49: .Op Fl c Ar host_certificate_file
1.268 dtucker 50: .Op Fl E Ar log_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 .
1.275 tedu 63: Together these programs replace rlogin and rsh,
1.235 jmc 64: and provide secure encrypted communications between two untrusted hosts
1.36 aaron 65: over an insecure network.
1.2 deraadt 66: .Pp
67: .Nm
1.212 jmc 68: listens for connections from clients.
1.40 aaron 69: It is normally started at boot from
1.2 deraadt 70: .Pa /etc/rc .
71: It forks a new
1.36 aaron 72: daemon for each incoming connection.
73: The forked daemons handle
1.1 deraadt 74: key exchange, encryption, authentication, command execution,
75: and data exchange.
1.2 deraadt 76: .Pp
77: .Nm
1.200 jmc 78: can be configured using command-line options or a configuration file
79: (by default
1.212 jmc 80: .Xr sshd_config 5 ) ;
81: command-line options override values specified in the
1.1 deraadt 82: configuration file.
1.25 markus 83: .Nm
84: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 85: .Dv SIGHUP ,
1.230 jmc 86: by executing itself with the name and options it was started with, e.g.\&
1.97 deraadt 87: .Pa /usr/sbin/sshd .
1.18 aaron 88: .Pp
89: The options are as follows:
1.2 deraadt 90: .Bl -tag -width Ds
1.200 jmc 91: .It Fl 4
92: Forces
93: .Nm
94: to use IPv4 addresses only.
95: .It Fl 6
96: Forces
97: .Nm
98: to use IPv6 addresses only.
1.2 deraadt 99: .It Fl b Ar bits
1.120 markus 100: Specifies the number of bits in the ephemeral protocol version 1
1.246 djm 101: server key (default 1024).
1.243 jmc 102: .It Fl C Ar connection_spec
103: Specify the connection parameters to use for the
104: .Fl T
105: extended test mode.
106: If provided, any
107: .Cm Match
108: directives in the configuration file
109: that would apply to the specified user, host, and address will be set before
110: the configuration is written to standard output.
111: The connection parameters are supplied as keyword=value pairs.
112: The keywords are
113: .Dq user ,
114: .Dq host ,
1.265 dtucker 115: .Dq laddr ,
116: .Dq lport ,
1.243 jmc 117: and
118: .Dq addr .
119: All are required and may be supplied in any order, either with multiple
120: .Fl C
121: options or as a comma-separated list.
1.252 jmc 122: .It Fl c Ar host_certificate_file
123: Specifies a path to a certificate file to identify
124: .Nm
125: during key exchange.
126: The certificate file must match a host key file specified using the
127: .Fl h
128: option or the
129: .Cm HostKey
130: configuration directive.
1.200 jmc 131: .It Fl D
132: When this option is specified,
133: .Nm
134: will not detach and does not become a daemon.
135: This allows easy monitoring of
136: .Nm sshd .
1.2 deraadt 137: .It Fl d
1.36 aaron 138: Debug mode.
1.250 djm 139: The server sends verbose debug output to standard error,
140: and does not put itself in the background.
1.36 aaron 141: The server also will not fork and will only process one connection.
142: This option is only intended for debugging for the server.
1.194 jmc 143: Multiple
144: .Fl d
145: options increase the debugging level.
1.67 aaron 146: Maximum is 3.
1.268 dtucker 147: .It Fl E Ar log_file
148: Append debug logs to
149: .Ar log_file
150: instead of the system log.
1.120 markus 151: .It Fl e
1.269 dtucker 152: Write debug logs to standard error instead of the system log.
1.235 jmc 153: .It Fl f Ar config_file
1.36 aaron 154: Specifies the name of the configuration file.
155: The default is
1.167 deraadt 156: .Pa /etc/ssh/sshd_config .
1.16 markus 157: .Nm
158: refuses to start if there is no configuration file.
1.2 deraadt 159: .It Fl g Ar login_grace_time
1.1 deraadt 160: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 161: 120 seconds).
1.36 aaron 162: If the client fails to authenticate the user within
163: this many seconds, the server disconnects and exits.
164: A value of zero indicates no limit.
1.2 deraadt 165: .It Fl h Ar host_key_file
1.160 stevesk 166: Specifies a file from which a host key is read.
1.7 markus 167: This option must be given if
168: .Nm
169: is not run as root (as the normal
1.160 stevesk 170: host key files are normally not readable by anyone but root).
171: The default is
1.167 deraadt 172: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 173: for protocol version 1, and
1.259 djm 174: .Pa /etc/ssh/ssh_host_dsa_key ,
1.273 naddy 175: .Pa /etc/ssh/ssh_host_ecdsa_key .
176: .Pa /etc/ssh/ssh_host_ed25519_key
1.259 djm 177: and
1.167 deraadt 178: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 179: for protocol version 2.
1.75 markus 180: It is possible to have multiple host key files for
1.120 markus 181: the different protocol versions and host key algorithms.
1.2 deraadt 182: .It Fl i
1.7 markus 183: Specifies that
184: .Nm
1.194 jmc 185: is being run from
186: .Xr inetd 8 .
1.280 djm 187: If SSH protocol 1 is enabled,
1.7 markus 188: .Nm
1.280 djm 189: should not normally be run
1.1 deraadt 190: from inetd because it needs to generate the server key before it can
1.280 djm 191: respond to the client, and this may take some time.
192: Clients may have to wait too long if the key was regenerated every time.
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 .
1.284 ! jmc 278: Protocol 1 should not be used
! 279: and is only offered to support legacy devices.
! 280: .Pp
! 281: Each host has a host-specific key,
1.214 jmc 282: used to identify the host.
1.284 ! jmc 283: Partial forward security for protocol 1 is provided through
1.214 jmc 284: an additional server key,
1.280 djm 285: normally 1024 bits,
1.214 jmc 286: generated when the server starts.
1.212 jmc 287: This key is normally regenerated every hour if it has been used, and
288: is never stored on disk.
289: Whenever a client connects, the daemon responds with its public
290: host and server keys.
291: The client compares the
292: RSA host key against its own database to verify that it has not changed.
293: The client then generates a 256-bit random number.
294: It encrypts this
295: random number using both the host key and the server key, and sends
296: the encrypted number to the server.
297: Both sides then use this
298: random number as a session key which is used to encrypt all further
299: communications in the session.
300: The rest of the session is encrypted
301: using a conventional cipher, currently Blowfish or 3DES, with 3DES
302: being used by default.
303: The client selects the encryption algorithm
304: to use from those offered by the server.
305: .Pp
1.214 jmc 306: For protocol 2,
307: forward security is provided through a Diffie-Hellman key agreement.
308: This key agreement results in a shared session key.
309: The rest of the session is encrypted using a symmetric cipher, currently
310: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
311: The client selects the encryption algorithm
312: to use from those offered by the server.
313: Additionally, session integrity is provided
314: through a cryptographic message authentication code
1.267 markus 315: (hmac-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160,
1.263 djm 316: hmac-sha2-256 or hmac-sha2-512).
1.214 jmc 317: .Pp
318: Finally, the server and the client enter an authentication dialog.
1.212 jmc 319: The client tries to authenticate itself using
1.214 jmc 320: host-based authentication,
321: public key authentication,
322: challenge-response authentication,
323: or password authentication.
1.212 jmc 324: .Pp
325: If the client successfully authenticates itself, a dialog for
326: preparing the session is entered.
327: At this time the client may request
328: things like allocating a pseudo-tty, forwarding X11 connections,
329: forwarding TCP connections, or forwarding the authentication agent
330: connection over the secure channel.
331: .Pp
1.216 jmc 332: After this, the client either requests a shell or execution of a command.
1.212 jmc 333: The sides then enter session mode.
334: In this mode, either side may send
335: data at any time, and such data is forwarded to/from the shell or
336: command on the server side, and the user terminal in the client side.
337: .Pp
338: When the user program terminates and all forwarded X11 and other
339: connections have been closed, the server sends command exit status to
340: the client, and both sides exit.
1.2 deraadt 341: .Sh LOGIN PROCESS
1.1 deraadt 342: When a user successfully logs in,
1.2 deraadt 343: .Nm
1.1 deraadt 344: does the following:
1.2 deraadt 345: .Bl -enum -offset indent
346: .It
1.1 deraadt 347: If the login is on a tty, and no command has been specified,
1.40 aaron 348: prints last login time and
1.2 deraadt 349: .Pa /etc/motd
1.1 deraadt 350: (unless prevented in the configuration file or by
1.207 djm 351: .Pa ~/.hushlogin ;
1.2 deraadt 352: see the
1.40 aaron 353: .Sx FILES
1.2 deraadt 354: section).
355: .It
1.1 deraadt 356: If the login is on a tty, records login time.
1.2 deraadt 357: .It
358: Checks
359: .Pa /etc/nologin ;
360: if it exists, prints contents and quits
1.1 deraadt 361: (unless root).
1.2 deraadt 362: .It
1.1 deraadt 363: Changes to run with normal user privileges.
1.2 deraadt 364: .It
1.1 deraadt 365: Sets up basic environment.
1.2 deraadt 366: .It
1.200 jmc 367: Reads the file
1.207 djm 368: .Pa ~/.ssh/environment ,
1.200 jmc 369: if it exists, and users are allowed to change their environment.
1.188 stevesk 370: See the
1.187 marc 371: .Cm PermitUserEnvironment
1.188 stevesk 372: option in
1.187 marc 373: .Xr sshd_config 5 .
1.2 deraadt 374: .It
1.1 deraadt 375: Changes to user's home directory.
1.2 deraadt 376: .It
377: If
1.207 djm 378: .Pa ~/.ssh/rc
1.276 djm 379: exists and the
380: .Xr sshd_config 5
381: .Cm PermitUserRC
382: option is set, runs it; else if
1.168 deraadt 383: .Pa /etc/ssh/sshrc
1.2 deraadt 384: exists, runs
1.36 aaron 385: it; otherwise runs xauth.
386: The
1.2 deraadt 387: .Dq rc
388: files are given the X11
1.1 deraadt 389: authentication protocol and cookie in standard input.
1.226 jmc 390: See
391: .Sx SSHRC ,
392: below.
1.2 deraadt 393: .It
1.1 deraadt 394: Runs user's shell or command.
1.279 djm 395: All commands are run under the user's login shell as specified in the
396: system password database.
1.2 deraadt 397: .El
1.226 jmc 398: .Sh SSHRC
399: If the file
400: .Pa ~/.ssh/rc
401: exists,
402: .Xr sh 1
403: runs it after reading the
404: environment files but before starting the user's shell or command.
405: It must not produce any output on stdout; stderr must be used
406: instead.
407: If X11 forwarding is in use, it will receive the "proto cookie" pair in
408: its standard input (and
409: .Ev DISPLAY
410: in its environment).
411: The script must call
412: .Xr xauth 1
413: because
414: .Nm
415: will not run xauth automatically to add X11 cookies.
416: .Pp
417: The primary purpose of this file is to run any initialization routines
418: which may be needed before the user's home directory becomes
419: accessible; AFS is a particular example of such an environment.
420: .Pp
421: This file will probably contain some initialization code followed by
422: something similar to:
423: .Bd -literal -offset 3n
424: if read proto cookie && [ -n "$DISPLAY" ]; then
425: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
426: # X11UseLocalhost=yes
427: echo add unix:`echo $DISPLAY |
428: cut -c11-` $proto $cookie
429: else
430: # X11UseLocalhost=no
431: echo add $DISPLAY $proto $cookie
432: fi | xauth -q -
433: fi
434: .Ed
435: .Pp
436: If this file does not exist,
437: .Pa /etc/ssh/sshrc
438: is run, and if that
439: does not exist either, xauth is used to add the cookie.
1.2 deraadt 440: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 441: .Cm AuthorizedKeysFile
1.262 jmc 442: specifies the files containing public keys for
1.217 jmc 443: public key authentication;
1.281 djm 444: if this option is not specified, the default is
1.261 djm 445: .Pa ~/.ssh/authorized_keys
446: and
447: .Pa ~/.ssh/authorized_keys2 .
1.36 aaron 448: Each line of the file contains one
1.2 deraadt 449: key (empty lines and lines starting with a
450: .Ql #
451: are ignored as
1.36 aaron 452: comments).
1.217 jmc 453: Protocol 1 public keys consist of the following space-separated fields:
454: options, bits, exponent, modulus, comment.
455: Protocol 2 public key consist of:
456: options, keytype, base64-encoded key, comment.
457: The options field is optional;
458: its presence is determined by whether the line starts
1.189 espie 459: with a number or not (the options field never starts with a number).
1.217 jmc 460: The bits, exponent, modulus, and comment fields give the RSA key for
1.75 markus 461: protocol version 1; the
1.1 deraadt 462: comment field is not used for anything (but may be convenient for the
463: user to identify the key).
1.75 markus 464: For protocol version 2 the keytype is
1.259 djm 465: .Dq ecdsa-sha2-nistp256 ,
466: .Dq ecdsa-sha2-nistp384 ,
467: .Dq ecdsa-sha2-nistp521 ,
1.272 jmc 468: .Dq ssh-ed25519 ,
1.75 markus 469: .Dq ssh-dss
470: or
471: .Dq ssh-rsa .
1.2 deraadt 472: .Pp
1.1 deraadt 473: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 474: (because of the size of the public key encoding) up to a limit of
475: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
476: keys up to 16 kilobits.
1.36 aaron 477: You don't want to type them in; instead, copy the
1.113 itojun 478: .Pa identity.pub ,
1.217 jmc 479: .Pa id_dsa.pub ,
1.259 djm 480: .Pa id_ecdsa.pub ,
1.273 naddy 481: .Pa id_ed25519.pub ,
1.75 markus 482: or the
1.113 itojun 483: .Pa id_rsa.pub
1.1 deraadt 484: file and edit it.
1.175 stevesk 485: .Pp
486: .Nm
487: enforces a minimum RSA key modulus size for protocol 1
488: and protocol 2 keys of 768 bits.
1.2 deraadt 489: .Pp
1.58 deraadt 490: The options (if present) consist of comma-separated option
1.36 aaron 491: specifications.
492: No spaces are permitted, except within double quotes.
1.141 stevesk 493: The following option specifications are supported (note
494: that option keywords are case-insensitive):
1.2 deraadt 495: .Bl -tag -width Ds
1.282 djm 496: .It Cm agent-forwarding
497: Enable authentication agent forwarding previously disabled by the
498: .Cm restrict
499: option.
1.253 djm 500: .It Cm cert-authority
1.251 djm 501: Specifies that the listed key is a certification authority (CA) that is
502: trusted to validate signed certificates for user authentication.
503: .Pp
504: Certificates may encode access restrictions similar to these key options.
505: If both certificate restrictions and key options are present, the most
506: restrictive union of the two is applied.
1.2 deraadt 507: .It Cm command="command"
1.1 deraadt 508: Specifies that the command is executed whenever this key is used for
1.36 aaron 509: authentication.
510: The command supplied by the user (if any) is ignored.
1.148 markus 511: The command is run on a pty if the client requests a pty;
1.36 aaron 512: otherwise it is run without a tty.
1.194 jmc 513: If an 8-bit clean channel is required,
1.147 deraadt 514: one must not request a pty or should specify
1.89 markus 515: .Cm no-pty .
1.36 aaron 516: A quote may be included in the command by quoting it with a backslash.
517: This option might be useful
1.189 espie 518: to restrict certain public keys to perform just a specific operation.
1.36 aaron 519: An example might be a key that permits remote backups but nothing else.
1.211 jmc 520: Note that the client may specify TCP and/or X11
1.51 hugh 521: forwarding unless they are explicitly prohibited.
1.233 dtucker 522: The command originally supplied by the client is available in the
523: .Ev SSH_ORIGINAL_COMMAND
524: environment variable.
1.149 markus 525: Note that this option applies to shell, command or subsystem execution.
1.251 djm 526: Also note that this command may be superseded by either a
527: .Xr sshd_config 5
528: .Cm ForceCommand
529: directive or a command embedded in a certificate.
1.2 deraadt 530: .It Cm environment="NAME=value"
1.1 deraadt 531: Specifies that the string is to be added to the environment when
1.36 aaron 532: logging in using this key.
533: Environment variables set this way
534: override other default environment values.
535: Multiple options of this type are permitted.
1.188 stevesk 536: Environment processing is disabled by default and is
537: controlled via the
538: .Cm PermitUserEnvironment
539: option.
1.155 markus 540: This option is automatically disabled if
541: .Cm UseLogin
542: is enabled.
1.218 jmc 543: .It Cm from="pattern-list"
1.244 djm 544: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 545: name of the remote host or its IP address must be present in the
1.244 djm 546: comma-separated list of patterns.
1.270 jmc 547: See PATTERNS in
1.229 jmc 548: .Xr ssh_config 5
549: for more information on patterns.
1.244 djm 550: .Pp
551: In addition to the wildcard matching that may be applied to hostnames or
552: addresses, a
553: .Cm from
1.248 sobrado 554: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 555: .Pp
556: The purpose of this option is to optionally increase security: public key
557: authentication by itself does not trust the network or name servers or
558: anything (but the key); however, if somebody somehow steals the key, the key
559: permits an intruder to log in from anywhere in the world.
560: This additional option makes using a stolen key more difficult (name
561: servers and/or routers would have to be compromised in addition to
562: just the key).
1.218 jmc 563: .It Cm no-agent-forwarding
564: Forbids authentication agent forwarding when this key is used for
565: authentication.
1.2 deraadt 566: .It Cm no-port-forwarding
1.211 jmc 567: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 568: Any port forward requests by the client will return an error.
1.230 jmc 569: This might be used, e.g. in connection with the
1.2 deraadt 570: .Cm command
1.1 deraadt 571: option.
1.218 jmc 572: .It Cm no-pty
573: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 574: .It Cm no-user-rc
1.241 jmc 575: Disables execution of
1.240 djm 576: .Pa ~/.ssh/rc .
1.2 deraadt 577: .It Cm no-X11-forwarding
1.1 deraadt 578: Forbids X11 forwarding when this key is used for authentication.
579: Any X11 forward requests by the client will return an error.
1.107 djm 580: .It Cm permitopen="host:port"
1.278 bentley 581: Limit local port forwarding with
582: .Xr ssh 1
583: .Fl L
584: such that it may only connect to the specified host and port.
1.257 djm 585: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 586: Multiple
1.107 djm 587: .Cm permitopen
1.197 jmc 588: options may be applied separated by commas.
589: No pattern matching is performed on the specified hostnames,
590: they must be literal domains or addresses.
1.264 dtucker 591: A port specification of
592: .Cm *
593: matches any port.
1.282 djm 594: .It Cm port-forwarding
595: Enable port forwarding previously disabled by the
596: .Cm restrict
1.256 djm 597: .It Cm principals="principals"
598: On a
599: .Cm cert-authority
600: line, specifies allowed principals for certificate authentication as a
601: comma-separated list.
602: At least one name from the list must appear in the certificate's
603: list of principals for the certificate to be accepted.
604: This option is ignored for keys that are not marked as trusted certificate
605: signers using the
606: .Cm cert-authority
607: option.
1.282 djm 608: .It Cm pty
609: Permits tty allocation previously disabled by the
610: .Cm restrict
611: option.
612: .It Cm restrict
613: Enable all restrictions, i.e. disable port, agent and X11 forwarding,
614: as well as disabling PTY allocation
615: and execution of
616: .Pa ~/.ssh/rc .
617: If any future restriction capabilities are added to authorized_keys files
618: they will be included in this set.
1.209 reyk 619: .It Cm tunnel="n"
620: Force a
621: .Xr tun 4
622: device on the server.
623: Without this option, the next available device will be used if
624: the client requests a tunnel.
1.282 djm 625: .It Cm user-rc
626: Enables execution of
627: .Pa ~/.ssh/rc
628: previously disabled by the
629: .Cm restrict
630: option.
631: .It Cm X11-forwarding
632: Permits X11 forwarding previously disabled by the
633: .Cm restrict
634: option.
1.2 deraadt 635: .El
636: .Pp
1.219 jmc 637: An example authorized_keys file:
1.222 jmc 638: .Bd -literal -offset 3n
1.219 jmc 639: # Comments allowed at start of line
640: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 641: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
642: AAAAB2...19Q== john@example.net
643: command="dump /home",no-pty,no-port-forwarding ssh-dss
644: AAAAC3...51R== example.net
645: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
646: AAAAB5...21S==
647: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
648: jane@example.net
1.282 djm 649: restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
650: user@example.net
651: restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==
652: user@example.net
1.219 jmc 653: .Ed
1.2 deraadt 654: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 655: The
1.194 jmc 656: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 657: and
1.207 djm 658: .Pa ~/.ssh/known_hosts
1.36 aaron 659: files contain host public keys for all known hosts.
660: The global file should
1.37 brad 661: be prepared by the administrator (optional), and the per-user file is
1.220 jmc 662: maintained automatically: whenever the user connects from an unknown host,
1.36 aaron 663: its key is added to the per-user file.
1.2 deraadt 664: .Pp
1.254 djm 665: Each line in these files contains the following fields: markers (optional),
666: hostnames, bits, exponent, modulus, comment.
1.36 aaron 667: The fields are separated by spaces.
1.2 deraadt 668: .Pp
1.254 djm 669: The marker is optional, but if it is present then it must be one of
670: .Dq @cert-authority ,
671: to indicate that the line contains a certification authority (CA) key,
672: or
673: .Dq @revoked ,
674: to indicate that the key contained on the line is revoked and must not ever
675: be accepted.
676: Only one marker should be used on a key line.
677: .Pp
1.196 david 678: Hostnames is a comma-separated list of patterns
1.220 jmc 679: .Pf ( Ql *
1.197 jmc 680: and
681: .Ql \&?
1.195 mouring 682: act as
1.1 deraadt 683: wildcards); each pattern in turn is matched against the canonical host
684: name (when authenticating a client) or against the user-supplied
1.36 aaron 685: name (when authenticating a server).
686: A pattern may also be preceded by
1.195 mouring 687: .Ql \&!
1.2 deraadt 688: to indicate negation: if the host name matches a negated
1.1 deraadt 689: pattern, it is not accepted (by that line) even if it matched another
690: pattern on the line.
1.231 dtucker 691: A hostname or address may optionally be enclosed within
692: .Ql \&[
693: and
694: .Ql \&]
695: brackets then followed by
696: .Ql \&:
1.232 jmc 697: and a non-standard port number.
1.2 deraadt 698: .Pp
1.205 djm 699: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 700: and addresses should the file's contents be disclosed.
701: Hashed hostnames start with a
702: .Ql |
1.205 djm 703: character.
704: Only one hashed hostname may appear on a single line and none of the above
705: negation or wildcard operators may be applied.
706: .Pp
1.49 markus 707: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.220 jmc 708: can be obtained, for example, from
1.167 deraadt 709: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 710: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 711: .Pp
712: Lines starting with
713: .Ql #
714: and empty lines are ignored as comments.
715: .Pp
1.1 deraadt 716: When performing host authentication, authentication is accepted if any
1.254 djm 717: matching line has the proper key; either one that matches exactly or,
718: if the server has presented a certificate for authentication, the key
719: of the certification authority that signed the certificate.
720: For a key to be trusted as a certification authority, it must use the
721: .Dq @cert-authority
722: marker described above.
723: .Pp
724: The known hosts file also provides a facility to mark keys as revoked,
725: for example when it is known that the associated private key has been
726: stolen.
727: Revoked keys are specified by including the
728: .Dq @revoked
729: marker at the beginning of the key line, and are never accepted for
730: authentication or as certification authorities, but instead will
731: produce a warning from
732: .Xr ssh 1
733: when they are encountered.
734: .Pp
735: It is permissible (but not
1.1 deraadt 736: recommended) to have several lines or different host keys for the same
1.36 aaron 737: names.
738: This will inevitably happen when short forms of host names
739: from different domains are put in the file.
740: It is possible
1.1 deraadt 741: that the files contain conflicting information; authentication is
742: accepted if valid information can be found from either file.
1.2 deraadt 743: .Pp
1.1 deraadt 744: Note that the lines in these files are typically hundreds of characters
745: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 746: Rather, generate them by a script,
747: .Xr ssh-keyscan 1
1.40 aaron 748: or by taking
1.167 deraadt 749: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 750: and adding the host names at the front.
1.254 djm 751: .Xr ssh-keygen 1
1.255 jmc 752: also offers some basic automated editing for
1.254 djm 753: .Pa ~/.ssh/known_hosts
754: including removing hosts matching a host name and converting all host
755: names to their hashed representations.
1.221 jmc 756: .Pp
757: An example ssh_known_hosts file:
758: .Bd -literal -offset 3n
759: # Comments allowed at start of line
760: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
761: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 762: # A hashed hostname
763: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
764: AAAA1234.....=
1.254 djm 765: # A revoked key
766: @revoked * ssh-rsa AAAAB5W...
767: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
768: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 769: .Ed
1.2 deraadt 770: .Sh FILES
1.223 jmc 771: .Bl -tag -width Ds -compact
1.258 jmc 772: .It Pa ~/.hushlogin
1.204 jmc 773: This file is used to suppress printing the last login time and
774: .Pa /etc/motd ,
775: if
776: .Cm PrintLastLog
777: and
778: .Cm PrintMotd ,
779: respectively,
780: are enabled.
781: It does not suppress printing of the banner specified by
782: .Cm Banner .
1.223 jmc 783: .Pp
1.258 jmc 784: .It Pa ~/.rhosts
1.225 jmc 785: This file is used for host-based authentication (see
786: .Xr ssh 1
787: for more information).
788: On some machines this file may need to be
789: world-readable if the user's home directory is on an NFS partition,
790: because
791: .Nm
792: reads it as root.
793: Additionally, this file must be owned by the user,
794: and must not have write permissions for anyone else.
795: The recommended
796: permission for most machines is read/write for the user, and not
1.1 deraadt 797: accessible by others.
1.2 deraadt 798: .Pp
1.258 jmc 799: .It Pa ~/.shosts
1.225 jmc 800: This file is used in exactly the same way as
801: .Pa .rhosts ,
802: but allows host-based authentication without permitting login with
803: rlogin/rsh.
1.238 mcbride 804: .Pp
1.258 jmc 805: .It Pa ~/.ssh/
1.238 mcbride 806: This directory is the default location for all user-specific configuration
807: and authentication information.
808: There is no general requirement to keep the entire contents of this directory
809: secret, but the recommended permissions are read/write/execute for the user,
810: and not accessible by others.
1.2 deraadt 811: .Pp
1.258 jmc 812: .It Pa ~/.ssh/authorized_keys
1.277 sobrado 813: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.273 naddy 814: that can be used for logging in as this user.
1.223 jmc 815: The format of this file is described above.
1.234 dtucker 816: The content of the file is not highly sensitive, but the recommended
1.225 jmc 817: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 818: .Pp
819: If this file, the
820: .Pa ~/.ssh
821: directory, or the user's home directory are writable
822: by other users, then the file could be modified or replaced by unauthorized
823: users.
824: In this case,
825: .Nm
826: will not allow it to be used unless the
827: .Cm StrictModes
828: option has been set to
829: .Dq no .
1.2 deraadt 830: .Pp
1.258 jmc 831: .It Pa ~/.ssh/environment
1.36 aaron 832: This file is read into the environment at login (if it exists).
833: It can only contain empty lines, comment lines (that start with
1.2 deraadt 834: .Ql # ) ,
1.36 aaron 835: and assignment lines of the form name=value.
836: The file should be writable
1.6 aaron 837: only by the user; it need not be readable by anyone else.
1.188 stevesk 838: Environment processing is disabled by default and is
839: controlled via the
840: .Cm PermitUserEnvironment
841: option.
1.223 jmc 842: .Pp
1.258 jmc 843: .It Pa ~/.ssh/known_hosts
1.225 jmc 844: Contains a list of host keys for all hosts the user has logged into
845: that are not already in the systemwide list of known host keys.
846: The format of this file is described above.
847: This file should be writable only by root/the owner and
1.223 jmc 848: can, but need not be, world-readable.
849: .Pp
1.258 jmc 850: .It Pa ~/.ssh/rc
1.226 jmc 851: Contains initialization routines to be run before
852: the user's home directory becomes accessible.
1.1 deraadt 853: This file should be writable only by the user, and need not be
854: readable by anyone else.
1.223 jmc 855: .Pp
1.258 jmc 856: .It Pa /etc/hosts.equiv
1.227 jmc 857: This file is for host-based authentication (see
858: .Xr ssh 1 ) .
859: It should only be writable by root.
1.223 jmc 860: .Pp
1.258 jmc 861: .It Pa /etc/moduli
1.283 djm 862: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange"
863: key exchange method.
1.223 jmc 864: The file format is described in
865: .Xr moduli 5 .
1.283 djm 866: If no usable groups are found in this file then fixed internal groups will
867: be used.
1.223 jmc 868: .Pp
1.258 jmc 869: .It Pa /etc/motd
1.223 jmc 870: See
871: .Xr motd 5 .
872: .Pp
1.258 jmc 873: .It Pa /etc/nologin
1.223 jmc 874: If this file exists,
875: .Nm
876: refuses to let anyone except root log in.
877: The contents of the file
878: are displayed to anyone trying to log in, and non-root connections are
879: refused.
880: The file should be world-readable.
881: .Pp
1.258 jmc 882: .It Pa /etc/shosts.equiv
1.227 jmc 883: This file is used in exactly the same way as
884: .Pa hosts.equiv ,
885: but allows host-based authentication without permitting login with
886: rlogin/rsh.
1.225 jmc 887: .Pp
1.258 jmc 888: .It Pa /etc/ssh/ssh_host_key
889: .It Pa /etc/ssh/ssh_host_dsa_key
1.259 djm 890: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.273 naddy 891: .It Pa /etc/ssh/ssh_host_ed25519_key
1.258 jmc 892: .It Pa /etc/ssh/ssh_host_rsa_key
1.266 dtucker 893: These files contain the private parts of the host keys.
1.223 jmc 894: These files should only be owned by root, readable only by root, and not
895: accessible to others.
896: Note that
897: .Nm
1.228 jmc 898: does not start if these files are group/world-accessible.
1.223 jmc 899: .Pp
1.258 jmc 900: .It Pa /etc/ssh/ssh_host_key.pub
901: .It Pa /etc/ssh/ssh_host_dsa_key.pub
1.259 djm 902: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.273 naddy 903: .It Pa /etc/ssh/ssh_host_ed25519_key.pub
1.258 jmc 904: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.266 dtucker 905: These files contain the public parts of the host keys.
1.223 jmc 906: These files should be world-readable but writable only by
907: root.
908: Their contents should match the respective private parts.
909: These files are not
910: really used for anything; they are provided for the convenience of
911: the user so their contents can be copied to known hosts files.
912: These files are created using
913: .Xr ssh-keygen 1 .
914: .Pp
1.258 jmc 915: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 916: Systemwide list of known host keys.
917: This file should be prepared by the
918: system administrator to contain the public host keys of all machines in the
919: organization.
920: The format of this file is described above.
921: This file should be writable only by root/the owner and
922: should be world-readable.
923: .Pp
1.258 jmc 924: .It Pa /etc/ssh/sshd_config
1.223 jmc 925: Contains configuration data for
926: .Nm sshd .
927: The file format and configuration options are described in
928: .Xr sshd_config 5 .
929: .Pp
1.258 jmc 930: .It Pa /etc/ssh/sshrc
1.226 jmc 931: Similar to
932: .Pa ~/.ssh/rc ,
933: it can be used to specify
1.36 aaron 934: machine-specific login-time initializations globally.
935: This file should be writable only by root, and should be world-readable.
1.223 jmc 936: .Pp
1.258 jmc 937: .It Pa /var/empty
1.223 jmc 938: .Xr chroot 2
939: directory used by
940: .Nm
941: during privilege separation in the pre-authentication phase.
942: The directory should not contain any files and must be owned by root
943: and not group or world-writable.
944: .Pp
1.258 jmc 945: .It Pa /var/run/sshd.pid
1.223 jmc 946: Contains the process ID of the
947: .Nm
948: listening for connections (if there are several daemons running
949: concurrently for different ports, this contains the process ID of the one
950: started last).
951: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 952: .El
1.2 deraadt 953: .Sh SEE ALSO
954: .Xr scp 1 ,
1.90 djm 955: .Xr sftp 1 ,
1.2 deraadt 956: .Xr ssh 1 ,
1.5 deraadt 957: .Xr ssh-add 1 ,
1.2 deraadt 958: .Xr ssh-agent 1 ,
1.5 deraadt 959: .Xr ssh-keygen 1 ,
1.235 jmc 960: .Xr ssh-keyscan 1 ,
1.200 jmc 961: .Xr chroot 2 ,
1.136 markus 962: .Xr login.conf 5 ,
963: .Xr moduli 5 ,
1.184 stevesk 964: .Xr sshd_config 5 ,
1.200 jmc 965: .Xr inetd 8 ,
1.128 mpech 966: .Xr sftp-server 8
1.198 jmc 967: .Sh AUTHORS
968: OpenSSH is a derivative of the original and free
969: ssh 1.2.12 release by Tatu Ylonen.
970: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
971: Theo de Raadt and Dug Song
972: removed many bugs, re-added newer features and
973: created OpenSSH.
974: Markus Friedl contributed the support for SSH
975: protocol versions 1.5 and 2.0.
976: Niels Provos and Markus Friedl contributed support
977: for privilege separation.
![[BACK]](/icons/back.gif){kind=icon}
Return to sshd.8 CVS log ![[TXT]](/icons/text.gif){kind=icon}
![[DIR]](/icons/dir.gif){kind=icon}
Up to [local] / src / usr.bin / ssh