Annotation of src/usr.bin/ssh/sshd.8, Revision 1.292
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: .\"
1.292 ! djm 36: .\" $OpenBSD: sshd.8,v 1.291 2017/06/24 06:28:50 jmc Exp $
1.291 jmc 37: .Dd $Mdocdate: June 24 2017 $
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
1.242 dtucker 47: .Op Fl C Ar connection_spec
1.251 djm 48: .Op Fl c Ar host_certificate_file
1.268 dtucker 49: .Op Fl E Ar log_file
1.2 deraadt 50: .Op Fl f Ar config_file
51: .Op Fl g Ar login_grace_time
52: .Op Fl h Ar host_key_file
1.156 markus 53: .Op Fl o Ar option
1.2 deraadt 54: .Op Fl p Ar port
1.61 markus 55: .Op Fl u Ar len
1.194 jmc 56: .Ek
1.40 aaron 57: .Sh DESCRIPTION
1.2 deraadt 58: .Nm
1.212 jmc 59: (OpenSSH Daemon) is the daemon program for
1.2 deraadt 60: .Xr ssh 1 .
1.275 tedu 61: Together these programs replace rlogin and rsh,
1.235 jmc 62: and provide secure encrypted communications between two untrusted hosts
1.36 aaron 63: over an insecure network.
1.2 deraadt 64: .Pp
65: .Nm
1.212 jmc 66: listens for connections from clients.
1.40 aaron 67: It is normally started at boot from
1.2 deraadt 68: .Pa /etc/rc .
69: It forks a new
1.36 aaron 70: daemon for each incoming connection.
71: The forked daemons handle
1.1 deraadt 72: key exchange, encryption, authentication, command execution,
73: and data exchange.
1.2 deraadt 74: .Pp
75: .Nm
1.200 jmc 76: can be configured using command-line options or a configuration file
77: (by default
1.212 jmc 78: .Xr sshd_config 5 ) ;
79: command-line options override values specified in the
1.1 deraadt 80: configuration file.
1.25 markus 81: .Nm
82: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 83: .Dv SIGHUP ,
1.230 jmc 84: by executing itself with the name and options it was started with, e.g.\&
1.97 deraadt 85: .Pa /usr/sbin/sshd .
1.18 aaron 86: .Pp
87: The options are as follows:
1.2 deraadt 88: .Bl -tag -width Ds
1.200 jmc 89: .It Fl 4
90: Forces
91: .Nm
92: to use IPv4 addresses only.
93: .It Fl 6
94: Forces
95: .Nm
96: to use IPv6 addresses only.
1.243 jmc 97: .It Fl C Ar connection_spec
98: Specify the connection parameters to use for the
99: .Fl T
100: extended test mode.
101: If provided, any
102: .Cm Match
103: directives in the configuration file
104: that would apply to the specified user, host, and address will be set before
105: the configuration is written to standard output.
106: The connection parameters are supplied as keyword=value pairs.
107: The keywords are
108: .Dq user ,
109: .Dq host ,
1.265 dtucker 110: .Dq laddr ,
111: .Dq lport ,
1.292 ! djm 112: .Dq rdomain
1.243 jmc 113: and
114: .Dq addr .
115: All are required and may be supplied in any order, either with multiple
116: .Fl C
117: options or as a comma-separated list.
1.252 jmc 118: .It Fl c Ar host_certificate_file
119: Specifies a path to a certificate file to identify
120: .Nm
121: during key exchange.
122: The certificate file must match a host key file specified using the
123: .Fl h
124: option or the
125: .Cm HostKey
126: configuration directive.
1.200 jmc 127: .It Fl D
128: When this option is specified,
129: .Nm
130: will not detach and does not become a daemon.
131: This allows easy monitoring of
132: .Nm sshd .
1.2 deraadt 133: .It Fl d
1.36 aaron 134: Debug mode.
1.250 djm 135: The server sends verbose debug output to standard error,
136: and does not put itself in the background.
1.36 aaron 137: The server also will not fork and will only process one connection.
138: This option is only intended for debugging for the server.
1.194 jmc 139: Multiple
140: .Fl d
141: options increase the debugging level.
1.67 aaron 142: Maximum is 3.
1.268 dtucker 143: .It Fl E Ar log_file
144: Append debug logs to
145: .Ar log_file
146: instead of the system log.
1.120 markus 147: .It Fl e
1.269 dtucker 148: Write debug logs to standard error instead of the system log.
1.235 jmc 149: .It Fl f Ar config_file
1.36 aaron 150: Specifies the name of the configuration file.
151: The default is
1.167 deraadt 152: .Pa /etc/ssh/sshd_config .
1.16 markus 153: .Nm
154: refuses to start if there is no configuration file.
1.2 deraadt 155: .It Fl g Ar login_grace_time
1.1 deraadt 156: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 157: 120 seconds).
1.36 aaron 158: If the client fails to authenticate the user within
159: this many seconds, the server disconnects and exits.
160: A value of zero indicates no limit.
1.2 deraadt 161: .It Fl h Ar host_key_file
1.160 stevesk 162: Specifies a file from which a host key is read.
1.7 markus 163: This option must be given if
164: .Nm
165: is not run as root (as the normal
1.160 stevesk 166: host key files are normally not readable by anyone but root).
167: The default is
1.259 djm 168: .Pa /etc/ssh/ssh_host_dsa_key ,
1.285 naddy 169: .Pa /etc/ssh/ssh_host_ecdsa_key ,
1.273 naddy 170: .Pa /etc/ssh/ssh_host_ed25519_key
1.259 djm 171: and
1.285 naddy 172: .Pa /etc/ssh/ssh_host_rsa_key .
1.75 markus 173: It is possible to have multiple host key files for
1.285 naddy 174: the different host key algorithms.
1.2 deraadt 175: .It Fl i
1.7 markus 176: Specifies that
177: .Nm
1.194 jmc 178: is being run from
179: .Xr inetd 8 .
1.156 markus 180: .It Fl o Ar option
181: Can be used to give options in the format used in the configuration file.
182: This is useful for specifying options for which there is no separate
183: command-line flag.
1.200 jmc 184: For full details of the options, and their values, see
185: .Xr sshd_config 5 .
1.2 deraadt 186: .It Fl p Ar port
1.1 deraadt 187: Specifies the port on which the server listens for connections
188: (default 22).
1.158 stevesk 189: Multiple port options are permitted.
1.210 stevesk 190: Ports specified in the configuration file with the
191: .Cm Port
192: option are ignored when a command-line port is specified.
193: Ports specified using the
194: .Cm ListenAddress
195: option override command-line ports.
1.2 deraadt 196: .It Fl q
1.36 aaron 197: Quiet mode.
198: Nothing is sent to the system log.
199: Normally the beginning,
1.1 deraadt 200: authentication, and termination of each connection is logged.
1.242 dtucker 201: .It Fl T
202: Extended test mode.
203: Check the validity of the configuration file, output the effective configuration
204: to stdout and then exit.
205: Optionally,
206: .Cm Match
207: rules may be applied by specifying the connection parameters using one or more
208: .Fl C
209: options.
1.243 jmc 210: .It Fl t
211: Test mode.
212: Only check the validity of the configuration file and sanity of the keys.
213: This is useful for updating
214: .Nm
215: reliably as configuration options may change.
1.61 markus 216: .It Fl u Ar len
217: This option is used to specify the size of the field
218: in the
219: .Li utmp
220: structure that holds the remote host name.
221: If the resolved host name is longer than
222: .Ar len ,
223: the dotted decimal value will be used instead.
224: This allows hosts with very long host names that
225: overflow this field to still be uniquely identified.
226: Specifying
227: .Fl u0
228: indicates that only dotted decimal addresses
229: should be put into the
230: .Pa utmp
231: file.
1.144 stevesk 232: .Fl u0
1.194 jmc 233: may also be used to prevent
1.144 stevesk 234: .Nm
235: from making DNS requests unless the authentication
236: mechanism or configuration requires it.
237: Authentication mechanisms that may require DNS include
1.285 naddy 238: .Cm HostbasedAuthentication
1.144 stevesk 239: and using a
240: .Cm from="pattern-list"
241: option in a key file.
1.170 stevesk 242: Configuration options that require DNS include using a
243: USER@HOST pattern in
244: .Cm AllowUsers
245: or
246: .Cm DenyUsers .
1.2 deraadt 247: .El
1.214 jmc 248: .Sh AUTHENTICATION
1.285 naddy 249: The OpenSSH SSH daemon supports SSH protocol 2 only.
1.284 jmc 250: Each host has a host-specific key,
1.214 jmc 251: used to identify the host.
1.212 jmc 252: Whenever a client connects, the daemon responds with its public
1.285 naddy 253: host key.
1.212 jmc 254: The client compares the
1.285 naddy 255: host key against its own database to verify that it has not changed.
256: Forward security is provided through a Diffie-Hellman key agreement.
1.214 jmc 257: This key agreement results in a shared session key.
258: The rest of the session is encrypted using a symmetric cipher, currently
259: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
260: The client selects the encryption algorithm
261: to use from those offered by the server.
262: Additionally, session integrity is provided
263: through a cryptographic message authentication code
1.289 djm 264: (hmac-md5, hmac-sha1, umac-64, umac-128,
1.263 djm 265: hmac-sha2-256 or hmac-sha2-512).
1.214 jmc 266: .Pp
267: Finally, the server and the client enter an authentication dialog.
1.212 jmc 268: The client tries to authenticate itself using
1.214 jmc 269: host-based authentication,
270: public key authentication,
271: challenge-response authentication,
272: or password authentication.
1.212 jmc 273: .Pp
274: If the client successfully authenticates itself, a dialog for
275: preparing the session is entered.
276: At this time the client may request
277: things like allocating a pseudo-tty, forwarding X11 connections,
278: forwarding TCP connections, or forwarding the authentication agent
279: connection over the secure channel.
280: .Pp
1.216 jmc 281: After this, the client either requests a shell or execution of a command.
1.212 jmc 282: The sides then enter session mode.
283: In this mode, either side may send
284: data at any time, and such data is forwarded to/from the shell or
285: command on the server side, and the user terminal in the client side.
286: .Pp
287: When the user program terminates and all forwarded X11 and other
288: connections have been closed, the server sends command exit status to
289: the client, and both sides exit.
1.2 deraadt 290: .Sh LOGIN PROCESS
1.1 deraadt 291: When a user successfully logs in,
1.2 deraadt 292: .Nm
1.1 deraadt 293: does the following:
1.2 deraadt 294: .Bl -enum -offset indent
295: .It
1.1 deraadt 296: If the login is on a tty, and no command has been specified,
1.40 aaron 297: prints last login time and
1.2 deraadt 298: .Pa /etc/motd
1.1 deraadt 299: (unless prevented in the configuration file or by
1.207 djm 300: .Pa ~/.hushlogin ;
1.2 deraadt 301: see the
1.40 aaron 302: .Sx FILES
1.2 deraadt 303: section).
304: .It
1.1 deraadt 305: If the login is on a tty, records login time.
1.2 deraadt 306: .It
307: Checks
308: .Pa /etc/nologin ;
309: if it exists, prints contents and quits
1.1 deraadt 310: (unless root).
1.2 deraadt 311: .It
1.1 deraadt 312: Changes to run with normal user privileges.
1.2 deraadt 313: .It
1.1 deraadt 314: Sets up basic environment.
1.2 deraadt 315: .It
1.200 jmc 316: Reads the file
1.207 djm 317: .Pa ~/.ssh/environment ,
1.200 jmc 318: if it exists, and users are allowed to change their environment.
1.188 stevesk 319: See the
1.187 marc 320: .Cm PermitUserEnvironment
1.188 stevesk 321: option in
1.187 marc 322: .Xr sshd_config 5 .
1.2 deraadt 323: .It
1.1 deraadt 324: Changes to user's home directory.
1.2 deraadt 325: .It
326: If
1.207 djm 327: .Pa ~/.ssh/rc
1.276 djm 328: exists and the
329: .Xr sshd_config 5
330: .Cm PermitUserRC
331: option is set, runs it; else if
1.168 deraadt 332: .Pa /etc/ssh/sshrc
1.2 deraadt 333: exists, runs
1.36 aaron 334: it; otherwise runs xauth.
335: The
1.2 deraadt 336: .Dq rc
337: files are given the X11
1.1 deraadt 338: authentication protocol and cookie in standard input.
1.226 jmc 339: See
340: .Sx SSHRC ,
341: below.
1.2 deraadt 342: .It
1.1 deraadt 343: Runs user's shell or command.
1.279 djm 344: All commands are run under the user's login shell as specified in the
345: system password database.
1.2 deraadt 346: .El
1.226 jmc 347: .Sh SSHRC
348: If the file
349: .Pa ~/.ssh/rc
350: exists,
351: .Xr sh 1
352: runs it after reading the
353: environment files but before starting the user's shell or command.
354: It must not produce any output on stdout; stderr must be used
355: instead.
356: If X11 forwarding is in use, it will receive the "proto cookie" pair in
357: its standard input (and
358: .Ev DISPLAY
359: in its environment).
360: The script must call
361: .Xr xauth 1
362: because
363: .Nm
364: will not run xauth automatically to add X11 cookies.
365: .Pp
366: The primary purpose of this file is to run any initialization routines
367: which may be needed before the user's home directory becomes
368: accessible; AFS is a particular example of such an environment.
369: .Pp
370: This file will probably contain some initialization code followed by
371: something similar to:
372: .Bd -literal -offset 3n
373: if read proto cookie && [ -n "$DISPLAY" ]; then
374: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
375: # X11UseLocalhost=yes
376: echo add unix:`echo $DISPLAY |
377: cut -c11-` $proto $cookie
378: else
379: # X11UseLocalhost=no
380: echo add $DISPLAY $proto $cookie
381: fi | xauth -q -
382: fi
383: .Ed
384: .Pp
385: If this file does not exist,
386: .Pa /etc/ssh/sshrc
387: is run, and if that
388: does not exist either, xauth is used to add the cookie.
1.2 deraadt 389: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 390: .Cm AuthorizedKeysFile
1.262 jmc 391: specifies the files containing public keys for
1.217 jmc 392: public key authentication;
1.281 djm 393: if this option is not specified, the default is
1.261 djm 394: .Pa ~/.ssh/authorized_keys
395: and
396: .Pa ~/.ssh/authorized_keys2 .
1.36 aaron 397: Each line of the file contains one
1.2 deraadt 398: key (empty lines and lines starting with a
399: .Ql #
400: are ignored as
1.36 aaron 401: comments).
1.285 naddy 402: Public keys consist of the following space-separated fields:
1.217 jmc 403: options, keytype, base64-encoded key, comment.
1.285 naddy 404: The options field is optional.
405: The keytype is
1.259 djm 406: .Dq ecdsa-sha2-nistp256 ,
407: .Dq ecdsa-sha2-nistp384 ,
408: .Dq ecdsa-sha2-nistp521 ,
1.272 jmc 409: .Dq ssh-ed25519 ,
1.75 markus 410: .Dq ssh-dss
411: or
1.285 naddy 412: .Dq ssh-rsa ;
413: the comment field is not used for anything (but may be convenient for the
414: user to identify the key).
1.2 deraadt 415: .Pp
1.285 naddy 416: Note that lines in this file can be several hundred bytes long
1.203 dtucker 417: (because of the size of the public key encoding) up to a limit of
418: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
419: keys up to 16 kilobits.
1.36 aaron 420: You don't want to type them in; instead, copy the
1.217 jmc 421: .Pa id_dsa.pub ,
1.259 djm 422: .Pa id_ecdsa.pub ,
1.273 naddy 423: .Pa id_ed25519.pub ,
1.75 markus 424: or the
1.113 itojun 425: .Pa id_rsa.pub
1.1 deraadt 426: file and edit it.
1.175 stevesk 427: .Pp
428: .Nm
1.285 naddy 429: enforces a minimum RSA key modulus size of 768 bits.
1.2 deraadt 430: .Pp
1.58 deraadt 431: The options (if present) consist of comma-separated option
1.36 aaron 432: specifications.
433: No spaces are permitted, except within double quotes.
1.141 stevesk 434: The following option specifications are supported (note
435: that option keywords are case-insensitive):
1.2 deraadt 436: .Bl -tag -width Ds
1.282 djm 437: .It Cm agent-forwarding
438: Enable authentication agent forwarding previously disabled by the
439: .Cm restrict
440: option.
1.253 djm 441: .It Cm cert-authority
1.251 djm 442: Specifies that the listed key is a certification authority (CA) that is
443: trusted to validate signed certificates for user authentication.
444: .Pp
445: Certificates may encode access restrictions similar to these key options.
446: If both certificate restrictions and key options are present, the most
447: restrictive union of the two is applied.
1.2 deraadt 448: .It Cm command="command"
1.1 deraadt 449: Specifies that the command is executed whenever this key is used for
1.36 aaron 450: authentication.
451: The command supplied by the user (if any) is ignored.
1.148 markus 452: The command is run on a pty if the client requests a pty;
1.36 aaron 453: otherwise it is run without a tty.
1.194 jmc 454: If an 8-bit clean channel is required,
1.147 deraadt 455: one must not request a pty or should specify
1.89 markus 456: .Cm no-pty .
1.36 aaron 457: A quote may be included in the command by quoting it with a backslash.
1.287 djm 458: .Pp
1.36 aaron 459: This option might be useful
1.189 espie 460: to restrict certain public keys to perform just a specific operation.
1.36 aaron 461: An example might be a key that permits remote backups but nothing else.
1.211 jmc 462: Note that the client may specify TCP and/or X11
1.287 djm 463: forwarding unless they are explicitly prohibited, e.g. using the
464: .Cm restrict
465: key option.
466: .Pp
1.233 dtucker 467: The command originally supplied by the client is available in the
468: .Ev SSH_ORIGINAL_COMMAND
469: environment variable.
1.149 markus 470: Note that this option applies to shell, command or subsystem execution.
1.287 djm 471: Also note that this command may be superseded by a
1.251 djm 472: .Xr sshd_config 5
473: .Cm ForceCommand
1.287 djm 474: directive.
475: .Pp
476: If a command is specified and a forced-command is embedded in a certificate
477: used for authentication, then the certificate will be accepted only if the
478: two commands are identical.
1.2 deraadt 479: .It Cm environment="NAME=value"
1.1 deraadt 480: Specifies that the string is to be added to the environment when
1.36 aaron 481: logging in using this key.
482: Environment variables set this way
483: override other default environment values.
484: Multiple options of this type are permitted.
1.188 stevesk 485: Environment processing is disabled by default and is
486: controlled via the
487: .Cm PermitUserEnvironment
488: option.
1.218 jmc 489: .It Cm from="pattern-list"
1.244 djm 490: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 491: name of the remote host or its IP address must be present in the
1.244 djm 492: comma-separated list of patterns.
1.270 jmc 493: See PATTERNS in
1.229 jmc 494: .Xr ssh_config 5
495: for more information on patterns.
1.244 djm 496: .Pp
497: In addition to the wildcard matching that may be applied to hostnames or
498: addresses, a
499: .Cm from
1.248 sobrado 500: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 501: .Pp
502: The purpose of this option is to optionally increase security: public key
503: authentication by itself does not trust the network or name servers or
504: anything (but the key); however, if somebody somehow steals the key, the key
505: permits an intruder to log in from anywhere in the world.
506: This additional option makes using a stolen key more difficult (name
507: servers and/or routers would have to be compromised in addition to
508: just the key).
1.218 jmc 509: .It Cm no-agent-forwarding
510: Forbids authentication agent forwarding when this key is used for
511: authentication.
1.2 deraadt 512: .It Cm no-port-forwarding
1.211 jmc 513: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 514: Any port forward requests by the client will return an error.
1.230 jmc 515: This might be used, e.g. in connection with the
1.2 deraadt 516: .Cm command
1.1 deraadt 517: option.
1.218 jmc 518: .It Cm no-pty
519: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 520: .It Cm no-user-rc
1.241 jmc 521: Disables execution of
1.240 djm 522: .Pa ~/.ssh/rc .
1.2 deraadt 523: .It Cm no-X11-forwarding
1.1 deraadt 524: Forbids X11 forwarding when this key is used for authentication.
525: Any X11 forward requests by the client will return an error.
1.107 djm 526: .It Cm permitopen="host:port"
1.278 bentley 527: Limit local port forwarding with
528: .Xr ssh 1
529: .Fl L
530: such that it may only connect to the specified host and port.
1.257 djm 531: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 532: Multiple
1.107 djm 533: .Cm permitopen
1.197 jmc 534: options may be applied separated by commas.
535: No pattern matching is performed on the specified hostnames,
536: they must be literal domains or addresses.
1.264 dtucker 537: A port specification of
538: .Cm *
539: matches any port.
1.282 djm 540: .It Cm port-forwarding
541: Enable port forwarding previously disabled by the
542: .Cm restrict
1.256 djm 543: .It Cm principals="principals"
544: On a
545: .Cm cert-authority
546: line, specifies allowed principals for certificate authentication as a
547: comma-separated list.
548: At least one name from the list must appear in the certificate's
549: list of principals for the certificate to be accepted.
550: This option is ignored for keys that are not marked as trusted certificate
551: signers using the
552: .Cm cert-authority
553: option.
1.282 djm 554: .It Cm pty
555: Permits tty allocation previously disabled by the
556: .Cm restrict
557: option.
558: .It Cm restrict
559: Enable all restrictions, i.e. disable port, agent and X11 forwarding,
560: as well as disabling PTY allocation
561: and execution of
562: .Pa ~/.ssh/rc .
563: If any future restriction capabilities are added to authorized_keys files
564: they will be included in this set.
1.209 reyk 565: .It Cm tunnel="n"
566: Force a
567: .Xr tun 4
568: device on the server.
569: Without this option, the next available device will be used if
570: the client requests a tunnel.
1.282 djm 571: .It Cm user-rc
572: Enables execution of
573: .Pa ~/.ssh/rc
574: previously disabled by the
575: .Cm restrict
576: option.
577: .It Cm X11-forwarding
578: Permits X11 forwarding previously disabled by the
579: .Cm restrict
580: option.
1.2 deraadt 581: .El
582: .Pp
1.219 jmc 583: An example authorized_keys file:
1.222 jmc 584: .Bd -literal -offset 3n
1.219 jmc 585: # Comments allowed at start of line
586: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 587: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
588: AAAAB2...19Q== john@example.net
589: command="dump /home",no-pty,no-port-forwarding ssh-dss
590: AAAAC3...51R== example.net
591: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
592: AAAAB5...21S==
593: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
594: jane@example.net
1.282 djm 595: restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
596: user@example.net
597: restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==
598: user@example.net
1.219 jmc 599: .Ed
1.2 deraadt 600: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 601: The
1.194 jmc 602: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 603: and
1.207 djm 604: .Pa ~/.ssh/known_hosts
1.36 aaron 605: files contain host public keys for all known hosts.
606: The global file should
1.37 brad 607: be prepared by the administrator (optional), and the per-user file is
1.288 dtucker 608: maintained automatically: whenever the user connects to an unknown host,
1.36 aaron 609: its key is added to the per-user file.
1.2 deraadt 610: .Pp
1.254 djm 611: Each line in these files contains the following fields: markers (optional),
1.285 naddy 612: hostnames, keytype, base64-encoded key, comment.
1.36 aaron 613: The fields are separated by spaces.
1.2 deraadt 614: .Pp
1.254 djm 615: The marker is optional, but if it is present then it must be one of
616: .Dq @cert-authority ,
617: to indicate that the line contains a certification authority (CA) key,
618: or
619: .Dq @revoked ,
620: to indicate that the key contained on the line is revoked and must not ever
621: be accepted.
622: Only one marker should be used on a key line.
623: .Pp
1.196 david 624: Hostnames is a comma-separated list of patterns
1.220 jmc 625: .Pf ( Ql *
1.197 jmc 626: and
627: .Ql \&?
1.195 mouring 628: act as
1.290 djm 629: wildcards); each pattern in turn is matched against the host name.
630: When
631: .Nm sshd
632: is authenticating a client, such as when using
633: .Cm HostbasedAuthentication ,
634: this will be the canonical client host name.
635: When
636: .Xr ssh 1
1.291 jmc 637: is authenticating a server, this will be the host name
1.290 djm 638: given by the user, the value of the
639: .Xr ssh 1
640: .Cm HostkeyAlias
641: if it was specified, or the canonical server hostname if the
642: .Xr ssh 1
643: .Cm CanonicalizeHostname
644: option was used.
645: .Pp
1.36 aaron 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
1.285 naddy 667: The keytype and base64-encoded key are taken directly from the host key; they
1.220 jmc 668: can be obtained, for example, from
1.285 naddy 669: .Pa /etc/ssh/ssh_host_rsa_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
1.285 naddy 708: or by taking, for example,
709: .Pa /etc/ssh/ssh_host_rsa_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
1.277 sobrado 773: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.273 naddy 774: that can be used for logging in 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.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
1.258 jmc 821: .It Pa /etc/moduli
1.283 djm 822: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange"
823: key exchange method.
1.223 jmc 824: The file format is described in
825: .Xr moduli 5 .
1.283 djm 826: If no usable groups are found in this file then fixed internal groups will
827: be used.
1.223 jmc 828: .Pp
1.258 jmc 829: .It Pa /etc/motd
1.223 jmc 830: See
831: .Xr motd 5 .
832: .Pp
1.258 jmc 833: .It Pa /etc/nologin
1.223 jmc 834: If this file exists,
835: .Nm
836: refuses to let anyone except root log in.
837: The contents of the file
838: are displayed to anyone trying to log in, and non-root connections are
839: refused.
840: The file should be world-readable.
841: .Pp
1.258 jmc 842: .It Pa /etc/shosts.equiv
1.227 jmc 843: This file is used in exactly the same way as
844: .Pa hosts.equiv ,
845: but allows host-based authentication without permitting login with
846: rlogin/rsh.
1.225 jmc 847: .Pp
1.258 jmc 848: .It Pa /etc/ssh/ssh_host_dsa_key
1.259 djm 849: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.273 naddy 850: .It Pa /etc/ssh/ssh_host_ed25519_key
1.258 jmc 851: .It Pa /etc/ssh/ssh_host_rsa_key
1.266 dtucker 852: These files contain the private parts of the host keys.
1.223 jmc 853: These files should only be owned by root, readable only by root, and not
854: accessible to others.
855: Note that
856: .Nm
1.228 jmc 857: does not start if these files are group/world-accessible.
1.223 jmc 858: .Pp
1.258 jmc 859: .It Pa /etc/ssh/ssh_host_dsa_key.pub
1.259 djm 860: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.273 naddy 861: .It Pa /etc/ssh/ssh_host_ed25519_key.pub
1.258 jmc 862: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.266 dtucker 863: These files contain the public parts of the host keys.
1.223 jmc 864: These files should be world-readable but writable only by
865: root.
866: Their contents should match the respective private parts.
867: These files are not
868: really used for anything; they are provided for the convenience of
869: the user so their contents can be copied to known hosts files.
870: These files are created using
871: .Xr ssh-keygen 1 .
872: .Pp
1.258 jmc 873: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 874: Systemwide list of known host keys.
875: This file should be prepared by the
876: system administrator to contain the public host keys of all machines in the
877: organization.
878: The format of this file is described above.
879: This file should be writable only by root/the owner and
880: should be world-readable.
881: .Pp
1.258 jmc 882: .It Pa /etc/ssh/sshd_config
1.223 jmc 883: Contains configuration data for
884: .Nm sshd .
885: The file format and configuration options are described in
886: .Xr sshd_config 5 .
887: .Pp
1.258 jmc 888: .It Pa /etc/ssh/sshrc
1.226 jmc 889: Similar to
890: .Pa ~/.ssh/rc ,
891: it can be used to specify
1.36 aaron 892: machine-specific login-time initializations globally.
893: This file should be writable only by root, and should be world-readable.
1.223 jmc 894: .Pp
1.258 jmc 895: .It Pa /var/empty
1.223 jmc 896: .Xr chroot 2
897: directory used by
898: .Nm
899: during privilege separation in the pre-authentication phase.
900: The directory should not contain any files and must be owned by root
901: and not group or world-writable.
902: .Pp
1.258 jmc 903: .It Pa /var/run/sshd.pid
1.223 jmc 904: Contains the process ID of the
905: .Nm
906: listening for connections (if there are several daemons running
907: concurrently for different ports, this contains the process ID of the one
908: started last).
909: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 910: .El
1.2 deraadt 911: .Sh SEE ALSO
912: .Xr scp 1 ,
1.90 djm 913: .Xr sftp 1 ,
1.2 deraadt 914: .Xr ssh 1 ,
1.5 deraadt 915: .Xr ssh-add 1 ,
1.2 deraadt 916: .Xr ssh-agent 1 ,
1.5 deraadt 917: .Xr ssh-keygen 1 ,
1.235 jmc 918: .Xr ssh-keyscan 1 ,
1.200 jmc 919: .Xr chroot 2 ,
1.136 markus 920: .Xr login.conf 5 ,
921: .Xr moduli 5 ,
1.184 stevesk 922: .Xr sshd_config 5 ,
1.200 jmc 923: .Xr inetd 8 ,
1.128 mpech 924: .Xr sftp-server 8
1.198 jmc 925: .Sh AUTHORS
926: OpenSSH is a derivative of the original and free
927: ssh 1.2.12 release by Tatu Ylonen.
928: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
929: Theo de Raadt and Dug Song
930: removed many bugs, re-added newer features and
931: created OpenSSH.
932: Markus Friedl contributed the support for SSH
933: protocol versions 1.5 and 2.0.
934: Niels Provos and Markus Friedl contributed support
935: for privilege separation.
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