Annotation of src/usr.bin/ssh/sshd.8, Revision 1.314
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: .\"
1.314 ! djm 36: .\" $OpenBSD: sshd.8,v 1.313 2020/08/27 01:07:10 djm Exp $
! 37: .Dd $Mdocdate: August 27 2020 $
1.2 deraadt 38: .Dt SSHD 8
39: .Os
40: .Sh NAME
41: .Nm sshd
1.308 jmc 42: .Nd OpenSSH 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
1.293 dtucker 103: directives in the configuration file that would apply are applied before the
104: configuration is written to standard output.
105: The connection parameters are supplied as keyword=value pairs and may be
106: supplied in any order, either with multiple
107: .Fl C
108: options or as a comma-separated list.
1.243 jmc 109: The keywords are
1.293 dtucker 110: .Dq addr,
1.243 jmc 111: .Dq user ,
112: .Dq host ,
1.265 dtucker 113: .Dq laddr ,
114: .Dq lport ,
1.293 dtucker 115: and
1.292 djm 116: .Dq rdomain
1.293 dtucker 117: and correspond to source address, user, resolved source host name,
118: local address, local port number and routing domain respectively.
1.252 jmc 119: .It Fl c Ar host_certificate_file
120: Specifies a path to a certificate file to identify
121: .Nm
122: during key exchange.
123: The certificate file must match a host key file specified using the
124: .Fl h
125: option or the
126: .Cm HostKey
127: configuration directive.
1.200 jmc 128: .It Fl D
129: When this option is specified,
130: .Nm
131: will not detach and does not become a daemon.
132: This allows easy monitoring of
133: .Nm sshd .
1.2 deraadt 134: .It Fl d
1.36 aaron 135: Debug mode.
1.250 djm 136: The server sends verbose debug output to standard error,
137: and does not put itself in the background.
1.36 aaron 138: The server also will not fork and will only process one connection.
139: This option is only intended for debugging for the server.
1.194 jmc 140: Multiple
141: .Fl d
142: options increase the debugging level.
1.67 aaron 143: Maximum is 3.
1.268 dtucker 144: .It Fl E Ar log_file
145: Append debug logs to
146: .Ar log_file
147: instead of the system log.
1.120 markus 148: .It Fl e
1.269 dtucker 149: Write debug logs to standard error instead of the system log.
1.235 jmc 150: .It Fl f Ar config_file
1.36 aaron 151: Specifies the name of the configuration file.
152: The default is
1.167 deraadt 153: .Pa /etc/ssh/sshd_config .
1.16 markus 154: .Nm
155: refuses to start if there is no configuration file.
1.2 deraadt 156: .It Fl g Ar login_grace_time
1.1 deraadt 157: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 158: 120 seconds).
1.36 aaron 159: If the client fails to authenticate the user within
160: this many seconds, the server disconnects and exits.
161: A value of zero indicates no limit.
1.2 deraadt 162: .It Fl h Ar host_key_file
1.160 stevesk 163: Specifies a file from which a host key is read.
1.7 markus 164: This option must be given if
165: .Nm
166: is not run as root (as the normal
1.160 stevesk 167: host key files are normally not readable by anyone but root).
168: The default is
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.
1.310 dtucker 256: Forward secrecy is provided through a Diffie-Hellman key agreement.
1.214 jmc 257: This key agreement results in a shared session key.
1.309 naddy 258: The rest of the session is encrypted using a symmetric cipher.
1.214 jmc 259: The client selects the encryption algorithm
260: to use from those offered by the server.
261: Additionally, session integrity is provided
1.310 dtucker 262: through a cryptographic message authentication code (MAC).
1.214 jmc 263: .Pp
264: Finally, the server and the client enter an authentication dialog.
1.212 jmc 265: The client tries to authenticate itself using
1.214 jmc 266: host-based authentication,
267: public key authentication,
268: challenge-response authentication,
269: or password authentication.
1.212 jmc 270: .Pp
271: If the client successfully authenticates itself, a dialog for
272: preparing the session is entered.
273: At this time the client may request
274: things like allocating a pseudo-tty, forwarding X11 connections,
275: forwarding TCP connections, or forwarding the authentication agent
276: connection over the secure channel.
277: .Pp
1.216 jmc 278: After this, the client either requests a shell or execution of a command.
1.212 jmc 279: The sides then enter session mode.
280: In this mode, either side may send
281: data at any time, and such data is forwarded to/from the shell or
282: command on the server side, and the user terminal in the client side.
283: .Pp
284: When the user program terminates and all forwarded X11 and other
285: connections have been closed, the server sends command exit status to
286: the client, and both sides exit.
1.2 deraadt 287: .Sh LOGIN PROCESS
1.1 deraadt 288: When a user successfully logs in,
1.2 deraadt 289: .Nm
1.1 deraadt 290: does the following:
1.2 deraadt 291: .Bl -enum -offset indent
292: .It
1.1 deraadt 293: If the login is on a tty, and no command has been specified,
1.40 aaron 294: prints last login time and
1.2 deraadt 295: .Pa /etc/motd
1.1 deraadt 296: (unless prevented in the configuration file or by
1.207 djm 297: .Pa ~/.hushlogin ;
1.2 deraadt 298: see the
1.40 aaron 299: .Sx FILES
1.2 deraadt 300: section).
301: .It
1.1 deraadt 302: If the login is on a tty, records login time.
1.2 deraadt 303: .It
304: Checks
305: .Pa /etc/nologin ;
306: if it exists, prints contents and quits
1.1 deraadt 307: (unless root).
1.2 deraadt 308: .It
1.1 deraadt 309: Changes to run with normal user privileges.
1.2 deraadt 310: .It
1.1 deraadt 311: Sets up basic environment.
1.2 deraadt 312: .It
1.200 jmc 313: Reads the file
1.207 djm 314: .Pa ~/.ssh/environment ,
1.200 jmc 315: if it exists, and users are allowed to change their environment.
1.188 stevesk 316: See the
1.187 marc 317: .Cm PermitUserEnvironment
1.188 stevesk 318: option in
1.187 marc 319: .Xr sshd_config 5 .
1.2 deraadt 320: .It
1.1 deraadt 321: Changes to user's home directory.
1.2 deraadt 322: .It
323: If
1.207 djm 324: .Pa ~/.ssh/rc
1.276 djm 325: exists and the
326: .Xr sshd_config 5
327: .Cm PermitUserRC
328: option is set, runs it; else if
1.168 deraadt 329: .Pa /etc/ssh/sshrc
1.2 deraadt 330: exists, runs
1.36 aaron 331: it; otherwise runs xauth.
332: The
1.2 deraadt 333: .Dq rc
334: files are given the X11
1.1 deraadt 335: authentication protocol and cookie in standard input.
1.226 jmc 336: See
337: .Sx SSHRC ,
338: below.
1.2 deraadt 339: .It
1.1 deraadt 340: Runs user's shell or command.
1.279 djm 341: All commands are run under the user's login shell as specified in the
342: system password database.
1.2 deraadt 343: .El
1.226 jmc 344: .Sh SSHRC
345: If the file
346: .Pa ~/.ssh/rc
347: exists,
348: .Xr sh 1
349: runs it after reading the
350: environment files but before starting the user's shell or command.
351: It must not produce any output on stdout; stderr must be used
352: instead.
353: If X11 forwarding is in use, it will receive the "proto cookie" pair in
354: its standard input (and
355: .Ev DISPLAY
356: in its environment).
357: The script must call
358: .Xr xauth 1
359: because
360: .Nm
361: will not run xauth automatically to add X11 cookies.
362: .Pp
363: The primary purpose of this file is to run any initialization routines
364: which may be needed before the user's home directory becomes
365: accessible; AFS is a particular example of such an environment.
366: .Pp
367: This file will probably contain some initialization code followed by
368: something similar to:
369: .Bd -literal -offset 3n
370: if read proto cookie && [ -n "$DISPLAY" ]; then
371: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
372: # X11UseLocalhost=yes
373: echo add unix:`echo $DISPLAY |
374: cut -c11-` $proto $cookie
375: else
376: # X11UseLocalhost=no
377: echo add $DISPLAY $proto $cookie
378: fi | xauth -q -
379: fi
380: .Ed
381: .Pp
382: If this file does not exist,
383: .Pa /etc/ssh/sshrc
384: is run, and if that
385: does not exist either, xauth is used to add the cookie.
1.2 deraadt 386: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 387: .Cm AuthorizedKeysFile
1.262 jmc 388: specifies the files containing public keys for
1.217 jmc 389: public key authentication;
1.281 djm 390: if this option is not specified, the default is
1.261 djm 391: .Pa ~/.ssh/authorized_keys
392: and
393: .Pa ~/.ssh/authorized_keys2 .
1.36 aaron 394: Each line of the file contains one
1.2 deraadt 395: key (empty lines and lines starting with a
396: .Ql #
397: are ignored as
1.36 aaron 398: comments).
1.285 naddy 399: Public keys consist of the following space-separated fields:
1.217 jmc 400: options, keytype, base64-encoded key, comment.
1.285 naddy 401: The options field is optional.
1.305 naddy 402: The supported key types are:
403: .Pp
404: .Bl -item -compact -offset indent
405: .It
406: sk-ecdsa-sha2-nistp256@openssh.com
407: .It
408: ecdsa-sha2-nistp256
409: .It
410: ecdsa-sha2-nistp384
411: .It
412: ecdsa-sha2-nistp521
413: .It
1.306 djm 414: sk-ssh-ed25519@openssh.com
415: .It
1.305 naddy 416: ssh-ed25519
417: .It
418: ssh-dss
419: .It
420: ssh-rsa
421: .El
422: .Pp
423: The comment field is not used for anything (but may be convenient for the
1.285 naddy 424: user to identify the key).
1.2 deraadt 425: .Pp
1.285 naddy 426: Note that lines in this file can be several hundred bytes long
1.203 dtucker 427: (because of the size of the public key encoding) up to a limit of
1.305 naddy 428: 8 kilobytes, which permits RSA keys up to 16 kilobits.
1.36 aaron 429: You don't want to type them in; instead, copy the
1.217 jmc 430: .Pa id_dsa.pub ,
1.259 djm 431: .Pa id_ecdsa.pub ,
1.305 naddy 432: .Pa id_ecdsa_sk.pub ,
1.273 naddy 433: .Pa id_ed25519.pub ,
1.306 djm 434: .Pa id_ed25519_sk.pub ,
1.75 markus 435: or the
1.113 itojun 436: .Pa id_rsa.pub
1.1 deraadt 437: file and edit it.
1.175 stevesk 438: .Pp
439: .Nm
1.296 dtucker 440: enforces a minimum RSA key modulus size of 1024 bits.
1.2 deraadt 441: .Pp
1.58 deraadt 442: The options (if present) consist of comma-separated option
1.36 aaron 443: specifications.
444: No spaces are permitted, except within double quotes.
1.141 stevesk 445: The following option specifications are supported (note
446: that option keywords are case-insensitive):
1.2 deraadt 447: .Bl -tag -width Ds
1.282 djm 448: .It Cm agent-forwarding
449: Enable authentication agent forwarding previously disabled by the
450: .Cm restrict
451: option.
1.253 djm 452: .It Cm cert-authority
1.251 djm 453: Specifies that the listed key is a certification authority (CA) that is
454: trusted to validate signed certificates for user authentication.
455: .Pp
456: Certificates may encode access restrictions similar to these key options.
457: If both certificate restrictions and key options are present, the most
458: restrictive union of the two is applied.
1.2 deraadt 459: .It Cm command="command"
1.1 deraadt 460: Specifies that the command is executed whenever this key is used for
1.36 aaron 461: authentication.
462: The command supplied by the user (if any) is ignored.
1.148 markus 463: The command is run on a pty if the client requests a pty;
1.36 aaron 464: otherwise it is run without a tty.
1.194 jmc 465: If an 8-bit clean channel is required,
1.147 deraadt 466: one must not request a pty or should specify
1.89 markus 467: .Cm no-pty .
1.36 aaron 468: A quote may be included in the command by quoting it with a backslash.
1.287 djm 469: .Pp
1.36 aaron 470: This option might be useful
1.189 espie 471: to restrict certain public keys to perform just a specific operation.
1.36 aaron 472: An example might be a key that permits remote backups but nothing else.
1.211 jmc 473: Note that the client may specify TCP and/or X11
1.287 djm 474: forwarding unless they are explicitly prohibited, e.g. using the
475: .Cm restrict
476: key option.
477: .Pp
1.233 dtucker 478: The command originally supplied by the client is available in the
479: .Ev SSH_ORIGINAL_COMMAND
480: environment variable.
1.149 markus 481: Note that this option applies to shell, command or subsystem execution.
1.287 djm 482: Also note that this command may be superseded by a
1.251 djm 483: .Xr sshd_config 5
484: .Cm ForceCommand
1.287 djm 485: directive.
486: .Pp
487: If a command is specified and a forced-command is embedded in a certificate
488: used for authentication, then the certificate will be accepted only if the
489: two commands are identical.
1.2 deraadt 490: .It Cm environment="NAME=value"
1.1 deraadt 491: Specifies that the string is to be added to the environment when
1.36 aaron 492: logging in using this key.
493: Environment variables set this way
494: override other default environment values.
495: Multiple options of this type are permitted.
1.188 stevesk 496: Environment processing is disabled by default and is
497: controlled via the
498: .Cm PermitUserEnvironment
499: option.
1.299 jmc 500: .It Cm expiry-time="timespec"
501: Specifies a time after which the key will not be accepted.
502: The time may be specified as a YYYYMMDD date or a YYYYMMDDHHMM[SS] time
503: in the system time-zone.
1.218 jmc 504: .It Cm from="pattern-list"
1.244 djm 505: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 506: name of the remote host or its IP address must be present in the
1.244 djm 507: comma-separated list of patterns.
1.270 jmc 508: See PATTERNS in
1.229 jmc 509: .Xr ssh_config 5
510: for more information on patterns.
1.244 djm 511: .Pp
512: In addition to the wildcard matching that may be applied to hostnames or
513: addresses, a
514: .Cm from
1.248 sobrado 515: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 516: .Pp
517: The purpose of this option is to optionally increase security: public key
518: authentication by itself does not trust the network or name servers or
519: anything (but the key); however, if somebody somehow steals the key, the key
520: permits an intruder to log in from anywhere in the world.
521: This additional option makes using a stolen key more difficult (name
522: servers and/or routers would have to be compromised in addition to
523: just the key).
1.218 jmc 524: .It Cm no-agent-forwarding
525: Forbids authentication agent forwarding when this key is used for
526: authentication.
1.2 deraadt 527: .It Cm no-port-forwarding
1.211 jmc 528: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 529: Any port forward requests by the client will return an error.
1.230 jmc 530: This might be used, e.g. in connection with the
1.2 deraadt 531: .Cm command
1.1 deraadt 532: option.
1.218 jmc 533: .It Cm no-pty
534: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 535: .It Cm no-user-rc
1.241 jmc 536: Disables execution of
1.240 djm 537: .Pa ~/.ssh/rc .
1.2 deraadt 538: .It Cm no-X11-forwarding
1.1 deraadt 539: Forbids X11 forwarding when this key is used for authentication.
540: Any X11 forward requests by the client will return an error.
1.302 djm 541: .It Cm permitlisten="[host:]port"
1.301 jmc 542: Limit remote port forwarding with the
1.300 djm 543: .Xr ssh 1
544: .Fl R
1.302 djm 545: option such that it may only listen on the specified host (optional) and port.
1.300 djm 546: IPv6 addresses can be specified by enclosing the address in square brackets.
547: Multiple
548: .Cm permitlisten
549: options may be applied separated by commas.
550: Hostnames may include wildcards as described in the PATTERNS section in
551: .Xr ssh_config 5 .
552: A port specification of
553: .Cm *
554: matches any port.
555: Note that the setting of
556: .Cm GatewayPorts
557: may further restrict listen addresses.
1.302 djm 558: Note that
559: .Xr ssh 1
560: will send a hostname of
561: .Dq localhost
562: if a listen host was not specified when the forwarding was requested, and
1.303 jmc 563: that this name is treated differently to the explicit localhost addresses
1.302 djm 564: .Dq 127.0.0.1
565: and
566: .Dq ::1 .
1.107 djm 567: .It Cm permitopen="host:port"
1.301 jmc 568: Limit local port forwarding with the
1.278 bentley 569: .Xr ssh 1
570: .Fl L
1.300 djm 571: option such that it may only connect to the specified host and port.
1.257 djm 572: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 573: Multiple
1.107 djm 574: .Cm permitopen
1.197 jmc 575: options may be applied separated by commas.
1.312 djm 576: No pattern matching or name lookup is performed on the
577: specified hostnames, they must be literal host names and/or addresses.
1.264 dtucker 578: A port specification of
579: .Cm *
580: matches any port.
1.282 djm 581: .It Cm port-forwarding
582: Enable port forwarding previously disabled by the
583: .Cm restrict
1.294 djm 584: option.
1.256 djm 585: .It Cm principals="principals"
586: On a
587: .Cm cert-authority
588: line, specifies allowed principals for certificate authentication as a
589: comma-separated list.
590: At least one name from the list must appear in the certificate's
591: list of principals for the certificate to be accepted.
592: This option is ignored for keys that are not marked as trusted certificate
593: signers using the
594: .Cm cert-authority
595: option.
1.282 djm 596: .It Cm pty
597: Permits tty allocation previously disabled by the
598: .Cm restrict
599: option.
1.307 djm 600: .It Cm no-touch-required
601: Do not require demonstration of user presence
602: for signatures made using this key.
1.313 djm 603: This option only makes sense for the FIDO authenticator algorithms
604: .Cm ecdsa-sk
605: and
606: .Cm ed25519-sk .
607: .It Cm verify-required
608: Require that signatures made using this key attest that they verified
609: the user, e.g. via a PIN.
1.311 naddy 610: This option only makes sense for the FIDO authenticator algorithms
1.307 djm 611: .Cm ecdsa-sk
612: and
613: .Cm ed25519-sk .
1.282 djm 614: .It Cm restrict
615: Enable all restrictions, i.e. disable port, agent and X11 forwarding,
616: as well as disabling PTY allocation
617: and execution of
618: .Pa ~/.ssh/rc .
619: If any future restriction capabilities are added to authorized_keys files
620: they will be included in this set.
1.209 reyk 621: .It Cm tunnel="n"
622: Force a
623: .Xr tun 4
624: device on the server.
625: Without this option, the next available device will be used if
626: the client requests a tunnel.
1.282 djm 627: .It Cm user-rc
628: Enables execution of
629: .Pa ~/.ssh/rc
630: previously disabled by the
631: .Cm restrict
632: option.
633: .It Cm X11-forwarding
634: Permits X11 forwarding previously disabled by the
635: .Cm restrict
636: option.
1.2 deraadt 637: .El
638: .Pp
1.219 jmc 639: An example authorized_keys file:
1.222 jmc 640: .Bd -literal -offset 3n
1.219 jmc 641: # Comments allowed at start of line
642: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 643: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
644: AAAAB2...19Q== john@example.net
1.304 dtucker 645: command="dump /home",no-pty,no-port-forwarding ssh-rsa
1.222 jmc 646: AAAAC3...51R== example.net
1.304 dtucker 647: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa
1.302 djm 648: AAAAB5...21S==
1.304 dtucker 649: permitlisten="localhost:8080",permitopen="localhost:22000" ssh-rsa
1.222 jmc 650: AAAAB5...21S==
651: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
652: jane@example.net
1.282 djm 653: restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
654: user@example.net
655: restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==
1.307 djm 656: user@example.net
657: no-touch-required sk-ecdsa-sha2-nistp256@openssh.com AAAAInN...Ko==
1.282 djm 658: user@example.net
1.219 jmc 659: .Ed
1.2 deraadt 660: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 661: The
1.194 jmc 662: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 663: and
1.207 djm 664: .Pa ~/.ssh/known_hosts
1.36 aaron 665: files contain host public keys for all known hosts.
666: The global file should
1.37 brad 667: be prepared by the administrator (optional), and the per-user file is
1.288 dtucker 668: maintained automatically: whenever the user connects to an unknown host,
1.36 aaron 669: its key is added to the per-user file.
1.2 deraadt 670: .Pp
1.314 ! djm 671: Each line in these files contains the following fields: marker (optional),
1.285 naddy 672: hostnames, keytype, base64-encoded key, comment.
1.36 aaron 673: The fields are separated by spaces.
1.2 deraadt 674: .Pp
1.254 djm 675: The marker is optional, but if it is present then it must be one of
676: .Dq @cert-authority ,
677: to indicate that the line contains a certification authority (CA) key,
678: or
679: .Dq @revoked ,
680: to indicate that the key contained on the line is revoked and must not ever
681: be accepted.
682: Only one marker should be used on a key line.
683: .Pp
1.196 david 684: Hostnames is a comma-separated list of patterns
1.220 jmc 685: .Pf ( Ql *
1.197 jmc 686: and
687: .Ql \&?
1.195 mouring 688: act as
1.290 djm 689: wildcards); each pattern in turn is matched against the host name.
690: When
691: .Nm sshd
692: is authenticating a client, such as when using
693: .Cm HostbasedAuthentication ,
694: this will be the canonical client host name.
695: When
696: .Xr ssh 1
1.291 jmc 697: is authenticating a server, this will be the host name
1.290 djm 698: given by the user, the value of the
699: .Xr ssh 1
700: .Cm HostkeyAlias
701: if it was specified, or the canonical server hostname if the
702: .Xr ssh 1
703: .Cm CanonicalizeHostname
704: option was used.
705: .Pp
1.36 aaron 706: A pattern may also be preceded by
1.195 mouring 707: .Ql \&!
1.2 deraadt 708: to indicate negation: if the host name matches a negated
1.1 deraadt 709: pattern, it is not accepted (by that line) even if it matched another
710: pattern on the line.
1.231 dtucker 711: A hostname or address may optionally be enclosed within
712: .Ql \&[
713: and
714: .Ql \&]
715: brackets then followed by
716: .Ql \&:
1.232 jmc 717: and a non-standard port number.
1.2 deraadt 718: .Pp
1.205 djm 719: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 720: and addresses should the file's contents be disclosed.
721: Hashed hostnames start with a
722: .Ql |
1.205 djm 723: character.
724: Only one hashed hostname may appear on a single line and none of the above
725: negation or wildcard operators may be applied.
726: .Pp
1.285 naddy 727: The keytype and base64-encoded key are taken directly from the host key; they
1.220 jmc 728: can be obtained, for example, from
1.285 naddy 729: .Pa /etc/ssh/ssh_host_rsa_key.pub .
1.1 deraadt 730: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 731: .Pp
732: Lines starting with
733: .Ql #
734: and empty lines are ignored as comments.
735: .Pp
1.1 deraadt 736: When performing host authentication, authentication is accepted if any
1.254 djm 737: matching line has the proper key; either one that matches exactly or,
738: if the server has presented a certificate for authentication, the key
739: of the certification authority that signed the certificate.
740: For a key to be trusted as a certification authority, it must use the
741: .Dq @cert-authority
742: marker described above.
743: .Pp
744: The known hosts file also provides a facility to mark keys as revoked,
745: for example when it is known that the associated private key has been
746: stolen.
747: Revoked keys are specified by including the
748: .Dq @revoked
749: marker at the beginning of the key line, and are never accepted for
750: authentication or as certification authorities, but instead will
751: produce a warning from
752: .Xr ssh 1
753: when they are encountered.
754: .Pp
755: It is permissible (but not
1.1 deraadt 756: recommended) to have several lines or different host keys for the same
1.36 aaron 757: names.
758: This will inevitably happen when short forms of host names
759: from different domains are put in the file.
760: It is possible
1.1 deraadt 761: that the files contain conflicting information; authentication is
762: accepted if valid information can be found from either file.
1.2 deraadt 763: .Pp
1.1 deraadt 764: Note that the lines in these files are typically hundreds of characters
765: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 766: Rather, generate them by a script,
767: .Xr ssh-keyscan 1
1.285 naddy 768: or by taking, for example,
769: .Pa /etc/ssh/ssh_host_rsa_key.pub
1.1 deraadt 770: and adding the host names at the front.
1.254 djm 771: .Xr ssh-keygen 1
1.255 jmc 772: also offers some basic automated editing for
1.254 djm 773: .Pa ~/.ssh/known_hosts
774: including removing hosts matching a host name and converting all host
775: names to their hashed representations.
1.221 jmc 776: .Pp
777: An example ssh_known_hosts file:
778: .Bd -literal -offset 3n
779: # Comments allowed at start of line
780: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
781: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 782: # A hashed hostname
783: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
784: AAAA1234.....=
1.254 djm 785: # A revoked key
786: @revoked * ssh-rsa AAAAB5W...
787: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
788: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 789: .Ed
1.2 deraadt 790: .Sh FILES
1.223 jmc 791: .Bl -tag -width Ds -compact
1.258 jmc 792: .It Pa ~/.hushlogin
1.204 jmc 793: This file is used to suppress printing the last login time and
794: .Pa /etc/motd ,
795: if
796: .Cm PrintLastLog
797: and
798: .Cm PrintMotd ,
799: respectively,
800: are enabled.
801: It does not suppress printing of the banner specified by
802: .Cm Banner .
1.223 jmc 803: .Pp
1.258 jmc 804: .It Pa ~/.rhosts
1.225 jmc 805: This file is used for host-based authentication (see
806: .Xr ssh 1
807: for more information).
808: On some machines this file may need to be
809: world-readable if the user's home directory is on an NFS partition,
810: because
811: .Nm
812: reads it as root.
813: Additionally, this file must be owned by the user,
814: and must not have write permissions for anyone else.
815: The recommended
816: permission for most machines is read/write for the user, and not
1.1 deraadt 817: accessible by others.
1.2 deraadt 818: .Pp
1.258 jmc 819: .It Pa ~/.shosts
1.225 jmc 820: This file is used in exactly the same way as
821: .Pa .rhosts ,
822: but allows host-based authentication without permitting login with
823: rlogin/rsh.
1.238 mcbride 824: .Pp
1.258 jmc 825: .It Pa ~/.ssh/
1.238 mcbride 826: This directory is the default location for all user-specific configuration
827: and authentication information.
828: There is no general requirement to keep the entire contents of this directory
829: secret, but the recommended permissions are read/write/execute for the user,
830: and not accessible by others.
1.2 deraadt 831: .Pp
1.258 jmc 832: .It Pa ~/.ssh/authorized_keys
1.277 sobrado 833: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.273 naddy 834: that can be used for logging in as this user.
1.223 jmc 835: The format of this file is described above.
1.234 dtucker 836: The content of the file is not highly sensitive, but the recommended
1.225 jmc 837: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 838: .Pp
839: If this file, the
840: .Pa ~/.ssh
841: directory, or the user's home directory are writable
842: by other users, then the file could be modified or replaced by unauthorized
843: users.
844: In this case,
845: .Nm
846: will not allow it to be used unless the
847: .Cm StrictModes
848: option has been set to
849: .Dq no .
1.2 deraadt 850: .Pp
1.258 jmc 851: .It Pa ~/.ssh/environment
1.36 aaron 852: This file is read into the environment at login (if it exists).
853: It can only contain empty lines, comment lines (that start with
1.2 deraadt 854: .Ql # ) ,
1.36 aaron 855: and assignment lines of the form name=value.
856: The file should be writable
1.6 aaron 857: only by the user; it need not be readable by anyone else.
1.188 stevesk 858: Environment processing is disabled by default and is
859: controlled via the
860: .Cm PermitUserEnvironment
861: option.
1.223 jmc 862: .Pp
1.258 jmc 863: .It Pa ~/.ssh/known_hosts
1.225 jmc 864: Contains a list of host keys for all hosts the user has logged into
865: that are not already in the systemwide list of known host keys.
866: The format of this file is described above.
867: This file should be writable only by root/the owner and
1.223 jmc 868: can, but need not be, world-readable.
869: .Pp
1.258 jmc 870: .It Pa ~/.ssh/rc
1.226 jmc 871: Contains initialization routines to be run before
872: the user's home directory becomes accessible.
1.1 deraadt 873: This file should be writable only by the user, and need not be
874: readable by anyone else.
1.223 jmc 875: .Pp
1.258 jmc 876: .It Pa /etc/hosts.equiv
1.227 jmc 877: This file is for host-based authentication (see
878: .Xr ssh 1 ) .
879: It should only be writable by root.
1.223 jmc 880: .Pp
1.258 jmc 881: .It Pa /etc/moduli
1.283 djm 882: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange"
883: key exchange method.
1.223 jmc 884: The file format is described in
885: .Xr moduli 5 .
1.283 djm 886: If no usable groups are found in this file then fixed internal groups will
887: be used.
1.223 jmc 888: .Pp
1.258 jmc 889: .It Pa /etc/motd
1.223 jmc 890: See
891: .Xr motd 5 .
892: .Pp
1.258 jmc 893: .It Pa /etc/nologin
1.223 jmc 894: If this file exists,
895: .Nm
896: refuses to let anyone except root log in.
897: The contents of the file
898: are displayed to anyone trying to log in, and non-root connections are
899: refused.
900: The file should be world-readable.
901: .Pp
1.258 jmc 902: .It Pa /etc/shosts.equiv
1.227 jmc 903: This file is used in exactly the same way as
904: .Pa hosts.equiv ,
905: but allows host-based authentication without permitting login with
906: rlogin/rsh.
1.225 jmc 907: .Pp
1.259 djm 908: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.273 naddy 909: .It Pa /etc/ssh/ssh_host_ed25519_key
1.258 jmc 910: .It Pa /etc/ssh/ssh_host_rsa_key
1.266 dtucker 911: These files contain the private parts of the host keys.
1.223 jmc 912: These files should only be owned by root, readable only by root, and not
913: accessible to others.
914: Note that
915: .Nm
1.228 jmc 916: does not start if these files are group/world-accessible.
1.223 jmc 917: .Pp
1.259 djm 918: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.273 naddy 919: .It Pa /etc/ssh/ssh_host_ed25519_key.pub
1.258 jmc 920: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.266 dtucker 921: These files contain the public parts of the host keys.
1.223 jmc 922: These files should be world-readable but writable only by
923: root.
924: Their contents should match the respective private parts.
925: These files are not
926: really used for anything; they are provided for the convenience of
927: the user so their contents can be copied to known hosts files.
928: These files are created using
929: .Xr ssh-keygen 1 .
930: .Pp
1.258 jmc 931: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 932: Systemwide list of known host keys.
933: This file should be prepared by the
934: system administrator to contain the public host keys of all machines in the
935: organization.
936: The format of this file is described above.
937: This file should be writable only by root/the owner and
938: should be world-readable.
939: .Pp
1.258 jmc 940: .It Pa /etc/ssh/sshd_config
1.223 jmc 941: Contains configuration data for
942: .Nm sshd .
943: The file format and configuration options are described in
944: .Xr sshd_config 5 .
945: .Pp
1.258 jmc 946: .It Pa /etc/ssh/sshrc
1.226 jmc 947: Similar to
948: .Pa ~/.ssh/rc ,
949: it can be used to specify
1.36 aaron 950: machine-specific login-time initializations globally.
951: This file should be writable only by root, and should be world-readable.
1.223 jmc 952: .Pp
1.258 jmc 953: .It Pa /var/empty
1.223 jmc 954: .Xr chroot 2
955: directory used by
956: .Nm
957: during privilege separation in the pre-authentication phase.
958: The directory should not contain any files and must be owned by root
959: and not group or world-writable.
960: .Pp
1.258 jmc 961: .It Pa /var/run/sshd.pid
1.223 jmc 962: Contains the process ID of the
963: .Nm
964: listening for connections (if there are several daemons running
965: concurrently for different ports, this contains the process ID of the one
966: started last).
967: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 968: .El
1.2 deraadt 969: .Sh SEE ALSO
970: .Xr scp 1 ,
1.90 djm 971: .Xr sftp 1 ,
1.2 deraadt 972: .Xr ssh 1 ,
1.5 deraadt 973: .Xr ssh-add 1 ,
1.2 deraadt 974: .Xr ssh-agent 1 ,
1.5 deraadt 975: .Xr ssh-keygen 1 ,
1.235 jmc 976: .Xr ssh-keyscan 1 ,
1.200 jmc 977: .Xr chroot 2 ,
1.136 markus 978: .Xr login.conf 5 ,
979: .Xr moduli 5 ,
1.184 stevesk 980: .Xr sshd_config 5 ,
1.200 jmc 981: .Xr inetd 8 ,
1.128 mpech 982: .Xr sftp-server 8
1.198 jmc 983: .Sh AUTHORS
984: OpenSSH is a derivative of the original and free
985: ssh 1.2.12 release by Tatu Ylonen.
986: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
987: Theo de Raadt and Dug Song
988: removed many bugs, re-added newer features and
989: created OpenSSH.
990: Markus Friedl contributed the support for SSH
991: protocol versions 1.5 and 2.0.
992: Niels Provos and Markus Friedl contributed support
993: for privilege separation.
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