Annotation of src/usr.bin/ssh/sshd.8, Revision 1.316
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: .\"
1.316 ! jmc 36: .\" $OpenBSD: sshd.8,v 1.315 2021/06/04 05:59:18 djm Exp $
! 37: .Dd $Mdocdate: June 4 2021 $
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 .
1.316 ! jmc 61: It provides secure encrypted communications between two untrusted hosts
1.36 aaron 62: over an insecure network.
1.2 deraadt 63: .Pp
64: .Nm
1.212 jmc 65: listens for connections from clients.
1.40 aaron 66: It is normally started at boot from
1.2 deraadt 67: .Pa /etc/rc .
68: It forks a new
1.36 aaron 69: daemon for each incoming connection.
70: The forked daemons handle
1.1 deraadt 71: key exchange, encryption, authentication, command execution,
72: and data exchange.
1.2 deraadt 73: .Pp
74: .Nm
1.200 jmc 75: can be configured using command-line options or a configuration file
76: (by default
1.212 jmc 77: .Xr sshd_config 5 ) ;
78: command-line options override values specified in the
1.1 deraadt 79: configuration file.
1.25 markus 80: .Nm
81: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 82: .Dv SIGHUP ,
1.230 jmc 83: by executing itself with the name and options it was started with, e.g.\&
1.97 deraadt 84: .Pa /usr/sbin/sshd .
1.18 aaron 85: .Pp
86: The options are as follows:
1.2 deraadt 87: .Bl -tag -width Ds
1.200 jmc 88: .It Fl 4
89: Forces
90: .Nm
91: to use IPv4 addresses only.
92: .It Fl 6
93: Forces
94: .Nm
95: to use IPv6 addresses only.
1.243 jmc 96: .It Fl C Ar connection_spec
97: Specify the connection parameters to use for the
98: .Fl T
99: extended test mode.
100: If provided, any
101: .Cm Match
1.293 dtucker 102: directives in the configuration file that would apply are applied before the
103: configuration is written to standard output.
104: The connection parameters are supplied as keyword=value pairs and may be
105: supplied in any order, either with multiple
106: .Fl C
107: options or as a comma-separated list.
1.243 jmc 108: The keywords are
1.316 ! jmc 109: .Dq addr ,
1.243 jmc 110: .Dq user ,
111: .Dq host ,
1.265 dtucker 112: .Dq laddr ,
113: .Dq lport ,
1.293 dtucker 114: and
1.292 djm 115: .Dq rdomain
1.293 dtucker 116: and correspond to source address, user, resolved source host name,
117: local address, local port number and routing domain respectively.
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.
1.316 ! jmc 137: The server also will not
! 138: .Xr fork 2
! 139: and will only process one connection.
1.36 aaron 140: This option is only intended for debugging for the server.
1.194 jmc 141: Multiple
142: .Fl d
143: options increase the debugging level.
1.67 aaron 144: Maximum is 3.
1.268 dtucker 145: .It Fl E Ar log_file
146: Append debug logs to
147: .Ar log_file
148: instead of the system log.
1.120 markus 149: .It Fl e
1.269 dtucker 150: Write debug logs to standard error instead of the system log.
1.235 jmc 151: .It Fl f Ar config_file
1.36 aaron 152: Specifies the name of the configuration file.
153: The default is
1.167 deraadt 154: .Pa /etc/ssh/sshd_config .
1.16 markus 155: .Nm
156: refuses to start if there is no configuration file.
1.2 deraadt 157: .It Fl g Ar login_grace_time
1.1 deraadt 158: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 159: 120 seconds).
1.36 aaron 160: If the client fails to authenticate the user within
161: this many seconds, the server disconnects and exits.
162: A value of zero indicates no limit.
1.2 deraadt 163: .It Fl h Ar host_key_file
1.160 stevesk 164: Specifies a file from which a host key is read.
1.7 markus 165: This option must be given if
166: .Nm
167: is not run as root (as the normal
1.160 stevesk 168: host key files are normally not readable by anyone but root).
169: The default is
1.285 naddy 170: .Pa /etc/ssh/ssh_host_ecdsa_key ,
1.273 naddy 171: .Pa /etc/ssh/ssh_host_ed25519_key
1.259 djm 172: and
1.285 naddy 173: .Pa /etc/ssh/ssh_host_rsa_key .
1.75 markus 174: It is possible to have multiple host key files for
1.285 naddy 175: the different host key algorithms.
1.2 deraadt 176: .It Fl i
1.7 markus 177: Specifies that
178: .Nm
1.194 jmc 179: is being run from
180: .Xr inetd 8 .
1.156 markus 181: .It Fl o Ar option
182: Can be used to give options in the format used in the configuration file.
183: This is useful for specifying options for which there is no separate
184: command-line flag.
1.200 jmc 185: For full details of the options, and their values, see
186: .Xr sshd_config 5 .
1.2 deraadt 187: .It Fl p Ar port
1.1 deraadt 188: Specifies the port on which the server listens for connections
189: (default 22).
1.158 stevesk 190: Multiple port options are permitted.
1.210 stevesk 191: Ports specified in the configuration file with the
192: .Cm Port
193: option are ignored when a command-line port is specified.
194: Ports specified using the
195: .Cm ListenAddress
196: option override command-line ports.
1.2 deraadt 197: .It Fl q
1.36 aaron 198: Quiet mode.
199: Nothing is sent to the system log.
200: Normally the beginning,
1.1 deraadt 201: authentication, and termination of each connection is logged.
1.242 dtucker 202: .It Fl T
203: Extended test mode.
204: Check the validity of the configuration file, output the effective configuration
205: to stdout and then exit.
206: Optionally,
207: .Cm Match
208: rules may be applied by specifying the connection parameters using one or more
209: .Fl C
210: options.
1.243 jmc 211: .It Fl t
212: Test mode.
213: Only check the validity of the configuration file and sanity of the keys.
214: This is useful for updating
215: .Nm
216: reliably as configuration options may change.
1.61 markus 217: .It Fl u Ar len
218: This option is used to specify the size of the field
219: in the
220: .Li utmp
221: structure that holds the remote host name.
222: If the resolved host name is longer than
223: .Ar len ,
224: the dotted decimal value will be used instead.
225: This allows hosts with very long host names that
226: overflow this field to still be uniquely identified.
227: Specifying
228: .Fl u0
229: indicates that only dotted decimal addresses
230: should be put into the
231: .Pa utmp
232: file.
1.144 stevesk 233: .Fl u0
1.194 jmc 234: may also be used to prevent
1.144 stevesk 235: .Nm
236: from making DNS requests unless the authentication
237: mechanism or configuration requires it.
238: Authentication mechanisms that may require DNS include
1.285 naddy 239: .Cm HostbasedAuthentication
1.144 stevesk 240: and using a
241: .Cm from="pattern-list"
242: option in a key file.
1.170 stevesk 243: Configuration options that require DNS include using a
244: USER@HOST pattern in
245: .Cm AllowUsers
246: or
247: .Cm DenyUsers .
1.2 deraadt 248: .El
1.214 jmc 249: .Sh AUTHENTICATION
1.285 naddy 250: The OpenSSH SSH daemon supports SSH protocol 2 only.
1.284 jmc 251: Each host has a host-specific key,
1.214 jmc 252: used to identify the host.
1.212 jmc 253: Whenever a client connects, the daemon responds with its public
1.285 naddy 254: host key.
1.212 jmc 255: The client compares the
1.285 naddy 256: host key against its own database to verify that it has not changed.
1.310 dtucker 257: Forward secrecy is provided through a Diffie-Hellman key agreement.
1.214 jmc 258: This key agreement results in a shared session key.
1.309 naddy 259: The rest of the session is encrypted using a symmetric cipher.
1.214 jmc 260: The client selects the encryption algorithm
261: to use from those offered by the server.
262: Additionally, session integrity is provided
1.310 dtucker 263: through a cryptographic message authentication code (MAC).
1.214 jmc 264: .Pp
265: Finally, the server and the client enter an authentication dialog.
1.212 jmc 266: The client tries to authenticate itself using
1.214 jmc 267: host-based authentication,
268: public key authentication,
269: challenge-response authentication,
270: or password authentication.
1.212 jmc 271: .Pp
272: If the client successfully authenticates itself, a dialog for
273: preparing the session is entered.
274: At this time the client may request
275: things like allocating a pseudo-tty, forwarding X11 connections,
276: forwarding TCP connections, or forwarding the authentication agent
277: connection over the secure channel.
278: .Pp
1.216 jmc 279: After this, the client either requests a shell or execution of a command.
1.212 jmc 280: The sides then enter session mode.
281: In this mode, either side may send
282: data at any time, and such data is forwarded to/from the shell or
283: command on the server side, and the user terminal in the client side.
284: .Pp
285: When the user program terminates and all forwarded X11 and other
286: connections have been closed, the server sends command exit status to
287: the client, and both sides exit.
1.2 deraadt 288: .Sh LOGIN PROCESS
1.1 deraadt 289: When a user successfully logs in,
1.2 deraadt 290: .Nm
1.1 deraadt 291: does the following:
1.2 deraadt 292: .Bl -enum -offset indent
293: .It
1.1 deraadt 294: If the login is on a tty, and no command has been specified,
1.40 aaron 295: prints last login time and
1.2 deraadt 296: .Pa /etc/motd
1.1 deraadt 297: (unless prevented in the configuration file or by
1.207 djm 298: .Pa ~/.hushlogin ;
1.2 deraadt 299: see the
1.40 aaron 300: .Sx FILES
1.2 deraadt 301: section).
302: .It
1.1 deraadt 303: If the login is on a tty, records login time.
1.2 deraadt 304: .It
305: Checks
306: .Pa /etc/nologin ;
307: if it exists, prints contents and quits
1.1 deraadt 308: (unless root).
1.2 deraadt 309: .It
1.1 deraadt 310: Changes to run with normal user privileges.
1.2 deraadt 311: .It
1.1 deraadt 312: Sets up basic environment.
1.2 deraadt 313: .It
1.200 jmc 314: Reads the file
1.207 djm 315: .Pa ~/.ssh/environment ,
1.200 jmc 316: if it exists, and users are allowed to change their environment.
1.188 stevesk 317: See the
1.187 marc 318: .Cm PermitUserEnvironment
1.188 stevesk 319: option in
1.187 marc 320: .Xr sshd_config 5 .
1.2 deraadt 321: .It
1.1 deraadt 322: Changes to user's home directory.
1.2 deraadt 323: .It
324: If
1.207 djm 325: .Pa ~/.ssh/rc
1.276 djm 326: exists and the
327: .Xr sshd_config 5
328: .Cm PermitUserRC
329: option is set, runs it; else if
1.168 deraadt 330: .Pa /etc/ssh/sshrc
1.2 deraadt 331: exists, runs
1.316 ! jmc 332: it; otherwise runs
! 333: .Xr xauth 1 .
1.36 aaron 334: The
1.2 deraadt 335: .Dq rc
336: files are given the X11
1.1 deraadt 337: authentication protocol and cookie in standard input.
1.226 jmc 338: See
339: .Sx SSHRC ,
340: below.
1.2 deraadt 341: .It
1.1 deraadt 342: Runs user's shell or command.
1.279 djm 343: All commands are run under the user's login shell as specified in the
344: system password database.
1.2 deraadt 345: .El
1.226 jmc 346: .Sh SSHRC
347: If the file
348: .Pa ~/.ssh/rc
349: exists,
350: .Xr sh 1
351: runs it after reading the
352: environment files but before starting the user's shell or command.
353: It must not produce any output on stdout; stderr must be used
354: instead.
355: If X11 forwarding is in use, it will receive the "proto cookie" pair in
356: its standard input (and
357: .Ev DISPLAY
358: in its environment).
359: The script must call
360: .Xr xauth 1
361: because
362: .Nm
363: will not run xauth automatically to add X11 cookies.
364: .Pp
365: The primary purpose of this file is to run any initialization routines
366: which may be needed before the user's home directory becomes
367: accessible; AFS is a particular example of such an environment.
368: .Pp
369: This file will probably contain some initialization code followed by
370: something similar to:
371: .Bd -literal -offset 3n
372: if read proto cookie && [ -n "$DISPLAY" ]; then
373: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
374: # X11UseLocalhost=yes
375: echo add unix:`echo $DISPLAY |
376: cut -c11-` $proto $cookie
377: else
378: # X11UseLocalhost=no
379: echo add $DISPLAY $proto $cookie
380: fi | xauth -q -
381: fi
382: .Ed
383: .Pp
384: If this file does not exist,
385: .Pa /etc/ssh/sshrc
386: is run, and if that
387: does not exist either, xauth is used to add the cookie.
1.2 deraadt 388: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 389: .Cm AuthorizedKeysFile
1.262 jmc 390: specifies the files containing public keys for
1.217 jmc 391: public key authentication;
1.281 djm 392: if this option is not specified, the default is
1.261 djm 393: .Pa ~/.ssh/authorized_keys
394: and
395: .Pa ~/.ssh/authorized_keys2 .
1.36 aaron 396: Each line of the file contains one
1.2 deraadt 397: key (empty lines and lines starting with a
398: .Ql #
399: are ignored as
1.36 aaron 400: comments).
1.285 naddy 401: Public keys consist of the following space-separated fields:
1.217 jmc 402: options, keytype, base64-encoded key, comment.
1.285 naddy 403: The options field is optional.
1.305 naddy 404: The supported key types are:
405: .Pp
406: .Bl -item -compact -offset indent
407: .It
408: sk-ecdsa-sha2-nistp256@openssh.com
409: .It
410: ecdsa-sha2-nistp256
411: .It
412: ecdsa-sha2-nistp384
413: .It
414: ecdsa-sha2-nistp521
415: .It
1.306 djm 416: sk-ssh-ed25519@openssh.com
417: .It
1.305 naddy 418: ssh-ed25519
419: .It
420: ssh-dss
421: .It
422: ssh-rsa
423: .El
424: .Pp
425: The comment field is not used for anything (but may be convenient for the
1.285 naddy 426: user to identify the key).
1.2 deraadt 427: .Pp
1.285 naddy 428: Note that lines in this file can be several hundred bytes long
1.203 dtucker 429: (because of the size of the public key encoding) up to a limit of
1.305 naddy 430: 8 kilobytes, which permits RSA keys up to 16 kilobits.
1.36 aaron 431: You don't want to type them in; instead, copy the
1.217 jmc 432: .Pa id_dsa.pub ,
1.259 djm 433: .Pa id_ecdsa.pub ,
1.305 naddy 434: .Pa id_ecdsa_sk.pub ,
1.273 naddy 435: .Pa id_ed25519.pub ,
1.306 djm 436: .Pa id_ed25519_sk.pub ,
1.75 markus 437: or the
1.113 itojun 438: .Pa id_rsa.pub
1.1 deraadt 439: file and edit it.
1.175 stevesk 440: .Pp
441: .Nm
1.296 dtucker 442: enforces a minimum RSA key modulus size of 1024 bits.
1.2 deraadt 443: .Pp
1.58 deraadt 444: The options (if present) consist of comma-separated option
1.36 aaron 445: specifications.
446: No spaces are permitted, except within double quotes.
1.141 stevesk 447: The following option specifications are supported (note
448: that option keywords are case-insensitive):
1.2 deraadt 449: .Bl -tag -width Ds
1.282 djm 450: .It Cm agent-forwarding
451: Enable authentication agent forwarding previously disabled by the
452: .Cm restrict
453: option.
1.253 djm 454: .It Cm cert-authority
1.251 djm 455: Specifies that the listed key is a certification authority (CA) that is
456: trusted to validate signed certificates for user authentication.
457: .Pp
458: Certificates may encode access restrictions similar to these key options.
459: If both certificate restrictions and key options are present, the most
460: restrictive union of the two is applied.
1.2 deraadt 461: .It Cm command="command"
1.1 deraadt 462: Specifies that the command is executed whenever this key is used for
1.36 aaron 463: authentication.
464: The command supplied by the user (if any) is ignored.
1.148 markus 465: The command is run on a pty if the client requests a pty;
1.36 aaron 466: otherwise it is run without a tty.
1.194 jmc 467: If an 8-bit clean channel is required,
1.147 deraadt 468: one must not request a pty or should specify
1.89 markus 469: .Cm no-pty .
1.36 aaron 470: A quote may be included in the command by quoting it with a backslash.
1.287 djm 471: .Pp
1.36 aaron 472: This option might be useful
1.189 espie 473: to restrict certain public keys to perform just a specific operation.
1.36 aaron 474: An example might be a key that permits remote backups but nothing else.
1.211 jmc 475: Note that the client may specify TCP and/or X11
1.287 djm 476: forwarding unless they are explicitly prohibited, e.g. using the
477: .Cm restrict
478: key option.
479: .Pp
1.233 dtucker 480: The command originally supplied by the client is available in the
481: .Ev SSH_ORIGINAL_COMMAND
482: environment variable.
1.149 markus 483: Note that this option applies to shell, command or subsystem execution.
1.287 djm 484: Also note that this command may be superseded by a
1.251 djm 485: .Xr sshd_config 5
486: .Cm ForceCommand
1.287 djm 487: directive.
488: .Pp
489: If a command is specified and a forced-command is embedded in a certificate
490: used for authentication, then the certificate will be accepted only if the
491: two commands are identical.
1.2 deraadt 492: .It Cm environment="NAME=value"
1.1 deraadt 493: Specifies that the string is to be added to the environment when
1.36 aaron 494: logging in using this key.
495: Environment variables set this way
496: override other default environment values.
497: Multiple options of this type are permitted.
1.188 stevesk 498: Environment processing is disabled by default and is
499: controlled via the
500: .Cm PermitUserEnvironment
501: option.
1.299 jmc 502: .It Cm expiry-time="timespec"
503: Specifies a time after which the key will not be accepted.
504: The time may be specified as a YYYYMMDD date or a YYYYMMDDHHMM[SS] time
505: in the system time-zone.
1.218 jmc 506: .It Cm from="pattern-list"
1.244 djm 507: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 508: name of the remote host or its IP address must be present in the
1.244 djm 509: comma-separated list of patterns.
1.270 jmc 510: See PATTERNS in
1.229 jmc 511: .Xr ssh_config 5
512: for more information on patterns.
1.244 djm 513: .Pp
514: In addition to the wildcard matching that may be applied to hostnames or
515: addresses, a
516: .Cm from
1.248 sobrado 517: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 518: .Pp
519: The purpose of this option is to optionally increase security: public key
520: authentication by itself does not trust the network or name servers or
521: anything (but the key); however, if somebody somehow steals the key, the key
522: permits an intruder to log in from anywhere in the world.
523: This additional option makes using a stolen key more difficult (name
524: servers and/or routers would have to be compromised in addition to
525: just the key).
1.218 jmc 526: .It Cm no-agent-forwarding
527: Forbids authentication agent forwarding when this key is used for
528: authentication.
1.2 deraadt 529: .It Cm no-port-forwarding
1.211 jmc 530: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 531: Any port forward requests by the client will return an error.
1.230 jmc 532: This might be used, e.g. in connection with the
1.2 deraadt 533: .Cm command
1.1 deraadt 534: option.
1.218 jmc 535: .It Cm no-pty
536: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 537: .It Cm no-user-rc
1.241 jmc 538: Disables execution of
1.240 djm 539: .Pa ~/.ssh/rc .
1.2 deraadt 540: .It Cm no-X11-forwarding
1.1 deraadt 541: Forbids X11 forwarding when this key is used for authentication.
542: Any X11 forward requests by the client will return an error.
1.302 djm 543: .It Cm permitlisten="[host:]port"
1.301 jmc 544: Limit remote port forwarding with the
1.300 djm 545: .Xr ssh 1
546: .Fl R
1.302 djm 547: option such that it may only listen on the specified host (optional) and port.
1.300 djm 548: IPv6 addresses can be specified by enclosing the address in square brackets.
549: Multiple
550: .Cm permitlisten
551: options may be applied separated by commas.
552: Hostnames may include wildcards as described in the PATTERNS section in
553: .Xr ssh_config 5 .
554: A port specification of
555: .Cm *
556: matches any port.
557: Note that the setting of
558: .Cm GatewayPorts
559: may further restrict listen addresses.
1.302 djm 560: Note that
561: .Xr ssh 1
562: will send a hostname of
563: .Dq localhost
564: if a listen host was not specified when the forwarding was requested, and
1.303 jmc 565: that this name is treated differently to the explicit localhost addresses
1.302 djm 566: .Dq 127.0.0.1
567: and
568: .Dq ::1 .
1.107 djm 569: .It Cm permitopen="host:port"
1.301 jmc 570: Limit local port forwarding with the
1.278 bentley 571: .Xr ssh 1
572: .Fl L
1.300 djm 573: option such that it may only connect to the specified host and port.
1.257 djm 574: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 575: Multiple
1.107 djm 576: .Cm permitopen
1.197 jmc 577: options may be applied separated by commas.
1.312 djm 578: No pattern matching or name lookup is performed on the
579: specified hostnames, they must be literal host names and/or addresses.
1.264 dtucker 580: A port specification of
581: .Cm *
582: matches any port.
1.282 djm 583: .It Cm port-forwarding
584: Enable port forwarding previously disabled by the
585: .Cm restrict
1.294 djm 586: option.
1.256 djm 587: .It Cm principals="principals"
588: On a
589: .Cm cert-authority
590: line, specifies allowed principals for certificate authentication as a
591: comma-separated list.
592: At least one name from the list must appear in the certificate's
593: list of principals for the certificate to be accepted.
594: This option is ignored for keys that are not marked as trusted certificate
595: signers using the
596: .Cm cert-authority
597: option.
1.282 djm 598: .It Cm pty
599: Permits tty allocation previously disabled by the
600: .Cm restrict
601: option.
1.307 djm 602: .It Cm no-touch-required
603: Do not require demonstration of user presence
604: for signatures made using this key.
1.313 djm 605: This option only makes sense for the FIDO authenticator algorithms
606: .Cm ecdsa-sk
607: and
608: .Cm ed25519-sk .
609: .It Cm verify-required
610: Require that signatures made using this key attest that they verified
611: the user, e.g. via a PIN.
1.311 naddy 612: This option only makes sense for the FIDO authenticator algorithms
1.307 djm 613: .Cm ecdsa-sk
614: and
615: .Cm ed25519-sk .
1.282 djm 616: .It Cm restrict
617: Enable all restrictions, i.e. disable port, agent and X11 forwarding,
618: as well as disabling PTY allocation
619: and execution of
620: .Pa ~/.ssh/rc .
621: If any future restriction capabilities are added to authorized_keys files
622: they will be included in this set.
1.209 reyk 623: .It Cm tunnel="n"
624: Force a
625: .Xr tun 4
626: device on the server.
627: Without this option, the next available device will be used if
628: the client requests a tunnel.
1.282 djm 629: .It Cm user-rc
630: Enables execution of
631: .Pa ~/.ssh/rc
632: previously disabled by the
633: .Cm restrict
634: option.
635: .It Cm X11-forwarding
636: Permits X11 forwarding previously disabled by the
637: .Cm restrict
638: option.
1.2 deraadt 639: .El
640: .Pp
1.219 jmc 641: An example authorized_keys file:
1.222 jmc 642: .Bd -literal -offset 3n
1.315 djm 643: # Comments are allowed at start of line. Blank lines are allowed.
644: # Plain key, no restrictions
645: ssh-rsa ...
646: # Forced command, disable PTY and all forwarding
647: restrict,command="dump /home" ssh-rsa ...
648: # Restriction of ssh -L forwarding destinations
649: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa ...
650: # Restriction of ssh -R forwarding listeners
651: permitlisten="localhost:8080",permitlisten="[::1]:22000" ssh-rsa ...
652: # Configuration for tunnel forwarding
653: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa ...
654: # Override of restriction to allow PTY allocation
655: restrict,pty,command="nethack" ssh-rsa ...
656: # Allow FIDO key without requiring touch
657: no-touch-required sk-ecdsa-sha2-nistp256@openssh.com ...
658: # Require user-verification (e.g. PIN or biometric) for FIDO key
659: verify-required sk-ecdsa-sha2-nistp256@openssh.com ...
660: # Trust CA key, allow touch-less FIDO if requested in certificate
661: cert-authority,no-touch-required,principals="user_a" ssh-rsa ...
1.219 jmc 662: .Ed
1.2 deraadt 663: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 664: The
1.194 jmc 665: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 666: and
1.207 djm 667: .Pa ~/.ssh/known_hosts
1.36 aaron 668: files contain host public keys for all known hosts.
669: The global file should
1.37 brad 670: be prepared by the administrator (optional), and the per-user file is
1.288 dtucker 671: maintained automatically: whenever the user connects to an unknown host,
1.36 aaron 672: its key is added to the per-user file.
1.2 deraadt 673: .Pp
1.314 djm 674: Each line in these files contains the following fields: marker (optional),
1.285 naddy 675: hostnames, keytype, base64-encoded key, comment.
1.36 aaron 676: The fields are separated by spaces.
1.2 deraadt 677: .Pp
1.254 djm 678: The marker is optional, but if it is present then it must be one of
679: .Dq @cert-authority ,
680: to indicate that the line contains a certification authority (CA) key,
681: or
682: .Dq @revoked ,
683: to indicate that the key contained on the line is revoked and must not ever
684: be accepted.
685: Only one marker should be used on a key line.
686: .Pp
1.196 david 687: Hostnames is a comma-separated list of patterns
1.220 jmc 688: .Pf ( Ql *
1.197 jmc 689: and
690: .Ql \&?
1.195 mouring 691: act as
1.290 djm 692: wildcards); each pattern in turn is matched against the host name.
693: When
694: .Nm sshd
695: is authenticating a client, such as when using
696: .Cm HostbasedAuthentication ,
697: this will be the canonical client host name.
698: When
699: .Xr ssh 1
1.291 jmc 700: is authenticating a server, this will be the host name
1.290 djm 701: given by the user, the value of the
702: .Xr ssh 1
703: .Cm HostkeyAlias
704: if it was specified, or the canonical server hostname if the
705: .Xr ssh 1
706: .Cm CanonicalizeHostname
707: option was used.
708: .Pp
1.36 aaron 709: A pattern may also be preceded by
1.195 mouring 710: .Ql \&!
1.2 deraadt 711: to indicate negation: if the host name matches a negated
1.1 deraadt 712: pattern, it is not accepted (by that line) even if it matched another
713: pattern on the line.
1.231 dtucker 714: A hostname or address may optionally be enclosed within
715: .Ql \&[
716: and
717: .Ql \&]
718: brackets then followed by
719: .Ql \&:
1.232 jmc 720: and a non-standard port number.
1.2 deraadt 721: .Pp
1.205 djm 722: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 723: and addresses should the file's contents be disclosed.
724: Hashed hostnames start with a
725: .Ql |
1.205 djm 726: character.
727: Only one hashed hostname may appear on a single line and none of the above
728: negation or wildcard operators may be applied.
729: .Pp
1.285 naddy 730: The keytype and base64-encoded key are taken directly from the host key; they
1.220 jmc 731: can be obtained, for example, from
1.285 naddy 732: .Pa /etc/ssh/ssh_host_rsa_key.pub .
1.1 deraadt 733: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 734: .Pp
735: Lines starting with
736: .Ql #
737: and empty lines are ignored as comments.
738: .Pp
1.1 deraadt 739: When performing host authentication, authentication is accepted if any
1.254 djm 740: matching line has the proper key; either one that matches exactly or,
741: if the server has presented a certificate for authentication, the key
742: of the certification authority that signed the certificate.
743: For a key to be trusted as a certification authority, it must use the
744: .Dq @cert-authority
745: marker described above.
746: .Pp
747: The known hosts file also provides a facility to mark keys as revoked,
748: for example when it is known that the associated private key has been
749: stolen.
750: Revoked keys are specified by including the
751: .Dq @revoked
752: marker at the beginning of the key line, and are never accepted for
753: authentication or as certification authorities, but instead will
754: produce a warning from
755: .Xr ssh 1
756: when they are encountered.
757: .Pp
758: It is permissible (but not
1.1 deraadt 759: recommended) to have several lines or different host keys for the same
1.36 aaron 760: names.
761: This will inevitably happen when short forms of host names
762: from different domains are put in the file.
763: It is possible
1.1 deraadt 764: that the files contain conflicting information; authentication is
765: accepted if valid information can be found from either file.
1.2 deraadt 766: .Pp
1.1 deraadt 767: Note that the lines in these files are typically hundreds of characters
768: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 769: Rather, generate them by a script,
770: .Xr ssh-keyscan 1
1.285 naddy 771: or by taking, for example,
772: .Pa /etc/ssh/ssh_host_rsa_key.pub
1.1 deraadt 773: and adding the host names at the front.
1.254 djm 774: .Xr ssh-keygen 1
1.255 jmc 775: also offers some basic automated editing for
1.254 djm 776: .Pa ~/.ssh/known_hosts
777: including removing hosts matching a host name and converting all host
778: names to their hashed representations.
1.221 jmc 779: .Pp
780: An example ssh_known_hosts file:
781: .Bd -literal -offset 3n
782: # Comments allowed at start of line
783: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
784: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 785: # A hashed hostname
786: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
787: AAAA1234.....=
1.254 djm 788: # A revoked key
789: @revoked * ssh-rsa AAAAB5W...
790: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
791: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 792: .Ed
1.2 deraadt 793: .Sh FILES
1.223 jmc 794: .Bl -tag -width Ds -compact
1.258 jmc 795: .It Pa ~/.hushlogin
1.204 jmc 796: This file is used to suppress printing the last login time and
797: .Pa /etc/motd ,
798: if
799: .Cm PrintLastLog
800: and
801: .Cm PrintMotd ,
802: respectively,
803: are enabled.
804: It does not suppress printing of the banner specified by
805: .Cm Banner .
1.223 jmc 806: .Pp
1.258 jmc 807: .It Pa ~/.rhosts
1.225 jmc 808: This file is used for host-based authentication (see
809: .Xr ssh 1
810: for more information).
811: On some machines this file may need to be
812: world-readable if the user's home directory is on an NFS partition,
813: because
814: .Nm
815: reads it as root.
816: Additionally, this file must be owned by the user,
817: and must not have write permissions for anyone else.
818: The recommended
819: permission for most machines is read/write for the user, and not
1.1 deraadt 820: accessible by others.
1.2 deraadt 821: .Pp
1.258 jmc 822: .It Pa ~/.shosts
1.225 jmc 823: This file is used in exactly the same way as
824: .Pa .rhosts ,
825: but allows host-based authentication without permitting login with
826: rlogin/rsh.
1.238 mcbride 827: .Pp
1.258 jmc 828: .It Pa ~/.ssh/
1.238 mcbride 829: This directory is the default location for all user-specific configuration
830: and authentication information.
831: There is no general requirement to keep the entire contents of this directory
832: secret, but the recommended permissions are read/write/execute for the user,
833: and not accessible by others.
1.2 deraadt 834: .Pp
1.258 jmc 835: .It Pa ~/.ssh/authorized_keys
1.277 sobrado 836: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.273 naddy 837: that can be used for logging in as this user.
1.223 jmc 838: The format of this file is described above.
1.234 dtucker 839: The content of the file is not highly sensitive, but the recommended
1.225 jmc 840: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 841: .Pp
842: If this file, the
843: .Pa ~/.ssh
844: directory, or the user's home directory are writable
845: by other users, then the file could be modified or replaced by unauthorized
846: users.
847: In this case,
848: .Nm
849: will not allow it to be used unless the
850: .Cm StrictModes
851: option has been set to
852: .Dq no .
1.2 deraadt 853: .Pp
1.258 jmc 854: .It Pa ~/.ssh/environment
1.36 aaron 855: This file is read into the environment at login (if it exists).
856: It can only contain empty lines, comment lines (that start with
1.2 deraadt 857: .Ql # ) ,
1.36 aaron 858: and assignment lines of the form name=value.
859: The file should be writable
1.6 aaron 860: only by the user; it need not be readable by anyone else.
1.188 stevesk 861: Environment processing is disabled by default and is
862: controlled via the
863: .Cm PermitUserEnvironment
864: option.
1.223 jmc 865: .Pp
1.258 jmc 866: .It Pa ~/.ssh/known_hosts
1.225 jmc 867: Contains a list of host keys for all hosts the user has logged into
868: that are not already in the systemwide list of known host keys.
869: The format of this file is described above.
870: This file should be writable only by root/the owner and
1.223 jmc 871: can, but need not be, world-readable.
872: .Pp
1.258 jmc 873: .It Pa ~/.ssh/rc
1.226 jmc 874: Contains initialization routines to be run before
875: the user's home directory becomes accessible.
1.1 deraadt 876: This file should be writable only by the user, and need not be
877: readable by anyone else.
1.223 jmc 878: .Pp
1.258 jmc 879: .It Pa /etc/hosts.equiv
1.227 jmc 880: This file is for host-based authentication (see
881: .Xr ssh 1 ) .
882: It should only be writable by root.
1.223 jmc 883: .Pp
1.258 jmc 884: .It Pa /etc/moduli
1.283 djm 885: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange"
886: key exchange method.
1.223 jmc 887: The file format is described in
888: .Xr moduli 5 .
1.283 djm 889: If no usable groups are found in this file then fixed internal groups will
890: be used.
1.223 jmc 891: .Pp
1.258 jmc 892: .It Pa /etc/motd
1.223 jmc 893: See
894: .Xr motd 5 .
895: .Pp
1.258 jmc 896: .It Pa /etc/nologin
1.223 jmc 897: If this file exists,
898: .Nm
899: refuses to let anyone except root log in.
900: The contents of the file
901: are displayed to anyone trying to log in, and non-root connections are
902: refused.
903: The file should be world-readable.
904: .Pp
1.258 jmc 905: .It Pa /etc/shosts.equiv
1.227 jmc 906: This file is used in exactly the same way as
907: .Pa hosts.equiv ,
908: but allows host-based authentication without permitting login with
909: rlogin/rsh.
1.225 jmc 910: .Pp
1.259 djm 911: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.273 naddy 912: .It Pa /etc/ssh/ssh_host_ed25519_key
1.258 jmc 913: .It Pa /etc/ssh/ssh_host_rsa_key
1.266 dtucker 914: These files contain the private parts of the host keys.
1.223 jmc 915: These files should only be owned by root, readable only by root, and not
916: accessible to others.
917: Note that
918: .Nm
1.228 jmc 919: does not start if these files are group/world-accessible.
1.223 jmc 920: .Pp
1.259 djm 921: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.273 naddy 922: .It Pa /etc/ssh/ssh_host_ed25519_key.pub
1.258 jmc 923: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.266 dtucker 924: These files contain the public parts of the host keys.
1.223 jmc 925: These files should be world-readable but writable only by
926: root.
927: Their contents should match the respective private parts.
928: These files are not
929: really used for anything; they are provided for the convenience of
930: the user so their contents can be copied to known hosts files.
931: These files are created using
932: .Xr ssh-keygen 1 .
933: .Pp
1.258 jmc 934: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 935: Systemwide list of known host keys.
936: This file should be prepared by the
937: system administrator to contain the public host keys of all machines in the
938: organization.
939: The format of this file is described above.
940: This file should be writable only by root/the owner and
941: should be world-readable.
942: .Pp
1.258 jmc 943: .It Pa /etc/ssh/sshd_config
1.223 jmc 944: Contains configuration data for
945: .Nm sshd .
946: The file format and configuration options are described in
947: .Xr sshd_config 5 .
948: .Pp
1.258 jmc 949: .It Pa /etc/ssh/sshrc
1.226 jmc 950: Similar to
951: .Pa ~/.ssh/rc ,
952: it can be used to specify
1.36 aaron 953: machine-specific login-time initializations globally.
954: This file should be writable only by root, and should be world-readable.
1.223 jmc 955: .Pp
1.258 jmc 956: .It Pa /var/empty
1.223 jmc 957: .Xr chroot 2
958: directory used by
959: .Nm
960: during privilege separation in the pre-authentication phase.
961: The directory should not contain any files and must be owned by root
962: and not group or world-writable.
963: .Pp
1.258 jmc 964: .It Pa /var/run/sshd.pid
1.223 jmc 965: Contains the process ID of the
966: .Nm
967: listening for connections (if there are several daemons running
968: concurrently for different ports, this contains the process ID of the one
969: started last).
970: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 971: .El
1.2 deraadt 972: .Sh SEE ALSO
973: .Xr scp 1 ,
1.90 djm 974: .Xr sftp 1 ,
1.2 deraadt 975: .Xr ssh 1 ,
1.5 deraadt 976: .Xr ssh-add 1 ,
1.2 deraadt 977: .Xr ssh-agent 1 ,
1.5 deraadt 978: .Xr ssh-keygen 1 ,
1.235 jmc 979: .Xr ssh-keyscan 1 ,
1.200 jmc 980: .Xr chroot 2 ,
1.136 markus 981: .Xr login.conf 5 ,
982: .Xr moduli 5 ,
1.184 stevesk 983: .Xr sshd_config 5 ,
1.200 jmc 984: .Xr inetd 8 ,
1.128 mpech 985: .Xr sftp-server 8
1.198 jmc 986: .Sh AUTHORS
987: OpenSSH is a derivative of the original and free
988: ssh 1.2.12 release by Tatu Ylonen.
989: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
990: Theo de Raadt and Dug Song
991: removed many bugs, re-added newer features and
992: created OpenSSH.
993: Markus Friedl contributed the support for SSH
994: protocol versions 1.5 and 2.0.
995: Niels Provos and Markus Friedl contributed support
996: for privilege separation.
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