Annotation of src/usr.bin/ssh/sshd.8, Revision 1.282
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: .\"
1.282 ! djm 36: .\" $OpenBSD: sshd.8,v 1.281 2015/09/11 03:13:36 djm Exp $
! 37: .Dd $Mdocdate: September 11 2015 $
1.2 deraadt 38: .Dt SSHD 8
39: .Os
40: .Sh NAME
41: .Nm sshd
1.120 markus 42: .Nd OpenSSH SSH daemon
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm sshd
1.194 jmc 45: .Bk -words
1.243 jmc 46: .Op Fl 46DdeiqTt
1.2 deraadt 47: .Op Fl b Ar bits
1.242 dtucker 48: .Op Fl C Ar connection_spec
1.251 djm 49: .Op Fl c Ar host_certificate_file
1.268 dtucker 50: .Op Fl E Ar log_file
1.2 deraadt 51: .Op Fl f Ar config_file
52: .Op Fl g Ar login_grace_time
53: .Op Fl h Ar host_key_file
54: .Op Fl k Ar key_gen_time
1.156 markus 55: .Op Fl o Ar option
1.2 deraadt 56: .Op Fl p Ar port
1.61 markus 57: .Op Fl u Ar len
1.194 jmc 58: .Ek
1.40 aaron 59: .Sh DESCRIPTION
1.2 deraadt 60: .Nm
1.212 jmc 61: (OpenSSH Daemon) is the daemon program for
1.2 deraadt 62: .Xr ssh 1 .
1.275 tedu 63: Together these programs replace rlogin and rsh,
1.235 jmc 64: and provide secure encrypted communications between two untrusted hosts
1.36 aaron 65: over an insecure network.
1.2 deraadt 66: .Pp
67: .Nm
1.212 jmc 68: listens for connections from clients.
1.40 aaron 69: It is normally started at boot from
1.2 deraadt 70: .Pa /etc/rc .
71: It forks a new
1.36 aaron 72: daemon for each incoming connection.
73: The forked daemons handle
1.1 deraadt 74: key exchange, encryption, authentication, command execution,
75: and data exchange.
1.2 deraadt 76: .Pp
77: .Nm
1.200 jmc 78: can be configured using command-line options or a configuration file
79: (by default
1.212 jmc 80: .Xr sshd_config 5 ) ;
81: command-line options override values specified in the
1.1 deraadt 82: configuration file.
1.25 markus 83: .Nm
84: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 85: .Dv SIGHUP ,
1.230 jmc 86: by executing itself with the name and options it was started with, e.g.\&
1.97 deraadt 87: .Pa /usr/sbin/sshd .
1.18 aaron 88: .Pp
89: The options are as follows:
1.2 deraadt 90: .Bl -tag -width Ds
1.200 jmc 91: .It Fl 4
92: Forces
93: .Nm
94: to use IPv4 addresses only.
95: .It Fl 6
96: Forces
97: .Nm
98: to use IPv6 addresses only.
1.2 deraadt 99: .It Fl b Ar bits
1.120 markus 100: Specifies the number of bits in the ephemeral protocol version 1
1.246 djm 101: server key (default 1024).
1.243 jmc 102: .It Fl C Ar connection_spec
103: Specify the connection parameters to use for the
104: .Fl T
105: extended test mode.
106: If provided, any
107: .Cm Match
108: directives in the configuration file
109: that would apply to the specified user, host, and address will be set before
110: the configuration is written to standard output.
111: The connection parameters are supplied as keyword=value pairs.
112: The keywords are
113: .Dq user ,
114: .Dq host ,
1.265 dtucker 115: .Dq laddr ,
116: .Dq lport ,
1.243 jmc 117: and
118: .Dq addr .
119: All are required and may be supplied in any order, either with multiple
120: .Fl C
121: options or as a comma-separated list.
1.252 jmc 122: .It Fl c Ar host_certificate_file
123: Specifies a path to a certificate file to identify
124: .Nm
125: during key exchange.
126: The certificate file must match a host key file specified using the
127: .Fl h
128: option or the
129: .Cm HostKey
130: configuration directive.
1.200 jmc 131: .It Fl D
132: When this option is specified,
133: .Nm
134: will not detach and does not become a daemon.
135: This allows easy monitoring of
136: .Nm sshd .
1.2 deraadt 137: .It Fl d
1.36 aaron 138: Debug mode.
1.250 djm 139: The server sends verbose debug output to standard error,
140: and does not put itself in the background.
1.36 aaron 141: The server also will not fork and will only process one connection.
142: This option is only intended for debugging for the server.
1.194 jmc 143: Multiple
144: .Fl d
145: options increase the debugging level.
1.67 aaron 146: Maximum is 3.
1.268 dtucker 147: .It Fl E Ar log_file
148: Append debug logs to
149: .Ar log_file
150: instead of the system log.
1.120 markus 151: .It Fl e
1.269 dtucker 152: Write debug logs to standard error instead of the system log.
1.235 jmc 153: .It Fl f Ar config_file
1.36 aaron 154: Specifies the name of the configuration file.
155: The default is
1.167 deraadt 156: .Pa /etc/ssh/sshd_config .
1.16 markus 157: .Nm
158: refuses to start if there is no configuration file.
1.2 deraadt 159: .It Fl g Ar login_grace_time
1.1 deraadt 160: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 161: 120 seconds).
1.36 aaron 162: If the client fails to authenticate the user within
163: this many seconds, the server disconnects and exits.
164: A value of zero indicates no limit.
1.2 deraadt 165: .It Fl h Ar host_key_file
1.160 stevesk 166: Specifies a file from which a host key is read.
1.7 markus 167: This option must be given if
168: .Nm
169: is not run as root (as the normal
1.160 stevesk 170: host key files are normally not readable by anyone but root).
171: The default is
1.167 deraadt 172: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 173: for protocol version 1, and
1.259 djm 174: .Pa /etc/ssh/ssh_host_dsa_key ,
1.273 naddy 175: .Pa /etc/ssh/ssh_host_ecdsa_key .
176: .Pa /etc/ssh/ssh_host_ed25519_key
1.259 djm 177: and
1.167 deraadt 178: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 179: for protocol version 2.
1.75 markus 180: It is possible to have multiple host key files for
1.120 markus 181: the different protocol versions and host key algorithms.
1.2 deraadt 182: .It Fl i
1.7 markus 183: Specifies that
184: .Nm
1.194 jmc 185: is being run from
186: .Xr inetd 8 .
1.280 djm 187: If SSH protocol 1 is enabled,
1.7 markus 188: .Nm
1.280 djm 189: should not normally be run
1.1 deraadt 190: from inetd because it needs to generate the server key before it can
1.280 djm 191: respond to the client, and this may take some time.
192: Clients may have to wait too long if the key was regenerated every time.
1.2 deraadt 193: .It Fl k Ar key_gen_time
1.120 markus 194: Specifies how often the ephemeral protocol version 1 server key is
195: regenerated (default 3600 seconds, or one hour).
1.36 aaron 196: The motivation for regenerating the key fairly
1.200 jmc 197: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 198: it becomes impossible to recover the key for decrypting intercepted
199: communications even if the machine is cracked into or physically
1.36 aaron 200: seized.
201: A value of zero indicates that the key will never be regenerated.
1.156 markus 202: .It Fl o Ar option
203: Can be used to give options in the format used in the configuration file.
204: This is useful for specifying options for which there is no separate
205: command-line flag.
1.200 jmc 206: For full details of the options, and their values, see
207: .Xr sshd_config 5 .
1.2 deraadt 208: .It Fl p Ar port
1.1 deraadt 209: Specifies the port on which the server listens for connections
210: (default 22).
1.158 stevesk 211: Multiple port options are permitted.
1.210 stevesk 212: Ports specified in the configuration file with the
213: .Cm Port
214: option are ignored when a command-line port is specified.
215: Ports specified using the
216: .Cm ListenAddress
217: option override command-line ports.
1.2 deraadt 218: .It Fl q
1.36 aaron 219: Quiet mode.
220: Nothing is sent to the system log.
221: Normally the beginning,
1.1 deraadt 222: authentication, and termination of each connection is logged.
1.242 dtucker 223: .It Fl T
224: Extended test mode.
225: Check the validity of the configuration file, output the effective configuration
226: to stdout and then exit.
227: Optionally,
228: .Cm Match
229: rules may be applied by specifying the connection parameters using one or more
230: .Fl C
231: options.
1.243 jmc 232: .It Fl t
233: Test mode.
234: Only check the validity of the configuration file and sanity of the keys.
235: This is useful for updating
236: .Nm
237: reliably as configuration options may change.
1.61 markus 238: .It Fl u Ar len
239: This option is used to specify the size of the field
240: in the
241: .Li utmp
242: structure that holds the remote host name.
243: If the resolved host name is longer than
244: .Ar len ,
245: the dotted decimal value will be used instead.
246: This allows hosts with very long host names that
247: overflow this field to still be uniquely identified.
248: Specifying
249: .Fl u0
250: indicates that only dotted decimal addresses
251: should be put into the
252: .Pa utmp
253: file.
1.144 stevesk 254: .Fl u0
1.194 jmc 255: may also be used to prevent
1.144 stevesk 256: .Nm
257: from making DNS requests unless the authentication
258: mechanism or configuration requires it.
259: Authentication mechanisms that may require DNS include
260: .Cm RhostsRSAAuthentication ,
1.215 jmc 261: .Cm HostbasedAuthentication ,
1.144 stevesk 262: and using a
263: .Cm from="pattern-list"
264: option in a key file.
1.170 stevesk 265: Configuration options that require DNS include using a
266: USER@HOST pattern in
267: .Cm AllowUsers
268: or
269: .Cm DenyUsers .
1.2 deraadt 270: .El
1.214 jmc 271: .Sh AUTHENTICATION
272: The OpenSSH SSH daemon supports SSH protocols 1 and 2.
1.249 jmc 273: The default is to use protocol 2 only,
1.214 jmc 274: though this can be changed via the
275: .Cm Protocol
276: option in
277: .Xr sshd_config 5 .
1.277 sobrado 278: Protocol 2 supports DSA, ECDSA, Ed25519 and RSA keys;
1.214 jmc 279: protocol 1 only supports RSA keys.
280: For both protocols,
281: each host has a host-specific key,
282: normally 2048 bits,
283: used to identify the host.
284: .Pp
285: Forward security for protocol 1 is provided through
286: an additional server key,
1.280 djm 287: normally 1024 bits,
1.214 jmc 288: generated when the server starts.
1.212 jmc 289: This key is normally regenerated every hour if it has been used, and
290: is never stored on disk.
291: Whenever a client connects, the daemon responds with its public
292: host and server keys.
293: The client compares the
294: RSA host key against its own database to verify that it has not changed.
295: The client then generates a 256-bit random number.
296: It encrypts this
297: random number using both the host key and the server key, and sends
298: the encrypted number to the server.
299: Both sides then use this
300: random number as a session key which is used to encrypt all further
301: communications in the session.
302: The rest of the session is encrypted
303: using a conventional cipher, currently Blowfish or 3DES, with 3DES
304: being used by default.
305: The client selects the encryption algorithm
306: to use from those offered by the server.
307: .Pp
1.214 jmc 308: For protocol 2,
309: forward security is provided through a Diffie-Hellman key agreement.
310: This key agreement results in a shared session key.
311: The rest of the session is encrypted using a symmetric cipher, currently
312: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
313: The client selects the encryption algorithm
314: to use from those offered by the server.
315: Additionally, session integrity is provided
316: through a cryptographic message authentication code
1.267 markus 317: (hmac-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160,
1.263 djm 318: hmac-sha2-256 or hmac-sha2-512).
1.214 jmc 319: .Pp
320: Finally, the server and the client enter an authentication dialog.
1.212 jmc 321: The client tries to authenticate itself using
1.214 jmc 322: host-based authentication,
323: public key authentication,
324: challenge-response authentication,
325: or password authentication.
1.212 jmc 326: .Pp
327: If the client successfully authenticates itself, a dialog for
328: preparing the session is entered.
329: At this time the client may request
330: things like allocating a pseudo-tty, forwarding X11 connections,
331: forwarding TCP connections, or forwarding the authentication agent
332: connection over the secure channel.
333: .Pp
1.216 jmc 334: After this, the client either requests a shell or execution of a command.
1.212 jmc 335: The sides then enter session mode.
336: In this mode, either side may send
337: data at any time, and such data is forwarded to/from the shell or
338: command on the server side, and the user terminal in the client side.
339: .Pp
340: When the user program terminates and all forwarded X11 and other
341: connections have been closed, the server sends command exit status to
342: the client, and both sides exit.
1.2 deraadt 343: .Sh LOGIN PROCESS
1.1 deraadt 344: When a user successfully logs in,
1.2 deraadt 345: .Nm
1.1 deraadt 346: does the following:
1.2 deraadt 347: .Bl -enum -offset indent
348: .It
1.1 deraadt 349: If the login is on a tty, and no command has been specified,
1.40 aaron 350: prints last login time and
1.2 deraadt 351: .Pa /etc/motd
1.1 deraadt 352: (unless prevented in the configuration file or by
1.207 djm 353: .Pa ~/.hushlogin ;
1.2 deraadt 354: see the
1.40 aaron 355: .Sx FILES
1.2 deraadt 356: section).
357: .It
1.1 deraadt 358: If the login is on a tty, records login time.
1.2 deraadt 359: .It
360: Checks
361: .Pa /etc/nologin ;
362: if it exists, prints contents and quits
1.1 deraadt 363: (unless root).
1.2 deraadt 364: .It
1.1 deraadt 365: Changes to run with normal user privileges.
1.2 deraadt 366: .It
1.1 deraadt 367: Sets up basic environment.
1.2 deraadt 368: .It
1.200 jmc 369: Reads the file
1.207 djm 370: .Pa ~/.ssh/environment ,
1.200 jmc 371: if it exists, and users are allowed to change their environment.
1.188 stevesk 372: See the
1.187 marc 373: .Cm PermitUserEnvironment
1.188 stevesk 374: option in
1.187 marc 375: .Xr sshd_config 5 .
1.2 deraadt 376: .It
1.1 deraadt 377: Changes to user's home directory.
1.2 deraadt 378: .It
379: If
1.207 djm 380: .Pa ~/.ssh/rc
1.276 djm 381: exists and the
382: .Xr sshd_config 5
383: .Cm PermitUserRC
384: option is set, runs it; else if
1.168 deraadt 385: .Pa /etc/ssh/sshrc
1.2 deraadt 386: exists, runs
1.36 aaron 387: it; otherwise runs xauth.
388: The
1.2 deraadt 389: .Dq rc
390: files are given the X11
1.1 deraadt 391: authentication protocol and cookie in standard input.
1.226 jmc 392: See
393: .Sx SSHRC ,
394: below.
1.2 deraadt 395: .It
1.1 deraadt 396: Runs user's shell or command.
1.279 djm 397: All commands are run under the user's login shell as specified in the
398: system password database.
1.2 deraadt 399: .El
1.226 jmc 400: .Sh SSHRC
401: If the file
402: .Pa ~/.ssh/rc
403: exists,
404: .Xr sh 1
405: runs it after reading the
406: environment files but before starting the user's shell or command.
407: It must not produce any output on stdout; stderr must be used
408: instead.
409: If X11 forwarding is in use, it will receive the "proto cookie" pair in
410: its standard input (and
411: .Ev DISPLAY
412: in its environment).
413: The script must call
414: .Xr xauth 1
415: because
416: .Nm
417: will not run xauth automatically to add X11 cookies.
418: .Pp
419: The primary purpose of this file is to run any initialization routines
420: which may be needed before the user's home directory becomes
421: accessible; AFS is a particular example of such an environment.
422: .Pp
423: This file will probably contain some initialization code followed by
424: something similar to:
425: .Bd -literal -offset 3n
426: if read proto cookie && [ -n "$DISPLAY" ]; then
427: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
428: # X11UseLocalhost=yes
429: echo add unix:`echo $DISPLAY |
430: cut -c11-` $proto $cookie
431: else
432: # X11UseLocalhost=no
433: echo add $DISPLAY $proto $cookie
434: fi | xauth -q -
435: fi
436: .Ed
437: .Pp
438: If this file does not exist,
439: .Pa /etc/ssh/sshrc
440: is run, and if that
441: does not exist either, xauth is used to add the cookie.
1.2 deraadt 442: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 443: .Cm AuthorizedKeysFile
1.262 jmc 444: specifies the files containing public keys for
1.217 jmc 445: public key authentication;
1.281 djm 446: if this option is not specified, the default is
1.261 djm 447: .Pa ~/.ssh/authorized_keys
448: and
449: .Pa ~/.ssh/authorized_keys2 .
1.36 aaron 450: Each line of the file contains one
1.2 deraadt 451: key (empty lines and lines starting with a
452: .Ql #
453: are ignored as
1.36 aaron 454: comments).
1.217 jmc 455: Protocol 1 public keys consist of the following space-separated fields:
456: options, bits, exponent, modulus, comment.
457: Protocol 2 public key consist of:
458: options, keytype, base64-encoded key, comment.
459: The options field is optional;
460: its presence is determined by whether the line starts
1.189 espie 461: with a number or not (the options field never starts with a number).
1.217 jmc 462: The bits, exponent, modulus, and comment fields give the RSA key for
1.75 markus 463: protocol version 1; the
1.1 deraadt 464: comment field is not used for anything (but may be convenient for the
465: user to identify the key).
1.75 markus 466: For protocol version 2 the keytype is
1.259 djm 467: .Dq ecdsa-sha2-nistp256 ,
468: .Dq ecdsa-sha2-nistp384 ,
469: .Dq ecdsa-sha2-nistp521 ,
1.272 jmc 470: .Dq ssh-ed25519 ,
1.75 markus 471: .Dq ssh-dss
472: or
473: .Dq ssh-rsa .
1.2 deraadt 474: .Pp
1.1 deraadt 475: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 476: (because of the size of the public key encoding) up to a limit of
477: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
478: keys up to 16 kilobits.
1.36 aaron 479: You don't want to type them in; instead, copy the
1.113 itojun 480: .Pa identity.pub ,
1.217 jmc 481: .Pa id_dsa.pub ,
1.259 djm 482: .Pa id_ecdsa.pub ,
1.273 naddy 483: .Pa id_ed25519.pub ,
1.75 markus 484: or the
1.113 itojun 485: .Pa id_rsa.pub
1.1 deraadt 486: file and edit it.
1.175 stevesk 487: .Pp
488: .Nm
489: enforces a minimum RSA key modulus size for protocol 1
490: and protocol 2 keys of 768 bits.
1.2 deraadt 491: .Pp
1.58 deraadt 492: The options (if present) consist of comma-separated option
1.36 aaron 493: specifications.
494: No spaces are permitted, except within double quotes.
1.141 stevesk 495: The following option specifications are supported (note
496: that option keywords are case-insensitive):
1.2 deraadt 497: .Bl -tag -width Ds
1.282 ! djm 498: .It Cm agent-forwarding
! 499: Enable authentication agent forwarding previously disabled by the
! 500: .Cm restrict
! 501: option.
1.253 djm 502: .It Cm cert-authority
1.251 djm 503: Specifies that the listed key is a certification authority (CA) that is
504: trusted to validate signed certificates for user authentication.
505: .Pp
506: Certificates may encode access restrictions similar to these key options.
507: If both certificate restrictions and key options are present, the most
508: restrictive union of the two is applied.
1.2 deraadt 509: .It Cm command="command"
1.1 deraadt 510: Specifies that the command is executed whenever this key is used for
1.36 aaron 511: authentication.
512: The command supplied by the user (if any) is ignored.
1.148 markus 513: The command is run on a pty if the client requests a pty;
1.36 aaron 514: otherwise it is run without a tty.
1.194 jmc 515: If an 8-bit clean channel is required,
1.147 deraadt 516: one must not request a pty or should specify
1.89 markus 517: .Cm no-pty .
1.36 aaron 518: A quote may be included in the command by quoting it with a backslash.
519: This option might be useful
1.189 espie 520: to restrict certain public keys to perform just a specific operation.
1.36 aaron 521: An example might be a key that permits remote backups but nothing else.
1.211 jmc 522: Note that the client may specify TCP and/or X11
1.51 hugh 523: forwarding unless they are explicitly prohibited.
1.233 dtucker 524: The command originally supplied by the client is available in the
525: .Ev SSH_ORIGINAL_COMMAND
526: environment variable.
1.149 markus 527: Note that this option applies to shell, command or subsystem execution.
1.251 djm 528: Also note that this command may be superseded by either a
529: .Xr sshd_config 5
530: .Cm ForceCommand
531: directive or a command embedded in a certificate.
1.2 deraadt 532: .It Cm environment="NAME=value"
1.1 deraadt 533: Specifies that the string is to be added to the environment when
1.36 aaron 534: logging in using this key.
535: Environment variables set this way
536: override other default environment values.
537: Multiple options of this type are permitted.
1.188 stevesk 538: Environment processing is disabled by default and is
539: controlled via the
540: .Cm PermitUserEnvironment
541: option.
1.155 markus 542: This option is automatically disabled if
543: .Cm UseLogin
544: is enabled.
1.218 jmc 545: .It Cm from="pattern-list"
1.244 djm 546: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 547: name of the remote host or its IP address must be present in the
1.244 djm 548: comma-separated list of patterns.
1.270 jmc 549: See PATTERNS in
1.229 jmc 550: .Xr ssh_config 5
551: for more information on patterns.
1.244 djm 552: .Pp
553: In addition to the wildcard matching that may be applied to hostnames or
554: addresses, a
555: .Cm from
1.248 sobrado 556: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 557: .Pp
558: The purpose of this option is to optionally increase security: public key
559: authentication by itself does not trust the network or name servers or
560: anything (but the key); however, if somebody somehow steals the key, the key
561: permits an intruder to log in from anywhere in the world.
562: This additional option makes using a stolen key more difficult (name
563: servers and/or routers would have to be compromised in addition to
564: just the key).
1.218 jmc 565: .It Cm no-agent-forwarding
566: Forbids authentication agent forwarding when this key is used for
567: authentication.
1.2 deraadt 568: .It Cm no-port-forwarding
1.211 jmc 569: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 570: Any port forward requests by the client will return an error.
1.230 jmc 571: This might be used, e.g. in connection with the
1.2 deraadt 572: .Cm command
1.1 deraadt 573: option.
1.218 jmc 574: .It Cm no-pty
575: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 576: .It Cm no-user-rc
1.241 jmc 577: Disables execution of
1.240 djm 578: .Pa ~/.ssh/rc .
1.2 deraadt 579: .It Cm no-X11-forwarding
1.1 deraadt 580: Forbids X11 forwarding when this key is used for authentication.
581: Any X11 forward requests by the client will return an error.
1.107 djm 582: .It Cm permitopen="host:port"
1.278 bentley 583: Limit local port forwarding with
584: .Xr ssh 1
585: .Fl L
586: such that it may only connect to the specified host and port.
1.257 djm 587: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 588: Multiple
1.107 djm 589: .Cm permitopen
1.197 jmc 590: options may be applied separated by commas.
591: No pattern matching is performed on the specified hostnames,
592: they must be literal domains or addresses.
1.264 dtucker 593: A port specification of
594: .Cm *
595: matches any port.
1.282 ! djm 596: .It Cm port-forwarding
! 597: Enable port forwarding previously disabled by the
! 598: .Cm restrict
1.256 djm 599: .It Cm principals="principals"
600: On a
601: .Cm cert-authority
602: line, specifies allowed principals for certificate authentication as a
603: comma-separated list.
604: At least one name from the list must appear in the certificate's
605: list of principals for the certificate to be accepted.
606: This option is ignored for keys that are not marked as trusted certificate
607: signers using the
608: .Cm cert-authority
609: option.
1.282 ! djm 610: .It Cm pty
! 611: Permits tty allocation previously disabled by the
! 612: .Cm restrict
! 613: option.
! 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
645: command="dump /home",no-pty,no-port-forwarding ssh-dss
646: AAAAC3...51R== example.net
647: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
648: AAAAB5...21S==
649: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
650: jane@example.net
1.282 ! djm 651: restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
! 652: user@example.net
! 653: restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==
! 654: user@example.net
1.219 jmc 655: .Ed
1.2 deraadt 656: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 657: The
1.194 jmc 658: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 659: and
1.207 djm 660: .Pa ~/.ssh/known_hosts
1.36 aaron 661: files contain host public keys for all known hosts.
662: The global file should
1.37 brad 663: be prepared by the administrator (optional), and the per-user file is
1.220 jmc 664: maintained automatically: whenever the user connects from an unknown host,
1.36 aaron 665: its key is added to the per-user file.
1.2 deraadt 666: .Pp
1.254 djm 667: Each line in these files contains the following fields: markers (optional),
668: hostnames, bits, exponent, modulus, comment.
1.36 aaron 669: The fields are separated by spaces.
1.2 deraadt 670: .Pp
1.254 djm 671: The marker is optional, but if it is present then it must be one of
672: .Dq @cert-authority ,
673: to indicate that the line contains a certification authority (CA) key,
674: or
675: .Dq @revoked ,
676: to indicate that the key contained on the line is revoked and must not ever
677: be accepted.
678: Only one marker should be used on a key line.
679: .Pp
1.196 david 680: Hostnames is a comma-separated list of patterns
1.220 jmc 681: .Pf ( Ql *
1.197 jmc 682: and
683: .Ql \&?
1.195 mouring 684: act as
1.1 deraadt 685: wildcards); each pattern in turn is matched against the canonical host
686: name (when authenticating a client) or against the user-supplied
1.36 aaron 687: name (when authenticating a server).
688: A pattern may also be preceded by
1.195 mouring 689: .Ql \&!
1.2 deraadt 690: to indicate negation: if the host name matches a negated
1.1 deraadt 691: pattern, it is not accepted (by that line) even if it matched another
692: pattern on the line.
1.231 dtucker 693: A hostname or address may optionally be enclosed within
694: .Ql \&[
695: and
696: .Ql \&]
697: brackets then followed by
698: .Ql \&:
1.232 jmc 699: and a non-standard port number.
1.2 deraadt 700: .Pp
1.205 djm 701: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 702: and addresses should the file's contents be disclosed.
703: Hashed hostnames start with a
704: .Ql |
1.205 djm 705: character.
706: Only one hashed hostname may appear on a single line and none of the above
707: negation or wildcard operators may be applied.
708: .Pp
1.49 markus 709: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.220 jmc 710: can be obtained, for example, from
1.167 deraadt 711: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 712: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 713: .Pp
714: Lines starting with
715: .Ql #
716: and empty lines are ignored as comments.
717: .Pp
1.1 deraadt 718: When performing host authentication, authentication is accepted if any
1.254 djm 719: matching line has the proper key; either one that matches exactly or,
720: if the server has presented a certificate for authentication, the key
721: of the certification authority that signed the certificate.
722: For a key to be trusted as a certification authority, it must use the
723: .Dq @cert-authority
724: marker described above.
725: .Pp
726: The known hosts file also provides a facility to mark keys as revoked,
727: for example when it is known that the associated private key has been
728: stolen.
729: Revoked keys are specified by including the
730: .Dq @revoked
731: marker at the beginning of the key line, and are never accepted for
732: authentication or as certification authorities, but instead will
733: produce a warning from
734: .Xr ssh 1
735: when they are encountered.
736: .Pp
737: It is permissible (but not
1.1 deraadt 738: recommended) to have several lines or different host keys for the same
1.36 aaron 739: names.
740: This will inevitably happen when short forms of host names
741: from different domains are put in the file.
742: It is possible
1.1 deraadt 743: that the files contain conflicting information; authentication is
744: accepted if valid information can be found from either file.
1.2 deraadt 745: .Pp
1.1 deraadt 746: Note that the lines in these files are typically hundreds of characters
747: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 748: Rather, generate them by a script,
749: .Xr ssh-keyscan 1
1.40 aaron 750: or by taking
1.167 deraadt 751: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 752: and adding the host names at the front.
1.254 djm 753: .Xr ssh-keygen 1
1.255 jmc 754: also offers some basic automated editing for
1.254 djm 755: .Pa ~/.ssh/known_hosts
756: including removing hosts matching a host name and converting all host
757: names to their hashed representations.
1.221 jmc 758: .Pp
759: An example ssh_known_hosts file:
760: .Bd -literal -offset 3n
761: # Comments allowed at start of line
762: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
763: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 764: # A hashed hostname
765: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
766: AAAA1234.....=
1.254 djm 767: # A revoked key
768: @revoked * ssh-rsa AAAAB5W...
769: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
770: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 771: .Ed
1.2 deraadt 772: .Sh FILES
1.223 jmc 773: .Bl -tag -width Ds -compact
1.258 jmc 774: .It Pa ~/.hushlogin
1.204 jmc 775: This file is used to suppress printing the last login time and
776: .Pa /etc/motd ,
777: if
778: .Cm PrintLastLog
779: and
780: .Cm PrintMotd ,
781: respectively,
782: are enabled.
783: It does not suppress printing of the banner specified by
784: .Cm Banner .
1.223 jmc 785: .Pp
1.258 jmc 786: .It Pa ~/.rhosts
1.225 jmc 787: This file is used for host-based authentication (see
788: .Xr ssh 1
789: for more information).
790: On some machines this file may need to be
791: world-readable if the user's home directory is on an NFS partition,
792: because
793: .Nm
794: reads it as root.
795: Additionally, this file must be owned by the user,
796: and must not have write permissions for anyone else.
797: The recommended
798: permission for most machines is read/write for the user, and not
1.1 deraadt 799: accessible by others.
1.2 deraadt 800: .Pp
1.258 jmc 801: .It Pa ~/.shosts
1.225 jmc 802: This file is used in exactly the same way as
803: .Pa .rhosts ,
804: but allows host-based authentication without permitting login with
805: rlogin/rsh.
1.238 mcbride 806: .Pp
1.258 jmc 807: .It Pa ~/.ssh/
1.238 mcbride 808: This directory is the default location for all user-specific configuration
809: and authentication information.
810: There is no general requirement to keep the entire contents of this directory
811: secret, but the recommended permissions are read/write/execute for the user,
812: and not accessible by others.
1.2 deraadt 813: .Pp
1.258 jmc 814: .It Pa ~/.ssh/authorized_keys
1.277 sobrado 815: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.273 naddy 816: that can be used for logging in as this user.
1.223 jmc 817: The format of this file is described above.
1.234 dtucker 818: The content of the file is not highly sensitive, but the recommended
1.225 jmc 819: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 820: .Pp
821: If this file, the
822: .Pa ~/.ssh
823: directory, or the user's home directory are writable
824: by other users, then the file could be modified or replaced by unauthorized
825: users.
826: In this case,
827: .Nm
828: will not allow it to be used unless the
829: .Cm StrictModes
830: option has been set to
831: .Dq no .
1.2 deraadt 832: .Pp
1.258 jmc 833: .It Pa ~/.ssh/environment
1.36 aaron 834: This file is read into the environment at login (if it exists).
835: It can only contain empty lines, comment lines (that start with
1.2 deraadt 836: .Ql # ) ,
1.36 aaron 837: and assignment lines of the form name=value.
838: The file should be writable
1.6 aaron 839: only by the user; it need not be readable by anyone else.
1.188 stevesk 840: Environment processing is disabled by default and is
841: controlled via the
842: .Cm PermitUserEnvironment
843: option.
1.223 jmc 844: .Pp
1.258 jmc 845: .It Pa ~/.ssh/known_hosts
1.225 jmc 846: Contains a list of host keys for all hosts the user has logged into
847: that are not already in the systemwide list of known host keys.
848: The format of this file is described above.
849: This file should be writable only by root/the owner and
1.223 jmc 850: can, but need not be, world-readable.
851: .Pp
1.258 jmc 852: .It Pa ~/.ssh/rc
1.226 jmc 853: Contains initialization routines to be run before
854: the user's home directory becomes accessible.
1.1 deraadt 855: This file should be writable only by the user, and need not be
856: readable by anyone else.
1.223 jmc 857: .Pp
1.258 jmc 858: .It Pa /etc/hosts.equiv
1.227 jmc 859: This file is for host-based authentication (see
860: .Xr ssh 1 ) .
861: It should only be writable by root.
1.223 jmc 862: .Pp
1.258 jmc 863: .It Pa /etc/moduli
1.223 jmc 864: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
865: The file format is described in
866: .Xr moduli 5 .
867: .Pp
1.258 jmc 868: .It Pa /etc/motd
1.223 jmc 869: See
870: .Xr motd 5 .
871: .Pp
1.258 jmc 872: .It Pa /etc/nologin
1.223 jmc 873: If this file exists,
874: .Nm
875: refuses to let anyone except root log in.
876: The contents of the file
877: are displayed to anyone trying to log in, and non-root connections are
878: refused.
879: The file should be world-readable.
880: .Pp
1.258 jmc 881: .It Pa /etc/shosts.equiv
1.227 jmc 882: This file is used in exactly the same way as
883: .Pa hosts.equiv ,
884: but allows host-based authentication without permitting login with
885: rlogin/rsh.
1.225 jmc 886: .Pp
1.258 jmc 887: .It Pa /etc/ssh/ssh_host_key
888: .It Pa /etc/ssh/ssh_host_dsa_key
1.259 djm 889: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.273 naddy 890: .It Pa /etc/ssh/ssh_host_ed25519_key
1.258 jmc 891: .It Pa /etc/ssh/ssh_host_rsa_key
1.266 dtucker 892: These files contain the private parts of the host keys.
1.223 jmc 893: These files should only be owned by root, readable only by root, and not
894: accessible to others.
895: Note that
896: .Nm
1.228 jmc 897: does not start if these files are group/world-accessible.
1.223 jmc 898: .Pp
1.258 jmc 899: .It Pa /etc/ssh/ssh_host_key.pub
900: .It Pa /etc/ssh/ssh_host_dsa_key.pub
1.259 djm 901: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.273 naddy 902: .It Pa /etc/ssh/ssh_host_ed25519_key.pub
1.258 jmc 903: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.266 dtucker 904: These files contain the public parts of the host keys.
1.223 jmc 905: These files should be world-readable but writable only by
906: root.
907: Their contents should match the respective private parts.
908: These files are not
909: really used for anything; they are provided for the convenience of
910: the user so their contents can be copied to known hosts files.
911: These files are created using
912: .Xr ssh-keygen 1 .
913: .Pp
1.258 jmc 914: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 915: Systemwide list of known host keys.
916: This file should be prepared by the
917: system administrator to contain the public host keys of all machines in the
918: organization.
919: The format of this file is described above.
920: This file should be writable only by root/the owner and
921: should be world-readable.
922: .Pp
1.258 jmc 923: .It Pa /etc/ssh/sshd_config
1.223 jmc 924: Contains configuration data for
925: .Nm sshd .
926: The file format and configuration options are described in
927: .Xr sshd_config 5 .
928: .Pp
1.258 jmc 929: .It Pa /etc/ssh/sshrc
1.226 jmc 930: Similar to
931: .Pa ~/.ssh/rc ,
932: it can be used to specify
1.36 aaron 933: machine-specific login-time initializations globally.
934: This file should be writable only by root, and should be world-readable.
1.223 jmc 935: .Pp
1.258 jmc 936: .It Pa /var/empty
1.223 jmc 937: .Xr chroot 2
938: directory used by
939: .Nm
940: during privilege separation in the pre-authentication phase.
941: The directory should not contain any files and must be owned by root
942: and not group or world-writable.
943: .Pp
1.258 jmc 944: .It Pa /var/run/sshd.pid
1.223 jmc 945: Contains the process ID of the
946: .Nm
947: listening for connections (if there are several daemons running
948: concurrently for different ports, this contains the process ID of the one
949: started last).
950: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 951: .El
1.2 deraadt 952: .Sh SEE ALSO
953: .Xr scp 1 ,
1.90 djm 954: .Xr sftp 1 ,
1.2 deraadt 955: .Xr ssh 1 ,
1.5 deraadt 956: .Xr ssh-add 1 ,
1.2 deraadt 957: .Xr ssh-agent 1 ,
1.5 deraadt 958: .Xr ssh-keygen 1 ,
1.235 jmc 959: .Xr ssh-keyscan 1 ,
1.200 jmc 960: .Xr chroot 2 ,
1.136 markus 961: .Xr login.conf 5 ,
962: .Xr moduli 5 ,
1.184 stevesk 963: .Xr sshd_config 5 ,
1.200 jmc 964: .Xr inetd 8 ,
1.128 mpech 965: .Xr sftp-server 8
1.198 jmc 966: .Sh AUTHORS
967: OpenSSH is a derivative of the original and free
968: ssh 1.2.12 release by Tatu Ylonen.
969: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
970: Theo de Raadt and Dug Song
971: removed many bugs, re-added newer features and
972: created OpenSSH.
973: Markus Friedl contributed the support for SSH
974: protocol versions 1.5 and 2.0.
975: Niels Provos and Markus Friedl contributed support
976: for privilege separation.
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