Annotation of src/usr.bin/ssh/sshd.8, Revision 1.277
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: .\"
1.277 ! sobrado 36: .\" $OpenBSD: sshd.8,v 1.276 2014/07/03 22:40:43 djm Exp $
! 37: .Dd $Mdocdate: July 3 2014 $
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 .
1.7 markus 187: .Nm
188: is normally not run
1.1 deraadt 189: from inetd because it needs to generate the server key before it can
1.36 aaron 190: respond to the client, and this may take tens of seconds.
191: Clients would have to wait too long if the key was regenerated every time.
1.230 jmc 192: However, with small key sizes (e.g. 512) using
1.7 markus 193: .Nm
194: from inetd may
1.1 deraadt 195: be feasible.
1.2 deraadt 196: .It Fl k Ar key_gen_time
1.120 markus 197: Specifies how often the ephemeral protocol version 1 server key is
198: regenerated (default 3600 seconds, or one hour).
1.36 aaron 199: The motivation for regenerating the key fairly
1.200 jmc 200: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 201: it becomes impossible to recover the key for decrypting intercepted
202: communications even if the machine is cracked into or physically
1.36 aaron 203: seized.
204: A value of zero indicates that the key will never be regenerated.
1.156 markus 205: .It Fl o Ar option
206: Can be used to give options in the format used in the configuration file.
207: This is useful for specifying options for which there is no separate
208: command-line flag.
1.200 jmc 209: For full details of the options, and their values, see
210: .Xr sshd_config 5 .
1.2 deraadt 211: .It Fl p Ar port
1.1 deraadt 212: Specifies the port on which the server listens for connections
213: (default 22).
1.158 stevesk 214: Multiple port options are permitted.
1.210 stevesk 215: Ports specified in the configuration file with the
216: .Cm Port
217: option are ignored when a command-line port is specified.
218: Ports specified using the
219: .Cm ListenAddress
220: option override command-line ports.
1.2 deraadt 221: .It Fl q
1.36 aaron 222: Quiet mode.
223: Nothing is sent to the system log.
224: Normally the beginning,
1.1 deraadt 225: authentication, and termination of each connection is logged.
1.242 dtucker 226: .It Fl T
227: Extended test mode.
228: Check the validity of the configuration file, output the effective configuration
229: to stdout and then exit.
230: Optionally,
231: .Cm Match
232: rules may be applied by specifying the connection parameters using one or more
233: .Fl C
234: options.
1.243 jmc 235: .It Fl t
236: Test mode.
237: Only check the validity of the configuration file and sanity of the keys.
238: This is useful for updating
239: .Nm
240: reliably as configuration options may change.
1.61 markus 241: .It Fl u Ar len
242: This option is used to specify the size of the field
243: in the
244: .Li utmp
245: structure that holds the remote host name.
246: If the resolved host name is longer than
247: .Ar len ,
248: the dotted decimal value will be used instead.
249: This allows hosts with very long host names that
250: overflow this field to still be uniquely identified.
251: Specifying
252: .Fl u0
253: indicates that only dotted decimal addresses
254: should be put into the
255: .Pa utmp
256: file.
1.144 stevesk 257: .Fl u0
1.194 jmc 258: may also be used to prevent
1.144 stevesk 259: .Nm
260: from making DNS requests unless the authentication
261: mechanism or configuration requires it.
262: Authentication mechanisms that may require DNS include
263: .Cm RhostsRSAAuthentication ,
1.215 jmc 264: .Cm HostbasedAuthentication ,
1.144 stevesk 265: and using a
266: .Cm from="pattern-list"
267: option in a key file.
1.170 stevesk 268: Configuration options that require DNS include using a
269: USER@HOST pattern in
270: .Cm AllowUsers
271: or
272: .Cm DenyUsers .
1.2 deraadt 273: .El
1.214 jmc 274: .Sh AUTHENTICATION
275: The OpenSSH SSH daemon supports SSH protocols 1 and 2.
1.249 jmc 276: The default is to use protocol 2 only,
1.214 jmc 277: though this can be changed via the
278: .Cm Protocol
279: option in
280: .Xr sshd_config 5 .
1.277 ! sobrado 281: Protocol 2 supports DSA, ECDSA, Ed25519 and RSA keys;
1.214 jmc 282: protocol 1 only supports RSA keys.
283: For both protocols,
284: each host has a host-specific key,
285: normally 2048 bits,
286: used to identify the host.
287: .Pp
288: Forward security for protocol 1 is provided through
289: an additional server key,
290: normally 768 bits,
291: generated when the server starts.
1.212 jmc 292: This key is normally regenerated every hour if it has been used, and
293: is never stored on disk.
294: Whenever a client connects, the daemon responds with its public
295: host and server keys.
296: The client compares the
297: RSA host key against its own database to verify that it has not changed.
298: The client then generates a 256-bit random number.
299: It encrypts this
300: random number using both the host key and the server key, and sends
301: the encrypted number to the server.
302: Both sides then use this
303: random number as a session key which is used to encrypt all further
304: communications in the session.
305: The rest of the session is encrypted
306: using a conventional cipher, currently Blowfish or 3DES, with 3DES
307: being used by default.
308: The client selects the encryption algorithm
309: to use from those offered by the server.
310: .Pp
1.214 jmc 311: For protocol 2,
312: forward security is provided through a Diffie-Hellman key agreement.
313: This key agreement results in a shared session key.
314: The rest of the session is encrypted using a symmetric cipher, currently
315: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
316: The client selects the encryption algorithm
317: to use from those offered by the server.
318: Additionally, session integrity is provided
319: through a cryptographic message authentication code
1.267 markus 320: (hmac-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160,
1.263 djm 321: hmac-sha2-256 or hmac-sha2-512).
1.214 jmc 322: .Pp
323: Finally, the server and the client enter an authentication dialog.
1.212 jmc 324: The client tries to authenticate itself using
1.214 jmc 325: host-based authentication,
326: public key authentication,
327: challenge-response authentication,
328: or password authentication.
1.212 jmc 329: .Pp
330: If the client successfully authenticates itself, a dialog for
331: preparing the session is entered.
332: At this time the client may request
333: things like allocating a pseudo-tty, forwarding X11 connections,
334: forwarding TCP connections, or forwarding the authentication agent
335: connection over the secure channel.
336: .Pp
1.216 jmc 337: After this, the client either requests a shell or execution of a command.
1.212 jmc 338: The sides then enter session mode.
339: In this mode, either side may send
340: data at any time, and such data is forwarded to/from the shell or
341: command on the server side, and the user terminal in the client side.
342: .Pp
343: When the user program terminates and all forwarded X11 and other
344: connections have been closed, the server sends command exit status to
345: the client, and both sides exit.
1.2 deraadt 346: .Sh LOGIN PROCESS
1.1 deraadt 347: When a user successfully logs in,
1.2 deraadt 348: .Nm
1.1 deraadt 349: does the following:
1.2 deraadt 350: .Bl -enum -offset indent
351: .It
1.1 deraadt 352: If the login is on a tty, and no command has been specified,
1.40 aaron 353: prints last login time and
1.2 deraadt 354: .Pa /etc/motd
1.1 deraadt 355: (unless prevented in the configuration file or by
1.207 djm 356: .Pa ~/.hushlogin ;
1.2 deraadt 357: see the
1.40 aaron 358: .Sx FILES
1.2 deraadt 359: section).
360: .It
1.1 deraadt 361: If the login is on a tty, records login time.
1.2 deraadt 362: .It
363: Checks
364: .Pa /etc/nologin ;
365: if it exists, prints contents and quits
1.1 deraadt 366: (unless root).
1.2 deraadt 367: .It
1.1 deraadt 368: Changes to run with normal user privileges.
1.2 deraadt 369: .It
1.1 deraadt 370: Sets up basic environment.
1.2 deraadt 371: .It
1.200 jmc 372: Reads the file
1.207 djm 373: .Pa ~/.ssh/environment ,
1.200 jmc 374: if it exists, and users are allowed to change their environment.
1.188 stevesk 375: See the
1.187 marc 376: .Cm PermitUserEnvironment
1.188 stevesk 377: option in
1.187 marc 378: .Xr sshd_config 5 .
1.2 deraadt 379: .It
1.1 deraadt 380: Changes to user's home directory.
1.2 deraadt 381: .It
382: If
1.207 djm 383: .Pa ~/.ssh/rc
1.276 djm 384: exists and the
385: .Xr sshd_config 5
386: .Cm PermitUserRC
387: option is set, runs it; else if
1.168 deraadt 388: .Pa /etc/ssh/sshrc
1.2 deraadt 389: exists, runs
1.36 aaron 390: it; otherwise runs xauth.
391: The
1.2 deraadt 392: .Dq rc
393: files are given the X11
1.1 deraadt 394: authentication protocol and cookie in standard input.
1.226 jmc 395: See
396: .Sx SSHRC ,
397: below.
1.2 deraadt 398: .It
1.1 deraadt 399: Runs user's shell or command.
1.2 deraadt 400: .El
1.226 jmc 401: .Sh SSHRC
402: If the file
403: .Pa ~/.ssh/rc
404: exists,
405: .Xr sh 1
406: runs it after reading the
407: environment files but before starting the user's shell or command.
408: It must not produce any output on stdout; stderr must be used
409: instead.
410: If X11 forwarding is in use, it will receive the "proto cookie" pair in
411: its standard input (and
412: .Ev DISPLAY
413: in its environment).
414: The script must call
415: .Xr xauth 1
416: because
417: .Nm
418: will not run xauth automatically to add X11 cookies.
419: .Pp
420: The primary purpose of this file is to run any initialization routines
421: which may be needed before the user's home directory becomes
422: accessible; AFS is a particular example of such an environment.
423: .Pp
424: This file will probably contain some initialization code followed by
425: something similar to:
426: .Bd -literal -offset 3n
427: if read proto cookie && [ -n "$DISPLAY" ]; then
428: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
429: # X11UseLocalhost=yes
430: echo add unix:`echo $DISPLAY |
431: cut -c11-` $proto $cookie
432: else
433: # X11UseLocalhost=no
434: echo add $DISPLAY $proto $cookie
435: fi | xauth -q -
436: fi
437: .Ed
438: .Pp
439: If this file does not exist,
440: .Pa /etc/ssh/sshrc
441: is run, and if that
442: does not exist either, xauth is used to add the cookie.
1.2 deraadt 443: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 444: .Cm AuthorizedKeysFile
1.262 jmc 445: specifies the files containing public keys for
1.217 jmc 446: public key authentication;
1.262 jmc 447: if none is specified, the default is
1.261 djm 448: .Pa ~/.ssh/authorized_keys
449: and
450: .Pa ~/.ssh/authorized_keys2 .
1.36 aaron 451: Each line of the file contains one
1.2 deraadt 452: key (empty lines and lines starting with a
453: .Ql #
454: are ignored as
1.36 aaron 455: comments).
1.217 jmc 456: Protocol 1 public keys consist of the following space-separated fields:
457: options, bits, exponent, modulus, comment.
458: Protocol 2 public key consist of:
459: options, keytype, base64-encoded key, comment.
460: The options field is optional;
461: its presence is determined by whether the line starts
1.189 espie 462: with a number or not (the options field never starts with a number).
1.217 jmc 463: The bits, exponent, modulus, and comment fields give the RSA key for
1.75 markus 464: protocol version 1; the
1.1 deraadt 465: comment field is not used for anything (but may be convenient for the
466: user to identify the key).
1.75 markus 467: For protocol version 2 the keytype is
1.259 djm 468: .Dq ecdsa-sha2-nistp256 ,
469: .Dq ecdsa-sha2-nistp384 ,
470: .Dq ecdsa-sha2-nistp521 ,
1.272 jmc 471: .Dq ssh-ed25519 ,
1.75 markus 472: .Dq ssh-dss
473: or
474: .Dq ssh-rsa .
1.2 deraadt 475: .Pp
1.1 deraadt 476: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 477: (because of the size of the public key encoding) up to a limit of
478: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
479: keys up to 16 kilobits.
1.36 aaron 480: You don't want to type them in; instead, copy the
1.113 itojun 481: .Pa identity.pub ,
1.217 jmc 482: .Pa id_dsa.pub ,
1.259 djm 483: .Pa id_ecdsa.pub ,
1.273 naddy 484: .Pa id_ed25519.pub ,
1.75 markus 485: or the
1.113 itojun 486: .Pa id_rsa.pub
1.1 deraadt 487: file and edit it.
1.175 stevesk 488: .Pp
489: .Nm
490: enforces a minimum RSA key modulus size for protocol 1
491: and protocol 2 keys of 768 bits.
1.2 deraadt 492: .Pp
1.58 deraadt 493: The options (if present) consist of comma-separated option
1.36 aaron 494: specifications.
495: No spaces are permitted, except within double quotes.
1.141 stevesk 496: The following option specifications are supported (note
497: that option keywords are case-insensitive):
1.2 deraadt 498: .Bl -tag -width Ds
1.253 djm 499: .It Cm cert-authority
1.251 djm 500: Specifies that the listed key is a certification authority (CA) that is
501: trusted to validate signed certificates for user authentication.
502: .Pp
503: Certificates may encode access restrictions similar to these key options.
504: If both certificate restrictions and key options are present, the most
505: restrictive union of the two is applied.
1.2 deraadt 506: .It Cm command="command"
1.1 deraadt 507: Specifies that the command is executed whenever this key is used for
1.36 aaron 508: authentication.
509: The command supplied by the user (if any) is ignored.
1.148 markus 510: The command is run on a pty if the client requests a pty;
1.36 aaron 511: otherwise it is run without a tty.
1.194 jmc 512: If an 8-bit clean channel is required,
1.147 deraadt 513: one must not request a pty or should specify
1.89 markus 514: .Cm no-pty .
1.36 aaron 515: A quote may be included in the command by quoting it with a backslash.
516: This option might be useful
1.189 espie 517: to restrict certain public keys to perform just a specific operation.
1.36 aaron 518: An example might be a key that permits remote backups but nothing else.
1.211 jmc 519: Note that the client may specify TCP and/or X11
1.51 hugh 520: forwarding unless they are explicitly prohibited.
1.233 dtucker 521: The command originally supplied by the client is available in the
522: .Ev SSH_ORIGINAL_COMMAND
523: environment variable.
1.149 markus 524: Note that this option applies to shell, command or subsystem execution.
1.251 djm 525: Also note that this command may be superseded by either a
526: .Xr sshd_config 5
527: .Cm ForceCommand
528: directive or a command embedded in a certificate.
1.2 deraadt 529: .It Cm environment="NAME=value"
1.1 deraadt 530: Specifies that the string is to be added to the environment when
1.36 aaron 531: logging in using this key.
532: Environment variables set this way
533: override other default environment values.
534: Multiple options of this type are permitted.
1.188 stevesk 535: Environment processing is disabled by default and is
536: controlled via the
537: .Cm PermitUserEnvironment
538: option.
1.155 markus 539: This option is automatically disabled if
540: .Cm UseLogin
541: is enabled.
1.218 jmc 542: .It Cm from="pattern-list"
1.244 djm 543: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 544: name of the remote host or its IP address must be present in the
1.244 djm 545: comma-separated list of patterns.
1.270 jmc 546: See PATTERNS in
1.229 jmc 547: .Xr ssh_config 5
548: for more information on patterns.
1.244 djm 549: .Pp
550: In addition to the wildcard matching that may be applied to hostnames or
551: addresses, a
552: .Cm from
1.248 sobrado 553: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 554: .Pp
555: The purpose of this option is to optionally increase security: public key
556: authentication by itself does not trust the network or name servers or
557: anything (but the key); however, if somebody somehow steals the key, the key
558: permits an intruder to log in from anywhere in the world.
559: This additional option makes using a stolen key more difficult (name
560: servers and/or routers would have to be compromised in addition to
561: just the key).
1.218 jmc 562: .It Cm no-agent-forwarding
563: Forbids authentication agent forwarding when this key is used for
564: authentication.
1.2 deraadt 565: .It Cm no-port-forwarding
1.211 jmc 566: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 567: Any port forward requests by the client will return an error.
1.230 jmc 568: This might be used, e.g. in connection with the
1.2 deraadt 569: .Cm command
1.1 deraadt 570: option.
1.218 jmc 571: .It Cm no-pty
572: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 573: .It Cm no-user-rc
1.241 jmc 574: Disables execution of
1.240 djm 575: .Pa ~/.ssh/rc .
1.2 deraadt 576: .It Cm no-X11-forwarding
1.1 deraadt 577: Forbids X11 forwarding when this key is used for authentication.
578: Any X11 forward requests by the client will return an error.
1.107 djm 579: .It Cm permitopen="host:port"
1.133 itojun 580: Limit local
1.107 djm 581: .Li ``ssh -L''
1.111 stevesk 582: port forwarding such that it may only connect to the specified host and
1.146 stevesk 583: port.
1.257 djm 584: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 585: Multiple
1.107 djm 586: .Cm permitopen
1.197 jmc 587: options may be applied separated by commas.
588: No pattern matching is performed on the specified hostnames,
589: they must be literal domains or addresses.
1.264 dtucker 590: A port specification of
591: .Cm *
592: matches any port.
1.256 djm 593: .It Cm principals="principals"
594: On a
595: .Cm cert-authority
596: line, specifies allowed principals for certificate authentication as a
597: comma-separated list.
598: At least one name from the list must appear in the certificate's
599: list of principals for the certificate to be accepted.
600: This option is ignored for keys that are not marked as trusted certificate
601: signers using the
602: .Cm cert-authority
603: option.
1.209 reyk 604: .It Cm tunnel="n"
605: Force a
606: .Xr tun 4
607: device on the server.
608: Without this option, the next available device will be used if
609: the client requests a tunnel.
1.2 deraadt 610: .El
611: .Pp
1.219 jmc 612: An example authorized_keys file:
1.222 jmc 613: .Bd -literal -offset 3n
1.219 jmc 614: # Comments allowed at start of line
615: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 616: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
617: AAAAB2...19Q== john@example.net
618: command="dump /home",no-pty,no-port-forwarding ssh-dss
619: AAAAC3...51R== example.net
620: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
621: AAAAB5...21S==
622: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
623: jane@example.net
1.219 jmc 624: .Ed
1.2 deraadt 625: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 626: The
1.194 jmc 627: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 628: and
1.207 djm 629: .Pa ~/.ssh/known_hosts
1.36 aaron 630: files contain host public keys for all known hosts.
631: The global file should
1.37 brad 632: be prepared by the administrator (optional), and the per-user file is
1.220 jmc 633: maintained automatically: whenever the user connects from an unknown host,
1.36 aaron 634: its key is added to the per-user file.
1.2 deraadt 635: .Pp
1.254 djm 636: Each line in these files contains the following fields: markers (optional),
637: hostnames, bits, exponent, modulus, comment.
1.36 aaron 638: The fields are separated by spaces.
1.2 deraadt 639: .Pp
1.254 djm 640: The marker is optional, but if it is present then it must be one of
641: .Dq @cert-authority ,
642: to indicate that the line contains a certification authority (CA) key,
643: or
644: .Dq @revoked ,
645: to indicate that the key contained on the line is revoked and must not ever
646: be accepted.
647: Only one marker should be used on a key line.
648: .Pp
1.196 david 649: Hostnames is a comma-separated list of patterns
1.220 jmc 650: .Pf ( Ql *
1.197 jmc 651: and
652: .Ql \&?
1.195 mouring 653: act as
1.1 deraadt 654: wildcards); each pattern in turn is matched against the canonical host
655: name (when authenticating a client) or against the user-supplied
1.36 aaron 656: name (when authenticating a server).
657: A pattern may also be preceded by
1.195 mouring 658: .Ql \&!
1.2 deraadt 659: to indicate negation: if the host name matches a negated
1.1 deraadt 660: pattern, it is not accepted (by that line) even if it matched another
661: pattern on the line.
1.231 dtucker 662: A hostname or address may optionally be enclosed within
663: .Ql \&[
664: and
665: .Ql \&]
666: brackets then followed by
667: .Ql \&:
1.232 jmc 668: and a non-standard port number.
1.2 deraadt 669: .Pp
1.205 djm 670: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 671: and addresses should the file's contents be disclosed.
672: Hashed hostnames start with a
673: .Ql |
1.205 djm 674: character.
675: Only one hashed hostname may appear on a single line and none of the above
676: negation or wildcard operators may be applied.
677: .Pp
1.49 markus 678: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.220 jmc 679: can be obtained, for example, from
1.167 deraadt 680: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 681: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 682: .Pp
683: Lines starting with
684: .Ql #
685: and empty lines are ignored as comments.
686: .Pp
1.1 deraadt 687: When performing host authentication, authentication is accepted if any
1.254 djm 688: matching line has the proper key; either one that matches exactly or,
689: if the server has presented a certificate for authentication, the key
690: of the certification authority that signed the certificate.
691: For a key to be trusted as a certification authority, it must use the
692: .Dq @cert-authority
693: marker described above.
694: .Pp
695: The known hosts file also provides a facility to mark keys as revoked,
696: for example when it is known that the associated private key has been
697: stolen.
698: Revoked keys are specified by including the
699: .Dq @revoked
700: marker at the beginning of the key line, and are never accepted for
701: authentication or as certification authorities, but instead will
702: produce a warning from
703: .Xr ssh 1
704: when they are encountered.
705: .Pp
706: It is permissible (but not
1.1 deraadt 707: recommended) to have several lines or different host keys for the same
1.36 aaron 708: names.
709: This will inevitably happen when short forms of host names
710: from different domains are put in the file.
711: It is possible
1.1 deraadt 712: that the files contain conflicting information; authentication is
713: accepted if valid information can be found from either file.
1.2 deraadt 714: .Pp
1.1 deraadt 715: Note that the lines in these files are typically hundreds of characters
716: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 717: Rather, generate them by a script,
718: .Xr ssh-keyscan 1
1.40 aaron 719: or by taking
1.167 deraadt 720: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 721: and adding the host names at the front.
1.254 djm 722: .Xr ssh-keygen 1
1.255 jmc 723: also offers some basic automated editing for
1.254 djm 724: .Pa ~/.ssh/known_hosts
725: including removing hosts matching a host name and converting all host
726: names to their hashed representations.
1.221 jmc 727: .Pp
728: An example ssh_known_hosts file:
729: .Bd -literal -offset 3n
730: # Comments allowed at start of line
731: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
732: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 733: # A hashed hostname
734: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
735: AAAA1234.....=
1.254 djm 736: # A revoked key
737: @revoked * ssh-rsa AAAAB5W...
738: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
739: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 740: .Ed
1.2 deraadt 741: .Sh FILES
1.223 jmc 742: .Bl -tag -width Ds -compact
1.258 jmc 743: .It Pa ~/.hushlogin
1.204 jmc 744: This file is used to suppress printing the last login time and
745: .Pa /etc/motd ,
746: if
747: .Cm PrintLastLog
748: and
749: .Cm PrintMotd ,
750: respectively,
751: are enabled.
752: It does not suppress printing of the banner specified by
753: .Cm Banner .
1.223 jmc 754: .Pp
1.258 jmc 755: .It Pa ~/.rhosts
1.225 jmc 756: This file is used for host-based authentication (see
757: .Xr ssh 1
758: for more information).
759: On some machines this file may need to be
760: world-readable if the user's home directory is on an NFS partition,
761: because
762: .Nm
763: reads it as root.
764: Additionally, this file must be owned by the user,
765: and must not have write permissions for anyone else.
766: The recommended
767: permission for most machines is read/write for the user, and not
1.1 deraadt 768: accessible by others.
1.2 deraadt 769: .Pp
1.258 jmc 770: .It Pa ~/.shosts
1.225 jmc 771: This file is used in exactly the same way as
772: .Pa .rhosts ,
773: but allows host-based authentication without permitting login with
774: rlogin/rsh.
1.238 mcbride 775: .Pp
1.258 jmc 776: .It Pa ~/.ssh/
1.238 mcbride 777: This directory is the default location for all user-specific configuration
778: and authentication information.
779: There is no general requirement to keep the entire contents of this directory
780: secret, but the recommended permissions are read/write/execute for the user,
781: and not accessible by others.
1.2 deraadt 782: .Pp
1.258 jmc 783: .It Pa ~/.ssh/authorized_keys
1.277 ! sobrado 784: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.273 naddy 785: that can be used for logging in as this user.
1.223 jmc 786: The format of this file is described above.
1.234 dtucker 787: The content of the file is not highly sensitive, but the recommended
1.225 jmc 788: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 789: .Pp
790: If this file, the
791: .Pa ~/.ssh
792: directory, or the user's home directory are writable
793: by other users, then the file could be modified or replaced by unauthorized
794: users.
795: In this case,
796: .Nm
797: will not allow it to be used unless the
798: .Cm StrictModes
799: option has been set to
800: .Dq no .
1.2 deraadt 801: .Pp
1.258 jmc 802: .It Pa ~/.ssh/environment
1.36 aaron 803: This file is read into the environment at login (if it exists).
804: It can only contain empty lines, comment lines (that start with
1.2 deraadt 805: .Ql # ) ,
1.36 aaron 806: and assignment lines of the form name=value.
807: The file should be writable
1.6 aaron 808: only by the user; it need not be readable by anyone else.
1.188 stevesk 809: Environment processing is disabled by default and is
810: controlled via the
811: .Cm PermitUserEnvironment
812: option.
1.223 jmc 813: .Pp
1.258 jmc 814: .It Pa ~/.ssh/known_hosts
1.225 jmc 815: Contains a list of host keys for all hosts the user has logged into
816: that are not already in the systemwide list of known host keys.
817: The format of this file is described above.
818: This file should be writable only by root/the owner and
1.223 jmc 819: can, but need not be, world-readable.
820: .Pp
1.258 jmc 821: .It Pa ~/.ssh/rc
1.226 jmc 822: Contains initialization routines to be run before
823: the user's home directory becomes accessible.
1.1 deraadt 824: This file should be writable only by the user, and need not be
825: readable by anyone else.
1.223 jmc 826: .Pp
1.258 jmc 827: .It Pa /etc/hosts.equiv
1.227 jmc 828: This file is for host-based authentication (see
829: .Xr ssh 1 ) .
830: It should only be writable by root.
1.223 jmc 831: .Pp
1.258 jmc 832: .It Pa /etc/moduli
1.223 jmc 833: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
834: The file format is described in
835: .Xr moduli 5 .
836: .Pp
1.258 jmc 837: .It Pa /etc/motd
1.223 jmc 838: See
839: .Xr motd 5 .
840: .Pp
1.258 jmc 841: .It Pa /etc/nologin
1.223 jmc 842: If this file exists,
843: .Nm
844: refuses to let anyone except root log in.
845: The contents of the file
846: are displayed to anyone trying to log in, and non-root connections are
847: refused.
848: The file should be world-readable.
849: .Pp
1.258 jmc 850: .It Pa /etc/shosts.equiv
1.227 jmc 851: This file is used in exactly the same way as
852: .Pa hosts.equiv ,
853: but allows host-based authentication without permitting login with
854: rlogin/rsh.
1.225 jmc 855: .Pp
1.258 jmc 856: .It Pa /etc/ssh/ssh_host_key
857: .It Pa /etc/ssh/ssh_host_dsa_key
1.259 djm 858: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.273 naddy 859: .It Pa /etc/ssh/ssh_host_ed25519_key
1.258 jmc 860: .It Pa /etc/ssh/ssh_host_rsa_key
1.266 dtucker 861: These files contain the private parts of the host keys.
1.223 jmc 862: These files should only be owned by root, readable only by root, and not
863: accessible to others.
864: Note that
865: .Nm
1.228 jmc 866: does not start if these files are group/world-accessible.
1.223 jmc 867: .Pp
1.258 jmc 868: .It Pa /etc/ssh/ssh_host_key.pub
869: .It Pa /etc/ssh/ssh_host_dsa_key.pub
1.259 djm 870: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.273 naddy 871: .It Pa /etc/ssh/ssh_host_ed25519_key.pub
1.258 jmc 872: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.266 dtucker 873: These files contain the public parts of the host keys.
1.223 jmc 874: These files should be world-readable but writable only by
875: root.
876: Their contents should match the respective private parts.
877: These files are not
878: really used for anything; they are provided for the convenience of
879: the user so their contents can be copied to known hosts files.
880: These files are created using
881: .Xr ssh-keygen 1 .
882: .Pp
1.258 jmc 883: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 884: Systemwide list of known host keys.
885: This file should be prepared by the
886: system administrator to contain the public host keys of all machines in the
887: organization.
888: The format of this file is described above.
889: This file should be writable only by root/the owner and
890: should be world-readable.
891: .Pp
1.258 jmc 892: .It Pa /etc/ssh/sshd_config
1.223 jmc 893: Contains configuration data for
894: .Nm sshd .
895: The file format and configuration options are described in
896: .Xr sshd_config 5 .
897: .Pp
1.258 jmc 898: .It Pa /etc/ssh/sshrc
1.226 jmc 899: Similar to
900: .Pa ~/.ssh/rc ,
901: it can be used to specify
1.36 aaron 902: machine-specific login-time initializations globally.
903: This file should be writable only by root, and should be world-readable.
1.223 jmc 904: .Pp
1.258 jmc 905: .It Pa /var/empty
1.223 jmc 906: .Xr chroot 2
907: directory used by
908: .Nm
909: during privilege separation in the pre-authentication phase.
910: The directory should not contain any files and must be owned by root
911: and not group or world-writable.
912: .Pp
1.258 jmc 913: .It Pa /var/run/sshd.pid
1.223 jmc 914: Contains the process ID of the
915: .Nm
916: listening for connections (if there are several daemons running
917: concurrently for different ports, this contains the process ID of the one
918: started last).
919: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 920: .El
1.2 deraadt 921: .Sh SEE ALSO
922: .Xr scp 1 ,
1.90 djm 923: .Xr sftp 1 ,
1.2 deraadt 924: .Xr ssh 1 ,
1.5 deraadt 925: .Xr ssh-add 1 ,
1.2 deraadt 926: .Xr ssh-agent 1 ,
1.5 deraadt 927: .Xr ssh-keygen 1 ,
1.235 jmc 928: .Xr ssh-keyscan 1 ,
1.200 jmc 929: .Xr chroot 2 ,
1.136 markus 930: .Xr login.conf 5 ,
931: .Xr moduli 5 ,
1.184 stevesk 932: .Xr sshd_config 5 ,
1.200 jmc 933: .Xr inetd 8 ,
1.128 mpech 934: .Xr sftp-server 8
1.198 jmc 935: .Sh AUTHORS
936: OpenSSH is a derivative of the original and free
937: ssh 1.2.12 release by Tatu Ylonen.
938: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
939: Theo de Raadt and Dug Song
940: removed many bugs, re-added newer features and
941: created OpenSSH.
942: Markus Friedl contributed the support for SSH
943: protocol versions 1.5 and 2.0.
944: Niels Provos and Markus Friedl contributed support
945: for privilege separation.
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