Annotation of src/usr.bin/ssh/sshd.8, Revision 1.260
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: .\"
1.260 ! jmc 36: .\" $OpenBSD: sshd.8,v 1.259 2010/08/31 11:54:45 djm Exp $
! 37: .Dd $Mdocdate: August 31 2010 $
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
1.2 deraadt 50: .Op Fl f Ar config_file
51: .Op Fl g Ar login_grace_time
52: .Op Fl h Ar host_key_file
53: .Op Fl k Ar key_gen_time
1.156 markus 54: .Op Fl o Ar option
1.2 deraadt 55: .Op Fl p Ar port
1.61 markus 56: .Op Fl u Ar len
1.194 jmc 57: .Ek
1.40 aaron 58: .Sh DESCRIPTION
1.2 deraadt 59: .Nm
1.212 jmc 60: (OpenSSH Daemon) is the daemon program for
1.2 deraadt 61: .Xr ssh 1 .
1.235 jmc 62: Together these programs replace
63: .Xr rlogin 1
64: and
65: .Xr rsh 1 ,
66: and provide secure encrypted communications between two untrusted hosts
1.36 aaron 67: over an insecure network.
1.2 deraadt 68: .Pp
69: .Nm
1.212 jmc 70: listens for connections from clients.
1.40 aaron 71: It is normally started at boot from
1.2 deraadt 72: .Pa /etc/rc .
73: It forks a new
1.36 aaron 74: daemon for each incoming connection.
75: The forked daemons handle
1.1 deraadt 76: key exchange, encryption, authentication, command execution,
77: and data exchange.
1.2 deraadt 78: .Pp
79: .Nm
1.200 jmc 80: can be configured using command-line options or a configuration file
81: (by default
1.212 jmc 82: .Xr sshd_config 5 ) ;
83: command-line options override values specified in the
1.1 deraadt 84: configuration file.
1.25 markus 85: .Nm
86: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 87: .Dv SIGHUP ,
1.230 jmc 88: by executing itself with the name and options it was started with, e.g.\&
1.97 deraadt 89: .Pa /usr/sbin/sshd .
1.18 aaron 90: .Pp
91: The options are as follows:
1.2 deraadt 92: .Bl -tag -width Ds
1.200 jmc 93: .It Fl 4
94: Forces
95: .Nm
96: to use IPv4 addresses only.
97: .It Fl 6
98: Forces
99: .Nm
100: to use IPv6 addresses only.
1.2 deraadt 101: .It Fl b Ar bits
1.120 markus 102: Specifies the number of bits in the ephemeral protocol version 1
1.246 djm 103: server key (default 1024).
1.243 jmc 104: .It Fl C Ar connection_spec
105: Specify the connection parameters to use for the
106: .Fl T
107: extended test mode.
108: If provided, any
109: .Cm Match
110: directives in the configuration file
111: that would apply to the specified user, host, and address will be set before
112: the configuration is written to standard output.
113: The connection parameters are supplied as keyword=value pairs.
114: The keywords are
115: .Dq user ,
116: .Dq host ,
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.
1.120 markus 147: .It Fl e
148: When this option is specified,
149: .Nm
150: will send the output to the 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
1.167 deraadt 170: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 171: for protocol version 1, and
1.259 djm 172: .Pa /etc/ssh/ssh_host_dsa_key ,
173: .Pa /etc/ssh/ssh_host_ecdsa_key
174: and
1.167 deraadt 175: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 176: for protocol version 2.
1.75 markus 177: It is possible to have multiple host key files for
1.120 markus 178: the different protocol versions and host key algorithms.
1.2 deraadt 179: .It Fl i
1.7 markus 180: Specifies that
181: .Nm
1.194 jmc 182: is being run from
183: .Xr inetd 8 .
1.7 markus 184: .Nm
185: is normally not run
1.1 deraadt 186: from inetd because it needs to generate the server key before it can
1.36 aaron 187: respond to the client, and this may take tens of seconds.
188: Clients would have to wait too long if the key was regenerated every time.
1.230 jmc 189: However, with small key sizes (e.g. 512) using
1.7 markus 190: .Nm
191: from inetd may
1.1 deraadt 192: be feasible.
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 .
1.259 djm 278: Protocol 2 supports DSA, ECDSA 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,
287: normally 768 bits,
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
1.237 pvalchev 317: (hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160).
1.214 jmc 318: .Pp
319: Finally, the server and the client enter an authentication dialog.
1.212 jmc 320: The client tries to authenticate itself using
1.214 jmc 321: host-based authentication,
322: public key authentication,
323: challenge-response authentication,
324: or password authentication.
1.212 jmc 325: .Pp
326: If the client successfully authenticates itself, a dialog for
327: preparing the session is entered.
328: At this time the client may request
329: things like allocating a pseudo-tty, forwarding X11 connections,
330: forwarding TCP connections, or forwarding the authentication agent
331: connection over the secure channel.
332: .Pp
1.216 jmc 333: After this, the client either requests a shell or execution of a command.
1.212 jmc 334: The sides then enter session mode.
335: In this mode, either side may send
336: data at any time, and such data is forwarded to/from the shell or
337: command on the server side, and the user terminal in the client side.
338: .Pp
339: When the user program terminates and all forwarded X11 and other
340: connections have been closed, the server sends command exit status to
341: the client, and both sides exit.
1.2 deraadt 342: .Sh LOGIN PROCESS
1.1 deraadt 343: When a user successfully logs in,
1.2 deraadt 344: .Nm
1.1 deraadt 345: does the following:
1.2 deraadt 346: .Bl -enum -offset indent
347: .It
1.1 deraadt 348: If the login is on a tty, and no command has been specified,
1.40 aaron 349: prints last login time and
1.2 deraadt 350: .Pa /etc/motd
1.1 deraadt 351: (unless prevented in the configuration file or by
1.207 djm 352: .Pa ~/.hushlogin ;
1.2 deraadt 353: see the
1.40 aaron 354: .Sx FILES
1.2 deraadt 355: section).
356: .It
1.1 deraadt 357: If the login is on a tty, records login time.
1.2 deraadt 358: .It
359: Checks
360: .Pa /etc/nologin ;
361: if it exists, prints contents and quits
1.1 deraadt 362: (unless root).
1.2 deraadt 363: .It
1.1 deraadt 364: Changes to run with normal user privileges.
1.2 deraadt 365: .It
1.1 deraadt 366: Sets up basic environment.
1.2 deraadt 367: .It
1.200 jmc 368: Reads the file
1.207 djm 369: .Pa ~/.ssh/environment ,
1.200 jmc 370: if it exists, and users are allowed to change their environment.
1.188 stevesk 371: See the
1.187 marc 372: .Cm PermitUserEnvironment
1.188 stevesk 373: option in
1.187 marc 374: .Xr sshd_config 5 .
1.2 deraadt 375: .It
1.1 deraadt 376: Changes to user's home directory.
1.2 deraadt 377: .It
378: If
1.207 djm 379: .Pa ~/.ssh/rc
1.2 deraadt 380: exists, runs it; else if
1.168 deraadt 381: .Pa /etc/ssh/sshrc
1.2 deraadt 382: exists, runs
1.36 aaron 383: it; otherwise runs xauth.
384: The
1.2 deraadt 385: .Dq rc
386: files are given the X11
1.1 deraadt 387: authentication protocol and cookie in standard input.
1.226 jmc 388: See
389: .Sx SSHRC ,
390: below.
1.2 deraadt 391: .It
1.1 deraadt 392: Runs user's shell or command.
1.2 deraadt 393: .El
1.226 jmc 394: .Sh SSHRC
395: If the file
396: .Pa ~/.ssh/rc
397: exists,
398: .Xr sh 1
399: runs it after reading the
400: environment files but before starting the user's shell or command.
401: It must not produce any output on stdout; stderr must be used
402: instead.
403: If X11 forwarding is in use, it will receive the "proto cookie" pair in
404: its standard input (and
405: .Ev DISPLAY
406: in its environment).
407: The script must call
408: .Xr xauth 1
409: because
410: .Nm
411: will not run xauth automatically to add X11 cookies.
412: .Pp
413: The primary purpose of this file is to run any initialization routines
414: which may be needed before the user's home directory becomes
415: accessible; AFS is a particular example of such an environment.
416: .Pp
417: This file will probably contain some initialization code followed by
418: something similar to:
419: .Bd -literal -offset 3n
420: if read proto cookie && [ -n "$DISPLAY" ]; then
421: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
422: # X11UseLocalhost=yes
423: echo add unix:`echo $DISPLAY |
424: cut -c11-` $proto $cookie
425: else
426: # X11UseLocalhost=no
427: echo add $DISPLAY $proto $cookie
428: fi | xauth -q -
429: fi
430: .Ed
431: .Pp
432: If this file does not exist,
433: .Pa /etc/ssh/sshrc
434: is run, and if that
435: does not exist either, xauth is used to add the cookie.
1.2 deraadt 436: .Sh AUTHORIZED_KEYS FILE FORMAT
1.125 markus 437: .Cm AuthorizedKeysFile
1.217 jmc 438: specifies the file containing public keys for
439: public key authentication;
440: if none is specified, the default is
441: .Pa ~/.ssh/authorized_keys .
1.36 aaron 442: Each line of the file contains one
1.2 deraadt 443: key (empty lines and lines starting with a
444: .Ql #
445: are ignored as
1.36 aaron 446: comments).
1.217 jmc 447: Protocol 1 public keys consist of the following space-separated fields:
448: options, bits, exponent, modulus, comment.
449: Protocol 2 public key consist of:
450: options, keytype, base64-encoded key, comment.
451: The options field is optional;
452: its presence is determined by whether the line starts
1.189 espie 453: with a number or not (the options field never starts with a number).
1.217 jmc 454: The bits, exponent, modulus, and comment fields give the RSA key for
1.75 markus 455: protocol version 1; the
1.1 deraadt 456: comment field is not used for anything (but may be convenient for the
457: user to identify the key).
1.75 markus 458: For protocol version 2 the keytype is
1.259 djm 459: .Dq ecdsa-sha2-nistp256 ,
460: .Dq ecdsa-sha2-nistp384 ,
461: .Dq ecdsa-sha2-nistp521 ,
1.75 markus 462: .Dq ssh-dss
463: or
464: .Dq ssh-rsa .
1.2 deraadt 465: .Pp
1.1 deraadt 466: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 467: (because of the size of the public key encoding) up to a limit of
468: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
469: keys up to 16 kilobits.
1.36 aaron 470: You don't want to type them in; instead, copy the
1.113 itojun 471: .Pa identity.pub ,
1.217 jmc 472: .Pa id_dsa.pub ,
1.259 djm 473: .Pa id_ecdsa.pub ,
1.75 markus 474: or the
1.113 itojun 475: .Pa id_rsa.pub
1.1 deraadt 476: file and edit it.
1.175 stevesk 477: .Pp
478: .Nm
479: enforces a minimum RSA key modulus size for protocol 1
480: and protocol 2 keys of 768 bits.
1.2 deraadt 481: .Pp
1.58 deraadt 482: The options (if present) consist of comma-separated option
1.36 aaron 483: specifications.
484: No spaces are permitted, except within double quotes.
1.141 stevesk 485: The following option specifications are supported (note
486: that option keywords are case-insensitive):
1.2 deraadt 487: .Bl -tag -width Ds
1.253 djm 488: .It Cm cert-authority
1.251 djm 489: Specifies that the listed key is a certification authority (CA) that is
490: trusted to validate signed certificates for user authentication.
491: .Pp
492: Certificates may encode access restrictions similar to these key options.
493: If both certificate restrictions and key options are present, the most
494: restrictive union of the two is applied.
1.2 deraadt 495: .It Cm command="command"
1.1 deraadt 496: Specifies that the command is executed whenever this key is used for
1.36 aaron 497: authentication.
498: The command supplied by the user (if any) is ignored.
1.148 markus 499: The command is run on a pty if the client requests a pty;
1.36 aaron 500: otherwise it is run without a tty.
1.194 jmc 501: If an 8-bit clean channel is required,
1.147 deraadt 502: one must not request a pty or should specify
1.89 markus 503: .Cm no-pty .
1.36 aaron 504: A quote may be included in the command by quoting it with a backslash.
505: This option might be useful
1.189 espie 506: to restrict certain public keys to perform just a specific operation.
1.36 aaron 507: An example might be a key that permits remote backups but nothing else.
1.211 jmc 508: Note that the client may specify TCP and/or X11
1.51 hugh 509: forwarding unless they are explicitly prohibited.
1.233 dtucker 510: The command originally supplied by the client is available in the
511: .Ev SSH_ORIGINAL_COMMAND
512: environment variable.
1.149 markus 513: Note that this option applies to shell, command or subsystem execution.
1.251 djm 514: Also note that this command may be superseded by either a
515: .Xr sshd_config 5
516: .Cm ForceCommand
517: directive or a command embedded in a certificate.
1.2 deraadt 518: .It Cm environment="NAME=value"
1.1 deraadt 519: Specifies that the string is to be added to the environment when
1.36 aaron 520: logging in using this key.
521: Environment variables set this way
522: override other default environment values.
523: Multiple options of this type are permitted.
1.188 stevesk 524: Environment processing is disabled by default and is
525: controlled via the
526: .Cm PermitUserEnvironment
527: option.
1.155 markus 528: This option is automatically disabled if
529: .Cm UseLogin
530: is enabled.
1.218 jmc 531: .It Cm from="pattern-list"
1.244 djm 532: Specifies that in addition to public key authentication, either the canonical
1.245 jmc 533: name of the remote host or its IP address must be present in the
1.244 djm 534: comma-separated list of patterns.
1.229 jmc 535: See
536: .Sx PATTERNS
537: in
538: .Xr ssh_config 5
539: for more information on patterns.
1.244 djm 540: .Pp
541: In addition to the wildcard matching that may be applied to hostnames or
542: addresses, a
543: .Cm from
1.248 sobrado 544: stanza may match IP addresses using CIDR address/masklen notation.
1.244 djm 545: .Pp
546: The purpose of this option is to optionally increase security: public key
547: authentication by itself does not trust the network or name servers or
548: anything (but the key); however, if somebody somehow steals the key, the key
549: permits an intruder to log in from anywhere in the world.
550: This additional option makes using a stolen key more difficult (name
551: servers and/or routers would have to be compromised in addition to
552: just the key).
1.218 jmc 553: .It Cm no-agent-forwarding
554: Forbids authentication agent forwarding when this key is used for
555: authentication.
1.2 deraadt 556: .It Cm no-port-forwarding
1.211 jmc 557: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 558: Any port forward requests by the client will return an error.
1.230 jmc 559: This might be used, e.g. in connection with the
1.2 deraadt 560: .Cm command
1.1 deraadt 561: option.
1.218 jmc 562: .It Cm no-pty
563: Prevents tty allocation (a request to allocate a pty will fail).
1.240 djm 564: .It Cm no-user-rc
1.241 jmc 565: Disables execution of
1.240 djm 566: .Pa ~/.ssh/rc .
1.2 deraadt 567: .It Cm no-X11-forwarding
1.1 deraadt 568: Forbids X11 forwarding when this key is used for authentication.
569: Any X11 forward requests by the client will return an error.
1.107 djm 570: .It Cm permitopen="host:port"
1.133 itojun 571: Limit local
1.107 djm 572: .Li ``ssh -L''
1.111 stevesk 573: port forwarding such that it may only connect to the specified host and
1.146 stevesk 574: port.
1.257 djm 575: IPv6 addresses can be specified by enclosing the address in square brackets.
1.146 stevesk 576: Multiple
1.107 djm 577: .Cm permitopen
1.197 jmc 578: options may be applied separated by commas.
579: No pattern matching is performed on the specified hostnames,
580: they must be literal domains or addresses.
1.256 djm 581: .It Cm principals="principals"
582: On a
583: .Cm cert-authority
584: line, specifies allowed principals for certificate authentication as a
585: comma-separated list.
586: At least one name from the list must appear in the certificate's
587: list of principals for the certificate to be accepted.
588: This option is ignored for keys that are not marked as trusted certificate
589: signers using the
590: .Cm cert-authority
591: option.
1.209 reyk 592: .It Cm tunnel="n"
593: Force a
594: .Xr tun 4
595: device on the server.
596: Without this option, the next available device will be used if
597: the client requests a tunnel.
1.2 deraadt 598: .El
599: .Pp
1.219 jmc 600: An example authorized_keys file:
1.222 jmc 601: .Bd -literal -offset 3n
1.219 jmc 602: # Comments allowed at start of line
603: ssh-rsa AAAAB3Nza...LiPk== user@example.net
1.222 jmc 604: from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
605: AAAAB2...19Q== john@example.net
606: command="dump /home",no-pty,no-port-forwarding ssh-dss
607: AAAAC3...51R== example.net
608: permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
609: AAAAB5...21S==
610: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
611: jane@example.net
1.219 jmc 612: .Ed
1.2 deraadt 613: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 614: The
1.194 jmc 615: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 616: and
1.207 djm 617: .Pa ~/.ssh/known_hosts
1.36 aaron 618: files contain host public keys for all known hosts.
619: The global file should
1.37 brad 620: be prepared by the administrator (optional), and the per-user file is
1.220 jmc 621: maintained automatically: whenever the user connects from an unknown host,
1.36 aaron 622: its key is added to the per-user file.
1.2 deraadt 623: .Pp
1.254 djm 624: Each line in these files contains the following fields: markers (optional),
625: hostnames, bits, exponent, modulus, comment.
1.36 aaron 626: The fields are separated by spaces.
1.2 deraadt 627: .Pp
1.254 djm 628: The marker is optional, but if it is present then it must be one of
629: .Dq @cert-authority ,
630: to indicate that the line contains a certification authority (CA) key,
631: or
632: .Dq @revoked ,
633: to indicate that the key contained on the line is revoked and must not ever
634: be accepted.
635: Only one marker should be used on a key line.
636: .Pp
1.196 david 637: Hostnames is a comma-separated list of patterns
1.220 jmc 638: .Pf ( Ql *
1.197 jmc 639: and
640: .Ql \&?
1.195 mouring 641: act as
1.1 deraadt 642: wildcards); each pattern in turn is matched against the canonical host
643: name (when authenticating a client) or against the user-supplied
1.36 aaron 644: name (when authenticating a server).
645: A pattern may also be preceded by
1.195 mouring 646: .Ql \&!
1.2 deraadt 647: to indicate negation: if the host name matches a negated
1.1 deraadt 648: pattern, it is not accepted (by that line) even if it matched another
649: pattern on the line.
1.231 dtucker 650: A hostname or address may optionally be enclosed within
651: .Ql \&[
652: and
653: .Ql \&]
654: brackets then followed by
655: .Ql \&:
1.232 jmc 656: and a non-standard port number.
1.2 deraadt 657: .Pp
1.205 djm 658: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 659: and addresses should the file's contents be disclosed.
660: Hashed hostnames start with a
661: .Ql |
1.205 djm 662: character.
663: Only one hashed hostname may appear on a single line and none of the above
664: negation or wildcard operators may be applied.
665: .Pp
1.49 markus 666: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.220 jmc 667: can be obtained, for example, from
1.167 deraadt 668: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 669: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 670: .Pp
671: Lines starting with
672: .Ql #
673: and empty lines are ignored as comments.
674: .Pp
1.1 deraadt 675: When performing host authentication, authentication is accepted if any
1.254 djm 676: matching line has the proper key; either one that matches exactly or,
677: if the server has presented a certificate for authentication, the key
678: of the certification authority that signed the certificate.
679: For a key to be trusted as a certification authority, it must use the
680: .Dq @cert-authority
681: marker described above.
682: .Pp
683: The known hosts file also provides a facility to mark keys as revoked,
684: for example when it is known that the associated private key has been
685: stolen.
686: Revoked keys are specified by including the
687: .Dq @revoked
688: marker at the beginning of the key line, and are never accepted for
689: authentication or as certification authorities, but instead will
690: produce a warning from
691: .Xr ssh 1
692: when they are encountered.
693: .Pp
694: It is permissible (but not
1.1 deraadt 695: recommended) to have several lines or different host keys for the same
1.36 aaron 696: names.
697: This will inevitably happen when short forms of host names
698: from different domains are put in the file.
699: It is possible
1.1 deraadt 700: that the files contain conflicting information; authentication is
701: accepted if valid information can be found from either file.
1.2 deraadt 702: .Pp
1.1 deraadt 703: Note that the lines in these files are typically hundreds of characters
704: long, and you definitely don't want to type in the host keys by hand.
1.254 djm 705: Rather, generate them by a script,
706: .Xr ssh-keyscan 1
1.40 aaron 707: or by taking
1.167 deraadt 708: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 709: and adding the host names at the front.
1.254 djm 710: .Xr ssh-keygen 1
1.255 jmc 711: also offers some basic automated editing for
1.254 djm 712: .Pa ~/.ssh/known_hosts
713: including removing hosts matching a host name and converting all host
714: names to their hashed representations.
1.221 jmc 715: .Pp
716: An example ssh_known_hosts file:
717: .Bd -literal -offset 3n
718: # Comments allowed at start of line
719: closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
720: cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
1.205 djm 721: # A hashed hostname
722: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
723: AAAA1234.....=
1.254 djm 724: # A revoked key
725: @revoked * ssh-rsa AAAAB5W...
726: # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
727: @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1.120 markus 728: .Ed
1.2 deraadt 729: .Sh FILES
1.223 jmc 730: .Bl -tag -width Ds -compact
1.258 jmc 731: .It Pa ~/.hushlogin
1.204 jmc 732: This file is used to suppress printing the last login time and
733: .Pa /etc/motd ,
734: if
735: .Cm PrintLastLog
736: and
737: .Cm PrintMotd ,
738: respectively,
739: are enabled.
740: It does not suppress printing of the banner specified by
741: .Cm Banner .
1.223 jmc 742: .Pp
1.258 jmc 743: .It Pa ~/.rhosts
1.225 jmc 744: This file is used for host-based authentication (see
745: .Xr ssh 1
746: for more information).
747: On some machines this file may need to be
748: world-readable if the user's home directory is on an NFS partition,
749: because
750: .Nm
751: reads it as root.
752: Additionally, this file must be owned by the user,
753: and must not have write permissions for anyone else.
754: The recommended
755: permission for most machines is read/write for the user, and not
1.1 deraadt 756: accessible by others.
1.2 deraadt 757: .Pp
1.258 jmc 758: .It Pa ~/.shosts
1.225 jmc 759: This file is used in exactly the same way as
760: .Pa .rhosts ,
761: but allows host-based authentication without permitting login with
762: rlogin/rsh.
1.238 mcbride 763: .Pp
1.258 jmc 764: .It Pa ~/.ssh/
1.238 mcbride 765: This directory is the default location for all user-specific configuration
766: and authentication information.
767: There is no general requirement to keep the entire contents of this directory
768: secret, but the recommended permissions are read/write/execute for the user,
769: and not accessible by others.
1.2 deraadt 770: .Pp
1.258 jmc 771: .It Pa ~/.ssh/authorized_keys
1.259 djm 772: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in
773: as this user.
1.223 jmc 774: The format of this file is described above.
1.234 dtucker 775: The content of the file is not highly sensitive, but the recommended
1.225 jmc 776: permissions are read/write for the user, and not accessible by others.
1.234 dtucker 777: .Pp
778: If this file, the
779: .Pa ~/.ssh
780: directory, or the user's home directory are writable
781: by other users, then the file could be modified or replaced by unauthorized
782: users.
783: In this case,
784: .Nm
785: will not allow it to be used unless the
786: .Cm StrictModes
787: option has been set to
788: .Dq no .
1.2 deraadt 789: .Pp
1.258 jmc 790: .It Pa ~/.ssh/environment
1.36 aaron 791: This file is read into the environment at login (if it exists).
792: It can only contain empty lines, comment lines (that start with
1.2 deraadt 793: .Ql # ) ,
1.36 aaron 794: and assignment lines of the form name=value.
795: The file should be writable
1.6 aaron 796: only by the user; it need not be readable by anyone else.
1.188 stevesk 797: Environment processing is disabled by default and is
798: controlled via the
799: .Cm PermitUserEnvironment
800: option.
1.223 jmc 801: .Pp
1.258 jmc 802: .It Pa ~/.ssh/known_hosts
1.225 jmc 803: Contains a list of host keys for all hosts the user has logged into
804: that are not already in the systemwide list of known host keys.
805: The format of this file is described above.
806: This file should be writable only by root/the owner and
1.223 jmc 807: can, but need not be, world-readable.
808: .Pp
1.258 jmc 809: .It Pa ~/.ssh/rc
1.226 jmc 810: Contains initialization routines to be run before
811: the user's home directory becomes accessible.
1.1 deraadt 812: This file should be writable only by the user, and need not be
813: readable by anyone else.
1.223 jmc 814: .Pp
1.258 jmc 815: .It Pa /etc/hosts.allow
816: .It Pa /etc/hosts.deny
1.223 jmc 817: Access controls that should be enforced by tcp-wrappers are defined here.
818: Further details are described in
819: .Xr hosts_access 5 .
820: .Pp
1.258 jmc 821: .It Pa /etc/hosts.equiv
1.227 jmc 822: This file is for host-based authentication (see
823: .Xr ssh 1 ) .
824: It should only be writable by root.
1.223 jmc 825: .Pp
1.258 jmc 826: .It Pa /etc/moduli
1.223 jmc 827: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
828: The file format is described in
829: .Xr moduli 5 .
830: .Pp
1.258 jmc 831: .It Pa /etc/motd
1.223 jmc 832: See
833: .Xr motd 5 .
834: .Pp
1.258 jmc 835: .It Pa /etc/nologin
1.223 jmc 836: If this file exists,
837: .Nm
838: refuses to let anyone except root log in.
839: The contents of the file
840: are displayed to anyone trying to log in, and non-root connections are
841: refused.
842: The file should be world-readable.
843: .Pp
1.258 jmc 844: .It Pa /etc/shosts.equiv
1.227 jmc 845: This file is used in exactly the same way as
846: .Pa hosts.equiv ,
847: but allows host-based authentication without permitting login with
848: rlogin/rsh.
1.225 jmc 849: .Pp
1.258 jmc 850: .It Pa /etc/ssh/ssh_host_key
851: .It Pa /etc/ssh/ssh_host_dsa_key
1.259 djm 852: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.258 jmc 853: .It Pa /etc/ssh/ssh_host_rsa_key
1.223 jmc 854: These three files contain the private parts of the host keys.
855: These files should only be owned by root, readable only by root, and not
856: accessible to others.
857: Note that
858: .Nm
1.228 jmc 859: does not start if these files are group/world-accessible.
1.223 jmc 860: .Pp
1.258 jmc 861: .It Pa /etc/ssh/ssh_host_key.pub
862: .It Pa /etc/ssh/ssh_host_dsa_key.pub
1.259 djm 863: .It Pa /etc/ssh/ssh_host_ecdsa_key.pub
1.258 jmc 864: .It Pa /etc/ssh/ssh_host_rsa_key.pub
1.223 jmc 865: These three files contain the public parts of the host keys.
866: These files should be world-readable but writable only by
867: root.
868: Their contents should match the respective private parts.
869: These files are not
870: really used for anything; they are provided for the convenience of
871: the user so their contents can be copied to known hosts files.
872: These files are created using
873: .Xr ssh-keygen 1 .
874: .Pp
1.258 jmc 875: .It Pa /etc/ssh/ssh_known_hosts
1.235 jmc 876: Systemwide list of known host keys.
877: This file should be prepared by the
878: system administrator to contain the public host keys of all machines in the
879: organization.
880: The format of this file is described above.
881: This file should be writable only by root/the owner and
882: should be world-readable.
883: .Pp
1.258 jmc 884: .It Pa /etc/ssh/sshd_config
1.223 jmc 885: Contains configuration data for
886: .Nm sshd .
887: The file format and configuration options are described in
888: .Xr sshd_config 5 .
889: .Pp
1.258 jmc 890: .It Pa /etc/ssh/sshrc
1.226 jmc 891: Similar to
892: .Pa ~/.ssh/rc ,
893: it can be used to specify
1.36 aaron 894: machine-specific login-time initializations globally.
895: This file should be writable only by root, and should be world-readable.
1.223 jmc 896: .Pp
1.258 jmc 897: .It Pa /var/empty
1.223 jmc 898: .Xr chroot 2
899: directory used by
900: .Nm
901: during privilege separation in the pre-authentication phase.
902: The directory should not contain any files and must be owned by root
903: and not group or world-writable.
904: .Pp
1.258 jmc 905: .It Pa /var/run/sshd.pid
1.223 jmc 906: Contains the process ID of the
907: .Nm
908: listening for connections (if there are several daemons running
909: concurrently for different ports, this contains the process ID of the one
910: started last).
911: The content of this file is not sensitive; it can be world-readable.
1.56 aaron 912: .El
1.2 deraadt 913: .Sh SEE ALSO
914: .Xr scp 1 ,
1.90 djm 915: .Xr sftp 1 ,
1.2 deraadt 916: .Xr ssh 1 ,
1.5 deraadt 917: .Xr ssh-add 1 ,
1.2 deraadt 918: .Xr ssh-agent 1 ,
1.5 deraadt 919: .Xr ssh-keygen 1 ,
1.235 jmc 920: .Xr ssh-keyscan 1 ,
1.200 jmc 921: .Xr chroot 2 ,
922: .Xr hosts_access 5 ,
1.136 markus 923: .Xr login.conf 5 ,
924: .Xr moduli 5 ,
1.184 stevesk 925: .Xr sshd_config 5 ,
1.200 jmc 926: .Xr inetd 8 ,
1.128 mpech 927: .Xr sftp-server 8
1.198 jmc 928: .Sh AUTHORS
929: OpenSSH is a derivative of the original and free
930: ssh 1.2.12 release by Tatu Ylonen.
931: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
932: Theo de Raadt and Dug Song
933: removed many bugs, re-added newer features and
934: created OpenSSH.
935: Markus Friedl contributed the support for SSH
936: protocol versions 1.5 and 2.0.
937: Niels Provos and Markus Friedl contributed support
938: for privilege separation.
1.216 jmc 939: .Sh CAVEATS
940: System security is not improved unless
941: .Nm rshd ,
942: .Nm rlogind ,
943: and
944: .Nm rexecd
945: are disabled (thus completely disabling
946: .Xr rlogin
947: and
948: .Xr rsh
949: into the machine).
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