Annotation of src/usr.bin/ssh/sshd.8, Revision 1.201
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
1.64 deraadt 7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
1.99 deraadt 13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.64 deraadt 16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 25: .\"
1.64 deraadt 26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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34: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.201 ! dtucker 37: .\" $OpenBSD: sshd.8,v 1.200 2003/10/08 08:27:36 jmc Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSHD 8
40: .Os
41: .Sh NAME
42: .Nm sshd
1.120 markus 43: .Nd OpenSSH SSH daemon
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm sshd
1.194 jmc 46: .Bk -words
1.200 jmc 47: .Op Fl 46Ddeiqt
1.2 deraadt 48: .Op Fl b Ar bits
49: .Op Fl f Ar config_file
50: .Op Fl g Ar login_grace_time
51: .Op Fl h Ar host_key_file
52: .Op Fl k Ar key_gen_time
1.156 markus 53: .Op Fl o Ar option
1.2 deraadt 54: .Op Fl p Ar port
1.61 markus 55: .Op Fl u Ar len
1.194 jmc 56: .Ek
1.40 aaron 57: .Sh DESCRIPTION
1.2 deraadt 58: .Nm
1.106 deraadt 59: (SSH Daemon) is the daemon program for
1.2 deraadt 60: .Xr ssh 1 .
1.42 hugh 61: Together these programs replace rlogin and rsh, and
1.1 deraadt 62: provide secure encrypted communications between two untrusted hosts
1.36 aaron 63: over an insecure network.
64: The programs are intended to be as easy to
1.1 deraadt 65: install and use as possible.
1.2 deraadt 66: .Pp
67: .Nm
1.36 aaron 68: is the daemon that 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.
1.49 markus 76: This implementation of
77: .Nm
78: supports both SSH protocol version 1 and 2 simultaneously.
1.2 deraadt 79: .Nm
1.194 jmc 80: works as follows:
1.49 markus 81: .Ss SSH protocol version 1
1.36 aaron 82: Each host has a host-specific RSA key
83: (normally 1024 bits) used to identify the host.
84: Additionally, when
1.1 deraadt 85: the daemon starts, it generates a server RSA key (normally 768 bits).
86: This key is normally regenerated every hour if it has been used, and
87: is never stored on disk.
1.2 deraadt 88: .Pp
1.194 jmc 89: Whenever a client connects, the daemon responds with its public
1.42 hugh 90: host and server keys.
1.36 aaron 91: The client compares the
1.49 markus 92: RSA host key against its own database to verify that it has not changed.
1.200 jmc 93: The client then generates a 256-bit random number.
1.36 aaron 94: It encrypts this
1.1 deraadt 95: random number using both the host key and the server key, and sends
1.36 aaron 96: the encrypted number to the server.
1.42 hugh 97: Both sides then use this
1.1 deraadt 98: random number as a session key which is used to encrypt all further
1.36 aaron 99: communications in the session.
100: The rest of the session is encrypted
1.42 hugh 101: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 102: being used by default.
1.36 aaron 103: The client selects the encryption algorithm
1.5 deraadt 104: to use from those offered by the server.
1.2 deraadt 105: .Pp
1.36 aaron 106: Next, the server and the client enter an authentication dialog.
107: The client tries to authenticate itself using
1.200 jmc 108: .Em rhosts
1.2 deraadt 109: authentication,
1.200 jmc 110: .Em rhosts
1.2 deraadt 111: authentication combined with RSA host
1.1 deraadt 112: authentication, RSA challenge-response authentication, or password
113: based authentication.
1.2 deraadt 114: .Pp
1.200 jmc 115: .Em rhosts
116: authentication is normally disabled
1.1 deraadt 117: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 118: configuration file if desired.
119: System security is not improved unless
1.180 millert 120: .Nm rshd ,
121: .Nm rlogind ,
1.2 deraadt 122: and
1.194 jmc 123: .Nm rexecd
1.1 deraadt 124: are disabled (thus completely disabling
1.180 millert 125: .Xr rlogin
1.1 deraadt 126: and
1.180 millert 127: .Xr rsh
1.42 hugh 128: into the machine).
1.49 markus 129: .Ss SSH protocol version 2
1.58 deraadt 130: Version 2 works similarly:
1.138 markus 131: Each host has a host-specific key (RSA or DSA) used to identify the host.
1.49 markus 132: However, when the daemon starts, it does not generate a server key.
133: Forward security is provided through a Diffie-Hellman key agreement.
134: This key agreement results in a shared session key.
1.120 markus 135: .Pp
1.103 deraadt 136: The rest of the session is encrypted using a symmetric cipher, currently
1.200 jmc 137: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
1.49 markus 138: The client selects the encryption algorithm
139: to use from those offered by the server.
140: Additionally, session integrity is provided
1.51 hugh 141: through a cryptographic message authentication code
1.49 markus 142: (hmac-sha1 or hmac-md5).
143: .Pp
144: Protocol version 2 provides a public key based
1.120 markus 145: user (PubkeyAuthentication) or
146: client host (HostbasedAuthentication) authentication method,
147: conventional password authentication and challenge response based methods.
1.49 markus 148: .Ss Command execution and data forwarding
1.1 deraadt 149: If the client successfully authenticates itself, a dialog for
1.36 aaron 150: preparing the session is entered.
151: At this time the client may request
1.1 deraadt 152: things like allocating a pseudo-tty, forwarding X11 connections,
153: forwarding TCP/IP connections, or forwarding the authentication agent
154: connection over the secure channel.
1.2 deraadt 155: .Pp
1.1 deraadt 156: Finally, the client either requests a shell or execution of a command.
1.36 aaron 157: The sides then enter session mode.
158: In this mode, either side may send
1.1 deraadt 159: data at any time, and such data is forwarded to/from the shell or
160: command on the server side, and the user terminal in the client side.
1.2 deraadt 161: .Pp
1.1 deraadt 162: When the user program terminates and all forwarded X11 and other
163: connections have been closed, the server sends command exit status to
164: the client, and both sides exit.
1.2 deraadt 165: .Pp
166: .Nm
1.200 jmc 167: can be configured using command-line options or a configuration file
168: (by default
169: .Xr sshd_config 5 ) .
1.36 aaron 170: Command-line options override values specified in the
1.1 deraadt 171: configuration file.
1.25 markus 172: .Pp
173: .Nm
174: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 175: .Dv SIGHUP ,
1.201 ! dtucker 176: by executing itself with the name and options it was started with, e.g.,
1.97 deraadt 177: .Pa /usr/sbin/sshd .
1.18 aaron 178: .Pp
179: The options are as follows:
1.2 deraadt 180: .Bl -tag -width Ds
1.200 jmc 181: .It Fl 4
182: Forces
183: .Nm
184: to use IPv4 addresses only.
185: .It Fl 6
186: Forces
187: .Nm
188: to use IPv6 addresses only.
1.2 deraadt 189: .It Fl b Ar bits
1.120 markus 190: Specifies the number of bits in the ephemeral protocol version 1
191: server key (default 768).
1.200 jmc 192: .It Fl D
193: When this option is specified,
194: .Nm
195: will not detach and does not become a daemon.
196: This allows easy monitoring of
197: .Nm sshd .
1.2 deraadt 198: .It Fl d
1.36 aaron 199: Debug mode.
200: The server sends verbose debug output to the system
201: log, and does not put itself in the background.
202: The server also will not fork and will only process one connection.
203: This option is only intended for debugging for the server.
1.194 jmc 204: Multiple
205: .Fl d
206: options increase the debugging level.
1.67 aaron 207: Maximum is 3.
1.120 markus 208: .It Fl e
209: When this option is specified,
210: .Nm
211: will send the output to the standard error instead of the system log.
1.2 deraadt 212: .It Fl f Ar configuration_file
1.36 aaron 213: Specifies the name of the configuration file.
214: The default is
1.167 deraadt 215: .Pa /etc/ssh/sshd_config .
1.16 markus 216: .Nm
217: refuses to start if there is no configuration file.
1.2 deraadt 218: .It Fl g Ar login_grace_time
1.1 deraadt 219: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 220: 120 seconds).
1.36 aaron 221: If the client fails to authenticate the user within
222: this many seconds, the server disconnects and exits.
223: A value of zero indicates no limit.
1.2 deraadt 224: .It Fl h Ar host_key_file
1.160 stevesk 225: Specifies a file from which a host key is read.
1.7 markus 226: This option must be given if
227: .Nm
228: is not run as root (as the normal
1.160 stevesk 229: host key files are normally not readable by anyone but root).
230: The default is
1.167 deraadt 231: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 232: for protocol version 1, and
1.167 deraadt 233: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 234: and
1.167 deraadt 235: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 236: for protocol version 2.
1.75 markus 237: It is possible to have multiple host key files for
1.120 markus 238: the different protocol versions and host key algorithms.
1.2 deraadt 239: .It Fl i
1.7 markus 240: Specifies that
241: .Nm
1.194 jmc 242: is being run from
243: .Xr inetd 8 .
1.7 markus 244: .Nm
245: is normally not run
1.1 deraadt 246: from inetd because it needs to generate the server key before it can
1.36 aaron 247: respond to the client, and this may take tens of seconds.
248: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 249: However, with small key sizes (e.g., 512) using
1.7 markus 250: .Nm
251: from inetd may
1.1 deraadt 252: be feasible.
1.2 deraadt 253: .It Fl k Ar key_gen_time
1.120 markus 254: Specifies how often the ephemeral protocol version 1 server key is
255: regenerated (default 3600 seconds, or one hour).
1.36 aaron 256: The motivation for regenerating the key fairly
1.200 jmc 257: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 258: it becomes impossible to recover the key for decrypting intercepted
259: communications even if the machine is cracked into or physically
1.36 aaron 260: seized.
261: A value of zero indicates that the key will never be regenerated.
1.156 markus 262: .It Fl o Ar option
263: Can be used to give options in the format used in the configuration file.
264: This is useful for specifying options for which there is no separate
265: command-line flag.
1.200 jmc 266: For full details of the options, and their values, see
267: .Xr sshd_config 5 .
1.2 deraadt 268: .It Fl p Ar port
1.1 deraadt 269: Specifies the port on which the server listens for connections
270: (default 22).
1.158 stevesk 271: Multiple port options are permitted.
272: Ports specified in the configuration file are ignored when a
273: command-line port is specified.
1.2 deraadt 274: .It Fl q
1.36 aaron 275: Quiet mode.
276: Nothing is sent to the system log.
277: Normally the beginning,
1.1 deraadt 278: authentication, and termination of each connection is logged.
1.137 stevesk 279: .It Fl t
280: Test mode.
281: Only check the validity of the configuration file and sanity of the keys.
1.157 deraadt 282: This is useful for updating
1.137 stevesk 283: .Nm
284: reliably as configuration options may change.
1.61 markus 285: .It Fl u Ar len
286: This option is used to specify the size of the field
287: in the
288: .Li utmp
289: structure that holds the remote host name.
290: If the resolved host name is longer than
291: .Ar len ,
292: the dotted decimal value will be used instead.
293: This allows hosts with very long host names that
294: overflow this field to still be uniquely identified.
295: Specifying
296: .Fl u0
297: indicates that only dotted decimal addresses
298: should be put into the
299: .Pa utmp
300: file.
1.144 stevesk 301: .Fl u0
1.194 jmc 302: may also be used to prevent
1.144 stevesk 303: .Nm
304: from making DNS requests unless the authentication
305: mechanism or configuration requires it.
306: Authentication mechanisms that may require DNS include
307: .Cm RhostsRSAAuthentication ,
308: .Cm HostbasedAuthentication
309: and using a
310: .Cm from="pattern-list"
311: option in a key file.
1.170 stevesk 312: Configuration options that require DNS include using a
313: USER@HOST pattern in
314: .Cm AllowUsers
315: or
316: .Cm DenyUsers .
1.2 deraadt 317: .El
318: .Sh CONFIGURATION FILE
319: .Nm
1.40 aaron 320: reads configuration data from
1.167 deraadt 321: .Pa /etc/ssh/sshd_config
1.2 deraadt 322: (or the file specified with
323: .Fl f
1.36 aaron 324: on the command line).
1.184 stevesk 325: The file format and configuration options are described in
326: .Xr sshd_config 5 .
1.2 deraadt 327: .Sh LOGIN PROCESS
1.1 deraadt 328: When a user successfully logs in,
1.2 deraadt 329: .Nm
1.1 deraadt 330: does the following:
1.2 deraadt 331: .Bl -enum -offset indent
332: .It
1.1 deraadt 333: If the login is on a tty, and no command has been specified,
1.40 aaron 334: prints last login time and
1.2 deraadt 335: .Pa /etc/motd
1.1 deraadt 336: (unless prevented in the configuration file or by
1.2 deraadt 337: .Pa $HOME/.hushlogin ;
338: see the
1.40 aaron 339: .Sx FILES
1.2 deraadt 340: section).
341: .It
1.1 deraadt 342: If the login is on a tty, records login time.
1.2 deraadt 343: .It
344: Checks
345: .Pa /etc/nologin ;
346: if it exists, prints contents and quits
1.1 deraadt 347: (unless root).
1.2 deraadt 348: .It
1.1 deraadt 349: Changes to run with normal user privileges.
1.2 deraadt 350: .It
1.1 deraadt 351: Sets up basic environment.
1.2 deraadt 352: .It
1.200 jmc 353: Reads the file
354: .Pa $HOME/.ssh/environment ,
355: if it exists, and users are allowed to change their environment.
1.188 stevesk 356: See the
1.187 marc 357: .Cm PermitUserEnvironment
1.188 stevesk 358: option in
1.187 marc 359: .Xr sshd_config 5 .
1.2 deraadt 360: .It
1.1 deraadt 361: Changes to user's home directory.
1.2 deraadt 362: .It
363: If
364: .Pa $HOME/.ssh/rc
365: exists, runs it; else if
1.168 deraadt 366: .Pa /etc/ssh/sshrc
1.2 deraadt 367: exists, runs
1.36 aaron 368: it; otherwise runs xauth.
369: The
1.2 deraadt 370: .Dq rc
371: files are given the X11
1.1 deraadt 372: authentication protocol and cookie in standard input.
1.2 deraadt 373: .It
1.1 deraadt 374: Runs user's shell or command.
1.2 deraadt 375: .El
376: .Sh AUTHORIZED_KEYS FILE FORMAT
377: .Pa $HOME/.ssh/authorized_keys
1.130 markus 378: is the default file that lists the public keys that are
379: permitted for RSA authentication in protocol version 1
380: and for public key authentication (PubkeyAuthentication)
381: in protocol version 2.
1.125 markus 382: .Cm AuthorizedKeysFile
383: may be used to specify an alternative file.
1.75 markus 384: .Pp
1.36 aaron 385: Each line of the file contains one
1.2 deraadt 386: key (empty lines and lines starting with a
387: .Ql #
388: are ignored as
1.36 aaron 389: comments).
1.75 markus 390: Each RSA public key consists of the following fields, separated by
1.36 aaron 391: spaces: options, bits, exponent, modulus, comment.
1.75 markus 392: Each protocol version 2 public key consists of:
393: options, keytype, base64 encoded key, comment.
1.189 espie 394: The options field
395: is optional; its presence is determined by whether the line starts
396: with a number or not (the options field never starts with a number).
1.75 markus 397: The bits, exponent, modulus and comment fields give the RSA key for
398: protocol version 1; the
1.1 deraadt 399: comment field is not used for anything (but may be convenient for the
400: user to identify the key).
1.75 markus 401: For protocol version 2 the keytype is
402: .Dq ssh-dss
403: or
404: .Dq ssh-rsa .
1.2 deraadt 405: .Pp
1.1 deraadt 406: Note that lines in this file are usually several hundred bytes long
1.189 espie 407: (because of the size of the public key encoding).
1.36 aaron 408: You don't want to type them in; instead, copy the
1.113 itojun 409: .Pa identity.pub ,
410: .Pa id_dsa.pub
1.75 markus 411: or the
1.113 itojun 412: .Pa id_rsa.pub
1.1 deraadt 413: file and edit it.
1.175 stevesk 414: .Pp
415: .Nm
416: enforces a minimum RSA key modulus size for protocol 1
417: and protocol 2 keys of 768 bits.
1.2 deraadt 418: .Pp
1.58 deraadt 419: The options (if present) consist of comma-separated option
1.36 aaron 420: specifications.
421: No spaces are permitted, except within double quotes.
1.141 stevesk 422: The following option specifications are supported (note
423: that option keywords are case-insensitive):
1.2 deraadt 424: .Bl -tag -width Ds
425: .It Cm from="pattern-list"
1.189 espie 426: Specifies that in addition to public key authentication, the canonical name
1.1 deraadt 427: of the remote host must be present in the comma-separated list of
1.36 aaron 428: patterns
1.196 david 429: .Pf ( Ql \&*
1.36 aaron 430: and
1.195 mouring 431: .Ql \&?
1.36 aaron 432: serve as wildcards).
433: The list may also contain
434: patterns negated by prefixing them with
1.195 mouring 435: .Ql \&! ;
1.36 aaron 436: if the canonical host name matches a negated pattern, the key is not accepted.
437: The purpose
1.189 espie 438: of this option is to optionally increase security: public key authentication
1.1 deraadt 439: by itself does not trust the network or name servers or anything (but
440: the key); however, if somebody somehow steals the key, the key
1.36 aaron 441: permits an intruder to log in from anywhere in the world.
442: This additional option makes using a stolen key more difficult (name
1.1 deraadt 443: servers and/or routers would have to be compromised in addition to
444: just the key).
1.2 deraadt 445: .It Cm command="command"
1.1 deraadt 446: Specifies that the command is executed whenever this key is used for
1.36 aaron 447: authentication.
448: The command supplied by the user (if any) is ignored.
1.148 markus 449: The command is run on a pty if the client requests a pty;
1.36 aaron 450: otherwise it is run without a tty.
1.194 jmc 451: If an 8-bit clean channel is required,
1.147 deraadt 452: one must not request a pty or should specify
1.89 markus 453: .Cm no-pty .
1.36 aaron 454: A quote may be included in the command by quoting it with a backslash.
455: This option might be useful
1.189 espie 456: to restrict certain public keys to perform just a specific operation.
1.36 aaron 457: An example might be a key that permits remote backups but nothing else.
1.51 hugh 458: Note that the client may specify TCP/IP and/or X11
459: forwarding unless they are explicitly prohibited.
1.149 markus 460: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 461: .It Cm environment="NAME=value"
1.1 deraadt 462: Specifies that the string is to be added to the environment when
1.36 aaron 463: logging in using this key.
464: Environment variables set this way
465: override other default environment values.
466: Multiple options of this type are permitted.
1.188 stevesk 467: Environment processing is disabled by default and is
468: controlled via the
469: .Cm PermitUserEnvironment
470: option.
1.155 markus 471: This option is automatically disabled if
472: .Cm UseLogin
473: is enabled.
1.2 deraadt 474: .It Cm no-port-forwarding
1.1 deraadt 475: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 476: Any port forward requests by the client will return an error.
477: This might be used, e.g., in connection with the
1.2 deraadt 478: .Cm command
1.1 deraadt 479: option.
1.2 deraadt 480: .It Cm no-X11-forwarding
1.1 deraadt 481: Forbids X11 forwarding when this key is used for authentication.
482: Any X11 forward requests by the client will return an error.
1.2 deraadt 483: .It Cm no-agent-forwarding
1.1 deraadt 484: Forbids authentication agent forwarding when this key is used for
485: authentication.
1.2 deraadt 486: .It Cm no-pty
1.1 deraadt 487: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 488: .It Cm permitopen="host:port"
1.133 itojun 489: Limit local
1.107 djm 490: .Li ``ssh -L''
1.111 stevesk 491: port forwarding such that it may only connect to the specified host and
1.146 stevesk 492: port.
493: IPv6 addresses can be specified with an alternative syntax:
1.200 jmc 494: .Ar host Ns / Ns Ar port .
1.146 stevesk 495: Multiple
1.107 djm 496: .Cm permitopen
1.197 jmc 497: options may be applied separated by commas.
498: No pattern matching is performed on the specified hostnames,
499: they must be literal domains or addresses.
1.2 deraadt 500: .El
501: .Ss Examples
1.200 jmc 502: 1024 33 12121...312314325 ylo@foo.bar
1.2 deraadt 503: .Pp
1.200 jmc 504: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
1.2 deraadt 505: .Pp
1.200 jmc 506: command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
1.107 djm 507: .Pp
1.200 jmc 508: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
1.2 deraadt 509: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 510: The
1.194 jmc 511: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 512: and
1.131 markus 513: .Pa $HOME/.ssh/known_hosts
1.36 aaron 514: files contain host public keys for all known hosts.
515: The global file should
1.37 brad 516: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 517: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 518: its key is added to the per-user file.
1.2 deraadt 519: .Pp
1.1 deraadt 520: Each line in these files contains the following fields: hostnames,
1.36 aaron 521: bits, exponent, modulus, comment.
522: The fields are separated by spaces.
1.2 deraadt 523: .Pp
1.196 david 524: Hostnames is a comma-separated list of patterns
1.197 jmc 525: .Pf ( Ql \&*
526: and
527: .Ql \&?
1.195 mouring 528: act as
1.1 deraadt 529: wildcards); each pattern in turn is matched against the canonical host
530: name (when authenticating a client) or against the user-supplied
1.36 aaron 531: name (when authenticating a server).
532: A pattern may also be preceded by
1.195 mouring 533: .Ql \&!
1.2 deraadt 534: to indicate negation: if the host name matches a negated
1.1 deraadt 535: pattern, it is not accepted (by that line) even if it matched another
536: pattern on the line.
1.2 deraadt 537: .Pp
1.49 markus 538: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 539: can be obtained, e.g., from
1.167 deraadt 540: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 541: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 542: .Pp
543: Lines starting with
544: .Ql #
545: and empty lines are ignored as comments.
546: .Pp
1.1 deraadt 547: When performing host authentication, authentication is accepted if any
1.36 aaron 548: matching line has the proper key.
549: It is thus permissible (but not
1.1 deraadt 550: recommended) to have several lines or different host keys for the same
1.36 aaron 551: names.
552: This will inevitably happen when short forms of host names
553: from different domains are put in the file.
554: It is possible
1.1 deraadt 555: that the files contain conflicting information; authentication is
556: accepted if valid information can be found from either file.
1.2 deraadt 557: .Pp
1.1 deraadt 558: Note that the lines in these files are typically hundreds of characters
559: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 560: Rather, generate them by a script
1.40 aaron 561: or by taking
1.167 deraadt 562: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 563: and adding the host names at the front.
1.2 deraadt 564: .Ss Examples
1.120 markus 565: .Bd -literal
1.200 jmc 566: closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
1.120 markus 567: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
568: .Ed
1.2 deraadt 569: .Sh FILES
570: .Bl -tag -width Ds
1.167 deraadt 571: .It Pa /etc/ssh/sshd_config
1.1 deraadt 572: Contains configuration data for
1.2 deraadt 573: .Nm sshd .
1.184 stevesk 574: The file format and configuration options are described in
575: .Xr sshd_config 5 .
1.167 deraadt 576: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.120 markus 577: These three files contain the private parts of the host keys.
1.98 deraadt 578: These files should only be owned by root, readable only by root, and not
1.1 deraadt 579: accessible to others.
1.14 markus 580: Note that
581: .Nm
582: does not start if this file is group/world-accessible.
1.167 deraadt 583: .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
1.120 markus 584: These three files contain the public parts of the host keys.
1.98 deraadt 585: These files should be world-readable but writable only by
1.36 aaron 586: root.
1.98 deraadt 587: Their contents should match the respective private parts.
588: These files are not
589: really used for anything; they are provided for the convenience of
590: the user so their contents can be copied to known hosts files.
591: These files are created using
1.7 markus 592: .Xr ssh-keygen 1 .
1.129 provos 593: .It Pa /etc/moduli
1.73 provos 594: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.192 stevesk 595: The file format is described in
596: .Xr moduli 5 .
1.185 stevesk 597: .It Pa /var/empty
598: .Xr chroot 2
599: directory used by
600: .Nm
601: during privilege separation in the pre-authentication phase.
602: The directory should not contain any files and must be owned by root
603: and not group or world-writable.
1.2 deraadt 604: .It Pa /var/run/sshd.pid
605: Contains the process ID of the
606: .Nm
1.1 deraadt 607: listening for connections (if there are several daemons running
1.186 stevesk 608: concurrently for different ports, this contains the process ID of the one
1.36 aaron 609: started last).
1.58 deraadt 610: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 611: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 612: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 613: This file must be readable by root (which may on some machines imply
614: it being world-readable if the user's home directory resides on an NFS
615: volume).
616: It is recommended that it not be accessible by others.
617: The format of this file is described above.
618: Users will place the contents of their
1.130 markus 619: .Pa identity.pub ,
1.44 deraadt 620: .Pa id_dsa.pub
1.113 itojun 621: and/or
622: .Pa id_rsa.pub
1.44 deraadt 623: files into this file, as described in
624: .Xr ssh-keygen 1 .
1.200 jmc 625: .It Pa "/etc/ssh/ssh_known_hosts", "$HOME/.ssh/known_hosts"
1.22 markus 626: These files are consulted when using rhosts with RSA host
1.131 markus 627: authentication or protocol version 2 hostbased authentication
628: to check the public key of the host.
1.36 aaron 629: The key must be listed in one of these files to be accepted.
1.22 markus 630: The client uses the same files
1.96 markus 631: to verify that it is connecting to the correct remote host.
1.36 aaron 632: These files should be writable only by root/the owner.
1.167 deraadt 633: .Pa /etc/ssh/ssh_known_hosts
1.2 deraadt 634: should be world-readable, and
635: .Pa $HOME/.ssh/known_hosts
1.194 jmc 636: can, but need not be, world-readable.
1.6 aaron 637: .It Pa /etc/nologin
1.40 aaron 638: If this file exists,
1.2 deraadt 639: .Nm
1.36 aaron 640: refuses to let anyone except root log in.
641: The contents of the file
1.1 deraadt 642: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 643: refused.
644: The file should be world-readable.
1.19 dugsong 645: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 646: Access controls that should be enforced by tcp-wrappers are defined here.
647: Further details are described in
1.19 dugsong 648: .Xr hosts_access 5 .
1.6 aaron 649: .It Pa $HOME/.rhosts
1.1 deraadt 650: This file contains host-username pairs, separated by a space, one per
1.36 aaron 651: line.
652: The given user on the corresponding host is permitted to log in
1.194 jmc 653: without a password.
1.36 aaron 654: The same file is used by rlogind and rshd.
1.6 aaron 655: The file must
1.1 deraadt 656: be writable only by the user; it is recommended that it not be
657: accessible by others.
1.2 deraadt 658: .Pp
1.200 jmc 659: It is also possible to use netgroups in the file.
1.36 aaron 660: Either host or user
1.1 deraadt 661: name may be of the form +@groupname to specify all hosts or all users
662: in the group.
1.2 deraadt 663: .It Pa $HOME/.shosts
664: For ssh,
665: this file is exactly the same as for
666: .Pa .rhosts .
667: However, this file is
668: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 669: .It Pa /etc/hosts.equiv
1.2 deraadt 670: This file is used during
1.200 jmc 671: .Em rhosts
1.36 aaron 672: authentication.
673: In the simplest form, this file contains host names, one per line.
674: Users on
1.1 deraadt 675: those hosts are permitted to log in without a password, provided they
1.36 aaron 676: have the same user name on both machines.
677: The host name may also be
1.1 deraadt 678: followed by a user name; such users are permitted to log in as
1.2 deraadt 679: .Em any
1.36 aaron 680: user on this machine (except root).
681: Additionally, the syntax
1.2 deraadt 682: .Dq +@group
1.36 aaron 683: can be used to specify netgroups.
684: Negated entries start with
1.2 deraadt 685: .Ql \&- .
686: .Pp
1.1 deraadt 687: If the client host/user is successfully matched in this file, login is
688: automatically permitted provided the client and server user names are the
1.36 aaron 689: same.
690: Additionally, successful RSA host authentication is normally required.
691: This file must be writable only by root; it is recommended
1.1 deraadt 692: that it be world-readable.
1.2 deraadt 693: .Pp
1.6 aaron 694: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 695: .Pa hosts.equiv .
1.1 deraadt 696: Beware that it really means that the named user(s) can log in as
1.2 deraadt 697: .Em anybody ,
1.1 deraadt 698: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 699: binaries and directories.
700: Using a user name practically grants the user root access.
701: The only valid use for user names that I can think
1.1 deraadt 702: of is in negative entries.
1.2 deraadt 703: .Pp
704: Note that this warning also applies to rsh/rlogin.
705: .It Pa /etc/shosts.equiv
1.1 deraadt 706: This is processed exactly as
1.2 deraadt 707: .Pa /etc/hosts.equiv .
1.1 deraadt 708: However, this file may be useful in environments that want to run both
1.2 deraadt 709: rsh/rlogin and ssh.
1.6 aaron 710: .It Pa $HOME/.ssh/environment
1.36 aaron 711: This file is read into the environment at login (if it exists).
712: It can only contain empty lines, comment lines (that start with
1.2 deraadt 713: .Ql # ) ,
1.36 aaron 714: and assignment lines of the form name=value.
715: The file should be writable
1.6 aaron 716: only by the user; it need not be readable by anyone else.
1.188 stevesk 717: Environment processing is disabled by default and is
718: controlled via the
719: .Cm PermitUserEnvironment
720: option.
1.2 deraadt 721: .It Pa $HOME/.ssh/rc
1.194 jmc 722: If this file exists, it is run with
723: .Pa /bin/sh
724: after reading the
1.36 aaron 725: environment files but before starting the user's shell or command.
1.176 stevesk 726: It must not produce any output on stdout; stderr must be used
727: instead.
728: If X11 forwarding is in use, it will receive the "proto cookie" pair in
729: its standard input (and
1.2 deraadt 730: .Ev DISPLAY
1.176 stevesk 731: in its environment).
732: The script must call
1.2 deraadt 733: .Xr xauth 1
1.176 stevesk 734: because
735: .Nm
736: will not run xauth automatically to add X11 cookies.
1.2 deraadt 737: .Pp
1.1 deraadt 738: The primary purpose of this file is to run any initialization routines
739: which may be needed before the user's home directory becomes
740: accessible; AFS is a particular example of such an environment.
1.2 deraadt 741: .Pp
1.1 deraadt 742: This file will probably contain some initialization code followed by
1.120 markus 743: something similar to:
744: .Bd -literal
1.176 stevesk 745: if read proto cookie && [ -n "$DISPLAY" ]; then
746: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
747: # X11UseLocalhost=yes
1.193 todd 748: echo add unix:`echo $DISPLAY |
1.176 stevesk 749: cut -c11-` $proto $cookie
750: else
751: # X11UseLocalhost=no
1.193 todd 752: echo add $DISPLAY $proto $cookie
753: fi | xauth -q -
1.176 stevesk 754: fi
1.120 markus 755: .Ed
1.2 deraadt 756: .Pp
757: If this file does not exist,
1.167 deraadt 758: .Pa /etc/ssh/sshrc
1.2 deraadt 759: is run, and if that
1.176 stevesk 760: does not exist either, xauth is used to add the cookie.
1.2 deraadt 761: .Pp
1.1 deraadt 762: This file should be writable only by the user, and need not be
763: readable by anyone else.
1.167 deraadt 764: .It Pa /etc/ssh/sshrc
1.2 deraadt 765: Like
766: .Pa $HOME/.ssh/rc .
767: This can be used to specify
1.36 aaron 768: machine-specific login-time initializations globally.
769: This file should be writable only by root, and should be world-readable.
1.56 aaron 770: .El
1.2 deraadt 771: .Sh SEE ALSO
772: .Xr scp 1 ,
1.90 djm 773: .Xr sftp 1 ,
1.2 deraadt 774: .Xr ssh 1 ,
1.5 deraadt 775: .Xr ssh-add 1 ,
1.2 deraadt 776: .Xr ssh-agent 1 ,
1.5 deraadt 777: .Xr ssh-keygen 1 ,
1.200 jmc 778: .Xr chroot 2 ,
779: .Xr hosts_access 5 ,
1.136 markus 780: .Xr login.conf 5 ,
781: .Xr moduli 5 ,
1.184 stevesk 782: .Xr sshd_config 5 ,
1.200 jmc 783: .Xr inetd 8 ,
1.128 mpech 784: .Xr sftp-server 8
1.119 markus 785: .Rs
786: .%A T. Ylonen
787: .%A T. Kivinen
788: .%A M. Saarinen
789: .%A T. Rinne
790: .%A S. Lehtinen
791: .%T "SSH Protocol Architecture"
1.174 markus 792: .%N draft-ietf-secsh-architecture-12.txt
793: .%D January 2002
1.120 markus 794: .%O work in progress material
795: .Re
796: .Rs
797: .%A M. Friedl
798: .%A N. Provos
799: .%A W. A. Simpson
800: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.174 markus 801: .%N draft-ietf-secsh-dh-group-exchange-02.txt
802: .%D January 2002
1.119 markus 803: .%O work in progress material
804: .Re
1.198 jmc 805: .Sh AUTHORS
806: OpenSSH is a derivative of the original and free
807: ssh 1.2.12 release by Tatu Ylonen.
808: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
809: Theo de Raadt and Dug Song
810: removed many bugs, re-added newer features and
811: created OpenSSH.
812: Markus Friedl contributed the support for SSH
813: protocol versions 1.5 and 2.0.
814: Niels Provos and Markus Friedl contributed support
815: for privilege separation.
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