Annotation of src/usr.bin/ssh/sshd.8, Revision 1.199
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,
32: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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: .\"
1.199 ! markus 37: .\" $OpenBSD: sshd.8,v 1.198 2003/06/10 09:12:12 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
1.150 stevesk 47: .Op Fl deiqtD46
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: .Pp
82: .Ss SSH protocol version 1
83: .Pp
1.36 aaron 84: Each host has a host-specific RSA key
85: (normally 1024 bits) used to identify the host.
86: Additionally, when
1.1 deraadt 87: the daemon starts, it generates a server RSA key (normally 768 bits).
88: This key is normally regenerated every hour if it has been used, and
89: is never stored on disk.
1.2 deraadt 90: .Pp
1.194 jmc 91: Whenever a client connects, the daemon responds with its public
1.42 hugh 92: host and server keys.
1.36 aaron 93: The client compares the
1.49 markus 94: RSA host key against its own database to verify that it has not changed.
1.36 aaron 95: The client then generates a 256 bit random number.
96: It encrypts this
1.1 deraadt 97: random number using both the host key and the server key, and sends
1.36 aaron 98: the encrypted number to the server.
1.42 hugh 99: Both sides then use this
1.1 deraadt 100: random number as a session key which is used to encrypt all further
1.36 aaron 101: communications in the session.
102: The rest of the session is encrypted
1.42 hugh 103: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 104: being used by default.
1.36 aaron 105: The client selects the encryption algorithm
1.5 deraadt 106: to use from those offered by the server.
1.2 deraadt 107: .Pp
1.36 aaron 108: Next, the server and the client enter an authentication dialog.
109: The client tries to authenticate itself using
1.2 deraadt 110: .Pa .rhosts
111: authentication,
112: .Pa .rhosts
113: authentication combined with RSA host
1.1 deraadt 114: authentication, RSA challenge-response authentication, or password
115: based authentication.
1.2 deraadt 116: .Pp
1.1 deraadt 117: Rhosts authentication is normally disabled
118: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 119: configuration file if desired.
120: System security is not improved unless
1.180 millert 121: .Nm rshd ,
122: .Nm rlogind ,
1.2 deraadt 123: and
1.194 jmc 124: .Nm rexecd
1.1 deraadt 125: are disabled (thus completely disabling
1.180 millert 126: .Xr rlogin
1.1 deraadt 127: and
1.180 millert 128: .Xr rsh
1.42 hugh 129: into the machine).
1.2 deraadt 130: .Pp
1.49 markus 131: .Ss SSH protocol version 2
132: .Pp
1.58 deraadt 133: Version 2 works similarly:
1.138 markus 134: Each host has a host-specific key (RSA or DSA) used to identify the host.
1.49 markus 135: However, when the daemon starts, it does not generate a server key.
136: Forward security is provided through a Diffie-Hellman key agreement.
137: This key agreement results in a shared session key.
1.120 markus 138: .Pp
1.103 deraadt 139: The rest of the session is encrypted using a symmetric cipher, currently
1.120 markus 140: 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
1.49 markus 141: The client selects the encryption algorithm
142: to use from those offered by the server.
143: Additionally, session integrity is provided
1.51 hugh 144: through a cryptographic message authentication code
1.49 markus 145: (hmac-sha1 or hmac-md5).
146: .Pp
147: Protocol version 2 provides a public key based
1.120 markus 148: user (PubkeyAuthentication) or
149: client host (HostbasedAuthentication) authentication method,
150: conventional password authentication and challenge response based methods.
1.49 markus 151: .Pp
152: .Ss Command execution and data forwarding
153: .Pp
1.1 deraadt 154: If the client successfully authenticates itself, a dialog for
1.36 aaron 155: preparing the session is entered.
156: At this time the client may request
1.1 deraadt 157: things like allocating a pseudo-tty, forwarding X11 connections,
158: forwarding TCP/IP connections, or forwarding the authentication agent
159: connection over the secure channel.
1.2 deraadt 160: .Pp
1.1 deraadt 161: Finally, the client either requests a shell or execution of a command.
1.36 aaron 162: The sides then enter session mode.
163: In this mode, either side may send
1.1 deraadt 164: data at any time, and such data is forwarded to/from the shell or
165: command on the server side, and the user terminal in the client side.
1.2 deraadt 166: .Pp
1.1 deraadt 167: When the user program terminates and all forwarded X11 and other
168: connections have been closed, the server sends command exit status to
169: the client, and both sides exit.
1.2 deraadt 170: .Pp
171: .Nm
1.1 deraadt 172: can be configured using command-line options or a configuration
1.36 aaron 173: file.
174: Command-line options override values specified in the
1.1 deraadt 175: configuration file.
1.25 markus 176: .Pp
177: .Nm
178: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 179: .Dv SIGHUP ,
1.128 mpech 180: by executing itself with the name it was started as, i.e.,
1.97 deraadt 181: .Pa /usr/sbin/sshd .
1.18 aaron 182: .Pp
183: The options are as follows:
1.2 deraadt 184: .Bl -tag -width Ds
185: .It Fl b Ar bits
1.120 markus 186: Specifies the number of bits in the ephemeral protocol version 1
187: server key (default 768).
1.2 deraadt 188: .It Fl d
1.36 aaron 189: Debug mode.
190: The server sends verbose debug output to the system
191: log, and does not put itself in the background.
192: The server also will not fork and will only process one connection.
193: This option is only intended for debugging for the server.
1.194 jmc 194: Multiple
195: .Fl d
196: options increase the debugging level.
1.67 aaron 197: Maximum is 3.
1.120 markus 198: .It Fl e
199: When this option is specified,
200: .Nm
201: will send the output to the standard error instead of the system log.
1.2 deraadt 202: .It Fl f Ar configuration_file
1.36 aaron 203: Specifies the name of the configuration file.
204: The default is
1.167 deraadt 205: .Pa /etc/ssh/sshd_config .
1.16 markus 206: .Nm
207: refuses to start if there is no configuration file.
1.2 deraadt 208: .It Fl g Ar login_grace_time
1.1 deraadt 209: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 210: 120 seconds).
1.36 aaron 211: If the client fails to authenticate the user within
212: this many seconds, the server disconnects and exits.
213: A value of zero indicates no limit.
1.2 deraadt 214: .It Fl h Ar host_key_file
1.160 stevesk 215: Specifies a file from which a host key is read.
1.7 markus 216: This option must be given if
217: .Nm
218: is not run as root (as the normal
1.160 stevesk 219: host key files are normally not readable by anyone but root).
220: The default is
1.167 deraadt 221: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 222: for protocol version 1, and
1.167 deraadt 223: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 224: and
1.167 deraadt 225: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 226: for protocol version 2.
1.75 markus 227: It is possible to have multiple host key files for
1.120 markus 228: the different protocol versions and host key algorithms.
1.2 deraadt 229: .It Fl i
1.7 markus 230: Specifies that
231: .Nm
1.194 jmc 232: is being run from
233: .Xr inetd 8 .
1.7 markus 234: .Nm
235: is normally not run
1.1 deraadt 236: from inetd because it needs to generate the server key before it can
1.36 aaron 237: respond to the client, and this may take tens of seconds.
238: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 239: However, with small key sizes (e.g., 512) using
1.7 markus 240: .Nm
241: from inetd may
1.1 deraadt 242: be feasible.
1.2 deraadt 243: .It Fl k Ar key_gen_time
1.120 markus 244: Specifies how often the ephemeral protocol version 1 server key is
245: regenerated (default 3600 seconds, or one hour).
1.36 aaron 246: The motivation for regenerating the key fairly
1.1 deraadt 247: often is that the key is not stored anywhere, and after about an hour,
248: it becomes impossible to recover the key for decrypting intercepted
249: communications even if the machine is cracked into or physically
1.36 aaron 250: seized.
251: A value of zero indicates that the key will never be regenerated.
1.156 markus 252: .It Fl o Ar option
253: Can be used to give options in the format used in the configuration file.
254: This is useful for specifying options for which there is no separate
255: command-line flag.
1.2 deraadt 256: .It Fl p Ar port
1.1 deraadt 257: Specifies the port on which the server listens for connections
258: (default 22).
1.158 stevesk 259: Multiple port options are permitted.
260: Ports specified in the configuration file are ignored when a
261: command-line port is specified.
1.2 deraadt 262: .It Fl q
1.36 aaron 263: Quiet mode.
264: Nothing is sent to the system log.
265: Normally the beginning,
1.1 deraadt 266: authentication, and termination of each connection is logged.
1.137 stevesk 267: .It Fl t
268: Test mode.
269: Only check the validity of the configuration file and sanity of the keys.
1.157 deraadt 270: This is useful for updating
1.137 stevesk 271: .Nm
272: reliably as configuration options may change.
1.61 markus 273: .It Fl u Ar len
274: This option is used to specify the size of the field
275: in the
276: .Li utmp
277: structure that holds the remote host name.
278: If the resolved host name is longer than
279: .Ar len ,
280: the dotted decimal value will be used instead.
281: This allows hosts with very long host names that
282: overflow this field to still be uniquely identified.
283: Specifying
284: .Fl u0
285: indicates that only dotted decimal addresses
286: should be put into the
287: .Pa utmp
288: file.
1.144 stevesk 289: .Fl u0
1.194 jmc 290: may also be used to prevent
1.144 stevesk 291: .Nm
292: from making DNS requests unless the authentication
293: mechanism or configuration requires it.
294: Authentication mechanisms that may require DNS include
295: .Cm RhostsRSAAuthentication ,
296: .Cm HostbasedAuthentication
297: and using a
298: .Cm from="pattern-list"
299: option in a key file.
1.170 stevesk 300: Configuration options that require DNS include using a
301: USER@HOST pattern in
302: .Cm AllowUsers
303: or
304: .Cm DenyUsers .
1.74 markus 305: .It Fl D
306: When this option is specified
307: .Nm
308: will not detach and does not become a daemon.
309: This allows easy monitoring of
1.76 markus 310: .Nm sshd .
1.29 markus 311: .It Fl 4
312: Forces
313: .Nm
314: to use IPv4 addresses only.
315: .It Fl 6
316: Forces
317: .Nm
318: to use IPv6 addresses only.
1.2 deraadt 319: .El
320: .Sh CONFIGURATION FILE
321: .Nm
1.40 aaron 322: reads configuration data from
1.167 deraadt 323: .Pa /etc/ssh/sshd_config
1.2 deraadt 324: (or the file specified with
325: .Fl f
1.36 aaron 326: on the command line).
1.184 stevesk 327: The file format and configuration options are described in
328: .Xr sshd_config 5 .
1.2 deraadt 329: .Sh LOGIN PROCESS
1.1 deraadt 330: When a user successfully logs in,
1.2 deraadt 331: .Nm
1.1 deraadt 332: does the following:
1.2 deraadt 333: .Bl -enum -offset indent
334: .It
1.1 deraadt 335: If the login is on a tty, and no command has been specified,
1.40 aaron 336: prints last login time and
1.2 deraadt 337: .Pa /etc/motd
1.1 deraadt 338: (unless prevented in the configuration file or by
1.2 deraadt 339: .Pa $HOME/.hushlogin ;
340: see the
1.40 aaron 341: .Sx FILES
1.2 deraadt 342: section).
343: .It
1.1 deraadt 344: If the login is on a tty, records login time.
1.2 deraadt 345: .It
346: Checks
347: .Pa /etc/nologin ;
348: if it exists, prints contents and quits
1.1 deraadt 349: (unless root).
1.2 deraadt 350: .It
1.1 deraadt 351: Changes to run with normal user privileges.
1.2 deraadt 352: .It
1.1 deraadt 353: Sets up basic environment.
1.2 deraadt 354: .It
355: Reads
356: .Pa $HOME/.ssh/environment
1.187 marc 357: if it exists and users are allowed to change their environment.
1.188 stevesk 358: See the
1.187 marc 359: .Cm PermitUserEnvironment
1.188 stevesk 360: option in
1.187 marc 361: .Xr sshd_config 5 .
1.2 deraadt 362: .It
1.1 deraadt 363: Changes to user's home directory.
1.2 deraadt 364: .It
365: If
366: .Pa $HOME/.ssh/rc
367: exists, runs it; else if
1.168 deraadt 368: .Pa /etc/ssh/sshrc
1.2 deraadt 369: exists, runs
1.36 aaron 370: it; otherwise runs xauth.
371: The
1.2 deraadt 372: .Dq rc
373: files are given the X11
1.1 deraadt 374: authentication protocol and cookie in standard input.
1.2 deraadt 375: .It
1.1 deraadt 376: Runs user's shell or command.
1.2 deraadt 377: .El
378: .Sh AUTHORIZED_KEYS FILE FORMAT
379: .Pa $HOME/.ssh/authorized_keys
1.130 markus 380: is the default file that lists the public keys that are
381: permitted for RSA authentication in protocol version 1
382: and for public key authentication (PubkeyAuthentication)
383: in protocol version 2.
1.125 markus 384: .Cm AuthorizedKeysFile
385: may be used to specify an alternative file.
1.75 markus 386: .Pp
1.36 aaron 387: Each line of the file contains one
1.2 deraadt 388: key (empty lines and lines starting with a
389: .Ql #
390: are ignored as
1.36 aaron 391: comments).
1.75 markus 392: Each RSA public key consists of the following fields, separated by
1.36 aaron 393: spaces: options, bits, exponent, modulus, comment.
1.75 markus 394: Each protocol version 2 public key consists of:
395: options, keytype, base64 encoded key, comment.
1.189 espie 396: The options field
397: is optional; its presence is determined by whether the line starts
398: with a number or not (the options field never starts with a number).
1.75 markus 399: The bits, exponent, modulus and comment fields give the RSA key for
400: protocol version 1; the
1.1 deraadt 401: comment field is not used for anything (but may be convenient for the
402: user to identify the key).
1.75 markus 403: For protocol version 2 the keytype is
404: .Dq ssh-dss
405: or
406: .Dq ssh-rsa .
1.2 deraadt 407: .Pp
1.1 deraadt 408: Note that lines in this file are usually several hundred bytes long
1.189 espie 409: (because of the size of the public key encoding).
1.36 aaron 410: You don't want to type them in; instead, copy the
1.113 itojun 411: .Pa identity.pub ,
412: .Pa id_dsa.pub
1.75 markus 413: or the
1.113 itojun 414: .Pa id_rsa.pub
1.1 deraadt 415: file and edit it.
1.175 stevesk 416: .Pp
417: .Nm
418: enforces a minimum RSA key modulus size for protocol 1
419: and protocol 2 keys of 768 bits.
1.2 deraadt 420: .Pp
1.58 deraadt 421: The options (if present) consist of comma-separated option
1.36 aaron 422: specifications.
423: No spaces are permitted, except within double quotes.
1.141 stevesk 424: The following option specifications are supported (note
425: that option keywords are case-insensitive):
1.2 deraadt 426: .Bl -tag -width Ds
427: .It Cm from="pattern-list"
1.189 espie 428: Specifies that in addition to public key authentication, the canonical name
1.1 deraadt 429: of the remote host must be present in the comma-separated list of
1.36 aaron 430: patterns
1.196 david 431: .Pf ( Ql \&*
1.36 aaron 432: and
1.195 mouring 433: .Ql \&?
1.36 aaron 434: serve as wildcards).
435: The list may also contain
436: patterns negated by prefixing them with
1.195 mouring 437: .Ql \&! ;
1.36 aaron 438: if the canonical host name matches a negated pattern, the key is not accepted.
439: The purpose
1.189 espie 440: of this option is to optionally increase security: public key authentication
1.1 deraadt 441: by itself does not trust the network or name servers or anything (but
442: the key); however, if somebody somehow steals the key, the key
1.36 aaron 443: permits an intruder to log in from anywhere in the world.
444: This additional option makes using a stolen key more difficult (name
1.1 deraadt 445: servers and/or routers would have to be compromised in addition to
446: just the key).
1.2 deraadt 447: .It Cm command="command"
1.1 deraadt 448: Specifies that the command is executed whenever this key is used for
1.36 aaron 449: authentication.
450: The command supplied by the user (if any) is ignored.
1.148 markus 451: The command is run on a pty if the client requests a pty;
1.36 aaron 452: otherwise it is run without a tty.
1.194 jmc 453: If an 8-bit clean channel is required,
1.147 deraadt 454: one must not request a pty or should specify
1.89 markus 455: .Cm no-pty .
1.36 aaron 456: A quote may be included in the command by quoting it with a backslash.
457: This option might be useful
1.189 espie 458: to restrict certain public keys to perform just a specific operation.
1.36 aaron 459: An example might be a key that permits remote backups but nothing else.
1.51 hugh 460: Note that the client may specify TCP/IP and/or X11
461: forwarding unless they are explicitly prohibited.
1.149 markus 462: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 463: .It Cm environment="NAME=value"
1.1 deraadt 464: Specifies that the string is to be added to the environment when
1.36 aaron 465: logging in using this key.
466: Environment variables set this way
467: override other default environment values.
468: Multiple options of this type are permitted.
1.188 stevesk 469: Environment processing is disabled by default and is
470: controlled via the
471: .Cm PermitUserEnvironment
472: option.
1.155 markus 473: This option is automatically disabled if
474: .Cm UseLogin
475: is enabled.
1.2 deraadt 476: .It Cm no-port-forwarding
1.1 deraadt 477: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 478: Any port forward requests by the client will return an error.
479: This might be used, e.g., in connection with the
1.2 deraadt 480: .Cm command
1.1 deraadt 481: option.
1.2 deraadt 482: .It Cm no-X11-forwarding
1.1 deraadt 483: Forbids X11 forwarding when this key is used for authentication.
484: Any X11 forward requests by the client will return an error.
1.2 deraadt 485: .It Cm no-agent-forwarding
1.1 deraadt 486: Forbids authentication agent forwarding when this key is used for
487: authentication.
1.2 deraadt 488: .It Cm no-pty
1.1 deraadt 489: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 490: .It Cm permitopen="host:port"
1.133 itojun 491: Limit local
1.107 djm 492: .Li ``ssh -L''
1.111 stevesk 493: port forwarding such that it may only connect to the specified host and
1.146 stevesk 494: port.
495: IPv6 addresses can be specified with an alternative syntax:
496: .Ar host/port .
497: Multiple
1.107 djm 498: .Cm permitopen
1.197 jmc 499: options may be applied separated by commas.
500: No pattern matching is performed on the specified hostnames,
501: they must be literal domains or addresses.
1.2 deraadt 502: .El
503: .Ss Examples
1.1 deraadt 504: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 505: .Pp
1.1 deraadt 506: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 507: .Pp
1.1 deraadt 508: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 509: .Pp
510: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 511: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 512: The
1.194 jmc 513: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 514: and
1.131 markus 515: .Pa $HOME/.ssh/known_hosts
1.36 aaron 516: files contain host public keys for all known hosts.
517: The global file should
1.37 brad 518: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 519: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 520: its key is added to the per-user file.
1.2 deraadt 521: .Pp
1.1 deraadt 522: Each line in these files contains the following fields: hostnames,
1.36 aaron 523: bits, exponent, modulus, comment.
524: The fields are separated by spaces.
1.2 deraadt 525: .Pp
1.196 david 526: Hostnames is a comma-separated list of patterns
1.197 jmc 527: .Pf ( Ql \&*
528: and
529: .Ql \&?
1.195 mouring 530: act as
1.1 deraadt 531: wildcards); each pattern in turn is matched against the canonical host
532: name (when authenticating a client) or against the user-supplied
1.36 aaron 533: name (when authenticating a server).
534: A pattern may also be preceded by
1.195 mouring 535: .Ql \&!
1.2 deraadt 536: to indicate negation: if the host name matches a negated
1.1 deraadt 537: pattern, it is not accepted (by that line) even if it matched another
538: pattern on the line.
1.2 deraadt 539: .Pp
1.49 markus 540: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 541: can be obtained, e.g., from
1.167 deraadt 542: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 543: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 544: .Pp
545: Lines starting with
546: .Ql #
547: and empty lines are ignored as comments.
548: .Pp
1.1 deraadt 549: When performing host authentication, authentication is accepted if any
1.36 aaron 550: matching line has the proper key.
551: It is thus permissible (but not
1.1 deraadt 552: recommended) to have several lines or different host keys for the same
1.36 aaron 553: names.
554: This will inevitably happen when short forms of host names
555: from different domains are put in the file.
556: It is possible
1.1 deraadt 557: that the files contain conflicting information; authentication is
558: accepted if valid information can be found from either file.
1.2 deraadt 559: .Pp
1.1 deraadt 560: Note that the lines in these files are typically hundreds of characters
561: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 562: Rather, generate them by a script
1.40 aaron 563: or by taking
1.167 deraadt 564: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 565: and adding the host names at the front.
1.2 deraadt 566: .Ss Examples
1.120 markus 567: .Bd -literal
568: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
569: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
570: .Ed
1.2 deraadt 571: .Sh FILES
572: .Bl -tag -width Ds
1.167 deraadt 573: .It Pa /etc/ssh/sshd_config
1.1 deraadt 574: Contains configuration data for
1.2 deraadt 575: .Nm sshd .
1.184 stevesk 576: The file format and configuration options are described in
577: .Xr sshd_config 5 .
1.167 deraadt 578: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.120 markus 579: These three files contain the private parts of the host keys.
1.98 deraadt 580: These files should only be owned by root, readable only by root, and not
1.1 deraadt 581: accessible to others.
1.14 markus 582: Note that
583: .Nm
584: does not start if this file is group/world-accessible.
1.167 deraadt 585: .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 586: These three files contain the public parts of the host keys.
1.98 deraadt 587: These files should be world-readable but writable only by
1.36 aaron 588: root.
1.98 deraadt 589: Their contents should match the respective private parts.
590: These files are not
591: really used for anything; they are provided for the convenience of
592: the user so their contents can be copied to known hosts files.
593: These files are created using
1.7 markus 594: .Xr ssh-keygen 1 .
1.129 provos 595: .It Pa /etc/moduli
1.73 provos 596: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.192 stevesk 597: The file format is described in
598: .Xr moduli 5 .
1.185 stevesk 599: .It Pa /var/empty
600: .Xr chroot 2
601: directory used by
602: .Nm
603: during privilege separation in the pre-authentication phase.
604: The directory should not contain any files and must be owned by root
605: and not group or world-writable.
1.2 deraadt 606: .It Pa /var/run/sshd.pid
607: Contains the process ID of the
608: .Nm
1.1 deraadt 609: listening for connections (if there are several daemons running
1.186 stevesk 610: concurrently for different ports, this contains the process ID of the one
1.36 aaron 611: started last).
1.58 deraadt 612: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 613: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 614: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 615: This file must be readable by root (which may on some machines imply
616: it being world-readable if the user's home directory resides on an NFS
617: volume).
618: It is recommended that it not be accessible by others.
619: The format of this file is described above.
620: Users will place the contents of their
1.130 markus 621: .Pa identity.pub ,
1.44 deraadt 622: .Pa id_dsa.pub
1.113 itojun 623: and/or
624: .Pa id_rsa.pub
1.44 deraadt 625: files into this file, as described in
626: .Xr ssh-keygen 1 .
1.167 deraadt 627: .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1.22 markus 628: These files are consulted when using rhosts with RSA host
1.131 markus 629: authentication or protocol version 2 hostbased authentication
630: to check the public key of the host.
1.36 aaron 631: The key must be listed in one of these files to be accepted.
1.22 markus 632: The client uses the same files
1.96 markus 633: to verify that it is connecting to the correct remote host.
1.36 aaron 634: These files should be writable only by root/the owner.
1.167 deraadt 635: .Pa /etc/ssh/ssh_known_hosts
1.2 deraadt 636: should be world-readable, and
637: .Pa $HOME/.ssh/known_hosts
1.194 jmc 638: can, but need not be, world-readable.
1.6 aaron 639: .It Pa /etc/nologin
1.40 aaron 640: If this file exists,
1.2 deraadt 641: .Nm
1.36 aaron 642: refuses to let anyone except root log in.
643: The contents of the file
1.1 deraadt 644: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 645: refused.
646: The file should be world-readable.
1.19 dugsong 647: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 648: Access controls that should be enforced by tcp-wrappers are defined here.
649: Further details are described in
1.19 dugsong 650: .Xr hosts_access 5 .
1.6 aaron 651: .It Pa $HOME/.rhosts
1.1 deraadt 652: This file contains host-username pairs, separated by a space, one per
1.36 aaron 653: line.
654: The given user on the corresponding host is permitted to log in
1.194 jmc 655: without a password.
1.36 aaron 656: The same file is used by rlogind and rshd.
1.6 aaron 657: The file must
1.1 deraadt 658: be writable only by the user; it is recommended that it not be
659: accessible by others.
1.2 deraadt 660: .Pp
1.36 aaron 661: If is also possible to use netgroups in the file.
662: Either host or user
1.1 deraadt 663: name may be of the form +@groupname to specify all hosts or all users
664: in the group.
1.2 deraadt 665: .It Pa $HOME/.shosts
666: For ssh,
667: this file is exactly the same as for
668: .Pa .rhosts .
669: However, this file is
670: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 671: .It Pa /etc/hosts.equiv
1.2 deraadt 672: This file is used during
673: .Pa .rhosts
1.36 aaron 674: authentication.
675: In the simplest form, this file contains host names, one per line.
676: Users on
1.1 deraadt 677: those hosts are permitted to log in without a password, provided they
1.36 aaron 678: have the same user name on both machines.
679: The host name may also be
1.1 deraadt 680: followed by a user name; such users are permitted to log in as
1.2 deraadt 681: .Em any
1.36 aaron 682: user on this machine (except root).
683: Additionally, the syntax
1.2 deraadt 684: .Dq +@group
1.36 aaron 685: can be used to specify netgroups.
686: Negated entries start with
1.2 deraadt 687: .Ql \&- .
688: .Pp
1.1 deraadt 689: If the client host/user is successfully matched in this file, login is
690: automatically permitted provided the client and server user names are the
1.36 aaron 691: same.
692: Additionally, successful RSA host authentication is normally required.
693: This file must be writable only by root; it is recommended
1.1 deraadt 694: that it be world-readable.
1.2 deraadt 695: .Pp
1.6 aaron 696: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 697: .Pa hosts.equiv .
1.1 deraadt 698: Beware that it really means that the named user(s) can log in as
1.2 deraadt 699: .Em anybody ,
1.1 deraadt 700: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 701: binaries and directories.
702: Using a user name practically grants the user root access.
703: The only valid use for user names that I can think
1.1 deraadt 704: of is in negative entries.
1.2 deraadt 705: .Pp
706: Note that this warning also applies to rsh/rlogin.
707: .It Pa /etc/shosts.equiv
1.1 deraadt 708: This is processed exactly as
1.2 deraadt 709: .Pa /etc/hosts.equiv .
1.1 deraadt 710: However, this file may be useful in environments that want to run both
1.2 deraadt 711: rsh/rlogin and ssh.
1.6 aaron 712: .It Pa $HOME/.ssh/environment
1.36 aaron 713: This file is read into the environment at login (if it exists).
714: It can only contain empty lines, comment lines (that start with
1.2 deraadt 715: .Ql # ) ,
1.36 aaron 716: and assignment lines of the form name=value.
717: The file should be writable
1.6 aaron 718: only by the user; it need not be readable by anyone else.
1.188 stevesk 719: Environment processing is disabled by default and is
720: controlled via the
721: .Cm PermitUserEnvironment
722: option.
1.2 deraadt 723: .It Pa $HOME/.ssh/rc
1.194 jmc 724: If this file exists, it is run with
725: .Pa /bin/sh
726: after reading the
1.36 aaron 727: environment files but before starting the user's shell or command.
1.176 stevesk 728: It must not produce any output on stdout; stderr must be used
729: instead.
730: If X11 forwarding is in use, it will receive the "proto cookie" pair in
731: its standard input (and
1.2 deraadt 732: .Ev DISPLAY
1.176 stevesk 733: in its environment).
734: The script must call
1.2 deraadt 735: .Xr xauth 1
1.176 stevesk 736: because
737: .Nm
738: will not run xauth automatically to add X11 cookies.
1.2 deraadt 739: .Pp
1.1 deraadt 740: The primary purpose of this file is to run any initialization routines
741: which may be needed before the user's home directory becomes
742: accessible; AFS is a particular example of such an environment.
1.2 deraadt 743: .Pp
1.1 deraadt 744: This file will probably contain some initialization code followed by
1.120 markus 745: something similar to:
746: .Bd -literal
1.176 stevesk 747: if read proto cookie && [ -n "$DISPLAY" ]; then
748: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
749: # X11UseLocalhost=yes
1.193 todd 750: echo add unix:`echo $DISPLAY |
1.176 stevesk 751: cut -c11-` $proto $cookie
752: else
753: # X11UseLocalhost=no
1.193 todd 754: echo add $DISPLAY $proto $cookie
755: fi | xauth -q -
1.176 stevesk 756: fi
1.120 markus 757: .Ed
1.2 deraadt 758: .Pp
759: If this file does not exist,
1.167 deraadt 760: .Pa /etc/ssh/sshrc
1.2 deraadt 761: is run, and if that
1.176 stevesk 762: does not exist either, xauth is used to add the cookie.
1.2 deraadt 763: .Pp
1.1 deraadt 764: This file should be writable only by the user, and need not be
765: readable by anyone else.
1.167 deraadt 766: .It Pa /etc/ssh/sshrc
1.2 deraadt 767: Like
768: .Pa $HOME/.ssh/rc .
769: This can be used to specify
1.36 aaron 770: machine-specific login-time initializations globally.
771: This file should be writable only by root, and should be world-readable.
1.56 aaron 772: .El
1.2 deraadt 773: .Sh SEE ALSO
774: .Xr scp 1 ,
1.90 djm 775: .Xr sftp 1 ,
1.2 deraadt 776: .Xr ssh 1 ,
1.5 deraadt 777: .Xr ssh-add 1 ,
1.2 deraadt 778: .Xr ssh-agent 1 ,
1.5 deraadt 779: .Xr ssh-keygen 1 ,
1.136 markus 780: .Xr login.conf 5 ,
781: .Xr moduli 5 ,
1.184 stevesk 782: .Xr sshd_config 5 ,
1.128 mpech 783: .Xr sftp-server 8
1.119 markus 784: .Rs
785: .%A T. Ylonen
786: .%A T. Kivinen
787: .%A M. Saarinen
788: .%A T. Rinne
789: .%A S. Lehtinen
790: .%T "SSH Protocol Architecture"
1.174 markus 791: .%N draft-ietf-secsh-architecture-12.txt
792: .%D January 2002
1.120 markus 793: .%O work in progress material
794: .Re
795: .Rs
796: .%A M. Friedl
797: .%A N. Provos
798: .%A W. A. Simpson
799: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.174 markus 800: .%N draft-ietf-secsh-dh-group-exchange-02.txt
801: .%D January 2002
1.119 markus 802: .%O work in progress material
803: .Re
1.198 jmc 804: .Sh AUTHORS
805: OpenSSH is a derivative of the original and free
806: ssh 1.2.12 release by Tatu Ylonen.
807: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
808: Theo de Raadt and Dug Song
809: removed many bugs, re-added newer features and
810: created OpenSSH.
811: Markus Friedl contributed the support for SSH
812: protocol versions 1.5 and 2.0.
813: Niels Provos and Markus Friedl contributed support
814: for privilege separation.