Annotation of src/usr.bin/ssh/sshd.8, Revision 1.204
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: .\"
1.204 ! jmc 37: .\" $OpenBSD: sshd.8,v 1.203 2004/12/06 11:41:03 dtucker 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 combined with RSA host
1.1 deraadt 110: authentication, RSA challenge-response authentication, or password
111: based authentication.
1.2 deraadt 112: .Pp
1.36 aaron 113: System security is not improved unless
1.180 millert 114: .Nm rshd ,
115: .Nm rlogind ,
1.2 deraadt 116: and
1.194 jmc 117: .Nm rexecd
1.1 deraadt 118: are disabled (thus completely disabling
1.180 millert 119: .Xr rlogin
1.1 deraadt 120: and
1.180 millert 121: .Xr rsh
1.42 hugh 122: into the machine).
1.49 markus 123: .Ss SSH protocol version 2
1.58 deraadt 124: Version 2 works similarly:
1.138 markus 125: Each host has a host-specific key (RSA or DSA) used to identify the host.
1.49 markus 126: However, when the daemon starts, it does not generate a server key.
127: Forward security is provided through a Diffie-Hellman key agreement.
128: This key agreement results in a shared session key.
1.120 markus 129: .Pp
1.103 deraadt 130: The rest of the session is encrypted using a symmetric cipher, currently
1.200 jmc 131: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
1.49 markus 132: The client selects the encryption algorithm
133: to use from those offered by the server.
134: Additionally, session integrity is provided
1.51 hugh 135: through a cryptographic message authentication code
1.49 markus 136: (hmac-sha1 or hmac-md5).
137: .Pp
138: Protocol version 2 provides a public key based
1.120 markus 139: user (PubkeyAuthentication) or
140: client host (HostbasedAuthentication) authentication method,
141: conventional password authentication and challenge response based methods.
1.49 markus 142: .Ss Command execution and data forwarding
1.1 deraadt 143: If the client successfully authenticates itself, a dialog for
1.36 aaron 144: preparing the session is entered.
145: At this time the client may request
1.1 deraadt 146: things like allocating a pseudo-tty, forwarding X11 connections,
147: forwarding TCP/IP connections, or forwarding the authentication agent
148: connection over the secure channel.
1.2 deraadt 149: .Pp
1.1 deraadt 150: Finally, the client either requests a shell or execution of a command.
1.36 aaron 151: The sides then enter session mode.
152: In this mode, either side may send
1.1 deraadt 153: data at any time, and such data is forwarded to/from the shell or
154: command on the server side, and the user terminal in the client side.
1.2 deraadt 155: .Pp
1.1 deraadt 156: When the user program terminates and all forwarded X11 and other
157: connections have been closed, the server sends command exit status to
158: the client, and both sides exit.
1.2 deraadt 159: .Pp
160: .Nm
1.200 jmc 161: can be configured using command-line options or a configuration file
162: (by default
163: .Xr sshd_config 5 ) .
1.36 aaron 164: Command-line options override values specified in the
1.1 deraadt 165: configuration file.
1.25 markus 166: .Pp
167: .Nm
168: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 169: .Dv SIGHUP ,
1.201 dtucker 170: by executing itself with the name and options it was started with, e.g.,
1.97 deraadt 171: .Pa /usr/sbin/sshd .
1.18 aaron 172: .Pp
173: The options are as follows:
1.2 deraadt 174: .Bl -tag -width Ds
1.200 jmc 175: .It Fl 4
176: Forces
177: .Nm
178: to use IPv4 addresses only.
179: .It Fl 6
180: Forces
181: .Nm
182: to use IPv6 addresses only.
1.2 deraadt 183: .It Fl b Ar bits
1.120 markus 184: Specifies the number of bits in the ephemeral protocol version 1
185: server key (default 768).
1.200 jmc 186: .It Fl D
187: When this option is specified,
188: .Nm
189: will not detach and does not become a daemon.
190: This allows easy monitoring of
191: .Nm sshd .
1.2 deraadt 192: .It Fl d
1.36 aaron 193: Debug mode.
194: The server sends verbose debug output to the system
195: log, and does not put itself in the background.
196: The server also will not fork and will only process one connection.
197: This option is only intended for debugging for the server.
1.194 jmc 198: Multiple
199: .Fl d
200: options increase the debugging level.
1.67 aaron 201: Maximum is 3.
1.120 markus 202: .It Fl e
203: When this option is specified,
204: .Nm
205: will send the output to the standard error instead of the system log.
1.2 deraadt 206: .It Fl f Ar configuration_file
1.36 aaron 207: Specifies the name of the configuration file.
208: The default is
1.167 deraadt 209: .Pa /etc/ssh/sshd_config .
1.16 markus 210: .Nm
211: refuses to start if there is no configuration file.
1.2 deraadt 212: .It Fl g Ar login_grace_time
1.1 deraadt 213: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 214: 120 seconds).
1.36 aaron 215: If the client fails to authenticate the user within
216: this many seconds, the server disconnects and exits.
217: A value of zero indicates no limit.
1.2 deraadt 218: .It Fl h Ar host_key_file
1.160 stevesk 219: Specifies a file from which a host key is read.
1.7 markus 220: This option must be given if
221: .Nm
222: is not run as root (as the normal
1.160 stevesk 223: host key files are normally not readable by anyone but root).
224: The default is
1.167 deraadt 225: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 226: for protocol version 1, and
1.167 deraadt 227: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 228: and
1.167 deraadt 229: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 230: for protocol version 2.
1.75 markus 231: It is possible to have multiple host key files for
1.120 markus 232: the different protocol versions and host key algorithms.
1.2 deraadt 233: .It Fl i
1.7 markus 234: Specifies that
235: .Nm
1.194 jmc 236: is being run from
237: .Xr inetd 8 .
1.7 markus 238: .Nm
239: is normally not run
1.1 deraadt 240: from inetd because it needs to generate the server key before it can
1.36 aaron 241: respond to the client, and this may take tens of seconds.
242: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 243: However, with small key sizes (e.g., 512) using
1.7 markus 244: .Nm
245: from inetd may
1.1 deraadt 246: be feasible.
1.2 deraadt 247: .It Fl k Ar key_gen_time
1.120 markus 248: Specifies how often the ephemeral protocol version 1 server key is
249: regenerated (default 3600 seconds, or one hour).
1.36 aaron 250: The motivation for regenerating the key fairly
1.200 jmc 251: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 252: it becomes impossible to recover the key for decrypting intercepted
253: communications even if the machine is cracked into or physically
1.36 aaron 254: seized.
255: A value of zero indicates that the key will never be regenerated.
1.156 markus 256: .It Fl o Ar option
257: Can be used to give options in the format used in the configuration file.
258: This is useful for specifying options for which there is no separate
259: command-line flag.
1.200 jmc 260: For full details of the options, and their values, see
261: .Xr sshd_config 5 .
1.2 deraadt 262: .It Fl p Ar port
1.1 deraadt 263: Specifies the port on which the server listens for connections
264: (default 22).
1.158 stevesk 265: Multiple port options are permitted.
266: Ports specified in the configuration file are ignored when a
267: command-line port is specified.
1.2 deraadt 268: .It Fl q
1.36 aaron 269: Quiet mode.
270: Nothing is sent to the system log.
271: Normally the beginning,
1.1 deraadt 272: authentication, and termination of each connection is logged.
1.137 stevesk 273: .It Fl t
274: Test mode.
275: Only check the validity of the configuration file and sanity of the keys.
1.157 deraadt 276: This is useful for updating
1.137 stevesk 277: .Nm
278: reliably as configuration options may change.
1.61 markus 279: .It Fl u Ar len
280: This option is used to specify the size of the field
281: in the
282: .Li utmp
283: structure that holds the remote host name.
284: If the resolved host name is longer than
285: .Ar len ,
286: the dotted decimal value will be used instead.
287: This allows hosts with very long host names that
288: overflow this field to still be uniquely identified.
289: Specifying
290: .Fl u0
291: indicates that only dotted decimal addresses
292: should be put into the
293: .Pa utmp
294: file.
1.144 stevesk 295: .Fl u0
1.194 jmc 296: may also be used to prevent
1.144 stevesk 297: .Nm
298: from making DNS requests unless the authentication
299: mechanism or configuration requires it.
300: Authentication mechanisms that may require DNS include
301: .Cm RhostsRSAAuthentication ,
302: .Cm HostbasedAuthentication
303: and using a
304: .Cm from="pattern-list"
305: option in a key file.
1.170 stevesk 306: Configuration options that require DNS include using a
307: USER@HOST pattern in
308: .Cm AllowUsers
309: or
310: .Cm DenyUsers .
1.2 deraadt 311: .El
312: .Sh CONFIGURATION FILE
313: .Nm
1.40 aaron 314: reads configuration data from
1.167 deraadt 315: .Pa /etc/ssh/sshd_config
1.2 deraadt 316: (or the file specified with
317: .Fl f
1.36 aaron 318: on the command line).
1.184 stevesk 319: The file format and configuration options are described in
320: .Xr sshd_config 5 .
1.2 deraadt 321: .Sh LOGIN PROCESS
1.1 deraadt 322: When a user successfully logs in,
1.2 deraadt 323: .Nm
1.1 deraadt 324: does the following:
1.2 deraadt 325: .Bl -enum -offset indent
326: .It
1.1 deraadt 327: If the login is on a tty, and no command has been specified,
1.40 aaron 328: prints last login time and
1.2 deraadt 329: .Pa /etc/motd
1.1 deraadt 330: (unless prevented in the configuration file or by
1.2 deraadt 331: .Pa $HOME/.hushlogin ;
332: see the
1.40 aaron 333: .Sx FILES
1.2 deraadt 334: section).
335: .It
1.1 deraadt 336: If the login is on a tty, records login time.
1.2 deraadt 337: .It
338: Checks
339: .Pa /etc/nologin ;
340: if it exists, prints contents and quits
1.1 deraadt 341: (unless root).
1.2 deraadt 342: .It
1.1 deraadt 343: Changes to run with normal user privileges.
1.2 deraadt 344: .It
1.1 deraadt 345: Sets up basic environment.
1.2 deraadt 346: .It
1.200 jmc 347: Reads the file
348: .Pa $HOME/.ssh/environment ,
349: if it exists, and users are allowed to change their environment.
1.188 stevesk 350: See the
1.187 marc 351: .Cm PermitUserEnvironment
1.188 stevesk 352: option in
1.187 marc 353: .Xr sshd_config 5 .
1.2 deraadt 354: .It
1.1 deraadt 355: Changes to user's home directory.
1.2 deraadt 356: .It
357: If
358: .Pa $HOME/.ssh/rc
359: exists, runs it; else if
1.168 deraadt 360: .Pa /etc/ssh/sshrc
1.2 deraadt 361: exists, runs
1.36 aaron 362: it; otherwise runs xauth.
363: The
1.2 deraadt 364: .Dq rc
365: files are given the X11
1.1 deraadt 366: authentication protocol and cookie in standard input.
1.2 deraadt 367: .It
1.1 deraadt 368: Runs user's shell or command.
1.2 deraadt 369: .El
370: .Sh AUTHORIZED_KEYS FILE FORMAT
371: .Pa $HOME/.ssh/authorized_keys
1.130 markus 372: is the default file that lists the public keys that are
373: permitted for RSA authentication in protocol version 1
374: and for public key authentication (PubkeyAuthentication)
375: in protocol version 2.
1.125 markus 376: .Cm AuthorizedKeysFile
377: may be used to specify an alternative file.
1.75 markus 378: .Pp
1.36 aaron 379: Each line of the file contains one
1.2 deraadt 380: key (empty lines and lines starting with a
381: .Ql #
382: are ignored as
1.36 aaron 383: comments).
1.75 markus 384: Each RSA public key consists of the following fields, separated by
1.36 aaron 385: spaces: options, bits, exponent, modulus, comment.
1.75 markus 386: Each protocol version 2 public key consists of:
387: options, keytype, base64 encoded key, comment.
1.189 espie 388: The options field
389: is optional; its presence is determined by whether the line starts
390: with a number or not (the options field never starts with a number).
1.75 markus 391: The bits, exponent, modulus and comment fields give the RSA key for
392: protocol version 1; the
1.1 deraadt 393: comment field is not used for anything (but may be convenient for the
394: user to identify the key).
1.75 markus 395: For protocol version 2 the keytype is
396: .Dq ssh-dss
397: or
398: .Dq ssh-rsa .
1.2 deraadt 399: .Pp
1.1 deraadt 400: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 401: (because of the size of the public key encoding) up to a limit of
402: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
403: keys up to 16 kilobits.
1.36 aaron 404: You don't want to type them in; instead, copy the
1.113 itojun 405: .Pa identity.pub ,
406: .Pa id_dsa.pub
1.75 markus 407: or the
1.113 itojun 408: .Pa id_rsa.pub
1.1 deraadt 409: file and edit it.
1.175 stevesk 410: .Pp
411: .Nm
412: enforces a minimum RSA key modulus size for protocol 1
413: and protocol 2 keys of 768 bits.
1.2 deraadt 414: .Pp
1.58 deraadt 415: The options (if present) consist of comma-separated option
1.36 aaron 416: specifications.
417: No spaces are permitted, except within double quotes.
1.141 stevesk 418: The following option specifications are supported (note
419: that option keywords are case-insensitive):
1.2 deraadt 420: .Bl -tag -width Ds
421: .It Cm from="pattern-list"
1.189 espie 422: Specifies that in addition to public key authentication, the canonical name
1.1 deraadt 423: of the remote host must be present in the comma-separated list of
1.36 aaron 424: patterns
1.196 david 425: .Pf ( Ql \&*
1.36 aaron 426: and
1.195 mouring 427: .Ql \&?
1.36 aaron 428: serve as wildcards).
429: The list may also contain
430: patterns negated by prefixing them with
1.195 mouring 431: .Ql \&! ;
1.36 aaron 432: if the canonical host name matches a negated pattern, the key is not accepted.
433: The purpose
1.189 espie 434: of this option is to optionally increase security: public key authentication
1.1 deraadt 435: by itself does not trust the network or name servers or anything (but
436: the key); however, if somebody somehow steals the key, the key
1.36 aaron 437: permits an intruder to log in from anywhere in the world.
438: This additional option makes using a stolen key more difficult (name
1.1 deraadt 439: servers and/or routers would have to be compromised in addition to
440: just the key).
1.2 deraadt 441: .It Cm command="command"
1.1 deraadt 442: Specifies that the command is executed whenever this key is used for
1.36 aaron 443: authentication.
444: The command supplied by the user (if any) is ignored.
1.148 markus 445: The command is run on a pty if the client requests a pty;
1.36 aaron 446: otherwise it is run without a tty.
1.194 jmc 447: If an 8-bit clean channel is required,
1.147 deraadt 448: one must not request a pty or should specify
1.89 markus 449: .Cm no-pty .
1.36 aaron 450: A quote may be included in the command by quoting it with a backslash.
451: This option might be useful
1.189 espie 452: to restrict certain public keys to perform just a specific operation.
1.36 aaron 453: An example might be a key that permits remote backups but nothing else.
1.51 hugh 454: Note that the client may specify TCP/IP and/or X11
455: forwarding unless they are explicitly prohibited.
1.149 markus 456: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 457: .It Cm environment="NAME=value"
1.1 deraadt 458: Specifies that the string is to be added to the environment when
1.36 aaron 459: logging in using this key.
460: Environment variables set this way
461: override other default environment values.
462: Multiple options of this type are permitted.
1.188 stevesk 463: Environment processing is disabled by default and is
464: controlled via the
465: .Cm PermitUserEnvironment
466: option.
1.155 markus 467: This option is automatically disabled if
468: .Cm UseLogin
469: is enabled.
1.2 deraadt 470: .It Cm no-port-forwarding
1.1 deraadt 471: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 472: Any port forward requests by the client will return an error.
473: This might be used, e.g., in connection with the
1.2 deraadt 474: .Cm command
1.1 deraadt 475: option.
1.2 deraadt 476: .It Cm no-X11-forwarding
1.1 deraadt 477: Forbids X11 forwarding when this key is used for authentication.
478: Any X11 forward requests by the client will return an error.
1.2 deraadt 479: .It Cm no-agent-forwarding
1.1 deraadt 480: Forbids authentication agent forwarding when this key is used for
481: authentication.
1.2 deraadt 482: .It Cm no-pty
1.1 deraadt 483: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 484: .It Cm permitopen="host:port"
1.133 itojun 485: Limit local
1.107 djm 486: .Li ``ssh -L''
1.111 stevesk 487: port forwarding such that it may only connect to the specified host and
1.146 stevesk 488: port.
489: IPv6 addresses can be specified with an alternative syntax:
1.200 jmc 490: .Ar host Ns / Ns Ar port .
1.146 stevesk 491: Multiple
1.107 djm 492: .Cm permitopen
1.197 jmc 493: options may be applied separated by commas.
494: No pattern matching is performed on the specified hostnames,
495: they must be literal domains or addresses.
1.2 deraadt 496: .El
497: .Ss Examples
1.200 jmc 498: 1024 33 12121...312314325 ylo@foo.bar
1.2 deraadt 499: .Pp
1.200 jmc 500: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
1.2 deraadt 501: .Pp
1.200 jmc 502: command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
1.107 djm 503: .Pp
1.200 jmc 504: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
1.2 deraadt 505: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 506: The
1.194 jmc 507: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 508: and
1.131 markus 509: .Pa $HOME/.ssh/known_hosts
1.36 aaron 510: files contain host public keys for all known hosts.
511: The global file should
1.37 brad 512: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 513: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 514: its key is added to the per-user file.
1.2 deraadt 515: .Pp
1.1 deraadt 516: Each line in these files contains the following fields: hostnames,
1.36 aaron 517: bits, exponent, modulus, comment.
518: The fields are separated by spaces.
1.2 deraadt 519: .Pp
1.196 david 520: Hostnames is a comma-separated list of patterns
1.197 jmc 521: .Pf ( Ql \&*
522: and
523: .Ql \&?
1.195 mouring 524: act as
1.1 deraadt 525: wildcards); each pattern in turn is matched against the canonical host
526: name (when authenticating a client) or against the user-supplied
1.36 aaron 527: name (when authenticating a server).
528: A pattern may also be preceded by
1.195 mouring 529: .Ql \&!
1.2 deraadt 530: to indicate negation: if the host name matches a negated
1.1 deraadt 531: pattern, it is not accepted (by that line) even if it matched another
532: pattern on the line.
1.2 deraadt 533: .Pp
1.49 markus 534: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 535: can be obtained, e.g., from
1.167 deraadt 536: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 537: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 538: .Pp
539: Lines starting with
540: .Ql #
541: and empty lines are ignored as comments.
542: .Pp
1.1 deraadt 543: When performing host authentication, authentication is accepted if any
1.36 aaron 544: matching line has the proper key.
545: It is thus permissible (but not
1.1 deraadt 546: recommended) to have several lines or different host keys for the same
1.36 aaron 547: names.
548: This will inevitably happen when short forms of host names
549: from different domains are put in the file.
550: It is possible
1.1 deraadt 551: that the files contain conflicting information; authentication is
552: accepted if valid information can be found from either file.
1.2 deraadt 553: .Pp
1.1 deraadt 554: Note that the lines in these files are typically hundreds of characters
555: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 556: Rather, generate them by a script
1.40 aaron 557: or by taking
1.167 deraadt 558: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 559: and adding the host names at the front.
1.2 deraadt 560: .Ss Examples
1.120 markus 561: .Bd -literal
1.200 jmc 562: closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
1.120 markus 563: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
564: .Ed
1.2 deraadt 565: .Sh FILES
566: .Bl -tag -width Ds
1.167 deraadt 567: .It Pa /etc/ssh/sshd_config
1.1 deraadt 568: Contains configuration data for
1.2 deraadt 569: .Nm sshd .
1.184 stevesk 570: The file format and configuration options are described in
571: .Xr sshd_config 5 .
1.167 deraadt 572: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.120 markus 573: These three files contain the private parts of the host keys.
1.98 deraadt 574: These files should only be owned by root, readable only by root, and not
1.1 deraadt 575: accessible to others.
1.14 markus 576: Note that
577: .Nm
578: does not start if this file is group/world-accessible.
1.167 deraadt 579: .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 580: These three files contain the public parts of the host keys.
1.98 deraadt 581: These files should be world-readable but writable only by
1.36 aaron 582: root.
1.98 deraadt 583: Their contents should match the respective private parts.
584: These files are not
585: really used for anything; they are provided for the convenience of
586: the user so their contents can be copied to known hosts files.
587: These files are created using
1.7 markus 588: .Xr ssh-keygen 1 .
1.129 provos 589: .It Pa /etc/moduli
1.73 provos 590: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.192 stevesk 591: The file format is described in
592: .Xr moduli 5 .
1.185 stevesk 593: .It Pa /var/empty
594: .Xr chroot 2
595: directory used by
596: .Nm
597: during privilege separation in the pre-authentication phase.
598: The directory should not contain any files and must be owned by root
599: and not group or world-writable.
1.2 deraadt 600: .It Pa /var/run/sshd.pid
601: Contains the process ID of the
602: .Nm
1.1 deraadt 603: listening for connections (if there are several daemons running
1.186 stevesk 604: concurrently for different ports, this contains the process ID of the one
1.36 aaron 605: started last).
1.58 deraadt 606: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 607: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 608: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 609: This file must be readable by root (which may on some machines imply
610: it being world-readable if the user's home directory resides on an NFS
611: volume).
612: It is recommended that it not be accessible by others.
613: The format of this file is described above.
614: Users will place the contents of their
1.130 markus 615: .Pa identity.pub ,
1.44 deraadt 616: .Pa id_dsa.pub
1.113 itojun 617: and/or
618: .Pa id_rsa.pub
1.44 deraadt 619: files into this file, as described in
620: .Xr ssh-keygen 1 .
1.200 jmc 621: .It Pa "/etc/ssh/ssh_known_hosts", "$HOME/.ssh/known_hosts"
1.22 markus 622: These files are consulted when using rhosts with RSA host
1.131 markus 623: authentication or protocol version 2 hostbased authentication
624: to check the public key of the host.
1.36 aaron 625: The key must be listed in one of these files to be accepted.
1.22 markus 626: The client uses the same files
1.96 markus 627: to verify that it is connecting to the correct remote host.
1.36 aaron 628: These files should be writable only by root/the owner.
1.167 deraadt 629: .Pa /etc/ssh/ssh_known_hosts
1.2 deraadt 630: should be world-readable, and
631: .Pa $HOME/.ssh/known_hosts
1.194 jmc 632: can, but need not be, world-readable.
1.204 ! jmc 633: .It Pa /etc/motd
! 634: See
! 635: .Xr motd 5 .
! 636: .It Pa $HOME/.hushlogin
! 637: This file is used to suppress printing the last login time and
! 638: .Pa /etc/motd ,
! 639: if
! 640: .Cm PrintLastLog
! 641: and
! 642: .Cm PrintMotd ,
! 643: respectively,
! 644: are enabled.
! 645: It does not suppress printing of the banner specified by
! 646: .Cm Banner .
1.6 aaron 647: .It Pa /etc/nologin
1.40 aaron 648: If this file exists,
1.2 deraadt 649: .Nm
1.36 aaron 650: refuses to let anyone except root log in.
651: The contents of the file
1.1 deraadt 652: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 653: refused.
654: The file should be world-readable.
1.19 dugsong 655: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 656: Access controls that should be enforced by tcp-wrappers are defined here.
657: Further details are described in
1.19 dugsong 658: .Xr hosts_access 5 .
1.6 aaron 659: .It Pa $HOME/.rhosts
1.202 markus 660: This file is used during
661: .Cm RhostsRSAAuthentication
662: and
663: .Cm HostbasedAuthentication
664: and contains host-username pairs, separated by a space, one per
1.36 aaron 665: line.
666: The given user on the corresponding host is permitted to log in
1.194 jmc 667: without a password.
1.36 aaron 668: The same file is used by rlogind and rshd.
1.6 aaron 669: The file must
1.1 deraadt 670: be writable only by the user; it is recommended that it not be
671: accessible by others.
1.2 deraadt 672: .Pp
1.200 jmc 673: It is also possible to use netgroups in the file.
1.36 aaron 674: Either host or user
1.1 deraadt 675: name may be of the form +@groupname to specify all hosts or all users
676: in the group.
1.2 deraadt 677: .It Pa $HOME/.shosts
678: For ssh,
679: this file is exactly the same as for
680: .Pa .rhosts .
681: However, this file is
682: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 683: .It Pa /etc/hosts.equiv
1.2 deraadt 684: This file is used during
1.202 markus 685: .Cm RhostsRSAAuthentication
686: and
687: .Cm HostbasedAuthentication
1.36 aaron 688: authentication.
689: In the simplest form, this file contains host names, one per line.
690: Users on
1.1 deraadt 691: those hosts are permitted to log in without a password, provided they
1.36 aaron 692: have the same user name on both machines.
693: The host name may also be
1.1 deraadt 694: followed by a user name; such users are permitted to log in as
1.2 deraadt 695: .Em any
1.36 aaron 696: user on this machine (except root).
697: Additionally, the syntax
1.2 deraadt 698: .Dq +@group
1.36 aaron 699: can be used to specify netgroups.
700: Negated entries start with
1.2 deraadt 701: .Ql \&- .
702: .Pp
1.1 deraadt 703: If the client host/user is successfully matched in this file, login is
704: automatically permitted provided the client and server user names are the
1.36 aaron 705: same.
1.202 markus 706: Additionally, successful client host key authentication is required.
1.36 aaron 707: This file must be writable only by root; it is recommended
1.1 deraadt 708: that it be world-readable.
1.2 deraadt 709: .Pp
1.6 aaron 710: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 711: .Pa hosts.equiv .
1.1 deraadt 712: Beware that it really means that the named user(s) can log in as
1.2 deraadt 713: .Em anybody ,
1.1 deraadt 714: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 715: binaries and directories.
716: Using a user name practically grants the user root access.
717: The only valid use for user names that I can think
1.1 deraadt 718: of is in negative entries.
1.2 deraadt 719: .Pp
720: Note that this warning also applies to rsh/rlogin.
721: .It Pa /etc/shosts.equiv
1.1 deraadt 722: This is processed exactly as
1.2 deraadt 723: .Pa /etc/hosts.equiv .
1.1 deraadt 724: However, this file may be useful in environments that want to run both
1.2 deraadt 725: rsh/rlogin and ssh.
1.6 aaron 726: .It Pa $HOME/.ssh/environment
1.36 aaron 727: This file is read into the environment at login (if it exists).
728: It can only contain empty lines, comment lines (that start with
1.2 deraadt 729: .Ql # ) ,
1.36 aaron 730: and assignment lines of the form name=value.
731: The file should be writable
1.6 aaron 732: only by the user; it need not be readable by anyone else.
1.188 stevesk 733: Environment processing is disabled by default and is
734: controlled via the
735: .Cm PermitUserEnvironment
736: option.
1.2 deraadt 737: .It Pa $HOME/.ssh/rc
1.194 jmc 738: If this file exists, it is run with
739: .Pa /bin/sh
740: after reading the
1.36 aaron 741: environment files but before starting the user's shell or command.
1.176 stevesk 742: It must not produce any output on stdout; stderr must be used
743: instead.
744: If X11 forwarding is in use, it will receive the "proto cookie" pair in
745: its standard input (and
1.2 deraadt 746: .Ev DISPLAY
1.176 stevesk 747: in its environment).
748: The script must call
1.2 deraadt 749: .Xr xauth 1
1.176 stevesk 750: because
751: .Nm
752: will not run xauth automatically to add X11 cookies.
1.2 deraadt 753: .Pp
1.1 deraadt 754: The primary purpose of this file is to run any initialization routines
755: which may be needed before the user's home directory becomes
756: accessible; AFS is a particular example of such an environment.
1.2 deraadt 757: .Pp
1.1 deraadt 758: This file will probably contain some initialization code followed by
1.120 markus 759: something similar to:
760: .Bd -literal
1.176 stevesk 761: if read proto cookie && [ -n "$DISPLAY" ]; then
762: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
763: # X11UseLocalhost=yes
1.193 todd 764: echo add unix:`echo $DISPLAY |
1.176 stevesk 765: cut -c11-` $proto $cookie
766: else
767: # X11UseLocalhost=no
1.193 todd 768: echo add $DISPLAY $proto $cookie
769: fi | xauth -q -
1.176 stevesk 770: fi
1.120 markus 771: .Ed
1.2 deraadt 772: .Pp
773: If this file does not exist,
1.167 deraadt 774: .Pa /etc/ssh/sshrc
1.2 deraadt 775: is run, and if that
1.176 stevesk 776: does not exist either, xauth is used to add the cookie.
1.2 deraadt 777: .Pp
1.1 deraadt 778: This file should be writable only by the user, and need not be
779: readable by anyone else.
1.167 deraadt 780: .It Pa /etc/ssh/sshrc
1.2 deraadt 781: Like
782: .Pa $HOME/.ssh/rc .
783: This can be used to specify
1.36 aaron 784: machine-specific login-time initializations globally.
785: This file should be writable only by root, and should be world-readable.
1.56 aaron 786: .El
1.2 deraadt 787: .Sh SEE ALSO
788: .Xr scp 1 ,
1.90 djm 789: .Xr sftp 1 ,
1.2 deraadt 790: .Xr ssh 1 ,
1.5 deraadt 791: .Xr ssh-add 1 ,
1.2 deraadt 792: .Xr ssh-agent 1 ,
1.5 deraadt 793: .Xr ssh-keygen 1 ,
1.200 jmc 794: .Xr chroot 2 ,
795: .Xr hosts_access 5 ,
1.136 markus 796: .Xr login.conf 5 ,
797: .Xr moduli 5 ,
1.184 stevesk 798: .Xr sshd_config 5 ,
1.200 jmc 799: .Xr inetd 8 ,
1.128 mpech 800: .Xr sftp-server 8
1.119 markus 801: .Rs
802: .%A T. Ylonen
803: .%A T. Kivinen
804: .%A M. Saarinen
805: .%A T. Rinne
806: .%A S. Lehtinen
807: .%T "SSH Protocol Architecture"
1.174 markus 808: .%N draft-ietf-secsh-architecture-12.txt
809: .%D January 2002
1.120 markus 810: .%O work in progress material
811: .Re
812: .Rs
813: .%A M. Friedl
814: .%A N. Provos
815: .%A W. A. Simpson
816: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.174 markus 817: .%N draft-ietf-secsh-dh-group-exchange-02.txt
818: .%D January 2002
1.119 markus 819: .%O work in progress material
820: .Re
1.198 jmc 821: .Sh AUTHORS
822: OpenSSH is a derivative of the original and free
823: ssh 1.2.12 release by Tatu Ylonen.
824: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
825: Theo de Raadt and Dug Song
826: removed many bugs, re-added newer features and
827: created OpenSSH.
828: Markus Friedl contributed the support for SSH
829: protocol versions 1.5 and 2.0.
830: Niels Provos and Markus Friedl contributed support
831: for privilege separation.
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