Annotation of src/usr.bin/ssh/sshd.8, Revision 1.212
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: .\"
1.212 ! jmc 37: .\" $OpenBSD: sshd.8,v 1.211 2006/01/12 22:20:00 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
1.212 ! jmc 59: (OpenSSH 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.
1.2 deraadt 64: .Pp
65: .Nm
1.212 ! jmc 66: listens for connections from clients.
1.40 aaron 67: It is normally started at boot from
1.2 deraadt 68: .Pa /etc/rc .
69: It forks a new
1.36 aaron 70: daemon for each incoming connection.
71: The forked daemons handle
1.1 deraadt 72: key exchange, encryption, authentication, command execution,
73: and data exchange.
1.2 deraadt 74: .Pp
75: .Nm
1.200 jmc 76: can be configured using command-line options or a configuration file
77: (by default
1.212 ! jmc 78: .Xr sshd_config 5 ) ;
! 79: command-line options override values specified in the
1.1 deraadt 80: configuration file.
1.25 markus 81: .Nm
82: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 83: .Dv SIGHUP ,
1.201 dtucker 84: by executing itself with the name and options it was started with, e.g.,
1.97 deraadt 85: .Pa /usr/sbin/sshd .
1.18 aaron 86: .Pp
87: The options are as follows:
1.2 deraadt 88: .Bl -tag -width Ds
1.200 jmc 89: .It Fl 4
90: Forces
91: .Nm
92: to use IPv4 addresses only.
93: .It Fl 6
94: Forces
95: .Nm
96: to use IPv6 addresses only.
1.2 deraadt 97: .It Fl b Ar bits
1.120 markus 98: Specifies the number of bits in the ephemeral protocol version 1
99: server key (default 768).
1.200 jmc 100: .It Fl D
101: When this option is specified,
102: .Nm
103: will not detach and does not become a daemon.
104: This allows easy monitoring of
105: .Nm sshd .
1.2 deraadt 106: .It Fl d
1.36 aaron 107: Debug mode.
108: The server sends verbose debug output to the system
109: log, and does not put itself in the background.
110: The server also will not fork and will only process one connection.
111: This option is only intended for debugging for the server.
1.194 jmc 112: Multiple
113: .Fl d
114: options increase the debugging level.
1.67 aaron 115: Maximum is 3.
1.120 markus 116: .It Fl e
117: When this option is specified,
118: .Nm
119: will send the output to the standard error instead of the system log.
1.2 deraadt 120: .It Fl f Ar configuration_file
1.36 aaron 121: Specifies the name of the configuration file.
122: The default is
1.167 deraadt 123: .Pa /etc/ssh/sshd_config .
1.16 markus 124: .Nm
125: refuses to start if there is no configuration file.
1.2 deraadt 126: .It Fl g Ar login_grace_time
1.1 deraadt 127: Gives the grace time for clients to authenticate themselves (default
1.191 stevesk 128: 120 seconds).
1.36 aaron 129: If the client fails to authenticate the user within
130: this many seconds, the server disconnects and exits.
131: A value of zero indicates no limit.
1.2 deraadt 132: .It Fl h Ar host_key_file
1.160 stevesk 133: Specifies a file from which a host key is read.
1.7 markus 134: This option must be given if
135: .Nm
136: is not run as root (as the normal
1.160 stevesk 137: host key files are normally not readable by anyone but root).
138: The default is
1.167 deraadt 139: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 140: for protocol version 1, and
1.167 deraadt 141: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 142: and
1.167 deraadt 143: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 144: for protocol version 2.
1.75 markus 145: It is possible to have multiple host key files for
1.120 markus 146: the different protocol versions and host key algorithms.
1.2 deraadt 147: .It Fl i
1.7 markus 148: Specifies that
149: .Nm
1.194 jmc 150: is being run from
151: .Xr inetd 8 .
1.7 markus 152: .Nm
153: is normally not run
1.1 deraadt 154: from inetd because it needs to generate the server key before it can
1.36 aaron 155: respond to the client, and this may take tens of seconds.
156: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 157: However, with small key sizes (e.g., 512) using
1.7 markus 158: .Nm
159: from inetd may
1.1 deraadt 160: be feasible.
1.2 deraadt 161: .It Fl k Ar key_gen_time
1.120 markus 162: Specifies how often the ephemeral protocol version 1 server key is
163: regenerated (default 3600 seconds, or one hour).
1.36 aaron 164: The motivation for regenerating the key fairly
1.200 jmc 165: often is that the key is not stored anywhere, and after about an hour
1.1 deraadt 166: it becomes impossible to recover the key for decrypting intercepted
167: communications even if the machine is cracked into or physically
1.36 aaron 168: seized.
169: A value of zero indicates that the key will never be regenerated.
1.156 markus 170: .It Fl o Ar option
171: Can be used to give options in the format used in the configuration file.
172: This is useful for specifying options for which there is no separate
173: command-line flag.
1.200 jmc 174: For full details of the options, and their values, see
175: .Xr sshd_config 5 .
1.2 deraadt 176: .It Fl p Ar port
1.1 deraadt 177: Specifies the port on which the server listens for connections
178: (default 22).
1.158 stevesk 179: Multiple port options are permitted.
1.210 stevesk 180: Ports specified in the configuration file with the
181: .Cm Port
182: option are ignored when a command-line port is specified.
183: Ports specified using the
184: .Cm ListenAddress
185: option override command-line ports.
1.2 deraadt 186: .It Fl q
1.36 aaron 187: Quiet mode.
188: Nothing is sent to the system log.
189: Normally the beginning,
1.1 deraadt 190: authentication, and termination of each connection is logged.
1.137 stevesk 191: .It Fl t
192: Test mode.
193: Only check the validity of the configuration file and sanity of the keys.
1.157 deraadt 194: This is useful for updating
1.137 stevesk 195: .Nm
196: reliably as configuration options may change.
1.61 markus 197: .It Fl u Ar len
198: This option is used to specify the size of the field
199: in the
200: .Li utmp
201: structure that holds the remote host name.
202: If the resolved host name is longer than
203: .Ar len ,
204: the dotted decimal value will be used instead.
205: This allows hosts with very long host names that
206: overflow this field to still be uniquely identified.
207: Specifying
208: .Fl u0
209: indicates that only dotted decimal addresses
210: should be put into the
211: .Pa utmp
212: file.
1.144 stevesk 213: .Fl u0
1.194 jmc 214: may also be used to prevent
1.144 stevesk 215: .Nm
216: from making DNS requests unless the authentication
217: mechanism or configuration requires it.
218: Authentication mechanisms that may require DNS include
219: .Cm RhostsRSAAuthentication ,
220: .Cm HostbasedAuthentication
221: and using a
222: .Cm from="pattern-list"
223: option in a key file.
1.170 stevesk 224: Configuration options that require DNS include using a
225: USER@HOST pattern in
226: .Cm AllowUsers
227: or
228: .Cm DenyUsers .
1.2 deraadt 229: .El
1.212 ! jmc 230: .Pp
! 231: This implementation of
! 232: .Nm
! 233: supports both SSH protocol version 1 and 2 simultaneously.
! 234: .Nm
! 235: works as follows:
! 236: .Ss SSH protocol version 1
! 237: Each host has a host-specific RSA key
! 238: (normally 2048 bits) used to identify the host.
! 239: Additionally, when
! 240: the daemon starts, it generates a server RSA key (normally 768 bits).
! 241: This key is normally regenerated every hour if it has been used, and
! 242: is never stored on disk.
! 243: .Pp
! 244: Whenever a client connects, the daemon responds with its public
! 245: host and server keys.
! 246: The client compares the
! 247: RSA host key against its own database to verify that it has not changed.
! 248: The client then generates a 256-bit random number.
! 249: It encrypts this
! 250: random number using both the host key and the server key, and sends
! 251: the encrypted number to the server.
! 252: Both sides then use this
! 253: random number as a session key which is used to encrypt all further
! 254: communications in the session.
! 255: The rest of the session is encrypted
! 256: using a conventional cipher, currently Blowfish or 3DES, with 3DES
! 257: being used by default.
! 258: The client selects the encryption algorithm
! 259: to use from those offered by the server.
! 260: .Pp
! 261: Next, the server and the client enter an authentication dialog.
! 262: The client tries to authenticate itself using
! 263: .Em rhosts
! 264: authentication combined with RSA host
! 265: authentication, RSA challenge-response authentication, or password
! 266: based authentication.
! 267: .Pp
! 268: System security is not improved unless
! 269: .Nm rshd ,
! 270: .Nm rlogind ,
! 271: and
! 272: .Nm rexecd
! 273: are disabled (thus completely disabling
! 274: .Xr rlogin
! 275: and
! 276: .Xr rsh
! 277: into the machine).
! 278: .Ss SSH protocol version 2
! 279: Version 2 works similarly:
! 280: Each host has a host-specific key (RSA or DSA) used to identify the host.
! 281: However, when the daemon starts, it does not generate a server key.
! 282: Forward security is provided through a Diffie-Hellman key agreement.
! 283: This key agreement results in a shared session key.
! 284: .Pp
! 285: The rest of the session is encrypted using a symmetric cipher, currently
! 286: 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
! 287: The client selects the encryption algorithm
! 288: to use from those offered by the server.
! 289: Additionally, session integrity is provided
! 290: through a cryptographic message authentication code
! 291: (hmac-sha1 or hmac-md5).
! 292: .Pp
! 293: Protocol version 2 provides a public key based
! 294: user (PubkeyAuthentication) or
! 295: client host (HostbasedAuthentication) authentication method,
! 296: conventional password authentication and challenge response based methods.
! 297: .Ss Command execution and data forwarding
! 298: If the client successfully authenticates itself, a dialog for
! 299: preparing the session is entered.
! 300: At this time the client may request
! 301: things like allocating a pseudo-tty, forwarding X11 connections,
! 302: forwarding TCP connections, or forwarding the authentication agent
! 303: connection over the secure channel.
! 304: .Pp
! 305: Finally, the client either requests a shell or execution of a command.
! 306: The sides then enter session mode.
! 307: In this mode, either side may send
! 308: data at any time, and such data is forwarded to/from the shell or
! 309: command on the server side, and the user terminal in the client side.
! 310: .Pp
! 311: When the user program terminates and all forwarded X11 and other
! 312: connections have been closed, the server sends command exit status to
! 313: the client, and both sides exit.
1.2 deraadt 314: .Sh CONFIGURATION FILE
315: .Nm
1.40 aaron 316: reads configuration data from
1.167 deraadt 317: .Pa /etc/ssh/sshd_config
1.2 deraadt 318: (or the file specified with
319: .Fl f
1.36 aaron 320: on the command line).
1.184 stevesk 321: The file format and configuration options are described in
322: .Xr sshd_config 5 .
1.2 deraadt 323: .Sh LOGIN PROCESS
1.1 deraadt 324: When a user successfully logs in,
1.2 deraadt 325: .Nm
1.1 deraadt 326: does the following:
1.2 deraadt 327: .Bl -enum -offset indent
328: .It
1.1 deraadt 329: If the login is on a tty, and no command has been specified,
1.40 aaron 330: prints last login time and
1.2 deraadt 331: .Pa /etc/motd
1.1 deraadt 332: (unless prevented in the configuration file or by
1.207 djm 333: .Pa ~/.hushlogin ;
1.2 deraadt 334: see the
1.40 aaron 335: .Sx FILES
1.2 deraadt 336: section).
337: .It
1.1 deraadt 338: If the login is on a tty, records login time.
1.2 deraadt 339: .It
340: Checks
341: .Pa /etc/nologin ;
342: if it exists, prints contents and quits
1.1 deraadt 343: (unless root).
1.2 deraadt 344: .It
1.1 deraadt 345: Changes to run with normal user privileges.
1.2 deraadt 346: .It
1.1 deraadt 347: Sets up basic environment.
1.2 deraadt 348: .It
1.200 jmc 349: Reads the file
1.207 djm 350: .Pa ~/.ssh/environment ,
1.200 jmc 351: if it exists, and users are allowed to change their environment.
1.188 stevesk 352: See the
1.187 marc 353: .Cm PermitUserEnvironment
1.188 stevesk 354: option in
1.187 marc 355: .Xr sshd_config 5 .
1.2 deraadt 356: .It
1.1 deraadt 357: Changes to user's home directory.
1.2 deraadt 358: .It
359: If
1.207 djm 360: .Pa ~/.ssh/rc
1.2 deraadt 361: exists, runs it; else if
1.168 deraadt 362: .Pa /etc/ssh/sshrc
1.2 deraadt 363: exists, runs
1.36 aaron 364: it; otherwise runs xauth.
365: The
1.2 deraadt 366: .Dq rc
367: files are given the X11
1.1 deraadt 368: authentication protocol and cookie in standard input.
1.2 deraadt 369: .It
1.1 deraadt 370: Runs user's shell or command.
1.2 deraadt 371: .El
372: .Sh AUTHORIZED_KEYS FILE FORMAT
1.207 djm 373: .Pa ~/.ssh/authorized_keys
1.130 markus 374: is the default file that lists the public keys that are
375: permitted for RSA authentication in protocol version 1
376: and for public key authentication (PubkeyAuthentication)
377: in protocol version 2.
1.125 markus 378: .Cm AuthorizedKeysFile
379: may be used to specify an alternative file.
1.75 markus 380: .Pp
1.36 aaron 381: Each line of the file contains one
1.2 deraadt 382: key (empty lines and lines starting with a
383: .Ql #
384: are ignored as
1.36 aaron 385: comments).
1.75 markus 386: Each RSA public key consists of the following fields, separated by
1.36 aaron 387: spaces: options, bits, exponent, modulus, comment.
1.75 markus 388: Each protocol version 2 public key consists of:
389: options, keytype, base64 encoded key, comment.
1.189 espie 390: The options field
391: is optional; its presence is determined by whether the line starts
392: with a number or not (the options field never starts with a number).
1.75 markus 393: The bits, exponent, modulus and comment fields give the RSA key for
394: protocol version 1; the
1.1 deraadt 395: comment field is not used for anything (but may be convenient for the
396: user to identify the key).
1.75 markus 397: For protocol version 2 the keytype is
398: .Dq ssh-dss
399: or
400: .Dq ssh-rsa .
1.2 deraadt 401: .Pp
1.1 deraadt 402: Note that lines in this file are usually several hundred bytes long
1.203 dtucker 403: (because of the size of the public key encoding) up to a limit of
404: 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA
405: keys up to 16 kilobits.
1.36 aaron 406: You don't want to type them in; instead, copy the
1.113 itojun 407: .Pa identity.pub ,
408: .Pa id_dsa.pub
1.75 markus 409: or the
1.113 itojun 410: .Pa id_rsa.pub
1.1 deraadt 411: file and edit it.
1.175 stevesk 412: .Pp
413: .Nm
414: enforces a minimum RSA key modulus size for protocol 1
415: and protocol 2 keys of 768 bits.
1.2 deraadt 416: .Pp
1.58 deraadt 417: The options (if present) consist of comma-separated option
1.36 aaron 418: specifications.
419: No spaces are permitted, except within double quotes.
1.141 stevesk 420: The following option specifications are supported (note
421: that option keywords are case-insensitive):
1.2 deraadt 422: .Bl -tag -width Ds
423: .It Cm from="pattern-list"
1.189 espie 424: Specifies that in addition to public key authentication, the canonical name
1.1 deraadt 425: of the remote host must be present in the comma-separated list of
1.36 aaron 426: patterns
1.196 david 427: .Pf ( Ql \&*
1.36 aaron 428: and
1.195 mouring 429: .Ql \&?
1.36 aaron 430: serve as wildcards).
431: The list may also contain
432: patterns negated by prefixing them with
1.195 mouring 433: .Ql \&! ;
1.36 aaron 434: if the canonical host name matches a negated pattern, the key is not accepted.
435: The purpose
1.189 espie 436: of this option is to optionally increase security: public key authentication
1.1 deraadt 437: by itself does not trust the network or name servers or anything (but
438: the key); however, if somebody somehow steals the key, the key
1.36 aaron 439: permits an intruder to log in from anywhere in the world.
440: This additional option makes using a stolen key more difficult (name
1.1 deraadt 441: servers and/or routers would have to be compromised in addition to
442: just the key).
1.2 deraadt 443: .It Cm command="command"
1.1 deraadt 444: Specifies that the command is executed whenever this key is used for
1.36 aaron 445: authentication.
446: The command supplied by the user (if any) is ignored.
1.148 markus 447: The command is run on a pty if the client requests a pty;
1.36 aaron 448: otherwise it is run without a tty.
1.194 jmc 449: If an 8-bit clean channel is required,
1.147 deraadt 450: one must not request a pty or should specify
1.89 markus 451: .Cm no-pty .
1.36 aaron 452: A quote may be included in the command by quoting it with a backslash.
453: This option might be useful
1.189 espie 454: to restrict certain public keys to perform just a specific operation.
1.36 aaron 455: An example might be a key that permits remote backups but nothing else.
1.211 jmc 456: Note that the client may specify TCP and/or X11
1.51 hugh 457: forwarding unless they are explicitly prohibited.
1.149 markus 458: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 459: .It Cm environment="NAME=value"
1.1 deraadt 460: Specifies that the string is to be added to the environment when
1.36 aaron 461: logging in using this key.
462: Environment variables set this way
463: override other default environment values.
464: Multiple options of this type are permitted.
1.188 stevesk 465: Environment processing is disabled by default and is
466: controlled via the
467: .Cm PermitUserEnvironment
468: option.
1.155 markus 469: This option is automatically disabled if
470: .Cm UseLogin
471: is enabled.
1.2 deraadt 472: .It Cm no-port-forwarding
1.211 jmc 473: Forbids TCP forwarding when this key is used for authentication.
1.36 aaron 474: Any port forward requests by the client will return an error.
475: This might be used, e.g., in connection with the
1.2 deraadt 476: .Cm command
1.1 deraadt 477: option.
1.2 deraadt 478: .It Cm no-X11-forwarding
1.1 deraadt 479: Forbids X11 forwarding when this key is used for authentication.
480: Any X11 forward requests by the client will return an error.
1.2 deraadt 481: .It Cm no-agent-forwarding
1.1 deraadt 482: Forbids authentication agent forwarding when this key is used for
483: authentication.
1.2 deraadt 484: .It Cm no-pty
1.1 deraadt 485: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 486: .It Cm permitopen="host:port"
1.133 itojun 487: Limit local
1.107 djm 488: .Li ``ssh -L''
1.111 stevesk 489: port forwarding such that it may only connect to the specified host and
1.146 stevesk 490: port.
491: IPv6 addresses can be specified with an alternative syntax:
1.200 jmc 492: .Ar host Ns / Ns Ar port .
1.146 stevesk 493: Multiple
1.107 djm 494: .Cm permitopen
1.197 jmc 495: options may be applied separated by commas.
496: No pattern matching is performed on the specified hostnames,
497: they must be literal domains or addresses.
1.209 reyk 498: .It Cm tunnel="n"
499: Force a
500: .Xr tun 4
501: device on the server.
502: Without this option, the next available device will be used if
503: the client requests a tunnel.
1.2 deraadt 504: .El
505: .Ss Examples
1.200 jmc 506: 1024 33 12121...312314325 ylo@foo.bar
1.2 deraadt 507: .Pp
1.200 jmc 508: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
1.2 deraadt 509: .Pp
1.200 jmc 510: command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
1.107 djm 511: .Pp
1.200 jmc 512: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
1.209 reyk 513: .Pp
514: tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== reyk@openbsd.org
1.2 deraadt 515: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 516: The
1.194 jmc 517: .Pa /etc/ssh/ssh_known_hosts
1.40 aaron 518: and
1.207 djm 519: .Pa ~/.ssh/known_hosts
1.36 aaron 520: files contain host public keys for all known hosts.
521: The global file should
1.37 brad 522: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 523: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 524: its key is added to the per-user file.
1.2 deraadt 525: .Pp
1.1 deraadt 526: Each line in these files contains the following fields: hostnames,
1.36 aaron 527: bits, exponent, modulus, comment.
528: The fields are separated by spaces.
1.2 deraadt 529: .Pp
1.196 david 530: Hostnames is a comma-separated list of patterns
1.197 jmc 531: .Pf ( Ql \&*
532: and
533: .Ql \&?
1.195 mouring 534: act as
1.1 deraadt 535: wildcards); each pattern in turn is matched against the canonical host
536: name (when authenticating a client) or against the user-supplied
1.36 aaron 537: name (when authenticating a server).
538: A pattern may also be preceded by
1.195 mouring 539: .Ql \&!
1.2 deraadt 540: to indicate negation: if the host name matches a negated
1.1 deraadt 541: pattern, it is not accepted (by that line) even if it matched another
542: pattern on the line.
1.2 deraadt 543: .Pp
1.205 djm 544: Alternately, hostnames may be stored in a hashed form which hides host names
1.206 jmc 545: and addresses should the file's contents be disclosed.
546: Hashed hostnames start with a
547: .Ql |
1.205 djm 548: character.
549: Only one hashed hostname may appear on a single line and none of the above
550: negation or wildcard operators may be applied.
551: .Pp
1.49 markus 552: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 553: can be obtained, e.g., from
1.167 deraadt 554: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 555: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 556: .Pp
557: Lines starting with
558: .Ql #
559: and empty lines are ignored as comments.
560: .Pp
1.1 deraadt 561: When performing host authentication, authentication is accepted if any
1.36 aaron 562: matching line has the proper key.
563: It is thus permissible (but not
1.1 deraadt 564: recommended) to have several lines or different host keys for the same
1.36 aaron 565: names.
566: This will inevitably happen when short forms of host names
567: from different domains are put in the file.
568: It is possible
1.1 deraadt 569: that the files contain conflicting information; authentication is
570: accepted if valid information can be found from either file.
1.2 deraadt 571: .Pp
1.1 deraadt 572: Note that the lines in these files are typically hundreds of characters
573: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 574: Rather, generate them by a script
1.40 aaron 575: or by taking
1.167 deraadt 576: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 577: and adding the host names at the front.
1.2 deraadt 578: .Ss Examples
1.120 markus 579: .Bd -literal
1.200 jmc 580: closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
1.120 markus 581: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1.205 djm 582: .Ed
583: .Bd -literal
584: # A hashed hostname
585: |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
586: AAAA1234.....=
1.120 markus 587: .Ed
1.2 deraadt 588: .Sh FILES
589: .Bl -tag -width Ds
1.167 deraadt 590: .It Pa /etc/ssh/sshd_config
1.1 deraadt 591: Contains configuration data for
1.2 deraadt 592: .Nm sshd .
1.184 stevesk 593: The file format and configuration options are described in
594: .Xr sshd_config 5 .
1.167 deraadt 595: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.120 markus 596: These three files contain the private parts of the host keys.
1.98 deraadt 597: These files should only be owned by root, readable only by root, and not
1.1 deraadt 598: accessible to others.
1.14 markus 599: Note that
600: .Nm
601: does not start if this file is group/world-accessible.
1.167 deraadt 602: .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 603: These three files contain the public parts of the host keys.
1.98 deraadt 604: These files should be world-readable but writable only by
1.36 aaron 605: root.
1.98 deraadt 606: Their contents should match the respective private parts.
607: These files are not
608: really used for anything; they are provided for the convenience of
609: the user so their contents can be copied to known hosts files.
610: These files are created using
1.7 markus 611: .Xr ssh-keygen 1 .
1.129 provos 612: .It Pa /etc/moduli
1.73 provos 613: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.192 stevesk 614: The file format is described in
615: .Xr moduli 5 .
1.185 stevesk 616: .It Pa /var/empty
617: .Xr chroot 2
618: directory used by
619: .Nm
620: during privilege separation in the pre-authentication phase.
621: The directory should not contain any files and must be owned by root
622: and not group or world-writable.
1.2 deraadt 623: .It Pa /var/run/sshd.pid
624: Contains the process ID of the
625: .Nm
1.1 deraadt 626: listening for connections (if there are several daemons running
1.186 stevesk 627: concurrently for different ports, this contains the process ID of the one
1.36 aaron 628: started last).
1.58 deraadt 629: The content of this file is not sensitive; it can be world-readable.
1.207 djm 630: .It Pa ~/.ssh/authorized_keys
1.120 markus 631: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 632: This file must be readable by root (which may on some machines imply
633: it being world-readable if the user's home directory resides on an NFS
634: volume).
635: It is recommended that it not be accessible by others.
636: The format of this file is described above.
637: Users will place the contents of their
1.130 markus 638: .Pa identity.pub ,
1.44 deraadt 639: .Pa id_dsa.pub
1.113 itojun 640: and/or
641: .Pa id_rsa.pub
1.44 deraadt 642: files into this file, as described in
643: .Xr ssh-keygen 1 .
1.207 djm 644: .It Pa "/etc/ssh/ssh_known_hosts", "~/.ssh/known_hosts"
1.22 markus 645: These files are consulted when using rhosts with RSA host
1.131 markus 646: authentication or protocol version 2 hostbased authentication
647: to check the public key of the host.
1.36 aaron 648: The key must be listed in one of these files to be accepted.
1.22 markus 649: The client uses the same files
1.96 markus 650: to verify that it is connecting to the correct remote host.
1.36 aaron 651: These files should be writable only by root/the owner.
1.167 deraadt 652: .Pa /etc/ssh/ssh_known_hosts
1.2 deraadt 653: should be world-readable, and
1.207 djm 654: .Pa ~/.ssh/known_hosts
1.194 jmc 655: can, but need not be, world-readable.
1.204 jmc 656: .It Pa /etc/motd
657: See
658: .Xr motd 5 .
1.207 djm 659: .It Pa ~/.hushlogin
1.204 jmc 660: This file is used to suppress printing the last login time and
661: .Pa /etc/motd ,
662: if
663: .Cm PrintLastLog
664: and
665: .Cm PrintMotd ,
666: respectively,
667: are enabled.
668: It does not suppress printing of the banner specified by
669: .Cm Banner .
1.6 aaron 670: .It Pa /etc/nologin
1.40 aaron 671: If this file exists,
1.2 deraadt 672: .Nm
1.36 aaron 673: refuses to let anyone except root log in.
674: The contents of the file
1.1 deraadt 675: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 676: refused.
677: The file should be world-readable.
1.19 dugsong 678: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 679: Access controls that should be enforced by tcp-wrappers are defined here.
680: Further details are described in
1.19 dugsong 681: .Xr hosts_access 5 .
1.207 djm 682: .It Pa ~/.rhosts
1.202 markus 683: This file is used during
684: .Cm RhostsRSAAuthentication
685: and
686: .Cm HostbasedAuthentication
687: and contains host-username pairs, separated by a space, one per
1.36 aaron 688: line.
689: The given user on the corresponding host is permitted to log in
1.194 jmc 690: without a password.
1.36 aaron 691: The same file is used by rlogind and rshd.
1.6 aaron 692: The file must
1.1 deraadt 693: be writable only by the user; it is recommended that it not be
694: accessible by others.
1.2 deraadt 695: .Pp
1.200 jmc 696: It is also possible to use netgroups in the file.
1.36 aaron 697: Either host or user
1.1 deraadt 698: name may be of the form +@groupname to specify all hosts or all users
699: in the group.
1.207 djm 700: .It Pa ~/.shosts
1.2 deraadt 701: For ssh,
702: this file is exactly the same as for
703: .Pa .rhosts .
704: However, this file is
705: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 706: .It Pa /etc/hosts.equiv
1.2 deraadt 707: This file is used during
1.202 markus 708: .Cm RhostsRSAAuthentication
709: and
710: .Cm HostbasedAuthentication
1.36 aaron 711: authentication.
712: In the simplest form, this file contains host names, one per line.
713: Users on
1.1 deraadt 714: those hosts are permitted to log in without a password, provided they
1.36 aaron 715: have the same user name on both machines.
716: The host name may also be
1.1 deraadt 717: followed by a user name; such users are permitted to log in as
1.2 deraadt 718: .Em any
1.36 aaron 719: user on this machine (except root).
720: Additionally, the syntax
1.2 deraadt 721: .Dq +@group
1.36 aaron 722: can be used to specify netgroups.
723: Negated entries start with
1.2 deraadt 724: .Ql \&- .
725: .Pp
1.1 deraadt 726: If the client host/user is successfully matched in this file, login is
727: automatically permitted provided the client and server user names are the
1.36 aaron 728: same.
1.202 markus 729: Additionally, successful client host key authentication is required.
1.36 aaron 730: This file must be writable only by root; it is recommended
1.1 deraadt 731: that it be world-readable.
1.2 deraadt 732: .Pp
1.6 aaron 733: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 734: .Pa hosts.equiv .
1.1 deraadt 735: Beware that it really means that the named user(s) can log in as
1.2 deraadt 736: .Em anybody ,
1.1 deraadt 737: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 738: binaries and directories.
739: Using a user name practically grants the user root access.
740: The only valid use for user names that I can think
1.1 deraadt 741: of is in negative entries.
1.2 deraadt 742: .Pp
743: Note that this warning also applies to rsh/rlogin.
744: .It Pa /etc/shosts.equiv
1.1 deraadt 745: This is processed exactly as
1.2 deraadt 746: .Pa /etc/hosts.equiv .
1.1 deraadt 747: However, this file may be useful in environments that want to run both
1.2 deraadt 748: rsh/rlogin and ssh.
1.207 djm 749: .It Pa ~/.ssh/environment
1.36 aaron 750: This file is read into the environment at login (if it exists).
751: It can only contain empty lines, comment lines (that start with
1.2 deraadt 752: .Ql # ) ,
1.36 aaron 753: and assignment lines of the form name=value.
754: The file should be writable
1.6 aaron 755: only by the user; it need not be readable by anyone else.
1.188 stevesk 756: Environment processing is disabled by default and is
757: controlled via the
758: .Cm PermitUserEnvironment
759: option.
1.207 djm 760: .It Pa ~/.ssh/rc
1.194 jmc 761: If this file exists, it is run with
762: .Pa /bin/sh
763: after reading the
1.36 aaron 764: environment files but before starting the user's shell or command.
1.176 stevesk 765: It must not produce any output on stdout; stderr must be used
766: instead.
767: If X11 forwarding is in use, it will receive the "proto cookie" pair in
768: its standard input (and
1.2 deraadt 769: .Ev DISPLAY
1.176 stevesk 770: in its environment).
771: The script must call
1.2 deraadt 772: .Xr xauth 1
1.176 stevesk 773: because
774: .Nm
775: will not run xauth automatically to add X11 cookies.
1.2 deraadt 776: .Pp
1.1 deraadt 777: The primary purpose of this file is to run any initialization routines
778: which may be needed before the user's home directory becomes
779: accessible; AFS is a particular example of such an environment.
1.2 deraadt 780: .Pp
1.1 deraadt 781: This file will probably contain some initialization code followed by
1.120 markus 782: something similar to:
783: .Bd -literal
1.176 stevesk 784: if read proto cookie && [ -n "$DISPLAY" ]; then
785: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
786: # X11UseLocalhost=yes
1.193 todd 787: echo add unix:`echo $DISPLAY |
1.176 stevesk 788: cut -c11-` $proto $cookie
789: else
790: # X11UseLocalhost=no
1.193 todd 791: echo add $DISPLAY $proto $cookie
792: fi | xauth -q -
1.176 stevesk 793: fi
1.120 markus 794: .Ed
1.2 deraadt 795: .Pp
796: If this file does not exist,
1.167 deraadt 797: .Pa /etc/ssh/sshrc
1.2 deraadt 798: is run, and if that
1.176 stevesk 799: does not exist either, xauth is used to add the cookie.
1.2 deraadt 800: .Pp
1.1 deraadt 801: This file should be writable only by the user, and need not be
802: readable by anyone else.
1.167 deraadt 803: .It Pa /etc/ssh/sshrc
1.2 deraadt 804: Like
1.207 djm 805: .Pa ~/.ssh/rc .
1.2 deraadt 806: This can be used to specify
1.36 aaron 807: machine-specific login-time initializations globally.
808: This file should be writable only by root, and should be world-readable.
1.56 aaron 809: .El
1.2 deraadt 810: .Sh SEE ALSO
811: .Xr scp 1 ,
1.90 djm 812: .Xr sftp 1 ,
1.2 deraadt 813: .Xr ssh 1 ,
1.5 deraadt 814: .Xr ssh-add 1 ,
1.2 deraadt 815: .Xr ssh-agent 1 ,
1.5 deraadt 816: .Xr ssh-keygen 1 ,
1.200 jmc 817: .Xr chroot 2 ,
818: .Xr hosts_access 5 ,
1.136 markus 819: .Xr login.conf 5 ,
820: .Xr moduli 5 ,
1.184 stevesk 821: .Xr sshd_config 5 ,
1.200 jmc 822: .Xr inetd 8 ,
1.128 mpech 823: .Xr sftp-server 8
1.119 markus 824: .Rs
825: .%A T. Ylonen
826: .%A T. Kivinen
827: .%A M. Saarinen
828: .%A T. Rinne
829: .%A S. Lehtinen
830: .%T "SSH Protocol Architecture"
1.174 markus 831: .%N draft-ietf-secsh-architecture-12.txt
832: .%D January 2002
1.120 markus 833: .%O work in progress material
834: .Re
835: .Rs
836: .%A M. Friedl
837: .%A N. Provos
838: .%A W. A. Simpson
839: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.174 markus 840: .%N draft-ietf-secsh-dh-group-exchange-02.txt
841: .%D January 2002
1.119 markus 842: .%O work in progress material
843: .Re
1.198 jmc 844: .Sh AUTHORS
845: OpenSSH is a derivative of the original and free
846: ssh 1.2.12 release by Tatu Ylonen.
847: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
848: Theo de Raadt and Dug Song
849: removed many bugs, re-added newer features and
850: created OpenSSH.
851: Markus Friedl contributed the support for SSH
852: protocol versions 1.5 and 2.0.
853: Niels Provos and Markus Friedl contributed support
854: for privilege separation.
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