Annotation of src/usr.bin/ssh/sshd.8, Revision 1.64
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: .\"
! 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.
! 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: .\"
1.2 deraadt 37: .Dd September 25, 1999
38: .Dt SSHD 8
39: .Os
40: .Sh NAME
41: .Nm sshd
42: .Nd secure shell daemon
43: .Sh SYNOPSIS
44: .Nm sshd
1.29 markus 45: .Op Fl diqQ46
1.2 deraadt 46: .Op Fl b Ar bits
47: .Op Fl f Ar config_file
48: .Op Fl g Ar login_grace_time
49: .Op Fl h Ar host_key_file
50: .Op Fl k Ar key_gen_time
51: .Op Fl p Ar port
1.61 markus 52: .Op Fl u Ar len
1.26 markus 53: .Op Fl V Ar client_protocol_id
1.40 aaron 54: .Sh DESCRIPTION
1.2 deraadt 55: .Nm
1.40 aaron 56: (Secure Shell Daemon) is the daemon program for
1.2 deraadt 57: .Xr ssh 1 .
1.42 hugh 58: Together these programs replace rlogin and rsh, and
1.1 deraadt 59: provide secure encrypted communications between two untrusted hosts
1.36 aaron 60: over an insecure network.
61: The programs are intended to be as easy to
1.1 deraadt 62: install and use as possible.
1.2 deraadt 63: .Pp
64: .Nm
1.36 aaron 65: is the daemon that listens for connections from clients.
1.40 aaron 66: It is normally started at boot from
1.2 deraadt 67: .Pa /etc/rc .
68: It forks a new
1.36 aaron 69: daemon for each incoming connection.
70: The forked daemons handle
1.1 deraadt 71: key exchange, encryption, authentication, command execution,
72: and data exchange.
1.49 markus 73: This implementation of
74: .Nm
75: supports both SSH protocol version 1 and 2 simultaneously.
1.2 deraadt 76: .Nm
1.36 aaron 77: works as follows.
1.49 markus 78: .Pp
79: .Ss SSH protocol version 1
80: .Pp
1.36 aaron 81: Each host has a host-specific RSA key
82: (normally 1024 bits) used to identify the host.
83: Additionally, when
1.1 deraadt 84: the daemon starts, it generates a server RSA key (normally 768 bits).
85: This key is normally regenerated every hour if it has been used, and
86: is never stored on disk.
1.2 deraadt 87: .Pp
1.42 hugh 88: Whenever a client connects the daemon responds with its public
89: host and server keys.
1.36 aaron 90: The client compares the
1.49 markus 91: RSA host key against its own database to verify that it has not changed.
1.36 aaron 92: The client then generates a 256 bit random number.
93: It encrypts this
1.1 deraadt 94: random number using both the host key and the server key, and sends
1.36 aaron 95: the encrypted number to the server.
1.42 hugh 96: Both sides then use this
1.1 deraadt 97: random number as a session key which is used to encrypt all further
1.36 aaron 98: communications in the session.
99: The rest of the session is encrypted
1.42 hugh 100: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 101: being used by default.
1.36 aaron 102: The client selects the encryption algorithm
1.5 deraadt 103: to use from those offered by the server.
1.2 deraadt 104: .Pp
1.36 aaron 105: Next, the server and the client enter an authentication dialog.
106: The client tries to authenticate itself using
1.2 deraadt 107: .Pa .rhosts
108: authentication,
109: .Pa .rhosts
110: authentication combined with RSA host
1.1 deraadt 111: authentication, RSA challenge-response authentication, or password
112: based authentication.
1.2 deraadt 113: .Pp
1.1 deraadt 114: Rhosts authentication is normally disabled
115: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 116: configuration file if desired.
117: System security is not improved unless
1.2 deraadt 118: .Xr rshd 8 ,
119: .Xr rlogind 8 ,
120: .Xr rexecd 8 ,
121: and
122: .Xr rexd 8
1.1 deraadt 123: are disabled (thus completely disabling
1.2 deraadt 124: .Xr rlogin 1
1.1 deraadt 125: and
1.2 deraadt 126: .Xr rsh 1
1.42 hugh 127: into the machine).
1.2 deraadt 128: .Pp
1.49 markus 129: .Ss SSH protocol version 2
130: .Pp
1.58 deraadt 131: Version 2 works similarly:
1.49 markus 132: Each host has a host-specific DSA key used to identify the host.
133: However, when the daemon starts, it does not generate a server key.
134: Forward security is provided through a Diffie-Hellman key agreement.
135: This key agreement results in a shared session key.
136: The rest of the session is encrypted
137: using a symmetric cipher, currently
138: Blowfish, 3DES or CAST128 in CBC mode or Arcfour.
139: The client selects the encryption algorithm
140: to use from those offered by the server.
141: Additionally, session integrity is provided
1.51 hugh 142: through a cryptographic message authentication code
1.49 markus 143: (hmac-sha1 or hmac-md5).
144: .Pp
145: Protocol version 2 provides a public key based
146: user authentication method (DSAAuthentication)
147: and conventional password authentication.
148: .Pp
149: .Ss Command execution and data forwarding
150: .Pp
1.1 deraadt 151: If the client successfully authenticates itself, a dialog for
1.36 aaron 152: preparing the session is entered.
153: At this time the client may request
1.1 deraadt 154: things like allocating a pseudo-tty, forwarding X11 connections,
155: forwarding TCP/IP connections, or forwarding the authentication agent
156: connection over the secure channel.
1.2 deraadt 157: .Pp
1.1 deraadt 158: Finally, the client either requests a shell or execution of a command.
1.36 aaron 159: The sides then enter session mode.
160: In this mode, either side may send
1.1 deraadt 161: data at any time, and such data is forwarded to/from the shell or
162: command on the server side, and the user terminal in the client side.
1.2 deraadt 163: .Pp
1.1 deraadt 164: When the user program terminates and all forwarded X11 and other
165: connections have been closed, the server sends command exit status to
166: the client, and both sides exit.
1.2 deraadt 167: .Pp
168: .Nm
1.1 deraadt 169: can be configured using command-line options or a configuration
1.36 aaron 170: file.
171: Command-line options override values specified in the
1.1 deraadt 172: configuration file.
1.25 markus 173: .Pp
174: .Nm
175: rereads its configuration file when it receives a hangup signal,
176: .Dv SIGHUP .
1.18 aaron 177: .Pp
178: The options are as follows:
1.2 deraadt 179: .Bl -tag -width Ds
180: .It Fl b Ar bits
1.1 deraadt 181: Specifies the number of bits in the server key (default 768).
1.2 deraadt 182: .Pp
183: .It Fl d
1.36 aaron 184: Debug mode.
185: The server sends verbose debug output to the system
186: log, and does not put itself in the background.
187: The server also will not fork and will only process one connection.
188: This option is only intended for debugging for the server.
1.2 deraadt 189: .It Fl f Ar configuration_file
1.36 aaron 190: Specifies the name of the configuration file.
191: The default is
1.2 deraadt 192: .Pa /etc/sshd_config .
1.16 markus 193: .Nm
194: refuses to start if there is no configuration file.
1.2 deraadt 195: .It Fl g Ar login_grace_time
1.1 deraadt 196: Gives the grace time for clients to authenticate themselves (default
1.36 aaron 197: 300 seconds).
198: If the client fails to authenticate the user within
199: this many seconds, the server disconnects and exits.
200: A value of zero indicates no limit.
1.2 deraadt 201: .It Fl h Ar host_key_file
1.49 markus 202: Specifies the file from which the RSA host key is read (default
1.2 deraadt 203: .Pa /etc/ssh_host_key ) .
1.7 markus 204: This option must be given if
205: .Nm
206: is not run as root (as the normal
1.1 deraadt 207: host file is normally not readable by anyone but root).
1.2 deraadt 208: .It Fl i
1.7 markus 209: Specifies that
210: .Nm
1.40 aaron 211: is being run from inetd.
1.7 markus 212: .Nm
213: is normally not run
1.1 deraadt 214: from inetd because it needs to generate the server key before it can
1.36 aaron 215: respond to the client, and this may take tens of seconds.
216: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 217: However, with small key sizes (e.g., 512) using
1.7 markus 218: .Nm
219: from inetd may
1.1 deraadt 220: be feasible.
1.2 deraadt 221: .It Fl k Ar key_gen_time
1.1 deraadt 222: Specifies how often the server key is regenerated (default 3600
1.36 aaron 223: seconds, or one hour).
224: The motivation for regenerating the key fairly
1.1 deraadt 225: often is that the key is not stored anywhere, and after about an hour,
226: it becomes impossible to recover the key for decrypting intercepted
227: communications even if the machine is cracked into or physically
1.36 aaron 228: seized.
229: A value of zero indicates that the key will never be regenerated.
1.2 deraadt 230: .It Fl p Ar port
1.1 deraadt 231: Specifies the port on which the server listens for connections
232: (default 22).
1.2 deraadt 233: .It Fl q
1.36 aaron 234: Quiet mode.
235: Nothing is sent to the system log.
236: Normally the beginning,
1.1 deraadt 237: authentication, and termination of each connection is logged.
1.61 markus 238: .It Fl u Ar len
239: This option is used to specify the size of the field
240: in the
241: .Li utmp
242: structure that holds the remote host name.
243: If the resolved host name is longer than
244: .Ar len ,
245: the dotted decimal value will be used instead.
246: This allows hosts with very long host names that
247: overflow this field to still be uniquely identified.
248: Specifying
249: .Fl u0
250: indicates that only dotted decimal addresses
251: should be put into the
252: .Pa utmp
253: file.
1.18 aaron 254: .It Fl Q
255: Do not print an error message if RSA support is missing.
1.26 markus 256: .It Fl V Ar client_protocol_id
257: SSH2 compatibility mode.
1.42 hugh 258: When this option is specified
1.26 markus 259: .Nm
1.42 hugh 260: assumes the client has sent the supplied version string
1.26 markus 261: and skips the
262: Protocol Version Identification Exchange.
1.29 markus 263: .It Fl 4
264: Forces
265: .Nm
266: to use IPv4 addresses only.
267: .It Fl 6
268: Forces
269: .Nm
270: to use IPv6 addresses only.
1.2 deraadt 271: .El
272: .Sh CONFIGURATION FILE
273: .Nm
1.40 aaron 274: reads configuration data from
1.2 deraadt 275: .Pa /etc/sshd_config
276: (or the file specified with
277: .Fl f
1.36 aaron 278: on the command line).
279: The file contains keyword-value pairs, one per line.
280: Lines starting with
1.2 deraadt 281: .Ql #
1.1 deraadt 282: and empty lines are interpreted as comments.
1.2 deraadt 283: .Pp
1.1 deraadt 284: The following keywords are possible.
1.2 deraadt 285: .Bl -tag -width Ds
286: .It Cm AFSTokenPassing
1.36 aaron 287: Specifies whether an AFS token may be forwarded to the server.
288: Default is
1.2 deraadt 289: .Dq yes .
1.11 markus 290: .It Cm AllowGroups
291: This keyword can be followed by a number of group names, separated
1.36 aaron 292: by spaces.
293: If specified, login is allowed only for users whose primary
1.11 markus 294: group matches one of the patterns.
295: .Ql \&*
296: and
297: .Ql ?
298: can be used as
1.36 aaron 299: wildcards in the patterns.
1.58 deraadt 300: Only group names are valid; a numerical group ID isn't recognized.
1.36 aaron 301: By default login is allowed regardless of the primary group.
1.3 dugsong 302: .Pp
1.11 markus 303: .It Cm AllowUsers
304: This keyword can be followed by a number of user names, separated
1.36 aaron 305: by spaces.
306: If specified, login is allowed only for users names that
1.11 markus 307: match one of the patterns.
308: .Ql \&*
309: and
310: .Ql ?
311: can be used as
1.36 aaron 312: wildcards in the patterns.
1.58 deraadt 313: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 314: By default login is allowed regardless of the user name.
1.11 markus 315: .Pp
1.41 markus 316: .It Cm Ciphers
317: Specifies the ciphers allowed for protocol version 2.
318: Multiple ciphers must be comma-separated.
319: The default is
1.50 markus 320: .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
1.8 markus 321: .It Cm CheckMail
322: Specifies whether
323: .Nm
324: should check for new mail for interactive logins.
325: The default is
326: .Dq no .
1.11 markus 327: .It Cm DenyGroups
328: This keyword can be followed by a number of group names, separated
1.36 aaron 329: by spaces.
330: Users whose primary group matches one of the patterns
1.11 markus 331: aren't allowed to log in.
332: .Ql \&*
333: and
334: .Ql ?
335: can be used as
1.36 aaron 336: wildcards in the patterns.
1.58 deraadt 337: Only group names are valid; a numerical group ID isn't recognized.
1.36 aaron 338: By default login is allowed regardless of the primary group.
1.11 markus 339: .Pp
340: .It Cm DenyUsers
341: This keyword can be followed by a number of user names, separated
1.36 aaron 342: by spaces.
343: Login is disallowed for user names that match one of the patterns.
1.11 markus 344: .Ql \&*
345: and
346: .Ql ?
1.36 aaron 347: can be used as wildcards in the patterns.
1.58 deraadt 348: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 349: By default login is allowed regardless of the user name.
1.49 markus 350: .It Cm DSAAuthentication
351: Specifies whether DSA authentication is allowed.
352: The default is
353: .Dq yes .
354: Note that this option applies to protocol version 2 only.
1.47 markus 355: .It Cm GatewayPorts
356: Specifies whether remote hosts are allowed to connect to ports
357: forwarded for the client.
358: The argument must be
359: .Dq yes
360: or
361: .Dq no .
362: The default is
363: .Dq no .
1.58 deraadt 364: .It Cm HostDSAKey
1.46 markus 365: Specifies the file containing the private DSA host key (default
366: .Pa /etc/ssh_host_dsa_key )
367: used by SSH protocol 2.0.
368: Note that
369: .Nm
1.51 hugh 370: disables protocol 2.0 if this file is group/world-accessible.
1.2 deraadt 371: .It Cm HostKey
1.46 markus 372: Specifies the file containing the private RSA host key (default
373: .Pa /etc/ssh_host_key )
374: used by SSH protocols 1.3 and 1.5.
1.9 markus 375: Note that
376: .Nm
1.51 hugh 377: disables protocols 1.3 and 1.5 if this file is group/world-accessible.
1.2 deraadt 378: .It Cm IgnoreRhosts
1.34 markus 379: Specifies that
380: .Pa .rhosts
1.40 aaron 381: and
1.34 markus 382: .Pa .shosts
383: files will not be used in authentication.
1.2 deraadt 384: .Pa /etc/hosts.equiv
1.1 deraadt 385: and
1.40 aaron 386: .Pa /etc/shosts.equiv
1.36 aaron 387: are still used.
1.40 aaron 388: The default is
1.34 markus 389: .Dq yes .
1.24 markus 390: .It Cm IgnoreUserKnownHosts
391: Specifies whether
392: .Nm
393: should ignore the user's
394: .Pa $HOME/.ssh/known_hosts
1.45 markus 395: during
1.24 markus 396: .Cm RhostsRSAAuthentication .
397: The default is
1.2 deraadt 398: .Dq no .
399: .It Cm KeepAlive
1.1 deraadt 400: Specifies whether the system should send keepalive messages to the
1.36 aaron 401: other side.
402: If they are sent, death of the connection or crash of one
403: of the machines will be properly noticed.
404: However, this means that
1.1 deraadt 405: connections will die if the route is down temporarily, and some people
1.36 aaron 406: find it annoying.
1.51 hugh 407: On the other hand, if keepalives are not sent,
1.2 deraadt 408: sessions may hang indefinitely on the server, leaving
409: .Dq ghost
410: users and consuming server resources.
411: .Pp
412: The default is
413: .Dq yes
414: (to send keepalives), and the server will notice
1.36 aaron 415: if the network goes down or the client host reboots.
416: This avoids infinitely hanging sessions.
1.2 deraadt 417: .Pp
418: To disable keepalives, the value should be set to
419: .Dq no
420: in both the server and the client configuration files.
421: .It Cm KerberosAuthentication
1.36 aaron 422: Specifies whether Kerberos authentication is allowed.
423: This can be in the form of a Kerberos ticket, or if
1.7 markus 424: .Cm PasswordAuthentication
1.1 deraadt 425: is yes, the password provided by the user will be validated through
1.59 provos 426: the Kerberos KDC. To use this option, the server needs a
427: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 428: Default is
1.60 provos 429: .Dq yes .
1.2 deraadt 430: .It Cm KerberosOrLocalPasswd
1.1 deraadt 431: If set then if password authentication through Kerberos fails then
432: the password will be validated via any additional local mechanism
1.2 deraadt 433: such as
434: .Pa /etc/passwd
1.36 aaron 435: or SecurID.
436: Default is
1.20 dugsong 437: .Dq yes .
1.2 deraadt 438: .It Cm KerberosTgtPassing
1.1 deraadt 439: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 440: Default is
1.3 dugsong 441: .Dq no ,
442: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 443: .It Cm KerberosTicketCleanup
1.7 markus 444: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 445: file on logout.
446: Default is
1.3 dugsong 447: .Dq yes .
1.2 deraadt 448: .It Cm KeyRegenerationInterval
1.1 deraadt 449: The server key is automatically regenerated after this many seconds
1.36 aaron 450: (if it has been used).
451: The purpose of regeneration is to prevent
1.1 deraadt 452: decrypting captured sessions by later breaking into the machine and
1.36 aaron 453: stealing the keys.
454: The key is never stored anywhere.
455: If the value is 0, the key is never regenerated.
456: The default is 3600 (seconds).
1.7 markus 457: .It Cm ListenAddress
458: Specifies what local address
459: .Nm
460: should listen on.
461: The default is to listen to all local addresses.
1.28 markus 462: Multiple options of this type are permitted.
463: Additionally, the
464: .Cm Ports
465: options must precede this option.
1.2 deraadt 466: .It Cm LoginGraceTime
1.1 deraadt 467: The server disconnects after this time if the user has not
1.36 aaron 468: successfully logged in.
469: If the value is 0, there is no time limit.
1.1 deraadt 470: The default is 600 (seconds).
1.23 markus 471: .It Cm LogLevel
472: Gives the verbosity level that is used when logging messages from
473: .Nm sshd .
474: The possible values are:
1.27 markus 475: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.23 markus 476: The default is INFO.
477: Logging with level DEBUG violates the privacy of users
478: and is not recommended.
1.55 markus 479: .It Cm MaxStartups
480: Specifies the maximum number of concurrent unauthenticated connections to the
481: .Nm
482: daemon.
483: Additional connections will be dropped until authentication succeeds or the
484: .Cm LoginGraceTime
485: expires for a connection.
486: The default is 10.
1.57 markus 487: .Pp
488: Alternatively, random early drop can be enabled by specifying
489: the three colon separated values
490: .Dq start:rate:full
491: (e.g. "10:30:60").
492: .Nm
493: will refuse connection attempts with a probabillity of
494: .Dq rate/100
495: (30%)
496: if there are currently
497: .Dq start
498: (10)
499: unauthenticated connections.
500: The probabillity increases linearly and all connection attempts
501: are refused if the number of unauthenticated connections reaches
502: .Dq full
503: (60).
1.2 deraadt 504: .It Cm PasswordAuthentication
1.1 deraadt 505: Specifies whether password authentication is allowed.
1.2 deraadt 506: The default is
507: .Dq yes .
1.58 deraadt 508: Note that this option applies to both protocol versions 1 and 2.
1.2 deraadt 509: .It Cm PermitEmptyPasswords
1.1 deraadt 510: When password authentication is allowed, it specifies whether the
1.36 aaron 511: server allows login to accounts with empty password strings.
512: The default is
1.34 markus 513: .Dq no .
1.2 deraadt 514: .It Cm PermitRootLogin
1.1 deraadt 515: Specifies whether the root can log in using
1.2 deraadt 516: .Xr ssh 1 .
1.15 markus 517: The argument must be
518: .Dq yes ,
519: .Dq without-password
520: or
521: .Dq no .
1.2 deraadt 522: The default is
523: .Dq yes .
1.15 markus 524: If this options is set to
525: .Dq without-password
526: only password authentication is disabled for root.
1.2 deraadt 527: .Pp
528: Root login with RSA authentication when the
529: .Ar command
530: option has been
1.1 deraadt 531: specified will be allowed regardless of the value of this setting
532: (which may be useful for taking remote backups even if root login is
533: normally not allowed).
1.43 markus 534: .It Cm PidFile
535: Specifies the file that contains the process identifier of the
536: .Nm
537: daemon.
538: The default is
539: .Pa /var/run/sshd.pid .
1.2 deraadt 540: .It Cm Port
1.1 deraadt 541: Specifies the port number that
1.2 deraadt 542: .Nm
1.36 aaron 543: listens on.
544: The default is 22.
1.28 markus 545: Multiple options of this type are permitted.
1.2 deraadt 546: .It Cm PrintMotd
1.1 deraadt 547: Specifies whether
1.2 deraadt 548: .Nm
1.40 aaron 549: should print
1.2 deraadt 550: .Pa /etc/motd
1.36 aaron 551: when a user logs in interactively.
552: (On some systems it is also printed by the shell,
1.2 deraadt 553: .Pa /etc/profile ,
1.36 aaron 554: or equivalent.)
555: The default is
1.2 deraadt 556: .Dq yes .
1.41 markus 557: .It Cm Protocol
558: Specifies the protocol versions
559: .Nm
560: should support.
561: The possible values are
562: .Dq 1
563: and
564: .Dq 2 .
565: Multiple versions must be comma-separated.
566: The default is
567: .Dq 1 .
1.2 deraadt 568: .It Cm RandomSeed
1.36 aaron 569: Obsolete.
570: Random number generation uses other techniques.
1.2 deraadt 571: .It Cm RhostsAuthentication
1.1 deraadt 572: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 573: files is sufficient.
574: Normally, this method should not be permitted because it is insecure.
1.7 markus 575: .Cm RhostsRSAAuthentication
576: should be used
1.1 deraadt 577: instead, because it performs RSA-based host authentication in addition
578: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 579: The default is
580: .Dq no .
581: .It Cm RhostsRSAAuthentication
1.1 deraadt 582: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 583: with successful RSA host authentication is allowed.
584: The default is
1.34 markus 585: .Dq no .
1.2 deraadt 586: .It Cm RSAAuthentication
1.36 aaron 587: Specifies whether pure RSA authentication is allowed.
588: The default is
1.2 deraadt 589: .Dq yes .
1.49 markus 590: Note that this option applies to protocol version 1 only.
1.2 deraadt 591: .It Cm ServerKeyBits
1.36 aaron 592: Defines the number of bits in the server key.
593: The minimum value is 512, and the default is 768.
1.7 markus 594: .It Cm SkeyAuthentication
595: Specifies whether
1.40 aaron 596: .Xr skey 1
1.36 aaron 597: authentication is allowed.
598: The default is
1.7 markus 599: .Dq yes .
600: Note that s/key authentication is enabled only if
601: .Cm PasswordAuthentication
602: is allowed, too.
1.2 deraadt 603: .It Cm StrictModes
1.12 markus 604: Specifies whether
605: .Nm
606: should check file modes and ownership of the
1.36 aaron 607: user's files and home directory before accepting login.
608: This is normally desirable because novices sometimes accidentally leave their
609: directory or files world-writable.
610: The default is
1.7 markus 611: .Dq yes .
1.54 jakob 612: .It Cm Subsystem
613: Configures an external subsystem (e.g. file transfer daemon).
614: Arguments should be a subsystem name and a command to execute upon subsystem request.
1.63 markus 615: The command
616: .Xr sftp-server 8
617: implements the
618: .Dq sftp
619: file transfer subsystem.
1.54 jakob 620: By default no subsystems are defined.
621: Note that this option applies to protocol version 2 only.
1.2 deraadt 622: .It Cm SyslogFacility
1.1 deraadt 623: Gives the facility code that is used when logging messages from
1.2 deraadt 624: .Nm sshd .
1.1 deraadt 625: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 626: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
627: The default is AUTH.
1.10 markus 628: .It Cm UseLogin
629: Specifies whether
630: .Xr login 1
1.53 markus 631: is used for interactive login sessions.
632: Note that
633: .Xr login 1
1.58 deraadt 634: is never used for remote command execution.
1.36 aaron 635: The default is
1.10 markus 636: .Dq no .
1.6 aaron 637: .It Cm X11DisplayOffset
638: Specifies the first display number available for
639: .Nm sshd Ns 's
1.36 aaron 640: X11 forwarding.
641: This prevents
1.6 aaron 642: .Nm
643: from interfering with real X11 servers.
1.34 markus 644: The default is 10.
1.30 markus 645: .It Cm X11Forwarding
1.36 aaron 646: Specifies whether X11 forwarding is permitted.
647: The default is
1.34 markus 648: .Dq no .
1.30 markus 649: Note that disabling X11 forwarding does not improve security in any
650: way, as users can always install their own forwarders.
1.52 markus 651: .It Cm XAuthLocation
652: Specifies the location of the
653: .Xr xauth 1
654: program.
655: The default is
656: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 657: .El
658: .Sh LOGIN PROCESS
1.1 deraadt 659: When a user successfully logs in,
1.2 deraadt 660: .Nm
1.1 deraadt 661: does the following:
1.2 deraadt 662: .Bl -enum -offset indent
663: .It
1.1 deraadt 664: If the login is on a tty, and no command has been specified,
1.40 aaron 665: prints last login time and
1.2 deraadt 666: .Pa /etc/motd
1.1 deraadt 667: (unless prevented in the configuration file or by
1.2 deraadt 668: .Pa $HOME/.hushlogin ;
669: see the
1.40 aaron 670: .Sx FILES
1.2 deraadt 671: section).
672: .It
1.1 deraadt 673: If the login is on a tty, records login time.
1.2 deraadt 674: .It
675: Checks
676: .Pa /etc/nologin ;
677: if it exists, prints contents and quits
1.1 deraadt 678: (unless root).
1.2 deraadt 679: .It
1.1 deraadt 680: Changes to run with normal user privileges.
1.2 deraadt 681: .It
1.1 deraadt 682: Sets up basic environment.
1.2 deraadt 683: .It
684: Reads
685: .Pa $HOME/.ssh/environment
686: if it exists.
687: .It
1.1 deraadt 688: Changes to user's home directory.
1.2 deraadt 689: .It
690: If
691: .Pa $HOME/.ssh/rc
692: exists, runs it; else if
693: .Pa /etc/sshrc
694: exists, runs
1.36 aaron 695: it; otherwise runs xauth.
696: The
1.2 deraadt 697: .Dq rc
698: files are given the X11
1.1 deraadt 699: authentication protocol and cookie in standard input.
1.2 deraadt 700: .It
1.1 deraadt 701: Runs user's shell or command.
1.2 deraadt 702: .El
703: .Sh AUTHORIZED_KEYS FILE FORMAT
1.40 aaron 704: The
1.2 deraadt 705: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 706: file lists the RSA keys that are
1.44 deraadt 707: permitted for RSA authentication in SSH protocols 1.3 and 1.5
1.51 hugh 708: Similarly, the
1.44 deraadt 709: .Pa $HOME/.ssh/authorized_keys2
710: file lists the DSA keys that are
711: permitted for DSA authentication in SSH protocol 2.0.
1.36 aaron 712: Each line of the file contains one
1.2 deraadt 713: key (empty lines and lines starting with a
714: .Ql #
715: are ignored as
1.36 aaron 716: comments).
717: Each line consists of the following fields, separated by
718: spaces: options, bits, exponent, modulus, comment.
719: The options field
1.1 deraadt 720: is optional; its presence is determined by whether the line starts
721: with a number or not (the option field never starts with a number).
722: The bits, exponent, modulus and comment fields give the RSA key; the
723: comment field is not used for anything (but may be convenient for the
724: user to identify the key).
1.2 deraadt 725: .Pp
1.1 deraadt 726: Note that lines in this file are usually several hundred bytes long
1.36 aaron 727: (because of the size of the RSA key modulus).
728: You don't want to type them in; instead, copy the
1.2 deraadt 729: .Pa identity.pub
1.1 deraadt 730: file and edit it.
1.2 deraadt 731: .Pp
1.58 deraadt 732: The options (if present) consist of comma-separated option
1.36 aaron 733: specifications.
734: No spaces are permitted, except within double quotes.
1.1 deraadt 735: The following option specifications are supported:
1.2 deraadt 736: .Bl -tag -width Ds
737: .It Cm from="pattern-list"
1.1 deraadt 738: Specifies that in addition to RSA authentication, the canonical name
739: of the remote host must be present in the comma-separated list of
1.36 aaron 740: patterns
741: .Pf ( Ql *
742: and
743: .Ql ?
744: serve as wildcards).
745: The list may also contain
746: patterns negated by prefixing them with
747: .Ql ! ;
748: if the canonical host name matches a negated pattern, the key is not accepted.
749: The purpose
1.1 deraadt 750: of this option is to optionally increase security: RSA authentication
751: by itself does not trust the network or name servers or anything (but
752: the key); however, if somebody somehow steals the key, the key
1.36 aaron 753: permits an intruder to log in from anywhere in the world.
754: This additional option makes using a stolen key more difficult (name
1.1 deraadt 755: servers and/or routers would have to be compromised in addition to
756: just the key).
1.2 deraadt 757: .It Cm command="command"
1.1 deraadt 758: Specifies that the command is executed whenever this key is used for
1.36 aaron 759: authentication.
760: The command supplied by the user (if any) is ignored.
1.1 deraadt 761: The command is run on a pty if the connection requests a pty;
1.36 aaron 762: otherwise it is run without a tty.
763: A quote may be included in the command by quoting it with a backslash.
764: This option might be useful
765: to restrict certain RSA keys to perform just a specific operation.
766: An example might be a key that permits remote backups but nothing else.
1.51 hugh 767: Note that the client may specify TCP/IP and/or X11
768: forwarding unless they are explicitly prohibited.
1.2 deraadt 769: .It Cm environment="NAME=value"
1.1 deraadt 770: Specifies that the string is to be added to the environment when
1.36 aaron 771: logging in using this key.
772: Environment variables set this way
773: override other default environment values.
774: Multiple options of this type are permitted.
1.2 deraadt 775: .It Cm no-port-forwarding
1.1 deraadt 776: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 777: Any port forward requests by the client will return an error.
778: This might be used, e.g., in connection with the
1.2 deraadt 779: .Cm command
1.1 deraadt 780: option.
1.2 deraadt 781: .It Cm no-X11-forwarding
1.1 deraadt 782: Forbids X11 forwarding when this key is used for authentication.
783: Any X11 forward requests by the client will return an error.
1.2 deraadt 784: .It Cm no-agent-forwarding
1.1 deraadt 785: Forbids authentication agent forwarding when this key is used for
786: authentication.
1.2 deraadt 787: .It Cm no-pty
1.1 deraadt 788: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 789: .El
790: .Ss Examples
1.1 deraadt 791: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 792: .Pp
1.1 deraadt 793: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 794: .Pp
1.1 deraadt 795: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 796: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 797: The
1.44 deraadt 798: .Pa /etc/ssh_known_hosts ,
799: .Pa /etc/ssh_known_hosts2 ,
800: .Pa $HOME/.ssh/known_hosts ,
1.40 aaron 801: and
1.44 deraadt 802: .Pa $HOME/.ssh/known_hosts2
1.36 aaron 803: files contain host public keys for all known hosts.
804: The global file should
1.37 brad 805: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 806: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 807: its key is added to the per-user file.
1.2 deraadt 808: .Pp
1.1 deraadt 809: Each line in these files contains the following fields: hostnames,
1.36 aaron 810: bits, exponent, modulus, comment.
811: The fields are separated by spaces.
1.2 deraadt 812: .Pp
1.1 deraadt 813: Hostnames is a comma-separated list of patterns ('*' and '?' act as
814: wildcards); each pattern in turn is matched against the canonical host
815: name (when authenticating a client) or against the user-supplied
1.36 aaron 816: name (when authenticating a server).
817: A pattern may also be preceded by
1.2 deraadt 818: .Ql !
819: to indicate negation: if the host name matches a negated
1.1 deraadt 820: pattern, it is not accepted (by that line) even if it matched another
821: pattern on the line.
1.2 deraadt 822: .Pp
1.49 markus 823: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 824: can be obtained, e.g., from
825: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 826: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 827: .Pp
828: Lines starting with
829: .Ql #
830: and empty lines are ignored as comments.
831: .Pp
1.1 deraadt 832: When performing host authentication, authentication is accepted if any
1.36 aaron 833: matching line has the proper key.
834: It is thus permissible (but not
1.1 deraadt 835: recommended) to have several lines or different host keys for the same
1.36 aaron 836: names.
837: This will inevitably happen when short forms of host names
838: from different domains are put in the file.
839: It is possible
1.1 deraadt 840: that the files contain conflicting information; authentication is
841: accepted if valid information can be found from either file.
1.2 deraadt 842: .Pp
1.1 deraadt 843: Note that the lines in these files are typically hundreds of characters
844: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 845: Rather, generate them by a script
1.40 aaron 846: or by taking
1.2 deraadt 847: .Pa /etc/ssh_host_key.pub
1.1 deraadt 848: and adding the host names at the front.
1.2 deraadt 849: .Ss Examples
1.1 deraadt 850: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 851: .Sh FILES
852: .Bl -tag -width Ds
853: .It Pa /etc/sshd_config
1.1 deraadt 854: Contains configuration data for
1.2 deraadt 855: .Nm sshd .
1.1 deraadt 856: This file should be writable by root only, but it is recommended
857: (though not necessary) that it be world-readable.
1.2 deraadt 858: .It Pa /etc/ssh_host_key
1.7 markus 859: Contains the private part of the host key.
1.1 deraadt 860: This file should only be owned by root, readable only by root, and not
861: accessible to others.
1.14 markus 862: Note that
863: .Nm
864: does not start if this file is group/world-accessible.
1.2 deraadt 865: .It Pa /etc/ssh_host_key.pub
1.7 markus 866: Contains the public part of the host key.
867: This file should be world-readable but writable only by
1.36 aaron 868: root.
869: Its contents should match the private part.
870: This file is not
1.1 deraadt 871: really used for anything; it is only provided for the convenience of
872: the user so its contents can be copied to known hosts files.
1.7 markus 873: These two files are created using
874: .Xr ssh-keygen 1 .
1.2 deraadt 875: .It Pa /var/run/sshd.pid
876: Contains the process ID of the
877: .Nm
1.1 deraadt 878: listening for connections (if there are several daemons running
879: concurrently for different ports, this contains the pid of the one
1.36 aaron 880: started last).
1.58 deraadt 881: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 882: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 883: Lists the RSA keys that can be used to log into the user's account.
884: This file must be readable by root (which may on some machines imply
885: it being world-readable if the user's home directory resides on an NFS
1.36 aaron 886: volume).
887: It is recommended that it not be accessible by others.
888: The format of this file is described above.
1.44 deraadt 889: Users will place the contents of their
890: .Pa identity.pub
891: files into this file, as described in
892: .Xr ssh-keygen 1 .
893: .It Pa $HOME/.ssh/authorized_keys2
894: Lists the DSA keys that can be used to log into the user's account.
895: This file must be readable by root (which may on some machines imply
896: it being world-readable if the user's home directory resides on an NFS
897: volume).
898: It is recommended that it not be accessible by others.
899: The format of this file is described above.
900: Users will place the contents of their
901: .Pa id_dsa.pub
902: files into this file, as described in
903: .Xr ssh-keygen 1 .
1.22 markus 904: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
905: These files are consulted when using rhosts with RSA host
1.36 aaron 906: authentication to check the public key of the host.
907: The key must be listed in one of these files to be accepted.
1.22 markus 908: The client uses the same files
1.58 deraadt 909: to verify that the remote host is the one it intended to connect.
1.36 aaron 910: These files should be writable only by root/the owner.
1.2 deraadt 911: .Pa /etc/ssh_known_hosts
912: should be world-readable, and
913: .Pa $HOME/.ssh/known_hosts
914: can but need not be world-readable.
1.6 aaron 915: .It Pa /etc/nologin
1.40 aaron 916: If this file exists,
1.2 deraadt 917: .Nm
1.36 aaron 918: refuses to let anyone except root log in.
919: The contents of the file
1.1 deraadt 920: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 921: refused.
922: The file should be world-readable.
1.19 dugsong 923: .It Pa /etc/hosts.allow, /etc/hosts.deny
924: If compiled with
925: .Sy LIBWRAP
926: support, tcp-wrappers access controls may be defined here as described in
927: .Xr hosts_access 5 .
1.6 aaron 928: .It Pa $HOME/.rhosts
1.1 deraadt 929: This file contains host-username pairs, separated by a space, one per
1.36 aaron 930: line.
931: The given user on the corresponding host is permitted to log in
932: without password.
933: The same file is used by rlogind and rshd.
1.6 aaron 934: The file must
1.1 deraadt 935: be writable only by the user; it is recommended that it not be
936: accessible by others.
1.2 deraadt 937: .Pp
1.36 aaron 938: If is also possible to use netgroups in the file.
939: Either host or user
1.1 deraadt 940: name may be of the form +@groupname to specify all hosts or all users
941: in the group.
1.2 deraadt 942: .It Pa $HOME/.shosts
943: For ssh,
944: this file is exactly the same as for
945: .Pa .rhosts .
946: However, this file is
947: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 948: .It Pa /etc/hosts.equiv
1.2 deraadt 949: This file is used during
950: .Pa .rhosts
1.36 aaron 951: authentication.
952: In the simplest form, this file contains host names, one per line.
953: Users on
1.1 deraadt 954: those hosts are permitted to log in without a password, provided they
1.36 aaron 955: have the same user name on both machines.
956: The host name may also be
1.1 deraadt 957: followed by a user name; such users are permitted to log in as
1.2 deraadt 958: .Em any
1.36 aaron 959: user on this machine (except root).
960: Additionally, the syntax
1.2 deraadt 961: .Dq +@group
1.36 aaron 962: can be used to specify netgroups.
963: Negated entries start with
1.2 deraadt 964: .Ql \&- .
965: .Pp
1.1 deraadt 966: If the client host/user is successfully matched in this file, login is
967: automatically permitted provided the client and server user names are the
1.36 aaron 968: same.
969: Additionally, successful RSA host authentication is normally required.
970: This file must be writable only by root; it is recommended
1.1 deraadt 971: that it be world-readable.
1.2 deraadt 972: .Pp
1.6 aaron 973: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 974: .Pa hosts.equiv .
1.1 deraadt 975: Beware that it really means that the named user(s) can log in as
1.2 deraadt 976: .Em anybody ,
1.1 deraadt 977: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 978: binaries and directories.
979: Using a user name practically grants the user root access.
980: The only valid use for user names that I can think
1.1 deraadt 981: of is in negative entries.
1.2 deraadt 982: .Pp
983: Note that this warning also applies to rsh/rlogin.
984: .It Pa /etc/shosts.equiv
1.1 deraadt 985: This is processed exactly as
1.2 deraadt 986: .Pa /etc/hosts.equiv .
1.1 deraadt 987: However, this file may be useful in environments that want to run both
1.2 deraadt 988: rsh/rlogin and ssh.
1.6 aaron 989: .It Pa $HOME/.ssh/environment
1.36 aaron 990: This file is read into the environment at login (if it exists).
991: It can only contain empty lines, comment lines (that start with
1.2 deraadt 992: .Ql # ) ,
1.36 aaron 993: and assignment lines of the form name=value.
994: The file should be writable
1.6 aaron 995: only by the user; it need not be readable by anyone else.
1.2 deraadt 996: .It Pa $HOME/.ssh/rc
1.1 deraadt 997: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 998: environment files but before starting the user's shell or command.
999: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1000: standard input (and
1001: .Ev DISPLAY
1.36 aaron 1002: in environment).
1003: This must call
1.2 deraadt 1004: .Xr xauth 1
1005: in that case.
1006: .Pp
1.1 deraadt 1007: The primary purpose of this file is to run any initialization routines
1008: which may be needed before the user's home directory becomes
1009: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1010: .Pp
1.1 deraadt 1011: This file will probably contain some initialization code followed by
1012: something similar to: "if read proto cookie; then echo add $DISPLAY
1013: $proto $cookie | xauth -q -; fi".
1.2 deraadt 1014: .Pp
1015: If this file does not exist,
1016: .Pa /etc/sshrc
1017: is run, and if that
1.1 deraadt 1018: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1019: .Pp
1.1 deraadt 1020: This file should be writable only by the user, and need not be
1021: readable by anyone else.
1.2 deraadt 1022: .It Pa /etc/sshrc
1023: Like
1024: .Pa $HOME/.ssh/rc .
1025: This can be used to specify
1.36 aaron 1026: machine-specific login-time initializations globally.
1027: This file should be writable only by root, and should be world-readable.
1.56 aaron 1028: .El
1.2 deraadt 1029: .Sh AUTHOR
1.17 deraadt 1030: OpenSSH
1.33 deraadt 1031: is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1.36 aaron 1032: but with bugs removed and newer features re-added.
1033: Rapidly after the
1.33 deraadt 1034: 1.2.12 release, newer versions of the original ssh bore successively
1035: more restrictive licenses, and thus demand for a free version was born.
1.48 markus 1036: .Pp
1.33 deraadt 1037: This version of OpenSSH
1.17 deraadt 1038: .Bl -bullet
1039: .It
1.32 aaron 1040: has all components of a restrictive nature (i.e., patents, see
1.17 deraadt 1041: .Xr ssl 8 )
1042: directly removed from the source code; any licensed or patented components
1043: are chosen from
1044: external libraries.
1045: .It
1.48 markus 1046: has been updated to support SSH protocol 1.5 and 2, making it compatible with
1047: all other SSH clients and servers.
1.17 deraadt 1048: .It
1.40 aaron 1049: contains added support for
1.17 deraadt 1050: .Xr kerberos 8
1051: authentication and ticket passing.
1052: .It
1053: supports one-time password authentication with
1054: .Xr skey 1 .
1055: .El
1.48 markus 1056: .Pp
1057: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1058: Niels Provos, Theo de Raadt, and Dug Song.
1059: .Pp
1060: The support for SSH protocol 2 was written by Markus Friedl.
1.2 deraadt 1061: .Sh SEE ALSO
1062: .Xr scp 1 ,
1.63 markus 1063: .Xr sftp-server 8 ,
1.2 deraadt 1064: .Xr ssh 1 ,
1.5 deraadt 1065: .Xr ssh-add 1 ,
1.2 deraadt 1066: .Xr ssh-agent 1 ,
1.5 deraadt 1067: .Xr ssh-keygen 1 ,
1.38 deraadt 1068: .Xr ssl 8 ,
1069: .Xr rlogin 1 ,
1070: .Xr rsh 1
![[BACK]](/icons/back.gif){kind=icon}
Return to sshd.8 CVS log ![[TXT]](/icons/text.gif){kind=icon}
![[DIR]](/icons/dir.gif){kind=icon}
Up to [local] / src / usr.bin / ssh