Annotation of src/usr.bin/ssh/sshd.8, Revision 1.56
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" sshd.8.in
4: .\"
5: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
6: .\"
7: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
8: .\" All rights reserved
9: .\"
10: .\" Created: Sat Apr 22 21:55:14 1995 ylo
11: .\"
1.56 ! aaron 12: .\" $Id: sshd.8,v 1.55 2000/06/26 21:59:18 markus Exp $
1.1 deraadt 13: .\"
1.2 deraadt 14: .Dd September 25, 1999
15: .Dt SSHD 8
16: .Os
17: .Sh NAME
18: .Nm sshd
19: .Nd secure shell daemon
20: .Sh SYNOPSIS
21: .Nm sshd
1.29 markus 22: .Op Fl diqQ46
1.2 deraadt 23: .Op Fl b Ar bits
24: .Op Fl f Ar config_file
25: .Op Fl g Ar login_grace_time
26: .Op Fl h Ar host_key_file
27: .Op Fl k Ar key_gen_time
28: .Op Fl p Ar port
1.26 markus 29: .Op Fl V Ar client_protocol_id
1.40 aaron 30: .Sh DESCRIPTION
1.2 deraadt 31: .Nm
1.40 aaron 32: (Secure Shell Daemon) is the daemon program for
1.2 deraadt 33: .Xr ssh 1 .
1.42 hugh 34: Together these programs replace rlogin and rsh, and
1.1 deraadt 35: provide secure encrypted communications between two untrusted hosts
1.36 aaron 36: over an insecure network.
37: The programs are intended to be as easy to
1.1 deraadt 38: install and use as possible.
1.2 deraadt 39: .Pp
40: .Nm
1.36 aaron 41: is the daemon that listens for connections from clients.
1.40 aaron 42: It is normally started at boot from
1.2 deraadt 43: .Pa /etc/rc .
44: It forks a new
1.36 aaron 45: daemon for each incoming connection.
46: The forked daemons handle
1.1 deraadt 47: key exchange, encryption, authentication, command execution,
48: and data exchange.
1.49 markus 49: This implementation of
50: .Nm
51: supports both SSH protocol version 1 and 2 simultaneously.
1.2 deraadt 52: .Nm
1.36 aaron 53: works as follows.
1.49 markus 54: .Pp
55: .Ss SSH protocol version 1
56: .Pp
1.36 aaron 57: Each host has a host-specific RSA key
58: (normally 1024 bits) used to identify the host.
59: Additionally, when
1.1 deraadt 60: the daemon starts, it generates a server RSA key (normally 768 bits).
61: This key is normally regenerated every hour if it has been used, and
62: is never stored on disk.
1.2 deraadt 63: .Pp
1.42 hugh 64: Whenever a client connects the daemon responds with its public
65: host and server keys.
1.36 aaron 66: The client compares the
1.49 markus 67: RSA host key against its own database to verify that it has not changed.
1.36 aaron 68: The client then generates a 256 bit random number.
69: It encrypts this
1.1 deraadt 70: random number using both the host key and the server key, and sends
1.36 aaron 71: the encrypted number to the server.
1.42 hugh 72: Both sides then use this
1.1 deraadt 73: random number as a session key which is used to encrypt all further
1.36 aaron 74: communications in the session.
75: The rest of the session is encrypted
1.42 hugh 76: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 77: being used by default.
1.36 aaron 78: The client selects the encryption algorithm
1.5 deraadt 79: to use from those offered by the server.
1.2 deraadt 80: .Pp
1.36 aaron 81: Next, the server and the client enter an authentication dialog.
82: The client tries to authenticate itself using
1.2 deraadt 83: .Pa .rhosts
84: authentication,
85: .Pa .rhosts
86: authentication combined with RSA host
1.1 deraadt 87: authentication, RSA challenge-response authentication, or password
88: based authentication.
1.2 deraadt 89: .Pp
1.1 deraadt 90: Rhosts authentication is normally disabled
91: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 92: configuration file if desired.
93: System security is not improved unless
1.2 deraadt 94: .Xr rshd 8 ,
95: .Xr rlogind 8 ,
96: .Xr rexecd 8 ,
97: and
98: .Xr rexd 8
1.1 deraadt 99: are disabled (thus completely disabling
1.2 deraadt 100: .Xr rlogin 1
1.1 deraadt 101: and
1.2 deraadt 102: .Xr rsh 1
1.42 hugh 103: into the machine).
1.2 deraadt 104: .Pp
1.49 markus 105: .Ss SSH protocol version 2
106: .Pp
107: Version 2 works similar:
108: Each host has a host-specific DSA key used to identify the host.
109: However, when the daemon starts, it does not generate a server key.
110: Forward security is provided through a Diffie-Hellman key agreement.
111: This key agreement results in a shared session key.
112: The rest of the session is encrypted
113: using a symmetric cipher, currently
114: Blowfish, 3DES or CAST128 in CBC mode or Arcfour.
115: The client selects the encryption algorithm
116: to use from those offered by the server.
117: Additionally, session integrity is provided
1.51 hugh 118: through a cryptographic message authentication code
1.49 markus 119: (hmac-sha1 or hmac-md5).
120: .Pp
121: Protocol version 2 provides a public key based
122: user authentication method (DSAAuthentication)
123: and conventional password authentication.
124: .Pp
125: .Ss Command execution and data forwarding
126: .Pp
1.1 deraadt 127: If the client successfully authenticates itself, a dialog for
1.36 aaron 128: preparing the session is entered.
129: At this time the client may request
1.1 deraadt 130: things like allocating a pseudo-tty, forwarding X11 connections,
131: forwarding TCP/IP connections, or forwarding the authentication agent
132: connection over the secure channel.
1.2 deraadt 133: .Pp
1.1 deraadt 134: Finally, the client either requests a shell or execution of a command.
1.36 aaron 135: The sides then enter session mode.
136: In this mode, either side may send
1.1 deraadt 137: data at any time, and such data is forwarded to/from the shell or
138: command on the server side, and the user terminal in the client side.
1.2 deraadt 139: .Pp
1.1 deraadt 140: When the user program terminates and all forwarded X11 and other
141: connections have been closed, the server sends command exit status to
142: the client, and both sides exit.
1.2 deraadt 143: .Pp
144: .Nm
1.1 deraadt 145: can be configured using command-line options or a configuration
1.36 aaron 146: file.
147: Command-line options override values specified in the
1.1 deraadt 148: configuration file.
1.25 markus 149: .Pp
150: .Nm
151: rereads its configuration file when it receives a hangup signal,
152: .Dv SIGHUP .
1.18 aaron 153: .Pp
154: The options are as follows:
1.2 deraadt 155: .Bl -tag -width Ds
156: .It Fl b Ar bits
1.1 deraadt 157: Specifies the number of bits in the server key (default 768).
1.2 deraadt 158: .Pp
159: .It Fl d
1.36 aaron 160: Debug mode.
161: The server sends verbose debug output to the system
162: log, and does not put itself in the background.
163: The server also will not fork and will only process one connection.
164: This option is only intended for debugging for the server.
1.2 deraadt 165: .It Fl f Ar configuration_file
1.36 aaron 166: Specifies the name of the configuration file.
167: The default is
1.2 deraadt 168: .Pa /etc/sshd_config .
1.16 markus 169: .Nm
170: refuses to start if there is no configuration file.
1.2 deraadt 171: .It Fl g Ar login_grace_time
1.1 deraadt 172: Gives the grace time for clients to authenticate themselves (default
1.36 aaron 173: 300 seconds).
174: If the client fails to authenticate the user within
175: this many seconds, the server disconnects and exits.
176: A value of zero indicates no limit.
1.2 deraadt 177: .It Fl h Ar host_key_file
1.49 markus 178: Specifies the file from which the RSA host key is read (default
1.2 deraadt 179: .Pa /etc/ssh_host_key ) .
1.7 markus 180: This option must be given if
181: .Nm
182: is not run as root (as the normal
1.1 deraadt 183: host file is normally not readable by anyone but root).
1.2 deraadt 184: .It Fl i
1.7 markus 185: Specifies that
186: .Nm
1.40 aaron 187: is being run from inetd.
1.7 markus 188: .Nm
189: is normally not run
1.1 deraadt 190: from inetd because it needs to generate the server key before it can
1.36 aaron 191: respond to the client, and this may take tens of seconds.
192: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 193: However, with small key sizes (e.g., 512) using
1.7 markus 194: .Nm
195: from inetd may
1.1 deraadt 196: be feasible.
1.2 deraadt 197: .It Fl k Ar key_gen_time
1.1 deraadt 198: Specifies how often the server key is regenerated (default 3600
1.36 aaron 199: seconds, or one hour).
200: The motivation for regenerating the key fairly
1.1 deraadt 201: often is that the key is not stored anywhere, and after about an hour,
202: it becomes impossible to recover the key for decrypting intercepted
203: communications even if the machine is cracked into or physically
1.36 aaron 204: seized.
205: A value of zero indicates that the key will never be regenerated.
1.2 deraadt 206: .It Fl p Ar port
1.1 deraadt 207: Specifies the port on which the server listens for connections
208: (default 22).
1.2 deraadt 209: .It Fl q
1.36 aaron 210: Quiet mode.
211: Nothing is sent to the system log.
212: Normally the beginning,
1.1 deraadt 213: authentication, and termination of each connection is logged.
1.18 aaron 214: .It Fl Q
215: Do not print an error message if RSA support is missing.
1.26 markus 216: .It Fl V Ar client_protocol_id
217: SSH2 compatibility mode.
1.42 hugh 218: When this option is specified
1.26 markus 219: .Nm
1.42 hugh 220: assumes the client has sent the supplied version string
1.26 markus 221: and skips the
222: Protocol Version Identification Exchange.
1.29 markus 223: .It Fl 4
224: Forces
225: .Nm
226: to use IPv4 addresses only.
227: .It Fl 6
228: Forces
229: .Nm
230: to use IPv6 addresses only.
1.2 deraadt 231: .El
232: .Sh CONFIGURATION FILE
233: .Nm
1.40 aaron 234: reads configuration data from
1.2 deraadt 235: .Pa /etc/sshd_config
236: (or the file specified with
237: .Fl f
1.36 aaron 238: on the command line).
239: The file contains keyword-value pairs, one per line.
240: Lines starting with
1.2 deraadt 241: .Ql #
1.1 deraadt 242: and empty lines are interpreted as comments.
1.2 deraadt 243: .Pp
1.1 deraadt 244: The following keywords are possible.
1.2 deraadt 245: .Bl -tag -width Ds
246: .It Cm AFSTokenPassing
1.36 aaron 247: Specifies whether an AFS token may be forwarded to the server.
248: Default is
1.2 deraadt 249: .Dq yes .
1.11 markus 250: .It Cm AllowGroups
251: This keyword can be followed by a number of group names, separated
1.36 aaron 252: by spaces.
253: If specified, login is allowed only for users whose primary
1.11 markus 254: group matches one of the patterns.
255: .Ql \&*
256: and
257: .Ql ?
258: can be used as
1.36 aaron 259: wildcards in the patterns.
260: Only group names are valid, a numerical group ID isn't recognized.
261: By default login is allowed regardless of the primary group.
1.3 dugsong 262: .Pp
1.11 markus 263: .It Cm AllowUsers
264: This keyword can be followed by a number of user names, separated
1.36 aaron 265: by spaces.
266: If specified, login is allowed only for users names that
1.11 markus 267: match one of the patterns.
268: .Ql \&*
269: and
270: .Ql ?
271: can be used as
1.36 aaron 272: wildcards in the patterns.
273: Only user names are valid, a numerical user ID isn't recognized.
274: By default login is allowed regardless of the user name.
1.11 markus 275: .Pp
1.41 markus 276: .It Cm Ciphers
277: Specifies the ciphers allowed for protocol version 2.
278: Multiple ciphers must be comma-separated.
279: The default is
1.50 markus 280: .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
1.8 markus 281: .It Cm CheckMail
282: Specifies whether
283: .Nm
284: should check for new mail for interactive logins.
285: The default is
286: .Dq no .
1.11 markus 287: .It Cm DenyGroups
288: This keyword can be followed by a number of group names, separated
1.36 aaron 289: by spaces.
290: Users whose primary group matches one of the patterns
1.11 markus 291: aren't allowed to log in.
292: .Ql \&*
293: and
294: .Ql ?
295: can be used as
1.36 aaron 296: wildcards in the patterns.
297: Only group names are valid, a numerical group ID isn't recognized.
298: By default login is allowed regardless of the primary group.
1.11 markus 299: .Pp
300: .It Cm DenyUsers
301: This keyword can be followed by a number of user names, separated
1.36 aaron 302: by spaces.
303: Login is disallowed for user names that match one of the patterns.
1.11 markus 304: .Ql \&*
305: and
306: .Ql ?
1.36 aaron 307: can be used as wildcards in the patterns.
308: Only user names are valid, a numerical user ID isn't recognized.
309: By default login is allowed regardless of the user name.
1.49 markus 310: .It Cm DSAAuthentication
311: Specifies whether DSA authentication is allowed.
312: The default is
313: .Dq yes .
314: Note that this option applies to protocol version 2 only.
1.47 markus 315: .It Cm GatewayPorts
316: Specifies whether remote hosts are allowed to connect to ports
317: forwarded for the client.
318: The argument must be
319: .Dq yes
320: or
321: .Dq no .
322: The default is
323: .Dq no .
1.46 markus 324: .It Cm HostDsaKey
325: Specifies the file containing the private DSA host key (default
326: .Pa /etc/ssh_host_dsa_key )
327: used by SSH protocol 2.0.
328: Note that
329: .Nm
1.51 hugh 330: disables protocol 2.0 if this file is group/world-accessible.
1.2 deraadt 331: .It Cm HostKey
1.46 markus 332: Specifies the file containing the private RSA host key (default
333: .Pa /etc/ssh_host_key )
334: used by SSH protocols 1.3 and 1.5.
1.9 markus 335: Note that
336: .Nm
1.51 hugh 337: disables protocols 1.3 and 1.5 if this file is group/world-accessible.
1.2 deraadt 338: .It Cm IgnoreRhosts
1.34 markus 339: Specifies that
340: .Pa .rhosts
1.40 aaron 341: and
1.34 markus 342: .Pa .shosts
343: files will not be used in authentication.
1.2 deraadt 344: .Pa /etc/hosts.equiv
1.1 deraadt 345: and
1.40 aaron 346: .Pa /etc/shosts.equiv
1.36 aaron 347: are still used.
1.40 aaron 348: The default is
1.34 markus 349: .Dq yes .
1.24 markus 350: .It Cm IgnoreUserKnownHosts
351: Specifies whether
352: .Nm
353: should ignore the user's
354: .Pa $HOME/.ssh/known_hosts
1.45 markus 355: during
1.24 markus 356: .Cm RhostsRSAAuthentication .
357: The default is
1.2 deraadt 358: .Dq no .
359: .It Cm KeepAlive
1.1 deraadt 360: Specifies whether the system should send keepalive messages to the
1.36 aaron 361: other side.
362: If they are sent, death of the connection or crash of one
363: of the machines will be properly noticed.
364: However, this means that
1.1 deraadt 365: connections will die if the route is down temporarily, and some people
1.36 aaron 366: find it annoying.
1.51 hugh 367: On the other hand, if keepalives are not sent,
1.2 deraadt 368: sessions may hang indefinitely on the server, leaving
369: .Dq ghost
370: users and consuming server resources.
371: .Pp
372: The default is
373: .Dq yes
374: (to send keepalives), and the server will notice
1.36 aaron 375: if the network goes down or the client host reboots.
376: This avoids infinitely hanging sessions.
1.2 deraadt 377: .Pp
378: To disable keepalives, the value should be set to
379: .Dq no
380: in both the server and the client configuration files.
381: .It Cm KerberosAuthentication
1.36 aaron 382: Specifies whether Kerberos authentication is allowed.
383: This can be in the form of a Kerberos ticket, or if
1.7 markus 384: .Cm PasswordAuthentication
1.1 deraadt 385: is yes, the password provided by the user will be validated through
1.36 aaron 386: the Kerberos KDC.
387: Default is
1.3 dugsong 388: .Dq yes .
1.2 deraadt 389: .It Cm KerberosOrLocalPasswd
1.1 deraadt 390: If set then if password authentication through Kerberos fails then
391: the password will be validated via any additional local mechanism
1.2 deraadt 392: such as
393: .Pa /etc/passwd
1.36 aaron 394: or SecurID.
395: Default is
1.20 dugsong 396: .Dq yes .
1.2 deraadt 397: .It Cm KerberosTgtPassing
1.1 deraadt 398: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 399: Default is
1.3 dugsong 400: .Dq no ,
401: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 402: .It Cm KerberosTicketCleanup
1.7 markus 403: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 404: file on logout.
405: Default is
1.3 dugsong 406: .Dq yes .
1.2 deraadt 407: .It Cm KeyRegenerationInterval
1.1 deraadt 408: The server key is automatically regenerated after this many seconds
1.36 aaron 409: (if it has been used).
410: The purpose of regeneration is to prevent
1.1 deraadt 411: decrypting captured sessions by later breaking into the machine and
1.36 aaron 412: stealing the keys.
413: The key is never stored anywhere.
414: If the value is 0, the key is never regenerated.
415: The default is 3600 (seconds).
1.7 markus 416: .It Cm ListenAddress
417: Specifies what local address
418: .Nm
419: should listen on.
420: The default is to listen to all local addresses.
1.28 markus 421: Multiple options of this type are permitted.
422: Additionally, the
423: .Cm Ports
424: options must precede this option.
1.2 deraadt 425: .It Cm LoginGraceTime
1.1 deraadt 426: The server disconnects after this time if the user has not
1.36 aaron 427: successfully logged in.
428: If the value is 0, there is no time limit.
1.1 deraadt 429: The default is 600 (seconds).
1.23 markus 430: .It Cm LogLevel
431: Gives the verbosity level that is used when logging messages from
432: .Nm sshd .
433: The possible values are:
1.27 markus 434: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.23 markus 435: The default is INFO.
436: Logging with level DEBUG violates the privacy of users
437: and is not recommended.
1.55 markus 438: .It Cm MaxStartups
439: Specifies the maximum number of concurrent unauthenticated connections to the
440: .Nm
441: daemon.
442: Additional connections will be dropped until authentication succeeds or the
443: .Cm LoginGraceTime
444: expires for a connection.
445: The default is 10.
1.2 deraadt 446: .It Cm PasswordAuthentication
1.1 deraadt 447: Specifies whether password authentication is allowed.
1.2 deraadt 448: The default is
449: .Dq yes .
1.49 markus 450: Note that this option applies to both protocol version 1 and 2.
1.2 deraadt 451: .It Cm PermitEmptyPasswords
1.1 deraadt 452: When password authentication is allowed, it specifies whether the
1.36 aaron 453: server allows login to accounts with empty password strings.
454: The default is
1.34 markus 455: .Dq no .
1.2 deraadt 456: .It Cm PermitRootLogin
1.1 deraadt 457: Specifies whether the root can log in using
1.2 deraadt 458: .Xr ssh 1 .
1.15 markus 459: The argument must be
460: .Dq yes ,
461: .Dq without-password
462: or
463: .Dq no .
1.2 deraadt 464: The default is
465: .Dq yes .
1.15 markus 466: If this options is set to
467: .Dq without-password
468: only password authentication is disabled for root.
1.2 deraadt 469: .Pp
470: Root login with RSA authentication when the
471: .Ar command
472: option has been
1.1 deraadt 473: specified will be allowed regardless of the value of this setting
474: (which may be useful for taking remote backups even if root login is
475: normally not allowed).
1.43 markus 476: .It Cm PidFile
477: Specifies the file that contains the process identifier of the
478: .Nm
479: daemon.
480: The default is
481: .Pa /var/run/sshd.pid .
1.2 deraadt 482: .It Cm Port
1.1 deraadt 483: Specifies the port number that
1.2 deraadt 484: .Nm
1.36 aaron 485: listens on.
486: The default is 22.
1.28 markus 487: Multiple options of this type are permitted.
1.2 deraadt 488: .It Cm PrintMotd
1.1 deraadt 489: Specifies whether
1.2 deraadt 490: .Nm
1.40 aaron 491: should print
1.2 deraadt 492: .Pa /etc/motd
1.36 aaron 493: when a user logs in interactively.
494: (On some systems it is also printed by the shell,
1.2 deraadt 495: .Pa /etc/profile ,
1.36 aaron 496: or equivalent.)
497: The default is
1.2 deraadt 498: .Dq yes .
1.41 markus 499: .It Cm Protocol
500: Specifies the protocol versions
501: .Nm
502: should support.
503: The possible values are
504: .Dq 1
505: and
506: .Dq 2 .
507: Multiple versions must be comma-separated.
508: The default is
509: .Dq 1 .
1.2 deraadt 510: .It Cm RandomSeed
1.36 aaron 511: Obsolete.
512: Random number generation uses other techniques.
1.2 deraadt 513: .It Cm RhostsAuthentication
1.1 deraadt 514: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 515: files is sufficient.
516: Normally, this method should not be permitted because it is insecure.
1.7 markus 517: .Cm RhostsRSAAuthentication
518: should be used
1.1 deraadt 519: instead, because it performs RSA-based host authentication in addition
520: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 521: The default is
522: .Dq no .
523: .It Cm RhostsRSAAuthentication
1.1 deraadt 524: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 525: with successful RSA host authentication is allowed.
526: The default is
1.34 markus 527: .Dq no .
1.2 deraadt 528: .It Cm RSAAuthentication
1.36 aaron 529: Specifies whether pure RSA authentication is allowed.
530: The default is
1.2 deraadt 531: .Dq yes .
1.49 markus 532: Note that this option applies to protocol version 1 only.
1.2 deraadt 533: .It Cm ServerKeyBits
1.36 aaron 534: Defines the number of bits in the server key.
535: The minimum value is 512, and the default is 768.
1.7 markus 536: .It Cm SkeyAuthentication
537: Specifies whether
1.40 aaron 538: .Xr skey 1
1.36 aaron 539: authentication is allowed.
540: The default is
1.7 markus 541: .Dq yes .
542: Note that s/key authentication is enabled only if
543: .Cm PasswordAuthentication
544: is allowed, too.
1.2 deraadt 545: .It Cm StrictModes
1.12 markus 546: Specifies whether
547: .Nm
548: should check file modes and ownership of the
1.36 aaron 549: user's files and home directory before accepting login.
550: This is normally desirable because novices sometimes accidentally leave their
551: directory or files world-writable.
552: The default is
1.7 markus 553: .Dq yes .
1.54 jakob 554: .It Cm Subsystem
555: Configures an external subsystem (e.g. file transfer daemon).
556: Arguments should be a subsystem name and a command to execute upon subsystem request.
557: By default no subsystems are defined.
558: Note that this option applies to protocol version 2 only.
1.2 deraadt 559: .It Cm SyslogFacility
1.1 deraadt 560: Gives the facility code that is used when logging messages from
1.2 deraadt 561: .Nm sshd .
1.1 deraadt 562: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 563: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
564: The default is AUTH.
1.10 markus 565: .It Cm UseLogin
566: Specifies whether
567: .Xr login 1
1.53 markus 568: is used for interactive login sessions.
569: Note that
570: .Xr login 1
571: is not never for remote command execution.
1.36 aaron 572: The default is
1.10 markus 573: .Dq no .
1.6 aaron 574: .It Cm X11DisplayOffset
575: Specifies the first display number available for
576: .Nm sshd Ns 's
1.36 aaron 577: X11 forwarding.
578: This prevents
1.6 aaron 579: .Nm
580: from interfering with real X11 servers.
1.34 markus 581: The default is 10.
1.30 markus 582: .It Cm X11Forwarding
1.36 aaron 583: Specifies whether X11 forwarding is permitted.
584: The default is
1.34 markus 585: .Dq no .
1.30 markus 586: Note that disabling X11 forwarding does not improve security in any
587: way, as users can always install their own forwarders.
1.52 markus 588: .It Cm XAuthLocation
589: Specifies the location of the
590: .Xr xauth 1
591: program.
592: The default is
593: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 594: .El
595: .Sh LOGIN PROCESS
1.1 deraadt 596: When a user successfully logs in,
1.2 deraadt 597: .Nm
1.1 deraadt 598: does the following:
1.2 deraadt 599: .Bl -enum -offset indent
600: .It
1.1 deraadt 601: If the login is on a tty, and no command has been specified,
1.40 aaron 602: prints last login time and
1.2 deraadt 603: .Pa /etc/motd
1.1 deraadt 604: (unless prevented in the configuration file or by
1.2 deraadt 605: .Pa $HOME/.hushlogin ;
606: see the
1.40 aaron 607: .Sx FILES
1.2 deraadt 608: section).
609: .It
1.1 deraadt 610: If the login is on a tty, records login time.
1.2 deraadt 611: .It
612: Checks
613: .Pa /etc/nologin ;
614: if it exists, prints contents and quits
1.1 deraadt 615: (unless root).
1.2 deraadt 616: .It
1.1 deraadt 617: Changes to run with normal user privileges.
1.2 deraadt 618: .It
1.1 deraadt 619: Sets up basic environment.
1.2 deraadt 620: .It
621: Reads
622: .Pa $HOME/.ssh/environment
623: if it exists.
624: .It
1.1 deraadt 625: Changes to user's home directory.
1.2 deraadt 626: .It
627: If
628: .Pa $HOME/.ssh/rc
629: exists, runs it; else if
630: .Pa /etc/sshrc
631: exists, runs
1.36 aaron 632: it; otherwise runs xauth.
633: The
1.2 deraadt 634: .Dq rc
635: files are given the X11
1.1 deraadt 636: authentication protocol and cookie in standard input.
1.2 deraadt 637: .It
1.1 deraadt 638: Runs user's shell or command.
1.2 deraadt 639: .El
640: .Sh AUTHORIZED_KEYS FILE FORMAT
1.40 aaron 641: The
1.2 deraadt 642: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 643: file lists the RSA keys that are
1.44 deraadt 644: permitted for RSA authentication in SSH protocols 1.3 and 1.5
1.51 hugh 645: Similarly, the
1.44 deraadt 646: .Pa $HOME/.ssh/authorized_keys2
647: file lists the DSA keys that are
648: permitted for DSA authentication in SSH protocol 2.0.
1.36 aaron 649: Each line of the file contains one
1.2 deraadt 650: key (empty lines and lines starting with a
651: .Ql #
652: are ignored as
1.36 aaron 653: comments).
654: Each line consists of the following fields, separated by
655: spaces: options, bits, exponent, modulus, comment.
656: The options field
1.1 deraadt 657: is optional; its presence is determined by whether the line starts
658: with a number or not (the option field never starts with a number).
659: The bits, exponent, modulus and comment fields give the RSA key; the
660: comment field is not used for anything (but may be convenient for the
661: user to identify the key).
1.2 deraadt 662: .Pp
1.1 deraadt 663: Note that lines in this file are usually several hundred bytes long
1.36 aaron 664: (because of the size of the RSA key modulus).
665: You don't want to type them in; instead, copy the
1.2 deraadt 666: .Pa identity.pub
1.1 deraadt 667: file and edit it.
1.2 deraadt 668: .Pp
1.1 deraadt 669: The options (if present) consists of comma-separated option
1.36 aaron 670: specifications.
671: No spaces are permitted, except within double quotes.
1.1 deraadt 672: The following option specifications are supported:
1.2 deraadt 673: .Bl -tag -width Ds
674: .It Cm from="pattern-list"
1.1 deraadt 675: Specifies that in addition to RSA authentication, the canonical name
676: of the remote host must be present in the comma-separated list of
1.36 aaron 677: patterns
678: .Pf ( Ql *
679: and
680: .Ql ?
681: serve as wildcards).
682: The list may also contain
683: patterns negated by prefixing them with
684: .Ql ! ;
685: if the canonical host name matches a negated pattern, the key is not accepted.
686: The purpose
1.1 deraadt 687: of this option is to optionally increase security: RSA authentication
688: by itself does not trust the network or name servers or anything (but
689: the key); however, if somebody somehow steals the key, the key
1.36 aaron 690: permits an intruder to log in from anywhere in the world.
691: This additional option makes using a stolen key more difficult (name
1.1 deraadt 692: servers and/or routers would have to be compromised in addition to
693: just the key).
1.2 deraadt 694: .It Cm command="command"
1.1 deraadt 695: Specifies that the command is executed whenever this key is used for
1.36 aaron 696: authentication.
697: The command supplied by the user (if any) is ignored.
1.1 deraadt 698: The command is run on a pty if the connection requests a pty;
1.36 aaron 699: otherwise it is run without a tty.
700: A quote may be included in the command by quoting it with a backslash.
701: This option might be useful
702: to restrict certain RSA keys to perform just a specific operation.
703: An example might be a key that permits remote backups but nothing else.
1.51 hugh 704: Note that the client may specify TCP/IP and/or X11
705: forwarding unless they are explicitly prohibited.
1.2 deraadt 706: .It Cm environment="NAME=value"
1.1 deraadt 707: Specifies that the string is to be added to the environment when
1.36 aaron 708: logging in using this key.
709: Environment variables set this way
710: override other default environment values.
711: Multiple options of this type are permitted.
1.2 deraadt 712: .It Cm no-port-forwarding
1.1 deraadt 713: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 714: Any port forward requests by the client will return an error.
715: This might be used, e.g., in connection with the
1.2 deraadt 716: .Cm command
1.1 deraadt 717: option.
1.2 deraadt 718: .It Cm no-X11-forwarding
1.1 deraadt 719: Forbids X11 forwarding when this key is used for authentication.
720: Any X11 forward requests by the client will return an error.
1.2 deraadt 721: .It Cm no-agent-forwarding
1.1 deraadt 722: Forbids authentication agent forwarding when this key is used for
723: authentication.
1.2 deraadt 724: .It Cm no-pty
1.1 deraadt 725: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 726: .El
727: .Ss Examples
1.1 deraadt 728: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 729: .Pp
1.1 deraadt 730: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 731: .Pp
1.1 deraadt 732: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 733: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 734: The
1.44 deraadt 735: .Pa /etc/ssh_known_hosts ,
736: .Pa /etc/ssh_known_hosts2 ,
737: .Pa $HOME/.ssh/known_hosts ,
1.40 aaron 738: and
1.44 deraadt 739: .Pa $HOME/.ssh/known_hosts2
1.36 aaron 740: files contain host public keys for all known hosts.
741: The global file should
1.37 brad 742: be prepared by the administrator (optional), and the per-user file is
1.1 deraadt 743: maintained automatically: whenever the user connects an unknown host
1.36 aaron 744: its key is added to the per-user file.
1.2 deraadt 745: .Pp
1.1 deraadt 746: Each line in these files contains the following fields: hostnames,
1.36 aaron 747: bits, exponent, modulus, comment.
748: The fields are separated by spaces.
1.2 deraadt 749: .Pp
1.1 deraadt 750: Hostnames is a comma-separated list of patterns ('*' and '?' act as
751: wildcards); each pattern in turn is matched against the canonical host
752: name (when authenticating a client) or against the user-supplied
1.36 aaron 753: name (when authenticating a server).
754: A pattern may also be preceded by
1.2 deraadt 755: .Ql !
756: to indicate negation: if the host name matches a negated
1.1 deraadt 757: pattern, it is not accepted (by that line) even if it matched another
758: pattern on the line.
1.2 deraadt 759: .Pp
1.49 markus 760: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 761: can be obtained, e.g., from
762: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 763: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 764: .Pp
765: Lines starting with
766: .Ql #
767: and empty lines are ignored as comments.
768: .Pp
1.1 deraadt 769: When performing host authentication, authentication is accepted if any
1.36 aaron 770: matching line has the proper key.
771: It is thus permissible (but not
1.1 deraadt 772: recommended) to have several lines or different host keys for the same
1.36 aaron 773: names.
774: This will inevitably happen when short forms of host names
775: from different domains are put in the file.
776: It is possible
1.1 deraadt 777: that the files contain conflicting information; authentication is
778: accepted if valid information can be found from either file.
1.2 deraadt 779: .Pp
1.1 deraadt 780: Note that the lines in these files are typically hundreds of characters
781: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 782: Rather, generate them by a script
1.40 aaron 783: or by taking
1.2 deraadt 784: .Pa /etc/ssh_host_key.pub
1.1 deraadt 785: and adding the host names at the front.
1.2 deraadt 786: .Ss Examples
1.1 deraadt 787: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 788: .Sh FILES
789: .Bl -tag -width Ds
790: .It Pa /etc/sshd_config
1.1 deraadt 791: Contains configuration data for
1.2 deraadt 792: .Nm sshd .
1.1 deraadt 793: This file should be writable by root only, but it is recommended
794: (though not necessary) that it be world-readable.
1.2 deraadt 795: .It Pa /etc/ssh_host_key
1.7 markus 796: Contains the private part of the host key.
1.1 deraadt 797: This file should only be owned by root, readable only by root, and not
798: accessible to others.
1.14 markus 799: Note that
800: .Nm
801: does not start if this file is group/world-accessible.
1.2 deraadt 802: .It Pa /etc/ssh_host_key.pub
1.7 markus 803: Contains the public part of the host key.
804: This file should be world-readable but writable only by
1.36 aaron 805: root.
806: Its contents should match the private part.
807: This file is not
1.1 deraadt 808: really used for anything; it is only provided for the convenience of
809: the user so its contents can be copied to known hosts files.
1.7 markus 810: These two files are created using
811: .Xr ssh-keygen 1 .
1.2 deraadt 812: .It Pa /var/run/sshd.pid
813: Contains the process ID of the
814: .Nm
1.1 deraadt 815: listening for connections (if there are several daemons running
816: concurrently for different ports, this contains the pid of the one
1.36 aaron 817: started last).
818: The contents of this file are not sensitive; it can be world-readable.
1.2 deraadt 819: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 820: Lists the RSA keys that can be used to log into the user's account.
821: This file must be readable by root (which may on some machines imply
822: it being world-readable if the user's home directory resides on an NFS
1.36 aaron 823: volume).
824: It is recommended that it not be accessible by others.
825: The format of this file is described above.
1.44 deraadt 826: Users will place the contents of their
827: .Pa identity.pub
828: files into this file, as described in
829: .Xr ssh-keygen 1 .
830: .It Pa $HOME/.ssh/authorized_keys2
831: Lists the DSA keys that can be used to log into the user's account.
832: This file must be readable by root (which may on some machines imply
833: it being world-readable if the user's home directory resides on an NFS
834: volume).
835: It is recommended that it not be accessible by others.
836: The format of this file is described above.
837: Users will place the contents of their
838: .Pa id_dsa.pub
839: files into this file, as described in
840: .Xr ssh-keygen 1 .
1.22 markus 841: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
842: These files are consulted when using rhosts with RSA host
1.36 aaron 843: authentication to check the public key of the host.
844: The key must be listed in one of these files to be accepted.
1.22 markus 845: The client uses the same files
1.36 aaron 846: to verify that the remote host is the one we intended to connect.
847: These files should be writable only by root/the owner.
1.2 deraadt 848: .Pa /etc/ssh_known_hosts
849: should be world-readable, and
850: .Pa $HOME/.ssh/known_hosts
851: can but need not be world-readable.
1.6 aaron 852: .It Pa /etc/nologin
1.40 aaron 853: If this file exists,
1.2 deraadt 854: .Nm
1.36 aaron 855: refuses to let anyone except root log in.
856: The contents of the file
1.1 deraadt 857: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 858: refused.
859: The file should be world-readable.
1.19 dugsong 860: .It Pa /etc/hosts.allow, /etc/hosts.deny
861: If compiled with
862: .Sy LIBWRAP
863: support, tcp-wrappers access controls may be defined here as described in
864: .Xr hosts_access 5 .
1.6 aaron 865: .It Pa $HOME/.rhosts
1.1 deraadt 866: This file contains host-username pairs, separated by a space, one per
1.36 aaron 867: line.
868: The given user on the corresponding host is permitted to log in
869: without password.
870: The same file is used by rlogind and rshd.
1.6 aaron 871: The file must
1.1 deraadt 872: be writable only by the user; it is recommended that it not be
873: accessible by others.
1.2 deraadt 874: .Pp
1.36 aaron 875: If is also possible to use netgroups in the file.
876: Either host or user
1.1 deraadt 877: name may be of the form +@groupname to specify all hosts or all users
878: in the group.
1.2 deraadt 879: .It Pa $HOME/.shosts
880: For ssh,
881: this file is exactly the same as for
882: .Pa .rhosts .
883: However, this file is
884: not used by rlogin and rshd, so using this permits access using SSH only.
885: .Pa /etc/hosts.equiv
886: This file is used during
887: .Pa .rhosts
1.36 aaron 888: authentication.
889: In the simplest form, this file contains host names, one per line.
890: Users on
1.1 deraadt 891: those hosts are permitted to log in without a password, provided they
1.36 aaron 892: have the same user name on both machines.
893: The host name may also be
1.1 deraadt 894: followed by a user name; such users are permitted to log in as
1.2 deraadt 895: .Em any
1.36 aaron 896: user on this machine (except root).
897: Additionally, the syntax
1.2 deraadt 898: .Dq +@group
1.36 aaron 899: can be used to specify netgroups.
900: Negated entries start with
1.2 deraadt 901: .Ql \&- .
902: .Pp
1.1 deraadt 903: If the client host/user is successfully matched in this file, login is
904: automatically permitted provided the client and server user names are the
1.36 aaron 905: same.
906: Additionally, successful RSA host authentication is normally required.
907: This file must be writable only by root; it is recommended
1.1 deraadt 908: that it be world-readable.
1.2 deraadt 909: .Pp
1.6 aaron 910: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 911: .Pa hosts.equiv .
1.1 deraadt 912: Beware that it really means that the named user(s) can log in as
1.2 deraadt 913: .Em anybody ,
1.1 deraadt 914: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 915: binaries and directories.
916: Using a user name practically grants the user root access.
917: The only valid use for user names that I can think
1.1 deraadt 918: of is in negative entries.
1.2 deraadt 919: .Pp
920: Note that this warning also applies to rsh/rlogin.
921: .It Pa /etc/shosts.equiv
1.1 deraadt 922: This is processed exactly as
1.2 deraadt 923: .Pa /etc/hosts.equiv .
1.1 deraadt 924: However, this file may be useful in environments that want to run both
1.2 deraadt 925: rsh/rlogin and ssh.
1.6 aaron 926: .It Pa $HOME/.ssh/environment
1.36 aaron 927: This file is read into the environment at login (if it exists).
928: It can only contain empty lines, comment lines (that start with
1.2 deraadt 929: .Ql # ) ,
1.36 aaron 930: and assignment lines of the form name=value.
931: The file should be writable
1.6 aaron 932: only by the user; it need not be readable by anyone else.
1.2 deraadt 933: .It Pa $HOME/.ssh/rc
1.1 deraadt 934: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 935: environment files but before starting the user's shell or command.
936: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 937: standard input (and
938: .Ev DISPLAY
1.36 aaron 939: in environment).
940: This must call
1.2 deraadt 941: .Xr xauth 1
942: in that case.
943: .Pp
1.1 deraadt 944: The primary purpose of this file is to run any initialization routines
945: which may be needed before the user's home directory becomes
946: accessible; AFS is a particular example of such an environment.
1.2 deraadt 947: .Pp
1.1 deraadt 948: This file will probably contain some initialization code followed by
949: something similar to: "if read proto cookie; then echo add $DISPLAY
950: $proto $cookie | xauth -q -; fi".
1.2 deraadt 951: .Pp
952: If this file does not exist,
953: .Pa /etc/sshrc
954: is run, and if that
1.1 deraadt 955: does not exist either, xauth is used to store the cookie.
1.2 deraadt 956: .Pp
1.1 deraadt 957: This file should be writable only by the user, and need not be
958: readable by anyone else.
1.2 deraadt 959: .It Pa /etc/sshrc
960: Like
961: .Pa $HOME/.ssh/rc .
962: This can be used to specify
1.36 aaron 963: machine-specific login-time initializations globally.
964: This file should be writable only by root, and should be world-readable.
1.56 ! aaron 965: .El
1.2 deraadt 966: .Sh AUTHOR
1.17 deraadt 967: OpenSSH
1.33 deraadt 968: is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1.36 aaron 969: but with bugs removed and newer features re-added.
970: Rapidly after the
1.33 deraadt 971: 1.2.12 release, newer versions of the original ssh bore successively
972: more restrictive licenses, and thus demand for a free version was born.
1.48 markus 973: .Pp
1.33 deraadt 974: This version of OpenSSH
1.17 deraadt 975: .Bl -bullet
976: .It
1.32 aaron 977: has all components of a restrictive nature (i.e., patents, see
1.17 deraadt 978: .Xr ssl 8 )
979: directly removed from the source code; any licensed or patented components
980: are chosen from
981: external libraries.
982: .It
1.48 markus 983: has been updated to support SSH protocol 1.5 and 2, making it compatible with
984: all other SSH clients and servers.
1.17 deraadt 985: .It
1.40 aaron 986: contains added support for
1.17 deraadt 987: .Xr kerberos 8
988: authentication and ticket passing.
989: .It
990: supports one-time password authentication with
991: .Xr skey 1 .
992: .El
993: .Pp
994: The libraries described in
1.5 deraadt 995: .Xr ssl 8
996: are required for proper operation.
1.48 markus 997: .Pp
998: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
999: Niels Provos, Theo de Raadt, and Dug Song.
1000: .Pp
1001: The support for SSH protocol 2 was written by Markus Friedl.
1.2 deraadt 1002: .Sh SEE ALSO
1003: .Xr scp 1 ,
1004: .Xr ssh 1 ,
1.5 deraadt 1005: .Xr ssh-add 1 ,
1.2 deraadt 1006: .Xr ssh-agent 1 ,
1.5 deraadt 1007: .Xr ssh-keygen 1 ,
1.38 deraadt 1008: .Xr ssl 8 ,
1009: .Xr rlogin 1 ,
1010: .Xr rsh 1
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