Annotation of src/usr.bin/ssh/sshd.8, Revision 1.42
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: .\"
1.42 ! hugh 12: .\" $Id: sshd.8,v 1.41 2000/04/12 23:00:28 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.
1.2 deraadt 49: .Pp
50: .Nm
1.36 aaron 51: works as follows.
52: Each host has a host-specific RSA key
53: (normally 1024 bits) used to identify the host.
54: Additionally, when
1.1 deraadt 55: the daemon starts, it generates a server RSA key (normally 768 bits).
56: This key is normally regenerated every hour if it has been used, and
57: is never stored on disk.
1.2 deraadt 58: .Pp
1.42 ! hugh 59: Whenever a client connects the daemon responds with its public
! 60: host and server keys.
1.36 aaron 61: The client compares the
1.1 deraadt 62: host key against its own database to verify that it has not changed.
1.36 aaron 63: The client then generates a 256 bit random number.
64: It encrypts this
1.1 deraadt 65: random number using both the host key and the server key, and sends
1.36 aaron 66: the encrypted number to the server.
1.42 ! hugh 67: Both sides then use this
1.1 deraadt 68: random number as a session key which is used to encrypt all further
1.36 aaron 69: communications in the session.
70: The rest of the session is encrypted
1.42 ! hugh 71: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 72: being used by default.
1.36 aaron 73: The client selects the encryption algorithm
1.5 deraadt 74: to use from those offered by the server.
1.2 deraadt 75: .Pp
1.36 aaron 76: Next, the server and the client enter an authentication dialog.
77: The client tries to authenticate itself using
1.2 deraadt 78: .Pa .rhosts
79: authentication,
80: .Pa .rhosts
81: authentication combined with RSA host
1.1 deraadt 82: authentication, RSA challenge-response authentication, or password
83: based authentication.
1.2 deraadt 84: .Pp
1.1 deraadt 85: Rhosts authentication is normally disabled
86: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 87: configuration file if desired.
88: System security is not improved unless
1.2 deraadt 89: .Xr rshd 8 ,
90: .Xr rlogind 8 ,
91: .Xr rexecd 8 ,
92: and
93: .Xr rexd 8
1.1 deraadt 94: are disabled (thus completely disabling
1.2 deraadt 95: .Xr rlogin 1
1.1 deraadt 96: and
1.2 deraadt 97: .Xr rsh 1
1.42 ! hugh 98: into the machine).
1.2 deraadt 99: .Pp
1.1 deraadt 100: If the client successfully authenticates itself, a dialog for
1.36 aaron 101: preparing the session is entered.
102: At this time the client may request
1.1 deraadt 103: things like allocating a pseudo-tty, forwarding X11 connections,
104: forwarding TCP/IP connections, or forwarding the authentication agent
105: connection over the secure channel.
1.2 deraadt 106: .Pp
1.1 deraadt 107: Finally, the client either requests a shell or execution of a command.
1.36 aaron 108: The sides then enter session mode.
109: In this mode, either side may send
1.1 deraadt 110: data at any time, and such data is forwarded to/from the shell or
111: command on the server side, and the user terminal in the client side.
1.2 deraadt 112: .Pp
1.1 deraadt 113: When the user program terminates and all forwarded X11 and other
114: connections have been closed, the server sends command exit status to
115: the client, and both sides exit.
1.2 deraadt 116: .Pp
117: .Nm
1.1 deraadt 118: can be configured using command-line options or a configuration
1.36 aaron 119: file.
120: Command-line options override values specified in the
1.1 deraadt 121: configuration file.
1.25 markus 122: .Pp
123: .Nm
124: rereads its configuration file when it receives a hangup signal,
125: .Dv SIGHUP .
1.18 aaron 126: .Pp
127: The options are as follows:
1.2 deraadt 128: .Bl -tag -width Ds
129: .It Fl b Ar bits
1.1 deraadt 130: Specifies the number of bits in the server key (default 768).
1.2 deraadt 131: .Pp
132: .It Fl d
1.36 aaron 133: Debug mode.
134: The server sends verbose debug output to the system
135: log, and does not put itself in the background.
136: The server also will not fork and will only process one connection.
137: This option is only intended for debugging for the server.
1.2 deraadt 138: .It Fl f Ar configuration_file
1.36 aaron 139: Specifies the name of the configuration file.
140: The default is
1.2 deraadt 141: .Pa /etc/sshd_config .
1.16 markus 142: .Nm
143: refuses to start if there is no configuration file.
1.2 deraadt 144: .It Fl g Ar login_grace_time
1.1 deraadt 145: Gives the grace time for clients to authenticate themselves (default
1.36 aaron 146: 300 seconds).
147: If the client fails to authenticate the user within
148: this many seconds, the server disconnects and exits.
149: A value of zero indicates no limit.
1.2 deraadt 150: .It Fl h Ar host_key_file
1.1 deraadt 151: Specifies the file from which the host key is read (default
1.2 deraadt 152: .Pa /etc/ssh_host_key ) .
1.7 markus 153: This option must be given if
154: .Nm
155: is not run as root (as the normal
1.1 deraadt 156: host file is normally not readable by anyone but root).
1.2 deraadt 157: .It Fl i
1.7 markus 158: Specifies that
159: .Nm
1.40 aaron 160: is being run from inetd.
1.7 markus 161: .Nm
162: is normally not run
1.1 deraadt 163: from inetd because it needs to generate the server key before it can
1.36 aaron 164: respond to the client, and this may take tens of seconds.
165: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 166: However, with small key sizes (e.g., 512) using
1.7 markus 167: .Nm
168: from inetd may
1.1 deraadt 169: be feasible.
1.2 deraadt 170: .It Fl k Ar key_gen_time
1.1 deraadt 171: Specifies how often the server key is regenerated (default 3600
1.36 aaron 172: seconds, or one hour).
173: The motivation for regenerating the key fairly
1.1 deraadt 174: often is that the key is not stored anywhere, and after about an hour,
175: it becomes impossible to recover the key for decrypting intercepted
176: communications even if the machine is cracked into or physically
1.36 aaron 177: seized.
178: A value of zero indicates that the key will never be regenerated.
1.2 deraadt 179: .It Fl p Ar port
1.1 deraadt 180: Specifies the port on which the server listens for connections
181: (default 22).
1.2 deraadt 182: .It Fl q
1.36 aaron 183: Quiet mode.
184: Nothing is sent to the system log.
185: Normally the beginning,
1.1 deraadt 186: authentication, and termination of each connection is logged.
1.18 aaron 187: .It Fl Q
188: Do not print an error message if RSA support is missing.
1.26 markus 189: .It Fl V Ar client_protocol_id
190: SSH2 compatibility mode.
1.42 ! hugh 191: When this option is specified
1.26 markus 192: .Nm
1.42 ! hugh 193: assumes the client has sent the supplied version string
1.26 markus 194: and skips the
195: Protocol Version Identification Exchange.
1.29 markus 196: .It Fl 4
197: Forces
198: .Nm
199: to use IPv4 addresses only.
200: .It Fl 6
201: Forces
202: .Nm
203: to use IPv6 addresses only.
1.2 deraadt 204: .El
205: .Sh CONFIGURATION FILE
206: .Nm
1.40 aaron 207: reads configuration data from
1.2 deraadt 208: .Pa /etc/sshd_config
209: (or the file specified with
210: .Fl f
1.36 aaron 211: on the command line).
212: The file contains keyword-value pairs, one per line.
213: Lines starting with
1.2 deraadt 214: .Ql #
1.1 deraadt 215: and empty lines are interpreted as comments.
1.2 deraadt 216: .Pp
1.1 deraadt 217: The following keywords are possible.
1.2 deraadt 218: .Bl -tag -width Ds
219: .It Cm AFSTokenPassing
1.36 aaron 220: Specifies whether an AFS token may be forwarded to the server.
221: Default is
1.2 deraadt 222: .Dq yes .
1.11 markus 223: .It Cm AllowGroups
224: This keyword can be followed by a number of group names, separated
1.36 aaron 225: by spaces.
226: If specified, login is allowed only for users whose primary
1.11 markus 227: group matches one of the patterns.
228: .Ql \&*
229: and
230: .Ql ?
231: can be used as
1.36 aaron 232: wildcards in the patterns.
233: Only group names are valid, a numerical group ID isn't recognized.
234: By default login is allowed regardless of the primary group.
1.3 dugsong 235: .Pp
1.11 markus 236: .It Cm AllowUsers
237: This keyword can be followed by a number of user names, separated
1.36 aaron 238: by spaces.
239: If specified, login is allowed only for users names that
1.11 markus 240: match one of the patterns.
241: .Ql \&*
242: and
243: .Ql ?
244: can be used as
1.36 aaron 245: wildcards in the patterns.
246: Only user names are valid, a numerical user ID isn't recognized.
247: By default login is allowed regardless of the user name.
1.11 markus 248: .Pp
1.41 markus 249: .It Cm Ciphers
250: Specifies the ciphers allowed for protocol version 2.
251: Multiple ciphers must be comma-separated.
252: The default is
253: .Dq blowfish-cbc,3des-cbc,arcfour,cast128-cbc .
1.8 markus 254: .It Cm CheckMail
255: Specifies whether
256: .Nm
257: should check for new mail for interactive logins.
258: The default is
259: .Dq no .
1.11 markus 260: .It Cm DenyGroups
261: This keyword can be followed by a number of group names, separated
1.36 aaron 262: by spaces.
263: Users whose primary group matches one of the patterns
1.11 markus 264: aren't allowed to log in.
265: .Ql \&*
266: and
267: .Ql ?
268: can be used as
1.36 aaron 269: wildcards in the patterns.
270: Only group names are valid, a numerical group ID isn't recognized.
271: By default login is allowed regardless of the primary group.
1.11 markus 272: .Pp
273: .It Cm DenyUsers
274: This keyword can be followed by a number of user names, separated
1.36 aaron 275: by spaces.
276: Login is disallowed for user names that match one of the patterns.
1.11 markus 277: .Ql \&*
278: and
279: .Ql ?
1.36 aaron 280: can be used as wildcards in the patterns.
281: Only user names are valid, a numerical user ID isn't recognized.
282: By default login is allowed regardless of the user name.
1.2 deraadt 283: .It Cm HostKey
1.1 deraadt 284: Specifies the file containing the private host key (default
1.2 deraadt 285: .Pa /etc/ssh_host_key ) .
1.9 markus 286: Note that
287: .Nm
1.14 markus 288: does not start if this file is group/world-accessible.
1.2 deraadt 289: .It Cm IgnoreRhosts
1.34 markus 290: Specifies that
291: .Pa .rhosts
1.40 aaron 292: and
1.34 markus 293: .Pa .shosts
294: files will not be used in authentication.
1.2 deraadt 295: .Pa /etc/hosts.equiv
1.1 deraadt 296: and
1.40 aaron 297: .Pa /etc/shosts.equiv
1.36 aaron 298: are still used.
1.40 aaron 299: The default is
1.34 markus 300: .Dq yes .
1.24 markus 301: .It Cm IgnoreUserKnownHosts
302: Specifies whether
303: .Nm
304: should ignore the user's
305: .Pa $HOME/.ssh/known_hosts
306: during
307: .Cm RhostsRSAAuthentication .
308: The default is
1.2 deraadt 309: .Dq no .
310: .It Cm KeepAlive
1.1 deraadt 311: Specifies whether the system should send keepalive messages to the
1.36 aaron 312: other side.
313: If they are sent, death of the connection or crash of one
314: of the machines will be properly noticed.
315: However, this means that
1.1 deraadt 316: connections will die if the route is down temporarily, and some people
1.36 aaron 317: find it annoying.
318: On the other hand, if keepalives are not send,
1.2 deraadt 319: sessions may hang indefinitely on the server, leaving
320: .Dq ghost
321: users and consuming server resources.
322: .Pp
323: The default is
324: .Dq yes
325: (to send keepalives), and the server will notice
1.36 aaron 326: if the network goes down or the client host reboots.
327: This avoids infinitely hanging sessions.
1.2 deraadt 328: .Pp
329: To disable keepalives, the value should be set to
330: .Dq no
331: in both the server and the client configuration files.
332: .It Cm KerberosAuthentication
1.36 aaron 333: Specifies whether Kerberos authentication is allowed.
334: This can be in the form of a Kerberos ticket, or if
1.7 markus 335: .Cm PasswordAuthentication
1.1 deraadt 336: is yes, the password provided by the user will be validated through
1.36 aaron 337: the Kerberos KDC.
338: Default is
1.3 dugsong 339: .Dq yes .
1.2 deraadt 340: .It Cm KerberosOrLocalPasswd
1.1 deraadt 341: If set then if password authentication through Kerberos fails then
342: the password will be validated via any additional local mechanism
1.2 deraadt 343: such as
344: .Pa /etc/passwd
1.36 aaron 345: or SecurID.
346: Default is
1.20 dugsong 347: .Dq yes .
1.2 deraadt 348: .It Cm KerberosTgtPassing
1.1 deraadt 349: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 350: Default is
1.3 dugsong 351: .Dq no ,
352: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 353: .It Cm KerberosTicketCleanup
1.7 markus 354: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 355: file on logout.
356: Default is
1.3 dugsong 357: .Dq yes .
1.2 deraadt 358: .It Cm KeyRegenerationInterval
1.1 deraadt 359: The server key is automatically regenerated after this many seconds
1.36 aaron 360: (if it has been used).
361: The purpose of regeneration is to prevent
1.1 deraadt 362: decrypting captured sessions by later breaking into the machine and
1.36 aaron 363: stealing the keys.
364: The key is never stored anywhere.
365: If the value is 0, the key is never regenerated.
366: The default is 3600 (seconds).
1.7 markus 367: .It Cm ListenAddress
368: Specifies what local address
369: .Nm
370: should listen on.
371: The default is to listen to all local addresses.
1.28 markus 372: Multiple options of this type are permitted.
373: Additionally, the
374: .Cm Ports
375: options must precede this option.
1.2 deraadt 376: .It Cm LoginGraceTime
1.1 deraadt 377: The server disconnects after this time if the user has not
1.36 aaron 378: successfully logged in.
379: If the value is 0, there is no time limit.
1.1 deraadt 380: The default is 600 (seconds).
1.23 markus 381: .It Cm LogLevel
382: Gives the verbosity level that is used when logging messages from
383: .Nm sshd .
384: The possible values are:
1.27 markus 385: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.23 markus 386: The default is INFO.
387: Logging with level DEBUG violates the privacy of users
388: and is not recommended.
1.2 deraadt 389: .It Cm PasswordAuthentication
1.1 deraadt 390: Specifies whether password authentication is allowed.
1.2 deraadt 391: The default is
392: .Dq yes .
393: .It Cm PermitEmptyPasswords
1.1 deraadt 394: When password authentication is allowed, it specifies whether the
1.36 aaron 395: server allows login to accounts with empty password strings.
396: The default is
1.34 markus 397: .Dq no .
1.2 deraadt 398: .It Cm PermitRootLogin
1.1 deraadt 399: Specifies whether the root can log in using
1.2 deraadt 400: .Xr ssh 1 .
1.15 markus 401: The argument must be
402: .Dq yes ,
403: .Dq without-password
404: or
405: .Dq no .
1.2 deraadt 406: The default is
407: .Dq yes .
1.15 markus 408: If this options is set to
409: .Dq without-password
410: only password authentication is disabled for root.
1.2 deraadt 411: .Pp
412: Root login with RSA authentication when the
413: .Ar command
414: option has been
1.1 deraadt 415: specified will be allowed regardless of the value of this setting
416: (which may be useful for taking remote backups even if root login is
417: normally not allowed).
1.2 deraadt 418: .It Cm Port
1.1 deraadt 419: Specifies the port number that
1.2 deraadt 420: .Nm
1.36 aaron 421: listens on.
422: The default is 22.
1.28 markus 423: Multiple options of this type are permitted.
1.2 deraadt 424: .It Cm PrintMotd
1.1 deraadt 425: Specifies whether
1.2 deraadt 426: .Nm
1.40 aaron 427: should print
1.2 deraadt 428: .Pa /etc/motd
1.36 aaron 429: when a user logs in interactively.
430: (On some systems it is also printed by the shell,
1.2 deraadt 431: .Pa /etc/profile ,
1.36 aaron 432: or equivalent.)
433: The default is
1.2 deraadt 434: .Dq yes .
1.41 markus 435: .It Cm Protocol
436: Specifies the protocol versions
437: .Nm
438: should support.
439: The possible values are
440: .Dq 1
441: and
442: .Dq 2 .
443: Multiple versions must be comma-separated.
444: The default is
445: .Dq 1 .
1.2 deraadt 446: .It Cm RandomSeed
1.36 aaron 447: Obsolete.
448: Random number generation uses other techniques.
1.2 deraadt 449: .It Cm RhostsAuthentication
1.1 deraadt 450: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 451: files is sufficient.
452: Normally, this method should not be permitted because it is insecure.
1.7 markus 453: .Cm RhostsRSAAuthentication
454: should be used
1.1 deraadt 455: instead, because it performs RSA-based host authentication in addition
456: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 457: The default is
458: .Dq no .
459: .It Cm RhostsRSAAuthentication
1.1 deraadt 460: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 461: with successful RSA host authentication is allowed.
462: The default is
1.34 markus 463: .Dq no .
1.2 deraadt 464: .It Cm RSAAuthentication
1.36 aaron 465: Specifies whether pure RSA authentication is allowed.
466: The default is
1.2 deraadt 467: .Dq yes .
468: .It Cm ServerKeyBits
1.36 aaron 469: Defines the number of bits in the server key.
470: The minimum value is 512, and the default is 768.
1.7 markus 471: .It Cm SkeyAuthentication
472: Specifies whether
1.40 aaron 473: .Xr skey 1
1.36 aaron 474: authentication is allowed.
475: The default is
1.7 markus 476: .Dq yes .
477: Note that s/key authentication is enabled only if
478: .Cm PasswordAuthentication
479: is allowed, too.
1.2 deraadt 480: .It Cm StrictModes
1.12 markus 481: Specifies whether
482: .Nm
483: should check file modes and ownership of the
1.36 aaron 484: user's files and home directory before accepting login.
485: This is normally desirable because novices sometimes accidentally leave their
486: directory or files world-writable.
487: The default is
1.7 markus 488: .Dq yes .
1.2 deraadt 489: .It Cm SyslogFacility
1.1 deraadt 490: Gives the facility code that is used when logging messages from
1.2 deraadt 491: .Nm sshd .
1.1 deraadt 492: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 493: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
494: The default is AUTH.
1.10 markus 495: .It Cm UseLogin
496: Specifies whether
497: .Xr login 1
1.36 aaron 498: is used.
499: The default is
1.10 markus 500: .Dq no .
1.6 aaron 501: .It Cm X11DisplayOffset
502: Specifies the first display number available for
503: .Nm sshd Ns 's
1.36 aaron 504: X11 forwarding.
505: This prevents
1.6 aaron 506: .Nm
507: from interfering with real X11 servers.
1.34 markus 508: The default is 10.
1.30 markus 509: .It Cm X11Forwarding
1.36 aaron 510: Specifies whether X11 forwarding is permitted.
511: The default is
1.34 markus 512: .Dq no .
1.30 markus 513: Note that disabling X11 forwarding does not improve security in any
514: way, as users can always install their own forwarders.
1.2 deraadt 515: .El
516: .Sh LOGIN PROCESS
1.1 deraadt 517: When a user successfully logs in,
1.2 deraadt 518: .Nm
1.1 deraadt 519: does the following:
1.2 deraadt 520: .Bl -enum -offset indent
521: .It
1.1 deraadt 522: If the login is on a tty, and no command has been specified,
1.40 aaron 523: prints last login time and
1.2 deraadt 524: .Pa /etc/motd
1.1 deraadt 525: (unless prevented in the configuration file or by
1.2 deraadt 526: .Pa $HOME/.hushlogin ;
527: see the
1.40 aaron 528: .Sx FILES
1.2 deraadt 529: section).
530: .It
1.1 deraadt 531: If the login is on a tty, records login time.
1.2 deraadt 532: .It
533: Checks
534: .Pa /etc/nologin ;
535: if it exists, prints contents and quits
1.1 deraadt 536: (unless root).
1.2 deraadt 537: .It
1.1 deraadt 538: Changes to run with normal user privileges.
1.2 deraadt 539: .It
1.1 deraadt 540: Sets up basic environment.
1.2 deraadt 541: .It
542: Reads
543: .Pa $HOME/.ssh/environment
544: if it exists.
545: .It
1.1 deraadt 546: Changes to user's home directory.
1.2 deraadt 547: .It
548: If
549: .Pa $HOME/.ssh/rc
550: exists, runs it; else if
551: .Pa /etc/sshrc
552: exists, runs
1.36 aaron 553: it; otherwise runs xauth.
554: The
1.2 deraadt 555: .Dq rc
556: files are given the X11
1.1 deraadt 557: authentication protocol and cookie in standard input.
1.2 deraadt 558: .It
1.1 deraadt 559: Runs user's shell or command.
1.2 deraadt 560: .El
561: .Sh AUTHORIZED_KEYS FILE FORMAT
1.40 aaron 562: The
1.2 deraadt 563: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 564: file lists the RSA keys that are
1.36 aaron 565: permitted for RSA authentication.
566: Each line of the file contains one
1.2 deraadt 567: key (empty lines and lines starting with a
568: .Ql #
569: are ignored as
1.36 aaron 570: comments).
571: Each line consists of the following fields, separated by
572: spaces: options, bits, exponent, modulus, comment.
573: The options field
1.1 deraadt 574: is optional; its presence is determined by whether the line starts
575: with a number or not (the option field never starts with a number).
576: The bits, exponent, modulus and comment fields give the RSA key; the
577: comment field is not used for anything (but may be convenient for the
578: user to identify the key).
1.2 deraadt 579: .Pp
1.1 deraadt 580: Note that lines in this file are usually several hundred bytes long
1.36 aaron 581: (because of the size of the RSA key modulus).
582: You don't want to type them in; instead, copy the
1.2 deraadt 583: .Pa identity.pub
1.1 deraadt 584: file and edit it.
1.2 deraadt 585: .Pp
1.1 deraadt 586: The options (if present) consists of comma-separated option
1.36 aaron 587: specifications.
588: No spaces are permitted, except within double quotes.
1.1 deraadt 589: The following option specifications are supported:
1.2 deraadt 590: .Bl -tag -width Ds
591: .It Cm from="pattern-list"
1.1 deraadt 592: Specifies that in addition to RSA authentication, the canonical name
593: of the remote host must be present in the comma-separated list of
1.36 aaron 594: patterns
595: .Pf ( Ql *
596: and
597: .Ql ?
598: serve as wildcards).
599: The list may also contain
600: patterns negated by prefixing them with
601: .Ql ! ;
602: if the canonical host name matches a negated pattern, the key is not accepted.
603: The purpose
1.1 deraadt 604: of this option is to optionally increase security: RSA authentication
605: by itself does not trust the network or name servers or anything (but
606: the key); however, if somebody somehow steals the key, the key
1.36 aaron 607: permits an intruder to log in from anywhere in the world.
608: This additional option makes using a stolen key more difficult (name
1.1 deraadt 609: servers and/or routers would have to be compromised in addition to
610: just the key).
1.2 deraadt 611: .It Cm command="command"
1.1 deraadt 612: Specifies that the command is executed whenever this key is used for
1.36 aaron 613: authentication.
614: The command supplied by the user (if any) is ignored.
1.1 deraadt 615: The command is run on a pty if the connection requests a pty;
1.36 aaron 616: otherwise it is run without a tty.
617: A quote may be included in the command by quoting it with a backslash.
618: This option might be useful
619: to restrict certain RSA keys to perform just a specific operation.
620: An example might be a key that permits remote backups but nothing else.
621: Notice that the client may specify TCP/IP and/or X11
1.1 deraadt 622: forwardings unless they are explicitly prohibited.
1.2 deraadt 623: .It Cm environment="NAME=value"
1.1 deraadt 624: Specifies that the string is to be added to the environment when
1.36 aaron 625: logging in using this key.
626: Environment variables set this way
627: override other default environment values.
628: Multiple options of this type are permitted.
1.2 deraadt 629: .It Cm no-port-forwarding
1.1 deraadt 630: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 631: Any port forward requests by the client will return an error.
632: This might be used, e.g., in connection with the
1.2 deraadt 633: .Cm command
1.1 deraadt 634: option.
1.2 deraadt 635: .It Cm no-X11-forwarding
1.1 deraadt 636: Forbids X11 forwarding when this key is used for authentication.
637: Any X11 forward requests by the client will return an error.
1.2 deraadt 638: .It Cm no-agent-forwarding
1.1 deraadt 639: Forbids authentication agent forwarding when this key is used for
640: authentication.
1.2 deraadt 641: .It Cm no-pty
1.1 deraadt 642: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 643: .El
644: .Ss Examples
1.1 deraadt 645: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 646: .Pp
1.1 deraadt 647: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 648: .Pp
1.1 deraadt 649: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 650: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 651: The
1.2 deraadt 652: .Pa /etc/ssh_known_hosts
1.40 aaron 653: and
1.2 deraadt 654: .Pa $HOME/.ssh/known_hosts
1.36 aaron 655: files contain host public keys for all known hosts.
656: The global file should
1.37 brad 657: be prepared by the administrator (optional), and the per-user file is
1.1 deraadt 658: maintained automatically: whenever the user connects an unknown host
1.36 aaron 659: its key is added to the per-user file.
1.2 deraadt 660: .Pp
1.1 deraadt 661: Each line in these files contains the following fields: hostnames,
1.36 aaron 662: bits, exponent, modulus, comment.
663: The fields are separated by spaces.
1.2 deraadt 664: .Pp
1.1 deraadt 665: Hostnames is a comma-separated list of patterns ('*' and '?' act as
666: wildcards); each pattern in turn is matched against the canonical host
667: name (when authenticating a client) or against the user-supplied
1.36 aaron 668: name (when authenticating a server).
669: A pattern may also be preceded by
1.2 deraadt 670: .Ql !
671: to indicate negation: if the host name matches a negated
1.1 deraadt 672: pattern, it is not accepted (by that line) even if it matched another
673: pattern on the line.
1.2 deraadt 674: .Pp
1.1 deraadt 675: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 676: can be obtained, e.g., from
677: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 678: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 679: .Pp
680: Lines starting with
681: .Ql #
682: and empty lines are ignored as comments.
683: .Pp
1.1 deraadt 684: When performing host authentication, authentication is accepted if any
1.36 aaron 685: matching line has the proper key.
686: It is thus permissible (but not
1.1 deraadt 687: recommended) to have several lines or different host keys for the same
1.36 aaron 688: names.
689: This will inevitably happen when short forms of host names
690: from different domains are put in the file.
691: It is possible
1.1 deraadt 692: that the files contain conflicting information; authentication is
693: accepted if valid information can be found from either file.
1.2 deraadt 694: .Pp
1.1 deraadt 695: Note that the lines in these files are typically hundreds of characters
696: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 697: Rather, generate them by a script
1.40 aaron 698: or by taking
1.2 deraadt 699: .Pa /etc/ssh_host_key.pub
1.1 deraadt 700: and adding the host names at the front.
1.2 deraadt 701: .Ss Examples
1.1 deraadt 702: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 703: .Sh FILES
704: .Bl -tag -width Ds
705: .It Pa /etc/sshd_config
1.1 deraadt 706: Contains configuration data for
1.2 deraadt 707: .Nm sshd .
1.1 deraadt 708: This file should be writable by root only, but it is recommended
709: (though not necessary) that it be world-readable.
1.2 deraadt 710: .It Pa /etc/ssh_host_key
1.7 markus 711: Contains the private part of the host key.
1.1 deraadt 712: This file should only be owned by root, readable only by root, and not
713: accessible to others.
1.14 markus 714: Note that
715: .Nm
716: does not start if this file is group/world-accessible.
1.2 deraadt 717: .It Pa /etc/ssh_host_key.pub
1.7 markus 718: Contains the public part of the host key.
719: This file should be world-readable but writable only by
1.36 aaron 720: root.
721: Its contents should match the private part.
722: This file is not
1.1 deraadt 723: really used for anything; it is only provided for the convenience of
724: the user so its contents can be copied to known hosts files.
1.7 markus 725: These two files are created using
726: .Xr ssh-keygen 1 .
1.2 deraadt 727: .It Pa /var/run/sshd.pid
728: Contains the process ID of the
729: .Nm
1.1 deraadt 730: listening for connections (if there are several daemons running
731: concurrently for different ports, this contains the pid of the one
1.36 aaron 732: started last).
733: The contents of this file are not sensitive; it can be world-readable.
1.2 deraadt 734: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 735: Lists the RSA keys that can be used to log into the user's account.
736: This file must be readable by root (which may on some machines imply
737: it being world-readable if the user's home directory resides on an NFS
1.36 aaron 738: volume).
739: It is recommended that it not be accessible by others.
740: The format of this file is described above.
1.22 markus 741: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
742: These files are consulted when using rhosts with RSA host
1.36 aaron 743: authentication to check the public key of the host.
744: The key must be listed in one of these files to be accepted.
1.22 markus 745: The client uses the same files
1.36 aaron 746: to verify that the remote host is the one we intended to connect.
747: These files should be writable only by root/the owner.
1.2 deraadt 748: .Pa /etc/ssh_known_hosts
749: should be world-readable, and
750: .Pa $HOME/.ssh/known_hosts
751: can but need not be world-readable.
1.6 aaron 752: .It Pa /etc/nologin
1.40 aaron 753: If this file exists,
1.2 deraadt 754: .Nm
1.36 aaron 755: refuses to let anyone except root log in.
756: The contents of the file
1.1 deraadt 757: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 758: refused.
759: The file should be world-readable.
1.19 dugsong 760: .It Pa /etc/hosts.allow, /etc/hosts.deny
761: If compiled with
762: .Sy LIBWRAP
763: support, tcp-wrappers access controls may be defined here as described in
764: .Xr hosts_access 5 .
1.6 aaron 765: .It Pa $HOME/.rhosts
1.1 deraadt 766: This file contains host-username pairs, separated by a space, one per
1.36 aaron 767: line.
768: The given user on the corresponding host is permitted to log in
769: without password.
770: The same file is used by rlogind and rshd.
1.6 aaron 771: The file must
1.1 deraadt 772: be writable only by the user; it is recommended that it not be
773: accessible by others.
1.2 deraadt 774: .Pp
1.36 aaron 775: If is also possible to use netgroups in the file.
776: Either host or user
1.1 deraadt 777: name may be of the form +@groupname to specify all hosts or all users
778: in the group.
1.2 deraadt 779: .It Pa $HOME/.shosts
780: For ssh,
781: this file is exactly the same as for
782: .Pa .rhosts .
783: However, this file is
784: not used by rlogin and rshd, so using this permits access using SSH only.
785: .Pa /etc/hosts.equiv
786: This file is used during
787: .Pa .rhosts
1.36 aaron 788: authentication.
789: In the simplest form, this file contains host names, one per line.
790: Users on
1.1 deraadt 791: those hosts are permitted to log in without a password, provided they
1.36 aaron 792: have the same user name on both machines.
793: The host name may also be
1.1 deraadt 794: followed by a user name; such users are permitted to log in as
1.2 deraadt 795: .Em any
1.36 aaron 796: user on this machine (except root).
797: Additionally, the syntax
1.2 deraadt 798: .Dq +@group
1.36 aaron 799: can be used to specify netgroups.
800: Negated entries start with
1.2 deraadt 801: .Ql \&- .
802: .Pp
1.1 deraadt 803: If the client host/user is successfully matched in this file, login is
804: automatically permitted provided the client and server user names are the
1.36 aaron 805: same.
806: Additionally, successful RSA host authentication is normally required.
807: This file must be writable only by root; it is recommended
1.1 deraadt 808: that it be world-readable.
1.2 deraadt 809: .Pp
1.6 aaron 810: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 811: .Pa hosts.equiv .
1.1 deraadt 812: Beware that it really means that the named user(s) can log in as
1.2 deraadt 813: .Em anybody ,
1.1 deraadt 814: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 815: binaries and directories.
816: Using a user name practically grants the user root access.
817: The only valid use for user names that I can think
1.1 deraadt 818: of is in negative entries.
1.2 deraadt 819: .Pp
820: Note that this warning also applies to rsh/rlogin.
821: .It Pa /etc/shosts.equiv
1.1 deraadt 822: This is processed exactly as
1.2 deraadt 823: .Pa /etc/hosts.equiv .
1.1 deraadt 824: However, this file may be useful in environments that want to run both
1.2 deraadt 825: rsh/rlogin and ssh.
1.6 aaron 826: .It Pa $HOME/.ssh/environment
1.36 aaron 827: This file is read into the environment at login (if it exists).
828: It can only contain empty lines, comment lines (that start with
1.2 deraadt 829: .Ql # ) ,
1.36 aaron 830: and assignment lines of the form name=value.
831: The file should be writable
1.6 aaron 832: only by the user; it need not be readable by anyone else.
1.2 deraadt 833: .It Pa $HOME/.ssh/rc
1.1 deraadt 834: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 835: environment files but before starting the user's shell or command.
836: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 837: standard input (and
838: .Ev DISPLAY
1.36 aaron 839: in environment).
840: This must call
1.2 deraadt 841: .Xr xauth 1
842: in that case.
843: .Pp
1.1 deraadt 844: The primary purpose of this file is to run any initialization routines
845: which may be needed before the user's home directory becomes
846: accessible; AFS is a particular example of such an environment.
1.2 deraadt 847: .Pp
1.1 deraadt 848: This file will probably contain some initialization code followed by
849: something similar to: "if read proto cookie; then echo add $DISPLAY
850: $proto $cookie | xauth -q -; fi".
1.2 deraadt 851: .Pp
852: If this file does not exist,
853: .Pa /etc/sshrc
854: is run, and if that
1.1 deraadt 855: does not exist either, xauth is used to store the cookie.
1.2 deraadt 856: .Pp
1.1 deraadt 857: This file should be writable only by the user, and need not be
858: readable by anyone else.
1.2 deraadt 859: .It Pa /etc/sshrc
860: Like
861: .Pa $HOME/.ssh/rc .
862: This can be used to specify
1.36 aaron 863: machine-specific login-time initializations globally.
864: This file should be writable only by root, and should be world-readable.
1.2 deraadt 865: .Sh AUTHOR
1.17 deraadt 866: OpenSSH
1.33 deraadt 867: is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1.36 aaron 868: but with bugs removed and newer features re-added.
869: Rapidly after the
1.33 deraadt 870: 1.2.12 release, newer versions of the original ssh bore successively
871: more restrictive licenses, and thus demand for a free version was born.
872: This version of OpenSSH
1.17 deraadt 873: .Bl -bullet
874: .It
1.32 aaron 875: has all components of a restrictive nature (i.e., patents, see
1.17 deraadt 876: .Xr ssl 8 )
877: directly removed from the source code; any licensed or patented components
878: are chosen from
879: external libraries.
880: .It
1.33 deraadt 881: has been updated to support ssh protocol 1.5, making it compatible with
882: all other ssh protocol 1 clients and servers.
1.17 deraadt 883: .It
1.40 aaron 884: contains added support for
1.17 deraadt 885: .Xr kerberos 8
886: authentication and ticket passing.
887: .It
888: supports one-time password authentication with
889: .Xr skey 1 .
890: .El
891: .Pp
892: The libraries described in
1.5 deraadt 893: .Xr ssl 8
894: are required for proper operation.
1.2 deraadt 895: .Sh SEE ALSO
896: .Xr scp 1 ,
897: .Xr ssh 1 ,
1.5 deraadt 898: .Xr ssh-add 1 ,
1.2 deraadt 899: .Xr ssh-agent 1 ,
1.5 deraadt 900: .Xr ssh-keygen 1 ,
1.38 deraadt 901: .Xr ssl 8 ,
902: .Xr rlogin 1 ,
903: .Xr rsh 1