Annotation of src/usr.bin/ssh/sshd.8, Revision 1.48
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: .\"
1.48 ! markus 12: .\" $Id: sshd.8,v 1.47 2000/05/03 18:03:07 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.
1.47 markus 283: .It Cm GatewayPorts
284: Specifies whether remote hosts are allowed to connect to ports
285: forwarded for the client.
286: The argument must be
287: .Dq yes
288: or
289: .Dq no .
290: The default is
291: .Dq no .
1.46 markus 292: .It Cm HostDsaKey
293: Specifies the file containing the private DSA host key (default
294: .Pa /etc/ssh_host_dsa_key )
295: used by SSH protocol 2.0.
296: Note that
297: .Nm
298: disables protcol 2.0 if this file is group/world-accessible.
1.2 deraadt 299: .It Cm HostKey
1.46 markus 300: Specifies the file containing the private RSA host key (default
301: .Pa /etc/ssh_host_key )
302: used by SSH protocols 1.3 and 1.5.
1.9 markus 303: Note that
304: .Nm
1.46 markus 305: disables protcols 1.3 and 1.5 if this file is group/world-accessible.
1.2 deraadt 306: .It Cm IgnoreRhosts
1.34 markus 307: Specifies that
308: .Pa .rhosts
1.40 aaron 309: and
1.34 markus 310: .Pa .shosts
311: files will not be used in authentication.
1.2 deraadt 312: .Pa /etc/hosts.equiv
1.1 deraadt 313: and
1.40 aaron 314: .Pa /etc/shosts.equiv
1.36 aaron 315: are still used.
1.40 aaron 316: The default is
1.34 markus 317: .Dq yes .
1.24 markus 318: .It Cm IgnoreUserKnownHosts
319: Specifies whether
320: .Nm
321: should ignore the user's
322: .Pa $HOME/.ssh/known_hosts
1.45 markus 323: during
1.24 markus 324: .Cm RhostsRSAAuthentication .
325: The default is
1.2 deraadt 326: .Dq no .
327: .It Cm KeepAlive
1.1 deraadt 328: Specifies whether the system should send keepalive messages to the
1.36 aaron 329: other side.
330: If they are sent, death of the connection or crash of one
331: of the machines will be properly noticed.
332: However, this means that
1.1 deraadt 333: connections will die if the route is down temporarily, and some people
1.36 aaron 334: find it annoying.
335: On the other hand, if keepalives are not send,
1.2 deraadt 336: sessions may hang indefinitely on the server, leaving
337: .Dq ghost
338: users and consuming server resources.
339: .Pp
340: The default is
341: .Dq yes
342: (to send keepalives), and the server will notice
1.36 aaron 343: if the network goes down or the client host reboots.
344: This avoids infinitely hanging sessions.
1.2 deraadt 345: .Pp
346: To disable keepalives, the value should be set to
347: .Dq no
348: in both the server and the client configuration files.
349: .It Cm KerberosAuthentication
1.36 aaron 350: Specifies whether Kerberos authentication is allowed.
351: This can be in the form of a Kerberos ticket, or if
1.7 markus 352: .Cm PasswordAuthentication
1.1 deraadt 353: is yes, the password provided by the user will be validated through
1.36 aaron 354: the Kerberos KDC.
355: Default is
1.3 dugsong 356: .Dq yes .
1.2 deraadt 357: .It Cm KerberosOrLocalPasswd
1.1 deraadt 358: If set then if password authentication through Kerberos fails then
359: the password will be validated via any additional local mechanism
1.2 deraadt 360: such as
361: .Pa /etc/passwd
1.36 aaron 362: or SecurID.
363: Default is
1.20 dugsong 364: .Dq yes .
1.2 deraadt 365: .It Cm KerberosTgtPassing
1.1 deraadt 366: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 367: Default is
1.3 dugsong 368: .Dq no ,
369: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 370: .It Cm KerberosTicketCleanup
1.7 markus 371: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 372: file on logout.
373: Default is
1.3 dugsong 374: .Dq yes .
1.2 deraadt 375: .It Cm KeyRegenerationInterval
1.1 deraadt 376: The server key is automatically regenerated after this many seconds
1.36 aaron 377: (if it has been used).
378: The purpose of regeneration is to prevent
1.1 deraadt 379: decrypting captured sessions by later breaking into the machine and
1.36 aaron 380: stealing the keys.
381: The key is never stored anywhere.
382: If the value is 0, the key is never regenerated.
383: The default is 3600 (seconds).
1.7 markus 384: .It Cm ListenAddress
385: Specifies what local address
386: .Nm
387: should listen on.
388: The default is to listen to all local addresses.
1.28 markus 389: Multiple options of this type are permitted.
390: Additionally, the
391: .Cm Ports
392: options must precede this option.
1.2 deraadt 393: .It Cm LoginGraceTime
1.1 deraadt 394: The server disconnects after this time if the user has not
1.36 aaron 395: successfully logged in.
396: If the value is 0, there is no time limit.
1.1 deraadt 397: The default is 600 (seconds).
1.23 markus 398: .It Cm LogLevel
399: Gives the verbosity level that is used when logging messages from
400: .Nm sshd .
401: The possible values are:
1.27 markus 402: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.23 markus 403: The default is INFO.
404: Logging with level DEBUG violates the privacy of users
405: and is not recommended.
1.2 deraadt 406: .It Cm PasswordAuthentication
1.1 deraadt 407: Specifies whether password authentication is allowed.
1.2 deraadt 408: The default is
409: .Dq yes .
410: .It Cm PermitEmptyPasswords
1.1 deraadt 411: When password authentication is allowed, it specifies whether the
1.36 aaron 412: server allows login to accounts with empty password strings.
413: The default is
1.34 markus 414: .Dq no .
1.2 deraadt 415: .It Cm PermitRootLogin
1.1 deraadt 416: Specifies whether the root can log in using
1.2 deraadt 417: .Xr ssh 1 .
1.15 markus 418: The argument must be
419: .Dq yes ,
420: .Dq without-password
421: or
422: .Dq no .
1.2 deraadt 423: The default is
424: .Dq yes .
1.15 markus 425: If this options is set to
426: .Dq without-password
427: only password authentication is disabled for root.
1.2 deraadt 428: .Pp
429: Root login with RSA authentication when the
430: .Ar command
431: option has been
1.1 deraadt 432: specified will be allowed regardless of the value of this setting
433: (which may be useful for taking remote backups even if root login is
434: normally not allowed).
1.43 markus 435: .It Cm PidFile
436: Specifies the file that contains the process identifier of the
437: .Nm
438: daemon.
439: The default is
440: .Pa /var/run/sshd.pid .
1.2 deraadt 441: .It Cm Port
1.1 deraadt 442: Specifies the port number that
1.2 deraadt 443: .Nm
1.36 aaron 444: listens on.
445: The default is 22.
1.28 markus 446: Multiple options of this type are permitted.
1.2 deraadt 447: .It Cm PrintMotd
1.1 deraadt 448: Specifies whether
1.2 deraadt 449: .Nm
1.40 aaron 450: should print
1.2 deraadt 451: .Pa /etc/motd
1.36 aaron 452: when a user logs in interactively.
453: (On some systems it is also printed by the shell,
1.2 deraadt 454: .Pa /etc/profile ,
1.36 aaron 455: or equivalent.)
456: The default is
1.2 deraadt 457: .Dq yes .
1.41 markus 458: .It Cm Protocol
459: Specifies the protocol versions
460: .Nm
461: should support.
462: The possible values are
463: .Dq 1
464: and
465: .Dq 2 .
466: Multiple versions must be comma-separated.
467: The default is
468: .Dq 1 .
1.2 deraadt 469: .It Cm RandomSeed
1.36 aaron 470: Obsolete.
471: Random number generation uses other techniques.
1.2 deraadt 472: .It Cm RhostsAuthentication
1.1 deraadt 473: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 474: files is sufficient.
475: Normally, this method should not be permitted because it is insecure.
1.7 markus 476: .Cm RhostsRSAAuthentication
477: should be used
1.1 deraadt 478: instead, because it performs RSA-based host authentication in addition
479: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 480: The default is
481: .Dq no .
482: .It Cm RhostsRSAAuthentication
1.1 deraadt 483: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 484: with successful RSA host authentication is allowed.
485: The default is
1.34 markus 486: .Dq no .
1.2 deraadt 487: .It Cm RSAAuthentication
1.36 aaron 488: Specifies whether pure RSA authentication is allowed.
489: The default is
1.2 deraadt 490: .Dq yes .
491: .It Cm ServerKeyBits
1.36 aaron 492: Defines the number of bits in the server key.
493: The minimum value is 512, and the default is 768.
1.7 markus 494: .It Cm SkeyAuthentication
495: Specifies whether
1.40 aaron 496: .Xr skey 1
1.36 aaron 497: authentication is allowed.
498: The default is
1.7 markus 499: .Dq yes .
500: Note that s/key authentication is enabled only if
501: .Cm PasswordAuthentication
502: is allowed, too.
1.2 deraadt 503: .It Cm StrictModes
1.12 markus 504: Specifies whether
505: .Nm
506: should check file modes and ownership of the
1.36 aaron 507: user's files and home directory before accepting login.
508: This is normally desirable because novices sometimes accidentally leave their
509: directory or files world-writable.
510: The default is
1.7 markus 511: .Dq yes .
1.2 deraadt 512: .It Cm SyslogFacility
1.1 deraadt 513: Gives the facility code that is used when logging messages from
1.2 deraadt 514: .Nm sshd .
1.1 deraadt 515: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 516: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
517: The default is AUTH.
1.10 markus 518: .It Cm UseLogin
519: Specifies whether
520: .Xr login 1
1.36 aaron 521: is used.
522: The default is
1.10 markus 523: .Dq no .
1.6 aaron 524: .It Cm X11DisplayOffset
525: Specifies the first display number available for
526: .Nm sshd Ns 's
1.36 aaron 527: X11 forwarding.
528: This prevents
1.6 aaron 529: .Nm
530: from interfering with real X11 servers.
1.34 markus 531: The default is 10.
1.30 markus 532: .It Cm X11Forwarding
1.36 aaron 533: Specifies whether X11 forwarding is permitted.
534: The default is
1.34 markus 535: .Dq no .
1.30 markus 536: Note that disabling X11 forwarding does not improve security in any
537: way, as users can always install their own forwarders.
1.2 deraadt 538: .El
539: .Sh LOGIN PROCESS
1.1 deraadt 540: When a user successfully logs in,
1.2 deraadt 541: .Nm
1.1 deraadt 542: does the following:
1.2 deraadt 543: .Bl -enum -offset indent
544: .It
1.1 deraadt 545: If the login is on a tty, and no command has been specified,
1.40 aaron 546: prints last login time and
1.2 deraadt 547: .Pa /etc/motd
1.1 deraadt 548: (unless prevented in the configuration file or by
1.2 deraadt 549: .Pa $HOME/.hushlogin ;
550: see the
1.40 aaron 551: .Sx FILES
1.2 deraadt 552: section).
553: .It
1.1 deraadt 554: If the login is on a tty, records login time.
1.2 deraadt 555: .It
556: Checks
557: .Pa /etc/nologin ;
558: if it exists, prints contents and quits
1.1 deraadt 559: (unless root).
1.2 deraadt 560: .It
1.1 deraadt 561: Changes to run with normal user privileges.
1.2 deraadt 562: .It
1.1 deraadt 563: Sets up basic environment.
1.2 deraadt 564: .It
565: Reads
566: .Pa $HOME/.ssh/environment
567: if it exists.
568: .It
1.1 deraadt 569: Changes to user's home directory.
1.2 deraadt 570: .It
571: If
572: .Pa $HOME/.ssh/rc
573: exists, runs it; else if
574: .Pa /etc/sshrc
575: exists, runs
1.36 aaron 576: it; otherwise runs xauth.
577: The
1.2 deraadt 578: .Dq rc
579: files are given the X11
1.1 deraadt 580: authentication protocol and cookie in standard input.
1.2 deraadt 581: .It
1.1 deraadt 582: Runs user's shell or command.
1.2 deraadt 583: .El
584: .Sh AUTHORIZED_KEYS FILE FORMAT
1.40 aaron 585: The
1.2 deraadt 586: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 587: file lists the RSA keys that are
1.44 deraadt 588: permitted for RSA authentication in SSH protocols 1.3 and 1.5
589: Similarily, the
590: .Pa $HOME/.ssh/authorized_keys2
591: file lists the DSA keys that are
592: permitted for DSA authentication in SSH protocol 2.0.
1.36 aaron 593: Each line of the file contains one
1.2 deraadt 594: key (empty lines and lines starting with a
595: .Ql #
596: are ignored as
1.36 aaron 597: comments).
598: Each line consists of the following fields, separated by
599: spaces: options, bits, exponent, modulus, comment.
600: The options field
1.1 deraadt 601: is optional; its presence is determined by whether the line starts
602: with a number or not (the option field never starts with a number).
603: The bits, exponent, modulus and comment fields give the RSA key; the
604: comment field is not used for anything (but may be convenient for the
605: user to identify the key).
1.2 deraadt 606: .Pp
1.1 deraadt 607: Note that lines in this file are usually several hundred bytes long
1.36 aaron 608: (because of the size of the RSA key modulus).
609: You don't want to type them in; instead, copy the
1.2 deraadt 610: .Pa identity.pub
1.1 deraadt 611: file and edit it.
1.2 deraadt 612: .Pp
1.1 deraadt 613: The options (if present) consists of comma-separated option
1.36 aaron 614: specifications.
615: No spaces are permitted, except within double quotes.
1.1 deraadt 616: The following option specifications are supported:
1.2 deraadt 617: .Bl -tag -width Ds
618: .It Cm from="pattern-list"
1.1 deraadt 619: Specifies that in addition to RSA authentication, the canonical name
620: of the remote host must be present in the comma-separated list of
1.36 aaron 621: patterns
622: .Pf ( Ql *
623: and
624: .Ql ?
625: serve as wildcards).
626: The list may also contain
627: patterns negated by prefixing them with
628: .Ql ! ;
629: if the canonical host name matches a negated pattern, the key is not accepted.
630: The purpose
1.1 deraadt 631: of this option is to optionally increase security: RSA authentication
632: by itself does not trust the network or name servers or anything (but
633: the key); however, if somebody somehow steals the key, the key
1.36 aaron 634: permits an intruder to log in from anywhere in the world.
635: This additional option makes using a stolen key more difficult (name
1.1 deraadt 636: servers and/or routers would have to be compromised in addition to
637: just the key).
1.2 deraadt 638: .It Cm command="command"
1.1 deraadt 639: Specifies that the command is executed whenever this key is used for
1.36 aaron 640: authentication.
641: The command supplied by the user (if any) is ignored.
1.1 deraadt 642: The command is run on a pty if the connection requests a pty;
1.36 aaron 643: otherwise it is run without a tty.
644: A quote may be included in the command by quoting it with a backslash.
645: This option might be useful
646: to restrict certain RSA keys to perform just a specific operation.
647: An example might be a key that permits remote backups but nothing else.
648: Notice that the client may specify TCP/IP and/or X11
1.1 deraadt 649: forwardings unless they are explicitly prohibited.
1.2 deraadt 650: .It Cm environment="NAME=value"
1.1 deraadt 651: Specifies that the string is to be added to the environment when
1.36 aaron 652: logging in using this key.
653: Environment variables set this way
654: override other default environment values.
655: Multiple options of this type are permitted.
1.2 deraadt 656: .It Cm no-port-forwarding
1.1 deraadt 657: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 658: Any port forward requests by the client will return an error.
659: This might be used, e.g., in connection with the
1.2 deraadt 660: .Cm command
1.1 deraadt 661: option.
1.2 deraadt 662: .It Cm no-X11-forwarding
1.1 deraadt 663: Forbids X11 forwarding when this key is used for authentication.
664: Any X11 forward requests by the client will return an error.
1.2 deraadt 665: .It Cm no-agent-forwarding
1.1 deraadt 666: Forbids authentication agent forwarding when this key is used for
667: authentication.
1.2 deraadt 668: .It Cm no-pty
1.1 deraadt 669: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 670: .El
671: .Ss Examples
1.1 deraadt 672: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 673: .Pp
1.1 deraadt 674: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 675: .Pp
1.1 deraadt 676: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 677: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 678: The
1.44 deraadt 679: .Pa /etc/ssh_known_hosts ,
680: .Pa /etc/ssh_known_hosts2 ,
681: .Pa $HOME/.ssh/known_hosts ,
1.40 aaron 682: and
1.44 deraadt 683: .Pa $HOME/.ssh/known_hosts2
1.36 aaron 684: files contain host public keys for all known hosts.
685: The global file should
1.37 brad 686: be prepared by the administrator (optional), and the per-user file is
1.1 deraadt 687: maintained automatically: whenever the user connects an unknown host
1.36 aaron 688: its key is added to the per-user file.
1.2 deraadt 689: .Pp
1.1 deraadt 690: Each line in these files contains the following fields: hostnames,
1.36 aaron 691: bits, exponent, modulus, comment.
692: The fields are separated by spaces.
1.2 deraadt 693: .Pp
1.1 deraadt 694: Hostnames is a comma-separated list of patterns ('*' and '?' act as
695: wildcards); each pattern in turn is matched against the canonical host
696: name (when authenticating a client) or against the user-supplied
1.36 aaron 697: name (when authenticating a server).
698: A pattern may also be preceded by
1.2 deraadt 699: .Ql !
700: to indicate negation: if the host name matches a negated
1.1 deraadt 701: pattern, it is not accepted (by that line) even if it matched another
702: pattern on the line.
1.2 deraadt 703: .Pp
1.1 deraadt 704: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 705: can be obtained, e.g., from
706: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 707: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 708: .Pp
709: Lines starting with
710: .Ql #
711: and empty lines are ignored as comments.
712: .Pp
1.1 deraadt 713: When performing host authentication, authentication is accepted if any
1.36 aaron 714: matching line has the proper key.
715: It is thus permissible (but not
1.1 deraadt 716: recommended) to have several lines or different host keys for the same
1.36 aaron 717: names.
718: This will inevitably happen when short forms of host names
719: from different domains are put in the file.
720: It is possible
1.1 deraadt 721: that the files contain conflicting information; authentication is
722: accepted if valid information can be found from either file.
1.2 deraadt 723: .Pp
1.1 deraadt 724: Note that the lines in these files are typically hundreds of characters
725: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 726: Rather, generate them by a script
1.40 aaron 727: or by taking
1.2 deraadt 728: .Pa /etc/ssh_host_key.pub
1.1 deraadt 729: and adding the host names at the front.
1.2 deraadt 730: .Ss Examples
1.1 deraadt 731: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 732: .Sh FILES
733: .Bl -tag -width Ds
734: .It Pa /etc/sshd_config
1.1 deraadt 735: Contains configuration data for
1.2 deraadt 736: .Nm sshd .
1.1 deraadt 737: This file should be writable by root only, but it is recommended
738: (though not necessary) that it be world-readable.
1.2 deraadt 739: .It Pa /etc/ssh_host_key
1.7 markus 740: Contains the private part of the host key.
1.1 deraadt 741: This file should only be owned by root, readable only by root, and not
742: accessible to others.
1.14 markus 743: Note that
744: .Nm
745: does not start if this file is group/world-accessible.
1.2 deraadt 746: .It Pa /etc/ssh_host_key.pub
1.7 markus 747: Contains the public part of the host key.
748: This file should be world-readable but writable only by
1.36 aaron 749: root.
750: Its contents should match the private part.
751: This file is not
1.1 deraadt 752: really used for anything; it is only provided for the convenience of
753: the user so its contents can be copied to known hosts files.
1.7 markus 754: These two files are created using
755: .Xr ssh-keygen 1 .
1.2 deraadt 756: .It Pa /var/run/sshd.pid
757: Contains the process ID of the
758: .Nm
1.1 deraadt 759: listening for connections (if there are several daemons running
760: concurrently for different ports, this contains the pid of the one
1.36 aaron 761: started last).
762: The contents of this file are not sensitive; it can be world-readable.
1.2 deraadt 763: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 764: Lists the RSA keys that can be used to log into the user's account.
765: This file must be readable by root (which may on some machines imply
766: it being world-readable if the user's home directory resides on an NFS
1.36 aaron 767: volume).
768: It is recommended that it not be accessible by others.
769: The format of this file is described above.
1.44 deraadt 770: Users will place the contents of their
771: .Pa identity.pub
772: files into this file, as described in
773: .Xr ssh-keygen 1 .
774: .It Pa $HOME/.ssh/authorized_keys2
775: Lists the DSA keys that can be used to log into the user's account.
776: This file must be readable by root (which may on some machines imply
777: it being world-readable if the user's home directory resides on an NFS
778: volume).
779: It is recommended that it not be accessible by others.
780: The format of this file is described above.
781: Users will place the contents of their
782: .Pa id_dsa.pub
783: files into this file, as described in
784: .Xr ssh-keygen 1 .
1.22 markus 785: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
786: These files are consulted when using rhosts with RSA host
1.36 aaron 787: authentication to check the public key of the host.
788: The key must be listed in one of these files to be accepted.
1.22 markus 789: The client uses the same files
1.36 aaron 790: to verify that the remote host is the one we intended to connect.
791: These files should be writable only by root/the owner.
1.2 deraadt 792: .Pa /etc/ssh_known_hosts
793: should be world-readable, and
794: .Pa $HOME/.ssh/known_hosts
795: can but need not be world-readable.
1.6 aaron 796: .It Pa /etc/nologin
1.40 aaron 797: If this file exists,
1.2 deraadt 798: .Nm
1.36 aaron 799: refuses to let anyone except root log in.
800: The contents of the file
1.1 deraadt 801: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 802: refused.
803: The file should be world-readable.
1.19 dugsong 804: .It Pa /etc/hosts.allow, /etc/hosts.deny
805: If compiled with
806: .Sy LIBWRAP
807: support, tcp-wrappers access controls may be defined here as described in
808: .Xr hosts_access 5 .
1.6 aaron 809: .It Pa $HOME/.rhosts
1.1 deraadt 810: This file contains host-username pairs, separated by a space, one per
1.36 aaron 811: line.
812: The given user on the corresponding host is permitted to log in
813: without password.
814: The same file is used by rlogind and rshd.
1.6 aaron 815: The file must
1.1 deraadt 816: be writable only by the user; it is recommended that it not be
817: accessible by others.
1.2 deraadt 818: .Pp
1.36 aaron 819: If is also possible to use netgroups in the file.
820: Either host or user
1.1 deraadt 821: name may be of the form +@groupname to specify all hosts or all users
822: in the group.
1.2 deraadt 823: .It Pa $HOME/.shosts
824: For ssh,
825: this file is exactly the same as for
826: .Pa .rhosts .
827: However, this file is
828: not used by rlogin and rshd, so using this permits access using SSH only.
829: .Pa /etc/hosts.equiv
830: This file is used during
831: .Pa .rhosts
1.36 aaron 832: authentication.
833: In the simplest form, this file contains host names, one per line.
834: Users on
1.1 deraadt 835: those hosts are permitted to log in without a password, provided they
1.36 aaron 836: have the same user name on both machines.
837: The host name may also be
1.1 deraadt 838: followed by a user name; such users are permitted to log in as
1.2 deraadt 839: .Em any
1.36 aaron 840: user on this machine (except root).
841: Additionally, the syntax
1.2 deraadt 842: .Dq +@group
1.36 aaron 843: can be used to specify netgroups.
844: Negated entries start with
1.2 deraadt 845: .Ql \&- .
846: .Pp
1.1 deraadt 847: If the client host/user is successfully matched in this file, login is
848: automatically permitted provided the client and server user names are the
1.36 aaron 849: same.
850: Additionally, successful RSA host authentication is normally required.
851: This file must be writable only by root; it is recommended
1.1 deraadt 852: that it be world-readable.
1.2 deraadt 853: .Pp
1.6 aaron 854: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 855: .Pa hosts.equiv .
1.1 deraadt 856: Beware that it really means that the named user(s) can log in as
1.2 deraadt 857: .Em anybody ,
1.1 deraadt 858: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 859: binaries and directories.
860: Using a user name practically grants the user root access.
861: The only valid use for user names that I can think
1.1 deraadt 862: of is in negative entries.
1.2 deraadt 863: .Pp
864: Note that this warning also applies to rsh/rlogin.
865: .It Pa /etc/shosts.equiv
1.1 deraadt 866: This is processed exactly as
1.2 deraadt 867: .Pa /etc/hosts.equiv .
1.1 deraadt 868: However, this file may be useful in environments that want to run both
1.2 deraadt 869: rsh/rlogin and ssh.
1.6 aaron 870: .It Pa $HOME/.ssh/environment
1.36 aaron 871: This file is read into the environment at login (if it exists).
872: It can only contain empty lines, comment lines (that start with
1.2 deraadt 873: .Ql # ) ,
1.36 aaron 874: and assignment lines of the form name=value.
875: The file should be writable
1.6 aaron 876: only by the user; it need not be readable by anyone else.
1.2 deraadt 877: .It Pa $HOME/.ssh/rc
1.1 deraadt 878: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 879: environment files but before starting the user's shell or command.
880: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 881: standard input (and
882: .Ev DISPLAY
1.36 aaron 883: in environment).
884: This must call
1.2 deraadt 885: .Xr xauth 1
886: in that case.
887: .Pp
1.1 deraadt 888: The primary purpose of this file is to run any initialization routines
889: which may be needed before the user's home directory becomes
890: accessible; AFS is a particular example of such an environment.
1.2 deraadt 891: .Pp
1.1 deraadt 892: This file will probably contain some initialization code followed by
893: something similar to: "if read proto cookie; then echo add $DISPLAY
894: $proto $cookie | xauth -q -; fi".
1.2 deraadt 895: .Pp
896: If this file does not exist,
897: .Pa /etc/sshrc
898: is run, and if that
1.1 deraadt 899: does not exist either, xauth is used to store the cookie.
1.2 deraadt 900: .Pp
1.1 deraadt 901: This file should be writable only by the user, and need not be
902: readable by anyone else.
1.2 deraadt 903: .It Pa /etc/sshrc
904: Like
905: .Pa $HOME/.ssh/rc .
906: This can be used to specify
1.36 aaron 907: machine-specific login-time initializations globally.
908: This file should be writable only by root, and should be world-readable.
1.2 deraadt 909: .Sh AUTHOR
1.17 deraadt 910: OpenSSH
1.33 deraadt 911: is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1.36 aaron 912: but with bugs removed and newer features re-added.
913: Rapidly after the
1.33 deraadt 914: 1.2.12 release, newer versions of the original ssh bore successively
915: more restrictive licenses, and thus demand for a free version was born.
1.48 ! markus 916: .Pp
1.33 deraadt 917: This version of OpenSSH
1.17 deraadt 918: .Bl -bullet
919: .It
1.32 aaron 920: has all components of a restrictive nature (i.e., patents, see
1.17 deraadt 921: .Xr ssl 8 )
922: directly removed from the source code; any licensed or patented components
923: are chosen from
924: external libraries.
925: .It
1.48 ! markus 926: has been updated to support SSH protocol 1.5 and 2, making it compatible with
! 927: all other SSH clients and servers.
1.17 deraadt 928: .It
1.40 aaron 929: contains added support for
1.17 deraadt 930: .Xr kerberos 8
931: authentication and ticket passing.
932: .It
933: supports one-time password authentication with
934: .Xr skey 1 .
935: .El
936: .Pp
937: The libraries described in
1.5 deraadt 938: .Xr ssl 8
939: are required for proper operation.
1.48 ! markus 940: .Pp
! 941: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
! 942: Niels Provos, Theo de Raadt, and Dug Song.
! 943: .Pp
! 944: The support for SSH protocol 2 was written by Markus Friedl.
1.2 deraadt 945: .Sh SEE ALSO
946: .Xr scp 1 ,
947: .Xr ssh 1 ,
1.5 deraadt 948: .Xr ssh-add 1 ,
1.2 deraadt 949: .Xr ssh-agent 1 ,
1.5 deraadt 950: .Xr ssh-keygen 1 ,
1.38 deraadt 951: .Xr ssl 8 ,
952: .Xr rlogin 1 ,
953: .Xr rsh 1
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