Annotation of src/usr.bin/ssh/sshd.8, Revision 1.60
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: .\"
1.60 ! provos 12: .\" $Id: sshd.8,v 1.59 2000/08/02 16:27:16 provos 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
1.58 deraadt 107: Version 2 works similarly:
1.49 markus 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.
1.58 deraadt 260: Only group names are valid; a numerical group ID isn't recognized.
1.36 aaron 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.
1.58 deraadt 273: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 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.
1.58 deraadt 297: Only group names are valid; a numerical group ID isn't recognized.
1.36 aaron 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.
1.58 deraadt 308: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 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 .
1.58 deraadt 324: .It Cm HostDSAKey
1.46 markus 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
1.59 provos 386: the Kerberos KDC. To use this option, the server needs a
387: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 388: Default is
1.60 ! provos 389: .Dq yes .
1.2 deraadt 390: .It Cm KerberosOrLocalPasswd
1.1 deraadt 391: If set then if password authentication through Kerberos fails then
392: the password will be validated via any additional local mechanism
1.2 deraadt 393: such as
394: .Pa /etc/passwd
1.36 aaron 395: or SecurID.
396: Default is
1.20 dugsong 397: .Dq yes .
1.2 deraadt 398: .It Cm KerberosTgtPassing
1.1 deraadt 399: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 400: Default is
1.3 dugsong 401: .Dq no ,
402: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 403: .It Cm KerberosTicketCleanup
1.7 markus 404: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 405: file on logout.
406: Default is
1.3 dugsong 407: .Dq yes .
1.2 deraadt 408: .It Cm KeyRegenerationInterval
1.1 deraadt 409: The server key is automatically regenerated after this many seconds
1.36 aaron 410: (if it has been used).
411: The purpose of regeneration is to prevent
1.1 deraadt 412: decrypting captured sessions by later breaking into the machine and
1.36 aaron 413: stealing the keys.
414: The key is never stored anywhere.
415: If the value is 0, the key is never regenerated.
416: The default is 3600 (seconds).
1.7 markus 417: .It Cm ListenAddress
418: Specifies what local address
419: .Nm
420: should listen on.
421: The default is to listen to all local addresses.
1.28 markus 422: Multiple options of this type are permitted.
423: Additionally, the
424: .Cm Ports
425: options must precede this option.
1.2 deraadt 426: .It Cm LoginGraceTime
1.1 deraadt 427: The server disconnects after this time if the user has not
1.36 aaron 428: successfully logged in.
429: If the value is 0, there is no time limit.
1.1 deraadt 430: The default is 600 (seconds).
1.23 markus 431: .It Cm LogLevel
432: Gives the verbosity level that is used when logging messages from
433: .Nm sshd .
434: The possible values are:
1.27 markus 435: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.23 markus 436: The default is INFO.
437: Logging with level DEBUG violates the privacy of users
438: and is not recommended.
1.55 markus 439: .It Cm MaxStartups
440: Specifies the maximum number of concurrent unauthenticated connections to the
441: .Nm
442: daemon.
443: Additional connections will be dropped until authentication succeeds or the
444: .Cm LoginGraceTime
445: expires for a connection.
446: The default is 10.
1.57 markus 447: .Pp
448: Alternatively, random early drop can be enabled by specifying
449: the three colon separated values
450: .Dq start:rate:full
451: (e.g. "10:30:60").
452: .Nm
453: will refuse connection attempts with a probabillity of
454: .Dq rate/100
455: (30%)
456: if there are currently
457: .Dq start
458: (10)
459: unauthenticated connections.
460: The probabillity increases linearly and all connection attempts
461: are refused if the number of unauthenticated connections reaches
462: .Dq full
463: (60).
1.2 deraadt 464: .It Cm PasswordAuthentication
1.1 deraadt 465: Specifies whether password authentication is allowed.
1.2 deraadt 466: The default is
467: .Dq yes .
1.58 deraadt 468: Note that this option applies to both protocol versions 1 and 2.
1.2 deraadt 469: .It Cm PermitEmptyPasswords
1.1 deraadt 470: When password authentication is allowed, it specifies whether the
1.36 aaron 471: server allows login to accounts with empty password strings.
472: The default is
1.34 markus 473: .Dq no .
1.2 deraadt 474: .It Cm PermitRootLogin
1.1 deraadt 475: Specifies whether the root can log in using
1.2 deraadt 476: .Xr ssh 1 .
1.15 markus 477: The argument must be
478: .Dq yes ,
479: .Dq without-password
480: or
481: .Dq no .
1.2 deraadt 482: The default is
483: .Dq yes .
1.15 markus 484: If this options is set to
485: .Dq without-password
486: only password authentication is disabled for root.
1.2 deraadt 487: .Pp
488: Root login with RSA authentication when the
489: .Ar command
490: option has been
1.1 deraadt 491: specified will be allowed regardless of the value of this setting
492: (which may be useful for taking remote backups even if root login is
493: normally not allowed).
1.43 markus 494: .It Cm PidFile
495: Specifies the file that contains the process identifier of the
496: .Nm
497: daemon.
498: The default is
499: .Pa /var/run/sshd.pid .
1.2 deraadt 500: .It Cm Port
1.1 deraadt 501: Specifies the port number that
1.2 deraadt 502: .Nm
1.36 aaron 503: listens on.
504: The default is 22.
1.28 markus 505: Multiple options of this type are permitted.
1.2 deraadt 506: .It Cm PrintMotd
1.1 deraadt 507: Specifies whether
1.2 deraadt 508: .Nm
1.40 aaron 509: should print
1.2 deraadt 510: .Pa /etc/motd
1.36 aaron 511: when a user logs in interactively.
512: (On some systems it is also printed by the shell,
1.2 deraadt 513: .Pa /etc/profile ,
1.36 aaron 514: or equivalent.)
515: The default is
1.2 deraadt 516: .Dq yes .
1.41 markus 517: .It Cm Protocol
518: Specifies the protocol versions
519: .Nm
520: should support.
521: The possible values are
522: .Dq 1
523: and
524: .Dq 2 .
525: Multiple versions must be comma-separated.
526: The default is
527: .Dq 1 .
1.2 deraadt 528: .It Cm RandomSeed
1.36 aaron 529: Obsolete.
530: Random number generation uses other techniques.
1.2 deraadt 531: .It Cm RhostsAuthentication
1.1 deraadt 532: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 533: files is sufficient.
534: Normally, this method should not be permitted because it is insecure.
1.7 markus 535: .Cm RhostsRSAAuthentication
536: should be used
1.1 deraadt 537: instead, because it performs RSA-based host authentication in addition
538: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 539: The default is
540: .Dq no .
541: .It Cm RhostsRSAAuthentication
1.1 deraadt 542: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 543: with successful RSA host authentication is allowed.
544: The default is
1.34 markus 545: .Dq no .
1.2 deraadt 546: .It Cm RSAAuthentication
1.36 aaron 547: Specifies whether pure RSA authentication is allowed.
548: The default is
1.2 deraadt 549: .Dq yes .
1.49 markus 550: Note that this option applies to protocol version 1 only.
1.2 deraadt 551: .It Cm ServerKeyBits
1.36 aaron 552: Defines the number of bits in the server key.
553: The minimum value is 512, and the default is 768.
1.7 markus 554: .It Cm SkeyAuthentication
555: Specifies whether
1.40 aaron 556: .Xr skey 1
1.36 aaron 557: authentication is allowed.
558: The default is
1.7 markus 559: .Dq yes .
560: Note that s/key authentication is enabled only if
561: .Cm PasswordAuthentication
562: is allowed, too.
1.2 deraadt 563: .It Cm StrictModes
1.12 markus 564: Specifies whether
565: .Nm
566: should check file modes and ownership of the
1.36 aaron 567: user's files and home directory before accepting login.
568: This is normally desirable because novices sometimes accidentally leave their
569: directory or files world-writable.
570: The default is
1.7 markus 571: .Dq yes .
1.54 jakob 572: .It Cm Subsystem
573: Configures an external subsystem (e.g. file transfer daemon).
574: Arguments should be a subsystem name and a command to execute upon subsystem request.
575: By default no subsystems are defined.
576: Note that this option applies to protocol version 2 only.
1.2 deraadt 577: .It Cm SyslogFacility
1.1 deraadt 578: Gives the facility code that is used when logging messages from
1.2 deraadt 579: .Nm sshd .
1.1 deraadt 580: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 581: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
582: The default is AUTH.
1.10 markus 583: .It Cm UseLogin
584: Specifies whether
585: .Xr login 1
1.53 markus 586: is used for interactive login sessions.
587: Note that
588: .Xr login 1
1.58 deraadt 589: is never used for remote command execution.
1.36 aaron 590: The default is
1.10 markus 591: .Dq no .
1.6 aaron 592: .It Cm X11DisplayOffset
593: Specifies the first display number available for
594: .Nm sshd Ns 's
1.36 aaron 595: X11 forwarding.
596: This prevents
1.6 aaron 597: .Nm
598: from interfering with real X11 servers.
1.34 markus 599: The default is 10.
1.30 markus 600: .It Cm X11Forwarding
1.36 aaron 601: Specifies whether X11 forwarding is permitted.
602: The default is
1.34 markus 603: .Dq no .
1.30 markus 604: Note that disabling X11 forwarding does not improve security in any
605: way, as users can always install their own forwarders.
1.52 markus 606: .It Cm XAuthLocation
607: Specifies the location of the
608: .Xr xauth 1
609: program.
610: The default is
611: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 612: .El
613: .Sh LOGIN PROCESS
1.1 deraadt 614: When a user successfully logs in,
1.2 deraadt 615: .Nm
1.1 deraadt 616: does the following:
1.2 deraadt 617: .Bl -enum -offset indent
618: .It
1.1 deraadt 619: If the login is on a tty, and no command has been specified,
1.40 aaron 620: prints last login time and
1.2 deraadt 621: .Pa /etc/motd
1.1 deraadt 622: (unless prevented in the configuration file or by
1.2 deraadt 623: .Pa $HOME/.hushlogin ;
624: see the
1.40 aaron 625: .Sx FILES
1.2 deraadt 626: section).
627: .It
1.1 deraadt 628: If the login is on a tty, records login time.
1.2 deraadt 629: .It
630: Checks
631: .Pa /etc/nologin ;
632: if it exists, prints contents and quits
1.1 deraadt 633: (unless root).
1.2 deraadt 634: .It
1.1 deraadt 635: Changes to run with normal user privileges.
1.2 deraadt 636: .It
1.1 deraadt 637: Sets up basic environment.
1.2 deraadt 638: .It
639: Reads
640: .Pa $HOME/.ssh/environment
641: if it exists.
642: .It
1.1 deraadt 643: Changes to user's home directory.
1.2 deraadt 644: .It
645: If
646: .Pa $HOME/.ssh/rc
647: exists, runs it; else if
648: .Pa /etc/sshrc
649: exists, runs
1.36 aaron 650: it; otherwise runs xauth.
651: The
1.2 deraadt 652: .Dq rc
653: files are given the X11
1.1 deraadt 654: authentication protocol and cookie in standard input.
1.2 deraadt 655: .It
1.1 deraadt 656: Runs user's shell or command.
1.2 deraadt 657: .El
658: .Sh AUTHORIZED_KEYS FILE FORMAT
1.40 aaron 659: The
1.2 deraadt 660: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 661: file lists the RSA keys that are
1.44 deraadt 662: permitted for RSA authentication in SSH protocols 1.3 and 1.5
1.51 hugh 663: Similarly, the
1.44 deraadt 664: .Pa $HOME/.ssh/authorized_keys2
665: file lists the DSA keys that are
666: permitted for DSA authentication in SSH protocol 2.0.
1.36 aaron 667: Each line of the file contains one
1.2 deraadt 668: key (empty lines and lines starting with a
669: .Ql #
670: are ignored as
1.36 aaron 671: comments).
672: Each line consists of the following fields, separated by
673: spaces: options, bits, exponent, modulus, comment.
674: The options field
1.1 deraadt 675: is optional; its presence is determined by whether the line starts
676: with a number or not (the option field never starts with a number).
677: The bits, exponent, modulus and comment fields give the RSA key; the
678: comment field is not used for anything (but may be convenient for the
679: user to identify the key).
1.2 deraadt 680: .Pp
1.1 deraadt 681: Note that lines in this file are usually several hundred bytes long
1.36 aaron 682: (because of the size of the RSA key modulus).
683: You don't want to type them in; instead, copy the
1.2 deraadt 684: .Pa identity.pub
1.1 deraadt 685: file and edit it.
1.2 deraadt 686: .Pp
1.58 deraadt 687: The options (if present) consist of comma-separated option
1.36 aaron 688: specifications.
689: No spaces are permitted, except within double quotes.
1.1 deraadt 690: The following option specifications are supported:
1.2 deraadt 691: .Bl -tag -width Ds
692: .It Cm from="pattern-list"
1.1 deraadt 693: Specifies that in addition to RSA authentication, the canonical name
694: of the remote host must be present in the comma-separated list of
1.36 aaron 695: patterns
696: .Pf ( Ql *
697: and
698: .Ql ?
699: serve as wildcards).
700: The list may also contain
701: patterns negated by prefixing them with
702: .Ql ! ;
703: if the canonical host name matches a negated pattern, the key is not accepted.
704: The purpose
1.1 deraadt 705: of this option is to optionally increase security: RSA authentication
706: by itself does not trust the network or name servers or anything (but
707: the key); however, if somebody somehow steals the key, the key
1.36 aaron 708: permits an intruder to log in from anywhere in the world.
709: This additional option makes using a stolen key more difficult (name
1.1 deraadt 710: servers and/or routers would have to be compromised in addition to
711: just the key).
1.2 deraadt 712: .It Cm command="command"
1.1 deraadt 713: Specifies that the command is executed whenever this key is used for
1.36 aaron 714: authentication.
715: The command supplied by the user (if any) is ignored.
1.1 deraadt 716: The command is run on a pty if the connection requests a pty;
1.36 aaron 717: otherwise it is run without a tty.
718: A quote may be included in the command by quoting it with a backslash.
719: This option might be useful
720: to restrict certain RSA keys to perform just a specific operation.
721: An example might be a key that permits remote backups but nothing else.
1.51 hugh 722: Note that the client may specify TCP/IP and/or X11
723: forwarding unless they are explicitly prohibited.
1.2 deraadt 724: .It Cm environment="NAME=value"
1.1 deraadt 725: Specifies that the string is to be added to the environment when
1.36 aaron 726: logging in using this key.
727: Environment variables set this way
728: override other default environment values.
729: Multiple options of this type are permitted.
1.2 deraadt 730: .It Cm no-port-forwarding
1.1 deraadt 731: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 732: Any port forward requests by the client will return an error.
733: This might be used, e.g., in connection with the
1.2 deraadt 734: .Cm command
1.1 deraadt 735: option.
1.2 deraadt 736: .It Cm no-X11-forwarding
1.1 deraadt 737: Forbids X11 forwarding when this key is used for authentication.
738: Any X11 forward requests by the client will return an error.
1.2 deraadt 739: .It Cm no-agent-forwarding
1.1 deraadt 740: Forbids authentication agent forwarding when this key is used for
741: authentication.
1.2 deraadt 742: .It Cm no-pty
1.1 deraadt 743: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 744: .El
745: .Ss Examples
1.1 deraadt 746: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 747: .Pp
1.1 deraadt 748: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 749: .Pp
1.1 deraadt 750: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 751: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 752: The
1.44 deraadt 753: .Pa /etc/ssh_known_hosts ,
754: .Pa /etc/ssh_known_hosts2 ,
755: .Pa $HOME/.ssh/known_hosts ,
1.40 aaron 756: and
1.44 deraadt 757: .Pa $HOME/.ssh/known_hosts2
1.36 aaron 758: files contain host public keys for all known hosts.
759: The global file should
1.37 brad 760: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 761: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 762: its key is added to the per-user file.
1.2 deraadt 763: .Pp
1.1 deraadt 764: Each line in these files contains the following fields: hostnames,
1.36 aaron 765: bits, exponent, modulus, comment.
766: The fields are separated by spaces.
1.2 deraadt 767: .Pp
1.1 deraadt 768: Hostnames is a comma-separated list of patterns ('*' and '?' act as
769: wildcards); each pattern in turn is matched against the canonical host
770: name (when authenticating a client) or against the user-supplied
1.36 aaron 771: name (when authenticating a server).
772: A pattern may also be preceded by
1.2 deraadt 773: .Ql !
774: to indicate negation: if the host name matches a negated
1.1 deraadt 775: pattern, it is not accepted (by that line) even if it matched another
776: pattern on the line.
1.2 deraadt 777: .Pp
1.49 markus 778: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 779: can be obtained, e.g., from
780: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 781: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 782: .Pp
783: Lines starting with
784: .Ql #
785: and empty lines are ignored as comments.
786: .Pp
1.1 deraadt 787: When performing host authentication, authentication is accepted if any
1.36 aaron 788: matching line has the proper key.
789: It is thus permissible (but not
1.1 deraadt 790: recommended) to have several lines or different host keys for the same
1.36 aaron 791: names.
792: This will inevitably happen when short forms of host names
793: from different domains are put in the file.
794: It is possible
1.1 deraadt 795: that the files contain conflicting information; authentication is
796: accepted if valid information can be found from either file.
1.2 deraadt 797: .Pp
1.1 deraadt 798: Note that the lines in these files are typically hundreds of characters
799: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 800: Rather, generate them by a script
1.40 aaron 801: or by taking
1.2 deraadt 802: .Pa /etc/ssh_host_key.pub
1.1 deraadt 803: and adding the host names at the front.
1.2 deraadt 804: .Ss Examples
1.1 deraadt 805: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 806: .Sh FILES
807: .Bl -tag -width Ds
808: .It Pa /etc/sshd_config
1.1 deraadt 809: Contains configuration data for
1.2 deraadt 810: .Nm sshd .
1.1 deraadt 811: This file should be writable by root only, but it is recommended
812: (though not necessary) that it be world-readable.
1.2 deraadt 813: .It Pa /etc/ssh_host_key
1.7 markus 814: Contains the private part of the host key.
1.1 deraadt 815: This file should only be owned by root, readable only by root, and not
816: accessible to others.
1.14 markus 817: Note that
818: .Nm
819: does not start if this file is group/world-accessible.
1.2 deraadt 820: .It Pa /etc/ssh_host_key.pub
1.7 markus 821: Contains the public part of the host key.
822: This file should be world-readable but writable only by
1.36 aaron 823: root.
824: Its contents should match the private part.
825: This file is not
1.1 deraadt 826: really used for anything; it is only provided for the convenience of
827: the user so its contents can be copied to known hosts files.
1.7 markus 828: These two files are created using
829: .Xr ssh-keygen 1 .
1.2 deraadt 830: .It Pa /var/run/sshd.pid
831: Contains the process ID of the
832: .Nm
1.1 deraadt 833: listening for connections (if there are several daemons running
834: concurrently for different ports, this contains the pid of the one
1.36 aaron 835: started last).
1.58 deraadt 836: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 837: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 838: Lists the RSA keys that can be used to log into the user's account.
839: This file must be readable by root (which may on some machines imply
840: it being world-readable if the user's home directory resides on an NFS
1.36 aaron 841: volume).
842: It is recommended that it not be accessible by others.
843: The format of this file is described above.
1.44 deraadt 844: Users will place the contents of their
845: .Pa identity.pub
846: files into this file, as described in
847: .Xr ssh-keygen 1 .
848: .It Pa $HOME/.ssh/authorized_keys2
849: Lists the DSA keys that can be used to log into the user's account.
850: This file must be readable by root (which may on some machines imply
851: it being world-readable if the user's home directory resides on an NFS
852: volume).
853: It is recommended that it not be accessible by others.
854: The format of this file is described above.
855: Users will place the contents of their
856: .Pa id_dsa.pub
857: files into this file, as described in
858: .Xr ssh-keygen 1 .
1.22 markus 859: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
860: These files are consulted when using rhosts with RSA host
1.36 aaron 861: authentication to check the public key of the host.
862: The key must be listed in one of these files to be accepted.
1.22 markus 863: The client uses the same files
1.58 deraadt 864: to verify that the remote host is the one it intended to connect.
1.36 aaron 865: These files should be writable only by root/the owner.
1.2 deraadt 866: .Pa /etc/ssh_known_hosts
867: should be world-readable, and
868: .Pa $HOME/.ssh/known_hosts
869: can but need not be world-readable.
1.6 aaron 870: .It Pa /etc/nologin
1.40 aaron 871: If this file exists,
1.2 deraadt 872: .Nm
1.36 aaron 873: refuses to let anyone except root log in.
874: The contents of the file
1.1 deraadt 875: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 876: refused.
877: The file should be world-readable.
1.19 dugsong 878: .It Pa /etc/hosts.allow, /etc/hosts.deny
879: If compiled with
880: .Sy LIBWRAP
881: support, tcp-wrappers access controls may be defined here as described in
882: .Xr hosts_access 5 .
1.6 aaron 883: .It Pa $HOME/.rhosts
1.1 deraadt 884: This file contains host-username pairs, separated by a space, one per
1.36 aaron 885: line.
886: The given user on the corresponding host is permitted to log in
887: without password.
888: The same file is used by rlogind and rshd.
1.6 aaron 889: The file must
1.1 deraadt 890: be writable only by the user; it is recommended that it not be
891: accessible by others.
1.2 deraadt 892: .Pp
1.36 aaron 893: If is also possible to use netgroups in the file.
894: Either host or user
1.1 deraadt 895: name may be of the form +@groupname to specify all hosts or all users
896: in the group.
1.2 deraadt 897: .It Pa $HOME/.shosts
898: For ssh,
899: this file is exactly the same as for
900: .Pa .rhosts .
901: However, this file is
902: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 903: .It Pa /etc/hosts.equiv
1.2 deraadt 904: This file is used during
905: .Pa .rhosts
1.36 aaron 906: authentication.
907: In the simplest form, this file contains host names, one per line.
908: Users on
1.1 deraadt 909: those hosts are permitted to log in without a password, provided they
1.36 aaron 910: have the same user name on both machines.
911: The host name may also be
1.1 deraadt 912: followed by a user name; such users are permitted to log in as
1.2 deraadt 913: .Em any
1.36 aaron 914: user on this machine (except root).
915: Additionally, the syntax
1.2 deraadt 916: .Dq +@group
1.36 aaron 917: can be used to specify netgroups.
918: Negated entries start with
1.2 deraadt 919: .Ql \&- .
920: .Pp
1.1 deraadt 921: If the client host/user is successfully matched in this file, login is
922: automatically permitted provided the client and server user names are the
1.36 aaron 923: same.
924: Additionally, successful RSA host authentication is normally required.
925: This file must be writable only by root; it is recommended
1.1 deraadt 926: that it be world-readable.
1.2 deraadt 927: .Pp
1.6 aaron 928: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 929: .Pa hosts.equiv .
1.1 deraadt 930: Beware that it really means that the named user(s) can log in as
1.2 deraadt 931: .Em anybody ,
1.1 deraadt 932: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 933: binaries and directories.
934: Using a user name practically grants the user root access.
935: The only valid use for user names that I can think
1.1 deraadt 936: of is in negative entries.
1.2 deraadt 937: .Pp
938: Note that this warning also applies to rsh/rlogin.
939: .It Pa /etc/shosts.equiv
1.1 deraadt 940: This is processed exactly as
1.2 deraadt 941: .Pa /etc/hosts.equiv .
1.1 deraadt 942: However, this file may be useful in environments that want to run both
1.2 deraadt 943: rsh/rlogin and ssh.
1.6 aaron 944: .It Pa $HOME/.ssh/environment
1.36 aaron 945: This file is read into the environment at login (if it exists).
946: It can only contain empty lines, comment lines (that start with
1.2 deraadt 947: .Ql # ) ,
1.36 aaron 948: and assignment lines of the form name=value.
949: The file should be writable
1.6 aaron 950: only by the user; it need not be readable by anyone else.
1.2 deraadt 951: .It Pa $HOME/.ssh/rc
1.1 deraadt 952: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 953: environment files but before starting the user's shell or command.
954: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 955: standard input (and
956: .Ev DISPLAY
1.36 aaron 957: in environment).
958: This must call
1.2 deraadt 959: .Xr xauth 1
960: in that case.
961: .Pp
1.1 deraadt 962: The primary purpose of this file is to run any initialization routines
963: which may be needed before the user's home directory becomes
964: accessible; AFS is a particular example of such an environment.
1.2 deraadt 965: .Pp
1.1 deraadt 966: This file will probably contain some initialization code followed by
967: something similar to: "if read proto cookie; then echo add $DISPLAY
968: $proto $cookie | xauth -q -; fi".
1.2 deraadt 969: .Pp
970: If this file does not exist,
971: .Pa /etc/sshrc
972: is run, and if that
1.1 deraadt 973: does not exist either, xauth is used to store the cookie.
1.2 deraadt 974: .Pp
1.1 deraadt 975: This file should be writable only by the user, and need not be
976: readable by anyone else.
1.2 deraadt 977: .It Pa /etc/sshrc
978: Like
979: .Pa $HOME/.ssh/rc .
980: This can be used to specify
1.36 aaron 981: machine-specific login-time initializations globally.
982: This file should be writable only by root, and should be world-readable.
1.56 aaron 983: .El
1.2 deraadt 984: .Sh AUTHOR
1.17 deraadt 985: OpenSSH
1.33 deraadt 986: is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1.36 aaron 987: but with bugs removed and newer features re-added.
988: Rapidly after the
1.33 deraadt 989: 1.2.12 release, newer versions of the original ssh bore successively
990: more restrictive licenses, and thus demand for a free version was born.
1.48 markus 991: .Pp
1.33 deraadt 992: This version of OpenSSH
1.17 deraadt 993: .Bl -bullet
994: .It
1.32 aaron 995: has all components of a restrictive nature (i.e., patents, see
1.17 deraadt 996: .Xr ssl 8 )
997: directly removed from the source code; any licensed or patented components
998: are chosen from
999: external libraries.
1000: .It
1.48 markus 1001: has been updated to support SSH protocol 1.5 and 2, making it compatible with
1002: all other SSH clients and servers.
1.17 deraadt 1003: .It
1.40 aaron 1004: contains added support for
1.17 deraadt 1005: .Xr kerberos 8
1006: authentication and ticket passing.
1007: .It
1008: supports one-time password authentication with
1009: .Xr skey 1 .
1010: .El
1011: .Pp
1012: The libraries described in
1.5 deraadt 1013: .Xr ssl 8
1014: are required for proper operation.
1.48 markus 1015: .Pp
1016: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1017: Niels Provos, Theo de Raadt, and Dug Song.
1018: .Pp
1019: The support for SSH protocol 2 was written by Markus Friedl.
1.2 deraadt 1020: .Sh SEE ALSO
1021: .Xr scp 1 ,
1022: .Xr ssh 1 ,
1.5 deraadt 1023: .Xr ssh-add 1 ,
1.2 deraadt 1024: .Xr ssh-agent 1 ,
1.5 deraadt 1025: .Xr ssh-keygen 1 ,
1.38 deraadt 1026: .Xr ssl 8 ,
1027: .Xr rlogin 1 ,
1028: .Xr rsh 1
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