Annotation of src/usr.bin/ssh/sshd.8, Revision 1.63
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: .\"
1.63 ! markus 12: .\" $Id: sshd.8,v 1.62 2000/08/25 16:16:15 deraadt 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
1.61 markus 29: .Op Fl u Ar len
1.26 markus 30: .Op Fl V Ar client_protocol_id
1.40 aaron 31: .Sh DESCRIPTION
1.2 deraadt 32: .Nm
1.40 aaron 33: (Secure Shell Daemon) is the daemon program for
1.2 deraadt 34: .Xr ssh 1 .
1.42 hugh 35: Together these programs replace rlogin and rsh, and
1.1 deraadt 36: provide secure encrypted communications between two untrusted hosts
1.36 aaron 37: over an insecure network.
38: The programs are intended to be as easy to
1.1 deraadt 39: install and use as possible.
1.2 deraadt 40: .Pp
41: .Nm
1.36 aaron 42: is the daemon that listens for connections from clients.
1.40 aaron 43: It is normally started at boot from
1.2 deraadt 44: .Pa /etc/rc .
45: It forks a new
1.36 aaron 46: daemon for each incoming connection.
47: The forked daemons handle
1.1 deraadt 48: key exchange, encryption, authentication, command execution,
49: and data exchange.
1.49 markus 50: This implementation of
51: .Nm
52: supports both SSH protocol version 1 and 2 simultaneously.
1.2 deraadt 53: .Nm
1.36 aaron 54: works as follows.
1.49 markus 55: .Pp
56: .Ss SSH protocol version 1
57: .Pp
1.36 aaron 58: Each host has a host-specific RSA key
59: (normally 1024 bits) used to identify the host.
60: Additionally, when
1.1 deraadt 61: the daemon starts, it generates a server RSA key (normally 768 bits).
62: This key is normally regenerated every hour if it has been used, and
63: is never stored on disk.
1.2 deraadt 64: .Pp
1.42 hugh 65: Whenever a client connects the daemon responds with its public
66: host and server keys.
1.36 aaron 67: The client compares the
1.49 markus 68: RSA host key against its own database to verify that it has not changed.
1.36 aaron 69: The client then generates a 256 bit random number.
70: It encrypts this
1.1 deraadt 71: random number using both the host key and the server key, and sends
1.36 aaron 72: the encrypted number to the server.
1.42 hugh 73: Both sides then use this
1.1 deraadt 74: random number as a session key which is used to encrypt all further
1.36 aaron 75: communications in the session.
76: The rest of the session is encrypted
1.42 hugh 77: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 78: being used by default.
1.36 aaron 79: The client selects the encryption algorithm
1.5 deraadt 80: to use from those offered by the server.
1.2 deraadt 81: .Pp
1.36 aaron 82: Next, the server and the client enter an authentication dialog.
83: The client tries to authenticate itself using
1.2 deraadt 84: .Pa .rhosts
85: authentication,
86: .Pa .rhosts
87: authentication combined with RSA host
1.1 deraadt 88: authentication, RSA challenge-response authentication, or password
89: based authentication.
1.2 deraadt 90: .Pp
1.1 deraadt 91: Rhosts authentication is normally disabled
92: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 93: configuration file if desired.
94: System security is not improved unless
1.2 deraadt 95: .Xr rshd 8 ,
96: .Xr rlogind 8 ,
97: .Xr rexecd 8 ,
98: and
99: .Xr rexd 8
1.1 deraadt 100: are disabled (thus completely disabling
1.2 deraadt 101: .Xr rlogin 1
1.1 deraadt 102: and
1.2 deraadt 103: .Xr rsh 1
1.42 hugh 104: into the machine).
1.2 deraadt 105: .Pp
1.49 markus 106: .Ss SSH protocol version 2
107: .Pp
1.58 deraadt 108: Version 2 works similarly:
1.49 markus 109: Each host has a host-specific DSA key used to identify the host.
110: However, when the daemon starts, it does not generate a server key.
111: Forward security is provided through a Diffie-Hellman key agreement.
112: This key agreement results in a shared session key.
113: The rest of the session is encrypted
114: using a symmetric cipher, currently
115: Blowfish, 3DES or CAST128 in CBC mode or Arcfour.
116: The client selects the encryption algorithm
117: to use from those offered by the server.
118: Additionally, session integrity is provided
1.51 hugh 119: through a cryptographic message authentication code
1.49 markus 120: (hmac-sha1 or hmac-md5).
121: .Pp
122: Protocol version 2 provides a public key based
123: user authentication method (DSAAuthentication)
124: and conventional password authentication.
125: .Pp
126: .Ss Command execution and data forwarding
127: .Pp
1.1 deraadt 128: If the client successfully authenticates itself, a dialog for
1.36 aaron 129: preparing the session is entered.
130: At this time the client may request
1.1 deraadt 131: things like allocating a pseudo-tty, forwarding X11 connections,
132: forwarding TCP/IP connections, or forwarding the authentication agent
133: connection over the secure channel.
1.2 deraadt 134: .Pp
1.1 deraadt 135: Finally, the client either requests a shell or execution of a command.
1.36 aaron 136: The sides then enter session mode.
137: In this mode, either side may send
1.1 deraadt 138: data at any time, and such data is forwarded to/from the shell or
139: command on the server side, and the user terminal in the client side.
1.2 deraadt 140: .Pp
1.1 deraadt 141: When the user program terminates and all forwarded X11 and other
142: connections have been closed, the server sends command exit status to
143: the client, and both sides exit.
1.2 deraadt 144: .Pp
145: .Nm
1.1 deraadt 146: can be configured using command-line options or a configuration
1.36 aaron 147: file.
148: Command-line options override values specified in the
1.1 deraadt 149: configuration file.
1.25 markus 150: .Pp
151: .Nm
152: rereads its configuration file when it receives a hangup signal,
153: .Dv SIGHUP .
1.18 aaron 154: .Pp
155: The options are as follows:
1.2 deraadt 156: .Bl -tag -width Ds
157: .It Fl b Ar bits
1.1 deraadt 158: Specifies the number of bits in the server key (default 768).
1.2 deraadt 159: .Pp
160: .It Fl d
1.36 aaron 161: Debug mode.
162: The server sends verbose debug output to the system
163: log, and does not put itself in the background.
164: The server also will not fork and will only process one connection.
165: This option is only intended for debugging for the server.
1.2 deraadt 166: .It Fl f Ar configuration_file
1.36 aaron 167: Specifies the name of the configuration file.
168: The default is
1.2 deraadt 169: .Pa /etc/sshd_config .
1.16 markus 170: .Nm
171: refuses to start if there is no configuration file.
1.2 deraadt 172: .It Fl g Ar login_grace_time
1.1 deraadt 173: Gives the grace time for clients to authenticate themselves (default
1.36 aaron 174: 300 seconds).
175: If the client fails to authenticate the user within
176: this many seconds, the server disconnects and exits.
177: A value of zero indicates no limit.
1.2 deraadt 178: .It Fl h Ar host_key_file
1.49 markus 179: Specifies the file from which the RSA host key is read (default
1.2 deraadt 180: .Pa /etc/ssh_host_key ) .
1.7 markus 181: This option must be given if
182: .Nm
183: is not run as root (as the normal
1.1 deraadt 184: host file is normally not readable by anyone but root).
1.2 deraadt 185: .It Fl i
1.7 markus 186: Specifies that
187: .Nm
1.40 aaron 188: is being run from inetd.
1.7 markus 189: .Nm
190: is normally not run
1.1 deraadt 191: from inetd because it needs to generate the server key before it can
1.36 aaron 192: respond to the client, and this may take tens of seconds.
193: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 194: However, with small key sizes (e.g., 512) using
1.7 markus 195: .Nm
196: from inetd may
1.1 deraadt 197: be feasible.
1.2 deraadt 198: .It Fl k Ar key_gen_time
1.1 deraadt 199: Specifies how often the server key is regenerated (default 3600
1.36 aaron 200: seconds, or one hour).
201: The motivation for regenerating the key fairly
1.1 deraadt 202: often is that the key is not stored anywhere, and after about an hour,
203: it becomes impossible to recover the key for decrypting intercepted
204: communications even if the machine is cracked into or physically
1.36 aaron 205: seized.
206: A value of zero indicates that the key will never be regenerated.
1.2 deraadt 207: .It Fl p Ar port
1.1 deraadt 208: Specifies the port on which the server listens for connections
209: (default 22).
1.2 deraadt 210: .It Fl q
1.36 aaron 211: Quiet mode.
212: Nothing is sent to the system log.
213: Normally the beginning,
1.1 deraadt 214: authentication, and termination of each connection is logged.
1.61 markus 215: .It Fl u Ar len
216: This option is used to specify the size of the field
217: in the
218: .Li utmp
219: structure that holds the remote host name.
220: If the resolved host name is longer than
221: .Ar len ,
222: the dotted decimal value will be used instead.
223: This allows hosts with very long host names that
224: overflow this field to still be uniquely identified.
225: Specifying
226: .Fl u0
227: indicates that only dotted decimal addresses
228: should be put into the
229: .Pa utmp
230: file.
1.18 aaron 231: .It Fl Q
232: Do not print an error message if RSA support is missing.
1.26 markus 233: .It Fl V Ar client_protocol_id
234: SSH2 compatibility mode.
1.42 hugh 235: When this option is specified
1.26 markus 236: .Nm
1.42 hugh 237: assumes the client has sent the supplied version string
1.26 markus 238: and skips the
239: Protocol Version Identification Exchange.
1.29 markus 240: .It Fl 4
241: Forces
242: .Nm
243: to use IPv4 addresses only.
244: .It Fl 6
245: Forces
246: .Nm
247: to use IPv6 addresses only.
1.2 deraadt 248: .El
249: .Sh CONFIGURATION FILE
250: .Nm
1.40 aaron 251: reads configuration data from
1.2 deraadt 252: .Pa /etc/sshd_config
253: (or the file specified with
254: .Fl f
1.36 aaron 255: on the command line).
256: The file contains keyword-value pairs, one per line.
257: Lines starting with
1.2 deraadt 258: .Ql #
1.1 deraadt 259: and empty lines are interpreted as comments.
1.2 deraadt 260: .Pp
1.1 deraadt 261: The following keywords are possible.
1.2 deraadt 262: .Bl -tag -width Ds
263: .It Cm AFSTokenPassing
1.36 aaron 264: Specifies whether an AFS token may be forwarded to the server.
265: Default is
1.2 deraadt 266: .Dq yes .
1.11 markus 267: .It Cm AllowGroups
268: This keyword can be followed by a number of group names, separated
1.36 aaron 269: by spaces.
270: If specified, login is allowed only for users whose primary
1.11 markus 271: group matches one of the patterns.
272: .Ql \&*
273: and
274: .Ql ?
275: can be used as
1.36 aaron 276: wildcards in the patterns.
1.58 deraadt 277: Only group names are valid; a numerical group ID isn't recognized.
1.36 aaron 278: By default login is allowed regardless of the primary group.
1.3 dugsong 279: .Pp
1.11 markus 280: .It Cm AllowUsers
281: This keyword can be followed by a number of user names, separated
1.36 aaron 282: by spaces.
283: If specified, login is allowed only for users names that
1.11 markus 284: match one of the patterns.
285: .Ql \&*
286: and
287: .Ql ?
288: can be used as
1.36 aaron 289: wildcards in the patterns.
1.58 deraadt 290: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 291: By default login is allowed regardless of the user name.
1.11 markus 292: .Pp
1.41 markus 293: .It Cm Ciphers
294: Specifies the ciphers allowed for protocol version 2.
295: Multiple ciphers must be comma-separated.
296: The default is
1.50 markus 297: .Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
1.8 markus 298: .It Cm CheckMail
299: Specifies whether
300: .Nm
301: should check for new mail for interactive logins.
302: The default is
303: .Dq no .
1.11 markus 304: .It Cm DenyGroups
305: This keyword can be followed by a number of group names, separated
1.36 aaron 306: by spaces.
307: Users whose primary group matches one of the patterns
1.11 markus 308: aren't allowed to log in.
309: .Ql \&*
310: and
311: .Ql ?
312: can be used as
1.36 aaron 313: wildcards in the patterns.
1.58 deraadt 314: Only group names are valid; a numerical group ID isn't recognized.
1.36 aaron 315: By default login is allowed regardless of the primary group.
1.11 markus 316: .Pp
317: .It Cm DenyUsers
318: This keyword can be followed by a number of user names, separated
1.36 aaron 319: by spaces.
320: Login is disallowed for user names that match one of the patterns.
1.11 markus 321: .Ql \&*
322: and
323: .Ql ?
1.36 aaron 324: can be used as wildcards in the patterns.
1.58 deraadt 325: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 326: By default login is allowed regardless of the user name.
1.49 markus 327: .It Cm DSAAuthentication
328: Specifies whether DSA authentication is allowed.
329: The default is
330: .Dq yes .
331: Note that this option applies to protocol version 2 only.
1.47 markus 332: .It Cm GatewayPorts
333: Specifies whether remote hosts are allowed to connect to ports
334: forwarded for the client.
335: The argument must be
336: .Dq yes
337: or
338: .Dq no .
339: The default is
340: .Dq no .
1.58 deraadt 341: .It Cm HostDSAKey
1.46 markus 342: Specifies the file containing the private DSA host key (default
343: .Pa /etc/ssh_host_dsa_key )
344: used by SSH protocol 2.0.
345: Note that
346: .Nm
1.51 hugh 347: disables protocol 2.0 if this file is group/world-accessible.
1.2 deraadt 348: .It Cm HostKey
1.46 markus 349: Specifies the file containing the private RSA host key (default
350: .Pa /etc/ssh_host_key )
351: used by SSH protocols 1.3 and 1.5.
1.9 markus 352: Note that
353: .Nm
1.51 hugh 354: disables protocols 1.3 and 1.5 if this file is group/world-accessible.
1.2 deraadt 355: .It Cm IgnoreRhosts
1.34 markus 356: Specifies that
357: .Pa .rhosts
1.40 aaron 358: and
1.34 markus 359: .Pa .shosts
360: files will not be used in authentication.
1.2 deraadt 361: .Pa /etc/hosts.equiv
1.1 deraadt 362: and
1.40 aaron 363: .Pa /etc/shosts.equiv
1.36 aaron 364: are still used.
1.40 aaron 365: The default is
1.34 markus 366: .Dq yes .
1.24 markus 367: .It Cm IgnoreUserKnownHosts
368: Specifies whether
369: .Nm
370: should ignore the user's
371: .Pa $HOME/.ssh/known_hosts
1.45 markus 372: during
1.24 markus 373: .Cm RhostsRSAAuthentication .
374: The default is
1.2 deraadt 375: .Dq no .
376: .It Cm KeepAlive
1.1 deraadt 377: Specifies whether the system should send keepalive messages to the
1.36 aaron 378: other side.
379: If they are sent, death of the connection or crash of one
380: of the machines will be properly noticed.
381: However, this means that
1.1 deraadt 382: connections will die if the route is down temporarily, and some people
1.36 aaron 383: find it annoying.
1.51 hugh 384: On the other hand, if keepalives are not sent,
1.2 deraadt 385: sessions may hang indefinitely on the server, leaving
386: .Dq ghost
387: users and consuming server resources.
388: .Pp
389: The default is
390: .Dq yes
391: (to send keepalives), and the server will notice
1.36 aaron 392: if the network goes down or the client host reboots.
393: This avoids infinitely hanging sessions.
1.2 deraadt 394: .Pp
395: To disable keepalives, the value should be set to
396: .Dq no
397: in both the server and the client configuration files.
398: .It Cm KerberosAuthentication
1.36 aaron 399: Specifies whether Kerberos authentication is allowed.
400: This can be in the form of a Kerberos ticket, or if
1.7 markus 401: .Cm PasswordAuthentication
1.1 deraadt 402: is yes, the password provided by the user will be validated through
1.59 provos 403: the Kerberos KDC. To use this option, the server needs a
404: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 405: Default is
1.60 provos 406: .Dq yes .
1.2 deraadt 407: .It Cm KerberosOrLocalPasswd
1.1 deraadt 408: If set then if password authentication through Kerberos fails then
409: the password will be validated via any additional local mechanism
1.2 deraadt 410: such as
411: .Pa /etc/passwd
1.36 aaron 412: or SecurID.
413: Default is
1.20 dugsong 414: .Dq yes .
1.2 deraadt 415: .It Cm KerberosTgtPassing
1.1 deraadt 416: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 417: Default is
1.3 dugsong 418: .Dq no ,
419: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 420: .It Cm KerberosTicketCleanup
1.7 markus 421: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 422: file on logout.
423: Default is
1.3 dugsong 424: .Dq yes .
1.2 deraadt 425: .It Cm KeyRegenerationInterval
1.1 deraadt 426: The server key is automatically regenerated after this many seconds
1.36 aaron 427: (if it has been used).
428: The purpose of regeneration is to prevent
1.1 deraadt 429: decrypting captured sessions by later breaking into the machine and
1.36 aaron 430: stealing the keys.
431: The key is never stored anywhere.
432: If the value is 0, the key is never regenerated.
433: The default is 3600 (seconds).
1.7 markus 434: .It Cm ListenAddress
435: Specifies what local address
436: .Nm
437: should listen on.
438: The default is to listen to all local addresses.
1.28 markus 439: Multiple options of this type are permitted.
440: Additionally, the
441: .Cm Ports
442: options must precede this option.
1.2 deraadt 443: .It Cm LoginGraceTime
1.1 deraadt 444: The server disconnects after this time if the user has not
1.36 aaron 445: successfully logged in.
446: If the value is 0, there is no time limit.
1.1 deraadt 447: The default is 600 (seconds).
1.23 markus 448: .It Cm LogLevel
449: Gives the verbosity level that is used when logging messages from
450: .Nm sshd .
451: The possible values are:
1.27 markus 452: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.23 markus 453: The default is INFO.
454: Logging with level DEBUG violates the privacy of users
455: and is not recommended.
1.55 markus 456: .It Cm MaxStartups
457: Specifies the maximum number of concurrent unauthenticated connections to the
458: .Nm
459: daemon.
460: Additional connections will be dropped until authentication succeeds or the
461: .Cm LoginGraceTime
462: expires for a connection.
463: The default is 10.
1.57 markus 464: .Pp
465: Alternatively, random early drop can be enabled by specifying
466: the three colon separated values
467: .Dq start:rate:full
468: (e.g. "10:30:60").
469: .Nm
470: will refuse connection attempts with a probabillity of
471: .Dq rate/100
472: (30%)
473: if there are currently
474: .Dq start
475: (10)
476: unauthenticated connections.
477: The probabillity increases linearly and all connection attempts
478: are refused if the number of unauthenticated connections reaches
479: .Dq full
480: (60).
1.2 deraadt 481: .It Cm PasswordAuthentication
1.1 deraadt 482: Specifies whether password authentication is allowed.
1.2 deraadt 483: The default is
484: .Dq yes .
1.58 deraadt 485: Note that this option applies to both protocol versions 1 and 2.
1.2 deraadt 486: .It Cm PermitEmptyPasswords
1.1 deraadt 487: When password authentication is allowed, it specifies whether the
1.36 aaron 488: server allows login to accounts with empty password strings.
489: The default is
1.34 markus 490: .Dq no .
1.2 deraadt 491: .It Cm PermitRootLogin
1.1 deraadt 492: Specifies whether the root can log in using
1.2 deraadt 493: .Xr ssh 1 .
1.15 markus 494: The argument must be
495: .Dq yes ,
496: .Dq without-password
497: or
498: .Dq no .
1.2 deraadt 499: The default is
500: .Dq yes .
1.15 markus 501: If this options is set to
502: .Dq without-password
503: only password authentication is disabled for root.
1.2 deraadt 504: .Pp
505: Root login with RSA authentication when the
506: .Ar command
507: option has been
1.1 deraadt 508: specified will be allowed regardless of the value of this setting
509: (which may be useful for taking remote backups even if root login is
510: normally not allowed).
1.43 markus 511: .It Cm PidFile
512: Specifies the file that contains the process identifier of the
513: .Nm
514: daemon.
515: The default is
516: .Pa /var/run/sshd.pid .
1.2 deraadt 517: .It Cm Port
1.1 deraadt 518: Specifies the port number that
1.2 deraadt 519: .Nm
1.36 aaron 520: listens on.
521: The default is 22.
1.28 markus 522: Multiple options of this type are permitted.
1.2 deraadt 523: .It Cm PrintMotd
1.1 deraadt 524: Specifies whether
1.2 deraadt 525: .Nm
1.40 aaron 526: should print
1.2 deraadt 527: .Pa /etc/motd
1.36 aaron 528: when a user logs in interactively.
529: (On some systems it is also printed by the shell,
1.2 deraadt 530: .Pa /etc/profile ,
1.36 aaron 531: or equivalent.)
532: The default is
1.2 deraadt 533: .Dq yes .
1.41 markus 534: .It Cm Protocol
535: Specifies the protocol versions
536: .Nm
537: should support.
538: The possible values are
539: .Dq 1
540: and
541: .Dq 2 .
542: Multiple versions must be comma-separated.
543: The default is
544: .Dq 1 .
1.2 deraadt 545: .It Cm RandomSeed
1.36 aaron 546: Obsolete.
547: Random number generation uses other techniques.
1.2 deraadt 548: .It Cm RhostsAuthentication
1.1 deraadt 549: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 550: files is sufficient.
551: Normally, this method should not be permitted because it is insecure.
1.7 markus 552: .Cm RhostsRSAAuthentication
553: should be used
1.1 deraadt 554: instead, because it performs RSA-based host authentication in addition
555: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 556: The default is
557: .Dq no .
558: .It Cm RhostsRSAAuthentication
1.1 deraadt 559: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 560: with successful RSA host authentication is allowed.
561: The default is
1.34 markus 562: .Dq no .
1.2 deraadt 563: .It Cm RSAAuthentication
1.36 aaron 564: Specifies whether pure RSA authentication is allowed.
565: The default is
1.2 deraadt 566: .Dq yes .
1.49 markus 567: Note that this option applies to protocol version 1 only.
1.2 deraadt 568: .It Cm ServerKeyBits
1.36 aaron 569: Defines the number of bits in the server key.
570: The minimum value is 512, and the default is 768.
1.7 markus 571: .It Cm SkeyAuthentication
572: Specifies whether
1.40 aaron 573: .Xr skey 1
1.36 aaron 574: authentication is allowed.
575: The default is
1.7 markus 576: .Dq yes .
577: Note that s/key authentication is enabled only if
578: .Cm PasswordAuthentication
579: is allowed, too.
1.2 deraadt 580: .It Cm StrictModes
1.12 markus 581: Specifies whether
582: .Nm
583: should check file modes and ownership of the
1.36 aaron 584: user's files and home directory before accepting login.
585: This is normally desirable because novices sometimes accidentally leave their
586: directory or files world-writable.
587: The default is
1.7 markus 588: .Dq yes .
1.54 jakob 589: .It Cm Subsystem
590: Configures an external subsystem (e.g. file transfer daemon).
591: Arguments should be a subsystem name and a command to execute upon subsystem request.
1.63 ! markus 592: The command
! 593: .Xr sftp-server 8
! 594: implements the
! 595: .Dq sftp
! 596: file transfer subsystem.
1.54 jakob 597: By default no subsystems are defined.
598: Note that this option applies to protocol version 2 only.
1.2 deraadt 599: .It Cm SyslogFacility
1.1 deraadt 600: Gives the facility code that is used when logging messages from
1.2 deraadt 601: .Nm sshd .
1.1 deraadt 602: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 603: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
604: The default is AUTH.
1.10 markus 605: .It Cm UseLogin
606: Specifies whether
607: .Xr login 1
1.53 markus 608: is used for interactive login sessions.
609: Note that
610: .Xr login 1
1.58 deraadt 611: is never used for remote command execution.
1.36 aaron 612: The default is
1.10 markus 613: .Dq no .
1.6 aaron 614: .It Cm X11DisplayOffset
615: Specifies the first display number available for
616: .Nm sshd Ns 's
1.36 aaron 617: X11 forwarding.
618: This prevents
1.6 aaron 619: .Nm
620: from interfering with real X11 servers.
1.34 markus 621: The default is 10.
1.30 markus 622: .It Cm X11Forwarding
1.36 aaron 623: Specifies whether X11 forwarding is permitted.
624: The default is
1.34 markus 625: .Dq no .
1.30 markus 626: Note that disabling X11 forwarding does not improve security in any
627: way, as users can always install their own forwarders.
1.52 markus 628: .It Cm XAuthLocation
629: Specifies the location of the
630: .Xr xauth 1
631: program.
632: The default is
633: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 634: .El
635: .Sh LOGIN PROCESS
1.1 deraadt 636: When a user successfully logs in,
1.2 deraadt 637: .Nm
1.1 deraadt 638: does the following:
1.2 deraadt 639: .Bl -enum -offset indent
640: .It
1.1 deraadt 641: If the login is on a tty, and no command has been specified,
1.40 aaron 642: prints last login time and
1.2 deraadt 643: .Pa /etc/motd
1.1 deraadt 644: (unless prevented in the configuration file or by
1.2 deraadt 645: .Pa $HOME/.hushlogin ;
646: see the
1.40 aaron 647: .Sx FILES
1.2 deraadt 648: section).
649: .It
1.1 deraadt 650: If the login is on a tty, records login time.
1.2 deraadt 651: .It
652: Checks
653: .Pa /etc/nologin ;
654: if it exists, prints contents and quits
1.1 deraadt 655: (unless root).
1.2 deraadt 656: .It
1.1 deraadt 657: Changes to run with normal user privileges.
1.2 deraadt 658: .It
1.1 deraadt 659: Sets up basic environment.
1.2 deraadt 660: .It
661: Reads
662: .Pa $HOME/.ssh/environment
663: if it exists.
664: .It
1.1 deraadt 665: Changes to user's home directory.
1.2 deraadt 666: .It
667: If
668: .Pa $HOME/.ssh/rc
669: exists, runs it; else if
670: .Pa /etc/sshrc
671: exists, runs
1.36 aaron 672: it; otherwise runs xauth.
673: The
1.2 deraadt 674: .Dq rc
675: files are given the X11
1.1 deraadt 676: authentication protocol and cookie in standard input.
1.2 deraadt 677: .It
1.1 deraadt 678: Runs user's shell or command.
1.2 deraadt 679: .El
680: .Sh AUTHORIZED_KEYS FILE FORMAT
1.40 aaron 681: The
1.2 deraadt 682: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 683: file lists the RSA keys that are
1.44 deraadt 684: permitted for RSA authentication in SSH protocols 1.3 and 1.5
1.51 hugh 685: Similarly, the
1.44 deraadt 686: .Pa $HOME/.ssh/authorized_keys2
687: file lists the DSA keys that are
688: permitted for DSA authentication in SSH protocol 2.0.
1.36 aaron 689: Each line of the file contains one
1.2 deraadt 690: key (empty lines and lines starting with a
691: .Ql #
692: are ignored as
1.36 aaron 693: comments).
694: Each line consists of the following fields, separated by
695: spaces: options, bits, exponent, modulus, comment.
696: The options field
1.1 deraadt 697: is optional; its presence is determined by whether the line starts
698: with a number or not (the option field never starts with a number).
699: The bits, exponent, modulus and comment fields give the RSA key; the
700: comment field is not used for anything (but may be convenient for the
701: user to identify the key).
1.2 deraadt 702: .Pp
1.1 deraadt 703: Note that lines in this file are usually several hundred bytes long
1.36 aaron 704: (because of the size of the RSA key modulus).
705: You don't want to type them in; instead, copy the
1.2 deraadt 706: .Pa identity.pub
1.1 deraadt 707: file and edit it.
1.2 deraadt 708: .Pp
1.58 deraadt 709: The options (if present) consist of comma-separated option
1.36 aaron 710: specifications.
711: No spaces are permitted, except within double quotes.
1.1 deraadt 712: The following option specifications are supported:
1.2 deraadt 713: .Bl -tag -width Ds
714: .It Cm from="pattern-list"
1.1 deraadt 715: Specifies that in addition to RSA authentication, the canonical name
716: of the remote host must be present in the comma-separated list of
1.36 aaron 717: patterns
718: .Pf ( Ql *
719: and
720: .Ql ?
721: serve as wildcards).
722: The list may also contain
723: patterns negated by prefixing them with
724: .Ql ! ;
725: if the canonical host name matches a negated pattern, the key is not accepted.
726: The purpose
1.1 deraadt 727: of this option is to optionally increase security: RSA authentication
728: by itself does not trust the network or name servers or anything (but
729: the key); however, if somebody somehow steals the key, the key
1.36 aaron 730: permits an intruder to log in from anywhere in the world.
731: This additional option makes using a stolen key more difficult (name
1.1 deraadt 732: servers and/or routers would have to be compromised in addition to
733: just the key).
1.2 deraadt 734: .It Cm command="command"
1.1 deraadt 735: Specifies that the command is executed whenever this key is used for
1.36 aaron 736: authentication.
737: The command supplied by the user (if any) is ignored.
1.1 deraadt 738: The command is run on a pty if the connection requests a pty;
1.36 aaron 739: otherwise it is run without a tty.
740: A quote may be included in the command by quoting it with a backslash.
741: This option might be useful
742: to restrict certain RSA keys to perform just a specific operation.
743: An example might be a key that permits remote backups but nothing else.
1.51 hugh 744: Note that the client may specify TCP/IP and/or X11
745: forwarding unless they are explicitly prohibited.
1.2 deraadt 746: .It Cm environment="NAME=value"
1.1 deraadt 747: Specifies that the string is to be added to the environment when
1.36 aaron 748: logging in using this key.
749: Environment variables set this way
750: override other default environment values.
751: Multiple options of this type are permitted.
1.2 deraadt 752: .It Cm no-port-forwarding
1.1 deraadt 753: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 754: Any port forward requests by the client will return an error.
755: This might be used, e.g., in connection with the
1.2 deraadt 756: .Cm command
1.1 deraadt 757: option.
1.2 deraadt 758: .It Cm no-X11-forwarding
1.1 deraadt 759: Forbids X11 forwarding when this key is used for authentication.
760: Any X11 forward requests by the client will return an error.
1.2 deraadt 761: .It Cm no-agent-forwarding
1.1 deraadt 762: Forbids authentication agent forwarding when this key is used for
763: authentication.
1.2 deraadt 764: .It Cm no-pty
1.1 deraadt 765: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 766: .El
767: .Ss Examples
1.1 deraadt 768: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 769: .Pp
1.1 deraadt 770: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 771: .Pp
1.1 deraadt 772: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 773: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 774: The
1.44 deraadt 775: .Pa /etc/ssh_known_hosts ,
776: .Pa /etc/ssh_known_hosts2 ,
777: .Pa $HOME/.ssh/known_hosts ,
1.40 aaron 778: and
1.44 deraadt 779: .Pa $HOME/.ssh/known_hosts2
1.36 aaron 780: files contain host public keys for all known hosts.
781: The global file should
1.37 brad 782: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 783: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 784: its key is added to the per-user file.
1.2 deraadt 785: .Pp
1.1 deraadt 786: Each line in these files contains the following fields: hostnames,
1.36 aaron 787: bits, exponent, modulus, comment.
788: The fields are separated by spaces.
1.2 deraadt 789: .Pp
1.1 deraadt 790: Hostnames is a comma-separated list of patterns ('*' and '?' act as
791: wildcards); each pattern in turn is matched against the canonical host
792: name (when authenticating a client) or against the user-supplied
1.36 aaron 793: name (when authenticating a server).
794: A pattern may also be preceded by
1.2 deraadt 795: .Ql !
796: to indicate negation: if the host name matches a negated
1.1 deraadt 797: pattern, it is not accepted (by that line) even if it matched another
798: pattern on the line.
1.2 deraadt 799: .Pp
1.49 markus 800: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 801: can be obtained, e.g., from
802: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 803: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 804: .Pp
805: Lines starting with
806: .Ql #
807: and empty lines are ignored as comments.
808: .Pp
1.1 deraadt 809: When performing host authentication, authentication is accepted if any
1.36 aaron 810: matching line has the proper key.
811: It is thus permissible (but not
1.1 deraadt 812: recommended) to have several lines or different host keys for the same
1.36 aaron 813: names.
814: This will inevitably happen when short forms of host names
815: from different domains are put in the file.
816: It is possible
1.1 deraadt 817: that the files contain conflicting information; authentication is
818: accepted if valid information can be found from either file.
1.2 deraadt 819: .Pp
1.1 deraadt 820: Note that the lines in these files are typically hundreds of characters
821: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 822: Rather, generate them by a script
1.40 aaron 823: or by taking
1.2 deraadt 824: .Pa /etc/ssh_host_key.pub
1.1 deraadt 825: and adding the host names at the front.
1.2 deraadt 826: .Ss Examples
1.1 deraadt 827: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 828: .Sh FILES
829: .Bl -tag -width Ds
830: .It Pa /etc/sshd_config
1.1 deraadt 831: Contains configuration data for
1.2 deraadt 832: .Nm sshd .
1.1 deraadt 833: This file should be writable by root only, but it is recommended
834: (though not necessary) that it be world-readable.
1.2 deraadt 835: .It Pa /etc/ssh_host_key
1.7 markus 836: Contains the private part of the host key.
1.1 deraadt 837: This file should only be owned by root, readable only by root, and not
838: accessible to others.
1.14 markus 839: Note that
840: .Nm
841: does not start if this file is group/world-accessible.
1.2 deraadt 842: .It Pa /etc/ssh_host_key.pub
1.7 markus 843: Contains the public part of the host key.
844: This file should be world-readable but writable only by
1.36 aaron 845: root.
846: Its contents should match the private part.
847: This file is not
1.1 deraadt 848: really used for anything; it is only provided for the convenience of
849: the user so its contents can be copied to known hosts files.
1.7 markus 850: These two files are created using
851: .Xr ssh-keygen 1 .
1.2 deraadt 852: .It Pa /var/run/sshd.pid
853: Contains the process ID of the
854: .Nm
1.1 deraadt 855: listening for connections (if there are several daemons running
856: concurrently for different ports, this contains the pid of the one
1.36 aaron 857: started last).
1.58 deraadt 858: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 859: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 860: Lists the RSA keys that can be used to log into the user's account.
861: This file must be readable by root (which may on some machines imply
862: it being world-readable if the user's home directory resides on an NFS
1.36 aaron 863: volume).
864: It is recommended that it not be accessible by others.
865: The format of this file is described above.
1.44 deraadt 866: Users will place the contents of their
867: .Pa identity.pub
868: files into this file, as described in
869: .Xr ssh-keygen 1 .
870: .It Pa $HOME/.ssh/authorized_keys2
871: Lists the DSA keys that can be used to log into the user's account.
872: This file must be readable by root (which may on some machines imply
873: it being world-readable if the user's home directory resides on an NFS
874: volume).
875: It is recommended that it not be accessible by others.
876: The format of this file is described above.
877: Users will place the contents of their
878: .Pa id_dsa.pub
879: files into this file, as described in
880: .Xr ssh-keygen 1 .
1.22 markus 881: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
882: These files are consulted when using rhosts with RSA host
1.36 aaron 883: authentication to check the public key of the host.
884: The key must be listed in one of these files to be accepted.
1.22 markus 885: The client uses the same files
1.58 deraadt 886: to verify that the remote host is the one it intended to connect.
1.36 aaron 887: These files should be writable only by root/the owner.
1.2 deraadt 888: .Pa /etc/ssh_known_hosts
889: should be world-readable, and
890: .Pa $HOME/.ssh/known_hosts
891: can but need not be world-readable.
1.6 aaron 892: .It Pa /etc/nologin
1.40 aaron 893: If this file exists,
1.2 deraadt 894: .Nm
1.36 aaron 895: refuses to let anyone except root log in.
896: The contents of the file
1.1 deraadt 897: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 898: refused.
899: The file should be world-readable.
1.19 dugsong 900: .It Pa /etc/hosts.allow, /etc/hosts.deny
901: If compiled with
902: .Sy LIBWRAP
903: support, tcp-wrappers access controls may be defined here as described in
904: .Xr hosts_access 5 .
1.6 aaron 905: .It Pa $HOME/.rhosts
1.1 deraadt 906: This file contains host-username pairs, separated by a space, one per
1.36 aaron 907: line.
908: The given user on the corresponding host is permitted to log in
909: without password.
910: The same file is used by rlogind and rshd.
1.6 aaron 911: The file must
1.1 deraadt 912: be writable only by the user; it is recommended that it not be
913: accessible by others.
1.2 deraadt 914: .Pp
1.36 aaron 915: If is also possible to use netgroups in the file.
916: Either host or user
1.1 deraadt 917: name may be of the form +@groupname to specify all hosts or all users
918: in the group.
1.2 deraadt 919: .It Pa $HOME/.shosts
920: For ssh,
921: this file is exactly the same as for
922: .Pa .rhosts .
923: However, this file is
924: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 925: .It Pa /etc/hosts.equiv
1.2 deraadt 926: This file is used during
927: .Pa .rhosts
1.36 aaron 928: authentication.
929: In the simplest form, this file contains host names, one per line.
930: Users on
1.1 deraadt 931: those hosts are permitted to log in without a password, provided they
1.36 aaron 932: have the same user name on both machines.
933: The host name may also be
1.1 deraadt 934: followed by a user name; such users are permitted to log in as
1.2 deraadt 935: .Em any
1.36 aaron 936: user on this machine (except root).
937: Additionally, the syntax
1.2 deraadt 938: .Dq +@group
1.36 aaron 939: can be used to specify netgroups.
940: Negated entries start with
1.2 deraadt 941: .Ql \&- .
942: .Pp
1.1 deraadt 943: If the client host/user is successfully matched in this file, login is
944: automatically permitted provided the client and server user names are the
1.36 aaron 945: same.
946: Additionally, successful RSA host authentication is normally required.
947: This file must be writable only by root; it is recommended
1.1 deraadt 948: that it be world-readable.
1.2 deraadt 949: .Pp
1.6 aaron 950: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 951: .Pa hosts.equiv .
1.1 deraadt 952: Beware that it really means that the named user(s) can log in as
1.2 deraadt 953: .Em anybody ,
1.1 deraadt 954: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 955: binaries and directories.
956: Using a user name practically grants the user root access.
957: The only valid use for user names that I can think
1.1 deraadt 958: of is in negative entries.
1.2 deraadt 959: .Pp
960: Note that this warning also applies to rsh/rlogin.
961: .It Pa /etc/shosts.equiv
1.1 deraadt 962: This is processed exactly as
1.2 deraadt 963: .Pa /etc/hosts.equiv .
1.1 deraadt 964: However, this file may be useful in environments that want to run both
1.2 deraadt 965: rsh/rlogin and ssh.
1.6 aaron 966: .It Pa $HOME/.ssh/environment
1.36 aaron 967: This file is read into the environment at login (if it exists).
968: It can only contain empty lines, comment lines (that start with
1.2 deraadt 969: .Ql # ) ,
1.36 aaron 970: and assignment lines of the form name=value.
971: The file should be writable
1.6 aaron 972: only by the user; it need not be readable by anyone else.
1.2 deraadt 973: .It Pa $HOME/.ssh/rc
1.1 deraadt 974: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 975: environment files but before starting the user's shell or command.
976: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 977: standard input (and
978: .Ev DISPLAY
1.36 aaron 979: in environment).
980: This must call
1.2 deraadt 981: .Xr xauth 1
982: in that case.
983: .Pp
1.1 deraadt 984: The primary purpose of this file is to run any initialization routines
985: which may be needed before the user's home directory becomes
986: accessible; AFS is a particular example of such an environment.
1.2 deraadt 987: .Pp
1.1 deraadt 988: This file will probably contain some initialization code followed by
989: something similar to: "if read proto cookie; then echo add $DISPLAY
990: $proto $cookie | xauth -q -; fi".
1.2 deraadt 991: .Pp
992: If this file does not exist,
993: .Pa /etc/sshrc
994: is run, and if that
1.1 deraadt 995: does not exist either, xauth is used to store the cookie.
1.2 deraadt 996: .Pp
1.1 deraadt 997: This file should be writable only by the user, and need not be
998: readable by anyone else.
1.2 deraadt 999: .It Pa /etc/sshrc
1000: Like
1001: .Pa $HOME/.ssh/rc .
1002: This can be used to specify
1.36 aaron 1003: machine-specific login-time initializations globally.
1004: This file should be writable only by root, and should be world-readable.
1.56 aaron 1005: .El
1.2 deraadt 1006: .Sh AUTHOR
1.17 deraadt 1007: OpenSSH
1.33 deraadt 1008: is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1.36 aaron 1009: but with bugs removed and newer features re-added.
1010: Rapidly after the
1.33 deraadt 1011: 1.2.12 release, newer versions of the original ssh bore successively
1012: more restrictive licenses, and thus demand for a free version was born.
1.48 markus 1013: .Pp
1.33 deraadt 1014: This version of OpenSSH
1.17 deraadt 1015: .Bl -bullet
1016: .It
1.32 aaron 1017: has all components of a restrictive nature (i.e., patents, see
1.17 deraadt 1018: .Xr ssl 8 )
1019: directly removed from the source code; any licensed or patented components
1020: are chosen from
1021: external libraries.
1022: .It
1.48 markus 1023: has been updated to support SSH protocol 1.5 and 2, making it compatible with
1024: all other SSH clients and servers.
1.17 deraadt 1025: .It
1.40 aaron 1026: contains added support for
1.17 deraadt 1027: .Xr kerberos 8
1028: authentication and ticket passing.
1029: .It
1030: supports one-time password authentication with
1031: .Xr skey 1 .
1032: .El
1.48 markus 1033: .Pp
1034: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1035: Niels Provos, Theo de Raadt, and Dug Song.
1036: .Pp
1037: The support for SSH protocol 2 was written by Markus Friedl.
1.2 deraadt 1038: .Sh SEE ALSO
1039: .Xr scp 1 ,
1.63 ! markus 1040: .Xr sftp-server 8 ,
1.2 deraadt 1041: .Xr ssh 1 ,
1.5 deraadt 1042: .Xr ssh-add 1 ,
1.2 deraadt 1043: .Xr ssh-agent 1 ,
1.5 deraadt 1044: .Xr ssh-keygen 1 ,
1.38 deraadt 1045: .Xr ssl 8 ,
1046: .Xr rlogin 1 ,
1047: .Xr rsh 1
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