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