Annotation of src/usr.bin/ssh/sshd.8, Revision 1.4
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: .\"
1.4 ! deraadt 12: .\" $Id: sshd.8,v 1.3 1999/09/29 18:16:21 dugsong 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
22: .Op Fl diq
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
62: using a conventional cipher. Currently,
1.2 deraadt 63: Blowfish, IDEA, DES, and 3DES.
64: 3DES
1.1 deraadt 65: is used by default. The client selects the encryption algorithm to use
66: from those offered by the server.
1.2 deraadt 67: .Pp
1.1 deraadt 68: Next, the server and the client enter an authentication dialog. The
1.2 deraadt 69: client tries to authenticate itself using
70: .Pa .rhosts
71: authentication,
72: .Pa .rhosts
73: authentication combined with RSA host
1.1 deraadt 74: authentication, RSA challenge-response authentication, or password
75: based authentication.
1.2 deraadt 76: .Pp
1.1 deraadt 77: Rhosts authentication is normally disabled
78: because it is fundamentally insecure, but can be enabled in the server
79: configuration file if desired. System security is not improved unless
1.2 deraadt 80: .Xr rshd 8 ,
81: .Xr rlogind 8 ,
82: .Xr rexecd 8 ,
83: and
84: .Xr rexd 8
1.1 deraadt 85: are disabled (thus completely disabling
1.2 deraadt 86: .Xr rlogin 1
1.1 deraadt 87: and
1.2 deraadt 88: .Xr rsh 1
1.1 deraadt 89: into that machine).
1.2 deraadt 90: .Pp
1.1 deraadt 91: If the client successfully authenticates itself, a dialog for
92: preparing the session is entered. At this time the client may request
93: things like allocating a pseudo-tty, forwarding X11 connections,
94: forwarding TCP/IP connections, or forwarding the authentication agent
95: connection over the secure channel.
1.2 deraadt 96: .Pp
1.1 deraadt 97: Finally, the client either requests a shell or execution of a command.
98: The sides then enter session mode. In this mode, either side may send
99: data at any time, and such data is forwarded to/from the shell or
100: command on the server side, and the user terminal in the client side.
1.2 deraadt 101: .Pp
1.1 deraadt 102: When the user program terminates and all forwarded X11 and other
103: connections have been closed, the server sends command exit status to
104: the client, and both sides exit.
1.2 deraadt 105: .Pp
106: .Nm
1.1 deraadt 107: can be configured using command-line options or a configuration
108: file. Command-line options override values specified in the
109: configuration file.
1.2 deraadt 110: .Sh OPTIONS
111: .Bl -tag -width Ds
112: .It Fl b Ar bits
1.1 deraadt 113: Specifies the number of bits in the server key (default 768).
1.2 deraadt 114: .Pp
115: .It Fl d
1.1 deraadt 116: Debug mode. The server sends verbose debug output to the system
117: log, and does not put itself in the background. The server also will
118: not fork and will only process one connection. This option is only
119: intended for debugging for the server.
1.2 deraadt 120: .It Fl f Ar configuration_file
1.1 deraadt 121: Specifies the name of the configuration file. The default is
1.2 deraadt 122: .Pa /etc/sshd_config .
123: .It Fl g Ar login_grace_time
1.1 deraadt 124: Gives the grace time for clients to authenticate themselves (default
125: 300 seconds). If the client fails to authenticate the user within
126: this many seconds, the server disconnects and exits. A value of zero
127: indicates no limit.
1.2 deraadt 128: .It Fl h Ar host_key_file
1.1 deraadt 129: Specifies the file from which the host key is read (default
1.2 deraadt 130: .Pa /etc/ssh_host_key ) .
1.1 deraadt 131: This option must be given if sshd is not run as root (as the normal
132: host file is normally not readable by anyone but root).
1.2 deraadt 133: .It Fl i
1.1 deraadt 134: Specifies that sshd is being run from inetd. Sshd is normally not run
135: from inetd because it needs to generate the server key before it can
136: respond to the client, and this may take tens of seconds. Clients
137: would have to wait too long if the key was regenerated every time.
138: However, with small key sizes (e.g. 512) using sshd from inetd may
139: be feasible.
1.2 deraadt 140: .It Fl k Ar key_gen_time
1.1 deraadt 141: Specifies how often the server key is regenerated (default 3600
142: seconds, or one hour). The motivation for regenerating the key fairly
143: often is that the key is not stored anywhere, and after about an hour,
144: it becomes impossible to recover the key for decrypting intercepted
145: communications even if the machine is cracked into or physically
146: seized. A value of zero indicates that the key will never be regenerated.
1.2 deraadt 147: .It Fl p Ar port
1.1 deraadt 148: Specifies the port on which the server listens for connections
149: (default 22).
1.2 deraadt 150: .It Fl q
1.1 deraadt 151: Quiet mode. Nothing is sent to the system log. Normally the beginning,
152: authentication, and termination of each connection is logged.
1.2 deraadt 153: .El
154: .Sh CONFIGURATION FILE
155: .Nm
1.1 deraadt 156: reads configuration data from
1.2 deraadt 157: .Pa /etc/sshd_config
158: (or the file specified with
159: .Fl f
160: on the command line). The file
161: contains keyword-value pairs, one per line. Lines starting with
162: .Ql #
1.1 deraadt 163: and empty lines are interpreted as comments.
1.2 deraadt 164: .Pp
1.1 deraadt 165: The following keywords are possible.
1.2 deraadt 166: .Bl -tag -width Ds
167: .It Cm AFSTokenPassing
1.3 dugsong 168: Specifies whether an AFS token may be forwarded to the server. Default is
1.2 deraadt 169: .Dq yes .
1.3 dugsong 170: .Pp
1.2 deraadt 171: .It Cm AllowHosts
1.1 deraadt 172: This keyword can be followed by any number of host name patterns,
173: separated by spaces. If specified, login is allowed only from hosts
1.2 deraadt 174: whose name matches one of the patterns.
175: .Ql \&*
176: and
177: .Ql ?
178: can be used as
1.1 deraadt 179: wildcards in the patterns. Normal name servers are used to map the
180: client's host into a canonical host name. If the name cannot be
181: mapped, its IP-address is used as the host name. By default all hosts
182: are allowed to connect.
1.2 deraadt 183: .Pp
1.1 deraadt 184: Note that
1.2 deraadt 185: .Nm
186: can also be configured to use tcp_wrappers using the
187: .Fl -with-libwrap
1.1 deraadt 188: compile-time configuration option.
1.2 deraadt 189: .It Cm DenyHosts
1.1 deraadt 190: This keyword can be followed by any number of host name patterns,
191: separated by spaces. If specified, login is disallowed from the hosts
192: whose name matches any of the patterns.
1.2 deraadt 193: .It Cm FascistLogging
1.1 deraadt 194: Specifies whether to use verbose logging. Verbose logging violates
195: the privacy of users and is not recommended. The argument must be
1.2 deraadt 196: .Dq yes
197: or
198: .Dq no .
199: The default is
200: .Dq no .
201: .It Cm HostKey
1.1 deraadt 202: Specifies the file containing the private host key (default
1.2 deraadt 203: .Pa /etc/ssh_host_key ) .
204: .It Cm IgnoreRhosts
1.1 deraadt 205: Specifies that rhosts and shosts files will not be used in
206: authentication.
1.2 deraadt 207: .Pa /etc/hosts.equiv
1.1 deraadt 208: and
1.2 deraadt 209: .Pa /etc/shosts.equiv
210: are still used. The default is
211: .Dq no .
212: .It Cm KeepAlive
1.1 deraadt 213: Specifies whether the system should send keepalive messages to the
214: other side. If they are sent, death of the connection or crash of one
215: of the machines will be properly noticed. However, this means that
216: connections will die if the route is down temporarily, and some people
217: find it annoying. On the other hand, if keepalives are not send,
1.2 deraadt 218: sessions may hang indefinitely on the server, leaving
219: .Dq ghost
220: users and consuming server resources.
221: .Pp
222: The default is
223: .Dq yes
224: (to send keepalives), and the server will notice
1.1 deraadt 225: if the network goes down or the client host reboots. This avoids
226: infinitely hanging sessions.
1.2 deraadt 227: .Pp
228: To disable keepalives, the value should be set to
229: .Dq no
230: in both the server and the client configuration files.
231: .It Cm KerberosAuthentication
1.1 deraadt 232: Specifies whether Kerberos authentication is allowed. This can
233: be in the form of a Kerberos ticket, or if PasswordAuthentication
234: is yes, the password provided by the user will be validated through
1.3 dugsong 235: the Kerberos KDC. Default is
236: .Dq yes .
1.2 deraadt 237: .It Cm KerberosOrLocalPasswd
1.1 deraadt 238: If set then if password authentication through Kerberos fails then
239: the password will be validated via any additional local mechanism
1.2 deraadt 240: such as
241: .Pa /etc/passwd
242: or SecurID. Default is
243: .Dq no .
244: .It Cm KerberosTgtPassing
1.1 deraadt 245: Specifies whether a Kerberos TGT may be forwarded to the server.
1.3 dugsong 246: Default is
247: .Dq no ,
248: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 249: .It Cm KerberosTicketCleanup
1.1 deraadt 250: Specifies whether to automatically destroy the user's
1.3 dugsong 251: ticket cache file on logout. Default is
252: .Dq yes .
1.2 deraadt 253: .It Cm KeyRegenerationInterval
1.1 deraadt 254: The server key is automatically regenerated after this many seconds
255: (if it has been used). The purpose of regeneration is to prevent
256: decrypting captured sessions by later breaking into the machine and
257: stealing the keys. The key is never stored anywhere. If the value is
258: 0, the key is never regenerated. The default is 3600
259: (seconds).
1.2 deraadt 260: .It Cm LoginGraceTime
1.1 deraadt 261: The server disconnects after this time if the user has not
262: successfully logged in. If the value is 0, there is no time limit.
263: The default is 600 (seconds).
1.2 deraadt 264: .It Cm PasswordAuthentication
1.1 deraadt 265: Specifies whether password authentication is allowed.
1.2 deraadt 266: The default is
267: .Dq yes .
268: .It Cm PermitEmptyPasswords
1.1 deraadt 269: When password authentication is allowed, it specifies whether the
270: server allows login to accounts with empty password strings. The default
1.2 deraadt 271: is
272: .Dq yes .
273: .It Cm PermitRootLogin
1.1 deraadt 274: Specifies whether the root can log in using
1.2 deraadt 275: .Xr ssh 1 .
276: The default is
277: .Dq yes .
278: .Pp
279: Root login with RSA authentication when the
280: .Ar command
281: option has been
1.1 deraadt 282: specified will be allowed regardless of the value of this setting
283: (which may be useful for taking remote backups even if root login is
284: normally not allowed).
1.2 deraadt 285: .It Cm Port
1.1 deraadt 286: Specifies the port number that
1.2 deraadt 287: .Nm
1.1 deraadt 288: listens on. The default is 22.
1.2 deraadt 289: .It Cm PrintMotd
1.1 deraadt 290: Specifies whether
1.2 deraadt 291: .Nm
1.1 deraadt 292: should print
1.2 deraadt 293: .Pa /etc/motd
1.1 deraadt 294: when a user logs in interactively. (On some systems it is also
1.2 deraadt 295: printed by the shell,
296: .Pa /etc/profile ,
297: or equivalent.) The default is
298: .Dq yes .
299: .It Cm QuietMode
1.1 deraadt 300: Specifies whether the system runs in quiet mode. In quiet mode,
301: nothing is logged in the system log, except fatal errors. The default
1.2 deraadt 302: is
303: .Dq no .
304: .It Cm RandomSeed
1.4 ! deraadt 305: Obsolete. Random number generation uses other techniques.
1.2 deraadt 306: .It Cm RhostsAuthentication
1.1 deraadt 307: Specifies whether authentication using rhosts or /etc/hosts.equiv
308: files is sufficient. Normally, this method should not be permitted
309: because it is insecure. RhostsRSAAuthentication should be used
310: instead, because it performs RSA-based host authentication in addition
311: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 312: The default is
313: .Dq no .
314: .It Cm RhostsRSAAuthentication
1.1 deraadt 315: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 deraadt 316: with successful RSA host authentication is allowed. The default is
317: .Dq yes .
318: .It Cm RSAAuthentication
319: Specifies whether pure RSA authentication is allowed. The default is
320: .Dq yes .
321: .It Cm ServerKeyBits
1.1 deraadt 322: Defines the number of bits in the server key. The minimum value is
323: 512, and the default is 768.
1.2 deraadt 324: .It Cm StrictModes
325: Specifies whether SSH should check file modes and ownership of the
1.1 deraadt 326: user's home directory and rhosts files before accepting login. This
327: is normally desirable because novices sometimes accidentally leave their
328: directory or files world-writable. The default is "yes".
1.2 deraadt 329: .It Cm SyslogFacility
1.1 deraadt 330: Gives the facility code that is used when logging messages from
1.2 deraadt 331: .Nm sshd .
1.1 deraadt 332: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
333: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is DAEMON.
1.2 deraadt 334: .It Cm X11Forwarding
335: Specifies whether X11 forwarding is permitted. The default is
336: .Dq yes .
1.1 deraadt 337: Note that disabling X11 forwarding does not improve security in any
338: way, as users can always install their own forwarders.
1.2 deraadt 339: .El
340: .Sh LOGIN PROCESS
1.1 deraadt 341: When a user successfully logs in,
1.2 deraadt 342: .Nm
1.1 deraadt 343: does the following:
1.2 deraadt 344: .Bl -enum -offset indent
345: .It
1.1 deraadt 346: If the login is on a tty, and no command has been specified,
347: prints last login time and
1.2 deraadt 348: .Pa /etc/motd
1.1 deraadt 349: (unless prevented in the configuration file or by
1.2 deraadt 350: .Pa $HOME/.hushlogin ;
351: see the
352: .Sx FILES
353: section).
354: .It
1.1 deraadt 355: If the login is on a tty, records login time.
1.2 deraadt 356: .It
357: Checks
358: .Pa /etc/nologin ;
359: if it exists, prints contents and quits
1.1 deraadt 360: (unless root).
1.2 deraadt 361: .It
1.1 deraadt 362: Changes to run with normal user privileges.
1.2 deraadt 363: .It
1.1 deraadt 364: Sets up basic environment.
1.2 deraadt 365: .It
366: Reads
367: .Pa /etc/environment
368: if it exists.
369: .It
370: Reads
371: .Pa $HOME/.ssh/environment
372: if it exists.
373: .It
1.1 deraadt 374: Changes to user's home directory.
1.2 deraadt 375: .It
376: If
377: .Pa $HOME/.ssh/rc
378: exists, runs it; else if
379: .Pa /etc/sshrc
380: exists, runs
381: it; otherwise runs xauth. The
382: .Dq rc
383: files are given the X11
1.1 deraadt 384: authentication protocol and cookie in standard input.
1.2 deraadt 385: .It
1.1 deraadt 386: Runs user's shell or command.
1.2 deraadt 387: .El
388: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 389: The
1.2 deraadt 390: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 391: file lists the RSA keys that are
392: permitted for RSA authentication. Each line of the file contains one
1.2 deraadt 393: key (empty lines and lines starting with a
394: .Ql #
395: are ignored as
1.1 deraadt 396: comments). Each line consists of the following fields, separated by
397: spaces: options, bits, exponent, modulus, comment. The options field
398: is optional; its presence is determined by whether the line starts
399: with a number or not (the option field never starts with a number).
400: The bits, exponent, modulus and comment fields give the RSA key; the
401: comment field is not used for anything (but may be convenient for the
402: user to identify the key).
1.2 deraadt 403: .Pp
1.1 deraadt 404: Note that lines in this file are usually several hundred bytes long
405: (because of the size of the RSA key modulus). You don't want to type
406: them in; instead, copy the
1.2 deraadt 407: .Pa identity.pub
1.1 deraadt 408: file and edit it.
1.2 deraadt 409: .Pp
1.1 deraadt 410: The options (if present) consists of comma-separated option
411: specifications. No spaces are permitted, except within double quotes.
412: The following option specifications are supported:
1.2 deraadt 413: .Bl -tag -width Ds
414: .It Cm from="pattern-list"
1.1 deraadt 415: Specifies that in addition to RSA authentication, the canonical name
416: of the remote host must be present in the comma-separated list of
417: patterns ('*' and '?' serve as wildcards). The list may also contain
418: patterns negated by prefixing them with '!'; if the canonical host
419: name matches a negated pattern, the key is not accepted. The purpose
420: of this option is to optionally increase security: RSA authentication
421: by itself does not trust the network or name servers or anything (but
422: the key); however, if somebody somehow steals the key, the key
423: permits an intruder to log in from anywhere in the world. This
424: additional option makes using a stolen key more difficult (name
425: servers and/or routers would have to be compromised in addition to
426: just the key).
1.2 deraadt 427: .It Cm command="command"
1.1 deraadt 428: Specifies that the command is executed whenever this key is used for
429: authentication. The command supplied by the user (if any) is ignored.
430: The command is run on a pty if the connection requests a pty;
431: otherwise it is run without a tty. A quote may be included in the
432: command by quoting it with a backslash. This option might be useful
433: to restrict certain RSA keys to perform just a specific operation. An
434: example might be a key that permits remote backups but nothing
435: else. Notice that the client may specify TCP/IP and/or X11
436: forwardings unless they are explicitly prohibited.
1.2 deraadt 437: .It Cm environment="NAME=value"
1.1 deraadt 438: Specifies that the string is to be added to the environment when
439: logging in using this key. Environment variables set this way
440: override other default environment values. Multiple options of this
441: type are permitted.
1.2 deraadt 442: .It Cm no-port-forwarding
1.1 deraadt 443: Forbids TCP/IP forwarding when this key is used for authentication.
444: Any port forward requests by the client will return an error. This
1.2 deraadt 445: might be used, e.g., in connection with the
446: .Cm command
1.1 deraadt 447: option.
1.2 deraadt 448: .It Cm no-X11-forwarding
1.1 deraadt 449: Forbids X11 forwarding when this key is used for authentication.
450: Any X11 forward requests by the client will return an error.
1.2 deraadt 451: .It Cm no-agent-forwarding
1.1 deraadt 452: Forbids authentication agent forwarding when this key is used for
453: authentication.
1.2 deraadt 454: .It Cm no-pty
1.1 deraadt 455: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 456: .El
457: .Ss Examples
1.1 deraadt 458: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 459: .Pp
1.1 deraadt 460: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 461: .Pp
1.1 deraadt 462: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 463: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 464: The
1.2 deraadt 465: .Pa /etc/ssh_known_hosts
1.1 deraadt 466: and
1.2 deraadt 467: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 468: files contain host public keys for all known hosts. The global file should
469: be prepared by the admistrator (optional), and the per-user file is
470: maintained automatically: whenever the user connects an unknown host
471: its key is added to the per-user file. The recommended way to create
1.2 deraadt 472: .Pa /etc/ssh_known_hosts
1.1 deraadt 473: is to use the
1.2 deraadt 474: .Xr make-ssh-known-hosts 1
1.1 deraadt 475: command.
1.2 deraadt 476: .Pp
1.1 deraadt 477: Each line in these files contains the following fields: hostnames,
478: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 deraadt 479: .Pp
1.1 deraadt 480: Hostnames is a comma-separated list of patterns ('*' and '?' act as
481: wildcards); each pattern in turn is matched against the canonical host
482: name (when authenticating a client) or against the user-supplied
483: name (when authenticating a server). A pattern may also be preceded
1.2 deraadt 484: by
485: .Ql !
486: to indicate negation: if the host name matches a negated
1.1 deraadt 487: pattern, it is not accepted (by that line) even if it matched another
488: pattern on the line.
1.2 deraadt 489: .Pp
1.1 deraadt 490: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 491: can be obtained, e.g., from
492: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 493: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 494: .Pp
495: Lines starting with
496: .Ql #
497: and empty lines are ignored as comments.
498: .Pp
1.1 deraadt 499: When performing host authentication, authentication is accepted if any
500: matching line has the proper key. It is thus permissible (but not
501: recommended) to have several lines or different host keys for the same
502: names. This will inevitably happen when short forms of host names
503: from different domains are put in the file. It is possible
504: that the files contain conflicting information; authentication is
505: accepted if valid information can be found from either file.
1.2 deraadt 506: .Pp
1.1 deraadt 507: Note that the lines in these files are typically hundreds of characters
508: long, and you definitely don't want to type in the host keys by hand.
509: Rather, generate them by a script (see
1.2 deraadt 510: .Xr make-ssh-known-hosts 1 )
1.1 deraadt 511: or by taking
1.2 deraadt 512: .Pa /etc/ssh_host_key.pub
1.1 deraadt 513: and adding the host names at the front.
1.2 deraadt 514: .Ss Examples
1.1 deraadt 515: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 516: .Sh FILES
517: .Bl -tag -width Ds
518: .It Pa /etc/sshd_config
1.1 deraadt 519: Contains configuration data for
1.2 deraadt 520: .Nm sshd .
1.1 deraadt 521: This file should be writable by root only, but it is recommended
522: (though not necessary) that it be world-readable.
1.2 deraadt 523: .It Pa /etc/ssh_host_key
1.1 deraadt 524: Contains the private part of the host key. This file is normally
525: created automatically by "make install", but can also be created
526: manually using
1.2 deraadt 527: .Xr ssh-keygen 1 .
1.1 deraadt 528: This file should only be owned by root, readable only by root, and not
529: accessible to others.
1.2 deraadt 530: .It Pa /etc/ssh_host_key.pub
1.1 deraadt 531: Contains the public part of the host key. This file is normally
532: created automatically by "make install", but can also be created
533: manually. This file should be world-readable but writable only by
534: root. Its contents should match the private part. This file is not
535: really used for anything; it is only provided for the convenience of
536: the user so its contents can be copied to known hosts files.
1.2 deraadt 537: .It Pa /var/run/sshd.pid
538: Contains the process ID of the
539: .Nm
1.1 deraadt 540: listening for connections (if there are several daemons running
541: concurrently for different ports, this contains the pid of the one
542: started last). The contents of this file are not sensitive; it can be
543: world-readable.
1.2 deraadt 544: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 545: Lists the RSA keys that can be used to log into the user's account.
546: This file must be readable by root (which may on some machines imply
547: it being world-readable if the user's home directory resides on an NFS
548: volume). It is recommended that it not be accessible by others. The
549: format of this file is described above.
1.2 deraadt 550: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1.1 deraadt 551: These files are consulted when using rhosts with RSA host
552: authentication to check the public key of the host. The key must be
553: listed in one of these files to be accepted. (The client uses the
554: same files to verify that the remote host is the one we intended to
555: connect.) These files should be writable only by root/the owner.
1.2 deraadt 556: .Pa /etc/ssh_known_hosts
557: should be world-readable, and
558: .Pa $HOME/.ssh/known_hosts
559: can but need not be world-readable.
560: .Pa /etc/nologin
1.1 deraadt 561: If this file exists,
1.2 deraadt 562: .Nm
1.1 deraadt 563: refuses to let anyone except root log in. The contents of the file
564: are displayed to anyone trying to log in, and non-root connections are
565: refused. The file should be world-readable.
1.2 deraadt 566: .Pa $HOME/.rhosts
1.1 deraadt 567: This file contains host-username pairs, separated by a space, one per
568: line. The given user on the corresponding host is permitted to log in
569: without password. The same file is used by rlogind and rshd.
1.2 deraadt 570: Ssh differs from rlogind
1.1 deraadt 571: and rshd in that it requires RSA host authentication in addition to
572: validating the host name retrieved from domain name servers (unless
1.2 deraadt 573: compiled with the
574: .Fl -with-rhosts
575: configuration option). The file must
1.1 deraadt 576: be writable only by the user; it is recommended that it not be
577: accessible by others.
1.2 deraadt 578: .Pp
1.1 deraadt 579: If is also possible to use netgroups in the file. Either host or user
580: name may be of the form +@groupname to specify all hosts or all users
581: in the group.
1.2 deraadt 582: .It Pa $HOME/.shosts
583: For ssh,
584: this file is exactly the same as for
585: .Pa .rhosts .
586: However, this file is
587: not used by rlogin and rshd, so using this permits access using SSH only.
588: .Pa /etc/hosts.equiv
589: This file is used during
590: .Pa .rhosts
591: authentication. In the
1.1 deraadt 592: simplest form, this file contains host names, one per line. Users on
593: those hosts are permitted to log in without a password, provided they
594: have the same user name on both machines. The host name may also be
595: followed by a user name; such users are permitted to log in as
1.2 deraadt 596: .Em any
597: user on this machine (except root). Additionally, the syntax
598: .Dq +@group
599: can be used to specify netgroups. Negated entries start with
600: .Ql \&- .
601: .Pp
1.1 deraadt 602: If the client host/user is successfully matched in this file, login is
603: automatically permitted provided the client and server user names are the
604: same. Additionally, successful RSA host authentication is normally
605: required. This file must be writable only by root; it is recommended
606: that it be world-readable.
1.2 deraadt 607: .Pp
608: .Sy Warning: It is almost never a good idea to use user names in
609: .Pa hosts.equiv .
1.1 deraadt 610: Beware that it really means that the named user(s) can log in as
1.2 deraadt 611: .Em anybody ,
1.1 deraadt 612: which includes bin, daemon, adm, and other accounts that own critical
613: binaries and directories. Using a user name practically grants the
614: user root access. The only valid use for user names that I can think
615: of is in negative entries.
1.2 deraadt 616: .Pp
617: Note that this warning also applies to rsh/rlogin.
618: .It Pa /etc/shosts.equiv
1.1 deraadt 619: This is processed exactly as
1.2 deraadt 620: .Pa /etc/hosts.equiv .
1.1 deraadt 621: However, this file may be useful in environments that want to run both
1.2 deraadt 622: rsh/rlogin and ssh.
623: .It Pa /etc/environment
1.1 deraadt 624: This file is read into the environment at login (if it exists). It
1.2 deraadt 625: can only contain empty lines, comment lines (that start with
626: .Ql # ) ,
627: and assignment lines of the form name=value. This file is processed in
1.1 deraadt 628: all environments (normal rsh/rlogin only process it on AIX and
629: potentially some other systems). The file should be writable only by
630: root, and should be world-readable.
1.2 deraadt 631: .It Pa $HOME/.ssh/environment
1.1 deraadt 632: This file is read into the environment after /etc/environment. It has
633: the same format. The file should be writable only by the user; it
634: need not be readable by anyone else.
1.2 deraadt 635: .It Pa $HOME/.ssh/rc
1.1 deraadt 636: If this file exists, it is run with /bin/sh after reading the
637: environment files but before starting the user's shell or command. If
638: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 639: standard input (and
640: .Ev DISPLAY
641: in environment). This must call
642: .Xr xauth 1
643: in that case.
644: .Pp
1.1 deraadt 645: The primary purpose of this file is to run any initialization routines
646: which may be needed before the user's home directory becomes
647: accessible; AFS is a particular example of such an environment.
1.2 deraadt 648: .Pp
1.1 deraadt 649: This file will probably contain some initialization code followed by
650: something similar to: "if read proto cookie; then echo add $DISPLAY
651: $proto $cookie | xauth -q -; fi".
1.2 deraadt 652: .Pp
653: If this file does not exist,
654: .Pa /etc/sshrc
655: is run, and if that
1.1 deraadt 656: does not exist either, xauth is used to store the cookie.
1.2 deraadt 657: .Pp
1.1 deraadt 658: This file should be writable only by the user, and need not be
659: readable by anyone else.
1.2 deraadt 660: .It Pa /etc/sshrc
661: Like
662: .Pa $HOME/.ssh/rc .
663: This can be used to specify
1.1 deraadt 664: machine-specific login-time initializations globally. This file
665: should be writable only by root, and should be world-readable.
1.2 deraadt 666: .Sh INSTALLATION
667: .Nm
1.1 deraadt 668: is normally run as root. If it is not run as root, it can
669: only log in as the user it is running as, and password authentication
670: may not work if the system uses shadow passwords. An alternative
671: host key file must also be used.
1.2 deraadt 672: .Pp
673: .Nm
1.1 deraadt 674: is normally started from
1.2 deraadt 675: .Pa /etc/rc
1.1 deraadt 676: or equivalent at system boot.
1.2 deraadt 677: .Pp
1.1 deraadt 678: Considerable work has been put to making
1.2 deraadt 679: .Nm
1.1 deraadt 680: secure. However, if you find a security problem, please report it
681: immediately to <ssh-bugs@cs.hut.fi>.
1.2 deraadt 682: .Sh AUTHOR
1.1 deraadt 683: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 684: .Pp
1.1 deraadt 685: Information about new releases, mailing lists, and other related
1.2 deraadt 686: issues can be found from the SSH WWW home page:
687: .Pp
688: .Dl http://www.cs.hut.fi/ssh.
689: .Sh SEE ALSO
690: .Xr make-ssh-known-hosts 1 ,
691: .Xr rlogin 1 ,
692: .Xr rsh 1 ,
693: .Xr scp 1 ,
694: .Xr ssh 1 ,
695: .Xr ssh-add x 1 ,
696: .Xr ssh-agent 1 ,
697: .Xr ssh-keygen 1
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