5 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
7 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
8 .\" All rights reserved
10 .\" Created: Sat Apr 22 21:55:14 1995 ylo
14 .Dd September 25, 1999
19 .Nd secure shell daemon
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
29 .Op Fl V Ar client_protocol_id
32 (Secure Shell Daemon) is the daemon program for
34 Together these programs replace rlogin and rsh programs, and
35 provide secure encrypted communications between two untrusted hosts
36 over an insecure network. The programs are intended to be as easy to
37 install and use as possible.
40 is the daemon that listens for connections from clients. It is
41 normally started at boot from
44 daemon for each incoming connection. The forked daemons handle
45 key exchange, encryption, authentication, command execution,
49 works as follows. Each host has a host-specific RSA key
50 (normally 1024 bits) used to identify the host. Additionally, when
51 the daemon starts, it generates a server RSA key (normally 768 bits).
52 This key is normally regenerated every hour if it has been used, and
53 is never stored on disk.
55 Whenever a client connects the daemon, the daemon sends its host
56 and server public keys to the client. The client compares the
57 host key against its own database to verify that it has not changed.
58 The client then generates a 256 bit random number. It encrypts this
59 random number using both the host key and the server key, and sends
60 the encrypted number to the server. Both sides then start to use this
61 random number as a session key which is used to encrypt all further
62 communications in the session. The rest of the session is encrypted
63 using a conventional cipher, currently Blowfish and 3DES, with 3DES
64 being is used by default. The client selects the encryption algorithm
65 to use from those offered by the server.
67 Next, the server and the client enter an authentication dialog. The
68 client tries to authenticate itself using
72 authentication combined with RSA host
73 authentication, RSA challenge-response authentication, or password
76 Rhosts authentication is normally disabled
77 because it is fundamentally insecure, but can be enabled in the server
78 configuration file if desired. System security is not improved unless
84 are disabled (thus completely disabling
90 If the client successfully authenticates itself, a dialog for
91 preparing the session is entered. At this time the client may request
92 things like allocating a pseudo-tty, forwarding X11 connections,
93 forwarding TCP/IP connections, or forwarding the authentication agent
94 connection over the secure channel.
96 Finally, the client either requests a shell or execution of a command.
97 The sides then enter session mode. In this mode, either side may send
98 data at any time, and such data is forwarded to/from the shell or
99 command on the server side, and the user terminal in the client side.
101 When the user program terminates and all forwarded X11 and other
102 connections have been closed, the server sends command exit status to
103 the client, and both sides exit.
106 can be configured using command-line options or a configuration
107 file. Command-line options override values specified in the
111 rereads its configuration file when it receives a hangup signal,
114 The options are as follows:
117 Specifies the number of bits in the server key (default 768).
120 Debug mode. The server sends verbose debug output to the system
121 log, and does not put itself in the background. The server also will
122 not fork and will only process one connection. This option is only
123 intended for debugging for the server.
124 .It Fl f Ar configuration_file
125 Specifies the name of the configuration file. The default is
126 .Pa /etc/sshd_config .
128 refuses to start if there is no configuration file.
129 .It Fl g Ar login_grace_time
130 Gives the grace time for clients to authenticate themselves (default
131 300 seconds). If the client fails to authenticate the user within
132 this many seconds, the server disconnects and exits. A value of zero
134 .It Fl h Ar host_key_file
135 Specifies the file from which the host key is read (default
136 .Pa /etc/ssh_host_key ) .
137 This option must be given if
139 is not run as root (as the normal
140 host file is normally not readable by anyone but root).
144 is being run from inetd.
147 from inetd because it needs to generate the server key before it can
148 respond to the client, and this may take tens of seconds. Clients
149 would have to wait too long if the key was regenerated every time.
150 However, with small key sizes (e.g. 512) using
154 .It Fl k Ar key_gen_time
155 Specifies how often the server key is regenerated (default 3600
156 seconds, or one hour). The motivation for regenerating the key fairly
157 often is that the key is not stored anywhere, and after about an hour,
158 it becomes impossible to recover the key for decrypting intercepted
159 communications even if the machine is cracked into or physically
160 seized. A value of zero indicates that the key will never be regenerated.
162 Specifies the port on which the server listens for connections
165 Quiet mode. Nothing is sent to the system log. Normally the beginning,
166 authentication, and termination of each connection is logged.
168 Do not print an error message if RSA support is missing.
169 .It Fl V Ar client_protocol_id
170 SSH2 compatibility mode.
171 When this options is specified
173 assumes the client has sent the given version string
175 Protocol Version Identification Exchange.
179 to use IPv4 addresses only.
183 to use IPv6 addresses only.
185 .Sh CONFIGURATION FILE
187 reads configuration data from
189 (or the file specified with
191 on the command line). The file
192 contains keyword-value pairs, one per line. Lines starting with
194 and empty lines are interpreted as comments.
196 The following keywords are possible.
198 .It Cm AFSTokenPassing
199 Specifies whether an AFS token may be forwarded to the server. Default is
202 This keyword can be followed by a number of group names, separated
203 by spaces. If specified, login is allowed only for users whose primary
204 group matches one of the patterns.
209 wildcards in the patterns. Only group names are valid, a numerical group
210 id isn't recognized. By default login is allowed regardless of
214 This keyword can be followed by a number of user names, separated
215 by spaces. If specified, login is allowed only for users names that
216 match one of the patterns.
221 wildcards in the patterns. Only user names are valid, a numerical user
222 id isn't recognized. By default login is allowed regardless of
228 should check for new mail for interactive logins.
232 This keyword can be followed by a number of group names, separated
233 by spaces. Users whose primary group matches one of the patterns
234 aren't allowed to log in.
239 wildcards in the patterns. Only group names are valid, a numerical group
240 id isn't recognized. By default login is allowed regardless of
244 This keyword can be followed by a number of user names, separated
245 by spaces. Login is disallowed for user names that match
251 wildcards in the patterns. Only user names are valid, a numerical user
252 id isn't recognized. By default login is allowed regardless of
255 Specifies the file containing the private host key (default
256 .Pa /etc/ssh_host_key ) .
259 does not start if this file is group/world-accessible.
265 files will not be used in authentication.
268 .Pa /etc/shosts.equiv
269 are still used. The default is
271 .It Cm IgnoreUserKnownHosts
274 should ignore the user's
275 .Pa $HOME/.ssh/known_hosts
277 .Cm RhostsRSAAuthentication .
281 Specifies whether the system should send keepalive messages to the
282 other side. If they are sent, death of the connection or crash of one
283 of the machines will be properly noticed. However, this means that
284 connections will die if the route is down temporarily, and some people
285 find it annoying. On the other hand, if keepalives are not send,
286 sessions may hang indefinitely on the server, leaving
288 users and consuming server resources.
292 (to send keepalives), and the server will notice
293 if the network goes down or the client host reboots. This avoids
294 infinitely hanging sessions.
296 To disable keepalives, the value should be set to
298 in both the server and the client configuration files.
299 .It Cm KerberosAuthentication
300 Specifies whether Kerberos authentication is allowed. This can
301 be in the form of a Kerberos ticket, or if
302 .Cm PasswordAuthentication
303 is yes, the password provided by the user will be validated through
304 the Kerberos KDC. Default is
306 .It Cm KerberosOrLocalPasswd
307 If set then if password authentication through Kerberos fails then
308 the password will be validated via any additional local mechanism
311 or SecurID. Default is
313 .It Cm KerberosTgtPassing
314 Specifies whether a Kerberos TGT may be forwarded to the server.
317 as this only works when the Kerberos KDC is actually an AFS kaserver.
318 .It Cm KerberosTicketCleanup
319 Specifies whether to automatically destroy the user's ticket cache
320 file on logout. Default is
322 .It Cm KeyRegenerationInterval
323 The server key is automatically regenerated after this many seconds
324 (if it has been used). The purpose of regeneration is to prevent
325 decrypting captured sessions by later breaking into the machine and
326 stealing the keys. The key is never stored anywhere. If the value is
327 0, the key is never regenerated. The default is 3600
330 Specifies what local address
333 The default is to listen to all local addresses.
334 Multiple options of this type are permitted.
337 options must precede this option.
338 .It Cm LoginGraceTime
339 The server disconnects after this time if the user has not
340 successfully logged in. If the value is 0, there is no time limit.
341 The default is 600 (seconds).
343 Gives the verbosity level that is used when logging messages from
345 The possible values are:
346 QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
348 Logging with level DEBUG violates the privacy of users
349 and is not recommended.
350 .It Cm PasswordAuthentication
351 Specifies whether password authentication is allowed.
354 .It Cm PermitEmptyPasswords
355 When password authentication is allowed, it specifies whether the
356 server allows login to accounts with empty password strings. The default
359 .It Cm PermitRootLogin
360 Specifies whether the root can log in using
369 If this options is set to
371 only password authentication is disabled for root.
373 Root login with RSA authentication when the
376 specified will be allowed regardless of the value of this setting
377 (which may be useful for taking remote backups even if root login is
378 normally not allowed).
380 Specifies the port number that
382 listens on. The default is 22.
383 Multiple options of this type are permitted.
389 when a user logs in interactively. (On some systems it is also
390 printed by the shell,
392 or equivalent.) The default is
395 Obsolete. Random number generation uses other techniques.
396 .It Cm RhostsAuthentication
397 Specifies whether authentication using rhosts or /etc/hosts.equiv
398 files is sufficient. Normally, this method should not be permitted
399 because it is insecure.
400 .Cm RhostsRSAAuthentication
402 instead, because it performs RSA-based host authentication in addition
403 to normal rhosts or /etc/hosts.equiv authentication.
406 .It Cm RhostsRSAAuthentication
407 Specifies whether rhosts or /etc/hosts.equiv authentication together
408 with successful RSA host authentication is allowed. The default is
410 .It Cm RSAAuthentication
411 Specifies whether pure RSA authentication is allowed. The default is
414 Defines the number of bits in the server key. The minimum value is
415 512, and the default is 768.
416 .It Cm SkeyAuthentication
419 authentication is allowed. The default is
421 Note that s/key authentication is enabled only if
422 .Cm PasswordAuthentication
427 should check file modes and ownership of the
428 user's files and home directory before accepting login. This
429 is normally desirable because novices sometimes accidentally leave their
430 directory or files world-writable. The default is
432 .It Cm SyslogFacility
433 Gives the facility code that is used when logging messages from
435 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
436 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH.
440 is used. The default is
442 .It Cm X11DisplayOffset
443 Specifies the first display number available for
445 X11 forwarding. This prevents
447 from interfering with real X11 servers.
450 Specifies whether X11 forwarding is permitted. The default is
452 Note that disabling X11 forwarding does not improve security in any
453 way, as users can always install their own forwarders.
456 When a user successfully logs in,
459 .Bl -enum -offset indent
461 If the login is on a tty, and no command has been specified,
462 prints last login time and
464 (unless prevented in the configuration file or by
465 .Pa $HOME/.hushlogin ;
470 If the login is on a tty, records login time.
474 if it exists, prints contents and quits
477 Changes to run with normal user privileges.
479 Sets up basic environment.
482 .Pa $HOME/.ssh/environment
485 Changes to user's home directory.
489 exists, runs it; else if
492 it; otherwise runs xauth. The
494 files are given the X11
495 authentication protocol and cookie in standard input.
497 Runs user's shell or command.
499 .Sh AUTHORIZED_KEYS FILE FORMAT
501 .Pa $HOME/.ssh/authorized_keys
502 file lists the RSA keys that are
503 permitted for RSA authentication. Each line of the file contains one
504 key (empty lines and lines starting with a
507 comments). Each line consists of the following fields, separated by
508 spaces: options, bits, exponent, modulus, comment. The options field
509 is optional; its presence is determined by whether the line starts
510 with a number or not (the option field never starts with a number).
511 The bits, exponent, modulus and comment fields give the RSA key; the
512 comment field is not used for anything (but may be convenient for the
513 user to identify the key).
515 Note that lines in this file are usually several hundred bytes long
516 (because of the size of the RSA key modulus). You don't want to type
517 them in; instead, copy the
521 The options (if present) consists of comma-separated option
522 specifications. No spaces are permitted, except within double quotes.
523 The following option specifications are supported:
525 .It Cm from="pattern-list"
526 Specifies that in addition to RSA authentication, the canonical name
527 of the remote host must be present in the comma-separated list of
528 patterns ('*' and '?' serve as wildcards). The list may also contain
529 patterns negated by prefixing them with '!'; if the canonical host
530 name matches a negated pattern, the key is not accepted. The purpose
531 of this option is to optionally increase security: RSA authentication
532 by itself does not trust the network or name servers or anything (but
533 the key); however, if somebody somehow steals the key, the key
534 permits an intruder to log in from anywhere in the world. This
535 additional option makes using a stolen key more difficult (name
536 servers and/or routers would have to be compromised in addition to
538 .It Cm command="command"
539 Specifies that the command is executed whenever this key is used for
540 authentication. The command supplied by the user (if any) is ignored.
541 The command is run on a pty if the connection requests a pty;
542 otherwise it is run without a tty. A quote may be included in the
543 command by quoting it with a backslash. This option might be useful
544 to restrict certain RSA keys to perform just a specific operation. An
545 example might be a key that permits remote backups but nothing
546 else. Notice that the client may specify TCP/IP and/or X11
547 forwardings unless they are explicitly prohibited.
548 .It Cm environment="NAME=value"
549 Specifies that the string is to be added to the environment when
550 logging in using this key. Environment variables set this way
551 override other default environment values. Multiple options of this
553 .It Cm no-port-forwarding
554 Forbids TCP/IP forwarding when this key is used for authentication.
555 Any port forward requests by the client will return an error. This
556 might be used, e.g., in connection with the
559 .It Cm no-X11-forwarding
560 Forbids X11 forwarding when this key is used for authentication.
561 Any X11 forward requests by the client will return an error.
562 .It Cm no-agent-forwarding
563 Forbids authentication agent forwarding when this key is used for
566 Prevents tty allocation (a request to allocate a pty will fail).
569 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
571 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
573 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
574 .Sh SSH_KNOWN_HOSTS FILE FORMAT
576 .Pa /etc/ssh_known_hosts
578 .Pa $HOME/.ssh/known_hosts
579 files contain host public keys for all known hosts. The global file should
580 be prepared by the admistrator (optional), and the per-user file is
581 maintained automatically: whenever the user connects an unknown host
582 its key is added to the per-user file.
584 Each line in these files contains the following fields: hostnames,
585 bits, exponent, modulus, comment. The fields are separated by spaces.
587 Hostnames is a comma-separated list of patterns ('*' and '?' act as
588 wildcards); each pattern in turn is matched against the canonical host
589 name (when authenticating a client) or against the user-supplied
590 name (when authenticating a server). A pattern may also be preceded
593 to indicate negation: if the host name matches a negated
594 pattern, it is not accepted (by that line) even if it matched another
597 Bits, exponent, and modulus are taken directly from the host key; they
598 can be obtained, e.g., from
599 .Pa /etc/ssh_host_key.pub .
600 The optional comment field continues to the end of the line, and is not used.
604 and empty lines are ignored as comments.
606 When performing host authentication, authentication is accepted if any
607 matching line has the proper key. It is thus permissible (but not
608 recommended) to have several lines or different host keys for the same
609 names. This will inevitably happen when short forms of host names
610 from different domains are put in the file. It is possible
611 that the files contain conflicting information; authentication is
612 accepted if valid information can be found from either file.
614 Note that the lines in these files are typically hundreds of characters
615 long, and you definitely don't want to type in the host keys by hand.
616 Rather, generate them by a script
618 .Pa /etc/ssh_host_key.pub
619 and adding the host names at the front.
621 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
624 .It Pa /etc/sshd_config
625 Contains configuration data for
627 This file should be writable by root only, but it is recommended
628 (though not necessary) that it be world-readable.
629 .It Pa /etc/ssh_host_key
630 Contains the private part of the host key.
631 This file should only be owned by root, readable only by root, and not
632 accessible to others.
635 does not start if this file is group/world-accessible.
636 .It Pa /etc/ssh_host_key.pub
637 Contains the public part of the host key.
638 This file should be world-readable but writable only by
639 root. Its contents should match the private part. This file is not
640 really used for anything; it is only provided for the convenience of
641 the user so its contents can be copied to known hosts files.
642 These two files are created using
644 .It Pa /var/run/sshd.pid
645 Contains the process ID of the
647 listening for connections (if there are several daemons running
648 concurrently for different ports, this contains the pid of the one
649 started last). The contents of this file are not sensitive; it can be
651 .It Pa $HOME/.ssh/authorized_keys
652 Lists the RSA keys that can be used to log into the user's account.
653 This file must be readable by root (which may on some machines imply
654 it being world-readable if the user's home directory resides on an NFS
655 volume). It is recommended that it not be accessible by others. The
656 format of this file is described above.
657 .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
658 These files are consulted when using rhosts with RSA host
659 authentication to check the public key of the host. The key must be
660 listed in one of these files to be accepted.
661 The client uses the same files
662 to verify that the remote host is the one we intended to
663 connect. These files should be writable only by root/the owner.
664 .Pa /etc/ssh_known_hosts
665 should be world-readable, and
666 .Pa $HOME/.ssh/known_hosts
667 can but need not be world-readable.
671 refuses to let anyone except root log in. The contents of the file
672 are displayed to anyone trying to log in, and non-root connections are
673 refused. The file should be world-readable.
674 .It Pa /etc/hosts.allow, /etc/hosts.deny
677 support, tcp-wrappers access controls may be defined here as described in
680 This file contains host-username pairs, separated by a space, one per
681 line. The given user on the corresponding host is permitted to log in
682 without password. The same file is used by rlogind and rshd.
684 be writable only by the user; it is recommended that it not be
685 accessible by others.
687 If is also possible to use netgroups in the file. Either host or user
688 name may be of the form +@groupname to specify all hosts or all users
692 this file is exactly the same as for
694 However, this file is
695 not used by rlogin and rshd, so using this permits access using SSH only.
697 This file is used during
699 authentication. In the
700 simplest form, this file contains host names, one per line. Users on
701 those hosts are permitted to log in without a password, provided they
702 have the same user name on both machines. The host name may also be
703 followed by a user name; such users are permitted to log in as
705 user on this machine (except root). Additionally, the syntax
707 can be used to specify netgroups. Negated entries start with
710 If the client host/user is successfully matched in this file, login is
711 automatically permitted provided the client and server user names are the
712 same. Additionally, successful RSA host authentication is normally
713 required. This file must be writable only by root; it is recommended
714 that it be world-readable.
716 .Sy "Warning: It is almost never a good idea to use user names in"
718 Beware that it really means that the named user(s) can log in as
720 which includes bin, daemon, adm, and other accounts that own critical
721 binaries and directories. Using a user name practically grants the
722 user root access. The only valid use for user names that I can think
723 of is in negative entries.
725 Note that this warning also applies to rsh/rlogin.
726 .It Pa /etc/shosts.equiv
727 This is processed exactly as
728 .Pa /etc/hosts.equiv .
729 However, this file may be useful in environments that want to run both
731 .It Pa $HOME/.ssh/environment
732 This file is read into the environment at login (if it exists). It
733 can only contain empty lines, comment lines (that start with
735 and assignment lines of the form name=value. The file should be writable
736 only by the user; it need not be readable by anyone else.
738 If this file exists, it is run with /bin/sh after reading the
739 environment files but before starting the user's shell or command. If
740 X11 spoofing is in use, this will receive the "proto cookie" pair in
743 in environment). This must call
747 The primary purpose of this file is to run any initialization routines
748 which may be needed before the user's home directory becomes
749 accessible; AFS is a particular example of such an environment.
751 This file will probably contain some initialization code followed by
752 something similar to: "if read proto cookie; then echo add $DISPLAY
753 $proto $cookie | xauth -q -; fi".
755 If this file does not exist,
758 does not exist either, xauth is used to store the cookie.
760 This file should be writable only by the user, and need not be
761 readable by anyone else.
765 This can be used to specify
766 machine-specific login-time initializations globally. This file
767 should be writable only by root, and should be world-readable.
770 is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
771 but with bugs removed and newer features re-added. Rapidly after the
772 1.2.12 release, newer versions of the original ssh bore successively
773 more restrictive licenses, and thus demand for a free version was born.
774 This version of OpenSSH
777 has all components of a restrictive nature (i.e., patents, see
779 directly removed from the source code; any licensed or patented components
783 has been updated to support ssh protocol 1.5, making it compatible with
784 all other ssh protocol 1 clients and servers.
786 contains added support for
788 authentication and ticket passing.
790 supports one-time password authentication with
794 The libraries described in
796 are required for proper operation.