2 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 .\" All rights reserved
6 .\" As far as I am concerned, the code I have written for this software
7 .\" can be used freely for any purpose. Any derived versions of this
8 .\" software must be clearly marked as such, and if the derived work is
9 .\" incompatible with the protocol description in the RFC file, it must be
10 .\" called by a name other than "ssh" or "Secure Shell".
12 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16 .\" Redistribution and use in source and binary forms, with or without
17 .\" modification, are permitted provided that the following conditions
19 .\" 1. Redistributions of source code must retain the above copyright
20 .\" notice, this list of conditions and the following disclaimer.
21 .\" 2. Redistributions in binary form must reproduce the above copyright
22 .\" notice, this list of conditions and the following disclaimer in the
23 .\" documentation and/or other materials provided with the distribution.
25 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
36 .\" $OpenBSD: sshd_config.5,v 1.350 2023/07/28 05:42:36 jmc Exp $
37 .Dd $Mdocdate: July 28 2023 $
42 .Nd OpenSSH daemon configuration file
45 reads configuration data from
46 .Pa /etc/ssh/sshd_config
47 (or the file specified with
50 The file contains keyword-argument pairs, one per line.
51 Unless noted otherwise, for each keyword, the first obtained value will be used.
54 and empty lines are interpreted as comments.
55 Arguments may optionally be enclosed in double quotes
57 in order to represent arguments containing spaces.
60 keywords and their meanings are as follows (note that
61 keywords are case-insensitive and arguments are case-sensitive):
64 Specifies what environment variables sent by the client will be copied into
73 for how to configure the client.
76 environment variable is always accepted whenever the client
77 requests a pseudo-terminal as it is required by the protocol.
78 Variables are specified by name, which may contain the wildcard characters
82 Multiple environment variables may be separated by whitespace or spread
86 Be warned that some environment variables could be used to bypass restricted
88 For this reason, care should be taken in the use of this directive.
89 The default is not to accept any environment variables.
91 Specifies which address family should be used by
100 .It Cm AllowAgentForwarding
103 forwarding is permitted.
106 Note that disabling agent forwarding does not improve security
107 unless users are also denied shell access, as they can always install
108 their own forwarders.
110 This keyword can be followed by a list of group name patterns, separated
112 If specified, login is allowed only for users whose primary
113 group or supplementary group list matches one of the patterns.
114 Only group names are valid; a numerical group ID is not recognized.
115 By default, login is allowed for all groups.
116 The allow/deny groups directives are processed in the following order:
122 for more information on patterns.
123 This keyword may appear multiple times in
125 with each instance appending to the list.
126 .It Cm AllowStreamLocalForwarding
127 Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
128 The available options are
133 to allow StreamLocal forwarding,
135 to prevent all StreamLocal forwarding,
137 to allow local (from the perspective of
141 to allow remote forwarding only.
142 Note that disabling StreamLocal forwarding does not improve security unless
143 users are also denied shell access, as they can always install their
145 .It Cm AllowTcpForwarding
146 Specifies whether TCP forwarding is permitted.
147 The available options are
152 to allow TCP forwarding,
154 to prevent all TCP forwarding,
156 to allow local (from the perspective of
160 to allow remote forwarding only.
161 Note that disabling TCP forwarding does not improve security unless
162 users are also denied shell access, as they can always install their
165 This keyword can be followed by a list of user name patterns, separated
167 If specified, login is allowed only for user names that
168 match one of the patterns.
169 Only user names are valid; a numerical user ID is not recognized.
170 By default, login is allowed for all users.
171 If the pattern takes the form USER@HOST then USER and HOST
172 are separately checked, restricting logins to particular
173 users from particular hosts.
174 HOST criteria may additionally contain addresses to match in CIDR
175 address/masklen format.
176 The allow/deny users directives are processed in the following order:
182 for more information on patterns.
183 This keyword may appear multiple times in
185 with each instance appending to the list.
186 .It Cm AuthenticationMethods
187 Specifies the authentication methods that must be successfully completed
188 for a user to be granted access.
189 This option must be followed by one or more lists of comma-separated
190 authentication method names, or by the single string
192 to indicate the default behaviour of accepting any single authentication
194 If the default is overridden, then successful authentication requires
195 completion of every method in at least one of these lists.
198 .Qq publickey,password publickey,keyboard-interactive
199 would require the user to complete public key authentication, followed by
200 either password or keyboard interactive authentication.
201 Only methods that are next in one or more lists are offered at each stage,
202 so for this example it would not be possible to attempt password or
203 keyboard-interactive authentication before public key.
205 For keyboard interactive authentication it is also possible to
206 restrict authentication to a specific device by appending a
207 colon followed by the device identifier
211 depending on the server configuration.
213 .Qq keyboard-interactive:bsdauth
214 would restrict keyboard interactive authentication to the
218 If the publickey method is listed more than once,
220 verifies that keys that have been used successfully are not reused for
221 subsequent authentications.
223 .Qq publickey,publickey
224 requires successful authentication using two different public keys.
226 Note that each authentication method listed should also be explicitly enabled
227 in the configuration.
229 The available authentication methods are:
230 .Qq gssapi-with-mic ,
232 .Qq keyboard-interactive ,
234 (used for access to password-less accounts when
235 .Cm PermitEmptyPasswords
240 .It Cm AuthorizedKeysCommand
241 Specifies a program to be used to look up the user's public keys.
242 The program must be owned by root, not writable by group or others and
243 specified by an absolute path.
245 .Cm AuthorizedKeysCommand
246 accept the tokens described in the
249 If no arguments are specified then the username of the target user is used.
251 The program should produce on standard output zero or
252 more lines of authorized_keys output (see
256 .Cm AuthorizedKeysCommand
257 is tried after the usual
258 .Cm AuthorizedKeysFile
259 files and will not be executed if a matching key is found there.
261 .Cm AuthorizedKeysCommand
263 .It Cm AuthorizedKeysCommandUser
264 Specifies the user under whose account the
265 .Cm AuthorizedKeysCommand
267 It is recommended to use a dedicated user that has no other role on the host
268 than running authorized keys commands.
270 .Cm AuthorizedKeysCommand
272 .Cm AuthorizedKeysCommandUser
275 will refuse to start.
276 .It Cm AuthorizedKeysFile
277 Specifies the file that contains the public keys used for user authentication.
278 The format is described in the AUTHORIZED_KEYS FILE FORMAT section of
281 .Cm AuthorizedKeysFile
282 accept the tokens described in the
286 .Cm AuthorizedKeysFile
287 is taken to be an absolute path or one relative to the user's home
289 Multiple files may be listed, separated by whitespace.
290 Alternately this option may be set to
292 to skip checking for user keys in files.
294 .Qq .ssh/authorized_keys .ssh/authorized_keys2 .
295 .It Cm AuthorizedPrincipalsCommand
296 Specifies a program to be used to generate the list of allowed
297 certificate principals as per
298 .Cm AuthorizedPrincipalsFile .
299 The program must be owned by root, not writable by group or others and
300 specified by an absolute path.
302 .Cm AuthorizedPrincipalsCommand
303 accept the tokens described in the
306 If no arguments are specified then the username of the target user is used.
308 The program should produce on standard output zero or
310 .Cm AuthorizedPrincipalsFile
313 .Cm AuthorizedPrincipalsCommand
315 .Cm AuthorizedPrincipalsFile
316 is specified, then certificates offered by the client for authentication
317 must contain a principal that is listed.
319 .Cm AuthorizedPrincipalsCommand
321 .It Cm AuthorizedPrincipalsCommandUser
322 Specifies the user under whose account the
323 .Cm AuthorizedPrincipalsCommand
325 It is recommended to use a dedicated user that has no other role on the host
326 than running authorized principals commands.
328 .Cm AuthorizedPrincipalsCommand
330 .Cm AuthorizedPrincipalsCommandUser
333 will refuse to start.
334 .It Cm AuthorizedPrincipalsFile
335 Specifies a file that lists principal names that are accepted for
336 certificate authentication.
337 When using certificates signed by a key listed in
338 .Cm TrustedUserCAKeys ,
339 this file lists names, one of which must appear in the certificate for it
340 to be accepted for authentication.
341 Names are listed one per line preceded by key options (as described in
342 .Sx AUTHORIZED_KEYS FILE FORMAT
345 Empty lines and comments starting with
350 .Cm AuthorizedPrincipalsFile
351 accept the tokens described in the
355 .Cm AuthorizedPrincipalsFile
356 is taken to be an absolute path or one relative to the user's home directory.
359 i.e. not to use a principals file \(en in this case, the username
360 of the user must appear in a certificate's principals list for it to be
364 .Cm AuthorizedPrincipalsFile
365 is only used when authentication proceeds using a CA listed in
366 .Cm TrustedUserCAKeys
367 and is not consulted for certification authorities trusted via
368 .Pa ~/.ssh/authorized_keys ,
371 key option offers a similar facility (see
375 The contents of the specified file are sent to the remote user before
376 authentication is allowed.
379 then no banner is displayed.
380 By default, no banner is displayed.
381 .It Cm CASignatureAlgorithms
382 Specifies which algorithms are allowed for signing of certificates
383 by certificate authorities (CAs).
385 .Bd -literal -offset indent
386 ssh-ed25519,ecdsa-sha2-nistp256,
387 ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
388 sk-ssh-ed25519@openssh.com,
389 sk-ecdsa-sha2-nistp256@openssh.com,
390 rsa-sha2-512,rsa-sha2-256
393 If the specified list begins with a
395 character, then the specified algorithms will be appended to the default set
396 instead of replacing them.
397 If the specified list begins with a
399 character, then the specified algorithms (including wildcards) will be removed
400 from the default set instead of replacing them.
402 Certificates signed using other algorithms will not be accepted for
403 public key or host-based authentication.
404 .It Cm ChannelTimeout
405 Specifies whether and how quickly
407 should close inactive channels.
408 Timeouts are specified as one or more
410 pairs separated by whitespace, where the
412 must be a channel type name (as described in the table below), optionally
413 containing wildcard characters.
417 is specified in seconds or may use any of the units documented in the
422 would cause all sessions to terminate after five minutes of inactivity.
423 Specifying a zero value disables the inactivity timeout.
425 The available channel types include:
427 .It Cm agent-connection
430 .It Cm direct-tcpip , Cm direct-streamlocal@openssh.com
431 Open TCP or Unix socket (respectively) connections that have
432 been established from a
434 local forwarding, i.e.\&
438 .It Cm forwarded-tcpip , Cm forwarded-streamlocal@openssh.com
439 Open TCP or Unix socket (respectively) connections that have been
442 listening on behalf of a
444 remote forwarding, i.e.\&
446 .It Cm session:command
447 Command execution sessions.
449 Interactive shell sessions.
450 .It Cm session:subsystem:...
451 Subsystem sessions, e.g. for
453 which could be identified as
454 .Cm session:subsystem:sftp .
455 .It Cm x11-connection
456 Open X11 forwarding sessions.
459 Note that in all the above cases, terminating an inactive session does not
460 guarantee to remove all resources associated with the session, e.g. shell
461 processes or X11 clients relating to the session may continue to execute.
463 Moreover, terminating an inactive channel or session does not necessarily
464 close the SSH connection, nor does it prevent a client from
465 requesting another channel of the same type.
466 In particular, expiring an inactive forwarding session does not prevent
467 another identical forwarding from being subsequently created.
469 .Cm UnusedConnectionTimeout ,
470 which may be used in conjunction with this option.
472 The default is not to expire channels of any type for inactivity.
473 .It Cm ChrootDirectory
474 Specifies the pathname of a directory to
476 to after authentication.
479 checks that all components of the pathname are root-owned directories
480 which are not writable by any other user or group.
483 changes the working directory to the user's home directory.
486 accept the tokens described in the
492 must contain the necessary files and directories to support the
494 For an interactive session this requires at least a shell, typically
507 For file transfer sessions using SFTP
508 no additional configuration of the environment is necessary if the in-process
510 though sessions which use logging may require
512 inside the chroot directory on some operating systems (see
516 For safety, it is very important that the directory hierarchy be
517 prevented from modification by other processes on the system (especially
518 those outside the jail).
519 Misconfiguration can lead to unsafe environments which
528 Specifies the ciphers allowed.
529 Multiple ciphers must be comma-separated.
530 If the specified list begins with a
532 character, then the specified ciphers will be appended to the default set
533 instead of replacing them.
534 If the specified list begins with a
536 character, then the specified ciphers (including wildcards) will be removed
537 from the default set instead of replacing them.
538 If the specified list begins with a
540 character, then the specified ciphers will be placed at the head of the
543 The supported ciphers are:
545 .Bl -item -compact -offset indent
561 aes128-gcm@openssh.com
563 aes256-gcm@openssh.com
565 chacha20-poly1305@openssh.com
569 .Bd -literal -offset indent
570 chacha20-poly1305@openssh.com,
571 aes128-ctr,aes192-ctr,aes256-ctr,
572 aes128-gcm@openssh.com,aes256-gcm@openssh.com
575 The list of available ciphers may also be obtained using
577 .It Cm ClientAliveCountMax
578 Sets the number of client alive messages which may be sent without
580 receiving any messages back from the client.
581 If this threshold is reached while client alive messages are being sent,
582 sshd will disconnect the client, terminating the session.
583 It is important to note that the use of client alive messages is very
586 The client alive messages are sent through the encrypted channel
587 and therefore will not be spoofable.
588 The TCP keepalive option enabled by
591 The client alive mechanism is valuable when the client or
592 server depend on knowing when a connection has become unresponsive.
594 The default value is 3.
596 .Cm ClientAliveInterval
598 .Cm ClientAliveCountMax
599 is left at the default, unresponsive SSH clients
600 will be disconnected after approximately 45 seconds.
602 .Cm ClientAliveCountMax
603 disables connection termination.
604 .It Cm ClientAliveInterval
605 Sets a timeout interval in seconds after which if no data has been received
608 will send a message through the encrypted
609 channel to request a response from the client.
611 is 0, indicating that these messages will not be sent to the client.
613 Specifies whether compression is enabled after
614 the user has authenticated successfully.
618 (a legacy synonym for
625 This keyword can be followed by a list of group name patterns, separated
627 Login is disallowed for users whose primary group or supplementary
628 group list matches one of the patterns.
629 Only group names are valid; a numerical group ID is not recognized.
630 By default, login is allowed for all groups.
631 The allow/deny groups directives are processed in the following order:
637 for more information on patterns.
638 This keyword may appear multiple times in
640 with each instance appending to the list.
642 This keyword can be followed by a list of user name patterns, separated
644 Login is disallowed for user names that match one of the patterns.
645 Only user names are valid; a numerical user ID is not recognized.
646 By default, login is allowed for all users.
647 If the pattern takes the form USER@HOST then USER and HOST
648 are separately checked, restricting logins to particular
649 users from particular hosts.
650 HOST criteria may additionally contain addresses to match in CIDR
651 address/masklen format.
652 The allow/deny users directives are processed in the following order:
658 for more information on patterns.
659 This keyword may appear multiple times in
661 with each instance appending to the list.
662 .It Cm DisableForwarding
663 Disables all forwarding features, including X11,
666 This option overrides all other forwarding-related options and may
667 simplify restricted configurations.
668 .It Cm ExposeAuthInfo
669 Writes a temporary file containing a list of authentication methods and
670 public credentials (e.g. keys) used to authenticate the user.
671 The location of the file is exposed to the user session through the
673 environment variable.
676 .It Cm FingerprintHash
677 Specifies the hash algorithm used when logging key fingerprints.
685 Forces the execution of the command specified by
687 ignoring any command supplied by the client and
690 The command is invoked by using the user's login shell with the -c option.
691 This applies to shell, command, or subsystem execution.
692 It is most useful inside a
695 The command originally supplied by the client is available in the
696 .Ev SSH_ORIGINAL_COMMAND
697 environment variable.
698 Specifying a command of
700 will force the use of an in-process SFTP server that requires no support
702 .Cm ChrootDirectory .
706 Specifies whether remote hosts are allowed to connect to ports
707 forwarded for the client.
710 binds remote port forwardings to the loopback address.
711 This prevents other remote hosts from connecting to forwarded ports.
713 can be used to specify that sshd
714 should allow remote port forwardings to bind to non-loopback addresses, thus
715 allowing other hosts to connect.
718 to force remote port forwardings to be available to the local host only,
720 to force remote port forwardings to bind to the wildcard address, or
722 to allow the client to select the address to which the forwarding is bound.
725 .It Cm GSSAPIAuthentication
726 Specifies whether user authentication based on GSSAPI is allowed.
729 .It Cm GSSAPICleanupCredentials
730 Specifies whether to automatically destroy the user's credentials cache
734 .It Cm GSSAPIStrictAcceptorCheck
735 Determines whether to be strict about the identity of the GSSAPI acceptor
736 a client authenticates against.
739 then the client must authenticate against the host
740 service on the current hostname.
743 then the client may authenticate against any service key stored in the
744 machine's default store.
745 This facility is provided to assist with operation on multi homed machines.
748 .It Cm HostbasedAcceptedAlgorithms
749 Specifies the signature algorithms that will be accepted for hostbased
750 authentication as a list of comma-separated patterns.
751 Alternately if the specified list begins with a
753 character, then the specified signature algorithms will be appended to
754 the default set instead of replacing them.
755 If the specified list begins with a
757 character, then the specified signature algorithms (including wildcards)
758 will be removed from the default set instead of replacing them.
759 If the specified list begins with a
761 character, then the specified signature algorithms will be placed at
762 the head of the default set.
763 The default for this option is:
764 .Bd -literal -offset 3n
765 ssh-ed25519-cert-v01@openssh.com,
766 ecdsa-sha2-nistp256-cert-v01@openssh.com,
767 ecdsa-sha2-nistp384-cert-v01@openssh.com,
768 ecdsa-sha2-nistp521-cert-v01@openssh.com,
769 sk-ssh-ed25519-cert-v01@openssh.com,
770 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
771 rsa-sha2-512-cert-v01@openssh.com,
772 rsa-sha2-256-cert-v01@openssh.com,
774 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
775 sk-ssh-ed25519@openssh.com,
776 sk-ecdsa-sha2-nistp256@openssh.com,
777 rsa-sha2-512,rsa-sha2-256
780 The list of available signature algorithms may also be obtained using
781 .Qq ssh -Q HostbasedAcceptedAlgorithms .
782 This was formerly named HostbasedAcceptedKeyTypes.
783 .It Cm HostbasedAuthentication
784 Specifies whether rhosts or /etc/hosts.equiv authentication together
785 with successful public key client host authentication is allowed
786 (host-based authentication).
789 .It Cm HostbasedUsesNameFromPacketOnly
790 Specifies whether or not the server will attempt to perform a reverse
791 name lookup when matching the name in the
797 .Cm HostbasedAuthentication .
802 uses the name supplied by the client rather than
803 attempting to resolve the name from the TCP connection itself.
806 .It Cm HostCertificate
807 Specifies a file containing a public host certificate.
808 The certificate's public key must match a private host key already specified
811 The default behaviour of
813 is not to load any certificates.
815 Specifies a file containing a private host key
818 .Pa /etc/ssh/ssh_host_ecdsa_key ,
819 .Pa /etc/ssh/ssh_host_ed25519_key
821 .Pa /etc/ssh/ssh_host_rsa_key .
825 will refuse to use a file if it is group/world-accessible
827 .Cm HostKeyAlgorithms
828 option restricts which of the keys are actually used by
831 It is possible to have multiple host key files.
832 It is also possible to specify public host key files instead.
833 In this case operations on the private key will be delegated
837 Identifies the UNIX-domain socket used to communicate
838 with an agent that has access to the private host keys.
841 is specified, the location of the socket will be read from the
843 environment variable.
844 .It Cm HostKeyAlgorithms
845 Specifies the host key signature algorithms
846 that the server offers.
847 The default for this option is:
848 .Bd -literal -offset 3n
849 ssh-ed25519-cert-v01@openssh.com,
850 ecdsa-sha2-nistp256-cert-v01@openssh.com,
851 ecdsa-sha2-nistp384-cert-v01@openssh.com,
852 ecdsa-sha2-nistp521-cert-v01@openssh.com,
853 sk-ssh-ed25519-cert-v01@openssh.com,
854 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
855 rsa-sha2-512-cert-v01@openssh.com,
856 rsa-sha2-256-cert-v01@openssh.com,
858 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
859 sk-ssh-ed25519@openssh.com,
860 sk-ecdsa-sha2-nistp256@openssh.com,
861 rsa-sha2-512,rsa-sha2-256
864 The list of available signature algorithms may also be obtained using
865 .Qq ssh -Q HostKeyAlgorithms .
867 Specifies whether to ignore per-user
872 .Cm HostbasedAuthentication .
876 .Pa /etc/ssh/shosts.equiv
877 are still used regardless of this setting.
881 (the default) to ignore all per-user files,
893 .It Cm IgnoreUserKnownHosts
896 should ignore the user's
897 .Pa ~/.ssh/known_hosts
899 .Cm HostbasedAuthentication
900 and use only the system-wide known hosts file
901 .Pa /etc/ssh/ssh_known_hosts .
905 Include the specified configuration file(s).
906 Multiple pathnames may be specified and each pathname may contain
908 wildcards that will be expanded and processed in lexical order.
909 Files without absolute paths are assumed to be in
913 directive may appear inside a
916 to perform conditional inclusion.
918 Specifies the IPv4 type-of-service or DSCP class for the connection.
947 to use the operating system default.
948 This option may take one or two arguments, separated by whitespace.
949 If one argument is specified, it is used as the packet class unconditionally.
950 If two values are specified, the first is automatically selected for
951 interactive sessions and the second for non-interactive sessions.
955 for interactive sessions and
958 for non-interactive sessions.
959 .It Cm KbdInteractiveAuthentication
960 Specifies whether to allow keyboard-interactive authentication.
961 All authentication styles from
966 The argument to this keyword must be
970 .Cm ChallengeResponseAuthentication
971 is a deprecated alias for this.
972 .It Cm KerberosAuthentication
973 Specifies whether the password provided by the user for
974 .Cm PasswordAuthentication
975 will be validated through the Kerberos KDC.
976 To use this option, the server needs a
977 Kerberos servtab which allows the verification of the KDC's identity.
980 .It Cm KerberosGetAFSToken
981 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
982 an AFS token before accessing the user's home directory.
985 .It Cm KerberosOrLocalPasswd
986 If password authentication through Kerberos fails then
987 the password will be validated via any additional local mechanism
992 .It Cm KerberosTicketCleanup
993 Specifies whether to automatically destroy the user's ticket cache
998 Specifies the available KEX (Key Exchange) algorithms.
999 Multiple algorithms must be comma-separated.
1000 Alternately if the specified list begins with a
1002 character, then the specified algorithms will be appended to the default set
1003 instead of replacing them.
1004 If the specified list begins with a
1006 character, then the specified algorithms (including wildcards) will be removed
1007 from the default set instead of replacing them.
1008 If the specified list begins with a
1010 character, then the specified algorithms will be placed at the head of the
1012 The supported algorithms are:
1014 .Bl -item -compact -offset indent
1018 curve25519-sha256@libssh.org
1020 diffie-hellman-group1-sha1
1022 diffie-hellman-group14-sha1
1024 diffie-hellman-group14-sha256
1026 diffie-hellman-group16-sha512
1028 diffie-hellman-group18-sha512
1030 diffie-hellman-group-exchange-sha1
1032 diffie-hellman-group-exchange-sha256
1040 sntrup761x25519-sha512@openssh.com
1044 .Bd -literal -offset indent
1045 sntrup761x25519-sha512@openssh.com,
1046 curve25519-sha256,curve25519-sha256@libssh.org,
1047 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1048 diffie-hellman-group-exchange-sha256,
1049 diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
1050 diffie-hellman-group14-sha256
1053 The list of available key exchange algorithms may also be obtained using
1054 .Qq ssh -Q KexAlgorithms .
1055 .It Cm ListenAddress
1056 Specifies the local addresses
1059 The following forms may be used:
1061 .Bl -item -offset indent -compact
1065 .Ar hostname | address
1067 .Op Cm rdomain Ar domain
1073 .Op Cm rdomain Ar domain
1077 .Ar IPv4_address : port
1079 .Op Cm rdomain Ar domain
1083 .Oo Ar hostname | address Oc : Ar port
1085 .Op Cm rdomain Ar domain
1092 listen in an explicit routing domain.
1096 sshd will listen on the address and all
1099 The default is to listen on all local addresses on the current default
1103 options are permitted.
1104 For more information on routing domains, see
1106 .It Cm LoginGraceTime
1107 The server disconnects after this time if the user has not
1108 successfully logged in.
1109 If the value is 0, there is no time limit.
1110 The default is 120 seconds.
1112 Gives the verbosity level that is used when logging messages from
1114 The possible values are:
1115 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1116 The default is INFO.
1117 DEBUG and DEBUG1 are equivalent.
1118 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
1119 Logging with a DEBUG level violates the privacy of users and is not recommended.
1121 Specify one or more overrides to LogLevel.
1122 An override consists of a pattern lists that matches the source file, function
1123 and line number to force detailed logging for.
1124 For example, an override pattern of:
1125 .Bd -literal -offset indent
1126 kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1129 would enable detailed logging for line 1000 of
1132 .Fn kex_exchange_identification
1133 function, and all code in the
1136 This option is intended for debugging and no overrides are enabled by default.
1138 Specifies the available MAC (message authentication code) algorithms.
1139 The MAC algorithm is used for data integrity protection.
1140 Multiple algorithms must be comma-separated.
1141 If the specified list begins with a
1143 character, then the specified algorithms will be appended to the default set
1144 instead of replacing them.
1145 If the specified list begins with a
1147 character, then the specified algorithms (including wildcards) will be removed
1148 from the default set instead of replacing them.
1149 If the specified list begins with a
1151 character, then the specified algorithms will be placed at the head of the
1154 The algorithms that contain
1156 calculate the MAC after encryption (encrypt-then-mac).
1157 These are considered safer and their use recommended.
1158 The supported MACs are:
1160 .Bl -item -compact -offset indent
1176 umac-128@openssh.com
1178 hmac-md5-etm@openssh.com
1180 hmac-md5-96-etm@openssh.com
1182 hmac-sha1-etm@openssh.com
1184 hmac-sha1-96-etm@openssh.com
1186 hmac-sha2-256-etm@openssh.com
1188 hmac-sha2-512-etm@openssh.com
1190 umac-64-etm@openssh.com
1192 umac-128-etm@openssh.com
1196 .Bd -literal -offset indent
1197 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1198 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1199 hmac-sha1-etm@openssh.com,
1200 umac-64@openssh.com,umac-128@openssh.com,
1201 hmac-sha2-256,hmac-sha2-512,hmac-sha1
1204 The list of available MAC algorithms may also be obtained using
1207 Introduces a conditional block.
1208 If all of the criteria on the
1210 line are satisfied, the keywords on the following lines override those
1211 set in the global section of the config file, until either another
1213 line or the end of the file.
1214 If a keyword appears in multiple
1216 blocks that are satisfied, only the first instance of the keyword is
1221 are one or more criteria-pattern pairs or the single token
1223 which matches all criteria.
1224 The available criteria are
1237 on which the connection was received).
1239 The match patterns may consist of single entries or comma-separated
1240 lists and may use the wildcard and negation operators described in the
1247 criteria may additionally contain addresses to match in CIDR
1248 address/masklen format,
1249 such as 192.0.2.0/24 or 2001:db8::/32.
1250 Note that the mask length provided must be consistent with the address -
1251 it is an error to specify a mask length that is too long for the address
1252 or one with bits set in this host portion of the address.
1253 For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1255 Only a subset of keywords may be used on the lines following a
1258 Available keywords are
1260 .Cm AllowAgentForwarding ,
1262 .Cm AllowStreamLocalForwarding ,
1263 .Cm AllowTcpForwarding ,
1265 .Cm AuthenticationMethods ,
1266 .Cm AuthorizedKeysCommand ,
1267 .Cm AuthorizedKeysCommandUser ,
1268 .Cm AuthorizedKeysFile ,
1269 .Cm AuthorizedPrincipalsCommand ,
1270 .Cm AuthorizedPrincipalsCommandUser ,
1271 .Cm AuthorizedPrincipalsFile ,
1273 .Cm CASignatureAlgorithms ,
1274 .Cm ChannelTimeout ,
1275 .Cm ChrootDirectory ,
1276 .Cm ClientAliveCountMax ,
1277 .Cm ClientAliveInterval ,
1280 .Cm DisableForwarding ,
1281 .Cm ExposeAuthInfo ,
1284 .Cm GSSAPIAuthentication ,
1285 .Cm HostbasedAcceptedAlgorithms ,
1286 .Cm HostbasedAuthentication ,
1287 .Cm HostbasedUsesNameFromPacketOnly ,
1291 .Cm KbdInteractiveAuthentication ,
1292 .Cm KerberosAuthentication ,
1296 .Cm PasswordAuthentication ,
1297 .Cm PermitEmptyPasswords ,
1300 .Cm PermitRootLogin ,
1304 .Cm PubkeyAcceptedAlgorithms ,
1305 .Cm PubkeyAuthentication ,
1306 .Cm PubkeyAuthOptions ,
1311 .Cm StreamLocalBindMask ,
1312 .Cm StreamLocalBindUnlink ,
1313 .Cm TrustedUserCAKeys ,
1314 .Cm UnusedConnectionTimeout ,
1315 .Cm X11DisplayOffset ,
1318 .Cm X11UseLocalhost .
1320 Specifies the maximum number of authentication attempts permitted per
1322 Once the number of failures reaches half this value,
1323 additional failures are logged.
1326 Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1327 sessions permitted per network connection.
1328 Multiple sessions may be established by clients that support connection
1332 to 1 will effectively disable session multiplexing, whereas setting it to 0
1333 will prevent all shell, login and subsystem sessions while still permitting
1337 Specifies the maximum number of concurrent unauthenticated connections to the
1339 Additional connections will be dropped until authentication succeeds or the
1341 expires for a connection.
1342 The default is 10:30:100.
1344 Alternatively, random early drop can be enabled by specifying
1345 the three colon separated values
1346 start:rate:full (e.g. "10:30:60").
1348 will refuse connection attempts with a probability of rate/100 (30%)
1349 if there are currently start (10) unauthenticated connections.
1350 The probability increases linearly and all connection attempts
1351 are refused if the number of unauthenticated connections reaches full (60).
1355 file that contains the Diffie-Hellman groups used for the
1356 .Dq diffie-hellman-group-exchange-sha1
1358 .Dq diffie-hellman-group-exchange-sha256
1359 key exchange methods.
1362 .It Cm PasswordAuthentication
1363 Specifies whether password authentication is allowed.
1364 Note that passwords may also be accepted via
1365 .Cm KbdInteractiveAuthentication .
1370 .It Cm PermitEmptyPasswords
1371 When password authentication is allowed, it specifies whether the
1372 server allows login to accounts with empty password strings.
1376 Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1377 The listen specification must be one of the following forms:
1379 .Bl -item -offset indent -compact
1392 Multiple permissions may be specified by separating them with whitespace.
1395 can be used to remove all restrictions and permit any listen requests.
1398 can be used to prohibit all listen requests.
1399 The host name may contain wildcards as described in the PATTERNS section in
1403 can also be used in place of a port number to allow all ports.
1404 By default all port forwarding listen requests are permitted.
1407 option may further restrict which addresses may be listened on.
1410 will request a listen host of
1412 if no listen host was specifically requested, and this name is
1413 treated differently to explicit localhost addresses of
1418 Specifies the destinations to which TCP port forwarding is permitted.
1419 The forwarding specification must be one of the following forms:
1421 .Bl -item -offset indent -compact
1430 .Ar IPv4_addr : port
1435 .Ar \&[ IPv6_addr \&] : port
1439 Multiple forwards may be specified by separating them with whitespace.
1442 can be used to remove all restrictions and permit any forwarding requests.
1445 can be used to prohibit all forwarding requests.
1448 can be used for host or port to allow all hosts or ports respectively.
1449 Otherwise, no pattern matching or address lookups are performed on supplied
1451 By default all port forwarding requests are permitted.
1452 .It Cm PermitRootLogin
1453 Specifies whether root can log in using
1455 The argument must be
1457 .Cm prohibit-password ,
1458 .Cm forced-commands-only ,
1464 .Cm ChallengeResponseAuthentication
1469 this setting may be overridden by the PAM policy.
1471 If this option is set to
1472 .Cm prohibit-password
1473 (or its deprecated alias,
1474 .Cm without-password ) ,
1475 password and keyboard-interactive authentication are disabled for root.
1477 If this option is set to
1478 .Cm forced-commands-only ,
1479 root login with public key authentication will be allowed,
1482 option has been specified
1483 (which may be useful for taking remote backups even if root login is
1484 normally not allowed).
1485 All other authentication methods are disabled for root.
1487 If this option is set to
1489 root is not allowed to log in.
1493 allocation is permitted.
1499 device forwarding is allowed.
1500 The argument must be
1516 Independent of this setting, the permissions of the selected
1518 device must allow access to the user.
1519 .It Cm PermitUserEnvironment
1521 .Pa ~/.ssh/environment
1525 .Pa ~/.ssh/authorized_keys
1531 or a pattern-list specifying which environment variable names to accept
1536 Enabling environment processing may enable users to bypass access
1537 restrictions in some configurations using mechanisms such as
1540 Specifies whether any
1545 .It Cm PerSourceMaxStartups
1546 Specifies the number of unauthenticated connections allowed from a
1547 given source address, or
1549 if there is no limit.
1550 This limit is applied in addition to
1555 .It Cm PerSourceNetBlockSize
1556 Specifies the number of bits of source address that are grouped together
1557 for the purposes of applying PerSourceMaxStartups limits.
1558 Values for IPv4 and optionally IPv6 may be specified, separated by a colon.
1561 which means each address is considered individually.
1563 Specifies the file that contains the process ID of the
1568 .Pa /var/run/sshd.pid .
1570 Specifies the port number that
1574 Multiple options of this type are permitted.
1580 should print the date and time of the last user login when a user logs
1589 when a user logs in interactively.
1590 (On some systems it is also printed by the shell,
1595 .It Cm PubkeyAcceptedAlgorithms
1596 Specifies the signature algorithms that will be accepted for public key
1597 authentication as a list of comma-separated patterns.
1598 Alternately if the specified list begins with a
1600 character, then the specified algorithms will be appended to the default set
1601 instead of replacing them.
1602 If the specified list begins with a
1604 character, then the specified algorithms (including wildcards) will be removed
1605 from the default set instead of replacing them.
1606 If the specified list begins with a
1608 character, then the specified algorithms will be placed at the head of the
1610 The default for this option is:
1611 .Bd -literal -offset 3n
1612 ssh-ed25519-cert-v01@openssh.com,
1613 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1614 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1615 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1616 sk-ssh-ed25519-cert-v01@openssh.com,
1617 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1618 rsa-sha2-512-cert-v01@openssh.com,
1619 rsa-sha2-256-cert-v01@openssh.com,
1621 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1622 sk-ssh-ed25519@openssh.com,
1623 sk-ecdsa-sha2-nistp256@openssh.com,
1624 rsa-sha2-512,rsa-sha2-256
1627 The list of available signature algorithms may also be obtained using
1628 .Qq ssh -Q PubkeyAcceptedAlgorithms .
1629 .It Cm PubkeyAuthOptions
1630 Sets one or more public key authentication options.
1631 The supported keywords are:
1633 (the default; indicating no additional options are enabled),
1636 .Cm verify-required .
1640 option causes public key authentication using a FIDO authenticator algorithm
1645 to always require the signature to attest that a physically present user
1646 explicitly confirmed the authentication (usually by touching the authenticator).
1649 requires user presence unless overridden with an authorized_keys option.
1652 flag disables this override.
1656 option requires a FIDO key signature attest that the user was verified,
1663 options have any effect for other, non-FIDO, public key types.
1664 .It Cm PubkeyAuthentication
1665 Specifies whether public key authentication is allowed.
1669 Specifies the maximum amount of data that may be transmitted or received
1670 before the session key is renegotiated, optionally followed by a maximum
1671 amount of time that may pass before the session key is renegotiated.
1672 The first argument is specified in bytes and may have a suffix of
1677 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1678 The default is between
1682 depending on the cipher.
1683 The optional second value is specified in seconds and may use any of the
1684 units documented in the
1687 The default value for
1691 which means that rekeying is performed after the cipher's default amount
1692 of data has been sent or received and no time based rekeying is done.
1693 .It Cm RequiredRSASize
1694 Specifies the minimum RSA key size (in bits) that
1697 User and host-based authentication keys smaller than this limit will be
1702 Note that this limit may only be raised from the default.
1704 Specifies revoked public keys file, or
1707 Keys listed in this file will be refused for public key authentication.
1708 Note that if this file is not readable, then public key authentication will
1709 be refused for all users.
1710 Keys may be specified as a text file, listing one public key per line, or as
1711 an OpenSSH Key Revocation List (KRL) as generated by
1713 For more information on KRLs, see the KEY REVOCATION LISTS section in
1716 Specifies an explicit routing domain that is applied after authentication
1718 The user session, as well as any forwarded or listening IP sockets,
1719 will be bound to this
1721 If the routing domain is set to
1723 then the domain in which the incoming connection was received will be applied.
1724 .It Cm SecurityKeyProvider
1725 Specifies a path to a library that will be used when loading
1726 FIDO authenticator-hosted keys, overriding the default of using
1727 the built-in USB HID support.
1729 Specifies one or more environment variables to set in child sessions started
1734 The environment value may be quoted (e.g. if it contains whitespace
1736 Environment variables set by
1738 override the default environment and any variables specified by the user
1742 .Cm PermitUserEnvironment .
1743 .It Cm StreamLocalBindMask
1744 Sets the octal file creation mode mask
1746 used when creating a Unix-domain socket file for local or remote
1748 This option is only used for port forwarding to a Unix-domain socket file.
1750 The default value is 0177, which creates a Unix-domain socket file that is
1751 readable and writable only by the owner.
1752 Note that not all operating systems honor the file mode on Unix-domain
1754 .It Cm StreamLocalBindUnlink
1755 Specifies whether to remove an existing Unix-domain socket file for local
1756 or remote port forwarding before creating a new one.
1757 If the socket file already exists and
1758 .Cm StreamLocalBindUnlink
1761 will be unable to forward the port to the Unix-domain socket file.
1762 This option is only used for port forwarding to a Unix-domain socket file.
1764 The argument must be
1773 should check file modes and ownership of the
1774 user's files and home directory before accepting login.
1775 This is normally desirable because novices sometimes accidentally leave their
1776 directory or files world-writable.
1779 Note that this does not apply to
1780 .Cm ChrootDirectory ,
1781 whose permissions and ownership are checked unconditionally.
1783 Configures an external subsystem (e.g. file transfer daemon).
1784 Arguments should be a subsystem name and a command (with optional arguments)
1785 to execute upon subsystem request.
1789 implements the SFTP file transfer subsystem.
1791 Alternately the name
1793 implements an in-process SFTP server.
1794 This may simplify configurations using
1796 to force a different filesystem root on clients.
1798 By default no subsystems are defined.
1799 .It Cm SyslogFacility
1800 Gives the facility code that is used when logging messages from
1802 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1803 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1804 The default is AUTH.
1806 Specifies whether the system should send TCP keepalive messages to the
1808 If they are sent, death of the connection or crash of one
1809 of the machines will be properly noticed.
1810 However, this means that
1811 connections will die if the route is down temporarily, and some people
1813 On the other hand, if TCP keepalives are not sent,
1814 sessions may hang indefinitely on the server, leaving
1816 users and consuming server resources.
1820 (to send TCP keepalive messages), and the server will notice
1821 if the network goes down or the client host crashes.
1822 This avoids infinitely hanging sessions.
1824 To disable TCP keepalive messages, the value should be set to
1826 .It Cm TrustedUserCAKeys
1827 Specifies a file containing public keys of certificate authorities that are
1828 trusted to sign user certificates for authentication, or
1831 Keys are listed one per line; empty lines and comments starting with
1834 If a certificate is presented for authentication and has its signing CA key
1835 listed in this file, then it may be used for authentication for any user
1836 listed in the certificate's principals list.
1837 Note that certificates that lack a list of principals will not be permitted
1838 for authentication using
1839 .Cm TrustedUserCAKeys .
1840 For more details on certificates, see the CERTIFICATES section in
1842 .It Cm UnusedConnectionTimeout
1843 Specifies whether and how quickly
1845 should close client connections with no open channels.
1846 Open channels include active shell, command execution or subsystem
1847 sessions, connected network, socket, agent or X11 forwardings.
1848 Forwarding listeners, such as those from the
1851 flag, are not considered as open channels and do not prevent the timeout.
1853 is specified in seconds or may use any of the units documented in the
1857 Note that this timeout starts when the client connection completes
1858 user authentication but before the client has an opportunity to open any
1860 Caution should be used when using short timeout values, as they may not
1861 provide sufficient time for the client to request and open its channels
1862 before terminating the connection.
1866 is to never expire connections for having no open channels.
1867 This option may be useful in conjunction with
1868 .Cm ChannelTimeout .
1872 attempts to send authentication success and failure messages
1878 For forward compatibility with an upcoming
1882 alias can be used instead.
1886 should look up the remote host name, and to check that
1887 the resolved host name for the remote IP address maps back to the
1888 very same IP address.
1890 If this option is set to
1892 then only addresses and not host names may be used in
1893 .Pa ~/.ssh/authorized_keys
1903 Enables the Pluggable Authentication Module interface.
1906 this will enable PAM authentication using
1907 .Cm KbdInteractiveAuthentication
1909 .Cm PasswordAuthentication
1910 in addition to PAM account and session module processing for all
1911 authentication types.
1913 Because PAM keyboard-interactive authentication usually serves an equivalent
1914 role to password authentication, you should disable either
1915 .Cm PasswordAuthentication
1917 .Cm KbdInteractiveAuthentication .
1921 is enabled, you will not be able to run
1926 .It Cm VersionAddendum
1927 Optionally specifies additional text to append to the SSH protocol banner
1928 sent by the server upon connection.
1930 .Qq FreeBSD-20240104 .
1933 may be used to disable this.
1934 .It Cm X11DisplayOffset
1935 Specifies the first display number available for
1938 This prevents sshd from interfering with real X11 servers.
1940 .It Cm X11Forwarding
1941 Specifies whether X11 forwarding is permitted.
1942 The argument must be
1949 When X11 forwarding is enabled, there may be additional exposure to
1950 the server and to client displays if the
1952 proxy display is configured to listen on the wildcard address (see
1953 .Cm X11UseLocalhost ) ,
1954 though this is not the default.
1955 Additionally, the authentication spoofing and authentication data
1956 verification and substitution occur on the client side.
1957 The security risk of using X11 forwarding is that the client's X11
1958 display server may be exposed to attack when the SSH client requests
1959 forwarding (see the warnings for
1962 .Xr ssh_config 5 ) .
1963 A system administrator may have a stance in which they want to
1964 protect clients that may expose themselves to attack by unwittingly
1965 requesting X11 forwarding, which can warrant a
1969 Note that disabling X11 forwarding does not prevent users from
1970 forwarding X11 traffic, as users can always install their own forwarders.
1971 .It Cm X11UseLocalhost
1974 should bind the X11 forwarding server to the loopback address or to
1975 the wildcard address.
1977 sshd binds the forwarding server to the loopback address and sets the
1978 hostname part of the
1980 environment variable to
1982 This prevents remote hosts from connecting to the proxy display.
1983 However, some older X11 clients may not function with this
1988 to specify that the forwarding server should be bound to the wildcard
1990 The argument must be
1996 .It Cm XAuthLocation
1997 Specifies the full pathname of the
2003 .Pa /usr/local/bin/xauth .
2007 command-line arguments and configuration file options that specify time
2008 may be expressed using a sequence of the form:
2010 .Ar time Op Ar qualifier ,
2014 is a positive integer value and
2016 is one of the following:
2018 .Bl -tag -width Ds -compact -offset indent
2033 Each member of the sequence is added together to calculate
2034 the total time value.
2036 Time format examples:
2038 .Bl -tag -width Ds -compact -offset indent
2040 600 seconds (10 minutes)
2044 1 hour 30 minutes (90 minutes)
2047 Arguments to some keywords can make use of tokens,
2048 which are expanded at runtime:
2050 .Bl -tag -width XXXX -offset indent -compact
2055 Identifies the connection endpoints, containing
2056 four space-separated values: client address, client port number,
2057 server address, and server port number.
2059 The routing domain in which the incoming connection was received.
2061 The fingerprint of the CA key.
2063 The fingerprint of the key or certificate.
2065 The home directory of the user.
2067 The key ID in the certificate.
2069 The base64-encoded CA key.
2071 The base64-encoded key or certificate for authentication.
2073 The serial number of the certificate.
2075 The type of the CA key.
2077 The key or certificate type.
2079 The numeric user ID of the target user.
2084 .Cm AuthorizedKeysCommand
2085 accepts the tokens %%, %C, %D, %f, %h, %k, %t, %U, and %u.
2087 .Cm AuthorizedKeysFile
2088 accepts the tokens %%, %h, %U, and %u.
2090 .Cm AuthorizedPrincipalsCommand
2091 accepts the tokens %%, %C, %D, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
2093 .Cm AuthorizedPrincipalsFile
2094 accepts the tokens %%, %h, %U, and %u.
2097 accepts the tokens %%, %h, %U, and %u.
2100 accepts the token %D.
2103 .It Pa /etc/ssh/sshd_config
2104 Contains configuration data for
2106 This file should be writable by root only, but it is recommended
2107 (though not necessary) that it be world-readable.
2114 OpenSSH is a derivative of the original and free
2115 ssh 1.2.12 release by
2117 .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
2121 removed many bugs, re-added newer features and
2124 contributed the support for SSH protocol versions 1.5 and 2.0.
2128 contributed support for privilege separation.