Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.347
1.1 stevesk 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
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".
11: .\"
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.
15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
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.
24: .\"
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
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33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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1.347 ! jmc 36: .\" $OpenBSD: sshd_config.5,v 1.346 2023/01/17 09:44:48 djm Exp $
! 37: .Dd $Mdocdate: January 17 2023 $
1.1 stevesk 38: .Dt SSHD_CONFIG 5
39: .Os
40: .Sh NAME
41: .Nm sshd_config
1.295 jmc 42: .Nd OpenSSH daemon configuration file
1.1 stevesk 43: .Sh DESCRIPTION
1.53 jmc 44: .Xr sshd 8
1.1 stevesk 45: reads configuration data from
46: .Pa /etc/ssh/sshd_config
47: (or the file specified with
48: .Fl f
49: on the command line).
50: The file contains keyword-argument pairs, one per line.
1.254 benno 51: For each keyword, the first obtained value will be used.
1.1 stevesk 52: Lines starting with
53: .Ql #
54: and empty lines are interpreted as comments.
1.56 dtucker 55: Arguments may optionally be enclosed in double quotes
56: .Pq \&"
57: in order to represent arguments containing spaces.
1.1 stevesk 58: .Pp
59: The possible
60: keywords and their meanings are as follows (note that
61: keywords are case-insensitive and arguments are case-sensitive):
62: .Bl -tag -width Ds
1.30 djm 63: .It Cm AcceptEnv
64: Specifies what environment variables sent by the client will be copied into
65: the session's
66: .Xr environ 7 .
67: See
68: .Cm SendEnv
1.273 djm 69: and
70: .Cm SetEnv
1.30 djm 71: in
72: .Xr ssh_config 5
73: for how to configure the client.
1.219 jmc 74: The
1.197 dtucker 75: .Ev TERM
1.275 djm 76: environment variable is always accepted whenever the client
1.204 djm 77: requests a pseudo-terminal as it is required by the protocol.
1.30 djm 78: Variables are specified by name, which may contain the wildcard characters
1.51 jmc 79: .Ql *
1.30 djm 80: and
81: .Ql \&? .
1.31 djm 82: Multiple environment variables may be separated by whitespace or spread
1.30 djm 83: across multiple
84: .Cm AcceptEnv
85: directives.
1.31 djm 86: Be warned that some environment variables could be used to bypass restricted
1.30 djm 87: user environments.
88: For this reason, care should be taken in the use of this directive.
89: The default is not to accept any environment variables.
1.37 djm 90: .It Cm AddressFamily
91: Specifies which address family should be used by
1.53 jmc 92: .Xr sshd 8 .
1.37 djm 93: Valid arguments are
1.237 jmc 94: .Cm any
95: (the default),
96: .Cm inet
1.52 jmc 97: (use IPv4 only), or
1.237 jmc 98: .Cm inet6
1.37 djm 99: (use IPv6 only).
1.89 jmc 100: .It Cm AllowAgentForwarding
101: Specifies whether
102: .Xr ssh-agent 1
103: forwarding is permitted.
104: The default is
1.237 jmc 105: .Cm yes .
1.89 jmc 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.
1.1 stevesk 109: .It Cm AllowGroups
110: This keyword can be followed by a list of group name patterns, separated
111: by spaces.
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.
1.301 djm 116: The allow/deny groups directives are processed in the following order:
1.54 jmc 117: .Cm DenyGroups ,
118: .Cm AllowGroups .
1.49 jmc 119: .Pp
1.161 jmc 120: See PATTERNS in
1.49 jmc 121: .Xr ssh_config 5
122: for more information on patterns.
1.231 jmc 123: .It Cm AllowStreamLocalForwarding
124: Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.
1.150 djm 125: The available options are
1.237 jmc 126: .Cm yes
127: (the default)
1.150 djm 128: or
1.237 jmc 129: .Cm all
1.231 jmc 130: to allow StreamLocal forwarding,
1.237 jmc 131: .Cm no
1.231 jmc 132: to prevent all StreamLocal forwarding,
1.237 jmc 133: .Cm local
1.150 djm 134: to allow local (from the perspective of
1.151 jmc 135: .Xr ssh 1 )
136: forwarding only or
1.237 jmc 137: .Cm remote
1.150 djm 138: to allow remote forwarding only.
1.231 jmc 139: Note that disabling StreamLocal forwarding does not improve security unless
1.1 stevesk 140: users are also denied shell access, as they can always install their
141: own forwarders.
1.231 jmc 142: .It Cm AllowTcpForwarding
143: Specifies whether TCP forwarding is permitted.
1.175 millert 144: The available options are
1.237 jmc 145: .Cm yes
146: (the default)
1.175 millert 147: or
1.237 jmc 148: .Cm all
1.231 jmc 149: to allow TCP forwarding,
1.237 jmc 150: .Cm no
1.231 jmc 151: to prevent all TCP forwarding,
1.237 jmc 152: .Cm local
1.175 millert 153: to allow local (from the perspective of
154: .Xr ssh 1 )
155: forwarding only or
1.237 jmc 156: .Cm remote
1.175 millert 157: to allow remote forwarding only.
1.231 jmc 158: Note that disabling TCP forwarding does not improve security unless
1.175 millert 159: users are also denied shell access, as they can always install their
160: own forwarders.
1.1 stevesk 161: .It Cm AllowUsers
162: This keyword can be followed by a list of user name patterns, separated
163: by spaces.
1.14 jmc 164: If specified, login is allowed only for user names that
1.1 stevesk 165: match one of the patterns.
166: Only user names are valid; a numerical user ID is not recognized.
167: By default, login is allowed for all users.
168: If the pattern takes the form USER@HOST then USER and HOST
169: are separately checked, restricting logins to particular
170: users from particular hosts.
1.222 jmc 171: HOST criteria may additionally contain addresses to match in CIDR
172: address/masklen format.
1.301 djm 173: The allow/deny users directives are processed in the following order:
1.54 jmc 174: .Cm DenyUsers ,
1.301 djm 175: .Cm AllowUsers .
1.49 jmc 176: .Pp
1.161 jmc 177: See PATTERNS in
1.49 jmc 178: .Xr ssh_config 5
179: for more information on patterns.
1.149 djm 180: .It Cm AuthenticationMethods
181: Specifies the authentication methods that must be successfully completed
182: for a user to be granted access.
1.281 djm 183: This option must be followed by one or more lists of comma-separated
1.224 djm 184: authentication method names, or by the single string
1.237 jmc 185: .Cm any
1.224 djm 186: to indicate the default behaviour of accepting any single authentication
1.225 jmc 187: method.
1.237 jmc 188: If the default is overridden, then successful authentication requires
1.224 djm 189: completion of every method in at least one of these lists.
1.149 djm 190: .Pp
1.237 jmc 191: For example,
192: .Qq publickey,password publickey,keyboard-interactive
1.149 djm 193: would require the user to complete public key authentication, followed by
194: either password or keyboard interactive authentication.
195: Only methods that are next in one or more lists are offered at each stage,
1.237 jmc 196: so for this example it would not be possible to attempt password or
1.149 djm 197: keyboard-interactive authentication before public key.
1.157 markus 198: .Pp
199: For keyboard interactive authentication it is also possible to
200: restrict authentication to a specific device by appending a
201: colon followed by the device identifier
1.237 jmc 202: .Cm bsdauth ,
203: .Cm pam ,
1.157 markus 204: or
1.237 jmc 205: .Cm skey ,
1.157 markus 206: depending on the server configuration.
207: For example,
1.237 jmc 208: .Qq keyboard-interactive:bsdauth
1.157 markus 209: would restrict keyboard interactive authentication to the
1.237 jmc 210: .Cm bsdauth
1.157 markus 211: device.
1.185 djm 212: .Pp
1.237 jmc 213: If the publickey method is listed more than once,
1.185 djm 214: .Xr sshd 8
215: verifies that keys that have been used successfully are not reused for
216: subsequent authentications.
1.237 jmc 217: For example,
218: .Qq publickey,publickey
219: requires successful authentication using two different public keys.
1.181 djm 220: .Pp
1.149 djm 221: Note that each authentication method listed should also be explicitly enabled
222: in the configuration.
1.251 djm 223: .Pp
224: The available authentication methods are:
225: .Qq gssapi-with-mic ,
226: .Qq hostbased ,
227: .Qq keyboard-interactive ,
228: .Qq none
229: (used for access to password-less accounts when
1.266 jmc 230: .Cm PermitEmptyPasswords
1.251 djm 231: is enabled),
232: .Qq password
233: and
234: .Qq publickey .
1.146 djm 235: .It Cm AuthorizedKeysCommand
1.147 jmc 236: Specifies a program to be used to look up the user's public keys.
1.201 djm 237: The program must be owned by root, not writable by group or others and
238: specified by an absolute path.
239: Arguments to
240: .Cm AuthorizedKeysCommand
1.235 jmc 241: accept the tokens described in the
242: .Sx TOKENS
243: section.
244: If no arguments are specified then the username of the target user is used.
1.201 djm 245: .Pp
246: The program should produce on standard output zero or
1.235 jmc 247: more lines of authorized_keys output (see
248: .Sx AUTHORIZED_KEYS
249: in
1.147 jmc 250: .Xr sshd 8 ) .
1.235 jmc 251: .Cm AuthorizedKeysCommand
1.310 djm 252: is tried after the usual
1.146 djm 253: .Cm AuthorizedKeysFile
1.310 djm 254: files and will not be executed if a matching key is found there.
1.237 jmc 255: By default, no
256: .Cm AuthorizedKeysCommand
257: is run.
1.146 djm 258: .It Cm AuthorizedKeysCommandUser
1.237 jmc 259: Specifies the user under whose account the
260: .Cm AuthorizedKeysCommand
261: is run.
1.146 djm 262: It is recommended to use a dedicated user that has no other role on the host
263: than running authorized keys commands.
1.186 djm 264: If
1.181 djm 265: .Cm AuthorizedKeysCommand
1.186 djm 266: is specified but
267: .Cm AuthorizedKeysCommandUser
268: is not, then
269: .Xr sshd 8
270: will refuse to start.
1.1 stevesk 271: .It Cm AuthorizedKeysFile
1.235 jmc 272: Specifies the file that contains the public keys used for user authentication.
1.285 schwarze 273: The format is described in the AUTHORIZED_KEYS FILE FORMAT section of
1.124 djm 274: .Xr sshd 8 .
1.235 jmc 275: Arguments to
1.1 stevesk 276: .Cm AuthorizedKeysFile
1.235 jmc 277: accept the tokens described in the
278: .Sx TOKENS
279: section.
1.1 stevesk 280: After expansion,
281: .Cm AuthorizedKeysFile
282: is taken to be an absolute path or one relative to the user's home
283: directory.
1.133 jmc 284: Multiple files may be listed, separated by whitespace.
1.212 djm 285: Alternately this option may be set to
1.237 jmc 286: .Cm none
1.212 djm 287: to skip checking for user keys in files.
1.133 jmc 288: The default is
1.237 jmc 289: .Qq .ssh/authorized_keys .ssh/authorized_keys2 .
1.202 djm 290: .It Cm AuthorizedPrincipalsCommand
291: Specifies a program to be used to generate the list of allowed
292: certificate principals as per
293: .Cm AuthorizedPrincipalsFile .
294: The program must be owned by root, not writable by group or others and
295: specified by an absolute path.
296: Arguments to
297: .Cm AuthorizedPrincipalsCommand
1.235 jmc 298: accept the tokens described in the
299: .Sx TOKENS
300: section.
301: If no arguments are specified then the username of the target user is used.
1.202 djm 302: .Pp
303: The program should produce on standard output zero or
304: more lines of
305: .Cm AuthorizedPrincipalsFile
306: output.
307: If either
308: .Cm AuthorizedPrincipalsCommand
309: or
310: .Cm AuthorizedPrincipalsFile
311: is specified, then certificates offered by the client for authentication
312: must contain a principal that is listed.
1.235 jmc 313: By default, no
314: .Cm AuthorizedPrincipalsCommand
315: is run.
1.202 djm 316: .It Cm AuthorizedPrincipalsCommandUser
1.235 jmc 317: Specifies the user under whose account the
318: .Cm AuthorizedPrincipalsCommand
319: is run.
1.202 djm 320: It is recommended to use a dedicated user that has no other role on the host
321: than running authorized principals commands.
322: If
323: .Cm AuthorizedPrincipalsCommand
324: is specified but
325: .Cm AuthorizedPrincipalsCommandUser
326: is not, then
327: .Xr sshd 8
328: will refuse to start.
1.121 djm 329: .It Cm AuthorizedPrincipalsFile
330: Specifies a file that lists principal names that are accepted for
331: certificate authentication.
332: When using certificates signed by a key listed in
333: .Cm TrustedUserCAKeys ,
334: this file lists names, one of which must appear in the certificate for it
335: to be accepted for authentication.
1.235 jmc 336: Names are listed one per line preceded by key options (as described in
337: .Sx AUTHORIZED_KEYS FILE FORMAT
338: in
1.125 jmc 339: .Xr sshd 8 ) .
1.124 djm 340: Empty lines and comments starting with
1.121 djm 341: .Ql #
342: are ignored.
343: .Pp
1.235 jmc 344: Arguments to
1.121 djm 345: .Cm AuthorizedPrincipalsFile
1.235 jmc 346: accept the tokens described in the
347: .Sx TOKENS
348: section.
1.121 djm 349: After expansion,
350: .Cm AuthorizedPrincipalsFile
1.235 jmc 351: is taken to be an absolute path or one relative to the user's home directory.
1.138 djm 352: The default is
1.237 jmc 353: .Cm none ,
1.138 djm 354: i.e. not to use a principals file \(en in this case, the username
1.121 djm 355: of the user must appear in a certificate's principals list for it to be
356: accepted.
1.235 jmc 357: .Pp
1.121 djm 358: Note that
359: .Cm AuthorizedPrincipalsFile
360: is only used when authentication proceeds using a CA listed in
361: .Cm TrustedUserCAKeys
362: and is not consulted for certification authorities trusted via
363: .Pa ~/.ssh/authorized_keys ,
364: though the
365: .Cm principals=
366: key option offers a similar facility (see
367: .Xr sshd 8
368: for details).
1.1 stevesk 369: .It Cm Banner
370: The contents of the specified file are sent to the remote user before
371: authentication is allowed.
1.78 djm 372: If the argument is
1.237 jmc 373: .Cm none
1.78 djm 374: then no banner is displayed.
1.1 stevesk 375: By default, no banner is displayed.
1.282 djm 376: .It Cm CASignatureAlgorithms
377: Specifies which algorithms are allowed for signing of certificates
378: by certificate authorities (CAs).
379: The default is:
380: .Bd -literal -offset indent
1.334 djm 381: ssh-ed25519,ecdsa-sha2-nistp256,
382: ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
383: sk-ssh-ed25519@openssh.com,
384: sk-ecdsa-sha2-nistp256@openssh.com,
1.330 djm 385: rsa-sha2-512,rsa-sha2-256
1.282 djm 386: .Ed
1.334 djm 387: .Pp
388: If the specified list begins with a
389: .Sq +
390: character, then the specified algorithms will be appended to the default set
391: instead of replacing them.
392: If the specified list begins with a
393: .Sq -
394: character, then the specified algorithms (including wildcards) will be removed
395: from the default set instead of replacing them.
1.282 djm 396: .Pp
397: Certificates signed using other algorithms will not be accepted for
398: public key or host-based authentication.
1.344 djm 399: .It Cm ChannelTimeout
400: Specifies whether and how quickly
401: .Xr sshd 8
402: should close inactive channels.
1.345 jmc 403: Timeouts are specified as one or more
1.344 djm 404: .Dq type=interval
405: pairs separated by whitespace, where the
406: .Dq type
407: must be a channel type name (as described in the table below), optionally
408: containing wildcard characters.
409: .Pp
410: The timeout value
411: .Dq interval
412: is specified in seconds or may use any of the units documented in the
413: .Sx TIME FORMATS
414: section.
415: For example,
416: .Dq session:*=5m
417: would cause all sessions to terminate after five minutes of inactivity.
418: Specifying a zero value disables the inactivity timeout.
419: .Pp
420: The available channel types include:
421: .Bl -tag -width Ds
422: .It Cm agent-connection
423: Open connections to
424: .Xr ssh-agent 1 .
1.345 jmc 425: .It Cm direct-tcpip , Cm direct-streamlocal@openssh.com
1.344 djm 426: Open TCP or Unix socket (respectively) connections that have
427: been established from a
428: .Xr ssh 1
1.345 jmc 429: local forwarding, i.e.\&
430: .Cm LocalForward
431: or
1.344 djm 432: .Cm DynamicForward .
1.345 jmc 433: .It Cm forwarded-tcpip , Cm forwarded-streamlocal@openssh.com
1.344 djm 434: Open TCP or Unix socket (respectively) connections that have been
435: established to a
436: .Xr sshd 8
437: listening on behalf of a
438: .Xr ssh 1
1.345 jmc 439: remote forwarding, i.e.\&
1.344 djm 440: .Cm RemoteForward .
441: .It Cm session:command
442: Command execution sessions.
443: .It Cm session:shell
444: Interactive shell sessions.
445: .It Cm session:subsystem:...
446: Subsystem sessions, e.g. for
447: .Xr sftp 1 ,
448: which could be identified as
449: .Cm session:subsystem:sftp .
450: .It Cm x11-connection
451: Open X11 forwarding sessions.
452: .El
453: .Pp
1.345 jmc 454: Note that in all the above cases, terminating an inactive session does not
1.344 djm 455: guarantee to remove all resources associated with the session, e.g. shell
456: processes or X11 clients relating to the session may continue to execute.
457: .Pp
1.345 jmc 458: Moreover, terminating an inactive channel or session does not necessarily
1.344 djm 459: close the SSH connection, nor does it prevent a client from
460: requesting another channel of the same type.
461: In particular, expiring an inactive forwarding session does not prevent
462: another identical forwarding from being subsequently created.
1.346 djm 463: See also
464: .Cm UnusedConnectionTimeout ,
465: which may be used in conjunction with this option.
1.344 djm 466: .Pp
467: The default is not to expire channels of any type for inactivity.
1.80 djm 468: .It Cm ChrootDirectory
1.113 stevesk 469: Specifies the pathname of a directory to
1.80 djm 470: .Xr chroot 2
471: to after authentication.
1.190 deraadt 472: At session startup
473: .Xr sshd 8
474: checks that all components of the pathname are root-owned directories
475: which are not writable by any other user or group.
1.106 stevesk 476: After the chroot,
477: .Xr sshd 8
478: changes the working directory to the user's home directory.
1.235 jmc 479: Arguments to
480: .Cm ChrootDirectory
481: accept the tokens described in the
482: .Sx TOKENS
483: section.
1.80 djm 484: .Pp
485: The
486: .Cm ChrootDirectory
487: must contain the necessary files and directories to support the
1.103 stevesk 488: user's session.
1.80 djm 489: For an interactive session this requires at least a shell, typically
490: .Xr sh 1 ,
491: and basic
492: .Pa /dev
493: nodes such as
494: .Xr null 4 ,
495: .Xr zero 4 ,
496: .Xr stdin 4 ,
497: .Xr stdout 4 ,
498: .Xr stderr 4 ,
1.180 jmc 499: and
1.80 djm 500: .Xr tty 4
501: devices.
1.237 jmc 502: For file transfer sessions using SFTP
503: no additional configuration of the environment is necessary if the in-process
504: sftp-server is used,
1.176 schwarze 505: though sessions which use logging may require
1.104 stevesk 506: .Pa /dev/log
1.176 schwarze 507: inside the chroot directory on some operating systems (see
1.104 stevesk 508: .Xr sftp-server 8
1.81 jmc 509: for details).
1.190 deraadt 510: .Pp
1.191 jmc 511: For safety, it is very important that the directory hierarchy be
1.190 deraadt 512: prevented from modification by other processes on the system (especially
513: those outside the jail).
514: Misconfiguration can lead to unsafe environments which
515: .Xr sshd 8
516: cannot detect.
1.80 djm 517: .Pp
1.215 djm 518: The default is
1.237 jmc 519: .Cm none ,
1.215 djm 520: indicating not to
1.80 djm 521: .Xr chroot 2 .
1.1 stevesk 522: .It Cm Ciphers
1.219 jmc 523: Specifies the ciphers allowed.
1.1 stevesk 524: Multiple ciphers must be comma-separated.
1.289 naddy 525: If the specified list begins with a
1.208 djm 526: .Sq +
527: character, then the specified ciphers will be appended to the default set
528: instead of replacing them.
1.289 naddy 529: If the specified list begins with a
1.242 djm 530: .Sq -
531: character, then the specified ciphers (including wildcards) will be removed
532: from the default set instead of replacing them.
1.290 naddy 533: If the specified list begins with a
534: .Sq ^
535: character, then the specified ciphers will be placed at the head of the
536: default set.
1.208 djm 537: .Pp
1.167 djm 538: The supported ciphers are:
539: .Pp
1.173 naddy 540: .Bl -item -compact -offset indent
541: .It
542: 3des-cbc
543: .It
544: aes128-cbc
545: .It
546: aes192-cbc
547: .It
548: aes256-cbc
549: .It
550: aes128-ctr
551: .It
552: aes192-ctr
553: .It
554: aes256-ctr
555: .It
556: aes128-gcm@openssh.com
557: .It
558: aes256-gcm@openssh.com
559: .It
560: chacha20-poly1305@openssh.com
561: .El
1.167 djm 562: .Pp
1.52 jmc 563: The default is:
1.173 naddy 564: .Bd -literal -offset indent
1.211 jmc 565: chacha20-poly1305@openssh.com,
1.173 naddy 566: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 567: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 568: .Ed
1.167 djm 569: .Pp
1.237 jmc 570: The list of available ciphers may also be obtained using
571: .Qq ssh -Q cipher .
1.1 stevesk 572: .It Cm ClientAliveCountMax
1.237 jmc 573: Sets the number of client alive messages which may be sent without
1.52 jmc 574: .Xr sshd 8
1.17 jmc 575: receiving any messages back from the client.
576: If this threshold is reached while client alive messages are being sent,
1.52 jmc 577: sshd will disconnect the client, terminating the session.
1.17 jmc 578: It is important to note that the use of client alive messages is very
579: different from
1.237 jmc 580: .Cm TCPKeepAlive .
1.17 jmc 581: The client alive messages are sent through the encrypted channel
582: and therefore will not be spoofable.
583: The TCP keepalive option enabled by
1.27 markus 584: .Cm TCPKeepAlive
1.17 jmc 585: is spoofable.
586: The client alive mechanism is valuable when the client or
1.288 dtucker 587: server depend on knowing when a connection has become unresponsive.
1.1 stevesk 588: .Pp
1.17 jmc 589: The default value is 3.
590: If
1.1 stevesk 591: .Cm ClientAliveInterval
1.237 jmc 592: is set to 15, and
1.1 stevesk 593: .Cm ClientAliveCountMax
1.52 jmc 594: is left at the default, unresponsive SSH clients
1.1 stevesk 595: will be disconnected after approximately 45 seconds.
1.302 djm 596: Setting a zero
597: .Cm ClientAliveCountMax
598: disables connection termination.
1.42 djm 599: .It Cm ClientAliveInterval
600: Sets a timeout interval in seconds after which if no data has been received
601: from the client,
1.52 jmc 602: .Xr sshd 8
1.42 djm 603: will send a message through the encrypted
604: channel to request a response from the client.
605: The default
606: is 0, indicating that these messages will not be sent to the client.
1.3 markus 607: .It Cm Compression
1.236 djm 608: Specifies whether compression is enabled after
1.44 markus 609: the user has authenticated successfully.
1.3 markus 610: The argument must be
1.237 jmc 611: .Cm yes ,
612: .Cm delayed
1.236 djm 613: (a legacy synonym for
1.237 jmc 614: .Cm yes )
1.3 markus 615: or
1.237 jmc 616: .Cm no .
1.3 markus 617: The default is
1.237 jmc 618: .Cm yes .
1.1 stevesk 619: .It Cm DenyGroups
620: This keyword can be followed by a list of group name patterns, separated
621: by spaces.
622: Login is disallowed for users whose primary group or supplementary
623: group list matches one of the patterns.
624: Only group names are valid; a numerical group ID is not recognized.
625: By default, login is allowed for all groups.
1.301 djm 626: The allow/deny groups directives are processed in the following order:
1.54 jmc 627: .Cm DenyGroups ,
628: .Cm AllowGroups .
1.49 jmc 629: .Pp
1.161 jmc 630: See PATTERNS in
1.49 jmc 631: .Xr ssh_config 5
632: for more information on patterns.
1.1 stevesk 633: .It Cm DenyUsers
634: This keyword can be followed by a list of user name patterns, separated
635: by spaces.
636: Login is disallowed for user names that match one of the patterns.
637: Only user names are valid; a numerical user ID is not recognized.
638: By default, login is allowed for all users.
639: If the pattern takes the form USER@HOST then USER and HOST
640: are separately checked, restricting logins to particular
641: users from particular hosts.
1.222 jmc 642: HOST criteria may additionally contain addresses to match in CIDR
643: address/masklen format.
1.301 djm 644: The allow/deny users directives are processed in the following order:
1.54 jmc 645: .Cm DenyUsers ,
1.301 djm 646: .Cm AllowUsers .
1.49 jmc 647: .Pp
1.161 jmc 648: See PATTERNS in
1.49 jmc 649: .Xr ssh_config 5
650: for more information on patterns.
1.239 djm 651: .It Cm DisableForwarding
652: Disables all forwarding features, including X11,
653: .Xr ssh-agent 1 ,
654: TCP and StreamLocal.
655: This option overrides all other forwarding-related options and may
656: simplify restricted configurations.
1.246 djm 657: .It Cm ExposeAuthInfo
1.253 jmc 658: Writes a temporary file containing a list of authentication methods and
1.246 djm 659: public credentials (e.g. keys) used to authenticate the user.
1.247 jmc 660: The location of the file is exposed to the user session through the
1.248 djm 661: .Ev SSH_USER_AUTH
1.247 jmc 662: environment variable.
1.253 jmc 663: The default is
664: .Cm no .
1.183 djm 665: .It Cm FingerprintHash
666: Specifies the hash algorithm used when logging key fingerprints.
667: Valid options are:
1.237 jmc 668: .Cm md5
1.183 djm 669: and
1.237 jmc 670: .Cm sha256 .
1.183 djm 671: The default is
1.237 jmc 672: .Cm sha256 .
1.67 dtucker 673: .It Cm ForceCommand
674: Forces the execution of the command specified by
675: .Cm ForceCommand ,
1.84 djm 676: ignoring any command supplied by the client and
677: .Pa ~/.ssh/rc
678: if present.
1.67 dtucker 679: The command is invoked by using the user's login shell with the -c option.
680: This applies to shell, command, or subsystem execution.
681: It is most useful inside a
682: .Cm Match
683: block.
684: The command originally supplied by the client is available in the
685: .Ev SSH_ORIGINAL_COMMAND
686: environment variable.
1.82 djm 687: Specifying a command of
1.237 jmc 688: .Cm internal-sftp
689: will force the use of an in-process SFTP server that requires no support
1.82 djm 690: files when used with
691: .Cm ChrootDirectory .
1.215 djm 692: The default is
1.237 jmc 693: .Cm none .
1.1 stevesk 694: .It Cm GatewayPorts
695: Specifies whether remote hosts are allowed to connect to ports
696: forwarded for the client.
697: By default,
1.52 jmc 698: .Xr sshd 8
1.15 jmc 699: binds remote port forwardings to the loopback address.
700: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 701: .Cm GatewayPorts
1.52 jmc 702: can be used to specify that sshd
1.39 djm 703: should allow remote port forwardings to bind to non-loopback addresses, thus
704: allowing other hosts to connect.
705: The argument may be
1.237 jmc 706: .Cm no
1.39 djm 707: to force remote port forwardings to be available to the local host only,
1.237 jmc 708: .Cm yes
1.39 djm 709: to force remote port forwardings to bind to the wildcard address, or
1.237 jmc 710: .Cm clientspecified
1.39 djm 711: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 712: The default is
1.237 jmc 713: .Cm no .
1.23 markus 714: .It Cm GSSAPIAuthentication
1.25 markus 715: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 716: The default is
1.237 jmc 717: .Cm no .
1.23 markus 718: .It Cm GSSAPICleanupCredentials
719: Specifies whether to automatically destroy the user's credentials cache
720: on logout.
721: The default is
1.237 jmc 722: .Cm yes .
1.203 djm 723: .It Cm GSSAPIStrictAcceptorCheck
724: Determines whether to be strict about the identity of the GSSAPI acceptor
725: a client authenticates against.
726: If set to
1.237 jmc 727: .Cm yes
728: then the client must authenticate against the host
1.203 djm 729: service on the current hostname.
730: If set to
1.237 jmc 731: .Cm no
1.203 djm 732: then the client may authenticate against any service key stored in the
733: machine's default store.
734: This facility is provided to assist with operation on multi homed machines.
735: The default is
1.237 jmc 736: .Cm yes .
1.325 dtucker 737: .It Cm HostbasedAcceptedAlgorithms
1.327 djm 738: Specifies the signature algorithms that will be accepted for hostbased
739: authentication as a list of comma-separated patterns.
1.289 naddy 740: Alternately if the specified list begins with a
1.208 djm 741: .Sq +
1.327 djm 742: character, then the specified signature algorithms will be appended to
743: the default set instead of replacing them.
1.289 naddy 744: If the specified list begins with a
1.242 djm 745: .Sq -
1.327 djm 746: character, then the specified signature algorithms (including wildcards)
747: will be removed from the default set instead of replacing them.
1.290 naddy 748: If the specified list begins with a
749: .Sq ^
1.327 djm 750: character, then the specified signature algorithms will be placed at
751: the head of the default set.
1.206 markus 752: The default for this option is:
753: .Bd -literal -offset 3n
1.316 djm 754: ssh-ed25519-cert-v01@openssh.com,
1.206 markus 755: ecdsa-sha2-nistp256-cert-v01@openssh.com,
756: ecdsa-sha2-nistp384-cert-v01@openssh.com,
757: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.316 djm 758: sk-ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 759: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.291 naddy 760: rsa-sha2-512-cert-v01@openssh.com,
761: rsa-sha2-256-cert-v01@openssh.com,
1.316 djm 762: ssh-ed25519,
1.206 markus 763: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.316 djm 764: sk-ssh-ed25519@openssh.com,
1.296 naddy 765: sk-ecdsa-sha2-nistp256@openssh.com,
1.339 naddy 766: rsa-sha2-512,rsa-sha2-256
1.206 markus 767: .Ed
768: .Pp
1.327 djm 769: The list of available signature algorithms may also be obtained using
1.325 dtucker 770: .Qq ssh -Q HostbasedAcceptedAlgorithms .
771: This was formerly named HostbasedAcceptedKeyTypes.
1.1 stevesk 772: .It Cm HostbasedAuthentication
773: Specifies whether rhosts or /etc/hosts.equiv authentication together
774: with successful public key client host authentication is allowed
1.50 jmc 775: (host-based authentication).
1.70 dtucker 776: The default is
1.237 jmc 777: .Cm no .
1.70 dtucker 778: .It Cm HostbasedUsesNameFromPacketOnly
779: Specifies whether or not the server will attempt to perform a reverse
780: name lookup when matching the name in the
781: .Pa ~/.shosts ,
782: .Pa ~/.rhosts ,
783: and
784: .Pa /etc/hosts.equiv
785: files during
786: .Cm HostbasedAuthentication .
787: A setting of
1.237 jmc 788: .Cm yes
1.70 dtucker 789: means that
790: .Xr sshd 8
791: uses the name supplied by the client rather than
792: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 793: The default is
1.237 jmc 794: .Cm no .
1.117 djm 795: .It Cm HostCertificate
796: Specifies a file containing a public host certificate.
797: The certificate's public key must match a private host key already specified
798: by
799: .Cm HostKey .
800: The default behaviour of
801: .Xr sshd 8
802: is not to load any certificates.
1.1 stevesk 803: .It Cm HostKey
804: Specifies a file containing a private host key
805: used by SSH.
1.229 naddy 806: The defaults are
1.169 naddy 807: .Pa /etc/ssh/ssh_host_ecdsa_key ,
808: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 809: and
1.229 naddy 810: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 811: .Pp
1.1 stevesk 812: Note that
1.52 jmc 813: .Xr sshd 8
1.206 markus 814: will refuse to use a file if it is group/world-accessible
815: and that the
816: .Cm HostKeyAlgorithms
817: option restricts which of the keys are actually used by
818: .Xr sshd 8 .
819: .Pp
1.1 stevesk 820: It is possible to have multiple host key files.
1.162 markus 821: It is also possible to specify public host key files instead.
822: In this case operations on the private key will be delegated
823: to an
824: .Xr ssh-agent 1 .
825: .It Cm HostKeyAgent
826: Identifies the UNIX-domain socket used to communicate
827: with an agent that has access to the private host keys.
1.223 markus 828: If the string
1.237 jmc 829: .Qq SSH_AUTH_SOCK
1.162 markus 830: is specified, the location of the socket will be read from the
831: .Ev SSH_AUTH_SOCK
832: environment variable.
1.206 markus 833: .It Cm HostKeyAlgorithms
1.327 djm 834: Specifies the host key signature algorithms
1.206 markus 835: that the server offers.
836: The default for this option is:
837: .Bd -literal -offset 3n
1.316 djm 838: ssh-ed25519-cert-v01@openssh.com,
1.206 markus 839: ecdsa-sha2-nistp256-cert-v01@openssh.com,
840: ecdsa-sha2-nistp384-cert-v01@openssh.com,
841: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.316 djm 842: sk-ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 843: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.291 naddy 844: rsa-sha2-512-cert-v01@openssh.com,
845: rsa-sha2-256-cert-v01@openssh.com,
1.316 djm 846: ssh-ed25519,
1.206 markus 847: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.316 djm 848: sk-ssh-ed25519@openssh.com,
1.296 naddy 849: sk-ecdsa-sha2-nistp256@openssh.com,
1.339 naddy 850: rsa-sha2-512,rsa-sha2-256
1.206 markus 851: .Ed
852: .Pp
1.327 djm 853: The list of available signature algorithms may also be obtained using
1.307 dtucker 854: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 855: .It Cm IgnoreRhosts
1.309 djm 856: Specifies whether to ignore per-user
1.1 stevesk 857: .Pa .rhosts
858: and
859: .Pa .shosts
1.309 djm 860: files during
1.1 stevesk 861: .Cm HostbasedAuthentication .
1.309 djm 862: The system-wide
1.1 stevesk 863: .Pa /etc/hosts.equiv
864: and
865: .Pa /etc/shosts.equiv
1.309 djm 866: are still used regardless of this setting.
867: .Pp
868: Accepted values are
869: .Cm yes
870: (the default) to ignore all per-user files,
871: .Cm shosts-only
872: to allow the use of
873: .Pa .shosts
874: but to ignore
875: .Pa .rhosts
876: or
877: .Cm no
878: to allow both
879: .Pa .shosts
880: and
1.311 jmc 881: .Pa rhosts .
1.1 stevesk 882: .It Cm IgnoreUserKnownHosts
883: Specifies whether
1.52 jmc 884: .Xr sshd 8
1.1 stevesk 885: should ignore the user's
1.41 djm 886: .Pa ~/.ssh/known_hosts
1.1 stevesk 887: during
1.261 djm 888: .Cm HostbasedAuthentication
889: and use only the system-wide known hosts file
1.341 dtucker 890: .Pa /etc/ssh/ssh_known_hosts .
1.1 stevesk 891: The default is
1.304 djm 892: .Dq no .
893: .It Cm Include
894: Include the specified configuration file(s).
1.305 jmc 895: Multiple pathnames may be specified and each pathname may contain
1.304 djm 896: .Xr glob 7
1.312 djm 897: wildcards that will be expanded and processed in lexical order.
1.304 djm 898: Files without absolute paths are assumed to be in
899: .Pa /etc/ssh .
1.305 jmc 900: An
1.304 djm 901: .Cm Include
902: directive may appear inside a
903: .Cm Match
904: block
905: to perform conditional inclusion.
1.129 djm 906: .It Cm IPQoS
907: Specifies the IPv4 type-of-service or DSCP class for the connection.
908: Accepted values are
1.237 jmc 909: .Cm af11 ,
910: .Cm af12 ,
911: .Cm af13 ,
912: .Cm af21 ,
913: .Cm af22 ,
914: .Cm af23 ,
915: .Cm af31 ,
916: .Cm af32 ,
917: .Cm af33 ,
918: .Cm af41 ,
919: .Cm af42 ,
920: .Cm af43 ,
921: .Cm cs0 ,
922: .Cm cs1 ,
923: .Cm cs2 ,
924: .Cm cs3 ,
925: .Cm cs4 ,
926: .Cm cs5 ,
927: .Cm cs6 ,
928: .Cm cs7 ,
929: .Cm ef ,
1.303 djm 930: .Cm le ,
1.237 jmc 931: .Cm lowdelay ,
932: .Cm throughput ,
933: .Cm reliability ,
1.250 djm 934: a numeric value, or
935: .Cm none
936: to use the operating system default.
1.131 djm 937: This option may take one or two arguments, separated by whitespace.
1.129 djm 938: If one argument is specified, it is used as the packet class unconditionally.
939: If two values are specified, the first is automatically selected for
940: interactive sessions and the second for non-interactive sessions.
941: The default is
1.264 job 942: .Cm af21
1.265 jmc 943: (Low-Latency Data)
1.129 djm 944: for interactive sessions and
1.264 job 945: .Cm cs1
1.265 jmc 946: (Lower Effort)
1.129 djm 947: for non-interactive sessions.
1.171 jmc 948: .It Cm KbdInteractiveAuthentication
949: Specifies whether to allow keyboard-interactive authentication.
1.332 dtucker 950: All authentication styles from
951: .Xr login.conf 5
952: are supported.
953: The default is
954: .Cm yes .
1.171 jmc 955: The argument to this keyword must be
1.237 jmc 956: .Cm yes
1.171 jmc 957: or
1.237 jmc 958: .Cm no .
1.171 jmc 959: .Cm ChallengeResponseAuthentication
1.332 dtucker 960: is a deprecated alias for this.
1.1 stevesk 961: .It Cm KerberosAuthentication
1.24 markus 962: Specifies whether the password provided by the user for
1.1 stevesk 963: .Cm PasswordAuthentication
1.24 markus 964: will be validated through the Kerberos KDC.
1.1 stevesk 965: To use this option, the server needs a
966: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 967: The default is
1.237 jmc 968: .Cm no .
1.29 dtucker 969: .It Cm KerberosGetAFSToken
1.45 djm 970: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 971: an AFS token before accessing the user's home directory.
1.52 jmc 972: The default is
1.237 jmc 973: .Cm no .
1.1 stevesk 974: .It Cm KerberosOrLocalPasswd
1.52 jmc 975: If password authentication through Kerberos fails then
1.1 stevesk 976: the password will be validated via any additional local mechanism
977: such as
978: .Pa /etc/passwd .
1.52 jmc 979: The default is
1.237 jmc 980: .Cm yes .
1.1 stevesk 981: .It Cm KerberosTicketCleanup
982: Specifies whether to automatically destroy the user's ticket cache
983: file on logout.
1.52 jmc 984: The default is
1.237 jmc 985: .Cm yes .
1.127 djm 986: .It Cm KexAlgorithms
987: Specifies the available KEX (Key Exchange) algorithms.
988: Multiple algorithms must be comma-separated.
1.289 naddy 989: Alternately if the specified list begins with a
1.208 djm 990: .Sq +
1.335 dtucker 991: character, then the specified algorithms will be appended to the default set
1.208 djm 992: instead of replacing them.
1.289 naddy 993: If the specified list begins with a
1.242 djm 994: .Sq -
1.335 dtucker 995: character, then the specified algorithms (including wildcards) will be removed
1.242 djm 996: from the default set instead of replacing them.
1.290 naddy 997: If the specified list begins with a
998: .Sq ^
1.335 dtucker 999: character, then the specified algorithms will be placed at the head of the
1.290 naddy 1000: default set.
1.173 naddy 1001: The supported algorithms are:
1002: .Pp
1003: .Bl -item -compact -offset indent
1004: .It
1.234 djm 1005: curve25519-sha256
1006: .It
1.173 naddy 1007: curve25519-sha256@libssh.org
1008: .It
1009: diffie-hellman-group1-sha1
1010: .It
1011: diffie-hellman-group14-sha1
1012: .It
1.263 djm 1013: diffie-hellman-group14-sha256
1014: .It
1015: diffie-hellman-group16-sha512
1016: .It
1017: diffie-hellman-group18-sha512
1018: .It
1.173 naddy 1019: diffie-hellman-group-exchange-sha1
1020: .It
1021: diffie-hellman-group-exchange-sha256
1022: .It
1023: ecdh-sha2-nistp256
1024: .It
1025: ecdh-sha2-nistp384
1026: .It
1027: ecdh-sha2-nistp521
1.298 dtucker 1028: .It
1.319 djm 1029: sntrup761x25519-sha512@openssh.com
1.173 naddy 1030: .El
1031: .Pp
1032: The default is:
1.170 dtucker 1033: .Bd -literal -offset indent
1.340 naddy 1034: sntrup761x25519-sha512@openssh.com,
1.234 djm 1035: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 1036: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1037: diffie-hellman-group-exchange-sha256,
1.263 djm 1038: diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
1.300 tedu 1039: diffie-hellman-group14-sha256
1.170 dtucker 1040: .Ed
1.188 djm 1041: .Pp
1.237 jmc 1042: The list of available key exchange algorithms may also be obtained using
1.307 dtucker 1043: .Qq ssh -Q KexAlgorithms .
1.1 stevesk 1044: .It Cm ListenAddress
1045: Specifies the local addresses
1.52 jmc 1046: .Xr sshd 8
1.1 stevesk 1047: should listen on.
1048: The following forms may be used:
1049: .Pp
1050: .Bl -item -offset indent -compact
1051: .It
1052: .Cm ListenAddress
1053: .Sm off
1.259 jmc 1054: .Ar hostname | address
1.1 stevesk 1055: .Sm on
1.260 jmc 1056: .Op Cm rdomain Ar domain
1.1 stevesk 1057: .It
1058: .Cm ListenAddress
1059: .Sm off
1.259 jmc 1060: .Ar hostname : port
1.1 stevesk 1061: .Sm on
1.260 jmc 1062: .Op Cm rdomain Ar domain
1.256 djm 1063: .It
1064: .Cm ListenAddress
1065: .Sm off
1.259 jmc 1066: .Ar IPv4_address : port
1.256 djm 1067: .Sm on
1.260 jmc 1068: .Op Cm rdomain Ar domain
1.1 stevesk 1069: .It
1070: .Cm ListenAddress
1071: .Sm off
1.259 jmc 1072: .Oo Ar hostname | address Oc : Ar port
1.1 stevesk 1073: .Sm on
1.260 jmc 1074: .Op Cm rdomain Ar domain
1.1 stevesk 1075: .El
1076: .Pp
1.256 djm 1077: The optional
1078: .Cm rdomain
1079: qualifier requests
1080: .Xr sshd 8
1081: listen in an explicit routing domain.
1.1 stevesk 1082: If
1083: .Ar port
1084: is not specified,
1.200 dtucker 1085: sshd will listen on the address and all
1.1 stevesk 1086: .Cm Port
1.17 jmc 1087: options specified.
1.256 djm 1088: The default is to listen on all local addresses on the current default
1089: routing domain.
1.15 jmc 1090: Multiple
1.1 stevesk 1091: .Cm ListenAddress
1.17 jmc 1092: options are permitted.
1.256 djm 1093: For more information on routing domains, see
1.259 jmc 1094: .Xr rdomain 4 .
1.1 stevesk 1095: .It Cm LoginGraceTime
1096: The server disconnects after this time if the user has not
1097: successfully logged in.
1098: If the value is 0, there is no time limit.
1.12 stevesk 1099: The default is 120 seconds.
1.1 stevesk 1100: .It Cm LogLevel
1101: Gives the verbosity level that is used when logging messages from
1.53 jmc 1102: .Xr sshd 8 .
1.1 stevesk 1103: The possible values are:
1.52 jmc 1104: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 1105: The default is INFO.
1106: DEBUG and DEBUG1 are equivalent.
1107: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
1108: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.317 djm 1109: .It Cm LogVerbose
1110: Specify one or more overrides to LogLevel.
1111: An override consists of a pattern lists that matches the source file, function
1112: and line number to force detailed logging for.
1113: For example, an override pattern of:
1114: .Bd -literal -offset indent
1115: kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1116: .Ed
1117: .Pp
1118: would enable detailed logging for line 1000 of
1.318 jmc 1119: .Pa kex.c ,
1.317 djm 1120: everything in the
1121: .Fn kex_exchange_identification
1122: function, and all code in the
1123: .Pa packet.c
1124: file.
1125: This option is intended for debugging and no overrides are enabled by default.
1.1 stevesk 1126: .It Cm MACs
1127: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 1128: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1129: Multiple algorithms must be comma-separated.
1.289 naddy 1130: If the specified list begins with a
1.208 djm 1131: .Sq +
1132: character, then the specified algorithms will be appended to the default set
1133: instead of replacing them.
1.289 naddy 1134: If the specified list begins with a
1.242 djm 1135: .Sq -
1136: character, then the specified algorithms (including wildcards) will be removed
1137: from the default set instead of replacing them.
1.290 naddy 1138: If the specified list begins with a
1139: .Sq ^
1140: character, then the specified algorithms will be placed at the head of the
1141: default set.
1.208 djm 1142: .Pp
1.152 markus 1143: The algorithms that contain
1.237 jmc 1144: .Qq -etm
1.152 markus 1145: calculate the MAC after encryption (encrypt-then-mac).
1146: These are considered safer and their use recommended.
1.173 naddy 1147: The supported MACs are:
1148: .Pp
1149: .Bl -item -compact -offset indent
1150: .It
1151: hmac-md5
1152: .It
1153: hmac-md5-96
1154: .It
1155: hmac-sha1
1156: .It
1157: hmac-sha1-96
1158: .It
1159: hmac-sha2-256
1160: .It
1161: hmac-sha2-512
1162: .It
1163: umac-64@openssh.com
1164: .It
1165: umac-128@openssh.com
1166: .It
1167: hmac-md5-etm@openssh.com
1168: .It
1169: hmac-md5-96-etm@openssh.com
1170: .It
1171: hmac-sha1-etm@openssh.com
1172: .It
1173: hmac-sha1-96-etm@openssh.com
1174: .It
1175: hmac-sha2-256-etm@openssh.com
1176: .It
1177: hmac-sha2-512-etm@openssh.com
1178: .It
1179: umac-64-etm@openssh.com
1180: .It
1181: umac-128-etm@openssh.com
1182: .El
1183: .Pp
1.52 jmc 1184: The default is:
1.77 jmc 1185: .Bd -literal -offset indent
1.152 markus 1186: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1187: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1188: hmac-sha1-etm@openssh.com,
1.173 naddy 1189: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1190: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1191: .Ed
1.188 djm 1192: .Pp
1.237 jmc 1193: The list of available MAC algorithms may also be obtained using
1194: .Qq ssh -Q mac .
1.60 dtucker 1195: .It Cm Match
1.61 jmc 1196: Introduces a conditional block.
1.65 dtucker 1197: If all of the criteria on the
1.60 dtucker 1198: .Cm Match
1.65 dtucker 1199: line are satisfied, the keywords on the following lines override those
1200: set in the global section of the config file, until either another
1.60 dtucker 1201: .Cm Match
1.65 dtucker 1202: line or the end of the file.
1.172 djm 1203: If a keyword appears in multiple
1204: .Cm Match
1.177 sobrado 1205: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1206: applied.
1.91 djm 1207: .Pp
1.61 jmc 1208: The arguments to
1.60 dtucker 1209: .Cm Match
1.163 dtucker 1210: are one or more criteria-pattern pairs or the single token
1211: .Cm All
1212: which matches all criteria.
1.60 dtucker 1213: The available criteria are
1214: .Cm User ,
1.69 dtucker 1215: .Cm Group ,
1.60 dtucker 1216: .Cm Host ,
1.139 dtucker 1217: .Cm LocalAddress ,
1218: .Cm LocalPort ,
1.258 djm 1219: .Cm RDomain ,
1.60 dtucker 1220: and
1.258 djm 1221: .Cm Address
1222: (with
1223: .Cm RDomain
1224: representing the
1225: .Xr rdomain 4
1.284 jmc 1226: on which the connection was received).
1.258 djm 1227: .Pp
1.91 djm 1228: The match patterns may consist of single entries or comma-separated
1229: lists and may use the wildcard and negation operators described in the
1.237 jmc 1230: .Sx PATTERNS
1231: section of
1.92 djm 1232: .Xr ssh_config 5 .
1.91 djm 1233: .Pp
1234: The patterns in an
1235: .Cm Address
1236: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1237: address/masklen format,
1238: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1239: Note that the mask length provided must be consistent with the address -
1240: it is an error to specify a mask length that is too long for the address
1.93 jmc 1241: or one with bits set in this host portion of the address.
1.237 jmc 1242: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1243: .Pp
1.60 dtucker 1244: Only a subset of keywords may be used on the lines following a
1245: .Cm Match
1246: keyword.
1247: Available keywords are
1.142 jmc 1248: .Cm AcceptEnv ,
1.99 okan 1249: .Cm AllowAgentForwarding ,
1.142 jmc 1250: .Cm AllowGroups ,
1.193 djm 1251: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1252: .Cm AllowTcpForwarding ,
1.141 markus 1253: .Cm AllowUsers ,
1.149 djm 1254: .Cm AuthenticationMethods ,
1.146 djm 1255: .Cm AuthorizedKeysCommand ,
1256: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1257: .Cm AuthorizedKeysFile ,
1.214 djm 1258: .Cm AuthorizedPrincipalsCommand ,
1259: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1260: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1261: .Cm Banner ,
1.336 dtucker 1262: .Cm CASignatureAlgorithms ,
1.346 djm 1263: .Cm ChannelTimeout ,
1.85 djm 1264: .Cm ChrootDirectory ,
1.238 markus 1265: .Cm ClientAliveCountMax ,
1266: .Cm ClientAliveInterval ,
1.141 markus 1267: .Cm DenyGroups ,
1268: .Cm DenyUsers ,
1.320 djm 1269: .Cm DisableForwarding ,
1.336 dtucker 1270: .Cm ExposeAuthInfo ,
1.67 dtucker 1271: .Cm ForceCommand ,
1.194 djm 1272: .Cm GatewayPorts ,
1.193 djm 1273: .Cm GSSAPIAuthentication ,
1.325 dtucker 1274: .Cm HostbasedAcceptedAlgorithms ,
1.87 djm 1275: .Cm HostbasedAuthentication ,
1.123 djm 1276: .Cm HostbasedUsesNameFromPacketOnly ,
1.308 djm 1277: .Cm IgnoreRhosts ,
1.306 naddy 1278: .Cm Include ,
1.193 djm 1279: .Cm IPQoS ,
1.74 jmc 1280: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1281: .Cm KerberosAuthentication ,
1.245 djm 1282: .Cm LogLevel ,
1.95 dtucker 1283: .Cm MaxAuthTries ,
1.94 dtucker 1284: .Cm MaxSessions ,
1.72 dtucker 1285: .Cm PasswordAuthentication ,
1.97 djm 1286: .Cm PermitEmptyPasswords ,
1.271 djm 1287: .Cm PermitListen ,
1.66 dtucker 1288: .Cm PermitOpen ,
1.79 dtucker 1289: .Cm PermitRootLogin ,
1.164 djm 1290: .Cm PermitTTY ,
1.123 djm 1291: .Cm PermitTunnel ,
1.174 djm 1292: .Cm PermitUserRC ,
1.324 dtucker 1293: .Cm PubkeyAcceptedAlgorithms ,
1.107 dtucker 1294: .Cm PubkeyAuthentication ,
1.336 dtucker 1295: .Cm PubkeyAuthOptions ,
1.159 dtucker 1296: .Cm RekeyLimit ,
1.193 djm 1297: .Cm RevokedKeys ,
1.257 djm 1298: .Cm RDomain ,
1.274 djm 1299: .Cm SetEnv ,
1.193 djm 1300: .Cm StreamLocalBindMask ,
1301: .Cm StreamLocalBindUnlink ,
1302: .Cm TrustedUserCAKeys ,
1.346 djm 1303: .Cm UnusedConnectionTimeout ,
1.66 dtucker 1304: .Cm X11DisplayOffset ,
1.101 djm 1305: .Cm X11Forwarding
1.60 dtucker 1306: and
1.286 jmc 1307: .Cm X11UseLocalhost .
1.33 dtucker 1308: .It Cm MaxAuthTries
1309: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1310: connection.
1311: Once the number of failures reaches half this value,
1312: additional failures are logged.
1313: The default is 6.
1.90 djm 1314: .It Cm MaxSessions
1.216 djm 1315: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1316: sessions permitted per network connection.
1317: Multiple sessions may be established by clients that support connection
1318: multiplexing.
1319: Setting
1320: .Cm MaxSessions
1321: to 1 will effectively disable session multiplexing, whereas setting it to 0
1322: will prevent all shell, login and subsystem sessions while still permitting
1323: forwarding.
1.90 djm 1324: The default is 10.
1.1 stevesk 1325: .It Cm MaxStartups
1326: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1327: SSH daemon.
1.1 stevesk 1328: Additional connections will be dropped until authentication succeeds or the
1329: .Cm LoginGraceTime
1330: expires for a connection.
1.156 dtucker 1331: The default is 10:30:100.
1.1 stevesk 1332: .Pp
1333: Alternatively, random early drop can be enabled by specifying
1334: the three colon separated values
1.237 jmc 1335: start:rate:full (e.g. "10:30:60").
1.53 jmc 1336: .Xr sshd 8
1.237 jmc 1337: will refuse connection attempts with a probability of rate/100 (30%)
1338: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1339: The probability increases linearly and all connection attempts
1.237 jmc 1340: are refused if the number of unauthenticated connections reaches full (60).
1.329 dtucker 1341: .It Cm ModuliFile
1342: Specifies the
1343: .Xr moduli 5
1344: file that contains the Diffie-Hellman groups used for the
1345: .Dq diffie-hellman-group-exchange-sha1
1346: and
1347: .Dq diffie-hellman-group-exchange-sha256
1348: key exchange methods.
1349: The default is
1350: .Pa /etc/moduli .
1.1 stevesk 1351: .It Cm PasswordAuthentication
1352: Specifies whether password authentication is allowed.
1353: The default is
1.237 jmc 1354: .Cm yes .
1.1 stevesk 1355: .It Cm PermitEmptyPasswords
1356: When password authentication is allowed, it specifies whether the
1357: server allows login to accounts with empty password strings.
1358: The default is
1.237 jmc 1359: .Cm no .
1.271 djm 1360: .It Cm PermitListen
1361: Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1362: The listen specification must be one of the following forms:
1363: .Pp
1364: .Bl -item -offset indent -compact
1365: .It
1366: .Cm PermitListen
1367: .Sm off
1.276 djm 1368: .Ar port
1.271 djm 1369: .Sm on
1370: .It
1371: .Cm PermitListen
1372: .Sm off
1.276 djm 1373: .Ar host : port
1.271 djm 1374: .Sm on
1375: .El
1376: .Pp
1377: Multiple permissions may be specified by separating them with whitespace.
1378: An argument of
1379: .Cm any
1380: can be used to remove all restrictions and permit any listen requests.
1381: An argument of
1382: .Cm none
1383: can be used to prohibit all listen requests.
1384: The host name may contain wildcards as described in the PATTERNS section in
1385: .Xr ssh_config 5 .
1386: The wildcard
1387: .Sq *
1388: can also be used in place of a port number to allow all ports.
1389: By default all port forwarding listen requests are permitted.
1.272 jmc 1390: Note that the
1.271 djm 1391: .Cm GatewayPorts
1392: option may further restrict which addresses may be listened on.
1.276 djm 1393: Note also that
1394: .Xr ssh 1
1395: will request a listen host of
1396: .Dq localhost
1.283 dtucker 1397: if no listen host was specifically requested, and this name is
1.277 jmc 1398: treated differently to explicit localhost addresses of
1.276 djm 1399: .Dq 127.0.0.1
1400: and
1401: .Dq ::1 .
1.62 dtucker 1402: .It Cm PermitOpen
1403: Specifies the destinations to which TCP port forwarding is permitted.
1404: The forwarding specification must be one of the following forms:
1405: .Pp
1406: .Bl -item -offset indent -compact
1407: .It
1408: .Cm PermitOpen
1409: .Sm off
1410: .Ar host : port
1411: .Sm on
1412: .It
1413: .Cm PermitOpen
1414: .Sm off
1415: .Ar IPv4_addr : port
1416: .Sm on
1417: .It
1418: .Cm PermitOpen
1419: .Sm off
1420: .Ar \&[ IPv6_addr \&] : port
1421: .Sm on
1422: .El
1423: .Pp
1.68 dtucker 1424: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1425: An argument of
1.237 jmc 1426: .Cm any
1.62 dtucker 1427: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1428: An argument of
1.237 jmc 1429: .Cm none
1.140 dtucker 1430: can be used to prohibit all forwarding requests.
1.227 jmc 1431: The wildcard
1.237 jmc 1432: .Sq *
1.299 djm 1433: can be used for host or port to allow all hosts or ports respectively.
1434: Otherwise, no pattern matching or address lookups are performed on supplied
1435: names.
1.63 jmc 1436: By default all port forwarding requests are permitted.
1.1 stevesk 1437: .It Cm PermitRootLogin
1.38 jmc 1438: Specifies whether root can log in using
1.1 stevesk 1439: .Xr ssh 1 .
1440: The argument must be
1.237 jmc 1441: .Cm yes ,
1442: .Cm prohibit-password ,
1443: .Cm forced-commands-only ,
1.1 stevesk 1444: or
1.237 jmc 1445: .Cm no .
1.1 stevesk 1446: The default is
1.237 jmc 1447: .Cm prohibit-password .
1.1 stevesk 1448: .Pp
1449: If this option is set to
1.237 jmc 1450: .Cm prohibit-password
1.255 jmc 1451: (or its deprecated alias,
1452: .Cm without-password ) ,
1.210 deraadt 1453: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1454: .Pp
1455: If this option is set to
1.237 jmc 1456: .Cm forced-commands-only ,
1.1 stevesk 1457: root login with public key authentication will be allowed,
1458: but only if the
1459: .Ar command
1460: option has been specified
1461: (which may be useful for taking remote backups even if root login is
1.17 jmc 1462: normally not allowed).
1463: All other authentication methods are disabled for root.
1.1 stevesk 1464: .Pp
1465: If this option is set to
1.237 jmc 1466: .Cm no ,
1.38 jmc 1467: root is not allowed to log in.
1.231 jmc 1468: .It Cm PermitTTY
1469: Specifies whether
1470: .Xr pty 4
1471: allocation is permitted.
1472: The default is
1.237 jmc 1473: .Cm yes .
1.46 reyk 1474: .It Cm PermitTunnel
1475: Specifies whether
1476: .Xr tun 4
1477: device forwarding is allowed.
1.47 reyk 1478: The argument must be
1.237 jmc 1479: .Cm yes ,
1480: .Cm point-to-point
1.58 stevesk 1481: (layer 3),
1.237 jmc 1482: .Cm ethernet
1.58 stevesk 1483: (layer 2), or
1.237 jmc 1484: .Cm no .
1.58 stevesk 1485: Specifying
1.237 jmc 1486: .Cm yes
1.58 stevesk 1487: permits both
1.237 jmc 1488: .Cm point-to-point
1.58 stevesk 1489: and
1.237 jmc 1490: .Cm ethernet .
1.46 reyk 1491: The default is
1.237 jmc 1492: .Cm no .
1.178 djm 1493: .Pp
1494: Independent of this setting, the permissions of the selected
1495: .Xr tun 4
1496: device must allow access to the user.
1.6 markus 1497: .It Cm PermitUserEnvironment
1498: Specifies whether
1499: .Pa ~/.ssh/environment
1.9 stevesk 1500: and
1.6 markus 1501: .Cm environment=
1502: options in
1503: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1504: are processed by
1.52 jmc 1505: .Xr sshd 8 .
1.278 djm 1506: Valid options are
1507: .Cm yes ,
1508: .Cm no
1509: or a pattern-list specifying which environment variable names to accept
1510: (for example
1511: .Qq LANG,LC_* ) .
1.6 markus 1512: The default is
1.237 jmc 1513: .Cm no .
1.9 stevesk 1514: Enabling environment processing may enable users to bypass access
1515: restrictions in some configurations using mechanisms such as
1516: .Ev LD_PRELOAD .
1.174 djm 1517: .It Cm PermitUserRC
1518: Specifies whether any
1519: .Pa ~/.ssh/rc
1520: file is executed.
1521: The default is
1.237 jmc 1522: .Cm yes .
1.321 dtucker 1523: .It Cm PerSourceMaxStartups
1524: Specifies the number of unauthenticated connections allowed from a
1525: given source address, or
1526: .Dq none
1527: if there is no limit.
1528: This limit is applied in addition to
1529: .Cm MaxStartups ,
1530: whichever is lower.
1531: The default is
1532: .Cm none .
1533: .It Cm PerSourceNetBlockSize
1534: Specifies the number of bits of source address that are grouped together
1535: for the purposes of applying PerSourceMaxStartups limits.
1536: Values for IPv4 and optionally IPv6 may be specified, separated by a colon.
1537: The default is
1.322 jmc 1538: .Cm 32:128 ,
1.321 dtucker 1539: which means each address is considered individually.
1.328 djm 1540: .It Cm PidFile
1541: Specifies the file that contains the process ID of the
1542: SSH daemon, or
1543: .Cm none
1544: to not write one.
1545: The default is
1546: .Pa /var/run/sshd.pid .
1.1 stevesk 1547: .It Cm Port
1548: Specifies the port number that
1.52 jmc 1549: .Xr sshd 8
1.1 stevesk 1550: listens on.
1551: The default is 22.
1552: Multiple options of this type are permitted.
1553: See also
1554: .Cm ListenAddress .
1555: .It Cm PrintLastLog
1556: Specifies whether
1.52 jmc 1557: .Xr sshd 8
1.36 jaredy 1558: should print the date and time of the last user login when a user logs
1559: in interactively.
1.1 stevesk 1560: The default is
1.237 jmc 1561: .Cm yes .
1.1 stevesk 1562: .It Cm PrintMotd
1563: Specifies whether
1.52 jmc 1564: .Xr sshd 8
1.1 stevesk 1565: should print
1566: .Pa /etc/motd
1567: when a user logs in interactively.
1568: (On some systems it is also printed by the shell,
1569: .Pa /etc/profile ,
1570: or equivalent.)
1571: The default is
1.237 jmc 1572: .Cm yes .
1.324 dtucker 1573: .It Cm PubkeyAcceptedAlgorithms
1574: Specifies the signature algorithms that will be accepted for public key
1575: authentication as a list of comma-separated patterns.
1.289 naddy 1576: Alternately if the specified list begins with a
1.208 djm 1577: .Sq +
1.324 dtucker 1578: character, then the specified algorithms will be appended to the default set
1.208 djm 1579: instead of replacing them.
1.289 naddy 1580: If the specified list begins with a
1.242 djm 1581: .Sq -
1.324 dtucker 1582: character, then the specified algorithms (including wildcards) will be removed
1.242 djm 1583: from the default set instead of replacing them.
1.290 naddy 1584: If the specified list begins with a
1585: .Sq ^
1.324 dtucker 1586: character, then the specified algorithms will be placed at the head of the
1.290 naddy 1587: default set.
1.206 markus 1588: The default for this option is:
1589: .Bd -literal -offset 3n
1.316 djm 1590: ssh-ed25519-cert-v01@openssh.com,
1.206 markus 1591: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1592: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1593: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.316 djm 1594: sk-ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 1595: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.291 naddy 1596: rsa-sha2-512-cert-v01@openssh.com,
1597: rsa-sha2-256-cert-v01@openssh.com,
1.316 djm 1598: ssh-ed25519,
1.296 naddy 1599: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.316 djm 1600: sk-ssh-ed25519@openssh.com,
1.291 naddy 1601: sk-ecdsa-sha2-nistp256@openssh.com,
1.338 dtucker 1602: rsa-sha2-512,rsa-sha2-256
1.206 markus 1603: .Ed
1604: .Pp
1.326 djm 1605: The list of available signature algorithms may also be obtained using
1.324 dtucker 1606: .Qq ssh -Q PubkeyAcceptedAlgorithms .
1.293 djm 1607: .It Cm PubkeyAuthOptions
1608: Sets one or more public key authentication options.
1.313 djm 1609: The supported keywords are:
1.294 jmc 1610: .Cm none
1.313 djm 1611: (the default; indicating no additional options are enabled),
1612: .Cm touch-required
1.293 djm 1613: and
1.313 djm 1614: .Cm verify-required .
1.293 djm 1615: .Pp
1616: The
1617: .Cm touch-required
1.297 naddy 1618: option causes public key authentication using a FIDO authenticator algorithm
1.294 jmc 1619: (i.e.\&
1.293 djm 1620: .Cm ecdsa-sk
1621: or
1622: .Cm ed25519-sk )
1623: to always require the signature to attest that a physically present user
1.297 naddy 1624: explicitly confirmed the authentication (usually by touching the authenticator).
1.293 djm 1625: By default,
1626: .Xr sshd 8
1.297 naddy 1627: requires user presence unless overridden with an authorized_keys option.
1.293 djm 1628: The
1629: .Cm touch-required
1630: flag disables this override.
1.313 djm 1631: .Pp
1632: The
1633: .Cm verify-required
1.315 jmc 1634: option requires a FIDO key signature attest that the user was verified,
1.314 jmc 1635: e.g. via a PIN.
1.313 djm 1636: .Pp
1637: Neither the
1638: .Cm touch-required
1639: or
1640: .Cm verify-required
1.314 jmc 1641: options have any effect for other, non-FIDO, public key types.
1.1 stevesk 1642: .It Cm PubkeyAuthentication
1643: Specifies whether public key authentication is allowed.
1644: The default is
1.237 jmc 1645: .Cm yes .
1.159 dtucker 1646: .It Cm RekeyLimit
1.342 djm 1647: Specifies the maximum amount of data that may be transmitted or received
1648: before the session key is renegotiated, optionally followed by a maximum
1649: amount of time that may pass before the session key is renegotiated.
1.159 dtucker 1650: The first argument is specified in bytes and may have a suffix of
1651: .Sq K ,
1652: .Sq M ,
1653: or
1654: .Sq G
1655: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1656: The default is between
1657: .Sq 1G
1658: and
1659: .Sq 4G ,
1660: depending on the cipher.
1661: The optional second value is specified in seconds and may use any of the
1662: units documented in the
1663: .Sx TIME FORMATS
1.160 jmc 1664: section.
1.159 dtucker 1665: The default value for
1666: .Cm RekeyLimit
1667: is
1.237 jmc 1668: .Cm default none ,
1.159 dtucker 1669: which means that rekeying is performed after the cipher's default amount
1670: of data has been sent or received and no time based rekeying is done.
1.343 djm 1671: .It Cm RequiredRSASize
1672: Specifies the minimum RSA key size (in bits) that
1673: .Xr sshd 8
1674: will accept.
1675: User and host-based authentication keys smaller than this limit will be
1676: refused.
1677: The default is
1678: .Cm 1024
1679: bits.
1680: Note that this limit may only be raised from the default.
1.118 djm 1681: .It Cm RevokedKeys
1.195 dtucker 1682: Specifies revoked public keys file, or
1.237 jmc 1683: .Cm none
1.195 dtucker 1684: to not use one.
1.118 djm 1685: Keys listed in this file will be refused for public key authentication.
1686: Note that if this file is not readable, then public key authentication will
1687: be refused for all users.
1.154 djm 1688: Keys may be specified as a text file, listing one public key per line, or as
1689: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1690: .Xr ssh-keygen 1 .
1.161 jmc 1691: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1692: .Xr ssh-keygen 1 .
1.257 djm 1693: .It Cm RDomain
1694: Specifies an explicit routing domain that is applied after authentication
1695: has completed.
1.333 jmc 1696: The user session, as well as any forwarded or listening IP sockets,
1.259 jmc 1697: will be bound to this
1.257 djm 1698: .Xr rdomain 4 .
1699: If the routing domain is set to
1700: .Cm \&%D ,
1.259 jmc 1701: then the domain in which the incoming connection was received will be applied.
1.296 naddy 1702: .It Cm SecurityKeyProvider
1.297 naddy 1703: Specifies a path to a library that will be used when loading
1704: FIDO authenticator-hosted keys, overriding the default of using
1705: the built-in USB HID support.
1.274 djm 1706: .It Cm SetEnv
1707: Specifies one or more environment variables to set in child sessions started
1708: by
1709: .Xr sshd 8
1710: as
1711: .Dq NAME=VALUE .
1712: The environment value may be quoted (e.g. if it contains whitespace
1713: characters).
1714: Environment variables set by
1715: .Cm SetEnv
1716: override the default environment and any variables specified by the user
1717: via
1718: .Cm AcceptEnv
1719: or
1720: .Cm PermitUserEnvironment .
1.175 millert 1721: .It Cm StreamLocalBindMask
1722: Sets the octal file creation mode mask
1723: .Pq umask
1724: used when creating a Unix-domain socket file for local or remote
1725: port forwarding.
1726: This option is only used for port forwarding to a Unix-domain socket file.
1727: .Pp
1728: The default value is 0177, which creates a Unix-domain socket file that is
1729: readable and writable only by the owner.
1730: Note that not all operating systems honor the file mode on Unix-domain
1731: socket files.
1732: .It Cm StreamLocalBindUnlink
1733: Specifies whether to remove an existing Unix-domain socket file for local
1734: or remote port forwarding before creating a new one.
1735: If the socket file already exists and
1736: .Cm StreamLocalBindUnlink
1737: is not enabled,
1738: .Nm sshd
1739: will be unable to forward the port to the Unix-domain socket file.
1740: This option is only used for port forwarding to a Unix-domain socket file.
1741: .Pp
1742: The argument must be
1.237 jmc 1743: .Cm yes
1.175 millert 1744: or
1.237 jmc 1745: .Cm no .
1.175 millert 1746: The default is
1.237 jmc 1747: .Cm no .
1.1 stevesk 1748: .It Cm StrictModes
1749: Specifies whether
1.52 jmc 1750: .Xr sshd 8
1.1 stevesk 1751: should check file modes and ownership of the
1752: user's files and home directory before accepting login.
1753: This is normally desirable because novices sometimes accidentally leave their
1754: directory or files world-writable.
1755: The default is
1.237 jmc 1756: .Cm yes .
1.112 djm 1757: Note that this does not apply to
1758: .Cm ChrootDirectory ,
1759: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1760: .It Cm Subsystem
1.51 jmc 1761: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1762: Arguments should be a subsystem name and a command (with optional arguments)
1763: to execute upon subsystem request.
1.80 djm 1764: .Pp
1.1 stevesk 1765: The command
1.237 jmc 1766: .Cm sftp-server
1767: implements the SFTP file transfer subsystem.
1.80 djm 1768: .Pp
1769: Alternately the name
1.237 jmc 1770: .Cm internal-sftp
1771: implements an in-process SFTP server.
1.80 djm 1772: This may simplify configurations using
1773: .Cm ChrootDirectory
1774: to force a different filesystem root on clients.
1775: .Pp
1.1 stevesk 1776: By default no subsystems are defined.
1777: .It Cm SyslogFacility
1778: Gives the facility code that is used when logging messages from
1.53 jmc 1779: .Xr sshd 8 .
1.1 stevesk 1780: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1781: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1782: The default is AUTH.
1.27 markus 1783: .It Cm TCPKeepAlive
1784: Specifies whether the system should send TCP keepalive messages to the
1785: other side.
1786: If they are sent, death of the connection or crash of one
1787: of the machines will be properly noticed.
1788: However, this means that
1789: connections will die if the route is down temporarily, and some people
1790: find it annoying.
1791: On the other hand, if TCP keepalives are not sent,
1792: sessions may hang indefinitely on the server, leaving
1.237 jmc 1793: .Qq ghost
1.27 markus 1794: users and consuming server resources.
1795: .Pp
1796: The default is
1.237 jmc 1797: .Cm yes
1.27 markus 1798: (to send TCP keepalive messages), and the server will notice
1799: if the network goes down or the client host crashes.
1800: This avoids infinitely hanging sessions.
1801: .Pp
1802: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1803: .Cm no .
1.118 djm 1804: .It Cm TrustedUserCAKeys
1805: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1806: trusted to sign user certificates for authentication, or
1.237 jmc 1807: .Cm none
1.195 dtucker 1808: to not use one.
1.119 jmc 1809: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1810: .Ql #
1811: are allowed.
1812: If a certificate is presented for authentication and has its signing CA key
1813: listed in this file, then it may be used for authentication for any user
1814: listed in the certificate's principals list.
1815: Note that certificates that lack a list of principals will not be permitted
1816: for authentication using
1817: .Cm TrustedUserCAKeys .
1.161 jmc 1818: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1819: .Xr ssh-keygen 1 .
1.346 djm 1820: .It Cm UnusedConnectionTimeout
1821: Specifies whether and how quickly
1822: .Xr sshd 8
1823: should close client connections with no open channels.
1824: Open channels include active shell, command execution or subsystem
1.347 ! jmc 1825: sessions, connected network, socket, agent or X11 forwardings.
1.346 djm 1826: Forwarding listeners, such as those from the
1827: .Xr ssh 1
1828: .Fl R
1.347 ! jmc 1829: flag, are not considered as open channels and do not prevent the timeout.
1.346 djm 1830: The timeout value
1831: is specified in seconds or may use any of the units documented in the
1832: .Sx TIME FORMATS
1833: section.
1834: .Pp
1835: Note that this timeout starts when the client connection completes
1836: user authentication but before the client has an opportunity to open any
1837: channels.
1838: Caution should be used when using short timeout values, as they may not
1839: provide sufficient time for the client to request and open its channels
1840: before terminating the connection.
1841: .Pp
1842: The default
1843: .Cm none
1844: is to never expire connections for having no open channels.
1845: This option may be useful in conjunction with
1846: .Cm ChannelTimeout .
1.18 markus 1847: .It Cm UseDNS
1848: Specifies whether
1.52 jmc 1849: .Xr sshd 8
1.207 djm 1850: should look up the remote host name, and to check that
1.18 markus 1851: the resolved host name for the remote IP address maps back to the
1852: very same IP address.
1.207 djm 1853: .Pp
1854: If this option is set to
1.237 jmc 1855: .Cm no
1.207 djm 1856: (the default) then only addresses and not host names may be used in
1.221 djm 1857: .Pa ~/.ssh/authorized_keys
1.207 djm 1858: .Cm from
1859: and
1.211 jmc 1860: .Nm
1.207 djm 1861: .Cm Match
1862: .Cm Host
1863: directives.
1.137 djm 1864: .It Cm VersionAddendum
1865: Optionally specifies additional text to append to the SSH protocol banner
1866: sent by the server upon connection.
1867: The default is
1.237 jmc 1868: .Cm none .
1.1 stevesk 1869: .It Cm X11DisplayOffset
1870: Specifies the first display number available for
1.52 jmc 1871: .Xr sshd 8 Ns 's
1.1 stevesk 1872: X11 forwarding.
1.52 jmc 1873: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1874: The default is 10.
1875: .It Cm X11Forwarding
1876: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1877: The argument must be
1.237 jmc 1878: .Cm yes
1.13 stevesk 1879: or
1.237 jmc 1880: .Cm no .
1.1 stevesk 1881: The default is
1.237 jmc 1882: .Cm no .
1.13 stevesk 1883: .Pp
1884: When X11 forwarding is enabled, there may be additional exposure to
1885: the server and to client displays if the
1.52 jmc 1886: .Xr sshd 8
1.13 stevesk 1887: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1888: .Cm X11UseLocalhost ) ,
1889: though this is not the default.
1.13 stevesk 1890: Additionally, the authentication spoofing and authentication data
1891: verification and substitution occur on the client side.
1892: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1893: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1894: forwarding (see the warnings for
1895: .Cm ForwardX11
1896: in
1.19 jmc 1897: .Xr ssh_config 5 ) .
1.13 stevesk 1898: A system administrator may have a stance in which they want to
1899: protect clients that may expose themselves to attack by unwittingly
1900: requesting X11 forwarding, which can warrant a
1.237 jmc 1901: .Cm no
1.13 stevesk 1902: setting.
1903: .Pp
1904: Note that disabling X11 forwarding does not prevent users from
1905: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1906: .It Cm X11UseLocalhost
1907: Specifies whether
1.52 jmc 1908: .Xr sshd 8
1.1 stevesk 1909: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1910: the wildcard address.
1911: By default,
1.52 jmc 1912: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1913: hostname part of the
1914: .Ev DISPLAY
1915: environment variable to
1.237 jmc 1916: .Cm localhost .
1.8 stevesk 1917: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1918: However, some older X11 clients may not function with this
1919: configuration.
1920: .Cm X11UseLocalhost
1921: may be set to
1.237 jmc 1922: .Cm no
1.1 stevesk 1923: to specify that the forwarding server should be bound to the wildcard
1924: address.
1925: The argument must be
1.237 jmc 1926: .Cm yes
1.1 stevesk 1927: or
1.237 jmc 1928: .Cm no .
1.1 stevesk 1929: The default is
1.237 jmc 1930: .Cm yes .
1.1 stevesk 1931: .It Cm XAuthLocation
1.11 stevesk 1932: Specifies the full pathname of the
1.1 stevesk 1933: .Xr xauth 1
1.195 dtucker 1934: program, or
1.237 jmc 1935: .Cm none
1.195 dtucker 1936: to not use one.
1.1 stevesk 1937: The default is
1938: .Pa /usr/X11R6/bin/xauth .
1939: .El
1.55 jmc 1940: .Sh TIME FORMATS
1.53 jmc 1941: .Xr sshd 8
1.1 stevesk 1942: command-line arguments and configuration file options that specify time
1943: may be expressed using a sequence of the form:
1944: .Sm off
1.7 stevesk 1945: .Ar time Op Ar qualifier ,
1.1 stevesk 1946: .Sm on
1947: where
1948: .Ar time
1949: is a positive integer value and
1950: .Ar qualifier
1951: is one of the following:
1952: .Pp
1953: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1954: .It Aq Cm none
1.1 stevesk 1955: seconds
1956: .It Cm s | Cm S
1957: seconds
1958: .It Cm m | Cm M
1959: minutes
1960: .It Cm h | Cm H
1961: hours
1962: .It Cm d | Cm D
1963: days
1964: .It Cm w | Cm W
1965: weeks
1966: .El
1967: .Pp
1968: Each member of the sequence is added together to calculate
1969: the total time value.
1970: .Pp
1971: Time format examples:
1972: .Pp
1973: .Bl -tag -width Ds -compact -offset indent
1974: .It 600
1975: 600 seconds (10 minutes)
1976: .It 10m
1977: 10 minutes
1978: .It 1h30m
1979: 1 hour 30 minutes (90 minutes)
1980: .El
1.235 jmc 1981: .Sh TOKENS
1982: Arguments to some keywords can make use of tokens,
1983: which are expanded at runtime:
1984: .Pp
1985: .Bl -tag -width XXXX -offset indent -compact
1986: .It %%
1987: A literal
1988: .Sq % .
1.257 djm 1989: .It \&%D
1990: The routing domain in which the incoming connection was received.
1.235 jmc 1991: .It %F
1992: The fingerprint of the CA key.
1993: .It %f
1994: The fingerprint of the key or certificate.
1995: .It %h
1996: The home directory of the user.
1997: .It %i
1998: The key ID in the certificate.
1999: .It %K
2000: The base64-encoded CA key.
2001: .It %k
2002: The base64-encoded key or certificate for authentication.
2003: .It %s
2004: The serial number of the certificate.
2005: .It \&%T
2006: The type of the CA key.
2007: .It %t
2008: The key or certificate type.
1.268 jmc 2009: .It \&%U
2010: The numeric user ID of the target user.
1.235 jmc 2011: .It %u
2012: The username.
2013: .El
2014: .Pp
2015: .Cm AuthorizedKeysCommand
1.270 jmc 2016: accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1.235 jmc 2017: .Pp
2018: .Cm AuthorizedKeysFile
1.270 jmc 2019: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 2020: .Pp
2021: .Cm AuthorizedPrincipalsCommand
1.270 jmc 2022: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
1.235 jmc 2023: .Pp
2024: .Cm AuthorizedPrincipalsFile
1.270 jmc 2025: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 2026: .Pp
2027: .Cm ChrootDirectory
1.270 jmc 2028: accepts the tokens %%, %h, %U, and %u.
1.257 djm 2029: .Pp
2030: .Cm RoutingDomain
2031: accepts the token %D.
1.1 stevesk 2032: .Sh FILES
2033: .Bl -tag -width Ds
2034: .It Pa /etc/ssh/sshd_config
2035: Contains configuration data for
1.53 jmc 2036: .Xr sshd 8 .
1.1 stevesk 2037: This file should be writable by root only, but it is recommended
2038: (though not necessary) that it be world-readable.
2039: .El
1.19 jmc 2040: .Sh SEE ALSO
1.237 jmc 2041: .Xr sftp-server 8 ,
1.19 jmc 2042: .Xr sshd 8
1.1 stevesk 2043: .Sh AUTHORS
1.237 jmc 2044: .An -nosplit
1.1 stevesk 2045: OpenSSH is a derivative of the original and free
1.237 jmc 2046: ssh 1.2.12 release by
2047: .An Tatu Ylonen .
2048: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
2049: .An Theo de Raadt
2050: and
2051: .An Dug Song
1.1 stevesk 2052: removed many bugs, re-added newer features and
2053: created OpenSSH.
1.237 jmc 2054: .An Markus Friedl
2055: contributed the support for SSH protocol versions 1.5 and 2.0.
2056: .An Niels Provos
2057: and
2058: .An Markus Friedl
2059: contributed support for privilege separation.
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