Annotation of src/usr.bin/ssh/sshd_config.5, Revision 1.344
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
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.
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1.344 ! djm 36: .\" $OpenBSD: sshd_config.5,v 1.343 2022/09/17 10:34:29 djm Exp $
! 37: .Dd $Mdocdate: September 17 2022 $
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.
! 403: Timeouts for specified as one or more
! 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 .
! 425: .It Cm direct-tcpip Cm direct-streamlocal@openssh.com
! 426: Open TCP or Unix socket (respectively) connections that have
! 427: been established from a
! 428: .Xr ssh 1
! 429: local forwarding, i.e.
! 430: .Cm LocalForward or
! 431: .Cm DynamicForward .
! 432: .It Cm forwarded-tcpip Cm forwarded-streamlocal@openssh.com
! 433: Open TCP or Unix socket (respectively) connections that have been
! 434: established to a
! 435: .Xr sshd 8
! 436: listening on behalf of a
! 437: .Xr ssh 1
! 438: remote forwarding, i.e.
! 439: .Cm RemoteForward .
! 440: .It Cm session:command
! 441: Command execution sessions.
! 442: .It Cm session:shell
! 443: Interactive shell sessions.
! 444: .It Cm session:subsystem:...
! 445: Subsystem sessions, e.g. for
! 446: .Xr sftp 1 ,
! 447: which could be identified as
! 448: .Cm session:subsystem:sftp .
! 449: .It Cm x11-connection
! 450: Open X11 forwarding sessions.
! 451: .El
! 452: .Pp
! 453: Note that, in all the above cases, terminating an inactive session does not
! 454: guarantee to remove all resources associated with the session, e.g. shell
! 455: processes or X11 clients relating to the session may continue to execute.
! 456: .Pp
! 457: Moreover, terminating an inactive channel or session does necessarily
! 458: close the SSH connection, nor does it prevent a client from
! 459: requesting another channel of the same type.
! 460: In particular, expiring an inactive forwarding session does not prevent
! 461: another identical forwarding from being subsequently created.
! 462: .Pp
! 463: The default is not to expire channels of any type for inactivity.
1.80 djm 464: .It Cm ChrootDirectory
1.113 stevesk 465: Specifies the pathname of a directory to
1.80 djm 466: .Xr chroot 2
467: to after authentication.
1.190 deraadt 468: At session startup
469: .Xr sshd 8
470: checks that all components of the pathname are root-owned directories
471: which are not writable by any other user or group.
1.106 stevesk 472: After the chroot,
473: .Xr sshd 8
474: changes the working directory to the user's home directory.
1.235 jmc 475: Arguments to
476: .Cm ChrootDirectory
477: accept the tokens described in the
478: .Sx TOKENS
479: section.
1.80 djm 480: .Pp
481: The
482: .Cm ChrootDirectory
483: must contain the necessary files and directories to support the
1.103 stevesk 484: user's session.
1.80 djm 485: For an interactive session this requires at least a shell, typically
486: .Xr sh 1 ,
487: and basic
488: .Pa /dev
489: nodes such as
490: .Xr null 4 ,
491: .Xr zero 4 ,
492: .Xr stdin 4 ,
493: .Xr stdout 4 ,
494: .Xr stderr 4 ,
1.180 jmc 495: and
1.80 djm 496: .Xr tty 4
497: devices.
1.237 jmc 498: For file transfer sessions using SFTP
499: no additional configuration of the environment is necessary if the in-process
500: sftp-server is used,
1.176 schwarze 501: though sessions which use logging may require
1.104 stevesk 502: .Pa /dev/log
1.176 schwarze 503: inside the chroot directory on some operating systems (see
1.104 stevesk 504: .Xr sftp-server 8
1.81 jmc 505: for details).
1.190 deraadt 506: .Pp
1.191 jmc 507: For safety, it is very important that the directory hierarchy be
1.190 deraadt 508: prevented from modification by other processes on the system (especially
509: those outside the jail).
510: Misconfiguration can lead to unsafe environments which
511: .Xr sshd 8
512: cannot detect.
1.80 djm 513: .Pp
1.215 djm 514: The default is
1.237 jmc 515: .Cm none ,
1.215 djm 516: indicating not to
1.80 djm 517: .Xr chroot 2 .
1.1 stevesk 518: .It Cm Ciphers
1.219 jmc 519: Specifies the ciphers allowed.
1.1 stevesk 520: Multiple ciphers must be comma-separated.
1.289 naddy 521: If the specified list begins with a
1.208 djm 522: .Sq +
523: character, then the specified ciphers will be appended to the default set
524: instead of replacing them.
1.289 naddy 525: If the specified list begins with a
1.242 djm 526: .Sq -
527: character, then the specified ciphers (including wildcards) will be removed
528: from the default set instead of replacing them.
1.290 naddy 529: If the specified list begins with a
530: .Sq ^
531: character, then the specified ciphers will be placed at the head of the
532: default set.
1.208 djm 533: .Pp
1.167 djm 534: The supported ciphers are:
535: .Pp
1.173 naddy 536: .Bl -item -compact -offset indent
537: .It
538: 3des-cbc
539: .It
540: aes128-cbc
541: .It
542: aes192-cbc
543: .It
544: aes256-cbc
545: .It
546: aes128-ctr
547: .It
548: aes192-ctr
549: .It
550: aes256-ctr
551: .It
552: aes128-gcm@openssh.com
553: .It
554: aes256-gcm@openssh.com
555: .It
556: chacha20-poly1305@openssh.com
557: .El
1.167 djm 558: .Pp
1.52 jmc 559: The default is:
1.173 naddy 560: .Bd -literal -offset indent
1.211 jmc 561: chacha20-poly1305@openssh.com,
1.173 naddy 562: aes128-ctr,aes192-ctr,aes256-ctr,
1.211 jmc 563: aes128-gcm@openssh.com,aes256-gcm@openssh.com
1.1 stevesk 564: .Ed
1.167 djm 565: .Pp
1.237 jmc 566: The list of available ciphers may also be obtained using
567: .Qq ssh -Q cipher .
1.1 stevesk 568: .It Cm ClientAliveCountMax
1.237 jmc 569: Sets the number of client alive messages which may be sent without
1.52 jmc 570: .Xr sshd 8
1.17 jmc 571: receiving any messages back from the client.
572: If this threshold is reached while client alive messages are being sent,
1.52 jmc 573: sshd will disconnect the client, terminating the session.
1.17 jmc 574: It is important to note that the use of client alive messages is very
575: different from
1.237 jmc 576: .Cm TCPKeepAlive .
1.17 jmc 577: The client alive messages are sent through the encrypted channel
578: and therefore will not be spoofable.
579: The TCP keepalive option enabled by
1.27 markus 580: .Cm TCPKeepAlive
1.17 jmc 581: is spoofable.
582: The client alive mechanism is valuable when the client or
1.288 dtucker 583: server depend on knowing when a connection has become unresponsive.
1.1 stevesk 584: .Pp
1.17 jmc 585: The default value is 3.
586: If
1.1 stevesk 587: .Cm ClientAliveInterval
1.237 jmc 588: is set to 15, and
1.1 stevesk 589: .Cm ClientAliveCountMax
1.52 jmc 590: is left at the default, unresponsive SSH clients
1.1 stevesk 591: will be disconnected after approximately 45 seconds.
1.302 djm 592: Setting a zero
593: .Cm ClientAliveCountMax
594: disables connection termination.
1.42 djm 595: .It Cm ClientAliveInterval
596: Sets a timeout interval in seconds after which if no data has been received
597: from the client,
1.52 jmc 598: .Xr sshd 8
1.42 djm 599: will send a message through the encrypted
600: channel to request a response from the client.
601: The default
602: is 0, indicating that these messages will not be sent to the client.
1.3 markus 603: .It Cm Compression
1.236 djm 604: Specifies whether compression is enabled after
1.44 markus 605: the user has authenticated successfully.
1.3 markus 606: The argument must be
1.237 jmc 607: .Cm yes ,
608: .Cm delayed
1.236 djm 609: (a legacy synonym for
1.237 jmc 610: .Cm yes )
1.3 markus 611: or
1.237 jmc 612: .Cm no .
1.3 markus 613: The default is
1.237 jmc 614: .Cm yes .
1.1 stevesk 615: .It Cm DenyGroups
616: This keyword can be followed by a list of group name patterns, separated
617: by spaces.
618: Login is disallowed for users whose primary group or supplementary
619: group list matches one of the patterns.
620: Only group names are valid; a numerical group ID is not recognized.
621: By default, login is allowed for all groups.
1.301 djm 622: The allow/deny groups directives are processed in the following order:
1.54 jmc 623: .Cm DenyGroups ,
624: .Cm AllowGroups .
1.49 jmc 625: .Pp
1.161 jmc 626: See PATTERNS in
1.49 jmc 627: .Xr ssh_config 5
628: for more information on patterns.
1.1 stevesk 629: .It Cm DenyUsers
630: This keyword can be followed by a list of user name patterns, separated
631: by spaces.
632: Login is disallowed for user names that match one of the patterns.
633: Only user names are valid; a numerical user ID is not recognized.
634: By default, login is allowed for all users.
635: If the pattern takes the form USER@HOST then USER and HOST
636: are separately checked, restricting logins to particular
637: users from particular hosts.
1.222 jmc 638: HOST criteria may additionally contain addresses to match in CIDR
639: address/masklen format.
1.301 djm 640: The allow/deny users directives are processed in the following order:
1.54 jmc 641: .Cm DenyUsers ,
1.301 djm 642: .Cm AllowUsers .
1.49 jmc 643: .Pp
1.161 jmc 644: See PATTERNS in
1.49 jmc 645: .Xr ssh_config 5
646: for more information on patterns.
1.239 djm 647: .It Cm DisableForwarding
648: Disables all forwarding features, including X11,
649: .Xr ssh-agent 1 ,
650: TCP and StreamLocal.
651: This option overrides all other forwarding-related options and may
652: simplify restricted configurations.
1.246 djm 653: .It Cm ExposeAuthInfo
1.253 jmc 654: Writes a temporary file containing a list of authentication methods and
1.246 djm 655: public credentials (e.g. keys) used to authenticate the user.
1.247 jmc 656: The location of the file is exposed to the user session through the
1.248 djm 657: .Ev SSH_USER_AUTH
1.247 jmc 658: environment variable.
1.253 jmc 659: The default is
660: .Cm no .
1.183 djm 661: .It Cm FingerprintHash
662: Specifies the hash algorithm used when logging key fingerprints.
663: Valid options are:
1.237 jmc 664: .Cm md5
1.183 djm 665: and
1.237 jmc 666: .Cm sha256 .
1.183 djm 667: The default is
1.237 jmc 668: .Cm sha256 .
1.67 dtucker 669: .It Cm ForceCommand
670: Forces the execution of the command specified by
671: .Cm ForceCommand ,
1.84 djm 672: ignoring any command supplied by the client and
673: .Pa ~/.ssh/rc
674: if present.
1.67 dtucker 675: The command is invoked by using the user's login shell with the -c option.
676: This applies to shell, command, or subsystem execution.
677: It is most useful inside a
678: .Cm Match
679: block.
680: The command originally supplied by the client is available in the
681: .Ev SSH_ORIGINAL_COMMAND
682: environment variable.
1.82 djm 683: Specifying a command of
1.237 jmc 684: .Cm internal-sftp
685: will force the use of an in-process SFTP server that requires no support
1.82 djm 686: files when used with
687: .Cm ChrootDirectory .
1.215 djm 688: The default is
1.237 jmc 689: .Cm none .
1.1 stevesk 690: .It Cm GatewayPorts
691: Specifies whether remote hosts are allowed to connect to ports
692: forwarded for the client.
693: By default,
1.52 jmc 694: .Xr sshd 8
1.15 jmc 695: binds remote port forwardings to the loopback address.
696: This prevents other remote hosts from connecting to forwarded ports.
1.1 stevesk 697: .Cm GatewayPorts
1.52 jmc 698: can be used to specify that sshd
1.39 djm 699: should allow remote port forwardings to bind to non-loopback addresses, thus
700: allowing other hosts to connect.
701: The argument may be
1.237 jmc 702: .Cm no
1.39 djm 703: to force remote port forwardings to be available to the local host only,
1.237 jmc 704: .Cm yes
1.39 djm 705: to force remote port forwardings to bind to the wildcard address, or
1.237 jmc 706: .Cm clientspecified
1.39 djm 707: to allow the client to select the address to which the forwarding is bound.
1.1 stevesk 708: The default is
1.237 jmc 709: .Cm no .
1.23 markus 710: .It Cm GSSAPIAuthentication
1.25 markus 711: Specifies whether user authentication based on GSSAPI is allowed.
1.26 djm 712: The default is
1.237 jmc 713: .Cm no .
1.23 markus 714: .It Cm GSSAPICleanupCredentials
715: Specifies whether to automatically destroy the user's credentials cache
716: on logout.
717: The default is
1.237 jmc 718: .Cm yes .
1.203 djm 719: .It Cm GSSAPIStrictAcceptorCheck
720: Determines whether to be strict about the identity of the GSSAPI acceptor
721: a client authenticates against.
722: If set to
1.237 jmc 723: .Cm yes
724: then the client must authenticate against the host
1.203 djm 725: service on the current hostname.
726: If set to
1.237 jmc 727: .Cm no
1.203 djm 728: then the client may authenticate against any service key stored in the
729: machine's default store.
730: This facility is provided to assist with operation on multi homed machines.
731: The default is
1.237 jmc 732: .Cm yes .
1.325 dtucker 733: .It Cm HostbasedAcceptedAlgorithms
1.327 djm 734: Specifies the signature algorithms that will be accepted for hostbased
735: authentication as a list of comma-separated patterns.
1.289 naddy 736: Alternately if the specified list begins with a
1.208 djm 737: .Sq +
1.327 djm 738: character, then the specified signature algorithms will be appended to
739: the default set instead of replacing them.
1.289 naddy 740: If the specified list begins with a
1.242 djm 741: .Sq -
1.327 djm 742: character, then the specified signature algorithms (including wildcards)
743: will be removed from the default set instead of replacing them.
1.290 naddy 744: If the specified list begins with a
745: .Sq ^
1.327 djm 746: character, then the specified signature algorithms will be placed at
747: the head of the default set.
1.206 markus 748: The default for this option is:
749: .Bd -literal -offset 3n
1.316 djm 750: ssh-ed25519-cert-v01@openssh.com,
1.206 markus 751: ecdsa-sha2-nistp256-cert-v01@openssh.com,
752: ecdsa-sha2-nistp384-cert-v01@openssh.com,
753: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.316 djm 754: sk-ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 755: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.291 naddy 756: rsa-sha2-512-cert-v01@openssh.com,
757: rsa-sha2-256-cert-v01@openssh.com,
1.316 djm 758: ssh-ed25519,
1.206 markus 759: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.316 djm 760: sk-ssh-ed25519@openssh.com,
1.296 naddy 761: sk-ecdsa-sha2-nistp256@openssh.com,
1.339 naddy 762: rsa-sha2-512,rsa-sha2-256
1.206 markus 763: .Ed
764: .Pp
1.327 djm 765: The list of available signature algorithms may also be obtained using
1.325 dtucker 766: .Qq ssh -Q HostbasedAcceptedAlgorithms .
767: This was formerly named HostbasedAcceptedKeyTypes.
1.1 stevesk 768: .It Cm HostbasedAuthentication
769: Specifies whether rhosts or /etc/hosts.equiv authentication together
770: with successful public key client host authentication is allowed
1.50 jmc 771: (host-based authentication).
1.70 dtucker 772: The default is
1.237 jmc 773: .Cm no .
1.70 dtucker 774: .It Cm HostbasedUsesNameFromPacketOnly
775: Specifies whether or not the server will attempt to perform a reverse
776: name lookup when matching the name in the
777: .Pa ~/.shosts ,
778: .Pa ~/.rhosts ,
779: and
780: .Pa /etc/hosts.equiv
781: files during
782: .Cm HostbasedAuthentication .
783: A setting of
1.237 jmc 784: .Cm yes
1.70 dtucker 785: means that
786: .Xr sshd 8
787: uses the name supplied by the client rather than
788: attempting to resolve the name from the TCP connection itself.
1.1 stevesk 789: The default is
1.237 jmc 790: .Cm no .
1.117 djm 791: .It Cm HostCertificate
792: Specifies a file containing a public host certificate.
793: The certificate's public key must match a private host key already specified
794: by
795: .Cm HostKey .
796: The default behaviour of
797: .Xr sshd 8
798: is not to load any certificates.
1.1 stevesk 799: .It Cm HostKey
800: Specifies a file containing a private host key
801: used by SSH.
1.229 naddy 802: The defaults are
1.169 naddy 803: .Pa /etc/ssh/ssh_host_ecdsa_key ,
804: .Pa /etc/ssh/ssh_host_ed25519_key
1.126 djm 805: and
1.229 naddy 806: .Pa /etc/ssh/ssh_host_rsa_key .
1.206 markus 807: .Pp
1.1 stevesk 808: Note that
1.52 jmc 809: .Xr sshd 8
1.206 markus 810: will refuse to use a file if it is group/world-accessible
811: and that the
812: .Cm HostKeyAlgorithms
813: option restricts which of the keys are actually used by
814: .Xr sshd 8 .
815: .Pp
1.1 stevesk 816: It is possible to have multiple host key files.
1.162 markus 817: It is also possible to specify public host key files instead.
818: In this case operations on the private key will be delegated
819: to an
820: .Xr ssh-agent 1 .
821: .It Cm HostKeyAgent
822: Identifies the UNIX-domain socket used to communicate
823: with an agent that has access to the private host keys.
1.223 markus 824: If the string
1.237 jmc 825: .Qq SSH_AUTH_SOCK
1.162 markus 826: is specified, the location of the socket will be read from the
827: .Ev SSH_AUTH_SOCK
828: environment variable.
1.206 markus 829: .It Cm HostKeyAlgorithms
1.327 djm 830: Specifies the host key signature algorithms
1.206 markus 831: that the server offers.
832: The default for this option is:
833: .Bd -literal -offset 3n
1.316 djm 834: ssh-ed25519-cert-v01@openssh.com,
1.206 markus 835: ecdsa-sha2-nistp256-cert-v01@openssh.com,
836: ecdsa-sha2-nistp384-cert-v01@openssh.com,
837: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.316 djm 838: sk-ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 839: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.291 naddy 840: rsa-sha2-512-cert-v01@openssh.com,
841: rsa-sha2-256-cert-v01@openssh.com,
1.316 djm 842: ssh-ed25519,
1.206 markus 843: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.316 djm 844: sk-ssh-ed25519@openssh.com,
1.296 naddy 845: sk-ecdsa-sha2-nistp256@openssh.com,
1.339 naddy 846: rsa-sha2-512,rsa-sha2-256
1.206 markus 847: .Ed
848: .Pp
1.327 djm 849: The list of available signature algorithms may also be obtained using
1.307 dtucker 850: .Qq ssh -Q HostKeyAlgorithms .
1.1 stevesk 851: .It Cm IgnoreRhosts
1.309 djm 852: Specifies whether to ignore per-user
1.1 stevesk 853: .Pa .rhosts
854: and
855: .Pa .shosts
1.309 djm 856: files during
1.1 stevesk 857: .Cm HostbasedAuthentication .
1.309 djm 858: The system-wide
1.1 stevesk 859: .Pa /etc/hosts.equiv
860: and
861: .Pa /etc/shosts.equiv
1.309 djm 862: are still used regardless of this setting.
863: .Pp
864: Accepted values are
865: .Cm yes
866: (the default) to ignore all per-user files,
867: .Cm shosts-only
868: to allow the use of
869: .Pa .shosts
870: but to ignore
871: .Pa .rhosts
872: or
873: .Cm no
874: to allow both
875: .Pa .shosts
876: and
1.311 jmc 877: .Pa rhosts .
1.1 stevesk 878: .It Cm IgnoreUserKnownHosts
879: Specifies whether
1.52 jmc 880: .Xr sshd 8
1.1 stevesk 881: should ignore the user's
1.41 djm 882: .Pa ~/.ssh/known_hosts
1.1 stevesk 883: during
1.261 djm 884: .Cm HostbasedAuthentication
885: and use only the system-wide known hosts file
1.341 dtucker 886: .Pa /etc/ssh/ssh_known_hosts .
1.1 stevesk 887: The default is
1.304 djm 888: .Dq no .
889: .It Cm Include
890: Include the specified configuration file(s).
1.305 jmc 891: Multiple pathnames may be specified and each pathname may contain
1.304 djm 892: .Xr glob 7
1.312 djm 893: wildcards that will be expanded and processed in lexical order.
1.304 djm 894: Files without absolute paths are assumed to be in
895: .Pa /etc/ssh .
1.305 jmc 896: An
1.304 djm 897: .Cm Include
898: directive may appear inside a
899: .Cm Match
900: block
901: to perform conditional inclusion.
1.129 djm 902: .It Cm IPQoS
903: Specifies the IPv4 type-of-service or DSCP class for the connection.
904: Accepted values are
1.237 jmc 905: .Cm af11 ,
906: .Cm af12 ,
907: .Cm af13 ,
908: .Cm af21 ,
909: .Cm af22 ,
910: .Cm af23 ,
911: .Cm af31 ,
912: .Cm af32 ,
913: .Cm af33 ,
914: .Cm af41 ,
915: .Cm af42 ,
916: .Cm af43 ,
917: .Cm cs0 ,
918: .Cm cs1 ,
919: .Cm cs2 ,
920: .Cm cs3 ,
921: .Cm cs4 ,
922: .Cm cs5 ,
923: .Cm cs6 ,
924: .Cm cs7 ,
925: .Cm ef ,
1.303 djm 926: .Cm le ,
1.237 jmc 927: .Cm lowdelay ,
928: .Cm throughput ,
929: .Cm reliability ,
1.250 djm 930: a numeric value, or
931: .Cm none
932: to use the operating system default.
1.131 djm 933: This option may take one or two arguments, separated by whitespace.
1.129 djm 934: If one argument is specified, it is used as the packet class unconditionally.
935: If two values are specified, the first is automatically selected for
936: interactive sessions and the second for non-interactive sessions.
937: The default is
1.264 job 938: .Cm af21
1.265 jmc 939: (Low-Latency Data)
1.129 djm 940: for interactive sessions and
1.264 job 941: .Cm cs1
1.265 jmc 942: (Lower Effort)
1.129 djm 943: for non-interactive sessions.
1.171 jmc 944: .It Cm KbdInteractiveAuthentication
945: Specifies whether to allow keyboard-interactive authentication.
1.332 dtucker 946: All authentication styles from
947: .Xr login.conf 5
948: are supported.
949: The default is
950: .Cm yes .
1.171 jmc 951: The argument to this keyword must be
1.237 jmc 952: .Cm yes
1.171 jmc 953: or
1.237 jmc 954: .Cm no .
1.171 jmc 955: .Cm ChallengeResponseAuthentication
1.332 dtucker 956: is a deprecated alias for this.
1.1 stevesk 957: .It Cm KerberosAuthentication
1.24 markus 958: Specifies whether the password provided by the user for
1.1 stevesk 959: .Cm PasswordAuthentication
1.24 markus 960: will be validated through the Kerberos KDC.
1.1 stevesk 961: To use this option, the server needs a
962: Kerberos servtab which allows the verification of the KDC's identity.
1.52 jmc 963: The default is
1.237 jmc 964: .Cm no .
1.29 dtucker 965: .It Cm KerberosGetAFSToken
1.45 djm 966: If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
1.29 dtucker 967: an AFS token before accessing the user's home directory.
1.52 jmc 968: The default is
1.237 jmc 969: .Cm no .
1.1 stevesk 970: .It Cm KerberosOrLocalPasswd
1.52 jmc 971: If password authentication through Kerberos fails then
1.1 stevesk 972: the password will be validated via any additional local mechanism
973: such as
974: .Pa /etc/passwd .
1.52 jmc 975: The default is
1.237 jmc 976: .Cm yes .
1.1 stevesk 977: .It Cm KerberosTicketCleanup
978: Specifies whether to automatically destroy the user's ticket cache
979: file on logout.
1.52 jmc 980: The default is
1.237 jmc 981: .Cm yes .
1.127 djm 982: .It Cm KexAlgorithms
983: Specifies the available KEX (Key Exchange) algorithms.
984: Multiple algorithms must be comma-separated.
1.289 naddy 985: Alternately if the specified list begins with a
1.208 djm 986: .Sq +
1.335 dtucker 987: character, then the specified algorithms will be appended to the default set
1.208 djm 988: instead of replacing them.
1.289 naddy 989: If the specified list begins with a
1.242 djm 990: .Sq -
1.335 dtucker 991: character, then the specified algorithms (including wildcards) will be removed
1.242 djm 992: from the default set instead of replacing them.
1.290 naddy 993: If the specified list begins with a
994: .Sq ^
1.335 dtucker 995: character, then the specified algorithms will be placed at the head of the
1.290 naddy 996: default set.
1.173 naddy 997: The supported algorithms are:
998: .Pp
999: .Bl -item -compact -offset indent
1000: .It
1.234 djm 1001: curve25519-sha256
1002: .It
1.173 naddy 1003: curve25519-sha256@libssh.org
1004: .It
1005: diffie-hellman-group1-sha1
1006: .It
1007: diffie-hellman-group14-sha1
1008: .It
1.263 djm 1009: diffie-hellman-group14-sha256
1010: .It
1011: diffie-hellman-group16-sha512
1012: .It
1013: diffie-hellman-group18-sha512
1014: .It
1.173 naddy 1015: diffie-hellman-group-exchange-sha1
1016: .It
1017: diffie-hellman-group-exchange-sha256
1018: .It
1019: ecdh-sha2-nistp256
1020: .It
1021: ecdh-sha2-nistp384
1022: .It
1023: ecdh-sha2-nistp521
1.298 dtucker 1024: .It
1.319 djm 1025: sntrup761x25519-sha512@openssh.com
1.173 naddy 1026: .El
1027: .Pp
1028: The default is:
1.170 dtucker 1029: .Bd -literal -offset indent
1.340 naddy 1030: sntrup761x25519-sha512@openssh.com,
1.234 djm 1031: curve25519-sha256,curve25519-sha256@libssh.org,
1.170 dtucker 1032: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1033: diffie-hellman-group-exchange-sha256,
1.263 djm 1034: diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
1.300 tedu 1035: diffie-hellman-group14-sha256
1.170 dtucker 1036: .Ed
1.188 djm 1037: .Pp
1.237 jmc 1038: The list of available key exchange algorithms may also be obtained using
1.307 dtucker 1039: .Qq ssh -Q KexAlgorithms .
1.1 stevesk 1040: .It Cm ListenAddress
1041: Specifies the local addresses
1.52 jmc 1042: .Xr sshd 8
1.1 stevesk 1043: should listen on.
1044: The following forms may be used:
1045: .Pp
1046: .Bl -item -offset indent -compact
1047: .It
1048: .Cm ListenAddress
1049: .Sm off
1.259 jmc 1050: .Ar hostname | address
1.1 stevesk 1051: .Sm on
1.260 jmc 1052: .Op Cm rdomain Ar domain
1.1 stevesk 1053: .It
1054: .Cm ListenAddress
1055: .Sm off
1.259 jmc 1056: .Ar hostname : port
1.1 stevesk 1057: .Sm on
1.260 jmc 1058: .Op Cm rdomain Ar domain
1.256 djm 1059: .It
1060: .Cm ListenAddress
1061: .Sm off
1.259 jmc 1062: .Ar IPv4_address : port
1.256 djm 1063: .Sm on
1.260 jmc 1064: .Op Cm rdomain Ar domain
1.1 stevesk 1065: .It
1066: .Cm ListenAddress
1067: .Sm off
1.259 jmc 1068: .Oo Ar hostname | address Oc : Ar port
1.1 stevesk 1069: .Sm on
1.260 jmc 1070: .Op Cm rdomain Ar domain
1.1 stevesk 1071: .El
1072: .Pp
1.256 djm 1073: The optional
1074: .Cm rdomain
1075: qualifier requests
1076: .Xr sshd 8
1077: listen in an explicit routing domain.
1.1 stevesk 1078: If
1079: .Ar port
1080: is not specified,
1.200 dtucker 1081: sshd will listen on the address and all
1.1 stevesk 1082: .Cm Port
1.17 jmc 1083: options specified.
1.256 djm 1084: The default is to listen on all local addresses on the current default
1085: routing domain.
1.15 jmc 1086: Multiple
1.1 stevesk 1087: .Cm ListenAddress
1.17 jmc 1088: options are permitted.
1.256 djm 1089: For more information on routing domains, see
1.259 jmc 1090: .Xr rdomain 4 .
1.1 stevesk 1091: .It Cm LoginGraceTime
1092: The server disconnects after this time if the user has not
1093: successfully logged in.
1094: If the value is 0, there is no time limit.
1.12 stevesk 1095: The default is 120 seconds.
1.1 stevesk 1096: .It Cm LogLevel
1097: Gives the verbosity level that is used when logging messages from
1.53 jmc 1098: .Xr sshd 8 .
1.1 stevesk 1099: The possible values are:
1.52 jmc 1100: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1.15 jmc 1101: The default is INFO.
1102: DEBUG and DEBUG1 are equivalent.
1103: DEBUG2 and DEBUG3 each specify higher levels of debugging output.
1104: Logging with a DEBUG level violates the privacy of users and is not recommended.
1.317 djm 1105: .It Cm LogVerbose
1106: Specify one or more overrides to LogLevel.
1107: An override consists of a pattern lists that matches the source file, function
1108: and line number to force detailed logging for.
1109: For example, an override pattern of:
1110: .Bd -literal -offset indent
1111: kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1112: .Ed
1113: .Pp
1114: would enable detailed logging for line 1000 of
1.318 jmc 1115: .Pa kex.c ,
1.317 djm 1116: everything in the
1117: .Fn kex_exchange_identification
1118: function, and all code in the
1119: .Pa packet.c
1120: file.
1121: This option is intended for debugging and no overrides are enabled by default.
1.1 stevesk 1122: .It Cm MACs
1123: Specifies the available MAC (message authentication code) algorithms.
1.219 jmc 1124: The MAC algorithm is used for data integrity protection.
1.1 stevesk 1125: Multiple algorithms must be comma-separated.
1.289 naddy 1126: If the specified list begins with a
1.208 djm 1127: .Sq +
1128: character, then the specified algorithms will be appended to the default set
1129: instead of replacing them.
1.289 naddy 1130: If the specified list begins with a
1.242 djm 1131: .Sq -
1132: character, then the specified algorithms (including wildcards) will be removed
1133: from the default set instead of replacing them.
1.290 naddy 1134: If the specified list begins with a
1135: .Sq ^
1136: character, then the specified algorithms will be placed at the head of the
1137: default set.
1.208 djm 1138: .Pp
1.152 markus 1139: The algorithms that contain
1.237 jmc 1140: .Qq -etm
1.152 markus 1141: calculate the MAC after encryption (encrypt-then-mac).
1142: These are considered safer and their use recommended.
1.173 naddy 1143: The supported MACs are:
1144: .Pp
1145: .Bl -item -compact -offset indent
1146: .It
1147: hmac-md5
1148: .It
1149: hmac-md5-96
1150: .It
1151: hmac-sha1
1152: .It
1153: hmac-sha1-96
1154: .It
1155: hmac-sha2-256
1156: .It
1157: hmac-sha2-512
1158: .It
1159: umac-64@openssh.com
1160: .It
1161: umac-128@openssh.com
1162: .It
1163: hmac-md5-etm@openssh.com
1164: .It
1165: hmac-md5-96-etm@openssh.com
1166: .It
1167: hmac-sha1-etm@openssh.com
1168: .It
1169: hmac-sha1-96-etm@openssh.com
1170: .It
1171: hmac-sha2-256-etm@openssh.com
1172: .It
1173: hmac-sha2-512-etm@openssh.com
1174: .It
1175: umac-64-etm@openssh.com
1176: .It
1177: umac-128-etm@openssh.com
1178: .El
1179: .Pp
1.52 jmc 1180: The default is:
1.77 jmc 1181: .Bd -literal -offset indent
1.152 markus 1182: umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1183: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1.217 djm 1184: hmac-sha1-etm@openssh.com,
1.173 naddy 1185: umac-64@openssh.com,umac-128@openssh.com,
1.217 djm 1186: hmac-sha2-256,hmac-sha2-512,hmac-sha1
1.77 jmc 1187: .Ed
1.188 djm 1188: .Pp
1.237 jmc 1189: The list of available MAC algorithms may also be obtained using
1190: .Qq ssh -Q mac .
1.60 dtucker 1191: .It Cm Match
1.61 jmc 1192: Introduces a conditional block.
1.65 dtucker 1193: If all of the criteria on the
1.60 dtucker 1194: .Cm Match
1.65 dtucker 1195: line are satisfied, the keywords on the following lines override those
1196: set in the global section of the config file, until either another
1.60 dtucker 1197: .Cm Match
1.65 dtucker 1198: line or the end of the file.
1.172 djm 1199: If a keyword appears in multiple
1200: .Cm Match
1.177 sobrado 1201: blocks that are satisfied, only the first instance of the keyword is
1.172 djm 1202: applied.
1.91 djm 1203: .Pp
1.61 jmc 1204: The arguments to
1.60 dtucker 1205: .Cm Match
1.163 dtucker 1206: are one or more criteria-pattern pairs or the single token
1207: .Cm All
1208: which matches all criteria.
1.60 dtucker 1209: The available criteria are
1210: .Cm User ,
1.69 dtucker 1211: .Cm Group ,
1.60 dtucker 1212: .Cm Host ,
1.139 dtucker 1213: .Cm LocalAddress ,
1214: .Cm LocalPort ,
1.258 djm 1215: .Cm RDomain ,
1.60 dtucker 1216: and
1.258 djm 1217: .Cm Address
1218: (with
1219: .Cm RDomain
1220: representing the
1221: .Xr rdomain 4
1.284 jmc 1222: on which the connection was received).
1.258 djm 1223: .Pp
1.91 djm 1224: The match patterns may consist of single entries or comma-separated
1225: lists and may use the wildcard and negation operators described in the
1.237 jmc 1226: .Sx PATTERNS
1227: section of
1.92 djm 1228: .Xr ssh_config 5 .
1.91 djm 1229: .Pp
1230: The patterns in an
1231: .Cm Address
1232: criteria may additionally contain addresses to match in CIDR
1.237 jmc 1233: address/masklen format,
1234: such as 192.0.2.0/24 or 2001:db8::/32.
1.91 djm 1235: Note that the mask length provided must be consistent with the address -
1236: it is an error to specify a mask length that is too long for the address
1.93 jmc 1237: or one with bits set in this host portion of the address.
1.237 jmc 1238: For example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
1.91 djm 1239: .Pp
1.60 dtucker 1240: Only a subset of keywords may be used on the lines following a
1241: .Cm Match
1242: keyword.
1243: Available keywords are
1.142 jmc 1244: .Cm AcceptEnv ,
1.99 okan 1245: .Cm AllowAgentForwarding ,
1.142 jmc 1246: .Cm AllowGroups ,
1.193 djm 1247: .Cm AllowStreamLocalForwarding ,
1.62 dtucker 1248: .Cm AllowTcpForwarding ,
1.141 markus 1249: .Cm AllowUsers ,
1.149 djm 1250: .Cm AuthenticationMethods ,
1.146 djm 1251: .Cm AuthorizedKeysCommand ,
1252: .Cm AuthorizedKeysCommandUser ,
1.147 jmc 1253: .Cm AuthorizedKeysFile ,
1.214 djm 1254: .Cm AuthorizedPrincipalsCommand ,
1255: .Cm AuthorizedPrincipalsCommandUser ,
1.123 djm 1256: .Cm AuthorizedPrincipalsFile ,
1.72 dtucker 1257: .Cm Banner ,
1.336 dtucker 1258: .Cm CASignatureAlgorithms ,
1.85 djm 1259: .Cm ChrootDirectory ,
1.238 markus 1260: .Cm ClientAliveCountMax ,
1261: .Cm ClientAliveInterval ,
1.141 markus 1262: .Cm DenyGroups ,
1263: .Cm DenyUsers ,
1.320 djm 1264: .Cm DisableForwarding ,
1.336 dtucker 1265: .Cm ExposeAuthInfo ,
1.67 dtucker 1266: .Cm ForceCommand ,
1.194 djm 1267: .Cm GatewayPorts ,
1.193 djm 1268: .Cm GSSAPIAuthentication ,
1.325 dtucker 1269: .Cm HostbasedAcceptedAlgorithms ,
1.87 djm 1270: .Cm HostbasedAuthentication ,
1.123 djm 1271: .Cm HostbasedUsesNameFromPacketOnly ,
1.308 djm 1272: .Cm IgnoreRhosts ,
1.306 naddy 1273: .Cm Include ,
1.193 djm 1274: .Cm IPQoS ,
1.74 jmc 1275: .Cm KbdInteractiveAuthentication ,
1.72 dtucker 1276: .Cm KerberosAuthentication ,
1.245 djm 1277: .Cm LogLevel ,
1.95 dtucker 1278: .Cm MaxAuthTries ,
1.94 dtucker 1279: .Cm MaxSessions ,
1.72 dtucker 1280: .Cm PasswordAuthentication ,
1.97 djm 1281: .Cm PermitEmptyPasswords ,
1.271 djm 1282: .Cm PermitListen ,
1.66 dtucker 1283: .Cm PermitOpen ,
1.79 dtucker 1284: .Cm PermitRootLogin ,
1.164 djm 1285: .Cm PermitTTY ,
1.123 djm 1286: .Cm PermitTunnel ,
1.174 djm 1287: .Cm PermitUserRC ,
1.324 dtucker 1288: .Cm PubkeyAcceptedAlgorithms ,
1.107 dtucker 1289: .Cm PubkeyAuthentication ,
1.336 dtucker 1290: .Cm PubkeyAuthOptions ,
1.159 dtucker 1291: .Cm RekeyLimit ,
1.193 djm 1292: .Cm RevokedKeys ,
1.257 djm 1293: .Cm RDomain ,
1.274 djm 1294: .Cm SetEnv ,
1.193 djm 1295: .Cm StreamLocalBindMask ,
1296: .Cm StreamLocalBindUnlink ,
1297: .Cm TrustedUserCAKeys ,
1.66 dtucker 1298: .Cm X11DisplayOffset ,
1.101 djm 1299: .Cm X11Forwarding
1.60 dtucker 1300: and
1.286 jmc 1301: .Cm X11UseLocalhost .
1.33 dtucker 1302: .It Cm MaxAuthTries
1303: Specifies the maximum number of authentication attempts permitted per
1.35 jmc 1304: connection.
1305: Once the number of failures reaches half this value,
1306: additional failures are logged.
1307: The default is 6.
1.90 djm 1308: .It Cm MaxSessions
1.216 djm 1309: Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
1310: sessions permitted per network connection.
1311: Multiple sessions may be established by clients that support connection
1312: multiplexing.
1313: Setting
1314: .Cm MaxSessions
1315: to 1 will effectively disable session multiplexing, whereas setting it to 0
1316: will prevent all shell, login and subsystem sessions while still permitting
1317: forwarding.
1.90 djm 1318: The default is 10.
1.1 stevesk 1319: .It Cm MaxStartups
1320: Specifies the maximum number of concurrent unauthenticated connections to the
1.52 jmc 1321: SSH daemon.
1.1 stevesk 1322: Additional connections will be dropped until authentication succeeds or the
1323: .Cm LoginGraceTime
1324: expires for a connection.
1.156 dtucker 1325: The default is 10:30:100.
1.1 stevesk 1326: .Pp
1327: Alternatively, random early drop can be enabled by specifying
1328: the three colon separated values
1.237 jmc 1329: start:rate:full (e.g. "10:30:60").
1.53 jmc 1330: .Xr sshd 8
1.237 jmc 1331: will refuse connection attempts with a probability of rate/100 (30%)
1332: if there are currently start (10) unauthenticated connections.
1.1 stevesk 1333: The probability increases linearly and all connection attempts
1.237 jmc 1334: are refused if the number of unauthenticated connections reaches full (60).
1.329 dtucker 1335: .It Cm ModuliFile
1336: Specifies the
1337: .Xr moduli 5
1338: file that contains the Diffie-Hellman groups used for the
1339: .Dq diffie-hellman-group-exchange-sha1
1340: and
1341: .Dq diffie-hellman-group-exchange-sha256
1342: key exchange methods.
1343: The default is
1344: .Pa /etc/moduli .
1.1 stevesk 1345: .It Cm PasswordAuthentication
1346: Specifies whether password authentication is allowed.
1347: The default is
1.237 jmc 1348: .Cm yes .
1.1 stevesk 1349: .It Cm PermitEmptyPasswords
1350: When password authentication is allowed, it specifies whether the
1351: server allows login to accounts with empty password strings.
1352: The default is
1.237 jmc 1353: .Cm no .
1.271 djm 1354: .It Cm PermitListen
1355: Specifies the addresses/ports on which a remote TCP port forwarding may listen.
1356: The listen specification must be one of the following forms:
1357: .Pp
1358: .Bl -item -offset indent -compact
1359: .It
1360: .Cm PermitListen
1361: .Sm off
1.276 djm 1362: .Ar port
1.271 djm 1363: .Sm on
1364: .It
1365: .Cm PermitListen
1366: .Sm off
1.276 djm 1367: .Ar host : port
1.271 djm 1368: .Sm on
1369: .El
1370: .Pp
1371: Multiple permissions may be specified by separating them with whitespace.
1372: An argument of
1373: .Cm any
1374: can be used to remove all restrictions and permit any listen requests.
1375: An argument of
1376: .Cm none
1377: can be used to prohibit all listen requests.
1378: The host name may contain wildcards as described in the PATTERNS section in
1379: .Xr ssh_config 5 .
1380: The wildcard
1381: .Sq *
1382: can also be used in place of a port number to allow all ports.
1383: By default all port forwarding listen requests are permitted.
1.272 jmc 1384: Note that the
1.271 djm 1385: .Cm GatewayPorts
1386: option may further restrict which addresses may be listened on.
1.276 djm 1387: Note also that
1388: .Xr ssh 1
1389: will request a listen host of
1390: .Dq localhost
1.283 dtucker 1391: if no listen host was specifically requested, and this name is
1.277 jmc 1392: treated differently to explicit localhost addresses of
1.276 djm 1393: .Dq 127.0.0.1
1394: and
1395: .Dq ::1 .
1.62 dtucker 1396: .It Cm PermitOpen
1397: Specifies the destinations to which TCP port forwarding is permitted.
1398: The forwarding specification must be one of the following forms:
1399: .Pp
1400: .Bl -item -offset indent -compact
1401: .It
1402: .Cm PermitOpen
1403: .Sm off
1404: .Ar host : port
1405: .Sm on
1406: .It
1407: .Cm PermitOpen
1408: .Sm off
1409: .Ar IPv4_addr : port
1410: .Sm on
1411: .It
1412: .Cm PermitOpen
1413: .Sm off
1414: .Ar \&[ IPv6_addr \&] : port
1415: .Sm on
1416: .El
1417: .Pp
1.68 dtucker 1418: Multiple forwards may be specified by separating them with whitespace.
1.62 dtucker 1419: An argument of
1.237 jmc 1420: .Cm any
1.62 dtucker 1421: can be used to remove all restrictions and permit any forwarding requests.
1.140 dtucker 1422: An argument of
1.237 jmc 1423: .Cm none
1.140 dtucker 1424: can be used to prohibit all forwarding requests.
1.227 jmc 1425: The wildcard
1.237 jmc 1426: .Sq *
1.299 djm 1427: can be used for host or port to allow all hosts or ports respectively.
1428: Otherwise, no pattern matching or address lookups are performed on supplied
1429: names.
1.63 jmc 1430: By default all port forwarding requests are permitted.
1.1 stevesk 1431: .It Cm PermitRootLogin
1.38 jmc 1432: Specifies whether root can log in using
1.1 stevesk 1433: .Xr ssh 1 .
1434: The argument must be
1.237 jmc 1435: .Cm yes ,
1436: .Cm prohibit-password ,
1437: .Cm forced-commands-only ,
1.1 stevesk 1438: or
1.237 jmc 1439: .Cm no .
1.1 stevesk 1440: The default is
1.237 jmc 1441: .Cm prohibit-password .
1.1 stevesk 1442: .Pp
1443: If this option is set to
1.237 jmc 1444: .Cm prohibit-password
1.255 jmc 1445: (or its deprecated alias,
1446: .Cm without-password ) ,
1.210 deraadt 1447: password and keyboard-interactive authentication are disabled for root.
1.1 stevesk 1448: .Pp
1449: If this option is set to
1.237 jmc 1450: .Cm forced-commands-only ,
1.1 stevesk 1451: root login with public key authentication will be allowed,
1452: but only if the
1453: .Ar command
1454: option has been specified
1455: (which may be useful for taking remote backups even if root login is
1.17 jmc 1456: normally not allowed).
1457: All other authentication methods are disabled for root.
1.1 stevesk 1458: .Pp
1459: If this option is set to
1.237 jmc 1460: .Cm no ,
1.38 jmc 1461: root is not allowed to log in.
1.231 jmc 1462: .It Cm PermitTTY
1463: Specifies whether
1464: .Xr pty 4
1465: allocation is permitted.
1466: The default is
1.237 jmc 1467: .Cm yes .
1.46 reyk 1468: .It Cm PermitTunnel
1469: Specifies whether
1470: .Xr tun 4
1471: device forwarding is allowed.
1.47 reyk 1472: The argument must be
1.237 jmc 1473: .Cm yes ,
1474: .Cm point-to-point
1.58 stevesk 1475: (layer 3),
1.237 jmc 1476: .Cm ethernet
1.58 stevesk 1477: (layer 2), or
1.237 jmc 1478: .Cm no .
1.58 stevesk 1479: Specifying
1.237 jmc 1480: .Cm yes
1.58 stevesk 1481: permits both
1.237 jmc 1482: .Cm point-to-point
1.58 stevesk 1483: and
1.237 jmc 1484: .Cm ethernet .
1.46 reyk 1485: The default is
1.237 jmc 1486: .Cm no .
1.178 djm 1487: .Pp
1488: Independent of this setting, the permissions of the selected
1489: .Xr tun 4
1490: device must allow access to the user.
1.6 markus 1491: .It Cm PermitUserEnvironment
1492: Specifies whether
1493: .Pa ~/.ssh/environment
1.9 stevesk 1494: and
1.6 markus 1495: .Cm environment=
1496: options in
1497: .Pa ~/.ssh/authorized_keys
1.9 stevesk 1498: are processed by
1.52 jmc 1499: .Xr sshd 8 .
1.278 djm 1500: Valid options are
1501: .Cm yes ,
1502: .Cm no
1503: or a pattern-list specifying which environment variable names to accept
1504: (for example
1505: .Qq LANG,LC_* ) .
1.6 markus 1506: The default is
1.237 jmc 1507: .Cm no .
1.9 stevesk 1508: Enabling environment processing may enable users to bypass access
1509: restrictions in some configurations using mechanisms such as
1510: .Ev LD_PRELOAD .
1.174 djm 1511: .It Cm PermitUserRC
1512: Specifies whether any
1513: .Pa ~/.ssh/rc
1514: file is executed.
1515: The default is
1.237 jmc 1516: .Cm yes .
1.321 dtucker 1517: .It Cm PerSourceMaxStartups
1518: Specifies the number of unauthenticated connections allowed from a
1519: given source address, or
1520: .Dq none
1521: if there is no limit.
1522: This limit is applied in addition to
1523: .Cm MaxStartups ,
1524: whichever is lower.
1525: The default is
1526: .Cm none .
1527: .It Cm PerSourceNetBlockSize
1528: Specifies the number of bits of source address that are grouped together
1529: for the purposes of applying PerSourceMaxStartups limits.
1530: Values for IPv4 and optionally IPv6 may be specified, separated by a colon.
1531: The default is
1.322 jmc 1532: .Cm 32:128 ,
1.321 dtucker 1533: which means each address is considered individually.
1.328 djm 1534: .It Cm PidFile
1535: Specifies the file that contains the process ID of the
1536: SSH daemon, or
1537: .Cm none
1538: to not write one.
1539: The default is
1540: .Pa /var/run/sshd.pid .
1.1 stevesk 1541: .It Cm Port
1542: Specifies the port number that
1.52 jmc 1543: .Xr sshd 8
1.1 stevesk 1544: listens on.
1545: The default is 22.
1546: Multiple options of this type are permitted.
1547: See also
1548: .Cm ListenAddress .
1549: .It Cm PrintLastLog
1550: Specifies whether
1.52 jmc 1551: .Xr sshd 8
1.36 jaredy 1552: should print the date and time of the last user login when a user logs
1553: in interactively.
1.1 stevesk 1554: The default is
1.237 jmc 1555: .Cm yes .
1.1 stevesk 1556: .It Cm PrintMotd
1557: Specifies whether
1.52 jmc 1558: .Xr sshd 8
1.1 stevesk 1559: should print
1560: .Pa /etc/motd
1561: when a user logs in interactively.
1562: (On some systems it is also printed by the shell,
1563: .Pa /etc/profile ,
1564: or equivalent.)
1565: The default is
1.237 jmc 1566: .Cm yes .
1.324 dtucker 1567: .It Cm PubkeyAcceptedAlgorithms
1568: Specifies the signature algorithms that will be accepted for public key
1569: authentication as a list of comma-separated patterns.
1.289 naddy 1570: Alternately if the specified list begins with a
1.208 djm 1571: .Sq +
1.324 dtucker 1572: character, then the specified algorithms will be appended to the default set
1.208 djm 1573: instead of replacing them.
1.289 naddy 1574: If the specified list begins with a
1.242 djm 1575: .Sq -
1.324 dtucker 1576: character, then the specified algorithms (including wildcards) will be removed
1.242 djm 1577: from the default set instead of replacing them.
1.290 naddy 1578: If the specified list begins with a
1579: .Sq ^
1.324 dtucker 1580: character, then the specified algorithms will be placed at the head of the
1.290 naddy 1581: default set.
1.206 markus 1582: The default for this option is:
1583: .Bd -literal -offset 3n
1.316 djm 1584: ssh-ed25519-cert-v01@openssh.com,
1.206 markus 1585: ecdsa-sha2-nistp256-cert-v01@openssh.com,
1586: ecdsa-sha2-nistp384-cert-v01@openssh.com,
1587: ecdsa-sha2-nistp521-cert-v01@openssh.com,
1.316 djm 1588: sk-ssh-ed25519-cert-v01@openssh.com,
1.296 naddy 1589: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1.291 naddy 1590: rsa-sha2-512-cert-v01@openssh.com,
1591: rsa-sha2-256-cert-v01@openssh.com,
1.316 djm 1592: ssh-ed25519,
1.296 naddy 1593: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1.316 djm 1594: sk-ssh-ed25519@openssh.com,
1.291 naddy 1595: sk-ecdsa-sha2-nistp256@openssh.com,
1.338 dtucker 1596: rsa-sha2-512,rsa-sha2-256
1.206 markus 1597: .Ed
1598: .Pp
1.326 djm 1599: The list of available signature algorithms may also be obtained using
1.324 dtucker 1600: .Qq ssh -Q PubkeyAcceptedAlgorithms .
1.293 djm 1601: .It Cm PubkeyAuthOptions
1602: Sets one or more public key authentication options.
1.313 djm 1603: The supported keywords are:
1.294 jmc 1604: .Cm none
1.313 djm 1605: (the default; indicating no additional options are enabled),
1606: .Cm touch-required
1.293 djm 1607: and
1.313 djm 1608: .Cm verify-required .
1.293 djm 1609: .Pp
1610: The
1611: .Cm touch-required
1.297 naddy 1612: option causes public key authentication using a FIDO authenticator algorithm
1.294 jmc 1613: (i.e.\&
1.293 djm 1614: .Cm ecdsa-sk
1615: or
1616: .Cm ed25519-sk )
1617: to always require the signature to attest that a physically present user
1.297 naddy 1618: explicitly confirmed the authentication (usually by touching the authenticator).
1.293 djm 1619: By default,
1620: .Xr sshd 8
1.297 naddy 1621: requires user presence unless overridden with an authorized_keys option.
1.293 djm 1622: The
1623: .Cm touch-required
1624: flag disables this override.
1.313 djm 1625: .Pp
1626: The
1627: .Cm verify-required
1.315 jmc 1628: option requires a FIDO key signature attest that the user was verified,
1.314 jmc 1629: e.g. via a PIN.
1.313 djm 1630: .Pp
1631: Neither the
1632: .Cm touch-required
1633: or
1634: .Cm verify-required
1.314 jmc 1635: options have any effect for other, non-FIDO, public key types.
1.1 stevesk 1636: .It Cm PubkeyAuthentication
1637: Specifies whether public key authentication is allowed.
1638: The default is
1.237 jmc 1639: .Cm yes .
1.159 dtucker 1640: .It Cm RekeyLimit
1.342 djm 1641: Specifies the maximum amount of data that may be transmitted or received
1642: before the session key is renegotiated, optionally followed by a maximum
1643: amount of time that may pass before the session key is renegotiated.
1.159 dtucker 1644: The first argument is specified in bytes and may have a suffix of
1645: .Sq K ,
1646: .Sq M ,
1647: or
1648: .Sq G
1649: to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1650: The default is between
1651: .Sq 1G
1652: and
1653: .Sq 4G ,
1654: depending on the cipher.
1655: The optional second value is specified in seconds and may use any of the
1656: units documented in the
1657: .Sx TIME FORMATS
1.160 jmc 1658: section.
1.159 dtucker 1659: The default value for
1660: .Cm RekeyLimit
1661: is
1.237 jmc 1662: .Cm default none ,
1.159 dtucker 1663: which means that rekeying is performed after the cipher's default amount
1664: of data has been sent or received and no time based rekeying is done.
1.343 djm 1665: .It Cm RequiredRSASize
1666: Specifies the minimum RSA key size (in bits) that
1667: .Xr sshd 8
1668: will accept.
1669: User and host-based authentication keys smaller than this limit will be
1670: refused.
1671: The default is
1672: .Cm 1024
1673: bits.
1674: Note that this limit may only be raised from the default.
1.118 djm 1675: .It Cm RevokedKeys
1.195 dtucker 1676: Specifies revoked public keys file, or
1.237 jmc 1677: .Cm none
1.195 dtucker 1678: to not use one.
1.118 djm 1679: Keys listed in this file will be refused for public key authentication.
1680: Note that if this file is not readable, then public key authentication will
1681: be refused for all users.
1.154 djm 1682: Keys may be specified as a text file, listing one public key per line, or as
1683: an OpenSSH Key Revocation List (KRL) as generated by
1.155 jmc 1684: .Xr ssh-keygen 1 .
1.161 jmc 1685: For more information on KRLs, see the KEY REVOCATION LISTS section in
1.154 djm 1686: .Xr ssh-keygen 1 .
1.257 djm 1687: .It Cm RDomain
1688: Specifies an explicit routing domain that is applied after authentication
1689: has completed.
1.333 jmc 1690: The user session, as well as any forwarded or listening IP sockets,
1.259 jmc 1691: will be bound to this
1.257 djm 1692: .Xr rdomain 4 .
1693: If the routing domain is set to
1694: .Cm \&%D ,
1.259 jmc 1695: then the domain in which the incoming connection was received will be applied.
1.296 naddy 1696: .It Cm SecurityKeyProvider
1.297 naddy 1697: Specifies a path to a library that will be used when loading
1698: FIDO authenticator-hosted keys, overriding the default of using
1699: the built-in USB HID support.
1.274 djm 1700: .It Cm SetEnv
1701: Specifies one or more environment variables to set in child sessions started
1702: by
1703: .Xr sshd 8
1704: as
1705: .Dq NAME=VALUE .
1706: The environment value may be quoted (e.g. if it contains whitespace
1707: characters).
1708: Environment variables set by
1709: .Cm SetEnv
1710: override the default environment and any variables specified by the user
1711: via
1712: .Cm AcceptEnv
1713: or
1714: .Cm PermitUserEnvironment .
1.175 millert 1715: .It Cm StreamLocalBindMask
1716: Sets the octal file creation mode mask
1717: .Pq umask
1718: used when creating a Unix-domain socket file for local or remote
1719: port forwarding.
1720: This option is only used for port forwarding to a Unix-domain socket file.
1721: .Pp
1722: The default value is 0177, which creates a Unix-domain socket file that is
1723: readable and writable only by the owner.
1724: Note that not all operating systems honor the file mode on Unix-domain
1725: socket files.
1726: .It Cm StreamLocalBindUnlink
1727: Specifies whether to remove an existing Unix-domain socket file for local
1728: or remote port forwarding before creating a new one.
1729: If the socket file already exists and
1730: .Cm StreamLocalBindUnlink
1731: is not enabled,
1732: .Nm sshd
1733: will be unable to forward the port to the Unix-domain socket file.
1734: This option is only used for port forwarding to a Unix-domain socket file.
1735: .Pp
1736: The argument must be
1.237 jmc 1737: .Cm yes
1.175 millert 1738: or
1.237 jmc 1739: .Cm no .
1.175 millert 1740: The default is
1.237 jmc 1741: .Cm no .
1.1 stevesk 1742: .It Cm StrictModes
1743: Specifies whether
1.52 jmc 1744: .Xr sshd 8
1.1 stevesk 1745: should check file modes and ownership of the
1746: user's files and home directory before accepting login.
1747: This is normally desirable because novices sometimes accidentally leave their
1748: directory or files world-writable.
1749: The default is
1.237 jmc 1750: .Cm yes .
1.112 djm 1751: Note that this does not apply to
1752: .Cm ChrootDirectory ,
1753: whose permissions and ownership are checked unconditionally.
1.1 stevesk 1754: .It Cm Subsystem
1.51 jmc 1755: Configures an external subsystem (e.g. file transfer daemon).
1.59 djm 1756: Arguments should be a subsystem name and a command (with optional arguments)
1757: to execute upon subsystem request.
1.80 djm 1758: .Pp
1.1 stevesk 1759: The command
1.237 jmc 1760: .Cm sftp-server
1761: implements the SFTP file transfer subsystem.
1.80 djm 1762: .Pp
1763: Alternately the name
1.237 jmc 1764: .Cm internal-sftp
1765: implements an in-process SFTP server.
1.80 djm 1766: This may simplify configurations using
1767: .Cm ChrootDirectory
1768: to force a different filesystem root on clients.
1769: .Pp
1.1 stevesk 1770: By default no subsystems are defined.
1771: .It Cm SyslogFacility
1772: Gives the facility code that is used when logging messages from
1.53 jmc 1773: .Xr sshd 8 .
1.1 stevesk 1774: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1775: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1776: The default is AUTH.
1.27 markus 1777: .It Cm TCPKeepAlive
1778: Specifies whether the system should send TCP keepalive messages to the
1779: other side.
1780: If they are sent, death of the connection or crash of one
1781: of the machines will be properly noticed.
1782: However, this means that
1783: connections will die if the route is down temporarily, and some people
1784: find it annoying.
1785: On the other hand, if TCP keepalives are not sent,
1786: sessions may hang indefinitely on the server, leaving
1.237 jmc 1787: .Qq ghost
1.27 markus 1788: users and consuming server resources.
1789: .Pp
1790: The default is
1.237 jmc 1791: .Cm yes
1.27 markus 1792: (to send TCP keepalive messages), and the server will notice
1793: if the network goes down or the client host crashes.
1794: This avoids infinitely hanging sessions.
1795: .Pp
1796: To disable TCP keepalive messages, the value should be set to
1.237 jmc 1797: .Cm no .
1.118 djm 1798: .It Cm TrustedUserCAKeys
1799: Specifies a file containing public keys of certificate authorities that are
1.195 dtucker 1800: trusted to sign user certificates for authentication, or
1.237 jmc 1801: .Cm none
1.195 dtucker 1802: to not use one.
1.119 jmc 1803: Keys are listed one per line; empty lines and comments starting with
1.118 djm 1804: .Ql #
1805: are allowed.
1806: If a certificate is presented for authentication and has its signing CA key
1807: listed in this file, then it may be used for authentication for any user
1808: listed in the certificate's principals list.
1809: Note that certificates that lack a list of principals will not be permitted
1810: for authentication using
1811: .Cm TrustedUserCAKeys .
1.161 jmc 1812: For more details on certificates, see the CERTIFICATES section in
1.118 djm 1813: .Xr ssh-keygen 1 .
1.18 markus 1814: .It Cm UseDNS
1815: Specifies whether
1.52 jmc 1816: .Xr sshd 8
1.207 djm 1817: should look up the remote host name, and to check that
1.18 markus 1818: the resolved host name for the remote IP address maps back to the
1819: very same IP address.
1.207 djm 1820: .Pp
1821: If this option is set to
1.237 jmc 1822: .Cm no
1.207 djm 1823: (the default) then only addresses and not host names may be used in
1.221 djm 1824: .Pa ~/.ssh/authorized_keys
1.207 djm 1825: .Cm from
1826: and
1.211 jmc 1827: .Nm
1.207 djm 1828: .Cm Match
1829: .Cm Host
1830: directives.
1.137 djm 1831: .It Cm VersionAddendum
1832: Optionally specifies additional text to append to the SSH protocol banner
1833: sent by the server upon connection.
1834: The default is
1.237 jmc 1835: .Cm none .
1.1 stevesk 1836: .It Cm X11DisplayOffset
1837: Specifies the first display number available for
1.52 jmc 1838: .Xr sshd 8 Ns 's
1.1 stevesk 1839: X11 forwarding.
1.52 jmc 1840: This prevents sshd from interfering with real X11 servers.
1.1 stevesk 1841: The default is 10.
1842: .It Cm X11Forwarding
1843: Specifies whether X11 forwarding is permitted.
1.13 stevesk 1844: The argument must be
1.237 jmc 1845: .Cm yes
1.13 stevesk 1846: or
1.237 jmc 1847: .Cm no .
1.1 stevesk 1848: The default is
1.237 jmc 1849: .Cm no .
1.13 stevesk 1850: .Pp
1851: When X11 forwarding is enabled, there may be additional exposure to
1852: the server and to client displays if the
1.52 jmc 1853: .Xr sshd 8
1.13 stevesk 1854: proxy display is configured to listen on the wildcard address (see
1.237 jmc 1855: .Cm X11UseLocalhost ) ,
1856: though this is not the default.
1.13 stevesk 1857: Additionally, the authentication spoofing and authentication data
1858: verification and substitution occur on the client side.
1859: The security risk of using X11 forwarding is that the client's X11
1.52 jmc 1860: display server may be exposed to attack when the SSH client requests
1.13 stevesk 1861: forwarding (see the warnings for
1862: .Cm ForwardX11
1863: in
1.19 jmc 1864: .Xr ssh_config 5 ) .
1.13 stevesk 1865: A system administrator may have a stance in which they want to
1866: protect clients that may expose themselves to attack by unwittingly
1867: requesting X11 forwarding, which can warrant a
1.237 jmc 1868: .Cm no
1.13 stevesk 1869: setting.
1870: .Pp
1871: Note that disabling X11 forwarding does not prevent users from
1872: forwarding X11 traffic, as users can always install their own forwarders.
1.1 stevesk 1873: .It Cm X11UseLocalhost
1874: Specifies whether
1.52 jmc 1875: .Xr sshd 8
1.1 stevesk 1876: should bind the X11 forwarding server to the loopback address or to
1.15 jmc 1877: the wildcard address.
1878: By default,
1.52 jmc 1879: sshd binds the forwarding server to the loopback address and sets the
1.1 stevesk 1880: hostname part of the
1881: .Ev DISPLAY
1882: environment variable to
1.237 jmc 1883: .Cm localhost .
1.8 stevesk 1884: This prevents remote hosts from connecting to the proxy display.
1.1 stevesk 1885: However, some older X11 clients may not function with this
1886: configuration.
1887: .Cm X11UseLocalhost
1888: may be set to
1.237 jmc 1889: .Cm no
1.1 stevesk 1890: to specify that the forwarding server should be bound to the wildcard
1891: address.
1892: The argument must be
1.237 jmc 1893: .Cm yes
1.1 stevesk 1894: or
1.237 jmc 1895: .Cm no .
1.1 stevesk 1896: The default is
1.237 jmc 1897: .Cm yes .
1.1 stevesk 1898: .It Cm XAuthLocation
1.11 stevesk 1899: Specifies the full pathname of the
1.1 stevesk 1900: .Xr xauth 1
1.195 dtucker 1901: program, or
1.237 jmc 1902: .Cm none
1.195 dtucker 1903: to not use one.
1.1 stevesk 1904: The default is
1905: .Pa /usr/X11R6/bin/xauth .
1906: .El
1.55 jmc 1907: .Sh TIME FORMATS
1.53 jmc 1908: .Xr sshd 8
1.1 stevesk 1909: command-line arguments and configuration file options that specify time
1910: may be expressed using a sequence of the form:
1911: .Sm off
1.7 stevesk 1912: .Ar time Op Ar qualifier ,
1.1 stevesk 1913: .Sm on
1914: where
1915: .Ar time
1916: is a positive integer value and
1917: .Ar qualifier
1918: is one of the following:
1919: .Pp
1920: .Bl -tag -width Ds -compact -offset indent
1.64 jmc 1921: .It Aq Cm none
1.1 stevesk 1922: seconds
1923: .It Cm s | Cm S
1924: seconds
1925: .It Cm m | Cm M
1926: minutes
1927: .It Cm h | Cm H
1928: hours
1929: .It Cm d | Cm D
1930: days
1931: .It Cm w | Cm W
1932: weeks
1933: .El
1934: .Pp
1935: Each member of the sequence is added together to calculate
1936: the total time value.
1937: .Pp
1938: Time format examples:
1939: .Pp
1940: .Bl -tag -width Ds -compact -offset indent
1941: .It 600
1942: 600 seconds (10 minutes)
1943: .It 10m
1944: 10 minutes
1945: .It 1h30m
1946: 1 hour 30 minutes (90 minutes)
1947: .El
1.235 jmc 1948: .Sh TOKENS
1949: Arguments to some keywords can make use of tokens,
1950: which are expanded at runtime:
1951: .Pp
1952: .Bl -tag -width XXXX -offset indent -compact
1953: .It %%
1954: A literal
1955: .Sq % .
1.257 djm 1956: .It \&%D
1957: The routing domain in which the incoming connection was received.
1.235 jmc 1958: .It %F
1959: The fingerprint of the CA key.
1960: .It %f
1961: The fingerprint of the key or certificate.
1962: .It %h
1963: The home directory of the user.
1964: .It %i
1965: The key ID in the certificate.
1966: .It %K
1967: The base64-encoded CA key.
1968: .It %k
1969: The base64-encoded key or certificate for authentication.
1970: .It %s
1971: The serial number of the certificate.
1972: .It \&%T
1973: The type of the CA key.
1974: .It %t
1975: The key or certificate type.
1.268 jmc 1976: .It \&%U
1977: The numeric user ID of the target user.
1.235 jmc 1978: .It %u
1979: The username.
1980: .El
1981: .Pp
1982: .Cm AuthorizedKeysCommand
1.270 jmc 1983: accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1.235 jmc 1984: .Pp
1985: .Cm AuthorizedKeysFile
1.270 jmc 1986: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1987: .Pp
1988: .Cm AuthorizedPrincipalsCommand
1.270 jmc 1989: accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
1.235 jmc 1990: .Pp
1991: .Cm AuthorizedPrincipalsFile
1.270 jmc 1992: accepts the tokens %%, %h, %U, and %u.
1.235 jmc 1993: .Pp
1994: .Cm ChrootDirectory
1.270 jmc 1995: accepts the tokens %%, %h, %U, and %u.
1.257 djm 1996: .Pp
1997: .Cm RoutingDomain
1998: accepts the token %D.
1.1 stevesk 1999: .Sh FILES
2000: .Bl -tag -width Ds
2001: .It Pa /etc/ssh/sshd_config
2002: Contains configuration data for
1.53 jmc 2003: .Xr sshd 8 .
1.1 stevesk 2004: This file should be writable by root only, but it is recommended
2005: (though not necessary) that it be world-readable.
2006: .El
1.19 jmc 2007: .Sh SEE ALSO
1.237 jmc 2008: .Xr sftp-server 8 ,
1.19 jmc 2009: .Xr sshd 8
1.1 stevesk 2010: .Sh AUTHORS
1.237 jmc 2011: .An -nosplit
1.1 stevesk 2012: OpenSSH is a derivative of the original and free
1.237 jmc 2013: ssh 1.2.12 release by
2014: .An Tatu Ylonen .
2015: .An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos ,
2016: .An Theo de Raadt
2017: and
2018: .An Dug Song
1.1 stevesk 2019: removed many bugs, re-added newer features and
2020: created OpenSSH.
1.237 jmc 2021: .An Markus Friedl
2022: contributed the support for SSH protocol versions 1.5 and 2.0.
2023: .An Niels Provos
2024: and
2025: .An Markus Friedl
2026: contributed support for privilege separation.
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