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. 35.\" 36.\" $OpenBSD: sshd_config.5,v 1.355 2024/02/21 06:17:29 djm Exp $ 37.Dd $Mdocdate: February 21 2024 $ 38.Dt SSHD_CONFIG 5 39.Os 40.Sh NAME 41.Nm sshd_config 42.Nd OpenSSH daemon configuration file 43.Sh DESCRIPTION 44.Xr sshd 8 45reads configuration data from 46.Pa /etc/ssh/sshd_config 47(or the file specified with 48.Fl f 49on the command line). 50The file contains keyword-argument pairs, one per line. 51Unless noted otherwise, for each keyword, the first obtained value will be used. 52Lines starting with 53.Ql # 54and empty lines are interpreted as comments. 55Arguments may optionally be enclosed in double quotes 56.Pq \&" 57in order to represent arguments containing spaces. 58.Pp 59The possible 60keywords and their meanings are as follows (note that 61keywords are case-insensitive and arguments are case-sensitive): 62.Bl -tag -width Ds 63.It Cm AcceptEnv 64Specifies what environment variables sent by the client will be copied into 65the session's 66.Xr environ 7 . 67See 68.Cm SendEnv 69and 70.Cm SetEnv 71in 72.Xr ssh_config 5 73for how to configure the client. 74The 75.Ev TERM 76environment variable is always accepted whenever the client 77requests a pseudo-terminal as it is required by the protocol. 78Variables are specified by name, which may contain the wildcard characters 79.Ql * 80and 81.Ql \&? . 82Multiple environment variables may be separated by whitespace or spread 83across multiple 84.Cm AcceptEnv 85directives. 86Be warned that some environment variables could be used to bypass restricted 87user environments. 88For this reason, care should be taken in the use of this directive. 89The default is not to accept any environment variables. 90.It Cm AddressFamily 91Specifies which address family should be used by 92.Xr sshd 8 . 93Valid arguments are 94.Cm any 95(the default), 96.Cm inet 97(use IPv4 only), or 98.Cm inet6 99(use IPv6 only). 100.It Cm AllowAgentForwarding 101Specifies whether 102.Xr ssh-agent 1 103forwarding is permitted. 104The default is 105.Cm yes . 106Note that disabling agent forwarding does not improve security 107unless users are also denied shell access, as they can always install 108their own forwarders. 109.It Cm AllowGroups 110This keyword can be followed by a list of group name patterns, separated 111by spaces. 112If specified, login is allowed only for users whose primary 113group or supplementary group list matches one of the patterns. 114Only group names are valid; a numerical group ID is not recognized. 115By default, login is allowed for all groups. 116The allow/deny groups directives are processed in the following order: 117.Cm DenyGroups , 118.Cm AllowGroups . 119.Pp 120See PATTERNS in 121.Xr ssh_config 5 122for more information on patterns. 123This keyword may appear multiple times in 124.Nm 125with each instance appending to the list. 126.It Cm AllowStreamLocalForwarding 127Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 128The available options are 129.Cm yes 130(the default) 131or 132.Cm all 133to allow StreamLocal forwarding, 134.Cm no 135to prevent all StreamLocal forwarding, 136.Cm local 137to allow local (from the perspective of 138.Xr ssh 1 ) 139forwarding only or 140.Cm remote 141to allow remote forwarding only. 142Note that disabling StreamLocal forwarding does not improve security unless 143users are also denied shell access, as they can always install their 144own forwarders. 145.It Cm AllowTcpForwarding 146Specifies whether TCP forwarding is permitted. 147The available options are 148.Cm yes 149(the default) 150or 151.Cm all 152to allow TCP forwarding, 153.Cm no 154to prevent all TCP forwarding, 155.Cm local 156to allow local (from the perspective of 157.Xr ssh 1 ) 158forwarding only or 159.Cm remote 160to allow remote forwarding only. 161Note that disabling TCP forwarding does not improve security unless 162users are also denied shell access, as they can always install their 163own forwarders. 164.It Cm AllowUsers 165This keyword can be followed by a list of user name patterns, separated 166by spaces. 167If specified, login is allowed only for user names that 168match one of the patterns. 169Only user names are valid; a numerical user ID is not recognized. 170By default, login is allowed for all users. 171If the pattern takes the form USER@HOST then USER and HOST 172are separately checked, restricting logins to particular 173users from particular hosts. 174HOST criteria may additionally contain addresses to match in CIDR 175address/masklen format. 176The allow/deny users directives are processed in the following order: 177.Cm DenyUsers , 178.Cm AllowUsers . 179.Pp 180See PATTERNS in 181.Xr ssh_config 5 182for more information on patterns. 183This keyword may appear multiple times in 184.Nm 185with each instance appending to the list. 186.It Cm AuthenticationMethods 187Specifies the authentication methods that must be successfully completed 188for a user to be granted access. 189This option must be followed by one or more lists of comma-separated 190authentication method names, or by the single string 191.Cm any 192to indicate the default behaviour of accepting any single authentication 193method. 194If the default is overridden, then successful authentication requires 195completion of every method in at least one of these lists. 196.Pp 197For example, 198.Qq publickey,password publickey,keyboard-interactive 199would require the user to complete public key authentication, followed by 200either password or keyboard interactive authentication. 201Only methods that are next in one or more lists are offered at each stage, 202so for this example it would not be possible to attempt password or 203keyboard-interactive authentication before public key. 204.Pp 205For keyboard interactive authentication it is also possible to 206restrict authentication to a specific device by appending a 207colon followed by the device identifier 208.Cm bsdauth , 209.Cm pam , 210or 211.Cm skey , 212depending on the server configuration. 213For example, 214.Qq keyboard-interactive:bsdauth 215would restrict keyboard interactive authentication to the 216.Cm bsdauth 217device. 218.Pp 219If the publickey method is listed more than once, 220.Xr sshd 8 221verifies that keys that have been used successfully are not reused for 222subsequent authentications. 223For example, 224.Qq publickey,publickey 225requires successful authentication using two different public keys. 226.Pp 227Note that each authentication method listed should also be explicitly enabled 228in the configuration. 229.Pp 230The available authentication methods are: 231.Qq gssapi-with-mic , 232.Qq hostbased , 233.Qq keyboard-interactive , 234.Qq none 235(used for access to password-less accounts when 236.Cm PermitEmptyPasswords 237is enabled), 238.Qq password 239and 240.Qq publickey . 241.It Cm AuthorizedKeysCommand 242Specifies a program to be used to look up the user's public keys. 243The program must be owned by root, not writable by group or others and 244specified by an absolute path. 245Arguments to 246.Cm AuthorizedKeysCommand 247accept the tokens described in the 248.Sx TOKENS 249section. 250If no arguments are specified then the username of the target user is used. 251.Pp 252The program should produce on standard output zero or 253more lines of authorized_keys output (see 254.Sx AUTHORIZED_KEYS 255in 256.Xr sshd 8 ) . 257.Cm AuthorizedKeysCommand 258is tried after the usual 259.Cm AuthorizedKeysFile 260files and will not be executed if a matching key is found there. 261By default, no 262.Cm AuthorizedKeysCommand 263is run. 264.It Cm AuthorizedKeysCommandUser 265Specifies the user under whose account the 266.Cm AuthorizedKeysCommand 267is run. 268It is recommended to use a dedicated user that has no other role on the host 269than running authorized keys commands. 270If 271.Cm AuthorizedKeysCommand 272is specified but 273.Cm AuthorizedKeysCommandUser 274is not, then 275.Xr sshd 8 276will refuse to start. 277.It Cm AuthorizedKeysFile 278Specifies the file that contains the public keys used for user authentication. 279The format is described in the AUTHORIZED_KEYS FILE FORMAT section of 280.Xr sshd 8 . 281Arguments to 282.Cm AuthorizedKeysFile 283accept the tokens described in the 284.Sx TOKENS 285section. 286After expansion, 287.Cm AuthorizedKeysFile 288is taken to be an absolute path or one relative to the user's home 289directory. 290Multiple files may be listed, separated by whitespace. 291Alternately this option may be set to 292.Cm none 293to skip checking for user keys in files. 294The default is 295.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 296.It Cm AuthorizedPrincipalsCommand 297Specifies a program to be used to generate the list of allowed 298certificate principals as per 299.Cm AuthorizedPrincipalsFile . 300The program must be owned by root, not writable by group or others and 301specified by an absolute path. 302Arguments to 303.Cm AuthorizedPrincipalsCommand 304accept the tokens described in the 305.Sx TOKENS 306section. 307If no arguments are specified then the username of the target user is used. 308.Pp 309The program should produce on standard output zero or 310more lines of 311.Cm AuthorizedPrincipalsFile 312output. 313If either 314.Cm AuthorizedPrincipalsCommand 315or 316.Cm AuthorizedPrincipalsFile 317is specified, then certificates offered by the client for authentication 318must contain a principal that is listed. 319By default, no 320.Cm AuthorizedPrincipalsCommand 321is run. 322.It Cm AuthorizedPrincipalsCommandUser 323Specifies the user under whose account the 324.Cm AuthorizedPrincipalsCommand 325is run. 326It is recommended to use a dedicated user that has no other role on the host 327than running authorized principals commands. 328If 329.Cm AuthorizedPrincipalsCommand 330is specified but 331.Cm AuthorizedPrincipalsCommandUser 332is not, then 333.Xr sshd 8 334will refuse to start. 335.It Cm AuthorizedPrincipalsFile 336Specifies a file that lists principal names that are accepted for 337certificate authentication. 338When using certificates signed by a key listed in 339.Cm TrustedUserCAKeys , 340this file lists names, one of which must appear in the certificate for it 341to be accepted for authentication. 342Names are listed one per line preceded by key options (as described in 343.Sx AUTHORIZED_KEYS FILE FORMAT 344in 345.Xr sshd 8 ) . 346Empty lines and comments starting with 347.Ql # 348are ignored. 349.Pp 350Arguments to 351.Cm AuthorizedPrincipalsFile 352accept the tokens described in the 353.Sx TOKENS 354section. 355After expansion, 356.Cm AuthorizedPrincipalsFile 357is taken to be an absolute path or one relative to the user's home directory. 358The default is 359.Cm none , 360i.e. not to use a principals file \(en in this case, the username 361of the user must appear in a certificate's principals list for it to be 362accepted. 363.Pp 364Note that 365.Cm AuthorizedPrincipalsFile 366is only used when authentication proceeds using a CA listed in 367.Cm TrustedUserCAKeys 368and is not consulted for certification authorities trusted via 369.Pa ~/.ssh/authorized_keys , 370though the 371.Cm principals= 372key option offers a similar facility (see 373.Xr sshd 8 374for details). 375.It Cm Banner 376The contents of the specified file are sent to the remote user before 377authentication is allowed. 378If the argument is 379.Cm none 380then no banner is displayed. 381By default, no banner is displayed. 382.It Cm CASignatureAlgorithms 383Specifies which algorithms are allowed for signing of certificates 384by certificate authorities (CAs). 385The default is: 386.Bd -literal -offset indent 387ssh-ed25519,ecdsa-sha2-nistp256, 388ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 389sk-ssh-ed25519@openssh.com, 390sk-ecdsa-sha2-nistp256@openssh.com, 391rsa-sha2-512,rsa-sha2-256 392.Ed 393.Pp 394If the specified list begins with a 395.Sq + 396character, then the specified algorithms will be appended to the default set 397instead of replacing them. 398If the specified list begins with a 399.Sq - 400character, then the specified algorithms (including wildcards) will be removed 401from the default set instead of replacing them. 402.Pp 403Certificates signed using other algorithms will not be accepted for 404public key or host-based authentication. 405.It Cm ChannelTimeout 406Specifies whether and how quickly 407.Xr sshd 8 408should close inactive channels. 409Timeouts are specified as one or more 410.Dq type=interval 411pairs separated by whitespace, where the 412.Dq type 413must be the special keyword 414.Dq global 415or a channel type name from the list below, optionally containing 416wildcard characters. 417.Pp 418The timeout value 419.Dq interval 420is specified in seconds or may use any of the units documented in the 421.Sx TIME FORMATS 422section. 423For example, 424.Dq session=5m 425would cause interactive sessions to terminate after five minutes of 426inactivity. 427Specifying a zero value disables the inactivity timeout. 428.Pp 429The special timeout 430.Dq global 431applies to all active channels, taken together. 432Traffic on any active channel will reset the timeout, but when the timeout 433expires then all open channels will be closed. 434Note that this global timeout is not matched by wildcards and must be 435specified explicitly. 436.Pp 437The available channel type names include: 438.Bl -tag -width Ds 439.It Cm agent-connection 440Open connections to 441.Xr ssh-agent 1 . 442.It Cm direct-tcpip , Cm direct-streamlocal@openssh.com 443Open TCP or Unix socket (respectively) connections that have 444been established from a 445.Xr ssh 1 446local forwarding, i.e.\& 447.Cm LocalForward 448or 449.Cm DynamicForward . 450.It Cm forwarded-tcpip , Cm forwarded-streamlocal@openssh.com 451Open TCP or Unix socket (respectively) connections that have been 452established to a 453.Xr sshd 8 454listening on behalf of a 455.Xr ssh 1 456remote forwarding, i.e.\& 457.Cm RemoteForward . 458.It Cm session 459The interactive main session, including shell session, command execution, 460.Xr scp 1 , 461.Xr sftp 1 , 462etc. 463.It Cm tun-connection 464Open 465.Cm TunnelForward 466connections. 467.It Cm x11-connection 468Open X11 forwarding sessions. 469.El 470.Pp 471Note that in all the above cases, terminating an inactive session does not 472guarantee to remove all resources associated with the session, e.g. shell 473processes or X11 clients relating to the session may continue to execute. 474.Pp 475Moreover, terminating an inactive channel or session does not necessarily 476close the SSH connection, nor does it prevent a client from 477requesting another channel of the same type. 478In particular, expiring an inactive forwarding session does not prevent 479another identical forwarding from being subsequently created. 480.Pp 481The default is not to expire channels of any type for inactivity. 482.It Cm ChrootDirectory 483Specifies the pathname of a directory to 484.Xr chroot 2 485to after authentication. 486At session startup 487.Xr sshd 8 488checks that all components of the pathname are root-owned directories 489which are not writable by group or others. 490After the chroot, 491.Xr sshd 8 492changes the working directory to the user's home directory. 493Arguments to 494.Cm ChrootDirectory 495accept the tokens described in the 496.Sx TOKENS 497section. 498.Pp 499The 500.Cm ChrootDirectory 501must contain the necessary files and directories to support the 502user's session. 503For an interactive session this requires at least a shell, typically 504.Xr sh 1 , 505and basic 506.Pa /dev 507nodes such as 508.Xr null 4 , 509.Xr zero 4 , 510.Xr stdin 4 , 511.Xr stdout 4 , 512.Xr stderr 4 , 513and 514.Xr tty 4 515devices. 516For file transfer sessions using SFTP 517no additional configuration of the environment is necessary if the in-process 518sftp-server is used, 519though sessions which use logging may require 520.Pa /dev/log 521inside the chroot directory on some operating systems (see 522.Xr sftp-server 8 523for details). 524.Pp 525For safety, it is very important that the directory hierarchy be 526prevented from modification by other processes on the system (especially 527those outside the jail). 528Misconfiguration can lead to unsafe environments which 529.Xr sshd 8 530cannot detect. 531.Pp 532The default is 533.Cm none , 534indicating not to 535.Xr chroot 2 . 536.It Cm Ciphers 537Specifies the ciphers allowed. 538Multiple ciphers must be comma-separated. 539If the specified list begins with a 540.Sq + 541character, then the specified ciphers will be appended to the default set 542instead of replacing them. 543If the specified list begins with a 544.Sq - 545character, then the specified ciphers (including wildcards) will be removed 546from the default set instead of replacing them. 547If the specified list begins with a 548.Sq ^ 549character, then the specified ciphers will be placed at the head of the 550default set. 551.Pp 552The supported ciphers are: 553.Pp 554.Bl -item -compact -offset indent 555.It 5563des-cbc 557.It 558aes128-cbc 559.It 560aes192-cbc 561.It 562aes256-cbc 563.It 564aes128-ctr 565.It 566aes192-ctr 567.It 568aes256-ctr 569.It 570aes128-gcm@openssh.com 571.It 572aes256-gcm@openssh.com 573.It 574chacha20-poly1305@openssh.com 575.El 576.Pp 577The default is: 578.Bd -literal -offset indent 579chacha20-poly1305@openssh.com, 580aes128-ctr,aes192-ctr,aes256-ctr, 581aes128-gcm@openssh.com,aes256-gcm@openssh.com 582.Ed 583.Pp 584The list of available ciphers may also be obtained using 585.Qq ssh -Q cipher . 586.It Cm ClientAliveCountMax 587Sets the number of client alive messages which may be sent without 588.Xr sshd 8 589receiving any messages back from the client. 590If this threshold is reached while client alive messages are being sent, 591sshd will disconnect the client, terminating the session. 592It is important to note that the use of client alive messages is very 593different from 594.Cm TCPKeepAlive . 595The client alive messages are sent through the encrypted channel 596and therefore will not be spoofable. 597The TCP keepalive option enabled by 598.Cm TCPKeepAlive 599is spoofable. 600The client alive mechanism is valuable when the client or 601server depend on knowing when a connection has become unresponsive. 602.Pp 603The default value is 3. 604If 605.Cm ClientAliveInterval 606is set to 15, and 607.Cm ClientAliveCountMax 608is left at the default, unresponsive SSH clients 609will be disconnected after approximately 45 seconds. 610Setting a zero 611.Cm ClientAliveCountMax 612disables connection termination. 613.It Cm ClientAliveInterval 614Sets a timeout interval in seconds after which if no data has been received 615from the client, 616.Xr sshd 8 617will send a message through the encrypted 618channel to request a response from the client. 619The default 620is 0, indicating that these messages will not be sent to the client. 621.It Cm Compression 622Specifies whether compression is enabled after 623the user has authenticated successfully. 624The argument must be 625.Cm yes , 626.Cm delayed 627(a legacy synonym for 628.Cm yes ) 629or 630.Cm no . 631The default is 632.Cm yes . 633.It Cm DenyGroups 634This keyword can be followed by a list of group name patterns, separated 635by spaces. 636Login is disallowed for users whose primary group or supplementary 637group list matches one of the patterns. 638Only group names are valid; a numerical group ID is not recognized. 639By default, login is allowed for all groups. 640The allow/deny groups directives are processed in the following order: 641.Cm DenyGroups , 642.Cm AllowGroups . 643.Pp 644See PATTERNS in 645.Xr ssh_config 5 646for more information on patterns. 647This keyword may appear multiple times in 648.Nm 649with each instance appending to the list. 650.It Cm DenyUsers 651This keyword can be followed by a list of user name patterns, separated 652by spaces. 653Login is disallowed for user names that match one of the patterns. 654Only user names are valid; a numerical user ID is not recognized. 655By default, login is allowed for all users. 656If the pattern takes the form USER@HOST then USER and HOST 657are separately checked, restricting logins to particular 658users from particular hosts. 659HOST criteria may additionally contain addresses to match in CIDR 660address/masklen format. 661The allow/deny users directives are processed in the following order: 662.Cm DenyUsers , 663.Cm AllowUsers . 664.Pp 665See PATTERNS in 666.Xr ssh_config 5 667for more information on patterns. 668This keyword may appear multiple times in 669.Nm 670with each instance appending to the list. 671.It Cm DisableForwarding 672Disables all forwarding features, including X11, 673.Xr ssh-agent 1 , 674TCP and StreamLocal. 675This option overrides all other forwarding-related options and may 676simplify restricted configurations. 677.It Cm ExposeAuthInfo 678Writes a temporary file containing a list of authentication methods and 679public credentials (e.g. keys) used to authenticate the user. 680The location of the file is exposed to the user session through the 681.Ev SSH_USER_AUTH 682environment variable. 683The default is 684.Cm no . 685.It Cm FingerprintHash 686Specifies the hash algorithm used when logging key fingerprints. 687Valid options are: 688.Cm md5 689and 690.Cm sha256 . 691The default is 692.Cm sha256 . 693.It Cm ForceCommand 694Forces the execution of the command specified by 695.Cm ForceCommand , 696ignoring any command supplied by the client and 697.Pa ~/.ssh/rc 698if present. 699The command is invoked by using the user's login shell with the -c option. 700This applies to shell, command, or subsystem execution. 701It is most useful inside a 702.Cm Match 703block. 704The command originally supplied by the client is available in the 705.Ev SSH_ORIGINAL_COMMAND 706environment variable. 707Specifying a command of 708.Cm internal-sftp 709will force the use of an in-process SFTP server that requires no support 710files when used with 711.Cm ChrootDirectory . 712The default is 713.Cm none . 714.It Cm GatewayPorts 715Specifies whether remote hosts are allowed to connect to ports 716forwarded for the client. 717By default, 718.Xr sshd 8 719binds remote port forwardings to the loopback address. 720This prevents other remote hosts from connecting to forwarded ports. 721.Cm GatewayPorts 722can be used to specify that sshd 723should allow remote port forwardings to bind to non-loopback addresses, thus 724allowing other hosts to connect. 725The argument may be 726.Cm no 727to force remote port forwardings to be available to the local host only, 728.Cm yes 729to force remote port forwardings to bind to the wildcard address, or 730.Cm clientspecified 731to allow the client to select the address to which the forwarding is bound. 732The default is 733.Cm no . 734.It Cm GSSAPIAuthentication 735Specifies whether user authentication based on GSSAPI is allowed. 736The default is 737.Cm no . 738.It Cm GSSAPICleanupCredentials 739Specifies whether to automatically destroy the user's credentials cache 740on logout. 741The default is 742.Cm yes . 743.It Cm GSSAPIStrictAcceptorCheck 744Determines whether to be strict about the identity of the GSSAPI acceptor 745a client authenticates against. 746If set to 747.Cm yes 748then the client must authenticate against the host 749service on the current hostname. 750If set to 751.Cm no 752then the client may authenticate against any service key stored in the 753machine's default store. 754This facility is provided to assist with operation on multi homed machines. 755The default is 756.Cm yes . 757.It Cm HostbasedAcceptedAlgorithms 758Specifies the signature algorithms that will be accepted for hostbased 759authentication as a list of comma-separated patterns. 760Alternately if the specified list begins with a 761.Sq + 762character, then the specified signature algorithms will be appended to 763the default set instead of replacing them. 764If the specified list begins with a 765.Sq - 766character, then the specified signature algorithms (including wildcards) 767will be removed from the default set instead of replacing them. 768If the specified list begins with a 769.Sq ^ 770character, then the specified signature algorithms will be placed at 771the head of the default set. 772The default for this option is: 773.Bd -literal -offset 3n 774ssh-ed25519-cert-v01@openssh.com, 775ecdsa-sha2-nistp256-cert-v01@openssh.com, 776ecdsa-sha2-nistp384-cert-v01@openssh.com, 777ecdsa-sha2-nistp521-cert-v01@openssh.com, 778sk-ssh-ed25519-cert-v01@openssh.com, 779sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 780rsa-sha2-512-cert-v01@openssh.com, 781rsa-sha2-256-cert-v01@openssh.com, 782ssh-ed25519, 783ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 784sk-ssh-ed25519@openssh.com, 785sk-ecdsa-sha2-nistp256@openssh.com, 786rsa-sha2-512,rsa-sha2-256 787.Ed 788.Pp 789The list of available signature algorithms may also be obtained using 790.Qq ssh -Q HostbasedAcceptedAlgorithms . 791This was formerly named HostbasedAcceptedKeyTypes. 792.It Cm HostbasedAuthentication 793Specifies whether rhosts or /etc/hosts.equiv authentication together 794with successful public key client host authentication is allowed 795(host-based authentication). 796The default is 797.Cm no . 798.It Cm HostbasedUsesNameFromPacketOnly 799Specifies whether or not the server will attempt to perform a reverse 800name lookup when matching the name in the 801.Pa ~/.shosts , 802.Pa ~/.rhosts , 803and 804.Pa /etc/hosts.equiv 805files during 806.Cm HostbasedAuthentication . 807A setting of 808.Cm yes 809means that 810.Xr sshd 8 811uses the name supplied by the client rather than 812attempting to resolve the name from the TCP connection itself. 813The default is 814.Cm no . 815.It Cm HostCertificate 816Specifies a file containing a public host certificate. 817The certificate's public key must match a private host key already specified 818by 819.Cm HostKey . 820The default behaviour of 821.Xr sshd 8 822is not to load any certificates. 823.It Cm HostKey 824Specifies a file containing a private host key 825used by SSH. 826The defaults are 827.Pa /etc/ssh/ssh_host_ecdsa_key , 828.Pa /etc/ssh/ssh_host_ed25519_key 829and 830.Pa /etc/ssh/ssh_host_rsa_key . 831.Pp 832Note that 833.Xr sshd 8 834will refuse to use a file if it is group/world-accessible 835and that the 836.Cm HostKeyAlgorithms 837option restricts which of the keys are actually used by 838.Xr sshd 8 . 839.Pp 840It is possible to have multiple host key files. 841It is also possible to specify public host key files instead. 842In this case operations on the private key will be delegated 843to an 844.Xr ssh-agent 1 . 845.It Cm HostKeyAgent 846Identifies the UNIX-domain socket used to communicate 847with an agent that has access to the private host keys. 848If the string 849.Qq SSH_AUTH_SOCK 850is specified, the location of the socket will be read from the 851.Ev SSH_AUTH_SOCK 852environment variable. 853.It Cm HostKeyAlgorithms 854Specifies the host key signature algorithms 855that the server offers. 856The default for this option is: 857.Bd -literal -offset 3n 858ssh-ed25519-cert-v01@openssh.com, 859ecdsa-sha2-nistp256-cert-v01@openssh.com, 860ecdsa-sha2-nistp384-cert-v01@openssh.com, 861ecdsa-sha2-nistp521-cert-v01@openssh.com, 862sk-ssh-ed25519-cert-v01@openssh.com, 863sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 864rsa-sha2-512-cert-v01@openssh.com, 865rsa-sha2-256-cert-v01@openssh.com, 866ssh-ed25519, 867ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 868sk-ssh-ed25519@openssh.com, 869sk-ecdsa-sha2-nistp256@openssh.com, 870rsa-sha2-512,rsa-sha2-256 871.Ed 872.Pp 873The list of available signature algorithms may also be obtained using 874.Qq ssh -Q HostKeyAlgorithms . 875.It Cm IgnoreRhosts 876Specifies whether to ignore per-user 877.Pa .rhosts 878and 879.Pa .shosts 880files during 881.Cm HostbasedAuthentication . 882The system-wide 883.Pa /etc/hosts.equiv 884and 885.Pa /etc/shosts.equiv 886are still used regardless of this setting. 887.Pp 888Accepted values are 889.Cm yes 890(the default) to ignore all per-user files, 891.Cm shosts-only 892to allow the use of 893.Pa .shosts 894but to ignore 895.Pa .rhosts 896or 897.Cm no 898to allow both 899.Pa .shosts 900and 901.Pa rhosts . 902.It Cm IgnoreUserKnownHosts 903Specifies whether 904.Xr sshd 8 905should ignore the user's 906.Pa ~/.ssh/known_hosts 907during 908.Cm HostbasedAuthentication 909and use only the system-wide known hosts file 910.Pa /etc/ssh/ssh_known_hosts . 911The default is 912.Dq no . 913.It Cm Include 914Include the specified configuration file(s). 915Multiple pathnames may be specified and each pathname may contain 916.Xr glob 7 917wildcards that will be expanded and processed in lexical order. 918Files without absolute paths are assumed to be in 919.Pa /etc/ssh . 920An 921.Cm Include 922directive may appear inside a 923.Cm Match 924block 925to perform conditional inclusion. 926.It Cm IPQoS 927Specifies the IPv4 type-of-service or DSCP class for the connection. 928Accepted values are 929.Cm af11 , 930.Cm af12 , 931.Cm af13 , 932.Cm af21 , 933.Cm af22 , 934.Cm af23 , 935.Cm af31 , 936.Cm af32 , 937.Cm af33 , 938.Cm af41 , 939.Cm af42 , 940.Cm af43 , 941.Cm cs0 , 942.Cm cs1 , 943.Cm cs2 , 944.Cm cs3 , 945.Cm cs4 , 946.Cm cs5 , 947.Cm cs6 , 948.Cm cs7 , 949.Cm ef , 950.Cm le , 951.Cm lowdelay , 952.Cm throughput , 953.Cm reliability , 954a numeric value, or 955.Cm none 956to use the operating system default. 957This option may take one or two arguments, separated by whitespace. 958If one argument is specified, it is used as the packet class unconditionally. 959If two values are specified, the first is automatically selected for 960interactive sessions and the second for non-interactive sessions. 961The default is 962.Cm af21 963(Low-Latency Data) 964for interactive sessions and 965.Cm cs1 966(Lower Effort) 967for non-interactive sessions. 968.It Cm KbdInteractiveAuthentication 969Specifies whether to allow keyboard-interactive authentication. 970All authentication styles from 971.Xr login.conf 5 972are supported. 973The default is 974.Cm yes . 975The argument to this keyword must be 976.Cm yes 977or 978.Cm no . 979.Cm ChallengeResponseAuthentication 980is a deprecated alias for this. 981.It Cm KerberosAuthentication 982Specifies whether the password provided by the user for 983.Cm PasswordAuthentication 984will be validated through the Kerberos KDC. 985To use this option, the server needs a 986Kerberos servtab which allows the verification of the KDC's identity. 987The default is 988.Cm no . 989.It Cm KerberosGetAFSToken 990If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 991an AFS token before accessing the user's home directory. 992The default is 993.Cm no . 994.It Cm KerberosOrLocalPasswd 995If password authentication through Kerberos fails then 996the password will be validated via any additional local mechanism 997such as 998.Pa /etc/passwd . 999The default is 1000.Cm yes . 1001.It Cm KerberosTicketCleanup 1002Specifies whether to automatically destroy the user's ticket cache 1003file on logout. 1004The default is 1005.Cm yes . 1006.It Cm KexAlgorithms 1007Specifies the available KEX (Key Exchange) algorithms. 1008Multiple algorithms must be comma-separated. 1009Alternately if the specified list begins with a 1010.Sq + 1011character, then the specified algorithms will be appended to the default set 1012instead of replacing them. 1013If the specified list begins with a 1014.Sq - 1015character, then the specified algorithms (including wildcards) will be removed 1016from the default set instead of replacing them. 1017If the specified list begins with a 1018.Sq ^ 1019character, then the specified algorithms will be placed at the head of the 1020default set. 1021The supported algorithms are: 1022.Pp 1023.Bl -item -compact -offset indent 1024.It 1025curve25519-sha256 1026.It 1027curve25519-sha256@libssh.org 1028.It 1029diffie-hellman-group1-sha1 1030.It 1031diffie-hellman-group14-sha1 1032.It 1033diffie-hellman-group14-sha256 1034.It 1035diffie-hellman-group16-sha512 1036.It 1037diffie-hellman-group18-sha512 1038.It 1039diffie-hellman-group-exchange-sha1 1040.It 1041diffie-hellman-group-exchange-sha256 1042.It 1043ecdh-sha2-nistp256 1044.It 1045ecdh-sha2-nistp384 1046.It 1047ecdh-sha2-nistp521 1048.It 1049sntrup761x25519-sha512@openssh.com 1050.El 1051.Pp 1052The default is: 1053.Bd -literal -offset indent 1054sntrup761x25519-sha512@openssh.com, 1055curve25519-sha256,curve25519-sha256@libssh.org, 1056ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1057diffie-hellman-group-exchange-sha256, 1058diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 1059diffie-hellman-group14-sha256 1060.Ed 1061.Pp 1062The list of available key exchange algorithms may also be obtained using 1063.Qq ssh -Q KexAlgorithms . 1064.It Cm ListenAddress 1065Specifies the local addresses 1066.Xr sshd 8 1067should listen on. 1068The following forms may be used: 1069.Pp 1070.Bl -item -offset indent -compact 1071.It 1072.Cm ListenAddress 1073.Sm off 1074.Ar hostname | address 1075.Sm on 1076.Op Cm rdomain Ar domain 1077.It 1078.Cm ListenAddress 1079.Sm off 1080.Ar hostname : port 1081.Sm on 1082.Op Cm rdomain Ar domain 1083.It 1084.Cm ListenAddress 1085.Sm off 1086.Ar IPv4_address : port 1087.Sm on 1088.Op Cm rdomain Ar domain 1089.It 1090.Cm ListenAddress 1091.Sm off 1092.Oo Ar hostname | address Oc : Ar port 1093.Sm on 1094.Op Cm rdomain Ar domain 1095.El 1096.Pp 1097The optional 1098.Cm rdomain 1099qualifier requests 1100.Xr sshd 8 1101listen in an explicit routing domain. 1102If 1103.Ar port 1104is not specified, 1105sshd will listen on the address and all 1106.Cm Port 1107options specified. 1108The default is to listen on all local addresses on the current default 1109routing domain. 1110Multiple 1111.Cm ListenAddress 1112options are permitted. 1113For more information on routing domains, see 1114.Xr rdomain 4 . 1115.It Cm LoginGraceTime 1116The server disconnects after this time if the user has not 1117successfully logged in. 1118If the value is 0, there is no time limit. 1119The default is 120 seconds. 1120.It Cm LogLevel 1121Gives the verbosity level that is used when logging messages from 1122.Xr sshd 8 . 1123The possible values are: 1124QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1125The default is INFO. 1126DEBUG and DEBUG1 are equivalent. 1127DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1128Logging with a DEBUG level violates the privacy of users and is not recommended. 1129.It Cm LogVerbose 1130Specify one or more overrides to 1131.Cm LogLevel . 1132An override consists of a pattern lists that matches the source file, function 1133and line number to force detailed logging for. 1134For example, an override pattern of: 1135.Bd -literal -offset indent 1136kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1137.Ed 1138.Pp 1139would enable detailed logging for line 1000 of 1140.Pa kex.c , 1141everything in the 1142.Fn kex_exchange_identification 1143function, and all code in the 1144.Pa packet.c 1145file. 1146This option is intended for debugging and no overrides are enabled by default. 1147.It Cm MACs 1148Specifies the available MAC (message authentication code) algorithms. 1149The MAC algorithm is used for data integrity protection. 1150Multiple algorithms must be comma-separated. 1151If the specified list begins with a 1152.Sq + 1153character, then the specified algorithms will be appended to the default set 1154instead of replacing them. 1155If the specified list begins with a 1156.Sq - 1157character, then the specified algorithms (including wildcards) will be removed 1158from the default set instead of replacing them. 1159If the specified list begins with a 1160.Sq ^ 1161character, then the specified algorithms will be placed at the head of the 1162default set. 1163.Pp 1164The algorithms that contain 1165.Qq -etm 1166calculate the MAC after encryption (encrypt-then-mac). 1167These are considered safer and their use recommended. 1168The supported MACs are: 1169.Pp 1170.Bl -item -compact -offset indent 1171.It 1172hmac-md5 1173.It 1174hmac-md5-96 1175.It 1176hmac-sha1 1177.It 1178hmac-sha1-96 1179.It 1180hmac-sha2-256 1181.It 1182hmac-sha2-512 1183.It 1184umac-64@openssh.com 1185.It 1186umac-128@openssh.com 1187.It 1188hmac-md5-etm@openssh.com 1189.It 1190hmac-md5-96-etm@openssh.com 1191.It 1192hmac-sha1-etm@openssh.com 1193.It 1194hmac-sha1-96-etm@openssh.com 1195.It 1196hmac-sha2-256-etm@openssh.com 1197.It 1198hmac-sha2-512-etm@openssh.com 1199.It 1200umac-64-etm@openssh.com 1201.It 1202umac-128-etm@openssh.com 1203.El 1204.Pp 1205The default is: 1206.Bd -literal -offset indent 1207umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1208hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1209hmac-sha1-etm@openssh.com, 1210umac-64@openssh.com,umac-128@openssh.com, 1211hmac-sha2-256,hmac-sha2-512,hmac-sha1 1212.Ed 1213.Pp 1214The list of available MAC algorithms may also be obtained using 1215.Qq ssh -Q mac . 1216.It Cm Match 1217Introduces a conditional block. 1218If all of the criteria on the 1219.Cm Match 1220line are satisfied, the keywords on the following lines override those 1221set in the global section of the config file, until either another 1222.Cm Match 1223line or the end of the file. 1224If a keyword appears in multiple 1225.Cm Match 1226blocks that are satisfied, only the first instance of the keyword is 1227applied. 1228.Pp 1229The arguments to 1230.Cm Match 1231are one or more criteria-pattern pairs or the single token 1232.Cm All 1233which matches all criteria. 1234The available criteria are 1235.Cm User , 1236.Cm Group , 1237.Cm Host , 1238.Cm LocalAddress , 1239.Cm LocalPort , 1240.Cm RDomain , 1241and 1242.Cm Address 1243(with 1244.Cm RDomain 1245representing the 1246.Xr rdomain 4 1247on which the connection was received). 1248.Pp 1249The match patterns may consist of single entries or comma-separated 1250lists and may use the wildcard and negation operators described in the 1251.Sx PATTERNS 1252section of 1253.Xr ssh_config 5 . 1254.Pp 1255The patterns in an 1256.Cm Address 1257criteria may additionally contain addresses to match in CIDR 1258address/masklen format, 1259such as 192.0.2.0/24 or 2001:db8::/32. 1260Note that the mask length provided must be consistent with the address - 1261it is an error to specify a mask length that is too long for the address 1262or one with bits set in this host portion of the address. 1263For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1264.Pp 1265Only a subset of keywords may be used on the lines following a 1266.Cm Match 1267keyword. 1268Available keywords are 1269.Cm AcceptEnv , 1270.Cm AllowAgentForwarding , 1271.Cm AllowGroups , 1272.Cm AllowStreamLocalForwarding , 1273.Cm AllowTcpForwarding , 1274.Cm AllowUsers , 1275.Cm AuthenticationMethods , 1276.Cm AuthorizedKeysCommand , 1277.Cm AuthorizedKeysCommandUser , 1278.Cm AuthorizedKeysFile , 1279.Cm AuthorizedPrincipalsCommand , 1280.Cm AuthorizedPrincipalsCommandUser , 1281.Cm AuthorizedPrincipalsFile , 1282.Cm Banner , 1283.Cm CASignatureAlgorithms , 1284.Cm ChannelTimeout , 1285.Cm ChrootDirectory , 1286.Cm ClientAliveCountMax , 1287.Cm ClientAliveInterval , 1288.Cm DenyGroups , 1289.Cm DenyUsers , 1290.Cm DisableForwarding , 1291.Cm ExposeAuthInfo , 1292.Cm ForceCommand , 1293.Cm GatewayPorts , 1294.Cm GSSAPIAuthentication , 1295.Cm HostbasedAcceptedAlgorithms , 1296.Cm HostbasedAuthentication , 1297.Cm HostbasedUsesNameFromPacketOnly , 1298.Cm IgnoreRhosts , 1299.Cm Include , 1300.Cm IPQoS , 1301.Cm KbdInteractiveAuthentication , 1302.Cm KerberosAuthentication , 1303.Cm LogLevel , 1304.Cm MaxAuthTries , 1305.Cm MaxSessions , 1306.Cm PasswordAuthentication , 1307.Cm PermitEmptyPasswords , 1308.Cm PermitListen , 1309.Cm PermitOpen , 1310.Cm PermitRootLogin , 1311.Cm PermitTTY , 1312.Cm PermitTunnel , 1313.Cm PermitUserRC , 1314.Cm PubkeyAcceptedAlgorithms , 1315.Cm PubkeyAuthentication , 1316.Cm PubkeyAuthOptions , 1317.Cm RekeyLimit , 1318.Cm RevokedKeys , 1319.Cm RDomain , 1320.Cm SetEnv , 1321.Cm StreamLocalBindMask , 1322.Cm StreamLocalBindUnlink , 1323.Cm TrustedUserCAKeys , 1324.Cm UnusedConnectionTimeout , 1325.Cm X11DisplayOffset , 1326.Cm X11Forwarding 1327and 1328.Cm X11UseLocalhost . 1329.It Cm MaxAuthTries 1330Specifies the maximum number of authentication attempts permitted per 1331connection. 1332Once the number of failures reaches half this value, 1333additional failures are logged. 1334The default is 6. 1335.It Cm MaxSessions 1336Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1337sessions permitted per network connection. 1338Multiple sessions may be established by clients that support connection 1339multiplexing. 1340Setting 1341.Cm MaxSessions 1342to 1 will effectively disable session multiplexing, whereas setting it to 0 1343will prevent all shell, login and subsystem sessions while still permitting 1344forwarding. 1345The default is 10. 1346.It Cm MaxStartups 1347Specifies the maximum number of concurrent unauthenticated connections to the 1348SSH daemon. 1349Additional connections will be dropped until authentication succeeds or the 1350.Cm LoginGraceTime 1351expires for a connection. 1352The default is 10:30:100. 1353.Pp 1354Alternatively, random early drop can be enabled by specifying 1355the three colon separated values 1356start:rate:full (e.g. "10:30:60"). 1357.Xr sshd 8 1358will refuse connection attempts with a probability of rate/100 (30%) 1359if there are currently start (10) unauthenticated connections. 1360The probability increases linearly and all connection attempts 1361are refused if the number of unauthenticated connections reaches full (60). 1362.It Cm ModuliFile 1363Specifies the 1364.Xr moduli 5 1365file that contains the Diffie-Hellman groups used for the 1366.Dq diffie-hellman-group-exchange-sha1 1367and 1368.Dq diffie-hellman-group-exchange-sha256 1369key exchange methods. 1370The default is 1371.Pa /etc/moduli . 1372.It Cm PasswordAuthentication 1373Specifies whether password authentication is allowed. 1374The default is 1375.Cm yes . 1376.It Cm PermitEmptyPasswords 1377When password authentication is allowed, it specifies whether the 1378server allows login to accounts with empty password strings. 1379The default is 1380.Cm no . 1381.It Cm PermitListen 1382Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1383The listen specification must be one of the following forms: 1384.Pp 1385.Bl -item -offset indent -compact 1386.It 1387.Cm PermitListen 1388.Sm off 1389.Ar port 1390.Sm on 1391.It 1392.Cm PermitListen 1393.Sm off 1394.Ar host : port 1395.Sm on 1396.El 1397.Pp 1398Multiple permissions may be specified by separating them with whitespace. 1399An argument of 1400.Cm any 1401can be used to remove all restrictions and permit any listen requests. 1402An argument of 1403.Cm none 1404can be used to prohibit all listen requests. 1405The host name may contain wildcards as described in the PATTERNS section in 1406.Xr ssh_config 5 . 1407The wildcard 1408.Sq * 1409can also be used in place of a port number to allow all ports. 1410By default all port forwarding listen requests are permitted. 1411Note that the 1412.Cm GatewayPorts 1413option may further restrict which addresses may be listened on. 1414Note also that 1415.Xr ssh 1 1416will request a listen host of 1417.Dq localhost 1418if no listen host was specifically requested, and this name is 1419treated differently to explicit localhost addresses of 1420.Dq 127.0.0.1 1421and 1422.Dq ::1 . 1423.It Cm PermitOpen 1424Specifies the destinations to which TCP port forwarding is permitted. 1425The forwarding specification must be one of the following forms: 1426.Pp 1427.Bl -item -offset indent -compact 1428.It 1429.Cm PermitOpen 1430.Sm off 1431.Ar host : port 1432.Sm on 1433.It 1434.Cm PermitOpen 1435.Sm off 1436.Ar IPv4_addr : port 1437.Sm on 1438.It 1439.Cm PermitOpen 1440.Sm off 1441.Ar \&[ IPv6_addr \&] : port 1442.Sm on 1443.El 1444.Pp 1445Multiple forwards may be specified by separating them with whitespace. 1446An argument of 1447.Cm any 1448can be used to remove all restrictions and permit any forwarding requests. 1449An argument of 1450.Cm none 1451can be used to prohibit all forwarding requests. 1452The wildcard 1453.Sq * 1454can be used for host or port to allow all hosts or ports respectively. 1455Otherwise, no pattern matching or address lookups are performed on supplied 1456names. 1457By default all port forwarding requests are permitted. 1458.It Cm PermitRootLogin 1459Specifies whether root can log in using 1460.Xr ssh 1 . 1461The argument must be 1462.Cm yes , 1463.Cm prohibit-password , 1464.Cm forced-commands-only , 1465or 1466.Cm no . 1467The default is 1468.Cm prohibit-password . 1469.Pp 1470If this option is set to 1471.Cm prohibit-password 1472(or its deprecated alias, 1473.Cm without-password ) , 1474password and keyboard-interactive authentication are disabled for root. 1475.Pp 1476If this option is set to 1477.Cm forced-commands-only , 1478root login with public key authentication will be allowed, 1479but only if the 1480.Ar command 1481option has been specified 1482(which may be useful for taking remote backups even if root login is 1483normally not allowed). 1484All other authentication methods are disabled for root. 1485.Pp 1486If this option is set to 1487.Cm no , 1488root is not allowed to log in. 1489.It Cm PermitTTY 1490Specifies whether 1491.Xr pty 4 1492allocation is permitted. 1493The default is 1494.Cm yes . 1495.It Cm PermitTunnel 1496Specifies whether 1497.Xr tun 4 1498device forwarding is allowed. 1499The argument must be 1500.Cm yes , 1501.Cm point-to-point 1502(layer 3), 1503.Cm ethernet 1504(layer 2), or 1505.Cm no . 1506Specifying 1507.Cm yes 1508permits both 1509.Cm point-to-point 1510and 1511.Cm ethernet . 1512The default is 1513.Cm no . 1514.Pp 1515Independent of this setting, the permissions of the selected 1516.Xr tun 4 1517device must allow access to the user. 1518.It Cm PermitUserEnvironment 1519Specifies whether 1520.Pa ~/.ssh/environment 1521and 1522.Cm environment= 1523options in 1524.Pa ~/.ssh/authorized_keys 1525are processed by 1526.Xr sshd 8 . 1527Valid options are 1528.Cm yes , 1529.Cm no 1530or a pattern-list specifying which environment variable names to accept 1531(for example 1532.Qq LANG,LC_* ) . 1533The default is 1534.Cm no . 1535Enabling environment processing may enable users to bypass access 1536restrictions in some configurations using mechanisms such as 1537.Ev LD_PRELOAD . 1538.It Cm PermitUserRC 1539Specifies whether any 1540.Pa ~/.ssh/rc 1541file is executed. 1542The default is 1543.Cm yes . 1544.It Cm PerSourceMaxStartups 1545Specifies the number of unauthenticated connections allowed from a 1546given source address, or 1547.Dq none 1548if there is no limit. 1549This limit is applied in addition to 1550.Cm MaxStartups , 1551whichever is lower. 1552The default is 1553.Cm none . 1554.It Cm PerSourceNetBlockSize 1555Specifies the number of bits of source address that are grouped together 1556for the purposes of applying PerSourceMaxStartups limits. 1557Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1558The default is 1559.Cm 32:128 , 1560which means each address is considered individually. 1561.It Cm PidFile 1562Specifies the file that contains the process ID of the 1563SSH daemon, or 1564.Cm none 1565to not write one. 1566The default is 1567.Pa /var/run/sshd.pid . 1568.It Cm Port 1569Specifies the port number that 1570.Xr sshd 8 1571listens on. 1572The default is 22. 1573Multiple options of this type are permitted. 1574See also 1575.Cm ListenAddress . 1576.It Cm PrintLastLog 1577Specifies whether 1578.Xr sshd 8 1579should print the date and time of the last user login when a user logs 1580in interactively. 1581The default is 1582.Cm yes . 1583.It Cm PrintMotd 1584Specifies whether 1585.Xr sshd 8 1586should print 1587.Pa /etc/motd 1588when a user logs in interactively. 1589(On some systems it is also printed by the shell, 1590.Pa /etc/profile , 1591or equivalent.) 1592The default is 1593.Cm yes . 1594.It Cm PubkeyAcceptedAlgorithms 1595Specifies the signature algorithms that will be accepted for public key 1596authentication as a list of comma-separated patterns. 1597Alternately if the specified list begins with a 1598.Sq + 1599character, then the specified algorithms will be appended to the default set 1600instead of replacing them. 1601If the specified list begins with a 1602.Sq - 1603character, then the specified algorithms (including wildcards) will be removed 1604from the default set instead of replacing them. 1605If the specified list begins with a 1606.Sq ^ 1607character, then the specified algorithms will be placed at the head of the 1608default set. 1609The default for this option is: 1610.Bd -literal -offset 3n 1611ssh-ed25519-cert-v01@openssh.com, 1612ecdsa-sha2-nistp256-cert-v01@openssh.com, 1613ecdsa-sha2-nistp384-cert-v01@openssh.com, 1614ecdsa-sha2-nistp521-cert-v01@openssh.com, 1615sk-ssh-ed25519-cert-v01@openssh.com, 1616sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1617rsa-sha2-512-cert-v01@openssh.com, 1618rsa-sha2-256-cert-v01@openssh.com, 1619ssh-ed25519, 1620ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1621sk-ssh-ed25519@openssh.com, 1622sk-ecdsa-sha2-nistp256@openssh.com, 1623rsa-sha2-512,rsa-sha2-256 1624.Ed 1625.Pp 1626The list of available signature algorithms may also be obtained using 1627.Qq ssh -Q PubkeyAcceptedAlgorithms . 1628.It Cm PubkeyAuthOptions 1629Sets one or more public key authentication options. 1630The supported keywords are: 1631.Cm none 1632(the default; indicating no additional options are enabled), 1633.Cm touch-required 1634and 1635.Cm verify-required . 1636.Pp 1637The 1638.Cm touch-required 1639option causes public key authentication using a FIDO authenticator algorithm 1640(i.e.\& 1641.Cm ecdsa-sk 1642or 1643.Cm ed25519-sk ) 1644to always require the signature to attest that a physically present user 1645explicitly confirmed the authentication (usually by touching the authenticator). 1646By default, 1647.Xr sshd 8 1648requires user presence unless overridden with an authorized_keys option. 1649The 1650.Cm touch-required 1651flag disables this override. 1652.Pp 1653The 1654.Cm verify-required 1655option requires a FIDO key signature attest that the user was verified, 1656e.g. via a PIN. 1657.Pp 1658Neither the 1659.Cm touch-required 1660or 1661.Cm verify-required 1662options have any effect for other, non-FIDO, public key types. 1663.It Cm PubkeyAuthentication 1664Specifies whether public key authentication is allowed. 1665The default is 1666.Cm yes . 1667.It Cm RekeyLimit 1668Specifies the maximum amount of data that may be transmitted or received 1669before the session key is renegotiated, optionally followed by a maximum 1670amount of time that may pass before the session key is renegotiated. 1671The first argument is specified in bytes and may have a suffix of 1672.Sq K , 1673.Sq M , 1674or 1675.Sq G 1676to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1677The default is between 1678.Sq 1G 1679and 1680.Sq 4G , 1681depending on the cipher. 1682The optional second value is specified in seconds and may use any of the 1683units documented in the 1684.Sx TIME FORMATS 1685section. 1686The default value for 1687.Cm RekeyLimit 1688is 1689.Cm default none , 1690which means that rekeying is performed after the cipher's default amount 1691of data has been sent or received and no time based rekeying is done. 1692.It Cm RequiredRSASize 1693Specifies the minimum RSA key size (in bits) that 1694.Xr sshd 8 1695will accept. 1696User and host-based authentication keys smaller than this limit will be 1697refused. 1698The default is 1699.Cm 1024 1700bits. 1701Note that this limit may only be raised from the default. 1702.It Cm RevokedKeys 1703Specifies revoked public keys file, or 1704.Cm none 1705to not use one. 1706Keys listed in this file will be refused for public key authentication. 1707Note that if this file is not readable, then public key authentication will 1708be refused for all users. 1709Keys may be specified as a text file, listing one public key per line, or as 1710an OpenSSH Key Revocation List (KRL) as generated by 1711.Xr ssh-keygen 1 . 1712For more information on KRLs, see the KEY REVOCATION LISTS section in 1713.Xr ssh-keygen 1 . 1714.It Cm RDomain 1715Specifies an explicit routing domain that is applied after authentication 1716has completed. 1717The user session, as well as any forwarded or listening IP sockets, 1718will be bound to this 1719.Xr rdomain 4 . 1720If the routing domain is set to 1721.Cm \&%D , 1722then the domain in which the incoming connection was received will be applied. 1723.It Cm SecurityKeyProvider 1724Specifies a path to a library that will be used when loading 1725FIDO authenticator-hosted keys, overriding the default of using 1726the built-in USB HID support. 1727.It Cm SetEnv 1728Specifies one or more environment variables to set in child sessions started 1729by 1730.Xr sshd 8 1731as 1732.Dq NAME=VALUE . 1733The environment value may be quoted (e.g. if it contains whitespace 1734characters). 1735Environment variables set by 1736.Cm SetEnv 1737override the default environment and any variables specified by the user 1738via 1739.Cm AcceptEnv 1740or 1741.Cm PermitUserEnvironment . 1742.It Cm StreamLocalBindMask 1743Sets the octal file creation mode mask 1744.Pq umask 1745used when creating a Unix-domain socket file for local or remote 1746port forwarding. 1747This option is only used for port forwarding to a Unix-domain socket file. 1748.Pp 1749The default value is 0177, which creates a Unix-domain socket file that is 1750readable and writable only by the owner. 1751Note that not all operating systems honor the file mode on Unix-domain 1752socket files. 1753.It Cm StreamLocalBindUnlink 1754Specifies whether to remove an existing Unix-domain socket file for local 1755or remote port forwarding before creating a new one. 1756If the socket file already exists and 1757.Cm StreamLocalBindUnlink 1758is not enabled, 1759.Nm sshd 1760will be unable to forward the port to the Unix-domain socket file. 1761This option is only used for port forwarding to a Unix-domain socket file. 1762.Pp 1763The argument must be 1764.Cm yes 1765or 1766.Cm no . 1767The default is 1768.Cm no . 1769.It Cm StrictModes 1770Specifies whether 1771.Xr sshd 8 1772should check file modes and ownership of the 1773user's files and home directory before accepting login. 1774This is normally desirable because novices sometimes accidentally leave their 1775directory or files world-writable. 1776The default is 1777.Cm yes . 1778Note that this does not apply to 1779.Cm ChrootDirectory , 1780whose permissions and ownership are checked unconditionally. 1781.It Cm Subsystem 1782Configures an external subsystem (e.g. file transfer daemon). 1783Arguments should be a subsystem name and a command (with optional arguments) 1784to execute upon subsystem request. 1785.Pp 1786The command 1787.Cm sftp-server 1788implements the SFTP file transfer subsystem. 1789.Pp 1790Alternately the name 1791.Cm internal-sftp 1792implements an in-process SFTP server. 1793This may simplify configurations using 1794.Cm ChrootDirectory 1795to force a different filesystem root on clients. 1796It accepts the same command line arguments as 1797.Cm sftp-server 1798and even though it is in-process, settings such as 1799.Cm LogLevel 1800or 1801.Cm SyslogFacility 1802do not apply to it and must be set explicitly via 1803command line arguments. 1804.Pp 1805By default no subsystems are defined. 1806.It Cm SyslogFacility 1807Gives the facility code that is used when logging messages from 1808.Xr sshd 8 . 1809The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1810LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1811The default is AUTH. 1812.It Cm TCPKeepAlive 1813Specifies whether the system should send TCP keepalive messages to the 1814other side. 1815If they are sent, death of the connection or crash of one 1816of the machines will be properly noticed. 1817However, this means that 1818connections will die if the route is down temporarily, and some people 1819find it annoying. 1820On the other hand, if TCP keepalives are not sent, 1821sessions may hang indefinitely on the server, leaving 1822.Qq ghost 1823users and consuming server resources. 1824.Pp 1825The default is 1826.Cm yes 1827(to send TCP keepalive messages), and the server will notice 1828if the network goes down or the client host crashes. 1829This avoids infinitely hanging sessions. 1830.Pp 1831To disable TCP keepalive messages, the value should be set to 1832.Cm no . 1833.It Cm TrustedUserCAKeys 1834Specifies a file containing public keys of certificate authorities that are 1835trusted to sign user certificates for authentication, or 1836.Cm none 1837to not use one. 1838Keys are listed one per line; empty lines and comments starting with 1839.Ql # 1840are allowed. 1841If a certificate is presented for authentication and has its signing CA key 1842listed in this file, then it may be used for authentication for any user 1843listed in the certificate's principals list. 1844Note that certificates that lack a list of principals will not be permitted 1845for authentication using 1846.Cm TrustedUserCAKeys . 1847For more details on certificates, see the CERTIFICATES section in 1848.Xr ssh-keygen 1 . 1849.It Cm UnusedConnectionTimeout 1850Specifies whether and how quickly 1851.Xr sshd 8 1852should close client connections with no open channels. 1853Open channels include active shell, command execution or subsystem 1854sessions, connected network, socket, agent or X11 forwardings. 1855Forwarding listeners, such as those from the 1856.Xr ssh 1 1857.Fl R 1858flag, are not considered as open channels and do not prevent the timeout. 1859The timeout value 1860is specified in seconds or may use any of the units documented in the 1861.Sx TIME FORMATS 1862section. 1863.Pp 1864Note that this timeout starts when the client connection completes 1865user authentication but before the client has an opportunity to open any 1866channels. 1867Caution should be used when using short timeout values, as they may not 1868provide sufficient time for the client to request and open its channels 1869before terminating the connection. 1870.Pp 1871The default 1872.Cm none 1873is to never expire connections for having no open channels. 1874This option may be useful in conjunction with 1875.Cm ChannelTimeout . 1876.It Cm UseDNS 1877Specifies whether 1878.Xr sshd 8 1879should look up the remote host name, and to check that 1880the resolved host name for the remote IP address maps back to the 1881very same IP address. 1882.Pp 1883If this option is set to 1884.Cm no 1885(the default) then only addresses and not host names may be used in 1886.Pa ~/.ssh/authorized_keys 1887.Cm from 1888and 1889.Nm 1890.Cm Match 1891.Cm Host 1892directives. 1893.It Cm VersionAddendum 1894Optionally specifies additional text to append to the SSH protocol banner 1895sent by the server upon connection. 1896The default is 1897.Cm none . 1898.It Cm X11DisplayOffset 1899Specifies the first display number available for 1900.Xr sshd 8 Ns 's 1901X11 forwarding. 1902This prevents sshd from interfering with real X11 servers. 1903The default is 10. 1904.It Cm X11Forwarding 1905Specifies whether X11 forwarding is permitted. 1906The argument must be 1907.Cm yes 1908or 1909.Cm no . 1910The default is 1911.Cm no . 1912.Pp 1913When X11 forwarding is enabled, there may be additional exposure to 1914the server and to client displays if the 1915.Xr sshd 8 1916proxy display is configured to listen on the wildcard address (see 1917.Cm X11UseLocalhost ) , 1918though this is not the default. 1919Additionally, the authentication spoofing and authentication data 1920verification and substitution occur on the client side. 1921The security risk of using X11 forwarding is that the client's X11 1922display server may be exposed to attack when the SSH client requests 1923forwarding (see the warnings for 1924.Cm ForwardX11 1925in 1926.Xr ssh_config 5 ) . 1927A system administrator may have a stance in which they want to 1928protect clients that may expose themselves to attack by unwittingly 1929requesting X11 forwarding, which can warrant a 1930.Cm no 1931setting. 1932.Pp 1933Note that disabling X11 forwarding does not prevent users from 1934forwarding X11 traffic, as users can always install their own forwarders. 1935.It Cm X11UseLocalhost 1936Specifies whether 1937.Xr sshd 8 1938should bind the X11 forwarding server to the loopback address or to 1939the wildcard address. 1940By default, 1941sshd binds the forwarding server to the loopback address and sets the 1942hostname part of the 1943.Ev DISPLAY 1944environment variable to 1945.Cm localhost . 1946This prevents remote hosts from connecting to the proxy display. 1947However, some older X11 clients may not function with this 1948configuration. 1949.Cm X11UseLocalhost 1950may be set to 1951.Cm no 1952to specify that the forwarding server should be bound to the wildcard 1953address. 1954The argument must be 1955.Cm yes 1956or 1957.Cm no . 1958The default is 1959.Cm yes . 1960.It Cm XAuthLocation 1961Specifies the full pathname of the 1962.Xr xauth 1 1963program, or 1964.Cm none 1965to not use one. 1966The default is 1967.Pa /usr/X11R6/bin/xauth . 1968.El 1969.Sh TIME FORMATS 1970.Xr sshd 8 1971command-line arguments and configuration file options that specify time 1972may be expressed using a sequence of the form: 1973.Sm off 1974.Ar time Op Ar qualifier , 1975.Sm on 1976where 1977.Ar time 1978is a positive integer value and 1979.Ar qualifier 1980is one of the following: 1981.Pp 1982.Bl -tag -width Ds -compact -offset indent 1983.It Aq Cm none 1984seconds 1985.It Cm s | Cm S 1986seconds 1987.It Cm m | Cm M 1988minutes 1989.It Cm h | Cm H 1990hours 1991.It Cm d | Cm D 1992days 1993.It Cm w | Cm W 1994weeks 1995.El 1996.Pp 1997Each member of the sequence is added together to calculate 1998the total time value. 1999.Pp 2000Time format examples: 2001.Pp 2002.Bl -tag -width Ds -compact -offset indent 2003.It 600 2004600 seconds (10 minutes) 2005.It 10m 200610 minutes 2007.It 1h30m 20081 hour 30 minutes (90 minutes) 2009.El 2010.Sh TOKENS 2011Arguments to some keywords can make use of tokens, 2012which are expanded at runtime: 2013.Pp 2014.Bl -tag -width XXXX -offset indent -compact 2015.It %% 2016A literal 2017.Sq % . 2018.It \&%C 2019Identifies the connection endpoints, containing 2020four space-separated values: client address, client port number, 2021server address, and server port number. 2022.It \&%D 2023The routing domain in which the incoming connection was received. 2024.It %F 2025The fingerprint of the CA key. 2026.It %f 2027The fingerprint of the key or certificate. 2028.It %h 2029The home directory of the user. 2030.It %i 2031The key ID in the certificate. 2032.It %K 2033The base64-encoded CA key. 2034.It %k 2035The base64-encoded key or certificate for authentication. 2036.It %s 2037The serial number of the certificate. 2038.It \&%T 2039The type of the CA key. 2040.It %t 2041The key or certificate type. 2042.It \&%U 2043The numeric user ID of the target user. 2044.It %u 2045The username. 2046.El 2047.Pp 2048.Cm AuthorizedKeysCommand 2049accepts the tokens %%, %C, %D, %f, %h, %k, %t, %U, and %u. 2050.Pp 2051.Cm AuthorizedKeysFile 2052accepts the tokens %%, %h, %U, and %u. 2053.Pp 2054.Cm AuthorizedPrincipalsCommand 2055accepts the tokens %%, %C, %D, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 2056.Pp 2057.Cm AuthorizedPrincipalsFile 2058accepts the tokens %%, %h, %U, and %u. 2059.Pp 2060.Cm ChrootDirectory 2061accepts the tokens %%, %h, %U, and %u. 2062.Pp 2063.Cm RoutingDomain 2064accepts the token %D. 2065.Sh FILES 2066.Bl -tag -width Ds 2067.It Pa /etc/ssh/sshd_config 2068Contains configuration data for 2069.Xr sshd 8 . 2070This file should be writable by root only, but it is recommended 2071(though not necessary) that it be world-readable. 2072.El 2073.Sh SEE ALSO 2074.Xr sftp-server 8 , 2075.Xr sshd 8 2076.Sh AUTHORS 2077.An -nosplit 2078OpenSSH is a derivative of the original and free 2079ssh 1.2.12 release by 2080.An Tatu Ylonen . 2081.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 2082.An Theo de Raadt 2083and 2084.An Dug Song 2085removed many bugs, re-added newer features and 2086created OpenSSH. 2087.An Markus Friedl 2088contributed the support for SSH protocol versions 1.5 and 2.0. 2089.An Niels Provos 2090and 2091.An Markus Friedl 2092contributed support for privilege separation. 2093