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 1.380 2024/12/06 16:24:27 djm Exp $ 37.Dd $Mdocdate: December 6 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.Cm 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 283may include wildcards and accept 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.Cm 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 352may include wildcards and accept 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-gcm@openssh.com,aes256-gcm@openssh.com, 581aes128-ctr,aes192-ctr,aes256-ctr 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 permitted KEX (Key Exchange) algorithms that the server will 1008offer to clients. 1009The ordering of this list is not important, as the client specifies the 1010preference order. 1011Multiple algorithms must be comma-separated. 1012.Pp 1013If the specified list begins with a 1014.Sq + 1015character, then the specified algorithms will be appended to the default set 1016instead of replacing them. 1017If the specified list begins with a 1018.Sq - 1019character, then the specified algorithms (including wildcards) will be removed 1020from the default set instead of replacing them. 1021If the specified list begins with a 1022.Sq ^ 1023character, then the specified algorithms will be placed at the head of the 1024default set. 1025.Pp 1026The supported algorithms are: 1027.Pp 1028.Bl -item -compact -offset indent 1029.It 1030curve25519-sha256 1031.It 1032curve25519-sha256@libssh.org 1033.It 1034diffie-hellman-group1-sha1 1035.It 1036diffie-hellman-group14-sha1 1037.It 1038diffie-hellman-group14-sha256 1039.It 1040diffie-hellman-group16-sha512 1041.It 1042diffie-hellman-group18-sha512 1043.It 1044diffie-hellman-group-exchange-sha1 1045.It 1046diffie-hellman-group-exchange-sha256 1047.It 1048ecdh-sha2-nistp256 1049.It 1050ecdh-sha2-nistp384 1051.It 1052ecdh-sha2-nistp521 1053.It 1054mlkem768x25519-sha256 1055.It 1056sntrup761x25519-sha512 1057.It 1058sntrup761x25519-sha512@openssh.com 1059.El 1060.Pp 1061The default is: 1062.Bd -literal -offset indent 1063mlkem768x25519-sha256, 1064sntrup761x25519-sha512,sntrup761x25519-sha512@openssh.com, 1065curve25519-sha256,curve25519-sha256@libssh.org, 1066ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521 1067.Ed 1068.Pp 1069The list of supported key exchange algorithms may also be obtained using 1070.Qq ssh -Q KexAlgorithms . 1071.It Cm ListenAddress 1072Specifies the local addresses 1073.Xr sshd 8 1074should listen on. 1075The following forms may be used: 1076.Pp 1077.Bl -item -offset indent -compact 1078.It 1079.Cm ListenAddress 1080.Sm off 1081.Ar hostname | address 1082.Sm on 1083.Op Cm rdomain Ar domain 1084.It 1085.Cm ListenAddress 1086.Sm off 1087.Ar hostname : port 1088.Sm on 1089.Op Cm rdomain Ar domain 1090.It 1091.Cm ListenAddress 1092.Sm off 1093.Ar IPv4_address : port 1094.Sm on 1095.Op Cm rdomain Ar domain 1096.It 1097.Cm ListenAddress 1098.Sm off 1099.Oo Ar hostname | address Oc : Ar port 1100.Sm on 1101.Op Cm rdomain Ar domain 1102.El 1103.Pp 1104The optional 1105.Cm rdomain 1106qualifier requests 1107.Xr sshd 8 1108listen in an explicit routing domain. 1109If 1110.Ar port 1111is not specified, 1112sshd will listen on the address and all 1113.Cm Port 1114options specified. 1115The default is to listen on all local addresses on the current default 1116routing domain. 1117Multiple 1118.Cm ListenAddress 1119options are permitted. 1120For more information on routing domains, see 1121.Xr rdomain 4 . 1122.It Cm LoginGraceTime 1123The server disconnects after this time if the user has not 1124successfully logged in. 1125If the value is 0, there is no time limit. 1126The default is 120 seconds. 1127.It Cm LogLevel 1128Gives the verbosity level that is used when logging messages from 1129.Xr sshd 8 . 1130The possible values are: 1131QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1132The default is INFO. 1133DEBUG and DEBUG1 are equivalent. 1134DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1135Logging with a DEBUG level violates the privacy of users and is not recommended. 1136.It Cm LogVerbose 1137Specify one or more overrides to 1138.Cm LogLevel . 1139An override consists of one or more pattern lists that matches the 1140source file, function and line number to force detailed logging for. 1141For example, an override pattern of: 1142.Bd -literal -offset indent 1143kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1144.Ed 1145.Pp 1146would enable detailed logging for line 1000 of 1147.Pa kex.c , 1148everything in the 1149.Fn kex_exchange_identification 1150function, and all code in the 1151.Pa packet.c 1152file. 1153This option is intended for debugging and no overrides are enabled by default. 1154.It Cm MACs 1155Specifies the available MAC (message authentication code) algorithms. 1156The MAC algorithm is used for data integrity protection. 1157Multiple algorithms must be comma-separated. 1158If the specified list begins with a 1159.Sq + 1160character, then the specified algorithms will be appended to the default set 1161instead of replacing them. 1162If the specified list begins with a 1163.Sq - 1164character, then the specified algorithms (including wildcards) will be removed 1165from the default set instead of replacing them. 1166If the specified list begins with a 1167.Sq ^ 1168character, then the specified algorithms will be placed at the head of the 1169default set. 1170.Pp 1171The algorithms that contain 1172.Qq -etm 1173calculate the MAC after encryption (encrypt-then-mac). 1174These are considered safer and their use recommended. 1175The supported MACs are: 1176.Pp 1177.Bl -item -compact -offset indent 1178.It 1179hmac-md5 1180.It 1181hmac-md5-96 1182.It 1183hmac-sha1 1184.It 1185hmac-sha1-96 1186.It 1187hmac-sha2-256 1188.It 1189hmac-sha2-512 1190.It 1191umac-64@openssh.com 1192.It 1193umac-128@openssh.com 1194.It 1195hmac-md5-etm@openssh.com 1196.It 1197hmac-md5-96-etm@openssh.com 1198.It 1199hmac-sha1-etm@openssh.com 1200.It 1201hmac-sha1-96-etm@openssh.com 1202.It 1203hmac-sha2-256-etm@openssh.com 1204.It 1205hmac-sha2-512-etm@openssh.com 1206.It 1207umac-64-etm@openssh.com 1208.It 1209umac-128-etm@openssh.com 1210.El 1211.Pp 1212The default is: 1213.Bd -literal -offset indent 1214umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1215hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1216hmac-sha1-etm@openssh.com, 1217umac-64@openssh.com,umac-128@openssh.com, 1218hmac-sha2-256,hmac-sha2-512,hmac-sha1 1219.Ed 1220.Pp 1221The list of available MAC algorithms may also be obtained using 1222.Qq ssh -Q mac . 1223.It Cm Match 1224Introduces a conditional block. 1225If all of the criteria on the 1226.Cm Match 1227line are satisfied, the keywords on the following lines override those 1228set in the global section of the config file, until either another 1229.Cm Match 1230line or the end of the file. 1231If a keyword appears in multiple 1232.Cm Match 1233blocks that are satisfied, only the first instance of the keyword is 1234applied. 1235.Pp 1236The arguments to 1237.Cm Match 1238are one or more criteria-pattern pairs or one of the single token criteria: 1239.Cm All , 1240which matches all criteria, or 1241.Cm Invalid-User , 1242which matches when the requested user-name does not match any known account. 1243The available criteria are 1244.Cm User , 1245.Cm Group , 1246.Cm Host , 1247.Cm LocalAddress , 1248.Cm LocalPort , 1249.Cm RDomain , 1250and 1251.Cm Address 1252(with 1253.Cm RDomain 1254representing the 1255.Xr rdomain 4 1256on which the connection was received). 1257.Pp 1258The match patterns may consist of single entries or comma-separated 1259lists and may use the wildcard and negation operators described in the 1260.Sx PATTERNS 1261section of 1262.Xr ssh_config 5 . 1263.Pp 1264The patterns in an 1265.Cm Address 1266criteria may additionally contain addresses to match in CIDR 1267address/masklen format, 1268such as 192.0.2.0/24 or 2001:db8::/32. 1269Note that the mask length provided must be consistent with the address - 1270it is an error to specify a mask length that is too long for the address 1271or one with bits set in this host portion of the address. 1272For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1273.Pp 1274Only a subset of keywords may be used on the lines following a 1275.Cm Match 1276keyword. 1277Available keywords are 1278.Cm AcceptEnv , 1279.Cm AllowAgentForwarding , 1280.Cm AllowGroups , 1281.Cm AllowStreamLocalForwarding , 1282.Cm AllowTcpForwarding , 1283.Cm AllowUsers , 1284.Cm AuthenticationMethods , 1285.Cm AuthorizedKeysCommand , 1286.Cm AuthorizedKeysCommandUser , 1287.Cm AuthorizedKeysFile , 1288.Cm AuthorizedPrincipalsCommand , 1289.Cm AuthorizedPrincipalsCommandUser , 1290.Cm AuthorizedPrincipalsFile , 1291.Cm Banner , 1292.Cm CASignatureAlgorithms , 1293.Cm ChannelTimeout , 1294.Cm ChrootDirectory , 1295.Cm ClientAliveCountMax , 1296.Cm ClientAliveInterval , 1297.Cm DenyGroups , 1298.Cm DenyUsers , 1299.Cm DisableForwarding , 1300.Cm ExposeAuthInfo , 1301.Cm ForceCommand , 1302.Cm GatewayPorts , 1303.Cm GSSAPIAuthentication , 1304.Cm HostbasedAcceptedAlgorithms , 1305.Cm HostbasedAuthentication , 1306.Cm HostbasedUsesNameFromPacketOnly , 1307.Cm IgnoreRhosts , 1308.Cm Include , 1309.Cm IPQoS , 1310.Cm KbdInteractiveAuthentication , 1311.Cm KerberosAuthentication , 1312.Cm LogLevel , 1313.Cm MaxAuthTries , 1314.Cm MaxSessions , 1315.Cm PasswordAuthentication , 1316.Cm PermitEmptyPasswords , 1317.Cm PermitListen , 1318.Cm PermitOpen , 1319.Cm PermitRootLogin , 1320.Cm PermitTTY , 1321.Cm PermitTunnel , 1322.Cm PermitUserRC , 1323.Cm PubkeyAcceptedAlgorithms , 1324.Cm PubkeyAuthentication , 1325.Cm PubkeyAuthOptions , 1326.Cm RefuseConnection , 1327.Cm RekeyLimit , 1328.Cm RevokedKeys , 1329.Cm RDomain , 1330.Cm SetEnv , 1331.Cm StreamLocalBindMask , 1332.Cm StreamLocalBindUnlink , 1333.Cm TrustedUserCAKeys , 1334.Cm UnusedConnectionTimeout , 1335.Cm X11DisplayOffset , 1336.Cm X11Forwarding 1337and 1338.Cm X11UseLocalhost . 1339.It Cm MaxAuthTries 1340Specifies the maximum number of authentication attempts permitted per 1341connection. 1342Once the number of failures reaches half this value, 1343additional failures are logged. 1344The default is 6. 1345.It Cm MaxSessions 1346Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1347sessions permitted per network connection. 1348Multiple sessions may be established by clients that support connection 1349multiplexing. 1350Setting 1351.Cm MaxSessions 1352to 1 will effectively disable session multiplexing, whereas setting it to 0 1353will prevent all shell, login and subsystem sessions while still permitting 1354forwarding. 1355The default is 10. 1356.It Cm MaxStartups 1357Specifies the maximum number of concurrent unauthenticated connections to the 1358SSH daemon. 1359Additional connections will be dropped until authentication succeeds or the 1360.Cm LoginGraceTime 1361expires for a connection. 1362The default is 10:30:100. 1363.Pp 1364Alternatively, random early drop can be enabled by specifying 1365the three colon separated values 1366start:rate:full (e.g. "10:30:60"). 1367.Xr sshd 8 1368will refuse connection attempts with a probability of rate/100 (30%) 1369if there are currently start (10) unauthenticated connections. 1370The probability increases linearly and all connection attempts 1371are refused if the number of unauthenticated connections reaches full (60). 1372.It Cm ModuliFile 1373Specifies the 1374.Xr moduli 5 1375file that contains the Diffie-Hellman groups used for the 1376.Dq diffie-hellman-group-exchange-sha1 1377and 1378.Dq diffie-hellman-group-exchange-sha256 1379key exchange methods. 1380The default is 1381.Pa /etc/moduli . 1382.It Cm PasswordAuthentication 1383Specifies whether password authentication is allowed. 1384The default is 1385.Cm yes . 1386.It Cm PermitEmptyPasswords 1387When password authentication is allowed, it specifies whether the 1388server allows login to accounts with empty password strings. 1389The default is 1390.Cm no . 1391.It Cm PermitListen 1392Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1393The listen specification must be one of the following forms: 1394.Pp 1395.Bl -item -offset indent -compact 1396.It 1397.Cm PermitListen 1398.Sm off 1399.Ar port 1400.Sm on 1401.It 1402.Cm PermitListen 1403.Sm off 1404.Ar host : port 1405.Sm on 1406.El 1407.Pp 1408Multiple permissions may be specified by separating them with whitespace. 1409An argument of 1410.Cm any 1411can be used to remove all restrictions and permit any listen requests. 1412An argument of 1413.Cm none 1414can be used to prohibit all listen requests. 1415The host name may contain wildcards as described in the PATTERNS section in 1416.Xr ssh_config 5 . 1417The wildcard 1418.Sq * 1419can also be used in place of a port number to allow all ports. 1420By default all port forwarding listen requests are permitted. 1421Note that the 1422.Cm GatewayPorts 1423option may further restrict which addresses may be listened on. 1424Note also that 1425.Xr ssh 1 1426will request a listen host of 1427.Dq localhost 1428if no listen host was specifically requested, and this name is 1429treated differently to explicit localhost addresses of 1430.Dq 127.0.0.1 1431and 1432.Dq ::1 . 1433.It Cm PermitOpen 1434Specifies the destinations to which TCP port forwarding is permitted. 1435The forwarding specification must be one of the following forms: 1436.Pp 1437.Bl -item -offset indent -compact 1438.It 1439.Cm PermitOpen 1440.Sm off 1441.Ar host : port 1442.Sm on 1443.It 1444.Cm PermitOpen 1445.Sm off 1446.Ar IPv4_addr : port 1447.Sm on 1448.It 1449.Cm PermitOpen 1450.Sm off 1451.Ar \&[ IPv6_addr \&] : port 1452.Sm on 1453.El 1454.Pp 1455Multiple forwards may be specified by separating them with whitespace. 1456An argument of 1457.Cm any 1458can be used to remove all restrictions and permit any forwarding requests. 1459An argument of 1460.Cm none 1461can be used to prohibit all forwarding requests. 1462The wildcard 1463.Sq * 1464can be used for host or port to allow all hosts or ports respectively. 1465Otherwise, no pattern matching or address lookups are performed on supplied 1466names. 1467By default all port forwarding requests are permitted. 1468.It Cm PermitRootLogin 1469Specifies whether root can log in using 1470.Xr ssh 1 . 1471The argument must be 1472.Cm yes , 1473.Cm prohibit-password , 1474.Cm forced-commands-only , 1475or 1476.Cm no . 1477The default is 1478.Cm prohibit-password . 1479.Pp 1480If this option is set to 1481.Cm prohibit-password 1482(or its deprecated alias, 1483.Cm without-password ) , 1484password and keyboard-interactive authentication are disabled for root. 1485.Pp 1486If this option is set to 1487.Cm forced-commands-only , 1488root login with public key authentication will be allowed, 1489but only if the 1490.Ar command 1491option has been specified 1492(which may be useful for taking remote backups even if root login is 1493normally not allowed). 1494All other authentication methods are disabled for root. 1495.Pp 1496If this option is set to 1497.Cm no , 1498root is not allowed to log in. 1499.It Cm PermitTTY 1500Specifies whether 1501.Xr pty 4 1502allocation is permitted. 1503The default is 1504.Cm yes . 1505.It Cm PermitTunnel 1506Specifies whether 1507.Xr tun 4 1508device forwarding is allowed. 1509The argument must be 1510.Cm yes , 1511.Cm point-to-point 1512(layer 3), 1513.Cm ethernet 1514(layer 2), or 1515.Cm no . 1516Specifying 1517.Cm yes 1518permits both 1519.Cm point-to-point 1520and 1521.Cm ethernet . 1522The default is 1523.Cm no . 1524.Pp 1525Independent of this setting, the permissions of the selected 1526.Xr tun 4 1527device must allow access to the user. 1528.It Cm PermitUserEnvironment 1529Specifies whether 1530.Pa ~/.ssh/environment 1531and 1532.Cm environment= 1533options in 1534.Pa ~/.ssh/authorized_keys 1535are processed by 1536.Xr sshd 8 . 1537Valid options are 1538.Cm yes , 1539.Cm no 1540or a pattern-list specifying which environment variable names to accept 1541(for example 1542.Qq LANG,LC_* ) . 1543The default is 1544.Cm no . 1545Enabling environment processing may enable users to bypass access 1546restrictions in some configurations using mechanisms such as 1547.Ev LD_PRELOAD . 1548.It Cm PermitUserRC 1549Specifies whether any 1550.Pa ~/.ssh/rc 1551file is executed. 1552The default is 1553.Cm yes . 1554.It Cm PerSourceMaxStartups 1555Specifies the number of unauthenticated connections allowed from a 1556given source address, or 1557.Dq none 1558if there is no limit. 1559This limit is applied in addition to 1560.Cm MaxStartups , 1561whichever is lower. 1562The default is 1563.Cm none . 1564.It Cm PerSourceNetBlockSize 1565Specifies the number of bits of source address that are grouped together 1566for the purposes of applying PerSourceMaxStartups limits. 1567Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1568The default is 1569.Cm 32:128 , 1570which means each address is considered individually. 1571.It Cm PerSourcePenalties 1572Controls penalties for various conditions that may represent attacks on 1573.Xr sshd 8 . 1574If a penalty is enforced against a client then its source address and any 1575others in the same network, as defined by 1576.Cm PerSourceNetBlockSize , 1577will be refused connection for a period. 1578.Pp 1579A penalty doesn't affect concurrent connections in progress, but multiple 1580penalties from the same source from concurrent connections will accumulate 1581up to a maximum. 1582Conversely, penalties are not applied until a minimum threshold time has been 1583accumulated. 1584.Pp 1585Penalties are enabled by default with the default settings listed below 1586but may disabled using the 1587.Cm no 1588keyword. 1589The defaults may be overridden by specifying one or more of the keywords below, 1590separated by whitespace. 1591All keywords accept arguments, e.g.\& 1592.Qq crash:2m . 1593.Bl -tag -width Ds 1594.It Cm crash:duration 1595Specifies how long to refuse clients that cause a crash of 1596.Xr sshd 8 (default: 90s). 1597.It Cm authfail:duration 1598Specifies how long to refuse clients that disconnect after making one or more 1599unsuccessful authentication attempts (default: 5s). 1600.It Cm refuseconnection:duration 1601Specifies how long to refuse clients that were administratively prohibited 1602connection via the 1603.Cm RefuseConnection 1604option (default: 10s). 1605.It Cm noauth:duration 1606Specifies how long to refuse clients that disconnect without attempting 1607authentication (default: 1s). 1608This timeout should be used cautiously otherwise it may penalise legitimate 1609scanning tools such as 1610.Xr ssh-keyscan 1 . 1611.It Cm grace-exceeded:duration 1612Specifies how long to refuse clients that fail to authenticate after 1613.Cm LoginGraceTime 1614(default: 10s). 1615.It Cm max:duration 1616Specifies the maximum time a particular source address range will be refused 1617access for (default: 10m). 1618Repeated penalties will accumulate up to this maximum. 1619.It Cm min:duration 1620Specifies the minimum penalty that must accrue before enforcement begins 1621(default: 15s). 1622.It Cm max-sources4:number , max-sources6:number 1623Specifies the maximum number of client IPv4 and IPv6 address ranges to 1624track for penalties (default: 65536 for both). 1625.It Cm overflow:mode 1626Controls how the server behaves when 1627.Cm max-sources4 1628or 1629.Cm max-sources6 1630is exceeded. 1631There are two operating modes: 1632.Cm deny-all , 1633which denies all incoming connections other than those exempted via 1634.Cm PerSourcePenaltyExemptList 1635until a penalty expires, and 1636.Cm permissive , 1637which allows new connections by removing existing penalties early 1638(default: permissive). 1639Note that client penalties below the 1640.Cm min 1641threshold count against the total number of tracked penalties. 1642IPv4 and IPv6 addresses are tracked separately, so an overflow in one will 1643not affect the other. 1644.It Cm overflow6:mode 1645Allows specifying a different overflow mode for IPv6 addresses. 1646The default it to use the same overflow mode as was specified for IPv4. 1647.El 1648.It Cm PerSourcePenaltyExemptList 1649Specifies a comma-separated list of addresses to exempt from penalties. 1650This list may contain wildcards and CIDR address/masklen ranges. 1651Note that the mask length provided must be consistent with the address - 1652it is an error to specify a mask length that is too long for the address 1653or one with bits set in this host portion of the address. 1654For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1655The default is not to exempt any addresses. 1656.It Cm PidFile 1657Specifies the file that contains the process ID of the 1658SSH daemon, or 1659.Cm none 1660to not write one. 1661The default is 1662.Pa /var/run/sshd.pid . 1663.It Cm Port 1664Specifies the port number that 1665.Xr sshd 8 1666listens on. 1667The default is 22. 1668Multiple options of this type are permitted. 1669See also 1670.Cm ListenAddress . 1671.It Cm PrintLastLog 1672Specifies whether 1673.Xr sshd 8 1674should print the date and time of the last user login when a user logs 1675in interactively. 1676The default is 1677.Cm yes . 1678.It Cm PrintMotd 1679Specifies whether 1680.Xr sshd 8 1681should print 1682.Pa /etc/motd 1683when a user logs in interactively. 1684(On some systems it is also printed by the shell, 1685.Pa /etc/profile , 1686or equivalent.) 1687The default is 1688.Cm yes . 1689.It Cm PubkeyAcceptedAlgorithms 1690Specifies the signature algorithms that will be accepted for public key 1691authentication as a list of comma-separated patterns. 1692Alternately if the specified list begins with a 1693.Sq + 1694character, then the specified algorithms will be appended to the default set 1695instead of replacing them. 1696If the specified list begins with a 1697.Sq - 1698character, then the specified algorithms (including wildcards) will be removed 1699from the default set instead of replacing them. 1700If the specified list begins with a 1701.Sq ^ 1702character, then the specified algorithms will be placed at the head of the 1703default set. 1704The default for this option is: 1705.Bd -literal -offset 3n 1706ssh-ed25519-cert-v01@openssh.com, 1707ecdsa-sha2-nistp256-cert-v01@openssh.com, 1708ecdsa-sha2-nistp384-cert-v01@openssh.com, 1709ecdsa-sha2-nistp521-cert-v01@openssh.com, 1710sk-ssh-ed25519-cert-v01@openssh.com, 1711sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1712rsa-sha2-512-cert-v01@openssh.com, 1713rsa-sha2-256-cert-v01@openssh.com, 1714ssh-ed25519, 1715ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1716sk-ssh-ed25519@openssh.com, 1717sk-ecdsa-sha2-nistp256@openssh.com, 1718rsa-sha2-512,rsa-sha2-256 1719.Ed 1720.Pp 1721The list of available signature algorithms may also be obtained using 1722.Qq ssh -Q PubkeyAcceptedAlgorithms . 1723.It Cm PubkeyAuthOptions 1724Sets one or more public key authentication options. 1725The supported keywords are: 1726.Cm none 1727(the default; indicating no additional options are enabled), 1728.Cm touch-required 1729and 1730.Cm verify-required . 1731.Pp 1732The 1733.Cm touch-required 1734option causes public key authentication using a FIDO authenticator algorithm 1735(i.e.\& 1736.Cm ecdsa-sk 1737or 1738.Cm ed25519-sk ) 1739to always require the signature to attest that a physically present user 1740explicitly confirmed the authentication (usually by touching the authenticator). 1741By default, 1742.Xr sshd 8 1743requires user presence unless overridden with an authorized_keys option. 1744The 1745.Cm touch-required 1746flag disables this override. 1747.Pp 1748The 1749.Cm verify-required 1750option requires a FIDO key signature attest that the user was verified, 1751e.g. via a PIN. 1752.Pp 1753Neither the 1754.Cm touch-required 1755or 1756.Cm verify-required 1757options have any effect for other, non-FIDO, public key types. 1758.It Cm PubkeyAuthentication 1759Specifies whether public key authentication is allowed. 1760The default is 1761.Cm yes . 1762.It Cm RefuseConnection 1763Indicates that 1764.Xr sshd 8 1765should unconditionally terminate the connection. 1766Additionally, a 1767.Cm refuseconnection 1768penalty may be recorded against the source of the connection if 1769.Cm PerSourcePenalties 1770are enabled. 1771This option is only really useful in a 1772.Cm Match 1773block. 1774.It Cm RekeyLimit 1775Specifies the maximum amount of data that may be transmitted or received 1776before the session key is renegotiated, optionally followed by a maximum 1777amount of time that may pass before the session key is renegotiated. 1778The first argument is specified in bytes and may have a suffix of 1779.Sq K , 1780.Sq M , 1781or 1782.Sq G 1783to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1784The default is between 1785.Sq 1G 1786and 1787.Sq 4G , 1788depending on the cipher. 1789The optional second value is specified in seconds and may use any of the 1790units documented in the 1791.Sx TIME FORMATS 1792section. 1793The default value for 1794.Cm RekeyLimit 1795is 1796.Cm default none , 1797which means that rekeying is performed after the cipher's default amount 1798of data has been sent or received and no time based rekeying is done. 1799.It Cm RequiredRSASize 1800Specifies the minimum RSA key size (in bits) that 1801.Xr sshd 8 1802will accept. 1803User and host-based authentication keys smaller than this limit will be 1804refused. 1805The default is 1806.Cm 1024 1807bits. 1808Note that this limit may only be raised from the default. 1809.It Cm RevokedKeys 1810Specifies revoked public keys file, or 1811.Cm none 1812to not use one. 1813Keys listed in this file will be refused for public key authentication. 1814Note that if this file is not readable, then public key authentication will 1815be refused for all users. 1816Keys may be specified as a text file, listing one public key per line, or as 1817an OpenSSH Key Revocation List (KRL) as generated by 1818.Xr ssh-keygen 1 . 1819For more information on KRLs, see the KEY REVOCATION LISTS section in 1820.Xr ssh-keygen 1 . 1821.It Cm RDomain 1822Specifies an explicit routing domain that is applied after authentication 1823has completed. 1824The user session, as well as any forwarded or listening IP sockets, 1825will be bound to this 1826.Xr rdomain 4 . 1827If the routing domain is set to 1828.Cm \&%D , 1829then the domain in which the incoming connection was received will be applied. 1830.It Cm SecurityKeyProvider 1831Specifies a path to a library that will be used when loading 1832FIDO authenticator-hosted keys, overriding the default of using 1833the built-in USB HID support. 1834.It Cm SetEnv 1835Specifies one or more environment variables to set in child sessions started 1836by 1837.Xr sshd 8 1838as 1839.Dq NAME=VALUE . 1840The environment value may be quoted (e.g. if it contains whitespace 1841characters). 1842Environment variables set by 1843.Cm SetEnv 1844override the default environment and any variables specified by the user 1845via 1846.Cm AcceptEnv 1847or 1848.Cm PermitUserEnvironment . 1849.It Cm SshdAuthPath 1850Overrides the default path to the 1851.Cm sshd-auth 1852binary that is invoked to complete user authentication. 1853The default is 1854.Pa /usr/libexec/sshd-auth . 1855This option is intended for use by tests. 1856.It Cm SshdSessionPath 1857Overrides the default path to the 1858.Cm sshd-session 1859binary that is invoked to handle each connection. 1860The default is 1861.Pa /usr/libexec/sshd-session . 1862This option is intended for use by tests. 1863.It Cm StreamLocalBindMask 1864Sets the octal file creation mode mask 1865.Pq umask 1866used when creating a Unix-domain socket file for local or remote 1867port forwarding. 1868This option is only used for port forwarding to a Unix-domain socket file. 1869.Pp 1870The default value is 0177, which creates a Unix-domain socket file that is 1871readable and writable only by the owner. 1872Note that not all operating systems honor the file mode on Unix-domain 1873socket files. 1874.It Cm StreamLocalBindUnlink 1875Specifies whether to remove an existing Unix-domain socket file for local 1876or remote port forwarding before creating a new one. 1877If the socket file already exists and 1878.Cm StreamLocalBindUnlink 1879is not enabled, 1880.Nm sshd 1881will be unable to forward the port to the Unix-domain socket file. 1882This option is only used for port forwarding to a Unix-domain socket file. 1883.Pp 1884The argument must be 1885.Cm yes 1886or 1887.Cm no . 1888The default is 1889.Cm no . 1890.It Cm StrictModes 1891Specifies whether 1892.Xr sshd 8 1893should check file modes and ownership of the 1894user's files and home directory before accepting login. 1895This is normally desirable because novices sometimes accidentally leave their 1896directory or files world-writable. 1897The default is 1898.Cm yes . 1899Note that this does not apply to 1900.Cm ChrootDirectory , 1901whose permissions and ownership are checked unconditionally. 1902.It Cm Subsystem 1903Configures an external subsystem (e.g. file transfer daemon). 1904Arguments should be a subsystem name and a command (with optional arguments) 1905to execute upon subsystem request. 1906.Pp 1907The command 1908.Cm sftp-server 1909implements the SFTP file transfer subsystem. 1910.Pp 1911Alternately the name 1912.Cm internal-sftp 1913implements an in-process SFTP server. 1914This may simplify configurations using 1915.Cm ChrootDirectory 1916to force a different filesystem root on clients. 1917It accepts the same command line arguments as 1918.Cm sftp-server 1919and even though it is in-process, settings such as 1920.Cm LogLevel 1921or 1922.Cm SyslogFacility 1923do not apply to it and must be set explicitly via 1924command line arguments. 1925.Pp 1926By default no subsystems are defined. 1927.It Cm SyslogFacility 1928Gives the facility code that is used when logging messages from 1929.Xr sshd 8 . 1930The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1931LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1932The default is AUTH. 1933.It Cm TCPKeepAlive 1934Specifies whether the system should send TCP keepalive messages to the 1935other side. 1936If they are sent, death of the connection or crash of one 1937of the machines will be properly noticed. 1938However, this means that 1939connections will die if the route is down temporarily, and some people 1940find it annoying. 1941On the other hand, if TCP keepalives are not sent, 1942sessions may hang indefinitely on the server, leaving 1943.Qq ghost 1944users and consuming server resources. 1945.Pp 1946The default is 1947.Cm yes 1948(to send TCP keepalive messages), and the server will notice 1949if the network goes down or the client host crashes. 1950This avoids infinitely hanging sessions. 1951.Pp 1952To disable TCP keepalive messages, the value should be set to 1953.Cm no . 1954.It Cm TrustedUserCAKeys 1955Specifies a file containing public keys of certificate authorities that are 1956trusted to sign user certificates for authentication, or 1957.Cm none 1958to not use one. 1959Keys are listed one per line; empty lines and comments starting with 1960.Ql # 1961are allowed. 1962If a certificate is presented for authentication and has its signing CA key 1963listed in this file, then it may be used for authentication for any user 1964listed in the certificate's principals list. 1965Note that certificates that lack a list of principals will not be permitted 1966for authentication using 1967.Cm TrustedUserCAKeys . 1968For more details on certificates, see the CERTIFICATES section in 1969.Xr ssh-keygen 1 . 1970.It Cm UnusedConnectionTimeout 1971Specifies whether and how quickly 1972.Xr sshd 8 1973should close client connections with no open channels. 1974Open channels include active shell, command execution or subsystem 1975sessions, connected network, socket, agent or X11 forwardings. 1976Forwarding listeners, such as those from the 1977.Xr ssh 1 1978.Fl R 1979flag, are not considered as open channels and do not prevent the timeout. 1980The timeout value 1981is specified in seconds or may use any of the units documented in the 1982.Sx TIME FORMATS 1983section. 1984.Pp 1985Note that this timeout starts when the client connection completes 1986user authentication but before the client has an opportunity to open any 1987channels. 1988Caution should be used when using short timeout values, as they may not 1989provide sufficient time for the client to request and open its channels 1990before terminating the connection. 1991.Pp 1992The default 1993.Cm none 1994is to never expire connections for having no open channels. 1995This option may be useful in conjunction with 1996.Cm ChannelTimeout . 1997.It Cm UseDNS 1998Specifies whether 1999.Xr sshd 8 2000should look up the remote host name, and to check that 2001the resolved host name for the remote IP address maps back to the 2002very same IP address. 2003.Pp 2004If this option is set to 2005.Cm no 2006(the default) then only addresses and not host names may be used in 2007.Pa ~/.ssh/authorized_keys 2008.Cm from 2009and 2010.Nm 2011.Cm Match 2012.Cm Host 2013directives. 2014.It Cm VersionAddendum 2015Optionally specifies additional text to append to the SSH protocol banner 2016sent by the server upon connection. 2017The default is 2018.Cm none . 2019.It Cm X11DisplayOffset 2020Specifies the first display number available for 2021.Xr sshd 8 Ns 's 2022X11 forwarding. 2023This prevents sshd from interfering with real X11 servers. 2024The default is 10. 2025.It Cm X11Forwarding 2026Specifies whether X11 forwarding is permitted. 2027The argument must be 2028.Cm yes 2029or 2030.Cm no . 2031The default is 2032.Cm no . 2033.Pp 2034When X11 forwarding is enabled, there may be additional exposure to 2035the server and to client displays if the 2036.Xr sshd 8 2037proxy display is configured to listen on the wildcard address (see 2038.Cm X11UseLocalhost ) , 2039though this is not the default. 2040Additionally, the authentication spoofing and authentication data 2041verification and substitution occur on the client side. 2042The security risk of using X11 forwarding is that the client's X11 2043display server may be exposed to attack when the SSH client requests 2044forwarding (see the warnings for 2045.Cm ForwardX11 2046in 2047.Xr ssh_config 5 ) . 2048A system administrator may have a stance in which they want to 2049protect clients that may expose themselves to attack by unwittingly 2050requesting X11 forwarding, which can warrant a 2051.Cm no 2052setting. 2053.Pp 2054Note that disabling X11 forwarding does not prevent users from 2055forwarding X11 traffic, as users can always install their own forwarders. 2056.It Cm X11UseLocalhost 2057Specifies whether 2058.Xr sshd 8 2059should bind the X11 forwarding server to the loopback address or to 2060the wildcard address. 2061By default, 2062sshd binds the forwarding server to the loopback address and sets the 2063hostname part of the 2064.Ev DISPLAY 2065environment variable to 2066.Cm localhost . 2067This prevents remote hosts from connecting to the proxy display. 2068However, some older X11 clients may not function with this 2069configuration. 2070.Cm X11UseLocalhost 2071may be set to 2072.Cm no 2073to specify that the forwarding server should be bound to the wildcard 2074address. 2075The argument must be 2076.Cm yes 2077or 2078.Cm no . 2079The default is 2080.Cm yes . 2081.It Cm XAuthLocation 2082Specifies the full pathname of the 2083.Xr xauth 1 2084program, or 2085.Cm none 2086to not use one. 2087The default is 2088.Pa /usr/X11R6/bin/xauth . 2089.El 2090.Sh TIME FORMATS 2091.Xr sshd 8 2092command-line arguments and configuration file options that specify time 2093may be expressed using a sequence of the form: 2094.Sm off 2095.Ar time Op Ar qualifier , 2096.Sm on 2097where 2098.Ar time 2099is a positive integer value and 2100.Ar qualifier 2101is one of the following: 2102.Pp 2103.Bl -tag -width Ds -compact -offset indent 2104.It Aq Cm none 2105seconds 2106.It Cm s | Cm S 2107seconds 2108.It Cm m | Cm M 2109minutes 2110.It Cm h | Cm H 2111hours 2112.It Cm d | Cm D 2113days 2114.It Cm w | Cm W 2115weeks 2116.El 2117.Pp 2118Each member of the sequence is added together to calculate 2119the total time value. 2120.Pp 2121Time format examples: 2122.Pp 2123.Bl -tag -width Ds -compact -offset indent 2124.It 600 2125600 seconds (10 minutes) 2126.It 10m 212710 minutes 2128.It 1h30m 21291 hour 30 minutes (90 minutes) 2130.El 2131.Sh TOKENS 2132Arguments to some keywords can make use of tokens, 2133which are expanded at runtime: 2134.Pp 2135.Bl -tag -width XXXX -offset indent -compact 2136.It %% 2137A literal 2138.Sq % . 2139.It \&%C 2140Identifies the connection endpoints, containing 2141four space-separated values: client address, client port number, 2142server address, and server port number. 2143.It \&%D 2144The routing domain in which the incoming connection was received. 2145.It %F 2146The fingerprint of the CA key. 2147.It %f 2148The fingerprint of the key or certificate. 2149.It %h 2150The home directory of the user. 2151.It %i 2152The key ID in the certificate. 2153.It %K 2154The base64-encoded CA key. 2155.It %k 2156The base64-encoded key or certificate for authentication. 2157.It %s 2158The serial number of the certificate. 2159.It \&%T 2160The type of the CA key. 2161.It %t 2162The key or certificate type. 2163.It \&%U 2164The numeric user ID of the target user. 2165.It %u 2166The username. 2167.El 2168.Pp 2169.Cm AuthorizedKeysCommand 2170accepts the tokens %%, %C, %D, %f, %h, %k, %t, %U, and %u. 2171.Pp 2172.Cm AuthorizedKeysFile 2173accepts the tokens %%, %h, %U, and %u. 2174.Pp 2175.Cm AuthorizedPrincipalsCommand 2176accepts the tokens %%, %C, %D, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 2177.Pp 2178.Cm AuthorizedPrincipalsFile 2179accepts the tokens %%, %h, %U, and %u. 2180.Pp 2181.Cm ChrootDirectory 2182accepts the tokens %%, %h, %U, and %u. 2183.Pp 2184.Cm RoutingDomain 2185accepts the token %D. 2186.Sh FILES 2187.Bl -tag -width Ds 2188.It Pa /etc/ssh/sshd_config 2189Contains configuration data for 2190.Xr sshd 8 . 2191This file should be writable by root only, but it is recommended 2192(though not necessary) that it be world-readable. 2193.El 2194.Sh SEE ALSO 2195.Xr sftp-server 8 , 2196.Xr sshd 8 2197.Sh AUTHORS 2198.An -nosplit 2199OpenSSH is a derivative of the original and free 2200ssh 1.2.12 release by 2201.An Tatu Ylonen . 2202.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 2203.An Theo de Raadt 2204and 2205.An Dug Song 2206removed many bugs, re-added newer features and 2207created OpenSSH. 2208.An Markus Friedl 2209contributed the support for SSH protocol versions 1.5 and 2.0. 2210.An Niels Provos 2211and 2212.An Markus Friedl 2213contributed support for privilege separation. 2214