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 1.335 2021/09/03 05:25:50 dtucker Exp $ 37.Dd $Mdocdate: September 3 2021 $ 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. 51For 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. 123.It Cm AllowStreamLocalForwarding 124Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 125The available options are 126.Cm yes 127(the default) 128or 129.Cm all 130to allow StreamLocal forwarding, 131.Cm no 132to prevent all StreamLocal forwarding, 133.Cm local 134to allow local (from the perspective of 135.Xr ssh 1 ) 136forwarding only or 137.Cm remote 138to allow remote forwarding only. 139Note that disabling StreamLocal forwarding does not improve security unless 140users are also denied shell access, as they can always install their 141own forwarders. 142.It Cm AllowTcpForwarding 143Specifies whether TCP forwarding is permitted. 144The available options are 145.Cm yes 146(the default) 147or 148.Cm all 149to allow TCP forwarding, 150.Cm no 151to prevent all TCP forwarding, 152.Cm local 153to allow local (from the perspective of 154.Xr ssh 1 ) 155forwarding only or 156.Cm remote 157to allow remote forwarding only. 158Note that disabling TCP forwarding does not improve security unless 159users are also denied shell access, as they can always install their 160own forwarders. 161.It Cm AllowUsers 162This keyword can be followed by a list of user name patterns, separated 163by spaces. 164If specified, login is allowed only for user names that 165match one of the patterns. 166Only user names are valid; a numerical user ID is not recognized. 167By default, login is allowed for all users. 168If the pattern takes the form USER@HOST then USER and HOST 169are separately checked, restricting logins to particular 170users from particular hosts. 171HOST criteria may additionally contain addresses to match in CIDR 172address/masklen format. 173The allow/deny users directives are processed in the following order: 174.Cm DenyUsers , 175.Cm AllowUsers . 176.Pp 177See PATTERNS in 178.Xr ssh_config 5 179for more information on patterns. 180.It Cm AuthenticationMethods 181Specifies the authentication methods that must be successfully completed 182for a user to be granted access. 183This option must be followed by one or more lists of comma-separated 184authentication method names, or by the single string 185.Cm any 186to indicate the default behaviour of accepting any single authentication 187method. 188If the default is overridden, then successful authentication requires 189completion of every method in at least one of these lists. 190.Pp 191For example, 192.Qq publickey,password publickey,keyboard-interactive 193would require the user to complete public key authentication, followed by 194either password or keyboard interactive authentication. 195Only methods that are next in one or more lists are offered at each stage, 196so for this example it would not be possible to attempt password or 197keyboard-interactive authentication before public key. 198.Pp 199For keyboard interactive authentication it is also possible to 200restrict authentication to a specific device by appending a 201colon followed by the device identifier 202.Cm bsdauth , 203.Cm pam , 204or 205.Cm skey , 206depending on the server configuration. 207For example, 208.Qq keyboard-interactive:bsdauth 209would restrict keyboard interactive authentication to the 210.Cm bsdauth 211device. 212.Pp 213If the publickey method is listed more than once, 214.Xr sshd 8 215verifies that keys that have been used successfully are not reused for 216subsequent authentications. 217For example, 218.Qq publickey,publickey 219requires successful authentication using two different public keys. 220.Pp 221Note that each authentication method listed should also be explicitly enabled 222in the configuration. 223.Pp 224The available authentication methods are: 225.Qq gssapi-with-mic , 226.Qq hostbased , 227.Qq keyboard-interactive , 228.Qq none 229(used for access to password-less accounts when 230.Cm PermitEmptyPasswords 231is enabled), 232.Qq password 233and 234.Qq publickey . 235.It Cm AuthorizedKeysCommand 236Specifies a program to be used to look up the user's public keys. 237The program must be owned by root, not writable by group or others and 238specified by an absolute path. 239Arguments to 240.Cm AuthorizedKeysCommand 241accept the tokens described in the 242.Sx TOKENS 243section. 244If no arguments are specified then the username of the target user is used. 245.Pp 246The program should produce on standard output zero or 247more lines of authorized_keys output (see 248.Sx AUTHORIZED_KEYS 249in 250.Xr sshd 8 ) . 251.Cm AuthorizedKeysCommand 252is tried after the usual 253.Cm AuthorizedKeysFile 254files and will not be executed if a matching key is found there. 255By default, no 256.Cm AuthorizedKeysCommand 257is run. 258.It Cm AuthorizedKeysCommandUser 259Specifies the user under whose account the 260.Cm AuthorizedKeysCommand 261is run. 262It is recommended to use a dedicated user that has no other role on the host 263than running authorized keys commands. 264If 265.Cm AuthorizedKeysCommand 266is specified but 267.Cm AuthorizedKeysCommandUser 268is not, then 269.Xr sshd 8 270will refuse to start. 271.It Cm AuthorizedKeysFile 272Specifies the file that contains the public keys used for user authentication. 273The format is described in the AUTHORIZED_KEYS FILE FORMAT section of 274.Xr sshd 8 . 275Arguments to 276.Cm AuthorizedKeysFile 277accept the tokens described in the 278.Sx TOKENS 279section. 280After expansion, 281.Cm AuthorizedKeysFile 282is taken to be an absolute path or one relative to the user's home 283directory. 284Multiple files may be listed, separated by whitespace. 285Alternately this option may be set to 286.Cm none 287to skip checking for user keys in files. 288The default is 289.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 290.It Cm AuthorizedPrincipalsCommand 291Specifies a program to be used to generate the list of allowed 292certificate principals as per 293.Cm AuthorizedPrincipalsFile . 294The program must be owned by root, not writable by group or others and 295specified by an absolute path. 296Arguments to 297.Cm AuthorizedPrincipalsCommand 298accept the tokens described in the 299.Sx TOKENS 300section. 301If no arguments are specified then the username of the target user is used. 302.Pp 303The program should produce on standard output zero or 304more lines of 305.Cm AuthorizedPrincipalsFile 306output. 307If either 308.Cm AuthorizedPrincipalsCommand 309or 310.Cm AuthorizedPrincipalsFile 311is specified, then certificates offered by the client for authentication 312must contain a principal that is listed. 313By default, no 314.Cm AuthorizedPrincipalsCommand 315is run. 316.It Cm AuthorizedPrincipalsCommandUser 317Specifies the user under whose account the 318.Cm AuthorizedPrincipalsCommand 319is run. 320It is recommended to use a dedicated user that has no other role on the host 321than running authorized principals commands. 322If 323.Cm AuthorizedPrincipalsCommand 324is specified but 325.Cm AuthorizedPrincipalsCommandUser 326is not, then 327.Xr sshd 8 328will refuse to start. 329.It Cm AuthorizedPrincipalsFile 330Specifies a file that lists principal names that are accepted for 331certificate authentication. 332When using certificates signed by a key listed in 333.Cm TrustedUserCAKeys , 334this file lists names, one of which must appear in the certificate for it 335to be accepted for authentication. 336Names are listed one per line preceded by key options (as described in 337.Sx AUTHORIZED_KEYS FILE FORMAT 338in 339.Xr sshd 8 ) . 340Empty lines and comments starting with 341.Ql # 342are ignored. 343.Pp 344Arguments to 345.Cm AuthorizedPrincipalsFile 346accept the tokens described in the 347.Sx TOKENS 348section. 349After expansion, 350.Cm AuthorizedPrincipalsFile 351is taken to be an absolute path or one relative to the user's home directory. 352The default is 353.Cm none , 354i.e. not to use a principals file \(en in this case, the username 355of the user must appear in a certificate's principals list for it to be 356accepted. 357.Pp 358Note that 359.Cm AuthorizedPrincipalsFile 360is only used when authentication proceeds using a CA listed in 361.Cm TrustedUserCAKeys 362and is not consulted for certification authorities trusted via 363.Pa ~/.ssh/authorized_keys , 364though the 365.Cm principals= 366key option offers a similar facility (see 367.Xr sshd 8 368for details). 369.It Cm Banner 370The contents of the specified file are sent to the remote user before 371authentication is allowed. 372If the argument is 373.Cm none 374then no banner is displayed. 375By default, no banner is displayed. 376.It Cm CASignatureAlgorithms 377Specifies which algorithms are allowed for signing of certificates 378by certificate authorities (CAs). 379The default is: 380.Bd -literal -offset indent 381ssh-ed25519,ecdsa-sha2-nistp256, 382ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 383sk-ssh-ed25519@openssh.com, 384sk-ecdsa-sha2-nistp256@openssh.com, 385rsa-sha2-512,rsa-sha2-256 386.Ed 387.Pp 388If the specified list begins with a 389.Sq + 390character, then the specified algorithms will be appended to the default set 391instead of replacing them. 392If the specified list begins with a 393.Sq - 394character, then the specified algorithms (including wildcards) will be removed 395from the default set instead of replacing them. 396.Pp 397Certificates signed using other algorithms will not be accepted for 398public key or host-based authentication. 399.It Cm ChrootDirectory 400Specifies the pathname of a directory to 401.Xr chroot 2 402to after authentication. 403At session startup 404.Xr sshd 8 405checks that all components of the pathname are root-owned directories 406which are not writable by any other user or group. 407After the chroot, 408.Xr sshd 8 409changes the working directory to the user's home directory. 410Arguments to 411.Cm ChrootDirectory 412accept the tokens described in the 413.Sx TOKENS 414section. 415.Pp 416The 417.Cm ChrootDirectory 418must contain the necessary files and directories to support the 419user's session. 420For an interactive session this requires at least a shell, typically 421.Xr sh 1 , 422and basic 423.Pa /dev 424nodes such as 425.Xr null 4 , 426.Xr zero 4 , 427.Xr stdin 4 , 428.Xr stdout 4 , 429.Xr stderr 4 , 430and 431.Xr tty 4 432devices. 433For file transfer sessions using SFTP 434no additional configuration of the environment is necessary if the in-process 435sftp-server is used, 436though sessions which use logging may require 437.Pa /dev/log 438inside the chroot directory on some operating systems (see 439.Xr sftp-server 8 440for details). 441.Pp 442For safety, it is very important that the directory hierarchy be 443prevented from modification by other processes on the system (especially 444those outside the jail). 445Misconfiguration can lead to unsafe environments which 446.Xr sshd 8 447cannot detect. 448.Pp 449The default is 450.Cm none , 451indicating not to 452.Xr chroot 2 . 453.It Cm Ciphers 454Specifies the ciphers allowed. 455Multiple ciphers must be comma-separated. 456If the specified list begins with a 457.Sq + 458character, then the specified ciphers will be appended to the default set 459instead of replacing them. 460If the specified list begins with a 461.Sq - 462character, then the specified ciphers (including wildcards) will be removed 463from the default set instead of replacing them. 464If the specified list begins with a 465.Sq ^ 466character, then the specified ciphers will be placed at the head of the 467default set. 468.Pp 469The supported ciphers are: 470.Pp 471.Bl -item -compact -offset indent 472.It 4733des-cbc 474.It 475aes128-cbc 476.It 477aes192-cbc 478.It 479aes256-cbc 480.It 481aes128-ctr 482.It 483aes192-ctr 484.It 485aes256-ctr 486.It 487aes128-gcm@openssh.com 488.It 489aes256-gcm@openssh.com 490.It 491chacha20-poly1305@openssh.com 492.El 493.Pp 494The default is: 495.Bd -literal -offset indent 496chacha20-poly1305@openssh.com, 497aes128-ctr,aes192-ctr,aes256-ctr, 498aes128-gcm@openssh.com,aes256-gcm@openssh.com 499.Ed 500.Pp 501The list of available ciphers may also be obtained using 502.Qq ssh -Q cipher . 503.It Cm ClientAliveCountMax 504Sets the number of client alive messages which may be sent without 505.Xr sshd 8 506receiving any messages back from the client. 507If this threshold is reached while client alive messages are being sent, 508sshd will disconnect the client, terminating the session. 509It is important to note that the use of client alive messages is very 510different from 511.Cm TCPKeepAlive . 512The client alive messages are sent through the encrypted channel 513and therefore will not be spoofable. 514The TCP keepalive option enabled by 515.Cm TCPKeepAlive 516is spoofable. 517The client alive mechanism is valuable when the client or 518server depend on knowing when a connection has become unresponsive. 519.Pp 520The default value is 3. 521If 522.Cm ClientAliveInterval 523is set to 15, and 524.Cm ClientAliveCountMax 525is left at the default, unresponsive SSH clients 526will be disconnected after approximately 45 seconds. 527Setting a zero 528.Cm ClientAliveCountMax 529disables connection termination. 530.It Cm ClientAliveInterval 531Sets a timeout interval in seconds after which if no data has been received 532from the client, 533.Xr sshd 8 534will send a message through the encrypted 535channel to request a response from the client. 536The default 537is 0, indicating that these messages will not be sent to the client. 538.It Cm Compression 539Specifies whether compression is enabled after 540the user has authenticated successfully. 541The argument must be 542.Cm yes , 543.Cm delayed 544(a legacy synonym for 545.Cm yes ) 546or 547.Cm no . 548The default is 549.Cm yes . 550.It Cm DenyGroups 551This keyword can be followed by a list of group name patterns, separated 552by spaces. 553Login is disallowed for users whose primary group or supplementary 554group list matches one of the patterns. 555Only group names are valid; a numerical group ID is not recognized. 556By default, login is allowed for all groups. 557The allow/deny groups directives are processed in the following order: 558.Cm DenyGroups , 559.Cm AllowGroups . 560.Pp 561See PATTERNS in 562.Xr ssh_config 5 563for more information on patterns. 564.It Cm DenyUsers 565This keyword can be followed by a list of user name patterns, separated 566by spaces. 567Login is disallowed for user names that match one of the patterns. 568Only user names are valid; a numerical user ID is not recognized. 569By default, login is allowed for all users. 570If the pattern takes the form USER@HOST then USER and HOST 571are separately checked, restricting logins to particular 572users from particular hosts. 573HOST criteria may additionally contain addresses to match in CIDR 574address/masklen format. 575The allow/deny users directives are processed in the following order: 576.Cm DenyUsers , 577.Cm AllowUsers . 578.Pp 579See PATTERNS in 580.Xr ssh_config 5 581for more information on patterns. 582.It Cm DisableForwarding 583Disables all forwarding features, including X11, 584.Xr ssh-agent 1 , 585TCP and StreamLocal. 586This option overrides all other forwarding-related options and may 587simplify restricted configurations. 588.It Cm ExposeAuthInfo 589Writes a temporary file containing a list of authentication methods and 590public credentials (e.g. keys) used to authenticate the user. 591The location of the file is exposed to the user session through the 592.Ev SSH_USER_AUTH 593environment variable. 594The default is 595.Cm no . 596.It Cm FingerprintHash 597Specifies the hash algorithm used when logging key fingerprints. 598Valid options are: 599.Cm md5 600and 601.Cm sha256 . 602The default is 603.Cm sha256 . 604.It Cm ForceCommand 605Forces the execution of the command specified by 606.Cm ForceCommand , 607ignoring any command supplied by the client and 608.Pa ~/.ssh/rc 609if present. 610The command is invoked by using the user's login shell with the -c option. 611This applies to shell, command, or subsystem execution. 612It is most useful inside a 613.Cm Match 614block. 615The command originally supplied by the client is available in the 616.Ev SSH_ORIGINAL_COMMAND 617environment variable. 618Specifying a command of 619.Cm internal-sftp 620will force the use of an in-process SFTP server that requires no support 621files when used with 622.Cm ChrootDirectory . 623The default is 624.Cm none . 625.It Cm GatewayPorts 626Specifies whether remote hosts are allowed to connect to ports 627forwarded for the client. 628By default, 629.Xr sshd 8 630binds remote port forwardings to the loopback address. 631This prevents other remote hosts from connecting to forwarded ports. 632.Cm GatewayPorts 633can be used to specify that sshd 634should allow remote port forwardings to bind to non-loopback addresses, thus 635allowing other hosts to connect. 636The argument may be 637.Cm no 638to force remote port forwardings to be available to the local host only, 639.Cm yes 640to force remote port forwardings to bind to the wildcard address, or 641.Cm clientspecified 642to allow the client to select the address to which the forwarding is bound. 643The default is 644.Cm no . 645.It Cm GSSAPIAuthentication 646Specifies whether user authentication based on GSSAPI is allowed. 647The default is 648.Cm no . 649.It Cm GSSAPICleanupCredentials 650Specifies whether to automatically destroy the user's credentials cache 651on logout. 652The default is 653.Cm yes . 654.It Cm GSSAPIStrictAcceptorCheck 655Determines whether to be strict about the identity of the GSSAPI acceptor 656a client authenticates against. 657If set to 658.Cm yes 659then the client must authenticate against the host 660service on the current hostname. 661If set to 662.Cm no 663then the client may authenticate against any service key stored in the 664machine's default store. 665This facility is provided to assist with operation on multi homed machines. 666The default is 667.Cm yes . 668.It Cm HostbasedAcceptedAlgorithms 669Specifies the signature algorithms that will be accepted for hostbased 670authentication as a list of comma-separated patterns. 671Alternately if the specified list begins with a 672.Sq + 673character, then the specified signature algorithms will be appended to 674the default set instead of replacing them. 675If the specified list begins with a 676.Sq - 677character, then the specified signature algorithms (including wildcards) 678will be removed from the default set instead of replacing them. 679If the specified list begins with a 680.Sq ^ 681character, then the specified signature algorithms will be placed at 682the head of the default set. 683The default for this option is: 684.Bd -literal -offset 3n 685ssh-ed25519-cert-v01@openssh.com, 686ecdsa-sha2-nistp256-cert-v01@openssh.com, 687ecdsa-sha2-nistp384-cert-v01@openssh.com, 688ecdsa-sha2-nistp521-cert-v01@openssh.com, 689sk-ssh-ed25519-cert-v01@openssh.com, 690sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 691rsa-sha2-512-cert-v01@openssh.com, 692rsa-sha2-256-cert-v01@openssh.com, 693ssh-rsa-cert-v01@openssh.com, 694ssh-ed25519, 695ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 696sk-ssh-ed25519@openssh.com, 697sk-ecdsa-sha2-nistp256@openssh.com, 698rsa-sha2-512,rsa-sha2-256,ssh-rsa 699.Ed 700.Pp 701The list of available signature algorithms may also be obtained using 702.Qq ssh -Q HostbasedAcceptedAlgorithms . 703This was formerly named HostbasedAcceptedKeyTypes. 704.It Cm HostbasedAuthentication 705Specifies whether rhosts or /etc/hosts.equiv authentication together 706with successful public key client host authentication is allowed 707(host-based authentication). 708The default is 709.Cm no . 710.It Cm HostbasedUsesNameFromPacketOnly 711Specifies whether or not the server will attempt to perform a reverse 712name lookup when matching the name in the 713.Pa ~/.shosts , 714.Pa ~/.rhosts , 715and 716.Pa /etc/hosts.equiv 717files during 718.Cm HostbasedAuthentication . 719A setting of 720.Cm yes 721means that 722.Xr sshd 8 723uses the name supplied by the client rather than 724attempting to resolve the name from the TCP connection itself. 725The default is 726.Cm no . 727.It Cm HostCertificate 728Specifies a file containing a public host certificate. 729The certificate's public key must match a private host key already specified 730by 731.Cm HostKey . 732The default behaviour of 733.Xr sshd 8 734is not to load any certificates. 735.It Cm HostKey 736Specifies a file containing a private host key 737used by SSH. 738The defaults are 739.Pa /etc/ssh/ssh_host_ecdsa_key , 740.Pa /etc/ssh/ssh_host_ed25519_key 741and 742.Pa /etc/ssh/ssh_host_rsa_key . 743.Pp 744Note that 745.Xr sshd 8 746will refuse to use a file if it is group/world-accessible 747and that the 748.Cm HostKeyAlgorithms 749option restricts which of the keys are actually used by 750.Xr sshd 8 . 751.Pp 752It is possible to have multiple host key files. 753It is also possible to specify public host key files instead. 754In this case operations on the private key will be delegated 755to an 756.Xr ssh-agent 1 . 757.It Cm HostKeyAgent 758Identifies the UNIX-domain socket used to communicate 759with an agent that has access to the private host keys. 760If the string 761.Qq SSH_AUTH_SOCK 762is specified, the location of the socket will be read from the 763.Ev SSH_AUTH_SOCK 764environment variable. 765.It Cm HostKeyAlgorithms 766Specifies the host key signature algorithms 767that the server offers. 768The default for this option is: 769.Bd -literal -offset 3n 770ssh-ed25519-cert-v01@openssh.com, 771ecdsa-sha2-nistp256-cert-v01@openssh.com, 772ecdsa-sha2-nistp384-cert-v01@openssh.com, 773ecdsa-sha2-nistp521-cert-v01@openssh.com, 774sk-ssh-ed25519-cert-v01@openssh.com, 775sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 776rsa-sha2-512-cert-v01@openssh.com, 777rsa-sha2-256-cert-v01@openssh.com, 778ssh-rsa-cert-v01@openssh.com, 779ssh-ed25519, 780ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 781sk-ssh-ed25519@openssh.com, 782sk-ecdsa-sha2-nistp256@openssh.com, 783rsa-sha2-512,rsa-sha2-256,ssh-rsa 784.Ed 785.Pp 786The list of available signature algorithms may also be obtained using 787.Qq ssh -Q HostKeyAlgorithms . 788.It Cm IgnoreRhosts 789Specifies whether to ignore per-user 790.Pa .rhosts 791and 792.Pa .shosts 793files during 794.Cm HostbasedAuthentication . 795The system-wide 796.Pa /etc/hosts.equiv 797and 798.Pa /etc/shosts.equiv 799are still used regardless of this setting. 800.Pp 801Accepted values are 802.Cm yes 803(the default) to ignore all per-user files, 804.Cm shosts-only 805to allow the use of 806.Pa .shosts 807but to ignore 808.Pa .rhosts 809or 810.Cm no 811to allow both 812.Pa .shosts 813and 814.Pa rhosts . 815.It Cm IgnoreUserKnownHosts 816Specifies whether 817.Xr sshd 8 818should ignore the user's 819.Pa ~/.ssh/known_hosts 820during 821.Cm HostbasedAuthentication 822and use only the system-wide known hosts file 823.Pa /etc/ssh/known_hosts . 824The default is 825.Dq no . 826.It Cm Include 827Include the specified configuration file(s). 828Multiple pathnames may be specified and each pathname may contain 829.Xr glob 7 830wildcards that will be expanded and processed in lexical order. 831Files without absolute paths are assumed to be in 832.Pa /etc/ssh . 833An 834.Cm Include 835directive may appear inside a 836.Cm Match 837block 838to perform conditional inclusion. 839.It Cm IPQoS 840Specifies the IPv4 type-of-service or DSCP class for the connection. 841Accepted values are 842.Cm af11 , 843.Cm af12 , 844.Cm af13 , 845.Cm af21 , 846.Cm af22 , 847.Cm af23 , 848.Cm af31 , 849.Cm af32 , 850.Cm af33 , 851.Cm af41 , 852.Cm af42 , 853.Cm af43 , 854.Cm cs0 , 855.Cm cs1 , 856.Cm cs2 , 857.Cm cs3 , 858.Cm cs4 , 859.Cm cs5 , 860.Cm cs6 , 861.Cm cs7 , 862.Cm ef , 863.Cm le , 864.Cm lowdelay , 865.Cm throughput , 866.Cm reliability , 867a numeric value, or 868.Cm none 869to use the operating system default. 870This option may take one or two arguments, separated by whitespace. 871If one argument is specified, it is used as the packet class unconditionally. 872If two values are specified, the first is automatically selected for 873interactive sessions and the second for non-interactive sessions. 874The default is 875.Cm af21 876(Low-Latency Data) 877for interactive sessions and 878.Cm cs1 879(Lower Effort) 880for non-interactive sessions. 881.It Cm KbdInteractiveAuthentication 882Specifies whether to allow keyboard-interactive authentication. 883All authentication styles from 884.Xr login.conf 5 885are supported. 886The default is 887.Cm yes . 888The argument to this keyword must be 889.Cm yes 890or 891.Cm no . 892.Cm ChallengeResponseAuthentication 893is a deprecated alias for this. 894.It Cm KerberosAuthentication 895Specifies whether the password provided by the user for 896.Cm PasswordAuthentication 897will be validated through the Kerberos KDC. 898To use this option, the server needs a 899Kerberos servtab which allows the verification of the KDC's identity. 900The default is 901.Cm no . 902.It Cm KerberosGetAFSToken 903If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 904an AFS token before accessing the user's home directory. 905The default is 906.Cm no . 907.It Cm KerberosOrLocalPasswd 908If password authentication through Kerberos fails then 909the password will be validated via any additional local mechanism 910such as 911.Pa /etc/passwd . 912The default is 913.Cm yes . 914.It Cm KerberosTicketCleanup 915Specifies whether to automatically destroy the user's ticket cache 916file on logout. 917The default is 918.Cm yes . 919.It Cm KexAlgorithms 920Specifies the available KEX (Key Exchange) algorithms. 921Multiple algorithms must be comma-separated. 922Alternately if the specified list begins with a 923.Sq + 924character, then the specified algorithms will be appended to the default set 925instead of replacing them. 926If the specified list begins with a 927.Sq - 928character, then the specified algorithms (including wildcards) will be removed 929from the default set instead of replacing them. 930If the specified list begins with a 931.Sq ^ 932character, then the specified algorithms will be placed at the head of the 933default set. 934The supported algorithms are: 935.Pp 936.Bl -item -compact -offset indent 937.It 938curve25519-sha256 939.It 940curve25519-sha256@libssh.org 941.It 942diffie-hellman-group1-sha1 943.It 944diffie-hellman-group14-sha1 945.It 946diffie-hellman-group14-sha256 947.It 948diffie-hellman-group16-sha512 949.It 950diffie-hellman-group18-sha512 951.It 952diffie-hellman-group-exchange-sha1 953.It 954diffie-hellman-group-exchange-sha256 955.It 956ecdh-sha2-nistp256 957.It 958ecdh-sha2-nistp384 959.It 960ecdh-sha2-nistp521 961.It 962sntrup761x25519-sha512@openssh.com 963.El 964.Pp 965The default is: 966.Bd -literal -offset indent 967curve25519-sha256,curve25519-sha256@libssh.org, 968ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 969diffie-hellman-group-exchange-sha256, 970diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 971diffie-hellman-group14-sha256 972.Ed 973.Pp 974The list of available key exchange algorithms may also be obtained using 975.Qq ssh -Q KexAlgorithms . 976.It Cm ListenAddress 977Specifies the local addresses 978.Xr sshd 8 979should listen on. 980The following forms may be used: 981.Pp 982.Bl -item -offset indent -compact 983.It 984.Cm ListenAddress 985.Sm off 986.Ar hostname | address 987.Sm on 988.Op Cm rdomain Ar domain 989.It 990.Cm ListenAddress 991.Sm off 992.Ar hostname : port 993.Sm on 994.Op Cm rdomain Ar domain 995.It 996.Cm ListenAddress 997.Sm off 998.Ar IPv4_address : port 999.Sm on 1000.Op Cm rdomain Ar domain 1001.It 1002.Cm ListenAddress 1003.Sm off 1004.Oo Ar hostname | address Oc : Ar port 1005.Sm on 1006.Op Cm rdomain Ar domain 1007.El 1008.Pp 1009The optional 1010.Cm rdomain 1011qualifier requests 1012.Xr sshd 8 1013listen in an explicit routing domain. 1014If 1015.Ar port 1016is not specified, 1017sshd will listen on the address and all 1018.Cm Port 1019options specified. 1020The default is to listen on all local addresses on the current default 1021routing domain. 1022Multiple 1023.Cm ListenAddress 1024options are permitted. 1025For more information on routing domains, see 1026.Xr rdomain 4 . 1027.It Cm LoginGraceTime 1028The server disconnects after this time if the user has not 1029successfully logged in. 1030If the value is 0, there is no time limit. 1031The default is 120 seconds. 1032.It Cm LogLevel 1033Gives the verbosity level that is used when logging messages from 1034.Xr sshd 8 . 1035The possible values are: 1036QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1037The default is INFO. 1038DEBUG and DEBUG1 are equivalent. 1039DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1040Logging with a DEBUG level violates the privacy of users and is not recommended. 1041.It Cm LogVerbose 1042Specify one or more overrides to LogLevel. 1043An override consists of a pattern lists that matches the source file, function 1044and line number to force detailed logging for. 1045For example, an override pattern of: 1046.Bd -literal -offset indent 1047kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1048.Ed 1049.Pp 1050would enable detailed logging for line 1000 of 1051.Pa kex.c , 1052everything in the 1053.Fn kex_exchange_identification 1054function, and all code in the 1055.Pa packet.c 1056file. 1057This option is intended for debugging and no overrides are enabled by default. 1058.It Cm MACs 1059Specifies the available MAC (message authentication code) algorithms. 1060The MAC algorithm is used for data integrity protection. 1061Multiple algorithms must be comma-separated. 1062If the specified list begins with a 1063.Sq + 1064character, then the specified algorithms will be appended to the default set 1065instead of replacing them. 1066If the specified list begins with a 1067.Sq - 1068character, then the specified algorithms (including wildcards) will be removed 1069from the default set instead of replacing them. 1070If the specified list begins with a 1071.Sq ^ 1072character, then the specified algorithms will be placed at the head of the 1073default set. 1074.Pp 1075The algorithms that contain 1076.Qq -etm 1077calculate the MAC after encryption (encrypt-then-mac). 1078These are considered safer and their use recommended. 1079The supported MACs are: 1080.Pp 1081.Bl -item -compact -offset indent 1082.It 1083hmac-md5 1084.It 1085hmac-md5-96 1086.It 1087hmac-sha1 1088.It 1089hmac-sha1-96 1090.It 1091hmac-sha2-256 1092.It 1093hmac-sha2-512 1094.It 1095umac-64@openssh.com 1096.It 1097umac-128@openssh.com 1098.It 1099hmac-md5-etm@openssh.com 1100.It 1101hmac-md5-96-etm@openssh.com 1102.It 1103hmac-sha1-etm@openssh.com 1104.It 1105hmac-sha1-96-etm@openssh.com 1106.It 1107hmac-sha2-256-etm@openssh.com 1108.It 1109hmac-sha2-512-etm@openssh.com 1110.It 1111umac-64-etm@openssh.com 1112.It 1113umac-128-etm@openssh.com 1114.El 1115.Pp 1116The default is: 1117.Bd -literal -offset indent 1118umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1119hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1120hmac-sha1-etm@openssh.com, 1121umac-64@openssh.com,umac-128@openssh.com, 1122hmac-sha2-256,hmac-sha2-512,hmac-sha1 1123.Ed 1124.Pp 1125The list of available MAC algorithms may also be obtained using 1126.Qq ssh -Q mac . 1127.It Cm Match 1128Introduces a conditional block. 1129If all of the criteria on the 1130.Cm Match 1131line are satisfied, the keywords on the following lines override those 1132set in the global section of the config file, until either another 1133.Cm Match 1134line or the end of the file. 1135If a keyword appears in multiple 1136.Cm Match 1137blocks that are satisfied, only the first instance of the keyword is 1138applied. 1139.Pp 1140The arguments to 1141.Cm Match 1142are one or more criteria-pattern pairs or the single token 1143.Cm All 1144which matches all criteria. 1145The available criteria are 1146.Cm User , 1147.Cm Group , 1148.Cm Host , 1149.Cm LocalAddress , 1150.Cm LocalPort , 1151.Cm RDomain , 1152and 1153.Cm Address 1154(with 1155.Cm RDomain 1156representing the 1157.Xr rdomain 4 1158on which the connection was received). 1159.Pp 1160The match patterns may consist of single entries or comma-separated 1161lists and may use the wildcard and negation operators described in the 1162.Sx PATTERNS 1163section of 1164.Xr ssh_config 5 . 1165.Pp 1166The patterns in an 1167.Cm Address 1168criteria may additionally contain addresses to match in CIDR 1169address/masklen format, 1170such as 192.0.2.0/24 or 2001:db8::/32. 1171Note that the mask length provided must be consistent with the address - 1172it is an error to specify a mask length that is too long for the address 1173or one with bits set in this host portion of the address. 1174For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1175.Pp 1176Only a subset of keywords may be used on the lines following a 1177.Cm Match 1178keyword. 1179Available keywords are 1180.Cm AcceptEnv , 1181.Cm AllowAgentForwarding , 1182.Cm AllowGroups , 1183.Cm AllowStreamLocalForwarding , 1184.Cm AllowTcpForwarding , 1185.Cm AllowUsers , 1186.Cm AuthenticationMethods , 1187.Cm AuthorizedKeysCommand , 1188.Cm AuthorizedKeysCommandUser , 1189.Cm AuthorizedKeysFile , 1190.Cm AuthorizedPrincipalsCommand , 1191.Cm AuthorizedPrincipalsCommandUser , 1192.Cm AuthorizedPrincipalsFile , 1193.Cm Banner , 1194.Cm ChrootDirectory , 1195.Cm ClientAliveCountMax , 1196.Cm ClientAliveInterval , 1197.Cm DenyGroups , 1198.Cm DenyUsers , 1199.Cm DisableForwarding , 1200.Cm ForceCommand , 1201.Cm GatewayPorts , 1202.Cm GSSAPIAuthentication , 1203.Cm HostbasedAcceptedAlgorithms , 1204.Cm HostbasedAuthentication , 1205.Cm HostbasedUsesNameFromPacketOnly , 1206.Cm IgnoreRhosts , 1207.Cm Include , 1208.Cm IPQoS , 1209.Cm KbdInteractiveAuthentication , 1210.Cm KerberosAuthentication , 1211.Cm LogLevel , 1212.Cm MaxAuthTries , 1213.Cm MaxSessions , 1214.Cm PasswordAuthentication , 1215.Cm PermitEmptyPasswords , 1216.Cm PermitListen , 1217.Cm PermitOpen , 1218.Cm PermitRootLogin , 1219.Cm PermitTTY , 1220.Cm PermitTunnel , 1221.Cm PermitUserRC , 1222.Cm PubkeyAcceptedAlgorithms , 1223.Cm PubkeyAuthentication , 1224.Cm RekeyLimit , 1225.Cm RevokedKeys , 1226.Cm RDomain , 1227.Cm SetEnv , 1228.Cm StreamLocalBindMask , 1229.Cm StreamLocalBindUnlink , 1230.Cm TrustedUserCAKeys , 1231.Cm X11DisplayOffset , 1232.Cm X11Forwarding 1233and 1234.Cm X11UseLocalhost . 1235.It Cm MaxAuthTries 1236Specifies the maximum number of authentication attempts permitted per 1237connection. 1238Once the number of failures reaches half this value, 1239additional failures are logged. 1240The default is 6. 1241.It Cm MaxSessions 1242Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1243sessions permitted per network connection. 1244Multiple sessions may be established by clients that support connection 1245multiplexing. 1246Setting 1247.Cm MaxSessions 1248to 1 will effectively disable session multiplexing, whereas setting it to 0 1249will prevent all shell, login and subsystem sessions while still permitting 1250forwarding. 1251The default is 10. 1252.It Cm MaxStartups 1253Specifies the maximum number of concurrent unauthenticated connections to the 1254SSH daemon. 1255Additional connections will be dropped until authentication succeeds or the 1256.Cm LoginGraceTime 1257expires for a connection. 1258The default is 10:30:100. 1259.Pp 1260Alternatively, random early drop can be enabled by specifying 1261the three colon separated values 1262start:rate:full (e.g. "10:30:60"). 1263.Xr sshd 8 1264will refuse connection attempts with a probability of rate/100 (30%) 1265if there are currently start (10) unauthenticated connections. 1266The probability increases linearly and all connection attempts 1267are refused if the number of unauthenticated connections reaches full (60). 1268.It Cm ModuliFile 1269Specifies the 1270.Xr moduli 5 1271file that contains the Diffie-Hellman groups used for the 1272.Dq diffie-hellman-group-exchange-sha1 1273and 1274.Dq diffie-hellman-group-exchange-sha256 1275key exchange methods. 1276The default is 1277.Pa /etc/moduli . 1278.It Cm PasswordAuthentication 1279Specifies whether password authentication is allowed. 1280The default is 1281.Cm yes . 1282.It Cm PermitEmptyPasswords 1283When password authentication is allowed, it specifies whether the 1284server allows login to accounts with empty password strings. 1285The default is 1286.Cm no . 1287.It Cm PermitListen 1288Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1289The listen specification must be one of the following forms: 1290.Pp 1291.Bl -item -offset indent -compact 1292.It 1293.Cm PermitListen 1294.Sm off 1295.Ar port 1296.Sm on 1297.It 1298.Cm PermitListen 1299.Sm off 1300.Ar host : port 1301.Sm on 1302.El 1303.Pp 1304Multiple permissions may be specified by separating them with whitespace. 1305An argument of 1306.Cm any 1307can be used to remove all restrictions and permit any listen requests. 1308An argument of 1309.Cm none 1310can be used to prohibit all listen requests. 1311The host name may contain wildcards as described in the PATTERNS section in 1312.Xr ssh_config 5 . 1313The wildcard 1314.Sq * 1315can also be used in place of a port number to allow all ports. 1316By default all port forwarding listen requests are permitted. 1317Note that the 1318.Cm GatewayPorts 1319option may further restrict which addresses may be listened on. 1320Note also that 1321.Xr ssh 1 1322will request a listen host of 1323.Dq localhost 1324if no listen host was specifically requested, and this name is 1325treated differently to explicit localhost addresses of 1326.Dq 127.0.0.1 1327and 1328.Dq ::1 . 1329.It Cm PermitOpen 1330Specifies the destinations to which TCP port forwarding is permitted. 1331The forwarding specification must be one of the following forms: 1332.Pp 1333.Bl -item -offset indent -compact 1334.It 1335.Cm PermitOpen 1336.Sm off 1337.Ar host : port 1338.Sm on 1339.It 1340.Cm PermitOpen 1341.Sm off 1342.Ar IPv4_addr : port 1343.Sm on 1344.It 1345.Cm PermitOpen 1346.Sm off 1347.Ar \&[ IPv6_addr \&] : port 1348.Sm on 1349.El 1350.Pp 1351Multiple forwards may be specified by separating them with whitespace. 1352An argument of 1353.Cm any 1354can be used to remove all restrictions and permit any forwarding requests. 1355An argument of 1356.Cm none 1357can be used to prohibit all forwarding requests. 1358The wildcard 1359.Sq * 1360can be used for host or port to allow all hosts or ports respectively. 1361Otherwise, no pattern matching or address lookups are performed on supplied 1362names. 1363By default all port forwarding requests are permitted. 1364.It Cm PermitRootLogin 1365Specifies whether root can log in using 1366.Xr ssh 1 . 1367The argument must be 1368.Cm yes , 1369.Cm prohibit-password , 1370.Cm forced-commands-only , 1371or 1372.Cm no . 1373The default is 1374.Cm prohibit-password . 1375.Pp 1376If this option is set to 1377.Cm prohibit-password 1378(or its deprecated alias, 1379.Cm without-password ) , 1380password and keyboard-interactive authentication are disabled for root. 1381.Pp 1382If this option is set to 1383.Cm forced-commands-only , 1384root login with public key authentication will be allowed, 1385but only if the 1386.Ar command 1387option has been specified 1388(which may be useful for taking remote backups even if root login is 1389normally not allowed). 1390All other authentication methods are disabled for root. 1391.Pp 1392If this option is set to 1393.Cm no , 1394root is not allowed to log in. 1395.It Cm PermitTTY 1396Specifies whether 1397.Xr pty 4 1398allocation is permitted. 1399The default is 1400.Cm yes . 1401.It Cm PermitTunnel 1402Specifies whether 1403.Xr tun 4 1404device forwarding is allowed. 1405The argument must be 1406.Cm yes , 1407.Cm point-to-point 1408(layer 3), 1409.Cm ethernet 1410(layer 2), or 1411.Cm no . 1412Specifying 1413.Cm yes 1414permits both 1415.Cm point-to-point 1416and 1417.Cm ethernet . 1418The default is 1419.Cm no . 1420.Pp 1421Independent of this setting, the permissions of the selected 1422.Xr tun 4 1423device must allow access to the user. 1424.It Cm PermitUserEnvironment 1425Specifies whether 1426.Pa ~/.ssh/environment 1427and 1428.Cm environment= 1429options in 1430.Pa ~/.ssh/authorized_keys 1431are processed by 1432.Xr sshd 8 . 1433Valid options are 1434.Cm yes , 1435.Cm no 1436or a pattern-list specifying which environment variable names to accept 1437(for example 1438.Qq LANG,LC_* ) . 1439The default is 1440.Cm no . 1441Enabling environment processing may enable users to bypass access 1442restrictions in some configurations using mechanisms such as 1443.Ev LD_PRELOAD . 1444.It Cm PermitUserRC 1445Specifies whether any 1446.Pa ~/.ssh/rc 1447file is executed. 1448The default is 1449.Cm yes . 1450.It Cm PerSourceMaxStartups 1451Specifies the number of unauthenticated connections allowed from a 1452given source address, or 1453.Dq none 1454if there is no limit. 1455This limit is applied in addition to 1456.Cm MaxStartups , 1457whichever is lower. 1458The default is 1459.Cm none . 1460.It Cm PerSourceNetBlockSize 1461Specifies the number of bits of source address that are grouped together 1462for the purposes of applying PerSourceMaxStartups limits. 1463Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1464The default is 1465.Cm 32:128 , 1466which means each address is considered individually. 1467.It Cm PidFile 1468Specifies the file that contains the process ID of the 1469SSH daemon, or 1470.Cm none 1471to not write one. 1472The default is 1473.Pa /var/run/sshd.pid . 1474.It Cm Port 1475Specifies the port number that 1476.Xr sshd 8 1477listens on. 1478The default is 22. 1479Multiple options of this type are permitted. 1480See also 1481.Cm ListenAddress . 1482.It Cm PrintLastLog 1483Specifies whether 1484.Xr sshd 8 1485should print the date and time of the last user login when a user logs 1486in interactively. 1487The default is 1488.Cm yes . 1489.It Cm PrintMotd 1490Specifies whether 1491.Xr sshd 8 1492should print 1493.Pa /etc/motd 1494when a user logs in interactively. 1495(On some systems it is also printed by the shell, 1496.Pa /etc/profile , 1497or equivalent.) 1498The default is 1499.Cm yes . 1500.It Cm PubkeyAcceptedAlgorithms 1501Specifies the signature algorithms that will be accepted for public key 1502authentication as a list of comma-separated patterns. 1503Alternately if the specified list begins with a 1504.Sq + 1505character, then the specified algorithms will be appended to the default set 1506instead of replacing them. 1507If the specified list begins with a 1508.Sq - 1509character, then the specified algorithms (including wildcards) will be removed 1510from the default set instead of replacing them. 1511If the specified list begins with a 1512.Sq ^ 1513character, then the specified algorithms will be placed at the head of the 1514default set. 1515The default for this option is: 1516.Bd -literal -offset 3n 1517ssh-ed25519-cert-v01@openssh.com, 1518ecdsa-sha2-nistp256-cert-v01@openssh.com, 1519ecdsa-sha2-nistp384-cert-v01@openssh.com, 1520ecdsa-sha2-nistp521-cert-v01@openssh.com, 1521sk-ssh-ed25519-cert-v01@openssh.com, 1522sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1523rsa-sha2-512-cert-v01@openssh.com, 1524rsa-sha2-256-cert-v01@openssh.com, 1525ssh-rsa-cert-v01@openssh.com, 1526ssh-ed25519, 1527ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1528sk-ssh-ed25519@openssh.com, 1529sk-ecdsa-sha2-nistp256@openssh.com, 1530rsa-sha2-512,rsa-sha2-256,ssh-rsa 1531.Ed 1532.Pp 1533The list of available signature algorithms may also be obtained using 1534.Qq ssh -Q PubkeyAcceptedAlgorithms . 1535.It Cm PubkeyAuthOptions 1536Sets one or more public key authentication options. 1537The supported keywords are: 1538.Cm none 1539(the default; indicating no additional options are enabled), 1540.Cm touch-required 1541and 1542.Cm verify-required . 1543.Pp 1544The 1545.Cm touch-required 1546option causes public key authentication using a FIDO authenticator algorithm 1547(i.e.\& 1548.Cm ecdsa-sk 1549or 1550.Cm ed25519-sk ) 1551to always require the signature to attest that a physically present user 1552explicitly confirmed the authentication (usually by touching the authenticator). 1553By default, 1554.Xr sshd 8 1555requires user presence unless overridden with an authorized_keys option. 1556The 1557.Cm touch-required 1558flag disables this override. 1559.Pp 1560The 1561.Cm verify-required 1562option requires a FIDO key signature attest that the user was verified, 1563e.g. via a PIN. 1564.Pp 1565Neither the 1566.Cm touch-required 1567or 1568.Cm verify-required 1569options have any effect for other, non-FIDO, public key types. 1570.It Cm PubkeyAuthentication 1571Specifies whether public key authentication is allowed. 1572The default is 1573.Cm yes . 1574.It Cm RekeyLimit 1575Specifies the maximum amount of data that may be transmitted before the 1576session key is renegotiated, optionally followed by a maximum amount of 1577time that may pass before the session key is renegotiated. 1578The first argument is specified in bytes and may have a suffix of 1579.Sq K , 1580.Sq M , 1581or 1582.Sq G 1583to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1584The default is between 1585.Sq 1G 1586and 1587.Sq 4G , 1588depending on the cipher. 1589The optional second value is specified in seconds and may use any of the 1590units documented in the 1591.Sx TIME FORMATS 1592section. 1593The default value for 1594.Cm RekeyLimit 1595is 1596.Cm default none , 1597which means that rekeying is performed after the cipher's default amount 1598of data has been sent or received and no time based rekeying is done. 1599.It Cm RevokedKeys 1600Specifies revoked public keys file, or 1601.Cm none 1602to not use one. 1603Keys listed in this file will be refused for public key authentication. 1604Note that if this file is not readable, then public key authentication will 1605be refused for all users. 1606Keys may be specified as a text file, listing one public key per line, or as 1607an OpenSSH Key Revocation List (KRL) as generated by 1608.Xr ssh-keygen 1 . 1609For more information on KRLs, see the KEY REVOCATION LISTS section in 1610.Xr ssh-keygen 1 . 1611.It Cm RDomain 1612Specifies an explicit routing domain that is applied after authentication 1613has completed. 1614The user session, as well as any forwarded or listening IP sockets, 1615will be bound to this 1616.Xr rdomain 4 . 1617If the routing domain is set to 1618.Cm \&%D , 1619then the domain in which the incoming connection was received will be applied. 1620.It Cm SecurityKeyProvider 1621Specifies a path to a library that will be used when loading 1622FIDO authenticator-hosted keys, overriding the default of using 1623the built-in USB HID support. 1624.It Cm SetEnv 1625Specifies one or more environment variables to set in child sessions started 1626by 1627.Xr sshd 8 1628as 1629.Dq NAME=VALUE . 1630The environment value may be quoted (e.g. if it contains whitespace 1631characters). 1632Environment variables set by 1633.Cm SetEnv 1634override the default environment and any variables specified by the user 1635via 1636.Cm AcceptEnv 1637or 1638.Cm PermitUserEnvironment . 1639.It Cm StreamLocalBindMask 1640Sets the octal file creation mode mask 1641.Pq umask 1642used when creating a Unix-domain socket file for local or remote 1643port forwarding. 1644This option is only used for port forwarding to a Unix-domain socket file. 1645.Pp 1646The default value is 0177, which creates a Unix-domain socket file that is 1647readable and writable only by the owner. 1648Note that not all operating systems honor the file mode on Unix-domain 1649socket files. 1650.It Cm StreamLocalBindUnlink 1651Specifies whether to remove an existing Unix-domain socket file for local 1652or remote port forwarding before creating a new one. 1653If the socket file already exists and 1654.Cm StreamLocalBindUnlink 1655is not enabled, 1656.Nm sshd 1657will be unable to forward the port to the Unix-domain socket file. 1658This option is only used for port forwarding to a Unix-domain socket file. 1659.Pp 1660The argument must be 1661.Cm yes 1662or 1663.Cm no . 1664The default is 1665.Cm no . 1666.It Cm StrictModes 1667Specifies whether 1668.Xr sshd 8 1669should check file modes and ownership of the 1670user's files and home directory before accepting login. 1671This is normally desirable because novices sometimes accidentally leave their 1672directory or files world-writable. 1673The default is 1674.Cm yes . 1675Note that this does not apply to 1676.Cm ChrootDirectory , 1677whose permissions and ownership are checked unconditionally. 1678.It Cm Subsystem 1679Configures an external subsystem (e.g. file transfer daemon). 1680Arguments should be a subsystem name and a command (with optional arguments) 1681to execute upon subsystem request. 1682.Pp 1683The command 1684.Cm sftp-server 1685implements the SFTP file transfer subsystem. 1686.Pp 1687Alternately the name 1688.Cm internal-sftp 1689implements an in-process SFTP server. 1690This may simplify configurations using 1691.Cm ChrootDirectory 1692to force a different filesystem root on clients. 1693.Pp 1694By default no subsystems are defined. 1695.It Cm SyslogFacility 1696Gives the facility code that is used when logging messages from 1697.Xr sshd 8 . 1698The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1699LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1700The default is AUTH. 1701.It Cm TCPKeepAlive 1702Specifies whether the system should send TCP keepalive messages to the 1703other side. 1704If they are sent, death of the connection or crash of one 1705of the machines will be properly noticed. 1706However, this means that 1707connections will die if the route is down temporarily, and some people 1708find it annoying. 1709On the other hand, if TCP keepalives are not sent, 1710sessions may hang indefinitely on the server, leaving 1711.Qq ghost 1712users and consuming server resources. 1713.Pp 1714The default is 1715.Cm yes 1716(to send TCP keepalive messages), and the server will notice 1717if the network goes down or the client host crashes. 1718This avoids infinitely hanging sessions. 1719.Pp 1720To disable TCP keepalive messages, the value should be set to 1721.Cm no . 1722.It Cm TrustedUserCAKeys 1723Specifies a file containing public keys of certificate authorities that are 1724trusted to sign user certificates for authentication, or 1725.Cm none 1726to not use one. 1727Keys are listed one per line; empty lines and comments starting with 1728.Ql # 1729are allowed. 1730If a certificate is presented for authentication and has its signing CA key 1731listed in this file, then it may be used for authentication for any user 1732listed in the certificate's principals list. 1733Note that certificates that lack a list of principals will not be permitted 1734for authentication using 1735.Cm TrustedUserCAKeys . 1736For more details on certificates, see the CERTIFICATES section in 1737.Xr ssh-keygen 1 . 1738.It Cm UseDNS 1739Specifies whether 1740.Xr sshd 8 1741should look up the remote host name, and to check that 1742the resolved host name for the remote IP address maps back to the 1743very same IP address. 1744.Pp 1745If this option is set to 1746.Cm no 1747(the default) then only addresses and not host names may be used in 1748.Pa ~/.ssh/authorized_keys 1749.Cm from 1750and 1751.Nm 1752.Cm Match 1753.Cm Host 1754directives. 1755.It Cm VersionAddendum 1756Optionally specifies additional text to append to the SSH protocol banner 1757sent by the server upon connection. 1758The default is 1759.Cm none . 1760.It Cm X11DisplayOffset 1761Specifies the first display number available for 1762.Xr sshd 8 Ns 's 1763X11 forwarding. 1764This prevents sshd from interfering with real X11 servers. 1765The default is 10. 1766.It Cm X11Forwarding 1767Specifies whether X11 forwarding is permitted. 1768The argument must be 1769.Cm yes 1770or 1771.Cm no . 1772The default is 1773.Cm no . 1774.Pp 1775When X11 forwarding is enabled, there may be additional exposure to 1776the server and to client displays if the 1777.Xr sshd 8 1778proxy display is configured to listen on the wildcard address (see 1779.Cm X11UseLocalhost ) , 1780though this is not the default. 1781Additionally, the authentication spoofing and authentication data 1782verification and substitution occur on the client side. 1783The security risk of using X11 forwarding is that the client's X11 1784display server may be exposed to attack when the SSH client requests 1785forwarding (see the warnings for 1786.Cm ForwardX11 1787in 1788.Xr ssh_config 5 ) . 1789A system administrator may have a stance in which they want to 1790protect clients that may expose themselves to attack by unwittingly 1791requesting X11 forwarding, which can warrant a 1792.Cm no 1793setting. 1794.Pp 1795Note that disabling X11 forwarding does not prevent users from 1796forwarding X11 traffic, as users can always install their own forwarders. 1797.It Cm X11UseLocalhost 1798Specifies whether 1799.Xr sshd 8 1800should bind the X11 forwarding server to the loopback address or to 1801the wildcard address. 1802By default, 1803sshd binds the forwarding server to the loopback address and sets the 1804hostname part of the 1805.Ev DISPLAY 1806environment variable to 1807.Cm localhost . 1808This prevents remote hosts from connecting to the proxy display. 1809However, some older X11 clients may not function with this 1810configuration. 1811.Cm X11UseLocalhost 1812may be set to 1813.Cm no 1814to specify that the forwarding server should be bound to the wildcard 1815address. 1816The argument must be 1817.Cm yes 1818or 1819.Cm no . 1820The default is 1821.Cm yes . 1822.It Cm XAuthLocation 1823Specifies the full pathname of the 1824.Xr xauth 1 1825program, or 1826.Cm none 1827to not use one. 1828The default is 1829.Pa /usr/X11R6/bin/xauth . 1830.El 1831.Sh TIME FORMATS 1832.Xr sshd 8 1833command-line arguments and configuration file options that specify time 1834may be expressed using a sequence of the form: 1835.Sm off 1836.Ar time Op Ar qualifier , 1837.Sm on 1838where 1839.Ar time 1840is a positive integer value and 1841.Ar qualifier 1842is one of the following: 1843.Pp 1844.Bl -tag -width Ds -compact -offset indent 1845.It Aq Cm none 1846seconds 1847.It Cm s | Cm S 1848seconds 1849.It Cm m | Cm M 1850minutes 1851.It Cm h | Cm H 1852hours 1853.It Cm d | Cm D 1854days 1855.It Cm w | Cm W 1856weeks 1857.El 1858.Pp 1859Each member of the sequence is added together to calculate 1860the total time value. 1861.Pp 1862Time format examples: 1863.Pp 1864.Bl -tag -width Ds -compact -offset indent 1865.It 600 1866600 seconds (10 minutes) 1867.It 10m 186810 minutes 1869.It 1h30m 18701 hour 30 minutes (90 minutes) 1871.El 1872.Sh TOKENS 1873Arguments to some keywords can make use of tokens, 1874which are expanded at runtime: 1875.Pp 1876.Bl -tag -width XXXX -offset indent -compact 1877.It %% 1878A literal 1879.Sq % . 1880.It \&%D 1881The routing domain in which the incoming connection was received. 1882.It %F 1883The fingerprint of the CA key. 1884.It %f 1885The fingerprint of the key or certificate. 1886.It %h 1887The home directory of the user. 1888.It %i 1889The key ID in the certificate. 1890.It %K 1891The base64-encoded CA key. 1892.It %k 1893The base64-encoded key or certificate for authentication. 1894.It %s 1895The serial number of the certificate. 1896.It \&%T 1897The type of the CA key. 1898.It %t 1899The key or certificate type. 1900.It \&%U 1901The numeric user ID of the target user. 1902.It %u 1903The username. 1904.El 1905.Pp 1906.Cm AuthorizedKeysCommand 1907accepts the tokens %%, %f, %h, %k, %t, %U, and %u. 1908.Pp 1909.Cm AuthorizedKeysFile 1910accepts the tokens %%, %h, %U, and %u. 1911.Pp 1912.Cm AuthorizedPrincipalsCommand 1913accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 1914.Pp 1915.Cm AuthorizedPrincipalsFile 1916accepts the tokens %%, %h, %U, and %u. 1917.Pp 1918.Cm ChrootDirectory 1919accepts the tokens %%, %h, %U, and %u. 1920.Pp 1921.Cm RoutingDomain 1922accepts the token %D. 1923.Sh FILES 1924.Bl -tag -width Ds 1925.It Pa /etc/ssh/sshd_config 1926Contains configuration data for 1927.Xr sshd 8 . 1928This file should be writable by root only, but it is recommended 1929(though not necessary) that it be world-readable. 1930.El 1931.Sh SEE ALSO 1932.Xr sftp-server 8 , 1933.Xr sshd 8 1934.Sh AUTHORS 1935.An -nosplit 1936OpenSSH is a derivative of the original and free 1937ssh 1.2.12 release by 1938.An Tatu Ylonen . 1939.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1940.An Theo de Raadt 1941and 1942.An Dug Song 1943removed many bugs, re-added newer features and 1944created OpenSSH. 1945.An Markus Friedl 1946contributed the support for SSH protocol versions 1.5 and 2.0. 1947.An Niels Provos 1948and 1949.An Markus Friedl 1950contributed support for privilege separation. 1951