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 1.340 2022/03/31 17:58:44 naddy Exp $ 37.Dd $Mdocdate: March 31 2022 $ 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 203or 204.Cm pam . 205depending on the server configuration. 206For example, 207.Qq keyboard-interactive:bsdauth 208would restrict keyboard interactive authentication to the 209.Cm bsdauth 210device. 211.Pp 212If the publickey method is listed more than once, 213.Xr sshd 8 214verifies that keys that have been used successfully are not reused for 215subsequent authentications. 216For example, 217.Qq publickey,publickey 218requires successful authentication using two different public keys. 219.Pp 220Note that each authentication method listed should also be explicitly enabled 221in the configuration. 222.Pp 223The available authentication methods are: 224.Qq gssapi-with-mic , 225.Qq hostbased , 226.Qq keyboard-interactive , 227.Qq none 228(used for access to password-less accounts when 229.Cm PermitEmptyPasswords 230is enabled), 231.Qq password 232and 233.Qq publickey . 234.It Cm AuthorizedKeysCommand 235Specifies a program to be used to look up the user's public keys. 236The program must be owned by root, not writable by group or others and 237specified by an absolute path. 238Arguments to 239.Cm AuthorizedKeysCommand 240accept the tokens described in the 241.Sx TOKENS 242section. 243If no arguments are specified then the username of the target user is used. 244.Pp 245The program should produce on standard output zero or 246more lines of authorized_keys output (see 247.Sx AUTHORIZED_KEYS 248in 249.Xr sshd 8 ) . 250.Cm AuthorizedKeysCommand 251is tried after the usual 252.Cm AuthorizedKeysFile 253files and will not be executed if a matching key is found there. 254By default, no 255.Cm AuthorizedKeysCommand 256is run. 257.It Cm AuthorizedKeysCommandUser 258Specifies the user under whose account the 259.Cm AuthorizedKeysCommand 260is run. 261It is recommended to use a dedicated user that has no other role on the host 262than running authorized keys commands. 263If 264.Cm AuthorizedKeysCommand 265is specified but 266.Cm AuthorizedKeysCommandUser 267is not, then 268.Xr sshd 8 269will refuse to start. 270.It Cm AuthorizedKeysFile 271Specifies the file that contains the public keys used for user authentication. 272The format is described in the AUTHORIZED_KEYS FILE FORMAT section of 273.Xr sshd 8 . 274Arguments to 275.Cm AuthorizedKeysFile 276accept the tokens described in the 277.Sx TOKENS 278section. 279After expansion, 280.Cm AuthorizedKeysFile 281is taken to be an absolute path or one relative to the user's home 282directory. 283Multiple files may be listed, separated by whitespace. 284Alternately this option may be set to 285.Cm none 286to skip checking for user keys in files. 287The default is 288.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 289.It Cm AuthorizedPrincipalsCommand 290Specifies a program to be used to generate the list of allowed 291certificate principals as per 292.Cm AuthorizedPrincipalsFile . 293The program must be owned by root, not writable by group or others and 294specified by an absolute path. 295Arguments to 296.Cm AuthorizedPrincipalsCommand 297accept the tokens described in the 298.Sx TOKENS 299section. 300If no arguments are specified then the username of the target user is used. 301.Pp 302The program should produce on standard output zero or 303more lines of 304.Cm AuthorizedPrincipalsFile 305output. 306If either 307.Cm AuthorizedPrincipalsCommand 308or 309.Cm AuthorizedPrincipalsFile 310is specified, then certificates offered by the client for authentication 311must contain a principal that is listed. 312By default, no 313.Cm AuthorizedPrincipalsCommand 314is run. 315.It Cm AuthorizedPrincipalsCommandUser 316Specifies the user under whose account the 317.Cm AuthorizedPrincipalsCommand 318is run. 319It is recommended to use a dedicated user that has no other role on the host 320than running authorized principals commands. 321If 322.Cm AuthorizedPrincipalsCommand 323is specified but 324.Cm AuthorizedPrincipalsCommandUser 325is not, then 326.Xr sshd 8 327will refuse to start. 328.It Cm AuthorizedPrincipalsFile 329Specifies a file that lists principal names that are accepted for 330certificate authentication. 331When using certificates signed by a key listed in 332.Cm TrustedUserCAKeys , 333this file lists names, one of which must appear in the certificate for it 334to be accepted for authentication. 335Names are listed one per line preceded by key options (as described in 336.Sx AUTHORIZED_KEYS FILE FORMAT 337in 338.Xr sshd 8 ) . 339Empty lines and comments starting with 340.Ql # 341are ignored. 342.Pp 343Arguments to 344.Cm AuthorizedPrincipalsFile 345accept the tokens described in the 346.Sx TOKENS 347section. 348After expansion, 349.Cm AuthorizedPrincipalsFile 350is taken to be an absolute path or one relative to the user's home directory. 351The default is 352.Cm none , 353i.e. not to use a principals file \(en in this case, the username 354of the user must appear in a certificate's principals list for it to be 355accepted. 356.Pp 357Note that 358.Cm AuthorizedPrincipalsFile 359is only used when authentication proceeds using a CA listed in 360.Cm TrustedUserCAKeys 361and is not consulted for certification authorities trusted via 362.Pa ~/.ssh/authorized_keys , 363though the 364.Cm principals= 365key option offers a similar facility (see 366.Xr sshd 8 367for details). 368.It Cm Banner 369The contents of the specified file are sent to the remote user before 370authentication is allowed. 371If the argument is 372.Cm none 373then no banner is displayed. 374By default, no banner is displayed. 375.It Cm CASignatureAlgorithms 376Specifies which algorithms are allowed for signing of certificates 377by certificate authorities (CAs). 378The default is: 379.Bd -literal -offset indent 380ssh-ed25519,ecdsa-sha2-nistp256, 381ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 382sk-ssh-ed25519@openssh.com, 383sk-ecdsa-sha2-nistp256@openssh.com, 384rsa-sha2-512,rsa-sha2-256 385.Ed 386.Pp 387If the specified list begins with a 388.Sq + 389character, then the specified algorithms will be appended to the default set 390instead of replacing them. 391If the specified list begins with a 392.Sq - 393character, then the specified algorithms (including wildcards) will be removed 394from the default set instead of replacing them. 395.Pp 396Certificates signed using other algorithms will not be accepted for 397public key or host-based authentication. 398.It Cm ChrootDirectory 399Specifies the pathname of a directory to 400.Xr chroot 2 401to after authentication. 402At session startup 403.Xr sshd 8 404checks that all components of the pathname are root-owned directories 405which are not writable by any other user or group. 406After the chroot, 407.Xr sshd 8 408changes the working directory to the user's home directory. 409Arguments to 410.Cm ChrootDirectory 411accept the tokens described in the 412.Sx TOKENS 413section. 414.Pp 415The 416.Cm ChrootDirectory 417must contain the necessary files and directories to support the 418user's session. 419For an interactive session this requires at least a shell, typically 420.Xr sh 1 , 421and basic 422.Pa /dev 423nodes such as 424.Xr null 4 , 425.Xr zero 4 , 426.Xr stdin 4 , 427.Xr stdout 4 , 428.Xr stderr 4 , 429and 430.Xr tty 4 431devices. 432For file transfer sessions using SFTP 433no additional configuration of the environment is necessary if the in-process 434sftp-server is used, 435though sessions which use logging may require 436.Pa /dev/log 437inside the chroot directory on some operating systems (see 438.Xr sftp-server 8 439for details). 440.Pp 441For safety, it is very important that the directory hierarchy be 442prevented from modification by other processes on the system (especially 443those outside the jail). 444Misconfiguration can lead to unsafe environments which 445.Xr sshd 8 446cannot detect. 447.Pp 448The default is 449.Cm none , 450indicating not to 451.Xr chroot 2 . 452.It Cm Ciphers 453Specifies the ciphers allowed. 454Multiple ciphers must be comma-separated. 455If the specified list begins with a 456.Sq + 457character, then the specified ciphers will be appended to the default set 458instead of replacing them. 459If the specified list begins with a 460.Sq - 461character, then the specified ciphers (including wildcards) will be removed 462from the default set instead of replacing them. 463If the specified list begins with a 464.Sq ^ 465character, then the specified ciphers will be placed at the head of the 466default set. 467.Pp 468The supported ciphers are: 469.Pp 470.Bl -item -compact -offset indent 471.It 4723des-cbc 473.It 474aes128-cbc 475.It 476aes192-cbc 477.It 478aes256-cbc 479.It 480aes128-ctr 481.It 482aes192-ctr 483.It 484aes256-ctr 485.It 486aes128-gcm@openssh.com 487.It 488aes256-gcm@openssh.com 489.It 490chacha20-poly1305@openssh.com 491.El 492.Pp 493The default is: 494.Bd -literal -offset indent 495chacha20-poly1305@openssh.com, 496aes128-ctr,aes192-ctr,aes256-ctr, 497aes128-gcm@openssh.com,aes256-gcm@openssh.com 498.Ed 499.Pp 500The list of available ciphers may also be obtained using 501.Qq ssh -Q cipher . 502.It Cm ClientAliveCountMax 503Sets the number of client alive messages which may be sent without 504.Xr sshd 8 505receiving any messages back from the client. 506If this threshold is reached while client alive messages are being sent, 507sshd will disconnect the client, terminating the session. 508It is important to note that the use of client alive messages is very 509different from 510.Cm TCPKeepAlive . 511The client alive messages are sent through the encrypted channel 512and therefore will not be spoofable. 513The TCP keepalive option enabled by 514.Cm TCPKeepAlive 515is spoofable. 516The client alive mechanism is valuable when the client or 517server depend on knowing when a connection has become unresponsive. 518.Pp 519The default value is 3. 520If 521.Cm ClientAliveInterval 522is set to 15, and 523.Cm ClientAliveCountMax 524is left at the default, unresponsive SSH clients 525will be disconnected after approximately 45 seconds. 526Setting a zero 527.Cm ClientAliveCountMax 528disables connection termination. 529.It Cm ClientAliveInterval 530Sets a timeout interval in seconds after which if no data has been received 531from the client, 532.Xr sshd 8 533will send a message through the encrypted 534channel to request a response from the client. 535The default 536is 0, indicating that these messages will not be sent to the client. 537.It Cm Compression 538Specifies whether compression is enabled after 539the user has authenticated successfully. 540The argument must be 541.Cm yes , 542.Cm delayed 543(a legacy synonym for 544.Cm yes ) 545or 546.Cm no . 547The default is 548.Cm yes . 549.It Cm DenyGroups 550This keyword can be followed by a list of group name patterns, separated 551by spaces. 552Login is disallowed for users whose primary group or supplementary 553group list matches one of the patterns. 554Only group names are valid; a numerical group ID is not recognized. 555By default, login is allowed for all groups. 556The allow/deny groups directives are processed in the following order: 557.Cm DenyGroups , 558.Cm AllowGroups . 559.Pp 560See PATTERNS in 561.Xr ssh_config 5 562for more information on patterns. 563.It Cm DenyUsers 564This keyword can be followed by a list of user name patterns, separated 565by spaces. 566Login is disallowed for user names that match one of the patterns. 567Only user names are valid; a numerical user ID is not recognized. 568By default, login is allowed for all users. 569If the pattern takes the form USER@HOST then USER and HOST 570are separately checked, restricting logins to particular 571users from particular hosts. 572HOST criteria may additionally contain addresses to match in CIDR 573address/masklen format. 574The allow/deny users directives are processed in the following order: 575.Cm DenyUsers , 576.Cm AllowUsers . 577.Pp 578See PATTERNS in 579.Xr ssh_config 5 580for more information on patterns. 581.It Cm DisableForwarding 582Disables all forwarding features, including X11, 583.Xr ssh-agent 1 , 584TCP and StreamLocal. 585This option overrides all other forwarding-related options and may 586simplify restricted configurations. 587.It Cm ExposeAuthInfo 588Writes a temporary file containing a list of authentication methods and 589public credentials (e.g. keys) used to authenticate the user. 590The location of the file is exposed to the user session through the 591.Ev SSH_USER_AUTH 592environment variable. 593The default is 594.Cm no . 595.It Cm FingerprintHash 596Specifies the hash algorithm used when logging key fingerprints. 597Valid options are: 598.Cm md5 599and 600.Cm sha256 . 601The default is 602.Cm sha256 . 603.It Cm ForceCommand 604Forces the execution of the command specified by 605.Cm ForceCommand , 606ignoring any command supplied by the client and 607.Pa ~/.ssh/rc 608if present. 609The command is invoked by using the user's login shell with the -c option. 610This applies to shell, command, or subsystem execution. 611It is most useful inside a 612.Cm Match 613block. 614The command originally supplied by the client is available in the 615.Ev SSH_ORIGINAL_COMMAND 616environment variable. 617Specifying a command of 618.Cm internal-sftp 619will force the use of an in-process SFTP server that requires no support 620files when used with 621.Cm ChrootDirectory . 622The default is 623.Cm none . 624.It Cm GatewayPorts 625Specifies whether remote hosts are allowed to connect to ports 626forwarded for the client. 627By default, 628.Xr sshd 8 629binds remote port forwardings to the loopback address. 630This prevents other remote hosts from connecting to forwarded ports. 631.Cm GatewayPorts 632can be used to specify that sshd 633should allow remote port forwardings to bind to non-loopback addresses, thus 634allowing other hosts to connect. 635The argument may be 636.Cm no 637to force remote port forwardings to be available to the local host only, 638.Cm yes 639to force remote port forwardings to bind to the wildcard address, or 640.Cm clientspecified 641to allow the client to select the address to which the forwarding is bound. 642The default is 643.Cm no . 644.It Cm GSSAPIAuthentication 645Specifies whether user authentication based on GSSAPI is allowed. 646The default is 647.Cm no . 648.It Cm GSSAPICleanupCredentials 649Specifies whether to automatically destroy the user's credentials cache 650on logout. 651The default is 652.Cm yes . 653.It Cm GSSAPIStrictAcceptorCheck 654Determines whether to be strict about the identity of the GSSAPI acceptor 655a client authenticates against. 656If set to 657.Cm yes 658then the client must authenticate against the host 659service on the current hostname. 660If set to 661.Cm no 662then the client may authenticate against any service key stored in the 663machine's default store. 664This facility is provided to assist with operation on multi homed machines. 665The default is 666.Cm yes . 667.It Cm HostbasedAcceptedAlgorithms 668Specifies the signature algorithms that will be accepted for hostbased 669authentication as a list of comma-separated patterns. 670Alternately if the specified list begins with a 671.Sq + 672character, then the specified signature algorithms will be appended to 673the default set instead of replacing them. 674If the specified list begins with a 675.Sq - 676character, then the specified signature algorithms (including wildcards) 677will be removed from the default set instead of replacing them. 678If the specified list begins with a 679.Sq ^ 680character, then the specified signature algorithms will be placed at 681the head of the default set. 682The default for this option is: 683.Bd -literal -offset 3n 684ssh-ed25519-cert-v01@openssh.com, 685ecdsa-sha2-nistp256-cert-v01@openssh.com, 686ecdsa-sha2-nistp384-cert-v01@openssh.com, 687ecdsa-sha2-nistp521-cert-v01@openssh.com, 688sk-ssh-ed25519-cert-v01@openssh.com, 689sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 690rsa-sha2-512-cert-v01@openssh.com, 691rsa-sha2-256-cert-v01@openssh.com, 692ssh-ed25519, 693ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 694sk-ssh-ed25519@openssh.com, 695sk-ecdsa-sha2-nistp256@openssh.com, 696rsa-sha2-512,rsa-sha2-256 697.Ed 698.Pp 699The list of available signature algorithms may also be obtained using 700.Qq ssh -Q HostbasedAcceptedAlgorithms . 701This was formerly named HostbasedAcceptedKeyTypes. 702.It Cm HostbasedAuthentication 703Specifies whether rhosts or /etc/hosts.equiv authentication together 704with successful public key client host authentication is allowed 705(host-based authentication). 706The default is 707.Cm no . 708.It Cm HostbasedUsesNameFromPacketOnly 709Specifies whether or not the server will attempt to perform a reverse 710name lookup when matching the name in the 711.Pa ~/.shosts , 712.Pa ~/.rhosts , 713and 714.Pa /etc/hosts.equiv 715files during 716.Cm HostbasedAuthentication . 717A setting of 718.Cm yes 719means that 720.Xr sshd 8 721uses the name supplied by the client rather than 722attempting to resolve the name from the TCP connection itself. 723The default is 724.Cm no . 725.It Cm HostCertificate 726Specifies a file containing a public host certificate. 727The certificate's public key must match a private host key already specified 728by 729.Cm HostKey . 730The default behaviour of 731.Xr sshd 8 732is not to load any certificates. 733.It Cm HostKey 734Specifies a file containing a private host key 735used by SSH. 736The defaults are 737.Pa /etc/ssh/ssh_host_ecdsa_key , 738.Pa /etc/ssh/ssh_host_ed25519_key 739and 740.Pa /etc/ssh/ssh_host_rsa_key . 741.Pp 742Note that 743.Xr sshd 8 744will refuse to use a file if it is group/world-accessible 745and that the 746.Cm HostKeyAlgorithms 747option restricts which of the keys are actually used by 748.Xr sshd 8 . 749.Pp 750It is possible to have multiple host key files. 751It is also possible to specify public host key files instead. 752In this case operations on the private key will be delegated 753to an 754.Xr ssh-agent 1 . 755.It Cm HostKeyAgent 756Identifies the UNIX-domain socket used to communicate 757with an agent that has access to the private host keys. 758If the string 759.Qq SSH_AUTH_SOCK 760is specified, the location of the socket will be read from the 761.Ev SSH_AUTH_SOCK 762environment variable. 763.It Cm HostKeyAlgorithms 764Specifies the host key signature algorithms 765that the server offers. 766The default for this option is: 767.Bd -literal -offset 3n 768ssh-ed25519-cert-v01@openssh.com, 769ecdsa-sha2-nistp256-cert-v01@openssh.com, 770ecdsa-sha2-nistp384-cert-v01@openssh.com, 771ecdsa-sha2-nistp521-cert-v01@openssh.com, 772sk-ssh-ed25519-cert-v01@openssh.com, 773sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 774rsa-sha2-512-cert-v01@openssh.com, 775rsa-sha2-256-cert-v01@openssh.com, 776ssh-ed25519, 777ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 778sk-ssh-ed25519@openssh.com, 779sk-ecdsa-sha2-nistp256@openssh.com, 780rsa-sha2-512,rsa-sha2-256 781.Ed 782.Pp 783The list of available signature algorithms may also be obtained using 784.Qq ssh -Q HostKeyAlgorithms . 785.It Cm IgnoreRhosts 786Specifies whether to ignore per-user 787.Pa .rhosts 788and 789.Pa .shosts 790files during 791.Cm HostbasedAuthentication . 792The system-wide 793.Pa /etc/hosts.equiv 794and 795.Pa /etc/ssh/shosts.equiv 796are still used regardless of this setting. 797.Pp 798Accepted values are 799.Cm yes 800(the default) to ignore all per-user files, 801.Cm shosts-only 802to allow the use of 803.Pa .shosts 804but to ignore 805.Pa .rhosts 806or 807.Cm no 808to allow both 809.Pa .shosts 810and 811.Pa rhosts . 812.It Cm IgnoreUserKnownHosts 813Specifies whether 814.Xr sshd 8 815should ignore the user's 816.Pa ~/.ssh/known_hosts 817during 818.Cm HostbasedAuthentication 819and use only the system-wide known hosts file 820.Pa /etc/ssh/known_hosts . 821The default is 822.Dq no . 823.It Cm Include 824Include the specified configuration file(s). 825Multiple pathnames may be specified and each pathname may contain 826.Xr glob 7 827wildcards that will be expanded and processed in lexical order. 828Files without absolute paths are assumed to be in 829.Pa /etc/ssh . 830An 831.Cm Include 832directive may appear inside a 833.Cm Match 834block 835to perform conditional inclusion. 836.It Cm IPQoS 837Specifies the IPv4 type-of-service or DSCP class for the connection. 838Accepted values are 839.Cm af11 , 840.Cm af12 , 841.Cm af13 , 842.Cm af21 , 843.Cm af22 , 844.Cm af23 , 845.Cm af31 , 846.Cm af32 , 847.Cm af33 , 848.Cm af41 , 849.Cm af42 , 850.Cm af43 , 851.Cm cs0 , 852.Cm cs1 , 853.Cm cs2 , 854.Cm cs3 , 855.Cm cs4 , 856.Cm cs5 , 857.Cm cs6 , 858.Cm cs7 , 859.Cm ef , 860.Cm le , 861.Cm lowdelay , 862.Cm throughput , 863.Cm reliability , 864a numeric value, or 865.Cm none 866to use the operating system default. 867This option may take one or two arguments, separated by whitespace. 868If one argument is specified, it is used as the packet class unconditionally. 869If two values are specified, the first is automatically selected for 870interactive sessions and the second for non-interactive sessions. 871The default is 872.Cm af21 873(Low-Latency Data) 874for interactive sessions and 875.Cm cs1 876(Lower Effort) 877for non-interactive sessions. 878.It Cm KbdInteractiveAuthentication 879Specifies whether to allow keyboard-interactive authentication. 880All authentication styles from 881.Xr login.conf 5 882are supported. 883The default is 884.Cm yes . 885The argument to this keyword must be 886.Cm yes 887or 888.Cm no . 889.Cm ChallengeResponseAuthentication 890is a deprecated alias for this. 891.It Cm KerberosAuthentication 892Specifies whether the password provided by the user for 893.Cm PasswordAuthentication 894will be validated through the Kerberos KDC. 895To use this option, the server needs a 896Kerberos servtab which allows the verification of the KDC's identity. 897The default is 898.Cm no . 899.It Cm KerberosGetAFSToken 900If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 901an AFS token before accessing the user's home directory. 902The default is 903.Cm no . 904.It Cm KerberosOrLocalPasswd 905If password authentication through Kerberos fails then 906the password will be validated via any additional local mechanism 907such as 908.Pa /etc/passwd . 909The default is 910.Cm yes . 911.It Cm KerberosTicketCleanup 912Specifies whether to automatically destroy the user's ticket cache 913file on logout. 914The default is 915.Cm yes . 916.It Cm KexAlgorithms 917Specifies the available KEX (Key Exchange) algorithms. 918Multiple algorithms must be comma-separated. 919Alternately if the specified list begins with a 920.Sq + 921character, then the specified algorithms will be appended to the default set 922instead of replacing them. 923If the specified list begins with a 924.Sq - 925character, then the specified algorithms (including wildcards) will be removed 926from the default set instead of replacing them. 927If the specified list begins with a 928.Sq ^ 929character, then the specified algorithms will be placed at the head of the 930default set. 931The supported algorithms are: 932.Pp 933.Bl -item -compact -offset indent 934.It 935curve25519-sha256 936.It 937curve25519-sha256@libssh.org 938.It 939diffie-hellman-group1-sha1 940.It 941diffie-hellman-group14-sha1 942.It 943diffie-hellman-group14-sha256 944.It 945diffie-hellman-group16-sha512 946.It 947diffie-hellman-group18-sha512 948.It 949diffie-hellman-group-exchange-sha1 950.It 951diffie-hellman-group-exchange-sha256 952.It 953ecdh-sha2-nistp256 954.It 955ecdh-sha2-nistp384 956.It 957ecdh-sha2-nistp521 958.It 959sntrup761x25519-sha512@openssh.com 960.El 961.Pp 962The default is: 963.Bd -literal -offset indent 964sntrup761x25519-sha512@openssh.com, 965curve25519-sha256,curve25519-sha256@libssh.org, 966ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 967diffie-hellman-group-exchange-sha256, 968diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 969diffie-hellman-group14-sha256 970.Ed 971.Pp 972The list of available key exchange algorithms may also be obtained using 973.Qq ssh -Q KexAlgorithms . 974.It Cm ListenAddress 975Specifies the local addresses 976.Xr sshd 8 977should listen on. 978The following forms may be used: 979.Pp 980.Bl -item -offset indent -compact 981.It 982.Cm ListenAddress 983.Sm off 984.Ar hostname | address 985.Sm on 986.Op Cm rdomain Ar domain 987.It 988.Cm ListenAddress 989.Sm off 990.Ar hostname : port 991.Sm on 992.Op Cm rdomain Ar domain 993.It 994.Cm ListenAddress 995.Sm off 996.Ar IPv4_address : port 997.Sm on 998.Op Cm rdomain Ar domain 999.It 1000.Cm ListenAddress 1001.Sm off 1002.Oo Ar hostname | address Oc : Ar port 1003.Sm on 1004.Op Cm rdomain Ar domain 1005.El 1006.Pp 1007The optional 1008.Cm rdomain 1009qualifier requests 1010.Xr sshd 8 1011listen in an explicit routing domain. 1012If 1013.Ar port 1014is not specified, 1015sshd will listen on the address and all 1016.Cm Port 1017options specified. 1018The default is to listen on all local addresses on the current default 1019routing domain. 1020Multiple 1021.Cm ListenAddress 1022options are permitted. 1023For more information on routing domains, see 1024.Xr rdomain 4 . 1025.It Cm LoginGraceTime 1026The server disconnects after this time if the user has not 1027successfully logged in. 1028If the value is 0, there is no time limit. 1029The default is 120 seconds. 1030.It Cm LogLevel 1031Gives the verbosity level that is used when logging messages from 1032.Xr sshd 8 . 1033The possible values are: 1034QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1035The default is INFO. 1036DEBUG and DEBUG1 are equivalent. 1037DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1038Logging with a DEBUG level violates the privacy of users and is not recommended. 1039.It Cm LogVerbose 1040Specify one or more overrides to LogLevel. 1041An override consists of a pattern lists that matches the source file, function 1042and line number to force detailed logging for. 1043For example, an override pattern of: 1044.Bd -literal -offset indent 1045kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1046.Ed 1047.Pp 1048would enable detailed logging for line 1000 of 1049.Pa kex.c , 1050everything in the 1051.Fn kex_exchange_identification 1052function, and all code in the 1053.Pa packet.c 1054file. 1055This option is intended for debugging and no overrides are enabled by default. 1056.It Cm MACs 1057Specifies the available MAC (message authentication code) algorithms. 1058The MAC algorithm is used for data integrity protection. 1059Multiple algorithms must be comma-separated. 1060If the specified list begins with a 1061.Sq + 1062character, then the specified algorithms will be appended to the default set 1063instead of replacing them. 1064If the specified list begins with a 1065.Sq - 1066character, then the specified algorithms (including wildcards) will be removed 1067from the default set instead of replacing them. 1068If the specified list begins with a 1069.Sq ^ 1070character, then the specified algorithms will be placed at the head of the 1071default set. 1072.Pp 1073The algorithms that contain 1074.Qq -etm 1075calculate the MAC after encryption (encrypt-then-mac). 1076These are considered safer and their use recommended. 1077The supported MACs are: 1078.Pp 1079.Bl -item -compact -offset indent 1080.It 1081hmac-md5 1082.It 1083hmac-md5-96 1084.It 1085hmac-sha1 1086.It 1087hmac-sha1-96 1088.It 1089hmac-sha2-256 1090.It 1091hmac-sha2-512 1092.It 1093umac-64@openssh.com 1094.It 1095umac-128@openssh.com 1096.It 1097hmac-md5-etm@openssh.com 1098.It 1099hmac-md5-96-etm@openssh.com 1100.It 1101hmac-sha1-etm@openssh.com 1102.It 1103hmac-sha1-96-etm@openssh.com 1104.It 1105hmac-sha2-256-etm@openssh.com 1106.It 1107hmac-sha2-512-etm@openssh.com 1108.It 1109umac-64-etm@openssh.com 1110.It 1111umac-128-etm@openssh.com 1112.El 1113.Pp 1114The default is: 1115.Bd -literal -offset indent 1116umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1117hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1118hmac-sha1-etm@openssh.com, 1119umac-64@openssh.com,umac-128@openssh.com, 1120hmac-sha2-256,hmac-sha2-512,hmac-sha1 1121.Ed 1122.Pp 1123The list of available MAC algorithms may also be obtained using 1124.Qq ssh -Q mac . 1125.It Cm Match 1126Introduces a conditional block. 1127If all of the criteria on the 1128.Cm Match 1129line are satisfied, the keywords on the following lines override those 1130set in the global section of the config file, until either another 1131.Cm Match 1132line or the end of the file. 1133If a keyword appears in multiple 1134.Cm Match 1135blocks that are satisfied, only the first instance of the keyword is 1136applied. 1137.Pp 1138The arguments to 1139.Cm Match 1140are one or more criteria-pattern pairs or the single token 1141.Cm All 1142which matches all criteria. 1143The available criteria are 1144.Cm User , 1145.Cm Group , 1146.Cm Host , 1147.Cm LocalAddress , 1148.Cm LocalPort , 1149.Cm RDomain , 1150and 1151.Cm Address 1152(with 1153.Cm RDomain 1154representing the 1155.Xr rdomain 4 1156on which the connection was received). 1157.Pp 1158The match patterns may consist of single entries or comma-separated 1159lists and may use the wildcard and negation operators described in the 1160.Sx PATTERNS 1161section of 1162.Xr ssh_config 5 . 1163.Pp 1164The patterns in an 1165.Cm Address 1166criteria may additionally contain addresses to match in CIDR 1167address/masklen format, 1168such as 192.0.2.0/24 or 2001:db8::/32. 1169Note that the mask length provided must be consistent with the address - 1170it is an error to specify a mask length that is too long for the address 1171or one with bits set in this host portion of the address. 1172For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1173.Pp 1174Only a subset of keywords may be used on the lines following a 1175.Cm Match 1176keyword. 1177Available keywords are 1178.Cm AcceptEnv , 1179.Cm AllowAgentForwarding , 1180.Cm AllowGroups , 1181.Cm AllowStreamLocalForwarding , 1182.Cm AllowTcpForwarding , 1183.Cm AllowUsers , 1184.Cm AuthenticationMethods , 1185.Cm AuthorizedKeysCommand , 1186.Cm AuthorizedKeysCommandUser , 1187.Cm AuthorizedKeysFile , 1188.Cm AuthorizedPrincipalsCommand , 1189.Cm AuthorizedPrincipalsCommandUser , 1190.Cm AuthorizedPrincipalsFile , 1191.Cm Banner , 1192.Cm CASignatureAlgorithms , 1193.Cm ChrootDirectory , 1194.Cm ClientAliveCountMax , 1195.Cm ClientAliveInterval , 1196.Cm DenyGroups , 1197.Cm DenyUsers , 1198.Cm DisableForwarding , 1199.Cm ExposeAuthInfo , 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 PubkeyAuthOptions , 1225.Cm RekeyLimit , 1226.Cm RevokedKeys , 1227.Cm RDomain , 1228.Cm SetEnv , 1229.Cm StreamLocalBindMask , 1230.Cm StreamLocalBindUnlink , 1231.Cm TrustedUserCAKeys , 1232.Cm X11DisplayOffset , 1233.Cm X11Forwarding 1234and 1235.Cm X11UseLocalhost . 1236.It Cm MaxAuthTries 1237Specifies the maximum number of authentication attempts permitted per 1238connection. 1239Once the number of failures reaches half this value, 1240additional failures are logged. 1241The default is 6. 1242.It Cm MaxSessions 1243Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1244sessions permitted per network connection. 1245Multiple sessions may be established by clients that support connection 1246multiplexing. 1247Setting 1248.Cm MaxSessions 1249to 1 will effectively disable session multiplexing, whereas setting it to 0 1250will prevent all shell, login and subsystem sessions while still permitting 1251forwarding. 1252The default is 10. 1253.It Cm MaxStartups 1254Specifies the maximum number of concurrent unauthenticated connections to the 1255SSH daemon. 1256Additional connections will be dropped until authentication succeeds or the 1257.Cm LoginGraceTime 1258expires for a connection. 1259The default is 10:30:100. 1260.Pp 1261Alternatively, random early drop can be enabled by specifying 1262the three colon separated values 1263start:rate:full (e.g. "10:30:60"). 1264.Xr sshd 8 1265will refuse connection attempts with a probability of rate/100 (30%) 1266if there are currently start (10) unauthenticated connections. 1267The probability increases linearly and all connection attempts 1268are refused if the number of unauthenticated connections reaches full (60). 1269.It Cm ModuliFile 1270Specifies the 1271.Xr moduli 5 1272file that contains the Diffie-Hellman groups used for the 1273.Dq diffie-hellman-group-exchange-sha1 1274and 1275.Dq diffie-hellman-group-exchange-sha256 1276key exchange methods. 1277The default is 1278.Pa /etc/moduli . 1279.It Cm PasswordAuthentication 1280Specifies whether password authentication is allowed. 1281See also 1282.Cm UsePAM . 1283The default is 1284.Cm no . 1285.It Cm PermitEmptyPasswords 1286When password authentication is allowed, it specifies whether the 1287server allows login to accounts with empty password strings. 1288The default is 1289.Cm no . 1290.It Cm PermitListen 1291Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1292The listen specification must be one of the following forms: 1293.Pp 1294.Bl -item -offset indent -compact 1295.It 1296.Cm PermitListen 1297.Sm off 1298.Ar port 1299.Sm on 1300.It 1301.Cm PermitListen 1302.Sm off 1303.Ar host : port 1304.Sm on 1305.El 1306.Pp 1307Multiple permissions may be specified by separating them with whitespace. 1308An argument of 1309.Cm any 1310can be used to remove all restrictions and permit any listen requests. 1311An argument of 1312.Cm none 1313can be used to prohibit all listen requests. 1314The host name may contain wildcards as described in the PATTERNS section in 1315.Xr ssh_config 5 . 1316The wildcard 1317.Sq * 1318can also be used in place of a port number to allow all ports. 1319By default all port forwarding listen requests are permitted. 1320Note that the 1321.Cm GatewayPorts 1322option may further restrict which addresses may be listened on. 1323Note also that 1324.Xr ssh 1 1325will request a listen host of 1326.Dq localhost 1327if no listen host was specifically requested, and this name is 1328treated differently to explicit localhost addresses of 1329.Dq 127.0.0.1 1330and 1331.Dq ::1 . 1332.It Cm PermitOpen 1333Specifies the destinations to which TCP port forwarding is permitted. 1334The forwarding specification must be one of the following forms: 1335.Pp 1336.Bl -item -offset indent -compact 1337.It 1338.Cm PermitOpen 1339.Sm off 1340.Ar host : port 1341.Sm on 1342.It 1343.Cm PermitOpen 1344.Sm off 1345.Ar IPv4_addr : port 1346.Sm on 1347.It 1348.Cm PermitOpen 1349.Sm off 1350.Ar \&[ IPv6_addr \&] : port 1351.Sm on 1352.El 1353.Pp 1354Multiple forwards may be specified by separating them with whitespace. 1355An argument of 1356.Cm any 1357can be used to remove all restrictions and permit any forwarding requests. 1358An argument of 1359.Cm none 1360can be used to prohibit all forwarding requests. 1361The wildcard 1362.Sq * 1363can be used for host or port to allow all hosts or ports respectively. 1364Otherwise, no pattern matching or address lookups are performed on supplied 1365names. 1366By default all port forwarding requests are permitted. 1367.It Cm PermitRootLogin 1368Specifies whether root can log in using 1369.Xr ssh 1 . 1370The argument must be 1371.Cm yes , 1372.Cm prohibit-password , 1373.Cm forced-commands-only , 1374or 1375.Cm no . 1376The default is 1377.Cm no . 1378Note that if 1379.Cm ChallengeResponseAuthentication 1380and 1381.Cm UsePAM 1382are both 1383.Cm yes , 1384this setting may be overridden by the PAM policy. 1385.Pp 1386If this option is set to 1387.Cm prohibit-password 1388(or its deprecated alias, 1389.Cm without-password ) , 1390password and keyboard-interactive authentication are disabled for root. 1391.Pp 1392If this option is set to 1393.Cm forced-commands-only , 1394root login with public key authentication will be allowed, 1395but only if the 1396.Ar command 1397option has been specified 1398(which may be useful for taking remote backups even if root login is 1399normally not allowed). 1400All other authentication methods are disabled for root. 1401.Pp 1402If this option is set to 1403.Cm no , 1404root is not allowed to log in. 1405.It Cm PermitTTY 1406Specifies whether 1407.Xr pty 4 1408allocation is permitted. 1409The default is 1410.Cm yes . 1411.It Cm PermitTunnel 1412Specifies whether 1413.Xr tun 4 1414device forwarding is allowed. 1415The argument must be 1416.Cm yes , 1417.Cm point-to-point 1418(layer 3), 1419.Cm ethernet 1420(layer 2), or 1421.Cm no . 1422Specifying 1423.Cm yes 1424permits both 1425.Cm point-to-point 1426and 1427.Cm ethernet . 1428The default is 1429.Cm no . 1430.Pp 1431Independent of this setting, the permissions of the selected 1432.Xr tun 4 1433device must allow access to the user. 1434.It Cm PermitUserEnvironment 1435Specifies whether 1436.Pa ~/.ssh/environment 1437and 1438.Cm environment= 1439options in 1440.Pa ~/.ssh/authorized_keys 1441are processed by 1442.Xr sshd 8 . 1443Valid options are 1444.Cm yes , 1445.Cm no 1446or a pattern-list specifying which environment variable names to accept 1447(for example 1448.Qq LANG,LC_* ) . 1449The default is 1450.Cm no . 1451Enabling environment processing may enable users to bypass access 1452restrictions in some configurations using mechanisms such as 1453.Ev LD_PRELOAD . 1454.It Cm PermitUserRC 1455Specifies whether any 1456.Pa ~/.ssh/rc 1457file is executed. 1458The default is 1459.Cm yes . 1460.It Cm PerSourceMaxStartups 1461Specifies the number of unauthenticated connections allowed from a 1462given source address, or 1463.Dq none 1464if there is no limit. 1465This limit is applied in addition to 1466.Cm MaxStartups , 1467whichever is lower. 1468The default is 1469.Cm none . 1470.It Cm PerSourceNetBlockSize 1471Specifies the number of bits of source address that are grouped together 1472for the purposes of applying PerSourceMaxStartups limits. 1473Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1474The default is 1475.Cm 32:128 , 1476which means each address is considered individually. 1477.It Cm PidFile 1478Specifies the file that contains the process ID of the 1479SSH daemon, or 1480.Cm none 1481to not write one. 1482The default is 1483.Pa /var/run/sshd.pid . 1484.It Cm Port 1485Specifies the port number that 1486.Xr sshd 8 1487listens on. 1488The default is 22. 1489Multiple options of this type are permitted. 1490See also 1491.Cm ListenAddress . 1492.It Cm PrintLastLog 1493Specifies whether 1494.Xr sshd 8 1495should print the date and time of the last user login when a user logs 1496in interactively. 1497The default is 1498.Cm yes . 1499.It Cm PrintMotd 1500Specifies whether 1501.Xr sshd 8 1502should print 1503.Pa /etc/motd 1504when a user logs in interactively. 1505(On some systems it is also printed by the shell, 1506.Pa /etc/profile , 1507or equivalent.) 1508The default is 1509.Cm yes . 1510.It Cm PubkeyAcceptedAlgorithms 1511Specifies the signature algorithms that will be accepted for public key 1512authentication as a list of comma-separated patterns. 1513Alternately if the specified list begins with a 1514.Sq + 1515character, then the specified algorithms will be appended to the default set 1516instead of replacing them. 1517If the specified list begins with a 1518.Sq - 1519character, then the specified algorithms (including wildcards) will be removed 1520from the default set instead of replacing them. 1521If the specified list begins with a 1522.Sq ^ 1523character, then the specified algorithms will be placed at the head of the 1524default set. 1525The default for this option is: 1526.Bd -literal -offset 3n 1527ssh-ed25519-cert-v01@openssh.com, 1528ecdsa-sha2-nistp256-cert-v01@openssh.com, 1529ecdsa-sha2-nistp384-cert-v01@openssh.com, 1530ecdsa-sha2-nistp521-cert-v01@openssh.com, 1531sk-ssh-ed25519-cert-v01@openssh.com, 1532sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1533rsa-sha2-512-cert-v01@openssh.com, 1534rsa-sha2-256-cert-v01@openssh.com, 1535ssh-ed25519, 1536ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1537sk-ssh-ed25519@openssh.com, 1538sk-ecdsa-sha2-nistp256@openssh.com, 1539rsa-sha2-512,rsa-sha2-256 1540.Ed 1541.Pp 1542The list of available signature algorithms may also be obtained using 1543.Qq ssh -Q PubkeyAcceptedAlgorithms . 1544.It Cm PubkeyAuthOptions 1545Sets one or more public key authentication options. 1546The supported keywords are: 1547.Cm none 1548(the default; indicating no additional options are enabled), 1549.Cm touch-required 1550and 1551.Cm verify-required . 1552.Pp 1553The 1554.Cm touch-required 1555option causes public key authentication using a FIDO authenticator algorithm 1556(i.e.\& 1557.Cm ecdsa-sk 1558or 1559.Cm ed25519-sk ) 1560to always require the signature to attest that a physically present user 1561explicitly confirmed the authentication (usually by touching the authenticator). 1562By default, 1563.Xr sshd 8 1564requires user presence unless overridden with an authorized_keys option. 1565The 1566.Cm touch-required 1567flag disables this override. 1568.Pp 1569The 1570.Cm verify-required 1571option requires a FIDO key signature attest that the user was verified, 1572e.g. via a PIN. 1573.Pp 1574Neither the 1575.Cm touch-required 1576or 1577.Cm verify-required 1578options have any effect for other, non-FIDO, public key types. 1579.It Cm PubkeyAuthentication 1580Specifies whether public key authentication is allowed. 1581The default is 1582.Cm yes . 1583.It Cm RekeyLimit 1584Specifies the maximum amount of data that may be transmitted before the 1585session key is renegotiated, optionally followed by a maximum amount of 1586time that may pass before the session key is renegotiated. 1587The first argument is specified in bytes and may have a suffix of 1588.Sq K , 1589.Sq M , 1590or 1591.Sq G 1592to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1593The default is between 1594.Sq 1G 1595and 1596.Sq 4G , 1597depending on the cipher. 1598The optional second value is specified in seconds and may use any of the 1599units documented in the 1600.Sx TIME FORMATS 1601section. 1602The default value for 1603.Cm RekeyLimit 1604is 1605.Cm default none , 1606which means that rekeying is performed after the cipher's default amount 1607of data has been sent or received and no time based rekeying is done. 1608.It Cm RevokedKeys 1609Specifies revoked public keys file, or 1610.Cm none 1611to not use one. 1612Keys listed in this file will be refused for public key authentication. 1613Note that if this file is not readable, then public key authentication will 1614be refused for all users. 1615Keys may be specified as a text file, listing one public key per line, or as 1616an OpenSSH Key Revocation List (KRL) as generated by 1617.Xr ssh-keygen 1 . 1618For more information on KRLs, see the KEY REVOCATION LISTS section in 1619.Xr ssh-keygen 1 . 1620.It Cm RDomain 1621Specifies an explicit routing domain that is applied after authentication 1622has completed. 1623The user session, as well as any forwarded or listening IP sockets, 1624will be bound to this 1625.Xr rdomain 4 . 1626If the routing domain is set to 1627.Cm \&%D , 1628then the domain in which the incoming connection was received will be applied. 1629.It Cm SecurityKeyProvider 1630Specifies a path to a library that will be used when loading 1631FIDO authenticator-hosted keys, overriding the default of using 1632the built-in USB HID support. 1633.It Cm SetEnv 1634Specifies one or more environment variables to set in child sessions started 1635by 1636.Xr sshd 8 1637as 1638.Dq NAME=VALUE . 1639The environment value may be quoted (e.g. if it contains whitespace 1640characters). 1641Environment variables set by 1642.Cm SetEnv 1643override the default environment and any variables specified by the user 1644via 1645.Cm AcceptEnv 1646or 1647.Cm PermitUserEnvironment . 1648.It Cm StreamLocalBindMask 1649Sets the octal file creation mode mask 1650.Pq umask 1651used when creating a Unix-domain socket file for local or remote 1652port forwarding. 1653This option is only used for port forwarding to a Unix-domain socket file. 1654.Pp 1655The default value is 0177, which creates a Unix-domain socket file that is 1656readable and writable only by the owner. 1657Note that not all operating systems honor the file mode on Unix-domain 1658socket files. 1659.It Cm StreamLocalBindUnlink 1660Specifies whether to remove an existing Unix-domain socket file for local 1661or remote port forwarding before creating a new one. 1662If the socket file already exists and 1663.Cm StreamLocalBindUnlink 1664is not enabled, 1665.Nm sshd 1666will be unable to forward the port to the Unix-domain socket file. 1667This option is only used for port forwarding to a Unix-domain socket file. 1668.Pp 1669The argument must be 1670.Cm yes 1671or 1672.Cm no . 1673The default is 1674.Cm no . 1675.It Cm StrictModes 1676Specifies whether 1677.Xr sshd 8 1678should check file modes and ownership of the 1679user's files and home directory before accepting login. 1680This is normally desirable because novices sometimes accidentally leave their 1681directory or files world-writable. 1682The default is 1683.Cm yes . 1684Note that this does not apply to 1685.Cm ChrootDirectory , 1686whose permissions and ownership are checked unconditionally. 1687.It Cm Subsystem 1688Configures an external subsystem (e.g. file transfer daemon). 1689Arguments should be a subsystem name and a command (with optional arguments) 1690to execute upon subsystem request. 1691.Pp 1692The command 1693.Cm sftp-server 1694implements the SFTP file transfer subsystem. 1695.Pp 1696Alternately the name 1697.Cm internal-sftp 1698implements an in-process SFTP server. 1699This may simplify configurations using 1700.Cm ChrootDirectory 1701to force a different filesystem root on clients. 1702.Pp 1703By default no subsystems are defined. 1704.It Cm SyslogFacility 1705Gives the facility code that is used when logging messages from 1706.Xr sshd 8 . 1707The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1708LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1709The default is AUTH. 1710.It Cm TCPKeepAlive 1711Specifies whether the system should send TCP keepalive messages to the 1712other side. 1713If they are sent, death of the connection or crash of one 1714of the machines will be properly noticed. 1715However, this means that 1716connections will die if the route is down temporarily, and some people 1717find it annoying. 1718On the other hand, if TCP keepalives are not sent, 1719sessions may hang indefinitely on the server, leaving 1720.Qq ghost 1721users and consuming server resources. 1722.Pp 1723The default is 1724.Cm yes 1725(to send TCP keepalive messages), and the server will notice 1726if the network goes down or the client host crashes. 1727This avoids infinitely hanging sessions. 1728.Pp 1729To disable TCP keepalive messages, the value should be set to 1730.Cm no . 1731.It Cm TrustedUserCAKeys 1732Specifies a file containing public keys of certificate authorities that are 1733trusted to sign user certificates for authentication, or 1734.Cm none 1735to not use one. 1736Keys are listed one per line; empty lines and comments starting with 1737.Ql # 1738are allowed. 1739If a certificate is presented for authentication and has its signing CA key 1740listed in this file, then it may be used for authentication for any user 1741listed in the certificate's principals list. 1742Note that certificates that lack a list of principals will not be permitted 1743for authentication using 1744.Cm TrustedUserCAKeys . 1745For more details on certificates, see the CERTIFICATES section in 1746.Xr ssh-keygen 1 . 1747.It Cm UseBlacklist 1748Specifies whether 1749.Xr sshd 8 1750attempts to send authentication success and failure messages 1751to the 1752.Xr blacklistd 8 1753daemon. 1754The default is 1755.Cm no . 1756For forward compatibility with an upcoming 1757.Xr blacklistd 1758rename, the 1759.Cm UseBlocklist 1760alias can be used instead. 1761.It Cm UseDNS 1762Specifies whether 1763.Xr sshd 8 1764should look up the remote host name, and to check that 1765the resolved host name for the remote IP address maps back to the 1766very same IP address. 1767.Pp 1768If this option is set to 1769.Cm no , 1770then only addresses and not host names may be used in 1771.Pa ~/.ssh/authorized_keys 1772.Cm from 1773and 1774.Nm 1775.Cm Match 1776.Cm Host 1777directives. 1778The default is 1779.Dq yes . 1780.It Cm UsePAM 1781Enables the Pluggable Authentication Module interface. 1782If set to 1783.Cm yes 1784this will enable PAM authentication using 1785.Cm KbdInteractiveAuthentication 1786and 1787.Cm PasswordAuthentication 1788in addition to PAM account and session module processing for all 1789authentication types. 1790.Pp 1791Because PAM keyboard-interactive authentication usually serves an equivalent 1792role to password authentication, you should disable either 1793.Cm PasswordAuthentication 1794or 1795.Cm KbdInteractiveAuthentication . 1796.Pp 1797If 1798.Cm UsePAM 1799is enabled, you will not be able to run 1800.Xr sshd 8 1801as a non-root user. 1802The default is 1803.Cm yes . 1804.It Cm VersionAddendum 1805Optionally specifies additional text to append to the SSH protocol banner 1806sent by the server upon connection. 1807The default is 1808.Qq FreeBSD-20220415 . 1809The value 1810.Cm none 1811may be used to disable this. 1812.It Cm X11DisplayOffset 1813Specifies the first display number available for 1814.Xr sshd 8 Ns 's 1815X11 forwarding. 1816This prevents sshd from interfering with real X11 servers. 1817The default is 10. 1818.It Cm X11Forwarding 1819Specifies whether X11 forwarding is permitted. 1820The argument must be 1821.Cm yes 1822or 1823.Cm no . 1824The default is 1825.Cm yes . 1826.Pp 1827When X11 forwarding is enabled, there may be additional exposure to 1828the server and to client displays if the 1829.Xr sshd 8 1830proxy display is configured to listen on the wildcard address (see 1831.Cm X11UseLocalhost ) , 1832though this is not the default. 1833Additionally, the authentication spoofing and authentication data 1834verification and substitution occur on the client side. 1835The security risk of using X11 forwarding is that the client's X11 1836display server may be exposed to attack when the SSH client requests 1837forwarding (see the warnings for 1838.Cm ForwardX11 1839in 1840.Xr ssh_config 5 ) . 1841A system administrator may have a stance in which they want to 1842protect clients that may expose themselves to attack by unwittingly 1843requesting X11 forwarding, which can warrant a 1844.Cm no 1845setting. 1846.Pp 1847Note that disabling X11 forwarding does not prevent users from 1848forwarding X11 traffic, as users can always install their own forwarders. 1849.It Cm X11UseLocalhost 1850Specifies whether 1851.Xr sshd 8 1852should bind the X11 forwarding server to the loopback address or to 1853the wildcard address. 1854By default, 1855sshd binds the forwarding server to the loopback address and sets the 1856hostname part of the 1857.Ev DISPLAY 1858environment variable to 1859.Cm localhost . 1860This prevents remote hosts from connecting to the proxy display. 1861However, some older X11 clients may not function with this 1862configuration. 1863.Cm X11UseLocalhost 1864may be set to 1865.Cm no 1866to specify that the forwarding server should be bound to the wildcard 1867address. 1868The argument must be 1869.Cm yes 1870or 1871.Cm no . 1872The default is 1873.Cm yes . 1874.It Cm XAuthLocation 1875Specifies the full pathname of the 1876.Xr xauth 1 1877program, or 1878.Cm none 1879to not use one. 1880The default is 1881.Pa /usr/local/bin/xauth . 1882.El 1883.Sh TIME FORMATS 1884.Xr sshd 8 1885command-line arguments and configuration file options that specify time 1886may be expressed using a sequence of the form: 1887.Sm off 1888.Ar time Op Ar qualifier , 1889.Sm on 1890where 1891.Ar time 1892is a positive integer value and 1893.Ar qualifier 1894is one of the following: 1895.Pp 1896.Bl -tag -width Ds -compact -offset indent 1897.It Aq Cm none 1898seconds 1899.It Cm s | Cm S 1900seconds 1901.It Cm m | Cm M 1902minutes 1903.It Cm h | Cm H 1904hours 1905.It Cm d | Cm D 1906days 1907.It Cm w | Cm W 1908weeks 1909.El 1910.Pp 1911Each member of the sequence is added together to calculate 1912the total time value. 1913.Pp 1914Time format examples: 1915.Pp 1916.Bl -tag -width Ds -compact -offset indent 1917.It 600 1918600 seconds (10 minutes) 1919.It 10m 192010 minutes 1921.It 1h30m 19221 hour 30 minutes (90 minutes) 1923.El 1924.Sh TOKENS 1925Arguments to some keywords can make use of tokens, 1926which are expanded at runtime: 1927.Pp 1928.Bl -tag -width XXXX -offset indent -compact 1929.It %% 1930A literal 1931.Sq % . 1932.It \&%D 1933The routing domain in which the incoming connection was received. 1934.It %F 1935The fingerprint of the CA key. 1936.It %f 1937The fingerprint of the key or certificate. 1938.It %h 1939The home directory of the user. 1940.It %i 1941The key ID in the certificate. 1942.It %K 1943The base64-encoded CA key. 1944.It %k 1945The base64-encoded key or certificate for authentication. 1946.It %s 1947The serial number of the certificate. 1948.It \&%T 1949The type of the CA key. 1950.It %t 1951The key or certificate type. 1952.It \&%U 1953The numeric user ID of the target user. 1954.It %u 1955The username. 1956.El 1957.Pp 1958.Cm AuthorizedKeysCommand 1959accepts the tokens %%, %f, %h, %k, %t, %U, and %u. 1960.Pp 1961.Cm AuthorizedKeysFile 1962accepts the tokens %%, %h, %U, and %u. 1963.Pp 1964.Cm AuthorizedPrincipalsCommand 1965accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 1966.Pp 1967.Cm AuthorizedPrincipalsFile 1968accepts the tokens %%, %h, %U, and %u. 1969.Pp 1970.Cm ChrootDirectory 1971accepts the tokens %%, %h, %U, and %u. 1972.Pp 1973.Cm RoutingDomain 1974accepts the token %D. 1975.Sh FILES 1976.Bl -tag -width Ds 1977.It Pa /etc/ssh/sshd_config 1978Contains configuration data for 1979.Xr sshd 8 . 1980This file should be writable by root only, but it is recommended 1981(though not necessary) that it be world-readable. 1982.El 1983.Sh SEE ALSO 1984.Xr sftp-server 8 , 1985.Xr sshd 8 1986.Sh AUTHORS 1987.An -nosplit 1988OpenSSH is a derivative of the original and free 1989ssh 1.2.12 release by 1990.An Tatu Ylonen . 1991.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1992.An Theo de Raadt 1993and 1994.An Dug Song 1995removed many bugs, re-added newer features and 1996created OpenSSH. 1997.An Markus Friedl 1998contributed the support for SSH protocol versions 1.5 and 2.0. 1999.An Niels Provos 2000and 2001.An Markus Friedl 2002contributed support for privilege separation. 2003