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 1.342 2022/06/24 04:27:14 djm Exp $ 37.Dd $Mdocdate: June 24 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 , 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-ed25519, 694ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 695sk-ssh-ed25519@openssh.com, 696sk-ecdsa-sha2-nistp256@openssh.com, 697rsa-sha2-512,rsa-sha2-256 698.Ed 699.Pp 700The list of available signature algorithms may also be obtained using 701.Qq ssh -Q HostbasedAcceptedAlgorithms . 702This was formerly named HostbasedAcceptedKeyTypes. 703.It Cm HostbasedAuthentication 704Specifies whether rhosts or /etc/hosts.equiv authentication together 705with successful public key client host authentication is allowed 706(host-based authentication). 707The default is 708.Cm no . 709.It Cm HostbasedUsesNameFromPacketOnly 710Specifies whether or not the server will attempt to perform a reverse 711name lookup when matching the name in the 712.Pa ~/.shosts , 713.Pa ~/.rhosts , 714and 715.Pa /etc/hosts.equiv 716files during 717.Cm HostbasedAuthentication . 718A setting of 719.Cm yes 720means that 721.Xr sshd 8 722uses the name supplied by the client rather than 723attempting to resolve the name from the TCP connection itself. 724The default is 725.Cm no . 726.It Cm HostCertificate 727Specifies a file containing a public host certificate. 728The certificate's public key must match a private host key already specified 729by 730.Cm HostKey . 731The default behaviour of 732.Xr sshd 8 733is not to load any certificates. 734.It Cm HostKey 735Specifies a file containing a private host key 736used by SSH. 737The defaults are 738.Pa /etc/ssh/ssh_host_ecdsa_key , 739.Pa /etc/ssh/ssh_host_ed25519_key 740and 741.Pa /etc/ssh/ssh_host_rsa_key . 742.Pp 743Note that 744.Xr sshd 8 745will refuse to use a file if it is group/world-accessible 746and that the 747.Cm HostKeyAlgorithms 748option restricts which of the keys are actually used by 749.Xr sshd 8 . 750.Pp 751It is possible to have multiple host key files. 752It is also possible to specify public host key files instead. 753In this case operations on the private key will be delegated 754to an 755.Xr ssh-agent 1 . 756.It Cm HostKeyAgent 757Identifies the UNIX-domain socket used to communicate 758with an agent that has access to the private host keys. 759If the string 760.Qq SSH_AUTH_SOCK 761is specified, the location of the socket will be read from the 762.Ev SSH_AUTH_SOCK 763environment variable. 764.It Cm HostKeyAlgorithms 765Specifies the host key signature algorithms 766that the server offers. 767The default for this option is: 768.Bd -literal -offset 3n 769ssh-ed25519-cert-v01@openssh.com, 770ecdsa-sha2-nistp256-cert-v01@openssh.com, 771ecdsa-sha2-nistp384-cert-v01@openssh.com, 772ecdsa-sha2-nistp521-cert-v01@openssh.com, 773sk-ssh-ed25519-cert-v01@openssh.com, 774sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 775rsa-sha2-512-cert-v01@openssh.com, 776rsa-sha2-256-cert-v01@openssh.com, 777ssh-ed25519, 778ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 779sk-ssh-ed25519@openssh.com, 780sk-ecdsa-sha2-nistp256@openssh.com, 781rsa-sha2-512,rsa-sha2-256 782.Ed 783.Pp 784The list of available signature algorithms may also be obtained using 785.Qq ssh -Q HostKeyAlgorithms . 786.It Cm IgnoreRhosts 787Specifies whether to ignore per-user 788.Pa .rhosts 789and 790.Pa .shosts 791files during 792.Cm HostbasedAuthentication . 793The system-wide 794.Pa /etc/hosts.equiv 795and 796.Pa /etc/shosts.equiv 797are still used regardless of this setting. 798.Pp 799Accepted values are 800.Cm yes 801(the default) to ignore all per-user files, 802.Cm shosts-only 803to allow the use of 804.Pa .shosts 805but to ignore 806.Pa .rhosts 807or 808.Cm no 809to allow both 810.Pa .shosts 811and 812.Pa rhosts . 813.It Cm IgnoreUserKnownHosts 814Specifies whether 815.Xr sshd 8 816should ignore the user's 817.Pa ~/.ssh/known_hosts 818during 819.Cm HostbasedAuthentication 820and use only the system-wide known hosts file 821.Pa /etc/ssh/ssh_known_hosts . 822The default is 823.Dq no . 824.It Cm Include 825Include the specified configuration file(s). 826Multiple pathnames may be specified and each pathname may contain 827.Xr glob 7 828wildcards that will be expanded and processed in lexical order. 829Files without absolute paths are assumed to be in 830.Pa /etc/ssh . 831An 832.Cm Include 833directive may appear inside a 834.Cm Match 835block 836to perform conditional inclusion. 837.It Cm IPQoS 838Specifies the IPv4 type-of-service or DSCP class for the connection. 839Accepted values are 840.Cm af11 , 841.Cm af12 , 842.Cm af13 , 843.Cm af21 , 844.Cm af22 , 845.Cm af23 , 846.Cm af31 , 847.Cm af32 , 848.Cm af33 , 849.Cm af41 , 850.Cm af42 , 851.Cm af43 , 852.Cm cs0 , 853.Cm cs1 , 854.Cm cs2 , 855.Cm cs3 , 856.Cm cs4 , 857.Cm cs5 , 858.Cm cs6 , 859.Cm cs7 , 860.Cm ef , 861.Cm le , 862.Cm lowdelay , 863.Cm throughput , 864.Cm reliability , 865a numeric value, or 866.Cm none 867to use the operating system default. 868This option may take one or two arguments, separated by whitespace. 869If one argument is specified, it is used as the packet class unconditionally. 870If two values are specified, the first is automatically selected for 871interactive sessions and the second for non-interactive sessions. 872The default is 873.Cm af21 874(Low-Latency Data) 875for interactive sessions and 876.Cm cs1 877(Lower Effort) 878for non-interactive sessions. 879.It Cm KbdInteractiveAuthentication 880Specifies whether to allow keyboard-interactive authentication. 881All authentication styles from 882.Xr login.conf 5 883are supported. 884The default is 885.Cm yes . 886The argument to this keyword must be 887.Cm yes 888or 889.Cm no . 890.Cm ChallengeResponseAuthentication 891is a deprecated alias for this. 892.It Cm KerberosAuthentication 893Specifies whether the password provided by the user for 894.Cm PasswordAuthentication 895will be validated through the Kerberos KDC. 896To use this option, the server needs a 897Kerberos servtab which allows the verification of the KDC's identity. 898The default is 899.Cm no . 900.It Cm KerberosGetAFSToken 901If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 902an AFS token before accessing the user's home directory. 903The default is 904.Cm no . 905.It Cm KerberosOrLocalPasswd 906If password authentication through Kerberos fails then 907the password will be validated via any additional local mechanism 908such as 909.Pa /etc/passwd . 910The default is 911.Cm yes . 912.It Cm KerberosTicketCleanup 913Specifies whether to automatically destroy the user's ticket cache 914file on logout. 915The default is 916.Cm yes . 917.It Cm KexAlgorithms 918Specifies the available KEX (Key Exchange) algorithms. 919Multiple algorithms must be comma-separated. 920Alternately if the specified list begins with a 921.Sq + 922character, then the specified algorithms will be appended to the default set 923instead of replacing them. 924If the specified list begins with a 925.Sq - 926character, then the specified algorithms (including wildcards) will be removed 927from the default set instead of replacing them. 928If the specified list begins with a 929.Sq ^ 930character, then the specified algorithms will be placed at the head of the 931default set. 932The supported algorithms are: 933.Pp 934.Bl -item -compact -offset indent 935.It 936curve25519-sha256 937.It 938curve25519-sha256@libssh.org 939.It 940diffie-hellman-group1-sha1 941.It 942diffie-hellman-group14-sha1 943.It 944diffie-hellman-group14-sha256 945.It 946diffie-hellman-group16-sha512 947.It 948diffie-hellman-group18-sha512 949.It 950diffie-hellman-group-exchange-sha1 951.It 952diffie-hellman-group-exchange-sha256 953.It 954ecdh-sha2-nistp256 955.It 956ecdh-sha2-nistp384 957.It 958ecdh-sha2-nistp521 959.It 960sntrup761x25519-sha512@openssh.com 961.El 962.Pp 963The default is: 964.Bd -literal -offset indent 965sntrup761x25519-sha512@openssh.com, 966curve25519-sha256,curve25519-sha256@libssh.org, 967ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 968diffie-hellman-group-exchange-sha256, 969diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 970diffie-hellman-group14-sha256 971.Ed 972.Pp 973The list of available key exchange algorithms may also be obtained using 974.Qq ssh -Q KexAlgorithms . 975.It Cm ListenAddress 976Specifies the local addresses 977.Xr sshd 8 978should listen on. 979The following forms may be used: 980.Pp 981.Bl -item -offset indent -compact 982.It 983.Cm ListenAddress 984.Sm off 985.Ar hostname | address 986.Sm on 987.Op Cm rdomain Ar domain 988.It 989.Cm ListenAddress 990.Sm off 991.Ar hostname : port 992.Sm on 993.Op Cm rdomain Ar domain 994.It 995.Cm ListenAddress 996.Sm off 997.Ar IPv4_address : port 998.Sm on 999.Op Cm rdomain Ar domain 1000.It 1001.Cm ListenAddress 1002.Sm off 1003.Oo Ar hostname | address Oc : Ar port 1004.Sm on 1005.Op Cm rdomain Ar domain 1006.El 1007.Pp 1008The optional 1009.Cm rdomain 1010qualifier requests 1011.Xr sshd 8 1012listen in an explicit routing domain. 1013If 1014.Ar port 1015is not specified, 1016sshd will listen on the address and all 1017.Cm Port 1018options specified. 1019The default is to listen on all local addresses on the current default 1020routing domain. 1021Multiple 1022.Cm ListenAddress 1023options are permitted. 1024For more information on routing domains, see 1025.Xr rdomain 4 . 1026.It Cm LoginGraceTime 1027The server disconnects after this time if the user has not 1028successfully logged in. 1029If the value is 0, there is no time limit. 1030The default is 120 seconds. 1031.It Cm LogLevel 1032Gives the verbosity level that is used when logging messages from 1033.Xr sshd 8 . 1034The possible values are: 1035QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1036The default is INFO. 1037DEBUG and DEBUG1 are equivalent. 1038DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1039Logging with a DEBUG level violates the privacy of users and is not recommended. 1040.It Cm LogVerbose 1041Specify one or more overrides to LogLevel. 1042An override consists of a pattern lists that matches the source file, function 1043and line number to force detailed logging for. 1044For example, an override pattern of: 1045.Bd -literal -offset indent 1046kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1047.Ed 1048.Pp 1049would enable detailed logging for line 1000 of 1050.Pa kex.c , 1051everything in the 1052.Fn kex_exchange_identification 1053function, and all code in the 1054.Pa packet.c 1055file. 1056This option is intended for debugging and no overrides are enabled by default. 1057.It Cm MACs 1058Specifies the available MAC (message authentication code) algorithms. 1059The MAC algorithm is used for data integrity protection. 1060Multiple algorithms must be comma-separated. 1061If the specified list begins with a 1062.Sq + 1063character, then the specified algorithms will be appended to the default set 1064instead of replacing them. 1065If the specified list begins with a 1066.Sq - 1067character, then the specified algorithms (including wildcards) will be removed 1068from the default set instead of replacing them. 1069If the specified list begins with a 1070.Sq ^ 1071character, then the specified algorithms will be placed at the head of the 1072default set. 1073.Pp 1074The algorithms that contain 1075.Qq -etm 1076calculate the MAC after encryption (encrypt-then-mac). 1077These are considered safer and their use recommended. 1078The supported MACs are: 1079.Pp 1080.Bl -item -compact -offset indent 1081.It 1082hmac-md5 1083.It 1084hmac-md5-96 1085.It 1086hmac-sha1 1087.It 1088hmac-sha1-96 1089.It 1090hmac-sha2-256 1091.It 1092hmac-sha2-512 1093.It 1094umac-64@openssh.com 1095.It 1096umac-128@openssh.com 1097.It 1098hmac-md5-etm@openssh.com 1099.It 1100hmac-md5-96-etm@openssh.com 1101.It 1102hmac-sha1-etm@openssh.com 1103.It 1104hmac-sha1-96-etm@openssh.com 1105.It 1106hmac-sha2-256-etm@openssh.com 1107.It 1108hmac-sha2-512-etm@openssh.com 1109.It 1110umac-64-etm@openssh.com 1111.It 1112umac-128-etm@openssh.com 1113.El 1114.Pp 1115The default is: 1116.Bd -literal -offset indent 1117umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1118hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1119hmac-sha1-etm@openssh.com, 1120umac-64@openssh.com,umac-128@openssh.com, 1121hmac-sha2-256,hmac-sha2-512,hmac-sha1 1122.Ed 1123.Pp 1124The list of available MAC algorithms may also be obtained using 1125.Qq ssh -Q mac . 1126.It Cm Match 1127Introduces a conditional block. 1128If all of the criteria on the 1129.Cm Match 1130line are satisfied, the keywords on the following lines override those 1131set in the global section of the config file, until either another 1132.Cm Match 1133line or the end of the file. 1134If a keyword appears in multiple 1135.Cm Match 1136blocks that are satisfied, only the first instance of the keyword is 1137applied. 1138.Pp 1139The arguments to 1140.Cm Match 1141are one or more criteria-pattern pairs or the single token 1142.Cm All 1143which matches all criteria. 1144The available criteria are 1145.Cm User , 1146.Cm Group , 1147.Cm Host , 1148.Cm LocalAddress , 1149.Cm LocalPort , 1150.Cm RDomain , 1151and 1152.Cm Address 1153(with 1154.Cm RDomain 1155representing the 1156.Xr rdomain 4 1157on which the connection was received). 1158.Pp 1159The match patterns may consist of single entries or comma-separated 1160lists and may use the wildcard and negation operators described in the 1161.Sx PATTERNS 1162section of 1163.Xr ssh_config 5 . 1164.Pp 1165The patterns in an 1166.Cm Address 1167criteria may additionally contain addresses to match in CIDR 1168address/masklen format, 1169such as 192.0.2.0/24 or 2001:db8::/32. 1170Note that the mask length provided must be consistent with the address - 1171it is an error to specify a mask length that is too long for the address 1172or one with bits set in this host portion of the address. 1173For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1174.Pp 1175Only a subset of keywords may be used on the lines following a 1176.Cm Match 1177keyword. 1178Available keywords are 1179.Cm AcceptEnv , 1180.Cm AllowAgentForwarding , 1181.Cm AllowGroups , 1182.Cm AllowStreamLocalForwarding , 1183.Cm AllowTcpForwarding , 1184.Cm AllowUsers , 1185.Cm AuthenticationMethods , 1186.Cm AuthorizedKeysCommand , 1187.Cm AuthorizedKeysCommandUser , 1188.Cm AuthorizedKeysFile , 1189.Cm AuthorizedPrincipalsCommand , 1190.Cm AuthorizedPrincipalsCommandUser , 1191.Cm AuthorizedPrincipalsFile , 1192.Cm Banner , 1193.Cm CASignatureAlgorithms , 1194.Cm ChrootDirectory , 1195.Cm ClientAliveCountMax , 1196.Cm ClientAliveInterval , 1197.Cm DenyGroups , 1198.Cm DenyUsers , 1199.Cm DisableForwarding , 1200.Cm ExposeAuthInfo , 1201.Cm ForceCommand , 1202.Cm GatewayPorts , 1203.Cm GSSAPIAuthentication , 1204.Cm HostbasedAcceptedAlgorithms , 1205.Cm HostbasedAuthentication , 1206.Cm HostbasedUsesNameFromPacketOnly , 1207.Cm IgnoreRhosts , 1208.Cm Include , 1209.Cm IPQoS , 1210.Cm KbdInteractiveAuthentication , 1211.Cm KerberosAuthentication , 1212.Cm LogLevel , 1213.Cm MaxAuthTries , 1214.Cm MaxSessions , 1215.Cm PasswordAuthentication , 1216.Cm PermitEmptyPasswords , 1217.Cm PermitListen , 1218.Cm PermitOpen , 1219.Cm PermitRootLogin , 1220.Cm PermitTTY , 1221.Cm PermitTunnel , 1222.Cm PermitUserRC , 1223.Cm PubkeyAcceptedAlgorithms , 1224.Cm PubkeyAuthentication , 1225.Cm PubkeyAuthOptions , 1226.Cm RekeyLimit , 1227.Cm RevokedKeys , 1228.Cm RDomain , 1229.Cm SetEnv , 1230.Cm StreamLocalBindMask , 1231.Cm StreamLocalBindUnlink , 1232.Cm TrustedUserCAKeys , 1233.Cm X11DisplayOffset , 1234.Cm X11Forwarding 1235and 1236.Cm X11UseLocalhost . 1237.It Cm MaxAuthTries 1238Specifies the maximum number of authentication attempts permitted per 1239connection. 1240Once the number of failures reaches half this value, 1241additional failures are logged. 1242The default is 6. 1243.It Cm MaxSessions 1244Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1245sessions permitted per network connection. 1246Multiple sessions may be established by clients that support connection 1247multiplexing. 1248Setting 1249.Cm MaxSessions 1250to 1 will effectively disable session multiplexing, whereas setting it to 0 1251will prevent all shell, login and subsystem sessions while still permitting 1252forwarding. 1253The default is 10. 1254.It Cm MaxStartups 1255Specifies the maximum number of concurrent unauthenticated connections to the 1256SSH daemon. 1257Additional connections will be dropped until authentication succeeds or the 1258.Cm LoginGraceTime 1259expires for a connection. 1260The default is 10:30:100. 1261.Pp 1262Alternatively, random early drop can be enabled by specifying 1263the three colon separated values 1264start:rate:full (e.g. "10:30:60"). 1265.Xr sshd 8 1266will refuse connection attempts with a probability of rate/100 (30%) 1267if there are currently start (10) unauthenticated connections. 1268The probability increases linearly and all connection attempts 1269are refused if the number of unauthenticated connections reaches full (60). 1270.It Cm ModuliFile 1271Specifies the 1272.Xr moduli 5 1273file that contains the Diffie-Hellman groups used for the 1274.Dq diffie-hellman-group-exchange-sha1 1275and 1276.Dq diffie-hellman-group-exchange-sha256 1277key exchange methods. 1278The default is 1279.Pa /etc/moduli . 1280.It Cm PasswordAuthentication 1281Specifies whether password authentication is allowed. 1282The default is 1283.Cm yes . 1284.It Cm PermitEmptyPasswords 1285When password authentication is allowed, it specifies whether the 1286server allows login to accounts with empty password strings. 1287The default is 1288.Cm no . 1289.It Cm PermitListen 1290Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1291The listen specification must be one of the following forms: 1292.Pp 1293.Bl -item -offset indent -compact 1294.It 1295.Cm PermitListen 1296.Sm off 1297.Ar port 1298.Sm on 1299.It 1300.Cm PermitListen 1301.Sm off 1302.Ar host : port 1303.Sm on 1304.El 1305.Pp 1306Multiple permissions may be specified by separating them with whitespace. 1307An argument of 1308.Cm any 1309can be used to remove all restrictions and permit any listen requests. 1310An argument of 1311.Cm none 1312can be used to prohibit all listen requests. 1313The host name may contain wildcards as described in the PATTERNS section in 1314.Xr ssh_config 5 . 1315The wildcard 1316.Sq * 1317can also be used in place of a port number to allow all ports. 1318By default all port forwarding listen requests are permitted. 1319Note that the 1320.Cm GatewayPorts 1321option may further restrict which addresses may be listened on. 1322Note also that 1323.Xr ssh 1 1324will request a listen host of 1325.Dq localhost 1326if no listen host was specifically requested, and this name is 1327treated differently to explicit localhost addresses of 1328.Dq 127.0.0.1 1329and 1330.Dq ::1 . 1331.It Cm PermitOpen 1332Specifies the destinations to which TCP port forwarding is permitted. 1333The forwarding specification must be one of the following forms: 1334.Pp 1335.Bl -item -offset indent -compact 1336.It 1337.Cm PermitOpen 1338.Sm off 1339.Ar host : port 1340.Sm on 1341.It 1342.Cm PermitOpen 1343.Sm off 1344.Ar IPv4_addr : port 1345.Sm on 1346.It 1347.Cm PermitOpen 1348.Sm off 1349.Ar \&[ IPv6_addr \&] : port 1350.Sm on 1351.El 1352.Pp 1353Multiple forwards may be specified by separating them with whitespace. 1354An argument of 1355.Cm any 1356can be used to remove all restrictions and permit any forwarding requests. 1357An argument of 1358.Cm none 1359can be used to prohibit all forwarding requests. 1360The wildcard 1361.Sq * 1362can be used for host or port to allow all hosts or ports respectively. 1363Otherwise, no pattern matching or address lookups are performed on supplied 1364names. 1365By default all port forwarding requests are permitted. 1366.It Cm PermitRootLogin 1367Specifies whether root can log in using 1368.Xr ssh 1 . 1369The argument must be 1370.Cm yes , 1371.Cm prohibit-password , 1372.Cm forced-commands-only , 1373or 1374.Cm no . 1375The default is 1376.Cm prohibit-password . 1377.Pp 1378If this option is set to 1379.Cm prohibit-password 1380(or its deprecated alias, 1381.Cm without-password ) , 1382password and keyboard-interactive authentication are disabled for root. 1383.Pp 1384If this option is set to 1385.Cm forced-commands-only , 1386root login with public key authentication will be allowed, 1387but only if the 1388.Ar command 1389option has been specified 1390(which may be useful for taking remote backups even if root login is 1391normally not allowed). 1392All other authentication methods are disabled for root. 1393.Pp 1394If this option is set to 1395.Cm no , 1396root is not allowed to log in. 1397.It Cm PermitTTY 1398Specifies whether 1399.Xr pty 4 1400allocation is permitted. 1401The default is 1402.Cm yes . 1403.It Cm PermitTunnel 1404Specifies whether 1405.Xr tun 4 1406device forwarding is allowed. 1407The argument must be 1408.Cm yes , 1409.Cm point-to-point 1410(layer 3), 1411.Cm ethernet 1412(layer 2), or 1413.Cm no . 1414Specifying 1415.Cm yes 1416permits both 1417.Cm point-to-point 1418and 1419.Cm ethernet . 1420The default is 1421.Cm no . 1422.Pp 1423Independent of this setting, the permissions of the selected 1424.Xr tun 4 1425device must allow access to the user. 1426.It Cm PermitUserEnvironment 1427Specifies whether 1428.Pa ~/.ssh/environment 1429and 1430.Cm environment= 1431options in 1432.Pa ~/.ssh/authorized_keys 1433are processed by 1434.Xr sshd 8 . 1435Valid options are 1436.Cm yes , 1437.Cm no 1438or a pattern-list specifying which environment variable names to accept 1439(for example 1440.Qq LANG,LC_* ) . 1441The default is 1442.Cm no . 1443Enabling environment processing may enable users to bypass access 1444restrictions in some configurations using mechanisms such as 1445.Ev LD_PRELOAD . 1446.It Cm PermitUserRC 1447Specifies whether any 1448.Pa ~/.ssh/rc 1449file is executed. 1450The default is 1451.Cm yes . 1452.It Cm PerSourceMaxStartups 1453Specifies the number of unauthenticated connections allowed from a 1454given source address, or 1455.Dq none 1456if there is no limit. 1457This limit is applied in addition to 1458.Cm MaxStartups , 1459whichever is lower. 1460The default is 1461.Cm none . 1462.It Cm PerSourceNetBlockSize 1463Specifies the number of bits of source address that are grouped together 1464for the purposes of applying PerSourceMaxStartups limits. 1465Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1466The default is 1467.Cm 32:128 , 1468which means each address is considered individually. 1469.It Cm PidFile 1470Specifies the file that contains the process ID of the 1471SSH daemon, or 1472.Cm none 1473to not write one. 1474The default is 1475.Pa /var/run/sshd.pid . 1476.It Cm Port 1477Specifies the port number that 1478.Xr sshd 8 1479listens on. 1480The default is 22. 1481Multiple options of this type are permitted. 1482See also 1483.Cm ListenAddress . 1484.It Cm PrintLastLog 1485Specifies whether 1486.Xr sshd 8 1487should print the date and time of the last user login when a user logs 1488in interactively. 1489The default is 1490.Cm yes . 1491.It Cm PrintMotd 1492Specifies whether 1493.Xr sshd 8 1494should print 1495.Pa /etc/motd 1496when a user logs in interactively. 1497(On some systems it is also printed by the shell, 1498.Pa /etc/profile , 1499or equivalent.) 1500The default is 1501.Cm yes . 1502.It Cm PubkeyAcceptedAlgorithms 1503Specifies the signature algorithms that will be accepted for public key 1504authentication as a list of comma-separated patterns. 1505Alternately if the specified list begins with a 1506.Sq + 1507character, then the specified algorithms will be appended to the default set 1508instead of replacing them. 1509If the specified list begins with a 1510.Sq - 1511character, then the specified algorithms (including wildcards) will be removed 1512from the default set instead of replacing them. 1513If the specified list begins with a 1514.Sq ^ 1515character, then the specified algorithms will be placed at the head of the 1516default set. 1517The default for this option is: 1518.Bd -literal -offset 3n 1519ssh-ed25519-cert-v01@openssh.com, 1520ecdsa-sha2-nistp256-cert-v01@openssh.com, 1521ecdsa-sha2-nistp384-cert-v01@openssh.com, 1522ecdsa-sha2-nistp521-cert-v01@openssh.com, 1523sk-ssh-ed25519-cert-v01@openssh.com, 1524sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1525rsa-sha2-512-cert-v01@openssh.com, 1526rsa-sha2-256-cert-v01@openssh.com, 1527ssh-ed25519, 1528ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1529sk-ssh-ed25519@openssh.com, 1530sk-ecdsa-sha2-nistp256@openssh.com, 1531rsa-sha2-512,rsa-sha2-256 1532.Ed 1533.Pp 1534The list of available signature algorithms may also be obtained using 1535.Qq ssh -Q PubkeyAcceptedAlgorithms . 1536.It Cm PubkeyAuthOptions 1537Sets one or more public key authentication options. 1538The supported keywords are: 1539.Cm none 1540(the default; indicating no additional options are enabled), 1541.Cm touch-required 1542and 1543.Cm verify-required . 1544.Pp 1545The 1546.Cm touch-required 1547option causes public key authentication using a FIDO authenticator algorithm 1548(i.e.\& 1549.Cm ecdsa-sk 1550or 1551.Cm ed25519-sk ) 1552to always require the signature to attest that a physically present user 1553explicitly confirmed the authentication (usually by touching the authenticator). 1554By default, 1555.Xr sshd 8 1556requires user presence unless overridden with an authorized_keys option. 1557The 1558.Cm touch-required 1559flag disables this override. 1560.Pp 1561The 1562.Cm verify-required 1563option requires a FIDO key signature attest that the user was verified, 1564e.g. via a PIN. 1565.Pp 1566Neither the 1567.Cm touch-required 1568or 1569.Cm verify-required 1570options have any effect for other, non-FIDO, public key types. 1571.It Cm PubkeyAuthentication 1572Specifies whether public key authentication is allowed. 1573The default is 1574.Cm yes . 1575.It Cm RekeyLimit 1576Specifies the maximum amount of data that may be transmitted or received 1577before the session key is renegotiated, optionally followed by a maximum 1578amount of time that may pass before the session key is renegotiated. 1579The first argument is specified in bytes and may have a suffix of 1580.Sq K , 1581.Sq M , 1582or 1583.Sq G 1584to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1585The default is between 1586.Sq 1G 1587and 1588.Sq 4G , 1589depending on the cipher. 1590The optional second value is specified in seconds and may use any of the 1591units documented in the 1592.Sx TIME FORMATS 1593section. 1594The default value for 1595.Cm RekeyLimit 1596is 1597.Cm default none , 1598which means that rekeying is performed after the cipher's default amount 1599of data has been sent or received and no time based rekeying is done. 1600.It Cm RevokedKeys 1601Specifies revoked public keys file, or 1602.Cm none 1603to not use one. 1604Keys listed in this file will be refused for public key authentication. 1605Note that if this file is not readable, then public key authentication will 1606be refused for all users. 1607Keys may be specified as a text file, listing one public key per line, or as 1608an OpenSSH Key Revocation List (KRL) as generated by 1609.Xr ssh-keygen 1 . 1610For more information on KRLs, see the KEY REVOCATION LISTS section in 1611.Xr ssh-keygen 1 . 1612.It Cm RDomain 1613Specifies an explicit routing domain that is applied after authentication 1614has completed. 1615The user session, as well as any forwarded or listening IP sockets, 1616will be bound to this 1617.Xr rdomain 4 . 1618If the routing domain is set to 1619.Cm \&%D , 1620then the domain in which the incoming connection was received will be applied. 1621.It Cm SecurityKeyProvider 1622Specifies a path to a library that will be used when loading 1623FIDO authenticator-hosted keys, overriding the default of using 1624the built-in USB HID support. 1625.It Cm SetEnv 1626Specifies one or more environment variables to set in child sessions started 1627by 1628.Xr sshd 8 1629as 1630.Dq NAME=VALUE . 1631The environment value may be quoted (e.g. if it contains whitespace 1632characters). 1633Environment variables set by 1634.Cm SetEnv 1635override the default environment and any variables specified by the user 1636via 1637.Cm AcceptEnv 1638or 1639.Cm PermitUserEnvironment . 1640.It Cm StreamLocalBindMask 1641Sets the octal file creation mode mask 1642.Pq umask 1643used when creating a Unix-domain socket file for local or remote 1644port forwarding. 1645This option is only used for port forwarding to a Unix-domain socket file. 1646.Pp 1647The default value is 0177, which creates a Unix-domain socket file that is 1648readable and writable only by the owner. 1649Note that not all operating systems honor the file mode on Unix-domain 1650socket files. 1651.It Cm StreamLocalBindUnlink 1652Specifies whether to remove an existing Unix-domain socket file for local 1653or remote port forwarding before creating a new one. 1654If the socket file already exists and 1655.Cm StreamLocalBindUnlink 1656is not enabled, 1657.Nm sshd 1658will be unable to forward the port to the Unix-domain socket file. 1659This option is only used for port forwarding to a Unix-domain socket file. 1660.Pp 1661The argument must be 1662.Cm yes 1663or 1664.Cm no . 1665The default is 1666.Cm no . 1667.It Cm StrictModes 1668Specifies whether 1669.Xr sshd 8 1670should check file modes and ownership of the 1671user's files and home directory before accepting login. 1672This is normally desirable because novices sometimes accidentally leave their 1673directory or files world-writable. 1674The default is 1675.Cm yes . 1676Note that this does not apply to 1677.Cm ChrootDirectory , 1678whose permissions and ownership are checked unconditionally. 1679.It Cm Subsystem 1680Configures an external subsystem (e.g. file transfer daemon). 1681Arguments should be a subsystem name and a command (with optional arguments) 1682to execute upon subsystem request. 1683.Pp 1684The command 1685.Cm sftp-server 1686implements the SFTP file transfer subsystem. 1687.Pp 1688Alternately the name 1689.Cm internal-sftp 1690implements an in-process SFTP server. 1691This may simplify configurations using 1692.Cm ChrootDirectory 1693to force a different filesystem root on clients. 1694.Pp 1695By default no subsystems are defined. 1696.It Cm SyslogFacility 1697Gives the facility code that is used when logging messages from 1698.Xr sshd 8 . 1699The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1700LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1701The default is AUTH. 1702.It Cm TCPKeepAlive 1703Specifies whether the system should send TCP keepalive messages to the 1704other side. 1705If they are sent, death of the connection or crash of one 1706of the machines will be properly noticed. 1707However, this means that 1708connections will die if the route is down temporarily, and some people 1709find it annoying. 1710On the other hand, if TCP keepalives are not sent, 1711sessions may hang indefinitely on the server, leaving 1712.Qq ghost 1713users and consuming server resources. 1714.Pp 1715The default is 1716.Cm yes 1717(to send TCP keepalive messages), and the server will notice 1718if the network goes down or the client host crashes. 1719This avoids infinitely hanging sessions. 1720.Pp 1721To disable TCP keepalive messages, the value should be set to 1722.Cm no . 1723.It Cm TrustedUserCAKeys 1724Specifies a file containing public keys of certificate authorities that are 1725trusted to sign user certificates for authentication, or 1726.Cm none 1727to not use one. 1728Keys are listed one per line; empty lines and comments starting with 1729.Ql # 1730are allowed. 1731If a certificate is presented for authentication and has its signing CA key 1732listed in this file, then it may be used for authentication for any user 1733listed in the certificate's principals list. 1734Note that certificates that lack a list of principals will not be permitted 1735for authentication using 1736.Cm TrustedUserCAKeys . 1737For more details on certificates, see the CERTIFICATES section in 1738.Xr ssh-keygen 1 . 1739.It Cm UseDNS 1740Specifies whether 1741.Xr sshd 8 1742should look up the remote host name, and to check that 1743the resolved host name for the remote IP address maps back to the 1744very same IP address. 1745.Pp 1746If this option is set to 1747.Cm no 1748(the default) then only addresses and not host names may be used in 1749.Pa ~/.ssh/authorized_keys 1750.Cm from 1751and 1752.Nm 1753.Cm Match 1754.Cm Host 1755directives. 1756.It Cm VersionAddendum 1757Optionally specifies additional text to append to the SSH protocol banner 1758sent by the server upon connection. 1759The default is 1760.Cm none . 1761.It Cm X11DisplayOffset 1762Specifies the first display number available for 1763.Xr sshd 8 Ns 's 1764X11 forwarding. 1765This prevents sshd from interfering with real X11 servers. 1766The default is 10. 1767.It Cm X11Forwarding 1768Specifies whether X11 forwarding is permitted. 1769The argument must be 1770.Cm yes 1771or 1772.Cm no . 1773The default is 1774.Cm no . 1775.Pp 1776When X11 forwarding is enabled, there may be additional exposure to 1777the server and to client displays if the 1778.Xr sshd 8 1779proxy display is configured to listen on the wildcard address (see 1780.Cm X11UseLocalhost ) , 1781though this is not the default. 1782Additionally, the authentication spoofing and authentication data 1783verification and substitution occur on the client side. 1784The security risk of using X11 forwarding is that the client's X11 1785display server may be exposed to attack when the SSH client requests 1786forwarding (see the warnings for 1787.Cm ForwardX11 1788in 1789.Xr ssh_config 5 ) . 1790A system administrator may have a stance in which they want to 1791protect clients that may expose themselves to attack by unwittingly 1792requesting X11 forwarding, which can warrant a 1793.Cm no 1794setting. 1795.Pp 1796Note that disabling X11 forwarding does not prevent users from 1797forwarding X11 traffic, as users can always install their own forwarders. 1798.It Cm X11UseLocalhost 1799Specifies whether 1800.Xr sshd 8 1801should bind the X11 forwarding server to the loopback address or to 1802the wildcard address. 1803By default, 1804sshd binds the forwarding server to the loopback address and sets the 1805hostname part of the 1806.Ev DISPLAY 1807environment variable to 1808.Cm localhost . 1809This prevents remote hosts from connecting to the proxy display. 1810However, some older X11 clients may not function with this 1811configuration. 1812.Cm X11UseLocalhost 1813may be set to 1814.Cm no 1815to specify that the forwarding server should be bound to the wildcard 1816address. 1817The argument must be 1818.Cm yes 1819or 1820.Cm no . 1821The default is 1822.Cm yes . 1823.It Cm XAuthLocation 1824Specifies the full pathname of the 1825.Xr xauth 1 1826program, or 1827.Cm none 1828to not use one. 1829The default is 1830.Pa /usr/X11R6/bin/xauth . 1831.El 1832.Sh TIME FORMATS 1833.Xr sshd 8 1834command-line arguments and configuration file options that specify time 1835may be expressed using a sequence of the form: 1836.Sm off 1837.Ar time Op Ar qualifier , 1838.Sm on 1839where 1840.Ar time 1841is a positive integer value and 1842.Ar qualifier 1843is one of the following: 1844.Pp 1845.Bl -tag -width Ds -compact -offset indent 1846.It Aq Cm none 1847seconds 1848.It Cm s | Cm S 1849seconds 1850.It Cm m | Cm M 1851minutes 1852.It Cm h | Cm H 1853hours 1854.It Cm d | Cm D 1855days 1856.It Cm w | Cm W 1857weeks 1858.El 1859.Pp 1860Each member of the sequence is added together to calculate 1861the total time value. 1862.Pp 1863Time format examples: 1864.Pp 1865.Bl -tag -width Ds -compact -offset indent 1866.It 600 1867600 seconds (10 minutes) 1868.It 10m 186910 minutes 1870.It 1h30m 18711 hour 30 minutes (90 minutes) 1872.El 1873.Sh TOKENS 1874Arguments to some keywords can make use of tokens, 1875which are expanded at runtime: 1876.Pp 1877.Bl -tag -width XXXX -offset indent -compact 1878.It %% 1879A literal 1880.Sq % . 1881.It \&%D 1882The routing domain in which the incoming connection was received. 1883.It %F 1884The fingerprint of the CA key. 1885.It %f 1886The fingerprint of the key or certificate. 1887.It %h 1888The home directory of the user. 1889.It %i 1890The key ID in the certificate. 1891.It %K 1892The base64-encoded CA key. 1893.It %k 1894The base64-encoded key or certificate for authentication. 1895.It %s 1896The serial number of the certificate. 1897.It \&%T 1898The type of the CA key. 1899.It %t 1900The key or certificate type. 1901.It \&%U 1902The numeric user ID of the target user. 1903.It %u 1904The username. 1905.El 1906.Pp 1907.Cm AuthorizedKeysCommand 1908accepts the tokens %%, %f, %h, %k, %t, %U, and %u. 1909.Pp 1910.Cm AuthorizedKeysFile 1911accepts the tokens %%, %h, %U, and %u. 1912.Pp 1913.Cm AuthorizedPrincipalsCommand 1914accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 1915.Pp 1916.Cm AuthorizedPrincipalsFile 1917accepts the tokens %%, %h, %U, and %u. 1918.Pp 1919.Cm ChrootDirectory 1920accepts the tokens %%, %h, %U, and %u. 1921.Pp 1922.Cm RoutingDomain 1923accepts the token %D. 1924.Sh FILES 1925.Bl -tag -width Ds 1926.It Pa /etc/ssh/sshd_config 1927Contains configuration data for 1928.Xr sshd 8 . 1929This file should be writable by root only, but it is recommended 1930(though not necessary) that it be world-readable. 1931.El 1932.Sh SEE ALSO 1933.Xr sftp-server 8 , 1934.Xr sshd 8 1935.Sh AUTHORS 1936.An -nosplit 1937OpenSSH is a derivative of the original and free 1938ssh 1.2.12 release by 1939.An Tatu Ylonen . 1940.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1941.An Theo de Raadt 1942and 1943.An Dug Song 1944removed many bugs, re-added newer features and 1945created OpenSSH. 1946.An Markus Friedl 1947contributed the support for SSH protocol versions 1.5 and 2.0. 1948.An Niels Provos 1949and 1950.An Markus Friedl 1951contributed support for privilege separation. 1952