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 1.334 2021/08/12 23:59:25 djm Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: August 12 2021 $ 39.Dt SSHD_CONFIG 5 40.Os 41.Sh NAME 42.Nm sshd_config 43.Nd OpenSSH daemon configuration file 44.Sh DESCRIPTION 45.Xr sshd 8 46reads configuration data from 47.Pa /etc/ssh/sshd_config 48(or the file specified with 49.Fl f 50on the command line). 51The file contains keyword-argument pairs, one per line. 52For each keyword, the first obtained value will be used. 53Lines starting with 54.Ql # 55and empty lines are interpreted as comments. 56Arguments may optionally be enclosed in double quotes 57.Pq \&" 58in order to represent arguments containing spaces. 59.Pp 60The possible 61keywords and their meanings are as follows (note that 62keywords are case-insensitive and arguments are case-sensitive): 63.Bl -tag -width Ds 64.It Cm AcceptEnv 65Specifies what environment variables sent by the client will be copied into 66the session's 67.Xr environ 7 . 68See 69.Cm SendEnv 70and 71.Cm SetEnv 72in 73.Xr ssh_config 5 74for how to configure the client. 75The 76.Ev TERM 77environment variable is always accepted whenever the client 78requests a pseudo-terminal as it is required by the protocol. 79Variables are specified by name, which may contain the wildcard characters 80.Ql * 81and 82.Ql \&? . 83Multiple environment variables may be separated by whitespace or spread 84across multiple 85.Cm AcceptEnv 86directives. 87Be warned that some environment variables could be used to bypass restricted 88user environments. 89For this reason, care should be taken in the use of this directive. 90The default is not to accept any environment variables. 91.It Cm AddressFamily 92Specifies which address family should be used by 93.Xr sshd 8 . 94Valid arguments are 95.Cm any 96(the default), 97.Cm inet 98(use IPv4 only), or 99.Cm inet6 100(use IPv6 only). 101.It Cm AllowAgentForwarding 102Specifies whether 103.Xr ssh-agent 1 104forwarding is permitted. 105The default is 106.Cm yes . 107Note that disabling agent forwarding does not improve security 108unless users are also denied shell access, as they can always install 109their own forwarders. 110.It Cm AllowGroups 111This keyword can be followed by a list of group name patterns, separated 112by spaces. 113If specified, login is allowed only for users whose primary 114group or supplementary group list matches one of the patterns. 115Only group names are valid; a numerical group ID is not recognized. 116By default, login is allowed for all groups. 117The allow/deny groups directives are processed in the following order: 118.Cm DenyGroups , 119.Cm AllowGroups . 120.Pp 121See PATTERNS in 122.Xr ssh_config 5 123for more information on patterns. 124.It Cm AllowStreamLocalForwarding 125Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 126The available options are 127.Cm yes 128(the default) 129or 130.Cm all 131to allow StreamLocal forwarding, 132.Cm no 133to prevent all StreamLocal forwarding, 134.Cm local 135to allow local (from the perspective of 136.Xr ssh 1 ) 137forwarding only or 138.Cm remote 139to allow remote forwarding only. 140Note that disabling StreamLocal forwarding does not improve security unless 141users are also denied shell access, as they can always install their 142own forwarders. 143.It Cm AllowTcpForwarding 144Specifies whether TCP forwarding is permitted. 145The available options are 146.Cm yes 147(the default) 148or 149.Cm all 150to allow TCP forwarding, 151.Cm no 152to prevent all TCP forwarding, 153.Cm local 154to allow local (from the perspective of 155.Xr ssh 1 ) 156forwarding only or 157.Cm remote 158to allow remote forwarding only. 159Note that disabling TCP forwarding does not improve security unless 160users are also denied shell access, as they can always install their 161own forwarders. 162.It Cm AllowUsers 163This keyword can be followed by a list of user name patterns, separated 164by spaces. 165If specified, login is allowed only for user names that 166match one of the patterns. 167Only user names are valid; a numerical user ID is not recognized. 168By default, login is allowed for all users. 169If the pattern takes the form USER@HOST then USER and HOST 170are separately checked, restricting logins to particular 171users from particular hosts. 172HOST criteria may additionally contain addresses to match in CIDR 173address/masklen format. 174The allow/deny users directives are processed in the following order: 175.Cm DenyUsers , 176.Cm AllowUsers . 177.Pp 178See PATTERNS in 179.Xr ssh_config 5 180for more information on patterns. 181.It Cm AuthenticationMethods 182Specifies the authentication methods that must be successfully completed 183for a user to be granted access. 184This option must be followed by one or more lists of comma-separated 185authentication method names, or by the single string 186.Cm any 187to indicate the default behaviour of accepting any single authentication 188method. 189If the default is overridden, then successful authentication requires 190completion of every method in at least one of these lists. 191.Pp 192For example, 193.Qq publickey,password publickey,keyboard-interactive 194would require the user to complete public key authentication, followed by 195either password or keyboard interactive authentication. 196Only methods that are next in one or more lists are offered at each stage, 197so for this example it would not be possible to attempt password or 198keyboard-interactive authentication before public key. 199.Pp 200For keyboard interactive authentication it is also possible to 201restrict authentication to a specific device by appending a 202colon followed by the device identifier 203.Cm bsdauth 204or 205.Cm pam . 206depending on the server configuration. 207For example, 208.Qq keyboard-interactive:bsdauth 209would restrict keyboard interactive authentication to the 210.Cm bsdauth 211device. 212.Pp 213If the publickey method is listed more than once, 214.Xr sshd 8 215verifies that keys that have been used successfully are not reused for 216subsequent authentications. 217For example, 218.Qq publickey,publickey 219requires successful authentication using two different public keys. 220.Pp 221Note that each authentication method listed should also be explicitly enabled 222in the configuration. 223.Pp 224The available authentication methods are: 225.Qq gssapi-with-mic , 226.Qq hostbased , 227.Qq keyboard-interactive , 228.Qq none 229(used for access to password-less accounts when 230.Cm PermitEmptyPasswords 231is enabled), 232.Qq password 233and 234.Qq publickey . 235.It Cm AuthorizedKeysCommand 236Specifies a program to be used to look up the user's public keys. 237The program must be owned by root, not writable by group or others and 238specified by an absolute path. 239Arguments to 240.Cm AuthorizedKeysCommand 241accept the tokens described in the 242.Sx TOKENS 243section. 244If no arguments are specified then the username of the target user is used. 245.Pp 246The program should produce on standard output zero or 247more lines of authorized_keys output (see 248.Sx AUTHORIZED_KEYS 249in 250.Xr sshd 8 ) . 251.Cm AuthorizedKeysCommand 252is tried after the usual 253.Cm AuthorizedKeysFile 254files and will not be executed if a matching key is found there. 255By default, no 256.Cm AuthorizedKeysCommand 257is run. 258.It Cm AuthorizedKeysCommandUser 259Specifies the user under whose account the 260.Cm AuthorizedKeysCommand 261is run. 262It is recommended to use a dedicated user that has no other role on the host 263than running authorized keys commands. 264If 265.Cm AuthorizedKeysCommand 266is specified but 267.Cm AuthorizedKeysCommandUser 268is not, then 269.Xr sshd 8 270will refuse to start. 271.It Cm AuthorizedKeysFile 272Specifies the file that contains the public keys used for user authentication. 273The format is described in the AUTHORIZED_KEYS FILE FORMAT section of 274.Xr sshd 8 . 275Arguments to 276.Cm AuthorizedKeysFile 277accept the tokens described in the 278.Sx TOKENS 279section. 280After expansion, 281.Cm AuthorizedKeysFile 282is taken to be an absolute path or one relative to the user's home 283directory. 284Multiple files may be listed, separated by whitespace. 285Alternately this option may be set to 286.Cm none 287to skip checking for user keys in files. 288The default is 289.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 290.It Cm AuthorizedPrincipalsCommand 291Specifies a program to be used to generate the list of allowed 292certificate principals as per 293.Cm AuthorizedPrincipalsFile . 294The program must be owned by root, not writable by group or others and 295specified by an absolute path. 296Arguments to 297.Cm AuthorizedPrincipalsCommand 298accept the tokens described in the 299.Sx TOKENS 300section. 301If no arguments are specified then the username of the target user is used. 302.Pp 303The program should produce on standard output zero or 304more lines of 305.Cm AuthorizedPrincipalsFile 306output. 307If either 308.Cm AuthorizedPrincipalsCommand 309or 310.Cm AuthorizedPrincipalsFile 311is specified, then certificates offered by the client for authentication 312must contain a principal that is listed. 313By default, no 314.Cm AuthorizedPrincipalsCommand 315is run. 316.It Cm AuthorizedPrincipalsCommandUser 317Specifies the user under whose account the 318.Cm AuthorizedPrincipalsCommand 319is run. 320It is recommended to use a dedicated user that has no other role on the host 321than running authorized principals commands. 322If 323.Cm AuthorizedPrincipalsCommand 324is specified but 325.Cm AuthorizedPrincipalsCommandUser 326is not, then 327.Xr sshd 8 328will refuse to start. 329.It Cm AuthorizedPrincipalsFile 330Specifies a file that lists principal names that are accepted for 331certificate authentication. 332When using certificates signed by a key listed in 333.Cm TrustedUserCAKeys , 334this file lists names, one of which must appear in the certificate for it 335to be accepted for authentication. 336Names are listed one per line preceded by key options (as described in 337.Sx AUTHORIZED_KEYS FILE FORMAT 338in 339.Xr sshd 8 ) . 340Empty lines and comments starting with 341.Ql # 342are ignored. 343.Pp 344Arguments to 345.Cm AuthorizedPrincipalsFile 346accept the tokens described in the 347.Sx TOKENS 348section. 349After expansion, 350.Cm AuthorizedPrincipalsFile 351is taken to be an absolute path or one relative to the user's home directory. 352The default is 353.Cm none , 354i.e. not to use a principals file \(en in this case, the username 355of the user must appear in a certificate's principals list for it to be 356accepted. 357.Pp 358Note that 359.Cm AuthorizedPrincipalsFile 360is only used when authentication proceeds using a CA listed in 361.Cm TrustedUserCAKeys 362and is not consulted for certification authorities trusted via 363.Pa ~/.ssh/authorized_keys , 364though the 365.Cm principals= 366key option offers a similar facility (see 367.Xr sshd 8 368for details). 369.It Cm Banner 370The contents of the specified file are sent to the remote user before 371authentication is allowed. 372If the argument is 373.Cm none 374then no banner is displayed. 375By default, no banner is displayed. 376.It Cm CASignatureAlgorithms 377Specifies which algorithms are allowed for signing of certificates 378by certificate authorities (CAs). 379The default is: 380.Bd -literal -offset indent 381ssh-ed25519,ecdsa-sha2-nistp256, 382ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 383sk-ssh-ed25519@openssh.com, 384sk-ecdsa-sha2-nistp256@openssh.com, 385rsa-sha2-512,rsa-sha2-256 386.Ed 387.Pp 388If the specified list begins with a 389.Sq + 390character, then the specified algorithms will be appended to the default set 391instead of replacing them. 392If the specified list begins with a 393.Sq - 394character, then the specified algorithms (including wildcards) will be removed 395from the default set instead of replacing them. 396.Pp 397Certificates signed using other algorithms will not be accepted for 398public key or host-based authentication. 399.It Cm ChrootDirectory 400Specifies the pathname of a directory to 401.Xr chroot 2 402to after authentication. 403At session startup 404.Xr sshd 8 405checks that all components of the pathname are root-owned directories 406which are not writable by any other user or group. 407After the chroot, 408.Xr sshd 8 409changes the working directory to the user's home directory. 410Arguments to 411.Cm ChrootDirectory 412accept the tokens described in the 413.Sx TOKENS 414section. 415.Pp 416The 417.Cm ChrootDirectory 418must contain the necessary files and directories to support the 419user's session. 420For an interactive session this requires at least a shell, typically 421.Xr sh 1 , 422and basic 423.Pa /dev 424nodes such as 425.Xr null 4 , 426.Xr zero 4 , 427.Xr stdin 4 , 428.Xr stdout 4 , 429.Xr stderr 4 , 430and 431.Xr tty 4 432devices. 433For file transfer sessions using SFTP 434no additional configuration of the environment is necessary if the in-process 435sftp-server is used, 436though sessions which use logging may require 437.Pa /dev/log 438inside the chroot directory on some operating systems (see 439.Xr sftp-server 8 440for details). 441.Pp 442For safety, it is very important that the directory hierarchy be 443prevented from modification by other processes on the system (especially 444those outside the jail). 445Misconfiguration can lead to unsafe environments which 446.Xr sshd 8 447cannot detect. 448.Pp 449The default is 450.Cm none , 451indicating not to 452.Xr chroot 2 . 453.It Cm Ciphers 454Specifies the ciphers allowed. 455Multiple ciphers must be comma-separated. 456If the specified list begins with a 457.Sq + 458character, then the specified ciphers will be appended to the default set 459instead of replacing them. 460If the specified list begins with a 461.Sq - 462character, then the specified ciphers (including wildcards) will be removed 463from the default set instead of replacing them. 464If the specified list begins with a 465.Sq ^ 466character, then the specified ciphers will be placed at the head of the 467default set. 468.Pp 469The supported ciphers are: 470.Pp 471.Bl -item -compact -offset indent 472.It 4733des-cbc 474.It 475aes128-cbc 476.It 477aes192-cbc 478.It 479aes256-cbc 480.It 481aes128-ctr 482.It 483aes192-ctr 484.It 485aes256-ctr 486.It 487aes128-gcm@openssh.com 488.It 489aes256-gcm@openssh.com 490.It 491chacha20-poly1305@openssh.com 492.El 493.Pp 494The default is: 495.Bd -literal -offset indent 496chacha20-poly1305@openssh.com, 497aes128-ctr,aes192-ctr,aes256-ctr, 498aes128-gcm@openssh.com,aes256-gcm@openssh.com 499.Ed 500.Pp 501The list of available ciphers may also be obtained using 502.Qq ssh -Q cipher . 503.It Cm ClientAliveCountMax 504Sets the number of client alive messages which may be sent without 505.Xr sshd 8 506receiving any messages back from the client. 507If this threshold is reached while client alive messages are being sent, 508sshd will disconnect the client, terminating the session. 509It is important to note that the use of client alive messages is very 510different from 511.Cm TCPKeepAlive . 512The client alive messages are sent through the encrypted channel 513and therefore will not be spoofable. 514The TCP keepalive option enabled by 515.Cm TCPKeepAlive 516is spoofable. 517The client alive mechanism is valuable when the client or 518server depend on knowing when a connection has become unresponsive. 519.Pp 520The default value is 3. 521If 522.Cm ClientAliveInterval 523is set to 15, and 524.Cm ClientAliveCountMax 525is left at the default, unresponsive SSH clients 526will be disconnected after approximately 45 seconds. 527Setting a zero 528.Cm ClientAliveCountMax 529disables connection termination. 530.It Cm ClientAliveInterval 531Sets a timeout interval in seconds after which if no data has been received 532from the client, 533.Xr sshd 8 534will send a message through the encrypted 535channel to request a response from the client. 536The default 537is 0, indicating that these messages will not be sent to the client. 538.It Cm Compression 539Specifies whether compression is enabled after 540the user has authenticated successfully. 541The argument must be 542.Cm yes , 543.Cm delayed 544(a legacy synonym for 545.Cm yes ) 546or 547.Cm no . 548The default is 549.Cm yes . 550.It Cm DenyGroups 551This keyword can be followed by a list of group name patterns, separated 552by spaces. 553Login is disallowed for users whose primary group or supplementary 554group list matches one of the patterns. 555Only group names are valid; a numerical group ID is not recognized. 556By default, login is allowed for all groups. 557The allow/deny groups directives are processed in the following order: 558.Cm DenyGroups , 559.Cm AllowGroups . 560.Pp 561See PATTERNS in 562.Xr ssh_config 5 563for more information on patterns. 564.It Cm DenyUsers 565This keyword can be followed by a list of user name patterns, separated 566by spaces. 567Login is disallowed for user names that match one of the patterns. 568Only user names are valid; a numerical user ID is not recognized. 569By default, login is allowed for all users. 570If the pattern takes the form USER@HOST then USER and HOST 571are separately checked, restricting logins to particular 572users from particular hosts. 573HOST criteria may additionally contain addresses to match in CIDR 574address/masklen format. 575The allow/deny users directives are processed in the following order: 576.Cm DenyUsers , 577.Cm AllowUsers . 578.Pp 579See PATTERNS in 580.Xr ssh_config 5 581for more information on patterns. 582.It Cm DisableForwarding 583Disables all forwarding features, including X11, 584.Xr ssh-agent 1 , 585TCP and StreamLocal. 586This option overrides all other forwarding-related options and may 587simplify restricted configurations. 588.It Cm ExposeAuthInfo 589Writes a temporary file containing a list of authentication methods and 590public credentials (e.g. keys) used to authenticate the user. 591The location of the file is exposed to the user session through the 592.Ev SSH_USER_AUTH 593environment variable. 594The default is 595.Cm no . 596.It Cm FingerprintHash 597Specifies the hash algorithm used when logging key fingerprints. 598Valid options are: 599.Cm md5 600and 601.Cm sha256 . 602The default is 603.Cm sha256 . 604.It Cm ForceCommand 605Forces the execution of the command specified by 606.Cm ForceCommand , 607ignoring any command supplied by the client and 608.Pa ~/.ssh/rc 609if present. 610The command is invoked by using the user's login shell with the -c option. 611This applies to shell, command, or subsystem execution. 612It is most useful inside a 613.Cm Match 614block. 615The command originally supplied by the client is available in the 616.Ev SSH_ORIGINAL_COMMAND 617environment variable. 618Specifying a command of 619.Cm internal-sftp 620will force the use of an in-process SFTP server that requires no support 621files when used with 622.Cm ChrootDirectory . 623The default is 624.Cm none . 625.It Cm GatewayPorts 626Specifies whether remote hosts are allowed to connect to ports 627forwarded for the client. 628By default, 629.Xr sshd 8 630binds remote port forwardings to the loopback address. 631This prevents other remote hosts from connecting to forwarded ports. 632.Cm GatewayPorts 633can be used to specify that sshd 634should allow remote port forwardings to bind to non-loopback addresses, thus 635allowing other hosts to connect. 636The argument may be 637.Cm no 638to force remote port forwardings to be available to the local host only, 639.Cm yes 640to force remote port forwardings to bind to the wildcard address, or 641.Cm clientspecified 642to allow the client to select the address to which the forwarding is bound. 643The default is 644.Cm no . 645.It Cm GSSAPIAuthentication 646Specifies whether user authentication based on GSSAPI is allowed. 647The default is 648.Cm no . 649.It Cm GSSAPICleanupCredentials 650Specifies whether to automatically destroy the user's credentials cache 651on logout. 652The default is 653.Cm yes . 654.It Cm GSSAPIStrictAcceptorCheck 655Determines whether to be strict about the identity of the GSSAPI acceptor 656a client authenticates against. 657If set to 658.Cm yes 659then the client must authenticate against the host 660service on the current hostname. 661If set to 662.Cm no 663then the client may authenticate against any service key stored in the 664machine's default store. 665This facility is provided to assist with operation on multi homed machines. 666The default is 667.Cm yes . 668.It Cm HostbasedAcceptedAlgorithms 669Specifies the signature algorithms that will be accepted for hostbased 670authentication as a list of comma-separated patterns. 671Alternately if the specified list begins with a 672.Sq + 673character, then the specified signature algorithms will be appended to 674the default set instead of replacing them. 675If the specified list begins with a 676.Sq - 677character, then the specified signature algorithms (including wildcards) 678will be removed from the default set instead of replacing them. 679If the specified list begins with a 680.Sq ^ 681character, then the specified signature algorithms will be placed at 682the head of the default set. 683The default for this option is: 684.Bd -literal -offset 3n 685ssh-ed25519-cert-v01@openssh.com, 686ecdsa-sha2-nistp256-cert-v01@openssh.com, 687ecdsa-sha2-nistp384-cert-v01@openssh.com, 688ecdsa-sha2-nistp521-cert-v01@openssh.com, 689sk-ssh-ed25519-cert-v01@openssh.com, 690sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 691rsa-sha2-512-cert-v01@openssh.com, 692rsa-sha2-256-cert-v01@openssh.com, 693ssh-rsa-cert-v01@openssh.com, 694ssh-ed25519, 695ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 696sk-ssh-ed25519@openssh.com, 697sk-ecdsa-sha2-nistp256@openssh.com, 698rsa-sha2-512,rsa-sha2-256,ssh-rsa 699.Ed 700.Pp 701The list of available signature algorithms may also be obtained using 702.Qq ssh -Q HostbasedAcceptedAlgorithms . 703This was formerly named HostbasedAcceptedKeyTypes. 704.It Cm HostbasedAuthentication 705Specifies whether rhosts or /etc/hosts.equiv authentication together 706with successful public key client host authentication is allowed 707(host-based authentication). 708The default is 709.Cm no . 710.It Cm HostbasedUsesNameFromPacketOnly 711Specifies whether or not the server will attempt to perform a reverse 712name lookup when matching the name in the 713.Pa ~/.shosts , 714.Pa ~/.rhosts , 715and 716.Pa /etc/hosts.equiv 717files during 718.Cm HostbasedAuthentication . 719A setting of 720.Cm yes 721means that 722.Xr sshd 8 723uses the name supplied by the client rather than 724attempting to resolve the name from the TCP connection itself. 725The default is 726.Cm no . 727.It Cm HostCertificate 728Specifies a file containing a public host certificate. 729The certificate's public key must match a private host key already specified 730by 731.Cm HostKey . 732The default behaviour of 733.Xr sshd 8 734is not to load any certificates. 735.It Cm HostKey 736Specifies a file containing a private host key 737used by SSH. 738The defaults are 739.Pa /etc/ssh/ssh_host_ecdsa_key , 740.Pa /etc/ssh/ssh_host_ed25519_key 741and 742.Pa /etc/ssh/ssh_host_rsa_key . 743.Pp 744Note that 745.Xr sshd 8 746will refuse to use a file if it is group/world-accessible 747and that the 748.Cm HostKeyAlgorithms 749option restricts which of the keys are actually used by 750.Xr sshd 8 . 751.Pp 752It is possible to have multiple host key files. 753It is also possible to specify public host key files instead. 754In this case operations on the private key will be delegated 755to an 756.Xr ssh-agent 1 . 757.It Cm HostKeyAgent 758Identifies the UNIX-domain socket used to communicate 759with an agent that has access to the private host keys. 760If the string 761.Qq SSH_AUTH_SOCK 762is specified, the location of the socket will be read from the 763.Ev SSH_AUTH_SOCK 764environment variable. 765.It Cm HostKeyAlgorithms 766Specifies the host key signature algorithms 767that the server offers. 768The default for this option is: 769.Bd -literal -offset 3n 770ssh-ed25519-cert-v01@openssh.com, 771ecdsa-sha2-nistp256-cert-v01@openssh.com, 772ecdsa-sha2-nistp384-cert-v01@openssh.com, 773ecdsa-sha2-nistp521-cert-v01@openssh.com, 774sk-ssh-ed25519-cert-v01@openssh.com, 775sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 776rsa-sha2-512-cert-v01@openssh.com, 777rsa-sha2-256-cert-v01@openssh.com, 778ssh-rsa-cert-v01@openssh.com, 779ssh-ed25519, 780ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 781sk-ssh-ed25519@openssh.com, 782sk-ecdsa-sha2-nistp256@openssh.com, 783rsa-sha2-512,rsa-sha2-256,ssh-rsa 784.Ed 785.Pp 786The list of available signature algorithms may also be obtained using 787.Qq ssh -Q HostKeyAlgorithms . 788.It Cm IgnoreRhosts 789Specifies whether to ignore per-user 790.Pa .rhosts 791and 792.Pa .shosts 793files during 794.Cm HostbasedAuthentication . 795The system-wide 796.Pa /etc/hosts.equiv 797and 798.Pa /etc/ssh/shosts.equiv 799are still used regardless of this setting. 800.Pp 801Accepted values are 802.Cm yes 803(the default) to ignore all per-user files, 804.Cm shosts-only 805to allow the use of 806.Pa .shosts 807but to ignore 808.Pa .rhosts 809or 810.Cm no 811to allow both 812.Pa .shosts 813and 814.Pa rhosts . 815.It Cm IgnoreUserKnownHosts 816Specifies whether 817.Xr sshd 8 818should ignore the user's 819.Pa ~/.ssh/known_hosts 820during 821.Cm HostbasedAuthentication 822and use only the system-wide known hosts file 823.Pa /etc/ssh/known_hosts . 824The default is 825.Dq no . 826.It Cm Include 827Include the specified configuration file(s). 828Multiple pathnames may be specified and each pathname may contain 829.Xr glob 7 830wildcards that will be expanded and processed in lexical order. 831Files without absolute paths are assumed to be in 832.Pa /etc/ssh . 833An 834.Cm Include 835directive may appear inside a 836.Cm Match 837block 838to perform conditional inclusion. 839.It Cm IPQoS 840Specifies the IPv4 type-of-service or DSCP class for the connection. 841Accepted values are 842.Cm af11 , 843.Cm af12 , 844.Cm af13 , 845.Cm af21 , 846.Cm af22 , 847.Cm af23 , 848.Cm af31 , 849.Cm af32 , 850.Cm af33 , 851.Cm af41 , 852.Cm af42 , 853.Cm af43 , 854.Cm cs0 , 855.Cm cs1 , 856.Cm cs2 , 857.Cm cs3 , 858.Cm cs4 , 859.Cm cs5 , 860.Cm cs6 , 861.Cm cs7 , 862.Cm ef , 863.Cm le , 864.Cm lowdelay , 865.Cm throughput , 866.Cm reliability , 867a numeric value, or 868.Cm none 869to use the operating system default. 870This option may take one or two arguments, separated by whitespace. 871If one argument is specified, it is used as the packet class unconditionally. 872If two values are specified, the first is automatically selected for 873interactive sessions and the second for non-interactive sessions. 874The default is 875.Cm af21 876(Low-Latency Data) 877for interactive sessions and 878.Cm cs1 879(Lower Effort) 880for non-interactive sessions. 881.It Cm KbdInteractiveAuthentication 882Specifies whether to allow keyboard-interactive authentication. 883All authentication styles from 884.Xr login.conf 5 885are supported. 886The default is 887.Cm yes . 888The argument to this keyword must be 889.Cm yes 890or 891.Cm no . 892.Cm ChallengeResponseAuthentication 893is a deprecated alias for this. 894.It Cm KerberosAuthentication 895Specifies whether the password provided by the user for 896.Cm PasswordAuthentication 897will be validated through the Kerberos KDC. 898To use this option, the server needs a 899Kerberos servtab which allows the verification of the KDC's identity. 900The default is 901.Cm no . 902.It Cm KerberosGetAFSToken 903If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 904an AFS token before accessing the user's home directory. 905The default is 906.Cm no . 907.It Cm KerberosOrLocalPasswd 908If password authentication through Kerberos fails then 909the password will be validated via any additional local mechanism 910such as 911.Pa /etc/passwd . 912The default is 913.Cm yes . 914.It Cm KerberosTicketCleanup 915Specifies whether to automatically destroy the user's ticket cache 916file on logout. 917The default is 918.Cm yes . 919.It Cm KexAlgorithms 920Specifies the available KEX (Key Exchange) algorithms. 921Multiple algorithms must be comma-separated. 922Alternately if the specified list begins with a 923.Sq + 924character, then the specified methods will be appended to the default set 925instead of replacing them. 926If the specified list begins with a 927.Sq - 928character, then the specified methods (including wildcards) will be removed 929from the default set instead of replacing them. 930If the specified list begins with a 931.Sq ^ 932character, then the specified methods will be placed at the head of the 933default set. 934The supported algorithms are: 935.Pp 936.Bl -item -compact -offset indent 937.It 938curve25519-sha256 939.It 940curve25519-sha256@libssh.org 941.It 942diffie-hellman-group1-sha1 943.It 944diffie-hellman-group14-sha1 945.It 946diffie-hellman-group14-sha256 947.It 948diffie-hellman-group16-sha512 949.It 950diffie-hellman-group18-sha512 951.It 952diffie-hellman-group-exchange-sha1 953.It 954diffie-hellman-group-exchange-sha256 955.It 956ecdh-sha2-nistp256 957.It 958ecdh-sha2-nistp384 959.It 960ecdh-sha2-nistp521 961.It 962sntrup761x25519-sha512@openssh.com 963.El 964.Pp 965The default is: 966.Bd -literal -offset indent 967curve25519-sha256,curve25519-sha256@libssh.org, 968ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 969diffie-hellman-group-exchange-sha256, 970diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 971diffie-hellman-group14-sha256 972.Ed 973.Pp 974The list of available key exchange algorithms may also be obtained using 975.Qq ssh -Q KexAlgorithms . 976.It Cm ListenAddress 977Specifies the local addresses 978.Xr sshd 8 979should listen on. 980The following forms may be used: 981.Pp 982.Bl -item -offset indent -compact 983.It 984.Cm ListenAddress 985.Sm off 986.Ar hostname | address 987.Sm on 988.Op Cm rdomain Ar domain 989.It 990.Cm ListenAddress 991.Sm off 992.Ar hostname : port 993.Sm on 994.Op Cm rdomain Ar domain 995.It 996.Cm ListenAddress 997.Sm off 998.Ar IPv4_address : port 999.Sm on 1000.Op Cm rdomain Ar domain 1001.It 1002.Cm ListenAddress 1003.Sm off 1004.Oo Ar hostname | address Oc : Ar port 1005.Sm on 1006.Op Cm rdomain Ar domain 1007.El 1008.Pp 1009The optional 1010.Cm rdomain 1011qualifier requests 1012.Xr sshd 8 1013listen in an explicit routing domain. 1014If 1015.Ar port 1016is not specified, 1017sshd will listen on the address and all 1018.Cm Port 1019options specified. 1020The default is to listen on all local addresses on the current default 1021routing domain. 1022Multiple 1023.Cm ListenAddress 1024options are permitted. 1025For more information on routing domains, see 1026.Xr rdomain 4 . 1027.It Cm LoginGraceTime 1028The server disconnects after this time if the user has not 1029successfully logged in. 1030If the value is 0, there is no time limit. 1031The default is 120 seconds. 1032.It Cm LogLevel 1033Gives the verbosity level that is used when logging messages from 1034.Xr sshd 8 . 1035The possible values are: 1036QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1037The default is INFO. 1038DEBUG and DEBUG1 are equivalent. 1039DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1040Logging with a DEBUG level violates the privacy of users and is not recommended. 1041.It Cm LogVerbose 1042Specify one or more overrides to LogLevel. 1043An override consists of a pattern lists that matches the source file, function 1044and line number to force detailed logging for. 1045For example, an override pattern of: 1046.Bd -literal -offset indent 1047kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1048.Ed 1049.Pp 1050would enable detailed logging for line 1000 of 1051.Pa kex.c , 1052everything in the 1053.Fn kex_exchange_identification 1054function, and all code in the 1055.Pa packet.c 1056file. 1057This option is intended for debugging and no overrides are enabled by default. 1058.It Cm MACs 1059Specifies the available MAC (message authentication code) algorithms. 1060The MAC algorithm is used for data integrity protection. 1061Multiple algorithms must be comma-separated. 1062If the specified list begins with a 1063.Sq + 1064character, then the specified algorithms will be appended to the default set 1065instead of replacing them. 1066If the specified list begins with a 1067.Sq - 1068character, then the specified algorithms (including wildcards) will be removed 1069from the default set instead of replacing them. 1070If the specified list begins with a 1071.Sq ^ 1072character, then the specified algorithms will be placed at the head of the 1073default set. 1074.Pp 1075The algorithms that contain 1076.Qq -etm 1077calculate the MAC after encryption (encrypt-then-mac). 1078These are considered safer and their use recommended. 1079The supported MACs are: 1080.Pp 1081.Bl -item -compact -offset indent 1082.It 1083hmac-md5 1084.It 1085hmac-md5-96 1086.It 1087hmac-sha1 1088.It 1089hmac-sha1-96 1090.It 1091hmac-sha2-256 1092.It 1093hmac-sha2-512 1094.It 1095umac-64@openssh.com 1096.It 1097umac-128@openssh.com 1098.It 1099hmac-md5-etm@openssh.com 1100.It 1101hmac-md5-96-etm@openssh.com 1102.It 1103hmac-sha1-etm@openssh.com 1104.It 1105hmac-sha1-96-etm@openssh.com 1106.It 1107hmac-sha2-256-etm@openssh.com 1108.It 1109hmac-sha2-512-etm@openssh.com 1110.It 1111umac-64-etm@openssh.com 1112.It 1113umac-128-etm@openssh.com 1114.El 1115.Pp 1116The default is: 1117.Bd -literal -offset indent 1118umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1119hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1120hmac-sha1-etm@openssh.com, 1121umac-64@openssh.com,umac-128@openssh.com, 1122hmac-sha2-256,hmac-sha2-512,hmac-sha1 1123.Ed 1124.Pp 1125The list of available MAC algorithms may also be obtained using 1126.Qq ssh -Q mac . 1127.It Cm Match 1128Introduces a conditional block. 1129If all of the criteria on the 1130.Cm Match 1131line are satisfied, the keywords on the following lines override those 1132set in the global section of the config file, until either another 1133.Cm Match 1134line or the end of the file. 1135If a keyword appears in multiple 1136.Cm Match 1137blocks that are satisfied, only the first instance of the keyword is 1138applied. 1139.Pp 1140The arguments to 1141.Cm Match 1142are one or more criteria-pattern pairs or the single token 1143.Cm All 1144which matches all criteria. 1145The available criteria are 1146.Cm User , 1147.Cm Group , 1148.Cm Host , 1149.Cm LocalAddress , 1150.Cm LocalPort , 1151.Cm RDomain , 1152and 1153.Cm Address 1154(with 1155.Cm RDomain 1156representing the 1157.Xr rdomain 4 1158on which the connection was received). 1159.Pp 1160The match patterns may consist of single entries or comma-separated 1161lists and may use the wildcard and negation operators described in the 1162.Sx PATTERNS 1163section of 1164.Xr ssh_config 5 . 1165.Pp 1166The patterns in an 1167.Cm Address 1168criteria may additionally contain addresses to match in CIDR 1169address/masklen format, 1170such as 192.0.2.0/24 or 2001:db8::/32. 1171Note that the mask length provided must be consistent with the address - 1172it is an error to specify a mask length that is too long for the address 1173or one with bits set in this host portion of the address. 1174For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1175.Pp 1176Only a subset of keywords may be used on the lines following a 1177.Cm Match 1178keyword. 1179Available keywords are 1180.Cm AcceptEnv , 1181.Cm AllowAgentForwarding , 1182.Cm AllowGroups , 1183.Cm AllowStreamLocalForwarding , 1184.Cm AllowTcpForwarding , 1185.Cm AllowUsers , 1186.Cm AuthenticationMethods , 1187.Cm AuthorizedKeysCommand , 1188.Cm AuthorizedKeysCommandUser , 1189.Cm AuthorizedKeysFile , 1190.Cm AuthorizedPrincipalsCommand , 1191.Cm AuthorizedPrincipalsCommandUser , 1192.Cm AuthorizedPrincipalsFile , 1193.Cm Banner , 1194.Cm ChrootDirectory , 1195.Cm ClientAliveCountMax , 1196.Cm ClientAliveInterval , 1197.Cm DenyGroups , 1198.Cm DenyUsers , 1199.Cm DisableForwarding , 1200.Cm ForceCommand , 1201.Cm GatewayPorts , 1202.Cm GSSAPIAuthentication , 1203.Cm HostbasedAcceptedAlgorithms , 1204.Cm HostbasedAuthentication , 1205.Cm HostbasedUsesNameFromPacketOnly , 1206.Cm IgnoreRhosts , 1207.Cm Include , 1208.Cm IPQoS , 1209.Cm KbdInteractiveAuthentication , 1210.Cm KerberosAuthentication , 1211.Cm LogLevel , 1212.Cm MaxAuthTries , 1213.Cm MaxSessions , 1214.Cm PasswordAuthentication , 1215.Cm PermitEmptyPasswords , 1216.Cm PermitListen , 1217.Cm PermitOpen , 1218.Cm PermitRootLogin , 1219.Cm PermitTTY , 1220.Cm PermitTunnel , 1221.Cm PermitUserRC , 1222.Cm PubkeyAcceptedAlgorithms , 1223.Cm PubkeyAuthentication , 1224.Cm RekeyLimit , 1225.Cm RevokedKeys , 1226.Cm RDomain , 1227.Cm SetEnv , 1228.Cm StreamLocalBindMask , 1229.Cm StreamLocalBindUnlink , 1230.Cm TrustedUserCAKeys , 1231.Cm X11DisplayOffset , 1232.Cm X11Forwarding 1233and 1234.Cm X11UseLocalhost . 1235.It Cm MaxAuthTries 1236Specifies the maximum number of authentication attempts permitted per 1237connection. 1238Once the number of failures reaches half this value, 1239additional failures are logged. 1240The default is 6. 1241.It Cm MaxSessions 1242Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1243sessions permitted per network connection. 1244Multiple sessions may be established by clients that support connection 1245multiplexing. 1246Setting 1247.Cm MaxSessions 1248to 1 will effectively disable session multiplexing, whereas setting it to 0 1249will prevent all shell, login and subsystem sessions while still permitting 1250forwarding. 1251The default is 10. 1252.It Cm MaxStartups 1253Specifies the maximum number of concurrent unauthenticated connections to the 1254SSH daemon. 1255Additional connections will be dropped until authentication succeeds or the 1256.Cm LoginGraceTime 1257expires for a connection. 1258The default is 10:30:100. 1259.Pp 1260Alternatively, random early drop can be enabled by specifying 1261the three colon separated values 1262start:rate:full (e.g. "10:30:60"). 1263.Xr sshd 8 1264will refuse connection attempts with a probability of rate/100 (30%) 1265if there are currently start (10) unauthenticated connections. 1266The probability increases linearly and all connection attempts 1267are refused if the number of unauthenticated connections reaches full (60). 1268.It Cm ModuliFile 1269Specifies the 1270.Xr moduli 5 1271file that contains the Diffie-Hellman groups used for the 1272.Dq diffie-hellman-group-exchange-sha1 1273and 1274.Dq diffie-hellman-group-exchange-sha256 1275key exchange methods. 1276The default is 1277.Pa /etc/moduli . 1278.It Cm PasswordAuthentication 1279Specifies whether password authentication is allowed. 1280See also 1281.Cm UsePAM . 1282The default is 1283.Cm no . 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 no . 1377Note that if 1378.Cm ChallengeResponseAuthentication 1379and 1380.Cm UsePAM 1381are both 1382.Cm yes , 1383this setting may be overridden by the PAM policy. 1384.Pp 1385If this option is set to 1386.Cm prohibit-password 1387(or its deprecated alias, 1388.Cm without-password ) , 1389password and keyboard-interactive authentication are disabled for root. 1390.Pp 1391If this option is set to 1392.Cm forced-commands-only , 1393root login with public key authentication will be allowed, 1394but only if the 1395.Ar command 1396option has been specified 1397(which may be useful for taking remote backups even if root login is 1398normally not allowed). 1399All other authentication methods are disabled for root. 1400.Pp 1401If this option is set to 1402.Cm no , 1403root is not allowed to log in. 1404.It Cm PermitTTY 1405Specifies whether 1406.Xr pty 4 1407allocation is permitted. 1408The default is 1409.Cm yes . 1410.It Cm PermitTunnel 1411Specifies whether 1412.Xr tun 4 1413device forwarding is allowed. 1414The argument must be 1415.Cm yes , 1416.Cm point-to-point 1417(layer 3), 1418.Cm ethernet 1419(layer 2), or 1420.Cm no . 1421Specifying 1422.Cm yes 1423permits both 1424.Cm point-to-point 1425and 1426.Cm ethernet . 1427The default is 1428.Cm no . 1429.Pp 1430Independent of this setting, the permissions of the selected 1431.Xr tun 4 1432device must allow access to the user. 1433.It Cm PermitUserEnvironment 1434Specifies whether 1435.Pa ~/.ssh/environment 1436and 1437.Cm environment= 1438options in 1439.Pa ~/.ssh/authorized_keys 1440are processed by 1441.Xr sshd 8 . 1442Valid options are 1443.Cm yes , 1444.Cm no 1445or a pattern-list specifying which environment variable names to accept 1446(for example 1447.Qq LANG,LC_* ) . 1448The default is 1449.Cm no . 1450Enabling environment processing may enable users to bypass access 1451restrictions in some configurations using mechanisms such as 1452.Ev LD_PRELOAD . 1453.It Cm PermitUserRC 1454Specifies whether any 1455.Pa ~/.ssh/rc 1456file is executed. 1457The default is 1458.Cm yes . 1459.It Cm PerSourceMaxStartups 1460Specifies the number of unauthenticated connections allowed from a 1461given source address, or 1462.Dq none 1463if there is no limit. 1464This limit is applied in addition to 1465.Cm MaxStartups , 1466whichever is lower. 1467The default is 1468.Cm none . 1469.It Cm PerSourceNetBlockSize 1470Specifies the number of bits of source address that are grouped together 1471for the purposes of applying PerSourceMaxStartups limits. 1472Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1473The default is 1474.Cm 32:128 , 1475which means each address is considered individually. 1476.It Cm PidFile 1477Specifies the file that contains the process ID of the 1478SSH daemon, or 1479.Cm none 1480to not write one. 1481The default is 1482.Pa /var/run/sshd.pid . 1483.It Cm Port 1484Specifies the port number that 1485.Xr sshd 8 1486listens on. 1487The default is 22. 1488Multiple options of this type are permitted. 1489See also 1490.Cm ListenAddress . 1491.It Cm PrintLastLog 1492Specifies whether 1493.Xr sshd 8 1494should print the date and time of the last user login when a user logs 1495in interactively. 1496The default is 1497.Cm yes . 1498.It Cm PrintMotd 1499Specifies whether 1500.Xr sshd 8 1501should print 1502.Pa /etc/motd 1503when a user logs in interactively. 1504(On some systems it is also printed by the shell, 1505.Pa /etc/profile , 1506or equivalent.) 1507The default is 1508.Cm yes . 1509.It Cm PubkeyAcceptedAlgorithms 1510Specifies the signature algorithms that will be accepted for public key 1511authentication as a list of comma-separated patterns. 1512Alternately if the specified list begins with a 1513.Sq + 1514character, then the specified algorithms will be appended to the default set 1515instead of replacing them. 1516If the specified list begins with a 1517.Sq - 1518character, then the specified algorithms (including wildcards) will be removed 1519from the default set instead of replacing them. 1520If the specified list begins with a 1521.Sq ^ 1522character, then the specified algorithms will be placed at the head of the 1523default set. 1524The default for this option is: 1525.Bd -literal -offset 3n 1526ssh-ed25519-cert-v01@openssh.com, 1527ecdsa-sha2-nistp256-cert-v01@openssh.com, 1528ecdsa-sha2-nistp384-cert-v01@openssh.com, 1529ecdsa-sha2-nistp521-cert-v01@openssh.com, 1530sk-ssh-ed25519-cert-v01@openssh.com, 1531sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1532rsa-sha2-512-cert-v01@openssh.com, 1533rsa-sha2-256-cert-v01@openssh.com, 1534ssh-rsa-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,ssh-rsa 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-20210907 . 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