1.\" 2.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 3.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 4.\" All rights reserved 5.\" 6.\" As far as I am concerned, the code I have written for this software 7.\" can be used freely for any purpose. Any derived versions of this 8.\" software must be clearly marked as such, and if the derived work is 9.\" incompatible with the protocol description in the RFC file, it must be 10.\" called by a name other than "ssh" or "Secure Shell". 11.\" 12.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 13.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 14.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 15.\" 16.\" Redistribution and use in source and binary forms, with or without 17.\" modification, are permitted provided that the following conditions 18.\" are met: 19.\" 1. Redistributions of source code must retain the above copyright 20.\" notice, this list of conditions and the following disclaimer. 21.\" 2. Redistributions in binary form must reproduce the above copyright 22.\" notice, this list of conditions and the following disclaimer in the 23.\" documentation and/or other materials provided with the distribution. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 26.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 27.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 28.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 29.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 30.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 31.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 32.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 33.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 34.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 35.\" 36.\" $OpenBSD: sshd_config.5,v 1.340 2022/03/31 17:58:44 naddy Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: March 31 2022 $ 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-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/ssh/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/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. 1282See also 1283.Cm UsePAM . 1284The default is 1285.Cm no . 1286.It Cm PermitEmptyPasswords 1287When password authentication is allowed, it specifies whether the 1288server allows login to accounts with empty password strings. 1289The default is 1290.Cm no . 1291.It Cm PermitListen 1292Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1293The listen specification must be one of the following forms: 1294.Pp 1295.Bl -item -offset indent -compact 1296.It 1297.Cm PermitListen 1298.Sm off 1299.Ar port 1300.Sm on 1301.It 1302.Cm PermitListen 1303.Sm off 1304.Ar host : port 1305.Sm on 1306.El 1307.Pp 1308Multiple permissions may be specified by separating them with whitespace. 1309An argument of 1310.Cm any 1311can be used to remove all restrictions and permit any listen requests. 1312An argument of 1313.Cm none 1314can be used to prohibit all listen requests. 1315The host name may contain wildcards as described in the PATTERNS section in 1316.Xr ssh_config 5 . 1317The wildcard 1318.Sq * 1319can also be used in place of a port number to allow all ports. 1320By default all port forwarding listen requests are permitted. 1321Note that the 1322.Cm GatewayPorts 1323option may further restrict which addresses may be listened on. 1324Note also that 1325.Xr ssh 1 1326will request a listen host of 1327.Dq localhost 1328if no listen host was specifically requested, and this name is 1329treated differently to explicit localhost addresses of 1330.Dq 127.0.0.1 1331and 1332.Dq ::1 . 1333.It Cm PermitOpen 1334Specifies the destinations to which TCP port forwarding is permitted. 1335The forwarding specification must be one of the following forms: 1336.Pp 1337.Bl -item -offset indent -compact 1338.It 1339.Cm PermitOpen 1340.Sm off 1341.Ar host : port 1342.Sm on 1343.It 1344.Cm PermitOpen 1345.Sm off 1346.Ar IPv4_addr : port 1347.Sm on 1348.It 1349.Cm PermitOpen 1350.Sm off 1351.Ar \&[ IPv6_addr \&] : port 1352.Sm on 1353.El 1354.Pp 1355Multiple forwards may be specified by separating them with whitespace. 1356An argument of 1357.Cm any 1358can be used to remove all restrictions and permit any forwarding requests. 1359An argument of 1360.Cm none 1361can be used to prohibit all forwarding requests. 1362The wildcard 1363.Sq * 1364can be used for host or port to allow all hosts or ports respectively. 1365Otherwise, no pattern matching or address lookups are performed on supplied 1366names. 1367By default all port forwarding requests are permitted. 1368.It Cm PermitRootLogin 1369Specifies whether root can log in using 1370.Xr ssh 1 . 1371The argument must be 1372.Cm yes , 1373.Cm prohibit-password , 1374.Cm forced-commands-only , 1375or 1376.Cm no . 1377The default is 1378.Cm no . 1379Note that if 1380.Cm ChallengeResponseAuthentication 1381and 1382.Cm UsePAM 1383are both 1384.Cm yes , 1385this setting may be overridden by the PAM policy. 1386.Pp 1387If this option is set to 1388.Cm prohibit-password 1389(or its deprecated alias, 1390.Cm without-password ) , 1391password and keyboard-interactive authentication are disabled for root. 1392.Pp 1393If this option is set to 1394.Cm forced-commands-only , 1395root login with public key authentication will be allowed, 1396but only if the 1397.Ar command 1398option has been specified 1399(which may be useful for taking remote backups even if root login is 1400normally not allowed). 1401All other authentication methods are disabled for root. 1402.Pp 1403If this option is set to 1404.Cm no , 1405root is not allowed to log in. 1406.It Cm PermitTTY 1407Specifies whether 1408.Xr pty 4 1409allocation is permitted. 1410The default is 1411.Cm yes . 1412.It Cm PermitTunnel 1413Specifies whether 1414.Xr tun 4 1415device forwarding is allowed. 1416The argument must be 1417.Cm yes , 1418.Cm point-to-point 1419(layer 3), 1420.Cm ethernet 1421(layer 2), or 1422.Cm no . 1423Specifying 1424.Cm yes 1425permits both 1426.Cm point-to-point 1427and 1428.Cm ethernet . 1429The default is 1430.Cm no . 1431.Pp 1432Independent of this setting, the permissions of the selected 1433.Xr tun 4 1434device must allow access to the user. 1435.It Cm PermitUserEnvironment 1436Specifies whether 1437.Pa ~/.ssh/environment 1438and 1439.Cm environment= 1440options in 1441.Pa ~/.ssh/authorized_keys 1442are processed by 1443.Xr sshd 8 . 1444Valid options are 1445.Cm yes , 1446.Cm no 1447or a pattern-list specifying which environment variable names to accept 1448(for example 1449.Qq LANG,LC_* ) . 1450The default is 1451.Cm no . 1452Enabling environment processing may enable users to bypass access 1453restrictions in some configurations using mechanisms such as 1454.Ev LD_PRELOAD . 1455.It Cm PermitUserRC 1456Specifies whether any 1457.Pa ~/.ssh/rc 1458file is executed. 1459The default is 1460.Cm yes . 1461.It Cm PerSourceMaxStartups 1462Specifies the number of unauthenticated connections allowed from a 1463given source address, or 1464.Dq none 1465if there is no limit. 1466This limit is applied in addition to 1467.Cm MaxStartups , 1468whichever is lower. 1469The default is 1470.Cm none . 1471.It Cm PerSourceNetBlockSize 1472Specifies the number of bits of source address that are grouped together 1473for the purposes of applying PerSourceMaxStartups limits. 1474Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1475The default is 1476.Cm 32:128 , 1477which means each address is considered individually. 1478.It Cm PidFile 1479Specifies the file that contains the process ID of the 1480SSH daemon, or 1481.Cm none 1482to not write one. 1483The default is 1484.Pa /var/run/sshd.pid . 1485.It Cm Port 1486Specifies the port number that 1487.Xr sshd 8 1488listens on. 1489The default is 22. 1490Multiple options of this type are permitted. 1491See also 1492.Cm ListenAddress . 1493.It Cm PrintLastLog 1494Specifies whether 1495.Xr sshd 8 1496should print the date and time of the last user login when a user logs 1497in interactively. 1498The default is 1499.Cm yes . 1500.It Cm PrintMotd 1501Specifies whether 1502.Xr sshd 8 1503should print 1504.Pa /etc/motd 1505when a user logs in interactively. 1506(On some systems it is also printed by the shell, 1507.Pa /etc/profile , 1508or equivalent.) 1509The default is 1510.Cm yes . 1511.It Cm PubkeyAcceptedAlgorithms 1512Specifies the signature algorithms that will be accepted for public key 1513authentication as a list of comma-separated patterns. 1514Alternately if the specified list begins with a 1515.Sq + 1516character, then the specified algorithms will be appended to the default set 1517instead of replacing them. 1518If the specified list begins with a 1519.Sq - 1520character, then the specified algorithms (including wildcards) will be removed 1521from the default set instead of replacing them. 1522If the specified list begins with a 1523.Sq ^ 1524character, then the specified algorithms will be placed at the head of the 1525default set. 1526The default for this option is: 1527.Bd -literal -offset 3n 1528ssh-ed25519-cert-v01@openssh.com, 1529ecdsa-sha2-nistp256-cert-v01@openssh.com, 1530ecdsa-sha2-nistp384-cert-v01@openssh.com, 1531ecdsa-sha2-nistp521-cert-v01@openssh.com, 1532sk-ssh-ed25519-cert-v01@openssh.com, 1533sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1534rsa-sha2-512-cert-v01@openssh.com, 1535rsa-sha2-256-cert-v01@openssh.com, 1536ssh-ed25519, 1537ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1538sk-ssh-ed25519@openssh.com, 1539sk-ecdsa-sha2-nistp256@openssh.com, 1540rsa-sha2-512,rsa-sha2-256 1541.Ed 1542.Pp 1543The list of available signature algorithms may also be obtained using 1544.Qq ssh -Q PubkeyAcceptedAlgorithms . 1545.It Cm PubkeyAuthOptions 1546Sets one or more public key authentication options. 1547The supported keywords are: 1548.Cm none 1549(the default; indicating no additional options are enabled), 1550.Cm touch-required 1551and 1552.Cm verify-required . 1553.Pp 1554The 1555.Cm touch-required 1556option causes public key authentication using a FIDO authenticator algorithm 1557(i.e.\& 1558.Cm ecdsa-sk 1559or 1560.Cm ed25519-sk ) 1561to always require the signature to attest that a physically present user 1562explicitly confirmed the authentication (usually by touching the authenticator). 1563By default, 1564.Xr sshd 8 1565requires user presence unless overridden with an authorized_keys option. 1566The 1567.Cm touch-required 1568flag disables this override. 1569.Pp 1570The 1571.Cm verify-required 1572option requires a FIDO key signature attest that the user was verified, 1573e.g. via a PIN. 1574.Pp 1575Neither the 1576.Cm touch-required 1577or 1578.Cm verify-required 1579options have any effect for other, non-FIDO, public key types. 1580.It Cm PubkeyAuthentication 1581Specifies whether public key authentication is allowed. 1582The default is 1583.Cm yes . 1584.It Cm RekeyLimit 1585Specifies the maximum amount of data that may be transmitted before the 1586session key is renegotiated, optionally followed by a maximum amount of 1587time that may pass before the session key is renegotiated. 1588The first argument is specified in bytes and may have a suffix of 1589.Sq K , 1590.Sq M , 1591or 1592.Sq G 1593to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1594The default is between 1595.Sq 1G 1596and 1597.Sq 4G , 1598depending on the cipher. 1599The optional second value is specified in seconds and may use any of the 1600units documented in the 1601.Sx TIME FORMATS 1602section. 1603The default value for 1604.Cm RekeyLimit 1605is 1606.Cm default none , 1607which means that rekeying is performed after the cipher's default amount 1608of data has been sent or received and no time based rekeying is done. 1609.It Cm RevokedKeys 1610Specifies revoked public keys file, or 1611.Cm none 1612to not use one. 1613Keys listed in this file will be refused for public key authentication. 1614Note that if this file is not readable, then public key authentication will 1615be refused for all users. 1616Keys may be specified as a text file, listing one public key per line, or as 1617an OpenSSH Key Revocation List (KRL) as generated by 1618.Xr ssh-keygen 1 . 1619For more information on KRLs, see the KEY REVOCATION LISTS section in 1620.Xr ssh-keygen 1 . 1621.It Cm RDomain 1622Specifies an explicit routing domain that is applied after authentication 1623has completed. 1624The user session, as well as any forwarded or listening IP sockets, 1625will be bound to this 1626.Xr rdomain 4 . 1627If the routing domain is set to 1628.Cm \&%D , 1629then the domain in which the incoming connection was received will be applied. 1630.It Cm SecurityKeyProvider 1631Specifies a path to a library that will be used when loading 1632FIDO authenticator-hosted keys, overriding the default of using 1633the built-in USB HID support. 1634.It Cm SetEnv 1635Specifies one or more environment variables to set in child sessions started 1636by 1637.Xr sshd 8 1638as 1639.Dq NAME=VALUE . 1640The environment value may be quoted (e.g. if it contains whitespace 1641characters). 1642Environment variables set by 1643.Cm SetEnv 1644override the default environment and any variables specified by the user 1645via 1646.Cm AcceptEnv 1647or 1648.Cm PermitUserEnvironment . 1649.It Cm StreamLocalBindMask 1650Sets the octal file creation mode mask 1651.Pq umask 1652used when creating a Unix-domain socket file for local or remote 1653port forwarding. 1654This option is only used for port forwarding to a Unix-domain socket file. 1655.Pp 1656The default value is 0177, which creates a Unix-domain socket file that is 1657readable and writable only by the owner. 1658Note that not all operating systems honor the file mode on Unix-domain 1659socket files. 1660.It Cm StreamLocalBindUnlink 1661Specifies whether to remove an existing Unix-domain socket file for local 1662or remote port forwarding before creating a new one. 1663If the socket file already exists and 1664.Cm StreamLocalBindUnlink 1665is not enabled, 1666.Nm sshd 1667will be unable to forward the port to the Unix-domain socket file. 1668This option is only used for port forwarding to a Unix-domain socket file. 1669.Pp 1670The argument must be 1671.Cm yes 1672or 1673.Cm no . 1674The default is 1675.Cm no . 1676.It Cm StrictModes 1677Specifies whether 1678.Xr sshd 8 1679should check file modes and ownership of the 1680user's files and home directory before accepting login. 1681This is normally desirable because novices sometimes accidentally leave their 1682directory or files world-writable. 1683The default is 1684.Cm yes . 1685Note that this does not apply to 1686.Cm ChrootDirectory , 1687whose permissions and ownership are checked unconditionally. 1688.It Cm Subsystem 1689Configures an external subsystem (e.g. file transfer daemon). 1690Arguments should be a subsystem name and a command (with optional arguments) 1691to execute upon subsystem request. 1692.Pp 1693The command 1694.Cm sftp-server 1695implements the SFTP file transfer subsystem. 1696.Pp 1697Alternately the name 1698.Cm internal-sftp 1699implements an in-process SFTP server. 1700This may simplify configurations using 1701.Cm ChrootDirectory 1702to force a different filesystem root on clients. 1703.Pp 1704By default no subsystems are defined. 1705.It Cm SyslogFacility 1706Gives the facility code that is used when logging messages from 1707.Xr sshd 8 . 1708The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1709LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1710The default is AUTH. 1711.It Cm TCPKeepAlive 1712Specifies whether the system should send TCP keepalive messages to the 1713other side. 1714If they are sent, death of the connection or crash of one 1715of the machines will be properly noticed. 1716However, this means that 1717connections will die if the route is down temporarily, and some people 1718find it annoying. 1719On the other hand, if TCP keepalives are not sent, 1720sessions may hang indefinitely on the server, leaving 1721.Qq ghost 1722users and consuming server resources. 1723.Pp 1724The default is 1725.Cm yes 1726(to send TCP keepalive messages), and the server will notice 1727if the network goes down or the client host crashes. 1728This avoids infinitely hanging sessions. 1729.Pp 1730To disable TCP keepalive messages, the value should be set to 1731.Cm no . 1732.It Cm TrustedUserCAKeys 1733Specifies a file containing public keys of certificate authorities that are 1734trusted to sign user certificates for authentication, or 1735.Cm none 1736to not use one. 1737Keys are listed one per line; empty lines and comments starting with 1738.Ql # 1739are allowed. 1740If a certificate is presented for authentication and has its signing CA key 1741listed in this file, then it may be used for authentication for any user 1742listed in the certificate's principals list. 1743Note that certificates that lack a list of principals will not be permitted 1744for authentication using 1745.Cm TrustedUserCAKeys . 1746For more details on certificates, see the CERTIFICATES section in 1747.Xr ssh-keygen 1 . 1748.It Cm UseBlacklist 1749Specifies whether 1750.Xr sshd 8 1751attempts to send authentication success and failure messages 1752to the 1753.Xr blacklistd 8 1754daemon. 1755The default is 1756.Cm no . 1757For forward compatibility with an upcoming 1758.Xr blacklistd 1759rename, the 1760.Cm UseBlocklist 1761alias can be used instead. 1762.It Cm UseDNS 1763Specifies whether 1764.Xr sshd 8 1765should look up the remote host name, and to check that 1766the resolved host name for the remote IP address maps back to the 1767very same IP address. 1768.Pp 1769If this option is set to 1770.Cm no , 1771then only addresses and not host names may be used in 1772.Pa ~/.ssh/authorized_keys 1773.Cm from 1774and 1775.Nm 1776.Cm Match 1777.Cm Host 1778directives. 1779The default is 1780.Dq yes . 1781.It Cm UsePAM 1782Enables the Pluggable Authentication Module interface. 1783If set to 1784.Cm yes 1785this will enable PAM authentication using 1786.Cm KbdInteractiveAuthentication 1787and 1788.Cm PasswordAuthentication 1789in addition to PAM account and session module processing for all 1790authentication types. 1791.Pp 1792Because PAM keyboard-interactive authentication usually serves an equivalent 1793role to password authentication, you should disable either 1794.Cm PasswordAuthentication 1795or 1796.Cm KbdInteractiveAuthentication . 1797.Pp 1798If 1799.Cm UsePAM 1800is enabled, you will not be able to run 1801.Xr sshd 8 1802as a non-root user. 1803The default is 1804.Cm yes . 1805.It Cm VersionAddendum 1806Optionally specifies additional text to append to the SSH protocol banner 1807sent by the server upon connection. 1808The default is 1809.Qq FreeBSD-20220415 . 1810The value 1811.Cm none 1812may be used to disable this. 1813.It Cm X11DisplayOffset 1814Specifies the first display number available for 1815.Xr sshd 8 Ns 's 1816X11 forwarding. 1817This prevents sshd from interfering with real X11 servers. 1818The default is 10. 1819.It Cm X11Forwarding 1820Specifies whether X11 forwarding is permitted. 1821The argument must be 1822.Cm yes 1823or 1824.Cm no . 1825The default is 1826.Cm yes . 1827.Pp 1828When X11 forwarding is enabled, there may be additional exposure to 1829the server and to client displays if the 1830.Xr sshd 8 1831proxy display is configured to listen on the wildcard address (see 1832.Cm X11UseLocalhost ) , 1833though this is not the default. 1834Additionally, the authentication spoofing and authentication data 1835verification and substitution occur on the client side. 1836The security risk of using X11 forwarding is that the client's X11 1837display server may be exposed to attack when the SSH client requests 1838forwarding (see the warnings for 1839.Cm ForwardX11 1840in 1841.Xr ssh_config 5 ) . 1842A system administrator may have a stance in which they want to 1843protect clients that may expose themselves to attack by unwittingly 1844requesting X11 forwarding, which can warrant a 1845.Cm no 1846setting. 1847.Pp 1848Note that disabling X11 forwarding does not prevent users from 1849forwarding X11 traffic, as users can always install their own forwarders. 1850.It Cm X11UseLocalhost 1851Specifies whether 1852.Xr sshd 8 1853should bind the X11 forwarding server to the loopback address or to 1854the wildcard address. 1855By default, 1856sshd binds the forwarding server to the loopback address and sets the 1857hostname part of the 1858.Ev DISPLAY 1859environment variable to 1860.Cm localhost . 1861This prevents remote hosts from connecting to the proxy display. 1862However, some older X11 clients may not function with this 1863configuration. 1864.Cm X11UseLocalhost 1865may be set to 1866.Cm no 1867to specify that the forwarding server should be bound to the wildcard 1868address. 1869The argument must be 1870.Cm yes 1871or 1872.Cm no . 1873The default is 1874.Cm yes . 1875.It Cm XAuthLocation 1876Specifies the full pathname of the 1877.Xr xauth 1 1878program, or 1879.Cm none 1880to not use one. 1881The default is 1882.Pa /usr/local/bin/xauth . 1883.El 1884.Sh TIME FORMATS 1885.Xr sshd 8 1886command-line arguments and configuration file options that specify time 1887may be expressed using a sequence of the form: 1888.Sm off 1889.Ar time Op Ar qualifier , 1890.Sm on 1891where 1892.Ar time 1893is a positive integer value and 1894.Ar qualifier 1895is one of the following: 1896.Pp 1897.Bl -tag -width Ds -compact -offset indent 1898.It Aq Cm none 1899seconds 1900.It Cm s | Cm S 1901seconds 1902.It Cm m | Cm M 1903minutes 1904.It Cm h | Cm H 1905hours 1906.It Cm d | Cm D 1907days 1908.It Cm w | Cm W 1909weeks 1910.El 1911.Pp 1912Each member of the sequence is added together to calculate 1913the total time value. 1914.Pp 1915Time format examples: 1916.Pp 1917.Bl -tag -width Ds -compact -offset indent 1918.It 600 1919600 seconds (10 minutes) 1920.It 10m 192110 minutes 1922.It 1h30m 19231 hour 30 minutes (90 minutes) 1924.El 1925.Sh TOKENS 1926Arguments to some keywords can make use of tokens, 1927which are expanded at runtime: 1928.Pp 1929.Bl -tag -width XXXX -offset indent -compact 1930.It %% 1931A literal 1932.Sq % . 1933.It \&%D 1934The routing domain in which the incoming connection was received. 1935.It %F 1936The fingerprint of the CA key. 1937.It %f 1938The fingerprint of the key or certificate. 1939.It %h 1940The home directory of the user. 1941.It %i 1942The key ID in the certificate. 1943.It %K 1944The base64-encoded CA key. 1945.It %k 1946The base64-encoded key or certificate for authentication. 1947.It %s 1948The serial number of the certificate. 1949.It \&%T 1950The type of the CA key. 1951.It %t 1952The key or certificate type. 1953.It \&%U 1954The numeric user ID of the target user. 1955.It %u 1956The username. 1957.El 1958.Pp 1959.Cm AuthorizedKeysCommand 1960accepts the tokens %%, %f, %h, %k, %t, %U, and %u. 1961.Pp 1962.Cm AuthorizedKeysFile 1963accepts the tokens %%, %h, %U, and %u. 1964.Pp 1965.Cm AuthorizedPrincipalsCommand 1966accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 1967.Pp 1968.Cm AuthorizedPrincipalsFile 1969accepts the tokens %%, %h, %U, and %u. 1970.Pp 1971.Cm ChrootDirectory 1972accepts the tokens %%, %h, %U, and %u. 1973.Pp 1974.Cm RoutingDomain 1975accepts the token %D. 1976.Sh FILES 1977.Bl -tag -width Ds 1978.It Pa /etc/ssh/sshd_config 1979Contains configuration data for 1980.Xr sshd 8 . 1981This file should be writable by root only, but it is recommended 1982(though not necessary) that it be world-readable. 1983.El 1984.Sh SEE ALSO 1985.Xr sftp-server 8 , 1986.Xr sshd 8 1987.Sh AUTHORS 1988.An -nosplit 1989OpenSSH is a derivative of the original and free 1990ssh 1.2.12 release by 1991.An Tatu Ylonen . 1992.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1993.An Theo de Raadt 1994and 1995.An Dug Song 1996removed many bugs, re-added newer features and 1997created OpenSSH. 1998.An Markus Friedl 1999contributed the support for SSH protocol versions 1.5 and 2.0. 2000.An Niels Provos 2001and 2002.An Markus Friedl 2003contributed support for privilege separation. 2004