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 1.350 2023/07/28 05:42:36 jmc Exp $ 37.Dd $Mdocdate: July 28 2023 $ 38.Dt SSHD_CONFIG 5 39.Os 40.Sh NAME 41.Nm sshd_config 42.Nd OpenSSH daemon configuration file 43.Sh DESCRIPTION 44.Xr sshd 8 45reads configuration data from 46.Pa /etc/ssh/sshd_config 47(or the file specified with 48.Fl f 49on the command line). 50The file contains keyword-argument pairs, one per line. 51Unless noted otherwise, for each keyword, the first obtained value will be used. 52Lines starting with 53.Ql # 54and empty lines are interpreted as comments. 55Arguments may optionally be enclosed in double quotes 56.Pq \&" 57in order to represent arguments containing spaces. 58.Pp 59The possible 60keywords and their meanings are as follows (note that 61keywords are case-insensitive and arguments are case-sensitive): 62.Bl -tag -width Ds 63.It Cm AcceptEnv 64Specifies what environment variables sent by the client will be copied into 65the session's 66.Xr environ 7 . 67See 68.Cm SendEnv 69and 70.Cm SetEnv 71in 72.Xr ssh_config 5 73for how to configure the client. 74The 75.Ev TERM 76environment variable is always accepted whenever the client 77requests a pseudo-terminal as it is required by the protocol. 78Variables are specified by name, which may contain the wildcard characters 79.Ql * 80and 81.Ql \&? . 82Multiple environment variables may be separated by whitespace or spread 83across multiple 84.Cm AcceptEnv 85directives. 86Be warned that some environment variables could be used to bypass restricted 87user environments. 88For this reason, care should be taken in the use of this directive. 89The default is not to accept any environment variables. 90.It Cm AddressFamily 91Specifies which address family should be used by 92.Xr sshd 8 . 93Valid arguments are 94.Cm any 95(the default), 96.Cm inet 97(use IPv4 only), or 98.Cm inet6 99(use IPv6 only). 100.It Cm AllowAgentForwarding 101Specifies whether 102.Xr ssh-agent 1 103forwarding is permitted. 104The default is 105.Cm yes . 106Note that disabling agent forwarding does not improve security 107unless users are also denied shell access, as they can always install 108their own forwarders. 109.It Cm AllowGroups 110This keyword can be followed by a list of group name patterns, separated 111by spaces. 112If specified, login is allowed only for users whose primary 113group or supplementary group list matches one of the patterns. 114Only group names are valid; a numerical group ID is not recognized. 115By default, login is allowed for all groups. 116The allow/deny groups directives are processed in the following order: 117.Cm DenyGroups , 118.Cm AllowGroups . 119.Pp 120See PATTERNS in 121.Xr ssh_config 5 122for more information on patterns. 123This keyword may appear multiple times in 124.Nm 125with each instance appending to the list. 126.It Cm AllowStreamLocalForwarding 127Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 128The available options are 129.Cm yes 130(the default) 131or 132.Cm all 133to allow StreamLocal forwarding, 134.Cm no 135to prevent all StreamLocal forwarding, 136.Cm local 137to allow local (from the perspective of 138.Xr ssh 1 ) 139forwarding only or 140.Cm remote 141to allow remote forwarding only. 142Note that disabling StreamLocal forwarding does not improve security unless 143users are also denied shell access, as they can always install their 144own forwarders. 145.It Cm AllowTcpForwarding 146Specifies whether TCP forwarding is permitted. 147The available options are 148.Cm yes 149(the default) 150or 151.Cm all 152to allow TCP forwarding, 153.Cm no 154to prevent all TCP forwarding, 155.Cm local 156to allow local (from the perspective of 157.Xr ssh 1 ) 158forwarding only or 159.Cm remote 160to allow remote forwarding only. 161Note that disabling TCP forwarding does not improve security unless 162users are also denied shell access, as they can always install their 163own forwarders. 164.It Cm AllowUsers 165This keyword can be followed by a list of user name patterns, separated 166by spaces. 167If specified, login is allowed only for user names that 168match one of the patterns. 169Only user names are valid; a numerical user ID is not recognized. 170By default, login is allowed for all users. 171If the pattern takes the form USER@HOST then USER and HOST 172are separately checked, restricting logins to particular 173users from particular hosts. 174HOST criteria may additionally contain addresses to match in CIDR 175address/masklen format. 176The allow/deny users directives are processed in the following order: 177.Cm DenyUsers , 178.Cm AllowUsers . 179.Pp 180See PATTERNS in 181.Xr ssh_config 5 182for more information on patterns. 183This keyword may appear multiple times in 184.Nm 185with each instance appending to the list. 186.It Cm AuthenticationMethods 187Specifies the authentication methods that must be successfully completed 188for a user to be granted access. 189This option must be followed by one or more lists of comma-separated 190authentication method names, or by the single string 191.Cm any 192to indicate the default behaviour of accepting any single authentication 193method. 194If the default is overridden, then successful authentication requires 195completion of every method in at least one of these lists. 196.Pp 197For example, 198.Qq publickey,password publickey,keyboard-interactive 199would require the user to complete public key authentication, followed by 200either password or keyboard interactive authentication. 201Only methods that are next in one or more lists are offered at each stage, 202so for this example it would not be possible to attempt password or 203keyboard-interactive authentication before public key. 204.Pp 205For keyboard interactive authentication it is also possible to 206restrict authentication to a specific device by appending a 207colon followed by the device identifier 208.Cm bsdauth 209or 210.Cm pam . 211depending on the server configuration. 212For example, 213.Qq keyboard-interactive:bsdauth 214would restrict keyboard interactive authentication to the 215.Cm bsdauth 216device. 217.Pp 218If the publickey method is listed more than once, 219.Xr sshd 8 220verifies that keys that have been used successfully are not reused for 221subsequent authentications. 222For example, 223.Qq publickey,publickey 224requires successful authentication using two different public keys. 225.Pp 226Note that each authentication method listed should also be explicitly enabled 227in the configuration. 228.Pp 229The available authentication methods are: 230.Qq gssapi-with-mic , 231.Qq hostbased , 232.Qq keyboard-interactive , 233.Qq none 234(used for access to password-less accounts when 235.Cm PermitEmptyPasswords 236is enabled), 237.Qq password 238and 239.Qq publickey . 240.It Cm AuthorizedKeysCommand 241Specifies a program to be used to look up the user's public keys. 242The program must be owned by root, not writable by group or others and 243specified by an absolute path. 244Arguments to 245.Cm AuthorizedKeysCommand 246accept the tokens described in the 247.Sx TOKENS 248section. 249If no arguments are specified then the username of the target user is used. 250.Pp 251The program should produce on standard output zero or 252more lines of authorized_keys output (see 253.Sx AUTHORIZED_KEYS 254in 255.Xr sshd 8 ) . 256.Cm AuthorizedKeysCommand 257is tried after the usual 258.Cm AuthorizedKeysFile 259files and will not be executed if a matching key is found there. 260By default, no 261.Cm AuthorizedKeysCommand 262is run. 263.It Cm AuthorizedKeysCommandUser 264Specifies the user under whose account the 265.Cm AuthorizedKeysCommand 266is run. 267It is recommended to use a dedicated user that has no other role on the host 268than running authorized keys commands. 269If 270.Cm AuthorizedKeysCommand 271is specified but 272.Cm AuthorizedKeysCommandUser 273is not, then 274.Xr sshd 8 275will refuse to start. 276.It Cm AuthorizedKeysFile 277Specifies the file that contains the public keys used for user authentication. 278The format is described in the AUTHORIZED_KEYS FILE FORMAT section of 279.Xr sshd 8 . 280Arguments to 281.Cm AuthorizedKeysFile 282accept the tokens described in the 283.Sx TOKENS 284section. 285After expansion, 286.Cm AuthorizedKeysFile 287is taken to be an absolute path or one relative to the user's home 288directory. 289Multiple files may be listed, separated by whitespace. 290Alternately this option may be set to 291.Cm none 292to skip checking for user keys in files. 293The default is 294.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 295.It Cm AuthorizedPrincipalsCommand 296Specifies a program to be used to generate the list of allowed 297certificate principals as per 298.Cm AuthorizedPrincipalsFile . 299The program must be owned by root, not writable by group or others and 300specified by an absolute path. 301Arguments to 302.Cm AuthorizedPrincipalsCommand 303accept the tokens described in the 304.Sx TOKENS 305section. 306If no arguments are specified then the username of the target user is used. 307.Pp 308The program should produce on standard output zero or 309more lines of 310.Cm AuthorizedPrincipalsFile 311output. 312If either 313.Cm AuthorizedPrincipalsCommand 314or 315.Cm AuthorizedPrincipalsFile 316is specified, then certificates offered by the client for authentication 317must contain a principal that is listed. 318By default, no 319.Cm AuthorizedPrincipalsCommand 320is run. 321.It Cm AuthorizedPrincipalsCommandUser 322Specifies the user under whose account the 323.Cm AuthorizedPrincipalsCommand 324is run. 325It is recommended to use a dedicated user that has no other role on the host 326than running authorized principals commands. 327If 328.Cm AuthorizedPrincipalsCommand 329is specified but 330.Cm AuthorizedPrincipalsCommandUser 331is not, then 332.Xr sshd 8 333will refuse to start. 334.It Cm AuthorizedPrincipalsFile 335Specifies a file that lists principal names that are accepted for 336certificate authentication. 337When using certificates signed by a key listed in 338.Cm TrustedUserCAKeys , 339this file lists names, one of which must appear in the certificate for it 340to be accepted for authentication. 341Names are listed one per line preceded by key options (as described in 342.Sx AUTHORIZED_KEYS FILE FORMAT 343in 344.Xr sshd 8 ) . 345Empty lines and comments starting with 346.Ql # 347are ignored. 348.Pp 349Arguments to 350.Cm AuthorizedPrincipalsFile 351accept the tokens described in the 352.Sx TOKENS 353section. 354After expansion, 355.Cm AuthorizedPrincipalsFile 356is taken to be an absolute path or one relative to the user's home directory. 357The default is 358.Cm none , 359i.e. not to use a principals file \(en in this case, the username 360of the user must appear in a certificate's principals list for it to be 361accepted. 362.Pp 363Note that 364.Cm AuthorizedPrincipalsFile 365is only used when authentication proceeds using a CA listed in 366.Cm TrustedUserCAKeys 367and is not consulted for certification authorities trusted via 368.Pa ~/.ssh/authorized_keys , 369though the 370.Cm principals= 371key option offers a similar facility (see 372.Xr sshd 8 373for details). 374.It Cm Banner 375The contents of the specified file are sent to the remote user before 376authentication is allowed. 377If the argument is 378.Cm none 379then no banner is displayed. 380By default, no banner is displayed. 381.It Cm CASignatureAlgorithms 382Specifies which algorithms are allowed for signing of certificates 383by certificate authorities (CAs). 384The default is: 385.Bd -literal -offset indent 386ssh-ed25519,ecdsa-sha2-nistp256, 387ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 388sk-ssh-ed25519@openssh.com, 389sk-ecdsa-sha2-nistp256@openssh.com, 390rsa-sha2-512,rsa-sha2-256 391.Ed 392.Pp 393If the specified list begins with a 394.Sq + 395character, then the specified algorithms will be appended to the default set 396instead of replacing them. 397If the specified list begins with a 398.Sq - 399character, then the specified algorithms (including wildcards) will be removed 400from the default set instead of replacing them. 401.Pp 402Certificates signed using other algorithms will not be accepted for 403public key or host-based authentication. 404.It Cm ChannelTimeout 405Specifies whether and how quickly 406.Xr sshd 8 407should close inactive channels. 408Timeouts are specified as one or more 409.Dq type=interval 410pairs separated by whitespace, where the 411.Dq type 412must be a channel type name (as described in the table below), optionally 413containing wildcard characters. 414.Pp 415The timeout value 416.Dq interval 417is specified in seconds or may use any of the units documented in the 418.Sx TIME FORMATS 419section. 420For example, 421.Dq session:*=5m 422would cause all sessions to terminate after five minutes of inactivity. 423Specifying a zero value disables the inactivity timeout. 424.Pp 425The available channel types include: 426.Bl -tag -width Ds 427.It Cm agent-connection 428Open connections to 429.Xr ssh-agent 1 . 430.It Cm direct-tcpip , Cm direct-streamlocal@openssh.com 431Open TCP or Unix socket (respectively) connections that have 432been established from a 433.Xr ssh 1 434local forwarding, i.e.\& 435.Cm LocalForward 436or 437.Cm DynamicForward . 438.It Cm forwarded-tcpip , Cm forwarded-streamlocal@openssh.com 439Open TCP or Unix socket (respectively) connections that have been 440established to a 441.Xr sshd 8 442listening on behalf of a 443.Xr ssh 1 444remote forwarding, i.e.\& 445.Cm RemoteForward . 446.It Cm session:command 447Command execution sessions. 448.It Cm session:shell 449Interactive shell sessions. 450.It Cm session:subsystem:... 451Subsystem sessions, e.g. for 452.Xr sftp 1 , 453which could be identified as 454.Cm session:subsystem:sftp . 455.It Cm x11-connection 456Open X11 forwarding sessions. 457.El 458.Pp 459Note that in all the above cases, terminating an inactive session does not 460guarantee to remove all resources associated with the session, e.g. shell 461processes or X11 clients relating to the session may continue to execute. 462.Pp 463Moreover, terminating an inactive channel or session does not necessarily 464close the SSH connection, nor does it prevent a client from 465requesting another channel of the same type. 466In particular, expiring an inactive forwarding session does not prevent 467another identical forwarding from being subsequently created. 468See also 469.Cm UnusedConnectionTimeout , 470which may be used in conjunction with this option. 471.Pp 472The default is not to expire channels of any type for inactivity. 473.It Cm ChrootDirectory 474Specifies the pathname of a directory to 475.Xr chroot 2 476to after authentication. 477At session startup 478.Xr sshd 8 479checks that all components of the pathname are root-owned directories 480which are not writable by any other user or group. 481After the chroot, 482.Xr sshd 8 483changes the working directory to the user's home directory. 484Arguments to 485.Cm ChrootDirectory 486accept the tokens described in the 487.Sx TOKENS 488section. 489.Pp 490The 491.Cm ChrootDirectory 492must contain the necessary files and directories to support the 493user's session. 494For an interactive session this requires at least a shell, typically 495.Xr sh 1 , 496and basic 497.Pa /dev 498nodes such as 499.Xr null 4 , 500.Xr zero 4 , 501.Xr stdin 4 , 502.Xr stdout 4 , 503.Xr stderr 4 , 504and 505.Xr tty 4 506devices. 507For file transfer sessions using SFTP 508no additional configuration of the environment is necessary if the in-process 509sftp-server is used, 510though sessions which use logging may require 511.Pa /dev/log 512inside the chroot directory on some operating systems (see 513.Xr sftp-server 8 514for details). 515.Pp 516For safety, it is very important that the directory hierarchy be 517prevented from modification by other processes on the system (especially 518those outside the jail). 519Misconfiguration can lead to unsafe environments which 520.Xr sshd 8 521cannot detect. 522.Pp 523The default is 524.Cm none , 525indicating not to 526.Xr chroot 2 . 527.It Cm Ciphers 528Specifies the ciphers allowed. 529Multiple ciphers must be comma-separated. 530If the specified list begins with a 531.Sq + 532character, then the specified ciphers will be appended to the default set 533instead of replacing them. 534If the specified list begins with a 535.Sq - 536character, then the specified ciphers (including wildcards) will be removed 537from the default set instead of replacing them. 538If the specified list begins with a 539.Sq ^ 540character, then the specified ciphers will be placed at the head of the 541default set. 542.Pp 543The supported ciphers are: 544.Pp 545.Bl -item -compact -offset indent 546.It 5473des-cbc 548.It 549aes128-cbc 550.It 551aes192-cbc 552.It 553aes256-cbc 554.It 555aes128-ctr 556.It 557aes192-ctr 558.It 559aes256-ctr 560.It 561aes128-gcm@openssh.com 562.It 563aes256-gcm@openssh.com 564.It 565chacha20-poly1305@openssh.com 566.El 567.Pp 568The default is: 569.Bd -literal -offset indent 570chacha20-poly1305@openssh.com, 571aes128-ctr,aes192-ctr,aes256-ctr, 572aes128-gcm@openssh.com,aes256-gcm@openssh.com 573.Ed 574.Pp 575The list of available ciphers may also be obtained using 576.Qq ssh -Q cipher . 577.It Cm ClientAliveCountMax 578Sets the number of client alive messages which may be sent without 579.Xr sshd 8 580receiving any messages back from the client. 581If this threshold is reached while client alive messages are being sent, 582sshd will disconnect the client, terminating the session. 583It is important to note that the use of client alive messages is very 584different from 585.Cm TCPKeepAlive . 586The client alive messages are sent through the encrypted channel 587and therefore will not be spoofable. 588The TCP keepalive option enabled by 589.Cm TCPKeepAlive 590is spoofable. 591The client alive mechanism is valuable when the client or 592server depend on knowing when a connection has become unresponsive. 593.Pp 594The default value is 3. 595If 596.Cm ClientAliveInterval 597is set to 15, and 598.Cm ClientAliveCountMax 599is left at the default, unresponsive SSH clients 600will be disconnected after approximately 45 seconds. 601Setting a zero 602.Cm ClientAliveCountMax 603disables connection termination. 604.It Cm ClientAliveInterval 605Sets a timeout interval in seconds after which if no data has been received 606from the client, 607.Xr sshd 8 608will send a message through the encrypted 609channel to request a response from the client. 610The default 611is 0, indicating that these messages will not be sent to the client. 612.It Cm Compression 613Specifies whether compression is enabled after 614the user has authenticated successfully. 615The argument must be 616.Cm yes , 617.Cm delayed 618(a legacy synonym for 619.Cm yes ) 620or 621.Cm no . 622The default is 623.Cm yes . 624.It Cm DenyGroups 625This keyword can be followed by a list of group name patterns, separated 626by spaces. 627Login is disallowed for users whose primary group or supplementary 628group list matches one of the patterns. 629Only group names are valid; a numerical group ID is not recognized. 630By default, login is allowed for all groups. 631The allow/deny groups directives are processed in the following order: 632.Cm DenyGroups , 633.Cm AllowGroups . 634.Pp 635See PATTERNS in 636.Xr ssh_config 5 637for more information on patterns. 638This keyword may appear multiple times in 639.Nm 640with each instance appending to the list. 641.It Cm DenyUsers 642This keyword can be followed by a list of user name patterns, separated 643by spaces. 644Login is disallowed for user names that match one of the patterns. 645Only user names are valid; a numerical user ID is not recognized. 646By default, login is allowed for all users. 647If the pattern takes the form USER@HOST then USER and HOST 648are separately checked, restricting logins to particular 649users from particular hosts. 650HOST criteria may additionally contain addresses to match in CIDR 651address/masklen format. 652The allow/deny users directives are processed in the following order: 653.Cm DenyUsers , 654.Cm AllowUsers . 655.Pp 656See PATTERNS in 657.Xr ssh_config 5 658for more information on patterns. 659This keyword may appear multiple times in 660.Nm 661with each instance appending to the list. 662.It Cm DisableForwarding 663Disables all forwarding features, including X11, 664.Xr ssh-agent 1 , 665TCP and StreamLocal. 666This option overrides all other forwarding-related options and may 667simplify restricted configurations. 668.It Cm ExposeAuthInfo 669Writes a temporary file containing a list of authentication methods and 670public credentials (e.g. keys) used to authenticate the user. 671The location of the file is exposed to the user session through the 672.Ev SSH_USER_AUTH 673environment variable. 674The default is 675.Cm no . 676.It Cm FingerprintHash 677Specifies the hash algorithm used when logging key fingerprints. 678Valid options are: 679.Cm md5 680and 681.Cm sha256 . 682The default is 683.Cm sha256 . 684.It Cm ForceCommand 685Forces the execution of the command specified by 686.Cm ForceCommand , 687ignoring any command supplied by the client and 688.Pa ~/.ssh/rc 689if present. 690The command is invoked by using the user's login shell with the -c option. 691This applies to shell, command, or subsystem execution. 692It is most useful inside a 693.Cm Match 694block. 695The command originally supplied by the client is available in the 696.Ev SSH_ORIGINAL_COMMAND 697environment variable. 698Specifying a command of 699.Cm internal-sftp 700will force the use of an in-process SFTP server that requires no support 701files when used with 702.Cm ChrootDirectory . 703The default is 704.Cm none . 705.It Cm GatewayPorts 706Specifies whether remote hosts are allowed to connect to ports 707forwarded for the client. 708By default, 709.Xr sshd 8 710binds remote port forwardings to the loopback address. 711This prevents other remote hosts from connecting to forwarded ports. 712.Cm GatewayPorts 713can be used to specify that sshd 714should allow remote port forwardings to bind to non-loopback addresses, thus 715allowing other hosts to connect. 716The argument may be 717.Cm no 718to force remote port forwardings to be available to the local host only, 719.Cm yes 720to force remote port forwardings to bind to the wildcard address, or 721.Cm clientspecified 722to allow the client to select the address to which the forwarding is bound. 723The default is 724.Cm no . 725.It Cm GSSAPIAuthentication 726Specifies whether user authentication based on GSSAPI is allowed. 727The default is 728.Cm no . 729.It Cm GSSAPICleanupCredentials 730Specifies whether to automatically destroy the user's credentials cache 731on logout. 732The default is 733.Cm yes . 734.It Cm GSSAPIStrictAcceptorCheck 735Determines whether to be strict about the identity of the GSSAPI acceptor 736a client authenticates against. 737If set to 738.Cm yes 739then the client must authenticate against the host 740service on the current hostname. 741If set to 742.Cm no 743then the client may authenticate against any service key stored in the 744machine's default store. 745This facility is provided to assist with operation on multi homed machines. 746The default is 747.Cm yes . 748.It Cm HostbasedAcceptedAlgorithms 749Specifies the signature algorithms that will be accepted for hostbased 750authentication as a list of comma-separated patterns. 751Alternately if the specified list begins with a 752.Sq + 753character, then the specified signature algorithms will be appended to 754the default set instead of replacing them. 755If the specified list begins with a 756.Sq - 757character, then the specified signature algorithms (including wildcards) 758will be removed from the default set instead of replacing them. 759If the specified list begins with a 760.Sq ^ 761character, then the specified signature algorithms will be placed at 762the head of the default set. 763The default for this option is: 764.Bd -literal -offset 3n 765ssh-ed25519-cert-v01@openssh.com, 766ecdsa-sha2-nistp256-cert-v01@openssh.com, 767ecdsa-sha2-nistp384-cert-v01@openssh.com, 768ecdsa-sha2-nistp521-cert-v01@openssh.com, 769sk-ssh-ed25519-cert-v01@openssh.com, 770sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 771rsa-sha2-512-cert-v01@openssh.com, 772rsa-sha2-256-cert-v01@openssh.com, 773ssh-ed25519, 774ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 775sk-ssh-ed25519@openssh.com, 776sk-ecdsa-sha2-nistp256@openssh.com, 777rsa-sha2-512,rsa-sha2-256 778.Ed 779.Pp 780The list of available signature algorithms may also be obtained using 781.Qq ssh -Q HostbasedAcceptedAlgorithms . 782This was formerly named HostbasedAcceptedKeyTypes. 783.It Cm HostbasedAuthentication 784Specifies whether rhosts or /etc/hosts.equiv authentication together 785with successful public key client host authentication is allowed 786(host-based authentication). 787The default is 788.Cm no . 789.It Cm HostbasedUsesNameFromPacketOnly 790Specifies whether or not the server will attempt to perform a reverse 791name lookup when matching the name in the 792.Pa ~/.shosts , 793.Pa ~/.rhosts , 794and 795.Pa /etc/hosts.equiv 796files during 797.Cm HostbasedAuthentication . 798A setting of 799.Cm yes 800means that 801.Xr sshd 8 802uses the name supplied by the client rather than 803attempting to resolve the name from the TCP connection itself. 804The default is 805.Cm no . 806.It Cm HostCertificate 807Specifies a file containing a public host certificate. 808The certificate's public key must match a private host key already specified 809by 810.Cm HostKey . 811The default behaviour of 812.Xr sshd 8 813is not to load any certificates. 814.It Cm HostKey 815Specifies a file containing a private host key 816used by SSH. 817The defaults are 818.Pa /etc/ssh/ssh_host_ecdsa_key , 819.Pa /etc/ssh/ssh_host_ed25519_key 820and 821.Pa /etc/ssh/ssh_host_rsa_key . 822.Pp 823Note that 824.Xr sshd 8 825will refuse to use a file if it is group/world-accessible 826and that the 827.Cm HostKeyAlgorithms 828option restricts which of the keys are actually used by 829.Xr sshd 8 . 830.Pp 831It is possible to have multiple host key files. 832It is also possible to specify public host key files instead. 833In this case operations on the private key will be delegated 834to an 835.Xr ssh-agent 1 . 836.It Cm HostKeyAgent 837Identifies the UNIX-domain socket used to communicate 838with an agent that has access to the private host keys. 839If the string 840.Qq SSH_AUTH_SOCK 841is specified, the location of the socket will be read from the 842.Ev SSH_AUTH_SOCK 843environment variable. 844.It Cm HostKeyAlgorithms 845Specifies the host key signature algorithms 846that the server offers. 847The default for this option is: 848.Bd -literal -offset 3n 849ssh-ed25519-cert-v01@openssh.com, 850ecdsa-sha2-nistp256-cert-v01@openssh.com, 851ecdsa-sha2-nistp384-cert-v01@openssh.com, 852ecdsa-sha2-nistp521-cert-v01@openssh.com, 853sk-ssh-ed25519-cert-v01@openssh.com, 854sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 855rsa-sha2-512-cert-v01@openssh.com, 856rsa-sha2-256-cert-v01@openssh.com, 857ssh-ed25519, 858ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 859sk-ssh-ed25519@openssh.com, 860sk-ecdsa-sha2-nistp256@openssh.com, 861rsa-sha2-512,rsa-sha2-256 862.Ed 863.Pp 864The list of available signature algorithms may also be obtained using 865.Qq ssh -Q HostKeyAlgorithms . 866.It Cm IgnoreRhosts 867Specifies whether to ignore per-user 868.Pa .rhosts 869and 870.Pa .shosts 871files during 872.Cm HostbasedAuthentication . 873The system-wide 874.Pa /etc/hosts.equiv 875and 876.Pa /etc/ssh/shosts.equiv 877are still used regardless of this setting. 878.Pp 879Accepted values are 880.Cm yes 881(the default) to ignore all per-user files, 882.Cm shosts-only 883to allow the use of 884.Pa .shosts 885but to ignore 886.Pa .rhosts 887or 888.Cm no 889to allow both 890.Pa .shosts 891and 892.Pa rhosts . 893.It Cm IgnoreUserKnownHosts 894Specifies whether 895.Xr sshd 8 896should ignore the user's 897.Pa ~/.ssh/known_hosts 898during 899.Cm HostbasedAuthentication 900and use only the system-wide known hosts file 901.Pa /etc/ssh/ssh_known_hosts . 902The default is 903.Dq no . 904.It Cm Include 905Include the specified configuration file(s). 906Multiple pathnames may be specified and each pathname may contain 907.Xr glob 7 908wildcards that will be expanded and processed in lexical order. 909Files without absolute paths are assumed to be in 910.Pa /etc/ssh . 911An 912.Cm Include 913directive may appear inside a 914.Cm Match 915block 916to perform conditional inclusion. 917.It Cm IPQoS 918Specifies the IPv4 type-of-service or DSCP class for the connection. 919Accepted values are 920.Cm af11 , 921.Cm af12 , 922.Cm af13 , 923.Cm af21 , 924.Cm af22 , 925.Cm af23 , 926.Cm af31 , 927.Cm af32 , 928.Cm af33 , 929.Cm af41 , 930.Cm af42 , 931.Cm af43 , 932.Cm cs0 , 933.Cm cs1 , 934.Cm cs2 , 935.Cm cs3 , 936.Cm cs4 , 937.Cm cs5 , 938.Cm cs6 , 939.Cm cs7 , 940.Cm ef , 941.Cm le , 942.Cm lowdelay , 943.Cm throughput , 944.Cm reliability , 945a numeric value, or 946.Cm none 947to use the operating system default. 948This option may take one or two arguments, separated by whitespace. 949If one argument is specified, it is used as the packet class unconditionally. 950If two values are specified, the first is automatically selected for 951interactive sessions and the second for non-interactive sessions. 952The default is 953.Cm af21 954(Low-Latency Data) 955for interactive sessions and 956.Cm cs1 957(Lower Effort) 958for non-interactive sessions. 959.It Cm KbdInteractiveAuthentication 960Specifies whether to allow keyboard-interactive authentication. 961All authentication styles from 962.Xr login.conf 5 963are supported. 964The default is 965.Cm yes . 966The argument to this keyword must be 967.Cm yes 968or 969.Cm no . 970.Cm ChallengeResponseAuthentication 971is a deprecated alias for this. 972.It Cm KerberosAuthentication 973Specifies whether the password provided by the user for 974.Cm PasswordAuthentication 975will be validated through the Kerberos KDC. 976To use this option, the server needs a 977Kerberos servtab which allows the verification of the KDC's identity. 978The default is 979.Cm no . 980.It Cm KerberosGetAFSToken 981If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 982an AFS token before accessing the user's home directory. 983The default is 984.Cm no . 985.It Cm KerberosOrLocalPasswd 986If password authentication through Kerberos fails then 987the password will be validated via any additional local mechanism 988such as 989.Pa /etc/passwd . 990The default is 991.Cm yes . 992.It Cm KerberosTicketCleanup 993Specifies whether to automatically destroy the user's ticket cache 994file on logout. 995The default is 996.Cm yes . 997.It Cm KexAlgorithms 998Specifies the available KEX (Key Exchange) algorithms. 999Multiple algorithms must be comma-separated. 1000Alternately if the specified list begins with a 1001.Sq + 1002character, then the specified algorithms will be appended to the default set 1003instead of replacing them. 1004If the specified list begins with a 1005.Sq - 1006character, then the specified algorithms (including wildcards) will be removed 1007from the default set instead of replacing them. 1008If the specified list begins with a 1009.Sq ^ 1010character, then the specified algorithms will be placed at the head of the 1011default set. 1012The supported algorithms are: 1013.Pp 1014.Bl -item -compact -offset indent 1015.It 1016curve25519-sha256 1017.It 1018curve25519-sha256@libssh.org 1019.It 1020diffie-hellman-group1-sha1 1021.It 1022diffie-hellman-group14-sha1 1023.It 1024diffie-hellman-group14-sha256 1025.It 1026diffie-hellman-group16-sha512 1027.It 1028diffie-hellman-group18-sha512 1029.It 1030diffie-hellman-group-exchange-sha1 1031.It 1032diffie-hellman-group-exchange-sha256 1033.It 1034ecdh-sha2-nistp256 1035.It 1036ecdh-sha2-nistp384 1037.It 1038ecdh-sha2-nistp521 1039.It 1040sntrup761x25519-sha512@openssh.com 1041.El 1042.Pp 1043The default is: 1044.Bd -literal -offset indent 1045sntrup761x25519-sha512@openssh.com, 1046curve25519-sha256,curve25519-sha256@libssh.org, 1047ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1048diffie-hellman-group-exchange-sha256, 1049diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 1050diffie-hellman-group14-sha256 1051.Ed 1052.Pp 1053The list of available key exchange algorithms may also be obtained using 1054.Qq ssh -Q KexAlgorithms . 1055.It Cm ListenAddress 1056Specifies the local addresses 1057.Xr sshd 8 1058should listen on. 1059The following forms may be used: 1060.Pp 1061.Bl -item -offset indent -compact 1062.It 1063.Cm ListenAddress 1064.Sm off 1065.Ar hostname | address 1066.Sm on 1067.Op Cm rdomain Ar domain 1068.It 1069.Cm ListenAddress 1070.Sm off 1071.Ar hostname : port 1072.Sm on 1073.Op Cm rdomain Ar domain 1074.It 1075.Cm ListenAddress 1076.Sm off 1077.Ar IPv4_address : port 1078.Sm on 1079.Op Cm rdomain Ar domain 1080.It 1081.Cm ListenAddress 1082.Sm off 1083.Oo Ar hostname | address Oc : Ar port 1084.Sm on 1085.Op Cm rdomain Ar domain 1086.El 1087.Pp 1088The optional 1089.Cm rdomain 1090qualifier requests 1091.Xr sshd 8 1092listen in an explicit routing domain. 1093If 1094.Ar port 1095is not specified, 1096sshd will listen on the address and all 1097.Cm Port 1098options specified. 1099The default is to listen on all local addresses on the current default 1100routing domain. 1101Multiple 1102.Cm ListenAddress 1103options are permitted. 1104For more information on routing domains, see 1105.Xr rdomain 4 . 1106.It Cm LoginGraceTime 1107The server disconnects after this time if the user has not 1108successfully logged in. 1109If the value is 0, there is no time limit. 1110The default is 120 seconds. 1111.It Cm LogLevel 1112Gives the verbosity level that is used when logging messages from 1113.Xr sshd 8 . 1114The possible values are: 1115QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1116The default is INFO. 1117DEBUG and DEBUG1 are equivalent. 1118DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1119Logging with a DEBUG level violates the privacy of users and is not recommended. 1120.It Cm LogVerbose 1121Specify one or more overrides to LogLevel. 1122An override consists of a pattern lists that matches the source file, function 1123and line number to force detailed logging for. 1124For example, an override pattern of: 1125.Bd -literal -offset indent 1126kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1127.Ed 1128.Pp 1129would enable detailed logging for line 1000 of 1130.Pa kex.c , 1131everything in the 1132.Fn kex_exchange_identification 1133function, and all code in the 1134.Pa packet.c 1135file. 1136This option is intended for debugging and no overrides are enabled by default. 1137.It Cm MACs 1138Specifies the available MAC (message authentication code) algorithms. 1139The MAC algorithm is used for data integrity protection. 1140Multiple algorithms must be comma-separated. 1141If the specified list begins with a 1142.Sq + 1143character, then the specified algorithms will be appended to the default set 1144instead of replacing them. 1145If the specified list begins with a 1146.Sq - 1147character, then the specified algorithms (including wildcards) will be removed 1148from the default set instead of replacing them. 1149If the specified list begins with a 1150.Sq ^ 1151character, then the specified algorithms will be placed at the head of the 1152default set. 1153.Pp 1154The algorithms that contain 1155.Qq -etm 1156calculate the MAC after encryption (encrypt-then-mac). 1157These are considered safer and their use recommended. 1158The supported MACs are: 1159.Pp 1160.Bl -item -compact -offset indent 1161.It 1162hmac-md5 1163.It 1164hmac-md5-96 1165.It 1166hmac-sha1 1167.It 1168hmac-sha1-96 1169.It 1170hmac-sha2-256 1171.It 1172hmac-sha2-512 1173.It 1174umac-64@openssh.com 1175.It 1176umac-128@openssh.com 1177.It 1178hmac-md5-etm@openssh.com 1179.It 1180hmac-md5-96-etm@openssh.com 1181.It 1182hmac-sha1-etm@openssh.com 1183.It 1184hmac-sha1-96-etm@openssh.com 1185.It 1186hmac-sha2-256-etm@openssh.com 1187.It 1188hmac-sha2-512-etm@openssh.com 1189.It 1190umac-64-etm@openssh.com 1191.It 1192umac-128-etm@openssh.com 1193.El 1194.Pp 1195The default is: 1196.Bd -literal -offset indent 1197umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1198hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1199hmac-sha1-etm@openssh.com, 1200umac-64@openssh.com,umac-128@openssh.com, 1201hmac-sha2-256,hmac-sha2-512,hmac-sha1 1202.Ed 1203.Pp 1204The list of available MAC algorithms may also be obtained using 1205.Qq ssh -Q mac . 1206.It Cm Match 1207Introduces a conditional block. 1208If all of the criteria on the 1209.Cm Match 1210line are satisfied, the keywords on the following lines override those 1211set in the global section of the config file, until either another 1212.Cm Match 1213line or the end of the file. 1214If a keyword appears in multiple 1215.Cm Match 1216blocks that are satisfied, only the first instance of the keyword is 1217applied. 1218.Pp 1219The arguments to 1220.Cm Match 1221are one or more criteria-pattern pairs or the single token 1222.Cm All 1223which matches all criteria. 1224The available criteria are 1225.Cm User , 1226.Cm Group , 1227.Cm Host , 1228.Cm LocalAddress , 1229.Cm LocalPort , 1230.Cm RDomain , 1231and 1232.Cm Address 1233(with 1234.Cm RDomain 1235representing the 1236.Xr rdomain 4 1237on which the connection was received). 1238.Pp 1239The match patterns may consist of single entries or comma-separated 1240lists and may use the wildcard and negation operators described in the 1241.Sx PATTERNS 1242section of 1243.Xr ssh_config 5 . 1244.Pp 1245The patterns in an 1246.Cm Address 1247criteria may additionally contain addresses to match in CIDR 1248address/masklen format, 1249such as 192.0.2.0/24 or 2001:db8::/32. 1250Note that the mask length provided must be consistent with the address - 1251it is an error to specify a mask length that is too long for the address 1252or one with bits set in this host portion of the address. 1253For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1254.Pp 1255Only a subset of keywords may be used on the lines following a 1256.Cm Match 1257keyword. 1258Available keywords are 1259.Cm AcceptEnv , 1260.Cm AllowAgentForwarding , 1261.Cm AllowGroups , 1262.Cm AllowStreamLocalForwarding , 1263.Cm AllowTcpForwarding , 1264.Cm AllowUsers , 1265.Cm AuthenticationMethods , 1266.Cm AuthorizedKeysCommand , 1267.Cm AuthorizedKeysCommandUser , 1268.Cm AuthorizedKeysFile , 1269.Cm AuthorizedPrincipalsCommand , 1270.Cm AuthorizedPrincipalsCommandUser , 1271.Cm AuthorizedPrincipalsFile , 1272.Cm Banner , 1273.Cm CASignatureAlgorithms , 1274.Cm ChannelTimeout , 1275.Cm ChrootDirectory , 1276.Cm ClientAliveCountMax , 1277.Cm ClientAliveInterval , 1278.Cm DenyGroups , 1279.Cm DenyUsers , 1280.Cm DisableForwarding , 1281.Cm ExposeAuthInfo , 1282.Cm ForceCommand , 1283.Cm GatewayPorts , 1284.Cm GSSAPIAuthentication , 1285.Cm HostbasedAcceptedAlgorithms , 1286.Cm HostbasedAuthentication , 1287.Cm HostbasedUsesNameFromPacketOnly , 1288.Cm IgnoreRhosts , 1289.Cm Include , 1290.Cm IPQoS , 1291.Cm KbdInteractiveAuthentication , 1292.Cm KerberosAuthentication , 1293.Cm LogLevel , 1294.Cm MaxAuthTries , 1295.Cm MaxSessions , 1296.Cm PasswordAuthentication , 1297.Cm PermitEmptyPasswords , 1298.Cm PermitListen , 1299.Cm PermitOpen , 1300.Cm PermitRootLogin , 1301.Cm PermitTTY , 1302.Cm PermitTunnel , 1303.Cm PermitUserRC , 1304.Cm PubkeyAcceptedAlgorithms , 1305.Cm PubkeyAuthentication , 1306.Cm PubkeyAuthOptions , 1307.Cm RekeyLimit , 1308.Cm RevokedKeys , 1309.Cm RDomain , 1310.Cm SetEnv , 1311.Cm StreamLocalBindMask , 1312.Cm StreamLocalBindUnlink , 1313.Cm TrustedUserCAKeys , 1314.Cm UnusedConnectionTimeout , 1315.Cm X11DisplayOffset , 1316.Cm X11Forwarding 1317and 1318.Cm X11UseLocalhost . 1319.It Cm MaxAuthTries 1320Specifies the maximum number of authentication attempts permitted per 1321connection. 1322Once the number of failures reaches half this value, 1323additional failures are logged. 1324The default is 6. 1325.It Cm MaxSessions 1326Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1327sessions permitted per network connection. 1328Multiple sessions may be established by clients that support connection 1329multiplexing. 1330Setting 1331.Cm MaxSessions 1332to 1 will effectively disable session multiplexing, whereas setting it to 0 1333will prevent all shell, login and subsystem sessions while still permitting 1334forwarding. 1335The default is 10. 1336.It Cm MaxStartups 1337Specifies the maximum number of concurrent unauthenticated connections to the 1338SSH daemon. 1339Additional connections will be dropped until authentication succeeds or the 1340.Cm LoginGraceTime 1341expires for a connection. 1342The default is 10:30:100. 1343.Pp 1344Alternatively, random early drop can be enabled by specifying 1345the three colon separated values 1346start:rate:full (e.g. "10:30:60"). 1347.Xr sshd 8 1348will refuse connection attempts with a probability of rate/100 (30%) 1349if there are currently start (10) unauthenticated connections. 1350The probability increases linearly and all connection attempts 1351are refused if the number of unauthenticated connections reaches full (60). 1352.It Cm ModuliFile 1353Specifies the 1354.Xr moduli 5 1355file that contains the Diffie-Hellman groups used for the 1356.Dq diffie-hellman-group-exchange-sha1 1357and 1358.Dq diffie-hellman-group-exchange-sha256 1359key exchange methods. 1360The default is 1361.Pa /etc/moduli . 1362.It Cm PasswordAuthentication 1363Specifies whether password authentication is allowed. 1364Note that passwords may also be accepted via 1365.Cm KbdInteractiveAuthentication . 1366See also 1367.Cm UsePAM . 1368The default is 1369.Cm no . 1370.It Cm PermitEmptyPasswords 1371When password authentication is allowed, it specifies whether the 1372server allows login to accounts with empty password strings. 1373The default is 1374.Cm no . 1375.It Cm PermitListen 1376Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1377The listen specification must be one of the following forms: 1378.Pp 1379.Bl -item -offset indent -compact 1380.It 1381.Cm PermitListen 1382.Sm off 1383.Ar port 1384.Sm on 1385.It 1386.Cm PermitListen 1387.Sm off 1388.Ar host : port 1389.Sm on 1390.El 1391.Pp 1392Multiple permissions may be specified by separating them with whitespace. 1393An argument of 1394.Cm any 1395can be used to remove all restrictions and permit any listen requests. 1396An argument of 1397.Cm none 1398can be used to prohibit all listen requests. 1399The host name may contain wildcards as described in the PATTERNS section in 1400.Xr ssh_config 5 . 1401The wildcard 1402.Sq * 1403can also be used in place of a port number to allow all ports. 1404By default all port forwarding listen requests are permitted. 1405Note that the 1406.Cm GatewayPorts 1407option may further restrict which addresses may be listened on. 1408Note also that 1409.Xr ssh 1 1410will request a listen host of 1411.Dq localhost 1412if no listen host was specifically requested, and this name is 1413treated differently to explicit localhost addresses of 1414.Dq 127.0.0.1 1415and 1416.Dq ::1 . 1417.It Cm PermitOpen 1418Specifies the destinations to which TCP port forwarding is permitted. 1419The forwarding specification must be one of the following forms: 1420.Pp 1421.Bl -item -offset indent -compact 1422.It 1423.Cm PermitOpen 1424.Sm off 1425.Ar host : port 1426.Sm on 1427.It 1428.Cm PermitOpen 1429.Sm off 1430.Ar IPv4_addr : port 1431.Sm on 1432.It 1433.Cm PermitOpen 1434.Sm off 1435.Ar \&[ IPv6_addr \&] : port 1436.Sm on 1437.El 1438.Pp 1439Multiple forwards may be specified by separating them with whitespace. 1440An argument of 1441.Cm any 1442can be used to remove all restrictions and permit any forwarding requests. 1443An argument of 1444.Cm none 1445can be used to prohibit all forwarding requests. 1446The wildcard 1447.Sq * 1448can be used for host or port to allow all hosts or ports respectively. 1449Otherwise, no pattern matching or address lookups are performed on supplied 1450names. 1451By default all port forwarding requests are permitted. 1452.It Cm PermitRootLogin 1453Specifies whether root can log in using 1454.Xr ssh 1 . 1455The argument must be 1456.Cm yes , 1457.Cm prohibit-password , 1458.Cm forced-commands-only , 1459or 1460.Cm no . 1461The default is 1462.Cm no . 1463Note that if 1464.Cm ChallengeResponseAuthentication 1465and 1466.Cm UsePAM 1467are both 1468.Cm yes , 1469this setting may be overridden by the PAM policy. 1470.Pp 1471If this option is set to 1472.Cm prohibit-password 1473(or its deprecated alias, 1474.Cm without-password ) , 1475password and keyboard-interactive authentication are disabled for root. 1476.Pp 1477If this option is set to 1478.Cm forced-commands-only , 1479root login with public key authentication will be allowed, 1480but only if the 1481.Ar command 1482option has been specified 1483(which may be useful for taking remote backups even if root login is 1484normally not allowed). 1485All other authentication methods are disabled for root. 1486.Pp 1487If this option is set to 1488.Cm no , 1489root is not allowed to log in. 1490.It Cm PermitTTY 1491Specifies whether 1492.Xr pty 4 1493allocation is permitted. 1494The default is 1495.Cm yes . 1496.It Cm PermitTunnel 1497Specifies whether 1498.Xr tun 4 1499device forwarding is allowed. 1500The argument must be 1501.Cm yes , 1502.Cm point-to-point 1503(layer 3), 1504.Cm ethernet 1505(layer 2), or 1506.Cm no . 1507Specifying 1508.Cm yes 1509permits both 1510.Cm point-to-point 1511and 1512.Cm ethernet . 1513The default is 1514.Cm no . 1515.Pp 1516Independent of this setting, the permissions of the selected 1517.Xr tun 4 1518device must allow access to the user. 1519.It Cm PermitUserEnvironment 1520Specifies whether 1521.Pa ~/.ssh/environment 1522and 1523.Cm environment= 1524options in 1525.Pa ~/.ssh/authorized_keys 1526are processed by 1527.Xr sshd 8 . 1528Valid options are 1529.Cm yes , 1530.Cm no 1531or a pattern-list specifying which environment variable names to accept 1532(for example 1533.Qq LANG,LC_* ) . 1534The default is 1535.Cm no . 1536Enabling environment processing may enable users to bypass access 1537restrictions in some configurations using mechanisms such as 1538.Ev LD_PRELOAD . 1539.It Cm PermitUserRC 1540Specifies whether any 1541.Pa ~/.ssh/rc 1542file is executed. 1543The default is 1544.Cm yes . 1545.It Cm PerSourceMaxStartups 1546Specifies the number of unauthenticated connections allowed from a 1547given source address, or 1548.Dq none 1549if there is no limit. 1550This limit is applied in addition to 1551.Cm MaxStartups , 1552whichever is lower. 1553The default is 1554.Cm none . 1555.It Cm PerSourceNetBlockSize 1556Specifies the number of bits of source address that are grouped together 1557for the purposes of applying PerSourceMaxStartups limits. 1558Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1559The default is 1560.Cm 32:128 , 1561which means each address is considered individually. 1562.It Cm PidFile 1563Specifies the file that contains the process ID of the 1564SSH daemon, or 1565.Cm none 1566to not write one. 1567The default is 1568.Pa /var/run/sshd.pid . 1569.It Cm Port 1570Specifies the port number that 1571.Xr sshd 8 1572listens on. 1573The default is 22. 1574Multiple options of this type are permitted. 1575See also 1576.Cm ListenAddress . 1577.It Cm PrintLastLog 1578Specifies whether 1579.Xr sshd 8 1580should print the date and time of the last user login when a user logs 1581in interactively. 1582The default is 1583.Cm yes . 1584.It Cm PrintMotd 1585Specifies whether 1586.Xr sshd 8 1587should print 1588.Pa /etc/motd 1589when a user logs in interactively. 1590(On some systems it is also printed by the shell, 1591.Pa /etc/profile , 1592or equivalent.) 1593The default is 1594.Cm yes . 1595.It Cm PubkeyAcceptedAlgorithms 1596Specifies the signature algorithms that will be accepted for public key 1597authentication as a list of comma-separated patterns. 1598Alternately if the specified list begins with a 1599.Sq + 1600character, then the specified algorithms will be appended to the default set 1601instead of replacing them. 1602If the specified list begins with a 1603.Sq - 1604character, then the specified algorithms (including wildcards) will be removed 1605from the default set instead of replacing them. 1606If the specified list begins with a 1607.Sq ^ 1608character, then the specified algorithms will be placed at the head of the 1609default set. 1610The default for this option is: 1611.Bd -literal -offset 3n 1612ssh-ed25519-cert-v01@openssh.com, 1613ecdsa-sha2-nistp256-cert-v01@openssh.com, 1614ecdsa-sha2-nistp384-cert-v01@openssh.com, 1615ecdsa-sha2-nistp521-cert-v01@openssh.com, 1616sk-ssh-ed25519-cert-v01@openssh.com, 1617sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1618rsa-sha2-512-cert-v01@openssh.com, 1619rsa-sha2-256-cert-v01@openssh.com, 1620ssh-ed25519, 1621ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1622sk-ssh-ed25519@openssh.com, 1623sk-ecdsa-sha2-nistp256@openssh.com, 1624rsa-sha2-512,rsa-sha2-256 1625.Ed 1626.Pp 1627The list of available signature algorithms may also be obtained using 1628.Qq ssh -Q PubkeyAcceptedAlgorithms . 1629.It Cm PubkeyAuthOptions 1630Sets one or more public key authentication options. 1631The supported keywords are: 1632.Cm none 1633(the default; indicating no additional options are enabled), 1634.Cm touch-required 1635and 1636.Cm verify-required . 1637.Pp 1638The 1639.Cm touch-required 1640option causes public key authentication using a FIDO authenticator algorithm 1641(i.e.\& 1642.Cm ecdsa-sk 1643or 1644.Cm ed25519-sk ) 1645to always require the signature to attest that a physically present user 1646explicitly confirmed the authentication (usually by touching the authenticator). 1647By default, 1648.Xr sshd 8 1649requires user presence unless overridden with an authorized_keys option. 1650The 1651.Cm touch-required 1652flag disables this override. 1653.Pp 1654The 1655.Cm verify-required 1656option requires a FIDO key signature attest that the user was verified, 1657e.g. via a PIN. 1658.Pp 1659Neither the 1660.Cm touch-required 1661or 1662.Cm verify-required 1663options have any effect for other, non-FIDO, public key types. 1664.It Cm PubkeyAuthentication 1665Specifies whether public key authentication is allowed. 1666The default is 1667.Cm yes . 1668.It Cm RekeyLimit 1669Specifies the maximum amount of data that may be transmitted or received 1670before the session key is renegotiated, optionally followed by a maximum 1671amount of time that may pass before the session key is renegotiated. 1672The first argument is specified in bytes and may have a suffix of 1673.Sq K , 1674.Sq M , 1675or 1676.Sq G 1677to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1678The default is between 1679.Sq 1G 1680and 1681.Sq 4G , 1682depending on the cipher. 1683The optional second value is specified in seconds and may use any of the 1684units documented in the 1685.Sx TIME FORMATS 1686section. 1687The default value for 1688.Cm RekeyLimit 1689is 1690.Cm default none , 1691which means that rekeying is performed after the cipher's default amount 1692of data has been sent or received and no time based rekeying is done. 1693.It Cm RequiredRSASize 1694Specifies the minimum RSA key size (in bits) that 1695.Xr sshd 8 1696will accept. 1697User and host-based authentication keys smaller than this limit will be 1698refused. 1699The default is 1700.Cm 1024 1701bits. 1702Note that this limit may only be raised from the default. 1703.It Cm RevokedKeys 1704Specifies revoked public keys file, or 1705.Cm none 1706to not use one. 1707Keys listed in this file will be refused for public key authentication. 1708Note that if this file is not readable, then public key authentication will 1709be refused for all users. 1710Keys may be specified as a text file, listing one public key per line, or as 1711an OpenSSH Key Revocation List (KRL) as generated by 1712.Xr ssh-keygen 1 . 1713For more information on KRLs, see the KEY REVOCATION LISTS section in 1714.Xr ssh-keygen 1 . 1715.It Cm RDomain 1716Specifies an explicit routing domain that is applied after authentication 1717has completed. 1718The user session, as well as any forwarded or listening IP sockets, 1719will be bound to this 1720.Xr rdomain 4 . 1721If the routing domain is set to 1722.Cm \&%D , 1723then the domain in which the incoming connection was received will be applied. 1724.It Cm SecurityKeyProvider 1725Specifies a path to a library that will be used when loading 1726FIDO authenticator-hosted keys, overriding the default of using 1727the built-in USB HID support. 1728.It Cm SetEnv 1729Specifies one or more environment variables to set in child sessions started 1730by 1731.Xr sshd 8 1732as 1733.Dq NAME=VALUE . 1734The environment value may be quoted (e.g. if it contains whitespace 1735characters). 1736Environment variables set by 1737.Cm SetEnv 1738override the default environment and any variables specified by the user 1739via 1740.Cm AcceptEnv 1741or 1742.Cm PermitUserEnvironment . 1743.It Cm StreamLocalBindMask 1744Sets the octal file creation mode mask 1745.Pq umask 1746used when creating a Unix-domain socket file for local or remote 1747port forwarding. 1748This option is only used for port forwarding to a Unix-domain socket file. 1749.Pp 1750The default value is 0177, which creates a Unix-domain socket file that is 1751readable and writable only by the owner. 1752Note that not all operating systems honor the file mode on Unix-domain 1753socket files. 1754.It Cm StreamLocalBindUnlink 1755Specifies whether to remove an existing Unix-domain socket file for local 1756or remote port forwarding before creating a new one. 1757If the socket file already exists and 1758.Cm StreamLocalBindUnlink 1759is not enabled, 1760.Nm sshd 1761will be unable to forward the port to the Unix-domain socket file. 1762This option is only used for port forwarding to a Unix-domain socket file. 1763.Pp 1764The argument must be 1765.Cm yes 1766or 1767.Cm no . 1768The default is 1769.Cm no . 1770.It Cm StrictModes 1771Specifies whether 1772.Xr sshd 8 1773should check file modes and ownership of the 1774user's files and home directory before accepting login. 1775This is normally desirable because novices sometimes accidentally leave their 1776directory or files world-writable. 1777The default is 1778.Cm yes . 1779Note that this does not apply to 1780.Cm ChrootDirectory , 1781whose permissions and ownership are checked unconditionally. 1782.It Cm Subsystem 1783Configures an external subsystem (e.g. file transfer daemon). 1784Arguments should be a subsystem name and a command (with optional arguments) 1785to execute upon subsystem request. 1786.Pp 1787The command 1788.Cm sftp-server 1789implements the SFTP file transfer subsystem. 1790.Pp 1791Alternately the name 1792.Cm internal-sftp 1793implements an in-process SFTP server. 1794This may simplify configurations using 1795.Cm ChrootDirectory 1796to force a different filesystem root on clients. 1797.Pp 1798By default no subsystems are defined. 1799.It Cm SyslogFacility 1800Gives the facility code that is used when logging messages from 1801.Xr sshd 8 . 1802The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1803LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1804The default is AUTH. 1805.It Cm TCPKeepAlive 1806Specifies whether the system should send TCP keepalive messages to the 1807other side. 1808If they are sent, death of the connection or crash of one 1809of the machines will be properly noticed. 1810However, this means that 1811connections will die if the route is down temporarily, and some people 1812find it annoying. 1813On the other hand, if TCP keepalives are not sent, 1814sessions may hang indefinitely on the server, leaving 1815.Qq ghost 1816users and consuming server resources. 1817.Pp 1818The default is 1819.Cm yes 1820(to send TCP keepalive messages), and the server will notice 1821if the network goes down or the client host crashes. 1822This avoids infinitely hanging sessions. 1823.Pp 1824To disable TCP keepalive messages, the value should be set to 1825.Cm no . 1826.It Cm TrustedUserCAKeys 1827Specifies a file containing public keys of certificate authorities that are 1828trusted to sign user certificates for authentication, or 1829.Cm none 1830to not use one. 1831Keys are listed one per line; empty lines and comments starting with 1832.Ql # 1833are allowed. 1834If a certificate is presented for authentication and has its signing CA key 1835listed in this file, then it may be used for authentication for any user 1836listed in the certificate's principals list. 1837Note that certificates that lack a list of principals will not be permitted 1838for authentication using 1839.Cm TrustedUserCAKeys . 1840For more details on certificates, see the CERTIFICATES section in 1841.Xr ssh-keygen 1 . 1842.It Cm UnusedConnectionTimeout 1843Specifies whether and how quickly 1844.Xr sshd 8 1845should close client connections with no open channels. 1846Open channels include active shell, command execution or subsystem 1847sessions, connected network, socket, agent or X11 forwardings. 1848Forwarding listeners, such as those from the 1849.Xr ssh 1 1850.Fl R 1851flag, are not considered as open channels and do not prevent the timeout. 1852The timeout value 1853is specified in seconds or may use any of the units documented in the 1854.Sx TIME FORMATS 1855section. 1856.Pp 1857Note that this timeout starts when the client connection completes 1858user authentication but before the client has an opportunity to open any 1859channels. 1860Caution should be used when using short timeout values, as they may not 1861provide sufficient time for the client to request and open its channels 1862before terminating the connection. 1863.Pp 1864The default 1865.Cm none 1866is to never expire connections for having no open channels. 1867This option may be useful in conjunction with 1868.Cm ChannelTimeout . 1869.It Cm UseBlacklist 1870Specifies whether 1871.Xr sshd 8 1872attempts to send authentication success and failure messages 1873to the 1874.Xr blacklistd 8 1875daemon. 1876The default is 1877.Cm no . 1878For forward compatibility with an upcoming 1879.Xr blacklistd 1880rename, the 1881.Cm UseBlocklist 1882alias can be used instead. 1883.It Cm UseDNS 1884Specifies whether 1885.Xr sshd 8 1886should look up the remote host name, and to check that 1887the resolved host name for the remote IP address maps back to the 1888very same IP address. 1889.Pp 1890If this option is set to 1891.Cm no , 1892then only addresses and not host names may be used in 1893.Pa ~/.ssh/authorized_keys 1894.Cm from 1895and 1896.Nm 1897.Cm Match 1898.Cm Host 1899directives. 1900The default is 1901.Dq yes . 1902.It Cm UsePAM 1903Enables the Pluggable Authentication Module interface. 1904If set to 1905.Cm yes 1906this will enable PAM authentication using 1907.Cm KbdInteractiveAuthentication 1908and 1909.Cm PasswordAuthentication 1910in addition to PAM account and session module processing for all 1911authentication types. 1912.Pp 1913Because PAM keyboard-interactive authentication usually serves an equivalent 1914role to password authentication, you should disable either 1915.Cm PasswordAuthentication 1916or 1917.Cm KbdInteractiveAuthentication . 1918.Pp 1919If 1920.Cm UsePAM 1921is enabled, you will not be able to run 1922.Xr sshd 8 1923as a non-root user. 1924The default is 1925.Cm yes . 1926.It Cm VersionAddendum 1927Optionally specifies additional text to append to the SSH protocol banner 1928sent by the server upon connection. 1929The default is 1930.Qq FreeBSD-20240104 . 1931The value 1932.Cm none 1933may be used to disable this. 1934.It Cm X11DisplayOffset 1935Specifies the first display number available for 1936.Xr sshd 8 Ns 's 1937X11 forwarding. 1938This prevents sshd from interfering with real X11 servers. 1939The default is 10. 1940.It Cm X11Forwarding 1941Specifies whether X11 forwarding is permitted. 1942The argument must be 1943.Cm yes 1944or 1945.Cm no . 1946The default is 1947.Cm no . 1948.Pp 1949When X11 forwarding is enabled, there may be additional exposure to 1950the server and to client displays if the 1951.Xr sshd 8 1952proxy display is configured to listen on the wildcard address (see 1953.Cm X11UseLocalhost ) , 1954though this is not the default. 1955Additionally, the authentication spoofing and authentication data 1956verification and substitution occur on the client side. 1957The security risk of using X11 forwarding is that the client's X11 1958display server may be exposed to attack when the SSH client requests 1959forwarding (see the warnings for 1960.Cm ForwardX11 1961in 1962.Xr ssh_config 5 ) . 1963A system administrator may have a stance in which they want to 1964protect clients that may expose themselves to attack by unwittingly 1965requesting X11 forwarding, which can warrant a 1966.Cm no 1967setting. 1968.Pp 1969Note that disabling X11 forwarding does not prevent users from 1970forwarding X11 traffic, as users can always install their own forwarders. 1971.It Cm X11UseLocalhost 1972Specifies whether 1973.Xr sshd 8 1974should bind the X11 forwarding server to the loopback address or to 1975the wildcard address. 1976By default, 1977sshd binds the forwarding server to the loopback address and sets the 1978hostname part of the 1979.Ev DISPLAY 1980environment variable to 1981.Cm localhost . 1982This prevents remote hosts from connecting to the proxy display. 1983However, some older X11 clients may not function with this 1984configuration. 1985.Cm X11UseLocalhost 1986may be set to 1987.Cm no 1988to specify that the forwarding server should be bound to the wildcard 1989address. 1990The argument must be 1991.Cm yes 1992or 1993.Cm no . 1994The default is 1995.Cm yes . 1996.It Cm XAuthLocation 1997Specifies the full pathname of the 1998.Xr xauth 1 1999program, or 2000.Cm none 2001to not use one. 2002The default is 2003.Pa /usr/local/bin/xauth . 2004.El 2005.Sh TIME FORMATS 2006.Xr sshd 8 2007command-line arguments and configuration file options that specify time 2008may be expressed using a sequence of the form: 2009.Sm off 2010.Ar time Op Ar qualifier , 2011.Sm on 2012where 2013.Ar time 2014is a positive integer value and 2015.Ar qualifier 2016is one of the following: 2017.Pp 2018.Bl -tag -width Ds -compact -offset indent 2019.It Aq Cm none 2020seconds 2021.It Cm s | Cm S 2022seconds 2023.It Cm m | Cm M 2024minutes 2025.It Cm h | Cm H 2026hours 2027.It Cm d | Cm D 2028days 2029.It Cm w | Cm W 2030weeks 2031.El 2032.Pp 2033Each member of the sequence is added together to calculate 2034the total time value. 2035.Pp 2036Time format examples: 2037.Pp 2038.Bl -tag -width Ds -compact -offset indent 2039.It 600 2040600 seconds (10 minutes) 2041.It 10m 204210 minutes 2043.It 1h30m 20441 hour 30 minutes (90 minutes) 2045.El 2046.Sh TOKENS 2047Arguments to some keywords can make use of tokens, 2048which are expanded at runtime: 2049.Pp 2050.Bl -tag -width XXXX -offset indent -compact 2051.It %% 2052A literal 2053.Sq % . 2054.It \&%C 2055Identifies the connection endpoints, containing 2056four space-separated values: client address, client port number, 2057server address, and server port number. 2058.It \&%D 2059The routing domain in which the incoming connection was received. 2060.It %F 2061The fingerprint of the CA key. 2062.It %f 2063The fingerprint of the key or certificate. 2064.It %h 2065The home directory of the user. 2066.It %i 2067The key ID in the certificate. 2068.It %K 2069The base64-encoded CA key. 2070.It %k 2071The base64-encoded key or certificate for authentication. 2072.It %s 2073The serial number of the certificate. 2074.It \&%T 2075The type of the CA key. 2076.It %t 2077The key or certificate type. 2078.It \&%U 2079The numeric user ID of the target user. 2080.It %u 2081The username. 2082.El 2083.Pp 2084.Cm AuthorizedKeysCommand 2085accepts the tokens %%, %C, %D, %f, %h, %k, %t, %U, and %u. 2086.Pp 2087.Cm AuthorizedKeysFile 2088accepts the tokens %%, %h, %U, and %u. 2089.Pp 2090.Cm AuthorizedPrincipalsCommand 2091accepts the tokens %%, %C, %D, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 2092.Pp 2093.Cm AuthorizedPrincipalsFile 2094accepts the tokens %%, %h, %U, and %u. 2095.Pp 2096.Cm ChrootDirectory 2097accepts the tokens %%, %h, %U, and %u. 2098.Pp 2099.Cm RoutingDomain 2100accepts the token %D. 2101.Sh FILES 2102.Bl -tag -width Ds 2103.It Pa /etc/ssh/sshd_config 2104Contains configuration data for 2105.Xr sshd 8 . 2106This file should be writable by root only, but it is recommended 2107(though not necessary) that it be world-readable. 2108.El 2109.Sh SEE ALSO 2110.Xr sftp-server 8 , 2111.Xr sshd 8 2112.Sh AUTHORS 2113.An -nosplit 2114OpenSSH is a derivative of the original and free 2115ssh 1.2.12 release by 2116.An Tatu Ylonen . 2117.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 2118.An Theo de Raadt 2119and 2120.An Dug Song 2121removed many bugs, re-added newer features and 2122created OpenSSH. 2123.An Markus Friedl 2124contributed the support for SSH protocol versions 1.5 and 2.0. 2125.An Niels Provos 2126and 2127.An Markus Friedl 2128contributed support for privilege separation. 2129