1.\" $NetBSD: sshd_config.5,v 1.43 2024/06/25 16:36:54 christos Exp $ 2.\" 3.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 4.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 5.\" All rights reserved 6.\" 7.\" As far as I am concerned, the code I have written for this software 8.\" can be used freely for any purpose. Any derived versions of this 9.\" software must be clearly marked as such, and if the derived work is 10.\" incompatible with the protocol description in the RFC file, it must be 11.\" called by a name other than "ssh" or "Secure Shell". 12.\" 13.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 14.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 15.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 16.\" 17.\" Redistribution and use in source and binary forms, with or without 18.\" modification, are permitted provided that the following conditions 19.\" are met: 20.\" 1. Redistributions of source code must retain the above copyright 21.\" notice, this list of conditions and the following disclaimer. 22.\" 2. Redistributions in binary form must reproduce the above copyright 23.\" notice, this list of conditions and the following disclaimer in the 24.\" documentation and/or other materials provided with the distribution. 25.\" 26.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 27.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 28.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 29.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 30.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 31.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 32.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 33.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 34.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 35.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 36.\" 37.\" $OpenBSD: sshd_config.5,v 1.355 2024/02/21 06:17:29 djm Exp $ 38.Dd February 21 2024 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. 52Unless noted otherwise, for 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. 124This keyword may appear multiple times in 125.Nm 126with each instance appending to the list. 127.It Cm AllowStreamLocalForwarding 128Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 129The available options are 130.Cm yes 131(the default) 132or 133.Cm all 134to allow StreamLocal forwarding, 135.Cm no 136to prevent all StreamLocal forwarding, 137.Cm local 138to allow local (from the perspective of 139.Xr ssh 1 ) 140forwarding only or 141.Cm remote 142to allow remote forwarding only. 143Note that disabling StreamLocal forwarding does not improve security unless 144users are also denied shell access, as they can always install their 145own forwarders. 146.It Cm AllowTcpForwarding 147Specifies whether TCP forwarding is permitted. 148The available options are 149.Cm yes 150(the default) 151or 152.Cm all 153to allow TCP forwarding, 154.Cm no 155to prevent all TCP forwarding, 156.Cm local 157to allow local (from the perspective of 158.Xr ssh 1 ) 159forwarding only or 160.Cm remote 161to allow remote forwarding only. 162Note that disabling TCP forwarding does not improve security unless 163users are also denied shell access, as they can always install their 164own forwarders. 165.It Cm AllowUsers 166This keyword can be followed by a list of user name patterns, separated 167by spaces. 168If specified, login is allowed only for user names that 169match one of the patterns. 170Only user names are valid; a numerical user ID is not recognized. 171By default, login is allowed for all users. 172If the pattern takes the form USER@HOST then USER and HOST 173are separately checked, restricting logins to particular 174users from particular hosts. 175HOST criteria may additionally contain addresses to match in CIDR 176address/masklen format. 177The allow/deny users directives are processed in the following order: 178.Cm DenyUsers , 179.Cm AllowUsers . 180.Pp 181See PATTERNS in 182.Xr ssh_config 5 183for more information on patterns. 184This keyword may appear multiple times in 185.Nm 186with each instance appending to the list. 187.It Cm AuthenticationMethods 188Specifies the authentication methods that must be successfully completed 189for a user to be granted access. 190This option must be followed by one or more lists of comma-separated 191authentication method names, or by the single string 192.Cm any 193to indicate the default behaviour of accepting any single authentication 194method. 195If the default is overridden, then successful authentication requires 196completion of every method in at least one of these lists. 197.Pp 198For example, 199.Qq publickey,password publickey,keyboard-interactive 200would require the user to complete public key authentication, followed by 201either password or keyboard interactive authentication. 202Only methods that are next in one or more lists are offered at each stage, 203so for this example it would not be possible to attempt password or 204keyboard-interactive authentication before public key. 205.Pp 206For keyboard interactive authentication it is also possible to 207restrict authentication to a specific device by appending a 208colon followed by the device identifier 209.Cm bsdauth , 210.Cm pam , 211or 212.Cm skey , 213depending on the server configuration. 214For example, 215.Qq keyboard-interactive:bsdauth 216would restrict keyboard interactive authentication to the 217.Cm bsdauth 218device. 219.Pp 220If the publickey method is listed more than once, 221.Xr sshd 8 222verifies that keys that have been used successfully are not reused for 223subsequent authentications. 224For example, 225.Qq publickey,publickey 226requires successful authentication using two different public keys. 227.Pp 228Note that each authentication method listed should also be explicitly enabled 229in the configuration. 230.Pp 231The available authentication methods are: 232.Qq gssapi-with-mic , 233.Qq hostbased , 234.Qq keyboard-interactive , 235.Qq none 236(used for access to password-less accounts when 237.Cm PermitEmptyPasswords 238is enabled), 239.Qq password 240and 241.Qq publickey . 242.It Cm AuthorizedKeysCommand 243Specifies a program to be used to look up the user's public keys. 244The program must be owned by root, not writable by group or others and 245specified by an absolute path. 246Arguments to 247.Cm AuthorizedKeysCommand 248accept the tokens described in the 249.Sx TOKENS 250section. 251If no arguments are specified then the username of the target user is used. 252.Pp 253The program should produce on standard output zero or 254more lines of authorized_keys output (see 255.Sx AUTHORIZED_KEYS 256in 257.Xr sshd 8 ) . 258.Cm AuthorizedKeysCommand 259is tried after the usual 260.Cm AuthorizedKeysFile 261files and will not be executed if a matching key is found there. 262By default, no 263.Cm AuthorizedKeysCommand 264is run. 265.It Cm AuthorizedKeysCommandUser 266Specifies the user under whose account the 267.Cm AuthorizedKeysCommand 268is run. 269It is recommended to use a dedicated user that has no other role on the host 270than running authorized keys commands. 271If 272.Cm AuthorizedKeysCommand 273is specified but 274.Cm AuthorizedKeysCommandUser 275is not, then 276.Xr sshd 8 277will refuse to start. 278.It Cm AuthorizedKeysFile 279Specifies the file that contains the public keys used for user authentication. 280The format is described in the AUTHORIZED_KEYS FILE FORMAT section of 281.Xr sshd 8 . 282Arguments to 283.Cm AuthorizedKeysFile 284accept the tokens described in the 285.Sx TOKENS 286section. 287After expansion, 288.Cm AuthorizedKeysFile 289is taken to be an absolute path or one relative to the user's home 290directory. 291Multiple files may be listed, separated by whitespace. 292Alternately this option may be set to 293.Cm none 294to skip checking for user keys in files. 295The default is 296.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 297.It Cm AuthorizedPrincipalsCommand 298Specifies a program to be used to generate the list of allowed 299certificate principals as per 300.Cm AuthorizedPrincipalsFile . 301The program must be owned by root, not writable by group or others and 302specified by an absolute path. 303Arguments to 304.Cm AuthorizedPrincipalsCommand 305accept the tokens described in the 306.Sx TOKENS 307section. 308If no arguments are specified then the username of the target user is used. 309.Pp 310The program should produce on standard output zero or 311more lines of 312.Cm AuthorizedPrincipalsFile 313output. 314If either 315.Cm AuthorizedPrincipalsCommand 316or 317.Cm AuthorizedPrincipalsFile 318is specified, then certificates offered by the client for authentication 319must contain a principal that is listed. 320By default, no 321.Cm AuthorizedPrincipalsCommand 322is run. 323.It Cm AuthorizedPrincipalsCommandUser 324Specifies the user under whose account the 325.Cm AuthorizedPrincipalsCommand 326is run. 327It is recommended to use a dedicated user that has no other role on the host 328than running authorized principals commands. 329If 330.Cm AuthorizedPrincipalsCommand 331is specified but 332.Cm AuthorizedPrincipalsCommandUser 333is not, then 334.Xr sshd 8 335will refuse to start. 336.It Cm AuthorizedPrincipalsFile 337Specifies a file that lists principal names that are accepted for 338certificate authentication. 339When using certificates signed by a key listed in 340.Cm TrustedUserCAKeys , 341this file lists names, one of which must appear in the certificate for it 342to be accepted for authentication. 343Names are listed one per line preceded by key options (as described in 344.Sx AUTHORIZED_KEYS FILE FORMAT 345in 346.Xr sshd 8 ) . 347Empty lines and comments starting with 348.Ql # 349are ignored. 350.Pp 351Arguments to 352.Cm AuthorizedPrincipalsFile 353accept the tokens described in the 354.Sx TOKENS 355section. 356After expansion, 357.Cm AuthorizedPrincipalsFile 358is taken to be an absolute path or one relative to the user's home directory. 359The default is 360.Cm none , 361i.e. not to use a principals file \(en in this case, the username 362of the user must appear in a certificate's principals list for it to be 363accepted. 364.Pp 365Note that 366.Cm AuthorizedPrincipalsFile 367is only used when authentication proceeds using a CA listed in 368.Cm TrustedUserCAKeys 369and is not consulted for certification authorities trusted via 370.Pa ~/.ssh/authorized_keys , 371though the 372.Cm principals= 373key option offers a similar facility (see 374.Xr sshd 8 375for details). 376.It Cm Banner 377The contents of the specified file are sent to the remote user before 378authentication is allowed. 379If the argument is 380.Cm none 381then no banner is displayed. 382By default, no banner is displayed. 383.It Cm CASignatureAlgorithms 384Specifies which algorithms are allowed for signing of certificates 385by certificate authorities (CAs). 386The default is: 387.Bd -literal -offset indent 388ssh-ed25519,ecdsa-sha2-nistp256, 389ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 390sk-ssh-ed25519@openssh.com, 391sk-ecdsa-sha2-nistp256@openssh.com, 392rsa-sha2-512,rsa-sha2-256 393.Ed 394.Pp 395If the specified list begins with a 396.Sq + 397character, then the specified algorithms will be appended to the default set 398instead of replacing them. 399If the specified list begins with a 400.Sq - 401character, then the specified algorithms (including wildcards) will be removed 402from the default set instead of replacing them. 403.Pp 404Certificates signed using other algorithms will not be accepted for 405public key or host-based authentication. 406.It Cm ChannelTimeout 407Specifies whether and how quickly 408.Xr sshd 8 409should close inactive channels. 410Timeouts are specified as one or more 411.Dq type=interval 412pairs separated by whitespace, where the 413.Dq type 414must be the special keyword 415.Dq global 416or a channel type name from the list below, optionally containing 417wildcard characters. 418.Pp 419The timeout value 420.Dq interval 421is specified in seconds or may use any of the units documented in the 422.Sx TIME FORMATS 423section. 424For example, 425.Dq session=5m 426would cause interactive sessions to terminate after five minutes of 427inactivity. 428Specifying a zero value disables the inactivity timeout. 429.Pp 430The special timeout 431.Dq global 432applies to all active channels, taken together. 433Traffic on any active channel will reset the timeout, but when the timeout 434expires then all open channels will be closed. 435Note that this global timeout is not matched by wildcards and must be 436specified explicitly. 437.Pp 438The available channel type names include: 439.Bl -tag -width Ds 440.It Cm agent-connection 441Open connections to 442.Xr ssh-agent 1 . 443.It Cm direct-tcpip , Cm direct-streamlocal@openssh.com 444Open TCP or Unix socket (respectively) connections that have 445been established from a 446.Xr ssh 1 447local forwarding, i.e.\& 448.Cm LocalForward 449or 450.Cm DynamicForward . 451.It Cm forwarded-tcpip , Cm forwarded-streamlocal@openssh.com 452Open TCP or Unix socket (respectively) connections that have been 453established to a 454.Xr sshd 8 455listening on behalf of a 456.Xr ssh 1 457remote forwarding, i.e.\& 458.Cm RemoteForward . 459.It Cm session 460The interactive main session, including shell session, command execution, 461.Xr scp 1 , 462.Xr sftp 1 , 463etc. 464.It Cm tun-connection 465Open 466.Cm TunnelForward 467connections. 468.It Cm x11-connection 469Open X11 forwarding sessions. 470.El 471.Pp 472Note that in all the above cases, terminating an inactive session does not 473guarantee to remove all resources associated with the session, e.g. shell 474processes or X11 clients relating to the session may continue to execute. 475.Pp 476Moreover, terminating an inactive channel or session does not necessarily 477close the SSH connection, nor does it prevent a client from 478requesting another channel of the same type. 479In particular, expiring an inactive forwarding session does not prevent 480another identical forwarding from being subsequently created. 481.Pp 482The default is not to expire channels of any type for inactivity. 483.It Cm ChrootDirectory 484Specifies the pathname of a directory to 485.Xr chroot 2 486to after authentication. 487At session startup 488.Xr sshd 8 489checks that all components of the pathname are root-owned directories 490which are not writable by group or others. 491After the chroot, 492.Xr sshd 8 493changes the working directory to the user's home directory. 494Arguments to 495.Cm ChrootDirectory 496accept the tokens described in the 497.Sx TOKENS 498section. 499.Pp 500The 501.Cm ChrootDirectory 502must contain the necessary files and directories to support the 503user's session. 504For an interactive session this requires at least a shell, typically 505.Xr sh 1 , 506and basic 507.Pa /dev 508nodes such as 509.Xr null 4 , 510.Xr zero 4 , 511.Xr stdin 4 , 512.Xr stdout 4 , 513.Xr stderr 4 , 514and 515.Xr tty 4 516devices. 517For file transfer sessions using SFTP 518no additional configuration of the environment is necessary if the in-process 519sftp-server is used, 520though sessions which use logging may require 521.Pa /dev/log 522inside the chroot directory on some operating systems (see 523.Xr sftp-server 8 524for details). 525.Pp 526For safety, it is very important that the directory hierarchy be 527prevented from modification by other processes on the system (especially 528those outside the jail). 529Misconfiguration can lead to unsafe environments which 530.Xr sshd 8 531cannot detect. 532.Pp 533The default is 534.Cm none , 535indicating not to 536.Xr chroot 2 . 537.It Cm Ciphers 538Specifies the ciphers allowed. 539Multiple ciphers must be comma-separated. 540If the specified list begins with a 541.Sq + 542character, then the specified ciphers will be appended to the default set 543instead of replacing them. 544If the specified list begins with a 545.Sq - 546character, then the specified ciphers (including wildcards) will be removed 547from the default set instead of replacing them. 548If the specified list begins with a 549.Sq ^ 550character, then the specified ciphers will be placed at the head of the 551default set. 552.Pp 553The supported ciphers are: 554.Pp 555.Bl -item -compact -offset indent 556.It 5573des-cbc 558.It 559aes128-cbc 560.It 561aes192-cbc 562.It 563aes256-cbc 564.It 565aes128-ctr 566.It 567aes192-ctr 568.It 569aes256-ctr 570.It 571aes128-gcm@openssh.com 572.It 573aes256-gcm@openssh.com 574.It 575chacha20-poly1305@openssh.com 576.El 577.Pp 578The default is: 579.Bd -literal -offset indent 580chacha20-poly1305@openssh.com, 581aes128-ctr,aes192-ctr,aes256-ctr, 582aes128-gcm@openssh.com,aes256-gcm@openssh.com 583.Ed 584.Pp 585The list of available ciphers may also be obtained using 586.Qq ssh -Q cipher . 587.It Cm ClientAliveCountMax 588Sets the number of client alive messages which may be sent without 589.Xr sshd 8 590receiving any messages back from the client. 591If this threshold is reached while client alive messages are being sent, 592sshd will disconnect the client, terminating the session. 593It is important to note that the use of client alive messages is very 594different from 595.Cm TCPKeepAlive . 596The client alive messages are sent through the encrypted channel 597and therefore will not be spoofable. 598The TCP keepalive option enabled by 599.Cm TCPKeepAlive 600is spoofable. 601The client alive mechanism is valuable when the client or 602server depend on knowing when a connection has become unresponsive. 603.Pp 604The default value is 3. 605If 606.Cm ClientAliveInterval 607is set to 15, and 608.Cm ClientAliveCountMax 609is left at the default, unresponsive SSH clients 610will be disconnected after approximately 45 seconds. 611Setting a zero 612.Cm ClientAliveCountMax 613disables connection termination. 614.It Cm ClientAliveInterval 615Sets a timeout interval in seconds after which if no data has been received 616from the client, 617.Xr sshd 8 618will send a message through the encrypted 619channel to request a response from the client. 620The default 621is 0, indicating that these messages will not be sent to the client. 622.It Cm Compression 623Specifies whether compression is enabled after 624the user has authenticated successfully. 625The argument must be 626.Cm yes , 627.Cm delayed 628(a legacy synonym for 629.Cm yes ) 630or 631.Cm no . 632The default is 633.Cm yes . 634.It Cm DenyGroups 635This keyword can be followed by a list of group name patterns, separated 636by spaces. 637Login is disallowed for users whose primary group or supplementary 638group list matches one of the patterns. 639Only group names are valid; a numerical group ID is not recognized. 640By default, login is allowed for all groups. 641The allow/deny groups directives are processed in the following order: 642.Cm DenyGroups , 643.Cm AllowGroups . 644.Pp 645See PATTERNS in 646.Xr ssh_config 5 647for more information on patterns. 648This keyword may appear multiple times in 649.Nm 650with each instance appending to the list. 651.It Cm DenyUsers 652This keyword can be followed by a list of user name patterns, separated 653by spaces. 654Login is disallowed for user names that match one of the patterns. 655Only user names are valid; a numerical user ID is not recognized. 656By default, login is allowed for all users. 657If the pattern takes the form USER@HOST then USER and HOST 658are separately checked, restricting logins to particular 659users from particular hosts. 660HOST criteria may additionally contain addresses to match in CIDR 661address/masklen format. 662The allow/deny users directives are processed in the following order: 663.Cm DenyUsers , 664.Cm AllowUsers . 665.Pp 666See PATTERNS in 667.Xr ssh_config 5 668for more information on patterns. 669This keyword may appear multiple times in 670.Nm 671with each instance appending to the list. 672.It Cm DisableForwarding 673Disables all forwarding features, including X11, 674.Xr ssh-agent 1 , 675TCP and StreamLocal. 676This option overrides all other forwarding-related options and may 677simplify restricted configurations. 678.It Cm ExposeAuthInfo 679Writes a temporary file containing a list of authentication methods and 680public credentials (e.g. keys) used to authenticate the user. 681The location of the file is exposed to the user session through the 682.Ev SSH_USER_AUTH 683environment variable. 684The default is 685.Cm no . 686.It Cm FingerprintHash 687Specifies the hash algorithm used when logging key fingerprints. 688Valid options are: 689.Cm md5 690and 691.Cm sha256 . 692The default is 693.Cm sha256 . 694.It Cm ForceCommand 695Forces the execution of the command specified by 696.Cm ForceCommand , 697ignoring any command supplied by the client and 698.Pa ~/.ssh/rc 699if present. 700The command is invoked by using the user's login shell with the -c option. 701This applies to shell, command, or subsystem execution. 702It is most useful inside a 703.Cm Match 704block. 705The command originally supplied by the client is available in the 706.Ev SSH_ORIGINAL_COMMAND 707environment variable. 708Specifying a command of 709.Cm internal-sftp 710will force the use of an in-process SFTP server that requires no support 711files when used with 712.Cm ChrootDirectory . 713The default is 714.Cm none . 715.It Cm GatewayPorts 716Specifies whether remote hosts are allowed to connect to ports 717forwarded for the client. 718By default, 719.Xr sshd 8 720binds remote port forwardings to the loopback address. 721This prevents other remote hosts from connecting to forwarded ports. 722.Cm GatewayPorts 723can be used to specify that sshd 724should allow remote port forwardings to bind to non-loopback addresses, thus 725allowing other hosts to connect. 726The argument may be 727.Cm no 728to force remote port forwardings to be available to the local host only, 729.Cm yes 730to force remote port forwardings to bind to the wildcard address, or 731.Cm clientspecified 732to allow the client to select the address to which the forwarding is bound. 733The default is 734.Cm no . 735.It Cm GSSAPIAuthentication 736Specifies whether user authentication based on GSSAPI is allowed. 737The default is 738.Cm no . 739.It Cm GSSAPICleanupCredentials 740Specifies whether to automatically destroy the user's credentials cache 741on logout. 742The default is 743.Cm yes . 744.It Cm GSSAPIStrictAcceptorCheck 745Determines whether to be strict about the identity of the GSSAPI acceptor 746a client authenticates against. 747If set to 748.Cm yes 749then the client must authenticate against the host 750service on the current hostname. 751If set to 752.Cm no 753then the client may authenticate against any service key stored in the 754machine's default store. 755This facility is provided to assist with operation on multi homed machines. 756The default is 757.Cm yes . 758.It Cm HostbasedAcceptedAlgorithms 759Specifies the signature algorithms that will be accepted for hostbased 760authentication as a list of comma-separated patterns. 761Alternately if the specified list begins with a 762.Sq + 763character, then the specified signature algorithms will be appended to 764the default set instead of replacing them. 765If the specified list begins with a 766.Sq - 767character, then the specified signature algorithms (including wildcards) 768will be removed from the default set instead of replacing them. 769If the specified list begins with a 770.Sq ^ 771character, then the specified signature algorithms will be placed at 772the head of the default set. 773The default for this option is: 774.Bd -literal -offset 3n 775ssh-ed25519-cert-v01@openssh.com, 776ecdsa-sha2-nistp256-cert-v01@openssh.com, 777ecdsa-sha2-nistp384-cert-v01@openssh.com, 778ecdsa-sha2-nistp521-cert-v01@openssh.com, 779sk-ssh-ed25519-cert-v01@openssh.com, 780sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 781rsa-sha2-512-cert-v01@openssh.com, 782rsa-sha2-256-cert-v01@openssh.com, 783ssh-ed25519, 784ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 785sk-ssh-ed25519@openssh.com, 786sk-ecdsa-sha2-nistp256@openssh.com, 787rsa-sha2-512,rsa-sha2-256 788.Ed 789.Pp 790The list of available signature algorithms may also be obtained using 791.Qq ssh -Q HostbasedAcceptedAlgorithms . 792This was formerly named HostbasedAcceptedKeyTypes. 793.It Cm HostbasedAuthentication 794Specifies whether rhosts or /etc/hosts.equiv authentication together 795with successful public key client host authentication is allowed 796(host-based authentication). 797The default is 798.Cm no . 799.It Cm HostbasedUsesNameFromPacketOnly 800Specifies whether or not the server will attempt to perform a reverse 801name lookup when matching the name in the 802.Pa ~/.shosts , 803.Pa ~/.rhosts , 804and 805.Pa /etc/hosts.equiv 806files during 807.Cm HostbasedAuthentication . 808A setting of 809.Cm yes 810means that 811.Xr sshd 8 812uses the name supplied by the client rather than 813attempting to resolve the name from the TCP connection itself. 814The default is 815.Cm no . 816.It Cm HostCertificate 817Specifies a file containing a public host certificate. 818The certificate's public key must match a private host key already specified 819by 820.Cm HostKey . 821The default behaviour of 822.Xr sshd 8 823is not to load any certificates. 824.It Cm HostKey 825Specifies a file containing a private host key 826used by SSH. 827The defaults are 828.Pa /etc/ssh/ssh_host_ecdsa_key , 829.Pa /etc/ssh/ssh_host_ed25519_key 830and 831.Pa /etc/ssh/ssh_host_rsa_key . 832.Pp 833Note that 834.Xr sshd 8 835will refuse to use a file if it is group/world-accessible 836and that the 837.Cm HostKeyAlgorithms 838option restricts which of the keys are actually used by 839.Xr sshd 8 . 840.Pp 841It is possible to have multiple host key files. 842It is also possible to specify public host key files instead. 843In this case operations on the private key will be delegated 844to an 845.Xr ssh-agent 1 . 846.It Cm HostKeyAgent 847Identifies the UNIX-domain socket used to communicate 848with an agent that has access to the private host keys. 849If the string 850.Qq SSH_AUTH_SOCK 851is specified, the location of the socket will be read from the 852.Ev SSH_AUTH_SOCK 853environment variable. 854.It Cm HostKeyAlgorithms 855Specifies the host key signature algorithms 856that the server offers. 857The default for this option is: 858.Bd -literal -offset 3n 859ssh-ed25519-cert-v01@openssh.com, 860ecdsa-sha2-nistp256-cert-v01@openssh.com, 861ecdsa-sha2-nistp384-cert-v01@openssh.com, 862ecdsa-sha2-nistp521-cert-v01@openssh.com, 863sk-ssh-ed25519-cert-v01@openssh.com, 864sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 865rsa-sha2-512-cert-v01@openssh.com, 866rsa-sha2-256-cert-v01@openssh.com, 867ssh-ed25519, 868ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 869sk-ssh-ed25519@openssh.com, 870sk-ecdsa-sha2-nistp256@openssh.com, 871rsa-sha2-512,rsa-sha2-256 872.Ed 873.Pp 874The list of available signature algorithms may also be obtained using 875.Qq ssh -Q HostKeyAlgorithms . 876.It Cm IgnoreRhosts 877Specifies whether to ignore per-user 878.Pa .rhosts 879and 880.Pa .shosts 881files during 882.Cm HostbasedAuthentication . 883The system-wide 884.Pa /etc/hosts.equiv 885and 886.Pa /etc/shosts.equiv 887are still used regardless of this setting. 888.Pp 889Accepted values are 890.Cm yes 891(the default) to ignore all per-user files, 892.Cm shosts-only 893to allow the use of 894.Pa .shosts 895but to ignore 896.Pa .rhosts 897or 898.Cm no 899to allow both 900.Pa .shosts 901and 902.Pa rhosts . 903.It Cm IgnoreUserKnownHosts 904Specifies whether 905.Xr sshd 8 906should ignore the user's 907.Pa ~/.ssh/known_hosts 908during 909.Cm HostbasedAuthentication 910and use only the system-wide known hosts file 911.Pa /etc/ssh/ssh_known_hosts . 912The default is 913.Dq no . 914.It Cm Include 915Include the specified configuration file(s). 916Multiple pathnames may be specified and each pathname may contain 917.Xr glob 7 918wildcards that will be expanded and processed in lexical order. 919Files without absolute paths are assumed to be in 920.Pa /etc/ssh . 921An 922.Cm Include 923directive may appear inside a 924.Cm Match 925block 926to perform conditional inclusion. 927.It Cm IPQoS 928Specifies the IPv4 type-of-service or DSCP class for the connection. 929Accepted values are 930.Cm af11 , 931.Cm af12 , 932.Cm af13 , 933.Cm af21 , 934.Cm af22 , 935.Cm af23 , 936.Cm af31 , 937.Cm af32 , 938.Cm af33 , 939.Cm af41 , 940.Cm af42 , 941.Cm af43 , 942.Cm cs0 , 943.Cm cs1 , 944.Cm cs2 , 945.Cm cs3 , 946.Cm cs4 , 947.Cm cs5 , 948.Cm cs6 , 949.Cm cs7 , 950.Cm ef , 951.Cm le , 952.Cm lowdelay , 953.Cm throughput , 954.Cm reliability , 955a numeric value, or 956.Cm none 957to use the operating system default. 958This option may take one or two arguments, separated by whitespace. 959If one argument is specified, it is used as the packet class unconditionally. 960If two values are specified, the first is automatically selected for 961interactive sessions and the second for non-interactive sessions. 962The default is 963.Cm af21 964(Low-Latency Data) 965for interactive sessions and 966.Cm cs1 967(Lower Effort) 968for non-interactive sessions. 969.It Cm KbdInteractiveAuthentication 970Specifies whether to allow keyboard-interactive authentication. 971All authentication styles from 972.Xr login.conf 5 973are supported. 974The default is 975.Cm yes . 976The argument to this keyword must be 977.Cm yes 978or 979.Cm no . 980.Cm ChallengeResponseAuthentication 981is a deprecated alias for this. 982.It Cm KerberosAuthentication 983Specifies whether the password provided by the user for 984.Cm PasswordAuthentication 985will be validated through the Kerberos KDC. 986To use this option, the server needs a 987Kerberos servtab which allows the verification of the KDC's identity. 988The default is 989.Cm no . 990.It Cm KerberosGetAFSToken 991If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 992an AFS token before accessing the user's home directory. 993The default is 994.Cm no . 995.It Cm KerberosOrLocalPasswd 996If password authentication through Kerberos fails then 997the password will be validated via any additional local mechanism 998such as 999.Pa /etc/passwd . 1000The default is 1001.Cm yes . 1002.It Cm KerberosTicketCleanup 1003Specifies whether to automatically destroy the user's ticket cache 1004file on logout. 1005The default is 1006.Cm yes . 1007.It Cm KexAlgorithms 1008Specifies the available KEX (Key Exchange) algorithms. 1009Multiple algorithms must be comma-separated. 1010Alternately if the specified list begins with a 1011.Sq + 1012character, then the specified algorithms will be appended to the default set 1013instead of replacing them. 1014If the specified list begins with a 1015.Sq - 1016character, then the specified algorithms (including wildcards) will be removed 1017from the default set instead of replacing them. 1018If the specified list begins with a 1019.Sq ^ 1020character, then the specified algorithms will be placed at the head of the 1021default set. 1022The supported algorithms are: 1023.Pp 1024.Bl -item -compact -offset indent 1025.It 1026curve25519-sha256 1027.It 1028curve25519-sha256@libssh.org 1029.It 1030diffie-hellman-group1-sha1 1031.It 1032diffie-hellman-group14-sha1 1033.It 1034diffie-hellman-group14-sha256 1035.It 1036diffie-hellman-group16-sha512 1037.It 1038diffie-hellman-group18-sha512 1039.It 1040diffie-hellman-group-exchange-sha1 1041.It 1042diffie-hellman-group-exchange-sha256 1043.It 1044ecdh-sha2-nistp256 1045.It 1046ecdh-sha2-nistp384 1047.It 1048ecdh-sha2-nistp521 1049.It 1050sntrup761x25519-sha512@openssh.com 1051.El 1052.Pp 1053The default is: 1054.Bd -literal -offset indent 1055sntrup761x25519-sha512@openssh.com, 1056curve25519-sha256,curve25519-sha256@libssh.org, 1057ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1058diffie-hellman-group-exchange-sha256, 1059diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 1060diffie-hellman-group14-sha256 1061.Ed 1062.Pp 1063The list of available key exchange algorithms may also be obtained using 1064.Qq ssh -Q KexAlgorithms . 1065.It Cm ListenAddress 1066Specifies the local addresses 1067.Xr sshd 8 1068should listen on. 1069The following forms may be used: 1070.Pp 1071.Bl -item -offset indent -compact 1072.It 1073.Cm ListenAddress 1074.Sm off 1075.Ar hostname | address 1076.Sm on 1077.Op Cm rdomain Ar domain 1078.It 1079.Cm ListenAddress 1080.Sm off 1081.Ar hostname : port 1082.Sm on 1083.Op Cm rdomain Ar domain 1084.It 1085.Cm ListenAddress 1086.Sm off 1087.Ar IPv4_address : port 1088.Sm on 1089.Op Cm rdomain Ar domain 1090.It 1091.Cm ListenAddress 1092.Sm off 1093.Oo Ar hostname | address Oc : Ar port 1094.Sm on 1095.Op Cm rdomain Ar domain 1096.El 1097.Pp 1098The optional 1099.Cm rdomain 1100qualifier requests 1101.Xr sshd 8 1102listen in an explicit routing domain. 1103If 1104.Ar port 1105is not specified, 1106sshd will listen on the address and all 1107.Cm Port 1108options specified. 1109The default is to listen on all local addresses on the current default 1110routing domain. 1111Multiple 1112.Cm ListenAddress 1113options are permitted. 1114For more information on routing domains, see 1115.Xr rdomain 4 . 1116.It Cm LoginGraceTime 1117The server disconnects after this time if the user has not 1118successfully logged in. 1119If the value is 0, there is no time limit. 1120The default is 120 seconds but the default 1121.Pa /etc/ssh/sshd_config 1122overwrites it to 600 seconds. 1123.It Cm LogLevel 1124Gives the verbosity level that is used when logging messages from 1125.Xr sshd 8 . 1126The possible values are: 1127QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1128The default is INFO. 1129DEBUG and DEBUG1 are equivalent. 1130DEBUG2 and DEBUG3 each specify higher levels of debugging output. 1131Logging with a DEBUG level violates the privacy of users and is not recommended. 1132.It Cm LogVerbose 1133Specify one or more overrides to 1134.Cm LogLevel . 1135An override consists of a pattern lists that matches the source file, function 1136and line number to force detailed logging for. 1137For example, an override pattern of: 1138.Bd -literal -offset indent 1139kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1140.Ed 1141.Pp 1142would enable detailed logging for line 1000 of 1143.Pa kex.c , 1144everything in the 1145.Fn kex_exchange_identification 1146function, and all code in the 1147.Pa packet.c 1148file. 1149This option is intended for debugging and no overrides are enabled by default. 1150.It Cm MACs 1151Specifies the available MAC (message authentication code) algorithms. 1152The MAC algorithm is used for data integrity protection. 1153Multiple algorithms must be comma-separated. 1154If the specified list begins with a 1155.Sq + 1156character, then the specified algorithms will be appended to the default set 1157instead of replacing them. 1158If the specified list begins with a 1159.Sq - 1160character, then the specified algorithms (including wildcards) will be removed 1161from the default set instead of replacing them. 1162If the specified list begins with a 1163.Sq ^ 1164character, then the specified algorithms will be placed at the head of the 1165default set. 1166.Pp 1167The algorithms that contain 1168.Qq -etm 1169calculate the MAC after encryption (encrypt-then-mac). 1170These are considered safer and their use recommended. 1171The supported MACs are: 1172.Pp 1173.Bl -item -compact -offset indent 1174.It 1175hmac-md5 1176.It 1177hmac-md5-96 1178.It 1179hmac-sha1 1180.It 1181hmac-sha1-96 1182.It 1183hmac-sha2-256 1184.It 1185hmac-sha2-512 1186.It 1187umac-64@openssh.com 1188.It 1189umac-128@openssh.com 1190.It 1191hmac-md5-etm@openssh.com 1192.It 1193hmac-md5-96-etm@openssh.com 1194.It 1195hmac-sha1-etm@openssh.com 1196.It 1197hmac-sha1-96-etm@openssh.com 1198.It 1199hmac-sha2-256-etm@openssh.com 1200.It 1201hmac-sha2-512-etm@openssh.com 1202.It 1203umac-64-etm@openssh.com 1204.It 1205umac-128-etm@openssh.com 1206.El 1207.Pp 1208The default is: 1209.Bd -literal -offset indent 1210umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1211hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1212hmac-sha1-etm@openssh.com, 1213umac-64@openssh.com,umac-128@openssh.com, 1214hmac-sha2-256,hmac-sha2-512,hmac-sha1 1215.Ed 1216.Pp 1217The list of available MAC algorithms may also be obtained using 1218.Qq ssh -Q mac . 1219.It Cm Match 1220Introduces a conditional block. 1221If all of the criteria on the 1222.Cm Match 1223line are satisfied, the keywords on the following lines override those 1224set in the global section of the config file, until either another 1225.Cm Match 1226line or the end of the file. 1227If a keyword appears in multiple 1228.Cm Match 1229blocks that are satisfied, only the first instance of the keyword is 1230applied. 1231.Pp 1232The arguments to 1233.Cm Match 1234are one or more criteria-pattern pairs or the single token 1235.Cm All 1236which matches all criteria. 1237The available criteria are 1238.Cm User , 1239.Cm Group , 1240.Cm Host , 1241.Cm LocalAddress , 1242.Cm LocalPort , 1243.Cm RDomain , 1244and 1245.Cm Address 1246(with 1247.Cm RDomain 1248representing the 1249.Xr rdomain 4 1250on which the connection was received). 1251.Pp 1252The match patterns may consist of single entries or comma-separated 1253lists and may use the wildcard and negation operators described in the 1254.Sx PATTERNS 1255section of 1256.Xr ssh_config 5 . 1257.Pp 1258The patterns in an 1259.Cm Address 1260criteria may additionally contain addresses to match in CIDR 1261address/masklen format, 1262such as 192.0.2.0/24 or 2001:db8::/32. 1263Note that the mask length provided must be consistent with the address - 1264it is an error to specify a mask length that is too long for the address 1265or one with bits set in this host portion of the address. 1266For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1267.Pp 1268Only a subset of keywords may be used on the lines following a 1269.Cm Match 1270keyword. 1271Available keywords are 1272.Cm AcceptEnv , 1273.Cm AllowAgentForwarding , 1274.Cm AllowGroups , 1275.Cm AllowStreamLocalForwarding , 1276.Cm AllowTcpForwarding , 1277.Cm AllowUsers , 1278.Cm AuthenticationMethods , 1279.Cm AuthorizedKeysCommand , 1280.Cm AuthorizedKeysCommandUser , 1281.Cm AuthorizedKeysFile , 1282.Cm AuthorizedPrincipalsCommand , 1283.Cm AuthorizedPrincipalsCommandUser , 1284.Cm AuthorizedPrincipalsFile , 1285.Cm Banner , 1286.Cm CASignatureAlgorithms , 1287.Cm ChannelTimeout , 1288.Cm ChrootDirectory , 1289.Cm ClientAliveCountMax , 1290.Cm ClientAliveInterval , 1291.Cm DenyGroups , 1292.Cm DenyUsers , 1293.Cm DisableForwarding , 1294.Cm ExposeAuthInfo , 1295.Cm ForceCommand , 1296.Cm GatewayPorts , 1297.Cm GSSAPIAuthentication , 1298.Cm HostbasedAcceptedAlgorithms , 1299.Cm HostbasedAuthentication , 1300.Cm HostbasedUsesNameFromPacketOnly , 1301.Cm IgnoreRhosts , 1302.Cm Include , 1303.Cm IPQoS , 1304.Cm KbdInteractiveAuthentication , 1305.Cm KerberosAuthentication , 1306.Cm LogLevel , 1307.Cm MaxAuthTries , 1308.Cm MaxSessions , 1309.Cm PasswordAuthentication , 1310.Cm PermitEmptyPasswords , 1311.Cm PermitListen , 1312.Cm PermitOpen , 1313.Cm PermitRootLogin , 1314.Cm PermitTTY , 1315.Cm PermitTunnel , 1316.Cm PermitUserRC , 1317.Cm PubkeyAcceptedAlgorithms , 1318.Cm PubkeyAuthentication , 1319.Cm PubkeyAuthOptions , 1320.Cm RekeyLimit , 1321.Cm RevokedKeys , 1322.Cm RDomain , 1323.Cm SetEnv , 1324.Cm StreamLocalBindMask , 1325.Cm StreamLocalBindUnlink , 1326.Cm TrustedUserCAKeys , 1327.Cm UnusedConnectionTimeout , 1328.Cm X11DisplayOffset , 1329.Cm X11Forwarding 1330and 1331.Cm X11UseLocalhost . 1332.It Cm MaxAuthTries 1333Specifies the maximum number of authentication attempts permitted per 1334connection. 1335Once the number of failures reaches half this value, 1336additional failures are logged. 1337The default is 6. 1338.It Cm MaxSessions 1339Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1340sessions permitted per network connection. 1341Multiple sessions may be established by clients that support connection 1342multiplexing. 1343Setting 1344.Cm MaxSessions 1345to 1 will effectively disable session multiplexing, whereas setting it to 0 1346will prevent all shell, login and subsystem sessions while still permitting 1347forwarding. 1348The default is 10. 1349.It Cm MaxStartups 1350Specifies the maximum number of concurrent unauthenticated connections to the 1351SSH daemon. 1352Additional connections will be dropped until authentication succeeds or the 1353.Cm LoginGraceTime 1354expires for a connection. 1355The default is 10:30:100. 1356.Pp 1357Alternatively, random early drop can be enabled by specifying 1358the three colon separated values 1359start:rate:full (e.g. "10:30:60"). 1360.Xr sshd 8 1361will refuse connection attempts with a probability of rate/100 (30%) 1362if there are currently start (10) unauthenticated connections. 1363The probability increases linearly and all connection attempts 1364are refused if the number of unauthenticated connections reaches full (60). 1365.It Cm ModuliFile 1366Specifies the 1367.Xr moduli 5 1368file that contains the Diffie-Hellman groups used for the 1369.Dq diffie-hellman-group-exchange-sha1 1370and 1371.Dq diffie-hellman-group-exchange-sha256 1372key exchange methods. 1373The default is 1374.Pa /etc/moduli . 1375.It Cm PasswordAuthentication 1376Specifies whether password authentication is allowed. 1377The default is 1378.Cm yes . 1379.It Cm PermitEmptyPasswords 1380When password authentication is allowed, it specifies whether the 1381server allows login to accounts with empty password strings. 1382The default is 1383.Cm no . 1384.It Cm PermitListen 1385Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1386The listen specification must be one of the following forms: 1387.Pp 1388.Bl -item -offset indent -compact 1389.It 1390.Cm PermitListen 1391.Sm off 1392.Ar port 1393.Sm on 1394.It 1395.Cm PermitListen 1396.Sm off 1397.Ar host : port 1398.Sm on 1399.El 1400.Pp 1401Multiple permissions may be specified by separating them with whitespace. 1402An argument of 1403.Cm any 1404can be used to remove all restrictions and permit any listen requests. 1405An argument of 1406.Cm none 1407can be used to prohibit all listen requests. 1408The host name may contain wildcards as described in the PATTERNS section in 1409.Xr ssh_config 5 . 1410The wildcard 1411.Sq * 1412can also be used in place of a port number to allow all ports. 1413By default all port forwarding listen requests are permitted. 1414Note that the 1415.Cm GatewayPorts 1416option may further restrict which addresses may be listened on. 1417Note also that 1418.Xr ssh 1 1419will request a listen host of 1420.Dq localhost 1421if no listen host was specifically requested, and this name is 1422treated differently to explicit localhost addresses of 1423.Dq 127.0.0.1 1424and 1425.Dq ::1 . 1426.It Cm PermitOpen 1427Specifies the destinations to which TCP port forwarding is permitted. 1428The forwarding specification must be one of the following forms: 1429.Pp 1430.Bl -item -offset indent -compact 1431.It 1432.Cm PermitOpen 1433.Sm off 1434.Ar host : port 1435.Sm on 1436.It 1437.Cm PermitOpen 1438.Sm off 1439.Ar IPv4_addr : port 1440.Sm on 1441.It 1442.Cm PermitOpen 1443.Sm off 1444.Ar \&[ IPv6_addr \&] : port 1445.Sm on 1446.El 1447.Pp 1448Multiple forwards may be specified by separating them with whitespace. 1449An argument of 1450.Cm any 1451can be used to remove all restrictions and permit any forwarding requests. 1452An argument of 1453.Cm none 1454can be used to prohibit all forwarding requests. 1455The wildcard 1456.Sq * 1457can be used for host or port to allow all hosts or ports respectively. 1458Otherwise, no pattern matching or address lookups are performed on supplied 1459names. 1460By default all port forwarding requests are permitted. 1461.It Cm PermitRootLogin 1462Specifies whether root can log in using 1463.Xr ssh 1 . 1464The argument must be 1465.Cm yes , 1466.Cm prohibit-password , 1467.Cm forced-commands-only , 1468or 1469.Cm no . 1470The default is 1471.Cm prohibit-password . 1472.Pp 1473If this option is set to 1474.Cm prohibit-password 1475(or its deprecated alias, 1476.Cm without-password ) , 1477password and keyboard-interactive authentication are disabled for root. 1478.Pp 1479If this option is set to 1480.Cm forced-commands-only , 1481root login with public key authentication will be allowed, 1482but only if the 1483.Ar command 1484option has been specified 1485(which may be useful for taking remote backups even if root login is 1486normally not allowed). 1487All other authentication methods are disabled for root. 1488.Pp 1489If this option is set to 1490.Cm no , 1491root is not allowed to log in. 1492.It Cm PermitTTY 1493Specifies whether 1494.Xr pty 4 1495allocation is permitted. 1496The default is 1497.Cm yes . 1498.It Cm PermitTunnel 1499Specifies whether 1500.Xr tun 4 1501device forwarding is allowed. 1502The argument must be 1503.Cm yes , 1504.Cm point-to-point 1505(layer 3), 1506.Cm ethernet 1507(layer 2), or 1508.Cm no . 1509Specifying 1510.Cm yes 1511permits both 1512.Cm point-to-point 1513and 1514.Cm ethernet . 1515The default is 1516.Cm no . 1517.Pp 1518Independent of this setting, the permissions of the selected 1519.Xr tun 4 1520device must allow access to the user. 1521.It Cm PermitUserEnvironment 1522Specifies whether 1523.Pa ~/.ssh/environment 1524and 1525.Cm environment= 1526options in 1527.Pa ~/.ssh/authorized_keys 1528are processed by 1529.Xr sshd 8 . 1530Valid options are 1531.Cm yes , 1532.Cm no 1533or a pattern-list specifying which environment variable names to accept 1534(for example 1535.Qq LANG,LC_* ) . 1536The default is 1537.Cm no . 1538Enabling environment processing may enable users to bypass access 1539restrictions in some configurations using mechanisms such as 1540.Ev LD_PRELOAD . 1541.It Cm PermitUserRC 1542Specifies whether any 1543.Pa ~/.ssh/rc 1544file is executed. 1545The default is 1546.Cm yes . 1547.It Cm PerSourceMaxStartups 1548Specifies the number of unauthenticated connections allowed from a 1549given source address, or 1550.Dq none 1551if there is no limit. 1552This limit is applied in addition to 1553.Cm MaxStartups , 1554whichever is lower. 1555The default is 1556.Cm none . 1557.It Cm PerSourceNetBlockSize 1558Specifies the number of bits of source address that are grouped together 1559for the purposes of applying PerSourceMaxStartups limits. 1560Values for IPv4 and optionally IPv6 may be specified, separated by a colon. 1561The default is 1562.Cm 32:128 , 1563which means each address is considered individually. 1564.It Cm PidFile 1565Specifies the file that contains the process ID of the 1566SSH daemon, or 1567.Cm none 1568to not write one. 1569The default is 1570.Pa /var/run/sshd.pid . 1571.It Cm Port 1572Specifies the port number that 1573.Xr sshd 8 1574listens on. 1575The default is 22. 1576Multiple options of this type are permitted. 1577See also 1578.Cm ListenAddress . 1579.It Cm PrintLastLog 1580Specifies whether 1581.Xr sshd 8 1582should print the date and time of the last user login when a user logs 1583in interactively. 1584The default is 1585.Cm yes . 1586.It Cm PrintMotd 1587Specifies whether 1588.Xr sshd 8 1589should print 1590.Pa /etc/motd 1591when a user logs in interactively. 1592(On some systems it is also printed by the shell, 1593.Pa /etc/profile , 1594or equivalent.) 1595The default is 1596.Cm yes . 1597.It Cm PubkeyAcceptedAlgorithms 1598Specifies the signature algorithms that will be accepted for public key 1599authentication as a list of comma-separated patterns. 1600Alternately if the specified list begins with a 1601.Sq + 1602character, then the specified algorithms will be appended to the default set 1603instead of replacing them. 1604If the specified list begins with a 1605.Sq - 1606character, then the specified algorithms (including wildcards) will be removed 1607from the default set instead of replacing them. 1608If the specified list begins with a 1609.Sq ^ 1610character, then the specified algorithms will be placed at the head of the 1611default set. 1612The default for this option is: 1613.Bd -literal -offset 3n 1614ssh-ed25519-cert-v01@openssh.com, 1615ecdsa-sha2-nistp256-cert-v01@openssh.com, 1616ecdsa-sha2-nistp384-cert-v01@openssh.com, 1617ecdsa-sha2-nistp521-cert-v01@openssh.com, 1618sk-ssh-ed25519-cert-v01@openssh.com, 1619sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1620rsa-sha2-512-cert-v01@openssh.com, 1621rsa-sha2-256-cert-v01@openssh.com, 1622ssh-ed25519, 1623ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1624sk-ssh-ed25519@openssh.com, 1625sk-ecdsa-sha2-nistp256@openssh.com, 1626rsa-sha2-512,rsa-sha2-256 1627.Ed 1628.Pp 1629The list of available signature algorithms may also be obtained using 1630.Qq ssh -Q PubkeyAcceptedAlgorithms . 1631.It Cm PubkeyAuthOptions 1632Sets one or more public key authentication options. 1633The supported keywords are: 1634.Cm none 1635(the default; indicating no additional options are enabled), 1636.Cm touch-required 1637and 1638.Cm verify-required . 1639.Pp 1640The 1641.Cm touch-required 1642option causes public key authentication using a FIDO authenticator algorithm 1643(i.e.\& 1644.Cm ecdsa-sk 1645or 1646.Cm ed25519-sk ) 1647to always require the signature to attest that a physically present user 1648explicitly confirmed the authentication (usually by touching the authenticator). 1649By default, 1650.Xr sshd 8 1651requires user presence unless overridden with an authorized_keys option. 1652The 1653.Cm touch-required 1654flag disables this override. 1655.Pp 1656The 1657.Cm verify-required 1658option requires a FIDO key signature attest that the user was verified, 1659e.g. via a PIN. 1660.Pp 1661Neither the 1662.Cm touch-required 1663or 1664.Cm verify-required 1665options have any effect for other, non-FIDO, public key types. 1666.It Cm PubkeyAuthentication 1667Specifies whether public key authentication is allowed. 1668The default is 1669.Cm yes . 1670.It Cm RekeyLimit 1671Specifies the maximum amount of data that may be transmitted or received 1672before the session key is renegotiated, optionally followed by a maximum 1673amount of time that may pass before the session key is renegotiated. 1674The first argument is specified in bytes and may have a suffix of 1675.Sq K , 1676.Sq M , 1677or 1678.Sq G 1679to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1680The default is between 1681.Sq 1G 1682and 1683.Sq 4G , 1684depending on the cipher. 1685The optional second value is specified in seconds and may use any of the 1686units documented in the 1687.Sx TIME FORMATS 1688section. 1689The default value for 1690.Cm RekeyLimit 1691is 1692.Cm default none , 1693which means that rekeying is performed after the cipher's default amount 1694of data has been sent or received and no time based rekeying is done. 1695.It Cm RequiredRSASize 1696Specifies the minimum RSA key size (in bits) that 1697.Xr sshd 8 1698will accept. 1699User and host-based authentication keys smaller than this limit will be 1700refused. 1701The default is 1702.Cm 1024 1703bits. 1704Note that this limit may only be raised from the default. 1705.It Cm RevokedKeys 1706Specifies revoked public keys file, or 1707.Cm none 1708to not use one. 1709Keys listed in this file will be refused for public key authentication. 1710Note that if this file is not readable, then public key authentication will 1711be refused for all users. 1712Keys may be specified as a text file, listing one public key per line, or as 1713an OpenSSH Key Revocation List (KRL) as generated by 1714.Xr ssh-keygen 1 . 1715For more information on KRLs, see the KEY REVOCATION LISTS section in 1716.Xr ssh-keygen 1 . 1717.It Cm RDomain 1718Specifies an explicit routing domain that is applied after authentication 1719has completed. 1720The user session, as well as any forwarded or listening IP sockets, 1721will be bound to this 1722.Xr rdomain 4 . 1723If the routing domain is set to 1724.Cm \&%D , 1725then the domain in which the incoming connection was received will be applied. 1726.It Cm SecurityKeyProvider 1727Specifies a path to a library that will be used when loading 1728FIDO authenticator-hosted keys, overriding the default of using 1729the built-in USB HID support. 1730.It Cm SetEnv 1731Specifies one or more environment variables to set in child sessions started 1732by 1733.Xr sshd 8 1734as 1735.Dq NAME=VALUE . 1736The environment value may be quoted (e.g. if it contains whitespace 1737characters). 1738Environment variables set by 1739.Cm SetEnv 1740override the default environment and any variables specified by the user 1741via 1742.Cm AcceptEnv 1743or 1744.Cm PermitUserEnvironment . 1745.It Cm StreamLocalBindMask 1746Sets the octal file creation mode mask 1747.Pq umask 1748used when creating a Unix-domain socket file for local or remote 1749port forwarding. 1750This option is only used for port forwarding to a Unix-domain socket file. 1751.Pp 1752The default value is 0177, which creates a Unix-domain socket file that is 1753readable and writable only by the owner. 1754Note that not all operating systems honor the file mode on Unix-domain 1755socket files. 1756.It Cm StreamLocalBindUnlink 1757Specifies whether to remove an existing Unix-domain socket file for local 1758or remote port forwarding before creating a new one. 1759If the socket file already exists and 1760.Cm StreamLocalBindUnlink 1761is not enabled, 1762.Nm sshd 1763will be unable to forward the port to the Unix-domain socket file. 1764This option is only used for port forwarding to a Unix-domain socket file. 1765.Pp 1766The argument must be 1767.Cm yes 1768or 1769.Cm no . 1770The default is 1771.Cm no . 1772.It Cm StrictModes 1773Specifies whether 1774.Xr sshd 8 1775should check file modes and ownership of the 1776user's files and home directory before accepting login. 1777This is normally desirable because novices sometimes accidentally leave their 1778directory or files world-writable. 1779The default is 1780.Cm yes . 1781Note that this does not apply to 1782.Cm ChrootDirectory , 1783whose permissions and ownership are checked unconditionally. 1784.It Cm Subsystem 1785Configures an external subsystem (e.g. file transfer daemon). 1786Arguments should be a subsystem name and a command (with optional arguments) 1787to execute upon subsystem request. 1788.Pp 1789The command 1790.Cm sftp-server 1791implements the SFTP file transfer subsystem. 1792.Pp 1793Alternately the name 1794.Cm internal-sftp 1795implements an in-process SFTP server. 1796This may simplify configurations using 1797.Cm ChrootDirectory 1798to force a different filesystem root on clients. 1799It accepts the same command line arguments as 1800.Cm sftp-server 1801and even though it is in-process, settings such as 1802.Cm LogLevel 1803or 1804.Cm SyslogFacility 1805do not apply to it and must be set explicitly via 1806command line arguments. 1807.Pp 1808By default no subsystems are defined. 1809.It Cm SyslogFacility 1810Gives the facility code that is used when logging messages from 1811.Xr sshd 8 . 1812The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1813LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1814The default is AUTH. 1815.It Cm TCPKeepAlive 1816Specifies whether the system should send TCP keepalive messages to the 1817other side. 1818If they are sent, death of the connection or crash of one 1819of the machines will be properly noticed. 1820However, this means that 1821connections will die if the route is down temporarily, and some people 1822find it annoying. 1823On the other hand, if TCP keepalives are not sent, 1824sessions may hang indefinitely on the server, leaving 1825.Qq ghost 1826users and consuming server resources. 1827.Pp 1828The default is 1829.Cm yes 1830(to send TCP keepalive messages), and the server will notice 1831if the network goes down or the client host crashes. 1832This avoids infinitely hanging sessions. 1833.Pp 1834To disable TCP keepalive messages, the value should be set to 1835.Cm no . 1836.It Cm TrustedUserCAKeys 1837Specifies a file containing public keys of certificate authorities that are 1838trusted to sign user certificates for authentication, or 1839.Cm none 1840to not use one. 1841Keys are listed one per line; empty lines and comments starting with 1842.Ql # 1843are allowed. 1844If a certificate is presented for authentication and has its signing CA key 1845listed in this file, then it may be used for authentication for any user 1846listed in the certificate's principals list. 1847Note that certificates that lack a list of principals will not be permitted 1848for authentication using 1849.Cm TrustedUserCAKeys . 1850For more details on certificates, see the CERTIFICATES section in 1851.Xr ssh-keygen 1 . 1852.It Cm UnusedConnectionTimeout 1853Specifies whether and how quickly 1854.Xr sshd 8 1855should close client connections with no open channels. 1856Open channels include active shell, command execution or subsystem 1857sessions, connected network, socket, agent or X11 forwardings. 1858Forwarding listeners, such as those from the 1859.Xr ssh 1 1860.Fl R 1861flag, are not considered as open channels and do not prevent the timeout. 1862The timeout value 1863is specified in seconds or may use any of the units documented in the 1864.Sx TIME FORMATS 1865section. 1866.Pp 1867Note that this timeout starts when the client connection completes 1868user authentication but before the client has an opportunity to open any 1869channels. 1870Caution should be used when using short timeout values, as they may not 1871provide sufficient time for the client to request and open its channels 1872before terminating the connection. 1873.Pp 1874The default 1875.Cm none 1876is to never expire connections for having no open channels. 1877This option may be useful in conjunction with 1878.Cm ChannelTimeout . 1879.It Cm UseDNS 1880Specifies whether 1881.Xr sshd 8 1882should look up the remote host name, and to check that 1883the resolved host name for the remote IP address maps back to the 1884very same IP address. 1885.Pp 1886If this option is set to 1887.Cm no 1888(the default) then only addresses and not host names may be used in 1889.Pa ~/.ssh/authorized_keys 1890.Cm from 1891and 1892.Nm 1893.Cm Match 1894.Cm Host 1895directives. 1896.It Cm VersionAddendum 1897Optionally specifies additional text to append to the SSH protocol banner 1898sent by the server upon connection. 1899The default is 1900.Cm none . 1901.It Cm X11DisplayOffset 1902Specifies the first display number available for 1903.Xr sshd 8 Ns 's 1904X11 forwarding. 1905This prevents sshd from interfering with real X11 servers. 1906The default is 10. 1907.It Cm X11Forwarding 1908Specifies whether X11 forwarding is permitted. 1909The argument must be 1910.Cm yes 1911or 1912.Cm no . 1913The default is 1914.Cm no . 1915.Pp 1916When X11 forwarding is enabled, there may be additional exposure to 1917the server and to client displays if the 1918.Xr sshd 8 1919proxy display is configured to listen on the wildcard address (see 1920.Cm X11UseLocalhost ) , 1921though this is not the default. 1922Additionally, the authentication spoofing and authentication data 1923verification and substitution occur on the client side. 1924The security risk of using X11 forwarding is that the client's X11 1925display server may be exposed to attack when the SSH client requests 1926forwarding (see the warnings for 1927.Cm ForwardX11 1928in 1929.Xr ssh_config 5 ) . 1930A system administrator may have a stance in which they want to 1931protect clients that may expose themselves to attack by unwittingly 1932requesting X11 forwarding, which can warrant a 1933.Cm no 1934setting. 1935.Pp 1936Note that disabling X11 forwarding does not prevent users from 1937forwarding X11 traffic, as users can always install their own forwarders. 1938.It Cm X11UseLocalhost 1939Specifies whether 1940.Xr sshd 8 1941should bind the X11 forwarding server to the loopback address or to 1942the wildcard address. 1943By default, 1944sshd binds the forwarding server to the loopback address and sets the 1945hostname part of the 1946.Ev DISPLAY 1947environment variable to 1948.Cm localhost . 1949This prevents remote hosts from connecting to the proxy display. 1950However, some older X11 clients may not function with this 1951configuration. 1952.Cm X11UseLocalhost 1953may be set to 1954.Cm no 1955to specify that the forwarding server should be bound to the wildcard 1956address. 1957The argument must be 1958.Cm yes 1959or 1960.Cm no . 1961The default is 1962.Cm yes . 1963.It Cm XAuthLocation 1964Specifies the full pathname of the 1965.Xr xauth 1 1966program, or 1967.Cm none 1968to not use one. 1969The default is 1970.Pa /usr/X11R6/bin/xauth . 1971.It Cm UseLPK 1972Specifies whether LDAP public key retrieval must be used or not. It allow 1973an easy centralisation of public keys within an LDAP directory. The argument must be 1974.Dq yes 1975or 1976.Dq no . 1977.It Cm LpkLdapConf 1978Specifies whether LDAP Public keys should parse the specified ldap.conf file 1979instead of sshd_config Tokens. The argument must be a valid path to an ldap.conf 1980file like 1981.Pa /etc/ldap.conf 1982.It Cm LpkServers 1983Specifies LDAP one or more [:space:] separated server's url the following form may be used: 1984.Pp 1985LpkServers ldaps://127.0.0.1 ldap://127.0.0.2 ldap://127.0.0.3 1986.It Cm LpkUserDN 1987Specifies the LDAP user DN. 1988.Pp 1989LpkUserDN ou=users,dc=phear,dc=org 1990.It Cm LpkGroupDN 1991Specifies the LDAP groups DN. 1992.Pp 1993LpkGroupDN ou=groups,dc=phear,dc=org 1994.It Cm LpkBindDN 1995Specifies the LDAP bind DN to use if necessary. 1996.Pp 1997LpkBindDN cn=Manager,dc=phear,dc=org 1998.It Cm LpkBindPw 1999Specifies the LDAP bind credential. 2000.Pp 2001LpkBindPw secret 2002.It Cm LpkServerGroup 2003Specifies one or more [:space:] separated group the server is part of. 2004.Pp 2005LpkServerGroup unix mail prod 2006.It Cm LpkFilter 2007Specifies an additional LDAP filter to use for finding SSH keys 2008.Pp 2009LpkFilter (hostAccess=master.phear.org) 2010.It Cm LpkForceTLS 2011Specifies if the LDAP server connection must be tried, forced or not used. The argument must be 2012.Dq yes 2013or 2014.Dq no 2015or 2016.Dq try . 2017.It Cm LpkSearchTimelimit 2018Specifies the search time limit before the search is considered over. value is 2019in seconds. 2020.Pp 2021LpkSearchTimelimit 3 2022.It Cm LpkBindTimelimit 2023Specifies the bind time limit before the connection is considered dead. value is 2024in seconds. 2025.Pp 2026LpkBindTimelimit 3 2027.El 2028.Sh TIME FORMATS 2029.Xr sshd 8 2030command-line arguments and configuration file options that specify time 2031may be expressed using a sequence of the form: 2032.Sm off 2033.Ar time Op Ar qualifier , 2034.Sm on 2035where 2036.Ar time 2037is a positive integer value and 2038.Ar qualifier 2039is one of the following: 2040.Pp 2041.Bl -tag -width Ds -compact -offset indent 2042.It Aq Cm none 2043seconds 2044.It Cm s | Cm S 2045seconds 2046.It Cm m | Cm M 2047minutes 2048.It Cm h | Cm H 2049hours 2050.It Cm d | Cm D 2051days 2052.It Cm w | Cm W 2053weeks 2054.El 2055.Pp 2056Each member of the sequence is added together to calculate 2057the total time value. 2058.Pp 2059Time format examples: 2060.Pp 2061.Bl -tag -width Ds -compact -offset indent 2062.It 600 2063600 seconds (10 minutes) 2064.It 10m 206510 minutes 2066.It 1h30m 20671 hour 30 minutes (90 minutes) 2068.El 2069.Sh TOKENS 2070Arguments to some keywords can make use of tokens, 2071which are expanded at runtime: 2072.Pp 2073.Bl -tag -width XXXX -offset indent -compact 2074.It %% 2075A literal 2076.Sq % . 2077.It \&%C 2078Identifies the connection endpoints, containing 2079four space-separated values: client address, client port number, 2080server address, and server port number. 2081.It \&%D 2082The routing domain in which the incoming connection was received. 2083.It %F 2084The fingerprint of the CA key. 2085.It %f 2086The fingerprint of the key or certificate. 2087.It %h 2088The home directory of the user. 2089.It %i 2090The key ID in the certificate. 2091.It %K 2092The base64-encoded CA key. 2093.It %k 2094The base64-encoded key or certificate for authentication. 2095.It %s 2096The serial number of the certificate. 2097.It \&%T 2098The type of the CA key. 2099.It %t 2100The key or certificate type. 2101.It \&%U 2102The numeric user ID of the target user. 2103.It %u 2104The username. 2105.El 2106.Pp 2107.Cm AuthorizedKeysCommand 2108accepts the tokens %%, %C, %D, %f, %h, %k, %t, %U, and %u. 2109.Pp 2110.Cm AuthorizedKeysFile 2111accepts the tokens %%, %h, %U, and %u. 2112.Pp 2113.Cm AuthorizedPrincipalsCommand 2114accepts the tokens %%, %C, %D, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 2115.Pp 2116.Cm AuthorizedPrincipalsFile 2117accepts the tokens %%, %h, %U, and %u. 2118.Pp 2119.Cm ChrootDirectory 2120accepts the tokens %%, %h, %U, and %u. 2121.Pp 2122.Cm RoutingDomain 2123accepts the token %D. 2124.Sh FILES 2125.Bl -tag -width Ds 2126.It Pa /etc/ssh/sshd_config 2127Contains configuration data for 2128.Xr sshd 8 . 2129This file should be writable by root only, but it is recommended 2130(though not necessary) that it be world-readable. 2131.El 2132.Sh SEE ALSO 2133.Xr sftp-server 8 , 2134.Xr sshd 8 2135.Sh AUTHORS 2136.An -nosplit 2137OpenSSH is a derivative of the original and free 2138ssh 1.2.12 release by 2139.An Tatu Ylonen . 2140.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 2141.An Theo de Raadt 2142and 2143.An Dug Song 2144removed many bugs, re-added newer features and 2145created OpenSSH. 2146.An Markus Friedl 2147contributed the support for SSH protocol versions 1.5 and 2.0. 2148.An Niels Provos 2149and 2150.An Markus Friedl 2151contributed support for privilege separation. 2152