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 1.282 2018/09/20 03:28:06 djm Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: September 20 2018 $ 39.Dt SSHD_CONFIG 5 40.Os 41.Sh NAME 42.Nm sshd_config 43.Nd OpenSSH SSH daemon configuration file 44.Sh DESCRIPTION 45.Xr sshd 8 46reads configuration data from 47.Pa /etc/ssh/sshd_config 48(or the file specified with 49.Fl f 50on the command line). 51The file contains keyword-argument pairs, one per line. 52For each keyword, the first obtained value will be used. 53Lines starting with 54.Ql # 55and empty lines are interpreted as comments. 56Arguments may optionally be enclosed in double quotes 57.Pq \&" 58in order to represent arguments containing spaces. 59.Pp 60The possible 61keywords and their meanings are as follows (note that 62keywords are case-insensitive and arguments are case-sensitive): 63.Bl -tag -width Ds 64.It Cm AcceptEnv 65Specifies what environment variables sent by the client will be copied into 66the session's 67.Xr environ 7 . 68See 69.Cm SendEnv 70and 71.Cm SetEnv 72in 73.Xr ssh_config 5 74for how to configure the client. 75The 76.Ev TERM 77environment variable is always accepted whenever the client 78requests a pseudo-terminal as it is required by the protocol. 79Variables are specified by name, which may contain the wildcard characters 80.Ql * 81and 82.Ql \&? . 83Multiple environment variables may be separated by whitespace or spread 84across multiple 85.Cm AcceptEnv 86directives. 87Be warned that some environment variables could be used to bypass restricted 88user environments. 89For this reason, care should be taken in the use of this directive. 90The default is not to accept any environment variables. 91.It Cm AddressFamily 92Specifies which address family should be used by 93.Xr sshd 8 . 94Valid arguments are 95.Cm any 96(the default), 97.Cm inet 98(use IPv4 only), or 99.Cm inet6 100(use IPv6 only). 101.It Cm AllowAgentForwarding 102Specifies whether 103.Xr ssh-agent 1 104forwarding is permitted. 105The default is 106.Cm yes . 107Note that disabling agent forwarding does not improve security 108unless users are also denied shell access, as they can always install 109their own forwarders. 110.It Cm AllowGroups 111This keyword can be followed by a list of group name patterns, separated 112by spaces. 113If specified, login is allowed only for users whose primary 114group or supplementary group list matches one of the patterns. 115Only group names are valid; a numerical group ID is not recognized. 116By default, login is allowed for all groups. 117The allow/deny directives are processed in the following order: 118.Cm DenyUsers , 119.Cm AllowUsers , 120.Cm DenyGroups , 121and finally 122.Cm AllowGroups . 123.Pp 124See PATTERNS in 125.Xr ssh_config 5 126for more information on patterns. 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 directives are processed in the following order: 178.Cm DenyUsers , 179.Cm AllowUsers , 180.Cm DenyGroups , 181and finally 182.Cm AllowGroups . 183.Pp 184See PATTERNS in 185.Xr ssh_config 5 186for more information on patterns. 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 210or 211.Cm pam . 212depending on the server configuration. 213For example, 214.Qq keyboard-interactive:bsdauth 215would restrict keyboard interactive authentication to the 216.Cm bsdauth 217device. 218.Pp 219If the publickey method is listed more than once, 220.Xr sshd 8 221verifies that keys that have been used successfully are not reused for 222subsequent authentications. 223For example, 224.Qq publickey,publickey 225requires successful authentication using two different public keys. 226.Pp 227Note that each authentication method listed should also be explicitly enabled 228in the configuration. 229.Pp 230The available authentication methods are: 231.Qq gssapi-with-mic , 232.Qq hostbased , 233.Qq keyboard-interactive , 234.Qq none 235(used for access to password-less accounts when 236.Cm PermitEmptyPasswords 237is enabled), 238.Qq password 239and 240.Qq publickey . 241.It Cm AuthorizedKeysCommand 242Specifies a program to be used to look up the user's public keys. 243The program must be owned by root, not writable by group or others and 244specified by an absolute path. 245Arguments to 246.Cm AuthorizedKeysCommand 247accept the tokens described in the 248.Sx TOKENS 249section. 250If no arguments are specified then the username of the target user is used. 251.Pp 252The program should produce on standard output zero or 253more lines of authorized_keys output (see 254.Sx AUTHORIZED_KEYS 255in 256.Xr sshd 8 ) . 257If a key supplied by 258.Cm AuthorizedKeysCommand 259does not successfully authenticate 260and authorize the user then public key authentication continues using the usual 261.Cm AuthorizedKeysFile 262files. 263By default, no 264.Cm AuthorizedKeysCommand 265is run. 266.It Cm AuthorizedKeysCommandUser 267Specifies the user under whose account the 268.Cm AuthorizedKeysCommand 269is run. 270It is recommended to use a dedicated user that has no other role on the host 271than running authorized keys commands. 272If 273.Cm AuthorizedKeysCommand 274is specified but 275.Cm AuthorizedKeysCommandUser 276is not, then 277.Xr sshd 8 278will refuse to start. 279.It Cm AuthorizedKeysFile 280Specifies the file that contains the public keys used for user authentication. 281The format is described in the 282.Sx AUTHORIZED_KEYS FILE FORMAT 283section of 284.Xr sshd 8 . 285Arguments to 286.Cm AuthorizedKeysFile 287accept the tokens described in the 288.Sx TOKENS 289section. 290After expansion, 291.Cm AuthorizedKeysFile 292is taken to be an absolute path or one relative to the user's home 293directory. 294Multiple files may be listed, separated by whitespace. 295Alternately this option may be set to 296.Cm none 297to skip checking for user keys in files. 298The default is 299.Qq .ssh/authorized_keys .ssh/authorized_keys2 . 300.It Cm AuthorizedPrincipalsCommand 301Specifies a program to be used to generate the list of allowed 302certificate principals as per 303.Cm AuthorizedPrincipalsFile . 304The program must be owned by root, not writable by group or others and 305specified by an absolute path. 306Arguments to 307.Cm AuthorizedPrincipalsCommand 308accept the tokens described in the 309.Sx TOKENS 310section. 311If no arguments are specified then the username of the target user is used. 312.Pp 313The program should produce on standard output zero or 314more lines of 315.Cm AuthorizedPrincipalsFile 316output. 317If either 318.Cm AuthorizedPrincipalsCommand 319or 320.Cm AuthorizedPrincipalsFile 321is specified, then certificates offered by the client for authentication 322must contain a principal that is listed. 323By default, no 324.Cm AuthorizedPrincipalsCommand 325is run. 326.It Cm AuthorizedPrincipalsCommandUser 327Specifies the user under whose account the 328.Cm AuthorizedPrincipalsCommand 329is run. 330It is recommended to use a dedicated user that has no other role on the host 331than running authorized principals commands. 332If 333.Cm AuthorizedPrincipalsCommand 334is specified but 335.Cm AuthorizedPrincipalsCommandUser 336is not, then 337.Xr sshd 8 338will refuse to start. 339.It Cm AuthorizedPrincipalsFile 340Specifies a file that lists principal names that are accepted for 341certificate authentication. 342When using certificates signed by a key listed in 343.Cm TrustedUserCAKeys , 344this file lists names, one of which must appear in the certificate for it 345to be accepted for authentication. 346Names are listed one per line preceded by key options (as described in 347.Sx AUTHORIZED_KEYS FILE FORMAT 348in 349.Xr sshd 8 ) . 350Empty lines and comments starting with 351.Ql # 352are ignored. 353.Pp 354Arguments to 355.Cm AuthorizedPrincipalsFile 356accept the tokens described in the 357.Sx TOKENS 358section. 359After expansion, 360.Cm AuthorizedPrincipalsFile 361is taken to be an absolute path or one relative to the user's home directory. 362The default is 363.Cm none , 364i.e. not to use a principals file \(en in this case, the username 365of the user must appear in a certificate's principals list for it to be 366accepted. 367.Pp 368Note that 369.Cm AuthorizedPrincipalsFile 370is only used when authentication proceeds using a CA listed in 371.Cm TrustedUserCAKeys 372and is not consulted for certification authorities trusted via 373.Pa ~/.ssh/authorized_keys , 374though the 375.Cm principals= 376key option offers a similar facility (see 377.Xr sshd 8 378for details). 379.It Cm Banner 380The contents of the specified file are sent to the remote user before 381authentication is allowed. 382If the argument is 383.Cm none 384then no banner is displayed. 385By default, no banner is displayed. 386.It Cm CASignatureAlgorithms 387Specifies which algorithms are allowed for signing of certificates 388by certificate authorities (CAs). 389The default is: 390.Bd -literal -offset indent 391ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 392ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 393.Ed 394.Pp 395Certificates signed using other algorithms will not be accepted for 396public key or host-based authentication. 397.It Cm ChallengeResponseAuthentication 398Specifies whether challenge-response authentication is allowed (e.g. via 399PAM or through authentication styles supported in 400.Xr login.conf 5 ) 401The default is 402.Cm yes . 403.It Cm ChrootDirectory 404Specifies the pathname of a directory to 405.Xr chroot 2 406to after authentication. 407At session startup 408.Xr sshd 8 409checks that all components of the pathname are root-owned directories 410which are not writable by any other user or group. 411After the chroot, 412.Xr sshd 8 413changes the working directory to the user's home directory. 414Arguments to 415.Cm ChrootDirectory 416accept the tokens described in the 417.Sx TOKENS 418section. 419.Pp 420The 421.Cm ChrootDirectory 422must contain the necessary files and directories to support the 423user's session. 424For an interactive session this requires at least a shell, typically 425.Xr sh 1 , 426and basic 427.Pa /dev 428nodes such as 429.Xr null 4 , 430.Xr zero 4 , 431.Xr stdin 4 , 432.Xr stdout 4 , 433.Xr stderr 4 , 434and 435.Xr tty 4 436devices. 437For file transfer sessions using SFTP 438no additional configuration of the environment is necessary if the in-process 439sftp-server is used, 440though sessions which use logging may require 441.Pa /dev/log 442inside the chroot directory on some operating systems (see 443.Xr sftp-server 8 444for details). 445.Pp 446For safety, it is very important that the directory hierarchy be 447prevented from modification by other processes on the system (especially 448those outside the jail). 449Misconfiguration can lead to unsafe environments which 450.Xr sshd 8 451cannot detect. 452.Pp 453The default is 454.Cm none , 455indicating not to 456.Xr chroot 2 . 457.It Cm Ciphers 458Specifies the ciphers allowed. 459Multiple ciphers must be comma-separated. 460If the specified value begins with a 461.Sq + 462character, then the specified ciphers will be appended to the default set 463instead of replacing them. 464If the specified value begins with a 465.Sq - 466character, then the specified ciphers (including wildcards) will be removed 467from the default set instead of replacing them. 468.Pp 469The supported ciphers are: 470.Pp 471.Bl -item -compact -offset indent 472.It 4733des-cbc 474.It 475aes128-cbc 476.It 477aes192-cbc 478.It 479aes256-cbc 480.It 481aes128-ctr 482.It 483aes192-ctr 484.It 485aes256-ctr 486.It 487aes128-gcm@openssh.com 488.It 489aes256-gcm@openssh.com 490.It 491chacha20-poly1305@openssh.com 492.El 493.Pp 494The default is: 495.Bd -literal -offset indent 496chacha20-poly1305@openssh.com, 497aes128-ctr,aes192-ctr,aes256-ctr, 498aes128-gcm@openssh.com,aes256-gcm@openssh.com, 499aes128-cbc,aes192-cbc,aes256-cbc 500.Ed 501.Pp 502The list of available ciphers may also be obtained using 503.Qq ssh -Q cipher . 504.It Cm ClientAliveCountMax 505Sets the number of client alive messages which may be sent without 506.Xr sshd 8 507receiving any messages back from the client. 508If this threshold is reached while client alive messages are being sent, 509sshd will disconnect the client, terminating the session. 510It is important to note that the use of client alive messages is very 511different from 512.Cm TCPKeepAlive . 513The client alive messages are sent through the encrypted channel 514and therefore will not be spoofable. 515The TCP keepalive option enabled by 516.Cm TCPKeepAlive 517is spoofable. 518The client alive mechanism is valuable when the client or 519server depend on knowing when a connection has become inactive. 520.Pp 521The default value is 3. 522If 523.Cm ClientAliveInterval 524is set to 15, and 525.Cm ClientAliveCountMax 526is left at the default, unresponsive SSH clients 527will be disconnected after approximately 45 seconds. 528.It Cm ClientAliveInterval 529Sets a timeout interval in seconds after which if no data has been received 530from the client, 531.Xr sshd 8 532will send a message through the encrypted 533channel to request a response from the client. 534The default 535is 0, indicating that these messages will not be sent to the client. 536.It Cm Compression 537Specifies whether compression is enabled after 538the user has authenticated successfully. 539The argument must be 540.Cm yes , 541.Cm delayed 542(a legacy synonym for 543.Cm yes ) 544or 545.Cm no . 546The default is 547.Cm yes . 548.It Cm DenyGroups 549This keyword can be followed by a list of group name patterns, separated 550by spaces. 551Login is disallowed for users whose primary group or supplementary 552group list matches one of the patterns. 553Only group names are valid; a numerical group ID is not recognized. 554By default, login is allowed for all groups. 555The allow/deny directives are processed in the following order: 556.Cm DenyUsers , 557.Cm AllowUsers , 558.Cm DenyGroups , 559and finally 560.Cm AllowGroups . 561.Pp 562See PATTERNS in 563.Xr ssh_config 5 564for more information on patterns. 565.It Cm DenyUsers 566This keyword can be followed by a list of user name patterns, separated 567by spaces. 568Login is disallowed for user names that match one of the patterns. 569Only user names are valid; a numerical user ID is not recognized. 570By default, login is allowed for all users. 571If the pattern takes the form USER@HOST then USER and HOST 572are separately checked, restricting logins to particular 573users from particular hosts. 574HOST criteria may additionally contain addresses to match in CIDR 575address/masklen format. 576The allow/deny directives are processed in the following order: 577.Cm DenyUsers , 578.Cm AllowUsers , 579.Cm DenyGroups , 580and finally 581.Cm AllowGroups . 582.Pp 583See PATTERNS in 584.Xr ssh_config 5 585for more information on patterns. 586.It Cm DisableForwarding 587Disables all forwarding features, including X11, 588.Xr ssh-agent 1 , 589TCP and StreamLocal. 590This option overrides all other forwarding-related options and may 591simplify restricted configurations. 592.It Cm ExposeAuthInfo 593Writes a temporary file containing a list of authentication methods and 594public credentials (e.g. keys) used to authenticate the user. 595The location of the file is exposed to the user session through the 596.Ev SSH_USER_AUTH 597environment variable. 598The default is 599.Cm no . 600.It Cm FingerprintHash 601Specifies the hash algorithm used when logging key fingerprints. 602Valid options are: 603.Cm md5 604and 605.Cm sha256 . 606The default is 607.Cm sha256 . 608.It Cm ForceCommand 609Forces the execution of the command specified by 610.Cm ForceCommand , 611ignoring any command supplied by the client and 612.Pa ~/.ssh/rc 613if present. 614The command is invoked by using the user's login shell with the -c option. 615This applies to shell, command, or subsystem execution. 616It is most useful inside a 617.Cm Match 618block. 619The command originally supplied by the client is available in the 620.Ev SSH_ORIGINAL_COMMAND 621environment variable. 622Specifying a command of 623.Cm internal-sftp 624will force the use of an in-process SFTP server that requires no support 625files when used with 626.Cm ChrootDirectory . 627The default is 628.Cm none . 629.It Cm GatewayPorts 630Specifies whether remote hosts are allowed to connect to ports 631forwarded for the client. 632By default, 633.Xr sshd 8 634binds remote port forwardings to the loopback address. 635This prevents other remote hosts from connecting to forwarded ports. 636.Cm GatewayPorts 637can be used to specify that sshd 638should allow remote port forwardings to bind to non-loopback addresses, thus 639allowing other hosts to connect. 640The argument may be 641.Cm no 642to force remote port forwardings to be available to the local host only, 643.Cm yes 644to force remote port forwardings to bind to the wildcard address, or 645.Cm clientspecified 646to allow the client to select the address to which the forwarding is bound. 647The default is 648.Cm no . 649.It Cm GSSAPIAuthentication 650Specifies whether user authentication based on GSSAPI is allowed. 651The default is 652.Cm no . 653.It Cm GSSAPICleanupCredentials 654Specifies whether to automatically destroy the user's credentials cache 655on logout. 656The default is 657.Cm yes . 658.It Cm GSSAPIStrictAcceptorCheck 659Determines whether to be strict about the identity of the GSSAPI acceptor 660a client authenticates against. 661If set to 662.Cm yes 663then the client must authenticate against the host 664service on the current hostname. 665If set to 666.Cm no 667then the client may authenticate against any service key stored in the 668machine's default store. 669This facility is provided to assist with operation on multi homed machines. 670The default is 671.Cm yes . 672.It Cm HostbasedAcceptedKeyTypes 673Specifies the key types that will be accepted for hostbased authentication 674as a list of comma-separated patterns. 675Alternately if the specified value begins with a 676.Sq + 677character, then the specified key types will be appended to the default set 678instead of replacing them. 679If the specified value begins with a 680.Sq - 681character, then the specified key types (including wildcards) will be removed 682from the default set instead of replacing them. 683The default for this option is: 684.Bd -literal -offset 3n 685ecdsa-sha2-nistp256-cert-v01@openssh.com, 686ecdsa-sha2-nistp384-cert-v01@openssh.com, 687ecdsa-sha2-nistp521-cert-v01@openssh.com, 688ssh-ed25519-cert-v01@openssh.com, 689rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 690ssh-rsa-cert-v01@openssh.com, 691ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 692ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 693.Ed 694.Pp 695The list of available key types may also be obtained using 696.Qq ssh -Q key . 697.It Cm HostbasedAuthentication 698Specifies whether rhosts or /etc/hosts.equiv authentication together 699with successful public key client host authentication is allowed 700(host-based authentication). 701The default is 702.Cm no . 703.It Cm HostbasedUsesNameFromPacketOnly 704Specifies whether or not the server will attempt to perform a reverse 705name lookup when matching the name in the 706.Pa ~/.shosts , 707.Pa ~/.rhosts , 708and 709.Pa /etc/hosts.equiv 710files during 711.Cm HostbasedAuthentication . 712A setting of 713.Cm yes 714means that 715.Xr sshd 8 716uses the name supplied by the client rather than 717attempting to resolve the name from the TCP connection itself. 718The default is 719.Cm no . 720.It Cm HostCertificate 721Specifies a file containing a public host certificate. 722The certificate's public key must match a private host key already specified 723by 724.Cm HostKey . 725The default behaviour of 726.Xr sshd 8 727is not to load any certificates. 728.It Cm HostKey 729Specifies a file containing a private host key 730used by SSH. 731The defaults are 732.Pa /etc/ssh/ssh_host_ecdsa_key , 733.Pa /etc/ssh/ssh_host_ed25519_key 734and 735.Pa /etc/ssh/ssh_host_rsa_key . 736.Pp 737Note that 738.Xr sshd 8 739will refuse to use a file if it is group/world-accessible 740and that the 741.Cm HostKeyAlgorithms 742option restricts which of the keys are actually used by 743.Xr sshd 8 . 744.Pp 745It is possible to have multiple host key files. 746It is also possible to specify public host key files instead. 747In this case operations on the private key will be delegated 748to an 749.Xr ssh-agent 1 . 750.It Cm HostKeyAgent 751Identifies the UNIX-domain socket used to communicate 752with an agent that has access to the private host keys. 753If the string 754.Qq SSH_AUTH_SOCK 755is specified, the location of the socket will be read from the 756.Ev SSH_AUTH_SOCK 757environment variable. 758.It Cm HostKeyAlgorithms 759Specifies the host key algorithms 760that the server offers. 761The default for this option is: 762.Bd -literal -offset 3n 763ecdsa-sha2-nistp256-cert-v01@openssh.com, 764ecdsa-sha2-nistp384-cert-v01@openssh.com, 765ecdsa-sha2-nistp521-cert-v01@openssh.com, 766ssh-ed25519-cert-v01@openssh.com, 767rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 768ssh-rsa-cert-v01@openssh.com, 769ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 770ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 771.Ed 772.Pp 773The list of available key types may also be obtained using 774.Qq ssh -Q key . 775.It Cm IgnoreRhosts 776Specifies that 777.Pa .rhosts 778and 779.Pa .shosts 780files will not be used in 781.Cm HostbasedAuthentication . 782.Pp 783.Pa /etc/hosts.equiv 784and 785.Pa /etc/ssh/shosts.equiv 786are still used. 787The default is 788.Cm yes . 789.It Cm IgnoreUserKnownHosts 790Specifies whether 791.Xr sshd 8 792should ignore the user's 793.Pa ~/.ssh/known_hosts 794during 795.Cm HostbasedAuthentication 796and use only the system-wide known hosts file 797.Pa /etc/ssh/known_hosts . 798The default is 799.Cm no . 800.It Cm IPQoS 801Specifies the IPv4 type-of-service or DSCP class for the connection. 802Accepted values are 803.Cm af11 , 804.Cm af12 , 805.Cm af13 , 806.Cm af21 , 807.Cm af22 , 808.Cm af23 , 809.Cm af31 , 810.Cm af32 , 811.Cm af33 , 812.Cm af41 , 813.Cm af42 , 814.Cm af43 , 815.Cm cs0 , 816.Cm cs1 , 817.Cm cs2 , 818.Cm cs3 , 819.Cm cs4 , 820.Cm cs5 , 821.Cm cs6 , 822.Cm cs7 , 823.Cm ef , 824.Cm lowdelay , 825.Cm throughput , 826.Cm reliability , 827a numeric value, or 828.Cm none 829to use the operating system default. 830This option may take one or two arguments, separated by whitespace. 831If one argument is specified, it is used as the packet class unconditionally. 832If two values are specified, the first is automatically selected for 833interactive sessions and the second for non-interactive sessions. 834The default is 835.Cm af21 836(Low-Latency Data) 837for interactive sessions and 838.Cm cs1 839(Lower Effort) 840for non-interactive sessions. 841.It Cm KbdInteractiveAuthentication 842Specifies whether to allow keyboard-interactive authentication. 843The argument to this keyword must be 844.Cm yes 845or 846.Cm no . 847The default is to use whatever value 848.Cm ChallengeResponseAuthentication 849is set to 850(by default 851.Cm yes ) . 852.It Cm KerberosAuthentication 853Specifies whether the password provided by the user for 854.Cm PasswordAuthentication 855will be validated through the Kerberos KDC. 856To use this option, the server needs a 857Kerberos servtab which allows the verification of the KDC's identity. 858The default is 859.Cm no . 860.It Cm KerberosGetAFSToken 861If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 862an AFS token before accessing the user's home directory. 863The default is 864.Cm no . 865.It Cm KerberosOrLocalPasswd 866If password authentication through Kerberos fails then 867the password will be validated via any additional local mechanism 868such as 869.Pa /etc/passwd . 870The default is 871.Cm yes . 872.It Cm KerberosTicketCleanup 873Specifies whether to automatically destroy the user's ticket cache 874file on logout. 875The default is 876.Cm yes . 877.It Cm KexAlgorithms 878Specifies the available KEX (Key Exchange) algorithms. 879Multiple algorithms must be comma-separated. 880Alternately if the specified value begins with a 881.Sq + 882character, then the specified methods will be appended to the default set 883instead of replacing them. 884If the specified value begins with a 885.Sq - 886character, then the specified methods (including wildcards) will be removed 887from the default set instead of replacing them. 888The supported algorithms are: 889.Pp 890.Bl -item -compact -offset indent 891.It 892curve25519-sha256 893.It 894curve25519-sha256@libssh.org 895.It 896diffie-hellman-group1-sha1 897.It 898diffie-hellman-group14-sha1 899.It 900diffie-hellman-group14-sha256 901.It 902diffie-hellman-group16-sha512 903.It 904diffie-hellman-group18-sha512 905.It 906diffie-hellman-group-exchange-sha1 907.It 908diffie-hellman-group-exchange-sha256 909.It 910ecdh-sha2-nistp256 911.It 912ecdh-sha2-nistp384 913.It 914ecdh-sha2-nistp521 915.El 916.Pp 917The default is: 918.Bd -literal -offset indent 919curve25519-sha256,curve25519-sha256@libssh.org, 920ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 921diffie-hellman-group-exchange-sha256, 922diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, 923diffie-hellman-group14-sha256,diffie-hellman-group14-sha1 924.Ed 925.Pp 926The list of available key exchange algorithms may also be obtained using 927.Qq ssh -Q kex . 928.It Cm ListenAddress 929Specifies the local addresses 930.Xr sshd 8 931should listen on. 932The following forms may be used: 933.Pp 934.Bl -item -offset indent -compact 935.It 936.Cm ListenAddress 937.Sm off 938.Ar hostname | address 939.Sm on 940.Op Cm rdomain Ar domain 941.It 942.Cm ListenAddress 943.Sm off 944.Ar hostname : port 945.Sm on 946.Op Cm rdomain Ar domain 947.It 948.Cm ListenAddress 949.Sm off 950.Ar IPv4_address : port 951.Sm on 952.Op Cm rdomain Ar domain 953.It 954.Cm ListenAddress 955.Sm off 956.Oo Ar hostname | address Oc : Ar port 957.Sm on 958.Op Cm rdomain Ar domain 959.El 960.Pp 961The optional 962.Cm rdomain 963qualifier requests 964.Xr sshd 8 965listen in an explicit routing domain. 966If 967.Ar port 968is not specified, 969sshd will listen on the address and all 970.Cm Port 971options specified. 972The default is to listen on all local addresses on the current default 973routing domain. 974Multiple 975.Cm ListenAddress 976options are permitted. 977For more information on routing domains, see 978.Xr rdomain 4 . 979.It Cm LoginGraceTime 980The server disconnects after this time if the user has not 981successfully logged in. 982If the value is 0, there is no time limit. 983The default is 120 seconds. 984.It Cm LogLevel 985Gives the verbosity level that is used when logging messages from 986.Xr sshd 8 . 987The possible values are: 988QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 989The default is INFO. 990DEBUG and DEBUG1 are equivalent. 991DEBUG2 and DEBUG3 each specify higher levels of debugging output. 992Logging with a DEBUG level violates the privacy of users and is not recommended. 993.It Cm MACs 994Specifies the available MAC (message authentication code) algorithms. 995The MAC algorithm is used for data integrity protection. 996Multiple algorithms must be comma-separated. 997If the specified value begins with a 998.Sq + 999character, then the specified algorithms will be appended to the default set 1000instead of replacing them. 1001If the specified value begins with a 1002.Sq - 1003character, then the specified algorithms (including wildcards) will be removed 1004from the default set instead of replacing them. 1005.Pp 1006The algorithms that contain 1007.Qq -etm 1008calculate the MAC after encryption (encrypt-then-mac). 1009These are considered safer and their use recommended. 1010The supported MACs are: 1011.Pp 1012.Bl -item -compact -offset indent 1013.It 1014hmac-md5 1015.It 1016hmac-md5-96 1017.It 1018hmac-sha1 1019.It 1020hmac-sha1-96 1021.It 1022hmac-sha2-256 1023.It 1024hmac-sha2-512 1025.It 1026umac-64@openssh.com 1027.It 1028umac-128@openssh.com 1029.It 1030hmac-md5-etm@openssh.com 1031.It 1032hmac-md5-96-etm@openssh.com 1033.It 1034hmac-sha1-etm@openssh.com 1035.It 1036hmac-sha1-96-etm@openssh.com 1037.It 1038hmac-sha2-256-etm@openssh.com 1039.It 1040hmac-sha2-512-etm@openssh.com 1041.It 1042umac-64-etm@openssh.com 1043.It 1044umac-128-etm@openssh.com 1045.El 1046.Pp 1047The default is: 1048.Bd -literal -offset indent 1049umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1050hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1051hmac-sha1-etm@openssh.com, 1052umac-64@openssh.com,umac-128@openssh.com, 1053hmac-sha2-256,hmac-sha2-512,hmac-sha1 1054.Ed 1055.Pp 1056The list of available MAC algorithms may also be obtained using 1057.Qq ssh -Q mac . 1058.It Cm Match 1059Introduces a conditional block. 1060If all of the criteria on the 1061.Cm Match 1062line are satisfied, the keywords on the following lines override those 1063set in the global section of the config file, until either another 1064.Cm Match 1065line or the end of the file. 1066If a keyword appears in multiple 1067.Cm Match 1068blocks that are satisfied, only the first instance of the keyword is 1069applied. 1070.Pp 1071The arguments to 1072.Cm Match 1073are one or more criteria-pattern pairs or the single token 1074.Cm All 1075which matches all criteria. 1076The available criteria are 1077.Cm User , 1078.Cm Group , 1079.Cm Host , 1080.Cm LocalAddress , 1081.Cm LocalPort , 1082.Cm RDomain , 1083and 1084.Cm Address 1085(with 1086.Cm RDomain 1087representing the 1088.Xr rdomain 4 1089on which the connection was received.) 1090.Pp 1091The match patterns may consist of single entries or comma-separated 1092lists and may use the wildcard and negation operators described in the 1093.Sx PATTERNS 1094section of 1095.Xr ssh_config 5 . 1096.Pp 1097The patterns in an 1098.Cm Address 1099criteria may additionally contain addresses to match in CIDR 1100address/masklen format, 1101such as 192.0.2.0/24 or 2001:db8::/32. 1102Note that the mask length provided must be consistent with the address - 1103it is an error to specify a mask length that is too long for the address 1104or one with bits set in this host portion of the address. 1105For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. 1106.Pp 1107Only a subset of keywords may be used on the lines following a 1108.Cm Match 1109keyword. 1110Available keywords are 1111.Cm AcceptEnv , 1112.Cm AllowAgentForwarding , 1113.Cm AllowGroups , 1114.Cm AllowStreamLocalForwarding , 1115.Cm AllowTcpForwarding , 1116.Cm AllowUsers , 1117.Cm AuthenticationMethods , 1118.Cm AuthorizedKeysCommand , 1119.Cm AuthorizedKeysCommandUser , 1120.Cm AuthorizedKeysFile , 1121.Cm AuthorizedPrincipalsCommand , 1122.Cm AuthorizedPrincipalsCommandUser , 1123.Cm AuthorizedPrincipalsFile , 1124.Cm Banner , 1125.Cm ChrootDirectory , 1126.Cm ClientAliveCountMax , 1127.Cm ClientAliveInterval , 1128.Cm DenyGroups , 1129.Cm DenyUsers , 1130.Cm ForceCommand , 1131.Cm GatewayPorts , 1132.Cm GSSAPIAuthentication , 1133.Cm HostbasedAcceptedKeyTypes , 1134.Cm HostbasedAuthentication , 1135.Cm HostbasedUsesNameFromPacketOnly , 1136.Cm IPQoS , 1137.Cm KbdInteractiveAuthentication , 1138.Cm KerberosAuthentication , 1139.Cm LogLevel , 1140.Cm MaxAuthTries , 1141.Cm MaxSessions , 1142.Cm PasswordAuthentication , 1143.Cm PermitEmptyPasswords , 1144.Cm PermitListen , 1145.Cm PermitOpen , 1146.Cm PermitRootLogin , 1147.Cm PermitTTY , 1148.Cm PermitTunnel , 1149.Cm PermitUserRC , 1150.Cm PubkeyAcceptedKeyTypes , 1151.Cm PubkeyAuthentication , 1152.Cm RekeyLimit , 1153.Cm RevokedKeys , 1154.Cm RDomain , 1155.Cm SetEnv , 1156.Cm StreamLocalBindMask , 1157.Cm StreamLocalBindUnlink , 1158.Cm TrustedUserCAKeys , 1159.Cm X11DisplayOffset , 1160.Cm X11Forwarding 1161and 1162.Cm X11UseLocalHost . 1163.It Cm MaxAuthTries 1164Specifies the maximum number of authentication attempts permitted per 1165connection. 1166Once the number of failures reaches half this value, 1167additional failures are logged. 1168The default is 6. 1169.It Cm MaxSessions 1170Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1171sessions permitted per network connection. 1172Multiple sessions may be established by clients that support connection 1173multiplexing. 1174Setting 1175.Cm MaxSessions 1176to 1 will effectively disable session multiplexing, whereas setting it to 0 1177will prevent all shell, login and subsystem sessions while still permitting 1178forwarding. 1179The default is 10. 1180.It Cm MaxStartups 1181Specifies the maximum number of concurrent unauthenticated connections to the 1182SSH daemon. 1183Additional connections will be dropped until authentication succeeds or the 1184.Cm LoginGraceTime 1185expires for a connection. 1186The default is 10:30:100. 1187.Pp 1188Alternatively, random early drop can be enabled by specifying 1189the three colon separated values 1190start:rate:full (e.g. "10:30:60"). 1191.Xr sshd 8 1192will refuse connection attempts with a probability of rate/100 (30%) 1193if there are currently start (10) unauthenticated connections. 1194The probability increases linearly and all connection attempts 1195are refused if the number of unauthenticated connections reaches full (60). 1196.It Cm PasswordAuthentication 1197Specifies whether password authentication is allowed. 1198See also 1199.Cm UsePAM . 1200The default is 1201.Cm no . 1202.It Cm PermitEmptyPasswords 1203When password authentication is allowed, it specifies whether the 1204server allows login to accounts with empty password strings. 1205The default is 1206.Cm no . 1207.It Cm PermitListen 1208Specifies the addresses/ports on which a remote TCP port forwarding may listen. 1209The listen specification must be one of the following forms: 1210.Pp 1211.Bl -item -offset indent -compact 1212.It 1213.Cm PermitListen 1214.Sm off 1215.Ar port 1216.Sm on 1217.It 1218.Cm PermitListen 1219.Sm off 1220.Ar host : port 1221.Sm on 1222.El 1223.Pp 1224Multiple permissions may be specified by separating them with whitespace. 1225An argument of 1226.Cm any 1227can be used to remove all restrictions and permit any listen requests. 1228An argument of 1229.Cm none 1230can be used to prohibit all listen requests. 1231The host name may contain wildcards as described in the PATTERNS section in 1232.Xr ssh_config 5 . 1233The wildcard 1234.Sq * 1235can also be used in place of a port number to allow all ports. 1236By default all port forwarding listen requests are permitted. 1237Note that the 1238.Cm GatewayPorts 1239option may further restrict which addresses may be listened on. 1240Note also that 1241.Xr ssh 1 1242will request a listen host of 1243.Dq localhost 1244if no listen host was specifically requested, and this this name is 1245treated differently to explicit localhost addresses of 1246.Dq 127.0.0.1 1247and 1248.Dq ::1 . 1249.It Cm PermitOpen 1250Specifies the destinations to which TCP port forwarding is permitted. 1251The forwarding specification must be one of the following forms: 1252.Pp 1253.Bl -item -offset indent -compact 1254.It 1255.Cm PermitOpen 1256.Sm off 1257.Ar host : port 1258.Sm on 1259.It 1260.Cm PermitOpen 1261.Sm off 1262.Ar IPv4_addr : port 1263.Sm on 1264.It 1265.Cm PermitOpen 1266.Sm off 1267.Ar \&[ IPv6_addr \&] : port 1268.Sm on 1269.El 1270.Pp 1271Multiple forwards may be specified by separating them with whitespace. 1272An argument of 1273.Cm any 1274can be used to remove all restrictions and permit any forwarding requests. 1275An argument of 1276.Cm none 1277can be used to prohibit all forwarding requests. 1278The wildcard 1279.Sq * 1280can be used for host or port to allow all hosts or ports, respectively. 1281By default all port forwarding requests are permitted. 1282.It Cm PermitRootLogin 1283Specifies whether root can log in using 1284.Xr ssh 1 . 1285The argument must be 1286.Cm yes , 1287.Cm prohibit-password , 1288.Cm forced-commands-only , 1289or 1290.Cm no . 1291The default is 1292.Cm no . 1293Note that if 1294.Cm ChallengeResponseAuthentication 1295and 1296.Cm UsePAM 1297are both 1298.Cm yes , 1299this setting may be overridden by the PAM policy. 1300.Pp 1301If this option is set to 1302.Cm prohibit-password 1303(or its deprecated alias, 1304.Cm without-password ) , 1305password and keyboard-interactive authentication are disabled for root. 1306.Pp 1307If this option is set to 1308.Cm forced-commands-only , 1309root login with public key authentication will be allowed, 1310but only if the 1311.Ar command 1312option has been specified 1313(which may be useful for taking remote backups even if root login is 1314normally not allowed). 1315All other authentication methods are disabled for root. 1316.Pp 1317If this option is set to 1318.Cm no , 1319root is not allowed to log in. 1320.It Cm PermitTTY 1321Specifies whether 1322.Xr pty 4 1323allocation is permitted. 1324The default is 1325.Cm yes . 1326.It Cm PermitTunnel 1327Specifies whether 1328.Xr tun 4 1329device forwarding is allowed. 1330The argument must be 1331.Cm yes , 1332.Cm point-to-point 1333(layer 3), 1334.Cm ethernet 1335(layer 2), or 1336.Cm no . 1337Specifying 1338.Cm yes 1339permits both 1340.Cm point-to-point 1341and 1342.Cm ethernet . 1343The default is 1344.Cm no . 1345.Pp 1346Independent of this setting, the permissions of the selected 1347.Xr tun 4 1348device must allow access to the user. 1349.It Cm PermitUserEnvironment 1350Specifies whether 1351.Pa ~/.ssh/environment 1352and 1353.Cm environment= 1354options in 1355.Pa ~/.ssh/authorized_keys 1356are processed by 1357.Xr sshd 8 . 1358Valid options are 1359.Cm yes , 1360.Cm no 1361or a pattern-list specifying which environment variable names to accept 1362(for example 1363.Qq LANG,LC_* ) . 1364The default is 1365.Cm no . 1366Enabling environment processing may enable users to bypass access 1367restrictions in some configurations using mechanisms such as 1368.Ev LD_PRELOAD . 1369.It Cm PermitUserRC 1370Specifies whether any 1371.Pa ~/.ssh/rc 1372file is executed. 1373The default is 1374.Cm yes . 1375.It Cm PidFile 1376Specifies the file that contains the process ID of the 1377SSH daemon, or 1378.Cm none 1379to not write one. 1380The default is 1381.Pa /var/run/sshd.pid . 1382.It Cm Port 1383Specifies the port number that 1384.Xr sshd 8 1385listens on. 1386The default is 22. 1387Multiple options of this type are permitted. 1388See also 1389.Cm ListenAddress . 1390.It Cm PrintLastLog 1391Specifies whether 1392.Xr sshd 8 1393should print the date and time of the last user login when a user logs 1394in interactively. 1395The default is 1396.Cm yes . 1397.It Cm PrintMotd 1398Specifies whether 1399.Xr sshd 8 1400should print 1401.Pa /etc/motd 1402when a user logs in interactively. 1403(On some systems it is also printed by the shell, 1404.Pa /etc/profile , 1405or equivalent.) 1406The default is 1407.Cm yes . 1408.It Cm PubkeyAcceptedKeyTypes 1409Specifies the key types that will be accepted for public key authentication 1410as a list of comma-separated patterns. 1411Alternately if the specified value begins with a 1412.Sq + 1413character, then the specified key types will be appended to the default set 1414instead of replacing them. 1415If the specified value begins with a 1416.Sq - 1417character, then the specified key types (including wildcards) will be removed 1418from the default set instead of replacing them. 1419The default for this option is: 1420.Bd -literal -offset 3n 1421ecdsa-sha2-nistp256-cert-v01@openssh.com, 1422ecdsa-sha2-nistp384-cert-v01@openssh.com, 1423ecdsa-sha2-nistp521-cert-v01@openssh.com, 1424ssh-ed25519-cert-v01@openssh.com, 1425rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 1426ssh-rsa-cert-v01@openssh.com, 1427ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1428ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 1429.Ed 1430.Pp 1431The list of available key types may also be obtained using 1432.Qq ssh -Q key . 1433.It Cm PubkeyAuthentication 1434Specifies whether public key authentication is allowed. 1435The default is 1436.Cm yes . 1437.It Cm RekeyLimit 1438Specifies the maximum amount of data that may be transmitted before the 1439session key is renegotiated, optionally followed a maximum amount of 1440time that may pass before the session key is renegotiated. 1441The first argument is specified in bytes and may have a suffix of 1442.Sq K , 1443.Sq M , 1444or 1445.Sq G 1446to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1447The default is between 1448.Sq 1G 1449and 1450.Sq 4G , 1451depending on the cipher. 1452The optional second value is specified in seconds and may use any of the 1453units documented in the 1454.Sx TIME FORMATS 1455section. 1456The default value for 1457.Cm RekeyLimit 1458is 1459.Cm default none , 1460which means that rekeying is performed after the cipher's default amount 1461of data has been sent or received and no time based rekeying is done. 1462.It Cm RevokedKeys 1463Specifies revoked public keys file, or 1464.Cm none 1465to not use one. 1466Keys listed in this file will be refused for public key authentication. 1467Note that if this file is not readable, then public key authentication will 1468be refused for all users. 1469Keys may be specified as a text file, listing one public key per line, or as 1470an OpenSSH Key Revocation List (KRL) as generated by 1471.Xr ssh-keygen 1 . 1472For more information on KRLs, see the KEY REVOCATION LISTS section in 1473.Xr ssh-keygen 1 . 1474.It Cm RDomain 1475Specifies an explicit routing domain that is applied after authentication 1476has completed. 1477The user session, as well and any forwarded or listening IP sockets, 1478will be bound to this 1479.Xr rdomain 4 . 1480If the routing domain is set to 1481.Cm \&%D , 1482then the domain in which the incoming connection was received will be applied. 1483.It Cm SetEnv 1484Specifies one or more environment variables to set in child sessions started 1485by 1486.Xr sshd 8 1487as 1488.Dq NAME=VALUE . 1489The environment value may be quoted (e.g. if it contains whitespace 1490characters). 1491Environment variables set by 1492.Cm SetEnv 1493override the default environment and any variables specified by the user 1494via 1495.Cm AcceptEnv 1496or 1497.Cm PermitUserEnvironment . 1498.It Cm StreamLocalBindMask 1499Sets the octal file creation mode mask 1500.Pq umask 1501used when creating a Unix-domain socket file for local or remote 1502port forwarding. 1503This option is only used for port forwarding to a Unix-domain socket file. 1504.Pp 1505The default value is 0177, which creates a Unix-domain socket file that is 1506readable and writable only by the owner. 1507Note that not all operating systems honor the file mode on Unix-domain 1508socket files. 1509.It Cm StreamLocalBindUnlink 1510Specifies whether to remove an existing Unix-domain socket file for local 1511or remote port forwarding before creating a new one. 1512If the socket file already exists and 1513.Cm StreamLocalBindUnlink 1514is not enabled, 1515.Nm sshd 1516will be unable to forward the port to the Unix-domain socket file. 1517This option is only used for port forwarding to a Unix-domain socket file. 1518.Pp 1519The argument must be 1520.Cm yes 1521or 1522.Cm no . 1523The default is 1524.Cm no . 1525.It Cm StrictModes 1526Specifies whether 1527.Xr sshd 8 1528should check file modes and ownership of the 1529user's files and home directory before accepting login. 1530This is normally desirable because novices sometimes accidentally leave their 1531directory or files world-writable. 1532The default is 1533.Cm yes . 1534Note that this does not apply to 1535.Cm ChrootDirectory , 1536whose permissions and ownership are checked unconditionally. 1537.It Cm Subsystem 1538Configures an external subsystem (e.g. file transfer daemon). 1539Arguments should be a subsystem name and a command (with optional arguments) 1540to execute upon subsystem request. 1541.Pp 1542The command 1543.Cm sftp-server 1544implements the SFTP file transfer subsystem. 1545.Pp 1546Alternately the name 1547.Cm internal-sftp 1548implements an in-process SFTP server. 1549This may simplify configurations using 1550.Cm ChrootDirectory 1551to force a different filesystem root on clients. 1552.Pp 1553By default no subsystems are defined. 1554.It Cm SyslogFacility 1555Gives the facility code that is used when logging messages from 1556.Xr sshd 8 . 1557The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1558LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1559The default is AUTH. 1560.It Cm TCPKeepAlive 1561Specifies whether the system should send TCP keepalive messages to the 1562other side. 1563If they are sent, death of the connection or crash of one 1564of the machines will be properly noticed. 1565However, this means that 1566connections will die if the route is down temporarily, and some people 1567find it annoying. 1568On the other hand, if TCP keepalives are not sent, 1569sessions may hang indefinitely on the server, leaving 1570.Qq ghost 1571users and consuming server resources. 1572.Pp 1573The default is 1574.Cm yes 1575(to send TCP keepalive messages), and the server will notice 1576if the network goes down or the client host crashes. 1577This avoids infinitely hanging sessions. 1578.Pp 1579To disable TCP keepalive messages, the value should be set to 1580.Cm no . 1581.It Cm TrustedUserCAKeys 1582Specifies a file containing public keys of certificate authorities that are 1583trusted to sign user certificates for authentication, or 1584.Cm none 1585to not use one. 1586Keys are listed one per line; empty lines and comments starting with 1587.Ql # 1588are allowed. 1589If a certificate is presented for authentication and has its signing CA key 1590listed in this file, then it may be used for authentication for any user 1591listed in the certificate's principals list. 1592Note that certificates that lack a list of principals will not be permitted 1593for authentication using 1594.Cm TrustedUserCAKeys . 1595For more details on certificates, see the CERTIFICATES section in 1596.Xr ssh-keygen 1 . 1597.It Cm UseBlacklist 1598Specifies whether 1599.Xr sshd 8 1600attempts to send authentication success and failure messages 1601to the 1602.Xr blacklistd 8 1603daemon. 1604The default is 1605.Cm no . 1606.It Cm UseDNS 1607Specifies whether 1608.Xr sshd 8 1609should look up the remote host name, and to check that 1610the resolved host name for the remote IP address maps back to the 1611very same IP address. 1612.Pp 1613If this option is set to 1614.Cm no , 1615then only addresses and not host names may be used in 1616.Pa ~/.ssh/authorized_keys 1617.Cm from 1618and 1619.Nm 1620.Cm Match 1621.Cm Host 1622directives. 1623The default is 1624.Dq yes . 1625.It Cm UsePAM 1626Enables the Pluggable Authentication Module interface. 1627If set to 1628.Cm yes 1629this will enable PAM authentication using 1630.Cm ChallengeResponseAuthentication 1631and 1632.Cm PasswordAuthentication 1633in addition to PAM account and session module processing for all 1634authentication types. 1635.Pp 1636Because PAM challenge-response authentication usually serves an equivalent 1637role to password authentication, you should disable either 1638.Cm PasswordAuthentication 1639or 1640.Cm ChallengeResponseAuthentication. 1641.Pp 1642If 1643.Cm UsePAM 1644is enabled, you will not be able to run 1645.Xr sshd 8 1646as a non-root user. 1647The default is 1648.Cm yes . 1649.It Cm VersionAddendum 1650Optionally specifies additional text to append to the SSH protocol banner 1651sent by the server upon connection. 1652The default is 1653.Qq FreeBSD-20180909 . 1654The value 1655.Cm none 1656may be used to disable this. 1657.It Cm X11DisplayOffset 1658Specifies the first display number available for 1659.Xr sshd 8 Ns 's 1660X11 forwarding. 1661This prevents sshd from interfering with real X11 servers. 1662The default is 10. 1663.It Cm X11Forwarding 1664Specifies whether X11 forwarding is permitted. 1665The argument must be 1666.Cm yes 1667or 1668.Cm no . 1669The default is 1670.Cm yes . 1671.Pp 1672When X11 forwarding is enabled, there may be additional exposure to 1673the server and to client displays if the 1674.Xr sshd 8 1675proxy display is configured to listen on the wildcard address (see 1676.Cm X11UseLocalhost ) , 1677though this is not the default. 1678Additionally, the authentication spoofing and authentication data 1679verification and substitution occur on the client side. 1680The security risk of using X11 forwarding is that the client's X11 1681display server may be exposed to attack when the SSH client requests 1682forwarding (see the warnings for 1683.Cm ForwardX11 1684in 1685.Xr ssh_config 5 ) . 1686A system administrator may have a stance in which they want to 1687protect clients that may expose themselves to attack by unwittingly 1688requesting X11 forwarding, which can warrant a 1689.Cm no 1690setting. 1691.Pp 1692Note that disabling X11 forwarding does not prevent users from 1693forwarding X11 traffic, as users can always install their own forwarders. 1694.It Cm X11UseLocalhost 1695Specifies whether 1696.Xr sshd 8 1697should bind the X11 forwarding server to the loopback address or to 1698the wildcard address. 1699By default, 1700sshd binds the forwarding server to the loopback address and sets the 1701hostname part of the 1702.Ev DISPLAY 1703environment variable to 1704.Cm localhost . 1705This prevents remote hosts from connecting to the proxy display. 1706However, some older X11 clients may not function with this 1707configuration. 1708.Cm X11UseLocalhost 1709may be set to 1710.Cm no 1711to specify that the forwarding server should be bound to the wildcard 1712address. 1713The argument must be 1714.Cm yes 1715or 1716.Cm no . 1717The default is 1718.Cm yes . 1719.It Cm XAuthLocation 1720Specifies the full pathname of the 1721.Xr xauth 1 1722program, or 1723.Cm none 1724to not use one. 1725The default is 1726.Pa /usr/local/bin/xauth . 1727.El 1728.Sh TIME FORMATS 1729.Xr sshd 8 1730command-line arguments and configuration file options that specify time 1731may be expressed using a sequence of the form: 1732.Sm off 1733.Ar time Op Ar qualifier , 1734.Sm on 1735where 1736.Ar time 1737is a positive integer value and 1738.Ar qualifier 1739is one of the following: 1740.Pp 1741.Bl -tag -width Ds -compact -offset indent 1742.It Aq Cm none 1743seconds 1744.It Cm s | Cm S 1745seconds 1746.It Cm m | Cm M 1747minutes 1748.It Cm h | Cm H 1749hours 1750.It Cm d | Cm D 1751days 1752.It Cm w | Cm W 1753weeks 1754.El 1755.Pp 1756Each member of the sequence is added together to calculate 1757the total time value. 1758.Pp 1759Time format examples: 1760.Pp 1761.Bl -tag -width Ds -compact -offset indent 1762.It 600 1763600 seconds (10 minutes) 1764.It 10m 176510 minutes 1766.It 1h30m 17671 hour 30 minutes (90 minutes) 1768.El 1769.Sh TOKENS 1770Arguments to some keywords can make use of tokens, 1771which are expanded at runtime: 1772.Pp 1773.Bl -tag -width XXXX -offset indent -compact 1774.It %% 1775A literal 1776.Sq % . 1777.It \&%D 1778The routing domain in which the incoming connection was received. 1779.It %F 1780The fingerprint of the CA key. 1781.It %f 1782The fingerprint of the key or certificate. 1783.It %h 1784The home directory of the user. 1785.It %i 1786The key ID in the certificate. 1787.It %K 1788The base64-encoded CA key. 1789.It %k 1790The base64-encoded key or certificate for authentication. 1791.It %s 1792The serial number of the certificate. 1793.It \&%T 1794The type of the CA key. 1795.It %t 1796The key or certificate type. 1797.It \&%U 1798The numeric user ID of the target user. 1799.It %u 1800The username. 1801.El 1802.Pp 1803.Cm AuthorizedKeysCommand 1804accepts the tokens %%, %f, %h, %k, %t, %U, and %u. 1805.Pp 1806.Cm AuthorizedKeysFile 1807accepts the tokens %%, %h, %U, and %u. 1808.Pp 1809.Cm AuthorizedPrincipalsCommand 1810accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. 1811.Pp 1812.Cm AuthorizedPrincipalsFile 1813accepts the tokens %%, %h, %U, and %u. 1814.Pp 1815.Cm ChrootDirectory 1816accepts the tokens %%, %h, %U, and %u. 1817.Pp 1818.Cm RoutingDomain 1819accepts the token %D. 1820.Sh FILES 1821.Bl -tag -width Ds 1822.It Pa /etc/ssh/sshd_config 1823Contains configuration data for 1824.Xr sshd 8 . 1825This file should be writable by root only, but it is recommended 1826(though not necessary) that it be world-readable. 1827.El 1828.Sh SEE ALSO 1829.Xr sftp-server 8 , 1830.Xr sshd 8 1831.Sh AUTHORS 1832.An -nosplit 1833OpenSSH is a derivative of the original and free 1834ssh 1.2.12 release by 1835.An Tatu Ylonen . 1836.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , 1837.An Theo de Raadt 1838and 1839.An Dug Song 1840removed many bugs, re-added newer features and 1841created OpenSSH. 1842.An Markus Friedl 1843contributed the support for SSH protocol versions 1.5 and 2.0. 1844.An Niels Provos 1845and 1846.An Markus Friedl 1847contributed support for privilege separation. 1848