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 1.220 2016/02/17 08:57:34 djm Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: February 17 2016 $ 39.Dt SSHD_CONFIG 5 40.Os 41.Sh NAME 42.Nm sshd_config 43.Nd OpenSSH SSH daemon configuration file 44.Sh SYNOPSIS 45.Nm /etc/ssh/sshd_config 46.Sh DESCRIPTION 47.Xr sshd 8 48reads configuration data from 49.Pa /etc/ssh/sshd_config 50(or the file specified with 51.Fl f 52on the command line). 53The file contains keyword-argument pairs, one per line. 54Lines starting with 55.Ql # 56and empty lines are interpreted as comments. 57Arguments may optionally be enclosed in double quotes 58.Pq \&" 59in order to represent arguments containing spaces. 60.Pp 61The possible 62keywords and their meanings are as follows (note that 63keywords are case-insensitive and arguments are case-sensitive): 64.Bl -tag -width Ds 65.It Cm AcceptEnv 66Specifies what environment variables sent by the client will be copied into 67the session's 68.Xr environ 7 . 69See 70.Cm SendEnv 71in 72.Xr ssh_config 5 73for how to configure the client. 74The 75.Ev TERM 76environment variable is always sent whenever the client 77requests a pseudo-terminal as it is required by the protocol. 78Variables are specified by name, which may contain the wildcard characters 79.Ql * 80and 81.Ql \&? . 82Multiple environment variables may be separated by whitespace or spread 83across multiple 84.Cm AcceptEnv 85directives. 86Be warned that some environment variables could be used to bypass restricted 87user environments. 88For this reason, care should be taken in the use of this directive. 89The default is not to accept any environment variables. 90.It Cm AddressFamily 91Specifies which address family should be used by 92.Xr sshd 8 . 93Valid arguments are 94.Dq any , 95.Dq inet 96(use IPv4 only), or 97.Dq inet6 98(use IPv6 only). 99The default is 100.Dq any . 101.It Cm AllowAgentForwarding 102Specifies whether 103.Xr ssh-agent 1 104forwarding is permitted. 105The default is 106.Dq 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 AllowTcpForwarding 128Specifies whether TCP forwarding is permitted. 129The available options are 130.Dq yes 131or 132.Dq all 133to allow TCP forwarding, 134.Dq no 135to prevent all TCP forwarding, 136.Dq local 137to allow local (from the perspective of 138.Xr ssh 1 ) 139forwarding only or 140.Dq remote 141to allow remote forwarding only. 142The default is 143.Dq yes . 144Note that disabling TCP forwarding does not improve security unless 145users are also denied shell access, as they can always install their 146own forwarders. 147.It Cm AllowStreamLocalForwarding 148Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 149The available options are 150.Dq yes 151or 152.Dq all 153to allow StreamLocal forwarding, 154.Dq no 155to prevent all StreamLocal forwarding, 156.Dq local 157to allow local (from the perspective of 158.Xr ssh 1 ) 159forwarding only or 160.Dq remote 161to allow remote forwarding only. 162The default is 163.Dq yes . 164Note that disabling StreamLocal forwarding does not improve security unless 165users are also denied shell access, as they can always install their 166own forwarders. 167.It Cm AllowUsers 168This keyword can be followed by a list of user name patterns, separated 169by spaces. 170If specified, login is allowed only for user names that 171match one of the patterns. 172Only user names are valid; a numerical user ID is not recognized. 173By default, login is allowed for all users. 174If the pattern takes the form USER@HOST then USER and HOST 175are separately checked, restricting logins to particular 176users from particular hosts. 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 comma-separated lists of 191authentication method names. 192Successful authentication requires completion of every method in at least 193one of these lists. 194.Pp 195For example, an argument of 196.Dq publickey,password publickey,keyboard-interactive 197would require the user to complete public key authentication, followed by 198either password or keyboard interactive authentication. 199Only methods that are next in one or more lists are offered at each stage, 200so for this example, it would not be possible to attempt password or 201keyboard-interactive authentication before public key. 202.Pp 203For keyboard interactive authentication it is also possible to 204restrict authentication to a specific device by appending a 205colon followed by the device identifier 206.Dq bsdauth , 207.Dq pam , 208or 209.Dq skey , 210depending on the server configuration. 211For example, 212.Dq keyboard-interactive:bsdauth 213would restrict keyboard interactive authentication to the 214.Dq bsdauth 215device. 216.Pp 217If the 218.Dq publickey 219method 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, an 224.Cm AuthenticationMethods 225of 226.Dq publickey,publickey 227will require successful authentication using two different public keys. 228.Pp 229This option will yield a fatal 230error if enabled if protocol 1 is also enabled. 231Note that each authentication method listed should also be explicitly enabled 232in the configuration. 233The default is not to require multiple authentication; successful completion 234of a single authentication method is sufficient. 235.It Cm AuthorizedKeysCommand 236Specifies a program to be used to look up the user's public keys. 237The program must be owned by root, not writable by group or others and 238specified by an absolute path. 239.Pp 240Arguments to 241.Cm AuthorizedKeysCommand 242may be provided using the following tokens, which will be expanded 243at runtime: %% is replaced by a literal '%', %u is replaced by the 244username being authenticated, %h is replaced by the home directory 245of the user being authenticated, %t is replaced with the key type 246offered for authentication, %f is replaced with the fingerprint of 247the key, and %k is replaced with the key being offered for authentication. 248If no arguments are specified then the username of the target user 249will be supplied. 250.Pp 251The program should produce on standard output zero or 252more lines of authorized_keys output (see AUTHORIZED_KEYS in 253.Xr sshd 8 ) . 254If a key supplied by AuthorizedKeysCommand does not successfully authenticate 255and authorize the user then public key authentication continues using the usual 256.Cm AuthorizedKeysFile 257files. 258By default, no AuthorizedKeysCommand is run. 259.It Cm AuthorizedKeysCommandUser 260Specifies the user under whose account the AuthorizedKeysCommand is run. 261It is recommended to use a dedicated user that has no other role on the host 262than running authorized keys commands. 263If 264.Cm AuthorizedKeysCommand 265is specified but 266.Cm AuthorizedKeysCommandUser 267is not, then 268.Xr sshd 8 269will refuse to start. 270.It Cm AuthorizedKeysFile 271Specifies the file that contains the public keys that can be used 272for user authentication. 273The format is described in the 274AUTHORIZED_KEYS FILE FORMAT 275section of 276.Xr sshd 8 . 277.Cm AuthorizedKeysFile 278may contain tokens of the form %T which are substituted during connection 279setup. 280The following tokens are defined: %% is replaced by a literal '%', 281%h is replaced by the home directory of the user being authenticated, and 282%u is replaced by the username of that user. 283After expansion, 284.Cm AuthorizedKeysFile 285is taken to be an absolute path or one relative to the user's home 286directory. 287Multiple files may be listed, separated by whitespace. 288Alternately this option may be set to 289.Dq none 290to skip checking for user keys in files. 291The default is 292.Dq .ssh/authorized_keys .ssh/authorized_keys2 . 293.It Cm AuthorizedPrincipalsCommand 294Specifies a program to be used to generate the list of allowed 295certificate principals as per 296.Cm AuthorizedPrincipalsFile . 297The program must be owned by root, not writable by group or others and 298specified by an absolute path. 299.Pp 300Arguments to 301.Cm AuthorizedPrincipalsCommand 302may be provided using the following tokens, which will be expanded 303at runtime: %% is replaced by a literal '%', %u is replaced by the 304username being authenticated and %h is replaced by the home directory 305of the user being authenticated. 306.Pp 307The program should produce on standard output zero or 308more lines of 309.Cm AuthorizedPrincipalsFile 310output. 311If either 312.Cm AuthorizedPrincipalsCommand 313or 314.Cm AuthorizedPrincipalsFile 315is specified, then certificates offered by the client for authentication 316must contain a principal that is listed. 317By default, no AuthorizedPrincipalsCommand is run. 318.It Cm AuthorizedPrincipalsCommandUser 319Specifies the user under whose account the AuthorizedPrincipalsCommand is run. 320It is recommended to use a dedicated user that has no other role on the host 321than running authorized principals commands. 322If 323.Cm AuthorizedPrincipalsCommand 324is specified but 325.Cm AuthorizedPrincipalsCommandUser 326is not, then 327.Xr sshd 8 328will refuse to start. 329.It Cm AuthorizedPrincipalsFile 330Specifies a file that lists principal names that are accepted for 331certificate authentication. 332When using certificates signed by a key listed in 333.Cm TrustedUserCAKeys , 334this file lists names, one of which must appear in the certificate for it 335to be accepted for authentication. 336Names are listed one per line preceded by key options (as described 337in AUTHORIZED_KEYS FILE FORMAT in 338.Xr sshd 8 ) . 339Empty lines and comments starting with 340.Ql # 341are ignored. 342.Pp 343.Cm AuthorizedPrincipalsFile 344may contain tokens of the form %T which are substituted during connection 345setup. 346The following tokens are defined: %% is replaced by a literal '%', 347%h is replaced by the home directory of the user being authenticated, and 348%u is replaced by the username of that user. 349After expansion, 350.Cm AuthorizedPrincipalsFile 351is taken to be an absolute path or one relative to the user's home 352directory. 353.Pp 354The default is 355.Dq none , 356i.e. not to use a principals file \(en in this case, the username 357of the user must appear in a certificate's principals list for it to be 358accepted. 359Note that 360.Cm AuthorizedPrincipalsFile 361is only used when authentication proceeds using a CA listed in 362.Cm TrustedUserCAKeys 363and is not consulted for certification authorities trusted via 364.Pa ~/.ssh/authorized_keys , 365though the 366.Cm principals= 367key option offers a similar facility (see 368.Xr sshd 8 369for details). 370.It Cm Banner 371The contents of the specified file are sent to the remote user before 372authentication is allowed. 373If the argument is 374.Dq none 375then no banner is displayed. 376By default, no banner is displayed. 377.It Cm ChallengeResponseAuthentication 378Specifies whether challenge-response authentication is allowed (e.g. via 379PAM or through authentication styles supported in 380.Xr login.conf 5 ) 381The default is 382.Dq yes . 383.It Cm ChrootDirectory 384Specifies the pathname of a directory to 385.Xr chroot 2 386to after authentication. 387At session startup 388.Xr sshd 8 389checks that all components of the pathname are root-owned directories 390which are not writable by any other user or group. 391After the chroot, 392.Xr sshd 8 393changes the working directory to the user's home directory. 394.Pp 395The pathname may contain the following tokens that are expanded at runtime once 396the connecting user has been authenticated: %% is replaced by a literal '%', 397%h is replaced by the home directory of the user being authenticated, and 398%u is replaced by the username of that user. 399.Pp 400The 401.Cm ChrootDirectory 402must contain the necessary files and directories to support the 403user's session. 404For an interactive session this requires at least a shell, typically 405.Xr sh 1 , 406and basic 407.Pa /dev 408nodes such as 409.Xr null 4 , 410.Xr zero 4 , 411.Xr stdin 4 , 412.Xr stdout 4 , 413.Xr stderr 4 , 414and 415.Xr tty 4 416devices. 417For file transfer sessions using 418.Dq sftp , 419no additional configuration of the environment is necessary if the 420in-process sftp server is used, 421though sessions which use logging may require 422.Pa /dev/log 423inside the chroot directory on some operating systems (see 424.Xr sftp-server 8 425for details). 426.Pp 427For safety, it is very important that the directory hierarchy be 428prevented from modification by other processes on the system (especially 429those outside the jail). 430Misconfiguration can lead to unsafe environments which 431.Xr sshd 8 432cannot detect. 433.Pp 434The default is 435.Dq none , 436indicating not to 437.Xr chroot 2 . 438.It Cm Ciphers 439Specifies the ciphers allowed. 440Multiple ciphers must be comma-separated. 441If the specified value begins with a 442.Sq + 443character, then the specified ciphers will be appended to the default set 444instead of replacing them. 445.Pp 446The supported ciphers are: 447.Pp 448.Bl -item -compact -offset indent 449.It 4503des-cbc 451.It 452aes128-cbc 453.It 454aes192-cbc 455.It 456aes256-cbc 457.It 458aes128-ctr 459.It 460aes192-ctr 461.It 462aes256-ctr 463.It 464aes128-gcm@openssh.com 465.It 466aes256-gcm@openssh.com 467.It 468arcfour 469.It 470arcfour128 471.It 472arcfour256 473.It 474blowfish-cbc 475.It 476cast128-cbc 477.It 478chacha20-poly1305@openssh.com 479.El 480.Pp 481The default is: 482.Bd -literal -offset indent 483chacha20-poly1305@openssh.com, 484aes128-ctr,aes192-ctr,aes256-ctr, 485aes128-gcm@openssh.com,aes256-gcm@openssh.com, 486aes128-cbc,aes192-cbc,aes256-cbc 487.Ed 488.Pp 489The list of available ciphers may also be obtained using the 490.Fl Q 491option of 492.Xr ssh 1 493with an argument of 494.Dq cipher . 495.It Cm ClientAliveCountMax 496Sets the number of client alive messages (see below) which may be 497sent without 498.Xr sshd 8 499receiving any messages back from the client. 500If this threshold is reached while client alive messages are being sent, 501sshd will disconnect the client, terminating the session. 502It is important to note that the use of client alive messages is very 503different from 504.Cm TCPKeepAlive 505(below). 506The client alive messages are sent through the encrypted channel 507and therefore will not be spoofable. 508The TCP keepalive option enabled by 509.Cm TCPKeepAlive 510is spoofable. 511The client alive mechanism is valuable when the client or 512server depend on knowing when a connection has become inactive. 513.Pp 514The default value is 3. 515If 516.Cm ClientAliveInterval 517(see below) is set to 15, and 518.Cm ClientAliveCountMax 519is left at the default, unresponsive SSH clients 520will be disconnected after approximately 45 seconds. 521.It Cm ClientAliveInterval 522Sets a timeout interval in seconds after which if no data has been received 523from the client, 524.Xr sshd 8 525will send a message through the encrypted 526channel to request a response from the client. 527The default 528is 0, indicating that these messages will not be sent to the client. 529.It Cm Compression 530Specifies whether compression is allowed, or delayed until 531the user has authenticated successfully. 532The argument must be 533.Dq yes , 534.Dq delayed , 535or 536.Dq no . 537The default is 538.Dq delayed . 539.It Cm DenyGroups 540This keyword can be followed by a list of group name patterns, separated 541by spaces. 542Login is disallowed for users whose primary group or supplementary 543group list matches one of the patterns. 544Only group names are valid; a numerical group ID is not recognized. 545By default, login is allowed for all groups. 546The allow/deny directives are processed in the following order: 547.Cm DenyUsers , 548.Cm AllowUsers , 549.Cm DenyGroups , 550and finally 551.Cm AllowGroups . 552.Pp 553See PATTERNS in 554.Xr ssh_config 5 555for more information on patterns. 556.It Cm DenyUsers 557This keyword can be followed by a list of user name patterns, separated 558by spaces. 559Login is disallowed for user names that match one of the patterns. 560Only user names are valid; a numerical user ID is not recognized. 561By default, login is allowed for all users. 562If the pattern takes the form USER@HOST then USER and HOST 563are separately checked, restricting logins to particular 564users from particular hosts. 565The allow/deny directives are processed in the following order: 566.Cm DenyUsers , 567.Cm AllowUsers , 568.Cm DenyGroups , 569and finally 570.Cm AllowGroups . 571.Pp 572See PATTERNS in 573.Xr ssh_config 5 574for more information on patterns. 575.It Cm FingerprintHash 576Specifies the hash algorithm used when logging key fingerprints. 577Valid options are: 578.Dq md5 579and 580.Dq sha256 . 581The default is 582.Dq sha256 . 583.It Cm ForceCommand 584Forces the execution of the command specified by 585.Cm ForceCommand , 586ignoring any command supplied by the client and 587.Pa ~/.ssh/rc 588if present. 589The command is invoked by using the user's login shell with the -c option. 590This applies to shell, command, or subsystem execution. 591It is most useful inside a 592.Cm Match 593block. 594The command originally supplied by the client is available in the 595.Ev SSH_ORIGINAL_COMMAND 596environment variable. 597Specifying a command of 598.Dq internal-sftp 599will force the use of an in-process sftp server that requires no support 600files when used with 601.Cm ChrootDirectory . 602The default is 603.Dq none . 604.It Cm GatewayPorts 605Specifies whether remote hosts are allowed to connect to ports 606forwarded for the client. 607By default, 608.Xr sshd 8 609binds remote port forwardings to the loopback address. 610This prevents other remote hosts from connecting to forwarded ports. 611.Cm GatewayPorts 612can be used to specify that sshd 613should allow remote port forwardings to bind to non-loopback addresses, thus 614allowing other hosts to connect. 615The argument may be 616.Dq no 617to force remote port forwardings to be available to the local host only, 618.Dq yes 619to force remote port forwardings to bind to the wildcard address, or 620.Dq clientspecified 621to allow the client to select the address to which the forwarding is bound. 622The default is 623.Dq no . 624.It Cm GSSAPIAuthentication 625Specifies whether user authentication based on GSSAPI is allowed. 626The default is 627.Dq no . 628.It Cm GSSAPICleanupCredentials 629Specifies whether to automatically destroy the user's credentials cache 630on logout. 631The default is 632.Dq yes . 633.It Cm GSSAPIStrictAcceptorCheck 634Determines whether to be strict about the identity of the GSSAPI acceptor 635a client authenticates against. 636If set to 637.Dq yes 638then the client must authenticate against the 639.Pa host 640service on the current hostname. 641If set to 642.Dq no 643then the client may authenticate against any service key stored in the 644machine's default store. 645This facility is provided to assist with operation on multi homed machines. 646The default is 647.Dq yes . 648.It Cm HostbasedAcceptedKeyTypes 649Specifies the key types that will be accepted for hostbased authentication 650as a comma-separated pattern list. 651Alternately if the specified value begins with a 652.Sq + 653character, then the specified key types will be appended to the default set 654instead of replacing them. 655The default for this option is: 656.Bd -literal -offset 3n 657ecdsa-sha2-nistp256-cert-v01@openssh.com, 658ecdsa-sha2-nistp384-cert-v01@openssh.com, 659ecdsa-sha2-nistp521-cert-v01@openssh.com, 660ssh-ed25519-cert-v01@openssh.com, 661ssh-rsa-cert-v01@openssh.com, 662ssh-dss-cert-v01@openssh.com, 663ecdsa-sha2-nistp256,ecdsa-sha2-nistp384, 664ecdsa-sha2-nistp521,ssh-ed25519, 665ssh-rsa,ssh-dss 666.Ed 667.Pp 668The 669.Fl Q 670option of 671.Xr ssh 1 672may be used to list supported key types. 673.It Cm HostbasedAuthentication 674Specifies whether rhosts or /etc/hosts.equiv authentication together 675with successful public key client host authentication is allowed 676(host-based authentication). 677The default is 678.Dq no . 679.It Cm HostbasedUsesNameFromPacketOnly 680Specifies whether or not the server will attempt to perform a reverse 681name lookup when matching the name in the 682.Pa ~/.shosts , 683.Pa ~/.rhosts , 684and 685.Pa /etc/hosts.equiv 686files during 687.Cm HostbasedAuthentication . 688A setting of 689.Dq yes 690means that 691.Xr sshd 8 692uses the name supplied by the client rather than 693attempting to resolve the name from the TCP connection itself. 694The default is 695.Dq no . 696.It Cm HostCertificate 697Specifies a file containing a public host certificate. 698The certificate's public key must match a private host key already specified 699by 700.Cm HostKey . 701The default behaviour of 702.Xr sshd 8 703is not to load any certificates. 704.It Cm HostKey 705Specifies a file containing a private host key 706used by SSH. 707The default is 708.Pa /etc/ssh/ssh_host_key 709for protocol version 1, and 710.Pa /etc/ssh/ssh_host_dsa_key , 711.Pa /etc/ssh/ssh_host_ecdsa_key , 712.Pa /etc/ssh/ssh_host_ed25519_key 713and 714.Pa /etc/ssh/ssh_host_rsa_key 715for protocol version 2. 716.Pp 717Note that 718.Xr sshd 8 719will refuse to use a file if it is group/world-accessible 720and that the 721.Cm HostKeyAlgorithms 722option restricts which of the keys are actually used by 723.Xr sshd 8 . 724.Pp 725It is possible to have multiple host key files. 726.Dq rsa1 727keys are used for version 1 and 728.Dq dsa , 729.Dq ecdsa , 730.Dq ed25519 731or 732.Dq rsa 733are used for version 2 of the SSH protocol. 734It is also possible to specify public host key files instead. 735In this case operations on the private key will be delegated 736to an 737.Xr ssh-agent 1 . 738.It Cm HostKeyAgent 739Identifies the UNIX-domain socket used to communicate 740with an agent that has access to the private host keys. 741If 742.Dq SSH_AUTH_SOCK 743is specified, the location of the socket will be read from the 744.Ev SSH_AUTH_SOCK 745environment variable. 746.It Cm HostKeyAlgorithms 747Specifies the host key algorithms 748that the server offers. 749The default for this option is: 750.Bd -literal -offset 3n 751ecdsa-sha2-nistp256-cert-v01@openssh.com, 752ecdsa-sha2-nistp384-cert-v01@openssh.com, 753ecdsa-sha2-nistp521-cert-v01@openssh.com, 754ssh-ed25519-cert-v01@openssh.com, 755ssh-rsa-cert-v01@openssh.com, 756ssh-dss-cert-v01@openssh.com, 757ecdsa-sha2-nistp256,ecdsa-sha2-nistp384, 758ecdsa-sha2-nistp521,ssh-ed25519, 759ssh-rsa,ssh-dss 760.Ed 761.Pp 762The list of available key types may also be obtained using the 763.Fl Q 764option of 765.Xr ssh 1 766with an argument of 767.Dq key . 768.It Cm IgnoreRhosts 769Specifies that 770.Pa .rhosts 771and 772.Pa .shosts 773files will not be used in 774.Cm RhostsRSAAuthentication 775or 776.Cm HostbasedAuthentication . 777.Pp 778.Pa /etc/hosts.equiv 779and 780.Pa /etc/ssh/shosts.equiv 781are still used. 782The default is 783.Dq yes . 784.It Cm IgnoreUserKnownHosts 785Specifies whether 786.Xr sshd 8 787should ignore the user's 788.Pa ~/.ssh/known_hosts 789during 790.Cm RhostsRSAAuthentication 791or 792.Cm HostbasedAuthentication . 793The default is 794.Dq no . 795.It Cm IPQoS 796Specifies the IPv4 type-of-service or DSCP class for the connection. 797Accepted values are 798.Dq af11 , 799.Dq af12 , 800.Dq af13 , 801.Dq af21 , 802.Dq af22 , 803.Dq af23 , 804.Dq af31 , 805.Dq af32 , 806.Dq af33 , 807.Dq af41 , 808.Dq af42 , 809.Dq af43 , 810.Dq cs0 , 811.Dq cs1 , 812.Dq cs2 , 813.Dq cs3 , 814.Dq cs4 , 815.Dq cs5 , 816.Dq cs6 , 817.Dq cs7 , 818.Dq ef , 819.Dq lowdelay , 820.Dq throughput , 821.Dq reliability , 822or a numeric value. 823This option may take one or two arguments, separated by whitespace. 824If one argument is specified, it is used as the packet class unconditionally. 825If two values are specified, the first is automatically selected for 826interactive sessions and the second for non-interactive sessions. 827The default is 828.Dq lowdelay 829for interactive sessions and 830.Dq throughput 831for non-interactive sessions. 832.It Cm KbdInteractiveAuthentication 833Specifies whether to allow keyboard-interactive authentication. 834The argument to this keyword must be 835.Dq yes 836or 837.Dq no . 838The default is to use whatever value 839.Cm ChallengeResponseAuthentication 840is set to 841(by default 842.Dq yes ) . 843.It Cm KerberosAuthentication 844Specifies whether the password provided by the user for 845.Cm PasswordAuthentication 846will be validated through the Kerberos KDC. 847To use this option, the server needs a 848Kerberos servtab which allows the verification of the KDC's identity. 849The default is 850.Dq no . 851.It Cm KerberosGetAFSToken 852If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 853an AFS token before accessing the user's home directory. 854The default is 855.Dq no . 856.It Cm KerberosOrLocalPasswd 857If password authentication through Kerberos fails then 858the password will be validated via any additional local mechanism 859such as 860.Pa /etc/passwd . 861The default is 862.Dq yes . 863.It Cm KerberosTicketCleanup 864Specifies whether to automatically destroy the user's ticket cache 865file on logout. 866The default is 867.Dq yes . 868.It Cm KexAlgorithms 869Specifies the available KEX (Key Exchange) algorithms. 870Multiple algorithms must be comma-separated. 871Alternately if the specified value begins with a 872.Sq + 873character, then the specified methods will be appended to the default set 874instead of replacing them. 875The supported algorithms are: 876.Pp 877.Bl -item -compact -offset indent 878.It 879curve25519-sha256@libssh.org 880.It 881diffie-hellman-group1-sha1 882.It 883diffie-hellman-group14-sha1 884.It 885diffie-hellman-group-exchange-sha1 886.It 887diffie-hellman-group-exchange-sha256 888.It 889ecdh-sha2-nistp256 890.It 891ecdh-sha2-nistp384 892.It 893ecdh-sha2-nistp521 894.El 895.Pp 896The default is: 897.Bd -literal -offset indent 898curve25519-sha256@libssh.org, 899ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 900diffie-hellman-group-exchange-sha256, 901diffie-hellman-group14-sha1 902.Ed 903.Pp 904The list of available key exchange algorithms may also be obtained using the 905.Fl Q 906option of 907.Xr ssh 1 908with an argument of 909.Dq kex . 910.It Cm KeyRegenerationInterval 911In protocol version 1, the ephemeral server key is automatically regenerated 912after this many seconds (if it has been used). 913The purpose of regeneration is to prevent 914decrypting captured sessions by later breaking into the machine and 915stealing the keys. 916The key is never stored anywhere. 917If the value is 0, the key is never regenerated. 918The default is 3600 (seconds). 919.It Cm ListenAddress 920Specifies the local addresses 921.Xr sshd 8 922should listen on. 923The following forms may be used: 924.Pp 925.Bl -item -offset indent -compact 926.It 927.Cm ListenAddress 928.Sm off 929.Ar host | Ar IPv4_addr | Ar IPv6_addr 930.Sm on 931.It 932.Cm ListenAddress 933.Sm off 934.Ar host | Ar IPv4_addr : Ar port 935.Sm on 936.It 937.Cm ListenAddress 938.Sm off 939.Oo 940.Ar host | Ar IPv6_addr Oc : Ar port 941.Sm on 942.El 943.Pp 944If 945.Ar port 946is not specified, 947sshd will listen on the address and all 948.Cm Port 949options specified. 950The default is to listen on all local addresses. 951Multiple 952.Cm ListenAddress 953options are permitted. 954.It Cm LoginGraceTime 955The server disconnects after this time if the user has not 956successfully logged in. 957If the value is 0, there is no time limit. 958The default is 120 seconds. 959.It Cm LogLevel 960Gives the verbosity level that is used when logging messages from 961.Xr sshd 8 . 962The possible values are: 963QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 964The default is INFO. 965DEBUG and DEBUG1 are equivalent. 966DEBUG2 and DEBUG3 each specify higher levels of debugging output. 967Logging with a DEBUG level violates the privacy of users and is not recommended. 968.It Cm MACs 969Specifies the available MAC (message authentication code) algorithms. 970The MAC algorithm is used for data integrity protection. 971Multiple algorithms must be comma-separated. 972If the specified value begins with a 973.Sq + 974character, then the specified algorithms will be appended to the default set 975instead of replacing them. 976.Pp 977The algorithms that contain 978.Dq -etm 979calculate the MAC after encryption (encrypt-then-mac). 980These are considered safer and their use recommended. 981The supported MACs are: 982.Pp 983.Bl -item -compact -offset indent 984.It 985hmac-md5 986.It 987hmac-md5-96 988.It 989hmac-ripemd160 990.It 991hmac-sha1 992.It 993hmac-sha1-96 994.It 995hmac-sha2-256 996.It 997hmac-sha2-512 998.It 999umac-64@openssh.com 1000.It 1001umac-128@openssh.com 1002.It 1003hmac-md5-etm@openssh.com 1004.It 1005hmac-md5-96-etm@openssh.com 1006.It 1007hmac-ripemd160-etm@openssh.com 1008.It 1009hmac-sha1-etm@openssh.com 1010.It 1011hmac-sha1-96-etm@openssh.com 1012.It 1013hmac-sha2-256-etm@openssh.com 1014.It 1015hmac-sha2-512-etm@openssh.com 1016.It 1017umac-64-etm@openssh.com 1018.It 1019umac-128-etm@openssh.com 1020.El 1021.Pp 1022The default is: 1023.Bd -literal -offset indent 1024umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1025hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1026hmac-sha1-etm@openssh.com, 1027umac-64@openssh.com,umac-128@openssh.com, 1028hmac-sha2-256,hmac-sha2-512,hmac-sha1 1029.Ed 1030.Pp 1031The list of available MAC algorithms may also be obtained using the 1032.Fl Q 1033option of 1034.Xr ssh 1 1035with an argument of 1036.Dq mac . 1037.It Cm Match 1038Introduces a conditional block. 1039If all of the criteria on the 1040.Cm Match 1041line are satisfied, the keywords on the following lines override those 1042set in the global section of the config file, until either another 1043.Cm Match 1044line or the end of the file. 1045If a keyword appears in multiple 1046.Cm Match 1047blocks that are satisfied, only the first instance of the keyword is 1048applied. 1049.Pp 1050The arguments to 1051.Cm Match 1052are one or more criteria-pattern pairs or the single token 1053.Cm All 1054which matches all criteria. 1055The available criteria are 1056.Cm User , 1057.Cm Group , 1058.Cm Host , 1059.Cm LocalAddress , 1060.Cm LocalPort , 1061and 1062.Cm Address . 1063The match patterns may consist of single entries or comma-separated 1064lists and may use the wildcard and negation operators described in the 1065PATTERNS section of 1066.Xr ssh_config 5 . 1067.Pp 1068The patterns in an 1069.Cm Address 1070criteria may additionally contain addresses to match in CIDR 1071address/masklen format, e.g.\& 1072.Dq 192.0.2.0/24 1073or 1074.Dq 3ffe:ffff::/32 . 1075Note that the mask length provided must be consistent with the address - 1076it is an error to specify a mask length that is too long for the address 1077or one with bits set in this host portion of the address. 1078For example, 1079.Dq 192.0.2.0/33 1080and 1081.Dq 192.0.2.0/8 1082respectively. 1083.Pp 1084Only a subset of keywords may be used on the lines following a 1085.Cm Match 1086keyword. 1087Available keywords are 1088.Cm AcceptEnv , 1089.Cm AllowAgentForwarding , 1090.Cm AllowGroups , 1091.Cm AllowStreamLocalForwarding , 1092.Cm AllowTcpForwarding , 1093.Cm AllowUsers , 1094.Cm AuthenticationMethods , 1095.Cm AuthorizedKeysCommand , 1096.Cm AuthorizedKeysCommandUser , 1097.Cm AuthorizedKeysFile , 1098.Cm AuthorizedPrincipalsCommand , 1099.Cm AuthorizedPrincipalsCommandUser , 1100.Cm AuthorizedPrincipalsFile , 1101.Cm Banner , 1102.Cm ChrootDirectory , 1103.Cm DenyGroups , 1104.Cm DenyUsers , 1105.Cm ForceCommand , 1106.Cm GatewayPorts , 1107.Cm GSSAPIAuthentication , 1108.Cm HostbasedAcceptedKeyTypes , 1109.Cm HostbasedAuthentication , 1110.Cm HostbasedUsesNameFromPacketOnly , 1111.Cm IPQoS , 1112.Cm KbdInteractiveAuthentication , 1113.Cm KerberosAuthentication , 1114.Cm MaxAuthTries , 1115.Cm MaxSessions , 1116.Cm PasswordAuthentication , 1117.Cm PermitEmptyPasswords , 1118.Cm PermitOpen , 1119.Cm PermitRootLogin , 1120.Cm PermitTTY , 1121.Cm PermitTunnel , 1122.Cm PermitUserRC , 1123.Cm PubkeyAcceptedKeyTypes , 1124.Cm PubkeyAuthentication , 1125.Cm RekeyLimit , 1126.Cm RevokedKeys , 1127.Cm RhostsRSAAuthentication , 1128.Cm RSAAuthentication , 1129.Cm StreamLocalBindMask , 1130.Cm StreamLocalBindUnlink , 1131.Cm TrustedUserCAKeys , 1132.Cm X11DisplayOffset , 1133.Cm X11Forwarding 1134and 1135.Cm X11UseLocalHost . 1136.It Cm MaxAuthTries 1137Specifies the maximum number of authentication attempts permitted per 1138connection. 1139Once the number of failures reaches half this value, 1140additional failures are logged. 1141The default is 6. 1142.It Cm MaxSessions 1143Specifies the maximum number of open shell, login or subsystem (e.g. sftp) 1144sessions permitted per network connection. 1145Multiple sessions may be established by clients that support connection 1146multiplexing. 1147Setting 1148.Cm MaxSessions 1149to 1 will effectively disable session multiplexing, whereas setting it to 0 1150will prevent all shell, login and subsystem sessions while still permitting 1151forwarding. 1152The default is 10. 1153.It Cm MaxStartups 1154Specifies the maximum number of concurrent unauthenticated connections to the 1155SSH daemon. 1156Additional connections will be dropped until authentication succeeds or the 1157.Cm LoginGraceTime 1158expires for a connection. 1159The default is 10:30:100. 1160.Pp 1161Alternatively, random early drop can be enabled by specifying 1162the three colon separated values 1163.Dq start:rate:full 1164(e.g. "10:30:60"). 1165.Xr sshd 8 1166will refuse connection attempts with a probability of 1167.Dq rate/100 1168(30%) 1169if there are currently 1170.Dq start 1171(10) 1172unauthenticated connections. 1173The probability increases linearly and all connection attempts 1174are refused if the number of unauthenticated connections reaches 1175.Dq full 1176(60). 1177.It Cm PasswordAuthentication 1178Specifies whether password authentication is allowed. 1179See also 1180.Cm UsePAM . 1181The default is 1182.Dq no . 1183.It Cm PermitEmptyPasswords 1184When password authentication is allowed, it specifies whether the 1185server allows login to accounts with empty password strings. 1186The default is 1187.Dq no . 1188.It Cm PermitOpen 1189Specifies the destinations to which TCP port forwarding is permitted. 1190The forwarding specification must be one of the following forms: 1191.Pp 1192.Bl -item -offset indent -compact 1193.It 1194.Cm PermitOpen 1195.Sm off 1196.Ar host : port 1197.Sm on 1198.It 1199.Cm PermitOpen 1200.Sm off 1201.Ar IPv4_addr : port 1202.Sm on 1203.It 1204.Cm PermitOpen 1205.Sm off 1206.Ar \&[ IPv6_addr \&] : port 1207.Sm on 1208.El 1209.Pp 1210Multiple forwards may be specified by separating them with whitespace. 1211An argument of 1212.Dq any 1213can be used to remove all restrictions and permit any forwarding requests. 1214An argument of 1215.Dq none 1216can be used to prohibit all forwarding requests. 1217By default all port forwarding requests are permitted. 1218.It Cm PermitRootLogin 1219Specifies whether root can log in using 1220.Xr ssh 1 . 1221The argument must be 1222.Dq yes , 1223.Dq prohibit-password , 1224.Dq without-password , 1225.Dq forced-commands-only , 1226or 1227.Dq no . 1228The default is 1229.Dq no . 1230Note that if 1231.Cm ChallengeResponseAuthentication 1232is 1233.Dq yes , 1234the root user may be allowed in with its password even if 1235.Cm PermitRootLogin is set to 1236.Dq without-password . 1237.Pp 1238If this option is set to 1239.Dq prohibit-password 1240or 1241.Dq without-password , 1242password and keyboard-interactive authentication are disabled for root. 1243.Pp 1244If this option is set to 1245.Dq forced-commands-only , 1246root login with public key authentication will be allowed, 1247but only if the 1248.Ar command 1249option has been specified 1250(which may be useful for taking remote backups even if root login is 1251normally not allowed). 1252All other authentication methods are disabled for root. 1253.Pp 1254If this option is set to 1255.Dq no , 1256root is not allowed to log in. 1257.It Cm PermitTunnel 1258Specifies whether 1259.Xr tun 4 1260device forwarding is allowed. 1261The argument must be 1262.Dq yes , 1263.Dq point-to-point 1264(layer 3), 1265.Dq ethernet 1266(layer 2), or 1267.Dq no . 1268Specifying 1269.Dq yes 1270permits both 1271.Dq point-to-point 1272and 1273.Dq ethernet . 1274The default is 1275.Dq no . 1276.Pp 1277Independent of this setting, the permissions of the selected 1278.Xr tun 4 1279device must allow access to the user. 1280.It Cm PermitTTY 1281Specifies whether 1282.Xr pty 4 1283allocation is permitted. 1284The default is 1285.Dq yes . 1286.It Cm PermitUserEnvironment 1287Specifies whether 1288.Pa ~/.ssh/environment 1289and 1290.Cm environment= 1291options in 1292.Pa ~/.ssh/authorized_keys 1293are processed by 1294.Xr sshd 8 . 1295The default is 1296.Dq no . 1297Enabling environment processing may enable users to bypass access 1298restrictions in some configurations using mechanisms such as 1299.Ev LD_PRELOAD . 1300.It Cm PermitUserRC 1301Specifies whether any 1302.Pa ~/.ssh/rc 1303file is executed. 1304The default is 1305.Dq yes . 1306.It Cm PidFile 1307Specifies the file that contains the process ID of the 1308SSH daemon, or 1309.Dq none 1310to not write one. 1311The default is 1312.Pa /var/run/sshd.pid . 1313.It Cm Port 1314Specifies the port number that 1315.Xr sshd 8 1316listens on. 1317The default is 22. 1318Multiple options of this type are permitted. 1319See also 1320.Cm ListenAddress . 1321.It Cm PrintLastLog 1322Specifies whether 1323.Xr sshd 8 1324should print the date and time of the last user login when a user logs 1325in interactively. 1326The default is 1327.Dq yes . 1328.It Cm PrintMotd 1329Specifies whether 1330.Xr sshd 8 1331should print 1332.Pa /etc/motd 1333when a user logs in interactively. 1334(On some systems it is also printed by the shell, 1335.Pa /etc/profile , 1336or equivalent.) 1337The default is 1338.Dq yes . 1339.It Cm Protocol 1340Specifies the protocol versions 1341.Xr sshd 8 1342supports. 1343The possible values are 1344.Sq 1 1345and 1346.Sq 2 . 1347Multiple versions must be comma-separated. 1348The default is 1349.Sq 2 . 1350Protocol 1 suffers from a number of cryptographic weaknesses and should 1351not be used. 1352It is only offered to support legacy devices. 1353.Pp 1354Note that the order of the protocol list does not indicate preference, 1355because the client selects among multiple protocol versions offered 1356by the server. 1357Specifying 1358.Dq 2,1 1359is identical to 1360.Dq 1,2 . 1361.It Cm PubkeyAcceptedKeyTypes 1362Specifies the key types that will be accepted for public key authentication 1363as a comma-separated pattern list. 1364Alternately if the specified value begins with a 1365.Sq + 1366character, then the specified key types will be appended to the default set 1367instead of replacing them. 1368The default for this option is: 1369.Bd -literal -offset 3n 1370ecdsa-sha2-nistp256-cert-v01@openssh.com, 1371ecdsa-sha2-nistp384-cert-v01@openssh.com, 1372ecdsa-sha2-nistp521-cert-v01@openssh.com, 1373ssh-ed25519-cert-v01@openssh.com, 1374ssh-rsa-cert-v01@openssh.com, 1375ssh-dss-cert-v01@openssh.com, 1376ecdsa-sha2-nistp256,ecdsa-sha2-nistp384, 1377ecdsa-sha2-nistp521,ssh-ed25519, 1378ssh-rsa,ssh-dss 1379.Ed 1380.Pp 1381The 1382.Fl Q 1383option of 1384.Xr ssh 1 1385may be used to list supported key types. 1386.It Cm PubkeyAuthentication 1387Specifies whether public key authentication is allowed. 1388The default is 1389.Dq yes . 1390.It Cm RekeyLimit 1391Specifies the maximum amount of data that may be transmitted before the 1392session key is renegotiated, optionally followed a maximum amount of 1393time that may pass before the session key is renegotiated. 1394The first argument is specified in bytes and may have a suffix of 1395.Sq K , 1396.Sq M , 1397or 1398.Sq G 1399to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1400The default is between 1401.Sq 1G 1402and 1403.Sq 4G , 1404depending on the cipher. 1405The optional second value is specified in seconds and may use any of the 1406units documented in the 1407.Sx TIME FORMATS 1408section. 1409The default value for 1410.Cm RekeyLimit 1411is 1412.Dq default none , 1413which means that rekeying is performed after the cipher's default amount 1414of data has been sent or received and no time based rekeying is done. 1415.It Cm RevokedKeys 1416Specifies revoked public keys file, or 1417.Dq none 1418to not use one. 1419Keys listed in this file will be refused for public key authentication. 1420Note that if this file is not readable, then public key authentication will 1421be refused for all users. 1422Keys may be specified as a text file, listing one public key per line, or as 1423an OpenSSH Key Revocation List (KRL) as generated by 1424.Xr ssh-keygen 1 . 1425For more information on KRLs, see the KEY REVOCATION LISTS section in 1426.Xr ssh-keygen 1 . 1427.It Cm RhostsRSAAuthentication 1428Specifies whether rhosts or 1429.Pa /etc/hosts.equiv 1430authentication together 1431with successful RSA host authentication is allowed. 1432The default is 1433.Dq no . 1434This option applies to protocol version 1 only. 1435.It Cm RSAAuthentication 1436Specifies whether pure RSA authentication is allowed. 1437The default is 1438.Dq yes . 1439This option applies to protocol version 1 only. 1440.It Cm ServerKeyBits 1441Defines the number of bits in the ephemeral protocol version 1 server key. 1442The default and minimum value is 1024. 1443.It Cm StreamLocalBindMask 1444Sets the octal file creation mode mask 1445.Pq umask 1446used when creating a Unix-domain socket file for local or remote 1447port forwarding. 1448This option is only used for port forwarding to a Unix-domain socket file. 1449.Pp 1450The default value is 0177, which creates a Unix-domain socket file that is 1451readable and writable only by the owner. 1452Note that not all operating systems honor the file mode on Unix-domain 1453socket files. 1454.It Cm StreamLocalBindUnlink 1455Specifies whether to remove an existing Unix-domain socket file for local 1456or remote port forwarding before creating a new one. 1457If the socket file already exists and 1458.Cm StreamLocalBindUnlink 1459is not enabled, 1460.Nm sshd 1461will be unable to forward the port to the Unix-domain socket file. 1462This option is only used for port forwarding to a Unix-domain socket file. 1463.Pp 1464The argument must be 1465.Dq yes 1466or 1467.Dq no . 1468The default is 1469.Dq no . 1470.It Cm StrictModes 1471Specifies whether 1472.Xr sshd 8 1473should check file modes and ownership of the 1474user's files and home directory before accepting login. 1475This is normally desirable because novices sometimes accidentally leave their 1476directory or files world-writable. 1477The default is 1478.Dq yes . 1479Note that this does not apply to 1480.Cm ChrootDirectory , 1481whose permissions and ownership are checked unconditionally. 1482.It Cm Subsystem 1483Configures an external subsystem (e.g. file transfer daemon). 1484Arguments should be a subsystem name and a command (with optional arguments) 1485to execute upon subsystem request. 1486.Pp 1487The command 1488.Xr sftp-server 8 1489implements the 1490.Dq sftp 1491file transfer subsystem. 1492.Pp 1493Alternately the name 1494.Dq internal-sftp 1495implements an in-process 1496.Dq sftp 1497server. 1498This may simplify configurations using 1499.Cm ChrootDirectory 1500to force a different filesystem root on clients. 1501.Pp 1502By default no subsystems are defined. 1503.It Cm SyslogFacility 1504Gives the facility code that is used when logging messages from 1505.Xr sshd 8 . 1506The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1507LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1508The default is AUTH. 1509.It Cm TCPKeepAlive 1510Specifies whether the system should send TCP keepalive messages to the 1511other side. 1512If they are sent, death of the connection or crash of one 1513of the machines will be properly noticed. 1514However, this means that 1515connections will die if the route is down temporarily, and some people 1516find it annoying. 1517On the other hand, if TCP keepalives are not sent, 1518sessions may hang indefinitely on the server, leaving 1519.Dq ghost 1520users and consuming server resources. 1521.Pp 1522The default is 1523.Dq yes 1524(to send TCP keepalive messages), and the server will notice 1525if the network goes down or the client host crashes. 1526This avoids infinitely hanging sessions. 1527.Pp 1528To disable TCP keepalive messages, the value should be set to 1529.Dq no . 1530.It Cm TrustedUserCAKeys 1531Specifies a file containing public keys of certificate authorities that are 1532trusted to sign user certificates for authentication, or 1533.Dq none 1534to not use one. 1535Keys are listed one per line; empty lines and comments starting with 1536.Ql # 1537are allowed. 1538If a certificate is presented for authentication and has its signing CA key 1539listed in this file, then it may be used for authentication for any user 1540listed in the certificate's principals list. 1541Note that certificates that lack a list of principals will not be permitted 1542for authentication using 1543.Cm TrustedUserCAKeys . 1544For more details on certificates, see the CERTIFICATES section in 1545.Xr ssh-keygen 1 . 1546.It Cm UseDNS 1547Specifies whether 1548.Xr sshd 8 1549should look up the remote host name, and to check that 1550the resolved host name for the remote IP address maps back to the 1551very same IP address. 1552.Pp 1553If this option is set to 1554.Dq no , 1555then only addresses and not host names may be used in 1556.Pa ~/.ssh/known_hosts 1557.Cm from 1558and 1559.Nm 1560.Cm Match 1561.Cm Host 1562directives. 1563The default is 1564.Dq yes . 1565.It Cm UseLogin 1566Specifies whether 1567.Xr login 1 1568is used for interactive login sessions. 1569The default is 1570.Dq no . 1571Note that 1572.Xr login 1 1573is never used for remote command execution. 1574Note also, that if this is enabled, 1575.Cm X11Forwarding 1576will be disabled because 1577.Xr login 1 1578does not know how to handle 1579.Xr xauth 1 1580cookies. 1581If 1582.Cm UsePrivilegeSeparation 1583is specified, it will be disabled after authentication. 1584.It Cm UsePAM 1585Enables the Pluggable Authentication Module interface. 1586If set to 1587.Dq yes 1588this will enable PAM authentication using 1589.Cm ChallengeResponseAuthentication 1590and 1591.Cm PasswordAuthentication 1592in addition to PAM account and session module processing for all 1593authentication types. 1594.Pp 1595Because PAM challenge-response authentication usually serves an equivalent 1596role to password authentication, you should disable either 1597.Cm PasswordAuthentication 1598or 1599.Cm ChallengeResponseAuthentication. 1600.Pp 1601If 1602.Cm UsePAM 1603is enabled, you will not be able to run 1604.Xr sshd 8 1605as a non-root user. 1606The default is 1607.Dq yes . 1608.It Cm UsePrivilegeSeparation 1609Specifies whether 1610.Xr sshd 8 1611separates privileges by creating an unprivileged child process 1612to deal with incoming network traffic. 1613After successful authentication, another process will be created that has 1614the privilege of the authenticated user. 1615The goal of privilege separation is to prevent privilege 1616escalation by containing any corruption within the unprivileged processes. 1617The argument must be 1618.Dq yes , 1619.Dq no , 1620or 1621.Dq sandbox . 1622If 1623.Cm UsePrivilegeSeparation 1624is set to 1625.Dq sandbox 1626then the pre-authentication unprivileged process is subject to additional 1627restrictions. 1628The default is 1629.Dq sandbox . 1630.It Cm VersionAddendum 1631Optionally specifies additional text to append to the SSH protocol banner 1632sent by the server upon connection. 1633The default is 1634.Dq FreeBSD-20160310 . 1635The value 1636.Dq none 1637may be used to disable this. 1638.It Cm X11DisplayOffset 1639Specifies the first display number available for 1640.Xr sshd 8 Ns 's 1641X11 forwarding. 1642This prevents sshd from interfering with real X11 servers. 1643The default is 10. 1644.It Cm X11Forwarding 1645Specifies whether X11 forwarding is permitted. 1646The argument must be 1647.Dq yes 1648or 1649.Dq no . 1650The default is 1651.Dq yes . 1652.Pp 1653When X11 forwarding is enabled, there may be additional exposure to 1654the server and to client displays if the 1655.Xr sshd 8 1656proxy display is configured to listen on the wildcard address (see 1657.Cm X11UseLocalhost 1658below), though this is not the default. 1659Additionally, the authentication spoofing and authentication data 1660verification and substitution occur on the client side. 1661The security risk of using X11 forwarding is that the client's X11 1662display server may be exposed to attack when the SSH client requests 1663forwarding (see the warnings for 1664.Cm ForwardX11 1665in 1666.Xr ssh_config 5 ) . 1667A system administrator may have a stance in which they want to 1668protect clients that may expose themselves to attack by unwittingly 1669requesting X11 forwarding, which can warrant a 1670.Dq no 1671setting. 1672.Pp 1673Note that disabling X11 forwarding does not prevent users from 1674forwarding X11 traffic, as users can always install their own forwarders. 1675X11 forwarding is automatically disabled if 1676.Cm UseLogin 1677is enabled. 1678.It Cm X11UseLocalhost 1679Specifies whether 1680.Xr sshd 8 1681should bind the X11 forwarding server to the loopback address or to 1682the wildcard address. 1683By default, 1684sshd binds the forwarding server to the loopback address and sets the 1685hostname part of the 1686.Ev DISPLAY 1687environment variable to 1688.Dq localhost . 1689This prevents remote hosts from connecting to the proxy display. 1690However, some older X11 clients may not function with this 1691configuration. 1692.Cm X11UseLocalhost 1693may be set to 1694.Dq no 1695to specify that the forwarding server should be bound to the wildcard 1696address. 1697The argument must be 1698.Dq yes 1699or 1700.Dq no . 1701The default is 1702.Dq yes . 1703.It Cm XAuthLocation 1704Specifies the full pathname of the 1705.Xr xauth 1 1706program, or 1707.Dq none 1708to not use one. 1709The default is 1710.Pa /usr/local/bin/xauth . 1711.El 1712.Sh TIME FORMATS 1713.Xr sshd 8 1714command-line arguments and configuration file options that specify time 1715may be expressed using a sequence of the form: 1716.Sm off 1717.Ar time Op Ar qualifier , 1718.Sm on 1719where 1720.Ar time 1721is a positive integer value and 1722.Ar qualifier 1723is one of the following: 1724.Pp 1725.Bl -tag -width Ds -compact -offset indent 1726.It Aq Cm none 1727seconds 1728.It Cm s | Cm S 1729seconds 1730.It Cm m | Cm M 1731minutes 1732.It Cm h | Cm H 1733hours 1734.It Cm d | Cm D 1735days 1736.It Cm w | Cm W 1737weeks 1738.El 1739.Pp 1740Each member of the sequence is added together to calculate 1741the total time value. 1742.Pp 1743Time format examples: 1744.Pp 1745.Bl -tag -width Ds -compact -offset indent 1746.It 600 1747600 seconds (10 minutes) 1748.It 10m 174910 minutes 1750.It 1h30m 17511 hour 30 minutes (90 minutes) 1752.El 1753.Sh FILES 1754.Bl -tag -width Ds 1755.It Pa /etc/ssh/sshd_config 1756Contains configuration data for 1757.Xr sshd 8 . 1758This file should be writable by root only, but it is recommended 1759(though not necessary) that it be world-readable. 1760.El 1761.Sh SEE ALSO 1762.Xr sshd 8 1763.Sh AUTHORS 1764OpenSSH is a derivative of the original and free 1765ssh 1.2.12 release by Tatu Ylonen. 1766Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1767Theo de Raadt and Dug Song 1768removed many bugs, re-added newer features and 1769created OpenSSH. 1770Markus Friedl contributed the support for SSH 1771protocol versions 1.5 and 2.0. 1772Niels Provos and Markus Friedl contributed support 1773for privilege separation. 1774