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 1.144 2012/06/29 13:57:25 naddy Exp $ 37.\" $FreeBSD$ 38.Dd June 29 2012 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. 74Note that environment passing is only supported for protocol 2. 75Variables are specified by name, which may contain the wildcard characters 76.Ql * 77and 78.Ql \&? . 79Multiple environment variables may be separated by whitespace or spread 80across multiple 81.Cm AcceptEnv 82directives. 83Be warned that some environment variables could be used to bypass restricted 84user environments. 85For this reason, care should be taken in the use of this directive. 86The default is not to accept any environment variables. 87.It Cm AddressFamily 88Specifies which address family should be used by 89.Xr sshd 8 . 90Valid arguments are 91.Dq any , 92.Dq inet 93(use IPv4 only), or 94.Dq inet6 95(use IPv6 only). 96The default is 97.Dq any . 98.It Cm AllowAgentForwarding 99Specifies whether 100.Xr ssh-agent 1 101forwarding is permitted. 102The default is 103.Dq yes . 104Note that disabling agent forwarding does not improve security 105unless users are also denied shell access, as they can always install 106their own forwarders. 107.It Cm AllowGroups 108This keyword can be followed by a list of group name patterns, separated 109by spaces. 110If specified, login is allowed only for users whose primary 111group or supplementary group list matches one of the patterns. 112Only group names are valid; a numerical group ID is not recognized. 113By default, login is allowed for all groups. 114The allow/deny directives are processed in the following order: 115.Cm DenyUsers , 116.Cm AllowUsers , 117.Cm DenyGroups , 118and finally 119.Cm AllowGroups . 120.Pp 121See 122.Sx PATTERNS 123in 124.Xr ssh_config 5 125for more information on patterns. 126.It Cm AllowTcpForwarding 127Specifies whether TCP forwarding is permitted. 128The default is 129.Dq yes . 130Note that disabling TCP forwarding does not improve security unless 131users are also denied shell access, as they can always install their 132own forwarders. 133.It Cm AllowUsers 134This keyword can be followed by a list of user name patterns, separated 135by spaces. 136If specified, login is allowed only for user names that 137match one of the patterns. 138Only user names are valid; a numerical user ID is not recognized. 139By default, login is allowed for all users. 140If the pattern takes the form USER@HOST then USER and HOST 141are separately checked, restricting logins to particular 142users from particular hosts. 143The allow/deny directives are processed in the following order: 144.Cm DenyUsers , 145.Cm AllowUsers , 146.Cm DenyGroups , 147and finally 148.Cm AllowGroups . 149.Pp 150See 151.Sx PATTERNS 152in 153.Xr ssh_config 5 154for more information on patterns. 155.It Cm AuthorizedKeysFile 156Specifies the file that contains the public keys that can be used 157for user authentication. 158The format is described in the 159.Sx AUTHORIZED_KEYS FILE FORMAT 160section of 161.Xr sshd 8 . 162.Cm AuthorizedKeysFile 163may contain tokens of the form %T which are substituted during connection 164setup. 165The following tokens are defined: %% is replaced by a literal '%', 166%h is replaced by the home directory of the user being authenticated, and 167%u is replaced by the username of that user. 168After expansion, 169.Cm AuthorizedKeysFile 170is taken to be an absolute path or one relative to the user's home 171directory. 172Multiple files may be listed, separated by whitespace. 173The default is 174.Dq .ssh/authorized_keys .ssh/authorized_keys2 . 175.It Cm AuthorizedPrincipalsFile 176Specifies a file that lists principal names that are accepted for 177certificate authentication. 178When using certificates signed by a key listed in 179.Cm TrustedUserCAKeys , 180this file lists names, one of which must appear in the certificate for it 181to be accepted for authentication. 182Names are listed one per line preceded by key options (as described 183in 184.Sx AUTHORIZED_KEYS FILE FORMAT 185in 186.Xr sshd 8 ) . 187Empty lines and comments starting with 188.Ql # 189are ignored. 190.Pp 191.Cm AuthorizedPrincipalsFile 192may contain tokens of the form %T which are substituted during connection 193setup. 194The following tokens are defined: %% is replaced by a literal '%', 195%h is replaced by the home directory of the user being authenticated, and 196%u is replaced by the username of that user. 197After expansion, 198.Cm AuthorizedPrincipalsFile 199is taken to be an absolute path or one relative to the user's home 200directory. 201.Pp 202The default is 203.Dq none , 204i.e. not to use a principals file \(en in this case, the username 205of the user must appear in a certificate's principals list for it to be 206accepted. 207Note that 208.Cm AuthorizedPrincipalsFile 209is only used when authentication proceeds using a CA listed in 210.Cm TrustedUserCAKeys 211and is not consulted for certification authorities trusted via 212.Pa ~/.ssh/authorized_keys , 213though the 214.Cm principals= 215key option offers a similar facility (see 216.Xr sshd 8 217for details). 218.It Cm Banner 219The contents of the specified file are sent to the remote user before 220authentication is allowed. 221If the argument is 222.Dq none 223then no banner is displayed. 224This option is only available for protocol version 2. 225By default, no banner is displayed. 226.It Cm ChallengeResponseAuthentication 227Specifies whether challenge-response authentication is allowed (e.g. via 228PAM or though authentication styles supported in 229.Xr login.conf 5 ) 230The default is 231.Dq yes . 232.It Cm ChrootDirectory 233Specifies the pathname of a directory to 234.Xr chroot 2 235to after authentication. 236All components of the pathname must be root-owned directories that are 237not writable by any other user or group. 238After the chroot, 239.Xr sshd 8 240changes the working directory to the user's home directory. 241.Pp 242The pathname may contain the following tokens that are expanded at runtime once 243the connecting user has been authenticated: %% is replaced by a literal '%', 244%h is replaced by the home directory of the user being authenticated, and 245%u is replaced by the username of that user. 246.Pp 247The 248.Cm ChrootDirectory 249must contain the necessary files and directories to support the 250user's session. 251For an interactive session this requires at least a shell, typically 252.Xr sh 1 , 253and basic 254.Pa /dev 255nodes such as 256.Xr null 4 , 257.Xr zero 4 , 258.Xr stdin 4 , 259.Xr stdout 4 , 260.Xr stderr 4 , 261.Xr arandom 4 262and 263.Xr tty 4 264devices. 265For file transfer sessions using 266.Dq sftp , 267no additional configuration of the environment is necessary if the 268in-process sftp server is used, 269though sessions which use logging do require 270.Pa /dev/log 271inside the chroot directory (see 272.Xr sftp-server 8 273for details). 274.Pp 275The default is not to 276.Xr chroot 2 . 277.It Cm Ciphers 278Specifies the ciphers allowed for protocol version 2. 279Multiple ciphers must be comma-separated. 280The supported ciphers are 281.Dq 3des-cbc , 282.Dq aes128-cbc , 283.Dq aes192-cbc , 284.Dq aes256-cbc , 285.Dq aes128-ctr , 286.Dq aes192-ctr , 287.Dq aes256-ctr , 288.Dq arcfour128 , 289.Dq arcfour256 , 290.Dq arcfour , 291.Dq blowfish-cbc , 292and 293.Dq cast128-cbc . 294The default is: 295.Bd -literal -offset 3n 296aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 297aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 298aes256-cbc,arcfour 299.Ed 300.It Cm ClientAliveCountMax 301Sets the number of client alive messages (see below) which may be 302sent without 303.Xr sshd 8 304receiving any messages back from the client. 305If this threshold is reached while client alive messages are being sent, 306sshd will disconnect the client, terminating the session. 307It is important to note that the use of client alive messages is very 308different from 309.Cm TCPKeepAlive 310(below). 311The client alive messages are sent through the encrypted channel 312and therefore will not be spoofable. 313The TCP keepalive option enabled by 314.Cm TCPKeepAlive 315is spoofable. 316The client alive mechanism is valuable when the client or 317server depend on knowing when a connection has become inactive. 318.Pp 319The default value is 3. 320If 321.Cm ClientAliveInterval 322(see below) is set to 15, and 323.Cm ClientAliveCountMax 324is left at the default, unresponsive SSH clients 325will be disconnected after approximately 45 seconds. 326This option applies to protocol version 2 only. 327.It Cm ClientAliveInterval 328Sets a timeout interval in seconds after which if no data has been received 329from the client, 330.Xr sshd 8 331will send a message through the encrypted 332channel to request a response from the client. 333The default 334is 0, indicating that these messages will not be sent to the client. 335This option applies to protocol version 2 only. 336.It Cm Compression 337Specifies whether compression is allowed, or delayed until 338the user has authenticated successfully. 339The argument must be 340.Dq yes , 341.Dq delayed , 342or 343.Dq no . 344The default is 345.Dq delayed . 346.It Cm DenyGroups 347This keyword can be followed by a list of group name patterns, separated 348by spaces. 349Login is disallowed for users whose primary group or supplementary 350group list matches one of the patterns. 351Only group names are valid; a numerical group ID is not recognized. 352By default, login is allowed for all groups. 353The allow/deny directives are processed in the following order: 354.Cm DenyUsers , 355.Cm AllowUsers , 356.Cm DenyGroups , 357and finally 358.Cm AllowGroups . 359.Pp 360See 361.Sx PATTERNS 362in 363.Xr ssh_config 5 364for more information on patterns. 365.It Cm DenyUsers 366This keyword can be followed by a list of user name patterns, separated 367by spaces. 368Login is disallowed for user names that match one of the patterns. 369Only user names are valid; a numerical user ID is not recognized. 370By default, login is allowed for all users. 371If the pattern takes the form USER@HOST then USER and HOST 372are separately checked, restricting logins to particular 373users from particular hosts. 374The allow/deny directives are processed in the following order: 375.Cm DenyUsers , 376.Cm AllowUsers , 377.Cm DenyGroups , 378and finally 379.Cm AllowGroups . 380.Pp 381See 382.Sx PATTERNS 383in 384.Xr ssh_config 5 385for more information on patterns. 386.It Cm ForceCommand 387Forces the execution of the command specified by 388.Cm ForceCommand , 389ignoring any command supplied by the client and 390.Pa ~/.ssh/rc 391if present. 392The command is invoked by using the user's login shell with the -c option. 393This applies to shell, command, or subsystem execution. 394It is most useful inside a 395.Cm Match 396block. 397The command originally supplied by the client is available in the 398.Ev SSH_ORIGINAL_COMMAND 399environment variable. 400Specifying a command of 401.Dq internal-sftp 402will force the use of an in-process sftp server that requires no support 403files when used with 404.Cm ChrootDirectory . 405.It Cm GatewayPorts 406Specifies whether remote hosts are allowed to connect to ports 407forwarded for the client. 408By default, 409.Xr sshd 8 410binds remote port forwardings to the loopback address. 411This prevents other remote hosts from connecting to forwarded ports. 412.Cm GatewayPorts 413can be used to specify that sshd 414should allow remote port forwardings to bind to non-loopback addresses, thus 415allowing other hosts to connect. 416The argument may be 417.Dq no 418to force remote port forwardings to be available to the local host only, 419.Dq yes 420to force remote port forwardings to bind to the wildcard address, or 421.Dq clientspecified 422to allow the client to select the address to which the forwarding is bound. 423The default is 424.Dq no . 425.It Cm GSSAPIAuthentication 426Specifies whether user authentication based on GSSAPI is allowed. 427The default is 428.Dq no . 429Note that this option applies to protocol version 2 only. 430.It Cm GSSAPICleanupCredentials 431Specifies whether to automatically destroy the user's credentials cache 432on logout. 433The default is 434.Dq yes . 435Note that this option applies to protocol version 2 only. 436.It Cm HostbasedAuthentication 437Specifies whether rhosts or /etc/hosts.equiv authentication together 438with successful public key client host authentication is allowed 439(host-based authentication). 440This option is similar to 441.Cm RhostsRSAAuthentication 442and applies to protocol version 2 only. 443The default is 444.Dq no . 445.It Cm HostbasedUsesNameFromPacketOnly 446Specifies whether or not the server will attempt to perform a reverse 447name lookup when matching the name in the 448.Pa ~/.shosts , 449.Pa ~/.rhosts , 450and 451.Pa /etc/hosts.equiv 452files during 453.Cm HostbasedAuthentication . 454A setting of 455.Dq yes 456means that 457.Xr sshd 8 458uses the name supplied by the client rather than 459attempting to resolve the name from the TCP connection itself. 460The default is 461.Dq no . 462.It Cm HostCertificate 463Specifies a file containing a public host certificate. 464The certificate's public key must match a private host key already specified 465by 466.Cm HostKey . 467The default behaviour of 468.Xr sshd 8 469is not to load any certificates. 470.It Cm HostKey 471Specifies a file containing a private host key 472used by SSH. 473The default is 474.Pa /etc/ssh/ssh_host_key 475for protocol version 1, and 476.Pa /etc/ssh/ssh_host_dsa_key , 477.Pa /etc/ssh/ssh_host_ecdsa_key 478and 479.Pa /etc/ssh/ssh_host_rsa_key 480for protocol version 2. 481Note that 482.Xr sshd 8 483will refuse to use a file if it is group/world-accessible. 484It is possible to have multiple host key files. 485.Dq rsa1 486keys are used for version 1 and 487.Dq dsa , 488.Dq ecdsa 489or 490.Dq rsa 491are used for version 2 of the SSH protocol. 492.It Cm IgnoreRhosts 493Specifies that 494.Pa .rhosts 495and 496.Pa .shosts 497files will not be used in 498.Cm RhostsRSAAuthentication 499or 500.Cm HostbasedAuthentication . 501.Pp 502.Pa /etc/hosts.equiv 503and 504.Pa /etc/ssh/shosts.equiv 505are still used. 506The default is 507.Dq yes . 508.It Cm IgnoreUserKnownHosts 509Specifies whether 510.Xr sshd 8 511should ignore the user's 512.Pa ~/.ssh/known_hosts 513during 514.Cm RhostsRSAAuthentication 515or 516.Cm HostbasedAuthentication . 517The default is 518.Dq no . 519.It Cm IPQoS 520Specifies the IPv4 type-of-service or DSCP class for the connection. 521Accepted values are 522.Dq af11 , 523.Dq af12 , 524.Dq af13 , 525.Dq af21 , 526.Dq af22 , 527.Dq af23 , 528.Dq af31 , 529.Dq af32 , 530.Dq af33 , 531.Dq af41 , 532.Dq af42 , 533.Dq af43 , 534.Dq cs0 , 535.Dq cs1 , 536.Dq cs2 , 537.Dq cs3 , 538.Dq cs4 , 539.Dq cs5 , 540.Dq cs6 , 541.Dq cs7 , 542.Dq ef , 543.Dq lowdelay , 544.Dq throughput , 545.Dq reliability , 546or a numeric value. 547This option may take one or two arguments, separated by whitespace. 548If one argument is specified, it is used as the packet class unconditionally. 549If two values are specified, the first is automatically selected for 550interactive sessions and the second for non-interactive sessions. 551The default is 552.Dq lowdelay 553for interactive sessions and 554.Dq throughput 555for non-interactive sessions. 556.It Cm KerberosAuthentication 557Specifies whether the password provided by the user for 558.Cm PasswordAuthentication 559will be validated through the Kerberos KDC. 560To use this option, the server needs a 561Kerberos servtab which allows the verification of the KDC's identity. 562The default is 563.Dq no . 564.It Cm KerberosGetAFSToken 565If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 566an AFS token before accessing the user's home directory. 567The default is 568.Dq no . 569.It Cm KerberosOrLocalPasswd 570If password authentication through Kerberos fails then 571the password will be validated via any additional local mechanism 572such as 573.Pa /etc/passwd . 574The default is 575.Dq yes . 576.It Cm KerberosTicketCleanup 577Specifies whether to automatically destroy the user's ticket cache 578file on logout. 579The default is 580.Dq yes . 581.It Cm KexAlgorithms 582Specifies the available KEX (Key Exchange) algorithms. 583Multiple algorithms must be comma-separated. 584The default is 585.Dq ecdh-sha2-nistp256 , 586.Dq ecdh-sha2-nistp384 , 587.Dq ecdh-sha2-nistp521 , 588.Dq diffie-hellman-group-exchange-sha256 , 589.Dq diffie-hellman-group-exchange-sha1 , 590.Dq diffie-hellman-group14-sha1 , 591.Dq diffie-hellman-group1-sha1 . 592.It Cm KeyRegenerationInterval 593In protocol version 1, the ephemeral server key is automatically regenerated 594after this many seconds (if it has been used). 595The purpose of regeneration is to prevent 596decrypting captured sessions by later breaking into the machine and 597stealing the keys. 598The key is never stored anywhere. 599If the value is 0, the key is never regenerated. 600The default is 3600 (seconds). 601.It Cm ListenAddress 602Specifies the local addresses 603.Xr sshd 8 604should listen on. 605The following forms may be used: 606.Pp 607.Bl -item -offset indent -compact 608.It 609.Cm ListenAddress 610.Sm off 611.Ar host No | Ar IPv4_addr No | Ar IPv6_addr 612.Sm on 613.It 614.Cm ListenAddress 615.Sm off 616.Ar host No | Ar IPv4_addr No : Ar port 617.Sm on 618.It 619.Cm ListenAddress 620.Sm off 621.Oo 622.Ar host No | Ar IPv6_addr Oc : Ar port 623.Sm on 624.El 625.Pp 626If 627.Ar port 628is not specified, 629sshd will listen on the address and all prior 630.Cm Port 631options specified. 632The default is to listen on all local addresses. 633Multiple 634.Cm ListenAddress 635options are permitted. 636Additionally, any 637.Cm Port 638options must precede this option for non-port qualified addresses. 639.It Cm LoginGraceTime 640The server disconnects after this time if the user has not 641successfully logged in. 642If the value is 0, there is no time limit. 643The default is 120 seconds. 644.It Cm LogLevel 645Gives the verbosity level that is used when logging messages from 646.Xr sshd 8 . 647The possible values are: 648QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 649The default is INFO. 650DEBUG and DEBUG1 are equivalent. 651DEBUG2 and DEBUG3 each specify higher levels of debugging output. 652Logging with a DEBUG level violates the privacy of users and is not recommended. 653.It Cm MACs 654Specifies the available MAC (message authentication code) algorithms. 655The MAC algorithm is used in protocol version 2 656for data integrity protection. 657Multiple algorithms must be comma-separated. 658The default is: 659.Bd -literal -offset indent 660hmac-md5,hmac-sha1,umac-64@openssh.com, 661hmac-sha2-256,hmac-sha2-512,hmac-ripemd160, 662hmac-sha1-96,hmac-md5-96 663.Ed 664.It Cm Match 665Introduces a conditional block. 666If all of the criteria on the 667.Cm Match 668line are satisfied, the keywords on the following lines override those 669set in the global section of the config file, until either another 670.Cm Match 671line or the end of the file. 672.Pp 673The arguments to 674.Cm Match 675are one or more criteria-pattern pairs. 676The available criteria are 677.Cm User , 678.Cm Group , 679.Cm Host , 680.Cm LocalAddress , 681.Cm LocalPort , 682and 683.Cm Address . 684The match patterns may consist of single entries or comma-separated 685lists and may use the wildcard and negation operators described in the 686.Sx PATTERNS 687section of 688.Xr ssh_config 5 . 689.Pp 690The patterns in an 691.Cm Address 692criteria may additionally contain addresses to match in CIDR 693address/masklen format, e.g.\& 694.Dq 192.0.2.0/24 695or 696.Dq 3ffe:ffff::/32 . 697Note that the mask length provided must be consistent with the address - 698it is an error to specify a mask length that is too long for the address 699or one with bits set in this host portion of the address. 700For example, 701.Dq 192.0.2.0/33 702and 703.Dq 192.0.2.0/8 704respectively. 705.Pp 706Only a subset of keywords may be used on the lines following a 707.Cm Match 708keyword. 709Available keywords are 710.Cm AcceptEnv , 711.Cm AllowAgentForwarding , 712.Cm AllowGroups , 713.Cm AllowTcpForwarding , 714.Cm AllowUsers , 715.Cm AuthorizedKeysFile , 716.Cm AuthorizedPrincipalsFile , 717.Cm Banner , 718.Cm ChrootDirectory , 719.Cm DenyGroups , 720.Cm DenyUsers , 721.Cm ForceCommand , 722.Cm GatewayPorts , 723.Cm GSSAPIAuthentication , 724.Cm HostbasedAuthentication , 725.Cm HostbasedUsesNameFromPacketOnly , 726.Cm KbdInteractiveAuthentication , 727.Cm KerberosAuthentication , 728.Cm MaxAuthTries , 729.Cm MaxSessions , 730.Cm PasswordAuthentication , 731.Cm PermitEmptyPasswords , 732.Cm PermitOpen , 733.Cm PermitRootLogin , 734.Cm PermitTunnel , 735.Cm PubkeyAuthentication , 736.Cm RhostsRSAAuthentication , 737.Cm RSAAuthentication , 738.Cm X11DisplayOffset , 739.Cm X11Forwarding 740and 741.Cm X11UseLocalHost . 742.It Cm MaxAuthTries 743Specifies the maximum number of authentication attempts permitted per 744connection. 745Once the number of failures reaches half this value, 746additional failures are logged. 747The default is 6. 748.It Cm MaxSessions 749Specifies the maximum number of open sessions permitted per network connection. 750The default is 10. 751.It Cm MaxStartups 752Specifies the maximum number of concurrent unauthenticated connections to the 753SSH daemon. 754Additional connections will be dropped until authentication succeeds or the 755.Cm LoginGraceTime 756expires for a connection. 757The default is 10. 758.Pp 759Alternatively, random early drop can be enabled by specifying 760the three colon separated values 761.Dq start:rate:full 762(e.g. "10:30:60"). 763.Xr sshd 8 764will refuse connection attempts with a probability of 765.Dq rate/100 766(30%) 767if there are currently 768.Dq start 769(10) 770unauthenticated connections. 771The probability increases linearly and all connection attempts 772are refused if the number of unauthenticated connections reaches 773.Dq full 774(60). 775.It Cm PasswordAuthentication 776Specifies whether password authentication is allowed. 777See also 778.Cm UsePAM . 779The default is 780.Dq no . 781.It Cm PermitEmptyPasswords 782When password authentication is allowed, it specifies whether the 783server allows login to accounts with empty password strings. 784The default is 785.Dq no . 786.It Cm PermitOpen 787Specifies the destinations to which TCP port forwarding is permitted. 788The forwarding specification must be one of the following forms: 789.Pp 790.Bl -item -offset indent -compact 791.It 792.Cm PermitOpen 793.Sm off 794.Ar host : port 795.Sm on 796.It 797.Cm PermitOpen 798.Sm off 799.Ar IPv4_addr : port 800.Sm on 801.It 802.Cm PermitOpen 803.Sm off 804.Ar \&[ IPv6_addr \&] : port 805.Sm on 806.El 807.Pp 808Multiple forwards may be specified by separating them with whitespace. 809An argument of 810.Dq any 811can be used to remove all restrictions and permit any forwarding requests. 812An argument of 813.Dq none 814can be used to prohibit all forwarding requests. 815By default all port forwarding requests are permitted. 816.It Cm PermitRootLogin 817Specifies whether root can log in using 818.Xr ssh 1 . 819The argument must be 820.Dq yes , 821.Dq without-password , 822.Dq forced-commands-only , 823or 824.Dq no . 825The default is 826.Dq no . 827Note that if 828.Cm ChallengeResponseAuthentication 829is 830.Dq yes , 831the root user may be allowed in with its password even if 832.Cm PermitRootLogin is set to 833.Dq without-password . 834.Pp 835If this option is set to 836.Dq without-password , 837password authentication is disabled for root. 838.Pp 839If this option is set to 840.Dq forced-commands-only , 841root login with public key authentication will be allowed, 842but only if the 843.Ar command 844option has been specified 845(which may be useful for taking remote backups even if root login is 846normally not allowed). 847All other authentication methods are disabled for root. 848.Pp 849If this option is set to 850.Dq no , 851root is not allowed to log in. 852.It Cm PermitTunnel 853Specifies whether 854.Xr tun 4 855device forwarding is allowed. 856The argument must be 857.Dq yes , 858.Dq point-to-point 859(layer 3), 860.Dq ethernet 861(layer 2), or 862.Dq no . 863Specifying 864.Dq yes 865permits both 866.Dq point-to-point 867and 868.Dq ethernet . 869The default is 870.Dq no . 871.It Cm PermitUserEnvironment 872Specifies whether 873.Pa ~/.ssh/environment 874and 875.Cm environment= 876options in 877.Pa ~/.ssh/authorized_keys 878are processed by 879.Xr sshd 8 . 880The default is 881.Dq no . 882Enabling environment processing may enable users to bypass access 883restrictions in some configurations using mechanisms such as 884.Ev LD_PRELOAD . 885.It Cm PidFile 886Specifies the file that contains the process ID of the 887SSH daemon. 888The default is 889.Pa /var/run/sshd.pid . 890.It Cm Port 891Specifies the port number that 892.Xr sshd 8 893listens on. 894The default is 22. 895Multiple options of this type are permitted. 896See also 897.Cm ListenAddress . 898.It Cm PrintLastLog 899Specifies whether 900.Xr sshd 8 901should print the date and time of the last user login when a user logs 902in interactively. 903The default is 904.Dq yes . 905.It Cm PrintMotd 906Specifies whether 907.Xr sshd 8 908should print 909.Pa /etc/motd 910when a user logs in interactively. 911(On some systems it is also printed by the shell, 912.Pa /etc/profile , 913or equivalent.) 914The default is 915.Dq yes . 916.It Cm Protocol 917Specifies the protocol versions 918.Xr sshd 8 919supports. 920The possible values are 921.Sq 1 922and 923.Sq 2 . 924Multiple versions must be comma-separated. 925The default is 926.Sq 2 . 927Note that the order of the protocol list does not indicate preference, 928because the client selects among multiple protocol versions offered 929by the server. 930Specifying 931.Dq 2,1 932is identical to 933.Dq 1,2 . 934.It Cm PubkeyAuthentication 935Specifies whether public key authentication is allowed. 936The default is 937.Dq yes . 938Note that this option applies to protocol version 2 only. 939.It Cm RevokedKeys 940Specifies a list of revoked public keys. 941Keys listed in this file will be refused for public key authentication. 942Note that if this file is not readable, then public key authentication will 943be refused for all users. 944.It Cm RhostsRSAAuthentication 945Specifies whether rhosts or 946.Pa /etc/hosts.equiv 947authentication together 948with successful RSA host authentication is allowed. 949The default is 950.Dq no . 951This option applies to protocol version 1 only. 952.It Cm RSAAuthentication 953Specifies whether pure RSA authentication is allowed. 954The default is 955.Dq yes . 956This option applies to protocol version 1 only. 957.It Cm ServerKeyBits 958Defines the number of bits in the ephemeral protocol version 1 server key. 959The minimum value is 512, and the default is 1024. 960.It Cm StrictModes 961Specifies whether 962.Xr sshd 8 963should check file modes and ownership of the 964user's files and home directory before accepting login. 965This is normally desirable because novices sometimes accidentally leave their 966directory or files world-writable. 967The default is 968.Dq yes . 969Note that this does not apply to 970.Cm ChrootDirectory , 971whose permissions and ownership are checked unconditionally. 972.It Cm Subsystem 973Configures an external subsystem (e.g. file transfer daemon). 974Arguments should be a subsystem name and a command (with optional arguments) 975to execute upon subsystem request. 976.Pp 977The command 978.Xr sftp-server 8 979implements the 980.Dq sftp 981file transfer subsystem. 982.Pp 983Alternately the name 984.Dq internal-sftp 985implements an in-process 986.Dq sftp 987server. 988This may simplify configurations using 989.Cm ChrootDirectory 990to force a different filesystem root on clients. 991.Pp 992By default no subsystems are defined. 993Note that this option applies to protocol version 2 only. 994.It Cm SyslogFacility 995Gives the facility code that is used when logging messages from 996.Xr sshd 8 . 997The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 998LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 999The default is AUTH. 1000.It Cm TCPKeepAlive 1001Specifies whether the system should send TCP keepalive messages to the 1002other side. 1003If they are sent, death of the connection or crash of one 1004of the machines will be properly noticed. 1005However, this means that 1006connections will die if the route is down temporarily, and some people 1007find it annoying. 1008On the other hand, if TCP keepalives are not sent, 1009sessions may hang indefinitely on the server, leaving 1010.Dq ghost 1011users and consuming server resources. 1012.Pp 1013The default is 1014.Dq yes 1015(to send TCP keepalive messages), and the server will notice 1016if the network goes down or the client host crashes. 1017This avoids infinitely hanging sessions. 1018.Pp 1019To disable TCP keepalive messages, the value should be set to 1020.Dq no . 1021.It Cm TrustedUserCAKeys 1022Specifies a file containing public keys of certificate authorities that are 1023trusted to sign user certificates for authentication. 1024Keys are listed one per line; empty lines and comments starting with 1025.Ql # 1026are allowed. 1027If a certificate is presented for authentication and has its signing CA key 1028listed in this file, then it may be used for authentication for any user 1029listed in the certificate's principals list. 1030Note that certificates that lack a list of principals will not be permitted 1031for authentication using 1032.Cm TrustedUserCAKeys . 1033For more details on certificates, see the 1034.Sx CERTIFICATES 1035section in 1036.Xr ssh-keygen 1 . 1037.It Cm UseDNS 1038Specifies whether 1039.Xr sshd 8 1040should look up the remote host name and check that 1041the resolved host name for the remote IP address maps back to the 1042very same IP address. 1043The default is 1044.Dq yes . 1045.It Cm UseLogin 1046Specifies whether 1047.Xr login 1 1048is used for interactive login sessions. 1049The default is 1050.Dq no . 1051Note that 1052.Xr login 1 1053is never used for remote command execution. 1054Note also, that if this is enabled, 1055.Cm X11Forwarding 1056will be disabled because 1057.Xr login 1 1058does not know how to handle 1059.Xr xauth 1 1060cookies. 1061If 1062.Cm UsePrivilegeSeparation 1063is specified, it will be disabled after authentication. 1064.It Cm UsePAM 1065Enables the Pluggable Authentication Module interface. 1066If set to 1067.Dq yes 1068this will enable PAM authentication using 1069.Cm ChallengeResponseAuthentication 1070and 1071.Cm PasswordAuthentication 1072in addition to PAM account and session module processing for all 1073authentication types. 1074.Pp 1075Because PAM challenge-response authentication usually serves an equivalent 1076role to password authentication, you should disable either 1077.Cm PasswordAuthentication 1078or 1079.Cm ChallengeResponseAuthentication. 1080.Pp 1081If 1082.Cm UsePAM 1083is enabled, you will not be able to run 1084.Xr sshd 8 1085as a non-root user. 1086The default is 1087.Dq yes . 1088.It Cm UsePrivilegeSeparation 1089Specifies whether 1090.Xr sshd 8 1091separates privileges by creating an unprivileged child process 1092to deal with incoming network traffic. 1093After successful authentication, another process will be created that has 1094the privilege of the authenticated user. 1095The goal of privilege separation is to prevent privilege 1096escalation by containing any corruption within the unprivileged processes. 1097The default is 1098.Dq sandbox . 1099If 1100.Cm UsePrivilegeSeparation 1101is set to 1102.Dq sandbox 1103then the pre-authentication unprivileged process is subject to additional 1104restrictions. 1105.It Cm VersionAddendum 1106Optionally specifies additional text to append to the SSH protocol banner 1107sent by the server upon connection. 1108The default is 1109.Dq FreeBSD-20120901 . 1110.It Cm X11DisplayOffset 1111Specifies the first display number available for 1112.Xr sshd 8 Ns 's 1113X11 forwarding. 1114This prevents sshd from interfering with real X11 servers. 1115The default is 10. 1116.It Cm X11Forwarding 1117Specifies whether X11 forwarding is permitted. 1118The argument must be 1119.Dq yes 1120or 1121.Dq no . 1122The default is 1123.Dq yes . 1124.Pp 1125When X11 forwarding is enabled, there may be additional exposure to 1126the server and to client displays if the 1127.Xr sshd 8 1128proxy display is configured to listen on the wildcard address (see 1129.Cm X11UseLocalhost 1130below), though this is not the default. 1131Additionally, the authentication spoofing and authentication data 1132verification and substitution occur on the client side. 1133The security risk of using X11 forwarding is that the client's X11 1134display server may be exposed to attack when the SSH client requests 1135forwarding (see the warnings for 1136.Cm ForwardX11 1137in 1138.Xr ssh_config 5 ) . 1139A system administrator may have a stance in which they want to 1140protect clients that may expose themselves to attack by unwittingly 1141requesting X11 forwarding, which can warrant a 1142.Dq no 1143setting. 1144.Pp 1145Note that disabling X11 forwarding does not prevent users from 1146forwarding X11 traffic, as users can always install their own forwarders. 1147X11 forwarding is automatically disabled if 1148.Cm UseLogin 1149is enabled. 1150.It Cm X11UseLocalhost 1151Specifies whether 1152.Xr sshd 8 1153should bind the X11 forwarding server to the loopback address or to 1154the wildcard address. 1155By default, 1156sshd binds the forwarding server to the loopback address and sets the 1157hostname part of the 1158.Ev DISPLAY 1159environment variable to 1160.Dq localhost . 1161This prevents remote hosts from connecting to the proxy display. 1162However, some older X11 clients may not function with this 1163configuration. 1164.Cm X11UseLocalhost 1165may be set to 1166.Dq no 1167to specify that the forwarding server should be bound to the wildcard 1168address. 1169The argument must be 1170.Dq yes 1171or 1172.Dq no . 1173The default is 1174.Dq yes . 1175.It Cm XAuthLocation 1176Specifies the full pathname of the 1177.Xr xauth 1 1178program. 1179The default is 1180.Pa /usr/local/bin/xauth . 1181.El 1182.Sh TIME FORMATS 1183.Xr sshd 8 1184command-line arguments and configuration file options that specify time 1185may be expressed using a sequence of the form: 1186.Sm off 1187.Ar time Op Ar qualifier , 1188.Sm on 1189where 1190.Ar time 1191is a positive integer value and 1192.Ar qualifier 1193is one of the following: 1194.Pp 1195.Bl -tag -width Ds -compact -offset indent 1196.It Aq Cm none 1197seconds 1198.It Cm s | Cm S 1199seconds 1200.It Cm m | Cm M 1201minutes 1202.It Cm h | Cm H 1203hours 1204.It Cm d | Cm D 1205days 1206.It Cm w | Cm W 1207weeks 1208.El 1209.Pp 1210Each member of the sequence is added together to calculate 1211the total time value. 1212.Pp 1213Time format examples: 1214.Pp 1215.Bl -tag -width Ds -compact -offset indent 1216.It 600 1217600 seconds (10 minutes) 1218.It 10m 121910 minutes 1220.It 1h30m 12211 hour 30 minutes (90 minutes) 1222.El 1223.Sh FILES 1224.Bl -tag -width Ds 1225.It Pa /etc/ssh/sshd_config 1226Contains configuration data for 1227.Xr sshd 8 . 1228This file should be writable by root only, but it is recommended 1229(though not necessary) that it be world-readable. 1230.El 1231.Sh SEE ALSO 1232.Xr sshd 8 1233.Sh AUTHORS 1234OpenSSH is a derivative of the original and free 1235ssh 1.2.12 release by Tatu Ylonen. 1236Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1237Theo de Raadt and Dug Song 1238removed many bugs, re-added newer features and 1239created OpenSSH. 1240Markus Friedl contributed the support for SSH 1241protocol versions 1.5 and 2.0. 1242Niels Provos and Markus Friedl contributed support 1243for privilege separation. 1244