1.\" -*- nroff -*- 2.\" 3.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 4.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 5.\" All rights reserved 6.\" 7.\" As far as I am concerned, the code I have written for this software 8.\" can be used freely for any purpose. Any derived versions of this 9.\" software must be clearly marked as such, and if the derived work is 10.\" incompatible with the protocol description in the RFC file, it must be 11.\" called by a name other than "ssh" or "Secure Shell". 12.\" 13.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 14.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 15.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 16.\" 17.\" Redistribution and use in source and binary forms, with or without 18.\" modification, are permitted provided that the following conditions 19.\" are met: 20.\" 1. Redistributions of source code must retain the above copyright 21.\" notice, this list of conditions and the following disclaimer. 22.\" 2. Redistributions in binary form must reproduce the above copyright 23.\" notice, this list of conditions and the following disclaimer in the 24.\" documentation and/or other materials provided with the distribution. 25.\" 26.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 27.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 28.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 29.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 30.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 31.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 32.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 33.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 34.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 35.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 36.\" 37.\" $OpenBSD: sshd_config.5,v 1.96 2008/07/02 02:24:18 djm Exp $ 38.Dd $Mdocdate: July 2 2008 $ 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. 158.Cm AuthorizedKeysFile 159may contain tokens of the form %T which are substituted during connection 160setup. 161The following tokens are defined: %% is replaced by a literal '%', 162%h is replaced by the home directory of the user being authenticated, and 163%u is replaced by the username of that user. 164After expansion, 165.Cm AuthorizedKeysFile 166is taken to be an absolute path or one relative to the user's home 167directory. 168The default is 169.Dq .ssh/authorized_keys . 170.It Cm Banner 171The contents of the specified file are sent to the remote user before 172authentication is allowed. 173If the argument is 174.Dq none 175then no banner is displayed. 176This option is only available for protocol version 2. 177By default, no banner is displayed. 178.It Cm ChallengeResponseAuthentication 179Specifies whether challenge-response authentication is allowed. 180All authentication styles from 181.Xr login.conf 5 182are supported. 183The default is 184.Dq yes . 185.It Cm ChrootDirectory 186Specifies a path to 187.Xr chroot 2 188to after authentication. 189This path, and all its components, must be root-owned directories that are 190not writable by any other user or group. 191.Pp 192The path may contain the following tokens that are expanded at runtime once 193the connecting user has been authenticated: %% is replaced by a literal '%', 194%h is replaced by the home directory of the user being authenticated, and 195%u is replaced by the username of that user. 196.Pp 197The 198.Cm ChrootDirectory 199must contain the necessary files and directories to support the 200users' session. 201For an interactive session this requires at least a shell, typically 202.Xr sh 1 , 203and basic 204.Pa /dev 205nodes such as 206.Xr null 4 , 207.Xr zero 4 , 208.Xr stdin 4 , 209.Xr stdout 4 , 210.Xr stderr 4 , 211.Xr arandom 4 212and 213.Xr tty 4 214devices. 215For file transfer sessions using 216.Dq sftp , 217no additional configuration of the environment is necessary if the 218in-process sftp server is used (see 219.Cm Subsystem 220for details). 221.Pp 222The default is not to 223.Xr chroot 2 . 224.It Cm Ciphers 225Specifies the ciphers allowed for protocol version 2. 226Multiple ciphers must be comma-separated. 227The supported ciphers are 228.Dq 3des-cbc , 229.Dq aes128-cbc , 230.Dq aes192-cbc , 231.Dq aes256-cbc , 232.Dq aes128-ctr , 233.Dq aes192-ctr , 234.Dq aes256-ctr , 235.Dq arcfour128 , 236.Dq arcfour256 , 237.Dq arcfour , 238.Dq blowfish-cbc , 239and 240.Dq cast128-cbc . 241The default is: 242.Bd -literal -offset 3n 243aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, 244arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, 245aes192-ctr,aes256-ctr 246.Ed 247.It Cm ClientAliveCountMax 248Sets the number of client alive messages (see below) which may be 249sent without 250.Xr sshd 8 251receiving any messages back from the client. 252If this threshold is reached while client alive messages are being sent, 253sshd will disconnect the client, terminating the session. 254It is important to note that the use of client alive messages is very 255different from 256.Cm TCPKeepAlive 257(below). 258The client alive messages are sent through the encrypted channel 259and therefore will not be spoofable. 260The TCP keepalive option enabled by 261.Cm TCPKeepAlive 262is spoofable. 263The client alive mechanism is valuable when the client or 264server depend on knowing when a connection has become inactive. 265.Pp 266The default value is 3. 267If 268.Cm ClientAliveInterval 269(see below) is set to 15, and 270.Cm ClientAliveCountMax 271is left at the default, unresponsive SSH clients 272will be disconnected after approximately 45 seconds. 273This option applies to protocol version 2 only. 274.It Cm ClientAliveInterval 275Sets a timeout interval in seconds after which if no data has been received 276from the client, 277.Xr sshd 8 278will send a message through the encrypted 279channel to request a response from the client. 280The default 281is 0, indicating that these messages will not be sent to the client. 282This option applies to protocol version 2 only. 283.It Cm Compression 284Specifies whether compression is allowed, or delayed until 285the user has authenticated successfully. 286The argument must be 287.Dq yes , 288.Dq delayed , 289or 290.Dq no . 291The default is 292.Dq delayed . 293.It Cm DenyGroups 294This keyword can be followed by a list of group name patterns, separated 295by spaces. 296Login is disallowed for users whose primary group or supplementary 297group list matches one of the patterns. 298Only group names are valid; a numerical group ID is not recognized. 299By default, login is allowed for all groups. 300The allow/deny directives are processed in the following order: 301.Cm DenyUsers , 302.Cm AllowUsers , 303.Cm DenyGroups , 304and finally 305.Cm AllowGroups . 306.Pp 307See 308.Sx PATTERNS 309in 310.Xr ssh_config 5 311for more information on patterns. 312.It Cm DenyUsers 313This keyword can be followed by a list of user name patterns, separated 314by spaces. 315Login is disallowed for user names that match one of the patterns. 316Only user names are valid; a numerical user ID is not recognized. 317By default, login is allowed for all users. 318If the pattern takes the form USER@HOST then USER and HOST 319are separately checked, restricting logins to particular 320users from particular hosts. 321The allow/deny directives are processed in the following order: 322.Cm DenyUsers , 323.Cm AllowUsers , 324.Cm DenyGroups , 325and finally 326.Cm AllowGroups . 327.Pp 328See 329.Sx PATTERNS 330in 331.Xr ssh_config 5 332for more information on patterns. 333.It Cm ForceCommand 334Forces the execution of the command specified by 335.Cm ForceCommand , 336ignoring any command supplied by the client and 337.Pa ~/.ssh/rc 338if present. 339The command is invoked by using the user's login shell with the -c option. 340This applies to shell, command, or subsystem execution. 341It is most useful inside a 342.Cm Match 343block. 344The command originally supplied by the client is available in the 345.Ev SSH_ORIGINAL_COMMAND 346environment variable. 347Specifying a command of 348.Dq internal-sftp 349will force the use of an in-process sftp server that requires no support 350files when used with 351.Cm ChrootDirectory . 352.It Cm GatewayPorts 353Specifies whether remote hosts are allowed to connect to ports 354forwarded for the client. 355By default, 356.Xr sshd 8 357binds remote port forwardings to the loopback address. 358This prevents other remote hosts from connecting to forwarded ports. 359.Cm GatewayPorts 360can be used to specify that sshd 361should allow remote port forwardings to bind to non-loopback addresses, thus 362allowing other hosts to connect. 363The argument may be 364.Dq no 365to force remote port forwardings to be available to the local host only, 366.Dq yes 367to force remote port forwardings to bind to the wildcard address, or 368.Dq clientspecified 369to allow the client to select the address to which the forwarding is bound. 370The default is 371.Dq no . 372.It Cm GSSAPIAuthentication 373Specifies whether user authentication based on GSSAPI is allowed. 374The default is 375.Dq no . 376Note that this option applies to protocol version 2 only. 377.It Cm GSSAPICleanupCredentials 378Specifies whether to automatically destroy the user's credentials cache 379on logout. 380The default is 381.Dq yes . 382Note that this option applies to protocol version 2 only. 383.It Cm HostbasedAuthentication 384Specifies whether rhosts or /etc/hosts.equiv authentication together 385with successful public key client host authentication is allowed 386(host-based authentication). 387This option is similar to 388.Cm RhostsRSAAuthentication 389and applies to protocol version 2 only. 390The default is 391.Dq no . 392.It Cm HostbasedUsesNameFromPacketOnly 393Specifies whether or not the server will attempt to perform a reverse 394name lookup when matching the name in the 395.Pa ~/.shosts , 396.Pa ~/.rhosts , 397and 398.Pa /etc/hosts.equiv 399files during 400.Cm HostbasedAuthentication . 401A setting of 402.Dq yes 403means that 404.Xr sshd 8 405uses the name supplied by the client rather than 406attempting to resolve the name from the TCP connection itself. 407The default is 408.Dq no . 409.It Cm HostKey 410Specifies a file containing a private host key 411used by SSH. 412The default is 413.Pa /etc/ssh/ssh_host_key 414for protocol version 1, and 415.Pa /etc/ssh/ssh_host_rsa_key 416and 417.Pa /etc/ssh/ssh_host_dsa_key 418for protocol version 2. 419Note that 420.Xr sshd 8 421will refuse to use a file if it is group/world-accessible. 422It is possible to have multiple host key files. 423.Dq rsa1 424keys are used for version 1 and 425.Dq dsa 426or 427.Dq rsa 428are used for version 2 of the SSH protocol. 429.It Cm IgnoreRhosts 430Specifies that 431.Pa .rhosts 432and 433.Pa .shosts 434files will not be used in 435.Cm RhostsRSAAuthentication 436or 437.Cm HostbasedAuthentication . 438.Pp 439.Pa /etc/hosts.equiv 440and 441.Pa /etc/shosts.equiv 442are still used. 443The default is 444.Dq yes . 445.It Cm IgnoreUserKnownHosts 446Specifies whether 447.Xr sshd 8 448should ignore the user's 449.Pa ~/.ssh/known_hosts 450during 451.Cm RhostsRSAAuthentication 452or 453.Cm HostbasedAuthentication . 454The default is 455.Dq no . 456.It Cm KerberosAuthentication 457Specifies whether the password provided by the user for 458.Cm PasswordAuthentication 459will be validated through the Kerberos KDC. 460To use this option, the server needs a 461Kerberos servtab which allows the verification of the KDC's identity. 462The default is 463.Dq no . 464.It Cm KerberosGetAFSToken 465If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 466an AFS token before accessing the user's home directory. 467The default is 468.Dq no . 469.It Cm KerberosOrLocalPasswd 470If password authentication through Kerberos fails then 471the password will be validated via any additional local mechanism 472such as 473.Pa /etc/passwd . 474The default is 475.Dq yes . 476.It Cm KerberosTicketCleanup 477Specifies whether to automatically destroy the user's ticket cache 478file on logout. 479The default is 480.Dq yes . 481.It Cm KeyRegenerationInterval 482In protocol version 1, the ephemeral server key is automatically regenerated 483after this many seconds (if it has been used). 484The purpose of regeneration is to prevent 485decrypting captured sessions by later breaking into the machine and 486stealing the keys. 487The key is never stored anywhere. 488If the value is 0, the key is never regenerated. 489The default is 3600 (seconds). 490.It Cm ListenAddress 491Specifies the local addresses 492.Xr sshd 8 493should listen on. 494The following forms may be used: 495.Pp 496.Bl -item -offset indent -compact 497.It 498.Cm ListenAddress 499.Sm off 500.Ar host No | Ar IPv4_addr No | Ar IPv6_addr 501.Sm on 502.It 503.Cm ListenAddress 504.Sm off 505.Ar host No | Ar IPv4_addr No : Ar port 506.Sm on 507.It 508.Cm ListenAddress 509.Sm off 510.Oo 511.Ar host No | Ar IPv6_addr Oc : Ar port 512.Sm on 513.El 514.Pp 515If 516.Ar port 517is not specified, 518sshd will listen on the address and all prior 519.Cm Port 520options specified. 521The default is to listen on all local addresses. 522Multiple 523.Cm ListenAddress 524options are permitted. 525Additionally, any 526.Cm Port 527options must precede this option for non-port qualified addresses. 528.It Cm LoginGraceTime 529The server disconnects after this time if the user has not 530successfully logged in. 531If the value is 0, there is no time limit. 532The default is 120 seconds. 533.It Cm LogLevel 534Gives the verbosity level that is used when logging messages from 535.Xr sshd 8 . 536The possible values are: 537QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 538The default is INFO. 539DEBUG and DEBUG1 are equivalent. 540DEBUG2 and DEBUG3 each specify higher levels of debugging output. 541Logging with a DEBUG level violates the privacy of users and is not recommended. 542.It Cm MACs 543Specifies the available MAC (message authentication code) algorithms. 544The MAC algorithm is used in protocol version 2 545for data integrity protection. 546Multiple algorithms must be comma-separated. 547The default is: 548.Bd -literal -offset indent 549hmac-md5,hmac-sha1,umac-64@openssh.com, 550hmac-ripemd160,hmac-sha1-96,hmac-md5-96 551.Ed 552.It Cm Match 553Introduces a conditional block. 554If all of the criteria on the 555.Cm Match 556line are satisfied, the keywords on the following lines override those 557set in the global section of the config file, until either another 558.Cm Match 559line or the end of the file. 560.Pp 561The arguments to 562.Cm Match 563are one or more criteria-pattern pairs. 564The available criteria are 565.Cm User , 566.Cm Group , 567.Cm Host , 568and 569.Cm Address . 570The match patterns may consist of single entries or comma-separated 571lists and may use the wildcard and negation operators described in the 572.Sx PATTERNS 573section of 574.Xr ssh_config 5 . 575.Pp 576The patterns in an 577.Cm Address 578criteria may additionally contain addresses to match in CIDR 579address/masklen format, e.g.\& 580.Dq 192.0.2.0/24 581or 582.Dq 3ffe:ffff::/32 . 583Note that the mask length provided must be consistent with the address - 584it is an error to specify a mask length that is too long for the address 585or one with bits set in this host portion of the address. 586For example, 587.Dq 192.0.2.0/33 588and 589.Dq 192.0.2.0/8 590respectively. 591.Pp 592Only a subset of keywords may be used on the lines following a 593.Cm Match 594keyword. 595Available keywords are 596.Cm AllowTcpForwarding , 597.Cm Banner , 598.Cm ChrootDirectory , 599.Cm ForceCommand , 600.Cm GatewayPorts , 601.Cm GSSAPIAuthentication , 602.Cm HostbasedAuthentication , 603.Cm KbdInteractiveAuthentication , 604.Cm KerberosAuthentication , 605.Cm MaxAuthTries , 606.Cm MaxSessions , 607.Cm PasswordAuthentication , 608.Cm PermitOpen , 609.Cm PermitRootLogin , 610.Cm RhostsRSAAuthentication , 611.Cm RSAAuthentication , 612.Cm X11DisplayOffset , 613.Cm X11Forwarding , 614and 615.Cm X11UseLocalHost . 616.It Cm MaxAuthTries 617Specifies the maximum number of authentication attempts permitted per 618connection. 619Once the number of failures reaches half this value, 620additional failures are logged. 621The default is 6. 622.It Cm MaxSessions 623Specifies the maximum number of open sessions permitted per network connection. 624The default is 10. 625.It Cm MaxStartups 626Specifies the maximum number of concurrent unauthenticated connections to the 627SSH daemon. 628Additional connections will be dropped until authentication succeeds or the 629.Cm LoginGraceTime 630expires for a connection. 631The default is 10. 632.Pp 633Alternatively, random early drop can be enabled by specifying 634the three colon separated values 635.Dq start:rate:full 636(e.g. "10:30:60"). 637.Xr sshd 8 638will refuse connection attempts with a probability of 639.Dq rate/100 640(30%) 641if there are currently 642.Dq start 643(10) 644unauthenticated connections. 645The probability increases linearly and all connection attempts 646are refused if the number of unauthenticated connections reaches 647.Dq full 648(60). 649.It Cm PasswordAuthentication 650Specifies whether password authentication is allowed. 651The default is 652.Dq yes . 653.It Cm PermitEmptyPasswords 654When password authentication is allowed, it specifies whether the 655server allows login to accounts with empty password strings. 656The default is 657.Dq no . 658.It Cm PermitOpen 659Specifies the destinations to which TCP port forwarding is permitted. 660The forwarding specification must be one of the following forms: 661.Pp 662.Bl -item -offset indent -compact 663.It 664.Cm PermitOpen 665.Sm off 666.Ar host : port 667.Sm on 668.It 669.Cm PermitOpen 670.Sm off 671.Ar IPv4_addr : port 672.Sm on 673.It 674.Cm PermitOpen 675.Sm off 676.Ar \&[ IPv6_addr \&] : port 677.Sm on 678.El 679.Pp 680Multiple forwards may be specified by separating them with whitespace. 681An argument of 682.Dq any 683can be used to remove all restrictions and permit any forwarding requests. 684By default all port forwarding requests are permitted. 685.It Cm PermitRootLogin 686Specifies whether root can log in using 687.Xr ssh 1 . 688The argument must be 689.Dq yes , 690.Dq without-password , 691.Dq forced-commands-only , 692or 693.Dq no . 694The default is 695.Dq yes . 696.Pp 697If this option is set to 698.Dq without-password , 699password authentication is disabled for root. 700.Pp 701If this option is set to 702.Dq forced-commands-only , 703root login with public key authentication will be allowed, 704but only if the 705.Ar command 706option has been specified 707(which may be useful for taking remote backups even if root login is 708normally not allowed). 709All other authentication methods are disabled for root. 710.Pp 711If this option is set to 712.Dq no , 713root is not allowed to log in. 714.It Cm PermitTunnel 715Specifies whether 716.Xr tun 4 717device forwarding is allowed. 718The argument must be 719.Dq yes , 720.Dq point-to-point 721(layer 3), 722.Dq ethernet 723(layer 2), or 724.Dq no . 725Specifying 726.Dq yes 727permits both 728.Dq point-to-point 729and 730.Dq ethernet . 731The default is 732.Dq no . 733.It Cm PermitUserEnvironment 734Specifies whether 735.Pa ~/.ssh/environment 736and 737.Cm environment= 738options in 739.Pa ~/.ssh/authorized_keys 740are processed by 741.Xr sshd 8 . 742The default is 743.Dq no . 744Enabling environment processing may enable users to bypass access 745restrictions in some configurations using mechanisms such as 746.Ev LD_PRELOAD . 747.It Cm PidFile 748Specifies the file that contains the process ID of the 749SSH daemon. 750The default is 751.Pa /var/run/sshd.pid . 752.It Cm Port 753Specifies the port number that 754.Xr sshd 8 755listens on. 756The default is 22. 757Multiple options of this type are permitted. 758See also 759.Cm ListenAddress . 760.It Cm PrintLastLog 761Specifies whether 762.Xr sshd 8 763should print the date and time of the last user login when a user logs 764in interactively. 765The default is 766.Dq yes . 767.It Cm PrintMotd 768Specifies whether 769.Xr sshd 8 770should print 771.Pa /etc/motd 772when a user logs in interactively. 773(On some systems it is also printed by the shell, 774.Pa /etc/profile , 775or equivalent.) 776The default is 777.Dq yes . 778.It Cm Protocol 779Specifies the protocol versions 780.Xr sshd 8 781supports. 782The possible values are 783.Sq 1 784and 785.Sq 2 . 786Multiple versions must be comma-separated. 787The default is 788.Dq 2,1 . 789Note that the order of the protocol list does not indicate preference, 790because the client selects among multiple protocol versions offered 791by the server. 792Specifying 793.Dq 2,1 794is identical to 795.Dq 1,2 . 796.It Cm PubkeyAuthentication 797Specifies whether public key authentication is allowed. 798The default is 799.Dq yes . 800Note that this option applies to protocol version 2 only. 801.It Cm RhostsRSAAuthentication 802Specifies whether rhosts or /etc/hosts.equiv authentication together 803with successful RSA host authentication is allowed. 804The default is 805.Dq no . 806This option applies to protocol version 1 only. 807.It Cm RSAAuthentication 808Specifies whether pure RSA authentication is allowed. 809The default is 810.Dq yes . 811This option applies to protocol version 1 only. 812.It Cm ServerKeyBits 813Defines the number of bits in the ephemeral protocol version 1 server key. 814The minimum value is 512, and the default is 1024. 815.It Cm StrictModes 816Specifies whether 817.Xr sshd 8 818should check file modes and ownership of the 819user's files and home directory before accepting login. 820This is normally desirable because novices sometimes accidentally leave their 821directory or files world-writable. 822The default is 823.Dq yes . 824.It Cm Subsystem 825Configures an external subsystem (e.g. file transfer daemon). 826Arguments should be a subsystem name and a command (with optional arguments) 827to execute upon subsystem request. 828.Pp 829The command 830.Xr sftp-server 8 831implements the 832.Dq sftp 833file transfer subsystem. 834.Pp 835Alternately the name 836.Dq internal-sftp 837implements an in-process 838.Dq sftp 839server. 840This may simplify configurations using 841.Cm ChrootDirectory 842to force a different filesystem root on clients. 843.Pp 844By default no subsystems are defined. 845Note that this option applies to protocol version 2 only. 846.It Cm SyslogFacility 847Gives the facility code that is used when logging messages from 848.Xr sshd 8 . 849The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 850LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 851The default is AUTH. 852.It Cm TCPKeepAlive 853Specifies whether the system should send TCP keepalive messages to the 854other side. 855If they are sent, death of the connection or crash of one 856of the machines will be properly noticed. 857However, this means that 858connections will die if the route is down temporarily, and some people 859find it annoying. 860On the other hand, if TCP keepalives are not sent, 861sessions may hang indefinitely on the server, leaving 862.Dq ghost 863users and consuming server resources. 864.Pp 865The default is 866.Dq yes 867(to send TCP keepalive messages), and the server will notice 868if the network goes down or the client host crashes. 869This avoids infinitely hanging sessions. 870.Pp 871To disable TCP keepalive messages, the value should be set to 872.Dq no . 873.It Cm UseDNS 874Specifies whether 875.Xr sshd 8 876should look up the remote host name and check that 877the resolved host name for the remote IP address maps back to the 878very same IP address. 879The default is 880.Dq yes . 881.It Cm UseLogin 882Specifies whether 883.Xr login 1 884is used for interactive login sessions. 885The default is 886.Dq no . 887Note that 888.Xr login 1 889is never used for remote command execution. 890Note also, that if this is enabled, 891.Cm X11Forwarding 892will be disabled because 893.Xr login 1 894does not know how to handle 895.Xr xauth 1 896cookies. 897If 898.Cm UsePrivilegeSeparation 899is specified, it will be disabled after authentication. 900.It Cm UsePrivilegeSeparation 901Specifies whether 902.Xr sshd 8 903separates privileges by creating an unprivileged child process 904to deal with incoming network traffic. 905After successful authentication, another process will be created that has 906the privilege of the authenticated user. 907The goal of privilege separation is to prevent privilege 908escalation by containing any corruption within the unprivileged processes. 909The default is 910.Dq yes . 911.It Cm X11DisplayOffset 912Specifies the first display number available for 913.Xr sshd 8 Ns 's 914X11 forwarding. 915This prevents sshd from interfering with real X11 servers. 916The default is 10. 917.It Cm X11Forwarding 918Specifies whether X11 forwarding is permitted. 919The argument must be 920.Dq yes 921or 922.Dq no . 923The default is 924.Dq no . 925.Pp 926When X11 forwarding is enabled, there may be additional exposure to 927the server and to client displays if the 928.Xr sshd 8 929proxy display is configured to listen on the wildcard address (see 930.Cm X11UseLocalhost 931below), though this is not the default. 932Additionally, the authentication spoofing and authentication data 933verification and substitution occur on the client side. 934The security risk of using X11 forwarding is that the client's X11 935display server may be exposed to attack when the SSH client requests 936forwarding (see the warnings for 937.Cm ForwardX11 938in 939.Xr ssh_config 5 ) . 940A system administrator may have a stance in which they want to 941protect clients that may expose themselves to attack by unwittingly 942requesting X11 forwarding, which can warrant a 943.Dq no 944setting. 945.Pp 946Note that disabling X11 forwarding does not prevent users from 947forwarding X11 traffic, as users can always install their own forwarders. 948X11 forwarding is automatically disabled if 949.Cm UseLogin 950is enabled. 951.It Cm X11UseLocalhost 952Specifies whether 953.Xr sshd 8 954should bind the X11 forwarding server to the loopback address or to 955the wildcard address. 956By default, 957sshd binds the forwarding server to the loopback address and sets the 958hostname part of the 959.Ev DISPLAY 960environment variable to 961.Dq localhost . 962This prevents remote hosts from connecting to the proxy display. 963However, some older X11 clients may not function with this 964configuration. 965.Cm X11UseLocalhost 966may be set to 967.Dq no 968to specify that the forwarding server should be bound to the wildcard 969address. 970The argument must be 971.Dq yes 972or 973.Dq no . 974The default is 975.Dq yes . 976.It Cm XAuthLocation 977Specifies the full pathname of the 978.Xr xauth 1 979program. 980The default is 981.Pa /usr/X11R6/bin/xauth . 982.El 983.Sh TIME FORMATS 984.Xr sshd 8 985command-line arguments and configuration file options that specify time 986may be expressed using a sequence of the form: 987.Sm off 988.Ar time Op Ar qualifier , 989.Sm on 990where 991.Ar time 992is a positive integer value and 993.Ar qualifier 994is one of the following: 995.Pp 996.Bl -tag -width Ds -compact -offset indent 997.It Aq Cm none 998seconds 999.It Cm s | Cm S 1000seconds 1001.It Cm m | Cm M 1002minutes 1003.It Cm h | Cm H 1004hours 1005.It Cm d | Cm D 1006days 1007.It Cm w | Cm W 1008weeks 1009.El 1010.Pp 1011Each member of the sequence is added together to calculate 1012the total time value. 1013.Pp 1014Time format examples: 1015.Pp 1016.Bl -tag -width Ds -compact -offset indent 1017.It 600 1018600 seconds (10 minutes) 1019.It 10m 102010 minutes 1021.It 1h30m 10221 hour 30 minutes (90 minutes) 1023.El 1024.Sh FILES 1025.Bl -tag -width Ds 1026.It Pa /etc/ssh/sshd_config 1027Contains configuration data for 1028.Xr sshd 8 . 1029This file should be writable by root only, but it is recommended 1030(though not necessary) that it be world-readable. 1031.El 1032.Sh SEE ALSO 1033.Xr sshd 8 1034.Sh AUTHORS 1035OpenSSH is a derivative of the original and free 1036ssh 1.2.12 release by Tatu Ylonen. 1037Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1038Theo de Raadt and Dug Song 1039removed many bugs, re-added newer features and 1040created OpenSSH. 1041Markus Friedl contributed the support for SSH 1042protocol versions 1.5 and 2.0. 1043Niels Provos and Markus Friedl contributed support 1044for privilege separation. 1045