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.8,v 1.248 2009/03/26 08:38:39 sobrado Exp $ 38.Dd $Mdocdate: March 26 2009 $ 39.Dt SSHD 8 40.Os 41.Sh NAME 42.Nm sshd 43.Nd OpenSSH SSH daemon 44.Sh SYNOPSIS 45.Nm sshd 46.Bk -words 47.Op Fl 46DdeiqTt 48.Op Fl b Ar bits 49.Op Fl C Ar connection_spec 50.Op Fl f Ar config_file 51.Op Fl g Ar login_grace_time 52.Op Fl h Ar host_key_file 53.Op Fl k Ar key_gen_time 54.Op Fl o Ar option 55.Op Fl p Ar port 56.Op Fl u Ar len 57.Ek 58.Sh DESCRIPTION 59.Nm 60(OpenSSH Daemon) is the daemon program for 61.Xr ssh 1 . 62Together these programs replace 63.Xr rlogin 1 64and 65.Xr rsh 1 , 66and provide secure encrypted communications between two untrusted hosts 67over an insecure network. 68.Pp 69.Nm 70listens for connections from clients. 71It is normally started at boot from 72.Pa /etc/rc . 73It forks a new 74daemon for each incoming connection. 75The forked daemons handle 76key exchange, encryption, authentication, command execution, 77and data exchange. 78.Pp 79.Nm 80can be configured using command-line options or a configuration file 81(by default 82.Xr sshd_config 5 ) ; 83command-line options override values specified in the 84configuration file. 85.Nm 86rereads its configuration file when it receives a hangup signal, 87.Dv SIGHUP , 88by executing itself with the name and options it was started with, e.g.\& 89.Pa /usr/sbin/sshd . 90.Pp 91The options are as follows: 92.Bl -tag -width Ds 93.It Fl 4 94Forces 95.Nm 96to use IPv4 addresses only. 97.It Fl 6 98Forces 99.Nm 100to use IPv6 addresses only. 101.It Fl b Ar bits 102Specifies the number of bits in the ephemeral protocol version 1 103server key (default 1024). 104.It Fl C Ar connection_spec 105Specify the connection parameters to use for the 106.Fl T 107extended test mode. 108If provided, any 109.Cm Match 110directives in the configuration file 111that would apply to the specified user, host, and address will be set before 112the configuration is written to standard output. 113The connection parameters are supplied as keyword=value pairs. 114The keywords are 115.Dq user , 116.Dq host , 117and 118.Dq addr . 119All are required and may be supplied in any order, either with multiple 120.Fl C 121options or as a comma-separated list. 122.It Fl D 123When this option is specified, 124.Nm 125will not detach and does not become a daemon. 126This allows easy monitoring of 127.Nm sshd . 128.It Fl d 129Debug mode. 130The server sends verbose debug output to the system 131log, and does not put itself in the background. 132The server also will not fork and will only process one connection. 133This option is only intended for debugging for the server. 134Multiple 135.Fl d 136options increase the debugging level. 137Maximum is 3. 138.It Fl e 139When this option is specified, 140.Nm 141will send the output to the standard error instead of the system log. 142.It Fl f Ar config_file 143Specifies the name of the configuration file. 144The default is 145.Pa /etc/ssh/sshd_config . 146.Nm 147refuses to start if there is no configuration file. 148.It Fl g Ar login_grace_time 149Gives the grace time for clients to authenticate themselves (default 150120 seconds). 151If the client fails to authenticate the user within 152this many seconds, the server disconnects and exits. 153A value of zero indicates no limit. 154.It Fl h Ar host_key_file 155Specifies a file from which a host key is read. 156This option must be given if 157.Nm 158is not run as root (as the normal 159host key files are normally not readable by anyone but root). 160The default is 161.Pa /etc/ssh/ssh_host_key 162for protocol version 1, and 163.Pa /etc/ssh/ssh_host_rsa_key 164and 165.Pa /etc/ssh/ssh_host_dsa_key 166for protocol version 2. 167It is possible to have multiple host key files for 168the different protocol versions and host key algorithms. 169.It Fl i 170Specifies that 171.Nm 172is being run from 173.Xr inetd 8 . 174.Nm 175is normally not run 176from inetd because it needs to generate the server key before it can 177respond to the client, and this may take tens of seconds. 178Clients would have to wait too long if the key was regenerated every time. 179However, with small key sizes (e.g. 512) using 180.Nm 181from inetd may 182be feasible. 183.It Fl k Ar key_gen_time 184Specifies how often the ephemeral protocol version 1 server key is 185regenerated (default 3600 seconds, or one hour). 186The motivation for regenerating the key fairly 187often is that the key is not stored anywhere, and after about an hour 188it becomes impossible to recover the key for decrypting intercepted 189communications even if the machine is cracked into or physically 190seized. 191A value of zero indicates that the key will never be regenerated. 192.It Fl o Ar option 193Can be used to give options in the format used in the configuration file. 194This is useful for specifying options for which there is no separate 195command-line flag. 196For full details of the options, and their values, see 197.Xr sshd_config 5 . 198.It Fl p Ar port 199Specifies the port on which the server listens for connections 200(default 22). 201Multiple port options are permitted. 202Ports specified in the configuration file with the 203.Cm Port 204option are ignored when a command-line port is specified. 205Ports specified using the 206.Cm ListenAddress 207option override command-line ports. 208.It Fl q 209Quiet mode. 210Nothing is sent to the system log. 211Normally the beginning, 212authentication, and termination of each connection is logged. 213.It Fl T 214Extended test mode. 215Check the validity of the configuration file, output the effective configuration 216to stdout and then exit. 217Optionally, 218.Cm Match 219rules may be applied by specifying the connection parameters using one or more 220.Fl C 221options. 222.It Fl t 223Test mode. 224Only check the validity of the configuration file and sanity of the keys. 225This is useful for updating 226.Nm 227reliably as configuration options may change. 228.It Fl u Ar len 229This option is used to specify the size of the field 230in the 231.Li utmp 232structure that holds the remote host name. 233If the resolved host name is longer than 234.Ar len , 235the dotted decimal value will be used instead. 236This allows hosts with very long host names that 237overflow this field to still be uniquely identified. 238Specifying 239.Fl u0 240indicates that only dotted decimal addresses 241should be put into the 242.Pa utmp 243file. 244.Fl u0 245may also be used to prevent 246.Nm 247from making DNS requests unless the authentication 248mechanism or configuration requires it. 249Authentication mechanisms that may require DNS include 250.Cm RhostsRSAAuthentication , 251.Cm HostbasedAuthentication , 252and using a 253.Cm from="pattern-list" 254option in a key file. 255Configuration options that require DNS include using a 256USER@HOST pattern in 257.Cm AllowUsers 258or 259.Cm DenyUsers . 260.El 261.Sh AUTHENTICATION 262The OpenSSH SSH daemon supports SSH protocols 1 and 2. 263Both protocols are supported by default, 264though this can be changed via the 265.Cm Protocol 266option in 267.Xr sshd_config 5 . 268Protocol 2 supports both RSA and DSA keys; 269protocol 1 only supports RSA keys. 270For both protocols, 271each host has a host-specific key, 272normally 2048 bits, 273used to identify the host. 274.Pp 275Forward security for protocol 1 is provided through 276an additional server key, 277normally 768 bits, 278generated when the server starts. 279This key is normally regenerated every hour if it has been used, and 280is never stored on disk. 281Whenever a client connects, the daemon responds with its public 282host and server keys. 283The client compares the 284RSA host key against its own database to verify that it has not changed. 285The client then generates a 256-bit random number. 286It encrypts this 287random number using both the host key and the server key, and sends 288the encrypted number to the server. 289Both sides then use this 290random number as a session key which is used to encrypt all further 291communications in the session. 292The rest of the session is encrypted 293using a conventional cipher, currently Blowfish or 3DES, with 3DES 294being used by default. 295The client selects the encryption algorithm 296to use from those offered by the server. 297.Pp 298For protocol 2, 299forward security is provided through a Diffie-Hellman key agreement. 300This key agreement results in a shared session key. 301The rest of the session is encrypted using a symmetric cipher, currently 302128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. 303The client selects the encryption algorithm 304to use from those offered by the server. 305Additionally, session integrity is provided 306through a cryptographic message authentication code 307(hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160). 308.Pp 309Finally, the server and the client enter an authentication dialog. 310The client tries to authenticate itself using 311host-based authentication, 312public key authentication, 313challenge-response authentication, 314or password authentication. 315.Pp 316If the client successfully authenticates itself, a dialog for 317preparing the session is entered. 318At this time the client may request 319things like allocating a pseudo-tty, forwarding X11 connections, 320forwarding TCP connections, or forwarding the authentication agent 321connection over the secure channel. 322.Pp 323After this, the client either requests a shell or execution of a command. 324The sides then enter session mode. 325In this mode, either side may send 326data at any time, and such data is forwarded to/from the shell or 327command on the server side, and the user terminal in the client side. 328.Pp 329When the user program terminates and all forwarded X11 and other 330connections have been closed, the server sends command exit status to 331the client, and both sides exit. 332.Sh LOGIN PROCESS 333When a user successfully logs in, 334.Nm 335does the following: 336.Bl -enum -offset indent 337.It 338If the login is on a tty, and no command has been specified, 339prints last login time and 340.Pa /etc/motd 341(unless prevented in the configuration file or by 342.Pa ~/.hushlogin ; 343see the 344.Sx FILES 345section). 346.It 347If the login is on a tty, records login time. 348.It 349Checks 350.Pa /etc/nologin ; 351if it exists, prints contents and quits 352(unless root). 353.It 354Changes to run with normal user privileges. 355.It 356Sets up basic environment. 357.It 358Reads the file 359.Pa ~/.ssh/environment , 360if it exists, and users are allowed to change their environment. 361See the 362.Cm PermitUserEnvironment 363option in 364.Xr sshd_config 5 . 365.It 366Changes to user's home directory. 367.It 368If 369.Pa ~/.ssh/rc 370exists, runs it; else if 371.Pa /etc/ssh/sshrc 372exists, runs 373it; otherwise runs xauth. 374The 375.Dq rc 376files are given the X11 377authentication protocol and cookie in standard input. 378See 379.Sx SSHRC , 380below. 381.It 382Runs user's shell or command. 383.El 384.Sh SSHRC 385If the file 386.Pa ~/.ssh/rc 387exists, 388.Xr sh 1 389runs it after reading the 390environment files but before starting the user's shell or command. 391It must not produce any output on stdout; stderr must be used 392instead. 393If X11 forwarding is in use, it will receive the "proto cookie" pair in 394its standard input (and 395.Ev DISPLAY 396in its environment). 397The script must call 398.Xr xauth 1 399because 400.Nm 401will not run xauth automatically to add X11 cookies. 402.Pp 403The primary purpose of this file is to run any initialization routines 404which may be needed before the user's home directory becomes 405accessible; AFS is a particular example of such an environment. 406.Pp 407This file will probably contain some initialization code followed by 408something similar to: 409.Bd -literal -offset 3n 410if read proto cookie && [ -n "$DISPLAY" ]; then 411 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 412 # X11UseLocalhost=yes 413 echo add unix:`echo $DISPLAY | 414 cut -c11-` $proto $cookie 415 else 416 # X11UseLocalhost=no 417 echo add $DISPLAY $proto $cookie 418 fi | xauth -q - 419fi 420.Ed 421.Pp 422If this file does not exist, 423.Pa /etc/ssh/sshrc 424is run, and if that 425does not exist either, xauth is used to add the cookie. 426.Sh AUTHORIZED_KEYS FILE FORMAT 427.Cm AuthorizedKeysFile 428specifies the file containing public keys for 429public key authentication; 430if none is specified, the default is 431.Pa ~/.ssh/authorized_keys . 432Each line of the file contains one 433key (empty lines and lines starting with a 434.Ql # 435are ignored as 436comments). 437Protocol 1 public keys consist of the following space-separated fields: 438options, bits, exponent, modulus, comment. 439Protocol 2 public key consist of: 440options, keytype, base64-encoded key, comment. 441The options field is optional; 442its presence is determined by whether the line starts 443with a number or not (the options field never starts with a number). 444The bits, exponent, modulus, and comment fields give the RSA key for 445protocol version 1; the 446comment field is not used for anything (but may be convenient for the 447user to identify the key). 448For protocol version 2 the keytype is 449.Dq ssh-dss 450or 451.Dq ssh-rsa . 452.Pp 453Note that lines in this file are usually several hundred bytes long 454(because of the size of the public key encoding) up to a limit of 4558 kilobytes, which permits DSA keys up to 8 kilobits and RSA 456keys up to 16 kilobits. 457You don't want to type them in; instead, copy the 458.Pa identity.pub , 459.Pa id_dsa.pub , 460or the 461.Pa id_rsa.pub 462file and edit it. 463.Pp 464.Nm 465enforces a minimum RSA key modulus size for protocol 1 466and protocol 2 keys of 768 bits. 467.Pp 468The options (if present) consist of comma-separated option 469specifications. 470No spaces are permitted, except within double quotes. 471The following option specifications are supported (note 472that option keywords are case-insensitive): 473.Bl -tag -width Ds 474.It Cm command="command" 475Specifies that the command is executed whenever this key is used for 476authentication. 477The command supplied by the user (if any) is ignored. 478The command is run on a pty if the client requests a pty; 479otherwise it is run without a tty. 480If an 8-bit clean channel is required, 481one must not request a pty or should specify 482.Cm no-pty . 483A quote may be included in the command by quoting it with a backslash. 484This option might be useful 485to restrict certain public keys to perform just a specific operation. 486An example might be a key that permits remote backups but nothing else. 487Note that the client may specify TCP and/or X11 488forwarding unless they are explicitly prohibited. 489The command originally supplied by the client is available in the 490.Ev SSH_ORIGINAL_COMMAND 491environment variable. 492Note that this option applies to shell, command or subsystem execution. 493.It Cm environment="NAME=value" 494Specifies that the string is to be added to the environment when 495logging in using this key. 496Environment variables set this way 497override other default environment values. 498Multiple options of this type are permitted. 499Environment processing is disabled by default and is 500controlled via the 501.Cm PermitUserEnvironment 502option. 503This option is automatically disabled if 504.Cm UseLogin 505is enabled. 506.It Cm from="pattern-list" 507Specifies that in addition to public key authentication, either the canonical 508name of the remote host or its IP address must be present in the 509comma-separated list of patterns. 510See 511.Sx PATTERNS 512in 513.Xr ssh_config 5 514for more information on patterns. 515.Pp 516In addition to the wildcard matching that may be applied to hostnames or 517addresses, a 518.Cm from 519stanza may match IP addresses using CIDR address/masklen notation. 520.Pp 521The purpose of this option is to optionally increase security: public key 522authentication by itself does not trust the network or name servers or 523anything (but the key); however, if somebody somehow steals the key, the key 524permits an intruder to log in from anywhere in the world. 525This additional option makes using a stolen key more difficult (name 526servers and/or routers would have to be compromised in addition to 527just the key). 528.It Cm no-agent-forwarding 529Forbids authentication agent forwarding when this key is used for 530authentication. 531.It Cm no-port-forwarding 532Forbids TCP forwarding when this key is used for authentication. 533Any port forward requests by the client will return an error. 534This might be used, e.g. in connection with the 535.Cm command 536option. 537.It Cm no-pty 538Prevents tty allocation (a request to allocate a pty will fail). 539.It Cm no-user-rc 540Disables execution of 541.Pa ~/.ssh/rc . 542.It Cm no-X11-forwarding 543Forbids X11 forwarding when this key is used for authentication. 544Any X11 forward requests by the client will return an error. 545.It Cm permitopen="host:port" 546Limit local 547.Li ``ssh -L'' 548port forwarding such that it may only connect to the specified host and 549port. 550IPv6 addresses can be specified with an alternative syntax: 551.Ar host Ns / Ns Ar port . 552Multiple 553.Cm permitopen 554options may be applied separated by commas. 555No pattern matching is performed on the specified hostnames, 556they must be literal domains or addresses. 557.It Cm tunnel="n" 558Force a 559.Xr tun 4 560device on the server. 561Without this option, the next available device will be used if 562the client requests a tunnel. 563.El 564.Pp 565An example authorized_keys file: 566.Bd -literal -offset 3n 567# Comments allowed at start of line 568ssh-rsa AAAAB3Nza...LiPk== user@example.net 569from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 570AAAAB2...19Q== john@example.net 571command="dump /home",no-pty,no-port-forwarding ssh-dss 572AAAAC3...51R== example.net 573permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 574AAAAB5...21S== 575tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 576jane@example.net 577.Ed 578.Sh SSH_KNOWN_HOSTS FILE FORMAT 579The 580.Pa /etc/ssh/ssh_known_hosts 581and 582.Pa ~/.ssh/known_hosts 583files contain host public keys for all known hosts. 584The global file should 585be prepared by the administrator (optional), and the per-user file is 586maintained automatically: whenever the user connects from an unknown host, 587its key is added to the per-user file. 588.Pp 589Each line in these files contains the following fields: hostnames, 590bits, exponent, modulus, comment. 591The fields are separated by spaces. 592.Pp 593Hostnames is a comma-separated list of patterns 594.Pf ( Ql * 595and 596.Ql \&? 597act as 598wildcards); each pattern in turn is matched against the canonical host 599name (when authenticating a client) or against the user-supplied 600name (when authenticating a server). 601A pattern may also be preceded by 602.Ql \&! 603to indicate negation: if the host name matches a negated 604pattern, it is not accepted (by that line) even if it matched another 605pattern on the line. 606A hostname or address may optionally be enclosed within 607.Ql \&[ 608and 609.Ql \&] 610brackets then followed by 611.Ql \&: 612and a non-standard port number. 613.Pp 614Alternately, hostnames may be stored in a hashed form which hides host names 615and addresses should the file's contents be disclosed. 616Hashed hostnames start with a 617.Ql | 618character. 619Only one hashed hostname may appear on a single line and none of the above 620negation or wildcard operators may be applied. 621.Pp 622Bits, exponent, and modulus are taken directly from the RSA host key; they 623can be obtained, for example, from 624.Pa /etc/ssh/ssh_host_key.pub . 625The optional comment field continues to the end of the line, and is not used. 626.Pp 627Lines starting with 628.Ql # 629and empty lines are ignored as comments. 630.Pp 631When performing host authentication, authentication is accepted if any 632matching line has the proper key. 633It is thus permissible (but not 634recommended) to have several lines or different host keys for the same 635names. 636This will inevitably happen when short forms of host names 637from different domains are put in the file. 638It is possible 639that the files contain conflicting information; authentication is 640accepted if valid information can be found from either file. 641.Pp 642Note that the lines in these files are typically hundreds of characters 643long, and you definitely don't want to type in the host keys by hand. 644Rather, generate them by a script 645or by taking 646.Pa /etc/ssh/ssh_host_key.pub 647and adding the host names at the front. 648.Pp 649An example ssh_known_hosts file: 650.Bd -literal -offset 3n 651# Comments allowed at start of line 652closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 653cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 654# A hashed hostname 655|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 656AAAA1234.....= 657.Ed 658.Sh FILES 659.Bl -tag -width Ds -compact 660.It ~/.hushlogin 661This file is used to suppress printing the last login time and 662.Pa /etc/motd , 663if 664.Cm PrintLastLog 665and 666.Cm PrintMotd , 667respectively, 668are enabled. 669It does not suppress printing of the banner specified by 670.Cm Banner . 671.Pp 672.It ~/.rhosts 673This file is used for host-based authentication (see 674.Xr ssh 1 675for more information). 676On some machines this file may need to be 677world-readable if the user's home directory is on an NFS partition, 678because 679.Nm 680reads it as root. 681Additionally, this file must be owned by the user, 682and must not have write permissions for anyone else. 683The recommended 684permission for most machines is read/write for the user, and not 685accessible by others. 686.Pp 687.It ~/.shosts 688This file is used in exactly the same way as 689.Pa .rhosts , 690but allows host-based authentication without permitting login with 691rlogin/rsh. 692.Pp 693.It ~/.ssh/ 694This directory is the default location for all user-specific configuration 695and authentication information. 696There is no general requirement to keep the entire contents of this directory 697secret, but the recommended permissions are read/write/execute for the user, 698and not accessible by others. 699.Pp 700.It ~/.ssh/authorized_keys 701Lists the public keys (RSA/DSA) that can be used for logging in as this user. 702The format of this file is described above. 703The content of the file is not highly sensitive, but the recommended 704permissions are read/write for the user, and not accessible by others. 705.Pp 706If this file, the 707.Pa ~/.ssh 708directory, or the user's home directory are writable 709by other users, then the file could be modified or replaced by unauthorized 710users. 711In this case, 712.Nm 713will not allow it to be used unless the 714.Cm StrictModes 715option has been set to 716.Dq no . 717.Pp 718.It ~/.ssh/environment 719This file is read into the environment at login (if it exists). 720It can only contain empty lines, comment lines (that start with 721.Ql # ) , 722and assignment lines of the form name=value. 723The file should be writable 724only by the user; it need not be readable by anyone else. 725Environment processing is disabled by default and is 726controlled via the 727.Cm PermitUserEnvironment 728option. 729.Pp 730.It ~/.ssh/known_hosts 731Contains a list of host keys for all hosts the user has logged into 732that are not already in the systemwide list of known host keys. 733The format of this file is described above. 734This file should be writable only by root/the owner and 735can, but need not be, world-readable. 736.Pp 737.It ~/.ssh/rc 738Contains initialization routines to be run before 739the user's home directory becomes accessible. 740This file should be writable only by the user, and need not be 741readable by anyone else. 742.Pp 743.It /etc/hosts.allow 744.It /etc/hosts.deny 745Access controls that should be enforced by tcp-wrappers are defined here. 746Further details are described in 747.Xr hosts_access 5 . 748.Pp 749.It /etc/hosts.equiv 750This file is for host-based authentication (see 751.Xr ssh 1 ) . 752It should only be writable by root. 753.Pp 754.It /etc/moduli 755Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". 756The file format is described in 757.Xr moduli 5 . 758.Pp 759.It /etc/motd 760See 761.Xr motd 5 . 762.Pp 763.It /etc/nologin 764If this file exists, 765.Nm 766refuses to let anyone except root log in. 767The contents of the file 768are displayed to anyone trying to log in, and non-root connections are 769refused. 770The file should be world-readable. 771.Pp 772.It /etc/shosts.equiv 773This file is used in exactly the same way as 774.Pa hosts.equiv , 775but allows host-based authentication without permitting login with 776rlogin/rsh. 777.Pp 778.It /etc/ssh/ssh_host_key 779.It /etc/ssh/ssh_host_dsa_key 780.It /etc/ssh/ssh_host_rsa_key 781These three files contain the private parts of the host keys. 782These files should only be owned by root, readable only by root, and not 783accessible to others. 784Note that 785.Nm 786does not start if these files are group/world-accessible. 787.Pp 788.It /etc/ssh/ssh_host_key.pub 789.It /etc/ssh/ssh_host_dsa_key.pub 790.It /etc/ssh/ssh_host_rsa_key.pub 791These three files contain the public parts of the host keys. 792These files should be world-readable but writable only by 793root. 794Their contents should match the respective private parts. 795These files are not 796really used for anything; they are provided for the convenience of 797the user so their contents can be copied to known hosts files. 798These files are created using 799.Xr ssh-keygen 1 . 800.Pp 801.It /etc/ssh/ssh_known_hosts 802Systemwide list of known host keys. 803This file should be prepared by the 804system administrator to contain the public host keys of all machines in the 805organization. 806The format of this file is described above. 807This file should be writable only by root/the owner and 808should be world-readable. 809.Pp 810.It /etc/ssh/sshd_config 811Contains configuration data for 812.Nm sshd . 813The file format and configuration options are described in 814.Xr sshd_config 5 . 815.Pp 816.It /etc/ssh/sshrc 817Similar to 818.Pa ~/.ssh/rc , 819it can be used to specify 820machine-specific login-time initializations globally. 821This file should be writable only by root, and should be world-readable. 822.Pp 823.It /var/empty 824.Xr chroot 2 825directory used by 826.Nm 827during privilege separation in the pre-authentication phase. 828The directory should not contain any files and must be owned by root 829and not group or world-writable. 830.Pp 831.It /var/run/sshd.pid 832Contains the process ID of the 833.Nm 834listening for connections (if there are several daemons running 835concurrently for different ports, this contains the process ID of the one 836started last). 837The content of this file is not sensitive; it can be world-readable. 838.El 839.Sh SEE ALSO 840.Xr scp 1 , 841.Xr sftp 1 , 842.Xr ssh 1 , 843.Xr ssh-add 1 , 844.Xr ssh-agent 1 , 845.Xr ssh-keygen 1 , 846.Xr ssh-keyscan 1 , 847.Xr chroot 2 , 848.Xr hosts_access 5 , 849.Xr login.conf 5 , 850.Xr moduli 5 , 851.Xr sshd_config 5 , 852.Xr inetd 8 , 853.Xr sftp-server 8 854.Sh AUTHORS 855OpenSSH is a derivative of the original and free 856ssh 1.2.12 release by Tatu Ylonen. 857Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 858Theo de Raadt and Dug Song 859removed many bugs, re-added newer features and 860created OpenSSH. 861Markus Friedl contributed the support for SSH 862protocol versions 1.5 and 2.0. 863Niels Provos and Markus Friedl contributed support 864for privilege separation. 865.Sh CAVEATS 866System security is not improved unless 867.Nm rshd , 868.Nm rlogind , 869and 870.Nm rexecd 871are disabled (thus completely disabling 872.Xr rlogin 873and 874.Xr rsh 875into the machine). 876