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 1.234 2006/08/21 08:15:57 dtucker Exp $ 38.\" $FreeBSD$ 39.Dd September 25, 1999 40.Dt SSHD 8 41.Os 42.Sh NAME 43.Nm sshd 44.Nd OpenSSH SSH daemon 45.Sh SYNOPSIS 46.Nm sshd 47.Bk -words 48.Op Fl 46Ddeiqt 49.Op Fl b Ar bits 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 rlogin and rsh, and 63provide secure encrypted communications between two untrusted hosts 64over an insecure network. 65.Pp 66.Nm 67listens for connections from clients. 68It is normally started at boot from 69.Pa /etc/rc.d/sshd . 70It forks a new 71daemon for each incoming connection. 72The forked daemons handle 73key exchange, encryption, authentication, command execution, 74and data exchange. 75.Pp 76.Nm 77can be configured using command-line options or a configuration file 78(by default 79.Xr sshd_config 5 ) ; 80command-line options override values specified in the 81configuration file. 82.Nm 83rereads its configuration file when it receives a hangup signal, 84.Dv SIGHUP , 85by executing itself with the name and options it was started with, e.g.\& 86.Pa /usr/sbin/sshd . 87.Pp 88The options are as follows: 89.Bl -tag -width Ds 90.It Fl 4 91Forces 92.Nm 93to use IPv4 addresses only. 94.It Fl 6 95Forces 96.Nm 97to use IPv6 addresses only. 98.It Fl b Ar bits 99Specifies the number of bits in the ephemeral protocol version 1 100server key (default 768). 101.It Fl D 102When this option is specified, 103.Nm 104will not detach and does not become a daemon. 105This allows easy monitoring of 106.Nm sshd . 107.It Fl d 108Debug mode. 109The server sends verbose debug output to the system 110log, and does not put itself in the background. 111The server also will not fork and will only process one connection. 112This option is only intended for debugging for the server. 113Multiple 114.Fl d 115options increase the debugging level. 116Maximum is 3. 117.It Fl e 118When this option is specified, 119.Nm 120will send the output to the standard error instead of the system log. 121.It Fl f Ar configuration_file 122Specifies the name of the configuration file. 123The default is 124.Pa /etc/ssh/sshd_config . 125.Nm 126refuses to start if there is no configuration file. 127.It Fl g Ar login_grace_time 128Gives the grace time for clients to authenticate themselves (default 129120 seconds). 130If the client fails to authenticate the user within 131this many seconds, the server disconnects and exits. 132A value of zero indicates no limit. 133.It Fl h Ar host_key_file 134Specifies a file from which a host key is read. 135This option must be given if 136.Nm 137is not run as root (as the normal 138host key files are normally not readable by anyone but root). 139The default is 140.Pa /etc/ssh/ssh_host_key 141for protocol version 1, and 142.Pa /etc/ssh/ssh_host_dsa_key 143for protocol version 2. 144It is possible to have multiple host key files for 145the different protocol versions and host key algorithms. 146.It Fl i 147Specifies that 148.Nm 149is being run from 150.Xr inetd 8 . 151.Nm 152is normally not run 153from inetd because it needs to generate the server key before it can 154respond to the client, and this may take tens of seconds. 155Clients would have to wait too long if the key was regenerated every time. 156However, with small key sizes (e.g. 512) using 157.Nm 158from inetd may 159be feasible. 160.It Fl k Ar key_gen_time 161Specifies how often the ephemeral protocol version 1 server key is 162regenerated (default 3600 seconds, or one hour). 163The motivation for regenerating the key fairly 164often is that the key is not stored anywhere, and after about an hour 165it becomes impossible to recover the key for decrypting intercepted 166communications even if the machine is cracked into or physically 167seized. 168A value of zero indicates that the key will never be regenerated. 169.It Fl o Ar option 170Can be used to give options in the format used in the configuration file. 171This is useful for specifying options for which there is no separate 172command-line flag. 173For full details of the options, and their values, see 174.Xr sshd_config 5 . 175.It Fl p Ar port 176Specifies the port on which the server listens for connections 177(default 22). 178Multiple port options are permitted. 179Ports specified in the configuration file with the 180.Cm Port 181option are ignored when a command-line port is specified. 182Ports specified using the 183.Cm ListenAddress 184option override command-line ports. 185.It Fl q 186Quiet mode. 187Nothing is sent to the system log. 188Normally the beginning, 189authentication, and termination of each connection is logged. 190.It Fl t 191Test mode. 192Only check the validity of the configuration file and sanity of the keys. 193This is useful for updating 194.Nm 195reliably as configuration options may change. 196.It Fl u Ar len 197This option is used to specify the size of the field 198in the 199.Li utmp 200structure that holds the remote host name. 201If the resolved host name is longer than 202.Ar len , 203the dotted decimal value will be used instead. 204This allows hosts with very long host names that 205overflow this field to still be uniquely identified. 206Specifying 207.Fl u0 208indicates that only dotted decimal addresses 209should be put into the 210.Pa utmp 211file. 212.Fl u0 213may also be used to prevent 214.Nm 215from making DNS requests unless the authentication 216mechanism or configuration requires it. 217Authentication mechanisms that may require DNS include 218.Cm RhostsRSAAuthentication , 219.Cm HostbasedAuthentication , 220and using a 221.Cm from="pattern-list" 222option in a key file. 223Configuration options that require DNS include using a 224USER@HOST pattern in 225.Cm AllowUsers 226or 227.Cm DenyUsers . 228.El 229.Sh AUTHENTICATION 230The OpenSSH SSH daemon supports SSH protocols 1 and 2. 231Both protocols are supported by default, 232though this can be changed via the 233.Cm Protocol 234option in 235.Xr sshd_config 5 . 236Protocol 2 supports both RSA and DSA keys; 237protocol 1 only supports RSA keys. 238For both protocols, 239each host has a host-specific key, 240normally 2048 bits, 241used to identify the host. 242.Pp 243Forward security for protocol 1 is provided through 244an additional server key, 245normally 768 bits, 246generated when the server starts. 247This key is normally regenerated every hour if it has been used, and 248is never stored on disk. 249Whenever a client connects, the daemon responds with its public 250host and server keys. 251The client compares the 252RSA host key against its own database to verify that it has not changed. 253The client then generates a 256-bit random number. 254It encrypts this 255random number using both the host key and the server key, and sends 256the encrypted number to the server. 257Both sides then use this 258random number as a session key which is used to encrypt all further 259communications in the session. 260The rest of the session is encrypted 261using a conventional cipher, currently Blowfish or 3DES, with 3DES 262being used by default. 263The client selects the encryption algorithm 264to use from those offered by the server. 265.Pp 266For protocol 2, 267forward security is provided through a Diffie-Hellman key agreement. 268This key agreement results in a shared session key. 269The rest of the session is encrypted using a symmetric cipher, currently 270128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. 271The client selects the encryption algorithm 272to use from those offered by the server. 273Additionally, session integrity is provided 274through a cryptographic message authentication code 275(hmac-sha1 or hmac-md5). 276.Pp 277Finally, the server and the client enter an authentication dialog. 278The client tries to authenticate itself using 279host-based authentication, 280public key authentication, 281challenge-response authentication, 282or password authentication. 283.Pp 284Regardless of the authentication type, the account is checked to 285ensure that it is accessible. An account is not accessible if it is 286locked, listed in 287.Cm DenyUsers 288or its group is listed in 289.Cm DenyGroups 290\&. The definition of a locked account is system dependant. Some platforms 291have their own account database (eg AIX) and some modify the passwd field ( 292.Ql \&*LK\&* 293on Solaris and UnixWare, 294.Ql \&* 295on HP-UX, containing 296.Ql Nologin 297on Tru64, 298a leading 299.Ql \&*LOCKED\&* 300on FreeBSD and a leading 301.Ql \&!! 302on Linux). If there is a requirement to disable password authentication 303for the account while allowing still public-key, then the passwd field 304should be set to something other than these values (eg 305.Ql NP 306or 307.Ql \&*NP\&* 308). 309.Pp 310If the client successfully authenticates itself, a dialog for 311preparing the session is entered. 312At this time the client may request 313things like allocating a pseudo-tty, forwarding X11 connections, 314forwarding TCP connections, or forwarding the authentication agent 315connection over the secure channel. 316.Pp 317After this, the client either requests a shell or execution of a command. 318The sides then enter session mode. 319In this mode, either side may send 320data at any time, and such data is forwarded to/from the shell or 321command on the server side, and the user terminal in the client side. 322.Pp 323When the user program terminates and all forwarded X11 and other 324connections have been closed, the server sends command exit status to 325the client, and both sides exit. 326.Sh LOGIN PROCESS 327When a user successfully logs in, 328.Nm 329does the following: 330.Bl -enum -offset indent 331.It 332If the login is on a tty, and no command has been specified, 333prints last login time and 334.Pa /etc/motd 335(unless prevented in the configuration file or by 336.Pa ~/.hushlogin ; 337see the 338.Sx FILES 339section). 340.It 341If the login is on a tty, records login time. 342.It 343Checks 344.Pa /etc/nologin and 345.Pa /var/run/nologin ; 346if one exists, it prints the contents and quits 347(unless root). 348.It 349Changes to run with normal user privileges. 350.It 351Sets up basic environment. 352.It 353Reads the file 354.Pa ~/.ssh/environment , 355if it exists, and users are allowed to change their environment. 356See the 357.Cm PermitUserEnvironment 358option in 359.Xr sshd_config 5 . 360.It 361Changes to user's home directory. 362.It 363If 364.Pa ~/.ssh/rc 365exists, runs it; else if 366.Pa /etc/ssh/sshrc 367exists, runs 368it; otherwise runs 369.Xr xauth 1 . 370The 371.Dq rc 372files are given the X11 373authentication protocol and cookie in standard input. 374See 375.Sx SSHRC , 376below. 377.It 378Runs user's shell or command. 379.El 380.Sh SSHRC 381If the file 382.Pa ~/.ssh/rc 383exists, 384.Xr sh 1 385runs it after reading the 386environment files but before starting the user's shell or command. 387It must not produce any output on stdout; stderr must be used 388instead. 389If X11 forwarding is in use, it will receive the "proto cookie" pair in 390its standard input (and 391.Ev DISPLAY 392in its environment). 393The script must call 394.Xr xauth 1 395because 396.Nm 397will not run xauth automatically to add X11 cookies. 398.Pp 399The primary purpose of this file is to run any initialization routines 400which may be needed before the user's home directory becomes 401accessible; AFS is a particular example of such an environment. 402.Pp 403This file will probably contain some initialization code followed by 404something similar to: 405.Bd -literal -offset 3n 406if read proto cookie && [ -n "$DISPLAY" ]; then 407 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 408 # X11UseLocalhost=yes 409 echo add unix:`echo $DISPLAY | 410 cut -c11-` $proto $cookie 411 else 412 # X11UseLocalhost=no 413 echo add $DISPLAY $proto $cookie 414 fi | xauth -q - 415fi 416.Ed 417.Pp 418If this file does not exist, 419.Pa /etc/ssh/sshrc 420is run, and if that 421does not exist either, xauth is used to add the cookie. 422.Sh AUTHORIZED_KEYS FILE FORMAT 423.Cm AuthorizedKeysFile 424specifies the file containing public keys for 425public key authentication; 426if none is specified, the default is 427.Pa ~/.ssh/authorized_keys . 428Each line of the file contains one 429key (empty lines and lines starting with a 430.Ql # 431are ignored as 432comments). 433Protocol 1 public keys consist of the following space-separated fields: 434options, bits, exponent, modulus, comment. 435Protocol 2 public key consist of: 436options, keytype, base64-encoded key, comment. 437The options field is optional; 438its presence is determined by whether the line starts 439with a number or not (the options field never starts with a number). 440The bits, exponent, modulus, and comment fields give the RSA key for 441protocol version 1; the 442comment field is not used for anything (but may be convenient for the 443user to identify the key). 444For protocol version 2 the keytype is 445.Dq ssh-dss 446or 447.Dq ssh-rsa . 448.Pp 449Note that lines in this file are usually several hundred bytes long 450(because of the size of the public key encoding) up to a limit of 4518 kilobytes, which permits DSA keys up to 8 kilobits and RSA 452keys up to 16 kilobits. 453You don't want to type them in; instead, copy the 454.Pa identity.pub , 455.Pa id_dsa.pub , 456or the 457.Pa id_rsa.pub 458file and edit it. 459.Pp 460.Nm 461enforces a minimum RSA key modulus size for protocol 1 462and protocol 2 keys of 768 bits. 463.Pp 464The options (if present) consist of comma-separated option 465specifications. 466No spaces are permitted, except within double quotes. 467The following option specifications are supported (note 468that option keywords are case-insensitive): 469.Bl -tag -width Ds 470.It Cm command="command" 471Specifies that the command is executed whenever this key is used for 472authentication. 473The command supplied by the user (if any) is ignored. 474The command is run on a pty if the client requests a pty; 475otherwise it is run without a tty. 476If an 8-bit clean channel is required, 477one must not request a pty or should specify 478.Cm no-pty . 479A quote may be included in the command by quoting it with a backslash. 480This option might be useful 481to restrict certain public keys to perform just a specific operation. 482An example might be a key that permits remote backups but nothing else. 483Note that the client may specify TCP and/or X11 484forwarding unless they are explicitly prohibited. 485The command originally supplied by the client is available in the 486.Ev SSH_ORIGINAL_COMMAND 487environment variable. 488Note that this option applies to shell, command or subsystem execution. 489.It Cm environment="NAME=value" 490Specifies that the string is to be added to the environment when 491logging in using this key. 492Environment variables set this way 493override other default environment values. 494Multiple options of this type are permitted. 495Environment processing is disabled by default and is 496controlled via the 497.Cm PermitUserEnvironment 498option. 499This option is automatically disabled if 500.Cm UseLogin 501is enabled. 502.It Cm from="pattern-list" 503Specifies that in addition to public key authentication, the canonical name 504of the remote host must be present in the comma-separated list of 505patterns. 506The purpose 507of this option is to optionally increase security: public key authentication 508by itself does not trust the network or name servers or anything (but 509the key); however, if somebody somehow steals the key, the key 510permits an intruder to log in from anywhere in the world. 511This additional option makes using a stolen key more difficult (name 512servers and/or routers would have to be compromised in addition to 513just the key). 514.Pp 515See 516.Sx PATTERNS 517in 518.Xr ssh_config 5 519for more information on patterns. 520.It Cm no-agent-forwarding 521Forbids authentication agent forwarding when this key is used for 522authentication. 523.It Cm no-port-forwarding 524Forbids TCP forwarding when this key is used for authentication. 525Any port forward requests by the client will return an error. 526This might be used, e.g. in connection with the 527.Cm command 528option. 529.It Cm no-pty 530Prevents tty allocation (a request to allocate a pty will fail). 531.It Cm no-X11-forwarding 532Forbids X11 forwarding when this key is used for authentication. 533Any X11 forward requests by the client will return an error. 534.It Cm permitopen="host:port" 535Limit local 536.Li ``ssh -L'' 537port forwarding such that it may only connect to the specified host and 538port. 539IPv6 addresses can be specified with an alternative syntax: 540.Ar host Ns / Ns Ar port . 541Multiple 542.Cm permitopen 543options may be applied separated by commas. 544No pattern matching is performed on the specified hostnames, 545they must be literal domains or addresses. 546.It Cm tunnel="n" 547Force a 548.Xr tun 4 549device on the server. 550Without this option, the next available device will be used if 551the client requests a tunnel. 552.El 553.Pp 554An example authorized_keys file: 555.Bd -literal -offset 3n 556# Comments allowed at start of line 557ssh-rsa AAAAB3Nza...LiPk== user@example.net 558from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 559AAAAB2...19Q== john@example.net 560command="dump /home",no-pty,no-port-forwarding ssh-dss 561AAAAC3...51R== example.net 562permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 563AAAAB5...21S== 564tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 565jane@example.net 566.Ed 567.Sh SSH_KNOWN_HOSTS FILE FORMAT 568The 569.Pa /etc/ssh/ssh_known_hosts 570and 571.Pa ~/.ssh/known_hosts 572files contain host public keys for all known hosts. 573The global file should 574be prepared by the administrator (optional), and the per-user file is 575maintained automatically: whenever the user connects from an unknown host, 576its key is added to the per-user file. 577.Pp 578Each line in these files contains the following fields: hostnames, 579bits, exponent, modulus, comment. 580The fields are separated by spaces. 581.Pp 582Hostnames is a comma-separated list of patterns 583.Pf ( Ql * 584and 585.Ql \&? 586act as 587wildcards); each pattern in turn is matched against the canonical host 588name (when authenticating a client) or against the user-supplied 589name (when authenticating a server). 590A pattern may also be preceded by 591.Ql \&! 592to indicate negation: if the host name matches a negated 593pattern, it is not accepted (by that line) even if it matched another 594pattern on the line. 595A hostname or address may optionally be enclosed within 596.Ql \&[ 597and 598.Ql \&] 599brackets then followed by 600.Ql \&: 601and a non-standard port number. 602.Pp 603Alternately, hostnames may be stored in a hashed form which hides host names 604and addresses should the file's contents be disclosed. 605Hashed hostnames start with a 606.Ql | 607character. 608Only one hashed hostname may appear on a single line and none of the above 609negation or wildcard operators may be applied. 610.Pp 611Bits, exponent, and modulus are taken directly from the RSA host key; they 612can be obtained, for example, from 613.Pa /etc/ssh/ssh_host_key.pub . 614The optional comment field continues to the end of the line, and is not used. 615.Pp 616Lines starting with 617.Ql # 618and empty lines are ignored as comments. 619.Pp 620When performing host authentication, authentication is accepted if any 621matching line has the proper key. 622It is thus permissible (but not 623recommended) to have several lines or different host keys for the same 624names. 625This will inevitably happen when short forms of host names 626from different domains are put in the file. 627It is possible 628that the files contain conflicting information; authentication is 629accepted if valid information can be found from either file. 630.Pp 631Note that the lines in these files are typically hundreds of characters 632long, and you definitely don't want to type in the host keys by hand. 633Rather, generate them by a script 634or by taking 635.Pa /etc/ssh/ssh_host_key.pub 636and adding the host names at the front. 637.Pp 638An example ssh_known_hosts file: 639.Bd -literal -offset 3n 640# Comments allowed at start of line 641closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 642cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 643# A hashed hostname 644|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 645AAAA1234.....= 646.Ed 647.Sh FILES 648.Bl -tag -width Ds -compact 649.It ~/.hushlogin 650This file is used to suppress printing the last login time and 651.Pa /etc/motd , 652if 653.Cm PrintLastLog 654and 655.Cm PrintMotd , 656respectively, 657are enabled. 658It does not suppress printing of the banner specified by 659.Cm Banner . 660.Pp 661.It ~/.rhosts 662This file is used for host-based authentication (see 663.Xr ssh 1 664for more information). 665On some machines this file may need to be 666world-readable if the user's home directory is on an NFS partition, 667because 668.Nm 669reads it as root. 670Additionally, this file must be owned by the user, 671and must not have write permissions for anyone else. 672The recommended 673permission for most machines is read/write for the user, and not 674accessible by others. 675.Pp 676.It ~/.shosts 677This file is used in exactly the same way as 678.Pa .rhosts , 679but allows host-based authentication without permitting login with 680rlogin/rsh. 681.Pp 682.It ~/.ssh/authorized_keys 683Lists the public keys (RSA/DSA) that can be used for logging in as this user. 684The format of this file is described above. 685The content of the file is not highly sensitive, but the recommended 686permissions are read/write for the user, and not accessible by others. 687.Pp 688If this file, the 689.Pa ~/.ssh 690directory, or the user's home directory are writable 691by other users, then the file could be modified or replaced by unauthorized 692users. 693In this case, 694.Nm 695will not allow it to be used unless the 696.Cm StrictModes 697option has been set to 698.Dq no . 699The recommended permissions can be set by executing 700.Dq chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys . 701.Pp 702.It ~/.ssh/environment 703This file is read into the environment at login (if it exists). 704It can only contain empty lines, comment lines (that start with 705.Ql # ) , 706and assignment lines of the form name=value. 707The file should be writable 708only by the user; it need not be readable by anyone else. 709Environment processing is disabled by default and is 710controlled via the 711.Cm PermitUserEnvironment 712option. 713.Pp 714.It ~/.ssh/known_hosts 715Contains a list of host keys for all hosts the user has logged into 716that are not already in the systemwide list of known host keys. 717The format of this file is described above. 718This file should be writable only by root/the owner and 719can, but need not be, world-readable. 720.Pp 721.It ~/.ssh/rc 722Contains initialization routines to be run before 723the user's home directory becomes accessible. 724This file should be writable only by the user, and need not be 725readable by anyone else. 726.Pp 727.It /etc/hosts.allow 728.It /etc/hosts.deny 729Access controls that should be enforced by tcp-wrappers are defined here. 730Further details are described in 731.Xr hosts_access 5 . 732.Pp 733.It /etc/hosts.equiv 734This file is for host-based authentication (see 735.Xr ssh 1 ) . 736It should only be writable by root. 737.Pp 738.It /etc/moduli 739Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". 740The file format is described in 741.Xr moduli 5 . 742.Pp 743.It /etc/motd 744See 745.Xr motd 5 . 746.Pp 747.It /etc/nologin 748If this file exists, 749.Nm 750refuses to let anyone except root log in. 751The contents of the file 752are displayed to anyone trying to log in, and non-root connections are 753refused. 754The file should be world-readable. 755.Pp 756.It /etc/shosts.equiv 757This file is used in exactly the same way as 758.Pa hosts.equiv , 759but allows host-based authentication without permitting login with 760rlogin/rsh. 761.Pp 762.It /etc/ssh/ssh_known_hosts 763Systemwide list of known host keys. 764This file should be prepared by the 765system administrator to contain the public host keys of all machines in the 766organization. 767The format of this file is described above. 768This file should be writable only by root/the owner and 769should be world-readable. 770.Pp 771.It /etc/ssh/ssh_host_key 772.It /etc/ssh/ssh_host_dsa_key 773.It /etc/ssh/ssh_host_rsa_key 774These three files contain the private parts of the host keys. 775These files should only be owned by root, readable only by root, and not 776accessible to others. 777Note that 778.Nm 779does not start if these files are group/world-accessible. 780.Pp 781.It /etc/ssh/ssh_host_key.pub 782.It /etc/ssh/ssh_host_dsa_key.pub 783.It /etc/ssh/ssh_host_rsa_key.pub 784These three files contain the public parts of the host keys. 785These files should be world-readable but writable only by 786root. 787Their contents should match the respective private parts. 788These files are not 789really used for anything; they are provided for the convenience of 790the user so their contents can be copied to known hosts files. 791These files are created using 792.Xr ssh-keygen 1 . 793.Pp 794.It /etc/ssh/sshd_config 795Contains configuration data for 796.Nm sshd . 797The file format and configuration options are described in 798.Xr sshd_config 5 . 799.Pp 800.It /etc/ssh/sshrc 801Similar to 802.Pa ~/.ssh/rc , 803it can be used to specify 804machine-specific login-time initializations globally. 805This file should be writable only by root, and should be world-readable. 806.Pp 807.It /var/empty 808.Xr chroot 2 809directory used by 810.Nm 811during privilege separation in the pre-authentication phase. 812The directory should not contain any files and must be owned by root 813and not group or world-writable. 814.Pp 815.It /var/run/sshd.pid 816Contains the process ID of the 817.Nm 818listening for connections (if there are several daemons running 819concurrently for different ports, this contains the process ID of the one 820started last). 821The content of this file is not sensitive; it can be world-readable. 822.El 823.Sh SEE ALSO 824.Xr scp 1 , 825.Xr sftp 1 , 826.Xr ssh 1 , 827.Xr ssh-add 1 , 828.Xr ssh-agent 1 , 829.Xr ssh-keygen 1 , 830.Xr chroot 2 , 831.Xr hosts_access 5 , 832.Xr login.conf 5 , 833.Xr moduli 5 , 834.Xr sshd_config 5 , 835.Xr inetd 8 , 836.Xr sftp-server 8 837.Sh AUTHORS 838OpenSSH is a derivative of the original and free 839ssh 1.2.12 release by Tatu Ylonen. 840Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 841Theo de Raadt and Dug Song 842removed many bugs, re-added newer features and 843created OpenSSH. 844Markus Friedl contributed the support for SSH 845protocol versions 1.5 and 2.0. 846Niels Provos and Markus Friedl contributed support 847for privilege separation. 848.Sh CAVEATS 849System security is not improved unless 850.Nm rshd , 851.Nm rlogind , 852and 853.Nm rexecd 854are disabled (thus completely disabling 855.Xr rlogin 856and 857.Xr rsh 858into the machine). 859