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.\" $FreeBSD$ 39.Dd March 26 2009 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 46DdeiqTt 49.Op Fl b Ar bits 50.Op Fl C Ar connection_spec 51.Op Fl f Ar config_file 52.Op Fl g Ar login_grace_time 53.Op Fl h Ar host_key_file 54.Op Fl k Ar key_gen_time 55.Op Fl o Ar option 56.Op Fl p Ar port 57.Op Fl u Ar len 58.Ek 59.Sh DESCRIPTION 60.Nm 61(OpenSSH Daemon) is the daemon program for 62.Xr ssh 1 . 63Together these programs replace 64.Xr rlogin 1 65and 66.Xr rsh 1 , 67and provide secure encrypted communications between two untrusted hosts 68over an insecure network. 69.Pp 70.Nm 71listens for connections from clients. 72It is normally started at boot from 73.Pa /etc/rc.d/sshd . 74It forks a new 75daemon for each incoming connection. 76The forked daemons handle 77key exchange, encryption, authentication, command execution, 78and data exchange. 79.Pp 80.Nm 81can be configured using command-line options or a configuration file 82(by default 83.Xr sshd_config 5 ) ; 84command-line options override values specified in the 85configuration file. 86.Nm 87rereads its configuration file when it receives a hangup signal, 88.Dv SIGHUP , 89by executing itself with the name and options it was started with, e.g.\& 90.Pa /usr/sbin/sshd . 91.Pp 92The options are as follows: 93.Bl -tag -width Ds 94.It Fl 4 95Forces 96.Nm 97to use IPv4 addresses only. 98.It Fl 6 99Forces 100.Nm 101to use IPv6 addresses only. 102.It Fl b Ar bits 103Specifies the number of bits in the ephemeral protocol version 1 104server key (default 1024). 105.It Fl C Ar connection_spec 106Specify the connection parameters to use for the 107.Fl T 108extended test mode. 109If provided, any 110.Cm Match 111directives in the configuration file 112that would apply to the specified user, host, and address will be set before 113the configuration is written to standard output. 114The connection parameters are supplied as keyword=value pairs. 115The keywords are 116.Dq user , 117.Dq host , 118and 119.Dq addr . 120All are required and may be supplied in any order, either with multiple 121.Fl C 122options or as a comma-separated list. 123.It Fl D 124When this option is specified, 125.Nm 126will not detach and does not become a daemon. 127This allows easy monitoring of 128.Nm sshd . 129.It Fl d 130Debug mode. 131The server sends verbose debug output to the system 132log, and does not put itself in the background. 133The server also will not fork and will only process one connection. 134This option is only intended for debugging for the server. 135Multiple 136.Fl d 137options increase the debugging level. 138Maximum is 3. 139.It Fl e 140When this option is specified, 141.Nm 142will send the output to the standard error instead of the system log. 143.It Fl f Ar config_file 144Specifies the name of the configuration file. 145The default is 146.Pa /etc/ssh/sshd_config . 147.Nm 148refuses to start if there is no configuration file. 149.It Fl g Ar login_grace_time 150Gives the grace time for clients to authenticate themselves (default 151120 seconds). 152If the client fails to authenticate the user within 153this many seconds, the server disconnects and exits. 154A value of zero indicates no limit. 155.It Fl h Ar host_key_file 156Specifies a file from which a host key is read. 157This option must be given if 158.Nm 159is not run as root (as the normal 160host key files are normally not readable by anyone but root). 161The default is 162.Pa /etc/ssh/ssh_host_key 163for protocol version 1, and 164.Pa /etc/ssh/ssh_host_rsa_key 165and 166.Pa /etc/ssh/ssh_host_dsa_key 167for protocol version 2. 168It is possible to have multiple host key files for 169the different protocol versions and host key algorithms. 170.It Fl i 171Specifies that 172.Nm 173is being run from 174.Xr inetd 8 . 175.Nm 176is normally not run 177from inetd because it needs to generate the server key before it can 178respond to the client, and this may take tens of seconds. 179Clients would have to wait too long if the key was regenerated every time. 180However, with small key sizes (e.g. 512) using 181.Nm 182from inetd may 183be feasible. 184.It Fl k Ar key_gen_time 185Specifies how often the ephemeral protocol version 1 server key is 186regenerated (default 3600 seconds, or one hour). 187The motivation for regenerating the key fairly 188often is that the key is not stored anywhere, and after about an hour 189it becomes impossible to recover the key for decrypting intercepted 190communications even if the machine is cracked into or physically 191seized. 192A value of zero indicates that the key will never be regenerated. 193.It Fl o Ar option 194Can be used to give options in the format used in the configuration file. 195This is useful for specifying options for which there is no separate 196command-line flag. 197For full details of the options, and their values, see 198.Xr sshd_config 5 . 199.It Fl p Ar port 200Specifies the port on which the server listens for connections 201(default 22). 202Multiple port options are permitted. 203Ports specified in the configuration file with the 204.Cm Port 205option are ignored when a command-line port is specified. 206Ports specified using the 207.Cm ListenAddress 208option override command-line ports. 209.It Fl q 210Quiet mode. 211Nothing is sent to the system log. 212Normally the beginning, 213authentication, and termination of each connection is logged. 214.It Fl T 215Extended test mode. 216Check the validity of the configuration file, output the effective configuration 217to stdout and then exit. 218Optionally, 219.Cm Match 220rules may be applied by specifying the connection parameters using one or more 221.Fl C 222options. 223.It Fl t 224Test mode. 225Only check the validity of the configuration file and sanity of the keys. 226This is useful for updating 227.Nm 228reliably as configuration options may change. 229.It Fl u Ar len 230This option is used to specify the size of the field 231in the 232.Li utmp 233structure that holds the remote host name. 234If the resolved host name is longer than 235.Ar len , 236the dotted decimal value will be used instead. 237This allows hosts with very long host names that 238overflow this field to still be uniquely identified. 239Specifying 240.Fl u0 241indicates that only dotted decimal addresses 242should be put into the 243.Pa utmp 244file. 245.Fl u0 246may also be used to prevent 247.Nm 248from making DNS requests unless the authentication 249mechanism or configuration requires it. 250Authentication mechanisms that may require DNS include 251.Cm RhostsRSAAuthentication , 252.Cm HostbasedAuthentication , 253and using a 254.Cm from="pattern-list" 255option in a key file. 256Configuration options that require DNS include using a 257USER@HOST pattern in 258.Cm AllowUsers 259or 260.Cm DenyUsers . 261.El 262.Sh AUTHENTICATION 263The OpenSSH SSH daemon supports SSH protocols 1 and 2. 264Both protocols are supported by default, 265though this can be changed via the 266.Cm Protocol 267option in 268.Xr sshd_config 5 . 269Protocol 2 supports both RSA and DSA keys; 270protocol 1 only supports RSA keys. 271For both protocols, 272each host has a host-specific key, 273normally 2048 bits, 274used to identify the host. 275.Pp 276Forward security for protocol 1 is provided through 277an additional server key, 278normally 768 bits, 279generated when the server starts. 280This key is normally regenerated every hour if it has been used, and 281is never stored on disk. 282Whenever a client connects, the daemon responds with its public 283host and server keys. 284The client compares the 285RSA host key against its own database to verify that it has not changed. 286The client then generates a 256-bit random number. 287It encrypts this 288random number using both the host key and the server key, and sends 289the encrypted number to the server. 290Both sides then use this 291random number as a session key which is used to encrypt all further 292communications in the session. 293The rest of the session is encrypted 294using a conventional cipher, currently Blowfish or 3DES, with 3DES 295being used by default. 296The client selects the encryption algorithm 297to use from those offered by the server. 298.Pp 299For protocol 2, 300forward security is provided through a Diffie-Hellman key agreement. 301This key agreement results in a shared session key. 302The rest of the session is encrypted using a symmetric cipher, currently 303128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. 304The client selects the encryption algorithm 305to use from those offered by the server. 306Additionally, session integrity is provided 307through a cryptographic message authentication code 308(hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160). 309.Pp 310Finally, the server and the client enter an authentication dialog. 311The client tries to authenticate itself using 312host-based authentication, 313public key authentication, 314challenge-response authentication, 315or password authentication. 316.Pp 317Regardless of the authentication type, the account is checked to 318ensure that it is accessible. An account is not accessible if it is 319locked, listed in 320.Cm DenyUsers 321or its group is listed in 322.Cm DenyGroups 323\&. The definition of a locked account is system dependant. Some platforms 324have their own account database (eg AIX) and some modify the passwd field ( 325.Ql \&*LK\&* 326on Solaris and UnixWare, 327.Ql \&* 328on HP-UX, containing 329.Ql Nologin 330on Tru64, 331a leading 332.Ql \&*LOCKED\&* 333on FreeBSD and a leading 334.Ql \&! 335on most Linuxes). 336If there is a requirement to disable password authentication 337for the account while allowing still public-key, then the passwd field 338should be set to something other than these values (eg 339.Ql NP 340or 341.Ql \&*NP\&* 342). 343.Pp 344If the client successfully authenticates itself, a dialog for 345preparing the session is entered. 346At this time the client may request 347things like allocating a pseudo-tty, forwarding X11 connections, 348forwarding TCP connections, or forwarding the authentication agent 349connection over the secure channel. 350.Pp 351After this, the client either requests a shell or execution of a command. 352The sides then enter session mode. 353In this mode, either side may send 354data at any time, and such data is forwarded to/from the shell or 355command on the server side, and the user terminal in the client side. 356.Pp 357When the user program terminates and all forwarded X11 and other 358connections have been closed, the server sends command exit status to 359the client, and both sides exit. 360.Sh LOGIN PROCESS 361When a user successfully logs in, 362.Nm 363does the following: 364.Bl -enum -offset indent 365.It 366If the login is on a tty, and no command has been specified, 367prints last login time and 368.Pa /etc/motd 369(unless prevented in the configuration file or by 370.Pa ~/.hushlogin ; 371see the 372.Sx FILES 373section). 374.It 375If the login is on a tty, records login time. 376.It 377Checks 378.Pa /etc/nologin and 379.Pa /var/run/nologin ; 380if one exists, it prints the contents and quits 381(unless root). 382.It 383Changes to run with normal user privileges. 384.It 385Sets up basic environment. 386.It 387Reads the file 388.Pa ~/.ssh/environment , 389if it exists, and users are allowed to change their environment. 390See the 391.Cm PermitUserEnvironment 392option in 393.Xr sshd_config 5 . 394.It 395Changes to user's home directory. 396.It 397If 398.Pa ~/.ssh/rc 399exists, runs it; else if 400.Pa /etc/ssh/sshrc 401exists, runs 402it; otherwise runs 403.Xr xauth 1 . 404The 405.Dq rc 406files are given the X11 407authentication protocol and cookie in standard input. 408See 409.Sx SSHRC , 410below. 411.It 412Runs user's shell or command. 413.El 414.Sh SSHRC 415If the file 416.Pa ~/.ssh/rc 417exists, 418.Xr sh 1 419runs it after reading the 420environment files but before starting the user's shell or command. 421It must not produce any output on stdout; stderr must be used 422instead. 423If X11 forwarding is in use, it will receive the "proto cookie" pair in 424its standard input (and 425.Ev DISPLAY 426in its environment). 427The script must call 428.Xr xauth 1 429because 430.Nm 431will not run xauth automatically to add X11 cookies. 432.Pp 433The primary purpose of this file is to run any initialization routines 434which may be needed before the user's home directory becomes 435accessible; AFS is a particular example of such an environment. 436.Pp 437This file will probably contain some initialization code followed by 438something similar to: 439.Bd -literal -offset 3n 440if read proto cookie && [ -n "$DISPLAY" ]; then 441 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 442 # X11UseLocalhost=yes 443 echo add unix:`echo $DISPLAY | 444 cut -c11-` $proto $cookie 445 else 446 # X11UseLocalhost=no 447 echo add $DISPLAY $proto $cookie 448 fi | xauth -q - 449fi 450.Ed 451.Pp 452If this file does not exist, 453.Pa /etc/ssh/sshrc 454is run, and if that 455does not exist either, xauth is used to add the cookie. 456.Sh AUTHORIZED_KEYS FILE FORMAT 457.Cm AuthorizedKeysFile 458specifies the file containing public keys for 459public key authentication; 460if none is specified, the default is 461.Pa ~/.ssh/authorized_keys . 462Each line of the file contains one 463key (empty lines and lines starting with a 464.Ql # 465are ignored as 466comments). 467Protocol 1 public keys consist of the following space-separated fields: 468options, bits, exponent, modulus, comment. 469Protocol 2 public key consist of: 470options, keytype, base64-encoded key, comment. 471The options field is optional; 472its presence is determined by whether the line starts 473with a number or not (the options field never starts with a number). 474The bits, exponent, modulus, and comment fields give the RSA key for 475protocol version 1; the 476comment field is not used for anything (but may be convenient for the 477user to identify the key). 478For protocol version 2 the keytype is 479.Dq ssh-dss 480or 481.Dq ssh-rsa . 482.Pp 483Note that lines in this file are usually several hundred bytes long 484(because of the size of the public key encoding) up to a limit of 4858 kilobytes, which permits DSA keys up to 8 kilobits and RSA 486keys up to 16 kilobits. 487You don't want to type them in; instead, copy the 488.Pa identity.pub , 489.Pa id_dsa.pub , 490or the 491.Pa id_rsa.pub 492file and edit it. 493.Pp 494.Nm 495enforces a minimum RSA key modulus size for protocol 1 496and protocol 2 keys of 768 bits. 497.Pp 498The options (if present) consist of comma-separated option 499specifications. 500No spaces are permitted, except within double quotes. 501The following option specifications are supported (note 502that option keywords are case-insensitive): 503.Bl -tag -width Ds 504.It Cm command="command" 505Specifies that the command is executed whenever this key is used for 506authentication. 507The command supplied by the user (if any) is ignored. 508The command is run on a pty if the client requests a pty; 509otherwise it is run without a tty. 510If an 8-bit clean channel is required, 511one must not request a pty or should specify 512.Cm no-pty . 513A quote may be included in the command by quoting it with a backslash. 514This option might be useful 515to restrict certain public keys to perform just a specific operation. 516An example might be a key that permits remote backups but nothing else. 517Note that the client may specify TCP and/or X11 518forwarding unless they are explicitly prohibited. 519The command originally supplied by the client is available in the 520.Ev SSH_ORIGINAL_COMMAND 521environment variable. 522Note that this option applies to shell, command or subsystem execution. 523.It Cm environment="NAME=value" 524Specifies that the string is to be added to the environment when 525logging in using this key. 526Environment variables set this way 527override other default environment values. 528Multiple options of this type are permitted. 529Environment processing is disabled by default and is 530controlled via the 531.Cm PermitUserEnvironment 532option. 533This option is automatically disabled if 534.Cm UseLogin 535is enabled. 536.It Cm from="pattern-list" 537Specifies that in addition to public key authentication, either the canonical 538name of the remote host or its IP address must be present in the 539comma-separated list of patterns. 540See 541.Sx PATTERNS 542in 543.Xr ssh_config 5 544for more information on patterns. 545.Pp 546In addition to the wildcard matching that may be applied to hostnames or 547addresses, a 548.Cm from 549stanza may match IP addresses using CIDR address/masklen notation. 550.Pp 551The purpose of this option is to optionally increase security: public key 552authentication by itself does not trust the network or name servers or 553anything (but the key); however, if somebody somehow steals the key, the key 554permits an intruder to log in from anywhere in the world. 555This additional option makes using a stolen key more difficult (name 556servers and/or routers would have to be compromised in addition to 557just the key). 558.It Cm no-agent-forwarding 559Forbids authentication agent forwarding when this key is used for 560authentication. 561.It Cm no-port-forwarding 562Forbids TCP forwarding when this key is used for authentication. 563Any port forward requests by the client will return an error. 564This might be used, e.g. in connection with the 565.Cm command 566option. 567.It Cm no-pty 568Prevents tty allocation (a request to allocate a pty will fail). 569.It Cm no-user-rc 570Disables execution of 571.Pa ~/.ssh/rc . 572.It Cm no-X11-forwarding 573Forbids X11 forwarding when this key is used for authentication. 574Any X11 forward requests by the client will return an error. 575.It Cm permitopen="host:port" 576Limit local 577.Li ``ssh -L'' 578port forwarding such that it may only connect to the specified host and 579port. 580IPv6 addresses can be specified with an alternative syntax: 581.Ar host Ns / Ns Ar port . 582Multiple 583.Cm permitopen 584options may be applied separated by commas. 585No pattern matching is performed on the specified hostnames, 586they must be literal domains or addresses. 587.It Cm tunnel="n" 588Force a 589.Xr tun 4 590device on the server. 591Without this option, the next available device will be used if 592the client requests a tunnel. 593.El 594.Pp 595An example authorized_keys file: 596.Bd -literal -offset 3n 597# Comments allowed at start of line 598ssh-rsa AAAAB3Nza...LiPk== user@example.net 599from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 600AAAAB2...19Q== john@example.net 601command="dump /home",no-pty,no-port-forwarding ssh-dss 602AAAAC3...51R== example.net 603permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 604AAAAB5...21S== 605tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 606jane@example.net 607.Ed 608.Sh SSH_KNOWN_HOSTS FILE FORMAT 609The 610.Pa /etc/ssh/ssh_known_hosts 611and 612.Pa ~/.ssh/known_hosts 613files contain host public keys for all known hosts. 614The global file should 615be prepared by the administrator (optional), and the per-user file is 616maintained automatically: whenever the user connects from an unknown host, 617its key is added to the per-user file. 618.Pp 619Each line in these files contains the following fields: hostnames, 620bits, exponent, modulus, comment. 621The fields are separated by spaces. 622.Pp 623Hostnames is a comma-separated list of patterns 624.Pf ( Ql * 625and 626.Ql \&? 627act as 628wildcards); each pattern in turn is matched against the canonical host 629name (when authenticating a client) or against the user-supplied 630name (when authenticating a server). 631A pattern may also be preceded by 632.Ql \&! 633to indicate negation: if the host name matches a negated 634pattern, it is not accepted (by that line) even if it matched another 635pattern on the line. 636A hostname or address may optionally be enclosed within 637.Ql \&[ 638and 639.Ql \&] 640brackets then followed by 641.Ql \&: 642and a non-standard port number. 643.Pp 644Alternately, hostnames may be stored in a hashed form which hides host names 645and addresses should the file's contents be disclosed. 646Hashed hostnames start with a 647.Ql | 648character. 649Only one hashed hostname may appear on a single line and none of the above 650negation or wildcard operators may be applied. 651.Pp 652Bits, exponent, and modulus are taken directly from the RSA host key; they 653can be obtained, for example, from 654.Pa /etc/ssh/ssh_host_key.pub . 655The optional comment field continues to the end of the line, and is not used. 656.Pp 657Lines starting with 658.Ql # 659and empty lines are ignored as comments. 660.Pp 661When performing host authentication, authentication is accepted if any 662matching line has the proper key. 663It is thus permissible (but not 664recommended) to have several lines or different host keys for the same 665names. 666This will inevitably happen when short forms of host names 667from different domains are put in the file. 668It is possible 669that the files contain conflicting information; authentication is 670accepted if valid information can be found from either file. 671.Pp 672Note that the lines in these files are typically hundreds of characters 673long, and you definitely don't want to type in the host keys by hand. 674Rather, generate them by a script 675or by taking 676.Pa /etc/ssh/ssh_host_key.pub 677and adding the host names at the front. 678.Pp 679An example ssh_known_hosts file: 680.Bd -literal -offset 3n 681# Comments allowed at start of line 682closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 683cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 684# A hashed hostname 685|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 686AAAA1234.....= 687.Ed 688.Sh FILES 689.Bl -tag -width Ds -compact 690.It ~/.hushlogin 691This file is used to suppress printing the last login time and 692.Pa /etc/motd , 693if 694.Cm PrintLastLog 695and 696.Cm PrintMotd , 697respectively, 698are enabled. 699It does not suppress printing of the banner specified by 700.Cm Banner . 701.Pp 702.It ~/.rhosts 703This file is used for host-based authentication (see 704.Xr ssh 1 705for more information). 706On some machines this file may need to be 707world-readable if the user's home directory is on an NFS partition, 708because 709.Nm 710reads it as root. 711Additionally, this file must be owned by the user, 712and must not have write permissions for anyone else. 713The recommended 714permission for most machines is read/write for the user, and not 715accessible by others. 716.Pp 717.It ~/.shosts 718This file is used in exactly the same way as 719.Pa .rhosts , 720but allows host-based authentication without permitting login with 721rlogin/rsh. 722.Pp 723.It ~/.ssh/ 724This directory is the default location for all user-specific configuration 725and authentication information. 726There is no general requirement to keep the entire contents of this directory 727secret, but the recommended permissions are read/write/execute for the user, 728and not accessible by others. 729.Pp 730.It ~/.ssh/authorized_keys 731Lists the public keys (RSA/DSA) that can be used for logging in as this user. 732The format of this file is described above. 733The content of the file is not highly sensitive, but the recommended 734permissions are read/write for the user, and not accessible by others. 735.Pp 736If this file, the 737.Pa ~/.ssh 738directory, or the user's home directory are writable 739by other users, then the file could be modified or replaced by unauthorized 740users. 741In this case, 742.Nm 743will not allow it to be used unless the 744.Cm StrictModes 745option has been set to 746.Dq no . 747.Pp 748.It ~/.ssh/environment 749This file is read into the environment at login (if it exists). 750It can only contain empty lines, comment lines (that start with 751.Ql # ) , 752and assignment lines of the form name=value. 753The file should be writable 754only by the user; it need not be readable by anyone else. 755Environment processing is disabled by default and is 756controlled via the 757.Cm PermitUserEnvironment 758option. 759.Pp 760.It ~/.ssh/known_hosts 761Contains a list of host keys for all hosts the user has logged into 762that are not already in the systemwide list of known host keys. 763The format of this file is described above. 764This file should be writable only by root/the owner and 765can, but need not be, world-readable. 766.Pp 767.It ~/.ssh/rc 768Contains initialization routines to be run before 769the user's home directory becomes accessible. 770This file should be writable only by the user, and need not be 771readable by anyone else. 772.Pp 773.It /etc/hosts.allow 774.It /etc/hosts.deny 775Access controls that should be enforced by tcp-wrappers are defined here. 776Further details are described in 777.Xr hosts_access 5 . 778.Pp 779.It /etc/hosts.equiv 780This file is for host-based authentication (see 781.Xr ssh 1 ) . 782It should only be writable by root. 783.Pp 784.It /etc/moduli 785Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". 786The file format is described in 787.Xr moduli 5 . 788.Pp 789.It /etc/motd 790See 791.Xr motd 5 . 792.Pp 793.It /etc/nologin 794If this file exists, 795.Nm 796refuses to let anyone except root log in. 797The contents of the file 798are displayed to anyone trying to log in, and non-root connections are 799refused. 800The file should be world-readable. 801.Pp 802.It /etc/shosts.equiv 803This file is used in exactly the same way as 804.Pa hosts.equiv , 805but allows host-based authentication without permitting login with 806rlogin/rsh. 807.Pp 808.It /etc/ssh/ssh_host_key 809.It /etc/ssh/ssh_host_dsa_key 810.It /etc/ssh/ssh_host_rsa_key 811These three files contain the private parts of the host keys. 812These files should only be owned by root, readable only by root, and not 813accessible to others. 814Note that 815.Nm 816does not start if these files are group/world-accessible. 817.Pp 818.It /etc/ssh/ssh_host_key.pub 819.It /etc/ssh/ssh_host_dsa_key.pub 820.It /etc/ssh/ssh_host_rsa_key.pub 821These three files contain the public parts of the host keys. 822These files should be world-readable but writable only by 823root. 824Their contents should match the respective private parts. 825These files are not 826really used for anything; they are provided for the convenience of 827the user so their contents can be copied to known hosts files. 828These files are created using 829.Xr ssh-keygen 1 . 830.Pp 831.It /etc/ssh/ssh_known_hosts 832Systemwide list of known host keys. 833This file should be prepared by the 834system administrator to contain the public host keys of all machines in the 835organization. 836The format of this file is described above. 837This file should be writable only by root/the owner and 838should be world-readable. 839.Pp 840.It /etc/ssh/sshd_config 841Contains configuration data for 842.Nm sshd . 843The file format and configuration options are described in 844.Xr sshd_config 5 . 845.Pp 846.It /etc/ssh/sshrc 847Similar to 848.Pa ~/.ssh/rc , 849it can be used to specify 850machine-specific login-time initializations globally. 851This file should be writable only by root, and should be world-readable. 852.Pp 853.It /var/empty 854.Xr chroot 2 855directory used by 856.Nm 857during privilege separation in the pre-authentication phase. 858The directory should not contain any files and must be owned by root 859and not group or world-writable. 860.Pp 861.It /var/run/sshd.pid 862Contains the process ID of the 863.Nm 864listening for connections (if there are several daemons running 865concurrently for different ports, this contains the process ID of the one 866started last). 867The content of this file is not sensitive; it can be world-readable. 868.El 869.Sh SEE ALSO 870.Xr scp 1 , 871.Xr sftp 1 , 872.Xr ssh 1 , 873.Xr ssh-add 1 , 874.Xr ssh-agent 1 , 875.Xr ssh-keygen 1 , 876.Xr ssh-keyscan 1 , 877.Xr chroot 2 , 878.Xr hosts_access 5 , 879.Xr login.conf 5 , 880.Xr moduli 5 , 881.Xr sshd_config 5 , 882.Xr inetd 8 , 883.Xr sftp-server 8 884.Sh AUTHORS 885OpenSSH is a derivative of the original and free 886ssh 1.2.12 release by Tatu Ylonen. 887Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 888Theo de Raadt and Dug Song 889removed many bugs, re-added newer features and 890created OpenSSH. 891Markus Friedl contributed the support for SSH 892protocol versions 1.5 and 2.0. 893Niels Provos and Markus Friedl contributed support 894for privilege separation. 895.Sh CAVEATS 896System security is not improved unless 897.Nm rshd , 898.Nm rlogind , 899and 900.Nm rexecd 901are disabled (thus completely disabling 902.Xr rlogin 903and 904.Xr rsh 905into the machine). 906