1.\" 2.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 3.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 4.\" All rights reserved 5.\" 6.\" As far as I am concerned, the code I have written for this software 7.\" can be used freely for any purpose. Any derived versions of this 8.\" software must be clearly marked as such, and if the derived work is 9.\" incompatible with the protocol description in the RFC file, it must be 10.\" called by a name other than "ssh" or "Secure Shell". 11.\" 12.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 13.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 14.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 15.\" 16.\" Redistribution and use in source and binary forms, with or without 17.\" modification, are permitted provided that the following conditions 18.\" are met: 19.\" 1. Redistributions of source code must retain the above copyright 20.\" notice, this list of conditions and the following disclaimer. 21.\" 2. Redistributions in binary form must reproduce the above copyright 22.\" notice, this list of conditions and the following disclaimer in the 23.\" documentation and/or other materials provided with the distribution. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 26.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 27.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 28.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 29.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 30.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 31.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 32.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 33.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 34.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 35.\" 36.\" $OpenBSD: sshd.8,v 1.260 2010/10/28 18:33:28 jmc Exp $ 37.Dd $Mdocdate: October 28 2010 $ 38.Dt SSHD 8 39.Os 40.Sh NAME 41.Nm sshd 42.Nd OpenSSH SSH daemon 43.Sh SYNOPSIS 44.Nm sshd 45.Bk -words 46.Op Fl 46DdeiqTt 47.Op Fl b Ar bits 48.Op Fl C Ar connection_spec 49.Op Fl c Ar host_certificate_file 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.d/sshd . 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 c Ar host_certificate_file 123Specifies a path to a certificate file to identify 124.Nm 125during key exchange. 126The certificate file must match a host key file specified using the 127.Fl h 128option or the 129.Cm HostKey 130configuration directive. 131.It Fl D 132When this option is specified, 133.Nm 134will not detach and does not become a daemon. 135This allows easy monitoring of 136.Nm sshd . 137.It Fl d 138Debug mode. 139The server sends verbose debug output to standard error, 140and does not put itself in the background. 141The server also will not fork and will only process one connection. 142This option is only intended for debugging for the server. 143Multiple 144.Fl d 145options increase the debugging level. 146Maximum is 3. 147.It Fl e 148When this option is specified, 149.Nm 150will send the output to the standard error instead of the system log. 151.It Fl f Ar config_file 152Specifies the name of the configuration file. 153The default is 154.Pa /etc/ssh/sshd_config . 155.Nm 156refuses to start if there is no configuration file. 157.It Fl g Ar login_grace_time 158Gives the grace time for clients to authenticate themselves (default 159120 seconds). 160If the client fails to authenticate the user within 161this many seconds, the server disconnects and exits. 162A value of zero indicates no limit. 163.It Fl h Ar host_key_file 164Specifies a file from which a host key is read. 165This option must be given if 166.Nm 167is not run as root (as the normal 168host key files are normally not readable by anyone but root). 169The default is 170.Pa /etc/ssh/ssh_host_key 171for protocol version 1, and 172.Pa /etc/ssh/ssh_host_dsa_key , 173.Pa /etc/ssh/ssh_host_ecdsa_key 174and 175.Pa /etc/ssh/ssh_host_rsa_key 176for protocol version 2. 177It is possible to have multiple host key files for 178the different protocol versions and host key algorithms. 179.It Fl i 180Specifies that 181.Nm 182is being run from 183.Xr inetd 8 . 184.Nm 185is normally not run 186from inetd because it needs to generate the server key before it can 187respond to the client, and this may take tens of seconds. 188Clients would have to wait too long if the key was regenerated every time. 189However, with small key sizes (e.g. 512) using 190.Nm 191from inetd may 192be feasible. 193.It Fl k Ar key_gen_time 194Specifies how often the ephemeral protocol version 1 server key is 195regenerated (default 3600 seconds, or one hour). 196The motivation for regenerating the key fairly 197often is that the key is not stored anywhere, and after about an hour 198it becomes impossible to recover the key for decrypting intercepted 199communications even if the machine is cracked into or physically 200seized. 201A value of zero indicates that the key will never be regenerated. 202.It Fl o Ar option 203Can be used to give options in the format used in the configuration file. 204This is useful for specifying options for which there is no separate 205command-line flag. 206For full details of the options, and their values, see 207.Xr sshd_config 5 . 208.It Fl p Ar port 209Specifies the port on which the server listens for connections 210(default 22). 211Multiple port options are permitted. 212Ports specified in the configuration file with the 213.Cm Port 214option are ignored when a command-line port is specified. 215Ports specified using the 216.Cm ListenAddress 217option override command-line ports. 218.It Fl q 219Quiet mode. 220Nothing is sent to the system log. 221Normally the beginning, 222authentication, and termination of each connection is logged. 223.It Fl T 224Extended test mode. 225Check the validity of the configuration file, output the effective configuration 226to stdout and then exit. 227Optionally, 228.Cm Match 229rules may be applied by specifying the connection parameters using one or more 230.Fl C 231options. 232.It Fl t 233Test mode. 234Only check the validity of the configuration file and sanity of the keys. 235This is useful for updating 236.Nm 237reliably as configuration options may change. 238.It Fl u Ar len 239This option is used to specify the size of the field 240in the 241.Li utmp 242structure that holds the remote host name. 243If the resolved host name is longer than 244.Ar len , 245the dotted decimal value will be used instead. 246This allows hosts with very long host names that 247overflow this field to still be uniquely identified. 248Specifying 249.Fl u0 250indicates that only dotted decimal addresses 251should be put into the 252.Pa utmp 253file. 254.Fl u0 255may also be used to prevent 256.Nm 257from making DNS requests unless the authentication 258mechanism or configuration requires it. 259Authentication mechanisms that may require DNS include 260.Cm RhostsRSAAuthentication , 261.Cm HostbasedAuthentication , 262and using a 263.Cm from="pattern-list" 264option in a key file. 265Configuration options that require DNS include using a 266USER@HOST pattern in 267.Cm AllowUsers 268or 269.Cm DenyUsers . 270.El 271.Sh AUTHENTICATION 272The OpenSSH SSH daemon supports SSH protocols 1 and 2. 273The default is to use protocol 2 only, 274though this can be changed via the 275.Cm Protocol 276option in 277.Xr sshd_config 5 . 278Protocol 2 supports DSA, ECDSA and RSA keys; 279protocol 1 only supports RSA keys. 280For both protocols, 281each host has a host-specific key, 282normally 2048 bits, 283used to identify the host. 284.Pp 285Forward security for protocol 1 is provided through 286an additional server key, 287normally 768 bits, 288generated when the server starts. 289This key is normally regenerated every hour if it has been used, and 290is never stored on disk. 291Whenever a client connects, the daemon responds with its public 292host and server keys. 293The client compares the 294RSA host key against its own database to verify that it has not changed. 295The client then generates a 256-bit random number. 296It encrypts this 297random number using both the host key and the server key, and sends 298the encrypted number to the server. 299Both sides then use this 300random number as a session key which is used to encrypt all further 301communications in the session. 302The rest of the session is encrypted 303using a conventional cipher, currently Blowfish or 3DES, with 3DES 304being used by default. 305The client selects the encryption algorithm 306to use from those offered by the server. 307.Pp 308For protocol 2, 309forward security is provided through a Diffie-Hellman key agreement. 310This key agreement results in a shared session key. 311The rest of the session is encrypted using a symmetric cipher, currently 312128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. 313The client selects the encryption algorithm 314to use from those offered by the server. 315Additionally, session integrity is provided 316through a cryptographic message authentication code 317(hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160). 318.Pp 319Finally, the server and the client enter an authentication dialog. 320The client tries to authenticate itself using 321host-based authentication, 322public key authentication, 323challenge-response authentication, 324or password authentication. 325.Pp 326Regardless of the authentication type, the account is checked to 327ensure that it is accessible. An account is not accessible if it is 328locked, listed in 329.Cm DenyUsers 330or its group is listed in 331.Cm DenyGroups 332\&. The definition of a locked account is system dependant. Some platforms 333have their own account database (eg AIX) and some modify the passwd field ( 334.Ql \&*LK\&* 335on Solaris and UnixWare, 336.Ql \&* 337on HP-UX, containing 338.Ql Nologin 339on Tru64, 340a leading 341.Ql \&*LOCKED\&* 342on FreeBSD and a leading 343.Ql \&! 344on most Linuxes). 345If there is a requirement to disable password authentication 346for the account while allowing still public-key, then the passwd field 347should be set to something other than these values (eg 348.Ql NP 349or 350.Ql \&*NP\&* 351). 352.Pp 353If the client successfully authenticates itself, a dialog for 354preparing the session is entered. 355At this time the client may request 356things like allocating a pseudo-tty, forwarding X11 connections, 357forwarding TCP connections, or forwarding the authentication agent 358connection over the secure channel. 359.Pp 360After this, the client either requests a shell or execution of a command. 361The sides then enter session mode. 362In this mode, either side may send 363data at any time, and such data is forwarded to/from the shell or 364command on the server side, and the user terminal in the client side. 365.Pp 366When the user program terminates and all forwarded X11 and other 367connections have been closed, the server sends command exit status to 368the client, and both sides exit. 369.Sh LOGIN PROCESS 370When a user successfully logs in, 371.Nm 372does the following: 373.Bl -enum -offset indent 374.It 375If the login is on a tty, and no command has been specified, 376prints last login time and 377.Pa /etc/motd 378(unless prevented in the configuration file or by 379.Pa ~/.hushlogin ; 380see the 381.Sx FILES 382section). 383.It 384If the login is on a tty, records login time. 385.It 386Checks 387.Pa /etc/nologin and 388.Pa /var/run/nologin ; 389if one exists, it prints the contents and quits 390(unless root). 391.It 392Changes to run with normal user privileges. 393.It 394Sets up basic environment. 395.It 396Reads the file 397.Pa ~/.ssh/environment , 398if it exists, and users are allowed to change their environment. 399See the 400.Cm PermitUserEnvironment 401option in 402.Xr sshd_config 5 . 403.It 404Changes to user's home directory. 405.It 406If 407.Pa ~/.ssh/rc 408exists, runs it; else if 409.Pa /etc/ssh/sshrc 410exists, runs 411it; otherwise runs 412.Xr xauth 1 . 413The 414.Dq rc 415files are given the X11 416authentication protocol and cookie (if applicable) in standard input. 417See 418.Sx SSHRC , 419below. 420.It 421Runs user's shell or command. 422.El 423.Sh SSHRC 424If the file 425.Pa ~/.ssh/rc 426exists, 427.Xr sh 1 428runs it after reading the 429environment files but before starting the user's shell or command. 430It must not produce any output on stdout; stderr must be used 431instead. 432If X11 forwarding is in use, it will receive the "proto cookie" pair in 433its standard input (and 434.Ev DISPLAY 435in its environment). 436The script must call 437.Xr xauth 1 438because 439.Nm 440will not run xauth automatically to add X11 cookies. 441.Pp 442The primary purpose of this file is to run any initialization routines 443which may be needed before the user's home directory becomes 444accessible; AFS is a particular example of such an environment. 445.Pp 446This file will probably contain some initialization code followed by 447something similar to: 448.Bd -literal -offset 3n 449if read proto cookie && [ -n "$DISPLAY" ]; then 450 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 451 # X11UseLocalhost=yes 452 echo add unix:`echo $DISPLAY | 453 cut -c11-` $proto $cookie 454 else 455 # X11UseLocalhost=no 456 echo add $DISPLAY $proto $cookie 457 fi | xauth -q - 458fi 459.Ed 460.Pp 461If this file does not exist, 462.Pa /etc/ssh/sshrc 463is run, and if that 464does not exist either, xauth is used to add the cookie. 465.Sh AUTHORIZED_KEYS FILE FORMAT 466.Cm AuthorizedKeysFile 467specifies the file containing public keys for 468public key authentication; 469if none is specified, the default is 470.Pa ~/.ssh/authorized_keys . 471Each line of the file contains one 472key (empty lines and lines starting with a 473.Ql # 474are ignored as 475comments). 476Protocol 1 public keys consist of the following space-separated fields: 477options, bits, exponent, modulus, comment. 478Protocol 2 public key consist of: 479options, keytype, base64-encoded key, comment. 480The options field is optional; 481its presence is determined by whether the line starts 482with a number or not (the options field never starts with a number). 483The bits, exponent, modulus, and comment fields give the RSA key for 484protocol version 1; the 485comment field is not used for anything (but may be convenient for the 486user to identify the key). 487For protocol version 2 the keytype is 488.Dq ecdsa-sha2-nistp256 , 489.Dq ecdsa-sha2-nistp384 , 490.Dq ecdsa-sha2-nistp521 , 491.Dq ssh-dss 492or 493.Dq ssh-rsa . 494.Pp 495Note that lines in this file are usually several hundred bytes long 496(because of the size of the public key encoding) up to a limit of 4978 kilobytes, which permits DSA keys up to 8 kilobits and RSA 498keys up to 16 kilobits. 499You don't want to type them in; instead, copy the 500.Pa identity.pub , 501.Pa id_dsa.pub , 502.Pa id_ecdsa.pub , 503or the 504.Pa id_rsa.pub 505file and edit it. 506.Pp 507.Nm 508enforces a minimum RSA key modulus size for protocol 1 509and protocol 2 keys of 768 bits. 510.Pp 511The options (if present) consist of comma-separated option 512specifications. 513No spaces are permitted, except within double quotes. 514The following option specifications are supported (note 515that option keywords are case-insensitive): 516.Bl -tag -width Ds 517.It Cm cert-authority 518Specifies that the listed key is a certification authority (CA) that is 519trusted to validate signed certificates for user authentication. 520.Pp 521Certificates may encode access restrictions similar to these key options. 522If both certificate restrictions and key options are present, the most 523restrictive union of the two is applied. 524.It Cm command="command" 525Specifies that the command is executed whenever this key is used for 526authentication. 527The command supplied by the user (if any) is ignored. 528The command is run on a pty if the client requests a pty; 529otherwise it is run without a tty. 530If an 8-bit clean channel is required, 531one must not request a pty or should specify 532.Cm no-pty . 533A quote may be included in the command by quoting it with a backslash. 534This option might be useful 535to restrict certain public keys to perform just a specific operation. 536An example might be a key that permits remote backups but nothing else. 537Note that the client may specify TCP and/or X11 538forwarding unless they are explicitly prohibited. 539The command originally supplied by the client is available in the 540.Ev SSH_ORIGINAL_COMMAND 541environment variable. 542Note that this option applies to shell, command or subsystem execution. 543Also note that this command may be superseded by either a 544.Xr sshd_config 5 545.Cm ForceCommand 546directive or a command embedded in a certificate. 547.It Cm environment="NAME=value" 548Specifies that the string is to be added to the environment when 549logging in using this key. 550Environment variables set this way 551override other default environment values. 552Multiple options of this type are permitted. 553Environment processing is disabled by default and is 554controlled via the 555.Cm PermitUserEnvironment 556option. 557This option is automatically disabled if 558.Cm UseLogin 559is enabled. 560.It Cm from="pattern-list" 561Specifies that in addition to public key authentication, either the canonical 562name of the remote host or its IP address must be present in the 563comma-separated list of patterns. 564See 565.Sx PATTERNS 566in 567.Xr ssh_config 5 568for more information on patterns. 569.Pp 570In addition to the wildcard matching that may be applied to hostnames or 571addresses, a 572.Cm from 573stanza may match IP addresses using CIDR address/masklen notation. 574.Pp 575The purpose of this option is to optionally increase security: public key 576authentication by itself does not trust the network or name servers or 577anything (but the key); however, if somebody somehow steals the key, the key 578permits an intruder to log in from anywhere in the world. 579This additional option makes using a stolen key more difficult (name 580servers and/or routers would have to be compromised in addition to 581just the key). 582.It Cm no-agent-forwarding 583Forbids authentication agent forwarding when this key is used for 584authentication. 585.It Cm no-port-forwarding 586Forbids TCP forwarding when this key is used for authentication. 587Any port forward requests by the client will return an error. 588This might be used, e.g. in connection with the 589.Cm command 590option. 591.It Cm no-pty 592Prevents tty allocation (a request to allocate a pty will fail). 593.It Cm no-user-rc 594Disables execution of 595.Pa ~/.ssh/rc . 596.It Cm no-X11-forwarding 597Forbids X11 forwarding when this key is used for authentication. 598Any X11 forward requests by the client will return an error. 599.It Cm permitopen="host:port" 600Limit local 601.Li ``ssh -L'' 602port forwarding such that it may only connect to the specified host and 603port. 604IPv6 addresses can be specified by enclosing the address in square brackets. 605Multiple 606.Cm permitopen 607options may be applied separated by commas. 608No pattern matching is performed on the specified hostnames, 609they must be literal domains or addresses. 610.It Cm principals="principals" 611On a 612.Cm cert-authority 613line, specifies allowed principals for certificate authentication as a 614comma-separated list. 615At least one name from the list must appear in the certificate's 616list of principals for the certificate to be accepted. 617This option is ignored for keys that are not marked as trusted certificate 618signers using the 619.Cm cert-authority 620option. 621.It Cm tunnel="n" 622Force a 623.Xr tun 4 624device on the server. 625Without this option, the next available device will be used if 626the client requests a tunnel. 627.El 628.Pp 629An example authorized_keys file: 630.Bd -literal -offset 3n 631# Comments allowed at start of line 632ssh-rsa AAAAB3Nza...LiPk== user@example.net 633from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 634AAAAB2...19Q== john@example.net 635command="dump /home",no-pty,no-port-forwarding ssh-dss 636AAAAC3...51R== example.net 637permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 638AAAAB5...21S== 639tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 640jane@example.net 641.Ed 642.Sh SSH_KNOWN_HOSTS FILE FORMAT 643The 644.Pa /etc/ssh/ssh_known_hosts 645and 646.Pa ~/.ssh/known_hosts 647files contain host public keys for all known hosts. 648The global file should 649be prepared by the administrator (optional), and the per-user file is 650maintained automatically: whenever the user connects from an unknown host, 651its key is added to the per-user file. 652.Pp 653Each line in these files contains the following fields: markers (optional), 654hostnames, bits, exponent, modulus, comment. 655The fields are separated by spaces. 656.Pp 657The marker is optional, but if it is present then it must be one of 658.Dq @cert-authority , 659to indicate that the line contains a certification authority (CA) key, 660or 661.Dq @revoked , 662to indicate that the key contained on the line is revoked and must not ever 663be accepted. 664Only one marker should be used on a key line. 665.Pp 666Hostnames is a comma-separated list of patterns 667.Pf ( Ql * 668and 669.Ql \&? 670act as 671wildcards); each pattern in turn is matched against the canonical host 672name (when authenticating a client) or against the user-supplied 673name (when authenticating a server). 674A pattern may also be preceded by 675.Ql \&! 676to indicate negation: if the host name matches a negated 677pattern, it is not accepted (by that line) even if it matched another 678pattern on the line. 679A hostname or address may optionally be enclosed within 680.Ql \&[ 681and 682.Ql \&] 683brackets then followed by 684.Ql \&: 685and a non-standard port number. 686.Pp 687Alternately, hostnames may be stored in a hashed form which hides host names 688and addresses should the file's contents be disclosed. 689Hashed hostnames start with a 690.Ql | 691character. 692Only one hashed hostname may appear on a single line and none of the above 693negation or wildcard operators may be applied. 694.Pp 695Bits, exponent, and modulus are taken directly from the RSA host key; they 696can be obtained, for example, from 697.Pa /etc/ssh/ssh_host_key.pub . 698The optional comment field continues to the end of the line, and is not used. 699.Pp 700Lines starting with 701.Ql # 702and empty lines are ignored as comments. 703.Pp 704When performing host authentication, authentication is accepted if any 705matching line has the proper key; either one that matches exactly or, 706if the server has presented a certificate for authentication, the key 707of the certification authority that signed the certificate. 708For a key to be trusted as a certification authority, it must use the 709.Dq @cert-authority 710marker described above. 711.Pp 712The known hosts file also provides a facility to mark keys as revoked, 713for example when it is known that the associated private key has been 714stolen. 715Revoked keys are specified by including the 716.Dq @revoked 717marker at the beginning of the key line, and are never accepted for 718authentication or as certification authorities, but instead will 719produce a warning from 720.Xr ssh 1 721when they are encountered. 722.Pp 723It is permissible (but not 724recommended) to have several lines or different host keys for the same 725names. 726This will inevitably happen when short forms of host names 727from different domains are put in the file. 728It is possible 729that the files contain conflicting information; authentication is 730accepted if valid information can be found from either file. 731.Pp 732Note that the lines in these files are typically hundreds of characters 733long, and you definitely don't want to type in the host keys by hand. 734Rather, generate them by a script, 735.Xr ssh-keyscan 1 736or by taking 737.Pa /etc/ssh/ssh_host_key.pub 738and adding the host names at the front. 739.Xr ssh-keygen 1 740also offers some basic automated editing for 741.Pa ~/.ssh/known_hosts 742including removing hosts matching a host name and converting all host 743names to their hashed representations. 744.Pp 745An example ssh_known_hosts file: 746.Bd -literal -offset 3n 747# Comments allowed at start of line 748closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 749cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 750# A hashed hostname 751|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 752AAAA1234.....= 753# A revoked key 754@revoked * ssh-rsa AAAAB5W... 755# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org 756@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... 757.Ed 758.Sh FILES 759.Bl -tag -width Ds -compact 760.It Pa ~/.hushlogin 761This file is used to suppress printing the last login time and 762.Pa /etc/motd , 763if 764.Cm PrintLastLog 765and 766.Cm PrintMotd , 767respectively, 768are enabled. 769It does not suppress printing of the banner specified by 770.Cm Banner . 771.Pp 772.It Pa ~/.rhosts 773This file is used for host-based authentication (see 774.Xr ssh 1 775for more information). 776On some machines this file may need to be 777world-readable if the user's home directory is on an NFS partition, 778because 779.Nm 780reads it as root. 781Additionally, this file must be owned by the user, 782and must not have write permissions for anyone else. 783The recommended 784permission for most machines is read/write for the user, and not 785accessible by others. 786.Pp 787.It Pa ~/.shosts 788This file is used in exactly the same way as 789.Pa .rhosts , 790but allows host-based authentication without permitting login with 791rlogin/rsh. 792.Pp 793.It Pa ~/.ssh/ 794This directory is the default location for all user-specific configuration 795and authentication information. 796There is no general requirement to keep the entire contents of this directory 797secret, but the recommended permissions are read/write/execute for the user, 798and not accessible by others. 799.Pp 800.It Pa ~/.ssh/authorized_keys 801Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in 802as this user. 803The format of this file is described above. 804The content of the file is not highly sensitive, but the recommended 805permissions are read/write for the user, and not accessible by others. 806.Pp 807If this file, the 808.Pa ~/.ssh 809directory, or the user's home directory are writable 810by other users, then the file could be modified or replaced by unauthorized 811users. 812In this case, 813.Nm 814will not allow it to be used unless the 815.Cm StrictModes 816option has been set to 817.Dq no . 818.Pp 819.It Pa ~/.ssh/environment 820This file is read into the environment at login (if it exists). 821It can only contain empty lines, comment lines (that start with 822.Ql # ) , 823and assignment lines of the form name=value. 824The file should be writable 825only by the user; it need not be readable by anyone else. 826Environment processing is disabled by default and is 827controlled via the 828.Cm PermitUserEnvironment 829option. 830.Pp 831.It Pa ~/.ssh/known_hosts 832Contains a list of host keys for all hosts the user has logged into 833that are not already in the systemwide list of known host keys. 834The format of this file is described above. 835This file should be writable only by root/the owner and 836can, but need not be, world-readable. 837.Pp 838.It Pa ~/.ssh/rc 839Contains initialization routines to be run before 840the user's home directory becomes accessible. 841This file should be writable only by the user, and need not be 842readable by anyone else. 843.Pp 844.It Pa /etc/hosts.allow 845.It Pa /etc/hosts.deny 846Access controls that should be enforced by tcp-wrappers are defined here. 847Further details are described in 848.Xr hosts_access 5 . 849.Pp 850.It Pa /etc/hosts.equiv 851This file is for host-based authentication (see 852.Xr ssh 1 ) . 853It should only be writable by root. 854.Pp 855.It Pa /etc/ssh/moduli 856Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". 857The file format is described in 858.Xr moduli 5 . 859.Pp 860.It Pa /etc/motd 861See 862.Xr motd 5 . 863.Pp 864.It Pa /etc/nologin 865If this file exists, 866.Nm 867refuses to let anyone except root log in. 868The contents of the file 869are displayed to anyone trying to log in, and non-root connections are 870refused. 871The file should be world-readable. 872.Pp 873.It Pa /etc/ssh/shosts.equiv 874This file is used in exactly the same way as 875.Pa hosts.equiv , 876but allows host-based authentication without permitting login with 877rlogin/rsh. 878.Pp 879.It Pa /etc/ssh/ssh_host_key 880.It Pa /etc/ssh/ssh_host_dsa_key 881.It Pa /etc/ssh/ssh_host_ecdsa_key 882.It Pa /etc/ssh/ssh_host_rsa_key 883These three files contain the private parts of the host keys. 884These files should only be owned by root, readable only by root, and not 885accessible to others. 886Note that 887.Nm 888does not start if these files are group/world-accessible. 889.Pp 890.It Pa /etc/ssh/ssh_host_key.pub 891.It Pa /etc/ssh/ssh_host_dsa_key.pub 892.It Pa /etc/ssh/ssh_host_ecdsa_key.pub 893.It Pa /etc/ssh/ssh_host_rsa_key.pub 894These three files contain the public parts of the host keys. 895These files should be world-readable but writable only by 896root. 897Their contents should match the respective private parts. 898These files are not 899really used for anything; they are provided for the convenience of 900the user so their contents can be copied to known hosts files. 901These files are created using 902.Xr ssh-keygen 1 . 903.Pp 904.It Pa /etc/ssh/ssh_known_hosts 905Systemwide list of known host keys. 906This file should be prepared by the 907system administrator to contain the public host keys of all machines in the 908organization. 909The format of this file is described above. 910This file should be writable only by root/the owner and 911should be world-readable. 912.Pp 913.It Pa /etc/ssh/sshd_config 914Contains configuration data for 915.Nm sshd . 916The file format and configuration options are described in 917.Xr sshd_config 5 . 918.Pp 919.It Pa /etc/ssh/sshrc 920Similar to 921.Pa ~/.ssh/rc , 922it can be used to specify 923machine-specific login-time initializations globally. 924This file should be writable only by root, and should be world-readable. 925.Pp 926.It Pa /var/empty 927.Xr chroot 2 928directory used by 929.Nm 930during privilege separation in the pre-authentication phase. 931The directory should not contain any files and must be owned by root 932and not group or world-writable. 933.Pp 934.It Pa /var/run/sshd.pid 935Contains the process ID of the 936.Nm 937listening for connections (if there are several daemons running 938concurrently for different ports, this contains the process ID of the one 939started last). 940The content of this file is not sensitive; it can be world-readable. 941.El 942.Sh SEE ALSO 943.Xr scp 1 , 944.Xr sftp 1 , 945.Xr ssh 1 , 946.Xr ssh-add 1 , 947.Xr ssh-agent 1 , 948.Xr ssh-keygen 1 , 949.Xr ssh-keyscan 1 , 950.Xr chroot 2 , 951.Xr hosts_access 5 , 952.Xr login.conf 5 , 953.Xr moduli 5 , 954.Xr sshd_config 5 , 955.Xr inetd 8 , 956.Xr sftp-server 8 957.Sh AUTHORS 958OpenSSH is a derivative of the original and free 959ssh 1.2.12 release by Tatu Ylonen. 960Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 961Theo de Raadt and Dug Song 962removed many bugs, re-added newer features and 963created OpenSSH. 964Markus Friedl contributed the support for SSH 965protocol versions 1.5 and 2.0. 966Niels Provos and Markus Friedl contributed support 967for privilege separation. 968.Sh CAVEATS 969System security is not improved unless 970.Nm rshd , 971.Nm rlogind , 972and 973.Nm rexecd 974are disabled (thus completely disabling 975.Xr rlogin 976and 977.Xr rsh 978into the machine). 979