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 1.291 2017/06/24 06:28:50 jmc Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: June 24 2017 $ 39.Dt SSHD 8 40.Os 41.Sh NAME 42.Nm sshd 43.Nd OpenSSH SSH daemon 44.Sh SYNOPSIS 45.Nm sshd 46.Bk -words 47.Op Fl 46DdeiqTt 48.Op Fl C Ar connection_spec 49.Op Fl c Ar host_certificate_file 50.Op Fl E Ar log_file 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 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, 63and provide 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 C Ar connection_spec 99Specify the connection parameters to use for the 100.Fl T 101extended test mode. 102If provided, any 103.Cm Match 104directives in the configuration file 105that would apply to the specified user, host, and address will be set before 106the configuration is written to standard output. 107The connection parameters are supplied as keyword=value pairs. 108The keywords are 109.Dq user , 110.Dq host , 111.Dq laddr , 112.Dq lport , 113and 114.Dq addr . 115All are required and may be supplied in any order, either with multiple 116.Fl C 117options or as a comma-separated list. 118.It Fl c Ar host_certificate_file 119Specifies a path to a certificate file to identify 120.Nm 121during key exchange. 122The certificate file must match a host key file specified using the 123.Fl h 124option or the 125.Cm HostKey 126configuration directive. 127.It Fl D 128When this option is specified, 129.Nm 130will not detach and does not become a daemon. 131This allows easy monitoring of 132.Nm sshd . 133.It Fl d 134Debug mode. 135The server sends verbose debug output to standard error, 136and does not put itself in the background. 137The server also will not fork and will only process one connection. 138This option is only intended for debugging for the server. 139Multiple 140.Fl d 141options increase the debugging level. 142Maximum is 3. 143.It Fl E Ar log_file 144Append debug logs to 145.Ar log_file 146instead of the system log. 147.It Fl e 148Write debug logs to standard error instead of the system log. 149.It Fl f Ar config_file 150Specifies the name of the configuration file. 151The default is 152.Pa /etc/ssh/sshd_config . 153.Nm 154refuses to start if there is no configuration file. 155.It Fl g Ar login_grace_time 156Gives the grace time for clients to authenticate themselves (default 157120 seconds). 158If the client fails to authenticate the user within 159this many seconds, the server disconnects and exits. 160A value of zero indicates no limit. 161.It Fl h Ar host_key_file 162Specifies a file from which a host key is read. 163This option must be given if 164.Nm 165is not run as root (as the normal 166host key files are normally not readable by anyone but root). 167The default is 168.Pa /etc/ssh/ssh_host_dsa_key , 169.Pa /etc/ssh/ssh_host_ecdsa_key , 170.Pa /etc/ssh/ssh_host_ed25519_key 171and 172.Pa /etc/ssh/ssh_host_rsa_key . 173It is possible to have multiple host key files for 174the different host key algorithms. 175.It Fl i 176Specifies that 177.Nm 178is being run from 179.Xr inetd 8 . 180.It Fl o Ar option 181Can be used to give options in the format used in the configuration file. 182This is useful for specifying options for which there is no separate 183command-line flag. 184For full details of the options, and their values, see 185.Xr sshd_config 5 . 186.It Fl p Ar port 187Specifies the port on which the server listens for connections 188(default 22). 189Multiple port options are permitted. 190Ports specified in the configuration file with the 191.Cm Port 192option are ignored when a command-line port is specified. 193Ports specified using the 194.Cm ListenAddress 195option override command-line ports. 196.It Fl q 197Quiet mode. 198Nothing is sent to the system log. 199Normally the beginning, 200authentication, and termination of each connection is logged. 201.It Fl T 202Extended test mode. 203Check the validity of the configuration file, output the effective configuration 204to stdout and then exit. 205Optionally, 206.Cm Match 207rules may be applied by specifying the connection parameters using one or more 208.Fl C 209options. 210.It Fl t 211Test mode. 212Only check the validity of the configuration file and sanity of the keys. 213This is useful for updating 214.Nm 215reliably as configuration options may change. 216.It Fl u Ar len 217This option is used to specify the size of the field 218in the 219.Li utmp 220structure that holds the remote host name. 221If the resolved host name is longer than 222.Ar len , 223the dotted decimal value will be used instead. 224This allows hosts with very long host names that 225overflow this field to still be uniquely identified. 226Specifying 227.Fl u0 228indicates that only dotted decimal addresses 229should be put into the 230.Pa utmp 231file. 232.Fl u0 233may also be used to prevent 234.Nm 235from making DNS requests unless the authentication 236mechanism or configuration requires it. 237Authentication mechanisms that may require DNS include 238.Cm HostbasedAuthentication 239and using a 240.Cm from="pattern-list" 241option in a key file. 242Configuration options that require DNS include using a 243USER@HOST pattern in 244.Cm AllowUsers 245or 246.Cm DenyUsers . 247.El 248.Sh AUTHENTICATION 249The OpenSSH SSH daemon supports SSH protocol 2 only. 250Each host has a host-specific key, 251used to identify the host. 252Whenever a client connects, the daemon responds with its public 253host key. 254The client compares the 255host key against its own database to verify that it has not changed. 256Forward security is provided through a Diffie-Hellman key agreement. 257This key agreement results in a shared session key. 258The rest of the session is encrypted using a symmetric cipher, currently 259128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. 260The client selects the encryption algorithm 261to use from those offered by the server. 262Additionally, session integrity is provided 263through a cryptographic message authentication code 264(hmac-md5, hmac-sha1, umac-64, umac-128, 265hmac-sha2-256 or hmac-sha2-512). 266.Pp 267Finally, the server and the client enter an authentication dialog. 268The client tries to authenticate itself using 269host-based authentication, 270public key authentication, 271challenge-response authentication, 272or password authentication. 273.Pp 274Regardless of the authentication type, the account is checked to 275ensure that it is accessible. An account is not accessible if it is 276locked, listed in 277.Cm DenyUsers 278or its group is listed in 279.Cm DenyGroups 280\&. The definition of a locked account is system dependant. Some platforms 281have their own account database (eg AIX) and some modify the passwd field ( 282.Ql \&*LK\&* 283on Solaris and UnixWare, 284.Ql \&* 285on HP-UX, containing 286.Ql Nologin 287on Tru64, 288a leading 289.Ql \&*LOCKED\&* 290on FreeBSD and a leading 291.Ql \&! 292on most Linuxes). 293If there is a requirement to disable password authentication 294for the account while allowing still public-key, then the passwd field 295should be set to something other than these values (eg 296.Ql NP 297or 298.Ql \&*NP\&* 299). 300.Pp 301If the client successfully authenticates itself, a dialog for 302preparing the session is entered. 303At this time the client may request 304things like allocating a pseudo-tty, forwarding X11 connections, 305forwarding TCP connections, or forwarding the authentication agent 306connection over the secure channel. 307.Pp 308After this, the client either requests a shell or execution of a command. 309The sides then enter session mode. 310In this mode, either side may send 311data at any time, and such data is forwarded to/from the shell or 312command on the server side, and the user terminal in the client side. 313.Pp 314When the user program terminates and all forwarded X11 and other 315connections have been closed, the server sends command exit status to 316the client, and both sides exit. 317.Sh LOGIN PROCESS 318When a user successfully logs in, 319.Nm 320does the following: 321.Bl -enum -offset indent 322.It 323If the login is on a tty, and no command has been specified, 324prints last login time and 325.Pa /etc/motd 326(unless prevented in the configuration file or by 327.Pa ~/.hushlogin ; 328see the 329.Sx FILES 330section). 331.It 332If the login is on a tty, records login time. 333.It 334Checks 335.Pa /etc/nologin and 336.Pa /var/run/nologin ; 337if one exists, it prints the contents and quits 338(unless root). 339.It 340Changes to run with normal user privileges. 341.It 342Sets up basic environment. 343.It 344Reads the file 345.Pa ~/.ssh/environment , 346if it exists, and users are allowed to change their environment. 347See the 348.Cm PermitUserEnvironment 349option in 350.Xr sshd_config 5 . 351.It 352Changes to user's home directory. 353.It 354If 355.Pa ~/.ssh/rc 356exists and the 357.Xr sshd_config 5 358.Cm PermitUserRC 359option is set, runs it; else if 360.Pa /etc/ssh/sshrc 361exists, runs 362it; otherwise runs 363.Xr xauth 1 . 364The 365.Dq rc 366files are given the X11 367authentication protocol and cookie in standard input. 368See 369.Sx SSHRC , 370below. 371.It 372Runs user's shell or command. 373All commands are run under the user's login shell as specified in the 374system password database. 375.El 376.Sh SSHRC 377If the file 378.Pa ~/.ssh/rc 379exists, 380.Xr sh 1 381runs it after reading the 382environment files but before starting the user's shell or command. 383It must not produce any output on stdout; stderr must be used 384instead. 385If X11 forwarding is in use, it will receive the "proto cookie" pair in 386its standard input (and 387.Ev DISPLAY 388in its environment). 389The script must call 390.Xr xauth 1 391because 392.Nm 393will not run xauth automatically to add X11 cookies. 394.Pp 395The primary purpose of this file is to run any initialization routines 396which may be needed before the user's home directory becomes 397accessible; AFS is a particular example of such an environment. 398.Pp 399This file will probably contain some initialization code followed by 400something similar to: 401.Bd -literal -offset 3n 402if read proto cookie && [ -n "$DISPLAY" ]; then 403 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 404 # X11UseLocalhost=yes 405 echo add unix:`echo $DISPLAY | 406 cut -c11-` $proto $cookie 407 else 408 # X11UseLocalhost=no 409 echo add $DISPLAY $proto $cookie 410 fi | xauth -q - 411fi 412.Ed 413.Pp 414If this file does not exist, 415.Pa /etc/ssh/sshrc 416is run, and if that 417does not exist either, xauth is used to add the cookie. 418.Sh AUTHORIZED_KEYS FILE FORMAT 419.Cm AuthorizedKeysFile 420specifies the files containing public keys for 421public key authentication; 422if this option is not specified, the default is 423.Pa ~/.ssh/authorized_keys 424and 425.Pa ~/.ssh/authorized_keys2 . 426Each line of the file contains one 427key (empty lines and lines starting with a 428.Ql # 429are ignored as 430comments). 431Public keys consist of the following space-separated fields: 432options, keytype, base64-encoded key, comment. 433The options field is optional. 434The keytype is 435.Dq ecdsa-sha2-nistp256 , 436.Dq ecdsa-sha2-nistp384 , 437.Dq ecdsa-sha2-nistp521 , 438.Dq ssh-ed25519 , 439.Dq ssh-dss 440or 441.Dq ssh-rsa ; 442the comment field is not used for anything (but may be convenient for the 443user to identify the key). 444.Pp 445Note that lines in this file can be several hundred bytes long 446(because of the size of the public key encoding) up to a limit of 4478 kilobytes, which permits DSA keys up to 8 kilobits and RSA 448keys up to 16 kilobits. 449You don't want to type them in; instead, copy the 450.Pa id_dsa.pub , 451.Pa id_ecdsa.pub , 452.Pa id_ed25519.pub , 453or the 454.Pa id_rsa.pub 455file and edit it. 456.Pp 457.Nm 458enforces a minimum RSA key modulus size of 768 bits. 459.Pp 460The options (if present) consist of comma-separated option 461specifications. 462No spaces are permitted, except within double quotes. 463The following option specifications are supported (note 464that option keywords are case-insensitive): 465.Bl -tag -width Ds 466.It Cm agent-forwarding 467Enable authentication agent forwarding previously disabled by the 468.Cm restrict 469option. 470.It Cm cert-authority 471Specifies that the listed key is a certification authority (CA) that is 472trusted to validate signed certificates for user authentication. 473.Pp 474Certificates may encode access restrictions similar to these key options. 475If both certificate restrictions and key options are present, the most 476restrictive union of the two is applied. 477.It Cm command="command" 478Specifies that the command is executed whenever this key is used for 479authentication. 480The command supplied by the user (if any) is ignored. 481The command is run on a pty if the client requests a pty; 482otherwise it is run without a tty. 483If an 8-bit clean channel is required, 484one must not request a pty or should specify 485.Cm no-pty . 486A quote may be included in the command by quoting it with a backslash. 487.Pp 488This option might be useful 489to restrict certain public keys to perform just a specific operation. 490An example might be a key that permits remote backups but nothing else. 491Note that the client may specify TCP and/or X11 492forwarding unless they are explicitly prohibited, e.g. using the 493.Cm restrict 494key option. 495.Pp 496The command originally supplied by the client is available in the 497.Ev SSH_ORIGINAL_COMMAND 498environment variable. 499Note that this option applies to shell, command or subsystem execution. 500Also note that this command may be superseded by a 501.Xr sshd_config 5 502.Cm ForceCommand 503directive. 504.Pp 505If a command is specified and a forced-command is embedded in a certificate 506used for authentication, then the certificate will be accepted only if the 507two commands are identical. 508.It Cm environment="NAME=value" 509Specifies that the string is to be added to the environment when 510logging in using this key. 511Environment variables set this way 512override other default environment values. 513Multiple options of this type are permitted. 514Environment processing is disabled by default and is 515controlled via the 516.Cm PermitUserEnvironment 517option. 518.It Cm from="pattern-list" 519Specifies that in addition to public key authentication, either the canonical 520name of the remote host or its IP address must be present in the 521comma-separated list of patterns. 522See PATTERNS in 523.Xr ssh_config 5 524for more information on patterns. 525.Pp 526In addition to the wildcard matching that may be applied to hostnames or 527addresses, a 528.Cm from 529stanza may match IP addresses using CIDR address/masklen notation. 530.Pp 531The purpose of this option is to optionally increase security: public key 532authentication by itself does not trust the network or name servers or 533anything (but the key); however, if somebody somehow steals the key, the key 534permits an intruder to log in from anywhere in the world. 535This additional option makes using a stolen key more difficult (name 536servers and/or routers would have to be compromised in addition to 537just the key). 538.It Cm no-agent-forwarding 539Forbids authentication agent forwarding when this key is used for 540authentication. 541.It Cm no-port-forwarding 542Forbids TCP forwarding when this key is used for authentication. 543Any port forward requests by the client will return an error. 544This might be used, e.g. in connection with the 545.Cm command 546option. 547.It Cm no-pty 548Prevents tty allocation (a request to allocate a pty will fail). 549.It Cm no-user-rc 550Disables execution of 551.Pa ~/.ssh/rc . 552.It Cm no-X11-forwarding 553Forbids X11 forwarding when this key is used for authentication. 554Any X11 forward requests by the client will return an error. 555.It Cm permitopen="host:port" 556Limit local port forwarding with 557.Xr ssh 1 558.Fl L 559such that it may only connect to the specified host and port. 560IPv6 addresses can be specified by enclosing the address in square brackets. 561Multiple 562.Cm permitopen 563options may be applied separated by commas. 564No pattern matching is performed on the specified hostnames, 565they must be literal domains or addresses. 566A port specification of 567.Cm * 568matches any port. 569.It Cm port-forwarding 570Enable port forwarding previously disabled by the 571.Cm restrict 572.It Cm principals="principals" 573On a 574.Cm cert-authority 575line, specifies allowed principals for certificate authentication as a 576comma-separated list. 577At least one name from the list must appear in the certificate's 578list of principals for the certificate to be accepted. 579This option is ignored for keys that are not marked as trusted certificate 580signers using the 581.Cm cert-authority 582option. 583.It Cm pty 584Permits tty allocation previously disabled by the 585.Cm restrict 586option. 587.It Cm restrict 588Enable all restrictions, i.e. disable port, agent and X11 forwarding, 589as well as disabling PTY allocation 590and execution of 591.Pa ~/.ssh/rc . 592If any future restriction capabilities are added to authorized_keys files 593they will be included in this set. 594.It Cm tunnel="n" 595Force a 596.Xr tun 4 597device on the server. 598Without this option, the next available device will be used if 599the client requests a tunnel. 600.It Cm user-rc 601Enables execution of 602.Pa ~/.ssh/rc 603previously disabled by the 604.Cm restrict 605option. 606.It Cm X11-forwarding 607Permits X11 forwarding previously disabled by the 608.Cm restrict 609option. 610.El 611.Pp 612An example authorized_keys file: 613.Bd -literal -offset 3n 614# Comments allowed at start of line 615ssh-rsa AAAAB3Nza...LiPk== user@example.net 616from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 617AAAAB2...19Q== john@example.net 618command="dump /home",no-pty,no-port-forwarding ssh-dss 619AAAAC3...51R== example.net 620permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 621AAAAB5...21S== 622tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 623jane@example.net 624restrict,command="uptime" ssh-rsa AAAA1C8...32Tv== 625user@example.net 626restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5== 627user@example.net 628.Ed 629.Sh SSH_KNOWN_HOSTS FILE FORMAT 630The 631.Pa /etc/ssh/ssh_known_hosts 632and 633.Pa ~/.ssh/known_hosts 634files contain host public keys for all known hosts. 635The global file should 636be prepared by the administrator (optional), and the per-user file is 637maintained automatically: whenever the user connects to an unknown host, 638its key is added to the per-user file. 639.Pp 640Each line in these files contains the following fields: markers (optional), 641hostnames, keytype, base64-encoded key, comment. 642The fields are separated by spaces. 643.Pp 644The marker is optional, but if it is present then it must be one of 645.Dq @cert-authority , 646to indicate that the line contains a certification authority (CA) key, 647or 648.Dq @revoked , 649to indicate that the key contained on the line is revoked and must not ever 650be accepted. 651Only one marker should be used on a key line. 652.Pp 653Hostnames is a comma-separated list of patterns 654.Pf ( Ql * 655and 656.Ql \&? 657act as 658wildcards); each pattern in turn is matched against the host name. 659When 660.Nm sshd 661is authenticating a client, such as when using 662.Cm HostbasedAuthentication , 663this will be the canonical client host name. 664When 665.Xr ssh 1 666is authenticating a server, this will be the host name 667given by the user, the value of the 668.Xr ssh 1 669.Cm HostkeyAlias 670if it was specified, or the canonical server hostname if the 671.Xr ssh 1 672.Cm CanonicalizeHostname 673option was used. 674.Pp 675A pattern may also be preceded by 676.Ql \&! 677to indicate negation: if the host name matches a negated 678pattern, it is not accepted (by that line) even if it matched another 679pattern on the line. 680A hostname or address may optionally be enclosed within 681.Ql \&[ 682and 683.Ql \&] 684brackets then followed by 685.Ql \&: 686and a non-standard port number. 687.Pp 688Alternately, hostnames may be stored in a hashed form which hides host names 689and addresses should the file's contents be disclosed. 690Hashed hostnames start with a 691.Ql | 692character. 693Only one hashed hostname may appear on a single line and none of the above 694negation or wildcard operators may be applied. 695.Pp 696The keytype and base64-encoded key are taken directly from the host key; they 697can be obtained, for example, from 698.Pa /etc/ssh/ssh_host_rsa_key.pub . 699The optional comment field continues to the end of the line, and is not used. 700.Pp 701Lines starting with 702.Ql # 703and empty lines are ignored as comments. 704.Pp 705When performing host authentication, authentication is accepted if any 706matching line has the proper key; either one that matches exactly or, 707if the server has presented a certificate for authentication, the key 708of the certification authority that signed the certificate. 709For a key to be trusted as a certification authority, it must use the 710.Dq @cert-authority 711marker described above. 712.Pp 713The known hosts file also provides a facility to mark keys as revoked, 714for example when it is known that the associated private key has been 715stolen. 716Revoked keys are specified by including the 717.Dq @revoked 718marker at the beginning of the key line, and are never accepted for 719authentication or as certification authorities, but instead will 720produce a warning from 721.Xr ssh 1 722when they are encountered. 723.Pp 724It is permissible (but not 725recommended) to have several lines or different host keys for the same 726names. 727This will inevitably happen when short forms of host names 728from different domains are put in the file. 729It is possible 730that the files contain conflicting information; authentication is 731accepted if valid information can be found from either file. 732.Pp 733Note that the lines in these files are typically hundreds of characters 734long, and you definitely don't want to type in the host keys by hand. 735Rather, generate them by a script, 736.Xr ssh-keyscan 1 737or by taking, for example, 738.Pa /etc/ssh/ssh_host_rsa_key.pub 739and adding the host names at the front. 740.Xr ssh-keygen 1 741also offers some basic automated editing for 742.Pa ~/.ssh/known_hosts 743including removing hosts matching a host name and converting all host 744names to their hashed representations. 745.Pp 746An example ssh_known_hosts file: 747.Bd -literal -offset 3n 748# Comments allowed at start of line 749closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 750cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 751# A hashed hostname 752|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 753AAAA1234.....= 754# A revoked key 755@revoked * ssh-rsa AAAAB5W... 756# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org 757@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... 758.Ed 759.Sh FILES 760.Bl -tag -width Ds -compact 761.It Pa ~/.hushlogin 762This file is used to suppress printing the last login time and 763.Pa /etc/motd , 764if 765.Cm PrintLastLog 766and 767.Cm PrintMotd , 768respectively, 769are enabled. 770It does not suppress printing of the banner specified by 771.Cm Banner . 772.Pp 773.It Pa ~/.rhosts 774This file is used for host-based authentication (see 775.Xr ssh 1 776for more information). 777On some machines this file may need to be 778world-readable if the user's home directory is on an NFS partition, 779because 780.Nm 781reads it as root. 782Additionally, this file must be owned by the user, 783and must not have write permissions for anyone else. 784The recommended 785permission for most machines is read/write for the user, and not 786accessible by others. 787.Pp 788.It Pa ~/.shosts 789This file is used in exactly the same way as 790.Pa .rhosts , 791but allows host-based authentication without permitting login with 792rlogin/rsh. 793.Pp 794.It Pa ~/.ssh/ 795This directory is the default location for all user-specific configuration 796and authentication information. 797There is no general requirement to keep the entire contents of this directory 798secret, but the recommended permissions are read/write/execute for the user, 799and not accessible by others. 800.Pp 801.It Pa ~/.ssh/authorized_keys 802Lists the public keys (DSA, ECDSA, Ed25519, RSA) 803that can be used for logging in as this user. 804The format of this file is described above. 805The content of the file is not highly sensitive, but the recommended 806permissions are read/write for the user, and not accessible by others. 807.Pp 808If this file, the 809.Pa ~/.ssh 810directory, or the user's home directory are writable 811by other users, then the file could be modified or replaced by unauthorized 812users. 813In this case, 814.Nm 815will not allow it to be used unless the 816.Cm StrictModes 817option has been set to 818.Dq no . 819.Pp 820.It Pa ~/.ssh/environment 821This file is read into the environment at login (if it exists). 822It can only contain empty lines, comment lines (that start with 823.Ql # ) , 824and assignment lines of the form name=value. 825The file should be writable 826only by the user; it need not be readable by anyone else. 827Environment processing is disabled by default and is 828controlled via the 829.Cm PermitUserEnvironment 830option. 831.Pp 832.It Pa ~/.ssh/known_hosts 833Contains a list of host keys for all hosts the user has logged into 834that are not already in the systemwide list of known host keys. 835The format of this file is described above. 836This file should be writable only by root/the owner and 837can, but need not be, world-readable. 838.Pp 839.It Pa ~/.ssh/rc 840Contains initialization routines to be run before 841the user's home directory becomes accessible. 842This file should be writable only by the user, and need not be 843readable by anyone else. 844.Pp 845.It Pa /etc/hosts.allow 846.It Pa /etc/hosts.deny 847Access controls that should be enforced by tcp-wrappers are defined here. 848Further details are described in 849.Xr hosts_access 5 . 850.Pp 851.It Pa /etc/hosts.equiv 852This file is for host-based authentication (see 853.Xr ssh 1 ) . 854It should only be writable by root. 855.Pp 856.It Pa /etc/moduli 857Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange" 858key exchange method. 859The file format is described in 860.Xr moduli 5 . 861If no usable groups are found in this file then fixed internal groups will 862be used. 863.Pp 864.It Pa /etc/motd 865See 866.Xr motd 5 . 867.Pp 868.It Pa /etc/nologin 869If this file exists, 870.Nm 871refuses to let anyone except root log in. 872The contents of the file 873are displayed to anyone trying to log in, and non-root connections are 874refused. 875The file should be world-readable. 876.Pp 877.It Pa /etc/shosts.equiv 878This file is used in exactly the same way as 879.Pa hosts.equiv , 880but allows host-based authentication without permitting login with 881rlogin/rsh. 882.Pp 883.It Pa /etc/ssh/ssh_host_dsa_key 884.It Pa /etc/ssh/ssh_host_ecdsa_key 885.It Pa /etc/ssh/ssh_host_ed25519_key 886.It Pa /etc/ssh/ssh_host_rsa_key 887These files contain the private parts of the host keys. 888These files should only be owned by root, readable only by root, and not 889accessible to others. 890Note that 891.Nm 892does not start if these files are group/world-accessible. 893.Pp 894.It Pa /etc/ssh/ssh_host_dsa_key.pub 895.It Pa /etc/ssh/ssh_host_ecdsa_key.pub 896.It Pa /etc/ssh/ssh_host_ed25519_key.pub 897.It Pa /etc/ssh/ssh_host_rsa_key.pub 898These files contain the public parts of the host keys. 899These files should be world-readable but writable only by 900root. 901Their contents should match the respective private parts. 902These files are not 903really used for anything; they are provided for the convenience of 904the user so their contents can be copied to known hosts files. 905These files are created using 906.Xr ssh-keygen 1 . 907.Pp 908.It Pa /etc/ssh/ssh_known_hosts 909Systemwide list of known host keys. 910This file should be prepared by the 911system administrator to contain the public host keys of all machines in the 912organization. 913The format of this file is described above. 914This file should be writable only by root/the owner and 915should be world-readable. 916.Pp 917.It Pa /etc/ssh/sshd_config 918Contains configuration data for 919.Nm sshd . 920The file format and configuration options are described in 921.Xr sshd_config 5 . 922.Pp 923.It Pa /etc/ssh/sshrc 924Similar to 925.Pa ~/.ssh/rc , 926it can be used to specify 927machine-specific login-time initializations globally. 928This file should be writable only by root, and should be world-readable. 929.Pp 930.It Pa /var/empty 931.Xr chroot 2 932directory used by 933.Nm 934during privilege separation in the pre-authentication phase. 935The directory should not contain any files and must be owned by root 936and not group or world-writable. 937.Pp 938.It Pa /var/run/sshd.pid 939Contains the process ID of the 940.Nm 941listening for connections (if there are several daemons running 942concurrently for different ports, this contains the process ID of the one 943started last). 944The content of this file is not sensitive; it can be world-readable. 945.El 946.Sh SEE ALSO 947.Xr scp 1 , 948.Xr sftp 1 , 949.Xr ssh 1 , 950.Xr ssh-add 1 , 951.Xr ssh-agent 1 , 952.Xr ssh-keygen 1 , 953.Xr ssh-keyscan 1 , 954.Xr chroot 2 , 955.Xr hosts_access 5 , 956.Xr login.conf 5 , 957.Xr moduli 5 , 958.Xr sshd_config 5 , 959.Xr inetd 8 , 960.Xr sftp-server 8 961.Sh AUTHORS 962OpenSSH is a derivative of the original and free 963ssh 1.2.12 release by Tatu Ylonen. 964Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 965Theo de Raadt and Dug Song 966removed many bugs, re-added newer features and 967created OpenSSH. 968Markus Friedl contributed the support for SSH 969protocol versions 1.5 and 2.0. 970Niels Provos and Markus Friedl contributed support 971for privilege separation. 972