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.Dd $Mdocdate: June 24 2017 $ 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 C Ar connection_spec 48.Op Fl c Ar host_certificate_file 49.Op Fl E Ar log_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 o Ar option 54.Op Fl p Ar port 55.Op Fl u Ar len 56.Ek 57.Sh DESCRIPTION 58.Nm 59(OpenSSH Daemon) is the daemon program for 60.Xr ssh 1 . 61Together these programs replace rlogin and rsh, 62and provide secure encrypted communications between two untrusted hosts 63over an insecure network. 64.Pp 65.Nm 66listens for connections from clients. 67It is normally started at boot from 68.Pa /etc/rc . 69It forks a new 70daemon for each incoming connection. 71The forked daemons handle 72key exchange, encryption, authentication, command execution, 73and data exchange. 74.Pp 75.Nm 76can be configured using command-line options or a configuration file 77(by default 78.Xr sshd_config 5 ) ; 79command-line options override values specified in the 80configuration file. 81.Nm 82rereads its configuration file when it receives a hangup signal, 83.Dv SIGHUP , 84by executing itself with the name and options it was started with, e.g.\& 85.Pa /usr/sbin/sshd . 86.Pp 87The options are as follows: 88.Bl -tag -width Ds 89.It Fl 4 90Forces 91.Nm 92to use IPv4 addresses only. 93.It Fl 6 94Forces 95.Nm 96to use IPv6 addresses only. 97.It Fl C Ar connection_spec 98Specify the connection parameters to use for the 99.Fl T 100extended test mode. 101If provided, any 102.Cm Match 103directives in the configuration file 104that would apply to the specified user, host, and address will be set before 105the configuration is written to standard output. 106The connection parameters are supplied as keyword=value pairs. 107The keywords are 108.Dq user , 109.Dq host , 110.Dq laddr , 111.Dq lport , 112and 113.Dq addr . 114All are required and may be supplied in any order, either with multiple 115.Fl C 116options or as a comma-separated list. 117.It Fl c Ar host_certificate_file 118Specifies a path to a certificate file to identify 119.Nm 120during key exchange. 121The certificate file must match a host key file specified using the 122.Fl h 123option or the 124.Cm HostKey 125configuration directive. 126.It Fl D 127When this option is specified, 128.Nm 129will not detach and does not become a daemon. 130This allows easy monitoring of 131.Nm sshd . 132.It Fl d 133Debug mode. 134The server sends verbose debug output to standard error, 135and does not put itself in the background. 136The server also will not fork and will only process one connection. 137This option is only intended for debugging for the server. 138Multiple 139.Fl d 140options increase the debugging level. 141Maximum is 3. 142.It Fl E Ar log_file 143Append debug logs to 144.Ar log_file 145instead of the system log. 146.It Fl e 147Write debug logs to standard error instead of the system log. 148.It Fl f Ar config_file 149Specifies the name of the configuration file. 150The default is 151.Pa /etc/ssh/sshd_config . 152.Nm 153refuses to start if there is no configuration file. 154.It Fl g Ar login_grace_time 155Gives the grace time for clients to authenticate themselves (default 156120 seconds). 157If the client fails to authenticate the user within 158this many seconds, the server disconnects and exits. 159A value of zero indicates no limit. 160.It Fl h Ar host_key_file 161Specifies a file from which a host key is read. 162This option must be given if 163.Nm 164is not run as root (as the normal 165host key files are normally not readable by anyone but root). 166The default is 167.Pa /etc/ssh/ssh_host_dsa_key , 168.Pa /etc/ssh/ssh_host_ecdsa_key , 169.Pa /etc/ssh/ssh_host_ed25519_key 170and 171.Pa /etc/ssh/ssh_host_rsa_key . 172It is possible to have multiple host key files for 173the different host key algorithms. 174.It Fl i 175Specifies that 176.Nm 177is being run from 178.Xr inetd 8 . 179.It Fl o Ar option 180Can be used to give options in the format used in the configuration file. 181This is useful for specifying options for which there is no separate 182command-line flag. 183For full details of the options, and their values, see 184.Xr sshd_config 5 . 185.It Fl p Ar port 186Specifies the port on which the server listens for connections 187(default 22). 188Multiple port options are permitted. 189Ports specified in the configuration file with the 190.Cm Port 191option are ignored when a command-line port is specified. 192Ports specified using the 193.Cm ListenAddress 194option override command-line ports. 195.It Fl q 196Quiet mode. 197Nothing is sent to the system log. 198Normally the beginning, 199authentication, and termination of each connection is logged. 200.It Fl T 201Extended test mode. 202Check the validity of the configuration file, output the effective configuration 203to stdout and then exit. 204Optionally, 205.Cm Match 206rules may be applied by specifying the connection parameters using one or more 207.Fl C 208options. 209.It Fl t 210Test mode. 211Only check the validity of the configuration file and sanity of the keys. 212This is useful for updating 213.Nm 214reliably as configuration options may change. 215.It Fl u Ar len 216This option is used to specify the size of the field 217in the 218.Li utmp 219structure that holds the remote host name. 220If the resolved host name is longer than 221.Ar len , 222the dotted decimal value will be used instead. 223This allows hosts with very long host names that 224overflow this field to still be uniquely identified. 225Specifying 226.Fl u0 227indicates that only dotted decimal addresses 228should be put into the 229.Pa utmp 230file. 231.Fl u0 232may also be used to prevent 233.Nm 234from making DNS requests unless the authentication 235mechanism or configuration requires it. 236Authentication mechanisms that may require DNS include 237.Cm HostbasedAuthentication 238and using a 239.Cm from="pattern-list" 240option in a key file. 241Configuration options that require DNS include using a 242USER@HOST pattern in 243.Cm AllowUsers 244or 245.Cm DenyUsers . 246.El 247.Sh AUTHENTICATION 248The OpenSSH SSH daemon supports SSH protocol 2 only. 249Each host has a host-specific key, 250used to identify the host. 251Whenever a client connects, the daemon responds with its public 252host key. 253The client compares the 254host key against its own database to verify that it has not changed. 255Forward security is provided through a Diffie-Hellman key agreement. 256This key agreement results in a shared session key. 257The rest of the session is encrypted using a symmetric cipher, currently 258128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. 259The client selects the encryption algorithm 260to use from those offered by the server. 261Additionally, session integrity is provided 262through a cryptographic message authentication code 263(hmac-md5, hmac-sha1, umac-64, umac-128, 264hmac-sha2-256 or hmac-sha2-512). 265.Pp 266Finally, the server and the client enter an authentication dialog. 267The client tries to authenticate itself using 268host-based authentication, 269public key authentication, 270challenge-response authentication, 271or password authentication. 272.Pp 273Regardless of the authentication type, the account is checked to 274ensure that it is accessible. An account is not accessible if it is 275locked, listed in 276.Cm DenyUsers 277or its group is listed in 278.Cm DenyGroups 279\&. The definition of a locked account is system dependant. Some platforms 280have their own account database (eg AIX) and some modify the passwd field ( 281.Ql \&*LK\&* 282on Solaris and UnixWare, 283.Ql \&* 284on HP-UX, containing 285.Ql Nologin 286on Tru64, 287a leading 288.Ql \&*LOCKED\&* 289on FreeBSD and a leading 290.Ql \&! 291on most Linuxes). 292If there is a requirement to disable password authentication 293for the account while allowing still public-key, then the passwd field 294should be set to something other than these values (eg 295.Ql NP 296or 297.Ql \&*NP\&* 298). 299.Pp 300If the client successfully authenticates itself, a dialog for 301preparing the session is entered. 302At this time the client may request 303things like allocating a pseudo-tty, forwarding X11 connections, 304forwarding TCP connections, or forwarding the authentication agent 305connection over the secure channel. 306.Pp 307After this, the client either requests a shell or execution of a command. 308The sides then enter session mode. 309In this mode, either side may send 310data at any time, and such data is forwarded to/from the shell or 311command on the server side, and the user terminal in the client side. 312.Pp 313When the user program terminates and all forwarded X11 and other 314connections have been closed, the server sends command exit status to 315the client, and both sides exit. 316.Sh LOGIN PROCESS 317When a user successfully logs in, 318.Nm 319does the following: 320.Bl -enum -offset indent 321.It 322If the login is on a tty, and no command has been specified, 323prints last login time and 324.Pa /etc/motd 325(unless prevented in the configuration file or by 326.Pa ~/.hushlogin ; 327see the 328.Sx FILES 329section). 330.It 331If the login is on a tty, records login time. 332.It 333Checks 334.Pa /etc/nologin ; 335if it exists, prints contents and quits 336(unless root). 337.It 338Changes to run with normal user privileges. 339.It 340Sets up basic environment. 341.It 342Reads the file 343.Pa ~/.ssh/environment , 344if it exists, and users are allowed to change their environment. 345See the 346.Cm PermitUserEnvironment 347option in 348.Xr sshd_config 5 . 349.It 350Changes to user's home directory. 351.It 352If 353.Pa ~/.ssh/rc 354exists and the 355.Xr sshd_config 5 356.Cm PermitUserRC 357option is set, runs it; else if 358.Pa /etc/ssh/sshrc 359exists, runs 360it; otherwise runs xauth. 361The 362.Dq rc 363files are given the X11 364authentication protocol and cookie in standard input. 365See 366.Sx SSHRC , 367below. 368.It 369Runs user's shell or command. 370All commands are run under the user's login shell as specified in the 371system password database. 372.El 373.Sh SSHRC 374If the file 375.Pa ~/.ssh/rc 376exists, 377.Xr sh 1 378runs it after reading the 379environment files but before starting the user's shell or command. 380It must not produce any output on stdout; stderr must be used 381instead. 382If X11 forwarding is in use, it will receive the "proto cookie" pair in 383its standard input (and 384.Ev DISPLAY 385in its environment). 386The script must call 387.Xr xauth 1 388because 389.Nm 390will not run xauth automatically to add X11 cookies. 391.Pp 392The primary purpose of this file is to run any initialization routines 393which may be needed before the user's home directory becomes 394accessible; AFS is a particular example of such an environment. 395.Pp 396This file will probably contain some initialization code followed by 397something similar to: 398.Bd -literal -offset 3n 399if read proto cookie && [ -n "$DISPLAY" ]; then 400 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 401 # X11UseLocalhost=yes 402 echo add unix:`echo $DISPLAY | 403 cut -c11-` $proto $cookie 404 else 405 # X11UseLocalhost=no 406 echo add $DISPLAY $proto $cookie 407 fi | xauth -q - 408fi 409.Ed 410.Pp 411If this file does not exist, 412.Pa /etc/ssh/sshrc 413is run, and if that 414does not exist either, xauth is used to add the cookie. 415.Sh AUTHORIZED_KEYS FILE FORMAT 416.Cm AuthorizedKeysFile 417specifies the files containing public keys for 418public key authentication; 419if this option is not specified, the default is 420.Pa ~/.ssh/authorized_keys 421and 422.Pa ~/.ssh/authorized_keys2 . 423Each line of the file contains one 424key (empty lines and lines starting with a 425.Ql # 426are ignored as 427comments). 428Public keys consist of the following space-separated fields: 429options, keytype, base64-encoded key, comment. 430The options field is optional. 431The keytype is 432.Dq ecdsa-sha2-nistp256 , 433.Dq ecdsa-sha2-nistp384 , 434.Dq ecdsa-sha2-nistp521 , 435.Dq ssh-ed25519 , 436.Dq ssh-dss 437or 438.Dq ssh-rsa ; 439the comment field is not used for anything (but may be convenient for the 440user to identify the key). 441.Pp 442Note that lines in this file can be several hundred bytes long 443(because of the size of the public key encoding) up to a limit of 4448 kilobytes, which permits DSA keys up to 8 kilobits and RSA 445keys up to 16 kilobits. 446You don't want to type them in; instead, copy the 447.Pa id_dsa.pub , 448.Pa id_ecdsa.pub , 449.Pa id_ed25519.pub , 450or the 451.Pa id_rsa.pub 452file and edit it. 453.Pp 454.Nm 455enforces a minimum RSA key modulus size of 768 bits. 456.Pp 457The options (if present) consist of comma-separated option 458specifications. 459No spaces are permitted, except within double quotes. 460The following option specifications are supported (note 461that option keywords are case-insensitive): 462.Bl -tag -width Ds 463.It Cm agent-forwarding 464Enable authentication agent forwarding previously disabled by the 465.Cm restrict 466option. 467.It Cm cert-authority 468Specifies that the listed key is a certification authority (CA) that is 469trusted to validate signed certificates for user authentication. 470.Pp 471Certificates may encode access restrictions similar to these key options. 472If both certificate restrictions and key options are present, the most 473restrictive union of the two is applied. 474.It Cm command="command" 475Specifies that the command is executed whenever this key is used for 476authentication. 477The command supplied by the user (if any) is ignored. 478The command is run on a pty if the client requests a pty; 479otherwise it is run without a tty. 480If an 8-bit clean channel is required, 481one must not request a pty or should specify 482.Cm no-pty . 483A quote may be included in the command by quoting it with a backslash. 484.Pp 485This option might be useful 486to restrict certain public keys to perform just a specific operation. 487An example might be a key that permits remote backups but nothing else. 488Note that the client may specify TCP and/or X11 489forwarding unless they are explicitly prohibited, e.g. using the 490.Cm restrict 491key option. 492.Pp 493The command originally supplied by the client is available in the 494.Ev SSH_ORIGINAL_COMMAND 495environment variable. 496Note that this option applies to shell, command or subsystem execution. 497Also note that this command may be superseded by a 498.Xr sshd_config 5 499.Cm ForceCommand 500directive. 501.Pp 502If a command is specified and a forced-command is embedded in a certificate 503used for authentication, then the certificate will be accepted only if the 504two commands are identical. 505.It Cm environment="NAME=value" 506Specifies that the string is to be added to the environment when 507logging in using this key. 508Environment variables set this way 509override other default environment values. 510Multiple options of this type are permitted. 511Environment processing is disabled by default and is 512controlled via the 513.Cm PermitUserEnvironment 514option. 515.It Cm from="pattern-list" 516Specifies that in addition to public key authentication, either the canonical 517name of the remote host or its IP address must be present in the 518comma-separated list of patterns. 519See PATTERNS in 520.Xr ssh_config 5 521for more information on patterns. 522.Pp 523In addition to the wildcard matching that may be applied to hostnames or 524addresses, a 525.Cm from 526stanza may match IP addresses using CIDR address/masklen notation. 527.Pp 528The purpose of this option is to optionally increase security: public key 529authentication by itself does not trust the network or name servers or 530anything (but the key); however, if somebody somehow steals the key, the key 531permits an intruder to log in from anywhere in the world. 532This additional option makes using a stolen key more difficult (name 533servers and/or routers would have to be compromised in addition to 534just the key). 535.It Cm no-agent-forwarding 536Forbids authentication agent forwarding when this key is used for 537authentication. 538.It Cm no-port-forwarding 539Forbids TCP forwarding when this key is used for authentication. 540Any port forward requests by the client will return an error. 541This might be used, e.g. in connection with the 542.Cm command 543option. 544.It Cm no-pty 545Prevents tty allocation (a request to allocate a pty will fail). 546.It Cm no-user-rc 547Disables execution of 548.Pa ~/.ssh/rc . 549.It Cm no-X11-forwarding 550Forbids X11 forwarding when this key is used for authentication. 551Any X11 forward requests by the client will return an error. 552.It Cm permitopen="host:port" 553Limit local port forwarding with 554.Xr ssh 1 555.Fl L 556such that it may only connect to the specified host and port. 557IPv6 addresses can be specified by enclosing the address in square brackets. 558Multiple 559.Cm permitopen 560options may be applied separated by commas. 561No pattern matching is performed on the specified hostnames, 562they must be literal domains or addresses. 563A port specification of 564.Cm * 565matches any port. 566.It Cm port-forwarding 567Enable port forwarding previously disabled by the 568.Cm restrict 569.It Cm principals="principals" 570On a 571.Cm cert-authority 572line, specifies allowed principals for certificate authentication as a 573comma-separated list. 574At least one name from the list must appear in the certificate's 575list of principals for the certificate to be accepted. 576This option is ignored for keys that are not marked as trusted certificate 577signers using the 578.Cm cert-authority 579option. 580.It Cm pty 581Permits tty allocation previously disabled by the 582.Cm restrict 583option. 584.It Cm restrict 585Enable all restrictions, i.e. disable port, agent and X11 forwarding, 586as well as disabling PTY allocation 587and execution of 588.Pa ~/.ssh/rc . 589If any future restriction capabilities are added to authorized_keys files 590they will be included in this set. 591.It Cm tunnel="n" 592Force a 593.Xr tun 4 594device on the server. 595Without this option, the next available device will be used if 596the client requests a tunnel. 597.It Cm user-rc 598Enables execution of 599.Pa ~/.ssh/rc 600previously disabled by the 601.Cm restrict 602option. 603.It Cm X11-forwarding 604Permits X11 forwarding previously disabled by the 605.Cm restrict 606option. 607.El 608.Pp 609An example authorized_keys file: 610.Bd -literal -offset 3n 611# Comments allowed at start of line 612ssh-rsa AAAAB3Nza...LiPk== user@example.net 613from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 614AAAAB2...19Q== john@example.net 615command="dump /home",no-pty,no-port-forwarding ssh-dss 616AAAAC3...51R== example.net 617permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 618AAAAB5...21S== 619tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 620jane@example.net 621restrict,command="uptime" ssh-rsa AAAA1C8...32Tv== 622user@example.net 623restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5== 624user@example.net 625.Ed 626.Sh SSH_KNOWN_HOSTS FILE FORMAT 627The 628.Pa /etc/ssh/ssh_known_hosts 629and 630.Pa ~/.ssh/known_hosts 631files contain host public keys for all known hosts. 632The global file should 633be prepared by the administrator (optional), and the per-user file is 634maintained automatically: whenever the user connects to an unknown host, 635its key is added to the per-user file. 636.Pp 637Each line in these files contains the following fields: markers (optional), 638hostnames, keytype, base64-encoded key, comment. 639The fields are separated by spaces. 640.Pp 641The marker is optional, but if it is present then it must be one of 642.Dq @cert-authority , 643to indicate that the line contains a certification authority (CA) key, 644or 645.Dq @revoked , 646to indicate that the key contained on the line is revoked and must not ever 647be accepted. 648Only one marker should be used on a key line. 649.Pp 650Hostnames is a comma-separated list of patterns 651.Pf ( Ql * 652and 653.Ql \&? 654act as 655wildcards); each pattern in turn is matched against the host name. 656When 657.Nm sshd 658is authenticating a client, such as when using 659.Cm HostbasedAuthentication , 660this will be the canonical client host name. 661When 662.Xr ssh 1 663is authenticating a server, this will be the host name 664given by the user, the value of the 665.Xr ssh 1 666.Cm HostkeyAlias 667if it was specified, or the canonical server hostname if the 668.Xr ssh 1 669.Cm CanonicalizeHostname 670option was used. 671.Pp 672A pattern may also be preceded by 673.Ql \&! 674to indicate negation: if the host name matches a negated 675pattern, it is not accepted (by that line) even if it matched another 676pattern on the line. 677A hostname or address may optionally be enclosed within 678.Ql \&[ 679and 680.Ql \&] 681brackets then followed by 682.Ql \&: 683and a non-standard port number. 684.Pp 685Alternately, hostnames may be stored in a hashed form which hides host names 686and addresses should the file's contents be disclosed. 687Hashed hostnames start with a 688.Ql | 689character. 690Only one hashed hostname may appear on a single line and none of the above 691negation or wildcard operators may be applied. 692.Pp 693The keytype and base64-encoded key are taken directly from the host key; they 694can be obtained, for example, from 695.Pa /etc/ssh/ssh_host_rsa_key.pub . 696The optional comment field continues to the end of the line, and is not used. 697.Pp 698Lines starting with 699.Ql # 700and empty lines are ignored as comments. 701.Pp 702When performing host authentication, authentication is accepted if any 703matching line has the proper key; either one that matches exactly or, 704if the server has presented a certificate for authentication, the key 705of the certification authority that signed the certificate. 706For a key to be trusted as a certification authority, it must use the 707.Dq @cert-authority 708marker described above. 709.Pp 710The known hosts file also provides a facility to mark keys as revoked, 711for example when it is known that the associated private key has been 712stolen. 713Revoked keys are specified by including the 714.Dq @revoked 715marker at the beginning of the key line, and are never accepted for 716authentication or as certification authorities, but instead will 717produce a warning from 718.Xr ssh 1 719when they are encountered. 720.Pp 721It is permissible (but not 722recommended) to have several lines or different host keys for the same 723names. 724This will inevitably happen when short forms of host names 725from different domains are put in the file. 726It is possible 727that the files contain conflicting information; authentication is 728accepted if valid information can be found from either file. 729.Pp 730Note that the lines in these files are typically hundreds of characters 731long, and you definitely don't want to type in the host keys by hand. 732Rather, generate them by a script, 733.Xr ssh-keyscan 1 734or by taking, for example, 735.Pa /etc/ssh/ssh_host_rsa_key.pub 736and adding the host names at the front. 737.Xr ssh-keygen 1 738also offers some basic automated editing for 739.Pa ~/.ssh/known_hosts 740including removing hosts matching a host name and converting all host 741names to their hashed representations. 742.Pp 743An example ssh_known_hosts file: 744.Bd -literal -offset 3n 745# Comments allowed at start of line 746closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 747cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 748# A hashed hostname 749|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 750AAAA1234.....= 751# A revoked key 752@revoked * ssh-rsa AAAAB5W... 753# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org 754@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... 755.Ed 756.Sh FILES 757.Bl -tag -width Ds -compact 758.It Pa ~/.hushlogin 759This file is used to suppress printing the last login time and 760.Pa /etc/motd , 761if 762.Cm PrintLastLog 763and 764.Cm PrintMotd , 765respectively, 766are enabled. 767It does not suppress printing of the banner specified by 768.Cm Banner . 769.Pp 770.It Pa ~/.rhosts 771This file is used for host-based authentication (see 772.Xr ssh 1 773for more information). 774On some machines this file may need to be 775world-readable if the user's home directory is on an NFS partition, 776because 777.Nm 778reads it as root. 779Additionally, this file must be owned by the user, 780and must not have write permissions for anyone else. 781The recommended 782permission for most machines is read/write for the user, and not 783accessible by others. 784.Pp 785.It Pa ~/.shosts 786This file is used in exactly the same way as 787.Pa .rhosts , 788but allows host-based authentication without permitting login with 789rlogin/rsh. 790.Pp 791.It Pa ~/.ssh/ 792This directory is the default location for all user-specific configuration 793and authentication information. 794There is no general requirement to keep the entire contents of this directory 795secret, but the recommended permissions are read/write/execute for the user, 796and not accessible by others. 797.Pp 798.It Pa ~/.ssh/authorized_keys 799Lists the public keys (DSA, ECDSA, Ed25519, RSA) 800that can be used for logging in as this user. 801The format of this file is described above. 802The content of the file is not highly sensitive, but the recommended 803permissions are read/write for the user, and not accessible by others. 804.Pp 805If this file, the 806.Pa ~/.ssh 807directory, or the user's home directory are writable 808by other users, then the file could be modified or replaced by unauthorized 809users. 810In this case, 811.Nm 812will not allow it to be used unless the 813.Cm StrictModes 814option has been set to 815.Dq no . 816.Pp 817.It Pa ~/.ssh/environment 818This file is read into the environment at login (if it exists). 819It can only contain empty lines, comment lines (that start with 820.Ql # ) , 821and assignment lines of the form name=value. 822The file should be writable 823only by the user; it need not be readable by anyone else. 824Environment processing is disabled by default and is 825controlled via the 826.Cm PermitUserEnvironment 827option. 828.Pp 829.It Pa ~/.ssh/known_hosts 830Contains a list of host keys for all hosts the user has logged into 831that are not already in the systemwide list of known host keys. 832The format of this file is described above. 833This file should be writable only by root/the owner and 834can, but need not be, world-readable. 835.Pp 836.It Pa ~/.ssh/rc 837Contains initialization routines to be run before 838the user's home directory becomes accessible. 839This file should be writable only by the user, and need not be 840readable by anyone else. 841.Pp 842.It Pa /etc/hosts.equiv 843This file is for host-based authentication (see 844.Xr ssh 1 ) . 845It should only be writable by root. 846.Pp 847.It Pa /etc/moduli 848Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange" 849key exchange method. 850The file format is described in 851.Xr moduli 5 . 852If no usable groups are found in this file then fixed internal groups will 853be used. 854.Pp 855.It Pa /etc/motd 856See 857.Xr motd 5 . 858.Pp 859.It Pa /etc/nologin 860If this file exists, 861.Nm 862refuses to let anyone except root log in. 863The contents of the file 864are displayed to anyone trying to log in, and non-root connections are 865refused. 866The file should be world-readable. 867.Pp 868.It Pa /etc/shosts.equiv 869This file is used in exactly the same way as 870.Pa hosts.equiv , 871but allows host-based authentication without permitting login with 872rlogin/rsh. 873.Pp 874.It Pa /etc/ssh/ssh_host_dsa_key 875.It Pa /etc/ssh/ssh_host_ecdsa_key 876.It Pa /etc/ssh/ssh_host_ed25519_key 877.It Pa /etc/ssh/ssh_host_rsa_key 878These files contain the private parts of the host keys. 879These files should only be owned by root, readable only by root, and not 880accessible to others. 881Note that 882.Nm 883does not start if these files are group/world-accessible. 884.Pp 885.It Pa /etc/ssh/ssh_host_dsa_key.pub 886.It Pa /etc/ssh/ssh_host_ecdsa_key.pub 887.It Pa /etc/ssh/ssh_host_ed25519_key.pub 888.It Pa /etc/ssh/ssh_host_rsa_key.pub 889These files contain the public parts of the host keys. 890These files should be world-readable but writable only by 891root. 892Their contents should match the respective private parts. 893These files are not 894really used for anything; they are provided for the convenience of 895the user so their contents can be copied to known hosts files. 896These files are created using 897.Xr ssh-keygen 1 . 898.Pp 899.It Pa /etc/ssh/ssh_known_hosts 900Systemwide list of known host keys. 901This file should be prepared by the 902system administrator to contain the public host keys of all machines in the 903organization. 904The format of this file is described above. 905This file should be writable only by root/the owner and 906should be world-readable. 907.Pp 908.It Pa /etc/ssh/sshd_config 909Contains configuration data for 910.Nm sshd . 911The file format and configuration options are described in 912.Xr sshd_config 5 . 913.Pp 914.It Pa /etc/ssh/sshrc 915Similar to 916.Pa ~/.ssh/rc , 917it can be used to specify 918machine-specific login-time initializations globally. 919This file should be writable only by root, and should be world-readable. 920.Pp 921.It Pa /var/empty 922.Xr chroot 2 923directory used by 924.Nm 925during privilege separation in the pre-authentication phase. 926The directory should not contain any files and must be owned by root 927and not group or world-writable. 928.Pp 929.It Pa /var/run/sshd.pid 930Contains the process ID of the 931.Nm 932listening for connections (if there are several daemons running 933concurrently for different ports, this contains the process ID of the one 934started last). 935The content of this file is not sensitive; it can be world-readable. 936.El 937.Sh SEE ALSO 938.Xr scp 1 , 939.Xr sftp 1 , 940.Xr ssh 1 , 941.Xr ssh-add 1 , 942.Xr ssh-agent 1 , 943.Xr ssh-keygen 1 , 944.Xr ssh-keyscan 1 , 945.Xr chroot 2 , 946.Xr login.conf 5 , 947.Xr moduli 5 , 948.Xr sshd_config 5 , 949.Xr inetd 8 , 950.Xr sftp-server 8 951.Sh AUTHORS 952OpenSSH is a derivative of the original and free 953ssh 1.2.12 release by Tatu Ylonen. 954Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 955Theo de Raadt and Dug Song 956removed many bugs, re-added newer features and 957created OpenSSH. 958Markus Friedl contributed the support for SSH 959protocol versions 1.5 and 2.0. 960Niels Provos and Markus Friedl contributed support 961for privilege separation. 962