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 1.278 2014/11/15 14:41:03 bentley Exp $ 37.Dd $Mdocdate: November 15 2014 $ 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 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 k Ar key_gen_time 55.Op Fl o Ar option 56.Op Fl p Ar port 57.Op Fl u Ar len 58.Ek 59.Sh DESCRIPTION 60.Nm 61(OpenSSH Daemon) is the daemon program for 62.Xr ssh 1 . 63Together these programs replace rlogin and rsh, 64and provide secure encrypted communications between two untrusted hosts 65over an insecure network. 66.Pp 67.Nm 68listens for connections from clients. 69It is normally started at boot from 70.Pa /etc/rc . 71It forks a new 72daemon for each incoming connection. 73The forked daemons handle 74key exchange, encryption, authentication, command execution, 75and data exchange. 76.Pp 77.Nm 78can be configured using command-line options or a configuration file 79(by default 80.Xr sshd_config 5 ) ; 81command-line options override values specified in the 82configuration file. 83.Nm 84rereads its configuration file when it receives a hangup signal, 85.Dv SIGHUP , 86by executing itself with the name and options it was started with, e.g.\& 87.Pa /usr/sbin/sshd . 88.Pp 89The options are as follows: 90.Bl -tag -width Ds 91.It Fl 4 92Forces 93.Nm 94to use IPv4 addresses only. 95.It Fl 6 96Forces 97.Nm 98to use IPv6 addresses only. 99.It Fl b Ar bits 100Specifies the number of bits in the ephemeral protocol version 1 101server key (default 1024). 102.It Fl C Ar connection_spec 103Specify the connection parameters to use for the 104.Fl T 105extended test mode. 106If provided, any 107.Cm Match 108directives in the configuration file 109that would apply to the specified user, host, and address will be set before 110the configuration is written to standard output. 111The connection parameters are supplied as keyword=value pairs. 112The keywords are 113.Dq user , 114.Dq host , 115.Dq laddr , 116.Dq lport , 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 Ar log_file 148Append debug logs to 149.Ar log_file 150instead of the system log. 151.It Fl e 152Write debug logs to standard error instead of the system log. 153.It Fl f Ar config_file 154Specifies the name of the configuration file. 155The default is 156.Pa /etc/ssh/sshd_config . 157.Nm 158refuses to start if there is no configuration file. 159.It Fl g Ar login_grace_time 160Gives the grace time for clients to authenticate themselves (default 161120 seconds). 162If the client fails to authenticate the user within 163this many seconds, the server disconnects and exits. 164A value of zero indicates no limit. 165.It Fl h Ar host_key_file 166Specifies a file from which a host key is read. 167This option must be given if 168.Nm 169is not run as root (as the normal 170host key files are normally not readable by anyone but root). 171The default is 172.Pa /etc/ssh/ssh_host_key 173for protocol version 1, and 174.Pa /etc/ssh/ssh_host_dsa_key , 175.Pa /etc/ssh/ssh_host_ecdsa_key . 176.Pa /etc/ssh/ssh_host_ed25519_key 177and 178.Pa /etc/ssh/ssh_host_rsa_key 179for protocol version 2. 180It is possible to have multiple host key files for 181the different protocol versions and host key algorithms. 182.It Fl i 183Specifies that 184.Nm 185is being run from 186.Xr inetd 8 . 187.Nm 188is normally not run 189from inetd because it needs to generate the server key before it can 190respond to the client, and this may take tens of seconds. 191Clients would have to wait too long if the key was regenerated every time. 192However, with small key sizes (e.g. 512) using 193.Nm 194from inetd may 195be feasible. 196.It Fl k Ar key_gen_time 197Specifies how often the ephemeral protocol version 1 server key is 198regenerated (default 3600 seconds, or one hour). 199The motivation for regenerating the key fairly 200often is that the key is not stored anywhere, and after about an hour 201it becomes impossible to recover the key for decrypting intercepted 202communications even if the machine is cracked into or physically 203seized. 204A value of zero indicates that the key will never be regenerated. 205.It Fl o Ar option 206Can be used to give options in the format used in the configuration file. 207This is useful for specifying options for which there is no separate 208command-line flag. 209For full details of the options, and their values, see 210.Xr sshd_config 5 . 211.It Fl p Ar port 212Specifies the port on which the server listens for connections 213(default 22). 214Multiple port options are permitted. 215Ports specified in the configuration file with the 216.Cm Port 217option are ignored when a command-line port is specified. 218Ports specified using the 219.Cm ListenAddress 220option override command-line ports. 221.It Fl q 222Quiet mode. 223Nothing is sent to the system log. 224Normally the beginning, 225authentication, and termination of each connection is logged. 226.It Fl T 227Extended test mode. 228Check the validity of the configuration file, output the effective configuration 229to stdout and then exit. 230Optionally, 231.Cm Match 232rules may be applied by specifying the connection parameters using one or more 233.Fl C 234options. 235.It Fl t 236Test mode. 237Only check the validity of the configuration file and sanity of the keys. 238This is useful for updating 239.Nm 240reliably as configuration options may change. 241.It Fl u Ar len 242This option is used to specify the size of the field 243in the 244.Li utmp 245structure that holds the remote host name. 246If the resolved host name is longer than 247.Ar len , 248the dotted decimal value will be used instead. 249This allows hosts with very long host names that 250overflow this field to still be uniquely identified. 251Specifying 252.Fl u0 253indicates that only dotted decimal addresses 254should be put into the 255.Pa utmp 256file. 257.Fl u0 258may also be used to prevent 259.Nm 260from making DNS requests unless the authentication 261mechanism or configuration requires it. 262Authentication mechanisms that may require DNS include 263.Cm RhostsRSAAuthentication , 264.Cm HostbasedAuthentication , 265and using a 266.Cm from="pattern-list" 267option in a key file. 268Configuration options that require DNS include using a 269USER@HOST pattern in 270.Cm AllowUsers 271or 272.Cm DenyUsers . 273.El 274.Sh AUTHENTICATION 275The OpenSSH SSH daemon supports SSH protocols 1 and 2. 276The default is to use protocol 2 only, 277though this can be changed via the 278.Cm Protocol 279option in 280.Xr sshd_config 5 . 281Protocol 2 supports DSA, ECDSA, Ed25519 and RSA keys; 282protocol 1 only supports RSA keys. 283For both protocols, 284each host has a host-specific key, 285normally 2048 bits, 286used to identify the host. 287.Pp 288Forward security for protocol 1 is provided through 289an additional server key, 290normally 768 bits, 291generated when the server starts. 292This key is normally regenerated every hour if it has been used, and 293is never stored on disk. 294Whenever a client connects, the daemon responds with its public 295host and server keys. 296The client compares the 297RSA host key against its own database to verify that it has not changed. 298The client then generates a 256-bit random number. 299It encrypts this 300random number using both the host key and the server key, and sends 301the encrypted number to the server. 302Both sides then use this 303random number as a session key which is used to encrypt all further 304communications in the session. 305The rest of the session is encrypted 306using a conventional cipher, currently Blowfish or 3DES, with 3DES 307being used by default. 308The client selects the encryption algorithm 309to use from those offered by the server. 310.Pp 311For protocol 2, 312forward security is provided through a Diffie-Hellman key agreement. 313This key agreement results in a shared session key. 314The rest of the session is encrypted using a symmetric cipher, currently 315128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. 316The client selects the encryption algorithm 317to use from those offered by the server. 318Additionally, session integrity is provided 319through a cryptographic message authentication code 320(hmac-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160, 321hmac-sha2-256 or hmac-sha2-512). 322.Pp 323Finally, the server and the client enter an authentication dialog. 324The client tries to authenticate itself using 325host-based authentication, 326public key authentication, 327challenge-response authentication, 328or password authentication. 329.Pp 330Regardless of the authentication type, the account is checked to 331ensure that it is accessible. An account is not accessible if it is 332locked, listed in 333.Cm DenyUsers 334or its group is listed in 335.Cm DenyGroups 336\&. The definition of a locked account is system dependant. Some platforms 337have their own account database (eg AIX) and some modify the passwd field ( 338.Ql \&*LK\&* 339on Solaris and UnixWare, 340.Ql \&* 341on HP-UX, containing 342.Ql Nologin 343on Tru64, 344a leading 345.Ql \&*LOCKED\&* 346on FreeBSD and a leading 347.Ql \&! 348on most Linuxes). 349If there is a requirement to disable password authentication 350for the account while allowing still public-key, then the passwd field 351should be set to something other than these values (eg 352.Ql NP 353or 354.Ql \&*NP\&* 355). 356.Pp 357If the client successfully authenticates itself, a dialog for 358preparing the session is entered. 359At this time the client may request 360things like allocating a pseudo-tty, forwarding X11 connections, 361forwarding TCP connections, or forwarding the authentication agent 362connection over the secure channel. 363.Pp 364After this, the client either requests a shell or execution of a command. 365The sides then enter session mode. 366In this mode, either side may send 367data at any time, and such data is forwarded to/from the shell or 368command on the server side, and the user terminal in the client side. 369.Pp 370When the user program terminates and all forwarded X11 and other 371connections have been closed, the server sends command exit status to 372the client, and both sides exit. 373.Sh LOGIN PROCESS 374When a user successfully logs in, 375.Nm 376does the following: 377.Bl -enum -offset indent 378.It 379If the login is on a tty, and no command has been specified, 380prints last login time and 381.Pa /etc/motd 382(unless prevented in the configuration file or by 383.Pa ~/.hushlogin ; 384see the 385.Sx FILES 386section). 387.It 388If the login is on a tty, records login time. 389.It 390Checks 391.Pa /etc/nologin ; 392if it exists, prints contents and quits 393(unless root). 394.It 395Changes to run with normal user privileges. 396.It 397Sets up basic environment. 398.It 399Reads the file 400.Pa ~/.ssh/environment , 401if it exists, and users are allowed to change their environment. 402See the 403.Cm PermitUserEnvironment 404option in 405.Xr sshd_config 5 . 406.It 407Changes to user's home directory. 408.It 409If 410.Pa ~/.ssh/rc 411exists and the 412.Xr sshd_config 5 413.Cm PermitUserRC 414option is set, runs it; else if 415.Pa /etc/ssh/sshrc 416exists, runs 417it; otherwise runs xauth. 418The 419.Dq rc 420files are given the X11 421authentication protocol and cookie in standard input. 422See 423.Sx SSHRC , 424below. 425.It 426Runs user's shell or command. 427.El 428.Sh SSHRC 429If the file 430.Pa ~/.ssh/rc 431exists, 432.Xr sh 1 433runs it after reading the 434environment files but before starting the user's shell or command. 435It must not produce any output on stdout; stderr must be used 436instead. 437If X11 forwarding is in use, it will receive the "proto cookie" pair in 438its standard input (and 439.Ev DISPLAY 440in its environment). 441The script must call 442.Xr xauth 1 443because 444.Nm 445will not run xauth automatically to add X11 cookies. 446.Pp 447The primary purpose of this file is to run any initialization routines 448which may be needed before the user's home directory becomes 449accessible; AFS is a particular example of such an environment. 450.Pp 451This file will probably contain some initialization code followed by 452something similar to: 453.Bd -literal -offset 3n 454if read proto cookie && [ -n "$DISPLAY" ]; then 455 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 456 # X11UseLocalhost=yes 457 echo add unix:`echo $DISPLAY | 458 cut -c11-` $proto $cookie 459 else 460 # X11UseLocalhost=no 461 echo add $DISPLAY $proto $cookie 462 fi | xauth -q - 463fi 464.Ed 465.Pp 466If this file does not exist, 467.Pa /etc/ssh/sshrc 468is run, and if that 469does not exist either, xauth is used to add the cookie. 470.Sh AUTHORIZED_KEYS FILE FORMAT 471.Cm AuthorizedKeysFile 472specifies the files containing public keys for 473public key authentication; 474if none is specified, the default is 475.Pa ~/.ssh/authorized_keys 476and 477.Pa ~/.ssh/authorized_keys2 . 478Each line of the file contains one 479key (empty lines and lines starting with a 480.Ql # 481are ignored as 482comments). 483Protocol 1 public keys consist of the following space-separated fields: 484options, bits, exponent, modulus, comment. 485Protocol 2 public key consist of: 486options, keytype, base64-encoded key, comment. 487The options field is optional; 488its presence is determined by whether the line starts 489with a number or not (the options field never starts with a number). 490The bits, exponent, modulus, and comment fields give the RSA key for 491protocol version 1; the 492comment field is not used for anything (but may be convenient for the 493user to identify the key). 494For protocol version 2 the keytype is 495.Dq ecdsa-sha2-nistp256 , 496.Dq ecdsa-sha2-nistp384 , 497.Dq ecdsa-sha2-nistp521 , 498.Dq ssh-ed25519 , 499.Dq ssh-dss 500or 501.Dq ssh-rsa . 502.Pp 503Note that lines in this file are usually several hundred bytes long 504(because of the size of the public key encoding) up to a limit of 5058 kilobytes, which permits DSA keys up to 8 kilobits and RSA 506keys up to 16 kilobits. 507You don't want to type them in; instead, copy the 508.Pa identity.pub , 509.Pa id_dsa.pub , 510.Pa id_ecdsa.pub , 511.Pa id_ed25519.pub , 512or the 513.Pa id_rsa.pub 514file and edit it. 515.Pp 516.Nm 517enforces a minimum RSA key modulus size for protocol 1 518and protocol 2 keys of 768 bits. 519.Pp 520The options (if present) consist of comma-separated option 521specifications. 522No spaces are permitted, except within double quotes. 523The following option specifications are supported (note 524that option keywords are case-insensitive): 525.Bl -tag -width Ds 526.It Cm cert-authority 527Specifies that the listed key is a certification authority (CA) that is 528trusted to validate signed certificates for user authentication. 529.Pp 530Certificates may encode access restrictions similar to these key options. 531If both certificate restrictions and key options are present, the most 532restrictive union of the two is applied. 533.It Cm command="command" 534Specifies that the command is executed whenever this key is used for 535authentication. 536The command supplied by the user (if any) is ignored. 537The command is run on a pty if the client requests a pty; 538otherwise it is run without a tty. 539If an 8-bit clean channel is required, 540one must not request a pty or should specify 541.Cm no-pty . 542A quote may be included in the command by quoting it with a backslash. 543This option might be useful 544to restrict certain public keys to perform just a specific operation. 545An example might be a key that permits remote backups but nothing else. 546Note that the client may specify TCP and/or X11 547forwarding unless they are explicitly prohibited. 548The command originally supplied by the client is available in the 549.Ev SSH_ORIGINAL_COMMAND 550environment variable. 551Note that this option applies to shell, command or subsystem execution. 552Also note that this command may be superseded by either a 553.Xr sshd_config 5 554.Cm ForceCommand 555directive or a command embedded in a certificate. 556.It Cm environment="NAME=value" 557Specifies that the string is to be added to the environment when 558logging in using this key. 559Environment variables set this way 560override other default environment values. 561Multiple options of this type are permitted. 562Environment processing is disabled by default and is 563controlled via the 564.Cm PermitUserEnvironment 565option. 566This option is automatically disabled if 567.Cm UseLogin 568is enabled. 569.It Cm from="pattern-list" 570Specifies that in addition to public key authentication, either the canonical 571name of the remote host or its IP address must be present in the 572comma-separated list of patterns. 573See PATTERNS in 574.Xr ssh_config 5 575for more information on patterns. 576.Pp 577In addition to the wildcard matching that may be applied to hostnames or 578addresses, a 579.Cm from 580stanza may match IP addresses using CIDR address/masklen notation. 581.Pp 582The purpose of this option is to optionally increase security: public key 583authentication by itself does not trust the network or name servers or 584anything (but the key); however, if somebody somehow steals the key, the key 585permits an intruder to log in from anywhere in the world. 586This additional option makes using a stolen key more difficult (name 587servers and/or routers would have to be compromised in addition to 588just the key). 589.It Cm no-agent-forwarding 590Forbids authentication agent forwarding when this key is used for 591authentication. 592.It Cm no-port-forwarding 593Forbids TCP forwarding when this key is used for authentication. 594Any port forward requests by the client will return an error. 595This might be used, e.g. in connection with the 596.Cm command 597option. 598.It Cm no-pty 599Prevents tty allocation (a request to allocate a pty will fail). 600.It Cm no-user-rc 601Disables execution of 602.Pa ~/.ssh/rc . 603.It Cm no-X11-forwarding 604Forbids X11 forwarding when this key is used for authentication. 605Any X11 forward requests by the client will return an error. 606.It Cm permitopen="host:port" 607Limit local port forwarding with 608.Xr ssh 1 609.Fl L 610such that it may only connect to the specified host and port. 611IPv6 addresses can be specified by enclosing the address in square brackets. 612Multiple 613.Cm permitopen 614options may be applied separated by commas. 615No pattern matching is performed on the specified hostnames, 616they must be literal domains or addresses. 617A port specification of 618.Cm * 619matches any port. 620.It Cm principals="principals" 621On a 622.Cm cert-authority 623line, specifies allowed principals for certificate authentication as a 624comma-separated list. 625At least one name from the list must appear in the certificate's 626list of principals for the certificate to be accepted. 627This option is ignored for keys that are not marked as trusted certificate 628signers using the 629.Cm cert-authority 630option. 631.It Cm tunnel="n" 632Force a 633.Xr tun 4 634device on the server. 635Without this option, the next available device will be used if 636the client requests a tunnel. 637.El 638.Pp 639An example authorized_keys file: 640.Bd -literal -offset 3n 641# Comments allowed at start of line 642ssh-rsa AAAAB3Nza...LiPk== user@example.net 643from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 644AAAAB2...19Q== john@example.net 645command="dump /home",no-pty,no-port-forwarding ssh-dss 646AAAAC3...51R== example.net 647permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 648AAAAB5...21S== 649tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 650jane@example.net 651.Ed 652.Sh SSH_KNOWN_HOSTS FILE FORMAT 653The 654.Pa /etc/ssh/ssh_known_hosts 655and 656.Pa ~/.ssh/known_hosts 657files contain host public keys for all known hosts. 658The global file should 659be prepared by the administrator (optional), and the per-user file is 660maintained automatically: whenever the user connects from an unknown host, 661its key is added to the per-user file. 662.Pp 663Each line in these files contains the following fields: markers (optional), 664hostnames, bits, exponent, modulus, comment. 665The fields are separated by spaces. 666.Pp 667The marker is optional, but if it is present then it must be one of 668.Dq @cert-authority , 669to indicate that the line contains a certification authority (CA) key, 670or 671.Dq @revoked , 672to indicate that the key contained on the line is revoked and must not ever 673be accepted. 674Only one marker should be used on a key line. 675.Pp 676Hostnames is a comma-separated list of patterns 677.Pf ( Ql * 678and 679.Ql \&? 680act as 681wildcards); each pattern in turn is matched against the canonical host 682name (when authenticating a client) or against the user-supplied 683name (when authenticating a server). 684A pattern may also be preceded by 685.Ql \&! 686to indicate negation: if the host name matches a negated 687pattern, it is not accepted (by that line) even if it matched another 688pattern on the line. 689A hostname or address may optionally be enclosed within 690.Ql \&[ 691and 692.Ql \&] 693brackets then followed by 694.Ql \&: 695and a non-standard port number. 696.Pp 697Alternately, hostnames may be stored in a hashed form which hides host names 698and addresses should the file's contents be disclosed. 699Hashed hostnames start with a 700.Ql | 701character. 702Only one hashed hostname may appear on a single line and none of the above 703negation or wildcard operators may be applied. 704.Pp 705Bits, exponent, and modulus are taken directly from the RSA host key; they 706can be obtained, for example, from 707.Pa /etc/ssh/ssh_host_key.pub . 708The optional comment field continues to the end of the line, and is not used. 709.Pp 710Lines starting with 711.Ql # 712and empty lines are ignored as comments. 713.Pp 714When performing host authentication, authentication is accepted if any 715matching line has the proper key; either one that matches exactly or, 716if the server has presented a certificate for authentication, the key 717of the certification authority that signed the certificate. 718For a key to be trusted as a certification authority, it must use the 719.Dq @cert-authority 720marker described above. 721.Pp 722The known hosts file also provides a facility to mark keys as revoked, 723for example when it is known that the associated private key has been 724stolen. 725Revoked keys are specified by including the 726.Dq @revoked 727marker at the beginning of the key line, and are never accepted for 728authentication or as certification authorities, but instead will 729produce a warning from 730.Xr ssh 1 731when they are encountered. 732.Pp 733It is permissible (but not 734recommended) to have several lines or different host keys for the same 735names. 736This will inevitably happen when short forms of host names 737from different domains are put in the file. 738It is possible 739that the files contain conflicting information; authentication is 740accepted if valid information can be found from either file. 741.Pp 742Note that the lines in these files are typically hundreds of characters 743long, and you definitely don't want to type in the host keys by hand. 744Rather, generate them by a script, 745.Xr ssh-keyscan 1 746or by taking 747.Pa /etc/ssh/ssh_host_key.pub 748and adding the host names at the front. 749.Xr ssh-keygen 1 750also offers some basic automated editing for 751.Pa ~/.ssh/known_hosts 752including removing hosts matching a host name and converting all host 753names to their hashed representations. 754.Pp 755An example ssh_known_hosts file: 756.Bd -literal -offset 3n 757# Comments allowed at start of line 758closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 759cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 760# A hashed hostname 761|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 762AAAA1234.....= 763# A revoked key 764@revoked * ssh-rsa AAAAB5W... 765# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org 766@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... 767.Ed 768.Sh FILES 769.Bl -tag -width Ds -compact 770.It Pa ~/.hushlogin 771This file is used to suppress printing the last login time and 772.Pa /etc/motd , 773if 774.Cm PrintLastLog 775and 776.Cm PrintMotd , 777respectively, 778are enabled. 779It does not suppress printing of the banner specified by 780.Cm Banner . 781.Pp 782.It Pa ~/.rhosts 783This file is used for host-based authentication (see 784.Xr ssh 1 785for more information). 786On some machines this file may need to be 787world-readable if the user's home directory is on an NFS partition, 788because 789.Nm 790reads it as root. 791Additionally, this file must be owned by the user, 792and must not have write permissions for anyone else. 793The recommended 794permission for most machines is read/write for the user, and not 795accessible by others. 796.Pp 797.It Pa ~/.shosts 798This file is used in exactly the same way as 799.Pa .rhosts , 800but allows host-based authentication without permitting login with 801rlogin/rsh. 802.Pp 803.It Pa ~/.ssh/ 804This directory is the default location for all user-specific configuration 805and authentication information. 806There is no general requirement to keep the entire contents of this directory 807secret, but the recommended permissions are read/write/execute for the user, 808and not accessible by others. 809.Pp 810.It Pa ~/.ssh/authorized_keys 811Lists the public keys (DSA, ECDSA, Ed25519, RSA) 812that can be used for logging in as this user. 813The format of this file is described above. 814The content of the file is not highly sensitive, but the recommended 815permissions are read/write for the user, and not accessible by others. 816.Pp 817If this file, the 818.Pa ~/.ssh 819directory, or the user's home directory are writable 820by other users, then the file could be modified or replaced by unauthorized 821users. 822In this case, 823.Nm 824will not allow it to be used unless the 825.Cm StrictModes 826option has been set to 827.Dq no . 828.Pp 829.It Pa ~/.ssh/environment 830This file is read into the environment at login (if it exists). 831It can only contain empty lines, comment lines (that start with 832.Ql # ) , 833and assignment lines of the form name=value. 834The file should be writable 835only by the user; it need not be readable by anyone else. 836Environment processing is disabled by default and is 837controlled via the 838.Cm PermitUserEnvironment 839option. 840.Pp 841.It Pa ~/.ssh/known_hosts 842Contains a list of host keys for all hosts the user has logged into 843that are not already in the systemwide list of known host keys. 844The format of this file is described above. 845This file should be writable only by root/the owner and 846can, but need not be, world-readable. 847.Pp 848.It Pa ~/.ssh/rc 849Contains initialization routines to be run before 850the user's home directory becomes accessible. 851This file should be writable only by the user, and need not be 852readable by anyone else. 853.Pp 854.It Pa /etc/hosts.equiv 855This file is for host-based authentication (see 856.Xr ssh 1 ) . 857It should only be writable by root. 858.Pp 859.It Pa /etc/moduli 860Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". 861The file format is described in 862.Xr moduli 5 . 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_key 884.It Pa /etc/ssh/ssh_host_dsa_key 885.It Pa /etc/ssh/ssh_host_ecdsa_key 886.It Pa /etc/ssh/ssh_host_ed25519_key 887.It Pa /etc/ssh/ssh_host_rsa_key 888These files contain the private parts of the host keys. 889These files should only be owned by root, readable only by root, and not 890accessible to others. 891Note that 892.Nm 893does not start if these files are group/world-accessible. 894.Pp 895.It Pa /etc/ssh/ssh_host_key.pub 896.It Pa /etc/ssh/ssh_host_dsa_key.pub 897.It Pa /etc/ssh/ssh_host_ecdsa_key.pub 898.It Pa /etc/ssh/ssh_host_ed25519_key.pub 899.It Pa /etc/ssh/ssh_host_rsa_key.pub 900These files contain the public parts of the host keys. 901These files should be world-readable but writable only by 902root. 903Their contents should match the respective private parts. 904These files are not 905really used for anything; they are provided for the convenience of 906the user so their contents can be copied to known hosts files. 907These files are created using 908.Xr ssh-keygen 1 . 909.Pp 910.It Pa /etc/ssh/ssh_known_hosts 911Systemwide list of known host keys. 912This file should be prepared by the 913system administrator to contain the public host keys of all machines in the 914organization. 915The format of this file is described above. 916This file should be writable only by root/the owner and 917should be world-readable. 918.Pp 919.It Pa /etc/ssh/sshd_config 920Contains configuration data for 921.Nm sshd . 922The file format and configuration options are described in 923.Xr sshd_config 5 . 924.Pp 925.It Pa /etc/ssh/sshrc 926Similar to 927.Pa ~/.ssh/rc , 928it can be used to specify 929machine-specific login-time initializations globally. 930This file should be writable only by root, and should be world-readable. 931.Pp 932.It Pa /var/empty 933.Xr chroot 2 934directory used by 935.Nm 936during privilege separation in the pre-authentication phase. 937The directory should not contain any files and must be owned by root 938and not group or world-writable. 939.Pp 940.It Pa /var/run/sshd.pid 941Contains the process ID of the 942.Nm 943listening for connections (if there are several daemons running 944concurrently for different ports, this contains the process ID of the one 945started last). 946The content of this file is not sensitive; it can be world-readable. 947.El 948.Sh SEE ALSO 949.Xr scp 1 , 950.Xr sftp 1 , 951.Xr ssh 1 , 952.Xr ssh-add 1 , 953.Xr ssh-agent 1 , 954.Xr ssh-keygen 1 , 955.Xr ssh-keyscan 1 , 956.Xr chroot 2 , 957.Xr login.conf 5 , 958.Xr moduli 5 , 959.Xr sshd_config 5 , 960.Xr inetd 8 , 961.Xr sftp-server 8 962.Sh AUTHORS 963OpenSSH is a derivative of the original and free 964ssh 1.2.12 release by Tatu Ylonen. 965Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 966Theo de Raadt and Dug Song 967removed many bugs, re-added newer features and 968created OpenSSH. 969Markus Friedl contributed the support for SSH 970protocol versions 1.5 and 2.0. 971Niels Provos and Markus Friedl contributed support 972for privilege separation. 973