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 1.280 2015/07/03 03:49:45 djm Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: July 3 2015 $ 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 b Ar bits 49.Op Fl C Ar connection_spec 50.Op Fl c Ar host_certificate_file 51.Op Fl E Ar log_file 52.Op Fl f Ar config_file 53.Op Fl g Ar login_grace_time 54.Op Fl h Ar host_key_file 55.Op Fl k Ar key_gen_time 56.Op Fl o Ar option 57.Op Fl p Ar port 58.Op Fl u Ar len 59.Ek 60.Sh DESCRIPTION 61.Nm 62(OpenSSH Daemon) is the daemon program for 63.Xr ssh 1 . 64Together these programs replace rlogin and rsh, 65and provide secure encrypted communications between two untrusted hosts 66over an insecure network. 67.Pp 68.Nm 69listens for connections from clients. 70It is normally started at boot from 71.Pa /etc/rc.d/sshd . 72It forks a new 73daemon for each incoming connection. 74The forked daemons handle 75key exchange, encryption, authentication, command execution, 76and data exchange. 77.Pp 78.Nm 79can be configured using command-line options or a configuration file 80(by default 81.Xr sshd_config 5 ) ; 82command-line options override values specified in the 83configuration file. 84.Nm 85rereads its configuration file when it receives a hangup signal, 86.Dv SIGHUP , 87by executing itself with the name and options it was started with, e.g.\& 88.Pa /usr/sbin/sshd . 89.Pp 90The options are as follows: 91.Bl -tag -width Ds 92.It Fl 4 93Forces 94.Nm 95to use IPv4 addresses only. 96.It Fl 6 97Forces 98.Nm 99to use IPv6 addresses only. 100.It Fl b Ar bits 101Specifies the number of bits in the ephemeral protocol version 1 102server key (default 1024). 103.It Fl C Ar connection_spec 104Specify the connection parameters to use for the 105.Fl T 106extended test mode. 107If provided, any 108.Cm Match 109directives in the configuration file 110that would apply to the specified user, host, and address will be set before 111the configuration is written to standard output. 112The connection parameters are supplied as keyword=value pairs. 113The keywords are 114.Dq user , 115.Dq host , 116.Dq laddr , 117.Dq lport , 118and 119.Dq addr . 120All are required and may be supplied in any order, either with multiple 121.Fl C 122options or as a comma-separated list. 123.It Fl c Ar host_certificate_file 124Specifies a path to a certificate file to identify 125.Nm 126during key exchange. 127The certificate file must match a host key file specified using the 128.Fl h 129option or the 130.Cm HostKey 131configuration directive. 132.It Fl D 133When this option is specified, 134.Nm 135will not detach and does not become a daemon. 136This allows easy monitoring of 137.Nm sshd . 138.It Fl d 139Debug mode. 140The server sends verbose debug output to standard error, 141and does not put itself in the background. 142The server also will not fork and will only process one connection. 143This option is only intended for debugging for the server. 144Multiple 145.Fl d 146options increase the debugging level. 147Maximum is 3. 148.It Fl E Ar log_file 149Append debug logs to 150.Ar log_file 151instead of the system log. 152.It Fl e 153Write debug logs to standard error instead of the system log. 154.It Fl f Ar config_file 155Specifies the name of the configuration file. 156The default is 157.Pa /etc/ssh/sshd_config . 158.Nm 159refuses to start if there is no configuration file. 160.It Fl g Ar login_grace_time 161Gives the grace time for clients to authenticate themselves (default 162120 seconds). 163If the client fails to authenticate the user within 164this many seconds, the server disconnects and exits. 165A value of zero indicates no limit. 166.It Fl h Ar host_key_file 167Specifies a file from which a host key is read. 168This option must be given if 169.Nm 170is not run as root (as the normal 171host key files are normally not readable by anyone but root). 172The default is 173.Pa /etc/ssh/ssh_host_key 174for protocol version 1, and 175.Pa /etc/ssh/ssh_host_dsa_key , 176.Pa /etc/ssh/ssh_host_ecdsa_key . 177.Pa /etc/ssh/ssh_host_ed25519_key 178and 179.Pa /etc/ssh/ssh_host_rsa_key 180for protocol version 2. 181It is possible to have multiple host key files for 182the different protocol versions and host key algorithms. 183.It Fl i 184Specifies that 185.Nm 186is being run from 187.Xr inetd 8 . 188If SSH protocol 1 is enabled, 189.Nm 190should not normally be run 191from inetd because it needs to generate the server key before it can 192respond to the client, and this may take some time. 193Clients may have to wait too long if the key was regenerated every time. 194.It Fl k Ar key_gen_time 195Specifies how often the ephemeral protocol version 1 server key is 196regenerated (default 3600 seconds, or one hour). 197The motivation for regenerating the key fairly 198often is that the key is not stored anywhere, and after about an hour 199it becomes impossible to recover the key for decrypting intercepted 200communications even if the machine is cracked into or physically 201seized. 202A value of zero indicates that the key will never be regenerated. 203.It Fl o Ar option 204Can be used to give options in the format used in the configuration file. 205This is useful for specifying options for which there is no separate 206command-line flag. 207For full details of the options, and their values, see 208.Xr sshd_config 5 . 209.It Fl p Ar port 210Specifies the port on which the server listens for connections 211(default 22). 212Multiple port options are permitted. 213Ports specified in the configuration file with the 214.Cm Port 215option are ignored when a command-line port is specified. 216Ports specified using the 217.Cm ListenAddress 218option override command-line ports. 219.It Fl q 220Quiet mode. 221Nothing is sent to the system log. 222Normally the beginning, 223authentication, and termination of each connection is logged. 224.It Fl T 225Extended test mode. 226Check the validity of the configuration file, output the effective configuration 227to stdout and then exit. 228Optionally, 229.Cm Match 230rules may be applied by specifying the connection parameters using one or more 231.Fl C 232options. 233.It Fl t 234Test mode. 235Only check the validity of the configuration file and sanity of the keys. 236This is useful for updating 237.Nm 238reliably as configuration options may change. 239.It Fl u Ar len 240This option is used to specify the size of the field 241in the 242.Li utmp 243structure that holds the remote host name. 244If the resolved host name is longer than 245.Ar len , 246the dotted decimal value will be used instead. 247This allows hosts with very long host names that 248overflow this field to still be uniquely identified. 249Specifying 250.Fl u0 251indicates that only dotted decimal addresses 252should be put into the 253.Pa utmp 254file. 255.Fl u0 256may also be used to prevent 257.Nm 258from making DNS requests unless the authentication 259mechanism or configuration requires it. 260Authentication mechanisms that may require DNS include 261.Cm RhostsRSAAuthentication , 262.Cm HostbasedAuthentication , 263and using a 264.Cm from="pattern-list" 265option in a key file. 266Configuration options that require DNS include using a 267USER@HOST pattern in 268.Cm AllowUsers 269or 270.Cm DenyUsers . 271.El 272.Sh AUTHENTICATION 273The OpenSSH SSH daemon supports SSH protocols 1 and 2. 274The default is to use protocol 2 only, 275though this can be changed via the 276.Cm Protocol 277option in 278.Xr sshd_config 5 . 279Protocol 2 supports DSA, ECDSA, Ed25519 and RSA keys; 280protocol 1 only supports RSA keys. 281For both protocols, 282each host has a host-specific key, 283normally 2048 bits, 284used to identify the host. 285.Pp 286Forward security for protocol 1 is provided through 287an additional server key, 288normally 1024 bits, 289generated when the server starts. 290This key is normally regenerated every hour if it has been used, and 291is never stored on disk. 292Whenever a client connects, the daemon responds with its public 293host and server keys. 294The client compares the 295RSA host key against its own database to verify that it has not changed. 296The client then generates a 256-bit random number. 297It encrypts this 298random number using both the host key and the server key, and sends 299the encrypted number to the server. 300Both sides then use this 301random number as a session key which is used to encrypt all further 302communications in the session. 303The rest of the session is encrypted 304using a conventional cipher, currently Blowfish or 3DES, with 3DES 305being used by default. 306The client selects the encryption algorithm 307to use from those offered by the server. 308.Pp 309For protocol 2, 310forward security is provided through a Diffie-Hellman key agreement. 311This key agreement results in a shared session key. 312The rest of the session is encrypted using a symmetric cipher, currently 313128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. 314The client selects the encryption algorithm 315to use from those offered by the server. 316Additionally, session integrity is provided 317through a cryptographic message authentication code 318(hmac-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160, 319hmac-sha2-256 or hmac-sha2-512). 320.Pp 321Finally, the server and the client enter an authentication dialog. 322The client tries to authenticate itself using 323host-based authentication, 324public key authentication, 325challenge-response authentication, 326or password authentication. 327.Pp 328Regardless of the authentication type, the account is checked to 329ensure that it is accessible. An account is not accessible if it is 330locked, listed in 331.Cm DenyUsers 332or its group is listed in 333.Cm DenyGroups 334\&. The definition of a locked account is system dependant. Some platforms 335have their own account database (eg AIX) and some modify the passwd field ( 336.Ql \&*LK\&* 337on Solaris and UnixWare, 338.Ql \&* 339on HP-UX, containing 340.Ql Nologin 341on Tru64, 342a leading 343.Ql \&*LOCKED\&* 344on FreeBSD and a leading 345.Ql \&! 346on most Linuxes). 347If there is a requirement to disable password authentication 348for the account while allowing still public-key, then the passwd field 349should be set to something other than these values (eg 350.Ql NP 351or 352.Ql \&*NP\&* 353). 354.Pp 355If the client successfully authenticates itself, a dialog for 356preparing the session is entered. 357At this time the client may request 358things like allocating a pseudo-tty, forwarding X11 connections, 359forwarding TCP connections, or forwarding the authentication agent 360connection over the secure channel. 361.Pp 362After this, the client either requests a shell or execution of a command. 363The sides then enter session mode. 364In this mode, either side may send 365data at any time, and such data is forwarded to/from the shell or 366command on the server side, and the user terminal in the client side. 367.Pp 368When the user program terminates and all forwarded X11 and other 369connections have been closed, the server sends command exit status to 370the client, and both sides exit. 371.Sh LOGIN PROCESS 372When a user successfully logs in, 373.Nm 374does the following: 375.Bl -enum -offset indent 376.It 377If the login is on a tty, and no command has been specified, 378prints last login time and 379.Pa /etc/motd 380(unless prevented in the configuration file or by 381.Pa ~/.hushlogin ; 382see the 383.Sx FILES 384section). 385.It 386If the login is on a tty, records login time. 387.It 388Checks 389.Pa /etc/nologin and 390.Pa /var/run/nologin ; 391if one exists, it prints the contents and quits 392(unless root). 393.It 394Changes to run with normal user privileges. 395.It 396Sets up basic environment. 397.It 398Reads the file 399.Pa ~/.ssh/environment , 400if it exists, and users are allowed to change their environment. 401See the 402.Cm PermitUserEnvironment 403option in 404.Xr sshd_config 5 . 405.It 406Changes to user's home directory. 407.It 408If 409.Pa ~/.ssh/rc 410exists and the 411.Xr sshd_config 5 412.Cm PermitUserRC 413option is set, runs it; else if 414.Pa /etc/ssh/sshrc 415exists, runs 416it; otherwise runs 417.Xr xauth 1 . 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. 427All commands are run under the user's login shell as specified in the 428system password database. 429.El 430.Sh SSHRC 431If the file 432.Pa ~/.ssh/rc 433exists, 434.Xr sh 1 435runs it after reading the 436environment files but before starting the user's shell or command. 437It must not produce any output on stdout; stderr must be used 438instead. 439If X11 forwarding is in use, it will receive the "proto cookie" pair in 440its standard input (and 441.Ev DISPLAY 442in its environment). 443The script must call 444.Xr xauth 1 445because 446.Nm 447will not run xauth automatically to add X11 cookies. 448.Pp 449The primary purpose of this file is to run any initialization routines 450which may be needed before the user's home directory becomes 451accessible; AFS is a particular example of such an environment. 452.Pp 453This file will probably contain some initialization code followed by 454something similar to: 455.Bd -literal -offset 3n 456if read proto cookie && [ -n "$DISPLAY" ]; then 457 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 458 # X11UseLocalhost=yes 459 echo add unix:`echo $DISPLAY | 460 cut -c11-` $proto $cookie 461 else 462 # X11UseLocalhost=no 463 echo add $DISPLAY $proto $cookie 464 fi | xauth -q - 465fi 466.Ed 467.Pp 468If this file does not exist, 469.Pa /etc/ssh/sshrc 470is run, and if that 471does not exist either, xauth is used to add the cookie. 472.Sh AUTHORIZED_KEYS FILE FORMAT 473.Cm AuthorizedKeysFile 474specifies the files containing public keys for 475public key authentication; 476if none is specified, the default is 477.Pa ~/.ssh/authorized_keys 478and 479.Pa ~/.ssh/authorized_keys2 . 480Each line of the file contains one 481key (empty lines and lines starting with a 482.Ql # 483are ignored as 484comments). 485Protocol 1 public keys consist of the following space-separated fields: 486options, bits, exponent, modulus, comment. 487Protocol 2 public key consist of: 488options, keytype, base64-encoded key, comment. 489The options field is optional; 490its presence is determined by whether the line starts 491with a number or not (the options field never starts with a number). 492The bits, exponent, modulus, and comment fields give the RSA key for 493protocol version 1; the 494comment field is not used for anything (but may be convenient for the 495user to identify the key). 496For protocol version 2 the keytype is 497.Dq ecdsa-sha2-nistp256 , 498.Dq ecdsa-sha2-nistp384 , 499.Dq ecdsa-sha2-nistp521 , 500.Dq ssh-ed25519 , 501.Dq ssh-dss 502or 503.Dq ssh-rsa . 504.Pp 505Note that lines in this file are usually several hundred bytes long 506(because of the size of the public key encoding) up to a limit of 5078 kilobytes, which permits DSA keys up to 8 kilobits and RSA 508keys up to 16 kilobits. 509You don't want to type them in; instead, copy the 510.Pa identity.pub , 511.Pa id_dsa.pub , 512.Pa id_ecdsa.pub , 513.Pa id_ed25519.pub , 514or the 515.Pa id_rsa.pub 516file and edit it. 517.Pp 518.Nm 519enforces a minimum RSA key modulus size for protocol 1 520and protocol 2 keys of 768 bits. 521.Pp 522The options (if present) consist of comma-separated option 523specifications. 524No spaces are permitted, except within double quotes. 525The following option specifications are supported (note 526that option keywords are case-insensitive): 527.Bl -tag -width Ds 528.It Cm cert-authority 529Specifies that the listed key is a certification authority (CA) that is 530trusted to validate signed certificates for user authentication. 531.Pp 532Certificates may encode access restrictions similar to these key options. 533If both certificate restrictions and key options are present, the most 534restrictive union of the two is applied. 535.It Cm command="command" 536Specifies that the command is executed whenever this key is used for 537authentication. 538The command supplied by the user (if any) is ignored. 539The command is run on a pty if the client requests a pty; 540otherwise it is run without a tty. 541If an 8-bit clean channel is required, 542one must not request a pty or should specify 543.Cm no-pty . 544A quote may be included in the command by quoting it with a backslash. 545This option might be useful 546to restrict certain public keys to perform just a specific operation. 547An example might be a key that permits remote backups but nothing else. 548Note that the client may specify TCP and/or X11 549forwarding unless they are explicitly prohibited. 550The command originally supplied by the client is available in the 551.Ev SSH_ORIGINAL_COMMAND 552environment variable. 553Note that this option applies to shell, command or subsystem execution. 554Also note that this command may be superseded by either a 555.Xr sshd_config 5 556.Cm ForceCommand 557directive or a command embedded in a certificate. 558.It Cm environment="NAME=value" 559Specifies that the string is to be added to the environment when 560logging in using this key. 561Environment variables set this way 562override other default environment values. 563Multiple options of this type are permitted. 564Environment processing is disabled by default and is 565controlled via the 566.Cm PermitUserEnvironment 567option. 568This option is automatically disabled if 569.Cm UseLogin 570is enabled. 571.It Cm from="pattern-list" 572Specifies that in addition to public key authentication, either the canonical 573name of the remote host or its IP address must be present in the 574comma-separated list of patterns. 575See PATTERNS in 576.Xr ssh_config 5 577for more information on patterns. 578.Pp 579In addition to the wildcard matching that may be applied to hostnames or 580addresses, a 581.Cm from 582stanza may match IP addresses using CIDR address/masklen notation. 583.Pp 584The purpose of this option is to optionally increase security: public key 585authentication by itself does not trust the network or name servers or 586anything (but the key); however, if somebody somehow steals the key, the key 587permits an intruder to log in from anywhere in the world. 588This additional option makes using a stolen key more difficult (name 589servers and/or routers would have to be compromised in addition to 590just the key). 591.It Cm no-agent-forwarding 592Forbids authentication agent forwarding when this key is used for 593authentication. 594.It Cm no-port-forwarding 595Forbids TCP forwarding when this key is used for authentication. 596Any port forward requests by the client will return an error. 597This might be used, e.g. in connection with the 598.Cm command 599option. 600.It Cm no-pty 601Prevents tty allocation (a request to allocate a pty will fail). 602.It Cm no-user-rc 603Disables execution of 604.Pa ~/.ssh/rc . 605.It Cm no-X11-forwarding 606Forbids X11 forwarding when this key is used for authentication. 607Any X11 forward requests by the client will return an error. 608.It Cm permitopen="host:port" 609Limit local port forwarding with 610.Xr ssh 1 611.Fl L 612such that it may only connect to the specified host and port. 613IPv6 addresses can be specified by enclosing the address in square brackets. 614Multiple 615.Cm permitopen 616options may be applied separated by commas. 617No pattern matching is performed on the specified hostnames, 618they must be literal domains or addresses. 619A port specification of 620.Cm * 621matches any port. 622.It Cm principals="principals" 623On a 624.Cm cert-authority 625line, specifies allowed principals for certificate authentication as a 626comma-separated list. 627At least one name from the list must appear in the certificate's 628list of principals for the certificate to be accepted. 629This option is ignored for keys that are not marked as trusted certificate 630signers using the 631.Cm cert-authority 632option. 633.It Cm tunnel="n" 634Force a 635.Xr tun 4 636device on the server. 637Without this option, the next available device will be used if 638the client requests a tunnel. 639.El 640.Pp 641An example authorized_keys file: 642.Bd -literal -offset 3n 643# Comments allowed at start of line 644ssh-rsa AAAAB3Nza...LiPk== user@example.net 645from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 646AAAAB2...19Q== john@example.net 647command="dump /home",no-pty,no-port-forwarding ssh-dss 648AAAAC3...51R== example.net 649permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 650AAAAB5...21S== 651tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 652jane@example.net 653.Ed 654.Sh SSH_KNOWN_HOSTS FILE FORMAT 655The 656.Pa /etc/ssh/ssh_known_hosts 657and 658.Pa ~/.ssh/known_hosts 659files contain host public keys for all known hosts. 660The global file should 661be prepared by the administrator (optional), and the per-user file is 662maintained automatically: whenever the user connects from an unknown host, 663its key is added to the per-user file. 664.Pp 665Each line in these files contains the following fields: markers (optional), 666hostnames, bits, exponent, modulus, comment. 667The fields are separated by spaces. 668.Pp 669The marker is optional, but if it is present then it must be one of 670.Dq @cert-authority , 671to indicate that the line contains a certification authority (CA) key, 672or 673.Dq @revoked , 674to indicate that the key contained on the line is revoked and must not ever 675be accepted. 676Only one marker should be used on a key line. 677.Pp 678Hostnames is a comma-separated list of patterns 679.Pf ( Ql * 680and 681.Ql \&? 682act as 683wildcards); each pattern in turn is matched against the canonical host 684name (when authenticating a client) or against the user-supplied 685name (when authenticating a server). 686A pattern may also be preceded by 687.Ql \&! 688to indicate negation: if the host name matches a negated 689pattern, it is not accepted (by that line) even if it matched another 690pattern on the line. 691A hostname or address may optionally be enclosed within 692.Ql \&[ 693and 694.Ql \&] 695brackets then followed by 696.Ql \&: 697and a non-standard port number. 698.Pp 699Alternately, hostnames may be stored in a hashed form which hides host names 700and addresses should the file's contents be disclosed. 701Hashed hostnames start with a 702.Ql | 703character. 704Only one hashed hostname may appear on a single line and none of the above 705negation or wildcard operators may be applied. 706.Pp 707Bits, exponent, and modulus are taken directly from the RSA host key; they 708can be obtained, for example, from 709.Pa /etc/ssh/ssh_host_key.pub . 710The optional comment field continues to the end of the line, and is not used. 711.Pp 712Lines starting with 713.Ql # 714and empty lines are ignored as comments. 715.Pp 716When performing host authentication, authentication is accepted if any 717matching line has the proper key; either one that matches exactly or, 718if the server has presented a certificate for authentication, the key 719of the certification authority that signed the certificate. 720For a key to be trusted as a certification authority, it must use the 721.Dq @cert-authority 722marker described above. 723.Pp 724The known hosts file also provides a facility to mark keys as revoked, 725for example when it is known that the associated private key has been 726stolen. 727Revoked keys are specified by including the 728.Dq @revoked 729marker at the beginning of the key line, and are never accepted for 730authentication or as certification authorities, but instead will 731produce a warning from 732.Xr ssh 1 733when they are encountered. 734.Pp 735It is permissible (but not 736recommended) to have several lines or different host keys for the same 737names. 738This will inevitably happen when short forms of host names 739from different domains are put in the file. 740It is possible 741that the files contain conflicting information; authentication is 742accepted if valid information can be found from either file. 743.Pp 744Note that the lines in these files are typically hundreds of characters 745long, and you definitely don't want to type in the host keys by hand. 746Rather, generate them by a script, 747.Xr ssh-keyscan 1 748or by taking 749.Pa /etc/ssh/ssh_host_key.pub 750and adding the host names at the front. 751.Xr ssh-keygen 1 752also offers some basic automated editing for 753.Pa ~/.ssh/known_hosts 754including removing hosts matching a host name and converting all host 755names to their hashed representations. 756.Pp 757An example ssh_known_hosts file: 758.Bd -literal -offset 3n 759# Comments allowed at start of line 760closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 761cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 762# A hashed hostname 763|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 764AAAA1234.....= 765# A revoked key 766@revoked * ssh-rsa AAAAB5W... 767# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org 768@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... 769.Ed 770.Sh FILES 771.Bl -tag -width Ds -compact 772.It Pa ~/.hushlogin 773This file is used to suppress printing the last login time and 774.Pa /etc/motd , 775if 776.Cm PrintLastLog 777and 778.Cm PrintMotd , 779respectively, 780are enabled. 781It does not suppress printing of the banner specified by 782.Cm Banner . 783.Pp 784.It Pa ~/.rhosts 785This file is used for host-based authentication (see 786.Xr ssh 1 787for more information). 788On some machines this file may need to be 789world-readable if the user's home directory is on an NFS partition, 790because 791.Nm 792reads it as root. 793Additionally, this file must be owned by the user, 794and must not have write permissions for anyone else. 795The recommended 796permission for most machines is read/write for the user, and not 797accessible by others. 798.Pp 799.It Pa ~/.shosts 800This file is used in exactly the same way as 801.Pa .rhosts , 802but allows host-based authentication without permitting login with 803rlogin/rsh. 804.Pp 805.It Pa ~/.ssh/ 806This directory is the default location for all user-specific configuration 807and authentication information. 808There is no general requirement to keep the entire contents of this directory 809secret, but the recommended permissions are read/write/execute for the user, 810and not accessible by others. 811.Pp 812.It Pa ~/.ssh/authorized_keys 813Lists the public keys (DSA, ECDSA, Ed25519, RSA) 814that can be used for logging in as this user. 815The format of this file is described above. 816The content of the file is not highly sensitive, but the recommended 817permissions are read/write for the user, and not accessible by others. 818.Pp 819If this file, the 820.Pa ~/.ssh 821directory, or the user's home directory are writable 822by other users, then the file could be modified or replaced by unauthorized 823users. 824In this case, 825.Nm 826will not allow it to be used unless the 827.Cm StrictModes 828option has been set to 829.Dq no . 830.Pp 831.It Pa ~/.ssh/environment 832This file is read into the environment at login (if it exists). 833It can only contain empty lines, comment lines (that start with 834.Ql # ) , 835and assignment lines of the form name=value. 836The file should be writable 837only by the user; it need not be readable by anyone else. 838Environment processing is disabled by default and is 839controlled via the 840.Cm PermitUserEnvironment 841option. 842.Pp 843.It Pa ~/.ssh/known_hosts 844Contains a list of host keys for all hosts the user has logged into 845that are not already in the systemwide list of known host keys. 846The format of this file is described above. 847This file should be writable only by root/the owner and 848can, but need not be, world-readable. 849.Pp 850.It Pa ~/.ssh/rc 851Contains initialization routines to be run before 852the user's home directory becomes accessible. 853This file should be writable only by the user, and need not be 854readable by anyone else. 855.Pp 856.It Pa /etc/hosts.allow 857.It Pa /etc/hosts.deny 858Access controls that should be enforced by tcp-wrappers are defined here. 859Further details are described in 860.Xr hosts_access 5 . 861.Pp 862.It Pa /etc/hosts.equiv 863This file is for host-based authentication (see 864.Xr ssh 1 ) . 865It should only be writable by root. 866.Pp 867.It Pa /etc/moduli 868Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". 869The file format is described in 870.Xr moduli 5 . 871.Pp 872.It Pa /etc/motd 873See 874.Xr motd 5 . 875.Pp 876.It Pa /etc/nologin 877If this file exists, 878.Nm 879refuses to let anyone except root log in. 880The contents of the file 881are displayed to anyone trying to log in, and non-root connections are 882refused. 883The file should be world-readable. 884.Pp 885.It Pa /etc/shosts.equiv 886This file is used in exactly the same way as 887.Pa hosts.equiv , 888but allows host-based authentication without permitting login with 889rlogin/rsh. 890.Pp 891.It Pa /etc/ssh/ssh_host_key 892.It Pa /etc/ssh/ssh_host_dsa_key 893.It Pa /etc/ssh/ssh_host_ecdsa_key 894.It Pa /etc/ssh/ssh_host_ed25519_key 895.It Pa /etc/ssh/ssh_host_rsa_key 896These files contain the private parts of the host keys. 897These files should only be owned by root, readable only by root, and not 898accessible to others. 899Note that 900.Nm 901does not start if these files are group/world-accessible. 902.Pp 903.It Pa /etc/ssh/ssh_host_key.pub 904.It Pa /etc/ssh/ssh_host_dsa_key.pub 905.It Pa /etc/ssh/ssh_host_ecdsa_key.pub 906.It Pa /etc/ssh/ssh_host_ed25519_key.pub 907.It Pa /etc/ssh/ssh_host_rsa_key.pub 908These files contain the public parts of the host keys. 909These files should be world-readable but writable only by 910root. 911Their contents should match the respective private parts. 912These files are not 913really used for anything; they are provided for the convenience of 914the user so their contents can be copied to known hosts files. 915These files are created using 916.Xr ssh-keygen 1 . 917.Pp 918.It Pa /etc/ssh/ssh_known_hosts 919Systemwide list of known host keys. 920This file should be prepared by the 921system administrator to contain the public host keys of all machines in the 922organization. 923The format of this file is described above. 924This file should be writable only by root/the owner and 925should be world-readable. 926.Pp 927.It Pa /etc/ssh/sshd_config 928Contains configuration data for 929.Nm sshd . 930The file format and configuration options are described in 931.Xr sshd_config 5 . 932.Pp 933.It Pa /etc/ssh/sshrc 934Similar to 935.Pa ~/.ssh/rc , 936it can be used to specify 937machine-specific login-time initializations globally. 938This file should be writable only by root, and should be world-readable. 939.Pp 940.It Pa /var/empty 941.Xr chroot 2 942directory used by 943.Nm 944during privilege separation in the pre-authentication phase. 945The directory should not contain any files and must be owned by root 946and not group or world-writable. 947.Pp 948.It Pa /var/run/sshd.pid 949Contains the process ID of the 950.Nm 951listening for connections (if there are several daemons running 952concurrently for different ports, this contains the process ID of the one 953started last). 954The content of this file is not sensitive; it can be world-readable. 955.El 956.Sh SEE ALSO 957.Xr scp 1 , 958.Xr sftp 1 , 959.Xr ssh 1 , 960.Xr ssh-add 1 , 961.Xr ssh-agent 1 , 962.Xr ssh-keygen 1 , 963.Xr ssh-keyscan 1 , 964.Xr chroot 2 , 965.Xr hosts_access 5 , 966.Xr login.conf 5 , 967.Xr moduli 5 , 968.Xr sshd_config 5 , 969.Xr inetd 8 , 970.Xr sftp-server 8 971.Sh AUTHORS 972OpenSSH is a derivative of the original and free 973ssh 1.2.12 release by Tatu Ylonen. 974Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 975Theo de Raadt and Dug Song 976removed many bugs, re-added newer features and 977created OpenSSH. 978Markus Friedl contributed the support for SSH 979protocol versions 1.5 and 2.0. 980Niels Provos and Markus Friedl contributed support 981for privilege separation. 982