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 1.321 2022/09/11 06:38:11 jmc Exp $ 37.Dd $Mdocdate: September 11 2022 $ 38.Dt SSHD 8 39.Os 40.Sh NAME 41.Nm sshd 42.Nd OpenSSH 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 . 61It provides secure encrypted communications between two untrusted hosts 62over an insecure network. 63.Pp 64.Nm 65listens for connections from clients. 66It is normally started at boot from 67.Pa /etc/rc . 68It forks a new 69daemon for each incoming connection. 70The forked daemons handle 71key exchange, encryption, authentication, command execution, 72and data exchange. 73.Pp 74.Nm 75can be configured using command-line options or a configuration file 76(by default 77.Xr sshd_config 5 ) ; 78command-line options override values specified in the 79configuration file. 80.Nm 81rereads its configuration file when it receives a hangup signal, 82.Dv SIGHUP , 83by executing itself with the name and options it was started with, e.g.\& 84.Pa /usr/sbin/sshd . 85.Pp 86The options are as follows: 87.Bl -tag -width Ds 88.It Fl 4 89Forces 90.Nm 91to use IPv4 addresses only. 92.It Fl 6 93Forces 94.Nm 95to use IPv6 addresses only. 96.It Fl C Ar connection_spec 97Specify the connection parameters to use for the 98.Fl T 99extended test mode. 100If provided, any 101.Cm Match 102directives in the configuration file that would apply are applied before the 103configuration is written to standard output. 104The connection parameters are supplied as keyword=value pairs and may be 105supplied in any order, either with multiple 106.Fl C 107options or as a comma-separated list. 108The keywords are 109.Dq addr , 110.Dq user , 111.Dq host , 112.Dq laddr , 113.Dq lport , 114and 115.Dq rdomain 116and correspond to source address, user, resolved source host name, 117local address, local port number and routing domain respectively. 118.It Fl c Ar host_certificate_file 119Specifies a path to a certificate file to identify 120.Nm 121during key exchange. 122The certificate file must match a host key file specified using the 123.Fl h 124option or the 125.Cm HostKey 126configuration directive. 127.It Fl D 128When this option is specified, 129.Nm 130will not detach and does not become a daemon. 131This allows easy monitoring of 132.Nm sshd . 133.It Fl d 134Debug mode. 135The server sends verbose debug output to standard error, 136and does not put itself in the background. 137The server also will not 138.Xr fork 2 139and will only process one connection. 140This option is only intended for debugging for the server. 141Multiple 142.Fl d 143options increase the debugging level. 144Maximum is 3. 145.It Fl E Ar log_file 146Append debug logs to 147.Ar log_file 148instead of the system log. 149.It Fl e 150Write debug logs to standard error instead of the system log. 151.It Fl f Ar config_file 152Specifies the name of the configuration file. 153The default is 154.Pa /etc/ssh/sshd_config . 155.Nm 156refuses to start if there is no configuration file. 157.It Fl g Ar login_grace_time 158Gives the grace time for clients to authenticate themselves (default 159120 seconds). 160If the client fails to authenticate the user within 161this many seconds, the server disconnects and exits. 162A value of zero indicates no limit. 163.It Fl h Ar host_key_file 164Specifies a file from which a host key is read. 165This option must be given if 166.Nm 167is not run as root (as the normal 168host key files are normally not readable by anyone but root). 169The default is 170.Pa /etc/ssh/ssh_host_ecdsa_key , 171.Pa /etc/ssh/ssh_host_ed25519_key 172and 173.Pa /etc/ssh/ssh_host_rsa_key . 174It is possible to have multiple host key files for 175the different host key algorithms. 176.It Fl i 177Specifies that 178.Nm 179is being run from 180.Xr inetd 8 . 181.It Fl o Ar option 182Can be used to give options in the format used in the configuration file. 183This is useful for specifying options for which there is no separate 184command-line flag. 185For full details of the options, and their values, see 186.Xr sshd_config 5 . 187.It Fl p Ar port 188Specifies the port on which the server listens for connections 189(default 22). 190Multiple port options are permitted. 191Ports specified in the configuration file with the 192.Cm Port 193option are ignored when a command-line port is specified. 194Ports specified using the 195.Cm ListenAddress 196option override command-line ports. 197.It Fl q 198Quiet mode. 199Nothing is sent to the system log. 200Normally the beginning, 201authentication, and termination of each connection is logged. 202.It Fl T 203Extended test mode. 204Check the validity of the configuration file, output the effective configuration 205to stdout and then exit. 206Optionally, 207.Cm Match 208rules may be applied by specifying the connection parameters using one or more 209.Fl C 210options. 211.It Fl t 212Test mode. 213Only check the validity of the configuration file and sanity of the keys. 214This is useful for updating 215.Nm 216reliably as configuration options may change. 217.It Fl u Ar len 218This option is used to specify the size of the field 219in the 220.Vt utmp 221structure that holds the remote host name. 222If the resolved host name is longer than 223.Ar len , 224the dotted decimal value will be used instead. 225This allows hosts with very long host names that 226overflow this field to still be uniquely identified. 227Specifying 228.Fl u0 229indicates that only dotted decimal addresses 230should be put into the 231.Pa utmp 232file. 233.Fl u0 234may also be used to prevent 235.Nm 236from making DNS requests unless the authentication 237mechanism or configuration requires it. 238Authentication mechanisms that may require DNS include 239.Cm HostbasedAuthentication 240and using a 241.Cm from="pattern-list" 242option in a key file. 243Configuration options that require DNS include using a 244USER@HOST pattern in 245.Cm AllowUsers 246or 247.Cm DenyUsers . 248.El 249.Sh AUTHENTICATION 250The OpenSSH SSH daemon supports SSH protocol 2 only. 251Each host has a host-specific key, 252used to identify the host. 253Whenever a client connects, the daemon responds with its public 254host key. 255The client compares the 256host key against its own database to verify that it has not changed. 257Forward secrecy is provided through a Diffie-Hellman key agreement. 258This key agreement results in a shared session key. 259The rest of the session is encrypted using a symmetric cipher. 260The client selects the encryption algorithm 261to use from those offered by the server. 262Additionally, session integrity is provided 263through a cryptographic message authentication code (MAC). 264.Pp 265Finally, the server and the client enter an authentication dialog. 266The client tries to authenticate itself using 267host-based authentication, 268public key authentication, 269challenge-response authentication, 270or password authentication. 271.Pp 272Regardless of the authentication type, the account is checked to 273ensure that it is accessible. An account is not accessible if it is 274locked, listed in 275.Cm DenyUsers 276or its group is listed in 277.Cm DenyGroups 278\&. The definition of a locked account is system dependent. Some platforms 279have their own account database (eg AIX) and some modify the passwd field ( 280.Ql \&*LK\&* 281on Solaris and UnixWare, 282.Ql \&* 283on HP-UX, containing 284.Ql Nologin 285on Tru64, 286a leading 287.Ql \&*LOCKED\&* 288on FreeBSD and a leading 289.Ql \&! 290on most Linuxes). 291If there is a requirement to disable password authentication 292for the account while allowing still public-key, then the passwd field 293should be set to something other than these values (eg 294.Ql NP 295or 296.Ql \&*NP\&* 297). 298.Pp 299If the client successfully authenticates itself, a dialog for 300preparing the session is entered. 301At this time the client may request 302things like allocating a pseudo-tty, forwarding X11 connections, 303forwarding TCP connections, or forwarding the authentication agent 304connection over the secure channel. 305.Pp 306After this, the client either requests an interactive shell or execution 307or a non-interactive command, which 308.Nm 309will execute via the user's shell using its 310.Fl c 311option. 312The sides then enter session mode. 313In this mode, either side may send 314data at any time, and such data is forwarded to/from the shell or 315command on the server side, and the user terminal in the client side. 316.Pp 317When the user program terminates and all forwarded X11 and other 318connections have been closed, the server sends command exit status to 319the client, and both sides exit. 320.Sh LOGIN PROCESS 321When a user successfully logs in, 322.Nm 323does the following: 324.Bl -enum -offset indent 325.It 326If the login is on a tty, and no command has been specified, 327prints last login time and 328.Pa /etc/motd 329(unless prevented in the configuration file or by 330.Pa ~/.hushlogin ; 331see the 332.Sx FILES 333section). 334.It 335If the login is on a tty, records login time. 336.It 337Checks 338.Pa /etc/nologin ; 339if it exists, prints contents and quits 340(unless root). 341.It 342Changes to run with normal user privileges. 343.It 344Sets up basic environment. 345.It 346Reads the file 347.Pa ~/.ssh/environment , 348if it exists, and users are allowed to change their environment. 349See the 350.Cm PermitUserEnvironment 351option in 352.Xr sshd_config 5 . 353.It 354Changes to user's home directory. 355.It 356If 357.Pa ~/.ssh/rc 358exists and the 359.Xr sshd_config 5 360.Cm PermitUserRC 361option is set, runs it; else if 362.Pa /etc/ssh/sshrc 363exists, runs 364it; otherwise runs 365.Xr xauth 1 . 366The 367.Dq rc 368files are given the X11 369authentication protocol and cookie in standard input. 370See 371.Sx SSHRC , 372below. 373.It 374Runs user's shell or command. 375All commands are run under the user's login shell as specified in the 376system password database. 377.El 378.Sh SSHRC 379If the file 380.Pa ~/.ssh/rc 381exists, 382.Xr sh 1 383runs it after reading the 384environment files but before starting the user's shell or command. 385It must not produce any output on stdout; stderr must be used 386instead. 387If X11 forwarding is in use, it will receive the "proto cookie" pair in 388its standard input (and 389.Ev DISPLAY 390in its environment). 391The script must call 392.Xr xauth 1 393because 394.Nm 395will not run xauth automatically to add X11 cookies. 396.Pp 397The primary purpose of this file is to run any initialization routines 398which may be needed before the user's home directory becomes 399accessible; AFS is a particular example of such an environment. 400.Pp 401This file will probably contain some initialization code followed by 402something similar to: 403.Bd -literal -offset 3n 404if read proto cookie && [ -n "$DISPLAY" ]; then 405 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 406 # X11UseLocalhost=yes 407 echo add unix:`echo $DISPLAY | 408 cut -c11-` $proto $cookie 409 else 410 # X11UseLocalhost=no 411 echo add $DISPLAY $proto $cookie 412 fi | xauth -q - 413fi 414.Ed 415.Pp 416If this file does not exist, 417.Pa /etc/ssh/sshrc 418is run, and if that 419does not exist either, xauth is used to add the cookie. 420.Sh AUTHORIZED_KEYS FILE FORMAT 421.Cm AuthorizedKeysFile 422specifies the files containing public keys for 423public key authentication; 424if this option is not specified, the default is 425.Pa ~/.ssh/authorized_keys 426and 427.Pa ~/.ssh/authorized_keys2 . 428Each line of the file contains one 429key (empty lines and lines starting with a 430.Ql # 431are ignored as 432comments). 433Public keys consist of the following space-separated fields: 434options, keytype, base64-encoded key, comment. 435The options field is optional. 436The supported key types are: 437.Pp 438.Bl -item -compact -offset indent 439.It 440sk-ecdsa-sha2-nistp256@openssh.com 441.It 442ecdsa-sha2-nistp256 443.It 444ecdsa-sha2-nistp384 445.It 446ecdsa-sha2-nistp521 447.It 448sk-ssh-ed25519@openssh.com 449.It 450ssh-ed25519 451.It 452ssh-dss 453.It 454ssh-rsa 455.El 456.Pp 457The comment field is not used for anything (but may be convenient for the 458user to identify the key). 459.Pp 460Note that lines in this file can be several hundred bytes long 461(because of the size of the public key encoding) up to a limit of 4628 kilobytes, which permits RSA keys up to 16 kilobits. 463You don't want to type them in; instead, copy the 464.Pa id_dsa.pub , 465.Pa id_ecdsa.pub , 466.Pa id_ecdsa_sk.pub , 467.Pa id_ed25519.pub , 468.Pa id_ed25519_sk.pub , 469or the 470.Pa id_rsa.pub 471file and edit it. 472.Pp 473.Nm 474enforces a minimum RSA key modulus size of 1024 bits. 475.Pp 476The options (if present) consist of comma-separated option 477specifications. 478No spaces are permitted, except within double quotes. 479The following option specifications are supported (note 480that option keywords are case-insensitive): 481.Bl -tag -width Ds 482.It Cm agent-forwarding 483Enable authentication agent forwarding previously disabled by the 484.Cm restrict 485option. 486.It Cm cert-authority 487Specifies that the listed key is a certification authority (CA) that is 488trusted to validate signed certificates for user authentication. 489.Pp 490Certificates may encode access restrictions similar to these key options. 491If both certificate restrictions and key options are present, the most 492restrictive union of the two is applied. 493.It Cm command="command" 494Specifies that the command is executed whenever this key is used for 495authentication. 496The command supplied by the user (if any) is ignored. 497The command is run on a pty if the client requests a pty; 498otherwise it is run without a tty. 499If an 8-bit clean channel is required, 500one must not request a pty or should specify 501.Cm no-pty . 502A quote may be included in the command by quoting it with a backslash. 503.Pp 504This option might be useful 505to restrict certain public keys to perform just a specific operation. 506An example might be a key that permits remote backups but nothing else. 507Note that the client may specify TCP and/or X11 508forwarding unless they are explicitly prohibited, e.g. using the 509.Cm restrict 510key option. 511.Pp 512The command originally supplied by the client is available in the 513.Ev SSH_ORIGINAL_COMMAND 514environment variable. 515Note that this option applies to shell, command or subsystem execution. 516Also note that this command may be superseded by a 517.Xr sshd_config 5 518.Cm ForceCommand 519directive. 520.Pp 521If a command is specified and a forced-command is embedded in a certificate 522used for authentication, then the certificate will be accepted only if the 523two commands are identical. 524.It Cm environment="NAME=value" 525Specifies that the string is to be added to the environment when 526logging in using this key. 527Environment variables set this way 528override other default environment values. 529Multiple options of this type are permitted. 530Environment processing is disabled by default and is 531controlled via the 532.Cm PermitUserEnvironment 533option. 534.It Cm expiry-time="timespec" 535Specifies a time after which the key will not be accepted. 536The time may be specified as a YYYYMMDD[Z] date or a YYYYMMDDHHMM[SS][Z] time. 537Dates and times will be interpreted in the system time zone unless suffixed 538by a Z character, in which case they will be interpreted in the UTC time zone. 539.It Cm from="pattern-list" 540Specifies that in addition to public key authentication, either the canonical 541name of the remote host or its IP address must be present in the 542comma-separated list of patterns. 543See PATTERNS in 544.Xr ssh_config 5 545for more information on patterns. 546.Pp 547In addition to the wildcard matching that may be applied to hostnames or 548addresses, a 549.Cm from 550stanza may match IP addresses using CIDR address/masklen notation. 551.Pp 552The purpose of this option is to optionally increase security: public key 553authentication by itself does not trust the network or name servers or 554anything (but the key); however, if somebody somehow steals the key, the key 555permits an intruder to log in from anywhere in the world. 556This additional option makes using a stolen key more difficult (name 557servers and/or routers would have to be compromised in addition to 558just the key). 559.It Cm no-agent-forwarding 560Forbids authentication agent forwarding when this key is used for 561authentication. 562.It Cm no-port-forwarding 563Forbids TCP forwarding when this key is used for authentication. 564Any port forward requests by the client will return an error. 565This might be used, e.g. in connection with the 566.Cm command 567option. 568.It Cm no-pty 569Prevents tty allocation (a request to allocate a pty will fail). 570.It Cm no-user-rc 571Disables execution of 572.Pa ~/.ssh/rc . 573.It Cm no-X11-forwarding 574Forbids X11 forwarding when this key is used for authentication. 575Any X11 forward requests by the client will return an error. 576.It Cm permitlisten="[host:]port" 577Limit remote port forwarding with the 578.Xr ssh 1 579.Fl R 580option such that it may only listen on the specified host (optional) and port. 581IPv6 addresses can be specified by enclosing the address in square brackets. 582Multiple 583.Cm permitlisten 584options may be applied separated by commas. 585Hostnames may include wildcards as described in the PATTERNS section in 586.Xr ssh_config 5 . 587A port specification of 588.Cm * 589matches any port. 590Note that the setting of 591.Cm GatewayPorts 592may further restrict listen addresses. 593Note that 594.Xr ssh 1 595will send a hostname of 596.Dq localhost 597if a listen host was not specified when the forwarding was requested, and 598that this name is treated differently to the explicit localhost addresses 599.Dq 127.0.0.1 600and 601.Dq ::1 . 602.It Cm permitopen="host:port" 603Limit local port forwarding with the 604.Xr ssh 1 605.Fl L 606option such that it may only connect to the specified host and port. 607IPv6 addresses can be specified by enclosing the address in square brackets. 608Multiple 609.Cm permitopen 610options may be applied separated by commas. 611No pattern matching or name lookup is performed on the 612specified hostnames, they must be literal host names and/or addresses. 613A port specification of 614.Cm * 615matches any port. 616.It Cm port-forwarding 617Enable port forwarding previously disabled by the 618.Cm restrict 619option. 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 pty 632Permits tty allocation previously disabled by the 633.Cm restrict 634option. 635.It Cm no-touch-required 636Do not require demonstration of user presence 637for signatures made using this key. 638This option only makes sense for the FIDO authenticator algorithms 639.Cm ecdsa-sk 640and 641.Cm ed25519-sk . 642.It Cm verify-required 643Require that signatures made using this key attest that they verified 644the user, e.g. via a PIN. 645This option only makes sense for the FIDO authenticator algorithms 646.Cm ecdsa-sk 647and 648.Cm ed25519-sk . 649.It Cm restrict 650Enable all restrictions, i.e. disable port, agent and X11 forwarding, 651as well as disabling PTY allocation 652and execution of 653.Pa ~/.ssh/rc . 654If any future restriction capabilities are added to authorized_keys files, 655they will be included in this set. 656.It Cm tunnel="n" 657Force a 658.Xr tun 4 659device on the server. 660Without this option, the next available device will be used if 661the client requests a tunnel. 662.It Cm user-rc 663Enables execution of 664.Pa ~/.ssh/rc 665previously disabled by the 666.Cm restrict 667option. 668.It Cm X11-forwarding 669Permits X11 forwarding previously disabled by the 670.Cm restrict 671option. 672.El 673.Pp 674An example authorized_keys file: 675.Bd -literal -offset 3n 676# Comments are allowed at start of line. Blank lines are allowed. 677# Plain key, no restrictions 678ssh-rsa ... 679# Forced command, disable PTY and all forwarding 680restrict,command="dump /home" ssh-rsa ... 681# Restriction of ssh -L forwarding destinations 682permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa ... 683# Restriction of ssh -R forwarding listeners 684permitlisten="localhost:8080",permitlisten="[::1]:22000" ssh-rsa ... 685# Configuration for tunnel forwarding 686tunnel="0",command="sh /etc/netstart tun0" ssh-rsa ... 687# Override of restriction to allow PTY allocation 688restrict,pty,command="nethack" ssh-rsa ... 689# Allow FIDO key without requiring touch 690no-touch-required sk-ecdsa-sha2-nistp256@openssh.com ... 691# Require user-verification (e.g. PIN or biometric) for FIDO key 692verify-required sk-ecdsa-sha2-nistp256@openssh.com ... 693# Trust CA key, allow touch-less FIDO if requested in certificate 694cert-authority,no-touch-required,principals="user_a" ssh-rsa ... 695.Ed 696.Sh SSH_KNOWN_HOSTS FILE FORMAT 697The 698.Pa /etc/ssh/ssh_known_hosts 699and 700.Pa ~/.ssh/known_hosts 701files contain host public keys for all known hosts. 702The global file should 703be prepared by the administrator (optional), and the per-user file is 704maintained automatically: whenever the user connects to an unknown host, 705its key is added to the per-user file. 706.Pp 707Each line in these files contains the following fields: marker (optional), 708hostnames, keytype, base64-encoded key, comment. 709The fields are separated by spaces. 710.Pp 711The marker is optional, but if it is present then it must be one of 712.Dq @cert-authority , 713to indicate that the line contains a certification authority (CA) key, 714or 715.Dq @revoked , 716to indicate that the key contained on the line is revoked and must not ever 717be accepted. 718Only one marker should be used on a key line. 719.Pp 720Hostnames is a comma-separated list of patterns 721.Pf ( Ql * 722and 723.Ql \&? 724act as 725wildcards); each pattern in turn is matched against the host name. 726When 727.Nm sshd 728is authenticating a client, such as when using 729.Cm HostbasedAuthentication , 730this will be the canonical client host name. 731When 732.Xr ssh 1 733is authenticating a server, this will be the host name 734given by the user, the value of the 735.Xr ssh 1 736.Cm HostkeyAlias 737if it was specified, or the canonical server hostname if the 738.Xr ssh 1 739.Cm CanonicalizeHostname 740option was used. 741.Pp 742A pattern may also be preceded by 743.Ql \&! 744to indicate negation: if the host name matches a negated 745pattern, it is not accepted (by that line) even if it matched another 746pattern on the line. 747A hostname or address may optionally be enclosed within 748.Ql \&[ 749and 750.Ql \&] 751brackets then followed by 752.Ql \&: 753and a non-standard port number. 754.Pp 755Alternately, hostnames may be stored in a hashed form which hides host names 756and addresses should the file's contents be disclosed. 757Hashed hostnames start with a 758.Ql | 759character. 760Only one hashed hostname may appear on a single line and none of the above 761negation or wildcard operators may be applied. 762.Pp 763The keytype and base64-encoded key are taken directly from the host key; they 764can be obtained, for example, from 765.Pa /etc/ssh/ssh_host_rsa_key.pub . 766The optional comment field continues to the end of the line, and is not used. 767.Pp 768Lines starting with 769.Ql # 770and empty lines are ignored as comments. 771.Pp 772When performing host authentication, authentication is accepted if any 773matching line has the proper key; either one that matches exactly or, 774if the server has presented a certificate for authentication, the key 775of the certification authority that signed the certificate. 776For a key to be trusted as a certification authority, it must use the 777.Dq @cert-authority 778marker described above. 779.Pp 780The known hosts file also provides a facility to mark keys as revoked, 781for example when it is known that the associated private key has been 782stolen. 783Revoked keys are specified by including the 784.Dq @revoked 785marker at the beginning of the key line, and are never accepted for 786authentication or as certification authorities, but instead will 787produce a warning from 788.Xr ssh 1 789when they are encountered. 790.Pp 791It is permissible (but not 792recommended) to have several lines or different host keys for the same 793names. 794This will inevitably happen when short forms of host names 795from different domains are put in the file. 796It is possible 797that the files contain conflicting information; authentication is 798accepted if valid information can be found from either file. 799.Pp 800Note that the lines in these files are typically hundreds of characters 801long, and you definitely don't want to type in the host keys by hand. 802Rather, generate them by a script, 803.Xr ssh-keyscan 1 804or by taking, for example, 805.Pa /etc/ssh/ssh_host_rsa_key.pub 806and adding the host names at the front. 807.Xr ssh-keygen 1 808also offers some basic automated editing for 809.Pa ~/.ssh/known_hosts 810including removing hosts matching a host name and converting all host 811names to their hashed representations. 812.Pp 813An example ssh_known_hosts file: 814.Bd -literal -offset 3n 815# Comments allowed at start of line 816cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 817# A hashed hostname 818|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 819AAAA1234.....= 820# A revoked key 821@revoked * ssh-rsa AAAAB5W... 822# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org 823@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... 824.Ed 825.Sh FILES 826.Bl -tag -width Ds -compact 827.It Pa ~/.hushlogin 828This file is used to suppress printing the last login time and 829.Pa /etc/motd , 830if 831.Cm PrintLastLog 832and 833.Cm PrintMotd , 834respectively, 835are enabled. 836It does not suppress printing of the banner specified by 837.Cm Banner . 838.Pp 839.It Pa ~/.rhosts 840This file is used for host-based authentication (see 841.Xr ssh 1 842for more information). 843On some machines this file may need to be 844world-readable if the user's home directory is on an NFS partition, 845because 846.Nm 847reads it as root. 848Additionally, this file must be owned by the user, 849and must not have write permissions for anyone else. 850The recommended 851permission for most machines is read/write for the user, and not 852accessible by others. 853.Pp 854.It Pa ~/.shosts 855This file is used in exactly the same way as 856.Pa .rhosts , 857but allows host-based authentication without permitting login with 858rlogin/rsh. 859.Pp 860.It Pa ~/.ssh/ 861This directory is the default location for all user-specific configuration 862and authentication information. 863There is no general requirement to keep the entire contents of this directory 864secret, but the recommended permissions are read/write/execute for the user, 865and not accessible by others. 866.Pp 867.It Pa ~/.ssh/authorized_keys 868Lists the public keys (DSA, ECDSA, Ed25519, RSA) 869that can be used for logging in as this user. 870The format of this file is described above. 871The content of the file is not highly sensitive, but the recommended 872permissions are read/write for the user, and not accessible by others. 873.Pp 874If this file, the 875.Pa ~/.ssh 876directory, or the user's home directory are writable 877by other users, then the file could be modified or replaced by unauthorized 878users. 879In this case, 880.Nm 881will not allow it to be used unless the 882.Cm StrictModes 883option has been set to 884.Dq no . 885.Pp 886.It Pa ~/.ssh/environment 887This file is read into the environment at login (if it exists). 888It can only contain empty lines, comment lines (that start with 889.Ql # ) , 890and assignment lines of the form name=value. 891The file should be writable 892only by the user; it need not be readable by anyone else. 893Environment processing is disabled by default and is 894controlled via the 895.Cm PermitUserEnvironment 896option. 897.Pp 898.It Pa ~/.ssh/known_hosts 899Contains a list of host keys for all hosts the user has logged into 900that are not already in the systemwide list of known host keys. 901The format of this file is described above. 902This file should be writable only by root/the owner and 903can, but need not be, world-readable. 904.Pp 905.It Pa ~/.ssh/rc 906Contains initialization routines to be run before 907the user's home directory becomes accessible. 908This file should be writable only by the user, and need not be 909readable by anyone else. 910.Pp 911.It Pa /etc/hosts.equiv 912This file is for host-based authentication (see 913.Xr ssh 1 ) . 914It should only be writable by root. 915.Pp 916.It Pa /etc/moduli 917Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange" 918key exchange method. 919The file format is described in 920.Xr moduli 5 . 921If no usable groups are found in this file then fixed internal groups will 922be used. 923.Pp 924.It Pa /etc/motd 925See 926.Xr motd 5 . 927.Pp 928.It Pa /etc/nologin 929If this file exists, 930.Nm 931refuses to let anyone except root log in. 932The contents of the file 933are displayed to anyone trying to log in, and non-root connections are 934refused. 935The file should be world-readable. 936.Pp 937.It Pa /etc/shosts.equiv 938This file is used in exactly the same way as 939.Pa hosts.equiv , 940but allows host-based authentication without permitting login with 941rlogin/rsh. 942.Pp 943.It Pa /etc/ssh/ssh_host_ecdsa_key 944.It Pa /etc/ssh/ssh_host_ed25519_key 945.It Pa /etc/ssh/ssh_host_rsa_key 946These files contain the private parts of the host keys. 947These files should only be owned by root, readable only by root, and not 948accessible to others. 949Note that 950.Nm 951does not start if these files are group/world-accessible. 952.Pp 953.It Pa /etc/ssh/ssh_host_ecdsa_key.pub 954.It Pa /etc/ssh/ssh_host_ed25519_key.pub 955.It Pa /etc/ssh/ssh_host_rsa_key.pub 956These files contain the public parts of the host keys. 957These files should be world-readable but writable only by 958root. 959Their contents should match the respective private parts. 960These files are not 961really used for anything; they are provided for the convenience of 962the user so their contents can be copied to known hosts files. 963These files are created using 964.Xr ssh-keygen 1 . 965.Pp 966.It Pa /etc/ssh/ssh_known_hosts 967Systemwide list of known host keys. 968This file should be prepared by the 969system administrator to contain the public host keys of all machines in the 970organization. 971The format of this file is described above. 972This file should be writable only by root/the owner and 973should be world-readable. 974.Pp 975.It Pa /etc/ssh/sshd_config 976Contains configuration data for 977.Nm sshd . 978The file format and configuration options are described in 979.Xr sshd_config 5 . 980.Pp 981.It Pa /etc/ssh/sshrc 982Similar to 983.Pa ~/.ssh/rc , 984it can be used to specify 985machine-specific login-time initializations globally. 986This file should be writable only by root, and should be world-readable. 987.Pp 988.It Pa /var/empty 989.Xr chroot 2 990directory used by 991.Nm 992during privilege separation in the pre-authentication phase. 993The directory should not contain any files and must be owned by root 994and not group or world-writable. 995.Pp 996.It Pa /var/run/sshd.pid 997Contains the process ID of the 998.Nm 999listening for connections (if there are several daemons running 1000concurrently for different ports, this contains the process ID of the one 1001started last). 1002The content of this file is not sensitive; it can be world-readable. 1003.El 1004.Sh SEE ALSO 1005.Xr scp 1 , 1006.Xr sftp 1 , 1007.Xr ssh 1 , 1008.Xr ssh-add 1 , 1009.Xr ssh-agent 1 , 1010.Xr ssh-keygen 1 , 1011.Xr ssh-keyscan 1 , 1012.Xr chroot 2 , 1013.Xr login.conf 5 , 1014.Xr moduli 5 , 1015.Xr sshd_config 5 , 1016.Xr inetd 8 , 1017.Xr sftp-server 8 1018.Sh AUTHORS 1019OpenSSH is a derivative of the original and free 1020ssh 1.2.12 release by Tatu Ylonen. 1021Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1022Theo de Raadt and Dug Song 1023removed many bugs, re-added newer features and 1024created OpenSSH. 1025Markus Friedl contributed the support for SSH 1026protocol versions 1.5 and 2.0. 1027Niels Provos and Markus Friedl contributed support 1028for privilege separation. 1029