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 1.318 2022/03/31 17:27:27 naddy Exp $ 37.Dd $Mdocdate: March 31 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.d/sshd . 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.Li 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 and 339.Pa /var/run/nologin ; 340if one exists, it prints the contents and quits 341(unless root). 342.It 343Changes to run with normal user privileges. 344.It 345Sets up basic environment. 346.It 347Reads the file 348.Pa ~/.ssh/environment , 349if it exists, and users are allowed to change their environment. 350See the 351.Cm PermitUserEnvironment 352option in 353.Xr sshd_config 5 . 354.It 355Changes to user's home directory. 356.It 357If 358.Pa ~/.ssh/rc 359exists and the 360.Xr sshd_config 5 361.Cm PermitUserRC 362option is set, runs it; else if 363.Pa /etc/ssh/sshrc 364exists, runs 365it; otherwise runs 366.Xr xauth 1 . 367The 368.Dq rc 369files are given the X11 370authentication protocol and cookie in standard input. 371See 372.Sx SSHRC , 373below. 374.It 375Runs user's shell or command. 376All commands are run under the user's login shell as specified in the 377system password database. 378.El 379.Sh SSHRC 380If the file 381.Pa ~/.ssh/rc 382exists, 383.Xr sh 1 384runs it after reading the 385environment files but before starting the user's shell or command. 386It must not produce any output on stdout; stderr must be used 387instead. 388If X11 forwarding is in use, it will receive the "proto cookie" pair in 389its standard input (and 390.Ev DISPLAY 391in its environment). 392The script must call 393.Xr xauth 1 394because 395.Nm 396will not run xauth automatically to add X11 cookies. 397.Pp 398The primary purpose of this file is to run any initialization routines 399which may be needed before the user's home directory becomes 400accessible; AFS is a particular example of such an environment. 401.Pp 402This file will probably contain some initialization code followed by 403something similar to: 404.Bd -literal -offset 3n 405if read proto cookie && [ -n "$DISPLAY" ]; then 406 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 407 # X11UseLocalhost=yes 408 echo add unix:`echo $DISPLAY | 409 cut -c11-` $proto $cookie 410 else 411 # X11UseLocalhost=no 412 echo add $DISPLAY $proto $cookie 413 fi | xauth -q - 414fi 415.Ed 416.Pp 417If this file does not exist, 418.Pa /etc/ssh/sshrc 419is run, and if that 420does not exist either, xauth is used to add the cookie. 421.Sh AUTHORIZED_KEYS FILE FORMAT 422.Cm AuthorizedKeysFile 423specifies the files containing public keys for 424public key authentication; 425if this option is not specified, the default is 426.Pa ~/.ssh/authorized_keys 427and 428.Pa ~/.ssh/authorized_keys2 . 429Each line of the file contains one 430key (empty lines and lines starting with a 431.Ql # 432are ignored as 433comments). 434Public keys consist of the following space-separated fields: 435options, keytype, base64-encoded key, comment. 436The options field is optional. 437The supported key types are: 438.Pp 439.Bl -item -compact -offset indent 440.It 441sk-ecdsa-sha2-nistp256@openssh.com 442.It 443ecdsa-sha2-nistp256 444.It 445ecdsa-sha2-nistp384 446.It 447ecdsa-sha2-nistp521 448.It 449sk-ssh-ed25519@openssh.com 450.It 451ssh-ed25519 452.It 453ssh-dss 454.It 455ssh-rsa 456.El 457.Pp 458The comment field is not used for anything (but may be convenient for the 459user to identify the key). 460.Pp 461Note that lines in this file can be several hundred bytes long 462(because of the size of the public key encoding) up to a limit of 4638 kilobytes, which permits RSA keys up to 16 kilobits. 464You don't want to type them in; instead, copy the 465.Pa id_dsa.pub , 466.Pa id_ecdsa.pub , 467.Pa id_ecdsa_sk.pub , 468.Pa id_ed25519.pub , 469.Pa id_ed25519_sk.pub , 470or the 471.Pa id_rsa.pub 472file and edit it. 473.Pp 474.Nm 475enforces a minimum RSA key modulus size of 1024 bits. 476.Pp 477The options (if present) consist of comma-separated option 478specifications. 479No spaces are permitted, except within double quotes. 480The following option specifications are supported (note 481that option keywords are case-insensitive): 482.Bl -tag -width Ds 483.It Cm agent-forwarding 484Enable authentication agent forwarding previously disabled by the 485.Cm restrict 486option. 487.It Cm cert-authority 488Specifies that the listed key is a certification authority (CA) that is 489trusted to validate signed certificates for user authentication. 490.Pp 491Certificates may encode access restrictions similar to these key options. 492If both certificate restrictions and key options are present, the most 493restrictive union of the two is applied. 494.It Cm command="command" 495Specifies that the command is executed whenever this key is used for 496authentication. 497The command supplied by the user (if any) is ignored. 498The command is run on a pty if the client requests a pty; 499otherwise it is run without a tty. 500If an 8-bit clean channel is required, 501one must not request a pty or should specify 502.Cm no-pty . 503A quote may be included in the command by quoting it with a backslash. 504.Pp 505This option might be useful 506to restrict certain public keys to perform just a specific operation. 507An example might be a key that permits remote backups but nothing else. 508Note that the client may specify TCP and/or X11 509forwarding unless they are explicitly prohibited, e.g. using the 510.Cm restrict 511key option. 512.Pp 513The command originally supplied by the client is available in the 514.Ev SSH_ORIGINAL_COMMAND 515environment variable. 516Note that this option applies to shell, command or subsystem execution. 517Also note that this command may be superseded by a 518.Xr sshd_config 5 519.Cm ForceCommand 520directive. 521.Pp 522If a command is specified and a forced-command is embedded in a certificate 523used for authentication, then the certificate will be accepted only if the 524two commands are identical. 525.It Cm environment="NAME=value" 526Specifies that the string is to be added to the environment when 527logging in using this key. 528Environment variables set this way 529override other default environment values. 530Multiple options of this type are permitted. 531Environment processing is disabled by default and is 532controlled via the 533.Cm PermitUserEnvironment 534option. 535.It Cm expiry-time="timespec" 536Specifies a time after which the key will not be accepted. 537The time may be specified as a YYYYMMDD date or a YYYYMMDDHHMM[SS] time 538in the system 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 816closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 817cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 818# A hashed hostname 819|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 820AAAA1234.....= 821# A revoked key 822@revoked * ssh-rsa AAAAB5W... 823# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org 824@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... 825.Ed 826.Sh FILES 827.Bl -tag -width Ds -compact 828.It Pa ~/.hushlogin 829This file is used to suppress printing the last login time and 830.Pa /etc/motd , 831if 832.Cm PrintLastLog 833and 834.Cm PrintMotd , 835respectively, 836are enabled. 837It does not suppress printing of the banner specified by 838.Cm Banner . 839.Pp 840.It Pa ~/.rhosts 841This file is used for host-based authentication (see 842.Xr ssh 1 843for more information). 844On some machines this file may need to be 845world-readable if the user's home directory is on an NFS partition, 846because 847.Nm 848reads it as root. 849Additionally, this file must be owned by the user, 850and must not have write permissions for anyone else. 851The recommended 852permission for most machines is read/write for the user, and not 853accessible by others. 854.Pp 855.It Pa ~/.shosts 856This file is used in exactly the same way as 857.Pa .rhosts , 858but allows host-based authentication without permitting login with 859rlogin/rsh. 860.Pp 861.It Pa ~/.ssh/ 862This directory is the default location for all user-specific configuration 863and authentication information. 864There is no general requirement to keep the entire contents of this directory 865secret, but the recommended permissions are read/write/execute for the user, 866and not accessible by others. 867.Pp 868.It Pa ~/.ssh/authorized_keys 869Lists the public keys (DSA, ECDSA, Ed25519, RSA) 870that can be used for logging in as this user. 871The format of this file is described above. 872The content of the file is not highly sensitive, but the recommended 873permissions are read/write for the user, and not accessible by others. 874.Pp 875If this file, the 876.Pa ~/.ssh 877directory, or the user's home directory are writable 878by other users, then the file could be modified or replaced by unauthorized 879users. 880In this case, 881.Nm 882will not allow it to be used unless the 883.Cm StrictModes 884option has been set to 885.Dq no . 886.Pp 887.It Pa ~/.ssh/environment 888This file is read into the environment at login (if it exists). 889It can only contain empty lines, comment lines (that start with 890.Ql # ) , 891and assignment lines of the form name=value. 892The file should be writable 893only by the user; it need not be readable by anyone else. 894Environment processing is disabled by default and is 895controlled via the 896.Cm PermitUserEnvironment 897option. 898.Pp 899.It Pa ~/.ssh/known_hosts 900Contains a list of host keys for all hosts the user has logged into 901that are not already in the systemwide list of known host keys. 902The format of this file is described above. 903This file should be writable only by root/the owner and 904can, but need not be, world-readable. 905.Pp 906.It Pa ~/.ssh/rc 907Contains initialization routines to be run before 908the user's home directory becomes accessible. 909This file should be writable only by the user, and need not be 910readable by anyone else. 911.Pp 912.It Pa /etc/hosts.allow 913.It Pa /etc/hosts.deny 914Access controls that should be enforced by tcp-wrappers are defined here. 915Further details are described in 916.Xr hosts_access 5 . 917.Pp 918.It Pa /etc/hosts.equiv 919This file is for host-based authentication (see 920.Xr ssh 1 ) . 921It should only be writable by root. 922.Pp 923.It Pa /etc/moduli 924Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange" 925key exchange method. 926The file format is described in 927.Xr moduli 5 . 928If no usable groups are found in this file then fixed internal groups will 929be used. 930.Pp 931.It Pa /etc/motd 932See 933.Xr motd 5 . 934.Pp 935.It Pa /etc/nologin 936If this file exists, 937.Nm 938refuses to let anyone except root log in. 939The contents of the file 940are displayed to anyone trying to log in, and non-root connections are 941refused. 942The file should be world-readable. 943.Pp 944.It Pa /etc/shosts.equiv 945This file is used in exactly the same way as 946.Pa hosts.equiv , 947but allows host-based authentication without permitting login with 948rlogin/rsh. 949.Pp 950.It Pa /etc/ssh/ssh_host_ecdsa_key 951.It Pa /etc/ssh/ssh_host_ed25519_key 952.It Pa /etc/ssh/ssh_host_rsa_key 953These files contain the private parts of the host keys. 954These files should only be owned by root, readable only by root, and not 955accessible to others. 956Note that 957.Nm 958does not start if these files are group/world-accessible. 959.Pp 960.It Pa /etc/ssh/ssh_host_ecdsa_key.pub 961.It Pa /etc/ssh/ssh_host_ed25519_key.pub 962.It Pa /etc/ssh/ssh_host_rsa_key.pub 963These files contain the public parts of the host keys. 964These files should be world-readable but writable only by 965root. 966Their contents should match the respective private parts. 967These files are not 968really used for anything; they are provided for the convenience of 969the user so their contents can be copied to known hosts files. 970These files are created using 971.Xr ssh-keygen 1 . 972.Pp 973.It Pa /etc/ssh/ssh_known_hosts 974Systemwide list of known host keys. 975This file should be prepared by the 976system administrator to contain the public host keys of all machines in the 977organization. 978The format of this file is described above. 979This file should be writable only by root/the owner and 980should be world-readable. 981.Pp 982.It Pa /etc/ssh/sshd_config 983Contains configuration data for 984.Nm sshd . 985The file format and configuration options are described in 986.Xr sshd_config 5 . 987.Pp 988.It Pa /etc/ssh/sshrc 989Similar to 990.Pa ~/.ssh/rc , 991it can be used to specify 992machine-specific login-time initializations globally. 993This file should be writable only by root, and should be world-readable. 994.Pp 995.It Pa /var/empty 996.Xr chroot 2 997directory used by 998.Nm 999during privilege separation in the pre-authentication phase. 1000The directory should not contain any files and must be owned by root 1001and not group or world-writable. 1002.Pp 1003.It Pa /var/run/sshd.pid 1004Contains the process ID of the 1005.Nm 1006listening for connections (if there are several daemons running 1007concurrently for different ports, this contains the process ID of the one 1008started last). 1009The content of this file is not sensitive; it can be world-readable. 1010.El 1011.Sh SEE ALSO 1012.Xr scp 1 , 1013.Xr sftp 1 , 1014.Xr ssh 1 , 1015.Xr ssh-add 1 , 1016.Xr ssh-agent 1 , 1017.Xr ssh-keygen 1 , 1018.Xr ssh-keyscan 1 , 1019.Xr chroot 2 , 1020.Xr hosts_access 5 , 1021.Xr login.conf 5 , 1022.Xr moduli 5 , 1023.Xr sshd_config 5 , 1024.Xr inetd 8 , 1025.Xr sftp-server 8 1026.Sh AUTHORS 1027OpenSSH is a derivative of the original and free 1028ssh 1.2.12 release by Tatu Ylonen. 1029Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1030Theo de Raadt and Dug Song 1031removed many bugs, re-added newer features and 1032created OpenSSH. 1033Markus Friedl contributed the support for SSH 1034protocol versions 1.5 and 2.0. 1035Niels Provos and Markus Friedl contributed support 1036for privilege separation. 1037