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 1.263 2011/08/02 01:22:11 djm Exp $ 37.Dd $Mdocdate: August 2 2011 $ 38.Dt SSHD 8 39.Os 40.Sh NAME 41.Nm sshd 42.Nd OpenSSH SSH daemon 43.Sh SYNOPSIS 44.Nm sshd 45.Bk -words 46.Op Fl 46DdeiqTt 47.Op Fl b Ar bits 48.Op Fl C Ar connection_spec 49.Op Fl c Ar host_certificate_file 50.Op Fl f Ar config_file 51.Op Fl g Ar login_grace_time 52.Op Fl h Ar host_key_file 53.Op Fl k Ar key_gen_time 54.Op Fl o Ar option 55.Op Fl p Ar port 56.Op Fl u Ar len 57.Ek 58.Sh DESCRIPTION 59.Nm 60(OpenSSH Daemon) is the daemon program for 61.Xr ssh 1 . 62Together these programs replace 63.Xr rlogin 1 64and 65.Xr rsh 1 , 66and provide secure encrypted communications between two untrusted hosts 67over an insecure network. 68.Pp 69.Nm 70listens for connections from clients. 71It is normally started at boot from 72.Pa /etc/rc.d/sshd . 73It forks a new 74daemon for each incoming connection. 75The forked daemons handle 76key exchange, encryption, authentication, command execution, 77and data exchange. 78.Pp 79.Nm 80can be configured using command-line options or a configuration file 81(by default 82.Xr sshd_config 5 ) ; 83command-line options override values specified in the 84configuration file. 85.Nm 86rereads its configuration file when it receives a hangup signal, 87.Dv SIGHUP , 88by executing itself with the name and options it was started with, e.g.\& 89.Pa /usr/sbin/sshd . 90.Pp 91The options are as follows: 92.Bl -tag -width Ds 93.It Fl 4 94Forces 95.Nm 96to use IPv4 addresses only. 97.It Fl 6 98Forces 99.Nm 100to use IPv6 addresses only. 101.It Fl b Ar bits 102Specifies the number of bits in the ephemeral protocol version 1 103server key (default 1024). 104.It Fl C Ar connection_spec 105Specify the connection parameters to use for the 106.Fl T 107extended test mode. 108If provided, any 109.Cm Match 110directives in the configuration file 111that would apply to the specified user, host, and address will be set before 112the configuration is written to standard output. 113The connection parameters are supplied as keyword=value pairs. 114The keywords are 115.Dq user , 116.Dq host , 117and 118.Dq addr . 119All are required and may be supplied in any order, either with multiple 120.Fl C 121options or as a comma-separated list. 122.It Fl c Ar host_certificate_file 123Specifies a path to a certificate file to identify 124.Nm 125during key exchange. 126The certificate file must match a host key file specified using the 127.Fl h 128option or the 129.Cm HostKey 130configuration directive. 131.It Fl D 132When this option is specified, 133.Nm 134will not detach and does not become a daemon. 135This allows easy monitoring of 136.Nm sshd . 137.It Fl d 138Debug mode. 139The server sends verbose debug output to standard error, 140and does not put itself in the background. 141The server also will not fork and will only process one connection. 142This option is only intended for debugging for the server. 143Multiple 144.Fl d 145options increase the debugging level. 146Maximum is 3. 147.It Fl e 148When this option is specified, 149.Nm 150will send the output to the 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_key 171for protocol version 1, and 172.Pa /etc/ssh/ssh_host_dsa_key , 173.Pa /etc/ssh/ssh_host_ecdsa_key 174and 175.Pa /etc/ssh/ssh_host_rsa_key 176for protocol version 2. 177It is possible to have multiple host key files for 178the different protocol versions and host key algorithms. 179.It Fl i 180Specifies that 181.Nm 182is being run from 183.Xr inetd 8 . 184.Nm 185is normally not run 186from inetd because it needs to generate the server key before it can 187respond to the client, and this may take tens of seconds. 188Clients would have to wait too long if the key was regenerated every time. 189However, with small key sizes (e.g. 512) using 190.Nm 191from inetd may 192be feasible. 193.It Fl k Ar key_gen_time 194Specifies how often the ephemeral protocol version 1 server key is 195regenerated (default 3600 seconds, or one hour). 196The motivation for regenerating the key fairly 197often is that the key is not stored anywhere, and after about an hour 198it becomes impossible to recover the key for decrypting intercepted 199communications even if the machine is cracked into or physically 200seized. 201A value of zero indicates that the key will never be regenerated. 202.It Fl o Ar option 203Can be used to give options in the format used in the configuration file. 204This is useful for specifying options for which there is no separate 205command-line flag. 206For full details of the options, and their values, see 207.Xr sshd_config 5 . 208.It Fl p Ar port 209Specifies the port on which the server listens for connections 210(default 22). 211Multiple port options are permitted. 212Ports specified in the configuration file with the 213.Cm Port 214option are ignored when a command-line port is specified. 215Ports specified using the 216.Cm ListenAddress 217option override command-line ports. 218.It Fl q 219Quiet mode. 220Nothing is sent to the system log. 221Normally the beginning, 222authentication, and termination of each connection is logged. 223.It Fl T 224Extended test mode. 225Check the validity of the configuration file, output the effective configuration 226to stdout and then exit. 227Optionally, 228.Cm Match 229rules may be applied by specifying the connection parameters using one or more 230.Fl C 231options. 232.It Fl t 233Test mode. 234Only check the validity of the configuration file and sanity of the keys. 235This is useful for updating 236.Nm 237reliably as configuration options may change. 238.It Fl u Ar len 239This option is used to specify the size of the field 240in the 241.Li utmp 242structure that holds the remote host name. 243If the resolved host name is longer than 244.Ar len , 245the dotted decimal value will be used instead. 246This allows hosts with very long host names that 247overflow this field to still be uniquely identified. 248Specifying 249.Fl u0 250indicates that only dotted decimal addresses 251should be put into the 252.Pa utmp 253file. 254.Fl u0 255may also be used to prevent 256.Nm 257from making DNS requests unless the authentication 258mechanism or configuration requires it. 259Authentication mechanisms that may require DNS include 260.Cm RhostsRSAAuthentication , 261.Cm HostbasedAuthentication , 262and using a 263.Cm from="pattern-list" 264option in a key file. 265Configuration options that require DNS include using a 266USER@HOST pattern in 267.Cm AllowUsers 268or 269.Cm DenyUsers . 270.El 271.Sh AUTHENTICATION 272The OpenSSH SSH daemon supports SSH protocols 1 and 2. 273The default is to use protocol 2 only, 274though this can be changed via the 275.Cm Protocol 276option in 277.Xr sshd_config 5 . 278Protocol 2 supports DSA, ECDSA and RSA keys; 279protocol 1 only supports RSA keys. 280For both protocols, 281each host has a host-specific key, 282normally 2048 bits, 283used to identify the host. 284.Pp 285Forward security for protocol 1 is provided through 286an additional server key, 287normally 768 bits, 288generated when the server starts. 289This key is normally regenerated every hour if it has been used, and 290is never stored on disk. 291Whenever a client connects, the daemon responds with its public 292host and server keys. 293The client compares the 294RSA host key against its own database to verify that it has not changed. 295The client then generates a 256-bit random number. 296It encrypts this 297random number using both the host key and the server key, and sends 298the encrypted number to the server. 299Both sides then use this 300random number as a session key which is used to encrypt all further 301communications in the session. 302The rest of the session is encrypted 303using a conventional cipher, currently Blowfish or 3DES, with 3DES 304being used by default. 305The client selects the encryption algorithm 306to use from those offered by the server. 307.Pp 308For protocol 2, 309forward security is provided through a Diffie-Hellman key agreement. 310This key agreement results in a shared session key. 311The rest of the session is encrypted using a symmetric cipher, currently 312128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. 313The client selects the encryption algorithm 314to use from those offered by the server. 315Additionally, session integrity is provided 316through a cryptographic message authentication code 317(hmac-md5, hmac-sha1, umac-64, hmac-ripemd160, 318hmac-sha2-256 or hmac-sha2-512). 319.Pp 320Finally, the server and the client enter an authentication dialog. 321The client tries to authenticate itself using 322host-based authentication, 323public key authentication, 324challenge-response authentication, 325or password authentication. 326.Pp 327Regardless of the authentication type, the account is checked to 328ensure that it is accessible. An account is not accessible if it is 329locked, listed in 330.Cm DenyUsers 331or its group is listed in 332.Cm DenyGroups 333\&. The definition of a locked account is system dependant. Some platforms 334have their own account database (eg AIX) and some modify the passwd field ( 335.Ql \&*LK\&* 336on Solaris and UnixWare, 337.Ql \&* 338on HP-UX, containing 339.Ql Nologin 340on Tru64, 341a leading 342.Ql \&*LOCKED\&* 343on FreeBSD and a leading 344.Ql \&! 345on most Linuxes). 346If there is a requirement to disable password authentication 347for the account while allowing still public-key, then the passwd field 348should be set to something other than these values (eg 349.Ql NP 350or 351.Ql \&*NP\&* 352). 353.Pp 354If the client successfully authenticates itself, a dialog for 355preparing the session is entered. 356At this time the client may request 357things like allocating a pseudo-tty, forwarding X11 connections, 358forwarding TCP connections, or forwarding the authentication agent 359connection over the secure channel. 360.Pp 361After this, the client either requests a shell or execution of a command. 362The sides then enter session mode. 363In this mode, either side may send 364data at any time, and such data is forwarded to/from the shell or 365command on the server side, and the user terminal in the client side. 366.Pp 367When the user program terminates and all forwarded X11 and other 368connections have been closed, the server sends command exit status to 369the client, and both sides exit. 370.Sh LOGIN PROCESS 371When a user successfully logs in, 372.Nm 373does the following: 374.Bl -enum -offset indent 375.It 376If the login is on a tty, and no command has been specified, 377prints last login time and 378.Pa /etc/motd 379(unless prevented in the configuration file or by 380.Pa ~/.hushlogin ; 381see the 382.Sx FILES 383section). 384.It 385If the login is on a tty, records login time. 386.It 387Checks 388.Pa /etc/nologin and 389.Pa /var/run/nologin ; 390if one exists, it prints the contents and quits 391(unless root). 392.It 393Changes to run with normal user privileges. 394.It 395Sets up basic environment. 396.It 397Reads the file 398.Pa ~/.ssh/environment , 399if it exists, and users are allowed to change their environment. 400See the 401.Cm PermitUserEnvironment 402option in 403.Xr sshd_config 5 . 404.It 405Changes to user's home directory. 406.It 407If 408.Pa ~/.ssh/rc 409exists, runs it; else if 410.Pa /etc/ssh/sshrc 411exists, runs 412it; otherwise runs 413.Xr xauth 1 . 414The 415.Dq rc 416files are given the X11 417authentication protocol and cookie (if applicable) in standard input. 418See 419.Sx SSHRC , 420below. 421.It 422Runs user's shell or command. 423.El 424.Sh SSHRC 425If the file 426.Pa ~/.ssh/rc 427exists, 428.Xr sh 1 429runs it after reading the 430environment files but before starting the user's shell or command. 431It must not produce any output on stdout; stderr must be used 432instead. 433If X11 forwarding is in use, it will receive the "proto cookie" pair in 434its standard input (and 435.Ev DISPLAY 436in its environment). 437The script must call 438.Xr xauth 1 439because 440.Nm 441will not run xauth automatically to add X11 cookies. 442.Pp 443The primary purpose of this file is to run any initialization routines 444which may be needed before the user's home directory becomes 445accessible; AFS is a particular example of such an environment. 446.Pp 447This file will probably contain some initialization code followed by 448something similar to: 449.Bd -literal -offset 3n 450if read proto cookie && [ -n "$DISPLAY" ]; then 451 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 452 # X11UseLocalhost=yes 453 echo add unix:`echo $DISPLAY | 454 cut -c11-` $proto $cookie 455 else 456 # X11UseLocalhost=no 457 echo add $DISPLAY $proto $cookie 458 fi | xauth -q - 459fi 460.Ed 461.Pp 462If this file does not exist, 463.Pa /etc/ssh/sshrc 464is run, and if that 465does not exist either, xauth is used to add the cookie. 466.Sh AUTHORIZED_KEYS FILE FORMAT 467.Cm AuthorizedKeysFile 468specifies the files containing public keys for 469public key authentication; 470if none is specified, the default is 471.Pa ~/.ssh/authorized_keys 472and 473.Pa ~/.ssh/authorized_keys2 . 474Each line of the file contains one 475key (empty lines and lines starting with a 476.Ql # 477are ignored as 478comments). 479Protocol 1 public keys consist of the following space-separated fields: 480options, bits, exponent, modulus, comment. 481Protocol 2 public key consist of: 482options, keytype, base64-encoded key, comment. 483The options field is optional; 484its presence is determined by whether the line starts 485with a number or not (the options field never starts with a number). 486The bits, exponent, modulus, and comment fields give the RSA key for 487protocol version 1; the 488comment field is not used for anything (but may be convenient for the 489user to identify the key). 490For protocol version 2 the keytype is 491.Dq ecdsa-sha2-nistp256 , 492.Dq ecdsa-sha2-nistp384 , 493.Dq ecdsa-sha2-nistp521 , 494.Dq ssh-dss 495or 496.Dq ssh-rsa . 497.Pp 498Note that lines in this file are usually several hundred bytes long 499(because of the size of the public key encoding) up to a limit of 5008 kilobytes, which permits DSA keys up to 8 kilobits and RSA 501keys up to 16 kilobits. 502You don't want to type them in; instead, copy the 503.Pa identity.pub , 504.Pa id_dsa.pub , 505.Pa id_ecdsa.pub , 506or the 507.Pa id_rsa.pub 508file and edit it. 509.Pp 510.Nm 511enforces a minimum RSA key modulus size for protocol 1 512and protocol 2 keys of 768 bits. 513.Pp 514The options (if present) consist of comma-separated option 515specifications. 516No spaces are permitted, except within double quotes. 517The following option specifications are supported (note 518that option keywords are case-insensitive): 519.Bl -tag -width Ds 520.It Cm cert-authority 521Specifies that the listed key is a certification authority (CA) that is 522trusted to validate signed certificates for user authentication. 523.Pp 524Certificates may encode access restrictions similar to these key options. 525If both certificate restrictions and key options are present, the most 526restrictive union of the two is applied. 527.It Cm command="command" 528Specifies that the command is executed whenever this key is used for 529authentication. 530The command supplied by the user (if any) is ignored. 531The command is run on a pty if the client requests a pty; 532otherwise it is run without a tty. 533If an 8-bit clean channel is required, 534one must not request a pty or should specify 535.Cm no-pty . 536A quote may be included in the command by quoting it with a backslash. 537This option might be useful 538to restrict certain public keys to perform just a specific operation. 539An example might be a key that permits remote backups but nothing else. 540Note that the client may specify TCP and/or X11 541forwarding unless they are explicitly prohibited. 542The command originally supplied by the client is available in the 543.Ev SSH_ORIGINAL_COMMAND 544environment variable. 545Note that this option applies to shell, command or subsystem execution. 546Also note that this command may be superseded by either a 547.Xr sshd_config 5 548.Cm ForceCommand 549directive or a command embedded in a certificate. 550.It Cm environment="NAME=value" 551Specifies that the string is to be added to the environment when 552logging in using this key. 553Environment variables set this way 554override other default environment values. 555Multiple options of this type are permitted. 556Environment processing is disabled by default and is 557controlled via the 558.Cm PermitUserEnvironment 559option. 560This option is automatically disabled if 561.Cm UseLogin 562is enabled. 563.It Cm from="pattern-list" 564Specifies that in addition to public key authentication, either the canonical 565name of the remote host or its IP address must be present in the 566comma-separated list of patterns. 567See 568.Sx PATTERNS 569in 570.Xr ssh_config 5 571for more information on patterns. 572.Pp 573In addition to the wildcard matching that may be applied to hostnames or 574addresses, a 575.Cm from 576stanza may match IP addresses using CIDR address/masklen notation. 577.Pp 578The purpose of this option is to optionally increase security: public key 579authentication by itself does not trust the network or name servers or 580anything (but the key); however, if somebody somehow steals the key, the key 581permits an intruder to log in from anywhere in the world. 582This additional option makes using a stolen key more difficult (name 583servers and/or routers would have to be compromised in addition to 584just the key). 585.It Cm no-agent-forwarding 586Forbids authentication agent forwarding when this key is used for 587authentication. 588.It Cm no-port-forwarding 589Forbids TCP forwarding when this key is used for authentication. 590Any port forward requests by the client will return an error. 591This might be used, e.g. in connection with the 592.Cm command 593option. 594.It Cm no-pty 595Prevents tty allocation (a request to allocate a pty will fail). 596.It Cm no-user-rc 597Disables execution of 598.Pa ~/.ssh/rc . 599.It Cm no-X11-forwarding 600Forbids X11 forwarding when this key is used for authentication. 601Any X11 forward requests by the client will return an error. 602.It Cm permitopen="host:port" 603Limit local 604.Li ``ssh -L'' 605port forwarding such that it may only connect to the specified host and 606port. 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 is performed on the specified hostnames, 612they must be literal domains or addresses. 613.It Cm principals="principals" 614On a 615.Cm cert-authority 616line, specifies allowed principals for certificate authentication as a 617comma-separated list. 618At least one name from the list must appear in the certificate's 619list of principals for the certificate to be accepted. 620This option is ignored for keys that are not marked as trusted certificate 621signers using the 622.Cm cert-authority 623option. 624.It Cm tunnel="n" 625Force a 626.Xr tun 4 627device on the server. 628Without this option, the next available device will be used if 629the client requests a tunnel. 630.El 631.Pp 632An example authorized_keys file: 633.Bd -literal -offset 3n 634# Comments allowed at start of line 635ssh-rsa AAAAB3Nza...LiPk== user@example.net 636from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 637AAAAB2...19Q== john@example.net 638command="dump /home",no-pty,no-port-forwarding ssh-dss 639AAAAC3...51R== example.net 640permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 641AAAAB5...21S== 642tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 643jane@example.net 644.Ed 645.Sh SSH_KNOWN_HOSTS FILE FORMAT 646The 647.Pa /etc/ssh/ssh_known_hosts 648and 649.Pa ~/.ssh/known_hosts 650files contain host public keys for all known hosts. 651The global file should 652be prepared by the administrator (optional), and the per-user file is 653maintained automatically: whenever the user connects from an unknown host, 654its key is added to the per-user file. 655.Pp 656Each line in these files contains the following fields: markers (optional), 657hostnames, bits, exponent, modulus, comment. 658The fields are separated by spaces. 659.Pp 660The marker is optional, but if it is present then it must be one of 661.Dq @cert-authority , 662to indicate that the line contains a certification authority (CA) key, 663or 664.Dq @revoked , 665to indicate that the key contained on the line is revoked and must not ever 666be accepted. 667Only one marker should be used on a key line. 668.Pp 669Hostnames is a comma-separated list of patterns 670.Pf ( Ql * 671and 672.Ql \&? 673act as 674wildcards); each pattern in turn is matched against the canonical host 675name (when authenticating a client) or against the user-supplied 676name (when authenticating a server). 677A pattern may also be preceded by 678.Ql \&! 679to indicate negation: if the host name matches a negated 680pattern, it is not accepted (by that line) even if it matched another 681pattern on the line. 682A hostname or address may optionally be enclosed within 683.Ql \&[ 684and 685.Ql \&] 686brackets then followed by 687.Ql \&: 688and a non-standard port number. 689.Pp 690Alternately, hostnames may be stored in a hashed form which hides host names 691and addresses should the file's contents be disclosed. 692Hashed hostnames start with a 693.Ql | 694character. 695Only one hashed hostname may appear on a single line and none of the above 696negation or wildcard operators may be applied. 697.Pp 698Bits, exponent, and modulus are taken directly from the RSA host key; they 699can be obtained, for example, from 700.Pa /etc/ssh/ssh_host_key.pub . 701The optional comment field continues to the end of the line, and is not used. 702.Pp 703Lines starting with 704.Ql # 705and empty lines are ignored as comments. 706.Pp 707When performing host authentication, authentication is accepted if any 708matching line has the proper key; either one that matches exactly or, 709if the server has presented a certificate for authentication, the key 710of the certification authority that signed the certificate. 711For a key to be trusted as a certification authority, it must use the 712.Dq @cert-authority 713marker described above. 714.Pp 715The known hosts file also provides a facility to mark keys as revoked, 716for example when it is known that the associated private key has been 717stolen. 718Revoked keys are specified by including the 719.Dq @revoked 720marker at the beginning of the key line, and are never accepted for 721authentication or as certification authorities, but instead will 722produce a warning from 723.Xr ssh 1 724when they are encountered. 725.Pp 726It is permissible (but not 727recommended) to have several lines or different host keys for the same 728names. 729This will inevitably happen when short forms of host names 730from different domains are put in the file. 731It is possible 732that the files contain conflicting information; authentication is 733accepted if valid information can be found from either file. 734.Pp 735Note that the lines in these files are typically hundreds of characters 736long, and you definitely don't want to type in the host keys by hand. 737Rather, generate them by a script, 738.Xr ssh-keyscan 1 739or by taking 740.Pa /etc/ssh/ssh_host_key.pub 741and adding the host names at the front. 742.Xr ssh-keygen 1 743also offers some basic automated editing for 744.Pa ~/.ssh/known_hosts 745including removing hosts matching a host name and converting all host 746names to their hashed representations. 747.Pp 748An example ssh_known_hosts file: 749.Bd -literal -offset 3n 750# Comments allowed at start of line 751closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 752cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 753# A hashed hostname 754|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 755AAAA1234.....= 756# A revoked key 757@revoked * ssh-rsa AAAAB5W... 758# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org 759@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... 760.Ed 761.Sh FILES 762.Bl -tag -width Ds -compact 763.It Pa ~/.hushlogin 764This file is used to suppress printing the last login time and 765.Pa /etc/motd , 766if 767.Cm PrintLastLog 768and 769.Cm PrintMotd , 770respectively, 771are enabled. 772It does not suppress printing of the banner specified by 773.Cm Banner . 774.Pp 775.It Pa ~/.rhosts 776This file is used for host-based authentication (see 777.Xr ssh 1 778for more information). 779On some machines this file may need to be 780world-readable if the user's home directory is on an NFS partition, 781because 782.Nm 783reads it as root. 784Additionally, this file must be owned by the user, 785and must not have write permissions for anyone else. 786The recommended 787permission for most machines is read/write for the user, and not 788accessible by others. 789.Pp 790.It Pa ~/.shosts 791This file is used in exactly the same way as 792.Pa .rhosts , 793but allows host-based authentication without permitting login with 794rlogin/rsh. 795.Pp 796.It Pa ~/.ssh/ 797This directory is the default location for all user-specific configuration 798and authentication information. 799There is no general requirement to keep the entire contents of this directory 800secret, but the recommended permissions are read/write/execute for the user, 801and not accessible by others. 802.Pp 803.It Pa ~/.ssh/authorized_keys 804Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in 805as this user. 806The format of this file is described above. 807The content of the file is not highly sensitive, but the recommended 808permissions are read/write for the user, and not accessible by others. 809.Pp 810If this file, the 811.Pa ~/.ssh 812directory, or the user's home directory are writable 813by other users, then the file could be modified or replaced by unauthorized 814users. 815In this case, 816.Nm 817will not allow it to be used unless the 818.Cm StrictModes 819option has been set to 820.Dq no . 821.Pp 822.It Pa ~/.ssh/environment 823This file is read into the environment at login (if it exists). 824It can only contain empty lines, comment lines (that start with 825.Ql # ) , 826and assignment lines of the form name=value. 827The file should be writable 828only by the user; it need not be readable by anyone else. 829Environment processing is disabled by default and is 830controlled via the 831.Cm PermitUserEnvironment 832option. 833.Pp 834.It Pa ~/.ssh/known_hosts 835Contains a list of host keys for all hosts the user has logged into 836that are not already in the systemwide list of known host keys. 837The format of this file is described above. 838This file should be writable only by root/the owner and 839can, but need not be, world-readable. 840.Pp 841.It Pa ~/.ssh/rc 842Contains initialization routines to be run before 843the user's home directory becomes accessible. 844This file should be writable only by the user, and need not be 845readable by anyone else. 846.Pp 847.It Pa /etc/hosts.allow 848.It Pa /etc/hosts.deny 849Access controls that should be enforced by tcp-wrappers are defined here. 850Further details are described in 851.Xr hosts_access 5 . 852.Pp 853.It Pa /etc/hosts.equiv 854This file is for host-based authentication (see 855.Xr ssh 1 ) . 856It should only be writable by root. 857.Pp 858.It Pa /etc/ssh/moduli 859Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". 860The file format is described in 861.Xr moduli 5 . 862.Pp 863.It Pa /etc/motd 864See 865.Xr motd 5 . 866.Pp 867.It Pa /etc/nologin 868If this file exists, 869.Nm 870refuses to let anyone except root log in. 871The contents of the file 872are displayed to anyone trying to log in, and non-root connections are 873refused. 874The file should be world-readable. 875.Pp 876.It Pa /etc/ssh/shosts.equiv 877This file is used in exactly the same way as 878.Pa hosts.equiv , 879but allows host-based authentication without permitting login with 880rlogin/rsh. 881.Pp 882.It Pa /etc/ssh/ssh_host_key 883.It Pa /etc/ssh/ssh_host_dsa_key 884.It Pa /etc/ssh/ssh_host_ecdsa_key 885.It Pa /etc/ssh/ssh_host_rsa_key 886These three files contain the private parts of the host keys. 887These files should only be owned by root, readable only by root, and not 888accessible to others. 889Note that 890.Nm 891does not start if these files are group/world-accessible. 892.Pp 893.It Pa /etc/ssh/ssh_host_key.pub 894.It Pa /etc/ssh/ssh_host_dsa_key.pub 895.It Pa /etc/ssh/ssh_host_ecdsa_key.pub 896.It Pa /etc/ssh/ssh_host_rsa_key.pub 897These three files contain the public parts of the host keys. 898These files should be world-readable but writable only by 899root. 900Their contents should match the respective private parts. 901These files are not 902really used for anything; they are provided for the convenience of 903the user so their contents can be copied to known hosts files. 904These files are created using 905.Xr ssh-keygen 1 . 906.Pp 907.It Pa /etc/ssh/ssh_known_hosts 908Systemwide list of known host keys. 909This file should be prepared by the 910system administrator to contain the public host keys of all machines in the 911organization. 912The format of this file is described above. 913This file should be writable only by root/the owner and 914should be world-readable. 915.Pp 916.It Pa /etc/ssh/sshd_config 917Contains configuration data for 918.Nm sshd . 919The file format and configuration options are described in 920.Xr sshd_config 5 . 921.Pp 922.It Pa /etc/ssh/sshrc 923Similar to 924.Pa ~/.ssh/rc , 925it can be used to specify 926machine-specific login-time initializations globally. 927This file should be writable only by root, and should be world-readable. 928.Pp 929.It Pa /var/empty 930.Xr chroot 2 931directory used by 932.Nm 933during privilege separation in the pre-authentication phase. 934The directory should not contain any files and must be owned by root 935and not group or world-writable. 936.Pp 937.It Pa /var/run/sshd.pid 938Contains the process ID of the 939.Nm 940listening for connections (if there are several daemons running 941concurrently for different ports, this contains the process ID of the one 942started last). 943The content of this file is not sensitive; it can be world-readable. 944.El 945.Sh SEE ALSO 946.Xr scp 1 , 947.Xr sftp 1 , 948.Xr ssh 1 , 949.Xr ssh-add 1 , 950.Xr ssh-agent 1 , 951.Xr ssh-keygen 1 , 952.Xr ssh-keyscan 1 , 953.Xr chroot 2 , 954.Xr hosts_access 5 , 955.Xr login.conf 5 , 956.Xr moduli 5 , 957.Xr sshd_config 5 , 958.Xr inetd 8 , 959.Xr sftp-server 8 960.Sh AUTHORS 961OpenSSH is a derivative of the original and free 962ssh 1.2.12 release by Tatu Ylonen. 963Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 964Theo de Raadt and Dug Song 965removed many bugs, re-added newer features and 966created OpenSSH. 967Markus Friedl contributed the support for SSH 968protocol versions 1.5 and 2.0. 969Niels Provos and Markus Friedl contributed support 970for privilege separation. 971.Sh CAVEATS 972System security is not improved unless 973.Nm rshd , 974.Nm rlogind , 975and 976.Nm rexecd 977are disabled (thus completely disabling 978.Xr rlogin 979and 980.Xr rsh 981into the machine). 982