1.\" -*- nroff -*- 2.\" 3.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 4.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 5.\" All rights reserved 6.\" 7.\" As far as I am concerned, the code I have written for this software 8.\" can be used freely for any purpose. Any derived versions of this 9.\" software must be clearly marked as such, and if the derived work is 10.\" incompatible with the protocol description in the RFC file, it must be 11.\" called by a name other than "ssh" or "Secure Shell". 12.\" 13.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 14.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 15.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 16.\" 17.\" Redistribution and use in source and binary forms, with or without 18.\" modification, are permitted provided that the following conditions 19.\" are met: 20.\" 1. Redistributions of source code must retain the above copyright 21.\" notice, this list of conditions and the following disclaimer. 22.\" 2. Redistributions in binary form must reproduce the above copyright 23.\" notice, this list of conditions and the following disclaimer in the 24.\" documentation and/or other materials provided with the distribution. 25.\" 26.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 27.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 28.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 29.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 30.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 31.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 32.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 33.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 34.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 35.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 36.\" 37.\" $OpenBSD: sshd.8,v 1.257 2010/08/04 05:37:01 djm Exp $ 38.Dd $Mdocdate: August 4 2010 $ 39.Dt SSHD 8 40.Os 41.Sh NAME 42.Nm sshd 43.Nd OpenSSH SSH daemon 44.Sh SYNOPSIS 45.Nm sshd 46.Bk -words 47.Op Fl 46DdeiqTt 48.Op Fl b Ar bits 49.Op Fl C Ar connection_spec 50.Op Fl c Ar host_certificate_file 51.Op Fl f Ar config_file 52.Op Fl g Ar login_grace_time 53.Op Fl h Ar host_key_file 54.Op Fl k Ar key_gen_time 55.Op Fl o Ar option 56.Op Fl p Ar port 57.Op Fl u Ar len 58.Ek 59.Sh DESCRIPTION 60.Nm 61(OpenSSH Daemon) is the daemon program for 62.Xr ssh 1 . 63Together these programs replace 64.Xr rlogin 1 65and 66.Xr rsh 1 , 67and provide secure encrypted communications between two untrusted hosts 68over an insecure network. 69.Pp 70.Nm 71listens for connections from clients. 72It is normally started at boot from 73.Pa /etc/rc.d/sshd . 74It forks a new 75daemon for each incoming connection. 76The forked daemons handle 77key exchange, encryption, authentication, command execution, 78and data exchange. 79.Pp 80.Nm 81can be configured using command-line options or a configuration file 82(by default 83.Xr sshd_config 5 ) ; 84command-line options override values specified in the 85configuration file. 86.Nm 87rereads its configuration file when it receives a hangup signal, 88.Dv SIGHUP , 89by executing itself with the name and options it was started with, e.g.\& 90.Pa /usr/sbin/sshd . 91.Pp 92The options are as follows: 93.Bl -tag -width Ds 94.It Fl 4 95Forces 96.Nm 97to use IPv4 addresses only. 98.It Fl 6 99Forces 100.Nm 101to use IPv6 addresses only. 102.It Fl b Ar bits 103Specifies the number of bits in the ephemeral protocol version 1 104server key (default 1024). 105.It Fl C Ar connection_spec 106Specify the connection parameters to use for the 107.Fl T 108extended test mode. 109If provided, any 110.Cm Match 111directives in the configuration file 112that would apply to the specified user, host, and address will be set before 113the configuration is written to standard output. 114The connection parameters are supplied as keyword=value pairs. 115The keywords are 116.Dq user , 117.Dq host , 118and 119.Dq addr . 120All are required and may be supplied in any order, either with multiple 121.Fl C 122options or as a comma-separated list. 123.It Fl c Ar host_certificate_file 124Specifies a path to a certificate file to identify 125.Nm 126during key exchange. 127The certificate file must match a host key file specified using the 128.Fl h 129option or the 130.Cm HostKey 131configuration directive. 132.It Fl D 133When this option is specified, 134.Nm 135will not detach and does not become a daemon. 136This allows easy monitoring of 137.Nm sshd . 138.It Fl d 139Debug mode. 140The server sends verbose debug output to standard error, 141and does not put itself in the background. 142The server also will not fork and will only process one connection. 143This option is only intended for debugging for the server. 144Multiple 145.Fl d 146options increase the debugging level. 147Maximum is 3. 148.It Fl e 149When this option is specified, 150.Nm 151will send the output to the standard error instead of the system log. 152.It Fl f Ar config_file 153Specifies the name of the configuration file. 154The default is 155.Pa /etc/ssh/sshd_config . 156.Nm 157refuses to start if there is no configuration file. 158.It Fl g Ar login_grace_time 159Gives the grace time for clients to authenticate themselves (default 160120 seconds). 161If the client fails to authenticate the user within 162this many seconds, the server disconnects and exits. 163A value of zero indicates no limit. 164.It Fl h Ar host_key_file 165Specifies a file from which a host key is read. 166This option must be given if 167.Nm 168is not run as root (as the normal 169host key files are normally not readable by anyone but root). 170The default is 171.Pa /etc/ssh/ssh_host_key 172for protocol version 1, and 173.Pa /etc/ssh/ssh_host_rsa_key 174and 175.Pa /etc/ssh/ssh_host_dsa_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 both RSA and DSA 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 or hmac-ripemd160). 318.Pp 319Finally, the server and the client enter an authentication dialog. 320The client tries to authenticate itself using 321host-based authentication, 322public key authentication, 323challenge-response authentication, 324or password authentication. 325.Pp 326Regardless of the authentication type, the account is checked to 327ensure that it is accessible. An account is not accessible if it is 328locked, listed in 329.Cm DenyUsers 330or its group is listed in 331.Cm DenyGroups 332\&. The definition of a locked account is system dependant. Some platforms 333have their own account database (eg AIX) and some modify the passwd field ( 334.Ql \&*LK\&* 335on Solaris and UnixWare, 336.Ql \&* 337on HP-UX, containing 338.Ql Nologin 339on Tru64, 340a leading 341.Ql \&*LOCKED\&* 342on FreeBSD and a leading 343.Ql \&! 344on most Linuxes). 345If there is a requirement to disable password authentication 346for the account while allowing still public-key, then the passwd field 347should be set to something other than these values (eg 348.Ql NP 349or 350.Ql \&*NP\&* 351). 352.Pp 353If the client successfully authenticates itself, a dialog for 354preparing the session is entered. 355At this time the client may request 356things like allocating a pseudo-tty, forwarding X11 connections, 357forwarding TCP connections, or forwarding the authentication agent 358connection over the secure channel. 359.Pp 360After this, the client either requests a shell or execution of a command. 361The sides then enter session mode. 362In this mode, either side may send 363data at any time, and such data is forwarded to/from the shell or 364command on the server side, and the user terminal in the client side. 365.Pp 366When the user program terminates and all forwarded X11 and other 367connections have been closed, the server sends command exit status to 368the client, and both sides exit. 369.Sh LOGIN PROCESS 370When a user successfully logs in, 371.Nm 372does the following: 373.Bl -enum -offset indent 374.It 375If the login is on a tty, and no command has been specified, 376prints last login time and 377.Pa /etc/motd 378(unless prevented in the configuration file or by 379.Pa ~/.hushlogin ; 380see the 381.Sx FILES 382section). 383.It 384If the login is on a tty, records login time. 385.It 386Checks 387.Pa /etc/nologin and 388.Pa /var/run/nologin ; 389if one exists, it prints the contents and quits 390(unless root). 391.It 392Changes to run with normal user privileges. 393.It 394Sets up basic environment. 395.It 396Reads the file 397.Pa ~/.ssh/environment , 398if it exists, and users are allowed to change their environment. 399See the 400.Cm PermitUserEnvironment 401option in 402.Xr sshd_config 5 . 403.It 404Changes to user's home directory. 405.It 406If 407.Pa ~/.ssh/rc 408exists, runs it; else if 409.Pa /etc/ssh/sshrc 410exists, runs 411it; otherwise runs 412.Xr xauth 1 . 413The 414.Dq rc 415files are given the X11 416authentication protocol and cookie (if applicable) in standard input. 417See 418.Sx SSHRC , 419below. 420.It 421Runs user's shell or command. 422.El 423.Sh SSHRC 424If the file 425.Pa ~/.ssh/rc 426exists, 427.Xr sh 1 428runs it after reading the 429environment files but before starting the user's shell or command. 430It must not produce any output on stdout; stderr must be used 431instead. 432If X11 forwarding is in use, it will receive the "proto cookie" pair in 433its standard input (and 434.Ev DISPLAY 435in its environment). 436The script must call 437.Xr xauth 1 438because 439.Nm 440will not run xauth automatically to add X11 cookies. 441.Pp 442The primary purpose of this file is to run any initialization routines 443which may be needed before the user's home directory becomes 444accessible; AFS is a particular example of such an environment. 445.Pp 446This file will probably contain some initialization code followed by 447something similar to: 448.Bd -literal -offset 3n 449if read proto cookie && [ -n "$DISPLAY" ]; then 450 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 451 # X11UseLocalhost=yes 452 echo add unix:`echo $DISPLAY | 453 cut -c11-` $proto $cookie 454 else 455 # X11UseLocalhost=no 456 echo add $DISPLAY $proto $cookie 457 fi | xauth -q - 458fi 459.Ed 460.Pp 461If this file does not exist, 462.Pa /etc/ssh/sshrc 463is run, and if that 464does not exist either, xauth is used to add the cookie. 465.Sh AUTHORIZED_KEYS FILE FORMAT 466.Cm AuthorizedKeysFile 467specifies the file containing public keys for 468public key authentication; 469if none is specified, the default is 470.Pa ~/.ssh/authorized_keys . 471Each line of the file contains one 472key (empty lines and lines starting with a 473.Ql # 474are ignored as 475comments). 476Protocol 1 public keys consist of the following space-separated fields: 477options, bits, exponent, modulus, comment. 478Protocol 2 public key consist of: 479options, keytype, base64-encoded key, comment. 480The options field is optional; 481its presence is determined by whether the line starts 482with a number or not (the options field never starts with a number). 483The bits, exponent, modulus, and comment fields give the RSA key for 484protocol version 1; the 485comment field is not used for anything (but may be convenient for the 486user to identify the key). 487For protocol version 2 the keytype is 488.Dq ssh-dss 489or 490.Dq ssh-rsa . 491.Pp 492Note that lines in this file are usually several hundred bytes long 493(because of the size of the public key encoding) up to a limit of 4948 kilobytes, which permits DSA keys up to 8 kilobits and RSA 495keys up to 16 kilobits. 496You don't want to type them in; instead, copy the 497.Pa identity.pub , 498.Pa id_dsa.pub , 499or the 500.Pa id_rsa.pub 501file and edit it. 502.Pp 503.Nm 504enforces a minimum RSA key modulus size for protocol 1 505and protocol 2 keys of 768 bits. 506.Pp 507The options (if present) consist of comma-separated option 508specifications. 509No spaces are permitted, except within double quotes. 510The following option specifications are supported (note 511that option keywords are case-insensitive): 512.Bl -tag -width Ds 513.It Cm cert-authority 514Specifies that the listed key is a certification authority (CA) that is 515trusted to validate signed certificates for user authentication. 516.Pp 517Certificates may encode access restrictions similar to these key options. 518If both certificate restrictions and key options are present, the most 519restrictive union of the two is applied. 520.It Cm command="command" 521Specifies that the command is executed whenever this key is used for 522authentication. 523The command supplied by the user (if any) is ignored. 524The command is run on a pty if the client requests a pty; 525otherwise it is run without a tty. 526If an 8-bit clean channel is required, 527one must not request a pty or should specify 528.Cm no-pty . 529A quote may be included in the command by quoting it with a backslash. 530This option might be useful 531to restrict certain public keys to perform just a specific operation. 532An example might be a key that permits remote backups but nothing else. 533Note that the client may specify TCP and/or X11 534forwarding unless they are explicitly prohibited. 535The command originally supplied by the client is available in the 536.Ev SSH_ORIGINAL_COMMAND 537environment variable. 538Note that this option applies to shell, command or subsystem execution. 539Also note that this command may be superseded by either a 540.Xr sshd_config 5 541.Cm ForceCommand 542directive or a command embedded in a certificate. 543.It Cm environment="NAME=value" 544Specifies that the string is to be added to the environment when 545logging in using this key. 546Environment variables set this way 547override other default environment values. 548Multiple options of this type are permitted. 549Environment processing is disabled by default and is 550controlled via the 551.Cm PermitUserEnvironment 552option. 553This option is automatically disabled if 554.Cm UseLogin 555is enabled. 556.It Cm from="pattern-list" 557Specifies that in addition to public key authentication, either the canonical 558name of the remote host or its IP address must be present in the 559comma-separated list of patterns. 560See 561.Sx PATTERNS 562in 563.Xr ssh_config 5 564for more information on patterns. 565.Pp 566In addition to the wildcard matching that may be applied to hostnames or 567addresses, a 568.Cm from 569stanza may match IP addresses using CIDR address/masklen notation. 570.Pp 571The purpose of this option is to optionally increase security: public key 572authentication by itself does not trust the network or name servers or 573anything (but the key); however, if somebody somehow steals the key, the key 574permits an intruder to log in from anywhere in the world. 575This additional option makes using a stolen key more difficult (name 576servers and/or routers would have to be compromised in addition to 577just the key). 578.It Cm no-agent-forwarding 579Forbids authentication agent forwarding when this key is used for 580authentication. 581.It Cm no-port-forwarding 582Forbids TCP forwarding when this key is used for authentication. 583Any port forward requests by the client will return an error. 584This might be used, e.g. in connection with the 585.Cm command 586option. 587.It Cm no-pty 588Prevents tty allocation (a request to allocate a pty will fail). 589.It Cm no-user-rc 590Disables execution of 591.Pa ~/.ssh/rc . 592.It Cm no-X11-forwarding 593Forbids X11 forwarding when this key is used for authentication. 594Any X11 forward requests by the client will return an error. 595.It Cm permitopen="host:port" 596Limit local 597.Li ``ssh -L'' 598port forwarding such that it may only connect to the specified host and 599port. 600IPv6 addresses can be specified by enclosing the address in square brackets. 601Multiple 602.Cm permitopen 603options may be applied separated by commas. 604No pattern matching is performed on the specified hostnames, 605they must be literal domains or addresses. 606.It Cm principals="principals" 607On a 608.Cm cert-authority 609line, specifies allowed principals for certificate authentication as a 610comma-separated list. 611At least one name from the list must appear in the certificate's 612list of principals for the certificate to be accepted. 613This option is ignored for keys that are not marked as trusted certificate 614signers using the 615.Cm cert-authority 616option. 617.It Cm tunnel="n" 618Force a 619.Xr tun 4 620device on the server. 621Without this option, the next available device will be used if 622the client requests a tunnel. 623.El 624.Pp 625An example authorized_keys file: 626.Bd -literal -offset 3n 627# Comments allowed at start of line 628ssh-rsa AAAAB3Nza...LiPk== user@example.net 629from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 630AAAAB2...19Q== john@example.net 631command="dump /home",no-pty,no-port-forwarding ssh-dss 632AAAAC3...51R== example.net 633permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 634AAAAB5...21S== 635tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 636jane@example.net 637.Ed 638.Sh SSH_KNOWN_HOSTS FILE FORMAT 639The 640.Pa /etc/ssh/ssh_known_hosts 641and 642.Pa ~/.ssh/known_hosts 643files contain host public keys for all known hosts. 644The global file should 645be prepared by the administrator (optional), and the per-user file is 646maintained automatically: whenever the user connects from an unknown host, 647its key is added to the per-user file. 648.Pp 649Each line in these files contains the following fields: markers (optional), 650hostnames, bits, exponent, modulus, comment. 651The fields are separated by spaces. 652.Pp 653The marker is optional, but if it is present then it must be one of 654.Dq @cert-authority , 655to indicate that the line contains a certification authority (CA) key, 656or 657.Dq @revoked , 658to indicate that the key contained on the line is revoked and must not ever 659be accepted. 660Only one marker should be used on a key line. 661.Pp 662Hostnames is a comma-separated list of patterns 663.Pf ( Ql * 664and 665.Ql \&? 666act as 667wildcards); each pattern in turn is matched against the canonical host 668name (when authenticating a client) or against the user-supplied 669name (when authenticating a server). 670A pattern may also be preceded by 671.Ql \&! 672to indicate negation: if the host name matches a negated 673pattern, it is not accepted (by that line) even if it matched another 674pattern on the line. 675A hostname or address may optionally be enclosed within 676.Ql \&[ 677and 678.Ql \&] 679brackets then followed by 680.Ql \&: 681and a non-standard port number. 682.Pp 683Alternately, hostnames may be stored in a hashed form which hides host names 684and addresses should the file's contents be disclosed. 685Hashed hostnames start with a 686.Ql | 687character. 688Only one hashed hostname may appear on a single line and none of the above 689negation or wildcard operators may be applied. 690.Pp 691Bits, exponent, and modulus are taken directly from the RSA host key; they 692can be obtained, for example, from 693.Pa /etc/ssh/ssh_host_key.pub . 694The optional comment field continues to the end of the line, and is not used. 695.Pp 696Lines starting with 697.Ql # 698and empty lines are ignored as comments. 699.Pp 700When performing host authentication, authentication is accepted if any 701matching line has the proper key; either one that matches exactly or, 702if the server has presented a certificate for authentication, the key 703of the certification authority that signed the certificate. 704For a key to be trusted as a certification authority, it must use the 705.Dq @cert-authority 706marker described above. 707.Pp 708The known hosts file also provides a facility to mark keys as revoked, 709for example when it is known that the associated private key has been 710stolen. 711Revoked keys are specified by including the 712.Dq @revoked 713marker at the beginning of the key line, and are never accepted for 714authentication or as certification authorities, but instead will 715produce a warning from 716.Xr ssh 1 717when they are encountered. 718.Pp 719It is permissible (but not 720recommended) to have several lines or different host keys for the same 721names. 722This will inevitably happen when short forms of host names 723from different domains are put in the file. 724It is possible 725that the files contain conflicting information; authentication is 726accepted if valid information can be found from either file. 727.Pp 728Note that the lines in these files are typically hundreds of characters 729long, and you definitely don't want to type in the host keys by hand. 730Rather, generate them by a script, 731.Xr ssh-keyscan 1 732or by taking 733.Pa /etc/ssh/ssh_host_key.pub 734and adding the host names at the front. 735.Xr ssh-keygen 1 736also offers some basic automated editing for 737.Pa ~/.ssh/known_hosts 738including removing hosts matching a host name and converting all host 739names to their hashed representations. 740.Pp 741An example ssh_known_hosts file: 742.Bd -literal -offset 3n 743# Comments allowed at start of line 744closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 745cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 746# A hashed hostname 747|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 748AAAA1234.....= 749# A revoked key 750@revoked * ssh-rsa AAAAB5W... 751# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org 752@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... 753.Ed 754.Sh FILES 755.Bl -tag -width Ds -compact 756.It ~/.hushlogin 757This file is used to suppress printing the last login time and 758.Pa /etc/motd , 759if 760.Cm PrintLastLog 761and 762.Cm PrintMotd , 763respectively, 764are enabled. 765It does not suppress printing of the banner specified by 766.Cm Banner . 767.Pp 768.It ~/.rhosts 769This file is used for host-based authentication (see 770.Xr ssh 1 771for more information). 772On some machines this file may need to be 773world-readable if the user's home directory is on an NFS partition, 774because 775.Nm 776reads it as root. 777Additionally, this file must be owned by the user, 778and must not have write permissions for anyone else. 779The recommended 780permission for most machines is read/write for the user, and not 781accessible by others. 782.Pp 783.It ~/.shosts 784This file is used in exactly the same way as 785.Pa .rhosts , 786but allows host-based authentication without permitting login with 787rlogin/rsh. 788.Pp 789.It ~/.ssh/ 790This directory is the default location for all user-specific configuration 791and authentication information. 792There is no general requirement to keep the entire contents of this directory 793secret, but the recommended permissions are read/write/execute for the user, 794and not accessible by others. 795.Pp 796.It ~/.ssh/authorized_keys 797Lists the public keys (RSA/DSA) that can be used for logging in as this user. 798The format of this file is described above. 799The content of the file is not highly sensitive, but the recommended 800permissions are read/write for the user, and not accessible by others. 801.Pp 802If this file, the 803.Pa ~/.ssh 804directory, or the user's home directory are writable 805by other users, then the file could be modified or replaced by unauthorized 806users. 807In this case, 808.Nm 809will not allow it to be used unless the 810.Cm StrictModes 811option has been set to 812.Dq no . 813.Pp 814.It ~/.ssh/environment 815This file is read into the environment at login (if it exists). 816It can only contain empty lines, comment lines (that start with 817.Ql # ) , 818and assignment lines of the form name=value. 819The file should be writable 820only by the user; it need not be readable by anyone else. 821Environment processing is disabled by default and is 822controlled via the 823.Cm PermitUserEnvironment 824option. 825.Pp 826.It ~/.ssh/known_hosts 827Contains a list of host keys for all hosts the user has logged into 828that are not already in the systemwide list of known host keys. 829The format of this file is described above. 830This file should be writable only by root/the owner and 831can, but need not be, world-readable. 832.Pp 833.It ~/.ssh/rc 834Contains initialization routines to be run before 835the user's home directory becomes accessible. 836This file should be writable only by the user, and need not be 837readable by anyone else. 838.Pp 839.It /etc/hosts.allow 840.It /etc/hosts.deny 841Access controls that should be enforced by tcp-wrappers are defined here. 842Further details are described in 843.Xr hosts_access 5 . 844.Pp 845.It /etc/hosts.equiv 846This file is for host-based authentication (see 847.Xr ssh 1 ) . 848It should only be writable by root. 849.Pp 850.It /etc/ssh/moduli 851Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". 852The file format is described in 853.Xr moduli 5 . 854.Pp 855.It /etc/motd 856See 857.Xr motd 5 . 858.Pp 859.It /etc/nologin 860If this file exists, 861.Nm 862refuses to let anyone except root log in. 863The contents of the file 864are displayed to anyone trying to log in, and non-root connections are 865refused. 866The file should be world-readable. 867.Pp 868.It /etc/ssh/shosts.equiv 869This file is used in exactly the same way as 870.Pa hosts.equiv , 871but allows host-based authentication without permitting login with 872rlogin/rsh. 873.Pp 874.It /etc/ssh/ssh_host_key 875.It /etc/ssh/ssh_host_dsa_key 876.It /etc/ssh/ssh_host_rsa_key 877These three files contain the private parts of the host keys. 878These files should only be owned by root, readable only by root, and not 879accessible to others. 880Note that 881.Nm 882does not start if these files are group/world-accessible. 883.Pp 884.It /etc/ssh/ssh_host_key.pub 885.It /etc/ssh/ssh_host_dsa_key.pub 886.It /etc/ssh/ssh_host_rsa_key.pub 887These three files contain the public parts of the host keys. 888These files should be world-readable but writable only by 889root. 890Their contents should match the respective private parts. 891These files are not 892really used for anything; they are provided for the convenience of 893the user so their contents can be copied to known hosts files. 894These files are created using 895.Xr ssh-keygen 1 . 896.Pp 897.It /etc/ssh/ssh_known_hosts 898Systemwide list of known host keys. 899This file should be prepared by the 900system administrator to contain the public host keys of all machines in the 901organization. 902The format of this file is described above. 903This file should be writable only by root/the owner and 904should be world-readable. 905.Pp 906.It /etc/ssh/sshd_config 907Contains configuration data for 908.Nm sshd . 909The file format and configuration options are described in 910.Xr sshd_config 5 . 911.Pp 912.It /etc/ssh/sshrc 913Similar to 914.Pa ~/.ssh/rc , 915it can be used to specify 916machine-specific login-time initializations globally. 917This file should be writable only by root, and should be world-readable. 918.Pp 919.It /var/empty 920.Xr chroot 2 921directory used by 922.Nm 923during privilege separation in the pre-authentication phase. 924The directory should not contain any files and must be owned by root 925and not group or world-writable. 926.Pp 927.It /var/run/sshd.pid 928Contains the process ID of the 929.Nm 930listening for connections (if there are several daemons running 931concurrently for different ports, this contains the process ID of the one 932started last). 933The content of this file is not sensitive; it can be world-readable. 934.El 935.Sh SEE ALSO 936.Xr scp 1 , 937.Xr sftp 1 , 938.Xr ssh 1 , 939.Xr ssh-add 1 , 940.Xr ssh-agent 1 , 941.Xr ssh-keygen 1 , 942.Xr ssh-keyscan 1 , 943.Xr chroot 2 , 944.Xr hosts_access 5 , 945.Xr login.conf 5 , 946.Xr moduli 5 , 947.Xr sshd_config 5 , 948.Xr inetd 8 , 949.Xr sftp-server 8 950.Sh AUTHORS 951OpenSSH is a derivative of the original and free 952ssh 1.2.12 release by Tatu Ylonen. 953Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 954Theo de Raadt and Dug Song 955removed many bugs, re-added newer features and 956created OpenSSH. 957Markus Friedl contributed the support for SSH 958protocol versions 1.5 and 2.0. 959Niels Provos and Markus Friedl contributed support 960for privilege separation. 961.Sh CAVEATS 962System security is not improved unless 963.Nm rshd , 964.Nm rlogind , 965and 966.Nm rexecd 967are disabled (thus completely disabling 968.Xr rlogin 969and 970.Xr rsh 971into the machine). 972