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 1.260 2010/10/28 18:33:28 jmc Exp $ 37.\" $FreeBSD$ 38.Dd October 28, 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_dsa_key , 174.Pa /etc/ssh/ssh_host_ecdsa_key 175and 176.Pa /etc/ssh/ssh_host_rsa_key 177for protocol version 2. 178It is possible to have multiple host key files for 179the different protocol versions and host key algorithms. 180.It Fl i 181Specifies that 182.Nm 183is being run from 184.Xr inetd 8 . 185.Nm 186is normally not run 187from inetd because it needs to generate the server key before it can 188respond to the client, and this may take tens of seconds. 189Clients would have to wait too long if the key was regenerated every time. 190However, with small key sizes (e.g. 512) using 191.Nm 192from inetd may 193be feasible. 194.It Fl k Ar key_gen_time 195Specifies how often the ephemeral protocol version 1 server key is 196regenerated (default 3600 seconds, or one hour). 197The motivation for regenerating the key fairly 198often is that the key is not stored anywhere, and after about an hour 199it becomes impossible to recover the key for decrypting intercepted 200communications even if the machine is cracked into or physically 201seized. 202A value of zero indicates that the key will never be regenerated. 203.It Fl o Ar option 204Can be used to give options in the format used in the configuration file. 205This is useful for specifying options for which there is no separate 206command-line flag. 207For full details of the options, and their values, see 208.Xr sshd_config 5 . 209.It Fl p Ar port 210Specifies the port on which the server listens for connections 211(default 22). 212Multiple port options are permitted. 213Ports specified in the configuration file with the 214.Cm Port 215option are ignored when a command-line port is specified. 216Ports specified using the 217.Cm ListenAddress 218option override command-line ports. 219.It Fl q 220Quiet mode. 221Nothing is sent to the system log. 222Normally the beginning, 223authentication, and termination of each connection is logged. 224.It Fl T 225Extended test mode. 226Check the validity of the configuration file, output the effective configuration 227to stdout and then exit. 228Optionally, 229.Cm Match 230rules may be applied by specifying the connection parameters using one or more 231.Fl C 232options. 233.It Fl t 234Test mode. 235Only check the validity of the configuration file and sanity of the keys. 236This is useful for updating 237.Nm 238reliably as configuration options may change. 239.It Fl u Ar len 240This option is used to specify the size of the field 241in the 242.Li utmp 243structure that holds the remote host name. 244If the resolved host name is longer than 245.Ar len , 246the dotted decimal value will be used instead. 247This allows hosts with very long host names that 248overflow this field to still be uniquely identified. 249Specifying 250.Fl u0 251indicates that only dotted decimal addresses 252should be put into the 253.Pa utmp 254file. 255.Fl u0 256may also be used to prevent 257.Nm 258from making DNS requests unless the authentication 259mechanism or configuration requires it. 260Authentication mechanisms that may require DNS include 261.Cm RhostsRSAAuthentication , 262.Cm HostbasedAuthentication , 263and using a 264.Cm from="pattern-list" 265option in a key file. 266Configuration options that require DNS include using a 267USER@HOST pattern in 268.Cm AllowUsers 269or 270.Cm DenyUsers . 271.El 272.Sh AUTHENTICATION 273The OpenSSH SSH daemon supports SSH protocols 1 and 2. 274The default is to use protocol 2 only, 275though this can be changed via the 276.Cm Protocol 277option in 278.Xr sshd_config 5 . 279Protocol 2 supports DSA, ECDSA and RSA keys; 280protocol 1 only supports RSA keys. 281For both protocols, 282each host has a host-specific key, 283normally 2048 bits, 284used to identify the host. 285.Pp 286Forward security for protocol 1 is provided through 287an additional server key, 288normally 768 bits, 289generated when the server starts. 290This key is normally regenerated every hour if it has been used, and 291is never stored on disk. 292Whenever a client connects, the daemon responds with its public 293host and server keys. 294The client compares the 295RSA host key against its own database to verify that it has not changed. 296The client then generates a 256-bit random number. 297It encrypts this 298random number using both the host key and the server key, and sends 299the encrypted number to the server. 300Both sides then use this 301random number as a session key which is used to encrypt all further 302communications in the session. 303The rest of the session is encrypted 304using a conventional cipher, currently Blowfish or 3DES, with 3DES 305being used by default. 306The client selects the encryption algorithm 307to use from those offered by the server. 308.Pp 309For protocol 2, 310forward security is provided through a Diffie-Hellman key agreement. 311This key agreement results in a shared session key. 312The rest of the session is encrypted using a symmetric cipher, currently 313128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. 314The client selects the encryption algorithm 315to use from those offered by the server. 316Additionally, session integrity is provided 317through a cryptographic message authentication code 318(hmac-md5, hmac-sha1, umac-64 or hmac-ripemd160). 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 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 file containing public keys for 469public key authentication; 470if none is specified, the default is 471.Pa ~/.ssh/authorized_keys . 472Each line of the file contains one 473key (empty lines and lines starting with a 474.Ql # 475are ignored as 476comments). 477Protocol 1 public keys consist of the following space-separated fields: 478options, bits, exponent, modulus, comment. 479Protocol 2 public key consist of: 480options, keytype, base64-encoded key, comment. 481The options field is optional; 482its presence is determined by whether the line starts 483with a number or not (the options field never starts with a number). 484The bits, exponent, modulus, and comment fields give the RSA key for 485protocol version 1; the 486comment field is not used for anything (but may be convenient for the 487user to identify the key). 488For protocol version 2 the keytype is 489.Dq ecdsa-sha2-nistp256 , 490.Dq ecdsa-sha2-nistp384 , 491.Dq ecdsa-sha2-nistp521 , 492.Dq ssh-dss 493or 494.Dq ssh-rsa . 495.Pp 496Note that lines in this file are usually several hundred bytes long 497(because of the size of the public key encoding) up to a limit of 4988 kilobytes, which permits DSA keys up to 8 kilobits and RSA 499keys up to 16 kilobits. 500You don't want to type them in; instead, copy the 501.Pa identity.pub , 502.Pa id_dsa.pub , 503.Pa id_ecdsa.pub , 504or the 505.Pa id_rsa.pub 506file and edit it. 507.Pp 508.Nm 509enforces a minimum RSA key modulus size for protocol 1 510and protocol 2 keys of 768 bits. 511.Pp 512The options (if present) consist of comma-separated option 513specifications. 514No spaces are permitted, except within double quotes. 515The following option specifications are supported (note 516that option keywords are case-insensitive): 517.Bl -tag -width Ds 518.It Cm cert-authority 519Specifies that the listed key is a certification authority (CA) that is 520trusted to validate signed certificates for user authentication. 521.Pp 522Certificates may encode access restrictions similar to these key options. 523If both certificate restrictions and key options are present, the most 524restrictive union of the two is applied. 525.It Cm command="command" 526Specifies that the command is executed whenever this key is used for 527authentication. 528The command supplied by the user (if any) is ignored. 529The command is run on a pty if the client requests a pty; 530otherwise it is run without a tty. 531If an 8-bit clean channel is required, 532one must not request a pty or should specify 533.Cm no-pty . 534A quote may be included in the command by quoting it with a backslash. 535This option might be useful 536to restrict certain public keys to perform just a specific operation. 537An example might be a key that permits remote backups but nothing else. 538Note that the client may specify TCP and/or X11 539forwarding unless they are explicitly prohibited. 540The command originally supplied by the client is available in the 541.Ev SSH_ORIGINAL_COMMAND 542environment variable. 543Note that this option applies to shell, command or subsystem execution. 544Also note that this command may be superseded by either a 545.Xr sshd_config 5 546.Cm ForceCommand 547directive or a command embedded in a certificate. 548.It Cm environment="NAME=value" 549Specifies that the string is to be added to the environment when 550logging in using this key. 551Environment variables set this way 552override other default environment values. 553Multiple options of this type are permitted. 554Environment processing is disabled by default and is 555controlled via the 556.Cm PermitUserEnvironment 557option. 558This option is automatically disabled if 559.Cm UseLogin 560is enabled. 561.It Cm from="pattern-list" 562Specifies that in addition to public key authentication, either the canonical 563name of the remote host or its IP address must be present in the 564comma-separated list of patterns. 565See 566.Sx PATTERNS 567in 568.Xr ssh_config 5 569for more information on patterns. 570.Pp 571In addition to the wildcard matching that may be applied to hostnames or 572addresses, a 573.Cm from 574stanza may match IP addresses using CIDR address/masklen notation. 575.Pp 576The purpose of this option is to optionally increase security: public key 577authentication by itself does not trust the network or name servers or 578anything (but the key); however, if somebody somehow steals the key, the key 579permits an intruder to log in from anywhere in the world. 580This additional option makes using a stolen key more difficult (name 581servers and/or routers would have to be compromised in addition to 582just the key). 583.It Cm no-agent-forwarding 584Forbids authentication agent forwarding when this key is used for 585authentication. 586.It Cm no-port-forwarding 587Forbids TCP forwarding when this key is used for authentication. 588Any port forward requests by the client will return an error. 589This might be used, e.g. in connection with the 590.Cm command 591option. 592.It Cm no-pty 593Prevents tty allocation (a request to allocate a pty will fail). 594.It Cm no-user-rc 595Disables execution of 596.Pa ~/.ssh/rc . 597.It Cm no-X11-forwarding 598Forbids X11 forwarding when this key is used for authentication. 599Any X11 forward requests by the client will return an error. 600.It Cm permitopen="host:port" 601Limit local 602.Li ``ssh -L'' 603port forwarding such that it may only connect to the specified host and 604port. 605IPv6 addresses can be specified by enclosing the address in square brackets. 606Multiple 607.Cm permitopen 608options may be applied separated by commas. 609No pattern matching is performed on the specified hostnames, 610they must be literal domains or addresses. 611.It Cm principals="principals" 612On a 613.Cm cert-authority 614line, specifies allowed principals for certificate authentication as a 615comma-separated list. 616At least one name from the list must appear in the certificate's 617list of principals for the certificate to be accepted. 618This option is ignored for keys that are not marked as trusted certificate 619signers using the 620.Cm cert-authority 621option. 622.It Cm tunnel="n" 623Force a 624.Xr tun 4 625device on the server. 626Without this option, the next available device will be used if 627the client requests a tunnel. 628.El 629.Pp 630An example authorized_keys file: 631.Bd -literal -offset 3n 632# Comments allowed at start of line 633ssh-rsa AAAAB3Nza...LiPk== user@example.net 634from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 635AAAAB2...19Q== john@example.net 636command="dump /home",no-pty,no-port-forwarding ssh-dss 637AAAAC3...51R== example.net 638permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 639AAAAB5...21S== 640tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 641jane@example.net 642.Ed 643.Sh SSH_KNOWN_HOSTS FILE FORMAT 644The 645.Pa /etc/ssh/ssh_known_hosts 646and 647.Pa ~/.ssh/known_hosts 648files contain host public keys for all known hosts. 649The global file should 650be prepared by the administrator (optional), and the per-user file is 651maintained automatically: whenever the user connects from an unknown host, 652its key is added to the per-user file. 653.Pp 654Each line in these files contains the following fields: markers (optional), 655hostnames, bits, exponent, modulus, comment. 656The fields are separated by spaces. 657.Pp 658The marker is optional, but if it is present then it must be one of 659.Dq @cert-authority , 660to indicate that the line contains a certification authority (CA) key, 661or 662.Dq @revoked , 663to indicate that the key contained on the line is revoked and must not ever 664be accepted. 665Only one marker should be used on a key line. 666.Pp 667Hostnames is a comma-separated list of patterns 668.Pf ( Ql * 669and 670.Ql \&? 671act as 672wildcards); each pattern in turn is matched against the canonical host 673name (when authenticating a client) or against the user-supplied 674name (when authenticating a server). 675A pattern may also be preceded by 676.Ql \&! 677to indicate negation: if the host name matches a negated 678pattern, it is not accepted (by that line) even if it matched another 679pattern on the line. 680A hostname or address may optionally be enclosed within 681.Ql \&[ 682and 683.Ql \&] 684brackets then followed by 685.Ql \&: 686and a non-standard port number. 687.Pp 688Alternately, hostnames may be stored in a hashed form which hides host names 689and addresses should the file's contents be disclosed. 690Hashed hostnames start with a 691.Ql | 692character. 693Only one hashed hostname may appear on a single line and none of the above 694negation or wildcard operators may be applied. 695.Pp 696Bits, exponent, and modulus are taken directly from the RSA host key; they 697can be obtained, for example, from 698.Pa /etc/ssh/ssh_host_key.pub . 699The optional comment field continues to the end of the line, and is not used. 700.Pp 701Lines starting with 702.Ql # 703and empty lines are ignored as comments. 704.Pp 705When performing host authentication, authentication is accepted if any 706matching line has the proper key; either one that matches exactly or, 707if the server has presented a certificate for authentication, the key 708of the certification authority that signed the certificate. 709For a key to be trusted as a certification authority, it must use the 710.Dq @cert-authority 711marker described above. 712.Pp 713The known hosts file also provides a facility to mark keys as revoked, 714for example when it is known that the associated private key has been 715stolen. 716Revoked keys are specified by including the 717.Dq @revoked 718marker at the beginning of the key line, and are never accepted for 719authentication or as certification authorities, but instead will 720produce a warning from 721.Xr ssh 1 722when they are encountered. 723.Pp 724It is permissible (but not 725recommended) to have several lines or different host keys for the same 726names. 727This will inevitably happen when short forms of host names 728from different domains are put in the file. 729It is possible 730that the files contain conflicting information; authentication is 731accepted if valid information can be found from either file. 732.Pp 733Note that the lines in these files are typically hundreds of characters 734long, and you definitely don't want to type in the host keys by hand. 735Rather, generate them by a script, 736.Xr ssh-keyscan 1 737or by taking 738.Pa /etc/ssh/ssh_host_key.pub 739and adding the host names at the front. 740.Xr ssh-keygen 1 741also offers some basic automated editing for 742.Pa ~/.ssh/known_hosts 743including removing hosts matching a host name and converting all host 744names to their hashed representations. 745.Pp 746An example ssh_known_hosts file: 747.Bd -literal -offset 3n 748# Comments allowed at start of line 749closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 750cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 751# A hashed hostname 752|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 753AAAA1234.....= 754# A revoked key 755@revoked * ssh-rsa AAAAB5W... 756# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org 757@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... 758.Ed 759.Sh FILES 760.Bl -tag -width Ds -compact 761.It Pa ~/.hushlogin 762This file is used to suppress printing the last login time and 763.Pa /etc/motd , 764if 765.Cm PrintLastLog 766and 767.Cm PrintMotd , 768respectively, 769are enabled. 770It does not suppress printing of the banner specified by 771.Cm Banner . 772.Pp 773.It Pa ~/.rhosts 774This file is used for host-based authentication (see 775.Xr ssh 1 776for more information). 777On some machines this file may need to be 778world-readable if the user's home directory is on an NFS partition, 779because 780.Nm 781reads it as root. 782Additionally, this file must be owned by the user, 783and must not have write permissions for anyone else. 784The recommended 785permission for most machines is read/write for the user, and not 786accessible by others. 787.Pp 788.It Pa ~/.shosts 789This file is used in exactly the same way as 790.Pa .rhosts , 791but allows host-based authentication without permitting login with 792rlogin/rsh. 793.Pp 794.It Pa ~/.ssh/ 795This directory is the default location for all user-specific configuration 796and authentication information. 797There is no general requirement to keep the entire contents of this directory 798secret, but the recommended permissions are read/write/execute for the user, 799and not accessible by others. 800.Pp 801.It Pa ~/.ssh/authorized_keys 802Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in 803as this user. 804The format of this file is described above. 805The content of the file is not highly sensitive, but the recommended 806permissions are read/write for the user, and not accessible by others. 807.Pp 808If this file, the 809.Pa ~/.ssh 810directory, or the user's home directory are writable 811by other users, then the file could be modified or replaced by unauthorized 812users. 813In this case, 814.Nm 815will not allow it to be used unless the 816.Cm StrictModes 817option has been set to 818.Dq no . 819.Pp 820.It Pa ~/.ssh/environment 821This file is read into the environment at login (if it exists). 822It can only contain empty lines, comment lines (that start with 823.Ql # ) , 824and assignment lines of the form name=value. 825The file should be writable 826only by the user; it need not be readable by anyone else. 827Environment processing is disabled by default and is 828controlled via the 829.Cm PermitUserEnvironment 830option. 831.Pp 832.It Pa ~/.ssh/known_hosts 833Contains a list of host keys for all hosts the user has logged into 834that are not already in the systemwide list of known host keys. 835The format of this file is described above. 836This file should be writable only by root/the owner and 837can, but need not be, world-readable. 838.Pp 839.It Pa ~/.ssh/rc 840Contains initialization routines to be run before 841the user's home directory becomes accessible. 842This file should be writable only by the user, and need not be 843readable by anyone else. 844.Pp 845.It Pa /etc/hosts.allow 846.It Pa /etc/hosts.deny 847Access controls that should be enforced by tcp-wrappers are defined here. 848Further details are described in 849.Xr hosts_access 5 . 850.Pp 851.It Pa /etc/hosts.equiv 852This file is for host-based authentication (see 853.Xr ssh 1 ) . 854It should only be writable by root. 855.Pp 856.It Pa /etc/moduli 857Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". 858The file format is described in 859.Xr moduli 5 . 860.Pp 861.It Pa /etc/motd 862See 863.Xr motd 5 . 864.Pp 865.It Pa /etc/nologin 866If this file exists, 867.Nm 868refuses to let anyone except root log in. 869The contents of the file 870are displayed to anyone trying to log in, and non-root connections are 871refused. 872The file should be world-readable. 873.Pp 874.It Pa /etc/shosts.equiv 875This file is used in exactly the same way as 876.Pa hosts.equiv , 877but allows host-based authentication without permitting login with 878rlogin/rsh. 879.Pp 880.It Pa /etc/ssh/ssh_host_key 881.It Pa /etc/ssh/ssh_host_dsa_key 882.It Pa /etc/ssh/ssh_host_ecdsa_key 883.It Pa /etc/ssh/ssh_host_rsa_key 884These three files contain the private parts of the host keys. 885These files should only be owned by root, readable only by root, and not 886accessible to others. 887Note that 888.Nm 889does not start if these files are group/world-accessible. 890.Pp 891.It Pa /etc/ssh/ssh_host_key.pub 892.It Pa /etc/ssh/ssh_host_dsa_key.pub 893.It Pa /etc/ssh/ssh_host_ecdsa_key.pub 894.It Pa /etc/ssh/ssh_host_rsa_key.pub 895These three files contain the public parts of the host keys. 896These files should be world-readable but writable only by 897root. 898Their contents should match the respective private parts. 899These files are not 900really used for anything; they are provided for the convenience of 901the user so their contents can be copied to known hosts files. 902These files are created using 903.Xr ssh-keygen 1 . 904.Pp 905.It Pa /etc/ssh/ssh_known_hosts 906Systemwide list of known host keys. 907This file should be prepared by the 908system administrator to contain the public host keys of all machines in the 909organization. 910The format of this file is described above. 911This file should be writable only by root/the owner and 912should be world-readable. 913.Pp 914.It Pa /etc/ssh/sshd_config 915Contains configuration data for 916.Nm sshd . 917The file format and configuration options are described in 918.Xr sshd_config 5 . 919.Pp 920.It Pa /etc/ssh/sshrc 921Similar to 922.Pa ~/.ssh/rc , 923it can be used to specify 924machine-specific login-time initializations globally. 925This file should be writable only by root, and should be world-readable. 926.Pp 927.It Pa /var/empty 928.Xr chroot 2 929directory used by 930.Nm 931during privilege separation in the pre-authentication phase. 932The directory should not contain any files and must be owned by root 933and not group or world-writable. 934.Pp 935.It Pa /var/run/sshd.pid 936Contains the process ID of the 937.Nm 938listening for connections (if there are several daemons running 939concurrently for different ports, this contains the process ID of the one 940started last). 941The content of this file is not sensitive; it can be world-readable. 942.El 943.Sh SEE ALSO 944.Xr scp 1 , 945.Xr sftp 1 , 946.Xr ssh 1 , 947.Xr ssh-add 1 , 948.Xr ssh-agent 1 , 949.Xr ssh-keygen 1 , 950.Xr ssh-keyscan 1 , 951.Xr chroot 2 , 952.Xr hosts_access 5 , 953.Xr login.conf 5 , 954.Xr moduli 5 , 955.Xr sshd_config 5 , 956.Xr inetd 8 , 957.Xr sftp-server 8 958.Sh AUTHORS 959OpenSSH is a derivative of the original and free 960ssh 1.2.12 release by Tatu Ylonen. 961Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 962Theo de Raadt and Dug Song 963removed many bugs, re-added newer features and 964created OpenSSH. 965Markus Friedl contributed the support for SSH 966protocol versions 1.5 and 2.0. 967Niels Provos and Markus Friedl contributed support 968for privilege separation. 969.Sh CAVEATS 970System security is not improved unless 971.Nm rshd , 972.Nm rlogind , 973and 974.Nm rexecd 975are disabled (thus completely disabling 976.Xr rlogin 977and 978.Xr rsh 979into the machine). 980