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 1.215 2006/02/01 09:11:41 jmc Exp $ 38.\" $FreeBSD$ 39.Dd September 25, 1999 40.Dt SSHD 8 41.Os 42.Sh NAME 43.Nm sshd 44.Nd OpenSSH SSH daemon 45.Sh SYNOPSIS 46.Nm sshd 47.Bk -words 48.Op Fl 46Ddeiqt 49.Op Fl b Ar bits 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 rlogin and rsh, and 63provide secure encrypted communications between two untrusted hosts 64over an insecure network. 65.Pp 66.Nm 67listens for connections from clients. 68It is normally started at boot from 69.Pa /etc/rc.d/sshd . 70It forks a new 71daemon for each incoming connection. 72The forked daemons handle 73key exchange, encryption, authentication, command execution, 74and data exchange. 75.Pp 76.Nm 77can be configured using command-line options or a configuration file 78(by default 79.Xr sshd_config 5 ) ; 80command-line options override values specified in the 81configuration file. 82.Nm 83rereads its configuration file when it receives a hangup signal, 84.Dv SIGHUP , 85by executing itself with the name and options it was started with, e.g., 86.Pa /usr/sbin/sshd . 87.Pp 88The options are as follows: 89.Bl -tag -width Ds 90.It Fl 4 91Forces 92.Nm 93to use IPv4 addresses only. 94.It Fl 6 95Forces 96.Nm 97to use IPv6 addresses only. 98.It Fl b Ar bits 99Specifies the number of bits in the ephemeral protocol version 1 100server key (default 768). 101.It Fl D 102When this option is specified, 103.Nm 104will not detach and does not become a daemon. 105This allows easy monitoring of 106.Nm sshd . 107.It Fl d 108Debug mode. 109The server sends verbose debug output to the system 110log, and does not put itself in the background. 111The server also will not fork and will only process one connection. 112This option is only intended for debugging for the server. 113Multiple 114.Fl d 115options increase the debugging level. 116Maximum is 3. 117.It Fl e 118When this option is specified, 119.Nm 120will send the output to the standard error instead of the system log. 121.It Fl f Ar configuration_file 122Specifies the name of the configuration file. 123The default is 124.Pa /etc/ssh/sshd_config . 125.Nm 126refuses to start if there is no configuration file. 127.It Fl g Ar login_grace_time 128Gives the grace time for clients to authenticate themselves (default 129120 seconds). 130If the client fails to authenticate the user within 131this many seconds, the server disconnects and exits. 132A value of zero indicates no limit. 133.It Fl h Ar host_key_file 134Specifies a file from which a host key is read. 135This option must be given if 136.Nm 137is not run as root (as the normal 138host key files are normally not readable by anyone but root). 139The default is 140.Pa /etc/ssh/ssh_host_key 141for protocol version 1, and 142.Pa /etc/ssh/ssh_host_dsa_key 143for protocol version 2. 144It is possible to have multiple host key files for 145the different protocol versions and host key algorithms. 146.It Fl i 147Specifies that 148.Nm 149is being run from 150.Xr inetd 8 . 151.Nm 152is normally not run 153from inetd because it needs to generate the server key before it can 154respond to the client, and this may take tens of seconds. 155Clients would have to wait too long if the key was regenerated every time. 156However, with small key sizes (e.g., 512) using 157.Nm 158from inetd may 159be feasible. 160.It Fl k Ar key_gen_time 161Specifies how often the ephemeral protocol version 1 server key is 162regenerated (default 3600 seconds, or one hour). 163The motivation for regenerating the key fairly 164often is that the key is not stored anywhere, and after about an hour 165it becomes impossible to recover the key for decrypting intercepted 166communications even if the machine is cracked into or physically 167seized. 168A value of zero indicates that the key will never be regenerated. 169.It Fl o Ar option 170Can be used to give options in the format used in the configuration file. 171This is useful for specifying options for which there is no separate 172command-line flag. 173For full details of the options, and their values, see 174.Xr sshd_config 5 . 175.It Fl p Ar port 176Specifies the port on which the server listens for connections 177(default 22). 178Multiple port options are permitted. 179Ports specified in the configuration file with the 180.Cm Port 181option are ignored when a command-line port is specified. 182Ports specified using the 183.Cm ListenAddress 184option override command-line ports. 185.It Fl q 186Quiet mode. 187Nothing is sent to the system log. 188Normally the beginning, 189authentication, and termination of each connection is logged. 190.It Fl t 191Test mode. 192Only check the validity of the configuration file and sanity of the keys. 193This is useful for updating 194.Nm 195reliably as configuration options may change. 196.It Fl u Ar len 197This option is used to specify the size of the field 198in the 199.Li utmp 200structure that holds the remote host name. 201If the resolved host name is longer than 202.Ar len , 203the dotted decimal value will be used instead. 204This allows hosts with very long host names that 205overflow this field to still be uniquely identified. 206Specifying 207.Fl u0 208indicates that only dotted decimal addresses 209should be put into the 210.Pa utmp 211file. 212.Fl u0 213may also be used to prevent 214.Nm 215from making DNS requests unless the authentication 216mechanism or configuration requires it. 217Authentication mechanisms that may require DNS include 218.Cm RhostsRSAAuthentication , 219.Cm HostbasedAuthentication , 220and using a 221.Cm from="pattern-list" 222option in a key file. 223Configuration options that require DNS include using a 224USER@HOST pattern in 225.Cm AllowUsers 226or 227.Cm DenyUsers . 228.El 229.Sh AUTHENTICATION 230The OpenSSH SSH daemon supports SSH protocols 1 and 2. 231Both protocols are supported by default, 232though this can be changed via the 233.Cm Protocol 234option in 235.Xr sshd_config 5 . 236Protocol 2 supports both RSA and DSA keys; 237protocol 1 only supports RSA keys. 238For both protocols, 239each host has a host-specific key, 240normally 2048 bits, 241used to identify the host. 242.Pp 243Forward security for protocol 1 is provided through 244an additional server key, 245normally 768 bits, 246generated when the server starts. 247This key is normally regenerated every hour if it has been used, and 248is never stored on disk. 249Whenever a client connects, the daemon responds with its public 250host and server keys. 251The client compares the 252RSA host key against its own database to verify that it has not changed. 253The client then generates a 256-bit random number. 254It encrypts this 255random number using both the host key and the server key, and sends 256the encrypted number to the server. 257Both sides then use this 258random number as a session key which is used to encrypt all further 259communications in the session. 260The rest of the session is encrypted 261using a conventional cipher, currently Blowfish or 3DES, with 3DES 262being used by default. 263The client selects the encryption algorithm 264to use from those offered by the server. 265.Pp 266For protocol 2, 267forward security is provided through a Diffie-Hellman key agreement. 268This key agreement results in a shared session key. 269The rest of the session is encrypted using a symmetric cipher, currently 270128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. 271The client selects the encryption algorithm 272to use from those offered by the server. 273Additionally, session integrity is provided 274through a cryptographic message authentication code 275(hmac-sha1 or hmac-md5). 276.Pp 277Finally, the server and the client enter an authentication dialog. 278The client tries to authenticate itself using 279host-based authentication, 280public key authentication, 281challenge-response authentication, 282or password authentication. 283.Pp 284Regardless of the authentication type, the account is checked to 285ensure that it is accessible. An account is not accessible if it is 286locked, listed in 287.Cm DenyUsers 288or its group is listed in 289.Cm DenyGroups 290\&. The definition of a locked account is system dependant. Some platforms 291have their own account database (eg AIX) and some modify the passwd field ( 292.Ql \&*LK\&* 293on Solaris and UnixWare, 294.Ql \&* 295on HP-UX, containing 296.Ql Nologin 297on Tru64, 298a leading 299.Ql \&*LOCKED\&* 300on FreeBSD and a leading 301.Ql \&!! 302on Linux). If there is a requirement to disable password authentication 303for the account while allowing still public-key, then the passwd field 304should be set to something other than these values (eg 305.Ql NP 306or 307.Ql \&*NP\&* 308). 309.Pp 310System security is not improved unless 311.Nm rshd , 312.Nm rlogind , 313and 314.Nm rexecd 315are disabled (thus completely disabling 316.Xr rlogin 317and 318.Xr rsh 319into the machine). 320.Sh COMMAND EXECUTION AND DATA FORWARDING 321If the client successfully authenticates itself, a dialog for 322preparing the session is entered. 323At this time the client may request 324things like allocating a pseudo-tty, forwarding X11 connections, 325forwarding TCP connections, or forwarding the authentication agent 326connection over the secure channel. 327.Pp 328Finally, the client either requests a shell or execution of a command. 329The sides then enter session mode. 330In this mode, either side may send 331data at any time, and such data is forwarded to/from the shell or 332command on the server side, and the user terminal in the client side. 333.Pp 334When the user program terminates and all forwarded X11 and other 335connections have been closed, the server sends command exit status to 336the client, and both sides exit. 337.Sh LOGIN PROCESS 338When a user successfully logs in, 339.Nm 340does the following: 341.Bl -enum -offset indent 342.It 343If the login is on a tty, and no command has been specified, 344prints last login time and 345.Pa /etc/motd 346(unless prevented in the configuration file or by 347.Pa ~/.hushlogin ; 348see the 349.Sx FILES 350section). 351.It 352If the login is on a tty, records login time. 353.It 354Checks 355.Pa /etc/nologin and 356.Pa /var/run/nologin ; 357if one exists, it prints the contents and quits 358(unless root). 359.It 360Changes to run with normal user privileges. 361.It 362Sets up basic environment. 363.It 364Reads the file 365.Pa ~/.ssh/environment , 366if it exists, and users are allowed to change their environment. 367See the 368.Cm PermitUserEnvironment 369option in 370.Xr sshd_config 5 . 371.It 372Changes to user's home directory. 373.It 374If 375.Pa ~/.ssh/rc 376exists, runs it; else if 377.Pa /etc/ssh/sshrc 378exists, runs 379it; otherwise runs 380.Xr xauth 1 . 381The 382.Dq rc 383files are given the X11 384authentication protocol and cookie (if applicable) in standard input. 385.It 386Runs user's shell or command. 387.El 388.Sh AUTHORIZED_KEYS FILE FORMAT 389.Pa ~/.ssh/authorized_keys 390is the default file that lists the public keys that are 391permitted for RSA authentication in protocol version 1 392and for public key authentication (PubkeyAuthentication) 393in protocol version 2. 394.Cm AuthorizedKeysFile 395may be used to specify an alternative file. 396.Pp 397Each line of the file contains one 398key (empty lines and lines starting with a 399.Ql # 400are ignored as 401comments). 402Each RSA public key consists of the following fields, separated by 403spaces: options, bits, exponent, modulus, comment. 404Each protocol version 2 public key consists of: 405options, keytype, base64 encoded key, comment. 406The options field 407is optional; its presence is determined by whether the line starts 408with a number or not (the options field never starts with a number). 409The bits, exponent, modulus and comment fields give the RSA key for 410protocol version 1; the 411comment field is not used for anything (but may be convenient for the 412user to identify the key). 413For protocol version 2 the keytype is 414.Dq ssh-dss 415or 416.Dq ssh-rsa . 417.Pp 418Note that lines in this file are usually several hundred bytes long 419(because of the size of the public key encoding) up to a limit of 4208 kilobytes, which permits DSA keys up to 8 kilobits and RSA 421keys up to 16 kilobits. 422You don't want to type them in; instead, copy the 423.Pa identity.pub , 424.Pa id_dsa.pub 425or the 426.Pa id_rsa.pub 427file and edit it. 428.Pp 429.Nm 430enforces a minimum RSA key modulus size for protocol 1 431and protocol 2 keys of 768 bits. 432.Pp 433The options (if present) consist of comma-separated option 434specifications. 435No spaces are permitted, except within double quotes. 436The following option specifications are supported (note 437that option keywords are case-insensitive): 438.Bl -tag -width Ds 439.It Cm from="pattern-list" 440Specifies that in addition to public key authentication, the canonical name 441of the remote host must be present in the comma-separated list of 442patterns 443.Pf ( Ql \&* 444and 445.Ql \&? 446serve as wildcards). 447The list may also contain 448patterns negated by prefixing them with 449.Ql \&! ; 450if the canonical host name matches a negated pattern, the key is not accepted. 451The purpose 452of this option is to optionally increase security: public key authentication 453by itself does not trust the network or name servers or anything (but 454the key); however, if somebody somehow steals the key, the key 455permits an intruder to log in from anywhere in the world. 456This additional option makes using a stolen key more difficult (name 457servers and/or routers would have to be compromised in addition to 458just the key). 459.It Cm command="command" 460Specifies that the command is executed whenever this key is used for 461authentication. 462The command supplied by the user (if any) is ignored. 463The command is run on a pty if the client requests a pty; 464otherwise it is run without a tty. 465If an 8-bit clean channel is required, 466one must not request a pty or should specify 467.Cm no-pty . 468A quote may be included in the command by quoting it with a backslash. 469This option might be useful 470to restrict certain public keys to perform just a specific operation. 471An example might be a key that permits remote backups but nothing else. 472Note that the client may specify TCP and/or X11 473forwarding unless they are explicitly prohibited. 474Note that this option applies to shell, command or subsystem execution. 475.It Cm environment="NAME=value" 476Specifies that the string is to be added to the environment when 477logging in using this key. 478Environment variables set this way 479override other default environment values. 480Multiple options of this type are permitted. 481Environment processing is disabled by default and is 482controlled via the 483.Cm PermitUserEnvironment 484option. 485This option is automatically disabled if 486.Cm UseLogin 487is enabled. 488.It Cm no-port-forwarding 489Forbids TCP forwarding when this key is used for authentication. 490Any port forward requests by the client will return an error. 491This might be used, e.g., in connection with the 492.Cm command 493option. 494.It Cm no-X11-forwarding 495Forbids X11 forwarding when this key is used for authentication. 496Any X11 forward requests by the client will return an error. 497.It Cm no-agent-forwarding 498Forbids authentication agent forwarding when this key is used for 499authentication. 500.It Cm no-pty 501Prevents tty allocation (a request to allocate a pty will fail). 502.It Cm permitopen="host:port" 503Limit local 504.Li ``ssh -L'' 505port forwarding such that it may only connect to the specified host and 506port. 507IPv6 addresses can be specified with an alternative syntax: 508.Ar host Ns / Ns Ar port . 509Multiple 510.Cm permitopen 511options may be applied separated by commas. 512No pattern matching is performed on the specified hostnames, 513they must be literal domains or addresses. 514.It Cm tunnel="n" 515Force a 516.Xr tun 4 517device on the server. 518Without this option, the next available device will be used if 519the client requests a tunnel. 520.El 521.Ss Examples 5221024 33 12121...312314325 ylo@foo.bar 523.Pp 524from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula 525.Pp 526command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi 527.Pp 528permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323 529.Pp 530tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== reyk@openbsd.org 531.Sh SSH_KNOWN_HOSTS FILE FORMAT 532The 533.Pa /etc/ssh/ssh_known_hosts 534and 535.Pa ~/.ssh/known_hosts 536files contain host public keys for all known hosts. 537The global file should 538be prepared by the administrator (optional), and the per-user file is 539maintained automatically: whenever the user connects from an unknown host 540its key is added to the per-user file. 541.Pp 542Each line in these files contains the following fields: hostnames, 543bits, exponent, modulus, comment. 544The fields are separated by spaces. 545.Pp 546Hostnames is a comma-separated list of patterns 547.Pf ( Ql \&* 548and 549.Ql \&? 550act as 551wildcards); each pattern in turn is matched against the canonical host 552name (when authenticating a client) or against the user-supplied 553name (when authenticating a server). 554A pattern may also be preceded by 555.Ql \&! 556to indicate negation: if the host name matches a negated 557pattern, it is not accepted (by that line) even if it matched another 558pattern on the line. 559.Pp 560Alternately, hostnames may be stored in a hashed form which hides host names 561and addresses should the file's contents be disclosed. 562Hashed hostnames start with a 563.Ql | 564character. 565Only one hashed hostname may appear on a single line and none of the above 566negation or wildcard operators may be applied. 567.Pp 568Bits, exponent, and modulus are taken directly from the RSA host key; they 569can be obtained, e.g., from 570.Pa /etc/ssh/ssh_host_key.pub . 571The optional comment field continues to the end of the line, and is not used. 572.Pp 573Lines starting with 574.Ql # 575and empty lines are ignored as comments. 576.Pp 577When performing host authentication, authentication is accepted if any 578matching line has the proper key. 579It is thus permissible (but not 580recommended) to have several lines or different host keys for the same 581names. 582This will inevitably happen when short forms of host names 583from different domains are put in the file. 584It is possible 585that the files contain conflicting information; authentication is 586accepted if valid information can be found from either file. 587.Pp 588Note that the lines in these files are typically hundreds of characters 589long, and you definitely don't want to type in the host keys by hand. 590Rather, generate them by a script 591or by taking 592.Pa /etc/ssh/ssh_host_key.pub 593and adding the host names at the front. 594.Ss Examples 595.Bd -literal 596closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi 597cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= 598.Ed 599.Bd -literal 600# A hashed hostname 601|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 602AAAA1234.....= 603.Ed 604.Sh FILES 605.Bl -tag -width Ds 606.It Pa /etc/ssh/sshd_config 607Contains configuration data for 608.Nm sshd . 609The file format and configuration options are described in 610.Xr sshd_config 5 . 611.It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key 612These two files contain the private parts of the host keys. 613These files should only be owned by root, readable only by root, and not 614accessible to others. 615Note that 616.Nm 617does not start if this file is group/world-accessible. 618.It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub 619These two files contain the public parts of the host keys. 620These files should be world-readable but writable only by 621root. 622Their contents should match the respective private parts. 623These files are not 624really used for anything; they are provided for the convenience of 625the user so their contents can be copied to known hosts files. 626These files are created using 627.Xr ssh-keygen 1 . 628.It Pa /etc/ssh/moduli 629Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". 630The file format is described in 631.Xr moduli 5 . 632.It Pa /var/empty 633.Xr chroot 2 634directory used by 635.Nm 636during privilege separation in the pre-authentication phase. 637The directory should not contain any files and must be owned by root 638and not group or world-writable. 639.It Pa /var/run/sshd.pid 640Contains the process ID of the 641.Nm 642listening for connections (if there are several daemons running 643concurrently for different ports, this contains the process ID of the one 644started last). 645The content of this file is not sensitive; it can be world-readable. 646.It Pa ~/.ssh/authorized_keys 647Lists the public keys (RSA or DSA) that can be used to log into the user's account. 648This file must be readable by root (which may on some machines imply 649it being world-readable if the user's home directory resides on an NFS 650volume). 651It is recommended that it not be accessible by others. 652The format of this file is described above. 653Users will place the contents of their 654.Pa identity.pub , 655.Pa id_dsa.pub 656and/or 657.Pa id_rsa.pub 658files into this file, as described in 659.Xr ssh-keygen 1 . 660.It Pa "/etc/ssh/ssh_known_hosts", "~/.ssh/known_hosts" 661These files are consulted when using rhosts with RSA host 662authentication or protocol version 2 hostbased authentication 663to check the public key of the host. 664The key must be listed in one of these files to be accepted. 665The client uses the same files 666to verify that it is connecting to the correct remote host. 667These files should be writable only by root/the owner. 668.Pa /etc/ssh/ssh_known_hosts 669should be world-readable, and 670.Pa ~/.ssh/known_hosts 671can, but need not be, world-readable. 672.It Pa /etc/motd 673See 674.Xr motd 5 . 675.It Pa ~/.hushlogin 676This file is used to suppress printing the last login time and 677.Pa /etc/motd , 678if 679.Cm PrintLastLog 680and 681.Cm PrintMotd , 682respectively, 683are enabled. 684It does not suppress printing of the banner specified by 685.Cm Banner . 686.It Pa /etc/nologin 687If this file exists, 688.Nm 689refuses to let anyone except root log in. 690The contents of the file 691are displayed to anyone trying to log in, and non-root connections are 692refused. 693The file should be world-readable. 694.It Pa /etc/hosts.allow, /etc/hosts.deny 695Access controls that should be enforced by tcp-wrappers are defined here. 696Further details are described in 697.Xr hosts_access 5 . 698.It Pa ~/.rhosts 699This file is used during 700.Cm RhostsRSAAuthentication 701and 702.Cm HostbasedAuthentication 703and contains host-username pairs, separated by a space, one per 704line. 705The given user on the corresponding host is permitted to log in 706without a password. 707The same file is used by rlogind and rshd. 708The file must 709be writable only by the user; it is recommended that it not be 710accessible by others. 711.Pp 712It is also possible to use netgroups in the file. 713Either host or user 714name may be of the form +@groupname to specify all hosts or all users 715in the group. 716.It Pa ~/.shosts 717For ssh, 718this file is exactly the same as for 719.Pa .rhosts . 720However, this file is 721not used by rlogin and rshd, so using this permits access using SSH only. 722.It Pa /etc/hosts.equiv 723This file is used during 724.Cm RhostsRSAAuthentication 725and 726.Cm HostbasedAuthentication 727authentication. 728In the simplest form, this file contains host names, one per line. 729Users on 730those hosts are permitted to log in without a password, provided they 731have the same user name on both machines. 732The host name may also be 733followed by a user name; such users are permitted to log in as 734.Em any 735user on this machine (except root). 736Additionally, the syntax 737.Dq +@group 738can be used to specify netgroups. 739Negated entries start with 740.Ql \&- . 741.Pp 742If the client host/user is successfully matched in this file, login is 743automatically permitted provided the client and server user names are the 744same. 745Additionally, successful client host key authentication is required. 746This file must be writable only by root; it is recommended 747that it be world-readable. 748.Pp 749.Sy "Warning: It is almost never a good idea to use user names in" 750.Pa hosts.equiv . 751Beware that it really means that the named user(s) can log in as 752.Em anybody , 753which includes bin, daemon, adm, and other accounts that own critical 754binaries and directories. 755Using a user name practically grants the user root access. 756The only valid use for user names that I can think 757of is in negative entries. 758.Pp 759Note that this warning also applies to rsh/rlogin. 760.It Pa /etc/ssh/shosts.equiv 761This is processed exactly as 762.Pa /etc/hosts.equiv . 763However, this file may be useful in environments that want to run both 764rsh/rlogin and ssh. 765.It Pa ~/.ssh/environment 766This file is read into the environment at login (if it exists). 767It can only contain empty lines, comment lines (that start with 768.Ql # ) , 769and assignment lines of the form name=value. 770The file should be writable 771only by the user; it need not be readable by anyone else. 772Environment processing is disabled by default and is 773controlled via the 774.Cm PermitUserEnvironment 775option. 776.It Pa ~/.ssh/rc 777If this file exists, it is run with 778.Pa /bin/sh 779after reading the 780environment files but before starting the user's shell or command. 781It must not produce any output on stdout; stderr must be used 782instead. 783If X11 forwarding is in use, it will receive the "proto cookie" pair in 784its standard input (and 785.Ev DISPLAY 786in its environment). 787The script must call 788.Xr xauth 1 789because 790.Nm 791will not run xauth automatically to add X11 cookies. 792.Pp 793The primary purpose of this file is to run any initialization routines 794which may be needed before the user's home directory becomes 795accessible; AFS is a particular example of such an environment. 796.Pp 797This file will probably contain some initialization code followed by 798something similar to: 799.Bd -literal 800if read proto cookie && [ -n "$DISPLAY" ]; then 801 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 802 # X11UseLocalhost=yes 803 echo add unix:`echo $DISPLAY | 804 cut -c11-` $proto $cookie 805 else 806 # X11UseLocalhost=no 807 echo add $DISPLAY $proto $cookie 808 fi | xauth -q - 809fi 810.Ed 811.Pp 812If this file does not exist, 813.Pa /etc/ssh/sshrc 814is run, and if that 815does not exist either, xauth is used to add the cookie. 816.Pp 817This file should be writable only by the user, and need not be 818readable by anyone else. 819.It Pa /etc/ssh/sshrc 820Like 821.Pa ~/.ssh/rc . 822This can be used to specify 823machine-specific login-time initializations globally. 824This file should be writable only by root, and should be world-readable. 825.El 826.Sh SEE ALSO 827.Xr scp 1 , 828.Xr sftp 1 , 829.Xr ssh 1 , 830.Xr ssh-add 1 , 831.Xr ssh-agent 1 , 832.Xr ssh-keygen 1 , 833.Xr chroot 2 , 834.Xr hosts_access 5 , 835.Xr login.conf 5 , 836.Xr moduli 5 , 837.Xr sshd_config 5 , 838.Xr inetd 8 , 839.Xr sftp-server 8 840.Rs 841.%A T. Ylonen 842.%A T. Kivinen 843.%A M. Saarinen 844.%A T. Rinne 845.%A S. Lehtinen 846.%T "SSH Protocol Architecture" 847.%N draft-ietf-secsh-architecture-12.txt 848.%D January 2002 849.%O work in progress material 850.Re 851.Rs 852.%A M. Friedl 853.%A N. Provos 854.%A W. A. Simpson 855.%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" 856.%N draft-ietf-secsh-dh-group-exchange-02.txt 857.%D January 2002 858.%O work in progress material 859.Re 860.Sh AUTHORS 861OpenSSH is a derivative of the original and free 862ssh 1.2.12 release by Tatu Ylonen. 863Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 864Theo de Raadt and Dug Song 865removed many bugs, re-added newer features and 866created OpenSSH. 867Markus Friedl contributed the support for SSH 868protocol versions 1.5 and 2.0. 869Niels Provos and Markus Friedl contributed support 870for privilege separation. 871