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 1.201 2004/05/02 11:54:31 dtucker 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(SSH 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. 65The programs are intended to be as easy to 66install and use as possible. 67.Pp 68.Nm 69is the daemon that listens for connections from clients. 70It is normally started at boot from 71.Pa /etc/rc.d/sshd . 72It forks a new 73daemon for each incoming connection. 74The forked daemons handle 75key exchange, encryption, authentication, command execution, 76and data exchange. 77This implementation of 78.Nm 79supports both SSH protocol version 1 and 2 simultaneously. 80.Nm 81works as follows: 82.Ss SSH protocol version 1 83Each host has a host-specific RSA key 84(normally 1024 bits) used to identify the host. 85Additionally, when 86the daemon starts, it generates a server RSA key (normally 768 bits). 87This key is normally regenerated every hour if it has been used, and 88is never stored on disk. 89.Pp 90Whenever a client connects, the daemon responds with its public 91host and server keys. 92The client compares the 93RSA host key against its own database to verify that it has not changed. 94The client then generates a 256-bit random number. 95It encrypts this 96random number using both the host key and the server key, and sends 97the encrypted number to the server. 98Both sides then use this 99random number as a session key which is used to encrypt all further 100communications in the session. 101The rest of the session is encrypted 102using a conventional cipher, currently Blowfish or 3DES, with 3DES 103being used by default. 104The client selects the encryption algorithm 105to use from those offered by the server. 106.Pp 107Next, the server and the client enter an authentication dialog. 108The client tries to authenticate itself using 109.Em .rhosts 110authentication, 111.Em .rhosts 112authentication combined with RSA host 113authentication, RSA challenge-response authentication, or password 114based authentication. 115.Pp 116Regardless of the authentication type, the account is checked to 117ensure that it is accessible. An account is not accessible if it is 118locked, listed in 119.Cm DenyUsers 120or its group is listed in 121.Cm DenyGroups 122\&. The definition of a locked account is system dependant. Some platforms 123have their own account database (eg AIX) and some modify the passwd field ( 124.Ql \&*LK\&* 125on Solaris, 126.Ql \&* 127on HP-UX, containing 128.Ql Nologin 129on Tru64 and a leading 130.Ql \&!! 131on Linux). If there is a requirement to disable password authentication 132for the account while allowing still public-key, then the passwd field 133should be set to something other than these values (eg 134.Ql NP 135or 136.Ql \&*NP\&* 137). 138.Pp 139.Em rhosts 140authentication is normally disabled 141because it is fundamentally insecure, but can be enabled in the server 142configuration file if desired. 143System security is not improved unless 144.Nm rshd , 145.Nm rlogind , 146and 147.Nm rexecd 148are disabled (thus completely disabling 149.Xr rlogin 150and 151.Xr rsh 152into the machine). 153.Ss SSH protocol version 2 154Version 2 works similarly: 155Each host has a host-specific key (RSA or DSA) used to identify the host. 156However, when the daemon starts, it does not generate a server key. 157Forward security is provided through a Diffie-Hellman key agreement. 158This key agreement results in a shared session key. 159.Pp 160The rest of the session is encrypted using a symmetric cipher, currently 161128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. 162The client selects the encryption algorithm 163to use from those offered by the server. 164Additionally, session integrity is provided 165through a cryptographic message authentication code 166(hmac-sha1 or hmac-md5). 167.Pp 168Protocol version 2 provides a public key based 169user (PubkeyAuthentication) or 170client host (HostbasedAuthentication) authentication method, 171conventional password authentication and challenge response based methods. 172.Ss Command execution and data forwarding 173If the client successfully authenticates itself, a dialog for 174preparing the session is entered. 175At this time the client may request 176things like allocating a pseudo-tty, forwarding X11 connections, 177forwarding TCP/IP connections, or forwarding the authentication agent 178connection over the secure channel. 179.Pp 180Finally, the client either requests a shell or execution of a command. 181The sides then enter session mode. 182In this mode, either side may send 183data at any time, and such data is forwarded to/from the shell or 184command on the server side, and the user terminal in the client side. 185.Pp 186When the user program terminates and all forwarded X11 and other 187connections have been closed, the server sends command exit status to 188the client, and both sides exit. 189.Pp 190.Nm 191can be configured using command-line options or a configuration file 192(by default 193.Xr sshd_config 5 ) . 194Command-line options override values specified in the 195configuration file. 196.Pp 197.Nm 198rereads its configuration file when it receives a hangup signal, 199.Dv SIGHUP , 200by executing itself with the name and options it was started with, e.g., 201.Pa /usr/sbin/sshd . 202.Pp 203The options are as follows: 204.Bl -tag -width Ds 205.It Fl 4 206Forces 207.Nm 208to use IPv4 addresses only. 209.It Fl 6 210Forces 211.Nm 212to use IPv6 addresses only. 213.It Fl b Ar bits 214Specifies the number of bits in the ephemeral protocol version 1 215server key (default 768). 216.It Fl D 217When this option is specified, 218.Nm 219will not detach and does not become a daemon. 220This allows easy monitoring of 221.Nm sshd . 222.It Fl d 223Debug mode. 224The server sends verbose debug output to the system 225log, and does not put itself in the background. 226The server also will not fork and will only process one connection. 227This option is only intended for debugging for the server. 228Multiple 229.Fl d 230options increase the debugging level. 231Maximum is 3. 232.It Fl e 233When this option is specified, 234.Nm 235will send the output to the standard error instead of the system log. 236.It Fl f Ar configuration_file 237Specifies the name of the configuration file. 238The default is 239.Pa /etc/ssh/sshd_config . 240.Nm 241refuses to start if there is no configuration file. 242.It Fl g Ar login_grace_time 243Gives the grace time for clients to authenticate themselves (default 244120 seconds). 245If the client fails to authenticate the user within 246this many seconds, the server disconnects and exits. 247A value of zero indicates no limit. 248.It Fl h Ar host_key_file 249Specifies a file from which a host key is read. 250This option must be given if 251.Nm 252is not run as root (as the normal 253host key files are normally not readable by anyone but root). 254The default is 255.Pa /etc/ssh/ssh_host_key 256for protocol version 1, and 257.Pa /etc/ssh/ssh_host_dsa_key 258for protocol version 2. 259It is possible to have multiple host key files for 260the different protocol versions and host key algorithms. 261.It Fl i 262Specifies that 263.Nm 264is being run from 265.Xr inetd 8 . 266.Nm 267is normally not run 268from inetd because it needs to generate the server key before it can 269respond to the client, and this may take tens of seconds. 270Clients would have to wait too long if the key was regenerated every time. 271However, with small key sizes (e.g., 512) using 272.Nm 273from inetd may 274be feasible. 275.It Fl k Ar key_gen_time 276Specifies how often the ephemeral protocol version 1 server key is 277regenerated (default 3600 seconds, or one hour). 278The motivation for regenerating the key fairly 279often is that the key is not stored anywhere, and after about an hour 280it becomes impossible to recover the key for decrypting intercepted 281communications even if the machine is cracked into or physically 282seized. 283A value of zero indicates that the key will never be regenerated. 284.It Fl o Ar option 285Can be used to give options in the format used in the configuration file. 286This is useful for specifying options for which there is no separate 287command-line flag. 288For full details of the options, and their values, see 289.Xr sshd_config 5 . 290.It Fl p Ar port 291Specifies the port on which the server listens for connections 292(default 22). 293Multiple port options are permitted. 294Ports specified in the configuration file are ignored when a 295command-line port is specified. 296.It Fl q 297Quiet mode. 298Nothing is sent to the system log. 299Normally the beginning, 300authentication, and termination of each connection is logged. 301.It Fl t 302Test mode. 303Only check the validity of the configuration file and sanity of the keys. 304This is useful for updating 305.Nm 306reliably as configuration options may change. 307.It Fl u Ar len 308This option is used to specify the size of the field 309in the 310.Li utmp 311structure that holds the remote host name. 312If the resolved host name is longer than 313.Ar len , 314the dotted decimal value will be used instead. 315This allows hosts with very long host names that 316overflow this field to still be uniquely identified. 317Specifying 318.Fl u0 319indicates that only dotted decimal addresses 320should be put into the 321.Pa utmp 322file. 323.Fl u0 324may also be used to prevent 325.Nm 326from making DNS requests unless the authentication 327mechanism or configuration requires it. 328Authentication mechanisms that may require DNS include 329.Cm RhostsRSAAuthentication , 330.Cm HostbasedAuthentication 331and using a 332.Cm from="pattern-list" 333option in a key file. 334Configuration options that require DNS include using a 335USER@HOST pattern in 336.Cm AllowUsers 337or 338.Cm DenyUsers . 339.El 340.Sh CONFIGURATION FILE 341.Nm 342reads configuration data from 343.Pa /etc/ssh/sshd_config 344(or the file specified with 345.Fl f 346on the command line). 347The file format and configuration options are described in 348.Xr sshd_config 5 . 349.Sh LOGIN PROCESS 350When a user successfully logs in, 351.Nm 352does the following: 353.Bl -enum -offset indent 354.It 355If the login is on a tty, and no command has been specified, 356prints last login time and 357.Pa /etc/motd 358(unless prevented in the configuration file or by 359.Pa $HOME/.hushlogin ; 360see the 361.Sx FILES 362section). 363.It 364If the login is on a tty, records login time. 365.It 366Checks 367.Pa /etc/nologin and 368.Pa /var/run/nologin ; 369if one exists, it prints the contents and quits 370(unless root). 371.It 372Changes to run with normal user privileges. 373.It 374Sets up basic environment. 375.It 376Reads the file 377.Pa $HOME/.ssh/environment , 378if it exists, and users are allowed to change their environment. 379See the 380.Cm PermitUserEnvironment 381option in 382.Xr sshd_config 5 . 383.It 384Changes to user's home directory. 385.It 386If 387.Pa $HOME/.ssh/rc 388exists, runs it; else if 389.Pa /etc/ssh/sshrc 390exists, runs 391it; otherwise runs 392.Xr xauth 1 . 393The 394.Dq rc 395files are given the X11 396authentication protocol and cookie (if applicable) in standard input. 397.It 398Runs user's shell or command. 399.El 400.Sh AUTHORIZED_KEYS FILE FORMAT 401.Pa $HOME/.ssh/authorized_keys 402is the default file that lists the public keys that are 403permitted for RSA authentication in protocol version 1 404and for public key authentication (PubkeyAuthentication) 405in protocol version 2. 406.Cm AuthorizedKeysFile 407may be used to specify an alternative file. 408.Pp 409Each line of the file contains one 410key (empty lines and lines starting with a 411.Ql # 412are ignored as 413comments). 414Each RSA public key consists of the following fields, separated by 415spaces: options, bits, exponent, modulus, comment. 416Each protocol version 2 public key consists of: 417options, keytype, base64 encoded key, comment. 418The options field 419is optional; its presence is determined by whether the line starts 420with a number or not (the options field never starts with a number). 421The bits, exponent, modulus and comment fields give the RSA key for 422protocol version 1; the 423comment field is not used for anything (but may be convenient for the 424user to identify the key). 425For protocol version 2 the keytype is 426.Dq ssh-dss 427or 428.Dq ssh-rsa . 429.Pp 430Note that lines in this file are usually several hundred bytes long 431(because of the size of the public key encoding). 432You don't want to type them in; instead, copy the 433.Pa identity.pub , 434.Pa id_dsa.pub 435or the 436.Pa id_rsa.pub 437file and edit it. 438.Pp 439.Nm 440enforces a minimum RSA key modulus size for protocol 1 441and protocol 2 keys of 768 bits. 442.Pp 443The options (if present) consist of comma-separated option 444specifications. 445No spaces are permitted, except within double quotes. 446The following option specifications are supported (note 447that option keywords are case-insensitive): 448.Bl -tag -width Ds 449.It Cm from="pattern-list" 450Specifies that in addition to public key authentication, the canonical name 451of the remote host must be present in the comma-separated list of 452patterns 453.Pf ( Ql \&* 454and 455.Ql \&? 456serve as wildcards). 457The list may also contain 458patterns negated by prefixing them with 459.Ql \&! ; 460if the canonical host name matches a negated pattern, the key is not accepted. 461The purpose 462of this option is to optionally increase security: public key authentication 463by itself does not trust the network or name servers or anything (but 464the key); however, if somebody somehow steals the key, the key 465permits an intruder to log in from anywhere in the world. 466This additional option makes using a stolen key more difficult (name 467servers and/or routers would have to be compromised in addition to 468just the key). 469.It Cm command="command" 470Specifies that the command is executed whenever this key is used for 471authentication. 472The command supplied by the user (if any) is ignored. 473The command is run on a pty if the client requests a pty; 474otherwise it is run without a tty. 475If an 8-bit clean channel is required, 476one must not request a pty or should specify 477.Cm no-pty . 478A quote may be included in the command by quoting it with a backslash. 479This option might be useful 480to restrict certain public keys to perform just a specific operation. 481An example might be a key that permits remote backups but nothing else. 482Note that the client may specify TCP/IP and/or X11 483forwarding unless they are explicitly prohibited. 484Note that this option applies to shell, command or subsystem execution. 485.It Cm environment="NAME=value" 486Specifies that the string is to be added to the environment when 487logging in using this key. 488Environment variables set this way 489override other default environment values. 490Multiple options of this type are permitted. 491Environment processing is disabled by default and is 492controlled via the 493.Cm PermitUserEnvironment 494option. 495This option is automatically disabled if 496.Cm UseLogin 497is enabled. 498.It Cm no-port-forwarding 499Forbids TCP/IP forwarding when this key is used for authentication. 500Any port forward requests by the client will return an error. 501This might be used, e.g., in connection with the 502.Cm command 503option. 504.It Cm no-X11-forwarding 505Forbids X11 forwarding when this key is used for authentication. 506Any X11 forward requests by the client will return an error. 507.It Cm no-agent-forwarding 508Forbids authentication agent forwarding when this key is used for 509authentication. 510.It Cm no-pty 511Prevents tty allocation (a request to allocate a pty will fail). 512.It Cm permitopen="host:port" 513Limit local 514.Li ``ssh -L'' 515port forwarding such that it may only connect to the specified host and 516port. 517IPv6 addresses can be specified with an alternative syntax: 518.Ar host Ns / Ns Ar port . 519Multiple 520.Cm permitopen 521options may be applied separated by commas. 522No pattern matching is performed on the specified hostnames, 523they must be literal domains or addresses. 524.El 525.Ss Examples 5261024 33 12121...312314325 ylo@foo.bar 527.Pp 528from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula 529.Pp 530command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi 531.Pp 532permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323 533.Sh SSH_KNOWN_HOSTS FILE FORMAT 534The 535.Pa /etc/ssh/ssh_known_hosts 536and 537.Pa $HOME/.ssh/known_hosts 538files contain host public keys for all known hosts. 539The global file should 540be prepared by the administrator (optional), and the per-user file is 541maintained automatically: whenever the user connects from an unknown host 542its key is added to the per-user file. 543.Pp 544Each line in these files contains the following fields: hostnames, 545bits, exponent, modulus, comment. 546The fields are separated by spaces. 547.Pp 548Hostnames is a comma-separated list of patterns 549.Pf ( Ql \&* 550and 551.Ql \&? 552act as 553wildcards); each pattern in turn is matched against the canonical host 554name (when authenticating a client) or against the user-supplied 555name (when authenticating a server). 556A pattern may also be preceded by 557.Ql \&! 558to indicate negation: if the host name matches a negated 559pattern, it is not accepted (by that line) even if it matched another 560pattern on the line. 561.Pp 562Bits, exponent, and modulus are taken directly from the RSA host key; they 563can be obtained, e.g., from 564.Pa /etc/ssh/ssh_host_key.pub . 565The optional comment field continues to the end of the line, and is not used. 566.Pp 567Lines starting with 568.Ql # 569and empty lines are ignored as comments. 570.Pp 571When performing host authentication, authentication is accepted if any 572matching line has the proper key. 573It is thus permissible (but not 574recommended) to have several lines or different host keys for the same 575names. 576This will inevitably happen when short forms of host names 577from different domains are put in the file. 578It is possible 579that the files contain conflicting information; authentication is 580accepted if valid information can be found from either file. 581.Pp 582Note that the lines in these files are typically hundreds of characters 583long, and you definitely don't want to type in the host keys by hand. 584Rather, generate them by a script 585or by taking 586.Pa /etc/ssh/ssh_host_key.pub 587and adding the host names at the front. 588.Ss Examples 589.Bd -literal 590closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi 591cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= 592.Ed 593.Sh FILES 594.Bl -tag -width Ds 595.It Pa /etc/ssh/sshd_config 596Contains configuration data for 597.Nm sshd . 598The file format and configuration options are described in 599.Xr sshd_config 5 . 600.It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key 601These two files contain the private parts of the host keys. 602These files should only be owned by root, readable only by root, and not 603accessible to others. 604Note that 605.Nm 606does not start if this file is group/world-accessible. 607.It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub 608These two files contain the public parts of the host keys. 609These files should be world-readable but writable only by 610root. 611Their contents should match the respective private parts. 612These files are not 613really used for anything; they are provided for the convenience of 614the user so their contents can be copied to known hosts files. 615These files are created using 616.Xr ssh-keygen 1 . 617.It Pa /etc/ssh/moduli 618Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". 619The file format is described in 620.Xr moduli 5 . 621.It Pa /var/empty 622.Xr chroot 2 623directory used by 624.Nm 625during privilege separation in the pre-authentication phase. 626The directory should not contain any files and must be owned by root 627and not group or world-writable. 628.It Pa /var/run/sshd.pid 629Contains the process ID of the 630.Nm 631listening for connections (if there are several daemons running 632concurrently for different ports, this contains the process ID of the one 633started last). 634The content of this file is not sensitive; it can be world-readable. 635.It Pa $HOME/.ssh/authorized_keys 636Lists the public keys (RSA or DSA) that can be used to log into the user's account. 637This file must be readable by root (which may on some machines imply 638it being world-readable if the user's home directory resides on an NFS 639volume). 640It is recommended that it not be accessible by others. 641The format of this file is described above. 642Users will place the contents of their 643.Pa identity.pub , 644.Pa id_dsa.pub 645and/or 646.Pa id_rsa.pub 647files into this file, as described in 648.Xr ssh-keygen 1 . 649.It Pa "/etc/ssh/ssh_known_hosts", "$HOME/.ssh/known_hosts" 650These files are consulted when using rhosts with RSA host 651authentication or protocol version 2 hostbased authentication 652to check the public key of the host. 653The key must be listed in one of these files to be accepted. 654The client uses the same files 655to verify that it is connecting to the correct remote host. 656These files should be writable only by root/the owner. 657.Pa /etc/ssh/ssh_known_hosts 658should be world-readable, and 659.Pa $HOME/.ssh/known_hosts 660can, but need not be, world-readable. 661.It Pa /etc/nologin 662If this file exists, 663.Nm 664refuses to let anyone except root log in. 665The contents of the file 666are displayed to anyone trying to log in, and non-root connections are 667refused. 668The file should be world-readable. 669.It Pa /etc/hosts.allow, /etc/hosts.deny 670Access controls that should be enforced by tcp-wrappers are defined here. 671Further details are described in 672.Xr hosts_access 5 . 673.It Pa $HOME/.rhosts 674This file contains host-username pairs, separated by a space, one per 675line. 676The given user on the corresponding host is permitted to log in 677without a password. 678The same file is used by rlogind and rshd. 679The file must 680be writable only by the user; it is recommended that it not be 681accessible by others. 682.Pp 683It is also possible to use netgroups in the file. 684Either host or user 685name may be of the form +@groupname to specify all hosts or all users 686in the group. 687.It Pa $HOME/.shosts 688For ssh, 689this file is exactly the same as for 690.Pa .rhosts . 691However, this file is 692not used by rlogin and rshd, so using this permits access using SSH only. 693.It Pa /etc/hosts.equiv 694This file is used during 695.Em rhosts 696authentication. 697In the simplest form, this file contains host names, one per line. 698Users on 699those hosts are permitted to log in without a password, provided they 700have the same user name on both machines. 701The host name may also be 702followed by a user name; such users are permitted to log in as 703.Em any 704user on this machine (except root). 705Additionally, the syntax 706.Dq +@group 707can be used to specify netgroups. 708Negated entries start with 709.Ql \&- . 710.Pp 711If the client host/user is successfully matched in this file, login is 712automatically permitted provided the client and server user names are the 713same. 714Additionally, successful RSA host authentication is normally required. 715This file must be writable only by root; it is recommended 716that it be world-readable. 717.Pp 718.Sy "Warning: It is almost never a good idea to use user names in" 719.Pa hosts.equiv . 720Beware that it really means that the named user(s) can log in as 721.Em anybody , 722which includes bin, daemon, adm, and other accounts that own critical 723binaries and directories. 724Using a user name practically grants the user root access. 725The only valid use for user names that I can think 726of is in negative entries. 727.Pp 728Note that this warning also applies to rsh/rlogin. 729.It Pa /etc/ssh/shosts.equiv 730This is processed exactly as 731.Pa /etc/hosts.equiv . 732However, this file may be useful in environments that want to run both 733rsh/rlogin and ssh. 734.It Pa $HOME/.ssh/environment 735This file is read into the environment at login (if it exists). 736It can only contain empty lines, comment lines (that start with 737.Ql # ) , 738and assignment lines of the form name=value. 739The file should be writable 740only by the user; it need not be readable by anyone else. 741Environment processing is disabled by default and is 742controlled via the 743.Cm PermitUserEnvironment 744option. 745.It Pa $HOME/.ssh/rc 746If this file exists, it is run with 747.Pa /bin/sh 748after reading the 749environment files but before starting the user's shell or command. 750It must not produce any output on stdout; stderr must be used 751instead. 752If X11 forwarding is in use, it will receive the "proto cookie" pair in 753its standard input (and 754.Ev DISPLAY 755in its environment). 756The script must call 757.Xr xauth 1 758because 759.Nm 760will not run xauth automatically to add X11 cookies. 761.Pp 762The primary purpose of this file is to run any initialization routines 763which may be needed before the user's home directory becomes 764accessible; AFS is a particular example of such an environment. 765.Pp 766This file will probably contain some initialization code followed by 767something similar to: 768.Bd -literal 769if read proto cookie && [ -n "$DISPLAY" ]; then 770 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 771 # X11UseLocalhost=yes 772 echo add unix:`echo $DISPLAY | 773 cut -c11-` $proto $cookie 774 else 775 # X11UseLocalhost=no 776 echo add $DISPLAY $proto $cookie 777 fi | xauth -q - 778fi 779.Ed 780.Pp 781If this file does not exist, 782.Pa /etc/ssh/sshrc 783is run, and if that 784does not exist either, xauth is used to add the cookie. 785.Pp 786This file should be writable only by the user, and need not be 787readable by anyone else. 788.It Pa /etc/ssh/sshrc 789Like 790.Pa $HOME/.ssh/rc . 791This can be used to specify 792machine-specific login-time initializations globally. 793This file should be writable only by root, and should be world-readable. 794.El 795.Sh SEE ALSO 796.Xr scp 1 , 797.Xr sftp 1 , 798.Xr ssh 1 , 799.Xr ssh-add 1 , 800.Xr ssh-agent 1 , 801.Xr ssh-keygen 1 , 802.Xr chroot 2 , 803.Xr hosts_access 5 , 804.Xr login.conf 5 , 805.Xr moduli 5 , 806.Xr sshd_config 5 , 807.Xr inetd 8 , 808.Xr sftp-server 8 809.Rs 810.%A T. Ylonen 811.%A T. Kivinen 812.%A M. Saarinen 813.%A T. Rinne 814.%A S. Lehtinen 815.%T "SSH Protocol Architecture" 816.%N draft-ietf-secsh-architecture-12.txt 817.%D January 2002 818.%O work in progress material 819.Re 820.Rs 821.%A M. Friedl 822.%A N. Provos 823.%A W. A. Simpson 824.%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" 825.%N draft-ietf-secsh-dh-group-exchange-02.txt 826.%D January 2002 827.%O work in progress material 828.Re 829.Sh AUTHORS 830OpenSSH is a derivative of the original and free 831ssh 1.2.12 release by Tatu Ylonen. 832Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 833Theo de Raadt and Dug Song 834removed many bugs, re-added newer features and 835created OpenSSH. 836Markus Friedl contributed the support for SSH 837protocol versions 1.5 and 2.0. 838Niels Provos and Markus Friedl contributed support 839for privilege separation. 840