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: ssh.1,v 1.433 2022/11/28 01:37:36 djm Exp $ 37.Dd $Mdocdate: November 28 2022 $ 38.Dt SSH 1 39.Os 40.Sh NAME 41.Nm ssh 42.Nd OpenSSH remote login client 43.Sh SYNOPSIS 44.Nm ssh 45.Op Fl 46AaCfGgKkMNnqsTtVvXxYy 46.Op Fl B Ar bind_interface 47.Op Fl b Ar bind_address 48.Op Fl c Ar cipher_spec 49.Op Fl D Oo Ar bind_address : Oc Ns Ar port 50.Op Fl E Ar log_file 51.Op Fl e Ar escape_char 52.Op Fl F Ar configfile 53.Op Fl I Ar pkcs11 54.Op Fl i Ar identity_file 55.Op Fl J Ar destination 56.Op Fl L Ar address 57.Op Fl l Ar login_name 58.Op Fl m Ar mac_spec 59.Op Fl O Ar ctl_cmd 60.Op Fl o Ar option 61.Op Fl p Ar port 62.Op Fl Q Ar query_option 63.Op Fl R Ar address 64.Op Fl S Ar ctl_path 65.Op Fl W Ar host : Ns Ar port 66.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun 67.Ar destination 68.Op Ar command Op Ar argument ... 69.Sh DESCRIPTION 70.Nm 71(SSH client) is a program for logging into a remote machine and for 72executing commands on a remote machine. 73It is intended to provide secure encrypted communications between 74two untrusted hosts over an insecure network. 75X11 connections, arbitrary TCP ports and 76.Ux Ns -domain 77sockets can also be forwarded over the secure channel. 78.Pp 79.Nm 80connects and logs into the specified 81.Ar destination , 82which may be specified as either 83.Sm off 84.Oo user @ Oc hostname 85.Sm on 86or a URI of the form 87.Sm off 88.No ssh:// Oo user @ Oc hostname Op : port . 89.Sm on 90The user must prove 91their identity to the remote machine using one of several methods 92(see below). 93.Pp 94If a 95.Ar command 96is specified, 97it will be executed on the remote host instead of a login shell. 98A complete command line may be specified as 99.Ar command , 100or it may have additional arguments. 101If supplied, the arguments will be appended to the command, separated by 102spaces, before it is sent to the server to be executed. 103.Pp 104The options are as follows: 105.Pp 106.Bl -tag -width Ds -compact 107.It Fl 4 108Forces 109.Nm 110to use IPv4 addresses only. 111.Pp 112.It Fl 6 113Forces 114.Nm 115to use IPv6 addresses only. 116.Pp 117.It Fl A 118Enables forwarding of connections from an authentication agent such as 119.Xr ssh-agent 1 . 120This can also be specified on a per-host basis in a configuration file. 121.Pp 122Agent forwarding should be enabled with caution. 123Users with the ability to bypass file permissions on the remote host 124(for the agent's 125.Ux Ns -domain 126socket) can access the local agent through the forwarded connection. 127An attacker cannot obtain key material from the agent, 128however they can perform operations on the keys that enable them to 129authenticate using the identities loaded into the agent. 130A safer alternative may be to use a jump host 131(see 132.Fl J ) . 133.Pp 134.It Fl a 135Disables forwarding of the authentication agent connection. 136.Pp 137.It Fl B Ar bind_interface 138Bind to the address of 139.Ar bind_interface 140before attempting to connect to the destination host. 141This is only useful on systems with more than one address. 142.Pp 143.It Fl b Ar bind_address 144Use 145.Ar bind_address 146on the local machine as the source address 147of the connection. 148Only useful on systems with more than one address. 149.Pp 150.It Fl C 151Requests compression of all data (including stdin, stdout, stderr, and 152data for forwarded X11, TCP and 153.Ux Ns -domain 154connections). 155The compression algorithm is the same used by 156.Xr gzip 1 . 157Compression is desirable on modem lines and other 158slow connections, but will only slow down things on fast networks. 159The default value can be set on a host-by-host basis in the 160configuration files; see the 161.Cm Compression 162option in 163.Xr ssh_config 5 . 164.Pp 165.It Fl c Ar cipher_spec 166Selects the cipher specification for encrypting the session. 167.Ar cipher_spec 168is a comma-separated list of ciphers 169listed in order of preference. 170See the 171.Cm Ciphers 172keyword in 173.Xr ssh_config 5 174for more information. 175.Pp 176.It Fl D Xo 177.Sm off 178.Oo Ar bind_address : Oc 179.Ar port 180.Sm on 181.Xc 182Specifies a local 183.Dq dynamic 184application-level port forwarding. 185This works by allocating a socket to listen to 186.Ar port 187on the local side, optionally bound to the specified 188.Ar bind_address . 189Whenever a connection is made to this port, the 190connection is forwarded over the secure channel, and the application 191protocol is then used to determine where to connect to from the 192remote machine. 193Currently the SOCKS4 and SOCKS5 protocols are supported, and 194.Nm 195will act as a SOCKS server. 196Only root can forward privileged ports. 197Dynamic port forwardings can also be specified in the configuration file. 198.Pp 199IPv6 addresses can be specified by enclosing the address in square brackets. 200Only the superuser can forward privileged ports. 201By default, the local port is bound in accordance with the 202.Cm GatewayPorts 203setting. 204However, an explicit 205.Ar bind_address 206may be used to bind the connection to a specific address. 207The 208.Ar bind_address 209of 210.Dq localhost 211indicates that the listening port be bound for local use only, while an 212empty address or 213.Sq * 214indicates that the port should be available from all interfaces. 215.Pp 216.It Fl E Ar log_file 217Append debug logs to 218.Ar log_file 219instead of standard error. 220.Pp 221.It Fl e Ar escape_char 222Sets the escape character for sessions with a pty (default: 223.Ql ~ ) . 224The escape character is only recognized at the beginning of a line. 225The escape character followed by a dot 226.Pq Ql \&. 227closes the connection; 228followed by control-Z suspends the connection; 229and followed by itself sends the escape character once. 230Setting the character to 231.Dq none 232disables any escapes and makes the session fully transparent. 233.Pp 234.It Fl F Ar configfile 235Specifies an alternative per-user configuration file. 236If a configuration file is given on the command line, 237the system-wide configuration file 238.Pq Pa /etc/ssh/ssh_config 239will be ignored. 240The default for the per-user configuration file is 241.Pa ~/.ssh/config . 242If set to 243.Dq none , 244no configuration files will be read. 245.Pp 246.It Fl f 247Requests 248.Nm 249to go to background just before command execution. 250This is useful if 251.Nm 252is going to ask for passwords or passphrases, but the user 253wants it in the background. 254This implies 255.Fl n . 256The recommended way to start X11 programs at a remote site is with 257something like 258.Ic ssh -f host xterm . 259.Pp 260If the 261.Cm ExitOnForwardFailure 262configuration option is set to 263.Dq yes , 264then a client started with 265.Fl f 266will wait for all remote port forwards to be successfully established 267before placing itself in the background. 268Refer to the description of 269.Cm ForkAfterAuthentication 270in 271.Xr ssh_config 5 272for details. 273.Pp 274.It Fl G 275Causes 276.Nm 277to print its configuration after evaluating 278.Cm Host 279and 280.Cm Match 281blocks and exit. 282.Pp 283.It Fl g 284Allows remote hosts to connect to local forwarded ports. 285If used on a multiplexed connection, then this option must be specified 286on the master process. 287.Pp 288.It Fl I Ar pkcs11 289Specify the PKCS#11 shared library 290.Nm 291should use to communicate with a PKCS#11 token providing keys for user 292authentication. 293.Pp 294.It Fl i Ar identity_file 295Selects a file from which the identity (private key) for 296public key authentication is read. 297You can also specify a public key file to use the corresponding 298private key that is loaded in 299.Xr ssh-agent 1 300when the private key file is not present locally. 301The default is 302.Pa ~/.ssh/id_rsa , 303.Pa ~/.ssh/id_ecdsa , 304.Pa ~/.ssh/id_ecdsa_sk , 305.Pa ~/.ssh/id_ed25519 , 306.Pa ~/.ssh/id_ed25519_sk 307and 308.Pa ~/.ssh/id_dsa . 309Identity files may also be specified on 310a per-host basis in the configuration file. 311It is possible to have multiple 312.Fl i 313options (and multiple identities specified in 314configuration files). 315If no certificates have been explicitly specified by the 316.Cm CertificateFile 317directive, 318.Nm 319will also try to load certificate information from the filename obtained 320by appending 321.Pa -cert.pub 322to identity filenames. 323.Pp 324.It Fl J Ar destination 325Connect to the target host by first making a 326.Nm 327connection to the jump host described by 328.Ar destination 329and then establishing a TCP forwarding to the ultimate destination from 330there. 331Multiple jump hops may be specified separated by comma characters. 332This is a shortcut to specify a 333.Cm ProxyJump 334configuration directive. 335Note that configuration directives supplied on the command-line generally 336apply to the destination host and not any specified jump hosts. 337Use 338.Pa ~/.ssh/config 339to specify configuration for jump hosts. 340.Pp 341.It Fl K 342Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 343credentials to the server. 344.Pp 345.It Fl k 346Disables forwarding (delegation) of GSSAPI credentials to the server. 347.Pp 348.It Fl L Xo 349.Sm off 350.Oo Ar bind_address : Oc 351.Ar port : host : hostport 352.Sm on 353.Xc 354.It Fl L Xo 355.Sm off 356.Oo Ar bind_address : Oc 357.Ar port : remote_socket 358.Sm on 359.Xc 360.It Fl L Xo 361.Sm off 362.Ar local_socket : host : hostport 363.Sm on 364.Xc 365.It Fl L Xo 366.Sm off 367.Ar local_socket : remote_socket 368.Sm on 369.Xc 370Specifies that connections to the given TCP port or Unix socket on the local 371(client) host are to be forwarded to the given host and port, or Unix socket, 372on the remote side. 373This works by allocating a socket to listen to either a TCP 374.Ar port 375on the local side, optionally bound to the specified 376.Ar bind_address , 377or to a Unix socket. 378Whenever a connection is made to the local port or socket, the 379connection is forwarded over the secure channel, and a connection is 380made to either 381.Ar host 382port 383.Ar hostport , 384or the Unix socket 385.Ar remote_socket , 386from the remote machine. 387.Pp 388Port forwardings can also be specified in the configuration file. 389Only the superuser can forward privileged ports. 390IPv6 addresses can be specified by enclosing the address in square brackets. 391.Pp 392By default, the local port is bound in accordance with the 393.Cm GatewayPorts 394setting. 395However, an explicit 396.Ar bind_address 397may be used to bind the connection to a specific address. 398The 399.Ar bind_address 400of 401.Dq localhost 402indicates that the listening port be bound for local use only, while an 403empty address or 404.Sq * 405indicates that the port should be available from all interfaces. 406.Pp 407.It Fl l Ar login_name 408Specifies the user to log in as on the remote machine. 409This also may be specified on a per-host basis in the configuration file. 410.Pp 411.It Fl M 412Places the 413.Nm 414client into 415.Dq master 416mode for connection sharing. 417Multiple 418.Fl M 419options places 420.Nm 421into 422.Dq master 423mode but with confirmation required using 424.Xr ssh-askpass 1 425before each operation that changes the multiplexing state 426(e.g. opening a new session). 427Refer to the description of 428.Cm ControlMaster 429in 430.Xr ssh_config 5 431for details. 432.Pp 433.It Fl m Ar mac_spec 434A comma-separated list of MAC (message authentication code) algorithms, 435specified in order of preference. 436See the 437.Cm MACs 438keyword in 439.Xr ssh_config 5 440for more information. 441.Pp 442.It Fl N 443Do not execute a remote command. 444This is useful for just forwarding ports. 445Refer to the description of 446.Cm SessionType 447in 448.Xr ssh_config 5 449for details. 450.Pp 451.It Fl n 452Redirects stdin from 453.Pa /dev/null 454(actually, prevents reading from stdin). 455This must be used when 456.Nm 457is run in the background. 458A common trick is to use this to run X11 programs on a remote machine. 459For example, 460.Ic ssh -n shadows.cs.hut.fi emacs & 461will start an emacs on shadows.cs.hut.fi, and the X11 462connection will be automatically forwarded over an encrypted channel. 463The 464.Nm 465program will be put in the background. 466(This does not work if 467.Nm 468needs to ask for a password or passphrase; see also the 469.Fl f 470option.) 471Refer to the description of 472.Cm StdinNull 473in 474.Xr ssh_config 5 475for details. 476.Pp 477.It Fl O Ar ctl_cmd 478Control an active connection multiplexing master process. 479When the 480.Fl O 481option is specified, the 482.Ar ctl_cmd 483argument is interpreted and passed to the master process. 484Valid commands are: 485.Dq check 486(check that the master process is running), 487.Dq forward 488(request forwardings without command execution), 489.Dq cancel 490(cancel forwardings), 491.Dq exit 492(request the master to exit), and 493.Dq stop 494(request the master to stop accepting further multiplexing requests). 495.Pp 496.It Fl o Ar option 497Can be used to give options in the format used in the configuration file. 498This is useful for specifying options for which there is no separate 499command-line flag. 500For full details of the options listed below, and their possible values, see 501.Xr ssh_config 5 . 502.Pp 503.Bl -tag -width Ds -offset indent -compact 504.It AddKeysToAgent 505.It AddressFamily 506.It BatchMode 507.It BindAddress 508.It CanonicalDomains 509.It CanonicalizeFallbackLocal 510.It CanonicalizeHostname 511.It CanonicalizeMaxDots 512.It CanonicalizePermittedCNAMEs 513.It CASignatureAlgorithms 514.It CertificateFile 515.It CheckHostIP 516.It Ciphers 517.It ClearAllForwardings 518.It Compression 519.It ConnectionAttempts 520.It ConnectTimeout 521.It ControlMaster 522.It ControlPath 523.It ControlPersist 524.It DynamicForward 525.It EnableEscapeCommandline 526.It EscapeChar 527.It ExitOnForwardFailure 528.It FingerprintHash 529.It ForkAfterAuthentication 530.It ForwardAgent 531.It ForwardX11 532.It ForwardX11Timeout 533.It ForwardX11Trusted 534.It GatewayPorts 535.It GlobalKnownHostsFile 536.It GSSAPIAuthentication 537.It GSSAPIDelegateCredentials 538.It HashKnownHosts 539.It Host 540.It HostbasedAcceptedAlgorithms 541.It HostbasedAuthentication 542.It HostKeyAlgorithms 543.It HostKeyAlias 544.It Hostname 545.It IdentitiesOnly 546.It IdentityAgent 547.It IdentityFile 548.It IPQoS 549.It KbdInteractiveAuthentication 550.It KbdInteractiveDevices 551.It KexAlgorithms 552.It KnownHostsCommand 553.It LocalCommand 554.It LocalForward 555.It LogLevel 556.It MACs 557.It Match 558.It NoHostAuthenticationForLocalhost 559.It NumberOfPasswordPrompts 560.It PasswordAuthentication 561.It PermitLocalCommand 562.It PermitRemoteOpen 563.It PKCS11Provider 564.It Port 565.It PreferredAuthentications 566.It ProxyCommand 567.It ProxyJump 568.It ProxyUseFdpass 569.It PubkeyAcceptedAlgorithms 570.It PubkeyAuthentication 571.It RekeyLimit 572.It RemoteCommand 573.It RemoteForward 574.It RequestTTY 575.It RequiredRSASize 576.It SendEnv 577.It ServerAliveInterval 578.It ServerAliveCountMax 579.It SessionType 580.It SetEnv 581.It StdinNull 582.It StreamLocalBindMask 583.It StreamLocalBindUnlink 584.It StrictHostKeyChecking 585.It TCPKeepAlive 586.It Tunnel 587.It TunnelDevice 588.It UpdateHostKeys 589.It User 590.It UserKnownHostsFile 591.It VerifyHostKeyDNS 592.It VisualHostKey 593.It XAuthLocation 594.El 595.Pp 596.It Fl p Ar port 597Port to connect to on the remote host. 598This can be specified on a 599per-host basis in the configuration file. 600.Pp 601.It Fl Q Ar query_option 602Queries for the algorithms supported by one of the following features: 603.Ar cipher 604(supported symmetric ciphers), 605.Ar cipher-auth 606(supported symmetric ciphers that support authenticated encryption), 607.Ar help 608(supported query terms for use with the 609.Fl Q 610flag), 611.Ar mac 612(supported message integrity codes), 613.Ar kex 614(key exchange algorithms), 615.Ar key 616(key types), 617.Ar key-cert 618(certificate key types), 619.Ar key-plain 620(non-certificate key types), 621.Ar key-sig 622(all key types and signature algorithms), 623.Ar protocol-version 624(supported SSH protocol versions), and 625.Ar sig 626(supported signature algorithms). 627Alternatively, any keyword from 628.Xr ssh_config 5 629or 630.Xr sshd_config 5 631that takes an algorithm list may be used as an alias for the corresponding 632query_option. 633.Pp 634.It Fl q 635Quiet mode. 636Causes most warning and diagnostic messages to be suppressed. 637.Pp 638.It Fl R Xo 639.Sm off 640.Oo Ar bind_address : Oc 641.Ar port : host : hostport 642.Sm on 643.Xc 644.It Fl R Xo 645.Sm off 646.Oo Ar bind_address : Oc 647.Ar port : local_socket 648.Sm on 649.Xc 650.It Fl R Xo 651.Sm off 652.Ar remote_socket : host : hostport 653.Sm on 654.Xc 655.It Fl R Xo 656.Sm off 657.Ar remote_socket : local_socket 658.Sm on 659.Xc 660.It Fl R Xo 661.Sm off 662.Oo Ar bind_address : Oc 663.Ar port 664.Sm on 665.Xc 666Specifies that connections to the given TCP port or Unix socket on the remote 667(server) host are to be forwarded to the local side. 668.Pp 669This works by allocating a socket to listen to either a TCP 670.Ar port 671or to a Unix socket on the remote side. 672Whenever a connection is made to this port or Unix socket, the 673connection is forwarded over the secure channel, and a connection 674is made from the local machine to either an explicit destination specified by 675.Ar host 676port 677.Ar hostport , 678or 679.Ar local_socket , 680or, if no explicit destination was specified, 681.Nm 682will act as a SOCKS 4/5 proxy and forward connections to the destinations 683requested by the remote SOCKS client. 684.Pp 685Port forwardings can also be specified in the configuration file. 686Privileged ports can be forwarded only when 687logging in as root on the remote machine. 688IPv6 addresses can be specified by enclosing the address in square brackets. 689.Pp 690By default, TCP listening sockets on the server will be bound to the loopback 691interface only. 692This may be overridden by specifying a 693.Ar bind_address . 694An empty 695.Ar bind_address , 696or the address 697.Ql * , 698indicates that the remote socket should listen on all interfaces. 699Specifying a remote 700.Ar bind_address 701will only succeed if the server's 702.Cm GatewayPorts 703option is enabled (see 704.Xr sshd_config 5 ) . 705.Pp 706If the 707.Ar port 708argument is 709.Ql 0 , 710the listen port will be dynamically allocated on the server and reported 711to the client at run time. 712When used together with 713.Ic -O forward , 714the allocated port will be printed to the standard output. 715.Pp 716.It Fl S Ar ctl_path 717Specifies the location of a control socket for connection sharing, 718or the string 719.Dq none 720to disable connection sharing. 721Refer to the description of 722.Cm ControlPath 723and 724.Cm ControlMaster 725in 726.Xr ssh_config 5 727for details. 728.Pp 729.It Fl s 730May be used to request invocation of a subsystem on the remote system. 731Subsystems facilitate the use of SSH 732as a secure transport for other applications (e.g.\& 733.Xr sftp 1 ) . 734The subsystem is specified as the remote command. 735Refer to the description of 736.Cm SessionType 737in 738.Xr ssh_config 5 739for details. 740.Pp 741.It Fl T 742Disable pseudo-terminal allocation. 743.Pp 744.It Fl t 745Force pseudo-terminal allocation. 746This can be used to execute arbitrary 747screen-based programs on a remote machine, which can be very useful, 748e.g. when implementing menu services. 749Multiple 750.Fl t 751options force tty allocation, even if 752.Nm 753has no local tty. 754.Pp 755.It Fl V 756Display the version number and exit. 757.Pp 758.It Fl v 759Verbose mode. 760Causes 761.Nm 762to print debugging messages about its progress. 763This is helpful in 764debugging connection, authentication, and configuration problems. 765Multiple 766.Fl v 767options increase the verbosity. 768The maximum is 3. 769.Pp 770.It Fl W Ar host : Ns Ar port 771Requests that standard input and output on the client be forwarded to 772.Ar host 773on 774.Ar port 775over the secure channel. 776Implies 777.Fl N , 778.Fl T , 779.Cm ExitOnForwardFailure 780and 781.Cm ClearAllForwardings , 782though these can be overridden in the configuration file or using 783.Fl o 784command line options. 785.Pp 786.It Fl w Xo 787.Ar local_tun Ns Op : Ns Ar remote_tun 788.Xc 789Requests 790tunnel 791device forwarding with the specified 792.Xr tun 4 793devices between the client 794.Pq Ar local_tun 795and the server 796.Pq Ar remote_tun . 797.Pp 798The devices may be specified by numerical ID or the keyword 799.Dq any , 800which uses the next available tunnel device. 801If 802.Ar remote_tun 803is not specified, it defaults to 804.Dq any . 805See also the 806.Cm Tunnel 807and 808.Cm TunnelDevice 809directives in 810.Xr ssh_config 5 . 811.Pp 812If the 813.Cm Tunnel 814directive is unset, it will be set to the default tunnel mode, which is 815.Dq point-to-point . 816If a different 817.Cm Tunnel 818forwarding mode it desired, then it should be specified before 819.Fl w . 820.Pp 821.It Fl X 822Enables X11 forwarding. 823This can also be specified on a per-host basis in a configuration file. 824.Pp 825X11 forwarding should be enabled with caution. 826Users with the ability to bypass file permissions on the remote host 827(for the user's X authorization database) 828can access the local X11 display through the forwarded connection. 829An attacker may then be able to perform activities such as keystroke monitoring. 830.Pp 831For this reason, X11 forwarding is subjected to X11 SECURITY extension 832restrictions by default. 833Refer to the 834.Nm 835.Fl Y 836option and the 837.Cm ForwardX11Trusted 838directive in 839.Xr ssh_config 5 840for more information. 841.Pp 842.It Fl x 843Disables X11 forwarding. 844.Pp 845.It Fl Y 846Enables trusted X11 forwarding. 847Trusted X11 forwardings are not subjected to the X11 SECURITY extension 848controls. 849.Pp 850.It Fl y 851Send log information using the 852.Xr syslog 3 853system module. 854By default this information is sent to stderr. 855.El 856.Pp 857.Nm 858may additionally obtain configuration data from 859a per-user configuration file and a system-wide configuration file. 860The file format and configuration options are described in 861.Xr ssh_config 5 . 862.Sh AUTHENTICATION 863The OpenSSH SSH client supports SSH protocol 2. 864.Pp 865The methods available for authentication are: 866GSSAPI-based authentication, 867host-based authentication, 868public key authentication, 869keyboard-interactive authentication, 870and password authentication. 871Authentication methods are tried in the order specified above, 872though 873.Cm PreferredAuthentications 874can be used to change the default order. 875.Pp 876Host-based authentication works as follows: 877If the machine the user logs in from is listed in 878.Pa /etc/hosts.equiv 879or 880.Pa /etc/shosts.equiv 881on the remote machine, the user is non-root and the user names are 882the same on both sides, or if the files 883.Pa ~/.rhosts 884or 885.Pa ~/.shosts 886exist in the user's home directory on the 887remote machine and contain a line containing the name of the client 888machine and the name of the user on that machine, the user is 889considered for login. 890Additionally, the server 891.Em must 892be able to verify the client's 893host key (see the description of 894.Pa /etc/ssh/ssh_known_hosts 895and 896.Pa ~/.ssh/known_hosts , 897below) 898for login to be permitted. 899This authentication method closes security holes due to IP 900spoofing, DNS spoofing, and routing spoofing. 901[Note to the administrator: 902.Pa /etc/hosts.equiv , 903.Pa ~/.rhosts , 904and the rlogin/rsh protocol in general, are inherently insecure and should be 905disabled if security is desired.] 906.Pp 907Public key authentication works as follows: 908The scheme is based on public-key cryptography, 909using cryptosystems 910where encryption and decryption are done using separate keys, 911and it is unfeasible to derive the decryption key from the encryption key. 912The idea is that each user creates a public/private 913key pair for authentication purposes. 914The server knows the public key, and only the user knows the private key. 915.Nm 916implements public key authentication protocol automatically, 917using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 918The HISTORY section of 919.Xr ssl 8 920contains a brief discussion of the DSA and RSA algorithms. 921.Pp 922The file 923.Pa ~/.ssh/authorized_keys 924lists the public keys that are permitted for logging in. 925When the user logs in, the 926.Nm 927program tells the server which key pair it would like to use for 928authentication. 929The client proves that it has access to the private key 930and the server checks that the corresponding public key 931is authorized to accept the account. 932.Pp 933The server may inform the client of errors that prevented public key 934authentication from succeeding after authentication completes using a 935different method. 936These may be viewed by increasing the 937.Cm LogLevel 938to 939.Cm DEBUG 940or higher (e.g. by using the 941.Fl v 942flag). 943.Pp 944The user creates their key pair by running 945.Xr ssh-keygen 1 . 946This stores the private key in 947.Pa ~/.ssh/id_dsa 948(DSA), 949.Pa ~/.ssh/id_ecdsa 950(ECDSA), 951.Pa ~/.ssh/id_ecdsa_sk 952(authenticator-hosted ECDSA), 953.Pa ~/.ssh/id_ed25519 954(Ed25519), 955.Pa ~/.ssh/id_ed25519_sk 956(authenticator-hosted Ed25519), 957or 958.Pa ~/.ssh/id_rsa 959(RSA) 960and stores the public key in 961.Pa ~/.ssh/id_dsa.pub 962(DSA), 963.Pa ~/.ssh/id_ecdsa.pub 964(ECDSA), 965.Pa ~/.ssh/id_ecdsa_sk.pub 966(authenticator-hosted ECDSA), 967.Pa ~/.ssh/id_ed25519.pub 968(Ed25519), 969.Pa ~/.ssh/id_ed25519_sk.pub 970(authenticator-hosted Ed25519), 971or 972.Pa ~/.ssh/id_rsa.pub 973(RSA) 974in the user's home directory. 975The user should then copy the public key 976to 977.Pa ~/.ssh/authorized_keys 978in their home directory on the remote machine. 979The 980.Pa authorized_keys 981file corresponds to the conventional 982.Pa ~/.rhosts 983file, and has one key 984per line, though the lines can be very long. 985After this, the user can log in without giving the password. 986.Pp 987A variation on public key authentication 988is available in the form of certificate authentication: 989instead of a set of public/private keys, 990signed certificates are used. 991This has the advantage that a single trusted certification authority 992can be used in place of many public/private keys. 993See the CERTIFICATES section of 994.Xr ssh-keygen 1 995for more information. 996.Pp 997The most convenient way to use public key or certificate authentication 998may be with an authentication agent. 999See 1000.Xr ssh-agent 1 1001and (optionally) the 1002.Cm AddKeysToAgent 1003directive in 1004.Xr ssh_config 5 1005for more information. 1006.Pp 1007Keyboard-interactive authentication works as follows: 1008The server sends an arbitrary 1009.Qq challenge 1010text and prompts for a response, possibly multiple times. 1011Examples of keyboard-interactive authentication include 1012.Bx 1013Authentication (see 1014.Xr login.conf 5 ) 1015and PAM (some 1016.Pf non- Ox 1017systems). 1018.Pp 1019Finally, if other authentication methods fail, 1020.Nm 1021prompts the user for a password. 1022The password is sent to the remote 1023host for checking; however, since all communications are encrypted, 1024the password cannot be seen by someone listening on the network. 1025.Pp 1026.Nm 1027automatically maintains and checks a database containing 1028identification for all hosts it has ever been used with. 1029Host keys are stored in 1030.Pa ~/.ssh/known_hosts 1031in the user's home directory. 1032Additionally, the file 1033.Pa /etc/ssh/ssh_known_hosts 1034is automatically checked for known hosts. 1035Any new hosts are automatically added to the user's file. 1036If a host's identification ever changes, 1037.Nm 1038warns about this and disables password authentication to prevent 1039server spoofing or man-in-the-middle attacks, 1040which could otherwise be used to circumvent the encryption. 1041The 1042.Cm StrictHostKeyChecking 1043option can be used to control logins to machines whose 1044host key is not known or has changed. 1045.Pp 1046When the user's identity has been accepted by the server, the server 1047either executes the given command in a non-interactive session or, 1048if no command has been specified, logs into the machine and gives 1049the user a normal shell as an interactive session. 1050All communication with 1051the remote command or shell will be automatically encrypted. 1052.Pp 1053If an interactive session is requested, 1054.Nm 1055by default will only request a pseudo-terminal (pty) for interactive 1056sessions when the client has one. 1057The flags 1058.Fl T 1059and 1060.Fl t 1061can be used to override this behaviour. 1062.Pp 1063If a pseudo-terminal has been allocated, the 1064user may use the escape characters noted below. 1065.Pp 1066If no pseudo-terminal has been allocated, 1067the session is transparent and can be used to reliably transfer binary data. 1068On most systems, setting the escape character to 1069.Dq none 1070will also make the session transparent even if a tty is used. 1071.Pp 1072The session terminates when the command or shell on the remote 1073machine exits and all X11 and TCP connections have been closed. 1074.Sh ESCAPE CHARACTERS 1075When a pseudo-terminal has been requested, 1076.Nm 1077supports a number of functions through the use of an escape character. 1078.Pp 1079A single tilde character can be sent as 1080.Ic ~~ 1081or by following the tilde by a character other than those described below. 1082The escape character must always follow a newline to be interpreted as 1083special. 1084The escape character can be changed in configuration files using the 1085.Cm EscapeChar 1086configuration directive or on the command line by the 1087.Fl e 1088option. 1089.Pp 1090The supported escapes (assuming the default 1091.Ql ~ ) 1092are: 1093.Bl -tag -width Ds 1094.It Cm ~. 1095Disconnect. 1096.It Cm ~^Z 1097Background 1098.Nm . 1099.It Cm ~# 1100List forwarded connections. 1101.It Cm ~& 1102Background 1103.Nm 1104at logout when waiting for forwarded connection / X11 sessions to terminate. 1105.It Cm ~? 1106Display a list of escape characters. 1107.It Cm ~B 1108Send a BREAK to the remote system 1109(only useful if the peer supports it). 1110.It Cm ~C 1111Open command line. 1112Currently this allows the addition of port forwardings using the 1113.Fl L , 1114.Fl R 1115and 1116.Fl D 1117options (see above). 1118It also allows the cancellation of existing port-forwardings 1119with 1120.Sm off 1121.Fl KL Oo Ar bind_address : Oc Ar port 1122.Sm on 1123for local, 1124.Sm off 1125.Fl KR Oo Ar bind_address : Oc Ar port 1126.Sm on 1127for remote and 1128.Sm off 1129.Fl KD Oo Ar bind_address : Oc Ar port 1130.Sm on 1131for dynamic port-forwardings. 1132.Ic !\& Ns Ar command 1133allows the user to execute a local command if the 1134.Ic PermitLocalCommand 1135option is enabled in 1136.Xr ssh_config 5 . 1137Basic help is available, using the 1138.Fl h 1139option. 1140.It Cm ~R 1141Request rekeying of the connection 1142(only useful if the peer supports it). 1143.It Cm ~V 1144Decrease the verbosity 1145.Pq Ic LogLevel 1146when errors are being written to stderr. 1147.It Cm ~v 1148Increase the verbosity 1149.Pq Ic LogLevel 1150when errors are being written to stderr. 1151.El 1152.Sh TCP FORWARDING 1153Forwarding of arbitrary TCP connections over a secure channel 1154can be specified either on the command line or in a configuration file. 1155One possible application of TCP forwarding is a secure connection to a 1156mail server; another is going through firewalls. 1157.Pp 1158In the example below, we look at encrypting communication for an IRC client, 1159even though the IRC server it connects to does not directly 1160support encrypted communication. 1161This works as follows: 1162the user connects to the remote host using 1163.Nm , 1164specifying the ports to be used to forward the connection. 1165After that it is possible to start the program locally, 1166and 1167.Nm 1168will encrypt and forward the connection to the remote server. 1169.Pp 1170The following example tunnels an IRC session from the client 1171to an IRC server at 1172.Dq server.example.com , 1173joining channel 1174.Dq #users , 1175nickname 1176.Dq pinky , 1177using the standard IRC port, 6667: 1178.Bd -literal -offset 4n 1179$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1180$ irc -c '#users' pinky IRC/127.0.0.1 1181.Ed 1182.Pp 1183The 1184.Fl f 1185option backgrounds 1186.Nm 1187and the remote command 1188.Dq sleep 10 1189is specified to allow an amount of time 1190(10 seconds, in the example) 1191to start the program which is going to use the tunnel. 1192If no connections are made within the time specified, 1193.Nm 1194will exit. 1195.Sh X11 FORWARDING 1196If the 1197.Cm ForwardX11 1198variable is set to 1199.Dq yes 1200(or see the description of the 1201.Fl X , 1202.Fl x , 1203and 1204.Fl Y 1205options above) 1206and the user is using X11 (the 1207.Ev DISPLAY 1208environment variable is set), the connection to the X11 display is 1209automatically forwarded to the remote side in such a way that any X11 1210programs started from the shell (or command) will go through the 1211encrypted channel, and the connection to the real X server will be made 1212from the local machine. 1213The user should not manually set 1214.Ev DISPLAY . 1215Forwarding of X11 connections can be 1216configured on the command line or in configuration files. 1217.Pp 1218The 1219.Ev DISPLAY 1220value set by 1221.Nm 1222will point to the server machine, but with a display number greater than zero. 1223This is normal, and happens because 1224.Nm 1225creates a 1226.Dq proxy 1227X server on the server machine for forwarding the 1228connections over the encrypted channel. 1229.Pp 1230.Nm 1231will also automatically set up Xauthority data on the server machine. 1232For this purpose, it will generate a random authorization cookie, 1233store it in Xauthority on the server, and verify that any forwarded 1234connections carry this cookie and replace it by the real cookie when 1235the connection is opened. 1236The real authentication cookie is never 1237sent to the server machine (and no cookies are sent in the plain). 1238.Pp 1239If the 1240.Cm ForwardAgent 1241variable is set to 1242.Dq yes 1243(or see the description of the 1244.Fl A 1245and 1246.Fl a 1247options above) and 1248the user is using an authentication agent, the connection to the agent 1249is automatically forwarded to the remote side. 1250.Sh VERIFYING HOST KEYS 1251When connecting to a server for the first time, 1252a fingerprint of the server's public key is presented to the user 1253(unless the option 1254.Cm StrictHostKeyChecking 1255has been disabled). 1256Fingerprints can be determined using 1257.Xr ssh-keygen 1 : 1258.Pp 1259.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1260.Pp 1261If the fingerprint is already known, it can be matched 1262and the key can be accepted or rejected. 1263If only legacy (MD5) fingerprints for the server are available, the 1264.Xr ssh-keygen 1 1265.Fl E 1266option may be used to downgrade the fingerprint algorithm to match. 1267.Pp 1268Because of the difficulty of comparing host keys 1269just by looking at fingerprint strings, 1270there is also support to compare host keys visually, 1271using 1272.Em random art . 1273By setting the 1274.Cm VisualHostKey 1275option to 1276.Dq yes , 1277a small ASCII graphic gets displayed on every login to a server, no matter 1278if the session itself is interactive or not. 1279By learning the pattern a known server produces, a user can easily 1280find out that the host key has changed when a completely different pattern 1281is displayed. 1282Because these patterns are not unambiguous however, a pattern that looks 1283similar to the pattern remembered only gives a good probability that the 1284host key is the same, not guaranteed proof. 1285.Pp 1286To get a listing of the fingerprints along with their random art for 1287all known hosts, the following command line can be used: 1288.Pp 1289.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1290.Pp 1291If the fingerprint is unknown, 1292an alternative method of verification is available: 1293SSH fingerprints verified by DNS. 1294An additional resource record (RR), 1295SSHFP, 1296is added to a zonefile 1297and the connecting client is able to match the fingerprint 1298with that of the key presented. 1299.Pp 1300In this example, we are connecting a client to a server, 1301.Dq host.example.com . 1302The SSHFP resource records should first be added to the zonefile for 1303host.example.com: 1304.Bd -literal -offset indent 1305$ ssh-keygen -r host.example.com. 1306.Ed 1307.Pp 1308The output lines will have to be added to the zonefile. 1309To check that the zone is answering fingerprint queries: 1310.Pp 1311.Dl $ dig -t SSHFP host.example.com 1312.Pp 1313Finally the client connects: 1314.Bd -literal -offset indent 1315$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1316[...] 1317Matching host key fingerprint found in DNS. 1318Are you sure you want to continue connecting (yes/no)? 1319.Ed 1320.Pp 1321See the 1322.Cm VerifyHostKeyDNS 1323option in 1324.Xr ssh_config 5 1325for more information. 1326.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1327.Nm 1328contains support for Virtual Private Network (VPN) tunnelling 1329using the 1330.Xr tun 4 1331network pseudo-device, 1332allowing two networks to be joined securely. 1333The 1334.Xr sshd_config 5 1335configuration option 1336.Cm PermitTunnel 1337controls whether the server supports this, 1338and at what level (layer 2 or 3 traffic). 1339.Pp 1340The following example would connect client network 10.0.50.0/24 1341with remote network 10.0.99.0/24 using a point-to-point connection 1342from 10.1.1.1 to 10.1.1.2, 1343provided that the SSH server running on the gateway to the remote network, 1344at 192.168.1.15, allows it. 1345.Pp 1346On the client: 1347.Bd -literal -offset indent 1348# ssh -f -w 0:1 192.168.1.15 true 1349# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1350# route add 10.0.99.0/24 10.1.1.2 1351.Ed 1352.Pp 1353On the server: 1354.Bd -literal -offset indent 1355# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1356# route add 10.0.50.0/24 10.1.1.1 1357.Ed 1358.Pp 1359Client access may be more finely tuned via the 1360.Pa /root/.ssh/authorized_keys 1361file (see below) and the 1362.Cm PermitRootLogin 1363server option. 1364The following entry would permit connections on 1365.Xr tun 4 1366device 1 from user 1367.Dq jane 1368and on tun device 2 from user 1369.Dq john , 1370if 1371.Cm PermitRootLogin 1372is set to 1373.Dq forced-commands-only : 1374.Bd -literal -offset 2n 1375tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1376tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1377.Ed 1378.Pp 1379Since an SSH-based setup entails a fair amount of overhead, 1380it may be more suited to temporary setups, 1381such as for wireless VPNs. 1382More permanent VPNs are better provided by tools such as 1383.Xr ipsecctl 8 1384and 1385.Xr isakmpd 8 . 1386.Sh ENVIRONMENT 1387.Nm 1388will normally set the following environment variables: 1389.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1390.It Ev DISPLAY 1391The 1392.Ev DISPLAY 1393variable indicates the location of the X11 server. 1394It is automatically set by 1395.Nm 1396to point to a value of the form 1397.Dq hostname:n , 1398where 1399.Dq hostname 1400indicates the host where the shell runs, and 1401.Sq n 1402is an integer \*(Ge 1. 1403.Nm 1404uses this special value to forward X11 connections over the secure 1405channel. 1406The user should normally not set 1407.Ev DISPLAY 1408explicitly, as that 1409will render the X11 connection insecure (and will require the user to 1410manually copy any required authorization cookies). 1411.It Ev HOME 1412Set to the path of the user's home directory. 1413.It Ev LOGNAME 1414Synonym for 1415.Ev USER ; 1416set for compatibility with systems that use this variable. 1417.It Ev MAIL 1418Set to the path of the user's mailbox. 1419.It Ev PATH 1420Set to the default 1421.Ev PATH , 1422as specified when compiling 1423.Nm . 1424.It Ev SSH_ASKPASS 1425If 1426.Nm 1427needs a passphrase, it will read the passphrase from the current 1428terminal if it was run from a terminal. 1429If 1430.Nm 1431does not have a terminal associated with it but 1432.Ev DISPLAY 1433and 1434.Ev SSH_ASKPASS 1435are set, it will execute the program specified by 1436.Ev SSH_ASKPASS 1437and open an X11 window to read the passphrase. 1438This is particularly useful when calling 1439.Nm 1440from a 1441.Pa .xsession 1442or related script. 1443(Note that on some machines it 1444may be necessary to redirect the input from 1445.Pa /dev/null 1446to make this work.) 1447.It Ev SSH_ASKPASS_REQUIRE 1448Allows further control over the use of an askpass program. 1449If this variable is set to 1450.Dq never 1451then 1452.Nm 1453will never attempt to use one. 1454If it is set to 1455.Dq prefer , 1456then 1457.Nm 1458will prefer to use the askpass program instead of the TTY when requesting 1459passwords. 1460Finally, if the variable is set to 1461.Dq force , 1462then the askpass program will be used for all passphrase input regardless 1463of whether 1464.Ev DISPLAY 1465is set. 1466.It Ev SSH_AUTH_SOCK 1467Identifies the path of a 1468.Ux Ns -domain 1469socket used to communicate with the agent. 1470.It Ev SSH_CONNECTION 1471Identifies the client and server ends of the connection. 1472The variable contains 1473four space-separated values: client IP address, client port number, 1474server IP address, and server port number. 1475.It Ev SSH_ORIGINAL_COMMAND 1476This variable contains the original command line if a forced command 1477is executed. 1478It can be used to extract the original arguments. 1479.It Ev SSH_TTY 1480This is set to the name of the tty (path to the device) associated 1481with the current shell or command. 1482If the current session has no tty, 1483this variable is not set. 1484.It Ev SSH_TUNNEL 1485Optionally set by 1486.Xr sshd 8 1487to contain the interface names assigned if tunnel forwarding was 1488requested by the client. 1489.It Ev SSH_USER_AUTH 1490Optionally set by 1491.Xr sshd 8 , 1492this variable may contain a pathname to a file that lists the authentication 1493methods successfully used when the session was established, including any 1494public keys that were used. 1495.It Ev TZ 1496This variable is set to indicate the present time zone if it 1497was set when the daemon was started (i.e. the daemon passes the value 1498on to new connections). 1499.It Ev USER 1500Set to the name of the user logging in. 1501.El 1502.Pp 1503Additionally, 1504.Nm 1505reads 1506.Pa ~/.ssh/environment , 1507and adds lines of the format 1508.Dq VARNAME=value 1509to the environment if the file exists and users are allowed to 1510change their environment. 1511For more information, see the 1512.Cm PermitUserEnvironment 1513option in 1514.Xr sshd_config 5 . 1515.Sh FILES 1516.Bl -tag -width Ds -compact 1517.It Pa ~/.rhosts 1518This file is used for host-based authentication (see above). 1519On some machines this file may need to be 1520world-readable if the user's home directory is on an NFS partition, 1521because 1522.Xr sshd 8 1523reads it as root. 1524Additionally, this file must be owned by the user, 1525and must not have write permissions for anyone else. 1526The recommended 1527permission for most machines is read/write for the user, and not 1528accessible by others. 1529.Pp 1530.It Pa ~/.shosts 1531This file is used in exactly the same way as 1532.Pa .rhosts , 1533but allows host-based authentication without permitting login with 1534rlogin/rsh. 1535.Pp 1536.It Pa ~/.ssh/ 1537This directory is the default location for all user-specific configuration 1538and authentication information. 1539There is no general requirement to keep the entire contents of this directory 1540secret, but the recommended permissions are read/write/execute for the user, 1541and not accessible by others. 1542.Pp 1543.It Pa ~/.ssh/authorized_keys 1544Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1545that can be used for logging in as this user. 1546The format of this file is described in the 1547.Xr sshd 8 1548manual page. 1549This file is not highly sensitive, but the recommended 1550permissions are read/write for the user, and not accessible by others. 1551.Pp 1552.It Pa ~/.ssh/config 1553This is the per-user configuration file. 1554The file format and configuration options are described in 1555.Xr ssh_config 5 . 1556Because of the potential for abuse, this file must have strict permissions: 1557read/write for the user, and not writable by others. 1558.Pp 1559.It Pa ~/.ssh/environment 1560Contains additional definitions for environment variables; see 1561.Sx ENVIRONMENT , 1562above. 1563.Pp 1564.It Pa ~/.ssh/id_dsa 1565.It Pa ~/.ssh/id_ecdsa 1566.It Pa ~/.ssh/id_ecdsa_sk 1567.It Pa ~/.ssh/id_ed25519 1568.It Pa ~/.ssh/id_ed25519_sk 1569.It Pa ~/.ssh/id_rsa 1570Contains the private key for authentication. 1571These files 1572contain sensitive data and should be readable by the user but not 1573accessible by others (read/write/execute). 1574.Nm 1575will simply ignore a private key file if it is accessible by others. 1576It is possible to specify a passphrase when 1577generating the key which will be used to encrypt the 1578sensitive part of this file using AES-128. 1579.Pp 1580.It Pa ~/.ssh/id_dsa.pub 1581.It Pa ~/.ssh/id_ecdsa.pub 1582.It Pa ~/.ssh/id_ecdsa_sk.pub 1583.It Pa ~/.ssh/id_ed25519.pub 1584.It Pa ~/.ssh/id_ed25519_sk.pub 1585.It Pa ~/.ssh/id_rsa.pub 1586Contains the public key for authentication. 1587These files are not 1588sensitive and can (but need not) be readable by anyone. 1589.Pp 1590.It Pa ~/.ssh/known_hosts 1591Contains a list of host keys for all hosts the user has logged into 1592that are not already in the systemwide list of known host keys. 1593See 1594.Xr sshd 8 1595for further details of the format of this file. 1596.Pp 1597.It Pa ~/.ssh/rc 1598Commands in this file are executed by 1599.Nm 1600when the user logs in, just before the user's shell (or command) is 1601started. 1602See the 1603.Xr sshd 8 1604manual page for more information. 1605.Pp 1606.It Pa /etc/hosts.equiv 1607This file is for host-based authentication (see above). 1608It should only be writable by root. 1609.Pp 1610.It Pa /etc/shosts.equiv 1611This file is used in exactly the same way as 1612.Pa hosts.equiv , 1613but allows host-based authentication without permitting login with 1614rlogin/rsh. 1615.Pp 1616.It Pa /etc/ssh/ssh_config 1617Systemwide configuration file. 1618The file format and configuration options are described in 1619.Xr ssh_config 5 . 1620.Pp 1621.It Pa /etc/ssh/ssh_host_key 1622.It Pa /etc/ssh/ssh_host_dsa_key 1623.It Pa /etc/ssh/ssh_host_ecdsa_key 1624.It Pa /etc/ssh/ssh_host_ed25519_key 1625.It Pa /etc/ssh/ssh_host_rsa_key 1626These files contain the private parts of the host keys 1627and are used for host-based authentication. 1628.Pp 1629.It Pa /etc/ssh/ssh_known_hosts 1630Systemwide list of known host keys. 1631This file should be prepared by the 1632system administrator to contain the public host keys of all machines in the 1633organization. 1634It should be world-readable. 1635See 1636.Xr sshd 8 1637for further details of the format of this file. 1638.Pp 1639.It Pa /etc/ssh/sshrc 1640Commands in this file are executed by 1641.Nm 1642when the user logs in, just before the user's shell (or command) is started. 1643See the 1644.Xr sshd 8 1645manual page for more information. 1646.El 1647.Sh EXIT STATUS 1648.Nm 1649exits with the exit status of the remote command or with 255 1650if an error occurred. 1651.Sh SEE ALSO 1652.Xr scp 1 , 1653.Xr sftp 1 , 1654.Xr ssh-add 1 , 1655.Xr ssh-agent 1 , 1656.Xr ssh-keygen 1 , 1657.Xr ssh-keyscan 1 , 1658.Xr tun 4 , 1659.Xr ssh_config 5 , 1660.Xr ssh-keysign 8 , 1661.Xr sshd 8 1662.Sh STANDARDS 1663.Rs 1664.%A S. Lehtinen 1665.%A C. Lonvick 1666.%D January 2006 1667.%R RFC 4250 1668.%T The Secure Shell (SSH) Protocol Assigned Numbers 1669.Re 1670.Pp 1671.Rs 1672.%A T. Ylonen 1673.%A C. Lonvick 1674.%D January 2006 1675.%R RFC 4251 1676.%T The Secure Shell (SSH) Protocol Architecture 1677.Re 1678.Pp 1679.Rs 1680.%A T. Ylonen 1681.%A C. Lonvick 1682.%D January 2006 1683.%R RFC 4252 1684.%T The Secure Shell (SSH) Authentication Protocol 1685.Re 1686.Pp 1687.Rs 1688.%A T. Ylonen 1689.%A C. Lonvick 1690.%D January 2006 1691.%R RFC 4253 1692.%T The Secure Shell (SSH) Transport Layer Protocol 1693.Re 1694.Pp 1695.Rs 1696.%A T. Ylonen 1697.%A C. Lonvick 1698.%D January 2006 1699.%R RFC 4254 1700.%T The Secure Shell (SSH) Connection Protocol 1701.Re 1702.Pp 1703.Rs 1704.%A J. Schlyter 1705.%A W. Griffin 1706.%D January 2006 1707.%R RFC 4255 1708.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1709.Re 1710.Pp 1711.Rs 1712.%A F. Cusack 1713.%A M. Forssen 1714.%D January 2006 1715.%R RFC 4256 1716.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1717.Re 1718.Pp 1719.Rs 1720.%A J. Galbraith 1721.%A P. Remaker 1722.%D January 2006 1723.%R RFC 4335 1724.%T The Secure Shell (SSH) Session Channel Break Extension 1725.Re 1726.Pp 1727.Rs 1728.%A M. Bellare 1729.%A T. Kohno 1730.%A C. Namprempre 1731.%D January 2006 1732.%R RFC 4344 1733.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1734.Re 1735.Pp 1736.Rs 1737.%A B. Harris 1738.%D January 2006 1739.%R RFC 4345 1740.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1741.Re 1742.Pp 1743.Rs 1744.%A M. Friedl 1745.%A N. Provos 1746.%A W. Simpson 1747.%D March 2006 1748.%R RFC 4419 1749.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1750.Re 1751.Pp 1752.Rs 1753.%A J. Galbraith 1754.%A R. Thayer 1755.%D November 2006 1756.%R RFC 4716 1757.%T The Secure Shell (SSH) Public Key File Format 1758.Re 1759.Pp 1760.Rs 1761.%A D. Stebila 1762.%A J. Green 1763.%D December 2009 1764.%R RFC 5656 1765.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1766.Re 1767.Pp 1768.Rs 1769.%A A. Perrig 1770.%A D. Song 1771.%D 1999 1772.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1773.%T Hash Visualization: a New Technique to improve Real-World Security 1774.Re 1775.Sh AUTHORS 1776OpenSSH is a derivative of the original and free 1777ssh 1.2.12 release by Tatu Ylonen. 1778Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1779Theo de Raadt and Dug Song 1780removed many bugs, re-added newer features and 1781created OpenSSH. 1782Markus Friedl contributed the support for SSH 1783protocol versions 1.5 and 2.0. 1784