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 1.434 2023/06/21 05:08:32 djm Exp $ 37.Dd $Mdocdate: June 21 2023 $ 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-ca-sign 618(valid CA signature algorithms for certificates), 619.Ar key-cert 620(certificate key types), 621.Ar key-plain 622(non-certificate key types), 623.Ar key-sig 624(all key types and signature algorithms), 625.Ar protocol-version 626(supported SSH protocol versions), and 627.Ar sig 628(supported signature algorithms). 629Alternatively, any keyword from 630.Xr ssh_config 5 631or 632.Xr sshd_config 5 633that takes an algorithm list may be used as an alias for the corresponding 634query_option. 635.Pp 636.It Fl q 637Quiet mode. 638Causes most warning and diagnostic messages to be suppressed. 639.Pp 640.It Fl R Xo 641.Sm off 642.Oo Ar bind_address : Oc 643.Ar port : host : hostport 644.Sm on 645.Xc 646.It Fl R Xo 647.Sm off 648.Oo Ar bind_address : Oc 649.Ar port : local_socket 650.Sm on 651.Xc 652.It Fl R Xo 653.Sm off 654.Ar remote_socket : host : hostport 655.Sm on 656.Xc 657.It Fl R Xo 658.Sm off 659.Ar remote_socket : local_socket 660.Sm on 661.Xc 662.It Fl R Xo 663.Sm off 664.Oo Ar bind_address : Oc 665.Ar port 666.Sm on 667.Xc 668Specifies that connections to the given TCP port or Unix socket on the remote 669(server) host are to be forwarded to the local side. 670.Pp 671This works by allocating a socket to listen to either a TCP 672.Ar port 673or to a Unix socket on the remote side. 674Whenever a connection is made to this port or Unix socket, the 675connection is forwarded over the secure channel, and a connection 676is made from the local machine to either an explicit destination specified by 677.Ar host 678port 679.Ar hostport , 680or 681.Ar local_socket , 682or, if no explicit destination was specified, 683.Nm 684will act as a SOCKS 4/5 proxy and forward connections to the destinations 685requested by the remote SOCKS client. 686.Pp 687Port forwardings can also be specified in the configuration file. 688Privileged ports can be forwarded only when 689logging in as root on the remote machine. 690IPv6 addresses can be specified by enclosing the address in square brackets. 691.Pp 692By default, TCP listening sockets on the server will be bound to the loopback 693interface only. 694This may be overridden by specifying a 695.Ar bind_address . 696An empty 697.Ar bind_address , 698or the address 699.Ql * , 700indicates that the remote socket should listen on all interfaces. 701Specifying a remote 702.Ar bind_address 703will only succeed if the server's 704.Cm GatewayPorts 705option is enabled (see 706.Xr sshd_config 5 ) . 707.Pp 708If the 709.Ar port 710argument is 711.Ql 0 , 712the listen port will be dynamically allocated on the server and reported 713to the client at run time. 714When used together with 715.Ic -O forward , 716the allocated port will be printed to the standard output. 717.Pp 718.It Fl S Ar ctl_path 719Specifies the location of a control socket for connection sharing, 720or the string 721.Dq none 722to disable connection sharing. 723Refer to the description of 724.Cm ControlPath 725and 726.Cm ControlMaster 727in 728.Xr ssh_config 5 729for details. 730.Pp 731.It Fl s 732May be used to request invocation of a subsystem on the remote system. 733Subsystems facilitate the use of SSH 734as a secure transport for other applications (e.g.\& 735.Xr sftp 1 ) . 736The subsystem is specified as the remote command. 737Refer to the description of 738.Cm SessionType 739in 740.Xr ssh_config 5 741for details. 742.Pp 743.It Fl T 744Disable pseudo-terminal allocation. 745.Pp 746.It Fl t 747Force pseudo-terminal allocation. 748This can be used to execute arbitrary 749screen-based programs on a remote machine, which can be very useful, 750e.g. when implementing menu services. 751Multiple 752.Fl t 753options force tty allocation, even if 754.Nm 755has no local tty. 756.Pp 757.It Fl V 758Display the version number and exit. 759.Pp 760.It Fl v 761Verbose mode. 762Causes 763.Nm 764to print debugging messages about its progress. 765This is helpful in 766debugging connection, authentication, and configuration problems. 767Multiple 768.Fl v 769options increase the verbosity. 770The maximum is 3. 771.Pp 772.It Fl W Ar host : Ns Ar port 773Requests that standard input and output on the client be forwarded to 774.Ar host 775on 776.Ar port 777over the secure channel. 778Implies 779.Fl N , 780.Fl T , 781.Cm ExitOnForwardFailure 782and 783.Cm ClearAllForwardings , 784though these can be overridden in the configuration file or using 785.Fl o 786command line options. 787.Pp 788.It Fl w Xo 789.Ar local_tun Ns Op : Ns Ar remote_tun 790.Xc 791Requests 792tunnel 793device forwarding with the specified 794.Xr tun 4 795devices between the client 796.Pq Ar local_tun 797and the server 798.Pq Ar remote_tun . 799.Pp 800The devices may be specified by numerical ID or the keyword 801.Dq any , 802which uses the next available tunnel device. 803If 804.Ar remote_tun 805is not specified, it defaults to 806.Dq any . 807See also the 808.Cm Tunnel 809and 810.Cm TunnelDevice 811directives in 812.Xr ssh_config 5 . 813.Pp 814If the 815.Cm Tunnel 816directive is unset, it will be set to the default tunnel mode, which is 817.Dq point-to-point . 818If a different 819.Cm Tunnel 820forwarding mode it desired, then it should be specified before 821.Fl w . 822.Pp 823.It Fl X 824Enables X11 forwarding. 825This can also be specified on a per-host basis in a configuration file. 826.Pp 827X11 forwarding should be enabled with caution. 828Users with the ability to bypass file permissions on the remote host 829(for the user's X authorization database) 830can access the local X11 display through the forwarded connection. 831An attacker may then be able to perform activities such as keystroke monitoring. 832.Pp 833For this reason, X11 forwarding is subjected to X11 SECURITY extension 834restrictions by default. 835Refer to the 836.Nm 837.Fl Y 838option and the 839.Cm ForwardX11Trusted 840directive in 841.Xr ssh_config 5 842for more information. 843.Pp 844.It Fl x 845Disables X11 forwarding. 846.Pp 847.It Fl Y 848Enables trusted X11 forwarding. 849Trusted X11 forwardings are not subjected to the X11 SECURITY extension 850controls. 851.Pp 852.It Fl y 853Send log information using the 854.Xr syslog 3 855system module. 856By default this information is sent to stderr. 857.El 858.Pp 859.Nm 860may additionally obtain configuration data from 861a per-user configuration file and a system-wide configuration file. 862The file format and configuration options are described in 863.Xr ssh_config 5 . 864.Sh AUTHENTICATION 865The OpenSSH SSH client supports SSH protocol 2. 866.Pp 867The methods available for authentication are: 868GSSAPI-based authentication, 869host-based authentication, 870public key authentication, 871keyboard-interactive authentication, 872and password authentication. 873Authentication methods are tried in the order specified above, 874though 875.Cm PreferredAuthentications 876can be used to change the default order. 877.Pp 878Host-based authentication works as follows: 879If the machine the user logs in from is listed in 880.Pa /etc/hosts.equiv 881or 882.Pa /etc/shosts.equiv 883on the remote machine, the user is non-root and the user names are 884the same on both sides, or if the files 885.Pa ~/.rhosts 886or 887.Pa ~/.shosts 888exist in the user's home directory on the 889remote machine and contain a line containing the name of the client 890machine and the name of the user on that machine, the user is 891considered for login. 892Additionally, the server 893.Em must 894be able to verify the client's 895host key (see the description of 896.Pa /etc/ssh/ssh_known_hosts 897and 898.Pa ~/.ssh/known_hosts , 899below) 900for login to be permitted. 901This authentication method closes security holes due to IP 902spoofing, DNS spoofing, and routing spoofing. 903[Note to the administrator: 904.Pa /etc/hosts.equiv , 905.Pa ~/.rhosts , 906and the rlogin/rsh protocol in general, are inherently insecure and should be 907disabled if security is desired.] 908.Pp 909Public key authentication works as follows: 910The scheme is based on public-key cryptography, 911using cryptosystems 912where encryption and decryption are done using separate keys, 913and it is unfeasible to derive the decryption key from the encryption key. 914The idea is that each user creates a public/private 915key pair for authentication purposes. 916The server knows the public key, and only the user knows the private key. 917.Nm 918implements public key authentication protocol automatically, 919using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 920The HISTORY section of 921.Xr ssl 8 922contains a brief discussion of the DSA and RSA algorithms. 923.Pp 924The file 925.Pa ~/.ssh/authorized_keys 926lists the public keys that are permitted for logging in. 927When the user logs in, the 928.Nm 929program tells the server which key pair it would like to use for 930authentication. 931The client proves that it has access to the private key 932and the server checks that the corresponding public key 933is authorized to accept the account. 934.Pp 935The server may inform the client of errors that prevented public key 936authentication from succeeding after authentication completes using a 937different method. 938These may be viewed by increasing the 939.Cm LogLevel 940to 941.Cm DEBUG 942or higher (e.g. by using the 943.Fl v 944flag). 945.Pp 946The user creates their key pair by running 947.Xr ssh-keygen 1 . 948This stores the private key in 949.Pa ~/.ssh/id_dsa 950(DSA), 951.Pa ~/.ssh/id_ecdsa 952(ECDSA), 953.Pa ~/.ssh/id_ecdsa_sk 954(authenticator-hosted ECDSA), 955.Pa ~/.ssh/id_ed25519 956(Ed25519), 957.Pa ~/.ssh/id_ed25519_sk 958(authenticator-hosted Ed25519), 959or 960.Pa ~/.ssh/id_rsa 961(RSA) 962and stores the public key in 963.Pa ~/.ssh/id_dsa.pub 964(DSA), 965.Pa ~/.ssh/id_ecdsa.pub 966(ECDSA), 967.Pa ~/.ssh/id_ecdsa_sk.pub 968(authenticator-hosted ECDSA), 969.Pa ~/.ssh/id_ed25519.pub 970(Ed25519), 971.Pa ~/.ssh/id_ed25519_sk.pub 972(authenticator-hosted Ed25519), 973or 974.Pa ~/.ssh/id_rsa.pub 975(RSA) 976in the user's home directory. 977The user should then copy the public key 978to 979.Pa ~/.ssh/authorized_keys 980in their home directory on the remote machine. 981The 982.Pa authorized_keys 983file corresponds to the conventional 984.Pa ~/.rhosts 985file, and has one key 986per line, though the lines can be very long. 987After this, the user can log in without giving the password. 988.Pp 989A variation on public key authentication 990is available in the form of certificate authentication: 991instead of a set of public/private keys, 992signed certificates are used. 993This has the advantage that a single trusted certification authority 994can be used in place of many public/private keys. 995See the CERTIFICATES section of 996.Xr ssh-keygen 1 997for more information. 998.Pp 999The most convenient way to use public key or certificate authentication 1000may be with an authentication agent. 1001See 1002.Xr ssh-agent 1 1003and (optionally) the 1004.Cm AddKeysToAgent 1005directive in 1006.Xr ssh_config 5 1007for more information. 1008.Pp 1009Keyboard-interactive authentication works as follows: 1010The server sends an arbitrary 1011.Qq challenge 1012text and prompts for a response, possibly multiple times. 1013Examples of keyboard-interactive authentication include 1014.Bx 1015Authentication (see 1016.Xr login.conf 5 ) 1017and PAM (some 1018.Pf non- Ox 1019systems). 1020.Pp 1021Finally, if other authentication methods fail, 1022.Nm 1023prompts the user for a password. 1024The password is sent to the remote 1025host for checking; however, since all communications are encrypted, 1026the password cannot be seen by someone listening on the network. 1027.Pp 1028.Nm 1029automatically maintains and checks a database containing 1030identification for all hosts it has ever been used with. 1031Host keys are stored in 1032.Pa ~/.ssh/known_hosts 1033in the user's home directory. 1034Additionally, the file 1035.Pa /etc/ssh/ssh_known_hosts 1036is automatically checked for known hosts. 1037Any new hosts are automatically added to the user's file. 1038If a host's identification ever changes, 1039.Nm 1040warns about this and disables password authentication to prevent 1041server spoofing or man-in-the-middle attacks, 1042which could otherwise be used to circumvent the encryption. 1043The 1044.Cm StrictHostKeyChecking 1045option can be used to control logins to machines whose 1046host key is not known or has changed. 1047.Pp 1048When the user's identity has been accepted by the server, the server 1049either executes the given command in a non-interactive session or, 1050if no command has been specified, logs into the machine and gives 1051the user a normal shell as an interactive session. 1052All communication with 1053the remote command or shell will be automatically encrypted. 1054.Pp 1055If an interactive session is requested, 1056.Nm 1057by default will only request a pseudo-terminal (pty) for interactive 1058sessions when the client has one. 1059The flags 1060.Fl T 1061and 1062.Fl t 1063can be used to override this behaviour. 1064.Pp 1065If a pseudo-terminal has been allocated, the 1066user may use the escape characters noted below. 1067.Pp 1068If no pseudo-terminal has been allocated, 1069the session is transparent and can be used to reliably transfer binary data. 1070On most systems, setting the escape character to 1071.Dq none 1072will also make the session transparent even if a tty is used. 1073.Pp 1074The session terminates when the command or shell on the remote 1075machine exits and all X11 and TCP connections have been closed. 1076.Sh ESCAPE CHARACTERS 1077When a pseudo-terminal has been requested, 1078.Nm 1079supports a number of functions through the use of an escape character. 1080.Pp 1081A single tilde character can be sent as 1082.Ic ~~ 1083or by following the tilde by a character other than those described below. 1084The escape character must always follow a newline to be interpreted as 1085special. 1086The escape character can be changed in configuration files using the 1087.Cm EscapeChar 1088configuration directive or on the command line by the 1089.Fl e 1090option. 1091.Pp 1092The supported escapes (assuming the default 1093.Ql ~ ) 1094are: 1095.Bl -tag -width Ds 1096.It Cm ~. 1097Disconnect. 1098.It Cm ~^Z 1099Background 1100.Nm . 1101.It Cm ~# 1102List forwarded connections. 1103.It Cm ~& 1104Background 1105.Nm 1106at logout when waiting for forwarded connection / X11 sessions to terminate. 1107.It Cm ~? 1108Display a list of escape characters. 1109.It Cm ~B 1110Send a BREAK to the remote system 1111(only useful if the peer supports it). 1112.It Cm ~C 1113Open command line. 1114Currently this allows the addition of port forwardings using the 1115.Fl L , 1116.Fl R 1117and 1118.Fl D 1119options (see above). 1120It also allows the cancellation of existing port-forwardings 1121with 1122.Sm off 1123.Fl KL Oo Ar bind_address : Oc Ar port 1124.Sm on 1125for local, 1126.Sm off 1127.Fl KR Oo Ar bind_address : Oc Ar port 1128.Sm on 1129for remote and 1130.Sm off 1131.Fl KD Oo Ar bind_address : Oc Ar port 1132.Sm on 1133for dynamic port-forwardings. 1134.Ic !\& Ns Ar command 1135allows the user to execute a local command if the 1136.Ic PermitLocalCommand 1137option is enabled in 1138.Xr ssh_config 5 . 1139Basic help is available, using the 1140.Fl h 1141option. 1142.It Cm ~R 1143Request rekeying of the connection 1144(only useful if the peer supports it). 1145.It Cm ~V 1146Decrease the verbosity 1147.Pq Ic LogLevel 1148when errors are being written to stderr. 1149.It Cm ~v 1150Increase the verbosity 1151.Pq Ic LogLevel 1152when errors are being written to stderr. 1153.El 1154.Sh TCP FORWARDING 1155Forwarding of arbitrary TCP connections over a secure channel 1156can be specified either on the command line or in a configuration file. 1157One possible application of TCP forwarding is a secure connection to a 1158mail server; another is going through firewalls. 1159.Pp 1160In the example below, we look at encrypting communication for an IRC client, 1161even though the IRC server it connects to does not directly 1162support encrypted communication. 1163This works as follows: 1164the user connects to the remote host using 1165.Nm , 1166specifying the ports to be used to forward the connection. 1167After that it is possible to start the program locally, 1168and 1169.Nm 1170will encrypt and forward the connection to the remote server. 1171.Pp 1172The following example tunnels an IRC session from the client 1173to an IRC server at 1174.Dq server.example.com , 1175joining channel 1176.Dq #users , 1177nickname 1178.Dq pinky , 1179using the standard IRC port, 6667: 1180.Bd -literal -offset 4n 1181$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1182$ irc -c '#users' pinky IRC/127.0.0.1 1183.Ed 1184.Pp 1185The 1186.Fl f 1187option backgrounds 1188.Nm 1189and the remote command 1190.Dq sleep 10 1191is specified to allow an amount of time 1192(10 seconds, in the example) 1193to start the program which is going to use the tunnel. 1194If no connections are made within the time specified, 1195.Nm 1196will exit. 1197.Sh X11 FORWARDING 1198If the 1199.Cm ForwardX11 1200variable is set to 1201.Dq yes 1202(or see the description of the 1203.Fl X , 1204.Fl x , 1205and 1206.Fl Y 1207options above) 1208and the user is using X11 (the 1209.Ev DISPLAY 1210environment variable is set), the connection to the X11 display is 1211automatically forwarded to the remote side in such a way that any X11 1212programs started from the shell (or command) will go through the 1213encrypted channel, and the connection to the real X server will be made 1214from the local machine. 1215The user should not manually set 1216.Ev DISPLAY . 1217Forwarding of X11 connections can be 1218configured on the command line or in configuration files. 1219.Pp 1220The 1221.Ev DISPLAY 1222value set by 1223.Nm 1224will point to the server machine, but with a display number greater than zero. 1225This is normal, and happens because 1226.Nm 1227creates a 1228.Dq proxy 1229X server on the server machine for forwarding the 1230connections over the encrypted channel. 1231.Pp 1232.Nm 1233will also automatically set up Xauthority data on the server machine. 1234For this purpose, it will generate a random authorization cookie, 1235store it in Xauthority on the server, and verify that any forwarded 1236connections carry this cookie and replace it by the real cookie when 1237the connection is opened. 1238The real authentication cookie is never 1239sent to the server machine (and no cookies are sent in the plain). 1240.Pp 1241If the 1242.Cm ForwardAgent 1243variable is set to 1244.Dq yes 1245(or see the description of the 1246.Fl A 1247and 1248.Fl a 1249options above) and 1250the user is using an authentication agent, the connection to the agent 1251is automatically forwarded to the remote side. 1252.Sh VERIFYING HOST KEYS 1253When connecting to a server for the first time, 1254a fingerprint of the server's public key is presented to the user 1255(unless the option 1256.Cm StrictHostKeyChecking 1257has been disabled). 1258Fingerprints can be determined using 1259.Xr ssh-keygen 1 : 1260.Pp 1261.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1262.Pp 1263If the fingerprint is already known, it can be matched 1264and the key can be accepted or rejected. 1265If only legacy (MD5) fingerprints for the server are available, the 1266.Xr ssh-keygen 1 1267.Fl E 1268option may be used to downgrade the fingerprint algorithm to match. 1269.Pp 1270Because of the difficulty of comparing host keys 1271just by looking at fingerprint strings, 1272there is also support to compare host keys visually, 1273using 1274.Em random art . 1275By setting the 1276.Cm VisualHostKey 1277option to 1278.Dq yes , 1279a small ASCII graphic gets displayed on every login to a server, no matter 1280if the session itself is interactive or not. 1281By learning the pattern a known server produces, a user can easily 1282find out that the host key has changed when a completely different pattern 1283is displayed. 1284Because these patterns are not unambiguous however, a pattern that looks 1285similar to the pattern remembered only gives a good probability that the 1286host key is the same, not guaranteed proof. 1287.Pp 1288To get a listing of the fingerprints along with their random art for 1289all known hosts, the following command line can be used: 1290.Pp 1291.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1292.Pp 1293If the fingerprint is unknown, 1294an alternative method of verification is available: 1295SSH fingerprints verified by DNS. 1296An additional resource record (RR), 1297SSHFP, 1298is added to a zonefile 1299and the connecting client is able to match the fingerprint 1300with that of the key presented. 1301.Pp 1302In this example, we are connecting a client to a server, 1303.Dq host.example.com . 1304The SSHFP resource records should first be added to the zonefile for 1305host.example.com: 1306.Bd -literal -offset indent 1307$ ssh-keygen -r host.example.com. 1308.Ed 1309.Pp 1310The output lines will have to be added to the zonefile. 1311To check that the zone is answering fingerprint queries: 1312.Pp 1313.Dl $ dig -t SSHFP host.example.com 1314.Pp 1315Finally the client connects: 1316.Bd -literal -offset indent 1317$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1318[...] 1319Matching host key fingerprint found in DNS. 1320Are you sure you want to continue connecting (yes/no)? 1321.Ed 1322.Pp 1323See the 1324.Cm VerifyHostKeyDNS 1325option in 1326.Xr ssh_config 5 1327for more information. 1328.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1329.Nm 1330contains support for Virtual Private Network (VPN) tunnelling 1331using the 1332.Xr tun 4 1333network pseudo-device, 1334allowing two networks to be joined securely. 1335The 1336.Xr sshd_config 5 1337configuration option 1338.Cm PermitTunnel 1339controls whether the server supports this, 1340and at what level (layer 2 or 3 traffic). 1341.Pp 1342The following example would connect client network 10.0.50.0/24 1343with remote network 10.0.99.0/24 using a point-to-point connection 1344from 10.1.1.1 to 10.1.1.2, 1345provided that the SSH server running on the gateway to the remote network, 1346at 192.168.1.15, allows it. 1347.Pp 1348On the client: 1349.Bd -literal -offset indent 1350# ssh -f -w 0:1 192.168.1.15 true 1351# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1352# route add 10.0.99.0/24 10.1.1.2 1353.Ed 1354.Pp 1355On the server: 1356.Bd -literal -offset indent 1357# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1358# route add 10.0.50.0/24 10.1.1.1 1359.Ed 1360.Pp 1361Client access may be more finely tuned via the 1362.Pa /root/.ssh/authorized_keys 1363file (see below) and the 1364.Cm PermitRootLogin 1365server option. 1366The following entry would permit connections on 1367.Xr tun 4 1368device 1 from user 1369.Dq jane 1370and on tun device 2 from user 1371.Dq john , 1372if 1373.Cm PermitRootLogin 1374is set to 1375.Dq forced-commands-only : 1376.Bd -literal -offset 2n 1377tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1378tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1379.Ed 1380.Pp 1381Since an SSH-based setup entails a fair amount of overhead, 1382it may be more suited to temporary setups, 1383such as for wireless VPNs. 1384More permanent VPNs are better provided by tools such as 1385.Xr ipsecctl 8 1386and 1387.Xr isakmpd 8 . 1388.Sh ENVIRONMENT 1389.Nm 1390will normally set the following environment variables: 1391.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1392.It Ev DISPLAY 1393The 1394.Ev DISPLAY 1395variable indicates the location of the X11 server. 1396It is automatically set by 1397.Nm 1398to point to a value of the form 1399.Dq hostname:n , 1400where 1401.Dq hostname 1402indicates the host where the shell runs, and 1403.Sq n 1404is an integer \*(Ge 1. 1405.Nm 1406uses this special value to forward X11 connections over the secure 1407channel. 1408The user should normally not set 1409.Ev DISPLAY 1410explicitly, as that 1411will render the X11 connection insecure (and will require the user to 1412manually copy any required authorization cookies). 1413.It Ev HOME 1414Set to the path of the user's home directory. 1415.It Ev LOGNAME 1416Synonym for 1417.Ev USER ; 1418set for compatibility with systems that use this variable. 1419.It Ev MAIL 1420Set to the path of the user's mailbox. 1421.It Ev PATH 1422Set to the default 1423.Ev PATH , 1424as specified when compiling 1425.Nm . 1426.It Ev SSH_ASKPASS 1427If 1428.Nm 1429needs a passphrase, it will read the passphrase from the current 1430terminal if it was run from a terminal. 1431If 1432.Nm 1433does not have a terminal associated with it but 1434.Ev DISPLAY 1435and 1436.Ev SSH_ASKPASS 1437are set, it will execute the program specified by 1438.Ev SSH_ASKPASS 1439and open an X11 window to read the passphrase. 1440This is particularly useful when calling 1441.Nm 1442from a 1443.Pa .xsession 1444or related script. 1445(Note that on some machines it 1446may be necessary to redirect the input from 1447.Pa /dev/null 1448to make this work.) 1449.It Ev SSH_ASKPASS_REQUIRE 1450Allows further control over the use of an askpass program. 1451If this variable is set to 1452.Dq never 1453then 1454.Nm 1455will never attempt to use one. 1456If it is set to 1457.Dq prefer , 1458then 1459.Nm 1460will prefer to use the askpass program instead of the TTY when requesting 1461passwords. 1462Finally, if the variable is set to 1463.Dq force , 1464then the askpass program will be used for all passphrase input regardless 1465of whether 1466.Ev DISPLAY 1467is set. 1468.It Ev SSH_AUTH_SOCK 1469Identifies the path of a 1470.Ux Ns -domain 1471socket used to communicate with the agent. 1472.It Ev SSH_CONNECTION 1473Identifies the client and server ends of the connection. 1474The variable contains 1475four space-separated values: client IP address, client port number, 1476server IP address, and server port number. 1477.It Ev SSH_ORIGINAL_COMMAND 1478This variable contains the original command line if a forced command 1479is executed. 1480It can be used to extract the original arguments. 1481.It Ev SSH_TTY 1482This is set to the name of the tty (path to the device) associated 1483with the current shell or command. 1484If the current session has no tty, 1485this variable is not set. 1486.It Ev SSH_TUNNEL 1487Optionally set by 1488.Xr sshd 8 1489to contain the interface names assigned if tunnel forwarding was 1490requested by the client. 1491.It Ev SSH_USER_AUTH 1492Optionally set by 1493.Xr sshd 8 , 1494this variable may contain a pathname to a file that lists the authentication 1495methods successfully used when the session was established, including any 1496public keys that were used. 1497.It Ev TZ 1498This variable is set to indicate the present time zone if it 1499was set when the daemon was started (i.e. the daemon passes the value 1500on to new connections). 1501.It Ev USER 1502Set to the name of the user logging in. 1503.El 1504.Pp 1505Additionally, 1506.Nm 1507reads 1508.Pa ~/.ssh/environment , 1509and adds lines of the format 1510.Dq VARNAME=value 1511to the environment if the file exists and users are allowed to 1512change their environment. 1513For more information, see the 1514.Cm PermitUserEnvironment 1515option in 1516.Xr sshd_config 5 . 1517.Sh FILES 1518.Bl -tag -width Ds -compact 1519.It Pa ~/.rhosts 1520This file is used for host-based authentication (see above). 1521On some machines this file may need to be 1522world-readable if the user's home directory is on an NFS partition, 1523because 1524.Xr sshd 8 1525reads it as root. 1526Additionally, this file must be owned by the user, 1527and must not have write permissions for anyone else. 1528The recommended 1529permission for most machines is read/write for the user, and not 1530accessible by others. 1531.Pp 1532.It Pa ~/.shosts 1533This file is used in exactly the same way as 1534.Pa .rhosts , 1535but allows host-based authentication without permitting login with 1536rlogin/rsh. 1537.Pp 1538.It Pa ~/.ssh/ 1539This directory is the default location for all user-specific configuration 1540and authentication information. 1541There is no general requirement to keep the entire contents of this directory 1542secret, but the recommended permissions are read/write/execute for the user, 1543and not accessible by others. 1544.Pp 1545.It Pa ~/.ssh/authorized_keys 1546Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1547that can be used for logging in as this user. 1548The format of this file is described in the 1549.Xr sshd 8 1550manual page. 1551This file is not highly sensitive, but the recommended 1552permissions are read/write for the user, and not accessible by others. 1553.Pp 1554.It Pa ~/.ssh/config 1555This is the per-user configuration file. 1556The file format and configuration options are described in 1557.Xr ssh_config 5 . 1558Because of the potential for abuse, this file must have strict permissions: 1559read/write for the user, and not writable by others. 1560.Pp 1561.It Pa ~/.ssh/environment 1562Contains additional definitions for environment variables; see 1563.Sx ENVIRONMENT , 1564above. 1565.Pp 1566.It Pa ~/.ssh/id_dsa 1567.It Pa ~/.ssh/id_ecdsa 1568.It Pa ~/.ssh/id_ecdsa_sk 1569.It Pa ~/.ssh/id_ed25519 1570.It Pa ~/.ssh/id_ed25519_sk 1571.It Pa ~/.ssh/id_rsa 1572Contains the private key for authentication. 1573These files 1574contain sensitive data and should be readable by the user but not 1575accessible by others (read/write/execute). 1576.Nm 1577will simply ignore a private key file if it is accessible by others. 1578It is possible to specify a passphrase when 1579generating the key which will be used to encrypt the 1580sensitive part of this file using AES-128. 1581.Pp 1582.It Pa ~/.ssh/id_dsa.pub 1583.It Pa ~/.ssh/id_ecdsa.pub 1584.It Pa ~/.ssh/id_ecdsa_sk.pub 1585.It Pa ~/.ssh/id_ed25519.pub 1586.It Pa ~/.ssh/id_ed25519_sk.pub 1587.It Pa ~/.ssh/id_rsa.pub 1588Contains the public key for authentication. 1589These files are not 1590sensitive and can (but need not) be readable by anyone. 1591.Pp 1592.It Pa ~/.ssh/known_hosts 1593Contains a list of host keys for all hosts the user has logged into 1594that are not already in the systemwide list of known host keys. 1595See 1596.Xr sshd 8 1597for further details of the format of this file. 1598.Pp 1599.It Pa ~/.ssh/rc 1600Commands in this file are executed by 1601.Nm 1602when the user logs in, just before the user's shell (or command) is 1603started. 1604See the 1605.Xr sshd 8 1606manual page for more information. 1607.Pp 1608.It Pa /etc/hosts.equiv 1609This file is for host-based authentication (see above). 1610It should only be writable by root. 1611.Pp 1612.It Pa /etc/shosts.equiv 1613This file is used in exactly the same way as 1614.Pa hosts.equiv , 1615but allows host-based authentication without permitting login with 1616rlogin/rsh. 1617.Pp 1618.It Pa /etc/ssh/ssh_config 1619Systemwide configuration file. 1620The file format and configuration options are described in 1621.Xr ssh_config 5 . 1622.Pp 1623.It Pa /etc/ssh/ssh_host_key 1624.It Pa /etc/ssh/ssh_host_dsa_key 1625.It Pa /etc/ssh/ssh_host_ecdsa_key 1626.It Pa /etc/ssh/ssh_host_ed25519_key 1627.It Pa /etc/ssh/ssh_host_rsa_key 1628These files contain the private parts of the host keys 1629and are used for host-based authentication. 1630.Pp 1631.It Pa /etc/ssh/ssh_known_hosts 1632Systemwide list of known host keys. 1633This file should be prepared by the 1634system administrator to contain the public host keys of all machines in the 1635organization. 1636It should be world-readable. 1637See 1638.Xr sshd 8 1639for further details of the format of this file. 1640.Pp 1641.It Pa /etc/ssh/sshrc 1642Commands in this file are executed by 1643.Nm 1644when the user logs in, just before the user's shell (or command) is started. 1645See the 1646.Xr sshd 8 1647manual page for more information. 1648.El 1649.Sh EXIT STATUS 1650.Nm 1651exits with the exit status of the remote command or with 255 1652if an error occurred. 1653.Sh SEE ALSO 1654.Xr scp 1 , 1655.Xr sftp 1 , 1656.Xr ssh-add 1 , 1657.Xr ssh-agent 1 , 1658.Xr ssh-keygen 1 , 1659.Xr ssh-keyscan 1 , 1660.Xr tun 4 , 1661.Xr ssh_config 5 , 1662.Xr ssh-keysign 8 , 1663.Xr sshd 8 1664.Sh STANDARDS 1665.Rs 1666.%A S. Lehtinen 1667.%A C. Lonvick 1668.%D January 2006 1669.%R RFC 4250 1670.%T The Secure Shell (SSH) Protocol Assigned Numbers 1671.Re 1672.Pp 1673.Rs 1674.%A T. Ylonen 1675.%A C. Lonvick 1676.%D January 2006 1677.%R RFC 4251 1678.%T The Secure Shell (SSH) Protocol Architecture 1679.Re 1680.Pp 1681.Rs 1682.%A T. Ylonen 1683.%A C. Lonvick 1684.%D January 2006 1685.%R RFC 4252 1686.%T The Secure Shell (SSH) Authentication Protocol 1687.Re 1688.Pp 1689.Rs 1690.%A T. Ylonen 1691.%A C. Lonvick 1692.%D January 2006 1693.%R RFC 4253 1694.%T The Secure Shell (SSH) Transport Layer Protocol 1695.Re 1696.Pp 1697.Rs 1698.%A T. Ylonen 1699.%A C. Lonvick 1700.%D January 2006 1701.%R RFC 4254 1702.%T The Secure Shell (SSH) Connection Protocol 1703.Re 1704.Pp 1705.Rs 1706.%A J. Schlyter 1707.%A W. Griffin 1708.%D January 2006 1709.%R RFC 4255 1710.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1711.Re 1712.Pp 1713.Rs 1714.%A F. Cusack 1715.%A M. Forssen 1716.%D January 2006 1717.%R RFC 4256 1718.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1719.Re 1720.Pp 1721.Rs 1722.%A J. Galbraith 1723.%A P. Remaker 1724.%D January 2006 1725.%R RFC 4335 1726.%T The Secure Shell (SSH) Session Channel Break Extension 1727.Re 1728.Pp 1729.Rs 1730.%A M. Bellare 1731.%A T. Kohno 1732.%A C. Namprempre 1733.%D January 2006 1734.%R RFC 4344 1735.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1736.Re 1737.Pp 1738.Rs 1739.%A B. Harris 1740.%D January 2006 1741.%R RFC 4345 1742.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1743.Re 1744.Pp 1745.Rs 1746.%A M. Friedl 1747.%A N. Provos 1748.%A W. Simpson 1749.%D March 2006 1750.%R RFC 4419 1751.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1752.Re 1753.Pp 1754.Rs 1755.%A J. Galbraith 1756.%A R. Thayer 1757.%D November 2006 1758.%R RFC 4716 1759.%T The Secure Shell (SSH) Public Key File Format 1760.Re 1761.Pp 1762.Rs 1763.%A D. Stebila 1764.%A J. Green 1765.%D December 2009 1766.%R RFC 5656 1767.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1768.Re 1769.Pp 1770.Rs 1771.%A A. Perrig 1772.%A D. Song 1773.%D 1999 1774.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1775.%T Hash Visualization: a New Technique to improve Real-World Security 1776.Re 1777.Sh AUTHORS 1778OpenSSH is a derivative of the original and free 1779ssh 1.2.12 release by Tatu Ylonen. 1780Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1781Theo de Raadt and Dug Song 1782removed many bugs, re-added newer features and 1783created OpenSSH. 1784Markus Friedl contributed the support for SSH 1785protocol versions 1.5 and 2.0. 1786