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 1.431 2022/05/28 05:57:56 jmc Exp $ 37.Dd $Mdocdate: May 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 EscapeChar 526.It ExitOnForwardFailure 527.It FingerprintHash 528.It ForkAfterAuthentication 529.It ForwardAgent 530.It ForwardX11 531.It ForwardX11Timeout 532.It ForwardX11Trusted 533.It GatewayPorts 534.It GlobalKnownHostsFile 535.It GSSAPIAuthentication 536.It GSSAPIDelegateCredentials 537.It HashKnownHosts 538.It Host 539.It HostbasedAcceptedAlgorithms 540.It HostbasedAuthentication 541.It HostKeyAlgorithms 542.It HostKeyAlias 543.It Hostname 544.It IdentitiesOnly 545.It IdentityAgent 546.It IdentityFile 547.It IPQoS 548.It KbdInteractiveAuthentication 549.It KbdInteractiveDevices 550.It KexAlgorithms 551.It KnownHostsCommand 552.It LocalCommand 553.It LocalForward 554.It LogLevel 555.It MACs 556.It Match 557.It NoHostAuthenticationForLocalhost 558.It NumberOfPasswordPrompts 559.It PasswordAuthentication 560.It PermitLocalCommand 561.It PermitRemoteOpen 562.It PKCS11Provider 563.It Port 564.It PreferredAuthentications 565.It ProxyCommand 566.It ProxyJump 567.It ProxyUseFdpass 568.It PubkeyAcceptedAlgorithms 569.It PubkeyAuthentication 570.It RekeyLimit 571.It RemoteCommand 572.It RemoteForward 573.It RequestTTY 574.It SendEnv 575.It ServerAliveInterval 576.It ServerAliveCountMax 577.It SessionType 578.It SetEnv 579.It StdinNull 580.It StreamLocalBindMask 581.It StreamLocalBindUnlink 582.It StrictHostKeyChecking 583.It TCPKeepAlive 584.It Tunnel 585.It TunnelDevice 586.It UpdateHostKeys 587.It User 588.It UserKnownHostsFile 589.It VerifyHostKeyDNS 590.It VisualHostKey 591.It XAuthLocation 592.El 593.Pp 594.It Fl p Ar port 595Port to connect to on the remote host. 596This can be specified on a 597per-host basis in the configuration file. 598.Pp 599.It Fl Q Ar query_option 600Queries for the algorithms supported by one of the following features: 601.Ar cipher 602(supported symmetric ciphers), 603.Ar cipher-auth 604(supported symmetric ciphers that support authenticated encryption), 605.Ar help 606(supported query terms for use with the 607.Fl Q 608flag), 609.Ar mac 610(supported message integrity codes), 611.Ar kex 612(key exchange algorithms), 613.Ar key 614(key types), 615.Ar key-cert 616(certificate key types), 617.Ar key-plain 618(non-certificate key types), 619.Ar key-sig 620(all key types and signature algorithms), 621.Ar protocol-version 622(supported SSH protocol versions), and 623.Ar sig 624(supported signature algorithms). 625Alternatively, any keyword from 626.Xr ssh_config 5 627or 628.Xr sshd_config 5 629that takes an algorithm list may be used as an alias for the corresponding 630query_option. 631.Pp 632.It Fl q 633Quiet mode. 634Causes most warning and diagnostic messages to be suppressed. 635.Pp 636.It Fl R Xo 637.Sm off 638.Oo Ar bind_address : Oc 639.Ar port : host : hostport 640.Sm on 641.Xc 642.It Fl R Xo 643.Sm off 644.Oo Ar bind_address : Oc 645.Ar port : local_socket 646.Sm on 647.Xc 648.It Fl R Xo 649.Sm off 650.Ar remote_socket : host : hostport 651.Sm on 652.Xc 653.It Fl R Xo 654.Sm off 655.Ar remote_socket : local_socket 656.Sm on 657.Xc 658.It Fl R Xo 659.Sm off 660.Oo Ar bind_address : Oc 661.Ar port 662.Sm on 663.Xc 664Specifies that connections to the given TCP port or Unix socket on the remote 665(server) host are to be forwarded to the local side. 666.Pp 667This works by allocating a socket to listen to either a TCP 668.Ar port 669or to a Unix socket on the remote side. 670Whenever a connection is made to this port or Unix socket, the 671connection is forwarded over the secure channel, and a connection 672is made from the local machine to either an explicit destination specified by 673.Ar host 674port 675.Ar hostport , 676or 677.Ar local_socket , 678or, if no explicit destination was specified, 679.Nm 680will act as a SOCKS 4/5 proxy and forward connections to the destinations 681requested by the remote SOCKS client. 682.Pp 683Port forwardings can also be specified in the configuration file. 684Privileged ports can be forwarded only when 685logging in as root on the remote machine. 686IPv6 addresses can be specified by enclosing the address in square brackets. 687.Pp 688By default, TCP listening sockets on the server will be bound to the loopback 689interface only. 690This may be overridden by specifying a 691.Ar bind_address . 692An empty 693.Ar bind_address , 694or the address 695.Ql * , 696indicates that the remote socket should listen on all interfaces. 697Specifying a remote 698.Ar bind_address 699will only succeed if the server's 700.Cm GatewayPorts 701option is enabled (see 702.Xr sshd_config 5 ) . 703.Pp 704If the 705.Ar port 706argument is 707.Ql 0 , 708the listen port will be dynamically allocated on the server and reported 709to the client at run time. 710When used together with 711.Ic -O forward , 712the allocated port will be printed to the standard output. 713.Pp 714.It Fl S Ar ctl_path 715Specifies the location of a control socket for connection sharing, 716or the string 717.Dq none 718to disable connection sharing. 719Refer to the description of 720.Cm ControlPath 721and 722.Cm ControlMaster 723in 724.Xr ssh_config 5 725for details. 726.Pp 727.It Fl s 728May be used to request invocation of a subsystem on the remote system. 729Subsystems facilitate the use of SSH 730as a secure transport for other applications (e.g.\& 731.Xr sftp 1 ) . 732The subsystem is specified as the remote command. 733Refer to the description of 734.Cm SessionType 735in 736.Xr ssh_config 5 737for details. 738.Pp 739.It Fl T 740Disable pseudo-terminal allocation. 741.Pp 742.It Fl t 743Force pseudo-terminal allocation. 744This can be used to execute arbitrary 745screen-based programs on a remote machine, which can be very useful, 746e.g. when implementing menu services. 747Multiple 748.Fl t 749options force tty allocation, even if 750.Nm 751has no local tty. 752.Pp 753.It Fl V 754Display the version number and exit. 755.Pp 756.It Fl v 757Verbose mode. 758Causes 759.Nm 760to print debugging messages about its progress. 761This is helpful in 762debugging connection, authentication, and configuration problems. 763Multiple 764.Fl v 765options increase the verbosity. 766The maximum is 3. 767.Pp 768.It Fl W Ar host : Ns Ar port 769Requests that standard input and output on the client be forwarded to 770.Ar host 771on 772.Ar port 773over the secure channel. 774Implies 775.Fl N , 776.Fl T , 777.Cm ExitOnForwardFailure 778and 779.Cm ClearAllForwardings , 780though these can be overridden in the configuration file or using 781.Fl o 782command line options. 783.Pp 784.It Fl w Xo 785.Ar local_tun Ns Op : Ns Ar remote_tun 786.Xc 787Requests 788tunnel 789device forwarding with the specified 790.Xr tun 4 791devices between the client 792.Pq Ar local_tun 793and the server 794.Pq Ar remote_tun . 795.Pp 796The devices may be specified by numerical ID or the keyword 797.Dq any , 798which uses the next available tunnel device. 799If 800.Ar remote_tun 801is not specified, it defaults to 802.Dq any . 803See also the 804.Cm Tunnel 805and 806.Cm TunnelDevice 807directives in 808.Xr ssh_config 5 . 809.Pp 810If the 811.Cm Tunnel 812directive is unset, it will be set to the default tunnel mode, which is 813.Dq point-to-point . 814If a different 815.Cm Tunnel 816forwarding mode it desired, then it should be specified before 817.Fl w . 818.Pp 819.It Fl X 820Enables X11 forwarding. 821This can also be specified on a per-host basis in a configuration file. 822.Pp 823X11 forwarding should be enabled with caution. 824Users with the ability to bypass file permissions on the remote host 825(for the user's X authorization database) 826can access the local X11 display through the forwarded connection. 827An attacker may then be able to perform activities such as keystroke monitoring. 828.Pp 829For this reason, X11 forwarding is subjected to X11 SECURITY extension 830restrictions by default. 831Refer to the 832.Nm 833.Fl Y 834option and the 835.Cm ForwardX11Trusted 836directive in 837.Xr ssh_config 5 838for more information. 839.Pp 840.It Fl x 841Disables X11 forwarding. 842.Pp 843.It Fl Y 844Enables trusted X11 forwarding. 845Trusted X11 forwardings are not subjected to the X11 SECURITY extension 846controls. 847.Pp 848.It Fl y 849Send log information using the 850.Xr syslog 3 851system module. 852By default this information is sent to stderr. 853.El 854.Pp 855.Nm 856may additionally obtain configuration data from 857a per-user configuration file and a system-wide configuration file. 858The file format and configuration options are described in 859.Xr ssh_config 5 . 860.Sh AUTHENTICATION 861The OpenSSH SSH client supports SSH protocol 2. 862.Pp 863The methods available for authentication are: 864GSSAPI-based authentication, 865host-based authentication, 866public key authentication, 867keyboard-interactive authentication, 868and password authentication. 869Authentication methods are tried in the order specified above, 870though 871.Cm PreferredAuthentications 872can be used to change the default order. 873.Pp 874Host-based authentication works as follows: 875If the machine the user logs in from is listed in 876.Pa /etc/hosts.equiv 877or 878.Pa /etc/shosts.equiv 879on the remote machine, the user is non-root and the user names are 880the same on both sides, or if the files 881.Pa ~/.rhosts 882or 883.Pa ~/.shosts 884exist in the user's home directory on the 885remote machine and contain a line containing the name of the client 886machine and the name of the user on that machine, the user is 887considered for login. 888Additionally, the server 889.Em must 890be able to verify the client's 891host key (see the description of 892.Pa /etc/ssh/ssh_known_hosts 893and 894.Pa ~/.ssh/known_hosts , 895below) 896for login to be permitted. 897This authentication method closes security holes due to IP 898spoofing, DNS spoofing, and routing spoofing. 899[Note to the administrator: 900.Pa /etc/hosts.equiv , 901.Pa ~/.rhosts , 902and the rlogin/rsh protocol in general, are inherently insecure and should be 903disabled if security is desired.] 904.Pp 905Public key authentication works as follows: 906The scheme is based on public-key cryptography, 907using cryptosystems 908where encryption and decryption are done using separate keys, 909and it is unfeasible to derive the decryption key from the encryption key. 910The idea is that each user creates a public/private 911key pair for authentication purposes. 912The server knows the public key, and only the user knows the private key. 913.Nm 914implements public key authentication protocol automatically, 915using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 916The HISTORY section of 917.Xr ssl 8 918contains a brief discussion of the DSA and RSA algorithms. 919.Pp 920The file 921.Pa ~/.ssh/authorized_keys 922lists the public keys that are permitted for logging in. 923When the user logs in, the 924.Nm 925program tells the server which key pair it would like to use for 926authentication. 927The client proves that it has access to the private key 928and the server checks that the corresponding public key 929is authorized to accept the account. 930.Pp 931The server may inform the client of errors that prevented public key 932authentication from succeeding after authentication completes using a 933different method. 934These may be viewed by increasing the 935.Cm LogLevel 936to 937.Cm DEBUG 938or higher (e.g. by using the 939.Fl v 940flag). 941.Pp 942The user creates their key pair by running 943.Xr ssh-keygen 1 . 944This stores the private key in 945.Pa ~/.ssh/id_dsa 946(DSA), 947.Pa ~/.ssh/id_ecdsa 948(ECDSA), 949.Pa ~/.ssh/id_ecdsa_sk 950(authenticator-hosted ECDSA), 951.Pa ~/.ssh/id_ed25519 952(Ed25519), 953.Pa ~/.ssh/id_ed25519_sk 954(authenticator-hosted Ed25519), 955or 956.Pa ~/.ssh/id_rsa 957(RSA) 958and stores the public key in 959.Pa ~/.ssh/id_dsa.pub 960(DSA), 961.Pa ~/.ssh/id_ecdsa.pub 962(ECDSA), 963.Pa ~/.ssh/id_ecdsa_sk.pub 964(authenticator-hosted ECDSA), 965.Pa ~/.ssh/id_ed25519.pub 966(Ed25519), 967.Pa ~/.ssh/id_ed25519_sk.pub 968(authenticator-hosted Ed25519), 969or 970.Pa ~/.ssh/id_rsa.pub 971(RSA) 972in the user's home directory. 973The user should then copy the public key 974to 975.Pa ~/.ssh/authorized_keys 976in their home directory on the remote machine. 977The 978.Pa authorized_keys 979file corresponds to the conventional 980.Pa ~/.rhosts 981file, and has one key 982per line, though the lines can be very long. 983After this, the user can log in without giving the password. 984.Pp 985A variation on public key authentication 986is available in the form of certificate authentication: 987instead of a set of public/private keys, 988signed certificates are used. 989This has the advantage that a single trusted certification authority 990can be used in place of many public/private keys. 991See the CERTIFICATES section of 992.Xr ssh-keygen 1 993for more information. 994.Pp 995The most convenient way to use public key or certificate authentication 996may be with an authentication agent. 997See 998.Xr ssh-agent 1 999and (optionally) the 1000.Cm AddKeysToAgent 1001directive in 1002.Xr ssh_config 5 1003for more information. 1004.Pp 1005Keyboard-interactive authentication works as follows: 1006The server sends an arbitrary 1007.Qq challenge 1008text and prompts for a response, possibly multiple times. 1009Examples of keyboard-interactive authentication include 1010.Bx 1011Authentication (see 1012.Xr login.conf 5 ) 1013and PAM (some 1014.Pf non- Ox 1015systems). 1016.Pp 1017Finally, if other authentication methods fail, 1018.Nm 1019prompts the user for a password. 1020The password is sent to the remote 1021host for checking; however, since all communications are encrypted, 1022the password cannot be seen by someone listening on the network. 1023.Pp 1024.Nm 1025automatically maintains and checks a database containing 1026identification for all hosts it has ever been used with. 1027Host keys are stored in 1028.Pa ~/.ssh/known_hosts 1029in the user's home directory. 1030Additionally, the file 1031.Pa /etc/ssh/ssh_known_hosts 1032is automatically checked for known hosts. 1033Any new hosts are automatically added to the user's file. 1034If a host's identification ever changes, 1035.Nm 1036warns about this and disables password authentication to prevent 1037server spoofing or man-in-the-middle attacks, 1038which could otherwise be used to circumvent the encryption. 1039The 1040.Cm StrictHostKeyChecking 1041option can be used to control logins to machines whose 1042host key is not known or has changed. 1043.Pp 1044When the user's identity has been accepted by the server, the server 1045either executes the given command in a non-interactive session or, 1046if no command has been specified, logs into the machine and gives 1047the user a normal shell as an interactive session. 1048All communication with 1049the remote command or shell will be automatically encrypted. 1050.Pp 1051If an interactive session is requested, 1052.Nm 1053by default will only request a pseudo-terminal (pty) for interactive 1054sessions when the client has one. 1055The flags 1056.Fl T 1057and 1058.Fl t 1059can be used to override this behaviour. 1060.Pp 1061If a pseudo-terminal has been allocated, the 1062user may use the escape characters noted below. 1063.Pp 1064If no pseudo-terminal has been allocated, 1065the session is transparent and can be used to reliably transfer binary data. 1066On most systems, setting the escape character to 1067.Dq none 1068will also make the session transparent even if a tty is used. 1069.Pp 1070The session terminates when the command or shell on the remote 1071machine exits and all X11 and TCP connections have been closed. 1072.Sh ESCAPE CHARACTERS 1073When a pseudo-terminal has been requested, 1074.Nm 1075supports a number of functions through the use of an escape character. 1076.Pp 1077A single tilde character can be sent as 1078.Ic ~~ 1079or by following the tilde by a character other than those described below. 1080The escape character must always follow a newline to be interpreted as 1081special. 1082The escape character can be changed in configuration files using the 1083.Cm EscapeChar 1084configuration directive or on the command line by the 1085.Fl e 1086option. 1087.Pp 1088The supported escapes (assuming the default 1089.Ql ~ ) 1090are: 1091.Bl -tag -width Ds 1092.It Cm ~. 1093Disconnect. 1094.It Cm ~^Z 1095Background 1096.Nm . 1097.It Cm ~# 1098List forwarded connections. 1099.It Cm ~& 1100Background 1101.Nm 1102at logout when waiting for forwarded connection / X11 sessions to terminate. 1103.It Cm ~? 1104Display a list of escape characters. 1105.It Cm ~B 1106Send a BREAK to the remote system 1107(only useful if the peer supports it). 1108.It Cm ~C 1109Open command line. 1110Currently this allows the addition of port forwardings using the 1111.Fl L , 1112.Fl R 1113and 1114.Fl D 1115options (see above). 1116It also allows the cancellation of existing port-forwardings 1117with 1118.Sm off 1119.Fl KL Oo Ar bind_address : Oc Ar port 1120.Sm on 1121for local, 1122.Sm off 1123.Fl KR Oo Ar bind_address : Oc Ar port 1124.Sm on 1125for remote and 1126.Sm off 1127.Fl KD Oo Ar bind_address : Oc Ar port 1128.Sm on 1129for dynamic port-forwardings. 1130.Ic !\& Ns Ar command 1131allows the user to execute a local command if the 1132.Ic PermitLocalCommand 1133option is enabled in 1134.Xr ssh_config 5 . 1135Basic help is available, using the 1136.Fl h 1137option. 1138.It Cm ~R 1139Request rekeying of the connection 1140(only useful if the peer supports it). 1141.It Cm ~V 1142Decrease the verbosity 1143.Pq Ic LogLevel 1144when errors are being written to stderr. 1145.It Cm ~v 1146Increase the verbosity 1147.Pq Ic LogLevel 1148when errors are being written to stderr. 1149.El 1150.Sh TCP FORWARDING 1151Forwarding of arbitrary TCP connections over a secure channel 1152can be specified either on the command line or in a configuration file. 1153One possible application of TCP forwarding is a secure connection to a 1154mail server; another is going through firewalls. 1155.Pp 1156In the example below, we look at encrypting communication for an IRC client, 1157even though the IRC server it connects to does not directly 1158support encrypted communication. 1159This works as follows: 1160the user connects to the remote host using 1161.Nm , 1162specifying the ports to be used to forward the connection. 1163After that it is possible to start the program locally, 1164and 1165.Nm 1166will encrypt and forward the connection to the remote server. 1167.Pp 1168The following example tunnels an IRC session from the client 1169to an IRC server at 1170.Dq server.example.com , 1171joining channel 1172.Dq #users , 1173nickname 1174.Dq pinky , 1175using the standard IRC port, 6667: 1176.Bd -literal -offset 4n 1177$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1178$ irc -c '#users' pinky IRC/127.0.0.1 1179.Ed 1180.Pp 1181The 1182.Fl f 1183option backgrounds 1184.Nm 1185and the remote command 1186.Dq sleep 10 1187is specified to allow an amount of time 1188(10 seconds, in the example) 1189to start the program which is going to use the tunnel. 1190If no connections are made within the time specified, 1191.Nm 1192will exit. 1193.Sh X11 FORWARDING 1194If the 1195.Cm ForwardX11 1196variable is set to 1197.Dq yes 1198(or see the description of the 1199.Fl X , 1200.Fl x , 1201and 1202.Fl Y 1203options above) 1204and the user is using X11 (the 1205.Ev DISPLAY 1206environment variable is set), the connection to the X11 display is 1207automatically forwarded to the remote side in such a way that any X11 1208programs started from the shell (or command) will go through the 1209encrypted channel, and the connection to the real X server will be made 1210from the local machine. 1211The user should not manually set 1212.Ev DISPLAY . 1213Forwarding of X11 connections can be 1214configured on the command line or in configuration files. 1215.Pp 1216The 1217.Ev DISPLAY 1218value set by 1219.Nm 1220will point to the server machine, but with a display number greater than zero. 1221This is normal, and happens because 1222.Nm 1223creates a 1224.Dq proxy 1225X server on the server machine for forwarding the 1226connections over the encrypted channel. 1227.Pp 1228.Nm 1229will also automatically set up Xauthority data on the server machine. 1230For this purpose, it will generate a random authorization cookie, 1231store it in Xauthority on the server, and verify that any forwarded 1232connections carry this cookie and replace it by the real cookie when 1233the connection is opened. 1234The real authentication cookie is never 1235sent to the server machine (and no cookies are sent in the plain). 1236.Pp 1237If the 1238.Cm ForwardAgent 1239variable is set to 1240.Dq yes 1241(or see the description of the 1242.Fl A 1243and 1244.Fl a 1245options above) and 1246the user is using an authentication agent, the connection to the agent 1247is automatically forwarded to the remote side. 1248.Sh VERIFYING HOST KEYS 1249When connecting to a server for the first time, 1250a fingerprint of the server's public key is presented to the user 1251(unless the option 1252.Cm StrictHostKeyChecking 1253has been disabled). 1254Fingerprints can be determined using 1255.Xr ssh-keygen 1 : 1256.Pp 1257.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1258.Pp 1259If the fingerprint is already known, it can be matched 1260and the key can be accepted or rejected. 1261If only legacy (MD5) fingerprints for the server are available, the 1262.Xr ssh-keygen 1 1263.Fl E 1264option may be used to downgrade the fingerprint algorithm to match. 1265.Pp 1266Because of the difficulty of comparing host keys 1267just by looking at fingerprint strings, 1268there is also support to compare host keys visually, 1269using 1270.Em random art . 1271By setting the 1272.Cm VisualHostKey 1273option to 1274.Dq yes , 1275a small ASCII graphic gets displayed on every login to a server, no matter 1276if the session itself is interactive or not. 1277By learning the pattern a known server produces, a user can easily 1278find out that the host key has changed when a completely different pattern 1279is displayed. 1280Because these patterns are not unambiguous however, a pattern that looks 1281similar to the pattern remembered only gives a good probability that the 1282host key is the same, not guaranteed proof. 1283.Pp 1284To get a listing of the fingerprints along with their random art for 1285all known hosts, the following command line can be used: 1286.Pp 1287.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1288.Pp 1289If the fingerprint is unknown, 1290an alternative method of verification is available: 1291SSH fingerprints verified by DNS. 1292An additional resource record (RR), 1293SSHFP, 1294is added to a zonefile 1295and the connecting client is able to match the fingerprint 1296with that of the key presented. 1297.Pp 1298In this example, we are connecting a client to a server, 1299.Dq host.example.com . 1300The SSHFP resource records should first be added to the zonefile for 1301host.example.com: 1302.Bd -literal -offset indent 1303$ ssh-keygen -r host.example.com. 1304.Ed 1305.Pp 1306The output lines will have to be added to the zonefile. 1307To check that the zone is answering fingerprint queries: 1308.Pp 1309.Dl $ dig -t SSHFP host.example.com 1310.Pp 1311Finally the client connects: 1312.Bd -literal -offset indent 1313$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1314[...] 1315Matching host key fingerprint found in DNS. 1316Are you sure you want to continue connecting (yes/no)? 1317.Ed 1318.Pp 1319See the 1320.Cm VerifyHostKeyDNS 1321option in 1322.Xr ssh_config 5 1323for more information. 1324.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1325.Nm 1326contains support for Virtual Private Network (VPN) tunnelling 1327using the 1328.Xr tun 4 1329network pseudo-device, 1330allowing two networks to be joined securely. 1331The 1332.Xr sshd_config 5 1333configuration option 1334.Cm PermitTunnel 1335controls whether the server supports this, 1336and at what level (layer 2 or 3 traffic). 1337.Pp 1338The following example would connect client network 10.0.50.0/24 1339with remote network 10.0.99.0/24 using a point-to-point connection 1340from 10.1.1.1 to 10.1.1.2, 1341provided that the SSH server running on the gateway to the remote network, 1342at 192.168.1.15, allows it. 1343.Pp 1344On the client: 1345.Bd -literal -offset indent 1346# ssh -f -w 0:1 192.168.1.15 true 1347# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1348# route add 10.0.99.0/24 10.1.1.2 1349.Ed 1350.Pp 1351On the server: 1352.Bd -literal -offset indent 1353# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1354# route add 10.0.50.0/24 10.1.1.1 1355.Ed 1356.Pp 1357Client access may be more finely tuned via the 1358.Pa /root/.ssh/authorized_keys 1359file (see below) and the 1360.Cm PermitRootLogin 1361server option. 1362The following entry would permit connections on 1363.Xr tun 4 1364device 1 from user 1365.Dq jane 1366and on tun device 2 from user 1367.Dq john , 1368if 1369.Cm PermitRootLogin 1370is set to 1371.Dq forced-commands-only : 1372.Bd -literal -offset 2n 1373tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1374tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1375.Ed 1376.Pp 1377Since an SSH-based setup entails a fair amount of overhead, 1378it may be more suited to temporary setups, 1379such as for wireless VPNs. 1380More permanent VPNs are better provided by tools such as 1381.Xr ipsecctl 8 1382and 1383.Xr isakmpd 8 . 1384.Sh ENVIRONMENT 1385.Nm 1386will normally set the following environment variables: 1387.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1388.It Ev DISPLAY 1389The 1390.Ev DISPLAY 1391variable indicates the location of the X11 server. 1392It is automatically set by 1393.Nm 1394to point to a value of the form 1395.Dq hostname:n , 1396where 1397.Dq hostname 1398indicates the host where the shell runs, and 1399.Sq n 1400is an integer \*(Ge 1. 1401.Nm 1402uses this special value to forward X11 connections over the secure 1403channel. 1404The user should normally not set 1405.Ev DISPLAY 1406explicitly, as that 1407will render the X11 connection insecure (and will require the user to 1408manually copy any required authorization cookies). 1409.It Ev HOME 1410Set to the path of the user's home directory. 1411.It Ev LOGNAME 1412Synonym for 1413.Ev USER ; 1414set for compatibility with systems that use this variable. 1415.It Ev MAIL 1416Set to the path of the user's mailbox. 1417.It Ev PATH 1418Set to the default 1419.Ev PATH , 1420as specified when compiling 1421.Nm . 1422.It Ev SSH_ASKPASS 1423If 1424.Nm 1425needs a passphrase, it will read the passphrase from the current 1426terminal if it was run from a terminal. 1427If 1428.Nm 1429does not have a terminal associated with it but 1430.Ev DISPLAY 1431and 1432.Ev SSH_ASKPASS 1433are set, it will execute the program specified by 1434.Ev SSH_ASKPASS 1435and open an X11 window to read the passphrase. 1436This is particularly useful when calling 1437.Nm 1438from a 1439.Pa .xsession 1440or related script. 1441(Note that on some machines it 1442may be necessary to redirect the input from 1443.Pa /dev/null 1444to make this work.) 1445.It Ev SSH_ASKPASS_REQUIRE 1446Allows further control over the use of an askpass program. 1447If this variable is set to 1448.Dq never 1449then 1450.Nm 1451will never attempt to use one. 1452If it is set to 1453.Dq prefer , 1454then 1455.Nm 1456will prefer to use the askpass program instead of the TTY when requesting 1457passwords. 1458Finally, if the variable is set to 1459.Dq force , 1460then the askpass program will be used for all passphrase input regardless 1461of whether 1462.Ev DISPLAY 1463is set. 1464.It Ev SSH_AUTH_SOCK 1465Identifies the path of a 1466.Ux Ns -domain 1467socket used to communicate with the agent. 1468.It Ev SSH_CONNECTION 1469Identifies the client and server ends of the connection. 1470The variable contains 1471four space-separated values: client IP address, client port number, 1472server IP address, and server port number. 1473.It Ev SSH_ORIGINAL_COMMAND 1474This variable contains the original command line if a forced command 1475is executed. 1476It can be used to extract the original arguments. 1477.It Ev SSH_TTY 1478This is set to the name of the tty (path to the device) associated 1479with the current shell or command. 1480If the current session has no tty, 1481this variable is not set. 1482.It Ev SSH_TUNNEL 1483Optionally set by 1484.Xr sshd 8 1485to contain the interface names assigned if tunnel forwarding was 1486requested by the client. 1487.It Ev SSH_USER_AUTH 1488Optionally set by 1489.Xr sshd 8 , 1490this variable may contain a pathname to a file that lists the authentication 1491methods successfully used when the session was established, including any 1492public keys that were used. 1493.It Ev TZ 1494This variable is set to indicate the present time zone if it 1495was set when the daemon was started (i.e. the daemon passes the value 1496on to new connections). 1497.It Ev USER 1498Set to the name of the user logging in. 1499.El 1500.Pp 1501Additionally, 1502.Nm 1503reads 1504.Pa ~/.ssh/environment , 1505and adds lines of the format 1506.Dq VARNAME=value 1507to the environment if the file exists and users are allowed to 1508change their environment. 1509For more information, see the 1510.Cm PermitUserEnvironment 1511option in 1512.Xr sshd_config 5 . 1513.Sh FILES 1514.Bl -tag -width Ds -compact 1515.It Pa ~/.rhosts 1516This file is used for host-based authentication (see above). 1517On some machines this file may need to be 1518world-readable if the user's home directory is on an NFS partition, 1519because 1520.Xr sshd 8 1521reads it as root. 1522Additionally, this file must be owned by the user, 1523and must not have write permissions for anyone else. 1524The recommended 1525permission for most machines is read/write for the user, and not 1526accessible by others. 1527.Pp 1528.It Pa ~/.shosts 1529This file is used in exactly the same way as 1530.Pa .rhosts , 1531but allows host-based authentication without permitting login with 1532rlogin/rsh. 1533.Pp 1534.It Pa ~/.ssh/ 1535This directory is the default location for all user-specific configuration 1536and authentication information. 1537There is no general requirement to keep the entire contents of this directory 1538secret, but the recommended permissions are read/write/execute for the user, 1539and not accessible by others. 1540.Pp 1541.It Pa ~/.ssh/authorized_keys 1542Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1543that can be used for logging in as this user. 1544The format of this file is described in the 1545.Xr sshd 8 1546manual page. 1547This file is not highly sensitive, but the recommended 1548permissions are read/write for the user, and not accessible by others. 1549.Pp 1550.It Pa ~/.ssh/config 1551This is the per-user configuration file. 1552The file format and configuration options are described in 1553.Xr ssh_config 5 . 1554Because of the potential for abuse, this file must have strict permissions: 1555read/write for the user, and not writable by others. 1556.Pp 1557.It Pa ~/.ssh/environment 1558Contains additional definitions for environment variables; see 1559.Sx ENVIRONMENT , 1560above. 1561.Pp 1562.It Pa ~/.ssh/id_dsa 1563.It Pa ~/.ssh/id_ecdsa 1564.It Pa ~/.ssh/id_ecdsa_sk 1565.It Pa ~/.ssh/id_ed25519 1566.It Pa ~/.ssh/id_ed25519_sk 1567.It Pa ~/.ssh/id_rsa 1568Contains the private key for authentication. 1569These files 1570contain sensitive data and should be readable by the user but not 1571accessible by others (read/write/execute). 1572.Nm 1573will simply ignore a private key file if it is accessible by others. 1574It is possible to specify a passphrase when 1575generating the key which will be used to encrypt the 1576sensitive part of this file using AES-128. 1577.Pp 1578.It Pa ~/.ssh/id_dsa.pub 1579.It Pa ~/.ssh/id_ecdsa.pub 1580.It Pa ~/.ssh/id_ecdsa_sk.pub 1581.It Pa ~/.ssh/id_ed25519.pub 1582.It Pa ~/.ssh/id_ed25519_sk.pub 1583.It Pa ~/.ssh/id_rsa.pub 1584Contains the public key for authentication. 1585These files are not 1586sensitive and can (but need not) be readable by anyone. 1587.Pp 1588.It Pa ~/.ssh/known_hosts 1589Contains a list of host keys for all hosts the user has logged into 1590that are not already in the systemwide list of known host keys. 1591See 1592.Xr sshd 8 1593for further details of the format of this file. 1594.Pp 1595.It Pa ~/.ssh/rc 1596Commands in this file are executed by 1597.Nm 1598when the user logs in, just before the user's shell (or command) is 1599started. 1600See the 1601.Xr sshd 8 1602manual page for more information. 1603.Pp 1604.It Pa /etc/hosts.equiv 1605This file is for host-based authentication (see above). 1606It should only be writable by root. 1607.Pp 1608.It Pa /etc/shosts.equiv 1609This file is used in exactly the same way as 1610.Pa hosts.equiv , 1611but allows host-based authentication without permitting login with 1612rlogin/rsh. 1613.Pp 1614.It Pa /etc/ssh/ssh_config 1615Systemwide configuration file. 1616The file format and configuration options are described in 1617.Xr ssh_config 5 . 1618.Pp 1619.It Pa /etc/ssh/ssh_host_key 1620.It Pa /etc/ssh/ssh_host_dsa_key 1621.It Pa /etc/ssh/ssh_host_ecdsa_key 1622.It Pa /etc/ssh/ssh_host_ed25519_key 1623.It Pa /etc/ssh/ssh_host_rsa_key 1624These files contain the private parts of the host keys 1625and are used for host-based authentication. 1626.Pp 1627.It Pa /etc/ssh/ssh_known_hosts 1628Systemwide list of known host keys. 1629This file should be prepared by the 1630system administrator to contain the public host keys of all machines in the 1631organization. 1632It should be world-readable. 1633See 1634.Xr sshd 8 1635for further details of the format of this file. 1636.Pp 1637.It Pa /etc/ssh/sshrc 1638Commands in this file are executed by 1639.Nm 1640when the user logs in, just before the user's shell (or command) is started. 1641See the 1642.Xr sshd 8 1643manual page for more information. 1644.El 1645.Sh EXIT STATUS 1646.Nm 1647exits with the exit status of the remote command or with 255 1648if an error occurred. 1649.Sh SEE ALSO 1650.Xr scp 1 , 1651.Xr sftp 1 , 1652.Xr ssh-add 1 , 1653.Xr ssh-agent 1 , 1654.Xr ssh-keygen 1 , 1655.Xr ssh-keyscan 1 , 1656.Xr tun 4 , 1657.Xr ssh_config 5 , 1658.Xr ssh-keysign 8 , 1659.Xr sshd 8 1660.Sh STANDARDS 1661.Rs 1662.%A S. Lehtinen 1663.%A C. Lonvick 1664.%D January 2006 1665.%R RFC 4250 1666.%T The Secure Shell (SSH) Protocol Assigned Numbers 1667.Re 1668.Pp 1669.Rs 1670.%A T. Ylonen 1671.%A C. Lonvick 1672.%D January 2006 1673.%R RFC 4251 1674.%T The Secure Shell (SSH) Protocol Architecture 1675.Re 1676.Pp 1677.Rs 1678.%A T. Ylonen 1679.%A C. Lonvick 1680.%D January 2006 1681.%R RFC 4252 1682.%T The Secure Shell (SSH) Authentication Protocol 1683.Re 1684.Pp 1685.Rs 1686.%A T. Ylonen 1687.%A C. Lonvick 1688.%D January 2006 1689.%R RFC 4253 1690.%T The Secure Shell (SSH) Transport Layer Protocol 1691.Re 1692.Pp 1693.Rs 1694.%A T. Ylonen 1695.%A C. Lonvick 1696.%D January 2006 1697.%R RFC 4254 1698.%T The Secure Shell (SSH) Connection Protocol 1699.Re 1700.Pp 1701.Rs 1702.%A J. Schlyter 1703.%A W. Griffin 1704.%D January 2006 1705.%R RFC 4255 1706.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1707.Re 1708.Pp 1709.Rs 1710.%A F. Cusack 1711.%A M. Forssen 1712.%D January 2006 1713.%R RFC 4256 1714.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1715.Re 1716.Pp 1717.Rs 1718.%A J. Galbraith 1719.%A P. Remaker 1720.%D January 2006 1721.%R RFC 4335 1722.%T The Secure Shell (SSH) Session Channel Break Extension 1723.Re 1724.Pp 1725.Rs 1726.%A M. Bellare 1727.%A T. Kohno 1728.%A C. Namprempre 1729.%D January 2006 1730.%R RFC 4344 1731.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1732.Re 1733.Pp 1734.Rs 1735.%A B. Harris 1736.%D January 2006 1737.%R RFC 4345 1738.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1739.Re 1740.Pp 1741.Rs 1742.%A M. Friedl 1743.%A N. Provos 1744.%A W. Simpson 1745.%D March 2006 1746.%R RFC 4419 1747.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1748.Re 1749.Pp 1750.Rs 1751.%A J. Galbraith 1752.%A R. Thayer 1753.%D November 2006 1754.%R RFC 4716 1755.%T The Secure Shell (SSH) Public Key File Format 1756.Re 1757.Pp 1758.Rs 1759.%A D. Stebila 1760.%A J. Green 1761.%D December 2009 1762.%R RFC 5656 1763.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1764.Re 1765.Pp 1766.Rs 1767.%A A. Perrig 1768.%A D. Song 1769.%D 1999 1770.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1771.%T Hash Visualization: a New Technique to improve Real-World Security 1772.Re 1773.Sh AUTHORS 1774OpenSSH is a derivative of the original and free 1775ssh 1.2.12 release by Tatu Ylonen. 1776Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1777Theo de Raadt and Dug Song 1778removed many bugs, re-added newer features and 1779created OpenSSH. 1780Markus Friedl contributed the support for SSH 1781protocol versions 1.5 and 2.0. 1782