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 1.427 2021/09/10 10:26:02 dtucker Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: September 10 2021 $ 39.Dt SSH 1 40.Os 41.Sh NAME 42.Nm ssh 43.Nd OpenSSH remote login client 44.Sh SYNOPSIS 45.Nm ssh 46.Op Fl 46AaCfGgKkMNnqsTtVvXxYy 47.Op Fl B Ar bind_interface 48.Op Fl b Ar bind_address 49.Op Fl c Ar cipher_spec 50.Op Fl D Oo Ar bind_address : Oc Ns Ar port 51.Op Fl E Ar log_file 52.Op Fl e Ar escape_char 53.Op Fl F Ar configfile 54.Op Fl I Ar pkcs11 55.Op Fl i Ar identity_file 56.Op Fl J Ar destination 57.Op Fl L Ar address 58.Op Fl l Ar login_name 59.Op Fl m Ar mac_spec 60.Op Fl O Ar ctl_cmd 61.Op Fl o Ar option 62.Op Fl p Ar port 63.Op Fl Q Ar query_option 64.Op Fl R Ar address 65.Op Fl S Ar ctl_path 66.Op Fl W Ar host : Ns Ar port 67.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun 68.Ar destination 69.Op Ar command Op Ar argument ... 70.Sh DESCRIPTION 71.Nm 72(SSH client) is a program for logging into a remote machine and for 73executing commands on a remote machine. 74It is intended to provide secure encrypted communications between 75two untrusted hosts over an insecure network. 76X11 connections, arbitrary TCP ports and 77.Ux Ns -domain 78sockets can also be forwarded over the secure channel. 79.Pp 80.Nm 81connects and logs into the specified 82.Ar destination , 83which may be specified as either 84.Sm off 85.Oo user @ Oc hostname 86.Sm on 87or a URI of the form 88.Sm off 89.No ssh:// Oo user @ Oc hostname Op : port . 90.Sm on 91The user must prove 92their identity to the remote machine using one of several methods 93(see below). 94.Pp 95If a 96.Ar command 97is specified, 98it will be executed on the remote host instead of a login shell. 99A complete command line may be specified as 100.Ar command , 101or it may have additional arguments. 102If supplied, the arguments will be appended to the command, separated by 103spaces, before it is sent to the server to be executed. 104.Pp 105The options are as follows: 106.Pp 107.Bl -tag -width Ds -compact 108.It Fl 4 109Forces 110.Nm 111to use IPv4 addresses only. 112.Pp 113.It Fl 6 114Forces 115.Nm 116to use IPv6 addresses only. 117.Pp 118.It Fl A 119Enables forwarding of connections from an authentication agent such as 120.Xr ssh-agent 1 . 121This can also be specified on a per-host basis in a configuration file. 122.Pp 123Agent forwarding should be enabled with caution. 124Users with the ability to bypass file permissions on the remote host 125(for the agent's 126.Ux Ns -domain 127socket) can access the local agent through the forwarded connection. 128An attacker cannot obtain key material from the agent, 129however they can perform operations on the keys that enable them to 130authenticate using the identities loaded into the agent. 131A safer alternative may be to use a jump host 132(see 133.Fl J ) . 134.Pp 135.It Fl a 136Disables forwarding of the authentication agent connection. 137.Pp 138.It Fl B Ar bind_interface 139Bind to the address of 140.Ar bind_interface 141before attempting to connect to the destination host. 142This is only useful on systems with more than one address. 143.Pp 144.It Fl b Ar bind_address 145Use 146.Ar bind_address 147on the local machine as the source address 148of the connection. 149Only useful on systems with more than one address. 150.Pp 151.It Fl C 152Requests compression of all data (including stdin, stdout, stderr, and 153data for forwarded X11, TCP and 154.Ux Ns -domain 155connections). 156The compression algorithm is the same used by 157.Xr gzip 1 . 158Compression is desirable on modem lines and other 159slow connections, but will only slow down things on fast networks. 160The default value can be set on a host-by-host basis in the 161configuration files; see the 162.Cm Compression 163option. 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_dsa , 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_rsa . 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 for more information. 439.Pp 440.It Fl N 441Do not execute a remote command. 442This is useful for just forwarding ports. 443Refer to the description of 444.Cm SessionType 445in 446.Xr ssh_config 5 447for details. 448.Pp 449.It Fl n 450Redirects stdin from 451.Pa /dev/null 452(actually, prevents reading from stdin). 453This must be used when 454.Nm 455is run in the background. 456A common trick is to use this to run X11 programs on a remote machine. 457For example, 458.Ic ssh -n shadows.cs.hut.fi emacs & 459will start an emacs on shadows.cs.hut.fi, and the X11 460connection will be automatically forwarded over an encrypted channel. 461The 462.Nm 463program will be put in the background. 464(This does not work if 465.Nm 466needs to ask for a password or passphrase; see also the 467.Fl f 468option.) 469Refer to the description of 470.Cm StdinNull 471in 472.Xr ssh_config 5 473for details. 474.Pp 475.It Fl O Ar ctl_cmd 476Control an active connection multiplexing master process. 477When the 478.Fl O 479option is specified, the 480.Ar ctl_cmd 481argument is interpreted and passed to the master process. 482Valid commands are: 483.Dq check 484(check that the master process is running), 485.Dq forward 486(request forwardings without command execution), 487.Dq cancel 488(cancel forwardings), 489.Dq exit 490(request the master to exit), and 491.Dq stop 492(request the master to stop accepting further multiplexing requests). 493.Pp 494.It Fl o Ar option 495Can be used to give options in the format used in the configuration file. 496This is useful for specifying options for which there is no separate 497command-line flag. 498For full details of the options listed below, and their possible values, see 499.Xr ssh_config 5 . 500.Pp 501.Bl -tag -width Ds -offset indent -compact 502.It AddKeysToAgent 503.It AddressFamily 504.It BatchMode 505.It BindAddress 506.It CanonicalDomains 507.It CanonicalizeFallbackLocal 508.It CanonicalizeHostname 509.It CanonicalizeMaxDots 510.It CanonicalizePermittedCNAMEs 511.It CASignatureAlgorithms 512.It CertificateFile 513.It CheckHostIP 514.It Ciphers 515.It ClearAllForwardings 516.It Compression 517.It ConnectionAttempts 518.It ConnectTimeout 519.It ControlMaster 520.It ControlPath 521.It ControlPersist 522.It DynamicForward 523.It EscapeChar 524.It ExitOnForwardFailure 525.It FingerprintHash 526.It ForkAfterAuthentication 527.It ForwardAgent 528.It ForwardX11 529.It ForwardX11Timeout 530.It ForwardX11Trusted 531.It GatewayPorts 532.It GlobalKnownHostsFile 533.It GSSAPIAuthentication 534.It GSSAPIDelegateCredentials 535.It HashKnownHosts 536.It Host 537.It HostbasedAcceptedAlgorithms 538.It HostbasedAuthentication 539.It HostKeyAlgorithms 540.It HostKeyAlias 541.It Hostname 542.It IdentitiesOnly 543.It IdentityAgent 544.It IdentityFile 545.It IPQoS 546.It KbdInteractiveAuthentication 547.It KbdInteractiveDevices 548.It KexAlgorithms 549.It KnownHostsCommand 550.It LocalCommand 551.It LocalForward 552.It LogLevel 553.It MACs 554.It Match 555.It NoHostAuthenticationForLocalhost 556.It NumberOfPasswordPrompts 557.It PasswordAuthentication 558.It PermitLocalCommand 559.It PermitRemoteOpen 560.It PKCS11Provider 561.It Port 562.It PreferredAuthentications 563.It ProxyCommand 564.It ProxyJump 565.It ProxyUseFdpass 566.It PubkeyAcceptedAlgorithms 567.It PubkeyAuthentication 568.It RekeyLimit 569.It RemoteCommand 570.It RemoteForward 571.It RequestTTY 572.It SendEnv 573.It ServerAliveInterval 574.It ServerAliveCountMax 575.It SessionType 576.It SetEnv 577.It StdinNull 578.It StreamLocalBindMask 579.It StreamLocalBindUnlink 580.It StrictHostKeyChecking 581.It TCPKeepAlive 582.It Tunnel 583.It TunnelDevice 584.It UpdateHostKeys 585.It User 586.It UserKnownHostsFile 587.It VerifyHostKeyDNS 588.It VersionAddendum 589.It VisualHostKey 590.It XAuthLocation 591.El 592.Pp 593.It Fl p Ar port 594Port to connect to on the remote host. 595This can be specified on a 596per-host basis in the configuration file. 597.Pp 598.It Fl Q Ar query_option 599Queries for the algorithms supported by one of the following features: 600.Ar cipher 601(supported symmetric ciphers), 602.Ar cipher-auth 603(supported symmetric ciphers that support authenticated encryption), 604.Ar help 605(supported query terms for use with the 606.Fl Q 607flag), 608.Ar mac 609(supported message integrity codes), 610.Ar kex 611(key exchange algorithms), 612.Ar key 613(key types), 614.Ar key-cert 615(certificate key types), 616.Ar key-plain 617(non-certificate key types), 618.Ar key-sig 619(all key types and signature algorithms), 620.Ar protocol-version 621(supported SSH protocol versions), and 622.Ar sig 623(supported signature algorithms). 624Alternatively, any keyword from 625.Xr ssh_config 5 626or 627.Xr sshd_config 5 628that takes an algorithm list may be used as an alias for the corresponding 629query_option. 630.Pp 631.It Fl q 632Quiet mode. 633Causes most warning and diagnostic messages to be suppressed. 634.Pp 635.It Fl R Xo 636.Sm off 637.Oo Ar bind_address : Oc 638.Ar port : host : hostport 639.Sm on 640.Xc 641.It Fl R Xo 642.Sm off 643.Oo Ar bind_address : Oc 644.Ar port : local_socket 645.Sm on 646.Xc 647.It Fl R Xo 648.Sm off 649.Ar remote_socket : host : hostport 650.Sm on 651.Xc 652.It Fl R Xo 653.Sm off 654.Ar remote_socket : local_socket 655.Sm on 656.Xc 657.It Fl R Xo 658.Sm off 659.Oo Ar bind_address : Oc 660.Ar port 661.Sm on 662.Xc 663Specifies that connections to the given TCP port or Unix socket on the remote 664(server) host are to be forwarded to the local side. 665.Pp 666This works by allocating a socket to listen to either a TCP 667.Ar port 668or to a Unix socket on the remote side. 669Whenever a connection is made to this port or Unix socket, the 670connection is forwarded over the secure channel, and a connection 671is made from the local machine to either an explicit destination specified by 672.Ar host 673port 674.Ar hostport , 675or 676.Ar local_socket , 677or, if no explicit destination was specified, 678.Nm 679will act as a SOCKS 4/5 proxy and forward connections to the destinations 680requested by the remote SOCKS client. 681.Pp 682Port forwardings can also be specified in the configuration file. 683Privileged ports can be forwarded only when 684logging in as root on the remote machine. 685IPv6 addresses can be specified by enclosing the address in square brackets. 686.Pp 687By default, TCP listening sockets on the server will be bound to the loopback 688interface only. 689This may be overridden by specifying a 690.Ar bind_address . 691An empty 692.Ar bind_address , 693or the address 694.Ql * , 695indicates that the remote socket should listen on all interfaces. 696Specifying a remote 697.Ar bind_address 698will only succeed if the server's 699.Cm GatewayPorts 700option is enabled (see 701.Xr sshd_config 5 ) . 702.Pp 703If the 704.Ar port 705argument is 706.Ql 0 , 707the listen port will be dynamically allocated on the server and reported 708to the client at run time. 709When used together with 710.Ic -O forward 711the allocated port will be printed to the standard output. 712.Pp 713.It Fl S Ar ctl_path 714Specifies the location of a control socket for connection sharing, 715or the string 716.Dq none 717to disable connection sharing. 718Refer to the description of 719.Cm ControlPath 720and 721.Cm ControlMaster 722in 723.Xr ssh_config 5 724for details. 725.Pp 726.It Fl s 727May be used to request invocation of a subsystem on the remote system. 728Subsystems facilitate the use of SSH 729as a secure transport for other applications (e.g.\& 730.Xr sftp 1 ) . 731The subsystem is specified as the remote command. 732Refer to the description of 733.Cm SessionType 734in 735.Xr ssh_config 5 736for details. 737.Pp 738.It Fl T 739Disable pseudo-terminal allocation. 740.Pp 741.It Fl t 742Force pseudo-terminal allocation. 743This can be used to execute arbitrary 744screen-based programs on a remote machine, which can be very useful, 745e.g. when implementing menu services. 746Multiple 747.Fl t 748options force tty allocation, even if 749.Nm 750has no local tty. 751.Pp 752.It Fl V 753Display the version number and exit. 754.Pp 755.It Fl v 756Verbose mode. 757Causes 758.Nm 759to print debugging messages about its progress. 760This is helpful in 761debugging connection, authentication, and configuration problems. 762Multiple 763.Fl v 764options increase the verbosity. 765The maximum is 3. 766.Pp 767.It Fl W Ar host : Ns Ar port 768Requests that standard input and output on the client be forwarded to 769.Ar host 770on 771.Ar port 772over the secure channel. 773Implies 774.Fl N , 775.Fl T , 776.Cm ExitOnForwardFailure 777and 778.Cm ClearAllForwardings , 779though these can be overridden in the configuration file or using 780.Fl o 781command line options. 782.Pp 783.It Fl w Xo 784.Ar local_tun Ns Op : Ns Ar remote_tun 785.Xc 786Requests 787tunnel 788device forwarding with the specified 789.Xr tun 4 790devices between the client 791.Pq Ar local_tun 792and the server 793.Pq Ar remote_tun . 794.Pp 795The devices may be specified by numerical ID or the keyword 796.Dq any , 797which uses the next available tunnel device. 798If 799.Ar remote_tun 800is not specified, it defaults to 801.Dq any . 802See also the 803.Cm Tunnel 804and 805.Cm TunnelDevice 806directives in 807.Xr ssh_config 5 . 808.Pp 809If the 810.Cm Tunnel 811directive is unset, it will be set to the default tunnel mode, which is 812.Dq point-to-point . 813If a different 814.Cm Tunnel 815forwarding mode it desired, then it should be specified before 816.Fl w . 817.Pp 818.It Fl X 819Enables X11 forwarding. 820This can also be specified on a per-host basis in a configuration file. 821.Pp 822X11 forwarding should be enabled with caution. 823Users with the ability to bypass file permissions on the remote host 824(for the user's X authorization database) 825can access the local X11 display through the forwarded connection. 826An attacker may then be able to perform activities such as keystroke monitoring. 827.Pp 828For this reason, X11 forwarding is subjected to X11 SECURITY extension 829restrictions by default. 830Please refer to the 831.Nm 832.Fl Y 833option and the 834.Cm ForwardX11Trusted 835directive in 836.Xr ssh_config 5 837for more information. 838.Pp 839.It Fl x 840Disables X11 forwarding. 841.Pp 842.It Fl Y 843Enables trusted X11 forwarding. 844Trusted X11 forwardings are not subjected to the X11 SECURITY extension 845controls. 846.Pp 847.It Fl y 848Send log information using the 849.Xr syslog 3 850system module. 851By default this information is sent to stderr. 852.El 853.Pp 854.Nm 855may additionally obtain configuration data from 856a per-user configuration file and a system-wide configuration file. 857The file format and configuration options are described in 858.Xr ssh_config 5 . 859.Sh AUTHENTICATION 860The OpenSSH SSH client supports SSH protocol 2. 861.Pp 862The methods available for authentication are: 863GSSAPI-based authentication, 864host-based authentication, 865public key authentication, 866keyboard-interactive authentication, 867and password authentication. 868Authentication methods are tried in the order specified above, 869though 870.Cm PreferredAuthentications 871can be used to change the default order. 872.Pp 873Host-based authentication works as follows: 874If the machine the user logs in from is listed in 875.Pa /etc/hosts.equiv 876or 877.Pa /etc/shosts.equiv 878on the remote machine, the user is non-root and the user names are 879the same on both sides, or if the files 880.Pa ~/.rhosts 881or 882.Pa ~/.shosts 883exist in the user's home directory on the 884remote machine and contain a line containing the name of the client 885machine and the name of the user on that machine, the user is 886considered for login. 887Additionally, the server 888.Em must 889be able to verify the client's 890host key (see the description of 891.Pa /etc/ssh/ssh_known_hosts 892and 893.Pa ~/.ssh/known_hosts , 894below) 895for login to be permitted. 896This authentication method closes security holes due to IP 897spoofing, DNS spoofing, and routing spoofing. 898[Note to the administrator: 899.Pa /etc/hosts.equiv , 900.Pa ~/.rhosts , 901and the rlogin/rsh protocol in general, are inherently insecure and should be 902disabled if security is desired.] 903.Pp 904Public key authentication works as follows: 905The scheme is based on public-key cryptography, 906using cryptosystems 907where encryption and decryption are done using separate keys, 908and it is unfeasible to derive the decryption key from the encryption key. 909The idea is that each user creates a public/private 910key pair for authentication purposes. 911The server knows the public key, and only the user knows the private key. 912.Nm 913implements public key authentication protocol automatically, 914using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 915The HISTORY section of 916.Xr ssl 8 917contains a brief discussion of the DSA and RSA algorithms. 918.Pp 919The file 920.Pa ~/.ssh/authorized_keys 921lists the public keys that are permitted for logging in. 922When the user logs in, the 923.Nm 924program tells the server which key pair it would like to use for 925authentication. 926The client proves that it has access to the private key 927and the server checks that the corresponding public key 928is authorized to accept the account. 929.Pp 930The server may inform the client of errors that prevented public key 931authentication from succeeding after authentication completes using a 932different method. 933These may be viewed by increasing the 934.Cm LogLevel 935to 936.Cm DEBUG 937or higher (e.g. by using the 938.Fl v 939flag). 940.Pp 941The user creates their key pair by running 942.Xr ssh-keygen 1 . 943This stores the private key in 944.Pa ~/.ssh/id_dsa 945(DSA), 946.Pa ~/.ssh/id_ecdsa 947(ECDSA), 948.Pa ~/.ssh/id_ecdsa_sk 949(authenticator-hosted ECDSA), 950.Pa ~/.ssh/id_ed25519 951(Ed25519), 952.Pa ~/.ssh/id_ed25519_sk 953(authenticator-hosted Ed25519), 954or 955.Pa ~/.ssh/id_rsa 956(RSA) 957and stores the public key in 958.Pa ~/.ssh/id_dsa.pub 959(DSA), 960.Pa ~/.ssh/id_ecdsa.pub 961(ECDSA), 962.Pa ~/.ssh/id_ecdsa_sk.pub 963(authenticator-hosted ECDSA), 964.Pa ~/.ssh/id_ed25519.pub 965(Ed25519), 966.Pa ~/.ssh/id_ed25519_sk.pub 967(authenticator-hosted Ed25519), 968or 969.Pa ~/.ssh/id_rsa.pub 970(RSA) 971in the user's home directory. 972The user should then copy the public key 973to 974.Pa ~/.ssh/authorized_keys 975in their home directory on the remote machine. 976The 977.Pa authorized_keys 978file corresponds to the conventional 979.Pa ~/.rhosts 980file, and has one key 981per line, though the lines can be very long. 982After this, the user can log in without giving the password. 983.Pp 984A variation on public key authentication 985is available in the form of certificate authentication: 986instead of a set of public/private keys, 987signed certificates are used. 988This has the advantage that a single trusted certification authority 989can be used in place of many public/private keys. 990See the CERTIFICATES section of 991.Xr ssh-keygen 1 992for more information. 993.Pp 994The most convenient way to use public key or certificate authentication 995may be with an authentication agent. 996See 997.Xr ssh-agent 1 998and (optionally) the 999.Cm AddKeysToAgent 1000directive in 1001.Xr ssh_config 5 1002for more information. 1003.Pp 1004Keyboard-interactive authentication works as follows: 1005The server sends an arbitrary 1006.Qq challenge 1007text and prompts for a response, possibly multiple times. 1008Examples of keyboard-interactive authentication include 1009.Bx 1010Authentication (see 1011.Xr login.conf 5 ) 1012and PAM (some 1013.Pf non- Ox 1014systems). 1015.Pp 1016Finally, if other authentication methods fail, 1017.Nm 1018prompts the user for a password. 1019The password is sent to the remote 1020host for checking; however, since all communications are encrypted, 1021the password cannot be seen by someone listening on the network. 1022.Pp 1023.Nm 1024automatically maintains and checks a database containing 1025identification for all hosts it has ever been used with. 1026Host keys are stored in 1027.Pa ~/.ssh/known_hosts 1028in the user's home directory. 1029Additionally, the file 1030.Pa /etc/ssh/ssh_known_hosts 1031is automatically checked for known hosts. 1032Any new hosts are automatically added to the user's file. 1033If a host's identification ever changes, 1034.Nm 1035warns about this and disables password authentication to prevent 1036server spoofing or man-in-the-middle attacks, 1037which could otherwise be used to circumvent the encryption. 1038The 1039.Cm StrictHostKeyChecking 1040option can be used to control logins to machines whose 1041host key is not known or has changed. 1042.Pp 1043When the user's identity has been accepted by the server, the server 1044either executes the given command in a non-interactive session or, 1045if no command has been specified, logs into the machine and gives 1046the user a normal shell as an interactive session. 1047All communication with 1048the remote command or shell will be automatically encrypted. 1049.Pp 1050If an interactive session is requested 1051.Nm 1052by default will only request a pseudo-terminal (pty) for interactive 1053sessions when the client has one. 1054The flags 1055.Fl T 1056and 1057.Fl t 1058can be used to override this behaviour. 1059.Pp 1060If a pseudo-terminal has been allocated the 1061user may use the escape characters noted below. 1062.Pp 1063If no pseudo-terminal has been allocated, 1064the session is transparent and can be used to reliably transfer binary data. 1065On most systems, setting the escape character to 1066.Dq none 1067will also make the session transparent even if a tty is used. 1068.Pp 1069The session terminates when the command or shell on the remote 1070machine exits and all X11 and TCP connections have been closed. 1071.Sh ESCAPE CHARACTERS 1072When a pseudo-terminal has been requested, 1073.Nm 1074supports a number of functions through the use of an escape character. 1075.Pp 1076A single tilde character can be sent as 1077.Ic ~~ 1078or by following the tilde by a character other than those described below. 1079The escape character must always follow a newline to be interpreted as 1080special. 1081The escape character can be changed in configuration files using the 1082.Cm EscapeChar 1083configuration directive or on the command line by the 1084.Fl e 1085option. 1086.Pp 1087The supported escapes (assuming the default 1088.Ql ~ ) 1089are: 1090.Bl -tag -width Ds 1091.It Cm ~. 1092Disconnect. 1093.It Cm ~^Z 1094Background 1095.Nm . 1096.It Cm ~# 1097List forwarded connections. 1098.It Cm ~& 1099Background 1100.Nm 1101at logout when waiting for forwarded connection / X11 sessions to terminate. 1102.It Cm ~? 1103Display a list of escape characters. 1104.It Cm ~B 1105Send a BREAK to the remote system 1106(only useful if the peer supports it). 1107.It Cm ~C 1108Open command line. 1109Currently this allows the addition of port forwardings using the 1110.Fl L , 1111.Fl R 1112and 1113.Fl D 1114options (see above). 1115It also allows the cancellation of existing port-forwardings 1116with 1117.Sm off 1118.Fl KL Oo Ar bind_address : Oc Ar port 1119.Sm on 1120for local, 1121.Sm off 1122.Fl KR Oo Ar bind_address : Oc Ar port 1123.Sm on 1124for remote and 1125.Sm off 1126.Fl KD Oo Ar bind_address : Oc Ar port 1127.Sm on 1128for dynamic port-forwardings. 1129.Ic !\& Ns Ar command 1130allows the user to execute a local command if the 1131.Ic PermitLocalCommand 1132option is enabled in 1133.Xr ssh_config 5 . 1134Basic help is available, using the 1135.Fl h 1136option. 1137.It Cm ~R 1138Request rekeying of the connection 1139(only useful if the peer supports it). 1140.It Cm ~V 1141Decrease the verbosity 1142.Pq Ic LogLevel 1143when errors are being written to stderr. 1144.It Cm ~v 1145Increase the verbosity 1146.Pq Ic LogLevel 1147when errors are being written to stderr. 1148.El 1149.Sh TCP FORWARDING 1150Forwarding of arbitrary TCP connections over a secure channel 1151can be specified either on the command line or in a configuration file. 1152One possible application of TCP forwarding is a secure connection to a 1153mail server; another is going through firewalls. 1154.Pp 1155In the example below, we look at encrypting communication for an IRC client, 1156even though the IRC server it connects to does not directly 1157support encrypted communication. 1158This works as follows: 1159the user connects to the remote host using 1160.Nm , 1161specifying the ports to be used to forward the connection. 1162After that it is possible to start the program locally, 1163and 1164.Nm 1165will encrypt and forward the connection to the remote server. 1166.Pp 1167The following example tunnels an IRC session from the client 1168to an IRC server at 1169.Dq server.example.com , 1170joining channel 1171.Dq #users , 1172nickname 1173.Dq pinky , 1174using the standard IRC port, 6667: 1175.Bd -literal -offset 4n 1176$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1177$ irc -c '#users' pinky IRC/127.0.0.1 1178.Ed 1179.Pp 1180The 1181.Fl f 1182option backgrounds 1183.Nm 1184and the remote command 1185.Dq sleep 10 1186is specified to allow an amount of time 1187(10 seconds, in the example) 1188to start the program which is going to use the tunnel. 1189If no connections are made within the time specified, 1190.Nm 1191will exit. 1192.Sh X11 FORWARDING 1193If the 1194.Cm ForwardX11 1195variable is set to 1196.Dq yes 1197(or see the description of the 1198.Fl X , 1199.Fl x , 1200and 1201.Fl Y 1202options above) 1203and the user is using X11 (the 1204.Ev DISPLAY 1205environment variable is set), the connection to the X11 display is 1206automatically forwarded to the remote side in such a way that any X11 1207programs started from the shell (or command) will go through the 1208encrypted channel, and the connection to the real X server will be made 1209from the local machine. 1210The user should not manually set 1211.Ev DISPLAY . 1212Forwarding of X11 connections can be 1213configured on the command line or in configuration files. 1214.Pp 1215The 1216.Ev DISPLAY 1217value set by 1218.Nm 1219will point to the server machine, but with a display number greater than zero. 1220This is normal, and happens because 1221.Nm 1222creates a 1223.Dq proxy 1224X server on the server machine for forwarding the 1225connections over the encrypted channel. 1226.Pp 1227.Nm 1228will also automatically set up Xauthority data on the server machine. 1229For this purpose, it will generate a random authorization cookie, 1230store it in Xauthority on the server, and verify that any forwarded 1231connections carry this cookie and replace it by the real cookie when 1232the connection is opened. 1233The real authentication cookie is never 1234sent to the server machine (and no cookies are sent in the plain). 1235.Pp 1236If the 1237.Cm ForwardAgent 1238variable is set to 1239.Dq yes 1240(or see the description of the 1241.Fl A 1242and 1243.Fl a 1244options above) and 1245the user is using an authentication agent, the connection to the agent 1246is automatically forwarded to the remote side. 1247.Sh VERIFYING HOST KEYS 1248When connecting to a server for the first time, 1249a fingerprint of the server's public key is presented to the user 1250(unless the option 1251.Cm StrictHostKeyChecking 1252has been disabled). 1253Fingerprints can be determined using 1254.Xr ssh-keygen 1 : 1255.Pp 1256.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1257.Pp 1258If the fingerprint is already known, it can be matched 1259and the key can be accepted or rejected. 1260If only legacy (MD5) fingerprints for the server are available, the 1261.Xr ssh-keygen 1 1262.Fl E 1263option may be used to downgrade the fingerprint algorithm to match. 1264.Pp 1265Because of the difficulty of comparing host keys 1266just by looking at fingerprint strings, 1267there is also support to compare host keys visually, 1268using 1269.Em random art . 1270By setting the 1271.Cm VisualHostKey 1272option to 1273.Dq yes , 1274a small ASCII graphic gets displayed on every login to a server, no matter 1275if the session itself is interactive or not. 1276By learning the pattern a known server produces, a user can easily 1277find out that the host key has changed when a completely different pattern 1278is displayed. 1279Because these patterns are not unambiguous however, a pattern that looks 1280similar to the pattern remembered only gives a good probability that the 1281host key is the same, not guaranteed proof. 1282.Pp 1283To get a listing of the fingerprints along with their random art for 1284all known hosts, the following command line can be used: 1285.Pp 1286.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1287.Pp 1288If the fingerprint is unknown, 1289an alternative method of verification is available: 1290SSH fingerprints verified by DNS. 1291An additional resource record (RR), 1292SSHFP, 1293is added to a zonefile 1294and the connecting client is able to match the fingerprint 1295with that of the key presented. 1296.Pp 1297In this example, we are connecting a client to a server, 1298.Dq host.example.com . 1299The SSHFP resource records should first be added to the zonefile for 1300host.example.com: 1301.Bd -literal -offset indent 1302$ ssh-keygen -r host.example.com. 1303.Ed 1304.Pp 1305The output lines will have to be added to the zonefile. 1306To check that the zone is answering fingerprint queries: 1307.Pp 1308.Dl $ dig -t SSHFP host.example.com 1309.Pp 1310Finally the client connects: 1311.Bd -literal -offset indent 1312$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1313[...] 1314Matching host key fingerprint found in DNS. 1315Are you sure you want to continue connecting (yes/no)? 1316.Ed 1317.Pp 1318See the 1319.Cm VerifyHostKeyDNS 1320option in 1321.Xr ssh_config 5 1322for more information. 1323.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1324.Nm 1325contains support for Virtual Private Network (VPN) tunnelling 1326using the 1327.Xr tun 4 1328network pseudo-device, 1329allowing two networks to be joined securely. 1330The 1331.Xr sshd_config 5 1332configuration option 1333.Cm PermitTunnel 1334controls whether the server supports this, 1335and at what level (layer 2 or 3 traffic). 1336.Pp 1337The following example would connect client network 10.0.50.0/24 1338with remote network 10.0.99.0/24 using a point-to-point connection 1339from 10.1.1.1 to 10.1.1.2, 1340provided that the SSH server running on the gateway to the remote network, 1341at 192.168.1.15, allows it. 1342.Pp 1343On the client: 1344.Bd -literal -offset indent 1345# ssh -f -w 0:1 192.168.1.15 true 1346# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1347# route add 10.0.99.0/24 10.1.1.2 1348.Ed 1349.Pp 1350On the server: 1351.Bd -literal -offset indent 1352# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1353# route add 10.0.50.0/24 10.1.1.1 1354.Ed 1355.Pp 1356Client access may be more finely tuned via the 1357.Pa /root/.ssh/authorized_keys 1358file (see below) and the 1359.Cm PermitRootLogin 1360server option. 1361The following entry would permit connections on 1362.Xr tun 4 1363device 1 from user 1364.Dq jane 1365and on tun device 2 from user 1366.Dq john , 1367if 1368.Cm PermitRootLogin 1369is set to 1370.Dq forced-commands-only : 1371.Bd -literal -offset 2n 1372tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1373tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1374.Ed 1375.Pp 1376Since an SSH-based setup entails a fair amount of overhead, 1377it may be more suited to temporary setups, 1378such as for wireless VPNs. 1379More permanent VPNs are better provided by tools such as 1380.Xr ipsecctl 8 1381and 1382.Xr isakmpd 8 . 1383.Sh ENVIRONMENT 1384.Nm 1385will normally set the following environment variables: 1386.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1387.It Ev DISPLAY 1388The 1389.Ev DISPLAY 1390variable indicates the location of the X11 server. 1391It is automatically set by 1392.Nm 1393to point to a value of the form 1394.Dq hostname:n , 1395where 1396.Dq hostname 1397indicates the host where the shell runs, and 1398.Sq n 1399is an integer \*(Ge 1. 1400.Nm 1401uses this special value to forward X11 connections over the secure 1402channel. 1403The user should normally not set 1404.Ev DISPLAY 1405explicitly, as that 1406will render the X11 connection insecure (and will require the user to 1407manually copy any required authorization cookies). 1408.It Ev HOME 1409Set to the path of the user's home directory. 1410.It Ev LOGNAME 1411Synonym for 1412.Ev USER ; 1413set for compatibility with systems that use this variable. 1414.It Ev MAIL 1415Set to the path of the user's mailbox. 1416.It Ev PATH 1417Set to the default 1418.Ev PATH , 1419as specified when compiling 1420.Nm . 1421.It Ev SSH_ASKPASS 1422If 1423.Nm 1424needs a passphrase, it will read the passphrase from the current 1425terminal if it was run from a terminal. 1426If 1427.Nm 1428does not have a terminal associated with it but 1429.Ev DISPLAY 1430and 1431.Ev SSH_ASKPASS 1432are set, it will execute the program specified by 1433.Ev SSH_ASKPASS 1434and open an X11 window to read the passphrase. 1435This is particularly useful when calling 1436.Nm 1437from a 1438.Pa .xsession 1439or related script. 1440(Note that on some machines it 1441may be necessary to redirect the input from 1442.Pa /dev/null 1443to make this work.) 1444.It Ev SSH_ASKPASS_REQUIRE 1445Allows further control over the use of an askpass program. 1446If this variable is set to 1447.Dq never 1448then 1449.Nm 1450will never attempt to use one. 1451If it is set to 1452.Dq prefer , 1453then 1454.Nm 1455will prefer to use the askpass program instead of the TTY when requesting 1456passwords. 1457Finally, if the variable is set to 1458.Dq force , 1459then the askpass program will be used for all passphrase input regardless 1460of whether 1461.Ev DISPLAY 1462is set. 1463.It Ev SSH_AUTH_SOCK 1464Identifies the path of a 1465.Ux Ns -domain 1466socket used to communicate with the agent. 1467.It Ev SSH_CONNECTION 1468Identifies the client and server ends of the connection. 1469The variable contains 1470four space-separated values: client IP address, client port number, 1471server IP address, and server port number. 1472.It Ev SSH_ORIGINAL_COMMAND 1473This variable contains the original command line if a forced command 1474is executed. 1475It can be used to extract the original arguments. 1476.It Ev SSH_TTY 1477This is set to the name of the tty (path to the device) associated 1478with the current shell or command. 1479If the current session has no tty, 1480this variable is not set. 1481.It Ev SSH_TUNNEL 1482Optionally set by 1483.Xr sshd 8 1484to contain the interface names assigned if tunnel forwarding was 1485requested by the client. 1486.It Ev SSH_USER_AUTH 1487Optionally set by 1488.Xr sshd 8 , 1489this variable may contain a pathname to a file that lists the authentication 1490methods successfully used when the session was established, including any 1491public keys that were used. 1492.It Ev TZ 1493This variable is set to indicate the present time zone if it 1494was set when the daemon was started (i.e. the daemon passes the value 1495on to new connections). 1496.It Ev USER 1497Set to the name of the user logging in. 1498.El 1499.Pp 1500Additionally, 1501.Nm 1502reads 1503.Pa ~/.ssh/environment , 1504and adds lines of the format 1505.Dq VARNAME=value 1506to the environment if the file exists and users are allowed to 1507change their environment. 1508For more information, see the 1509.Cm PermitUserEnvironment 1510option in 1511.Xr sshd_config 5 . 1512.Sh FILES 1513.Bl -tag -width Ds -compact 1514.It Pa ~/.rhosts 1515This file is used for host-based authentication (see above). 1516On some machines this file may need to be 1517world-readable if the user's home directory is on an NFS partition, 1518because 1519.Xr sshd 8 1520reads it as root. 1521Additionally, this file must be owned by the user, 1522and must not have write permissions for anyone else. 1523The recommended 1524permission for most machines is read/write for the user, and not 1525accessible by others. 1526.Pp 1527.It Pa ~/.shosts 1528This file is used in exactly the same way as 1529.Pa .rhosts , 1530but allows host-based authentication without permitting login with 1531rlogin/rsh. 1532.Pp 1533.It Pa ~/.ssh/ 1534This directory is the default location for all user-specific configuration 1535and authentication information. 1536There is no general requirement to keep the entire contents of this directory 1537secret, but the recommended permissions are read/write/execute for the user, 1538and not accessible by others. 1539.Pp 1540.It Pa ~/.ssh/authorized_keys 1541Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1542that can be used for logging in as this user. 1543The format of this file is described in the 1544.Xr sshd 8 1545manual page. 1546This file is not highly sensitive, but the recommended 1547permissions are read/write for the user, and not accessible by others. 1548.Pp 1549.It Pa ~/.ssh/config 1550This is the per-user configuration file. 1551The file format and configuration options are described in 1552.Xr ssh_config 5 . 1553Because of the potential for abuse, this file must have strict permissions: 1554read/write for the user, and not writable by others. 1555.Pp 1556.It Pa ~/.ssh/environment 1557Contains additional definitions for environment variables; see 1558.Sx ENVIRONMENT , 1559above. 1560.Pp 1561.It Pa ~/.ssh/id_dsa 1562.It Pa ~/.ssh/id_ecdsa 1563.It Pa ~/.ssh/id_ecdsa_sk 1564.It Pa ~/.ssh/id_ed25519 1565.It Pa ~/.ssh/id_ed25519_sk 1566.It Pa ~/.ssh/id_rsa 1567Contains the private key for authentication. 1568These files 1569contain sensitive data and should be readable by the user but not 1570accessible by others (read/write/execute). 1571.Nm 1572will simply ignore a private key file if it is accessible by others. 1573It is possible to specify a passphrase when 1574generating the key which will be used to encrypt the 1575sensitive part of this file using AES-128. 1576.Pp 1577.It Pa ~/.ssh/id_dsa.pub 1578.It Pa ~/.ssh/id_ecdsa.pub 1579.It Pa ~/.ssh/id_ecdsa_sk.pub 1580.It Pa ~/.ssh/id_ed25519.pub 1581.It Pa ~/.ssh/id_ed25519_sk.pub 1582.It Pa ~/.ssh/id_rsa.pub 1583Contains the public key for authentication. 1584These files are not 1585sensitive and can (but need not) be readable by anyone. 1586.Pp 1587.It Pa ~/.ssh/known_hosts 1588Contains a list of host keys for all hosts the user has logged into 1589that are not already in the systemwide list of known host keys. 1590See 1591.Xr sshd 8 1592for further details of the format of this file. 1593.Pp 1594.It Pa ~/.ssh/rc 1595Commands in this file are executed by 1596.Nm 1597when the user logs in, just before the user's shell (or command) is 1598started. 1599See the 1600.Xr sshd 8 1601manual page for more information. 1602.Pp 1603.It Pa /etc/hosts.equiv 1604This file is for host-based authentication (see above). 1605It should only be writable by root. 1606.Pp 1607.It Pa /etc/shosts.equiv 1608This file is used in exactly the same way as 1609.Pa hosts.equiv , 1610but allows host-based authentication without permitting login with 1611rlogin/rsh. 1612.Pp 1613.It Pa /etc/ssh/ssh_config 1614Systemwide configuration file. 1615The file format and configuration options are described in 1616.Xr ssh_config 5 . 1617.Pp 1618.It Pa /etc/ssh/ssh_host_key 1619.It Pa /etc/ssh/ssh_host_dsa_key 1620.It Pa /etc/ssh/ssh_host_ecdsa_key 1621.It Pa /etc/ssh/ssh_host_ed25519_key 1622.It Pa /etc/ssh/ssh_host_rsa_key 1623These files contain the private parts of the host keys 1624and are used for host-based authentication. 1625.Pp 1626.It Pa /etc/ssh/ssh_known_hosts 1627Systemwide list of known host keys. 1628This file should be prepared by the 1629system administrator to contain the public host keys of all machines in the 1630organization. 1631It should be world-readable. 1632See 1633.Xr sshd 8 1634for further details of the format of this file. 1635.Pp 1636.It Pa /etc/ssh/sshrc 1637Commands in this file are executed by 1638.Nm 1639when the user logs in, just before the user's shell (or command) is started. 1640See the 1641.Xr sshd 8 1642manual page for more information. 1643.El 1644.Sh EXIT STATUS 1645.Nm 1646exits with the exit status of the remote command or with 255 1647if an error occurred. 1648.Sh SEE ALSO 1649.Xr scp 1 , 1650.Xr sftp 1 , 1651.Xr ssh-add 1 , 1652.Xr ssh-agent 1 , 1653.Xr ssh-keygen 1 , 1654.Xr ssh-keyscan 1 , 1655.Xr tun 4 , 1656.Xr ssh_config 5 , 1657.Xr ssh-keysign 8 , 1658.Xr sshd 8 1659.Sh STANDARDS 1660.Rs 1661.%A S. Lehtinen 1662.%A C. Lonvick 1663.%D January 2006 1664.%R RFC 4250 1665.%T The Secure Shell (SSH) Protocol Assigned Numbers 1666.Re 1667.Pp 1668.Rs 1669.%A T. Ylonen 1670.%A C. Lonvick 1671.%D January 2006 1672.%R RFC 4251 1673.%T The Secure Shell (SSH) Protocol Architecture 1674.Re 1675.Pp 1676.Rs 1677.%A T. Ylonen 1678.%A C. Lonvick 1679.%D January 2006 1680.%R RFC 4252 1681.%T The Secure Shell (SSH) Authentication Protocol 1682.Re 1683.Pp 1684.Rs 1685.%A T. Ylonen 1686.%A C. Lonvick 1687.%D January 2006 1688.%R RFC 4253 1689.%T The Secure Shell (SSH) Transport Layer Protocol 1690.Re 1691.Pp 1692.Rs 1693.%A T. Ylonen 1694.%A C. Lonvick 1695.%D January 2006 1696.%R RFC 4254 1697.%T The Secure Shell (SSH) Connection Protocol 1698.Re 1699.Pp 1700.Rs 1701.%A J. Schlyter 1702.%A W. Griffin 1703.%D January 2006 1704.%R RFC 4255 1705.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1706.Re 1707.Pp 1708.Rs 1709.%A F. Cusack 1710.%A M. Forssen 1711.%D January 2006 1712.%R RFC 4256 1713.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1714.Re 1715.Pp 1716.Rs 1717.%A J. Galbraith 1718.%A P. Remaker 1719.%D January 2006 1720.%R RFC 4335 1721.%T The Secure Shell (SSH) Session Channel Break Extension 1722.Re 1723.Pp 1724.Rs 1725.%A M. Bellare 1726.%A T. Kohno 1727.%A C. Namprempre 1728.%D January 2006 1729.%R RFC 4344 1730.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1731.Re 1732.Pp 1733.Rs 1734.%A B. Harris 1735.%D January 2006 1736.%R RFC 4345 1737.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1738.Re 1739.Pp 1740.Rs 1741.%A M. Friedl 1742.%A N. Provos 1743.%A W. Simpson 1744.%D March 2006 1745.%R RFC 4419 1746.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1747.Re 1748.Pp 1749.Rs 1750.%A J. Galbraith 1751.%A R. Thayer 1752.%D November 2006 1753.%R RFC 4716 1754.%T The Secure Shell (SSH) Public Key File Format 1755.Re 1756.Pp 1757.Rs 1758.%A D. Stebila 1759.%A J. Green 1760.%D December 2009 1761.%R RFC 5656 1762.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1763.Re 1764.Pp 1765.Rs 1766.%A A. Perrig 1767.%A D. Song 1768.%D 1999 1769.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1770.%T Hash Visualization: a New Technique to improve Real-World Security 1771.Re 1772.Sh AUTHORS 1773OpenSSH is a derivative of the original and free 1774ssh 1.2.12 release by Tatu Ylonen. 1775Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1776Theo de Raadt and Dug Song 1777removed many bugs, re-added newer features and 1778created OpenSSH. 1779Markus Friedl contributed the support for SSH 1780protocol versions 1.5 and 2.0. 1781