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 1.442 2024/06/27 21:02:16 jmc Exp $ 37.Dd $Mdocdate: June 27 2024 $ 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 tag 62.Op Fl p Ar port 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.Nm 70.Op Fl Q Ar query_option 71.Sh DESCRIPTION 72.Nm 73(SSH client) is a program for logging into a remote machine and for 74executing commands on a remote machine. 75It is intended to provide secure encrypted communications between 76two untrusted hosts over an insecure network. 77X11 connections, arbitrary TCP ports and 78.Ux Ns -domain 79sockets can also be forwarded over the secure channel. 80.Pp 81.Nm 82connects and logs into the specified 83.Ar destination , 84which may be specified as either 85.Sm off 86.Oo user @ Oc hostname 87.Sm on 88or a URI of the form 89.Sm off 90.No ssh:// Oo user @ Oc hostname Op : port . 91.Sm on 92The user must prove 93their identity to the remote machine using one of several methods 94(see below). 95.Pp 96If a 97.Ar command 98is specified, 99it will be executed on the remote host instead of a login shell. 100A complete command line may be specified as 101.Ar command , 102or it may have additional arguments. 103If supplied, the arguments will be appended to the command, separated by 104spaces, before it is sent to the server to be executed. 105.Pp 106The options are as follows: 107.Pp 108.Bl -tag -width Ds -compact 109.It Fl 4 110Forces 111.Nm 112to use IPv4 addresses only. 113.Pp 114.It Fl 6 115Forces 116.Nm 117to use IPv6 addresses only. 118.Pp 119.It Fl A 120Enables forwarding of connections from an authentication agent such as 121.Xr ssh-agent 1 . 122This can also be specified on a per-host basis in a configuration file. 123.Pp 124Agent forwarding should be enabled with caution. 125Users with the ability to bypass file permissions on the remote host 126(for the agent's 127.Ux Ns -domain 128socket) can access the local agent through the forwarded connection. 129An attacker cannot obtain key material from the agent, 130however they can perform operations on the keys that enable them to 131authenticate using the identities loaded into the agent. 132A safer alternative may be to use a jump host 133(see 134.Fl J ) . 135.Pp 136.It Fl a 137Disables forwarding of the authentication agent connection. 138.Pp 139.It Fl B Ar bind_interface 140Bind to the address of 141.Ar bind_interface 142before attempting to connect to the destination host. 143This is only useful on systems with more than one address. 144.Pp 145.It Fl b Ar bind_address 146Use 147.Ar bind_address 148on the local machine as the source address 149of the connection. 150Only useful on systems with more than one address. 151.Pp 152.It Fl C 153Requests compression of all data (including stdin, stdout, stderr, and 154data for forwarded X11, TCP and 155.Ux Ns -domain 156connections). 157The compression algorithm is the same used by 158.Xr gzip 1 . 159Compression is desirable on modem lines and other 160slow connections, but will only slow down things on fast networks. 161The default value can be set on a host-by-host basis in the 162configuration files; see the 163.Cm Compression 164option in 165.Xr ssh_config 5 . 166.Pp 167.It Fl c Ar cipher_spec 168Selects the cipher specification for encrypting the session. 169.Ar cipher_spec 170is a comma-separated list of ciphers 171listed in order of preference. 172See the 173.Cm Ciphers 174keyword in 175.Xr ssh_config 5 176for more information. 177.Pp 178.It Fl D Xo 179.Sm off 180.Oo Ar bind_address : Oc 181.Ar port 182.Sm on 183.Xc 184Specifies a local 185.Dq dynamic 186application-level port forwarding. 187This works by allocating a socket to listen to 188.Ar port 189on the local side, optionally bound to the specified 190.Ar bind_address . 191Whenever a connection is made to this port, the 192connection is forwarded over the secure channel, and the application 193protocol is then used to determine where to connect to from the 194remote machine. 195Currently the SOCKS4 and SOCKS5 protocols are supported, and 196.Nm 197will act as a SOCKS server. 198Only root can forward privileged ports. 199Dynamic port forwardings can also be specified in the configuration file. 200.Pp 201IPv6 addresses can be specified by enclosing the address in square brackets. 202Only the superuser can forward privileged ports. 203By default, the local port is bound in accordance with the 204.Cm GatewayPorts 205setting. 206However, an explicit 207.Ar bind_address 208may be used to bind the connection to a specific address. 209The 210.Ar bind_address 211of 212.Dq localhost 213indicates that the listening port be bound for local use only, while an 214empty address or 215.Sq * 216indicates that the port should be available from all interfaces. 217.Pp 218.It Fl E Ar log_file 219Append debug logs to 220.Ar log_file 221instead of standard error. 222.Pp 223.It Fl e Ar escape_char 224Sets the escape character for sessions with a pty (default: 225.Ql ~ ) . 226The escape character is only recognized at the beginning of a line. 227The escape character followed by a dot 228.Pq Ql \&. 229closes the connection; 230followed by control-Z suspends the connection; 231and followed by itself sends the escape character once. 232Setting the character to 233.Dq none 234disables any escapes and makes the session fully transparent. 235.Pp 236.It Fl F Ar configfile 237Specifies an alternative per-user configuration file. 238If a configuration file is given on the command line, 239the system-wide configuration file 240.Pq Pa /etc/ssh/ssh_config 241will be ignored. 242The default for the per-user configuration file is 243.Pa ~/.ssh/config . 244If set to 245.Dq none , 246no configuration files will be read. 247.Pp 248.It Fl f 249Requests 250.Nm 251to go to background just before command execution. 252This is useful if 253.Nm 254is going to ask for passwords or passphrases, but the user 255wants it in the background. 256This implies 257.Fl n . 258The recommended way to start X11 programs at a remote site is with 259something like 260.Ic ssh -f host xterm . 261.Pp 262If the 263.Cm ExitOnForwardFailure 264configuration option is set to 265.Dq yes , 266then a client started with 267.Fl f 268will wait for all remote port forwards to be successfully established 269before placing itself in the background. 270Refer to the description of 271.Cm ForkAfterAuthentication 272in 273.Xr ssh_config 5 274for details. 275.Pp 276.It Fl G 277Causes 278.Nm 279to print its configuration after evaluating 280.Cm Host 281and 282.Cm Match 283blocks and exit. 284.Pp 285.It Fl g 286Allows remote hosts to connect to local forwarded ports. 287If used on a multiplexed connection, then this option must be specified 288on the master process. 289.Pp 290.It Fl I Ar pkcs11 291Specify the PKCS#11 shared library 292.Nm 293should use to communicate with a PKCS#11 token providing keys for user 294authentication. 295.Pp 296.It Fl i Ar identity_file 297Selects a file from which the identity (private key) for 298public key authentication is read. 299You can also specify a public key file to use the corresponding 300private key that is loaded in 301.Xr ssh-agent 1 302when the private key file is not present locally. 303The default is 304.Pa ~/.ssh/id_rsa , 305.Pa ~/.ssh/id_ecdsa , 306.Pa ~/.ssh/id_ecdsa_sk , 307.Pa ~/.ssh/id_ed25519 308and 309.Pa ~/.ssh/id_ed25519_sk . 310Identity files may also be specified on 311a per-host basis in the configuration file. 312It is possible to have multiple 313.Fl i 314options (and multiple identities specified in 315configuration files). 316If no certificates have been explicitly specified by the 317.Cm CertificateFile 318directive, 319.Nm 320will also try to load certificate information from the filename obtained 321by appending 322.Pa -cert.pub 323to identity filenames. 324.Pp 325.It Fl J Ar destination 326Connect to the target host by first making an 327.Nm 328connection to the jump host described by 329.Ar destination 330and then establishing a TCP forwarding to the ultimate destination from 331there. 332Multiple jump hops may be specified separated by comma characters. 333IPv6 addresses can be specified by enclosing the address in square brackets. 334This is a shortcut to specify a 335.Cm ProxyJump 336configuration directive. 337Note that configuration directives supplied on the command-line generally 338apply to the destination host and not any specified jump hosts. 339Use 340.Pa ~/.ssh/config 341to specify configuration for jump hosts. 342.Pp 343.It Fl K 344Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 345credentials to the server. 346.Pp 347.It Fl k 348Disables forwarding (delegation) of GSSAPI credentials to the server. 349.Pp 350.It Fl L Xo 351.Sm off 352.Oo Ar bind_address : Oc 353.Ar port : host : hostport 354.Sm on 355.Xc 356.It Fl L Xo 357.Sm off 358.Oo Ar bind_address : Oc 359.Ar port : remote_socket 360.Sm on 361.Xc 362.It Fl L Xo 363.Sm off 364.Ar local_socket : host : hostport 365.Sm on 366.Xc 367.It Fl L Xo 368.Sm off 369.Ar local_socket : remote_socket 370.Sm on 371.Xc 372Specifies that connections to the given TCP port or Unix socket on the local 373(client) host are to be forwarded to the given host and port, or Unix socket, 374on the remote side. 375This works by allocating a socket to listen to either a TCP 376.Ar port 377on the local side, optionally bound to the specified 378.Ar bind_address , 379or to a Unix socket. 380Whenever a connection is made to the local port or socket, the 381connection is forwarded over the secure channel, and a connection is 382made to either 383.Ar host 384port 385.Ar hostport , 386or the Unix socket 387.Ar remote_socket , 388from the remote machine. 389.Pp 390Port forwardings can also be specified in the configuration file. 391Only the superuser can forward privileged ports. 392IPv6 addresses can be specified by enclosing the address in square brackets. 393.Pp 394By default, the local port is bound in accordance with the 395.Cm GatewayPorts 396setting. 397However, an explicit 398.Ar bind_address 399may be used to bind the connection to a specific address. 400The 401.Ar bind_address 402of 403.Dq localhost 404indicates that the listening port be bound for local use only, while an 405empty address or 406.Sq * 407indicates that the port should be available from all interfaces. 408.Pp 409.It Fl l Ar login_name 410Specifies the user to log in as on the remote machine. 411This also may be specified on a per-host basis in the configuration file. 412.Pp 413.It Fl M 414Places the 415.Nm 416client into 417.Dq master 418mode for connection sharing. 419Multiple 420.Fl M 421options places 422.Nm 423into 424.Dq master 425mode but with confirmation required using 426.Xr ssh-askpass 1 427before each operation that changes the multiplexing state 428(e.g. opening a new session). 429Refer to the description of 430.Cm ControlMaster 431in 432.Xr ssh_config 5 433for details. 434.Pp 435.It Fl m Ar mac_spec 436A comma-separated list of MAC (message authentication code) algorithms, 437specified in order of preference. 438See the 439.Cm MACs 440keyword in 441.Xr ssh_config 5 442for more information. 443.Pp 444.It Fl N 445Do not execute a remote command. 446This is useful for just forwarding ports. 447Refer to the description of 448.Cm SessionType 449in 450.Xr ssh_config 5 451for details. 452.Pp 453.It Fl n 454Redirects stdin from 455.Pa /dev/null 456(actually, prevents reading from stdin). 457This must be used when 458.Nm 459is run in the background. 460A common trick is to use this to run X11 programs on a remote machine. 461For example, 462.Ic ssh -n shadows.cs.hut.fi emacs & 463will start an emacs on shadows.cs.hut.fi, and the X11 464connection will be automatically forwarded over an encrypted channel. 465The 466.Nm 467program will be put in the background. 468(This does not work if 469.Nm 470needs to ask for a password or passphrase; see also the 471.Fl f 472option.) 473Refer to the description of 474.Cm StdinNull 475in 476.Xr ssh_config 5 477for details. 478.Pp 479.It Fl O Ar ctl_cmd 480Control an active connection multiplexing master process. 481When the 482.Fl O 483option is specified, the 484.Ar ctl_cmd 485argument is interpreted and passed to the master process. 486Valid commands are: 487.Dq check 488(check that the master process is running), 489.Dq forward 490(request forwardings without command execution), 491.Dq cancel 492(cancel forwardings), 493.Dq exit 494(request the master to exit), and 495.Dq stop 496(request the master to stop accepting further multiplexing requests). 497.Pp 498.It Fl o Ar option 499Can be used to give options in the format used in the configuration file. 500This is useful for specifying options for which there is no separate 501command-line flag. 502For full details of the options listed below, and their possible values, see 503.Xr ssh_config 5 . 504.Pp 505.Bl -tag -width Ds -offset indent -compact 506.It AddKeysToAgent 507.It AddressFamily 508.It BatchMode 509.It BindAddress 510.It CanonicalDomains 511.It CanonicalizeFallbackLocal 512.It CanonicalizeHostname 513.It CanonicalizeMaxDots 514.It CanonicalizePermittedCNAMEs 515.It CASignatureAlgorithms 516.It CertificateFile 517.It CheckHostIP 518.It Ciphers 519.It ClearAllForwardings 520.It Compression 521.It ConnectionAttempts 522.It ConnectTimeout 523.It ControlMaster 524.It ControlPath 525.It ControlPersist 526.It DynamicForward 527.It EnableEscapeCommandline 528.It EscapeChar 529.It ExitOnForwardFailure 530.It FingerprintHash 531.It ForkAfterAuthentication 532.It ForwardAgent 533.It ForwardX11 534.It ForwardX11Timeout 535.It ForwardX11Trusted 536.It GatewayPorts 537.It GlobalKnownHostsFile 538.It GSSAPIAuthentication 539.It GSSAPIDelegateCredentials 540.It HashKnownHosts 541.It Host 542.It HostbasedAcceptedAlgorithms 543.It HostbasedAuthentication 544.It HostKeyAlgorithms 545.It HostKeyAlias 546.It Hostname 547.It IdentitiesOnly 548.It IdentityAgent 549.It IdentityFile 550.It IPQoS 551.It KbdInteractiveAuthentication 552.It KbdInteractiveDevices 553.It KexAlgorithms 554.It KnownHostsCommand 555.It LocalCommand 556.It LocalForward 557.It LogLevel 558.It MACs 559.It Match 560.It NoHostAuthenticationForLocalhost 561.It NumberOfPasswordPrompts 562.It PasswordAuthentication 563.It PermitLocalCommand 564.It PermitRemoteOpen 565.It PKCS11Provider 566.It Port 567.It PreferredAuthentications 568.It ProxyCommand 569.It ProxyJump 570.It ProxyUseFdpass 571.It PubkeyAcceptedAlgorithms 572.It PubkeyAuthentication 573.It RekeyLimit 574.It RemoteCommand 575.It RemoteForward 576.It RequestTTY 577.It RequiredRSASize 578.It SendEnv 579.It ServerAliveInterval 580.It ServerAliveCountMax 581.It SessionType 582.It SetEnv 583.It StdinNull 584.It StreamLocalBindMask 585.It StreamLocalBindUnlink 586.It StrictHostKeyChecking 587.It TCPKeepAlive 588.It Tunnel 589.It TunnelDevice 590.It UpdateHostKeys 591.It User 592.It UserKnownHostsFile 593.It VerifyHostKeyDNS 594.It VisualHostKey 595.It XAuthLocation 596.El 597.Pp 598.It Fl P Ar tag 599Specify a tag name that may be used to select configuration in 600.Xr ssh_config 5 . 601Refer to the 602.Cm Tag 603and 604.Cm Match 605keywords in 606.Xr ssh_config 5 607for more information. 608.It Fl p Ar port 609Port to connect to on the remote host. 610This can be specified on a 611per-host basis in the configuration file. 612.Pp 613.It Fl Q Ar query_option 614Queries for the algorithms supported by one of the following features: 615.Ar cipher 616(supported symmetric ciphers), 617.Ar cipher-auth 618(supported symmetric ciphers that support authenticated encryption), 619.Ar help 620(supported query terms for use with the 621.Fl Q 622flag), 623.Ar mac 624(supported message integrity codes), 625.Ar kex 626(key exchange algorithms), 627.Ar key 628(key types), 629.Ar key-ca-sign 630(valid CA signature algorithms for certificates), 631.Ar key-cert 632(certificate key types), 633.Ar key-plain 634(non-certificate key types), 635.Ar key-sig 636(all key types and signature algorithms), 637.Ar protocol-version 638(supported SSH protocol versions), and 639.Ar sig 640(supported signature algorithms). 641Alternatively, any keyword from 642.Xr ssh_config 5 643or 644.Xr sshd_config 5 645that takes an algorithm list may be used as an alias for the corresponding 646query_option. 647.Pp 648.It Fl q 649Quiet mode. 650Causes most warning and diagnostic messages to be suppressed. 651.Pp 652.It Fl R Xo 653.Sm off 654.Oo Ar bind_address : Oc 655.Ar port : host : hostport 656.Sm on 657.Xc 658.It Fl R Xo 659.Sm off 660.Oo Ar bind_address : Oc 661.Ar port : local_socket 662.Sm on 663.Xc 664.It Fl R Xo 665.Sm off 666.Ar remote_socket : host : hostport 667.Sm on 668.Xc 669.It Fl R Xo 670.Sm off 671.Ar remote_socket : local_socket 672.Sm on 673.Xc 674.It Fl R Xo 675.Sm off 676.Oo Ar bind_address : Oc 677.Ar port 678.Sm on 679.Xc 680Specifies that connections to the given TCP port or Unix socket on the remote 681(server) host are to be forwarded to the local side. 682.Pp 683This works by allocating a socket to listen to either a TCP 684.Ar port 685or to a Unix socket on the remote side. 686Whenever a connection is made to this port or Unix socket, the 687connection is forwarded over the secure channel, and a connection 688is made from the local machine to either an explicit destination specified by 689.Ar host 690port 691.Ar hostport , 692or 693.Ar local_socket , 694or, if no explicit destination was specified, 695.Nm 696will act as a SOCKS 4/5 proxy and forward connections to the destinations 697requested by the remote SOCKS client. 698.Pp 699Port forwardings can also be specified in the configuration file. 700Privileged ports can be forwarded only when 701logging in as root on the remote machine. 702IPv6 addresses can be specified by enclosing the address in square brackets. 703.Pp 704By default, TCP listening sockets on the server will be bound to the loopback 705interface only. 706This may be overridden by specifying a 707.Ar bind_address . 708An empty 709.Ar bind_address , 710or the address 711.Ql * , 712indicates that the remote socket should listen on all interfaces. 713Specifying a remote 714.Ar bind_address 715will only succeed if the server's 716.Cm GatewayPorts 717option is enabled (see 718.Xr sshd_config 5 ) . 719.Pp 720If the 721.Ar port 722argument is 723.Ql 0 , 724the listen port will be dynamically allocated on the server and reported 725to the client at run time. 726When used together with 727.Ic -O forward , 728the allocated port will be printed to the standard output. 729.Pp 730.It Fl S Ar ctl_path 731Specifies the location of a control socket for connection sharing, 732or the string 733.Dq none 734to disable connection sharing. 735Refer to the description of 736.Cm ControlPath 737and 738.Cm ControlMaster 739in 740.Xr ssh_config 5 741for details. 742.Pp 743.It Fl s 744May be used to request invocation of a subsystem on the remote system. 745Subsystems facilitate the use of SSH 746as a secure transport for other applications (e.g.\& 747.Xr sftp 1 ) . 748The subsystem is specified as the remote command. 749Refer to the description of 750.Cm SessionType 751in 752.Xr ssh_config 5 753for details. 754.Pp 755.It Fl T 756Disable pseudo-terminal allocation. 757.Pp 758.It Fl t 759Force pseudo-terminal allocation. 760This can be used to execute arbitrary 761screen-based programs on a remote machine, which can be very useful, 762e.g. when implementing menu services. 763Multiple 764.Fl t 765options force tty allocation, even if 766.Nm 767has no local tty. 768.Pp 769.It Fl V 770Display the version number and exit. 771.Pp 772.It Fl v 773Verbose mode. 774Causes 775.Nm 776to print debugging messages about its progress. 777This is helpful in 778debugging connection, authentication, and configuration problems. 779Multiple 780.Fl v 781options increase the verbosity. 782The maximum is 3. 783.Pp 784.It Fl W Ar host : Ns Ar port 785Requests that standard input and output on the client be forwarded to 786.Ar host 787on 788.Ar port 789over the secure channel. 790Implies 791.Fl N , 792.Fl T , 793.Cm ExitOnForwardFailure 794and 795.Cm ClearAllForwardings , 796though these can be overridden in the configuration file or using 797.Fl o 798command line options. 799.Pp 800.It Fl w Xo 801.Ar local_tun Ns Op : Ns Ar remote_tun 802.Xc 803Requests 804tunnel 805device forwarding with the specified 806.Xr tun 4 807devices between the client 808.Pq Ar local_tun 809and the server 810.Pq Ar remote_tun . 811.Pp 812The devices may be specified by numerical ID or the keyword 813.Dq any , 814which uses the next available tunnel device. 815If 816.Ar remote_tun 817is not specified, it defaults to 818.Dq any . 819See also the 820.Cm Tunnel 821and 822.Cm TunnelDevice 823directives in 824.Xr ssh_config 5 . 825.Pp 826If the 827.Cm Tunnel 828directive is unset, it will be set to the default tunnel mode, which is 829.Dq point-to-point . 830If a different 831.Cm Tunnel 832forwarding mode it desired, then it should be specified before 833.Fl w . 834.Pp 835.It Fl X 836Enables X11 forwarding. 837This can also be specified on a per-host basis in a configuration file. 838.Pp 839X11 forwarding should be enabled with caution. 840Users with the ability to bypass file permissions on the remote host 841(for the user's X authorization database) 842can access the local X11 display through the forwarded connection. 843An attacker may then be able to perform activities such as keystroke monitoring. 844.Pp 845For this reason, X11 forwarding is subjected to X11 SECURITY extension 846restrictions by default. 847Refer to the 848.Nm 849.Fl Y 850option and the 851.Cm ForwardX11Trusted 852directive in 853.Xr ssh_config 5 854for more information. 855.Pp 856.It Fl x 857Disables X11 forwarding. 858.Pp 859.It Fl Y 860Enables trusted X11 forwarding. 861Trusted X11 forwardings are not subjected to the X11 SECURITY extension 862controls. 863.Pp 864.It Fl y 865Send log information using the 866.Xr syslog 3 867system module. 868By default this information is sent to stderr. 869.El 870.Pp 871.Nm 872may additionally obtain configuration data from 873a per-user configuration file and a system-wide configuration file. 874The file format and configuration options are described in 875.Xr ssh_config 5 . 876.Sh AUTHENTICATION 877The OpenSSH SSH client supports SSH protocol 2. 878.Pp 879The methods available for authentication are: 880GSSAPI-based authentication, 881host-based authentication, 882public key authentication, 883keyboard-interactive authentication, 884and password authentication. 885Authentication methods are tried in the order specified above, 886though 887.Cm PreferredAuthentications 888can be used to change the default order. 889.Pp 890Host-based authentication works as follows: 891If the machine the user logs in from is listed in 892.Pa /etc/hosts.equiv 893or 894.Pa /etc/shosts.equiv 895on the remote machine, the user is non-root and the user names are 896the same on both sides, or if the files 897.Pa ~/.rhosts 898or 899.Pa ~/.shosts 900exist in the user's home directory on the 901remote machine and contain a line containing the name of the client 902machine and the name of the user on that machine, the user is 903considered for login. 904Additionally, the server 905.Em must 906be able to verify the client's 907host key (see the description of 908.Pa /etc/ssh/ssh_known_hosts 909and 910.Pa ~/.ssh/known_hosts , 911below) 912for login to be permitted. 913This authentication method closes security holes due to IP 914spoofing, DNS spoofing, and routing spoofing. 915[Note to the administrator: 916.Pa /etc/hosts.equiv , 917.Pa ~/.rhosts , 918and the rlogin/rsh protocol in general, are inherently insecure and should be 919disabled if security is desired.] 920.Pp 921Public key authentication works as follows: 922The scheme is based on public-key cryptography, 923using cryptosystems 924where encryption and decryption are done using separate keys, 925and it is unfeasible to derive the decryption key from the encryption key. 926The idea is that each user creates a public/private 927key pair for authentication purposes. 928The server knows the public key, and only the user knows the private key. 929.Nm 930implements public key authentication protocol automatically, 931using one of the ECDSA, Ed25519 or RSA algorithms. 932.Pp 933The file 934.Pa ~/.ssh/authorized_keys 935lists the public keys that are permitted for logging in. 936When the user logs in, the 937.Nm 938program tells the server which key pair it would like to use for 939authentication. 940The client proves that it has access to the private key 941and the server checks that the corresponding public key 942is authorized to accept the account. 943.Pp 944The server may inform the client of errors that prevented public key 945authentication from succeeding after authentication completes using a 946different method. 947These may be viewed by increasing the 948.Cm LogLevel 949to 950.Cm DEBUG 951or higher (e.g. by using the 952.Fl v 953flag). 954.Pp 955The user creates their key pair by running 956.Xr ssh-keygen 1 . 957This stores the private key in 958.Pa ~/.ssh/id_ecdsa 959(ECDSA), 960.Pa ~/.ssh/id_ecdsa_sk 961(authenticator-hosted ECDSA), 962.Pa ~/.ssh/id_ed25519 963(Ed25519), 964.Pa ~/.ssh/id_ed25519_sk 965(authenticator-hosted Ed25519), 966or 967.Pa ~/.ssh/id_rsa 968(RSA) 969and stores the public key in 970.Pa ~/.ssh/id_ecdsa.pub 971(ECDSA), 972.Pa ~/.ssh/id_ecdsa_sk.pub 973(authenticator-hosted ECDSA), 974.Pa ~/.ssh/id_ed25519.pub 975(Ed25519), 976.Pa ~/.ssh/id_ed25519_sk.pub 977(authenticator-hosted Ed25519), 978or 979.Pa ~/.ssh/id_rsa.pub 980(RSA) 981in the user's home directory. 982The user should then copy the public key 983to 984.Pa ~/.ssh/authorized_keys 985in their home directory on the remote machine. 986The 987.Pa authorized_keys 988file corresponds to the conventional 989.Pa ~/.rhosts 990file, and has one key 991per line, though the lines can be very long. 992After this, the user can log in without giving the password. 993.Pp 994A variation on public key authentication 995is available in the form of certificate authentication: 996instead of a set of public/private keys, 997signed certificates are used. 998This has the advantage that a single trusted certification authority 999can be used in place of many public/private keys. 1000See the CERTIFICATES section of 1001.Xr ssh-keygen 1 1002for more information. 1003.Pp 1004The most convenient way to use public key or certificate authentication 1005may be with an authentication agent. 1006See 1007.Xr ssh-agent 1 1008and (optionally) the 1009.Cm AddKeysToAgent 1010directive in 1011.Xr ssh_config 5 1012for more information. 1013.Pp 1014Keyboard-interactive authentication works as follows: 1015The server sends an arbitrary 1016.Qq challenge 1017text and prompts for a response, possibly multiple times. 1018Examples of keyboard-interactive authentication include 1019.Bx 1020Authentication (see 1021.Xr login.conf 5 ) 1022and PAM (some 1023.Pf non- Ox 1024systems). 1025.Pp 1026Finally, if other authentication methods fail, 1027.Nm 1028prompts the user for a password. 1029The password is sent to the remote 1030host for checking; however, since all communications are encrypted, 1031the password cannot be seen by someone listening on the network. 1032.Pp 1033.Nm 1034automatically maintains and checks a database containing 1035identification for all hosts it has ever been used with. 1036Host keys are stored in 1037.Pa ~/.ssh/known_hosts 1038in the user's home directory. 1039Additionally, the file 1040.Pa /etc/ssh/ssh_known_hosts 1041is automatically checked for known hosts. 1042Any new hosts are automatically added to the user's file. 1043If a host's identification ever changes, 1044.Nm 1045warns about this and disables password authentication to prevent 1046server spoofing or man-in-the-middle attacks, 1047which could otherwise be used to circumvent the encryption. 1048The 1049.Cm StrictHostKeyChecking 1050option can be used to control logins to machines whose 1051host key is not known or has changed. 1052.Pp 1053When the user's identity has been accepted by the server, the server 1054either executes the given command in a non-interactive session or, 1055if no command has been specified, logs into the machine and gives 1056the user a normal shell as an interactive session. 1057All communication with 1058the remote command or shell will be automatically encrypted. 1059.Pp 1060If an interactive session is requested, 1061.Nm 1062by default will only request a pseudo-terminal (pty) for interactive 1063sessions when the client has one. 1064The flags 1065.Fl T 1066and 1067.Fl t 1068can be used to override this behaviour. 1069.Pp 1070If a pseudo-terminal has been allocated, the 1071user may use the escape characters noted below. 1072.Pp 1073If no pseudo-terminal has been allocated, 1074the session is transparent and can be used to reliably transfer binary data. 1075On most systems, setting the escape character to 1076.Dq none 1077will also make the session transparent even if a tty is used. 1078.Pp 1079The session terminates when the command or shell on the remote 1080machine exits and all X11 and TCP connections have been closed. 1081.Sh ESCAPE CHARACTERS 1082When a pseudo-terminal has been requested, 1083.Nm 1084supports a number of functions through the use of an escape character. 1085.Pp 1086A single tilde character can be sent as 1087.Ic ~~ 1088or by following the tilde by a character other than those described below. 1089The escape character must always follow a newline to be interpreted as 1090special. 1091The escape character can be changed in configuration files using the 1092.Cm EscapeChar 1093configuration directive or on the command line by the 1094.Fl e 1095option. 1096.Pp 1097The supported escapes (assuming the default 1098.Ql ~ ) 1099are: 1100.Bl -tag -width Ds 1101.It Cm ~. 1102Disconnect. 1103.It Cm ~^Z 1104Background 1105.Nm . 1106.It Cm ~# 1107List forwarded connections. 1108.It Cm ~& 1109Background 1110.Nm 1111at logout when waiting for forwarded connection / X11 sessions to terminate. 1112.It Cm ~? 1113Display a list of escape characters. 1114.It Cm ~B 1115Send a BREAK to the remote system 1116(only useful if the peer supports it). 1117.It Cm ~C 1118Open command line. 1119Currently this allows the addition of port forwardings using the 1120.Fl L , 1121.Fl R 1122and 1123.Fl D 1124options (see above). 1125It also allows the cancellation of existing port-forwardings 1126with 1127.Sm off 1128.Fl KL Oo Ar bind_address : Oc Ar port 1129.Sm on 1130for local, 1131.Sm off 1132.Fl KR Oo Ar bind_address : Oc Ar port 1133.Sm on 1134for remote and 1135.Sm off 1136.Fl KD Oo Ar bind_address : Oc Ar port 1137.Sm on 1138for dynamic port-forwardings. 1139.Ic !\& Ns Ar command 1140allows the user to execute a local command if the 1141.Ic PermitLocalCommand 1142option is enabled in 1143.Xr ssh_config 5 . 1144Basic help is available, using the 1145.Fl h 1146option. 1147.It Cm ~R 1148Request rekeying of the connection 1149(only useful if the peer supports it). 1150.It Cm ~V 1151Decrease the verbosity 1152.Pq Ic LogLevel 1153when errors are being written to stderr. 1154.It Cm ~v 1155Increase the verbosity 1156.Pq Ic LogLevel 1157when errors are being written to stderr. 1158.El 1159.Sh TCP FORWARDING 1160Forwarding of arbitrary TCP connections over a secure channel 1161can be specified either on the command line or in a configuration file. 1162One possible application of TCP forwarding is a secure connection to a 1163mail server; another is going through firewalls. 1164.Pp 1165In the example below, we look at encrypting communication for an IRC client, 1166even though the IRC server it connects to does not directly 1167support encrypted communication. 1168This works as follows: 1169the user connects to the remote host using 1170.Nm , 1171specifying the ports to be used to forward the connection. 1172After that it is possible to start the program locally, 1173and 1174.Nm 1175will encrypt and forward the connection to the remote server. 1176.Pp 1177The following example tunnels an IRC session from the client 1178to an IRC server at 1179.Dq server.example.com , 1180joining channel 1181.Dq #users , 1182nickname 1183.Dq pinky , 1184using the standard IRC port, 6667: 1185.Bd -literal -offset 4n 1186$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1187$ irc -c '#users' pinky IRC/127.0.0.1 1188.Ed 1189.Pp 1190The 1191.Fl f 1192option backgrounds 1193.Nm 1194and the remote command 1195.Dq sleep 10 1196is specified to allow an amount of time 1197(10 seconds, in the example) 1198to start the program which is going to use the tunnel. 1199If no connections are made within the time specified, 1200.Nm 1201will exit. 1202.Sh X11 FORWARDING 1203If the 1204.Cm ForwardX11 1205variable is set to 1206.Dq yes 1207(or see the description of the 1208.Fl X , 1209.Fl x , 1210and 1211.Fl Y 1212options above) 1213and the user is using X11 (the 1214.Ev DISPLAY 1215environment variable is set), the connection to the X11 display is 1216automatically forwarded to the remote side in such a way that any X11 1217programs started from the shell (or command) will go through the 1218encrypted channel, and the connection to the real X server will be made 1219from the local machine. 1220The user should not manually set 1221.Ev DISPLAY . 1222Forwarding of X11 connections can be 1223configured on the command line or in configuration files. 1224.Pp 1225The 1226.Ev DISPLAY 1227value set by 1228.Nm 1229will point to the server machine, but with a display number greater than zero. 1230This is normal, and happens because 1231.Nm 1232creates a 1233.Dq proxy 1234X server on the server machine for forwarding the 1235connections over the encrypted channel. 1236.Pp 1237.Nm 1238will also automatically set up Xauthority data on the server machine. 1239For this purpose, it will generate a random authorization cookie, 1240store it in Xauthority on the server, and verify that any forwarded 1241connections carry this cookie and replace it by the real cookie when 1242the connection is opened. 1243The real authentication cookie is never 1244sent to the server machine (and no cookies are sent in the plain). 1245.Pp 1246If the 1247.Cm ForwardAgent 1248variable is set to 1249.Dq yes 1250(or see the description of the 1251.Fl A 1252and 1253.Fl a 1254options above) and 1255the user is using an authentication agent, the connection to the agent 1256is automatically forwarded to the remote side. 1257.Sh VERIFYING HOST KEYS 1258When connecting to a server for the first time, 1259a fingerprint of the server's public key is presented to the user 1260(unless the option 1261.Cm StrictHostKeyChecking 1262has been disabled). 1263Fingerprints can be determined using 1264.Xr ssh-keygen 1 : 1265.Pp 1266.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1267.Pp 1268If the fingerprint is already known, it can be matched 1269and the key can be accepted or rejected. 1270If only legacy (MD5) fingerprints for the server are available, the 1271.Xr ssh-keygen 1 1272.Fl E 1273option may be used to downgrade the fingerprint algorithm to match. 1274.Pp 1275Because of the difficulty of comparing host keys 1276just by looking at fingerprint strings, 1277there is also support to compare host keys visually, 1278using 1279.Em random art . 1280By setting the 1281.Cm VisualHostKey 1282option to 1283.Dq yes , 1284a small ASCII graphic gets displayed on every login to a server, no matter 1285if the session itself is interactive or not. 1286By learning the pattern a known server produces, a user can easily 1287find out that the host key has changed when a completely different pattern 1288is displayed. 1289Because these patterns are not unambiguous however, a pattern that looks 1290similar to the pattern remembered only gives a good probability that the 1291host key is the same, not guaranteed proof. 1292.Pp 1293To get a listing of the fingerprints along with their random art for 1294all known hosts, the following command line can be used: 1295.Pp 1296.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1297.Pp 1298If the fingerprint is unknown, 1299an alternative method of verification is available: 1300SSH fingerprints verified by DNS. 1301An additional resource record (RR), 1302SSHFP, 1303is added to a zonefile 1304and the connecting client is able to match the fingerprint 1305with that of the key presented. 1306.Pp 1307In this example, we are connecting a client to a server, 1308.Dq host.example.com . 1309The SSHFP resource records should first be added to the zonefile for 1310host.example.com: 1311.Bd -literal -offset indent 1312$ ssh-keygen -r host.example.com. 1313.Ed 1314.Pp 1315The output lines will have to be added to the zonefile. 1316To check that the zone is answering fingerprint queries: 1317.Pp 1318.Dl $ dig -t SSHFP host.example.com 1319.Pp 1320Finally the client connects: 1321.Bd -literal -offset indent 1322$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1323[...] 1324Matching host key fingerprint found in DNS. 1325Are you sure you want to continue connecting (yes/no)? 1326.Ed 1327.Pp 1328See the 1329.Cm VerifyHostKeyDNS 1330option in 1331.Xr ssh_config 5 1332for more information. 1333.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1334.Nm 1335contains support for Virtual Private Network (VPN) tunnelling 1336using the 1337.Xr tun 4 1338network pseudo-device, 1339allowing two networks to be joined securely. 1340The 1341.Xr sshd_config 5 1342configuration option 1343.Cm PermitTunnel 1344controls whether the server supports this, 1345and at what level (layer 2 or 3 traffic). 1346.Pp 1347The following example would connect client network 10.0.50.0/24 1348with remote network 10.0.99.0/24 using a point-to-point connection 1349from 10.1.1.1 to 10.1.1.2, 1350provided that the SSH server running on the gateway to the remote network, 1351at 192.168.1.15, allows it. 1352.Pp 1353On the client: 1354.Bd -literal -offset indent 1355# ssh -f -w 0:1 192.168.1.15 true 1356# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1357# route add 10.0.99.0/24 10.1.1.2 1358.Ed 1359.Pp 1360On the server: 1361.Bd -literal -offset indent 1362# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1363# route add 10.0.50.0/24 10.1.1.1 1364.Ed 1365.Pp 1366Client access may be more finely tuned via the 1367.Pa /root/.ssh/authorized_keys 1368file (see below) and the 1369.Cm PermitRootLogin 1370server option. 1371The following entry would permit connections on 1372.Xr tun 4 1373device 1 from user 1374.Dq jane 1375and on tun device 2 from user 1376.Dq john , 1377if 1378.Cm PermitRootLogin 1379is set to 1380.Dq forced-commands-only : 1381.Bd -literal -offset 2n 1382tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1383tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1384.Ed 1385.Pp 1386Since an SSH-based setup entails a fair amount of overhead, 1387it may be more suited to temporary setups, 1388such as for wireless VPNs. 1389More permanent VPNs are better provided by tools such as 1390.Xr ipsecctl 8 1391and 1392.Xr isakmpd 8 . 1393.Sh ENVIRONMENT 1394.Nm 1395will normally set the following environment variables: 1396.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1397.It Ev DISPLAY 1398The 1399.Ev DISPLAY 1400variable indicates the location of the X11 server. 1401It is automatically set by 1402.Nm 1403to point to a value of the form 1404.Dq hostname:n , 1405where 1406.Dq hostname 1407indicates the host where the shell runs, and 1408.Sq n 1409is an integer \*(Ge 1. 1410.Nm 1411uses this special value to forward X11 connections over the secure 1412channel. 1413The user should normally not set 1414.Ev DISPLAY 1415explicitly, as that 1416will render the X11 connection insecure (and will require the user to 1417manually copy any required authorization cookies). 1418.It Ev HOME 1419Set to the path of the user's home directory. 1420.It Ev LOGNAME 1421Synonym for 1422.Ev USER ; 1423set for compatibility with systems that use this variable. 1424.It Ev MAIL 1425Set to the path of the user's mailbox. 1426.It Ev PATH 1427Set to the default 1428.Ev PATH , 1429as specified when compiling 1430.Nm . 1431.It Ev SSH_ASKPASS 1432If 1433.Nm 1434needs a passphrase, it will read the passphrase from the current 1435terminal if it was run from a terminal. 1436If 1437.Nm 1438does not have a terminal associated with it but 1439.Ev DISPLAY 1440and 1441.Ev SSH_ASKPASS 1442are set, it will execute the program specified by 1443.Ev SSH_ASKPASS 1444and open an X11 window to read the passphrase. 1445This is particularly useful when calling 1446.Nm 1447from a 1448.Pa .xsession 1449or related script. 1450(Note that on some machines it 1451may be necessary to redirect the input from 1452.Pa /dev/null 1453to make this work.) 1454.It Ev SSH_ASKPASS_REQUIRE 1455Allows further control over the use of an askpass program. 1456If this variable is set to 1457.Dq never 1458then 1459.Nm 1460will never attempt to use one. 1461If it is set to 1462.Dq prefer , 1463then 1464.Nm 1465will prefer to use the askpass program instead of the TTY when requesting 1466passwords. 1467Finally, if the variable is set to 1468.Dq force , 1469then the askpass program will be used for all passphrase input regardless 1470of whether 1471.Ev DISPLAY 1472is set. 1473.It Ev SSH_AUTH_SOCK 1474Identifies the path of a 1475.Ux Ns -domain 1476socket used to communicate with the agent. 1477.It Ev SSH_CONNECTION 1478Identifies the client and server ends of the connection. 1479The variable contains 1480four space-separated values: client IP address, client port number, 1481server IP address, and server port number. 1482.It Ev SSH_ORIGINAL_COMMAND 1483This variable contains the original command line if a forced command 1484is executed. 1485It can be used to extract the original arguments. 1486.It Ev SSH_TTY 1487This is set to the name of the tty (path to the device) associated 1488with the current shell or command. 1489If the current session has no tty, 1490this variable is not set. 1491.It Ev SSH_TUNNEL 1492Optionally set by 1493.Xr sshd 8 1494to contain the interface names assigned if tunnel forwarding was 1495requested by the client. 1496.It Ev SSH_USER_AUTH 1497Optionally set by 1498.Xr sshd 8 , 1499this variable may contain a pathname to a file that lists the authentication 1500methods successfully used when the session was established, including any 1501public keys that were used. 1502.It Ev TZ 1503This variable is set to indicate the present time zone if it 1504was set when the daemon was started (i.e. the daemon passes the value 1505on to new connections). 1506.It Ev USER 1507Set to the name of the user logging in. 1508.El 1509.Pp 1510Additionally, 1511.Nm 1512reads 1513.Pa ~/.ssh/environment , 1514and adds lines of the format 1515.Dq VARNAME=value 1516to the environment if the file exists and users are allowed to 1517change their environment. 1518For more information, see the 1519.Cm PermitUserEnvironment 1520option in 1521.Xr sshd_config 5 . 1522.Sh FILES 1523.Bl -tag -width Ds -compact 1524.It Pa ~/.rhosts 1525This file is used for host-based authentication (see above). 1526On some machines this file may need to be 1527world-readable if the user's home directory is on an NFS partition, 1528because 1529.Xr sshd 8 1530reads it as root. 1531Additionally, this file must be owned by the user, 1532and must not have write permissions for anyone else. 1533The recommended 1534permission for most machines is read/write for the user, and not 1535accessible by others. 1536.Pp 1537.It Pa ~/.shosts 1538This file is used in exactly the same way as 1539.Pa .rhosts , 1540but allows host-based authentication without permitting login with 1541rlogin/rsh. 1542.Pp 1543.It Pa ~/.ssh/ 1544This directory is the default location for all user-specific configuration 1545and authentication information. 1546There is no general requirement to keep the entire contents of this directory 1547secret, but the recommended permissions are read/write/execute for the user, 1548and not accessible by others. 1549.Pp 1550.It Pa ~/.ssh/authorized_keys 1551Lists the public keys (ECDSA, Ed25519, RSA) 1552that can be used for logging in as this user. 1553The format of this file is described in the 1554.Xr sshd 8 1555manual page. 1556This file is not highly sensitive, but the recommended 1557permissions are read/write for the user, and not accessible by others. 1558.Pp 1559.It Pa ~/.ssh/config 1560This is the per-user configuration file. 1561The file format and configuration options are described in 1562.Xr ssh_config 5 . 1563Because of the potential for abuse, this file must have strict permissions: 1564read/write for the user, and not writable by others. 1565.Pp 1566.It Pa ~/.ssh/environment 1567Contains additional definitions for environment variables; see 1568.Sx ENVIRONMENT , 1569above. 1570.Pp 1571.It Pa ~/.ssh/id_ecdsa 1572.It Pa ~/.ssh/id_ecdsa_sk 1573.It Pa ~/.ssh/id_ed25519 1574.It Pa ~/.ssh/id_ed25519_sk 1575.It Pa ~/.ssh/id_rsa 1576Contains the private key for authentication. 1577These files 1578contain sensitive data and should be readable by the user but not 1579accessible by others (read/write/execute). 1580.Nm 1581will simply ignore a private key file if it is accessible by others. 1582It is possible to specify a passphrase when 1583generating the key which will be used to encrypt the 1584sensitive part of this file using AES-128. 1585.Pp 1586.It Pa ~/.ssh/id_ecdsa.pub 1587.It Pa ~/.ssh/id_ecdsa_sk.pub 1588.It Pa ~/.ssh/id_ed25519.pub 1589.It Pa ~/.ssh/id_ed25519_sk.pub 1590.It Pa ~/.ssh/id_rsa.pub 1591Contains the public key for authentication. 1592These files are not 1593sensitive and can (but need not) be readable by anyone. 1594.Pp 1595.It Pa ~/.ssh/known_hosts 1596Contains a list of host keys for all hosts the user has logged into 1597that are not already in the systemwide list of known host keys. 1598See 1599.Xr sshd 8 1600for further details of the format of this file. 1601.Pp 1602.It Pa ~/.ssh/rc 1603Commands in this file are executed by 1604.Nm 1605when the user logs in, just before the user's shell (or command) is 1606started. 1607See the 1608.Xr sshd 8 1609manual page for more information. 1610.Pp 1611.It Pa /etc/hosts.equiv 1612This file is for host-based authentication (see above). 1613It should only be writable by root. 1614.Pp 1615.It Pa /etc/shosts.equiv 1616This file is used in exactly the same way as 1617.Pa hosts.equiv , 1618but allows host-based authentication without permitting login with 1619rlogin/rsh. 1620.Pp 1621.It Pa /etc/ssh/ssh_config 1622Systemwide configuration file. 1623The file format and configuration options are described in 1624.Xr ssh_config 5 . 1625.Pp 1626.It Pa /etc/ssh/ssh_host_ecdsa_key 1627.It Pa /etc/ssh/ssh_host_ed25519_key 1628.It Pa /etc/ssh/ssh_host_rsa_key 1629These files contain the private parts of the host keys 1630and are used for host-based authentication. 1631.Pp 1632.It Pa /etc/ssh/ssh_known_hosts 1633Systemwide list of known host keys. 1634This file should be prepared by the 1635system administrator to contain the public host keys of all machines in the 1636organization. 1637It should be world-readable. 1638See 1639.Xr sshd 8 1640for further details of the format of this file. 1641.Pp 1642.It Pa /etc/ssh/sshrc 1643Commands in this file are executed by 1644.Nm 1645when the user logs in, just before the user's shell (or command) is started. 1646See the 1647.Xr sshd 8 1648manual page for more information. 1649.El 1650.Sh EXIT STATUS 1651.Nm 1652exits with the exit status of the remote command or with 255 1653if an error occurred. 1654.Sh SEE ALSO 1655.Xr scp 1 , 1656.Xr sftp 1 , 1657.Xr ssh-add 1 , 1658.Xr ssh-agent 1 , 1659.Xr ssh-keygen 1 , 1660.Xr ssh-keyscan 1 , 1661.Xr tun 4 , 1662.Xr ssh_config 5 , 1663.Xr ssh-keysign 8 , 1664.Xr sshd 8 1665.Sh STANDARDS 1666.Rs 1667.%A S. Lehtinen 1668.%A C. Lonvick 1669.%D January 2006 1670.%R RFC 4250 1671.%T The Secure Shell (SSH) Protocol Assigned Numbers 1672.Re 1673.Pp 1674.Rs 1675.%A T. Ylonen 1676.%A C. Lonvick 1677.%D January 2006 1678.%R RFC 4251 1679.%T The Secure Shell (SSH) Protocol Architecture 1680.Re 1681.Pp 1682.Rs 1683.%A T. Ylonen 1684.%A C. Lonvick 1685.%D January 2006 1686.%R RFC 4252 1687.%T The Secure Shell (SSH) Authentication Protocol 1688.Re 1689.Pp 1690.Rs 1691.%A T. Ylonen 1692.%A C. Lonvick 1693.%D January 2006 1694.%R RFC 4253 1695.%T The Secure Shell (SSH) Transport Layer Protocol 1696.Re 1697.Pp 1698.Rs 1699.%A T. Ylonen 1700.%A C. Lonvick 1701.%D January 2006 1702.%R RFC 4254 1703.%T The Secure Shell (SSH) Connection Protocol 1704.Re 1705.Pp 1706.Rs 1707.%A J. Schlyter 1708.%A W. Griffin 1709.%D January 2006 1710.%R RFC 4255 1711.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1712.Re 1713.Pp 1714.Rs 1715.%A F. Cusack 1716.%A M. Forssen 1717.%D January 2006 1718.%R RFC 4256 1719.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1720.Re 1721.Pp 1722.Rs 1723.%A J. Galbraith 1724.%A P. Remaker 1725.%D January 2006 1726.%R RFC 4335 1727.%T The Secure Shell (SSH) Session Channel Break Extension 1728.Re 1729.Pp 1730.Rs 1731.%A M. Bellare 1732.%A T. Kohno 1733.%A C. Namprempre 1734.%D January 2006 1735.%R RFC 4344 1736.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1737.Re 1738.Pp 1739.Rs 1740.%A B. Harris 1741.%D January 2006 1742.%R RFC 4345 1743.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1744.Re 1745.Pp 1746.Rs 1747.%A M. Friedl 1748.%A N. Provos 1749.%A W. Simpson 1750.%D March 2006 1751.%R RFC 4419 1752.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1753.Re 1754.Pp 1755.Rs 1756.%A J. Galbraith 1757.%A R. Thayer 1758.%D November 2006 1759.%R RFC 4716 1760.%T The Secure Shell (SSH) Public Key File Format 1761.Re 1762.Pp 1763.Rs 1764.%A D. Stebila 1765.%A J. Green 1766.%D December 2009 1767.%R RFC 5656 1768.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1769.Re 1770.Pp 1771.Rs 1772.%A A. Perrig 1773.%A D. Song 1774.%D 1999 1775.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1776.%T Hash Visualization: a New Technique to improve Real-World Security 1777.Re 1778.Sh AUTHORS 1779OpenSSH is a derivative of the original and free 1780ssh 1.2.12 release by Tatu Ylonen. 1781Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1782Theo de Raadt and Dug Song 1783removed many bugs, re-added newer features and 1784created OpenSSH. 1785Markus Friedl contributed the support for SSH 1786protocol versions 1.5 and 2.0. 1787