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 1.449 2026/07/01 00:04:46 djm Exp $ 37.Dd $Mdocdate: July 1 2026 $ 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 conninfo 490(report information about the master connection), 491.Dq channels 492(report information about open channels), 493.Dq forward 494(request forwardings without command execution), 495.Dq cancel 496(cancel forwardings), 497.Dq proxy 498(connect to a running multiplexing master in proxy mode), 499.Dq exit 500(request the master to exit), and 501.Dq stop 502(request the master to stop accepting further multiplexing requests). 503.Pp 504.It Fl o Ar option 505Can be used to give options in the format used in the configuration file. 506This is useful for specifying options for which there is no separate 507command-line flag. 508For full details of the options listed below, and their possible values, see 509.Xr ssh_config 5 . 510.Pp 511.Bl -tag -width Ds -offset indent -compact 512.It AddKeysToAgent 513.It AddressFamily 514.It BatchMode 515.It BindAddress 516.It BindInterface 517.It CASignatureAlgorithms 518.It CanonicalDomains 519.It CanonicalizeFallbackLocal 520.It CanonicalizeHostname 521.It CanonicalizeMaxDots 522.It CanonicalizePermittedCNAMEs 523.It CertificateFile 524.It ChannelTimeout 525.It CheckHostIP 526.It Ciphers 527.It ClearAllForwardings 528.It Compression 529.It ConnectTimeout 530.It ConnectionAttempts 531.It ControlMaster 532.It ControlPath 533.It ControlPersist 534.It DynamicForward 535.It EnableEscapeCommandline 536.It EnableSSHKeysign 537.It EscapeChar 538.It ExitOnForwardFailure 539.It FingerprintHash 540.It ForkAfterAuthentication 541.It ForwardAgent 542.It ForwardX11 543.It ForwardX11Timeout 544.It ForwardX11Trusted 545.It GSSAPIAuthentication 546.It GSSAPIDelegateCredentials 547.It GatewayPorts 548.It GlobalKnownHostsFile 549.It HashKnownHosts 550.It HostKeyAlgorithms 551.It HostKeyAlias 552.It HostbasedAcceptedAlgorithms 553.It HostbasedAuthentication 554.It Hostname 555.It IPQoS 556.It IdentitiesOnly 557.It IdentityAgent 558.It IdentityFile 559.It IgnoreUnknown 560.It KbdInteractiveAuthentication 561.It KbdInteractiveDevices 562.It KexAlgorithms 563.It KnownHostsCommand 564.It LocalCommand 565.It LocalForward 566.It LogLevel 567.It LogVerbose 568.It MACs 569.It NoHostAuthenticationForLocalhost 570.It NumberOfPasswordPrompts 571.It ObscureKeystrokeTiming 572.It PKCS11Provider 573.It PasswordAuthentication 574.It PermitLocalCommand 575.It PermitRemoteOpen 576.It Port 577.It PreferredAuthentications 578.It ProxyCommand 579.It ProxyJump 580.It ProxyUseFdpass 581.It PubkeyAcceptedAlgorithms 582.It PubkeyAuthentication 583.It RefuseConnection 584.It RekeyLimit 585.It RemoteCommand 586.It RemoteForward 587.It RequestTTY 588.It RequiredRSASize 589.It RevokedHostKeys 590.It SecurityKeyProvider 591.It SendEnv 592.It ServerAliveCountMax 593.It ServerAliveInterval 594.It SessionType 595.It SetEnv 596.It StdinNull 597.It StreamLocalBindMask 598.It StreamLocalBindUnlink 599.It StrictHostKeyChecking 600.It SyslogFacility 601.It TCPKeepAlive 602.It Tag 603.It Tunnel 604.It TunnelDevice 605.It UpdateHostKeys 606.It User 607.It UserKnownHostsFile 608.It VerifyHostKeyDNS 609.It VersionAddendum 610.It VisualHostKey 611.It WarnWeakCrypto 612.It XAuthLocation 613.El 614.Pp 615.It Fl P Ar tag 616Specify a tag name that may be used to select configuration in 617.Xr ssh_config 5 . 618Refer to the 619.Cm Tag 620and 621.Cm Match 622keywords in 623.Xr ssh_config 5 624for more information. 625.It Fl p Ar port 626Port to connect to on the remote host. 627This can be specified on a 628per-host basis in the configuration file. 629.Pp 630.It Fl Q Ar query_option 631Queries for the algorithms supported by one of the following features: 632.Ar cipher 633(supported symmetric ciphers), 634.Ar cipher-auth 635(supported symmetric ciphers that support authenticated encryption), 636.Ar help 637(supported query terms for use with the 638.Fl Q 639flag), 640.Ar mac 641(supported message integrity codes), 642.Ar kex 643(key exchange algorithms), 644.Ar key 645(key types), 646.Ar key-ca-sign 647(valid CA signature algorithms for certificates), 648.Ar key-cert 649(certificate key types), 650.Ar key-plain 651(non-certificate key types), 652.Ar key-sig 653(all key types and signature algorithms), 654.Ar protocol-version 655(supported SSH protocol versions), and 656.Ar sig 657(supported signature algorithms). 658Alternatively, any keyword from 659.Xr ssh_config 5 660or 661.Xr sshd_config 5 662that takes an algorithm list may be used as an alias for the corresponding 663query_option. 664.Pp 665.It Fl q 666Quiet mode. 667Causes most warning and diagnostic messages to be suppressed. 668.Pp 669.It Fl R Xo 670.Sm off 671.Oo Ar bind_address : Oc 672.Ar port : host : hostport 673.Sm on 674.Xc 675.It Fl R Xo 676.Sm off 677.Oo Ar bind_address : Oc 678.Ar port : local_socket 679.Sm on 680.Xc 681.It Fl R Xo 682.Sm off 683.Ar remote_socket : host : hostport 684.Sm on 685.Xc 686.It Fl R Xo 687.Sm off 688.Ar remote_socket : local_socket 689.Sm on 690.Xc 691.It Fl R Xo 692.Sm off 693.Oo Ar bind_address : Oc 694.Ar port 695.Sm on 696.Xc 697Specifies that connections to the given TCP port or Unix socket on the remote 698(server) host are to be forwarded to the local side. 699.Pp 700This works by allocating a socket to listen to either a TCP 701.Ar port 702or to a Unix socket on the remote side. 703Whenever a connection is made to this port or Unix socket, the 704connection is forwarded over the secure channel, and a connection 705is made from the local machine to either an explicit destination specified by 706.Ar host 707port 708.Ar hostport , 709or 710.Ar local_socket , 711or, if no explicit destination was specified, 712.Nm 713will act as a SOCKS 4/5 proxy and forward connections to the destinations 714requested by the remote SOCKS client. 715.Pp 716Port forwardings can also be specified in the configuration file. 717Privileged ports can be forwarded only when 718logging in as root on the remote machine. 719IPv6 addresses can be specified by enclosing the address in square brackets. 720.Pp 721By default, TCP listening sockets on the server will be bound to the loopback 722interface only. 723This may be overridden by specifying a 724.Ar bind_address . 725An empty 726.Ar bind_address , 727or the address 728.Ql * , 729indicates that the remote socket should listen on all interfaces. 730Specifying a remote 731.Ar bind_address 732will only succeed if the server's 733.Cm GatewayPorts 734option is enabled (see 735.Xr sshd_config 5 ) . 736.Pp 737If the 738.Ar port 739argument is 740.Ql 0 , 741the listen port will be dynamically allocated on the server and reported 742to the client at run time. 743When used together with 744.Ic -O forward , 745the allocated port will be printed to the standard output. 746.Pp 747.It Fl S Ar ctl_path 748Specifies the location of a control socket for connection sharing, 749or the string 750.Dq none 751to disable connection sharing. 752Refer to the description of 753.Cm ControlPath 754and 755.Cm ControlMaster 756in 757.Xr ssh_config 5 758for details. 759.Pp 760.It Fl s 761May be used to request invocation of a subsystem on the remote system. 762Subsystems facilitate the use of SSH 763as a secure transport for other applications (e.g.\& 764.Xr sftp 1 ) . 765The subsystem is specified as the remote command. 766Refer to the description of 767.Cm SessionType 768in 769.Xr ssh_config 5 770for details. 771.Pp 772.It Fl T 773Disable pseudo-terminal allocation. 774.Pp 775.It Fl t 776Force pseudo-terminal allocation. 777This can be used to execute arbitrary 778screen-based programs on a remote machine, which can be very useful, 779e.g. when implementing menu services. 780Multiple 781.Fl t 782options force tty allocation, even if 783.Nm 784has no local tty. 785.Pp 786.It Fl V 787Display the version number and exit. 788.Pp 789.It Fl v 790Verbose mode. 791Causes 792.Nm 793to print debugging messages about its progress. 794This is helpful in 795debugging connection, authentication, and configuration problems. 796Multiple 797.Fl v 798options increase the verbosity. 799The maximum is 3. 800.Pp 801.It Fl W Ar host : Ns Ar port 802Requests that standard input and output on the client be forwarded to 803.Ar host 804on 805.Ar port 806over the secure channel. 807Implies 808.Fl N , 809.Fl T , 810.Cm ExitOnForwardFailure 811and 812.Cm ClearAllForwardings , 813though these can be overridden in the configuration file or using 814.Fl o 815command line options. 816.Pp 817.It Fl w Xo 818.Ar local_tun Ns Op : Ns Ar remote_tun 819.Xc 820Requests 821tunnel 822device forwarding with the specified 823.Xr tun 4 824devices between the client 825.Pq Ar local_tun 826and the server 827.Pq Ar remote_tun . 828.Pp 829The devices may be specified by numerical ID or the keyword 830.Dq any , 831which uses the next available tunnel device. 832If 833.Ar remote_tun 834is not specified, it defaults to 835.Dq any . 836See also the 837.Cm Tunnel 838and 839.Cm TunnelDevice 840directives in 841.Xr ssh_config 5 . 842.Pp 843If the 844.Cm Tunnel 845directive is unset, it will be set to the default tunnel mode, which is 846.Dq point-to-point . 847If a different 848.Cm Tunnel 849forwarding mode it desired, then it should be specified before 850.Fl w . 851.Pp 852.It Fl X 853Enables X11 forwarding. 854This can also be specified on a per-host basis in a configuration file. 855.Pp 856X11 forwarding should be enabled with caution. 857Users with the ability to bypass file permissions on the remote host 858(for the user's X authorization database) 859can access the local X11 display through the forwarded connection. 860An attacker may then be able to perform activities such as keystroke monitoring. 861.Pp 862For this reason, X11 forwarding is subjected to X11 SECURITY extension 863restrictions by default. 864Refer to the 865.Nm 866.Fl Y 867option and the 868.Cm ForwardX11Trusted 869directive in 870.Xr ssh_config 5 871for more information. 872.Pp 873.It Fl x 874Disables X11 forwarding. 875.Pp 876.It Fl Y 877Enables trusted X11 forwarding. 878Trusted X11 forwardings are not subjected to the X11 SECURITY extension 879controls. 880.Pp 881.It Fl y 882Send log information using the 883.Xr syslog 3 884system module. 885By default this information is sent to stderr. 886.El 887.Pp 888.Nm 889may additionally obtain configuration data from 890a per-user configuration file and a system-wide configuration file. 891The file format and configuration options are described in 892.Xr ssh_config 5 . 893.Sh AUTHENTICATION 894The OpenSSH SSH client supports SSH protocol 2. 895.Pp 896The methods available for authentication are: 897GSSAPI-based authentication, 898host-based authentication, 899public key authentication, 900keyboard-interactive authentication, 901and password authentication. 902Authentication methods are tried in the order specified above, 903though 904.Cm PreferredAuthentications 905can be used to change the default order. 906.Pp 907Host-based authentication works as follows: 908If the machine the user logs in from is listed in 909.Pa /etc/hosts.equiv 910or 911.Pa /etc/shosts.equiv 912on the remote machine, the user is non-root and the user names are 913the same on both sides, or if the files 914.Pa ~/.rhosts 915or 916.Pa ~/.shosts 917exist in the user's home directory on the 918remote machine and contain a line containing the name of the client 919machine and the name of the user on that machine, the user is 920considered for login. 921Additionally, the server 922.Em must 923be able to verify the client's 924host key (see the description of 925.Pa /etc/ssh/ssh_known_hosts 926and 927.Pa ~/.ssh/known_hosts , 928below) 929for login to be permitted. 930This authentication method closes security holes due to IP 931spoofing, DNS spoofing, and routing spoofing. 932[Note to the administrator: 933.Pa /etc/hosts.equiv , 934.Pa ~/.rhosts , 935and the rlogin/rsh protocol in general, are inherently insecure and should be 936disabled if security is desired.] 937.Pp 938Public key authentication works as follows: 939The scheme is based on public-key cryptography, 940using cryptosystems 941where encryption and decryption are done using separate keys, 942and it is unfeasible to derive the decryption key from the encryption key. 943The idea is that each user creates a public/private 944key pair for authentication purposes. 945The server knows the public key, and only the user knows the private key. 946.Nm 947implements public key authentication protocol automatically, 948using one of the ECDSA, Ed25519 or RSA algorithms. 949.Pp 950The file 951.Pa ~/.ssh/authorized_keys 952lists the public keys that are permitted for logging in. 953When the user logs in, the 954.Nm 955program tells the server which key pair it would like to use for 956authentication. 957The client proves that it has access to the private key 958and the server checks that the corresponding public key 959is authorized to accept the account. 960.Pp 961The server may inform the client of errors that prevented public key 962authentication from succeeding after authentication completes using a 963different method. 964These may be viewed by increasing the 965.Cm LogLevel 966to 967.Cm DEBUG 968or higher (e.g. by using the 969.Fl v 970flag). 971.Pp 972The user creates their key pair by running 973.Xr ssh-keygen 1 . 974This stores the private key in 975.Pa ~/.ssh/id_ecdsa 976(ECDSA), 977.Pa ~/.ssh/id_ecdsa_sk 978(authenticator-hosted ECDSA), 979.Pa ~/.ssh/id_ed25519 980(Ed25519), 981.Pa ~/.ssh/id_ed25519_sk 982(authenticator-hosted Ed25519), 983or 984.Pa ~/.ssh/id_rsa 985(RSA) 986and stores the public key in 987.Pa ~/.ssh/id_ecdsa.pub 988(ECDSA), 989.Pa ~/.ssh/id_ecdsa_sk.pub 990(authenticator-hosted ECDSA), 991.Pa ~/.ssh/id_ed25519.pub 992(Ed25519), 993.Pa ~/.ssh/id_ed25519_sk.pub 994(authenticator-hosted Ed25519), 995or 996.Pa ~/.ssh/id_rsa.pub 997(RSA) 998in the user's home directory. 999The user should then copy the public key 1000to 1001.Pa ~/.ssh/authorized_keys 1002in their home directory on the remote machine. 1003The 1004.Pa authorized_keys 1005file corresponds to the conventional 1006.Pa ~/.rhosts 1007file, and has one key 1008per line, though the lines can be very long. 1009After this, the user can log in without giving the password. 1010.Pp 1011A variation on public key authentication 1012is available in the form of certificate authentication: 1013instead of a set of public/private keys, 1014signed certificates are used. 1015This has the advantage that a single trusted certification authority 1016can be used in place of many public/private keys. 1017See the CERTIFICATES section of 1018.Xr ssh-keygen 1 1019for more information. 1020.Pp 1021The most convenient way to use public key or certificate authentication 1022may be with an authentication agent. 1023See 1024.Xr ssh-agent 1 1025and (optionally) the 1026.Cm AddKeysToAgent 1027directive in 1028.Xr ssh_config 5 1029for more information. 1030.Pp 1031Keyboard-interactive authentication works as follows: 1032The server sends an arbitrary 1033.Qq challenge 1034text and prompts for a response, possibly multiple times. 1035Examples of keyboard-interactive authentication include 1036.Bx 1037Authentication (see 1038.Xr login.conf 5 ) 1039and PAM (some 1040.Pf non- Ox 1041systems). 1042.Pp 1043Finally, if other authentication methods fail, 1044.Nm 1045prompts the user for a password. 1046The password is sent to the remote 1047host for checking; however, since all communications are encrypted, 1048the password cannot be seen by someone listening on the network. 1049.Pp 1050.Nm 1051automatically maintains and checks a database containing 1052identification for all hosts it has ever been used with. 1053Host keys are stored in 1054.Pa ~/.ssh/known_hosts 1055in the user's home directory. 1056Additionally, the file 1057.Pa /etc/ssh/ssh_known_hosts 1058is automatically checked for known hosts. 1059Any new hosts are automatically added to the user's file. 1060If a host's identification ever changes, 1061.Nm 1062warns about this and disables password authentication to prevent 1063server spoofing or man-in-the-middle attacks, 1064which could otherwise be used to circumvent the encryption. 1065The 1066.Cm StrictHostKeyChecking 1067option can be used to control logins to machines whose 1068host key is not known or has changed. 1069.Pp 1070When the user's identity has been accepted by the server, the server 1071either executes the given command in a non-interactive session or, 1072if no command has been specified, logs into the machine and gives 1073the user a normal shell as an interactive session. 1074All communication with 1075the remote command or shell will be automatically encrypted. 1076.Pp 1077If an interactive session is requested, 1078.Nm 1079by default will only request a pseudo-terminal (pty) for interactive 1080sessions when the client has one. 1081The flags 1082.Fl T 1083and 1084.Fl t 1085can be used to override this behaviour. 1086.Pp 1087If a pseudo-terminal has been allocated, the 1088user may use the escape characters noted below. 1089.Pp 1090If no pseudo-terminal has been allocated, 1091the session is transparent and can be used to reliably transfer binary data. 1092On most systems, setting the escape character to 1093.Dq none 1094will also make the session transparent even if a tty is used. 1095.Pp 1096The session terminates when the command or shell on the remote 1097machine exits and all X11 and TCP connections have been closed. 1098.Sh ESCAPE CHARACTERS 1099When a pseudo-terminal has been requested, 1100.Nm 1101supports a number of functions through the use of an escape character. 1102.Pp 1103A single tilde character can be sent as 1104.Ic ~~ 1105or by following the tilde by a character other than those described below. 1106The escape character must always follow a newline to be interpreted as 1107special. 1108The escape character can be changed in configuration files using the 1109.Cm EscapeChar 1110configuration directive or on the command line by the 1111.Fl e 1112option. 1113.Pp 1114The supported escapes (assuming the default 1115.Ql ~ ) 1116are: 1117.Bl -tag -width Ds 1118.It Cm ~. 1119Disconnect. 1120.It Cm ~^Z 1121Background 1122.Nm . 1123.It Cm ~# 1124List forwarded connections. 1125.It Cm ~& 1126Background 1127.Nm 1128at logout when waiting for forwarded connection / X11 sessions to terminate. 1129.It Cm ~? 1130Display a list of escape characters. 1131.It Cm ~B 1132Send a BREAK to the remote system 1133(only useful if the peer supports it). 1134.It Cm ~C 1135Open command line. 1136Currently this allows the addition of port forwardings using the 1137.Fl L , 1138.Fl R 1139and 1140.Fl D 1141options (see above). 1142It also allows the cancellation of existing port-forwardings 1143with 1144.Sm off 1145.Fl KL Oo Ar bind_address : Oc Ar port 1146.Sm on 1147for local, 1148.Sm off 1149.Fl KR Oo Ar bind_address : Oc Ar port 1150.Sm on 1151for remote and 1152.Sm off 1153.Fl KD Oo Ar bind_address : Oc Ar port 1154.Sm on 1155for dynamic port-forwardings. 1156.Ic !\& Ns Ar command 1157allows the user to execute a local command if the 1158.Ic PermitLocalCommand 1159option is enabled in 1160.Xr ssh_config 5 . 1161Basic help is available, using the 1162.Fl h 1163option. 1164.It Cm ~I 1165Show information about the current SSH connection. 1166.It Cm ~R 1167Request rekeying of the connection 1168(only useful if the peer supports it). 1169.It Cm ~V 1170Decrease the verbosity 1171.Pq Ic LogLevel 1172when errors are being written to stderr. 1173.It Cm ~v 1174Increase the verbosity 1175.Pq Ic LogLevel 1176when errors are being written to stderr. 1177.El 1178.Sh TCP FORWARDING 1179Forwarding of arbitrary TCP connections over a secure channel 1180can be specified either on the command line or in a configuration file. 1181One possible application of TCP forwarding is a secure connection to a 1182mail server; another is going through firewalls. 1183.Pp 1184In the example below, we look at encrypting communication for an IRC client, 1185even though the IRC server it connects to does not directly 1186support encrypted communication. 1187This works as follows: 1188the user connects to the remote host using 1189.Nm , 1190specifying the ports to be used to forward the connection. 1191After that it is possible to start the program locally, 1192and 1193.Nm 1194will encrypt and forward the connection to the remote server. 1195.Pp 1196The following example tunnels an IRC session from the client 1197to an IRC server at 1198.Dq server.example.com , 1199joining channel 1200.Dq #users , 1201nickname 1202.Dq pinky , 1203using the standard IRC port, 6667: 1204.Bd -literal -offset 4n 1205$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1206$ irc -c '#users' pinky IRC/127.0.0.1 1207.Ed 1208.Pp 1209The 1210.Fl f 1211option backgrounds 1212.Nm 1213and the remote command 1214.Dq sleep 10 1215is specified to allow an amount of time 1216(10 seconds, in the example) 1217to start the program which is going to use the tunnel. 1218If no connections are made within the time specified, 1219.Nm 1220will exit. 1221.Sh X11 FORWARDING 1222If the 1223.Cm ForwardX11 1224variable is set to 1225.Dq yes 1226(or see the description of the 1227.Fl X , 1228.Fl x , 1229and 1230.Fl Y 1231options above) 1232and the user is using X11 (the 1233.Ev DISPLAY 1234environment variable is set), the connection to the X11 display is 1235automatically forwarded to the remote side in such a way that any X11 1236programs started from the shell (or command) will go through the 1237encrypted channel, and the connection to the real X server will be made 1238from the local machine. 1239The user should not manually set 1240.Ev DISPLAY . 1241Forwarding of X11 connections can be 1242configured on the command line or in configuration files. 1243.Pp 1244The 1245.Ev DISPLAY 1246value set by 1247.Nm 1248will point to the server machine, but with a display number greater than zero. 1249This is normal, and happens because 1250.Nm 1251creates a 1252.Dq proxy 1253X server on the server machine for forwarding the 1254connections over the encrypted channel. 1255.Pp 1256.Nm 1257will also automatically set up Xauthority data on the server machine. 1258For this purpose, it will generate a random authorization cookie, 1259store it in Xauthority on the server, and verify that any forwarded 1260connections carry this cookie and replace it by the real cookie when 1261the connection is opened. 1262The real authentication cookie is never 1263sent to the server machine (and no cookies are sent in the plain). 1264.Pp 1265If the 1266.Cm ForwardAgent 1267variable is set to 1268.Dq yes 1269(or see the description of the 1270.Fl A 1271and 1272.Fl a 1273options above) and 1274the user is using an authentication agent, the connection to the agent 1275is automatically forwarded to the remote side. 1276.Sh VERIFYING HOST KEYS 1277When connecting to a server for the first time, 1278a fingerprint of the server's public key is presented to the user 1279(unless the option 1280.Cm StrictHostKeyChecking 1281has been disabled). 1282Fingerprints can be determined using 1283.Xr ssh-keygen 1 : 1284.Pp 1285.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1286.Pp 1287If the fingerprint is already known, it can be matched 1288and the key can be accepted or rejected. 1289If only legacy (MD5) fingerprints for the server are available, the 1290.Xr ssh-keygen 1 1291.Fl E 1292option may be used to downgrade the fingerprint algorithm to match. 1293.Pp 1294Because of the difficulty of comparing host keys 1295just by looking at fingerprint strings, 1296there is also support to compare host keys visually, 1297using 1298.Em random art . 1299By setting the 1300.Cm VisualHostKey 1301option to 1302.Dq yes , 1303a small ASCII graphic gets displayed on every login to a server, no matter 1304if the session itself is interactive or not. 1305By learning the pattern a known server produces, a user can easily 1306find out that the host key has changed when a completely different pattern 1307is displayed. 1308Because these patterns are not unambiguous however, a pattern that looks 1309similar to the pattern remembered only gives a good probability that the 1310host key is the same, not guaranteed proof. 1311.Pp 1312To get a listing of the fingerprints along with their random art for 1313all known hosts, the following command line can be used: 1314.Pp 1315.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1316.Pp 1317If the fingerprint is unknown, 1318an alternative method of verification is available: 1319SSH fingerprints verified by DNS. 1320An additional resource record (RR), 1321SSHFP, 1322is added to a zonefile 1323and the connecting client is able to match the fingerprint 1324with that of the key presented. 1325.Pp 1326In this example, we are connecting a client to a server, 1327.Dq host.example.com . 1328The SSHFP resource records should first be added to the zonefile for 1329host.example.com: 1330.Bd -literal -offset indent 1331$ ssh-keygen -r host.example.com. 1332.Ed 1333.Pp 1334The output lines will have to be added to the zonefile. 1335To check that the zone is answering fingerprint queries: 1336.Pp 1337.Dl $ dig -t SSHFP host.example.com 1338.Pp 1339Finally the client connects: 1340.Bd -literal -offset indent 1341$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1342[...] 1343Matching host key fingerprint found in DNS. 1344Are you sure you want to continue connecting (yes/no)? 1345.Ed 1346.Pp 1347See the 1348.Cm VerifyHostKeyDNS 1349option in 1350.Xr ssh_config 5 1351for more information. 1352.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1353.Nm 1354contains support for Virtual Private Network (VPN) tunnelling 1355using the 1356.Xr tun 4 1357network pseudo-device, 1358allowing two networks to be joined securely. 1359The 1360.Xr sshd_config 5 1361configuration option 1362.Cm PermitTunnel 1363controls whether the server supports this, 1364and at what level (layer 2 or 3 traffic). 1365.Pp 1366The following example would connect client network 10.0.50.0/24 1367with remote network 10.0.99.0/24 using a point-to-point connection 1368from 10.1.1.1 to 10.1.1.2, 1369provided that the SSH server running on the gateway to the remote network, 1370at 192.168.1.15, allows it. 1371.Pp 1372On the client: 1373.Bd -literal -offset indent 1374# ssh -f -w 0:1 192.168.1.15 true 1375# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1376# route add 10.0.99.0/24 10.1.1.2 1377.Ed 1378.Pp 1379On the server: 1380.Bd -literal -offset indent 1381# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1382# route add 10.0.50.0/24 10.1.1.1 1383.Ed 1384.Pp 1385Client access may be more finely tuned via the 1386.Pa /root/.ssh/authorized_keys 1387file (see below) and the 1388.Cm PermitRootLogin 1389server option. 1390The following entry would permit connections on 1391.Xr tun 4 1392device 1 from user 1393.Dq jane 1394and on tun device 2 from user 1395.Dq john , 1396if 1397.Cm PermitRootLogin 1398is set to 1399.Dq forced-commands-only : 1400.Bd -literal -offset 2n 1401tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1402tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1403.Ed 1404.Pp 1405Since an SSH-based setup entails a fair amount of overhead, 1406it may be more suited to temporary setups, 1407such as for wireless VPNs. 1408More permanent VPNs are better provided by tools such as 1409.Xr ipsecctl 8 1410and 1411.Xr isakmpd 8 . 1412.Sh ENVIRONMENT 1413.Nm 1414will normally set the following environment variables: 1415.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1416.It Ev DISPLAY 1417The 1418.Ev DISPLAY 1419variable indicates the location of the X11 server. 1420It is automatically set by 1421.Nm 1422to point to a value of the form 1423.Dq hostname:n , 1424where 1425.Dq hostname 1426indicates the host where the shell runs, and 1427.Sq n 1428is an integer \*(Ge 1. 1429.Nm 1430uses this special value to forward X11 connections over the secure 1431channel. 1432The user should normally not set 1433.Ev DISPLAY 1434explicitly, as that 1435will render the X11 connection insecure (and will require the user to 1436manually copy any required authorization cookies). 1437.It Ev HOME 1438Set to the path of the user's home directory. 1439.It Ev LOGNAME 1440Synonym for 1441.Ev USER ; 1442set for compatibility with systems that use this variable. 1443.It Ev MAIL 1444Set to the path of the user's mailbox. 1445.It Ev PATH 1446Set to the default 1447.Ev PATH , 1448as specified when compiling 1449.Nm . 1450.It Ev SSH_ASKPASS 1451If 1452.Nm 1453needs a passphrase, it will read the passphrase from the current 1454terminal if it was run from a terminal. 1455If 1456.Nm 1457does not have a terminal associated with it but 1458.Ev DISPLAY 1459and 1460.Ev SSH_ASKPASS 1461are set, it will execute the program specified by 1462.Ev SSH_ASKPASS 1463and open an X11 window to read the passphrase. 1464This is particularly useful when calling 1465.Nm 1466from a 1467.Pa .xsession 1468or related script. 1469(Note that on some machines it 1470may be necessary to redirect the input from 1471.Pa /dev/null 1472to make this work.) 1473.It Ev SSH_ASKPASS_REQUIRE 1474Allows further control over the use of an askpass program. 1475If this variable is set to 1476.Dq never 1477then 1478.Nm 1479will never attempt to use one. 1480If it is set to 1481.Dq prefer , 1482then 1483.Nm 1484will prefer to use the askpass program instead of the TTY when requesting 1485passwords. 1486Finally, if the variable is set to 1487.Dq force , 1488then the askpass program will be used for all passphrase input regardless 1489of whether 1490.Ev DISPLAY 1491is set. 1492.It Ev SSH_AUTH_SOCK 1493Identifies the path of a 1494.Ux Ns -domain 1495socket used to communicate with the agent. 1496.It Ev SSH_CONNECTION 1497Identifies the client and server ends of the connection. 1498The variable contains 1499four space-separated values: client IP address, client port number, 1500server IP address, and server port number. 1501.It Ev SSH_ORIGINAL_COMMAND 1502This variable contains the original command line if a forced command 1503is executed. 1504It can be used to extract the original arguments. 1505.It Ev SSH_TTY 1506This is set to the name of the tty (path to the device) associated 1507with the current shell or command. 1508If the current session has no tty, 1509this variable is not set. 1510.It Ev SSH_TUNNEL 1511Optionally set by 1512.Xr sshd 8 1513to contain the interface names assigned if tunnel forwarding was 1514requested by the client. 1515.It Ev SSH_USER_AUTH 1516Optionally set by 1517.Xr sshd 8 , 1518this variable may contain a pathname to a file that lists the authentication 1519methods successfully used when the session was established, including any 1520public keys that were used. 1521.It Ev TZ 1522This variable is set to indicate the present time zone if it 1523was set when the daemon was started (i.e. the daemon passes the value 1524on to new connections). 1525.It Ev USER 1526Set to the name of the user logging in. 1527.El 1528.Pp 1529Additionally, 1530.Nm 1531reads 1532.Pa ~/.ssh/environment , 1533and adds lines of the format 1534.Dq VARNAME=value 1535to the environment if the file exists and users are allowed to 1536change their environment. 1537For more information, see the 1538.Cm PermitUserEnvironment 1539option in 1540.Xr sshd_config 5 . 1541.Sh FILES 1542.Bl -tag -width Ds -compact 1543.It Pa ~/.rhosts 1544This file is used for host-based authentication (see above). 1545On some machines this file may need to be 1546world-readable if the user's home directory is on an NFS partition, 1547because 1548.Xr sshd 8 1549reads it as root. 1550Additionally, this file must be owned by the user, 1551and must not have write permissions for anyone else. 1552The recommended 1553permission for most machines is read/write for the user, and not 1554accessible by others. 1555.Pp 1556.It Pa ~/.shosts 1557This file is used in exactly the same way as 1558.Pa .rhosts , 1559but allows host-based authentication without permitting login with 1560rlogin/rsh. 1561.Pp 1562.It Pa ~/.ssh/ 1563This directory is the default location for all user-specific configuration 1564and authentication information. 1565There is no general requirement to keep the entire contents of this directory 1566secret, but the recommended permissions are read/write/execute for the user, 1567and not accessible by others. 1568.Pp 1569.It Pa ~/.ssh/authorized_keys 1570Lists the public keys (ECDSA, Ed25519, RSA) 1571that can be used for logging in as this user. 1572The format of this file is described in the 1573.Xr sshd 8 1574manual page. 1575This file is not highly sensitive, but the recommended 1576permissions are read/write for the user, and not accessible by others. 1577.Pp 1578.It Pa ~/.ssh/config 1579This is the per-user configuration file. 1580The file format and configuration options are described in 1581.Xr ssh_config 5 . 1582Because of the potential for abuse, this file must have strict permissions: 1583read/write for the user, and not writable by others. 1584.Pp 1585.It Pa ~/.ssh/environment 1586Contains additional definitions for environment variables; see 1587.Sx ENVIRONMENT , 1588above. 1589.Pp 1590.It Pa ~/.ssh/id_ecdsa 1591.It Pa ~/.ssh/id_ecdsa_sk 1592.It Pa ~/.ssh/id_ed25519 1593.It Pa ~/.ssh/id_ed25519_sk 1594.It Pa ~/.ssh/id_rsa 1595Contains the private key for authentication. 1596These files 1597contain sensitive data and should be readable by the user but not 1598accessible by others (read/write/execute). 1599.Nm 1600will simply ignore a private key file if it is accessible by others. 1601It is possible to specify a passphrase when 1602generating the key which will be used to encrypt the 1603sensitive part of this file using AES-128. 1604.Pp 1605.It Pa ~/.ssh/id_ecdsa.pub 1606.It Pa ~/.ssh/id_ecdsa_sk.pub 1607.It Pa ~/.ssh/id_ed25519.pub 1608.It Pa ~/.ssh/id_ed25519_sk.pub 1609.It Pa ~/.ssh/id_rsa.pub 1610Contains the public key for authentication. 1611These files are not 1612sensitive and can (but need not) be readable by anyone. 1613.Pp 1614.It Pa ~/.ssh/known_hosts 1615Contains a list of host keys for all hosts the user has logged into 1616that are not already in the systemwide list of known host keys. 1617See 1618.Xr sshd 8 1619for further details of the format of this file. 1620.Pp 1621.It Pa ~/.ssh/rc 1622Commands in this file are executed by 1623.Nm 1624when the user logs in, just before the user's shell (or command) is 1625started. 1626See the 1627.Xr sshd 8 1628manual page for more information. 1629.Pp 1630.It Pa /etc/hosts.equiv 1631This file is for host-based authentication (see above). 1632It should only be writable by root. 1633.Pp 1634.It Pa /etc/shosts.equiv 1635This file is used in exactly the same way as 1636.Pa hosts.equiv , 1637but allows host-based authentication without permitting login with 1638rlogin/rsh. 1639.Pp 1640.It Pa /etc/ssh/ssh_config 1641Systemwide configuration file. 1642The file format and configuration options are described in 1643.Xr ssh_config 5 . 1644.Pp 1645.It Pa /etc/ssh/ssh_host_ecdsa_key 1646.It Pa /etc/ssh/ssh_host_ed25519_key 1647.It Pa /etc/ssh/ssh_host_rsa_key 1648These files contain the private parts of the host keys 1649and are used for host-based authentication. 1650.Pp 1651.It Pa /etc/ssh/ssh_known_hosts 1652Systemwide list of known host keys. 1653This file should be prepared by the 1654system administrator to contain the public host keys of all machines in the 1655organization. 1656It should be world-readable. 1657See 1658.Xr sshd 8 1659for further details of the format of this file. 1660.Pp 1661.It Pa /etc/ssh/sshrc 1662Commands in this file are executed by 1663.Nm 1664when the user logs in, just before the user's shell (or command) is started. 1665See the 1666.Xr sshd 8 1667manual page for more information. 1668.El 1669.Sh EXIT STATUS 1670.Nm 1671exits with the exit status of the remote command or with 255 1672if an error occurred. 1673.Sh SEE ALSO 1674.Xr scp 1 , 1675.Xr sftp 1 , 1676.Xr ssh-add 1 , 1677.Xr ssh-agent 1 , 1678.Xr ssh-keygen 1 , 1679.Xr ssh-keyscan 1 , 1680.Xr tun 4 , 1681.Xr ssh_config 5 , 1682.Xr ssh-keysign 8 , 1683.Xr sshd 8 1684.Sh STANDARDS 1685.Rs 1686.%A S. Lehtinen 1687.%A C. Lonvick 1688.%D January 2006 1689.%R RFC 4250 1690.%T The Secure Shell (SSH) Protocol Assigned Numbers 1691.Re 1692.Pp 1693.Rs 1694.%A T. Ylonen 1695.%A C. Lonvick 1696.%D January 2006 1697.%R RFC 4251 1698.%T The Secure Shell (SSH) Protocol Architecture 1699.Re 1700.Pp 1701.Rs 1702.%A T. Ylonen 1703.%A C. Lonvick 1704.%D January 2006 1705.%R RFC 4252 1706.%T The Secure Shell (SSH) Authentication Protocol 1707.Re 1708.Pp 1709.Rs 1710.%A T. Ylonen 1711.%A C. Lonvick 1712.%D January 2006 1713.%R RFC 4253 1714.%T The Secure Shell (SSH) Transport Layer Protocol 1715.Re 1716.Pp 1717.Rs 1718.%A T. Ylonen 1719.%A C. Lonvick 1720.%D January 2006 1721.%R RFC 4254 1722.%T The Secure Shell (SSH) Connection Protocol 1723.Re 1724.Pp 1725.Rs 1726.%A J. Schlyter 1727.%A W. Griffin 1728.%D January 2006 1729.%R RFC 4255 1730.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1731.Re 1732.Pp 1733.Rs 1734.%A F. Cusack 1735.%A M. Forssen 1736.%D January 2006 1737.%R RFC 4256 1738.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1739.Re 1740.Pp 1741.Rs 1742.%A J. Galbraith 1743.%A P. Remaker 1744.%D January 2006 1745.%R RFC 4335 1746.%T The Secure Shell (SSH) Session Channel Break Extension 1747.Re 1748.Pp 1749.Rs 1750.%A M. Bellare 1751.%A T. Kohno 1752.%A C. Namprempre 1753.%D January 2006 1754.%R RFC 4344 1755.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1756.Re 1757.Pp 1758.Rs 1759.%A B. Harris 1760.%D January 2006 1761.%R RFC 4345 1762.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1763.Re 1764.Pp 1765.Rs 1766.%A M. Friedl 1767.%A N. Provos 1768.%A W. Simpson 1769.%D March 2006 1770.%R RFC 4419 1771.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1772.Re 1773.Pp 1774.Rs 1775.%A J. Galbraith 1776.%A R. Thayer 1777.%D November 2006 1778.%R RFC 4716 1779.%T The Secure Shell (SSH) Public Key File Format 1780.Re 1781.Pp 1782.Rs 1783.%A D. Stebila 1784.%A J. Green 1785.%D December 2009 1786.%R RFC 5656 1787.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1788.Re 1789.Pp 1790.Rs 1791.%A A. Perrig 1792.%A D. Song 1793.%D 1999 1794.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1795.%T Hash Visualization: a New Technique to improve Real-World Security 1796.Re 1797.Sh AUTHORS 1798OpenSSH is a derivative of the original and free 1799ssh 1.2.12 release by Tatu Ylonen. 1800Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1801Theo de Raadt and Dug Song 1802removed many bugs, re-added newer features and 1803created OpenSSH. 1804Markus Friedl contributed the support for SSH 1805protocol versions 1.5 and 2.0. 1806