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 1.444 2024/12/04 14:37:55 djm Exp $ 37.Dd $Mdocdate: December 4 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 proxy 494(connect to a running multiplexing master in proxy mode), 495.Dq exit 496(request the master to exit), and 497.Dq stop 498(request the master to stop accepting further multiplexing requests). 499.Pp 500.It Fl o Ar option 501Can be used to give options in the format used in the configuration file. 502This is useful for specifying options for which there is no separate 503command-line flag. 504For full details of the options listed below, and their possible values, see 505.Xr ssh_config 5 . 506.Pp 507.Bl -tag -width Ds -offset indent -compact 508.It AddKeysToAgent 509.It AddressFamily 510.It BatchMode 511.It BindAddress 512.It BindInterface 513.It CASignatureAlgorithms 514.It CanonicalDomains 515.It CanonicalizeFallbackLocal 516.It CanonicalizeHostname 517.It CanonicalizeMaxDots 518.It CanonicalizePermittedCNAMEs 519.It CertificateFile 520.It ChannelTimeout 521.It CheckHostIP 522.It Ciphers 523.It ClearAllForwardings 524.It Compression 525.It ConnectTimeout 526.It ConnectionAttempts 527.It ControlMaster 528.It ControlPath 529.It ControlPersist 530.It DynamicForward 531.It EnableEscapeCommandline 532.It EnableSSHKeysign 533.It EscapeChar 534.It ExitOnForwardFailure 535.It FingerprintHash 536.It ForkAfterAuthentication 537.It ForwardAgent 538.It ForwardX11 539.It ForwardX11Timeout 540.It ForwardX11Trusted 541.It GSSAPIAuthentication 542.It GSSAPIDelegateCredentials 543.It GatewayPorts 544.It GlobalKnownHostsFile 545.It HashKnownHosts 546.It Host 547.It HostKeyAlgorithms 548.It HostKeyAlias 549.It HostbasedAcceptedAlgorithms 550.It HostbasedAuthentication 551.It Hostname 552.It IPQoS 553.It IdentitiesOnly 554.It IdentityAgent 555.It IdentityFile 556.It IgnoreUnknown 557.It Include 558.It KbdInteractiveAuthentication 559.It KbdInteractiveDevices 560.It KexAlgorithms 561.It KnownHostsCommand 562.It LocalCommand 563.It LocalForward 564.It LogLevel 565.It LogVerbose 566.It MACs 567.It NoHostAuthenticationForLocalhost 568.It NumberOfPasswordPrompts 569.It ObscureKeystrokeTiming 570.It PKCS11Provider 571.It PasswordAuthentication 572.It PermitLocalCommand 573.It PermitRemoteOpen 574.It Port 575.It PreferredAuthentications 576.It ProxyCommand 577.It ProxyJump 578.It ProxyUseFdpass 579.It PubkeyAcceptedAlgorithms 580.It PubkeyAuthentication 581.It RekeyLimit 582.It RemoteCommand 583.It RemoteForward 584.It RequestTTY 585.It RequiredRSASize 586.It RevokedHostKeys 587.It SecurityKeyProvider 588.It SendEnv 589.It ServerAliveCountMax 590.It ServerAliveInterval 591.It SessionType 592.It SetEnv 593.It StdinNull 594.It StreamLocalBindMask 595.It StreamLocalBindUnlink 596.It StrictHostKeyChecking 597.It SyslogFacility 598.It TCPKeepAlive 599.It Tag 600.It Tunnel 601.It TunnelDevice 602.It UpdateHostKeys 603.It User 604.It UserKnownHostsFile 605.It VerifyHostKeyDNS 606.It VisualHostKey 607.It XAuthLocation 608.El 609.Pp 610.It Fl P Ar tag 611Specify a tag name that may be used to select configuration in 612.Xr ssh_config 5 . 613Refer to the 614.Cm Tag 615and 616.Cm Match 617keywords in 618.Xr ssh_config 5 619for more information. 620.It Fl p Ar port 621Port to connect to on the remote host. 622This can be specified on a 623per-host basis in the configuration file. 624.Pp 625.It Fl Q Ar query_option 626Queries for the algorithms supported by one of the following features: 627.Ar cipher 628(supported symmetric ciphers), 629.Ar cipher-auth 630(supported symmetric ciphers that support authenticated encryption), 631.Ar help 632(supported query terms for use with the 633.Fl Q 634flag), 635.Ar mac 636(supported message integrity codes), 637.Ar kex 638(key exchange algorithms), 639.Ar key 640(key types), 641.Ar key-ca-sign 642(valid CA signature algorithms for certificates), 643.Ar key-cert 644(certificate key types), 645.Ar key-plain 646(non-certificate key types), 647.Ar key-sig 648(all key types and signature algorithms), 649.Ar protocol-version 650(supported SSH protocol versions), and 651.Ar sig 652(supported signature algorithms). 653Alternatively, any keyword from 654.Xr ssh_config 5 655or 656.Xr sshd_config 5 657that takes an algorithm list may be used as an alias for the corresponding 658query_option. 659.Pp 660.It Fl q 661Quiet mode. 662Causes most warning and diagnostic messages to be suppressed. 663.Pp 664.It Fl R Xo 665.Sm off 666.Oo Ar bind_address : Oc 667.Ar port : host : hostport 668.Sm on 669.Xc 670.It Fl R Xo 671.Sm off 672.Oo Ar bind_address : Oc 673.Ar port : local_socket 674.Sm on 675.Xc 676.It Fl R Xo 677.Sm off 678.Ar remote_socket : host : hostport 679.Sm on 680.Xc 681.It Fl R Xo 682.Sm off 683.Ar remote_socket : local_socket 684.Sm on 685.Xc 686.It Fl R Xo 687.Sm off 688.Oo Ar bind_address : Oc 689.Ar port 690.Sm on 691.Xc 692Specifies that connections to the given TCP port or Unix socket on the remote 693(server) host are to be forwarded to the local side. 694.Pp 695This works by allocating a socket to listen to either a TCP 696.Ar port 697or to a Unix socket on the remote side. 698Whenever a connection is made to this port or Unix socket, the 699connection is forwarded over the secure channel, and a connection 700is made from the local machine to either an explicit destination specified by 701.Ar host 702port 703.Ar hostport , 704or 705.Ar local_socket , 706or, if no explicit destination was specified, 707.Nm 708will act as a SOCKS 4/5 proxy and forward connections to the destinations 709requested by the remote SOCKS client. 710.Pp 711Port forwardings can also be specified in the configuration file. 712Privileged ports can be forwarded only when 713logging in as root on the remote machine. 714IPv6 addresses can be specified by enclosing the address in square brackets. 715.Pp 716By default, TCP listening sockets on the server will be bound to the loopback 717interface only. 718This may be overridden by specifying a 719.Ar bind_address . 720An empty 721.Ar bind_address , 722or the address 723.Ql * , 724indicates that the remote socket should listen on all interfaces. 725Specifying a remote 726.Ar bind_address 727will only succeed if the server's 728.Cm GatewayPorts 729option is enabled (see 730.Xr sshd_config 5 ) . 731.Pp 732If the 733.Ar port 734argument is 735.Ql 0 , 736the listen port will be dynamically allocated on the server and reported 737to the client at run time. 738When used together with 739.Ic -O forward , 740the allocated port will be printed to the standard output. 741.Pp 742.It Fl S Ar ctl_path 743Specifies the location of a control socket for connection sharing, 744or the string 745.Dq none 746to disable connection sharing. 747Refer to the description of 748.Cm ControlPath 749and 750.Cm ControlMaster 751in 752.Xr ssh_config 5 753for details. 754.Pp 755.It Fl s 756May be used to request invocation of a subsystem on the remote system. 757Subsystems facilitate the use of SSH 758as a secure transport for other applications (e.g.\& 759.Xr sftp 1 ) . 760The subsystem is specified as the remote command. 761Refer to the description of 762.Cm SessionType 763in 764.Xr ssh_config 5 765for details. 766.Pp 767.It Fl T 768Disable pseudo-terminal allocation. 769.Pp 770.It Fl t 771Force pseudo-terminal allocation. 772This can be used to execute arbitrary 773screen-based programs on a remote machine, which can be very useful, 774e.g. when implementing menu services. 775Multiple 776.Fl t 777options force tty allocation, even if 778.Nm 779has no local tty. 780.Pp 781.It Fl V 782Display the version number and exit. 783.Pp 784.It Fl v 785Verbose mode. 786Causes 787.Nm 788to print debugging messages about its progress. 789This is helpful in 790debugging connection, authentication, and configuration problems. 791Multiple 792.Fl v 793options increase the verbosity. 794The maximum is 3. 795.Pp 796.It Fl W Ar host : Ns Ar port 797Requests that standard input and output on the client be forwarded to 798.Ar host 799on 800.Ar port 801over the secure channel. 802Implies 803.Fl N , 804.Fl T , 805.Cm ExitOnForwardFailure 806and 807.Cm ClearAllForwardings , 808though these can be overridden in the configuration file or using 809.Fl o 810command line options. 811.Pp 812.It Fl w Xo 813.Ar local_tun Ns Op : Ns Ar remote_tun 814.Xc 815Requests 816tunnel 817device forwarding with the specified 818.Xr tun 4 819devices between the client 820.Pq Ar local_tun 821and the server 822.Pq Ar remote_tun . 823.Pp 824The devices may be specified by numerical ID or the keyword 825.Dq any , 826which uses the next available tunnel device. 827If 828.Ar remote_tun 829is not specified, it defaults to 830.Dq any . 831See also the 832.Cm Tunnel 833and 834.Cm TunnelDevice 835directives in 836.Xr ssh_config 5 . 837.Pp 838If the 839.Cm Tunnel 840directive is unset, it will be set to the default tunnel mode, which is 841.Dq point-to-point . 842If a different 843.Cm Tunnel 844forwarding mode it desired, then it should be specified before 845.Fl w . 846.Pp 847.It Fl X 848Enables X11 forwarding. 849This can also be specified on a per-host basis in a configuration file. 850.Pp 851X11 forwarding should be enabled with caution. 852Users with the ability to bypass file permissions on the remote host 853(for the user's X authorization database) 854can access the local X11 display through the forwarded connection. 855An attacker may then be able to perform activities such as keystroke monitoring. 856.Pp 857For this reason, X11 forwarding is subjected to X11 SECURITY extension 858restrictions by default. 859Refer to the 860.Nm 861.Fl Y 862option and the 863.Cm ForwardX11Trusted 864directive in 865.Xr ssh_config 5 866for more information. 867.Pp 868.It Fl x 869Disables X11 forwarding. 870.Pp 871.It Fl Y 872Enables trusted X11 forwarding. 873Trusted X11 forwardings are not subjected to the X11 SECURITY extension 874controls. 875.Pp 876.It Fl y 877Send log information using the 878.Xr syslog 3 879system module. 880By default this information is sent to stderr. 881.El 882.Pp 883.Nm 884may additionally obtain configuration data from 885a per-user configuration file and a system-wide configuration file. 886The file format and configuration options are described in 887.Xr ssh_config 5 . 888.Sh AUTHENTICATION 889The OpenSSH SSH client supports SSH protocol 2. 890.Pp 891The methods available for authentication are: 892GSSAPI-based authentication, 893host-based authentication, 894public key authentication, 895keyboard-interactive authentication, 896and password authentication. 897Authentication methods are tried in the order specified above, 898though 899.Cm PreferredAuthentications 900can be used to change the default order. 901.Pp 902Host-based authentication works as follows: 903If the machine the user logs in from is listed in 904.Pa /etc/hosts.equiv 905or 906.Pa /etc/shosts.equiv 907on the remote machine, the user is non-root and the user names are 908the same on both sides, or if the files 909.Pa ~/.rhosts 910or 911.Pa ~/.shosts 912exist in the user's home directory on the 913remote machine and contain a line containing the name of the client 914machine and the name of the user on that machine, the user is 915considered for login. 916Additionally, the server 917.Em must 918be able to verify the client's 919host key (see the description of 920.Pa /etc/ssh/ssh_known_hosts 921and 922.Pa ~/.ssh/known_hosts , 923below) 924for login to be permitted. 925This authentication method closes security holes due to IP 926spoofing, DNS spoofing, and routing spoofing. 927[Note to the administrator: 928.Pa /etc/hosts.equiv , 929.Pa ~/.rhosts , 930and the rlogin/rsh protocol in general, are inherently insecure and should be 931disabled if security is desired.] 932.Pp 933Public key authentication works as follows: 934The scheme is based on public-key cryptography, 935using cryptosystems 936where encryption and decryption are done using separate keys, 937and it is unfeasible to derive the decryption key from the encryption key. 938The idea is that each user creates a public/private 939key pair for authentication purposes. 940The server knows the public key, and only the user knows the private key. 941.Nm 942implements public key authentication protocol automatically, 943using one of the ECDSA, Ed25519 or RSA algorithms. 944.Pp 945The file 946.Pa ~/.ssh/authorized_keys 947lists the public keys that are permitted for logging in. 948When the user logs in, the 949.Nm 950program tells the server which key pair it would like to use for 951authentication. 952The client proves that it has access to the private key 953and the server checks that the corresponding public key 954is authorized to accept the account. 955.Pp 956The server may inform the client of errors that prevented public key 957authentication from succeeding after authentication completes using a 958different method. 959These may be viewed by increasing the 960.Cm LogLevel 961to 962.Cm DEBUG 963or higher (e.g. by using the 964.Fl v 965flag). 966.Pp 967The user creates their key pair by running 968.Xr ssh-keygen 1 . 969This stores the private key in 970.Pa ~/.ssh/id_ecdsa 971(ECDSA), 972.Pa ~/.ssh/id_ecdsa_sk 973(authenticator-hosted ECDSA), 974.Pa ~/.ssh/id_ed25519 975(Ed25519), 976.Pa ~/.ssh/id_ed25519_sk 977(authenticator-hosted Ed25519), 978or 979.Pa ~/.ssh/id_rsa 980(RSA) 981and stores the public key in 982.Pa ~/.ssh/id_ecdsa.pub 983(ECDSA), 984.Pa ~/.ssh/id_ecdsa_sk.pub 985(authenticator-hosted ECDSA), 986.Pa ~/.ssh/id_ed25519.pub 987(Ed25519), 988.Pa ~/.ssh/id_ed25519_sk.pub 989(authenticator-hosted Ed25519), 990or 991.Pa ~/.ssh/id_rsa.pub 992(RSA) 993in the user's home directory. 994The user should then copy the public key 995to 996.Pa ~/.ssh/authorized_keys 997in their home directory on the remote machine. 998The 999.Pa authorized_keys 1000file corresponds to the conventional 1001.Pa ~/.rhosts 1002file, and has one key 1003per line, though the lines can be very long. 1004After this, the user can log in without giving the password. 1005.Pp 1006A variation on public key authentication 1007is available in the form of certificate authentication: 1008instead of a set of public/private keys, 1009signed certificates are used. 1010This has the advantage that a single trusted certification authority 1011can be used in place of many public/private keys. 1012See the CERTIFICATES section of 1013.Xr ssh-keygen 1 1014for more information. 1015.Pp 1016The most convenient way to use public key or certificate authentication 1017may be with an authentication agent. 1018See 1019.Xr ssh-agent 1 1020and (optionally) the 1021.Cm AddKeysToAgent 1022directive in 1023.Xr ssh_config 5 1024for more information. 1025.Pp 1026Keyboard-interactive authentication works as follows: 1027The server sends an arbitrary 1028.Qq challenge 1029text and prompts for a response, possibly multiple times. 1030Examples of keyboard-interactive authentication include 1031.Bx 1032Authentication (see 1033.Xr login.conf 5 ) 1034and PAM (some 1035.Pf non- Ox 1036systems). 1037.Pp 1038Finally, if other authentication methods fail, 1039.Nm 1040prompts the user for a password. 1041The password is sent to the remote 1042host for checking; however, since all communications are encrypted, 1043the password cannot be seen by someone listening on the network. 1044.Pp 1045.Nm 1046automatically maintains and checks a database containing 1047identification for all hosts it has ever been used with. 1048Host keys are stored in 1049.Pa ~/.ssh/known_hosts 1050in the user's home directory. 1051Additionally, the file 1052.Pa /etc/ssh/ssh_known_hosts 1053is automatically checked for known hosts. 1054Any new hosts are automatically added to the user's file. 1055If a host's identification ever changes, 1056.Nm 1057warns about this and disables password authentication to prevent 1058server spoofing or man-in-the-middle attacks, 1059which could otherwise be used to circumvent the encryption. 1060The 1061.Cm StrictHostKeyChecking 1062option can be used to control logins to machines whose 1063host key is not known or has changed. 1064.Pp 1065When the user's identity has been accepted by the server, the server 1066either executes the given command in a non-interactive session or, 1067if no command has been specified, logs into the machine and gives 1068the user a normal shell as an interactive session. 1069All communication with 1070the remote command or shell will be automatically encrypted. 1071.Pp 1072If an interactive session is requested, 1073.Nm 1074by default will only request a pseudo-terminal (pty) for interactive 1075sessions when the client has one. 1076The flags 1077.Fl T 1078and 1079.Fl t 1080can be used to override this behaviour. 1081.Pp 1082If a pseudo-terminal has been allocated, the 1083user may use the escape characters noted below. 1084.Pp 1085If no pseudo-terminal has been allocated, 1086the session is transparent and can be used to reliably transfer binary data. 1087On most systems, setting the escape character to 1088.Dq none 1089will also make the session transparent even if a tty is used. 1090.Pp 1091The session terminates when the command or shell on the remote 1092machine exits and all X11 and TCP connections have been closed. 1093.Sh ESCAPE CHARACTERS 1094When a pseudo-terminal has been requested, 1095.Nm 1096supports a number of functions through the use of an escape character. 1097.Pp 1098A single tilde character can be sent as 1099.Ic ~~ 1100or by following the tilde by a character other than those described below. 1101The escape character must always follow a newline to be interpreted as 1102special. 1103The escape character can be changed in configuration files using the 1104.Cm EscapeChar 1105configuration directive or on the command line by the 1106.Fl e 1107option. 1108.Pp 1109The supported escapes (assuming the default 1110.Ql ~ ) 1111are: 1112.Bl -tag -width Ds 1113.It Cm ~. 1114Disconnect. 1115.It Cm ~^Z 1116Background 1117.Nm . 1118.It Cm ~# 1119List forwarded connections. 1120.It Cm ~& 1121Background 1122.Nm 1123at logout when waiting for forwarded connection / X11 sessions to terminate. 1124.It Cm ~? 1125Display a list of escape characters. 1126.It Cm ~B 1127Send a BREAK to the remote system 1128(only useful if the peer supports it). 1129.It Cm ~C 1130Open command line. 1131Currently this allows the addition of port forwardings using the 1132.Fl L , 1133.Fl R 1134and 1135.Fl D 1136options (see above). 1137It also allows the cancellation of existing port-forwardings 1138with 1139.Sm off 1140.Fl KL Oo Ar bind_address : Oc Ar port 1141.Sm on 1142for local, 1143.Sm off 1144.Fl KR Oo Ar bind_address : Oc Ar port 1145.Sm on 1146for remote and 1147.Sm off 1148.Fl KD Oo Ar bind_address : Oc Ar port 1149.Sm on 1150for dynamic port-forwardings. 1151.Ic !\& Ns Ar command 1152allows the user to execute a local command if the 1153.Ic PermitLocalCommand 1154option is enabled in 1155.Xr ssh_config 5 . 1156Basic help is available, using the 1157.Fl h 1158option. 1159.It Cm ~R 1160Request rekeying of the connection 1161(only useful if the peer supports it). 1162.It Cm ~V 1163Decrease the verbosity 1164.Pq Ic LogLevel 1165when errors are being written to stderr. 1166.It Cm ~v 1167Increase the verbosity 1168.Pq Ic LogLevel 1169when errors are being written to stderr. 1170.El 1171.Sh TCP FORWARDING 1172Forwarding of arbitrary TCP connections over a secure channel 1173can be specified either on the command line or in a configuration file. 1174One possible application of TCP forwarding is a secure connection to a 1175mail server; another is going through firewalls. 1176.Pp 1177In the example below, we look at encrypting communication for an IRC client, 1178even though the IRC server it connects to does not directly 1179support encrypted communication. 1180This works as follows: 1181the user connects to the remote host using 1182.Nm , 1183specifying the ports to be used to forward the connection. 1184After that it is possible to start the program locally, 1185and 1186.Nm 1187will encrypt and forward the connection to the remote server. 1188.Pp 1189The following example tunnels an IRC session from the client 1190to an IRC server at 1191.Dq server.example.com , 1192joining channel 1193.Dq #users , 1194nickname 1195.Dq pinky , 1196using the standard IRC port, 6667: 1197.Bd -literal -offset 4n 1198$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1199$ irc -c '#users' pinky IRC/127.0.0.1 1200.Ed 1201.Pp 1202The 1203.Fl f 1204option backgrounds 1205.Nm 1206and the remote command 1207.Dq sleep 10 1208is specified to allow an amount of time 1209(10 seconds, in the example) 1210to start the program which is going to use the tunnel. 1211If no connections are made within the time specified, 1212.Nm 1213will exit. 1214.Sh X11 FORWARDING 1215If the 1216.Cm ForwardX11 1217variable is set to 1218.Dq yes 1219(or see the description of the 1220.Fl X , 1221.Fl x , 1222and 1223.Fl Y 1224options above) 1225and the user is using X11 (the 1226.Ev DISPLAY 1227environment variable is set), the connection to the X11 display is 1228automatically forwarded to the remote side in such a way that any X11 1229programs started from the shell (or command) will go through the 1230encrypted channel, and the connection to the real X server will be made 1231from the local machine. 1232The user should not manually set 1233.Ev DISPLAY . 1234Forwarding of X11 connections can be 1235configured on the command line or in configuration files. 1236.Pp 1237The 1238.Ev DISPLAY 1239value set by 1240.Nm 1241will point to the server machine, but with a display number greater than zero. 1242This is normal, and happens because 1243.Nm 1244creates a 1245.Dq proxy 1246X server on the server machine for forwarding the 1247connections over the encrypted channel. 1248.Pp 1249.Nm 1250will also automatically set up Xauthority data on the server machine. 1251For this purpose, it will generate a random authorization cookie, 1252store it in Xauthority on the server, and verify that any forwarded 1253connections carry this cookie and replace it by the real cookie when 1254the connection is opened. 1255The real authentication cookie is never 1256sent to the server machine (and no cookies are sent in the plain). 1257.Pp 1258If the 1259.Cm ForwardAgent 1260variable is set to 1261.Dq yes 1262(or see the description of the 1263.Fl A 1264and 1265.Fl a 1266options above) and 1267the user is using an authentication agent, the connection to the agent 1268is automatically forwarded to the remote side. 1269.Sh VERIFYING HOST KEYS 1270When connecting to a server for the first time, 1271a fingerprint of the server's public key is presented to the user 1272(unless the option 1273.Cm StrictHostKeyChecking 1274has been disabled). 1275Fingerprints can be determined using 1276.Xr ssh-keygen 1 : 1277.Pp 1278.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1279.Pp 1280If the fingerprint is already known, it can be matched 1281and the key can be accepted or rejected. 1282If only legacy (MD5) fingerprints for the server are available, the 1283.Xr ssh-keygen 1 1284.Fl E 1285option may be used to downgrade the fingerprint algorithm to match. 1286.Pp 1287Because of the difficulty of comparing host keys 1288just by looking at fingerprint strings, 1289there is also support to compare host keys visually, 1290using 1291.Em random art . 1292By setting the 1293.Cm VisualHostKey 1294option to 1295.Dq yes , 1296a small ASCII graphic gets displayed on every login to a server, no matter 1297if the session itself is interactive or not. 1298By learning the pattern a known server produces, a user can easily 1299find out that the host key has changed when a completely different pattern 1300is displayed. 1301Because these patterns are not unambiguous however, a pattern that looks 1302similar to the pattern remembered only gives a good probability that the 1303host key is the same, not guaranteed proof. 1304.Pp 1305To get a listing of the fingerprints along with their random art for 1306all known hosts, the following command line can be used: 1307.Pp 1308.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1309.Pp 1310If the fingerprint is unknown, 1311an alternative method of verification is available: 1312SSH fingerprints verified by DNS. 1313An additional resource record (RR), 1314SSHFP, 1315is added to a zonefile 1316and the connecting client is able to match the fingerprint 1317with that of the key presented. 1318.Pp 1319In this example, we are connecting a client to a server, 1320.Dq host.example.com . 1321The SSHFP resource records should first be added to the zonefile for 1322host.example.com: 1323.Bd -literal -offset indent 1324$ ssh-keygen -r host.example.com. 1325.Ed 1326.Pp 1327The output lines will have to be added to the zonefile. 1328To check that the zone is answering fingerprint queries: 1329.Pp 1330.Dl $ dig -t SSHFP host.example.com 1331.Pp 1332Finally the client connects: 1333.Bd -literal -offset indent 1334$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1335[...] 1336Matching host key fingerprint found in DNS. 1337Are you sure you want to continue connecting (yes/no)? 1338.Ed 1339.Pp 1340See the 1341.Cm VerifyHostKeyDNS 1342option in 1343.Xr ssh_config 5 1344for more information. 1345.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1346.Nm 1347contains support for Virtual Private Network (VPN) tunnelling 1348using the 1349.Xr tun 4 1350network pseudo-device, 1351allowing two networks to be joined securely. 1352The 1353.Xr sshd_config 5 1354configuration option 1355.Cm PermitTunnel 1356controls whether the server supports this, 1357and at what level (layer 2 or 3 traffic). 1358.Pp 1359The following example would connect client network 10.0.50.0/24 1360with remote network 10.0.99.0/24 using a point-to-point connection 1361from 10.1.1.1 to 10.1.1.2, 1362provided that the SSH server running on the gateway to the remote network, 1363at 192.168.1.15, allows it. 1364.Pp 1365On the client: 1366.Bd -literal -offset indent 1367# ssh -f -w 0:1 192.168.1.15 true 1368# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1369# route add 10.0.99.0/24 10.1.1.2 1370.Ed 1371.Pp 1372On the server: 1373.Bd -literal -offset indent 1374# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1375# route add 10.0.50.0/24 10.1.1.1 1376.Ed 1377.Pp 1378Client access may be more finely tuned via the 1379.Pa /root/.ssh/authorized_keys 1380file (see below) and the 1381.Cm PermitRootLogin 1382server option. 1383The following entry would permit connections on 1384.Xr tun 4 1385device 1 from user 1386.Dq jane 1387and on tun device 2 from user 1388.Dq john , 1389if 1390.Cm PermitRootLogin 1391is set to 1392.Dq forced-commands-only : 1393.Bd -literal -offset 2n 1394tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1395tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1396.Ed 1397.Pp 1398Since an SSH-based setup entails a fair amount of overhead, 1399it may be more suited to temporary setups, 1400such as for wireless VPNs. 1401More permanent VPNs are better provided by tools such as 1402.Xr ipsecctl 8 1403and 1404.Xr isakmpd 8 . 1405.Sh ENVIRONMENT 1406.Nm 1407will normally set the following environment variables: 1408.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1409.It Ev DISPLAY 1410The 1411.Ev DISPLAY 1412variable indicates the location of the X11 server. 1413It is automatically set by 1414.Nm 1415to point to a value of the form 1416.Dq hostname:n , 1417where 1418.Dq hostname 1419indicates the host where the shell runs, and 1420.Sq n 1421is an integer \*(Ge 1. 1422.Nm 1423uses this special value to forward X11 connections over the secure 1424channel. 1425The user should normally not set 1426.Ev DISPLAY 1427explicitly, as that 1428will render the X11 connection insecure (and will require the user to 1429manually copy any required authorization cookies). 1430.It Ev HOME 1431Set to the path of the user's home directory. 1432.It Ev LOGNAME 1433Synonym for 1434.Ev USER ; 1435set for compatibility with systems that use this variable. 1436.It Ev MAIL 1437Set to the path of the user's mailbox. 1438.It Ev PATH 1439Set to the default 1440.Ev PATH , 1441as specified when compiling 1442.Nm . 1443.It Ev SSH_ASKPASS 1444If 1445.Nm 1446needs a passphrase, it will read the passphrase from the current 1447terminal if it was run from a terminal. 1448If 1449.Nm 1450does not have a terminal associated with it but 1451.Ev DISPLAY 1452and 1453.Ev SSH_ASKPASS 1454are set, it will execute the program specified by 1455.Ev SSH_ASKPASS 1456and open an X11 window to read the passphrase. 1457This is particularly useful when calling 1458.Nm 1459from a 1460.Pa .xsession 1461or related script. 1462(Note that on some machines it 1463may be necessary to redirect the input from 1464.Pa /dev/null 1465to make this work.) 1466.It Ev SSH_ASKPASS_REQUIRE 1467Allows further control over the use of an askpass program. 1468If this variable is set to 1469.Dq never 1470then 1471.Nm 1472will never attempt to use one. 1473If it is set to 1474.Dq prefer , 1475then 1476.Nm 1477will prefer to use the askpass program instead of the TTY when requesting 1478passwords. 1479Finally, if the variable is set to 1480.Dq force , 1481then the askpass program will be used for all passphrase input regardless 1482of whether 1483.Ev DISPLAY 1484is set. 1485.It Ev SSH_AUTH_SOCK 1486Identifies the path of a 1487.Ux Ns -domain 1488socket used to communicate with the agent. 1489.It Ev SSH_CONNECTION 1490Identifies the client and server ends of the connection. 1491The variable contains 1492four space-separated values: client IP address, client port number, 1493server IP address, and server port number. 1494.It Ev SSH_ORIGINAL_COMMAND 1495This variable contains the original command line if a forced command 1496is executed. 1497It can be used to extract the original arguments. 1498.It Ev SSH_TTY 1499This is set to the name of the tty (path to the device) associated 1500with the current shell or command. 1501If the current session has no tty, 1502this variable is not set. 1503.It Ev SSH_TUNNEL 1504Optionally set by 1505.Xr sshd 8 1506to contain the interface names assigned if tunnel forwarding was 1507requested by the client. 1508.It Ev SSH_USER_AUTH 1509Optionally set by 1510.Xr sshd 8 , 1511this variable may contain a pathname to a file that lists the authentication 1512methods successfully used when the session was established, including any 1513public keys that were used. 1514.It Ev TZ 1515This variable is set to indicate the present time zone if it 1516was set when the daemon was started (i.e. the daemon passes the value 1517on to new connections). 1518.It Ev USER 1519Set to the name of the user logging in. 1520.El 1521.Pp 1522Additionally, 1523.Nm 1524reads 1525.Pa ~/.ssh/environment , 1526and adds lines of the format 1527.Dq VARNAME=value 1528to the environment if the file exists and users are allowed to 1529change their environment. 1530For more information, see the 1531.Cm PermitUserEnvironment 1532option in 1533.Xr sshd_config 5 . 1534.Sh FILES 1535.Bl -tag -width Ds -compact 1536.It Pa ~/.rhosts 1537This file is used for host-based authentication (see above). 1538On some machines this file may need to be 1539world-readable if the user's home directory is on an NFS partition, 1540because 1541.Xr sshd 8 1542reads it as root. 1543Additionally, this file must be owned by the user, 1544and must not have write permissions for anyone else. 1545The recommended 1546permission for most machines is read/write for the user, and not 1547accessible by others. 1548.Pp 1549.It Pa ~/.shosts 1550This file is used in exactly the same way as 1551.Pa .rhosts , 1552but allows host-based authentication without permitting login with 1553rlogin/rsh. 1554.Pp 1555.It Pa ~/.ssh/ 1556This directory is the default location for all user-specific configuration 1557and authentication information. 1558There is no general requirement to keep the entire contents of this directory 1559secret, but the recommended permissions are read/write/execute for the user, 1560and not accessible by others. 1561.Pp 1562.It Pa ~/.ssh/authorized_keys 1563Lists the public keys (ECDSA, Ed25519, RSA) 1564that can be used for logging in as this user. 1565The format of this file is described in the 1566.Xr sshd 8 1567manual page. 1568This file is not highly sensitive, but the recommended 1569permissions are read/write for the user, and not accessible by others. 1570.Pp 1571.It Pa ~/.ssh/config 1572This is the per-user configuration file. 1573The file format and configuration options are described in 1574.Xr ssh_config 5 . 1575Because of the potential for abuse, this file must have strict permissions: 1576read/write for the user, and not writable by others. 1577.Pp 1578.It Pa ~/.ssh/environment 1579Contains additional definitions for environment variables; see 1580.Sx ENVIRONMENT , 1581above. 1582.Pp 1583.It Pa ~/.ssh/id_ecdsa 1584.It Pa ~/.ssh/id_ecdsa_sk 1585.It Pa ~/.ssh/id_ed25519 1586.It Pa ~/.ssh/id_ed25519_sk 1587.It Pa ~/.ssh/id_rsa 1588Contains the private key for authentication. 1589These files 1590contain sensitive data and should be readable by the user but not 1591accessible by others (read/write/execute). 1592.Nm 1593will simply ignore a private key file if it is accessible by others. 1594It is possible to specify a passphrase when 1595generating the key which will be used to encrypt the 1596sensitive part of this file using AES-128. 1597.Pp 1598.It Pa ~/.ssh/id_ecdsa.pub 1599.It Pa ~/.ssh/id_ecdsa_sk.pub 1600.It Pa ~/.ssh/id_ed25519.pub 1601.It Pa ~/.ssh/id_ed25519_sk.pub 1602.It Pa ~/.ssh/id_rsa.pub 1603Contains the public key for authentication. 1604These files are not 1605sensitive and can (but need not) be readable by anyone. 1606.Pp 1607.It Pa ~/.ssh/known_hosts 1608Contains a list of host keys for all hosts the user has logged into 1609that are not already in the systemwide list of known host keys. 1610See 1611.Xr sshd 8 1612for further details of the format of this file. 1613.Pp 1614.It Pa ~/.ssh/rc 1615Commands in this file are executed by 1616.Nm 1617when the user logs in, just before the user's shell (or command) is 1618started. 1619See the 1620.Xr sshd 8 1621manual page for more information. 1622.Pp 1623.It Pa /etc/hosts.equiv 1624This file is for host-based authentication (see above). 1625It should only be writable by root. 1626.Pp 1627.It Pa /etc/shosts.equiv 1628This file is used in exactly the same way as 1629.Pa hosts.equiv , 1630but allows host-based authentication without permitting login with 1631rlogin/rsh. 1632.Pp 1633.It Pa /etc/ssh/ssh_config 1634Systemwide configuration file. 1635The file format and configuration options are described in 1636.Xr ssh_config 5 . 1637.Pp 1638.It Pa /etc/ssh/ssh_host_ecdsa_key 1639.It Pa /etc/ssh/ssh_host_ed25519_key 1640.It Pa /etc/ssh/ssh_host_rsa_key 1641These files contain the private parts of the host keys 1642and are used for host-based authentication. 1643.Pp 1644.It Pa /etc/ssh/ssh_known_hosts 1645Systemwide list of known host keys. 1646This file should be prepared by the 1647system administrator to contain the public host keys of all machines in the 1648organization. 1649It should be world-readable. 1650See 1651.Xr sshd 8 1652for further details of the format of this file. 1653.Pp 1654.It Pa /etc/ssh/sshrc 1655Commands in this file are executed by 1656.Nm 1657when the user logs in, just before the user's shell (or command) is started. 1658See the 1659.Xr sshd 8 1660manual page for more information. 1661.El 1662.Sh EXIT STATUS 1663.Nm 1664exits with the exit status of the remote command or with 255 1665if an error occurred. 1666.Sh SEE ALSO 1667.Xr scp 1 , 1668.Xr sftp 1 , 1669.Xr ssh-add 1 , 1670.Xr ssh-agent 1 , 1671.Xr ssh-keygen 1 , 1672.Xr ssh-keyscan 1 , 1673.Xr tun 4 , 1674.Xr ssh_config 5 , 1675.Xr ssh-keysign 8 , 1676.Xr sshd 8 1677.Sh STANDARDS 1678.Rs 1679.%A S. Lehtinen 1680.%A C. Lonvick 1681.%D January 2006 1682.%R RFC 4250 1683.%T The Secure Shell (SSH) Protocol Assigned Numbers 1684.Re 1685.Pp 1686.Rs 1687.%A T. Ylonen 1688.%A C. Lonvick 1689.%D January 2006 1690.%R RFC 4251 1691.%T The Secure Shell (SSH) Protocol Architecture 1692.Re 1693.Pp 1694.Rs 1695.%A T. Ylonen 1696.%A C. Lonvick 1697.%D January 2006 1698.%R RFC 4252 1699.%T The Secure Shell (SSH) Authentication Protocol 1700.Re 1701.Pp 1702.Rs 1703.%A T. Ylonen 1704.%A C. Lonvick 1705.%D January 2006 1706.%R RFC 4253 1707.%T The Secure Shell (SSH) Transport Layer Protocol 1708.Re 1709.Pp 1710.Rs 1711.%A T. Ylonen 1712.%A C. Lonvick 1713.%D January 2006 1714.%R RFC 4254 1715.%T The Secure Shell (SSH) Connection Protocol 1716.Re 1717.Pp 1718.Rs 1719.%A J. Schlyter 1720.%A W. Griffin 1721.%D January 2006 1722.%R RFC 4255 1723.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1724.Re 1725.Pp 1726.Rs 1727.%A F. Cusack 1728.%A M. Forssen 1729.%D January 2006 1730.%R RFC 4256 1731.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1732.Re 1733.Pp 1734.Rs 1735.%A J. Galbraith 1736.%A P. Remaker 1737.%D January 2006 1738.%R RFC 4335 1739.%T The Secure Shell (SSH) Session Channel Break Extension 1740.Re 1741.Pp 1742.Rs 1743.%A M. Bellare 1744.%A T. Kohno 1745.%A C. Namprempre 1746.%D January 2006 1747.%R RFC 4344 1748.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1749.Re 1750.Pp 1751.Rs 1752.%A B. Harris 1753.%D January 2006 1754.%R RFC 4345 1755.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1756.Re 1757.Pp 1758.Rs 1759.%A M. Friedl 1760.%A N. Provos 1761.%A W. Simpson 1762.%D March 2006 1763.%R RFC 4419 1764.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1765.Re 1766.Pp 1767.Rs 1768.%A J. Galbraith 1769.%A R. Thayer 1770.%D November 2006 1771.%R RFC 4716 1772.%T The Secure Shell (SSH) Public Key File Format 1773.Re 1774.Pp 1775.Rs 1776.%A D. Stebila 1777.%A J. Green 1778.%D December 2009 1779.%R RFC 5656 1780.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1781.Re 1782.Pp 1783.Rs 1784.%A A. Perrig 1785.%A D. Song 1786.%D 1999 1787.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1788.%T Hash Visualization: a New Technique to improve Real-World Security 1789.Re 1790.Sh AUTHORS 1791OpenSSH is a derivative of the original and free 1792ssh 1.2.12 release by Tatu Ylonen. 1793Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1794Theo de Raadt and Dug Song 1795removed many bugs, re-added newer features and 1796created OpenSSH. 1797Markus Friedl contributed the support for SSH 1798protocol versions 1.5 and 2.0. 1799