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 1.432 2022/09/17 10:33:18 djm Exp $ 37.Dd $Mdocdate: September 17 2022 $ 38.Dt SSH 1 39.Os 40.Sh NAME 41.Nm ssh 42.Nd OpenSSH remote login client 43.Sh SYNOPSIS 44.Nm ssh 45.Op Fl 46AaCfGgKkMNnqsTtVvXxYy 46.Op Fl B Ar bind_interface 47.Op Fl b Ar bind_address 48.Op Fl c Ar cipher_spec 49.Op Fl D Oo Ar bind_address : Oc Ns Ar port 50.Op Fl E Ar log_file 51.Op Fl e Ar escape_char 52.Op Fl F Ar configfile 53.Op Fl I Ar pkcs11 54.Op Fl i Ar identity_file 55.Op Fl J Ar destination 56.Op Fl L Ar address 57.Op Fl l Ar login_name 58.Op Fl m Ar mac_spec 59.Op Fl O Ar ctl_cmd 60.Op Fl o Ar option 61.Op Fl p Ar port 62.Op Fl Q Ar query_option 63.Op Fl R Ar address 64.Op Fl S Ar ctl_path 65.Op Fl W Ar host : Ns Ar port 66.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun 67.Ar destination 68.Op Ar command Op Ar argument ... 69.Sh DESCRIPTION 70.Nm 71(SSH client) is a program for logging into a remote machine and for 72executing commands on a remote machine. 73It is intended to provide secure encrypted communications between 74two untrusted hosts over an insecure network. 75X11 connections, arbitrary TCP ports and 76.Ux Ns -domain 77sockets can also be forwarded over the secure channel. 78.Pp 79.Nm 80connects and logs into the specified 81.Ar destination , 82which may be specified as either 83.Sm off 84.Oo user @ Oc hostname 85.Sm on 86or a URI of the form 87.Sm off 88.No ssh:// Oo user @ Oc hostname Op : port . 89.Sm on 90The user must prove 91their identity to the remote machine using one of several methods 92(see below). 93.Pp 94If a 95.Ar command 96is specified, 97it will be executed on the remote host instead of a login shell. 98A complete command line may be specified as 99.Ar command , 100or it may have additional arguments. 101If supplied, the arguments will be appended to the command, separated by 102spaces, before it is sent to the server to be executed. 103.Pp 104The options are as follows: 105.Pp 106.Bl -tag -width Ds -compact 107.It Fl 4 108Forces 109.Nm 110to use IPv4 addresses only. 111.Pp 112.It Fl 6 113Forces 114.Nm 115to use IPv6 addresses only. 116.Pp 117.It Fl A 118Enables forwarding of connections from an authentication agent such as 119.Xr ssh-agent 1 . 120This can also be specified on a per-host basis in a configuration file. 121.Pp 122Agent forwarding should be enabled with caution. 123Users with the ability to bypass file permissions on the remote host 124(for the agent's 125.Ux Ns -domain 126socket) can access the local agent through the forwarded connection. 127An attacker cannot obtain key material from the agent, 128however they can perform operations on the keys that enable them to 129authenticate using the identities loaded into the agent. 130A safer alternative may be to use a jump host 131(see 132.Fl J ) . 133.Pp 134.It Fl a 135Disables forwarding of the authentication agent connection. 136.Pp 137.It Fl B Ar bind_interface 138Bind to the address of 139.Ar bind_interface 140before attempting to connect to the destination host. 141This is only useful on systems with more than one address. 142.Pp 143.It Fl b Ar bind_address 144Use 145.Ar bind_address 146on the local machine as the source address 147of the connection. 148Only useful on systems with more than one address. 149.Pp 150.It Fl C 151Requests compression of all data (including stdin, stdout, stderr, and 152data for forwarded X11, TCP and 153.Ux Ns -domain 154connections). 155The compression algorithm is the same used by 156.Xr gzip 1 . 157Compression is desirable on modem lines and other 158slow connections, but will only slow down things on fast networks. 159The default value can be set on a host-by-host basis in the 160configuration files; see the 161.Cm Compression 162option in 163.Xr ssh_config 5 . 164.Pp 165.It Fl c Ar cipher_spec 166Selects the cipher specification for encrypting the session. 167.Ar cipher_spec 168is a comma-separated list of ciphers 169listed in order of preference. 170See the 171.Cm Ciphers 172keyword in 173.Xr ssh_config 5 174for more information. 175.Pp 176.It Fl D Xo 177.Sm off 178.Oo Ar bind_address : Oc 179.Ar port 180.Sm on 181.Xc 182Specifies a local 183.Dq dynamic 184application-level port forwarding. 185This works by allocating a socket to listen to 186.Ar port 187on the local side, optionally bound to the specified 188.Ar bind_address . 189Whenever a connection is made to this port, the 190connection is forwarded over the secure channel, and the application 191protocol is then used to determine where to connect to from the 192remote machine. 193Currently the SOCKS4 and SOCKS5 protocols are supported, and 194.Nm 195will act as a SOCKS server. 196Only root can forward privileged ports. 197Dynamic port forwardings can also be specified in the configuration file. 198.Pp 199IPv6 addresses can be specified by enclosing the address in square brackets. 200Only the superuser can forward privileged ports. 201By default, the local port is bound in accordance with the 202.Cm GatewayPorts 203setting. 204However, an explicit 205.Ar bind_address 206may be used to bind the connection to a specific address. 207The 208.Ar bind_address 209of 210.Dq localhost 211indicates that the listening port be bound for local use only, while an 212empty address or 213.Sq * 214indicates that the port should be available from all interfaces. 215.Pp 216.It Fl E Ar log_file 217Append debug logs to 218.Ar log_file 219instead of standard error. 220.Pp 221.It Fl e Ar escape_char 222Sets the escape character for sessions with a pty (default: 223.Ql ~ ) . 224The escape character is only recognized at the beginning of a line. 225The escape character followed by a dot 226.Pq Ql \&. 227closes the connection; 228followed by control-Z suspends the connection; 229and followed by itself sends the escape character once. 230Setting the character to 231.Dq none 232disables any escapes and makes the session fully transparent. 233.Pp 234.It Fl F Ar configfile 235Specifies an alternative per-user configuration file. 236If a configuration file is given on the command line, 237the system-wide configuration file 238.Pq Pa /etc/ssh/ssh_config 239will be ignored. 240The default for the per-user configuration file is 241.Pa ~/.ssh/config . 242If set to 243.Dq none , 244no configuration files will be read. 245.Pp 246.It Fl f 247Requests 248.Nm 249to go to background just before command execution. 250This is useful if 251.Nm 252is going to ask for passwords or passphrases, but the user 253wants it in the background. 254This implies 255.Fl n . 256The recommended way to start X11 programs at a remote site is with 257something like 258.Ic ssh -f host xterm . 259.Pp 260If the 261.Cm ExitOnForwardFailure 262configuration option is set to 263.Dq yes , 264then a client started with 265.Fl f 266will wait for all remote port forwards to be successfully established 267before placing itself in the background. 268Refer to the description of 269.Cm ForkAfterAuthentication 270in 271.Xr ssh_config 5 272for details. 273.Pp 274.It Fl G 275Causes 276.Nm 277to print its configuration after evaluating 278.Cm Host 279and 280.Cm Match 281blocks and exit. 282.Pp 283.It Fl g 284Allows remote hosts to connect to local forwarded ports. 285If used on a multiplexed connection, then this option must be specified 286on the master process. 287.Pp 288.It Fl I Ar pkcs11 289Specify the PKCS#11 shared library 290.Nm 291should use to communicate with a PKCS#11 token providing keys for user 292authentication. 293.Pp 294.It Fl i Ar identity_file 295Selects a file from which the identity (private key) for 296public key authentication is read. 297You can also specify a public key file to use the corresponding 298private key that is loaded in 299.Xr ssh-agent 1 300when the private key file is not present locally. 301The default is 302.Pa ~/.ssh/id_rsa , 303.Pa ~/.ssh/id_ecdsa , 304.Pa ~/.ssh/id_ecdsa_sk , 305.Pa ~/.ssh/id_ed25519 , 306.Pa ~/.ssh/id_ed25519_sk 307and 308.Pa ~/.ssh/id_dsa . 309Identity files may also be specified on 310a per-host basis in the configuration file. 311It is possible to have multiple 312.Fl i 313options (and multiple identities specified in 314configuration files). 315If no certificates have been explicitly specified by the 316.Cm CertificateFile 317directive, 318.Nm 319will also try to load certificate information from the filename obtained 320by appending 321.Pa -cert.pub 322to identity filenames. 323.Pp 324.It Fl J Ar destination 325Connect to the target host by first making a 326.Nm 327connection to the jump host described by 328.Ar destination 329and then establishing a TCP forwarding to the ultimate destination from 330there. 331Multiple jump hops may be specified separated by comma characters. 332This is a shortcut to specify a 333.Cm ProxyJump 334configuration directive. 335Note that configuration directives supplied on the command-line generally 336apply to the destination host and not any specified jump hosts. 337Use 338.Pa ~/.ssh/config 339to specify configuration for jump hosts. 340.Pp 341.It Fl K 342Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 343credentials to the server. 344.Pp 345.It Fl k 346Disables forwarding (delegation) of GSSAPI credentials to the server. 347.Pp 348.It Fl L Xo 349.Sm off 350.Oo Ar bind_address : Oc 351.Ar port : host : hostport 352.Sm on 353.Xc 354.It Fl L Xo 355.Sm off 356.Oo Ar bind_address : Oc 357.Ar port : remote_socket 358.Sm on 359.Xc 360.It Fl L Xo 361.Sm off 362.Ar local_socket : host : hostport 363.Sm on 364.Xc 365.It Fl L Xo 366.Sm off 367.Ar local_socket : remote_socket 368.Sm on 369.Xc 370Specifies that connections to the given TCP port or Unix socket on the local 371(client) host are to be forwarded to the given host and port, or Unix socket, 372on the remote side. 373This works by allocating a socket to listen to either a TCP 374.Ar port 375on the local side, optionally bound to the specified 376.Ar bind_address , 377or to a Unix socket. 378Whenever a connection is made to the local port or socket, the 379connection is forwarded over the secure channel, and a connection is 380made to either 381.Ar host 382port 383.Ar hostport , 384or the Unix socket 385.Ar remote_socket , 386from the remote machine. 387.Pp 388Port forwardings can also be specified in the configuration file. 389Only the superuser can forward privileged ports. 390IPv6 addresses can be specified by enclosing the address in square brackets. 391.Pp 392By default, the local port is bound in accordance with the 393.Cm GatewayPorts 394setting. 395However, an explicit 396.Ar bind_address 397may be used to bind the connection to a specific address. 398The 399.Ar bind_address 400of 401.Dq localhost 402indicates that the listening port be bound for local use only, while an 403empty address or 404.Sq * 405indicates that the port should be available from all interfaces. 406.Pp 407.It Fl l Ar login_name 408Specifies the user to log in as on the remote machine. 409This also may be specified on a per-host basis in the configuration file. 410.Pp 411.It Fl M 412Places the 413.Nm 414client into 415.Dq master 416mode for connection sharing. 417Multiple 418.Fl M 419options places 420.Nm 421into 422.Dq master 423mode but with confirmation required using 424.Xr ssh-askpass 1 425before each operation that changes the multiplexing state 426(e.g. opening a new session). 427Refer to the description of 428.Cm ControlMaster 429in 430.Xr ssh_config 5 431for details. 432.Pp 433.It Fl m Ar mac_spec 434A comma-separated list of MAC (message authentication code) algorithms, 435specified in order of preference. 436See the 437.Cm MACs 438keyword in 439.Xr ssh_config 5 440for more information. 441.Pp 442.It Fl N 443Do not execute a remote command. 444This is useful for just forwarding ports. 445Refer to the description of 446.Cm SessionType 447in 448.Xr ssh_config 5 449for details. 450.Pp 451.It Fl n 452Redirects stdin from 453.Pa /dev/null 454(actually, prevents reading from stdin). 455This must be used when 456.Nm 457is run in the background. 458A common trick is to use this to run X11 programs on a remote machine. 459For example, 460.Ic ssh -n shadows.cs.hut.fi emacs & 461will start an emacs on shadows.cs.hut.fi, and the X11 462connection will be automatically forwarded over an encrypted channel. 463The 464.Nm 465program will be put in the background. 466(This does not work if 467.Nm 468needs to ask for a password or passphrase; see also the 469.Fl f 470option.) 471Refer to the description of 472.Cm StdinNull 473in 474.Xr ssh_config 5 475for details. 476.Pp 477.It Fl O Ar ctl_cmd 478Control an active connection multiplexing master process. 479When the 480.Fl O 481option is specified, the 482.Ar ctl_cmd 483argument is interpreted and passed to the master process. 484Valid commands are: 485.Dq check 486(check that the master process is running), 487.Dq forward 488(request forwardings without command execution), 489.Dq cancel 490(cancel forwardings), 491.Dq exit 492(request the master to exit), and 493.Dq stop 494(request the master to stop accepting further multiplexing requests). 495.Pp 496.It Fl o Ar option 497Can be used to give options in the format used in the configuration file. 498This is useful for specifying options for which there is no separate 499command-line flag. 500For full details of the options listed below, and their possible values, see 501.Xr ssh_config 5 . 502.Pp 503.Bl -tag -width Ds -offset indent -compact 504.It AddKeysToAgent 505.It AddressFamily 506.It BatchMode 507.It BindAddress 508.It CanonicalDomains 509.It CanonicalizeFallbackLocal 510.It CanonicalizeHostname 511.It CanonicalizeMaxDots 512.It CanonicalizePermittedCNAMEs 513.It CASignatureAlgorithms 514.It CertificateFile 515.It CheckHostIP 516.It Ciphers 517.It ClearAllForwardings 518.It Compression 519.It ConnectionAttempts 520.It ConnectTimeout 521.It ControlMaster 522.It ControlPath 523.It ControlPersist 524.It DynamicForward 525.It EscapeChar 526.It ExitOnForwardFailure 527.It FingerprintHash 528.It ForkAfterAuthentication 529.It ForwardAgent 530.It ForwardX11 531.It ForwardX11Timeout 532.It ForwardX11Trusted 533.It GatewayPorts 534.It GlobalKnownHostsFile 535.It GSSAPIAuthentication 536.It GSSAPIDelegateCredentials 537.It HashKnownHosts 538.It Host 539.It HostbasedAcceptedAlgorithms 540.It HostbasedAuthentication 541.It HostKeyAlgorithms 542.It HostKeyAlias 543.It Hostname 544.It IdentitiesOnly 545.It IdentityAgent 546.It IdentityFile 547.It IPQoS 548.It KbdInteractiveAuthentication 549.It KbdInteractiveDevices 550.It KexAlgorithms 551.It KnownHostsCommand 552.It LocalCommand 553.It LocalForward 554.It LogLevel 555.It MACs 556.It Match 557.It NoHostAuthenticationForLocalhost 558.It NumberOfPasswordPrompts 559.It PasswordAuthentication 560.It PermitLocalCommand 561.It PermitRemoteOpen 562.It PKCS11Provider 563.It Port 564.It PreferredAuthentications 565.It ProxyCommand 566.It ProxyJump 567.It ProxyUseFdpass 568.It PubkeyAcceptedAlgorithms 569.It PubkeyAuthentication 570.It RekeyLimit 571.It RemoteCommand 572.It RemoteForward 573.It RequestTTY 574.It RequiredRSASize 575.It SendEnv 576.It ServerAliveInterval 577.It ServerAliveCountMax 578.It SessionType 579.It SetEnv 580.It StdinNull 581.It StreamLocalBindMask 582.It StreamLocalBindUnlink 583.It StrictHostKeyChecking 584.It TCPKeepAlive 585.It Tunnel 586.It TunnelDevice 587.It UpdateHostKeys 588.It User 589.It UserKnownHostsFile 590.It VerifyHostKeyDNS 591.It VisualHostKey 592.It XAuthLocation 593.El 594.Pp 595.It Fl p Ar port 596Port to connect to on the remote host. 597This can be specified on a 598per-host basis in the configuration file. 599.Pp 600.It Fl Q Ar query_option 601Queries for the algorithms supported by one of the following features: 602.Ar cipher 603(supported symmetric ciphers), 604.Ar cipher-auth 605(supported symmetric ciphers that support authenticated encryption), 606.Ar help 607(supported query terms for use with the 608.Fl Q 609flag), 610.Ar mac 611(supported message integrity codes), 612.Ar kex 613(key exchange algorithms), 614.Ar key 615(key types), 616.Ar key-cert 617(certificate key types), 618.Ar key-plain 619(non-certificate key types), 620.Ar key-sig 621(all key types and signature algorithms), 622.Ar protocol-version 623(supported SSH protocol versions), and 624.Ar sig 625(supported signature algorithms). 626Alternatively, any keyword from 627.Xr ssh_config 5 628or 629.Xr sshd_config 5 630that takes an algorithm list may be used as an alias for the corresponding 631query_option. 632.Pp 633.It Fl q 634Quiet mode. 635Causes most warning and diagnostic messages to be suppressed. 636.Pp 637.It Fl R Xo 638.Sm off 639.Oo Ar bind_address : Oc 640.Ar port : host : hostport 641.Sm on 642.Xc 643.It Fl R Xo 644.Sm off 645.Oo Ar bind_address : Oc 646.Ar port : local_socket 647.Sm on 648.Xc 649.It Fl R Xo 650.Sm off 651.Ar remote_socket : host : hostport 652.Sm on 653.Xc 654.It Fl R Xo 655.Sm off 656.Ar remote_socket : local_socket 657.Sm on 658.Xc 659.It Fl R Xo 660.Sm off 661.Oo Ar bind_address : Oc 662.Ar port 663.Sm on 664.Xc 665Specifies that connections to the given TCP port or Unix socket on the remote 666(server) host are to be forwarded to the local side. 667.Pp 668This works by allocating a socket to listen to either a TCP 669.Ar port 670or to a Unix socket on the remote side. 671Whenever a connection is made to this port or Unix socket, the 672connection is forwarded over the secure channel, and a connection 673is made from the local machine to either an explicit destination specified by 674.Ar host 675port 676.Ar hostport , 677or 678.Ar local_socket , 679or, if no explicit destination was specified, 680.Nm 681will act as a SOCKS 4/5 proxy and forward connections to the destinations 682requested by the remote SOCKS client. 683.Pp 684Port forwardings can also be specified in the configuration file. 685Privileged ports can be forwarded only when 686logging in as root on the remote machine. 687IPv6 addresses can be specified by enclosing the address in square brackets. 688.Pp 689By default, TCP listening sockets on the server will be bound to the loopback 690interface only. 691This may be overridden by specifying a 692.Ar bind_address . 693An empty 694.Ar bind_address , 695or the address 696.Ql * , 697indicates that the remote socket should listen on all interfaces. 698Specifying a remote 699.Ar bind_address 700will only succeed if the server's 701.Cm GatewayPorts 702option is enabled (see 703.Xr sshd_config 5 ) . 704.Pp 705If the 706.Ar port 707argument is 708.Ql 0 , 709the listen port will be dynamically allocated on the server and reported 710to the client at run time. 711When used together with 712.Ic -O forward , 713the allocated port will be printed to the standard output. 714.Pp 715.It Fl S Ar ctl_path 716Specifies the location of a control socket for connection sharing, 717or the string 718.Dq none 719to disable connection sharing. 720Refer to the description of 721.Cm ControlPath 722and 723.Cm ControlMaster 724in 725.Xr ssh_config 5 726for details. 727.Pp 728.It Fl s 729May be used to request invocation of a subsystem on the remote system. 730Subsystems facilitate the use of SSH 731as a secure transport for other applications (e.g.\& 732.Xr sftp 1 ) . 733The subsystem is specified as the remote command. 734Refer to the description of 735.Cm SessionType 736in 737.Xr ssh_config 5 738for details. 739.Pp 740.It Fl T 741Disable pseudo-terminal allocation. 742.Pp 743.It Fl t 744Force pseudo-terminal allocation. 745This can be used to execute arbitrary 746screen-based programs on a remote machine, which can be very useful, 747e.g. when implementing menu services. 748Multiple 749.Fl t 750options force tty allocation, even if 751.Nm 752has no local tty. 753.Pp 754.It Fl V 755Display the version number and exit. 756.Pp 757.It Fl v 758Verbose mode. 759Causes 760.Nm 761to print debugging messages about its progress. 762This is helpful in 763debugging connection, authentication, and configuration problems. 764Multiple 765.Fl v 766options increase the verbosity. 767The maximum is 3. 768.Pp 769.It Fl W Ar host : Ns Ar port 770Requests that standard input and output on the client be forwarded to 771.Ar host 772on 773.Ar port 774over the secure channel. 775Implies 776.Fl N , 777.Fl T , 778.Cm ExitOnForwardFailure 779and 780.Cm ClearAllForwardings , 781though these can be overridden in the configuration file or using 782.Fl o 783command line options. 784.Pp 785.It Fl w Xo 786.Ar local_tun Ns Op : Ns Ar remote_tun 787.Xc 788Requests 789tunnel 790device forwarding with the specified 791.Xr tun 4 792devices between the client 793.Pq Ar local_tun 794and the server 795.Pq Ar remote_tun . 796.Pp 797The devices may be specified by numerical ID or the keyword 798.Dq any , 799which uses the next available tunnel device. 800If 801.Ar remote_tun 802is not specified, it defaults to 803.Dq any . 804See also the 805.Cm Tunnel 806and 807.Cm TunnelDevice 808directives in 809.Xr ssh_config 5 . 810.Pp 811If the 812.Cm Tunnel 813directive is unset, it will be set to the default tunnel mode, which is 814.Dq point-to-point . 815If a different 816.Cm Tunnel 817forwarding mode it desired, then it should be specified before 818.Fl w . 819.Pp 820.It Fl X 821Enables X11 forwarding. 822This can also be specified on a per-host basis in a configuration file. 823.Pp 824X11 forwarding should be enabled with caution. 825Users with the ability to bypass file permissions on the remote host 826(for the user's X authorization database) 827can access the local X11 display through the forwarded connection. 828An attacker may then be able to perform activities such as keystroke monitoring. 829.Pp 830For this reason, X11 forwarding is subjected to X11 SECURITY extension 831restrictions by default. 832Refer to the 833.Nm 834.Fl Y 835option and the 836.Cm ForwardX11Trusted 837directive in 838.Xr ssh_config 5 839for more information. 840.Pp 841.It Fl x 842Disables X11 forwarding. 843.Pp 844.It Fl Y 845Enables trusted X11 forwarding. 846Trusted X11 forwardings are not subjected to the X11 SECURITY extension 847controls. 848.Pp 849.It Fl y 850Send log information using the 851.Xr syslog 3 852system module. 853By default this information is sent to stderr. 854.El 855.Pp 856.Nm 857may additionally obtain configuration data from 858a per-user configuration file and a system-wide configuration file. 859The file format and configuration options are described in 860.Xr ssh_config 5 . 861.Sh AUTHENTICATION 862The OpenSSH SSH client supports SSH protocol 2. 863.Pp 864The methods available for authentication are: 865GSSAPI-based authentication, 866host-based authentication, 867public key authentication, 868keyboard-interactive authentication, 869and password authentication. 870Authentication methods are tried in the order specified above, 871though 872.Cm PreferredAuthentications 873can be used to change the default order. 874.Pp 875Host-based authentication works as follows: 876If the machine the user logs in from is listed in 877.Pa /etc/hosts.equiv 878or 879.Pa /etc/shosts.equiv 880on the remote machine, the user is non-root and the user names are 881the same on both sides, or if the files 882.Pa ~/.rhosts 883or 884.Pa ~/.shosts 885exist in the user's home directory on the 886remote machine and contain a line containing the name of the client 887machine and the name of the user on that machine, the user is 888considered for login. 889Additionally, the server 890.Em must 891be able to verify the client's 892host key (see the description of 893.Pa /etc/ssh/ssh_known_hosts 894and 895.Pa ~/.ssh/known_hosts , 896below) 897for login to be permitted. 898This authentication method closes security holes due to IP 899spoofing, DNS spoofing, and routing spoofing. 900[Note to the administrator: 901.Pa /etc/hosts.equiv , 902.Pa ~/.rhosts , 903and the rlogin/rsh protocol in general, are inherently insecure and should be 904disabled if security is desired.] 905.Pp 906Public key authentication works as follows: 907The scheme is based on public-key cryptography, 908using cryptosystems 909where encryption and decryption are done using separate keys, 910and it is unfeasible to derive the decryption key from the encryption key. 911The idea is that each user creates a public/private 912key pair for authentication purposes. 913The server knows the public key, and only the user knows the private key. 914.Nm 915implements public key authentication protocol automatically, 916using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 917The HISTORY section of 918.Xr ssl 8 919contains a brief discussion of the DSA and RSA algorithms. 920.Pp 921The file 922.Pa ~/.ssh/authorized_keys 923lists the public keys that are permitted for logging in. 924When the user logs in, the 925.Nm 926program tells the server which key pair it would like to use for 927authentication. 928The client proves that it has access to the private key 929and the server checks that the corresponding public key 930is authorized to accept the account. 931.Pp 932The server may inform the client of errors that prevented public key 933authentication from succeeding after authentication completes using a 934different method. 935These may be viewed by increasing the 936.Cm LogLevel 937to 938.Cm DEBUG 939or higher (e.g. by using the 940.Fl v 941flag). 942.Pp 943The user creates their key pair by running 944.Xr ssh-keygen 1 . 945This stores the private key in 946.Pa ~/.ssh/id_dsa 947(DSA), 948.Pa ~/.ssh/id_ecdsa 949(ECDSA), 950.Pa ~/.ssh/id_ecdsa_sk 951(authenticator-hosted ECDSA), 952.Pa ~/.ssh/id_ed25519 953(Ed25519), 954.Pa ~/.ssh/id_ed25519_sk 955(authenticator-hosted Ed25519), 956or 957.Pa ~/.ssh/id_rsa 958(RSA) 959and stores the public key in 960.Pa ~/.ssh/id_dsa.pub 961(DSA), 962.Pa ~/.ssh/id_ecdsa.pub 963(ECDSA), 964.Pa ~/.ssh/id_ecdsa_sk.pub 965(authenticator-hosted ECDSA), 966.Pa ~/.ssh/id_ed25519.pub 967(Ed25519), 968.Pa ~/.ssh/id_ed25519_sk.pub 969(authenticator-hosted Ed25519), 970or 971.Pa ~/.ssh/id_rsa.pub 972(RSA) 973in the user's home directory. 974The user should then copy the public key 975to 976.Pa ~/.ssh/authorized_keys 977in their home directory on the remote machine. 978The 979.Pa authorized_keys 980file corresponds to the conventional 981.Pa ~/.rhosts 982file, and has one key 983per line, though the lines can be very long. 984After this, the user can log in without giving the password. 985.Pp 986A variation on public key authentication 987is available in the form of certificate authentication: 988instead of a set of public/private keys, 989signed certificates are used. 990This has the advantage that a single trusted certification authority 991can be used in place of many public/private keys. 992See the CERTIFICATES section of 993.Xr ssh-keygen 1 994for more information. 995.Pp 996The most convenient way to use public key or certificate authentication 997may be with an authentication agent. 998See 999.Xr ssh-agent 1 1000and (optionally) the 1001.Cm AddKeysToAgent 1002directive in 1003.Xr ssh_config 5 1004for more information. 1005.Pp 1006Keyboard-interactive authentication works as follows: 1007The server sends an arbitrary 1008.Qq challenge 1009text and prompts for a response, possibly multiple times. 1010Examples of keyboard-interactive authentication include 1011.Bx 1012Authentication (see 1013.Xr login.conf 5 ) 1014and PAM (some 1015.Pf non- Ox 1016systems). 1017.Pp 1018Finally, if other authentication methods fail, 1019.Nm 1020prompts the user for a password. 1021The password is sent to the remote 1022host for checking; however, since all communications are encrypted, 1023the password cannot be seen by someone listening on the network. 1024.Pp 1025.Nm 1026automatically maintains and checks a database containing 1027identification for all hosts it has ever been used with. 1028Host keys are stored in 1029.Pa ~/.ssh/known_hosts 1030in the user's home directory. 1031Additionally, the file 1032.Pa /etc/ssh/ssh_known_hosts 1033is automatically checked for known hosts. 1034Any new hosts are automatically added to the user's file. 1035If a host's identification ever changes, 1036.Nm 1037warns about this and disables password authentication to prevent 1038server spoofing or man-in-the-middle attacks, 1039which could otherwise be used to circumvent the encryption. 1040The 1041.Cm StrictHostKeyChecking 1042option can be used to control logins to machines whose 1043host key is not known or has changed. 1044.Pp 1045When the user's identity has been accepted by the server, the server 1046either executes the given command in a non-interactive session or, 1047if no command has been specified, logs into the machine and gives 1048the user a normal shell as an interactive session. 1049All communication with 1050the remote command or shell will be automatically encrypted. 1051.Pp 1052If an interactive session is requested, 1053.Nm 1054by default will only request a pseudo-terminal (pty) for interactive 1055sessions when the client has one. 1056The flags 1057.Fl T 1058and 1059.Fl t 1060can be used to override this behaviour. 1061.Pp 1062If a pseudo-terminal has been allocated, the 1063user may use the escape characters noted below. 1064.Pp 1065If no pseudo-terminal has been allocated, 1066the session is transparent and can be used to reliably transfer binary data. 1067On most systems, setting the escape character to 1068.Dq none 1069will also make the session transparent even if a tty is used. 1070.Pp 1071The session terminates when the command or shell on the remote 1072machine exits and all X11 and TCP connections have been closed. 1073.Sh ESCAPE CHARACTERS 1074When a pseudo-terminal has been requested, 1075.Nm 1076supports a number of functions through the use of an escape character. 1077.Pp 1078A single tilde character can be sent as 1079.Ic ~~ 1080or by following the tilde by a character other than those described below. 1081The escape character must always follow a newline to be interpreted as 1082special. 1083The escape character can be changed in configuration files using the 1084.Cm EscapeChar 1085configuration directive or on the command line by the 1086.Fl e 1087option. 1088.Pp 1089The supported escapes (assuming the default 1090.Ql ~ ) 1091are: 1092.Bl -tag -width Ds 1093.It Cm ~. 1094Disconnect. 1095.It Cm ~^Z 1096Background 1097.Nm . 1098.It Cm ~# 1099List forwarded connections. 1100.It Cm ~& 1101Background 1102.Nm 1103at logout when waiting for forwarded connection / X11 sessions to terminate. 1104.It Cm ~? 1105Display a list of escape characters. 1106.It Cm ~B 1107Send a BREAK to the remote system 1108(only useful if the peer supports it). 1109.It Cm ~C 1110Open command line. 1111Currently this allows the addition of port forwardings using the 1112.Fl L , 1113.Fl R 1114and 1115.Fl D 1116options (see above). 1117It also allows the cancellation of existing port-forwardings 1118with 1119.Sm off 1120.Fl KL Oo Ar bind_address : Oc Ar port 1121.Sm on 1122for local, 1123.Sm off 1124.Fl KR Oo Ar bind_address : Oc Ar port 1125.Sm on 1126for remote and 1127.Sm off 1128.Fl KD Oo Ar bind_address : Oc Ar port 1129.Sm on 1130for dynamic port-forwardings. 1131.Ic !\& Ns Ar command 1132allows the user to execute a local command if the 1133.Ic PermitLocalCommand 1134option is enabled in 1135.Xr ssh_config 5 . 1136Basic help is available, using the 1137.Fl h 1138option. 1139.It Cm ~R 1140Request rekeying of the connection 1141(only useful if the peer supports it). 1142.It Cm ~V 1143Decrease the verbosity 1144.Pq Ic LogLevel 1145when errors are being written to stderr. 1146.It Cm ~v 1147Increase the verbosity 1148.Pq Ic LogLevel 1149when errors are being written to stderr. 1150.El 1151.Sh TCP FORWARDING 1152Forwarding of arbitrary TCP connections over a secure channel 1153can be specified either on the command line or in a configuration file. 1154One possible application of TCP forwarding is a secure connection to a 1155mail server; another is going through firewalls. 1156.Pp 1157In the example below, we look at encrypting communication for an IRC client, 1158even though the IRC server it connects to does not directly 1159support encrypted communication. 1160This works as follows: 1161the user connects to the remote host using 1162.Nm , 1163specifying the ports to be used to forward the connection. 1164After that it is possible to start the program locally, 1165and 1166.Nm 1167will encrypt and forward the connection to the remote server. 1168.Pp 1169The following example tunnels an IRC session from the client 1170to an IRC server at 1171.Dq server.example.com , 1172joining channel 1173.Dq #users , 1174nickname 1175.Dq pinky , 1176using the standard IRC port, 6667: 1177.Bd -literal -offset 4n 1178$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 1179$ irc -c '#users' pinky IRC/127.0.0.1 1180.Ed 1181.Pp 1182The 1183.Fl f 1184option backgrounds 1185.Nm 1186and the remote command 1187.Dq sleep 10 1188is specified to allow an amount of time 1189(10 seconds, in the example) 1190to start the program which is going to use the tunnel. 1191If no connections are made within the time specified, 1192.Nm 1193will exit. 1194.Sh X11 FORWARDING 1195If the 1196.Cm ForwardX11 1197variable is set to 1198.Dq yes 1199(or see the description of the 1200.Fl X , 1201.Fl x , 1202and 1203.Fl Y 1204options above) 1205and the user is using X11 (the 1206.Ev DISPLAY 1207environment variable is set), the connection to the X11 display is 1208automatically forwarded to the remote side in such a way that any X11 1209programs started from the shell (or command) will go through the 1210encrypted channel, and the connection to the real X server will be made 1211from the local machine. 1212The user should not manually set 1213.Ev DISPLAY . 1214Forwarding of X11 connections can be 1215configured on the command line or in configuration files. 1216.Pp 1217The 1218.Ev DISPLAY 1219value set by 1220.Nm 1221will point to the server machine, but with a display number greater than zero. 1222This is normal, and happens because 1223.Nm 1224creates a 1225.Dq proxy 1226X server on the server machine for forwarding the 1227connections over the encrypted channel. 1228.Pp 1229.Nm 1230will also automatically set up Xauthority data on the server machine. 1231For this purpose, it will generate a random authorization cookie, 1232store it in Xauthority on the server, and verify that any forwarded 1233connections carry this cookie and replace it by the real cookie when 1234the connection is opened. 1235The real authentication cookie is never 1236sent to the server machine (and no cookies are sent in the plain). 1237.Pp 1238If the 1239.Cm ForwardAgent 1240variable is set to 1241.Dq yes 1242(or see the description of the 1243.Fl A 1244and 1245.Fl a 1246options above) and 1247the user is using an authentication agent, the connection to the agent 1248is automatically forwarded to the remote side. 1249.Sh VERIFYING HOST KEYS 1250When connecting to a server for the first time, 1251a fingerprint of the server's public key is presented to the user 1252(unless the option 1253.Cm StrictHostKeyChecking 1254has been disabled). 1255Fingerprints can be determined using 1256.Xr ssh-keygen 1 : 1257.Pp 1258.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1259.Pp 1260If the fingerprint is already known, it can be matched 1261and the key can be accepted or rejected. 1262If only legacy (MD5) fingerprints for the server are available, the 1263.Xr ssh-keygen 1 1264.Fl E 1265option may be used to downgrade the fingerprint algorithm to match. 1266.Pp 1267Because of the difficulty of comparing host keys 1268just by looking at fingerprint strings, 1269there is also support to compare host keys visually, 1270using 1271.Em random art . 1272By setting the 1273.Cm VisualHostKey 1274option to 1275.Dq yes , 1276a small ASCII graphic gets displayed on every login to a server, no matter 1277if the session itself is interactive or not. 1278By learning the pattern a known server produces, a user can easily 1279find out that the host key has changed when a completely different pattern 1280is displayed. 1281Because these patterns are not unambiguous however, a pattern that looks 1282similar to the pattern remembered only gives a good probability that the 1283host key is the same, not guaranteed proof. 1284.Pp 1285To get a listing of the fingerprints along with their random art for 1286all known hosts, the following command line can be used: 1287.Pp 1288.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1289.Pp 1290If the fingerprint is unknown, 1291an alternative method of verification is available: 1292SSH fingerprints verified by DNS. 1293An additional resource record (RR), 1294SSHFP, 1295is added to a zonefile 1296and the connecting client is able to match the fingerprint 1297with that of the key presented. 1298.Pp 1299In this example, we are connecting a client to a server, 1300.Dq host.example.com . 1301The SSHFP resource records should first be added to the zonefile for 1302host.example.com: 1303.Bd -literal -offset indent 1304$ ssh-keygen -r host.example.com. 1305.Ed 1306.Pp 1307The output lines will have to be added to the zonefile. 1308To check that the zone is answering fingerprint queries: 1309.Pp 1310.Dl $ dig -t SSHFP host.example.com 1311.Pp 1312Finally the client connects: 1313.Bd -literal -offset indent 1314$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1315[...] 1316Matching host key fingerprint found in DNS. 1317Are you sure you want to continue connecting (yes/no)? 1318.Ed 1319.Pp 1320See the 1321.Cm VerifyHostKeyDNS 1322option in 1323.Xr ssh_config 5 1324for more information. 1325.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1326.Nm 1327contains support for Virtual Private Network (VPN) tunnelling 1328using the 1329.Xr tun 4 1330network pseudo-device, 1331allowing two networks to be joined securely. 1332The 1333.Xr sshd_config 5 1334configuration option 1335.Cm PermitTunnel 1336controls whether the server supports this, 1337and at what level (layer 2 or 3 traffic). 1338.Pp 1339The following example would connect client network 10.0.50.0/24 1340with remote network 10.0.99.0/24 using a point-to-point connection 1341from 10.1.1.1 to 10.1.1.2, 1342provided that the SSH server running on the gateway to the remote network, 1343at 192.168.1.15, allows it. 1344.Pp 1345On the client: 1346.Bd -literal -offset indent 1347# ssh -f -w 0:1 192.168.1.15 true 1348# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1349# route add 10.0.99.0/24 10.1.1.2 1350.Ed 1351.Pp 1352On the server: 1353.Bd -literal -offset indent 1354# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1355# route add 10.0.50.0/24 10.1.1.1 1356.Ed 1357.Pp 1358Client access may be more finely tuned via the 1359.Pa /root/.ssh/authorized_keys 1360file (see below) and the 1361.Cm PermitRootLogin 1362server option. 1363The following entry would permit connections on 1364.Xr tun 4 1365device 1 from user 1366.Dq jane 1367and on tun device 2 from user 1368.Dq john , 1369if 1370.Cm PermitRootLogin 1371is set to 1372.Dq forced-commands-only : 1373.Bd -literal -offset 2n 1374tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1375tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1376.Ed 1377.Pp 1378Since an SSH-based setup entails a fair amount of overhead, 1379it may be more suited to temporary setups, 1380such as for wireless VPNs. 1381More permanent VPNs are better provided by tools such as 1382.Xr ipsecctl 8 1383and 1384.Xr isakmpd 8 . 1385.Sh ENVIRONMENT 1386.Nm 1387will normally set the following environment variables: 1388.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1389.It Ev DISPLAY 1390The 1391.Ev DISPLAY 1392variable indicates the location of the X11 server. 1393It is automatically set by 1394.Nm 1395to point to a value of the form 1396.Dq hostname:n , 1397where 1398.Dq hostname 1399indicates the host where the shell runs, and 1400.Sq n 1401is an integer \*(Ge 1. 1402.Nm 1403uses this special value to forward X11 connections over the secure 1404channel. 1405The user should normally not set 1406.Ev DISPLAY 1407explicitly, as that 1408will render the X11 connection insecure (and will require the user to 1409manually copy any required authorization cookies). 1410.It Ev HOME 1411Set to the path of the user's home directory. 1412.It Ev LOGNAME 1413Synonym for 1414.Ev USER ; 1415set for compatibility with systems that use this variable. 1416.It Ev MAIL 1417Set to the path of the user's mailbox. 1418.It Ev PATH 1419Set to the default 1420.Ev PATH , 1421as specified when compiling 1422.Nm . 1423.It Ev SSH_ASKPASS 1424If 1425.Nm 1426needs a passphrase, it will read the passphrase from the current 1427terminal if it was run from a terminal. 1428If 1429.Nm 1430does not have a terminal associated with it but 1431.Ev DISPLAY 1432and 1433.Ev SSH_ASKPASS 1434are set, it will execute the program specified by 1435.Ev SSH_ASKPASS 1436and open an X11 window to read the passphrase. 1437This is particularly useful when calling 1438.Nm 1439from a 1440.Pa .xsession 1441or related script. 1442(Note that on some machines it 1443may be necessary to redirect the input from 1444.Pa /dev/null 1445to make this work.) 1446.It Ev SSH_ASKPASS_REQUIRE 1447Allows further control over the use of an askpass program. 1448If this variable is set to 1449.Dq never 1450then 1451.Nm 1452will never attempt to use one. 1453If it is set to 1454.Dq prefer , 1455then 1456.Nm 1457will prefer to use the askpass program instead of the TTY when requesting 1458passwords. 1459Finally, if the variable is set to 1460.Dq force , 1461then the askpass program will be used for all passphrase input regardless 1462of whether 1463.Ev DISPLAY 1464is set. 1465.It Ev SSH_AUTH_SOCK 1466Identifies the path of a 1467.Ux Ns -domain 1468socket used to communicate with the agent. 1469.It Ev SSH_CONNECTION 1470Identifies the client and server ends of the connection. 1471The variable contains 1472four space-separated values: client IP address, client port number, 1473server IP address, and server port number. 1474.It Ev SSH_ORIGINAL_COMMAND 1475This variable contains the original command line if a forced command 1476is executed. 1477It can be used to extract the original arguments. 1478.It Ev SSH_TTY 1479This is set to the name of the tty (path to the device) associated 1480with the current shell or command. 1481If the current session has no tty, 1482this variable is not set. 1483.It Ev SSH_TUNNEL 1484Optionally set by 1485.Xr sshd 8 1486to contain the interface names assigned if tunnel forwarding was 1487requested by the client. 1488.It Ev SSH_USER_AUTH 1489Optionally set by 1490.Xr sshd 8 , 1491this variable may contain a pathname to a file that lists the authentication 1492methods successfully used when the session was established, including any 1493public keys that were used. 1494.It Ev TZ 1495This variable is set to indicate the present time zone if it 1496was set when the daemon was started (i.e. the daemon passes the value 1497on to new connections). 1498.It Ev USER 1499Set to the name of the user logging in. 1500.El 1501.Pp 1502Additionally, 1503.Nm 1504reads 1505.Pa ~/.ssh/environment , 1506and adds lines of the format 1507.Dq VARNAME=value 1508to the environment if the file exists and users are allowed to 1509change their environment. 1510For more information, see the 1511.Cm PermitUserEnvironment 1512option in 1513.Xr sshd_config 5 . 1514.Sh FILES 1515.Bl -tag -width Ds -compact 1516.It Pa ~/.rhosts 1517This file is used for host-based authentication (see above). 1518On some machines this file may need to be 1519world-readable if the user's home directory is on an NFS partition, 1520because 1521.Xr sshd 8 1522reads it as root. 1523Additionally, this file must be owned by the user, 1524and must not have write permissions for anyone else. 1525The recommended 1526permission for most machines is read/write for the user, and not 1527accessible by others. 1528.Pp 1529.It Pa ~/.shosts 1530This file is used in exactly the same way as 1531.Pa .rhosts , 1532but allows host-based authentication without permitting login with 1533rlogin/rsh. 1534.Pp 1535.It Pa ~/.ssh/ 1536This directory is the default location for all user-specific configuration 1537and authentication information. 1538There is no general requirement to keep the entire contents of this directory 1539secret, but the recommended permissions are read/write/execute for the user, 1540and not accessible by others. 1541.Pp 1542.It Pa ~/.ssh/authorized_keys 1543Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1544that can be used for logging in as this user. 1545The format of this file is described in the 1546.Xr sshd 8 1547manual page. 1548This file is not highly sensitive, but the recommended 1549permissions are read/write for the user, and not accessible by others. 1550.Pp 1551.It Pa ~/.ssh/config 1552This is the per-user configuration file. 1553The file format and configuration options are described in 1554.Xr ssh_config 5 . 1555Because of the potential for abuse, this file must have strict permissions: 1556read/write for the user, and not writable by others. 1557.Pp 1558.It Pa ~/.ssh/environment 1559Contains additional definitions for environment variables; see 1560.Sx ENVIRONMENT , 1561above. 1562.Pp 1563.It Pa ~/.ssh/id_dsa 1564.It Pa ~/.ssh/id_ecdsa 1565.It Pa ~/.ssh/id_ecdsa_sk 1566.It Pa ~/.ssh/id_ed25519 1567.It Pa ~/.ssh/id_ed25519_sk 1568.It Pa ~/.ssh/id_rsa 1569Contains the private key for authentication. 1570These files 1571contain sensitive data and should be readable by the user but not 1572accessible by others (read/write/execute). 1573.Nm 1574will simply ignore a private key file if it is accessible by others. 1575It is possible to specify a passphrase when 1576generating the key which will be used to encrypt the 1577sensitive part of this file using AES-128. 1578.Pp 1579.It Pa ~/.ssh/id_dsa.pub 1580.It Pa ~/.ssh/id_ecdsa.pub 1581.It Pa ~/.ssh/id_ecdsa_sk.pub 1582.It Pa ~/.ssh/id_ed25519.pub 1583.It Pa ~/.ssh/id_ed25519_sk.pub 1584.It Pa ~/.ssh/id_rsa.pub 1585Contains the public key for authentication. 1586These files are not 1587sensitive and can (but need not) be readable by anyone. 1588.Pp 1589.It Pa ~/.ssh/known_hosts 1590Contains a list of host keys for all hosts the user has logged into 1591that are not already in the systemwide list of known host keys. 1592See 1593.Xr sshd 8 1594for further details of the format of this file. 1595.Pp 1596.It Pa ~/.ssh/rc 1597Commands in this file are executed by 1598.Nm 1599when the user logs in, just before the user's shell (or command) is 1600started. 1601See the 1602.Xr sshd 8 1603manual page for more information. 1604.Pp 1605.It Pa /etc/hosts.equiv 1606This file is for host-based authentication (see above). 1607It should only be writable by root. 1608.Pp 1609.It Pa /etc/shosts.equiv 1610This file is used in exactly the same way as 1611.Pa hosts.equiv , 1612but allows host-based authentication without permitting login with 1613rlogin/rsh. 1614.Pp 1615.It Pa /etc/ssh/ssh_config 1616Systemwide configuration file. 1617The file format and configuration options are described in 1618.Xr ssh_config 5 . 1619.Pp 1620.It Pa /etc/ssh/ssh_host_key 1621.It Pa /etc/ssh/ssh_host_dsa_key 1622.It Pa /etc/ssh/ssh_host_ecdsa_key 1623.It Pa /etc/ssh/ssh_host_ed25519_key 1624.It Pa /etc/ssh/ssh_host_rsa_key 1625These files contain the private parts of the host keys 1626and are used for host-based authentication. 1627.Pp 1628.It Pa /etc/ssh/ssh_known_hosts 1629Systemwide list of known host keys. 1630This file should be prepared by the 1631system administrator to contain the public host keys of all machines in the 1632organization. 1633It should be world-readable. 1634See 1635.Xr sshd 8 1636for further details of the format of this file. 1637.Pp 1638.It Pa /etc/ssh/sshrc 1639Commands in this file are executed by 1640.Nm 1641when the user logs in, just before the user's shell (or command) is started. 1642See the 1643.Xr sshd 8 1644manual page for more information. 1645.El 1646.Sh EXIT STATUS 1647.Nm 1648exits with the exit status of the remote command or with 255 1649if an error occurred. 1650.Sh SEE ALSO 1651.Xr scp 1 , 1652.Xr sftp 1 , 1653.Xr ssh-add 1 , 1654.Xr ssh-agent 1 , 1655.Xr ssh-keygen 1 , 1656.Xr ssh-keyscan 1 , 1657.Xr tun 4 , 1658.Xr ssh_config 5 , 1659.Xr ssh-keysign 8 , 1660.Xr sshd 8 1661.Sh STANDARDS 1662.Rs 1663.%A S. Lehtinen 1664.%A C. Lonvick 1665.%D January 2006 1666.%R RFC 4250 1667.%T The Secure Shell (SSH) Protocol Assigned Numbers 1668.Re 1669.Pp 1670.Rs 1671.%A T. Ylonen 1672.%A C. Lonvick 1673.%D January 2006 1674.%R RFC 4251 1675.%T The Secure Shell (SSH) Protocol Architecture 1676.Re 1677.Pp 1678.Rs 1679.%A T. Ylonen 1680.%A C. Lonvick 1681.%D January 2006 1682.%R RFC 4252 1683.%T The Secure Shell (SSH) Authentication Protocol 1684.Re 1685.Pp 1686.Rs 1687.%A T. Ylonen 1688.%A C. Lonvick 1689.%D January 2006 1690.%R RFC 4253 1691.%T The Secure Shell (SSH) Transport Layer Protocol 1692.Re 1693.Pp 1694.Rs 1695.%A T. Ylonen 1696.%A C. Lonvick 1697.%D January 2006 1698.%R RFC 4254 1699.%T The Secure Shell (SSH) Connection Protocol 1700.Re 1701.Pp 1702.Rs 1703.%A J. Schlyter 1704.%A W. Griffin 1705.%D January 2006 1706.%R RFC 4255 1707.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1708.Re 1709.Pp 1710.Rs 1711.%A F. Cusack 1712.%A M. Forssen 1713.%D January 2006 1714.%R RFC 4256 1715.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1716.Re 1717.Pp 1718.Rs 1719.%A J. Galbraith 1720.%A P. Remaker 1721.%D January 2006 1722.%R RFC 4335 1723.%T The Secure Shell (SSH) Session Channel Break Extension 1724.Re 1725.Pp 1726.Rs 1727.%A M. Bellare 1728.%A T. Kohno 1729.%A C. Namprempre 1730.%D January 2006 1731.%R RFC 4344 1732.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1733.Re 1734.Pp 1735.Rs 1736.%A B. Harris 1737.%D January 2006 1738.%R RFC 4345 1739.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1740.Re 1741.Pp 1742.Rs 1743.%A M. Friedl 1744.%A N. Provos 1745.%A W. Simpson 1746.%D March 2006 1747.%R RFC 4419 1748.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1749.Re 1750.Pp 1751.Rs 1752.%A J. Galbraith 1753.%A R. Thayer 1754.%D November 2006 1755.%R RFC 4716 1756.%T The Secure Shell (SSH) Public Key File Format 1757.Re 1758.Pp 1759.Rs 1760.%A D. Stebila 1761.%A J. Green 1762.%D December 2009 1763.%R RFC 5656 1764.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1765.Re 1766.Pp 1767.Rs 1768.%A A. Perrig 1769.%A D. Song 1770.%D 1999 1771.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1772.%T Hash Visualization: a New Technique to improve Real-World Security 1773.Re 1774.Sh AUTHORS 1775OpenSSH is a derivative of the original and free 1776ssh 1.2.12 release by Tatu Ylonen. 1777Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1778Theo de Raadt and Dug Song 1779removed many bugs, re-added newer features and 1780created OpenSSH. 1781Markus Friedl contributed the support for SSH 1782protocol versions 1.5 and 2.0. 1783