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 1.396 2018/07/19 10:28:47 dtucker Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: July 19 2018 $ 39.Dt SSH 1 40.Os 41.Sh NAME 42.Nm ssh 43.Nd OpenSSH SSH client (remote login program) 44.Sh SYNOPSIS 45.Nm ssh 46.Op Fl 46AaCfGgKkMNnqsTtVvXxYy 47.Op Fl B Ar bind_interface 48.Op Fl b Ar bind_address 49.Op Fl c Ar cipher_spec 50.Op Fl D Oo Ar bind_address : Oc Ns Ar port 51.Op Fl E Ar log_file 52.Op Fl e Ar escape_char 53.Op Fl F Ar configfile 54.Op Fl I Ar pkcs11 55.Op Fl i Ar identity_file 56.Op Fl J Ar destination 57.Op Fl L Ar address 58.Op Fl l Ar login_name 59.Op Fl m Ar mac_spec 60.Op Fl O Ar ctl_cmd 61.Op Fl o Ar option 62.Op Fl p Ar port 63.Op Fl Q Ar query_option 64.Op Fl R Ar address 65.Op Fl S Ar ctl_path 66.Op Fl W Ar host : Ns Ar port 67.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun 68.Ar destination 69.Op Ar command 70.Sh DESCRIPTION 71.Nm 72(SSH client) is a program for logging into a remote machine and for 73executing commands on a remote machine. 74It is intended to provide secure encrypted communications between 75two untrusted hosts over an insecure network. 76X11 connections, arbitrary TCP ports and 77.Ux Ns -domain 78sockets can also be forwarded over the secure channel. 79.Pp 80.Nm 81connects and logs into the specified 82.Ar destination , 83which may be specified as either 84.Sm off 85.Oo user @ Oc hostname 86.Sm on 87or a URI of the form 88.Sm off 89.No ssh:// Oo user @ Oc hostname Op : port . 90.Sm on 91The user must prove 92his/her identity to the remote machine using one of several methods 93(see below). 94.Pp 95If a 96.Ar command 97is specified, 98it is executed on the remote host instead of a login shell. 99.Pp 100The options are as follows: 101.Pp 102.Bl -tag -width Ds -compact 103.It Fl 4 104Forces 105.Nm 106to use IPv4 addresses only. 107.Pp 108.It Fl 6 109Forces 110.Nm 111to use IPv6 addresses only. 112.Pp 113.It Fl A 114Enables forwarding of the authentication agent connection. 115This can also be specified on a per-host basis in a configuration file. 116.Pp 117Agent forwarding should be enabled with caution. 118Users with the ability to bypass file permissions on the remote host 119(for the agent's 120.Ux Ns -domain 121socket) can access the local agent through the forwarded connection. 122An attacker cannot obtain key material from the agent, 123however they can perform operations on the keys that enable them to 124authenticate using the identities loaded into the agent. 125.Pp 126.It Fl a 127Disables forwarding of the authentication agent connection. 128.Pp 129.It Fl B Ar bind_interface 130Bind to the address of 131.Ar bind_interface 132before attempting to connect to the destination host. 133This is only useful on systems with more than one address. 134.Pp 135.It Fl b Ar bind_address 136Use 137.Ar bind_address 138on the local machine as the source address 139of the connection. 140Only useful on systems with more than one address. 141.Pp 142.It Fl C 143Requests compression of all data (including stdin, stdout, stderr, and 144data for forwarded X11, TCP and 145.Ux Ns -domain 146connections). 147The compression algorithm is the same used by 148.Xr gzip 1 . 149Compression is desirable on modem lines and other 150slow connections, but will only slow down things on fast networks. 151The default value can be set on a host-by-host basis in the 152configuration files; see the 153.Cm Compression 154option. 155.Pp 156.It Fl c Ar cipher_spec 157Selects the cipher specification for encrypting the session. 158.Ar cipher_spec 159is a comma-separated list of ciphers 160listed in order of preference. 161See the 162.Cm Ciphers 163keyword in 164.Xr ssh_config 5 165for more information. 166.Pp 167.It Fl D Xo 168.Sm off 169.Oo Ar bind_address : Oc 170.Ar port 171.Sm on 172.Xc 173Specifies a local 174.Dq dynamic 175application-level port forwarding. 176This works by allocating a socket to listen to 177.Ar port 178on the local side, optionally bound to the specified 179.Ar bind_address . 180Whenever a connection is made to this port, the 181connection is forwarded over the secure channel, and the application 182protocol is then used to determine where to connect to from the 183remote machine. 184Currently the SOCKS4 and SOCKS5 protocols are supported, and 185.Nm 186will act as a SOCKS server. 187Only root can forward privileged ports. 188Dynamic port forwardings can also be specified in the configuration file. 189.Pp 190IPv6 addresses can be specified by enclosing the address in square brackets. 191Only the superuser can forward privileged ports. 192By default, the local port is bound in accordance with the 193.Cm GatewayPorts 194setting. 195However, an explicit 196.Ar bind_address 197may be used to bind the connection to a specific address. 198The 199.Ar bind_address 200of 201.Dq localhost 202indicates that the listening port be bound for local use only, while an 203empty address or 204.Sq * 205indicates that the port should be available from all interfaces. 206.Pp 207.It Fl E Ar log_file 208Append debug logs to 209.Ar log_file 210instead of standard error. 211.Pp 212.It Fl e Ar escape_char 213Sets the escape character for sessions with a pty (default: 214.Ql ~ ) . 215The escape character is only recognized at the beginning of a line. 216The escape character followed by a dot 217.Pq Ql \&. 218closes the connection; 219followed by control-Z suspends the connection; 220and followed by itself sends the escape character once. 221Setting the character to 222.Dq none 223disables any escapes and makes the session fully transparent. 224.Pp 225.It Fl F Ar configfile 226Specifies an alternative per-user configuration file. 227If a configuration file is given on the command line, 228the system-wide configuration file 229.Pq Pa /etc/ssh/ssh_config 230will be ignored. 231The default for the per-user configuration file is 232.Pa ~/.ssh/config . 233.Pp 234.It Fl f 235Requests 236.Nm 237to go to background just before command execution. 238This is useful if 239.Nm 240is going to ask for passwords or passphrases, but the user 241wants it in the background. 242This implies 243.Fl n . 244The recommended way to start X11 programs at a remote site is with 245something like 246.Ic ssh -f host xterm . 247.Pp 248If the 249.Cm ExitOnForwardFailure 250configuration option is set to 251.Dq yes , 252then a client started with 253.Fl f 254will wait for all remote port forwards to be successfully established 255before placing itself in the background. 256.Pp 257.It Fl G 258Causes 259.Nm 260to print its configuration after evaluating 261.Cm Host 262and 263.Cm Match 264blocks and exit. 265.Pp 266.It Fl g 267Allows remote hosts to connect to local forwarded ports. 268If used on a multiplexed connection, then this option must be specified 269on the master process. 270.Pp 271.It Fl I Ar pkcs11 272Specify the PKCS#11 shared library 273.Nm 274should use to communicate with a PKCS#11 token providing the user's 275private RSA key. 276.Pp 277.It Fl i Ar identity_file 278Selects a file from which the identity (private key) for 279public key authentication is read. 280The default is 281.Pa ~/.ssh/id_dsa , 282.Pa ~/.ssh/id_ecdsa , 283.Pa ~/.ssh/id_ed25519 284and 285.Pa ~/.ssh/id_rsa . 286Identity files may also be specified on 287a per-host basis in the configuration file. 288It is possible to have multiple 289.Fl i 290options (and multiple identities specified in 291configuration files). 292If no certificates have been explicitly specified by the 293.Cm CertificateFile 294directive, 295.Nm 296will also try to load certificate information from the filename obtained 297by appending 298.Pa -cert.pub 299to identity filenames. 300.Pp 301.It Fl J Ar destination 302Connect to the target host by first making a 303.Nm 304connection to the jump host described by 305.Ar destination 306and then establishing a TCP forwarding to the ultimate destination from 307there. 308Multiple jump hops may be specified separated by comma characters. 309This is a shortcut to specify a 310.Cm ProxyJump 311configuration directive. 312.Pp 313.It Fl K 314Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 315credentials to the server. 316.Pp 317.It Fl k 318Disables forwarding (delegation) of GSSAPI credentials to the server. 319.Pp 320.It Fl L Xo 321.Sm off 322.Oo Ar bind_address : Oc 323.Ar port : host : hostport 324.Sm on 325.Xc 326.It Fl L Xo 327.Sm off 328.Oo Ar bind_address : Oc 329.Ar port : remote_socket 330.Sm on 331.Xc 332.It Fl L Xo 333.Sm off 334.Ar local_socket : host : hostport 335.Sm on 336.Xc 337.It Fl L Xo 338.Sm off 339.Ar local_socket : remote_socket 340.Sm on 341.Xc 342Specifies that connections to the given TCP port or Unix socket on the local 343(client) host are to be forwarded to the given host and port, or Unix socket, 344on the remote side. 345This works by allocating a socket to listen to either a TCP 346.Ar port 347on the local side, optionally bound to the specified 348.Ar bind_address , 349or to a Unix socket. 350Whenever a connection is made to the local port or socket, the 351connection is forwarded over the secure channel, and a connection is 352made to either 353.Ar host 354port 355.Ar hostport , 356or the Unix socket 357.Ar remote_socket , 358from the remote machine. 359.Pp 360Port forwardings can also be specified in the configuration file. 361Only the superuser can forward privileged ports. 362IPv6 addresses can be specified by enclosing the address in square brackets. 363.Pp 364By default, the local port is bound in accordance with the 365.Cm GatewayPorts 366setting. 367However, an explicit 368.Ar bind_address 369may be used to bind the connection to a specific address. 370The 371.Ar bind_address 372of 373.Dq localhost 374indicates that the listening port be bound for local use only, while an 375empty address or 376.Sq * 377indicates that the port should be available from all interfaces. 378.Pp 379.It Fl l Ar login_name 380Specifies the user to log in as on the remote machine. 381This also may be specified on a per-host basis in the configuration file. 382.Pp 383.It Fl M 384Places the 385.Nm 386client into 387.Dq master 388mode for connection sharing. 389Multiple 390.Fl M 391options places 392.Nm 393into 394.Dq master 395mode with confirmation required before slave connections are accepted. 396Refer to the description of 397.Cm ControlMaster 398in 399.Xr ssh_config 5 400for details. 401.Pp 402.It Fl m Ar mac_spec 403A comma-separated list of MAC (message authentication code) algorithms, 404specified in order of preference. 405See the 406.Cm MACs 407keyword for more information. 408.Pp 409.It Fl N 410Do not execute a remote command. 411This is useful for just forwarding ports. 412.Pp 413.It Fl n 414Redirects stdin from 415.Pa /dev/null 416(actually, prevents reading from stdin). 417This must be used when 418.Nm 419is run in the background. 420A common trick is to use this to run X11 programs on a remote machine. 421For example, 422.Ic ssh -n shadows.cs.hut.fi emacs & 423will start an emacs on shadows.cs.hut.fi, and the X11 424connection will be automatically forwarded over an encrypted channel. 425The 426.Nm 427program will be put in the background. 428(This does not work if 429.Nm 430needs to ask for a password or passphrase; see also the 431.Fl f 432option.) 433.Pp 434.It Fl O Ar ctl_cmd 435Control an active connection multiplexing master process. 436When the 437.Fl O 438option is specified, the 439.Ar ctl_cmd 440argument is interpreted and passed to the master process. 441Valid commands are: 442.Dq check 443(check that the master process is running), 444.Dq forward 445(request forwardings without command execution), 446.Dq cancel 447(cancel forwardings), 448.Dq exit 449(request the master to exit), and 450.Dq stop 451(request the master to stop accepting further multiplexing requests). 452.Pp 453.It Fl o Ar option 454Can be used to give options in the format used in the configuration file. 455This is useful for specifying options for which there is no separate 456command-line flag. 457For full details of the options listed below, and their possible values, see 458.Xr ssh_config 5 . 459.Pp 460.Bl -tag -width Ds -offset indent -compact 461.It AddKeysToAgent 462.It AddressFamily 463.It BatchMode 464.It BindAddress 465.It CanonicalDomains 466.It CanonicalizeFallbackLocal 467.It CanonicalizeHostname 468.It CanonicalizeMaxDots 469.It CanonicalizePermittedCNAMEs 470.It CertificateFile 471.It ChallengeResponseAuthentication 472.It CheckHostIP 473.It Ciphers 474.It ClearAllForwardings 475.It Compression 476.It ConnectionAttempts 477.It ConnectTimeout 478.It ControlMaster 479.It ControlPath 480.It ControlPersist 481.It DynamicForward 482.It EscapeChar 483.It ExitOnForwardFailure 484.It FingerprintHash 485.It ForwardAgent 486.It ForwardX11 487.It ForwardX11Timeout 488.It ForwardX11Trusted 489.It GatewayPorts 490.It GlobalKnownHostsFile 491.It GSSAPIAuthentication 492.It GSSAPIDelegateCredentials 493.It HashKnownHosts 494.It Host 495.It HostbasedAuthentication 496.It HostbasedKeyTypes 497.It HostKeyAlgorithms 498.It HostKeyAlias 499.It HostName 500.It IdentitiesOnly 501.It IdentityAgent 502.It IdentityFile 503.It IPQoS 504.It KbdInteractiveAuthentication 505.It KbdInteractiveDevices 506.It KexAlgorithms 507.It LocalCommand 508.It LocalForward 509.It LogLevel 510.It MACs 511.It Match 512.It NoHostAuthenticationForLocalhost 513.It NumberOfPasswordPrompts 514.It PasswordAuthentication 515.It PermitLocalCommand 516.It PKCS11Provider 517.It Port 518.It PreferredAuthentications 519.It ProxyCommand 520.It ProxyJump 521.It ProxyUseFdpass 522.It PubkeyAcceptedKeyTypes 523.It PubkeyAuthentication 524.It RekeyLimit 525.It RemoteCommand 526.It RemoteForward 527.It RequestTTY 528.It SendEnv 529.It ServerAliveInterval 530.It ServerAliveCountMax 531.It SetEnv 532.It StreamLocalBindMask 533.It StreamLocalBindUnlink 534.It StrictHostKeyChecking 535.It TCPKeepAlive 536.It Tunnel 537.It TunnelDevice 538.It UpdateHostKeys 539.It User 540.It UserKnownHostsFile 541.It VerifyHostKeyDNS 542.It VersionAddendum 543.It VisualHostKey 544.It XAuthLocation 545.El 546.Pp 547.It Fl p Ar port 548Port to connect to on the remote host. 549This can be specified on a 550per-host basis in the configuration file. 551.Pp 552.It Fl Q Ar query_option 553Queries 554.Nm 555for the algorithms supported for the specified version 2. 556The available features are: 557.Ar cipher 558(supported symmetric ciphers), 559.Ar cipher-auth 560(supported symmetric ciphers that support authenticated encryption), 561.Ar mac 562(supported message integrity codes), 563.Ar kex 564(key exchange algorithms), 565.Ar key 566(key types), 567.Ar key-cert 568(certificate key types), 569.Ar key-plain 570(non-certificate key types), and 571.Ar protocol-version 572(supported SSH protocol versions). 573.Pp 574.It Fl q 575Quiet mode. 576Causes most warning and diagnostic messages to be suppressed. 577.Pp 578.It Fl R Xo 579.Sm off 580.Oo Ar bind_address : Oc 581.Ar port : host : hostport 582.Sm on 583.Xc 584.It Fl R Xo 585.Sm off 586.Oo Ar bind_address : Oc 587.Ar port : local_socket 588.Sm on 589.Xc 590.It Fl R Xo 591.Sm off 592.Ar remote_socket : host : hostport 593.Sm on 594.Xc 595.It Fl R Xo 596.Sm off 597.Ar remote_socket : local_socket 598.Sm on 599.Xc 600.It Fl R Xo 601.Sm off 602.Oo Ar bind_address : Oc 603.Ar port 604.Sm on 605.Xc 606Specifies that connections to the given TCP port or Unix socket on the remote 607(server) host are to be forwarded to the local side. 608.Pp 609This works by allocating a socket to listen to either a TCP 610.Ar port 611or to a Unix socket on the remote side. 612Whenever a connection is made to this port or Unix socket, the 613connection is forwarded over the secure channel, and a connection 614is made from the local machine to either an explicit destination specified by 615.Ar host 616port 617.Ar hostport , 618or 619.Ar local_socket , 620or, if no explicit destination was specified, 621.Nm 622will act as a SOCKS 4/5 proxy and forward connections to the destinations 623requested by the remote SOCKS client. 624.Pp 625Port forwardings can also be specified in the configuration file. 626Privileged ports can be forwarded only when 627logging in as root on the remote machine. 628IPv6 addresses can be specified by enclosing the address in square brackets. 629.Pp 630By default, TCP listening sockets on the server will be bound to the loopback 631interface only. 632This may be overridden by specifying a 633.Ar bind_address . 634An empty 635.Ar bind_address , 636or the address 637.Ql * , 638indicates that the remote socket should listen on all interfaces. 639Specifying a remote 640.Ar bind_address 641will only succeed if the server's 642.Cm GatewayPorts 643option is enabled (see 644.Xr sshd_config 5 ) . 645.Pp 646If the 647.Ar port 648argument is 649.Ql 0 , 650the listen port will be dynamically allocated on the server and reported 651to the client at run time. 652When used together with 653.Ic -O forward 654the allocated port will be printed to the standard output. 655.Pp 656.It Fl S Ar ctl_path 657Specifies the location of a control socket for connection sharing, 658or the string 659.Dq none 660to disable connection sharing. 661Refer to the description of 662.Cm ControlPath 663and 664.Cm ControlMaster 665in 666.Xr ssh_config 5 667for details. 668.Pp 669.It Fl s 670May be used to request invocation of a subsystem on the remote system. 671Subsystems facilitate the use of SSH 672as a secure transport for other applications (e.g.\& 673.Xr sftp 1 ) . 674The subsystem is specified as the remote command. 675.Pp 676.It Fl T 677Disable pseudo-terminal allocation. 678.Pp 679.It Fl t 680Force pseudo-terminal allocation. 681This can be used to execute arbitrary 682screen-based programs on a remote machine, which can be very useful, 683e.g. when implementing menu services. 684Multiple 685.Fl t 686options force tty allocation, even if 687.Nm 688has no local tty. 689.Pp 690.It Fl V 691Display the version number and exit. 692.Pp 693.It Fl v 694Verbose mode. 695Causes 696.Nm 697to print debugging messages about its progress. 698This is helpful in 699debugging connection, authentication, and configuration problems. 700Multiple 701.Fl v 702options increase the verbosity. 703The maximum is 3. 704.Pp 705.It Fl W Ar host : Ns Ar port 706Requests that standard input and output on the client be forwarded to 707.Ar host 708on 709.Ar port 710over the secure channel. 711Implies 712.Fl N , 713.Fl T , 714.Cm ExitOnForwardFailure 715and 716.Cm ClearAllForwardings , 717though these can be overridden in the configuration file or using 718.Fl o 719command line options. 720.Pp 721.It Fl w Xo 722.Ar local_tun Ns Op : Ns Ar remote_tun 723.Xc 724Requests 725tunnel 726device forwarding with the specified 727.Xr tun 4 728devices between the client 729.Pq Ar local_tun 730and the server 731.Pq Ar remote_tun . 732.Pp 733The devices may be specified by numerical ID or the keyword 734.Dq any , 735which uses the next available tunnel device. 736If 737.Ar remote_tun 738is not specified, it defaults to 739.Dq any . 740See also the 741.Cm Tunnel 742and 743.Cm TunnelDevice 744directives in 745.Xr ssh_config 5 . 746.Pp 747If the 748.Cm Tunnel 749directive is unset, it will be set to the default tunnel mode, which is 750.Dq point-to-point . 751If a different 752.Cm Tunnel 753forwarding mode it desired, then it should be specified before 754.Fl w . 755.Pp 756.It Fl X 757Enables X11 forwarding. 758This can also be specified on a per-host basis in a configuration file. 759.Pp 760X11 forwarding should be enabled with caution. 761Users with the ability to bypass file permissions on the remote host 762(for the user's X authorization database) 763can access the local X11 display through the forwarded connection. 764An attacker may then be able to perform activities such as keystroke monitoring. 765.Pp 766For this reason, X11 forwarding is subjected to X11 SECURITY extension 767restrictions by default. 768Please refer to the 769.Nm 770.Fl Y 771option and the 772.Cm ForwardX11Trusted 773directive in 774.Xr ssh_config 5 775for more information. 776.Pp 777.It Fl x 778Disables X11 forwarding. 779.Pp 780.It Fl Y 781Enables trusted X11 forwarding. 782Trusted X11 forwardings are not subjected to the X11 SECURITY extension 783controls. 784.Pp 785.It Fl y 786Send log information using the 787.Xr syslog 3 788system module. 789By default this information is sent to stderr. 790.El 791.Pp 792.Nm 793may additionally obtain configuration data from 794a per-user configuration file and a system-wide configuration file. 795The file format and configuration options are described in 796.Xr ssh_config 5 . 797.Sh AUTHENTICATION 798The OpenSSH SSH client supports SSH protocol 2. 799.Pp 800The methods available for authentication are: 801GSSAPI-based authentication, 802host-based authentication, 803public key authentication, 804challenge-response authentication, 805and password authentication. 806Authentication methods are tried in the order specified above, 807though 808.Cm PreferredAuthentications 809can be used to change the default order. 810.Pp 811Host-based authentication works as follows: 812If the machine the user logs in from is listed in 813.Pa /etc/hosts.equiv 814or 815.Pa /etc/shosts.equiv 816on the remote machine, and the user names are 817the same on both sides, or if the files 818.Pa ~/.rhosts 819or 820.Pa ~/.shosts 821exist in the user's home directory on the 822remote machine and contain a line containing the name of the client 823machine and the name of the user on that machine, the user is 824considered for login. 825Additionally, the server 826.Em must 827be able to verify the client's 828host key (see the description of 829.Pa /etc/ssh/ssh_known_hosts 830and 831.Pa ~/.ssh/known_hosts , 832below) 833for login to be permitted. 834This authentication method closes security holes due to IP 835spoofing, DNS spoofing, and routing spoofing. 836[Note to the administrator: 837.Pa /etc/hosts.equiv , 838.Pa ~/.rhosts , 839and the rlogin/rsh protocol in general, are inherently insecure and should be 840disabled if security is desired.] 841.Pp 842Public key authentication works as follows: 843The scheme is based on public-key cryptography, 844using cryptosystems 845where encryption and decryption are done using separate keys, 846and it is unfeasible to derive the decryption key from the encryption key. 847The idea is that each user creates a public/private 848key pair for authentication purposes. 849The server knows the public key, and only the user knows the private key. 850.Nm 851implements public key authentication protocol automatically, 852using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 853The HISTORY section of 854.Xr ssl 8 855contains a brief discussion of the DSA and RSA algorithms. 856.Pp 857The file 858.Pa ~/.ssh/authorized_keys 859lists the public keys that are permitted for logging in. 860When the user logs in, the 861.Nm 862program tells the server which key pair it would like to use for 863authentication. 864The client proves that it has access to the private key 865and the server checks that the corresponding public key 866is authorized to accept the account. 867.Pp 868The server may inform the client of errors that prevented public key 869authentication from succeeding after authentication completes using a 870different method. 871These may be viewed by increasing the 872.Cm LogLevel 873to 874.Cm DEBUG 875or higher (e.g. by using the 876.Fl v 877flag). 878.Pp 879The user creates his/her key pair by running 880.Xr ssh-keygen 1 . 881This stores the private key in 882.Pa ~/.ssh/id_dsa 883(DSA), 884.Pa ~/.ssh/id_ecdsa 885(ECDSA), 886.Pa ~/.ssh/id_ed25519 887(Ed25519), 888or 889.Pa ~/.ssh/id_rsa 890(RSA) 891and stores the public key in 892.Pa ~/.ssh/id_dsa.pub 893(DSA), 894.Pa ~/.ssh/id_ecdsa.pub 895(ECDSA), 896.Pa ~/.ssh/id_ed25519.pub 897(Ed25519), 898or 899.Pa ~/.ssh/id_rsa.pub 900(RSA) 901in the user's home directory. 902The user should then copy the public key 903to 904.Pa ~/.ssh/authorized_keys 905in his/her home directory on the remote machine. 906The 907.Pa authorized_keys 908file corresponds to the conventional 909.Pa ~/.rhosts 910file, and has one key 911per line, though the lines can be very long. 912After this, the user can log in without giving the password. 913.Pp 914A variation on public key authentication 915is available in the form of certificate authentication: 916instead of a set of public/private keys, 917signed certificates are used. 918This has the advantage that a single trusted certification authority 919can be used in place of many public/private keys. 920See the CERTIFICATES section of 921.Xr ssh-keygen 1 922for more information. 923.Pp 924The most convenient way to use public key or certificate authentication 925may be with an authentication agent. 926See 927.Xr ssh-agent 1 928and (optionally) the 929.Cm AddKeysToAgent 930directive in 931.Xr ssh_config 5 932for more information. 933.Pp 934Challenge-response authentication works as follows: 935The server sends an arbitrary 936.Qq challenge 937text, and prompts for a response. 938Examples of challenge-response authentication include 939.Bx 940Authentication (see 941.Xr login.conf 5 ) 942and PAM (some 943.Pf non- Ox 944systems). 945.Pp 946Finally, if other authentication methods fail, 947.Nm 948prompts the user for a password. 949The password is sent to the remote 950host for checking; however, since all communications are encrypted, 951the password cannot be seen by someone listening on the network. 952.Pp 953.Nm 954automatically maintains and checks a database containing 955identification for all hosts it has ever been used with. 956Host keys are stored in 957.Pa ~/.ssh/known_hosts 958in the user's home directory. 959Additionally, the file 960.Pa /etc/ssh/ssh_known_hosts 961is automatically checked for known hosts. 962Any new hosts are automatically added to the user's file. 963If a host's identification ever changes, 964.Nm 965warns about this and disables password authentication to prevent 966server spoofing or man-in-the-middle attacks, 967which could otherwise be used to circumvent the encryption. 968The 969.Cm StrictHostKeyChecking 970option can be used to control logins to machines whose 971host key is not known or has changed. 972.Pp 973When the user's identity has been accepted by the server, the server 974either executes the given command in a non-interactive session or, 975if no command has been specified, logs into the machine and gives 976the user a normal shell as an interactive session. 977All communication with 978the remote command or shell will be automatically encrypted. 979.Pp 980If an interactive session is requested 981.Nm 982by default will only request a pseudo-terminal (pty) for interactive 983sessions when the client has one. 984The flags 985.Fl T 986and 987.Fl t 988can be used to override this behaviour. 989.Pp 990If a pseudo-terminal has been allocated the 991user may use the escape characters noted below. 992.Pp 993If no pseudo-terminal has been allocated, 994the session is transparent and can be used to reliably transfer binary data. 995On most systems, setting the escape character to 996.Dq none 997will also make the session transparent even if a tty is used. 998.Pp 999The session terminates when the command or shell on the remote 1000machine exits and all X11 and TCP connections have been closed. 1001.Sh ESCAPE CHARACTERS 1002When a pseudo-terminal has been requested, 1003.Nm 1004supports a number of functions through the use of an escape character. 1005.Pp 1006A single tilde character can be sent as 1007.Ic ~~ 1008or by following the tilde by a character other than those described below. 1009The escape character must always follow a newline to be interpreted as 1010special. 1011The escape character can be changed in configuration files using the 1012.Cm EscapeChar 1013configuration directive or on the command line by the 1014.Fl e 1015option. 1016.Pp 1017The supported escapes (assuming the default 1018.Ql ~ ) 1019are: 1020.Bl -tag -width Ds 1021.It Cm ~. 1022Disconnect. 1023.It Cm ~^Z 1024Background 1025.Nm . 1026.It Cm ~# 1027List forwarded connections. 1028.It Cm ~& 1029Background 1030.Nm 1031at logout when waiting for forwarded connection / X11 sessions to terminate. 1032.It Cm ~? 1033Display a list of escape characters. 1034.It Cm ~B 1035Send a BREAK to the remote system 1036(only useful if the peer supports it). 1037.It Cm ~C 1038Open command line. 1039Currently this allows the addition of port forwardings using the 1040.Fl L , 1041.Fl R 1042and 1043.Fl D 1044options (see above). 1045It also allows the cancellation of existing port-forwardings 1046with 1047.Sm off 1048.Fl KL Oo Ar bind_address : Oc Ar port 1049.Sm on 1050for local, 1051.Sm off 1052.Fl KR Oo Ar bind_address : Oc Ar port 1053.Sm on 1054for remote and 1055.Sm off 1056.Fl KD Oo Ar bind_address : Oc Ar port 1057.Sm on 1058for dynamic port-forwardings. 1059.Ic !\& Ns Ar command 1060allows the user to execute a local command if the 1061.Ic PermitLocalCommand 1062option is enabled in 1063.Xr ssh_config 5 . 1064Basic help is available, using the 1065.Fl h 1066option. 1067.It Cm ~R 1068Request rekeying of the connection 1069(only useful if the peer supports it). 1070.It Cm ~V 1071Decrease the verbosity 1072.Pq Ic LogLevel 1073when errors are being written to stderr. 1074.It Cm ~v 1075Increase the verbosity 1076.Pq Ic LogLevel 1077when errors are being written to stderr. 1078.El 1079.Sh TCP FORWARDING 1080Forwarding of arbitrary TCP connections over the secure channel can 1081be specified either on the command line or in a configuration file. 1082One possible application of TCP forwarding is a secure connection to a 1083mail server; another is going through firewalls. 1084.Pp 1085In the example below, we look at encrypting communication between 1086an IRC client and server, even though the IRC server does not directly 1087support encrypted communications. 1088This works as follows: 1089the user connects to the remote host using 1090.Nm , 1091specifying a port to be used to forward connections 1092to the remote server. 1093After that it is possible to start the service which is to be encrypted 1094on the client machine, 1095connecting to the same local port, 1096and 1097.Nm 1098will encrypt and forward the connection. 1099.Pp 1100The following example tunnels an IRC session from client machine 1101.Dq 127.0.0.1 1102(localhost) 1103to remote server 1104.Dq server.example.com : 1105.Bd -literal -offset 4n 1106$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 1107$ irc -c '#users' -p 1234 pinky 127.0.0.1 1108.Ed 1109.Pp 1110This tunnels a connection to IRC server 1111.Dq server.example.com , 1112joining channel 1113.Dq #users , 1114nickname 1115.Dq pinky , 1116using port 1234. 1117It doesn't matter which port is used, 1118as long as it's greater than 1023 1119(remember, only root can open sockets on privileged ports) 1120and doesn't conflict with any ports already in use. 1121The connection is forwarded to port 6667 on the remote server, 1122since that's the standard port for IRC services. 1123.Pp 1124The 1125.Fl f 1126option backgrounds 1127.Nm 1128and the remote command 1129.Dq sleep 10 1130is specified to allow an amount of time 1131(10 seconds, in the example) 1132to start the service which is to be tunnelled. 1133If no connections are made within the time specified, 1134.Nm 1135will exit. 1136.Sh X11 FORWARDING 1137If the 1138.Cm ForwardX11 1139variable is set to 1140.Dq yes 1141(or see the description of the 1142.Fl X , 1143.Fl x , 1144and 1145.Fl Y 1146options above) 1147and the user is using X11 (the 1148.Ev DISPLAY 1149environment variable is set), the connection to the X11 display is 1150automatically forwarded to the remote side in such a way that any X11 1151programs started from the shell (or command) will go through the 1152encrypted channel, and the connection to the real X server will be made 1153from the local machine. 1154The user should not manually set 1155.Ev DISPLAY . 1156Forwarding of X11 connections can be 1157configured on the command line or in configuration files. 1158.Pp 1159The 1160.Ev DISPLAY 1161value set by 1162.Nm 1163will point to the server machine, but with a display number greater than zero. 1164This is normal, and happens because 1165.Nm 1166creates a 1167.Dq proxy 1168X server on the server machine for forwarding the 1169connections over the encrypted channel. 1170.Pp 1171.Nm 1172will also automatically set up Xauthority data on the server machine. 1173For this purpose, it will generate a random authorization cookie, 1174store it in Xauthority on the server, and verify that any forwarded 1175connections carry this cookie and replace it by the real cookie when 1176the connection is opened. 1177The real authentication cookie is never 1178sent to the server machine (and no cookies are sent in the plain). 1179.Pp 1180If the 1181.Cm ForwardAgent 1182variable is set to 1183.Dq yes 1184(or see the description of the 1185.Fl A 1186and 1187.Fl a 1188options above) and 1189the user is using an authentication agent, the connection to the agent 1190is automatically forwarded to the remote side. 1191.Sh VERIFYING HOST KEYS 1192When connecting to a server for the first time, 1193a fingerprint of the server's public key is presented to the user 1194(unless the option 1195.Cm StrictHostKeyChecking 1196has been disabled). 1197Fingerprints can be determined using 1198.Xr ssh-keygen 1 : 1199.Pp 1200.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1201.Pp 1202If the fingerprint is already known, it can be matched 1203and the key can be accepted or rejected. 1204If only legacy (MD5) fingerprints for the server are available, the 1205.Xr ssh-keygen 1 1206.Fl E 1207option may be used to downgrade the fingerprint algorithm to match. 1208.Pp 1209Because of the difficulty of comparing host keys 1210just by looking at fingerprint strings, 1211there is also support to compare host keys visually, 1212using 1213.Em random art . 1214By setting the 1215.Cm VisualHostKey 1216option to 1217.Dq yes , 1218a small ASCII graphic gets displayed on every login to a server, no matter 1219if the session itself is interactive or not. 1220By learning the pattern a known server produces, a user can easily 1221find out that the host key has changed when a completely different pattern 1222is displayed. 1223Because these patterns are not unambiguous however, a pattern that looks 1224similar to the pattern remembered only gives a good probability that the 1225host key is the same, not guaranteed proof. 1226.Pp 1227To get a listing of the fingerprints along with their random art for 1228all known hosts, the following command line can be used: 1229.Pp 1230.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1231.Pp 1232If the fingerprint is unknown, 1233an alternative method of verification is available: 1234SSH fingerprints verified by DNS. 1235An additional resource record (RR), 1236SSHFP, 1237is added to a zonefile 1238and the connecting client is able to match the fingerprint 1239with that of the key presented. 1240.Pp 1241In this example, we are connecting a client to a server, 1242.Dq host.example.com . 1243The SSHFP resource records should first be added to the zonefile for 1244host.example.com: 1245.Bd -literal -offset indent 1246$ ssh-keygen -r host.example.com. 1247.Ed 1248.Pp 1249The output lines will have to be added to the zonefile. 1250To check that the zone is answering fingerprint queries: 1251.Pp 1252.Dl $ dig -t SSHFP host.example.com 1253.Pp 1254Finally the client connects: 1255.Bd -literal -offset indent 1256$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1257[...] 1258Matching host key fingerprint found in DNS. 1259Are you sure you want to continue connecting (yes/no)? 1260.Ed 1261.Pp 1262See the 1263.Cm VerifyHostKeyDNS 1264option in 1265.Xr ssh_config 5 1266for more information. 1267.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1268.Nm 1269contains support for Virtual Private Network (VPN) tunnelling 1270using the 1271.Xr tun 4 1272network pseudo-device, 1273allowing two networks to be joined securely. 1274The 1275.Xr sshd_config 5 1276configuration option 1277.Cm PermitTunnel 1278controls whether the server supports this, 1279and at what level (layer 2 or 3 traffic). 1280.Pp 1281The following example would connect client network 10.0.50.0/24 1282with remote network 10.0.99.0/24 using a point-to-point connection 1283from 10.1.1.1 to 10.1.1.2, 1284provided that the SSH server running on the gateway to the remote network, 1285at 192.168.1.15, allows it. 1286.Pp 1287On the client: 1288.Bd -literal -offset indent 1289# ssh -f -w 0:1 192.168.1.15 true 1290# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1291# route add 10.0.99.0/24 10.1.1.2 1292.Ed 1293.Pp 1294On the server: 1295.Bd -literal -offset indent 1296# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1297# route add 10.0.50.0/24 10.1.1.1 1298.Ed 1299.Pp 1300Client access may be more finely tuned via the 1301.Pa /root/.ssh/authorized_keys 1302file (see below) and the 1303.Cm PermitRootLogin 1304server option. 1305The following entry would permit connections on 1306.Xr tun 4 1307device 1 from user 1308.Dq jane 1309and on tun device 2 from user 1310.Dq john , 1311if 1312.Cm PermitRootLogin 1313is set to 1314.Dq forced-commands-only : 1315.Bd -literal -offset 2n 1316tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1317tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1318.Ed 1319.Pp 1320Since an SSH-based setup entails a fair amount of overhead, 1321it may be more suited to temporary setups, 1322such as for wireless VPNs. 1323More permanent VPNs are better provided by tools such as 1324.Xr ipsecctl 8 1325and 1326.Xr isakmpd 8 . 1327.Sh ENVIRONMENT 1328.Nm 1329will normally set the following environment variables: 1330.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1331.It Ev DISPLAY 1332The 1333.Ev DISPLAY 1334variable indicates the location of the X11 server. 1335It is automatically set by 1336.Nm 1337to point to a value of the form 1338.Dq hostname:n , 1339where 1340.Dq hostname 1341indicates the host where the shell runs, and 1342.Sq n 1343is an integer \*(Ge 1. 1344.Nm 1345uses this special value to forward X11 connections over the secure 1346channel. 1347The user should normally not set 1348.Ev DISPLAY 1349explicitly, as that 1350will render the X11 connection insecure (and will require the user to 1351manually copy any required authorization cookies). 1352.It Ev HOME 1353Set to the path of the user's home directory. 1354.It Ev LOGNAME 1355Synonym for 1356.Ev USER ; 1357set for compatibility with systems that use this variable. 1358.It Ev MAIL 1359Set to the path of the user's mailbox. 1360.It Ev PATH 1361Set to the default 1362.Ev PATH , 1363as specified when compiling 1364.Nm . 1365.It Ev SSH_ASKPASS 1366If 1367.Nm 1368needs a passphrase, it will read the passphrase from the current 1369terminal if it was run from a terminal. 1370If 1371.Nm 1372does not have a terminal associated with it but 1373.Ev DISPLAY 1374and 1375.Ev SSH_ASKPASS 1376are set, it will execute the program specified by 1377.Ev SSH_ASKPASS 1378and open an X11 window to read the passphrase. 1379This is particularly useful when calling 1380.Nm 1381from a 1382.Pa .xsession 1383or related script. 1384(Note that on some machines it 1385may be necessary to redirect the input from 1386.Pa /dev/null 1387to make this work.) 1388.It Ev SSH_AUTH_SOCK 1389Identifies the path of a 1390.Ux Ns -domain 1391socket used to communicate with the agent. 1392.It Ev SSH_CONNECTION 1393Identifies the client and server ends of the connection. 1394The variable contains 1395four space-separated values: client IP address, client port number, 1396server IP address, and server port number. 1397.It Ev SSH_ORIGINAL_COMMAND 1398This variable contains the original command line if a forced command 1399is executed. 1400It can be used to extract the original arguments. 1401.It Ev SSH_TTY 1402This is set to the name of the tty (path to the device) associated 1403with the current shell or command. 1404If the current session has no tty, 1405this variable is not set. 1406.It Ev SSH_TUNNEL 1407Optionally set by 1408.Xr sshd 8 1409to contain the interface names assigned if tunnel forwarding was 1410requested by the client. 1411.It Ev SSH_USER_AUTH 1412Optionally set by 1413.Xr sshd 8 , 1414this variable may contain a pathname to a file that lists the authentication 1415methods successfully used when the session was established, including any 1416public keys that were used. 1417.It Ev TZ 1418This variable is set to indicate the present time zone if it 1419was set when the daemon was started (i.e. the daemon passes the value 1420on to new connections). 1421.It Ev USER 1422Set to the name of the user logging in. 1423.El 1424.Pp 1425Additionally, 1426.Nm 1427reads 1428.Pa ~/.ssh/environment , 1429and adds lines of the format 1430.Dq VARNAME=value 1431to the environment if the file exists and users are allowed to 1432change their environment. 1433For more information, see the 1434.Cm PermitUserEnvironment 1435option in 1436.Xr sshd_config 5 . 1437.Sh FILES 1438.Bl -tag -width Ds -compact 1439.It Pa ~/.rhosts 1440This file is used for host-based authentication (see above). 1441On some machines this file may need to be 1442world-readable if the user's home directory is on an NFS partition, 1443because 1444.Xr sshd 8 1445reads it as root. 1446Additionally, this file must be owned by the user, 1447and must not have write permissions for anyone else. 1448The recommended 1449permission for most machines is read/write for the user, and not 1450accessible by others. 1451.Pp 1452.It Pa ~/.shosts 1453This file is used in exactly the same way as 1454.Pa .rhosts , 1455but allows host-based authentication without permitting login with 1456rlogin/rsh. 1457.Pp 1458.It Pa ~/.ssh/ 1459This directory is the default location for all user-specific configuration 1460and authentication information. 1461There is no general requirement to keep the entire contents of this directory 1462secret, but the recommended permissions are read/write/execute for the user, 1463and not accessible by others. 1464.Pp 1465.It Pa ~/.ssh/authorized_keys 1466Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1467that can be used for logging in as this user. 1468The format of this file is described in the 1469.Xr sshd 8 1470manual page. 1471This file is not highly sensitive, but the recommended 1472permissions are read/write for the user, and not accessible by others. 1473.Pp 1474.It Pa ~/.ssh/config 1475This is the per-user configuration file. 1476The file format and configuration options are described in 1477.Xr ssh_config 5 . 1478Because of the potential for abuse, this file must have strict permissions: 1479read/write for the user, and not writable by others. 1480.Pp 1481.It Pa ~/.ssh/environment 1482Contains additional definitions for environment variables; see 1483.Sx ENVIRONMENT , 1484above. 1485.Pp 1486.It Pa ~/.ssh/id_dsa 1487.It Pa ~/.ssh/id_ecdsa 1488.It Pa ~/.ssh/id_ed25519 1489.It Pa ~/.ssh/id_rsa 1490Contains the private key for authentication. 1491These files 1492contain sensitive data and should be readable by the user but not 1493accessible by others (read/write/execute). 1494.Nm 1495will simply ignore a private key file if it is accessible by others. 1496It is possible to specify a passphrase when 1497generating the key which will be used to encrypt the 1498sensitive part of this file using AES-128. 1499.Pp 1500.It Pa ~/.ssh/id_dsa.pub 1501.It Pa ~/.ssh/id_ecdsa.pub 1502.It Pa ~/.ssh/id_ed25519.pub 1503.It Pa ~/.ssh/id_rsa.pub 1504Contains the public key for authentication. 1505These files are not 1506sensitive and can (but need not) be readable by anyone. 1507.Pp 1508.It Pa ~/.ssh/known_hosts 1509Contains a list of host keys for all hosts the user has logged into 1510that are not already in the systemwide list of known host keys. 1511See 1512.Xr sshd 8 1513for further details of the format of this file. 1514.Pp 1515.It Pa ~/.ssh/rc 1516Commands in this file are executed by 1517.Nm 1518when the user logs in, just before the user's shell (or command) is 1519started. 1520See the 1521.Xr sshd 8 1522manual page for more information. 1523.Pp 1524.It Pa /etc/hosts.equiv 1525This file is for host-based authentication (see above). 1526It should only be writable by root. 1527.Pp 1528.It Pa /etc/shosts.equiv 1529This file is used in exactly the same way as 1530.Pa hosts.equiv , 1531but allows host-based authentication without permitting login with 1532rlogin/rsh. 1533.Pp 1534.It Pa /etc/ssh/ssh_config 1535Systemwide configuration file. 1536The file format and configuration options are described in 1537.Xr ssh_config 5 . 1538.Pp 1539.It Pa /etc/ssh/ssh_host_key 1540.It Pa /etc/ssh/ssh_host_dsa_key 1541.It Pa /etc/ssh/ssh_host_ecdsa_key 1542.It Pa /etc/ssh/ssh_host_ed25519_key 1543.It Pa /etc/ssh/ssh_host_rsa_key 1544These files contain the private parts of the host keys 1545and are used for host-based authentication. 1546.Pp 1547.It Pa /etc/ssh/ssh_known_hosts 1548Systemwide list of known host keys. 1549This file should be prepared by the 1550system administrator to contain the public host keys of all machines in the 1551organization. 1552It should be world-readable. 1553See 1554.Xr sshd 8 1555for further details of the format of this file. 1556.Pp 1557.It Pa /etc/ssh/sshrc 1558Commands in this file are executed by 1559.Nm 1560when the user logs in, just before the user's shell (or command) is started. 1561See the 1562.Xr sshd 8 1563manual page for more information. 1564.El 1565.Sh EXIT STATUS 1566.Nm 1567exits with the exit status of the remote command or with 255 1568if an error occurred. 1569.Sh SEE ALSO 1570.Xr scp 1 , 1571.Xr sftp 1 , 1572.Xr ssh-add 1 , 1573.Xr ssh-agent 1 , 1574.Xr ssh-keygen 1 , 1575.Xr ssh-keyscan 1 , 1576.Xr tun 4 , 1577.Xr ssh_config 5 , 1578.Xr ssh-keysign 8 , 1579.Xr sshd 8 1580.Sh STANDARDS 1581.Rs 1582.%A S. Lehtinen 1583.%A C. Lonvick 1584.%D January 2006 1585.%R RFC 4250 1586.%T The Secure Shell (SSH) Protocol Assigned Numbers 1587.Re 1588.Pp 1589.Rs 1590.%A T. Ylonen 1591.%A C. Lonvick 1592.%D January 2006 1593.%R RFC 4251 1594.%T The Secure Shell (SSH) Protocol Architecture 1595.Re 1596.Pp 1597.Rs 1598.%A T. Ylonen 1599.%A C. Lonvick 1600.%D January 2006 1601.%R RFC 4252 1602.%T The Secure Shell (SSH) Authentication Protocol 1603.Re 1604.Pp 1605.Rs 1606.%A T. Ylonen 1607.%A C. Lonvick 1608.%D January 2006 1609.%R RFC 4253 1610.%T The Secure Shell (SSH) Transport Layer Protocol 1611.Re 1612.Pp 1613.Rs 1614.%A T. Ylonen 1615.%A C. Lonvick 1616.%D January 2006 1617.%R RFC 4254 1618.%T The Secure Shell (SSH) Connection Protocol 1619.Re 1620.Pp 1621.Rs 1622.%A J. Schlyter 1623.%A W. Griffin 1624.%D January 2006 1625.%R RFC 4255 1626.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1627.Re 1628.Pp 1629.Rs 1630.%A F. Cusack 1631.%A M. Forssen 1632.%D January 2006 1633.%R RFC 4256 1634.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1635.Re 1636.Pp 1637.Rs 1638.%A J. Galbraith 1639.%A P. Remaker 1640.%D January 2006 1641.%R RFC 4335 1642.%T The Secure Shell (SSH) Session Channel Break Extension 1643.Re 1644.Pp 1645.Rs 1646.%A M. Bellare 1647.%A T. Kohno 1648.%A C. Namprempre 1649.%D January 2006 1650.%R RFC 4344 1651.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1652.Re 1653.Pp 1654.Rs 1655.%A B. Harris 1656.%D January 2006 1657.%R RFC 4345 1658.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1659.Re 1660.Pp 1661.Rs 1662.%A M. Friedl 1663.%A N. Provos 1664.%A W. Simpson 1665.%D March 2006 1666.%R RFC 4419 1667.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1668.Re 1669.Pp 1670.Rs 1671.%A J. Galbraith 1672.%A R. Thayer 1673.%D November 2006 1674.%R RFC 4716 1675.%T The Secure Shell (SSH) Public Key File Format 1676.Re 1677.Pp 1678.Rs 1679.%A D. Stebila 1680.%A J. Green 1681.%D December 2009 1682.%R RFC 5656 1683.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1684.Re 1685.Pp 1686.Rs 1687.%A A. Perrig 1688.%A D. Song 1689.%D 1999 1690.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1691.%T Hash Visualization: a New Technique to improve Real-World Security 1692.Re 1693.Sh AUTHORS 1694OpenSSH is a derivative of the original and free 1695ssh 1.2.12 release by Tatu Ylonen. 1696Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1697Theo de Raadt and Dug Song 1698removed many bugs, re-added newer features and 1699created OpenSSH. 1700Markus Friedl contributed the support for SSH 1701protocol versions 1.5 and 2.0. 1702