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 1.399 2018/09/20 06:58:48 jmc Exp $ 37.Dd $Mdocdate: September 20 2018 $ 38.Dt SSH 1 39.Os 40.Sh NAME 41.Nm ssh 42.Nd OpenSSH SSH client (remote login program) 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 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 91his/her identity to the remote machine using one of several methods 92(see below). 93.Pp 94If a 95.Ar command 96is specified, 97it is executed on the remote host instead of a login shell. 98.Pp 99The options are as follows: 100.Pp 101.Bl -tag -width Ds -compact 102.It Fl 4 103Forces 104.Nm 105to use IPv4 addresses only. 106.Pp 107.It Fl 6 108Forces 109.Nm 110to use IPv6 addresses only. 111.Pp 112.It Fl A 113Enables forwarding of the authentication agent connection. 114This can also be specified on a per-host basis in a configuration file. 115.Pp 116Agent forwarding should be enabled with caution. 117Users with the ability to bypass file permissions on the remote host 118(for the agent's 119.Ux Ns -domain 120socket) can access the local agent through the forwarded connection. 121An attacker cannot obtain key material from the agent, 122however they can perform operations on the keys that enable them to 123authenticate using the identities loaded into the agent. 124.Pp 125.It Fl a 126Disables forwarding of the authentication agent connection. 127.Pp 128.It Fl B Ar bind_interface 129Bind to the address of 130.Ar bind_interface 131before attempting to connect to the destination host. 132This is only useful on systems with more than one address. 133.Pp 134.It Fl b Ar bind_address 135Use 136.Ar bind_address 137on the local machine as the source address 138of the connection. 139Only useful on systems with more than one address. 140.Pp 141.It Fl C 142Requests compression of all data (including stdin, stdout, stderr, and 143data for forwarded X11, TCP and 144.Ux Ns -domain 145connections). 146The compression algorithm is the same used by 147.Xr gzip 1 . 148Compression is desirable on modem lines and other 149slow connections, but will only slow down things on fast networks. 150The default value can be set on a host-by-host basis in the 151configuration files; see the 152.Cm Compression 153option. 154.Pp 155.It Fl c Ar cipher_spec 156Selects the cipher specification for encrypting the session. 157.Ar cipher_spec 158is a comma-separated list of ciphers 159listed in order of preference. 160See the 161.Cm Ciphers 162keyword in 163.Xr ssh_config 5 164for more information. 165.Pp 166.It Fl D Xo 167.Sm off 168.Oo Ar bind_address : Oc 169.Ar port 170.Sm on 171.Xc 172Specifies a local 173.Dq dynamic 174application-level port forwarding. 175This works by allocating a socket to listen to 176.Ar port 177on the local side, optionally bound to the specified 178.Ar bind_address . 179Whenever a connection is made to this port, the 180connection is forwarded over the secure channel, and the application 181protocol is then used to determine where to connect to from the 182remote machine. 183Currently the SOCKS4 and SOCKS5 protocols are supported, and 184.Nm 185will act as a SOCKS server. 186Only root can forward privileged ports. 187Dynamic port forwardings can also be specified in the configuration file. 188.Pp 189IPv6 addresses can be specified by enclosing the address in square brackets. 190Only the superuser can forward privileged ports. 191By default, the local port is bound in accordance with the 192.Cm GatewayPorts 193setting. 194However, an explicit 195.Ar bind_address 196may be used to bind the connection to a specific address. 197The 198.Ar bind_address 199of 200.Dq localhost 201indicates that the listening port be bound for local use only, while an 202empty address or 203.Sq * 204indicates that the port should be available from all interfaces. 205.Pp 206.It Fl E Ar log_file 207Append debug logs to 208.Ar log_file 209instead of standard error. 210.Pp 211.It Fl e Ar escape_char 212Sets the escape character for sessions with a pty (default: 213.Ql ~ ) . 214The escape character is only recognized at the beginning of a line. 215The escape character followed by a dot 216.Pq Ql \&. 217closes the connection; 218followed by control-Z suspends the connection; 219and followed by itself sends the escape character once. 220Setting the character to 221.Dq none 222disables any escapes and makes the session fully transparent. 223.Pp 224.It Fl F Ar configfile 225Specifies an alternative per-user configuration file. 226If a configuration file is given on the command line, 227the system-wide configuration file 228.Pq Pa /etc/ssh/ssh_config 229will be ignored. 230The default for the per-user configuration file is 231.Pa ~/.ssh/config . 232.Pp 233.It Fl f 234Requests 235.Nm 236to go to background just before command execution. 237This is useful if 238.Nm 239is going to ask for passwords or passphrases, but the user 240wants it in the background. 241This implies 242.Fl n . 243The recommended way to start X11 programs at a remote site is with 244something like 245.Ic ssh -f host xterm . 246.Pp 247If the 248.Cm ExitOnForwardFailure 249configuration option is set to 250.Dq yes , 251then a client started with 252.Fl f 253will wait for all remote port forwards to be successfully established 254before placing itself in the background. 255.Pp 256.It Fl G 257Causes 258.Nm 259to print its configuration after evaluating 260.Cm Host 261and 262.Cm Match 263blocks and exit. 264.Pp 265.It Fl g 266Allows remote hosts to connect to local forwarded ports. 267If used on a multiplexed connection, then this option must be specified 268on the master process. 269.Pp 270.It Fl I Ar pkcs11 271Specify the PKCS#11 shared library 272.Nm 273should use to communicate with a PKCS#11 token providing the user's 274private RSA key. 275.Pp 276.It Fl i Ar identity_file 277Selects a file from which the identity (private key) for 278public key authentication is read. 279The default is 280.Pa ~/.ssh/id_dsa , 281.Pa ~/.ssh/id_ecdsa , 282.Pa ~/.ssh/id_ed25519 283and 284.Pa ~/.ssh/id_rsa . 285Identity files may also be specified on 286a per-host basis in the configuration file. 287It is possible to have multiple 288.Fl i 289options (and multiple identities specified in 290configuration files). 291If no certificates have been explicitly specified by the 292.Cm CertificateFile 293directive, 294.Nm 295will also try to load certificate information from the filename obtained 296by appending 297.Pa -cert.pub 298to identity filenames. 299.Pp 300.It Fl J Ar destination 301Connect to the target host by first making a 302.Nm 303connection to the jump host described by 304.Ar destination 305and then establishing a TCP forwarding to the ultimate destination from 306there. 307Multiple jump hops may be specified separated by comma characters. 308This is a shortcut to specify a 309.Cm ProxyJump 310configuration directive. 311.Pp 312.It Fl K 313Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 314credentials to the server. 315.Pp 316.It Fl k 317Disables forwarding (delegation) of GSSAPI credentials to the server. 318.Pp 319.It Fl L Xo 320.Sm off 321.Oo Ar bind_address : Oc 322.Ar port : host : hostport 323.Sm on 324.Xc 325.It Fl L Xo 326.Sm off 327.Oo Ar bind_address : Oc 328.Ar port : remote_socket 329.Sm on 330.Xc 331.It Fl L Xo 332.Sm off 333.Ar local_socket : host : hostport 334.Sm on 335.Xc 336.It Fl L Xo 337.Sm off 338.Ar local_socket : remote_socket 339.Sm on 340.Xc 341Specifies that connections to the given TCP port or Unix socket on the local 342(client) host are to be forwarded to the given host and port, or Unix socket, 343on the remote side. 344This works by allocating a socket to listen to either a TCP 345.Ar port 346on the local side, optionally bound to the specified 347.Ar bind_address , 348or to a Unix socket. 349Whenever a connection is made to the local port or socket, the 350connection is forwarded over the secure channel, and a connection is 351made to either 352.Ar host 353port 354.Ar hostport , 355or the Unix socket 356.Ar remote_socket , 357from the remote machine. 358.Pp 359Port forwardings can also be specified in the configuration file. 360Only the superuser can forward privileged ports. 361IPv6 addresses can be specified by enclosing the address in square brackets. 362.Pp 363By default, the local port is bound in accordance with the 364.Cm GatewayPorts 365setting. 366However, an explicit 367.Ar bind_address 368may be used to bind the connection to a specific address. 369The 370.Ar bind_address 371of 372.Dq localhost 373indicates that the listening port be bound for local use only, while an 374empty address or 375.Sq * 376indicates that the port should be available from all interfaces. 377.Pp 378.It Fl l Ar login_name 379Specifies the user to log in as on the remote machine. 380This also may be specified on a per-host basis in the configuration file. 381.Pp 382.It Fl M 383Places the 384.Nm 385client into 386.Dq master 387mode for connection sharing. 388Multiple 389.Fl M 390options places 391.Nm 392into 393.Dq master 394mode but with confirmation required using 395.Xr ssh-askpass 1 396before each operation that changes the multiplexing state 397(e.g. opening a new session). 398Refer to the description of 399.Cm ControlMaster 400in 401.Xr ssh_config 5 402for details. 403.Pp 404.It Fl m Ar mac_spec 405A comma-separated list of MAC (message authentication code) algorithms, 406specified in order of preference. 407See the 408.Cm MACs 409keyword for more information. 410.Pp 411.It Fl N 412Do not execute a remote command. 413This is useful for just forwarding ports. 414.Pp 415.It Fl n 416Redirects stdin from 417.Pa /dev/null 418(actually, prevents reading from stdin). 419This must be used when 420.Nm 421is run in the background. 422A common trick is to use this to run X11 programs on a remote machine. 423For example, 424.Ic ssh -n shadows.cs.hut.fi emacs & 425will start an emacs on shadows.cs.hut.fi, and the X11 426connection will be automatically forwarded over an encrypted channel. 427The 428.Nm 429program will be put in the background. 430(This does not work if 431.Nm 432needs to ask for a password or passphrase; see also the 433.Fl f 434option.) 435.Pp 436.It Fl O Ar ctl_cmd 437Control an active connection multiplexing master process. 438When the 439.Fl O 440option is specified, the 441.Ar ctl_cmd 442argument is interpreted and passed to the master process. 443Valid commands are: 444.Dq check 445(check that the master process is running), 446.Dq forward 447(request forwardings without command execution), 448.Dq cancel 449(cancel forwardings), 450.Dq exit 451(request the master to exit), and 452.Dq stop 453(request the master to stop accepting further multiplexing requests). 454.Pp 455.It Fl o Ar option 456Can be used to give options in the format used in the configuration file. 457This is useful for specifying options for which there is no separate 458command-line flag. 459For full details of the options listed below, and their possible values, see 460.Xr ssh_config 5 . 461.Pp 462.Bl -tag -width Ds -offset indent -compact 463.It AddKeysToAgent 464.It AddressFamily 465.It BatchMode 466.It BindAddress 467.It CanonicalDomains 468.It CanonicalizeFallbackLocal 469.It CanonicalizeHostname 470.It CanonicalizeMaxDots 471.It CanonicalizePermittedCNAMEs 472.It CASignatureAlgorithms 473.It CertificateFile 474.It ChallengeResponseAuthentication 475.It CheckHostIP 476.It Ciphers 477.It ClearAllForwardings 478.It Compression 479.It ConnectionAttempts 480.It ConnectTimeout 481.It ControlMaster 482.It ControlPath 483.It ControlPersist 484.It DynamicForward 485.It EscapeChar 486.It ExitOnForwardFailure 487.It FingerprintHash 488.It ForwardAgent 489.It ForwardX11 490.It ForwardX11Timeout 491.It ForwardX11Trusted 492.It GatewayPorts 493.It GlobalKnownHostsFile 494.It GSSAPIAuthentication 495.It GSSAPIDelegateCredentials 496.It HashKnownHosts 497.It Host 498.It HostbasedAuthentication 499.It HostbasedKeyTypes 500.It HostKeyAlgorithms 501.It HostKeyAlias 502.It HostName 503.It IdentitiesOnly 504.It IdentityAgent 505.It IdentityFile 506.It IPQoS 507.It KbdInteractiveAuthentication 508.It KbdInteractiveDevices 509.It KexAlgorithms 510.It LocalCommand 511.It LocalForward 512.It LogLevel 513.It MACs 514.It Match 515.It NoHostAuthenticationForLocalhost 516.It NumberOfPasswordPrompts 517.It PasswordAuthentication 518.It PermitLocalCommand 519.It PKCS11Provider 520.It Port 521.It PreferredAuthentications 522.It ProxyCommand 523.It ProxyJump 524.It ProxyUseFdpass 525.It PubkeyAcceptedKeyTypes 526.It PubkeyAuthentication 527.It RekeyLimit 528.It RemoteCommand 529.It RemoteForward 530.It RequestTTY 531.It SendEnv 532.It ServerAliveInterval 533.It ServerAliveCountMax 534.It SetEnv 535.It StreamLocalBindMask 536.It StreamLocalBindUnlink 537.It StrictHostKeyChecking 538.It TCPKeepAlive 539.It Tunnel 540.It TunnelDevice 541.It UpdateHostKeys 542.It User 543.It UserKnownHostsFile 544.It VerifyHostKeyDNS 545.It VisualHostKey 546.It XAuthLocation 547.El 548.Pp 549.It Fl p Ar port 550Port to connect to on the remote host. 551This can be specified on a 552per-host basis in the configuration file. 553.Pp 554.It Fl Q Ar query_option 555Queries 556.Nm 557for the algorithms supported for the specified version 2. 558The available features are: 559.Ar cipher 560(supported symmetric ciphers), 561.Ar cipher-auth 562(supported symmetric ciphers that support authenticated encryption), 563.Ar help 564(supported query terms for use with the 565.Fl Q 566flag), 567.Ar mac 568(supported message integrity codes), 569.Ar kex 570(key exchange algorithms), 571.Ar key 572(key types), 573.Ar key-cert 574(certificate key types), 575.Ar key-plain 576(non-certificate key types), 577.Ar protocol-version 578(supported SSH protocol versions), and 579.Ar sig 580(supported signature algorithms). 581.Pp 582.It Fl q 583Quiet mode. 584Causes most warning and diagnostic messages to be suppressed. 585.Pp 586.It Fl R Xo 587.Sm off 588.Oo Ar bind_address : Oc 589.Ar port : host : hostport 590.Sm on 591.Xc 592.It Fl R Xo 593.Sm off 594.Oo Ar bind_address : Oc 595.Ar port : local_socket 596.Sm on 597.Xc 598.It Fl R Xo 599.Sm off 600.Ar remote_socket : host : hostport 601.Sm on 602.Xc 603.It Fl R Xo 604.Sm off 605.Ar remote_socket : local_socket 606.Sm on 607.Xc 608.It Fl R Xo 609.Sm off 610.Oo Ar bind_address : Oc 611.Ar port 612.Sm on 613.Xc 614Specifies that connections to the given TCP port or Unix socket on the remote 615(server) host are to be forwarded to the local side. 616.Pp 617This works by allocating a socket to listen to either a TCP 618.Ar port 619or to a Unix socket on the remote side. 620Whenever a connection is made to this port or Unix socket, the 621connection is forwarded over the secure channel, and a connection 622is made from the local machine to either an explicit destination specified by 623.Ar host 624port 625.Ar hostport , 626or 627.Ar local_socket , 628or, if no explicit destination was specified, 629.Nm 630will act as a SOCKS 4/5 proxy and forward connections to the destinations 631requested by the remote SOCKS client. 632.Pp 633Port forwardings can also be specified in the configuration file. 634Privileged ports can be forwarded only when 635logging in as root on the remote machine. 636IPv6 addresses can be specified by enclosing the address in square brackets. 637.Pp 638By default, TCP listening sockets on the server will be bound to the loopback 639interface only. 640This may be overridden by specifying a 641.Ar bind_address . 642An empty 643.Ar bind_address , 644or the address 645.Ql * , 646indicates that the remote socket should listen on all interfaces. 647Specifying a remote 648.Ar bind_address 649will only succeed if the server's 650.Cm GatewayPorts 651option is enabled (see 652.Xr sshd_config 5 ) . 653.Pp 654If the 655.Ar port 656argument is 657.Ql 0 , 658the listen port will be dynamically allocated on the server and reported 659to the client at run time. 660When used together with 661.Ic -O forward 662the allocated port will be printed to the standard output. 663.Pp 664.It Fl S Ar ctl_path 665Specifies the location of a control socket for connection sharing, 666or the string 667.Dq none 668to disable connection sharing. 669Refer to the description of 670.Cm ControlPath 671and 672.Cm ControlMaster 673in 674.Xr ssh_config 5 675for details. 676.Pp 677.It Fl s 678May be used to request invocation of a subsystem on the remote system. 679Subsystems facilitate the use of SSH 680as a secure transport for other applications (e.g.\& 681.Xr sftp 1 ) . 682The subsystem is specified as the remote command. 683.Pp 684.It Fl T 685Disable pseudo-terminal allocation. 686.Pp 687.It Fl t 688Force pseudo-terminal allocation. 689This can be used to execute arbitrary 690screen-based programs on a remote machine, which can be very useful, 691e.g. when implementing menu services. 692Multiple 693.Fl t 694options force tty allocation, even if 695.Nm 696has no local tty. 697.Pp 698.It Fl V 699Display the version number and exit. 700.Pp 701.It Fl v 702Verbose mode. 703Causes 704.Nm 705to print debugging messages about its progress. 706This is helpful in 707debugging connection, authentication, and configuration problems. 708Multiple 709.Fl v 710options increase the verbosity. 711The maximum is 3. 712.Pp 713.It Fl W Ar host : Ns Ar port 714Requests that standard input and output on the client be forwarded to 715.Ar host 716on 717.Ar port 718over the secure channel. 719Implies 720.Fl N , 721.Fl T , 722.Cm ExitOnForwardFailure 723and 724.Cm ClearAllForwardings , 725though these can be overridden in the configuration file or using 726.Fl o 727command line options. 728.Pp 729.It Fl w Xo 730.Ar local_tun Ns Op : Ns Ar remote_tun 731.Xc 732Requests 733tunnel 734device forwarding with the specified 735.Xr tun 4 736devices between the client 737.Pq Ar local_tun 738and the server 739.Pq Ar remote_tun . 740.Pp 741The devices may be specified by numerical ID or the keyword 742.Dq any , 743which uses the next available tunnel device. 744If 745.Ar remote_tun 746is not specified, it defaults to 747.Dq any . 748See also the 749.Cm Tunnel 750and 751.Cm TunnelDevice 752directives in 753.Xr ssh_config 5 . 754.Pp 755If the 756.Cm Tunnel 757directive is unset, it will be set to the default tunnel mode, which is 758.Dq point-to-point . 759If a different 760.Cm Tunnel 761forwarding mode it desired, then it should be specified before 762.Fl w . 763.Pp 764.It Fl X 765Enables X11 forwarding. 766This can also be specified on a per-host basis in a configuration file. 767.Pp 768X11 forwarding should be enabled with caution. 769Users with the ability to bypass file permissions on the remote host 770(for the user's X authorization database) 771can access the local X11 display through the forwarded connection. 772An attacker may then be able to perform activities such as keystroke monitoring. 773.Pp 774For this reason, X11 forwarding is subjected to X11 SECURITY extension 775restrictions by default. 776Please refer to the 777.Nm 778.Fl Y 779option and the 780.Cm ForwardX11Trusted 781directive in 782.Xr ssh_config 5 783for more information. 784.Pp 785.It Fl x 786Disables X11 forwarding. 787.Pp 788.It Fl Y 789Enables trusted X11 forwarding. 790Trusted X11 forwardings are not subjected to the X11 SECURITY extension 791controls. 792.Pp 793.It Fl y 794Send log information using the 795.Xr syslog 3 796system module. 797By default this information is sent to stderr. 798.El 799.Pp 800.Nm 801may additionally obtain configuration data from 802a per-user configuration file and a system-wide configuration file. 803The file format and configuration options are described in 804.Xr ssh_config 5 . 805.Sh AUTHENTICATION 806The OpenSSH SSH client supports SSH protocol 2. 807.Pp 808The methods available for authentication are: 809GSSAPI-based authentication, 810host-based authentication, 811public key authentication, 812challenge-response authentication, 813and password authentication. 814Authentication methods are tried in the order specified above, 815though 816.Cm PreferredAuthentications 817can be used to change the default order. 818.Pp 819Host-based authentication works as follows: 820If the machine the user logs in from is listed in 821.Pa /etc/hosts.equiv 822or 823.Pa /etc/shosts.equiv 824on the remote machine, and the user names are 825the same on both sides, or if the files 826.Pa ~/.rhosts 827or 828.Pa ~/.shosts 829exist in the user's home directory on the 830remote machine and contain a line containing the name of the client 831machine and the name of the user on that machine, the user is 832considered for login. 833Additionally, the server 834.Em must 835be able to verify the client's 836host key (see the description of 837.Pa /etc/ssh/ssh_known_hosts 838and 839.Pa ~/.ssh/known_hosts , 840below) 841for login to be permitted. 842This authentication method closes security holes due to IP 843spoofing, DNS spoofing, and routing spoofing. 844[Note to the administrator: 845.Pa /etc/hosts.equiv , 846.Pa ~/.rhosts , 847and the rlogin/rsh protocol in general, are inherently insecure and should be 848disabled if security is desired.] 849.Pp 850Public key authentication works as follows: 851The scheme is based on public-key cryptography, 852using cryptosystems 853where encryption and decryption are done using separate keys, 854and it is unfeasible to derive the decryption key from the encryption key. 855The idea is that each user creates a public/private 856key pair for authentication purposes. 857The server knows the public key, and only the user knows the private key. 858.Nm 859implements public key authentication protocol automatically, 860using one of the DSA, ECDSA, Ed25519 or RSA algorithms. 861The HISTORY section of 862.Xr ssl 8 863contains a brief discussion of the DSA and RSA algorithms. 864.Pp 865The file 866.Pa ~/.ssh/authorized_keys 867lists the public keys that are permitted for logging in. 868When the user logs in, the 869.Nm 870program tells the server which key pair it would like to use for 871authentication. 872The client proves that it has access to the private key 873and the server checks that the corresponding public key 874is authorized to accept the account. 875.Pp 876The server may inform the client of errors that prevented public key 877authentication from succeeding after authentication completes using a 878different method. 879These may be viewed by increasing the 880.Cm LogLevel 881to 882.Cm DEBUG 883or higher (e.g. by using the 884.Fl v 885flag). 886.Pp 887The user creates his/her key pair by running 888.Xr ssh-keygen 1 . 889This stores the private key in 890.Pa ~/.ssh/id_dsa 891(DSA), 892.Pa ~/.ssh/id_ecdsa 893(ECDSA), 894.Pa ~/.ssh/id_ed25519 895(Ed25519), 896or 897.Pa ~/.ssh/id_rsa 898(RSA) 899and stores the public key in 900.Pa ~/.ssh/id_dsa.pub 901(DSA), 902.Pa ~/.ssh/id_ecdsa.pub 903(ECDSA), 904.Pa ~/.ssh/id_ed25519.pub 905(Ed25519), 906or 907.Pa ~/.ssh/id_rsa.pub 908(RSA) 909in the user's home directory. 910The user should then copy the public key 911to 912.Pa ~/.ssh/authorized_keys 913in his/her home directory on the remote machine. 914The 915.Pa authorized_keys 916file corresponds to the conventional 917.Pa ~/.rhosts 918file, and has one key 919per line, though the lines can be very long. 920After this, the user can log in without giving the password. 921.Pp 922A variation on public key authentication 923is available in the form of certificate authentication: 924instead of a set of public/private keys, 925signed certificates are used. 926This has the advantage that a single trusted certification authority 927can be used in place of many public/private keys. 928See the CERTIFICATES section of 929.Xr ssh-keygen 1 930for more information. 931.Pp 932The most convenient way to use public key or certificate authentication 933may be with an authentication agent. 934See 935.Xr ssh-agent 1 936and (optionally) the 937.Cm AddKeysToAgent 938directive in 939.Xr ssh_config 5 940for more information. 941.Pp 942Challenge-response authentication works as follows: 943The server sends an arbitrary 944.Qq challenge 945text, and prompts for a response. 946Examples of challenge-response authentication include 947.Bx 948Authentication (see 949.Xr login.conf 5 ) 950and PAM (some 951.Pf non- Ox 952systems). 953.Pp 954Finally, if other authentication methods fail, 955.Nm 956prompts the user for a password. 957The password is sent to the remote 958host for checking; however, since all communications are encrypted, 959the password cannot be seen by someone listening on the network. 960.Pp 961.Nm 962automatically maintains and checks a database containing 963identification for all hosts it has ever been used with. 964Host keys are stored in 965.Pa ~/.ssh/known_hosts 966in the user's home directory. 967Additionally, the file 968.Pa /etc/ssh/ssh_known_hosts 969is automatically checked for known hosts. 970Any new hosts are automatically added to the user's file. 971If a host's identification ever changes, 972.Nm 973warns about this and disables password authentication to prevent 974server spoofing or man-in-the-middle attacks, 975which could otherwise be used to circumvent the encryption. 976The 977.Cm StrictHostKeyChecking 978option can be used to control logins to machines whose 979host key is not known or has changed. 980.Pp 981When the user's identity has been accepted by the server, the server 982either executes the given command in a non-interactive session or, 983if no command has been specified, logs into the machine and gives 984the user a normal shell as an interactive session. 985All communication with 986the remote command or shell will be automatically encrypted. 987.Pp 988If an interactive session is requested 989.Nm 990by default will only request a pseudo-terminal (pty) for interactive 991sessions when the client has one. 992The flags 993.Fl T 994and 995.Fl t 996can be used to override this behaviour. 997.Pp 998If a pseudo-terminal has been allocated the 999user may use the escape characters noted below. 1000.Pp 1001If no pseudo-terminal has been allocated, 1002the session is transparent and can be used to reliably transfer binary data. 1003On most systems, setting the escape character to 1004.Dq none 1005will also make the session transparent even if a tty is used. 1006.Pp 1007The session terminates when the command or shell on the remote 1008machine exits and all X11 and TCP connections have been closed. 1009.Sh ESCAPE CHARACTERS 1010When a pseudo-terminal has been requested, 1011.Nm 1012supports a number of functions through the use of an escape character. 1013.Pp 1014A single tilde character can be sent as 1015.Ic ~~ 1016or by following the tilde by a character other than those described below. 1017The escape character must always follow a newline to be interpreted as 1018special. 1019The escape character can be changed in configuration files using the 1020.Cm EscapeChar 1021configuration directive or on the command line by the 1022.Fl e 1023option. 1024.Pp 1025The supported escapes (assuming the default 1026.Ql ~ ) 1027are: 1028.Bl -tag -width Ds 1029.It Cm ~. 1030Disconnect. 1031.It Cm ~^Z 1032Background 1033.Nm . 1034.It Cm ~# 1035List forwarded connections. 1036.It Cm ~& 1037Background 1038.Nm 1039at logout when waiting for forwarded connection / X11 sessions to terminate. 1040.It Cm ~? 1041Display a list of escape characters. 1042.It Cm ~B 1043Send a BREAK to the remote system 1044(only useful if the peer supports it). 1045.It Cm ~C 1046Open command line. 1047Currently this allows the addition of port forwardings using the 1048.Fl L , 1049.Fl R 1050and 1051.Fl D 1052options (see above). 1053It also allows the cancellation of existing port-forwardings 1054with 1055.Sm off 1056.Fl KL Oo Ar bind_address : Oc Ar port 1057.Sm on 1058for local, 1059.Sm off 1060.Fl KR Oo Ar bind_address : Oc Ar port 1061.Sm on 1062for remote and 1063.Sm off 1064.Fl KD Oo Ar bind_address : Oc Ar port 1065.Sm on 1066for dynamic port-forwardings. 1067.Ic !\& Ns Ar command 1068allows the user to execute a local command if the 1069.Ic PermitLocalCommand 1070option is enabled in 1071.Xr ssh_config 5 . 1072Basic help is available, using the 1073.Fl h 1074option. 1075.It Cm ~R 1076Request rekeying of the connection 1077(only useful if the peer supports it). 1078.It Cm ~V 1079Decrease the verbosity 1080.Pq Ic LogLevel 1081when errors are being written to stderr. 1082.It Cm ~v 1083Increase the verbosity 1084.Pq Ic LogLevel 1085when errors are being written to stderr. 1086.El 1087.Sh TCP FORWARDING 1088Forwarding of arbitrary TCP connections over the secure channel can 1089be specified either on the command line or in a configuration file. 1090One possible application of TCP forwarding is a secure connection to a 1091mail server; another is going through firewalls. 1092.Pp 1093In the example below, we look at encrypting communication between 1094an IRC client and server, even though the IRC server does not directly 1095support encrypted communications. 1096This works as follows: 1097the user connects to the remote host using 1098.Nm , 1099specifying a port to be used to forward connections 1100to the remote server. 1101After that it is possible to start the service which is to be encrypted 1102on the client machine, 1103connecting to the same local port, 1104and 1105.Nm 1106will encrypt and forward the connection. 1107.Pp 1108The following example tunnels an IRC session from client machine 1109.Dq 127.0.0.1 1110(localhost) 1111to remote server 1112.Dq server.example.com : 1113.Bd -literal -offset 4n 1114$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 1115$ irc -c '#users' -p 1234 pinky 127.0.0.1 1116.Ed 1117.Pp 1118This tunnels a connection to IRC server 1119.Dq server.example.com , 1120joining channel 1121.Dq #users , 1122nickname 1123.Dq pinky , 1124using port 1234. 1125It doesn't matter which port is used, 1126as long as it's greater than 1023 1127(remember, only root can open sockets on privileged ports) 1128and doesn't conflict with any ports already in use. 1129The connection is forwarded to port 6667 on the remote server, 1130since that's the standard port for IRC services. 1131.Pp 1132The 1133.Fl f 1134option backgrounds 1135.Nm 1136and the remote command 1137.Dq sleep 10 1138is specified to allow an amount of time 1139(10 seconds, in the example) 1140to start the service which is to be tunnelled. 1141If no connections are made within the time specified, 1142.Nm 1143will exit. 1144.Sh X11 FORWARDING 1145If the 1146.Cm ForwardX11 1147variable is set to 1148.Dq yes 1149(or see the description of the 1150.Fl X , 1151.Fl x , 1152and 1153.Fl Y 1154options above) 1155and the user is using X11 (the 1156.Ev DISPLAY 1157environment variable is set), the connection to the X11 display is 1158automatically forwarded to the remote side in such a way that any X11 1159programs started from the shell (or command) will go through the 1160encrypted channel, and the connection to the real X server will be made 1161from the local machine. 1162The user should not manually set 1163.Ev DISPLAY . 1164Forwarding of X11 connections can be 1165configured on the command line or in configuration files. 1166.Pp 1167The 1168.Ev DISPLAY 1169value set by 1170.Nm 1171will point to the server machine, but with a display number greater than zero. 1172This is normal, and happens because 1173.Nm 1174creates a 1175.Dq proxy 1176X server on the server machine for forwarding the 1177connections over the encrypted channel. 1178.Pp 1179.Nm 1180will also automatically set up Xauthority data on the server machine. 1181For this purpose, it will generate a random authorization cookie, 1182store it in Xauthority on the server, and verify that any forwarded 1183connections carry this cookie and replace it by the real cookie when 1184the connection is opened. 1185The real authentication cookie is never 1186sent to the server machine (and no cookies are sent in the plain). 1187.Pp 1188If the 1189.Cm ForwardAgent 1190variable is set to 1191.Dq yes 1192(or see the description of the 1193.Fl A 1194and 1195.Fl a 1196options above) and 1197the user is using an authentication agent, the connection to the agent 1198is automatically forwarded to the remote side. 1199.Sh VERIFYING HOST KEYS 1200When connecting to a server for the first time, 1201a fingerprint of the server's public key is presented to the user 1202(unless the option 1203.Cm StrictHostKeyChecking 1204has been disabled). 1205Fingerprints can be determined using 1206.Xr ssh-keygen 1 : 1207.Pp 1208.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1209.Pp 1210If the fingerprint is already known, it can be matched 1211and the key can be accepted or rejected. 1212If only legacy (MD5) fingerprints for the server are available, the 1213.Xr ssh-keygen 1 1214.Fl E 1215option may be used to downgrade the fingerprint algorithm to match. 1216.Pp 1217Because of the difficulty of comparing host keys 1218just by looking at fingerprint strings, 1219there is also support to compare host keys visually, 1220using 1221.Em random art . 1222By setting the 1223.Cm VisualHostKey 1224option to 1225.Dq yes , 1226a small ASCII graphic gets displayed on every login to a server, no matter 1227if the session itself is interactive or not. 1228By learning the pattern a known server produces, a user can easily 1229find out that the host key has changed when a completely different pattern 1230is displayed. 1231Because these patterns are not unambiguous however, a pattern that looks 1232similar to the pattern remembered only gives a good probability that the 1233host key is the same, not guaranteed proof. 1234.Pp 1235To get a listing of the fingerprints along with their random art for 1236all known hosts, the following command line can be used: 1237.Pp 1238.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1239.Pp 1240If the fingerprint is unknown, 1241an alternative method of verification is available: 1242SSH fingerprints verified by DNS. 1243An additional resource record (RR), 1244SSHFP, 1245is added to a zonefile 1246and the connecting client is able to match the fingerprint 1247with that of the key presented. 1248.Pp 1249In this example, we are connecting a client to a server, 1250.Dq host.example.com . 1251The SSHFP resource records should first be added to the zonefile for 1252host.example.com: 1253.Bd -literal -offset indent 1254$ ssh-keygen -r host.example.com. 1255.Ed 1256.Pp 1257The output lines will have to be added to the zonefile. 1258To check that the zone is answering fingerprint queries: 1259.Pp 1260.Dl $ dig -t SSHFP host.example.com 1261.Pp 1262Finally the client connects: 1263.Bd -literal -offset indent 1264$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1265[...] 1266Matching host key fingerprint found in DNS. 1267Are you sure you want to continue connecting (yes/no)? 1268.Ed 1269.Pp 1270See the 1271.Cm VerifyHostKeyDNS 1272option in 1273.Xr ssh_config 5 1274for more information. 1275.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1276.Nm 1277contains support for Virtual Private Network (VPN) tunnelling 1278using the 1279.Xr tun 4 1280network pseudo-device, 1281allowing two networks to be joined securely. 1282The 1283.Xr sshd_config 5 1284configuration option 1285.Cm PermitTunnel 1286controls whether the server supports this, 1287and at what level (layer 2 or 3 traffic). 1288.Pp 1289The following example would connect client network 10.0.50.0/24 1290with remote network 10.0.99.0/24 using a point-to-point connection 1291from 10.1.1.1 to 10.1.1.2, 1292provided that the SSH server running on the gateway to the remote network, 1293at 192.168.1.15, allows it. 1294.Pp 1295On the client: 1296.Bd -literal -offset indent 1297# ssh -f -w 0:1 192.168.1.15 true 1298# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1299# route add 10.0.99.0/24 10.1.1.2 1300.Ed 1301.Pp 1302On the server: 1303.Bd -literal -offset indent 1304# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1305# route add 10.0.50.0/24 10.1.1.1 1306.Ed 1307.Pp 1308Client access may be more finely tuned via the 1309.Pa /root/.ssh/authorized_keys 1310file (see below) and the 1311.Cm PermitRootLogin 1312server option. 1313The following entry would permit connections on 1314.Xr tun 4 1315device 1 from user 1316.Dq jane 1317and on tun device 2 from user 1318.Dq john , 1319if 1320.Cm PermitRootLogin 1321is set to 1322.Dq forced-commands-only : 1323.Bd -literal -offset 2n 1324tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1325tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1326.Ed 1327.Pp 1328Since an SSH-based setup entails a fair amount of overhead, 1329it may be more suited to temporary setups, 1330such as for wireless VPNs. 1331More permanent VPNs are better provided by tools such as 1332.Xr ipsecctl 8 1333and 1334.Xr isakmpd 8 . 1335.Sh ENVIRONMENT 1336.Nm 1337will normally set the following environment variables: 1338.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1339.It Ev DISPLAY 1340The 1341.Ev DISPLAY 1342variable indicates the location of the X11 server. 1343It is automatically set by 1344.Nm 1345to point to a value of the form 1346.Dq hostname:n , 1347where 1348.Dq hostname 1349indicates the host where the shell runs, and 1350.Sq n 1351is an integer \*(Ge 1. 1352.Nm 1353uses this special value to forward X11 connections over the secure 1354channel. 1355The user should normally not set 1356.Ev DISPLAY 1357explicitly, as that 1358will render the X11 connection insecure (and will require the user to 1359manually copy any required authorization cookies). 1360.It Ev HOME 1361Set to the path of the user's home directory. 1362.It Ev LOGNAME 1363Synonym for 1364.Ev USER ; 1365set for compatibility with systems that use this variable. 1366.It Ev MAIL 1367Set to the path of the user's mailbox. 1368.It Ev PATH 1369Set to the default 1370.Ev PATH , 1371as specified when compiling 1372.Nm . 1373.It Ev SSH_ASKPASS 1374If 1375.Nm 1376needs a passphrase, it will read the passphrase from the current 1377terminal if it was run from a terminal. 1378If 1379.Nm 1380does not have a terminal associated with it but 1381.Ev DISPLAY 1382and 1383.Ev SSH_ASKPASS 1384are set, it will execute the program specified by 1385.Ev SSH_ASKPASS 1386and open an X11 window to read the passphrase. 1387This is particularly useful when calling 1388.Nm 1389from a 1390.Pa .xsession 1391or related script. 1392(Note that on some machines it 1393may be necessary to redirect the input from 1394.Pa /dev/null 1395to make this work.) 1396.It Ev SSH_AUTH_SOCK 1397Identifies the path of a 1398.Ux Ns -domain 1399socket used to communicate with the agent. 1400.It Ev SSH_CONNECTION 1401Identifies the client and server ends of the connection. 1402The variable contains 1403four space-separated values: client IP address, client port number, 1404server IP address, and server port number. 1405.It Ev SSH_ORIGINAL_COMMAND 1406This variable contains the original command line if a forced command 1407is executed. 1408It can be used to extract the original arguments. 1409.It Ev SSH_TTY 1410This is set to the name of the tty (path to the device) associated 1411with the current shell or command. 1412If the current session has no tty, 1413this variable is not set. 1414.It Ev SSH_TUNNEL 1415Optionally set by 1416.Xr sshd 8 1417to contain the interface names assigned if tunnel forwarding was 1418requested by the client. 1419.It Ev SSH_USER_AUTH 1420Optionally set by 1421.Xr sshd 8 , 1422this variable may contain a pathname to a file that lists the authentication 1423methods successfully used when the session was established, including any 1424public keys that were used. 1425.It Ev TZ 1426This variable is set to indicate the present time zone if it 1427was set when the daemon was started (i.e. the daemon passes the value 1428on to new connections). 1429.It Ev USER 1430Set to the name of the user logging in. 1431.El 1432.Pp 1433Additionally, 1434.Nm 1435reads 1436.Pa ~/.ssh/environment , 1437and adds lines of the format 1438.Dq VARNAME=value 1439to the environment if the file exists and users are allowed to 1440change their environment. 1441For more information, see the 1442.Cm PermitUserEnvironment 1443option in 1444.Xr sshd_config 5 . 1445.Sh FILES 1446.Bl -tag -width Ds -compact 1447.It Pa ~/.rhosts 1448This file is used for host-based authentication (see above). 1449On some machines this file may need to be 1450world-readable if the user's home directory is on an NFS partition, 1451because 1452.Xr sshd 8 1453reads it as root. 1454Additionally, this file must be owned by the user, 1455and must not have write permissions for anyone else. 1456The recommended 1457permission for most machines is read/write for the user, and not 1458accessible by others. 1459.Pp 1460.It Pa ~/.shosts 1461This file is used in exactly the same way as 1462.Pa .rhosts , 1463but allows host-based authentication without permitting login with 1464rlogin/rsh. 1465.Pp 1466.It Pa ~/.ssh/ 1467This directory is the default location for all user-specific configuration 1468and authentication information. 1469There is no general requirement to keep the entire contents of this directory 1470secret, but the recommended permissions are read/write/execute for the user, 1471and not accessible by others. 1472.Pp 1473.It Pa ~/.ssh/authorized_keys 1474Lists the public keys (DSA, ECDSA, Ed25519, RSA) 1475that can be used for logging in as this user. 1476The format of this file is described in the 1477.Xr sshd 8 1478manual page. 1479This file is not highly sensitive, but the recommended 1480permissions are read/write for the user, and not accessible by others. 1481.Pp 1482.It Pa ~/.ssh/config 1483This is the per-user configuration file. 1484The file format and configuration options are described in 1485.Xr ssh_config 5 . 1486Because of the potential for abuse, this file must have strict permissions: 1487read/write for the user, and not writable by others. 1488.Pp 1489.It Pa ~/.ssh/environment 1490Contains additional definitions for environment variables; see 1491.Sx ENVIRONMENT , 1492above. 1493.Pp 1494.It Pa ~/.ssh/id_dsa 1495.It Pa ~/.ssh/id_ecdsa 1496.It Pa ~/.ssh/id_ed25519 1497.It Pa ~/.ssh/id_rsa 1498Contains the private key for authentication. 1499These files 1500contain sensitive data and should be readable by the user but not 1501accessible by others (read/write/execute). 1502.Nm 1503will simply ignore a private key file if it is accessible by others. 1504It is possible to specify a passphrase when 1505generating the key which will be used to encrypt the 1506sensitive part of this file using AES-128. 1507.Pp 1508.It Pa ~/.ssh/id_dsa.pub 1509.It Pa ~/.ssh/id_ecdsa.pub 1510.It Pa ~/.ssh/id_ed25519.pub 1511.It Pa ~/.ssh/id_rsa.pub 1512Contains the public key for authentication. 1513These files are not 1514sensitive and can (but need not) be readable by anyone. 1515.Pp 1516.It Pa ~/.ssh/known_hosts 1517Contains a list of host keys for all hosts the user has logged into 1518that are not already in the systemwide list of known host keys. 1519See 1520.Xr sshd 8 1521for further details of the format of this file. 1522.Pp 1523.It Pa ~/.ssh/rc 1524Commands in this file are executed by 1525.Nm 1526when the user logs in, just before the user's shell (or command) is 1527started. 1528See the 1529.Xr sshd 8 1530manual page for more information. 1531.Pp 1532.It Pa /etc/hosts.equiv 1533This file is for host-based authentication (see above). 1534It should only be writable by root. 1535.Pp 1536.It Pa /etc/shosts.equiv 1537This file is used in exactly the same way as 1538.Pa hosts.equiv , 1539but allows host-based authentication without permitting login with 1540rlogin/rsh. 1541.Pp 1542.It Pa /etc/ssh/ssh_config 1543Systemwide configuration file. 1544The file format and configuration options are described in 1545.Xr ssh_config 5 . 1546.Pp 1547.It Pa /etc/ssh/ssh_host_key 1548.It Pa /etc/ssh/ssh_host_dsa_key 1549.It Pa /etc/ssh/ssh_host_ecdsa_key 1550.It Pa /etc/ssh/ssh_host_ed25519_key 1551.It Pa /etc/ssh/ssh_host_rsa_key 1552These files contain the private parts of the host keys 1553and are used for host-based authentication. 1554.Pp 1555.It Pa /etc/ssh/ssh_known_hosts 1556Systemwide list of known host keys. 1557This file should be prepared by the 1558system administrator to contain the public host keys of all machines in the 1559organization. 1560It should be world-readable. 1561See 1562.Xr sshd 8 1563for further details of the format of this file. 1564.Pp 1565.It Pa /etc/ssh/sshrc 1566Commands in this file are executed by 1567.Nm 1568when the user logs in, just before the user's shell (or command) is started. 1569See the 1570.Xr sshd 8 1571manual page for more information. 1572.El 1573.Sh EXIT STATUS 1574.Nm 1575exits with the exit status of the remote command or with 255 1576if an error occurred. 1577.Sh SEE ALSO 1578.Xr scp 1 , 1579.Xr sftp 1 , 1580.Xr ssh-add 1 , 1581.Xr ssh-agent 1 , 1582.Xr ssh-keygen 1 , 1583.Xr ssh-keyscan 1 , 1584.Xr tun 4 , 1585.Xr ssh_config 5 , 1586.Xr ssh-keysign 8 , 1587.Xr sshd 8 1588.Sh STANDARDS 1589.Rs 1590.%A S. Lehtinen 1591.%A C. Lonvick 1592.%D January 2006 1593.%R RFC 4250 1594.%T The Secure Shell (SSH) Protocol Assigned Numbers 1595.Re 1596.Pp 1597.Rs 1598.%A T. Ylonen 1599.%A C. Lonvick 1600.%D January 2006 1601.%R RFC 4251 1602.%T The Secure Shell (SSH) Protocol Architecture 1603.Re 1604.Pp 1605.Rs 1606.%A T. Ylonen 1607.%A C. Lonvick 1608.%D January 2006 1609.%R RFC 4252 1610.%T The Secure Shell (SSH) Authentication Protocol 1611.Re 1612.Pp 1613.Rs 1614.%A T. Ylonen 1615.%A C. Lonvick 1616.%D January 2006 1617.%R RFC 4253 1618.%T The Secure Shell (SSH) Transport Layer Protocol 1619.Re 1620.Pp 1621.Rs 1622.%A T. Ylonen 1623.%A C. Lonvick 1624.%D January 2006 1625.%R RFC 4254 1626.%T The Secure Shell (SSH) Connection Protocol 1627.Re 1628.Pp 1629.Rs 1630.%A J. Schlyter 1631.%A W. Griffin 1632.%D January 2006 1633.%R RFC 4255 1634.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1635.Re 1636.Pp 1637.Rs 1638.%A F. Cusack 1639.%A M. Forssen 1640.%D January 2006 1641.%R RFC 4256 1642.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1643.Re 1644.Pp 1645.Rs 1646.%A J. Galbraith 1647.%A P. Remaker 1648.%D January 2006 1649.%R RFC 4335 1650.%T The Secure Shell (SSH) Session Channel Break Extension 1651.Re 1652.Pp 1653.Rs 1654.%A M. Bellare 1655.%A T. Kohno 1656.%A C. Namprempre 1657.%D January 2006 1658.%R RFC 4344 1659.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1660.Re 1661.Pp 1662.Rs 1663.%A B. Harris 1664.%D January 2006 1665.%R RFC 4345 1666.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1667.Re 1668.Pp 1669.Rs 1670.%A M. Friedl 1671.%A N. Provos 1672.%A W. Simpson 1673.%D March 2006 1674.%R RFC 4419 1675.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1676.Re 1677.Pp 1678.Rs 1679.%A J. Galbraith 1680.%A R. Thayer 1681.%D November 2006 1682.%R RFC 4716 1683.%T The Secure Shell (SSH) Public Key File Format 1684.Re 1685.Pp 1686.Rs 1687.%A D. Stebila 1688.%A J. Green 1689.%D December 2009 1690.%R RFC 5656 1691.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1692.Re 1693.Pp 1694.Rs 1695.%A A. Perrig 1696.%A D. Song 1697.%D 1999 1698.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1699.%T Hash Visualization: a New Technique to improve Real-World Security 1700.Re 1701.Sh AUTHORS 1702OpenSSH is a derivative of the original and free 1703ssh 1.2.12 release by Tatu Ylonen. 1704Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1705Theo de Raadt and Dug Song 1706removed many bugs, re-added newer features and 1707created OpenSSH. 1708Markus Friedl contributed the support for SSH 1709protocol versions 1.5 and 2.0. 1710