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 1.334 2013/07/18 01:12:26 djm Exp $ 37.\" $FreeBSD$ 38.Dd July 18, 2013 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.Bk -words 47.Op Fl 1246AaCfgKkMNnqsTtVvXxYy 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 L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport 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 R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport 63.Op Fl S Ar ctl_path 64.Op Fl W Ar host : Ns Ar port 65.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun 66.Oo Ar user Ns @ Oc Ns Ar hostname 67.Op Ar command 68.Ek 69.Nm 70.Fl Q Ar protocol_feature 71.Sh DESCRIPTION 72.Nm 73(SSH client) is a program for logging into a remote machine and for 74executing commands on a remote machine. 75It is intended to replace rlogin and rsh, 76and provide secure encrypted communications between 77two untrusted hosts over an insecure network. 78X11 connections and arbitrary TCP ports 79can also be forwarded over the secure channel. 80.Pp 81.Nm 82connects and logs into the specified 83.Ar hostname 84(with optional 85.Ar user 86name). 87The user must prove 88his/her identity to the remote machine using one of several methods 89depending on the protocol version used (see below). 90.Pp 91If 92.Ar command 93is specified, 94it is executed on the remote host instead of a login shell. 95.Pp 96The options are as follows: 97.Bl -tag -width Ds 98.It Fl 1 99Forces 100.Nm 101to try protocol version 1 only. 102.It Fl 2 103Forces 104.Nm 105to try protocol version 2 only. 106.It Fl 4 107Forces 108.Nm 109to use IPv4 addresses only. 110.It Fl 6 111Forces 112.Nm 113to use IPv6 addresses only. 114.It Fl A 115Enables forwarding of the authentication agent connection. 116This can also be specified on a per-host basis in a configuration file. 117.Pp 118Agent forwarding should be enabled with caution. 119Users with the ability to bypass file permissions on the remote host 120(for the agent's 121.Ux Ns -domain 122socket) can access the local agent through the forwarded connection. 123An attacker cannot obtain key material from the agent, 124however they can perform operations on the keys that enable them to 125authenticate using the identities loaded into the agent. 126.It Fl a 127Disables forwarding of the authentication agent connection. 128.It Fl b Ar bind_address 129Use 130.Ar bind_address 131on the local machine as the source address 132of the connection. 133Only useful on systems with more than one address. 134.It Fl C 135Requests compression of all data (including stdin, stdout, stderr, and 136data for forwarded X11 and TCP connections). 137The compression algorithm is the same used by 138.Xr gzip 1 , 139and the 140.Dq level 141can be controlled by the 142.Cm CompressionLevel 143option for protocol version 1. 144Compression is desirable on modem lines and other 145slow connections, but will only slow down things on fast networks. 146The default value can be set on a host-by-host basis in the 147configuration files; see the 148.Cm Compression 149option. 150.It Fl c Ar cipher_spec 151Selects the cipher specification for encrypting the session. 152.Pp 153Protocol version 1 allows specification of a single cipher. 154The supported values are 155.Dq 3des , 156.Dq blowfish , 157and 158.Dq des . 159.Ar 3des 160(triple-des) is an encrypt-decrypt-encrypt triple with three different keys. 161It is believed to be secure. 162.Ar blowfish 163is a fast block cipher; it appears very secure and is much faster than 164.Ar 3des . 165.Ar des 166is only supported in the 167.Nm 168client for interoperability with legacy protocol 1 implementations 169that do not support the 170.Ar 3des 171cipher. 172Its use is strongly discouraged due to cryptographic weaknesses. 173The default is 174.Dq 3des . 175.Pp 176For protocol version 2, 177.Ar cipher_spec 178is a comma-separated list of ciphers 179listed in order of preference. 180See the 181.Cm Ciphers 182keyword in 183.Xr ssh_config 5 184for more information. 185.It Fl D Xo 186.Sm off 187.Oo Ar bind_address : Oc 188.Ar port 189.Sm on 190.Xc 191Specifies a local 192.Dq dynamic 193application-level port forwarding. 194This works by allocating a socket to listen to 195.Ar port 196on the local side, optionally bound to the specified 197.Ar bind_address . 198Whenever a connection is made to this port, the 199connection is forwarded over the secure channel, and the application 200protocol is then used to determine where to connect to from the 201remote machine. 202Currently the SOCKS4 and SOCKS5 protocols are supported, and 203.Nm 204will act as a SOCKS server. 205Only root can forward privileged ports. 206Dynamic port forwardings can also be specified in the configuration file. 207.Pp 208IPv6 addresses can be specified by enclosing the address in square brackets. 209Only the superuser can forward privileged ports. 210By default, the local port is bound in accordance with the 211.Cm GatewayPorts 212setting. 213However, an explicit 214.Ar bind_address 215may be used to bind the connection to a specific address. 216The 217.Ar bind_address 218of 219.Dq localhost 220indicates that the listening port be bound for local use only, while an 221empty address or 222.Sq * 223indicates that the port should be available from all interfaces. 224.It Fl E Ar log_file 225Append debug logs to 226.Ar log_file 227instead of standard error. 228.It Fl e Ar escape_char 229Sets the escape character for sessions with a pty (default: 230.Ql ~ ) . 231The escape character is only recognized at the beginning of a line. 232The escape character followed by a dot 233.Pq Ql \&. 234closes the connection; 235followed by control-Z suspends the connection; 236and followed by itself sends the escape character once. 237Setting the character to 238.Dq none 239disables any escapes and makes the session fully transparent. 240.It Fl F Ar configfile 241Specifies an alternative per-user configuration file. 242If a configuration file is given on the command line, 243the system-wide configuration file 244.Pq Pa /etc/ssh/ssh_config 245will be ignored. 246The default for the per-user configuration file is 247.Pa ~/.ssh/config . 248.It Fl f 249Requests 250.Nm 251to go to background just before command execution. 252This is useful if 253.Nm 254is going to ask for passwords or passphrases, but the user 255wants it in the background. 256This implies 257.Fl n . 258The recommended way to start X11 programs at a remote site is with 259something like 260.Ic ssh -f host xterm . 261.Pp 262If the 263.Cm ExitOnForwardFailure 264configuration option is set to 265.Dq yes , 266then a client started with 267.Fl f 268will wait for all remote port forwards to be successfully established 269before placing itself in the background. 270.It Fl g 271Allows remote hosts to connect to local forwarded ports. 272.It Fl I Ar pkcs11 273Specify the PKCS#11 shared library 274.Nm 275should use to communicate with a PKCS#11 token providing the user's 276private RSA key. 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/identity 282for protocol version 1, and 283.Pa ~/.ssh/id_dsa , 284.Pa ~/.ssh/id_ecdsa 285and 286.Pa ~/.ssh/id_rsa 287for protocol version 2. 288Identity files may also be specified on 289a per-host basis in the configuration file. 290It is possible to have multiple 291.Fl i 292options (and multiple identities specified in 293configuration files). 294.Nm 295will also try to load certificate information from the filename obtained 296by appending 297.Pa -cert.pub 298to identity filenames. 299.It Fl K 300Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 301credentials to the server. 302.It Fl k 303Disables forwarding (delegation) of GSSAPI credentials to the server. 304.It Fl L Xo 305.Sm off 306.Oo Ar bind_address : Oc 307.Ar port : host : hostport 308.Sm on 309.Xc 310Specifies that the given port on the local (client) host is to be 311forwarded to the given host and port on the remote side. 312This works by allocating a socket to listen to 313.Ar port 314on the local side, optionally bound to the specified 315.Ar bind_address . 316Whenever a connection is made to this port, the 317connection is forwarded over the secure channel, and a connection is 318made to 319.Ar host 320port 321.Ar hostport 322from the remote machine. 323Port forwardings can also be specified in the configuration file. 324IPv6 addresses can be specified by enclosing the address in square brackets. 325Only the superuser can forward privileged ports. 326By default, the local port is bound in accordance with the 327.Cm GatewayPorts 328setting. 329However, an explicit 330.Ar bind_address 331may be used to bind the connection to a specific address. 332The 333.Ar bind_address 334of 335.Dq localhost 336indicates that the listening port be bound for local use only, while an 337empty address or 338.Sq * 339indicates that the port should be available from all interfaces. 340.It Fl l Ar login_name 341Specifies the user to log in as on the remote machine. 342This also may be specified on a per-host basis in the configuration file. 343.It Fl M 344Places the 345.Nm 346client into 347.Dq master 348mode for connection sharing. 349Multiple 350.Fl M 351options places 352.Nm 353into 354.Dq master 355mode with confirmation required before slave connections are accepted. 356Refer to the description of 357.Cm ControlMaster 358in 359.Xr ssh_config 5 360for details. 361.It Fl m Ar mac_spec 362Additionally, for protocol version 2 a comma-separated list of MAC 363(message authentication code) algorithms can 364be specified in order of preference. 365See the 366.Cm MACs 367keyword for more information. 368.It Fl N 369Do not execute a remote command. 370This is useful for just forwarding ports 371(protocol version 2 only). 372.It Fl n 373Redirects stdin from 374.Pa /dev/null 375(actually, prevents reading from stdin). 376This must be used when 377.Nm 378is run in the background. 379A common trick is to use this to run X11 programs on a remote machine. 380For example, 381.Ic ssh -n shadows.cs.hut.fi emacs & 382will start an emacs on shadows.cs.hut.fi, and the X11 383connection will be automatically forwarded over an encrypted channel. 384The 385.Nm 386program will be put in the background. 387(This does not work if 388.Nm 389needs to ask for a password or passphrase; see also the 390.Fl f 391option.) 392.It Fl O Ar ctl_cmd 393Control an active connection multiplexing master process. 394When the 395.Fl O 396option is specified, the 397.Ar ctl_cmd 398argument is interpreted and passed to the master process. 399Valid commands are: 400.Dq check 401(check that the master process is running), 402.Dq forward 403(request forwardings without command execution), 404.Dq cancel 405(cancel forwardings), 406.Dq exit 407(request the master to exit), and 408.Dq stop 409(request the master to stop accepting further multiplexing requests). 410.It Fl o Ar option 411Can be used to give options in the format used in the configuration file. 412This is useful for specifying options for which there is no separate 413command-line flag. 414For full details of the options listed below, and their possible values, see 415.Xr ssh_config 5 . 416.Pp 417.Bl -tag -width Ds -offset indent -compact 418.It AddressFamily 419.It BatchMode 420.It BindAddress 421.It ChallengeResponseAuthentication 422.It CheckHostIP 423.It Cipher 424.It Ciphers 425.It ClearAllForwardings 426.It Compression 427.It CompressionLevel 428.It ConnectionAttempts 429.It ConnectTimeout 430.It ControlMaster 431.It ControlPath 432.It ControlPersist 433.It DynamicForward 434.It EscapeChar 435.It ExitOnForwardFailure 436.It ForwardAgent 437.It ForwardX11 438.It ForwardX11Timeout 439.It ForwardX11Trusted 440.It GatewayPorts 441.It GlobalKnownHostsFile 442.It GSSAPIAuthentication 443.It GSSAPIDelegateCredentials 444.It HashKnownHosts 445.It Host 446.It HostbasedAuthentication 447.It HostKeyAlgorithms 448.It HostKeyAlias 449.It HostName 450.It IdentityFile 451.It IdentitiesOnly 452.It IPQoS 453.It KbdInteractiveAuthentication 454.It KbdInteractiveDevices 455.It KexAlgorithms 456.It LocalCommand 457.It LocalForward 458.It LogLevel 459.It MACs 460.It NoHostAuthenticationForLocalhost 461.It NumberOfPasswordPrompts 462.It PasswordAuthentication 463.It PermitLocalCommand 464.It PKCS11Provider 465.It Port 466.It PreferredAuthentications 467.It Protocol 468.It ProxyCommand 469.It PubkeyAuthentication 470.It RekeyLimit 471.It RemoteForward 472.It RequestTTY 473.It RhostsRSAAuthentication 474.It RSAAuthentication 475.It SendEnv 476.It ServerAliveInterval 477.It ServerAliveCountMax 478.It StrictHostKeyChecking 479.It TCPKeepAlive 480.It Tunnel 481.It TunnelDevice 482.It UsePrivilegedPort 483.It User 484.It UserKnownHostsFile 485.It VerifyHostKeyDNS 486.It VersionAddendum 487.It VisualHostKey 488.It XAuthLocation 489.El 490.It Fl p Ar port 491Port to connect to on the remote host. 492This can be specified on a 493per-host basis in the configuration file. 494.It Fl Q Ar protocol_feature 495Queries 496.Nm 497for the algorithms supported for the specified version 2 498.Ar protocol_feature . 499The queriable features are: 500.Dq cipher 501(supported symmetric ciphers), 502.Dq MAC 503(supported message integrity codes), 504.Dq KEX 505(key exchange algorithms), 506.Dq key 507(key types). 508Protocol features are treated case-insensitively. 509.It Fl q 510Quiet mode. 511Causes most warning and diagnostic messages to be suppressed. 512.It Fl R Xo 513.Sm off 514.Oo Ar bind_address : Oc 515.Ar port : host : hostport 516.Sm on 517.Xc 518Specifies that the given port on the remote (server) host is to be 519forwarded to the given host and port on the local side. 520This works by allocating a socket to listen to 521.Ar port 522on the remote side, and whenever a connection is made to this port, the 523connection is forwarded over the secure channel, and a connection is 524made to 525.Ar host 526port 527.Ar hostport 528from the local machine. 529.Pp 530Port forwardings can also be specified in the configuration file. 531Privileged ports can be forwarded only when 532logging in as root on the remote machine. 533IPv6 addresses can be specified by enclosing the address in square brackets. 534.Pp 535By default, the listening socket on the server will be bound to the loopback 536interface only. 537This may be overridden by specifying a 538.Ar bind_address . 539An empty 540.Ar bind_address , 541or the address 542.Ql * , 543indicates that the remote socket should listen on all interfaces. 544Specifying a remote 545.Ar bind_address 546will only succeed if the server's 547.Cm GatewayPorts 548option is enabled (see 549.Xr sshd_config 5 ) . 550.Pp 551If the 552.Ar port 553argument is 554.Ql 0 , 555the listen port will be dynamically allocated on the server and reported 556to the client at run time. 557When used together with 558.Ic -O forward 559the allocated port will be printed to the standard output. 560.It Fl S Ar ctl_path 561Specifies the location of a control socket for connection sharing, 562or the string 563.Dq none 564to disable connection sharing. 565Refer to the description of 566.Cm ControlPath 567and 568.Cm ControlMaster 569in 570.Xr ssh_config 5 571for details. 572.It Fl s 573May be used to request invocation of a subsystem on the remote system. 574Subsystems are a feature of the SSH2 protocol which facilitate the use 575of SSH as a secure transport for other applications (eg.\& 576.Xr sftp 1 ) . 577The subsystem is specified as the remote command. 578.It Fl T 579Disable pseudo-tty allocation. 580.It Fl t 581Force pseudo-tty allocation. 582This can be used to execute arbitrary 583screen-based programs on a remote machine, which can be very useful, 584e.g. when implementing menu services. 585Multiple 586.Fl t 587options force tty allocation, even if 588.Nm 589has no local tty. 590.It Fl V 591Display the version number and exit. 592.It Fl v 593Verbose mode. 594Causes 595.Nm 596to print debugging messages about its progress. 597This is helpful in 598debugging connection, authentication, and configuration problems. 599Multiple 600.Fl v 601options increase the verbosity. 602The maximum is 3. 603.It Fl W Ar host : Ns Ar port 604Requests that standard input and output on the client be forwarded to 605.Ar host 606on 607.Ar port 608over the secure channel. 609Implies 610.Fl N , 611.Fl T , 612.Cm ExitOnForwardFailure 613and 614.Cm ClearAllForwardings . 615Works with Protocol version 2 only. 616.It Fl w Xo 617.Ar local_tun Ns Op : Ns Ar remote_tun 618.Xc 619Requests 620tunnel 621device forwarding with the specified 622.Xr tun 4 623devices between the client 624.Pq Ar local_tun 625and the server 626.Pq Ar remote_tun . 627.Pp 628The devices may be specified by numerical ID or the keyword 629.Dq any , 630which uses the next available tunnel device. 631If 632.Ar remote_tun 633is not specified, it defaults to 634.Dq any . 635See also the 636.Cm Tunnel 637and 638.Cm TunnelDevice 639directives in 640.Xr ssh_config 5 . 641If the 642.Cm Tunnel 643directive is unset, it is set to the default tunnel mode, which is 644.Dq point-to-point . 645.It Fl X 646Enables X11 forwarding. 647This can also be specified on a per-host basis in a configuration file. 648.Pp 649X11 forwarding should be enabled with caution. 650Users with the ability to bypass file permissions on the remote host 651(for the user's X authorization database) 652can access the local X11 display through the forwarded connection. 653An attacker may then be able to perform activities such as keystroke monitoring. 654.Pp 655For this reason, X11 forwarding is subjected to X11 SECURITY extension 656restrictions by default. 657Please refer to the 658.Nm 659.Fl Y 660option and the 661.Cm ForwardX11Trusted 662directive in 663.Xr ssh_config 5 664for more information. 665.It Fl x 666Disables X11 forwarding. 667.It Fl Y 668Enables trusted X11 forwarding. 669Trusted X11 forwardings are not subjected to the X11 SECURITY extension 670controls. 671.It Fl y 672Send log information using the 673.Xr syslog 3 674system module. 675By default this information is sent to stderr. 676.El 677.Pp 678.Nm 679may additionally obtain configuration data from 680a per-user configuration file and a system-wide configuration file. 681The file format and configuration options are described in 682.Xr ssh_config 5 . 683.Sh AUTHENTICATION 684The OpenSSH SSH client supports SSH protocols 1 and 2. 685The default is to use protocol 2 only, 686though this can be changed via the 687.Cm Protocol 688option in 689.Xr ssh_config 5 690or the 691.Fl 1 692and 693.Fl 2 694options (see above). 695Both protocols support similar authentication methods, 696but protocol 2 is the default since 697it provides additional mechanisms for confidentiality 698(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) 699and integrity (hmac-md5, hmac-sha1, 700hmac-sha2-256, hmac-sha2-512, 701umac-64, umac-128, hmac-ripemd160). 702Protocol 1 lacks a strong mechanism for ensuring the 703integrity of the connection. 704.Pp 705The methods available for authentication are: 706GSSAPI-based authentication, 707host-based authentication, 708public key authentication, 709challenge-response authentication, 710and password authentication. 711Authentication methods are tried in the order specified above, 712though protocol 2 has a configuration option to change the default order: 713.Cm PreferredAuthentications . 714.Pp 715Host-based authentication works as follows: 716If the machine the user logs in from is listed in 717.Pa /etc/hosts.equiv 718or 719.Pa /etc/shosts.equiv 720on the remote machine, and the user names are 721the same on both sides, or if the files 722.Pa ~/.rhosts 723or 724.Pa ~/.shosts 725exist in the user's home directory on the 726remote machine and contain a line containing the name of the client 727machine and the name of the user on that machine, the user is 728considered for login. 729Additionally, the server 730.Em must 731be able to verify the client's 732host key (see the description of 733.Pa /etc/ssh/ssh_known_hosts 734and 735.Pa ~/.ssh/known_hosts , 736below) 737for login to be permitted. 738This authentication method closes security holes due to IP 739spoofing, DNS spoofing, and routing spoofing. 740[Note to the administrator: 741.Pa /etc/hosts.equiv , 742.Pa ~/.rhosts , 743and the rlogin/rsh protocol in general, are inherently insecure and should be 744disabled if security is desired.] 745.Pp 746Public key authentication works as follows: 747The scheme is based on public-key cryptography, 748using cryptosystems 749where encryption and decryption are done using separate keys, 750and it is unfeasible to derive the decryption key from the encryption key. 751The idea is that each user creates a public/private 752key pair for authentication purposes. 753The server knows the public key, and only the user knows the private key. 754.Nm 755implements public key authentication protocol automatically, 756using one of the DSA, ECDSA or RSA algorithms. 757Protocol 1 is restricted to using only RSA keys, 758but protocol 2 may use any. 759The HISTORY section of 760.Xr ssl 8 761contains a brief discussion of the DSA and RSA algorithms. 762.Pp 763The file 764.Pa ~/.ssh/authorized_keys 765lists the public keys that are permitted for logging in. 766When the user logs in, the 767.Nm 768program tells the server which key pair it would like to use for 769authentication. 770The client proves that it has access to the private key 771and the server checks that the corresponding public key 772is authorized to accept the account. 773.Pp 774The user creates his/her key pair by running 775.Xr ssh-keygen 1 . 776This stores the private key in 777.Pa ~/.ssh/identity 778(protocol 1), 779.Pa ~/.ssh/id_dsa 780(protocol 2 DSA), 781.Pa ~/.ssh/id_ecdsa 782(protocol 2 ECDSA), 783or 784.Pa ~/.ssh/id_rsa 785(protocol 2 RSA) 786and stores the public key in 787.Pa ~/.ssh/identity.pub 788(protocol 1), 789.Pa ~/.ssh/id_dsa.pub 790(protocol 2 DSA), 791.Pa ~/.ssh/id_ecdsa.pub 792(protocol 2 ECDSA), 793or 794.Pa ~/.ssh/id_rsa.pub 795(protocol 2 RSA) 796in the user's home directory. 797The user should then copy the public key 798to 799.Pa ~/.ssh/authorized_keys 800in his/her home directory on the remote machine. 801The 802.Pa authorized_keys 803file corresponds to the conventional 804.Pa ~/.rhosts 805file, and has one key 806per line, though the lines can be very long. 807After this, the user can log in without giving the password. 808.Pp 809A variation on public key authentication 810is available in the form of certificate authentication: 811instead of a set of public/private keys, 812signed certificates are used. 813This has the advantage that a single trusted certification authority 814can be used in place of many public/private keys. 815See the CERTIFICATES section of 816.Xr ssh-keygen 1 817for more information. 818.Pp 819The most convenient way to use public key or certificate authentication 820may be with an authentication agent. 821See 822.Xr ssh-agent 1 823for more information. 824.Pp 825Challenge-response authentication works as follows: 826The server sends an arbitrary 827.Qq challenge 828text, and prompts for a response. 829Protocol 2 allows multiple challenges and responses; 830protocol 1 is restricted to just one challenge/response. 831Examples of challenge-response authentication include 832BSD Authentication (see 833.Xr login.conf 5 ) 834and PAM (some non-OpenBSD systems). 835.Pp 836Finally, if other authentication methods fail, 837.Nm 838prompts the user for a password. 839The password is sent to the remote 840host for checking; however, since all communications are encrypted, 841the password cannot be seen by someone listening on the network. 842.Pp 843.Nm 844automatically maintains and checks a database containing 845identification for all hosts it has ever been used with. 846Host keys are stored in 847.Pa ~/.ssh/known_hosts 848in the user's home directory. 849Additionally, the file 850.Pa /etc/ssh/ssh_known_hosts 851is automatically checked for known hosts. 852Any new hosts are automatically added to the user's file. 853If a host's identification ever changes, 854.Nm 855warns about this and disables password authentication to prevent 856server spoofing or man-in-the-middle attacks, 857which could otherwise be used to circumvent the encryption. 858The 859.Cm StrictHostKeyChecking 860option can be used to control logins to machines whose 861host key is not known or has changed. 862.Pp 863When the user's identity has been accepted by the server, the server 864either executes the given command, or logs into the machine and gives 865the user a normal shell on the remote machine. 866All communication with 867the remote command or shell will be automatically encrypted. 868.Pp 869If a pseudo-terminal has been allocated (normal login session), the 870user may use the escape characters noted below. 871.Pp 872If no pseudo-tty has been allocated, 873the session is transparent and can be used to reliably transfer binary data. 874On most systems, setting the escape character to 875.Dq none 876will also make the session transparent even if a tty is used. 877.Pp 878The session terminates when the command or shell on the remote 879machine exits and all X11 and TCP connections have been closed. 880.Sh ESCAPE CHARACTERS 881When a pseudo-terminal has been requested, 882.Nm 883supports a number of functions through the use of an escape character. 884.Pp 885A single tilde character can be sent as 886.Ic ~~ 887or by following the tilde by a character other than those described below. 888The escape character must always follow a newline to be interpreted as 889special. 890The escape character can be changed in configuration files using the 891.Cm EscapeChar 892configuration directive or on the command line by the 893.Fl e 894option. 895.Pp 896The supported escapes (assuming the default 897.Ql ~ ) 898are: 899.Bl -tag -width Ds 900.It Cm ~. 901Disconnect. 902.It Cm ~^Z 903Background 904.Nm . 905.It Cm ~# 906List forwarded connections. 907.It Cm ~& 908Background 909.Nm 910at logout when waiting for forwarded connection / X11 sessions to terminate. 911.It Cm ~? 912Display a list of escape characters. 913.It Cm ~B 914Send a BREAK to the remote system 915(only useful for SSH protocol version 2 and if the peer supports it). 916.It Cm ~C 917Open command line. 918Currently this allows the addition of port forwardings using the 919.Fl L , 920.Fl R 921and 922.Fl D 923options (see above). 924It also allows the cancellation of existing port-forwardings 925with 926.Sm off 927.Fl KL Oo Ar bind_address : Oc Ar port 928.Sm on 929for local, 930.Sm off 931.Fl KR Oo Ar bind_address : Oc Ar port 932.Sm on 933for remote and 934.Sm off 935.Fl KD Oo Ar bind_address : Oc Ar port 936.Sm on 937for dynamic port-forwardings. 938.Ic !\& Ns Ar command 939allows the user to execute a local command if the 940.Ic PermitLocalCommand 941option is enabled in 942.Xr ssh_config 5 . 943Basic help is available, using the 944.Fl h 945option. 946.It Cm ~R 947Request rekeying of the connection 948(only useful for SSH protocol version 2 and if the peer supports it). 949.It Cm ~V 950Decrease the verbosity 951.Pq Ic LogLevel 952when errors are being written to stderr. 953.It Cm ~v 954Increase the verbosity 955.Pq Ic LogLevel 956when errors are being written to stderr. 957.El 958.Sh TCP FORWARDING 959Forwarding of arbitrary TCP connections over the secure channel can 960be specified either on the command line or in a configuration file. 961One possible application of TCP forwarding is a secure connection to a 962mail server; another is going through firewalls. 963.Pp 964In the example below, we look at encrypting communication between 965an IRC client and server, even though the IRC server does not directly 966support encrypted communications. 967This works as follows: 968the user connects to the remote host using 969.Nm , 970specifying a port to be used to forward connections 971to the remote server. 972After that it is possible to start the service which is to be encrypted 973on the client machine, 974connecting to the same local port, 975and 976.Nm 977will encrypt and forward the connection. 978.Pp 979The following example tunnels an IRC session from client machine 980.Dq 127.0.0.1 981(localhost) 982to remote server 983.Dq server.example.com : 984.Bd -literal -offset 4n 985$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 986$ irc -c '#users' -p 1234 pinky 127.0.0.1 987.Ed 988.Pp 989This tunnels a connection to IRC server 990.Dq server.example.com , 991joining channel 992.Dq #users , 993nickname 994.Dq pinky , 995using port 1234. 996It doesn't matter which port is used, 997as long as it's greater than 1023 998(remember, only root can open sockets on privileged ports) 999and doesn't conflict with any ports already in use. 1000The connection is forwarded to port 6667 on the remote server, 1001since that's the standard port for IRC services. 1002.Pp 1003The 1004.Fl f 1005option backgrounds 1006.Nm 1007and the remote command 1008.Dq sleep 10 1009is specified to allow an amount of time 1010(10 seconds, in the example) 1011to start the service which is to be tunnelled. 1012If no connections are made within the time specified, 1013.Nm 1014will exit. 1015.Sh X11 FORWARDING 1016If the 1017.Cm ForwardX11 1018variable is set to 1019.Dq yes 1020(or see the description of the 1021.Fl X , 1022.Fl x , 1023and 1024.Fl Y 1025options above) 1026and the user is using X11 (the 1027.Ev DISPLAY 1028environment variable is set), the connection to the X11 display is 1029automatically forwarded to the remote side in such a way that any X11 1030programs started from the shell (or command) will go through the 1031encrypted channel, and the connection to the real X server will be made 1032from the local machine. 1033The user should not manually set 1034.Ev DISPLAY . 1035Forwarding of X11 connections can be 1036configured on the command line or in configuration files. 1037.Pp 1038The 1039.Ev DISPLAY 1040value set by 1041.Nm 1042will point to the server machine, but with a display number greater than zero. 1043This is normal, and happens because 1044.Nm 1045creates a 1046.Dq proxy 1047X server on the server machine for forwarding the 1048connections over the encrypted channel. 1049.Pp 1050.Nm 1051will also automatically set up Xauthority data on the server machine. 1052For this purpose, it will generate a random authorization cookie, 1053store it in Xauthority on the server, and verify that any forwarded 1054connections carry this cookie and replace it by the real cookie when 1055the connection is opened. 1056The real authentication cookie is never 1057sent to the server machine (and no cookies are sent in the plain). 1058.Pp 1059If the 1060.Cm ForwardAgent 1061variable is set to 1062.Dq yes 1063(or see the description of the 1064.Fl A 1065and 1066.Fl a 1067options above) and 1068the user is using an authentication agent, the connection to the agent 1069is automatically forwarded to the remote side. 1070.Sh VERIFYING HOST KEYS 1071When connecting to a server for the first time, 1072a fingerprint of the server's public key is presented to the user 1073(unless the option 1074.Cm StrictHostKeyChecking 1075has been disabled). 1076Fingerprints can be determined using 1077.Xr ssh-keygen 1 : 1078.Pp 1079.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1080.Pp 1081If the fingerprint is already known, it can be matched 1082and the key can be accepted or rejected. 1083Because of the difficulty of comparing host keys 1084just by looking at hex strings, 1085there is also support to compare host keys visually, 1086using 1087.Em random art . 1088By setting the 1089.Cm VisualHostKey 1090option to 1091.Dq yes , 1092a small ASCII graphic gets displayed on every login to a server, no matter 1093if the session itself is interactive or not. 1094By learning the pattern a known server produces, a user can easily 1095find out that the host key has changed when a completely different pattern 1096is displayed. 1097Because these patterns are not unambiguous however, a pattern that looks 1098similar to the pattern remembered only gives a good probability that the 1099host key is the same, not guaranteed proof. 1100.Pp 1101To get a listing of the fingerprints along with their random art for 1102all known hosts, the following command line can be used: 1103.Pp 1104.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1105.Pp 1106If the fingerprint is unknown, 1107an alternative method of verification is available: 1108SSH fingerprints verified by DNS. 1109An additional resource record (RR), 1110SSHFP, 1111is added to a zonefile 1112and the connecting client is able to match the fingerprint 1113with that of the key presented. 1114.Pp 1115In this example, we are connecting a client to a server, 1116.Dq host.example.com . 1117The SSHFP resource records should first be added to the zonefile for 1118host.example.com: 1119.Bd -literal -offset indent 1120$ ssh-keygen -r host.example.com. 1121.Ed 1122.Pp 1123The output lines will have to be added to the zonefile. 1124To check that the zone is answering fingerprint queries: 1125.Pp 1126.Dl $ dig -t SSHFP host.example.com 1127.Pp 1128Finally the client connects: 1129.Bd -literal -offset indent 1130$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1131[...] 1132Matching host key fingerprint found in DNS. 1133Are you sure you want to continue connecting (yes/no)? 1134.Ed 1135.Pp 1136See the 1137.Cm VerifyHostKeyDNS 1138option in 1139.Xr ssh_config 5 1140for more information. 1141.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1142.Nm 1143contains support for Virtual Private Network (VPN) tunnelling 1144using the 1145.Xr tun 4 1146network pseudo-device, 1147allowing two networks to be joined securely. 1148The 1149.Xr sshd_config 5 1150configuration option 1151.Cm PermitTunnel 1152controls whether the server supports this, 1153and at what level (layer 2 or 3 traffic). 1154.Pp 1155The following example would connect client network 10.0.50.0/24 1156with remote network 10.0.99.0/24 using a point-to-point connection 1157from 10.1.1.1 to 10.1.1.2, 1158provided that the SSH server running on the gateway to the remote network, 1159at 192.168.1.15, allows it. 1160.Pp 1161On the client: 1162.Bd -literal -offset indent 1163# ssh -f -w 0:1 192.168.1.15 true 1164# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1165# route add 10.0.99.0/24 10.1.1.2 1166.Ed 1167.Pp 1168On the server: 1169.Bd -literal -offset indent 1170# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1171# route add 10.0.50.0/24 10.1.1.1 1172.Ed 1173.Pp 1174Client access may be more finely tuned via the 1175.Pa /root/.ssh/authorized_keys 1176file (see below) and the 1177.Cm PermitRootLogin 1178server option. 1179The following entry would permit connections on 1180.Xr tun 4 1181device 1 from user 1182.Dq jane 1183and on tun device 2 from user 1184.Dq john , 1185if 1186.Cm PermitRootLogin 1187is set to 1188.Dq forced-commands-only : 1189.Bd -literal -offset 2n 1190tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1191tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1192.Ed 1193.Pp 1194Since an SSH-based setup entails a fair amount of overhead, 1195it may be more suited to temporary setups, 1196such as for wireless VPNs. 1197More permanent VPNs are better provided by tools such as 1198.Xr ipsecctl 8 1199and 1200.Xr isakmpd 8 . 1201.Sh ENVIRONMENT 1202.Nm 1203will normally set the following environment variables: 1204.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1205.It Ev DISPLAY 1206The 1207.Ev DISPLAY 1208variable indicates the location of the X11 server. 1209It is automatically set by 1210.Nm 1211to point to a value of the form 1212.Dq hostname:n , 1213where 1214.Dq hostname 1215indicates the host where the shell runs, and 1216.Sq n 1217is an integer \*(Ge 1. 1218.Nm 1219uses this special value to forward X11 connections over the secure 1220channel. 1221The user should normally not set 1222.Ev DISPLAY 1223explicitly, as that 1224will render the X11 connection insecure (and will require the user to 1225manually copy any required authorization cookies). 1226.It Ev HOME 1227Set to the path of the user's home directory. 1228.It Ev LOGNAME 1229Synonym for 1230.Ev USER ; 1231set for compatibility with systems that use this variable. 1232.It Ev MAIL 1233Set to the path of the user's mailbox. 1234.It Ev PATH 1235Set to the default 1236.Ev PATH , 1237as specified when compiling 1238.Nm . 1239.It Ev SSH_ASKPASS 1240If 1241.Nm 1242needs a passphrase, it will read the passphrase from the current 1243terminal if it was run from a terminal. 1244If 1245.Nm 1246does not have a terminal associated with it but 1247.Ev DISPLAY 1248and 1249.Ev SSH_ASKPASS 1250are set, it will execute the program specified by 1251.Ev SSH_ASKPASS 1252and open an X11 window to read the passphrase. 1253This is particularly useful when calling 1254.Nm 1255from a 1256.Pa .xsession 1257or related script. 1258(Note that on some machines it 1259may be necessary to redirect the input from 1260.Pa /dev/null 1261to make this work.) 1262.It Ev SSH_AUTH_SOCK 1263Identifies the path of a 1264.Ux Ns -domain 1265socket used to communicate with the agent. 1266.It Ev SSH_CONNECTION 1267Identifies the client and server ends of the connection. 1268The variable contains 1269four space-separated values: client IP address, client port number, 1270server IP address, and server port number. 1271.It Ev SSH_ORIGINAL_COMMAND 1272This variable contains the original command line if a forced command 1273is executed. 1274It can be used to extract the original arguments. 1275.It Ev SSH_TTY 1276This is set to the name of the tty (path to the device) associated 1277with the current shell or command. 1278If the current session has no tty, 1279this variable is not set. 1280.It Ev TZ 1281This variable is set to indicate the present time zone if it 1282was set when the daemon was started (i.e. the daemon passes the value 1283on to new connections). 1284.It Ev USER 1285Set to the name of the user logging in. 1286.El 1287.Pp 1288Additionally, 1289.Nm 1290reads 1291.Pa ~/.ssh/environment , 1292and adds lines of the format 1293.Dq VARNAME=value 1294to the environment if the file exists and users are allowed to 1295change their environment. 1296For more information, see the 1297.Cm PermitUserEnvironment 1298option in 1299.Xr sshd_config 5 . 1300.Sh FILES 1301.Bl -tag -width Ds -compact 1302.It Pa ~/.rhosts 1303This file is used for host-based authentication (see above). 1304On some machines this file may need to be 1305world-readable if the user's home directory is on an NFS partition, 1306because 1307.Xr sshd 8 1308reads it as root. 1309Additionally, this file must be owned by the user, 1310and must not have write permissions for anyone else. 1311The recommended 1312permission for most machines is read/write for the user, and not 1313accessible by others. 1314.Pp 1315.It Pa ~/.shosts 1316This file is used in exactly the same way as 1317.Pa .rhosts , 1318but allows host-based authentication without permitting login with 1319rlogin/rsh. 1320.Pp 1321.It Pa ~/.ssh/ 1322This directory is the default location for all user-specific configuration 1323and authentication information. 1324There is no general requirement to keep the entire contents of this directory 1325secret, but the recommended permissions are read/write/execute for the user, 1326and not accessible by others. 1327.Pp 1328.It Pa ~/.ssh/authorized_keys 1329Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as 1330this user. 1331The format of this file is described in the 1332.Xr sshd 8 1333manual page. 1334This file is not highly sensitive, but the recommended 1335permissions are read/write for the user, and not accessible by others. 1336.Pp 1337.It Pa ~/.ssh/config 1338This is the per-user configuration file. 1339The file format and configuration options are described in 1340.Xr ssh_config 5 . 1341Because of the potential for abuse, this file must have strict permissions: 1342read/write for the user, and not writable by others. 1343.Pp 1344.It Pa ~/.ssh/environment 1345Contains additional definitions for environment variables; see 1346.Sx ENVIRONMENT , 1347above. 1348.Pp 1349.It Pa ~/.ssh/identity 1350.It Pa ~/.ssh/id_dsa 1351.It Pa ~/.ssh/id_ecdsa 1352.It Pa ~/.ssh/id_rsa 1353Contains the private key for authentication. 1354These files 1355contain sensitive data and should be readable by the user but not 1356accessible by others (read/write/execute). 1357.Nm 1358will simply ignore a private key file if it is accessible by others. 1359It is possible to specify a passphrase when 1360generating the key which will be used to encrypt the 1361sensitive part of this file using 3DES. 1362.Pp 1363.It Pa ~/.ssh/identity.pub 1364.It Pa ~/.ssh/id_dsa.pub 1365.It Pa ~/.ssh/id_ecdsa.pub 1366.It Pa ~/.ssh/id_rsa.pub 1367Contains the public key for authentication. 1368These files are not 1369sensitive and can (but need not) be readable by anyone. 1370.Pp 1371.It Pa ~/.ssh/known_hosts 1372Contains a list of host keys for all hosts the user has logged into 1373that are not already in the systemwide list of known host keys. 1374See 1375.Xr sshd 8 1376for further details of the format of this file. 1377.Pp 1378.It Pa ~/.ssh/rc 1379Commands in this file are executed by 1380.Nm 1381when the user logs in, just before the user's shell (or command) is 1382started. 1383See the 1384.Xr sshd 8 1385manual page for more information. 1386.Pp 1387.It Pa /etc/hosts.equiv 1388This file is for host-based authentication (see above). 1389It should only be writable by root. 1390.Pp 1391.It Pa /etc/shosts.equiv 1392This file is used in exactly the same way as 1393.Pa hosts.equiv , 1394but allows host-based authentication without permitting login with 1395rlogin/rsh. 1396.Pp 1397.It Pa /etc/ssh/ssh_config 1398Systemwide configuration file. 1399The file format and configuration options are described in 1400.Xr ssh_config 5 . 1401.Pp 1402.It Pa /etc/ssh/ssh_host_key 1403.It Pa /etc/ssh/ssh_host_dsa_key 1404.It Pa /etc/ssh/ssh_host_ecdsa_key 1405.It Pa /etc/ssh/ssh_host_rsa_key 1406These files contain the private parts of the host keys 1407and are used for host-based authentication. 1408If protocol version 1 is used, 1409.Nm 1410must be setuid root, since the host key is readable only by root. 1411For protocol version 2, 1412.Nm 1413uses 1414.Xr ssh-keysign 8 1415to access the host keys, 1416eliminating the requirement that 1417.Nm 1418be setuid root when host-based authentication is used. 1419By default 1420.Nm 1421is not setuid root. 1422.Pp 1423.It Pa /etc/ssh/ssh_known_hosts 1424Systemwide list of known host keys. 1425This file should be prepared by the 1426system administrator to contain the public host keys of all machines in the 1427organization. 1428It should be world-readable. 1429See 1430.Xr sshd 8 1431for further details of the format of this file. 1432.Pp 1433.It Pa /etc/ssh/sshrc 1434Commands in this file are executed by 1435.Nm 1436when the user logs in, just before the user's shell (or command) is started. 1437See the 1438.Xr sshd 8 1439manual page for more information. 1440.El 1441.Sh EXIT STATUS 1442.Nm 1443exits with the exit status of the remote command or with 255 1444if an error occurred. 1445.Sh SEE ALSO 1446.Xr scp 1 , 1447.Xr sftp 1 , 1448.Xr ssh-add 1 , 1449.Xr ssh-agent 1 , 1450.Xr ssh-keygen 1 , 1451.Xr ssh-keyscan 1 , 1452.Xr tun 4 , 1453.Xr hosts.equiv 5 , 1454.Xr ssh_config 5 , 1455.Xr ssh-keysign 8 , 1456.Xr sshd 8 1457.Sh STANDARDS 1458.Rs 1459.%A S. Lehtinen 1460.%A C. Lonvick 1461.%D January 2006 1462.%R RFC 4250 1463.%T The Secure Shell (SSH) Protocol Assigned Numbers 1464.Re 1465.Pp 1466.Rs 1467.%A T. Ylonen 1468.%A C. Lonvick 1469.%D January 2006 1470.%R RFC 4251 1471.%T The Secure Shell (SSH) Protocol Architecture 1472.Re 1473.Pp 1474.Rs 1475.%A T. Ylonen 1476.%A C. Lonvick 1477.%D January 2006 1478.%R RFC 4252 1479.%T The Secure Shell (SSH) Authentication Protocol 1480.Re 1481.Pp 1482.Rs 1483.%A T. Ylonen 1484.%A C. Lonvick 1485.%D January 2006 1486.%R RFC 4253 1487.%T The Secure Shell (SSH) Transport Layer Protocol 1488.Re 1489.Pp 1490.Rs 1491.%A T. Ylonen 1492.%A C. Lonvick 1493.%D January 2006 1494.%R RFC 4254 1495.%T The Secure Shell (SSH) Connection Protocol 1496.Re 1497.Pp 1498.Rs 1499.%A J. Schlyter 1500.%A W. Griffin 1501.%D January 2006 1502.%R RFC 4255 1503.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 1504.Re 1505.Pp 1506.Rs 1507.%A F. Cusack 1508.%A M. Forssen 1509.%D January 2006 1510.%R RFC 4256 1511.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH) 1512.Re 1513.Pp 1514.Rs 1515.%A J. Galbraith 1516.%A P. Remaker 1517.%D January 2006 1518.%R RFC 4335 1519.%T The Secure Shell (SSH) Session Channel Break Extension 1520.Re 1521.Pp 1522.Rs 1523.%A M. Bellare 1524.%A T. Kohno 1525.%A C. Namprempre 1526.%D January 2006 1527.%R RFC 4344 1528.%T The Secure Shell (SSH) Transport Layer Encryption Modes 1529.Re 1530.Pp 1531.Rs 1532.%A B. Harris 1533.%D January 2006 1534.%R RFC 4345 1535.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol 1536.Re 1537.Pp 1538.Rs 1539.%A M. Friedl 1540.%A N. Provos 1541.%A W. Simpson 1542.%D March 2006 1543.%R RFC 4419 1544.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 1545.Re 1546.Pp 1547.Rs 1548.%A J. Galbraith 1549.%A R. Thayer 1550.%D November 2006 1551.%R RFC 4716 1552.%T The Secure Shell (SSH) Public Key File Format 1553.Re 1554.Pp 1555.Rs 1556.%A D. Stebila 1557.%A J. Green 1558.%D December 2009 1559.%R RFC 5656 1560.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer 1561.Re 1562.Pp 1563.Rs 1564.%A A. Perrig 1565.%A D. Song 1566.%D 1999 1567.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99) 1568.%T Hash Visualization: a New Technique to improve Real-World Security 1569.Re 1570.Sh AUTHORS 1571OpenSSH is a derivative of the original and free 1572ssh 1.2.12 release by Tatu Ylonen. 1573Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1574Theo de Raadt and Dug Song 1575removed many bugs, re-added newer features and 1576created OpenSSH. 1577Markus Friedl contributed the support for SSH 1578protocol versions 1.5 and 2.0. 1579