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 1.320 2011/08/02 01:22:11 djm Exp $ 37.Dd $Mdocdate: August 2 2011 $ 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.Bk -words 46.Op Fl 1246AaCfgKkMNnqsTtVvXxYy 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 escape_char 51.Op Fl F Ar configfile 52.Op Fl I Ar pkcs11 53.Op Fl i Ar identity_file 54.Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport 55.Op Fl l Ar login_name 56.Op Fl m Ar mac_spec 57.Op Fl O Ar ctl_cmd 58.Op Fl o Ar option 59.Op Fl p Ar port 60.Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport 61.Op Fl S Ar ctl_path 62.Op Fl W Ar host : Ns Ar port 63.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun 64.Oo Ar user Ns @ Oc Ns Ar hostname 65.Op Ar command 66.Ek 67.Sh DESCRIPTION 68.Nm 69(SSH client) is a program for logging into a remote machine and for 70executing commands on a remote machine. 71It is intended to replace rlogin and rsh, 72and provide secure encrypted communications between 73two untrusted hosts over an insecure network. 74X11 connections and arbitrary TCP ports 75can also be forwarded over the secure channel. 76.Pp 77.Nm 78connects and logs into the specified 79.Ar hostname 80(with optional 81.Ar user 82name). 83The user must prove 84his/her identity to the remote machine using one of several methods 85depending on the protocol version used (see below). 86.Pp 87If 88.Ar command 89is specified, 90it is executed on the remote host instead of a login shell. 91.Pp 92The options are as follows: 93.Bl -tag -width Ds 94.It Fl 1 95Forces 96.Nm 97to try protocol version 1 only. 98.It Fl 2 99Forces 100.Nm 101to try protocol version 2 only. 102.It Fl 4 103Forces 104.Nm 105to use IPv4 addresses only. 106.It Fl 6 107Forces 108.Nm 109to use IPv6 addresses only. 110.It Fl A 111Enables forwarding of the authentication agent connection. 112This can also be specified on a per-host basis in a configuration file. 113.Pp 114Agent forwarding should be enabled with caution. 115Users with the ability to bypass file permissions on the remote host 116(for the agent's 117.Ux Ns -domain 118socket) can access the local agent through the forwarded connection. 119An attacker cannot obtain key material from the agent, 120however they can perform operations on the keys that enable them to 121authenticate using the identities loaded into the agent. 122.It Fl a 123Disables forwarding of the authentication agent connection. 124.It Fl b Ar bind_address 125Use 126.Ar bind_address 127on the local machine as the source address 128of the connection. 129Only useful on systems with more than one address. 130.It Fl C 131Requests compression of all data (including stdin, stdout, stderr, and 132data for forwarded X11 and TCP connections). 133The compression algorithm is the same used by 134.Xr gzip 1 , 135and the 136.Dq level 137can be controlled by the 138.Cm CompressionLevel 139option for protocol version 1. 140Compression is desirable on modem lines and other 141slow connections, but will only slow down things on fast networks. 142The default value can be set on a host-by-host basis in the 143configuration files; see the 144.Cm Compression 145option. 146.It Fl c Ar cipher_spec 147Selects the cipher specification for encrypting the session. 148.Pp 149Protocol version 1 allows specification of a single cipher. 150The supported values are 151.Dq 3des , 152.Dq blowfish , 153and 154.Dq des . 155.Ar 3des 156(triple-des) is an encrypt-decrypt-encrypt triple with three different keys. 157It is believed to be secure. 158.Ar blowfish 159is a fast block cipher; it appears very secure and is much faster than 160.Ar 3des . 161.Ar des 162is only supported in the 163.Nm 164client for interoperability with legacy protocol 1 implementations 165that do not support the 166.Ar 3des 167cipher. 168Its use is strongly discouraged due to cryptographic weaknesses. 169The default is 170.Dq 3des . 171.Pp 172For protocol version 2, 173.Ar cipher_spec 174is a comma-separated list of ciphers 175listed in order of preference. 176See the 177.Cm Ciphers 178keyword in 179.Xr ssh_config 5 180for more information. 181.It Fl D Xo 182.Sm off 183.Oo Ar bind_address : Oc 184.Ar port 185.Sm on 186.Xc 187Specifies a local 188.Dq dynamic 189application-level port forwarding. 190This works by allocating a socket to listen to 191.Ar port 192on the local side, optionally bound to the specified 193.Ar bind_address . 194Whenever a connection is made to this port, the 195connection is forwarded over the secure channel, and the application 196protocol is then used to determine where to connect to from the 197remote machine. 198Currently the SOCKS4 and SOCKS5 protocols are supported, and 199.Nm 200will act as a SOCKS server. 201Only root can forward privileged ports. 202Dynamic port forwardings can also be specified in the configuration file. 203.Pp 204IPv6 addresses can be specified by enclosing the address in square brackets. 205Only the superuser can forward privileged ports. 206By default, the local port is bound in accordance with the 207.Cm GatewayPorts 208setting. 209However, an explicit 210.Ar bind_address 211may be used to bind the connection to a specific address. 212The 213.Ar bind_address 214of 215.Dq localhost 216indicates that the listening port be bound for local use only, while an 217empty address or 218.Sq * 219indicates that the port should be available from all interfaces. 220.It Fl e Ar escape_char 221Sets the escape character for sessions with a pty (default: 222.Ql ~ ) . 223The escape character is only recognized at the beginning of a line. 224The escape character followed by a dot 225.Pq Ql \&. 226closes the connection; 227followed by control-Z suspends the connection; 228and followed by itself sends the escape character once. 229Setting the character to 230.Dq none 231disables any escapes and makes the session fully transparent. 232.It Fl F Ar configfile 233Specifies an alternative per-user configuration file. 234If a configuration file is given on the command line, 235the system-wide configuration file 236.Pq Pa /etc/ssh/ssh_config 237will be ignored. 238The default for the per-user configuration file is 239.Pa ~/.ssh/config . 240.It Fl f 241Requests 242.Nm 243to go to background just before command execution. 244This is useful if 245.Nm 246is going to ask for passwords or passphrases, but the user 247wants it in the background. 248This implies 249.Fl n . 250The recommended way to start X11 programs at a remote site is with 251something like 252.Ic ssh -f host xterm . 253.Pp 254If the 255.Cm ExitOnForwardFailure 256configuration option is set to 257.Dq yes , 258then a client started with 259.Fl f 260will wait for all remote port forwards to be successfully established 261before placing itself in the background. 262.It Fl g 263Allows remote hosts to connect to local forwarded ports. 264.It Fl I Ar pkcs11 265Specify the PKCS#11 shared library 266.Nm 267should use to communicate with a PKCS#11 token providing the user's 268private RSA key. 269.It Fl i Ar identity_file 270Selects a file from which the identity (private key) for 271public key authentication is read. 272The default is 273.Pa ~/.ssh/identity 274for protocol version 1, and 275.Pa ~/.ssh/id_dsa , 276.Pa ~/.ssh/id_ecdsa 277and 278.Pa ~/.ssh/id_rsa 279for protocol version 2. 280Identity files may also be specified on 281a per-host basis in the configuration file. 282It is possible to have multiple 283.Fl i 284options (and multiple identities specified in 285configuration files). 286.Nm 287will also try to load certificate information from the filename obtained 288by appending 289.Pa -cert.pub 290to identity filenames. 291.It Fl K 292Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI 293credentials to the server. 294.It Fl k 295Disables forwarding (delegation) of GSSAPI credentials to the server. 296.It Fl L Xo 297.Sm off 298.Oo Ar bind_address : Oc 299.Ar port : host : hostport 300.Sm on 301.Xc 302Specifies that the given port on the local (client) host is to be 303forwarded to the given host and port on the remote side. 304This works by allocating a socket to listen to 305.Ar port 306on the local side, optionally bound to the specified 307.Ar bind_address . 308Whenever a connection is made to this port, the 309connection is forwarded over the secure channel, and a connection is 310made to 311.Ar host 312port 313.Ar hostport 314from the remote machine. 315Port forwardings can also be specified in the configuration file. 316IPv6 addresses can be specified by enclosing the address in square brackets. 317Only the superuser can forward privileged ports. 318By default, the local port is bound in accordance with the 319.Cm GatewayPorts 320setting. 321However, an explicit 322.Ar bind_address 323may be used to bind the connection to a specific address. 324The 325.Ar bind_address 326of 327.Dq localhost 328indicates that the listening port be bound for local use only, while an 329empty address or 330.Sq * 331indicates that the port should be available from all interfaces. 332.It Fl l Ar login_name 333Specifies the user to log in as on the remote machine. 334This also may be specified on a per-host basis in the configuration file. 335.It Fl M 336Places the 337.Nm 338client into 339.Dq master 340mode for connection sharing. 341Multiple 342.Fl M 343options places 344.Nm 345into 346.Dq master 347mode with confirmation required before slave connections are accepted. 348Refer to the description of 349.Cm ControlMaster 350in 351.Xr ssh_config 5 352for details. 353.It Fl m Ar mac_spec 354Additionally, for protocol version 2 a comma-separated list of MAC 355(message authentication code) algorithms can 356be specified in order of preference. 357See the 358.Cm MACs 359keyword for more information. 360.It Fl N 361Do not execute a remote command. 362This is useful for just forwarding ports 363(protocol version 2 only). 364.It Fl n 365Redirects stdin from 366.Pa /dev/null 367(actually, prevents reading from stdin). 368This must be used when 369.Nm 370is run in the background. 371A common trick is to use this to run X11 programs on a remote machine. 372For example, 373.Ic ssh -n shadows.cs.hut.fi emacs & 374will start an emacs on shadows.cs.hut.fi, and the X11 375connection will be automatically forwarded over an encrypted channel. 376The 377.Nm 378program will be put in the background. 379(This does not work if 380.Nm 381needs to ask for a password or passphrase; see also the 382.Fl f 383option.) 384.It Fl O Ar ctl_cmd 385Control an active connection multiplexing master process. 386When the 387.Fl O 388option is specified, the 389.Ar ctl_cmd 390argument is interpreted and passed to the master process. 391Valid commands are: 392.Dq check 393(check that the master process is running), 394.Dq forward 395(request forwardings without command execution), 396.Dq exit 397(request the master to exit), and 398.Dq stop 399(request the master to stop accepting further multiplexing requests). 400.It Fl o Ar option 401Can be used to give options in the format used in the configuration file. 402This is useful for specifying options for which there is no separate 403command-line flag. 404For full details of the options listed below, and their possible values, see 405.Xr ssh_config 5 . 406.Pp 407.Bl -tag -width Ds -offset indent -compact 408.It AddressFamily 409.It BatchMode 410.It BindAddress 411.It ChallengeResponseAuthentication 412.It CheckHostIP 413.It Cipher 414.It Ciphers 415.It ClearAllForwardings 416.It Compression 417.It CompressionLevel 418.It ConnectionAttempts 419.It ConnectTimeout 420.It ControlMaster 421.It ControlPath 422.It DynamicForward 423.It EscapeChar 424.It ExitOnForwardFailure 425.It ForwardAgent 426.It ForwardX11 427.It ForwardX11Trusted 428.It GatewayPorts 429.It GlobalKnownHostsFile 430.It GSSAPIAuthentication 431.It GSSAPIDelegateCredentials 432.It HashKnownHosts 433.It Host 434.It HostbasedAuthentication 435.It HostKeyAlgorithms 436.It HostKeyAlias 437.It HostName 438.It IdentityFile 439.It IdentitiesOnly 440.It IPQoS 441.It KbdInteractiveDevices 442.It KexAlgorithms 443.It LocalCommand 444.It LocalForward 445.It LogLevel 446.It MACs 447.It NoHostAuthenticationForLocalhost 448.It NumberOfPasswordPrompts 449.It PasswordAuthentication 450.It PermitLocalCommand 451.It PKCS11Provider 452.It Port 453.It PreferredAuthentications 454.It Protocol 455.It ProxyCommand 456.It PubkeyAuthentication 457.It RekeyLimit 458.It RemoteForward 459.It RequestTTY 460.It RhostsRSAAuthentication 461.It RSAAuthentication 462.It SendEnv 463.It ServerAliveInterval 464.It ServerAliveCountMax 465.It StrictHostKeyChecking 466.It TCPKeepAlive 467.It Tunnel 468.It TunnelDevice 469.It UsePrivilegedPort 470.It User 471.It UserKnownHostsFile 472.It VerifyHostKeyDNS 473.It VisualHostKey 474.It XAuthLocation 475.El 476.It Fl p Ar port 477Port to connect to on the remote host. 478This can be specified on a 479per-host basis in the configuration file. 480.It Fl q 481Quiet mode. 482Causes most warning and diagnostic messages to be suppressed. 483.It Fl R Xo 484.Sm off 485.Oo Ar bind_address : Oc 486.Ar port : host : hostport 487.Sm on 488.Xc 489Specifies that the given port on the remote (server) host is to be 490forwarded to the given host and port on the local side. 491This works by allocating a socket to listen to 492.Ar port 493on the remote side, and whenever a connection is made to this port, the 494connection is forwarded over the secure channel, and a connection is 495made to 496.Ar host 497port 498.Ar hostport 499from the local machine. 500.Pp 501Port forwardings can also be specified in the configuration file. 502Privileged ports can be forwarded only when 503logging in as root on the remote machine. 504IPv6 addresses can be specified by enclosing the address in square braces. 505.Pp 506By default, the listening socket on the server will be bound to the loopback 507interface only. 508This may be overridden by specifying a 509.Ar bind_address . 510An empty 511.Ar bind_address , 512or the address 513.Ql * , 514indicates that the remote socket should listen on all interfaces. 515Specifying a remote 516.Ar bind_address 517will only succeed if the server's 518.Cm GatewayPorts 519option is enabled (see 520.Xr sshd_config 5 ) . 521.Pp 522If the 523.Ar port 524argument is 525.Ql 0 , 526the listen port will be dynamically allocated on the server and reported 527to the client at run time. 528When used together with 529.Ic -O forward 530the allocated port will be printed to the standard output. 531.It Fl S Ar ctl_path 532Specifies the location of a control socket for connection sharing, 533or the string 534.Dq none 535to disable connection sharing. 536Refer to the description of 537.Cm ControlPath 538and 539.Cm ControlMaster 540in 541.Xr ssh_config 5 542for details. 543.It Fl s 544May be used to request invocation of a subsystem on the remote system. 545Subsystems are a feature of the SSH2 protocol which facilitate the use 546of SSH as a secure transport for other applications (eg.\& 547.Xr sftp 1 ) . 548The subsystem is specified as the remote command. 549.It Fl T 550Disable pseudo-tty allocation. 551.It Fl t 552Force pseudo-tty allocation. 553This can be used to execute arbitrary 554screen-based programs on a remote machine, which can be very useful, 555e.g. when implementing menu services. 556Multiple 557.Fl t 558options force tty allocation, even if 559.Nm 560has no local tty. 561.It Fl V 562Display the version number and exit. 563.It Fl v 564Verbose mode. 565Causes 566.Nm 567to print debugging messages about its progress. 568This is helpful in 569debugging connection, authentication, and configuration problems. 570Multiple 571.Fl v 572options increase the verbosity. 573The maximum is 3. 574.It Fl W Ar host : Ns Ar port 575Requests that standard input and output on the client be forwarded to 576.Ar host 577on 578.Ar port 579over the secure channel. 580Implies 581.Fl N , 582.Fl T , 583.Cm ExitOnForwardFailure 584and 585.Cm ClearAllForwardings 586and works with Protocol version 2 only. 587.It Fl w Xo 588.Ar local_tun Ns Op : Ns Ar remote_tun 589.Xc 590Requests 591tunnel 592device forwarding with the specified 593.Xr tun 4 594devices between the client 595.Pq Ar local_tun 596and the server 597.Pq Ar remote_tun . 598.Pp 599The devices may be specified by numerical ID or the keyword 600.Dq any , 601which uses the next available tunnel device. 602If 603.Ar remote_tun 604is not specified, it defaults to 605.Dq any . 606See also the 607.Cm Tunnel 608and 609.Cm TunnelDevice 610directives in 611.Xr ssh_config 5 . 612If the 613.Cm Tunnel 614directive is unset, it is set to the default tunnel mode, which is 615.Dq point-to-point . 616.It Fl X 617Enables X11 forwarding. 618This can also be specified on a per-host basis in a configuration file. 619.Pp 620X11 forwarding should be enabled with caution. 621Users with the ability to bypass file permissions on the remote host 622(for the user's X authorization database) 623can access the local X11 display through the forwarded connection. 624An attacker may then be able to perform activities such as keystroke monitoring. 625.Pp 626For this reason, X11 forwarding is subjected to X11 SECURITY extension 627restrictions by default. 628Please refer to the 629.Nm 630.Fl Y 631option and the 632.Cm ForwardX11Trusted 633directive in 634.Xr ssh_config 5 635for more information. 636.It Fl x 637Disables X11 forwarding. 638.It Fl Y 639Enables trusted X11 forwarding. 640Trusted X11 forwardings are not subjected to the X11 SECURITY extension 641controls. 642.It Fl y 643Send log information using the 644.Xr syslog 3 645system module. 646By default this information is sent to stderr. 647.El 648.Pp 649.Nm 650may additionally obtain configuration data from 651a per-user configuration file and a system-wide configuration file. 652The file format and configuration options are described in 653.Xr ssh_config 5 . 654.Sh AUTHENTICATION 655The OpenSSH SSH client supports SSH protocols 1 and 2. 656The default is to use protocol 2 only, 657though this can be changed via the 658.Cm Protocol 659option in 660.Xr ssh_config 5 661or the 662.Fl 1 663and 664.Fl 2 665options (see above). 666Both protocols support similar authentication methods, 667but protocol 2 is the default since 668it provides additional mechanisms for confidentiality 669(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) 670and integrity (hmac-md5, hmac-sha1, 671hmac-sha2-256, hmac-sha2-512, 672umac-64, hmac-ripemd160). 673Protocol 1 lacks a strong mechanism for ensuring the 674integrity of the connection. 675.Pp 676The methods available for authentication are: 677GSSAPI-based authentication, 678host-based authentication, 679public key authentication, 680challenge-response authentication, 681and password authentication. 682Authentication methods are tried in the order specified above, 683though protocol 2 has a configuration option to change the default order: 684.Cm PreferredAuthentications . 685.Pp 686Host-based authentication works as follows: 687If the machine the user logs in from is listed in 688.Pa /etc/hosts.equiv 689or 690.Pa /etc/shosts.equiv 691on the remote machine, and the user names are 692the same on both sides, or if the files 693.Pa ~/.rhosts 694or 695.Pa ~/.shosts 696exist in the user's home directory on the 697remote machine and contain a line containing the name of the client 698machine and the name of the user on that machine, the user is 699considered for login. 700Additionally, the server 701.Em must 702be able to verify the client's 703host key (see the description of 704.Pa /etc/ssh/ssh_known_hosts 705and 706.Pa ~/.ssh/known_hosts , 707below) 708for login to be permitted. 709This authentication method closes security holes due to IP 710spoofing, DNS spoofing, and routing spoofing. 711[Note to the administrator: 712.Pa /etc/hosts.equiv , 713.Pa ~/.rhosts , 714and the rlogin/rsh protocol in general, are inherently insecure and should be 715disabled if security is desired.] 716.Pp 717Public key authentication works as follows: 718The scheme is based on public-key cryptography, 719using cryptosystems 720where encryption and decryption are done using separate keys, 721and it is unfeasible to derive the decryption key from the encryption key. 722The idea is that each user creates a public/private 723key pair for authentication purposes. 724The server knows the public key, and only the user knows the private key. 725.Nm 726implements public key authentication protocol automatically, 727using one of the DSA, ECDSA or RSA algorithms. 728Protocol 1 is restricted to using only RSA keys, 729but protocol 2 may use any. 730The 731.Sx HISTORY 732section of 733.Xr ssl 8 734contains a brief discussion of the DSA and RSA algorithms. 735.Pp 736The file 737.Pa ~/.ssh/authorized_keys 738lists the public keys that are permitted for logging in. 739When the user logs in, the 740.Nm 741program tells the server which key pair it would like to use for 742authentication. 743The client proves that it has access to the private key 744and the server checks that the corresponding public key 745is authorized to accept the account. 746.Pp 747The user creates his/her key pair by running 748.Xr ssh-keygen 1 . 749This stores the private key in 750.Pa ~/.ssh/identity 751(protocol 1), 752.Pa ~/.ssh/id_dsa 753(protocol 2 DSA), 754.Pa ~/.ssh/id_ecdsa 755(protocol 2 ECDSA), 756or 757.Pa ~/.ssh/id_rsa 758(protocol 2 RSA) 759and stores the public key in 760.Pa ~/.ssh/identity.pub 761(protocol 1), 762.Pa ~/.ssh/id_dsa.pub 763(protocol 2 DSA), 764.Pa ~/.ssh/id_ecdsa.pub 765(protocol 2 ECDSA), 766or 767.Pa ~/.ssh/id_rsa.pub 768(protocol 2 RSA) 769in the user's home directory. 770The user should then copy the public key 771to 772.Pa ~/.ssh/authorized_keys 773in his/her home directory on the remote machine. 774The 775.Pa authorized_keys 776file corresponds to the conventional 777.Pa ~/.rhosts 778file, and has one key 779per line, though the lines can be very long. 780After this, the user can log in without giving the password. 781.Pp 782A variation on public key authentication 783is available in the form of certificate authentication: 784instead of a set of public/private keys, 785signed certificates are used. 786This has the advantage that a single trusted certification authority 787can be used in place of many public/private keys. 788See the 789.Sx CERTIFICATES 790section of 791.Xr ssh-keygen 1 792for more information. 793.Pp 794The most convenient way to use public key or certificate authentication 795may be with an authentication agent. 796See 797.Xr ssh-agent 1 798for more information. 799.Pp 800Challenge-response authentication works as follows: 801The server sends an arbitrary 802.Qq challenge 803text, and prompts for a response. 804Protocol 2 allows multiple challenges and responses; 805protocol 1 is restricted to just one challenge/response. 806Examples of challenge-response authentication include 807BSD Authentication (see 808.Xr login.conf 5 ) 809and PAM (some non-OpenBSD systems). 810.Pp 811Finally, if other authentication methods fail, 812.Nm 813prompts the user for a password. 814The password is sent to the remote 815host for checking; however, since all communications are encrypted, 816the password cannot be seen by someone listening on the network. 817.Pp 818.Nm 819automatically maintains and checks a database containing 820identification for all hosts it has ever been used with. 821Host keys are stored in 822.Pa ~/.ssh/known_hosts 823in the user's home directory. 824Additionally, the file 825.Pa /etc/ssh/ssh_known_hosts 826is automatically checked for known hosts. 827Any new hosts are automatically added to the user's file. 828If a host's identification ever changes, 829.Nm 830warns about this and disables password authentication to prevent 831server spoofing or man-in-the-middle attacks, 832which could otherwise be used to circumvent the encryption. 833The 834.Cm StrictHostKeyChecking 835option can be used to control logins to machines whose 836host key is not known or has changed. 837.Pp 838When the user's identity has been accepted by the server, the server 839either executes the given command, or logs into the machine and gives 840the user a normal shell on the remote machine. 841All communication with 842the remote command or shell will be automatically encrypted. 843.Pp 844If a pseudo-terminal has been allocated (normal login session), the 845user may use the escape characters noted below. 846.Pp 847If no pseudo-tty has been allocated, 848the session is transparent and can be used to reliably transfer binary data. 849On most systems, setting the escape character to 850.Dq none 851will also make the session transparent even if a tty is used. 852.Pp 853The session terminates when the command or shell on the remote 854machine exits and all X11 and TCP connections have been closed. 855.Sh ESCAPE CHARACTERS 856When a pseudo-terminal has been requested, 857.Nm 858supports a number of functions through the use of an escape character. 859.Pp 860A single tilde character can be sent as 861.Ic ~~ 862or by following the tilde by a character other than those described below. 863The escape character must always follow a newline to be interpreted as 864special. 865The escape character can be changed in configuration files using the 866.Cm EscapeChar 867configuration directive or on the command line by the 868.Fl e 869option. 870.Pp 871The supported escapes (assuming the default 872.Ql ~ ) 873are: 874.Bl -tag -width Ds 875.It Cm ~. 876Disconnect. 877.It Cm ~^Z 878Background 879.Nm . 880.It Cm ~# 881List forwarded connections. 882.It Cm ~& 883Background 884.Nm 885at logout when waiting for forwarded connection / X11 sessions to terminate. 886.It Cm ~? 887Display a list of escape characters. 888.It Cm ~B 889Send a BREAK to the remote system 890(only useful for SSH protocol version 2 and if the peer supports it). 891.It Cm ~C 892Open command line. 893Currently this allows the addition of port forwardings using the 894.Fl L , 895.Fl R 896and 897.Fl D 898options (see above). 899It also allows the cancellation of existing remote port-forwardings 900using 901.Sm off 902.Fl KR Oo Ar bind_address : Oc Ar port . 903.Sm on 904.Ic !\& Ns Ar command 905allows the user to execute a local command if the 906.Ic PermitLocalCommand 907option is enabled in 908.Xr ssh_config 5 . 909Basic help is available, using the 910.Fl h 911option. 912.It Cm ~R 913Request rekeying of the connection 914(only useful for SSH protocol version 2 and if the peer supports it). 915.El 916.Sh TCP FORWARDING 917Forwarding of arbitrary TCP connections over the secure channel can 918be specified either on the command line or in a configuration file. 919One possible application of TCP forwarding is a secure connection to a 920mail server; another is going through firewalls. 921.Pp 922In the example below, we look at encrypting communication between 923an IRC client and server, even though the IRC server does not directly 924support encrypted communications. 925This works as follows: 926the user connects to the remote host using 927.Nm , 928specifying a port to be used to forward connections 929to the remote server. 930After that it is possible to start the service which is to be encrypted 931on the client machine, 932connecting to the same local port, 933and 934.Nm 935will encrypt and forward the connection. 936.Pp 937The following example tunnels an IRC session from client machine 938.Dq 127.0.0.1 939(localhost) 940to remote server 941.Dq server.example.com : 942.Bd -literal -offset 4n 943$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 944$ irc -c '#users' -p 1234 pinky 127.0.0.1 945.Ed 946.Pp 947This tunnels a connection to IRC server 948.Dq server.example.com , 949joining channel 950.Dq #users , 951nickname 952.Dq pinky , 953using port 1234. 954It doesn't matter which port is used, 955as long as it's greater than 1023 956(remember, only root can open sockets on privileged ports) 957and doesn't conflict with any ports already in use. 958The connection is forwarded to port 6667 on the remote server, 959since that's the standard port for IRC services. 960.Pp 961The 962.Fl f 963option backgrounds 964.Nm 965and the remote command 966.Dq sleep 10 967is specified to allow an amount of time 968(10 seconds, in the example) 969to start the service which is to be tunnelled. 970If no connections are made within the time specified, 971.Nm 972will exit. 973.Sh X11 FORWARDING 974If the 975.Cm ForwardX11 976variable is set to 977.Dq yes 978(or see the description of the 979.Fl X , 980.Fl x , 981and 982.Fl Y 983options above) 984and the user is using X11 (the 985.Ev DISPLAY 986environment variable is set), the connection to the X11 display is 987automatically forwarded to the remote side in such a way that any X11 988programs started from the shell (or command) will go through the 989encrypted channel, and the connection to the real X server will be made 990from the local machine. 991The user should not manually set 992.Ev DISPLAY . 993Forwarding of X11 connections can be 994configured on the command line or in configuration files. 995.Pp 996The 997.Ev DISPLAY 998value set by 999.Nm 1000will point to the server machine, but with a display number greater than zero. 1001This is normal, and happens because 1002.Nm 1003creates a 1004.Dq proxy 1005X server on the server machine for forwarding the 1006connections over the encrypted channel. 1007.Pp 1008.Nm 1009will also automatically set up Xauthority data on the server machine. 1010For this purpose, it will generate a random authorization cookie, 1011store it in Xauthority on the server, and verify that any forwarded 1012connections carry this cookie and replace it by the real cookie when 1013the connection is opened. 1014The real authentication cookie is never 1015sent to the server machine (and no cookies are sent in the plain). 1016.Pp 1017If the 1018.Cm ForwardAgent 1019variable is set to 1020.Dq yes 1021(or see the description of the 1022.Fl A 1023and 1024.Fl a 1025options above) and 1026the user is using an authentication agent, the connection to the agent 1027is automatically forwarded to the remote side. 1028.Sh VERIFYING HOST KEYS 1029When connecting to a server for the first time, 1030a fingerprint of the server's public key is presented to the user 1031(unless the option 1032.Cm StrictHostKeyChecking 1033has been disabled). 1034Fingerprints can be determined using 1035.Xr ssh-keygen 1 : 1036.Pp 1037.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 1038.Pp 1039If the fingerprint is already known, it can be matched 1040and the key can be accepted or rejected. 1041Because of the difficulty of comparing host keys 1042just by looking at hex strings, 1043there is also support to compare host keys visually, 1044using 1045.Em random art . 1046By setting the 1047.Cm VisualHostKey 1048option to 1049.Dq yes , 1050a small ASCII graphic gets displayed on every login to a server, no matter 1051if the session itself is interactive or not. 1052By learning the pattern a known server produces, a user can easily 1053find out that the host key has changed when a completely different pattern 1054is displayed. 1055Because these patterns are not unambiguous however, a pattern that looks 1056similar to the pattern remembered only gives a good probability that the 1057host key is the same, not guaranteed proof. 1058.Pp 1059To get a listing of the fingerprints along with their random art for 1060all known hosts, the following command line can be used: 1061.Pp 1062.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts 1063.Pp 1064If the fingerprint is unknown, 1065an alternative method of verification is available: 1066SSH fingerprints verified by DNS. 1067An additional resource record (RR), 1068SSHFP, 1069is added to a zonefile 1070and the connecting client is able to match the fingerprint 1071with that of the key presented. 1072.Pp 1073In this example, we are connecting a client to a server, 1074.Dq host.example.com . 1075The SSHFP resource records should first be added to the zonefile for 1076host.example.com: 1077.Bd -literal -offset indent 1078$ ssh-keygen -r host.example.com. 1079.Ed 1080.Pp 1081The output lines will have to be added to the zonefile. 1082To check that the zone is answering fingerprint queries: 1083.Pp 1084.Dl $ dig -t SSHFP host.example.com 1085.Pp 1086Finally the client connects: 1087.Bd -literal -offset indent 1088$ ssh -o "VerifyHostKeyDNS ask" host.example.com 1089[...] 1090Matching host key fingerprint found in DNS. 1091Are you sure you want to continue connecting (yes/no)? 1092.Ed 1093.Pp 1094See the 1095.Cm VerifyHostKeyDNS 1096option in 1097.Xr ssh_config 5 1098for more information. 1099.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS 1100.Nm 1101contains support for Virtual Private Network (VPN) tunnelling 1102using the 1103.Xr tun 4 1104network pseudo-device, 1105allowing two networks to be joined securely. 1106The 1107.Xr sshd_config 5 1108configuration option 1109.Cm PermitTunnel 1110controls whether the server supports this, 1111and at what level (layer 2 or 3 traffic). 1112.Pp 1113The following example would connect client network 10.0.50.0/24 1114with remote network 10.0.99.0/24 using a point-to-point connection 1115from 10.1.1.1 to 10.1.1.2, 1116provided that the SSH server running on the gateway to the remote network, 1117at 192.168.1.15, allows it. 1118.Pp 1119On the client: 1120.Bd -literal -offset indent 1121# ssh -f -w 0:1 192.168.1.15 true 1122# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 1123# route add 10.0.99.0/24 10.1.1.2 1124.Ed 1125.Pp 1126On the server: 1127.Bd -literal -offset indent 1128# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 1129# route add 10.0.50.0/24 10.1.1.1 1130.Ed 1131.Pp 1132Client access may be more finely tuned via the 1133.Pa /root/.ssh/authorized_keys 1134file (see below) and the 1135.Cm PermitRootLogin 1136server option. 1137The following entry would permit connections on 1138.Xr tun 4 1139device 1 from user 1140.Dq jane 1141and on tun device 2 from user 1142.Dq john , 1143if 1144.Cm PermitRootLogin 1145is set to 1146.Dq forced-commands-only : 1147.Bd -literal -offset 2n 1148tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 1149tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 1150.Ed 1151.Pp 1152Since an SSH-based setup entails a fair amount of overhead, 1153it may be more suited to temporary setups, 1154such as for wireless VPNs. 1155More permanent VPNs are better provided by tools such as 1156.Xr ipsecctl 8 1157and 1158.Xr isakmpd 8 . 1159.Sh ENVIRONMENT 1160.Nm 1161will normally set the following environment variables: 1162.Bl -tag -width "SSH_ORIGINAL_COMMAND" 1163.It Ev DISPLAY 1164The 1165.Ev DISPLAY 1166variable indicates the location of the X11 server. 1167It is automatically set by 1168.Nm 1169to point to a value of the form 1170.Dq hostname:n , 1171where 1172.Dq hostname 1173indicates the host where the shell runs, and 1174.Sq n 1175is an integer \*(Ge 1. 1176.Nm 1177uses this special value to forward X11 connections over the secure 1178channel. 1179The user should normally not set 1180.Ev DISPLAY 1181explicitly, as that 1182will render the X11 connection insecure (and will require the user to 1183manually copy any required authorization cookies). 1184.It Ev HOME 1185Set to the path of the user's home directory. 1186.It Ev LOGNAME 1187Synonym for 1188.Ev USER ; 1189set for compatibility with systems that use this variable. 1190.It Ev MAIL 1191Set to the path of the user's mailbox. 1192.It Ev PATH 1193Set to the default 1194.Ev PATH , 1195as specified when compiling 1196.Nm . 1197.It Ev SSH_ASKPASS 1198If 1199.Nm 1200needs a passphrase, it will read the passphrase from the current 1201terminal if it was run from a terminal. 1202If 1203.Nm 1204does not have a terminal associated with it but 1205.Ev DISPLAY 1206and 1207.Ev SSH_ASKPASS 1208are set, it will execute the program specified by 1209.Ev SSH_ASKPASS 1210and open an X11 window to read the passphrase. 1211This is particularly useful when calling 1212.Nm 1213from a 1214.Pa .xsession 1215or related script. 1216(Note that on some machines it 1217may be necessary to redirect the input from 1218.Pa /dev/null 1219to make this work.) 1220.It Ev SSH_AUTH_SOCK 1221Identifies the path of a 1222.Ux Ns -domain 1223socket used to communicate with the agent. 1224.It Ev SSH_CONNECTION 1225Identifies the client and server ends of the connection. 1226The variable contains 1227four space-separated values: client IP address, client port number, 1228server IP address, and server port number. 1229.It Ev SSH_ORIGINAL_COMMAND 1230This variable contains the original command line if a forced command 1231is executed. 1232It can be used to extract the original arguments. 1233.It Ev SSH_TTY 1234This is set to the name of the tty (path to the device) associated 1235with the current shell or command. 1236If the current session has no tty, 1237this variable is not set. 1238.It Ev TZ 1239This variable is set to indicate the present time zone if it 1240was set when the daemon was started (i.e. the daemon passes the value 1241on to new connections). 1242.It Ev USER 1243Set to the name of the user logging in. 1244.El 1245.Pp 1246Additionally, 1247.Nm 1248reads 1249.Pa ~/.ssh/environment , 1250and adds lines of the format 1251.Dq VARNAME=value 1252to the environment if the file exists and users are allowed to 1253change their environment. 1254For more information, see the 1255.Cm PermitUserEnvironment 1256option in 1257.Xr sshd_config 5 . 1258.Sh FILES 1259.Bl -tag -width Ds -compact 1260.It Pa ~/.rhosts 1261This file is used for host-based authentication (see above). 1262On some machines this file may need to be 1263world-readable if the user's home directory is on an NFS partition, 1264because 1265.Xr sshd 8 1266reads it as root. 1267Additionally, this file must be owned by the user, 1268and must not have write permissions for anyone else. 1269The recommended 1270permission for most machines is read/write for the user, and not 1271accessible by others. 1272.Pp 1273.It Pa ~/.shosts 1274This file is used in exactly the same way as 1275.Pa .rhosts , 1276but allows host-based authentication without permitting login with 1277rlogin/rsh. 1278.Pp 1279.It Pa ~/.ssh/ 1280This directory is the default location for all user-specific configuration 1281and authentication information. 1282There is no general requirement to keep the entire contents of this directory 1283secret, but the recommended permissions are read/write/execute for the user, 1284and not accessible by others. 1285.Pp 1286.It Pa ~/.ssh/authorized_keys 1287Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as 1288this user. 1289The format of this file is described in the 1290.Xr sshd 8 1291manual page. 1292This file is not highly sensitive, but the recommended 1293permissions are read/write for the user, and not accessible by others. 1294.Pp 1295.It Pa ~/.ssh/config 1296This is the per-user configuration file. 1297The file format and configuration options are described in 1298.Xr ssh_config 5 . 1299Because of the potential for abuse, this file must have strict permissions: 1300read/write for the user, and not accessible by others. 1301.Pp 1302.It Pa ~/.ssh/environment 1303Contains additional definitions for environment variables; see 1304.Sx ENVIRONMENT , 1305above. 1306.Pp 1307.It Pa ~/.ssh/identity 1308.It Pa ~/.ssh/id_dsa 1309.It Pa ~/.ssh/id_ecdsa 1310.It Pa ~/.ssh/id_rsa 1311Contains the private key for authentication. 1312These files 1313contain sensitive data and should be readable by the user but not 1314accessible by others (read/write/execute). 1315.Nm 1316will simply ignore a private key file if it is accessible by others. 1317It is possible to specify a passphrase when 1318generating the key which will be used to encrypt the 1319sensitive part of this file using 3DES. 1320.Pp 1321.It Pa ~/.ssh/identity.pub 1322.It Pa ~/.ssh/id_dsa.pub 1323.It Pa ~/.ssh/id_ecdsa.pub 1324.It Pa ~/.ssh/id_rsa.pub 1325Contains the public key for authentication. 1326These files are not 1327sensitive and can (but need not) be readable by anyone. 1328.Pp 1329.It Pa ~/.ssh/known_hosts 1330Contains a list of host keys for all hosts the user has logged into 1331that are not already in the systemwide list of known host keys. 1332See 1333.Xr sshd 8 1334for further details of the format of this file. 1335.Pp 1336.It Pa ~/.ssh/rc 1337Commands in this file are executed by 1338.Nm 1339when the user logs in, just before the user's shell (or command) is 1340started. 1341See the 1342.Xr sshd 8 1343manual page for more information. 1344.Pp 1345.It Pa /etc/hosts.equiv 1346This file is for host-based authentication (see above). 1347It should only be writable by root. 1348.Pp 1349.It Pa /etc/shosts.equiv 1350This file is used in exactly the same way as 1351.Pa hosts.equiv , 1352but allows host-based authentication without permitting login with 1353rlogin/rsh. 1354.Pp 1355.It Pa /etc/ssh/ssh_config 1356Systemwide configuration file. 1357The file format and configuration options are described in 1358.Xr ssh_config 5 . 1359.Pp 1360.It Pa /etc/ssh/ssh_host_key 1361.It Pa /etc/ssh/ssh_host_dsa_key 1362.It Pa /etc/ssh/ssh_host_ecdsa_key 1363.It Pa /etc/ssh/ssh_host_rsa_key 1364These three files contain the private parts of the host keys 1365and are used for host-based authentication. 1366If protocol version 1 is used, 1367.Nm 1368must be setuid root, since the host key is readable only by root. 1369For protocol version 2, 1370.Nm 1371uses 1372.Xr ssh-keysign 8 1373to access the host keys, 1374eliminating the requirement that 1375.Nm 1376be setuid root when host-based authentication is used. 1377By default 1378.Nm 1379is not setuid root. 1380.Pp 1381.It Pa /etc/ssh/ssh_known_hosts 1382Systemwide list of known host keys. 1383This file should be prepared by the 1384system administrator to contain the public host keys of all machines in the 1385organization. 1386It should be world-readable. 1387See 1388.Xr sshd 8 1389for further details of the format of this file. 1390.Pp 1391.It Pa /etc/ssh/sshrc 1392Commands in this file are executed by 1393.Nm 1394when the user logs in, just before the user's shell (or command) is started. 1395See the 1396.Xr sshd 8 1397manual page for more information. 1398.El 1399.Sh EXIT STATUS 1400.Nm 1401exits with the exit status of the remote command or with 255 1402if an error occurred. 1403.Sh SEE ALSO 1404.Xr scp 1 , 1405.Xr sftp 1 , 1406.Xr ssh-add 1 , 1407.Xr ssh-agent 1 , 1408.Xr ssh-keygen 1 , 1409.Xr ssh-keyscan 1 , 1410.Xr tun 4 , 1411.Xr hosts.equiv 5 , 1412.Xr ssh_config 5 , 1413.Xr ssh-keysign 8 , 1414.Xr sshd 8 1415.Rs 1416.%R RFC 4250 1417.%T "The Secure Shell (SSH) Protocol Assigned Numbers" 1418.%D 2006 1419.Re 1420.Rs 1421.%R RFC 4251 1422.%T "The Secure Shell (SSH) Protocol Architecture" 1423.%D 2006 1424.Re 1425.Rs 1426.%R RFC 4252 1427.%T "The Secure Shell (SSH) Authentication Protocol" 1428.%D 2006 1429.Re 1430.Rs 1431.%R RFC 4253 1432.%T "The Secure Shell (SSH) Transport Layer Protocol" 1433.%D 2006 1434.Re 1435.Rs 1436.%R RFC 4254 1437.%T "The Secure Shell (SSH) Connection Protocol" 1438.%D 2006 1439.Re 1440.Rs 1441.%R RFC 4255 1442.%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints" 1443.%D 2006 1444.Re 1445.Rs 1446.%R RFC 4256 1447.%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)" 1448.%D 2006 1449.Re 1450.Rs 1451.%R RFC 4335 1452.%T "The Secure Shell (SSH) Session Channel Break Extension" 1453.%D 2006 1454.Re 1455.Rs 1456.%R RFC 4344 1457.%T "The Secure Shell (SSH) Transport Layer Encryption Modes" 1458.%D 2006 1459.Re 1460.Rs 1461.%R RFC 4345 1462.%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol" 1463.%D 2006 1464.Re 1465.Rs 1466.%R RFC 4419 1467.%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol" 1468.%D 2006 1469.Re 1470.Rs 1471.%R RFC 4716 1472.%T "The Secure Shell (SSH) Public Key File Format" 1473.%D 2006 1474.Re 1475.Rs 1476.%R RFC 5656 1477.%T "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer" 1478.%D 2009 1479.Re 1480.Rs 1481.%T "Hash Visualization: a New Technique to improve Real-World Security" 1482.%A A. Perrig 1483.%A D. Song 1484.%D 1999 1485.%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)" 1486.Re 1487.Sh AUTHORS 1488OpenSSH is a derivative of the original and free 1489ssh 1.2.12 release by Tatu Ylonen. 1490Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1491Theo de Raadt and Dug Song 1492removed many bugs, re-added newer features and 1493created OpenSSH. 1494Markus Friedl contributed the support for SSH 1495protocol versions 1.5 and 2.0. 1496