ssh.0 revision 180746
1SSH(1) OpenBSD Reference Manual SSH(1) 2 3NAME 4 ssh - OpenSSH SSH client (remote login program) 5 6SYNOPSIS 7 ssh [-1246AaCfgKkMNnqsTtVvXxY] [-b bind_address] [-c cipher_spec] 8 [-D [bind_address:]port] [-e escape_char] [-F configfile] 9 [-i identity_file] [-L [bind_address:]port:host:hostport] 10 [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] 11 [-R [bind_address:]port:host:hostport] [-S ctl_path] 12 [-w local_tun[:remote_tun]] [user@]hostname [command] 13 14DESCRIPTION 15 ssh (SSH client) is a program for logging into a remote machine and for 16 executing commands on a remote machine. It is intended to replace rlogin 17 and rsh, and provide secure encrypted communications between two untrust- 18 ed hosts over an insecure network. X11 connections and arbitrary TCP 19 ports can also be forwarded over the secure channel. 20 21 ssh connects and logs into the specified hostname (with optional user 22 name). The user must prove his/her identity to the remote machine using 23 one of several methods depending on the protocol version used (see be- 24 low). 25 26 If command is specified, it is executed on the remote host instead of a 27 login shell. 28 29 The options are as follows: 30 31 -1 Forces ssh to try protocol version 1 only. 32 33 -2 Forces ssh to try protocol version 2 only. 34 35 -4 Forces ssh to use IPv4 addresses only. 36 37 -6 Forces ssh to use IPv6 addresses only. 38 39 -A Enables forwarding of the authentication agent connection. This 40 can also be specified on a per-host basis in a configuration 41 file. 42 43 Agent forwarding should be enabled with caution. Users with the 44 ability to bypass file permissions on the remote host (for the 45 agent's Unix-domain socket) can access the local agent through 46 the forwarded connection. An attacker cannot obtain key material 47 from the agent, however they can perform operations on the keys 48 that enable them to authenticate using the identities loaded into 49 the agent. 50 51 -a Disables forwarding of the authentication agent connection. 52 53 -b bind_address 54 Use bind_address on the local machine as the source address of 55 the connection. Only useful on systems with more than one ad- 56 dress. 57 58 -C Requests compression of all data (including stdin, stdout, 59 stderr, and data for forwarded X11 and TCP connections). The 60 compression algorithm is the same used by gzip(1), and the 61 ``level'' can be controlled by the CompressionLevel option for 62 protocol version 1. Compression is desirable on modem lines and 63 other slow connections, but will only slow down things on fast 64 networks. The default value can be set on a host-by-host basis 65 in the configuration files; see the Compression option. 66 67 -c cipher_spec 68 Selects the cipher specification for encrypting the session. 69 70 Protocol version 1 allows specification of a single cipher. The 71 supported values are ``3des'', ``blowfish'', and ``des''. 3des 72 (triple-des) is an encrypt-decrypt-encrypt triple with three dif- 73 ferent keys. It is believed to be secure. blowfish is a fast 74 block cipher; it appears very secure and is much faster than 75 3des. des is only supported in the ssh client for interoperabil- 76 ity with legacy protocol 1 implementations that do not support 77 the 3des cipher. Its use is strongly discouraged due to crypto- 78 graphic weaknesses. The default is ``3des''. 79 80 For protocol version 2, cipher_spec is a comma-separated list of 81 ciphers listed in order of preference. The supported ciphers 82 are: 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr, 83 aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, blow- 84 fish-cbc, and cast128-cbc. The default is: 85 86 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, 87 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, 88 aes192-ctr,aes256-ctr 89 90 -D [bind_address:]port 91 Specifies a local ``dynamic'' application-level port forwarding. 92 This works by allocating a socket to listen to port on the local 93 side, optionally bound to the specified bind_address. Whenever a 94 connection is made to this port, the connection is forwarded over 95 the secure channel, and the application protocol is then used to 96 determine where to connect to from the remote machine. Currently 97 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act 98 as a SOCKS server. Only root can forward privileged ports. Dy- 99 namic port forwardings can also be specified in the configuration 100 file. 101 102 IPv6 addresses can be specified with an alternative syntax: 103 [bind_address/]port or by enclosing the address in square brack- 104 ets. Only the superuser can forward privileged ports. By de- 105 fault, the local port is bound in accordance with the 106 GatewayPorts setting. However, an explicit bind_address may be 107 used to bind the connection to a specific address. The 108 bind_address of ``localhost'' indicates that the listening port 109 be bound for local use only, while an empty address or `*' indi- 110 cates that the port should be available from all interfaces. 111 112 -e escape_char 113 Sets the escape character for sessions with a pty (default: `~'). 114 The escape character is only recognized at the beginning of a 115 line. The escape character followed by a dot (`.') closes the 116 connection; followed by control-Z suspends the connection; and 117 followed by itself sends the escape character once. Setting the 118 character to ``none'' disables any escapes and makes the session 119 fully transparent. 120 121 -F configfile 122 Specifies an alternative per-user configuration file. If a con- 123 figuration file is given on the command line, the system-wide 124 configuration file (/etc/ssh/ssh_config) will be ignored. The 125 default for the per-user configuration file is ~/.ssh/config. 126 127 -f Requests ssh to go to background just before command execution. 128 This is useful if ssh is going to ask for passwords or passphras- 129 es, but the user wants it in the background. This implies -n. 130 The recommended way to start X11 programs at a remote site is 131 with something like ssh -f host xterm. 132 133 -g Allows remote hosts to connect to local forwarded ports. 134 135 -I smartcard_device 136 Specify the device ssh should use to communicate with a smartcard 137 used for storing the user's private RSA key. This option is only 138 available if support for smartcard devices is compiled in (de- 139 fault is no support). 140 141 -i identity_file 142 Selects a file from which the identity (private key) for RSA or 143 DSA authentication is read. The default is ~/.ssh/identity for 144 protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for pro- 145 tocol version 2. Identity files may also be specified on a per- 146 host basis in the configuration file. It is possible to have 147 multiple -i options (and multiple identities specified in config- 148 uration files). 149 150 -K Enables GSSAPI-based authentication and forwarding (delegation) 151 of GSSAPI credentials to the server. 152 153 -k Disables forwarding (delegation) of GSSAPI credentials to the 154 server. 155 156 -L [bind_address:]port:host:hostport 157 Specifies that the given port on the local (client) host is to be 158 forwarded to the given host and port on the remote side. This 159 works by allocating a socket to listen to port on the local side, 160 optionally bound to the specified bind_address. Whenever a con- 161 nection is made to this port, the connection is forwarded over 162 the secure channel, and a connection is made to host port 163 hostport from the remote machine. Port forwardings can also be 164 specified in the configuration file. IPv6 addresses can be spec- 165 ified with an alternative syntax: [bind_address/]port/host/host- 166 port or by enclosing the address in square brackets. Only the 167 superuser can forward privileged ports. By default, the local 168 port is bound in accordance with the GatewayPorts setting. How- 169 ever, an explicit bind_address may be used to bind the connection 170 to a specific address. The bind_address of ``localhost'' indi- 171 cates that the listening port be bound for local use only, while 172 an empty address or `*' indicates that the port should be avail- 173 able from all interfaces. 174 175 -l login_name 176 Specifies the user to log in as on the remote machine. This also 177 may be specified on a per-host basis in the configuration file. 178 179 -M Places the ssh client into ``master'' mode for connection shar- 180 ing. Multiple -M options places ssh into ``master'' mode with 181 confirmation required before slave connections are accepted. Re- 182 fer to the description of ControlMaster in ssh_config(5) for de- 183 tails. 184 185 -m mac_spec 186 Additionally, for protocol version 2 a comma-separated list of 187 MAC (message authentication code) algorithms can be specified in 188 order of preference. See the MACs keyword for more information. 189 190 -N Do not execute a remote command. This is useful for just for- 191 warding ports (protocol version 2 only). 192 193 -n Redirects stdin from /dev/null (actually, prevents reading from 194 stdin). This must be used when ssh is run in the background. A 195 common trick is to use this to run X11 programs on a remote ma- 196 chine. For example, ssh -n shadows.cs.hut.fi emacs & will start 197 an emacs on shadows.cs.hut.fi, and the X11 connection will be au- 198 tomatically forwarded over an encrypted channel. The ssh program 199 will be put in the background. (This does not work if ssh needs 200 to ask for a password or passphrase; see also the -f option.) 201 202 -O ctl_cmd 203 Control an active connection multiplexing master process. When 204 the -O option is specified, the ctl_cmd argument is interpreted 205 and passed to the master process. Valid commands are: ``check'' 206 (check that the master process is running) and ``exit'' (request 207 the master to exit). 208 209 -o option 210 Can be used to give options in the format used in the configura- 211 tion file. This is useful for specifying options for which there 212 is no separate command-line flag. For full details of the op- 213 tions listed below, and their possible values, see ssh_config(5). 214 215 AddressFamily 216 BatchMode 217 BindAddress 218 ChallengeResponseAuthentication 219 CheckHostIP 220 Cipher 221 Ciphers 222 ClearAllForwardings 223 Compression 224 CompressionLevel 225 ConnectionAttempts 226 ConnectTimeout 227 ControlMaster 228 ControlPath 229 DynamicForward 230 EscapeChar 231 ExitOnForwardFailure 232 ForwardAgent 233 ForwardX11 234 ForwardX11Trusted 235 GatewayPorts 236 GlobalKnownHostsFile 237 GSSAPIAuthentication 238 GSSAPIDelegateCredentials 239 HashKnownHosts 240 Host 241 HostbasedAuthentication 242 HostKeyAlgorithms 243 HostKeyAlias 244 HostName 245 IdentityFile 246 IdentitiesOnly 247 KbdInteractiveDevices 248 LocalCommand 249 LocalForward 250 LogLevel 251 MACs 252 NoHostAuthenticationForLocalhost 253 NumberOfPasswordPrompts 254 PasswordAuthentication 255 PermitLocalCommand 256 Port 257 PreferredAuthentications 258 Protocol 259 ProxyCommand 260 PubkeyAuthentication 261 RekeyLimit 262 RemoteForward 263 RhostsRSAAuthentication 264 RSAAuthentication 265 SendEnv 266 ServerAliveInterval 267 ServerAliveCountMax 268 SmartcardDevice 269 StrictHostKeyChecking 270 TCPKeepAlive 271 Tunnel 272 TunnelDevice 273 UsePrivilegedPort 274 User 275 UserKnownHostsFile 276 VerifyHostKeyDNS 277 XAuthLocation 278 279 -p port 280 Port to connect to on the remote host. This can be specified on 281 a per-host basis in the configuration file. 282 283 -q Quiet mode. Causes most warning and diagnostic messages to be 284 suppressed. 285 286 -R [bind_address:]port:host:hostport 287 Specifies that the given port on the remote (server) host is to 288 be forwarded to the given host and port on the local side. This 289 works by allocating a socket to listen to port on the remote 290 side, and whenever a connection is made to this port, the connec- 291 tion is forwarded over the secure channel, and a connection is 292 made to host port hostport from the local machine. 293 294 Port forwardings can also be specified in the configuration file. 295 Privileged ports can be forwarded only when logging in as root on 296 the remote machine. IPv6 addresses can be specified by enclosing 297 the address in square braces or using an alternative syntax: 298 [bind_address/]host/port/hostport. 299 300 By default, the listening socket on the server will be bound to 301 the loopback interface only. This may be overriden by specifying 302 a bind_address. An empty bind_address, or the address `*', indi- 303 cates that the remote socket should listen on all interfaces. 304 Specifying a remote bind_address will only succeed if the serv- 305 er's GatewayPorts option is enabled (see sshd_config(5)). 306 307 -S ctl_path 308 Specifies the location of a control socket for connection shar- 309 ing. Refer to the description of ControlPath and ControlMaster 310 in ssh_config(5) for details. 311 312 -s May be used to request invocation of a subsystem on the remote 313 system. Subsystems are a feature of the SSH2 protocol which fa- 314 cilitate the use of SSH as a secure transport for other applica- 315 tions (eg. sftp(1)). The subsystem is specified as the remote 316 command. 317 318 -T Disable pseudo-tty allocation. 319 320 -t Force pseudo-tty allocation. This can be used to execute arbi- 321 trary screen-based programs on a remote machine, which can be 322 very useful, e.g. when implementing menu services. Multiple -t 323 options force tty allocation, even if ssh has no local tty. 324 325 -V Display the version number and exit. 326 327 -v Verbose mode. Causes ssh to print debugging messages about its 328 progress. This is helpful in debugging connection, authentica- 329 tion, and configuration problems. Multiple -v options increase 330 the verbosity. The maximum is 3. 331 332 -w local_tun[:remote_tun] 333 Requests tunnel device forwarding with the specified tun(4) de- 334 vices between the client (local_tun) and the server (remote_tun). 335 336 The devices may be specified by numerical ID or the keyword 337 ``any'', which uses the next available tunnel device. If 338 remote_tun is not specified, it defaults to ``any''. See also 339 the Tunnel and TunnelDevice directives in ssh_config(5). If the 340 Tunnel directive is unset, it is set to the default tunnel mode, 341 which is ``point-to-point''. 342 343 -X Enables X11 forwarding. This can also be specified on a per-host 344 basis in a configuration file. 345 346 X11 forwarding should be enabled with caution. Users with the 347 ability to bypass file permissions on the remote host (for the 348 user's X authorization database) can access the local X11 display 349 through the forwarded connection. An attacker may then be able 350 to perform activities such as keystroke monitoring. 351 352 For this reason, X11 forwarding is subjected to X11 SECURITY ex- 353 tension restrictions by default. Please refer to the ssh -Y op- 354 tion and the ForwardX11Trusted directive in ssh_config(5) for 355 more information. 356 357 -x Disables X11 forwarding. 358 359 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not 360 subjected to the X11 SECURITY extension controls. 361 362 ssh may additionally obtain configuration data from a per-user configura- 363 tion file and a system-wide configuration file. The file format and con- 364 figuration options are described in ssh_config(5). 365 366 ssh exits with the exit status of the remote command or with 255 if an 367 error occurred. 368 369AUTHENTICATION 370 The OpenSSH SSH client supports SSH protocols 1 and 2. Protocol 2 is the 371 default, with ssh falling back to protocol 1 if it detects protocol 2 is 372 unsupported. These settings may be altered using the Protocol option in 373 ssh_config(5), or enforced using the -1 and -2 options (see above). Both 374 protocols support similar authentication methods, but protocol 2 is pre- 375 ferred since it provides additional mechanisms for confidentiality (the 376 traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and 377 integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). Protocol 1 378 lacks a strong mechanism for ensuring the integrity of the connection. 379 380 The methods available for authentication are: GSSAPI-based authentica- 381 tion, host-based authentication, public key authentication, challenge-re- 382 sponse authentication, and password authentication. Authentication meth- 383 ods are tried in the order specified above, though protocol 2 has a con- 384 figuration option to change the default order: PreferredAuthentications. 385 386 Host-based authentication works as follows: If the machine the user logs 387 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote 388 machine, and the user names are the same on both sides, or if the files 389 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote 390 machine and contain a line containing the name of the client machine and 391 the name of the user on that machine, the user is considered for login. 392 Additionally, the server must be able to verify the client's host key 393 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, 394 below) for login to be permitted. This authentication method closes se- 395 curity holes due to IP spoofing, DNS spoofing, and routing spoofing. 396 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the 397 rlogin/rsh protocol in general, are inherently insecure and should be 398 disabled if security is desired.] 399 400 Public key authentication works as follows: The scheme is based on pub- 401 lic-key cryptography, using cryptosystems where encryption and decryption 402 are done using separate keys, and it is unfeasible to derive the decryp- 403 tion key from the encryption key. The idea is that each user creates a 404 public/private key pair for authentication purposes. The server knows 405 the public key, and only the user knows the private key. ssh implements 406 public key authentication protocol automatically, using either the RSA or 407 DSA algorithms. Protocol 1 is restricted to using only RSA keys, but 408 protocol 2 may use either. The HISTORY section of ssl(8) contains a 409 brief discussion of the two algorithms. 410 411 The file ~/.ssh/authorized_keys lists the public keys that are permitted 412 for logging in. When the user logs in, the ssh program tells the server 413 which key pair it would like to use for authentication. The client 414 proves that it has access to the private key and the server checks that 415 the corresponding public key is authorized to accept the account. 416 417 The user creates his/her key pair by running ssh-keygen(1). This stores 418 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 419 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in 420 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or 421 ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The us- 422 er should then copy the public key to ~/.ssh/authorized_keys in his/her 423 home directory on the remote machine. The authorized_keys file corre- 424 sponds to the conventional ~/.rhosts file, and has one key per line, 425 though the lines can be very long. After this, the user can log in with- 426 out giving the password. 427 428 The most convenient way to use public key authentication may be with an 429 authentication agent. See ssh-agent(1) for more information. 430 431 Challenge-response authentication works as follows: The server sends an 432 arbitrary "challenge" text, and prompts for a response. Protocol 2 al- 433 lows multiple challenges and responses; protocol 1 is restricted to just 434 one challenge/response. Examples of challenge-response authentication 435 include BSD Authentication (see login.conf(5)) and PAM (some non-OpenBSD 436 systems). 437 438 Finally, if other authentication methods fail, ssh prompts the user for a 439 password. The password is sent to the remote host for checking; however, 440 since all communications are encrypted, the password cannot be seen by 441 someone listening on the network. 442 443 ssh automatically maintains and checks a database containing identifica- 444 tion for all hosts it has ever been used with. Host keys are stored in 445 ~/.ssh/known_hosts in the user's home directory. Additionally, the file 446 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any 447 new hosts are automatically added to the user's file. If a host's iden- 448 tification ever changes, ssh warns about this and disables password au- 449 thentication to prevent server spoofing or man-in-the-middle attacks, 450 which could otherwise be used to circumvent the encryption. The 451 StrictHostKeyChecking option can be used to control logins to machines 452 whose host key is not known or has changed. 453 454 When the user's identity has been accepted by the server, the server ei- 455 ther executes the given command, or logs into the machine and gives the 456 user a normal shell on the remote machine. All communication with the 457 remote command or shell will be automatically encrypted. 458 459 If a pseudo-terminal has been allocated (normal login session), the user 460 may use the escape characters noted below. 461 462 If no pseudo-tty has been allocated, the session is transparent and can 463 be used to reliably transfer binary data. On most systems, setting the 464 escape character to ``none'' will also make the session transparent even 465 if a tty is used. 466 467 The session terminates when the command or shell on the remote machine 468 exits and all X11 and TCP connections have been closed. 469 470ESCAPE CHARACTERS 471 When a pseudo-terminal has been requested, ssh supports a number of func- 472 tions through the use of an escape character. 473 474 A single tilde character can be sent as ~~ or by following the tilde by a 475 character other than those described below. The escape character must 476 always follow a newline to be interpreted as special. The escape charac- 477 ter can be changed in configuration files using the EscapeChar configura- 478 tion directive or on the command line by the -e option. 479 480 The supported escapes (assuming the default `~') are: 481 482 ~. Disconnect. 483 484 ~^Z Background ssh. 485 486 ~# List forwarded connections. 487 488 ~& Background ssh at logout when waiting for forwarded connection / 489 X11 sessions to terminate. 490 491 ~? Display a list of escape characters. 492 493 ~B Send a BREAK to the remote system (only useful for SSH protocol 494 version 2 and if the peer supports it). 495 496 ~C Open command line. Currently this allows the addition of port 497 forwardings using the -L and -R options (see above). It also al- 498 lows the cancellation of existing remote port-forwardings using 499 -KR[bind_address:]port. !command allows the user to execute a 500 local command if the PermitLocalCommand option is enabled in 501 ssh_config(5). Basic help is available, using the -h option. 502 503 ~R Request rekeying of the connection (only useful for SSH protocol 504 version 2 and if the peer supports it). 505 506TCP FORWARDING 507 Forwarding of arbitrary TCP connections over the secure channel can be 508 specified either on the command line or in a configuration file. One 509 possible application of TCP forwarding is a secure connection to a mail 510 server; another is going through firewalls. 511 512 In the example below, we look at encrypting communication between an IRC 513 client and server, even though the IRC server does not directly support 514 encrypted communications. This works as follows: the user connects to 515 the remote host using ssh, specifying a port to be used to forward con- 516 nections to the remote server. After that it is possible to start the 517 service which is to be encrypted on the client machine, connecting to the 518 same local port, and ssh will encrypt and forward the connection. 519 520 The following example tunnels an IRC session from client machine 521 ``127.0.0.1'' (localhost) to remote server ``server.example.com'': 522 523 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 524 $ irc -c '#users' -p 1234 pinky 127.0.0.1 525 526 This tunnels a connection to IRC server ``server.example.com'', joining 527 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't mat- 528 ter which port is used, as long as it's greater than 1023 (remember, only 529 root can open sockets on privileged ports) and doesn't conflict with any 530 ports already in use. The connection is forwarded to port 6667 on the 531 remote server, since that's the standard port for IRC services. 532 533 The -f option backgrounds ssh and the remote command ``sleep 10'' is 534 specified to allow an amount of time (10 seconds, in the example) to 535 start the service which is to be tunnelled. If no connections are made 536 within the time specified, ssh will exit. 537 538X11 FORWARDING 539 If the ForwardX11 variable is set to ``yes'' (or see the description of 540 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY 541 environment variable is set), the connection to the X11 display is auto- 542 matically forwarded to the remote side in such a way that any X11 pro- 543 grams started from the shell (or command) will go through the encrypted 544 channel, and the connection to the real X server will be made from the 545 local machine. The user should not manually set DISPLAY. Forwarding of 546 X11 connections can be configured on the command line or in configuration 547 files. 548 549 The DISPLAY value set by ssh will point to the server machine, but with a 550 display number greater than zero. This is normal, and happens because 551 ssh creates a ``proxy'' X server on the server machine for forwarding the 552 connections over the encrypted channel. 553 554 ssh will also automatically set up Xauthority data on the server machine. 555 For this purpose, it will generate a random authorization cookie, store 556 it in Xauthority on the server, and verify that any forwarded connections 557 carry this cookie and replace it by the real cookie when the connection 558 is opened. The real authentication cookie is never sent to the server 559 machine (and no cookies are sent in the plain). 560 561 If the ForwardAgent variable is set to ``yes'' (or see the description of 562 the -A and -a options above) and the user is using an authentication 563 agent, the connection to the agent is automatically forwarded to the re- 564 mote side. 565 566VERIFYING HOST KEYS 567 When connecting to a server for the first time, a fingerprint of the 568 server's public key is presented to the user (unless the option 569 StrictHostKeyChecking has been disabled). Fingerprints can be determined 570 using ssh-keygen(1): 571 572 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 573 574 If the fingerprint is already known, it can be matched and verified, and 575 the key can be accepted. If the fingerprint is unknown, an alternative 576 method of verification is available: SSH fingerprints verified by DNS. 577 An additional resource record (RR), SSHFP, is added to a zonefile and the 578 connecting client is able to match the fingerprint with that of the key 579 presented. 580 581 In this example, we are connecting a client to a server, 582 ``host.example.com''. The SSHFP resource records should first be added 583 to the zonefile for host.example.com: 584 585 $ ssh-keygen -r host.example.com. 586 587 The output lines will have to be added to the zonefile. To check that 588 the zone is answering fingerprint queries: 589 590 $ dig -t SSHFP host.example.com 591 592 Finally the client connects: 593 594 $ ssh -o "VerifyHostKeyDNS ask" host.example.com 595 [...] 596 Matching host key fingerprint found in DNS. 597 Are you sure you want to continue connecting (yes/no)? 598 599 See the VerifyHostKeyDNS option in ssh_config(5) for more information. 600 601SSH-BASED VIRTUAL PRIVATE NETWORKS 602 ssh contains support for Virtual Private Network (VPN) tunnelling using 603 the tun(4) network pseudo-device, allowing two networks to be joined se- 604 curely. The sshd_config(5) configuration option PermitTunnel controls 605 whether the server supports this, and at what level (layer 2 or 3 traf- 606 fic). 607 608 The following example would connect client network 10.0.50.0/24 with re- 609 mote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1 610 to 10.1.1.2, provided that the SSH server running on the gateway to the 611 remote network, at 192.168.1.15, allows it. 612 613 On the client: 614 615 # ssh -f -w 0:1 192.168.1.15 true 616 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 617 # route add 10.0.99.0/24 10.1.1.2 618 619 On the server: 620 621 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 622 # route add 10.0.50.0/24 10.1.1.1 623 624 Client access may be more finely tuned via the /root/.ssh/authorized_keys 625 file (see below) and the PermitRootLogin server option. The following 626 entry would permit connections on tun(4) device 1 from user ``jane'' and 627 on tun device 2 from user ``john'', if PermitRootLogin is set to 628 ``forced-commands-only'': 629 630 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 631 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 632 633 Since an SSH-based setup entails a fair amount of overhead, it may be 634 more suited to temporary setups, such as for wireless VPNs. More perma- 635 nent VPNs are better provided by tools such as ipsecctl(8) and 636 isakmpd(8). 637 638ENVIRONMENT 639 ssh will normally set the following environment variables: 640 641 DISPLAY The DISPLAY variable indicates the location of the 642 X11 server. It is automatically set by ssh to 643 point to a value of the form ``hostname:n'', where 644 ``hostname'' indicates the host where the shell 645 runs, and `n' is an integer >= 1. ssh uses this 646 special value to forward X11 connections over the 647 secure channel. The user should normally not set 648 DISPLAY explicitly, as that will render the X11 649 connection insecure (and will require the user to 650 manually copy any required authorization cookies). 651 652 HOME Set to the path of the user's home directory. 653 654 LOGNAME Synonym for USER; set for compatibility with sys- 655 tems that use this variable. 656 657 MAIL Set to the path of the user's mailbox. 658 659 PATH Set to the default PATH, as specified when compil- 660 ing ssh. 661 662 SSH_ASKPASS If ssh needs a passphrase, it will read the 663 passphrase from the current terminal if it was run 664 from a terminal. If ssh does not have a terminal 665 associated with it but DISPLAY and SSH_ASKPASS are 666 set, it will execute the program specified by 667 SSH_ASKPASS and open an X11 window to read the 668 passphrase. This is particularly useful when call- 669 ing ssh from a .xsession or related script. (Note 670 that on some machines it may be necessary to redi- 671 rect the input from /dev/null to make this work.) 672 673 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to 674 communicate with the agent. 675 676 SSH_CONNECTION Identifies the client and server ends of the con- 677 nection. The variable contains four space-separat- 678 ed values: client IP address, client port number, 679 server IP address, and server port number. 680 681 SSH_ORIGINAL_COMMAND This variable contains the original command line if 682 a forced command is executed. It can be used to 683 extract the original arguments. 684 685 SSH_TTY This is set to the name of the tty (path to the de- 686 vice) associated with the current shell or command. 687 If the current session has no tty, this variable is 688 not set. 689 690 TZ This variable is set to indicate the present time 691 zone if it was set when the daemon was started 692 (i.e. the daemon passes the value on to new connec- 693 tions). 694 695 USER Set to the name of the user logging in. 696 697 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format 698 ``VARNAME=value'' to the environment if the file exists and users are al- 699 lowed to change their environment. For more information, see the 700 PermitUserEnvironment option in sshd_config(5). 701 702FILES 703 ~/.rhosts 704 This file is used for host-based authentication (see above). On 705 some machines this file may need to be world-readable if the us- 706 er's home directory is on an NFS partition, because sshd(8) reads 707 it as root. Additionally, this file must be owned by the user, 708 and must not have write permissions for anyone else. The recom- 709 mended permission for most machines is read/write for the user, 710 and not accessible by others. 711 712 ~/.shosts 713 This file is used in exactly the same way as .rhosts, but allows 714 host-based authentication without permitting login with 715 rlogin/rsh. 716 717 ~/.ssh/ 718 This directory is the default location for all user-specific con- 719 figuration and authentication information. There is no general 720 requirement to keep the entire contents of this directory secret, 721 but the recommended permissions are read/write/execute for the 722 user, and not accessible by others. 723 724 ~/.ssh/authorized_keys 725 Lists the public keys (RSA/DSA) that can be used for logging in 726 as this user. The format of this file is described in the 727 sshd(8) manual page. This file is not highly sensitive, but the 728 recommended permissions are read/write for the user, and not ac- 729 cessible by others. 730 731 ~/.ssh/config 732 This is the per-user configuration file. The file format and 733 configuration options are described in ssh_config(5). Because of 734 the potential for abuse, this file must have strict permissions: 735 read/write for the user, and not accessible by others. 736 737 ~/.ssh/environment 738 Contains additional definitions for environment variables; see 739 ENVIRONMENT, above. 740 741 ~/.ssh/identity 742 ~/.ssh/id_dsa 743 ~/.ssh/id_rsa 744 Contains the private key for authentication. These files contain 745 sensitive data and should be readable by the user but not acces- 746 sible by others (read/write/execute). ssh will simply ignore a 747 private key file if it is accessible by others. It is possible 748 to specify a passphrase when generating the key which will be 749 used to encrypt the sensitive part of this file using 3DES. 750 751 ~/.ssh/identity.pub 752 ~/.ssh/id_dsa.pub 753 ~/.ssh/id_rsa.pub 754 Contains the public key for authentication. These files are not 755 sensitive and can (but need not) be readable by anyone. 756 757 ~/.ssh/known_hosts 758 Contains a list of host keys for all hosts the user has logged 759 into that are not already in the systemwide list of known host 760 keys. See sshd(8) for further details of the format of this 761 file. 762 763 ~/.ssh/rc 764 Commands in this file are executed by ssh when the user logs in, 765 just before the user's shell (or command) is started. See the 766 sshd(8) manual page for more information. 767 768 /etc/hosts.equiv 769 This file is for host-based authentication (see above). It 770 should only be writable by root. 771 772 /etc/shosts.equiv 773 This file is used in exactly the same way as hosts.equiv, but al- 774 lows host-based authentication without permitting login with 775 rlogin/rsh. 776 777 /etc/ssh/ssh_config 778 Systemwide configuration file. The file format and configuration 779 options are described in ssh_config(5). 780 781 /etc/ssh/ssh_host_key 782 /etc/ssh/ssh_host_dsa_key 783 /etc/ssh/ssh_host_rsa_key 784 These three files contain the private parts of the host keys and 785 are used for host-based authentication. If protocol version 1 is 786 used, ssh must be setuid root, since the host key is readable on- 787 ly by root. For protocol version 2, ssh uses ssh-keysign(8) to 788 access the host keys, eliminating the requirement that ssh be se- 789 tuid root when host-based authentication is used. By default ssh 790 is not setuid root. 791 792 /etc/ssh/ssh_known_hosts 793 Systemwide list of known host keys. This file should be prepared 794 by the system administrator to contain the public host keys of 795 all machines in the organization. It should be world-readable. 796 See sshd(8) for further details of the format of this file. 797 798 /etc/ssh/sshrc 799 Commands in this file are executed by ssh when the user logs in, 800 just before the user's shell (or command) is started. See the 801 sshd(8) manual page for more information. 802 803SEE ALSO 804 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), 805 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8) 806 807 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006. 808 809 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006. 810 811 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006. 812 813 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006. 814 815 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006. 816 817 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 818 4255, 2006. 819 820 Generic Message Exchange Authentication for the Secure Shell Protocol 821 (SSH), RFC 4256, 2006. 822 823 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006. 824 825 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006. 826 827 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer 828 Protocol, RFC 4345, 2006. 829 830 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer 831 Protocol, RFC 4419, 2006. 832 833 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006. 834 835AUTHORS 836 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 837 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 838 de Raadt and Dug Song removed many bugs, re-added newer features and 839 created OpenSSH. Markus Friedl contributed the support for SSH protocol 840 versions 1.5 and 2.0. 841 842OpenBSD 4.3 March 26, 2008 13 843