Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1 | SSH(1) General Commands Manual SSH(1) |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 2 | |
| 3 | NAME |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 4 | ssh M-bM-^@M-^S OpenSSH SSH client (remote login program) |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 5 | |
| 6 | SYNOPSIS |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 7 | ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] |
| 8 | [-D [bind_address:]port] [-E log_file] [-e escape_char] |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 9 | [-F configfile] [-I pkcs11] [-i identity_file] |
| 10 | [-J [user@]host[:port]] [-L address] [-l login_name] [-m mac_spec] |
| 11 | [-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address] |
| 12 | [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] |
Greg Hartman | ccacbc9 | 2016-02-03 09:59:44 -0800 | [diff] [blame] | 13 | [user@]hostname [command] |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 14 | |
| 15 | DESCRIPTION |
| 16 | ssh (SSH client) is a program for logging into a remote machine and for |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 17 | executing commands on a remote machine. It is intended to provide secure |
| 18 | encrypted communications between two untrusted hosts over an insecure |
| 19 | network. X11 connections, arbitrary TCP ports and UNIX-domain sockets |
| 20 | can also be forwarded over the secure channel. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 21 | |
| 22 | ssh connects and logs into the specified hostname (with optional user |
| 23 | name). The user must prove his/her identity to the remote machine using |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 24 | one of several methods (see below). |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 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 |
| 56 | address. |
| 57 | |
| 58 | -C Requests compression of all data (including stdin, stdout, |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 59 | stderr, and data for forwarded X11, TCP and UNIX-domain |
| 60 | connections). The compression algorithm is the same used by |
| 61 | gzip(1), and the M-bM-^@M-^\levelM-bM-^@M-^] can be controlled by the |
| 62 | CompressionLevel option for protocol version 1. Compression is |
| 63 | desirable on modem lines and other slow connections, but will |
| 64 | only slow down things on fast networks. The default value can be |
| 65 | set on a host-by-host basis in the configuration files; see the |
| 66 | Compression option. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 67 | |
| 68 | -c cipher_spec |
| 69 | Selects the cipher specification for encrypting the session. |
| 70 | |
| 71 | Protocol version 1 allows specification of a single cipher. The |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 72 | supported values are M-bM-^@M-^\3desM-bM-^@M-^], M-bM-^@M-^\blowfishM-bM-^@M-^], and M-bM-^@M-^\desM-bM-^@M-^]. For protocol |
| 73 | version 2, cipher_spec is a comma-separated list of ciphers |
| 74 | listed in order of preference. See the Ciphers keyword in |
| 75 | ssh_config(5) for more information. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 76 | |
| 77 | -D [bind_address:]port |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 78 | Specifies a local M-bM-^@M-^\dynamicM-bM-^@M-^] application-level port forwarding. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 79 | This works by allocating a socket to listen to port on the local |
| 80 | side, optionally bound to the specified bind_address. Whenever a |
| 81 | connection is made to this port, the connection is forwarded over |
| 82 | the secure channel, and the application protocol is then used to |
| 83 | determine where to connect to from the remote machine. Currently |
| 84 | the SOCKS4 and SOCKS5 protocols are supported, and ssh will act |
| 85 | as a SOCKS server. Only root can forward privileged ports. |
| 86 | Dynamic port forwardings can also be specified in the |
| 87 | configuration file. |
| 88 | |
| 89 | IPv6 addresses can be specified by enclosing the address in |
| 90 | square brackets. Only the superuser can forward privileged |
| 91 | ports. By default, the local port is bound in accordance with |
| 92 | the GatewayPorts setting. However, an explicit bind_address may |
| 93 | be used to bind the connection to a specific address. The |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 94 | bind_address of M-bM-^@M-^\localhostM-bM-^@M-^] indicates that the listening port be |
| 95 | bound for local use only, while an empty address or M-bM-^@M-^X*M-bM-^@M-^Y indicates |
| 96 | that the port should be available from all interfaces. |
| 97 | |
| 98 | -E log_file |
| 99 | Append debug logs to log_file instead of standard error. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 100 | |
| 101 | -e escape_char |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 102 | Sets the escape character for sessions with a pty (default: M-bM-^@M-^X~M-bM-^@M-^Y). |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 103 | The escape character is only recognized at the beginning of a |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 104 | line. The escape character followed by a dot (M-bM-^@M-^X.M-bM-^@M-^Y) closes the |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 105 | connection; followed by control-Z suspends the connection; and |
| 106 | followed by itself sends the escape character once. Setting the |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 107 | character to M-bM-^@M-^\noneM-bM-^@M-^] disables any escapes and makes the session |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 108 | fully transparent. |
| 109 | |
| 110 | -F configfile |
| 111 | Specifies an alternative per-user configuration file. If a |
| 112 | configuration file is given on the command line, the system-wide |
| 113 | configuration file (/etc/ssh/ssh_config) will be ignored. The |
| 114 | default for the per-user configuration file is ~/.ssh/config. |
| 115 | |
| 116 | -f Requests ssh to go to background just before command execution. |
| 117 | This is useful if ssh is going to ask for passwords or |
| 118 | passphrases, but the user wants it in the background. This |
| 119 | implies -n. The recommended way to start X11 programs at a |
| 120 | remote site is with something like ssh -f host xterm. |
| 121 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 122 | If the ExitOnForwardFailure configuration option is set to M-bM-^@M-^\yesM-bM-^@M-^], |
| 123 | then a client started with -f will wait for all remote port |
| 124 | forwards to be successfully established before placing itself in |
| 125 | the background. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 126 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 127 | -G Causes ssh to print its configuration after evaluating Host and |
| 128 | Match blocks and exit. |
| 129 | |
| 130 | -g Allows remote hosts to connect to local forwarded ports. If used |
| 131 | on a multiplexed connection, then this option must be specified |
| 132 | on the master process. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 133 | |
| 134 | -I pkcs11 |
| 135 | Specify the PKCS#11 shared library ssh should use to communicate |
| 136 | with a PKCS#11 token providing the user's private RSA key. |
| 137 | |
| 138 | -i identity_file |
| 139 | Selects a file from which the identity (private key) for public |
| 140 | key authentication is read. The default is ~/.ssh/identity for |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 141 | protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, |
| 142 | ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol version 2. |
| 143 | Identity files may also be specified on a per-host basis in the |
| 144 | configuration file. It is possible to have multiple -i options |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 145 | (and multiple identities specified in configuration files). If |
| 146 | no certificates have been explicitly specified by the |
| 147 | CertificateFile directive, ssh will also try to load certificate |
| 148 | information from the filename obtained by appending -cert.pub to |
| 149 | identity filenames. |
| 150 | |
| 151 | -J [user@]host[:port] |
| 152 | Connect to the target host by first making a ssh connection to |
| 153 | the jump host and then establishing a TCP forwarding to the |
| 154 | ultimate destination from there. Multiple jump hops may be |
| 155 | specified separated by comma characters. This is a shortcut to |
| 156 | specify a ProxyJump configuration directive. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 157 | |
| 158 | -K Enables GSSAPI-based authentication and forwarding (delegation) |
| 159 | of GSSAPI credentials to the server. |
| 160 | |
| 161 | -k Disables forwarding (delegation) of GSSAPI credentials to the |
| 162 | server. |
| 163 | |
| 164 | -L [bind_address:]port:host:hostport |
Greg Hartman | ccacbc9 | 2016-02-03 09:59:44 -0800 | [diff] [blame] | 165 | -L [bind_address:]port:remote_socket |
| 166 | -L local_socket:host:hostport |
| 167 | -L local_socket:remote_socket |
| 168 | Specifies that connections to the given TCP port or Unix socket |
| 169 | on the local (client) host are to be forwarded to the given host |
| 170 | and port, or Unix socket, on the remote side. This works by |
| 171 | allocating a socket to listen to either a TCP port on the local |
| 172 | side, optionally bound to the specified bind_address, or to a |
| 173 | Unix socket. Whenever a connection is made to the local port or |
| 174 | socket, the connection is forwarded over the secure channel, and |
| 175 | a connection is made to either host port hostport, or the Unix |
| 176 | socket remote_socket, from the remote machine. |
| 177 | |
| 178 | Port forwardings can also be specified in the configuration file. |
| 179 | Only the superuser can forward privileged ports. IPv6 addresses |
| 180 | can be specified by enclosing the address in square brackets. |
| 181 | |
| 182 | By default, the local port is bound in accordance with the |
| 183 | GatewayPorts setting. However, an explicit bind_address may be |
| 184 | used to bind the connection to a specific address. The |
| 185 | bind_address of M-bM-^@M-^\localhostM-bM-^@M-^] indicates that the listening port be |
| 186 | bound for local use only, while an empty address or M-bM-^@M-^X*M-bM-^@M-^Y indicates |
| 187 | that the port should be available from all interfaces. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 188 | |
| 189 | -l login_name |
| 190 | Specifies the user to log in as on the remote machine. This also |
| 191 | may be specified on a per-host basis in the configuration file. |
| 192 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 193 | -M Places the ssh client into M-bM-^@M-^\masterM-bM-^@M-^] mode for connection sharing. |
| 194 | Multiple -M options places ssh into M-bM-^@M-^\masterM-bM-^@M-^] mode with |
| 195 | confirmation required before slave connections are accepted. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 196 | Refer to the description of ControlMaster in ssh_config(5) for |
| 197 | details. |
| 198 | |
| 199 | -m mac_spec |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 200 | A comma-separated list of MAC (message authentication code) |
| 201 | algorithms, specified in order of preference. See the MACs |
| 202 | keyword for more information. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 203 | |
| 204 | -N Do not execute a remote command. This is useful for just |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 205 | forwarding ports. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 206 | |
| 207 | -n Redirects stdin from /dev/null (actually, prevents reading from |
| 208 | stdin). This must be used when ssh is run in the background. A |
| 209 | common trick is to use this to run X11 programs on a remote |
| 210 | machine. For example, ssh -n shadows.cs.hut.fi emacs & will |
| 211 | start an emacs on shadows.cs.hut.fi, and the X11 connection will |
| 212 | be automatically forwarded over an encrypted channel. The ssh |
| 213 | program will be put in the background. (This does not work if |
| 214 | ssh needs to ask for a password or passphrase; see also the -f |
| 215 | option.) |
| 216 | |
| 217 | -O ctl_cmd |
| 218 | Control an active connection multiplexing master process. When |
| 219 | the -O option is specified, the ctl_cmd argument is interpreted |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 220 | and passed to the master process. Valid commands are: M-bM-^@M-^\checkM-bM-^@M-^] |
| 221 | (check that the master process is running), M-bM-^@M-^\forwardM-bM-^@M-^] (request |
| 222 | forwardings without command execution), M-bM-^@M-^\cancelM-bM-^@M-^] (cancel |
| 223 | forwardings), M-bM-^@M-^\exitM-bM-^@M-^] (request the master to exit), and M-bM-^@M-^\stopM-bM-^@M-^] |
| 224 | (request the master to stop accepting further multiplexing |
| 225 | requests). |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 226 | |
| 227 | -o option |
| 228 | Can be used to give options in the format used in the |
| 229 | configuration file. This is useful for specifying options for |
| 230 | which there is no separate command-line flag. For full details |
| 231 | of the options listed below, and their possible values, see |
| 232 | ssh_config(5). |
| 233 | |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 234 | AddKeysToAgent |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 235 | AddressFamily |
| 236 | BatchMode |
| 237 | BindAddress |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 238 | CanonicalDomains |
| 239 | CanonicalizeFallbackLocal |
| 240 | CanonicalizeHostname |
| 241 | CanonicalizeMaxDots |
| 242 | CanonicalizePermittedCNAMEs |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 243 | CertificateFile |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 244 | ChallengeResponseAuthentication |
| 245 | CheckHostIP |
| 246 | Cipher |
| 247 | Ciphers |
| 248 | ClearAllForwardings |
| 249 | Compression |
| 250 | CompressionLevel |
| 251 | ConnectionAttempts |
| 252 | ConnectTimeout |
| 253 | ControlMaster |
| 254 | ControlPath |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 255 | ControlPersist |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 256 | DynamicForward |
| 257 | EscapeChar |
| 258 | ExitOnForwardFailure |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 259 | FingerprintHash |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 260 | ForwardAgent |
| 261 | ForwardX11 |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 262 | ForwardX11Timeout |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 263 | ForwardX11Trusted |
| 264 | GatewayPorts |
| 265 | GlobalKnownHostsFile |
| 266 | GSSAPIAuthentication |
| 267 | GSSAPIDelegateCredentials |
| 268 | HashKnownHosts |
| 269 | Host |
| 270 | HostbasedAuthentication |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 271 | HostbasedKeyTypes |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 272 | HostKeyAlgorithms |
| 273 | HostKeyAlias |
| 274 | HostName |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 275 | IdentitiesOnly |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 276 | IdentityAgent |
| 277 | IdentityFile |
| 278 | Include |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 279 | IPQoS |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 280 | KbdInteractiveAuthentication |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 281 | KbdInteractiveDevices |
| 282 | KexAlgorithms |
| 283 | LocalCommand |
| 284 | LocalForward |
| 285 | LogLevel |
| 286 | MACs |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 287 | Match |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 288 | NoHostAuthenticationForLocalhost |
| 289 | NumberOfPasswordPrompts |
| 290 | PasswordAuthentication |
| 291 | PermitLocalCommand |
| 292 | PKCS11Provider |
| 293 | Port |
| 294 | PreferredAuthentications |
| 295 | Protocol |
| 296 | ProxyCommand |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 297 | ProxyJump |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 298 | ProxyUseFdpass |
Greg Hartman | ccacbc9 | 2016-02-03 09:59:44 -0800 | [diff] [blame] | 299 | PubkeyAcceptedKeyTypes |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 300 | PubkeyAuthentication |
| 301 | RekeyLimit |
| 302 | RemoteForward |
| 303 | RequestTTY |
| 304 | RhostsRSAAuthentication |
| 305 | RSAAuthentication |
| 306 | SendEnv |
| 307 | ServerAliveInterval |
| 308 | ServerAliveCountMax |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 309 | StreamLocalBindMask |
| 310 | StreamLocalBindUnlink |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 311 | StrictHostKeyChecking |
| 312 | TCPKeepAlive |
| 313 | Tunnel |
| 314 | TunnelDevice |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 315 | UpdateHostKeys |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 316 | UsePrivilegedPort |
| 317 | User |
| 318 | UserKnownHostsFile |
| 319 | VerifyHostKeyDNS |
| 320 | VisualHostKey |
| 321 | XAuthLocation |
| 322 | |
| 323 | -p port |
| 324 | Port to connect to on the remote host. This can be specified on |
| 325 | a per-host basis in the configuration file. |
| 326 | |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 327 | -Q query_option |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 328 | Queries ssh for the algorithms supported for the specified |
| 329 | version 2. The available features are: cipher (supported |
| 330 | symmetric ciphers), cipher-auth (supported symmetric ciphers that |
| 331 | support authenticated encryption), mac (supported message |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 332 | integrity codes), kex (key exchange algorithms), key (key types), |
| 333 | key-cert (certificate key types), key-plain (non-certificate key |
| 334 | types), and protocol-version (supported SSH protocol versions). |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 335 | |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 336 | -q Quiet mode. Causes most warning and diagnostic messages to be |
| 337 | suppressed. |
| 338 | |
| 339 | -R [bind_address:]port:host:hostport |
Greg Hartman | ccacbc9 | 2016-02-03 09:59:44 -0800 | [diff] [blame] | 340 | -R [bind_address:]port:local_socket |
| 341 | -R remote_socket:host:hostport |
| 342 | -R remote_socket:local_socket |
| 343 | Specifies that connections to the given TCP port or Unix socket |
| 344 | on the remote (server) host are to be forwarded to the given host |
| 345 | and port, or Unix socket, on the local side. This works by |
| 346 | allocating a socket to listen to either a TCP port or to a Unix |
| 347 | socket on the remote side. Whenever a connection is made to this |
| 348 | port or Unix socket, the connection is forwarded over the secure |
| 349 | channel, and a connection is made to either host port hostport, |
| 350 | or local_socket, from the local machine. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 351 | |
| 352 | Port forwardings can also be specified in the configuration file. |
| 353 | Privileged ports can be forwarded only when logging in as root on |
| 354 | the remote machine. IPv6 addresses can be specified by enclosing |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 355 | the address in square brackets. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 356 | |
Greg Hartman | ccacbc9 | 2016-02-03 09:59:44 -0800 | [diff] [blame] | 357 | By default, TCP listening sockets on the server will be bound to |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 358 | the loopback interface only. This may be overridden by |
| 359 | specifying a bind_address. An empty bind_address, or the address |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 360 | M-bM-^@M-^X*M-bM-^@M-^Y, indicates that the remote socket should listen on all |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 361 | interfaces. Specifying a remote bind_address will only succeed |
| 362 | if the server's GatewayPorts option is enabled (see |
| 363 | sshd_config(5)). |
| 364 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 365 | If the port argument is M-bM-^@M-^X0M-bM-^@M-^Y, the listen port will be dynamically |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 366 | allocated on the server and reported to the client at run time. |
| 367 | When used together with -O forward the allocated port will be |
| 368 | printed to the standard output. |
| 369 | |
| 370 | -S ctl_path |
| 371 | Specifies the location of a control socket for connection |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 372 | sharing, or the string M-bM-^@M-^\noneM-bM-^@M-^] to disable connection sharing. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 373 | Refer to the description of ControlPath and ControlMaster in |
| 374 | ssh_config(5) for details. |
| 375 | |
| 376 | -s May be used to request invocation of a subsystem on the remote |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 377 | system. Subsystems facilitate the use of SSH as a secure |
| 378 | transport for other applications (e.g. sftp(1)). The subsystem |
| 379 | is specified as the remote command. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 380 | |
Greg Hartman | ccacbc9 | 2016-02-03 09:59:44 -0800 | [diff] [blame] | 381 | -T Disable pseudo-terminal allocation. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 382 | |
Greg Hartman | ccacbc9 | 2016-02-03 09:59:44 -0800 | [diff] [blame] | 383 | -t Force pseudo-terminal allocation. This can be used to execute |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 384 | arbitrary screen-based programs on a remote machine, which can be |
| 385 | very useful, e.g. when implementing menu services. Multiple -t |
| 386 | options force tty allocation, even if ssh has no local tty. |
| 387 | |
| 388 | -V Display the version number and exit. |
| 389 | |
| 390 | -v Verbose mode. Causes ssh to print debugging messages about its |
| 391 | progress. This is helpful in debugging connection, |
| 392 | authentication, and configuration problems. Multiple -v options |
| 393 | increase the verbosity. The maximum is 3. |
| 394 | |
| 395 | -W host:port |
| 396 | Requests that standard input and output on the client be |
| 397 | forwarded to host on port over the secure channel. Implies -N, |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 398 | -T, ExitOnForwardFailure and ClearAllForwardings, though these |
| 399 | can be overridden in the configuration file or using -o command |
| 400 | line options. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 401 | |
| 402 | -w local_tun[:remote_tun] |
| 403 | Requests tunnel device forwarding with the specified tun(4) |
| 404 | devices between the client (local_tun) and the server |
| 405 | (remote_tun). |
| 406 | |
| 407 | The devices may be specified by numerical ID or the keyword |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 408 | M-bM-^@M-^\anyM-bM-^@M-^], which uses the next available tunnel device. If |
| 409 | remote_tun is not specified, it defaults to M-bM-^@M-^\anyM-bM-^@M-^]. See also the |
| 410 | Tunnel and TunnelDevice directives in ssh_config(5). If the |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 411 | Tunnel directive is unset, it is set to the default tunnel mode, |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 412 | which is M-bM-^@M-^\point-to-pointM-bM-^@M-^]. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 413 | |
| 414 | -X Enables X11 forwarding. This can also be specified on a per-host |
| 415 | basis in a configuration file. |
| 416 | |
| 417 | X11 forwarding should be enabled with caution. Users with the |
| 418 | ability to bypass file permissions on the remote host (for the |
| 419 | user's X authorization database) can access the local X11 display |
| 420 | through the forwarded connection. An attacker may then be able |
| 421 | to perform activities such as keystroke monitoring. |
| 422 | |
| 423 | For this reason, X11 forwarding is subjected to X11 SECURITY |
| 424 | extension restrictions by default. Please refer to the ssh -Y |
| 425 | option and the ForwardX11Trusted directive in ssh_config(5) for |
| 426 | more information. |
| 427 | |
| 428 | -x Disables X11 forwarding. |
| 429 | |
| 430 | -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not |
| 431 | subjected to the X11 SECURITY extension controls. |
| 432 | |
| 433 | -y Send log information using the syslog(3) system module. By |
| 434 | default this information is sent to stderr. |
| 435 | |
| 436 | ssh may additionally obtain configuration data from a per-user |
| 437 | configuration file and a system-wide configuration file. The file format |
| 438 | and configuration options are described in ssh_config(5). |
| 439 | |
| 440 | AUTHENTICATION |
| 441 | The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to |
| 442 | use protocol 2 only, though this can be changed via the Protocol option |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 443 | in ssh_config(5) or the -1 and -2 options (see above). Protocol 1 should |
| 444 | not be used and is only offered to support legacy devices. It suffers |
| 445 | from a number of cryptographic weaknesses and doesn't support many of the |
| 446 | advanced features available for protocol 2. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 447 | |
| 448 | The methods available for authentication are: GSSAPI-based |
| 449 | authentication, host-based authentication, public key authentication, |
| 450 | challenge-response authentication, and password authentication. |
| 451 | Authentication methods are tried in the order specified above, though |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 452 | PreferredAuthentications can be used to change the default order. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 453 | |
| 454 | Host-based authentication works as follows: If the machine the user logs |
| 455 | in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote |
| 456 | machine, and the user names are the same on both sides, or if the files |
| 457 | ~/.rhosts or ~/.shosts exist in the user's home directory on the remote |
| 458 | machine and contain a line containing the name of the client machine and |
| 459 | the name of the user on that machine, the user is considered for login. |
| 460 | Additionally, the server must be able to verify the client's host key |
| 461 | (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, |
| 462 | below) for login to be permitted. This authentication method closes |
| 463 | security holes due to IP spoofing, DNS spoofing, and routing spoofing. |
| 464 | [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the |
| 465 | rlogin/rsh protocol in general, are inherently insecure and should be |
| 466 | disabled if security is desired.] |
| 467 | |
| 468 | Public key authentication works as follows: The scheme is based on |
| 469 | public-key cryptography, using cryptosystems where encryption and |
| 470 | decryption are done using separate keys, and it is unfeasible to derive |
| 471 | the decryption key from the encryption key. The idea is that each user |
| 472 | creates a public/private key pair for authentication purposes. The |
| 473 | server knows the public key, and only the user knows the private key. |
| 474 | ssh implements public key authentication protocol automatically, using |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 475 | one of the DSA, ECDSA, Ed25519 or RSA algorithms. The HISTORY section of |
| 476 | ssl(8) contains a brief discussion of the DSA and RSA algorithms. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 477 | |
| 478 | The file ~/.ssh/authorized_keys lists the public keys that are permitted |
| 479 | for logging in. When the user logs in, the ssh program tells the server |
| 480 | which key pair it would like to use for authentication. The client |
| 481 | proves that it has access to the private key and the server checks that |
| 482 | the corresponding public key is authorized to accept the account. |
| 483 | |
| 484 | The user creates his/her key pair by running ssh-keygen(1). This stores |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 485 | the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (DSA), |
| 486 | ~/.ssh/id_ecdsa (ECDSA), ~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa |
| 487 | (RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1), |
| 488 | ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA), |
| 489 | ~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's |
| 490 | home directory. The user should then copy the public key to |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 491 | ~/.ssh/authorized_keys in his/her home directory on the remote machine. |
| 492 | The authorized_keys file corresponds to the conventional ~/.rhosts file, |
| 493 | and has one key per line, though the lines can be very long. After this, |
| 494 | the user can log in without giving the password. |
| 495 | |
| 496 | A variation on public key authentication is available in the form of |
| 497 | certificate authentication: instead of a set of public/private keys, |
| 498 | signed certificates are used. This has the advantage that a single |
| 499 | trusted certification authority can be used in place of many |
| 500 | public/private keys. See the CERTIFICATES section of ssh-keygen(1) for |
| 501 | more information. |
| 502 | |
| 503 | The most convenient way to use public key or certificate authentication |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 504 | may be with an authentication agent. See ssh-agent(1) and (optionally) |
| 505 | the AddKeysToAgent directive in ssh_config(5) for more information. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 506 | |
| 507 | Challenge-response authentication works as follows: The server sends an |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 508 | arbitrary "challenge" text, and prompts for a response. Examples of |
| 509 | challenge-response authentication include BSD Authentication (see |
| 510 | login.conf(5)) and PAM (some non-OpenBSD systems). |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 511 | |
| 512 | Finally, if other authentication methods fail, ssh prompts the user for a |
| 513 | password. The password is sent to the remote host for checking; however, |
| 514 | since all communications are encrypted, the password cannot be seen by |
| 515 | someone listening on the network. |
| 516 | |
| 517 | ssh automatically maintains and checks a database containing |
| 518 | identification for all hosts it has ever been used with. Host keys are |
| 519 | stored in ~/.ssh/known_hosts in the user's home directory. Additionally, |
| 520 | the file /etc/ssh/ssh_known_hosts is automatically checked for known |
| 521 | hosts. Any new hosts are automatically added to the user's file. If a |
| 522 | host's identification ever changes, ssh warns about this and disables |
| 523 | password authentication to prevent server spoofing or man-in-the-middle |
| 524 | attacks, which could otherwise be used to circumvent the encryption. The |
| 525 | StrictHostKeyChecking option can be used to control logins to machines |
| 526 | whose host key is not known or has changed. |
| 527 | |
| 528 | When the user's identity has been accepted by the server, the server |
Greg Hartman | ccacbc9 | 2016-02-03 09:59:44 -0800 | [diff] [blame] | 529 | either executes the given command in a non-interactive session or, if no |
| 530 | command has been specified, logs into the machine and gives the user a |
| 531 | normal shell as an interactive session. All communication with the |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 532 | remote command or shell will be automatically encrypted. |
| 533 | |
Greg Hartman | ccacbc9 | 2016-02-03 09:59:44 -0800 | [diff] [blame] | 534 | If an interactive session is requested ssh by default will only request a |
| 535 | pseudo-terminal (pty) for interactive sessions when the client has one. |
| 536 | The flags -T and -t can be used to override this behaviour. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 537 | |
Greg Hartman | ccacbc9 | 2016-02-03 09:59:44 -0800 | [diff] [blame] | 538 | If a pseudo-terminal has been allocated the user may use the escape |
| 539 | characters noted below. |
| 540 | |
| 541 | If no pseudo-terminal has been allocated, the session is transparent and |
| 542 | can be used to reliably transfer binary data. On most systems, setting |
| 543 | the escape character to M-bM-^@M-^\noneM-bM-^@M-^] will also make the session transparent |
| 544 | even if a tty is used. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 545 | |
| 546 | The session terminates when the command or shell on the remote machine |
| 547 | exits and all X11 and TCP connections have been closed. |
| 548 | |
| 549 | ESCAPE CHARACTERS |
| 550 | When a pseudo-terminal has been requested, ssh supports a number of |
| 551 | functions through the use of an escape character. |
| 552 | |
| 553 | A single tilde character can be sent as ~~ or by following the tilde by a |
| 554 | character other than those described below. The escape character must |
| 555 | always follow a newline to be interpreted as special. The escape |
| 556 | character can be changed in configuration files using the EscapeChar |
| 557 | configuration directive or on the command line by the -e option. |
| 558 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 559 | The supported escapes (assuming the default M-bM-^@M-^X~M-bM-^@M-^Y) are: |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 560 | |
| 561 | ~. Disconnect. |
| 562 | |
| 563 | ~^Z Background ssh. |
| 564 | |
| 565 | ~# List forwarded connections. |
| 566 | |
| 567 | ~& Background ssh at logout when waiting for forwarded connection / |
| 568 | X11 sessions to terminate. |
| 569 | |
| 570 | ~? Display a list of escape characters. |
| 571 | |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 572 | ~B Send a BREAK to the remote system (only useful if the peer |
| 573 | supports it). |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 574 | |
| 575 | ~C Open command line. Currently this allows the addition of port |
| 576 | forwardings using the -L, -R and -D options (see above). It also |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 577 | allows the cancellation of existing port-forwardings with |
| 578 | -KL[bind_address:]port for local, -KR[bind_address:]port for |
| 579 | remote and -KD[bind_address:]port for dynamic port-forwardings. |
| 580 | !command allows the user to execute a local command if the |
| 581 | PermitLocalCommand option is enabled in ssh_config(5). Basic |
| 582 | help is available, using the -h option. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 583 | |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 584 | ~R Request rekeying of the connection (only useful if the peer |
| 585 | supports it). |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 586 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 587 | ~V Decrease the verbosity (LogLevel) when errors are being written |
| 588 | to stderr. |
| 589 | |
| 590 | ~v Increase the verbosity (LogLevel) when errors are being written |
| 591 | to stderr. |
| 592 | |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 593 | TCP FORWARDING |
| 594 | Forwarding of arbitrary TCP connections over the secure channel can be |
| 595 | specified either on the command line or in a configuration file. One |
| 596 | possible application of TCP forwarding is a secure connection to a mail |
| 597 | server; another is going through firewalls. |
| 598 | |
| 599 | In the example below, we look at encrypting communication between an IRC |
| 600 | client and server, even though the IRC server does not directly support |
| 601 | encrypted communications. This works as follows: the user connects to |
| 602 | the remote host using ssh, specifying a port to be used to forward |
| 603 | connections to the remote server. After that it is possible to start the |
| 604 | service which is to be encrypted on the client machine, connecting to the |
| 605 | same local port, and ssh will encrypt and forward the connection. |
| 606 | |
| 607 | The following example tunnels an IRC session from client machine |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 608 | M-bM-^@M-^\127.0.0.1M-bM-^@M-^] (localhost) to remote server M-bM-^@M-^\server.example.comM-bM-^@M-^]: |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 609 | |
| 610 | $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 |
| 611 | $ irc -c '#users' -p 1234 pinky 127.0.0.1 |
| 612 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 613 | This tunnels a connection to IRC server M-bM-^@M-^\server.example.comM-bM-^@M-^], joining |
| 614 | channel M-bM-^@M-^\#usersM-bM-^@M-^], nickname M-bM-^@M-^\pinkyM-bM-^@M-^], using port 1234. It doesn't matter |
| 615 | which port is used, as long as it's greater than 1023 (remember, only |
| 616 | root can open sockets on privileged ports) and doesn't conflict with any |
| 617 | ports already in use. The connection is forwarded to port 6667 on the |
| 618 | remote server, since that's the standard port for IRC services. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 619 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 620 | The -f option backgrounds ssh and the remote command M-bM-^@M-^\sleep 10M-bM-^@M-^] is |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 621 | specified to allow an amount of time (10 seconds, in the example) to |
| 622 | start the service which is to be tunnelled. If no connections are made |
| 623 | within the time specified, ssh will exit. |
| 624 | |
| 625 | X11 FORWARDING |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 626 | If the ForwardX11 variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (or see the description of the |
| 627 | -X, -x, and -Y options above) and the user is using X11 (the DISPLAY |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 628 | environment variable is set), the connection to the X11 display is |
| 629 | automatically forwarded to the remote side in such a way that any X11 |
| 630 | programs started from the shell (or command) will go through the |
| 631 | encrypted channel, and the connection to the real X server will be made |
| 632 | from the local machine. The user should not manually set DISPLAY. |
| 633 | Forwarding of X11 connections can be configured on the command line or in |
| 634 | configuration files. |
| 635 | |
| 636 | The DISPLAY value set by ssh will point to the server machine, but with a |
| 637 | display number greater than zero. This is normal, and happens because |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 638 | ssh creates a M-bM-^@M-^\proxyM-bM-^@M-^] X server on the server machine for forwarding the |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 639 | connections over the encrypted channel. |
| 640 | |
| 641 | ssh will also automatically set up Xauthority data on the server machine. |
| 642 | For this purpose, it will generate a random authorization cookie, store |
| 643 | it in Xauthority on the server, and verify that any forwarded connections |
| 644 | carry this cookie and replace it by the real cookie when the connection |
| 645 | is opened. The real authentication cookie is never sent to the server |
| 646 | machine (and no cookies are sent in the plain). |
| 647 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 648 | If the ForwardAgent variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (or see the description of |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 649 | the -A and -a options above) and the user is using an authentication |
| 650 | agent, the connection to the agent is automatically forwarded to the |
| 651 | remote side. |
| 652 | |
| 653 | VERIFYING HOST KEYS |
| 654 | When connecting to a server for the first time, a fingerprint of the |
| 655 | server's public key is presented to the user (unless the option |
| 656 | StrictHostKeyChecking has been disabled). Fingerprints can be determined |
| 657 | using ssh-keygen(1): |
| 658 | |
| 659 | $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key |
| 660 | |
| 661 | If the fingerprint is already known, it can be matched and the key can be |
Greg Hartman | ccacbc9 | 2016-02-03 09:59:44 -0800 | [diff] [blame] | 662 | accepted or rejected. If only legacy (MD5) fingerprints for the server |
| 663 | are available, the ssh-keygen(1) -E option may be used to downgrade the |
| 664 | fingerprint algorithm to match. |
| 665 | |
| 666 | Because of the difficulty of comparing host keys just by looking at |
| 667 | fingerprint strings, there is also support to compare host keys visually, |
| 668 | using random art. By setting the VisualHostKey option to M-bM-^@M-^\yesM-bM-^@M-^], a small |
| 669 | ASCII graphic gets displayed on every login to a server, no matter if the |
| 670 | session itself is interactive or not. By learning the pattern a known |
| 671 | server produces, a user can easily find out that the host key has changed |
| 672 | when a completely different pattern is displayed. Because these patterns |
| 673 | are not unambiguous however, a pattern that looks similar to the pattern |
| 674 | remembered only gives a good probability that the host key is the same, |
| 675 | not guaranteed proof. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 676 | |
| 677 | To get a listing of the fingerprints along with their random art for all |
| 678 | known hosts, the following command line can be used: |
| 679 | |
| 680 | $ ssh-keygen -lv -f ~/.ssh/known_hosts |
| 681 | |
| 682 | If the fingerprint is unknown, an alternative method of verification is |
| 683 | available: SSH fingerprints verified by DNS. An additional resource |
| 684 | record (RR), SSHFP, is added to a zonefile and the connecting client is |
| 685 | able to match the fingerprint with that of the key presented. |
| 686 | |
| 687 | In this example, we are connecting a client to a server, |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 688 | M-bM-^@M-^\host.example.comM-bM-^@M-^]. The SSHFP resource records should first be added to |
| 689 | the zonefile for host.example.com: |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 690 | |
| 691 | $ ssh-keygen -r host.example.com. |
| 692 | |
| 693 | The output lines will have to be added to the zonefile. To check that |
| 694 | the zone is answering fingerprint queries: |
| 695 | |
| 696 | $ dig -t SSHFP host.example.com |
| 697 | |
| 698 | Finally the client connects: |
| 699 | |
| 700 | $ ssh -o "VerifyHostKeyDNS ask" host.example.com |
| 701 | [...] |
| 702 | Matching host key fingerprint found in DNS. |
| 703 | Are you sure you want to continue connecting (yes/no)? |
| 704 | |
| 705 | See the VerifyHostKeyDNS option in ssh_config(5) for more information. |
| 706 | |
| 707 | SSH-BASED VIRTUAL PRIVATE NETWORKS |
| 708 | ssh contains support for Virtual Private Network (VPN) tunnelling using |
| 709 | the tun(4) network pseudo-device, allowing two networks to be joined |
| 710 | securely. The sshd_config(5) configuration option PermitTunnel controls |
| 711 | whether the server supports this, and at what level (layer 2 or 3 |
| 712 | traffic). |
| 713 | |
| 714 | The following example would connect client network 10.0.50.0/24 with |
| 715 | remote network 10.0.99.0/24 using a point-to-point connection from |
| 716 | 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway |
| 717 | to the remote network, at 192.168.1.15, allows it. |
| 718 | |
| 719 | On the client: |
| 720 | |
| 721 | # ssh -f -w 0:1 192.168.1.15 true |
| 722 | # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 |
| 723 | # route add 10.0.99.0/24 10.1.1.2 |
| 724 | |
| 725 | On the server: |
| 726 | |
| 727 | # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 |
| 728 | # route add 10.0.50.0/24 10.1.1.1 |
| 729 | |
| 730 | Client access may be more finely tuned via the /root/.ssh/authorized_keys |
| 731 | file (see below) and the PermitRootLogin server option. The following |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 732 | entry would permit connections on tun(4) device 1 from user M-bM-^@M-^\janeM-bM-^@M-^] and on |
| 733 | tun device 2 from user M-bM-^@M-^\johnM-bM-^@M-^], if PermitRootLogin is set to |
| 734 | M-bM-^@M-^\forced-commands-onlyM-bM-^@M-^]: |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 735 | |
| 736 | tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane |
| 737 | tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john |
| 738 | |
| 739 | Since an SSH-based setup entails a fair amount of overhead, it may be |
| 740 | more suited to temporary setups, such as for wireless VPNs. More |
| 741 | permanent VPNs are better provided by tools such as ipsecctl(8) and |
| 742 | isakmpd(8). |
| 743 | |
| 744 | ENVIRONMENT |
| 745 | ssh will normally set the following environment variables: |
| 746 | |
| 747 | DISPLAY The DISPLAY variable indicates the location of the |
| 748 | X11 server. It is automatically set by ssh to |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 749 | point to a value of the form M-bM-^@M-^\hostname:nM-bM-^@M-^], where |
| 750 | M-bM-^@M-^\hostnameM-bM-^@M-^] indicates the host where the shell runs, |
| 751 | and M-bM-^@M-^XnM-bM-^@M-^Y is an integer M-bM-^IM-% 1. ssh uses this special |
| 752 | value to forward X11 connections over the secure |
| 753 | channel. The user should normally not set DISPLAY |
| 754 | explicitly, as that will render the X11 connection |
| 755 | insecure (and will require the user to manually |
| 756 | copy any required authorization cookies). |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 757 | |
| 758 | HOME Set to the path of the user's home directory. |
| 759 | |
| 760 | LOGNAME Synonym for USER; set for compatibility with |
| 761 | systems that use this variable. |
| 762 | |
| 763 | MAIL Set to the path of the user's mailbox. |
| 764 | |
| 765 | PATH Set to the default PATH, as specified when |
| 766 | compiling ssh. |
| 767 | |
| 768 | SSH_ASKPASS If ssh needs a passphrase, it will read the |
| 769 | passphrase from the current terminal if it was run |
| 770 | from a terminal. If ssh does not have a terminal |
| 771 | associated with it but DISPLAY and SSH_ASKPASS are |
| 772 | set, it will execute the program specified by |
| 773 | SSH_ASKPASS and open an X11 window to read the |
| 774 | passphrase. This is particularly useful when |
| 775 | calling ssh from a .xsession or related script. |
| 776 | (Note that on some machines it may be necessary to |
| 777 | redirect the input from /dev/null to make this |
| 778 | work.) |
| 779 | |
| 780 | SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to |
| 781 | communicate with the agent. |
| 782 | |
| 783 | SSH_CONNECTION Identifies the client and server ends of the |
| 784 | connection. The variable contains four space- |
| 785 | separated values: client IP address, client port |
| 786 | number, server IP address, and server port number. |
| 787 | |
| 788 | SSH_ORIGINAL_COMMAND This variable contains the original command line if |
| 789 | a forced command is executed. It can be used to |
| 790 | extract the original arguments. |
| 791 | |
| 792 | SSH_TTY This is set to the name of the tty (path to the |
| 793 | device) associated with the current shell or |
| 794 | command. If the current session has no tty, this |
| 795 | variable is not set. |
| 796 | |
| 797 | TZ This variable is set to indicate the present time |
| 798 | zone if it was set when the daemon was started |
| 799 | (i.e. the daemon passes the value on to new |
| 800 | connections). |
| 801 | |
| 802 | USER Set to the name of the user logging in. |
| 803 | |
| 804 | Additionally, ssh reads ~/.ssh/environment, and adds lines of the format |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 805 | M-bM-^@M-^\VARNAME=valueM-bM-^@M-^] to the environment if the file exists and users are |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 806 | allowed to change their environment. For more information, see the |
| 807 | PermitUserEnvironment option in sshd_config(5). |
| 808 | |
| 809 | FILES |
| 810 | ~/.rhosts |
| 811 | This file is used for host-based authentication (see above). On |
| 812 | some machines this file may need to be world-readable if the |
| 813 | user's home directory is on an NFS partition, because sshd(8) |
| 814 | reads it as root. Additionally, this file must be owned by the |
| 815 | user, and must not have write permissions for anyone else. The |
| 816 | recommended permission for most machines is read/write for the |
| 817 | user, and not accessible by others. |
| 818 | |
| 819 | ~/.shosts |
| 820 | This file is used in exactly the same way as .rhosts, but allows |
| 821 | host-based authentication without permitting login with |
| 822 | rlogin/rsh. |
| 823 | |
| 824 | ~/.ssh/ |
| 825 | This directory is the default location for all user-specific |
| 826 | configuration and authentication information. There is no |
| 827 | general requirement to keep the entire contents of this directory |
| 828 | secret, but the recommended permissions are read/write/execute |
| 829 | for the user, and not accessible by others. |
| 830 | |
| 831 | ~/.ssh/authorized_keys |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 832 | Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used |
| 833 | for logging in as this user. The format of this file is |
| 834 | described in the sshd(8) manual page. This file is not highly |
| 835 | sensitive, but the recommended permissions are read/write for the |
| 836 | user, and not accessible by others. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 837 | |
| 838 | ~/.ssh/config |
| 839 | This is the per-user configuration file. The file format and |
| 840 | configuration options are described in ssh_config(5). Because of |
| 841 | the potential for abuse, this file must have strict permissions: |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 842 | read/write for the user, and not writable by others. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 843 | |
| 844 | ~/.ssh/environment |
| 845 | Contains additional definitions for environment variables; see |
| 846 | ENVIRONMENT, above. |
| 847 | |
| 848 | ~/.ssh/identity |
| 849 | ~/.ssh/id_dsa |
| 850 | ~/.ssh/id_ecdsa |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 851 | ~/.ssh/id_ed25519 |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 852 | ~/.ssh/id_rsa |
| 853 | Contains the private key for authentication. These files contain |
| 854 | sensitive data and should be readable by the user but not |
| 855 | accessible by others (read/write/execute). ssh will simply |
| 856 | ignore a private key file if it is accessible by others. It is |
| 857 | possible to specify a passphrase when generating the key which |
| 858 | will be used to encrypt the sensitive part of this file using |
| 859 | 3DES. |
| 860 | |
| 861 | ~/.ssh/identity.pub |
| 862 | ~/.ssh/id_dsa.pub |
| 863 | ~/.ssh/id_ecdsa.pub |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 864 | ~/.ssh/id_ed25519.pub |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 865 | ~/.ssh/id_rsa.pub |
| 866 | Contains the public key for authentication. These files are not |
| 867 | sensitive and can (but need not) be readable by anyone. |
| 868 | |
| 869 | ~/.ssh/known_hosts |
| 870 | Contains a list of host keys for all hosts the user has logged |
| 871 | into that are not already in the systemwide list of known host |
| 872 | keys. See sshd(8) for further details of the format of this |
| 873 | file. |
| 874 | |
| 875 | ~/.ssh/rc |
| 876 | Commands in this file are executed by ssh when the user logs in, |
| 877 | just before the user's shell (or command) is started. See the |
| 878 | sshd(8) manual page for more information. |
| 879 | |
| 880 | /etc/hosts.equiv |
| 881 | This file is for host-based authentication (see above). It |
| 882 | should only be writable by root. |
| 883 | |
| 884 | /etc/shosts.equiv |
| 885 | This file is used in exactly the same way as hosts.equiv, but |
| 886 | allows host-based authentication without permitting login with |
| 887 | rlogin/rsh. |
| 888 | |
| 889 | /etc/ssh/ssh_config |
| 890 | Systemwide configuration file. The file format and configuration |
| 891 | options are described in ssh_config(5). |
| 892 | |
| 893 | /etc/ssh/ssh_host_key |
| 894 | /etc/ssh/ssh_host_dsa_key |
| 895 | /etc/ssh/ssh_host_ecdsa_key |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 896 | /etc/ssh/ssh_host_ed25519_key |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 897 | /etc/ssh/ssh_host_rsa_key |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 898 | These files contain the private parts of the host keys and are |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 899 | used for host-based authentication. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 900 | |
| 901 | /etc/ssh/ssh_known_hosts |
| 902 | Systemwide list of known host keys. This file should be prepared |
| 903 | by the system administrator to contain the public host keys of |
| 904 | all machines in the organization. It should be world-readable. |
| 905 | See sshd(8) for further details of the format of this file. |
| 906 | |
| 907 | /etc/ssh/sshrc |
| 908 | Commands in this file are executed by ssh when the user logs in, |
| 909 | just before the user's shell (or command) is started. See the |
| 910 | sshd(8) manual page for more information. |
| 911 | |
| 912 | EXIT STATUS |
| 913 | ssh exits with the exit status of the remote command or with 255 if an |
| 914 | error occurred. |
| 915 | |
| 916 | SEE ALSO |
| 917 | scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 918 | tun(4), ssh_config(5), ssh-keysign(8), sshd(8) |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 919 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 920 | STANDARDS |
| 921 | S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned |
| 922 | Numbers, RFC 4250, January 2006. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 923 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 924 | T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture, |
| 925 | RFC 4251, January 2006. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 926 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 927 | T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol, |
| 928 | RFC 4252, January 2006. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 929 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 930 | T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer |
| 931 | Protocol, RFC 4253, January 2006. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 932 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 933 | T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC |
| 934 | 4254, January 2006. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 935 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 936 | J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell |
| 937 | (SSH) Key Fingerprints, RFC 4255, January 2006. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 938 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 939 | F. Cusack and M. Forssen, Generic Message Exchange Authentication for the |
| 940 | Secure Shell Protocol (SSH), RFC 4256, January 2006. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 941 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 942 | J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break |
| 943 | Extension, RFC 4335, January 2006. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 944 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 945 | M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport |
| 946 | Layer Encryption Modes, RFC 4344, January 2006. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 947 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 948 | B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport |
| 949 | Layer Protocol, RFC 4345, January 2006. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 950 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 951 | M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for |
| 952 | the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 953 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 954 | J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File |
| 955 | Format, RFC 4716, November 2006. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 956 | |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 957 | D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the |
| 958 | Secure Shell Transport Layer, RFC 5656, December 2009. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 959 | |
| 960 | A. Perrig and D. Song, Hash Visualization: a New Technique to improve |
| 961 | Real-World Security, 1999, International Workshop on Cryptographic |
| 962 | Techniques and E-Commerce (CrypTEC '99). |
| 963 | |
| 964 | AUTHORS |
| 965 | OpenSSH is a derivative of the original and free ssh 1.2.12 release by |
| 966 | Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo |
| 967 | de Raadt and Dug Song removed many bugs, re-added newer features and |
| 968 | created OpenSSH. Markus Friedl contributed the support for SSH protocol |
| 969 | versions 1.5 and 2.0. |
| 970 | |
Greg Hartman | 9768ca4 | 2017-06-22 20:49:52 -0700 | [diff] [blame] | 971 | OpenBSD 6.0 July 16, 2016 OpenBSD 6.0 |