github.com/lmorg/murex@v0.0.0-20240217211045-e081c89cd4ef/utils/man/test_ssh.txt

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1 SSH(1) General Commands Manual SSH(1)
2
3 NAME
4 ssh — OpenSSH remote login client
5
6 SYNOPSIS
7 ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port] [-E log_file] [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file] [-J destination] [-L address]
8 [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-P tag] [-p port] [-Q query_option] [-R address] [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] destination [command [argument ...]]
9
10 DESCRIPTION
11 ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine. It is intended to provide secure encrypted communications between two untrusted hosts over an insecure network.
12 X11 connections, arbitrary TCP ports and Unix-domain sockets can also be forwarded over the secure channel.
13
14 ssh connects and logs into the specified destination, which may be specified as either [user@]hostname or a URI of the form ssh://[user@]hostname[:port]. The user must prove their identity to the remote machine using one of
15 several methods (see below).
16
17 If a command is specified, it will be executed on the remote host instead of a login shell. A complete command line may be specified as command, or it may have additional arguments. If supplied, the arguments will be ap‐
18 pended to the command, separated by spaces, before it is sent to the server to be executed.
19
20 The options are as follows:
21
22 -4 Forces ssh to use IPv4 addresses only.
23
24 -6 Forces ssh to use IPv6 addresses only.
25
26 -A Enables forwarding of connections from an authentication agent such as ssh-agent(1). This can also be specified on a per-host basis in a configuration file.
27
28 Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the agent's Unix-domain socket) can access the local agent through the forwarded connection.
29 An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent. A safer alternative may be to use a
30 jump host (see -J).
31
32 -a Disables forwarding of the authentication agent connection.
33
34 -B bind_interface
35 Bind to the address of bind_interface before attempting to connect to the destination host. This is only useful on systems with more than one address.
36
37 -b bind_address
38 Use bind_address on the local machine as the source address of the connection. Only useful on systems with more than one address.
39
40 -C Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11, TCP and Unix-domain connections). The compression algorithm is the same used by gzip(1). Compression is desirable on mo‐
41 dem lines and other slow connections, but will only slow down things on fast networks. The default value can be set on a host-by-host basis in the configuration files; see the Compression option in ssh_config(5).
42
43 -c cipher_spec
44 Selects the cipher specification for encrypting the session. cipher_spec is a comma-separated list of ciphers listed in order of preference. See the Ciphers keyword in ssh_config(5) for more information.
45
46 -D [bind_address:]port
47 Specifies a local “dynamic” application-level port forwarding. This works by allocating a socket to listen to port on the local side, optionally bound to the specified bind_address. Whenever a connection is made to
48 this port, the connection is forwarded over the secure channel, and the application protocol is then used to determine where to connect to from the remote machine. Currently the SOCKS4 and SOCKS5 protocols are sup‐
49 ported, and ssh will act as a SOCKS server. Only root can forward privileged ports. Dynamic port forwardings can also be specified in the configuration file.
50
51 IPv6 addresses can be specified by enclosing the address in square brackets. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the GatewayPorts setting. However,
52 an explicit bind_address may be used to bind the connection to a specific address. The bind_address of “localhost” indicates that the listening port be bound for local use only, while an empty address or ‘*’ indicates
53 that the port should be available from all interfaces.
54
55 -E log_file
56 Append debug logs to log_file instead of standard error.
57
58 -e escape_char
59 Sets the escape character for sessions with a pty (default: ‘˜’). The escape character is only recognized at the beginning of a line. The escape character followed by a dot (‘.’) closes the connection; followed by
60 control-Z suspends the connection; and followed by itself sends the escape character once. Setting the character to “none” disables any escapes and makes the session fully transparent.
61
62 -F configfile
63 Specifies an alternative per-user configuration file. If a configuration file is given on the command line, the system-wide configuration file (/etc/ssh/ssh_config) will be ignored. The default for the per-user con‐
64 figuration file is ˜/.ssh/config. If set to “none”, no configuration files will be read.
65
66 -f Requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the background. This implies -n. The recommended way to
67 start X11 programs at a remote site is with something like ssh -f host xterm.
68
69 If the ExitOnForwardFailure configuration option is set to “yes”, then a client started with -f will wait for all remote port forwards to be successfully established before placing itself in the background. Refer to
70 the description of ForkAfterAuthentication in ssh_config(5) for details.
71
72 -G Causes ssh to print its configuration after evaluating Host and Match blocks and exit.
73
74 -g Allows remote hosts to connect to local forwarded ports. If used on a multiplexed connection, then this option must be specified on the master process.
75
76 -I pkcs11
77 Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing keys for user authentication.
78
79 -i identity_file
80 Selects a file from which the identity (private key) for public key authentication is read. You can also specify a public key file to use the corresponding private key that is loaded in ssh-agent(1) when the private
81 key file is not present locally. The default is ˜/.ssh/id_rsa, ˜/.ssh/id_ecdsa, ˜/.ssh/id_ecdsa_sk, ˜/.ssh/id_ed25519, ˜/.ssh/id_ed25519_sk and ˜/.ssh/id_dsa. Identity files may also be specified on a per-host basis
82 in the configuration file. It is possible to have multiple -i options (and multiple identities specified in configuration files). If no certificates have been explicitly specified by the CertificateFile directive,
83 ssh will also try to load certificate information from the filename obtained by appending -cert.pub to identity filenames.
84
85 -J destination
86 Connect to the target host by first making an ssh connection to the jump host described by destination and then establishing a TCP forwarding to the ultimate destination from there. Multiple jump hops may be specified
87 separated by comma characters. This is a shortcut to specify a ProxyJump configuration directive. Note that configuration directives supplied on the command-line generally apply to the destination host and not any
88 specified jump hosts. Use ˜/.ssh/config to specify configuration for jump hosts.
89
90 -K Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI credentials to the server.
91
92 -k Disables forwarding (delegation) of GSSAPI credentials to the server.
93
94 -L [bind_address:]port:host:hostport
95 -L [bind_address:]port:remote_socket
96 -L local_socket:host:hostport
97 -L local_socket:remote_socket
98 Specifies that connections to the given TCP port or Unix socket on the local (client) host are to be forwarded to the given host and port, or Unix socket, on the remote side. This works by allocating a socket to lis‐
99 ten to either a TCP port on the local side, optionally bound to the specified bind_address, or to a Unix socket. Whenever a connection is made to the local port or socket, the connection is forwarded over the secure
100 channel, and a connection is made to either host port hostport, or the Unix socket remote_socket, from the remote machine.
101
102 Port forwardings can also be specified in the configuration file. Only the superuser can forward privileged ports. IPv6 addresses can be specified by enclosing the address in square brackets.
103
104 By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address of “localhost” indicates
105 that the listening port be bound for local use only, while an empty address or ‘*’ indicates that the port should be available from all interfaces.
106
107 -l login_name
108 Specifies the user to log in as on the remote machine. This also may be specified on a per-host basis in the configuration file.
109
110 -M Places the ssh client into “master” mode for connection sharing. Multiple -M options places ssh into “master” mode but with confirmation required using ssh-askpass(1) before each operation that changes the multiplex‐
111 ing state (e.g. opening a new session). Refer to the description of ControlMaster in ssh_config(5) for details.
112
113 -m mac_spec
114 A comma-separated list of MAC (message authentication code) algorithms, specified in order of preference. See the MACs keyword in ssh_config(5) for more information.
115
116 -N Do not execute a remote command. This is useful for just forwarding ports. Refer to the description of SessionType in ssh_config(5) for details.
117
118 -n Redirects stdin from /dev/null (actually, prevents reading from stdin). This must be used when ssh is run in the background. A common trick is to use this to run X11 programs on a remote machine. For example, ssh -n
119 shadows.cs.hut.fi emacs & will start an emacs on shadows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted channel. The ssh program will be put in the background. (This does not work
120 if ssh needs to ask for a password or passphrase; see also the -f option.) Refer to the description of StdinNull in ssh_config(5) for details.
121
122 -O ctl_cmd
123 Control an active connection multiplexing master process. When the -O option is specified, the ctl_cmd argument is interpreted and passed to the master process. Valid commands are: “check” (check that the master
124 process is running), “forward” (request forwardings without command execution), “cancel” (cancel forwardings), “exit” (request the master to exit), and “stop” (request the master to stop accepting further multiplexing
125 requests).
126
127 -o option
128 Can be used to give options in the format used in the configuration file. This is useful for specifying options for which there is no separate command-line flag. For full details of the options listed below, and
129 their possible values, see ssh_config(5).
130
131 AddKeysToAgent
132 AddressFamily
133 BatchMode
134 BindAddress
135 CanonicalDomains
136 CanonicalizeFallbackLocal
137 CanonicalizeHostname
138 CanonicalizeMaxDots
139 CanonicalizePermittedCNAMEs
140 CASignatureAlgorithms
141 CertificateFile
142 CheckHostIP
143 Ciphers
144 ClearAllForwardings
145 Compression
146 ConnectionAttempts
147 ConnectTimeout
148 ControlMaster
149 ControlPath
150 ControlPersist
151 DynamicForward
152 EnableEscapeCommandline
153 EscapeChar
154 ExitOnForwardFailure
155 FingerprintHash
156 ForkAfterAuthentication
157 ForwardAgent
158 ForwardX11
159 ForwardX11Timeout
160 ForwardX11Trusted
161 GatewayPorts
162 GlobalKnownHostsFile
163 GSSAPIAuthentication
164 GSSAPIDelegateCredentials
165 HashKnownHosts
166 Host
167 HostbasedAcceptedAlgorithms
168 HostbasedAuthentication
169 HostKeyAlgorithms
170 HostKeyAlias
171 Hostname
172 IdentitiesOnly
173 IdentityAgent
174 IdentityFile
175 IPQoS
176 KbdInteractiveAuthentication
177 KbdInteractiveDevices
178 KexAlgorithms
179 KnownHostsCommand
180 LocalCommand
181 LocalForward
182 LogLevel
183 MACs
184 Match
185 NoHostAuthenticationForLocalhost
186 NumberOfPasswordPrompts
187 PasswordAuthentication
188 PermitLocalCommand
189 PermitRemoteOpen
190 PKCS11Provider
191 Port
192 PreferredAuthentications
193 ProxyCommand
194 ProxyJump
195 ProxyUseFdpass
196 PubkeyAcceptedAlgorithms
197 PubkeyAuthentication
198 RekeyLimit
199 RemoteCommand
200 RemoteForward
201 RequestTTY
202 RequiredRSASize
203 SendEnv
204 ServerAliveInterval
205 ServerAliveCountMax
206 SessionType
207 SetEnv
208 StdinNull
209 StreamLocalBindMask
210 StreamLocalBindUnlink
211 StrictHostKeyChecking
212 TCPKeepAlive
213 Tunnel
214 TunnelDevice
215 UpdateHostKeys
216 User
217 UserKnownHostsFile
218 VerifyHostKeyDNS
219 VisualHostKey
220 XAuthLocation
221
222 -P tag Specify a tag name that may be used to select configuration in ssh_config(5). Refer to the Tag and Match keywords in ssh_config(5) for more information.
223 -p port
224 Port to connect to on the remote host. This can be specified on a per-host basis in the configuration file.
225
226 -Q query_option
227 Queries for the algorithms supported by one of the following features: cipher (supported symmetric ciphers), cipher-auth (supported symmetric ciphers that support authenticated encryption), help (supported query terms
228 for use with the -Q flag), mac (supported message integrity codes), kex (key exchange algorithms), key (key types), key-ca-sign (valid CA signature algorithms for certificates), key-cert (certificate key types),
229 key-plain (non-certificate key types), key-sig (all key types and signature algorithms), protocol-version (supported SSH protocol versions), and sig (supported signature algorithms). Alternatively, any keyword from
230 ssh_config(5) or sshd_config(5) that takes an algorithm list may be used as an alias for the corresponding query_option.
231
232 -q Quiet mode. Causes most warning and diagnostic messages to be suppressed.
233
234 -R [bind_address:]port:host:hostport
235 -R [bind_address:]port:local_socket
236 -R remote_socket:host:hostport
237 -R remote_socket:local_socket
238 -R [bind_address:]port
239 Specifies that connections to the given TCP port or Unix socket on the remote (server) host are to be forwarded to the local side.
240
241 This works by allocating a socket to listen to either a TCP port or to a Unix socket on the remote side. Whenever a connection is made to this port or Unix socket, the connection is forwarded over the secure channel,
242 and a connection is made from the local machine to either an explicit destination specified by host port hostport, or local_socket, or, if no explicit destination was specified, ssh will act as a SOCKS 4/5 proxy and
243 forward connections to the destinations requested by the remote SOCKS client.
244
245 Port forwardings can also be specified in the configuration file. Privileged ports can be forwarded only when logging in as root on the remote machine. IPv6 addresses can be specified by enclosing the address in
246 square brackets.
247
248 By default, TCP listening sockets on the server will be bound to the loopback interface only. This may be overridden by specifying a bind_address. An empty bind_address, or the address ‘*’, indicates that the remote
249 socket should listen on all interfaces. Specifying a remote bind_address will only succeed if the server's GatewayPorts option is enabled (see sshd_config(5)).
250
251 If the port argument is ‘0’, the listen port will be dynamically allocated on the server and reported to the client at run time. When used together with -O forward, the allocated port will be printed to the standard
252 output.
253
254 -S ctl_path
255 Specifies the location of a control socket for connection sharing, or the string “none” to disable connection sharing. Refer to the description of ControlPath and ControlMaster in ssh_config(5) for details.
256
257 -s May be used to request invocation of a subsystem on the remote system. Subsystems facilitate the use of SSH as a secure transport for other applications (e.g. sftp(1)). The subsystem is specified as the remote com‐
258 mand. Refer to the description of SessionType in ssh_config(5) for details.
259
260 -T Disable pseudo-terminal allocation.
261
262 -t Force pseudo-terminal allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, e.g. when implementing menu services. Multiple -t options force tty alloca‐
263 tion, even if ssh has no local tty.
264
265 -V Display the version number and exit.
266
267 -v Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple -v options increase the verbosity. The maximum
268 is 3.
269
270 -W host:port
271 Requests that standard input and output on the client be forwarded to host on port over the secure channel. Implies -N, -T, ExitOnForwardFailure and ClearAllForwardings, though these can be overridden in the configu‐
272 ration file or using -o command line options.
273
274 -w local_tun[:remote_tun]
275 Requests tunnel device forwarding with the specified tun(4) devices between the client (local_tun) and the server (remote_tun).
276
277 The devices may be specified by numerical ID or the keyword “any”, which uses the next available tunnel device. If remote_tun is not specified, it defaults to “any”. See also the Tunnel and TunnelDevice directives in
278 ssh_config(5).
279
280 If the Tunnel directive is unset, it will be set to the default tunnel mode, which is “point-to-point”. If a different Tunnel forwarding mode it desired, then it should be specified before -w.
281
282 -X Enables X11 forwarding. This can also be specified on a per-host basis in a configuration file.
283
284 X11 forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the user's X authorization database) can access the local X11 display through the forwarded con‐
285 nection. An attacker may then be able to perform activities such as keystroke monitoring.
286
287 For this reason, X11 forwarding is subjected to X11 SECURITY extension restrictions by default. Refer to the ssh -Y option and the ForwardX11Trusted directive in ssh_config(5) for more information.
288
289 -x Disables X11 forwarding.
290
291 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY extension controls.
292
293 -y Send log information using the syslog(3) system module. By default this information is sent to stderr.
294
295 ssh may additionally obtain configuration data from a per-user configuration file and a system-wide configuration file. The file format and configuration options are described in ssh_config(5).
296
297 AUTHENTICATION
298 The OpenSSH SSH client supports SSH protocol 2.
299
300 The methods available for authentication are: GSSAPI-based authentication, host-based authentication, public key authentication, keyboard-interactive authentication, and password authentication. Authentication methods are
301 tried in the order specified above, though PreferredAuthentications can be used to change the default order.
302
303 Host-based authentication works as follows: If the machine the user logs in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the remote machine, the user is non-root and the user names are the same on both sides,
304 or if the files ˜/.rhosts or ˜/.shosts exist in the user's home directory on the remote machine and contain a line containing the name of the client machine and the name of the user on that machine, the user is considered for
305 login. Additionally, the server must be able to verify the client's host key (see the description of /etc/ssh/ssh_known_hosts and ˜/.ssh/known_hosts, below) for login to be permitted. This authentication method closes secu‐
306 rity holes due to IP spoofing, DNS spoofing, and routing spoofing. [Note to the administrator: /etc/hosts.equiv, ˜/.rhosts, and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is
307 desired.]
308
309 Public key authentication works as follows: The scheme is based on public-key cryptography, using cryptosystems where encryption and decryption are done using separate keys, and it is unfeasible to derive the decryption key
310 from the encryption key. The idea is that each user creates a public/private key pair for authentication purposes. The server knows the public key, and only the user knows the private key. ssh implements public key authen‐
311 tication protocol automatically, using one of the DSA, ECDSA, Ed25519 or RSA algorithms. The HISTORY section of ssl(8) contains a brief discussion of the DSA and RSA algorithms.
312
313 The file ˜/.ssh/authorized_keys lists the public keys that are permitted for logging in. When the user logs in, the ssh program tells the server which key pair it would like to use for authentication. The client proves that
314 it has access to the private key and the server checks that the corresponding public key is authorized to accept the account.
315
316 The server may inform the client of errors that prevented public key authentication from succeeding after authentication completes using a different method. These may be viewed by increasing the LogLevel to DEBUG or higher
317 (e.g. by using the -v flag).
318
319 The user creates their key pair by running ssh-keygen(1). This stores the private key in ˜/.ssh/id_dsa (DSA), ˜/.ssh/id_ecdsa (ECDSA), ˜/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA), ˜/.ssh/id_ed25519 (Ed25519),
320 ˜/.ssh/id_ed25519_sk (authenticator-hosted Ed25519), or ˜/.ssh/id_rsa (RSA) and stores the public key in ˜/.ssh/id_dsa.pub (DSA), ˜/.ssh/id_ecdsa.pub (ECDSA), ˜/.ssh/id_ecdsa_sk.pub (authenticator-hosted ECDSA),
321 ˜/.ssh/id_ed25519.pub (Ed25519), ˜/.ssh/id_ed25519_sk.pub (authenticator-hosted Ed25519), or ˜/.ssh/id_rsa.pub (RSA) in the user's home directory. The user should then copy the public key to ˜/.ssh/authorized_keys in their
322 home directory on the remote machine. The authorized_keys file corresponds to the conventional ˜/.rhosts file, and has one key per line, though the lines can be very long. After this, the user can log in without giving the
323 password.
324
325 A variation on public key authentication is available in the form of certificate authentication: instead of a set of public/private keys, signed certificates are used. This has the advantage that a single trusted certifica‐
326 tion authority can be used in place of many public/private keys. See the CERTIFICATES section of ssh-keygen(1) for more information.
327
328 The most convenient way to use public key or certificate authentication may be with an authentication agent. See ssh-agent(1) and (optionally) the AddKeysToAgent directive in ssh_config(5) for more information.
329
330 Keyboard-interactive authentication works as follows: The server sends an arbitrary "challenge" text and prompts for a response, possibly multiple times. Examples of keyboard-interactive authentication include BSD Authentica‐
331 tion (see login.conf(5)) and PAM (some non-OpenBSD systems).
332
333 Finally, if other authentication methods fail, ssh prompts the user for a password. The password is sent to the remote host for checking; however, since all communications are encrypted, the password cannot be seen by someone
334 listening on the network.
335
336 ssh automatically maintains and checks a database containing identification for all hosts it has ever been used with. Host keys are stored in ˜/.ssh/known_hosts in the user's home directory. Additionally, the file
337 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any new hosts are automatically added to the user's file. If a host's identification ever changes, ssh warns about this and disables password authentication
338 to prevent server spoofing or man-in-the-middle attacks, which could otherwise be used to circumvent the encryption. The StrictHostKeyChecking option can be used to control logins to machines whose host key is not known or
339 has changed.
340
341 When the user's identity has been accepted by the server, the server either executes the given command in a non-interactive session or, if no command has been specified, logs into the machine and gives the user a normal shell
342 as an interactive session. All communication with the remote command or shell will be automatically encrypted.
343
344 If an interactive session is requested, ssh by default will only request a pseudo-terminal (pty) for interactive sessions when the client has one. The flags -T and -t can be used to override this behaviour.
345
346 If a pseudo-terminal has been allocated, the user may use the escape characters noted below.
347
348 If no pseudo-terminal has been allocated, the session is transparent and can be used to reliably transfer binary data. On most systems, setting the escape character to “none” will also make the session transparent even if a
349 tty is used.
350
351 The session terminates when the command or shell on the remote machine exits and all X11 and TCP connections have been closed.
352
353 ESCAPE CHARACTERS
354 When a pseudo-terminal has been requested, ssh supports a number of functions through the use of an escape character.
355
356 A single tilde character can be sent as ˜˜ or by following the tilde by a character other than those described below. The escape character must always follow a newline to be interpreted as special. The escape character can
357 be changed in configuration files using the EscapeChar configuration directive or on the command line by the -e option.
358
359 The supported escapes (assuming the default ‘˜’) are:
360
361 ˜. Disconnect.
362
363 ˜ˆZ Background ssh.
364
365 ˜# List forwarded connections.
366
367 ˜& Background ssh at logout when waiting for forwarded connection / X11 sessions to terminate.
368
369 ˜? Display a list of escape characters.
370
371 ˜B Send a BREAK to the remote system (only useful if the peer supports it).
372
373 ˜C Open command line. Currently this allows the addition of port forwardings using the -L, -R and -D options (see above). It also allows the cancellation of existing port-forwardings with -KL[bind_address:]port for lo‐
374 cal, -KR[bind_address:]port for remote and -KD[bind_address:]port for dynamic port-forwardings. !command allows the user to execute a local command if the PermitLocalCommand option is enabled in ssh_config(5). Basic
375 help is available, using the -h option.
376
377 ˜R Request rekeying of the connection (only useful if the peer supports it).
378
379 ˜V Decrease the verbosity (LogLevel) when errors are being written to stderr.
380
381 ˜v Increase the verbosity (LogLevel) when errors are being written to stderr.
382
383 TCP FORWARDING
384 Forwarding of arbitrary TCP connections over a secure channel can be specified either on the command line or in a configuration file. One possible application of TCP forwarding is a secure connection to a mail server; another
385 is going through firewalls.
386
387 In the example below, we look at encrypting communication for an IRC client, even though the IRC server it connects to does not directly support encrypted communication. This works as follows: the user connects to the remote
388 host using ssh, specifying the ports to be used to forward the connection. After that it is possible to start the program locally, and ssh will encrypt and forward the connection to the remote server.
389
390 The following example tunnels an IRC session from the client to an IRC server at “server.example.com”, joining channel “#users”, nickname “pinky”, using the standard IRC port, 6667:
391
392 $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
393 $ irc -c '#users' pinky IRC/127.0.0.1
394
395 The -f option backgrounds ssh and the remote command “sleep 10” is specified to allow an amount of time (10 seconds, in the example) to start the program which is going to use the tunnel. If no connections are made within the
396 time specified, ssh will exit.
397
398 X11 FORWARDING
399 If the ForwardX11 variable is set to “yes” (or see the description of the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY environment variable is set), the connection to the X11 display is automatically
400 forwarded to the remote side in such a way that any X11 programs started from the shell (or command) will go through the encrypted channel, and the connection to the real X server will be made from the local machine. The user
401 should not manually set DISPLAY. Forwarding of X11 connections can be configured on the command line or in configuration files.
402
403 The DISPLAY value set by ssh will point to the server machine, but with a display number greater than zero. This is normal, and happens because ssh creates a “proxy” X server on the server machine for forwarding the connec‐
404 tions over the encrypted channel.
405
406 ssh will also automatically set up Xauthority data on the server machine. For this purpose, it will generate a random authorization cookie, store it in Xauthority on the server, and verify that any forwarded connections carry
407 this cookie and replace it by the real cookie when the connection is opened. The real authentication cookie is never sent to the server machine (and no cookies are sent in the plain).
408
409 If the ForwardAgent variable is set to “yes” (or see the description of the -A and -a options above) and the user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side.
410
411 VERIFYING HOST KEYS
412 When connecting to a server for the first time, a fingerprint of the server's public key is presented to the user (unless the option StrictHostKeyChecking has been disabled). Fingerprints can be determined using
413 ssh-keygen(1):
414
415 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
416
417 If the fingerprint is already known, it can be matched and the key can be accepted or rejected. If only legacy (MD5) fingerprints for the server are available, the ssh-keygen(1) -E option may be used to downgrade the finger‐
418 print algorithm to match.
419
420 Because of the difficulty of comparing host keys just by looking at fingerprint strings, there is also support to compare host keys visually, using random art. By setting the VisualHostKey option to “yes”, a small ASCII
421 graphic gets displayed on every login to a server, no matter if the session itself is interactive or not. By learning the pattern a known server produces, a user can easily find out that the host key has changed when a com‐
422 pletely different pattern is displayed. Because these patterns are not unambiguous however, a pattern that looks similar to the pattern remembered only gives a good probability that the host key is the same, not guaranteed
423 proof.
424
425 To get a listing of the fingerprints along with their random art for all known hosts, the following command line can be used:
426
427 $ ssh-keygen -lv -f ˜/.ssh/known_hosts
428
429 If the fingerprint is unknown, an alternative method of verification is available: SSH fingerprints verified by DNS. An additional resource record (RR), SSHFP, is added to a zonefile and the connecting client is able to match
430 the fingerprint with that of the key presented.
431
432 In this example, we are connecting a client to a server, “host.example.com”. The SSHFP resource records should first be added to the zonefile for host.example.com:
433
434 $ ssh-keygen -r host.example.com.
435
436 The output lines will have to be added to the zonefile. To check that the zone is answering fingerprint queries:
437
438 $ dig -t SSHFP host.example.com
439
440 Finally the client connects:
441
442 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
443 [...]
444 Matching host key fingerprint found in DNS.
445 Are you sure you want to continue connecting (yes/no)?
446
447 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
448
449 SSH-BASED VIRTUAL PRIVATE NETWORKS
450 ssh contains support for Virtual Private Network (VPN) tunnelling using the tun(4) network pseudo-device, allowing two networks to be joined securely. The sshd_config(5) configuration option PermitTunnel controls whether the
451 server supports this, and at what level (layer 2 or 3 traffic).
452
453 The following example would connect client network 10.0.50.0/24 with remote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway to the remote
454 network, at 192.168.1.15, allows it.
455
456 On the client:
457
458 # ssh -f -w 0:1 192.168.1.15 true
459 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
460 # route add 10.0.99.0/24 10.1.1.2
461
462 On the server:
463
464 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
465 # route add 10.0.50.0/24 10.1.1.1
466
467 Client access may be more finely tuned via the /root/.ssh/authorized_keys file (see below) and the PermitRootLogin server option. The following entry would permit connections on tun(4) device 1 from user “jane” and on tun de‐
468 vice 2 from user “john”, if PermitRootLogin is set to “forced-commands-only”:
469
470 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
471 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
472
473 Since an SSH-based setup entails a fair amount of overhead, it may be more suited to temporary setups, such as for wireless VPNs. More permanent VPNs are better provided by tools such as ipsecctl(8) and isakmpd(8).
474
475 ENVIRONMENT
476 ssh will normally set the following environment variables:
477
478 DISPLAY The DISPLAY variable indicates the location of the X11 server. It is automatically set by ssh to point to a value of the form “hostname:n”, where “hostname” indicates the host where the shell runs, and
479 ‘n’ is an integer ≥ 1. ssh uses this special value to forward X11 connections over the secure channel. The user should normally not set DISPLAY explicitly, as that will render the X11 connection inse‐
480 cure (and will require the user to manually copy any required authorization cookies).
481
482 HOME Set to the path of the user's home directory.
483
484 LOGNAME Synonym for USER; set for compatibility with systems that use this variable.
485
486 MAIL Set to the path of the user's mailbox.
487
488 PATH Set to the default PATH, as specified when compiling ssh.
489
490 SSH_ASKPASS If ssh needs a passphrase, it will read the passphrase from the current terminal if it was run from a terminal. If ssh does not have a terminal associated with it but DISPLAY and SSH_ASKPASS are set, it
491 will execute the program specified by SSH_ASKPASS and open an X11 window to read the passphrase. This is particularly useful when calling ssh from a .xsession or related script. (Note that on some ma‐
492 chines it may be necessary to redirect the input from /dev/null to make this work.)
493
494 SSH_ASKPASS_REQUIRE Allows further control over the use of an askpass program. If this variable is set to “never” then ssh will never attempt to use one. If it is set to “prefer”, then ssh will prefer to use the askpass
495 program instead of the TTY when requesting passwords. Finally, if the variable is set to “force”, then the askpass program will be used for all passphrase input regardless of whether DISPLAY is set.
496
497 SSH_AUTH_SOCK Identifies the path of a Unix-domain socket used to communicate with the agent.
498
499 SSH_CONNECTION Identifies the client and server ends of the connection. The variable contains four space-separated values: client IP address, client port number, server IP address, and server port number.
500
501 SSH_ORIGINAL_COMMAND This variable contains the original command line if a forced command is executed. It can be used to extract the original arguments.
502
503 SSH_TTY This is set to the name of the tty (path to the device) associated with the current shell or command. If the current session has no tty, this variable is not set.
504
505 SSH_TUNNEL Optionally set by sshd(8) to contain the interface names assigned if tunnel forwarding was requested by the client.
506
507 SSH_USER_AUTH Optionally set by sshd(8), this variable may contain a pathname to a file that lists the authentication methods successfully used when the session was established, including any public keys that were
508 used.
509
510 TZ This variable is set to indicate the present time zone if it was set when the daemon was started (i.e. the daemon passes the value on to new connections).
511
512 USER Set to the name of the user logging in.
513
514 Additionally, ssh reads ˜/.ssh/environment, and adds lines of the format “VARNAME=value” to the environment if the file exists and users are allowed to change their environment. For more information, see the
515 PermitUserEnvironment option in sshd_config(5).
516
517 FILES
518 ˜/.rhosts
519 This file is used for host-based authentication (see above). On some machines this file may need to be world-readable if the user's home directory is on an NFS partition, because sshd(8) reads it as root. Addition‐
520 ally, this file must be owned by the user, and must not have write permissions for anyone else. The recommended permission for most machines is read/write for the user, and not accessible by others.
521
522 ˜/.shosts
523 This file is used in exactly the same way as .rhosts, but allows host-based authentication without permitting login with rlogin/rsh.
524
525 ˜/.ssh/
526 This directory is the default location for all user-specific configuration and authentication information. There is no general requirement to keep the entire contents of this directory secret, but the recommended per‐
527 missions are read/write/execute for the user, and not accessible by others.
528
529 ˜/.ssh/authorized_keys
530 Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used for logging in as this user. The format of this file is described in the sshd(8) manual page. This file is not highly sensitive, but the recommended
531 permissions are read/write for the user, and not accessible by others.
532
533 ˜/.ssh/config
534 This is the per-user configuration file. The file format and configuration options are described in ssh_config(5). Because of the potential for abuse, this file must have strict permissions: read/write for the user,
535 and not writable by others.
536
537 ˜/.ssh/environment
538 Contains additional definitions for environment variables; see “ENVIRONMENT”, above.
539
540 ˜/.ssh/id_dsa
541 ˜/.ssh/id_ecdsa
542 ˜/.ssh/id_ecdsa_sk
543 ˜/.ssh/id_ed25519
544 ˜/.ssh/id_ed25519_sk
545 ˜/.ssh/id_rsa
546 Contains the private key for authentication. These files contain sensitive data and should be readable by the user but not accessible by others (read/write/execute). ssh will simply ignore a private key file if it is
547 accessible by others. It is possible to specify a passphrase when generating the key which will be used to encrypt the sensitive part of this file using AES-128.
548
549 ˜/.ssh/id_dsa.pub
550 ˜/.ssh/id_ecdsa.pub
551 ˜/.ssh/id_ecdsa_sk.pub
552 ˜/.ssh/id_ed25519.pub
553 ˜/.ssh/id_ed25519_sk.pub
554 ˜/.ssh/id_rsa.pub
555 Contains the public key for authentication. These files are not sensitive and can (but need not) be readable by anyone.
556
557 ˜/.ssh/known_hosts
558 Contains a list of host keys for all hosts the user has logged into that are not already in the systemwide list of known host keys. See sshd(8) for further details of the format of this file.
559
560 ˜/.ssh/rc
561 Commands in this file are executed by ssh when the user logs in, just before the user's shell (or command) is started. See the sshd(8) manual page for more information.
562
563 /etc/hosts.equiv
564 This file is for host-based authentication (see above). It should only be writable by root.
565
566 /etc/ssh/shosts.equiv
567 This file is used in exactly the same way as hosts.equiv, but allows host-based authentication without permitting login with rlogin/rsh.
568
569 /etc/ssh/ssh_config
570 Systemwide configuration file. The file format and configuration options are described in ssh_config(5).
571
572 /etc/ssh/ssh_host_key
573 /etc/ssh/ssh_host_dsa_key
574 /etc/ssh/ssh_host_ecdsa_key
575 /etc/ssh/ssh_host_ed25519_key
576 /etc/ssh/ssh_host_rsa_key
577 These files contain the private parts of the host keys and are used for host-based authentication.
578
579 /etc/ssh/ssh_known_hosts
580 Systemwide list of known host keys. This file should be prepared by the system administrator to contain the public host keys of all machines in the organization. It should be world-readable. See sshd(8) for further
581 details of the format of this file.
582
583 /etc/ssh/sshrc
584 Commands in this file are executed by ssh when the user logs in, just before the user's shell (or command) is started. See the sshd(8) manual page for more information.
585
586 EXIT STATUS
587 ssh exits with the exit status of the remote command or with 255 if an error occurred.
588
589 SEE ALSO
590 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
591
592 STANDARDS
593 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, January 2006.
594
595 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture, RFC 4251, January 2006.
596
597 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol, RFC 4252, January 2006.
598
599 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, January 2006.
600
601 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC 4254, January 2006.
602
603 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, January 2006.
604
605 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the Secure Shell Protocol (SSH), RFC 4256, January 2006.
606
607 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, January 2006.
608
609 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, January 2006.
610
611 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol, RFC 4345, January 2006.
612
613 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
614
615 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File Format, RFC 4716, November 2006.
616
617 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer, RFC 5656, December 2009.
618
619 A. Perrig and D. Song, Hash Visualization: a New Technique to improve Real-World Security, 1999, International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99).
620
621 AUTHORS
622 OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created
623 OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.
624
625 GNU July 23, 2023 SSH(1)