1SSH_CONFIG(5) BSD File Formats Manual SSH_CONFIG(5)
2
4 ssh_config — OpenSSH client configuration file
5
7 ssh(1) obtains configuration data from the following sources in the fol‐
8 lowing order:
9 1. command-line options
11 2. user's configuration file (~/.ssh/config)
12 3. system-wide configuration file (/etc/ssh/ssh_config)
13 For each parameter, the first obtained value will be used. The configu‐
15 ration files contain sections separated by Host specifications, and that
16 section is only applied for hosts that match one of the patterns given in
17 the specification. The matched host name is usually the one given on the
18 command line (see the CanonicalizeHostname option for exceptions).
19 Since the first obtained value for each parameter is used, more host-spe‐
21 cific declarations should be given near the beginning of the file, and
22 general defaults at the end.
23 The file contains keyword-argument pairs, one per line. Lines starting
25 with ‘#’ and empty lines are interpreted as comments. Arguments may op‐
26 tionally be enclosed in double quotes (") in order to represent arguments
27 containing spaces. Configuration options may be separated by whitespace
28 or optional whitespace and exactly one ‘=’; the latter format is useful
29 to avoid the need to quote whitespace when specifying configuration op‐
30 tions using the ssh, scp, and sftp -o option.
31 The possible keywords and their meanings are as follows (note that key‐
33 words are case-insensitive and arguments are case-sensitive):
34 Host Restricts the following declarations (up to the next Host or
36 Match keyword) to be only for those hosts that match one of the
37 patterns given after the keyword. If more than one pattern is
38 provided, they should be separated by whitespace. A single ‘*’
39 as a pattern can be used to provide global defaults for all
40 hosts. The host is usually the hostname argument given on the
41 command line (see the CanonicalizeHostname keyword for excep‐
42 tions).
43 A pattern entry may be negated by prefixing it with an exclama‐
45 tion mark (‘!’). If a negated entry is matched, then the Host
46 entry is ignored, regardless of whether any other patterns on the
47 line match. Negated matches are therefore useful to provide ex‐
48 ceptions for wildcard matches.
49 See PATTERNS for more information on patterns.
51 Match Restricts the following declarations (up to the next Host or
53 Match keyword) to be used only when the conditions following the
54 Match keyword are satisfied. Match conditions are specified us‐
55 ing one or more criteria or the single token all which always
56 matches. The available criteria keywords are: canonical, final,
57 exec, host, originalhost, user, and localuser. The all criteria
58 must appear alone or immediately after canonical or final. Other
59 criteria may be combined arbitrarily. All criteria but all,
60 canonical, and final require an argument. Criteria may be
61 negated by prepending an exclamation mark (‘!’).
62 The canonical keyword matches only when the configuration file is
64 being re-parsed after hostname canonicalization (see the
65 CanonicalizeHostname option). This may be useful to specify con‐
66 ditions that work with canonical host names only.
67 The final keyword requests that the configuration be re-parsed
69 (regardless of whether CanonicalizeHostname is enabled), and
70 matches only during this final pass. If CanonicalizeHostname is
71 enabled, then canonical and final match during the same pass.
72 The exec keyword executes the specified command under the user's
74 shell. If the command returns a zero exit status then the condi‐
75 tion is considered true. Commands containing whitespace charac‐
76 ters must be quoted. Arguments to exec accept the tokens de‐
77 scribed in the TOKENS section.
78 The other keywords' criteria must be single entries or comma-sep‐
80 arated lists and may use the wildcard and negation operators de‐
81 scribed in the PATTERNS section. The criteria for the host key‐
82 word are matched against the target hostname, after any substitu‐
83 tion by the Hostname or CanonicalizeHostname options. The
84 originalhost keyword matches against the hostname as it was spec‐
85 ified on the command-line. The user keyword matches against the
86 target username on the remote host. The localuser keyword
87 matches against the name of the local user running ssh(1) (this
88 keyword may be useful in system-wide ssh_config files).
89 AddKeysToAgent
91 Specifies whether keys should be automatically added to a running
92 ssh-agent(1). If this option is set to yes and a key is loaded
93 from a file, the key and its passphrase are added to the agent
94 with the default lifetime, as if by ssh-add(1). If this option
95 is set to ask, ssh(1) will require confirmation using the
96 SSH_ASKPASS program before adding a key (see ssh-add(1) for de‐
97 tails). If this option is set to confirm, each use of the key
98 must be confirmed, as if the -c option was specified to
99 ssh-add(1). If this option is set to no, no keys are added to
100 the agent. Alternately, this option may be specified as a time
101 interval using the format described in the TIME FORMATS section
102 of sshd_config(5) to specify the key's lifetime in ssh-agent(1),
103 after which it will automatically be removed. The argument must
104 be no (the default), yes, confirm (optionally followed by a time
105 interval), ask or a time interval.
106 AddressFamily
108 Specifies which address family to use when connecting. Valid ar‐
109 guments are any (the default), inet (use IPv4 only), or inet6
110 (use IPv6 only).
111 BatchMode
113 If set to yes, user interaction such as password prompts and host
114 key confirmation requests will be disabled. This option is use‐
115 ful in scripts and other batch jobs where no user is present to
116 interact with ssh(1). The argument must be yes or no (the de‐
117 fault).
118 BindAddress
120 Use the specified address on the local machine as the source ad‐
121 dress of the connection. Only useful on systems with more than
122 one address.
123 BindInterface
125 Use the address of the specified interface on the local machine
126 as the source address of the connection.
127 CanonicalDomains
129 When CanonicalizeHostname is enabled, this option specifies the
130 list of domain suffixes in which to search for the specified des‐
131 tination host.
132 CanonicalizeFallbackLocal
134 Specifies whether to fail with an error when hostname canonical‐
135 ization fails. The default, yes, will attempt to look up the un‐
136 qualified hostname using the system resolver's search rules. A
137 value of no will cause ssh(1) to fail instantly if
138 CanonicalizeHostname is enabled and the target hostname cannot be
139 found in any of the domains specified by CanonicalDomains.
140 CanonicalizeHostname
142 Controls whether explicit hostname canonicalization is performed.
143 The default, no, is not to perform any name rewriting and let the
144 system resolver handle all hostname lookups. If set to yes then,
145 for connections that do not use a ProxyCommand or ProxyJump,
146 ssh(1) will attempt to canonicalize the hostname specified on the
147 command line using the CanonicalDomains suffixes and
148 CanonicalizePermittedCNAMEs rules. If CanonicalizeHostname is
149 set to always, then canonicalization is applied to proxied con‐
150 nections too.
151 If this option is enabled, then the configuration files are pro‐
153 cessed again using the new target name to pick up any new config‐
154 uration in matching Host and Match stanzas. A value of none dis‐
155 ables the use of a ProxyJump host.
156 CanonicalizeMaxDots
158 Specifies the maximum number of dot characters in a hostname be‐
159 fore canonicalization is disabled. The default, 1, allows a sin‐
160 gle dot (i.e. hostname.subdomain).
161 CanonicalizePermittedCNAMEs
163 Specifies rules to determine whether CNAMEs should be followed
164 when canonicalizing hostnames. The rules consist of one or more
165 arguments of source_domain_list:target_domain_list, where
166 source_domain_list is a pattern-list of domains that may follow
167 CNAMEs in canonicalization, and target_domain_list is a pattern-
168 list of domains that they may resolve to.
169 For example, "*.a.example.com:*.b.example.com,*.c.example.com"
171 will allow hostnames matching "*.a.example.com" to be canonical‐
172 ized to names in the "*.b.example.com" or "*.c.example.com" do‐
173 mains.
174 A single argument of "none" causes no CNAMEs to be considered for
176 canonicalization. This is the default behaviour.
177 CASignatureAlgorithms
179 The default is handled system-wide by crypto-policies(7). To see
180 the defaults and how to modify this default, see manual page
181 update-crypto-policies(8).
182 Specifies which algorithms are allowed for signing of certifi‐
184 cates by certificate authorities (CAs). If the specified list
185 begins with a ‘+’ character, then the specified algorithms will
186 be appended to the default set instead of replacing them. If the
187 specified list begins with a ‘-’ character, then the specified
188 algorithms (including wildcards) will be removed from the default
189 set instead of replacing them.
190 ssh(1) will not accept host certificates signed using algorithms
192 other than those specified.
193 CertificateFile
195 Specifies a file from which the user's certificate is read. A
196 corresponding private key must be provided separately in order to
197 use this certificate either from an IdentityFile directive or -i
198 flag to ssh(1), via ssh-agent(1), or via a PKCS11Provider or
199 SecurityKeyProvider.
200 Arguments to CertificateFile may use the tilde syntax to refer to
202 a user's home directory, the tokens described in the TOKENS sec‐
203 tion and environment variables as described in the ENVIRONMENT
204 VARIABLES section.
205 It is possible to have multiple certificate files specified in
207 configuration files; these certificates will be tried in se‐
208 quence. Multiple CertificateFile directives will add to the list
209 of certificates used for authentication.
210 CheckHostIP
212 If set to yes [22mssh(1) will additionally check the host IP address
213 in the known_hosts file. This allows it to detect if a host key
214 changed due to DNS spoofing and will add addresses of destination
215 hosts to ~/.ssh/known_hosts in the process, regardless of the
216 setting of StrictHostKeyChecking. If the option is set to no
217 (the default), the check will not be executed.
218 Ciphers
220 The default is handled system-wide by crypto-policies(7). To see
221 the defaults and how to modify this default, see manual page
222 update-crypto-policies(8).
223 Specifies the ciphers allowed and their order of preference.
225 Multiple ciphers must be comma-separated. If the specified list
226 begins with a ‘+’ character, then the specified ciphers will be
227 appended to the built-in openssh default set instead of replacing
228 them. If the specified list begins with a ‘-’ character, then
229 the specified ciphers (including wildcards) will be removed from
230 the built-in openssh default set instead of replacing them. If
231 the specified list begins with a ‘^’ character, then the speci‐
232 fied ciphers will be placed at the head of the built-in openssh
233 default set.
234 The supported ciphers are:
236 3des-cbc
238 aes128-cbc
239 aes192-cbc
240 aes256-cbc
241 aes128-ctr
242 aes192-ctr
243 aes256-ctr
244 aes128-gcm@openssh.com
245 aes256-gcm@openssh.com
246 chacha20-poly1305@openssh.com
247 The list of available ciphers may also be obtained using "ssh -Q
249 cipher".
250 ClearAllForwardings
252 Specifies that all local, remote, and dynamic port forwardings
253 specified in the configuration files or on the command line be
254 cleared. This option is primarily useful when used from the
255 ssh(1) command line to clear port forwardings set in configura‐
256 tion files, and is automatically set by scp(1) and sftp(1). The
257 argument must be yes or no (the default).
258 Compression
260 Specifies whether to use compression. The argument must be yes
261 or no (the default).
262 ConnectionAttempts
264 Specifies the number of tries (one per second) to make before ex‐
265 iting. The argument must be an integer. This may be useful in
266 scripts if the connection sometimes fails. The default is 1.
267 ConnectTimeout
269 Specifies the timeout (in seconds) used when connecting to the
270 SSH server, instead of using the default system TCP timeout.
271 This timeout is applied both to establishing the connection and
272 to performing the initial SSH protocol handshake and key ex‐
273 change.
274 ControlMaster
276 Enables the sharing of multiple sessions over a single network
277 connection. When set to yes, ssh(1) will listen for connections
278 on a control socket specified using the ControlPath argument.
279 Additional sessions can connect to this socket using the same
280 ControlPath with ControlMaster set to no (the default). These
281 sessions will try to reuse the master instance's network connec‐
282 tion rather than initiating new ones, but will fall back to con‐
283 necting normally if the control socket does not exist, or is not
284 listening.
285 Setting this to ask will cause ssh(1) to listen for control con‐
287 nections, but require confirmation using ssh-askpass(1). If the
288 ControlPath cannot be opened, ssh(1) will continue without con‐
289 necting to a master instance.
290 X11 and ssh-agent(1) forwarding is supported over these multi‐
292 plexed connections, however the display and agent forwarded will
293 be the one belonging to the master connection i.e. it is not pos‐
294 sible to forward multiple displays or agents.
295 Two additional options allow for opportunistic multiplexing: try
297 to use a master connection but fall back to creating a new one if
298 one does not already exist. These options are: auto and autoask.
299 The latter requires confirmation like the ask option.
300 ControlPath
302 Specify the path to the control socket used for connection shar‐
303 ing as described in the ControlMaster section above or the string
304 none to disable connection sharing. Arguments to ControlPath may
305 use the tilde syntax to refer to a user's home directory, the to‐
306 kens described in the TOKENS section and environment variables as
307 described in the ENVIRONMENT VARIABLES section. It is recom‐
308 mended that any ControlPath used for opportunistic connection
309 sharing include at least %h, %p, and %r (or alternatively %C) and
310 be placed in a directory that is not writable by other users.
311 This ensures that shared connections are uniquely identified.
312 ControlPersist
314 When used in conjunction with ControlMaster, specifies that the
315 master connection should remain open in the background (waiting
316 for future client connections) after the initial client connec‐
317 tion has been closed. If set to no (the default), then the mas‐
318 ter connection will not be placed into the background, and will
319 close as soon as the initial client connection is closed. If set
320 to yes or 0, then the master connection will remain in the back‐
321 ground indefinitely (until killed or closed via a mechanism such
322 as the "ssh -O exit"). If set to a time in seconds, or a time in
323 any of the formats documented in sshd_config(5), then the back‐
324 grounded master connection will automatically terminate after it
325 has remained idle (with no client connections) for the specified
326 time.
327 DynamicForward
329 Specifies that a TCP port on the local machine be forwarded over
330 the secure channel, and the application protocol is then used to
331 determine where to connect to from the remote machine.
332 The argument must be [bind_address:]port. IPv6 addresses can be
334 specified by enclosing addresses in square brackets. By default,
335 the local port is bound in accordance with the GatewayPorts set‐
336 ting. However, an explicit bind_address may be used to bind the
337 connection to a specific address. The bind_address of localhost
338 indicates that the listening port be bound for local use only,
339 while an empty address or ‘*’ indicates that the port should be
340 available from all interfaces.
341 Currently the SOCKS4 and SOCKS5 protocols are supported, and
343 ssh(1) will act as a SOCKS server. Multiple forwardings may be
344 specified, and additional forwardings can be given on the command
345 line. Only the superuser can forward privileged ports.
346 EnableSSHKeysign
348 Setting this option to yes in the global client configuration
349 file /etc/ssh/ssh_config enables the use of the helper program
350 ssh-keysign(8) during HostbasedAuthentication. The argument must
351 be yes or no (the default). This option should be placed in the
352 non-hostspecific section. See ssh-keysign(8) for more informa‐
353 tion.
354 EscapeChar
356 Sets the escape character (default: ‘~’). The escape character
357 can also be set on the command line. The argument should be a
358 single character, ‘^’ followed by a letter, or none to disable
359 the escape character entirely (making the connection transparent
360 for binary data).
361 ExitOnForwardFailure
363 Specifies whether ssh(1) should terminate the connection if it
364 cannot set up all requested dynamic, tunnel, local, and remote
365 port forwardings, (e.g. if either end is unable to bind and lis‐
366 ten on a specified port). Note that ExitOnForwardFailure does
367 not apply to connections made over port forwardings and will not,
368 for example, cause ssh(1) to exit if TCP connections to the ulti‐
369 mate forwarding destination fail. The argument must be yes or no
370 (the default).
371 FingerprintHash
373 Specifies the hash algorithm used when displaying key finger‐
374 prints. Valid options are: md5 and sha256 (the default).
375 ForkAfterAuthentication
377 Requests ssh to go to background just before command execution.
378 This is useful if ssh is going to ask for passwords or
379 passphrases, but the user wants it in the background. This im‐
380 plies the StdinNull configuration option being set to “yes”. The
381 recommended way to start X11 programs at a remote site is with
382 something like ssh -f host xterm, which is the same as ssh host
383 xterm if the ForkAfterAuthentication configuration option is set
384 to “yes”.
385 If the ExitOnForwardFailure configuration option is set to “yes”,
387 then a client started with the ForkAfterAuthentication configura‐
388 tion option being set to “yes” will wait for all remote port for‐
389 wards to be successfully established before placing itself in the
390 background. The argument to this keyword must be yes (same as
391 the -f option) or no (the default).
392 ForwardAgent
394 Specifies whether the connection to the authentication agent (if
395 any) will be forwarded to the remote machine. The argument may
396 be yes, no (the default), an explicit path to an agent socket or
397 the name of an environment variable (beginning with ‘$’) in which
398 to find the path.
399 Agent forwarding should be enabled with caution. Users with the
401 ability to bypass file permissions on the remote host (for the
402 agent's Unix-domain socket) can access the local agent through
403 the forwarded connection. An attacker cannot obtain key material
404 from the agent, however they can perform operations on the keys
405 that enable them to authenticate using the identities loaded into
406 the agent.
407 ForwardX11
409 Specifies whether X11 connections will be automatically redi‐
410 rected over the secure channel and DISPLAY set. The argument
411 must be yes or no (the default).
412 X11 forwarding should be enabled with caution. Users with the
414 ability to bypass file permissions on the remote host (for the
415 user's X11 authorization database) can access the local X11 dis‐
416 play through the forwarded connection. An attacker may then be
417 able to perform activities such as keystroke monitoring if the
418 ForwardX11Trusted option is also enabled.
419 ForwardX11Timeout
421 Specify a timeout for untrusted X11 forwarding using the format
422 described in the TIME FORMATS section of sshd_config(5). X11
423 connections received by ssh(1) after this time will be refused.
424 Setting ForwardX11Timeout to zero will disable the timeout and
425 permit X11 forwarding for the life of the connection. The de‐
426 fault is to disable untrusted X11 forwarding after twenty minutes
427 has elapsed.
428 ForwardX11Trusted
430 If this option is set to yes, remote X11 clients will have full
431 access to the original X11 display.
432 If this option is set to no (the default), remote X11 clients
434 will be considered untrusted and prevented from stealing or tam‐
435 pering with data belonging to trusted X11 clients. Furthermore,
436 the xauth(1) token used for the session will be set to expire af‐
437 ter 20 minutes. Remote clients will be refused access after this
438 time.
439 See the X11 SECURITY extension specification for full details on
441 the restrictions imposed on untrusted clients.
442 GatewayPorts
444 Specifies whether remote hosts are allowed to connect to local
445 forwarded ports. By default, ssh(1) binds local port forwardings
446 to the loopback address. This prevents other remote hosts from
447 connecting to forwarded ports. GatewayPorts can be used to spec‐
448 ify that ssh should bind local port forwardings to the wildcard
449 address, thus allowing remote hosts to connect to forwarded
450 ports. The argument must be yes or no (the default).
451 GlobalKnownHostsFile
453 Specifies one or more files to use for the global host key data‐
454 base, separated by whitespace. The default is
455 /etc/ssh/ssh_known_hosts, /etc/ssh/ssh_known_hosts2.
456 GSSAPIAuthentication
458 Specifies whether user authentication based on GSSAPI is allowed.
459 The default is no.
460 GSSAPIClientIdentity
462 If set, specifies the GSSAPI client identity that ssh should use
463 when connecting to the server. The default is unset, which means
464 that the default identity will be used.
465 GSSAPIDelegateCredentials
467 Forward (delegate) credentials to the server. The default is no.
468 GSSAPIKeyExchange
470 Specifies whether key exchange based on GSSAPI may be used. When
471 using GSSAPI key exchange the server need not have a host key.
472 The default is “no”.
473 GSSAPIRenewalForcesRekey
475 If set to “yes” then renewal of the client's GSSAPI credentials
476 will force the rekeying of the ssh connection. With a compatible
477 server, this will delegate the renewed credentials to a session
478 on the server.
479 Checks are made to ensure that credentials are only propagated
481 when the new credentials match the old ones on the originating
482 client and where the receiving server still has the old set in
483 its cache.
484 The default is “no”.
486 For this to work GSSAPIKeyExchange needs to be enabled in the
488 server and also used by the client.
489 GSSAPIServerIdentity
491 If set, specifies the GSSAPI server identity that ssh should ex‐
492 pect when connecting to the server. The default is unset, which
493 means that the expected GSSAPI server identity will be determined
494 from the target hostname.
495 GSSAPITrustDns
497 Set to “yes” to indicate that the DNS is trusted to securely
498 canonicalize the name of the host being connected to. If “no”,
499 the hostname entered on the command line will be passed untouched
500 to the GSSAPI library. The default is “no”.
501 GSSAPIKexAlgorithms
503 The default is handled system-wide by crypto-policies(7). To see
504 the defaults and how to modify this default, see manual page
505 update-crypto-policies(8).
506 The list of key exchange algorithms that are offered for GSSAPI
508 key exchange. Possible values are
509 gss-gex-sha1-,
511 gss-group1-sha1-,
512 gss-group14-sha1-,
513 gss-group14-sha256-,
514 gss-group16-sha512-,
515 gss-nistp256-sha256-,
516 gss-curve25519-sha256-
517 This option only applies to connections using GSSAPI.
519 HashKnownHosts
521 Indicates that ssh(1) should hash host names and addresses when
522 they are added to ~/.ssh/known_hosts. These hashed names may be
523 used normally by ssh(1) and sshd(8), but they do not visually re‐
524 veal identifying information if the file's contents are dis‐
525 closed. The default is no. Note that existing names and ad‐
526 dresses in known hosts files will not be converted automatically,
527 but may be manually hashed using ssh-keygen(1).
528 HostbasedAcceptedAlgorithms
530 Specifies the signature algorithms that will be used for host‐
531 based authentication as a comma-separated list of patterns. Al‐
532 ternately if the specified list begins with a ‘+’ character, then
533 the specified signature algorithms will be appended to the de‐
534 fault set instead of replacing them. If the specified list be‐
535 gins with a ‘-’ character, then the specified signature algo‐
536 rithms (including wildcards) will be removed from the default set
537 instead of replacing them. If the specified list begins with a
538 ‘^’ character, then the specified signature algorithms will be
539 placed at the head of the default set. The default for this op‐
540 tion is:
541 ssh-ed25519-cert-v01@openssh.com,
543 ecdsa-sha2-nistp256-cert-v01@openssh.com,
544 ecdsa-sha2-nistp384-cert-v01@openssh.com,
545 ecdsa-sha2-nistp521-cert-v01@openssh.com,
546 sk-ssh-ed25519-cert-v01@openssh.com,
547 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
548 rsa-sha2-512-cert-v01@openssh.com,
549 rsa-sha2-256-cert-v01@openssh.com,
550 ssh-ed25519,
551 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
552 sk-ssh-ed25519@openssh.com,
553 sk-ecdsa-sha2-nistp256@openssh.com,
554 rsa-sha2-512,rsa-sha2-256
555 The -Q option of ssh(1) may be used to list supported signature
557 algorithms. This was formerly named HostbasedKeyTypes.
558 HostbasedAuthentication
560 Specifies whether to try rhosts based authentication with public
561 key authentication. The argument must be yes or no (the de‐
562 fault).
563 HostKeyAlgorithms
565 Specifies the host key signature algorithms that the client wants
566 to use in order of preference. Alternately if the specified list
567 begins with a ‘+’ character, then the specified signature algo‐
568 rithms will be appended to the default set instead of replacing
569 them. If the specified list begins with a ‘-’ character, then
570 the specified signature algorithms (including wildcards) will be
571 removed from the default set instead of replacing them. If the
572 specified list begins with a ‘^’ character, then the specified
573 signature algorithms will be placed at the head of the default
574 set. The default for this option is:
575 ssh-ed25519-cert-v01@openssh.com,
577 ecdsa-sha2-nistp256-cert-v01@openssh.com,
578 ecdsa-sha2-nistp384-cert-v01@openssh.com,
579 ecdsa-sha2-nistp521-cert-v01@openssh.com,
580 sk-ssh-ed25519-cert-v01@openssh.com,
581 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
582 rsa-sha2-512-cert-v01@openssh.com,
583 rsa-sha2-256-cert-v01@openssh.com,
584 ssh-ed25519,
585 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
586 sk-ecdsa-sha2-nistp256@openssh.com,
587 sk-ssh-ed25519@openssh.com,
588 rsa-sha2-512,rsa-sha2-256
589 If hostkeys are known for the destination host then this default
591 is modified to prefer their algorithms.
592 The list of available signature algorithms may also be obtained
594 using "ssh -Q HostKeyAlgorithms".
595 HostKeyAlias
597 Specifies an alias that should be used instead of the real host
598 name when looking up or saving the host key in the host key data‐
599 base files and when validating host certificates. This option is
600 useful for tunneling SSH connections or for multiple servers run‐
601 ning on a single host.
602 Hostname
604 Specifies the real host name to log into. This can be used to
605 specify nicknames or abbreviations for hosts. Arguments to
606 Hostname accept the tokens described in the TOKENS section. Nu‐
607 meric IP addresses are also permitted (both on the command line
608 and in Hostname specifications). The default is the name given
609 on the command line.
610 IdentitiesOnly
612 Specifies that ssh(1) should only use the configured authentica‐
613 tion identity and certificate files (either the default files, or
614 those explicitly configured in the ssh_config files or passed on
615 the ssh(1) command-line), even if ssh-agent(1) or a
616 PKCS11Provider or SecurityKeyProvider offers more identities.
617 The argument to this keyword must be yes or no (the default).
618 This option is intended for situations where ssh-agent offers
619 many different identities.
620 IdentityAgent
622 Specifies the UNIX-domain socket used to communicate with the au‐
623 thentication agent.
624 This option overrides the SSH_AUTH_SOCK environment variable and
626 can be used to select a specific agent. Setting the socket name
627 to none disables the use of an authentication agent. If the
628 string "SSH_AUTH_SOCK" is specified, the location of the socket
629 will be read from the SSH_AUTH_SOCK environment variable. Other‐
630 wise if the specified value begins with a ‘$’ character, then it
631 will be treated as an environment variable containing the loca‐
632 tion of the socket.
633 Arguments to IdentityAgent may use the tilde syntax to refer to a
635 user's home directory, the tokens described in the TOKENS section
636 and environment variables as described in the ENVIRONMENT
637 VARIABLES section.
638 IdentityFile
640 Specifies a file from which the user's DSA, ECDSA, authenticator-
641 hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA au‐
642 thentication identity is read. The default is ~/.ssh/id_dsa,
643 ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519,
644 ~/.ssh/id_ed25519_sk and ~/.ssh/id_rsa. Additionally, any iden‐
645 tities represented by the authentication agent will be used for
646 authentication unless IdentitiesOnly is set. If no certificates
647 have been explicitly specified by CertificateFile, ssh(1) will
648 try to load certificate information from the filename obtained by
649 appending -cert.pub to the path of a specified IdentityFile.
650 Arguments to IdentityFile may use the tilde syntax to refer to a
652 user's home directory or the tokens described in the TOKENS sec‐
653 tion.
654 It is possible to have multiple identity files specified in con‐
656 figuration files; all these identities will be tried in sequence.
657 Multiple IdentityFile directives will add to the list of identi‐
658 ties tried (this behaviour differs from that of other configura‐
659 tion directives).
660 IdentityFile may be used in conjunction with IdentitiesOnly to
662 select which identities in an agent are offered during authenti‐
663 cation. IdentityFile may also be used in conjunction with
664 CertificateFile in order to provide any certificate also needed
665 for authentication with the identity.
666 The authentication identity can be also specified in a form of
668 PKCS#11 URI starting with a string pkcs11:. There is supported a
669 subset of the PKCS#11 URI as defined in RFC 7512 (implemented
670 path arguments id, manufacturer, object, token and query argu‐
671 ments module-path and pin-value ). The URI can not be in quotes.
672 IgnoreUnknown
674 Specifies a pattern-list of unknown options to be ignored if they
675 are encountered in configuration parsing. This may be used to
676 suppress errors if ssh_config contains options that are unrecog‐
677 nised by ssh(1). It is recommended that IgnoreUnknown be listed
678 early in the configuration file as it will not be applied to un‐
679 known options that appear before it.
680 Include
682 Include the specified configuration file(s). Multiple pathnames
683 may be specified and each pathname may contain glob(7) wildcards
684 and, for user configurations, shell-like ‘~’ references to user
685 home directories. Wildcards will be expanded and processed in
686 lexical order. Files without absolute paths are assumed to be in
687 ~/.ssh if included in a user configuration file or /etc/ssh if
688 included from the system configuration file. Include directive
689 may appear inside a Match or Host block to perform conditional
690 inclusion.
691 IPQoS Specifies the IPv4 type-of-service or DSCP class for connections.
693 Accepted values are af11, af12, af13, af21, af22, af23, af31,
694 af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5, cs6,
695 cs7, ef, le, lowdelay, throughput, reliability, a numeric value,
696 or none to use the operating system default. This option may
697 take one or two arguments, separated by whitespace. If one argu‐
698 ment is specified, it is used as the packet class uncondition‐
699 ally. If two values are specified, the first is automatically
700 selected for interactive sessions and the second for non-interac‐
701 tive sessions. The default is af21 (Low-Latency Data) for inter‐
702 active sessions and cs1 (Lower Effort) for non-interactive ses‐
703 sions.
704 KbdInteractiveAuthentication
706 Specifies whether to use keyboard-interactive authentication.
707 The argument to this keyword must be yes (the default) or no.
708 ChallengeResponseAuthentication is a deprecated alias for this.
709 KbdInteractiveDevices
711 Specifies the list of methods to use in keyboard-interactive au‐
712 thentication. Multiple method names must be comma-separated.
713 The default is to use the server specified list. The methods
714 available vary depending on what the server supports. For an
715 OpenSSH server, it may be zero or more of: bsdauth and pam.
716 KexAlgorithms
718 The default is handled system-wide by crypto-policies(7). To see
719 the defaults and how to modify this default, see manual page
720 update-crypto-policies(8).
721 Specifies the available KEX (Key Exchange) algorithms. Multiple
723 algorithms must be comma-separated. If the specified list begins
724 with a ‘+’ character, then the specified algorithms will be ap‐
725 pended to the built-in openssh default set instead of replacing
726 them. If the specified list begins with a ‘-’ character, then
727 the specified algorithms (including wildcards) will be removed
728 from the built-in openssh default set instead of replacing them.
729 If the specified list begins with a ‘^’ character, then the spec‐
730 ified algorithms will be placed at the head of the built-in
731 openssh default set.
732 The list of available key exchange algorithms may also be ob‐
734 tained using "ssh -Q kex".
735 KnownHostsCommand
737 Specifies a command to use to obtain a list of host keys, in ad‐
738 dition to those listed in UserKnownHostsFile and
739 GlobalKnownHostsFile. This command is executed after the files
740 have been read. It may write host key lines to standard output
741 in identical format to the usual files (described in the
742 VERIFYING HOST KEYS section in ssh(1)). Arguments to
743 KnownHostsCommand accept the tokens described in the TOKENS sec‐
744 tion. The command may be invoked multiple times per connection:
745 once when preparing the preference list of host key algorithms to
746 use, again to obtain the host key for the requested host name
747 and, if CheckHostIP is enabled, one more time to obtain the host
748 key matching the server's address. If the command exits abnor‐
749 mally or returns a non-zero exit status then the connection is
750 terminated.
751 LocalCommand
753 Specifies a command to execute on the local machine after suc‐
754 cessfully connecting to the server. The command string extends
755 to the end of the line, and is executed with the user's shell.
756 Arguments to LocalCommand accept the tokens described in the
757 TOKENS section.
758 The command is run synchronously and does not have access to the
760 session of the ssh(1) that spawned it. It should not be used for
761 interactive commands.
762 This directive is ignored unless PermitLocalCommand has been en‐
764 abled.
765 LocalForward
767 Specifies that a TCP port on the local machine be forwarded over
768 the secure channel to the specified host and port from the remote
769 machine. The first argument specifies the listener and may be
770 [bind_address:]port or a Unix domain socket path. The second ar‐
771 gument is the destination and may be host:hostport or a Unix do‐
772 main socket path if the remote host supports it.
773 IPv6 addresses can be specified by enclosing addresses in square
775 brackets. Multiple forwardings may be specified, and additional
776 forwardings can be given on the command line. Only the superuser
777 can forward privileged ports. By default, the local port is
778 bound in accordance with the GatewayPorts setting. However, an
779 explicit bind_address may be used to bind the connection to a
780 specific address. The bind_address of localhost indicates that
781 the listening port be bound for local use only, while an empty
782 address or ‘*’ indicates that the port should be available from
783 all interfaces. Unix domain socket paths may use the tokens de‐
784 scribed in the TOKENS section and environment variables as de‐
785 scribed in the ENVIRONMENT VARIABLES section.
786 LogLevel
788 Gives the verbosity level that is used when logging messages from
789 ssh(1). The possible values are: QUIET, FATAL, ERROR, INFO, VER‐
790 BOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
791 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
792 higher levels of verbose output.
793 LogVerbose
795 Specify one or more overrides to LogLevel. An override consists
796 of a pattern lists that matches the source file, function and
797 line number to force detailed logging for. For example, an over‐
798 ride pattern of:
799 kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
801 would enable detailed logging for line 1000 of kex.c, everything
803 in the kex_exchange_identification() function, and all code in
804 the packet.c file. This option is intended for debugging and no
805 overrides are enabled by default.
806 MACs The default is handled system-wide by crypto-policies(7). To see
808 the defaults and how to modify this default, see manual page
809 update-crypto-policies(8).
810 Specifies the MAC (message authentication code) algorithms in or‐
812 der of preference. The MAC algorithm is used for data integrity
813 protection. Multiple algorithms must be comma-separated. If the
814 specified list begins with a ‘+’ character, then the specified
815 algorithms will be appended to the built-in openssh default set
816 instead of replacing them. If the specified list begins with a
817 ‘-’ character, then the specified algorithms (including wild‐
818 cards) will be removed from the built-in openssh default set in‐
819 stead of replacing them. If the specified list begins with a ‘^’
820 character, then the specified algorithms will be placed at the
821 head of the built-in openssh default set.
822 The algorithms that contain "-etm" calculate the MAC after en‐
824 cryption (encrypt-then-mac). These are considered safer and
825 their use recommended.
826 The list of available MAC algorithms may also be obtained using
828 "ssh -Q mac".
829 NoHostAuthenticationForLocalhost
831 Disable host authentication for localhost (loopback addresses).
832 The argument to this keyword must be yes or no (the default).
833 NumberOfPasswordPrompts
835 Specifies the number of password prompts before giving up. The
836 argument to this keyword must be an integer. The default is 3.
837 PasswordAuthentication
839 Specifies whether to use password authentication. The argument
840 to this keyword must be yes (the default) or no.
841 PermitLocalCommand
843 Allow local command execution via the LocalCommand option or us‐
844 ing the !command escape sequence in ssh(1). The argument must be
845 yes or no (the default).
846 PermitRemoteOpen
848 Specifies the destinations to which remote TCP port forwarding is
849 permitted when RemoteForward is used as a SOCKS proxy. The for‐
850 warding specification must be one of the following forms:
851 PermitRemoteOpen host:port
853 PermitRemoteOpen IPv4_addr:port
854 PermitRemoteOpen [IPv6_addr]:port
855 Multiple forwards may be specified by separating them with white‐
857 space. An argument of any can be used to remove all restrictions
858 and permit any forwarding requests. An argument of none can be
859 used to prohibit all forwarding requests. The wildcard ‘*’ can
860 be used for host or port to allow all hosts or ports respec‐
861 tively. Otherwise, no pattern matching or address lookups are
862 performed on supplied names.
863 PKCS11Provider
865 Specifies which PKCS#11 provider to use or none to indicate that
866 no provider should be used (the default). The argument to this
867 keyword is a path to the PKCS#11 shared library ssh(1) should use
868 to communicate with a PKCS#11 token providing keys for user au‐
869 thentication.
870 Port Specifies the port number to connect on the remote host. The de‐
872 fault is 22.
873 PreferredAuthentications
875 Specifies the order in which the client should try authentication
876 methods. This allows a client to prefer one method (e.g.
877 keyboard-interactive) over another method (e.g. password). The
878 default is:
879 gssapi-with-mic,hostbased,publickey,
881 keyboard-interactive,password
882 ProxyCommand
884 Specifies the command to use to connect to the server. The com‐
885 mand string extends to the end of the line, and is executed using
886 the user's shell ‘exec’ directive to avoid a lingering shell
887 process.
888 Arguments to ProxyCommand accept the tokens described in the
890 TOKENS section. The command can be basically anything, and
891 should read from its standard input and write to its standard
892 output. It should eventually connect an sshd(8) server running
893 on some machine, or execute sshd -i somewhere. Host key manage‐
894 ment will be done using the Hostname of the host being connected
895 (defaulting to the name typed by the user). Setting the command
896 to none disables this option entirely. Note that CheckHostIP is
897 not available for connects with a proxy command.
898 This directive is useful in conjunction with nc(1) and its proxy
900 support. For example, the following directive would connect via
901 an HTTP proxy at 192.0.2.0:
902 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
904 ProxyJump
906 Specifies one or more jump proxies as either [user@]host[:port]
907 or an ssh URI. Multiple proxies may be separated by comma char‐
908 acters and will be visited sequentially. Setting this option
909 will cause ssh(1) to connect to the target host by first making a
910 ssh(1) connection to the specified ProxyJump host and then estab‐
911 lishing a TCP forwarding to the ultimate target from there. Set‐
912 ting the host to none disables this option entirely.
913 Note that this option will compete with the ProxyCommand option -
915 whichever is specified first will prevent later instances of the
916 other from taking effect.
917 Note also that the configuration for the destination host (either
919 supplied via the command-line or the configuration file) is not
920 generally applied to jump hosts. ~/.ssh/config should be used if
921 specific configuration is required for jump hosts.
922 ProxyUseFdpass
924 Specifies that ProxyCommand will pass a connected file descriptor
925 back to ssh(1) instead of continuing to execute and pass data.
926 The default is no.
927 PubkeyAcceptedAlgorithms
929 The default is handled system-wide by crypto-policies(7). To see
930 the defaults and how to modify this default, see manual page
931 update-crypto-policies(8).
932 Specifies the signature algorithms that will be used for public
934 key authentication as a comma-separated list of patterns. If the
935 specified list begins with a ‘+’ character, then the algorithms
936 after it will be appended to the built-in openssh default instead
937 of replacing it. If the specified list begins with a ‘-’ charac‐
938 ter, then the specified algorithms (including wildcards) will be
939 removed from the built-in openssh default set instead of replac‐
940 ing them. If the specified list begins with a ‘^’ character,
941 then the specified algorithms will be placed at the head of the
942 built-in openssh default set.
943 The list of available signature algorithms may also be obtained
945 using "ssh -Q PubkeyAcceptedAlgorithms".
946 PubkeyAuthentication
948 Specifies whether to try public key authentication. The argument
949 to this keyword must be yes (the default) or no.
950 RekeyLimit
952 Specifies the maximum amount of data that may be transmitted be‐
953 fore the session key is renegotiated, optionally followed by a
954 maximum amount of time that may pass before the session key is
955 renegotiated. The first argument is specified in bytes and may
956 have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes,
957 Megabytes, or Gigabytes, respectively. The default is between
958 ‘1G’ and ‘4G’, depending on the cipher. The optional second
959 value is specified in seconds and may use any of the units docu‐
960 mented in the TIME FORMATS section of sshd_config(5). The de‐
961 fault value for RekeyLimit is default none, which means that
962 rekeying is performed after the cipher's default amount of data
963 has been sent or received and no time based rekeying is done.
964 RemoteCommand
966 Specifies a command to execute on the remote machine after suc‐
967 cessfully connecting to the server. The command string extends
968 to the end of the line, and is executed with the user's shell.
969 Arguments to RemoteCommand accept the tokens described in the
970 TOKENS section.
971 RemoteForward
973 Specifies that a TCP port on the remote machine be forwarded over
974 the secure channel. The remote port may either be forwarded to a
975 specified host and port from the local machine, or may act as a
976 SOCKS 4/5 proxy that allows a remote client to connect to arbi‐
977 trary destinations from the local machine. The first argument is
978 the listening specification and may be [bind_address:]port or, if
979 the remote host supports it, a Unix domain socket path. If for‐
980 warding to a specific destination then the second argument must
981 be host:hostport or a Unix domain socket path, otherwise if no
982 destination argument is specified then the remote forwarding will
983 be established as a SOCKS proxy. When acting as a SOCKS proxy
984 the destination of the connection can be restricted by
985 PermitRemoteOpen.
986 IPv6 addresses can be specified by enclosing addresses in square
988 brackets. Multiple forwardings may be specified, and additional
989 forwardings can be given on the command line. Privileged ports
990 can be forwarded only when logging in as root on the remote ma‐
991 chine. Unix domain socket paths may use the tokens described in
992 the TOKENS section and environment variables as described in the
993 ENVIRONMENT VARIABLES section.
994 If the port argument is 0, the listen port will be dynamically
996 allocated on the server and reported to the client at run time.
997 If the bind_address is not specified, the default is to only bind
999 to loopback addresses. If the bind_address is ‘*’ or an empty
1000 string, then the forwarding is requested to listen on all inter‐
1001 faces. Specifying a remote bind_address will only succeed if the
1002 server's GatewayPorts option is enabled (see sshd_config(5)).
1003 RequestTTY
1005 Specifies whether to request a pseudo-tty for the session. The
1006 argument may be one of: no (never request a TTY), yes (always re‐
1007 quest a TTY when standard input is a TTY), force (always request
1008 a TTY) or auto (request a TTY when opening a login session).
1009 This option mirrors the -t and -T flags for ssh(1).
1010 RevokedHostKeys
1012 Specifies revoked host public keys. Keys listed in this file
1013 will be refused for host authentication. Note that if this file
1014 does not exist or is not readable, then host authentication will
1015 be refused for all hosts. Keys may be specified as a text file,
1016 listing one public key per line, or as an OpenSSH Key Revocation
1017 List (KRL) as generated by ssh-keygen(1). For more information
1018 on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).
1019 SecurityKeyProvider
1021 Specifies a path to a library that will be used when loading any
1022 FIDO authenticator-hosted keys, overriding the default of using
1023 the built-in USB HID support.
1024 If the specified value begins with a ‘$’ character, then it will
1026 be treated as an environment variable containing the path to the
1027 library.
1028 SendEnv
1030 Specifies what variables from the local environ(7) should be sent
1031 to the server. The server must also support it, and the server
1032 must be configured to accept these environment variables. Note
1033 that the TERM environment variable is always sent whenever a
1034 pseudo-terminal is requested as it is required by the protocol.
1035 Refer to AcceptEnv in sshd_config(5) for how to configure the
1036 server. Variables are specified by name, which may contain wild‐
1037 card characters. Multiple environment variables may be separated
1038 by whitespace or spread across multiple SendEnv directives.
1039 See PATTERNS for more information on patterns.
1041 It is possible to clear previously set SendEnv variable names by
1043 prefixing patterns with -. The default is not to send any envi‐
1044 ronment variables.
1045 ServerAliveCountMax
1047 Sets the number of server alive messages (see below) which may be
1048 sent without ssh(1) receiving any messages back from the server.
1049 If this threshold is reached while server alive messages are be‐
1050 ing sent, ssh will disconnect from the server, terminating the
1051 session. It is important to note that the use of server alive
1052 messages is very different from TCPKeepAlive (below). The server
1053 alive messages are sent through the encrypted channel and there‐
1054 fore will not be spoofable. The TCP keepalive option enabled by
1055 TCPKeepAlive is spoofable. The server alive mechanism is valu‐
1056 able when the client or server depend on knowing when a connec‐
1057 tion has become unresponsive.
1058 The default value is 3. If, for example, ServerAliveInterval
1060 (see below) is set to 15 and ServerAliveCountMax is left at the
1061 default, if the server becomes unresponsive, ssh will disconnect
1062 after approximately 45 seconds.
1063 ServerAliveInterval
1065 Sets a timeout interval in seconds after which if no data has
1066 been received from the server, ssh(1) will send a message through
1067 the encrypted channel to request a response from the server. The
1068 default is 0, indicating that these messages will not be sent to
1069 the server.
1070 SessionType
1072 May be used to either request invocation of a subsystem on the
1073 remote system, or to prevent the execution of a remote command at
1074 all. The latter is useful for just forwarding ports. The argu‐
1075 ment to this keyword must be none (same as the -N option),
1076 subsystem (same as the -s option) or default (shell or command
1077 execution).
1078 SetEnv Directly specify one or more environment variables and their con‐
1080 tents to be sent to the server. Similarly to SendEnv, with the
1081 exception of the TERM variable, the server must be prepared to
1082 accept the environment variable.
1083 StdinNull
1085 Redirects stdin from /dev/null (actually, prevents reading from
1086 stdin). Either this or the equivalent -n option must be used
1087 when ssh is run in the background. The argument to this keyword
1088 must be yes (same as the -n option) or no (the default).
1089 StreamLocalBindMask
1091 Sets the octal file creation mode mask (umask) used when creating
1092 a Unix-domain socket file for local or remote port forwarding.
1093 This option is only used for port forwarding to a Unix-domain
1094 socket file.
1095 The default value is 0177, which creates a Unix-domain socket
1097 file that is readable and writable only by the owner. Note that
1098 not all operating systems honor the file mode on Unix-domain
1099 socket files.
1100 StreamLocalBindUnlink
1102 Specifies whether to remove an existing Unix-domain socket file
1103 for local or remote port forwarding before creating a new one.
1104 If the socket file already exists and StreamLocalBindUnlink is
1105 not enabled, ssh will be unable to forward the port to the Unix-
1106 domain socket file. This option is only used for port forwarding
1107 to a Unix-domain socket file.
1108 The argument must be yes or no (the default).
1110 StrictHostKeyChecking
1112 If this flag is set to yes, ssh(1) will never automatically add
1113 host keys to the ~/.ssh/known_hosts file, and refuses to connect
1114 to hosts whose host key has changed. This provides maximum pro‐
1115 tection against man-in-the-middle (MITM) attacks, though it can
1116 be annoying when the /etc/ssh/ssh_known_hosts file is poorly
1117 maintained or when connections to new hosts are frequently made.
1118 This option forces the user to manually add all new hosts.
1119 If this flag is set to accept-new then ssh will automatically add
1121 new host keys to the user's known_hosts file, but will not permit
1122 connections to hosts with changed host keys. If this flag is set
1123 to no or off, ssh will automatically add new host keys to the
1124 user known hosts files and allow connections to hosts with
1125 changed hostkeys to proceed, subject to some restrictions. If
1126 this flag is set to ask (the default), new host keys will be
1127 added to the user known host files only after the user has con‐
1128 firmed that is what they really want to do, and ssh will refuse
1129 to connect to hosts whose host key has changed. The host keys of
1130 known hosts will be verified automatically in all cases.
1131 SyslogFacility
1133 Gives the facility code that is used when logging messages from
1134 ssh(1). The possible values are: DAEMON, USER, AUTH, LOCAL0, LO‐
1135 CAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The de‐
1136 fault is USER.
1137 TCPKeepAlive
1139 Specifies whether the system should send TCP keepalive messages
1140 to the other side. If they are sent, death of the connection or
1141 crash of one of the machines will be properly noticed. However,
1142 this means that connections will die if the route is down tempo‐
1143 rarily, and some people find it annoying.
1144 The default is yes (to send TCP keepalive messages), and the
1146 client will notice if the network goes down or the remote host
1147 dies. This is important in scripts, and many users want it too.
1148 To disable TCP keepalive messages, the value should be set to no.
1150 See also ServerAliveInterval for protocol-level keepalives.
1151 Tunnel Request tun(4) device forwarding between the client and the
1153 server. The argument must be yes, point-to-point (layer 3),
1154 ethernet (layer 2), or no (the default). Specifying yes requests
1155 the default tunnel mode, which is point-to-point.
1156 TunnelDevice
1158 Specifies the tun(4) devices to open on the client (local_tun)
1159 and the server (remote_tun).
1160 The argument must be local_tun[:remote_tun]. The devices may be
1162 specified by numerical ID or the keyword any, which uses the next
1163 available tunnel device. If remote_tun is not specified, it de‐
1164 faults to any. The default is any:any.
1165 UpdateHostKeys
1167 Specifies whether ssh(1) should accept notifications of addi‐
1168 tional hostkeys from the server sent after authentication has
1169 completed and add them to UserKnownHostsFile. The argument must
1170 be yes, no or ask. This option allows learning alternate
1171 hostkeys for a server and supports graceful key rotation by al‐
1172 lowing a server to send replacement public keys before old ones
1173 are removed.
1174 Additional hostkeys are only accepted if the key used to authen‐
1176 ticate the host was already trusted or explicitly accepted by the
1177 user, the host was authenticated via UserKnownHostsFile (i.e. not
1178 GlobalKnownHostsFile) and the host was authenticated using a
1179 plain key and not a certificate.
1180 UpdateHostKeys is enabled by default if the user has not overrid‐
1182 den the default UserKnownHostsFile setting and has not enabled
1183 VerifyHostKeyDNS, otherwise UpdateHostKeys will be set to no.
1184 If UpdateHostKeys is set to ask, then the user is asked to con‐
1186 firm the modifications to the known_hosts file. Confirmation is
1187 currently incompatible with ControlPersist, and will be disabled
1188 if it is enabled.
1189 Presently, only sshd(8) from OpenSSH 6.8 and greater support the
1191 "hostkeys@openssh.com" protocol extension used to inform the
1192 client of all the server's hostkeys.
1193 User Specifies the user to log in as. This can be useful when a dif‐
1195 ferent user name is used on different machines. This saves the
1196 trouble of having to remember to give the user name on the com‐
1197 mand line.
1198 UserKnownHostsFile
1200 Specifies one or more files to use for the user host key data‐
1201 base, separated by whitespace. Each filename may use tilde nota‐
1202 tion to refer to the user's home directory, the tokens described
1203 in the TOKENS section and environment variables as described in
1204 the ENVIRONMENT VARIABLES section. The default is
1205 ~/.ssh/known_hosts, ~/.ssh/known_hosts2.
1206 VerifyHostKeyDNS
1208 Specifies whether to verify the remote key using DNS and SSHFP
1209 resource records. If this option is set to yes, the client will
1210 implicitly trust keys that match a secure fingerprint from DNS.
1211 Insecure fingerprints will be handled as if this option was set
1212 to ask. If this option is set to ask, information on fingerprint
1213 match will be displayed, but the user will still need to confirm
1214 new host keys according to the StrictHostKeyChecking option. The
1215 default is no.
1216 See also VERIFYING HOST KEYS in ssh(1).
1218 VisualHostKey
1220 If this flag is set to yes, an ASCII art representation of the
1221 remote host key fingerprint is printed in addition to the finger‐
1222 print string at login and for unknown host keys. If this flag is
1223 set to no (the default), no fingerprint strings are printed at
1224 login and only the fingerprint string will be printed for unknown
1225 host keys.
1226 XAuthLocation
1228 Specifies the full pathname of the xauth(1) program. The default
1229 is /usr/bin/xauth.
1230
1232 A pattern consists of zero or more non-whitespace characters, ‘*’ (a
1233 wildcard that matches zero or more characters), or ‘?’ (a wildcard that
1234 matches exactly one character). For example, to specify a set of decla‐
1235 rations for any host in the ".co.uk" set of domains, the following pat‐
1236 tern could be used:
1237 Host *.co.uk
1239 The following pattern would match any host in the 192.168.0.[0-9] network
1241 range:
1242 Host 192.168.0.?
1244 A pattern-list is a comma-separated list of patterns. Patterns within
1246 pattern-lists may be negated by preceding them with an exclamation mark
1247 (‘!’). For example, to allow a key to be used from anywhere within an
1248 organization except from the "dialup" pool, the following entry (in au‐
1249 thorized_keys) could be used:
1250 from="!*.dialup.example.com,*.example.com"
1252 Note that a negated match will never produce a positive result by itself.
1254 For example, attempting to match "host3" against the following pattern-
1255 list will fail:
1256 from="!host1,!host2"
1258 The solution here is to include a term that will yield a positive match,
1260 such as a wildcard:
1261 from="!host1,!host2,*"
1263
1265 Arguments to some keywords can make use of tokens, which are expanded at
1266 runtime:
1267 %% A literal ‘%’.
1269 %C Hash of %l%h%p%r.
1270 %d Local user's home directory.
1271 %f The fingerprint of the server's host key.
1272 %H The known_hosts hostname or address that is being searched
1273 for.
1274 %h The remote hostname.
1275 %I A string describing the reason for a KnownHostsCommand execu‐
1276 tion: either ADDRESS when looking up a host by address (only
1277 when CheckHostIP is enabled), HOSTNAME when searching by
1278 hostname, or ORDER when preparing the host key algorithm
1279 preference list to use for the destination host.
1280 %i The local user ID.
1281 %K The base64 encoded host key.
1282 %k The host key alias if specified, otherwise the original re‐
1283 mote hostname given on the command line.
1284 %L The local hostname.
1285 %l The local hostname, including the domain name.
1286 %n The original remote hostname, as given on the command line.
1287 %p The remote port.
1288 %r The remote username.
1289 %T The local tun(4) or tap(4) network interface assigned if tun‐
1290 nel forwarding was requested, or "NONE" otherwise.
1291 %t The type of the server host key, e.g. ssh-ed25519.
1292 %u The local username.
1293 CertificateFile, ControlPath, IdentityAgent, IdentityFile,
1295 KnownHostsCommand, LocalForward, Match exec, RemoteCommand,
1296 RemoteForward, and UserKnownHostsFile accept the tokens %%, %C, %d, %h,
1297 %i, %k, %L, %l, %n, %p, %r, and %u.
1298 KnownHostsCommand additionally accepts the tokens %f, %H, %I, %K and %t.
1300 Hostname accepts the tokens %% and %h.
1302 LocalCommand accepts all tokens.
1304 ProxyCommand accepts the tokens %%, %h, %n, %p, and %r.
1306
1308 Arguments to some keywords can be expanded at runtime from environment
1309 variables on the client by enclosing them in ${}, for example
1310 ${HOME}/.ssh would refer to the user's .ssh directory. If a specified
1311 environment variable does not exist then an error will be returned and
1312 the setting for that keyword will be ignored.
1313 The keywords CertificateFile, ControlPath, IdentityAgent, IdentityFile,
1315 KnownHostsCommand, and UserKnownHostsFile support environment variables.
1316 The keywords LocalForward and RemoteForward support environment variables
1317 only for Unix domain socket paths.
1318
1320 ~/.ssh/config
1321 This is the per-user configuration file. The format of this file
1322 is described above. This file is used by the SSH client. Be‐
1323 cause of the potential for abuse, this file must have strict per‐
1324 missions: read/write for the user, and not writable by others.
1325 /etc/ssh/ssh_config
1327 Systemwide configuration file. This file provides defaults for
1328 those values that are not specified in the user's configuration
1329 file, and for those users who do not have a configuration file.
1330 This file must be world-readable.
1331
1333 ssh(1)
1334
1336 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1337 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1338 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1339 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
1340 versions 1.5 and 2.0.
1341BSD September 25, 2021 BSD