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. GSSAPI configuration file ($HOME/.ssh/config.gssapi)
13 4. Kerberos configuration file ($HOME/.ssh/config.krb)
14 5. system-wide configuration file (/etc/gsissh/ssh_config)
15 For each parameter, the first obtained value will be used. The configu‐
17 ration files contain sections separated by Host specifications, and that
18 section is only applied for hosts that match one of the patterns given in
19 the specification. The matched host name is usually the one given on the
20 command line (see the CanonicalizeHostname option for exceptions).
21 Since the first obtained value for each parameter is used, more host-spe‐
23 cific declarations should be given near the beginning of the file, and
24 general defaults at the end.
25 The file contains keyword-argument pairs, one per line. Lines starting
27 with ‘#’ and empty lines are interpreted as comments. Arguments may op‐
28 tionally be enclosed in double quotes (") in order to represent arguments
29 containing spaces. Configuration options may be separated by whitespace
30 or optional whitespace and exactly one ‘=’; the latter format is useful
31 to avoid the need to quote whitespace when specifying configuration op‐
32 tions using the ssh, scp, and sftp -o option.
33 The possible keywords and their meanings are as follows (note that key‐
35 words are case-insensitive and arguments are case-sensitive):
36 Host Restricts the following declarations (up to the next Host or
38 Match keyword) to be only for those hosts that match one of the
39 patterns given after the keyword. If more than one pattern is
40 provided, they should be separated by whitespace. A single ‘*’
41 as a pattern can be used to provide global defaults for all
42 hosts. The host is usually the hostname argument given on the
43 command line (see the CanonicalizeHostname keyword for excep‐
44 tions).
45 A pattern entry may be negated by prefixing it with an exclama‐
47 tion mark (‘!’). If a negated entry is matched, then the Host
48 entry is ignored, regardless of whether any other patterns on the
49 line match. Negated matches are therefore useful to provide ex‐
50 ceptions for wildcard matches.
51 See PATTERNS for more information on patterns.
53 Match Restricts the following declarations (up to the next Host or
55 Match keyword) to be used only when the conditions following the
56 Match keyword are satisfied. Match conditions are specified us‐
57 ing one or more criteria or the single token all which always
58 matches. The available criteria keywords are: canonical, final,
59 exec, host, originalhost, user, and localuser. The all criteria
60 must appear alone or immediately after canonical or final. Other
61 criteria may be combined arbitrarily. All criteria but all,
62 canonical, and final require an argument. Criteria may be
63 negated by prepending an exclamation mark (‘!’).
64 The canonical keyword matches only when the configuration file is
66 being re-parsed after hostname canonicalization (see the
67 CanonicalizeHostname option). This may be useful to specify con‐
68 ditions that work with canonical host names only.
69 The final keyword requests that the configuration be re-parsed
71 (regardless of whether CanonicalizeHostname is enabled), and
72 matches only during this final pass. If CanonicalizeHostname is
73 enabled, then canonical and final match during the same pass.
74 The exec keyword executes the specified command under the user's
76 shell. If the command returns a zero exit status then the condi‐
77 tion is considered true. Commands containing whitespace charac‐
78 ters must be quoted. Arguments to exec accept the tokens de‐
79 scribed in the TOKENS section.
80 The other keywords' criteria must be single entries or comma-sep‐
82 arated lists and may use the wildcard and negation operators de‐
83 scribed in the PATTERNS section. The criteria for the host key‐
84 word are matched against the target hostname, after any substitu‐
85 tion by the Hostname or CanonicalizeHostname options. The
86 originalhost keyword matches against the hostname as it was spec‐
87 ified on the command-line. The user keyword matches against the
88 target username on the remote host. The localuser keyword
89 matches against the name of the local user running ssh(1) (this
90 keyword may be useful in system-wide ssh_config files).
91 AddKeysToAgent
93 Specifies whether keys should be automatically added to a running
94 ssh-agent(1). If this option is set to yes and a key is loaded
95 from a file, the key and its passphrase are added to the agent
96 with the default lifetime, as if by ssh-add(1). If this option
97 is set to ask, ssh(1) will require confirmation using the
98 SSH_ASKPASS program before adding a key (see ssh-add(1) for de‐
99 tails). If this option is set to confirm, each use of the key
100 must be confirmed, as if the -c option was specified to
101 ssh-add(1). If this option is set to no, no keys are added to
102 the agent. Alternately, this option may be specified as a time
103 interval using the format described in the TIME FORMATS section
104 of sshd_config(5) to specify the key's lifetime in ssh-agent(1),
105 after which it will automatically be removed. The argument must
106 be no (the default), yes, confirm (optionally followed by a time
107 interval), ask or a time interval.
108 AddressFamily
110 Specifies which address family to use when connecting. Valid ar‐
111 guments are any (the default), inet (use IPv4 only), or inet6
112 (use IPv6 only).
113 BatchMode
115 If set to yes, user interaction such as password prompts and host
116 key confirmation requests will be disabled. This option is use‐
117 ful in scripts and other batch jobs where no user is present to
118 interact with ssh(1). The argument must be yes or no (the de‐
119 fault).
120 BindAddress
122 Use the specified address on the local machine as the source ad‐
123 dress of the connection. Only useful on systems with more than
124 one address.
125 BindInterface
127 Use the address of the specified interface on the local machine
128 as the source address of the connection.
129 CanonicalDomains
131 When CanonicalizeHostname is enabled, this option specifies the
132 list of domain suffixes in which to search for the specified des‐
133 tination host.
134 CanonicalizeFallbackLocal
136 Specifies whether to fail with an error when hostname canonical‐
137 ization fails. The default, yes, will attempt to look up the un‐
138 qualified hostname using the system resolver's search rules. A
139 value of no will cause ssh(1) to fail instantly if
140 CanonicalizeHostname is enabled and the target hostname cannot be
141 found in any of the domains specified by CanonicalDomains.
142 CanonicalizeHostname
144 Controls whether explicit hostname canonicalization is performed.
145 The default, no, is not to perform any name rewriting and let the
146 system resolver handle all hostname lookups. If set to yes then,
147 for connections that do not use a ProxyCommand or ProxyJump,
148 ssh(1) will attempt to canonicalize the hostname specified on the
149 command line using the CanonicalDomains suffixes and
150 CanonicalizePermittedCNAMEs rules. If CanonicalizeHostname is
151 set to always, then canonicalization is applied to proxied con‐
152 nections too.
153 If this option is enabled, then the configuration files are pro‐
155 cessed again using the new target name to pick up any new config‐
156 uration in matching Host and Match stanzas. A value of none dis‐
157 ables the use of a ProxyJump host.
158 CanonicalizeMaxDots
160 Specifies the maximum number of dot characters in a hostname be‐
161 fore canonicalization is disabled. The default, 1, allows a sin‐
162 gle dot (i.e. hostname.subdomain).
163 CanonicalizePermittedCNAMEs
165 Specifies rules to determine whether CNAMEs should be followed
166 when canonicalizing hostnames. The rules consist of one or more
167 arguments of source_domain_list:target_domain_list, where
168 source_domain_list is a pattern-list of domains that may follow
169 CNAMEs in canonicalization, and target_domain_list is a pattern-
170 list of domains that they may resolve to.
171 For example, "*.a.example.com:*.b.example.com,*.c.example.com"
173 will allow hostnames matching "*.a.example.com" to be canonical‐
174 ized to names in the "*.b.example.com" or "*.c.example.com" do‐
175 mains.
176 A single argument of "none" causes no CNAMEs to be considered for
178 canonicalization. This is the default behaviour.
179 CASignatureAlgorithms
181 The default is handled system-wide by crypto-policies(7). To see
182 the defaults and how to modify this default, see manual page
183 update-crypto-policies(8).
184 Specifies which algorithms are allowed for signing of certifi‐
186 cates by certificate authorities (CAs). If the specified list
187 begins with a ‘+’ character, then the specified algorithms will
188 be appended to the default set instead of replacing them. If the
189 specified list begins with a ‘-’ character, then the specified
190 algorithms (including wildcards) will be removed from the default
191 set instead of replacing them.
192 ssh(1) will not accept host certificates signed using algorithms
194 other than those specified.
195 CertificateFile
197 Specifies a file from which the user's certificate is read. A
198 corresponding private key must be provided separately in order to
199 use this certificate either from an IdentityFile directive or -i
200 flag to ssh(1), via ssh-agent(1), or via a PKCS11Provider or
201 SecurityKeyProvider.
202 Arguments to CertificateFile may use the tilde syntax to refer to
204 a user's home directory, the tokens described in the TOKENS sec‐
205 tion and environment variables as described in the ENVIRONMENT
206 VARIABLES section.
207 It is possible to have multiple certificate files specified in
209 configuration files; these certificates will be tried in se‐
210 quence. Multiple CertificateFile directives will add to the list
211 of certificates used for authentication.
212 CheckHostIP
214 If set to yes [22mssh(1) will additionally check the host IP address
215 in the known_hosts file. This allows it to detect if a host key
216 changed due to DNS spoofing and will add addresses of destination
217 hosts to ~/.ssh/known_hosts in the process, regardless of the
218 setting of StrictHostKeyChecking. If the option is set to no
219 (the default), the check will not be executed.
220 Ciphers
222 The default is handled system-wide by crypto-policies(7). To see
223 the defaults and how to modify this default, see manual page
224 update-crypto-policies(8).
225 Specifies the ciphers allowed and their order of preference.
227 Multiple ciphers must be comma-separated. If the specified list
228 begins with a ‘+’ character, then the specified ciphers will be
229 appended to the built-in openssh default set instead of replacing
230 them. If the specified list begins with a ‘-’ character, then
231 the specified ciphers (including wildcards) will be removed from
232 the built-in openssh default set instead of replacing them. If
233 the specified list begins with a ‘^’ character, then the speci‐
234 fied ciphers will be placed at the head of the built-in openssh
235 default set.
236 The supported ciphers are:
238 3des-cbc
240 aes128-cbc
241 aes192-cbc
242 aes256-cbc
243 aes128-ctr
244 aes192-ctr
245 aes256-ctr
246 aes128-gcm@openssh.com
247 aes256-gcm@openssh.com
248 chacha20-poly1305@openssh.com
249 The list of available ciphers may also be obtained using "ssh -Q
251 cipher".
252 ClearAllForwardings
254 Specifies that all local, remote, and dynamic port forwardings
255 specified in the configuration files or on the command line be
256 cleared. This option is primarily useful when used from the
257 ssh(1) command line to clear port forwardings set in configura‐
258 tion files, and is automatically set by scp(1) and sftp(1). The
259 argument must be yes or no (the default).
260 Compression
262 Specifies whether to use compression. The argument must be yes
263 or no (the default).
264 ConnectionAttempts
266 Specifies the number of tries (one per second) to make before ex‐
267 iting. The argument must be an integer. This may be useful in
268 scripts if the connection sometimes fails. The default is 1.
269 ConnectTimeout
271 Specifies the timeout (in seconds) used when connecting to the
272 SSH server, instead of using the default system TCP timeout.
273 This timeout is applied both to establishing the connection and
274 to performing the initial SSH protocol handshake and key ex‐
275 change.
276 ControlMaster
278 Enables the sharing of multiple sessions over a single network
279 connection. When set to yes, ssh(1) will listen for connections
280 on a control socket specified using the ControlPath argument.
281 Additional sessions can connect to this socket using the same
282 ControlPath with ControlMaster set to no (the default). These
283 sessions will try to reuse the master instance's network connec‐
284 tion rather than initiating new ones, but will fall back to con‐
285 necting normally if the control socket does not exist, or is not
286 listening.
287 Setting this to ask will cause ssh(1) to listen for control con‐
289 nections, but require confirmation using ssh-askpass(1). If the
290 ControlPath cannot be opened, ssh(1) will continue without con‐
291 necting to a master instance.
292 X11 and ssh-agent(1) forwarding is supported over these multi‐
294 plexed connections, however the display and agent forwarded will
295 be the one belonging to the master connection i.e. it is not pos‐
296 sible to forward multiple displays or agents.
297 Two additional options allow for opportunistic multiplexing: try
299 to use a master connection but fall back to creating a new one if
300 one does not already exist. These options are: auto and autoask.
301 The latter requires confirmation like the ask option.
302 ControlPath
304 Specify the path to the control socket used for connection shar‐
305 ing as described in the ControlMaster section above or the string
306 none to disable connection sharing. Arguments to ControlPath may
307 use the tilde syntax to refer to a user's home directory, the to‐
308 kens described in the TOKENS section and environment variables as
309 described in the ENVIRONMENT VARIABLES section. It is recom‐
310 mended that any ControlPath used for opportunistic connection
311 sharing include at least %h, %p, and %r (or alternatively %C) and
312 be placed in a directory that is not writable by other users.
313 This ensures that shared connections are uniquely identified.
314 ControlPersist
316 When used in conjunction with ControlMaster, specifies that the
317 master connection should remain open in the background (waiting
318 for future client connections) after the initial client connec‐
319 tion has been closed. If set to no (the default), then the mas‐
320 ter connection will not be placed into the background, and will
321 close as soon as the initial client connection is closed. If set
322 to yes or 0, then the master connection will remain in the back‐
323 ground indefinitely (until killed or closed via a mechanism such
324 as the "ssh -O exit"). If set to a time in seconds, or a time in
325 any of the formats documented in sshd_config(5), then the back‐
326 grounded master connection will automatically terminate after it
327 has remained idle (with no client connections) for the specified
328 time.
329 DynamicForward
331 Specifies that a TCP port on the local machine be forwarded over
332 the secure channel, and the application protocol is then used to
333 determine where to connect to from the remote machine.
334 The argument must be [bind_address:]port. IPv6 addresses can be
336 specified by enclosing addresses in square brackets. By default,
337 the local port is bound in accordance with the GatewayPorts set‐
338 ting. However, an explicit bind_address may be used to bind the
339 connection to a specific address. The bind_address of localhost
340 indicates that the listening port be bound for local use only,
341 while an empty address or ‘*’ indicates that the port should be
342 available from all interfaces.
343 Currently the SOCKS4 and SOCKS5 protocols are supported, and
345 ssh(1) will act as a SOCKS server. Multiple forwardings may be
346 specified, and additional forwardings can be given on the command
347 line. Only the superuser can forward privileged ports.
348 EnableSSHKeysign
350 Setting this option to yes in the global client configuration
351 file /etc/gsissh/ssh_config enables the use of the helper program
352 ssh-keysign(8) during HostbasedAuthentication. The argument must
353 be yes or no (the default). This option should be placed in the
354 non-hostspecific section. See ssh-keysign(8) for more informa‐
355 tion.
356 EscapeChar
358 Sets the escape character (default: ‘~’). The escape character
359 can also be set on the command line. The argument should be a
360 single character, ‘^’ followed by a letter, or none to disable
361 the escape character entirely (making the connection transparent
362 for binary data).
363 ExitOnForwardFailure
365 Specifies whether ssh(1) should terminate the connection if it
366 cannot set up all requested dynamic, tunnel, local, and remote
367 port forwardings, (e.g. if either end is unable to bind and lis‐
368 ten on a specified port). Note that ExitOnForwardFailure does
369 not apply to connections made over port forwardings and will not,
370 for example, cause ssh(1) to exit if TCP connections to the ulti‐
371 mate forwarding destination fail. The argument must be yes or no
372 (the default).
373 FingerprintHash
375 Specifies the hash algorithm used when displaying key finger‐
376 prints. Valid options are: md5 and sha256 (the default).
377 ForkAfterAuthentication
379 Requests ssh to go to background just before command execution.
380 This is useful if ssh is going to ask for passwords or
381 passphrases, but the user wants it in the background. This im‐
382 plies the StdinNull configuration option being set to “yes”. The
383 recommended way to start X11 programs at a remote site is with
384 something like ssh -f host xterm, which is the same as ssh host
385 xterm if the ForkAfterAuthentication configuration option is set
386 to “yes”.
387 If the ExitOnForwardFailure configuration option is set to “yes”,
389 then a client started with the ForkAfterAuthentication configura‐
390 tion option being set to “yes” will wait for all remote port for‐
391 wards to be successfully established before placing itself in the
392 background. The argument to this keyword must be yes (same as
393 the -f option) or no (the default).
394 ForwardAgent
396 Specifies whether the connection to the authentication agent (if
397 any) will be forwarded to the remote machine. The argument may
398 be yes, no (the default), an explicit path to an agent socket or
399 the name of an environment variable (beginning with ‘$’) in which
400 to find the path.
401 Agent forwarding should be enabled with caution. Users with the
403 ability to bypass file permissions on the remote host (for the
404 agent's Unix-domain socket) can access the local agent through
405 the forwarded connection. An attacker cannot obtain key material
406 from the agent, however they can perform operations on the keys
407 that enable them to authenticate using the identities loaded into
408 the agent.
409 ForwardX11
411 Specifies whether X11 connections will be automatically redi‐
412 rected over the secure channel and DISPLAY set. The argument
413 must be yes or no (the default).
414 X11 forwarding should be enabled with caution. Users with the
416 ability to bypass file permissions on the remote host (for the
417 user's X11 authorization database) can access the local X11 dis‐
418 play through the forwarded connection. An attacker may then be
419 able to perform activities such as keystroke monitoring if the
420 ForwardX11Trusted option is also enabled.
421 ForwardX11Timeout
423 Specify a timeout for untrusted X11 forwarding using the format
424 described in the TIME FORMATS section of sshd_config(5). X11
425 connections received by ssh(1) after this time will be refused.
426 Setting ForwardX11Timeout to zero will disable the timeout and
427 permit X11 forwarding for the life of the connection. The de‐
428 fault is to disable untrusted X11 forwarding after twenty minutes
429 has elapsed.
430 ForwardX11Trusted
432 If this option is set to yes, remote X11 clients will have full
433 access to the original X11 display.
434 If this option is set to no (the default), remote X11 clients
436 will be considered untrusted and prevented from stealing or tam‐
437 pering with data belonging to trusted X11 clients. Furthermore,
438 the xauth(1) token used for the session will be set to expire af‐
439 ter 20 minutes. Remote clients will be refused access after this
440 time.
441 See the X11 SECURITY extension specification for full details on
443 the restrictions imposed on untrusted clients.
444 GatewayPorts
446 Specifies whether remote hosts are allowed to connect to local
447 forwarded ports. By default, ssh(1) binds local port forwardings
448 to the loopback address. This prevents other remote hosts from
449 connecting to forwarded ports. GatewayPorts can be used to spec‐
450 ify that ssh should bind local port forwardings to the wildcard
451 address, thus allowing remote hosts to connect to forwarded
452 ports. The argument must be yes or no (the default).
453 GlobalKnownHostsFile
455 Specifies one or more files to use for the global host key data‐
456 base, separated by whitespace. The default is
457 /etc/gsissh/ssh_known_hosts, /etc/gsissh/ssh_known_hosts2.
458 GSSAPIAuthentication
460 Specifies whether user authentication based on GSSAPI is allowed.
461 The default is yes.
462 GSSAPIClientIdentity
464 If set, specifies the GSSAPI client identity that ssh should use
465 when connecting to the server. The default is unset, which means
466 that the default identity will be used.
467 GSSAPIDelegateCredentials
469 Forward (delegate) credentials to the server. The default is
470 yes.
471 GSSAPIKeyExchange
473 Specifies whether key exchange based on GSSAPI may be used. When
474 using GSSAPI key exchange the server need not have a host key.
475 The default is “yes”.
476 GSSAPIRenewalForcesRekey
478 If set to “yes” then renewal of the client's GSSAPI credentials
479 will force the rekeying of the ssh connection. With a compatible
480 server, this will delegate the renewed credentials to a session
481 on the server.
482 Checks are made to ensure that credentials are only propagated
484 when the new credentials match the old ones on the originating
485 client and where the receiving server still has the old set in
486 its cache.
487 The default is “no”.
489 For this to work GSSAPIKeyExchange needs to be enabled in the
491 server and also used by the client.
492 GSSAPIServerIdentity
494 If set, specifies the GSSAPI server identity that ssh should ex‐
495 pect when connecting to the server. The default is unset, which
496 means that the expected GSSAPI server identity will be determined
497 from the target hostname.
498 GSSAPITrustDns
500 Set to “yes” to indicate that the DNS is trusted to securely
501 canonicalize the name of the host being connected to. If “no”,
502 the hostname entered on the command line will be passed untouched
503 to the GSSAPI library. The default is “no”.
504 GSSAPIKexAlgorithms
506 The default is handled system-wide by crypto-policies(7). To see
507 the defaults and how to modify this default, see manual page
508 update-crypto-policies(8).
509 The list of key exchange algorithms that are offered for GSSAPI
511 key exchange. Possible values are
512 gss-gex-sha1-,
514 gss-group1-sha1-,
515 gss-group14-sha1-,
516 gss-group14-sha256-,
517 gss-group16-sha512-,
518 gss-nistp256-sha256-,
519 gss-curve25519-sha256-
520 This option only applies to connections using GSSAPI.
522 HashKnownHosts
524 Indicates that ssh(1) should hash host names and addresses when
525 they are added to ~/.ssh/known_hosts. These hashed names may be
526 used normally by ssh(1) and sshd(8), but they do not visually re‐
527 veal identifying information if the file's contents are dis‐
528 closed. The default is no. Note that existing names and ad‐
529 dresses in known hosts files will not be converted automatically,
530 but may be manually hashed using ssh-keygen(1).
531 HostbasedAcceptedAlgorithms
533 Specifies the signature algorithms that will be used for host‐
534 based authentication as a comma-separated list of patterns. Al‐
535 ternately if the specified list begins with a ‘+’ character, then
536 the specified signature algorithms will be appended to the de‐
537 fault set instead of replacing them. If the specified list be‐
538 gins with a ‘-’ character, then the specified signature algo‐
539 rithms (including wildcards) will be removed from the default set
540 instead of replacing them. If the specified list begins with a
541 ‘^’ character, then the specified signature algorithms will be
542 placed at the head of the default set. The default for this op‐
543 tion is:
544 ssh-ed25519-cert-v01@openssh.com,
546 ecdsa-sha2-nistp256-cert-v01@openssh.com,
547 ecdsa-sha2-nistp384-cert-v01@openssh.com,
548 ecdsa-sha2-nistp521-cert-v01@openssh.com,
549 sk-ssh-ed25519-cert-v01@openssh.com,
550 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
551 rsa-sha2-512-cert-v01@openssh.com,
552 rsa-sha2-256-cert-v01@openssh.com,
553 ssh-ed25519,
554 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
555 sk-ssh-ed25519@openssh.com,
556 sk-ecdsa-sha2-nistp256@openssh.com,
557 rsa-sha2-512,rsa-sha2-256
558 The -Q option of ssh(1) may be used to list supported signature
560 algorithms. This was formerly named HostbasedKeyTypes.
561 HostbasedAuthentication
563 Specifies whether to try rhosts based authentication with public
564 key authentication. The argument must be yes or no (the de‐
565 fault).
566 HostKeyAlgorithms
568 Specifies the host key signature algorithms that the client wants
569 to use in order of preference. Alternately if the specified list
570 begins with a ‘+’ character, then the specified signature algo‐
571 rithms will be appended to the default set instead of replacing
572 them. If the specified list begins with a ‘-’ character, then
573 the specified signature algorithms (including wildcards) will be
574 removed from the default set instead of replacing them. If the
575 specified list begins with a ‘^’ character, then the specified
576 signature algorithms will be placed at the head of the default
577 set. The default for this option is:
578 ssh-ed25519-cert-v01@openssh.com,
580 ecdsa-sha2-nistp256-cert-v01@openssh.com,
581 ecdsa-sha2-nistp384-cert-v01@openssh.com,
582 ecdsa-sha2-nistp521-cert-v01@openssh.com,
583 sk-ssh-ed25519-cert-v01@openssh.com,
584 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
585 rsa-sha2-512-cert-v01@openssh.com,
586 rsa-sha2-256-cert-v01@openssh.com,
587 ssh-ed25519,
588 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
589 sk-ecdsa-sha2-nistp256@openssh.com,
590 sk-ssh-ed25519@openssh.com,
591 rsa-sha2-512,rsa-sha2-256
592 If hostkeys are known for the destination host then this default
594 is modified to prefer their algorithms.
595 The list of available signature algorithms may also be obtained
597 using "ssh -Q HostKeyAlgorithms".
598 HostKeyAlias
600 Specifies an alias that should be used instead of the real host
601 name when looking up or saving the host key in the host key data‐
602 base files and when validating host certificates. This option is
603 useful for tunneling SSH connections or for multiple servers run‐
604 ning on a single host.
605 Hostname
607 Specifies the real host name to log into. This can be used to
608 specify nicknames or abbreviations for hosts. Arguments to
609 Hostname accept the tokens described in the TOKENS section. Nu‐
610 meric IP addresses are also permitted (both on the command line
611 and in Hostname specifications). The default is the name given
612 on the command line.
613 IdentitiesOnly
615 Specifies that ssh(1) should only use the configured authentica‐
616 tion identity and certificate files (either the default files, or
617 those explicitly configured in the ssh_config files or passed on
618 the ssh(1) command-line), even if ssh-agent(1) or a
619 PKCS11Provider or SecurityKeyProvider offers more identities.
620 The argument to this keyword must be yes or no (the default).
621 This option is intended for situations where ssh-agent offers
622 many different identities.
623 IdentityAgent
625 Specifies the UNIX-domain socket used to communicate with the au‐
626 thentication agent.
627 This option overrides the SSH_AUTH_SOCK environment variable and
629 can be used to select a specific agent. Setting the socket name
630 to none disables the use of an authentication agent. If the
631 string "SSH_AUTH_SOCK" is specified, the location of the socket
632 will be read from the SSH_AUTH_SOCK environment variable. Other‐
633 wise if the specified value begins with a ‘$’ character, then it
634 will be treated as an environment variable containing the loca‐
635 tion of the socket.
636 Arguments to IdentityAgent may use the tilde syntax to refer to a
638 user's home directory, the tokens described in the TOKENS section
639 and environment variables as described in the ENVIRONMENT
640 VARIABLES section.
641 IdentityFile
643 Specifies a file from which the user's DSA, ECDSA, authenticator-
644 hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA au‐
645 thentication identity is read. The default is ~/.ssh/id_dsa,
646 ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519,
647 ~/.ssh/id_ed25519_sk and ~/.ssh/id_rsa. Additionally, any iden‐
648 tities represented by the authentication agent will be used for
649 authentication unless IdentitiesOnly is set. If no certificates
650 have been explicitly specified by CertificateFile, ssh(1) will
651 try to load certificate information from the filename obtained by
652 appending -cert.pub to the path of a specified IdentityFile.
653 Arguments to IdentityFile may use the tilde syntax to refer to a
655 user's home directory or the tokens described in the TOKENS sec‐
656 tion.
657 It is possible to have multiple identity files specified in con‐
659 figuration files; all these identities will be tried in sequence.
660 Multiple IdentityFile directives will add to the list of identi‐
661 ties tried (this behaviour differs from that of other configura‐
662 tion directives).
663 IdentityFile may be used in conjunction with IdentitiesOnly to
665 select which identities in an agent are offered during authenti‐
666 cation. IdentityFile may also be used in conjunction with
667 CertificateFile in order to provide any certificate also needed
668 for authentication with the identity.
669 The authentication identity can be also specified in a form of
671 PKCS#11 URI starting with a string pkcs11:. There is supported a
672 subset of the PKCS#11 URI as defined in RFC 7512 (implemented
673 path arguments id, manufacturer, object, token and query argu‐
674 ments module-path and pin-value ). The URI can not be in quotes.
675 IgnoreUnknown
677 Specifies a pattern-list of unknown options to be ignored if they
678 are encountered in configuration parsing. This may be used to
679 suppress errors if ssh_config contains options that are unrecog‐
680 nised by ssh(1). It is recommended that IgnoreUnknown be listed
681 early in the configuration file as it will not be applied to un‐
682 known options that appear before it.
683 Include
685 Include the specified configuration file(s). Multiple pathnames
686 may be specified and each pathname may contain glob(7) wildcards
687 and, for user configurations, shell-like ‘~’ references to user
688 home directories. Wildcards will be expanded and processed in
689 lexical order. Files without absolute paths are assumed to be in
690 ~/.ssh if included in a user configuration file or /etc/ssh if
691 included from the system configuration file. Include directive
692 may appear inside a Match or Host block to perform conditional
693 inclusion.
694 IPQoS Specifies the IPv4 type-of-service or DSCP class for connections.
696 Accepted values are af11, af12, af13, af21, af22, af23, af31,
697 af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5, cs6,
698 cs7, ef, le, lowdelay, throughput, reliability, a numeric value,
699 or none to use the operating system default. This option may
700 take one or two arguments, separated by whitespace. If one argu‐
701 ment is specified, it is used as the packet class uncondition‐
702 ally. If two values are specified, the first is automatically
703 selected for interactive sessions and the second for non-interac‐
704 tive sessions. The default is af21 (Low-Latency Data) for inter‐
705 active sessions and cs1 (Lower Effort) for non-interactive ses‐
706 sions.
707 KbdInteractiveAuthentication
709 Specifies whether to use keyboard-interactive authentication.
710 The argument to this keyword must be yes (the default) or no.
711 ChallengeResponseAuthentication is a deprecated alias for this.
712 KbdInteractiveDevices
714 Specifies the list of methods to use in keyboard-interactive au‐
715 thentication. Multiple method names must be comma-separated.
716 The default is to use the server specified list. The methods
717 available vary depending on what the server supports. For an
718 OpenSSH server, it may be zero or more of: bsdauth and pam.
719 KexAlgorithms
721 The default is handled system-wide by crypto-policies(7). To see
722 the defaults and how to modify this default, see manual page
723 update-crypto-policies(8).
724 Specifies the available KEX (Key Exchange) algorithms. Multiple
726 algorithms must be comma-separated. If the specified list begins
727 with a ‘+’ character, then the specified algorithms will be ap‐
728 pended to the built-in openssh default set instead of replacing
729 them. If the specified list begins with a ‘-’ character, then
730 the specified algorithms (including wildcards) will be removed
731 from the built-in openssh default set instead of replacing them.
732 If the specified list begins with a ‘^’ character, then the spec‐
733 ified algorithms will be placed at the head of the built-in
734 openssh default set.
735 The list of available key exchange algorithms may also be ob‐
737 tained using "ssh -Q kex".
738 KnownHostsCommand
740 Specifies a command to use to obtain a list of host keys, in ad‐
741 dition to those listed in UserKnownHostsFile and
742 GlobalKnownHostsFile. This command is executed after the files
743 have been read. It may write host key lines to standard output
744 in identical format to the usual files (described in the
745 VERIFYING HOST KEYS section in ssh(1)). Arguments to
746 KnownHostsCommand accept the tokens described in the TOKENS sec‐
747 tion. The command may be invoked multiple times per connection:
748 once when preparing the preference list of host key algorithms to
749 use, again to obtain the host key for the requested host name
750 and, if CheckHostIP is enabled, one more time to obtain the host
751 key matching the server's address. If the command exits abnor‐
752 mally or returns a non-zero exit status then the connection is
753 terminated.
754 LocalCommand
756 Specifies a command to execute on the local machine after suc‐
757 cessfully connecting to the server. The command string extends
758 to the end of the line, and is executed with the user's shell.
759 Arguments to LocalCommand accept the tokens described in the
760 TOKENS section.
761 The command is run synchronously and does not have access to the
763 session of the ssh(1) that spawned it. It should not be used for
764 interactive commands.
765 This directive is ignored unless PermitLocalCommand has been en‐
767 abled.
768 LocalForward
770 Specifies that a TCP port on the local machine be forwarded over
771 the secure channel to the specified host and port from the remote
772 machine. The first argument specifies the listener and may be
773 [bind_address:]port or a Unix domain socket path. The second ar‐
774 gument is the destination and may be host:hostport or a Unix do‐
775 main socket path if the remote host supports it.
776 IPv6 addresses can be specified by enclosing addresses in square
778 brackets. Multiple forwardings may be specified, and additional
779 forwardings can be given on the command line. Only the superuser
780 can forward privileged ports. By default, the local port is
781 bound in accordance with the GatewayPorts setting. However, an
782 explicit bind_address may be used to bind the connection to a
783 specific address. The bind_address of localhost indicates that
784 the listening port be bound for local use only, while an empty
785 address or ‘*’ indicates that the port should be available from
786 all interfaces. Unix domain socket paths may use the tokens de‐
787 scribed in the TOKENS section and environment variables as de‐
788 scribed in the ENVIRONMENT VARIABLES section.
789 LogLevel
791 Gives the verbosity level that is used when logging messages from
792 ssh(1). The possible values are: QUIET, FATAL, ERROR, INFO, VER‐
793 BOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
794 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
795 higher levels of verbose output.
796 LogVerbose
798 Specify one or more overrides to LogLevel. An override consists
799 of a pattern lists that matches the source file, function and
800 line number to force detailed logging for. For example, an over‐
801 ride pattern of:
802 kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
804 would enable detailed logging for line 1000 of kex.c, everything
806 in the kex_exchange_identification() function, and all code in
807 the packet.c file. This option is intended for debugging and no
808 overrides are enabled by default.
809 MACs The default is handled system-wide by crypto-policies(7). To see
811 the defaults and how to modify this default, see manual page
812 update-crypto-policies(8).
813 Specifies the MAC (message authentication code) algorithms in or‐
815 der of preference. The MAC algorithm is used for data integrity
816 protection. Multiple algorithms must be comma-separated. If the
817 specified list begins with a ‘+’ character, then the specified
818 algorithms will be appended to the built-in openssh default set
819 instead of replacing them. If the specified list begins with a
820 ‘-’ character, then the specified algorithms (including wild‐
821 cards) will be removed from the built-in openssh default set in‐
822 stead of replacing them. If the specified list begins with a ‘^’
823 character, then the specified algorithms will be placed at the
824 head of the built-in openssh default set.
825 The algorithms that contain "-etm" calculate the MAC after en‐
827 cryption (encrypt-then-mac). These are considered safer and
828 their use recommended.
829 The list of available MAC algorithms may also be obtained using
831 "ssh -Q mac".
832 NoHostAuthenticationForLocalhost
834 Disable host authentication for localhost (loopback addresses).
835 The argument to this keyword must be yes or no (the default).
836 NumberOfPasswordPrompts
838 Specifies the number of password prompts before giving up. The
839 argument to this keyword must be an integer. The default is 3.
840 PasswordAuthentication
842 Specifies whether to use password authentication. The argument
843 to this keyword must be yes (the default) or no.
844 PermitLocalCommand
846 Allow local command execution via the LocalCommand option or us‐
847 ing the !command escape sequence in ssh(1). The argument must be
848 yes or no (the default).
849 PermitRemoteOpen
851 Specifies the destinations to which remote TCP port forwarding is
852 permitted when RemoteForward is used as a SOCKS proxy. The for‐
853 warding specification must be one of the following forms:
854 PermitRemoteOpen host:port
856 PermitRemoteOpen IPv4_addr:port
857 PermitRemoteOpen [IPv6_addr]:port
858 Multiple forwards may be specified by separating them with white‐
860 space. An argument of any can be used to remove all restrictions
861 and permit any forwarding requests. An argument of none can be
862 used to prohibit all forwarding requests. The wildcard ‘*’ can
863 be used for host or port to allow all hosts or ports respec‐
864 tively. Otherwise, no pattern matching or address lookups are
865 performed on supplied names.
866 PKCS11Provider
868 Specifies which PKCS#11 provider to use or none to indicate that
869 no provider should be used (the default). The argument to this
870 keyword is a path to the PKCS#11 shared library ssh(1) should use
871 to communicate with a PKCS#11 token providing keys for user au‐
872 thentication.
873 Port Specifies the port number to connect on the remote host. The de‐
875 fault is 22.
876 PreferredAuthentications
878 Specifies the order in which the client should try authentication
879 methods. This allows a client to prefer one method (e.g.
880 keyboard-interactive) over another method (e.g. password). The
881 default is:
882 gssapi-keyex,gssapi-with-mic,hostbased,publickey,
884 keyboard-interactive,password
885 ProxyCommand
887 Specifies the command to use to connect to the server. The com‐
888 mand string extends to the end of the line, and is executed using
889 the user's shell ‘exec’ directive to avoid a lingering shell
890 process.
891 Arguments to ProxyCommand accept the tokens described in the
893 TOKENS section. The command can be basically anything, and
894 should read from its standard input and write to its standard
895 output. It should eventually connect an sshd(8) server running
896 on some machine, or execute sshd -i somewhere. Host key manage‐
897 ment will be done using the Hostname of the host being connected
898 (defaulting to the name typed by the user). Setting the command
899 to none disables this option entirely. Note that CheckHostIP is
900 not available for connects with a proxy command.
901 This directive is useful in conjunction with nc(1) and its proxy
903 support. For example, the following directive would connect via
904 an HTTP proxy at 192.0.2.0:
905 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
907 ProxyJump
909 Specifies one or more jump proxies as either [user@]host[:port]
910 or an ssh URI. Multiple proxies may be separated by comma char‐
911 acters and will be visited sequentially. Setting this option
912 will cause ssh(1) to connect to the target host by first making a
913 ssh(1) connection to the specified ProxyJump host and then estab‐
914 lishing a TCP forwarding to the ultimate target from there. Set‐
915 ting the host to none disables this option entirely.
916 Note that this option will compete with the ProxyCommand option -
918 whichever is specified first will prevent later instances of the
919 other from taking effect.
920 Note also that the configuration for the destination host (either
922 supplied via the command-line or the configuration file) is not
923 generally applied to jump hosts. ~/.ssh/config should be used if
924 specific configuration is required for jump hosts.
925 ProxyUseFdpass
927 Specifies that ProxyCommand will pass a connected file descriptor
928 back to ssh(1) instead of continuing to execute and pass data.
929 The default is no.
930 PubkeyAcceptedAlgorithms
932 The default is handled system-wide by crypto-policies(7). To see
933 the defaults and how to modify this default, see manual page
934 update-crypto-policies(8).
935 Specifies the signature algorithms that will be used for public
937 key authentication as a comma-separated list of patterns. If the
938 specified list begins with a ‘+’ character, then the algorithms
939 after it will be appended to the built-in openssh default instead
940 of replacing it. If the specified list begins with a ‘-’ charac‐
941 ter, then the specified algorithms (including wildcards) will be
942 removed from the built-in openssh default set instead of replac‐
943 ing them. If the specified list begins with a ‘^’ character,
944 then the specified algorithms will be placed at the head of the
945 built-in openssh default set.
946 The list of available signature algorithms may also be obtained
948 using "ssh -Q PubkeyAcceptedAlgorithms".
949 PubkeyAuthentication
951 Specifies whether to try public key authentication. The argument
952 to this keyword must be yes (the default) or no.
953 RekeyLimit
955 Specifies the maximum amount of data that may be transmitted be‐
956 fore the session key is renegotiated, optionally followed by a
957 maximum amount of time that may pass before the session key is
958 renegotiated. The first argument is specified in bytes and may
959 have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes,
960 Megabytes, or Gigabytes, respectively. The default is between
961 ‘1G’ and ‘4G’, depending on the cipher. The optional second
962 value is specified in seconds and may use any of the units docu‐
963 mented in the TIME FORMATS section of sshd_config(5). The de‐
964 fault value for RekeyLimit is default none, which means that
965 rekeying is performed after the cipher's default amount of data
966 has been sent or received and no time based rekeying is done.
967 RemoteCommand
969 Specifies a command to execute on the remote machine after suc‐
970 cessfully connecting to the server. The command string extends
971 to the end of the line, and is executed with the user's shell.
972 Arguments to RemoteCommand accept the tokens described in the
973 TOKENS section.
974 RemoteForward
976 Specifies that a TCP port on the remote machine be forwarded over
977 the secure channel. The remote port may either be forwarded to a
978 specified host and port from the local machine, or may act as a
979 SOCKS 4/5 proxy that allows a remote client to connect to arbi‐
980 trary destinations from the local machine. The first argument is
981 the listening specification and may be [bind_address:]port or, if
982 the remote host supports it, a Unix domain socket path. If for‐
983 warding to a specific destination then the second argument must
984 be host:hostport or a Unix domain socket path, otherwise if no
985 destination argument is specified then the remote forwarding will
986 be established as a SOCKS proxy. When acting as a SOCKS proxy
987 the destination of the connection can be restricted by
988 PermitRemoteOpen.
989 IPv6 addresses can be specified by enclosing addresses in square
991 brackets. Multiple forwardings may be specified, and additional
992 forwardings can be given on the command line. Privileged ports
993 can be forwarded only when logging in as root on the remote ma‐
994 chine. Unix domain socket paths may use the tokens described in
995 the TOKENS section and environment variables as described in the
996 ENVIRONMENT VARIABLES section.
997 If the port argument is 0, the listen port will be dynamically
999 allocated on the server and reported to the client at run time.
1000 If the bind_address is not specified, the default is to only bind
1002 to loopback addresses. If the bind_address is ‘*’ or an empty
1003 string, then the forwarding is requested to listen on all inter‐
1004 faces. Specifying a remote bind_address will only succeed if the
1005 server's GatewayPorts option is enabled (see sshd_config(5)).
1006 RequestTTY
1008 Specifies whether to request a pseudo-tty for the session. The
1009 argument may be one of: no (never request a TTY), yes (always re‐
1010 quest a TTY when standard input is a TTY), force (always request
1011 a TTY) or auto (request a TTY when opening a login session).
1012 This option mirrors the -t and -T flags for ssh(1).
1013 RevokedHostKeys
1015 Specifies revoked host public keys. Keys listed in this file
1016 will be refused for host authentication. Note that if this file
1017 does not exist or is not readable, then host authentication will
1018 be refused for all hosts. Keys may be specified as a text file,
1019 listing one public key per line, or as an OpenSSH Key Revocation
1020 List (KRL) as generated by ssh-keygen(1). For more information
1021 on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).
1022 SecurityKeyProvider
1024 Specifies a path to a library that will be used when loading any
1025 FIDO authenticator-hosted keys, overriding the default of using
1026 the built-in USB HID support.
1027 If the specified value begins with a ‘$’ character, then it will
1029 be treated as an environment variable containing the path to the
1030 library.
1031 SendEnv
1033 Specifies what variables from the local environ(7) should be sent
1034 to the server. The server must also support it, and the server
1035 must be configured to accept these environment variables. Note
1036 that the TERM environment variable is always sent whenever a
1037 pseudo-terminal is requested as it is required by the protocol.
1038 Refer to AcceptEnv in sshd_config(5) for how to configure the
1039 server. Variables are specified by name, which may contain wild‐
1040 card characters. Multiple environment variables may be separated
1041 by whitespace or spread across multiple SendEnv directives.
1042 See PATTERNS for more information on patterns.
1044 It is possible to clear previously set SendEnv variable names by
1046 prefixing patterns with -. The default is not to send any envi‐
1047 ronment variables.
1048 ServerAliveCountMax
1050 Sets the number of server alive messages (see below) which may be
1051 sent without ssh(1) receiving any messages back from the server.
1052 If this threshold is reached while server alive messages are be‐
1053 ing sent, ssh will disconnect from the server, terminating the
1054 session. It is important to note that the use of server alive
1055 messages is very different from TCPKeepAlive (below). The server
1056 alive messages are sent through the encrypted channel and there‐
1057 fore will not be spoofable. The TCP keepalive option enabled by
1058 TCPKeepAlive is spoofable. The server alive mechanism is valu‐
1059 able when the client or server depend on knowing when a connec‐
1060 tion has become unresponsive.
1061 The default value is 3. If, for example, ServerAliveInterval
1063 (see below) is set to 15 and ServerAliveCountMax is left at the
1064 default, if the server becomes unresponsive, ssh will disconnect
1065 after approximately 45 seconds.
1066 ServerAliveInterval
1068 Sets a timeout interval in seconds after which if no data has
1069 been received from the server, ssh(1) will send a message through
1070 the encrypted channel to request a response from the server. The
1071 default is 0, indicating that these messages will not be sent to
1072 the server.
1073 SessionType
1075 May be used to either request invocation of a subsystem on the
1076 remote system, or to prevent the execution of a remote command at
1077 all. The latter is useful for just forwarding ports. The argu‐
1078 ment to this keyword must be none (same as the -N option),
1079 subsystem (same as the -s option) or default (shell or command
1080 execution).
1081 SetEnv Directly specify one or more environment variables and their con‐
1083 tents to be sent to the server. Similarly to SendEnv, with the
1084 exception of the TERM variable, the server must be prepared to
1085 accept the environment variable.
1086 StdinNull
1088 Redirects stdin from /dev/null (actually, prevents reading from
1089 stdin). Either this or the equivalent -n option must be used
1090 when ssh is run in the background. The argument to this keyword
1091 must be yes (same as the -n option) or no (the default).
1092 StreamLocalBindMask
1094 Sets the octal file creation mode mask (umask) used when creating
1095 a Unix-domain socket file for local or remote port forwarding.
1096 This option is only used for port forwarding to a Unix-domain
1097 socket file.
1098 The default value is 0177, which creates a Unix-domain socket
1100 file that is readable and writable only by the owner. Note that
1101 not all operating systems honor the file mode on Unix-domain
1102 socket files.
1103 StreamLocalBindUnlink
1105 Specifies whether to remove an existing Unix-domain socket file
1106 for local or remote port forwarding before creating a new one.
1107 If the socket file already exists and StreamLocalBindUnlink is
1108 not enabled, ssh will be unable to forward the port to the Unix-
1109 domain socket file. This option is only used for port forwarding
1110 to a Unix-domain socket file.
1111 The argument must be yes or no (the default).
1113 StrictHostKeyChecking
1115 If this flag is set to yes, ssh(1) will never automatically add
1116 host keys to the ~/.ssh/known_hosts file, and refuses to connect
1117 to hosts whose host key has changed. This provides maximum pro‐
1118 tection against man-in-the-middle (MITM) attacks, though it can
1119 be annoying when the /etc/gsissh/ssh_known_hosts file is poorly
1120 maintained or when connections to new hosts are frequently made.
1121 This option forces the user to manually add all new hosts.
1122 If this flag is set to accept-new then ssh will automatically add
1124 new host keys to the user's known_hosts file, but will not permit
1125 connections to hosts with changed host keys. If this flag is set
1126 to no or off, ssh will automatically add new host keys to the
1127 user known hosts files and allow connections to hosts with
1128 changed hostkeys to proceed, subject to some restrictions. If
1129 this flag is set to ask (the default), new host keys will be
1130 added to the user known host files only after the user has con‐
1131 firmed that is what they really want to do, and ssh will refuse
1132 to connect to hosts whose host key has changed. The host keys of
1133 known hosts will be verified automatically in all cases.
1134 SyslogFacility
1136 Gives the facility code that is used when logging messages from
1137 ssh(1). The possible values are: DAEMON, USER, AUTH, LOCAL0, LO‐
1138 CAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The de‐
1139 fault is USER.
1140 TCPKeepAlive
1142 Specifies whether the system should send TCP keepalive messages
1143 to the other side. If they are sent, death of the connection or
1144 crash of one of the machines will be properly noticed. However,
1145 this means that connections will die if the route is down tempo‐
1146 rarily, and some people find it annoying.
1147 The default is yes (to send TCP keepalive messages), and the
1149 client will notice if the network goes down or the remote host
1150 dies. This is important in scripts, and many users want it too.
1151 To disable TCP keepalive messages, the value should be set to no.
1153 See also ServerAliveInterval for protocol-level keepalives.
1154 Tunnel Request tun(4) device forwarding between the client and the
1156 server. The argument must be yes, point-to-point (layer 3),
1157 ethernet (layer 2), or no (the default). Specifying yes requests
1158 the default tunnel mode, which is point-to-point.
1159 TunnelDevice
1161 Specifies the tun(4) devices to open on the client (local_tun)
1162 and the server (remote_tun).
1163 The argument must be local_tun[:remote_tun]. The devices may be
1165 specified by numerical ID or the keyword any, which uses the next
1166 available tunnel device. If remote_tun is not specified, it de‐
1167 faults to any. The default is any:any.
1168 UpdateHostKeys
1170 Specifies whether ssh(1) should accept notifications of addi‐
1171 tional hostkeys from the server sent after authentication has
1172 completed and add them to UserKnownHostsFile. The argument must
1173 be yes, no or ask. This option allows learning alternate
1174 hostkeys for a server and supports graceful key rotation by al‐
1175 lowing a server to send replacement public keys before old ones
1176 are removed.
1177 Additional hostkeys are only accepted if the key used to authen‐
1179 ticate the host was already trusted or explicitly accepted by the
1180 user, the host was authenticated via UserKnownHostsFile (i.e. not
1181 GlobalKnownHostsFile) and the host was authenticated using a
1182 plain key and not a certificate.
1183 UpdateHostKeys is enabled by default if the user has not overrid‐
1185 den the default UserKnownHostsFile setting and has not enabled
1186 VerifyHostKeyDNS, otherwise UpdateHostKeys will be set to no.
1187 If UpdateHostKeys is set to ask, then the user is asked to con‐
1189 firm the modifications to the known_hosts file. Confirmation is
1190 currently incompatible with ControlPersist, and will be disabled
1191 if it is enabled.
1192 Presently, only sshd(8) from OpenSSH 6.8 and greater support the
1194 "hostkeys@openssh.com" protocol extension used to inform the
1195 client of all the server's hostkeys.
1196 User Specifies the user to log in as. This can be useful when a dif‐
1198 ferent user name is used on different machines. This saves the
1199 trouble of having to remember to give the user name on the com‐
1200 mand line.
1201 UserKnownHostsFile
1203 Specifies one or more files to use for the user host key data‐
1204 base, separated by whitespace. Each filename may use tilde nota‐
1205 tion to refer to the user's home directory, the tokens described
1206 in the TOKENS section and environment variables as described in
1207 the ENVIRONMENT VARIABLES section. The default is
1208 ~/.ssh/known_hosts, ~/.ssh/known_hosts2.
1209 VerifyHostKeyDNS
1211 Specifies whether to verify the remote key using DNS and SSHFP
1212 resource records. If this option is set to yes, the client will
1213 implicitly trust keys that match a secure fingerprint from DNS.
1214 Insecure fingerprints will be handled as if this option was set
1215 to ask. If this option is set to ask, information on fingerprint
1216 match will be displayed, but the user will still need to confirm
1217 new host keys according to the StrictHostKeyChecking option. The
1218 default is no.
1219 See also VERIFYING HOST KEYS in ssh(1).
1221 VisualHostKey
1223 If this flag is set to yes, an ASCII art representation of the
1224 remote host key fingerprint is printed in addition to the finger‐
1225 print string at login and for unknown host keys. If this flag is
1226 set to no (the default), no fingerprint strings are printed at
1227 login and only the fingerprint string will be printed for unknown
1228 host keys.
1229 XAuthLocation
1231 Specifies the full pathname of the xauth(1) program. The default
1232 is /usr/bin/xauth.
1233
1235 A pattern consists of zero or more non-whitespace characters, ‘*’ (a
1236 wildcard that matches zero or more characters), or ‘?’ (a wildcard that
1237 matches exactly one character). For example, to specify a set of decla‐
1238 rations for any host in the ".co.uk" set of domains, the following pat‐
1239 tern could be used:
1240 Host *.co.uk
1242 The following pattern would match any host in the 192.168.0.[0-9] network
1244 range:
1245 Host 192.168.0.?
1247 A pattern-list is a comma-separated list of patterns. Patterns within
1249 pattern-lists may be negated by preceding them with an exclamation mark
1250 (‘!’). For example, to allow a key to be used from anywhere within an
1251 organization except from the "dialup" pool, the following entry (in au‐
1252 thorized_keys) could be used:
1253 from="!*.dialup.example.com,*.example.com"
1255 Note that a negated match will never produce a positive result by itself.
1257 For example, attempting to match "host3" against the following pattern-
1258 list will fail:
1259 from="!host1,!host2"
1261 The solution here is to include a term that will yield a positive match,
1263 such as a wildcard:
1264 from="!host1,!host2,*"
1266
1268 Arguments to some keywords can make use of tokens, which are expanded at
1269 runtime:
1270 %% A literal ‘%’.
1272 %C Hash of %l%h%p%r.
1273 %d Local user's home directory.
1274 %f The fingerprint of the server's host key.
1275 %H The known_hosts hostname or address that is being searched
1276 for.
1277 %h The remote hostname.
1278 %I A string describing the reason for a KnownHostsCommand execu‐
1279 tion: either ADDRESS when looking up a host by address (only
1280 when CheckHostIP is enabled), HOSTNAME when searching by
1281 hostname, or ORDER when preparing the host key algorithm
1282 preference list to use for the destination host.
1283 %i The local user ID.
1284 %K The base64 encoded host key.
1285 %k The host key alias if specified, otherwise the original re‐
1286 mote hostname given on the command line.
1287 %L The local hostname.
1288 %l The local hostname, including the domain name.
1289 %n The original remote hostname, as given on the command line.
1290 %p The remote port.
1291 %r The remote username.
1292 %T The local tun(4) or tap(4) network interface assigned if tun‐
1293 nel forwarding was requested, or "NONE" otherwise.
1294 %t The type of the server host key, e.g. ssh-ed25519.
1295 %u The local username.
1296 CertificateFile, ControlPath, IdentityAgent, IdentityFile,
1298 KnownHostsCommand, LocalForward, Match exec, RemoteCommand,
1299 RemoteForward, and UserKnownHostsFile accept the tokens %%, %C, %d, %h,
1300 %i, %k, %L, %l, %n, %p, %r, and %u.
1301 KnownHostsCommand additionally accepts the tokens %f, %H, %I, %K and %t.
1303 Hostname accepts the tokens %% and %h.
1305 LocalCommand accepts all tokens.
1307 ProxyCommand accepts the tokens %%, %h, %n, %p, and %r.
1309
1311 Arguments to some keywords can be expanded at runtime from environment
1312 variables on the client by enclosing them in ${}, for example
1313 ${HOME}/.ssh would refer to the user's .ssh directory. If a specified
1314 environment variable does not exist then an error will be returned and
1315 the setting for that keyword will be ignored.
1316 The keywords CertificateFile, ControlPath, IdentityAgent, IdentityFile,
1318 KnownHostsCommand, and UserKnownHostsFile support environment variables.
1319 The keywords LocalForward and RemoteForward support environment variables
1320 only for Unix domain socket paths.
1321
1323 ~/.ssh/config
1324 This is the per-user configuration file. The format of this file
1325 is described above. This file is used by the SSH client. Be‐
1326 cause of the potential for abuse, this file must have strict per‐
1327 missions: read/write for the user, and not writable by others.
1328 /etc/gsissh/ssh_config
1330 Systemwide configuration file. This file provides defaults for
1331 those values that are not specified in the user's configuration
1332 file, and for those users who do not have a configuration file.
1333 This file must be world-readable.
1334
1336 ssh(1)
1337
1339 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1340 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1341 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1342 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
1343 versions 1.5 and 2.0.
1344BSD September 25, 2021 BSD