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
26 optionally be enclosed in double quotes (") in order to represent argu‐
27 ments containing spaces. Configuration options may be separated by
28 whitespace or optional whitespace and exactly one ‘=’; the latter format
29 is useful to avoid the need to quote whitespace when specifying configu‐
30 ration options 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
48 exceptions 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
55 using 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
77 described 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
81 described in the PATTERNS section. The criteria for the host
82 keyword are matched against the target hostname, after any sub‐
83 stitution 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
97 details). 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. The argument must be yes, confirm, ask, or no (the
101 default).
102 AddressFamily
104 Specifies which address family to use when connecting. Valid
105 arguments are any (the default), inet (use IPv4 only), or inet6
106 (use IPv6 only).
107 BatchMode
109 If set to yes, user interaction such as password prompts and host
110 key confirmation requests will be disabled. This option is use‐
111 ful in scripts and other batch jobs where no user is present to
112 interact with ssh(1). The argument must be yes or no (the
113 default).
114 BindAddress
116 Use the specified address on the local machine as the source
117 address of the connection. Only useful on systems with more than
118 one address.
119 BindInterface
121 Use the address of the specified interface on the local machine
122 as the source address of the connection.
123 CanonicalDomains
125 When CanonicalizeHostname is enabled, this option specifies the
126 list of domain suffixes in which to search for the specified des‐
127 tination host.
128 CanonicalizeFallbackLocal
130 Specifies whether to fail with an error when hostname canonical‐
131 ization fails. The default, yes, will attempt to look up the
132 unqualified hostname using the system resolver's search rules. A
133 value of no will cause ssh(1) to fail instantly if
134 CanonicalizeHostname is enabled and the target hostname cannot be
135 found in any of the domains specified by CanonicalDomains.
136 CanonicalizeHostname
138 Controls whether explicit hostname canonicalization is performed.
139 The default, no, is not to perform any name rewriting and let the
140 system resolver handle all hostname lookups. If set to yes then,
141 for connections that do not use a ProxyCommand or ProxyJump,
142 ssh(1) will attempt to canonicalize the hostname specified on the
143 command line using the CanonicalDomains suffixes and
144 CanonicalizePermittedCNAMEs rules. If CanonicalizeHostname is
145 set to always, then canonicalization is applied to proxied con‐
146 nections too.
147 If this option is enabled, then the configuration files are pro‐
149 cessed again using the new target name to pick up any new config‐
150 uration in matching Host and Match stanzas.
151 CanonicalizeMaxDots
153 Specifies the maximum number of dot characters in a hostname
154 before canonicalization is disabled. The default, 1, allows a
155 single dot (i.e. hostname.subdomain).
156 CanonicalizePermittedCNAMEs
158 Specifies rules to determine whether CNAMEs should be followed
159 when canonicalizing hostnames. The rules consist of one or more
160 arguments of source_domain_list:target_domain_list, where
161 source_domain_list is a pattern-list of domains that may follow
162 CNAMEs in canonicalization, and target_domain_list is a pattern-
163 list of domains that they may resolve to.
164 For example, "*.a.example.com:*.b.example.com,*.c.example.com"
166 will allow hostnames matching "*.a.example.com" to be canonical‐
167 ized to names in the "*.b.example.com" or "*.c.example.com"
168 domains.
169 CASignatureAlgorithms
171 The default is handled system-wide by crypto-policies(7). To see
172 the defaults and how to modify this default, see manual page
173 update-crypto-policies(8).
174 Specifies which algorithms are allowed for signing of certifi‐
176 cates by certificate authorities (CAs). ssh(1) will not accept
177 host certificates signed using algorithms other than those speci‐
178 fied.
179 CertificateFile
181 Specifies a file from which the user's certificate is read. A
182 corresponding private key must be provided separately in order to
183 use this certificate either from an IdentityFile directive or -i
184 flag to ssh(1), via ssh-agent(1), or via a PKCS11Provider or
185 SecurityKeyProvider.
186 Arguments to CertificateFile may use the tilde syntax to refer to
188 a user's home directory or the tokens described in the TOKENS
189 section.
190 It is possible to have multiple certificate files specified in
192 configuration files; these certificates will be tried in
193 sequence. Multiple CertificateFile directives will add to the
194 list of certificates used for authentication.
195 ChallengeResponseAuthentication
197 Specifies whether to use challenge-response authentication. The
198 argument to this keyword must be yes (the default) or no.
199 CheckHostIP
201 If set to yes (the default), ssh(1) will additionally check the
202 host IP address in the known_hosts file. This allows it to
203 detect if a host key changed due to DNS spoofing and will add
204 addresses of destination hosts to ~/.ssh/known_hosts in the
205 process, regardless of the setting of StrictHostKeyChecking. If
206 the option is set to no, the check will not be executed.
207 Ciphers
209 The default is handled system-wide by crypto-policies(7). To see
210 the defaults and how to modify this default, see manual page
211 update-crypto-policies(8).
212 Specifies the ciphers allowed and their order of preference.
214 Multiple ciphers must be comma-separated. If the specified list
215 begins with a ‘+’ character, then the specified ciphers will be
216 appended to the built-in default set instead of replacing them.
217 If the specified list begins with a ‘-’ character, then the spec‐
218 ified ciphers (including wildcards) will be removed from the
219 built-in default set instead of replacing them. If the specified
220 list begins with a ‘^’ character, then the specified ciphers will
221 be placed at the head of the built-in default set.
222 The supported ciphers are:
224 3des-cbc
226 aes128-cbc
227 aes192-cbc
228 aes256-cbc
229 aes128-ctr
230 aes192-ctr
231 aes256-ctr
232 aes128-gcm@openssh.com
233 aes256-gcm@openssh.com
234 chacha20-poly1305@openssh.com
235 The list of available ciphers may also be obtained using "ssh -Q
237 cipher".
238 ClearAllForwardings
240 Specifies that all local, remote, and dynamic port forwardings
241 specified in the configuration files or on the command line be
242 cleared. This option is primarily useful when used from the
243 ssh(1) command line to clear port forwardings set in configura‐
244 tion files, and is automatically set by scp(1) and sftp(1). The
245 argument must be yes or no (the default).
246 Compression
248 Specifies whether to use compression. The argument must be yes
249 or no (the default).
250 ConnectionAttempts
252 Specifies the number of tries (one per second) to make before
253 exiting. The argument must be an integer. This may be useful in
254 scripts if the connection sometimes fails. The default is 1.
255 ConnectTimeout
257 Specifies the timeout (in seconds) used when connecting to the
258 SSH server, instead of using the default system TCP timeout.
259 This timeout is applied both to establishing the connection and
260 to performing the initial SSH protocol handshake and key
261 exchange.
262 ControlMaster
264 Enables the sharing of multiple sessions over a single network
265 connection. When set to yes, ssh(1) will listen for connections
266 on a control socket specified using the ControlPath argument.
267 Additional sessions can connect to this socket using the same
268 ControlPath with ControlMaster set to no (the default). These
269 sessions will try to reuse the master instance's network connec‐
270 tion rather than initiating new ones, but will fall back to con‐
271 necting normally if the control socket does not exist, or is not
272 listening.
273 Setting this to ask will cause ssh(1) to listen for control con‐
275 nections, but require confirmation using ssh-askpass(1). If the
276 ControlPath cannot be opened, ssh(1) will continue without con‐
277 necting to a master instance.
278 X11 and ssh-agent(1) forwarding is supported over these multi‐
280 plexed connections, however the display and agent forwarded will
281 be the one belonging to the master connection i.e. it is not pos‐
282 sible to forward multiple displays or agents.
283 Two additional options allow for opportunistic multiplexing: try
285 to use a master connection but fall back to creating a new one if
286 one does not already exist. These options are: auto and autoask.
287 The latter requires confirmation like the ask option.
288 ControlPath
290 Specify the path to the control socket used for connection shar‐
291 ing as described in the ControlMaster section above or the string
292 none to disable connection sharing. Arguments to ControlPath may
293 use the tilde syntax to refer to a user's home directory or the
294 tokens described in the TOKENS section. It is recommended that
295 any ControlPath used for opportunistic connection sharing include
296 at least %h, %p, and %r (or alternatively %C) and be placed in a
297 directory that is not writable by other users. This ensures that
298 shared connections are uniquely identified.
299 ControlPersist
301 When used in conjunction with ControlMaster, specifies that the
302 master connection should remain open in the background (waiting
303 for future client connections) after the initial client connec‐
304 tion has been closed. If set to no (the default), then the mas‐
305 ter connection will not be placed into the background, and will
306 close as soon as the initial client connection is closed. If set
307 to yes or 0, then the master connection will remain in the back‐
308 ground indefinitely (until killed or closed via a mechanism such
309 as the "ssh -O exit"). If set to a time in seconds, or a time in
310 any of the formats documented in sshd_config(5), then the back‐
311 grounded master connection will automatically terminate after it
312 has remained idle (with no client connections) for the specified
313 time.
314 DynamicForward
316 Specifies that a TCP port on the local machine be forwarded over
317 the secure channel, and the application protocol is then used to
318 determine where to connect to from the remote machine.
319 The argument must be [bind_address:]port. IPv6 addresses can be
321 specified by enclosing addresses in square brackets. By default,
322 the local port is bound in accordance with the GatewayPorts set‐
323 ting. However, an explicit bind_address may be used to bind the
324 connection to a specific address. The bind_address of localhost
325 indicates that the listening port be bound for local use only,
326 while an empty address or ‘*’ indicates that the port should be
327 available from all interfaces.
328 Currently the SOCKS4 and SOCKS5 protocols are supported, and
330 ssh(1) will act as a SOCKS server. Multiple forwardings may be
331 specified, and additional forwardings can be given on the command
332 line. Only the superuser can forward privileged ports.
333 EnableSSHKeysign
335 Setting this option to yes in the global client configuration
336 file /etc/ssh/ssh_config enables the use of the helper program
337 ssh-keysign(8) during HostbasedAuthentication. The argument must
338 be yes or no (the default). This option should be placed in the
339 non-hostspecific section. See ssh-keysign(8) for more informa‐
340 tion.
341 EscapeChar
343 Sets the escape character (default: ‘~’). The escape character
344 can also be set on the command line. The argument should be a
345 single character, ‘^’ followed by a letter, or none to disable
346 the escape character entirely (making the connection transparent
347 for binary data).
348 ExitOnForwardFailure
350 Specifies whether ssh(1) should terminate the connection if it
351 cannot set up all requested dynamic, tunnel, local, and remote
352 port forwardings, (e.g. if either end is unable to bind and lis‐
353 ten on a specified port). Note that ExitOnForwardFailure does
354 not apply to connections made over port forwardings and will not,
355 for example, cause ssh(1) to exit if TCP connections to the ulti‐
356 mate forwarding destination fail. The argument must be yes or no
357 (the default).
358 FingerprintHash
360 Specifies the hash algorithm used when displaying key finger‐
361 prints. Valid options are: md5 and sha256 (the default).
362 ForwardAgent
364 Specifies whether the connection to the authentication agent (if
365 any) will be forwarded to the remote machine. The argument may
366 be yes, no (the default), an explicit path to an agent socket or
367 the name of an environment variable (beginning with ‘$’) in which
368 to find the path.
369 Agent forwarding should be enabled with caution. Users with the
371 ability to bypass file permissions on the remote host (for the
372 agent's Unix-domain socket) can access the local agent through
373 the forwarded connection. An attacker cannot obtain key material
374 from the agent, however they can perform operations on the keys
375 that enable them to authenticate using the identities loaded into
376 the agent.
377 ForwardX11
379 Specifies whether X11 connections will be automatically redi‐
380 rected over the secure channel and DISPLAY set. The argument
381 must be yes or no (the default).
382 X11 forwarding should be enabled with caution. Users with the
384 ability to bypass file permissions on the remote host (for the
385 user's X11 authorization database) can access the local X11 dis‐
386 play through the forwarded connection. An attacker may then be
387 able to perform activities such as keystroke monitoring if the
388 ForwardX11Trusted option is also enabled.
389 ForwardX11Timeout
391 Specify a timeout for untrusted X11 forwarding using the format
392 described in the TIME FORMATS section of sshd_config(5). X11
393 connections received by ssh(1) after this time will be refused.
394 Setting ForwardX11Timeout to zero will disable the timeout and
395 permit X11 forwarding for the life of the connection. The
396 default is to disable untrusted X11 forwarding after twenty min‐
397 utes has elapsed.
398 ForwardX11Trusted
400 If this option is set to yes, remote X11 clients will have full
401 access to the original X11 display.
402 If this option is set to no (the default), remote X11 clients
404 will be considered untrusted and prevented from stealing or tam‐
405 pering with data belonging to trusted X11 clients. Furthermore,
406 the xauth(1) token used for the session will be set to expire
407 after 20 minutes. Remote clients will be refused access after
408 this time.
409 See the X11 SECURITY extension specification for full details on
411 the restrictions imposed on untrusted clients.
412 GatewayPorts
414 Specifies whether remote hosts are allowed to connect to local
415 forwarded ports. By default, ssh(1) binds local port forwardings
416 to the loopback address. This prevents other remote hosts from
417 connecting to forwarded ports. GatewayPorts can be used to spec‐
418 ify that ssh should bind local port forwardings to the wildcard
419 address, thus allowing remote hosts to connect to forwarded
420 ports. The argument must be yes or no (the default).
421 GlobalKnownHostsFile
423 Specifies one or more files to use for the global host key data‐
424 base, separated by whitespace. The default is
425 /etc/ssh/ssh_known_hosts, /etc/ssh/ssh_known_hosts2.
426 GSSAPIAuthentication
428 Specifies whether user authentication based on GSSAPI is allowed.
429 The default is no.
430 GSSAPIClientIdentity
432 If set, specifies the GSSAPI client identity that ssh should use
433 when connecting to the server. The default is unset, which means
434 that the default identity will be used.
435 GSSAPIDelegateCredentials
437 Forward (delegate) credentials to the server. The default is no.
438 GSSAPIKeyExchange
440 Specifies whether key exchange based on GSSAPI may be used. When
441 using GSSAPI key exchange the server need not have a host key.
442 The default is “no”.
443 GSSAPIRenewalForcesRekey
445 If set to “yes” then renewal of the client's GSSAPI credentials
446 will force the rekeying of the ssh connection. With a compatible
447 server, this will delegate the renewed credentials to a session
448 on the server.
449 Checks are made to ensure that credentials are only propagated
451 when the new credentials match the old ones on the originating
452 client and where the receiving server still has the old set in
453 its cache.
454 The default is “no”.
456 For this to work GSSAPIKeyExchange needs to be enabled in the
458 server and also used by the client.
459 GSSAPIServerIdentity
461 If set, specifies the GSSAPI server identity that ssh should
462 expect when connecting to the server. The default is unset, which
463 means that the expected GSSAPI server identity will be determined
464 from the target hostname.
465 GSSAPITrustDns
467 Set to “yes” to indicate that the DNS is trusted to securely
468 canonicalize the name of the host being connected to. If “no”,
469 the hostname entered on the command line will be passed untouched
470 to the GSSAPI library. The default is “no”.
471 GSSAPIKexAlgorithms
473 The default is handled system-wide by crypto-policies(7). To see
474 the defaults and how to modify this default, see manual page
475 update-crypto-policies(8).
476 The list of key exchange algorithms that are offered for GSSAPI
478 key exchange. Possible values are
479 gss-gex-sha1-,
481 gss-group1-sha1-,
482 gss-group14-sha1-,
483 gss-group14-sha256-,
484 gss-group16-sha512-,
485 gss-nistp256-sha256-,
486 gss-curve25519-sha256-
487 This option only applies to connections using GSSAPI.
489 HashKnownHosts
491 Indicates that ssh(1) should hash host names and addresses when
492 they are added to ~/.ssh/known_hosts. These hashed names may be
493 used normally by ssh(1) and sshd(8), but they do not visually
494 reveal identifying information if the file's contents are dis‐
495 closed. The default is no. Note that existing names and
496 addresses in known hosts files will not be converted automati‐
497 cally, but may be manually hashed using ssh-keygen(1).
498 HostbasedAuthentication
500 Specifies whether to try rhosts based authentication with public
501 key authentication. The argument must be yes or no (the
502 default).
503 HostbasedKeyTypes
505 Specifies the key types that will be used for hostbased authenti‐
506 cation as a comma-separated list of patterns. Alternately if the
507 specified list begins with a ‘+’ character, then the specified
508 key types will be appended to the default set instead of replac‐
509 ing them. If the specified list begins with a ‘-’ character,
510 then the specified key types (including wildcards) will be
511 removed from the default set instead of replacing them. If the
512 specified list begins with a ‘^’ character, then the specified
513 key types will be placed at the head of the default set. The
514 default for this option is:
515 ecdsa-sha2-nistp256-cert-v01@openssh.com,
517 ecdsa-sha2-nistp384-cert-v01@openssh.com,
518 ecdsa-sha2-nistp521-cert-v01@openssh.com,
519 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
520 ssh-ed25519-cert-v01@openssh.com,
521 sk-ssh-ed25519-cert-v01@openssh.com,
522 rsa-sha2-512-cert-v01@openssh.com,
523 rsa-sha2-256-cert-v01@openssh.com,
524 ssh-rsa-cert-v01@openssh.com,
525 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
526 sk-ecdsa-sha2-nistp256@openssh.com,
527 ssh-ed25519,sk-ssh-ed25519@openssh.com,
528 rsa-sha2-512,rsa-sha2-256,ssh-rsa
529 The -Q option of ssh(1) may be used to list supported key types.
531 HostKeyAlgorithms
533 Specifies the host key algorithms that the client wants to use in
534 order of preference. Alternately if the specified list begins
535 with a ‘+’ character, then the specified key types will be
536 appended to the default set instead of replacing them. If the
537 specified list begins with a ‘-’ character, then the specified
538 key types (including wildcards) will be removed from the default
539 set instead of replacing them. If the specified list begins with
540 a ‘^’ character, then the specified key types will be placed at
541 the head of the default set. The default for this option is:
542 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-ecdsa-sha2-nistp256-cert-v01@openssh.com,
547 ssh-ed25519-cert-v01@openssh.com,
548 sk-ssh-ed25519-cert-v01@openssh.com,
549 rsa-sha2-512-cert-v01@openssh.com,
550 rsa-sha2-256-cert-v01@openssh.com,
551 ssh-rsa-cert-v01@openssh.com,
552 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
553 sk-ecdsa-sha2-nistp256@openssh.com,
554 ssh-ed25519,sk-ssh-ed25519@openssh.com,
555 rsa-sha2-512,rsa-sha2-256,ssh-rsa
556 If hostkeys are known for the destination host then this default
558 is modified to prefer their algorithms.
559 The list of available key types may also be obtained using "ssh
561 -Q HostKeyAlgorithms".
562 HostKeyAlias
564 Specifies an alias that should be used instead of the real host
565 name when looking up or saving the host key in the host key data‐
566 base files and when validating host certificates. This option is
567 useful for tunneling SSH connections or for multiple servers run‐
568 ning on a single host.
569 Hostname
571 Specifies the real host name to log into. This can be used to
572 specify nicknames or abbreviations for hosts. Arguments to
573 Hostname accept the tokens described in the TOKENS section.
574 Numeric IP addresses are also permitted (both on the command line
575 and in Hostname specifications). The default is the name given
576 on the command line.
577 IdentitiesOnly
579 Specifies that ssh(1) should only use the configured authentica‐
580 tion identity and certificate files (either the default files, or
581 those explicitly configured in the ssh_config files or passed on
582 the ssh(1) command-line), even if ssh-agent(1) or a
583 PKCS11Provider or SecurityKeyProvider offers more identities.
584 The argument to this keyword must be yes or no (the default).
585 This option is intended for situations where ssh-agent offers
586 many different identities.
587 IdentityAgent
589 Specifies the UNIX-domain socket used to communicate with the
590 authentication agent.
591 This option overrides the SSH_AUTH_SOCK environment variable and
593 can be used to select a specific agent. Setting the socket name
594 to none disables the use of an authentication agent. If the
595 string "SSH_AUTH_SOCK" is specified, the location of the socket
596 will be read from the SSH_AUTH_SOCK environment variable. Other‐
597 wise if the specified value begins with a ‘$’ character, then it
598 will be treated as an environment variable containing the loca‐
599 tion of the socket.
600 Arguments to IdentityAgent may use the tilde syntax to refer to a
602 user's home directory or the tokens described in the TOKENS sec‐
603 tion.
604 IdentityFile
606 Specifies a file from which the user's DSA, ECDSA, authenticator-
607 hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA
608 authentication identity is read. The default is ~/.ssh/id_dsa,
609 ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519,
610 ~/.ssh/id_ed25519_sk and ~/.ssh/id_rsa. Additionally, any iden‐
611 tities represented by the authentication agent will be used for
612 authentication unless IdentitiesOnly is set. If no certificates
613 have been explicitly specified by CertificateFile, ssh(1) will
614 try to load certificate information from the filename obtained by
615 appending -cert.pub to the path of a specified IdentityFile.
616 Arguments to IdentityFile may use the tilde syntax to refer to a
618 user's home directory or the tokens described in the TOKENS sec‐
619 tion.
620 It is possible to have multiple identity files specified in con‐
622 figuration files; all these identities will be tried in sequence.
623 Multiple IdentityFile directives will add to the list of identi‐
624 ties tried (this behaviour differs from that of other configura‐
625 tion directives).
626 IdentityFile may be used in conjunction with IdentitiesOnly to
628 select which identities in an agent are offered during authenti‐
629 cation. IdentityFile may also be used in conjunction with
630 CertificateFile in order to provide any certificate also needed
631 for authentication with the identity.
632 The authentication identity can be also specified in a form of
634 PKCS#11 URI starting with a string pkcs11:. There is supported a
635 subset of the PKCS#11 URI as defined in RFC 7512 (implemented
636 path arguments id, manufacturer, object, token and query argu‐
637 ments module-path and pin-value ). The URI can not be in quotes.
638 IgnoreUnknown
640 Specifies a pattern-list of unknown options to be ignored if they
641 are encountered in configuration parsing. This may be used to
642 suppress errors if ssh_config contains options that are unrecog‐
643 nised by ssh(1). It is recommended that IgnoreUnknown be listed
644 early in the configuration file as it will not be applied to
645 unknown options that appear before it.
646 Include
648 Include the specified configuration file(s). Multiple pathnames
649 may be specified and each pathname may contain glob(7) wildcards
650 and, for user configurations, shell-like ‘~’ references to user
651 home directories. Files without absolute paths are assumed to be
652 in ~/.ssh if included in a user configuration file or /etc/ssh if
653 included from the system configuration file. Include directive
654 may appear inside a Match or Host block to perform conditional
655 inclusion.
656 IPQoS Specifies the IPv4 type-of-service or DSCP class for connections.
658 Accepted values are af11, af12, af13, af21, af22, af23, af31,
659 af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5, cs6,
660 cs7, ef, le, lowdelay, throughput, reliability, a numeric value,
661 or none to use the operating system default. This option may
662 take one or two arguments, separated by whitespace. If one argu‐
663 ment is specified, it is used as the packet class uncondition‐
664 ally. If two values are specified, the first is automatically
665 selected for interactive sessions and the second for non-interac‐
666 tive sessions. The default is af21 (Low-Latency Data) for inter‐
667 active sessions and cs1 (Lower Effort) for non-interactive ses‐
668 sions.
669 KbdInteractiveAuthentication
671 Specifies whether to use keyboard-interactive authentication.
672 The argument to this keyword must be yes (the default) or no.
673 KbdInteractiveDevices
675 Specifies the list of methods to use in keyboard-interactive
676 authentication. Multiple method names must be comma-separated.
677 The default is to use the server specified list. The methods
678 available vary depending on what the server supports. For an
679 OpenSSH server, it may be zero or more of: bsdauth and pam.
680 KexAlgorithms
682 The default is handled system-wide by crypto-policies(7). To see
683 the defaults and how to modify this default, see manual page
684 update-crypto-policies(8).
685 Specifies the available KEX (Key Exchange) algorithms. Multiple
687 algorithms must be comma-separated. If the specified list begins
688 with a ‘+’ character, then the specified methods will be appended
689 to the built-in default set instead of replacing them. If the
690 specified list begins with a ‘-’ character, then the specified
691 methods (including wildcards) will be removed from the built-in
692 default set instead of replacing them. If the specified list
693 begins with a ‘^’ character, then the specified methods will be
694 placed at the head of the built-in default set.
695 The list of available key exchange algorithms may also be
697 obtained using "ssh -Q kex".
698 LocalCommand
700 Specifies a command to execute on the local machine after suc‐
701 cessfully connecting to the server. The command string extends
702 to the end of the line, and is executed with the user's shell.
703 Arguments to LocalCommand accept the tokens described in the
704 TOKENS section.
705 The command is run synchronously and does not have access to the
707 session of the ssh(1) that spawned it. It should not be used for
708 interactive commands.
709 This directive is ignored unless PermitLocalCommand has been
711 enabled.
712 LocalForward
714 Specifies that a TCP port on the local machine be forwarded over
715 the secure channel to the specified host and port from the remote
716 machine. The first argument must be [bind_address:]port and the
717 second argument must be host:hostport. IPv6 addresses can be
718 specified by enclosing addresses in square brackets. Multiple
719 forwardings may be specified, and additional forwardings can be
720 given on the command line. Only the superuser can forward privi‐
721 leged ports. By default, the local port is bound in accordance
722 with the GatewayPorts setting. However, an explicit bind_address
723 may be used to bind the connection to a specific address. The
724 bind_address of localhost indicates that the listening port be
725 bound for local use only, while an empty address or ‘*’ indicates
726 that the port should be available from all interfaces.
727 LogLevel
729 Gives the verbosity level that is used when logging messages from
730 ssh(1). The possible values are: QUIET, FATAL, ERROR, INFO, VER‐
731 BOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
732 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
733 higher levels of verbose output.
734 MACs The default is handled system-wide by crypto-policies(7). To see
736 the defaults and how to modify this default, see manual page
737 update-crypto-policies(8).
738 Specifies the MAC (message authentication code) algorithms in
740 order of preference. The MAC algorithm is used for data
741 integrity protection. Multiple algorithms must be comma-sepa‐
742 rated. If the specified list begins with a ‘+’ character, then
743 the specified algorithms will be appended to the built-in default
744 set instead of replacing them. If the specified list begins with
745 a ‘-’ character, then the specified algorithms (including wild‐
746 cards) will be removed from the built-in default set instead of
747 replacing them. If the specified list begins with a ‘^’ charac‐
748 ter, then the specified algorithms will be placed at the head of
749 the built-in default set.
750 The algorithms that contain "-etm" calculate the MAC after
752 encryption (encrypt-then-mac). These are considered safer and
753 their use recommended.
754 The list of available MAC algorithms may also be obtained using
756 "ssh -Q mac".
757 NoHostAuthenticationForLocalhost
759 Disable host authentication for localhost (loopback addresses).
760 The argument to this keyword must be yes or no (the default).
761 NumberOfPasswordPrompts
763 Specifies the number of password prompts before giving up. The
764 argument to this keyword must be an integer. The default is 3.
765 PasswordAuthentication
767 Specifies whether to use password authentication. The argument
768 to this keyword must be yes (the default) or no.
769 PermitLocalCommand
771 Allow local command execution via the LocalCommand option or
772 using the !command escape sequence in ssh(1). The argument must
773 be yes or no (the default).
774 PKCS11Provider
776 Specifies which PKCS#11 provider to use or none to indicate that
777 no provider should be used (the default). The argument to this
778 keyword is a path to the PKCS#11 shared library ssh(1) should use
779 to communicate with a PKCS#11 token providing keys for user
780 authentication.
781 Port Specifies the port number to connect on the remote host. The
783 default is 22.
784 PreferredAuthentications
786 Specifies the order in which the client should try authentication
787 methods. This allows a client to prefer one method (e.g.
788 keyboard-interactive) over another method (e.g. password). The
789 default is:
790 gssapi-with-mic,hostbased,publickey,
792 keyboard-interactive,password
793 ProxyCommand
795 Specifies the command to use to connect to the server. The com‐
796 mand string extends to the end of the line, and is executed using
797 the user's shell ‘exec’ directive to avoid a lingering shell
798 process.
799 Arguments to ProxyCommand accept the tokens described in the
801 TOKENS section. The command can be basically anything, and
802 should read from its standard input and write to its standard
803 output. It should eventually connect an sshd(8) server running
804 on some machine, or execute sshd -i somewhere. Host key manage‐
805 ment will be done using the Hostname of the host being connected
806 (defaulting to the name typed by the user). Setting the command
807 to none disables this option entirely. Note that CheckHostIP is
808 not available for connects with a proxy command.
809 This directive is useful in conjunction with nc(1) and its proxy
811 support. For example, the following directive would connect via
812 an HTTP proxy at 192.0.2.0:
813 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
815 ProxyJump
817 Specifies one or more jump proxies as either [user@]host[:port]
818 or an ssh URI. Multiple proxies may be separated by comma char‐
819 acters and will be visited sequentially. Setting this option
820 will cause ssh(1) to connect to the target host by first making a
821 ssh(1) connection to the specified ProxyJump host and then estab‐
822 lishing a TCP forwarding to the ultimate target from there.
823 Note that this option will compete with the ProxyCommand option -
825 whichever is specified first will prevent later instances of the
826 other from taking effect.
827 Note also that the configuration for the destination host (either
829 supplied via the command-line or the configuration file) is not
830 generally applied to jump hosts. ~/.ssh/config should be used if
831 specific configuration is required for jump hosts.
832 ProxyUseFdpass
834 Specifies that ProxyCommand will pass a connected file descriptor
835 back to ssh(1) instead of continuing to execute and pass data.
836 The default is no.
837 PubkeyAcceptedKeyTypes
839 The default is handled system-wide by crypto-policies(7). To see
840 the defaults and how to modify this default, see manual page
841 update-crypto-policies(8).
842 Specifies the key types that will be used for public key authen‐
844 tication as a comma-separated list of patterns. If the specified
845 list begins with a ‘+’ character, then the key types after it
846 will be appended to the built-in default instead of replacing it.
847 If the specified list begins with a ‘-’ character, then the spec‐
848 ified key types (including wildcards) will be removed from the
849 built-in default set instead of replacing them. If the specified
850 list begins with a ‘^’ character, then the specified key types
851 will be placed at the head of the built-in default set.
852 The list of available key types may also be obtained using "ssh
854 -Q PubkeyAcceptedKeyTypes".
855 PubkeyAuthentication
857 Specifies whether to try public key authentication. The argument
858 to this keyword must be yes (the default) or no.
859 RekeyLimit
861 Specifies the maximum amount of data that may be transmitted
862 before the session key is renegotiated, optionally followed a
863 maximum amount of time that may pass before the session key is
864 renegotiated. The first argument is specified in bytes and may
865 have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes,
866 Megabytes, or Gigabytes, respectively. The default is between
867 ‘1G’ and ‘4G’, depending on the cipher. The optional second
868 value is specified in seconds and may use any of the units docu‐
869 mented in the TIME FORMATS section of sshd_config(5). The
870 default value for RekeyLimit is default none, which means that
871 rekeying is performed after the cipher's default amount of data
872 has been sent or received and no time based rekeying is done.
873 RemoteCommand
875 Specifies a command to execute on the remote machine after suc‐
876 cessfully connecting to the server. The command string extends
877 to the end of the line, and is executed with the user's shell.
878 Arguments to RemoteCommand accept the tokens described in the
879 TOKENS section.
880 RemoteForward
882 Specifies that a TCP port on the remote machine be forwarded over
883 the secure channel. The remote port may either be forwarded to a
884 specified host and port from the local machine, or may act as a
885 SOCKS 4/5 proxy that allows a remote client to connect to arbi‐
886 trary destinations from the local machine. The first argument
887 must be [bind_address:]port If forwarding to a specific destina‐
888 tion then the second argument must be host:hostport, otherwise if
889 no destination argument is specified then the remote forwarding
890 will be established as a SOCKS proxy.
891 IPv6 addresses can be specified by enclosing addresses in square
893 brackets. Multiple forwardings may be specified, and additional
894 forwardings can be given on the command line. Privileged ports
895 can be forwarded only when logging in as root on the remote
896 machine.
897 If the port argument is 0, the listen port will be dynamically
899 allocated on the server and reported to the client at run time.
900 If the bind_address is not specified, the default is to only bind
902 to loopback addresses. If the bind_address is ‘*’ or an empty
903 string, then the forwarding is requested to listen on all inter‐
904 faces. Specifying a remote bind_address will only succeed if the
905 server's GatewayPorts option is enabled (see sshd_config(5)).
906 RequestTTY
908 Specifies whether to request a pseudo-tty for the session. The
909 argument may be one of: no (never request a TTY), yes (always
910 request a TTY when standard input is a TTY), force (always
911 request a TTY) or auto (request a TTY when opening a login ses‐
912 sion). This option mirrors the -t and -T flags for ssh(1).
913 RevokedHostKeys
915 Specifies revoked host public keys. Keys listed in this file
916 will be refused for host authentication. Note that if this file
917 does not exist or is not readable, then host authentication will
918 be refused for all hosts. Keys may be specified as a text file,
919 listing one public key per line, or as an OpenSSH Key Revocation
920 List (KRL) as generated by ssh-keygen(1). For more information
921 on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).
922 SecurityKeyProvider
924 Specifies a path to a library that will be used when loading any
925 FIDO authenticator-hosted keys, overriding the default of using
926 the built-in USB HID support.
927 If the specified value begins with a ‘$’ character, then it will
929 be treated as an environment variable containing the path to the
930 library.
931 SendEnv
933 Specifies what variables from the local environ(7) should be sent
934 to the server. The server must also support it, and the server
935 must be configured to accept these environment variables. Note
936 that the TERM environment variable is always sent whenever a
937 pseudo-terminal is requested as it is required by the protocol.
938 Refer to AcceptEnv in sshd_config(5) for how to configure the
939 server. Variables are specified by name, which may contain wild‐
940 card characters. Multiple environment variables may be separated
941 by whitespace or spread across multiple SendEnv directives.
942 See PATTERNS for more information on patterns.
944 It is possible to clear previously set SendEnv variable names by
946 prefixing patterns with -. The default is not to send any envi‐
947 ronment variables.
948 ServerAliveCountMax
950 Sets the number of server alive messages (see below) which may be
951 sent without ssh(1) receiving any messages back from the server.
952 If this threshold is reached while server alive messages are
953 being sent, ssh will disconnect from the server, terminating the
954 session. It is important to note that the use of server alive
955 messages is very different from TCPKeepAlive (below). The server
956 alive messages are sent through the encrypted channel and there‐
957 fore will not be spoofable. The TCP keepalive option enabled by
958 TCPKeepAlive is spoofable. The server alive mechanism is valu‐
959 able when the client or server depend on knowing when a connec‐
960 tion has become unresponsive.
961 The default value is 3. If, for example, ServerAliveInterval
963 (see below) is set to 15 and ServerAliveCountMax is left at the
964 default, if the server becomes unresponsive, ssh will disconnect
965 after approximately 45 seconds.
966 ServerAliveInterval
968 Sets a timeout interval in seconds after which if no data has
969 been received from the server, ssh(1) will send a message through
970 the encrypted channel to request a response from the server. The
971 default is 0, indicating that these messages will not be sent to
972 the server.
973 SetEnv Directly specify one or more environment variables and their con‐
975 tents to be sent to the server. Similarly to SendEnv, the server
976 must be prepared to accept the environment variable.
977 StreamLocalBindMask
979 Sets the octal file creation mode mask (umask) used when creating
980 a Unix-domain socket file for local or remote port forwarding.
981 This option is only used for port forwarding to a Unix-domain
982 socket file.
983 The default value is 0177, which creates a Unix-domain socket
985 file that is readable and writable only by the owner. Note that
986 not all operating systems honor the file mode on Unix-domain
987 socket files.
988 StreamLocalBindUnlink
990 Specifies whether to remove an existing Unix-domain socket file
991 for local or remote port forwarding before creating a new one.
992 If the socket file already exists and StreamLocalBindUnlink is
993 not enabled, ssh will be unable to forward the port to the Unix-
994 domain socket file. This option is only used for port forwarding
995 to a Unix-domain socket file.
996 The argument must be yes or no (the default).
998 StrictHostKeyChecking
1000 If this flag is set to yes, ssh(1) will never automatically add
1001 host keys to the ~/.ssh/known_hosts file, and refuses to connect
1002 to hosts whose host key has changed. This provides maximum pro‐
1003 tection against man-in-the-middle (MITM) attacks, though it can
1004 be annoying when the /etc/ssh/ssh_known_hosts file is poorly
1005 maintained or when connections to new hosts are frequently made.
1006 This option forces the user to manually add all new hosts.
1007 If this flag is set to “accept-new” then ssh will automatically
1009 add new host keys to the user known hosts files, but will not
1010 permit connections to hosts with changed host keys. If this flag
1011 is set to “no” or “off”, ssh will automatically add new host keys
1012 to the user known hosts files and allow connections to hosts with
1013 changed hostkeys to proceed, subject to some restrictions. If
1014 this flag is set to ask (the default), new host keys will be
1015 added to the user known host files only after the user has con‐
1016 firmed that is what they really want to do, and ssh will refuse
1017 to connect to hosts whose host key has changed. The host keys of
1018 known hosts will be verified automatically in all cases.
1019 SyslogFacility
1021 Gives the facility code that is used when logging messages from
1022 ssh(1). The possible values are: DAEMON, USER, AUTH, LOCAL0,
1023 LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The
1024 default is USER.
1025 TCPKeepAlive
1027 Specifies whether the system should send TCP keepalive messages
1028 to the other side. If they are sent, death of the connection or
1029 crash of one of the machines will be properly noticed. However,
1030 this means that connections will die if the route is down tempo‐
1031 rarily, and some people find it annoying.
1032 The default is yes (to send TCP keepalive messages), and the
1034 client will notice if the network goes down or the remote host
1035 dies. This is important in scripts, and many users want it too.
1036 To disable TCP keepalive messages, the value should be set to no.
1038 See also ServerAliveInterval for protocol-level keepalives.
1039 Tunnel Request tun(4) device forwarding between the client and the
1041 server. The argument must be yes, point-to-point (layer 3),
1042 ethernet (layer 2), or no (the default). Specifying yes requests
1043 the default tunnel mode, which is point-to-point.
1044 TunnelDevice
1046 Specifies the tun(4) devices to open on the client (local_tun)
1047 and the server (remote_tun).
1048 The argument must be local_tun[:remote_tun]. The devices may be
1050 specified by numerical ID or the keyword any, which uses the next
1051 available tunnel device. If remote_tun is not specified, it
1052 defaults to any. The default is any:any.
1053 UpdateHostKeys
1055 Specifies whether ssh(1) should accept notifications of addi‐
1056 tional hostkeys from the server sent after authentication has
1057 completed and add them to UserKnownHostsFile. The argument must
1058 be yes, no or ask. This option allows learning alternate
1059 hostkeys for a server and supports graceful key rotation by
1060 allowing a server to send replacement public keys before old ones
1061 are removed. Additional hostkeys are only accepted if the key
1062 used to authenticate the host was already trusted or explicitly
1063 accepted by the user.
1064 UpdateHostKeys is enabled by default if the user has not overrid‐
1066 den the default UserKnownHostsFile setting, otherwise
1067 UpdateHostKeys will be set to ask.
1068 If UpdateHostKeys is set to ask, then the user is asked to con‐
1070 firm the modifications to the known_hosts file. Confirmation is
1071 currently incompatible with ControlPersist, and will be disabled
1072 if it is enabled.
1073 Presently, only sshd(8) from OpenSSH 6.8 and greater support the
1075 "hostkeys@openssh.com" protocol extension used to inform the
1076 client of all the server's hostkeys.
1077 User Specifies the user to log in as. This can be useful when a dif‐
1079 ferent user name is used on different machines. This saves the
1080 trouble of having to remember to give the user name on the com‐
1081 mand line.
1082 UserKnownHostsFile
1084 Specifies one or more files to use for the user host key data‐
1085 base, separated by whitespace. The default is
1086 ~/.ssh/known_hosts, ~/.ssh/known_hosts2.
1087 VerifyHostKeyDNS
1089 Specifies whether to verify the remote key using DNS and SSHFP
1090 resource records. If this option is set to yes, the client will
1091 implicitly trust keys that match a secure fingerprint from DNS.
1092 Insecure fingerprints will be handled as if this option was set
1093 to ask. If this option is set to ask, information on fingerprint
1094 match will be displayed, but the user will still need to confirm
1095 new host keys according to the StrictHostKeyChecking option. The
1096 default is no.
1097 See also VERIFYING HOST KEYS in ssh(1).
1099 VisualHostKey
1101 If this flag is set to yes, an ASCII art representation of the
1102 remote host key fingerprint is printed in addition to the finger‐
1103 print string at login and for unknown host keys. If this flag is
1104 set to no (the default), no fingerprint strings are printed at
1105 login and only the fingerprint string will be printed for unknown
1106 host keys.
1107 XAuthLocation
1109 Specifies the full pathname of the xauth(1) program. The default
1110 is /usr/bin/xauth.
1111
1113 A pattern consists of zero or more non-whitespace characters, ‘*’ (a
1114 wildcard that matches zero or more characters), or ‘?’ (a wildcard that
1115 matches exactly one character). For example, to specify a set of decla‐
1116 rations for any host in the ".co.uk" set of domains, the following pat‐
1117 tern could be used:
1118 Host *.co.uk
1120 The following pattern would match any host in the 192.168.0.[0-9] network
1122 range:
1123 Host 192.168.0.?
1125 A pattern-list is a comma-separated list of patterns. Patterns within
1127 pattern-lists may be negated by preceding them with an exclamation mark
1128 (‘!’). For example, to allow a key to be used from anywhere within an
1129 organization except from the "dialup" pool, the following entry (in
1130 authorized_keys) could be used:
1131 from="!*.dialup.example.com,*.example.com"
1133 Note that a negated match will never produce a positive result by itself.
1135 For example, attempting to match "host3" against the following pattern-
1136 list will fail:
1137 from="!host1,!host2"
1139 The solution here is to include a term that will yield a positive match,
1141 such as a wildcard:
1142 from="!host1,!host2,*"
1144
1146 Arguments to some keywords can make use of tokens, which are expanded at
1147 runtime:
1148 %% A literal ‘%’.
1150 %C Hash of %l%h%p%r.
1151 %d Local user's home directory.
1152 %h The remote hostname.
1153 %i The local user ID.
1154 %L The local hostname.
1155 %l The local hostname, including the domain name.
1156 %n The original remote hostname, as given on the command line.
1157 %p The remote port.
1158 %r The remote username.
1159 %T The local tun(4) or tap(4) network interface assigned if tun‐
1160 nel forwarding was requested, or "NONE" otherwise.
1161 %u The local username.
1162 Match exec accepts the tokens %%, %h, %i, %L, %l, %n, %p, %r, and %u.
1164 CertificateFile accepts the tokens %%, %d, %h, %i, %l, %r, and %u.
1166 ControlPath accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and
1168 %u.
1169 Hostname accepts the tokens %% and %h.
1171 IdentityAgent and IdentityFile accept the tokens %%, %d, %h, %i, %l, %r,
1173 and %u.
1174 LocalCommand accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, %T,
1176 and %u.
1177 ProxyCommand accepts the tokens %%, %h, %n, %p, and %r.
1179 RemoteCommand accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, and
1181 %u.
1182
1184 ~/.ssh/config
1185 This is the per-user configuration file. The format of this file
1186 is described above. This file is used by the SSH client.
1187 Because of the potential for abuse, this file must have strict
1188 permissions: read/write for the user, and not writable by others.
1189 /etc/ssh/ssh_config
1191 Systemwide configuration file. This file provides defaults for
1192 those values that are not specified in the user's configuration
1193 file, and for those users who do not have a configuration file.
1194 This file must be world-readable.
1195
1197 ssh(1)
1198
1200 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1201 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1202 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1203 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
1204 versions 1.5 and 2.0.
1205BSD May 10, 2020 BSD