# ssh_config - OpenSSH client configuration file - man(5) - [phpMan] [_SSH_CONFIG_(5)](https://www.chedong.com/phpMan.php/man/SSHCONFIG/5/markdown) File Formats Manual [_SSH_CONFIG_(5)](https://www.chedong.com/phpMan.php/man/SSHCONFIG/5/markdown) ## NAME ssh_config — OpenSSH client configuration file ## DESCRIPTION [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) obtains configuration data from the following sources in the following order: 1. command-line options 2. user's configuration file (_~/.ssh/config_) 3. system-wide configuration file (_/etc/ssh/ssh_config_) Unless noted otherwise, for each parameter, the first obtained value will be used. The con‐ figuration files contain sections separated by **Host **specifications, and that section is only applied for hosts that match one of the patterns given in the specification. The matched host name is usually the one given on the command line (see the **CanonicalizeHostname **option for exceptions). Since the first obtained value for each parameter is used, more host-specific declarations should be given near the beginning of the file, and general defaults at the end. Note that the Debian **openssh-client **package sets several options as standard in _/etc/ssh/ssh_config_ which are not the default in [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown): **• Include /etc/ssh/ssh_config.d/*.conf** **• SendEnv **LANG LC_* **• HashKnownHosts **yes **• GSSAPIAuthentication **yes _/etc/ssh/ssh_config.d/*.conf_ files are included at the start of the system-wide configuration file, so options set there will override those in _/etc/ssh/ssh_config._ The file contains keyword-argument pairs, one per line. Lines starting with ‘#’ and empty lines are interpreted as comments. Arguments may optionally be enclosed in double quotes (") in order to represent arguments containing spaces. Configuration options may be separated by whitespace or optional whitespace and exactly one ‘=’; the latter format is useful to avoid the need to quote whitespace when specifying configuration options using the **ssh**, **scp**, and **sftp -o **option. The possible keywords and their meanings are as follows (note that keywords are case-insensi‐ tive and arguments are case-sensitive): **Host **Restricts the following declarations (up to the next **Host **or **Match **keyword) to be only for those hosts that match one of the patterns given after the keyword. If more than one pattern is provided, they should be separated by whitespace. A single ‘*’ as a pattern can be used to provide global defaults for all hosts. The host is usu‐ ally the _hostname_ argument given on the command line (see the **CanonicalizeHostname** keyword for exceptions). A pattern entry may be negated by prefixing it with an exclamation mark (‘!’). If a negated entry is matched, then the **Host **entry is ignored, regardless of whether any other patterns on the line match. Negated matches are therefore useful to provide exceptions for wildcard matches. See “PATTERNS” for more information on patterns. **Match **Restricts the following declarations (up to the next **Host **or **Match **keyword) to be used only when the conditions following the **Match **keyword are satisfied. Match con‐ ditions are specified using one or more criteria or the single token **all **which always matches. The available criteria keywords are: **canonical**, **final**, **exec**, **localnetwork**, **host**, **originalhost**, **Tag**, **user**, and **localuser**. The **all **criteria must appear alone or immediately after **canonical **or **final**. Other criteria may be combined arbitrarily. All criteria but **all**, **canonical**, and **final **require an argument. Criteria may be negated by prepending an exclamation mark (‘!’). The **canonical **keyword matches only when the configuration file is being re-parsed af‐ ter hostname canonicalization (see the **CanonicalizeHostname **option). This may be useful to specify conditions that work with canonical host names only. The **final **keyword requests that the configuration be re-parsed (regardless of whether **CanonicalizeHostname **is enabled), and matches only during this final pass. If **CanonicalizeHostname **is enabled, then **canonical **and **final **match during the same pass. The **exec **keyword executes the specified command under the user's shell. If the com‐ mand returns a zero exit status then the condition is considered true. Commands con‐ taining whitespace characters must be quoted. Arguments to **exec **accept the tokens described in the “TOKENS” section. The **localnetwork **keyword matches the addresses of active local network interfaces against the supplied list of networks in CIDR format. This may be convenient for varying the effective configuration on devices that roam between networks. Note that network address is not a trustworthy criteria in many situations (e.g. when the net‐ work is automatically configured using DHCP) and so caution should be applied if us‐ ing it to control security-sensitive configuration. The other keywords' criteria must be single entries or comma-separated lists and may use the wildcard and negation operators described in the “PATTERNS” section. The criteria for the **host **keyword are matched against the target hostname, after any sub‐ stitution by the **Hostname **or **CanonicalizeHostname **options. The **originalhost **keyword matches against the hostname as it was specified on the command-line. The **tagged** keyword matches a tag name specified by a prior **Tag **directive or on the [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) com‐ mand-line using the **-P **flag. The **user **keyword matches against the target username on the remote host. The **localuser **keyword matches against the name of the local user running [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) (this keyword may be useful in system-wide **ssh_config **files). ### AddKeysToAgent Specifies whether keys should be automatically added to a running [_ssh-agent_(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown). If this option is set to **yes **and a key is loaded from a file, the key and its passphrase are added to the agent with the default lifetime, as if by [_ssh-add_(1)](https://www.chedong.com/phpMan.php/man/ssh-add/1/markdown). If this op‐ tion is set to **ask**, [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will require confirmation using the SSH_ASKPASS program before adding a key (see [_ssh-add_(1)](https://www.chedong.com/phpMan.php/man/ssh-add/1/markdown) for details). If this option is set to **confirm**, each use of the key must be confirmed, as if the **-c **option was specified to [_ssh-add_(1)](https://www.chedong.com/phpMan.php/man/ssh-add/1/markdown). If this option is set to **no**, no keys are added to the agent. Alter‐ nately, this option may be specified as a time interval using the format described in the “TIME FORMATS” section of [_sshd_config_(5)](https://www.chedong.com/phpMan.php/man/sshdconfig/5/markdown) to specify the key's lifetime in [_ssh-agent_(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown), after which it will automatically be removed. The argument must be **no** (the default), **yes**, **confirm **(optionally followed by a time interval), **ask **or a time interval. ### AddressFamily Specifies which address family to use when connecting. Valid arguments are **any **(the default), **inet **(use IPv4 only), or **inet6 **(use IPv6 only). ### BatchMode If set to **yes**, user interaction such as password prompts and host key confirmation requests will be disabled. In addition, the **ServerAliveInterval **option will be set to 300 seconds by default (Debian-specific). This option is useful in scripts and other batch jobs where no user is present to interact with [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown), and where it is desirable to detect a broken network swiftly. The argument must be **yes **or **no **(the default). ### BindAddress Use the specified address on the local machine as the source address of the connec‐ tion. Only useful on systems with more than one address. ### BindInterface Use the address of the specified interface on the local machine as the source address of the connection. ### CanonicalDomains When **CanonicalizeHostname **is enabled, this option specifies the list of domain suf‐ fixes in which to search for the specified destination host. ### CanonicalizeFallbackLocal Specifies whether to fail with an error when hostname canonicalization fails. The default, **yes**, will attempt to look up the unqualified hostname using the system re‐ solver's search rules. A value of **no **will cause [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) to fail instantly if **CanonicalizeHostname **is enabled and the target hostname cannot be found in any of the domains specified by **CanonicalDomains**. ### CanonicalizeHostname Controls whether explicit hostname canonicalization is performed. The default, **no**, is not to perform any name rewriting and let the system resolver handle all hostname lookups. If set to **yes **then, for connections that do not use a **ProxyCommand **or **ProxyJump**, [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will attempt to canonicalize the hostname specified on the command line using the **CanonicalDomains **suffixes and **CanonicalizePermittedCNAMEs **rules. If **CanonicalizeHostname **is set to **always**, then canonicalization is applied to proxied connections too. If this option is enabled, then the configuration files are processed again using the new target name to pick up any new configuration in matching **Host **and **Match **stanzas. A value of **none **disables the use of a **ProxyJump **host. ### CanonicalizeMaxDots Specifies the maximum number of dot characters in a hostname before canonicalization is disabled. The default, 1, allows a single dot (i.e. hostname.subdomain). ### CanonicalizePermittedCNAMEs Specifies rules to determine whether CNAMEs should be followed when canonicalizing hostnames. The rules consist of one or more arguments of _source_domain_list_:_target_domain_list_, where _source_domain_list_ is a pattern-list of domains that may follow CNAMEs in canonicalization, and _target_domain_list_ is a pat‐ tern-list of domains that they may resolve to. For example, "*.a.example.com:*.b.example.com,*.c.example.com" will allow hostnames matching "*.a.example.com" to be canonicalized to names in the "*.b.example.com" or "*.c.example.com" domains. A single argument of "none" causes no CNAMEs to be considered for canonicalization. This is the default behaviour. ### CASignatureAlgorithms Specifies which algorithms are allowed for signing of certificates by certificate au‐ thorities (CAs). The default is: ssh-ed25519,ecdsa-sha2-nistp256, ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, , , rsa-sha2-512,rsa-sha2-256 If the specified list begins with a ‘+’ character, then the specified algorithms will be appended to the default set instead of replacing them. If the specified list be‐ gins with a ‘-’ character, then the specified algorithms (including wildcards) will be removed from the default set instead of replacing them. [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will not accept host certificates signed using algorithms other than those specified. ### CertificateFile Specifies a file from which the user's certificate is read. A corresponding private key must be provided separately in order to use this certificate either from an **IdentityFile **directive or **-i **flag to [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown), via [_ssh-agent_(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown), or via a **PKCS11Provider **or **SecurityKeyProvider**. Arguments to **CertificateFile **may use the tilde syntax to refer to a user's home di‐ rectory, the tokens described in the “TOKENS” section and environment variables as described in the “ENVIRONMENT VARIABLES” section. It is possible to have multiple certificate files specified in configuration files; these certificates will be tried in sequence. Multiple **CertificateFile **directives will add to the list of certificates used for authentication. ### ChannelTimeout Specifies whether and how quickly [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) should close inactive channels. Timeouts are specified as one or more “type=interval” pairs separated by whitespace, where the “type” must be a channel type name (as described in the table below), optionally con‐ taining wildcard characters. The timeout value “interval” is specified in seconds or may use any of the units doc‐ umented in the “TIME FORMATS” section. For example, “session=5m” would cause the in‐ teractive session to terminate after five minutes of inactivity. Specifying a zero value disables the inactivity timeout. The available channel types include: **agent-connection** Open connections to [_ssh-agent_(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown). **direct-tcpip**, **** Open TCP or Unix socket (respectively) connections that have been established from a [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) local forwarding, i.e. **LocalForward **or **DynamicForward**. **forwarded-tcpip**, **** Open TCP or Unix socket (respectively) connections that have been established to a [_sshd_(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) listening on behalf of a [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) remote forwarding, i.e. **RemoteForward**. **session** The interactive main session, including shell session, command execution, [_scp_(1)](https://www.chedong.com/phpMan.php/man/scp/1/markdown), [_sftp_(1)](https://www.chedong.com/phpMan.php/man/sftp/1/markdown), etc. **tun-connection** Open **TunnelForward **connections. **x11-connection** Open X11 forwarding sessions. Note that in all the above cases, terminating an inactive session does not guarantee to remove all resources associated with the session, e.g. shell processes or X11 clients relating to the session may continue to execute. Moreover, terminating an inactive channel or session does not necessarily close the SSH connection, nor does it prevent a client from requesting another channel of the same type. In particular, expiring an inactive forwarding session does not prevent another identical forwarding from being subsequently created. The default is not to expire channels of any type for inactivity. ### CheckHostIP If set to **yes**, [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will additionally check the host IP address in the _known_hosts_ file. This allows it to detect if a host key changed due to DNS spoofing and will add addresses of destination hosts to _~/.ssh/known_hosts_ in the process, regardless of the setting of **StrictHostKeyChecking**. If the option is set to **no **(the default), the check will not be executed. ### Ciphers Specifies the ciphers allowed and their order of preference. Multiple ciphers must be comma-separated. If the specified list begins with a ‘+’ character, then the specified ciphers will be appended to the default set instead of replacing them. If the specified list begins with a ‘-’ character, then the specified ciphers (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ‘^’ character, then the specified ciphers will be placed at the head of the default set. The supported ciphers are: 3des-cbc aes128-cbc aes192-cbc aes256-cbc aes128-ctr aes192-ctr aes256-ctr The default is: , aes128-ctr,aes192-ctr,aes256-ctr, , The list of available ciphers may also be obtained using "ssh -Q cipher". ### ClearAllForwardings Specifies that all local, remote, and dynamic port forwardings specified in the con‐ figuration files or on the command line be cleared. This option is primarily useful when used from the [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) command line to clear port forwardings set in configuration files, and is automatically set by [_scp_(1)](https://www.chedong.com/phpMan.php/man/scp/1/markdown) and [_sftp_(1)](https://www.chedong.com/phpMan.php/man/sftp/1/markdown). The argument must be **yes **or **no **(the default). ### Compression Specifies whether to use compression. The argument must be **yes **or **no **(the default). ### ConnectionAttempts Specifies the number of tries (one per second) to make before exiting. The argument must be an integer. This may be useful in scripts if the connection sometimes fails. The default is 1. ### ConnectTimeout Specifies the timeout (in seconds) used when connecting to the SSH server, instead of using the default system TCP timeout. This timeout is applied both to establishing the connection and to performing the initial SSH protocol handshake and key exchange. ### ControlMaster Enables the sharing of multiple sessions over a single network connection. When set to **yes**, [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will listen for connections on a control socket specified using the **ControlPath **argument. Additional sessions can connect to this socket using the same **ControlPath **with **ControlMaster **set to **no **(the default). These sessions will try to reuse the master instance's network connection rather than initiating new ones, but will fall back to connecting normally if the control socket does not exist, or is not listening. Setting this to **ask **will cause [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) to listen for control connections, but require confirmation using [_ssh-askpass_(1)](https://www.chedong.com/phpMan.php/man/ssh-askpass/1/markdown). If the **ControlPath **cannot be opened, [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will continue without connecting to a master instance. X11 and [_ssh-agent_(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown) forwarding is supported over these multiplexed connections, how‐ ever the display and agent forwarded will be the one belonging to the master connec‐ tion i.e. it is not possible to forward multiple displays or agents. Two additional options allow for opportunistic multiplexing: try to use a master con‐ nection but fall back to creating a new one if one does not already exist. These op‐ tions are: **auto **and **autoask**. The latter requires confirmation like the **ask **option. ### ControlPath Specify the path to the control socket used for connection sharing as described in the **ControlMaster **section above or the string **none **to disable connection sharing. Arguments to **ControlPath **may use the tilde syntax to refer to a user's home direc‐ tory, the tokens described in the “TOKENS” section and environment variables as de‐ scribed in the “ENVIRONMENT VARIABLES” section. It is recommended that any **ControlPath **used for opportunistic connection sharing include at least %h, %p, and %r (or alternatively %C) and be placed in a directory that is not writable by other users. This ensures that shared connections are uniquely identified. ### ControlPersist When used in conjunction with **ControlMaster**, specifies that the master connection should remain open in the background (waiting for future client connections) after the initial client connection has been closed. If set to **no **(the default), then the master connection will not be placed into the background, and will close as soon as the initial client connection is closed. If set to **yes **or 0, then the master connec‐ tion will remain in the background indefinitely (until killed or closed via a mecha‐ nism such as the "ssh -O exit"). If set to a time in seconds, or a time in any of the formats documented in [_sshd_config_(5)](https://www.chedong.com/phpMan.php/man/sshdconfig/5/markdown), then the backgrounded master connection will automatically terminate after it has remained idle (with no client connections) for the specified time. ### DynamicForward Specifies that a TCP port on the local machine be forwarded over the secure channel, and the application protocol is then used to determine where to connect to from the remote machine. The argument must be [_bind_address_:]_port_. IPv6 addresses can be specified by enclos‐ ing addresses in square brackets. By default, the local port is bound in accordance with the **GatewayPorts **setting. However, an explicit _bind_address_ may be used to bind the connection to a specific address. The _bind_address_ of **localhost **indicates that the listening port be bound for local use only, while an empty address or ‘*’ indi‐ cates that the port should be available from all interfaces. Currently the SOCKS4 and SOCKS5 protocols are supported, and [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will act as a SOCKS server. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports. ### EnableEscapeCommandline Enables the command line option in the **EscapeChar **menu for interactive sessions (de‐ fault ‘~C’). By default, the command line is disabled. ### EnableSSHKeysign Setting this option to **yes **in the global client configuration file _/etc/ssh/ssh_config_ enables the use of the helper program [_ssh-keysign_(8)](https://www.chedong.com/phpMan.php/man/ssh-keysign/8/markdown) during **HostbasedAuthentication**. The argument must be **yes **or **no **(the default). This option should be placed in the non-hostspecific section. See [_ssh-keysign_(8)](https://www.chedong.com/phpMan.php/man/ssh-keysign/8/markdown) for more infor‐ mation. ### EscapeChar Sets the escape character (default: ‘~’). The escape character can also be set on the command line. The argument should be a single character, ‘^’ followed by a let‐ ter, or **none **to disable the escape character entirely (making the connection trans‐ parent for binary data). ### ExitOnForwardFailure Specifies whether [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) should terminate the connection if it cannot set up all re‐ quested dynamic, tunnel, local, and remote port forwardings, (e.g. if either end is unable to bind and listen on a specified port). Note that **ExitOnForwardFailure **does not apply to connections made over port forwardings and will not, for example, cause [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) to exit if TCP connections to the ultimate forwarding destination fail. The argument must be **yes **or **no **(the default). ### FingerprintHash Specifies the hash algorithm used when displaying key fingerprints. Valid options are: **md5 **and **sha256 **(the default). ### ForkAfterAuthentication Requests **ssh **to go to background just before command execution. This is useful if **ssh **is going to ask for passwords or passphrases, but the user wants it in the back‐ ground. This implies the **StdinNull **configuration option being set to “yes”. The recommended way to start X11 programs at a remote site is with something like **ssh -f** **host xterm**, which is the same as **ssh host xterm **if the **ForkAfterAuthentication **con‐ figuration option is set to “yes”. If the **ExitOnForwardFailure **configuration option is set to “yes”, then a client started with the **ForkAfterAuthentication **configuration option being set to “yes” will wait for all remote port forwards to be successfully established before placing it‐ self in the background. The argument to this keyword must be **yes **(same as the **-f **op‐ tion) or **no **(the default). ### ForwardAgent Specifies whether the connection to the authentication agent (if any) will be for‐ warded to the remote machine. The argument may be **yes**, **no **(the default), an explicit path to an agent socket or the name of an environment variable (beginning with ‘$’) in which to find the path. Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the agent's Unix-domain socket) can access the local agent through the forwarded connection. An attacker cannot obtain key ma‐ terial from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent. ### ForwardX11 Specifies whether X11 connections will be automatically redirected over the secure channel and DISPLAY set. The argument must be **yes **or **no **(the default). X11 forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the user's X11 authorization database) can access the local X11 display through the forwarded connection. An attacker may then be able to perform activities such as keystroke monitoring if the **ForwardX11Trusted **option is also enabled. ### ForwardX11Timeout Specify a timeout for untrusted X11 forwarding using the format described in the “TIME FORMATS” section of [_sshd_config_(5)](https://www.chedong.com/phpMan.php/man/sshdconfig/5/markdown). X11 connections received by [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) after this time will be refused. Setting **ForwardX11Timeout **to zero will disable the time‐ out and permit X11 forwarding for the life of the connection. The default is to dis‐ able untrusted X11 forwarding after twenty minutes has elapsed. ### ForwardX11Trusted If this option is set to **yes**, (the Debian-specific default), remote X11 clients will have full access to the original X11 display. If this option is set to **no **(the upstream default), remote X11 clients will be con‐ sidered untrusted and prevented from stealing or tampering with data belonging to trusted X11 clients. Furthermore, the [_xauth_(1)](https://www.chedong.com/phpMan.php/man/xauth/1/markdown) token used for the session will be set to expire after 20 minutes. Remote clients will be refused access after this time. See the X11 SECURITY extension specification for full details on the restrictions im‐ posed on untrusted clients. ### GatewayPorts Specifies whether remote hosts are allowed to connect to local forwarded ports. By default, [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) binds local port forwardings to the loopback address. This prevents other remote hosts from connecting to forwarded ports. **GatewayPorts **can be used to specify that ssh should bind local port forwardings to the wildcard address, thus al‐ lowing remote hosts to connect to forwarded ports. The argument must be **yes **or **no** (the default). ### GlobalKnownHostsFile Specifies one or more files to use for the global host key database, separated by whitespace. The default is _/etc/ssh/ssh_known_hosts_, _/etc/ssh/ssh_known_hosts2_. ### GSSAPIAuthentication Specifies whether user authentication based on GSSAPI is allowed. The default is **no**. ### GSSAPIClientIdentity If set, specifies the GSSAPI client identity that ssh should use when connecting to the server. The default is unset, which means that the default identity will be used. ### GSSAPIDelegateCredentials Forward (delegate) credentials to the server. The default is **no**. ### GSSAPIKeyExchange Specifies whether key exchange based on GSSAPI may be used. When using GSSAPI key ex‐ change the server need not have a host key. The default is “no”. ### GSSAPIRenewalForcesRekey If set to “yes” then renewal of the client's GSSAPI credentials will force the rekey‐ ing of the ssh connection. With a compatible server, this will delegate the renewed credentials to a session on the server. Checks are made to ensure that credentials are only propagated when the new creden‐ tials match the old ones on the originating client and where the receiving server still has the old set in its cache. The default is “no”. For this to work **GSSAPIKeyExchange **needs to be enabled in the server and also used by the client. ### GSSAPIServerIdentity If set, specifies the GSSAPI server identity that ssh should expect when connecting to the server. The default is unset, which means that the expected GSSAPI server identity will be determined from the target hostname. ### GSSAPITrustDns Set to “yes” to indicate that the DNS is trusted to securely canonicalize the name of the host being connected to. If “no”, the hostname entered on the command line will be passed untouched to the GSSAPI library. The default is “no”. ### GSSAPIKexAlgorithms The list of key exchange algorithms that are offered for GSSAPI key exchange. Possi‐ ble values are gss-gex-sha1-, gss-group1-sha1-, gss-group14-sha1-, gss-group14-sha256-, gss-group16-sha512-, gss-nistp256-sha256-, gss-curve25519-sha256- The default is “gss-group14-sha256-,gss-group16-sha512-,gss-nistp256-sha256-,gss-curve25519-sha256-,gss-gex-sha1-,gss-group14-sha1-”. This option only applies to connections using GSSAPI. ### HashKnownHosts Indicates that [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) should hash host names and addresses when they are added to _~/.ssh/known_hosts_. These hashed names may be used normally by [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) and [_sshd_(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown), but they do not visually reveal identifying information if the file's contents are disclosed. The default is **no**. Note that existing names and addresses in known hosts files will not be converted automatically, but may be manually hashed using [_ssh-keygen_(1)](https://www.chedong.com/phpMan.php/man/ssh-keygen/1/markdown). Use of this option may break facilities such as tab-completion that rely on being able to read unhashed host names from _~/.ssh/known_hosts_. ### HostbasedAcceptedAlgorithms Specifies the signature algorithms that will be used for hostbased authentication as a comma-separated list of patterns. Alternately if the specified list begins with a ‘+’ character, then the specified signature algorithms will be appended to the de‐ fault set instead of replacing them. If the specified list begins with a ‘-’ charac‐ ter, then the specified signature algorithms (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ‘^’ character, then the specified signature algorithms will be placed at the head of the default set. The default for this option is: , , , , , , , , ssh-ed25519, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, , , rsa-sha2-512,rsa-sha2-256 The **-Q **option of [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) may be used to list supported signature algorithms. This was formerly named HostbasedKeyTypes. ### HostbasedAuthentication Specifies whether to try rhosts based authentication with public key authentication. The argument must be **yes **or **no **(the default). ### HostKeyAlgorithms Specifies the host key signature algorithms that the client wants to use in order of preference. Alternately if the specified list begins with a ‘+’ character, then the specified signature algorithms will be appended to the default set instead of replac‐ ing them. If the specified list begins with a ‘-’ character, then the specified sig‐ nature algorithms (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ‘^’ character, then the spec‐ ified signature algorithms will be placed at the head of the default set. The de‐ fault for this option is: , , , , , , , , ssh-ed25519, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, , , rsa-sha2-512,rsa-sha2-256 If hostkeys are known for the destination host then this default is modified to pre‐ fer their algorithms. The list of available signature algorithms may also be obtained using "ssh -Q HostKeyAlgorithms". ### HostKeyAlias Specifies an alias that should be used instead of the real host name when looking up or saving the host key in the host key database files and when validating host cer‐ tificates. This option is useful for tunneling SSH connections or for multiple servers running on a single host. ### Hostname Specifies the real host name to log into. This can be used to specify nicknames or abbreviations for hosts. Arguments to **Hostname **accept the tokens described in the “TOKENS” section. Numeric IP addresses are also permitted (both on the command line and in **Hostname **specifications). The default is the name given on the command line. ### IdentitiesOnly Specifies that [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) should only use the configured authentication identity and cer‐ tificate files (either the default files, or those explicitly configured in the **ssh_config **files or passed on the [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) command-line), even if [_ssh-agent_(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown) or a **PKCS11Provider **or **SecurityKeyProvider **offers more identities. The argument to this keyword must be **yes **or **no **(the default). This option is intended for situations where ssh-agent offers many different identities. ### IdentityAgent Specifies the Unix-domain socket used to communicate with the authentication agent. This option overrides the SSH_AUTH_SOCK environment variable and can be used to se‐ lect a specific agent. Setting the socket name to **none **disables the use of an au‐ thentication agent. If the string "SSH_AUTH_SOCK" is specified, the location of the socket will be read from the SSH_AUTH_SOCK environment variable. Otherwise if the specified value begins with a ‘$’ character, then it will be treated as an environ‐ ment variable containing the location of the socket. Arguments to **IdentityAgent **may use the tilde syntax to refer to a user's home direc‐ tory, the tokens described in the “TOKENS” section and environment variables as de‐ scribed in the “ENVIRONMENT VARIABLES” section. ### IdentityFile Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. You can also specify a public key file to use the corresponding private key that is loaded in [_ssh-agent_(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown) when the private key file is not present locally. The default is _~/.ssh/id_rsa_, _~/.ssh/id_ecdsa_, _~/.ssh/id_ecdsa_sk_, _~/.ssh/id_ed25519_, _~/.ssh/id_ed25519_sk_ and _~/.ssh/id_dsa_. Additionally, any identities represented by the authentication agent will be used for authentication unless **IdentitiesOnly **is set. If no certificates have been explicitly specified by **CertificateFile**, [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will try to load certificate information from the filename obtained by appending _-cert.pub_ to the path of a specified **IdentityFile**. Arguments to **IdentityFile **may use the tilde syntax to refer to a user's home direc‐ tory or the tokens described in the “TOKENS” section. Alternately an argument of **none **may be used to indicate no identity files should be loaded. It is possible to have multiple identity files specified in configuration files; all these identities will be tried in sequence. Multiple **IdentityFile **directives will add to the list of identities tried (this behaviour differs from that of other con‐ figuration directives). **IdentityFile **may be used in conjunction with **IdentitiesOnly **to select which identi‐ ties in an agent are offered during authentication. **IdentityFile **may also be used in conjunction with **CertificateFile **in order to provide any certificate also needed for authentication with the identity. ### IgnoreUnknown Specifies a pattern-list of unknown options to be ignored if they are encountered in configuration parsing. This may be used to suppress errors if **ssh_config **contains options that are unrecognised by [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown). It is recommended that **IgnoreUnknown **be listed early in the configuration file as it will not be applied to unknown options that appear before it. ### Include Include the specified configuration file(s). Multiple pathnames may be specified and each pathname may contain [_glob_(7)](https://www.chedong.com/phpMan.php/man/glob/7/markdown) wildcards and, for user configurations, shell-like ‘~’ references to user home directories. Wildcards will be expanded and processed in lexical order. Files without absolute paths are assumed to be in _~/.ssh_ if included in a user configuration file or _/etc/ssh_ if included from the system configuration file. **Include **directive may appear inside a **Match **or **Host **block to perform condi‐ tional inclusion. **IPQoS **Specifies the IPv4 type-of-service or DSCP class for connections. Accepted values are **af11**, **af12**, **af13**, **af21**, **af22**, **af23**, **af31**, **af32**, **af33**, **af41**, **af42**, **af43**, **cs0**, **cs1**, **cs2**, **cs3**, **cs4**, **cs5**, **cs6**, **cs7**, **ef**, **le**, **lowdelay**, **throughput**, **reliability**, a numeric value, or **none **to use the operating system default. This option may take one or two arguments, separated by whitespace. If one argument is specified, it is used as the packet class unconditionally. If two values are specified, the first is automati‐ cally selected for interactive sessions and the second for non-interactive sessions. The default is **lowdelay **for interactive sessions and **throughput **for non-interactive sessions. ### KbdInteractiveAuthentication Specifies whether to use keyboard-interactive authentication. The argument to this keyword must be **yes **(the default) or **no**. **ChallengeResponseAuthentication **is a depre‐ cated alias for this. ### KbdInteractiveDevices Specifies the list of methods to use in keyboard-interactive authentication. Multi‐ ple method names must be comma-separated. The default is to use the server specified list. The methods available vary depending on what the server supports. For an OpenSSH server, it may be zero or more of: **bsdauth **and **pam**. ### KexAlgorithms Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be comma-separated. If the specified list begins with a ‘+’ character, then the speci‐ fied algorithms will be appended to the default set instead of replacing them. If the specified list begins with a ‘-’ character, then the specified algorithms (in‐ cluding wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ‘^’ character, then the specified algorithms will be placed at the head of the default set. The default is: , curve25519-sha256,, ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, diffie-hellman-group-exchange-sha256, diffie-hellman-group16-sha512, diffie-hellman-group18-sha512, diffie-hellman-group14-sha256 The list of available key exchange algorithms may also be obtained using "ssh -Q kex". ### KnownHostsCommand Specifies a command to use to obtain a list of host keys, in addition to those listed in **UserKnownHostsFile **and **GlobalKnownHostsFile**. This command is executed after the files have been read. It may write host key lines to standard output in identical format to the usual files (described in the “VERIFYING HOST KEYS” section in [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown)). Arguments to **KnownHostsCommand **accept the tokens described in the “TOKENS” section. The command may be invoked multiple times per connection: once when preparing the preference list of host key algorithms to use, again to obtain the host key for the requested host name and, if **CheckHostIP **is enabled, one more time to obtain the host key matching the server's address. If the command exits abnormally or returns a non- zero exit status then the connection is terminated. ### LocalCommand Specifies a command to execute on the local machine after successfully connecting to the server. The command string extends to the end of the line, and is executed with the user's shell. Arguments to **LocalCommand **accept the tokens described in the “TOKENS” section. The command is run synchronously and does not have access to the session of the [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) that spawned it. It should not be used for interactive commands. This directive is ignored unless **PermitLocalCommand **has been enabled. ### LocalForward Specifies that a TCP port on the local machine be forwarded over the secure channel to the specified host and port from the remote machine. The first argument specifies the listener and may be [_bind_address_:]_port_ or a Unix domain socket path. The second argument is the destination and may be _host_:_hostport_ or a Unix domain socket path if the remote host supports it. IPv6 addresses can be specified by enclosing addresses in square brackets. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the **GatewayPorts **setting. However, an explicit _bind_address_ may be used to bind the connection to a specific address. The _bind_address_ of **localhost **indicates that the listening port be bound for local use only, while an empty address or ‘*’ indicates that the port should be available from all interfaces. Unix domain socket paths may use the tokens described in the “TOKENS” section and environment variables as described in the “ENVIRONMENT VARIABLES” section. ### LogLevel Gives the verbosity level that is used when logging messages from [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown). The possi‐ ble values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DE‐ BUG3. The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify higher levels of verbose output. ### LogVerbose Specify one or more overrides to LogLevel. An override consists of a pattern lists that matches the source file, function and line number to force detailed logging for. For example, an override pattern of: kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* would enable detailed logging for line 1000 of _kex.c_, everything in the **kex_exchange_identification**() function, and all code in the _packet.c_ file. This op‐ tion is intended for debugging and no overrides are enabled by default. **MACs **Specifies the MAC (message authentication code) algorithms in order of preference. The MAC algorithm is used for data integrity protection. Multiple algorithms must be comma-separated. If the specified list begins with a ‘+’ character, then the speci‐ fied algorithms will be appended to the default set instead of replacing them. If the specified list begins with a ‘-’ character, then the specified algorithms (in‐ cluding wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ‘^’ character, then the specified algorithms will be placed at the head of the default set. The algorithms that contain "-etm" calculate the MAC after encryption (encrypt-then- mac). These are considered safer and their use recommended. The default is: ,, ,, , ,, hmac-sha2-256,hmac-sha2-512,hmac-sha1 The list of available MAC algorithms may also be obtained using "ssh -Q mac". ### NoHostAuthenticationForLocalhost Disable host authentication for localhost (loopback addresses). The argument to this keyword must be **yes **or **no **(the default). ### NumberOfPasswordPrompts Specifies the number of password prompts before giving up. The argument to this key‐ word must be an integer. The default is 3. ### ObscureKeystrokeTiming Specifies whether [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) should try to obscure inter-keystroke timings from passive observers of network traffic. If enabled, then for interactive sessions, [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will send keystrokes at fixed intervals of a few tens of milliseconds and will send fake keystroke packets for some time after typing ceases. The argument to this keyword must be **yes**, **no **or an interval specifier of the form **interval:milliseconds **(e.g. **interval:80 **for 80 milliseconds). The default is to obscure keystrokes using a 20ms packet interval. Note that smaller intervals will result in higher fake keystroke packet rates. ### PasswordAuthentication Specifies whether to use password authentication. The argument to this keyword must be **yes **(the default) or **no**. ### PermitLocalCommand Allow local command execution via the **LocalCommand **option or using the **!_**command_ es‐ cape sequence in [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown). The argument must be **yes **or **no **(the default). ### PermitRemoteOpen Specifies the destinations to which remote TCP port forwarding is permitted when **RemoteForward **is used as a SOCKS proxy. The forwarding specification must be one of the following forms: **PermitRemoteOpen _**host_:_port_ **PermitRemoteOpen _**IPv4_addr_:_port_ **PermitRemoteOpen _**[IPv6_addr]_:_port_ Multiple forwards may be specified by separating them with whitespace. An argument of **any **can be used to remove all restrictions and permit any forwarding requests. An argument of **none **can be used to prohibit all forwarding requests. The wildcard ‘*’ can be used for host or port to allow all hosts or ports respectively. Otherwise, no pattern matching or address lookups are performed on supplied names. ### PKCS11Provider Specifies which PKCS#11 provider to use or **none **to indicate that no provider should be used (the default). The argument to this keyword is a path to the PKCS#11 shared library [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) should use to communicate with a PKCS#11 token providing keys for user authentication. **Port **Specifies the port number to connect on the remote host. The default is 22. ### PreferredAuthentications Specifies the order in which the client should try authentication methods. This al‐ lows a client to prefer one method (e.g. **keyboard-interactive**) over another method (e.g. **password**). The default is: gssapi-with-mic,hostbased,publickey, keyboard-interactive,password ### ProxyCommand Specifies the command to use to connect to the server. The command string extends to the end of the line, and is executed using the user's shell ‘exec’ directive to avoid a lingering shell process. Arguments to **ProxyCommand **accept the tokens described in the “TOKENS” section. The command can be basically anything, and should read from its standard input and write to its standard output. It should eventually connect an [_sshd_(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) server running on some machine, or execute **sshd -i **somewhere. Host key management will be done using the **Hostname **of the host being connected (defaulting to the name typed by the user). Setting the command to **none **disables this option entirely. Note that **CheckHostIP **is not available for connects with a proxy command. This directive is useful in conjunction with [_nc_(1)](https://www.chedong.com/phpMan.php/man/nc/1/markdown) and its proxy support. For exam‐ ple, the following directive would connect via an HTTP proxy at 192.0.2.0: ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p ### ProxyJump Specifies one or more jump proxies as either [_user_@]_host_[:_port_] or an ssh URI. Mul‐ tiple proxies may be separated by comma characters and will be visited sequentially. Setting this option will cause [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) to connect to the target host by first making a [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) connection to the specified **ProxyJump **host and then establishing a TCP for‐ warding to the ultimate target from there. Setting the host to **none **disables this option entirely. Note that this option will compete with the **ProxyCommand **option - whichever is speci‐ fied first will prevent later instances of the other from taking effect. Note also that the configuration for the destination host (either supplied via the command-line or the configuration file) is not generally applied to jump hosts. _~/.ssh/config_ should be used if specific configuration is required for jump hosts. ### ProxyUseFdpass Specifies that **ProxyCommand **will pass a connected file descriptor back to [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) in‐ stead of continuing to execute and pass data. The default is **no**. ### PubkeyAcceptedAlgorithms Specifies the signature algorithms that will be used for public key authentication as a comma-separated list of patterns. If the specified list begins with a ‘+’ charac‐ ter, then the algorithms after it will be appended to the default instead of replac‐ ing it. If the specified list begins with a ‘-’ character, then the specified algo‐ rithms (including wildcards) will be removed from the default set instead of replac‐ ing them. If the specified list begins with a ‘^’ character, then the specified al‐ gorithms will be placed at the head of the default set. The default for this option is: , , , , , , , , ssh-ed25519, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, , , rsa-sha2-512,rsa-sha2-256 The list of available signature algorithms may also be obtained using "ssh -Q PubkeyAcceptedAlgorithms". ### PubkeyAuthentication Specifies whether to try public key authentication. The argument to this keyword must be **yes **(the default), **no**, **unbound **or **host-bound**. The final two options enable public key authentication while respectively disabling or enabling the OpenSSH host- bound authentication protocol extension required for restricted [_ssh-agent_(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown) forward‐ ing. ### RekeyLimit Specifies the maximum amount of data that may be transmitted or received before the session key is renegotiated, optionally followed by a maximum amount of time that may pass before the session key is renegotiated. The first argument is specified in bytes and may have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes, Megabytes, or Gigabytes, respectively. The default is between ‘1G’ and ‘4G’, depending on the ci‐ pher. The optional second value is specified in seconds and may use any of the units documented in the TIME FORMATS section of [_sshd_config_(5)](https://www.chedong.com/phpMan.php/man/sshdconfig/5/markdown). The default value for **RekeyLimit **is **default none**, which means that rekeying is performed after the cipher's default amount of data has been sent or received and no time based rekeying is done. ### RemoteCommand Specifies a command to execute on the remote machine after successfully connecting to the server. The command string extends to the end of the line, and is executed with the user's shell. Arguments to **RemoteCommand **accept the tokens described in the “TOKENS” section. ### RemoteForward Specifies that a TCP port on the remote machine be forwarded over the secure channel. The remote port may either be forwarded to a specified host and port from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote client to connect to arbitrary destinations from the local machine. The first argument is the listening specification and may be [_bind_address_:]_port_ or, if the remote host supports it, a Unix domain socket path. If forwarding to a specific destination then the second ar‐ gument must be _host_:_hostport_ or a Unix domain socket path, otherwise if no destina‐ tion argument is specified then the remote forwarding will be established as a SOCKS proxy. When acting as a SOCKS proxy, the destination of the connection can be re‐ stricted by **PermitRemoteOpen**. IPv6 addresses can be specified by enclosing addresses in square brackets. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Privileged ports can be forwarded only when logging in as root on the remote machine. Unix domain socket paths may use the tokens described in the “TOKENS” sec‐ tion and environment variables as described in the “ENVIRONMENT VARIABLES” section. If the _port_ argument is 0, the listen port will be dynamically allocated on the server and reported to the client at run time. If the _bind_address_ is not specified, the default is to only bind to loopback ad‐ dresses. If the _bind_address_ is ‘*’ or an empty string, then the forwarding is re‐ quested to listen on all interfaces. Specifying a remote _bind_address_ will only suc‐ ceed if the server's **GatewayPorts **option is enabled (see [_sshd_config_(5)](https://www.chedong.com/phpMan.php/man/sshdconfig/5/markdown)). ### RequestTTY Specifies whether to request a pseudo-tty for the session. The argument may be one of: **no **(never request a TTY), **yes **(always request a TTY when standard input is a TTY), **force **(always request a TTY) or **auto **(request a TTY when opening a login ses‐ sion). This option mirrors the **-t **and **-T **flags for [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown). ### RequiredRSASize Specifies the minimum RSA key size (in bits) that [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will accept. User authenti‐ cation keys smaller than this limit will be ignored. Servers that present host keys smaller than this limit will cause the connection to be terminated. The default is **1024 **bits. Note that this limit may only be raised from the default. ### RevokedHostKeys Specifies revoked host public keys. Keys listed in this file will be refused for host authentication. Note that if this file does not exist or is not readable, then host authentication will be refused for all hosts. Keys may be specified as a text file, listing one public key per line, or as an OpenSSH Key Revocation List (KRL) as generated by [_ssh-keygen_(1)](https://www.chedong.com/phpMan.php/man/ssh-keygen/1/markdown). For more information on KRLs, see the KEY REVOCATION LISTS section in [_ssh-keygen_(1)](https://www.chedong.com/phpMan.php/man/ssh-keygen/1/markdown). Arguments to **RevokedHostKeys **may use the tilde syn‐ tax to refer to a user's home directory, the tokens described in the “TOKENS” section and environment variables as described in the “ENVIRONMENT VARIABLES” section. ### SecurityKeyProvider Specifies a path to a library that will be used when loading any FIDO authenticator- hosted keys, overriding the default of using the built-in USB HID support. If the specified value begins with a ‘$’ character, then it will be treated as an en‐ vironment variable containing the path to the library. ### SendEnv Specifies what variables from the local [_environ_(7)](https://www.chedong.com/phpMan.php/man/environ/7/markdown) should be sent to the server. The server must also support it, and the server must be configured to accept these envi‐ ronment variables. Note that the TERM environment variable is always sent whenever a pseudo-terminal is requested as it is required by the protocol. Refer to **AcceptEnv** in [_sshd_config_(5)](https://www.chedong.com/phpMan.php/man/sshdconfig/5/markdown) for how to configure the server. Variables are specified by name, which may contain wildcard characters. Multiple environment variables may be sepa‐ rated by whitespace or spread across multiple **SendEnv **directives. See “PATTERNS” for more information on patterns. It is possible to clear previously set **SendEnv **variable names by prefixing patterns with _-_. The default is not to send any environment variables. ### ServerAliveCountMax Sets the number of server alive messages (see below) which may be sent without [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) receiving any messages back from the server. If this threshold is reached while server alive messages are being sent, ssh will disconnect from the server, terminat‐ ing the session. It is important to note that the use of server alive messages is very different from **TCPKeepAlive **(below). The server alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by **TCPKeepAlive **is spoofable. The server alive mechanism is valuable when the client or server depend on knowing when a connection has become unresponsive. The default value is 3. If, for example, **ServerAliveInterval **(see below) is set to 15 and **ServerAliveCountMax **is left at the default, if the server becomes unrespon‐ sive, ssh will disconnect after approximately 45 seconds. ### ServerAliveInterval Sets a timeout interval in seconds after which if no data has been received from the server, [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will send a message through the encrypted channel to request a re‐ sponse from the server. The default is 0, indicating that these messages will not be sent to the server, or 300 if the **BatchMode **option is set (Debian-specific). **ProtocolKeepAlives **and **SetupTimeOut **are Debian-specific compatibility aliases for this option. ### SessionType May be used to either request invocation of a subsystem on the remote system, or to prevent the execution of a remote command at all. The latter is useful for just for‐ warding ports. The argument to this keyword must be **none **(same as the **-N **option), **subsystem **(same as the **-s **option) or **default **(shell or command execution). **SetEnv **Directly specify one or more environment variables and their contents to be sent to the server. Similarly to **SendEnv**, with the exception of the TERM variable, the server must be prepared to accept the environment variable. ### StdinNull Redirects stdin from _/dev/null_ (actually, prevents reading from stdin). Either this or the equivalent **-n **option must be used when **ssh **is run in the background. The ar‐ gument to this keyword must be **yes **(same as the **-n **option) or **no **(the default). ### StreamLocalBindMask Sets the octal file creation mode mask (umask) used when creating a Unix-domain socket file for local or remote port forwarding. This option is only used for port forwarding to a Unix-domain socket file. The default value is 0177, which creates a Unix-domain socket file that is readable and writable only by the owner. Note that not all operating systems honor the file mode on Unix-domain socket files. ### StreamLocalBindUnlink Specifies whether to remove an existing Unix-domain socket file for local or remote port forwarding before creating a new one. If the socket file already exists and **StreamLocalBindUnlink **is not enabled, **ssh **will be unable to forward the port to the Unix-domain socket file. This option is only used for port forwarding to a Unix-do‐ main socket file. The argument must be **yes **or **no **(the default). ### StrictHostKeyChecking If this flag is set to **yes**, [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will never automatically add host keys to the _~/.ssh/known_hosts_ file, and refuses to connect to hosts whose host key has changed. This provides maximum protection against man-in-the-middle (MITM) attacks, though it can be annoying when the _/etc/ssh/ssh_known_hosts_ file is poorly maintained or when connections to new hosts are frequently made. This option forces the user to manu‐ ally add all new hosts. If this flag is set to **accept-new **then ssh will automatically add new host keys to the user's _known_hosts_ file, but will not permit connections to hosts with changed host keys. If this flag is set to **no **or **off**, ssh will automatically add new host keys to the user known hosts files and allow connections to hosts with changed hostkeys to proceed, subject to some restrictions. If this flag is set to **ask **(the default), new host keys will be added to the user known host files only after the user has confirmed that is what they really want to do, and ssh will refuse to con‐ nect to hosts whose host key has changed. The host keys of known hosts will be veri‐ fied automatically in all cases. ### SyslogFacility Gives the facility code that is used when logging messages from [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown). The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LO‐ CAL6, LOCAL7. The default is USER. ### TCPKeepAlive Specifies whether the system should send TCP keepalive messages to the other side. If they are sent, death of the connection or crash of one of the machines will be properly noticed. This option only uses TCP keepalives (as opposed to using ssh level keepalives), so takes a long time to notice when the connection dies. As such, you probably want the **ServerAliveInterval **option as well. However, this means that connections will die if the route is down temporarily, and some people find it annoy‐ ing. The default is **yes **(to send TCP keepalive messages), and the client will notice if the network goes down or the remote host dies. This is important in scripts, and many users want it too. To disable TCP keepalive messages, the value should be set to **no**. See also **ServerAliveInterval **for protocol-level keepalives. **Tag **Specify a configuration tag name that may be later used by a **Match **directive to se‐ lect a block of configuration. **Tunnel **Request [_tun_(4)](https://www.chedong.com/phpMan.php/man/tun/4/markdown) device forwarding between the client and the server. The argument must be **yes**, **point-to-point **(layer 3), **ethernet **(layer 2), or **no **(the default). Specifying **yes **requests the default tunnel mode, which is **point-to-point**. ### TunnelDevice Specifies the [_tun_(4)](https://www.chedong.com/phpMan.php/man/tun/4/markdown) devices to open on the client (_local_tun_) and the server (_remote_tun_). The argument must be _local_tun_[:_remote_tun_]. The devices may be specified by numeri‐ cal ID or the keyword **any**, which uses the next available tunnel device. If _remote_tun_ is not specified, it defaults to **any**. The default is **any:any**. ### UpdateHostKeys Specifies whether [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) should accept notifications of additional hostkeys from the server sent after authentication has completed and add them to **UserKnownHostsFile**. The argument must be **yes**, **no **or **ask**. This option allows learning alternate hostkeys for a server and supports graceful key rotation by allowing a server to send replace‐ ment public keys before old ones are removed. Additional hostkeys are only accepted if the key used to authenticate the host was already trusted or explicitly accepted by the user, the host was authenticated via **UserKnownHostsFile **(i.e. not **GlobalKnownHostsFile**) and the host was authenticated us‐ ing a plain key and not a certificate. **UpdateHostKeys **is enabled by default if the user has not overridden the default **UserKnownHostsFile **setting and has not enabled **VerifyHostKeyDNS**, otherwise **UpdateHostKeys **will be set to **no**. If **UpdateHostKeys **is set to **ask**, then the user is asked to confirm the modifications to the known_hosts file. Confirmation is currently incompatible with **ControlPersist**, and will be disabled if it is enabled. Presently, only [_sshd_(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) from OpenSSH 6.8 and greater support the "" protocol extension used to inform the client of all the server's hostkeys. **User **Specifies the user to log in as. This can be useful when a different user name is used on different machines. This saves the trouble of having to remember to give the user name on the command line. ### UserKnownHostsFile Specifies one or more files to use for the user host key database, separated by whitespace. Each filename may use tilde notation to refer to the user's home direc‐ tory, the tokens described in the “TOKENS” section and environment variables as de‐ scribed in the “ENVIRONMENT VARIABLES” section. A value of **none **causes [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) to ig‐ nore any user-specific known hosts files. The default is _~/.ssh/known_hosts_, _~/.ssh/known_hosts2_. ### VerifyHostKeyDNS Specifies whether to verify the remote key using DNS and SSHFP resource records. If this option is set to **yes**, the client will implicitly trust keys that match a secure fingerprint from DNS. Insecure fingerprints will be handled as if this option was set to **ask**. If this option is set to **ask**, information on fingerprint match will be displayed, but the user will still need to confirm new host keys according to the **StrictHostKeyChecking **option. The default is **no**. See also “VERIFYING HOST KEYS” in [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown). ### VisualHostKey If this flag is set to **yes**, an ASCII art representation of the remote host key fin‐ gerprint is printed in addition to the fingerprint string at login and for unknown host keys. If this flag is set to **no **(the default), no fingerprint strings are printed at login and only the fingerprint string will be printed for unknown host keys. ### XAuthLocation Specifies the full pathname of the [_xauth_(1)](https://www.chedong.com/phpMan.php/man/xauth/1/markdown) program. The default is _/usr/bin/xauth_. ## PATTERNS A _pattern_ consists of zero or more non-whitespace characters, ‘*’ (a wildcard that matches zero or more characters), or ‘?’ (a wildcard that matches exactly one character). For exam‐ ple, to specify a set of declarations for any host in the ".co.uk" set of domains, the fol‐ lowing pattern could be used: **Host *.co.uk** The following pattern would match any host in the 192.168.0.[0-9] network range: **Host 192.168.0.?** A _pattern-list_ is a comma-separated list of patterns. Patterns within pattern-lists may be negated by preceding them with an exclamation mark (‘!’). For example, to allow a key to be used from anywhere within an organization except from the "dialup" pool, the following entry (in authorized_keys) could be used: **from="!*.dialup.example.com,*.example.com"** Note that a negated match will never produce a positive result by itself. For example, at‐ tempting to match "host3" against the following pattern-list will fail: **from="!host1,!host2"** The solution here is to include a term that will yield a positive match, such as a wildcard: **from="!host1,!host2,*"** ## TOKENS Arguments to some keywords can make use of tokens, which are expanded at runtime: %% A literal ‘%’. %C Hash of %l%h%p%r%j. %d Local user's home directory. %f The fingerprint of the server's host key. %H The _known_hosts_ hostname or address that is being searched for. %h The remote hostname. %I A string describing the reason for a **KnownHostsCommand **execution: either **ADDRESS** when looking up a host by address (only when **CheckHostIP **is enabled), **HOSTNAME** when searching by hostname, or **ORDER **when preparing the host key algorithm pref‐ erence list to use for the destination host. %i The local user ID. %j The contents of the ProxyJump option, or the empty string if this option is un‐ set. %K The base64 encoded host key. %k The host key alias if specified, otherwise the original remote hostname given on the command line. %L The local hostname. %l The local hostname, including the domain name. %n The original remote hostname, as given on the command line. %p The remote port. %r The remote username. %T The local [_tun_(4)](https://www.chedong.com/phpMan.php/man/tun/4/markdown) or [_tap_(4)](https://www.chedong.com/phpMan.php/man/tap/4/markdown) network interface assigned if tunnel forwarding was requested, or "NONE" otherwise. %t The type of the server host key, e.g. **ssh-ed25519**. %u The local username. **CertificateFile**, **ControlPath**, **IdentityAgent**, **IdentityFile**, **KnownHostsCommand**, **LocalForward**, **Match exec**, **RemoteCommand**, **RemoteForward**, **RevokedHostKeys**, and **UserKnownHostsFile **accept the tokens %%, %C, %d, %h, %i, %j, %k, %L, %l, %n, %p, %r, and %u. **KnownHostsCommand **additionally accepts the tokens %f, %H, %I, %K and %t. **Hostname **accepts the tokens %% and %h. **LocalCommand **accepts all tokens. **ProxyCommand **and **ProxyJump **accept the tokens %%, %h, %n, %p, and %r. Note that some of these directives build commands for execution via the shell. Because [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) performs no filtering or escaping of characters that have special meaning in shell commands (e.g. quotes), it is the user's responsibility to ensure that the arguments passed to [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) do not contain such characters and that tokens are appropriately quoted when used. ## ENVIRONMENT VARIABLES Arguments to some keywords can be expanded at runtime from environment variables on the client by enclosing them in **${}**, for example **${HOME}/.ssh **would refer to the user's .ssh di‐ rectory. If a specified environment variable does not exist then an error will be returned and the setting for that keyword will be ignored. The keywords **CertificateFile**, **ControlPath**, **IdentityAgent**, **IdentityFile**, **KnownHostsCommand**, and **UserKnownHostsFile **support environment variables. The keywords **LocalForward **and **RemoteForward **support environment variables only for Unix domain socket paths. ## FILES _~/.ssh/config_ This is the per-user configuration file. The format of this file is described above. This file is used by the SSH client. Because of the potential for abuse, this file must have strict permissions: read/write for the user, and not writable by others. It may be group-writable provided that the group in question contains only the user. _/etc/ssh/ssh_config_ Systemwide configuration file. This file provides defaults for those values that are not specified in the user's configuration file, and for those users who do not have a configuration file. This file must be world-readable. ## SEE ALSO [_ssh_(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) ## AUTHORS OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0. Debian October 12, 2023 [_SSH_CONFIG_(5)](https://www.chedong.com/phpMan.php/man/SSHCONFIG/5/markdown)