ssh_config (5) Linux Manual Page

For each parameter, the first obtained value will be used. The configuration 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):

/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-insensitive 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 usually 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 Sx 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 conditions are specified using one or more criteria or the single token all which always matches. The available criteria keywords are: canonical final exec host originalhost 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 after 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 command returns a zero exit status then the condition is considered true. Commands containing whitespace characters must be quoted. Arguments to exec accept the tokens described in the Sx TOKENS section. The other keywords’ criteria must be single entries or comma-separated lists and may use the wildcard and negation operators described in the Sx PATTERNS section. The criteria for the host keyword are matched against the target hostname, after any substitution by the Hostname or CanonicalizeHostname options. The originalhost keyword matches against the hostname as it was specified on the command-line. 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) (this keyword may be useful in system-wide ssh_config files).

AddKeysToAgent

Specifies whether keys should be automatically added to a running ssh-agent1. 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-add1. If this option is set to ask ssh(1) will require confirmation using the SSH_ASKPASS program before adding a key (see ssh-add1 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-add1. If this option is set to no no keys are added to the agent. Alternately, this option may be specified as a time interval using the format described in the Sx TIME FORMATS section of sshd_config5 to specify the key’s lifetime in ssh-agent1, 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), 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 connection. 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 suffixes 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 resolver’s search rules. A value of no will cause ssh(1) 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) 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 pattern-list of domains that they may resolve to. For example, Qq *.a.example.com:*.b.example.com,*.c.example.com will allow hostnames matching Qq *.a.example.com to be canonicalized to names in the Qq *.b.example.com or Qq *.c.example.com domains. A single argument of Qq 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 authorities (CAs). The default is:

ssh-ed25519,ecdsa-sha2-nistp256,
ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519 [at] openssh.com,
sk-ecdsa-sha2-nistp256 [at] openssh.com,
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 begins with a `-‘ character, then the specified algorithms (including wildcards) will be removed from the default set instead of replacing them. ssh(1) 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), via ssh-agent1, or via a PKCS11Provider or SecurityKeyProvider Arguments to CertificateFile may use the tilde syntax to refer to a user’s home directory, the tokens described in the Sx TOKENS section and environment variables as described in the Sx 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.

CheckHostIP

If set to yes ssh(1) 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
aes128-gcm [at] openssh.com
aes256-gcm [at] openssh.com
chacha20-poly1305 [at] openssh.com

The default is:

chacha20-poly1305 [at] openssh.com,
aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm [at] openssh.com,aes256-gcm [at] openssh.com

The list of available ciphers may also be obtained using Qq ssh -Q cipher .

ClearAllForwardings

Specifies that all local, remote, and dynamic port forwardings specified in the configuration files or on the command line be cleared. This option is primarily useful when used from the ssh(1) command line to clear port forwardings set in configuration files, and is automatically set by scp(1) and sftp(1). 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) 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) to listen for control connections, but require confirmation using ssh-askpass1. If the ControlPath cannot be opened, ssh(1) will continue without connecting to a master instance. X11 and ssh-agent1 forwarding is supported over these multiplexed connections, however the display and agent forwarded will be the one belonging to the master connection i.e. it is not possible to forward multiple displays or agents. Two additional options allow for opportunistic multiplexing: try to use a master connection but fall back to creating a new one if one does not already exist. These options 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 directory, the tokens described in the Sx TOKENS section and environment variables as described in the Sx 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 connection will remain in the background indefinitely (until killed or closed via a mechanism such as the Qq ssh -O exit ) . If set to a time in seconds, or a time in any of the formats documented in sshd_config5, 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 enclosing 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 `*’ indicates that the port should be available from all interfaces. Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh(1) 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.

EnableSSHKeysign

Setting this option to yes in the global client configuration file /etc/ssh/ssh_config enables the use of the helper program ssh-keysign8 during HostbasedAuthentication The argument must be yes or no (the default). This option should be placed in the non-hostspecific section. See ssh-keysign8 for more information.

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 letter, or none to disable the escape character entirely (making the connection transparent for binary data).

ExitOnForwardFailure

Specifies whether ssh(1) should terminate the connection if it cannot set up all requested 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) 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 background. 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 configuration 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 itself in the background. The argument to this keyword must be yes (same as the –f option) or no (the default).

ForwardAgent

Specifies whether the connection to the authentication agent (if any) will be forwarded 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 material 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 Sx TIME FORMATS section of sshd_config5. X11 connections received by ssh(1) after this time will be refused. Setting ForwardX11Timeout to zero will disable the timeout and permit X11 forwarding for the life of the connection. The default is to disable 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 considered untrusted and prevented from stealing or tampering with data belonging to trusted X11 clients. Furthermore, the xauth(1) 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 imposed on untrusted clients.

GatewayPorts

Specifies whether remote hosts are allowed to connect to local forwarded ports. By default, ssh(1) 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 allowing 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 exchange 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 rekeying 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 credentials 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. Possible 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) should hash host names and addresses when they are added to ~/.ssh/known_hosts These hashed names may be used normally by ssh(1) and sshd(8), 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-keygen1. 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 default set instead of replacing them. If the specified list begins with a `-‘ character, 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-cert-v01 [at] openssh.com,
ecdsa-sha2-nistp256-cert-v01 [at] openssh.com,
ecdsa-sha2-nistp384-cert-v01 [at] openssh.com,
ecdsa-sha2-nistp521-cert-v01 [at] openssh.com,
sk-ssh-ed25519-cert-v01 [at] openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01 [at] openssh.com,
rsa-sha2-512-cert-v01 [at] openssh.com,
rsa-sha2-256-cert-v01 [at] openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519 [at] openssh.com,
sk-ecdsa-sha2-nistp256 [at] openssh.com,
rsa-sha2-512,rsa-sha2-256

The –Q option of ssh(1) 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 replacing them. If the specified list begins with a `-‘ character, 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-cert-v01 [at] openssh.com,
ecdsa-sha2-nistp256-cert-v01 [at] openssh.com,
ecdsa-sha2-nistp384-cert-v01 [at] openssh.com,
ecdsa-sha2-nistp521-cert-v01 [at] openssh.com,
sk-ssh-ed25519-cert-v01 [at] openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01 [at] openssh.com,
rsa-sha2-512-cert-v01 [at] openssh.com,
rsa-sha2-256-cert-v01 [at] openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ecdsa-sha2-nistp256 [at] openssh.com,
sk-ssh-ed25519 [at] openssh.com,
rsa-sha2-512,rsa-sha2-256

If hostkeys are known for the destination host then this default is modified to prefer their algorithms. The list of available signature algorithms may also be obtained using Qq 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 certificates. 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 Sx 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) should only use the configured authentication identity and certificate files (either the default files, or those explicitly configured in the ssh_config files or passed on the ssh(1) command-line), even if ssh-agent1 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 socket used to communicate with the authentication agent. This option overrides the SSH_AUTH_SOCK environment variable and can be used to select a specific agent. Setting the socket name to none disables the use of an authentication agent. If the string Qq 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 environment variable containing the location of the socket. Arguments to IdentityAgent may use the tilde syntax to refer to a user’s home directory, the tokens described in the Sx TOKENS section and environment variables as described in the Sx 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. 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) 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 directory or the tokens described in the Sx TOKENS section. 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 configuration directives). IdentityFile may be used in conjunction with IdentitiesOnly to select which identities 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). 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) 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 conditional 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 automatically 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 deprecated alias for this.

KbdInteractiveDevices

Specifies the list of methods to use in keyboard-interactive authentication. Multiple 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 specified algorithms will be appended to the default set instead of replacing them. If the specified list begins with a `-‘ character, then the specified 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 algorithms will be placed at the head of the default set. The default is:

curve25519-sha256,curve25519-sha256 [at] libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
sntrup761x25519-sha512 [at] openssh.com,
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 Qq 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 Sx VERIFYING HOST KEYS section in ssh(1)). Arguments to KnownHostsCommand accept the tokens described in the Sx 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 Sx TOKENS section. The command is run synchronously and does not have access to the session of the ssh(1) 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 Sx TOKENS section and environment variables as described in the Sx ENVIRONMENT VARIABLES section.

LogLevel

Gives the verbosity level that is used when logging messages from ssh(1). The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 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 Fn kex_exchange_identification function, and all code in the packet.c file. This option 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 specified algorithms will be appended to the default set instead of replacing them. If the specified list begins with a `-‘ character, then the specified 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 algorithms will be placed at the head of the default set. The algorithms that contain Qq -etm calculate the MAC after encryption (encrypt-then-mac). These are considered safer and their use recommended. The default is:

umac-64-etm [at] openssh.com,umac-128-etm [at] openssh.com,
hmac-sha2-256-etm [at] openssh.com,hmac-sha2-512-etm [at] openssh.com,
hmac-sha1-etm [at] openssh.com,
umac-64 [at] openssh.com,umac-128 [at] openssh.com,
hmac-sha2-256,hmac-sha2-512,hmac-sha1

The list of available MAC algorithms may also be obtained using Qq 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 keyword must be an integer. The default is 3.

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 escape sequence in ssh(1). 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.

ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p

ssh-ed25519-cert-v01 [at] openssh.com,
ecdsa-sha2-nistp256-cert-v01 [at] openssh.com,
ecdsa-sha2-nistp384-cert-v01 [at] openssh.com,
ecdsa-sha2-nistp521-cert-v01 [at] openssh.com,
sk-ssh-ed25519-cert-v01 [at] openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01 [at] openssh.com,
rsa-sha2-512-cert-v01 [at] openssh.com,
rsa-sha2-256-cert-v01 [at] openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519 [at] openssh.com,
sk-ecdsa-sha2-nistp256 [at] openssh.com,
rsa-sha2-512,rsa-sha2-256