Enum ssh_cfg::SshOptionKey

pub enum SshOptionKey {
    Host,
    AcceptEnv,
    AddKeysToAgent,
    AddressFamily,
    AllowAgentForwarding,
    AllowGroups,
    AllowStreamLocalForwarding,
    AllowTcpForwarding,
    AllowUsers,
    AuthenticationMethods,
    AuthorizedKeysCommand,
    AuthorizedKeysCommandUser,
    AuthorizedKeysFile,
    AuthorizedPrincipalsCommand,
    AuthorizedPrincipalsCommandUser,
    AuthorizedPrincipalsFile,
    Banner,
    BatchMode,
    BindAddress,
    BindInterface,
    CanonicalDomains,
    CanonicalizeFallbackLocal,
    CanonicalizeHostname,
    CanonicalizeMaxDots,
    CanonicalizePermittedCNAMEs,
    CASignatureAlgorithms,
    CertificateFile,
    ChallengeResponseAuthentication,
    CheckHostIP,
    ChrootDirectory,
    Cipher,
    Ciphers,
    ClearAllForwardings,
    ClientAliveCountMax,
    ClientAliveInterval,
    Compression,
    CompressionLevel,
    ConnectionAttempts,
    ConnectTimeout,
    ControlMaster,
    ControlPath,
    ControlPersist,
    DenyGroups,
    DenyUsers,
    DisableForwarding,
    DynamicForward,
    EnableSSHKeysign,
    EscapeChar,
    ExitOnForwardFailure,
    ExposeAuthInfo,
    FingerprintHash,
    ForceCommand,
    ForkAfterAuthentication,
    ForwardAgent,
    ForwardX11,
    ForwardX11Timeout,
    ForwardX11Trusted,
    GatewayPorts,
    GlobalKnownHostsFile,
    GSSAPIAuthentication,
    GSSAPICleanupCredentials,
    GSSAPIClientIdentity,
    GSSAPIDelegateCredentials,
    GSSAPIKeyExchange,
    GSSAPIRenewalForcesRekey,
    GSSAPIStrictAcceptorCheck,
    GSSAPITrustDns,
    HashKnownHosts,
    HostbasedAcceptedAlgorithms,
    HostbasedAcceptedKeyTypes,
    HostbasedAuthentication,
    HostbasedUsesNameFromPacketOnly,
    HostCertificate,
    HostKey,
    HostKeyAgent,
    HostKeyAlgorithms,
    HostKeyAlias,
    Hostname,
    HostName,
    IdentitiesOnly,
    IdentityAgent,
    IdentityFile,
    IgnoreRhosts,
    IgnoreUnknown,
    IgnoreUserKnownHosts,
    Include,
    IPQoS,
    KbdInteractiveAuthentication,
    KbdInteractiveDevices,
    KerberosAuthentication,
    KerberosGetAFSToken,
    KerberosOrLocalPasswd,
    KerberosTicketCleanup,
    KexAlgorithms,
    KnownHostsCommand,
    ListenAddress,
    LocalCommand,
    LocalForward,
    LoginGraceTime,
    LogLevel,
    LogVerbose,
    MACs,
    Match,
    MaxAuthTries,
    MaxSessions,
    MaxStartups,
    NoHostAuthenticationForLocalhost,
    NumberOfPasswordPrompts,
    PasswordAuthentication,
    PermitEmptyPasswords,
    PermitListen,
    PermitLocalCommand,
    PermitOpen,
    PermitRemoteOpen,
    PermitRootLogin,
    PermitTTY,
    PermitTunnel,
    PermitUserEnvironment,
    PermitUserRC,
    PidFile,
    PKCS11Provider,
    Port,
    PreferredAuthentications,
    PrintLastLog,
    PrintMotd,
    Protocol,
    ProxyCommand,
    ProxyJump,
    ProxyUseFdpass,
    PubkeyAcceptedAlgorithms,
    PubkeyAcceptedKeyTypes,
    PubkeyAuthentication,
    RDomain,
    RekeyLimit,
    RemoteCommand,
    RemoteForward,
    RequestTTY,
    RevokedHostKeys,
    RevokedKeys,
    RhostsRSAAuthentication,
    RSAAuthentication,
    SecurityKeyProvider,
    SendEnv,
    ServerAliveCountMax,
    ServerAliveInterval,
    SessionType,
    SetEnv,
    SmartcardDevice,
    StdinNull,
    StreamLocalBindMask,
    StreamLocalBindUnlink,
    StrictHostKeyChecking,
    StrictModes,
    Subsystem,
    SyslogFacility,
    TCPKeepAlive,
    TrustedUserCAKeys,
    Tunnel,
    TunnelDevice,
    UpdateHostKeys,
    UseBlacklist,
    UseDNS,
    UsePAM,
    UsePrivilegedPort,
    User,
    UserKnownHostsFile,
    VerifyHostKeyDNS,
    VersionAddendum,
    VisualHostKey,
    X11DisplayOffset,
    X11Forwarding,
    X11UseLocalhost,
    XAuthLocation,
}

SSH option keys inside the SSH configuration file.

See https://linux.die.net/man/5/ssh_config

Variants

Host

Restricts the following declarations (up to the next Host 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 the hostname argument given on the command line (i.e. the name is not converted to a canonicalized host name before matching).

See Patterns for more information on patterns.

AcceptEnv

Specifies what environment variables sent by the client will be copied into the session’s environ(7).

See SendEnv and SetEnv in ssh_config(5) for how to configure the client. The TERM environment variable is always accepted whenever the client requests a pseudo-terminal as it is required by the protocol. Variables are specified by name, which may contain the wildcard characters * and ?. Multiple environment variables may be separated by whitespace or spread across multiple AcceptEnv directives. Be warned that some environment variables could be used to bypass restricted user environments. For this reason, care should be taken in the use of this directive. The default is not to accept any environment variables.

AddKeysToAgent

Specifies whether keys should be automatically added to a running ssh-agent(1).

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). If this option is set to ask, ssh(1) will require confirmation using the SSH_ASKPASS program before adding a key (see ssh-add(1) 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). 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 TIME FORMATS section of sshd_config(5) to specify the key’s lifetime in ssh-agent(1), 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, inet (use IPv4 only), or inet6 (use IPv6 only).

AllowAgentForwarding

Specifies whether ssh-agent(1) forwarding is permitted.

The default is yes. Note that disabling agent forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders.

AllowGroups

This keyword can be followed by a list of group name patterns, separated by spaces.

If specified, login is allowed only for users whose primary group or supplementary group list matches one of the patterns. Only group names are valid; a numerical group ID is not recognized. By default, login is allowed for all groups. The allow/deny directives are processed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.

See PATTERNS in ssh_config(5) for more information on patterns.

AllowStreamLocalForwarding

Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.

The available options are yes (the default) or all to allow StreamLocal forwarding, no to prevent all StreamLocal forwarding, local to allow local (from the perspective of ssh(1)) forwarding only or remote to allow remote forwarding only. Note that disabling StreamLocal forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders.

AllowTcpForwarding

Specifies whether TCP forwarding is permitted.

The available options are yes (the default) or all to allow TCP forwarding, no to prevent all TCP forwarding, local to allow local (from the perspective of ssh(1)) forwarding only or remote to allow remote forwarding only. Note that disabling TCP forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders.

AllowUsers

This keyword can be followed by a list of user name patterns, separated by spaces.

If specified, login is allowed only for user names that match one of the patterns. Only user names are valid; a numerical user ID is not recognized. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts. HOST criteria may additionally contain addresses to match in CIDR address/masklen format. The allow/deny directives are processed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.

See PATTERNS in ssh_config(5) for more information on patterns.

AuthenticationMethods

Specifies the authentication methods that must be successfully completed for a user to be granted access.

This option must be followed by one or more lists of comma-separated authentication method names, or by the single string any to indicate the default behaviour of accepting any single authentication method. If the default is overridden, then successful authentication requires completion of every method in at least one of these lists.

For example, "publickey,password publickey,keyboard-interactive" would require the user to complete public key authentication, followed by either password or keyboard interactive authentication. Only methods that are next in one or more lists are offered at each stage, so for this example it would not be possible to attempt password or keyboard-interactive authentication before public key.

For keyboard interactive authentication it is also possible to restrict authentication to a specific device by appending a colon followed by the device identifier bsdauth or pam. depending on the server configuration. For example, “keyboard-interactive:bsdauth” would restrict keyboard interactive authentication to the bsdauth device.

If the publickey method is listed more than once, sshd(8) verifies that keys that have been used successfully are not reused for subsequent authentications. For example, “publickey,publickey” requires successful authentication using two different public keys.

Note that each authentication method listed should also be explicitly enabled in the configuration.

The available authentication methods are: “gssapi-with-mic”, “hostbased”, “keyboard-interactive”, “none” (used for access to password-less accounts when PermitEmptyPasswords is enabled), “password” and “publickey”.

AuthorizedKeysCommand

Specifies a program to be used to look up the user’s public keys.

The program must be owned by root, not writable by group or others and specified by an absolute path. Arguments to AuthorizedKeysCommand accept the tokens described in the TOKENS section. If no arguments are specified then the username of the target user is used.

The program should produce on standard output zero or more lines of authorized_keys output (see AUTHORIZED_KEYS in sshd(8)). If a key supplied by AuthorizedKeysCommand does not successfully authenticate and authorize the user then public key authentication continues using the usual AuthorizedKeysFile files. By default, no AuthorizedKeysCommand is run.

AuthorizedKeysCommandUser

Specifies the user under whose account the AuthorizedKeysCommand is run.

It is recommended to use a dedicated user that has no other role on the host than running authorized keys commands. If AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser is not, then sshd(8) will refuse to start.

AuthorizedKeysFile

Specifies the file that contains the public keys used for user authentication.

The format is described in the AUTHORIZED_KEYS FILE FORMAT section of sshd(8). Arguments to AuthorizedKeysFile accept the tokens described in the TOKENS section. After expansion, AuthorizedKeysFile is taken to be an absolute path or one relative to the user’s home directory. Multiple files may be listed, separated by whitespace. Alternately this option may be set to none to skip checking for user keys in files. The default is “.ssh/authorized_keys .ssh/authorized_keys2”.

AuthorizedPrincipalsCommand

Specifies a program to be used to generate the list of allowed certificate principals as per AuthorizedPrincipalsFile.

The program must be owned by root, not writable by group or others and specified by an absolute path. Arguments to AuthorizedPrincipalsCommand accept the tokens described in the TOKENS section. If no arguments are specified then the username of the target user is used.

The program should produce on standard output zero or more lines of AuthorizedPrincipalsFile output. If either AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is specified, then certificates offered by the client for authentication must contain a principal that is listed. By default, no AuthorizedPrincipalsCommand is run.

AuthorizedPrincipalsCommandUser

Specifies the user under whose account the AuthorizedPrincipalsCommand is run.

It is recommended to use a dedicated user that has no other role on the host than running authorized principals commands. If AuthorizedPrincipalsCommand is specified but AuthorizedPrincipalsCommandUser is not, then sshd(8) will refuse to start.

AuthorizedPrincipalsFile

Specifies a file that lists principal names that are accepted for certificate authentication.

When using certificates signed by a key listed in TrustedUserCAKeys, this file lists names, one of which must appear in the certificate for it to be accepted for authentication. Names are listed one per line preceded by key options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)). Empty lines and comments starting with # are ignored.

Arguments to AuthorizedPrincipalsFile accept the tokens described in the TOKENS section. After expansion, AuthorizedPrincipalsFile is taken to be an absolute path or one relative to the user’s home directory. The default is none, i.e. not to use a principals file - in this case, the username of the user must appear in a certificate’s principals list for it to be accepted.

Note that AuthorizedPrincipalsFile is only used when authentication proceeds using a CA listed in TrustedUserCAKeys and is not consulted for certification authorities trusted via ~/.ssh/authorized_keys, though the principals=key option offers a similar facility (see sshd(8) for details).

Banner

The contents of the specified file are sent to the remote user before authentication is allowed.

If the argument is none then no banner is displayed. By default, no banner is displayed.

BatchMode

If set to yes, passphrase/password querying will be disabled.

This option is useful in scripts and other batch jobs where no user is present to supply the password. The argument must be yes or no. The default is no.

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. Note that this option does not work if UsePrivilegedPort is set to yes.

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, "*.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.

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@openssh.com,
sk-ecdsa-sha2-nistp256@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-agent(1), 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 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.

ChallengeResponseAuthentication

Specifies whether to use challenge-response authentication.

The argument to this keyword must be yes or no. The default is yes.

CheckHostIP

If this flag is set to yes, ssh(1) will additionally check the host IP address in the known_hosts file.

This allows ssh to detect if a host key changed due to DNS spoofing. If the option is set to no, the check will not be executed. The default is yes.

ChrootDirectory

Specifies the pathname of a directory to chroot(2) to after authentication.

At session startup sshd(8) checks that all components of the pathname are root-owned directories which are not writable by any other user or group. After the chroot, sshd(8) changes the working directory to the user’s home directory. Arguments to ChrootDirectory accept the tokens described in the TOKENS section.

The ChrootDirectory must contain the necessary files and directories to support the user’s session. For an interactive session this requires at least a shell, typically sh(1), and basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), and tty(4) devices. For file transfer sessions using SFTP no additional configuration of the environment is necessary if the inprocess sftp-server is used, though sessions which use logging may require /dev/log inside the chroot directory on some operating systems (see sftp-server(8) for details).

For safety, it is very important that the directory hierarchy be prevented from modification by other processes on the system (especially those outside the jail). Misconfiguration can lead to unsafe environments which sshd(8) cannot detect.

The default is none, indicating not to chroot(2).

Cipher

Specifies the cipher to use for encrypting the session in protocol version 1.

Currently, blowfish, 3des, and des are supported. des is only supported in the ssh(1) client for interoperability with legacy protocol 1 implementations that do not support the 3des cipher. Its use is strongly discouraged due to cryptographic weaknesses. The default is 3des.

Ciphers

Specifies the ciphers allowed for protocol version 2 in order of preference.

Multiple ciphers must be comma-separated. The supported ciphers are 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr, aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, blowfish-cbc, and cast128-cbc. The default is:

aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour

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 is no.

ClientAliveCountMax

Sets the number of client alive messages which may be sent without sshd(8) receiving any messages back from the client.

If this threshold is reached while client alive messages are being sent, sshd will disconnect the client, terminating the session. It is important to note that the use of client alive messages is very different from TCPKeepAlive. The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by TCPKeepAlive is spoofable. The client alive mechanism is valuable when the client or server depend on knowing when a connection has become inactive.

The default value is 3. If ClientAliveInterval is set to 15, and ClientAliveCountMax is left at the default, unresponsive SSH clients will be disconnected after approximately 45 seconds.

ClientAliveInterval

Sets a timeout interval in seconds after which if no data has been received from the client, sshd(8) will send a message through the encrypted channel to request a response from the client.

The default is 0, indicating that these messages will not be sent to the client.

Compression

Specifies whether to use compression.

The argument must be yes or no. The default is no.

CompressionLevel

Specifies the compression level to use if compression is enabled.

The argument must be an integer from 1 (fast) to 9 (slow, best). The default level is 6, which is good for most applications. The meaning of the values is the same as in gzip(1). Note that this option applies to protocol version 1 only.

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 value is used only when the target is down or really unreachable, not when it refuses the connection.

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 to listen for control connections, but require confirmation using the SSH_ASKPASS program before they are accepted (see ssh-add(1) for details). If the ControlPath cannot be opened, ssh will continue without connecting to a master instance.

X11 and ssh-agent(1) 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.

In the path, %l will be substituted by the local host name, %h will be substituted by the target host name, %p the port, and %r by the remote login username. It is recommended that any ControlPath used for opportunistic connection sharing include at least %h, %p, and `%r. 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 “ssh -O exit”). If set to a time in seconds, or a time in any of the formats documented in sshd_config(5), then the backgrounded master connection will automatically terminate after it has remained idle (with no client connections) for the specified time.

DenyGroups

This keyword can be followed by a list of group name patterns, separated by spaces.

Login is disallowed for users whose primary group or supplementary group list matches one of the patterns. Only group names are valid; a numerical group ID is not recognized. By default, login is allowed for all groups. The allow/deny directives are processed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.

See PATTERNS in ssh_config(5) for more information on patterns.

DenyUsers

This keyword can be followed by a list of user name patterns, separated by spaces.

Login is disallowed for user names that match one of the patterns. Only user names are valid; a numerical user ID is not recognized. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts. HOST criteria may additionally contain addresses to match in CIDR address/masklen format. The allow/deny directives are processed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.

See PATTERNS in ssh_config(5) for more information on patterns.

DisableForwarding

Disables all forwarding features, including X11, ssh-agent(1), TCP and StreamLocal.

This option overrides all other forwarding related options and may simplify restricted configurations.

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 or by using an alternative syntax: [bind_address/]port. 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-keysign(8) during HostbasedAuthentication.

The argument must be yes or no. The default is no. This option should be placed in the non-hostspecific section. See ssh-keysign(8) 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.

The argument must be yes or no. The default is no.

ExposeAuthInfo

Writes a temporary file containing a list of authentication methods and public credentials (e.g. keys) used to authenticate the user.

The location of the file is exposed to the user session through the SSH_USER_AUTH environment variable. The default is no.

FingerprintHash

Specifies the hash algorithm used when displaying key fingerprints. Valid options are: md5 and sha256 (the default).

ForceCommand

Forces the execution of the command specified by ForceCommand, ignoring any command supplied by the client and ~/.ssh/rc if present.

The command is invoked by using the user’s login shell with the -c option. This applies to shell, command, or subsystem execution. It is most useful inside a Match block. The command originally supplied by the client is available in the SSH_ORIGINAL_COMMAND environment variable. Specifying a command of internal-sftp will force the use of an in-process SFTP server that requires no support files when used with ChrootDirectory. The default is none.

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 must be yes or no. The default is no.

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 is no.

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). 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, remote X11 clients will have full access to the original X11 display.

If this option is set to no, 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.

The default is no.

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 is no.

GlobalKnownHostsFile

Specifies a file to use for the global host key database instead of /etc/ssh/ssh_known_hosts.

GSSAPIAuthentication

Specifies whether user authentication based on GSSAPI is allowed.

The default is no. Note that this option applies to protocol version 2 only.

GSSAPICleanupCredentials

Specifies whether to automatically destroy the user’s credentials cache on logout.

The default is yes.

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. Note that this option applies to protocol version 2 connections using GSSAPI.

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. Note that this option applies to protocol version 2 only.

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 can delegate the renewed credentials to a session on the server. The default is no.

GSSAPIStrictAcceptorCheck

Determines whether to be strict about the identity of the GSSAPI acceptor a client authenticates against.

If set to yes then the client must authenticate against the host service on the current hostname. If set to no then the client may authenticate against any service key stored in the machine’s default store. This facility is provided to assist with operation on multi homed machines. The default is yes.

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. This option only applies to protocol version 2 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 reveal identifying information should the file’s contents be 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).

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

The -Q option of ssh(1) may be used to list supported signature algorithms. This was formerly named HostbasedKeyTypes.

HostbasedAcceptedKeyTypes

Specifies the key types that will be accepted for hostbased authentication as a list of comma-separated patterns.

Alternately if the specified value begins with a + character, then the specified key types will be appended to the default set instead of replacing them. If the specified value begins with a - character, then the specified key types (including wildcards) will be removed from the default set instead of replacing them.

The default for this option is:

ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,
ecdsa-sha2-nistp384,
ecdsa-sha2-nistp521,
ssh-ed25519,
rsa-sha2-512,
rsa-sha2-256,
ssh-rsa

The list of available key types may also be obtained using ssh -Q key.

HostbasedAuthentication

Specifies whether to try rhosts based authentication with public key authentication.

The argument must be yes or no. The default is no. This option applies to protocol version 2 only and is similar to RhostsRSAAuthentication.

HostbasedUsesNameFromPacketOnly

Specifies whether or not the server will attempt to perform a reverse name lookup when matching the name in the ~/.shosts, rhosts, and /etc/hosts.equiv files during HostbasedAuthentication.

A setting of yes means that sshd(8) uses the name supplied by the client rather than attempting to resolve the name from the TCP connection itself. The default is no.

HostCertificate

Specifies a file containing a public host certificate.

The certificate’s public key must match a private host key already specified by HostKey. The default behaviour of sshd(8) is not to load any certificates.

HostKey

Specifies a file containing a private host key used by SSH.

The defaults are /etc/ssh/ssh_host_ecdsa_key, /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key.

Note that sshd(8) will refuse to use a file if it is group/world-accessible and that the HostKeyAlgorithms option restricts which of the keys are actually used by sshd(8).

It is possible to have multiple host key files. It is also possible to specify public host key files instead. In this case operations on the private key will be delegated to an ssh-agent(1).

HostKeyAgent

Identifies the UNIX-domain socket used to communicate with an agent that has access to the private host keys.

If the string SSH_AUTH_SOCK“ is specified, the location of the socket will be read from the SSH_AUTH_SOCK environment variable.

HostKeyAlgorithms

Specifies the protocol version 2 host key algorithms that the client wants to use in order of preference.

The default for this option is: ssh-rsa,ssh-dss.

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.

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.

HostName

Specifies the real host name to log into.

This can be used to specify nicknames or abbreviations for hosts. The default is the name given on the command line. Numeric IP addresses are also permitted (both on the command line and in HostName specifications).

IdentitiesOnly

Specifies that ssh(1) should only use the authentication identity files configured in the ssh_config files, even if ssh-agent(1) offers more identities.

The argument to this keyword must be yes or no. This option is intended for situations where ssh-agent offers many different identities. The default is no.

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 select a specific agent. Setting the socket name to none disables the use of an authentication 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 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 TOKENS section and environment variables as described in the ENVIRONMENT VARIABLES section.

IdentityFile

Specifies a file from which the user’s RSA or DSA authentication identity is read.

The default is ~/.ssh/identity for protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for protocol version 2. Additionally, any identities represented by the authentication agent will be used for authentication.

The file name may use the tilde syntax to refer to a user’s home directory or one of the following escape characters: %d (local user’s home directory), %u (local user name), %l (local host name), %h (remote host name) or %r (remote user name).

It is possible to have multiple identity files specified in configuration files; all these identities will be tried in sequence.

IgnoreRhosts

Specifies that .rhosts and .shosts files will not be used in HostbasedAuthentication.

/etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The default is yes.

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.

IgnoreUserKnownHosts

Specifies whether sshd(8) should ignore the user’s ~/.ssh/known_hosts during HostbasedAuthentication and use only the system-wide known hosts file /etc/ssh/known_hosts.

The default is no.

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 af21 (Low-Latency Data) for interactive sessions and cs1 (Lower Effort) for non-interactive sessions.

KbdInteractiveAuthentication

Specifies whether to use keyboard-interactive authentication.

The argument to this keyword must be yes or no. The default is yes.

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, pam, and skey.

KerberosAuthentication

Specifies whether the password provided by the user for PasswordAuthentication will be validated through the Kerberos KDC.

To use this option, the server needs a Kerberos servtab which allows the verification of the KDC’s identity. The default is no.

KerberosGetAFSToken

If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire an AFS token before accessing the user’s home directory.

The default is no.

KerberosOrLocalPasswd

If password authentication through Kerberos fails then the password will be validated via any additional local mechanism such as /etc/passwd.

The default is yes.

KerberosTicketCleanup

Specifies whether to automatically destroy the user’s ticket cache file on logout. The default is yes.

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 methods will be appended to the default set instead of replacing them. If the specified list begins with a - character, then the specified methods (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ^ character, then the specified methods will be placed at the head of the default set. The default is:

curve25519-sha256,
curve25519-sha256@libssh.org,
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)). 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.

ListenAddress

Specifies the local addresses sshd(8) should listen on.

The following forms may be used:

ListenAddress hostname|address [rdomain domain]
ListenAddress hostname:port [rdomain domain]
ListenAddress IPv4_address:port [rdomain domain]
ListenAddress [hostname|address]:port [rdomain domain]

The optional rdomain qualifier requests sshd(8) listen in an explicit routing domain. If port is not specified, sshd will listen on the address and all Port options specified. The default is to listen on all local addresses on the current default routing domain. Multiple ListenAddress options are permitted. For more information on routing domains, see rdomain(4).

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. The following escape character substitutions will be performed:

• %d (local user’s home directory)
• %h (remote host name)
• %l (local host name)
• %n (host name as provided on the command line)
• %p (remote port)
• %r (remote user name)
• %u (local user name)

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 must be [bind_address:]port and the second argument must be host:hostport. IPv6 addresses can be specified by enclosing addresses in square brackets or by using an alternative syntax: [bind_address/]port and host/hostport. 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.

LoginGraceTime

The server disconnects after this time if the user has not successfully logged in.

If the value is 0, there is no time limit. The default is 120 seconds.

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 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 in protocol version 2 for data integrity protection. Multiple algorithms must be comma-separated. The default is:

hmac-md5,hmac-sha1,umac-64@openssh.com,
hmac-ripemd160,hmac-sha1-96,hmac-md5-96

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 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 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).

MaxAuthTries

Specifies the maximum number of authentication attempts permitted per connection.

Once the number of failures reaches half this value, additional failures are logged. The default is 6.

MaxSessions

Specifies the maximum number of open shell, login or subsystem (e.g. sftp) sessions permitted per network connection.

Multiple sessions may be established by clients that support connection multiplexing. Setting MaxSessions to 1 will effectively disable session multiplexing, whereas setting it to 0 will prevent all shell, login and subsystem sessions while still permitting forwarding. The default is 10.

MaxStartups

Specifies the maximum number of concurrent unauthenticated connections to the SSH daemon.

Additional connections will be dropped until authentication succeeds or the LoginGraceTime expires for a connection. The default is 10:30:100.

Alternatively, random early drop can be enabled by specifying the three colon separated values start:rate:full (e.g. “10:30:60”). sshd(8) will refuse connection attempts with a probability of rate/100 (30%) if there are currently start (10) unauthenticated connections. The probability increases linearly and all connection attempts are refused if the number of unauthenticated connections reaches full (60).

NoHostAuthenticationForLocalhost

This option can be used if the home directory is shared across machines.

In this case localhost will refer to a different machine on each of the machines and the user will get many warnings about changed host keys. However, this option disables host authentication for localhost. The argument to this keyword must be yes or no. The default is to check the host key for localhost.

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 or no. The default is yes.

PermitEmptyPasswords

When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings.

The default is no.

PermitListen

Specifies the addresses/ports on which a remote TCP port forwarding may listen.

The listen specification must be one of the following forms:

PermitListen port
PermitListen host:port

Multiple permissions may be specified by separating them with whitespace. An argument of any can be used to remove all restrictions and permit any listen requests. An argument of none can be used to prohibit all listen requests. The host name may contain wildcards as described in the PATTERNS section in ssh_config(5). The wildcard * can also be used in place of a port number to allow all ports. By default all port forwarding listen requests are permitted. Note that the GatewayPorts option may further restrict which addresses may be listened on. Note also that ssh(1) will request a listen host of “localhost” if no listen host was specifically requested, and this this name is treated differently to explicit localhost addresses of 127.0.0.1“ and “::1”.

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 is no.

PermitOpen

Specifies the destinations to which TCP port forwarding is permitted.

The forwarding specification must be one of the following forms:

PermitOpen host:port
PermitOpen IPv4_addr:port
PermitOpen [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. By default all port forwarding requests are permitted.

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.

PermitRootLogin

Specifies whether root can log in using ssh(1).

The argument must be yes, prohibit-password, forced-commands-only, or no. The default is no. Note that if ChallengeResponseAuthentication and UsePAM are both yes, this setting may be overridden by the PAM policy.

If this option is set to prohibit-password (or its deprecated alias, without-password), password and keyboard-interactive authentication are disabled for root.

If this option is set to forced-commands-only, root login with public key authentication will be allowed, but only if the command option has been specified (which may be useful for taking remote backups even if root login is normally not allowed). All other authentication methods are disabled for root.

If this option is set to no, root is not allowed to log in.

PermitTTY

Specifies whether pty(4) allocation is permitted.

The default is yes.

PermitTunnel

Specifies whether tun(4) device forwarding is allowed.

The argument must be yes, point-to-point (layer 3), ethernet (layer 2), or no. Specifying yes permits both point-to-point and ethernet. The default is no.

Independent of this setting, the permissions of the selected tun(4) device must allow access to the user.

PermitUserEnvironment

Specifies whether ~/.ssh/environment and environment= options in ssh/authorized_keys are processed by sshd(8).

Valid options are yes, no or a pattern-list specifying which environment variable names to accept (for example “LANG,LC_*”). The default is no. Enabling environment processing may enable users to bypass access restrictions in some configurations using mechanisms such as LD_PRELOAD.

PermitUserRC

Specifies whether any ~/.ssh/rc file is executed.

The default is yes.

PidFile

Specifies the file that contains the process ID of the SSH daemon, or none to not write one. The default is /var/run/sshd.pid.

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) 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 protocol 2 authentication methods.

This allows a client to prefer one method (e.g. keyboard-interactive) over another method (e.g. password). The default for this option is: gssapi-with-mic, hostbased, publickey, keyboard-interactive, password.

PrintLastLog

Specifies whether sshd(8) should print the date and time of the last user login when a user logs in interactively.

The default is yes.

PrintMotd

Specifies whether sshd(8) should print /etc/motd when a user logs in interactively.

(On some systems it is also printed by the shell, /etc/profile, or equivalent.) The default is yes.

Protocol

Specifies the protocol versions ssh(1) should support in order of preference.

The possible values are ‘1’ and ‘2’. Multiple versions must be comma-separated. The default is 2,1. This means that ssh tries version 2 and falls back to version 1 if version 2 is not available.

ProxyCommand

Specifies the command to use to connect to the server.

The command string extends to the end of the line, and is executed with the user’s shell. In the command string, %h will be substituted by the host name to connect and %p by the port. 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) 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) and its proxy support. For example, 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.

Multiple proxies may be separated by comma characters and will be visited sequentially. Setting this option will cause ssh(1) to connect to the target host by first making a ssh(1) connection to the specified ProxyJump host and then establishing a TCP forwarding 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 specified 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) instead 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 + character, then the algorithms after it will be appended to the default instead of replacing it. 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 for this option is:

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

The list of available signature algorithms may also be obtained using ssh -Q PubkeyAcceptedAlgorithms.

PubkeyAcceptedKeyTypes

Specifies the key types that will be accepted for public key authentication as a list of comma-separated patterns.

Alternately if the specified value begins with a + character, then the specified key types will be appended to the default set instead of replacing them. If the specified value begins with a - character, then the specified key types (including wildcards) will be removed from the default set instead of replacing them.

The default for this option is:

ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,
ecdsa-sha2-nistp384,
ecdsa-sha2-nistp521,
ssh-ed25519,
rsa-sha2-512,
rsa-sha2-256,
ssh-rsa

The list of available key types may also be obtained using ssh -Q key.

PubkeyAuthentication

Specifies whether to try public key authentication.

The argument to this keyword must be yes or no. The default is yes. This option applies to protocol version 2 only.

RDomain

Specifies an explicit routing domain that is applied after authentication has completed.

The user session, as well and any forwarded or listening IP sockets, will be bound to this rdomain(4). If the routing domain is set to %D, then the domain in which the incoming connection was received will be applied.

RekeyLimit

Specifies the maximum amount of data that may be transmitted before the session key is renegotiated.

The argument is the number of bytes, with an optional suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes, Megabytes, or Gigabytes, respectively. The default is between ‘1G’ and ‘4G’, depending on the cipher. This option applies to protocol version 2 only.

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 to the specified host and port from the local machine.

The first argument must be [bind_address:]port and the second argument must be host:hostport. IPv6 addresses can be specified by enclosing addresses in square brackets or by using an alternative syntax: [bind_address/]port and host/hostport. 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.

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 addresses. If the bind_address is * or an empty string, then the forwarding is requested to listen on all interfaces. Specifying a remote bind_address will only succeed if the server’s GatewayPorts option is enabled (see sshd_config(5)).

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 session). This option mirrors the -t and -T flags for ssh(1).

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). For more information on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).

RevokedKeys

Specifies revoked public keys file, or none to not use one.

Keys listed in this file will be refused for public key authentication. Note that if this file is not readable, then public key authentication will be refused for all users. 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). For more information on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).

RhostsRSAAuthentication

Specifies whether to try rhosts based authentication with RSA host authentication.

The argument must be yes or no. The default is no. This option applies to protocol version 1 only and requires ssh(1) to be setuid root.

RSAAuthentication

Specifies whether to try RSA authentication.

The argument to this keyword must be yes or no. RSA authentication will only be attempted if the identity file exists, or an authentication agent is running. The default is yes. Note that this option applies to protocol version 1 only.

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 environment variable containing the path to the library.

SendEnv

Specifies what variables from the local environ(7) should be sent to the server.

Note that environment passing is only supported for protocol 2. The server must also support it, and the server must be configured to accept these environment variables. Refer to AcceptEnv in sshd_config(5) for how to configure the server. Variables are specified by name, which may contain wildcard characters. Multiple environment variables may be separated by whitespace or spread across multiple SendEnv directives. The default is not to send any environment variables.

See Patterns for more information on patterns.

ServerAliveCountMax

Sets the number of server alive messages (see below) which may be sent without ssh(1) 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, terminating 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 inactive.

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 unresponsive, ssh will disconnect after approximately 45 seconds. This option applies to protocol version 2 only.

ServerAliveInterval

Sets a timeout interval in seconds after which if no data has been received from the server, ssh(1) will send a message through the encrypted channel to request a response from the server.

The default is 0, indicating that these messages will not be sent to the server. This option applies to protocol version 2 only.

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 forwarding 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.

SmartcardDevice

Specifies which smartcard device to use.

The argument to this keyword is the device ssh(1) should use to communicate with a smartcard used for storing the user’s private RSA key. By default, no device is specified and smartcard support is not activated.

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 argument 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-domain socket file.

The argument must be yes or no (the default).

StrictHostKeyChecking

If this flag is set to yes, ssh(1) 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 trojan horse 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 manually add all new hosts. If this flag is set to no, ssh will automatically add new host keys to the user known hosts files. If this flag is set to ask, 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 connect to hosts whose host key has changed. The host keys of known hosts will be verified automatically in all cases. The argument must be yes, no, or ask. The default is ask.

StrictModes

Specifies whether sshd(8) should check file modes and ownership of the user’s files and home directory before accepting login.

This is normally desirable because novices sometimes accidentally leave their directory or files world-writable. The default is yes. Note that this does not apply to ChrootDirectory, whose permissions and ownership are checked unconditionally.

Subsystem

Configures an external subsystem (e.g. file transfer daemon).

Arguments should be a subsystem name and a command (with optional arguments) to execute upon subsystem request.

The command sftp-server implements the SFTP file transfer subsystem.

Alternately the name internal-sftp implements an in-process SFTP server. This may simplify configurations using ChrootDirectory to force a different filesystem root on clients.

By default no subsystems are defined.

SyslogFacility

Gives the facility code that is used when logging messages from ssh(1).

The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, 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. However, this means that connections will die if the route is down temporarily, and some people find it annoying.

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.

TrustedUserCAKeys

Specifies a file containing public keys of certificate authorities that are trusted to sign user certificates for authentication, or none to not use one.

Keys are listed one per line; empty lines and comments starting with # are allowed. If a certificate is presented for authentication and has its signing CA key listed in this file, then it may be used for authentication for any user listed in the certificate’s principals list. Note that certificates that lack a list of principals will not be permitted for authentication using TrustedUserCAKeys. For more details on certificates, see the CERTIFICATES section in ssh-keygen(1).

Tunnel

Request tun(4) device forwarding between the client and the server.

The argument must be yes, point-to-point (layer 3), ethernet (layer 2), or no. Specifying yes requests the default tunnel mode, which is point-to-point. The default is no.

TunnelDevice

Specifies the tun(4) 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 numerical 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) 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 replacement 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 using 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) from OpenSSH 6.8 and greater support the “hostkeys@openssh.com” protocol extension used to inform the client of all the server’s hostkeys.

UseBlacklist

Specifies whether sshd(8) attempts to send authentication success and failure messages to the blacklistd(8) daemon.

The default is no. For forward compatibility with an upcoming blacklisted rename, the UseBlocklist alias can be used instead.

UseDNS

Specifies whether sshd(8) should look up the remote host name, and to check that the resolved host name for the remote IP address maps back to the very same IP address.

If this option is set to no, then only addresses and not host names may be used in ~/.ssh/authorized_keys from and sshd_config Match Host directives. The default is “yes”.

UsePAM

Enables the Pluggable Authentication Module interface.

If set to yes this will enable PAM authentication using ChallengeResponseAuthentication and PasswordAuthentication in addition to PAM account and session module processing for all authentication types.

Because PAM challenge-response authentication usually serves an equivalent role to password authentication, you should disable either PasswordAuthentication or ChallengeResponseAuthentication.

If UsePAM is enabled, you will not be able to run sshd(8) as a non-root user. The default is yes.

UsePrivilegedPort

Specifies whether to use a privileged port for outgoing connections.

The argument must be yes or no. The default is no. If set to yes, ssh(1) must be setuid root. Note that this option must be set to yes for RhostsRSAAuthentication with older servers.

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 a file to use for the user host key database instead of ~/.ssh/known_hosts.

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 argument must be yes, no, or ask. The default is no. Note that this option applies to protocol version 2 only.

See also VERIFYING HOST KEYS in ssh(1).

VersionAddendum

Optionally specifies additional text to append to the SSH protocol banner sent by the server upon connection.

The default is FreeBSD-20200214. The value none may be used to disable this.

VisualHostKey

If this flag is set to yes, an ASCII art representation of the remote host key fingerprint is printed in addition to the hex fingerprint string at login and for unknown host keys.

If this flag is set to no, no fingerprint strings are printed at login and only the hex fingerprint string will be printed for unknown host keys. The default is no.

X11DisplayOffset

Specifies the first display number available for sshd(8)’s X11 forwarding.

This prevents sshd from interfering with real X11 servers. The default is 10.

X11Forwarding

Specifies whether X11 forwarding is permitted.

The argument must be yes or no. The default is yes.

When X11 forwarding is enabled, there may be additional exposure to the server and to client displays if the sshd(8) proxy display is configured to listen on the wildcard address (see X11UseLocalhost), though this is not the default. Additionally, the authentication spoofing and authentication data verification and substitution occur on the client side. The security risk of using X11 forwarding is that the client’s X11 display server may be exposed to attack when the SSH client requests forwarding (see the warnings for ForwardX11 in ssh_config(5)). A system administrator may have a stance in which they want to protect clients that may expose themselves to attack by unwittingly requesting X11 forwarding, which can warrant a no setting.

Note that disabling X11 forwarding does not prevent users from forwarding X11 traffic, as users can always install their own forwarders.

X11UseLocalhost

Specifies whether sshd(8) should bind the X11 forwarding server to the loopback address or to the wildcard address.

By default, sshd binds the forwarding server to the loopback address and sets the hostname part of the DISPLAY environment variable to localhost. This prevents remote hosts from connecting to the proxy display. However, some older X11 clients may not function with this configuration. X11UseLocalhost may be set to no to specify that the forwarding server should be bound to the wildcard address. The argument must be yes or no. The default is yes.

XAuthLocation

Specifies the full pathname of the xauth(1) program.

The default is /usr/bin/xauth.

Trait Implementations

impl Clone for SshOptionKey

fn clone(&self) -> SshOptionKey

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for SshOptionKey

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Display for SshOptionKey

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl FromStr for SshOptionKey

type Err = ConfigError

The associated error which can be returned from parsing.

fn from_str(s: &str) -> Result<Self, Self::Err>

Parses a string s to return a value of this type.

impl Hash for SshOptionKey

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given Hasher.

impl PartialEq<SshOptionKey> for SshOptionKey

fn eq(&self, other: &SshOptionKey) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

This method tests for !=.

impl Copy for SshOptionKey

impl Eq for SshOptionKey

impl StructuralEq for SshOptionKey

impl StructuralPartialEq for SshOptionKey