sshd_config

SSHD_CONFIG(5)               BSD File Formats Manual              SSHD_CONFIG(5)

NAME
     sshd_config — OpenSSH daemon configuration file

DESCRIPTION
     sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
     specified with -f on the command line).  The file contains keyword-argument
     pairs, one per line.  For each keyword, the first obtained value will be
     used.  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.

     The possible keywords and their meanings are as follows (note that keywords
     are case-insensitive and arguments are case-sensitive):

     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.

     AddressFamily
             Specifies which address family should be used by sshd(8).  Valid
             arguments are any (the default), 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 groups directives are processed in the following order:
             DenyGroups, 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 users directives are processed in the following order:
             DenyUsers, AllowUsers.

             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)).
             AuthorizedKeysCommand is tried after the usual AuthorizedKeysFile
             files and will not be executed if a matching key is found there.
             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.

     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

             Certificates signed using other algorithms will not be accepted for
             public key or host-based authentication.

     ChallengeResponseAuthentication
             Specifies whether challenge-response authentication is allowed
             (e.g. via PAM or through authentication styles supported in
             login.conf(5)) 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 in-process 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).

     Ciphers
             Specifies the ciphers allowed.  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@openssh.com
                   aes256-gcm@openssh.com
                   chacha20-poly1305@openssh.com

             The default is:

                   chacha20-poly1305@openssh.com,
                   aes128-ctr,aes192-ctr,aes256-ctr,
                   aes128-gcm@openssh.com,aes256-gcm@openssh.com

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

     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 unresponsive.

             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.
             Setting a zero ClientAliveCountMax disables connection termination.

     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 compression is enabled after the user has
             authenticated successfully.  The argument must be yes, delayed (a
             legacy synonym for yes) or no.  The default is yes.

     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 groups
             directives are processed in the following order: DenyGroups,
             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 users
             directives are processed in the following order: DenyUsers,
             AllowUsers.

             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.

     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 logging key fingerprints.
             Valid options are: md5 and sha256.  The default is sha256.

     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.

     GatewayPorts
             Specifies whether remote hosts are allowed to connect to ports
             forwarded for the client.  By default, sshd(8) binds remote port
             forwardings to the loopback address.  This prevents other remote
             hosts from connecting to forwarded ports.  GatewayPorts can be used
             to specify that sshd should allow remote port forwardings to bind
             to non-loopback addresses, thus allowing other hosts to connect.
             The argument may be no to force remote port forwardings to be
             available to the local host only, yes to force remote port
             forwardings to bind to the wildcard address, or clientspecified to
             allow the client to select the address to which the forwarding is
             bound.  The default is no.

     GSSAPIAuthentication
             Specifies whether user authentication based on GSSAPI is allowed.
             The default is no.

     GSSAPICleanupCredentials
             Specifies whether to automatically destroy the user's credentials
             cache on logout.  The default is yes.

     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.

     HostbasedAcceptedAlgorithms
             Specifies the signature algorithms that will be accepted for
             hostbased authentication as a list of comma-separated 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 list of available signature algorithms may also be obtained
             using "ssh -Q HostbasedAcceptedAlgorithms".  This was formerly
             named HostbasedAcceptedKeyTypes.

     HostbasedAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication
             together with successful public key client host authentication is
             allowed (host-based authentication).  The default is no.

     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 host key signature algorithms that the server offers.
             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 HostKeyAlgorithms".

     IgnoreRhosts
             Specifies whether to ignore per-user .rhosts and .shosts files
             during HostbasedAuthentication.  The system-wide /etc/hosts.equiv
             and /etc/ssh/shosts.equiv are still used regardless of this
             setting.

             Accepted values are yes (the default) to ignore all per-user files,
             shosts-only to allow the use of .shosts but to ignore .rhosts or no
             to allow both .shosts and rhosts.

     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
             that will be expanded and processed in lexical order.  Files
             without absolute paths are assumed to be in /etc/ssh.  An Include
             directive may appear inside a Match block to perform conditional
             inclusion.

     IPQoS   Specifies the IPv4 type-of-service or DSCP class for the
             connection.  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 allow keyboard-interactive authentication.
             The argument to this keyword must be yes or no.  The default is to
             use whatever value ChallengeResponseAuthentication is set to (by
             default yes).

     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.  Alternately 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 supported algorithms are:

                   curve25519-sha256
                   curve25519-sha256@libssh.org
                   diffie-hellman-group1-sha1
                   diffie-hellman-group14-sha1
                   diffie-hellman-group14-sha256
                   diffie-hellman-group16-sha512
                   diffie-hellman-group18-sha512
                   diffie-hellman-group-exchange-sha1
                   diffie-hellman-group-exchange-sha256
                   ecdh-sha2-nistp256
                   ecdh-sha2-nistp384
                   ecdh-sha2-nistp521
                   sntrup761x25519-sha512@openssh.com

             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 KexAlgorithms".

     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).

     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
             sshd(8).  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 debugging output.  Logging with a DEBUG level
             violates the privacy of users and is not recommended.

     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 available MAC (message authentication code)
             algorithms.  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 "-etm" calculate the MAC after
             encryption (encrypt-then-mac).  These are considered safer and
             their use recommended.  The supported MACs are:

                   hmac-md5
                   hmac-md5-96
                   hmac-sha1
                   hmac-sha1-96
                   hmac-sha2-256
                   hmac-sha2-512
                   umac-64@openssh.com
                   umac-128@openssh.com
                   hmac-md5-etm@openssh.com
                   hmac-md5-96-etm@openssh.com
                   hmac-sha1-etm@openssh.com
                   hmac-sha1-96-etm@openssh.com
                   hmac-sha2-256-etm@openssh.com
                   hmac-sha2-512-etm@openssh.com
                   umac-64-etm@openssh.com
                   umac-128-etm@openssh.com

             The default is:

                   umac-64-etm@openssh.com,umac-128-etm@openssh.com,
                   hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
                   hmac-sha1-etm@openssh.com,
                   umac-64@openssh.com,umac-128@openssh.com,
                   hmac-sha2-256,hmac-sha2-512,hmac-sha1

             The list of available MAC algorithms may also be obtained using
             "ssh -Q mac".

     Match   Introduces a conditional block.  If all of the criteria on the
             Match line are satisfied, the keywords on the following lines
             override those set in the global section of the config file, until
             either another Match line or the end of the file.  If a keyword
             appears in multiple Match blocks that are satisfied, only the first
             instance of the keyword is applied.

             The arguments to Match are one or more criteria-pattern pairs or
             the single token All which matches all criteria.  The available
             criteria are User, Group, Host, LocalAddress, LocalPort, RDomain,
             and Address (with RDomain representing the rdomain(4) on which the
             connection was received).

             The match patterns may consist of single entries or comma-separated
             lists and may use the wildcard and negation operators described in
             the PATTERNS section of ssh_config(5).

             The patterns in an Address criteria may additionally contain
             addresses to match in CIDR address/masklen format, such as
             192.0.2.0/24 or 2001:db8::/32.  Note that the mask length provided
             must be consistent with the address - it is an error to specify a
             mask length that is too long for the address or one with bits set
             in this host portion of the address.  For example, 192.0.2.0/33 and
             192.0.2.0/8, respectively.

             Only a subset of keywords may be used on the lines following a
             Match keyword.  Available keywords are AcceptEnv,
             AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
             AllowTcpForwarding, AllowUsers, AuthenticationMethods,
             AuthorizedKeysCommand, AuthorizedKeysCommandUser,
             AuthorizedKeysFile, AuthorizedPrincipalsCommand,
             AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile, Banner,
             ChrootDirectory, ClientAliveCountMax, ClientAliveInterval,
             DenyGroups, DenyUsers, DisableForwarding, ForceCommand,
             GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedAlgorithms,
             HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
             IgnoreRhosts, Include, IPQoS, KbdInteractiveAuthentication,
             KerberosAuthentication, LogLevel, MaxAuthTries, MaxSessions,
             PasswordAuthentication, PermitEmptyPasswords, PermitListen,
             PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel, PermitUserRC,
             PubkeyAcceptedAlgorithms, PubkeyAuthentication, RekeyLimit,
             RevokedKeys, RDomain, SetEnv, StreamLocalBindMask,
             StreamLocalBindUnlink, TrustedUserCAKeys, X11DisplayOffset,
             X11Forwarding and X11UseLocalhost.

     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).

     ModuliFile
             Specifies the moduli(5) file that contains the Diffie-Hellman
             groups used for the “diffie-hellman-group-exchange-sha1” and
             “diffie-hellman-group-exchange-sha256” key exchange methods.  The
             default is /etc/ssh/moduli.

     PasswordAuthentication
             Specifies whether password authentication is allowed.  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 name is treated
             differently to explicit localhost addresses of “127.0.0.1” and
             “::1”.

     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.  Otherwise, no pattern matching or address lookups
             are performed on supplied names.  By default all port forwarding
             requests are permitted.

     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 prohibit-password.

             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.

     PerSourceMaxStartups
             Specifies the number of unauthenticated connections allowed from a
             given source address, or “none” if there is no limit.  This limit
             is applied in addition to MaxStartups, whichever is lower.  The
             default is none.

     PerSourceNetBlockSize
             Specifies the number of bits of source address that are grouped
             together for the purposes of applying PerSourceMaxStartups limits.
             Values for IPv4 and optionally IPv6 may be specified, separated by
             a colon.  The default is 32:128, which means each address is
             considered individually.

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

     Port    Specifies the port number that sshd(8) listens on.  The default is
             22.  Multiple options of this type are permitted.  See also
             ListenAddress.

     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.

     PubkeyAcceptedAlgorithms
             Specifies the signature algorithms that will be accepted for public
             key authentication as a list of comma-separated patterns.
             Alternately 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 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".

     PubkeyAuthOptions
             Sets one or more public key authentication options.  The supported
             keywords are: none (the default; indicating no additional options
             are enabled), touch-required and verify-required.

             The touch-required option causes public key authentication using a
             FIDO authenticator algorithm (i.e. ecdsa-sk or ed25519-sk) to
             always require the signature to attest that a physically present
             user explicitly confirmed the authentication (usually by touching
             the authenticator).  By default, sshd(8) requires user presence
             unless overridden with an authorized_keys option.  The
             touch-required flag disables this override.

             The verify-required option requires a FIDO key signature attest
             that the user was verified, e.g. via a PIN.

             Neither the touch-required or verify-required options have any
             effect for other, non-FIDO, public key types.

     PubkeyAuthentication
             Specifies whether public key authentication is allowed.  The
             default is yes.

     RekeyLimit
             Specifies the maximum amount of data that may be transmitted before
             the session key is renegotiated, optionally followed by a maximum
             amount of time that may pass before the session key is
             renegotiated.  The first argument is specified in bytes and may
             have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes, Megabytes,
             or Gigabytes, respectively.  The default is between ‘1G’ and ‘4G’,
             depending on the cipher.  The optional second value is specified in
             seconds and may use any of the units documented in the TIME FORMATS
             section.  The default value for RekeyLimit is default none, which
             means that rekeying is performed after the cipher's default amount
             of data has been sent or received and no time based rekeying is
             done.

     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).

     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.

     SecurityKeyProvider
             Specifies a path to a library that will be used when loading FIDO
             authenticator-hosted keys, overriding the default of using the
             built-in USB HID support.

     SetEnv  Specifies one or more environment variables to set in child
             sessions started by sshd(8) as “NAME=VALUE”.  The environment value
             may be quoted (e.g. if it contains whitespace characters).
             Environment variables set by SetEnv override the default
             environment and any variables specified by the user via AcceptEnv
             or PermitUserEnvironment.

     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, sshd 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 is no.

     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
             sshd(8).  The possible values are: DAEMON, USER, AUTH, LOCAL0,
             LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.  The
             default is AUTH.

     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.  On the other hand, if TCP
             keepalives are not sent, sessions may hang indefinitely on the
             server, leaving "ghost" users and consuming server resources.

             The default is yes (to send TCP keepalive messages), and the server
             will notice if the network goes down or the client host crashes.
             This avoids infinitely hanging sessions.

             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).

     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 (the default) then only addresses and
             not host names may be used in ~/.ssh/authorized_keys from and
             sshd_config Match Host directives.

     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 no.

     VersionAddendum
             Optionally specifies additional text to append to the SSH protocol
             banner sent by the server upon connection.  The default is none.

     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 no.

             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, or none to not
             use one.  The default is /usr/bin/xauth.

TIME FORMATS
     sshd(8) command-line arguments and configuration file options that specify
     time may be expressed using a sequence of the form: time[qualifier], where
     time is a positive integer value and qualifier is one of the following:

           ⟨none⟩  seconds
           s | S   seconds
           m | M   minutes
           h | H   hours
           d | D   days
           w | W   weeks

     Each member of the sequence is added together to calculate the total time
     value.

     Time format examples:

           600     600 seconds (10 minutes)
           10m     10 minutes
           1h30m   1 hour 30 minutes (90 minutes)

TOKENS
     Arguments to some keywords can make use of tokens, which are expanded at
     runtime:

           %%    A literal ‘%’.
           %D    The routing domain in which the incoming connection was
                 received.
           %F    The fingerprint of the CA key.
           %f    The fingerprint of the key or certificate.
           %h    The home directory of the user.
           %i    The key ID in the certificate.
           %K    The base64-encoded CA key.
           %k    The base64-encoded key or certificate for authentication.
           %s    The serial number of the certificate.
           %T    The type of the CA key.
           %t    The key or certificate type.
           %U    The numeric user ID of the target user.
           %u    The username.

     AuthorizedKeysCommand accepts the tokens %%, %f, %h, %k, %t, %U, and %u.

     AuthorizedKeysFile accepts the tokens %%, %h, %U, and %u.

     AuthorizedPrincipalsCommand accepts the tokens %%, %F, %f, %h, %i, %K, %k,
     %s, %T, %t, %U, and %u.

     AuthorizedPrincipalsFile accepts the tokens %%, %h, %U, and %u.

     ChrootDirectory accepts the tokens %%, %h, %U, and %u.

     RoutingDomain accepts the token %D.

FILES
     /etc/ssh/sshd_config
             Contains configuration data for sshd(8).  This file should be
             writable by root only, but it is recommended (though not necessary)
             that it be world-readable.

SEE ALSO
     sftp-server(8), sshd(8)

AUTHORS
     OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu
     Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de
     Raadt and Dug Song removed many bugs, re-added newer features and created
     OpenSSH.  Markus Friedl contributed the support for SSH protocol versions
     1.5 and 2.0.  Niels Provos and Markus Friedl contributed support for
     privilege separation.

BSD                               April 4, 2021                              BSD