# sshd_config

## 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.  Unless noted otherwise, 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.  This keyword may appear multiple times in
                   **sshd_config** with each instance appending to the list.
    
           **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.  This keyword may appear multiple times in
                   **sshd_config** with each instance appending to the list.
    
           **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
    
                   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.
    
                   Certificates signed using other algorithms will not be
                   accepted for public key or host-based authentication.
    
           **ChannelTimeout**
                   Specifies whether and how quickly _sshd_(8) should close
                   inactive channels.  Timeouts are specified as one or more
                   “type=interval” pairs separated by whitespace, where the
                   “type” must be the special keyword “global” or a channel
                   type name from the list below, optionally containing
                   wildcard characters.
    
                   The timeout value “interval” is specified in seconds or
                   may use any of the units documented in the “TIME FORMATS”
                   section.  For example, “session=5m” would cause
                   interactive sessions to terminate after five minutes of
                   inactivity.  Specifying a zero value disables the
                   inactivity timeout.
    
                   The special timeout “global” applies to all active
                   channels, taken together.  Traffic on any active channel
                   will reset the timeout, but when the timeout expires then
                   all open channels will be closed.  Note that this global
                   timeout is not matched by wildcards and must be specified
                   explicitly.
    
                   The available channel type names include:
    
                   **agent-connection**
                           Open connections to _ssh-agent_(1).
    
                   **direct-tcpip** , **direct-streamlocal@openssh.com**
                           Open TCP or Unix socket (respectively) connections
                           that have been established from a _ssh_(1) local
                           forwarding, i.e. **LocalForward** or **DynamicForward**.
    
                   **forwarded-tcpip** , **forwarded-streamlocal@openssh.com**
                           Open TCP or Unix socket (respectively) connections
                           that have been established to a _sshd_(8) listening
                           on behalf of a _ssh_(1) remote forwarding, i.e.
                           **RemoteForward**.
    
                   **session**
                           The interactive main session, including shell
                           session, command execution, _scp_(1), _sftp_(1), etc.
    
                   **tun-connection**
                           Open **TunnelForward** connections.
    
                   **x11-connection**
                           Open X11 forwarding sessions.
    
                   Note that in all the above cases, terminating an inactive
                   session does not guarantee to remove all resources
                   associated with the session, e.g. shell processes or X11
                   clients relating to the session may continue to execute.
    
                   Moreover, terminating an inactive channel or session does
                   not necessarily close the SSH connection, nor does it
                   prevent a client from requesting another channel of the
                   same type.  In particular, expiring an inactive forwarding
                   session does not prevent another identical forwarding from
                   being subsequently created.
    
                   The default is not to expire channels of any type for
                   inactivity.
    
           **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 group or others.
                   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.  This keyword may appear multiple times in
                   **sshd_config** with each instance appending to the list.
    
           **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.  This keyword may appear multiple times in
                   **sshd_config** with each instance appending to the list.
    
           **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-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
    
                   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-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
    
                   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/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/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.  All authentication styles from
                   _login.conf_(5) are supported.  The default is **yes**.  The
                   argument to this keyword must be **yes** or **no**.
                   **ChallengeResponseAuthentication** is a deprecated alias for
                   this.
    
           **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 permitted KEX (Key Exchange) algorithms that
                   the server will offer to clients.  The ordering of this
                   list is not important, as the client specifies the
                   preference order.  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 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
                         mlkem768x25519-sha256
                         sntrup761x25519-sha512
                         sntrup761x25519-sha512@openssh.com
    
                   The default is:
    
                         sntrup761x25519-sha512,sntrup761x25519-sha512@openssh.com,
                         mlkem768x25519-sha256,
                         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 supported 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 one or more 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 one of the single token criteria: **All** , which
                   matches all criteria, or **Invalid-User** , which matches when
                   the requested user-name does not match any known account.
                   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** , **CASignatureAlgorithms** , **ChannelTimeout** ,
                   **ChrootDirectory** , **ClientAliveCountMax** , **ClientAliveInterval** ,
                   **DenyGroups** , **DenyUsers** , **DisableForwarding** , **ExposeAuthInfo** ,
                   **ForceCommand** , **GatewayPorts** , **GSSAPIAuthentication** ,
                   **HostbasedAcceptedAlgorithms** , **HostbasedAuthentication** ,
                   **HostbasedUsesNameFromPacketOnly** , **IgnoreRhosts** , **Include** ,
                   **IPQoS** , **KbdInteractiveAuthentication** ,
                   **KerberosAuthentication** , **LogLevel** , **MaxAuthTries** ,
                   **MaxSessions** , **PAMServiceName** , **PasswordAuthentication** ,
                   **PermitEmptyPasswords** , **PermitListen** , **PermitOpen** ,
                   **PermitRootLogin** , **PermitTTY** , **PermitTunnel** , **PermitUserRC** ,
                   **PubkeyAcceptedAlgorithms** , **PubkeyAuthentication** ,
                   **PubkeyAuthOptions** , **RefuseConnection** , **RekeyLimit** ,
                   **RevokedKeys** , **RDomain** , **SetEnv** , **StreamLocalBindMask** ,
                   **StreamLocalBindUnlink** , **TrustedUserCAKeys** ,
                   **UnusedConnectionTimeout** , **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/moduli_.
    
           **PAMServiceName**
                   Specifies the service name used for Pluggable
                   Authentication Modules (PAM) authentication, authorisation
                   and session controls when **UsePAM** is enabled.  The default
                   is **sshd**.
    
           **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.
    
           **PerSourcePenalties**
                   Controls penalties for various conditions that may
                   represent attacks on _sshd_(8).  If a penalty is enforced
                   against a client then its source address and any others in
                   the same network, as defined by **PerSourceNetBlockSize** ,
                   will be refused connection for a period.
    
                   A penalty doesn't affect concurrent connections in
                   progress, but multiple penalties from the same source from
                   concurrent connections will accumulate up to a maximum.
                   Conversely, penalties are not applied until a minimum
                   threshold time has been accumulated.
    
                   Penalties are enabled by default with the default settings
                   listed below but may disabled using the **no** keyword.  The
                   defaults may be overridden by specifying one or more of
                   the keywords below, separated by whitespace.  All keywords
                   accept arguments, e.g. "crash:2m".
    
                   **crash:duration**
                           Specifies how long to refuse clients that cause a
                           crash of _sshd_(8) (default: 90s).
    
                   **authfail:duration**
                           Specifies how long to refuse clients that
                           disconnect after making one or more unsuccessful
                           authentication attempts (default: 5s).
    
                   **refuseconnection:duration**
                           Specifies how long to refuse clients that were
                           administratively prohibited connection via the
                           **RefuseConnection** option (default: 10s).
    
                   **noauth:duration**
                           Specifies how long to refuse clients that
                           disconnect without attempting authentication
                           (default: 1s).  This timeout should be used
                           cautiously otherwise it may penalise legitimate
                           scanning tools such as _ssh-keyscan_(1).
    
                   **grace-exceeded:duration**
                           Specifies how long to refuse clients that fail to
                           authenticate after **LoginGraceTime**(default: 10s).
    
                   **max:duration**
                           Specifies the maximum time a particular source
                           address range will be refused access for (default:
                           10m).  Repeated penalties will accumulate up to
                           this maximum.
    
                   **min:duration**
                           Specifies the minimum penalty that must accrue
                           before enforcement begins (default: 15s).
    
                   **max-sources4:number** , **max-sources6:number**
                           Specifies the maximum number of client IPv4 and
                           IPv6 address ranges to track for penalties
                           (default: 65536 for both).
    
                   **overflow:mode**
                           Controls how the server behaves when **max-sources4**
                           or **max-sources6** is exceeded.  There are two
                           operating modes: **deny-all** , which denies all
                           incoming connections other than those exempted via
                           **PerSourcePenaltyExemptList** until a penalty
                           expires, and **permissive** , which allows new
                           connections by removing existing penalties early
                           (default: permissive).  Note that client penalties
                           below the **min** threshold count against the total
                           number of tracked penalties.  IPv4 and IPv6
                           addresses are tracked separately, so an overflow
                           in one will not affect the other.
    
                   **overflow6:mode**
                           Allows specifying a different overflow mode for
                           IPv6 addresses.  The default it to use the same
                           overflow mode as was specified for IPv4.
    
           **PerSourcePenaltyExemptList**
                   Specifies a comma-separated list of addresses to exempt
                   from penalties.  This list may contain wildcards and CIDR
                   address/masklen ranges.  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.  The default is not to exempt any addresses.
    
           **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_.
    
           **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-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
    
                   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**.
    
           **RefuseConnection**
                   Indicates that _sshd_(8) should unconditionally terminate
                   the connection.  Additionally, a **refuseconnection** penalty
                   may be recorded against the source of the connection if
                   **PerSourcePenalties** are enabled.  This option is only
                   really useful in a **Match** block.
    
           **RekeyLimit**
                   Specifies the maximum amount of data that may be
                   transmitted or received before the session key is
                   renegotiated, optionally followed by a maximum amount of
                   time that may pass before the session key is renegotiated.
                   The first argument is specified in bytes and may have a
                   suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes,
                   Megabytes, or Gigabytes, respectively.  The default is
                   between ‘1G’ and ‘4G’, depending on the 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.
    
           **RequiredRSASize**
                   Specifies the minimum RSA key size (in bits) that _sshd_(8)
                   will accept.  User and host-based authentication keys
                   smaller than this limit will be refused.  The default is
                   **1024** bits.  Note that this limit may only be raised from
                   the default.
    
           **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
                   as 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**.
    
           **SshdSessionPath**
                   Overrides the default path to the **sshd-session** binary that
                   is invoked to handle each connection.  The default is
                   _/usr/libexec/sshd-session_.  This option is intended for
                   use by tests.
    
           **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.  It accepts the same command line arguments as
                   **sftp-server** and even though it is in-process, settings
                   such as **LogLevel** or **SyslogFacility** do not apply to it and
                   must be set explicitly via command line arguments.
    
                   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).
    
           **UnusedConnectionTimeout**
                   Specifies whether and how quickly _sshd_(8) should close
                   client connections with no open channels.  Open channels
                   include active shell, command execution or subsystem
                   sessions, connected network, socket, agent or X11
                   forwardings.  Forwarding listeners, such as those from the
                   _ssh_(1) **-R** flag, are not considered as open channels and do
                   not prevent the timeout.  The timeout value is specified
                   in seconds or may use any of the units documented in the
                   “TIME FORMATS” section.
    
                   Note that this timeout starts when the client connection
                   completes user authentication but before the client has an
                   opportunity to open any channels.  Caution should be used
                   when using short timeout values, as they may not provide
                   sufficient time for the client to request and open its
                   channels before terminating the connection.
    
                   The default **none** is to never expire connections for having
                   no open channels.  This option may be useful in
                   conjunction with **ChannelTimeout**.
    
           **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
                   **KbdInteractiveAuthentication** and **PasswordAuthentication** in
                   addition to PAM account and session module processing for
                   all authentication types.
    
                   Because PAM keyboard-interactive authentication usually
                   serves an equivalent role to password authentication, you
                   should disable either **PasswordAuthentication** or
                   **KbdInteractiveAuthentication**.
    
                   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/X11R6/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 ‘%’.
                 %C    Identifies the connection endpoints, containing four
                       space-separated values: client address, client port
                       number, server address, and server port number.
                 %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 %%, %C, %D, %f, %h, %k,
           %t, %U, and %u.
    
           **AuthorizedKeysFile** accepts the tokens %%, %h, %U, and %u.
    
           **AuthorizedPrincipalsCommand** accepts the tokens %%, %C, %D, %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.
    


* * *