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NAME

     sshd_config — OpenSSH SSH daemon configuration file

SYNOPSIS

     /etc/ssh/sshd_config

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.  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.

     Note that the Debian openssh-server package sets several options as standard in /etc/ssh/sshd_config which
     are not the default in sshd(8).  The exact list depends on whether the package was installed fresh or
     upgraded from various possible previous versions, but includes at least the following:

              ChallengeResponseAuthentication no
              X11Forwarding yes
              PrintMotd no
              AcceptEnv LANG LC_*
              Subsystem sftp /usr/lib/openssh/sftp-server
              UsePAM yes

     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 in ssh_config(5) for how to configure the client.  Note that environment
             passing is only supported for protocol 2.  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”, “inet” (use
             IPv4 only), or “inet6” (use IPv6 only).  The default is “any”.

     AllowAgentForwarding
             Specifies whether ssh-agent(1) forwarding is permitted.  The default is “yes”.  Note that disabling
             agent forwarding does not improve security unless users are also denied shell access, as they can
             always install their own forwarders.

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

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

     AllowTcpForwarding
             Specifies whether TCP forwarding is permitted.  The available options are “yes” 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.  The default is “yes”.  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.  The allow/deny directives are processed in the following
             order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.

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

     AuthenticationMethods
             Specifies the authentication methods that must be successfully completed for a user to be granted
             access.  This option must be followed by one or more comma-separated lists of authentication method
             names.  Successful authentication requires completion of every method in at least one of these
             lists.

             For example, an argument of “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”, “pam”, or “skey”,
             depending on the server configuration.  For example, “keyboard-interactive:bsdauth” would restrict
             keyboard interactive authentication to the “bsdauth” device.

             This option is only available for SSH protocol 2 and will yield a fatal error if enabled if
             protocol 1 is also enabled.  Note that each authentication method listed should also be explicitly
             enabled in the configuration.  The default is not to require multiple authentication; successful
             completion of a single authentication method is sufficient.

     AuthorizedKeysCommand
             Specifies a program to be used to look up the user's public keys.  The program must be owned by
             root and not writable by group or others.  It will be invoked with a single argument of the
             username being authenticated, and should produce on standard output zero or more lines of
             authorized_keys output (see AUTHORIZED_KEYS in sshd(8)).  If a key supplied by
             AuthorizedKeysCommand does not successfully authenticate and authorize the user then public key
             authentication continues using the usual AuthorizedKeysFile files.  By default, no
             AuthorizedKeysCommand is run.

     AuthorizedKeysCommandUser
             Specifies the user under whose account the AuthorizedKeysCommand is run.  It is recommended to use
             a dedicated user that has no other role on the host than running authorized keys commands.

     AuthorizedKeysFile
             Specifies the file that contains the public keys that can be used for user authentication.  The
             format is described in the AUTHORIZED_KEYS FILE FORMAT section of sshd(8).  AuthorizedKeysFile may
             contain tokens of the form %T which are substituted during connection setup.  The following tokens
             are defined: %% is replaced by a literal '%', %h is replaced by the home directory of the user
             being authenticated, and %u is replaced by the username of that user.  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.  The default is “.ssh/authorized_keys
             .ssh/authorized_keys2”.

     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.

             AuthorizedPrincipalsFile may contain tokens of the form %T which are substituted during connection
             setup.  The following tokens are defined: %% is replaced by a literal '%', %h is replaced by the
             home directory of the user being authenticated, and %u is replaced by the username of that user.
             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.  This option is only available for protocol
             version 2.  By default, no banner is displayed.

     ChallengeResponseAuthentication
             Specifies whether challenge-response authentication is allowed (e.g. via PAM).  The default is
             “yes”.

     ChrootDirectory
             Specifies the pathname of a directory to chroot(2) to after authentication.  All components of the
             pathname must be root-owned directories that are not writable by any other user or group.  After
             the chroot, sshd(8) changes the working directory to the user's home directory.

             The pathname may contain the following tokens that are expanded at runtime once the connecting user
             has been authenticated: %% is replaced by a literal '%', %h is replaced by the home directory of
             the user being authenticated, and %u is replaced by the username of that user.

             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), arandom(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 do require /dev/log inside the
             chroot directory (see sftp-server(8) for details).

             The default is not to chroot(2).

     Ciphers
             Specifies the ciphers allowed for protocol version 2.  Multiple ciphers must be comma-separated.
             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”, “arcfour128”, “arcfour256”, “arcfour”,
             “blowfish-cbc”, “cast128-cbc”, and “chacha20-poly1305@openssh.com”.

             The default is:

                aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
                aes128-gcm@openssh.com,aes256-gcm@openssh.com,
                chacha20-poly1305@openssh.com,
                aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
                aes256-cbc,arcfour

             The list of available ciphers may also be obtained using the -Q option of ssh(1).

     ClientAliveCountMax
             Sets the number of client alive messages (see below) 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 (below).  The client alive
             messages are sent through the encrypted channel and therefore will not be spoofable.  The TCP
             keepalive option enabled by TCPKeepAlive is spoofable.  The client alive mechanism is valuable when
             the client or server depend on knowing when a connection has become inactive.

             The default value is 3.  If ClientAliveInterval (see below) is set to 15, and ClientAliveCountMax
             is left at the default, unresponsive SSH clients will be disconnected after approximately 45
             seconds.  This option applies to protocol version 2 only.

     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.  This option
             applies to protocol version 2 only.

     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”.

     DebianBanner
             Specifies whether the distribution-specified extra version suffix is included during initial
             protocol handshake.  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 directives are processed in the following order: DenyUsers,
             AllowUsers, DenyGroups, and finally AllowGroups.

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

     DenyUsers
             This keyword can be followed by a list of user name patterns, separated by spaces.  Login is
             disallowed for user names that match one of the patterns.  Only user names are valid; a numerical
             user ID is not recognized.  By default, login is allowed for all users.  If the pattern takes the
             form USER@HOST then USER and HOST are separately checked, restricting logins to particular users
             from particular hosts.  The allow/deny directives are processed in the following order: DenyUsers,
             AllowUsers, DenyGroups, and finally AllowGroups.

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

     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.

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

     GSSAPIKeyExchange
             Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange doesn't rely on ssh
             keys to verify host identity.  The default is “no”.  Note that this option applies to protocol
             version 2 only.

     GSSAPICleanupCredentials
             Specifies whether to automatically destroy the user's credentials cache on logout.  The default is
             “yes”.  Note that this option applies to protocol version 2 only.

     GSSAPIStrictAcceptorCheck
             Determines whether to be strict about the identity of the GSSAPI acceptor a client authenticates
             against. If “yes” then the client must authenticate against the host service on the current
             hostname. If “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”.  Note that this option applies only to protocol version 2 GSSAPI connections, and
             setting it to “no” may only work with recent Kerberos GSSAPI libraries.

     GSSAPIStoreCredentialsOnRekey
             Controls whether the user's GSSAPI credentials should be updated following a successful connection
             rekeying. This option can be used to accepted renewed or updated credentials from a compatible
             client. The default is “no”.

     HostbasedAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication together with successful public key
             client host authentication is allowed (host-based authentication).  This option is similar to
             RhostsRSAAuthentication and applies to protocol version 2 only.  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 default is /etc/ssh/ssh_host_key
             for protocol version 1, and /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key,
             /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for protocol version 2.  Note that
             sshd(8) will refuse to use a file if it is group/world-accessible.  It is possible to have multiple
             host key files.  “rsa1” keys are used for version 1 and “dsa”, “ecdsa”, “ed25519” or “rsa” are used
             for version 2 of the SSH protocol.  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 “SSH_AUTH_SOCK” is specified, the location of the socket will be read from the
             SSH_AUTH_SOCK environment variable.

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

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

     IgnoreUserKnownHosts
             Specifies whether sshd(8) should ignore the user's ~/.ssh/known_hosts during
             RhostsRSAAuthentication or HostbasedAuthentication.  The default is “no”.

     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”, “lowdelay”, “throughput”, “reliability”, or
             a numeric value.  This option may take one or two arguments, separated by whitespace.  If one
             argument is specified, it is used as the packet class unconditionally.  If two values are
             specified, the first is automatically selected for interactive sessions and the second for non-
             interactive sessions.  The default is “lowdelay” for interactive sessions and “throughput” for non-
             interactive sessions.

     KbdInteractiveAuthentication
             Specifies whether to 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.  The default is

                   curve25519-sha256@libssh.org,
                   ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
                   diffie-hellman-group-exchange-sha256,
                   diffie-hellman-group-exchange-sha1,
                   diffie-hellman-group14-sha1,
                   diffie-hellman-group1-sha1

     KeyRegenerationInterval
             In protocol version 1, the ephemeral server key is automatically regenerated after this many
             seconds (if it has been used).  The purpose of regeneration is to prevent decrypting captured
             sessions by later breaking into the machine and stealing the keys.  The key is never stored
             anywhere.  If the value is 0, the key is never regenerated.  The default is 3600 (seconds).

     ListenAddress
             Specifies the local addresses sshd(8) should listen on.  The following forms may be used:

                   ListenAddress host|IPv4_addr|IPv6_addr
                   ListenAddress host|IPv4_addr:port
                   ListenAddress [host|IPv6_addr]:port

             If port is not specified, sshd will listen on the address and all prior Port options specified.
             The default is to listen on all local addresses.  Multiple ListenAddress options are permitted.
             Additionally, any Port options must precede this option for non-port qualified addresses.

     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.

     MACs    Specifies the available MAC (message authentication code) algorithms.  The MAC algorithm is used in
             protocol version 2 for data integrity protection.  Multiple algorithms must be comma-separated.
             The algorithms that contain “-etm” calculate the MAC after encryption (encrypt-then-mac).  These
             are considered safer and their use recommended.  The default is:

                   hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
                   umac-64-etm@openssh.com,umac-128-etm@openssh.com,
                   hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
                   hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
                   hmac-md5-96-etm@openssh.com,
                   hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
                   hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
                   hmac-sha1-96,hmac-md5-96

     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 satisified, 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, and Address.
             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, e.g. “192.0.2.0/24” or “3ffe:ffff::/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, AllowTcpForwarding, AllowUsers,
             AuthenticationMethods, AuthorizedKeysCommand, AuthorizedKeysCommandUser, AuthorizedKeysFile,
             AuthorizedPrincipalsFile, Banner, ChrootDirectory, DenyGroups, DenyUsers, ForceCommand,
             GatewayPorts, GSSAPIAuthentication, HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
             KbdInteractiveAuthentication, KerberosAuthentication, MaxAuthTries, MaxSessions,
             PasswordAuthentication, PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel,
             PubkeyAuthentication, RekeyLimit, RhostsRSAAuthentication, RSAAuthentication, 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 sessions permitted per network connection.  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).

     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”.

     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.  By default all port forwarding requests are permitted.

     PermitRootLogin
             Specifies whether root can log in using ssh(1).  The argument must be “yes”, “without-password”,
             “forced-commands-only”, or “no”.  The default is “yes”.

             If this option is set to “without-password”, password authentication is 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.

     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”.

     PermitTTY
             Specifies whether pty(4) allocation is permitted.  The default is “yes”.

     PermitUserEnvironment
             Specifies whether ~/.ssh/environment and environment= options in ~/.ssh/authorized_keys are
             processed by sshd(8).  The default is “no”.  Enabling environment processing may enable users to
             bypass access restrictions in some configurations using mechanisms such as LD_PRELOAD.

     PidFile
             Specifies the file that contains the process ID of the SSH daemon.  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”.

     Protocol
             Specifies the protocol versions sshd(8) supports.  The possible values are ‘1’ and ‘2’.  Multiple
             versions must be comma-separated.  The default is ‘2’.  Note that the order of the protocol list
             does not indicate preference, because the client selects among multiple protocol versions offered
             by the server.  Specifying “2,1” is identical to “1,2”.

     PubkeyAuthentication
             Specifies whether public key authentication is allowed.  The default is “yes”.  Note that this
             option applies to protocol version 2 only.

     RekeyLimit
             Specifies the maximum amount of data that may be transmitted before the session key is
             renegotiated, optionally followed 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.  This option applies to protocol version 2
             only.

     RevokedKeys
             Specifies revoked public keys.  Keys listed in this file will be refused for public key
             authentication.  Note that if this file is not readable, then public key authentication will be
             refused for all users.  Keys may be specified as a text file, listing one public key per line, or
             as an OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1).  For more information on
             KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).

     RhostsRSAAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host
             authentication is allowed.  The default is “no”.  This option applies to protocol version 1 only.

     RSAAuthentication
             Specifies whether pure RSA authentication is allowed.  The default is “yes”.  This option applies
             to protocol version 1 only.

     ServerKeyBits
             Defines the number of bits in the ephemeral protocol version 1 server key.  The minimum value is
             512, and the default is 1024.

     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(8) 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.  Note that this option applies to protocol version 2 only.

     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”.

             This option was formerly called KeepAlive.

     TrustedUserCAKeys
             Specifies a file containing public keys of certificate authorities that are trusted to sign user
             certificates for authentication.  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 check that the resolved host name
             for the remote IP address maps back to the very same IP address.  The default is “yes”.

     UseLogin
             Specifies whether login(1) is used for interactive login sessions.  The default is “no”.  Note that
             login(1) is never used for remote command execution.  Note also, that if this is enabled,
             X11Forwarding will be disabled because login(1) does not know how to handle xauth(1) cookies.  If
             UsePrivilegeSeparation is specified, it will be disabled after authentication.

     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”.

     UsePrivilegeSeparation
             Specifies whether sshd(8) separates privileges by creating an unprivileged child process to deal
             with incoming network traffic.  After successful authentication, another process will be created
             that has the privilege of the authenticated user.  The goal of privilege separation is to prevent
             privilege escalation by containing any corruption within the unprivileged processes.  The default
             is “yes”.  If UsePrivilegeSeparation is set to “sandbox” then the pre-authentication unprivileged
             process is subject to additional restrictions.

     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 below), 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.  X11 forwarding is automatically disabled if UseLogin is
             enabled.

     X11UseLocalhost
             Specifies whether sshd(8) should bind the X11 forwarding server to the loopback address or to the
             wildcard address.  By default, sshd binds the forwarding server to the loopback address and sets
             the hostname part of the DISPLAY environment variable to “localhost”.  This prevents remote hosts
             from connecting to the proxy display.  However, some older X11 clients may not function with this
             configuration.  X11UseLocalhost may be set to “no” to specify that the forwarding server should be
             bound to the wildcard address.  The argument must be “yes” or “no”.  The default is “yes”.

     XAuthLocation
             Specifies the full pathname of the xauth(1) program.  The default is /usr/bin/xauth.

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)

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

     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.