man-debian-7.11

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

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:

           ·   Protocol 2
           ·   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 rea‐
             son, 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 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 recog‐
             nized.  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 pro‐
             cessed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally
             AllowGroups.

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

     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, sep‐
             arated 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 cer‐
             tificate 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 substi‐
             tuted 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 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 authen‐
             tication 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 sup‐
             port 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”,
             “arcfour128”, “arcfour256”, “arcfour”, “blowfish-cbc”, and “cast128-cbc”.  The
             default is:

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

     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 ver‐
             sion 2 only.

     Compression
             Specifies whether compression is allowed, or delayed until the user has
             authenticated successfully.  The argument must be “yes”, “delayed”, or “no”.
             The default is “delayed”.

     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 numeri‐
             cal 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 follow‐
             ing 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 for‐
             wardings to bind to non-loopback addresses, thus allowing other hosts to con‐
             nect.  The argument may be “no” to force remote port forwardings to be avail‐
             able 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 ver‐
             sion 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 Ker‐
             beros 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 suc‐
             cessful public key client host authentication is allowed (host-based authenti‐
             cation).  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 pub‐
             lic 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 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-accessi‐
             ble.  It is possible to have multiple host key files.  “rsa1” keys are used
             for version 1 and “dsa”, “ecdsa” or “rsa” are used for version 2 of the SSH
             protocol.

     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 interac‐
             tive sessions and the second for non-interactive sessions.  The default is
             “lowdelay” for interactive sessions and “throughput” for non-interactive ses‐
             sions.

     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 “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 pre‐
             cede 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.  Multi‐
             ple algorithms must be comma-separated.  The default is:

                   hmac-md5,hmac-sha1,umac-64@openssh.com,
                   hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
                   hmac-sha2-256,hmac-sha256-96,hmac-sha2-512,
                   hmac-sha2-512-96

     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.

             The arguments to Match are one or more criteria-pattern pairs.  The available
             criteria are User, Group, Host, and Address.  The match patterns may consist
             of single entries or comma-separated lists and may use the wildcard and nega‐
             tion 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 AllowAgentForwarding, AllowTcpForwarding,
             AuthorizedKeysFile, AuthorizedPrincipalsFile, Banner, ChrootDirectory,
             ForceCommand, GatewayPorts, GSSAPIAuthentication, HostbasedAuthentication,
             HostbasedUsesNameFromPacketOnly, KbdInteractiveAuthentication,
             KerberosAuthentication, MaxAuthTries, MaxSessions, PasswordAuthentication,
             PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTunnel,
             PubkeyAuthentication, RhostsRSAAuthentication, RSAAuthentication,
             X11DisplayOffset, X11Forwarding and X11UseLocalHost.

     MaxAuthTries
             Specifies the maximum number of authentication attempts permitted per connec‐
             tion.  Once the number of failures reaches half this value, additional fail‐
             ures are logged.  The default is 6.

     MaxSessions
             Specifies the maximum number of open sessions permitted per network connec‐
             tion.  The default is 10.

     MaxStartups
             Specifies the maximum number of concurrent unauthenticated connections to the
             SSH daemon.  Additional connections will be dropped until authentication suc‐
             ceeds 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 cur‐
             rently “start” (10) unauthenticated connections.  The probability increases
             linearly and all connection attempts are refused if the number of unauthenti‐
             cated connections reaches “full” (60).

     PasswordAuthentication
             Specifies whether password authentication is allowed.  The default is “yes”.

     PermitBlacklistedKeys
             Specifies whether sshd(8) should allow keys recorded in its blacklist of
             known-compromised keys (see ssh-vulnkey(1)).  If “yes”, then attempts to
             authenticate with compromised keys will be logged but accepted.  If “no”, then
             attempts to authenticate with compromised keys will be rejected.  The default
             is “no”.

     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 for‐
             warding 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 dis‐
             abled 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 speci‐
             fied (which may be useful for taking remote backups even if root login is nor‐
             mally 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”.

     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.  Multi‐
             ple 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 interac‐
             tively.  (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.

     RevokedKeys
             Specifies a list of revoked public keys.  Keys listed in this file will be
             refused for public key authentication.  Note that if this file is not read‐
             able, then public key authentication will be refused for all users.

     RhostsRSAAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication together with suc‐
             cessful 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 proto‐
             col 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 in‐
             finitely 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 certifi‐
             cate 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 cer‐
             tificate's principals list.  Note that certificates that lack a list of prin‐
             cipals 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 execu‐
             tion.  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 process‐
             ing 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 authen‐
             tication, 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.

     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 loop‐
             back address or to the wildcard address.  By default, sshd binds the forward‐
             ing server to the loopback address and sets the hostname part of the DISPLAY
             environment variable to “localhost”.  This prevents remote hosts from connect‐
             ing 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 con‐
     tributed the support for SSH protocol versions 1.5 and 2.0.  Niels Provos and Markus
     Friedl contributed support for privilege separation.