Provided by: openssh-server_7.4p1-10_amd64 bug

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):

           ·   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.  The TERM
             environment variable is always sent 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 directives are processed in the following
             order: DenyUsers, AllowUsers, DenyGroups, and finally
             AllowGroups.

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

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

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

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

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

     AuthenticationMethods
             Specifies the authentication methods that must be successfully
             completed for a user to be granted access.  This option must be
             followed by one or more comma-separated lists of 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, pam, or skey,
             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.

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

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

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

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

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

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

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

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

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

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

     Banner  The contents of the specified file are sent to the remote user
             before authentication is allowed.  If the argument is none then
             no banner is displayed.  By default, no banner is displayed.

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

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

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

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

     Ciphers
             Specifies the ciphers allowed.  Multiple ciphers must be comma-
             separated.  If the specified value begins with a ‘+’ character,
             then the specified ciphers will be appended to the default set
             instead of replacing them.

             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
                   arcfour
                   arcfour128
                   arcfour256
                   blowfish-cbc
                   cast128-cbc
                   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
             inactive.

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

     ClientAliveInterval
             Sets a timeout interval in seconds after which if no data has
             been received from the client, sshd(8) will send a message
             through the encrypted channel to request a response from the
             client.  The default is 0, indicating that these messages will
             not be sent to the client.

     Compression
             Specifies whether 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.  HOST criteria may
             additionally contain addresses to match in CIDR address/masklen
             format.  The allow/deny directives are processed in the following
             order: DenyUsers, AllowUsers, DenyGroups, and finally
             AllowGroups.

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

     DisableForwarding
             Disables all forwarding features, including X11, ssh-agent(1),
             TCP and StreamLocal.  This option overrides all other forwarding-
             related options and may simplify restricted configurations.

     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.

     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.

     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.

     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.

     HostbasedAcceptedKeyTypes
             Specifies the key types that will be accepted for hostbased
             authentication as a comma-separated pattern list.  Alternately if
             the specified value begins with a ‘+’ character, then the
             specified key types will be appended to the default set instead
             of replacing them.  The default for this option is:

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

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

     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_rsa_key,
             /etc/ssh/ssh_host_ecdsa_key and /etc/ssh/ssh_host_ed25519_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 algorithms that the server offers.  The
             default for this option is:

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

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

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

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

     IgnoreUserKnownHosts
             Specifies whether sshd(8) should ignore the user's
             ~/.ssh/known_hosts during 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.  Alternately if the specified
             value begins with a ‘+’ character, then the specified methods
             will be appended to the default set instead of replacing them.
             The supported algorithms are:

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

             The default is:

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

             The list of available key exchange algorithms may also be
             obtained using "ssh -Q kex".

     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
             Port options specified.  The default is to listen on all local
             addresses.  Multiple ListenAddress options are permitted.

     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 for data integrity
             protection.  Multiple algorithms must be comma-separated.  If the
             specified value begins with a ‘+’ character, then the specified
             algorithms will be appended to the default set instead of
             replacing them.

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

             The default is:

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

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

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

             The arguments to Match are one or more criteria-pattern pairs or
             the single token All which matches all criteria.  The available
             criteria are User, Group, Host, LocalAddress, LocalPort, 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, such as
             192.0.2.0/24 or 2001:db8::/32.  Note that the mask length
             provided must be consistent with the address - it is an error to
             specify a mask length that is too long for the address or one
             with bits set in this host portion of the address.  For example,
             192.0.2.0/33 and 192.0.2.0/8, respectively.

             Only a subset of keywords may be used on the lines following a
             Match keyword.  Available keywords are AcceptEnv,
             AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
             AllowTcpForwarding, AllowUsers, AuthenticationMethods,
             AuthorizedKeysCommand, AuthorizedKeysCommandUser,
             AuthorizedKeysFile, AuthorizedPrincipalsCommand,
             AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile,
             Banner, ChrootDirectory, ClientAliveCountMax,
             ClientAliveInterval, DenyGroups, DenyUsers, ForceCommand,
             GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
             HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS,
             KbdInteractiveAuthentication, KerberosAuthentication,
             MaxAuthTries, MaxSessions, PasswordAuthentication,
             PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY,
             PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes,
             PubkeyAuthentication, RekeyLimit, RevokedKeys,
             StreamLocalBindMask, StreamLocalBindUnlink, TrustedUserCAKeys,
             X11DisplayOffset, X11Forwarding and X11UseLocalHost.

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

     MaxSessions
             Specifies the maximum number of open shell, login or subsystem
             (e.g. sftp) sessions permitted per network connection.  Multiple
             sessions may be established by clients that support connection
             multiplexing.  Setting MaxSessions to 1 will effectively disable
             session multiplexing, whereas setting it to 0 will prevent all
             shell, login and subsystem sessions while still permitting
             forwarding.  The default is 10.

     MaxStartups
             Specifies the maximum number of concurrent unauthenticated
             connections to the SSH daemon.  Additional connections will be
             dropped until authentication succeeds or the LoginGraceTime
             expires for a connection.  The default is 10:30:100.

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

     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.  The
             wildcard ‘*’ can be used for host or port to allow all hosts or
             ports, respectively.  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, without-password,
             forced-commands-only, or no.  The default is prohibit-password.

             If this option is set to prohibit-password or 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).  The default is
             no.  Enabling environment processing may enable users to bypass
             access restrictions in some configurations using mechanisms such
             as LD_PRELOAD.

     PermitUserRC
             Specifies whether any ~/.ssh/rc file is executed.  The default is
             yes.

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

     PubkeyAcceptedKeyTypes
             Specifies the key types that will be accepted for public key
             authentication as a comma-separated pattern list.  Alternately if
             the specified value begins with a ‘+’ character, then the
             specified key types will be appended to the default set instead
             of replacing them.  The default for this option is:

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

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

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

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

     RevokedKeys
             Specifies revoked public keys file, or none to not use one.  Keys
             listed in this file will be refused for public key
             authentication.  Note that if this file is not readable, then
             public key authentication will be refused for all users.  Keys
             may be specified as a text file, listing one public key per line,
             or as an OpenSSH Key Revocation List (KRL) as generated by
             ssh-keygen(1).  For more information on KRLs, see the KEY
             REVOCATION LISTS section in ssh-keygen(1).

     StreamLocalBindMask
             Sets the octal file creation mode mask (umask) used when creating
             a Unix-domain socket file for local or remote port forwarding.
             This option is only used for port forwarding to a Unix-domain
             socket file.

             The default value is 0177, which creates a Unix-domain socket
             file that is readable and writable only by the owner.  Note that
             not all operating systems honor the file mode on Unix-domain
             socket files.

     StreamLocalBindUnlink
             Specifies whether to remove an existing Unix-domain socket file
             for local or remote port forwarding before creating a new one.
             If the socket file already exists and StreamLocalBindUnlink is
             not enabled, sshd will be unable to forward the port to the Unix-
             domain socket file.  This option is only used for port forwarding
             to a Unix-domain socket file.

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

     StrictModes
             Specifies whether sshd(8) should check file modes and ownership
             of the user's files and home directory before accepting login.
             This is normally desirable because novices sometimes accidentally
             leave their directory or files world-writable.  The default is
             yes.  Note that this does not apply to ChrootDirectory, whose
             permissions and ownership are checked unconditionally.

     Subsystem
             Configures an external subsystem (e.g. file transfer daemon).
             Arguments should be a subsystem name and a command (with optional
             arguments) to execute upon subsystem request.

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

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

             By default no subsystems are defined.

     SyslogFacility
             Gives the facility code that is used when logging messages from
             sshd(8).  The possible values are: DAEMON, USER, AUTH, LOCAL0,
             LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.  The
             default is AUTH.

     TCPKeepAlive
             Specifies whether the system should send TCP keepalive messages
             to the other side.  If they are sent, death of the connection or
             crash of one of the machines will be properly noticed.  However,
             this means that connections will die if the route is down
             temporarily, and some people find it annoying.  On the other
             hand, if TCP keepalives are not sent, sessions may hang
             indefinitely on the server, leaving "ghost" users and consuming
             server resources.

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

             To disable TCP keepalive messages, the value should be set to no.

             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, or none to not use one.  Keys are listed one per
             line; empty lines and comments starting with ‘#’ are allowed.  If
             a certificate is presented for authentication and has its signing
             CA key listed in this file, then it may be used for
             authentication for any user listed in the certificate's
             principals list.  Note that certificates that lack a list of
             principals will not be permitted for authentication using
             TrustedUserCAKeys.  For more details on certificates, see the
             CERTIFICATES section in ssh-keygen(1).

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

             If this option is set to no (the default) then only addresses and
             not host names may be used in ~/.ssh/authorized_keys from and
             sshd_config Match Host directives.

     UsePAM  Enables the Pluggable Authentication Module interface.  If set to
             yes this will enable PAM authentication using
             ChallengeResponseAuthentication and PasswordAuthentication in
             addition to PAM account and session module processing for all
             authentication types.

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

             If UsePAM is enabled, you will not be able to run sshd(8) as a
             non-root user.  The default is no.

     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
             argument must be yes, no, or sandbox.  If UsePrivilegeSeparation
             is set to sandbox then the pre-authentication unprivileged
             process is subject to additional restrictions.  The default is
             sandbox.

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

     X11DisplayOffset
             Specifies the first display number available for sshd(8)'s X11
             forwarding.  This prevents sshd from interfering with real X11
             servers.  The default is 10.

     X11Forwarding
             Specifies whether X11 forwarding is permitted.  The argument must
             be yes or no.  The default is no.

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

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

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

     XAuthLocation
             Specifies the full pathname of the xauth(1) program, or none to
             not use one.  The default is /usr/bin/xauth.

TIME FORMATS

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

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

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

     Time format examples:

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

TOKENS

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

           %%    A literal ‘%’.
           %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 username.

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

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

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

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

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

FILES

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

SEE ALSO

     sftp-server(8), sshd(8)

AUTHORS

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