NAME

       filter-a - filter A in DNS responses when AAAA is present

SYNOPSIS

       plugin query "filter-a.so" [{ parameters }];

DESCRIPTION

       filter-a.so is a query plugin module for named, enabling named to omit some IPv4 addresses
       when responding to clients.

       For example:

          plugin query "filter-a.so" {
                  filter-a-on-v6 yes;
                  filter-a-on-v4 yes;
                  filter-a { 192.0.2.1; 2001:db8:2::1; };
          };

       This module is intended to aid transition from IPv4 to IPv6 by withholding IPv4  addresses
       from  DNS clients which are not connected to the IPv4 Internet, when the name being looked
       up has an IPv6 address available. Use of this module is not recommended unless  absolutely
       necessary.

       Note:  This  mechanism  can erroneously cause other servers not to give A records to their
       clients. If a recursing server with both IPv6 and  IPv4  network  connections  queries  an
       authoritative  server  using  this  mechanism via IPv6, it is denied A records even if its
       client is using IPv4.

OPTIONS

       filter-a
              This option specifies a list of client addresses for which A  filtering  is  to  be
              applied. The default is any.

       filter-a-on-v6
              If  set to yes, this option indicates that the DNS client is at an IPv6 address, in
              filter-a. If the response does not include DNSSEC signatures, then  all  A  records
              are  deleted  from  the response. This filtering applies to all responses, not only
              authoritative ones.

              If set to break-dnssec, then A records are deleted even when DNSSEC is enabled.  As
              suggested  by  the  name,  this  causes the response to fail to verify, because the
              DNSSEC protocol is designed to detect deletions.

              This mechanism can erroneously cause other servers not to give A records  to  their
              clients.  If a recursing server with both IPv6 and IPv4 network connections queries
              an authoritative server using this mechanism via IPv6, it is denied A records  even
              if its client is using IPv4.

       filter-a-on-v4
              This  option  is identical to filter-a-on-v6, except that it filters A responses to
              queries from IPv4 clients instead of IPv6 clients. To  filter  all  responses,  set
              both options to yes.

SEE ALSO

       BIND 9 Administrator Reference Manual.

AUTHOR

       Internet Systems Consortium

COPYRIGHT

       2022, Internet Systems Consortium