FILTER-A(8)

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 <#std-iscman-named>, enabling named <#std-iscman-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
2026, Internet Systems Consortium