Update to BIND 9.11.18
    
    From Upstream Release notes:
    
    Security Fixes
    
        DNS rebinding protection was ineffective when BIND 9 is configured as a forwarding DNS server. Found and responsibly reported by Tobias Klein. [GL #1574]
    
    Known Issues
    
        We have received reports that in some circumstances, receipt of an IXFR can cause the processing of queries to slow significantly. Some of these were related to RPZ processing, which has been fixed in this release (see below). Others appear to occur where there are NSEC3-related changes (such as an operator changing the NSEC3 salt used in the hash calculation). These are being investigated. [GL #1685]
    
    (cherry picked from commit 6e3b160e3724afd2d118c7343684ce6c0f2925f4)