CVE-2019-6477

A flaw was found in the way bind limited the number of TCP clients that can be connected at any given time. A remote attacker could use one TCP client to send a large number of DNS requests over a single connection, causing exhaustion of the pool of file descriptors available to named, and potentially affecting network connections and the management of files such as log files or zone journal files.

Mitigation

The vulnerability can be mitigated by disabling server TCP-pipelining:

       keep-response-order { any; };  

and then restarting BIND. The server restart is necessary because neither a 'reload' nor a 'reconfig' operation will properly reset currently pipelining TCP clients.
Disabling TCP-pipelining entirely is completely effective at mitigating the vulnerability with minimal impact to clients that use pipelined TCP connections and with no impact to clients that do not support TCP-pipelining. The majority of Internet client DNS queries are transported over UDP or TCP without use of TCP-pipelining.
Note: This mitigation will only work with bind-9.11 and above.