trust-dns vulnerable to Remote Attackers causing Denial-of-Service (packet loops) with crafted DNS packets

Summary

trust-dns and trust-dns-server are vulnerable to remotely triggered denial-of-service attacks, consuming both network and CPU resources. DNS messages with the QR=1 bit set are responded to with a `FormErr` response. This allows creating a traffic loop, in which these `FormErr` responses are sent nonstop between vulnerable servers. There are two scenarios how this can be exploited: 1) Create a loop between two instances of trust-dns, consuming network resources, or 2) consuming the CPU of a single instance. With two instances *A* and *B* an attacker sends a DNS query with a spoofed source IP address to *A*. *A* replies with a `FormErr` to *B*. Now both servers with ping-pong the message back and forth until by chance the packet is dropped in the network. Multiple spoofed packets can be sent by the attacker, increasing resource consumption. A single server can get locked up replying to itself. Same setup as above, but now *A* sends the reply to itself. The packet is sent out as fast as the CPU and network stack manage. This locks up a CPU core. Multiple packets from the attacker consume multiple CPU cores.