TCP ACKs where you least expect them.
This simple tool is meant to test what happens when you send weird packets to servers during otherwise normal conversations. In particular, ackhole listens on stdin for IPs of servers to connect to (maybe obtained using zmap, for example), and connects to each one. Meanwhile, ackhole listens to packets sent/received in order to track the connection parameters of the connection. After a certain point (e.g. the client has sent some data), ackhole can then inject packets (ACKs, RSTs, out-of-sequence packets, etc), and observe what the server does.
This tool is mainly developed to test how servers will respond to TapDance behavior; incomplete HTTP requests, followed by spurious ACKs, and how long the server will remain silent before sending responses or tearing down the connection due to timeout.