Source code and details: https://github.com/umutcamliyurt/PortTripper

How it works

On startup PortTripper:

1. Scans the configured port range and builds a whitelist of ports already in use by real services (first run only).
2. Draws a cryptographically random sample of up to -maxports ports from the range, excluding whitelisted ports. Using crypto/rand for selection means the open set is unpredictable to an attacker even if they know the configured range.
3. Binds TCP and UDP listeners on every chosen port.
4. On TCP: accepts connections, holds them open for a configurable duration, then drops them, wasting the scanner's threads and file descriptors.
5. On UDP: reads and discards datagrams without replying, so ports appear open|filtered to scanners rather than closed.

All real service ports are untouched because they are already bound before PortTripper starts, and the auto-generated whitelist tells PortTripper to skip them.