ClientHello splitting in Bash

example.sh

# Author: Vinicius Fortuna
# Example practical usage at https://github.com/pypi/support/issues/2751#issuecomment-1535248888

mkfifo in.fifo
nc -l localhost 8443 < in.fifo | { dd bs=100 count=1; sleep 0.1; cat; } | nc files.pythonhosted.org 443 > in.fifo &
time curl --connect-to ::localhost:8443 'https://files.pythonhosted.org/packages/84/a9/2bf119f3f9cff1f376f924e39cfae18dec92a1514784046d185731301281/scipy-1.10.1.tar.gz' > /dev/null

This code is running a local server on port 8443 that intercepts the outgoing flow, splits it on byte 100 using dd, then forwards to files.pythonhosted.org:443. We use in.fifo to connect the two nc commands for the incoming flow.

Here is a diagram of what's going on: