OK, cool, that sorta works.

I added

#include <linux/netfilter_ipv4.h>
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in Socket.xs and re-installed it. Now, I get my original error, the same that I got if I did

getsockopt($server_socket, SOL_SOCKET, 80);  # 80 = SO_ORIGINAL_DST
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That error is "Protocol not available".

I am back to square one. :-(