easiest would be a vpn between the servers - however you need to be using a routed IP range on the source/originating server (this would likely be a secondary or additional IP range on the server) - you would use 2 ips in that range (a /31 subnet) for the openvpn tunnel, with 1 IP for each end of the tunnel.
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If you don't need to tunnel the traffic for security (i.e. if it's just a public website) something like Squid or Apache used as a reverse proxy would achieve what you need.
As someone else has pointed out HAProxy will also do the job.
You could also transmit the data over a ssl certificate with just nginx.
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