Well, TCP doesn't have anything directly equivalent to HTTP proxying. In HTTP, the client (generally) knows about the proxying - it talks to the proxy, and asks the proxy to connect to the real web server on its behalf.
TCP doesn't define that sort of thing, so any proxying would have to either be transparent (i.e. something that a router or the operating system does without the client knowing, e.g. with iptables) or as part of the protocol on top of TCP (HTTP proxying is a good example of this, as is SOCKS mentioned in a different answer).
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