Moles with SOCKS?

Question

Many applications that depend on network resources (such as games connecting to game servers) do not allow the user to set a proxy. There are a lot of ways to force the usage of a proxy regardless, I personally have always been a fan of a local SOCKS tunnel.

What's that?

Instead of connecting directly, an application on your local machine listens on a locally bound address and relays any connection to a fixed endpoint using a remote SOCKS proxy.

Example: Let our target be 11.22.33.44:5555, and our remote socks proxy be 99.88.77.66:5555. Our application listens on 127.0.0.1:5555. Our route now looks like this:

connect to 127.0.0.1:5555 - relays via SOCKS proxy > 99.88.77.66:5555 - to > 11.22.33.44:5555

Your goal is to implement the local application relaying connections to the target.

Input

Three address/port pairs representing the bind, proxy and target address.

• You may deviate from the order (but specify if you do).
• Address parsing is up to your language/tools of choice, but should at the very minimum support IPv4 (11.22.33.44).
• You may use builtin types of your language. (like InetSocketAddress in Java, struct in_addr in C, etc.)

Output

No rules for output due to networking.

Additional

• This is code-golf, shortest answer in bytes wins.
• Standard rules apply, default loopholes are forbidden.
• You do not have to support more than 1 connection. The program may terminate after the first connection finishes.
• Program behaviour under network failures is undefined; you may crash the entire program, reject the incoming connection, do whatever you want.
• You may assume the remote SOCKS proxy's version (4/4a/5).
• Using libraries/inbuilts/etc. is encouraged, but not required.
• Stay in input scope (f.e. don't proxy all connections on the machine).

Answer 1

Kotlin, 240 bytes

This was fun and taught me some things I didn't know about Kotlin.

import java.net.*fun a(vararg p:InetSocketAddress){ServerSocket().run{bind(p[0])
accept().let{Socket(Proxy(Proxy.Type.SOCKS,p[1])).run{connect(p[2])
Thread{it.inputStream.copyTo(outputStream)}.start()
inputStream.copyTo(it.outputStream)}}}}

Tested locally with:

fun main(args: Array<String>) {
    thread { a(InetSocketAddress(2000), InetSocketAddress(3000), InetSocketAddress(4000)) }
    thread { Socket().connect(InetSocketAddress(2000)) }
    ServerSocket(3000).accept().getInputStream().run { generateSequence { read() } }
            .forEach { print("$it,") }
}

Prints 5,2,0,2, which confirms an attempted SOCKS5 connection.