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I have two networks, one considered secure, lets call it A and one considered insecure, B. The software I need to use, uses HSMS, a protocol based on TCP with no security built in.
Now the problem is that the software has a server a in the secure network A and needs to connect to clients b1, b2, ... in B. The requirement from the software is that I open one port with full TCP access. But that is too risky as the whole network B and the clients can not be trusted.

My question now is, is there a way to use something like an application layer firewall to filter everything based on the protocol so that the open port is not used for anything else.
Especially is there a simple, read not too involved, way to create a protocol filter for HSMS in such a firewall.

Another requirement is that the clients are not allowed to talk to each other through that proxy.

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What's wrong with making a firewall only allow connections to server a on port p in the firewall? The port can't be used to connect to any other systems in network A; since the server on a is expecting that particular protocol, non-compliant clients won't be able to connect; and if you're really worried you can whitelist clients on the firewall so that only clients b1, b2, ... in B can connect to a.

Other options include VPN setups or SSH tunneling.

Another requirement is that the clients are not allowed to talk to each other through that proxy.

In a sane configuration, that's not going to be the case anyway. Beyond that, how do you intend to prevent clients from connecting to each other?

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That doesn't solve the problem. I can't trust the clients. I need to be able to control what content, read only hsms compliant data, is sent to the server a otherwise other programs on the clients could hijack the tcp connection. So if I only proxy the clients can still use to complete tcp connection. –  dignifiedquire Mar 20 '13 at 18:01
    
Generally speaking servers already don't trust clients when it comes to the internet. What can other programs running on the clients do that a proxy is going to prevent? They can't talk to other servers in A, they can't talk to other services on a, so the only thing left is if they send non-compliant data -- what can a proxy do that the service already in place can't? –  afrazier Mar 20 '13 at 18:39
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