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As the title says, I would like to send data using an existing tcp connection. Said connection has already been established by a 3rd party program. I haven't been able to find much information about this, and it's safe to say I don't know how this will work at all.

The operating system is Windows. My preferred programming language is python - I'd prefer not to use 3rd party python modules, but I will if they make my life easier.

Just to clarify, in case you aren't sure what I want to do: I want to send data as if it were sent by a different program; pretty much like WPE pro's send function does.

Update:

Technically, couldn't I manually design the TCP packet and then tell the network device (or operating system) to send that packet? Wouldn't that be exactly the same thing an injected socket would do?

Edit: Wikipedia says the receiving host acknowledges packets it receives, which makes this a bit more difficult. But if can drop that acknowledge-packet before the 3rd party program receives it, then this should work. Right?

Aran-Fey
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  • I think you'll find this is pretty close to impossible. Your 2 best bets would be to take control of the process that owns the connection (attach yourself as a debugger) or act as a proxy from the start (make 2 TCP connections with your software sitting in the middle). – Celada Nov 07 '12 at 16:41
  • Acting as a proxy would be acceptable. But how would I do that? – Aran-Fey Nov 07 '12 at 16:46
  • The client would connect to you, and then you connect to the server. You pass data back and forth between the two TCP connections faithfully *except* when you want to make intentional changes. – Celada Nov 07 '12 at 17:05
  • Yes, I got that. How do I code it? Specifically: How do I make the client connect to me? – Aran-Fey Nov 07 '12 at 17:34
  • Obviously, that depends on your "3rd party program". I guess this program must have a way to specify the IP address and port number that it connects to? – Celada Nov 07 '12 at 19:14
  • You mean like a config file where it reads the ip and port from? It doesn't have that, but I know the destination ip and port; the source port seems to change every time, though. – Aran-Fey Nov 08 '12 at 08:00
  • Well then obviously if the client isn't configurable then you'll have to see if you can arrange to have your code listen where the server normally listens and have the real server itself listen somewhere else. Or use NAT to change the IP address it connects to. There are lots of ways of configuring your system. This is getting into the specific configuration of your local network and 3rd party software, WAY out of scope. – Celada Nov 08 '12 at 16:45

2 Answers2

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Scapy/Pcapy are pretty powerful tools for monitoring and injecting packets into a live network interface. I've used them for several projects. These tools are ideal for stimulus/response low-level network protocols (ie DHCP, DNS, etc) and anything non-stateful sent over simple UDP.

Unfortunately, the TCP layer is very complicated and stateful. So injecting something that makes sense into the stream will be more difficult. Moreover, Scapy/Pcapy do not currently have support for tcp.

Celada
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Doug T.
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    Injecting 'something that makes sense' shouldn't be difficult in my case. There's no large data being transferred, so I won't have to mess with sequence numbers. The biggest problem would be dropping the receiving host's acknowledge packet. – Aran-Fey Nov 08 '12 at 08:20
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A TCP session is not intended to be a many-to-one connection. Its a point-to-point stateful protocol which keeps track of packets that have been sent versus those that have been received by the other end. I don't believe you can inject yourself into an already-established session. Your best bet, as was pointed out previously, is to create a proxy and act as a man-in-the-middle interloper. Still not a trivial thing but doable.

sizzzzlerz
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