Seeking Guidance: Intercepting, Modifying, and Forwarding IP Packets on macOS

Question

Prasanna_ OP

Created 3d

Replies 2

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Hello Apple Developer Community,

I am currently working on a macOS project where my primary goal is to intercept IP packets, modify them (specifically the TCP payload), and then forward them. My intended use case involves selectively intercepting outgoing packets based on their destination IP, altering their content, and sending them on their way to the original destination.

What I’ve Tried:

NEAppProxyProvider: • I explored using App Proxy Provider to handle new TCP and UDP flows. • While it allowed me to read the data, handling direct packet modification and forwarding without creating a new connection or proxy setup proved challenging, especially for maintaining TCP state and handling TLS traffic.
System Extension with NEFilterPacketProvider: • I considered NEFilterPacketProvider for intercepting and modifying network packets. • However, the documentation implies that packet filtering only supports allow/block actions, not modification and reinjection of packets back into the system.
I am planning to try NEPacketTunnelProvider: But the documentation states that this is not the right use case. Packets are expected to go into the tunnel. Since I don't have any requirement to create and maintain a tunnel, this doesn't look like an option for me.
Transparent proxy setups like NETransparentProxyProvider do not appear to offer direct packet modification capabilities without involving a user-space proxy approach.
Implementing packet-level interception outside of the Network Extension framework (e.g., Network Kernel Extension) seems unsupported in newer macOS versions (Sequoia and later).

My Questions:

Is there a recommended approach or combination of Network Extension capabilities that would allow intercepting and modifying IP packets directly?
Can NEFilterPacketProvider or any other extension be utilized in a way to modify and reinject packets back into the system?
Are there any examples or sample projects that achieve similar functionality, possibly using a blend of Network Extension and lower-level networking frameworks?

I appreciate any insights or pointers to documentation or examples that could help achieve this.

Thanks and Regards. Prasanna.

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Answer 1

DTS Engineer OP

Apple

2d

Your summary of the available facilities is pretty accurate. The one thing I’ll note in that it’s rare to see an app proxy provider on macOS. Rather, most folks use a transparent proxy. These are very similar though.

Is there a reason you have to do this at the IP level? As I said, most folks use a transparent proxy, which makes this task easier because you get the TCP flow, and thus you don’t have to mess around with all the low-level TCP gubbins.

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Answer 2

Prasanna_ OP

22h

Hi,

Thank you for your response. You are correct that modifying the TCP flow without dealing with low-level TCP intricacies is beneficial. However, we will face the following challenges and complexities:

Path-specific Modifications: The network path between the source and destination includes transparent devices that make decisions based on the modifications made to the TCP payload. Modifying only the TCP flow may not work for these devices, as it is not guaranteed that each TCP flow corresponds to a single TCP packet.
Complexities with TransparentProxyProvider: From what I understand, using NETransparentProxyProvider requires creating new TCP or UDP connections and acting as a proxy for the applications. This effectively means implementing a full proxy server, which comes with additional complexities such as handling TLS certificates and potentially other aspects of secure communication.

Any insights or guidance on these points would be greatly appreciated.

Thanks, Prasanna

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