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I have a basic idea of what a software-based packet filter does, and I'm not sure if it's entirely accurate. Basically, a packet filter is a loadable kernel module that is inserted into the network stack between the network layer and the transport layer and tells the kernel "Instead of sending incoming packets to the transport layer, send them to me first, and I will decide whether to forward them or not. Also, do the same thing with outgoing packets."

My two questions are:

  1. Is my idea accurate,
  2. How does the packet filter get the kernel to listen to it and bypass its normal packet forwarding behavior, sending all packets to the filter instead?

I'm curious about how such a firewall would actually be implemented (at an abstract level; I'm aware I could just look at the source code for a firewall, but that would be rather tedious).

2

It is not the packet filter which gets the kernel to listen to it, it is the kernel which entrust network data to the packet filter.

The packet filter and the kernel must be compatible, technically speaking the kernel must offer specifics hooks used by the packet filter to do its job.

On Linux for instance, behind iptable front-end you will find that the Linux kernel offers hooks allowing the Netfilter framework to register callback functions:

  • If the firewall is not enabled, there will be no callback registered so the kernel will handle everything by default,
  • If firewall is enabled, Netfilter will have registered its callback functions so the kernel will be able to call them when appropriate.
0

Electrical signals transmitted along the RX/TX wires are read by the network card and translated/transmuted upwards the stack into kernel space where it is interpreted by netfilter. If you lsmod, you can find the kernel modules that do this. Netfilter hooks provide a way to cross from kernel space to user space programs.

It's pretty well worn by now, so the concept of a firewall is a filter based ruleset that can interact with the packet device.

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