I would like to intercept incoming packets on the wifi link and do XOR operations on the TCP payload in transit.

Currently I am wondering if it's possible to use iptables or netfilter to get that type of functionality. I've seen scripts that let you modify TTL headers and compute checksums, but I want to be able to access the data payload itself. Is there an easy solution for ddwrt or openwrt?

Alternatively, is there any similar project with opensource code which I could borrow and tweak?

Read here
https://en.wikipedia.org/wiki/Berkeley_Packet_Filter

BPF runs in kernel space.
You need to write sone BPF code some samples can found in
samples/bpf
in the linux tree

Hello Hyseda,

I too am doing something similar.  Ultimately I want to control incoming data streams, TCP and possibly UDP (nonsensical since UDP is not a stream..) and I intend to do so by managing acks and offered windows in the outgoing TCP.  I actually feel fairly comfortable with that.

My intention is to build a Netfilter module so I can grab the packets going by.

I would be quite interested in sharing the experience with others - I am too tired of working solo.

However, I am have a really hard time getting started.  It has been extremely difficult to get a basic netfilter module built in OpenWrt, I'm lost in include path hell, a problem that I will be describing in a separate thread.

If you are interested, I would not mind starting a side conversation on our efforts.

best,
Andy