Hi... I am currently developing a userspace program using libipq to emulate a satellite link which would require me to inject extra delay and error on packets going through a bridge (with netfilter support). No problem on intercepting the packets going through the bridge.
Also, no problem on incurring extra delay on the packets. But, while i am implementing the extra satellite error, i suddenly found myself confuse over several points. What i need to do is to randomly decide an error in the packet and actually corrupt that bit in the packet. My questions: [1] In the struct ipq_packet_msg, the data_len field is referring to what? Is it referring to the length of the whole IP packet (header + payload) or just the header or the payload alone? And, is the payload[0] referring to the first byte of the IP packet? [2] I have already managed to randomly decide a bit to corrupt within a packet (which is just inverting the bit) but i'm unsure on how to actually access the packet (header + payload) and invert the bit. [3] I have read somewhere that the kernel would silently / automatically drop the packet if i change the packet without updating the checksum. What i need to know is that, which kernel will drop the packet? My bridge or the receiving workstation? I don't want my libipq program to drop it as i need it to be as transparent as possible. Please enlighten me on the above issues. Really, suddenly, suffering a brain crash. Luckily, Linux doesn't crash so easily! Thank you in advance. Wei Koong CHAI CCSR, University of Surrey, United Kingdom.
