Hi all,
Sorry about sending the question to both lists cause I am not sure which
one is the best list.
I am using the latest libnetfilter_queue and other dependencies
(downloaded a couple of days ago).
I've spent some time trying to rewrite the packets (specifically, the
source IP address or the destination IP address) and reinject it back to
the network. I think I got the code right. However, after I did the
modification on the packets, although with the correct checksum at both
IP and TCP layer (I have tried several fixed packet headers where the
checksum is known beforehand and make sure checksum is computed
correctly), the packet was not successfully reinjected (not captured by
tcpdump).
The weird thing is that they fail deterministically but randomly (to me)
for different IP addresses. For example, if I rewrite the destination IP
to 1.1.1.1, it succeeded, but if I rewrite the destination IP to
1.1.1.2, it then failed. And for source IPs, it always fails. I've
tested the other fields like TTL or source port and destination port, it
always succeeded.
One possibility that can partially explain this is that my machine is
not allowing spoofed IP address. But then I use hping to spoof other IPs
and the packets are indeed captured by tcpdump (I also checked my
iptables configuration). I then think if hping is using a different
mechanism to send out packets than libnetfilter_queue. But I couldn't
find the answer until I dive into the corresponding kernel module (I
think hping is just using raw socket). I am wondering if there's a way
to get the result of the injected packet (whether or not it is delivered
out). I don't know what the return value of nfq_set_verdict() means, it
is always some value larger than 0 (e.g. 32, 72 ...).
Can anybody shed some lights on this? Thanks a lot! I can post my code
here if needed.
Regards.
-Zhiyun
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