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Old 09-08-2008, 07:03 PM   #1
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Cannot send out packets with spoofed source address

Hello everyone,

I've just written a C program to send an UDP packet out to some other computers with the source IP of my choice (not necessarily the IP of my NIC).

I think the program works fine as I tried tcpdump from local host and see all the created packets in the NIC. However, they do not appear on the machine where the packets should be sent, as I used tcpdump there and did not capture anything.

The fact was that, if I use the source IP of the same subnet with my NIC, then the packets would appear in the remote machine, otherwise they disappear. So I suppose my local machine must be preventing packets to be sent out with spoofed source IP somehow. Tried to turn off iptables, SELinux as well as to change various options from /proc/sys/net/ipv4 but didn't get any further info, no log, no error messages, etc.

Can any one give a hint. I'm using Fedora

Thanks a lot for reading.

Old 09-09-2008, 07:48 PM   #2
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Have you tried checking other tools that do similar things (nmap and hping for example) and sniffing the packets with wireshark to see what's happening. I'm assuming you own this network (or at least have permission to do this).
Old 09-14-2008, 12:44 PM   #3
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Excellent hint Steve, finally I discovered that it was the intermediate router that dropped all the packets with spoofed source IP.

Thanks a lot for that. This problem has been irritating me for quite a few days.




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