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In order to understand how TCP works, I tried to forge my own TCP SYN/SYN-ACK/ACK (based on the tutorial: http://www.thice.nl/creating-ack-get-packets-with-scapy/ ).
The problem is that whenever my computer recieve the SYN-ACK from the server, it generates a RST packet that stops the connection process.
I tried on a OS X Lion and on a Ubuntu 10.10 Maverick Meerkat, both reset the connection. I found this: http://lkml.indiana.edu/hypermail/linux/net/0404.2/0021.html, I don't know if it is the reason.
Does anyone could tell me what could be the reason? And how to avoid this problem?
Thank you.
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In TCP, packets with the "Reset" (RST or R) flag are sent to abort a connection. Probably the most common reason you are seeing this is that an SYN packet is sent to a closed port. But RST packets may be sent in other cases to indicate that a connection should be closed.
A TCP Reset (RST) packet is used by a TCP sender to indicate that it will neither accept nor receive more data. Out-of-path network management devices may generate and inject TCP Reset packets in order to terminate undesired connections.
The TCP RST flag resets the connection. It indicates that the receiver should delete the connection. The receiver deletes the connection based on the sequence number and header information.
In the case of a RST/ACK, The device is acknowledging whatever data was sent in the previous packet(s) in the sequence with an ACK and then notifying the sender that the connection has closed with the RST. The device is simply combining the two packets into one, just like a SYN/ACK.
The article you cited makes this pretty clear...
Since you are not completing the full TCP handshake your operating system might try to take control and can start sending RST (reset) packets, to avoid this we can use iptables:
iptables -A OUTPUT -p tcp --tcp-flags RST RST -s 192.168.1.20 -j DROP Essentially, the problem is that scapy runs in user space, and the linux kernel will receive the SYN-ACK first. The kernel will send a RST because it won't have a socket open on the port number in question, before you have a chance to do anything with scapy.
The solution (as the blog mentions) is to firewall your kernel from sending a RST packet.
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