If I read your description correctly: - Attacker sends spoofed TCP SYN from your IP address(es) and different src ports, to some TCP servers (e.g. port 80) - TCP servers respond with SYN/ACK ; many servers resend the SYN/ACK hence amplification . - *** your system does not respond *** - Servers may think you're doing SYN-Flood against them, since connection remains in SYN_RCVD, and hence complain. In fact, we don't really know what is the goal of the attackers; they may in fact be trying to do SYN-Flood against these servers, and you're just a secondary victim and not the even the target, that's also possible.
Anyway, is this the case? If it is... may I ask, do you (or why don't you) respond to the unsolicited SYN/ACK with RST as per the RFC? I suspect you don't, maybe due to these packets being dropped by FW/NAT, that's quite common. But as you should understand by now from my text, this (non-standard) behavior is NOT recommended. The problem may disappear if you reconfigure your FW/NAT (or host) to respond with RST to unsolicited SYN/ACK. As I explained above, if my conjectures are true, then OVH as well as the remote servers may have a valid reason to consider you either as the attacker or as an (unknowning, perhaps) accomplice. I may be wrong - sorry if so - and would appreciate, in any case, if you can confirm or clarify, thanks. -- Amir Herzberg Comcast professor of Security Innovations, University of Connecticut Homepage: https://sites.google.com/site/amirherzberg/home Foundations of Cyber-Security (part I: applied crypto, part II: network-security): https://www.researchgate.net/project/Foundations-of-Cyber-Security On Thu, Feb 20, 2020 at 5:23 PM Octolus Development <ad...@octolus.net> wrote: > A very old attack method called TCP-AMP ( https://pastebin.com/jYhWdgHn ) > has been getting really popular recently. > > I've been a victim of it multiple times on many of my IP's and every time > it happens - My IP's end up getting blacklisted in major big databases. We > also receive tons of abuse reports for "Port Scanning". > > Example of the reports we're getting: > tcp: 51.81.XX.XX:19342 -> 209.208.XX.XX:80 (SYN_RECV) > tcp: 51.81.XX.XX:14066 -> 209.208.XX.XX:80 (SYN_RECV) > > OVH are threatening to kick us off their network, because we are victims > of this attack. And requesting us to do something about it, despite the > fact that there is nothing you can do when you are being victim of an DDoS > Attack. > > Anyone else had any problems with these kind of attacks? > > The attack basically works like this; > - The attacker scans the internet for TCP Services, i.e port 80. > - The attacker then sends spoofed requests from our IP to these TCP > Services, which makes the remote service attempt to connect to us to > initiate the handshake.. This clearly fails. > ... Which ends up with hundreds of request to these services, reporting us > for "port flood". > > >
