This is true for other out-of-state floods too. This flood could also be used as a smokescreen for more advanced attacks. By continuously sending TCP-SYN packets towards a target, stateful defenses can go down (In some cases into a fail-open mode). ![]() Instead, the connection remains pending until it times out, ultimately exhausting resources and causing the server to go offline. It begins with the attacker sending a message to the targeted server, that responds with a “SYN ACK” (synchronize acknowledgment) message signaling receipt and awaiting the connection to be completed by the requesting machine (the attacker). This leaves the TCP backlog saturated and the server and/or daemon attacked will not be able to receive any new connections. The attack is aimed at consuming connection resources on the backend servers and on stateful elements, like firewalls and load balancers by sending numerous TCP-SYN requests toward targeted services while spoofing the attack packets source IP (Internet Protocol). What is a SYN Flood DDoS Attack?Ī SYN (Synchronization) flood, generally caused by botnets, is a form of attack that targets server resources via the firewall or perimeter defenses. So let us start by aligning on what a SYN flood DDoS attack is. As DDoS attackers continue to change and vary their strategies and methods, it becomes important to truly understand one’s network vulnerabilities to damaging DDoS attack s. One of these vectors, a common one, is the SYN flood. When I saw the huge number connections between Chinese IPs and the outside interface of the ASA I assumed we were being hit and I overlooked the fact that the source and targets were pointing to outgoing connections not incoming.There are various DDoS vectors that cause networks to crash, resulting in downtime for enterprises. So I quickly discarded an infected PC on LAN as the problem. In this case the PAT table looked normal. Usually the FIRST thing I do in these situations is to check the PAT table to look for infected PCs. ![]() When the PC was taken offline and fixed the problem dissapeared. Further investigation found a compromised machine on the LAN with hacking tools installed. It looks like we were being used to launch attacks. It was _not_ the target as I was thinking because I was misreading the ASA info. The _source_ of the 10,000+ connections was the PAT address of the ASA. Just for grins, I did send an email and it promptly came back marked "mailbox full" which is quite funny I thought.Ģ) Will adding the IPS module help here? I am hoping that the processing of the dropped packets would move to the module and leave the main processor of the ASA free to do its usual NAT and firewall functions.Īfter working a bit more on this it turns out my diagnosis was incorrect. They suggest I email the abuse mailbox from the offending ISP. Besides, the feedback from people who have tried this doesn't seem too convincing.ġ) My ISP is unwilling and/or unable to do anything. Seems like just basic DoS of our service. The attacks don't seem to be pinpointing any particular server or service. I have seen some examples on using service policies to set connection and embryonic limits but I don't think they will work for me because the attacks come from several IPs and use several different ports. It converts the 10,000 connections into 12-15k dropped packets per second which doesn't crash the box but pretty much makes it unusable. I am currently using the shun command to try to mitigate the problem but it is not much help. The attack easily brings down the 5505 by hitting the 10,000 connection limit of the box. Our 5505 is currently being hit by a SYN attack from surprise, surprise, China.
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