On Sat, 20 Oct 2001, Fernando Gont wrote:
> Hi! > > I've read some explanations about the SYN flood DoS attack. > I understand that when the attacker fills the listening queue of the > attacked host with incomplete connections, the attacked host will not > reply to any SYN it receives after that. That's an old explanation; basically any OS released in the last few years will throw old/random connections out of the queue when it fills up. What you actually describe here is more "spoofing" than a syn flood; a syn flood just involves blasting large numbers of syn packets at someone, with no intent of actually spoofing a connection. When Mitnick / anyone else did spoofing, it was through weaknesses in the algorithm used to generate initial sequence numbers, not through simple brute force. (I'm assuming that's how Mitnick did it; I'm not aware that he has revealed exactly how he did anything, and I doubt I'd trust him even if he did explain.) > However, I don't understand why it will not even reply with an RST > when it receives a SYN-ACK from other machine. In the general case of spoofing, you could ensure that a syn-ack does not elicit a rst by spoofing the IP of a nonexistant host. I've also read that older tcp stacks could be caused to stop emitting packets by filling their SYN queue; I'm not sure when that stopped applying. These days, spoofing really isn't a concern; those looking to find a way into a system wouldn't gain anything by spoofing and would instead look for a buffer overflow to exploit. Syn floods are alive and well, though. Mike "Silby" Silbersack To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-net" in the body of the message
