How exactly proxy is supposed to behave when it "hangs onto" say 10.000 + unfinished TCP connections ? Will it deny new ones (because resources are always limited) ? Looks like it's the only thing it will be able to do and as soon as first packet is denied, hacker's won. As hard as it is to be implemented the only way to fight this is have every single ISP to filter outgoing packets. Assuming big players have enough desire they can do it quickly by making an offer smaller ISPs can't refuse, the same kind Sprint made when it started filtering small CIDRs. It's certainly harder to catch those who don't comply though ... Interestingly enough, the source IP's could be valid, then it becomes even harder to see if the TCP connection request is valid or bogus. And once again, source filtering by ea and every AS looks like the only solution.
On Mon, 16 Sep 1996, Craig A. Huegen wrote:
The SYN flood coming towards my host X looks like this, at approximately 2,000 PPS:
182.58.239.2.1526 -> 172.30.15.5.80 TCP SYN 19.23.212.4.10294 -> 172.30.15.5.80 TCP SYN 93.29.233.68.4355 -> 172.30.15.5.80 TCP SYN [... on and on ...]
Tell me how to filter this.
The only thing that comes close to the concept of "filtering" is to build a SYN proxy that replies with SYN-ACK and hangs onto SYN packets until the ACK is received from the net before actually letting the packets through to your server. This may require sequence number munging on every packet but that's generally the kind of thing proxies do.