Re: TCP SYN attacks - a simple solution
Original message <199610062314.TAA29781@merit.edu> From: rex@staff.cs.su.oz.au (Rex di Bona) Date: Oct 7, 8:10 Subject: TCP SYN attacks - a simple solution
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I propose a solution where the initial sequence number is calculated (not random), and is based on a cryptographic calculation of the senders Initial Sequence Number, the ports, and a "per boot" secret number. In this way the initial packet can be discarded, and on receipt of the third SYN packet can be recalculated. ...
The idea has been floated before, and I believe it to be the right solution to this problem. However, I have some suggested improvements: 1. The use of a "per boot" secret number allows an attacker to poll your machine to deduce the secret, and then attack you with that knowledge. A solution to this problem is to use a rapidly changing secret, the pattern of which cannot be easily deduced, and a sliding window of acceptance. (If the hash doesn't match the current scheme, but matches the scheme we were using in the past N seconds, then accept the packet) The change interval needs to be short enough that, by the time an attacker has been able to compute the next number, the window for accepting that has closed. 2. The TCP specification allows data to ride along with the initial SYN, and requires that after the three-way handshake, that the data be presented to the application layer. One solution is to realize that very few implementations take advantage of this, and simply not accept this form of SYN. A second solution is to NOT ack the data that is riding along, but the TCP state machine definition has some holes here which may not work for all implementations. The solution of buffering those SYN's which contain data is unacceptable, because attackers will simply switch to sending SYNs with data. I believe that switching to this sort of scheme would also thwart most attacks which rely on sequence number prediction, and save memory significantly over schemes which simply increase the number of TCBs allowed. -matthew kaufman matthew@scruz.net ps. I've been meaning to write this entire scheme, with the enhancements I propose here, as a draft specification, but I keep getting interrupted by flooded phone rooms and the like this weekend. *sigh*
The idea has been floated before, and I believe it to be the right solution to this problem. However, I have some suggested improvements:
1. The use of a "per boot" secret number allows an attacker to poll your machine to deduce the secret, and then attack you with that knowledge.
A solution to this problem is to use a rapidly changing secret, the pattern of which cannot be easily deduced, and a sliding window of acceptance. (If the hash doesn't match the current scheme, but matches the scheme we were using in the past N seconds, then accept the packet)
The change interval needs to be short enough that, by the time an attacker has been able to compute the next number, the window for accepting that has closed.
I figure that if you steal 4 to 12 bytes for mss storage, 2^20 is still enough possibilities that you can just rotate your secret every minutes and test against the old one for 30 seconds...
-matthew kaufman matthew@scruz.net
Yes.
ps. I've been meaning to write this entire scheme, with the enhancements I propose here, as a draft specification, but I keep getting interrupted by flooded phone rooms and the like this weekend. *sigh*
Hopefully there will be a working implementation of this by week's end. Jeff Weisberg has code which runs on sun3s and (soon, I hope) on other Suns under SunOS. This has always seemed to me to be the best way to do things, though an OS patch to go to hashed-entry into arrays of PCBs is a definite win to back-implement into SunOS (for example) in general. Avi
participants (2)
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Avi Freedman -
matthew＠scruz.net