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syn dos vs. http dos

Posted on 2014-01-21
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Last Modified: 2014-02-08
Experts,

What's the difference between a syn dos and an http dos? Or is there not a difference and http dos is just a form of syn dos?
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Question by:trojan81
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Dave Howe earned 1600 total points
ID: 39799546
A SYN dos sends only SYN packets. To understand what that means, you need to understand how TCP works - essentially, for each TCP connection, there is an exchange like this:

C->S  <syn>
S->C <syn-ack>
C->S <ack>

a SYN dos attack sends only the syn, and ignores the reply. as the server creates a table entry in its TCP stack for each connection on receipt of the syn, this attack eats up all available table entries until no further clients can connect.

a http dos attack is different. each will perform a full handshake as above, request a page, and then on the same channel, request a page (again and again) - preferably a dynamic page so that the server has to work to calculate the page again and again, but a static will do.

as each http server can only handle a limited number of threads (either due to cpu/memory constraints or configuration) the repeated requests occupy all the server's capacity and it cannot accept or process any further client connections until the attacker's sessions complete - and as they are continually querying on the same connection, they *never* complete.

the downside (for an attacker) is that http attacks require a full tcp connection, hence cannot be sent from a faked address. SYN attacks, by contrast, can be sent from a faked address (or multiple faked addresses), and hence the responses go elsewhere (making it harder to trace the attacker, harder to block, and saving the attacker bandwidth issues from the responses to its queries)

the classic defense against SYN attacks is the SYNCOOKIE - many vendors (such as cisco) have their own reimplementations of this.
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by:trojan81
ID: 39844835
well said! thank you
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