[ISN] Defending against SYN-flood DoS attacks

From: InfoSec News (isnat_private)
Date: Mon Aug 27 2001 - 05:54:14 PDT

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    By Thomas C Greene in Washington
    Posted: 25/08/2001 at 00:39 GMT
    A SYN flood is perhaps the most efficient packet attack, devouring the
    greatest amount service with the least effort. It fakes the initial
    handshake of a TCP connection with spoofed IPs which the target
    machine is unable to answer.
    Establishing a TCP connection requires the exchange of three packets:
    the first with a SYN (for SYNchronise) bit from the surfer, then
    SYN/ACK in return from the Web server, and finally ACK (for
    ACKnowledge) back from the surfer. The connection is then established;
    but if there's a delay in completing the handshake, the server
    re-tries (sending SYN/ACK) several times, and waits with the necessary
    resources to accept the connection already allocated.
    Re-try and timeout periods can add up to over three minutes per bogus
    connection, so it's easy to see how even a modest flood of
    unanswerable SYN packets can overwhelm a server in short order.
    Because the handshake is a necessary part of normal Net traffic,
    malicious SYN packets are difficult to filter. You can cope with an
    attack by changing the number of times your machine will re-try the
    SYN/ACK response, but you'll also deny legitimate connections if you
    get too aggressive.
    With this difficulty in mind, TechMavens' Ross Oliver decided to
    benchmark several hardware solutions, all in roughly the same price
    range, using a homebrew kit to simulate SYN floods against them. He
    released his results at last week's USENIX Security Symposium in
    He established a baseline for his test server (Apache over Red Hat
    7.1), which, when unprotected, crashed at 100 SYNs/sec. The worst
    performers were the Cisco PIX firewall and Checkpoint's Firewall-1
    equipped with the SYNDefender module.
    The Cisco kit showed no advantage whatsoever, crashing at the baseline
    100 SYNs/sec. Firewall-1 showed only marginally better results,
    breaking down (i.e., dropping connections) at a lame 500 SYNs/sec,
    which can be exceeded by only two or three boxes connected by T1,
    cable or DSL lines.
    It's fair to note that while one expects at least some protection from
    any firewall, the Cisco kit isn't marketed for SYN flood protection as
    the Checkpoint kit obviously is.
    Netscreen's Netscreen-100 fared better, breaking down after 14,000
    SYNs/sec for a 28-fold performance improvement at roughly the same
    Only the Top Layer AppSafe switch exceeded the test's limits, showing
    no sign of stress while sustaining 22,000 SYNs/sec, the maximum Oliver
    could throw at it with his rig. This would work out to about one
    dollar per SYN during a fairly severe attack, which strikes us as
    rather economical protection.
    Of course we asked Top Layer if they had any idea where the AppSafe's
    performance might top out. Marketing Director Dennis Anglin told us
    they're currently benchmarking it (and, we'll bet, tweaking it), but
    haven't got any solid numbers just yet.
    The switch distinguishes 'normal', 'suspicious' and 'malicious'
    traffic according to user-defined rules, and can be configured to lock
    out troublesome IPs for anywhere from fifteen seconds to over a week.
    We look forward to learning just how much punishment it can take. If
    any of our readers using it have anecdotal data to pass along, we'd
    love to see it.
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