Re: SYN Attacks - how i cant stop it

From: Anton Alin-Adrian (
Date: 02/13/04

  • Next message: Jacques A. Vidrine: "Re: XFree86 Font Information File Buffer Overflow"
    Date: Fri, 13 Feb 2004 18:33:14 +0200

    JJB wrote:
    > You talk about the net.inet.tcp.syncookies=1 knob,
    > how about an description on what it does and why you
    > are recommending using it.

    The net.inet.tcp.syncookies 'knob', if set to 1, enables syn cookies.
    Syn cookies were invented specifically for syn flood protection. A brief
    description of syncookies idea can be read here:

    > How would one go about mirroring back the attackers
    > syn packets to port 80 or 22?
    > Please describe this easy method of yours.

    Mirroring back packets to the attacker is, first of all, a nasty thing.
    Secondly, it is only possible if the attacker's IP is known. If it is
    not known, then obviously it's not possible.

    Knowing the attacker's IP does not necessarly mean that he is performing
    the current attacks from that IP.

    Packet redirection with ipfw is done using divert sockets. One needs to
    have it compiled into the kernel. Divert sockets are also used by ipfw
    nat redirection. It's all in the man pages of ipfw.

    If the flood is severly intense (from the point of stack memory
    exhaution), it might be a good improvement to drop 5% of incoming SYN
    packets. This can also be done with ipfw, and is described in the manual
    pages. However, I don't think one would ever come to this.

    Asking the ISP to put the server behind a decent cisco router, and
    implement syn cookies on hardware devices, is the best protection.

    Alin-Adrian Anton
    Reversed Hell Networks
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