Who Is at the Door: The SYN Denial of Service

How to survive the SYN attack on a TCP/IP protocol weakness.

To lessen the severity of this attack, all providers should install the proper filters to prevent packages from leaving their network with forged source addresses, known as IP spoofing. This can be done by preventing packets that have a source address from outside your network from leaving your network.

Because the Linux kernel source code is under the GNU Public License (GPL), anyone with a copy of Linux is entitled to the source code. Having the source code, a user can apply a fix to his kernel and recompile it. If you were using a proprietary operating system, you would be at the mercy of your operating system vendor.

One of the easiest ways around this problem is to increase the size of the queue. This has been done in the 2.0.x kernels. If the queue is made large enough, it becomes more difficult for hosts with slow connections to the Internet (dial up, dynamic IP connections) to flood enough packets to prevent normal connections.

For your network servers to take advantage of the larger queue, they must be recompiled with a larger value as the backlog argument for the listen() function. Sendmail and inetd (found in NetKit-B) are two important programs that must be recompiled to “SYN-proof” your system.

A patch from Alan Cox implements random dropping of uncompleted connections, which prevents the buffers from filling, although the number of partially completed connections in the listen queue can increase. This same patch, which has yet to be integrated into the 2.0.x kernels as of patch level 27, also disallows a single class C from using up more than 30% of the queue. This last method prevents attacks from providers who have installed the source filters discussed above and from exploiters who do not use random source addresses.

The patch for the current kernel (2.0.29) can be obtained from http://www.dna.lth.se/~erics/linux.html. To apply it, download and unzip the patch into the /usr/src subdirectory and type

patch < tcp-syncookies-patch-1. When you run make config (or menuconfig or xconfig), you will see two additions under “Networking Options”. Just compile them into the kernel.

Other methods of protection have been suggested on various Internet forums, including creative firewalls that establish the TCP connection and then pass it on. Several companies are marketing commercial products based on these ideas. These solutions are not necessary for Linux users. Network solutions such as those are for users who don't have the option of compiling a fixed kernel.


The Internet is undergoing a massive scaling, and as a result, it is no longer possible to identify a given network administrator. While the Internet protocols were designed for unreliable networks, they were not designed for untrusted networks.

Although the SYN attack has proven very effective in denying service to important servers, the problem is well under control in the Linux world. The combination of a larger queue and the random drop technique makes your Linux-based system relatively immune to this attack.

Douglas L. Stewart works for Pencom Systems Administration and graduated from the University of Mississippi in December. Douglas can be reached via e-mail at douglas@pobox.com.

P. Tobin Maginnis is an Associate Professor of Computer Science at the University of Mississippi.

Thomas Simpson is a graduate student in Computer Science at the University of Mississippi.