Malformed syn packet cause %100 cpu and interrupts FreeBSD 9.0 release

[Chuck Swiger]

On Mar 15, 2012, at 12:49 PM, Seyit Özgür wrote:
> Today we tried to see what happens Malformed syn packets on FreeBSD 9.0 release..
> 
> Those packets rise to CPU %100 and stucks..
> 
> listening on ix0, link-type EN10MB (Ethernet), capture size 65535 bytes
> 18:33:30.010215 IP vgn44-1-88-123-89-40.fbx.proxad.net > 85.xxx.xxx.90: tcp
> 18:33:30.010242 IP 225.74.196.88.sta.estpak.ee > 85.xxx.xxx.90: tcp
> 18:33:30.010269 IP Nnov-Prospekt.71.quantum.rn > 85.xxx.xxx.90: tcp
> 18:33:30.010296 IP host52-108-static.49-88-b.business.telecomitalia.it > 85.xxx.xxx.90: tcp
> 18:33:30.010325 IP 125.Red-88-1-75.dynamicIP.rima-tde.net > 85.xxx.xxx.90: tcp
> 
> i dont know which tool generate those packets.. but as we see i dont see seq, flag, lenth etc.. just this ouput on tcpdump...
> 
> Is there any kernel feature for do NOT process malformed syn packets ??

A firewall can block them before the system will see and try to process them as incoming traffic.

Also, running tcpdump with -X will give both hex and ASCII rendition of the packets, which would be helpful to identify what you mean by "malformed".

Regards,
-- 
-Chuck