Server not sending a SYN/ACK packet in response to a SYN packet

Posted by jeff on Server Fault See other posts from Server Fault or by jeff
Published on 2011-08-03T14:37:05Z Indexed on 2012/04/15 23:33 UTC
Read the original article Hit count: 353

Filed under:

linux

|

webserver

|

tcp

Using iptraf, tcpdump and wireshark I can see a SYN packet coming in but only the ACK FLAG is set in reply packet.

I'm running Debian 5 with kernel 2.6.36

I've turned off window_scaling and tcp_timestamps, tcp_tw_recycle and tcp_tw_reuse:

cat /etc/sysctl.conf 



net.ipv4.tcp_tw_recycle = 0
net.ipv4.tcp_tw_reuse = 0
net.ipv4.tcp_window_scaling = 0
net.ipv4.tcp_timestamps = 0

I've attached an image of the wireshark output.

http://imgur.com/pECG0.png

Output to netstat

netstat -natu | grep '72.23.130.104'

tcp        0      0 97.107.134.212:18000    72.23.130.104:42905     SYN_RECV

I've been doing everything possible to find a solution and have yet to figure out the problem, so any help/suggestions are much appreciated.

UPDATE 1: I've set tcp_syncookies = 0 and noticed I am now replying with 1 SYN+ACK for every 50 SYN requests. The host trying to connect is sending a SYN request about once every second.

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