SYN only packets

James Harrison jamesharrison at
Fri Aug 5 02:53:38 CEST 2005


I've been trying to calm down some DDoS attacks on my server, but i've 
been stymied on how to block them- however, as APF's AntiDOS plugin 
captured and reported to root, the really vigorous ones (The ones it 
catches) have no ACK in their headers. SYN, but no ACK. I've read that 
this is a common technique used by DDoSers, but i'm unsure if anything 
else depends on it.

The plan i'm looking at is possibly blocking all packets with SYN alone, 
no ACK.. would this be possible with iptables, and how would this affect 
other web services?

Here's one of the captured packet messages (MAC/IPs are removed obviously)

Aug  4 21:02:30 ukdsl21 kernel: ** IN_TCP DROP ** IN=eth0 OUT= MAC=** SRC=** DST=** LEN=48 TOS=0x04 PREC=0x00 TTL=116 ID=53191 DF PROTO=TCP SPT=4122 DPT=4899 WINDOW=64240 RES=0x00 SYN URGP=0 OPT (020405B401010402) 

Any ideas?

Sorry if this sounds completely barmy- if it does, do tell :-)

Thanks in advance,
James Harrison

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