Be careful rate-limiting SYNs too much as you can (ironically) degrade performance quite a bit. I would set a high burst limit and a reasonable average limit. I mention this because I did some SYN-limiting on a firewall once and while it protected me from DDoS attacks it also slowed down Apache somewhat. You may be better served attempting to identify SYN floods from specific hosts (-m recent?) and dropping those connections... Derick Anderson -----Original Message----- From: netfilter-bounces@xxxxxxxxxxxxxxxxxxx [mailto:netfilter-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Grant Taylor Sent: Friday, August 05, 2005 1:51 AM To: James Harrison Cc: netfilter@xxxxxxxxxxxxxxxxxxx Subject: Re: SYN only packets Try taking a look at SYN-Cookies in the Linux kernel. This code was written explicitly for this type of situation. You can not stop SYN packets as they are the very first packet in the three way handshake to start a TCP connection. About all you can do (other than SYN-Cookies) is to rate limit the number of packets per source IP. Grant. . . . James Harrison wrote: > Hi, > > 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 >
