SYN only packets

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

Hi,

The main problem with SYN dos's is the fact that, if is big enough, eats
machine resources, no matter what you can do at layer 3, it beats you down at layer 1, cos every time the nic receives a packet, generates an interrupt, and if you are receiving > 100.000 pps, simply your system collapses in a few seconds. Against that, but not yet well implemented in Linux, you can use polling (as, for example, freebsd can do), but this still won't save you if the attack is big enough. Your only solution is then asking your uplink to ratelimit the amount of syn per second arriving to you.

Regards,

- Ruben

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. . . .