Well dunno how to start but i will try to explain what is going on.
On my website i have lots of users and lots of haters. Some of the haters were trying to shutdown my website but was unsuccessful. Until now! . Some one figure out if they do Ddoss-dos attacks from irc with spoofed ip's and with a lot of bandwidth, they would bring my connection down to 0 . Which happened

Well i have tried to put some iptables rules and hoped that will help but no luck still.

I'm using debian with the latest 2.4.x kernel...

Here are some of the log's that i am getting from netstat...
tcp 0 0 my.url.com sh cncln.online.ln.cn:1945 SYN_RECV
tcp 0 0 my.url.com sh cncln.online.ln.cn:1986 SYN_RECV
tcp 0 0 my.url.com sh 198.105.47.159:1205 SYN_RECV
tcp 0 0 my.url.com sh cncln.online.ln.cn:1650 SYN_RECV
tcp 0 0 my.url.com sh 198.105.47.104:1919 SYN_RECV
tcp 0 0 my.url.com sh 198.105.47.104:1919 SYN_RECV


Thank You a big time.

Setup a firewall! Iptables with following enable in it
Enable flood control
Drop all Spoofing and bad addresses
Do Sanity check and drop
Make sure NEW incoming tcp connections are SYN packets
Drop all incoming fragments
Drop all incoming malformed XMAS packets
Drop all incoming malformed NULL packets
Bad incoming source ip address 0.0.0.0/8
Bad incoming source ip address 127.0.0.0/8
Bad incoming source ip address 10.0.0.0/8 and all other private ips
etc

May also want to check out Apache's mod_dosevasive. It has some tools to defend against DOS attacks. But the more important thing is to setup the firewall as mentioned above. Probably the most important rules would be:This will limit the amount of SYN packets coming into your server.

thanks for the quick reply,

i do have iptables set up and use all kinds of settings that i red of the internet

iptables -A INPUT -s any/0 -p tcp --syn -j DROP
echo 1 > /proc/sys/net/ipv4/tcp_syncookies
echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts /for smurf etc..
sysctl -w net.ipv4.tcp_max_syn_backlog="2048" to max the open pipes..

but im unsure how to setup the other things u mentioned.. Sanity check, xmas, null...

But what I've seen is that even if i drop the syn_ack they still get recieved, so there is no chance of saving that bw but it still succedes in killing my connection and makes it almost impossible to ssh to the box



TruckStuff:
* The apache is off, this is towards the ssh port or any port that they find open...

* should i really RETURN the syn requests? isn't it better to DROP that to?

Setup firewall using www.giptables.org

It provides good protection against all kind of TCP SYN-flooding Denial of Service attacks.

It provides good protections against IP spoofing.

I'm using same on more than 4 very very busy servers and they works

i have tried but i got some´errors when i run it..

etc/rc.d/init.d/giptables: line 39: /etc/rc.d/init.d/functions: No such file or directory
/etc/rc.d/init.d/giptables: line 42: /etc/sysconfig/network: No such file or directory
/etc/rc.d/init.d/giptables: line 45: [: =: unary operator expected
Starting up GIPTables Firewall: /etc/rc.d/init.d/giptables: line 92: success: command not found
/etc/rc.d/init.d/giptables: line 93: failure: command not found

The firewall still works tho, but i never like error messages when it comes to a fw that´s supposed to save my ass

any suggestions ?

mod_dosevasive is better , preventing web based attacks.

yea i bet it is if that was the problem

This works without problem on RHL/RHEL may be some code need to change on Debian

do you have syn_cookies enabled?
i don't know where it is in debian but in red hat you hav to edit /etc/sysctl.conf and add the following line:
net.ipv4.tcp_syncookies=1
or try: #sysctl -w net.ipv4.tcp_syncookies=1

if you limit from iptables the rate of the incoming requests (syn bit set) you block the legitimate users too

we'll see how this works out. I'm very thankful for this greate help.

And yes the syn cookies are enabled as I pasted it in my 2:nd post..

well i am still on it to check out

btw i am still open to take some suggestions from profesionals.

sorry, i haven't read it carefully
i really don't know other protection from synflooding but syn_cookies