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Please note I am NOT asking how to protect server from DDoS. What I mean is that if someone runs siege -c 1000 www.example.com on his VPS. That will result in my server serving thousands of requests and slowing down. If I run siege targeted to say, shared hosting, I don't see the extreme drop in responsivness (from 60ms to 2000ms/request) I do on my machine. I don't expect to be able to mitigate big DDoS, but I'd like to be safe from the first script-kiddie discovering siege :) On the other hand I'm OK with the fact that when big amounts of traffic comes from legitimate sources, the response may be slower.

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  • You're not considering differences in latency between a "local" DoS on your own machine and one across the internet on shared hosting. – EEAA Sep 10 '13 at 12:51
  • I used a VPS from different provider to issue the "attack" - so, yes I am... I used it both on my server and on shared server... – Tomáš Fejfar Sep 10 '13 at 17:54
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You can use a firewall to limit the number of concurrent connections and the rate of new connections coming from a network (e.g. a /32 for IPv4 and a /64 for IPv6). Example of what it may look like using iptables:

# Limit number of concurrent connections
-A INPUT -i eth0 -p tcp --syn -m connlimit --connlimit-above 50 -j DROP
# Limit rate of new connections
-A INPUT -i eth0 -p tcp --syn -m hashlimit --hashlimit-name tcp --hashlimit-mode srcip --hashlimit-above 3/sec --hashlimit-burst 7 --hashlimit-srcmask 32 -j DROP

(Same thing for ip6tables except adding --connlimit-mask 64 to the first and changing --hashlimit-srcmask to 64 in the second.)

You can also limit the rate of HTTP requests, for example with the limit_req module of nginx.

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    Just to make it absolutely clear, please make sure to use the DROP target and not REJECT for this. DROP will drop the packets and a sane TCP/IP implementation on a legitimate host will retry after some short delay, whereas REJECT will cause a connection reset on the remote end and cause a large "the web server refused the connection" to be displayed to the user (almost certainly not what you want). – user Sep 10 '13 at 15:17
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Aside from some basic firewalling, we rely on haproxy to do the heavy TCP lifting. We have found that it runs circles around an out of the box Apache HTTPd instance as far as any kind of slowdos attack goes.

-A INPUT -p tcp -m tcp -m state --state RELATED,ESTABLISHED -j ACCEPT

-A INPUT -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j SYNFLOOD

-A INPUT -p tcp -m tcp --dport 80 -m state --state NEW -j HTTPDGUARD
-A INPUT -p tcp -m tcp --dport 443 -m state --state NEW -j HTTPDGUARD

-A HTTPDGUARD -m connlimit --connlimit-above 25 --connlimit-mask 32 -j HTTPDENY
-A HTTPDENY -j LOG --log-prefix "HTTP Flood: "
-A HTTPDENY -p tcp -m tcp -j REJECT --reject-with tcp-reset

-A SYNFLOOD -m state --state NEW -m recent --set --name SYNRATE --rsource
-A SYNFLOOD ! -s 150.156.24.0/24 -m state --state NEW -m recent --update \
   --seconds 5 --hitcount 200 --name SYNRATE --rsource -j DROP

-A INPUT -p tcp -m tcp --dport 80 -m state --state NEW -j ACCEPT
-A INPUT -p tcp -m tcp --dport 443 -m state --state NEW -j ACCEPT

But basically, you either need to block the attack, fix the exploit, or do both.

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fail2ban is one of the many tools that are designed to handle such situation. fail2ban worked for me in stopping couple of brute force attack on ssh. I assume that your server is a linux.

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