use iptables to limit denial of server DoS

mfoley · 11-12-2017, 09:31 AM

I'm looking to use iptables to help limit denial of service attacks to my webserver. I'm no iptables expert, so I'd like the input of some experts on my idea. I'm thinking about the following. Will it work? Are there improvements to be had or a better idea generally? The idea is to block IPs having 50 or more hits per second.

Code:

/usr/sbin/iptables -N webdrop
/usr/sbin/iptables -A webdrop -j LOG --log-level 6 --log-prefix "Web DoS attempt "
/usr/sbin/iptables -A webdrop -j DROP
    
/usr/sbin/iptables -N webcount
/usr/sbin/iptables -A webcount -m recent --set
/usr/sbin/iptables -A webcount -m recent --rcheck --hitcount 50 -j webdrop
/usr/sbin/iptables -A webcount -j RETURN

/usr/sbin/iptables -A INPUT -p tcp --syn -m multiport --dports 80,443 -i eth0 -j webcount
/usr/sbin/iptables -A INPUT -p tcp --syn -m limit --limit 1/s --limit-burst 3 -i eth0 -m multiport --dports 22,443 -j ACCEPT

camp0 · 11-13-2017, 10:35 AM

The iptables rules looks fine but may be other people have better suggestions, on the other and if you have an apache server behind you can have a look to http://dominia.org/djao/limitipconn.html, im sure that nginx have something similar.

For have a good approach to DoS attacks is to act at network layer(iptables) and at application layer (mod on apache or whatever).

mfoley · 11-13-2017, 11:40 AM

Quote:

Originally Posted by camp0

The iptables rules looks fine but may be other people have better suggestions, on the other and if you have an apache server behind you can have a look to http://dominia.org/djao/limitipconn.html, im sure that nginx have something similar.

For have a good approach to DoS attacks is to act at network layer(iptables) and at application layer (mod on apache or whatever).

Thanks - I'm trying the mod_limitipconn now, although I'll have to contrive to get more than 10 simultaneous connections going.

I'm still looking for some feedback on the iptables rules before I try those.

lazydog · 11-13-2017, 10:33 PM

I would do something like this:

Code:

iptables -N webcount
iptables -A webcount -m recent --set --name webcount --rsource
iptables -A webcount -m recent --update --seconds 3600 --hitcount 50 --name webcount --rsource -j LOG --log-level 6 --log-prefix "Web MAX Connections Reached: "
iptables -A webcount -m recent --update --seconds 3600 --hitcount 50 --name webcount --rsource -j DROP
iptables -A webcount -j ACCEPT

iptables -A INPUT -i eth0 -p tcp -m conntrack --ctstate NEW -m tcp -m multiport --dports 80,443 -j webcount
iptables -A INPUT -i eth0 -p tcp -m conntrack --ctstate NEW -m tcp -m limit --limit 1/s --limit-burst 3 -m multiport --dport 22 -j ACCEPT

Don't forget to add your ESTABLISHED and RELATED rules also.

mfoley · 11-19-2017, 10:12 PM

lazydog: your rules look more succinct, only one new chain. What does the --update --seconds 3600 do? Does it un-DROP the blocked IP after one hour?

(I do have the ESTABLISHED and RELATED rules define before this snippet)

lazydog · 11-21-2017, 06:05 PM

--update: Updates the recent counter in webcount.
--seconds: This is the timer for the recent counter in webcount.

So in other words when a packet comes in for ports 80 or 443 they go to the webcount chain. This chain then looks at the counters in webcount for the ip address if it is not there it is added by update. If it is there the last time is updated with the present time and the the count is checked against the timer seconds. If the hit count is above hitcount and the time the ip was last seen is under seconds the rule is applied.