Tuesday, February 16, 2010

Iptables Limits Connections Per IP

How do I restrict the number of connections used by a single IP address to my server for port 80 and 25 using iptables?

You need to use the connlimit modules which allows you to restrict the number of parallel TCP connections to a server per client IP address (or address block).

This is useful to protect your server or vps box against flooding, spamming or content scraping.


Syntax
The syntax is as follows:
$ /sbin/iptables -A INPUT -p tcp --syn --dport $port -m connlimit --connlimit-above N -j REJECT --reject-with tcp-reset
# save the changes see iptables-save man page, the following is redhat and friends specific command
$ service iptables save

Example: Limit SSH Connections Per IP / Host
Only allow 3 ssg connections per client host:
$ /sbin/iptables  -A INPUT -p tcp --syn --dport 22 -m connlimit --connlimit-above 3 -j REJECT
# save the changes see iptables-save man page, the following is redhat and friends specific command
$ service iptables save

Example: Limit HTTP Connections Per IP / Host
Only allow 20 http connections per IP (MaxClients is set to 60 in httpd.conf):

WARNING! Please note that large proxy servers may legitimately create a large number of connections to your server. You can skip those ips using ! syntax
$ /sbin/iptables -A INPUT -p tcp --syn --dport 80 -m connlimit --connlimit-above 20 -j REJECT --reject-with tcp-reset
# save the changes see iptables-save man page, the following is redhat and friends specific command
$ service iptables save
Skip proxy server IP 1.2.3.4 from this kind of limitations:

$ /sbin/iptables -A INPUT -p tcp --syn --dport 80 -d ! 1.2.3.4 -m connlimit --connlimit-above 20 -j REJECT --reject-with tcp-reset

Example: Class C Limitations
In this example, limit the parallel http requests to 20 per class C sized network (24 bit netmask)
$ /sbin/iptables  -A INPUT -p tcp --syn --dport 80 -m connlimit --connlimit-above 20 --connlimit-mask 24 -j REJECT --reject-with tcp-reset
# save the changes see iptables-save man page
$ service iptables save

Example: Limit Connections Per Second
The following example will drop incoming connections if IP make more than 10 connection attempts to port 80 within 100 seconds (add rules to your iptables shell script)

#!/bin/bash
IPT=/sbin/iptables
# Max connection in seconds
SECONDS=100
# Max connections per IP
BLOCKCOUNT=10
# ....
# ..
# default action can be DROP or REJECT
DACTION="DROP"
$IPT -A INPUT -p tcp --dport 80 -i eth0 -m state --state NEW -m recent --set
$IPT -A INPUT -p tcp --dport 80 -i eth0 -m state --state NEW -m recent --update --seconds ${SECONDS} --hitcount ${BLOCKCOUNT} -j ${DACTION}
# ....
# ..

How Do I Test My Firewall Working?
Use the following shell script to connect to your web server hosted at 202.1.2.3:

#!/bin/bash
ip="202.1.2.3"
port="80"
for i in {1..100}
do
  # do nothing just connect and exit
  echo "exit" | nc ${ip} ${port};
done

References:

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