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 iptables connlimit module which allows you to restrict the number of parallel TCP connections to a server per client IP address (or address block).
You can use domain controller to 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