How restrict concurrent parallel session on Linux

You can restrict the parallel sessions made by external servers. Below the steps:

1. Log as root user

2. View the current rules:

iptables -L 

View the numbered rules:

iptables -L --line-numbers  view the numbered rules

3. Edit rc.local file(adding static rule):

vi /etc/rc.local

For example add rule which allowing only 5 concurrent sessions on port 80 from source IP 192.168.6.1:

iptables -A INPUT -s 192.168.6.1 -p tcp --syn --dport 80 -m connlimit --connlimit-above 5 -j REJECT --reject-with tcp-reset

4. Add in terminal (adding dynamic rule):

iptables -A INPUT -s 192.168.6.1 -p tcp --syn --dport 80 -m connlimit --connlimit-above 5 -j REJECT --reject-with tcp-reset

That's it!

If you delete any rule just use below command:

iptables -D INPUT N

where N is the number of rule. You can find the number of rule by using command:

iptables -L --line-numbers