Limiting number of simultaneous connections

Using connlimit module

We can use connlimit module of iptables to limit number of simultaneous connections to server in following manner

iptables -I INPUT -p tcp --syn --dport 22 -m connlimit --connlimit-above 1 -j REJECT

This will allow only one ssh connection to server per client.

To limit number of simultaneous connections based on subnets we can use below syntax

iptables -p tcp --syn --dport 22 -m connlimit --connlimit-above 16 --connlimit-mask 24 -j REJECT

This would allow only 16 connections from each /24 network.