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Restrict parallel connections Using Iptables

connlimit

Allows you to restrict the number of parallel connections to a server per client IP address (or client address block).

[!] --connlimit-above n Match if the number of existing connections is (not) above n.

--connlimit-mask prefix_length Group hosts using the prefix length. For IPv4, this must be a number between (including) 0 and 32. For IPv6, between 0 and 128.

Examples:

Allow 2 telnet connections per client host

iptables -A INPUT -p tcp --syn --dport 23 -m connlimit --connlimit-above 2 -j REJECT

You can also match the other way around:

iptables -A INPUT -p tcp --syn --dport 23 -m connlimit ! --connlimit-above 2 -j ACCEPT

Limit the number of parallel HTTP requests to 16 per class C sized network (24 bit netmask)

iptables -p tcp --syn --dport 80 -m connlimit --connlimit-above 16 --connlimit-mask 24 -j REJECT

Limit the number of parallel HTTP requests to 16 for the link local network (ipv6)

ip6tables -p tcp --syn --dport 80 -s fe80::/64 -m connlimit --connlimit-above 16 --connlimit-mask 64 -j REJECT