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.
Allow 2 telnet connections per client host
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iptables -A INPUT -p tcp --syn --dport 23 -m connlimit --connlimit-above 2 -j REJECT |
You can also match the other way around:
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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)
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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)
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ip6tables -p tcp --syn --dport 80 -s fe80::/64 -m connlimit --connlimit-above 16 --connlimit-mask 64 -j REJECT |