From
http://linux.die.net/man/8/iptables
hashlimit
This patch adds a new match called 'hashlimit'. The idea is to have something like 'limit', but either per destination-ip or per (destip,destport) tuple.
It gives you the ability to express
'1000 packets per second for every host in 192.168.0.0/16'
'100 packets per second for every service of 192.168.1.1'
with a single iptables rule.
--hashlimit rate
A rate just like the limit match
--hashlimit-burst num
Burst value, just like limit match
--hashlimit-mode destip | destip-destport
Limit per IP or per port
--hashlimit-name foo
The name for the /proc/net/ipt_hashlimit/foo entry
--hashlimit-htable-size num
The number of buckets of the hash table
--hashlimit-htable-max num
Maximum entries in the hash
--hashlimit-htable-expire num
After how many miliseconds do hash entries expire
--hashlimit-htable-gcinterval num
How many miliseconds between garbage collection intervals
So I think changing hashlimit-mode works, not entirely sure thou