srcds DOS attack?

[c400]

i have one problem
i have the next realization of my home net
internet-------(linux router with ipTables (NAT+port forwarding))--------CSS_Server
if i do this on CSS_Server then everything works like a charm
///////iptables -N logattacker
///////$IPT -A INPUT -p udp -m udp --dport 27015 -m length --length 0:32 -j logattacker
///////$IPT -A INPUT -p udp -m udp --dport 27035 -m length --length 0:32 -j logattacker
///////$IPT -A logattacker -j LOG --log-prefix "SRCDS:ATTACK: " --log-ip-options -m limit --limit 2/sec
///////$IPT -A logattacker -j DROP

but i want to use the rule on my router machine:
what should i change? Should i change INPUT on POSTROUTING or do something else?
Because jus using those rules without any changes on the router machine does not work