smoothwall iptables

[driechers]

i am building a firewall with smoothwall and i need to limit the packet rate so i made a script along the lines of

iptables -A INPUT -p tcp --syn -j syn-flood
iptables -N syn-flood
iptables -A syn-flood -p tcp -m multiport --dports 81 -m length --length 64 -j DROP
iptables -A syn-flood -m limit --limit 3/second --limit-burst 4 -j RETURN
iptables -A syn-flood -j DROP

i think that this should work but when i try to run it says

iptables: no chain/target/match by that name

i think that i have narrowed the error down to line 4 but i don't know how to fix it does anybody know what i should do to get this to work?