when will the netfilter conntrack helper will be called?

Hai,
when will the netfilter conntrack helper funtion will be called
for a router? It will be in input chain, forward chain or output chain
for the first packet detected matching the helper registration
parameters.

i printed skbuff contents for TFTP which i found after SNAT done .will
it not be before doing NAT?..can any one guide me if i am wrong

My setup is
client router server
192.168.0.1 10.145.2.23(wan) 10.145.6.78

c2b682a4: 17 192.168.0.1:32770 -> 10.145.6.78:69
rep tuple c2b682bc: 17 10.145.6.78:69 -> 10.145.2.23:32770
and skbuff contents as 10.145.2.23:32770 -> 10.145.6.78:69

can i directly change tupule contents to redirect the packet to any
ipaddress or port?

regards,
murugan