when will the netfilter conntrack helper will be called?

[murugan.venugopal at wipro.com]

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


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