Slair1 commented on issue #2680: Using Source NAT option on Private Gateway 
does not work
URL: https://github.com/apache/cloudstack/issues/2680#issuecomment-396407405
 
 
   @rhtyd /cc @ustcweizhou 
   
   I think you're right @rhtyd, it isn't obvious, but when a guest VM attempts 
to communicate with another guest VM's public IP, the packet flows into 
iptables and then hairpins back out.  Without the SNAT in place, the reply from 
the destination VM would go straight to source VM.  Which breaks any stateful, 
protocol such as TCP, since the source IP in the reply packet will be the 
private IP of the original destination VM while the original source VM is 
expecting the source IP to be the public IP...  If that makes sense... if not 
let me know and i can explain more.
   
   I'll take a look and update the code some!

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