TCP source port used by Nginx proxy

[See Hong Kwee]

Hi 

I have been looking to log source port used by nginx proxy to forward the traffic to upstream server. What is available is $remote_addr and $remote_port which capture the client IP address and port, the source port used by nginx proxy to upstream seems to be missing.  

Is there any variable for that ? The use case is for troubleshooting between the proxy and upstream.    
Thanks.
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