set port range for nginx (no replies)

Hi, all
I've got an nginx server in an internal network, and the server will
forward request to outer net using proxy_pass,
it seems like:
proxy_pass http://[public ip]:[port]
Unfortunately, the firewall block most of the port, so the proxy_pass
failed.
I have two questions:
How the proxy_pass use port to forward the request? pick a random port?
Would that be possible to set a port range (10000~20000 e.g) for proxy_pass
to use?

Regards,
Yao
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