Nginx returning 414 even when large_client_header_buffers is set

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It appears that no matter how big I set the value of
large_client_header_buffers to be, nginx just doesn't care of the
setting and still returns 414 on a long request. 

I tried to make it 16k, 32k, 256k, and 512k, etc and POSTing a request
with 1.5k long URL returns 414. It works when I reduce the request URI
length to about 1k, regardless of the large_client_header_buffers value
as well.

I also tried to set proxy_buffers to a large value and didn't help.

Is there any other settings I need to look at to make the nginx box take
longer request URI without returning 414?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,225093,225135#msg-225135