upstream status (no replies)

Hi,

I am using nginx as proxy with two upstream servers. In the access log, I
log the upstream_address, upstream_status, status (downstream), a special
response header from upstream, etc.

A few times I see in the log upstream_address: server1:port, server2:port
with upstream_status: 504, 502 status: 502 special header: -
Since there is the special response header, I assume nginx didn't get any
responses from either upstream servers. Why would upstream_status shows 504
and 502? Those code must not come from upstream, is it generated by nginx
self? I thought it would be -, - since there are no upstream_status
returned from upstream.

Thanks!
Frank
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