The server, while acting as a gateway or proxy, did not receive a
timely response from the upstream server specified by the URI (e.g.
HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
to access in attempting to complete the request.

Note: Note to implementors: some deployed proxies are known to
return 400 or 500 when DNS lookups time out.
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