got 502 error when accessing http sites about forwardproxy HOT 4 CLOSED

Bug is fixed (updated version is available on caddyserver.com)

from forwardproxy.

Does that happen for all HTTP hosts or for particular few?
Evidently, it fails to DNS resolve the host you're trying to access. You seem to have redacted the host away, so I can't tell why this would happen.
You can try to run host your-http-host.com or dig your-http-host.com on the server with forwardproxy, and see if it's resolvable and if the resolved address is correct. Perhaps, it gets DNS poisoned?

from forwardproxy.

Yes, as long as http sites it reports 502.
My host website on this server is absolutely accessible.
With your direction, I typed ./caddy -host my.host
It returned

Activating privacy features... done.
https://my.host
http://my.host

Looks good, right? Then I visit a http site, for example http://earthquake.usgs.gov/ over forwardproxy, the log are

02/Jul/2018:21:17:20 -0400 [ERROR 502 /] dial failed: Lookup of failed: lookup : no such host
02/Jul/2018:21:17:20 -0400 [ERROR 502 /favicon.ico] dial failed: Lookup of failed: lookup : no such host

from forwardproxy.

It seems like forwardproxy regards the http site I visit is under my domain, http://a.com/b.html as my.host/b.html

from forwardproxy.