HAProxy only returns error 500 in forward proxy mode

Consider this minimal HAProxy 2.0 configuration

defaults
    timeout connect 5s
    timeout client 5s
    timeout server 5s
    option http_proxy
    mode http

frontend proxy_in
    bind :8000
    default_backend proxy_out

backend proxy_out
    option http_proxy

I would have expected that HAProxy would listen on port 8000 for HTTP Proxy requests (with the limitation of not resolving IP addresses according to the documentation.

Unfortunately, I do only get errors 500 back:

$ curl -v -x localhost:8000 http://1.1.1.1
* Rebuilt URL to: http://1.1.1.1/
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 8000 (#0)
> GET http://1.1.1.1/ HTTP/1.1
> Host: 1.1.1.1
> User-Agent: curl/7.54.0
> Accept: */*
> Proxy-Connection: Keep-Alive
>
< HTTP/1.1 500 Internal Server Error
< content-length: 96
< cache-control: no-cache
< content-type: text/html
< connection: close
<
<html><body><h1>500 Internal Server Error</h1>
An internal server error occured.
</body></html>

Am I misunderstanding the http_proxy option or is there another issue in my configuration?

Solution

This looks like to the bug which was just fixed BUG/MEDIUM: http/htx: unbreak option http_proxy. You can try to build a version with this fix or wait to the next version.