Use haproxy as a reverse proxy with an application behind Internet proxy - haproxy

Question

I need to integrate several web applications on-premise and off-site under a common internally hosted URL. The on-premise applications are in the same data center as the haproxy, but the off-site applications can only be reached via a http proxy because the server on which haproxy is running has no direct Internet access. Therefore I have to use a http Internet proxy, SOCKS might be an option too.
How can I tell haproxy that a backend can only be reached via proxy ?
I would rather not use an additional component like socksify / proxifier / proxychains / tsocks / ... because this introduces additional overhead.
This picture shows the components involved in the setup:
When I run this on a machine with direct Internet connection I can use this config and it works just fine:
frontend main
bind *:8000
acl is_extweb1 path_beg -i /policies
acl is_extweb2 path_beg -i /produkte
use_backend externalweb1 if is_extweb1
use_backend externalweb2 if is_extweb2
backend externalweb1
server static www.google.com:80 check
backend externalweb2
server static www.gmx.net:80 check
(Obviously these are not the URLs I am talking to, this is just an example)
Haproxy is able to check the external applications and routes traffic to them:
In the safe environment of the company I work at I have to use a proxy and haproxy is unable to connect to the external applications.
How can I enable haproxy to use those external web application servers behind a http proxy (no authentication needed) while providing access to them through a common http page / via browser ?

Answer 1

How about to use delegate ( http://delegate.org/documents/ ) for this, just as an idea.
haproxy -> delegate -f -vv -P127.0.0.1:8081 PROXY=<your-proxy>
http://delegate9.org/delegate/Manual.shtml?PROXY
I know it's not that elegant but it could work.
I have tested this setup with a local squid and this curl call
echo 'GET http://www.php.net/' |curl -v telnet://127.0.0.1:8081
The curl call simluates the haproxy tcp call.

Answer 2

I was intrigued to make it work but i really could not find anything in the haproxy documentation, so i googled a bit and found that nginx might do the trick, but it didn't for me, after a bit more of googleing i ended up finding a configuration for apache that works.
here is the important part:
Listen 80
SSLProxyEngine on
ProxyPass /example/ https://www.example.com/
ProxyPassReverse /example/ https://www.example.com/
ProxyRemote https://www.example.com/ http://corporateproxy:port
ProxyPass /google/ https://www.google.com/
ProxyPassReverse /google/ https://www.google.com/
ProxyRemote https://www.google.com/ http://corporateproxy:port
i'm quite sure there should be a way to translate this configuration to nginx and even to haproxy... if i manage to find the time i will update the answer with my findings.
for apache to work you should also enable a few modules, i put up a github repository with a basic docker configuration that showcases feel free to have a look at that to see the full working configuration.