Android version may not use proxy if VPN is running

Android foxyproxy uses the configured proxy if system isn't running VPN, and Firefox will show a warning about proxy refusing connections if that proxy isn't running. But if Android is running a VPN, and I forgot to use additional proxy, Firefox may access internet but not saying proxy is refusing connections. E.g, using system VPN to connect to Tor, Freegate, or I2P, so Tor, Freegate, or I2P is hidden in VPN traffic, but since Android has battery optimization for 3rd-party apps, system may terminate additional proxy to optimize battery usage, or the user just forgot to launch additional proxy, in such case, Foxyproxy may allow Firefox to use Android VPN but not go through additional proxy like Tor, Freegate, or I2P.

[erosman]

VPNs run at system level. Extensions do not have access to such data. FoxyProxy wont be aware if a VPN running on the system or not.

But I tested it by looking up my IP address, system-level VPN with user-level proxy has different IP address shown on some websites, then I terminate user-level proxy but keep VPN on then the IP address changed. So Firefox doesn't go through user-level proxy when VPN is on.

[erosman]

So Firefox doesn't go through user-level proxy when VPN is on.

I will ask Mozilla engineers for verification.

[erosman]

As mentioned VPN is a system level proxy while FoxyProxy handles Firefox routing.

That is how it should work when FoxyProxy is actively proxying the connection.

No VPN
Firefox -> proxy-server (in FoxyProxy) -> destination
Destination should show the proxy-server's IP

With VPN
Firefox -> proxy-server (in FoxyProxy) -> VPN ->destination
Destination should show the VPN's IP

If Android puts the VPN app to sleep, it should behave like No VPN, until VPN is restarted.

No VPN
Firefox -> proxy-server (in FoxyProxy) -> destination
Destination should show the proxy-server's IP

I think it doesn't explain why Firefox can bypass proxy if user-level proxy isn't running while VPN is running. If I don't launch either user-level proxy or system VPN, foxyproxy trying to connect to proxy will result in proxy refused connection, then I launch VPN but don't launch user-level proxy, refresh the webpage then Firefox connects to website via VPN, not saying proxy refused connection. Then turn off VPN again, refreshing webpage will say proxy refused connection.

[erosman]

For clarity, VPN = VPN application

There is a difference between a VPN and a proxy.

• A VPN takes over all network connections for all applications and is independent of applications
• A proxy is a software that will forward the net requests that it receives specifically

Users can turn on a VPN and it does not require any configuration in any applications. In fact, applications are not aware of VPN's existence.

A proxy requires specific rules/configurations to tell the application where to send the net requests.

FoxyProxy can be set to tell Firefox where to send the net requests.
Firefox can also be set to send the request to a system proxy (via its Settings -> Network Connection).
A system proxy is not a VPN.

Neither FoxyProxy, nor Firefox have any control over the VPN and the VPN grabs the net request after Firefox and before it is sent out.

If the VPN is enabled but sleeping, the connections wont get through until it wakes up.

3rd-party proxy usually listens on 127.0.0.1 or [::1], there's the same issue. If proxy isn't running but VPN is running, foxyproxy allows Firefox to bypass proxy instead of saying proxy refused connection. I think if proxy listens on localhost, it doesn't go through VPN, but Firefox can directly use VPN. Is it related to ROM? If I use custom ROM like LineageOS? In my case, if VPN is running, I must ensure 3rd-party proxy, which listens on localhost, is also running, to ensure Firefox connects to local proxy, otherwise it directly uses VPN.

[erosman]

The issue appears to be related to Firefox as FoxyProxy has no control over the issue.

I will ask Mozilla engineers for verification

The issue appears to be related to Firefox

You can contact Mozilla engineers, will you report to Mozilla?

[erosman]

You can contact Mozilla engineers, will you report to Mozilla?

I did. They said there should not be any difference on behaviour between desktop and Android version in this regard.
Furthermore, you are using a very specific configuration.

Android foxyproxy uses the configured proxy if system isn't running VPN, and Firefox will show a warning about proxy refusing connections if that proxy isn't running.

Is that FoxyProxy extension installed in Firefox or FoxyProxy APP (that is a VPN app) installed on Android?
FoxyProxy extension does not have such settings.

But if Android is running a VPN, and I forgot to use additional proxy, Firefox may access internet but not saying proxy is refusing connections. E.g, using system VPN to connect to Tor, Freegate, or I2P, so Tor, Freegate, or I2P is hidden in VPN traffic,

Are you setting a proxy in Firefox's FoxyProxy extension as well as using a stand-alone VPN?
If you have forgotten to set a proxy, why would Firefox report proxy refusing connection?

but since Android has battery optimization for 3rd-party apps, system may terminate additional proxy to optimize battery usage, or the user just forgot to launch additional proxy, in such case, Foxyproxy may allow Firefox to use Android VPN but not go through additional proxy like Tor, Freegate, or I2P.

If a VPN is on on Android, everything goes through it (unless VPN configured to pass some) and FoxyProxy or Firefox have no control over it.

It is still unclear how your set-up is.
What is "user just forgot to launch additional proxy"?
Are you using a VPN app, and a separate e proxy App?

I installed the extension for Firefox, and set proxy in the extension, so it won't be conflict with VPN. Android doesn't allow 2 VPNs to run simultaneously, but obviously I use Foxyproxy extension for Firefox. I use VPN app and a separate proxy app, and set foxyproxy extension to connect to localhost proxy. So I assume the issue is related to ROM.

[erosman]

I use VPN app and a separate proxy app, and set foxyproxy extension to connect to localhost proxy.

When you say "connect to localhost proxy", do you mean "proxy app"?

I think if proxy listens on localhost, it doesn't go through VPN

If you have the following on Android, you can set FoxyProxy to forward to a proxy app (or not). It has no control over the VPN app. Whatever happens after Firefox forwards to the proxy app in out Firefox’s control.

• FoxyProxy on Firefox
• A proxy app on 127.0.0.1 or [::1]
• A VPN app

but Firefox can directly use VPN

That is incorrect. Firefox sends out the request. It is the VPN app that intercepts the outgoing request of Firefox or any other app.

if VPN is running, I must ensure 3rd-party proxy, which listens on localhost, is also running

If Firefox is set to forward to a proxy app on 127.0.0.1, it must be on or otherwise the connection fails.

FoxyProxy/Firefox -> 127.0.0.1 proxy app -> out

If there is a VPN app involved, it would intercept before going out

FoxyProxy/Firefox -> 127.0.0.1 proxy app -> VPN APP -> out

you can set FoxyProxy to forward to a proxy app

Foxyproxy extension for Android Firefox seemingly only accepts ip address or domain name, I didn't find setting to specify app.

When you say "connect to localhost proxy", do you mean "proxy app"?

My installed proxy app listens on localhost. Is it possible the VPN also forwards localhost traffic?