Libproxy – 1.0: Configuration Logic
Description: Reference for Libproxy-1.0: Configuration Logic

# Configuration Logic

## Introduction

As the proxy configuration predates libproxy, we need to consider previous implementation behavior to ensure consistency with user expectations. This page presents analyses of well known implementation base on the platform they run on.

## Linux

On Linux the pioneer of proxy support is Mozilla browsers (former Netscape). But other browsers do support proxy and has it’s own proxy configuration interpretation logic. Current GNOME proxy settings is a copy of Firefox settings.

### Firefox

When using Firefox internal manual settings, the proxy is selected base on the most specific proxy (e.g. HTTP before SOCKS). Only one proxy is selected, if connection to that proxy fails, then connection fails.

     IF protocol specific proxy is set THEN use it
     ELSE IF SOCKS proxy is set THEN use it
     ELSE use direct connection.

### Firefox and Chromium (with GNOME settings)

After some testing we found that Chromium mimics perfectly Firefox behavior when using system settings. When using manual proxy configuration mode, those browsers chooses a proxy base on the most generic solution (SOCKS) to the most specific (per protocol proxies), with an exception when a single proxy is set for all protocols. Only one protocol is selected and no fallback will occur in the case of failures. Those browsers support SOCKS, HTTP, HTTPS, FTP and Firefox also support Gopher. You can also set the configuration to use a specific PAC file or to automatically discover one (WPAD) but those does not contain any special logic. Next is the logic represent as pseudo code:

     IF not using same proxy for all protocols THEN
       IF SOCKS is set THEN use it
       ELSE IF protocol specific proxy is set THEN use it
       ELSE IF using same proxy for all protocols THEN
        IF SOCKS is set THEN use it
     IF no proxy has been set THEN use direct connection

## OS X

OS X uses it’s own way for proxy settings. It supports protocols including SOCKS, HTTP, HTTPS, FTP, Gopher, RTSP, and automatic configuration through PAC files. For sake of simplicity, we have tested the logic with the default browser Safari.

### Safari

Safari interpret proxy logic differently from Firefox. If multiple proxy are configured, it try each of them until a connection is established. From our testing the order seems to be from most specific to most generic (starting with manual configuration). Next is the logic represented as pseudo code:

     DEFINE proxy_list as list
     IF protocol specific proxy is set THEN add it to proxy_list
     IF SOCKS proxy is set THEN append it to proxy_list
     IF PAC auto-configuration is set THEN append it to proxy_list
     FOREACH proxy in proxy_list
       connect to proxy
       IF connection failed THEN continue
       ELSE stop

## Windows

Windows user most often use Internet Explorer, Firefox or Opera for browsing. Analyses as shown that Firefox acts exactly the same as on Linux, except that same logic is applied for internal settings and system setting (IE settings). Internet explorer also act the same way, and Opera only support protocol specific proxies (no SOCKS). So essentially, if chooses choose the most specific proxy, and if that one fails the own connection fails.

## Conclusion

Base on current result, we see that the most common logic is to select a proxy starting from the most specific (HTTP, FTP, etc.) to the least specific (SOCKS, PAC then WPAD). OS X pushes a bit further by trying all the configure proxy that match the protocol. This technique is interesting for libproxy since it warranty that connection will be possible for all cases covered by the others. The only difference with the GNOME environment is that OS X may connect through an HTTP server while Firefox and Chromium (on Gnome) would connect to a SOCKS server if both are available.