Using proxy auto config to get around the "squid does not know IPv6 yet" limitation

I like making stuff which just works from the users perspective, and they dont have to care about how it works. But i do not like transparent proxies, diverting HTTP traffic without the applications knowing about it just does not seem to be right. But I like the idea of proxy autoconfiguration in web browsers. I did set it up a long time ago, with squid proxy, but there were some problems with it:

• When you go to an address with no DNS records associated with, squid displays an error page. When not using a proxy, browsers redirect you to a search page, or display a much nicer error. (I'm really not into redesigning squid error pages.)


• IPv6. Squid does not know about IPv6 yet.

Well, actually squid does support IPv6, in version 3.1, but as of this writing it is still RC. The most important part of the Internet is the Web, so i'm reluctant to use RC. But we could use the PAC file, which is just a javascript code to not use proxy for IPv6 sites. I wanted to make this work when I first created the proxy autoconfig file, but I could not find a good way to test how it works, so my script always returned the same string: "PROXY proxy:3128; DIRECT". This means: use the proxy server at proxy:3128, and if that fails, use direct connection. Yesterday I stumbled upon pactester, so I tought I'll give it a try.

I wanted the following funcionality:

• If the hostname is not resolvable, use direct.


• If the hostname resolves only to IPv4 addresses, use proxy


• If the hostname resolves only to IPv6 addresses, use direct


• If the hostname resolves to both IPv4 and IPv6 address, use direct.

First, since I use gentoo, I wanted to create an ebuild for pactester. The packaging of pactester is a bit ugly, you'll get 1Mb of bundled dependencies and 15kb of useful code. I did put it into /usr/local/portage/net-proxy/pactester/.

The wpad.dat file should have a function FindProxyForURL(url, host), and newer Internet Exploer also supports a function FindProxyForURLEx(url, host). Here are some things to know:

• Internet Explorer 8 calls the function FindProxyForURLEx.


• pactester and Google Chrome are using the FindProxyForURL function


• There are additional functions which you can use in your script, see pac_utils.js


• There are even more additional functions for you to use in FindProxyForURLEx, see proxy_resolver_v8.cc


• in pactester the function dnsResolveEx() is not available


• Google Chrome and Internet Explorer gives different results when calling dnsResolve()

So with a little experimentation I wrote the following proxy autoconfiguration script: wpad.dat

I found the following links useful:
http://kate-ward.blogspot.com/2008/11/im-up-and-running-with-ipv6.html
http://users.telenet.be/mydotcom/library/network/pac.htm