proxy page 2 - Dave's Blog

WPAD Server Fiddler Extension

2010 Jan 5, 7:42

I've made a WPAD server Fiddler extension and in a fit of creativity I've named it: WPAD Server Fiddler Extension.

Of course you know about Fiddler, Eric's awesome HTTP debugger tool, the HTTP proxy that lets you inspect, visualize and modify the HTTP traffic that flows through it. And on the subject you've probably definitely heard of WPAD, the Web Proxy Auto Discovery protocol that allows web browsers like IE to use DHCP or DNS to automatically discover HTTP proxies on their network. While working on a particularly nasty WPAD bug towards the end of IE8 I really wished I had a way to see the WPAD requests and responses and modify PAC responses in Fiddler. Well the wishes of me of the past are now fulfilled by present day me as this Fiddler extension will respond to WPAD DHCP requests telling those clients (by default) that Fiddler is their proxy.

When I started working on this project I didn't really understand how DHCP worked especially with respect to WPAD. I won't bore you with my misconceptions: it works by having your one DHCP server on your network respond to regular DHCP requests as well as WPAD DHCP requests. And Windows I've found runs a DHCP client service (you can start/stop it via Start|Run|'services.msc', scroll to DHCP Client or via the command line with "net start/stop 'DHCP Client'") that caches DHCP server responses making it just slightly more difficult to test and debug my extension. If a Windows app uses the DHCP client APIs to ask for the WPAD option, this service will send out a DHCP request and take the first DHCP server response it gets. That means that if you're on a network with a DHCP server, my extension will be racing to respond to the client. If the DHCP server wins then the client ignores the WPAD response from my extension.

Various documents and tools I found useful while working on this:

• WPAD IETF draft document the never out of draft document describing WPAD
• DHCP RFC
• DHCP options RFC
• DHCP Wikipedia article which includes helpful byte layout tables for DHCP messages.
• Fiddler Wiki esp. the dev section but also the portion on the new preferences API.
• Fiddler Dev Docs and for those things not covered in the Fiddler docs, Visual Studio's name completion (or the .NET Reflector) usually served me well.
• Netmon a great network sniffing tool for all my non-HTTP network inspection needs.
• Tiny DHCP Server on CodeProject which implements a conventional DHCP server.

proxy fiddler http technical debug wpad pac tool dhcp

Second Life Translator

2007 Jul 4, 10:58Hackdiary
I really enjoy reading Matt Biddulph's blog hackdiary. An entry some time ago talked about his Second Life flickr screen which is a screen in Second Life that displays images from flickr.com based on viewers suggested tags. I'm a novice to the Second Life scripting API and so it was from this blog post I became aware of the llHTTPRequest. This is like the XMLHttpRequest for Second Life code in that it lets you make HTTP requests. I decided that I too could do something cool with this.

Translator
I decided to make a translator object that a Second Life user would wear that would translate anything said near them. The details aren't too surprising: The translator object keeps an owner modifiable list of translation instructions each consisting of who to listen to, the language they speak, who to tell the translation to, and into what language to translate. When the translator hears someone, it runs through its list of translation instructions and when it finds a match for the speaker uses the llHTTPRequest to send off what was said to Google translate. When the result comes back the translator simply says the response.

Issues
Unfortunately, the llHTTPRequest limits the response size to 2K and no translation site I can find has the translated text in the first 2K. There's a flag HTTP_BODY_MAXLENGTH provided but it defaults to 2K and you can't change its value. So I decided to setup a PHP script on my site to act as a translating proxy and parse the translated text out of the HTML response from Google translate. Through experimentation I found that their site can take parameters text and langpair queries in the query like so: http://translate.google.com/translate_t?text=car%20moi%20m%C3%AAme%20j%27en%20rit&langpair=fr|en. On the topic of non US-ASCII characters (which is important for a translator) I found that llHTTPRequest encodes non US-ASCII characters as percent-encoded UTF-8 when constructing the request URI. However, when Google translate takes parameters off the URI it only seems to interpret it as percent-encoded UTF-8 when the user-agent is IE's. So after changing my PHP script to use IE7's user-agent non US-ASCII character input worked.

In Use
Actually using it in practice is rather difficult. Between typos, slang, abbreviations, and the current state of the free online translators its very difficult to carry on a conversation. Additionally, I don't really like talking to random people on Second Life anyway. So... not too useful.personal translate second-life technical translator sl code google php llhttprequest