Web Proxy Autoconfiguration

Linux 1 Minute

Since the last time I looked up how to configure the Web Proxy, apparently somebody came up with WPAD – the Web Proxy Auto-Discovery Protocol (Or maybe I simply missed it).

The idea is quite neat: In the dhcp server, add an entry where the browser can request an url which in turn returns the information for Proxy configuration. Alternatively, instead of the dhcp server, the name server can have entries for wpad.company.com or wpad.department.company.com which are then used to request the configuration.

I decided I’d go with the dhcp entry (bust just in case I also added a wpad entry to bind). So, what exactly needs to be done?

1. Add an option local-pac-server with number 252 and type text to the dchp configuration and set its value to the download url of a wpad.dat file. This can be done for the isc-dchp-server with an entry like this to the /etc/dhcp/dhcpd.conf file:

option local-pac-server code 252 = text;
option local-pac-server "http://someserver.excample.com/wpad.dat";

2. Set up a web server on that computer that serves the file. I didn’t want to install a full blown Apache for this, so I went with mini_httpd. I simply installed the Ubuntu package for it and made two changes to the configuration file /etc/mini-httpd.conf:

# was:
# host=localhost
host=[the IP adress]
# was
# data_dir=/var/www/http
data_dir=/var/www

3. Create a wpad.dat file in the root of the data_dir like this:

function FindProxyForURL(url, host) {
return "PROXY proxy.example.com:8080; DIRECT";
}

4. Wonder WhyTF this was so complicated:

  1. Why not simply configure the string “PROXY proxy.example.com:8080; DIRECT” in the dhcp server?
  2. Why return a text file with a JavaScript function rather than a text file with just the string that JavaScript function returns?

This worked fine even for the Avira Antivirus updater.

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