Web Proxy Auto-Discovery
As I mentioned earlier, proxy auto-configuration usually requires the user to manually enter a URL in the proxy configuration window. Recent efforts by browser developers seek to eliminate that step. This new feature is called web proxy auto-discovery, or WPAD. WPAD is designed to let browsers automatically learn the auto-configuration URL with no assistance from the user.
WPAD uses a number of protocols and services to find the
auto-configuration URL. The preferred protocol is DHCP
(Dynamic Host Configuration Protocol). If DHCP fails, the
user-agent may try the Service Location Protocol (SLP, RFC
2068) next. If SLP fails as well, it queries the DNS for
A, SRV, and
TXT records. A client that implements WPAD
must support at least the DHCP and DNS A
record queries. Support for SLP is optional.
Using the DNS is likely to be the simplest and most
popular approach for network administrators. The first step is
to select an HTTP server and place a proxy auto-configuration
script there under the name /wpad.dat.
Then the administrator needs only to add a DNS entry for a
host named wpad. For example, if
wpad.company.com is an alias for
company.com’s web server, and the
wpad.dat file is installed there, all of
the company’s web browsers can automatically configure
themselves.
WPAD was submitted to the IETF as an Internet Draft in December of 1999. Unfortunately, it has not yet been updated or moved beyond that stage, and the draft is now expired. Even so, Microsoft’s ...