The field is User-Agent. However, blocking access to users of Navigator isn't a particularly useful thing to do. If you must do something, why not modify your GET handler to add a header to the start of all html pages informing people of the problem, and suggesting alternatives.
I haven't modified my GET, but at the top of all the standard c2.org web pages (http://www.c2.org/) if you are using pre-1.12 netscape, it barfs at you with a nasty message. I plan on adding a line for all netscape browsers, with a link to Lance's page, once it is ready. (Maybe it is ready now, I just haven't looked yet.)
Someone else [I can't remember, but I'll call them Alice] claimed that the security problems showing up were part of a deliberate conspiracy. To anyone who knows anything about the history of these things knows how absurd this is. The principals at Netscape are a nice bunch of really guys, but were not really up to speed on issues like security and networking- for example, the first incarnation of SSL had an RC4 stream running with no checksumming whatsoever. The security problems that resulted are due to the learning curve.
Simon
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