missing HTTP_HOST, HTTP/1.0 and rewrite rules (help needed!)

Hello,

I'm currently using Apache 1.3.x with a HTTP_HOST based rewrite system
and a have some problem with HTTP/1.0 requests.

Some HTTP 1/0 request does not send the "Host:" variable

ex:

GET http://www.test.com/test.html HTTP/1.0

But my system use

HTTP_HOST_BASE for rewriting

And if HTTP_HOST is empty, HTTP_HOST_BASE is empty too and I have a Bad
Request 400 Error.

Forcing host to a be numeric is not the solution, because I need the
domain host.

Does someone have an idea to get the host OR making a redirection to an
other webserver if HTTP_HOST is empty.

Thank you very much for your attention

Julien