I know NCSA httpd have this function. I think it could make other servers.
It's possible by "user-agent" line in the request. See http documentation:
ftp://ftp.yars.free.net/pub/doc/Network/Protocols/http/draft-ietf-http-v10-spec-00.txt.gz
Sorry for bad english.
Petr V.Kurkin << RUSSIA >>
joy@moon.uniyar.ac.ru