$r->header_out('Vary', join ", ",
qw(accept accept-language accept-encoding user-agent));
The header of a very cool page may greet the user with something like
Hallo Harri, Dein NutScrape versteht zwar PNG aber leider kein GZIP.
However, this header has the side effect of being expensive for a
caching proxy. As of this writing, Squid (Version 2.3.STABLE4) does
not cache resources that come with a Vary header
at all. So without a clever workaround, the Squid accelerator is of
no use for these documents.