I've been running varnish (3.0.1) in front of Apache and during a spike in traffic two days ago all visitors to the site started to receive a 403 error. From Googling about I've found one thread which says this is caused by Varnish's IP being passed to Apache instead of the user's who's making the request. (see https://www.varnish-cache.org/lists/pipermail/varnish-misc/2011-March/005730.html)
The suggestion was "You could try passing the ip as x-forwarded-for"... I'm not sure what IP is being referred to (the client's?) or in fact how I'd go about doing that. Has anyone seen this before or knows how to achieve what is being suggested?