How does Varnish cache handle different domains?

Question

I just started reading about Varnish and am considering using it as a frontend to my webservers. I have multiple domains on my webserver that fetch the same content when the same query strings are used. So lets say one client visits:

http://domain1.com/script.php?string1=abc&string2=123

And later on, someone else visits:

http://domain2.com/script.php?string1=abc&string2=123

Would Varnish cache the results from the first visit and use that for the second client? Thanks!

EDIT: After some more reading, it sounds like the following may work:

sub vcl_hash {
    set req.hash += req.url;
    return (hash);
}

So instead of added the http.host variable to the hash, it ignores it.

Solution

You can configure it to do so, sort-of. By default, it won't (and shouldn't).

From Varnish FAQ/HowDoI

I have a site with many hostnames, how do I keep them from multiplying the cache?

You can do this by normalizing the "Host" header for all your hostnames. Here's a VCL example:

if (req.http.host ~ "^(www.)?example.com") { set req.http.host = "example.com"; }

Comments

[Jeremy Stein]

That or it's probably possible to strip the hostname entirely and just use the remainder of the path. YMMV.

[brianjob]

Hmm, I added some code above that may work. If anyone can confirm, that'd be great!

[Lee B]

I don't think (or don't hope) that Lin is asking about how to share the cache. The problem is that sharing the same data across urls on different sites could be a security/integrity issue. example.com/bobs_friends is NOT the same as example2.com/bobs_friends. Serving one when the other is asked for could be disastrous.