Cookies

Varnish will, in the default configuration, not cache a object coming from the backend with a Set-Cookie header present. Also, if the client sends a Cookie header, Varnish will bypass the cache and go directly to the backend.

This can be overly conservative. A lot of sites use Google Analytics (GA) to analyze their traffic. GA sets a cookie to track you. This cookie is used by the client side javascript and is therefore of no interest to the server.

Cookies from the client

For a lot of web application it makes sense to completely disregard the cookies unless you are accessing a special part of the web site. This VCL snippet in vcl_recv will disregard cookies unless you are accessing /admin/:

if ( !( req.url ~ ^/admin/) ) {
  unset req.http.Cookie;
}

Quite simple. If, however, you need to do something more complicated, like removing one out of several cookies, things get difficult. Unfortunately Varnish doesn't have good tools for manipulating the Cookies. We have to use regular expressions to do the work. If you are familiar with regular expressions you'll understand whats going on. If you don't I suggest you either pick up a book on the subject, read through the pcrepattern man page or read through one of many online guides.

Let me show you what Varnish Software uses. We use some cookies for Google Analytics tracking and similar tools. The cookies are all set and used by Javascript. Varnish and Drupal doesn't need to see those cookies and since Varnish will cease caching of pages when the client sends cookies we will discard these unnecessary cookies in VCL.

In the following VCL we discard all cookies that start with a underscore:

// Remove has_js and Google Analytics __* cookies.
set req.http.Cookie = regsuball(req.http.Cookie, "(^|;\s*)(_[_a-z]+|has_js)=[^;]*", "");
// Remove a ";" prefix, if present.
set req.http.Cookie = regsub(req.http.Cookie, "^;\s*", "");

Let me show you an example where we remove everything except the cookies named COOKIE1 and COOKIE2 and you can marvel at it:

sub vcl_recv {
  if (req.http.Cookie) {
    set req.http.Cookie = ";" + req.http.Cookie;
    set req.http.Cookie = regsuball(req.http.Cookie, "; +", ";");
    set req.http.Cookie = regsuball(req.http.Cookie, ";(COOKIE1|COOKIE2)=", "; \1=");
    set req.http.Cookie = regsuball(req.http.Cookie, ";[^ ][^;]*", "");
    set req.http.Cookie = regsuball(req.http.Cookie, "^[; ]+|[; ]+$", "");

    if (req.http.Cookie == "") {
        remove req.http.Cookie;
    }
}

The example is taken from the Varnish Wiki, where you can find other scary examples of what can be done in VCL.

Cookies coming from the backend

If your backend server sets a cookie using the Set-Cookie header Varnish will not cache the page. A hit-for-pass object (see tutorial_vcl_fetch_actions) is created. So, if the backend server acts silly and sets unwanted cookies just unset the Set-Cookie header and all should be fine.