Varnish

Secure Socket Layer (SSL) is the protocol that allows web sites to serve traffic in HTTPS. This provides end to end encryption between the two end points (the browser and the web server). The benefits of using HTTPS is that traffic between the two end points cannot be deciphered by anyone snooping on the connection. This reduces the odds of exposing sensitive information such as passwords, or getting the web site hacked by malicious parties. Google has also indicated that sites serving content exclusively in HTTPS will get a small bump in Page Rank.

Using a Reverse Proxy and/or a Content Delivery Network (CDN) has become common practice for Drupal and other Content Management Systems.

One inconvenient aspect of this is that your web server no longer gets the correct IP address, and neither does your application. The IP address is that of the machine that the reverse proxy is running on.

In Drupal, there is code in core that tries to work around this, by looking up the IP address in the HTTP header HTTP_X_FORWARDED_FOR, or a custom header that you can set.

We have mentioned before that both Pressflow 6.x and Drupal 7.x (but not core Drupal 6.x), disable page caching when a session is created for an anonymous user.

An extreme case of this happened recently, because of a perfect storm.

Symptoms

The client sends a newsletter to site users, be they who have accounts on the site, or others who just entered their email to get the newsletter.

Ubuntu Server 12.04 LTS finally provides a stable long term support server distro that has a recent version of Varnish in its repositories.

Trouble is, the repository provided package of Varnish has some issues. Specifically, the command line tools, such as varnishhist, varnishstat, ...etc. do not report anything. Therefore one cannot know the hit/miss rates, hits per second, or other useful information. Moreover, monitoring Varnish using Munin for such statistics does not work either.

There are two ways you can overcome this, both are described below.