This page describes how to write rulesets for HTTPS Everywhere, a browser extension that switches sites over from HTTP to HTTPS automatically. HTTPS Everywhere comes with thousands of rulesets that tell HTTPS Everywhere which sites it should switch to HTTPS and how. If there is a site that offers HTTPS and is not handled by the extension, this guide will explain how to add that site.

#### Rulesets

A `ruleset` is an XML file describing behavior for a site or group of sites. A ruleset contains one or more `rules`. For example, here is `RabbitMQ.xml`, from the plugin distribution:

    <ruleset name="RabbitMQ">
            <target host="rabbitmq.com" />
            <target host="www.rabbitmq.com" />

            <rule from="^http:"
                    to="https:" />
    </ruleset>

The `target` tag specifies which web sites the ruleset applies to. The `rule` tag specifies how URLs on those web sites should be rewritten. This rule says that any URLs on `rabbitmq.com` and `www.rabbitmq.com` should be modified by replacing "http:" with "https:".

When the browser loads a URL, HTTPS Everywhere takes the host name (e.g. `www.rabbitmq.com`) and searches its ruleset database for rulesets that match that host name.

HTTPS Everywhere then tries each rule in those rulesets against the full URL. If the Regular Expression, or regexp, in one of those rules matches, HTTPS Everywhere rewrites the URL according the `to` attribute of the rule.

#### Wildcard Targets

To cover all of a domain's subdomains, you may want to specify a wildcard target like `*.twitter.com`. Specifying this type of left-side wildcard matches any host name with `.twitter.com` as a suffix, e.g. `www.twitter.com` or `urls.api.twitter.com`. You can also specify a right-side wildcard like `www.google.*`. Right-side wildcards, unlike left-side wildcards, apply only one level deep. So if you want to cover all countries you'll generally need to specify `www.google.*`, `www.google.co.*`, and `www.google.com.*` to cover domains like `www.google.co.uk` or `www.google.com.au`. You should use wildcard targets only when you have rules that apply to the entire wildcard space. If your rules only apply to specific hosts, you should list each host as a separate target.

#### Rules and Regular Expressions

The `rule` tags do the actual rewriting work. The `from` attribute of each rule is a regular expression matched against a full URL. You can use rules to rewrite URLs in simple or complicated ways. Here's a simplified (and now obsolete) example for Wikipedia:

    <ruleset name="Wikipedia">
      <target host="*.wikipedia.org" />

      <rule from="^http://(\w{2})\.wikipedia\.org/wiki/"
              to="https://secure.wikimedia.org/wikipedia/$1/wiki/"/>
    </ruleset>

The `to` attribute replaces the text matched by the `from` attribute. It can contain placeholders like `$1` that are replaced with the text matched inside the parentheses.

This rule rewrites a URL like `http://fr.wikipedia.org/wiki/Chose` to `https://secure.wikimedia.org/wikipedia/fr/wiki/Chose`. Notice, again, that the target is allowed to contain (just one) * as a wildcard meaning "any".

Rules are applied in the order they are listed within each ruleset. Order between rulesets is unspecified. Only the first rule or exception matching a given URL is applied.

Rules are evaluated using Javascript regular expressions, which are similar but not identical to Perl-style regular expressions. Note that if your rules include ampersands (&), they need to be appropriately XML-encoded: replace each occurrence of & with &#x26;.

#### Exclusions

An exclusion specifies a pattern, using a regular expression, for URLs where the rule should not be applied. The Stack Exchange rule contains an exclusion for the OpenID login path, which breaks logins if it is rewritten:

    <exclusion pattern="^http://(?:\w+\.)?stack(?:exchange|overflow)\.com/users/authenticate/" />

Exclusions are always evaluated before rules in a given ruleset. Matching any exclusion means that a URL won't match any rules within the same ruleset. However, if other rulesets match the same target hosts, the rules in those rulesets will still be tried.

#### Style Guide

There are many different ways you can write a ruleset, or regular expression within the ruleset. It's easier for everyone to understand the rulesets if they follow similar practices. You should read and follow the Ruleset style guide. Some of the guidelines in that document are intended to make Ruleset testing less cumbersome.

#### Secure Cookies

Many HTTPS websites fail to correctly set the secure flag on authentication and/or tracking cookies. HTTPS Everywhere provides a facility for turning this flag on. For instance:

    <securecookie host="^market\.android\.com$" name=".+" />

The "host" parameter is a regexp specifying which domains should have their cookies secured; the "name" parameter is a regexp specifying which cookies should be secured. For a cookie to be secured, it must be sent by a target host for that ruleset. It must also be sent over HTTPS and match the name regexp. For cookies set by Javascript in a web page, the Firefox extension can't tell which host set the cookie and instead uses the domain attribute of the cookie to check against target hosts. A cookie whose domain attribute starts with a "." (the default, if not specified by Javascript) will be matched as if it was sent from a host name made by stripping the leading dot.

#### Testing

We use an automated checker to run some basic tests on all rulesets. This is described in more detail in our Ruleset Testing document, but in short there are two parts: Your ruleset must have enough test URLs to cover all the various types of URL covered by your rules. And each of those test URLs must load, both before rewriting and after rewriting. Every target host tag generates an implicit test URL unless it contains a wildcard. You can add additional test URLs manually using the `<test url="..."/>` tag. The test URLs you add this way should be real pages loaded from the site, or real images, CSS, and Javascript if you have rules that specifically affect those resources.

You should also manually test your ruleset by placing it in the `HTTPSEverywhereUserRules/` subdirectory in your Firefox profile directory, and then restarting Firefox. While using the rule, check for messages in the Firefox Error Console to see if there are any issues with the way the site supports HTTPS.

If you've tested your rule and are sure it would be of use to the world at large, submit it as a pull request on our GitHub repository or send it to the rulesets mailing list at `https-everywhere-rules AT eff.org`. Please be aware that this is a public and publicly-archived mailing list.

#### make-trivial-rule

As an alternative to writing rules by hand, there are scripts you can run from a Unix command line to automate the process of creating a simple rule for a specified domain. These scripts are not included with HTTPS Everywhere releases but are available in our development repository and are described in our development documentation.

#### Disabling a ruleset by default

Sometimes rulesets are useful or interesting, but cause problems that make them unsuitable for being enabled by default in everyone's browsers. Typically when a ruleset has problems we will disable it by default until someone has time to fix it. You can do this by adding a `default_off` attribute to the ruleset element, with a value explaining why the rule is off.

    <ruleset name="Amazon (buggy)" default_off="breaks site">
       <target host="www.amazon.*" />
       <target host="amazon.*" />
    </ruleset>

You can add more details, like a link to a bug report, in the comments for the file.

#### Mixed Content Blocking (MCB)

Some rulesets may trigger active mixed content (i.e. scripts loaded over HTTP instead of HTTPS). This type of mixed content is blocked in both Chrome and Firefox, before HTTPS Everywhere has a chance to rewrite the URLs to an HTTPS version. This generally breaks the site. However, the Tor Browser doesn't block mixed content, in order to allow HTTPS Everywhere to try and rewrite the URLs to an HTTPS version.

To enable a rule only on platforms that allow mixed content (currently only the Tor Browser), you can add a `platform="mixedcontent"` attribute to the ruleset element.

#### HTTPS->HTTP downgrade rules

By default, HTTPS Everywhere will refuse to allow rules that would downgrade a URL from HTTPS to HTTP. Occasionally, this is necessary because the extension rewrites a page to HTTPS, and that page contains relative links to resources which do not exist on the HTTPS part of the site. This is very rare, especially because these resources will typically be blocked by Mixed Content Blocking. If it necessary, you can add a `downgrade="1"` attribute to the rule to make it easier to audit the ruleset library for such rules.