Dynamic serving is a setup where the server responds with different HTML (and CSS) on the same URL depending on which user agent requests the page (mobile, tablet, or desktop).
As it is not immediately apparent in this setup that the site alters the HTML for mobile user agents (the mobile content is "hidden" when crawled with a desktop user agent), we recommend that the server send a hint to request that Googlebot for smartphones also crawl the page, and thus discover the mobile content. This hint is implemented using the Vary HTTP header.
- Use the Vary HTTP header to signal your changes depending on the user agent.
- Detect user agent strings correctly.
The Vary HTTP Header
The Vary HTTP header has two important and useful implications:
- It signals to caching servers used in ISPs and elsewhere that they should consider the user agent when deciding whether to serve the page from cache or not. Without the Vary HTTP header, a cache may mistakenly serve mobile users the cache of the desktop HTML page or vice versa.
- It helps Googlebot discover your mobile-optimized content faster, as a valid Vary HTTP header is one of the signals we may use to crawl URLs that serve mobile-optimized content.
The Vary HTTP header is part of the server's response to a request, like this:
HTTP/1.1 200 OK Content-Type: text/html Vary: User-Agent Content-Length: 5710 (... rest of HTTP response headers...)
The Vary HTTP header tells the browser that the contents of the response varies
depending on the user agent that requests the page. If your server already uses
the Vary HTTP header, you can add
User-Agent to the list that's already
Correctly detecting user agents
Detecting user agents (sometimes called user agent "sniffing") is generally an error-prone technique. There are many reasons why, but three kinds of failures are common:
- User agent detection depends on having a list of user agent strings (or substrings) to match against. Such lists need constant maintenance and updating and don't match new user agents. In reality, many such lists are not maintained appropriately and are stale, giving your users a bad experience.
When matching user agents, it's common to mismatch, sometimes detecting a desktop user agent as a mobile one or detecting a mobile user agent as a desktop. Likewise, a common mistake for sites is to inadvertently treat tablet devices as smartphones. If you are detecting the user agent of browsers accessing your site, be sure the detection looks for smartphone-specific strings (such as checking for both the words "Android" and "Mobile") as opposed to generic mobile strings (checking for just "Android"). Learn more in our blog post.
Be very careful of cloaking when detecting user agents. When detecting the user agent, the site is detecting the device class or type by looking for the device name in the user agent string; it should not be looking specifically for Googlebot. All Googlebot user agents identify themselves as specific mobile devices, and you should treat these Googlebot user agents exactly like you would treat these devices.