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History

User-Agent: <something> is a string of characters sent by HTTP clients (browsers, bots, calendar applications, etc.) for each individual HTTP request to a server. The HTTP Protocol as defined in 1991 didn’t have this field, but the next version defined in 1992 added User-Agent in the HTTP requests headers. Its syntax was defined as “the software product name, with an optional slash and version designator“. The prose already invited people to use it for analytics and identify the products with implementation issues.

This line if present gives the software program used by the original client. This is for statistical purposes and the tracing of protocol violations. It should be included.

Fast forward to August 2013, the HTTP/1.1 specification is being revised and also defines User-Agent.

A user agent SHOULD NOT generate a User-Agent field containing
needlessly fine-grained detail and SHOULD limit the addition of
subproducts by third parties. Overly long and detailed User-Agent
field values increase request latency and the risk of a user being
identified against their wishes (“fingerprinting”).

Likewise, implementations are encouraged not to use the product
tokens of other implementations in order to declare compatibility
with them, as this circumvents the purpose of the field. If a user
agent masquerades as a different user agent, recipients can assume
that the user intentionally desires to see responses tailored for
that identified user agent, even if they might not work as well for
the actual user agent being used.

Explore User Agents We have a huge collection of over 62.5 million different types of user agents. Look around to find user agents for whatever browser, operating system, device, software type or layout engine you need. If you need to decode a user agent, use the User Agent Parser or the User Agent Parsing API. It is possible to change or 'fake' what your web browser sends as its user agent. Some mobile web browsers will let you change what the browser identifies itself as (ie 'Mobile Mode' or 'Desktop Mode') in order to access certain websites that only allow desktop computers. If you change this setting, the user agent is what is affected. User Agent strings come in all shapes and sizes, and the number of unique user agents is growing all the time. We've collected millions of user agents and have categorised them here for you, based on many of the things we detect about them - Operating System, Browser, Hardware Type, Browser Type, and so on. Browse the user agents database. Firefox user agent string reference This document describes the user agent string used in Firefox 4 and later and applications based on Gecko 2.0 and later. For a breakdown of changes to the string in Gecko 2.0, see Final User Agent string for Firefox 4 (blog post). See also this document on user agent sniffing and this Hacks blog post.

Basically, the HTTP specification discouraged since its inception the detection of the User-Agent string for tailoring the user experience. Currently, the user agent strings have become overly long. They are abused in every possible way. They include detailed information. They lie about what they really are and they are used for branding and advertising the devices they run on.

User-Agent Detection

User agent detection (or sniffing) is the mechanism used for parsing the User-Agent string and inferring physical and applicative properties about the device and its browser. But let get the record straight. User-Agent sniffing is a future fail strategy. By design, you will detect only what is known, not what will come. The space of small devices (smartphones, feature phones, tablets, watches, arduino, etc.) is a very fast-paced evolving space. The diversity in terms of physical characteristics will only increase. Updating databases and algorithms for identifying correctly is a very high maintenance task which is doomed to fail at a point in the future. Sites get abandoned, libraries are not maintained and Web sites will break just because they were not planned for the future coming devices. All of these have costs in resources and branding.

New solutions are being developed for helping people to adjust the user experience depending on the capabilities of the products, not its name. Responsive design helps to create Web sites that are adjusting for different screen sizes. Each time you detect a product or a feature, it is important to thoroughly understand why you are trying to detect this feature. You could fall in the same traps as the ones existing with user agent detection algorithms.

We have to deal on a daily basis with abusive user agent detection blocking Firefox OS and/or Firefox on Android. Provideo driver download for windows 10. It is not only Mozilla products, every product and brand has to deal at a point with the fact to be excluded because they didn’t have the right token to pass an ill-coded algorithm. User agent detection leads to situation where a new player can hardly enter the market even if it has the right set of technologies. Remember that there are huge benefits to create a system which is resilient to many situations.

Some companies will be using the User-Agent string as an identifier for bypassing a pay-wall or offering specific content for a group of users during a marketing campaign. It seems to be an easy solution at first but it creates an environment easy to by-pass in spoofing the user agent.

Firefox and Mobile

Firefox OS and Firefox on Android have very simple documented User-Agent strings.

Firefox OS

Firefox on Android

The most current case of user agent detection is to know if the device is a mobile to redirect the browser to a dedicated Web site tailored with mobile content. We recommend you to limit your detection to the simplest possible string by matching the substring mobi in lowercase.

If you are detecting on the client side with JavaScript, one possibility among many would be to do:

You might want to add more than one token in the if statement.

Remember that whatever the number of tokens you put there, you will fail at a point in the future. Some devices will not have JavaScript, will not have the right token. The pattern or the length of the token was not as you had initially planned. Download prolific driver. The stones on the path are plenty, choose the way of the simplicity.

Summary: UA detection Checklist Zen

1. Do not detect user agent strings
2. Use responsive design for your new mobile sites (media queries)
3. If you are using a specific feature, use feature detections to enhance, not block
4. And if finally you are using UA detection, just go with the most simple and generic strings.
5. Always provide a working fallback whatever the solutions you chose are.

Practice. Learn. Imagine. Modify. And start again. There will be many road blocks on the way depending on the context, the business requirements, the social infrastructure of your own company. Keep this checklist close to you and give the Web to more people.

About Karl Dubost

Always elsewhere.

Default User Agent Firefox

About Robert Nyman [Editor emeritus]

Random User Agent Firefox

Technical Evangelist & Editor of Mozilla Hacks. Gives talks & blogs about HTML5, JavaScript & the Open Web. Robert is a strong believer in HTML5 and the Open Web and has been working since 1999 with Front End development for the web - in Sweden and in New York City. He regularly also blogs at http://robertnyman.com and loves to travel and meet people.