Mobile device detection in Nginx with just 7 lines of configuration

gistfile1.txt

### Testing if the client is a mobile or a desktop.
### The selection is based on the usual UA strings for desktop browsers.

## Testing a user agent using a method that reverts the logic of the
## UA detection. Inspired by notnotmobile.appspot.com.
map $http_user_agent $is_desktop {
    default 0;
    ~*linux.*android|windows\s+(?:ce|phone) 0; # exceptions to the rule
    ~*spider|crawl|slurp|bot 1; # bots
    ~*windows|linux|os\s+x\s*[\d\._]+|solaris|bsd 1; # OSes
}

## Revert the logic.
map $is_desktop $is_mobile {
    1 0;
    0 1;
}

Usage example:
set $mobile_rewrite do_not_perform;

if ($is_mobile) {
set $mobile_rewrite perform;
}

if ($mobile_rewrite = perform) {
rewrite ^ http://m.domain.com$request_uri? redirect;
break;
}