Create a file named **config.ru**:
```ruby
run lambda { |env|
[200, {"Content-Type" => "text/html"}, [view(env)]]
}
def view(env)
res = ""
res << "
"
env.sort.each do |key, value|
res << "#{key}: #{value}"
res << "\n"
end
res << ""
res
end
```
Start the server with e.g. **rackup**, puma, or thin:
```ruby
$ rackup
```
Access http://localhost:9292 to see the request headers.