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A reader Aaron J. writes in response to the short article How to obtain the IP address of the current user:

Do you have any advice on how I can take this a step further? I am looking to limit a given user to one vote per session but I’m not sure how to achieve this. I’d appreciate any help you can offer. Thanks for your time.

Good question, Aaron. Remember that a session is simply tied to your browser cookie, so if we allow one vote per session, all the user has to do is clear their cookies and then vote again. And again. And again! So I think what you mean is how do we allow only one vote per IP address? To enforce that, we'll need to save a list of the IP addresses that have already sent us a vote. And before we record a vote, we need to make sure their IP address isn't already on that list. Further, we should remember which poll number they have voted in so we can have more than one poll in our application. We're going to need a simple table:

# lib/db/migrate/001_voter_log.rb
create_table :vote_log do |t|
  t.column :poll_id, :integer
  t.column :client_ip, :string
end

For simplicity of this example, we'll put all of our logic in the controller. (I won't show the VoteLog model class because it's empty.)

# RAILS_ROOT/app/controllers/poll_controller.rb
def record_a_vote
  poll_id = params[:poll_id]
  client_ip = request.remote_ip

  unless VoteLog.count(:all, :conditions => ['poll_id = ? AND client_ip = ?', poll_id, client_ip]) == 0
    redirect_to :already_voted
  end

  Poll.find(poll_id).record_vote(params[:candidate]) # Or however you count votes
  VoteLog.create(:poll_id => poll_id, :client_ip => client_ip)
end

def already_voted
  render :text => "You already voted, no cheating!"
end

Just keep in mind this approach might not be appropriate in all situations. Due to Network Address Translation (NAT) firewalls, many thousands of people will appear to have the same client_ip. This is particularly true in corporate environments. If that's a concern, you'll need to go with a full-blown registered-user approach.

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How to obtain the IP address of the current user

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If you've ever needed to implement user-friendly upload, you know intimately what a pain it is to get right. The web just isn't built for uploading files from a browser. I mean, it kinda works, but even then only with a dozen or so limitations. Even the major photo and video sites have tried various solutions to make this easier for users. So when I built Dibs.net, I decided rather quickly to abandon all hope of getting it working flawlessly with plain ol' Javascript and HTML, and instead looked into using a fairly nonintrusive Flash uploader component. (Without Flash installed, it just falls back to a simple HTML-based file-upload form.)

That's not to say it was perfectly simple to get working with Rails. Because Dibs.net accepts uploads only from logged-in users, I ran into two limitations that would not allow me to use this solution:

• Flash doesn't send the cookies from the browser (at least it doesn't in Firefox; it might in IE)
• Rails doesn't support non-cookie sessions

Because Flash doesn't send the session cookie, Rails thinks the request is coming from a new, logged-out user and creates a new session for it. Adding a cookies feature to Flash was well out of my hands since I don't work for Adobe, so I looked into a way to restore the session from a session key passed as a URL parameter. After some experimentation, I found a solution that works great.

Continue reading "Restoring Rails session data when cookies aren’t available"

Web applications can receive requests directly, via a CGI process, through proxy servers, relayed from front-end web servers, and so on. This can complicate how you might find out where the request originated if you, for example, wanted to limit an online poll to one vote per IP address. Luckily, Rails consolidates most of the ways to get this info into a single convenience method on the request object for us.

Continue reading "How to obtain the IP address of the current user"