Helper to find the final destination for a URL

lib/final_destination.rb

@@ -0,0 +1,119 @@
+require "socket"
+require "ipaddr"
+require 'excon'
+
+# Determine the final endpoint for a Web URI, following redirects
+class FinalDestination
+
+  attr_reader :status
+
+  def initialize(url, opts = nil)
+    @uri = URI(url) rescue nil
+    @opts = opts || {}
+    @opts[:max_redirects] ||= 5
+    @opts[:lookup_ip] ||= lambda do |host|
+      begin
+        IPSocket::getaddress(host)
+      rescue SocketError
+        nil
+      end
+    end
+    @limit = @opts[:max_redirects]
+    @status = :ready
+  end
+
+  def redirected?
+    @limit < @opts[:max_redirects]
+  end
+
+  def request_headers
+    { "User-Agent" => "Mozilla/5.0 (Windows NT 6.2; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36",
+      "Accept" => "text/html",
+      "Host" => @uri.hostname }
+  end
+
+  def resolve
+    if @limit < 0
+      @status = :too_many_redirects
+      return nil
+    end
+
+    return nil unless validate_uri
+    headers = request_headers
+    head = Excon.head(@uri.to_s, read_timeout: 20, headers: headers)
+
+    # If the site does not allow HEAD, just try the url
+    return @uri if head.status == 405
+
+    if head.status == 200
+      @uri = nil unless FinalDestination.header_for(head, 'content-type') =~ /text\/html/
+      @status = :resolved
+      return @uri
+    end
+
+    location = FinalDestination.header_for(head, 'location')
+    if location
+      location = "#{@uri.scheme}://#{@uri.host}#{location}" if location[0] == "/"
+      @uri = URI(location) rescue nil
+      @limit -= 1
+      return resolve
+    end
+
+    nil
+  end
+
+  def validate_uri
+    validate_uri_format && is_public?
+  end
+
+  def validate_uri_format
+    return false unless @uri
+    return false unless ['https', 'http'].include?(@uri.scheme)
+
+    if @uri.scheme == 'http'
+      return @uri.port == 80
+    elsif @uri.scheme == 'https'
+      return @uri.port == 443
+    end
+
+    false
+  end
+
+  def is_public?
+    return false unless @uri && @uri.host
+
+    address_s = @opts[:lookup_ip].call(@uri.hostname)
+    return false unless address_s
+
+    address = IPAddr.new(address_s)
+    private_match = FinalDestination.private_ranges.any? {|r| r === address }
+
+    if private_match
+      @status = :invalid_address
+      return false
+    end
+
+    true
+  end
+
+  def self.private_ranges
+    @private_ranges ||= [
+      IPAddr.new('127.0.0.1'),
+      IPAddr.new('172.16.0.0/12'),
+      IPAddr.new('192.168.0.0/16'),
+      IPAddr.new('10.0.0.0/8'),
+      IPAddr.new('fc00::/7')
+    ]
+  end
+
+  def self.lookup_ip(host)
+    IPSocket::getaddress(host)
+  end
+
+  def self.header_for(head, name)
+    header = head.headers.detect do |k, _|
+      name == k.downcase
+    end
+    header[1] if header
+  end
+end