Run Watir-Webdriver through a proxy to capture network traffic. Heavily inspired by https://web.archive.org/web/20110202071609/http://altentee.com/blogs/2008/httpwatch-a-free-alternative-using-ruby/

proxied_browser.rb

#!/usr/bin/env ruby
require 'watir-webdriver'
require 'webrick/httpproxy'

# USAGE: ./proxied_browser.rb server (port) [json]
#        ./proxied_browser.rb browser (port)

class ProxiedBrowser < Watir::Browser
  def initialize(port)
    proxy = ['localhost',port].join(?:)
    profile = Selenium::WebDriver::Firefox::Profile.new
    profile.proxy = Selenium::WebDriver::Proxy.new(
      http: proxy,
      ftp: proxy,
      ssl: proxy
    )
    super :firefox, profile: profile
  end
end

class Proxy < WEBrick::HTTPProxyServer
  def initialize(port)
    options = {
      Port: port,
      AccessLog: [],
      ProxyContentHandler: ->(req, res) do
        if ARGV.include? 'json'
          puts format_json(req, res)
          puts "JSONSEPARATOR\n"
        else
          puts format_text(req, res)
        end
      end
    }
    super options
  end
  
  def self.start(port)
    server = new(port)
    trap("INT") { server.shutdown }
    server.start
  end
  
  private
  
  def format_json(req, res)
    require 'oj'
    hash = {
      'request' => {
        'request_line' => req.request_line.chomp,
        'headers' => req.header
      },
      'response' => {
        'headers' => res.header,
        'body' => parse_body(res)
      }
    }
    Oj.dump hash
  end
  
  def format_text(req, res)
    puts "-" * 75
    puts ">>> #{req.request_line.chomp}\n"
    req.header.keys.each do |k|
      puts "#{k.capitalize}: #{req.header[k]}"
    end
    puts "<<<"
    puts res.status_line
    res.header.keys.each do |k|
      puts "#{k.capitalize}: #{res.header[k]}"
    end
    unless res.body.nil?
      body = parse_body(res)
      body.split("\n").each_with_index do |line, i|
        puts "\n<<< #{i} #{line}"
      end
    end
  end
  
  def parse_body(res)
    primitive_body = res.body
    case res.header['content-encoding']
    when 'gzip'
      Zlib::GzipReader.new(StringIO.new(primitive_body), encoding: 'ASCII-8BIT').read
    when 'deflate'
      Zlib::Inflate.inflate(primitive_body)
    else
      primitive_body
    end
  end
end

unless ARGV.empty?
  case ARGV[0]
  when 'server' then Proxy.start(ARGV[1])
  when 'browser'
    begin
      b = ProxiedBrowser.new(ARGV[1])
      require 'pry'
      pry
    ensure
      b.close
    end
  end
end