Index: lib/net/http.rb
===================================================================
--- lib/net/http.rb	(revision 36037)
+++ lib/net/http.rb	(working copy)
@@ -268,7 +268,18 @@ module Net   #:nodoc:
   # === Proxies
   #
   # Net::HTTP::Proxy has the same methods as Net::HTTP but its instances always
-  # connect via the proxy instead of directly to the given host.
+  # connect via the proxy instead of directly to the given host.  Net::HTTP
+  # can create a proxy from the +HTTP_PROXY+ environment variable if +:ENV+ is
+  # used for the proxy address:
+  #
+  #   Net::HTTP::Proxy(:ENV).start('www.example.com') { |http|
+  #     # connect with ENV['HTTP_PROXY'] if available
+  #   end
+  #
+  # Net::HTTP will be returned when no proxy is configured so there is no need
+  # for conditional code.
+  #
+  # You may also create a custom proxy:
   #
   #   proxy_addr = 'your.proxy.host'
   #   proxy_port = 8080
@@ -277,9 +288,6 @@ module Net   #:nodoc:
   #     # always connect to your.proxy.addr:8080
   #   }
   #
-  # Net::HTTP::Proxy returns a Net::HTTP instance when proxy_addr is nil so
-  # there is no need for conditional code.
-  #
   # See Net::HTTP::Proxy for further details and examples such as proxies that
   # require a username and password.
   #
@@ -878,15 +886,21 @@ module Net   #:nodoc:
     # Creates an HTTP proxy class which behaves like Net::HTTP, but
     # performs all access via the specified proxy.
     #
-    # The arguments are the DNS name or IP address of the proxy host,
-    # the port to use to access the proxy, and a username and password
-    # if authorization is required to use the proxy.
+    # If +:ENV+ is given for +p_addr+, the proxy host and port are taken from
+    # the +HTTP_PROXY+ environment variable (or its lowercase equivalent) if
+    # present.  If the proxy requires authentication you must supply it by
+    # hand.
+    #
+    # If you are connecting to a custom proxy, the arguments are the DNS name
+    # or IP address of the proxy host, the port to use to access the proxy,
+    # and a username and password if authorization is required to use the
+    # proxy.
+    #
+    # If +p_addr+ is nil, this method returns self (a Net::HTTP object).
     #
     # You can replace any use of the Net::HTTP class with use of the
     # proxy class created.
     #
-    # If +p_addr+ is nil, this method returns self (a Net::HTTP object).
-    #
     #   # Example
     #   proxy_class = Net::HTTP::Proxy('proxy.example.com', 8080)
     #
@@ -907,15 +921,21 @@ module Net   #:nodoc:
     #     # and password
     #   }
     #
-    # Note that net/http does not use the HTTP_PROXY environment variable.
-    # If you want to use a proxy, you must set it explicitly.
-    #
-    def HTTP.Proxy(p_addr, p_port = nil, p_user = nil, p_pass = nil)
+    def HTTP.Proxy(p_addr = nil, p_port = nil, p_user = nil, p_pass = nil)
       return self unless p_addr
-      delta = ProxyDelta
-      proxyclass = Class.new(self)
-      proxyclass.module_eval {
-        include delta
+
+      if p_addr == :ENV then
+        env_proxy = ENV['http_proxy'] || ENV['HTTP_PROXY']
+
+        return self unless env_proxy
+
+        env_proxy =~ /:([^\]]+)\z/ # handle IPv6 address like [::1]:8000
+        p_addr = $` || env_proxy
+        p_port = $1.to_i if $1
+      end
+
+      Class.new(self) {
+        include Net::HTTP::ProxyDelta
         # with proxy
         @is_proxy_class = true
         @proxy_address = p_addr
@@ -923,7 +943,6 @@ module Net   #:nodoc:
         @proxy_user    = p_user
         @proxy_pass    = p_pass
       }
-      proxyclass
     end
 
     class << HTTP
Index: test/net/http/test_http.rb
===================================================================
--- test/net/http/test_http.rb	(revision 36037)
+++ test/net/http/test_http.rb	(working copy)
@@ -5,6 +5,132 @@ require 'net/http'
 require 'stringio'
 require_relative 'utils'
 
+class TestNetHTTP < Test::Unit::TestCase
+
+  def test_class_Proxy
+    no_proxy_class = Net::HTTP.Proxy nil
+
+    assert_equal Net::HTTP, no_proxy_class
+
+    proxy_class = Net::HTTP.Proxy 'proxy.example', 8000, 'user', 'pass'
+
+    refute_equal Net::HTTP, proxy_class
+
+    assert_operator proxy_class, :<, Net::HTTP
+
+    assert_equal 'proxy.example', proxy_class.proxy_address
+    assert_equal 8000,            proxy_class.proxy_port
+    assert_equal 'user',          proxy_class.proxy_user
+    assert_equal 'pass',          proxy_class.proxy_pass
+  end
+
+  def test_class_Proxy_from_ENV
+    clean_http_proxy_env do
+      ENV['HTTP_PROXY']      = 'proxy.example:8000'
+
+      # These are ignored on purpose.  See Bug 4388 and Feature 6546
+      ENV['HTTP_PROXY_USER'] = 'user'
+      ENV['HTTP_PROXY_PASS'] = 'pass'
+
+      proxy_class = Net::HTTP.Proxy :ENV
+
+      refute_equal Net::HTTP, proxy_class
+
+      assert_operator proxy_class, :<, Net::HTTP
+
+      assert_equal 'proxy.example', proxy_class.proxy_address
+      assert_equal 8000,            proxy_class.proxy_port
+      assert_nil                    proxy_class.proxy_user
+      assert_nil                    proxy_class.proxy_pass
+    end
+  end
+
+  def test_class_Proxy_from_ENV_ipv6
+    clean_http_proxy_env do
+      ENV['HTTP_PROXY'] = '[::1]:8000'
+
+      proxy_class = Net::HTTP.Proxy :ENV
+
+      refute_equal Net::HTTP, proxy_class
+
+      assert_operator proxy_class, :<, Net::HTTP
+
+      assert_equal '[::1]', proxy_class.proxy_address
+      assert_equal 8000,    proxy_class.proxy_port
+      assert_nil            proxy_class.proxy_user
+      assert_nil            proxy_class.proxy_pass
+    end
+  end
+
+  def test_class_Proxy_from_ENV_lowercase
+    clean_http_proxy_env do
+      ENV['http_proxy']      = 'proxy.example:8000'
+
+      # These are ignored on purpose.  See Bug 4388 and Feature 6546
+      ENV['http_proxy_user'] = 'user'
+      ENV['http_proxy_pass'] = 'pass'
+
+      proxy_class = Net::HTTP.Proxy :ENV
+
+      refute_equal Net::HTTP, proxy_class
+
+      assert_operator proxy_class, :<, Net::HTTP
+
+      assert_equal 'proxy.example', proxy_class.proxy_address
+      assert_equal 8000,            proxy_class.proxy_port
+      assert_nil                    proxy_class.proxy_user
+      assert_nil                    proxy_class.proxy_pass
+    end
+  end
+
+  def test_class_Proxy_from_ENV_none
+    clean_http_proxy_env do
+      no_proxy_class = Net::HTTP.Proxy :ENV
+
+      assert_equal Net::HTTP, no_proxy_class
+    end
+  end
+
+  def test_class_Proxy_from_ENV_no_port
+    clean_http_proxy_env do
+      ENV['HTTP_PROXY'] = 'proxy.example'
+
+      proxy_class = Net::HTTP.Proxy :ENV
+
+      refute_equal Net::HTTP, proxy_class
+
+      assert_operator proxy_class, :<, Net::HTTP
+
+      assert_equal 'proxy.example', proxy_class.proxy_address
+      assert_equal 80,              proxy_class.proxy_port
+      assert_nil                    proxy_class.proxy_user
+      assert_nil                    proxy_class.proxy_pass
+    end
+  end
+
+  def clean_http_proxy_env
+    orig = {
+      'HTTP_PROXY'      => ENV['HTTP_PROXY'],
+      'http_proxy'      => ENV['HTTP_PROXY'],
+      'HTTP_PROXY_USER' => ENV['HTTP_PROXY_USER'],
+      'http_proxy_user' => ENV['http_proxy_user'],
+      'HTTP_PROXY_PASS' => ENV['HTTP_PROXY_PASS'],
+      'http_proxy_pass' => ENV['http_proxy_pass'],
+    }
+
+    orig.each_key do |key|
+      ENV.delete key
+    end
+
+    yield
+  ensure
+    orig.each do |key, value|
+      ENV[key] = value
+    end
+  end
+
+end
+
 module TestNetHTTP_version_1_1_methods
 
   def test_s_get