Add ipaddr optional parameter to Net::HTTP#start

to replace the address for TCP/IP connection [Feature #5180]

There're 3 layers of hostname:
* host address for TCP/IP
* TLS server name
* HTTP Host header value
To test DNS round robin or check server certificate from server local,
people sometimes want to connect server with given IP address but keep
TLS server name and HTTP Host header value.

closes [Feature #15215]
closes https://github.com/ruby/ruby/pull/1893
closes https://github.com/ruby/ruby/pull/1977

NEWS

@@ -508,6 +508,11 @@ Net::FTP::
   * Add Net::FTP#features to check available features, and Net::FTP#option to
     enable/disable each of them.  [Feature #15964]
 
+Net::HTTP::
+
+  * Add ipaddr optional parameter to Net::HTTP#start to replace the address for
+    TCP/IP connection [Feature #5180]
+
 Net::IMAP::
 
   * Add Server Name Indication (SNI) support.  [Feature #15594]

lib/net/http.rb

@@ -571,7 +571,7 @@ module Net   #:nodoc:
     # _opt_     :: optional hash
     #
     # _opt_ sets following values by its accessor.
-    # The keys are ca_file, ca_path, cert, cert_store, ciphers,
+    # The keys are ipaddr, ca_file, ca_path, cert, cert_store, ciphers,
     # close_on_empty_response, key, open_timeout, read_timeout, write_timeout, ssl_timeout,
     # ssl_version, use_ssl, verify_callback, verify_depth and verify_mode.
     # If you set :use_ssl as true, you can use https and default value of
@@ -590,6 +590,7 @@ module Net   #:nodoc:
       p_addr = :ENV if arg.size < 2
       port = https_default_port if !port && opt && opt[:use_ssl]
       http = new(address, port, p_addr, p_port, p_user, p_pass)
+      http.ipaddr = opt[:ipaddr] if opt[:ipaddr]
 
       if opt
         if opt[:use_ssl]
@@ -660,6 +661,7 @@ module Net   #:nodoc:
     def initialize(address, port = nil)
       @address = address
       @port    = (port || HTTP.default_port)
+      @ipaddr = nil
       @local_host = nil
       @local_port = nil
       @curr_http_version = HTTPVersion
@@ -727,6 +729,17 @@ module Net   #:nodoc:
     attr_writer :proxy_user
     attr_writer :proxy_pass
 
+    # The IP address to connect to/used to connect to
+    def ipaddr
+      started? ?  @socket.io.peeraddr[3] : @ipaddr
+    end
+
+    # Set the IP address to connect to
+    def ipaddr=(addr)
+      raise IOError, "ipaddr value changed, but session already started" if started?
+      @ipaddr = addr
+    end
+
     # Number of seconds to wait for the connection to open. Any number
     # may be used, including Floats for fractional seconds. If the HTTP
     # object cannot open a connection in this many seconds, it raises a
@@ -934,20 +947,20 @@ module Net   #:nodoc:
 
     def connect
       if proxy? then
-        conn_address = proxy_address
+        conn_addr = proxy_address
         conn_port    = proxy_port
       else
-        conn_address = address
+        conn_addr = conn_address
         conn_port    = port
       end
 
-      D "opening connection to #{conn_address}:#{conn_port}..."
+      D "opening connection to #{conn_addr}:#{conn_port}..."
       s = Timeout.timeout(@open_timeout, Net::OpenTimeout) {
         begin
-          TCPSocket.open(conn_address, conn_port, @local_host, @local_port)
+          TCPSocket.open(conn_addr, conn_port, @local_host, @local_port)
         rescue => e
           raise e, "Failed to open TCP connection to " +
-            "#{conn_address}:#{conn_port} (#{e.message})"
+            "#{conn_addr}:#{conn_port} (#{e.message})"
         end
       }
       s.setsockopt(Socket::IPPROTO_TCP, Socket::TCP_NODELAY, 1)
@@ -984,7 +997,7 @@ module Net   #:nodoc:
           OpenSSL::SSL::SSLContext::SESSION_CACHE_CLIENT |
           OpenSSL::SSL::SSLContext::SESSION_CACHE_NO_INTERNAL_STORE
         @ssl_context.session_new_cb = proc {|sock, sess| @ssl_session = sess }
-        D "starting SSL for #{conn_address}:#{conn_port}..."
+        D "starting SSL for #{conn_addr}:#{conn_port}..."
         s = OpenSSL::SSL::SSLSocket.new(s, @ssl_context)
         s.sync_close = true
         # Server Name Indication (SNI) RFC 3546
@@ -1161,7 +1174,7 @@ module Net   #:nodoc:
     # without proxy, obsolete
 
     def conn_address # :nodoc:
-      address()
+      @ipaddr || address()
     end
 
     def conn_port # :nodoc:

test/net/http/test_https.rb

@@ -50,6 +50,37 @@ class TestNetHTTPS < Test::Unit::TestCase
     skip $!
   end
 
+  def test_get_SNI
+    http = Net::HTTP.new("localhost", config("port"))
+    http.ipaddr = config('host')
+    http.use_ssl = true
+    http.cert_store = TEST_STORE
+    certs = []
+    http.verify_callback = Proc.new do |preverify_ok, store_ctx|
+      certs << store_ctx.current_cert
+      preverify_ok
+    end
+    http.request_get("/") {|res|
+      assert_equal($test_net_http_data, res.body)
+    }
+    assert_equal(CA_CERT.to_der, certs[0].to_der)
+    assert_equal(SERVER_CERT.to_der, certs[1].to_der)
+  end
+
+  def test_get_SNI_failure
+    http = Net::HTTP.new("invalid_servername", config("port"))
+    http.ipaddr = config('host')
+    http.use_ssl = true
+    http.cert_store = TEST_STORE
+    certs = []
+    http.verify_callback = Proc.new do |preverify_ok, store_ctx|
+      certs << store_ctx.current_cert
+      preverify_ok
+    end
+    @log_tester = lambda {|_| }
+    assert_raise(OpenSSL::SSL::SSLError){ http.start }
+  end
+
   def test_post
     http = Net::HTTP.new("localhost", config("port"))
     http.use_ssl = true