diff --git lib/rubygems/remote_fetcher.rb lib/rubygems/remote_fetcher.rb
index 157a308..facbc7a 100644
--- lib/rubygems/remote_fetcher.rb
+++ lib/rubygems/remote_fetcher.rb
@@ -103,7 +103,13 @@ class Gem::RemoteFetcher
     rescue Resolv::ResolvError
       uri
     else
-      URI.parse "#{res.target}#{uri.path}"
+      target = res.target.to_s.strip
+
+      if /\.#{Regexp.quote(host)}\z/ =~ target
+        return URI.parse "#{uri.scheme}://#{target}#{uri.path}"
+      end
+
+      uri
     end
   end

diff --git test/rubygems/test_gem_remote_fetcher.rb test/rubygems/test_gem_remote_fetcher.rb
index 24ab815..e3774d2 100644
--- test/rubygems/test_gem_remote_fetcher.rb
+++ test/rubygems/test_gem_remote_fetcher.rb
@@ -177,15 +177,60 @@ gems:
   end

   def test_api_endpoint
+    uri = URI.parse "http://example.com/foo"
+    target = MiniTest::Mock.new
+    target.expect :target, "gems.example.com"
+
+    dns = MiniTest::Mock.new
+    dns.expect :getresource, target, [String, Object]
+
+    fetch = Gem::RemoteFetcher.new nil, dns
+    assert_equal URI.parse("http://gems.example.com/foo"), fetch.api_endpoint(uri)
+
+    target.verify
+    dns.verify
+  end
+
+  def test_api_endpoint_ignores_trans_domain_values
     uri = URI.parse "http://gems.example.com/foo"
     target = MiniTest::Mock.new
-    target.expect :target, "http://blah.com"
+    target.expect :target, "blah.com"

     dns = MiniTest::Mock.new
     dns.expect :getresource, target, [String, Object]

     fetch = Gem::RemoteFetcher.new nil, dns
-    assert_equal URI.parse("http://blah.com/foo"), fetch.api_endpoint(uri)
+    assert_equal URI.parse("http://gems.example.com/foo"), fetch.api_endpoint(uri)
+
+    target.verify
+    dns.verify
+  end
+
+  def test_api_endpoint_ignores_trans_domain_values_that_starts_with_original
+    uri = URI.parse "http://example.com/foo"
+    target = MiniTest::Mock.new
+    target.expect :target, "example.combadguy.com"
+
+    dns = MiniTest::Mock.new
+    dns.expect :getresource, target, [String, Object]
+
+    fetch = Gem::RemoteFetcher.new nil, dns
+    assert_equal URI.parse("http://example.com/foo"), fetch.api_endpoint(uri)
+
+    target.verify
+    dns.verify
+  end
+
+  def test_api_endpoint_ignores_trans_domain_values_that_end_with_original
+    uri = URI.parse "http://example.com/foo"
+    target = MiniTest::Mock.new
+    target.expect :target, "badexample.com"
+
+    dns = MiniTest::Mock.new
+    dns.expect :getresource, target, [String, Object]
+
+    fetch = Gem::RemoteFetcher.new nil, dns
+    assert_equal URI.parse("http://example.com/foo"), fetch.api_endpoint(uri)

     target.verify
     dns.verify