diff --git a/lib/logstash/outputs/elasticsearch/http_client/pool.rb b/lib/logstash/outputs/elasticsearch/http_client/pool.rb
index cbafdb26..814c05cb 100644
--- a/lib/logstash/outputs/elasticsearch/http_client/pool.rb
+++ b/lib/logstash/outputs/elasticsearch/http_client/pool.rb
@@ -265,8 +265,12 @@ def healthcheck!
     end
 
     def elasticsearch?(url)
-      response = perform_request_to_url(url, :get, "/")
-      return false if response.code == 401 || response.code == 403
+      begin
+        response = perform_request_to_url(url, :get, "/")
+      rescue ::LogStash::Outputs::ElasticSearch::HttpClient::Pool::BadResponseCodeError => e
+        return false if response.code == 401 || response.code == 403
+        raise e
+      end
 
       version_info = LogStash::Json.load(response.body)
       return false if version_info['version'].nil?
@@ -281,7 +285,7 @@ def elasticsearch?(url)
         return false if build_flavour.nil? || build_flavour != 'default' || !valid_tagline?(version_info)
       else
         # case >= 7.14
-        product_header = response.headers['X-elastic-product']
+        product_header = response.headers['x-elastic-product']
         return false if product_header.nil? || product_header != 'Elasticsearch'
       end
       return true
diff --git a/logstash-output-elasticsearch.gemspec b/logstash-output-elasticsearch.gemspec
index 13422e2e..f680554f 100644
--- a/logstash-output-elasticsearch.gemspec
+++ b/logstash-output-elasticsearch.gemspec
@@ -30,6 +30,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'logstash-devutils'
   s.add_development_dependency 'flores'
   s.add_development_dependency 'cabin', ['~> 0.6']
+  s.add_development_dependency 'webmock'
   # Still used in some specs, we should remove this ASAP
   s.add_development_dependency 'elasticsearch'
 end
diff --git a/spec/unit/outputs/elasticsearch/http_client/pool_spec.rb b/spec/unit/outputs/elasticsearch/http_client/pool_spec.rb
index e89e835a..3636e717 100644
--- a/spec/unit/outputs/elasticsearch/http_client/pool_spec.rb
+++ b/spec/unit/outputs/elasticsearch/http_client/pool_spec.rb
@@ -1,6 +1,7 @@
 require "logstash/devutils/rspec/spec_helper"
 require "logstash/outputs/elasticsearch/http_client"
 require 'cabin'
+require 'webmock/rspec'
 
 describe LogStash::Outputs::ElasticSearch::HttpClient::Pool do
   let(:logger) { Cabin::Channel.get }
@@ -341,3 +342,125 @@ def code
     end
   end
 end
+
+describe "#elasticsearch?" do
+  let(:logger) { Cabin::Channel.get }
+  let(:adapter) { LogStash::Outputs::ElasticSearch::HttpClient::ManticoreAdapter.new(logger) }
+  let(:initial_urls) { [::LogStash::Util::SafeURI.new("http://localhost:9200")] }
+  let(:options) { {:resurrect_delay => 2, :url_normalizer => proc {|u| u}} } # Shorten the delay a bit to speed up tests
+  let(:es_node_versions) { [ "0.0.0" ] }
+  let(:license_status) { 'active' }
+
+  subject { LogStash::Outputs::ElasticSearch::HttpClient::Pool.new(logger, adapter, initial_urls, options) }
+
+  let(:url) { ::LogStash::Util::SafeURI.new("http://localhost:9200") }
+
+  context "in case HTTP error code" do
+    it "should fail for 401" do
+      stub_request(:get, "localhost:9200").to_return(status: 401)
+      expect(subject.elasticsearch?(url)).to be false
+
+   #   # stub_request(:post, "www.example.com").
+   #   #   with(body: "abc", headers: { 'Content-Length' => 3 })
+   #   #
+   #   uri = URI.parse("http://localhost:9300/")
+   #   # req = Net::HTTP::Get.new(uri.path)
+   #   # req['Content-Length'] = 3
+   #
+   #   #res = Net::HTTP.start(uri.host, uri.port) do |http|
+   #   #  http.request(req, "abc")
+   #   #end
+   #   res = Net::HTTP.get_response(uri)
+   #   expect(res.class).to eq(Net::HTTPUnauthorized)
+   #   expect(res).to be_a(Net::HTTPUnauthorized)
+   #   puts "DNADBG>> HTTP net response: #{res}"
+   #
+   #
+   #   # using Manticore
+   #   manticore = ::Manticore::Client.new()
+   #   #resp = manticore.get("http://localhost:9300/")
+   #   resp = manticore.send("get", "http://localhost:9300/", {})
+   #   resp.call
+   #   puts "DNADBG>> Manticore response: #{resp.class}"
+   #   puts "DNADBG>> resp.code: #{resp.code}"
+   #   expect(resp.code).to eq(401)
+
+    end
+
+    it "should fail for 403" do
+      stub_request(:get, "localhost:9200").to_return(status: 403)
+      expect(subject.elasticsearch?(url)).to be false
+    end
+  end
+
+  context "when connecting to a cluster which reply without 'version' field" do
+    it "should fail" do
+      stub_request(:get, "localhost:9200").to_return(body: '{"field": "funky"}')
+      expect(subject.elasticsearch?(url)).to be false
+    end
+  end
+
+  context "when connecting to a cluster with version < 6.0.0" do
+    it "should fail" do
+      stub_request(:get, "localhost:9200").to_return(body: '{"version": {"number": "5.0.0"}}')
+      expect(subject.elasticsearch?(url)).to be false
+    end
+  end
+
+  context "when connecting to a cluster with version in [6.0.0..7.0.0)" do
+    it "must be successful with valid 'tagline'" do
+      stub_request(:get, "localhost:9200").to_return(status: 200, body: '{"version": {"number": "6.5.0"}, "tagline": "You Know, for Search"}')
+      expect(subject.elasticsearch?(url)).to be true
+    end
+
+    it "should fail if invalid 'tagline'" do
+      stub_request(:get, "localhost:9200").to_return(body: '{"version": {"number": "6.5.0"}, "tagline": "You don\'t know"}')
+      expect(subject.elasticsearch?(url)).to be false
+    end
+
+    it "should fail if 'tagline' is not present" do
+      stub_request(:get, "localhost:9200").to_return(body: '{"version": {"number": "6.5.0"}}')
+      expect(subject.elasticsearch?(url)).to be false
+    end
+  end
+
+  context "when connecting to a cluster with version in [7.0.0..7.14.0)" do
+    it "must be successful is 'build_flavour' is 'default' and tagline is correct" do
+      stub_request(:get, "localhost:9200/").to_return(status: 200, body: '{"version": {"number": "7.5.0", "build_flavour": "default"}, "tagline": "You Know, for Search"}')
+      expect(subject.elasticsearch?(url)).to be true
+    end
+
+    it "should fail if 'build_flavour' is not 'default' and tagline is correct" do
+      stub_request(:get, "localhost:9200").to_return(status: 200, body: '{"version": {"number": "7.5.0", "build_flavour": "oss"}, "tagline": "You Know, for Search"}')
+      expect(subject.elasticsearch?(url)).to be false
+    end
+
+    it "should fail if 'build_flavour' is not present and tagline is correct" do
+      stub_request(:get, "localhost:9200").to_return(status: 200, body: '{"version": {"number": "7.5.0"}, "tagline": "You Know, for Search"}')
+      expect(subject.elasticsearch?(url)).to be false
+    end
+  end
+
+  context "when connecting to a cluster with version >= 7.14.0" do
+    it "should fail if 'X-elastic-product' header is not present" do
+      stub_request(:get, "localhost:9200").to_return(status: 200, body: '{"version": {"number": "7.14.0"}}')
+      expect(subject.elasticsearch?(url)).to be false
+    end
+
+    it "should fail if 'X-elastic-product' header is present but with bad value" do
+      stub_request(:get, "localhost:9200")
+            .to_return(status: 200,
+                       headers: {'X-elastic-product' => 'not good'},
+                       body: '{"version": {"number": "7.14.0"}}')
+      expect(subject.elasticsearch?(url)).to be false
+    end
+
+    it "must be successful when 'X-elastic-product' header is present with expected value" do
+      stub_request(:get, "localhost:9200")
+            .to_return(status: 200,
+                       headers: {'X-elastic-product': 'Elasticsearch'},
+                       body: '{"version": {"number": "7.14.0"}}')
+      expect(subject.elasticsearch?(url)).to be true
+    end
+  end
+end