diff --git a/test/command/test_cat.rb b/test/command/test_cat.rb
index 51c1a20bbb..f1e1067823 100644
--- a/test/command/test_cat.rb
+++ b/test/command/test_cat.rb
@@ -18,7 +18,7 @@ def setup
     @primary = create_primary
     metadata = @primary.buffer.new_metadata
     @chunk = create_chunk(@primary, metadata, @es)
-    @port = unused_port
+    @port = unused_port(protocol: :tcp)
   end
 
   def teardown
diff --git a/test/command/test_fluentd.rb b/test/command/test_fluentd.rb
index 60e0b131ed..0b2106f129 100644
--- a/test/command/test_fluentd.rb
+++ b/test/command/test_fluentd.rb
@@ -1175,7 +1175,7 @@ def multi_workers_ready?; true; end
     end
   end
 
-  sub_test_case 'sahred socket options' do
+  sub_test_case 'shared socket options' do
     test 'enable shared socket by default' do
       conf = ""
       conf_path = create_conf_file('empty.conf', conf)
diff --git a/test/helper.rb b/test/helper.rb
index 9dc8633423..4e9f028a9c 100644
--- a/test/helper.rb
+++ b/test/helper.rb
@@ -71,17 +71,31 @@ class Test::Unit::AssertionFailedError < StandardError
 
 include Fluent::Test::Helpers
 
-def unused_port(num = 1, protocol: :tcp, bind: "0.0.0.0")
+def unused_port(num = 1, protocol:, bind: "0.0.0.0")
   case protocol
-  when :tcp
+  when :tcp, :tls
     unused_port_tcp(num)
   when :udp
     unused_port_udp(num, bind: bind)
+  when :all
+    unused_port_tcp_udp(num)
   else
     raise ArgumentError, "unknown protocol: #{protocol}"
   end
 end
 
+def unused_port_tcp_udp(num = 1)
+  raise "not support num > 1" if num > 1
+
+  # The default maximum number of file descriptors in macOS is 256.
+  # It might need to set num to a smaller value than that.
+  tcp_ports = unused_port_tcp(200)
+  port = unused_port_udp(1, port_list: tcp_ports)
+  raise "can't find unused port" unless port
+
+  port
+end
+
 def unused_port_tcp(num = 1)
   ports = []
   sockets = []
@@ -90,7 +104,7 @@ def unused_port_tcp(num = 1)
     sockets << s
     ports << s.addr[1]
   end
-  sockets.each{|s| s.close }
+  sockets.each(&:close)
   if num == 1
     return ports.first
   else
@@ -100,12 +114,15 @@ def unused_port_tcp(num = 1)
 
 PORT_RANGE_AVAILABLE = (1024...65535)
 
-def unused_port_udp(num = 1, bind: "0.0.0.0")
+def unused_port_udp(num = 1, port_list: [], bind: "0.0.0.0")
   family = IPAddr.new(IPSocket.getaddress(bind)).ipv4? ? ::Socket::AF_INET : ::Socket::AF_INET6
   ports = []
   sockets = []
-  while ports.size < num
-    port = rand(PORT_RANGE_AVAILABLE)
+
+  use_random_port = port_list.empty?
+  i = 0
+  loop do
+    port = use_random_port ? rand(PORT_RANGE_AVAILABLE) : port_list[i]
     u = UDPSocket.new(family)
     if (u.bind(bind, port) rescue nil)
       ports << port
@@ -113,8 +130,11 @@ def unused_port_udp(num = 1, bind: "0.0.0.0")
     else
       u.close
     end
+    i += 1
+    break if ports.size >= num
+    break if !use_random_port && i >= port_list.size
   end
-  sockets.each{|s| s.close }
+  sockets.each(&:close)
   if num == 1
     return ports.first
   else
diff --git a/test/plugin/test_in_forward.rb b/test/plugin/test_in_forward.rb
index 9a3b46fd9a..cbebe91593 100644
--- a/test/plugin/test_in_forward.rb
+++ b/test/plugin/test_in_forward.rb
@@ -18,7 +18,8 @@ def setup
     Fluent::Test.setup
     @responses = []  # for testing responses after sending data
     @d = nil
-    @port = unused_port
+    # forward plugin uses TCP and UDP sockets on the same port number
+    @port = unused_port(protocol: :all)
   end
 
   def teardown
diff --git a/test/plugin/test_in_http.rb b/test/plugin/test_in_http.rb
index 3b9107f224..e60e30b402 100644
--- a/test/plugin/test_in_http.rb
+++ b/test/plugin/test_in_http.rb
@@ -18,7 +18,7 @@ def shutdown
 
   def setup
     Fluent::Test.setup
-    @port = unused_port
+    @port = unused_port(protocol: :tcp)
   end
 
   def teardown
diff --git a/test/plugin/test_in_monitor_agent.rb b/test/plugin/test_in_monitor_agent.rb
index 76e0a26a18..eb8d2dea7c 100644
--- a/test/plugin/test_in_monitor_agent.rb
+++ b/test/plugin/test_in_monitor_agent.rb
@@ -392,7 +392,7 @@ def test_enable_input_metrics(with_config)
     end
 
     test "emit" do
-      port = unused_port
+      port = unused_port(protocol: :tcp)
       d = create_driver("
   @type monitor_agent
   bind '127.0.0.1'
@@ -451,7 +451,7 @@ def get(uri, header = {})
 
   sub_test_case "servlets" do
     setup do
-      @port = unused_port
+      @port = unused_port(protocol: :tcp)
       # check @type and type in one configuration
       conf = <<-EOC
 <source>
@@ -759,7 +759,7 @@ def write(chunk)
     end
 
     setup do
-      @port = unused_port
+      @port = unused_port(protocol: :tcp)
       # check @type and type in one configuration
       conf = <<-EOC
 <source>
@@ -840,7 +840,7 @@ def write(chunk)
 
   sub_test_case "check the port number of http server" do
     test "on single worker environment" do
-      port = unused_port
+      port = unused_port(protocol: :tcp)
       d = create_driver("
   @type monitor_agent
   bind '127.0.0.1'
@@ -851,7 +851,7 @@ def write(chunk)
     end
 
     test "worker_id = 2 on multi worker environment" do
-      port = unused_port
+      port = unused_port(protocol: :tcp)
       Fluent::SystemConfig.overwrite_system_config('workers' => 4) do
         d = Fluent::Test::Driver::Input.new(Fluent::Plugin::MonitorAgentInput)
         d.instance.instance_eval{ @_fluentd_worker_id = 2 }
@@ -905,7 +905,7 @@ def filter(tag, time, record)
     end
 
     test "plugins have a variable named buffer does not throws NoMethodError" do
-      port = unused_port
+      port = unused_port(protocol: :tcp)
       d = create_driver("
   @type monitor_agent
   bind '127.0.0.1'
diff --git a/test/plugin/test_in_syslog.rb b/test/plugin/test_in_syslog.rb
index f715e1fef9..868e77bb04 100755
--- a/test/plugin/test_in_syslog.rb
+++ b/test/plugin/test_in_syslog.rb
@@ -5,24 +5,24 @@
 class SyslogInputTest < Test::Unit::TestCase
   def setup
     Fluent::Test.setup
-    @port = unused_port
+    @port = unused_port(protocol: :udp)
   end
 
   def teardown
     @port = nil
   end
 
-  def ipv4_config
+  def ipv4_config(port = @port)
     %[
-      port #{@port}
+      port #{port}
       bind 127.0.0.1
       tag syslog
     ]
   end
 
-  def ipv6_config
+  def ipv6_config(port = @port)
     %[
-      port #{@port}
+      port #{port}
       bind ::1
       tag syslog
     ]
@@ -69,7 +69,8 @@ def test_configure_resolve_hostname(param)
        'Use transport and protocol' => ["protocol_type udp\n<transport tcp>\n </transport>", :udp, :tcp])
   def test_configure_protocol(param)
     conf, proto_type, transport_proto_type = *param
-    d = create_driver([ipv4_config, conf].join("\n"))
+    port = unused_port(protocol: proto_type ? proto_type : transport_proto_type)
+    d = create_driver([ipv4_config(port), conf].join("\n"))
 
     assert_equal(d.instance.protocol_type, proto_type)
     assert_equal(d.instance.transport_config.protocol, transport_proto_type)
@@ -158,12 +159,13 @@ def test_msg_size_udp_for_large_msg
   end
 
   def test_msg_size_with_tcp
-    d = create_driver([ipv4_config, "<transport tcp> \n</transport>"].join("\n"))
+    port = unused_port(protocol: :tcp)
+    d = create_driver([ipv4_config(port), "<transport tcp> \n</transport>"].join("\n"))
     tests = create_test_case
 
     d.run(expect_emits: 2) do
       tests.each {|test|
-        TCPSocket.open('127.0.0.1', @port) do |s|
+        TCPSocket.open('127.0.0.1', port) do |s|
           s.send(test['msg'], 0)
         end
       }
@@ -189,11 +191,12 @@ def test_emit_rfc5452
   end
 
   def test_msg_size_with_same_tcp_connection
-    d = create_driver([ipv4_config, "<transport tcp> \n</transport>"].join("\n"))
+    port = unused_port(protocol: :tcp)
+    d = create_driver([ipv4_config(port), "<transport tcp> \n</transport>"].join("\n"))
     tests = create_test_case
 
     d.run(expect_emits: 2) do
-      TCPSocket.open('127.0.0.1', @port) do |s|
+      TCPSocket.open('127.0.0.1', port) do |s|
         tests.each {|test|
           s.send(test['msg'], 0)
         }
@@ -347,12 +350,13 @@ def compare_test_result(events, tests, options = {})
 
   sub_test_case 'octet counting frame' do
     def test_msg_size_with_tcp
-      d = create_driver([ipv4_config, "<transport tcp> \n</transport>", 'frame_type octet_count'].join("\n"))
+      port = unused_port(protocol: :tcp)
+      d = create_driver([ipv4_config(port), "<transport tcp> \n</transport>", 'frame_type octet_count'].join("\n"))
       tests = create_test_case
 
       d.run(expect_emits: 2) do
         tests.each {|test|
-          TCPSocket.open('127.0.0.1', @port) do |s|
+          TCPSocket.open('127.0.0.1', port) do |s|
             s.send(test['msg'], 0)
           end
         }
@@ -363,11 +367,12 @@ def test_msg_size_with_tcp
     end
 
     def test_msg_size_with_same_tcp_connection
-      d = create_driver([ipv4_config, "<transport tcp> \n</transport>", 'frame_type octet_count'].join("\n"))
+      port = unused_port(protocol: :tcp)
+      d = create_driver([ipv4_config(port), "<transport tcp> \n</transport>", 'frame_type octet_count'].join("\n"))
       tests = create_test_case
 
       d.run(expect_emits: 2) do
-        TCPSocket.open('127.0.0.1', @port) do |s|
+        TCPSocket.open('127.0.0.1', port) do |s|
           tests.each {|test|
             s.send(test['msg'], 0)
           }
@@ -469,7 +474,8 @@ def test_emit_unmatched_lines_with_address
   end
 
   def test_send_keepalive_packet_is_disabled_by_default
-    d = create_driver(ipv4_config + %[
+    port = unused_port(protocol: :tcp)
+    d = create_driver(ipv4_config(port) + %[
       <transport tcp>
       </transport>
       protocol tcp
@@ -479,19 +485,20 @@ def test_send_keepalive_packet_is_disabled_by_default
 
   def test_send_keepalive_packet_can_be_enabled
     addr = "127.0.0.1"
-    d = create_driver(ipv4_config + %[
+    port = unused_port(protocol: :tcp)
+    d = create_driver(ipv4_config(port) + %[
       <transport tcp>
       </transport>
       send_keepalive_packet true
     ])
     assert_true d.instance.send_keepalive_packet
     mock.proxy(d.instance).server_create_connection(
-      :in_syslog_tcp_server, @port,
+      :in_syslog_tcp_server, port,
       bind: addr,
       resolve_name: nil,
       send_keepalive_packet: true)
     d.run do
-      TCPSocket.open(addr, @port)
+      TCPSocket.open(addr, port)
     end
   end
 
diff --git a/test/plugin/test_in_tcp.rb b/test/plugin/test_in_tcp.rb
index c1d917332e..a27e00bba3 100755
--- a/test/plugin/test_in_tcp.rb
+++ b/test/plugin/test_in_tcp.rb
@@ -5,7 +5,7 @@
 class TcpInputTest < Test::Unit::TestCase
   def setup
     Fluent::Test.setup
-    @port = unused_port
+    @port = unused_port(protocol: :tcp)
   end
 
   def teardown
diff --git a/test/plugin/test_in_udp.rb b/test/plugin/test_in_udp.rb
index dcc63e1f8f..1f1bb457cb 100755
--- a/test/plugin/test_in_udp.rb
+++ b/test/plugin/test_in_udp.rb
@@ -5,7 +5,7 @@
 class UdpInputTest < Test::Unit::TestCase
   def setup
     Fluent::Test.setup
-    @port = unused_port
+    @port = unused_port(protocol: :udp)
   end
 
   def teardown
diff --git a/test/plugin/test_out_forward.rb b/test/plugin/test_out_forward.rb
index 438caf8fa2..306d9d8234 100644
--- a/test/plugin/test_out_forward.rb
+++ b/test/plugin/test_out_forward.rb
@@ -12,7 +12,8 @@ def setup
     FileUtils.rm_rf(TMP_DIR)
     FileUtils.mkdir_p(TMP_DIR)
     @d = nil
-    @target_port = unused_port
+    # forward plugin uses TCP and UDP sockets on the same port number
+    @target_port = unused_port(protocol: :all)
   end
 
   def teardown
diff --git a/test/plugin/test_out_stream.rb b/test/plugin/test_out_stream.rb
index 05e9011aab..f19328f5be 100644
--- a/test/plugin/test_out_stream.rb
+++ b/test/plugin/test_out_stream.rb
@@ -54,7 +54,7 @@ class TcpOutputTest < Test::Unit::TestCase
 
   def setup
     super
-    @port = unused_port
+    @port = unused_port(protocol: :tcp)
   end
 
   def teardown
diff --git a/test/plugin_helper/test_http_server_helper.rb b/test/plugin_helper/test_http_server_helper.rb
index 24312a9843..bc9853c013 100644
--- a/test/plugin_helper/test_http_server_helper.rb
+++ b/test/plugin_helper/test_http_server_helper.rb
@@ -14,7 +14,7 @@ class HttpHelperTest < Test::Unit::TestCase
   CERT_CA_DIR = File.expand_path(File.dirname(__FILE__) + '/data/cert/with_ca')
 
   def setup
-    @port = unused_port
+    @port = unused_port(protocol: :tcp)
   end
 
   def teardown
diff --git a/test/plugin_helper/test_server.rb b/test/plugin_helper/test_server.rb
index 347938eca3..63beb0bd70 100644
--- a/test/plugin_helper/test_server.rb
+++ b/test/plugin_helper/test_server.rb
@@ -15,7 +15,7 @@ class Dummy < Fluent::Plugin::TestBase
   TMP_DIR = File.expand_path(File.dirname(__FILE__) + "/../tmp/plugin_helper_server")
 
   setup do
-    @port = unused_port
+    @port = unused_port(protocol: :tcp)
     if Fluent.windows?
       @socket_manager_server = ServerEngine::SocketManager::Server.open
       @socket_manager_path = @socket_manager_server.path
@@ -233,11 +233,12 @@ class Dummy < Fluent::Plugin::TestBase
       # 'server_create_connection tcp' => [:server_create_connection, :unix],
     )
     test 'raise error if udp options specified for tcp/tls/unix' do |(m, proto)|
+      port = unused_port(protocol: proto)
       assert_raise ArgumentError do
-        @d.__send__(m, :myserver, @port, proto: proto, max_bytes: 128){|x| x }
+        @d.__send__(m, :myserver, port, proto: proto, max_bytes: 128){|x| x }
       end
       assert_raise ArgumentError do
-        @d.__send__(m, :myserver, @port, proto: proto, flags: 1){|x| x }
+        @d.__send__(m, :myserver, port, proto: proto, flags: 1){|x| x }
       end
     end
 
@@ -245,8 +246,9 @@ class Dummy < Fluent::Plugin::TestBase
       'server_create udp' => [:server_create, :udp],
     )
     test 'raise error if tcp/tls options specified for udp' do |(m, proto)|
+      port = unused_port(protocol: proto)
       assert_raise(ArgumentError.new("BUG: linger_timeout is available for tcp/tls")) do
-        @d.__send__(m, :myserver, @port, proto: proto, linger_timeout: 1, max_bytes: 128){|x| x }
+        @d.__send__(m, :myserver, port, proto: proto, linger_timeout: 1, max_bytes: 128){|x| x }
       end
     end
 
@@ -254,8 +256,9 @@ class Dummy < Fluent::Plugin::TestBase
       'server_create udp' => [:server_create, :udp],
     )
     test 'raise error if tcp/tls/unix backlog options specified for udp' do |(m, proto)|
+      port = unused_port(protocol: proto)
       assert_raise(ArgumentError.new("BUG: backlog is available for tcp/tls")) do
-        @d.__send__(m, :myserver, @port, proto: proto, backlog: 500){|x| x }
+        @d.__send__(m, :myserver, port, proto: proto, backlog: 500){|x| x }
       end
     end
 
@@ -263,8 +266,9 @@ class Dummy < Fluent::Plugin::TestBase
       'server_create udp' => [:server_create, :udp],
     )
     test 'raise error if tcp/tls send_keepalive_packet option is specified for udp' do |(m, proto)|
+      port = unused_port(protocol: proto)
       assert_raise(ArgumentError.new("BUG: send_keepalive_packet is available for tcp/tls")) do
-        @d.__send__(m, :myserver, @port, proto: proto, send_keepalive_packet: true){|x| x }
+        @d.__send__(m, :myserver, port, proto: proto, send_keepalive_packet: true){|x| x }
       end
     end
 
@@ -276,8 +280,9 @@ class Dummy < Fluent::Plugin::TestBase
       # 'server_create_connection unix' => [:server_create_connection, :unix, {}],
     )
     test 'raise error if tls options specified for tcp/udp/unix' do |(m, proto, kwargs)|
+      port = unused_port(protocol: proto)
       assert_raise(ArgumentError.new("BUG: tls_options is available only for tls")) do
-        @d.__send__(m, :myserver, @port, proto: proto, tls_options: {}, **kwargs){|x| x }
+        @d.__send__(m, :myserver, port, proto: proto, tls_options: {}, **kwargs){|x| x }
       end
     end
 
@@ -289,7 +294,8 @@ class Dummy < Fluent::Plugin::TestBase
       'server_create_connection tls' => [:server_create_connection, :tls, {tls_options: {insecure: true}}],
     )
     test 'can bind specified IPv4 address' do |(m, proto, kwargs)|
-      @d.__send__(m, :myserver, @port, proto: proto, bind: "127.0.0.1", **kwargs){|x| x }
+      port = unused_port(protocol: proto)
+      @d.__send__(m, :myserver, port, proto: proto, bind: "127.0.0.1", **kwargs){|x| x }
       assert_equal "127.0.0.1", @d._servers.first.bind
       assert_equal "127.0.0.1", @d._servers.first.server.instance_eval{ instance_variable_defined?(:@listen_socket) ? @listen_socket : @_io }.addr[3]
     end
@@ -303,7 +309,8 @@ class Dummy < Fluent::Plugin::TestBase
     )
     test 'can bind specified IPv6 address' do |(m, proto, kwargs)| # if available
       omit "IPv6 unavailable here" unless ipv6_enabled?
-      @d.__send__(m, :myserver, @port, proto: proto, bind: "::1", **kwargs){|x| x }
+      port = unused_port(protocol: proto)
+      @d.__send__(m, :myserver, port, proto: proto, bind: "::1", **kwargs){|x| x }
       assert_equal "::1", @d._servers.first.bind
       assert_equal "::1", @d._servers.first.server.instance_eval{ instance_variable_defined?(:@listen_socket) ? @listen_socket : @_io }.addr[3]
     end
@@ -320,10 +327,11 @@ class Dummy < Fluent::Plugin::TestBase
     test 'can create 2 or more servers which share same bind address and port if shared option is true' do |(m, proto, kwargs)|
       begin
         d2 = Dummy.new; d2.start; d2.after_start
+        port = unused_port(protocol: proto)
 
         assert_nothing_raised do
-          @d.__send__(m, :myserver, @port, proto: proto, **kwargs){|x| x }
-          d2.__send__(m, :myserver, @port, proto: proto, **kwargs){|x| x }
+          @d.__send__(m, :myserver, port, proto: proto, **kwargs){|x| x }
+          d2.__send__(m, :myserver, port, proto: proto, **kwargs){|x| x }
         end
       ensure
         d2.stop; d2.before_shutdown; d2.shutdown; d2.after_shutdown; d2.close; d2.terminate
@@ -344,12 +352,13 @@ class Dummy < Fluent::Plugin::TestBase
     test 'cannot create 2 or more servers using same bind address and port if shared option is false' do |(m, proto, kwargs)|
       begin
         d2 = Dummy.new; d2.start; d2.after_start
+        port = unused_port(protocol: proto)
 
         assert_nothing_raised do
-          @d.__send__(m, :myserver, @port, proto: proto, shared: false, **kwargs){|x| x }
+          @d.__send__(m, :myserver, port, proto: proto, shared: false, **kwargs){|x| x }
         end
         assert_raise(Errno::EADDRINUSE, Errno::EACCES) do
-          d2.__send__(m, :myserver, @port, proto: proto, **kwargs){|x| x }
+          d2.__send__(m, :myserver, port, proto: proto, **kwargs){|x| x }
         end
       ensure
         d2.stop; d2.before_shutdown; d2.shutdown; d2.after_shutdown; d2.close; d2.terminate
@@ -365,16 +374,18 @@ class Dummy < Fluent::Plugin::TestBase
       # 'unix' => [:unix, {}],
     )
     test 'raise error if block argument is not specified or too many' do |(proto, kwargs)|
+      port = unused_port(protocol: proto)
       assert_raise(ArgumentError.new("BUG: block must have 1 or 2 arguments")) do
-        @d.server_create(:myserver, @port, proto: proto, **kwargs){ 1 }
+        @d.server_create(:myserver, port, proto: proto, **kwargs){ 1 }
       end
       assert_raise(ArgumentError.new("BUG: block must have 1 or 2 arguments")) do
-        @d.server_create(:myserver, @port, proto: proto, **kwargs){|sock, conn, what_is_this| 1 }
+        @d.server_create(:myserver, port, proto: proto, **kwargs){|sock, conn, what_is_this| 1 }
       end
     end
 
     test 'creates udp server if specified in proto' do
-      @d.server_create(:myserver, @port, proto: :udp, max_bytes: 512){|x| x }
+      port = unused_port(protocol: :udp)
+      @d.server_create(:myserver, port, proto: :udp, max_bytes: 512){|x| x }
 
       created_server_info = @d._servers.first
       assert_equal :udp, created_server_info.proto
@@ -587,7 +598,8 @@ class Dummy < Fluent::Plugin::TestBase
   sub_test_case '#server_create_udp' do
     test 'can accept all keyword arguments valid for udp server' do
       assert_nothing_raised do
-        @d.server_create_udp(:s, @port, bind: '127.0.0.1', shared: false, resolve_name: true, max_bytes: 100, flags: 1) do |data, conn|
+        port = unused_port(protocol: :udp)
+        @d.server_create_udp(:s, port, bind: '127.0.0.1', shared: false, resolve_name: true, max_bytes: 100, flags: 1) do |data, conn|
           # ...
         end
       end
@@ -595,14 +607,15 @@ class Dummy < Fluent::Plugin::TestBase
 
     test 'creates a udp server just to read data' do
       received = ""
-      @d.server_create_udp(:s, @port, max_bytes: 128) do |data|
+      port = unused_port(protocol: :udp)
+      @d.server_create_udp(:s, port, max_bytes: 128) do |data|
         received << data
       end
       bind_port = unused_port(protocol: :udp, bind: "127.0.0.1")
       3.times do
         sock = UDPSocket.new(Socket::AF_INET)
         sock.bind("127.0.0.1", bind_port)
-        sock.connect("127.0.0.1", @port)
+        sock.connect("127.0.0.1", port)
         sock.puts "yay"
         sock.puts "foo"
         sock.close
@@ -614,16 +627,17 @@ class Dummy < Fluent::Plugin::TestBase
     test 'creates a udp server to read and write data' do
       received = ""
       responses = []
-      @d.server_create_udp(:s, @port, max_bytes: 128) do |data, sock|
+      port = unused_port(protocol: :udp)
+      @d.server_create_udp(:s, port, max_bytes: 128) do |data, sock|
         received << data
         sock.write "ack\n"
       end
-      bind_port = unused_port
+      bind_port = unused_port(protocol: :udp)
       3.times do
         begin
           sock = UDPSocket.new(Socket::AF_INET)
           sock.bind("127.0.0.1", bind_port)
-          sock.connect("127.0.0.1", @port)
+          sock.connect("127.0.0.1", port)
           th = Thread.new do
             while true
               begin
@@ -654,11 +668,12 @@ class Dummy < Fluent::Plugin::TestBase
 
       received = ""
       responses = []
-      @d.server_create_udp(:s, @port, bind: "::1", max_bytes: 128) do |data, sock|
+      port = unused_port(protocol: :udp)
+      @d.server_create_udp(:s, port, bind: "::1", max_bytes: 128) do |data, sock|
         received << data
         sock.write "ack\n"
       end
-      bind_port = unused_port
+      bind_port = unused_port(protocol: :udp)
       3.times do
         begin
           sock = UDPSocket.new(Socket::AF_INET6)
@@ -667,7 +682,7 @@ class Dummy < Fluent::Plugin::TestBase
             responses << sock.recv(16)
             true
           end
-          sock.connect("::1", @port)
+          sock.connect("::1", port)
           sock.write "yay\nfoo\n"
           th.join(5)
         ensure
@@ -682,13 +697,14 @@ class Dummy < Fluent::Plugin::TestBase
     test 'does not resolve name of client address in default' do
       received = ""
       sources = []
-      @d.server_create_udp(:s, @port, max_bytes: 128) do |data, sock|
+      port = unused_port(protocol: :udp)
+      @d.server_create_udp(:s, port, max_bytes: 128) do |data, sock|
         received << data
         sources << sock.remote_host
       end
       3.times do
         sock = UDPSocket.new(Socket::AF_INET)
-        sock.connect("127.0.0.1", @port)
+        sock.connect("127.0.0.1", port)
         sock.puts "yay"
         sock.close
       end
@@ -702,13 +718,14 @@ class Dummy < Fluent::Plugin::TestBase
 
       received = ""
       sources = []
-      @d.server_create_udp(:s, @port, resolve_name: true, max_bytes: 128) do |data, sock|
+      port = unused_port(protocol: :udp)
+      @d.server_create_udp(:s, port, resolve_name: true, max_bytes: 128) do |data, sock|
         received << data
         sources << sock.remote_host
       end
       3.times do
         sock = UDPSocket.new(Socket::AF_INET)
-        sock.connect("127.0.0.1", @port)
+        sock.connect("127.0.0.1", port)
         sock.puts "yay"
         sock.close
       end
@@ -720,7 +737,8 @@ class Dummy < Fluent::Plugin::TestBase
     test 'raises error if plugin registers data callback for connection object from #server_create' do
       received = ""
       errors = []
-      @d.server_create_udp(:s, @port, max_bytes: 128) do |data, sock|
+      port = unused_port(protocol: :udp)
+      @d.server_create_udp(:s, port, max_bytes: 128) do |data, sock|
         received << data
         begin
           sock.data{|d| received << d.upcase }
@@ -729,7 +747,7 @@ class Dummy < Fluent::Plugin::TestBase
         end
       end
       sock = UDPSocket.new(Socket::AF_INET)
-      sock.connect("127.0.0.1", @port)
+      sock.connect("127.0.0.1", port)
       sock.write "foo\n"
       sock.close
 
@@ -742,7 +760,8 @@ class Dummy < Fluent::Plugin::TestBase
     test 'raise error if plugin registers write_complete callback for udp' do
       received = ""
       errors = []
-      @d.server_create_udp(:s, @port, max_bytes: 128) do |data, sock|
+      port = unused_port(protocol: :udp)
+      @d.server_create_udp(:s, port, max_bytes: 128) do |data, sock|
         received << data
         begin
           sock.on(:write_complete){|conn| "" }
@@ -751,7 +770,7 @@ class Dummy < Fluent::Plugin::TestBase
         end
       end
       sock = UDPSocket.new(Socket::AF_INET)
-      sock.connect("127.0.0.1", @port)
+      sock.connect("127.0.0.1", port)
       sock.write "foo\n"
       sock.close
 
@@ -764,7 +783,8 @@ class Dummy < Fluent::Plugin::TestBase
     test 'raises error if plugin registers close callback for udp' do
       received = ""
       errors = []
-      @d.server_create_udp(:s, @port, max_bytes: 128) do |data, sock|
+      port = unused_port(protocol: :udp)
+      @d.server_create_udp(:s, port, max_bytes: 128) do |data, sock|
         received << data
         begin
           sock.on(:close){|d| "" }
@@ -773,7 +793,7 @@ class Dummy < Fluent::Plugin::TestBase
         end
       end
       sock = UDPSocket.new(Socket::AF_INET)
-      sock.connect("127.0.0.1", @port)
+      sock.connect("127.0.0.1", port)
       sock.write "foo\n"
       sock.close
 
@@ -786,11 +806,12 @@ class Dummy < Fluent::Plugin::TestBase
     test 'can bind IPv4 / IPv6 together' do
       omit "IPv6 unavailable here" unless ipv6_enabled?
 
+      port = unused_port(protocol: :udp)
       assert_nothing_raised do
-        @d.server_create_udp(:s_ipv4_udp, @port, bind: '0.0.0.0', shared: false, max_bytes: 128) do |data, sock|
+        @d.server_create_udp(:s_ipv4_udp, port, bind: '0.0.0.0', shared: false, max_bytes: 128) do |data, sock|
           # ...
         end
-        @d.server_create_udp(:s_ipv6_udp, @port, bind: '::', shared: false, max_bytes: 128) do |data, sock|
+        @d.server_create_udp(:s_ipv6_udp, port, bind: '::', shared: false, max_bytes: 128) do |data, sock|
           # ...
         end
       end
@@ -803,11 +824,12 @@ class Dummy < Fluent::Plugin::TestBase
         max_bytes, records, expected = data.values
 
         actual_records = []
-        @d.server_create_udp(:myserver, @port, max_bytes: max_bytes) do |data, sock|
+        port = unused_port(protocol: :udp)
+        @d.server_create_udp(:myserver, port, max_bytes: max_bytes) do |data, sock|
           actual_records << data
         end
 
-        open_client(:udp, "127.0.0.1", @port) do |sock|
+        open_client(:udp, "127.0.0.1", port) do |sock|
           records.each do |record|
             sock.send(record, 0)
           end
@@ -823,11 +845,12 @@ class Dummy < Fluent::Plugin::TestBase
         max_bytes, records, expected = data.values
 
         actual_records = []
-        @d.server_create_udp(:myserver, @port, max_bytes: max_bytes) do |data|
+        port = unused_port(protocol: :udp)
+        @d.server_create_udp(:myserver, port, max_bytes: max_bytes) do |data|
           actual_records << data
         end
 
-        open_client(:udp, "127.0.0.1", @port) do |sock|
+        open_client(:udp, "127.0.0.1", port) do |sock|
           records.each do |record|
             sock.send(record, 0)
           end
diff --git a/test/plugin_helper/test_socket.rb b/test/plugin_helper/test_socket.rb
index cb2ff32de0..624169f08b 100644
--- a/test/plugin_helper/test_socket.rb
+++ b/test/plugin_helper/test_socket.rb
@@ -11,7 +11,7 @@ class SocketHelperTest < Test::Unit::TestCase
   CERT_CHAINS_DIR = File.expand_path(File.dirname(__FILE__) + '/data/cert/cert_chains')
 
   def setup
-    @port = unused_port
+    @port = unused_port(protocol: :tcp)
   end
 
   def teardown