diff --git a/test/session_test.rb b/test/session_test.rb
index ce9005c3..6004d27f 100644
--- a/test/session_test.rb
+++ b/test/session_test.rb
@@ -58,7 +58,7 @@ class SessionTest < Minitest::Test
   end
 
   def test_session_timeout_connect_timeout
-    uri = build_uri("/", origin("127.0.0.1:9090"))
+    uri = build_uri("/", origin("127.0.0.1:#{CONNECT_TIMEOUT_PORT}"))
     session = HTTPX.timeout(connect_timeout: 0.5, operation_timeout: 30, total_timeout: 2)
     response = session.get(uri)
     assert response.is_a?(HTTPX::ErrorResponse), "response should have failed (#{response.class})"
diff --git a/test/support/ci/build.sh b/test/support/ci/build.sh
index fbcc8e1b..6f8ddcd1 100755
--- a/test/support/ci/build.sh
+++ b/test/support/ci/build.sh
@@ -11,8 +11,10 @@ else
 fi
 
 # use port 9090 to test connection timeouts
-iptables -A OUTPUT -p tcp -m tcp --tcp-flags SYN SYN --sport 9090 -j DROP
+CONNECT_TIMEOUT_PORT=9090
+iptables -A OUTPUT -p tcp -m tcp --tcp-flags SYN SYN --sport $CONNECT_TIMEOUT_PORT -j DROP
 
+export CONNECT_TIMEOUT_PORT=$CONNECT_TIMEOUT_PORT
 export PATH=$GEM_HOME/bin:$BUNDLE_PATH/gems/bin:$PATH
 mkdir -p "$GEM_HOME" && chmod 777 "$GEM_HOME"
 gem install bundler -v="1.17.3" --no-doc --conservative
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 53cabf3c..bc96ce8d 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -22,7 +22,9 @@ Dir[File.join(".", "test", "support", "**", "*.rb")].sort.each { |f| require f }
 OpenSSL::SSL::SSLContext::DEFAULT_CERT_STORE.add_file(ENV["SSL_CERT_FILE"]) if RUBY_VERSION.start_with?("2.3") && ENV.key?("SSL_CERT_FILE")
 
 # 9090 drops SYN packets for connect timeout tests, make sure there's a server binding there.
-server = TCPServer.new("127.0.0.1", 9090)
+CONNECT_TIMEOUT_PORT = ENV.fetch("CONNECT_TIMEOUT_PORT", 9090).to_i
+
+server = TCPServer.new("127.0.0.1", CONNECT_TIMEOUT_PORT)
 
 Thread.start do
   begin