diff --git a/build/docker/centos6/Dockerfile b/build/docker/centos6/Dockerfile
index d0dc51a..4df75de 100644
--- a/build/docker/centos6/Dockerfile
+++ b/build/docker/centos6/Dockerfile
@@ -40,6 +40,9 @@
       python-pip \
     && yum clean all
 
+# optional dependencies
+RUN pip install ipaddress backports.ssl_match_hostname tornado
+
 # CMake
 RUN curl -sSL https://cmake.org/files/v3.4/cmake-3.4.1.tar.gz | tar -xz && \
     cd cmake-3.4.1 && ./bootstrap && make -j4 && make install && \