Add mode-detecting iptables wrappers to the debian-iptables image

build/debian-iptables/Dockerfile

@@ -14,10 +14,28 @@
 
 FROM BASEIMAGE
 
+# Install latest iptables package from buster-backports
+RUN echo deb http://deb.debian.org/debian buster-backports main >> /etc/apt/sources.list; \
+    apt-get update; \
+    apt-get -t buster-backports -y --no-install-recommends install iptables
+
+# Install other dependencies and then clean up apt caches
 RUN clean-install \
     conntrack \
     ebtables \
     ipset \
-    iptables \
     kmod \
     netbase
+
+# Install iptables wrapper scripts to detect the correct iptables mode
+# the first time any of them is run
+COPY iptables-wrapper /usr/sbin/iptables-wrapper
+
+RUN update-alternatives \
+	--install /usr/sbin/iptables iptables /usr/sbin/iptables-wrapper 100 \
+	--slave /usr/sbin/iptables-restore iptables-restore /usr/sbin/iptables-wrapper \
+	--slave /usr/sbin/iptables-save iptables-save /usr/sbin/iptables-wrapper
+RUN update-alternatives \
+	--install /usr/sbin/ip6tables ip6tables /usr/sbin/iptables-wrapper 100 \
+	--slave /usr/sbin/ip6tables-restore ip6tables-restore /usr/sbin/iptables-wrapper \
+	--slave /usr/sbin/ip6tables-save ip6tables-save /usr/sbin/iptables-wrapper

build/debian-iptables/iptables-wrapper

@@ -0,0 +1,34 @@
+#!/bin/sh
+
+# Copyright 2019 The Kubernetes Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -e
+
+# Detect whether the base system is using iptables-legacy or
+# iptables-nft. This assumes that some non-containerized process (eg
+# kubelet) has already created some iptables rules.
+num_legacy_lines=$( (iptables-legacy-save || true; ip6tables-legacy-save || true) 2>/dev/null | grep '^-' | wc -l)
+num_nft_lines=$( (iptables-nft-save || true; ip6tables-nft-save || true) 2>/dev/null | grep '^-' | wc -l)
+if [ "${num_legacy_lines}" -ge "${num_nft_lines}" ]; then
+    mode=legacy
+else
+    mode=nft
+fi
+
+update-alternatives --set iptables "/usr/sbin/iptables-${mode}" > /dev/null
+update-alternatives --set ip6tables "/usr/sbin/ip6tables-${mode}" > /dev/null
+
+# Now re-exec the original command with the newly-selected alternative
+exec "$0" "$@"