#!/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.

# Bugs in iptables-nft 1.8.3 may cause it to get stuck in a loop in
# some circumstances, so we have to run the nft check in a timeout. To
# avoid hitting that timeout, we only bother to even check nft if
# legacy iptables was empty / mostly empty.

num_legacy_lines=$( (iptables-legacy-save || true; ip6tables-legacy-save || true) 2>/dev/null | grep '^-' | wc -l)
if [ "${num_legacy_lines}" -ge 10 ]; then
    mode=legacy
else
    num_nft_lines=$( (timeout 5 sh -c "iptables-nft-save; ip6tables-nft-save" || true) 2>/dev/null | grep '^-' | wc -l)
    if [ "${num_legacy_lines}" -ge "${num_nft_lines}" ]; then
	mode=legacy
    else
	mode=nft
    fi
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 "$0" "$@"