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+#!/bin/sh
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+
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+# Copyright 2020 The Kubernetes Authors.
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+#
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+# Licensed under the Apache License, Version 2.0 (the "License");
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+# you may not use this file except in compliance with the License.
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+# You may obtain a copy of the License at
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+#
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+# http://www.apache.org/licenses/LICENSE-2.0
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+#
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+# Unless required by applicable law or agreed to in writing, software
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+# distributed under the License is distributed on an "AS IS" BASIS,
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+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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+# See the License for the specific language governing permissions and
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+# limitations under the License.
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+
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+# Usage:
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+#
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+# iptables-wrapper-installer.sh [--no-sanity-check]
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+#
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+# Installs a wrapper iptables script in a container that will figure out
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+# whether iptables-legacy or iptables-nft is in use on the host and then
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+# replaces itself with the correct underlying iptables version.
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+#
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+# Unless "--no-sanity-check" is passed, it will first verify that the
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+# container already contains a suitable version of iptables.
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+
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+# NOTE: This can only use POSIX /bin/sh features; the build container
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+# might not contain bash.
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+
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+set -eu
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+
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+# Find iptables binary location
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+if [ -d /usr/sbin -a -e /usr/sbin/iptables ]; then
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+ sbin="/usr/sbin"
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+elif [ -d /sbin -a -e /sbin/iptables ]; then
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+ sbin="/sbin"
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+else
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+ echo "ERROR: iptables is not present in either /usr/sbin or /sbin" 1>&2
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+ exit 1
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+fi
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+
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+# Determine how the system selects between iptables-legacy and iptables-nft
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+if [ -x /usr/sbin/alternatives ]; then
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+ # Fedora/SUSE style alternatives
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+ altstyle="fedora"
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+elif [ -x /usr/sbin/update-alternatives ]; then
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+ # Debian style alternatives
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+ altstyle="debian"
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+else
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+ # No alternatives system
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+ altstyle="none"
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+fi
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+
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+if [ "${1:-}" != "--no-sanity-check" ]; then
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+ # Ensure dependencies are installed
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+ if ! version=$("${sbin}/iptables-nft" --version 2> /dev/null); then
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+ echo "ERROR: iptables-nft is not installed" 1>&2
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+ exit 1
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+ fi
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+ if ! "${sbin}/iptables-legacy" --version > /dev/null 2>&1; then
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+ echo "ERROR: iptables-legacy is not installed" 1>&2
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+ exit 1
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+ fi
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+
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+ case "${version}" in
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+ *v1.8.[012]\ *)
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+ echo "ERROR: iptables 1.8.0 - 1.8.2 have compatibility bugs." 1>&2
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+ echo " Upgrade to 1.8.3 or newer." 1>&2
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+ exit 1
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+ ;;
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+ *v1.8.3\ *)
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+ # 1.8.3 mostly works but can get stuck in an infinite loop if the nft
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+ # kernel modules are unavailable
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+ need_timeout=1
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+ ;;
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+ *)
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+ # 1.8.4+ are OK
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+ ;;
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+ esac
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+fi
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+
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+# Start creating the wrapper...
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+rm -f "${sbin}/iptables-wrapper"
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+cat > "${sbin}/iptables-wrapper" <<EOF
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+#!/bin/sh
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+
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+# Copyright 2020 The Kubernetes Authors.
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+#
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+# Licensed under the Apache License, Version 2.0 (the "License");
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+# you may not use this file except in compliance with the License.
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+# You may obtain a copy of the License at
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+#
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+# http://www.apache.org/licenses/LICENSE-2.0
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+#
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+# Unless required by applicable law or agreed to in writing, software
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+# distributed under the License is distributed on an "AS IS" BASIS,
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+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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+# See the License for the specific language governing permissions and
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+# limitations under the License.
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+
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+# NOTE: This can only use POSIX /bin/sh features; the container image
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+# might not contain bash.
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+
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+set -eu
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+
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+# Detect whether the base system is using iptables-legacy or
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+# iptables-nft. This assumes that some non-containerized process (eg
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+# kubelet) has already created some iptables rules.
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+EOF
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+
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+if [ "${need_timeout:-0}" = 0 ]; then
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+ # Write out the simpler version of legacy-vs-nft detection
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+ cat >> "${sbin}/iptables-wrapper" <<EOF
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+num_legacy_lines=\$( (iptables-legacy-save || true; ip6tables-legacy-save || true) 2>/dev/null | grep '^-' | wc -l)
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+num_nft_lines=\$( (iptables-nft-save || true; ip6tables-nft-save || true) 2>/dev/null | grep '^-' | wc -l)
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+if [ "\${num_legacy_lines}" -ge "\${num_nft_lines}" ]; then
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+ mode=legacy
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+else
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+ mode=nft
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+fi
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+EOF
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+else
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+ # Write out the version of legacy-vs-nft detection with an nft timeout
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+ cat >> "${sbin}/iptables-wrapper" <<EOF
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+# The iptables-nft binary in this image can get stuck in an infinite
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+# loop if nft is not available so we need to wrap a timeout around it
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+# (and to avoid that, we don't even bother calling iptables-nft if it
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+# looks like iptables-legacy is going to win).
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+num_legacy_lines=\$( (iptables-legacy-save || true; ip6tables-legacy-save || true) 2>/dev/null | grep '^-' | wc -l)
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+if [ "\${num_legacy_lines}" -ge 10 ]; then
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+ mode=legacy
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+else
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+ num_nft_lines=\$( (timeout 5 sh -c "iptables-nft-save; ip6tables-nft-save" || true) 2>/dev/null | grep '^-' | wc -l)
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+ if [ "\${num_legacy_lines}" -ge "\${num_nft_lines}" ]; then
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+ mode=legacy
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+ else
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+ mode=nft
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+ fi
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+fi
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+EOF
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+fi
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+
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+# Write out the appropriate alternatives-selection commands
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+case "${altstyle}" in
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+ fedora)
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+cat >> "${sbin}/iptables-wrapper" <<EOF
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+# Update links to point to the selected binaries
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+alternatives --set iptables "/usr/sbin/iptables-\${mode}" > /dev/null || failed=1
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+EOF
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+ ;;
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+
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+ debian)
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+cat >> "${sbin}/iptables-wrapper" <<EOF
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+# Update links to point to the selected binaries
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+update-alternatives --set iptables "/usr/sbin/iptables-\${mode}" > /dev/null || failed=1
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+update-alternatives --set ip6tables "/usr/sbin/ip6tables-\${mode}" > /dev/null || failed=1
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+EOF
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+ ;;
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+
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+ *)
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+cat >> "${sbin}/iptables-wrapper" <<EOF
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+# Update links to point to the selected binaries
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+for cmd in iptables iptables-save iptables-restore ip6tables ip6tables-save ip6tables-restore; do
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+ rm -f "${sbin}/\${cmd}"
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+ ln -s "${sbin}/xtables-\${mode}-multi" "${sbin}/\${cmd}"
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+done 2>/dev/null || failed=1
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+EOF
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+ ;;
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+esac
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+
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+# Write out the post-alternatives-selection error checking and final wrap-up
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+cat >> "${sbin}/iptables-wrapper" <<EOF
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+if [ "\${failed:-0}" = 1 ]; then
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+ echo "Unable to redirect iptables binaries. (Are you running in an unprivileged pod?)" 1>&2
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+ # fake it, though this will probably also fail if they aren't root
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+ exec "${sbin}/xtables-\${mode}-multi" "\$0" "\$@"
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+fi
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+
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+# Now re-exec the original command with the newly-selected alternative
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+exec "\$0" "\$@"
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+EOF
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+chmod +x "${sbin}/iptables-wrapper"
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+
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+# Now back in the installer script, point the iptables binaries at our
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+# wrapper
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+case "${altstyle}" in
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+ fedora)
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+ alternatives \
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+ --install /usr/sbin/iptables iptables /usr/sbin/iptables-wrapper 100 \
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+ --slave /usr/sbin/iptables-restore iptables-restore /usr/sbin/iptables-wrapper \
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+ --slave /usr/sbin/iptables-save iptables-save /usr/sbin/iptables-wrapper \
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+ --slave /usr/sbin/ip6tables iptables /usr/sbin/iptables-wrapper \
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+ --slave /usr/sbin/ip6tables-restore iptables-restore /usr/sbin/iptables-wrapper \
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+ --slave /usr/sbin/ip6tables-save iptables-save /usr/sbin/iptables-wrapper
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+ ;;
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+
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+ debian)
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+ update-alternatives \
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+ --install /usr/sbin/iptables iptables /usr/sbin/iptables-wrapper 100 \
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+ --slave /usr/sbin/iptables-restore iptables-restore /usr/sbin/iptables-wrapper \
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+ --slave /usr/sbin/iptables-save iptables-save /usr/sbin/iptables-wrapper
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+ update-alternatives \
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+ --install /usr/sbin/ip6tables ip6tables /usr/sbin/iptables-wrapper 100 \
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+ --slave /usr/sbin/ip6tables-restore ip6tables-restore /usr/sbin/iptables-wrapper \
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+ --slave /usr/sbin/ip6tables-save ip6tables-save /usr/sbin/iptables-wrapper
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+ ;;
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+
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+ *)
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+ for cmd in iptables iptables-save iptables-restore ip6tables ip6tables-save ip6tables-restore; do
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+ rm -f "${sbin}/${cmd}"
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+ ln -s "${sbin}/iptables-wrapper" "${sbin}/${cmd}"
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+ done
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+ ;;
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+esac
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+
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+# Cleanup
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+rm -f "$0"
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Chris Kim