posix-nftables-apply

#!/usr/bin/env sh
#
# This program is free software: you can redistribute it and/or modify it
# under the terms of the GNU General Public License as published by the
# Free Software Foundation, either version 3 of the License, or (at your
# option) any later version.  Please see LICENSE.txt at the top level of
# the source code distribution for details.
#
# @package nftables-apply
# @author <felix.bouynot@setenforce.one>
# @link https://github.com/fbouynot/scripts/blob/main/posix-nftables-apply
# @copyright <felix.bouynot@setenforce.one>
#
# Free adaptation for nftables of iptables-apply (https://github.com/wertarbyte/iptables/blob/master/iptables-apply)
#

# -e: When a command fails, bash exits instead of continuing with the rest of the script
# -u: This will make the script fail, when accessing an unset variable
set -eu

# Trap to remove TMPFILE if the script is stopped before the cleanup
trap 'rm -f "${TMPFILE}"' EXIT

# Replace the Internal Field Separator ' \n\t' by '\n\t' so you can loop through names with spaces 
IFS=$(printf '\n\t')

# Enable debug mode by running your script as TRACE=1 ./script.sh instead of ./script.sh
if [ "${TRACE-0}" = "1" ]
then
    set -o xtrace
fi

# Define constants
PROGNAME="${0##*/}"
VERSION='1.3.0'
RED="$(tput setaf 1)" || exit 1
NC="$(tput sgr0)" || exit 1 # No Color
TMPFILE=$(mktemp) || exit 1

DEFAULT_TIMEOUT=15
DEFAULT_DESTINATION_FILE='/etc/nftables.conf'
DEFAULT_SOURCE_FILE='/etc/nftables-candidate.conf'

readonly PROGNAME VERSION RED NC DEFAULT_TIMEOUT DEFAULT_DESTINATION_FILE DEFAULT_SOURCE_FILE
 
help() {
    cat << EOF
Usage: ${PROGNAME} [-Vh] [ { -s | --source-file } <source-file> ] [ { -d | --destination-file } <destination-file> ] [ { -t | --timeout } <timeout> ]
-h    --help                                                     Print this message.
-V    --version                                                  Print the version.
-s    --source-file        STRING                                The source file for candidate config.           (default: ${DEFAULT_SOURCE_FILE})
-d    --destination-file   STRING                                The destination file where to write the config. (default: ${DEFAULT_DESTINATION_FILE})
-t    --timeout            INT                                   The time to wait before rolling back.           (default: ${DEFAULT_TIMEOUT})
EOF

    exit 2
}

version() {
    cat << EOF
${PROGNAME} version ${VERSION} under GPLv3 licence.
EOF

    exit 2
}

# Deal with arguments
while [ $# -gt 0 ]
do
    key="${1}"

    case $key in
        -h|--help)
            help
            ;;
        -s|--source-file)
            export source_file="${2}"
            shift # consume -s
            ;;
        -d|--destination-file)
            export destination_file="${2}"
            shift # consume -d
            ;;
        -t|--timeout)
            export timeout="${2}"
            shift # consume -t
            ;;
        -V|--version)
            version
            ;;
        *)
            shift
            ;;
    esac
    shift # consume $1
done

# Set defaults if no options specified
source_file="${source_file:-$DEFAULT_SOURCE_FILE}"
destination_file="${destination_file:-$DEFAULT_DESTINATION_FILE}"
timeout="${timeout:-$DEFAULT_TIMEOUT}"

# Change directory to base script directory
cd "$(dirname "${0}")"

# Check root permissions
check_root() {
    # Check the command is run as root
    if [ "$(id -u)" -ne 0 ]
    then
        printf "%sE:%s please run as root\n" "${RED}" "${NC}" >&2
        exit 3
    fi

    return 0
}

restore() {
    nft flush ruleset
    nft -f "${TMPFILE}"

    # Start fail2ban
    if systemctl is-enabled fail2ban > /dev/null 2>&1
    then
        systemctl start fail2ban 2>/dev/null
    fi

    return 0
}

save() {
    cp "${source_file}" "${destination_file}"
    printf "\nConfiguration changed\n"

    return 0
}

# Main function
main() {
    # Check the command is run as root
    check_root

    # Check if we can read the destination file
    if [ ! -r "${destination_file}" ]
    then
        printf "%sE:%s cannot read %s\n" "${RED}" "${NC}" "${destination_file}" >&2
        exit 4
    fi

    # Backup current ruleset
    nft list ruleset > "${TMPFILE}"

    # Check if we can read the source file
    if [ ! -r "${source_file}" ]
    then
        printf "%sE:%s cannot read %s\n" "${RED}" "${NC}" "${source_file}" >&2
        exit 5
    fi

    # Dry run new ruleset, exit if failures
    nft -f "${source_file}" || (printf "%sE:%s Invalid rules, exiting\n" "${RED}" "${NC}" >&2 && exit 6)

    # Check the candidate configuration starts by flushing ruleset
    if [ "$(head -n 1 /etc/nftables-candidate.conf)" != "flush ruleset" ]
    then
        sed -i '1s/^/flush ruleset\n/' "${source_file}"
    fi

    # Stop fail2ban
    if systemctl is-active fail2ban > /dev/null 2>&1
    then
        systemctl stop fail2ban 2>/dev/null
    fi

    # Apply new ruleset, rollback if timeout
    timeout "${timeout}"s nft -f "${source_file}" || (printf "%sE:%s timeout while applying new configuration, rolling back to the previous ruleset\n" "${RED}" "${NC}" >&2 && restore && exit 7)

    # Ask the user if they can open a new connection
    # If they can't, rollback
    # If they can, save
    printf "Can you establish NEW connections to the machine? (y/N) "
    stty -icanon -echo
    timeout "${timeout}" answer="$(dd bs=1 count=1 2> /dev/null)" 2>&1 || :
    if [ "${answer:?}" = "y" ]
    then
        stty icanon echo
        save
    else
        stty icanon echo
        printf "%sE:%s rolling back to the previous ruleset\n" "${RED}" "${NC}" >&2
        restore
        exit 8
    fi

    # Start fail2ban
    if systemctl is-enabled fail2ban > /dev/null 2>&1
    then
        systemctl start fail2ban 2>/dev/null
    fi

    exit 0
}

main "$@"