furtka-gateway/examples/starter-stack/ops/firewall.sh

#!/usr/bin/env bash
# Restrict wg-easy's admin API (tcp/51821, or GATEWAY_WG_ADMIN_PORT if
# overridden) to loopback and the gateway's own docker bridge network. Run
# this once on the VPS after `docker compose up -d`, and again if the
# "internal" network's subnet ever changes.
#
# `sudo` does not inherit your shell's env or read .env for you — if you
# overrode GATEWAY_WG_ADMIN_PORT, run:
#   set -a; source .env; set +a; sudo -E ops/firewall.sh
#
# Why this exists: wg-easy runs with network_mode: host (see
# docker-compose.yaml's comment on why — Traefik needs a route to
# WireGuard peer addresses that only exists in whatever network namespace
# wg-easy's wg0 interface lives in). Host networking means Docker itself
# can no longer keep wg-easy's admin port off the VPS's public interface
# the way it does for every other container here — this script is that
# missing piece, done with iptables instead.
#
# NOT yet validated against a real multi-interface VPS. Review
# DOCKER_BRIDGE_SUBNET against your actual `docker network inspect
# furtka-gateway_internal` output before relying on this as your only line
# of defense. A cloud provider security group that blocks 51821/tcp
# entirely is a good belt-and-suspenders addition on top of this — wg-easy's
# admin API has no legitimate reason to ever be reached from outside this
# host.
#
# IPv4 only. If this VPS also has a public IPv6 address, ip6tables needs
# the equivalent rules added by hand — not yet handled here.

set -euo pipefail

# Must match GATEWAY_WG_ADMIN_PORT in .env if you overrode it (e.g.
# GATEWAY_MODE=single-parallel, where 51821 is already taken by another
# wg-easy on this host).
ADMIN_PORT="${GATEWAY_WG_ADMIN_PORT:-51821}"
# Must match docker-compose.yaml's networks.internal.ipam.config subnet.
DOCKER_BRIDGE_SUBNET="172.28.0.0/24"

if [[ $EUID -ne 0 ]]; then
  echo "must be run as root" >&2
  exit 1
fi

# Insert ACCEPT rules at the head of the chain, then append DROP at the
# tail — this ordering holds regardless of how many times the script runs,
# since -I always lands above whatever's already there (including a
# previous run's DROP) and -A always lands below.
iptables -C INPUT -p tcp --dport "$ADMIN_PORT" -s 127.0.0.1 -j ACCEPT 2>/dev/null \
  || iptables -I INPUT -p tcp --dport "$ADMIN_PORT" -s 127.0.0.1 -j ACCEPT

iptables -C INPUT -p tcp --dport "$ADMIN_PORT" -s "$DOCKER_BRIDGE_SUBNET" -j ACCEPT 2>/dev/null \
  || iptables -I INPUT -p tcp --dport "$ADMIN_PORT" -s "$DOCKER_BRIDGE_SUBNET" -j ACCEPT

iptables -C INPUT -p tcp --dport "$ADMIN_PORT" -j DROP 2>/dev/null \
  || iptables -A INPUT -p tcp --dport "$ADMIN_PORT" -j DROP

echo "wg-easy admin API (tcp/$ADMIN_PORT) now restricted to loopback + $DOCKER_BRIDGE_SUBNET"
echo "NOTE: these rules do not persist across reboot on most distros —"
echo "install iptables-persistent (Debian/Ubuntu) or an equivalent, or add"
echo "this script to a boot-time hook."