Forward a new host port to a running docker container.

forward.sh

#!/bin/bash

# Add a firewall NAT rule to expose a port open in a docker container, on a host.
#
# This is only really useful if the container is already running, and you don't
# want to/can't shut it down, but need a new incoming port open.
#
# Requires `jq` for parsing docker container information.
#
# 2019 @leonjza

ACTION=$1
CONTAINER_NAME=$2
CONTAINER_PORT=$3

if ! [[ "$ACTION" =~ ^(-A|-C|-D)$ ]] || [ -z "$CONTAINER_NAME" ] || [ -z "$CONTAINER_PORT" ]; then
    echo "Usage: $0 [action] [container name] [port]"
    echo " Actions can be -A (add rules); -C (check rules); -D (delete rules)"
    exit
fi

CONTAINER_IP=$(docker inspect nc-container | jq -r ".[0].NetworkSettings.IPAddress")
DOCKER_INTERFACE=docker0    # not sure if this is standard, but ok

echo "Will $ACTION rules for $CONTAINER_PORT to $CONTAINER_IP for container $CONTAINER_NAME..."

iptables -t nat $ACTION POSTROUTING --source $CONTAINER_IP --destination $CONTAINER_IP -p tcp --dport $CONTAINER_PORT -j MASQUERADE
iptables -t nat $ACTION DOCKER ! -i $DOCKER_INTERFACE -p tcp --dport $CONTAINER_PORT -j DNAT --to-destination $CONTAINER_IP:$CONTAINER_PORT
iptables $ACTION DOCKER ! -i $DOCKER_INTERFACE -o $DOCKER_INTERFACE --source 0.0.0.0/0 --destination $CONTAINER_IP -p tcp --dport $CONTAINER_PORT -j ACCEPT