feat: allow to set dns values to DOCKER_HOST environment variable

Dockerfile

@@ -1,6 +1,6 @@
 FROM alpine:latest
 
-RUN apk --update --no-cache add iptables
+RUN apk --update --no-cache add iptables libcap
 
 COPY ./entrypoint.sh /
 

entrypoint.sh

@@ -1,35 +1,56 @@
 #!/bin/sh
 set -e
 
-# determine docker host
-if [ $DOCKER_HOST ]; then
-  echo "Docker Host: $DOCKER_HOST"
+function cap_support {
+  capsh --print | grep "Current:" | cut -d' ' -f3 | grep -q "$1"
+}
+
+# ensure network capabilities
+if ! cap_support 'cap_net_admin' || ! cap_support 'cap_net_raw'; then
+  echo "[ERROR] docker-host container needs Linux capabilities NET_ADMIN and NET_RAW"
+  echo "  e.g 'docker run --cap-add=NET_ADMIN --cap-add=NET_RAW ...'"
+  exit 1
+fi
+
+if [ -n "$DOCKER_HOST" ]; then
+  docker_host_ipv4="$(getent ahostsv4 "$DOCKER_HOST" | head -n1 | cut -d' ' -f1)"
+  if [ "$docker_host_ipv4" != "$DOCKER_HOST" ]; then
+    echo "Docker Host: ${docker_host_ipv4:-'n/a'} ($DOCKER_HOST)"
+  else
+    echo "Docker Host: ${docker_host_ipv4:-'n/a'}"
+  fi
 else
-  DOCKER_HOST="$(getent hosts host.docker.internal | cut -d' ' -f1)"
-  if [ $DOCKER_HOST ]; then
-    echo "Docker Host: $DOCKER_HOST (host.docker.internal)"
+  DOCKER_HOST='host.docker.internal'
+  docker_host_ipv4="$(getent ahostsv4 "$DOCKER_HOST" | head -n1 | cut -d' ' -f1)"
+  if [ -n "$docker_host_ipv4" ]; then
+    echo "Docker Host: $docker_host_ipv4 ($DOCKER_HOST)"
   else
-    DOCKER_HOST=$(ip -4 route show default | cut -d' ' -f3)
-    echo "Docker Host: $DOCKER_HOST (default gateway)"
+    docker_host_ipv4=$(ip -4 route show default | cut -d' ' -f3)
+    echo "Docker Host: ${docker_host_ipv4:-'n/a'} (default gateway)"
   fi
 fi
 
+# exit if docker host ip could not be determined
+if [ -z "$docker_host_ipv4" ]; then
+  exit 1
+fi
+
 FORWARDING_PORTS=${PORTS:-'0:65535'}
 echo "Forwarding ports: $FORWARDING_PORTS"
 
 # setup forwarding rules
+iptables -t nat -I POSTROUTING -j MASQUERADE
 for forwarding_port in $(echo "$FORWARDING_PORTS" | tr ";" " ")
 do
   iptables --table nat --insert PREROUTING \
     --protocol tcp \
     --dport "$forwarding_port" \
-    --jump DNAT --to-destination $DOCKER_HOST
+    --jump DNAT --to-destination "$docker_host_ipv4"
   iptables --table nat --insert PREROUTING \
     --protocol udp \
     --dport "$forwarding_port" \
-    --jump DNAT --to-destination $DOCKER_HOST
+    --jump DNAT --to-destination "$docker_host_ipv4"
 done
-iptables -t nat -I POSTROUTING -j MASQUERADE
 
 # exit on ctrl+c
 trap "exit 0;" TERM INT