detect docker host IP based on the default route

we were using the well known name host.docker.internal to get the host IP address, however, it seems that in some systems it resolves to a loopback address. We use this address to replace the embedded docker dns IP address in the resolv.conf. We can't use a loopback address, because coredns obtains the upstream dns resolvers from the resolv.conf, and evidentelly, it crashes if the upstream DNS resolver is a loopback, because it meains that is itself, and that is a loop.
Huihuiyule-www · Mar 27, 2021 · c17ca16 · c17ca16
1 parent 83b25fd
commit c17ca16
Showing 1 changed file with 2 additions and 1 deletion.
diff --git a/images/base/files/usr/local/bin/entrypoint b/images/base/files/usr/local/bin/entrypoint
@@ -337,7 +337,8 @@ enable_network_magic(){
   # first we need to detect an IP to use for reaching the docker host
   local docker_host_ip
   docker_host_ip="$( (getent ahostsv4 'host.docker.internal' | head -n1 | cut -d' ' -f1) || true)"
-  if [[ -z "${docker_host_ip}" ]]; then
+  # if the ip doesn't exist or is a loopback address use the default gateway
+  if [[ -z "${docker_host_ip}" ]] || [[ $docker_host_ip =~ ^127\.[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
     docker_host_ip=$(ip -4 route show default | cut -d' ' -f3)
   fi