Allow access to OS X hosts from inside containers

This adds a script for creating a loopback alias and adding a corresponding entry to /etc/hosts on OS X machines. See the README additions for more details. The script can be run with a Make target, which in turn is run by `devstack.start` to avoid any manual steps.

ECOM-6586

Makefile

@@ -18,12 +18,15 @@ devstack.provision: ## Provision all services
 devstack.reset: ## Remove all service containers
 	docker-compose down
 
-devstack.start: clone ## Start all services
+devstack.start: clone loopback ## Start all services
 	docker-compose up
 
 devstack.stop: ## Stop all services
 	docker-compose stop
 
+loopback: ## Create loopback alias
+	./loopback.sh
+
 requirements: ## Install requirements
 	pip install -r requirements.txt
 

README.md

@@ -27,3 +27,28 @@ After the services have started, if you need shell access to one of the services
 ```
 $ make devstack.open.discovery
 ```
+
+## Loopback Alias
+
+Containers making requests to the LMS and Studio must communicate with ports exposed on the host system by the Vagrant VM.
+
+This is fine on Linux, but doesn't work out of the box on OS X. Attempting to access localhost on a container will result
+in talking to the Docker for Mac HyperKit VM, not the host machine.
+
+While it's true that you can get this to work by accessing your Mac's external IP from your containers, this isn't ideal because
+it won't work if you have no network access on your host. Your external IP also changes as you switch networks, meaning you'd have
+to change the IP accessed by your containers every time you changed networks.
+
+A better solution, borrowed from the [Docker forums](https://forums.docker.com/t/access-host-not-vm-from-inside-container/11747/10),
+is to give your host a fixed address by creating a [loopback](http://askubuntu.com/questions/247625/what-is-the-loopback-device-and-how-do-i-use-it)
+alias. This is done for you by the `devstack.start` target.
+
+The result is a fixed IP which your containers can use to access ports on your host machine. Note that the underlying script uses `sudo`;
+adding IP addresses requires root access. Also note that the alias will not survive a host reboot, which is why the `devstack.start` target
+always attempts to set up the loopback for you.
+
+Part of the loopback alias setup includes adding a line to the `/etc/hosts` file on your machine. If you want to stop using devstack, you can clean this up by opening your `/etc/hosts` file and removing this line:
+
+```
+10.254.254.254 docker.host
+```

docker-compose.yml

@@ -17,9 +17,10 @@ services:
       context: .
       dockerfile: ./docker/elasticsearch/Dockerfile
     container_name: edx.devstack.elasticsearch
-    ports:
-      - "9200:9200"
-      - "9300:9300"
+    # TODO: What to do about these forwarded ports? They'll conflict with ports forwarded by the Vagrant VM.
+    # ports:
+    #   - "9200:9200"
+    #   - "9300:9300"
     volumes:
       - ./.dev/volumes/elasticsearch/data:/usr/share/elasticsearch/data
       - ./.dev/volumes/elasticsearch/logs:/usr/share/elasticsearch/logs
@@ -27,8 +28,8 @@ services:
   memcached:
     container_name: edx.devstack.memcached
     image: memcached:1.4.24
-    ports:
-      - "11211:11211"
+    # ports:
+    #   - "11211:11211"
 
   mysql:
     command: mysqld --character-set-server=utf8 --collation-server=utf8_general_ci
@@ -37,8 +38,8 @@ services:
       MYSQL_ROOT_PASSWORD: ""
       MYSQL_ALLOW_EMPTY_PASSWORD: "yes"
     image: mysql:5.6
-    ports:
-      - "3306:3306"
+    # ports:
+    #   - "3306:3306"
     volumes:
       - ./.dev/volumes/mysql:/var/lib/mysql
 
@@ -68,6 +69,9 @@ services:
     environment:
       TEST_ELASTICSEARCH_URL: "http://edx.devstack.elasticsearch:9200"
       ENABLE_DJANGO_TOOLBAR: 1
+    extra_hosts:
+      # For server-to-server calls.
+      - "docker.host:10.254.254.254"
     image: edxops/discovery:devstack
     ports:
       - "18381:18381"

loopback.sh

+# It's not necessary to do this on Linux, so check that we're on OS X before continuing.
+if [ "$(uname)" == "Darwin" ]; then
+    # This loopback alias allows access to OS X hosts from inside a container. 10.254.254.254
+    # is a private IP likely to be unused. As is, the alias will not survive a reboot.
+    # Borrowed from https://forums.docker.com/t/access-host-not-vm-from-inside-container/11747/10.
+    sudo ifconfig lo0 alias 10.254.254.254
+
+    # This will check for a "docker.host" entry in your hosts file. It's created if it doesn't
+    # exist, so you can use URLs like "http://docker.host:8000" in your service configuration.
+    if ! grep -q "docker.host" /etc/hosts; then
+        # Using tee to write to /etc/hosts because using >> to append isn't allowed, even as root.
+        # See http://stackoverflow.com/a/550808.
+        echo "10.254.254.254 docker.host" | sudo tee -a /etc/hosts > /dev/null
+    fi
+elif [ "$(expr substr $(uname -s) 1 5)" == "Linux" ]; then
+    if ! grep -q "docker.host" /etc/hosts; then
+        echo "127.0.0.1 docker.host" | sudo tee -a /etc/hosts > /dev/null
+    fi
+fi