How to connect to a MongoDB running on an aws ec2 host (linux) out of docker - node.js

Question

Situation:
I prototyped a small web (node.js) app and dockerized it for deployment and replicability purposes.
The app speaks with a MongoDB running directly on the host.
Problem:
On the server (AWS EC2 instance, only Port 80 and 443 open), I am not able to interact with MongoDB and I am wondering why.
docker run --net="host" -e 'NODE_ENV=production' -e 'MONGO_URI=mongodb://USER:PASSWORD!#172.31.32.1:27017/test_db' DOCKER_IMAGE
MongoDB connected ...
Warning: connect.session() MemoryStore is not
designed for a production environment, as it will leak
memory, and will not scale past a single process.
HTTP Server started on port 80
(node:1) DeprecationWarning: current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
MongoNetworkError: failed to connect to server [172.31.32.1:27017] on first connect [MongoNetworkError: connection timed out
at connectionFailureError (/app/server/node_modules/mongodb/lib/core/connection/connect.js:377:14)
at Socket.<anonymous> (/app/server/node_modules/mongodb/lib/core/connection/connect.js:287:16)
at Object.onceWrapper (events.js:284:20)
at Socket.emit (events.js:196:13)
at Socket._onTimeout (net.js:432:8)
at listOnTimeout (internal/timers.js:531:17)
at processTimers (internal/timers.js:475:7) {
name: 'MongoNetworkError',
[Symbol(mongoErrorContextSymbol)]: {}
}]
at Pool.<anonymous> (/app/server/node_modules/mongodb/lib/core/topologies/server.js:433:11)
at Pool.emit (events.js:196:13)
at /app/server/node_modules/mongodb/lib/core/connection/pool.js:571:14
at /app/server/node_modules/mongodb/lib/core/connection/pool.js:994:11
at /app/server/node_modules/mongodb/lib/core/connection/connect.js:40:11
at callback (/app/server/node_modules/mongodb/lib/core/connection/connect.js:262:5)
at Socket.<anonymous> (/app/server/node_modules/mongodb/lib/core/connection/connect.js:287:7)
at Object.onceWrapper (events.js:284:20)
at Socket.emit (events.js:196:13)
at Socket._onTimeout (net.js:432:8)
at listOnTimeout (internal/timers.js:531:17)
at processTimers (internal/timers.js:475:7) {
name: 'MongoNetworkError',
[Symbol(mongoErrorContextSymbol)]: {}
}
I initially tried localhost instead of the IP address, but that does not work. It throws an authentication error (which is somewhat strange). Since there is no host.docker.internal for Linux I had to (temporarily) resolve to the explicit IP address. The IP address that I am using I got via:
netstat -nr | grep '^0\.0\.0\.0' | awk '{print $2}'
What I find very strange is that I am not getting an Authentication error but a Timeout error, so to me it seems like the app is able to connect to Mongo. Also, the "MonogDB connected ..." would indicate that, as it is produced by the following line in my server script.
mongoose
.connect(DB, { useNewUrlParser: true })
.then(console.log("MongoDB connected ..."))
.catch(err => console.log(err));
For completeness sake, the same setup (i.e. dockerized app and running MongoDB directly on the host) worked without a problem locally.
Also, I am able to enter the Mongo Shell on the server via mongo.
Any explanation or tip is appreciated!

Answer 1

If mongo DB is running outside of the Docker, you can use the private IP of the EC2 instance to connect with Mongo DB (running on Host) from inside Docker.
EC2_PRIVATE_IP=$(curl http://169.254.169.254/latest/meta-data/local-ipv4)
docker run --net="host" -e 'NODE_ENV=production' -e 'MONGO_URI=mongodb://USER:PASSWORD!#$EC_PRIVATE_IP:27017/test_db' DOCKER_IMAGE