cant connect to local database using external IP:Port

I have 2 Servers, A and B.

They both run the same replicated server-application (rest-API) written in go. On Server B, I also have a database running. Both server and database in docker-containers.

docker-compose.yml

...

 userDB:
      image: mysql:oracle
      restart: always
      container_name: userDB
      environment:
        MYSQL_DATABASE: 'mydb'
        MYSQL_USER: 'user'
        MYSQL_PASSWORD: 'mypw
        MYSQL_ROOT_PASSWORD: 'myrootpw'
      cap_add:
        - SYS_NICE
      ports:
        - '3307:3306'
      networks:
        - dbnet
...


go_rest:
    build:
      context: .
      dockerfile: ./goREST/Dockerfile
    container_name: go_rest
    command: ["./goREST"]
    restart: always
    ports:
      - 8081:8081
    networks:
      - dbnet

Inside the Server, I connect to the database like so:

	db, err := sql.Open("mysql", "root:myrootpw@tcp(bbb.bbb.bbb.bbb:3307)/mydb")


	var usr DBUser
	stmt, _ := db.Prepare(`SELECT * from user;`)
	_ = stmt.QueryRow().Scan(&usr.SteamId)
	log.Println(*usr.SteamId)

note, that the bbb.bbb.... is the public IP of my Server B.

Now the problem:

Running th server-application on Server A (which doesn't have the database), works fine. I can also connect to the database from my development machine using this command:

ssh -L 3307:127.0.0.1:3307 usr@bbb.bbb.bbb.bbb

But when using dockerized version on the machine which holds the database, I can't connect to the database? This only happens, when running inside docker, when I just run the executable like ./goRest on Server B, it also works fine. I am really confused.

In case my explanation was confusing:

A -----> B(docker) works
B -----> B(local) works
B -----> B(docker) doesn not work

(docker) referes to the server-application. The database is always dockerized.

OS: Ubuntu 20.04LTS

EDIT

The error that occurs looks like this:

rest  | 2022/08/29 22:54:00 http: panic serving ......6:45264: runtime error: invalid memory address or nil pointer dereference
rest  | goroutine 9 [running]:
rest  | net/http.(*conn).serve.func1(0xc0000eefa0)
rest  | 	/usr/local/go/src/net/http/server.go:1805 +0x153
rest  | panic(0x7d9900, 0xbb2e50)
rest  | 	/usr/local/go/src/runtime/panic.go:971 +0x499
rest  | database/sql.(*Stmt).QueryContext(0x0, 0x9b5b10, 0xc00001a088, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
rest  | 	/usr/local/go/src/database/sql/sql.go:2630 +0x63
rest  | database/sql.(*Stmt).QueryRowContext(0x0, 0x9b5b10, 0xc00001a088, 0x0, 0x0, 0x0, 0x9af7a0)
rest  | 	/usr/local/go/src/database/sql/sql.go:2705 +0x6d

You need to replace bbb.bbb.bbb.bbb:3307 with userDB:3306 when running inside Docker. Try to make that a parameter that you pass at runtime, this way you don't have to change the code when you run with or without Docker.

Also you don't have to map port 3306 unless you want to connect to the database with some external tool. Your dockerized application doesn't need it.

I now found a solution.

So the problem was with ufw. I had rules like this:

3307                       ALLOW       aaa.aaa.aaa.aaa
3307                       ALLOW       prod.local.ip.notebook
3307                       ALLOW       bbb.bbb.bbb.bbb

That's why the acces from my local development computer (aka. prod.local.ip.notebook) and the access from server A (aka aaa.aaa.aaa.aaa) works

The problem was bbb.bbb.bbb.bbb, because the container with the rest-API accessing the database was not allowed in ufw. To allow this IP, I had to get the container's IP.

So I ran: docker exec -it xyz /bin/sh, then inside the container: ifconfig which returned 172.22.0.5 for eth0.

This is the IP which had to be added to ufw. So my new ufw configuration looks like this:

3307                       ALLOW       aaa.aaa.aaa.aaa <- remote server
3307                       ALLOW       prod.local.ip.notebook <- development
3307                       ALLOW       bbb.bbb.bbb.bbb
3307                       ALLOW       172.22.0.5 <- container ip