My eventual goal here is to allow a container running a FastAPI app to communicate with a MySQL database on the host.

First I tried using host.docker.internal

Dockerfile

FROM debian:latest

RUN apt update && apt install -y \
    netcat \
    iputils-ping

CMD echo "tailing /dev/null" && tail -f /dev/null

docker-compose.yml

version: "3.2"

services:
  test:
    build:
      context: "."
    extra_hosts:
      - "host.docker.internal:host-gateway"

Expected behavior: ping works, nc -vz works

In particular, with nc -vz I'd expect to see something like:

root@9fe8de220d44:/# nc -vz host.docker.internal 80
Connection to host.docker.internal (172.17.0.1) port 80 (tcp) succeeded!

Actual behavior: ping works, nc -vz doesn't

root@5981bcfbf598:/# ping host.docker.internal
PING host.docker.internal (172.17.0.1) 56(84) bytes of data.
64 bytes from host.docker.internal (172.17.0.1): icmp_seq=1 ttl=64 time=0.079 ms
64 bytes from host.docker.internal (172.17.0.1): icmp_seq=2 ttl=64 time=0.067 ms
64 bytes from host.docker.internal (172.17.0.1): icmp_seq=3 ttl=64 time=0.068 ms
^C
--- host.docker.internal ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2013ms
rtt min/avg/max/mdev = 0.067/0.071/0.079/0.005 ms
root@5981bcfbf598:/# nc -vz host.docker.internal 80
nc: connect to host.docker.internal (172.17.0.1) port 80 (tcp) failed: Connection refused

On the host

I have apache running on port 80

$ netstat -tulpn
...
tcp6       0      0 :::80                   :::*                    LISTEN      1258/apache2

Additionally, my firewall is configured to allow all inbound requests to port 80: firewall says http port 80 allows all ipv4 and ipv6

OS and docker versions:

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.6 LTS
Release:    18.04
Codename:   bionic
$ docker --version
Docker version 20.10.21, build baeda1f

Manually specifying the network also fails

After host.docker.internal failed, I followed the instructions for connecting from a container to a Linux host (Ubuntu 18.04 in my case) here using a manually specified network: https://stackoverflow.com/a/70725882

Here's my setup:

Dockerfile

FROM debian:latest

RUN apt update && apt install -y \
    netcat \
    iputils-ping

CMD echo "tailing /dev/null" && tail -f /dev/null

docker-compose.yml

version: "3.2"

networks:
  test:
    name: test-network
    attachable: true
    ipam:
      driver: default
      config:
        - subnet: 172.42.0.0/16
          ip_range: 172.42.5.0/24
          gateway: 172.42.0.1

services:
  test:
    build:
      context: "."
    networks:
      - test

Confirm gateway

$ docker inspect test-test-1  -f '{{range .NetworkSettings.Networks}}{{.Gateway}}{{end}}'
172.42.0.1

ping works

root@07f81c211a0c:/# ping 172.42.0.1
PING 172.42.0.1 (172.42.0.1) 56(84) bytes of data.
64 bytes from 172.42.0.1: icmp_seq=1 ttl=64 time=0.056 ms
64 bytes from 172.42.0.1: icmp_seq=2 ttl=64 time=0.068 ms
64 bytes from 172.42.0.1: icmp_seq=3 ttl=64 time=0.065 ms

Expected behavior: nc -vz succeeds

From the instructions at https://stackoverflow.com/a/70725882:

root@9fe8de220d44:/# nc -vz 172.18.0.1 80
Connection to 172.18.0.1 80 port [tcp/http] succeeded!

Actual behavior: nc -vz fails

root@07f81c211a0c:/# nc -vz 172.42.0.1 80
nc: connect to 172.42.0.1 port 80 (tcp) failed: Connection refused

What am I doing wrong?

Thanks in advance for your help!

CodePudding user response：

I was able to solve this problem thanks to https://forums.docker.com/t/how-to-connect-from-docker-container-to-the-host/123318.

I ran

$ ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet XXX.XXX.XXX.XXX  netmask ...

and copied the address XXX.XXX.XXX.XXX, and replaced host-gateway with it in my docker-compose.yml:

version: "3.2"

services:
  test:
    build:
      context: "."
    extra_hosts:
      - "host.docker.internal:XXX.XXX.XXX.XXX"

Now, from within the container:

root@f5836a37815a:/# nc -vz host.docker.internal 80
Connection to host.docker.internal (104.248.221.215) 80 port [tcp/*] succeeded!

I'm not sure why all the advice to use host-gateway didn't work. I was under the impression it should work for my version of docker (compose).

$ docker version
Client: Docker Engine - Community
 Version:           20.10.21
...
$ docker compose version
Docker Compose version v2.12.2