Docker Compose: Mock external services - networking

Question

I have the following situation:
My application consists of a single web service that calls an
external API (say, some SaaS service, ElasticSearch or so). For non-unit-testing purposes we want to control the external service and later also inject faults. The application and the "mocked" API are dockerized and
now I want to use docker-compose to spin all containers up.
Because the application has several addresses hardcoded (e.g. the hostname of external services) I cannot change them and need to work around.
The service container makes a call to http://external-service.com/getsomestuff.
My idea was to use some features that are provided by docker to reroute all outgoing traffic to the external http://external-service.com/getsomestuff to the mock container without changing the URL.
My docker-compose.yaml looks like:
version: '2'
services:
service:
build: ./service
container_name: my-service1
ports:
- "5000:5000"
command: /bin/sh -c "python3 app.py"
api:
build: ./api-mock
container_name: my-api-mock
ports:
- "5001:5000"
command: /bin/sh -c "python3 app.py"
Finally, I have a driver that just does the following:
curl -XGET localhost:5000/
curl -XPUT localhost:5001/configure?delay=10
curl -XGET localhost:5000/
where the second curl just sets the delay in the mock to 10 seconds.
There are several options I have considered:
Using iptables-fu (would require modifying Dockerfiles to install it)
Using docker networks (this is really unclear to me)
Is there any simple option to achieve what I want?
Edit:
For clarity, here is the relevant part of the service code:
import requests
#app.route('/')
def do_stuff():
r = requests.get('http://external-service.com/getsomestuff')
return process_api_response(r.text())

Answer 1

Docker runs an internal DNS server for user defined networks. Any unknown host lookups are forwarded to you normal DNS servers.
Version 2+ compose files will automatically create a network for compose to use so there's a number of ways to control the hostnames it resolves.
The simplest way is to name your container with the hostname:
version: "2"
services:
external-service.com:
image: busybox
command: sleep 100
ping:
image: busybox
command: ping external-service.com
depends_on:
- external-service.com
If you want to keep container names you can use links
version: "2"
services:
api:
image: busybox
command: sleep 100
ping:
image: busybox
links:
- api:external-service.com
command: ping external-service.com
depends_on:
- api
Or network aliases
version: "2"
services:
api:
image: busybox
command: sleep 100
networks:
pingnet:
aliases:
- external-service.com
ping:
image: busybox
command: ping external-service.com
depends_on:
- api
networks:
- pingnet
networks:
pingnet:

Answer 2

I'm not entirely clear what the problem is you're trying to solve, but if you're trying to make external-service.com inside the container direct traffic to your "mock" service, I think you should be able to do that using the extra_hosts directive in your docker-compose.yml file. For example, if I have this:
version: "2"
services:
example:
image: myimage
extra_hosts:
- google.com:172.23.254.1
That will result in /etc/hosts in the container containing:
172.23.254.1 google.com
And attempts to access http://google.com will hit my web server at 172.23.254.1.

Answer 3

I was able to solve this with -links, is there a way to do networks in docker-compose?
version: '3'
services:
MOCK:
image: api-mock:latest
container_name: api-mock-container
ports:
- "8081:80"
api:
image: my-service1:latest
links:
- MOCK:external-service.com