Send mail from a container using a postfix container - bash

Question

I'm using an application hosted on a docker container.
This application executes bash scripts / instructions to send mails.
I made another container which executes Postfix as a SMTP Relay.
I want to send mails from my application container by using a bash script using my Postfix container as a relay.
I tried to connect with SSH from my application container to my Postfix container. But that doesn't seem to work.
How can i make it so a script executed in my application container can use my Postfix relay while not allowing anything outside of the docker network, or even better, to only allow some containers, to send mails from this relay.
EDIT 1 : Docker-compose files
Application docker compose :
version: "3.4"
volumes:
[...]
services:
application:
restart: always
build: ./application
depends_on:
- mariadb
container_name: application
volumes:
[...]
ports:
- "80:80"
- "443:443"
- "5669:5669"
deploy:
restart_policy:
window: 300s
links:
- mariadb
external_links:
- smtp-server
mariadb:
restart: always
image: mariadb
command: mysqld --sql-mode=ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
container_name: application-mariadb
volumes:
[...]
environment:
MYSQL_RANDOM_ROOT_PASSWORD: "yes"
deploy:
restart_policy:
window: 300s
Here's my docker compose for my SMTP server :
version: "3.4"
services:
postfix:
restart: always
build: ./postfix
container_name: smtp-server
deploy:
restart_policy:
window: 300s

Answer 1

{a quick response, because I "cicle" in my work ... and I'm taking 10 minutes of clear my mint, I hope it serves you}
Are you using "docker-compose" ?, could you give an example of your YML file? (a little more context)
[you can not connect to by ssh to a container unless you have "supervisor" installed,which I do not recommend at all.]
from what I see, you only need to make private networks; You could use this:
https://docs.docker.com/compose/networking/
to hide everything, I also recommend using a load balancer / Inverse Proxy like TRAEFIK (if they have access to port 80 or 443 in some clear way this ...)
so you only expose 1/2 port(s) (80 + 443 for example) and everything else is protected by your reverse proxy
Watch as I separate the networks as you need the different containers.
bash have access to db and smtp
db does not have access smtp neither nginx
nginx have access to bash
nginx have access to proxy network to expose 80 and 443
no other container is exposed to the outside more than nginx
--
version: "3"
services:
bash:
####### use hostname "smtp" as SMTP server
image: bash
depends_on:
- db
networks:
- smtp_internal_network
- internal_network
- data_network
volumes:
- ../html:/var/www/html
restart: always
db:
image: percona:5.7
# ports: # for debug connections and querys
# - 3306:3306
volumes:
- ../db_data:/var/lib/mysql
environment:
MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD}
MYSQL_DATABASE: ${MYSQL_DATABASE}
MYSQL_USER: ${MYSQL_USER}
MYSQL_PASSWORD: ${MYSQL_PASSWORD}
networks:
- data_network
restart: always
smtp:
image: mwader/postfix-relay
environment:
- POSTFIX_myhostname=smtp.domain.tld
networks:
- smtp_internal_network
restart: always
nginx:
image: nginx
volumes:
- ../html:/var/www/html
networks:
- external_network
- internal_network
labels:
- "traefik.backend=nginx_${COMPOSE_PROJECT_NAME}"
- "traefik.port=80"
- "traefik.frontend.rule=Host:${FRONTEND_RULE}"
- "traefik.frontend.passHostHeader=true"
- "traefik.enable=true"
- "traefik.docker.network=traefik_proxy"
restart: always
depends_on:
- db
- bash
networks:
external_network:
external:
name: traefik_proxy
internal_network:
driver: bridge
smtp_internal_network:
driver: bridge
data_network:
driver: bridge
Edit:
version: "3"
volumes:
[...]
services:
####### use hostname "smtp" as SMTP server in your application
application:
restart: always
build: ./application
depends_on:
- mariadb
volumes:
[...]
ports:
- "80:80"
- "443:443"
- "5669:5669"
deploy:
restart_policy:
window: 300s
networks:
- smtp_external_network
- data_network
mariadb:
restart: always
image: mariadb
command: mysqld --sql-mode=ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
networks:
- data_network
volumes:
[...]
environment:
MYSQL_RANDOM_ROOT_PASSWORD: "yes"
deploy:
restart_policy:
window: 300s
networks:
smtp_external_network:
external:
name: [ReplaceForFolderParentNameOfSmtpYmlWithoutSquareBrackets]_smtp
data_network:
driver: bridge
--- (in your other file)
services:
smtp:
restart: always
build: ./postfix
networks:
- smtp
deploy:
restart_policy:
window: 300s
networks:
smpt:
driver: bridge