Docker SMTP container to use in Jenkins-Docker container - docker

Question

This topic is about Docker networking, which I can't get to allow dockerised Jenkins to use a dockerised SMTP server.
Here's how I run my containers and connect them to a user-defined network, so that containers' name might be use as a target host:
# Run Jenkins image, with port binding, Docker sock sharing, and configuration sharing
docker run -d -p 8080:8080 -v /var/run/docker.sock:/var/run/docker.sock -v /root/jenkins_conf/:/var/jenkins_home/ --name jenkins jenkins/jenkins:lts
# Run SMTP image
docker run -d --name smtp namshi/smtp
# Create user-defined network
docker network create jenkins-net
# Connect both containers
docker network connect jenkins-net jenkins
docker network connect jenkins-net smtp
Within my jenkins container, I can reach the smtp service via the default 'bridge' network:
$ (echo >/dev/tcp/172.17.0.5/25) &>/dev/null && echo "open" || echo "close"
open
and I can also reach it via my user-defined network, both via IP and hostname:
$ (echo >/dev/tcp/172.18.0.3/25) &>/dev/null && echo "open" || echo "close"
open
$ (echo >/dev/tcp/smtp/25) &>/dev/null && echo "open" || echo "close"
open
So far, so good.
But then, in Jenkins > Manage Jenkins > Configure System > E-mail notification, trying to use the test email sending tool gives me following results:
SMTP server: 172.17.0.5 (SMTP container IP on the default 'bridge' network)
SMTP port: 25
=> the email is sent and duly received !
SMTP server: 172.18.0.3 (SMTP container IP on my user-defined 'jenkins-net' network)
SMTP port: 25
=> Failed to send out e-mail
com.sun.mail.smtp.SMTPAddressFailedException: 550 relay not permitted
SMTP server: smtp (SMTP container name on my user-defined 'jenkins-net' network)
SMTP port: 25
=> same error
Why the difference of behaviour from the SMTP server between the use of the 2 networks?
What am I missing about Docker networking?
Edit: So the quick solution was to run the smtp container with the --network option, instead of running it and then connecting it to the network. See Stefano's answer below for more details and adequacy.

Answer 1

The problem you described is unrelated to the docker networking. The namshi/smtp image uses exim4 as SMTP. In this specific exim4 setup, you're required to provide the neworks from where it's allowed to connect and send email.
Checking the entrypoint.sh file, I found the following command:
dc_relay_nets "$(ip addr show dev eth0 | awk '$1 == "inet" { print $2 }' | xargs | sed 's/ /:/g')${RELAY_NETWORKS}"
This means that by default, it'll accept the emails coming from the IP address associated with the interface eth0 and other possible RELAY_NETWORKS (if defined).
Since the container is not attached by default to the jenkins-net network at the creation, it won't recognize as valid the emails coming from that address.
Try to start the containers like this:
docker network create jenkins-net
SUBNET=$( docker network inspect \
-f '{{range .IPAM.Config}}{{.Subnet}}{{end}}' \
jenkins-net )
docker run -d \
--network jenkins-net \
-e RELAY_NETWORKS=":${SUBNET}" \
--name smtp \
namshi/smtp
docker run -d \
-p 8080:8080 \
-v /var/run/docker.sock:/var/run/docker.sock \
-v /root/jenkins_conf/:/var/jenkins_home/ \
--network jenkins-net \
--name jenkins \
jenkins/jenkins:lts