docker jenkins smtp docker-network jenkins-docker

Docker SMTP container to use in Jenkins-Docker container

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.

Solution

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