I have two containers on the same user-defined bridge network, but they can't ping each other by name. What are the common pitfalls in DNS resolution? I've checked my firewall settings but the service discovery still seems to be failing intermittently.
3 answers
Networking issues in usually boil down to one of three things: the network type, container naming, or host-level firewalls like UFW. Ensure you are using a "user-defined bridge" and not the default bridge, as the default one doesn't support automatic service discovery by name. Also, check if your containers have actually finished starting up; the internal DNS server only registers them once they are in a 'running' state. I spent weeks debugging a similar issue in 2023 only to find that my host's Docker daemon wasn't properly updating the iptables rules. Running docker network inspect will give you the ground truth on which containers are actually connected to the bridge and what their assigned IP addresses are.
Are you using 'aliases' in your network configuration, or are you strictly relying on the container names for your service-to-service communication?
Always check the logs of the dockerd service. Often, it will show errors related to network creation or iptables that aren't visible in the container logs.
That’s a great point, Gregory. Daemon logs are often overlooked but contain the most vital information for solving deep networking bugs.
I’m relying on container names, Donald. I thought that was the standard way for to handle it. If I switch to using aliases in my compose file, does that make the DNS resolution more stable, or is it just a way to provide multiple names for the same service? I'm trying to find the most "fail-proof" way to link my frontend and backend.