Docker Networking Fundamentals

Overview

Create a user-defined bridge network, connect containers by name, publish ports, and know when host/overlay/macvlan apply.

Default bridge isolates containers; user-defined bridges add DNS. Host shares the host stack. Overlay spans Swarm/multi-host. Port mapping publishes container ports to the host.

This is a core tutorial in Module 8 · Networking of the REBASH Academy Docker for Cloud & DevOps Engineers series — written for Cloud, DevOps, Platform, and SRE engineers.

Prerequisites

• Volumes and Persistent Storage
• Networking basics from the Networking track help

Learning Objectives

By the end of this tutorial, you will be able to:

• Create and inspect a bridge network
• Reach containers via DNS names
• Publish ports with -p
• Contrast bridge vs host
• Outline overlay / macvlan use cases

Architecture

This topic’s control points and relationships are shown below.

Theory

What

Docker networks connect containers to each other and to the outside world. The default bridge network suits single-host apps; host shares the host network namespace; none isolates; overlay serves multi-host Swarm; macvlan makes containers appear as LAN hosts. Published ports map container ports to the host.

Why

Most “container cannot connect” tickets are network misunderstandings: wrong network, unpublished ports, or DNS name mismatches. Compose and Swarm rely on user-defined networks so service names resolve via embedded DNS.

How it works

User-defined bridge networks give containers IP addresses and DNS entries based on container or Compose service names. docker run --network attaches at start; you can also connect later. Publishing -p 8080:80 forwards host port 8080 to container port 80. Troubleshoot with docker network inspect, docker exec plus wget/nc, and host tools such as ss or lsof for published ports. Firewall rules on the host can still block traffic after Docker’s iptables/nftables integration.

Driver	Typical use
bridge	Single-host apps (default for Compose)
host	Max performance / special networking
none	Locked down (no NIC)
overlay	Multi-host Swarm
macvlan	Appear as LAN hosts

Key concepts

• Embedded DNS — service discovery on user-defined networks
• Publish vs expose — publish creates host forwarding
• Hairpin / localhost — Desktop vs Linux differences
• Network policies — Docker alone is not Kubernetes NetworkPolicy

Common pitfalls

• Using the legacy default bridge without DNS service names
• Binding only to 127.0.0.1 then wondering why other hosts cannot connect
• Assuming containers share localhost with the host (they do not, except host mode)
• Overlapping subnet CIDRs with corporate VPNs

Hands-on Lab

Create a workspace for this tutorial.

mkdir -p ~/rebash-docker/module-08 && cd ~/rebash-docker/module-08

Focus: create a user-defined bridge network and connect two containers

Step 1 – Network and DNS by name

docker network create rebash-net
docker run -d --name rebash-web --network rebash-net nginx:alpine
docker run --rm --network rebash-net curlimages/curl:8.10.1 curl -sI http://rebash-web/ | head -n 5
docker network inspect rebash-net --format '{{json .Containers}}' | head -c 400; echo

Step 2 – Cleanup network resources

docker rm -f rebash-web
docker network rm rebash-net

Final step – Cleanup note

docker rm -f rebash-web 2>/dev/null || true
docker network rm rebash-net 2>/dev/null || true
# Keep ~/rebash-docker/ for later tutorials

Validation

• Lab commands run under ~/rebash-docker/module-08/
• You can explain each Theory section in your own words
• You used modern tooling where it applies to this topic
• You can describe one production failure mode for this topic

Code Walkthrough

Production practice for Docker Networking Fundamentals always combines:

1. Inspect before you change (status, plan, logs, dry-run)
2. Prefer reversible, documented changes (Git, IaC, drop-ins, version pins)
3. Capture evidence (command output, pipeline logs) for handovers
4. Prefer current tools and APIs over legacy shortcuts
5. Least privilege — escalate credentials only when required

Keep runbooks short enough to follow under pressure. Automate checks; keep humans for judgement.

Security Considerations

• Treat credentials and tokens for docker as privileged — never commit them
• Prefer short-lived auth (OIDC, roles, SSO) over long-lived keys
• Validate blast radius before apply/deploy/delete operations
• Restrict who can approve production changes
• Collect audit logs; limit who can read sensitive traces

Common Mistakes

Using the legacy default bridge without DNS service names

Validate assumptions against the Theory section and official docs before changing production.

Binding only to 127.0.0.1 then wondering why other hosts cannot connect

Lab shortcuts (open security groups, admin roles, skip approvals) must not ship unchanged.

Changing production without a rollback path

Always know how to revert (previous artefact, prior release, state rollback, DNS failback).

Best Practices

• Encode Docker Networking Fundamentals changes as code and review them in pull requests
• Pin versions (images, modules, actions, provider plugins)
• Separate environments with clear promotion gates
• Alert on symptoms with runbooks attached
• Destroy lab resources; tag everything with owner and expiry where possible

Troubleshooting

Symptom	Likely cause	Fix
Auth / permission denied	Wrong identity, policy, or scope	Check caller identity, roles, and least-privilege policies
Timeout / no route	Network, DNS, security group, or endpoint	Trace path, DNS, and allow-lists before retrying
Drift / unexpected plan	Manual change or wrong state/workspace	Reconcile desired vs actual; avoid click-ops on managed resources
Pipeline/job red	Flaky step, cache, or missing secret	Read failing step logs; bisect recent workflow/config changes
Cost spike	Idle load balancer, NAT, oversized compute	Inventory billable resources; stop/delete labs promptly

Summary

Docker Networking Fundamentals is essential for Cloud and DevOps engineers working with docker. Practise the lab until the inspection and change path is muscle memory, then continue the track.

Interview Questions

1. Bridge versus host versus none networks?
2. How do containers resolve each other by name?
3. Published ports versus container ports?
4. When do custom networks beat the default bridge?
5. How do you inspect connectivity failures?

Sample answer — question 2

Use docker network inspect and confirm both containers share the network.

Sample answer — question 4

Avoid host networking unless required; it weakens isolation.

Related Tutorials

• Course overview
• Docker Compose Fundamentals

References

• Docker networking overview