Docker Ep 3: Docker Networking Deep Dive

TL;DR (Quick Summary)

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• Default Bridge (docker0): Standard single-host network. Containers can reach each other via IP address, but NOT by container name!
• User-Defined Bridge: Custom user networks. Provide Embedded DNS Resolution (containers can communicate using container names e.g., http://database:5432).
• Host Network (--network host): Bypasses network isolation. Container shares the host IP directly (fastest performance, no port mapping needed).
• Overlay Network: Enables multi-host container networking across Swarm or Kubernetes clusters.

1. Docker Network Architecture

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graph TD
    subgraph HostNetworkNamespace ["Host Network Namespace"]
        Eth0["Host Physical Interface: eth0"]
        IPTables["iptables NAT Rules"] <--> Eth0
        DockerBridge["docker0 / Custom Bridge: 172.19.0.1"] <--> IPTables
    end

    subgraph Container1 ["Container 1"]
        Veth1["veth1a"] <--> DockerBridge
        App1["Web Container (172.19.0.2)"] <--> Veth1
    end

    subgraph Container2 ["Container 2"]
        Veth2["veth2a"] <--> DockerBridge
        App2["Database Container (172.19.0.3)"] <--> Veth2
    end

2. Comparison: Network Drivers

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3. Step-by-Step Lab: Building a Custom User-Defined Network

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Let’s prove that User-Defined Bridge networks provide automatic DNS resolution between microservices.

Step 1: Creating a Custom Network

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Create a custom bridge network named app-net:

docker network create --driver bridge app-net

Inspect the network configuration:

docker network inspect app-net

Step 2: Launching Containers on the Custom Network

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Launch a Redis database container:

docker run -d --name my-redis --network app-net redis:alpine

Launch a Node container connected to the same network:

docker run -d --name my-app --network app-net alpine sleep 3600

Step 3: Testing Embedded DNS Name Resolution

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Ping my-redis by container name from inside my-app:

docker exec my-app ping -c 2 my-redis

Expected Terminal Output:

PING my-redis (172.19.0.2): 56 data bytes
64 bytes from 172.19.0.2: seq=0 ttl=64 time=0.089 ms
64 bytes from 172.19.0.2: seq=1 ttl=64 time=0.095 ms

The embedded Docker DNS engine resolved my-redis to 172.19.0.2 automatically!

4. Troubleshooting & Common Errors

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Error 1: Name or service not known in Default Bridge

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The Cause: You ran two containers on the default docker0 network without creating a custom network, and tried to connect via container name (http://backend:8080). Default bridge does NOT support name resolution! The Fix: Create a custom bridge: docker network create my-net and run both containers with --network my-net.

Summary & Next Steps

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In this episode:

• We analyzed Docker network drivers (Bridge, Host, Overlay).
• We created user-defined bridge networks.
• We tested embedded container DNS resolution by container name.

Next, we move to Episode 4: Docker Volumes and Data Persistence!