Docker Networking Knowledge Base

Network configuration patterns and communication strategies for PHP Docker stacks.

Network Types

Type	Driver	Use Case	Isolation	Performance
Bridge	bridge	Default single-host	Container-level	Good
Host	host	Maximum performance	None (shares host)	Best
Overlay	overlay	Multi-host (Swarm/K8s)	Container-level	Moderate
None	none	Complete isolation	Full	N/A
Macvlan	macvlan	Direct LAN access	Network-level	Good

PHP-FPM + Nginx Communication

TCP Socket (Default in Docker)

┌─────────────────────────────────────────────────────────────────────────────┐
│                     TCP SOCKET COMMUNICATION                                 │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│   Client ──▶ :80 ──▶ ┌─────────┐  TCP :9000  ┌──────────┐                 │
│                       │  Nginx  │ ──────────▶ │ PHP-FPM  │                 │
│                       └─────────┘              └──────────┘                 │
│                                                                              │
│   Nginx config: fastcgi_pass php:9000;                                      │
│   PHP-FPM config: listen = 0.0.0.0:9000                                    │
│                                                                              │
│   Pros: Works across containers, simple setup, scalable                     │
│   Cons: Slight overhead vs Unix socket                                      │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘

Unix Socket (Shared Volume)

┌─────────────────────────────────────────────────────────────────────────────┐
│                    UNIX SOCKET COMMUNICATION                                 │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│   Client ──▶ :80 ──▶ ┌─────────┐  /sock/fpm  ┌──────────┐                 │
│                       │  Nginx  │ ──────────▶ │ PHP-FPM  │                 │
│                       └────┬────┘              └────┬─────┘                 │
│                            │     shared volume      │                       │
│                            └────────┐  ┌────────────┘                       │
│                                     ▼  ▼                                    │
│                              ┌──────────────┐                               │
│                              │ /var/run/php/ │                               │
│                              │  php-fpm.sock │                               │
│                              └──────────────┘                               │
│                                                                              │
│   Nginx config: fastcgi_pass unix:/var/run/php/php-fpm.sock;               │
│   PHP-FPM config: listen = /var/run/php/php-fpm.sock                       │
│                                                                              │
│   Pros: ~5-10% faster, no TCP overhead                                      │
│   Cons: Requires shared volume, harder to scale                             │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘

Compose configuration for Unix socket:

services:
  php:
    volumes:
      - php-socket:/var/run/php

  nginx:
    volumes:
      - php-socket:/var/run/php:ro

volumes:
  php-socket:

DNS Resolution in Compose

Docker Compose provides automatic DNS resolution using service names.

┌──────────────────────────────────────────────────────────┐
│              DNS RESOLUTION                                │
├──────────────────────────────────────────────────────────┤
│                                                           │
│   Service Name    ──▶  Container IP                       │
│   ────────────────────────────────────                    │
│   postgres        ──▶  172.20.0.2                         │
│   redis           ──▶  172.20.0.3                         │
│   rabbitmq        ──▶  172.20.0.4                         │
│   php             ──▶  172.20.0.5                         │
│   nginx           ──▶  172.20.0.6                         │
│                                                           │
│   Resolution scope: within same network only              │
│   Resolver: Docker embedded DNS (127.0.0.11)             │
│                                                           │
└──────────────────────────────────────────────────────────┘

PHP connection strings using service names:

// PostgreSQL
'postgresql://app:secret@postgres:5432/app'

// Redis
'redis://redis:6379'

// RabbitMQ
'amqp://guest:guest@rabbitmq:5672/%2f'

// Elasticsearch
'http://elasticsearch:9200'

DNS Aliases

services:
  postgres:
    networks:
      backend:
        aliases:
          - db
          - database
          - pg

Port Mapping Strategies

Pattern	Syntax	Visibility	Use Case
Published	ports: ["8080:80"]	Host + containers	Dev access, public services
Exposed	expose: ["9000"]	Containers only	Internal services (FPM)
Host binding	ports: ["127.0.0.1:5432:5432"]	Localhost only	Databases in dev
Random host	ports: ["5432"]	Random host port	CI, parallel testing

Recommended Port Mapping

services:
  # Public-facing — bind to all interfaces
  nginx:
    ports:
      - "80:80"
      - "443:443"

  # Internal service — expose only (no host port)
  php:
    expose:
      - "9000"

  # Database — localhost only (dev security)
  postgres:
    ports:
      - "127.0.0.1:${POSTGRES_PORT:-5432}:5432"

  # Management UI — localhost only
  rabbitmq:
    ports:
      - "127.0.0.1:${RABBITMQ_MGMT_PORT:-15672}:15672"

Network Segmentation

networks:
  frontend:
    driver: bridge
    # Public-facing services (Nginx, Mailhog UI)

  backend:
    driver: bridge
    internal: true
    # Database, cache, queue — no external access

  monitoring:
    driver: bridge
    # Prometheus, Grafana, Jaeger

Segmentation Matrix

┌─────────────────────────────────────────────────────────────────┐
│   Service         │ frontend │ backend │ monitoring │ internet  │
│   ────────────────┼──────────┼─────────┼────────────┼────────── │
│   Nginx           │    ✓     │         │            │    ✓      │
│   PHP-FPM         │    ✓     │    ✓    │            │           │
│   Worker          │          │    ✓    │            │           │
│   Cron            │          │    ✓    │            │           │
│   PostgreSQL      │          │    ✓    │            │           │
│   Redis           │          │    ✓    │            │           │
│   RabbitMQ        │          │    ✓    │            │           │
│   Elasticsearch   │          │    ✓    │            │           │
│   Prometheus      │          │    ✓    │     ✓      │           │
│   Grafana         │          │         │     ✓      │    ✓      │
│   Jaeger          │          │    ✓    │     ✓      │           │
└─────────────────────────────────────────────────────────────────┘

The internal: true flag on the backend network prevents containers from reaching the internet, ensuring databases and caches are not exposed.

Inter-Container Communication

Same Network (Direct)

// Containers on the same network communicate directly via service name
$redis = new \Redis();
$redis->connect('redis', 6379);

Cross-Network (Shared Service)

services:
  php:
    networks:
      - frontend
      - backend    # PHP needs both to talk to Nginx AND database

  postgres:
    networks:
      - backend    # Only backend — no direct external access

Service Discovery Pattern

// Use environment variables for service endpoints
$dbHost = getenv('DATABASE_HOST') ?: 'postgres';
$redisHost = getenv('REDIS_HOST') ?: 'redis';
$amqpHost = getenv('AMQP_HOST') ?: 'rabbitmq';

host.docker.internal for Local Development

Access host machine services from within containers:

services:
  php:
    extra_hosts:
      - "host.docker.internal:host-gateway"
    environment:
      # Connect to services running on host machine
      XDEBUG_CONFIG: "client_host=host.docker.internal"

Platform	host.docker.internal	Notes
Docker Desktop (Mac)	Built-in	Works automatically
Docker Desktop (Windows)	Built-in	Works automatically
Linux	Requires extra_hosts	Use host-gateway mapping

Troubleshooting

Issue	Diagnosis	Solution
Container cannot resolve service name	Wrong network	Ensure both services share a network
Connection refused between containers	Port not exposed	Add expose or check service is listening
Intermittent DNS failures (Alpine)	musl resolver	Add options ndots:0 to resolv.conf
Cannot reach host machine	Missing extra_hosts	Add host.docker.internal:host-gateway
External access to internal service	Missing internal: true	Mark backend network as internal
Port conflict on host	Port already in use	Use variable port mapping ${PORT:-5432}:5432

References

For service configuration examples, see acc-docker-compose-knowledge. For base image networking considerations, see acc-docker-base-images-knowledge.