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Dockerized Agent Can’t Connect to Service Running on Host — Connection Refused

Symptom

• psycopg2.OperationalError: could not connect to server: Connection refused (port 5432)
• redis.exceptions.ConnectionError: Error 111 connecting to localhost:6379
• Agent works fine locally but fails when run in Docker
• curl localhost:8080 inside container returns connection refused
• Service is definitely running and accessible from the host machine

Root Cause

Inside a Docker container, localhost refers to the container’s own network namespace — not the host machine. A service running on the host machine’s localhost:5432 is not accessible at localhost:5432 from within a container.

Fix

Option 1: Use host.docker.internal (Mac/Windows)

# WRONG — resolves to container itself
DATABASE_URL = "postgresql://user:pass@localhost:5432/db"

# RIGHT — resolves to host machine on Mac and Windows Docker Desktop
DATABASE_URL = "postgresql://user:pass@host.docker.internal:5432/db"

# docker-compose.yml — add extra_hosts for Linux compatibility
services:
  agent:
    image: my-agent
    extra_hosts:
      - "host.docker.internal:host-gateway"  # Linux: map to actual host IP
    environment:
      - DATABASE_URL=postgresql://user:pass@host.docker.internal:5432/db

Option 2: Use host network mode (Linux)

# Run container with host networking — no network isolation
docker run --network=host my-agent

# Now localhost inside container = host machine localhost
# Note: --network=host doesn't work on Mac/Windows Docker Desktop

# docker-compose.yml
services:
  agent:
    image: my-agent
    network_mode: host  # Linux only

Option 3: Move the service into Docker Compose (recommended)

# docker-compose.yml — run everything together
services:
  agent:
    build: .
    environment:
      - DATABASE_URL=postgresql://postgres:password@postgres:5432/agentdb
      - REDIS_URL=redis://redis:6379
    depends_on:
      postgres:
        condition: service_healthy
      redis:
        condition: service_healthy

  postgres:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: password
      POSTGRES_DB: agentdb
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 5s
      timeout: 5s
      retries: 5

  redis:
    image: redis:7-alpine
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
      interval: 5s

Inside the agent container, postgres and redis are the hostnames — Docker Compose creates a shared network automatically.

Option 4: Use the host machine’s actual IP

# Find host IP from inside container
docker run --rm alpine ip route | grep default | awk '{print $3}'
# e.g., 172.17.0.1

# Or get host IP on the host machine
hostname -I | awk '{print $1}'
# e.g., 192.168.1.100

import os, socket

def get_host_ip() -> str:
    """Get host machine IP from inside Docker container"""
    # Try host.docker.internal first (Mac/Windows)
    try:
        return socket.gethostbyname("host.docker.internal")
    except socket.gaierror:
        pass

    # Fall back to default gateway IP (Linux)
    import subprocess
    result = subprocess.run(
        ["ip", "route", "show", "default"],
        capture_output=True, text=True
    )
    # "default via 172.17.0.1 dev eth0"
    parts = result.stdout.split()
    if "via" in parts:
        return parts[parts.index("via") + 1]

    return os.environ.get("HOST_IP", "localhost")

DATABASE_HOST = get_host_ip()

Option 5: Environment variable for flexibility

import os

# Let the environment dictate the host
DB_HOST = os.environ.get("DB_HOST", "localhost")
DB_PORT = int(os.environ.get("DB_PORT", "5432"))

# When running locally: DB_HOST=localhost (default)
# When running in Docker: DB_HOST=host.docker.internal
# When in Docker Compose: DB_HOST=postgres (service name)

# Local development
python agent.py

# Docker run
docker run -e DB_HOST=host.docker.internal my-agent

# Docker Compose
# Set DB_HOST=postgres in docker-compose.yml environment

Connectivity Quick Reference

Where agent runs	Where service runs	Use this host
Local (no Docker)	Local	localhost
Docker container	Same Docker Compose	Service name (e.g., postgres)
Docker container	Host machine (Mac/Win)	host.docker.internal
Docker container	Host machine (Linux)	172.17.0.1 or --network=host
Docker container	Another container	Container name or shared network
Docker container	Remote server	IP address or hostname

Diagnosis

# Test connectivity from inside a container
docker run --rm alpine sh -c "
  apk add --no-cache curl netcat-openbsd &&
  nc -zv host.docker.internal 5432 && echo 'OK' || echo 'FAILED'
"

# Check what localhost resolves to inside container
docker exec -it my-container-name getent hosts localhost
# Should show 127.0.0.1 (container loopback, not host)

Expected Token Savings

Debugging Docker networking confusion: ~6,000 tokens Knowing host.docker.internal upfront: prevents the issue entirely

Environment

• Dockerized agents connecting to host-side services (databases, message queues, APIs)
• Source: direct experience, Docker networking documentation

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