Fix: Docker Container Network Connectivity Issues

!Symptoms

• Container cannot reach external hosts: 'Could not resolve host'
• Container-to-container communication fails on the same network
• Ports are not accessible from the host despite -p flag
• Docker compose services cannot find each other by name

?Root Causes

• Container not on the correct Docker network
• DNS resolution inside the container is broken
• iptables rules blocking Docker traffic
• Docker daemon DNS configuration is wrong
• Port mapping conflict with another container or host process
• Firewall (UFW, firewalld) interfering with Docker networking

#Diagnosis Steps

1. 1Check container network: `docker inspect <container> --format='{{json .NetworkSettings.Networks}}'`
2. 2Test DNS from inside: `docker exec <container> nslookup google.com`
3. 3Check port mappings: `docker port <container>`
4. 4List Docker networks: `docker network ls` and inspect: `docker network inspect <name>`
5. 5Check iptables: `iptables -L -n | grep -i docker`

>Fix

1. 1Connect container to the correct network: `docker network connect <network> <container>`
2. 2Fix DNS: add --dns=8.8.8.8 to docker run or set dns in daemon.json
3. 3For compose: ensure services are on the same network (default compose network usually works)
4. 4Restart Docker daemon to reset iptables rules: `systemctl restart docker`
5. 5Check for port conflicts: `ss -tlnp | grep <port>`

*Prevention

• Use Docker Compose for multi-container setups to manage networking automatically
• Create explicit Docker networks for application groups
• Configure Docker daemon DNS in /etc/docker/daemon.json
• Test network connectivity as part of container health checks
• Avoid using --network=host unless necessary

Related Error Messages