Debug:docker

Docker Debugging Guide

This guide provides a systematic approach to debugging Docker containers, images, networks, and volumes. Follow the four-phase methodology for efficient problem resolution.

The Four Phases of Docker Debugging

Phase 1: Quick Assessment (30 seconds)

Get immediate context about the issue.

# Check container status
docker ps -a

# View recent logs (last 50 lines)
docker logs --tail 50 <container>

# Check container exit code
docker inspect <container> --format='{{.State.ExitCode}}'

# Quick health check
docker inspect <container> --format='{{.State.Health.Status}}'

Phase 2: Log Analysis (2-5 minutes)

Deep dive into container logs and events.

# Follow logs in real-time
docker logs -f <container>

# View logs with timestamps
docker logs --timestamps <container>

# View logs since specific time
docker logs --since 30m <container>

# Check Docker daemon events
docker events --since 1h

# View system-wide logs
journalctl -u docker.service --since "1 hour ago"

Phase 3: Interactive Investigation (5-15 minutes)

Get hands-on access to the container environment.

# Open shell in running container
docker exec -it <container> /bin/sh
# or
docker exec -it <container> /bin/bash

# Run commands without shell
docker exec <container> cat /etc/hosts
docker exec <container> env

# Use docker debug for enhanced debugging (Docker Desktop 4.27+)
docker debug <container>

# Inspect container configuration
docker inspect <container>

# Check network configuration
docker network inspect <network>

Phase 4: Deep Analysis (15+ minutes)

Comprehensive investigation for complex issues.

# Monitor resource usage
docker stats <container>

# Check disk usage
docker system df -v

# Inspect image layers
docker history <image>

# Export container filesystem for analysis
docker export <container> -o container.tar

# View detailed container info
docker inspect <container> | jq '.'

Common Error Patterns and Solutions

Exit Codes

OOM Killed Containers (Exit Code 137)

# Check if container was OOM killed
docker inspect <container> --format='{{.State.OOMKilled}}'

# View memory limits
docker inspect <container> --format='{{.HostConfig.Memory}}'

# Run with increased memory
docker run -m 2g --memory-swap 4g <image>

# Monitor memory in real-time
docker stats --format "table {{.Name}}\t{{.MemUsage}}\t{{.MemPerc}}"

Image Build Failures

# Build with verbose output
docker build --progress=plain -t <image> .

# Build without cache
docker build --no-cache -t <image> .

# Build specific stage for debugging
docker build --target <stage-name> -t <image> .

# Inspect build cache
docker builder prune --dry-run

# Debug failed layer by running from previous successful layer
docker run -it <last-successful-layer-id> /bin/sh

Common Build Issues:

• COPY failed: file not found - Check paths are relative to build context
• RUN failed - Check command syntax, ensure dependencies are installed
• Permission denied - Add chmod or run as appropriate user

Networking Issues

# List networks
docker network ls

# Inspect network
docker network inspect <network>

# Check container network settings
docker inspect <container> --format='{{json .NetworkSettings.Networks}}'

# Test connectivity between containers
docker exec <container1> ping <container2>

# Test DNS resolution
docker exec <container> nslookup <hostname>

# Check exposed ports
docker port <container>

# Debug with network tools
docker run --rm --network=<network> nicolaka/netshoot ping <target>

Common Network Issues:

• Containers on different networks cannot communicate
• Port already in use: lsof -i :<port> to find conflicting process
• DNS resolution fails: Check Docker DNS settings

Volume Mount Problems

# List volumes
docker volume ls

# Inspect volume
docker volume inspect <volume>

# Check mount points
docker inspect <container> --format='{{json .Mounts}}'

# Verify host path exists
ls -la /path/to/host/directory

# Check permissions inside container
docker exec <container> ls -la /mount/path

# Test with simple container
docker run --rm -v /host/path:/container/path alpine ls -la /container/path

Common Volume Issues:

• Permission denied - Check UID/GID mapping, use :z or :Z for SELinux
• Path not found - Ensure host path exists before mounting
• Windows paths - Use forward slashes or escaped backslashes

Permission Denied Errors

# Add user to docker group (Linux)
sudo usermod -aG docker $USER
newgrp docker

# Check Docker socket permissions
ls -la /var/run/docker.sock

# Run container as specific user
docker run --user $(id -u):$(id -g) <image>

# Fix file permissions in Dockerfile
RUN chown -R appuser:appuser /app
USER appuser

Container Exits Immediately

# Check what command is running
docker inspect <container> --format='{{.Config.Cmd}}'
docker inspect <container> --format='{{.Config.Entrypoint}}'

# Keep container running for debugging
docker run -d <image> tail -f /dev/null
# or
docker run -d <image> sleep infinity

# Override entrypoint for debugging
docker run -it --entrypoint /bin/sh <image>

# Check if process is foreground
# (Docker needs a foreground process to keep running)

Docker Desktop Won't Start

Windows:

• Enable Hyper-V: Enable-WindowsOptionalFeature -Online -FeatureName Microsoft-Hyper-V -All
• Enable WSL2: wsl --install
• Check virtualization in BIOS (VT-x/AMD-V)
• Temporarily disable antivirus

Mac:

• Check available disk space
• Reset Docker Desktop: Delete ~/Library/Group\ Containers/group.com.docker/
• Reinstall Docker Desktop

Linux:

• Check Docker daemon: sudo systemctl status docker
• View daemon logs: journalctl -u docker.service

Debugging Tools Reference

Essential Commands

# Container lifecycle
docker ps -a                          # List all containers
docker logs <container>               # View logs
docker logs -f --tail 100 <container> # Follow last 100 lines
docker exec -it <container> sh        # Interactive shell
docker inspect <container>            # Full container details
docker top <container>                # Running processes

# Images
docker images                         # List images
docker history <image>                # Show image layers
docker inspect <image>                # Image details

# System
docker system df                      # Disk usage
docker system events                  # Real-time events
docker system info                    # System-wide info
docker system prune                   # Clean up unused resources

# Network
docker network ls                     # List networks
docker network inspect <network>      # Network details

# Volumes
docker volume ls                      # List volumes
docker volume inspect <volume>        # Volume details

Advanced Debugging

# Docker debug (Docker Desktop 4.27+)
docker debug <container>              # Enhanced shell with tools

# Process inspection
docker exec <container> ps aux        # List processes
docker exec <container> top           # Interactive process viewer

# Network debugging
docker exec <container> netstat -tlnp # Open ports
docker exec <container> ss -tlnp      # Socket statistics
docker exec <container> curl -v <url> # HTTP debugging

# File system
docker diff <container>               # Changed files
docker cp <container>:/path ./local   # Copy files out
docker cp ./local <container>:/path   # Copy files in

# Resource monitoring
docker stats                          # Live resource usage
docker stats --no-stream              # Single snapshot

Debug Container Image

For minimal images without debugging tools, use a sidecar approach:

# Use netshoot for network debugging
docker run -it --network container:<target> nicolaka/netshoot

# Use busybox for basic tools
docker run -it --pid container:<target> busybox

Quick Reference Commands

# Most common debugging sequence
docker ps -a                                  # 1. Check status
docker logs --tail 100 -f <container>         # 2. View logs
docker exec -it <container> sh                # 3. Interactive shell
docker inspect <container>                    # 4. Full details

# Performance debugging
docker stats                                  # Resource usage
docker system df                              # Disk usage
docker events                                 # System events

# Network debugging
docker network ls                             # List networks
docker network inspect <network>              # Network details
docker exec <container> ping <host>           # Test connectivity

# Clean up
docker system prune -af                       # Remove all unused data
docker volume prune                           # Remove unused volumes
docker builder prune                          # Remove build cache

Troubleshooting Checklist

• Check container status with docker ps -a
• Review logs with docker logs <container>
• Verify exit code with docker inspect --format='{{.State.ExitCode}}'
• Check for OOM: docker inspect --format='{{.State.OOMKilled}}'
• Verify network connectivity between containers
• Check volume mounts and permissions
• Ensure required ports are not in use
• Verify environment variables are set correctly
• Check available disk space with docker system df
• Review Docker daemon logs if system-level issue

Sources

• How to Fix and Debug Docker Containers Like a Superhero - Docker Blog
• Docker Debugging: Common Scenarios and 7 Practical Tips - Lumigo
• docker debug - Docker Docs
• Docker Logs: Complete Debugging Cheat Sheet - Atmosly
• 100 Common Docker Errors and Solutions - DEV Community
• How to Debug and Fix Common Docker Issues - DigitalOcean
• Common Docker Topics - Docker Docs
• 5 Methods to Keep Docker Container Running for Debugging - Spacelift