dvcrn Container Debug
Debug running Docker containers and Compose services. Covers logs, exec, networking, resource inspection, multi-stage builds, health checks, and common failure patterns.
When to Use
- Container exits immediately or crashes on start
- Application inside container behaves differently than on host
- Containers can't communicate with each other
- Container is using too much memory or CPU
- Multi-stage Docker build produces unexpected results
- Health checks are failing
- Compose services start in wrong order or can't connect
Container Logs
View and filter logs
# Last 100 lines
docker logs --tail 100 my-container
# Follow (stream) logs
docker logs -f my-container
# Follow with timestamps
docker logs -f -t my-container
# Logs since a time
docker logs --since 30m my-container
docker logs --since "2026-02-03T10:00:00" my-container
# Logs between times
docker logs --since 1h --until 30m my-container
# Compose: logs for all services
docker compose logs -f
# Compose: logs for specific service
docker compose logs -f api db
# Redirect logs to file for analysis
docker logs my-container > container.log 2>&1
# Separate stdout and stderr
docker logs my-container > stdout.log 2> stderr.log
Inspect log driver
# Check what log driver a container uses
docker inspect --format='{{.HostConfig.LogConfig.Type}}' my-container
# If json-file driver, find the actual log file
docker inspect --format='{{.LogPath}}' my-container
# Check log file size
ls -lh $(docker inspect --format='{{.LogPath}}' my-container)
Exec Into Containers
Interactive shell
# Bash (most common)
docker exec -it my-container bash
# If bash isn't available (Alpine, distroless)
docker exec -it my-container sh
# As root (even if container runs as non-root user)
docker exec -u root -it my-container bash
# With specific environment variables
docker exec -e DEBUG=1 -it my-container bash
# Run a single command (no interactive shell)
docker exec my-container cat /etc/os-release
docker exec my-container ls -la /app/
docker exec my-container env
Debug a crashed container
# Container exited? Check exit code
docker inspect --format='{{.State.ExitCode}}' my-container
docker inspect --format='{{.State.Error}}' my-container
# Common exit codes:
# 0 = clean exit
# 1 = application error
# 137 = killed (OOM or docker kill) — 128 + signal 9
# 139 = segfault — 128 + signal 11
# 143 = terminated (SIGTERM) — 128 + signal 15
# Start a stopped container to debug it
docker start -ai my-container
# Or override the entrypoint to get a shell
docker run -it --entrypoint sh my-image
# Copy files out of a stopped container
docker cp my-container:/app/error.log ./error.log
docker cp my-container:/etc/nginx/nginx.conf ./nginx.conf
Debug without a shell (distroless / scratch images)
# Use docker cp to extract files
docker cp my-container:/app/config.json ./
# Use nsenter to get a shell in the container's namespace (Linux)
PID=$(docker inspect --format='{{.State.Pid}}' my-container)
nsenter -t $PID -m -u -i -n -p -- /bin/sh
# Attach a debug container to the same namespace
docker run -it --pid=container:my-container --net=container:my-container busybox sh
# Docker Desktop: use debug extension
docker debug my-container
Networking
Inspect container networking
# Show container IP address
docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' my-container
# Show all network details
docker inspect -f '{{json .NetworkSettings.Networks}}' my-container | jq
# List all networks
docker network ls
# Inspect a network (see all connected containers)
docker network inspect bridge
docker network inspect my-compose-network
# Show port mappings
docker port my-container
Test connectivity between containers
# From inside container A, reach container B
docker exec container-a ping container-b
docker exec container-a curl http://container-b:8080/health
# DNS resolution inside container
docker exec my-container nslookup db
docker exec my-container cat /etc/resolv.conf
docker exec my-container cat /etc/hosts
# Test if port is reachable
docker exec my-container nc -zv db 5432
docker exec my-container wget -qO- http://api:3000/health
# If curl/ping not available in container, install or use a debug container:
docker run --rm --network container:my-container curlimages/curl curl -s http://localhost:8080
Common networking issues
# "Connection refused" between containers
# 1. Check the app binds to 0.0.0.0, not 127.0.0.1
docker exec my-container netstat -tlnp
# If listening on 127.0.0.1 — fix the app config
# 2. Check containers are on the same network
docker inspect -f '{{json .NetworkSettings.Networks}}' container-a | jq 'keys'
docker inspect -f '{{json .NetworkSettings.Networks}}' container-b | jq 'keys'
# 3. Check published ports vs exposed ports
# EXPOSE only documents, it doesn't publish
# Use -p host:container to publish
# "Name not found" — DNS not resolving container names
# Container names resolve only on user-defined networks, NOT the default bridge
docker network create my-net
docker run --network my-net --name api my-api-image
docker run --network my-net --name db postgres
# Now "api" and "db" resolve to each other
Capture network traffic
# tcpdump inside a container
docker exec my-container tcpdump -i eth0 -n port 8080
# If tcpdump not available, use a sidecar
docker run --rm --net=container:my-container nicolaka/netshoot tcpdump -i eth0 -n
# netshoot has: tcpdump, curl, nslookup, netstat, iperf, etc.
docker run --rm --net=container:my-container nicolaka/netshoot bash
Resource Usage
Real-time stats
# All containers
docker stats
# Specific containers
docker stats api db redis
# One-shot (no streaming)
docker stats --no-stream
# Formatted output
docker stats --format "table {{.Name}}\t{{.CPUPerc}}\t{{.MemUsage}}\t{{.NetIO}}"
Memory investigation
# Check memory limit
docker inspect --format='{{.HostConfig.Memory}}' my-container
# 0 means unlimited
# Check if container was OOM-killed
docker inspect --format='{{.State.OOMKilled}}' my-container
# Memory usage breakdown (Linux cgroups)
docker exec my-container cat /sys/fs/cgroup/memory.current 2>/dev/null || \
docker exec my-container cat /sys/fs/cgroup/memory/memory.usage_in_bytes
# Process memory inside container
docker exec my-container ps aux --sort=-%mem | head -10
docker exec my-container top -bn1
Disk usage
# Overall Docker disk usage
docker system df
docker system df -v
# Container filesystem size
docker inspect --format='{{.SizeRw}}' my-container
# Find large files inside container
docker exec my-container du -sh /* 2>/dev/null | sort -rh | head -10
docker exec my-container find /tmp -size +10M -type f
# Check for log file bloat
docker exec my-container ls -lh /var/log/
Dockerfile Debugging
Multi-stage build debugging
# Build up to a specific stage
docker build --target builder -t my-app:builder .
# Inspect what's in the builder stage
docker run --rm -it my-app:builder sh
docker run --rm my-app:builder ls -la /app/
docker run --rm my-app:builder cat /app/package.json
# Check which files made it to the final image
docker run --rm my-image ls -laR /app/
# Build with no cache (fresh build)
docker build --no-cache -t my-app .
# Build with progress output
docker build --progress=plain -t my-app .
Image inspection
# Show image layers (size of each)
docker history my-image
docker history --no-trunc my-image
# Inspect image config (entrypoint, cmd, env, ports)
docker inspect my-image | jq '.[0].Config | {Cmd, Entrypoint, Env, ExposedPorts, WorkingDir}'
# Compare two images
docker history image-a --format "{{.Size}}\t{{.CreatedBy}}" > layers-a.txt
docker history image-b --format "{{.Size}}\t{{.CreatedBy}}" > layers-b.txt
diff layers-a.txt layers-b.txt
# Find what changed between builds
docker diff my-container
# A = added, C = changed, D = deleted
Health Checks
Define and debug health checks
# In Dockerfile
HEALTHCHECK --interval=30s --timeout=10s --start-period=60s --retries=3 \
CMD curl -f http://localhost:8080/health || exit 1
# Check health status
docker inspect --format='{{.State.Health.Status}}' my-container
# "healthy", "unhealthy", or "starting"
# See health check log (last 5 results)
docker inspect --format='{{json .State.Health}}' my-container | jq
# Run health check manually
docker exec my-container curl -f http://localhost:8080/health
# Override health check at run time
docker run --health-cmd "curl -f http://localhost:8080/health || exit 1" \
--health-interval 10s my-image
# Disable health check
docker run --no-healthcheck my-image
Docker Compose Debugging
Service startup issues
# Check service status
docker compose ps
# See why a service failed
docker compose logs failed-service
# Start with verbose output
docker compose up --build 2>&1 | tee compose.log
# Start a single service (with dependencies)
docker compose up db
# Start without dependencies
docker compose up --no-deps api
# Recreate containers from scratch
docker compose up --force-recreate --build
# Check effective config (after variable substitution)
docker compose config
Service dependency and startup order
# docker-compose.yml
services:
api:
depends_on:
db:
condition: service_healthy
redis:
condition: service_started
db:
image: postgres:16
healthcheck:
test: ["CMD-SHELL", "pg_isready -U postgres"]
interval: 5s
timeout: 5s
retries: 5
redis:
image: redis:7
healthcheck:
test: ["CMD", "redis-cli", "ping"]
interval: 5s
timeout: 5s
retries: 5
# Wait for a service to be healthy before running commands
docker compose up -d db
docker compose exec db pg_isready # Polls until ready
docker compose up -d api
Cleanup
# Remove stopped containers
docker container prune
# Remove unused images
docker image prune
# Remove everything unused (containers, images, networks, volumes)
docker system prune -a
# Remove volumes too (WARNING: deletes data)
docker system prune -a --volumes
# Remove dangling build cache
docker builder prune
Tips
docker logs -fis the first thing to check. Most container failures are visible in the logs.- Exit code 137 means OOM-killed. Increase the memory limit or fix the memory leak.
- Apps inside containers must bind to
0.0.0.0, not127.0.0.1. Localhost inside a container is isolated. - Container names only resolve via DNS on user-defined networks, not the default
bridge. Always create a custom network for multi-container setups. docker execonly works on running containers. For crashed containers, usedocker cpto extract logs or override the entrypoint withdocker run --entrypoint sh.nicolaka/netshootis the Swiss Army knife for container networking. It has every networking tool pre-installed.--progress=plainduring builds shows full command output, which is essential for debugging build failures.- Health checks with
start-periodprevent false unhealthy status during slow application startup.