Compose Networking Debug

Advanced15 min

Diagnose and fix networking issues between Docker Compose services including DNS resolution, port conflicts, and connectivity problems.

Prerequisites

-Docker Compose installed
-A running Compose stack

Steps

List Compose networks

See which networks the Compose stack created.

$ docker network ls --filter 'label=com.docker.compose.project'

Inspect a network to see connected containers

View detailed info about a network including which containers are attached and their IP addresses.

$ docker network inspect <network-name> --format '{{range .Containers}}{{.Name}}: {{.IPv4Address}}{{println}}{{end}}'

Test DNS resolution between services

Verify that service discovery works by resolving a service name from within another container.

$ docker compose exec api nslookup db

In Compose, services can reach each other by service name. If DNS fails, check that both services are on the same network.

Test connectivity between services

Verify that one service can actually reach another on the expected port.

$ docker compose exec api sh -c 'nc -zv db 5432 || echo Connection failed'

If nc is not available, try: wget -qO- --spider http://db:5432 or install it with apk add netcat-openbsd.

Debug with netshoot on the Compose network

Attach a full networking toolkit to the Compose network for advanced debugging.

$ docker run -it --rm --network <compose-network-name> nicolaka/netshoot

Full Script

full-script.sh

#!/bin/bash
# Compose Networking Debug
set -euo pipefail

echo "=== Compose Networks ==="
docker network ls --filter 'label=com.docker.compose.project'

echo ""
echo "=== Service Connectivity ==="
for network in $(docker network ls --filter 'label=com.docker.compose.project' -q); do
  echo "Network: $(docker network inspect "$network" --format '{{.Name}}')"
  docker network inspect "$network" --format '{{range .Containers}}  {{.Name}}: {{.IPv4Address}}{{println}}{{end}}'
done

echo ""
echo "=== Debug Commands ==="
echo "DNS lookup:   docker compose exec <service> nslookup <target-service>"
echo "Port check:   docker compose exec <service> nc -zv <target-service> <port>"
echo "Netshoot:     docker run -it --rm --network <network-name> nicolaka/netshoot"
echo "Ping:         docker compose exec <service> ping -c 3 <target-service>"

FAQ

Discussion

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#!/bin/bash # Compose Networking Debug set -euo pipefail echo "=== Compose Networks ===" docker network ls --filter 'label=com.docker.compose.project' echo "" echo "=== Service Connectivity ===" for network in $(docker network ls --filter 'label=com.docker.compose.project' -q); do echo "Network: $(docker network inspect "$network" --format '{{.Name}}')" docker network inspect "$network" --format '{{range .Containers}} {{.Name}}: {{.IPv4Address}}{{println}}{{end}}' done echo "" echo "=== Debug Commands ===" echo "DNS lookup: docker compose exec <service> nslookup <target-service>" echo "Port check: docker compose exec <service> nc -zv <target-service> <port>" echo "Netshoot: docker run -it --rm --network <network-name> nicolaka/netshoot" echo "Ping: docker compose exec <service> ping -c 3 <target-service>"