Docker Cheatsheet

Networking

Use this Docker reference while you build software engineering projects, review code for technical interview prep, or polish examples for a software engineer resume.

Network Drivers

Driver	When to use
`bridge`	Default; isolated network on a single host
`host`	Container shares host network stack (Linux only)
`none`	No networking
`overlay`	Multi-host (Swarm / Kubernetes)
`macvlan`	Container gets its own MAC/IP on physical network
`ipvlan`	Like macvlan but shares host MAC

Managing Networks

docker network create mynet                              # bridge by default
docker network create --driver bridge mynet
docker network create --subnet 172.20.0.0/16 --gateway 172.20.0.1 mynet
docker network create --driver overlay --attachable swarmnet

docker network ls                                        # list networks
docker network inspect mynet                             # full JSON
docker network rm mynet
docker network prune                                     # remove all unused

Connecting Containers

# Attach at run time
docker run -d --network mynet --name api myapi
docker run -d --network mynet --name db postgres:17

# From inside the network, containers reach each other by name:
# api can connect to "db:5432"

# Connect / disconnect a running container
docker network connect mynet mycontainer
docker network disconnect mynet mycontainer

# Attach to multiple networks
docker run -d --network frontend --network backend myapp

Port Publishing

docker run -p 8080:80 nginx              # all interfaces
docker run -p 127.0.0.1:8080:80 nginx   # localhost only
docker run -p 8080:80/udp nginx          # UDP
docker run -P nginx                      # all EXPOSE ports → random host ports
docker port mycontainer                  # show published mappings

host and none Modes

docker run --network host nginx          # port 80 on the host directly (Linux)
docker run --network none alpine         # completely isolated

DNS and Service Discovery

Containers on a user-defined bridge resolve each other by container name.
The default bridge network does NOT provide automatic DNS — use named networks.
Custom DNS: docker run --dns 8.8.8.8 ... or set in daemon config.

# Verify DNS from inside a container
docker exec mycontainer nslookup db
docker exec mycontainer cat /etc/resolv.conf

Compose Networking

services:
  web:
    image: nginx
    networks:
      - frontend

  api:
    image: myapi
    networks:
      - frontend
      - backend

  db:
    image: postgres:17
    networks:
      - backend

networks:
  frontend:
  backend:
    internal: true      # no external access

In Compose, each service is reachable by its service name on any shared network (e.g., api can reach db:5432).

Inspecting Network Traffic

# Show container IP
docker inspect -f '{{.NetworkSettings.Networks.mynet.IPAddress}}' mycontainer

# Live packet capture (requires tcpdump in image)
docker run --rm --network container:mycontainer \
  nicolaka/netshoot tcpdump -i eth0 port 80

# Expose network namespace to host tools
pid=$(docker inspect -f '{{.State.Pid}}' mycontainer)
sudo nsenter -t $pid -n tcpdump

Common Network Patterns

# Sidecar shares network namespace
docker run -d --name app myapp
docker run -d --network container:app envoy

# Link containers (legacy — prefer named networks)
docker run --link db:database myapp   # deprecated