Day 43 · Docker

Docker Ports Mapping

By Published Docker / Linux~1h to complete

Technical Overview

By default, Docker containers run in isolated network environments. They are assigned an internal IP address that is only accessible from the Docker host itself or other containers connected to the same virtual network.

To allow external clients (e.g., users on the internet or systems in the corporate network) to access services running inside a container, you must use Port Mapping (also known as port publishing).

How Docker Port Mapping Works

When you start a container with the -p (or --publish) flag, you define a binding between a port on the host machine and a port inside the container.

Under the hood, when a port is mapped:

  1. The Docker daemon starts a userland proxy process (docker-proxy) on the host that listens on the specified host port.
  2. It configures host-level packet routing rules (iptables rules in the DOCKER NAT chain) to intercept incoming traffic on that host port and forward it directly to the container's internal IP address and port.
sequenceDiagram actor Client as External Client participant Host as Docker Host (Port 8086) participant Proxy as Docker Proxy / iptables participant Container as Nginx Container (Port 80) Client->>Host: Request http://stapp02:8086 Host->>Proxy: Intercept traffic on host port 8086 Proxy->>Container: Forward traffic to container IP on port 80 Container-->>Proxy: Return HTTP Response (200 OK) Proxy-->>Host: Pass response Host-->>Client: Respond to client

Port Mapping Command Reference

Standard Port Mapping Syntax

docker run -p <host_port>:<container_port> <image>
  • Example: -p 8086:80 routes incoming host traffic on port 8086 to port 80 inside the container.

Port Mapping Variations

1. IP-Specific Binding

By default, Docker binds the host port to all network interfaces (0.0.0.0 - any IP). You can restrict access by binding to a specific host IP address:

docker run -p 127.0.0.1:8086:80 <image>

This ensures the container is only reachable locally from the host itself, preventing external access.

2. Protocol Specification (TCP/UDP)

By default, Docker publishes TCP ports. To map UDP ports (such as for DNS or VPN containers), append /udp to the port argument:

docker run -p 53:53/udp -p 53:53/tcp <image>

3. Random Ephemeral Port Mapping

If you omit the host port, Docker will automatically map the container port to a random, unused ephemeral port on the host (typically in the range 32768 to 60999):

docker run -p 80 <image>

4. Publish All Exposed Ports

You can map all ports defined in the image's EXPOSE metadata instructions to random host ports automatically using the uppercase -P flag:

docker run -d -P nginx:alpine

Infrastructure & Configuration Requirements

  • Target Host: Nautilus App Server 2 (stapp02) (can vary in labs, e.g., stapp01, stapp02, stapp03)
  • SSH User: steve (associated with stapp02; tony for stapp01, banner for stapp03)
  • Container Name: ports_map (or custom container name)
  • Base Image: nginx:alpine
  • Host Port: 8086
  • Container Port: 80

Step-by-Step Implementation

Step 1: Connect to the Application Server

Establish an SSH connection from the Jump Host to App Server 2:

ssh steve@stapp02

Provide the server password when prompted.

Step 2: Verify Port Availability

Before launching the container, verify that host port 8086 is not already bound by another service on the host:

ss -tuln | grep 8086
# or
netstat -tuln | grep 8086

If this command returns no output, the port is available.

Step 3: Run the Container with Port Mapping

Run the nginx:alpine container in detached mode (-d), name it ports_map, and map host port 8086 to container port 80:

docker run -d --name ports_map -p 8086:80 nginx:alpine

Note: Prepend sudo if your user is not in the docker group:

sudo docker run -d --name ports_map -p 8086:80 nginx:alpine

Post-Deployment Verification

1. Verify Container Execution

Ensure that the container was launched successfully and is in the Up state:

docker ps

Expected Output:

CONTAINER ID   IMAGE          COMMAND                  CREATED         STATUS         PORTS                  NAMES
1fee9bf7d02e   nginx:alpine   "/docker-entrypoint.…"   5 seconds ago   Up 5 seconds   0.0.0.0:8086->80/tcp   ports_map

2. Verify Port Binding on the Host

Confirm Nginx is actively listening on the host's port 8086:

ss -tuln | grep 8086

Expected Output:

tcp   LISTEN   0        128              0.0.0.0:8086           0.0.0.0:*

3. Verify HTTP Connectivity

Perform an HTTP request using curl to the mapped port on localhost to verify the Nginx welcome response:

curl -I http://localhost:8086

Expected Output:

HTTP/1.1 200 OK
Server: nginx/1.25.1
Content-Type: text/html
...

Log out of the Application Server:

exit

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