How to Link Containers in Docker: Simple Guide

To link containers in Docker, create a user-defined network with docker network create and run containers attached to this network using --network. Containers on the same network can communicate by container name without extra linking flags.

Syntax

Linking containers is done by creating a network and attaching containers to it. The main commands are:

• docker network create <network-name>: Creates a new network.
• docker run --network <network-name> --name <container-name> <image>: Runs a container attached to the network.

Containers on the same network can reach each other by their --name.

bash

docker network create my-network
docker run -d --name container1 --network my-network nginx
docker run -d --name container2 --network my-network alpine sleep 1000

Example

This example creates a network, runs two containers on it, and shows how one container can ping the other by name.

bash

docker network create my-network

docker run -d --name webserver --network my-network nginx

docker run -it --rm --network my-network alpine sh -c "ping -c 3 webserver"

Output

PING webserver (172.18.0.2): 56 data bytes
64 bytes from 172.18.0.2: seq=0 ttl=64 time=0.123 ms
64 bytes from 172.18.0.2: seq=1 ttl=64 time=0.110 ms
64 bytes from 172.18.0.2: seq=2 ttl=64 time=0.105 ms

--- webserver ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss

Common Pitfalls

Common mistakes when linking containers include:

• Using the old --link flag, which is deprecated and less flexible.
• Not creating or attaching containers to the same user-defined network, so they cannot resolve each other's names.
• Expecting containers on the default bridge network to communicate by name without extra setup.

Always use user-defined networks for container communication.

bash

docker run -d --name old-nginx nginx

docker run -it --rm alpine ping old-nginx

# This will fail because containers are on default bridge network and name resolution is not automatic.

# Correct way:
docker network create my-net

docker run -d --name new-nginx --network my-net nginx

docker run -it --rm --network my-net alpine ping new-nginx

Quick Reference

Summary tips for linking containers:

• Create a user-defined network with docker network create.
• Run containers with --network <network-name> to connect them.
• Use container names to communicate between containers.
• Avoid the deprecated --link flag.

Key Takeaways

Use user-defined Docker networks to link containers for easy name-based communication.

Attach containers to the same network using the --network flag when running them.

Avoid the deprecated --link option; it is less flexible and not recommended.

Containers on the default bridge network cannot resolve each other by name automatically.

Ping or connect to containers by their --name within the same network.