Container connectivity

Learn how containers on the same network communicate.

Connecting containers

A running container can be connected to other networks later or removed from a network; a container can be on several networks at once. This is used, for example, to temporarily put a debugging tool into the back-end network:

docker network connect arka api
docker network disconnect arka api
docker inspect -f '{{range $ad, $_ := .NetworkSettings.Networks}}{{$ad}} {{end}}' api
# on arka

Alias

--network-alias gives the container an extra DNS name inside the network. Several containers can carry the same alias; the DNS query returns all their IPs (simple load distribution):

docker run -d --network uygulama-agi --network-alias arama elasticsearch:8.15.0
docker run -d --network uygulama-agi --network-alias arama elasticsearch:8.15.0
docker run --rm --network uygulama-agi alpine nslookup arama   # two addresses

Exercise Join and leave

Write three commands: connect the running api container to the arka network; run ping -c 1 db inside api; then disconnect api from the arka network.

Show example solution

docker network connect arka api
docker exec api ping -c 1 db
docker network disconnect arka api

Test yourself

If a container is attached to two networks, which containers can it reach?
- Only the host
- Containers on both networks
- Only those on the first network
- None
Show answer

Containers on both networks — It gets a separate interface and IP for each network.

Connect a running container to a network.

docker network ____ arka api

Show answer

docker network connect arka api

Match.

network connect

Join a network
network disconnect

Leave a network
--network-alias

Extra DNS name
nslookup

DNS query

Next lesson: Publishing ports

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