What does --net=host option in Docker command really do?
After the docker installation you have 3 networks by default:
docker network ls
NETWORK ID NAME DRIVER SCOPE
f3be8b1ef7ce bridge bridge local
fbff927877c1 host host local
023bb5940080 none null local
I'm trying to keep this simple. So if you start a container by default it will be created inside the bridge (docker0) network.
$ docker run -d jenkins
1498e581cdba jenkins "/bin/tini -- /usr..." 3 minutes ago Up 3 minutes 8080/tcp, 50000/tcp friendly_bell
In the dockerfile of jenkins the ports 8080
and 50000
are exposed. Those ports are opened for the container on its bridge network. So everything inside that bridge network can access the container on port 8080
and 50000
. Everything in the bridge network is in the private range of "Subnet": "172.17.0.0/16",
If you want to access them from the outside you have to map the ports with -p 8080:8080
. This will map the port of your container to the port of your real server (the host network). So accessing your server on 8080
will route to your bridgenetwork on port 8080
.
Now you also have your host network. Which does not containerize the containers networking. So if you start a container in the host network it will look like this (it's the first one):
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
1efd834949b2 jenkins "/bin/tini -- /usr..." 6 minutes ago Up 6 minutes eloquent_panini
1498e581cdba jenkins "/bin/tini -- /usr..." 10 minutes ago Up 10 minutes 8080/tcp, 50000/tcp friendly_bell
The difference is with the ports. Your container is now inside your host network. So if you open port 8080
on your host you will acces the container immediately.
$ sudo iptables -I INPUT 5 -p tcp -m tcp --dport 8080 -j ACCEPT
I've opened port 8080
in my firewall and when I'm now accesing my server on port 8080
I'm accessing my jenkins. I think this blog is also useful to understand it better.
The --net=host
option is used to make the programs inside the Docker container look like they are running on the host itself, from the perspective of the network. It allows the container greater network access than it can normally get.
Normally you have to forward ports from the host machine into a container, but when the containers share the host's network, any network activity happens directly on the host machine - just as it would if the program was running locally on the host instead of inside a container.
While this does mean you no longer have to expose ports and map them to container ports, it means you have to edit your Dockerfiles to adjust the ports each container listens on, to avoid conflicts as you can't have two containers operating on the same host port. However, the real reason for this option is for running apps that need network access that is difficult to forward through to a container at the port level.
For example, if you want to run a DHCP server then you need to be able to listen to broadcast traffic on the network, and extract the MAC address from the packet. This information is lost during the port forwarding process, so the only way to run a DHCP server inside Docker is to run the container as --net=host
.
Generally speaking, --net=host
is only needed when you are running programs with very specific, unusual network needs.
Lastly, from a security perspective, Docker containers can listen on many ports, even though they only advertise (expose) a single port. Normally this is fine as you only forward the single expected port, however if you use --net=host
then you'll get all the container's ports listening on the host, even those that aren't listed in the Dockerfile. This means you will need to check the container closely (especially if it's not yours, e.g. an official one provided by a software project) to make sure you don't inadvertently expose extra services on the machine.
- you can create your own new network like --net="anyname"
- this is done to isolate the services from different container.
- suppose the same service are running in different containers, but the port mapping remains same, the first container starts well , but the same service from second container will fail. so to avoid this, either change the port mappings or create a network.
Remember one point that the host networking driver only works on Linux hosts, and is not supported on Docker Desktop for Mac, Docker Desktop for Windows, or Docker EE for Windows Server