What does --net host option in Docker command really do

Question

I m a little bit beginner to Docker  I couldn t find any clear description of what this option does in docker run command in deep and bit confused about it   Can we use it to access the applications running on docker containers without specifying a port  As an example if I run a webapp deployed via a docker image in port 8080 by using option -p 8080 8080 in docker run command  I know I will have to access it on 8080 port on Docker containers ip  theWebAppName  But I cannot really think of a way how --net host option works

User · Accepted Answer

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.

User · Answer

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

User · Answer

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

[docker] What does --net=host option in Docker command really do?

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