I need to understand how TCP uses ephemeral port in a container. I understand network is namespaced, and TCP port in container would be NAT’d to host port. Does that mean for two containers running in the same host, if one container binds to 64000 ports(use up 64k ports available inside the container using TCP bind(), without binding to host port), the other container won’t be able to use any port as all ports in the host system are used up?
Assuming one IP per host ofcourse
Hi TheJoker if you'll try to use localhost docker engine and run two containers with port let's say 80 with simple nginx server you can run them without any problem as long as you are not binding them with host port. If you are binding port 80 of container with port 80 of host obviously you can do that only for one container.
If you will run this command twice
docker run -d -p 80:80 nginx
You'll receive similar message to this one
docker: Error response from daemon: driver failed programming external connectivity on endpoint angry_mclean (d8bbf5af6503b4d54d234f1bf69ee372a8ada6ef07a5ebd138479691d5679994): Bind for 0.0.0.0:80 failed: port is already allocated.
To sum up you can run as many containers as you want with exposed port but you can bind only one to host port.
If you'll run container with 64000 ports bind to your host (-P option to bind all exposed ports) than your container is occupying all ports (not possible as your host system use some ports but theoretically).
UPDATE:
For more information please see :
https://docs.docker.com/engine/reference/builder/#expose
https://docs.docker.com/network/iptables/
Related
One of my Docker container ports needs to be redirected to the host, like with this parameter: -p 1234:4321. This works great for something like port 80 which I open via browser.
But if I have ports I want to permanently bind on my host machine I get into trouble, because docker already bound that port on my host. If I want to bind that port on my host machine I get exceptions like: Address already in use: NET_Bind.
How can I tell Docker to just reroute traffic to the host port without binding that port?
I understand port mapping with -p. I understand I can only map my container port on one port on the host network:
$ docker run -d -p 8080:80 nginx
There can no other container map its port on 8080 because there is already running a container. This port 8080 will be mapped on docker0 port 80 and so on on docker-container-port 80.
But I don't really understand why I can have another nginx:
$ docker -run -d -p 8888:80
I have to map my port on a different port of the host (8888) but why can my docker0 network open port 80 2 times? there are 2 containers behind it with port 80. I know it works but I just don't understand why.
Each container runs in a separate network namespace. This is an isolated network environment that does not shared network resources (addresses, interfaces, routes, etc) with the host. When you start a service in a container, it is as if you have started it on another machine.
Just as you can have two different machines on your network with webservers running on port 80, you can have two different containers on your host with webservers running on port 80.
Because they are in different network namespaces, there is no conflict.
For more reading on network namespaces:
https://blog.scottlowe.org/2013/09/04/introducing-linux-network-namespaces/
https://lwn.net/Articles/580893/
I have a docker container, and also installed on the VM a daemon listening for UDP on port 8125. The container sends data with UDP protocol on this 8125 port.
I was trying to open the port by starting the container with the -p 8125:8125/udp, but I'm getting the following error:
Error starting userland proxy: listen udp 0.0.0.0:8125: bind: address already in use
Which makes sense because the daemon is already listening on this port.
So how can I configure Docker so that the container can send UDP payloads to the external daemon ?
Opening ports is only needed when you want to Listen for the requests not sending. By default Docker provides the necessary network namespace for your container to communicate to the host or outside world.
So, you could it in two ways:
use --net host in your docker run and send requests to localhost:8125 in this case you containerized app is effectively sharing the host's network stack. So localhost points to the daemon that's already running in your host.
talk to the container network gateway (which is usually 172.17.0.1) or your host's hostname from your container. Then your are able to send packets to your daemon in your host.
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.
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.
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
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.
From my docker container I want to access the MySQL server running on my host at 127.0.0.1. I want to access the web server running on my container container from the host. I tried this:
docker run -it --expose 8000 --expose 8001 --net='host' -P f29963c3b74f
But none of the ports show up as exposed:
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
093695f9bc58 f29963c3b74f "/bin/sh -c '/root/br" 4 minutes ago Up 4 minutes elated_volhard
$
$ docker port 093695f9bc58
If I don't have --net='host', the ports are exposed, and I can access the web server on the container.
How can the host and container mutually access each others ports?
When --expose you define:
The port number inside the container (where the service listens) does
not need to match the port number exposed on the outside of the
container (where clients connect). For example, inside the container
an HTTP service is listening on port 80 (and so the image developer
specifies EXPOSE 80 in the Dockerfile). At runtime, the port might be
bound to 42800 on the host. To find the mapping between the host ports
and the exposed ports, use docker port.
With --net=host
--network="host" gives the container full access to local system services such as D-bus and is therefore considered insecure.
Here you have nothing in "ports" because you have all ports opened for host.
If you dont want to use host network you can access host port from docker container with docker interface
- How to access host port from docker container
- From inside of a Docker container, how do I connect to the localhost of the machine?.
When you want to access container from host you need to publish ports to host interface.
The -P option publishes all the ports to the host interfaces. Docker
binds each exposed port to a random port on the host. The range of
ports are within an ephemeral port range defined by
/proc/sys/net/ipv4/ip_local_port_range. Use the -p flag to explicitly
map a single port or range of ports.
In short, when you define just --expose 8000 the port is not exposed to 8000 but to some random port. When you want to make port 8000 visible to host you need to map published port -p 8000:8000.
Docker's network model is to create a new network namespace for your container. That means that container gets its own 127.0.0.1. If you want a container to reach a mysql service that is only listening on 127.0.0.1 on the host, you won't be able to reach it.
--net=host will put your container into the same network namespace as the host, but this is not advisable since it is effectively turning off all of the other network features that docker has-- you don't get isolation, you don't get port expose/publishing, etc.
The best solution will probably be to make your mysql server listen on an interface that is routable from the docker containers.
If you don't want to make mysql listen to your public interface, you can create a bridge interface, give it a random ip (make sure you don't have any conflicts), connect it to nothing, and configure mysql to listen only on that ip and 127.0.0.1. For example:
sudo brctl addbr myownbridge
sudo ifconfig myownbridge 10.255.255.255
sudo docker run --rm -it alpine ping -c 1 10.255.255.255
That IP address will be routable from both your host and any container running on that host.
Another approach would be to containerize your mysql server. You could put it on the same network as your other containers and get to it that way. You can even publish its port 3306 to the host's 127.0.0.1 interface.