All articles tagged #iptables

If you're dealing with containers regularly, you've probably published ports many, many times already. A typical need for publishing arises like this: you're developing a web app, locally but in a container, and you want to test it using your laptop's browser. The next thing you do is docker run -p 8080:80 app and then open localhost:8080 in the browser. Easy-peasy!

But have you ever wondered what actually happens when you ask Docker to publish a port?

In this article, I'll try to connect the dots between port publishing, the term apparently coined by Docker, and a more traditional networking technique called port forwarding. I'll also take a look under the hood of different "single-host" container runtimes (Docker Engine, Docker Desktop, containerd, nerdclt, and Lima) to compare the port publishing implementations and capabilities.

As always, the ultimate goal is to gain a deeper understanding of the technology and get closer to becoming a power user of containers. Let the diving begin!

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Even when you have just one physical or virtual server, it's often a good idea to run multiple instances of your application on it. Luckily, when the application is containerized, it's actually relatively simple. With multiple application containers, you get horizontal scaling and a much-needed redundancy for a very little price. Thus, if there is a sudden need for handling more requests, you can adjust the number of containers accordingly. And if one of the containers dies, there are others to handle its traffic share, so your app isn't a SPOF anymore.

The tricky part here is how to expose such a multi-container application to the clients. Multiple containers mean multiple listening sockets. But most of the time, clients just want to have a single point of entry.

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Just kidding, it's not... But fear not and read on!

Working with containers always feels like magic. In a good way for those who understand the internals and in a terrifying - for those who don't. Luckily, we've been looking under the hood of the containerization technology for quite some time already and even managed to uncover that containers are just isolated and restricted Linux processes, that images aren't really needed to run containers, and on the contrary - to build an image we need to run some containers.

Now comes a time to tackle the container networking problem. Or, more precisely, a single-host container networking problem. In this article, we are going to answer the following questions:

• How to virtualize network resources to make containers think each of them has a dedicated network stack?
• How to turn containers into friendly neighbors, prevent them from interfering, and teach to communicate well?
• How to reach the outside world (e.g. the Internet) from inside the container?
• How to reach containers running on a machine from the outside world (aka port publishing)?

While answering these questions, we'll setup a container networking from scratch using standard Linux tools. As a result, it'll become apparent that the single-host container networking is nothing more than a simple combintion of the well-known Linux facilities:

• network namespaces;
• virtual Ethernet devices (veth);
• virtual network switches (bridge);
• IP routing and network address translation (NAT).

And for better or worse, no code is required to make the networking magic happen...

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Gee, it's my turn to throw some gloom light on iptables! There are hundreds or even thousands of articles on the topic out there, including introductory ones. I'm not going to put either formal and boring definitions here nor long lists of useful commands. I would rather try to use layman's terms and scribbling as much as possible to give you some insights about the domain before going to all these tables, rules, targets, and policies. By the way, the first time I faced this tool I was pretty much confused by the terminology too!

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