Bridge vs. Switch: What I Learned From a Data Center Tour

The difference between these two networking devices has been an unsolvable mystery to me for quite some time. For a while, I used to use the words "bridge" and "switch" interchangeably. But after getting more into networking, I started noticing that some people tend to see them as rather different devices... So, maybe I've been totally wrong? Maybe saying "bridge aka switch" is way too inaccurate?

Let's try to figure it out!

How network switch works

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Computer Networking Basics For Developers

As a software engineer, I need to deal with networking every now and then - be it configuring a SOHO network, setting up container networking, or troubleshooting connectivity between servers in a data center. The domain is pretty broad, and the terminology can get quite confusing quickly. This article is my layman's attempt to sort the basic things out with the minimum words and maximum drawings. The primary focus will be on the Data link layer (OSI L2) of wired networks where the Ethernet is the king nowadays. But I'll slightly touch upon its neighboring layers too.

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Networking Lab - L3 to L2 Segments Mapping

It's pretty common for an L2 segment to have a single IP subnet running atop. However, technically it's possible to configure multiple IP subnets over a single L2 broadcast domain. And although more complicated, configuring a single IP subnet over multiple disjoint L2 segments is also doable. In this lab, we'll cover the first two scenarios while the more advanced third case deserves its own lab - Proxy ARP.

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Container Networking Is Simple!

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

You can find a Russian translation of this article here.

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 combination 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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