Libinfo calls mdnsresponder on what it is

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You set up a Mac firewall or just check what's going on with the Activity Monitor when you notice something going on cryptic: mDNSResponder. What is this process and should you be concerned? No: this is a central part of macOS.

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This article is part of our ongoing series that explains various processes in the Activity Monitor. like kernel_task, hidd, mdsworker, installd, WindowServer, blued, launchd, backup, opendirectoryd, powerd, coreauthd, configd and many others. Don't know what these services are? Better start reading!

What is mDNSResponder?

Today's process, mDNSResponder, is a core part of the Bonjour protocol. Bonjour is Apple's zero-configuration network service, which means it's how Apple devices find each other on a network. Our mDNSResponder process regularly searches your local network for other Bonjour-enabled devices.

Why should you look for other devices? To simplify networking. An example of this work is library sharing from iTunes. Open iTunes and you can view and search other iTunes libraries over your local network. Bonjour is why this works: the protocol allows two computers on the same network to find each other easily, meaning the list of shared iTunes libraries is always up to date.

Bonjour does more than just iTunes sharing helps populate the list of "shared" devices in the Finder. Bonjour also fills the image transfer in photos, the list of Airplay compatible devices and the quick search for printers. Because the same process is performed on Windows, Bonjour can also be used to quickly connect to Windows computers that are running software like iTunes. Here's how iTunes libraries are shared between PCs and Macs.

Third-party software can also use Bonjour: you can stream audio from iTunes to Kodi even using Kodi on Windows if you have Bonjour installed. You can quickly browse all Bonjour-enabled devices on your network using a simple program called the Bonjour Browser.

If you are using a Mac firewall, you will see popups through mDNSResponder. If you prevent this process from accessing the network, Bonjour will stop working. This makes it difficult to use your local network. In some cases, disabling Bonjour can prevent you from even connecting to the internet. So it is probably best to only allow mDNSResponders to access your network.

You should remember that mDNSResponder shouldn't use a lot of CPU power. If you do, in most cases restarting your Mac should fix the problem.

Wait, Apple hasn't removed mDNSResponder?

You might be thinking that Apple removed mDNSResponder from macOS years ago, and you're kind of correct. According to Ars Technica, Apple temporarily dropped the mDNSResponder for Yosemite in 2014 only to discover that lots Things break without them. A year later, Apple brought back mDNSResponder for El Capitan, which fixed 300 different macOS bugs in a single swift move. This suggests that mDNSResponder won't disappear from macOS any time soon.

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