What is mDNSResponder?

mDNSResponder is one of the most important networking processes on your Mac. It appears in Activity Monitor on every macOS system.

What is mDNSResponder?

mDNSResponder handles multicast DNS and DNS Service Discovery. Put simply, it is the process behind Bonjour, Apple's zero-configuration networking. It lets devices on a local network find each other without a central server.

What does it do?

mDNSResponder handles several things:

• Local device discovery: finding printers, AirPlay devices, Apple TVs, and other Macs on your network
• AirDrop and AirPlay: finding nearby devices for file sharing and screen mirroring
• DNS caching: it also acts as the system DNS resolver, storing lookups for faster browsing
• Name resolution: turning .local hostnames (such as MyMac.local) into IP addresses
• Printer discovery: finding network printers without you typing in IP addresses by hand

Why does it use network resources?

mDNSResponder regularly sends and listens for multicast packets on your local network. This is how devices announce themselves and find each other. The traffic stays local and uses very little bandwidth.

Does it use a lot of CPU or memory?

Normally, no. If you see mDNSResponder using a lot of resources, common causes include:

• Many devices on the network
• Network setup problems causing repeated lookups
• DNS failures where lookups keep being retried

You can flush its DNS cache if you have name resolution problems:

sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

Can you turn it off?

You should not. mDNSResponder is needed for DNS resolution and local network discovery. Turning it off would break web browsing, AirDrop, AirPlay, printer discovery, and many other features.

Should you worry?

No. It is a core macOS networking part. Apple tried replacing it with discoveryd in OS X 10.10 Yosemite, but the replacement had so many problems that Apple brought mDNSResponder back in 10.10.4.

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