macOS processes › Networking and wireless
What is mDNSResponder on Mac?
mDNSResponder looks up web addresses (DNS) for every app on your Mac and runs Bonjour, which finds printers, AirPlay speakers and other devices on your local network.
What it does
When an app connects to a website, it asks mDNSResponder to turn the name into an address, and the answer is cached for next time. Bonjour advertises services on your Mac and discovers ones nearby without any setup. Because every app's lookups pass through it, its activity reflects your whole Mac's network use. A small companion process, mDNSResponderHelper, does privileged tasks for it.
Why it gets busy
- Apps making huge numbers of lookups, like browsers with many tabs, ad-heavy pages or network scanners.
- A slow or failing DNS server, which makes lookups retry.
- Many Bonjour devices on a busy network, like an office or dorm.
- VPN or DNS filter apps that redirect lookups.
Why is mDNSResponder using so much CPU?
mDNSResponder handles DNS lookups for all apps, so heavy use usually means an app is making a lot of requests or your DNS server is struggling. Close busy apps or switch DNS servers.
Is it safe to quit?
macOS restarts it right away. Lookups pause briefly and the DNS cache is cleared.
How to calm it down
- Close tabs and apps that are loading constantly.
- If your DNS server is slow, try a different one in System Settings › Network › your connection › Details… › DNS.
- Flush the DNS cache by running
sudo killall -HUP mDNSResponderin Terminal. It asks for your password and is harmless. - Check VPN and DNS filter apps, which can multiply lookups.
In SystemWatch
The website edition of SystemWatch shows which apps have open network connections, which helps you find the one sending mDNSResponder so many lookups.
