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I got a message from WOW platform saying my machine was generating suspicious activity. I run AED4 and MacAffee last versions with 0 threats results.

Then I turned to the Activity Monitor and checked what has been going on, just to find this _mdnsresponder process amidst regular activity.

Checked the web and apparently is some kind of bug at the service of an (eventual) unknown trojan. There is nothing to relax someone like these news.

Does anyone know how to get rid of this plague?


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mDNSResponder is an iTunes process. – Sathya Sep 13 '10 at 18:51
It might be related to iTunes on Windows (I have no idea), but on a Mac it's essential for networking. It's even one of the only three programs that are granted network access, even when you tell your firewall to block everything else. So, ensure it's not Apple's mDNSResponder before you remove it! – Arjan Sep 13 '10 at 20:15

You can't remove mDNSResponder from Mac OS X Snow Leopard and still have a functioning internet connection. mDNSResponder is used for DNS resolution and several other tasks in Mac OS X 10.6 and is not a trojan.

In Activity Monitor you should a process called mDNSResponder run by a user called _mdnsresponder. If the process itself is called _mdnsresponder then this is something else. I'm not aware (and can't find any evidence contrary) of any Mac OS X malware running a process called _mdnsresponder. Is the process actually called _mdnsresponder?

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AFAIK, mDNSResponder is only used for multicast DNS, not for "normal" unicast resolution. See mDNS. – grawity Jul 30 '11 at 22:33
@grawity: As of Snow Leopard and newer unicast resolution was rolled into mDNSResponder's process. See… for more info. – Chealion Aug 1 '11 at 17:26

The user is called _mdnsresponder, the process is mDNSResponder. On Mac OS X Lion, it may prevent you from accessing Internet. When this happens, mDNSResponder uses 100% of CPU, same for any application relying on DNS resolution (pretty much most applications nowadays). So use Activity Monitor to Force Quit it mDNSResponder. Then it should solve the issue as it restarts itself.

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