In an apparent effort to fix DNS errors seen in recent OS X 10.10 versions, Apple's latest Yosemite beta shows the buggy discoveryd service replaced with mDNSResponder, a process last implemented in OS X 10.9 Mavericks.
Source: Apple Support Communities userrwross1776
With the OS X 10.10.4 beta, released on Tuesday, Apple reinstated mDNSResponder to handle DNS processes previously assigned to discoveryd since the debut of OS X 10.10 Yosemite.
As reported by ArsTechnica earlier this year, discovered discoveryd would repeatedly fail to resolve device names, duplicate machine names and cause other network problems that led to general system slowdowns. The process was also attributed to various Wi-Fi issues.
The discoveryd service was thought to be linked with Handoff functionality, which debuted alongside Yosemite and iOS 8, but those features appear to be operational even with the transfer to mDNSResponder.
It is not clear if Apple plans to permanently fall back to mDNSResponsder when OS X 10.10.4 launches, or is merely using the older service as a stopgap solution as it works to fix discoveryd.