WARNING: This article may be obsoleteThis post was published in 2021-01-22. Obviously, expired content is less useful to users if it has already pasted its expiration date.
如果你是macOS上的Chrome用户（中国大陆地区），你可能会在2020年的某些时间发现它打开国内网页的速度非常非常慢，见：为什么 Google Chrome 打开网页有时极慢？
问题出在Chrome内置的secure DNS上：
把secure DNS关掉，Chrome就会遵循macOS->network->DNS里的设置（比如说8.8.8.8, 1.1.1.1）了吗？
大概是的。
2023年更新：关掉secure DNS（或者修改了系统的DNS）以后，你可能需要手动清理Chrome的DNS缓存：访问 chrome://net-internals/#dns ，然后点击 Clear host cache 按钮：
如果通过上面这个secure DNS案例来继续深究Chrome的DNS查询记录，你可能会发现一些看起来有些奇怪的53端口访问日志：
尽管你没有在macOS DNS里设置这个地址，Chrome仍然会坚持不懈地尝试连接它：[2001:4860:4860::8888]。
Google Chrome背叛了你的DNS设置吗？大概是没有的。事实上，这个对53端口的访问并不用于DNS解析：https://bugs.chromium.org/p/chromium/issues/detail?id=331746
Chrome's internal DNS resolver checks whether the host currently has global IPv6 connectivity by creating a socket connected to Google Public DNS' IPv6 address. These show up in the NetLog as UDP_SOCKET objects with a source dependency of the HOST_RESOLVER_IMPL_REQUEST.
This change adds an explicit IPV6_REACHABILITY_CHECK source type that will show up instead, to clarify the purpose of the UDP sockets.
(Note that no packets are actually sent as a result of the checks; UDP doesn't need to send any packets to connect, and the HostResolverImpl doesn't send any data over the socket explicitly.)
https://bugs.chromium.org/p/chromium/issues/detail?id=331746
