My eldest son was trying to enroll in college at a university in Florida. Perversely, his website refused to appear. He tried Safari, Firefox and Chrome on his Mac and Safari on his iPhone. Only his devices seemed to be affected. Switching to cellular worked, isolating the problem in our local network. Still, my Mac laptop could access the site just fine. Looking at our routers and other settings, I couldn’t find the reason for this failure.
What did I finally discover? Our main network and Wi-Fi router had outdated DNS server information. Our provider, CenturyLink, has at some point updated the servers it uses to provide searches for Domain Name System (DNS) records. When you type something like podunk.eduyour operating system must perform a DNS query to convert this address to a machine number (an Internet Protocol (IP) address) which it uses to create the actual end-to-end connection between a browser or other software and a waiter.
DNS servers are unloved utility items. By the late 2000s, many ISPs had overlooked the speed of these servers, which can perform billions of simple requests per day or more for a network of users. A slow DNS lookup can slow down everything on your devices as you browse. (Information is cached for minutes to days, so the first search is the most painful.)
Some third parties have thrived on providing high-quality freemium DNS lookups: lightning-fast responses for free, and you might pay extra for filtering and other services. Eventually, Google got into the business with Public DNS, a free service decoupled from ISPs. Others followed.
On most home networks, your ISP provides you with information to manually enter into your router’s configuration to initiate access. This almost always includes the IP addresses of two DNS servers – primary and secondary – which you must enter in numerical form. It’s a chicken and egg problem: you can’t use DNS to look up a name if your network or devices don’t know how to find a DNS server. You may never have to change these details. Over the past decade, some people have changed these settings to point to other free or paid Google DNS services.
When you connect a Mac, iPhone, or other Internet-enabled hardware to a local network via Wi-Fi or Ethernet, almost all home networks automatically assign it a local network address. This assignment points your device’s DNS queries to the router, which in turn relays them to the DNS servers it has configured in its settings.
In my case, CenturyLink is our provider, and I probably haven’t changed our DNS server numbers in as long as I can remember. But on my Mac laptop, I sometimes fiddled with them just for that computer, for testing, and for speed. (Go to System Preferences > Networkselect WirelessClick on AdvancedClick on DNSand click the + in the lower left corner to add one or more custom entries.) These custom entries override the DNS server information at the router in favor of the ones you selected.
At this point, I had a hunch. Did CenturyLink change the addresses of its DNS servers without, say, notifying its customers? Sure enough, CenturyLink’s help page on DNS server addresses showed ones that I had never seen before and were not configured on my router. I updated my router settings, applied them, and suddenly the “broken” college page loaded fine on all of our networked devices.
The only mystery that remains is how CenturyLink runs a semi-broken old DNS server that seemed to only omit a sites on the internet.
This Mac 911 article is in response to a question submitted by Macworld reader Benjamin.
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