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MAC network fix: manual DNS causes campus network authentication failure

349 words 1 min #OS

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Environment Setup

  • Device: MacBook Air M5
  • System: macOS Tahoe 26.4.1
  • Network: NJU campus network (Web Portal authentication, requires student/staff account login)

Problem Description

After connecting to NJU-WLAN, I cannot access the internet normally, and the authentication page also fails to open. However, the network works fine when connecting to a phone hotspot or other Wi-Fi.

Troubleshooting Process

Step Action Result
1 Connected to NJU-WLAN and tried to access a webpage Could not open any webpage, and the authentication page would not pop up either
2 Restarted the Mac, forgot the network, and reconnected Problem persisted
3 Checked IP address, proxy, and VPN status All normal; no proxy or VPN was enabled
4 Checked DNS settings Found manually configured public DNS: 8.8.8.8 and 114.114.114.114
5 Removed all manual DNS entries and switched to automatic assignment from the campus network The authentication page loaded normally
6 Disconnected from the campus network, reconfigured the public DNS, and reconnected The issue reproduced, confirming it was a DNS problem
7 Restored automatic DNS acquisition Problem resolved

Cause Analysis

DNS (Domain Name System): the phonebook of the internet, responsible for translating human-friendly domain names (such as www.google.com) into the IP addresses used by computers (such as 142.250.80.4).
User enters a URL → DNS query → obtain IP → access the target server

The campus network uses a Web Portal authentication mechanism. Before a device completes authentication, all network requests are forcibly redirected to the internal authentication server. The domain name of this authentication server can only be resolved by the DNS server on the campus intranet.

When the Mac's DNS is manually set to 8.8.8.8 (Google) or 114.114.114.114 (a domestic public DNS):

  1. The system queries the public DNS for the authentication server's domain name
  2. The public DNS cannot resolve the intranet domain name and returns a resolution failure
  3. The browser cannot obtain the IP address of the authentication page, so the authentication page cannot pop up
  4. The device can never complete authentication, and therefore cannot connect to the external network

So what looks like "no internet connection" is actually the DNS setting blocking the authentication process.