Dell Inspiron 3567（Ubuntu16.04）连WiFi能ping但无法上网求解

Hey there, let's work through this odd problem you're facing—being able to ping Google's servers and domain but having no luck with apt-get update or browsers. This usually points to issues with proxy settings, DNS caching, or application-specific network configurations. Here are the steps to troubleshoot:

1. Check System-Wide Proxy Settings #

First, make sure your system isn't stuck with an incorrect proxy configuration, which can block both browsers and APT:

• Graphical Interface: Go to System Settings > Network > Network Proxy. Set it to "None" if you aren't using a proxy, or verify the proxy details if you are.
• Command Line: Check for proxy environment variables with:

  echo $http_proxy $https_proxy
  

  If you see unexpected values, clear them with:

  unset http_proxy https_proxy
  

  Then try running sudo apt-get update again to test.

2. Reset DNS Cache & Network Manager #

Even though you can resolve domains, a corrupted DNS cache might interfere with application-level requests:

• Restart Network Manager to refresh network configurations:

  sudo service network-manager restart
  

• Flush DNS cache (Ubuntu 16.04 uses dnsmasq by default):

  sudo /etc/init.d/dns-clean restart
  sudo systemctl restart dnsmasq
  

3. Verify APT Proxy Configuration #

APT might have its own proxy settings that override system defaults. Check these files:

• Open /etc/apt/apt.conf with a text editor (e.g., sudo nano /etc/apt/apt.conf) and look for lines like:

  Acquire::http::Proxy "http://proxy.example.com:8080/";
  

  Delete or comment out these lines (add a # at the start) if you don't need a proxy.
• Also check files in /etc/apt/apt.conf.d/—any file with "proxy" in the name might contain similar settings.

4. Check Browser Proxy Settings #

Browsers sometimes use their own proxy configs, even if system settings are correct:

• For Chrome/Firefox: Go to the browser's settings, search for "proxy", and ensure it's set to "Use system proxy settings" or "No proxy" (whichever applies to your network).
• Avoid manual proxy entries unless you're certain they're correct.

5. Verify Firewall Rules #

The default UFW firewall might be blocking HTTP/HTTPS traffic by mistake:

• Check current firewall status:

  sudo ufw status
  

• If you see rules denying 80/tcp (HTTP) or 443/tcp (HTTPS), allow them:

  sudo ufw allow 80/tcp
  sudo ufw allow 443/tcp
  sudo ufw reload
  

6. Adjust MTU Value #

A mismatched MTU (Maximum Transmission Unit) can cause large packets to fail, even if ping works:

• Check your WiFi interface's current MTU (replace wlan0 with your actual interface name from ip link show):

  ip link show wlan0
  

• Try lowering the MTU to 1400:

  sudo ip link set wlan0 mtu 1400
  

  Test apt-get update and your browser after this change.

7. Reset Network Manager Configuration #

If all else fails, reset Network Manager to default settings:

sudo service network-manager stop
sudo rm /var/lib/NetworkManager/NetworkManager.state
sudo service network-manager start

Then reconnect your WiFi network and test again.

内容的提问来源于stack exchange，提问作者RAMESH Pqr