网络连接问题（无法访问此站点）：部分网站无法打开求助

Hey there! Let's work through this problem where some sites load fine (like Google, Gmail, Yahoo) but others (askubuntu, nvidia, Facebook) won't open. Here are actionable steps to fix this:

1. Check & Update Your DNS Settings #

Most selective access issues boil down to DNS problems. Try switching to a public, reliable DNS server:

• Google DNS: 8.8.8.8 (primary) and 8.8.4.4 (secondary)
• Cloudflare DNS: 1.1.1.1 (primary) and 1.0.0.1 (secondary)

On Ubuntu, you can update this via:

• Graphical interface: Go to Settings > Network > Click the gear icon next to your connection > IPv4 > Set DNS to "Manual" and add the addresses above.
• Command line: Use nmcli con mod <your-connection-name> ipv4.dns "8.8.8.8,8.8.4.4" then restart NetworkManager with sudo systemctl restart NetworkManager.

2. Flush DNS Cache #

Old cached DNS records might be causing conflicts. Flush them with these commands:

• Ubuntu/Debian: sudo systemd-resolve --flush-caches
• Windows: ipconfig /flushdns
• macOS: sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

3. Inspect Your Hosts File #

Malware or accidental edits could have blocked these sites in your hosts file. Check it with:

cat /etc/hosts

If you see entries for askubuntu.com, nvidia.com, or facebook.com pointing to wrong IPs (like 127.0.0.1), edit the file to remove them:

sudo nano /etc/hosts

Save changes (Ctrl+O, Enter) and exit (Ctrl+X).

4. Test Network Connectivity #

Run these commands to pinpoint where the connection fails:

• Ping the site: ping askubuntu.com — if you get no response, the issue is likely with routing or DNS.
• Trace the route: traceroute askubuntu.com — this shows each hop your request takes; if it stops at a specific point, that's where the block is happening.

5. Check Firewall & Proxy Settings #

• Proxy: Make sure you're not using a proxy that's blocking these sites. Go to your system's network settings and disable any active proxies temporarily.
• Firewall: On Ubuntu, check if UFW has any rules blocking these sites: sudo ufw status. If you see suspicious rules, remove them with sudo ufw delete <rule-number>.

6. Restart Network Hardware & Services #

• Restart your router/modem (unplug it for 30 seconds, then plug back in) to clear any cached routing issues.
• On your machine, restart the network service: sudo systemctl restart NetworkManager (Ubuntu) or equivalent for your OS.

If none of these steps work, it's possible your ISP is restricting access to these sites. You can reach out to your ISP to confirm, or try using a VPN to bypass the restriction.

内容的提问来源于stack exchange，提问作者Sridhar