Server network connection problems need to be investigated from four aspects: local interface, connectivity, DNS resolution, and firewall. 1. Check whether the network card status is up and whether there are error packets. The cloud server also needs to check the virtual network card and security group; 2. Use ping and traceroute to test connectivity and pay attention to the ICMP blocking situation; 3. Use nslookup or dig to confirm that the DNS resolution is normal, and replace the DNS server if necessary; 4. Check the system firewall rules and port monitoring status, and the cloud server should also check the platform security group settings.

Server network connection issues will affect service availability and user experience, and you cannot only look at the surface when troubleshooting. First, we need to confirm whether the problem lies in the server itself or the intermediate network link . Here are some practical check methods.

1. Check the local network interface status

Start from the server and see if the network card is working normally:

• Use ip link or ifconfig to see if the network card is up
• Check whether there are a large number of error packets, such as using ethtool -S eth0 to view packet loss and error packets
• If it is a cloud server, check whether the virtual network card or security group rules limit traffic

A common phenomenon is that the network card is down or intercepted by firewall rules. At this time, you need to restart the network card or adjust the iptables/firewalld rules.

2. Test basic connectivity (Ping and Traceroute)

Use basic commands to test whether the network is smooth:

• ping www.example.com to see if it can be connected to the external network
• traceroute www.example.com to see which jumps in the middle

Notice:

• Some websites block ICMP requests, and ping not working does not mean there is a problem.
• You can test it with another domain name or IP address
• If traceroute keeps at a certain step *, it may be a routing problem or an intermediate device prohibits ICMP

3. Check whether DNS resolution is normal

Sometimes the network is fine, but not being able to resolve the domain name can also lead to "unable to connect":

• Use nslookup example.com or dig example.com to test whether DNS returns the correct IP
• If it fails, you can temporarily modify /etc/resolv.conf to 8.8.8.8 or 114.114.114.114 to see if it is restored

If DNS is slow or unreliable, the entire application may get stuck, especially services that rely on external APIs.

4. Check the system firewall and port opening situation

Many connection problems are that the firewall blocks traffic:

• Use iptables -L -n or firewall-cmd --list-all to view the rules
• Use ss -tuln or netstat -tuln to see if the service listens for the correct port
• Try to shut down the firewall (such as systemctl stop firewalld ) for temporary verification

Especially for cloud servers, the security group settings of the system firewall and cloud platform must be checked at the same time.

Basically, if you encounter these troubleshooting steps, you can start from network card, DNS, routing, and firewall to narrow the scope step by step. It is not complicated but it is easy to ignore details. For example, if a small configuration is corrected incorrectly, it will cause the entire disk to be blocked.

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