When the CentOS server cannot be connected to the network, you can follow the following steps to check: 1. Check the status of the network interface, use ip link show to confirm whether the interface is enabled. If it is not enabled, use sudo ip link set <interface_name> up to start, and use ip addr show to view the IP allocation status; 2. If it is in DHCP mode, run sudo dhclient <interface_name> to obtain the IP. If it is static configuration, check the IP, gateway and DNS settings in /etc/sysconfig/network-scripts/ifcfg-<interface_name> and restart the network service; 3. Check the routing table ip route show to ensure that there is a default gateway. If there is no, add it temporarily or modify GATEWAY= in the configuration file; 4. Check the DNS configuration in /etc/resolv.conf, and manually add public DNS such as 8.8.8.8 for testing; 5. Check the firewalld status, open the port or temporarily close the firewall test connection if necessary; 6. Check the SELinux status. If it is enforcing mode, switch to permissive mode to eliminate interference. Most network problems can usually be solved through the above steps.

If your CentOS server has no network connection, there are a few common culprits—misconfigured settings, downed interfaces, or firewall issues. The good news is that most of these can be diagnosed and fixed quickly with a few basic commands and checks.

Check the Network Interface Status

First things first: make sure the network interface is actually up and recognized by the system. You can use the ip command to check:

 ip link show

Look for an interface like eth0 , ens33 , or something similar. If it says DOWN , bring it up with:

 sudo ip link set <interface_name> up

Also, verify if it received an IP address using:

 ip addr show

If there's no IP address assigned, you might have a problem with DHCP or static configuration.

• For DHCP , try restarting the service:

   sudo dhclient <interface_name>

• For static setups, double-check /etc/sysconfig/network-scripts/ifcfg-<interface_name> for correct IP, gateway, and DNS entries.

Don't forget to restart the network service after any changes:

 sudo systemctl restart NetworkManager
# or if you're using the older network service:
sudo systemctl restart network

Verify DNS and Routing Settings

Even if the interface looks fine, the server might still not be able to reach the outside world due to routing or DNS problems.

Start by checking the routing table:

 ip route show

Make sure there's a default route pointing to a valid gateway. If not, add one temporarily:

 sudo ip route add default via <gateway_ip> dev <interface_name>

To make this permanent, update the GATEWAY= line in your interface config file.

For DNS issues, look at /etc/resolv.conf . It should contain lines like:

 nameserver 8.8.8.8
nameserver 8.8.4.4

You can temporarily test different DNS servers by editing this file directly. Just keep in mind that some systems regenerate this file automatically based on other network settings.

Check Firewall and SELinux

Sometimes the system itself blocks outgoing traffic without you realizing it. The main suspects here are firewalld and SELinux.

To see if firewalld is running:

 sudo systemctl status firewalld

If it's active, allow specific services or ports as needed:

 sudo firewall-cmd --add-port=80/tcp --permanent
sudo firewall-cmd --reload

Or temporarily disable it to test connectivity:

 sudo systemctl stop firewalld

SELinux doesn't usually block network access outright, but it can interfere depending on what services you're running. To check its status:

 sestatus

If it's enforcing, try switching it to permit mode:

 sudo setenforce 0

Then test your network again. If it works now, dig into SELinux policies later—you don't want to leave it disabled permanently.

That's a solid starting point for troubleshooting network issues on a CentOS server. These steps cover physical layer problems, IP configuration, routing, DNS, and basic firewall interference. Fixing one or two of these areas often gets things back online.

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