How to troubleshoot DNS issues on a Linux machine?

When encountering DNS problems, first check the /etc/resolv.conf file to see if the correct nameserver is configured; secondly, you can manually add public DNS such as 8.8.8.8 for testing; then use nslookup and dig commands to verify whether DNS resolution is normal. If these tools are not installed, you can first install the dnsutils or bind-utils package; then check the systemd-resolved service status and configuration file /etc/systemd/resolved.conf, and set DNS and FallbackDNS as needed before restarting the service; finally check the network interface status and firewall rules, confirm that port 53 is not blocked and try to temporarily close the firewall to eliminate interference.

DNS problems are common in Linux systems, especially in servers or development environments. If you find that the web page cannot be opened, the ping domain name cannot be connected, but the IP address can be accessed, it is likely that there is something wrong with DNS. Below are some troubleshooting and solutions.

Check /etc/resolv.conf file

This file determines the DNS server used by the system. You can use cat /etc/resolv.conf to view the current configuration.

• If there is no nameserver line inside, or just 127.0.0.53 is written (this is the local address of systemd-resolved), you may need to check the network service further.
• You can manually add a public DNS, such as Google's 8.8.8.8 or Cloudflare's 1.1.1.1 to see if it solves the problem:

 nameserver 8.8.8.8

Note: Some distributions use NetworkManager or systemd-resolved to manage this file dynamically, and direct modifications may be overwritten.

Test DNS resolution using nslookup and dig

These two commands can help you determine whether it is a system configuration problem or a network-level problem.

• nslookup example:

 nslookup google.com

If server can't find google.com or timeout is returned, it means that DNS is not working properly.

• Dig example:

 dig google.com

Check whether ANSWER SECTION in the output has an IP address. If not, it may be a parsing failure.

Tips: If you don't install dig or nslookup , you can install dnsutils or bind-utils package.

Check systemd-resolved status (for systems using systemd)

Many modern Linux distributions use systemd-resolved by default to manage DNS.

• View status:

 systemctl status systemd-resolved

If the service is not running, you can start it with the following command:

 sudo systemctl start systemd-resolved
sudo systemctl enable systemd-resolved

• Check its configuration file /etc/systemd/resolved.conf , where you can set DNS and FallbackDNS:

 DNS=8.8.8.8
FallbackDNS=1.1.1.1

Restart the service after the modification:

 sudo systemctl restart systemd-resolved

Check network connections and firewalls

Sometimes DNS requests cannot be sent at all, maybe because:

• The network interface is not configured correctly (for example, DHCP cannot obtain DNS)
• Firewall blocks UDP 53 port (DNS default port)

You can try the following:

• Use ip a to see if the network card status is normal
• Use tcpdump -i eth0 port 53 to catch the packet to see if there is any DNS request issued
• Temporarily turn off firewall test:

 sudo ufw disable
# or sudo systemctl stop firewalld

Basically these are the investigation methods. Although DNS failures seem small, they have a big impact. The key is to confirm step by step whether it is a local configuration problem, network problem, or an upstream DNS service problem.

The above is the detailed content of How to troubleshoot DNS issues on a Linux machine?. For more information, please follow other related articles on the PHP Chinese website!