Why can't I access the site after the domain name has been resolved?

　　Many people encounter a very typical yet perplexing problem when building a website or deploying a service: the domain name has been resolved, but the website remains inaccessible. Some suspect a server issue, others think the domain name hasn't taken effect, and some even repeatedly modify the DNS records without success. In reality, this type of problem is often not a single point of failure, but rather caused by improper configuration in one or more of the multiple stages. To completely resolve this, it's essential to understand the entire chain from "entering the domain name" to "page loading."

　　You can imagine the entire access process as a path: browser enters domain name → DNS resolution → obtains IP address → connects to server → server response → page loads. If any step in this chain fails, it will result in "inaccessibility."

　　One of the most common scenarios is that the DNS resolution hasn't fully taken effect. Domain name resolution isn't real-time; there's a global DNS cache propagation time, a process commonly known as "DNS propagation." Sometimes, after modifying the DNS records, your computer can access the website normally, but others can't. This is because DNS cache refresh times differ across regions. Generally, the resolution takes anywhere from a few minutes to 24 hours to take effect.

　　You can check if the current parsing is working using the following command:

nslookup yourdomain.com

　　Or use:

dig yourdomain.com +short

　　If the returned IP address is not your server's IP, it means the DNS resolution is not fully effective or the configuration is incorrect.

　　Another very common problem is incorrect DNS record configuration. For example, an A record should be added but is incorrectly configured as a CNAME, or the IP address is entered incorrectly. A typical correct A record should look like this:

　　Type: A Host Record: @Record Value: Your Server IP

　　If you are using a subdomain, such as www, you need to add it separately:

　　Type: A Host Record: www Record Value: Your Server IP

　　Many beginners overlook this point, resulting in accessing example.com being possible, but www.example.com being inaccessible (or vice versa).

　　Next is a crucial issue that is often overlooked: whether the server is actually running the service. Domain name resolution only directs access requests to the server, but the server itself must have a web service listening on port (usually 80 or 443). You can execute the following on the server:

ss -tlnp

　　Check if any service is listening on port 80. If not, the web service (such as Nginx or Apache) is not running.

　　For example, to start Nginx:

sudo systemctl start nginx

　　And set it to start automatically on boot:

sudo systemctl enable nginx

　　If the service is already running but still inaccessible, the problem may lie in firewall or security group restrictions. Many cloud servers only open the SSH port (22) by default, while not opening HTTP (80) or HTTPS (443). The ports need to be allowed in the firewall:

sudo ufw allow 80
sudo ufw allow 443
sudo ufw reload

　　If you are using a cloud platform, you also need to allow the corresponding port in the "Security Group"; otherwise, external access will still be inaccessible.

　　Another scenario is that the server is accessible, but the browser fails to access it. This is usually related to the web server configuration. For example, incorrect Nginx configuration, unbound domain name, or configuration file not taking effect. A basic Nginx site configuration is as follows:

server {
    listen 80;
    server_name yourdomain.com www.yourdomain.com;

    root /var/www/html;
    index index.html;
}

　　After modifying the configuration, the service needs to be reloaded:

sudo nginx -t
sudo systemctl reload nginx

　　If the domain name is not correctly entered in the configuration file, the server may not be able to recognize the request, thus returning an error page.

　　Another more hidden problem is that the domain name is not registered or is restricted. If you are using a server in mainland China, the domain name must be registered; otherwise, access will be blocked. If you are using an overseas server (such as in Hong Kong, Japan, or the United States), registration is not required, but you may be affected by ISP or network policies, leading to access problems in some regions.

　　With the increasing prevalence of HTTPS, SSL certificate issues have also become a common cause. If you have enabled HTTPS, but the certificate is not configured correctly or has expired, the browser may directly block access. You can use tools to generate free certificates, for example:

sudo apt install certbot python3-certbot-nginx -y
sudo certbot --nginx

　　After configuration, accessing https://yourdomain.com should open normally.

　　Another possibility is domain poisoning or DNS hijacking. This issue is common in cross-border access, such as using a foreign server but targeting domestic users. If a node in the DNS resolution path is poisoned, users may be resolved to the wrong IP address. In this case, you can try using a public DNS (such as 8.8.8.8) to test:

nslookup yourdomain.com 8.8.8.8

　　If the returned IP address differs from the local DNS, a DNS pollution issue may exist.

　　For more complex situations, you can use curl to simulate a request and determine if the server is responding:

curl -I http://yourdomain.com

　　If an HTTP/1.1 200 OK response is returned, the server is functioning correctly, and the problem may lie in the browser or DNS layer. If there is no response, the server itself still has issues.

　　During troubleshooting, it's recommended to follow a simple logic: first check DNS resolution → then check the server → then check the network → finally check the application layer configuration. Don't modify a large number of configurations at once; instead, verify each step gradually.

　　To help beginners understand more clearly, you can summarize it in one sentence:

　　Domain name resolution is just the "direction," the server service is the "door," the firewall is the "access control," and the web configuration is the "room setup." If any link has a problem, users cannot access the site smoothly.

　　In practical experience, most "cannot access after DNS resolution" problems ultimately fall into the following categories: DNS resolution not taking effect, incorrect IP address, port not open, service not started, or misconfigured. By following the troubleshooting methods described in this article step by step, you can generally quickly locate the problem.