# How to force nginx to resolve DNS (of a dynamic hostname) every time when doing proxy_pass?

By default, Nginx caches DNS records for a certain period to enhance performance. However, if you have a dynamic hostname and you need Nginx to resolve DNS every time a request comes in, you can use the `resolver` and `set` directives in the Nginx configuration.

Here's an example of how you can force Nginx to resolve the DNS of a dynamic hostname for each request:

```
http {
    resolver <DNS_IP> valid=30s;  # Replace <DNS_IP> with your DNS server IP

    server {
        listen 80;

        location / {
            resolver <DNS_IP> valid=30s;  # Replace <DNS_IP> with your DNS server IP
            set $backend "your_dynamic_hostname";  # Replace with your dynamic hostname variable

            proxy_pass <http://$backend>;
            proxy_set_header Host $host;
            proxy_set_header X-Real-IP $remote_addr;
        }
    }
}
```

Explanation:

- The `resolver` directive specifies the DNS server Nginx should use to resolve domain names.
- The `valid` parameter in the `resolver` directive sets the time limit for caching DNS records. In this example, it's set to 30 seconds, meaning DNS records will expire and be re-resolved every 30 seconds.
- The `set` directive is used to create a variable (`$backend`) that holds your dynamic hostname.

This configuration tells Nginx to resolve the DNS for the dynamic hostname every 30 seconds and pass the requests to the resolved IP.

Remember to replace `<DNS_IP>` with the IP address of your DNS server and `"your_dynamic_hostname"` with the variable holding your actual dynamic hostname.

Please be cautious with the low caching time, as it might affect the server's performance due to frequent DNS lookups. Adjust the `valid` parameter as needed for your use case.