The core logic of DNS caching: balancing efficiency and risk.

DNS translates human-readable domain names into machine-readable IP addresses (such as `192.0.2.1`). When you first visit a website, your computer queries a DNS server, performing a series of recursive or iterative lookups to obtain the target IP address.

However, if a complete global lookup were performed every time you visit a website, the network would be overwhelmed, resulting in an extremely slow experience. Therefore, DNS caching mechanisms were developed. Your operating system (as well as your router and browser) temporarily stores the mapping between the queried domain name and IP address locally for a period of time. This "temporary" period is called TTL, set by the domain administrator, and can be a few minutes, hours, or even longer. Within the TTL's validity period, when accessing the same domain name again, the computer directly reads the IP address from the local cache, completing the resolution instantly without repeating the lookup.

Caching brings speed, but it also introduces a problem: when the IP address behind a domain name changes, your locally stored old record becomes an "expired map." The website server has moved, but your computer is still stubbornly searching for the old address, naturally failing to connect. This is the root of the problem described at the beginning. Refreshing the DNS cache manually clears this local, outdated DNS lookup, forcing the computer to perform a global DNS lookup again on the next visit to obtain the latest correct address.

Besides website migration, refreshing the cache is also useful in other scenarios. For example, when debugging a local development environment, you might need to temporarily point a domain name to the local server's IP address (`127.0.0.1`). After modifying the system's hosts file, refreshing the cache is necessary for the new rule to take effect immediately. Alternatively, when you suspect a network problem is due to an ISP's DNS server providing incorrect or polluted records, clearing the local cache is also a first step in troubleshooting.

Refresh Commands: Operation Guide for Different Systems

Performing a refresh operation requires using different commands depending on your operating system. Note that these commands need to be run in a terminal or command prompt with administrator privileges.

On Windows systems, the most commonly used command is `ipconfig /flushdns`. This command clears the cache maintained by the Windows DNS client service.

In Windows Command Prompt (run as administrator), execute:

ipconfig /flushdns

After successful execution, you will see a message saying "DNS resolution cache successfully flushed." For Windows 10 and later, you may sometimes need to restart the "DNS Client" service, which can be done with the following commands:

Stop the DNS Client service:

net stop dnscache

Start the DNS Client service:

net start dnscache

On macOS, the commands vary depending on the system version. On newer macOS systems (such as macOS Monterey, Ventura, Sonoma), you need to use the following commands:

Execute in macOS Terminal (you may need to enter your administrator password):

sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

On mainstream Linux distributions (such as Ubuntu, Debian, CentOS), the situation is slightly more complex, because Linux typically manages the DNS cache by default using services such as `systemd-resolved`, `dnsmasq`, or `NSCD`. The flush command depends on the service your system is actually using. The most common method is to use `systemd-resolved`. The refresh command is:

For systems using systemd-resolved:

sudo systemd-resolve --flush-caches

If your system uses `NSCD` (Name Service Cache Daemon), you need to execute:

For systems using NSCD:

sudo systemctl restart nscd

Or

sudo service nscd restart

If you are unsure which service your system is using, you can try executing `sudo systemctl status systemd-resolved nscd` to check which service is active.

After Refreshing: Verification and In-Depth Thinking

After executing the refresh command, how do you verify that it has indeed taken effect? ​​A direct method is to try accessing the website that had the problem before. Additionally, you can use network diagnostic commands for testing.

On Windows, you can use the `nslookup` command. Open a command prompt, type `nslookup yourdomain`, and it will display the DNS servers used to resolve that domain name and the resulting IP address. Comparing the results before and after the refresh will visually show the changes.

On macOS and Linux, the `dig` command is a more powerful tool. Executing `dig your_domain` returns detailed resolution process and information. If you want to see a simplified, `nslookup`-like result, you can use the `host` command: `host your_domain`.

Understanding the principles and operation of refreshing the DNS cache can help you solve most problems caused by delayed domain name resolution. However, it's not a panacea. If the problem persists after refreshing, the fault may not be in the local cache. You need to trace back along the DNS resolution chain: check if your local hosts file has been accidentally modified, confirm that your router's DNS settings are correct, try switching to a public DNS server (such as `8.8.8.8` or `1.1.1.1`), or use an online DNS propagation test tool to check if records on DNS servers around the world have been updated.

A deeper understanding is that DNS refresh solves the "last mile" problem—the records on your device's local machine. It cannot force other nodes on the internet (such as your ISP's caching server or recursive DNS servers) to expire their caches prematurely. Those nodes' caches adhere to the TTL (Time-To-Live) value set by the domain administrator. This is why large websites, when changing IP addresses, will reduce the TTL value beforehand, causing the global cache to expire quickly, and then adjust it back to the normal value after the change is complete, achieving a smooth migration.

In short, refreshing the DNS cache is an efficient and direct basic network troubleshooting skill. It resolves connection failures caused by outdated information by resetting the local "address book." Mastering its commands on different platforms and understanding its effective boundaries will allow you to handle website access anomalies, development environment configuration issues, and other problems with greater ease.