Linux flush dns cache ubuntu

How to Flush the DNS Cache on Ubuntu

The DNS or the Domain Name Server can be characterized as one of the most essential parts of your link to the internet. The DNS translates the domain names to and from the IP addresses so that we don’t need to remember or keep a list of all the IP addresses of the websites we ever want to access. Our systems also maintain a list of DNS records so that we can access our frequently visited websites faster through a quick resolution of IP addresses. This cache on our system needs to be flushed from time to time. This is required because websites may change their addresses time and again, so it is a good idea to avoid IP conflict by clearing the cache. Flushing the cache is also a good way to clear unnecessary data residing on our systems.

So let us see how we can flush the DNS cache on our Ubuntu Systems.

We have executed the commands and procedures mentioned in this article on an Ubuntu 22.04 LTS system. Since we are using the Linux command line-the Terminal to flush the cache, you can open it through the system Dash or the CTRL+ALT+T shortcut.

Method 1: Using systemd-resolve to flush DNS-Cache

Most Ubuntu and Debian systems are running the systemd-resolve daemon. Your system already uses it for a lot of things. So let us use it for flushing the cache through the following command:

$ sudo systemd-resolve --flush-caches

You can then check the statistics in order to make sure that your cache size is now zero, thus cleared. Run the following command in order to view the statistics:

$ sudo systemd-resolve --statistics

You cache size 0 in the above output shows that the DNS cache has been cleared.

Method 2: Using dns-clean to flush DNS-Cache

Another way to clear the DNS cache is by starting the dns-clean utility. Run the following command in your Terminal to do so:

$ sudo /etc/init.d/dns-clean start

You have now learned two effective methods to clear your DNS cache. In both the cases, the process is pretty simple and does not require you to restart your Linux system.

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Flush DNS Cache on Ubuntu

The full form of DNS is Domain Name System. It is used to translate domain names to IP addresses. It seems really simple, but without it the internet won’t be what it is today. Can you imagine memorizing thousands of IP addresses? Can you imagine a world without google.com or yourwebsite.com? DNS makes everything about the internet very easy for us.

This article is about flushing DNS on Ubuntu. But to understand why it is necessary, first you have to understand how DNS works and a little bit about DNS caching.

How DNS Works:

Simply, DNS can be thought of a table of domain names and IP addresses as given below:

Please know that, none of the data in this table is real. It’s just for the purpose of demonstration. So let’s get back to our original topic.

When you visit, let’s say, linuxhint.com, the browser asks the DNS server (set on your computer) a few questions.

Your Computer: Hey, do you know linuxhint.com?

DNS Server: No, I do not. But the DNS server 4.4.4.4 may know about it.

Your Computer: Contacts the DNS server 4.4.4.4 and asks, “hey, do you know linuxhint.com?”

DNS Server 2: Yes, I do. What can I do for you?

Your Computer: I need the IP address of linuxhint.com. Can I have it?

DNS Server 2: No fear, that’s why I am here. Here is the IP address of linuxhint.com 1.2.3.4.

Your Computer: You’re a life saver. Thanks.

Now your computer connects to 1.2.3.4 and your favorite website linuxhint.com shows up. That was really hard, wasn’t it?

The next time you visit linuxhint.com, the same thing happens again.

DNS Caching:

In the earlier section, you saw how a domain name is resolved to IP addresses. This journey through the DNS servers takes a while and till it’s complete and a domain name can be resolved to IP addresses, you won’t be able to connect to any website or server on the internet.

To solve this issue, DNS caching is used. Here, when you try to resolve a domain name to IP addresses for the first time, it takes a little bit longer. But once the domain name is resolved, the IP addresses are stored in your own computer. So, the next time you need to resolve the same domain name, it won’t take as long as it did on the first time.

Problems with DNS Caching:

DNS caching is good. How come DNS caching is problematic? Well, the world of internet is so dynamic that the DNS information is changing constantly. It may have changed a few times even while I am writing this article.

So, what happens when the DNS information changes and we are using the DNS information that is cached on our own computer? Well, that’s where it gets problematic. In that case, we will be using the old DNS information. We may have connectivity issues, false redirection issues and many other issues.

To solve this problem, we have to delete the cache (also called DNS flushing) and rebuild it. This is the topic of this article.

Checking If DNS Caching is Enabled:

You can check whether DNS caching is enabled very easily on Ubuntu. You need to have the nslookup or dig utility installed on your computer for this to work.

nslookup or dig command may not be available by default on your Ubuntu machine. But you can easily install it from the official package repository of Ubuntu.

First, update the APT package repository cache with the following command:

The APT package repository cache should be updated.

Now install the nslookup and dig with the following command:

Now press y and then press to continue.

sudo systemd-resolve --flush-caches

sudo system-resolve --statistics

sudo systemctl restart nscd