Linux remove dns cache

How do I clear the DNS cache?

I just updated the DNS record ( ns1 , ns2 , ns3.myhostingcompany.com ) for a site I’ve got hosted, but I still get the domain registrar parking page. I’d like to see if the problem is Ubuntu’s cached DNS records. Is there a way to clear Ubuntu’s DNS cache? (if such a thing exists?)

Also, check /etc/hosts . I’ve just been sure that the old IP address of my domain was being cached, but only strace ping example.com revealed that I forgot to remove the /etc/hosts record which I added a time ago because of lacking patience for DNS propagation.

a lot of these answers suggest caching is disabled by default, but they also refer older versions. It certainly appears to be on by default in my machine (18.04) and various answers below do show you how to flush it, just scroll down

17 Answers 17

Ubuntu 17.04 and higher (18.04)

From Ubuntu 17.04 and onwards, systemd-resolve is used for DNS. You can flush systemd’s caches like so:

sudo systemd-resolve --flush-caches 

Ubuntu 22.04 and higher

sudo resolvectl flush-caches 

This didn’t work for me on 16.04 LTS — but it was useful to see yet another way it might have been cacheing: sudo systemd-resolve —statistics

Any idea why it displays this error on Ubuntu 19.10? Failed to flush caches: Unit dbus-org.freedesktop.resolve1.service not found.

For ubuntu 22.04, we need to use: sudo resolvectl flush-caches . See answer from @codezalot in systemd-resolve-command-not-found-in-ubuntu-22-04-desktop.

For 18.04 and higher

For 11.10 and below

Ubuntu doesn’t cache dns records by default so unless you’ve installed a dns cache there isn’t anything to clear.

DNS records are likely cached by your provider’s DNS servers so if you want to check if the DNS changes you made were successful you can interrogate a DNS server from your domain hosting service with dig:

dig -t a ns1.myhostingcompany.com @domain_registrar_dns_server

It you want Ubuntu to start caching dns I recommend installing pdnsd together with resolvconf . nscd is buggy and not advisable.

Mike Shultz’s answer definitely doesn’t work on Ubuntu 20. You get this error message: sd_bus_open_system: No such file or directory

So, For 10 years later. If I disabled systemd-resolve for ubuntu and debian. Then there is no DNS cache, Each nslookup will request DNS server?

12.04

Ubuntu 12.04 uses dnsmasq which is built into network-manager , but it doesn’t cache dns so there is no need to flush it. Here is a sample line from my syslog to prove that point:

dnsmasq[2980]: started, version 2.59 cache disabled 

There is also no need for any configuration of dnsmasq . If you are running with stock settings it won’t be caching dns, as for it to do so you have to explicitly set it up as this Ubuntu article describes.

If you wanted to refresh your settings you could disable and then enable networking or run

sudo service network-manager restart 

This restarts dnsmasq because it is built in to network-manager ; check your syslog for the evidence for this.

If you are using a wired connection with dhcp network manager will be taking the settings direct from your router and your connection will be automatically established when you login to Ubuntu. You could check that the settings are correct in your router if you can access it via the web interface, and perhaps reboot it if necessary. If it is a general problem with dns, you could try using Google dns instead of your isp dns, and more information on that is detailed here.

Note that Ubuntu uses systemd-resolve from 17.04 and onwards so this answer doesn’t apply anymore to recent Ubuntu versions. See «flush DNS cache in Ubuntu 17.04 and higher (18.04)»

By default, DNS is not cached in Ubuntu < 17.04 (but it might be cached in the network or application)

To confirm one way or the other whether dnsmasq is caching, run ps ax | grep dnsmasq and look at the running command. Here’s a breakdown of my default 13.10 machine:

/usr/sbin/dnsmasq \ --no-resolv \ --keep-in-foreground \ --no-hosts \ --bind-interfaces \ --pid-file=/var/run/NetworkManager/dnsmasq.pid \ --listen-address=127.0.1.1 \ --conf-file=/var/run/NetworkManager/dnsmasq.conf \ --cache-size=0 \ --proxy-dnssec \ --enable-dbus=org.freedesktop.NetworkManager.dnsmasq \ --conf-dir=/etc/NetworkManager/dnsmasq.d

/etc/NetworkManager/dnsmasq.d is empty by default. So there aren’t any overrides coming in there and just to check —cache-size=0 means what we think it means (instead of an unlimited cache), man dnsmasq shows:

-c, --cache-size= Set the size of dnsmasq's cache. The default is 150 names. Setting the cache size to zero disables caching. 

So while dnsmasq can cache DNS, it isn’t caching out the box. You can check your machine and various configuration directories to check you’re on the same page.

If you are seeing cache issues, this is likely happening in one of a few places:

• Upstream from your computer. Some routers cache. Many corporate networks will cache DNS. Many ISP-run DNS servers and will use their own caches. The only way to guarantee against a network cache is to use a cache you can manually refresh. This is why I like OpenDNS.
• In the client application (notably browsers). Applications can do all sorts of their own caching that Ubuntu has no effect on. How Firefox caches DNS. How to clear Chrome’s DNS cache. Other browsers (and applications) might have their own mechanisms.
• I’m scraping the barrel here but perhaps you’ve installed a non-standard DNS server in Ubuntu instead of turning caching on in dnsmasq . There are many: nscd , DJBDNS dnscache (aka TinyDNS), pdns , pdnsd , Bind9 (and its variants), and more I can’t even remember. These will probably be evidenced in /etc/resolv.conf (with config in /etc/resolvconf/` to autogen that file). The following shows an locally intercepted DNS query:

$ nslookup askubuntu.com Server: 127.0.1.1 Address: 127.0.1.1#53 Non-authoritative answer: Name: askubuntu.com Address: 198.252.206.24 

If you’re not hitting 8.8.8.8 (or whatever you expect your DNS server to be), check what you are hitting instead. In my case I can see this is just dnsmasq set up to mirror DNS queries back for LXC, but in your case it might be doing bad cachey things. If you have done of the listed caches, the process for clearing each varies:

sudo /etc/init.d/nscd reload # nscd sudo /etc/init.d/named restart # bind9 

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Flush DNS Cache on Ubuntu 20.04 — Step by Step process ?

DNS server is used to convert domain names to IP addresses.

When you type a website name in to address bar of your browser, your system does not know where to find this address. It asks for this address from the DNS server.

The DNS server contains the mapping of the domain name to IP addresses.

This domain name to IP address mapping changes from time to time.

Our system also maintains a temporary database containing domain to IP mapping information about the sites we visited recently.

This temporary database is known as DNS cache. It is used to avoid fetching the frequently or recently visited sites more quickly from the locally stores DNS cache instead of retrieving them from DNS servers.

Sometimes, it is required to flush the DNS cache. This is required in cases like when the cache becomes corrupted, and it is redirecting you towards the wrong websites.

It is also needed when a site is moved to a different address but you the browser is still loading the old site.

Flushing the DNS cache deletes the entire domain to IP mapping entries including old and invalid records. The next time when the system tries to access the websites, DNS cache repopulates the new entries.

Here at LinuxAPT, as part of our Server Management Services, we regularly help our Customers to perform DNS configurations and related queries.

In this context, we shall look into how to flush the DNS cache on Ubuntu.

How to Flush the DNS Cache on Ubuntu 20.04 LTS ?

Most Ubuntu (18.04 and later) and Debian systems use systemd-resolved daemon to cache DNS queries. We can also use it to flush the DNS cache.

You can perform the following tasks.

1. Check «systemd-resolved» Status

Let’s first check if systemd-resolved is running. Execute the below command to do so:

$ sudo systemctl is-active systemd-resolved

If you see » active » in the above command’s output, it means systemd-resolved is running on your system.

2. View Current Cache Size

Now if you want to view the current cache size along with some other information, execute the below command in Terminal:

$ sudo systemd-resolve --statistics

Following is the output of the above command, which shows different information.

At present, we are only concerned with the » Current Cache Size «, which is almost full. We want to make the cache size 0 by flushing it.

3. Flush DNS Cache

Now in order to flush the DNS cache, execute the below command in Terminal:

$ sudo systemd-resolve --flush-caches

4. Verify the Cache Size

You can then look up the statistics to ensure that the size of your cache is now zero:

$ sudo systemd-resolve --statistics

The output will show the current cache size as «0» which confirms that the DNS cache has been cleared.

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Conclusion

This article covers how to flush the DNS cache on Ubuntu. If the sites are not loading properly or you are receiving outdated sites, try to flush the DNS cache to see if the issue resolves.

Remember, after the DNS cache is flushed, sites will first load slightly slower.

But once the DNS cache is re-populated, sites will be loaded faster.

Facts about flushing DNS cache on Ubuntu:

1. DNS cache is a temporary database that stores information about previous DNS lookups.

2. Whenever you visit a website, your OS and web browser will keep a record for the domain and the corresponding IP address.

3. Flushing DNS cache eliminates the need for repetitive queries to the remote DNS servers and allows your OS or browser to resolve the website’s URL quickly.

How to use dns-clean to flush DNS-Cache ?

You can 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

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