Linux update dns cache

How To Flush Linux / UNIX DNS Cache

I ‘m on a Dial UP Internet connection under Linux and frequent dial up disconnection causing dns problems. How do I flush DNS cache under UNIX / Linux distribution using a shell prompt?

Under MS-Windows you can use the ipconfig command to flush dns cache. However, Linux and UNIX provides various ways to flush cache. Linux can run nscd or BIND or dnsmasq as the name service caching daemon. Large and work-group servers may use BIND or dnsmasq as a dedicated caching server to speed up queries.

HowTo: Flush nscd dns cache

Nscd caches libc-issued requests to the Name Service. If retrieving NSS data is fairly expensive, nscd is able to speed up consecutive access to the same data dramatically and increase overall system performance. Just restart nscd:
$ sudo /etc/init.d/nscd restart
OR
# service nscd restart
OR
# service nscd reload
This daemon provides a cache for the most common name service requests. The default configuration file, /etc/nscd.conf, determines the behavior of the cache daemon.

Flush dnsmasq dns cache

dnsmasq is a lightweight DNS, TFTP and DHCP server. It is intended to provide coupled DNS and DHCP service to a LAN. Dnsmasq accepts DNS queries and either answers them from a small, local, cache or forwards them to a real, recursive, DNS server. This software is also installed many cheap routers to cache dns queries. Just restart the dnsmasq service to flush out dns cache:
$ sudo /etc/init.d/dnsmasq restart
OR
# service dnsmasq restart

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Flush caching BIND server dns cache

A caching BIND server obtains information from another server (a Zone Master) in response to a host query and then saves (caches) the data locally. All you have to do is restart bind to clear its cache:
# /etc/init.d/named restart
You can also use rndc command as follows flush out all cache:
# rndc restart
OR
# rndc exec
BIND v9.3.0 and above will support flushing all of the records attached to a particular domain name with rndc flushname command. In this example flush all records releated to cyberciti.biz domain:
# rndc flushname cyberciti.biz
It is also possible to flush out BIND views. For example, lan and wan views can be flushed using the following command:
# rndc flush lan
# rndc flush wan

A note about Mac OS X Unix users

Type the following command as root user:
# dscacheutil -flushcache
OR
$ sudo dscacheutil -flushcache
If you are using OS X 10.5 or earlier try the following command:
lookupd -flushcache

A note about /etc/hosts file

/etc/hosts act as the static table lookup for hostnames. You need to remove and/or update records as per your requirements under Unix like operating systems:
# vi /etc/hosts
Sample outputs:

See also:

Related: Windows Vista / XP Flush DNS Cache with ipconfig Command

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Как очистить / сбросить кеш DNS в Linux

В некоторых случаях вам трудно добраться до некоторых сайтов из-за проблемы с DNS, возможно, ваш локальный DNS-кеш поврежден.

Для такой ситуации вам необходимо перезапустить службу кэширования DNS операционной системы.

Кэш-память DNS для системы доменных имен – это временная база данных, которая поддерживается операционной системой компьютера, которая содержит IP-адрес сайтов, которые вы недавно просматривали.

См. Ниже список основных серверов кэширования DNS, используемых в дистрибутиве Linux.

• nscd DNS cache
• dnsmasq dns cache
• BIND server dns cache

nscd DNS Cache: nscd – демон кэша службы имен, Nscd – это демон, который обеспечивает кэш для наиболее распространенных запросов службы имен.

Файл конфигурации по умолчанию, /etc/nscd.conf

dnsmasq DNS Cache: Dnsmasq – легкий, небольшой в размере, простой в настройке, DNS-сервер и DHCP-сервер.

Он предназначен для предоставления DNS и опционально DHCP для небольшой сети и подходит для маршрутизаторов с ограниченным доступом и брандмауэров.

Он может обслуживать имена локальных машин, которые не входят в глобальный DNS.

Он предназначен для использования в персональных компьютерах и небольших сетях, а не для больших сетей.

BIND Server DNS Cache: BIND означает «Berkeley Internet Name Domain».

Наиболее широко используемое программное обеспечение сервера имен, BIND – это программное обеспечение с открытым исходным кодом, которое реализует протоколы доменных имен (DNS) для Интернета.

BIND на сегодняшний день является наиболее широко используемым программным обеспечением DNS в Интернете, обеспечивая надежную и стабильную платформу.

1) Очистить кеш DNS на Ubuntu / Debian / LinuxMint

Используйте приведенную ниже команду для очистки кеша DNS в системах Ubuntu / Debian / Mint.

2) Сброс кеша dns сервера BIND

Используйте приведенную ниже команду для сброса кеша BIND сервера dns

3) Очистить кэш nscd DNS

Используйте команду ниже:

4) Очистить dnsmasq dns кэш

Используйте приведенную ниже команду для стирания кеша dnsmasq server dns

5) Очистка кеша dns в Windows

Используйте приведенную ниже команду для очистки кеша dns в Windows:

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How to Flush the DNS Cache on Ubuntu

The DNS or the Domain Name Server can be characterized as the most essential part 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 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 18.04 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:

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:

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:

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.

Karim Buzdar

About the Author: Karim Buzdar holds a degree in telecommunication engineering and holds several sysadmin certifications. As an IT engineer and technical author, he writes for various web sites. You can reach Karim on LinkedIn

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Flush DNS Cache on Ubuntu

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:

Domain Name	IP Address
linuxhint.com	1.2.3.4
support.linuxhint.com	3.4.5.7
google.com	8.9.5.4
www.google.com	8.9.5.4

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.

nslookup and dig commands should now be available.

Now to test whether caching is enabled, run the following command:

As you can see, the DNS server used to resolve the domain name is 127.0.0.53, which is a loopback IP address. So DNS caching is enabled. If you have it disabled, then the DNS server should be anything other than 127.0.0.X.

You can check the same thing with the dig command as well as follows:

As you can see, the loopback IP addresses is used as the DNS server addresses here as well. So, DNS caching is enabled.

Flushing DNS on Ubuntu 18.04 LTS:

Ubuntu 18.04 LTS uses a local DNS server and caches DNS queries by default. Ubuntu 18.04 LTS uses systemd for this purpose.

You can run the following command to check how many DNS entries are cached and many other information on Ubuntu 18.04 LTS:

As you can see, information about DNS cache is listed in the marked section of the screenshot below.

To flush the DNS cache on Ubuntu 18.04 LTS, run the following command:

You can also restart the systemd-resolved service to flush the DNS caches on Ubuntu 18.04 LTS.

To restart the systemd-resolved service, run the following command:

As you can see, the caches are cleared.

Flushing DNS Cache on Ubuntu 16.04:

On Ubuntu 16.04 LTS, DNS cache is not enabled by default. But some applications like bind, dnsmasq, nscd etc. may cache it.

If you’re using nscd for caching DNS queries, then you can flush the DNS cache by simply restarting the nscd service.

You can restart the nscd service on Ubuntu 16.04 LTS to flush DNS caches with the following command:

If you’re using dnsmasq for caching DNS, then restarting dnsmasq service with the following command should flush the DNS cache.

So, that’s how you flush DNS cache on Ubuntu 18.04 LTS and 16.04 LTS. Thanks for reading this article.

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