Linux reset dns cache

How To Flush DNS Cache On Linux (For systemd-resolved, BIND, Dnsmasq Or nscd)

This article explains how to flush the DNS cache (clear it) on Linux, not only for systems that use systemd-resolved (e.g. Ubuntu 18.04 and newer), but also for those using nscd, BIND or Dnsmasq to cache DNS queries.

DNS, or Domain Name System, is like the Internet’s phonebook, translating human-friendly computer hostnames into IP addresses. For example, google.com is translated to the 216.58.214.238 IPv4 address, and 2a00:1450:400d:808::200e IPv6 address. This way the domain name can change the IP address without affecting its users, and without changing the its name.

The DNS cache (or DNS resolver cache) is a temporary database that stores these records, which is used to quickly lookup a website when you’re trying to visit it.

If the DNS is cached, you might not be able to access some hostnames that were changed recently — until your DNS cache is updated or cleared, that’s why it’s useful in some situations to flush / clear the DNS cache. After the DNS cache is cleared, the new addresses are taken from the DNS server set for your network.

How to flush the DNS cache when using systemd-resolved

systemd-resolved is a network name resolution manager. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR and MulticastDNS resolver and responder.

systemd-resolved is used by default on recent versions of many Linux distributions, including Ubuntu, Linux Mint and Fedora.

Don’t know if you system uses systemd-resolved? Use:

systemctl is-active systemd-resolved

In case this command returns active , then your system has systemd-resolved enabled and active. If anything else (e.g. inactive ) is shown, then systemd-resolved is not used on your system.

Flush the DNS cache when using systemd-resolved:

sudo systemd-resolve --flush-caches

Check the DNS cache using:

systemd-resolve --statistics

The Current Cache Size is the value you’re looking for. If it’s 0 , you just flushed the DNS cache.

Example with output (ran immediately after flushing the DNS cache, so the cache size is 0):

$ systemd-resolve --statistics DNSSEC supported by current servers: no Transactions Current Transactions: 0 Total Transactions: 5538 Cache Current Cache Size: 0 Cache Hits: 3482 Cache Misses: 2096 DNSSEC Verdicts Secure: 0 Insecure: 0 Bogus: 0 Indeterminate: 0

How to flush the DNS cache when using BIND

BIND or «named» acts as an authoritative name server for one or more specific domain, as well as a recursive resolver for the DNS system generally, and it can act as a caching DNS server. Bind had three major revisions, with BIND4 and BIND8 being technically obsolete, and BIND9 being the supported version, which features DNSSEC in addition to other features and enhancements.

To check if BIND (the service name is «named») is active on your system (though note that most cases it’s active only if you install and enable it explicitly), you could run:

When active is returned by this command, it means the service is active on your system.

On older systems, or systems without systemd, you could check the service status instead, using

To flush the DNS cache for BIND (named), use:

Checking if the DNS cache has been cleared is a bit more complicated for BIND. After flushing the DNS cache, run the command that follows below to dump the cache:

The cache file is now saved in /var/named/data/cache_dump.db . You can look at this file as root to check if there are any cached DNS entries:

sudo cat /var/named/data/cache_dump.db

If the cache was flushed (so there are no cached DNS entries), this file should look like this:

$ sudo cat /var/named/data/cache_dump.db ; ; Start view _default ; ; ; Cache dump of view '_default' (cache _default) ; $DATE 20190711141232 ; ; Address database dump ; ; [edns success/4096 timeout/1432 timeout/1232 timeout/512 timeout] ; [plain success/timeout] ; ; ; Unassociated entries ; ; ; Bad cache ;

If it contains entries like the ones that follow below, the DNS cache was not flushed:

102397 IN DNSKEY 256 3 8 ( AwEAAeVDC34GZILwsQJy97K6Fst4P3XYZrXL yrkausYzStEjSUulgh+iLgHg0y7FIF890+sI jXsk7KLJUmCOWfYWPorNKEOKLk5Zx/4M6D3I HZE3O3m/Eahrc28qQzmTLxiMZAW65MvR2UO3 LxVtYOPBEBiDgAQA47x2JLsJYtavCzNL5WiU k59OgvHmDqmcC7VXYBhK8V8Tic089XJgExGe plKWUt9yyc31ra1swJX51XsOaQz17+vyLVH8 AZP26KvKFiZeoRbaq6vl+hc8HQnI2ug5rA2z oz3MsSQBvP1f/HvqsWxLqwXXKyDD1QM639U+ XzVB8CYigyscRP22QFnwKIU= ) ; ZSK; alg = RSASHA256 ; key >

How to flush the DNS cache when using Dnsmasq

Dnsmasq is a DNS forwarder and DHCP server that can be used as a caching DNS server.

To check if dnsmasq is active on your system, you could run:

systemctl is-active dnsmasq

When active is returned by this command, it means the service is active on your system.

On older systems, or systems without systemd, you could check the service status instead, using:

To flush the DNS cache when using Dnsmasq, restart its service:

sudo systemctl restart dnsmasq

On older systems, or systems without systemd, use this command to flush the DNS cache for Dnsmasq:

sudo service dnsmasq restart

I couldn’t find a way of checking if the dnsmasq has been flushed.

How to flush the DNS cache when using nscd

nscd is a name service cache daemon.

To check if nscd is active on your system (though note that most cases it’s active only if you install and enable it explicitly), you could run:

When «active» is returned by this command, it means the service is active on your system.

On older systems, or systems without systemd, you could check the service status instead, using:

While most guides you’ll find online mention to restart the nscd service, that doesn’t actually flush the DNS cache, since the files are still stored in /var/db/nscd , and are used after the nscd service starts again. What you actually want to do to flush the DNS cache for nscd is to invalidate the cache.

To invalidate the hosts cache (to flush the DNS hosts cache) when using nscd, use:

sudo nscd --invalidate=hosts

To check if the DNS cache was flushed, run:

And look under the hosts section (if you have multiple enabled), and you should see 0 as the value for used data pool size if the DNS cache was just flushed. It will slowly increase as the DNS are used to access various hosts / domains names.

Example with output (after the DNS cache was flushed):

$ sudo nscd -g nscd configuration: 0 server debug level 1m 5s server runtime 5 current number of threads 32 maximum number of threads 0 number of times clients had to wait no paranoia mode enabled 3600 restart internal 5 reload count . hosts cache: yes cache is enabled yes cache is persistent yes cache is shared 211 suggested size 216064 total data pool size 0 used data pool size 3600 seconds time to live for positive entries 20 seconds time to live for negative entries 0 cache hits on positive entries 0 cache hits on negative entries 5 cache misses on positive entries . 

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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: 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.

$ sudo /etc/init.d/dns-clean start [sudo] password for magesh: [Enter your root password] * Restoring resolver state. [ OK ]

2) Сброс кеша dns сервера BIND

Используйте приведенную ниже команду для сброса кеша BIND сервера dns

# /etc/init.d/named restart Stopping named: . [ OK ] Starting named: [ OK ] # service named restart Stopping named: . [ OK ] Starting named: [ OK ]

3) Очистить кэш nscd DNS

# /etc/init.d/nscd restart # service nscd restart # service nscd reload # nscd -i hosts

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:

# ipconfig /flushdns Windows IP Configuration Successfully flushed the DNS Resolver Cache.

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