How to flush DNS cache in Linux

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This guide will go through how to flush DNS cache in Linux. On a computer’s operating system or web browser, the term “DNS cache” refers to the temporary storage of data about past DNS lookups.
A DNS lookup can be swiftly retrieved by your OS or browser if you keep a local copy of it, which makes it possible to resolve a website’s URL to its matching IP much more quickly.

How to flush DNS cache in Linux

  • Start by finding out which type of resolver your system is using i.e systemd-resolved or dnsmasq using the command below.
sudo lsof -i :53 -S

Sample output

COMMAND   PID            USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
systemd-r 709 systemd-resolve   13u  IPv4  35920      0t0  UDP localhost:domain 
systemd-r 709 systemd-resolve   14u  IPv4  35921      0t0  TCP localhost:domain (LISTEN)

In the example above the system uses systemd-resolved

How to flush DNS cache in Linux using systemd-resolved

  • To flush the DNS cache using systemd-resolved, use one of the following command.
sudo systemd-resolve --flush-caches

or

sudo resolvectl flush-caches
  • Next, check the current cache size using the following command.
sudo systemd-resolve --statistics

or

sudo resolvectl statistics

Sample output

DNSSEC supported by current servers: no

Transactions              
Current Transactions: 0
  Total Transactions: 2050

Cache                     
  Current Cache Size: 2
          Cache Hits: 389
        Cache Misses: 1671

DNSSEC Verdicts           
              Secure: 0
            Insecure: 0
               Bogus: 0
       Indeterminate: 0

Flush DNS cache in Linux using signals

  • Sending a USR2 signal to the systemd-resolved can also be used to clear the DNS cache.
sudo killall -USR2 systemd-resolved
  • Next, use the USR1 command below to dump the current state.
sudo killall -USR1 systemd-resolved
  • Print the journal on your terminal using the following command.
sudo journalctl -r -u systemd-resolved

Sample output

Nov 01 01:03:44 itnixpro systemd-resolved[709]:         Server dropped DO flag: no
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         Seen invalid packet: no
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         Seen RRSIG RR missing: no
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         Seen OPT RR getting lost: no
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         Seen truncated packet: no
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         Failed TCP attempts: 0
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         Failed UDP attempts: 0
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         Maximum UDP fragment size received: 512
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         Can do DNSSEC: no
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         DNSSEC Mode: no
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         Possible feature level: UDP+EDNS0
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         Verified feature level: UDP+EDNS0
Nov 01 01:03:44 itnixpro systemd-resolved[709]: [Server 192.168.171.2 type=link interface=ens33]
Nov 01 01:03:44 itnixpro systemd-resolved[709]: [Scope protocol=dns]
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         www.googleapis.com IN AAAA 2a00:1450:401a:805::200a
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         www.googleapis.com IN AAAA 2a00:1450:401a:800::200a
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         www.googleapis.com IN AAAA 2a00:1450:401a:801::200a
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         www.googleapis.com IN A 172.217.170.170
Nov 01 01:03:44 itnixpro systemd-resolved[709]:         www.googleapis.com IN A 172.217.170.202
Nov 01 01:03:44 itnixpro systemd-resolved[709]: CACHE:
Nov 01 01:03:44 itnixpro systemd-resolved[709]: [Scope protocol=dns interface=ens33]
Nov 01 00:53:12 itnixpro systemd-resolved[709]: Flushed all caches.
Nov 01 00:50:38 itnixpro systemd-resolved[709]: Flushed all caches.
Okt 31 11:57:58 itnixpro systemd-resolved[709]: Clock change detected. Flushing caches.
Okt 31 11:57:38 itnixpro systemd-resolved[709]: ens33: Bus client set DNS server list to: 192.168.171.2
Okt 31 11:57:38 itnixpro systemd-resolved[709]: ens33: Bus client set default route setting: yes
Okt 31 11:57:38 itnixpro systemd-resolved[709]: ens33: Bus client set search domain list to: localdomain
Okt 31 11:57:31 itnixpro systemd[1]: Started Network Name Resolution.
Okt 31 11:57:31 itnixpro systemd-resolved[709]: Using system hostname 'itnixpro'.
Okt 31 11:57:30 itnixpro systemd-resolved[709]: Negative trust anchors: home.arpa 10.in-addr.arpa 16.172.in-addr.arpa 17.172.in-addr.arpa 18.172.in-addr.arpa 19.172.in-addr.arpa 20.172.in-addr.arpa 21.1>
Okt 31 11:57:30 itnixpro systemd-resolved[709]: . IN DS 20326 8 2 e06d44b80b8f1d39a95c0b0d7c65d08458e880409bbc683457104237c7f8ec8d
Okt 31 11:57:30 itnixpro systemd-resolved[709]: Positive Trust Anchors:
Okt 31 11:57:29 itnixpro systemd[1]: Starting Network Name Resolution...
-- Boot 7f2caea837404e919cd6b59fcdd0a6fc --
Okt 30 01:15:00 itnixpro systemd[1]: systemd-resolved.service: Consumed 4.395s CPU time.
Okt 30 01:15:00 itnixpro systemd[1]: Stopped Network Name Resolution.
Okt 30 01:15:00 itnixpro systemd[1]: systemd-resolved.service: Deactivated successfully.
Okt 30 01:15:00 itnixpro systemd[1]: Stopping Network Name Resolution...
Okt 30 01:09:30 itnixpro systemd-resolved[536]: ens33: Bus client set DNS server list to: 192.168.171.2
Okt 30 01:09:30 itnixpro systemd-resolved[536]: ens33: Bus client set default route setting: yes
Okt 30 01:09:30 itnixpro systemd-resolved[536]: ens33: Bus client set search domain list to: localdomain
Okt 30 01:09:30 itnixpro systemd-resolved[536]: ens33: Bus client reset DNS server list.
Okt 30 01:09:30 itnixpro systemd-resolved[536]: ens33: Bus client set default route setting: no
Okt 30 01:09:30 itnixpro systemd-resolved[536]: ens33: Bus client reset search domain list.
Okt 30 01:08:09 itnixpro systemd-resolved[536]: ens33: Bus client set DNS server list to: 192.168.171.2
Okt 30 01:08:09 itnixpro systemd-resolved[536]: ens33: Bus client set default route setting: yes
Okt 30 01:08:09 itnixpro systemd-resolved[536]: ens33: Bus client set search domain list to: localdomain
Okt 30 01:08:09 itnixpro systemd-resolved[536]: ens33: Bus client reset DNS server list.
Okt 30 01:08:09 itnixpro systemd-resolved[536]: ens33: Bus client set default route setting: no
Okt 30 01:08:09 itnixpro systemd-resolved[536]: ens33: Bus client reset search domain list.
Okt 30 01:07:35 itnixpro systemd-resolved[536]: ens33: Bus client set DNS server list to: 192.168.171.2
Okt 30 01:07:35 itnixpro systemd-resolved[536]: ens33: Bus client set default route setting: yes
Okt 30 01:07:35 itnixpro systemd-resolved[536]: ens33: Bus client set search domain list to: localdomain
Okt 30 01:07:31 itnixpro systemd-resolved[536]: ens33: Bus client reset DNS server list.

How to flush DNS cache in Linux using dnsmasq

  • Run the command below to flush the DNS cache using dnsmasq. Note, your system should be using dnsmasq.
sudo killall -HUP dnsmasq
  • Next, check if dnsmasq is active using the command below.
sudo systemctl is-active dnsmasq
  • To check the status, use the following command.
sudo service dnsmasq status
  • Restart dnsmasq using the command below.
sudo systemctl restart dnsmasq

or

sudo service dnsmasq restart
  • You have reached the end of our article, we have gone through how to flush DNS cache in Linux.

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