Clearing DNS cache on vCenter Server Appliance 6.5/6.7


 

I came across a situation recently where I was building some nested ESXi hosts and had configured the incorrect IP address for their DNS record, only to realise this after attempting to add the host into vCenter:

Failure to add host due to incorrect IP configured in DNS record

By attempting to add the host, vCenter will query your DNS servers for the IP address associated with the FQDN you have supplied (assuming you did this method, not via IP address). If you have set the wrong IP on your DNS server for the above FQDN record, you have 2 options:

  1. Wait a short while for vCenter to clear the cache entry for this FQDN, thus allowing you to add the host with the correct IP from the DNS record
  2. Clear the DNS cache on your vCenter Server Appliance (vCSA)

vCSA 6.5 upwards runs on Photon OS with dnsmasq acting as a local caching nameserver (thank you to Brandon for the comment to clarify).

Enough rambling, so how do we do it?

If you’re anything like me, you will be too impatient to wait the 5-10 minutes (from my observations) to get the updated DNS record. Thus the following method can be used to clear the DNS cache on your vCSA:

  1. SSH to your vCenter Server appliance
  2. Enter the bash shell, ie shell
  3. Flush the DNS cache by means of restarting the dnsmasq service
    systemctl restart dnsmasq

vCenter will now perform a fresh query to your DNS server to obtain the correct IP, thus allowing you to add the host.

Successfully added after clearing DNS cache

21 Comments

  1. June 12, 2017 / 10:58 pm

    Nice work mate, looks like you’re going great guns !

    • admin
      Author
      June 13, 2017 / 6:32 am

      Thanks Pete!

  2. August 13, 2017 / 2:32 am

    Hey, what do you think about choosing vCenter on Windows vs Linux? I’m in the middle of that decision..

    • admin
      Author
      August 13, 2017 / 11:18 am

      Hi Jonathon,

      Where possible, it is always preferable to use the vCenter Server Appliance unless you have an explicit requirement to run your vCenter server on Windows. Feel free to reach out if you need further clarification.

      Kane.

  3. Francis Lee Mondia
    January 26, 2018 / 3:20 am

    For some reason, this doesn’t seem to work on our VCSA 6.5..

    • admin
      Author
      January 27, 2018 / 12:03 am

      Hi Francis, did you run the command from the VAMI shell or from the bash shell?
      If you can give me the build number or release number of the VCSA you’re using I’d be happy to do some digging.
      Kane.

  4. Francis
    January 27, 2018 / 12:42 am

    Hi Kane,

    Thanks for replying. The build number is VMware ESXi, 6.5.0, 7526125

    I was doing it from the bash shell, logged-in to the VCSA appliance.

    For some reason, DNS refreshed between 10-15mins since last check. The last time, we had to wait till the next day for the DNS to take effect even with those commands run.

    Kind regards,
    Francis

    • admin
      Author
      March 2, 2018 / 12:19 am

      Hi Francis,

      Please see update at the top of this post, give that a go and let me know how you go!

      Cheers,
      Kane.

  5. RT
    January 29, 2018 / 7:40 am

    Thank you ! Exactly what I needed.

  6. Tyson
    February 9, 2018 / 12:17 am

    This doesn’t work with my VCSA 6.5 either. I’m running the latest build (6.5.0.14000 Build Number 7515524)

    My issue was I messed up and added the new IP to DNS but forgot to remove the old record. Fixed it in DNS and now trying to get vCenter to recognize the change.

    root@VC01 [ ~ ]# nslookup geoff.foo.com
    Server: 127.0.0.1
    Address: 127.0.0.1#53

    Non-authoritative answer:
    Name: geoff.foo.com
    Address: 123.123.30.54
    Name: geoff.foo.com
    Address: 123.123.23.98

    root@VC01 [ ~ ]# systemctl restart systemd-resolved.service
    root@VC01 [ ~ ]# nslookup geoff.foo.com
    Server: 127.0.0.1
    Address: 127.0.0.1#53

    Non-authoritative answer:
    Name: geoff.foo.com
    Address: 123.123.23.98
    Name: geoff.foo.com
    Address: 123.123.30.54

    root@VC01 [ ~ ]#

    • admin
      Author
      March 2, 2018 / 12:19 am

      Hi Tyson,

      Please see update at the top of the post, give that a go and let me know how you go!

      Cheers,
      Kane.

  7. Jim Parrish
    February 16, 2018 / 4:10 pm

    Does restarting this service cause any other side effects? In a production environment, I wouldn’t want to lose connection to the other existing hosts. Just curious if there’s anything else to watch out for? Thanks!

    • admin
      Author
      March 2, 2018 / 12:18 am

      Hi Jim,

      Nothing else to worry about, this will not cause an outage.

      Cheers,
      Kane.

  8. Maciek
    March 1, 2018 / 10:37 am

    Hi,

    On my vCenter also doesn’t work. I was wating more than 1 hour.
    I restarted one more service:

    systemctl restart systemd-networkd.service

    After this vCetner obtains new IPs immediately.

    Kind Regards
    Maciek

    • admin
      Author
      March 2, 2018 / 12:17 am

      Thank you Maciek, post updated to reflect this!

  9. Brandon
    March 21, 2018 / 10:00 pm

    So, I took a look at the nslookup output from a VCSA 6.5.0 7801515 with integrated PSC to find the proper solution:
    nslookup host.domain.tld
    Server: 127.0.0.1
    Address: 127.0.0.1#53

    Non-authoritative answer:
    Name: host.domain.tld
    Address: a.b.c.d

    VCSA 6.5 runs dnsmasq as a local caching nameserver.
    The proper way to clear it is to restart that service, not the others.

    systemctl restart dnsmasq

    Here’s your proof (run at a bash shell on the VCSA):
    netstat -lpW | grep domain

    That’ll show you dnsmasq listening on port 53 (domain).
    And, if you look at the resolv.conf file, you’ll see it always sticks 127.0.0.1 as the first name server, regardless of what you’ve set for name servers (hence why nslookup defaulted to looking at loopback).

    • admin
      Author
      March 26, 2018 / 10:11 am

      Hi Brandon, thank you for clarification. I will make note of it in my post. Cheers, Kane.

    • March 31, 2018 / 9:11 pm

      Can confirm, this is the real solution here.

      Disclaimer: I am a VMware Product Manager 😉

    • admin
      Author
      November 30, 2018 / 11:33 pm

      Hi Demetrius,

      Thank you for clarifying. I have updated the post to reflect.

      Cheers,
      Kane.

  10. obwanken00by
    January 8, 2019 / 2:11 pm

    Thanks mate – you just spared me a looooooooooooooong vcsa reboot =)

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