On a vanilla ubuntu 10.04 install running nmap shows the following:

22/tcp  open  ssh
53/tcp  open  domain

It seems that bind9 is running and port 53 is open. I'd like to close as many ports as possible to harden the system, but I'm not sure about this one.

Does it affect the DNS system if I close port 53, or is it only required if I actually host a DNS server?

Can I safely close it?

  • What does secondary or slave mean, is it repeater (extender). Aug 13 '21 at 22:25
  • Where do you read secondary/slave?
    – Era
    Aug 14 '21 at 8:31
  • DNS requests are sent from a random port above 1023, so it won't hurt DNS to close the receive port.
    – stark
    Aug 14 '21 at 12:10

It is required only if you are hosting a DNS server, as primary/secondary/slave etc. If that is not the case, simply permanently stop the service and make sure incoming connection to port 53 is closed at the firewall level.

  • Most likely you don't have to do anything at "the firewall level" after simply stopping the nameserver. Feb 6 '13 at 22:34

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