[Dnsmasq-discuss] DNS set using dhcp-host expires?
Geert Stappers
stappers at stappers.nl
Tue Apr 21 07:14:05 BST 2020
On Mon, Apr 20, 2020 at 10:27:11PM -0400, Jiawen Chen wrote:
> Hi,
>
...
>
> I'm using dnsmasq for DHCP + DNS on my home LAN and use dhcp-host to
> statically assign an IP address and hostname. However, I'm discovering that
> while it works most of the time, sometimes, ssh for example, can't find the
> host by name, only IP. One instance is a Linux VM. Initially I can ssh into
> the VM. But if I power down the VM for say a few days, and start it back
> up, the name is missing (even though it is somehow assigned the same IP (it
> keeps its MAC)).
>
> dnsmasq.conf:
> ```
> # other irrelevant stuff
>
> no-resolv
> server=8.8.8.8
> server=8.8.4.4
> server=1.1.1.1
>
> expand-hosts
> domain-needed
> bogus-priv
> local=/internal/
>
> # Assign 192.168.0.150 to my living room pc
> dhcp-host=<livingroom_pc_MAC_redacted>,192.168.0.150,livingroom.internal,infinite
>
> # Assign 192.168.0.151 to my file server
> dhcp-host=<fileserver_MAC_redacted>,192.168.0.151,fileserver.internal,infinite
> ```
I suggest to change the ',infinite' into something like ',12h' for
the servers that are be powered-off of for several days.
What I think is that the DNS cache entry expires ( and I think that it
is valid behaviour.)
Expriment that can be done:
* Restart dnsmasq, so it forgets about dhcp-hosts in DNS cache
* Check cache dump with SIGUSR1
* Power-on an infinite DHCP client e.g. the Linux VM
* Check cache dump with SIGUSR1
* Regular work with the example Linux VM
* Power-off the infinite DHCP client
* Sample several days the cache dump
* Report when the DNS entry of the powered-off server is gone
Recipe to sabotage the expriment:
while true
do
nslookup linuxVM > output
process_output
if hostname_not_found
then
exit
fi
sleep 2 hour
done
because it keeps the DNS entry in cache.
>
> For these static DHCP leases, is the best practice to set them in
> /etc/hosts instead?
>
> I also discovered --host-record, which does something very similar. Is
> there an advantage in using one option vs the other?
>
Groeten
Geert Stappers
--
Silence is hard to parse
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