How can I flush the DNS cache of a USG LITE 60AX?

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Lohkamp
Lohkamp Posts: 33
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I had a client connected once directly via WLAN to the WAN gateway, an AVM FRITZ!Box which is now again connected to the Zyxel WLAN.

But nslookup of this client still lets the USG report [client].fritz.box with its old IPv6.
This is a show stopper, I expect I will have to reboot the USG LITE 60AX which breaks current internet connections.

For sure I rebooted my current other Windows client from which I issued the nslookup.

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  • Zyxel_James
    Zyxel_James Posts: 840
    Zyxel Certified Network Administrator - Security Zyxel Certified Network Administrator - Nebula Zyxel Certified Sales Associate 100 Answers
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    Currently, we can't flush the DNS cache for USG LITE 60AX.
    However, I’m more curious about the issue you’re experiencing because USG LITE 60AX does not have IPv6 function.
    So the client's DNS server is the USG LITE 60AX or the FRITZ!Box gateway? The FRITZ!Box is the front gateway of USG LITE 60AX or they're in different internet line?

  • Lohkamp
    Lohkamp Posts: 33
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    edited May 2025
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    All VM (and other) clients use DHCP from the USG LITE 60AX and thus use its DNS service.
    USG LITE 60AX has an AVM FRITZ!Box 6890 LTE at its WAN port (VDSL with automatic LTE fallback).

    I connected an ASUS notebook ("ZenbookOL" as device name) once directly to the WLAN of the FRITZ!Box. So it occurred in its DNS server.
    nslookup ZenbookOL [IP of FRITZ!Box]
    still reported the ZenbookOL even after I deleted it from its internal email client list, so deleting/flushing the DNS server within the FRITZ!Box was also not working.

    nslookup ZenbookOL (with USG LITE 60AX answering) only answered with the answer from the FRITZ!Box with its IPv6 address and as "ZenbookOL.fritz.box" and although the ZenbookOL was connected via WLAN only to the USB LITE 60AX and restarted several times, Firewall setup to "private network".

    So, I informed my users and restarted both the FRITZ!Box and the USG LITE 60AX. Only after that the correct IPv4 (only, for sure) was answered by the USG LITE 60AX and I could access the notebook's SMB shares e. g.
    USG LITE 60AX unfortunately provides no IPv6 to the internet, so from time to time and for testing I have to connect devices directly to the FRITZ!Box to have both IPv4 + IPv6 to the internet. But is needed only once every few months.

  • Lohkamp
    Lohkamp Posts: 33
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    Today, I had the same need.
    I created a Hyper-V VM on a Windows client, connected via LAN (Oracle Linux 10.2 to host Oracle XE) and gave this VM the name "oraclelinux".
    However, I had to install an older version of Oracle Linux (8.10) to be able to install the Oracle XE database.

    So, I stopped the old VM, created the new one and gave this Linux server the same name "oraclelinux".

    Both did never run at the same time!

    The old VM hat got the IPv4 .20 as 4th byte, the new VM got .21.

    Over the whole day (many hours)
    nslookup oraclelinux

    was answered by the USG LITE 60AX with the old .20 although the old VM was never started again,
    the new one many times. And it queries its IP by DHCP from the USB LITE 60AX.

    I even renamed the .20 in the NCC client list to "oraclelinux1" and the .21 to "oraclelinux" manually.

    After rebooting the USG LITE 60AX "nslookup oraclelinux" answered "unknown", even many minutes after starting the VM .21 again.
    It remains visible under "clients" in NCC but offline, with the correct MAC as reported by "ip a" within Oracle Linux.
    And I can connect to the internet from within this VM.

    Very strange. I could only help myself by temporarily adding oraclelinux to my local hosts file to be able to SSH into it and open web management pages in it.

    A DNS cache flush would help a lot in such cases!

    For sure, I tested ipconfig /flushdns on a Windows PC, but for sure this has no effektcon nslookup.
    And I got the wrong answer (.20) also from a completely different PC that was not used today except of this tests hours after switching to the new VM with .21

    So, this is an issue within the USG LITE 60AX.

    Kind regards
    Oliver

  • Lohkamp
    Lohkamp Posts: 33
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    I found out that this is a general problem with Oracle Linux which installs itself with

    computer name: oracle-linux
    real computer name: oraclelinux.local

    I changed via https://oraclelinux.:9090/system
    "computer name" to oraclelinux.local, too, rebootet, and it is working.

    Working:
    nslookup oraclelinux
    nslookup oraclelinux.

    Not working:
    nslookup oraclelinux.local

    But this is OK for me. Maybe, because I renamed this "client" in NCC to "oraclelinux" a few days ago.

    So, my problem is solved. Maybe, this is an Oracle Linux related special issue due to these two "settings" for the host name.

    Zyxel unrelated spoiler alert: if you change the host name of an OracleDdatabase do not forget to change it also in

    nano /opt/oracle/homes/OraDBHome21cXE/network/admin/listener.ora
    nano /opt/oracle/homes/OraDBHome21cXE/network/admin/tnsnames.ora
    systemctl restart oracle-xe-21c.service

    Kind regards
    Oliver

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