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<p class="MsoNormal">Hello there!<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">In our IPv6 network, the address of a Recursive DNS Server (RDNSS) is supplied in a Router Advertisement (RA), with a lifetime of 60 seconds.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">It appears that RDNSS lifetime is not being honoured (<a href="https://www.rfc-editor.org/rfc/rfc8106.html#section-5.1:~:text=in
the option.-,Lifetime,-32-bit unsigned">RFC 8106, section 5.1</a>). I reviewed the code and can see where the <a href="https://github.com/systemd/systemd-stable/blob/4a31fa2fb040005b73253da75cf84949b8485175/src/network/networkd-ndisc.c#L712">RDNSS lifetime is being saved</a>, though I was unable to determine how it was being handled upon expiry.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">How do I configure networkd so that the RA’s RDNSS lifetime is honoured?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Here is a summary of the simple protocol exchange:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<ol style="margin-top:0cm" start="1" type="1">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1">Router: Send RA [RDNSS address of “nameserver60s”, lifetime: “60”]<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1">Host: “resolvectl” shows the link’s DNS server now lists the RDNSS address of “nameserver60s”<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1">** Wait for more than 60 seconds – the RDNSS entry should expire **<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1">Host:<o:p></o:p></li><ol style="margin-top:0cm" start="1" type="a">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level2 lfo1">“resolvectl” continues to list the address of “nameserver60s” on the link.<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level2 lfo1">Using tcpdump to trace “ping test.example.com”, the “nameserver60s” is still being used. It never timed out.<o:p></o:p></li></ol>
</ol>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Here is my network configuration, showing UseDNS and UseDomains both set to “yes”:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">------------------------------------------------------------<o:p></o:p></p>
<p class="MsoNormal">$ cat /etc/systemd/network/10-eno0.network <o:p></o:p></p>
<p class="MsoNormal">[Match]<o:p></o:p></p>
<p class="MsoNormal">KernelCommandLine=!nfsroot<o:p></o:p></p>
<p class="MsoNormal">Name=eno0<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">[DHCP]<o:p></o:p></p>
<p class="MsoNormal">ClientIdentifier=mac<o:p></o:p></p>
<p class="MsoNormal">RouteMetric=10<o:p></o:p></p>
<p class="MsoNormal">UseDomains=yes<o:p></o:p></p>
<p class="MsoNormal">UseHostname=yes<o:p></o:p></p>
<p class="MsoNormal">UseMTU=yes<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">[Network]<o:p></o:p></p>
<p class="MsoNormal">#DHCP=ipv6<o:p></o:p></p>
<p class="MsoNormal">Address=10.1.1.1/24<o:p></o:p></p>
<p class="MsoNormal">#DNS=1.2.3.6<o:p></o:p></p>
<p class="MsoNormal">Gateway=1.1.1.2<o:p></o:p></p>
<p class="MsoNormal">IPv6AcceptRA=yes<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">[IPv6AcceptRA]<o:p></o:p></p>
<p class="MsoNormal">UseDNS=yes<o:p></o:p></p>
<p class="MsoNormal">UseDomains=yes<o:p></o:p></p>
<p class="MsoNormal">------------------------------------------------------------<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Grateful for any suggestions.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Kind regards,<o:p></o:p></p>
<p class="MsoNormal">Matt.<o:p></o:p></p>
<p class="MsoNormal">PS: We’re on systemd 250. I’ve searched later versions of the
<a href="https://github.com/systemd/systemd/releases">release notes</a> and it seems there have been no changes in this area.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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