[systemd-devel] IPv6AcceptRA: RDNSS Lifetime is not expiring

[Muggeridge, Matt]

Hello there!

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.

It appears that RDNSS lifetime is not being honoured (RFC 8106, section 5.1<https://www.rfc-editor.org/rfc/rfc8106.html#section-5.1:~:text=in
               the option.-,Lifetime,-32-bit unsigned>).  I reviewed the code and can see where the RDNSS lifetime is being saved<https://github.com/systemd/systemd-stable/blob/4a31fa2fb040005b73253da75cf84949b8485175/src/network/networkd-ndisc.c#L712>, though I was unable to determine how it was being handled upon expiry.

How do I configure networkd so that the RA's RDNSS lifetime is honoured?

Here is a summary of the simple protocol exchange:


  1.  Router:  Send RA [RDNSS address of "nameserver60s", lifetime: "60"]
  2.  Host: "resolvectl" shows the link's DNS server now lists the RDNSS address of "nameserver60s"
  3.  ** Wait for more than 60 seconds - the RDNSS entry should expire **
  4.  Host:
     *   "resolvectl" continues to list the address of "nameserver60s" on the link.
     *   Using tcpdump to trace "ping test.example.com", the "nameserver60s" is still being used.  It never timed out.

Here is my network configuration, showing UseDNS and UseDomains both set to "yes":

------------------------------------------------------------
$ cat /etc/systemd/network/10-eno0.network
[Match]
KernelCommandLine=!nfsroot
Name=eno0

[DHCP]
ClientIdentifier=mac
RouteMetric=10
UseDomains=yes
UseHostname=yes
UseMTU=yes

[Network]
#DHCP=ipv6
Address=10.1.1.1/24
#DNS=1.2.3.6
Gateway=1.1.1.2
IPv6AcceptRA=yes

[IPv6AcceptRA]
UseDNS=yes
UseDomains=yes
------------------------------------------------------------

Grateful for any suggestions.

Kind regards,
Matt.
PS: We're on systemd 250.  I've searched later versions of the release notes<https://github.com/systemd/systemd/releases> and it seems there have been no changes in this area.


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