CentOS - Oct 2021 - Unexpected /etc/resolv.conf updates on CentOS 7

Hi

Does here anyone know exactly when NetworkManager creates or is supposed 
to create /etc/resolv.conf for a network connection? Is there a way I 
can control it, or alternatively, is there a good way to debug the 
functionality?

I thought that there would simply be an update whenever a connection was 
established, and an addresses/network info was received (if using DHCP), 
and that the information would pretty much be left alone after that. 
However, I've lately found that a new file gets written every few hours 
even though there is no connection change (that I can detect), i.e. the 
same link is up all along. Does anyone have any ideas why that might 
happen? I thought at first that the updates could be linked to DHCP 
lease renewal, but on closer inspection, that does not seem to be the 
case. I don't see anything in the system log related to networking at 
the points where a new file gets written.

I get the above behaviour for my home Wifi net. It seemed to start after 
I switched to a new router, but that might be coincidental. It's a 
problem for me because I'm also using "commercial" VPN software
(not
integrated with NetworkManager) that will create it's own resolv.conf 
file; it replaces data e.g. from Network Manager when VPN is enabled, 
and restores it on disable. If NetworkManager "refreshes" the 
information in the mean time, the DNS config for VPN is lost, and the 
link doesn't work as expected...

This is on a CentOS 7 system with all the latest updates.

- Toralf

If you just want to tell NM to clear off and leave your resolv.conf 
alone do the following:

If you don't want it to touch the contents of the file then remove all 
DNSx= parameters from all ifcfg files and add PEERDNS=?no? instead. Now 
you are solely responsible for setting the content of /etc/resolv.conf 
yourself.

You may also have to edit /etc/NetworkManager/NetworkManager.conf and 
add dns=none to the [main] section.

On 13/10/2021 18:24, Toralf Lund wrote:
> Hi
> 
> Does here anyone know exactly when NetworkManager creates or is supposed 
> to create /etc/resolv.conf for a network connection? Is there a way I 
> can control it, or alternatively, is there a good way to debug the 
> functionality?
> 
> I thought that there would simply be an update whenever a connection was 
> established, and an addresses/network info was received (if using DHCP), 
> and that the information would pretty much be left alone after that. 
> However, I've lately found that a new file gets written every few hours
> even though there is no connection change (that I can detect), i.e. the 
> same link is up all along. Does anyone have any ideas why that might 
> happen? I thought at first that the updates could be linked to DHCP 
> lease renewal, but on closer inspection, that does not seem to be the 
> case. I don't see anything in the system log related to networking at 
> the points where a new file gets written.
> 
> I get the above behaviour for my home Wifi net. It seemed to start after 
> I switched to a new router, but that might be coincidental. It's a 
> problem for me because I'm also using "commercial" VPN
software (not
> integrated with NetworkManager) that will create it's own resolv.conf 
> file; it replaces data e.g. from Network Manager when VPN is enabled, 
> and restores it on disable. If NetworkManager "refreshes" the 
> information in the mean time, the DNS config for VPN is lost, and the 
> link doesn't work as expected...
> 
> This is on a CentOS 7 system with all the latest updates.
> 
> - Toralf
> 
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> https://lists.centos.org/mailman/listinfo/centos
-- 
J Martin Rushton MBCS

On 10/13/2021 10:24 AM, Toralf Lund wrote:
> Does here anyone know exactly when NetworkManager creates or is 
> supposed to create /etc/resolv.conf for a network connection? Is there 
> a way I can control it, or alternatively, is there a good way to debug 
> the functionality? 
Take a look at change_resolve_conf in 
/etc/sysconfig/network-scripts/network-functions. This is invoked in a 
few places in that directory, like in ifup-post.

I'm curious to hear what you find.