[Dnsmasq-discuss] No servers after coming back from sleep (was dnsmasq 2.86 crash)

[Eloy Paris]

On Tue, Oct 19, 2021 at 01:13:46PM -0400, Eloy Paris wrote:

> I am seeing the new issue happen very often -- the machine goes to sleep
> and when it comes back it seems like dnsmasq does not have upstream
> servers to forward requests to, so the virtual machine that relies on
> dnsmasq for DNS resolution cannot resolve anything.

I've done some troubleshooting of this and my /etc/resolv.conf seems
stable when the machine comes back from sleep and dnsmasq reads it.
However, for some reason the servers there don't seem to be added to
the daemon->servers linked list.

dnsmasq.c:poll_resolv() has:

----------------------------------------------------------------------
  if (latest)
    {
      static int warned = 0;
      if (reload_servers(latest->name))
        {
          my_syslog(LOG_INFO, _("reading %s"), latest->name);
          warned = 0;
          check_servers(0);
----------------------------------------------------------------------

I instrumented check_servers(), as that is what logs "using nameserver
xyz", and my syslog has this in the working case (before I put the
machine to sleep):

Oct 20 05:16:32 chapilu dnsmasq[167055]: /etc/resolv.conf: search example.com
Oct 20 05:16:32 chapilu dnsmasq[167055]: /etc/resolv.conf: nameserver 1.2.3.4
Oct 20 05:16:32 chapilu dnsmasq[167055]: /etc/resolv.conf: nameserver 1.2.3.5
Oct 20 05:16:32 chapilu dnsmasq[167055]: /etc/resolv.conf: nameserver 1.2.3.6
Oct 20 05:16:32 chapilu dnsmasq[167055]: reading /etc/resolv.conf
Oct 20 05:16:32 chapilu dnsmasq[167055]: check_servers(): Server #1: domain = , interface =
Oct 20 05:16:32 chapilu dnsmasq[167055]: using nameserver 1.2.3.4#53
Oct 20 05:16:32 chapilu dnsmasq[167055]: check_servers(): Server #2: domain = , interface =
Oct 20 05:16:32 chapilu dnsmasq[167055]: using nameserver 1.2.3.5#53
Oct 20 05:16:32 chapilu dnsmasq[167055]: check_servers(): Server #3: domain = , interface =
Oct 20 05:16:32 chapilu dnsmasq[167055]: using nameserver 1.2.3.6#53
Oct 20 05:16:32 chapilu dnsmasq[167055]: check_servers(): 3 servers in daemon->servers
Oct 20 05:16:32 chapilu dnsmasq[167055]: check_servers(): 0 servers in daemon->local_domains

(Doing "touch /etc/resolv.conf" when things are working [before I put
the machine to sleep], produces the above as well.)

However, when I put the machine to sleep, and later resume, I get this:

Oct 20 05:17:26 chapilu dnsmasq[167055]: /etc/resolv.conf: # Generated by NetworkManager
Oct 20 05:17:26 chapilu dnsmasq[167055]: /etc/resolv.conf: search example.com
Oct 20 05:17:26 chapilu dnsmasq[167055]: /etc/resolv.conf: nameserver 1.2.3.4
Oct 20 05:17:26 chapilu dnsmasq[167055]: /etc/resolv.conf: nameserver 1.2.3.5
Oct 20 05:17:26 chapilu dnsmasq[167055]: /etc/resolv.conf: nameserver 1.2.3.6
Oct 20 05:17:26 chapilu dnsmasq[167055]: reading /etc/resolv.conf
Oct 20 05:17:26 chapilu dnsmasq[167055]: check_servers(): 0 servers in daemon->servers
Oct 20 05:17:26 chapilu dnsmasq[167055]: check_servers(): 0 servers in daemon->local_domains

So it seems like after the resume from sleep, /etc/resolv.conf is
stable, has the correct upstream DNS servers, but somehow
daemon->servers ends up with nothing; what could cause this?

Cheers,

Eloy Paris.-