[systemd-devel] systemd-resolved service ignores UseDNS=false

[Mikhail Morfikov]

In the systemd-resolved manual we can read something like this:

  The DNS servers contacted are determined from the global settings in
  resolved.conf(5), the per-link static settings in .network files, and
  the per-link dynamic settings received over DHCP.

1. Let's say that I have set all the three settings, which one will be
applied?

2. If I set the global settings via the /etc/systemd/resolved.conf file,
for instance:

[Resolve]
DNS=127.0.2.1
FallbackDNS=208.67.222.222 208.67.220.220

will this local resolver be used all the time, even when dhcp
server sends an ip address of other resolver in the network to
the client?

I don't know how that is supposed to work, but it looks like it doesn't
work in the way I thought it would. So I added some entries to this
/etc/systemd/network/50-bond0-dhcp.network file:

[Match]
Name=bond0

[Network]
Description=Bonded network
DHCP=ipv4
LinkLocalAddressing=no
DNS=127.0.2.1
IPForward=true

[DHCP]
UseDNS=false
UseMTU=false
SendHostname=true
UseHostname=false
UseDomains=true
UseRoutes=true
CriticalConnection=true
RequestBroadcast=true

And according to the systemd-networkd manual:

  DNS -- a DNS server address, which must be in the format described
  in inet_pton(3). This option may be specified more than once.

  UseDNS -- when true (the default), the DNS servers received from the
  DHCP server will be used and take precedence over any statically
  configured ones.

(btw, the link to inet_pton is dead)

So UseDNS is set to false, and I thought the system will be using
the local resolver, but it sometimes uses the local settings and
sometimes not -- it depends on restarting the systemd-resolved
service, for example:

# ls -al /etc/resolv.conf
lrwxrwxrwx 1 root root 32 2015-02-27 23:52:39 /etc/resolv.conf -> /run/systemd/resolve/resolv.conf

# cat /etc/resolv.conf
nameserver 127.0.2.1
nameserver 192.168.1.1
search mhouse.lh

# systemctl restart systemd-resolved.service
# cat /etc/resolv.conf
nameserver 192.168.1.1
nameserver 127.0.2.1
search mhouse.lh

(I've cut the comments for readability)

As you can see, the two resolvers just switched, and it seems that my
system tries to use the 192.168.1.1 , but I disabled it for testing on
my router. After I ping a domain, I have 10s lag, and then ping go there
and back, so it falls back to the local resolver.

3. Shouldn't there be just one resolver in the /etc/resolv.conf file?

4. How to force the system to use the one particular resolver no matter
what? I know I could probably do that by creating a static file instead
of a link (and maybe chattr +i if necessary), but I want to do this
using the systemd native tools if that is possible of course.

5. Is the /etc/resolv.conf file necessary  when using systemd?
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