Syslog flooded with systemd-resolved messages "Using degraded feature set"

Lately my syslog is being flooded with these messages:

localhost systemd-resolved[1632]: Using degraded feature set (TCP) for DNS server 127.0.1.1.

I'm on Ubuntu 17.04. resolv.conf is auto-configured, i.e. I did not mess with it.

Question: How can I stop the flooding of my syslog? It grows to several gigabytes, filling up my root partition.

I had an old record lying in my /etc/resolvconf.conf that injected 127.0.0.1 whenever network went up:

# /etc/resolvconf.conf
# Configuration for resolvconf(8)
# See resolvconf.conf(5) for details

resolv_conf=/etc/resolv.conf
# If you run a local name server, you should uncomment the below line and
# configure your subscribers configuration files below.
name_servers=127.0.0.1

So commented it out, then ran resolvconf -u and my issue was fixed. Currently using systemd-networkd, so dont't know for sure if this will help OP's specific case.

However, NetworkManager has a setting to make it stop updating resolv.conf:

[main]
dns=none

More information in another question at network manager: how to stop nm updating /etc/resolv.conf

Update: The same kind of message appears when my upstream DNS servers are down/not responding. However, containing the upstream IP addresses on a round-robin basis between the server alternatives.

Update #2: When using systemd-networkd, if you've opted to use the following DNS settings for your interface in /etc/systemd/network/<iface>.network:

[DHCP]
UseDNS=true

which is the default setting, then it will not be using your local systemd-resolved instance, but instead use the a server from DHCP. Set UseDNS=false to use your systemd-resolver. Read man systemd.network for detailed information.