Prepend to search domains instead of replacing

Previously the resolv-prepender script overwrote the search domains
with a specific set of domains, notably including the cluster domain
so cluster addresses will resolve. However, this is not desirable as
it may miss some configured search domains if they don't happen to
come in via DHCP (for example, search domains configured on an
interface via nmcli/nmstate).

This change modifies the prepender logic to also prepend the cluster
domain to the search domain list rather than overwrite it completely.
This means that we don't have to manage the full list of search
domains like we did before. We just add on to the default ones
NetworkManager writes, which should result in less confusing DNS
behavior from the script.

One caveat is that I don't know if the same method can be used for
the resolved case and I don't have any way to test that locally, so
I've left the logic there alone for now. If this turns out to be a
problem for OKD as well then someone will have to work with us to
get that behavior changed too.

templates/common/on-prem/files/NetworkManager-resolv-prepender.yaml

@@ -67,9 +67,13 @@ contents:
                 fi
             else
                 >&2 echo "NM resolv-prepender: Prepending 'nameserver $NAMESERVER_IP' to /etc/resolv.conf (other nameservers from /var/run/NetworkManager/resolv.conf)"
-                sed -e "/^search/d" \
-                    -e "/Generated by/c# Generated by KNI resolv prepender NM dispatcher script\nsearch $DOMAINS\nnameserver $NAMESERVER_IP" \
+                sed -e "/Generated by/c# Generated by KNI resolv prepender NM dispatcher script" \
                     /var/run/NetworkManager/resolv.conf > /etc/resolv.tmp
+                sed -i "s/search.*/\0\nnameserver $NAMESERVER_IP/" /etc/resolv.tmp
+                # Make sure cluster domain is first in the search list
+                sed -i "s/search \(.*\)/search {{.DNS.Spec.BaseDomain}} \1/" /etc/resolv.tmp
+                # Remove duplicate cluster domain entries
+                sed -i "s/\(search {{.DNS.Spec.BaseDomain}}.*\) {{.DNS.Spec.BaseDomain}}\( .*\|$\)/\1\2/" /etc/resolv.tmp
                 # Only leave the first 3 nameservers in /etc/resolv.conf
                 sed -i ':a $!{N; ba}; s/\(^\|\n\)nameserver/\n# nameserver/4g' /etc/resolv.tmp
                 mv -f /etc/resolv.tmp /etc/resolv.conf