GCP: Prepend resolv.conf on the bootstrap node with custom-dns

When UserProvisionedDNS is enabled during an install on GCP, prepend
the NetworkManager generated resolv.conf file on the bootstrap node
with the IP of the localhost.

data/data/bootstrap/gcp/files/etc/NetworkManager/dispatcher.d/30-local-dns-prepender.template

@@ -0,0 +1,53 @@
+#!/bin/bash
+IFACE=$1
+STATUS=$2
+case "$STATUS" in
+    up|dhcp4-change|dhcp6-change|dns-change)
+{{if .PlatformData.GCP.UserProvisionedDNS}}
+    logger -s "NM local-dns-prepender triggered by ${1} ${2}."
+
+    # In DHCP connections, the resolv.conf content may be late, thus we wait for nameservers
+    timeout 45s /bin/bash <<EOF
+        if [[ "$STATUS" == dhcp* ]]; then
+            >&2 echo  "NM resolv-prepender: Checking for nameservers in /var/run/NetworkManager/resolv.conf"
+            while ! grep nameserver /var/run/NetworkManager/resolv.conf; do
+                >&2 echo  "NM resolv-prepender: NM resolv.conf still empty of nameserver"
+                sleep 0.5
+            done
+        fi
+EOF
+
+    DNS_IP="127.0.0.1"
+    set +e
+    if systemctl -q is-enabled systemd-resolved; then
+        >&2 echo "NM resolv-prepender: Setting up systemd-resolved for local DNS"
+        if [[ ! -f /etc/systemd/resolved.conf.d/60-kni.conf ]]; then
+            >&2 echo "NM resolv-prepender: Creating /etc/systemd/resolved.conf.d/60-kni.conf"
+            mkdir -p /etc/systemd/resolved.conf.d
+            echo "[Resolve]" > /etc/systemd/resolved.conf.d/60-kni.conf
+            echo "DNS=$DNS_IP" >> /etc/systemd/resolved.conf.d/60-kni.conf
+            echo "Domains=api.{{.ClusterDomain}} api-int.{{.ClusterDomain}} apps.{{.ClusterDomain}}" >> \
+                /etc/systemd/resolved.conf.d/60-kni.conf
+            if systemctl -q is-active systemd-resolved; then
+                >&2 echo "NM resolv-prepender: restarting systemd-resolved"
+                systemctl restart systemd-resolved
+            fi
+        fi
+    else
+	cp -f /var/run/NetworkManager/resolv.conf /etc/resolv.tmp
+        sed -i "/^# Generated by.*$/a nameserver $DNS_IP" /etc/resolv.tmp
+        if cmp -s /etc/resolv.tmp /etc/resolv.conf; then
+            logger -s "NM local-dns-prepender: /etc/resolv.conf is already up to date"
+            rm -f /etc/resolv.tmp
+            exit 0
+
+        else
+	    logger -s "NM local-dns-prepender: overwriting /etc/resolv.conf to add local DNS IP and DNS servers obtained by DHCP"
+            mv -f /etc/resolv.tmp /etc/resolv.conf
+        fi
+    fi
+{{end}}
+    ;;
+    *)
+    ;;
+esac

pkg/asset/ignition/bootstrap/common.go

@@ -29,6 +29,7 @@ import (
 	"github.com/openshift/installer/pkg/asset"
 	"github.com/openshift/installer/pkg/asset/ignition"
 	"github.com/openshift/installer/pkg/asset/ignition/bootstrap/baremetal"
+	"github.com/openshift/installer/pkg/asset/ignition/bootstrap/gcp"
 	"github.com/openshift/installer/pkg/asset/ignition/bootstrap/vsphere"
 	mcign "github.com/openshift/installer/pkg/asset/ignition/machine"
 	"github.com/openshift/installer/pkg/asset/installconfig"
@@ -40,6 +41,7 @@ import (
 	"github.com/openshift/installer/pkg/asset/tls"
 	"github.com/openshift/installer/pkg/types"
 	baremetaltypes "github.com/openshift/installer/pkg/types/baremetal"
+	gcptypes "github.com/openshift/installer/pkg/types/gcp"
 	nutanixtypes "github.com/openshift/installer/pkg/types/nutanix"
 	vspheretypes "github.com/openshift/installer/pkg/types/vsphere"
 )
@@ -99,6 +101,7 @@ type bootstrapTemplateData struct {
 type platformTemplateData struct {
 	BareMetal *baremetal.TemplateData
 	VSphere   *vsphere.TemplateData
+	GCP       *gcp.TemplateData
 }
 
 // Common is an asset that generates the ignition config for bootstrap nodes.
@@ -314,6 +317,8 @@ func (a *Common) getTemplateData(dependencies asset.Parents, bootstrapInPlace bo
 			ironicCreds.Password,
 			dependencies,
 		)
+	case gcptypes.Name:
+		platformData.GCP = gcp.GetTemplateData(installConfig.Config.Platform.GCP)
 	case vspheretypes.Name:
 		platformData.VSphere = vsphere.GetTemplateData(installConfig.Config.Platform.VSphere)
 	}

pkg/asset/ignition/bootstrap/gcp/template.go

@@ -0,0 +1,21 @@
+package gcp
+
+import (
+	"github.com/openshift/installer/pkg/types/dns"
+	"github.com/openshift/installer/pkg/types/gcp"
+)
+
+// TemplateData holds data specific to templates used for the gcp platform.
+type TemplateData struct {
+	// UserProvisionedDNS indicates whether this feature has been enabled on GCP
+	UserProvisionedDNS bool
+}
+
+// GetTemplateData returns platform-specific data for bootstrap templates.
+func GetTemplateData(config *gcp.Platform) *TemplateData {
+	var templateData TemplateData
+
+	templateData.UserProvisionedDNS = (config.UserProvisionedDNS == dns.UserProvisionedDNSEnabled)
+
+	return &templateData
+}