refactor(kuma-dp): put Corefile into embedded FS (#8117)

Signed-off-by: Mike Beaumont <mjboamail@gmail.com>

app/kuma-dp/pkg/dataplane/dnsserver/Corefile

@@ -0,0 +1,15 @@
+.:{{ .CoreDNSPort }} {
+    forward . 127.0.0.1:{{ .EnvoyDNSPort }}
+    # We want all requests to be sent to the Envoy DNS Filter, unsuccessful responses should be forwarded to the original DNS server.
+    # For example: requests other than A, AAAA and SRV will return NOTIMP when hitting the envoy filter and should be sent to the original DNS server.
+    # Codes from: https://github.com/miekg/dns/blob/master/msg.go#L138
+    alternate NOTIMP,FORMERR,NXDOMAIN,SERVFAIL,REFUSED . /etc/resolv.conf
+    prometheus localhost:{{ .PrometheusPort }}
+    errors
+}
+
+.:{{ .CoreDNSEmptyPort }} {
+    template ANY ANY . {
+      rcode NXDOMAIN
+    }
+}

app/kuma-dp/pkg/dataplane/dnsserver/config_file.go

@@ -1,6 +1,7 @@
 package dnsserver
 
 import (
+	"embed"
 	"os"
 	"path/filepath"
 
@@ -9,6 +10,9 @@ import (
 	kuma_dp "github.com/kumahq/kuma/pkg/config/app/kuma-dp"
 )
 
+//go:embed Corefile
+var config embed.FS
+
 func GenerateConfigFile(cfg kuma_dp.DNS, config []byte) (string, error) {
 	configFile := filepath.Join(cfg.ConfigDir, "Corefile")
 	if err := writeFile(configFile, config, 0o600); err != nil {

app/kuma-dp/pkg/dataplane/dnsserver/dnsserver.go

@@ -36,23 +36,6 @@ type Opts struct {
 	Quit   chan struct{}
 }
 
-// DefaultCoreFileTemplate defines the template to use to configure coreDNS to use the envoy dns filter.
-const DefaultCoreFileTemplate = `.:{{ .CoreDNSPort }} {
-    forward . 127.0.0.1:{{ .EnvoyDNSPort }}
-    # We want all requests to be sent to the Envoy DNS Filter, unsuccessful responses should be forwarded to the original DNS server.
-    # For example: requests other than A, AAAA and SRV will return NOTIMP when hitting the envoy filter and should be sent to the original DNS server.
-    # Codes from: https://github.com/miekg/dns/blob/master/msg.go#L138
-    alternate NOTIMP,FORMERR,NXDOMAIN,SERVFAIL,REFUSED . /etc/resolv.conf
-    prometheus localhost:{{ .PrometheusPort }}
-    errors
-}
-
-.:{{ .CoreDNSEmptyPort }} {
-    template ANY ANY . {
-      rcode NXDOMAIN
-    }
-}`
-
 func lookupDNSServerPath(configuredPath string) (string, error) {
 	return files.LookupBinaryPath(
 		files.LookupInPath(configuredPath),
@@ -108,7 +91,11 @@ func (s *DNSServer) Start(stop <-chan struct{}) error {
 
 		tmpl = t
 	} else {
-		t, err := template.New("Corefile").Parse(DefaultCoreFileTemplate)
+		corefile, err := config.ReadFile("Corefile")
+		if err != nil {
+			return errors.Wrap(err, "couldn't open embedded Corefile")
+		}
+		t, err := template.New("Corefile").Parse(string(corefile))
 		if err != nil {
 			return err
 		}

app/kuma-dp/pkg/dataplane/dnsserver/dnsserver_test.go

@@ -132,7 +132,8 @@ var _ = Describe("DNS Server", func() {
     template ANY ANY . {
       rcode NXDOMAIN
     }
-}`))
+}
+`))
 		}))
 
 		It("should return an error if DNS Server crashes", test.Within(10*time.Second, func() {