fix(8201): restore old behavior forward plugin continue on empty conf file (#8203)

plugin/forward/resolve.go

@@ -1,6 +1,7 @@
 package forward
 
 import (
+	"errors"
 	"fmt"
 	"net"
 	"strings"
@@ -39,9 +40,14 @@ func classifyToAddrs(toAddrs []string) ([]toEntry, error) {
 			continue
 		}
 
+		// Empty file are skip
+		if errors.Is(parseErr, parse.ErrNoNameservers) {
+			continue
+		}
+
 		// Only fall through to hostname parsing if the error specifically
 		// indicates the address is not an IP or file. Other errors (like
-		// "no nameservers found" from file parsing) should be propagated.
+		// "invalid address" from ip parsing) should be propagated.
 		if !strings.Contains(parseErr.Error(), "not an IP address or file") {
 			return nil, parseErr
 		}

plugin/forward/resolve_test.go

@@ -24,6 +24,13 @@ func TestClassifyToAddrs(t *testing.T) {
 	}
 	defer os.Remove(resolv)
 
+	// Create a commented.conf for file test
+	const commentedResolv = "commented_resolv.conf"
+	if err := os.WriteFile(commentedResolv, []byte("# nameserver 1.1.1.1\n# nameserver 1.0.0.1\n"), 0666); err != nil {
+		t.Fatal(err)
+	}
+	defer os.Remove(commentedResolv)
+
 	tests := []struct {
 		name        string
 		input       []string
@@ -84,10 +91,10 @@ func TestClassifyToAddrs(t *testing.T) {
 			wantDynamic: 1,
 		},
 		{
-			name:        "/dev/null returns file error",
-			input:       []string{"/dev/null"},
-			wantErr:     true,
-			errContains: "no nameservers",
+			name:        "an existing file not empty but without nameserver is skipped",
+			input:       []string{commentedResolv},
+			wantDynamic: 0,
+			wantStatic:  0,
 		},
 	}
 

plugin/forward/setup_test.go

@@ -242,6 +242,14 @@ nameserver 10.10.255.253`), 0666); err != nil {
 	}
 	defer os.Remove(resolvIPV6)
 
+	const emptyResolv = "empty.conf"
+	if err := os.WriteFile(emptyResolv,
+		[]byte(`# nameserver 1.1.1.1
+# nameserver 1.0.0.1`), 0666); err != nil {
+		t.Fatalf("Failed to write empty.conf file: %s", err)
+	}
+	defer os.Remove(emptyResolv)
+
 	tests := []struct {
 		input         string
 		shouldErr     bool
@@ -251,9 +259,11 @@ nameserver 10.10.255.253`), 0666); err != nil {
 		// pass
 		{`forward . ` + resolv, false, "", []string{"10.10.255.252:53", "10.10.255.253:53"}},
 		// fail
-		{`forward . /dev/null`, true, "no nameservers", nil},
+		{`forward . /dev/null`, true, "no valid upstream addresses found", nil},
 		// IPV6 with local zone
 		{`forward . ` + resolvIPV6, false, "", []string{"[0388:d254:7aec:6892:9f7f:e93b:5806:1b0f]:53"}},
+		// pass when empty forward file is found
+		{`forward . ` + emptyResolv + ` 127.0.0.1`, false, "", []string{"127.0.0.1:53"}},
 	}
 
 	for i, test := range tests {
@@ -517,6 +527,7 @@ func TestMultiForward(t *testing.T) {
 		t.Error("expected third plugin to be last, but Next is not nil")
 	}
 }
+
 func TestNextAlternate(t *testing.T) {
 	testsValid := []struct {
 		input    string

plugin/pkg/parse/host.go

@@ -54,7 +54,7 @@ func HostPortOrFile(s ...string) ([]string, error) {
 					servers = append(servers, ss...)
 					continue
 				}
-				return servers, fmt.Errorf("not an IP address or file: %q", host)
+				return servers, fmt.Errorf("not an IP address or file %q: %w", host, err)
 			}
 			var ss string
 			switch trans {
@@ -79,7 +79,7 @@ func HostPortOrFile(s ...string) ([]string, error) {
 				servers = append(servers, ss...)
 				continue
 			}
-			return servers, fmt.Errorf("not an IP address or file: %q", host)
+			return servers, fmt.Errorf("not an IP address or file %q: %w", host, err)
 		}
 		servers = append(servers, h)
 	}
@@ -92,13 +92,13 @@ func HostPortOrFile(s ...string) ([]string, error) {
 // Try to open this is a file first.
 func tryFile(s string) ([]string, error) {
 	c, err := dns.ClientConfigFromFile(s)
-	if err == os.ErrNotExist {
+	if errors.Is(err, os.ErrNotExist) {
 		return nil, fmt.Errorf("failed to open file %q: %q", s, err)
 	} else if err != nil {
 		return nil, err
 	}
 
-	servers := []string{}
+	var servers []string
 	for _, s := range c.Servers {
 		servers = append(servers, net.JoinHostPort(stripZone(s), c.Port))
 	}