Edit CLUSTER_DOMAIN environment variable support (#3283)

* Edit CLUSTER_DOMAIN environment variable support for custom cluster domains

This change modifies the getClusterDomainName() function to prioritize
the CLUSTER_DOMAIN environment variable over /etc/resolv.conf parsing,
enabling explicit configuration of cluster domains.

This is particularly useful when using service mesh configurations where
the internal DNS domain (e.g., mesh.net) differs from the Kubernetes
cluster domain.

Changes:
- Modified getClusterDomainName() to check CLUSTER_DOMAIN env var first
- Falls back to /etc/resolv.conf parsing if env var is not set
- Updated tests to validate precedence behavior
- Added test cases for custom domain scenarios

* Edit CLUSTER_DOMAIN environment variable support for custom cluster domains

This change modifies the getClusterDomainName() function to prioritize
the CLUSTER_DOMAIN environment variable over /etc/resolv.conf parsing,
enabling explicit configuration of cluster domains.

This is particularly useful when using service mesh configurations where
the internal DNS domain (e.g., mesh.net) differs from the Kubernetes
cluster domain.

Changes:
- Modified getClusterDomainName() to check CLUSTER_DOMAIN env var first
- Falls back to /etc/resolv.conf parsing if env var is not set
- Updated tests to validate precedence behavior
- Added test cases for custom domain scenarios

Author: prashanthjos

network/domain.go

@@ -56,7 +56,12 @@ func GetClusterDomainName() string {
 }
 
 func getClusterDomainName(r io.Reader) string {
-	// First look in the conf file.
+	// First check the ENV variable (allows override).
+	if domain := os.Getenv(clusterDomainEnvKey); len(domain) > 0 {
+		return domain
+	}
+
+	// Then look in the conf file.
 	for scanner := bufio.NewScanner(r); scanner.Scan(); {
 		elements := strings.Split(scanner.Text(), " ")
 		if elements[0] != "search" {
@@ -69,11 +74,6 @@ func getClusterDomainName(r io.Reader) string {
 		}
 	}
 
-	// Then look in the ENV.
-	if domain := os.Getenv(clusterDomainEnvKey); len(domain) > 0 {
-		return domain
-	}
-
 	// For all abnormal cases return default domain name.
 	return defaultDomainName
 }

network/domain_test.go

@@ -36,16 +36,16 @@ options ndots:5
 `,
 		want: "abc.com",
 	}, {
-		name: "all good with env set but ignored",
+		name: "env variable takes precedence over resolv.conf",
 		resolvConf: `
 nameserver 1.1.1.1
 search default.svc.abc.com svc.abc.com abc.com
 options ndots:5
 `,
-		env:  "ignored.com",
-		want: "abc.com",
+		env:  "mesh.net",
+		want: "mesh.net",
 	}, {
-		name:       "all good from env",
+		name:       "env variable used when resolv.conf is empty",
 		resolvConf: ``,
 		env:        "abc.com",
 		want:       "abc.com",