fix: Allow use of valid Port 0

Port 0 is a normal but reserved port.
See: https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml
or RFC6335 Section 6

CHANGELOG.next.asciidoc

@@ -46,6 +46,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 - Fix high IO and handling of a corrupted registry log file. {pull}35893[35893]
 - Filebeat, when running with Elastic-Agent, reports status for Filestream input. {pull}40121[40121]
 - Implement Elastic Agent status and health reporting for Winlog Filebeat input. {pull}40163[40163]
+- Fix the parsing of port 0
 
 *Heartbeat*
 

libbeat/processors/communityid/communityid.go

@@ -153,7 +153,7 @@ func (p *processor) buildFlow(event *beat.Event) *flowhash.Flow {
 			return nil
 		}
 		sp, ok := tryToUint(v)
-		if !ok || sp < 1 || sp > 65535 {
+		if !ok || sp > 65535 {
 			return nil
 		}
 		flow.SourcePort = uint16(sp)
@@ -164,7 +164,7 @@ func (p *processor) buildFlow(event *beat.Event) *flowhash.Flow {
 			return nil
 		}
 		dp, ok := tryToUint(v)
-		if !ok || dp < 1 || dp > 65535 {
+		if !ok || dp > 65535 {
 			return nil
 		}
 		flow.DestinationPort = uint16(dp)

libbeat/processors/communityid/communityid_test.go

@@ -67,12 +67,6 @@ func TestRun(t *testing.T) {
 		testProcessor(t, 0, e, nil)
 	})
 
-	t.Run("invalid source port", func(t *testing.T) {
-		e := evt()
-		e.Put("source.port", 0)
-		testProcessor(t, 0, e, nil)
-	})
-
 	t.Run("invalid source port1", func(t *testing.T) {
 		e := evt()
 		e.Put("source.port", 123456)
@@ -142,6 +136,12 @@ func TestRun(t *testing.T) {
 		testProcessor(t, 0, e, "1:D3t8Q1aFA6Ev0A/AO4i9PnU3AeI=")
 	})
 
+	t.Run("valid source port 0", func(t *testing.T) {
+		e := evt()
+		e.Put("source.port", 0)
+		testProcessor(t, 0, e, "1:yrNkRN7VyfVz1Wh12tjRHhxERxM=")
+	})
+
 	t.Run("iana number", func(t *testing.T) {
 		e := evt()
 		e.Delete("network.transport")