Avoid issue with compression of DNS messages causing overflow

agent/dns.go

@@ -718,7 +718,10 @@ func syncExtra(index map[string]dns.RR, resp *dns.Msg) {
 func (d *DNSServer) trimTCPResponse(req, resp *dns.Msg) (trimmed bool) {
 	hasExtra := len(resp.Extra) > 0
 	// There is some overhead, 65535 does not work
-	maxSize := 64000
+	maxSize := 65533 // 64k - 2 bytes
+	// In order to compute properly, we have to avoid compress first
+	compressed := resp.Compress
+	resp.Compress = false
 
 	// We avoid some function calls and allocations by only handling the
 	// extra data when necessary.
@@ -745,6 +748,8 @@ func (d *DNSServer) trimTCPResponse(req, resp *dns.Msg) (trimmed bool) {
 			len(resp.Answer), originalNumRecords, resp.Len(), originalSize)
 
 	}
+	// Restore compression if any
+	resp.Compress = compressed
 	return truncated
 }
 

agent/dns_test.go

@@ -2800,7 +2800,8 @@ func TestDNS_TCP_and_UDP_Truncate(t *testing.T) {
 		for _, qType := range []uint16{dns.TypeANY, dns.TypeA, dns.TypeSRV} {
 			for _, question := range questions {
 				for _, protocol := range protocols {
-					t.Run(fmt.Sprintf("lookup %s %s (qType:=%d)", question, protocol, qType), func(t *testing.T) {
+					for _, compress := range []bool{true, false} {
+						t.Run(fmt.Sprintf("lookup %s %s (qType:=%d) compressed=%b", question, protocol, qType, compress), func(t *testing.T) {
 							m := new(dns.Msg)
 							m.SetQuestion(question, dns.TypeANY)
 							if protocol == "udp" {
@@ -2808,6 +2809,7 @@ func TestDNS_TCP_and_UDP_Truncate(t *testing.T) {
 							}
 							c := new(dns.Client)
 							c.Net = protocol
+							m.Compress = compress
 							in, out, err := c.Exchange(m, a.DNSAddr())
 							if err != nil && err != dns.ErrTruncated {
 								t.Fatalf("err: %v", err)
@@ -2827,6 +2829,7 @@ func TestDNS_TCP_and_UDP_Truncate(t *testing.T) {
 			}
 		}
 	}
+}
 
 func TestDNS_ServiceLookup_Truncate(t *testing.T) {
 	t.Parallel()