fix(ddns): handle second-level domain correctly (#438)

pkg/ddns/ddns.go

@@ -108,7 +108,7 @@ func splitDomainSOA(domain string) (prefix string, zone string, err error) {
 			if len(r.Answer) > 0 {
 				if soa, ok := r.Answer[0].(*dns.SOA); ok {
 					zone = soa.Hdr.Name
-					prefix = domain[:len(domain)-len(zone)-1]
+					prefix = libdns.RelativeName(domain, zone)
 					return
 				}
 			}

pkg/ddns/ddns_test.go

@@ -27,6 +27,11 @@ func TestSplitDomainSOA(t *testing.T) {
 			zone:   "example.com.",
 			prefix: "abc",
 		},
+		{
+			domain: "example.com",
+			zone:   "example.com.",
+			prefix: "",
+		},
 	}
 
 	for _, c := range cases {