Add SIP/JABBER labels to underscore exception list (#453)

* Improve comments in checkLabel
* Reformat labelUnderscores to make it easier to add to
* Add to exception list for label warnings
* Add underscores in hostnames to the opinions list.

pkg/normalize/validate.go

@@ -86,7 +86,15 @@ func validateRecordTypes(rec *models.RecordConfig, domain string, pTypes []strin
 
 // underscores in names are often used erroneously. They are valid for dns records, but invalid for urls.
 // here we list common records expected to have underscores. Anything else containing an underscore will print a warning.
-var labelUnderscores = []string{"_domainkey", "_dmarc", "_amazonses", "_acme-challenge"}
+var labelUnderscores = []string{
+	"_acme-challenge",
+	"_amazonses",
+	"_dmarc",
+	"_domainkey",
+	"_jabber",
+	"_sip",
+	"_xmpp",
+}
 
 // these record types may contain underscores
 var rTypeUnderscores = []string{"SRV", "TLSA", "TXT"}
@@ -106,18 +114,23 @@ func checkLabel(label string, rType string, domain string, meta map[string]strin
 			return errors.Errorf(`label %s ends with domain name %s. Record names should not be fully qualified. Add {skip_fqdn_check:"true"} to this record if you really want to make %s.%s`, label, domain, label, domain)
 		}
 	}
-	// check for underscores last
+
+	// Underscores are permitted in labels, but we print a warning unless they
+	// are used in a way we consider typical.  Yes, we're opinionated here.
+
+	// Don't warn for certain rtypes:
 	for _, ex := range rTypeUnderscores {
 		if rType == ex {
 			return nil
 		}
 	}
+	// Don't warn for certain label substrings
 	for _, ex := range labelUnderscores {
 		if strings.Contains(label, ex) {
 			return nil
 		}
 	}
-	// underscores are warnings
+	// Otherwise, warn.
 	if strings.ContainsRune(label, '_') {
 		return Warning{errors.Errorf("label %s.%s contains an underscore", label, domain)}
 	}