Add basic checking of mx records for mailto links

htmltest/check-link.go

@@ -5,6 +5,7 @@ import (
 	"github.com/wjdp/htmltest/issues"
 	"github.com/wjdp/htmltest/output"
 	"golang.org/x/net/html"
+	"net"
 	"net/http"
 	"net/url"
 	"os"
@@ -349,6 +350,71 @@ func (hT *HTMLTest) checkMailto(ref *htmldoc.Reference) {
 		})
 		return
 	}
+
+	// split off domain, check mx, fallback to A or AAAA if that fails
+	var dnserr *net.DNSError
+	var ok bool
+
+	domain := strings.Split(ref.URL.Opaque, "@")[1]
+
+	// loop over the current domain until we have a valid result or have exhausted all possibilities
+	for domain != "" {
+		// if a simple MX lookup works, we are done, continue
+		if _, err := net.LookupMX(domain); err == nil {
+			break // success, time to exit
+		} else if dnserr, ok = err.(*net.DNSError); !ok || dnserr.Err != "no such host" {
+			// this isn't an error we are expecting to see here
+			hT.issueStore.AddIssue(issues.Issue{
+				Level:     issues.LevelWarning,
+				Message:   "unable to perform LookupMX, unknown error",
+				Reference: ref,
+			})
+			return
+		}
+
+		// do we have to restart because of a CNAME
+		if cname, err := net.LookupCNAME(domain); err == nil && cname != domain {
+			// we have a valid CNAME, try with that. Loops return NXDOMAIN by default
+			domain = cname
+			continue
+
+		} else if dnserr, ok = err.(*net.DNSError); !ok || dnserr.Err != "no such host" {
+			// this isn't an error we are expecting to see here
+			hT.issueStore.AddIssue(issues.Issue{
+				Level:     issues.LevelWarning,
+				Message:   "unable to perform LookupCNAME, unknown error",
+				Reference: ref,
+			})
+			return
+		}
+
+		// an A or AAAA record here would be valid
+		if _, err := net.LookupHost(domain); err == nil {
+			break // its not ideal, but a valid A/AAAA record is acceptable for email
+		} else {
+			dnserr, ok = err.(*net.DNSError)
+			if !ok || dnserr.Err != "no such host" {
+				// we shouldn't see this here
+				hT.issueStore.AddIssue(issues.Issue{
+					Level:     issues.LevelWarning,
+					Message:   "unable to perform LookupHost, unknown error",
+					Reference: ref,
+				})
+				return
+			}
+
+			if dnserr.Err == "no such host" {
+				// represents NXDOMAIN or no records
+				hT.issueStore.AddIssue(issues.Issue{
+					Level:     issues.LevelError,
+					Message:   "email domain could not be resolved correctly",
+					Reference: ref,
+				})
+				return
+			}
+		}
+
+	}
 }
 
 func (hT *HTMLTest) checkTel(ref *htmldoc.Reference) {

htmltest/check-link_test.go

@@ -383,13 +383,44 @@ func TestMailtoBlank(t *testing.T) {
 	tExpectIssue(t, hT, "mailto is empty", 1)
 }
 
-func TestMailtoInvalid(t *testing.T) {
+func TestMailtoInvalidFormat(t *testing.T) {
 	// fails for invalid mailto links
 	hT := tTestFile("fixtures/links/invalid_mailto_link.html")
 	tExpectIssueCount(t, hT, 1)
 	tExpectIssue(t, hT, "contains an invalid email address", 1)
 }
 
+func TestMailtoInvalidCname(t *testing.T) {
+	// rubdomain with just a CNAME record without MX pointers
+	// email is still deliverable so should be accepted
+	// the records have been carefully created to ensure this is valid and will later fall under one of the other
+	// pass conditions
+	hT := tTestFile("fixtures/links/invalid_mailto_cname.html")
+	tExpectIssueCount(t, hT, 0)
+}
+
+func TestMailtoInvalidCnameLoop(t *testing.T) {
+	// subdomain with just a CNAME record without MX records in a loop
+	// should be detected and fail as we can never resolve this
+	hT := tTestFile("fixtures/links/invalid_mailto_cname_loop.html")
+	tExpectIssueCount(t, hT, 1)
+	tExpectIssue(t, hT, "email domain could not be resolved correctly", 1)
+}
+
+func TestMailtoInvalidNoRecords(t *testing.T) {
+	// domain with no records associated at all, must fail as completely unroutable
+	hT := tTestFile("fixtures/links/invalid_mailto_norecords.html")
+	tExpectIssueCount(t, hT, 1)
+	tExpectIssue(t, hT, "email domain could not be resolved correctly", 1)
+}
+
+func TestMailtoInvalidAFallback(t *testing.T) {
+	// domain without MX records but with a valid A record
+	// email can be routed to the A record so should be accepted
+	hT := tTestFile("fixtures/links/invalid_mailto_fallback.html")
+	tExpectIssueCount(t, hT, 0)
+}
+
 func TestMailtoIgnore(t *testing.T) {
 	// ignores mailto links when told to
 	hT := tTestFileOpts("fixtures/links/blank_mailto_link.html",

htmltest/fixtures/links/invalid_mailto_cname.html

@@ -0,0 +1,10 @@
+<html>
+
+<body>
+
+<!-- www.golang.org CNAME golang.org - seems appropriate -->
+<a href="mailto:helloworld@cname_to_a_with_no_mx.dtaylor.uk">Meow me</a>
+
+</body>
+
+</html>

htmltest/fixtures/links/invalid_mailto_cname_loop.html

@@ -0,0 +1,10 @@
+<html>
+
+<body>
+
+<!-- www.golang.org CNAME golang.org - seems appropriate -->
+<a href="mailto:[email protected]">Meow me</a>
+
+</body>
+
+</html>

htmltest/fixtures/links/invalid_mailto_fallback.html

@@ -0,0 +1,9 @@
+<html>
+
+<body>
+
+<a href="mailto:[email protected]">Meow me</a>
+
+</body>
+
+</html>

htmltest/fixtures/links/invalid_mailto_norecords.html

@@ -0,0 +1,9 @@
+<html>
+
+<body>
+
+<a href="mailto:[email protected]">Meow me</a>
+
+</body>
+
+</html>