net: discard unrooted 254 byte names, not rooted ones

Fixes #54285

Change-Id: I20d4d6b9d0532d8a344582b99d446352ae94edcf
GitHub-Last-Rev: ad13374
GitHub-Pull-Request: #54293
Reviewed-on: https://go-review.googlesource.com/c/go/+/421674
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Run-TryBot: Damien Neil <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>

Author: mateusz834

src/net/dnsclient_unix.go

@@ -482,7 +482,7 @@ func (conf *dnsConfig) nameList(name string) []string {
 	// Check name length (see isDomainName).
 	l := len(name)
 	rooted := l > 0 && name[l-1] == '.'
-	if l > 254 || l == 254 && rooted {
+	if l > 254 || l == 254 && !rooted {
 		return nil
 	}
 

src/net/dnsclient_unix_test.go

@@ -2216,3 +2216,131 @@ func TestDNSPacketSize(t *testing.T) {
 		t.Errorf("lookup failed: %v", err)
 	}
 }
+
+func TestLongDNSNames(t *testing.T) {
+	const longDNSsuffix = ".go.dev."
+	const longDNSsuffixNoEndingDot = ".go.dev"
+
+	var longDNSPrefix = strings.Repeat("verylongdomainlabel.", 20)
+
+	var longDNSNamesTests = []struct {
+		req  string
+		fail bool
+	}{
+		{req: longDNSPrefix[:255-len(longDNSsuffix)] + longDNSsuffix, fail: true},
+		{req: longDNSPrefix[:254-len(longDNSsuffix)] + longDNSsuffix},
+		{req: longDNSPrefix[:253-len(longDNSsuffix)] + longDNSsuffix},
+
+		{req: longDNSPrefix[:253-len(longDNSsuffixNoEndingDot)] + longDNSsuffixNoEndingDot},
+		{req: longDNSPrefix[:254-len(longDNSsuffixNoEndingDot)] + longDNSsuffixNoEndingDot, fail: true},
+	}
+
+	fake := fakeDNSServer{
+		rh: func(_, _ string, q dnsmessage.Message, _ time.Time) (dnsmessage.Message, error) {
+			r := dnsmessage.Message{
+				Header: dnsmessage.Header{
+					ID:       q.Header.ID,
+					Response: true,
+					RCode:    dnsmessage.RCodeSuccess,
+				},
+				Questions: q.Questions,
+				Answers: []dnsmessage.Resource{
+					{
+						Header: dnsmessage.ResourceHeader{
+							Name:  q.Questions[0].Name,
+							Type:  q.Questions[0].Type,
+							Class: dnsmessage.ClassINET,
+						},
+					},
+				},
+			}
+
+			switch q.Questions[0].Type {
+			case dnsmessage.TypeA:
+				r.Answers[0].Body = &dnsmessage.AResource{A: TestAddr}
+			case dnsmessage.TypeAAAA:
+				r.Answers[0].Body = &dnsmessage.AAAAResource{AAAA: TestAddr6}
+			case dnsmessage.TypeTXT:
+				r.Answers[0].Body = &dnsmessage.TXTResource{TXT: []string{"."}}
+			case dnsmessage.TypeMX:
+				r.Answers[0].Body = &dnsmessage.MXResource{
+					MX: dnsmessage.MustNewName("go.dev."),
+				}
+			case dnsmessage.TypeNS:
+				r.Answers[0].Body = &dnsmessage.NSResource{
+					NS: dnsmessage.MustNewName("go.dev."),
+				}
+			case dnsmessage.TypeSRV:
+				r.Answers[0].Body = &dnsmessage.SRVResource{
+					Target: dnsmessage.MustNewName("go.dev."),
+				}
+			default:
+				panic("unknown dnsmessage type")
+			}
+
+			return r, nil
+		},
+	}
+
+	r := &Resolver{PreferGo: true, Dial: fake.DialContext}
+
+	methodTests := []string{"CNAME", "Host", "IP", "IPAddr", "MX", "NS", "NetIP", "SRV", "TXT"}
+	query := func(t string, req string) error {
+		switch t {
+		case "CNAME":
+			_, err := r.LookupCNAME(context.Background(), req)
+			return err
+		case "Host":
+			_, err := r.LookupHost(context.Background(), req)
+			return err
+		case "IP":
+			_, err := r.LookupIP(context.Background(), "ip", req)
+			return err
+		case "IPAddr":
+			_, err := r.LookupIPAddr(context.Background(), req)
+			return err
+		case "MX":
+			_, err := r.LookupMX(context.Background(), req)
+			return err
+		case "NS":
+			_, err := r.LookupNS(context.Background(), req)
+			return err
+		case "NetIP":
+			_, err := r.LookupNetIP(context.Background(), "ip", req)
+			return err
+		case "SRV":
+			const service = "service"
+			const proto = "proto"
+			req = req[len(service)+len(proto)+4:]
+			_, _, err := r.LookupSRV(context.Background(), service, proto, req)
+			return err
+		case "TXT":
+			_, err := r.LookupTXT(context.Background(), req)
+			return err
+		}
+		panic("unknown query method")
+	}
+
+	for i, v := range longDNSNamesTests {
+		for _, testName := range methodTests {
+			err := query(testName, v.req)
+			if v.fail {
+				if err == nil {
+					t.Errorf("%v: Lookup%v: unexpected success", i, testName)
+					break
+				}
+
+				expectedErr := DNSError{Err: errNoSuchHost.Error(), Name: v.req, IsNotFound: true}
+				var dnsErr *DNSError
+				errors.As(err, &dnsErr)
+				if dnsErr == nil || *dnsErr != expectedErr {
+					t.Errorf("%v: Lookup%v: unexpected error: %v", i, testName, err)
+				}
+				break
+			}
+			if err != nil {
+				t.Errorf("%v: Lookup%v: unexpected error: %v", i, testName, err)
+			}
+		}
+	}
+}