net: send EDNS(0) packet length in DNS query

Advertise to DNS resolvers that we are willing and able to accept up
to 1232 bytes in a DNS packet. The value 1232 was chosen based on
https://dnsflagday.net/2020/.

For #6464
For #21160
For #44135
For #51127
Fixes #51153

Change-Id: If9182d5210bfe047cf0a4d46163effc6812ab677
Reviewed-on: https://go-review.googlesource.com/c/go/+/386016
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/net/dnsclient_unix.go

@@ -60,6 +60,19 @@ func newRequest(q dnsmessage.Question) (id uint16, udpReq, tcpReq []byte, err er
 	if err := b.Question(q); err != nil {
 		return 0, nil, nil, err
 	}
+
+	// Accept packets up to maxDNSPacketSize.  RFC 6891.
+	if err := b.StartAdditionals(); err != nil {
+		return 0, nil, nil, err
+	}
+	var rh dnsmessage.ResourceHeader
+	if err := rh.SetEDNS0(maxDNSPacketSize, dnsmessage.RCodeSuccess, false); err != nil {
+		return 0, nil, nil, err
+	}
+	if err := b.OPTResource(rh, dnsmessage.OPTResource{}); err != nil {
+		return 0, nil, nil, err
+	}
+
 	tcpReq, err = b.Finish()
 	udpReq = tcpReq[2:]
 	l := len(tcpReq) - 2

src/net/dnsclient_unix_test.go

@@ -2161,3 +2161,58 @@ func TestRootNS(t *testing.T) {
 		t.Errorf("records = [%v]; want [%v]", strings.Join(records, " "), want[0])
 	}
 }
+
+// Test that we advertise support for a larger DNS packet size.
+// This isn't a great test as it just tests the dnsmessage package
+// against itself.
+func TestDNSPacketSize(t *testing.T) {
+	fake := fakeDNSServer{
+		rh: func(_, _ string, q dnsmessage.Message, _ time.Time) (dnsmessage.Message, error) {
+			if len(q.Additionals) == 0 {
+				t.Error("missing EDNS record")
+			} else if opt, ok := q.Additionals[0].Body.(*dnsmessage.OPTResource); !ok {
+				t.Errorf("additional record type %T, expected OPTResource", q.Additionals[0])
+			} else if len(opt.Options) != 0 {
+				t.Errorf("found %d Options, expected none", len(opt.Options))
+			} else {
+				got := int(q.Additionals[0].Header.Class)
+				t.Logf("EDNS packet size == %d", got)
+				if got != maxDNSPacketSize {
+					t.Errorf("EDNS packet size == %d, want %d", got, maxDNSPacketSize)
+				}
+			}
+
+			// Hand back a dummy answer to verify that
+			// LookupIPAddr completes.
+			r := dnsmessage.Message{
+				Header: dnsmessage.Header{
+					ID:       q.Header.ID,
+					Response: true,
+					RCode:    dnsmessage.RCodeSuccess,
+				},
+				Questions: q.Questions,
+			}
+			if q.Questions[0].Type == dnsmessage.TypeA {
+				r.Answers = []dnsmessage.Resource{
+					{
+						Header: dnsmessage.ResourceHeader{
+							Name:   q.Questions[0].Name,
+							Type:   dnsmessage.TypeA,
+							Class:  dnsmessage.ClassINET,
+							Length: 4,
+						},
+						Body: &dnsmessage.AResource{
+							A: TestAddr,
+						},
+					},
+				}
+			}
+			return r, nil
+		},
+	}
+
+	r := &Resolver{PreferGo: true, Dial: fake.DialContext}
+	if _, err := r.LookupIPAddr(context.Background(), "go.dev"); err != nil {
+		t.Errorf("lookup failed: %v", err)
+	}
+}