Checksum the header and refuse sequence numbers the sender never sent

frame_test.go

@@ -0,0 +1,66 @@
+package main
+
+import "testing"
+
+func testFrame() []byte {
+	buf := make([]byte, 512)
+	spec := newFrameSpec([6]byte{1, 2, 3, 4, 5, 6}, [6]byte{7, 8, 9, 10, 11, 12},
+		etherBase, []int{512})
+	spec.prefill(buf, 3)
+	putHeader(buf, 3, 5, 0x0123456789ab, 512-minFrame)
+	return buf
+}
+
+func TestHeaderRoundTrip(t *testing.T) {
+	p, st := parseHeader(testFrame())
+	if st != hdrOK {
+		t.Fatalf("status = %v, want hdrOK", st)
+	}
+	want := parsed{patIdx: 3, stream: 5, seq: 0x0123456789ab, payLen: 512 - minFrame}
+	if p != want {
+		t.Errorf("parsed = %+v, want %+v", p, want)
+	}
+}
+
+// The sequence number is the one field on the wire that nothing else checks, and
+// a single flipped bit in it is exactly what a marginal cable produces. Every
+// bit of the header has to be covered, including the checksum's own.
+func TestHeaderChecksumCatchesEverySingleBitFlip(t *testing.T) {
+	for byteIdx := 0; byteIdx < hdrLen; byteIdx++ {
+		for bit := 0; bit < 8; bit++ {
+			buf := testFrame()
+			buf[ethHdrLen+byteIdx] ^= 1 << bit
+
+			_, st := parseHeader(buf)
+			if st == hdrOK {
+				t.Errorf("header byte %d bit %d flipped and the frame was accepted",
+					byteIdx, bit)
+			}
+			// The magic is what says the frame is ours at all, so damage there is
+			// somebody else's traffic as far as we can tell.
+			wantForeign := byteIdx < 4
+			if wantForeign && st != hdrForeign {
+				t.Errorf("magic byte %d bit %d = %v, want hdrForeign", byteIdx, bit, st)
+			}
+			if !wantForeign && st != hdrDamaged {
+				t.Errorf("header byte %d bit %d = %v, want hdrDamaged", byteIdx, bit, st)
+			}
+		}
+	}
+}
+
+func TestHeaderShortFrameIsForeign(t *testing.T) {
+	if _, st := parseHeader(testFrame()[:minFrame-1]); st != hdrForeign {
+		t.Errorf("status = %v, want hdrForeign", st)
+	}
+}
+
+// A length that runs past the frame is damage rather than a foreign header: the
+// magic and the checksum both said the header was ours.
+func TestHeaderLengthPastFrameIsDamaged(t *testing.T) {
+	buf := testFrame()
+	putHeader(buf, 3, 5, 1, len(buf))
+	if _, st := parseHeader(buf); st != hdrDamaged {
+		t.Errorf("status = %v, want hdrDamaged", st)
+	}
+}