Add fuzzing framework (#580)

Add easy way to fuzz this dns library, put fuzz related code in fuzz.go
and have a small Makefile.fuzz to be used:

$ make -f Makefile.fuzz build
$ make -f Makefile.fuzz fuzz

Will build and fuzz the library. Both pack/unpack and NewRR are fuzz
targets, but we could open this up.

Makefile.fuzz

@@ -0,0 +1,33 @@
+# Makefile for fuzzing
+#
+# Use go-fuzz and needs the tools installed.
+# See https://blog.cloudflare.com/dns-parser-meet-go-fuzzer/
+#
+# Installing go-fuzz:
+# $ make -f Makefile.fuzz get
+# Installs:
+# * github.com/dvyukov/go-fuzz/go-fuzz
+# * get github.com/dvyukov/go-fuzz/go-fuzz-build
+
+all: build
+
+.PHONY: build
+build:
+	go-fuzz-build -tags fuzz github.com/miekg/dns
+
+.PHONY: build-newrr
+build-newrr:
+	go-fuzz-build -func FuzzNewRR -tags fuzz github.com/miekg/dns
+
+.PHONY: fuzz
+fuzz:
+	go-fuzz -bin=dns-fuzz.zip -workdir=fuzz
+
+.PHONY: get
+get:
+	go get github.com/dvyukov/go-fuzz/go-fuzz
+	go get github.com/dvyukov/go-fuzz/go-fuzz-build
+
+.PHONY: clean
+clean:
+	rm *-fuzz.zip

fuzz.go

@@ -0,0 +1,23 @@
+// +build fuzz
+
+package dns
+
+func Fuzz(data []byte) int {
+	msg := new(Msg)
+
+	if err := msg.Unpack(data); err != nil {
+		return 0
+	}
+	if _, err := msg.Pack(); err != nil {
+		return 0
+	}
+
+	return 1
+}
+
+func FuzzNewRR(data []byte) int {
+	if _, err := NewRR(string(data)); err != nil {
+		return 0
+	}
+	return 1
+}

fuzz_test.go

@@ -1,25 +0,0 @@
-package dns
-
-import "testing"
-
-func TestFuzzString(t *testing.T) {
-	testcases := []string{"", " MINFO ", "	RP ", "	NSEC 0 0", "	\" NSEC 0 0\"", "  \" MINFO \"",
-		";a ", ";a<><61><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>",
-		"	NSAP O ", "  NSAP N ",
-		" TYPE4 TYPE6a789a3bc0045c8a5fb42c7d1bd998f5444 IN 9579b47d46817afbd17273e6",
-		" TYPE45 3 3 4147994 TYPE\\(\\)\\)\\(\\)\\(\\(\\)\\(\\)\\)\\)\\(\\)\\(\\)\\(\\(\\R 948\"\")\\(\\)\\)\\)\\(\\ ",
-		"$GENERATE 0-3 ${441189,5039418474430,o}",
-		"$INCLUDE 00 TYPE00000000000n ",
-		"$INCLUDE PE4 TYPE061463623/727071511 \\(\\)\\$GENERATE 6-462/0",
-	}
-	for i, tc := range testcases {
-		rr, err := NewRR(tc)
-		if err == nil {
-			// rr can be nil because we can (for instance) just parse a comment
-			if rr == nil {
-				continue
-			}
-			t.Fatalf("parsed mailformed RR %d: %s", i, rr.String())
-		}
-	}
-}