Add debug.Hexdump (#1902)

Allow plugins to dump messages in text pcap to the log. The forward
plugin does this when a reply does not much the query.

If the debug plugin isn't loaded Hexdump and Hexdumpf are noop.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/debug/pcap.go

@@ -0,0 +1,72 @@
+package debug
+
+import (
+	"bytes"
+	"fmt"
+
+	"github.com/coredns/coredns/plugin/pkg/log"
+
+	"github.com/miekg/dns"
+)
+
+// Hexdump converts the dns message m to a hex dump Whireshark can import.
+// See https://www.wireshark.org/docs/man-pages/text2pcap.html.
+// This output looks like this:
+//
+// 00000 dc bd 01 00 00 01 00 00 00 00 00 01 07 65 78 61
+// 000010 6d 70 6c 65 05 6c 6f 63 61 6c 00 00 01 00 01 00
+// 000020 00 29 10 00 00 00 80 00 00 00
+// 00002a
+//
+// Hexdump will use log.Debug to write the dump to the log, each line
+// is prefixed with 'debug: ' so the data can be easily extracted.
+//
+// msg will prefix the pcap dump.
+func Hexdump(m *dns.Msg, v ...interface{}) {
+	if !log.D {
+		return
+	}
+
+	buf, _ := m.Pack()
+	if len(buf) == 0 {
+		return
+	}
+
+	out := "\n" + string(hexdump(buf))
+	v = append(v, out)
+	log.Debug(v...)
+}
+
+// Hexdumpf dumps a DNS message as Hexdump, but allows a format string.
+func Hexdumpf(m *dns.Msg, format string, v ...interface{}) {
+	if !log.D {
+		return
+	}
+
+	buf, _ := m.Pack()
+	if len(buf) == 0 {
+		return
+	}
+
+	format += "\n%s"
+	v = append(v, hexdump(buf))
+	log.Debugf(format, v...)
+}
+
+func hexdump(data []byte) []byte {
+	b := new(bytes.Buffer)
+
+	newline := ""
+	for i := 0; i < len(data); i++ {
+		if i%16 == 0 {
+			fmt.Fprintf(b, "%s%s%06x", newline, prefix, i)
+			newline = "\n"
+		}
+		fmt.Fprintf(b, " %02x", data[i])
+	}
+	fmt.Fprintf(b, "\n%s%06x", prefix, len(data))
+
+	return b.Bytes()
+}
+
+const prefix = "debug: "