Detect which name are applicable for compression

_examples/mx/mx.go

@@ -36,4 +36,8 @@ func main() {
 	for _, a := range r.Answer {
 		fmt.Printf("%v\n", a)
 	}
+        println(r.String())
+        buf, _ := r.Pack()
+
+        dns.Compress(buf)
 }

rawmsg.go

@@ -4,11 +4,95 @@
 
 package dns
 
+import (
+        "fmt"
+)
+
 // Function defined in this subpackage work on []byte and but still
 // provide some higher level functions.
 
 // RawSetId sets the message ID in buf.
-func RawSetId(buf []byte, off int, id uint16) bool {
-	buf[off], buf[off+1] = packUint16(id)
+func RawSetId(msg []byte, off int, id uint16) bool {
+	msg[off], msg[off+1] = packUint16(id)
 	return true
 }
+
+type rawlabel struct {
+        offset  int     // offset where this labels starts in the msg buf
+        str     string  // the label, not this includes the length at the start
+}
+
+// Compress performs name comression in the dns message contained in buf.
+// It returns the number of dnames compressed.
+func Compress(msg []byte) int {
+
+	// Map the labels to the offset in the message
+	table := make(map[string]int)
+	l := make([]rawlabel, 127) // Max labels?
+	i := 0
+
+	// Very much like the loop in msg.go
+	off := 12 // Start of the first name in the q section
+	question := true
+Loop:
+	for {
+		c := int(msg[off])
+		switch c & 0xC0 {
+		case 0x00:
+			if c == 0x00 {
+				// Do all of the bookkeeping
+
+                                name := ""
+                                poffset := 0      // Where to point to
+                                moffset := 0          // From where to insert the pointer
+				for j := i-1; j >= 0; j-- {
+                                        name = l[j].str + name
+                                        if idx, ok := table[name]; !ok {
+                                                table[name] = l[j].offset
+                                        } else {
+                                                poffset = idx
+                                                moffset = l[j].offset
+                                        }
+				}
+                                if poffset == 0 {
+                                        println("niks gevonden, nieuw!")
+                                } else {
+                                        println("We kunnen verwijzen naar", poffset, "vanaf", moffset)
+                                }
+
+				// end of the name
+				if question {
+					// In question section
+					off += 4 // type, class + 1
+					question = false
+				} else {
+					// In the "body" of the msg
+					off += 2 + 2 + 4 + 1 // type, class, ttl + 1     
+					// we are at the rdlength
+					rdlength, _ := unpackUint16(msg, off)
+					off += int(rdlength) + 1 // Skip the rdata
+				}
+                                off++
+				if off+1 > len(msg) {
+					break Loop
+				}
+				i = 0
+                                continue Loop
+			}
+
+			if off+c+1 > len(msg) {
+				break Loop
+			}
+			// c is the mount to scan forward
+                        l[i] = rawlabel{offset: off, str: string(msg[off : off+c+1])}
+			i++
+			// save the new names 
+			off += c + 1
+		default:
+			break Loop
+		}
+	}
+        fmt.Printf("table %v\n", table)
+
+	return 0
+}