144 lines
4.2 KiB
Go
144 lines
4.2 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package dns
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import (
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"fmt"
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)
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const maxPointer = 2 << 13 // We have 14 bits for the offset
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// Function defined in this subpackage work on []byte and but still
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// provide some higher level functions.
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// RawSetId sets the message ID in buf.
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func RawSetId(msg []byte, off int, id uint16) bool {
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msg[off], msg[off+1] = packUint16(id)
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return true
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}
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type rawlabel struct {
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offset int // offset where this labels starts in the msg buf
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name string // the label, not this includes the length at the start
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}
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type rawmove struct {
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offset int // where to point to
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from int // where in the buffer set the pointer
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length int // used in calculating how much to shrink the message
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}
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// Compress performs name comression in the dns message contained in buf.
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// It returns the number of bytes saved.
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func Compress(msg []byte) int {
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// First we create a table of domain names to which we
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// can link. This is stored in 'table'
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// Once for another name the longest possible link is
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// found we save how we must change msg to perform this
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// compression. This is saved in 'moves'. After we
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// traversed the entire message, we perform all the
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// moves.
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// TODO: Maybe it should be optimized.
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// Map the labels to the offset in the message
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table := make(map[string]int)
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moves := make([]rawmove, 0)
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l := make([]rawlabel, 127) // Max labels?
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i := 0
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// Very much like the loop in msg.go
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off := 12 // Start of the first name in the q section
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question := true
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Loop:
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for {
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c := int(msg[off])
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switch c & 0xC0 {
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case 0x00:
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if c == 0x00 {
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// Do all of the bookkeeping
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name := ""
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poffset := 0 // Where to point to
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moffset := 0 // From where to insert the pointer
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for j := i - 1; j >= 0; j-- {
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name = l[j].name + name
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if idx, ok := table[name]; !ok {
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table[name] = l[j].offset
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} else {
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poffset = idx
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moffset = l[j].offset
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}
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}
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if poffset != 0 {
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//println("We kunnen verwijzen naar", poffset, "vanaf", moffset, "voor", name)
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//println("met lengte", len(name)+1)
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// the +1 for the name is for the null byte at the end
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// Discount for previous moves, reset the poffset counter
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for i := len(moves)-1; i >= 0; i-- {
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if poffset > moves[i].from {
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poffset -= (moves[i].length - 2)
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}
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}
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// need to shorten the rdlength in the RR too!
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moves = append(moves, rawmove{offset: poffset, from: moffset, length: len(name) + 1})
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}
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// end of the name
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if question {
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// In question section
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off += 4 // type, class + 1
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question = false
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} else {
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// How to handle well known records here
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// NS, MX, CNAME? eatName() function?
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// In the "body" of the msg
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off += 2 + 2 + 4 + 1 // type, class, ttl + 1
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// we are at the rdlength
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rdlength, _ := unpackUint16(msg, off)
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off += int(rdlength) + 1 // Skip the rdata
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}
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off++
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if off+1 > len(msg) {
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break Loop
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}
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i = 0
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continue Loop
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}
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if off+c+1 > len(msg) {
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break Loop
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}
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// If we are too deep in the message we cannot point to
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// it, so skip this label.
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if off < maxPointer {
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l[i] = rawlabel{offset: off, name: string(msg[off : off+c+1])}
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i++
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}
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// save the new names
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off += c + 1
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default:
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break Loop
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}
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}
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// fmt.Printf("table %v\n", table)
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// fmt.Printf("moves %v\n", moves)
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saved := 0
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// Start at the back, easier to move
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for i := len(moves) - 1; i >= 0; i-- {
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fmt.Printf("%v\n", moves[i])
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// move the bytes
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copy(msg[moves[i].from+1:], msg[moves[i].from+moves[i].length-1:])
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// Now set the pointer at moves[i].from and moves[i].from+1
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fmt.Printf("bits %b\n", moves[i].offset^0xC000)
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msg[moves[i].from], msg[moves[i].from+1] = packUint16(uint16(moves[i].offset ^ 0xC000))
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saved += moves[i].length // minus something
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}
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return saved
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}
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