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package rfc1035label
import (
"errors"
"fmt"
"strings"
)
// Labels represents RFC1035 labels
//
// This implements RFC 1035 labels, including compression.
// https://tools.ietf.org/html/rfc1035#section-4.1.4
type Labels struct {
// original contains the original bytes if the object was parsed from a byte
// sequence, or nil otherwise. The `original` field is necessary to deal
// with compressed labels. If the labels are further modified, the original
// content is invalidated and no compression will be used.
original []byte
// Labels contains the parsed labels. A change here invalidates the
// `original` object.
Labels []string
}
// same compares two string arrays
func same(a, b []string) bool {
if len(a) != len(b) {
return false
}
for i := 0; i < len(a); i++ {
if a[i] != b[i] {
return false
}
}
return true
}
// String prints labels.
func (l *Labels) String() string {
return fmt.Sprintf("%v", l.Labels)
}
// ToBytes returns a byte sequence representing the labels. If the original
// sequence is modified, the labels are parsed again, otherwise the original
// byte sequence is returned.
func (l *Labels) ToBytes() []byte {
// if the original byte sequence has been modified, invalidate it and
// serialize again.
// NOTE: this function is not thread-safe. If multiple threads modify
// the `Labels` field, the result may be wrong.
originalLabels, err := labelsFromBytes(l.original)
// if the original object has not been modified, or we cannot parse it,
// return the original bytes.
if err != nil || (l.original != nil && same(originalLabels, l.Labels)) {
return l.original
}
return labelsToBytes(l.Labels)
}
// Length returns the length in bytes of the serialized labels
func (l *Labels) Length() int {
return len(l.ToBytes())
}
// NewLabels returns an initialized Labels object.
func NewLabels() *Labels {
return &Labels{
Labels: make([]string, 0),
}
}
// FromBytes reads labels from a bytes stream according to RFC 1035.
func (l *Labels) FromBytes(data []byte) error {
labs, err := labelsFromBytes(data)
if err != nil {
return err
}
l.original = data
l.Labels = labs
return nil
}
// FromBytes returns a Labels object from the given byte sequence, or an error if
// any.
func FromBytes(data []byte) (*Labels, error) {
var l Labels
if err := l.FromBytes(data); err != nil {
return nil, err
}
return &l, nil
}
// ErrBufferTooShort is returned when the label cannot be parsed due to a wrong
// length or missing bytes.
var ErrBufferTooShort = errors.New("rfc1035label: buffer too short")
// fromBytes decodes a serialized stream and returns a list of labels
func labelsFromBytes(buf []byte) ([]string, error) {
var (
labels = make([]string, 0)
pos, oldPos int
label string
handlingPointer bool
)
for {
if pos >= len(buf) {
// interpret label without trailing zero-length byte as a partial
// domain name field as per RFC 4704 Section 4.2
if label != "" {
labels = append(labels, label)
}
break
}
length := int(buf[pos])
pos++
var chunk string
if length == 0 {
labels = append(labels, label)
label = ""
if handlingPointer {
pos = oldPos
handlingPointer = false
}
} else if length&0xc0 == 0xc0 {
// compression pointer
if handlingPointer {
return nil, errors.New("rfc1035label: cannot handle nested pointers")
}
handlingPointer = true
if pos+1 > len(buf) {
return nil, errors.New("rfc1035label: pointer buffer too short")
}
off := int(buf[pos-1]&^0xc0)<<8 + int(buf[pos])
oldPos = pos + 1
pos = off
} else {
if pos+length > len(buf) {
return nil, ErrBufferTooShort
}
chunk = string(buf[pos : pos+length])
if label != "" {
label += "."
}
label += chunk
pos += length
}
}
return labels, nil
}
// labelToBytes encodes a label and returns a serialized stream of bytes
func labelToBytes(label string) []byte {
var encodedLabel []byte
if len(label) == 0 {
return []byte{0}
}
for _, part := range strings.Split(label, ".") {
encodedLabel = append(encodedLabel, byte(len(part)))
encodedLabel = append(encodedLabel, []byte(part)...)
}
return append(encodedLabel, 0)
}
// labelsToBytes encodes a list of labels and returns a serialized stream of
// bytes
func labelsToBytes(labels []string) []byte {
var encodedLabels []byte
for _, label := range labels {
encodedLabels = append(encodedLabels, labelToBytes(label)...)
}
return encodedLabels
}
|
