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// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package icmp
import "encoding/binary"
// An Extension represents an ICMP extension.
type Extension interface {
// Len returns the length of ICMP extension.
// Proto must be either the ICMPv4 or ICMPv6 protocol number.
Len(proto int) int
// Marshal returns the binary encoding of ICMP extension.
// Proto must be either the ICMPv4 or ICMPv6 protocol number.
Marshal(proto int) ([]byte, error)
}
const extensionVersion = 2
func validExtensionHeader(b []byte) bool {
v := int(b[0]&0xf0) >> 4
s := binary.BigEndian.Uint16(b[2:4])
if s != 0 {
s = checksum(b)
}
if v != extensionVersion || s != 0 {
return false
}
return true
}
// parseExtensions parses b as a list of ICMP extensions.
// The length attribute l must be the length attribute field in
// received icmp messages.
//
// It will return a list of ICMP extensions and an adjusted length
// attribute that represents the length of the padded original
// datagram field. Otherwise, it returns an error.
func parseExtensions(b []byte, l int) ([]Extension, int, error) {
// Still a lot of non-RFC 4884 compliant implementations are
// out there. Set the length attribute l to 128 when it looks
// inappropriate for backwards compatibility.
//
// A minimal extension at least requires 8 octets; 4 octets
// for an extension header, and 4 octets for a single object
// header.
//
// See RFC 4884 for further information.
if 128 > l || l+8 > len(b) {
l = 128
}
if l+8 > len(b) {
return nil, -1, errNoExtension
}
if !validExtensionHeader(b[l:]) {
if l == 128 {
return nil, -1, errNoExtension
}
l = 128
if !validExtensionHeader(b[l:]) {
return nil, -1, errNoExtension
}
}
var exts []Extension
for b = b[l+4:]; len(b) >= 4; {
ol := int(binary.BigEndian.Uint16(b[:2]))
if 4 > ol || ol > len(b) {
break
}
switch b[2] {
case classMPLSLabelStack:
ext, err := parseMPLSLabelStack(b[:ol])
if err != nil {
return nil, -1, err
}
exts = append(exts, ext)
case classInterfaceInfo:
ext, err := parseInterfaceInfo(b[:ol])
if err != nil {
return nil, -1, err
}
exts = append(exts, ext)
}
b = b[ol:]
}
return exts, l, nil
}
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