计算ICMPv6磁头的16位校验和

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1楼 · 发布于 2024-05-29 11:38:34

我会用一个例子来回答你的问题。在

让我们从Wireshark wiki获取this sample capture，这样我们就有了相同的包，在Wireshark中打开它，让我们获取第一个ICMPv6包（第3帧）。在

请注意，对于这个包，至少有一点很重要：IPv6层的有效负载长度为32（0x20）。在

注意：要在Wireshark上将数据包提取为字符串，请选择数据包和所需的层（例如Ipv6），然后：right click>；copy>；bytes>；hex stream

构建伪标题

要计算校验和，首先要根据RFC 2460 section 8.1构建伪头。在

校验和是在伪报头和ICMPv6数据包上计算的。在

The IPv6 version of ICMP [ICMPv6] includes the above pseudo-header in its checksum computation

我们需要构建伪标题：

源IP
目标IP
上层包长度
下一个标题

源IP和目标IP来自IPv6层。在

下一个标题字段固定为58：

The Next Header field in the pseudo-header for ICMP contains the value 58, which identifies the IPv6 version of ICMP.

上层包长度：

The Upper-Layer Packet Length in the pseudo-header is the length of the upper-layer header and data (e.g., TCP header plus TCP data). Some upper-layer protocols carry their own length information (e.g., the Length field in the UDP header); for such protocols, that is the length used in the pseudo- header. Other protocols (such as TCP) do not carry their own length information, in which case the length used in the pseudo-header is the Payload Length from the IPv6 header, minus the length of any extension headers present between the IPv6 header and the upper-layer header.

在我们的例子中，上层（ICMPv6）不携带长度字段，所以在这种情况下，我们必须使用IPv6层的有效负载长度字段，对于这个数据包，这个字段是32（0x20）。在

让我们尝试一些代码：

def build_pseudo_header(src_ip, dest_ip, payload_len):
    source_ip_bytes = bytearray.fromhex(src_ip)
    dest_ip_bytes = bytearray.fromhex(dest_ip)
    next_header = struct.pack(">I", 58)
    upper_layer_len = struct.pack(">I", payload_len)
    return source_ip_bytes + dest_ip_bytes + upper_layer_len + next_header

代码应该这样调用：

^{pr2}$

构建ICMPV6数据包

正如rfc 4443 section 2.3中提到的，在任何计算之前，校验和字段必须设置为0。在

For computing the checksum, the checksum field is first set to zero.

在本例中，我使用ICMPv6中的type和code字段作为一个16位的单值。校验和字段被删除，数据包的剩余部分被简单地称为“remainment”：

TYPE_CODE = "8700"
REMAINDER = "00000000fe80000000000000026097fffe0769ea01010000860580da"

正在为校验和计算构建数据包的ICMPv6部分：

def build_icmpv6_chunk(type_and_code, other):
    type_code_bytes = bytearray.fromhex(type_and_code)
    checksum = struct.pack(">I", 0)  # make sure checksum is set to 0 here
    other_bytes = bytearray.fromhex(other)
    return type_code_bytes + checksum + other_bytes

名称如下：

TYPE_CODE = "8700"
REMAINDER = "00000000fe80000000000000026097fffe0769ea01010000860580da"
icmpv6_chunk = build_icmpv6_chunk(TYPE_CODE, REMAINDER)

计算校验和

根据RFC 1701计算校验和。Python中的主要困难是将和包装成16位的数量。在

calc_checksum()函数的输入是伪报头和数据包的ICMPv6部分的连接（校验和设置为0）：

Python示例：

def calc_checksum(packet):
    total = 0

    # Add up 16-bit words
    num_words = len(packet) // 2
    for chunk in struct.unpack("!%sH" % num_words, packet[0:num_words*2]):
        total += chunk

    # Add any left over byte
    if len(packet) % 2:
        total += ord(packet[-1]) << 8

    # Fold 32-bits into 16-bits
    total = (total >> 16) + (total & 0xffff)
    total += total >> 16
    return (~total + 0x10000 & 0xffff)

代码示例

代码很难看，但返回了正确的校验和。在我们的示例中，此代码返回0x68db，根据wireshark的说法，这是正确的。在

#!/usr/local/bin/python3
# -*- coding: utf8 -*-

import struct

SOURCE_IP = "fe80000000000000020086fffe0580da"
DEST_IP = "fe80000000000000026097fffe0769ea"
TYPE_CODE = "8700"
REMAINDER = "00000000fe80000000000000026097fffe0769ea01010000860580da"


def calc_checksum(packet):
    total = 0

    # Add up 16-bit words
    num_words = len(packet) // 2
    for chunk in struct.unpack("!%sH" % num_words, packet[0:num_words*2]):
        total += chunk

    # Add any left over byte
    if len(packet) % 2:
        total += ord(packet[-1]) << 8

    # Fold 32-bits into 16-bits
    total = (total >> 16) + (total & 0xffff)
    total += total >> 16
    return (~total + 0x10000 & 0xffff)


def build_pseudo_header(src_ip, dest_ip, payload_len):
    source_ip_bytes = bytearray.fromhex(src_ip)
    dest_ip_bytes = bytearray.fromhex(dest_ip)
    next_header = struct.pack(">I", 58)
    upper_layer_len = struct.pack(">I", payload_len)
    return source_ip_bytes + dest_ip_bytes + upper_layer_len + next_header


def build_icmpv6_chunk(type_and_code, other):
    type_code_bytes = bytearray.fromhex(type_and_code)
    checksum = struct.pack(">I", 0)
    other_bytes = bytearray.fromhex(other)
    return type_code_bytes + checksum + other_bytes


def main():
    icmpv6_chunk = build_icmpv6_chunk(TYPE_CODE, REMAINDER)
    pseudo_header = build_pseudo_header(SOURCE_IP, DEST_IP, 32)
    icmpv6_packet = pseudo_header + icmpv6_chunk
    checksum = calc_checksum(icmpv6_packet)

    print("checksum: {:#x}".format(checksum))

if __name__ == '__main__':
    main()

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