Perl 中的 UDP 校验和函数

Question

我已经搜索了很多我需要的函数来在 Perl 中计算 UDP 标头校验和。

我在C中找到这个函数：http://www.netfor2.com/udpsum.htm 但我不知道如何使用它，我尝试将它转换为 Perl 但没有托管

这就是我尝试的：

sub udp_checksum(my $len_udp, my @src_addr, my @dest_addr, my $padding, my @buff)
{
    my $proto_udp = 17;
    my $padd = 0;
    my $word16;
    my $sum;

    # Find out if the length of data is even or odd number. If odd,
    # add a padding byte = 0 at the end of packet
    if ($padding&1==1){
        $padd = 1;
        $buff[$len_udp] = 0;
    }

    # initialize sum to zero
    $sum = 0;

    # make 16 bit words out of every two adjacent 8 bit words and 
    # calculate the sum of all 16 bit words
    for (my $i = 0; $i < $len_udp + $padd; $i = $i + 2){
        $word16 =(($buff[i] << 8) & 0xFF00) + ($buff[i+1] & 0xFF);
        $sum = $sum + $word16;
    }

    # add the UDP pseudo header which contains the IP source and destinationn addresses
    for (my $i = 0; $i < 4; $i = $i + 2){
        $word16 =(($src_addr[i] << 8) & 0xFF00) + ($src_addr[i+1] & 0xFF);
        $sum = $sum + $word16;
    }

    for (my $i = 0; $i < 4; $i = $i + 2){
        $word16 =(($dest_addr[i] << 8) & 0xFF00) + ($dest_addr[i+1] & 0xFF);
        $sum = $sum + $word16;
    }

    # the protocol number and the length of the UDP packet
    $sum = $sum + $prot_udp + $len_udp;

    # keep only the last 16 bits of the 32 bit calculated sum and add the carries
        while ($sum >> 16){
            $sum = ($sum & 0xFFFF) + ($sum >> 16);
        }

    # Take the one's complement of sum
    $sum = ~$sum;

    return $sum;
}

请帮助我。

Answer 1

use List::Util qw( sum );
use Socket     qw( IPPROTO_UDP );

sub udp_checksum {
    my $packed_src_addr = shift;  # As 4-char string, e.g. as returned by inet_aton.
    my $packed_dst_addr = shift;  # As 4-char string, e.g. as returned by inet_aton.
    my $udp_packet      = shift;  # UDP header and data as a string.

    my $sum = sum(
        IPPROTO_UDP,
        length($udp_packet),
        map({ unpack('n*', $_) }
            $packed_src_addr,
            $packed_dst_addr,
            $udp_packet."\0",  # Extra byte ignored if length($udp_packet) is even.
        ),
    );

    while (my $hi = $sum >> 16) {
        $sum = ($sum & 0xFFFF) + $hi;
    }

    return ~$sum & 0xFFFF;
}

要构建 UDP 数据包，

my $udp_packet = pack('nnnna*', $src_port, $dst_port, 8+length($body), 0, $body);
my $checksum = udp_checksum($packed_src_addr, $packed_dst_addr, $udp_packet);
substr($udp_packet, 6, 2, pack('n', $checksum));

要验证 UDP 数据包的校验和，

udp_checksum($packed_src_addr, $packed_dst_addr, $udp_packet)
    and die("Invalid checksum\n");

示例：

#!/usr/bin/perl

use strict;
use Socket;
use warnings;
use List::Util qw(sum);
use Socket     qw(IPPROTO_UDP);
use Socket     qw(inet_aton);

main();

sub main {
    my $src_port = 27005;
    my $dst_port = 27015;
    my $body = pack("H*", "92380000621f008063d5179000927df3a2e09bf319a66bf300c239aa9393d8eaa244119a");
    my $packed_src_addr = inet_aton("156.215.205.76");
    my $packed_dst_addr = inet_aton("31.186.251.163");

    my $udp_packet = pack('nnnna*', $src_port, $dst_port, 8+length($body), 0, $body);
    my $checksum = udp_checksum($packed_src_addr, $packed_dst_addr, $udp_packet);

    print "\n\nUDP Checksum: $checksum\n\n";
}

sub udp_checksum {
    my $packed_src_addr = shift;  # As 4-char string, e.g. as returned by inet_aton.
     my $packed_dst_addr = shift;  # As 4-char string, e.g. as returned by inet_aton.
    my $udp_packet      = shift;  # UDP header and data as a string.

    my $sum = sum(
    IPPROTO_UDP,
    length($udp_packet),
    map({ unpack('n*', $_) }
        $packed_src_addr,
        $packed_dst_addr,
        $udp_packet."\0",  # Extra byte ignored if length($udp_packet) is even.
        ),
    );

    while (my $hi = $sum >> 16) {
        $sum = ($sum & 0xFFFF) + $hi;
    }

    return ~$sum & 0xFFFF;
}

Answer 2

我用它来计算 Perl 中的 udp 校验和：

my $chk = ~((unpack "%32S*", $pkt) % 65535)

然后将其修补到数据包中，如下所示：

substr $msg, 12 + 16, 2, pack "S", $chk;

TCP 校验和的工作方式不同，表达式为：

my $chk = (~unpack "%32S*", $dstip) % 65535