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我正在为一个自学 Ruby 项目滚动我自己的 IP 数据包，并且需要计算 IP 标头校验和（如 RFC 791 p.14 中所述）。当我在这里输入我的问题时弹出的一个相关问题将我指向 RFC 1071，所以我可能大部分都在那里，但只是为了添加到 Stack Overflow，任何人（可能是 Future Josh）都可以提供一些 Ruby 代码用于计算校验和，假设以下 Ruby 位：

    def build_ip_packet(tos, ttl, protocol, dst_addr, data)
        len = (IP_HEADER_LEN * 4) + data.size

        ip_header = %w{ #{IP_VERSION} #{IP_HEADER_LEN} #{tos} #{len} #{IP_IDENTIFICATION} #{IP_FLAGS_BIT_0} #{IP_FLAGS_BIT_1} #{IP_FLAGS_BIT_2} #{IP_FRAG_OFFSET} #{ttl} #{protocol} #{hdr_checksum} #{src_addr} #{dst_addr} }

        [...]
    end

常量在文件顶部定义，但如果您正在查看 RFC791 p.11，它们应该是不言自明的。

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1 回答 1

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RFC 1071 provides the following sample implementation in C:

in 6
   {
       /* Compute Internet Checksum for "count" bytes
        *         beginning at location "addr".
        */
   register long sum = 0;

    while( count > 1 )  {
       /*  This is the inner loop */
           sum += * (unsigned short) addr++;
           count -= 2;
   }

       /*  Add left-over byte, if any */
   if( count > 0 )
           sum += * (unsigned char *) addr;

       /*  Fold 32-bit sum to 16 bits */
   while (sum>>16)
       sum = (sum & 0xffff) + (sum >> 16);

   checksum = ~sum;

}

I wrote the following C code to unit test it:

#include <stdio.h>
#include <stdlib.h>


unsigned short checksum (int count, unsigned short * addr) {
    unsigned long sum = 0;

    while (count > 1) {
        sum += *addr++;
        count -= 2;
    }

    // Add left-over byte, if any
    if (count > 0)
        sum += * (unsigned char *) addr;

    while (sum >> 16)
        sum = (sum & 0xffff) + (sum >> 16);

    return (unsigned short)sum;
}

void test (const unsigned short expected, const unsigned short got) {
    printf(
        "%s  expected 0x%04x, got 0x%04x\n",
        (expected == got ? "OK" : "NOK"),
        expected,
        got
    );
}

int main(void) {
    unsigned short *buf = calloc(1024, sizeof(unsigned short));

    buf[0] = 0x0000;

    test(
        0x0,
        checksum(2, buf)
    );

    buf[0] = 0x0001;
    buf[1] = 0xf203;
    buf[2] = 0xf4f5;
    buf[3] = 0xf6f7;

    test(
        0xddf2,
        checksum(8, buf)
    );

    return 0;
}

It is slightly disconcerting that my code does not take the bitwise NOT of sum in the last line of the checksum() function like the RFC implementation does, but adding the bitwise NOT broke my unit tests.

Running the code yields:

: jglover@jglover-3; /tmp/rfc-1071-checksum 
OK  expected 0x0000, got 0x0000
OK  expected 0xddf2, got 0xddf2

I ported this to Ruby as follows:

def rfc1071_checksum(header_fields)
    checksum = 0

    header_fields.each{|field| checksum += field}

    while (checksum >> 16) != 0
        checksum = (checksum & 0xffff) + (checksum >> 16)
    end

    return checksum
end

I call the method like this:

def build_ip_packet(tos, ttl, protocol, dst_addr, data)
    len = (IP_HEADER_LEN * 4) + data.size

    # Header checksum is set to 0 for computing the checksum; see RFC 791 p.14
    hdr_checksum = 0

    src_addr = IPAddr.new('127.0.0.1')

    header_fields = [
        ( ((IP_VERSION << 4) + IP_HEADER_LEN) << 8 ) + ( tos      ),
        len,
        IP_IDENTIFICATION,
        ( (IP_FLAGS_BIT_0 << 15) + (IP_FLAGS_BIT_1 << 14) + (IP_FLAGS_BIT_2 << 13) + (IP_FRAG_OFFSET << 12)),
        ( ttl                                 << 8 ) + ( protocol ),
        hdr_checksum,
        src_addr & 0xff00,  # 16 most significant bits
        src_addr & 0x00ff,  # 16 least significant bits
        dst_addr & 0xff00,  # 16 most significant bits
        dst_addr & 0x00ff,  # 16 least significant bits
    ]

    hdr_checksum = rfc1071_checksum(header_fields)

    return nil
end

And here are the unit tests:

require 'test/unit'
require 'lib/traceroute'

class TestTraceroute < MiniTest::Unit::TestCase
    def test_rfc1071_checksum
        [
            [ 0x0000, [ 0x0000 ] ],
            [
                0xddf2,
                [
                    0x0001f203,
                    0xf4f5f6f7,
                ]
            ],
        ].each{|input_record|
            got      = Traceroute.new.rfc1071_checksum(input_record[1])
            expected = input_record[0]

            assert_equal(got, expected, "rfc1071_checksum: #{input_record[1].inspect}")
        }
    end
end
于 2009-12-28T10:20:58.253 回答