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123 lines
2.4 KiB
C
123 lines
2.4 KiB
C
/*
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* BIRD Library -- IP One-Complement Checksum
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*
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* (c) 1999--2000 Martin Mares <mj@ucw.cz>
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*
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* Can be freely distributed and used under the terms of the GNU GPL.
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*/
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/**
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* DOC: Miscellaneous functions.
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*/
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#include <stdarg.h>
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#include "nest/bird.h"
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#include "checksum.h"
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static inline u32
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add32(u32 sum, u32 x)
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{
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u32 z = sum + x;
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// return z + (z < sum);
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/* add carry */
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if (z < x)
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z++;
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return z;
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}
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static u16
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ipsum_calc_block(u32 *buf, unsigned len, u16 isum)
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{
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/*
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* A few simple facts about the IP checksum (see RFC 1071 for detailed
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* discussion):
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*
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* o It's associative and commutative.
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* o It's byte order independent.
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* o It's word size independent.
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*
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* This gives us a neat 32-bits-at-a-time algorithm which respects
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* usual alignment requirements and is reasonably fast.
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*/
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ASSERT(!(len % 4));
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if (!len)
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return isum;
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u32 *end = buf + (len >> 2);
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u32 sum = isum;
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while (buf < end)
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sum = add32(sum, *buf++);
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sum = (sum >> 16) + (sum & 0xffff); /* add high-16 to low-16 */
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sum += (sum >> 16); /* add carry */
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return sum;
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}
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static u16
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ipsum_calc(void *frag, unsigned len, va_list args)
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{
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u16 sum = 0;
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for(;;)
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{
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sum = ipsum_calc_block(frag, len, sum);
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frag = va_arg(args, void *);
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if (!frag)
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break;
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len = va_arg(args, unsigned);
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}
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return sum;
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}
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/**
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* ipsum_verify - verify an IP checksum
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* @frag: first packet fragment
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* @len: length in bytes
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*
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* This function verifies whether a given fragmented packet
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* has correct one's complement checksum as used by the IP
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* protocol.
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*
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* It uses all the clever tricks described in RFC 1071 to speed
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* up checksum calculation as much as possible.
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*
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* Result: 1 if the checksum is correct, 0 else.
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*/
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int
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ipsum_verify(void *frag, unsigned len, ...)
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{
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va_list args;
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u16 sum;
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va_start(args, len);
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sum = ipsum_calc(frag, len, args);
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va_end(args);
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return sum == 0xffff;
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}
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/**
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* ipsum_calculate - compute an IP checksum
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* @frag: first packet fragment
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* @len: length in bytes
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*
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* This function calculates a one's complement checksum of a given fragmented
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* packet.
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*
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* It uses all the clever tricks described in RFC 1071 to speed
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* up checksum calculation as much as possible.
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*/
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u16
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ipsum_calculate(void *frag, unsigned len, ...)
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{
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va_list args;
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u16 sum;
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va_start(args, len);
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sum = ipsum_calc(frag, len, args);
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va_end(args);
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return 0xffff - sum;
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}
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