#### checksum.c3.0 KB History Raw

 123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124 ``````/* * Checksum routine for Internet Protocol family headers (C Version). * * Refer to "Computing the Internet Checksum" by R. Braden, D. Borman and * C. Partridge, Computer Communication Review, Vol. 19, No. 2, April 1989, * pp. 86-101, for additional details on computing this checksum. */ #include #include "checksum.h" int /* return checksum in low-order 16 bits */ in_cksum(void *parg, int nbytes) { u_short *ptr = parg; register long sum; /* assumes long == 32 bits */ u_short oddbyte; register u_short answer; /* assumes u_short == 16 bits */ /* * Our algorithm is simple, using a 32-bit accumulator (sum), * we add sequential 16-bit words to it, and at the end, fold back * all the carry bits from the top 16 bits into the lower 16 bits. */ sum = 0; while (nbytes > 1) { sum += *ptr++; nbytes -= 2; } /* mop up an odd byte, if necessary */ if (nbytes == 1) { oddbyte = 0; /* make sure top half is zero */ *((u_char *) &oddbyte) = *(u_char *)ptr; /* one byte only */ sum += oddbyte; } /* * Add back carry outs from top 16 bits to low 16 bits. */ sum = (sum >> 16) + (sum & 0xffff); /* add high-16 to low-16 */ sum += (sum >> 16); /* add carry */ answer = ~sum; /* ones-complement, then truncate to 16 bits */ return(answer); } /* Fletcher Checksum -- Refer to RFC1008. */ #define MODX 4102 /* 5802 should be fine */ /* To be consistent, offset is 0-based index, rather than the 1-based index required in the specification ISO 8473, Annex C.1 */ /* calling with offset == FLETCHER_CHECKSUM_VALIDATE will validate the checksum without modifying the buffer; a valid checksum returns 0 */ u_int16_t fletcher_checksum(u_char * buffer, const size_t len, const uint16_t offset) { u_int8_t *p; int x, y, c0, c1; u_int16_t checksum; u_int16_t *csum; size_t partial_len, i, left = len; checksum = 0; if (offset != FLETCHER_CHECKSUM_VALIDATE) /* Zero the csum in the packet. */ { assert (offset < (len - 1)); /* account for two bytes of checksum */ csum = (u_int16_t *) (buffer + offset); *(csum) = 0; } p = buffer; c0 = 0; c1 = 0; while (left != 0) { partial_len = MIN(left, MODX); for (i = 0; i < partial_len; i++) { c0 = c0 + *(p++); c1 += c0; } c0 = c0 % 255; c1 = c1 % 255; left -= partial_len; } /* The cast is important, to ensure the mod is taken as a signed value. */ x = (int)((len - offset - 1) * c0 - c1) % 255; if (x <= 0) x += 255; y = 510 - c0 - x; if (y > 255) y -= 255; if (offset == FLETCHER_CHECKSUM_VALIDATE) { checksum = (c1 << 8) + c0; } else { /* * Now we write this to the packet. * We could skip this step too, since the checksum returned would * be stored into the checksum field by the caller. */ buffer[offset] = x; buffer[offset + 1] = y; /* Take care of the endian issue */ checksum = htons((x << 8) | (y & 0xFF)); } return checksum; } ``````