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-rw-r--r--lib/checksum.c46
-rw-r--r--lib/checksum.h1
2 files changed, 29 insertions, 18 deletions
diff --git a/lib/checksum.c b/lib/checksum.c
index 3ddde815..43940b71 100644
--- a/lib/checksum.c
+++ b/lib/checksum.c
@@ -51,6 +51,8 @@ in_cksum(void *parg, int nbytes)
/* 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)
{
@@ -62,13 +64,14 @@ fletcher_checksum(u_char * buffer, const size_t len, const uint16_t offset)
checksum = 0;
- assert (offset < len);
- /*
- * Zero the csum in the packet.
- */
- csum = (u_int16_t *) (buffer + offset);
- *(csum) = 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;
@@ -89,7 +92,7 @@ fletcher_checksum(u_char * buffer, const size_t len, const uint16_t offset)
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;
@@ -98,17 +101,24 @@ fletcher_checksum(u_char * buffer, const size_t len, const uint16_t offset)
y = 510 - c0 - x;
if (y > 255)
y -= 255;
-
- /*
- * 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));
+
+ 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;
}
diff --git a/lib/checksum.h b/lib/checksum.h
index da1d3cba..b310f744 100644
--- a/lib/checksum.h
+++ b/lib/checksum.h
@@ -1,2 +1,3 @@
extern int in_cksum(void *, int);
+#define FLETCHER_CHECKSUM_VALIDATE 0xffff
extern u_int16_t fletcher_checksum(u_char *, const size_t len, const uint16_t offset);