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Make "ip_checksum()" take just pointer and length arguments, and make 

"ip_checksum_shouldbe()" compute the correct checksum given the computed
whole-packet checksum and the value of the checksum field; that scheme
can be better extended in the future to handle checksums other than the
IP header checksum, e.g. ICMP, UDP, and TCP checksums (although we'd
want a somewhat more optimized checksumming routine for that, and
perhaps have an option to control whether to do checksum checking on TCP
and UDP packets, as that could be expensive).

That requires that we remember the value of the computed checksum, not
just check it against 0; that renders "ip_checksum_state()"
uninteresting, as we can just compare the value against 0 in line.

svn path=/trunk/; revision=2210
This commit is contained in:
Guy Harris 2000-08-05 05:08:21 +00:00
parent 88f4dbf0e9
commit 6a4a59ea1e
1 changed files with 36 additions and 30 deletions
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66
packet-ip.c
View File

@ -1,7 +1,7 @@
/* packet-ip.c
* Routines for IP and miscellaneous IP protocol packet disassembly
*
* $Id: packet-ip.c,v 1.97 2000/08/04 22:43:45 guy Exp $
* $Id: packet-ip.c,v 1.98 2000/08/05 05:08:21 guy Exp $
*
* Ethereal - Network traffic analyzer
* By Gerald Combs <gerald@zing.org>
@ -776,14 +776,14 @@ static const true_false_string flags_set_truth = {
"Not set"
};
static guint16 ip_checksum(e_ip *iph)
static guint16 ip_checksum(const guint8 *ptr, int len)
{
unsigned long Sum;
unsigned char *Ptr, *PtrEnd;
const unsigned char *Ptr, *PtrEnd;
Sum = 0;
PtrEnd = (lo_nibble(iph->ip_v_hl) * 4 + (char *)iph);
for (Ptr = (unsigned char *) iph; Ptr < PtrEnd; Ptr += 2) {
PtrEnd = ptr + len;
for (Ptr = ptr; Ptr < PtrEnd; Ptr += 2) {
Sum += pntohs(Ptr);
}
Sum = (Sum & 0xFFFF) + (Sum >> 16);
@ -792,31 +792,35 @@ static guint16 ip_checksum(e_ip *iph)
return (guint16)~Sum;
}
static gboolean ip_checksum_state(e_ip *iph)
static guint16 ip_checksum_shouldbe(guint16 sum, guint16 computed_sum)
{
guint16 Sum;
guint32 shouldbe;
Sum = ip_checksum(iph);
if (Sum != 0)
return FALSE;
return TRUE;
}
static guint16 ip_checksum_shouldbe(e_ip *iph)
{
guint16 ipsum;
guint16 Sum;
ipsum = iph->ip_sum;
iph->ip_sum = 0;
Sum = ip_checksum(iph);
iph->ip_sum = ipsum;
return Sum;
/*
* The value that should have gone into the checksum field
* is the negative of the value gotten by summing up everything
* *but* the checksum field.
*
* We can compute that by subtracting the value of the checksum
* field from the sum of all the data in the packet, and then
* computing the negative of that value.
*
* "sum" is the value of the checksum field, and "computed_sum"
* is the negative of the sum of all the data in the packets,
* so that's -(-computed_sum - sum), or (sum + computed_sum).
*
* All the arithmetic in question is one's complement, so the
* addition must include an end-around carry; we do this by
* doing the arithmetic in 32 bits (with no sign-extension),
* and then adding the upper 16 bits of the sum, which contain
* the carry, to the lower 16 bits of the sum, and then do it
* again in case *that* sum produced a carry.
*/
shouldbe = sum;
shouldbe += computed_sum;
shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
return shouldbe;
}
void
@ -827,6 +831,7 @@ dissect_ip(const u_char *pd, int offset, frame_data *fd, proto_tree *tree) {
guint hlen, optlen, len;
guint16 flags;
guint8 nxt;
guint16 ipsum;
/* To do: check for errs, etc. */
if (!BYTES_ARE_IN_FRAME(offset, IPH_MIN_LEN)) {
@ -899,14 +904,15 @@ dissect_ip(const u_char *pd, int offset, frame_data *fd, proto_tree *tree) {
proto_tree_add_uint_format(ip_tree, hf_ip_proto, NullTVB, offset + 9, 1, iph.ip_p,
"Protocol: %s (0x%02x)", ipprotostr(iph.ip_p), iph.ip_p);
if (ip_checksum_state((e_ip*) &pd[offset])) {
ipsum = ip_checksum(&pd[offset], hlen);
if (ipsum == 0) {
proto_tree_add_uint_format(ip_tree, hf_ip_checksum, NullTVB, offset + 10, 2, iph.ip_sum,
"Header checksum: 0x%04x (correct)", iph.ip_sum);
}
else {
proto_tree_add_uint_format(ip_tree, hf_ip_checksum, NullTVB, offset + 10, 2, iph.ip_sum,
"Header checksum: 0x%04x (incorrect, should be 0x%04x)", iph.ip_sum,
ip_checksum_shouldbe((e_ip*) &pd[offset]));
ip_checksum_shouldbe(iph.ip_sum, ipsum));
}
proto_tree_add_ipv4(ip_tree, hf_ip_src, NullTVB, offset + 12, 4, iph.ip_src);