When adding the last byte of an odd number of bytes to a TCP or UDP

checksum, fetch it by casting the pointer to "const u_int8_t *" rather
than "const char *" - casting it to "const char *" causes it to be
sign-extended, perhaps causing 16 1 bits to be added in at the top and,
at least on big-endian platforms (where "htons()" does nothing, meaning
it won't trim off the extra 16 bits) with signed "char"s (e.g., SPARC),
causing the checksum to be computed incorrectly and causing it to
incorrectly be reported as bad.

1 files changed, 3 insertions, 3 deletions

diff --git a/print-tcp.c b/print-tcp.c
index 852300ce..9cc707fc 100644
--- a/print-tcp.c
+++ b/print-tcp.c
@@ -21,7 +21,7 @@
 
 #ifndef lint
 static const char rcsid[] =
-    "@(#) $Header: /tcpdump/master/tcpdump/print-tcp.c,v 1.80 2000-11-17 19:08:16 itojun Exp $ (LBL)";
+    "@(#) $Header: /tcpdump/master/tcpdump/print-tcp.c,v 1.81 2000-12-23 20:55:22 guy Exp $ (LBL)";
 #endif
 
 #ifdef HAVE_CONFIG_H
@@ -169,7 +169,7 @@ static int tcp_cksum(register const struct ip *ip,
 		sum += *sp++;
 
 	if (tlen & 1) {
-		sum += htons( (*(const char *)sp) << 8);
+		sum += htons( (*(const u_int8_t *)sp) << 8);
 	}
 
 	while (sum > 0xffff)
@@ -217,7 +217,7 @@ static int tcp6_cksum(const struct ip6_hdr *ip6, const struct tcphdr *tp,
 		sum += *sp++;
 
 	if (tlen & 1)
-		sum += htons((*(const char *)sp) << 8);
+		sum += htons((*(const u_int8_t *)sp) << 8);
 
 	while (sum > 0xffff)
 		sum = (sum & 0xffff) + (sum >> 16);