This looks like an uninitialized value in gencode.c . Try this patch.
Bill
Index: gencode.c
===================================================================
RCS file: /tcpdump/master/libpcap/gencode.c,v
retrieving revision 1.158
diff -u -r1.158 gencode.c
--- gencode.c 2001/09/20 00:24:24 1.158
+++ gencode.c 2001/11/12 17:26:33
@@ -3265,6 +3268,7 @@
if (sizeof(mask) * 8 < masklen)
bpf_error("mask length must be <= %u", (unsigned int)(sizeof(mask) *
8));
+ memset(&mask, 0, sizeof(mask));
memset(&mask, 0xff, masklen / 8);
if (masklen % 8) {
mask.s6_addr[masklen / 8] =
