--- libnet.orig/src/libnet_pblock.c
+++ libnet/src/libnet_pblock.c
@@ -389,7 +389,7 @@
             {
                 if ((q->flags) & LIBNET_PBLOCK_DO_CHECKSUM)
                 {
-                    int offset = (l->total_size + l->aligner) - q->ip_offset;
+                    int offset = l->aligner + q->ip_offset;
                     c = libnet_do_checksum(l, *packet + offset,
                             libnet_pblock_p2p(q->type), q->h_len);
                     if (c == -1)