diff --git a/checksum.c b/checksum.c
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+++ b/checksum.c
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+/*
+ * Copyright 2011 Daniel Drown
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * checksum.c - ipv4/ipv6 checksum calculation
+ */
+#include <netinet/in.h>
+#include <netinet/ip.h>
+#include <netinet/ip_icmp.h>
+#include <netinet/udp.h>
+#include <netinet/tcp.h>
+#include <netinet/ip6.h>
+#include <netinet/icmp6.h>
+#include <linux/icmp.h>
+
+#include "checksum.h"
+
+/* function: ip_checksum_add
+ * adds data to a checksum
+ * current_sum - the current checksum (or 0 to start a new checksum)
+ * data        - the data to add to the checksum
+ * len         - length of data
+ */
+uint32_t ip_checksum_add(uint32_t current_sum, const void *data, int len) {
+  uint32_t checksum = current_sum;
+  int left = len;
+  const uint16_t *data_16 = data;
+
+  while(left > 1) {
+    checksum += *data_16;
+    data_16++;
+    left -= 2;
+  }
+  if(left) {
+    checksum += *(uint8_t *)data_16;
+  }
+
+  return checksum;
+}
+
+/* function: ip_checksum_finish
+ * close the checksum
+ * temp_sum - sum from ip_checksum_add
+ */
+uint16_t ip_checksum_finish(uint32_t temp_sum) {
+  while(temp_sum > 0xffff)
+    temp_sum = (temp_sum >> 16) + (temp_sum & 0xFFFF);
+
+  temp_sum = (~temp_sum) & 0xffff;
+
+  return temp_sum;
+}
+
+/* function: ip_checksum
+ * combined ip_checksum_add and ip_checksum_finish
+ * data - data to checksum
+ * len  - length of data
+ */
+uint16_t ip_checksum(const void *data, int len) {
+  uint32_t temp_sum;
+
+  temp_sum = ip_checksum_add(0,data,len);
+  return ip_checksum_finish(temp_sum);
+}
+
+/* function: ipv6_pseudo_header_checksum
+ * calculate the pseudo header checksum for use in tcp/udp/icmp headers
+ * current_sum - the current checksum or 0 to start a new checksum
+ * ip6         - the ipv6 header
+ */
+uint32_t ipv6_pseudo_header_checksum(uint32_t current_sum, const struct ip6_hdr *ip6) {
+  uint32_t checksum_len, checksum_next;
+
+  checksum_len = htonl(ntohs(ip6->ip6_plen));
+  checksum_next = htonl(ip6->ip6_nxt);
+
+  current_sum = ip_checksum_add(current_sum,&(ip6->ip6_src),sizeof(struct in6_addr));
+  current_sum = ip_checksum_add(current_sum,&(ip6->ip6_dst),sizeof(struct in6_addr));
+  current_sum = ip_checksum_add(current_sum,&checksum_len,sizeof(checksum_len));
+  current_sum = ip_checksum_add(current_sum,&checksum_next,sizeof(checksum_next));
+
+  return current_sum;
+}
+
+/* function: ipv4_pseudo_header_checksum
+ * calculate the pseudo header checksum for use in tcp/udp headers
+ * current_sum - the current checksum or 0 to start a new checksum
+ * ip          - the ipv4 header
+ */
+uint32_t ipv4_pseudo_header_checksum(uint32_t current_sum, const struct iphdr *ip) {
+  uint16_t temp_protocol, temp_length;
+
+  temp_protocol = htons(ip->protocol);
+  temp_length = htons(ntohs(ip->tot_len) - ip->ihl*4);
+
+  current_sum = ip_checksum_add(current_sum, &(ip->saddr), sizeof(uint32_t));
+  current_sum = ip_checksum_add(current_sum, &(ip->daddr), sizeof(uint32_t));
+  current_sum = ip_checksum_add(current_sum, &temp_protocol, sizeof(uint16_t));
+  current_sum = ip_checksum_add(current_sum, &temp_length, sizeof(uint16_t));
+
+  return current_sum;
+}