1/*
2 * Copyright 2011 Daniel Drown
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 *
16 * checksum.c - ipv4/ipv6 checksum calculation
17 */
18#include <netinet/in.h>
19#include <netinet/ip.h>
20#include <netinet/ip_icmp.h>
21#include <netinet/udp.h>
22#include <netinet/tcp.h>
23#include <netinet/ip6.h>
24#include <netinet/icmp6.h>
25#include <linux/icmp.h>
26
27#include "checksum.h"
28
29/* function: ip_checksum_add
30 * adds data to a checksum
31 * current - the current checksum (or 0 to start a new checksum)
32 * data        - the data to add to the checksum
33 * len         - length of data
34 */
35uint32_t ip_checksum_add(uint32_t current, const void *data, int len) {
36  uint32_t checksum = current;
37  int left = len;
38  const uint16_t *data_16 = data;
39
40  while(left > 1) {
41    checksum += *data_16;
42    data_16++;
43    left -= 2;
44  }
45  if(left) {
46    checksum += *(uint8_t *)data_16;
47  }
48
49  return checksum;
50}
51
52/* function: ip_checksum_fold
53 * folds a 32-bit partial checksum into 16 bits
54 * temp_sum - sum from ip_checksum_add
55 * returns: the folded checksum in network byte order
56 */
57uint16_t ip_checksum_fold(uint32_t temp_sum) {
58  while(temp_sum > 0xffff)
59    temp_sum = (temp_sum >> 16) + (temp_sum & 0xFFFF);
60
61  return temp_sum;
62}
63
64/* function: ip_checksum_finish
65 * folds and closes the checksum
66 * temp_sum - sum from ip_checksum_add
67 * returns: a header checksum value in network byte order
68 */
69uint16_t ip_checksum_finish(uint32_t temp_sum) {
70  return ~ip_checksum_fold(temp_sum);
71}
72
73/* function: ip_checksum
74 * combined ip_checksum_add and ip_checksum_finish
75 * data - data to checksum
76 * len  - length of data
77 */
78uint16_t ip_checksum(const void *data, int len) {
79  uint32_t temp_sum;
80
81  temp_sum = ip_checksum_add(0,data,len);
82  return ip_checksum_finish(temp_sum);
83}
84
85/* function: ipv6_pseudo_header_checksum
86 * calculate the pseudo header checksum for use in tcp/udp/icmp headers
87 * ip6      - the ipv6 header
88 * len      - the transport length (transport header + payload)
89 * protocol - the transport layer protocol, can be different from ip6->ip6_nxt for fragments
90 */
91uint32_t ipv6_pseudo_header_checksum(const struct ip6_hdr *ip6, uint16_t len, uint8_t protocol) {
92  uint32_t checksum_len, checksum_next;
93  checksum_len = htonl((uint32_t) len);
94  checksum_next = htonl(protocol);
95
96  uint32_t current = 0;
97  current = ip_checksum_add(current, &(ip6->ip6_src), sizeof(struct in6_addr));
98  current = ip_checksum_add(current, &(ip6->ip6_dst), sizeof(struct in6_addr));
99  current = ip_checksum_add(current, &checksum_len, sizeof(checksum_len));
100  current = ip_checksum_add(current, &checksum_next, sizeof(checksum_next));
101
102  return current;
103}
104
105/* function: ipv4_pseudo_header_checksum
106 * calculate the pseudo header checksum for use in tcp/udp headers
107 * ip      - the ipv4 header
108 * len     - the transport length (transport header + payload)
109 */
110uint32_t ipv4_pseudo_header_checksum(const struct iphdr *ip, uint16_t len) {
111  uint16_t temp_protocol, temp_length;
112
113  temp_protocol = htons(ip->protocol);
114  temp_length = htons(len);
115
116  uint32_t current = 0;
117  current = ip_checksum_add(current, &(ip->saddr), sizeof(uint32_t));
118  current = ip_checksum_add(current, &(ip->daddr), sizeof(uint32_t));
119  current = ip_checksum_add(current, &temp_protocol, sizeof(uint16_t));
120  current = ip_checksum_add(current, &temp_length, sizeof(uint16_t));
121
122  return current;
123}
124
125/* function: ip_checksum_adjust
126 * calculates a new checksum given a previous checksum and the old and new pseudo-header checksums
127 * checksum    - the header checksum in the original packet in network byte order
128 * old_hdr_sum - the pseudo-header checksum of the original packet
129 * new_hdr_sum - the pseudo-header checksum of the translated packet
130 * returns: the new header checksum in network byte order
131 */
132uint16_t ip_checksum_adjust(uint16_t checksum, uint32_t old_hdr_sum, uint32_t new_hdr_sum) {
133  // Algorithm suggested in RFC 1624.
134  // http://tools.ietf.org/html/rfc1624#section-3
135  checksum = ~checksum;
136  uint16_t folded_sum = ip_checksum_fold(checksum + new_hdr_sum);
137  uint16_t folded_old = ip_checksum_fold(old_hdr_sum);
138  if (folded_sum > folded_old) {
139    return ~(folded_sum - folded_old);
140  } else {
141    return ~(folded_sum - folded_old - 1);  // end-around borrow
142  }
143}
144