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
26#include "checksum.h"
27
28/* function: ip_checksum_add
29 * adds data to a checksum
30 * current - the current checksum (or 0 to start a new checksum)
31 * data        - the data to add to the checksum
32 * len         - length of data
33 */
34uint32_t ip_checksum_add(uint32_t current, const void *data, int len) {
35  uint32_t checksum = current;
36  int left = len;
37  const uint16_t *data_16 = data;
38
39  while(left > 1) {
40    checksum += *data_16;
41    data_16++;
42    left -= 2;
43  }
44  if(left) {
45    checksum += *(uint8_t *)data_16;
46  }
47
48  return checksum;
49}
50
51/* function: ip_checksum_fold
52 * folds a 32-bit partial checksum into 16 bits
53 * temp_sum - sum from ip_checksum_add
54 * returns: the folded checksum in network byte order
55 */
56uint16_t ip_checksum_fold(uint32_t temp_sum) {
57  while(temp_sum > 0xffff)
58    temp_sum = (temp_sum >> 16) + (temp_sum & 0xFFFF);
59
60  return temp_sum;
61}
62
63/* function: ip_checksum_finish
64 * folds and closes the checksum
65 * temp_sum - sum from ip_checksum_add
66 * returns: a header checksum value in network byte order
67 */
68uint16_t ip_checksum_finish(uint32_t temp_sum) {
69  return ~ip_checksum_fold(temp_sum);
70}
71
72/* function: ip_checksum
73 * combined ip_checksum_add and ip_checksum_finish
74 * data - data to checksum
75 * len  - length of data
76 */
77uint16_t ip_checksum(const void *data, int len) {
78  uint32_t temp_sum;
79
80  temp_sum = ip_checksum_add(0,data,len);
81  return ip_checksum_finish(temp_sum);
82}
83
84/* function: ipv6_pseudo_header_checksum
85 * calculate the pseudo header checksum for use in tcp/udp/icmp headers
86 * ip6      - the ipv6 header
87 * len      - the transport length (transport header + payload)
88 * protocol - the transport layer protocol, can be different from ip6->ip6_nxt for fragments
89 */
90uint32_t ipv6_pseudo_header_checksum(const struct ip6_hdr *ip6, uint16_t len, uint8_t protocol) {
91  uint32_t checksum_len, checksum_next;
92  checksum_len = htonl((uint32_t) len);
93  checksum_next = htonl(protocol);
94
95  uint32_t current = 0;
96  current = ip_checksum_add(current, &(ip6->ip6_src), sizeof(struct in6_addr));
97  current = ip_checksum_add(current, &(ip6->ip6_dst), sizeof(struct in6_addr));
98  current = ip_checksum_add(current, &checksum_len, sizeof(checksum_len));
99  current = ip_checksum_add(current, &checksum_next, sizeof(checksum_next));
100
101  return current;
102}
103
104/* function: ipv4_pseudo_header_checksum
105 * calculate the pseudo header checksum for use in tcp/udp headers
106 * ip      - the ipv4 header
107 * len     - the transport length (transport header + payload)
108 */
109uint32_t ipv4_pseudo_header_checksum(const struct iphdr *ip, uint16_t len) {
110  uint16_t temp_protocol, temp_length;
111
112  temp_protocol = htons(ip->protocol);
113  temp_length = htons(len);
114
115  uint32_t current = 0;
116  current = ip_checksum_add(current, &(ip->saddr), sizeof(uint32_t));
117  current = ip_checksum_add(current, &(ip->daddr), sizeof(uint32_t));
118  current = ip_checksum_add(current, &temp_protocol, sizeof(uint16_t));
119  current = ip_checksum_add(current, &temp_length, sizeof(uint16_t));
120
121  return current;
122}
123
124/* function: ip_checksum_adjust
125 * calculates a new checksum given a previous checksum and the old and new pseudo-header checksums
126 * checksum    - the header checksum in the original packet in network byte order
127 * old_hdr_sum - the pseudo-header checksum of the original packet
128 * new_hdr_sum - the pseudo-header checksum of the translated packet
129 * returns: the new header checksum in network byte order
130 */
131uint16_t ip_checksum_adjust(uint16_t checksum, uint32_t old_hdr_sum, uint32_t new_hdr_sum) {
132  // Algorithm suggested in RFC 1624.
133  // http://tools.ietf.org/html/rfc1624#section-3
134  checksum = ~checksum;
135  uint16_t folded_sum = ip_checksum_fold(checksum + new_hdr_sum);
136  uint16_t folded_old = ip_checksum_fold(old_hdr_sum);
137  if (folded_sum > folded_old) {
138    return ~(folded_sum - folded_old);
139  } else {
140    return ~(folded_sum - folded_old - 1);  // end-around borrow
141  }
142}
143