一. 原理

將TCP相關內容（TCP偽頭部+TCP頭部+TCP內容）轉換成16比特的字符，然后進行累加，最后結果進行取反。TCP偽頭部是固定的，下文有相關代碼展示。

二. 源碼

1. 源碼

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/ether.h>// 偽頭部結構體
struct pseudo_header {uint32_t source_address;uint32_t dest_address;uint8_t zero;uint8_t protocol;uint16_t tcp_length;
};// 計算16位校驗和
uint16_t calculate_checksum(uint16_t *buff, int size) {uint32_t sum = 0;while (size > 1) {sum += *buff++;size -= 2;}if (size > 0) {sum += *buff & 0xFF;}while (sum >> 16) {sum = (sum & 0xFFFF) + (sum >> 16);}return (uint16_t)(~sum);
}// 計算TCP校驗和
uint16_t calculate_tcp_checksum(struct iphdr *ip_hdr, uint8_t *tcp_packet, int tcp_len) {struct pseudo_header pshdr;uint16_t *header_ptr;int total_len = sizeof(struct pseudo_header) + tcp_len;pshdr.source_address = ip_hdr->saddr;pshdr.dest_address = ip_hdr->daddr;pshdr.zero = 0;pshdr.protocol = IPPROTO_TCP; // TCP protocol number, =6pshdr.tcp_length = htons(tcp_len);// Allocate memory for pseudo header and TCP packetuint8_t *data = malloc(total_len);memcpy(data, (char *)&pshdr, sizeof(struct pseudo_header));memcpy(data + sizeof(struct pseudo_header), tcp_packet, tcp_len);// Calculate checksumuint16_t checksum = calculate_checksum((uint16_t *)data, total_len);free(data);return checksum;
}int main() {//IP信息char ip_head[] = {0x45, 0x00, 0x00, 0xc8, 0x14, 0x2b, 0x40, 0x00,0x3f, 0x06, 0xa3, 0x2f, 0xc0, 0xa8, 0x01, 0x21,0xc0, 0xa8, 0x01, 0x64};//TCP包信息（頭+內容）char tcp_packet[] = {0x05, 0x99, 0xd6, 0x83, 0x8a, 0x4a, 0x40, 0x80, 0x42, 0xa8, 0x40, 0x65, 0x80, 0x18, 0x01, 0xfa,0x65, 0x15, 0x00, 0x00, 0x01, 0x01, 0x08, 0x0a, 0xf3, 0x31, 0xdc, 0x0f, 0xa4, 0xd3, 0x50, 0x8d,0x04, 0x01, 0x00, 0x94, 0x00, 0x36, 0x01, 0x00, 0xaa, 0x7c, 0x00, 0xc3, 0x00, 0x00, 0x00, 0x0a,0x0f, 0x33, 0x00, 0x27, 0x00, 0x73, 0x00, 0x6c, 0x00, 0x65, 0x00, 0x65, 0x00, 0x70, 0x00, 0x27,0x00, 0x20, 0x00, 0x69, 0x00, 0x73, 0x00, 0x20, 0x00, 0x6e, 0x00, 0x6f, 0x00, 0x74, 0x00, 0x20,0x00, 0x61, 0x00, 0x20, 0x00, 0x72, 0x00, 0x65, 0x00, 0x63, 0x00, 0x6f, 0x00, 0x67, 0x00, 0x6e,0x00, 0x69, 0x00, 0x7a, 0x00, 0x65, 0x00, 0x64, 0x00, 0x20, 0x00, 0x62, 0x00, 0x75, 0x00, 0x69,0x00, 0x6c, 0x00, 0x74, 0x00, 0x2d, 0x00, 0x69, 0x00, 0x6e, 0x00, 0x20, 0x00, 0x66, 0x00, 0x75,0x00, 0x6e, 0x00, 0x63, 0x00, 0x74, 0x00, 0x69, 0x00, 0x6f, 0x00, 0x6e, 0x00, 0x20, 0x00, 0x6e,0x00, 0x61, 0x00, 0x6d, 0x00, 0x65, 0x00, 0x2e, 0x00, 0x04, 0x76, 0x00, 0x72, 0x00, 0x72, 0x00,0x70, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0xfd, 0x02, 0x00, 0xfd, 0x00, 0x00, 0x00, 0x00, 0x00,0x00, 0x00, 0x00, 0x00};// Calculate TCP checksumprintf("TCP old checksum: 0x%02X%02X\n", tcp_packet[16], tcp_packet[17]);tcp_packet[16] = tcp_packet[17] = 0; //先將checksum置空// uint16_t checksum = calculate_tcp_checksum(0x2101a8c0, 0x6401a8c0, tcp_packet, sizeof(tcp_packet)); uint16_t checksum = calculate_tcp_checksum((struct iphdr *)ip_head, tcp_packet, sizeof(tcp_packet)); printf("TCP new checksum: 0x%04X\n", htons(checksum)); //為了打印，需要將主機序換成網絡序號（真實賦值給tcp頭時不需要轉換）return 0;
}

gcc -o tcp_checksum_test tcp_checksum_test.c 即可以生成相應可執行程序（本測試運行環境ubuntu）
2. 結果