在Unix操作系统中，有一个路由表，其中包含一定数量的元组。这些元组由网络IP，子网掩码，网关IP和接口名称组成。这些详细信息用于将数据包转发到其网络外部以连接到Internet。因此，本文给出了一个想法，即当需要转发数据包时，系统如何做出决定。
例子：
Input: 201.2.2.2
Output: 12.23.44.1 eth9
Here, there is no network IP entry in the routing table
which starts with "201". In this case it will
choose default path(0.0.0.0, 0.0.0.0, 12.23.44.1, eth9).
Still, it will perform bitwise AND operation with
each entry and then chooses default path's interface
and gateway to send packet outside. Default path means
interface to which system is directly connected.
Input: 200.200.200.1
Output: 190.164.1.2 eth0
Here bitwise AND operation is performed
with each entry of routing table and correspondent
network's interface name and gateway IP is returned.
数据包转发如何工作?
借助一个小示例，可以很容易地理解这一点：
- 假设有一个IP地址为“ 20.129.0.1 ”的数据包，并且路由表具有以下条目：
Network’s IP Address Subnet Mask Gateway’s IP Address Interface Name 200.200.16.0 255.255.248.0 192.13.2.55 eth4 200.200.200.0 255.255.248.0 192.13.2.55 eth4 0.0.0.0 0.0.0.0 12.23.44.1 eth9 20.128.0.0 255.128.0.0 12.1.1.1 eth1 20.0.0.0 255.0.0.0 12.1.1.1 eth2 20.0.0.0 255.128.0.0 12.1.1.1 eth3 - 因此，当数据包进入系统内核以查找网关和接口时，它将首先对每个条目的子网掩码执行按位与运算，以找到最长前缀匹配。
- 然后，将按位AND运算的结果与网络的IP地址进行比较。因此它将返回网关的相应IP地址和接口名称，数据包可以通过该IP地址发出。
- 20.129.0.1的二进制表示形式是00010100.10000001.00000000.00000001 。然后，它为路由表的每个条目执行带有子网掩码的按位与运算。
- 在此表中，条目号为4(即20.128.0.0、255.128.0.0、12.1.1.1，eth1 )，该条目给出此数据包的最长前缀匹配。因此，数据包将从eth1接口发出，并选择网关12.1.1.1进行转发。
下面是使用链表数据结构的上述方法的实现。它以2个文件作为输入，并在上述另一个文件中返回输出。
输入文件和输出文件
- 文件“ input.txt”表示数据包的IP地址。
- 文件“ routing.txt”包含要与IP地址匹配的路由表条目。
- 文件“ output.txt”包含每个输入的输出。
程序：
// C code to implement IP forwarding table lookup
#include
#include
#include
#include
#include
#define M 15
#define N 150
// Declaration of structure of linked list
// to store ip address in each node
struct node {
char* data;
struct node* next;
} * head[15];
// This function fetch data from file
// and store them into different arrays
void storeData(FILE* fp,
char buf[M][N],
char net[M][N],
char mask[M][N],
char gateway[M][N],
char port[M][N])
{
char line[200];
int c, i = 0, j, k = 0, m = 0;
// Read data from the file line by line
// and each line is stored in array separately.
while (fgets(line, sizeof(line), fp)) {
j = 0;
for (int l = 0; l < strlen(line); l++) {
buf[i][j] = line[l];
j++;
}
i++;
}
// From each lines stored in buf,
// network id, subnet mask, gateway
// and port are extracted
// and stored into individual arrays.
for (i = 0; i < 15; i++) {
k = 0;
for (j = 0; buf[i][j] != ','; j++) {
net[i][k] = buf[i][j];
k++;
}
m = j + 2;
k = 0;
for (j = m; buf[i][j] != ','; j++) {
mask[i][k] = buf[i][j];
k++;
}
m = j + 2;
k = 0;
for (j = m; buf[i][j] != ','; j++) {
gateway[i][k] = buf[i][j];
k++;
}
m = j + 2;
k = 0;
for (j = m; buf[i][j] != '\0'; j++) {
port[i][k] = buf[i][j];
k++;
}
}
}
// Function to create routing table
// using arrays created by storeData() function
// using linked list data structure
void insert(char net[M][N], char mask[M][N],
char gateway[M][N], char port[M][N],
char buf[M][N])
{
char *temp1, *temp2, *temp3, *temp4;
struct node* new;
for (int i = 0; i < M; i++) {
// Initialize head of each
// linked list with NULL.
head[i] = NULL;
}
for (int i = 0; i < M; i++) {
for (int j = 0; j < 4; j++) {
// If head is null
// then first create new node
// and store network id into it.
if (head[i] == NULL) {
new = (struct node*)malloc(
sizeof(struct node));
new->data = net[i];
new->next = NULL;
head[i] = new;
}
// If head is not null
// and value of j is 1 then create new node
// which is pointed by head and it
// will contain subnet mask
else if (j == 1) {
new->next = (struct node*)malloc(
sizeof(struct node));
new = new->next;
new->data = mask[i];
new->next = NULL;
}
// If head is not null and value of j is 2
// then create new node
// which is pointed by subnet mask
// and it will contain gateway
else if (j == 2) {
new->next = (struct node*)malloc(
sizeof(struct node));
new = new->next;
new->data = gateway[i];
}
// If head is not null and value of j is 3
// then create new node
// which is pointed by gateway and
// it will contain port
else if (j == 3) {
new->next = (struct node*)malloc(
sizeof(struct node));
new = new->next;
new->data = port[i];
}
}
}
// Perform sorting on the basis
// of longest prefix of subnet mask
for (int i = 0; i < M; i++) {
for (int j = i; j < M; j++) {
// Longest prefix has been compared
// by using inet_addr() system call
// which gives decimal value of an ip address.
if (inet_addr(head[i]->next->data)
< inet_addr(head[j]->next->data)) {
struct node* temp = head[i];
head[i] = head[j];
head[j] = temp;
}
}
}
}
// This function will search for gateway ip
// and port number in routing table
// through which packet has been sent
// to next node/destination
void search(FILE* fp1, FILE* fp2)
{
char str[100];
struct in_addr addr;
unsigned int val;
fprintf(fp2, "%c", ' ');
// Read file 'input.txt' line by line
// and perform bitwise AND between subnet mask
// and input(destination) ip coming from file.
while (fgets(str, sizeof(str), fp1)) {
for (int i = 0; i < M; i++) {
// Perform bitwise AND operation on result
// (i.e. Decimal value of an ip address)
// coming from inet_addr() system call
val = inet_addr(str) & inet_addr(head[i]->next->data);
addr.s_addr = val;
char* str1 = inet_ntoa(addr);
char* str2 = head[i]->data;
int count = 0;
// Compare the network id string with result
// coming after performing AND operation
// and if they are same then increment count.
for (int i = 0; str1[i] != '\0'; i++) {
if (str1[i] == str2[i]) {
count++;
}
}
// If count is same as the string length
// of network id then find gateway ip
// and port number of that respective network id
// and write it into 'output.txt' file.
if (count == strlen(str1)) {
struct node* ptr = head[i]->next;
struct node* temp = ptr->next;
while (temp != NULL) {
fprintf(fp2, "%s ", temp->data);
temp = temp->next;
}
break;
}
}
}
}
// Driver code
int main(int argc, char* argv[])
{
FILE *fin, *fout, *fp;
char buf[M][N] = { { 0 } };
char net[M][N] = { { 0 } };
char mask[M][N] = { { 0 } };
char gateway[M][N] = { { 0 } };
char port[M][N] = { { 0 } };
// if command line argument is less than 3
// then it will show standard error.
if (argc < 3) {
fprintf(stderr, "File name:%s\n", argv[0]);
return 1;
}
// If 3 arguments are given then input
// and routing.txt files will be opened in read mode
// while output.txt file is opened in write mode.
else {
fin = fopen(argv[1], "r");
fout = fopen(argv[2], "w");
fp = fopen(argv[3], "r");
}
// If any of the file is not present
// then it will give an error.
if (fp == NULL || fin == NULL || fout == NULL) {
printf("Error");
return 0;
}
// This function will read the data
// of a file 'routing.txt' line by line
// and store them into one array named 'buf',
// after that the coma separated values in buf
// are stored into their respective array.
storeData(fp, buf, net, mask, gateway, port);
// It will create routing table using linked list
insert(net, mask, gateway, port, buf);
// It will take input from input.txt files
// which contains only destination ip address
// and search about the route through which
// packet has been sent in network
// and output is stored in to an output.txt file
search(fin, fout);
printf("Forwarding table has been implemented successfully");
printf("See the output in %s file\n", argv[2]);
/*Closes all the files*/
fclose(fin);
fclose(fp);
fclose(fout);
return 0;
}
// This code is written by Pooja Patel
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