Re: Creating a routing program - the basics

[Mateus Interciso <p.zarnick@gmail.com>]

On Tue, 23 Oct 2007 13:15:11 +0200, Steve Graegert wrote:

> On 10/23/07, Mateus Interciso <p.zarnick@gmail.com> wrote:
>> Hello, I'm currently on university, and for my graduation paper, I'll
>> be implementing a routing algorythm, by hand, in C. The routing
>> algorythm itself is not easy, but one of the starting parts of it, is
>> to find a way of discovering the MAC that it is currently connected to,
>> maybe is bether if I draw, since I'm not really that good in english:
>>
>> [PC with n NICs]-----(n cables)---->[n PCs]
>>
>> I have full control of the first PC, of course, since is the one I'm
>> implementing the router, so I do know the MAC of it, but how to
>> discover the MAC on the other end of the n lines, so I can start
>> sending packets?
> 
> Probably this is a starting point:  (not tested)
> 
> ___ BEGIN SOURCE ___
> 
> #include <iostream>
> #include <stdio.h>
> #include <netdb.h>
> #include <netinet/if_ether.h>
> 
> int read_mac(void) {
> 	int sock, sockfd, n, cnt;
> 	char buffer[2048];
> 	unsigned char *iphead, *ethhead;
> 	struct ether_addr ether;
> 
> 	if ((sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_IP))) < 0) {
> 		perror("socket");
> 		exit(1);
> 	}
> 
> 	while (1) {
> 		if ((n = recvfrom(sock, buffer, 2048, 0, NULL, NULL)) == 
-1) {
> 			perror("recvfrom");
> 			close(sock);
> 			exit(1);
> 		}
> 	
> 		ethhead = (unsigned char *)buffer;
> 		
> 		if (ethhead != NULL) {
> 			iphead = (unsigned char *)(buffer + 14); // Skip 
Ethernet header
> 			printf("Peer MAC: "
> 				"%02x:%02x:%02x:%02x:%02x:%02x\n",
> 				ethhead[0], ethhead[1], ethhead[2],
> 				ethhead[3], ethhead[4], ethhead[5]);
> 			printf("Source MAC: "
> 				"%02x:%02x:%02x:%02x:%02x:%02x\n",
> 				ethhead[6], ethhead[7], ethhead[8],
> 				ethhead[9], ethhead[10], ethhead[11]);
> 
> 			if (*iphead == 0x45) { // Check for IPv4, no 
options present
> 				printf("Peer IP: %d.%d.%d.%d\n",
> 					iphead[12], iphead[13],
> 					iphead[14], iphead[15]);
> 				printf("Source IP: %d.%d.%d.%d\n",
> 					iphead[16], iphead[17],
> 					iphead[18], iphead[19]);
> 				printf("Protocol (UDP = 11): %02x Hex\n", 
iphead[9]);
> 			}
> 		}
> 	}
> 	
> 	return 0;
> }
> 
> 
> int main(int argc, char **argv) {
>     read_mac();
>     return EXIT_SUCCESS;
> }
> 
> ___ END SOURCE ___
> 
> Honestly, you should make yourself acquainted with low-level network
> programming.  Otherwise you'll be asking a lot of theses questions in
> the future.  Anyway, hope this helps.
> 
> 	\Steve
> 
> --
> 
> Steve Grägert
> DigitalEther.de
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Thank you, this will help.
Do you have any book/site where I can take a deep look into how to do 
that stuff?

MAteus

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