Re: Creating a routing program - the basics

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

On Tue, 23 Oct 2007 15:56:22 +0200, Steve Graegert wrote:

> On 10/23/07, Mateus Interciso <p.zarnick@gmail.com> wrote:
>> 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
>> > -
>> > To unsubscribe from this list: send the line "unsubscribe
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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?
> 
> I can't make any recommendations beyond the well-known programming books
> like Steven's UNP and O'Reilly's "Understanding the Linux Kernel". 
> Additionally, you may find Sams Publishing 's "Linux(R) Socket
> Programming" (0672319357) useful.
> 
> 	\Steve
> 
> --
> 
> Steve Grägert
> DigitalEther.de
> -
> To unsubscribe from this list: send the line "unsubscribe
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Thank you, I'll take a look at those books :D

Mateus

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