Re: Creating a routing program - the basics

["Steve Graegert" <graegerts@gmail.com>]

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
> > linux-c-programming" in the body of a message to
> > majordomo@vger.kernel.org More majordomo info at
> > http://vger.kernel.org/majordomo-info.html
>
> 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
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