Reading the ARP cache table

I would like to read the ARP cache table programatically using Linux.

In Stevens Unix Network Programming Vol 1, he uses the sysctl function to
access the ARP cache entries.
But I can't find the required constants NET_RT_FLAGS AND RT_LLINFO!

Thanks!
Ramy

P.S. I found code that does this in ~FreeBSD~

<http://unix.derkeiler.com/Mailing-Lists/FreeBSD/net/2003-08/0133.html>

#include <sys/file.h>
#include <sys/socket.h>
#include <sys/sysctl.h>
#include <net/if_dl.h>
#include <net/route.h>
#include <netinet/in.h>
#include <netinet/if_ether.h>
#include <arpa/inet.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>


#define ROUNDUP(a) \
((a) > 0 ? (1 + (((a) - 1) | (sizeof(long) - 1))) : sizeof(long))
int
main(int argc, char *argv[])
{
int mib[6];
size_t needed;
char *lim, *buf, *next;
if (argc != 1) {
(void)fprintf(stderr, "Usage: %s\n", argv[0]);
exit(1);
}


mib[0] = CTL_NET;
mib[1] = PF_ROUTE;
mib[2] = 0;
mib[3] = AF_INET;
mib[4] = NET_RT_FLAGS;
mib[5] = RTF_LLINFO;
if (sysctl(mib, 6, NULL, &needed, NULL, 0) < 0)
errx(1, "route-sysctl-estimate");
if ((buf = malloc(needed)) == NULL)
errx(1, "malloc");
if (sysctl(mib, 6, buf, &needed, NULL, 0) < 0)
errx(1, "actual retrieval of routing table");


lim = buf + needed;
next = buf;
while (next < lim) {
struct rt_msghdr *rtm = (struct rt_msghdr *)next;
struct sockaddr_inarp *sinarp = (struct sockaddr_inarp
*)(rtm + 1);
struct sockaddr_dl *sdl =
(struct sockaddr_dl *)((char *)sinarp +
ROUNDUP(sinarp->sin_len));
if (sdl->sdl_alen) { /* complete ARP entry */
printf("%s at ", inet_ntoa(sinarp->sin_addr));
printf("%s", ether_ntoa((struct ether_addr
*)LLADDR(sdl)));
printf("\n");
}
next += rtm->rtm_msglen;
}
free(buf);
return(0);
}

Ramy Asselin wrote:
> I would like to read the ARP cache table programatically using Linux.
>
> In Stevens Unix Network Programming Vol 1, he uses the sysctl function
> to access the ARP cache entries.
> But I can't find the required constants NET_RT_FLAGS AND RT_LLINFO!
>
> Thanks!
> Ramy
>
> P.S. I found code that does this in ~FreeBSD~
>
> <http://unix.derkeiler.com/Mailing-Lists/FreeBSD/net/2003-08/0133.html>
>
> [snip]

A good place to start would be the source code for the arp(8) program
itself. On Linux boxes, the arp(8) program is usually installed via the
'net-tools' package. Here's a Freshmeat page for the 'net-tools' stuff:

http://freshmeat.net/projects/net-tools/?topic_id=150

--
Jim

To reply by email, remove "link" and change "now.here" to "yahoo"
jfischer_link5809{at}now.here.com

"Ramy Asselin" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> I would like to read the ARP cache table programatically using Linux.
>
> In Stevens Unix Network Programming Vol 1, he uses the sysctl function to
> access the ARP cache entries.
> But I can't find the required constants NET_RT_FLAGS AND RT_LLINFO!
>

A way to get started is to run 'strace arp '.
You can then see the system calls the arp command makes.

Or you can get the source code to the arp command.