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Full Discussion: SIOCGARP to display the entire ARP table.
Top Forums Programming SIOCGARP to display the entire ARP table. Post 302448191 by semash! on Wednesday 25th of August 2010 12:08:38 PM
Old 08-25-2010
SIOCGARP to display the entire ARP table.
Hello everybody,

I have a working code that displays an entry from a given IP address. but, how can i display the entire ARP table? like the option 'arp -a' of the standard linux program.

This is the code:
Code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <netdb.h>
#include <netinet/in.h>
#include <net/if.h>
#include <errno.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <net/if_arp.h>

char *mac_ntoa(unsigned char *ptr){
static char address[30];
sprintf(address, "%02X:%02X:%02X:%02X:%02X:%02X",
ptr[0], ptr[1], ptr[2], ptr[3], ptr[4], ptr[5]);
return(address);
}

int main(int argc, char* argv[]){
int s;

struct arpreq req;
struct hostent *hp;
struct sockaddr_in *sin;

char *host = argv[1];

bzero((caddr_t)&req, sizeof(req));

sin = (struct sockaddr_in *)&req.arp_pa;
sin->sin_family = AF_INET; /* Address Family: Internet */
sin->sin_addr.s_addr = inet_addr(host);

if(sin->sin_addr.s_addr ==-1){
if(!(hp = gethostbyname(host))){
fprintf(stderr, "arp: %s ", host);
herror((char *)NULL);
return(-1);
        }
bcopy((char *)hp->h_addr, (char *)&sin->sin_addr, sizeof(sin->sin_addr));
}

if((s = socket(AF_INET, SOCK_DGRAM, 0)) < 0){
perror("socket() failed.");
exit(-1);
        } /* Socket is opened.*/

strcpy(req.arp_dev, "eth0");

if(ioctl(s, SIOCGARP, (caddr_t)&req) <0){
if(errno == ENXIO){
printf("%s (%s) -- no entry.\n", host, inet_ntoa(sin->sin_addr));
exit(-1);
        } else {
perror("SIOCGARP");
exit(-1);
        }
}
close(s); /* Close the socket, we don't need it anymore. */

printf("%s (%s) at ", host, inet_ntoa(sin->sin_addr));

if(req.arp_flags & ATF_COM){
printf("%s ", mac_ntoa(req.arp_ha.sa_data));
        } else {
printf("incomplete");
}

if(req.arp_flags & ATF_PERM){
printf("ATF_PERM");
        }
if(req.arp_flags & ATF_PUBL){
printf("ATF_PUBL");
        }
if(req.arp_flags & ATF_USETRAILERS){
printf("ATF_USETRAILERS");
}

printf("\n");
return(0);
}

I've looked to /usr/src/linux/net/ipv4/arp.c code before posting and can't find anything that tells me how to do it.

Thanks in advance.
 

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LEARN ABOUT MINIX

rtnetlink

RTNETLINK(3)						     Linux Programmer's Manual						      RTNETLINK(3)

NAME

       rtnetlink - macros to manipulate rtnetlink messages

SYNOPSIS

       #include <asm/types.h>
       #include <linux/netlink.h>
       #include <linux/rtnetlink.h>
       #include <sys/socket.h>

       rtnetlink_socket = socket(AF_NETLINK, int socket_type, NETLINK_ROUTE);

       int RTA_OK(struct rtattr *rta, int rtabuflen);

       void *RTA_DATA(struct rtattr *rta);

       unsigned int RTA_PAYLOAD(struct rtattr *rta);

       struct rtattr *RTA_NEXT(struct rtattr *rta, unsigned int rtabuflen);

       unsigned int RTA_LENGTH(unsigned int length);

       unsigned int RTA_SPACE(unsigned int length);

DESCRIPTION

       All  rtnetlink(7) messages consist of a netlink(7) message header and appended attributes.  The attributes should be manipulated only using
       the macros provided here.

       RTA_OK(rta, attrlen) returns true if rta points to a valid routing attribute; attrlen is the running length of the attribute buffer.   When
       not true then you must assume there are no more attributes in the message, even if attrlen is nonzero.

       RTA_DATA(rta) returns a pointer to the start of this attribute's data.

       RTA_PAYLOAD(rta) returns the length of this attribute's data.

       RTA_NEXT(rta,  attrlen)	gets  the  next  attribute after rta.  Calling this macro will update attrlen.	You should use RTA_OK to check the
       validity of the returned pointer.

       RTA_LENGTH(len) returns the length which is required for len bytes of data plus the header.

       RTA_SPACE(len) returns the amount of space which will be needed in a message with len bytes of data.

CONFORMING TO

       These macros are nonstandard Linux extensions.

BUGS

       This manual page is incomplete.

EXAMPLE

       Creating a rtnetlink message to set the MTU of a device:

	   #include <linux/rtnetlink.h>

	   ...

	   struct {
	       struct nlmsghdr	nh;
	       struct ifinfomsg if;
	       char		attrbuf[512];
	   } req;

	   struct rtattr *rta;
	   unsigned int mtu = 1000;

	   int rtnetlink_sk = socket(AF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);

	   memset(&req, 0, sizeof(req));
	   req.nh.nlmsg_len = NLMSG_LENGTH(sizeof(struct ifinfomsg));
	   req.nh.nlmsg_flags = NLM_F_REQUEST;
	   req.nh.nlmsg_type = RTM_NEWLINK;
	   req.if.ifi_family = AF_UNSPEC;
	   req.if.ifi_index = INTERFACE_INDEX;
	   req.if.ifi_change = 0xffffffff; /* ??? */
	   rta = (struct rtattr *)(((char *) &req) +
				    NLMSG_ALIGN(req.nh.nlmsg_len));
	   rta->rta_type = IFLA_MTU;
	   rta->rta_len = RTA_LENGTH(sizeof(unsigned int));
	   req.nh.nlmsg_len = NLMSG_ALIGN(req.nh.nlmsg_len) +
					 RTA_LENGTH(sizeof(mtu));
	   memcpy(RTA_DATA(rta), &mtu, sizeof(mtu));
	   send(rtnetlink_sk, &req, req.nh.nlmsg_len, 0);

SEE ALSO

       netlink(3), netlink(7), rtnetlink(7)

COLOPHON

       This page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and  the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

GNU
								    2014-09-06							      RTNETLINK(3)
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