How can I determine the default gateway on iPhone?

Question

I need to obtain the IP address of the default gateway in an iPhone application. I cannot find the proper API. Anyone know if iPhone exposes this information?

Answer 1

I have solved this, though i dunno if this solution is valid (wont be rejected by apple)

first i'll give you the sources i used to build my solution upon:

http://code.google.com/p/doubango/source/browse/trunk/thirdparties/iphone/include/net/route.h?r=348

http://code.google.com/p/touchcode/source/browse/Experimental/TouchHTTPD/Externals/libnatpmp-20071213/getgateway.c?r=bc90b03205cbebb0633a7b5b203c2c6d5cb860dc

It would be great to get some feedback if you think apple will allow this kind of code.

getgateway.h

#ifndef __GETGATEWAY_H__
#define __GETGATEWAY_H__

/* getdefaultgateway() :
 * return value :
 *    0 : success
 *   -1 : failure    */
int getdefaultgateway(in_addr_t * addr);

#endif

getgateway.c

#include <stdio.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <sys/sysctl.h>
#include "getgateway.h"
#include "route.h"
#include <net/if.h>
#include <string.h>

#define CTL_NET         4               /* network, see socket.h */


#if defined(BSD) || defined(__APPLE__)

#define ROUNDUP(a) \
((a) > 0 ? (1 + (((a) - 1) | (sizeof(long) - 1))) : sizeof(long))

int getdefaultgateway(in_addr_t * addr)
{
    int mib[] = {CTL_NET, PF_ROUTE, 0, AF_INET,
        NET_RT_FLAGS, RTF_GATEWAY};
    size_t l;
    char * buf, * p;
    struct rt_msghdr * rt;
    struct sockaddr * sa;
    struct sockaddr * sa_tab[RTAX_MAX];
    int i;
    int r = -1;
    if(sysctl(mib, sizeof(mib)/sizeof(int), 0, &l, 0, 0) < 0) {
        return -1;
    }
    if(l>0) {
        buf = malloc(l);
        if(sysctl(mib, sizeof(mib)/sizeof(int), buf, &l, 0, 0) < 0) {
            return -1;
        }
        for(p=buf; p<buf+l; p+=rt->rtm_msglen) {
            rt = (struct rt_msghdr *)p;
            sa = (struct sockaddr *)(rt + 1);
            for(i=0; i<RTAX_MAX; i++) {
                if(rt->rtm_addrs & (1 << i)) {
                    sa_tab[i] = sa;
                    sa = (struct sockaddr *)((char *)sa + ROUNDUP(sa->sa_len));
                } else {
                    sa_tab[i] = NULL;
                }
            }

            if( ((rt->rtm_addrs & (RTA_DST|RTA_GATEWAY)) == (RTA_DST|RTA_GATEWAY))
               && sa_tab[RTAX_DST]->sa_family == AF_INET
               && sa_tab[RTAX_GATEWAY]->sa_family == AF_INET) {


                if(((struct sockaddr_in *)sa_tab[RTAX_DST])->sin_addr.s_addr == 0) {
                        char ifName[128];
                        if_indextoname(rt->rtm_index,ifName);

                        if(strcmp("en0",ifName)==0){

                                *addr = ((struct sockaddr_in *)(sa_tab[RTAX_GATEWAY]))->sin_addr.s_addr;
                                r = 0;                        
                        }
                }
            }
        }
        free(buf);
    }
    return r;
}
#endif