Get router mac (without system call for ARP) in Objective-C

Question

Ok - I am writing a daemon in Objective C that checks the connected router mac address every 5 seconds.

I am completely new to objective C, and I am looking for a better way to do what I'm already doing.

I'm currently calling "arp -a" and parsing the results via "Task":

NSTask *task;
task = [[NSTask alloc] init];
[task setLaunchPath: @"/usr/sbin/arp"];

NSArray *arguments;
arguments = [NSArray arrayWithObjects: @"-a", nil];
[task setArguments: arguments]; 

NSPipe *pipe;
pipe = [NSPipe pipe];
[task setStandardOutput: pipe];

NSFileHandle *file;
file = [pipe fileHandleForReading];

[task launch];

NSData *data;
data = [file readDataToEndOfFile];

I'm afraid that this isn't very efficient.

Any suggestions? I am running this codeblock once every 5 seconds.

Solution

Apple's implementation of arp is open source. Take a look at the file for an idea of its implementation... it's not terribly convoluted. It is pure ANSI C, though.

You should be able to simply copy-paste the majority of the functionality... and instead of printing the results, just store the raw address.

Edit: Here's a stripped down version of the source that just runs the equivalent of arp -a. This should compile without any special directives.

/*
 * Copyright (c) 1999 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this
 * file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_LICENSE_HEADER_END@
 */


#include <sys/param.h>
#include <sys/file.h>
#include <sys/socket.h>
#include <sys/sysctl.h>

#include <net/if.h>
#include <net/if_dl.h>
#include <net/if_types.h>
#include <net/route.h>

#include <netinet/in.h>
#include <netinet/if_ether.h>

#include <arpa/inet.h>

#include <err.h>
#include <errno.h>
#include <netdb.h>
#include <nlist.h>
#include <paths.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

static int nflag;

void
ether_print(cp)
    u_char *cp;
{
    printf("%x:%x:%x:%x:%x:%x", cp[0], cp[1], cp[2], cp[3], cp[4], cp[5]);
}


/*
 * Dump the entire arp table
 */
int
dump(addr)
    u_long addr;
{
    int mib[6];
    size_t needed;
    char *host, *lim, *buf, *next;
    struct rt_msghdr *rtm;
    struct sockaddr_inarp *sin;
    struct sockaddr_dl *sdl;
    extern int h_errno;
    struct hostent *hp;
    int found_entry = 0;

    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)
        err(1, "route-sysctl-estimate");
    if ((buf = malloc(needed)) == NULL)
        err(1, "malloc");
    if (sysctl(mib, 6, buf, &needed, NULL, 0) < 0)
        err(1, "actual retrieval of routing table");
    lim = buf + needed;
    for (next = buf; next < lim; next += rtm->rtm_msglen) {
        rtm = (struct rt_msghdr *)next;
        sin = (struct sockaddr_inarp *)(rtm + 1);
        sdl = (struct sockaddr_dl *)(sin + 1);
        if (addr) {
            if (addr != sin->sin_addr.s_addr)
                continue;
            found_entry = 1;
        }
        if (nflag == 0)
            hp = gethostbyaddr((caddr_t)&(sin->sin_addr),
                sizeof sin->sin_addr, AF_INET);
        else
            hp = 0;
        if (hp)
            host = hp->h_name;
        else {
            host = "?";
            if (h_errno == TRY_AGAIN)
                nflag = 1;
        }
        printf("%s (%s) at ", host, inet_ntoa(sin->sin_addr));
        if (sdl->sdl_alen)
            ether_print((u_char *)LLADDR(sdl));
        else
            printf("(incomplete)");
        if (rtm->rtm_rmx.rmx_expire == 0)
            printf(" permanent");
        if (sin->sin_other & SIN_PROXY)
            printf(" published (proxy only)");
        if (rtm->rtm_addrs & RTA_NETMASK) {
            sin = (struct sockaddr_inarp *)
                (sdl->sdl_len + (char *)sdl);
            if (sin->sin_addr.s_addr == 0xffffffff)
                printf(" published");
            if (sin->sin_len != 8)
                printf("(weird)");
        }
        printf("\n");
    }
    return (found_entry);
}

int main (int argc, char const *argv[])
{
    dump(0);
    return 0;
}