arpping.c

/* vi: set sw=4 ts=4: */
/*
 * Mostly stolen from: dhcpcd - DHCP client daemon
 * by Yoichi Hariguchi <yoichi@fore.com>
 *
 * Licensed under GPLv2, see file LICENSE in this source tree.
 */
#include <netinet/if_ether.h>
#include <net/if_arp.h>
#include <stdint.h>
#include <string.h>
#include <sys/time.h>
#include <poll.h>
#include <netinet/in.h>
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/ioctl.h>
#include <linux/if.h>

struct arpMsg {
    /* Ethernet header */
    uint8_t  h_dest[6];     /* 00 destination ether addr */
    uint8_t  h_source[6];   /* 06 source ether addr */
    uint16_t h_proto;       /* 0c packet type ID field */

    /* ARP packet */
    uint16_t htype;         /* 0e hardware type (must be ARPHRD_ETHER) */
    uint16_t ptype;         /* 10 protocol type (must be ETH_P_IP) */
    uint8_t  hlen;          /* 12 hardware address length (must be 6) */
    uint8_t  plen;          /* 13 protocol address length (must be 4) */
    uint16_t operation;     /* 14 ARP opcode */
    uint8_t  sHaddr[6];     /* 16 sender's hardware address */
    uint8_t  sInaddr[4];    /* 1c sender's IP address */
    uint8_t  tHaddr[6];     /* 20 target's hardware address */
    uint8_t  tInaddr[4];    /* 26 target's IP address */
    uint8_t  pad[18];       /* 2a pad for min. ethernet payload (60 bytes) */
} PACKED;

enum {
    ARP_MSG_SIZE = 0x2a
};

const int const_int_1 = 1;

/* Like strncpy but make sure the resulting string is always 0 terminated. */
static
char* safe_strncpy(char *dst, const char *src, size_t size)
{
        if (!size) return dst;
        dst[--size] = '\0';
        return strncpy(dst, src, size);
}

static
unsigned long long monotonic_ms(void)
{
        struct timeval tv;
        gettimeofday (&tv, NULL);
        return tv.tv_sec * 1000ULL + tv.tv_usec/1000;
}

/* Wrapper which restarts poll on EINTR or ENOMEM.
 * On other errors does perror("poll") and returns.
 * Warning! May take longer than timeout_ms to return! */
static
int safe_poll(struct pollfd *ufds, nfds_t nfds, int timeout)
{
        while (1) {
                int n = poll(ufds, nfds, timeout);
                if (n >= 0)
                        return n;
                /* Make sure we inch towards completion */
                if (timeout > 0)
                        timeout--;
                /* E.g. strace causes poll to return this */
                if (errno == EINTR)
                        continue;
                /* Kernel is very low on memory. Retry. */
                /* I doubt many callers would handle this correctly! */
                if (errno == ENOMEM)
                        continue;
                return n;
        }
}

static
ssize_t safe_read(int fd, void *buf, size_t count)
{
        ssize_t n;

        do {
                n = read(fd, buf, count);
        } while (n < 0 && errno == EINTR);

        return n;
}

static
char* strncpy_IFNAMSIZ(char *dst, const char *src)
{
#ifndef IFNAMSIZ
        enum { IFNAMSIZ = 16 };
#endif
        return strncpy(dst, src, IFNAMSIZ);
}

static
int udhcp_read_interface(const char *interface, int *ifindex, uint32_t *nip, uint8_t *mac)
{
    /* char buffer instead of bona-fide struct avoids aliasing warning */
    char ifr_buf[sizeof(struct ifreq)];
    struct ifreq *const ifr = (void *)ifr_buf;

    int fd;
    struct sockaddr_in *our_ip;

    memset(ifr, 0, sizeof(*ifr));
    fd = socket(AF_INET, SOCK_RAW, IPPROTO_RAW);

    ifr->ifr_addr.sa_family = AF_INET;
    strncpy_IFNAMSIZ(ifr->ifr_name, interface);
    if (nip) {
        if (ioctl(fd, SIOCGIFADDR, ifr)) {
            close(fd);
            return -1;
        }
        our_ip = (struct sockaddr_in *) &ifr->ifr_addr;
        *nip = our_ip->sin_addr.s_addr;
    }

    if (ifindex) {
        if (ioctl(fd, SIOCGIFINDEX, ifr) != 0) {
            close(fd);
            return -1;
        }
        *ifindex = ifr->ifr_ifindex;
    }

    if (mac) {
        if (ioctl(fd, SIOCGIFHWADDR, ifr) != 0) {
            close(fd);
            return -1;
        }
        memcpy(mac, ifr->ifr_hwaddr.sa_data, 6);
    }

    close(fd);
    return 0;
}

/* Returns 1 if no reply received */
int arpping(uint32_t test_nip,
        char *result_mac,
        const char *interface)
{
    int timeout_ms;
    struct pollfd pfd[1];
#define s (pfd[0].fd)           /* socket */
    int rv = 1;             /* "no reply received" yet */
    struct sockaddr addr;   /* for interface name */
    struct arpMsg arp;

    uint32_t from_ip;
    uint8_t from_mac[6];
    udhcp_read_interface (interface, NULL, &from_ip, from_mac);

    s = socket(PF_PACKET, SOCK_PACKET, htons(ETH_P_ARP));
    if (s == -1) {
        //bb_perror_msg(bb_msg_can_not_create_raw_socket);
        return -1;
    }

    if (setsockopt(s, SOL_SOCKET, SO_BROADCAST, &const_int_1, sizeof(const_int_1)) == -1) {
        //bb_perror_msg("can't enable bcast on raw socket");
        goto ret;
    }

    /* send arp request */
    memset(&arp, 0, sizeof(arp));
    memset(arp.h_dest, 0xff, 6);                    /* MAC DA */
    memcpy(arp.h_source, from_mac, 6);              /* MAC SA */
    arp.h_proto = htons(ETH_P_ARP);                 /* protocol type (Ethernet) */
    arp.htype = htons(ARPHRD_ETHER);                /* hardware type */
    arp.ptype = htons(ETH_P_IP);                    /* protocol type (ARP message) */
    arp.hlen = 6;                                   /* hardware address length */
    arp.plen = 4;                                   /* protocol address length */
    arp.operation = htons(ARPOP_REQUEST);           /* ARP op code */
    memcpy(arp.sHaddr, from_mac, 6);                /* source hardware address */
    memcpy(arp.sInaddr, &from_ip, sizeof(from_ip)); /* source IP address */
    /* tHaddr is zero-filled */                     /* target hardware address */
    memcpy(arp.tInaddr, &test_nip, sizeof(test_nip));/* target IP address */

    memset(&addr, 0, sizeof(addr));
    safe_strncpy(addr.sa_data, interface, sizeof(addr.sa_data));
    if (sendto(s, &arp, sizeof(arp), 0, &addr, sizeof(addr)) < 0) {
        // TODO: error message? caller didn't expect us to fail,
        // just returning 1 "no reply received" misleads it.
        goto ret;
    }

    /* wait for arp reply, and check it */
    timeout_ms = 1000;
    do {
        typedef uint32_t aliased_uint32_t;
        int r;
        unsigned prevTime = monotonic_ms();

        pfd[0].events = POLLIN;
        r = safe_poll(pfd, 1, timeout_ms);
        if (r < 0)
            break;
        if (r) {
            r = safe_read(s, &arp, sizeof(arp));
            if (r < 0)
                break;

            if (r >= ARP_MSG_SIZE
             && arp.operation == htons(ARPOP_REPLY)
             /* don't check it: Linux doesn't return proper tHaddr (fixed in 2.6.24?) */
             /* && memcmp(arp.tHaddr, from_mac, 6) == 0 */
             && *(aliased_uint32_t*)arp.sInaddr == test_nip
            ) {
                /* if ARP source MAC matches safe_mac
                 * (which is client's MAC), then it's not a conflict
                 * (client simply already has this IP and replies to ARPs!)
                 */
                sprintf(result_mac, "%02X:%02X:%02X:%02X:%02X:%02X",
                    arp.sHaddr[0], arp.sHaddr[1], arp.sHaddr[2],
                    arp.sHaddr[3], arp.sHaddr[4], arp.sHaddr[5]);
                break;
            }
        }
        timeout_ms -= (unsigned)monotonic_ms() - prevTime;
    } while (timeout_ms > 0);

 ret:
    close(s);
    return rv;
}