minimal icmp for ios

icmp.c

#include <arpa/inet.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>

#define PACKETSIZE    64

struct icmp_echo {
    u_char    icmp_type;
    u_char    icmp_code;
    u_short   icmp_cksum;
    u_short   icmp_echo_id;
    u_short   icmp_echo_seq;
};

struct icmp_pkt
{
    struct ip ip;
    struct icmp_echo icmp;
    char msg[PACKETSIZE-sizeof( struct icmp )];
};

static unsigned short checksum( void * b, int len )
{
    unsigned short * buf = b;
    unsigned int sum = 0;
    unsigned short result;
    
    for( sum = 0; len > 1; len -= 2 ) sum += *buf++;
    if( len == 1 )
        sum += *(unsigned char *)buf;
    sum = (sum >> 16) + (sum & 0xFFFF);
    sum += (sum >> 16);
    result = ~sum;
    return result;
}

static uint16_t g_ip_id;
static uint16_t g_icmp_id;
static uint16_t g_icmp_seq;

static void make_icmp_pkt( struct icmp_pkt * pkt, uint32_t src, uint32_t dst )
{
    pkt->ip.ip_v = 4;
    pkt->ip.ip_hl = sizeof( struct ip ) >> 2;
    pkt->ip.ip_tos = 0;
    pkt->ip.ip_len = htons( sizeof( struct icmp_pkt ) );
    pkt->ip.ip_id = htons( g_ip_id++ );
    pkt->ip.ip_off = htons( IP_DF );
    pkt->ip.ip_ttl = 64;
    pkt->ip.ip_p = IPPROTO_ICMP;
    pkt->ip.ip_sum = 0;
    pkt->ip.ip_src.s_addr = src;
    pkt->ip.ip_dst.s_addr = dst;
    
    pkt->ip.ip_sum = checksum( &pkt->ip, sizeof( pkt->ip ) );
    
    pkt->icmp.icmp_type = ICMP_ECHO;
    pkt->icmp.icmp_code = 0;
    pkt->icmp.icmp_cksum = 0;
    pkt->icmp.icmp_echo_id = htons( g_icmp_id );
    pkt->icmp.icmp_echo_seq = htons( g_icmp_seq );
    
    for( size_t i = 0; i < sizeof( pkt->msg ); i++ )
        pkt->msg[i] = i;
    
    pkt->icmp.icmp_cksum = checksum( &pkt->icmp, sizeof( pkt->icmp ) + sizeof( pkt->msg ) );
}

static uint32_t make_ip( const char * sz )
{
    struct sockaddr_in sin;
    inet_pton( AF_INET, sz, &sin.sin_addr);
    return sin.sin_addr.s_addr;
}