//********************************************************************************
//* Resolve an IP address to a hardware address; if not found,initiate query
//* and return NULL.  If an address is returned, the interface driver may  
//* send the packet; if NULL is returned,res_arp() will have saved the packet 
//* on its pending queue,so no further action (like freeing the packet) is 
//* necessary.

uint8 * res_arp(
     struct iface *iface,		//* Pointer to interface block */
     enum   arp_hwtype hardware,	//* Hardware type */
     int32  target,			//* Target IP address */
     struct mbuf **bpp)	    //* IP datagram to be queued if unresolved */                
{
    register struct arp_tab *arp;
    struct ip ip;
    
    if((arp = arp_lookup(hardware,target)) != NULL && arp->state == ARP_VALID)
    	return arp->hw_addr;
    if(arp != NULL){
    	/* Earlier packets are already pending, kick this one back
    	 * as a source quench
    	 */
    	ntohip(&ip,bpp);
    	icmp_output(&ip,*bpp,ICMP_QUENCH,0,NULL);
    	free_p(bpp);
    } else {
    	/* Create an entry and put the datagram on the
    	 * queue pending an answer
    	 */
    	arp = arp_add(target,hardware,NULL,0);
    	enqueue(&arp->pending,bpp);
    	arp_output(iface,hardware,target);
    }
    return NULL;
}
/* Look up the given IP address in the ARP table */
struct arp_tab *arp_lookup(hardware,ipaddr)
enum   arp_hwtype hardware;
int32  ipaddr;
{
	register struct arp_tab *ap;

	for(ap = Arp_tab[hash_ip(ipaddr)]; ap != NULL; ap = ap->next){
		if(ap->ip_addr == ipaddr && ap->hardware == hardware)
			break;
	}
	return ap;
}