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arp(4p)								       arp(4p)

       arp - Address Resolution Protocol

       pseudo-device ether

       The  ARP protocol is used to map dynamically between DARPA Internet and
       10Mb/s Ethernet addresses.  It is  used	by  all	 the  10Mb/s  Ethernet
       interface drivers.

       The  ARP	 protocol  caches Internet-Ethernet address mappings.  When an
       interface requests a mapping for an  address  not  in  the  cache,  ARP
       queues  the message which requires the mapping and broadcasts a message
       on the  associated  network  requesting	the  address  mapping.	 If  a
       response	 is  provided,	the new mapping is cached and any pending mes‐
       sages are transmitted.  The ARP protocol queues only the most  recently
       ``transmitted''	packet	while  waiting	for  a	mapping	 request to be
       responded to.

       To enable communications with systems which do not use ARP, ioctls  are
       provided	 to  enter  and	 delete	 entries  in  the Internet-to-Ethernet
       tables.	The usage is:
       #include <sys/ioctl.h>
       #include <sys/socket.h>
       #include <net/if.h>
       struct arpreq arpreq;

       ioctl(s, SIOCSARP, (caddr_t)&arpreq);
       ioctl(s, SIOCGARP, (caddr_t)&arpreq);
       ioctl(s, SIOCDARP, (caddr_t)&arpreq);

       Each ioctl takes the same structure as an argument.  SIOCSARP  sets  an
       ARP  entry,  SIOCGARP  gets  an	ARP entry, and SIOCDARP deletes an ARP
       entry.  These ioctls may be applied to any  socket  descriptor  s,  but
       only by the superuser.  The arpreq structure contains:
	* ARP ioctl request
       struct arpreq {
	   struct sockaddr   arp_pa;	 /* protocol address */
	   struct sockaddr   arp_ha;	 /* hardware address */
	   int		     arp_flags;	 /* flags */
       /*  arp_flags field values */
       #define ATF_COM	2   /* completed entry (arp_ha valid) */
       #define	 ATF_PERM 4   /* permanent entry */
       #define	 ATF_PUBL 8   /* publish (respond for other host) */

       The  address  family  for  the arp_pa sockaddr must be AF_INET; for the
       arp_ha sockaddr, it must be AF_UNSPEC.  The only flag bits that can  be
       written	are  ATF_PERM  and  ATF_PUBL.  ATF_PERM causes the entry to be
       permanent if the ioctl call succeeds.  The ioctl may fail if more  than
       four permanent Internet host addresses hash to the same slot.  ATF_PUBL
       specifies that the ARP code should respond  to  ARP  requests  for  the
       indicated  host	coming from other machines.  This lets a SUN act as an
       ARP server, which can be used to make an ARP-only  machine  talk	 to  a
       non-ARP machine.

       The  ARP	 protocol watches passively for a host that responds to an ARP
       mapping request for the local host's address.

       ARP packets on the Ethernet use only 42 bytes of	 data.	 The  smallest
       legal  Ethernet	packet	is 60 bytes, however, not including CRC.  Some
       systems may not enforce the minimum packet size.

       duplicate IP address!! sent from Ethernet address: %x:%x:%x:%x:%x:%x
       ARP has discovered another host on the local network that  responds  to
       mapping requests for its own Internet address.

See Also
       inet(4f), arp(8c), ifconfig(8c)


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