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arp(7)


NAME

  arp - Address Resolution Protocol


SYNOPSIS

  pseudo-device ether


DESCRIPTION

  The ARP protocol is used to map dynamically between DARPA Internet and
  Ethernet addresses.  It is used by all the 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 messages 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 the system 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.


RESTRICTIONS

  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.


ERRORS

  arp: local IP address nn.nn.nn.nn in use by hardware 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(7), arp(8), ifconfig(8)

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