arp(7)							 Miscellaneous Information Manual						    arp(7)

NAME

       arp - Address Resolution Protocol

SYNOPSIS

       pseudo-device ether

DESCRIPTION

       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 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 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.

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.

RELATED INFORMATION

       inet(7), arp(8), ifconfig(8) delim off

																	    arp(7)