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@(#)arp4.4 6.1 (Berkeley) 05/15/85

C 5 arp - Address Resolution Protocol "pseudo-device ether" ARP is a protocol used to dynamically map between DARPA Internet and 10Mb/s Ethernet addresses. It is used by all the 10Mb/s Ethernet interface drivers.

ARP 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. ARP will queue at most one packet while waiting for a mapping request to be responded to; only the most recently ``transmitted'' packet is kept.

To enable communications with systems which do not use ARP, ioctls are provided to enter and delete entries in the Internet-to-Ethernet tables. Usage:

 #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 super-user. 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 which may be written are ATF_PERM and ATF_PUBL. ATF_PERM causes the entry to be permanent if the ioctl call succeeds. The peculiar nature of the ARP tables may cause the ioctl to fail if more than 4 (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 allows a host to act as an "ARP server" which may be useful in convincing an ARP-only machine to talk to a non-ARP machine.

ARP watches passively for hosts impersonating the local host (i.e. a host which responds to an ARP mapping request for the local host's address).

"duplicate IP address!! sent from ethernet address: %x:%x:%x:%x:%x:%x." ARP has discovered another host on the local network which responds to mapping requests for its own Internet address. ec(4), de(4), il(4), inet(4F), arp(8C), ifconfig(8C)

An Ethernet Address Resolution Protocol, RFC826, Dave Plummer, MIT.

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