arp(7P)

NAME
     arp - Address Resolution Protocol

DESCRIPTION
     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.  It is not specific to Internet protocols or to 10Mb/s
     Ethernet, but this implementation currently supports only that
     combination.

     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 message is
     transmitted.  Each address mapping has a timer associated with it and
     completed address mappings are aged after 20 minutes.  The timer is reset
     each time the address mapping is updated by a SIOCSARP ioctl call or by
     sending a packet to the Internet address to which the mapping belongs.
     This timer value is not configurable.  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 facilitate 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        0x02 /* completed entry (arp_ha valid) */
     #define   ATF_PERM       0x04 /* permanent entry */
     #define   ATF_PUBL       0x08 /* publish (respond for other host) */
     #define   ATF_USETRAILERS     0x10 /* send trailer packets to 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, ATF_PUBL and ATF_USETRAILERS.  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 8 (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 is also used to negotiate the use of trailer IP encapsulations;
     trailers are an alternate encapsulation used to allow efficient packet
     alignment for large packets despite variable-sized headers.  Hosts which
     wish to receive trailer encapsulations so indicate by sending gratuitous
     ARP translation replies along with replies to IP requests; they are also
     sent in reply to IP translation replies.  The negotiation is thus fully
     symmetrical, in that either or both hosts may request trailers.  The
     ATF_USETRAILERS flag is used to record the receipt of such a reply, and
     enables the transmission of trailer packets to that host.

     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).

DIAGNOSTICS
     The following messages can appear on the console:
     arp: host with ether address %x:%x:%x:%x:%x:%x is using my IP address
     x.x.x.x
     ARP has discovered another host on the local network which responds to
     mapping requests for its own Internet address.
     arp: ether address is broadcast for IP address x.x.x.x
     ARP has discovered another host on the local network which maps that
     host's IP address onto the ethernet broadcast address.

SEE ALSO
     inet(7F), arp(1M), ifconfig(1M), intro(3)
     ``An Ethernet Address Resolution Protocol,'' RFC826, Dave Plummer,
     Network Information Center, SRI.
     ``Trailer Encapsulations,'' RFC893, S.J. Leffler and M.J. Karels, Network
     Information Center, SRI.

BUGS
     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.