ARP(7P)                                                   ARP(7P)



     NAME
          arp - Address Resolution Protocol - TCP

     SYNOPSIS
          pseudo-device ether

     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.
          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) */



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     ARP(7P)                                                   ARP(7P)



          #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
          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.

     SEE ALSO
          inet(7p), arp(1m), ifconfig(1m)
          ``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.




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     ARP(7P)                                                   ARP(7P)



     ORIGIN
          4.3 BSD





















































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