ARP(7)                 UNIX System V(Internet Utilities)                 ARP(7)


NAME
      ARP - Address Resolution Protocol

SYNOPSIS
      #include <sys/socket.h>
      #include <net/ifarp.h>
      #include <netinet/in.h>

      s = socket(AFINET, SOCKDGRAM, 0);

      d = open ("/dev/arp", ORDWR);

DESCRIPTION
      ARP is a protocol used to map dynamically between Internet Protocol (IP)
      and 10Mb/s Ethernet addresses.  It is used by all the 10Mb/s Ethernet
      datalink providers (interface drivers).  It is not specific to the
      Internet Protocol or to the 10Mb/s Ethernet, but this implementation
      currently supports only that combination.  The STREAMS device /dev/arp is
      not a Transport Level Interface (TLI) transport provider and may not be
      used with the TLI interface.

      ARP caches IP-to-Ethernet address mappings.  When an interface requests a
      mapping for an address not in the cache, ARP queues the message that
      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, ioctl()
      requests are provided to enter and delete entries in the IP-to-Ethernet
      tables.

USAGE
            #include <sys/sockio.h>
            #include <sys/socket.h>
            #include <net/if.h>
            #include <net/ifarp.h>
            struct arpreq arpreq;
            ioctl(s, SIOCSARP, (caddrt)&arpreq);
            ioctl(s, SIOCGARP, (caddrt)&arpreq);
            ioctl(s, SIOCDARP, (caddrt)&arpreq);

      Each ioctl() request takes the same structure as an argument.  SIOCSARP
      sets an ARP entry, SIOCGARP gets an ARP entry, and SIOCDARP deletes an
      ARP entry.  These ioctl() requests may be applied to any Internet family
      socket descriptor s, or to a descriptor for the ARP device, but only by
      the privileged user.  The arpreq structure contains:

      /*
      * ARP ioctl request
      */


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ARP(7)                 UNIX System V(Internet Utilities)                 ARP(7)


      struct arpreq {
            struct sockaddr arppa;     /* protocol address */
            struct sockaddr arpha;     /* hardware address */
            int             arpflags;  /* flags */
      };
      /*  arpflags field values */
      #define ATFCOM          0x2  /* completed entry (arpha valid) */
      #define ATFPERM         0x4  /* permanent entry */
      #define ATFPUBL         0x8  /* publish (respond for other host) */
      #define ATFUSETRAILERS  0x10 /* send trailer packets to host */

      The address family for the arppa sockaddr must be AFINET; for the
      arpha sockaddr it must be AFUNSPEC.  The only flag bits that may be
      written are ATFPERM, ATFPUBL and ATFUSETRAILERS.  ATFPERM makes the
      entry permanent if the ioctl() request succeeds.  The peculiar nature of
      the ARP tables may cause the ioctl() request to fail if too many
      permanent IP addresses hash to the same slot.  ATFPUBL 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 that
      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
      ATFUSETRAILERS 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 (that is, a
      host which responds to an ARP mapping request for the local host's
      address).

SEE ALSO
      arp(1M), ifconfig(1M), if(3N), inet(7)

      Plummer, Dave, ``An Ethernet Address Resolution Protocol -or- Converting
      Network Protocol Addresses to 48.bit Ethernet Addresses for Transmission
      on Ethernet Hardware,'' RFC 826, Network Information Center, SRI
      International, Menlo Park, Calif., November 1982

      Leffler, Sam, and Michael Karels, ``Trailer Encapsulations,'' RFC 893,
      Network Information Center, SRI International, Menlo Park, Calif., April
      1984







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