ARP(7)                           (TPC/IP)                            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:



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   ARP(7)                           (TPC/IP)                            ARP(7)


         /*
         * ARP ioctl request
         */
         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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