4.3BSD - /usr/man/cat4/arp.0

ARP(4P)                                                                ARP(4P)


NAME
       arp - Address Resolution Protocol

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 mes‐
       sage which requires the mapping and broadcasts a message on the associ‐
       ated network requesting the address mapping.  If  a  response  is  pro‐
       vided,  the  new mapping is cached and any pending message is transmit‐
       ted.  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
       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
       ec(4), de(4), il(4), inet(4F), arp(8C), ifconfig(8C)
       ‘‘An Ethernet Address Resolution Protocol,’’ RFC826, Dave Plummer, Net‐
       work Information Center, SRI.
       ‘‘Trailer Encapsulations,’’ RFC893, S.J. Leffler and M.J. Karels,  Net‐
       work 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.


4.2 Berkeley Distribution        May 15, 1986                          ARP(4P)