NAME    [Toc]    [Back]

       arp - Address Resolution Protocol

SYNOPSIS    [Toc]    [Back]

       pseudo-device ether

DESCRIPTION    [Toc]    [Back]

       The  ARP protocol is used to map dynamically between DARPA
       Internet and Ethernet addresses.  It is used  by  all  the
       Ethernet interface drivers.

       The  ARP  protocol  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 messages are transmitted.  The ARP protocol queues
       only the most recently ``transmitted'' packet while  waiting
 for a mapping request to be responded to.

       To  enable  communications  with  systems which do not use
       ARP, ioctls are provided to enter and  delete  entries  in
       the Internet-to-Ethernet tables. The usage is:

       #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  superuser.
 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   2    /*  completed
  entry  (arp_ha  valid)  */ #define ATF_PERM 4   /*
       permanent  entry  */  #define  ATF_PUBL  8    /*   publish
       (respond for other 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 that can be written are ATF_PERM and
       ATF_PUBL. ATF_PERM causes the entry to be permanent if the
       ioctl  call succeeds. The ioctl may fail if more than four
       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  lets  the  system  act as an ARP server,
       which can be used to make an ARP-only machine  talk  to  a
       non-ARP machine.

       The  ARP  protocol  watches  passively  for  a  host  that
       responds to an ARP mapping request for  the  local  host's
       address.

RESTRICTIONS    [Toc]    [Back]

       ARP  packets  on  the  Ethernet use only 42 bytes of data.
       The smallest legal Ethernet packet is 60  bytes,  however,
       not  including CRC. Some systems may not enforce the minimum
 packet size.

ERRORS    [Toc]    [Back]

       ARP has discovered another host on the local network  that
       responds to mapping requests for its own Internet address.

SEE ALSO    [Toc]    [Back]

       inet(7), arp(8), ifconfig(8)



                                                           arp(7)