arp(7P)

NAME

     arp, ARP - Address Resolution Protocol

SYNOPSIS

     #include <sys/fcntl.h>

     #include <sys/socket.h>

     #include <net/if_arp.h>

     #include <netinet/in.h>

     s = socket(AF_INET, SOCK_DGRAM, 0);

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

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)  and  it  can  be used by other  datalink providers
     that support broadcast, such as  FDDI and  Token  Ring.  The
     only  network  layer supported in this implementation is the
     Internet Protocol, although ARP is not specific to that pro-
     tocol.

     ARP caches IP-to-Ethernet address mappings. When  an  inter-
     face 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, ARP caches the new  map-
     ping  and  transmits  any pending message. ARP will queue at
     most four packets while waiting for a response to a  mapping
     request;   it  keeps only the four most recently transmitted
     packets.

APPLICATION PROGRAMMING INTERFACE

     The STREAMS device /dev/arp is not a Transport Level  Inter-
     face ("TLI)" transport provider and may not be used with the
     TLI interface.

     To facilitate communications with systems that  do  not  use
     ARP,  ioctl()  requests  are  provided  to  enter and delete
     entries in the IP-to-Ethernet tables.

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

     Each ioctl() request takes the same structure  as  an  argu-
     ment.  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
     */
     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 0x2           /* completed entry (arp_ha valid) */
     #define ATF_PERM 0x4          /* permanent entry */
     #define ATF_PUBL 0x8          /* 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 that may be written are ATF_PUBL  and  ATF_USETRAILERS.
     ATF_PERM  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. 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 encapsu-
     lations.  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  trans-
     lation  replies  along  with replies to IP requests; trailer
     encapsulations are also sent  in  reply  to  IP  translation
     replies.  The negotiation is thus fully symmetrical, in that
     either   host   or   both   may   request   trailers.    The
     ATF_USETRAILERS flag records 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_tcp(7P), inet(7P)

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

     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,
     California, November 1982.

DIAGNOSTICS

     IP: Hardware address '%x:%x:%x:%x:%x:%x'

     trying to be our address '%d.%d.%d.%d'!
           Duplicate IP address. ARP has discovered another  host
           on  the  local  network  which  responds  to   mapping
           requests for the Internet address of this system.

     IP: Proxy ARP problem?  Hardware address '%x:%x:%x:%x:%x:%x'

     thinks it is '%d.%d.%d.%d'
           This message will appear if  arp(1M) has been used  to
           create  a  published entry, and some other host on the
           local network responds to  mapping  requests  for  the
           published ARPentry.