NAME

arp − Address Resolution Protocol

DESCRIPTION

ARP is a protocol used to dynamically map between DARPA Internet and Ethernet addresses on a local area network.  It is used by the Ethernet interface drivers and is not directly accessible by users. 

ARP caches Internet-Ethernet address mappings.  When an interface requests a mapping for an unknown address, ARP drops that message and broadcasts a message on the associated network requesting the address mapping.  If a response is provided, the new mapping is cached and used with any subsequent messages to that address.  Until a response is obtained, ARP discards any messages to the unknown address.  ARP itself is not Internet or Ethernet specific, but this implementation of it is. 

ARP watches passively for hosts impersonating the local host (that is, a host that responds to an ARP mapping request for the local host’s address) and can periodically probe a network looking for imposters. 

The contents of the ARP cache can be accessed and manipulated via the following ioctl commands (on /dev/EXOS/admin):

EXIOCSARP
Set an ARP table entry.

EXOICGARP
Get an ARP table entry.

EXIOCDARP
Delete an ARP table entry.

The structure that is used by all of these commands is

struct EXarp_ioctl {
struct sockaddrarp_pa;/* protocol address */
struct sockaddrarp_ha;/* hardware address */
longarp_flags/* flags */
};

For the purpose of stepping through the ARP table, an EXOICGARP can specify a protocol address of type AF_COUNT.  Otherwise, only types AF_INET and AF_ETHER are currently supported for the protocol address and hardware address, respectively. 

The following flags are meaningful:

AFT_COM
ARP table entry has been completed.

AFT_PERM
ARP table entry is not to age out (that is, it is permanent).

AFT_PUBL
ARP requests for the protocol address should be replied to even if the protocol address is not that of the local host.

DIAGNOSTICS

With respect to ioctl commands the following diagnostics apply:

EACCESS
A non-super user has attempted to alter the contents of the ARP cache.

ENXIO
No entry exists for specified protocol address, or count exceeds table size.

FILES

/dev/EXOS/admin- special file
<EXOS/ex_ioctl.h>- ioctl definitions
<EXOS/sys/host_arp.h>- arp definitions
<EXOS/sys/socket.h>- socket definitions

SEE ALSO

arp(1M)

Version 2.4  —  May 08, 1986