arp(7)arp(7)NAMEarp - Address Resolution Protocol
SYNOPSIS
pseudo-device ether
DESCRIPTION
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 ``trans‐
mitted'' 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 /* pub‐
lish (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 per‐
manent if the ioctl call succeeds. The ioctl may fail if more than four
permanent Internet host addresses hash to the same slot. ATF_PUBL spec‐
ifies that the ARP code should respond to ARP requests for the indi‐
cated 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
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
ARP has discovered another host on the local network that responds to
mapping requests for its own Internet address.
SEE ALSOinet(7), arp(8), ifconfig(8)arp(7)
