| ARP(4) | Device Drivers Manual | ARP(4) | 
arp —
#include <netinet/if_ether.h>
ARP 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 message is transmitted. ARP will queue at
    most one packet while waiting for a response to a mapping request; only the
    most recently ``transmitted'' packet is kept. If the target host does not
    respond after several requests, the host is considered to be down for a
    short period (normally 20 seconds), allowing an error to be returned to
    transmission attempts during this interval. The error is
    EHOSTDOWN for a non-responding destination host, and
    EHOSTUNREACH for a non-responding router.
Each ARP cache entry is stored in a network interface which a response of ARP comes in. ARP cache entries time out periodically (normally 20 minutes after validated; entries are not validated when not in use).
ARP entries may be added, deleted or changed with the arp(8) utility. Manually-added entries may be temporary or permanent, and may be “published”, in which case the system will respond to ARP requests for that host as if it were the target of the request.
In the past, ARP was used to negotiate the use of a trailer encapsulation. This is no longer supported.
ARP will defend the host's active address when a conflicting message is received. However, if another conflicting message for the address is found within a 10 second period, then the address is marked duplicate and the host will stop using it.
For some systems such as a router or a server, it is desirable
    never to give up an assigned address. This can be achieved by setting the
    sysctl(7) variable
    net.inet.ip_dad_count to 0.
In all of the above cases, ARP will log diagnostic messages which include the hardware address of the conflicting host.
Plummer, D., RFC 826, An Ethernet Address Resolution Protocol.
Leffler, S.J. and Karels, M.J., RFC 893, Trailer Encapsulations.
Cheshire, S., RFC 5227, IPv4 Address Conflict Detection.
Address Conflict Detection was added in NetBSD 8.0.
| October 12, 2016 | NetBSD 10.1 |