ARP - Address Resolution Protocol

Address Resolution Protocol ou ARP é um protocolo usado para encontrar um endereço Ethernet (MAC) a partir do endereço IP. O emissor difunde em broadcast um pacote ARP contendo o endereço IP de outro host e espera uma resposta com um endereço MAC respectivo. Cada máquina mantém uma tabela de resolução em cache para reduzir a latência e carga na rede. O ARP permite que o endereço IP seja independente do endereço Ethernet, mas apenas funciona se todos os hosts o suportarem.
O ARP pode trabalhar com outras camadas de enlace, além do Ethernet e outras camadas de rede, além do Protocolo IP.A alternativa para as máquinas que não suportem ARP é um pré-mapeamento (estático) dos endereços.

Ao processo inverso dá-se o nome de RARP (Reverse ARP).

Nota: arp é também o nome de um comando Unix que permite consultar e manipular o cache de ARP do kernel do sistema operativo.
ARP(8) DragonFly System Manager's Manual ARP(8)

NAME
arp(4,8) -- address resolution display and control

SYNOPSIS
arp(4,8) [-n] [-c cpu] hostname(1,7,1 ghostname)
arp(4,8) [-n] [-c cpu] -a
arp(4,8) -d hostname(1,7,1 ghostname) [pub]
arp(4,8) -d -a
arp(4,8) -s hostname(1,7,1 ghostname) ether_addr [temp] [pub [only]]
arp(4,8) -S hostname(1,7,1 ghostname) ether_addr [temp] [pub [only]]
arp(4,8) -f filename

DESCRIPTION
The arp(4,8) utility displays and modifies the Internet-to-Ethernet address
translation tables used by the address resolution protocol (arp(4,8)(4)).
With no flags, the program displays the current ARP entry for hostname.
The host may be specified by name or by number, using Internet dot nota-
tion.

Available options:

-a The program displays or deletes all of the current ARP entries.

-c cpu On SMP systems the route(4,8) table is replicated. This option allows
the route(4,8) table for a specific cpu to be accessed and exists pri-
marily for debugging purposes.

-d A super-user may delete(7,n) an entry for the host called hostname(1,7,1 ghostname)
with the -d flag. If the pub keyword is specified, only the
``published'' ARP entry for this host will be deleted.

Alternatively, the -d flag may be combined with the -a flag to
delete(7,n) all entries.

-n Show network addresses as numbers (normally arp(4,8) attempts to dis-
play addresses symbolically).

-s hostname(1,7,1 ghostname) ether_addr
Create an ARP entry for the host called hostname(1,7,1 ghostname) with the Ether-
net address ether_addr. The Ethernet address is given as six hex
bytes separated by colons. The entry will be permanent unless
the word temp is given in the command. If the word pub is given,
the entry will be ``published''; i.e., this system will act as an
ARP server, responding to requests for hostname(1,7,1 ghostname) even though the
host address is not its own. In this case the ether_addr can be
given as auto in which case the interfaces on this host will be
examined, and if one of them is found to occupy the same subnet,
its Ethernet address will be used. If the only keyword is also
specified, this will create a ``published (proxy only)'' entry.
This type of entry is created automatically if arp(4,8) detects that a
routing table entry for hostname(1,7,1 ghostname) already exists.

-S hostname(1,7,1 ghostname) ether_addr
Is just like -s except any existing ARP entry for this host will
be deleted first.

-f filename
Cause the file filename to be read and multiple entries to be set
in the ARP tables. Entries in the file should be of the form

hostname(1,7,1 ghostname) ether_addr [temp] [pub]

with argument meanings as given above. Leading whitespace and
empty lines are ignored. A `#' character will mark the rest of
the line as a comment.

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