NAME

arp — address resolution display and control

SYNOPSIS

arp	[ -n][ -i interface] hostname

arp	[ -n][ -i interface] -a

arp	-d hostname [ pub]

arp	-d [ -i interface] -a

arp	-s hostname ether_addr [ temp][ blackhole | reject][ pub]

arp	-S hostname ether_addr [ temp][ blackhole | reject][ pub]

arp	-f filename

DESCRIPTION

The arp utility displays and modifies the Internet-to-Ethernet address translation tables used by the address resolution protocol ( arp(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 notation.

Available options:

-a

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

-d

A super-user may delete an entry for the host called hostname 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 all entries.

-i interface

Limit the operation scope to the ARP entries on interface. Applicable only to the following operations: display one, display all, delete all.

-n

Show network addresses as numbers (normally arp attempts to display addresses symbolically).

-s hostname ether_addr

Create an ARP entry for the host called hostname with the Ethernet 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 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 reject keyword is specified the entry will be marked so that traffic to the host will be discarded and the sender will be notified the host is unreachable. The blackhole keyword is similar in that traffic is discarded but the sender is not notified. These can be used to block external traffic to a host without using a firewall.

-S hostname 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 ether_addr [ temp][ blackhole | reject][ 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.

SEE ALSO

inet(3), arp(4), ifconfig(8), ndp(8)

HISTORY

The arp utility appeared in 4.3BSD.