NAME

arp — address resolution display and control

SYNOPSIS

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. If the ifscope keyword is specified, the entry specific to the interface 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.

-l

Show link-layer reachability information.

-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 only keyword is also specified, this will create a “published (proxy only)” entry. This type of entry is created automatically if arp detects that a routing table entry for hostname already exists.

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.

If the ifscope keyword is specified, the entry will set with an additional property that strictly associate the entry to the interface. This allows for the presence of multiple entries with the same destination on different interfaces.

-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] [pub [only]] [ifscope interface]

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.

-x

Show extended link-layer reachability information in addition to that shown by the -l flag.

SEE ALSO

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

HISTORY

The arp utility appeared in 4.3BSD.