arp, ARP - Address Resolution Protocol


#include <sys/fcntl.h>
#include <sys/socket.h>
#include <net/if_arp.h>
#include <netinet/in.h>

s = socket(PF_INET, SOCK_DGRAM, 0);

d = open ("/dev/arp", oflag);


ARP is a protocol used to map dynamically between Internet Protocol (IP)
and Ethernet addresses. It is used by all Ethernet datalink providers
(network drivers) and can be used by other datalink providers that
support broadcast, including FDDI and Token Ring. The only network
layer supported in this implementation is the Internet Protocol, although
ARP is not specific to that protocol.

ARP caches IP-to-link-layer address mappings. When an interface requests
a mapping for an address not in the cache, ARP queues the message that
requires the mapping and broadcasts a message on the associated network
requesting the address mapping. If a response is provided, ARP caches the
new mapping and transmits any pending message. ARP will queue a maximum
of four packets while awaiting a response to a mapping request. ARP
keeps only the first four transmitted packets.


The STREAMS device /dev/arp is not a Transport Level Interface (TLI)
transport provider and may not be used with the TLI interface.

To facilitate communications with systems that do not use ARP,
ioctl() requests are provided to enter and delete entries in the
IP-to-link address tables. Ioctls that change the table contents
require the PRIV_SYS_NET_CONFIG privilege. See privileges(7).

#include <sys/sockio.h>
#include <sys/socket.h>
#include <net/if.h>
#include <net/if_arp.h>
struct arpreq arpreq;
ioctl(s, SIOCSARP, (caddr_t)&arpreq);
ioctl(s, SIOCGARP, (caddr_t)&arpreq);
ioctl(s, SIOCDARP, (caddr_t)&arpreq);

SIOCSARP, SIOCGARP and SIOCDARP are BSD compatible ioctls. These ioctls
do not communicate the mac address length between the user and the kernel
(and thus only work for 6 byte wide Ethernet addresses). To manage the
ARP cache for media that has different sized mac addresses, use

#include <sys/sockio.h>
#include <sys/socket.h>
#include <net/if.h>
#include <net/if_dl.h>
#include <net/if_arp.h>
struct xarpreq xarpreq;
ioctl(s, SIOCSXARP, (caddr_t)&xarpreq);
ioctl(s, SIOCGXARP, (caddr_t)&xarpreq);
ioctl(s, SIOCDXARP, (caddr_t)&xarpreq);

Each ioctl() request takes the same structure as an argument.
SIOCS[X]ARP sets an ARP entry, SIOCG[X]ARP gets an ARP entry, and
SIOCD[X]ARP deletes an ARP entry. These ioctl() requests may be applied
to any Internet family socket descriptors, or to a descriptor for the ARP
device. Note that SIOCS[X]ARP and SIOCD[X]ARP require the user to have
the PRIV_SYS_NET_CONFIG privilege, while SIOCG[X]ARP does not.

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 */

The xarpreq structure contains:

* Extended ARP ioctl request
struct xarpreq {
struct sockaddr_storage xarp_pa; /* protocol address */
struct sockaddr_dl xarp_ha; /* hardware address */
int xarp_flags; /* arp_flags field values */
#define ATF_COM 0x2 /* completed entry (arp_ha valid) */
#define ATF_PERM 0x4 /* permanent (non-aging) entry */
#define ATF_PUBL 0x8 /* publish (respond for other host) */
#define ATF_USETRAILERS 0x10 /* send trailer pckts to host */
#define ATF_AUTHORITY 0x20 /* hardware address is authoritative */

The address family for the [x]arp_pa sockaddr must be AF_INET. The
ATF_COM flag bits ([x]arp_flags) cannot be altered. ATF_USETRAILERS is
not implemented by the operating system and is retained for compatibility
only. ATF_PERM makes the entry permanent (disables aging) if the ioctl()
request succeeds. ATF_PUBL specifies that the system should respond to
ARP requests for the indicated protocol address coming from other
machines. This allows a host to act as an ARP server, which may be useful
in convincing an ARP-only machine to talk to a non-ARP machine.
ATF_AUTHORITY indicates that this machine owns the address. ARP does not
update the entry based on received packets.

The address family for the arp_ha sockaddr must be AF_UNSPEC.

Before invoking any of the SIOC*XARP ioctls, user code must fill in the
xarp_pa field with the protocol (IP) address information, similar to the
BSD variant. The SIOC*XARP ioctls come in two (legal) varieties,
depending on xarp_ha.sdl_nlen:

1. if sdl_nlen = 0, it behaves as an extended BSD ioctl. The
kernel uses the IP address to determine the network interface.

2. if (sdl_nlen > 0) and (sdl_nlen < LIFNAMSIZ), the kernel uses
the interface name in sdl_data[0] to determine the network
interface; sdl_nlen represents the length of the string
(excluding terminating null character).

3. if (sdl_nlen >= LIFNAMSIZ), an error (EINVAL) is flagged from
the ioctl.

Other than the above, the xarp_ha structure should be 0-filled except for
SIOCSXARP, where the sdl_alen field must be set to the size of hardware
address length and the hardware address itself must be placed in the
LLADDR/sdl_data[] area. (EINVAL will be returned if user specified
sdl_alen does not match the address length of the identified interface).

On return from the kernel on a SIOCGXARP ioctl, the kernel fills in the
name of the interface (excluding terminating NULL) and its hardware
address, one after another, in the sdl_data/LLADDR area; if the two are
larger than can be held in the 244 byte sdl_data[] area, an EINVAL error
is returned. Assuming it fits, the kernel will also set sdl_alen with the
length of the hardware address, sdl_nlen with the length of the name of
the interface (excluding terminating NULL), sdl_type with an IFT_* value
to indicate the type of the media, sdl_slen with 0, sdl_family with
AF_LINK and sdl_index (which if not 0) with system given index for the
interface. The information returned is very similar to that returned via
routing sockets on an RTM_IFINFO message.

The ARP ioctls have several additional restrictions and enhancements
when used in conjunction with IPMP:

o ARP mappings for IPMP data and test addresses are managed by
the kernel and cannot be changed through ARP ioctls, though
they may be retrieved using SIOCGARP or SIOCGXARP.

o ARP mappings for a given IPMP group must be consistent across
the group. As a result, ARP mappings cannot be associated
with individual underlying IP interfaces in an IPMP group and
must instead be associated with the corresponding IPMP IP

o Proxy ARP mappings for an IPMP group are automatically managed
by the kernel. Specifically, if the hardware address in a
SIOCSARP or SIOCSXARP request matches the hardware address of
an IP interface in an IPMP group and the IP address is not
local to the system, the kernel regards this as a IPMP Proxy
ARP entry. This IPMP Proxy ARP entry will have its hardware
address automatically adjusted in order to keep the IP
address reachable (provided the IPMP group has not entirely

ARP performs duplicate address detection for local addresses. When a
logical interface is brought up (IFF_UP) or any time the hardware link
goes up (IFF_RUNNING), ARP sends probes (ar$spa == 0) for the assigned
address. If a conflict is found, the interface is torn down. See
ifconfig(8) for more details.

ARP watches for hosts impersonating the local host, that is, any host
that responds to an ARP request for the local host's address, and any
address for which the local host is an authority. ARP defends local
addresses and logs those with ATF_AUTHORITY set, and can tear down local
addresses on an excess of conflicts.

ARP also handles UNARP messages received from other nodes. It does not
generate these messages.


The arp driver registers itself with the netinfo interface. To gain
access to these events, a handle from net_protocol_lookup must be
acquired by passing it the value NHF_ARP. Through this interface, two
packet events are supported:

Physical in - ARP packets received via a network interface

Physical out - ARP packets to be sent out via a network interface

For ARP packets, the hook_pkt_event structure is filled out as follows:


Identifier indicating the inbound interface for packets received
with the physical in event.


Identifier indicating the outbound interface for packets received
with the physical out event.


Pointer to the start of the ARP header (not the Ethernet header).


Pointer to the start of the mblk_t chain containing the ARP packet.


Pointer to the mblk_t with the ARP header in it.


In addition to events describing packets as they move through the
system, it is also possible to receive notification of events relating to
network interfaces. These events are all reported back through the same
callback. The list of events is as follows:


A new network interface has been instantiated.


A network interface is no longer associated with ARP.


sockaddr(3SOCKET), if_tcp(4P), inet(4P), privileges(7), arp(8),
ifconfig(8), netinfo(9F)

Plummer, Dave, An Ethernet Address Resolution Protocol or Converting
Network Protocol Addresses to 48 .bit Ethernet Addresses for
Transmission on Ethernet Hardware, RFC 826, STD 0037, November 1982.

Malkin, Gary, ARP Extension - UNARP, RFC 1868, November, 1995


Several messages can be written to the system logs (by the IP module)
when errors occur. In the following examples, the hardware address
strings include colon (:) separated ASCII representations of the link
layer addresses, whose lengths depend on the underlying media (for
example, 6 bytes for Ethernet).

Node %x:%x ... %x:%x is using our IP address %d.%d.%d.%d on %s.

Duplicate IP address warning. ARP has discovered another host on a
local network that responds to mapping requests for the Internet
address of this system, and has defended the system against this node
by re-announcing the ARP entry.

%s has duplicate address %d.%d.%d.%d (in use by %x:%x ... %x:%x);

Duplicate IP address detected while performing initial probing. The
newly-configured interface has been shut down.

%s has duplicate address %d.%d.%d.%d (claimed by %x:%x ... %x:%x);

Duplicate IP address detected on a running IP interface. The conflict
cannot be resolved, and the interface has been disabled to protect
the network.

Recovered address %d.%d.%d.%d on %s.

An interface with a previously-conflicting IP address has been
recovered automatically and reenabled. The conflict has been

Proxy ARP problem? Node '%x:%x ... %x:%x' is using %d.%d.%d.%d on %s

This message appears if arp(8) has been used to create a published
permanent (ATF_AUTHORITY) entry, and some other host on the local
network responds to mapping requests for the published ARP entry.

September 2, 2015 ARP(4P)