ARP(7P) ARP(7P)
arp - Address Resolution Protocol
ARP is a protocol that provides a dynamic mapping from an IP address to
the corresponding physical network address. The 32-bit IP addresses only
make sense to the TCP/IP protocol suite. A physical network such as an
Ethernet or a token ring has it own addressing scheme (often 48-bit
addresses) to which any network layer using the physical network must
conform. Two machines on a given physical network can communicate only
if they know each other's physical network address. ARP provides a
mapping between the two different forms of addresses.
ARP caches IP-physical 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. Each address
mapping has a timer associated with it and completed address mappings are
aged after 20 minutes. The timer is reset each time the address mapping
is updated by a SIOCSARP ioctl call or by sending a packet to the IP
address to which the mapping belongs. This timer value is not
configurable. ARP will queue at most one packet while waiting for a
mapping request to be responded to; only the most recently
``transmitted'' packet is kept.
To facilitate communications with systems which do not use ARP, ioctls
are provided to enter and delete entries in the IP-to-physical address
tables. Usage:
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <net/if.h>
struct arpreq arpreq;
ioctl(s, SIOCSARP, (caddr_t)&arpreq);
ioctl(s, SIOCGARP, (caddr_t)&arpreq);
ioctl(s, SIOCDARP, (caddr_t)&arpreq);
Each ioctl takes the same structure as an argument. SIOCSARP sets an ARP
entry, SIOCGARP gets an ARP entry, and SIOCDARP deletes an ARP entry.
These ioctls may be applied to any socket descriptor s, but only by the
super-user. 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 */
};
/* arp_flags field values */
#define ATF_COM 0x02 /* completed entry (arp_ha valid) */
Page 1
ARP(7P) ARP(7P)
#define ATF_PERM 0x04 /* permanent entry */
#define ATF_PUBL 0x08 /* publish (respond for other host) */
#define ATF_USETRAILERS 0x10 /* send trailer packets to host */
The address family for the arp_pa sockaddr must be AF_INET; for the
arp_ha sockaddr it must be AF_UNSPEC. The only flag bits which may be
written are ATF_PERM, ATF_PUBL and ATF_USETRAILERS. ATF_PERM causes the
entry to be permanent if the ioctl call succeeds. The peculiar nature of
the ARP tables may cause the ioctl to fail if more than 8 (permanent)
Internet host addresses hash to the same slot. ATF_PUBL specifies that
the ARP code should respond to ARP requests for the indicated host 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 nonARP
machine.
ARP is also used to negotiate the use of trailer IP encapsulations;
trailers are an alternate encapsulation used to allow efficient packet
alignment for large packets despite variable-sized headers. Hosts which
wish to receive trailer encapsulations so indicate by sending gratuitous
ARP translation replies along with replies to IP requests; they are also
sent in reply to IP translation replies. The negotiation is thus fully
symmetrical, in that either or both hosts may request trailers. The
ATF_USETRAILERS flag is used to record the receipt of such a reply, and
enables the transmission of trailer packets to that host.
ARP watches passively for hosts impersonating the local host (i.e. a host
which responds to an ARP mapping request for the local host's address).
The following messages can appear on the console:
arp: host with ether address %x:%x:%x:%x:%x:%x is using my IP address
x.x.x.x
ARP has discovered another host on the local network which responds to
mapping requests for its own Internet address.
arp: ether address is broadcast for IP address x.x.x.x
ARP has discovered another host on the local network which maps that
host's IP address onto the ethernet broadcast address.
inet(7F), arp(1M), ifconfig(1M), intro(3)
``An Ethernet Address Resolution Protocol,'' RFC826, Dave Plummer,
Network Information Center, SRI.
``Trailer Encapsulations,'' RFC893, S.J. Leffler and M.J. Karels, Network
Information Center, SRI.
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