ARP(7) DEVICES AND MODULES ARP(7)
NAME
ARP - Address Resolution Protocol
SYNOPSIS
#include <sys/socket.h>
#include <net/ifarp.h>
#include <netinet/in.h>
s = socket(AFINET, SOCKDGRAM, 0);
d = open ("/dev/arp", ORDWR);
DESCRIPTION
ARP is a protocol used to map dynamically between Internet
Protocol (IP) and 10Mb/s Ethernet addresses. It is used by
all the 10Mb/s Ethernet datalink providers (interface
drivers). It is not specific to the Internet Protocol or to
the 10Mb/s Ethernet, but this implementation currently sup-
ports only that combination. The STREAMS device /dev/arp is
not a Transport Level Interface (TLI) transport provider and
may not be used with the TLI interface.
ARP caches IP-to-Ethernet address mappings. When an inter-
face 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, the new mapping is
cached and any pending message is transmitted. 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, ioctl() requests are provided to enter and delete
entries in the IP-to-Ethernet tables.
USAGE
#include <sys/sockio.h>
#include <sys/socket.h>
#include <net/if.h>
#include <net/ifarp.h>
struct arpreq arpreq;
ioctl(s, SIOCSARP, (caddrt)&arpreq);
ioctl(s, SIOCGARP, (caddrt)&arpreq);
ioctl(s, SIOCDARP, (caddrt)&arpreq);
Each ioctl() request takes the same structure as an argu-
ment. SIOCSARP sets an ARP entry, SIOCGARP gets an ARP
entry, and SIOCDARP deletes an ARP entry. These ioctl()
requests may be applied to any Internet family socket
descriptor s, or to a descriptor for the ARP device, but
only by the privileged user. The arpreq structure contains:
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ARP(7) DEVICES AND MODULES ARP(7)
/*
* ARP ioctl request
*/
struct arpreq {
struct sockaddr arppa; /* protocol address */
struct sockaddr arpha; /* hardware address */
int arpflags; /* flags */
};
/* arpflags field values */
#define ATFCOM 0x2 /* completed entry (arpha valid) */
#define ATFPERM 0x4 /* permanent entry */
#define ATFPUBL 0x8 /* publish (respond for other host) */
#define ATFUSETRAILERS 0x10 /* send trailer packets to host */
The address family for the arppa sockaddr must be AFINET;
for the arpha sockaddr it must be AFUNSPEC. The only flag
bits that may be written are ATFPERM, ATFPUBL and
ATFUSETRAILERS. ATFPERM makes the entry permanent if the
ioctl() request succeeds. The peculiar nature of the ARP
tables may cause the ioctl() request to fail if too many
permanent IP addresses hash to the same slot. ATFPUBL
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 non-ARP
machine.
ARP is also used to negotiate the use of trailer IP encapsu-
lations; trailers are an alternate encapsulation used to
allow efficient packet alignment for large packets despite
variable-sized headers. Hosts that wish to receive trailer
encapsulations so indicate by sending gratuitous ARP trans-
lation replies along with replies to IP requests; they are
also sent in reply to IP translation replies. The negotia-
tion is thus fully symmetrical, in that either or both hosts
may request trailers. The ATFUSETRAILERS 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
(that is, a host which responds to an ARP mapping request
for the local host's address).
SEE ALSO
arp(1M), ifconfig(1M), if(3N), inet(7).
Plummer, Dave, ``An Ethernet Address Resolution Protocol
-or- Converting Network Protocol Addresses to 48.bit Ether-
net Addresses for Transmission on Ethernet Hardware,'' RFC
826, Network Information Center, SRI International, Menlo
Park, Calif., November 1982.
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ARP(7) DEVICES AND MODULES ARP(7)
Leffler, Sam, and Michael Karels, ``Trailer Encapsula-
tions,'' RFC 893, Network Information Center, SRI Interna-
tional, Menlo Park, Calif., April 1984.
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