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@(#)arp4.4 6.3 (Berkeley) 09/19/88
ARP caches Internet-Ethernet 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. 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 \^s are provided to enter and delete entries in the Internet-to-Ethernet 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) */
#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 non-ARP 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).
``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.