1998-01-09 09:35:18 +03:00
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.\" $NetBSD: arp.4p,v 1.2 1998/01/09 06:35:19 perry Exp $
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.\"
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1994-06-19 04:07:16 +04:00
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.\" Copyright (c) 1983 Regents of the University of California.
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.\" All rights reserved. The Berkeley software License Agreement
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.\" specifies the terms and conditions for redistribution.
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.\"
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.\" @(#)arp.4p 6.2 (Berkeley) 5/15/86
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.\"
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.TH ARP 4P "May 15, 1986"
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.UC 5
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.SH NAME
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arp \- Address Resolution Protocol
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.SH SYNOPSIS
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.B "pseudo-device ether"
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.SH DESCRIPTION
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ARP is a protocol used to dynamically map between DARPA Internet
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and 10Mb/s Ethernet addresses. It is
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used by all the 10Mb/s Ethernet interface drivers.
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It is not specific to Internet protocols or to 10Mb/s Ethernet,
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but this implementation currently supports only that combination.
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.PP
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ARP caches Internet-Ethernet address mappings. When an interface
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requests a mapping for an address not in the cache, ARP queues the
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message which requires the mapping and broadcasts
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a message on the associated network requesting the address mapping.
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If a response is provided, the new mapping is cached and any pending
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message is transmitted.
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ARP will queue
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at most one packet while waiting for a mapping request to be responded to;
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only the most recently ``transmitted'' packet is kept.
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.PP
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To facilitate communications with systems which do not use ARP,
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.IR ioctl \^s
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are provided to enter and delete entries in the Internet-to-Ethernet tables.
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Usage:
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.LP
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.nf
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.ft B
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#include <sys/ioctl.h>
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#include <sys/socket.h>
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#include <net/if.h>
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struct arpreq arpreq;
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ioctl(s, SIOCSARP, (caddr_t)&arpreq);
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ioctl(s, SIOCGARP, (caddr_t)&arpreq);
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ioctl(s, SIOCDARP, (caddr_t)&arpreq);
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.fi
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.ft R
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Each ioctl takes the same structure as an argument.
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SIOCSARP sets an ARP entry, SIOCGARP gets an ARP entry, and SIOCDARP
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deletes an ARP entry. These ioctls may be applied to any socket descriptor
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.I s,
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but only by the super-user.
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The
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.I arpreq
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structure contains:
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.LP
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.RS
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.ta \w'#define\ \ 'u +\w'ATF_USETRAILERS\ \ 'u +\w'0x08\ \ \ \ 'u
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.nf
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/*
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* ARP ioctl request
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*/
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struct arpreq {
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struct sockaddr arp_pa; /* protocol address */
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struct sockaddr arp_ha; /* hardware address */
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int arp_flags; /* flags */
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};
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/* arp_flags field values */
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#define ATF_COM 0x02 /* completed entry (arp_ha valid) */
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#define ATF_PERM 0x04 /* permanent entry */
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#define ATF_PUBL 0x08 /* publish (respond for other host) */
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#define ATF_USETRAILERS 0x10 /* send trailer packets to host */
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.fi
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.RE
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.LP
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The address family for the
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.I arp_pa
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sockaddr must be AF_INET; for the
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.I arp_ha
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sockaddr it must be AF_UNSPEC.
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The only flag bits which may be written are ATF_PERM, ATF_PUBL
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and ATF_USETRAILERS.
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ATF_PERM causes the entry to be permanent if the ioctl call succeeds.
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The peculiar nature of the ARP tables may cause the ioctl to fail if more
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than 8 (permanent) Internet host addresses hash to the same slot.
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ATF_PUBL specifies that the ARP code should respond to ARP requests for the
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indicated host coming from other machines. This allows a host to act as an
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``ARP server,'' which may be useful in convincing an ARP-only machine to talk
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to a non-ARP machine.
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.PP
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ARP is also used to negotiate the use of trailer IP encapsulations;
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trailers are an alternate encapsulation used to allow efficient packet
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alignment for large packets despite variable-sized headers.
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Hosts which wish to receive trailer encapsulations so indicate
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by sending gratuitous ARP translation replies along with replies
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to IP requests; they are also sent in reply to IP translation replies.
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The negotiation is thus fully symmetrical, in that either or both hosts
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may request trailers.
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The ATF_USETRAILERS flag is used to record the receipt of such a reply,
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and enables the transmission of trailer packets to that host.
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.PP
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ARP watches passively for hosts impersonating the local host (i.e. a host
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which responds to an ARP mapping request for the local host's address).
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.SH DIAGNOSTICS
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.B "duplicate IP address!! sent from ethernet address: %x:%x:%x:%x:%x:%x."
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ARP has discovered another host on the local network which responds to
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mapping requests for its own Internet address.
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.SH SEE ALSO
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ec(4), de(4), il(4), inet(4F), arp(8C), ifconfig(8C)
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.br
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``An Ethernet Address Resolution Protocol,'' RFC826, Dave Plummer,
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Network Information Center, SRI.
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.br
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``Trailer Encapsulations,'' RFC893, S.J. Leffler and M.J. Karels,
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Network Information Center, SRI.
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.SH BUGS
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ARP packets on the Ethernet use only 42 bytes of data; however, the smallest
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legal Ethernet packet is 60 bytes (not including CRC).
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Some systems may not enforce the minimum packet size, others will.
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