Document our RFC 5227 implementation.

usr.sbin/arp/arp.4

@@ -1,4 +1,4 @@
-.\"	$NetBSD: arp.4,v 1.12 2016/04/04 13:32:37 wiz Exp $
+.\"	$NetBSD: arp.4,v 1.13 2016/10/12 08:43:17 roy Exp $
 .\"
 .\" Copyright (c) 1985, 1986, 1988, 1994
 .\"	The Regents of the University of California.  All rights reserved.
@@ -29,7 +29,7 @@
 .\"
 .\"	from: @(#)arp4.4	6.5 (Berkeley) 4/18/94
 .\"
-.Dd March 30, 2016
+.Dd October 12, 2016
 .Dt ARP 4
 .Os
 .Sh NAME
@@ -80,16 +80,31 @@ as if it were the target of the request.
 In the past,
 ARP was used to negotiate the use of a trailer encapsulation.
 This is no longer supported.
-.Pp
-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).
 .Sh DIAGNOSTICS
 .Bl -diag
-.It "duplicate IP address %x sent from ethernet address %x:%x:%x:%x:%x:%x."
-ARP has discovered another host on the local network which responds to
-mapping requests for its own Internet address with a different Ethernet
-address, generally indicating that two hosts are attempting to use the
-same Internet address.
+ARP implements Address Conflict Detection.
+When an address is first added to the host, it marked tentative and
+ARP probes the network to discover if another host has the address.
+If another host replies with the same address, then the local address is
+marked duplicate and the host will not use it. Otherwise the tentative
+mark is removed and the host can start using the address.
+.Pp
+ARP will defend the host's active address when a conflicting message is
+received.
+However, if another conflicting message for the address is found within
+a 10 second period, then the address is marked duplicate and the host will
+stop using it.
+.Pp
+For some systems such as a router or a server,
+it is desirable never to give up an assigned address.
+This can be achieved by setting the
+.Xr sysctl 7
+variable
+.Dv net.inet.ip_dad_count
+to 0.
+.Pp
+In all of the above cases, ARP will log diagnostic messages which include
+the hardware address of the conflicting host.
 .El
 .Sh SEE ALSO
 .Xr inet 4 ,
@@ -109,7 +124,15 @@ same Internet address.
 .%B "Trailer Encapsulations"
 .%T RFC 893
 .Re
+.Rs
+.%A Cheshire, S.
+.%B "IPv4 Address Conflict Detection"
+.%T RFC 5227
+.Re
 .Sh HISTORY
 Since
 .Nx 8.0 ,
 the ARP cache was not stored in the routing table.
+.Pp
+Address Conflict Detection was added in
+.Nx 8.0 .