.\"	$NetBSD: arp.9,v 1.27 2019/09/01 14:49:11 sevan Exp $
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Ignatios Souvatzis.
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.Dd September 1, 2019
.Dt ARP 9
.Os
.Sh NAME
.Nm arp ,
.Nm arp_ifinit ,
.Nm arpresolve ,
.Nm arpintr
.Nd externally visible ARP functions
.Sh SYNOPSIS
.In netinet/if_inarp.h
.Ft void
.Fn arp_ifinit "struct ifnet *ifp" "struct ifaddr *ifa"
.Ft int
.Fn arpresolve "struct ifnet *ifp" "const struct rtentry *rt" "struct mbuf *m" "const struct sockaddr *dst" "void *desten" "size_t destlen"
.Ft void
.Fn arpintr
.Sh DESCRIPTION
The
.Nm
functions provide the interface between the
.Nm
module and the network drivers which need
.Nm
functionality.
Such drivers must request the
.Ar arp
attribute in their "files" declaration.
.Bl -tag -width "arp_ifinit()"
.It Fn arp_ifinit
Sets up the
.Nm
specific fields in
.Fa ifa .
Additionally, it sends out a gratuitous
.Nm
request on
.Fa ifp ,
so that other machines are warned that we have a (new) address and
duplicate addresses can be detected.
.Pp
You must call this in your drivers' ioctl function when you get a
SIOCSIFADDR request with an AF_INET address family.
.It Fn arpresolve
is called by network output functions to resolve an IPv4 address.
If no
.Fa rt
is given, a new one is looked up or created.
If the passed or found
.Fa rt
does not contain a valid gateway link level address, a pointer to the packet
in
.Fa m
is stored in the route entry, possibly replacing older stored packets, and an
.Nm
request is sent instead.
When an
.Nm
reply is received, the last held packet is send.
Otherwise, the looked up address is returned and written into the storage
.Fa desten
points to.
.Fn arpresolve
returns 1, if a valid address was stored to
.Fa desten ,
and the packet can be sent immediately.
Else a 0 is returned.
.It Fn arpintr
When an
.Nm
packet is received, the network driver (class) input interrupt handler queues
the packet on the arpintrq queue, and requests an
.Fn arpintr
soft interrupt callback.
.Fn arpintr
dequeues the packets, performs sanity checks and calls (for IPv4
.Nm
packets, which are the only ones supported currently) the
.Fn in_arpinput
function.
.Fn in_arpinput
either generates a reply to request packets, and adds the sender address
translation to the routing table, if a matching route entry is found.
If the route entry contained a pointer to a held packet, that packet is
sent.
.El
.Sh CODE REFERENCES
The ARP code is implemented in
.Pa sys/net/if_arp.h ,
.Pa sys/netinet/if_inarp.h
and
.Pa sys/netinet/if_arp.c .
.Sh SEE ALSO
.Xr ether_ifattach 9
.\" .Xr fddi_ifattach 9 ,
.\" .Xr arc_ifattach 9
.Pp
Plummer, D., "RFC826", An Ethernet Address Resolution Protocol.
.Sh STANDARDS
RFC 826
.Sh HISTORY
Rewritten to support other than Ethernet link level addresses in
.Nx 1.3 .
.Sh AUTHORS
.An UCB CSRG
(original implementation)
.An Ignatios Souvatzis
(support for non-Ethernet)